Asif Ahmad

Suvam Saha

Dr Mohammad Zafar

Dr Thierry Roisnel

Dr Prasanta Ghosh
email: sghosh@iitm.ac.in

Sundargopal Ghosh

Syntheses and Structures of Facial and Meridional Stereoisomers of kappa(2)-N,S-Chelated Ruthenium Borate Complexes

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

Introduction

The synthesis and study of ruthenium complexes having boroncentered polyfunctional ligands have become a significant research interest over the past few decades. [1] These polyfunctional ligands can stabilize the metal center with different coordination modes occupying preferred geometrical position. For example, Sabo-etienne et al. synthesized a phospinobenzyl-(amino) borane ruthenium complex exhibiting two adjacent C-H and B-H agostic interactions that participate in selective B-H, C-H, and B-C bond activation. [2] Recently, in many reports it have been found that borate ligands can show versatile coordination modes at ruthenium center, [1a,3] where the metal center is stabilized by the 1,3-N,S donor ligand either in chelating (κ 2 -N,S) or in bridging (µ 2 -N,S/µ 1 -S) coordination modes. [1b,3,4,5] In fact, these 1,3-N,S chelated ligands have shown the potential hemilabile character to offer coordinative unsaturation at the metal center for further functionalization and to provide access to a large variety of functional groups with different steric and electronic properties.

The coordination chemistry of stable tridentate pincer-type ligands have also played an dominant role in catalysis owing to their ability to inhabit a particular coordination geometry in which the ligand occupies facial (fac) and meridional (mer) sites around metal center. [6,7] For example, "POP" type ligands have been utilised in catalysis for decades, especially the ligand frameworks [8,9] Xantphos or DPEphos [DPEphos = Bis{ (2diphenylphosphino)phenyl}ether] have been used in rhodium catalyzed hydroformylation reaction due to its wide bite angle. [START_REF] Kranenburg | [END_REF] For example, Balakrishna and co-workers synthesized and structurally characterized DPEphos complex of ruthenium, fac-10.1002/ejoc.202201283

Accepted Manuscript

European Journal of Organic Chemistry [Ru(S(O)Me2)(Cl)2(k 3 -P,O,P-DPEphos)] which upon addition of phosphine led to the formation of mer-[Ru(S(O)Me2)(Cl)2(k 3 -P,O,P-DPEphos)]. [11] Weller and co-workers reported rhodium complex, fac-[{PCyp2(η 2 -C5H7)}Rh(k 3 -P,O,P-DPEphos)][BAr F 4] (Cyp = cyclopentanyl) which undergoes alkene hydrogenation in H2 atmosphere generate to fac-[( PCyp3)Rh(H)2(k 3 -P,O,P-DPEphos)][BAr F 4] complex 1 (Chart a). [12a] cation Using exchange chromatography, Fletcher and co-workers were able to isolate various new fac-and mer-isomers of 5-functionalized-2,2'-bipyridine complexes. The relative yield of the fac isomer was decreased significantly with increasing the steric bulk of the substituted group at 5-position of the bipyridine ligand (Chart 1b). [12b] The kinetic and thermodynamic stability of transition metal complexes with cis-trans facial and meridional stereoisomers are of considerable interest. [13,14] According to theoretical studies, the trans isomers of these octahedral complexes are kinetically stable and they can be converted to cis form when subjected to thermolysis which has been further supported by the experimental studies. [15] In the course of our recent studies on transtion metal borate complexes, we have synthesized various κ 2 -N,S-chelated borate complexes trans-mer-[(κ 2 -N,S-NS2C7H4)PR3Ru{κ 3 -H,S′,S-H2B(NS2C7H4)2}], (trans-mer-1a: R = PCy3, trans-mer-1b: R = PPh3) and exploited their reactivity towards various small molecules such as boranes silanes, alkynes. [16] While working on these κ 2 -N,S-chelated species, trans-mer-1a-b, we have observed that the aerial oxidation of these Ru(II) complexes generate redox-active Ru(III) species, mer-[(κ 2 -N,S-NS2C7H4)2PR3Ru{κ 1 -S-(NS2C7H4 ( )), mer-I = R : PCy3, mer-II: R = PPh3) (Scheme 1). [17] This is mainly due to the hemilabilaity of the κ 2 -N,S-ligand benzothiazole chelated coordinated to the ruthenium center. This observation prompted us to investigate the thermodynamic stability of trans-mer-1a-b at various temperatures. The main goal of this study was to examine the capability of the flexible borate moieties and hemilabile N,S-chelating mercaptobenzothiazolyl ligand to adopt different spatial arrangements at ruthenium center. Based on the coordination modes at the ruthenium center, we describe the novel isomerization reaction of trans-meridional to cis-merdional and cis-facial isomers of ruthenium borate species along with their electronic properties. To gain an insight into the electronic structure of the complexes, density functional theory (DFT) calculations were also carried out.

