Murat Gül

Giorgio Ivan Russo

Hussein Kandil

Florence Boitrelle

Ramadan Saleh

Eric Chung

Parviz Kavoussi

Taymour Mostafa

Rupin Shah

Ashok Agarwal

Male Infertility: New Developments, Current Challenges, and Future Directions

Keywords: New Developments, Current Challenges, and Future Directions DNA fragmentation, Epigenomics, Infertility, male, Spermatozoa

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Infertility is a common medical problem, with the male factor being involved in approximately 50% of cases [START_REF]WHO laboratory manual for the examination and processing of human semen[END_REF][START_REF] Inhorn | Infertility around the globe: new thinking on gender, reproductive technologies and global movements in the 21st century[END_REF]. Male infertility has been linked to numerous genetic and lifestyle factors, but approximately 30% of cases are still recognized as idiopathic [START_REF] Sinclair | Male infertility: nutritional and environmental considerations[END_REF]. Over the last 40 years, developments in male infertility diagnostics, such as genomics and molecular testing, have helped elucidate etiologies for what was previously considered unexplained infertility. However, there is a need for new research to fill the gaps in knowledge on the etiologies of male infertility and to offer definite solutions for many of these cases that are still untreatable. This review aims to identify areas of deficiency in the knowledge, discuss new diagnostic methods and thera-

TOWARDS A BETTER UNDERSTANDING OF THE ETIOPATHOGENESIS OF MALE INFERTILITY

Different factors are implicated in spermatogenic aberration and consequently male infertility, including environmental, genetic, inflammatory, infective, drug-induced, and hormonal disorders, in addition to anatomic etiologies such as varicoceles and reproductive tract obstructions [START_REF] Vander Borght | Fertility and infertility: definition and epidemiology[END_REF]. Varicocele is considered the most common correctable cause of male infertility with an incidence of approximately 35% among men with primary infertility and 70% to 80% among men with secondary infertility [START_REF] Alsaikhan | Epidemiology of varicocele[END_REF]. Currently, many studies indicate a correlation between varicocele and the progressive decline in testicular function. Hypotheses for the induction of spermatogenic dysfunction induced by varicocele include elevated intra-scrotal temparature, oxidative stress (OS), seminiferous tubules hypoxia, venous reflux, backflow of adrenal metabolites, and sperm DNA fragmentation (SDF) [START_REF] Baazeem | Varicocele: how this condition and its management affects men's health[END_REF]. A recent systematic review and meta-analysis supported the relationship between varicocele repair and improved semen parameters [START_REF] Agarwal | Impact of varicocele repair on semen parameters in infertile men: a systematic review and meta-analysis[END_REF], while a study conducted by Panach-Navarrete et al [START_REF] Panach-Navarrete | Is there a relationship between varicocele and testosterone levels?[END_REF] concluded that varicocele repair improves semen analysis (SA) parameters only in patients with decreased baseline parameters. Further, a metaanalysis from Chen et al [START_REF] Chen | Efficacy of varicocelectomy in the treatment of hypogonadism in subfertile males with clinical varicocele: a meta-analysis[END_REF] reported increased testosterone levels in hypogonadal males who underwent varicocele repair but there is still controversy about the impact of the grades of varicocele, size of testes, presence of other medical comorbidities or duration of varicocele on testosterone production.

An increase in endocrine-mediated environmental and lifestyle factors having a detrimental impact on male fertility has been reported [START_REF] Fallara | Male factor infertility trends throughout the last 10 years: report from a tertiary-referral academic andrology centre[END_REF]. A significantly lower total sperm count has been reported among e-cigarette and cigarette smokers [START_REF] Sharma | Cigarette smoking and semen quality: a new meta-analysis examining the effect of the 2010 World Health Organization laboratory methods for the examination of human semen[END_REF]. A study by Holmboe et al [12] revealed that compared to non-smokers, both daily e-cigarette users, and daily cigarette smokers had significantly lower total sperm counts (147 million vs. 91 million and 139 million vs. 103 million respectively) in an adjusted analysis. However, higher levels of total and free testosterone were observed in cigarette users only and no association was observed in e-cigarette smokers [12]. Although clinically it is well accepted that smoking has an adverse impact on spermatozoa, more research on the impact of smoking and e-cigarettes and also of passive smoking on male fertility is warranted. Similarly, several population-based surveys reported an increasing percentage of altered semen parameters among overweight males [START_REF] Amiri | Potential adverse effects of female and male obesity on fertility: a narrative review[END_REF]. Few mechanisms that alter reproductive function among obese males have been reported, including increased serum estradiol levels and a higher level of serum leptin, which directly results in testosterone downregulation, erectile dysfunction, and higher inflammatory mediators [START_REF] Liu | Obesity, a serious etiologic factor for male subfertility in modern society[END_REF][START_REF] Katib | Mechanisms linking obesity to male infertility[END_REF]. This data has brought attention to the consideration of nutrigenomics in male infertility where interaction between nutrients, diet, and various genes expression may play an important role in health and development. In rats, a high-fat diet has been shown to influence preimplantation embryo gene expression, fetal growth of the offspring, and the metabolism of the adult [START_REF] De Castro Barbosa | High-fat diet reprograms the epigenome of rat spermatozoa and transgenerationally affects metabolism of the offspring[END_REF]. In fact, according to a recent study by Cannarella et al [START_REF] Cannarella | Testicular volume in 268 children and adolescents followed-up for childhood obesitya retrospective cross-sectional study[END_REF], male obesity is related to a mutated sperm DNA methylation sequence that seems to involve reprogramming fidelity in a set of genes. Another recent study suggests that overweight boys are more likely to be infertile men. This study revealed that obese pre-pubertal boys had smaller testicles than normal pre-pubertal boys who were not obese [START_REF] Cannarella | Testicular volume in 268 children and adolescents followed-up for childhood obesitya retrospective cross-sectional study[END_REF].

Together with the lifestyle impact on male factor fertility, there is a growing concern about the global reduction of semen quality due to environmental pollution influenced by different endocrine-disrupting factors affecting male reproductive system [START_REF] Pizzorno | Environmental toxins and infertility[END_REF][START_REF] Cannarella | Temporal decline of sperm concentration: role of endocrine disruptors[END_REF]. Environmental factors can also cause epigenomic changes through DNA methylation, histone modifications, and non-coding (ncRNAs) that play a key role in the proper functioning of cells, including spermatozoa [START_REF] Cescon | Environmental impact on male (in)fertility via epigenetic route[END_REF].

Lately, data emerging from the COVID-19 pandemic indicate that male patients account for 56% to 73% of the infected population [START_REF] Chen | Clini-cal characteristics of 113 deceased patients with coronavirus disease 2019: retrospective study[END_REF][START_REF] Huang | Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China[END_REF][START_REF] Xu | Clinical findings in a group of patients infected with the 2019 novel coronavirus (SARS-Cov-2) outside of Wuhan, China: retrospective case series[END_REF]. SARS-CoV-2-infected males were found to have higher morbidity and mortality rates than age-matched females, suggesting sexbased differences in the prevalence and severity of COVID-19 [START_REF] Peckham | Male sex identified by global COVID-19 meta-analysis as a risk factor for death and ITU admission[END_REF]. For the SARS-CoV-2 virus, the mechanism of cellular entry has been identified as the interaction between the SARS-CoV-2 viral spike protein www.wjmh.org and angiotensin-converting enzyme 2 (ACE2) on cells co-expressing ACE2 and the cellular transmembrane protease serine-2 [START_REF] Jackson | Mechanisms of SARS-CoV-2 entry into cells[END_REF][START_REF] Collins | Impact of COVID-19 on male fertility[END_REF]. The testis has high levels of ACE2 expression [START_REF] Tur-Kaspa | COV-ID-19 may affect male fertility but is not sexually transmitted: a systematic review[END_REF]. The ACE2 converts Angiotensin II to Angiotensin 1-7 in Leydig cells and adjusts the production of testosterone and consequently may contribute to spermatogenesis modulation, which suggests a potential for the negative influence of the virus on male fertility [START_REF] Pan | Angiotensin-converting enzymes play a dominant role in fertility[END_REF]. A Belgian study suggests that even men who contract COVID-19 with very mild illness and are afebrile may have an impact on their semen parameters and SDF for 3 to 6 months due to an inflammatory response [START_REF] Donders | Sperm quality and absence of SARS-CoV-2 RNA in semen after COVID-19 infection: a prospective, observational study and validation of the SpermCOVID test[END_REF]. In a small number of fatal cases of COVID-19, pathology at the time of autopsy has demonstrated orchitis, basal membrane thickening, vascular changes, scarcity of Leydig cells and Sertoli cells, and reduced spermatogenesis. Additional research is needed to detect the long-term impact of COVID-19 infection on male reproductive health, especially in cases that were not severe.

Existing evidence suggest that OS may play a role in male infertility. The term Male Oxidative Stress Infertility (MOSI) has been suggested for diagnosing a subset of infertile males with abnormal semen parameters, previously described as idiopathic [START_REF] Tremellen | Oxidative stress and male infertility--a clinical perspective[END_REF]. So far, although the measurement of OS in semen is not used routinely, the introduction of novel technologies that promptly detect seminal OS via assessment of oxidation-reduction potential (ORP) using a bench-top analyzer permits an accurate and cost-effective diagnosis of MOSI [START_REF] Agarwal | Role of oxidative stress, infection and inflammation in male infertility[END_REF]. Varicoceles are known to induce OS and varicocele repair has been demonstrated to reduce OS [START_REF] Kavoussi | Varicocele repair improves static oxidation reduction potential as a measure of seminal oxidative stress levels in infertile men: a prospective clinical trial using the MiOXSYS system[END_REF].

MALE INFERTILITY DIAGNOSTICS 1. The 6th edition of the WHO manual for semen analysis

The recommendations for the initial assessment of the infertile couple have been recently updated [START_REF] Schlegel | Diagnosis and treatment of infertility in men: AUA/ASRM guideline part I[END_REF]. Along with a thorough history and physical examination, it is recommended that a SA be ordered. Since 1980, the WHO has attempted to standardize the methodology of semen examination through six editions of laboratory manuals to examine human semen with the latest (6th) edition being published in July 2021 [34].

The main novelty in the 6th edition lies in the absence of recommended SA reference values. In the 6th edition, the 5th percentile values of basic semen param-eters are provided and vary very slightly from the 5th edition. These 5th percentile values were established by analyzing the semen parameters of 3,989 males who were able to initiate a natural pregnancy with a time to pregnancy of less than 12 months [START_REF]WHO laboratory manual for the examination and processing of human semen[END_REF]. However, the 6th edition specifies that these 5th percentile values are only one way to assess the male fertility potential [START_REF]WHO laboratory manual for the examination and processing of human semen[END_REF]34]. Certainly, semen parameters and their thresholds alone are not sufficient to predict the couples' fertility potential due to the complex and multifactorial nature of fertility.