Results and Discussion

Thermolysis of trans-mer-1a. As shown in Scheme 1, room temperature aerial oxidation of trans-mer-1a-b CDCl in 3 redox to converted -complexes, Ru(III) active mer-I-II. [17] However, when trans-mer-1a was thermolysed at 90°C for 24h, it led to the formation of 2a and 3a (Scheme 1). This reaction mixture did not benefit from thin-layer chromatographic (TLC) separation, which was attempted. This is mainly because of the fact that the species 2a and 3a are interconvertible in solution. Consequently, due to our inability to separate these complexes into their pure forms, we could not obtain pure spectral data for them. Therefore, we have attempted to fractionally crystalize 2a and 3a in order to isolate them in pure form. Fortunately, few needle-shaped yellow crystals was produced by a chloroform/hexane solution of 2a and 3a within the J-young NMR tube which were identified as a [(κ 2 -N,S NS2C7H4)PPh3Ru{κ 3 -H,S,S′-H2B(NS2C7H4)2}] 2a that shows cismer geometry (Figure 1b). Several attempts to get the crystals of 3a were unsuccessful. As a result, in an attempt to get the analogous molecule of 3a, we have carried out the thermolysis of trans-mer-1b that led to the formation of 2b and 3b, which are also under equillibrium. Fortunately, few needle-shaped yellow crystals was produced by a chloroform/hexane solution of 2b and 3b which were identified as a [(κ 2 -N,S-NS2C7H4)PPh3Ru{κ 3 -H,S,S′-H2B(NS2C7H4)2}] 3b that shows cis-fac geometry, (Figure 1c).

In parallel to the formation of 2b and 3b, thermolysis of trans-mer-1b also yielded 4. The 31 P{ 1 H} NMR spectrum of 4 displays a peak at δ = 52.3 ppm. The 1 H NMR spectrum shows peaks in the aromatic region in the range of δ = 6.88-7.94 ppm that denote the presence of two benzothiazole ligand and two phosphine ligands. The ESI-MS spectra of 4 shows peak at m/z = 958.9921 having isotopic distribution patterns. To get a clear picture for molecule 4, single crystals appropriate for X-ray diffraction studies were grown from the slow diffusion of a CH2Cl2/hexane solution at -5 °C. Based on the X-ray diffraction studies along with the spectroscopic data, we have identified 4 as a cis-[(κ 2 -N,S-C7H4NS2)2Ru(PPh3)2] (cis-4, Figure 2, vide infra). [18] Note that, in 2015, cis-4 and cis-[(κ 2 -N,S-C3H4NS2)2Ru(PPh3)2] complexes were isolated from the thermolysis of [Ru(PPh3)3Cl2] with 2-mercaptobenzothiazole and 2-mercaptothiazoline, respectively in presence of NEt3 and was characterized spectroscopically. [19] In addition, earlier Sousa and co-workers isolated a series of analogous complexes with various heterocyclic bidentate ligands. [20] Scheme 2. Synthesis of trans-4 and cis-4.