Testing for SDF is indicated to complete the diagnostic evaluation in specific circumstances. SDF testing has been described as an "extended examination" of semen in the 6th edition of the WHO Manual of Human Semen Analysis [START_REF]WHO laboratory manual for the examination and processing of human semen[END_REF]. However, the 6th edition neither provides indications for SDF testing nor does it address the variability of test results with different assays. Therefore, clinicians need to rely on the recommendations available in recent systematic reviews, meta-analyses, and guidelines regarding the causes of SDF, the indications for SDF testing, and possible treatments in patients with high SDF levels [START_REF] Agarwal | Sperm DNA fragmentation: a new guideline for clinicians[END_REF][START_REF] Agarwal | Sperm DNA fragmentation: a critical assessment of clinical practice guidelines[END_REF][START_REF] Esteves | Sperm DNA fragmentation testing: summary evidence and clinical practice recommendations[END_REF][START_REF] Farkouh | Beyond conventional sperm parameters: the role of sperm DNA fragmentation in male infertility[END_REF][START_REF] Tharakan | EAU Working Panel on Male Sexual Reproductive Health. European Association of Urology guidelines panel on male sexual and reproductive health: a clinical consultation guide on the indications for performing sperm DNA fragmentation testing in men with infertility and testicular sperm extraction in nonazoospermic men[END_REF].

In the research section (advanced SA), the 6th edition of the WHO manual lists some tests to assess seminal OS [START_REF]WHO laboratory manual for the examination and processing of human semen[END_REF]. Despite a growing number of publications, international societies have not yet taken up this subject and the role of OS on male fertility is still not given due importance, perhaps because testing of OS is variable and may lack standardization. Well-designed studies will undoubtedly improve the utility of these tests and define their usefulness in managing male infertility in the future.

Finally, the new 6th edition of the WHO manual of SA does not detail all the new tests available for genetic and epigenetic diagnosis. This is likely due to the uncertainty of the indications and clinical utility of these tests currently. Given the cost and the complexity of implementation and clinical interpretation of these genetic and epigenetic tests, recommendations are difficult to be established from the literature. While recent guidelines recommend karyotyping and Y-chromosome microdeletions in the work-up of nonobstructive causes of severe oligozoospermia and azoospermia [START_REF] Schlegel | Diagnosis and treatment of infertility in men: AUA/ASRM guideline part I[END_REF], the clinical indications for whole exome or genome studies, seminal microRNA, DNA methylation, or histone post-translational modification tests have not yet been determined. Regarding genetic test-ing alone, literature reviews are regularly published, and a multiple number of genes have been described in male infertility [START_REF] Krenz | The male fertility gene atlas: a web tool for collecting and integrating OMICS data in the context of male infertility[END_REF][START_REF] Omolaoye | Omics and male infertility: highlighting the application of transcriptomic data[END_REF]. However, it seems premature to recommend these genomic or pan-genomic analyses to the "general" infertile population. Their routine screening is not yet applicable in andrology and assisted reproductive technology (ART) centers around the world, but with further advances in the field, these tests may become routine in the evaluation of infertile males in future.

The strength of the WHO 6th edition manual is in its excellence as a technical guide. It is important that the technical recommendations are actually followed by andrology laboratories around the world to ensure the quality, consistency, and reproducibility of testing from one laboratory to another. Clinically, what is considered a weakness of the manual is its lack of criteria for the clinical interpretation of these tests. It is clear that the literature does not allow the determination of reference standards/thresholds according to, for example, the andrological pathology presented by the infertile man, nor standards/thresholds for the selection of a particular ART technique.

Use of artificial intelligence in sperm analysis

Evaluation of the infertile male is governed by the data obtained from conventional semen parameters, which have limited ability to assess male fertility despite the intensive laboratory skills needed. This has prompted scientists to develop computational methods to replace manual alternatives while studying the possibility of incorporating artificial intelligence (AI) within the scope of andrology [START_REF] Wang | Artificial intelligence in reproductive medicine[END_REF][START_REF] Yu | Emerging technologies for home-based semen analysis[END_REF][START_REF] Holt | Reproducibility of computer-aided semen analysis: comparison of five different systems used in a practical workshop[END_REF][START_REF] Lamb | Artificial intelligence in medicine and male infertility[END_REF][START_REF] Mahmoud | Performance of the sperm quality analyser in predicting the outcome of assisted reproduction[END_REF]. The unprecedented increase in complex medical data surpasses the capacity of the basic statistical models to deduce the desired information. Hence, AI has been considered for using different complex algorithms in appraising a relationship between different variables related to fertility [START_REF] Wang | Artificial intelligence in reproductive medicine[END_REF]. In a multi-institutional study by Ory et al [START_REF] Ory | Artificial intelligence based machine learning models predict sperm parameter upgrading after varicocele repair: a multi-institutional analysis[END_REF], a machine learning (ML) model successfully predicted subsequent upgrades in sperm parameters in 87% of men (area under the curve [AUC]=0.72) following varicocele repair. Another example of an AI model is Bemaner (Shenzhen Createcare Technology Co.), a smartphone application that measures sperm motility at home, capturing and uploading videos assessed by an AI algorithm of image recognition. Bemaner's results of sperm analysis were compared to grades offered by experienced andrologists and showed a strong correlation between total and motile sperm concentration (r=0.65, p<0.001; r=0.84, p<0.001, respectively) and percentage of motility (r=0.90, p<0.001) [START_REF] Tsai | Weband artificial intelligence-based image recognition for sperm motility analysis: verification study[END_REF]. However, there is a concern of providing false reassurance without a complete formal SA including findings such as the presence of leukocytes, sperm agglutination, or other microscopic clues of potential pathology. The same principle has been explored by Kobori et al [START_REF] Kobori | Novel device for male infertility screening with single-ball lens microscope and smartphone[END_REF], who used a computer-assisted semen analyzer for both comparison and validation.

Another recent advance is the use of AI methods which are particularly objective and suitable for video images [START_REF] Hicks | Machine learning-based analysis of sperm videos and participant data for male fertility prediction[END_REF][START_REF] Riegler | Artificial intelligence in the fertility clinic: status, pitfalls and possibilities[END_REF]. These correspond to ML, a sub-field of AI. These methods promise to improve intracytoplasmic sperm injection (ICSI) by guiding clinicians to objectively select the optimum sperm [START_REF] Riegler | Artificial intelligence in the fertility clinic: status, pitfalls and possibilities[END_REF][START_REF] You | Machine learning for sperm selection[END_REF]. Some AI devices have been developed to analyze sperm morphology [START_REF] Chandra | Prolificacy assessment of spermatozoan via state-of-theart deep learning frameworks[END_REF][START_REF] Ilhan | Sperm morphology analysis by using the fusion of two-stage fine-tuned deep networks[END_REF][START_REF] Ilhan | A fully automated hybrid human sperm detection and classification system based on mobile-net and the performance comparison with conventional methods[END_REF][START_REF] Movahed | Automatic segmentation of sperm's parts in microscopic images of human semen smears using concatenated learning approaches[END_REF]. Recent morphological assessment models based on unstained sperm images are being developed to improve the ICSI technique by classifying images in real-time [START_REF] Abbasi | Effect of deep transfer and multi-task learning on sperm abnormality detection[END_REF][START_REF] Javadi | A novel deep learning method for automatic assessment of human sperm images[END_REF]. In a recent review, it was stated that the currently existing models are restricted in their capabilities, necessitating the need to develop specific models tailored to andrology [START_REF] Ghayda | Global Andrology Forum. Artificial intelligence in andrology: from semen analysis to image diagnostics[END_REF]. While the implementation of AI in andrology holds promise, it encounters various challenges. Firstly, data sources often lack accuracy or completeness. Secondly, the absence of standardized protocols hinders the deployment of the limited number of available AI models. Moreover, the approval process for AI in medical applications lacks consistency across different governing bodies. Thirdly, there is a concern that AI might restrict patient care autonomy. Currently, most clinicians place importance on a patient-focused and evidence-based approach to decision-making. However, a recent survey involving German healthcare providers indicates an inclination to embrace AI technology in medical care [START_REF] Maassen | Future medical artificial intelligence application requirements and expectations of physicians in German university hospitals: web-based survey[END_REF]. Fourthly, the cost of developing and validating AI models in andrology poses a significant challenge due to limited funding availability. This is especially pronounced considering the relatively small size of the andrology specialty compared to other medical fields. Lastly, ethical concerns arise regarding the suitability of AI models for all patients, particularly when considerable individual variations are expected. Indeed, an ethical dilemma arises with the introduction of novel and exwww.wjmh.org pensive tools, for example, in concerning the fair allocation of payment and beneficiaries of this technology, and in considering disparities in insurance and financial resources across the medical landscape in various countries [START_REF] Ghayda | Global Andrology Forum. Artificial intelligence in andrology: from semen analysis to image diagnostics[END_REF].

Home testing of semen

Many males experience significant pressure when asked to deliver a semen sample in the laboratory premises, a fact that encouraged the idea of developing semen home collection kits, offering a more convenient way of collection. Many products have been implemented and studied to replace the conventional lab-based collection model [START_REF] Yu | Emerging technologies for home-based semen analysis[END_REF]. The Food and Drug Administration has approved many at-home sperm testing products based on their accuracy and ease of use, including SpermCheck ® , YO ® , and Trak ® . The Sperm-Check ® utilizes sperm-specific monoclonal antibodies and offers an accuracy of 97% to 98% when compared with trained laboratory professionals [START_REF] Yu | Emerging technologies for home-based semen analysis[END_REF][START_REF] Coppola | SpermCheck fertility, an immunodiagnostic home test that detects normozoospermia and severe oligozoospermia[END_REF]. The YO ® system connects a smartphone camera to the sample examination station and evaluates motile sperm concentration with an accuracy between 97.2% and 98.3%, according to the type of smartphone used [START_REF] Onofre | Simplified sperm testing devices: a possible tool to overcome lack of accessibility and inconsistency in male factor infertility diagnosis. An opportunity for lowand middle-income countries[END_REF]. The Trak ® system comprises of a portable device used to assess sperm count using centrifugal motion and claims an accuracy of 93.3%, 82.4%, and 95.5% for results categorized as ≤15 million/mL, 15-55 million/mL, and >55 million/mL, respectively [START_REF] Yu | Emerging technologies for home-based semen analysis[END_REF][START_REF] Schaff | Novel centrifugal technology for measuring sperm concentration in the home[END_REF]. On the other hand, the Micra Sperm Test is a thirty-minute home-based sperm testing product measuring semen volume, sperm count and motility, but the results are subjected to variability [START_REF] Yu | Emerging technologies for home-based semen analysis[END_REF][START_REF] Onofre | Simplified sperm testing devices: a possible tool to overcome lack of accessibility and inconsistency in male factor infertility diagnosis. An opportunity for lowand middle-income countries[END_REF].

Whole genome testing

Many genetic defects have been identified following the innovative emergence of next-generation sequencing (NGS) [START_REF] Sun | Next-generation diagnostics: gene panel, exome, or whole genome?[END_REF]. The exome, which represents 1% of the human genome, consists of 180,000 exons [START_REF] Kolmykov | Whole-exome sequencing analysis of human semen quality in Russian multiethnic population[END_REF]. Currently, complex whole exome sequencing represents the diagnostic tool of choice, but it is believed that whole exome sequencing (WES) will be replaced by whole genome sequencing (WGS), which has a lower cost and incorporates practical software facilitating the interpretation of results [START_REF] Ghieh | Will whole-genome sequencing become the first-line genetic analysis for male infertility in the near future?[END_REF]. Practically, this has helped in the understanding of several male infertility states; for instance, several genes have been identified in the context of non-obstructive azoospermia (NOA), includ-ing FANCA, PLK4, WKN3, MEI1, ADAD2, and TEX11 [START_REF] Ghieh | Will whole-genome sequencing become the first-line genetic analysis for male infertility in the near future?[END_REF][START_REF] Cioppi | Genetics of azoospermia[END_REF].

Epigenetic markers

Many epigenetic markers have been considered in assessing the presence of active spermatogenesis among NOA patients. For instance, ESX1 transcript was identified in approximately 95% (62 out of 65 samples) of males with the presence of spermatogenesis in testicular tissue [START_REF] Bonaparte | ESX1 gene expression as a robust marker of residual spermatogenesis in azoospermic men[END_REF]. Additionally, a study by Yao et al [START_REF] Yao | Distinct expression profiles and novel targets of MicroRNAs in human spermatogonia, pachytene spermatocytes, and round spermatids between OA patients and NOA patients[END_REF] demonstrated 396, 395, and 378 microRNAs that were differentially expressed in the spermatogonia, pachytene spermatocytes, and round spermatids, respectively, between NOA and patients with obstructive azoospermia. It is suggested that epigenetic markers may help resolve much of the current uncertainty that surrounds the prediction of the presence or absence of spermatogenesis in azoospermic men.

Seminal proteomics

Studying the seminal plasma is a novel approach that supports the management of male infertility since it is rich in protein biomarkers at a concentration of 35-55 mg/mL, with semenogelins and kallikrein 3 being two examples of highly abundant seminal proteins [START_REF] Pilch | Large-scale and high-confidence proteomic analysis of human seminal plasma[END_REF][START_REF] Gilany | Exploring the human seminal plasma proteome: an unexplored gold mine of biomarker for male infertility and male reproduction disorder[END_REF]. This is, however, challenged by the variability in protein concentrations between individuals [START_REF] Gilany | Exploring the human seminal plasma proteome: an unexplored gold mine of biomarker for male infertility and male reproduction disorder[END_REF]. From a clinical perspective, Batruch et al [START_REF] Batruch | Analysis of seminal plasma from patients with nonobstructive azoospermia and identification of candidate biomarkers of male infertility[END_REF] identified different seminal protein expression profiles between patients with NOA and their fertile counterparts. An altered seminal plasma proteomic profile is also found in patients with varicocele, correlated with increased ROS generation and up-regulation of antioxidant systems [START_REF] Selvam | Proteomic analysis of seminal plasma from bilateral varicocele patients indicates an oxidative state and increased inflammatory response[END_REF][START_REF] Selvam | Functional analysis of differentially expressed acetylated spermatozoal proteins in infertile men with unilateral and bilateral varicocele[END_REF]. Sixty-four and 31 proteins were expressed in bilateral and unilateral varicocele patients, respectively, which reflected the varicocele severity and its impact on seminal parameters [START_REF] Agarwal | Differential proteomic profiling of spermatozoal proteins of infertile men with unilateral or bilateral varicocele[END_REF]. Seminal protein RNAs could predict the presence of spermatozoa in patients with NOA in certain instances, as demonstrated for miR-192a [START_REF] Zhi | Seminal plasma miR-192a: a biomarker predicting successful resolution of nonobstructive azoospermia following varicocele repair[END_REF] and hsa-circ-0000116 [START_REF] Lv | Over-expression of hsa_circ_0000116 in patients with non-obstructive azoospermia and its predictive value in testicular sperm retrieval[END_REF]. Therefore, RNAs that regulate germ cell apoptosis and are involved in spermatogenesis may also play a role in predicting sperm retrieval. The efficacy of ECM1, TEX101, and LGALS3BP in predicting TESE outcomes in patients with NOA has already been examined, providing more support for SP proteomics. Very intriguingly, ECM1 could also be important for ART result prediction [START_REF] Cannarella | Seminal plasma transcriptome and proteome: towards a molecular approach in the diagnosis of idiopathic male infertility[END_REF]. Additional studies on larger populations are warranted to validate the role of protein biomarkers in the clinical practice of male infertility [START_REF] Bieniek | Seminal biomarkers for the evaluation of male infertility[END_REF].

Radiomics

Radiomics involves the extraction of numerical values from radiological images, thus offering a more comprehensive analysis that is beyond the simple visual capacity [START_REF] Lambin | Radiomics: extracting more information from medical images using advanced feature analysis[END_REF][START_REF] Gillies | Radiomics: images are more than pictures, they are data[END_REF]. A pilot study by De Santi et al [START_REF] Santi | From subjective to objective: a pilot study on testicular radiomics analysis as a measure of gonadal function[END_REF] compared scrotal ultrasonographic findings and testicular function represented by semen parameters (sperm concentration, total sperm number, total motility, progressive motility, and sperm morphology) and reproductive hormones (luteinizing hormone [LH], follicle-stimulating hormone [FSH], and total testosterone). The results showed that ultrasound-related textural features were correlated and predicted sperm concentrations and total counts, total and progressive motility, morphology, and serum gonadotropins but did not correlate with serum total testosterone levels.

In a recent study on ten males with NOA, it was observed that choline and creatine were the most pronounced metabolite peaks seen following spectroscopic examination of five males with NOA who were positive for sperm during microTESE [START_REF] Karakus | Correlation between high choline metabolite signal in spectroscopy and sperm retrieval chance at micro-TESE[END_REF]. Testicular normalized apparent diffusion coefficient (ADC) derived from the conventional mono-exponential model is a parameter that reflects the water diffusion motion, which is primarily associated with the cell density of the tissue and extracellular space [START_REF] Hatakenaka | Apparent diffusion coefficients of breast tumors: clinical application[END_REF]. "In the testis, compact interstitial and connective tissue, and seminiferous tubules restrict water diffusion, thus affecting the ADC. This can make ADC a useful diagnostic tool. A study on 20 subjects with NOA found a significanty higher ADC in NOA males who had foci of advanced spermatogenesis with a Johnsen score ≥8 [START_REF] Tsili | Diffusion tensor imaging parameters in tes-tes with nonobstructive azoospermia[END_REF]."

RELATIONSHIP BETWEEN A MALE'S FERTILITY STATUS AND GENERAL HEALTH

In recent years, general health status is gaining increasing clinical attention in the male reproductive setting. Looking beyond the scope of reproduction and assessing the general well-being of the patient is, therefore, a crucial aspect of the management of infertile males. A Swedish population-based study compared a total of 101,331 males diagnosed with infertility or infertility-related diagnosis with 2,762,254 fertile males and found that the risk of death below the age of 30 years was higher among males diagnosed with infertility (adjusted hazard ratio [HR]=3.26; 95% confidence interval [CI]=2.42-4.41), which was explained by the occurrence of malignancy that was diagnosed before infertility [START_REF] Lundberg | Mortality in 43,598 men with infertility -a Swedish nationwide population-based cohort study[END_REF]. Thus, it is suggested that infertility can be regarded as a predictor of mortality and morbidity among males [START_REF] Del Giudice | Clinical correlation among male infertility and overall male health: a systematic review of the literature[END_REF][START_REF] Glazer | Male infertility and risk of nonmalignant chronic diseases: a systematic review of the epidemiological evidence[END_REF]. In a systematic review and metaanalysis comparing fertile to infertile males, male infertility has been associated with an increased risk of death [START_REF] Del Giudice | The association between mortality and male infertility: systematic review and meta-analysis[END_REF]. In another study, medical comorbidities, including hypertension and hyperlipidemia, were significantly higher in infertile subjects (21.7%) compared to fertile counterparts (9.1%) [START_REF] Shiraishi | Effects of medical comorbidity on male infertility and comorbidity treatment on spermatogenesis[END_REF].

A cross-sectional study on more than 9,000 males has shown that comorbidities, including cardiovascular diseases, were higher among patients with low sperm count, motility, and volume [START_REF] Eisenberg | Relationship between semen production and medical comorbidity[END_REF][START_REF] Eisenberg | Increased risk of incident chronic medical conditions in infertile men: analysis of United States claims data[END_REF]. Patients with hypertension can be more prone to having abnormal semen parameters [START_REF] Guo | Hypertension and male fertility[END_REF]. Another study on 32,442 males observed an association between abnormal sperm parameters and testicular malignancy [START_REF] Jacobsen | Risk of testicular cancer in men with abnormal semen characteristics: cohort study[END_REF]. The development of testicular malignancy was three times higher among infertile males compared to fertile subjects (HR=2.8; 95% CI=1.3-6.0) [START_REF] Walsh | Increased risk of testicular germ cell cancer among infertile men[END_REF]. Therefore, clinicians assessing infertile men must also consider their general health [START_REF] Ventimiglia | Comorbidities and male infertility: a worrisome picture[END_REF]. Management of the infertile male should be directed towards not only specific therapy to improve his fertility but also therapy for nonspecific medical comorbidities that may influence his fertility, general health, and life expectancy as well.

PERSONALIZED MEDICINE AND MALE INFERTILITY

Personalized medicine is increasingly used for the treatment of various diseases and disorders. For male infertility, this approach can be delivered in different ways, including stem cell therapy, gene therapy, and nanoparticle drug delivery.