As complex cis-4 turned out to be interesting and we have obtained solid-state X-ray structure, we wanted to make it in good quantity for further chemistry, in praticular small molecule activation. [16] Thus in an attempt to synthesize cis-4 in a different route, one equivalent of [Ru(PPh3)3Cl2] was treated with two equivalents of potassium salt of 2-mercaptobenzothiazole ligand in THF at room temperature (Scheme 2). Within 4 hours, the colour of the reaction mixture changed from brown to yellow. The 31 P{ 1 H} NMR spectrum of the reaction mixture showed one peak at δ =52.3 ppm that corresponds to cis-4 and the peak at δ = 41.6 ppm indicates the formation of a new species trans-4 (Figure S21). The mass spectrum of this reaction mixture displays a peak at m/z = 958.9921. The chromatographic separation of this mixture allowed us to isolate cis-4 along with the unknown species, which was converted to cis-4 over time. Systematic evaluation of multinuclear NMR spectroscopic data and mass spectrometric data of the reaction mixture suggested it as the isomer of the cis-4, that is trans-4 (Figures S18-S21). Note that, thermolysis of cis-4 at 90 °C for 24 hours does not show any conversion of cis-4 to trans-4. Recently, we have reported redox-active complexes, mer-I-II that underwent unusual dual site B-H bond activation with boranes. [16] Similarly, the presence of dual active sites in cis-4 prompted us to explore the reactivity of it towards boranes. As shown in Scheme 3, the reaction of cis-4 with BH3.THF at room temperature also led to the formation of trans-mer-1b. X-ray crystallography and Geometry Assignment. As shown in Figures 1a-b, the solid state X-ray diffraction analyses of suitable crystals of 2a and 3b confirmed their molecular structures. Complex 2a crystallizes in a triclinic system with space group P ͞ 1 and 3b crystallizes in the monoclinic system with P21/c space group. The molecular structure of 2a shows a distorted octahedral geometry in which the tris-homoleptic mercaptobenzothizolyl sulfur moieties are coordinated to the ruthenium center in meridional fashion. The phosphine is cis to the hydride ligand and the N,S-donor mercaptobenzothaizolyl ligand is coordinated to ruthenium that formed four-membered ruthenacycle with bite angle 67.60 (11)°. Based on the coordination of the donor atoms to ruthenium center, 2a now can be defined as cis-mer-[(κ 2 -N,S-NS2C7H4)PCy3Ru{κ 3 -H,S,S′-H2B(NS2C7H4)2}] (cis-mer-2a). The Ru1-S3 bond distance of 2.3217( 14) Å in cis-mer-2a is significantly shorter than Ru1-S1 and Ru1-S5 bond distances of 2.4618(15) and 2.4396( 14) Å, respectively. The Ru1-S1-C21-N1 ring is almost flat with a torsion angle of -0.7( 5)˚ (Figures 1a). 6), Ru1-P1 2.3485( 14), Ru1-S1 2.4618( 15), Ru1-S3 2.3217( 14), Ru1-S5 2.4396( 14), N1-Ru1-S1 67.60 (11), S3-Ru1-S5 87.43( 5), S3-Ru1-P1 94.46 (5). . cis-fac-3b (b): Ru1-B51 2.578(9), Ru1-P1 2.279( 2), Ru1-N22 2.131(7), Ru1-S21 2.317( 2), Ru1-S1 2.454( 2), Ru1-S11 2.486( 2), N22-Ru1-S1 67.6( 2), S21-Ru1-S1 103.06( 8), S21-Ru1-S11 87.37(]7), S1-Ru1-S11 84.11 (8).