There is growing interest in induced pluripotent stem cells (iPSCs) and mesenchymal stem cells for their potential application in reproductive medicine, particularly in cases of azoospermia-related infertility [START_REF] Saha | Application of stem cell therapy for infertility[END_REF]. The embryonic stem cells (ESC) represent a turning point in regenerative medicine thanks to their unlimited self-renewal properties and, above all, differentiation into ectoderm, endoderm, and mesoderm. An interestwww.wjmh.org ing study documented the possibility of developing functional sperm using a sperm-deficient mouse model (Kit w /Kitw v ) through gene-repaired ESC isolated from cloned blastocysts originating from nuclear transferred somatic cells (ntESC) using gene repair technology [START_REF] Yuan | Generation of fertile offspring from Kit(w)/Kit(wv) mice through differentiation of gene corrected nuclear transfer embryonic stem cells[END_REF].

The CRISPR/Cas9 system has enabled reproductive research on gene repair. The CRISPR/Cas9 method allows for modifying the nucleic acids that constitute the genome of all living organisms. The CRISPR/Cas9 system can be used, to generate knockout mouse (KO) mice rapidly and for more complex gene manipulations [START_REF] Lu | Impact of antioxidant therapy on natural pregnancy outcomes and semen parameters in infertile men: a systematic review and meta-analysis of randomized controlled trials[END_REF].

The cut-and-sew mechanism of the genome makes it possible to identify a faulty DNA locus and substitute it with a functioning sequence, thereby reverting the infertility condition. By generating KO mouse lines using the CRISPR/Cas9 enzyme, Lu et al [START_REF] Lu | Impact of antioxidant therapy on natural pregnancy outcomes and semen parameters in infertile men: a systematic review and meta-analysis of randomized controlled trials[END_REF] were able to analyze the function of 30 testis-enriched genes and four ubiquitously expressed genes involved in male reproduction. The KO males exhibited normal fecundity, suggesting that these 34 genes are expendable on their own for male fertility.

In addition to the genetic approaches being developed, some men will benefit from supplementary medical therapy. The action of antioxidants and their effectiveness in improving the functional capacity of spermatozoa and other parameters of semen is controversial. However, a recent systematic review and meta-analysis of randomized controlled trials demonstrated that antioxidant therapies seem to improve spontaneous pregnancy rate and conventional sperm parameters [100] and measurement of ROS levels may help determine appropriate candidates for antioxidant therapy.

In recent years, advances in nanotechnology have allowed the administration of specific drugs. Solid lipid nanoparticles were first developed in 1990 and are characterized by their submicron size [101]. Together with other characteristics relating to the composition of the matrix, these nanoparticles have the potential to be released in the target area in a precise and prolonged manner [102]. These scientific advances are widely utilized in the veterinary field, but their use for treating infertility in humans has not yet been fully established.

MALE FERTILITY PRESERVATION

Fertility preservation has progressed extensively using innovations in cryobiology for cryopreserving spermatozoa [103]. Different techniques of sperm cryopreservation have been used for ICSI including slowfreezing and vitrification. Slow-freezing is the conventional technique and can result in the formation of ice crystals which may cause damage to the sperm cytoskeleton, membrane, and DNA [104-107]. The technique of vitrification for sperm cryopreservation refers to ultrafast freezing of a small volume of semen with direct contact with contaminant-free liquid nitrogen which reduces osmotic damage by preventing ice formation. Vitrification has been correlated with higher recovery rates and motility [108,109] and lower SDF [110] as compared to slow freeze. However, the 6th edition of the WHO manual for SA recommends that sperm vitrification should be considered an experimental procedure as improved post-thaw semen parameters after vitrification in comparison to conventional cryopreservation techniques have limited data to support it [34].

Despite these innovations, it has been argued that a series of epigenetic modifications may occur secondary to cryopreservation, including changes in mRNA expression [111]. Cryopreservation is used following testicular sperm retrieval in infertile azoospermic males, where a limited number of sperm are retrieved [112], or as fertility preservation before chemo-radiotherapy. Fertility preservation is much more challenging in the context of pre-pubertal boys suffering from cancer who will need different oncologic therapies. Since spermatogenesis occurs at puberty, harvesting and cryopreserving spermatogonial stem cells before undergoing therapy is being studied as a possible fertility-preserving option for pre-pubertal patients, where the preserved tissue could either be used for autologous transplantation or for in vitro induction of spermatogenesis [113]. These techniques have been successfully performed in a mouse model, where the cryopreserved harvested tissues were cultured after thawing and resulted in complete spermatogenesis, and sperm were successfully used for ICSI [114,115].

Cryopreservation of testicular tissue in pre-pubertal boys has been discussed for more than 20 years [116,117]. The rationale behind the cryopreservation of testicular tissue is to restore spermatogenesis in adulthood. Overall, patient survival from childhood cancer has dramat-ically increased with the development of chemo-and radiotherapy regimens. In most European countries, overall five-year childhood cancer survival is estimated at more than 80% and approximately 500,000 childhood cancer survivors will expect to be a father [118]. It is simple to collect spermatozoa in adults before any gonadotoxic treatment and use them in future ARTs [119]; however, for pre-pubertal boys, there is no established method defined to preserve and restore spermatogenesis [120].

Autologously engrafted frozen-thawed ovarian cortex has successfully led to more than 100 live births in female cancer survivors worldwide [121]. Theoretically, the analog method with testicular tissue engraftment in pre-pubertal boys is also expected to restore spermatogenesis and lead to successful pregnancies. Achieving spermatogenesis by orthotopic and ectopic transplantation of pre-pubertal testicular tissue has been demonstrated in mice [122]. In another study, from two juvenile monkey testis xenografts, six healthy monkeys were produced by ICSI [123]. In 2019, a milestone paper showed the possibility of spermatogenesis restoration when cryopreserved and fresh pre-pubertal testis tissues from rhesus monkeys were subjected to autologous grafting beneath the skin of the back or scrotum [124]. In the latter study, sperm derived from scrotal skin was used in ICSI, and a Grady baby ('graft-derived baby') was born. Although this method seems more promising for advancing fertility preservation in pre-pubertal boys to a clinical stage, several concerns, including optimal testicular tissue size to be transplanted, the ideal age of transplantation, and post-grafting time for sperm extraction, should first be eliminated [120].

FUTURE OF STEM CELLS IN MALE INFERTILITY

Strategies to restore fertility for pre-pubertal boys include spermatogonial stem cell transplantation (SSCT), testicular tissue engraftment, and in vitro spermatogenesis [120,125]. Autologous SSCT is one of the most studied methods for restoring fertility in prepubertal boys. The proof of concept study in mice was published in 1994 [126]; thereafter, in many species, SSCT was shown successfully to restore spermatogenesis [127]. The first study applying SSCT to humans was performed in 1999; however, the results of this study have never been reported [128]. Since then, no SSCT attempts for humans have been reported as several major concerns limit the application of SSCT in a real-life setting.

Culturing human SSCs without any Xeno product is the first issue that needs to be overcome. Using animal components in cell culture introduces the risk of contaminating cells with pathogens, making them inappropriate for medical use. Recently, a xeno-free culture method has been developed to propagate SSCs from infant boys using human platelet lysate and human serum albumin replacing fetal bovine serum and bovine serum albumin [129]. Some other components have also been proposed to replace animal-derived components, such as: bovine serum albumin or fetal bovine serum [130]. However, further studies are required to establish an optimum xeno-free media to expand SSCs and use them in future cultural practices. In addition to optimizing SSC culture with xeno-free components, the risk of potential malignant cell transmission and defining the optimum technique are the other leading limitations of SSCT. Although several sorting strategies have been proposed, evidence is inconsistent and unconvincing regarding eliminating malignant cells from SSCs [131,132]. Similarly, data are inconclusive regarding the optimal SSCT technique, as more studies are needed regarding the optimal transplantation site, optimum cell count, and ideal hydrostatic pressure [133][134][135].

In vitro maturation of SSCs to spermatozoa has been studied for over a century. In 1999, in vitro maturation of testicular samples from males with premeiotic maturation arrest led to successful pregnancy and live birth [136]. However, other groups could not replicate this study due to a lack of definitive protocols. Considering that the SSCs' self-renewal and differentiation need a stem cell niche (testicular architecture and somatic support), establishing the optimum culture system is difficult. The development of biomaterials and nanotechnology may help the progress of in vitro spermatogenesis into the clinical stage [137][138][139].

It is possible to derive human induced pluripotent stem cells, also known as hiPSCs, from the somatic cells of patients. The in vitro development of functional germ cells from patient-specific induced pluripotent stem cells (iPSCs) may give new therapeutic strategies for couples who are unable to have children [140]. Idiopathic infertility patients' somatic cells (such as skin www.wjmh.org fibroblast, keratinocyte, peripheral blood mononuclear cells, or renal tubular cells in urine) are reprogrammed into iPSCs, and these iPSCs are subsequently differentiated into male germ cells using a variety of techniques [141-145]. The process is called "differentiation." iPSCs may, if necessary, undergo genome editing to fix known genetic flaws. These cells have the potential to be utilized for in vitro disease modeling, research on tissue regeneration, and cell-based therapeutic treatment. In disease modeling, comparing cells obtained from patients to those derived from normal individuals can reveal unique insights into the underlying mechanisms that cause idiopathic male infertility. However, the chemical pathways underpinning the formation of male germ cells remain poorly known. The employment of hiPSCs in reproductive medicine and fundamental research would benefit greatly from a greater understanding of human germ cell development [140].

THE ART OF ART

Among the most widely used treatment methods, ARTs have become the gold standard in medically assisted reproductive medicine. Not surprisingly, the keyword intracytoplasmic sperm injection "ICSI" has been cited over 13,000 times in 30 years, according to a Scopus search as of June 2022.

Despite the continuous progress made by ART clinics and laboratories, ICSI live birth rates vary between 12.3% and 46.5% per oocyte retrieval cycle (attempt), with an average cumulative retrieval live birth rate of less than 30% [146]. For this reason, over the past 30 years, several "variants" of ICSI have been described. Researchers have focused on methods of sperm selection before injection. These include intracytoplasmic morphologically selected sperm injection (IMSI) using differential interferential contrast and high magnification, physiologic ICSI (PICSI) with hyaluronic acid/ hyaluronan, magnetic-activated cell sorting (MACs) techniques, the zeta potential sperm selection process, zona pellucida-bound sperm selection, and microfluidic sperm sorting cells. Recent meta-analyses are inconclusive about the efficacy of these ICSI variants (compared to conventional ICSI), as the quality of the studies included in these meta-analyses are ranked to be moderate to low [147,148]. It should be noted, however, that the trend is towards improved pregnancy rates with certain variants of ICSI, such as IMSI [148].

Additionally, some of these techniques can be useful in certain indications that can be determined after careful andrological evaluation of the male partner. Indeed, selecting the best spermatozoon, the one with the most intact and least fragmented DNA in a patient with non-treated risk factors for SDF, for example, is pointless. The initial evaluation of infertile males and the management of the causes of infertility will undoubtedly be important aspects of the post-ICSI era.

Laser-assisted (LA) sperm selection technique selects immotile but living sperm cells and results in a healthy birth [149]. It was also effective in pentoxifyllineresistant immotile spermatozoa in males with Kartagener's syndrome [150]. Birefringence-based selection technique is another advancement where a light wave is split into two unequally reflected waves using an optically anisotropic medium. With the use of polarized light microscopy, mature and viable spermatozoa can be selected. The technique has been found superior to the hypo-osmotic swelling test with higher pregnancy rates in TESE/ICSI cycles [151,152]. On the other hand, fluorescence-activated cell sorting (FACS) isolates living sperm cells labeled with fluorophore-conjugated antibodies from seminal fluid when excited by a laser beam [153]. The technology has been implemented recently in sperm isolation after TESE from males with NOA. However, the cost of the procedure, the possible sperm loss and the time constraint have limited this technology [154].

AI FOR ANDROLOGICAL SURGERIES

AI has the potential to play a significant role in andrological surgeries as well. AI can be used in andrological surgery in several ways. One way is through the use of ML models, which can be guided by AI algorithms to predict surgery outcomes. In a study by Ory et al [START_REF] Ory | Artificial intelligence based machine learning models predict sperm parameter upgrading after varicocele repair: a multi-institutional analysis[END_REF], using pre-operative hormonal, clinical, and sperm analysis data, a ML model successfully predicted clinically significant improvement in post-varicocelectomy sperm parameters. Also, there were some attempts for predicting sperm retrieval in mTESE for NOA patients [155,156]. In a paper by Zeadna et al [156], a model including LH, FSH, testosterone, testicular size, semen volume, age, BMI, and ethnicity as candidate predictors were able to predict sperm retrieval rate with moderate accuracy (AUC=0.8). However, this paper was criticized for its low number of candidate predictors, small sample size, selection bias, and surgical technique used for sperm retrieval [157]. Also, deep neural networks are used to segment the penile shaft before assessment and it provides accuracy on par with manual examination for patients with penile curvature [158]. It seems that the ML models in andrological surgeries have a long way to be used in the andrological surgeries.

THE FUTURE OF ANDROLOGISTS

Undoubtedly, the role of the andrologist will expand in the coming years with international initiatives already underway. For example, under the auspices of the European Society of Human Reproduction and Embryology (ESHRE), a group has been created to improve global research and management of male fertility and infertility [159]. In the same year, a global group of andrologists was created, collectively known as the Global Andrology Forum (GAF) (https://globalandrologyforum.com) [160]. This group currently has about 700 members from 84 countries. The goals of the GAF include collaborations between andrologists from around the world, addressing questions and controversies in andrology, training researchers and physicians in scientific research in andrology, and making andrology a field of research care and training on its own. Several training courses were held online during the COVID pandemic, which attracted hundreds of researchers and clinicians interested in andrology [161,162].

CONCLUSIONS

Male infertility is a common problem and a significant source of stressful life. To determine the optimal treatment for male infertility, a comprehensive, individualized diagnostic workup is warranted to determine the underlying cause. The development of genetic testing and the use of epigenetic markers, seminal proteomes, and radiomics offers hope for understanding the etiopathogenesis of male infertility. In addition, advancements in male fertility preservation tools provide hope to restore the fertility potential of male patients with cancer undergoing gonadotoxic therapies. In addition, the future application of AI in the infertility practice may help establish a definitive diagnosis for many infertility cases and provide prognostic value for sperm extraction and reproductive outcome under natural and assisted conditions. Sincere efforts of the professional organizations of andrology will enhance the spread of andrological knowledge and reduce the gap between the research field and clinical practice.

Acknowledgements

We are grateful to Dr. Manaf Al Hashimi, MD (UAE), Dr.

Wael Zohdy, MD (Egypt), and Prof. Umut Çağın Arı, PhD, DVM (Turkey) for critically reviewing our manuscript. The help of Dr.

Damayanthi Durairajanayagam, PhD (Malaysia) in editing our manuscript is appreciated.

Funding

None.

Conflict of Interest

The authors have nothing to disclose.

Author Contribution


Ashoka Suchithra

Alexandre Sahadevan

Noemi Abhervé

Cristina Monni

José Ramón Sáenz De Pipaón

João C Galán-Mascarós

Bruno Waerenborgh

Pascale Vieira

Sébastien Auban-Senzier

El-Eulmi Pillet

Bendeif

Sahadevan

Alexandre Abhervé

Noemi Monni

Cristina Sáenz De Pipaón

José Ramón Galán-Mascarós

João C Waerenborgh

Bruno J C Vieira

Pascale Auban-Senzier

Sébastien Pillet

El-Eulmi Bendeif

Pere Alemany

Enric Canadell

Maria Laura Mercuri

Narcis Avarvari

Conducting Anilate-Based Mixed-Valence Fe(II)Fe(III) Coordination Polymer: Small-Polaron Hopping Model for Oxalate-Type Fe(II)Fe(III) 2D Networks

come   L'archive ouverte pluridisciplinaire

Introduction

Molecular materials combining conducting (π-type, delocalized) and magnetic (d-type, localized) electrons have attracted major interest in molecular science since they can exhibit the coexistence of two distinct physical properties, furnished by the two independent networks, or novel and improved properties when they interact. 1 In this context, heterobimetallic oxalate-bridged compounds have been thoroughly used as magnetic lattice of multifunctional magnetic materials. 2 They are formed by anionic networks [M II M III (ox)3] - (ox = oxalate) with magnetic ions linked through bis-bidentate bridging oxalate ligands. The second property is provided by the charge-compensating cation, thus combining the longrange magnetic ordering of the oxalate network with paramagnetism, 3 photochromism, 4 electrical conductivity, 5 proton conductivity, 6 ferroelectricity, 7 chirality, 8 or single-molecule magnet behavior. 9 In the last years, coordination polymers based on the 3,5-disubstituted-2,6-dihydroxy-1,4-benzoquinone (H2dhbq, see Scheme 1), also called anilate ligand in its dianionic form (dhbq 2-or X2An 2-), have been thoroughly developed. 10 They usually present 2D layered 11 or 3D extended networks 12 with larger cavities than those obtained in oxalate-based coordination polymers due to the larger size of the bridging ligand. More recently, Miyasaka et al. have been able to increase the magnetic ordering temperature in the previously reported ferrimagnet (NBu4)[Mn II Cr III (Cl2An)3] (Cl2An 2-= chloranilate) from 10 to 40 K. 13 Taking advantage of the porosity of the material, they have inserted Li + ions into the pores of the 2D network in order to generate a radical Cl2An •3-(S=1/2) and produce a new exchange interaction between the radical ligand and the metal centers, thus giving a further proof of the potential of the anilate ligand to enhance the magnetic coupling in the extended network. In this context, coexistence of electrical conductivity and magnetic ordering in Fe III anilate-based coordination polymers have been reported simultaneously by two research groups in 2015. 14 In both cases, the conducting properties were attributed to the presence of radical anilate bridging ligand species. The role played by the Fe II /Fe III and L 2-/L 3-mixed valency was further discussed by Robson et al. with a more recent result based on an interpenetrated 3D network of formula (NBu4)[Fe2(F2An)3] (F2An 2-= fluoranilate). 12d Another advantage of the anilate ligand is that it can be substituted on the 3 and 5 positions of the aromatic ring by a large variety of substituents. 15 Recently, the asymmetric chlorocyananilate (ClCNAn 2-, see scheme 1) 16 ligand has been combined to the redox-active molecule bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) to prepare an organic semiconductor with formula [HClCNAn]2[BEDT-TTF], 17 and to lanthanide ions in a series of coordination polymers of general formula [Ln2(ClCNAn)3(DMF)6]•(CH2Cl2)x (Ln = Yb, x = 0; Ln = Nd or Er, x = 1) which present NIR emission properties. 18 In this work, we have been investigating this non-symmetric anilate ligand to prepare the mixed-valence coordination polymer of formula [TAG][Fe II Fe III (ClCNAn)3]•(solvate) (1), where TAG is the C3 symmetric tris(amino)-guanidinium cation, never used so far in such coordination polymers. Thorough structural characterization and study of the magnetic and conducting properties of this crystalline material are described. Since mixed-valence Fe II Fe III oxalate-based coordination polymers were previously reported to present a very poor conductivity, we have also investigated the origin and mechanism of the transport properties in mixed-valence Fe II Fe III networks based on oxalate related bridging ligands, i.e. oxalate, squarate, dhbq 2-and ClCNAn 2-. Scheme 1. 2,6-dihydroxy-1,4-benzoquinone (H2dhbq) and potassium chlorocyananilate (KHClCNAn).

Experimental Section

Tris(amino)-guanidinium chloride (TAGCl) 19 and potassium chlorocyananilate (KHClCNAn) 16 were prepared according to the reported procedures. All other chemicals were commercially available and used as received without further purification.

Synthesis of [TAG][Fe II Fe III (ClCNAn)3]•(solvate) (1)

An aqueous solution (8 mL) of KHClCNAn (64 mg, 0.27 mmol) was first placed in the bottom of a test tube, then was carefully layered a solution of TAGCl (42 mg, 0.3 mmol) in a mixture of water (2 mL) and THF (2 mL) in the middle, and then a solution of Fe(ClO4)2 •xH2O (51 mg, 0.2 mmol) in acetone (3 mL) on the top. After one week black hexagonal crystals suitable for XRD measurement appeared at the interface. Elemental Anal. C22H67N9O41Cl3Fe2 calcd. C, 19.84; H, 5.07; N, 9.47 % found: C, 19.32; H, 5.23; N, 9.22

Structural characterization

Data collection was performed at 150 K on an Agilent Supernova with CuKα (λ = 1.54184 Å). A single crystal of 1 was mounted on a glass fiber loop using a viscous hydrocarbon oil to coat the crystal and then transferred directly to the cold nitrogen stream for data collection. The structure was solved by direct methods with the SIR97 program and refined against all F 2 values with the SHELXL-97 program using the WinGX graphical user interface. All nonhydrogen atoms were refined anisotropically except for the C and N atoms from the cationic entity CN6H9 + , and hydrogen atoms were placed in calculated positions and refined isotropically with a riding model. The program SQUEEZE from PLATON was used to calculate the potential solvent accessible void volume and the nature of the disordered solvent molecules. It has indicated a total void space of 885 Å 3 and 293 electrons/cell. This corresponds to 29 molecules of water that have been inserted in the formula of the compound. A summary of crystallographic data and refinement results are listed in Table 1.

Data collection was performed at 10 K on a SuperNova Microfocus diffractometer equipped with a two-dimensional ATLAS detector, using Mo Kα radiation (λ = 0.71073 Å) and a Helijet He open-flow cryosystem. The unit-cell determination and data reduction were performed using the CrysAlisPRO program suite (Rigaku Oxford Diffraction, 2017) on the full data set. The data have been indexed using the trigonal setting with cell parameters a = b = 13.5493( 16) Å, and c = 9.484(3) Å. In addition, the diffraction pattern showed the presence of very weak (1/3 1/3 1/3) superstructure reflections, which were not taken into account in the structure determination. The corresponding crystal structure was refined on F 2 by weighted full matrix least-squares methods using the SHELXL program (Sheldrick, 2008). All non-hydrogen atoms were refined anisotropically except for the C and N atoms from the cationic entity CN6H9 + , and hydrogen atoms were placed in calculated positions and refined isotropically with a riding model. The program SQUEEZE from PLATON was used to calculate the potential solvent accessible void volume and the nature of the disordered solvent molecules.

CCDC-1858728 (150 K) and 1858527 (10 K) contain the supplementary crystallographic data for this paper.

R1(F) = ΣǁF0|-|FCǁ/Σ|F0|; b wR2(F 2 ) = [Σw(F0 2 -FC 2 ) 2 /ΣwF0 4 ] 1/2 ; c S(F 2 ) = [Σw(F0 2 -FC 2 ) 2 /(n+r-p)] 1/2 .

Raman Measurements

Raman spectrum of 1 was carried out at room temperature on single crystals by using a micro Raman spectrometer (Horiba Labram 300) equipped with a He-Ne laser (λ = 632.81 nm) laser in the 80-2000 cm -1 range, with a 20 LWD objective (<0.25 mW/μm 2 on the crystal). A 180° reflective geometry was adopted. The samples were mounted on a glass microscope slide, and the scattering peaks were calibrated against a Si standard (n = 520.7 cm -1 ). A typical spectrum was collected with a 300 s time constant at <1 cm -1 resolution and was averaged over three scans. KHClCNAn FT-Raman spectrum was carried out at room temperature on a capillary tube by using a Bruker RFS/100 FT-Raman spectrometer equipped with a Nd-YAG laser (λ = 1064 nm), on a back-scattering geometry. No sample decomposition was observed during the experiments. The choice of the FT-Raman spectrometer was required because of the fluorescence of the ligand under He-Ne laser.

Magnetic measurements

Magnetic measurements were carried out on polycrystalline samples with a Quantum Design MPMS-XL-7T SQUID magnetometer (Quantum Design, Inc, San Diego, CA, USA). Magnetic measurements (dc) were carried out under an applied field of 1000 Oe. Zero-field cooled/field cooled/remanent magnetization (ZFC/FC/RM) were collected under an applied field of 25 Oe. AC susceptibility measurements were carried out with an alternating magnetic field of 3.95 Oe, in the 1-1500 Hz frequency range.

Mössbauer spectroscopy

Mössbauer spectra were collected in transmission mode using a conventional constantacceleration spectrometer and a 25 mCi 57 Co source in a Rh matrix. The velocity scale was calibrated using α-Fe foil. Isomer shifts, IS, are given relative to this standard at room temperature. The low temperature spectrum was collected in a bath cryostat with the sample in He exchange gas. The absorber was obtained by gently packing the powdered sample into a perspex holder. The spectra were fitted to Lorentzian lines using a non-linear least-squares method.

Single-crystal transport measurements

Electrical transport measurements were performed on hexagonal-shaped single crystals. Gold wires (17 μm diameter) were glued with silver paste either on two edges or on both faces of the crystals (for conductivity measurements parallel and perpendicular to the 2D planes, respectively). Two-probe DC measurements were performed applying a constant voltage in the range 0-5V and measuring the current using a Keithley 6487 Picoammeter/Voltage Source.

Capacitance measurement was performed applying a constant voltage of 5V at a frequency of 2 kHz, using a RLC bridge. Low temperature was provided by a homemade cryostat equipped with a 4 K pulse-tube.

Theoretical calculations

Density functional theory (DFT) based calculations were carried out adopting the hybrid TPSSh functional 20 which has been shown to give good high spin-low spin relative energies for spincrossover complexes involving iron 21 and the standard double-+ polarization basis set 6-31G (d). 22 Geometries were optimized forcing a D3 symmetry and a high-spin configuration using the Gaussian 09 code. 23

Results and Discussion

Synthesis

The synthesis of the mixed-valence compound 1 differs from that of the heterobimetallic oxalate and anilate-based coordination polymers. In such cases, the tetrabutylammonium salt of (tris-oxalato)metal(III) or (tris-anilato)metal(III) complex was first prepared and isolated. The precursor thus obtained was then reacted with the second metal salt by diffusion techniques to grow the bimetallic extended network. Here we have slowly diffused a solution of Fe II ClO4,xH2O and a solution of tris(amino)-guanidinium chloride (TAGCl) into a solution of KHClCNAn. Due to the partial oxidation of the Fe II ions under aerobic conditions, black hexagonal crystals of the Fe II Fe III compound 1 were obtained after one week.

Crystal Structure

Compound 1 crystallizes in the trigonal polar space group P3. The structure is formed by anionic 2D layers of formula [Fe II Fe III (ClCNAn)3] -in the ab plane, alternating with cationic layers of TAG. The anionic layer presents the well-known honeycomb structure, which is similar to other extended oxalate 24 and anilate-based 2D networks. 11a-e It consists of a hexagonal layer with Fe II and Fe III ions linked through the anionic bis-bidentate ClCNAn 2-ligands (Figure 1, S1 and S2). As usual for this type of 2D networks, the two crystallographically independent metal centers present alternated chirality (-configuration for Fe1 and -configuration for Fe2 in the crystal used to solve the structure). The intralayer metal-metal distance is 7.860 Å. The average Fe-O distances are very similar between both metal centres (2.037( 12) Å for Fe1 and 2.047( 13) Å for Fe2, see Table 2), which may indicate an electron delocalization and an average oxidation state of +2.5 for each Fe center (vide infra). The cationic layer is formed by one crystallographically independent TAG cation and water molecules. The TAG cation has an occupancy of 1/3, which is half of the Fe atoms, and is located only on half of the vertices of the hexagons. The structure of the cation is planar, with the C8-N2 and N2-N3 distances (1.394 (15) and 1.558 (17) Å respectively, see Table S1) in agreement with distances reported in the literature for other TAG based compounds. 25 Anionic and cationic layers present intermolecular H-bonding interactions between the terminal amino groups of the TAG cation and the chloro substituents of the anilate ligands (Figure 1). The distance between two anionic layers corresponds to the value of the c parameter ( 9.430(4) Å). The cationic and anionic 2D layers are eclipsed, leading to hexagonal channels along the c axis which are filled only by solvent molecules. When compared to the previously reported anilate-based layered coordination polymers, 11a-h here the use of the smaller cation results in a drastic increase of the void space inside the hexagonal channels (885 Å 3 ). This represents 58% of the total volume thus increasing the porosity of the 2D material (Figure S3). As a consequence, the compound shows a fast release of the solvent molecules after filtration, and the nature of these solvent molecules couldn't be attributed without ambiguity. In order to reach a good reliability factor, and since 293 electrons per hexagonal cavity were determined by the SQUEEZE program, 29 molecules of water have been integrated in the empirical formula (see Table 1). However, TGA analysis couldn't confirm the precise nature of the solvent molecules (Figure S4). Since a mixture of three solvents have been used during the synthesis (water, THF and acetone), the formula of the compound should be defined as [TAG][Fe II Fe III (ClCNAn)3]•(solvate). By comparison with the 150 K crystal structure, the overall honeycomb 2D structural architecture in the trigonal P3 space group is retained at 10 K. Accordingly, the asymmetric unit still contains two symmetrically independent iron sites (Fe1 and Fe2). The unit cell parameters and unit cell volumes are slightly lower at 10 K, owing to usual thermal contraction effects. The intralayer metal-metal distance (7.8243(7) Å) and average Fe-O distances (2.020( 14) Å for Fe1 and 2.043( 14) Å for Fe2) are not significantly different from the 150 K values. Accordingly, the 10 K crystal structure does not evidence a specific ordering of the Fe II and Fe III ions on the symmetrically independent Fe1 and Fe2 sites. We can therefore consider that in this description, the Fe II and Fe III ions are spatially distributed and disordered over the two sites. As mentioned in the experimental section, very weak superstructure reflections were detected on the diffraction pattern, which indicates that the exact structural ordering may be more complex than this description in the P3 space group. However, the quality of the X-ray diffraction data, and especially the weakness of the superstructure reflections, does not allow going further. The current description leads to two Fe II and two Fe III different local environments in the crystal, which is consistent with the results from Mössbauer spectroscopy (vide supra).

Raman Spectroscopy

Raman spectra are valuable probes to investigate the oxidation state of coordinated benzoquinone derivatives. 26 Therefore In order to confirm the oxidation state of the bridging ligand a Raman study at room temperature was performed and a comparison between the Raman spectra of 1 and the free KHClCNAn ligand is reported in Figure 2. The strong and broad band centered at ca. 1574 cm -1 can be assigned to a ν(C=C) + ν(C=O) combination band and the significant observed downshift from 1617 cm -1 for the free ligand can be attributed to a weakened double bond character of these terminal groups because of the coordination with the metal ion; the weak band centered at ca. 1675 cm -1 present in the free ligand spectrum can be assigned to the ν(C=O) vibration mode for the uncoordinated C=O groups of the free ligand and in fact is not observed in 1. The two bands observed in the 1400-1250 cm -1 region are assigned to the ν(C-C) + ν(C-O) combination band and ν(C-C) vibration, respectively, and according to Harris et al. 14c confirm the assignment of ligand oxidation state as dianionic ClCNAn 2-, supporting structural findings. The observed band at 594 cm -1 can be assigned to ν(Fe-O) + ν(C-C) combination stretching mode, as already found in previously reported dianionic anilate-based honeycomb-like networks. 14a No bands can be unambiguously assigned to Fe II -O and Fe III -O vibrational modes thus supporting extensive electron delocalization between the Fe centers in 1 at room temperature, as clearly shown by Mössbauer spectra (vide infra).

Magnetic Properties

The magnetic properties were measured on a polycrystalline sample of 1. The product of the molar magnetic susceptibility times the temperature (χm T) presents a value of 9.7 emu.K.mol - 1 at 300 K, which corresponds to the expected spin only value (7.38 emu.K.mol -1 ) with a g = 2.2 (Figure 3). When the temperature is lowered, χmT slightly decreases, suggesting weak antiferromagnetic interactions between paramagnetic centres through the anilate bridges. Below 50 K, χmT increases until reaching a value of ≈27 emu.K.mol -1 at 7 K, followed by a sharp decrease at lower temperatures. This suggests a magnetic ordering, which was confirmed by zero-field-cooled/field-cooled (ZFC/FC) and remnant magnetization measurements under a very low magnetic field (Figure S5). The ZFC and FC plots diverge below 3.5 K, indicating the appearance of an irreversibility or memory effect. The remnant magnetization becomes nonnegligible at the same temperature, confirming the existence of spontaneous magnetization below this temperature. The magnetic ordering was also confirmed by susceptibility measurements under alternating magnetic field (AC susceptibility, Figure 4a). This shows the appearance of an out-of-phase signal and an ordering temperature Tc slightly dependent on the frequency, which can be observed for both superparamagnets and spin glasses. 27 The fitting of this frequency dependent behavior to a simple Arrhenius model (Figure 4b) yields parameters with no physical meaning, including a τ0 = 10 -12 very different to what is found in superparamagnets (or single molecule magnets, with values of τ0 between 10 -8 and 10 -10 s). 28 This τ0 value falls within the range reported for magnetic spin-glass systems (10 -12 -10 -14 s) 29 and is then consistent with a glassy magnet behavior as observed in many 2D magnetic materials. Isothermal magnetization measurements at low temperatures show a fast increase of the magnetization at low fields that become more gradual at higher fields (Figure 5). The sharp increase at low fields (H < 1000 Oe) also supports the appearance of spontaneous magnetization due to strong interactions between metal centers. The magnetization saturates at higher fields reaching ≈ 5 μB, far from the expected 9 μB for parallel alignment of spin carriers. This confirms the ferrimagnetic nature of the spontaneous magnetization that stabilizes a ground state with an intermediate spin and is characteristic of a glassy ferrimagnet as suggested by the initial decrease in the χmT at high temperatures. An additional proof of the magnetic ordering is the presence of a hysteresis loop at 2 K with a coercive field of 60 Oe. Mössbauer spectroscopy has been used to confirm the oxidation state of the Fe metal centers (Figure 6). At 10 K three broad absorption peaks are observed. They may be interpreted by two quadrupole doublets. However due to the large width of the absorption peaks a significantly better fit is obtained with four quadrupole doublets. The estimated isomer shift, IS, and quadrupole splitting, QS, are consistent with the presence of high-spin Fe 3+ and high spin Fe 2+ in octahedral coordination by anionic oxygen atoms. [START_REF] Greenwood | Mössbauer Spectroscopy[END_REF] The estimated relative areas indicate that approximately half of the Fe cations are in the +3 state and the other half in the +2 state. The two doublets observed for each oxidation state are consistent with the occupation of Fe(1) and Fe(2) crystallographic sites by both Fe 2+ and Fe 3+ . The room temperature spectrum shows only one asymmetric doublet with IS and QS consistent with an average oxidation state of +2.5. [START_REF] Shilov | [END_REF] The temperature dependence of the Mössbauer spectra of the anilate is similar to the behavior observed for other mixed valence iron compounds, namely molecular complexes. 33 The intermediate isomer shift at room temperature corresponds to a charge delocalized state on the Mössbauer spectroscopy time scale of ~ 10 - 7 s, i.e. a charge transfer frequency ≥ 10 8 s -1 . As the temperatures decreases, the frequency of charge delocalization gradually decreases and at 10 K the Fe 2+ and Fe 3+ states are localized when compared to the Mössbauer effect time window (i.e. the life-time of the Fe 2+ and Fe 3+ states becomes longer than 10 -7 s).

Transport properties

Since mixed-valence state and electron delocalization have been evidenced in 1, we could expect this coordination polymer to present transport properties, therefore electrical conductivity measurements have been carried out on single crystals. The hexagonal shape of the crystals allows determining the direction of the {001} plane and then to measure the conductivity parallel (σ⁄⁄) and perpendicular (σ ┴ ) to the 2D layers, corresponding to the ab plane. The temperature dependence of the resistivity indicates that 1 is a semiconductor (Figure 7). The parallel room-temperature conductivity value σ⁄⁄ is about 2.10 -3 S/cm, almost three orders of magnitude higher than the perpendicular room-temperature σꓕ (7.10 -6 S/cm).

Relationship between the electrical conductivity and the nature of the bridging ligand

The fairly good conductivity of compound 1 as well as of the few recently reported anilatobased Fe II Fe III coordination polymers 11f,14b is in sharp contrast with the low conductivity found for the oxalate-based ones. 34 To gain an insight into the origin of the good conductivity in our anilate-based coordination polymer, and to point out the crucial role of the bridging ligand, we have undertaken a theoretical study on the electron transfer in 2D Fe II Fe III networks based on bis(bidentate) oxalate-type ligands in which we consider the conductivity dominated by thermally activated small-polaron hopping. We are interested in a simple, pragmatic approach, highlighting the role of the bridging ligand and in this comparative work, we have decided to focus on four ligands, oxalate, squarate (C4O4 2-, dianion of 3,4-dihydroxycyclobut-3-ene-1,2-dione), dhbq 2-and ClCNAn 2-, with different electron delocalization capabilities.

The basic reasoning behind the small-polaron hopping approach to the electron transfer process in either discrete or extended mixed-valence systems [START_REF] Launay | Electrons in Molecules: From Basic Principles to Molecular Electronics[END_REF][START_REF] Austin | [END_REF] is qualitatively illustrated in Figure 8. Consider a system formed by two separated high-spin Fe II L6 (t2g 4 eg 2 ) and high-spin Fe III L6 (t2g 3 eg 2 ) complexes in close proximity. Although the electron transfer between them is between levels of the Fe t2g orbital set, which are formally non-bonding, such transfer leads, in general, to an increase/decrease of the M-L distances in the Fe II /Fe III L6 units due to the expansion/contraction of the electron cloud. As a consequence of the fact that electrons move much faster than nuclei, the much faster electron transfer occurs in such a way that the geometry cannot change during the process and the system cannot exchange thermal energy with the surroundings. In other words, before the electron transfer can actually take place a Fe III species with the Fe II geometry (and viceversa) must be created. This rearrangement is however energetically unfavorable, so that the electron transfer will only occur when as a result of some vibrational process the two Fe centers reach equal coordination geometries (see Figure 8a). Thus, to understand the differences in thermally activated conductivity one must focus on the evaluation of the energetic cost of such "equalization" of the two sites. A simple yet usually very insightful analysis relies on the assumption that the structural distortion around each center may be described by a simple harmonic oscillator Ei = ½ ki(di -d0)², where ki is the force constant, di the Fe-L distance and d0 its equilibrium value. Within this approximation, the energy of the whole system is: Etot = kIII(dA -dIII) 2 + kII(dB -dII) 2 where for the sake of simplicity the ½ factors have been included in the force constants. Here let us assume that complex A is in the Fe III state and complex B in the Fe II state, but there is a totally equivalent expression interchanging A and B. If we plot the two energy surfaces Etot(dA,dB) we see that they cross along the line dA = dB (Figure 8b). The initial and final configurations (respectively top and bottom configurations in Figure 8a) correspond to minima in the lower surface, and the structures for which dA = dB to the crossing seam. The situation is more conveniently analyzed by using a contour plot of the bottom surface (Figure 8c), obtained by joining the two bottom halves of the two intersecting surfaces. The two minima correspond to the configurations Fe III A ••• Fe II B (zone noted as F in Figure 8c) and Fe II A ••• Fe III B (noted as I).

The point on the dA = dB line for which the total energy is minimal (M) is found by taking the derivative of the energy function with dA = dB = dM and equating it to zero. It is found that dM = (kIIdII + kIIIdIII) / (kII + kIII) and the corresponding energy EM = (kIIkIII / (kII + kIII))(dII -dIII) 2 . In other words, the minimal energy required to make the coordination environments on both Fe centers equal, that is, the height of the barrier for thermally activated electron transfer, depends basically on the difference squared between the radii of the Fe III and Fe II coordination environments in their equilibrium geometries.

The model can be more simply depicted by using a cut through the two surfaces in which we plot the energy along the dotted path joining the two minima through the M point, which is taken as the origin (Figure 9). This gives two parabolas corresponding to the energy of the whole system with either an Fe II Fe III or an Fe III Fe II configuration and crossing at the M point.

Note that when the two complexes are not totally isolated from their surroundings, the energy necessary to distort the complexes has an additional contribution from the environment. This is particularly important in the case of 2D lattices where the distortion of one site contributes to the distortion of all its neighboring sites. This contribution is usually included considering a generic parameter λ, which is the vertical ionization energy from one minimum to the other curve, or in other words, the energy necessary to transfer the electron from A to B without considering a previous structural equalization of both centers. The barrier for thermal electron transfer in the absence of interaction (i.e. the energy difference between the minima and the crossing point of the two curves) is then simply λ/4. In the real case there is always some degree of electronic interaction between the two sites and a gap, 2VAB, is opened at point M and the barrier for thermal electron transfer is consequently lowered to λ/4 -VAB.

Unfortunately, there is no simple way to evaluate VAB without having recourse to long and costly computations, but since we are here interested only in looking for trends when the bridging ligand is changed, it seems safe to consider that λ/4 will be the leading term in the energy barrier, so that we can make our comparisons neglecting the effects of the nature of the ligands on VAB.

It is now easy to relate the main parameters (mainly structural) of this simple model to the transport measurements. The diagram in Figure 9 is completely general and applicable to any electron transfer process. In an extended system, when the dimensions of the zone where the necessary atomic rearrangement controlling the electron transfer occurs are of the order of the coordination sphere of a single site as in the present case, one talks about a system with small-polarons. [START_REF] Austin | [END_REF] The polaron energy, WP, is the energy gained when the system relaxes after addition of one electron. In this small-polaron scenario the conductivity is dominated by thermally activated electron hopping with the mobility given by the equation μ = μ0e -WH/k , where WH is the electron hopping barrier, i.e. the energy cost to reach the geometry under which the electron transfer is possible. As far as it is assumed that the variation of energy is a quadratic function of the structural parameters, the energy cost per site to reach the "equalization" geometry is 1/4 WP and, taking into account that there are two sites involved in the transfer, it follows that WH = 1/2 WP. Since according to the Franck-Condon principle the energy to optically excite one electron from one to the other site (i.e. in Figure 9) is twice the polaron energy it follows that WH = 1/2 WP = λ/4. Therefore WH = EM = F(dII -dIII) 2 , with F = kIIkIII / (kII + kIII) and dM = (kIIdII + kIIIdIII) / (kII + kIII). In that way it is possible to correlate the transport (WH) and structural (kII, kIII, dII and dIII) features for a series of compounds.

How are these parameters tuned by the nature of the bridging ligand? The values of kII, kIII, dII and dIII can be evaluated from density functional calculations (DFT). We carried out structural optimizations of Fe II L3 and Fe III L3 complexes with the four ligands oxalate, squarate, dhbq 2-and ClCNAn 2-using the TPSSh functional, which is known to give good high spin-low spin relative energies for spin-crossover complexes involving iron. 21 The geometries were optimized forcing a D3 symmetry and a high-spin configuration. In that way we obtained the dM-L parameter which is the distance between the Fe atom and the midpoint to the closest C-C bonds (between the C atoms bonded to the coordinating O atoms) which corresponds to dII or dIII of the above discussion. Once the optimal dM-L parameter for each Fe II and each Fe III complex was found the values of kII or kIII were obtained by reoptimizing the structure for fixed dM-L ±δ values and fitting the energy to a second order polynomial. The results for the four ligands are reported in Figure 9b. According to the data in Figure 8b WH = 0.214, 0.057, 0.044 and 0.037 eV for oxalate, squarate, dhbq 2-and ClCNAn 2-, respectively. Despite the simplicity of the approach, these numbers are very reasonable when compared with WH estimations based on experimental data for Fe II Fe III solids like magnetite ( ~0.15 eV) [START_REF] Austin | [END_REF] and partially substituted ferrites ( ~0.11 eV) [START_REF] Tsuda | Electronic conduction in Oxides[END_REF] which are in between those calculated for oxalate and the other three ligands. Thus, we believe that the simple approach captures the essence of the electron transfer process although the activation energies are smaller than the experimental ones. This is not unexpected since the Fe sites are immersed in a quite rigid 2D network, which must lead to a noticeable increase in the parameter, and hence in the barrier estimated here by /4. According to the values in Figure 8b electrons should be much less mobile in oxalate networks than in anilate ones, which is in good agreement with experimental observations. 34,11f,14b What is the reason for this finding? The F values for the oxalate ligands, are in between those for both the dhbq 9b it is clear that whereas the dIII values are fairly similar, dII for the oxalate ligand is markedly larger than those for the dhbq 2-and ClCNAn 2-ligands. Thus, the determining factor for the lower conductivity of the oxalate systems lies in the difficulty to cope with the extra electron of the high spin Fe II situation. The occurrence of extensive delocalization through the benzene ring of the dhbq 2-and ClCNAn 2-acts as a buffer for the electronic rearrangement needed by the presence of the extra electron. Note that when the ligands are more similar, the F factor depending on the force constants may become the key factor.

According to Figure 8b this is for instance the case when comparing the dhbq 2-and ClCNAn 2- ligands. The squarate ligand is also associated with a small WH value, not much different from those of the dhbq 2-and ClCNAn 2-ligands. However, the (Δd) 2 term is intermediate between those of the oxalate and dhbq 2-and ClCNAn 2-pair of ligands. Clearly, in that case the F term plays an important role. Both the geometrical constraints imposed by the four-member ring as well as the delocalization in the central part of the ligand influence the electron transfer tendency.

We thus conclude that the present approach provides a simple, yet insightful, model to rationalize the transport results for these Fe II Fe III mixed-valence coordination polymers and that the polymers based on the dhbq 2-and ClCNAn 2-ligands rank among the most effective ones in promoting the electron delocalization through a small-polaron hopping mechanism.

Conclusions

The mixed-valence Fe II Fe III 2D coordination polymer [TAG][Fe II Fe III (ClCNAn)3] based on the asymmetric chloro-cyanoanilate ligand and containing, for the first time in such 2D networks, the tris(amino)-guanidinium (TAG) cation has been synthesized and crystallized in the polar non-centrosymmetric space group P3 thanks to the C3 symmetry of the cation and its ability to engage in intermolecular hydrogen bonding with the chlorine atoms of the ligand. Magnetic susceptibility measurements in combination with Mössbauer spectroscopy are indicative of a spin-glass behavior with magnetic ordering below 4 K and the presence of intermediate Fe(+2.5) oxidation state at RT and charge localization Fe II Fe III at 10 K, with a statistic occupational crystallographic site according to the 10 K Mössbauer spectra, also confirmed by the X-ray structure at 10 K. Single crystal electron transport measurements in the 2D plane and perpendicular on it show semiconducting behavior of the material, with a rather high RT value of 2.10 -3 S/cm for the in-plane conductivity, much higher than the one reported in the oxalate based 2D Fe II Fe III coordination polymers. In order to shed light on this difference and on the electron transport mechanism on these fast developing multifunctional families of 2D coordination polymers, the small-polaron hopping approach to the electron transfer process has been applied to a series of mixed-valence Fe II Fe III oxalate related coordination polymers containing as bridging ligands oxalate 2-, squarate 2-, dhbq 2-and ClCNAn 2-. The results are clearly indicative of a much lower electron hopping barrier in the anilate complexes than in oxalates.

These results open the way towards the use of the TAG cation in such multifunctional binuclear/bimetallic transition metal or lanthanide coordination polymers, possibly endowed with multiferroic properties thanks to the crystallization in polar space groups. Moreover, the mechanism of electron transport in the mixed-valence Fe II Fe III coordination polymers with bridging oxalate type ligands has been disclosed, pointing out the higher "elasticity" of the anilate network compared to the oxalate one.

  %. FT-IR ( max/cm -1 ): 2221 (C≡N), 1631, 1492 (C-O +C-C), 869 (C-Cl +C-O).

Figure 1 .

 1 Figure 1. Structure of 1 in the ac plane (top left) and in the ab plane (top right). H-bonding intermolecular interactions (dashed lines) between the cationic and anionic layers (bottom). Colour code: C, black; H, cyan; O, red; N, blue; Cl, yellow; Fe, green.

Figure 2 .

 2 Figure 2. Comparison of Raman spectra of 1 and the KHClCNAn ligand, performed at room temperature with a He-Ne laser (= 632,81 nm) and a Nd-YAG (= 1064 nm), respectively.

Figure 3 .

 3 Figure 3. Temperature dependence of the product of the molar magnetic susceptibility times the temperature (χmT) of 1 under an applied field of 1000 Oe.

30

 30

Figure 4 .

 4 Figure 4. (a) Temperature dependence of the in-phase (χ') and out-of-phase (χ'') AC susceptibility of 1 and (b) Arrhenius plot for the frequency dependence of the position of the peak in χ'' vs 1/T.

Figure 5 .

 5 Figure 5. (a) Isothermal magnetization of 1 at different temperatures and (b) magnetic hysteresis loop at 2K.

Figure 6 .

 6 Figure 6. (a) Mössbauer spectra of 1 taken at 295 and 10 K and (b) estimated parameters from the spectra.The lines over the experimental points are the calculated functions. On the spectrum taken at 10 K this function is the sum of four quadrupole doublets shown slightly shifted for clarity. IS, isomer shift relative to metallic α-Fe at 295 K; QS, quadrupole splitting; I, relative areas. Estimated errors are < 0.02 mm/s for IS, QS, < 2% for I.

Figure 7 .

 7 Figure 7. (a) Electrical resistivity ρ// plotted as log ρ// versus the inverse temperature, measured with a 4V voltage applied in the ab plane. The red line is the fit to the data with the law ρ = ρ0 exp (Ea/T) giving the activation energy Ea. (b) Room-temperature resistivity (ρ) and conductivity (σ) values measured along (⁄⁄) and perpendicular ( ┴ ) to the 2D layers.

Figure 8 .

 8 Figure 8. (a) Schematic representation of the three consecutive steps during the electron transfer with two neighboring pseudooctahedral Fe(chelate)3 centers shown along the C3 axis. (b) 3D plot of the energy surface Etot as a function of dA and dB. (c) Contour plot of the bottom surface from the Etot plot displayed in (b).

Figure 9 .

 9 Figure 9. (a) Energy plot for a Fe II Fe III to Fe III Fe II electron transfer process. (b) Calculated parameters for the four studied bridging ligands. Distances are expressed in Å, energies in kcal.mol -1 , k and F in kcal.mol -1 .Å -2 .

  triennale tra la Fondazione di Sardegna e gli Atenei Sardi, Regione Sardegna-L.R. 7/2007 annualità 2016-DGR 28/21 del 17.05.2015 "Innovative Molecular Functional Materials for Environmental and Biomedical Applications" and INSTM. Work in Spain was supported by the Spanish Ministerio de Economía y Competitividad (Grants FIS2012-37549-C05-05, FIS2015-64886-C5-4-P, CTQ2015-64579-C3-3-P and CTQ2015-71287-R) and Generalitat de Catalunya (2014SGR301, 2017SGR797 and XRQTC, and the CERCA program). E.C. acknowledges support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant SEV-2015-0496. J.C.W. acknowledges Fundação para a Ciência e a Tecnologia (FCT, Portugal) through the project UID/Multi/04349/2013.

Table 1 .

 1 Crystallographic data for compound 1 at 150 K and 10 K.

	150 K	10 K

Table 2 .

 2 Selected Fe-O bond distances (Å) in compound 1.

	Fe-O distances	150 K (Å)	10 K (Å)
	Fe1-O1	2.000(6)	1.937(7)
	Fe1-O2	2.086(6)	2.103(7)
	Fe2-O3	2.073(7)	2.117(7)
	Fe2-O4	2.018(6)	1.974(7)

  2-and ClCNAn2-ones, yet WH is five times larger. It is clear that the factor determining the large difference is (Δd) 2 , i.e. the square of the difference in the d values for Fe in the two oxidation states. Because of the quadratic dependence, this term has an overwhelming influence on the final value of WH. Comparing the dII and dIII distances for the oxalate, dhbq 2-and ClCNAn 2- ligands in Figure

ACKNOWLEDGMENTS

The work in France was supported by the CNRS, the University of Angers, the Erasmus program (mobility grant to N.M.), and the RFI Regional project LUMOMAT (grant to A.A., project ASCO MMM). This work was supported in Italy by the Fondazione di Sardegna-Convenzione

ASSOCIATED CONTENT

Supporting Information. Full experimental section including description of the synthesis and characterization of all new materials and all the techniques employed in the research reported here. This information is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Authors

p.alemany@ub.edu canadell@icmab.es mercuri@unica.it narcis.avarvari@univ-angers.fr

Notes

The authors declare no competing financial interest.