The molecular geometries of 3b depicted in Figures 1c revealed distorted octahedral geometry in which the mercaptobezothiazolyl sulpur atoms are coordinated to ruthenium center in a facial fashion. However, the hydride, phosphine and nitrogen donor sites are trans to each of the mercaptobenzothizolyl sulphur atoms. Four equatorial atoms of them are arranged in a square plane (H-S-S-N) and their structures are similar to trans-mer-1b with different orientation of the borate and phosphine ligand around ruthenium center. Interestingly, in both the cases, the hydride ligand is cis-oriented to the phosphine ligand. Three sulfur atoms of tris-homoleptic mercaptobenzothizolyl are coordinated to the ruthenium center in facial fashion. As a result of the donor atoms coordination with the ruthenium centre, 3b may now be described as cis-fac-3b. The bite angles (N-Ru-S) for four membered chelating ruthenacycle are 67.6(2)° for cis-fac-3b. The Ru-B bond distance of 2.575(3) Å (cis-fac-3b) are comparatively shorter as compared to trans-mer-1b (2.775 Å). The Ru-P bond distances for cis-fac-3b (2.2740(7)Å) are significantly longer as compared to trans-mer-1b 2.2696(15) Å. The molecular structure of 4, shown in Figure 2, displays an octahedral geometry around Ru center comprising two κ 2 -1,3-N,S-chelated rings along with two phosphine ligands. Considering the fact that these phosphine ligands are cis to each other, 4 is defined as cis-4. In addition, the phosphine ligands are trans to the N atoms of the κ 2 -1,3-N,Smercaptobenzothiazolyl donor ligands. The torsion angles of both the four-membered (S-C-N-Ru) rings (1.6(6)° and -0.4(6)°) Having all these molecules with different phosphines, we are now able to assign the 1 H, 11 B{ 1 H} and 31 P{ 1 H} chemical shifts from their combined NMR spectra. The 11 B{ 1 H} NMR spectrum of the mixture of 2b and 3b shows one broad resonance in the upfield region at δ = -4.6 ppm. Besides the 1 H chemical shifts for mercaptobenzothiazolyl and phosphine ligands, the 1 H NMR spectra of them show chemical shifts in the range of δ = -16.84 to -15.12 ppm likely due to the presence of Ru-H-B protons. In cis-mer-2b, the hydride is trans to N, whereas in case of cis-fac-3b, the hydride is trans to S atom. Due to the greater electronegativity of N atom, donation of electrons from the B-H bond to ruthenium is expected to be more as compared to the Ru-H-B proton, which is trans to S atom. As a result, it is reasonable to assume that the metal hydride character for Ru-H-B proton in cis-mer-2b is greater than the cis-fac-3b isomer. Thus, the 1 H chemical shift of thehydride in cis-mer-2b appears in the upfield region as compared to cis-fac-3b. Based on the trans effect and DFT studies (Table S1), the peak at δ = -16.84 ppm is assigned to Ru-H-B of cis-mer-2b and the chemical shift at δ = -15.12 ppm is allocated of cis-fac-3b (Figure 3). The 31 P chemical shifts at δ = 55.8 ppm has been assigned to cis-fac-3b and δ = 59.9 ppm for cis-mer-2b(Table S1). An ESI-MS spectrum shows an isotropic pattern at m/z 874.9563. In a similar fashion, the chemical shifts for cis-mer-2a and cis-fac-3a have been assigned (Figures S4-S17). The cis-trans nomenclature for molecules trans-mer-1a-b, cis-mer-2a-b, and cis-fac-3a-b, has been established based on the J(H,P) coupling constant values obtained from 1 H{ 11 B} NMR. In the 1 H NMR spectrum for trans-mer-1b, the broad hydride peak at δ = -3.71 ppm appeared as a doublet of doublet when decoupled with 11 B ( 2 JHP = 33.2 Hz, 2 JHH = 16.3 Hz) (Figure 4). This indicates that the hydride is trans to phosphine ligand. [21] Similarly, for cis-fac-3b, the broad hydride peak at δ = -15.6 ppm appears as a doublet of doublet with coupling constants, 2 JHP = 9.1 Hz and 2 JHH = 16.3 Hz. This may be due to the adjacent cisoriented phosphine group. This looks like a triplet as a consequence of minimal difference in coupling constant values of 2 JHP and 2 JHH. The 1 H NMR spectra for cis-mer-2b and cis-fac-3b appeared significantly in the upfield region as compared to trans-mer-1b (δ = -3.4 ppm). This is due to the trans effect of phosphine ligands. The 1 H chemical shift of the aromatic protons of 1,3-N,S-chelated mercaptobenzothizole ligand of cis-fac-3b has been shifted significantly to upfield region as compared to trans-mer-1b isomer. This may due to the cis orientation of the phosphine moeiety. [22] 10.1002/ejoc.202201283

Accepted Manuscript

European Journal of Organic Chemistry DFT Calculations. To gain some insight into the bonding nature and to understand the differences and similarities of the transmer-1a-1b, cis-mer-2a, cis-fac-3a stereoisomers, various computational simulations at the bp86/def2-tzvp [23] level of theory-based in DFT were executed. The HOMOs of the trans isomers are mainly localized on the d-orbital of ruthenium and the p-orbitals of nitrogen and sulfur atoms of the borate ligand, whereas in the case of the cis isomers, the HOMOs involve the p-orbitals of sulfur atoms of the chelated benzothiazole moiety along with the ruthenium d-orbital and p-orbitals nitrogen and sulfur atoms of the borate ligand (Figure S23). The LUMOs are mainly found to be localized over benzothiazole moieties of borate ligand for both the isomers (Figure S23). The HOMO-LUMO energy gap increases in the order trans-mer < cis-fac < cis-mer (Table S2). The natural bonding orbital (NBO) analysis shows that charge transfer from phosphorus to ruthenium follows the trend trans-mer<cis-mer<cis-fac, [24] which is further supported by the natural valence population analysis (Table S2). The NBO analysis dictates very strong Ru-H-B bonding interaction (Figure 5a-c). In addition, the contour of the laplacian of the electron density(∇ 2 ρ) shows the presence of a ring-critical point (RCP) in the RuSCN plane of cis-mer-2a (Figure 5d). Electronic spectra. Conversion of trans-mer-1a-b to cis-mer-2a-b and cis-fac-3a-b has prompted us to explore the photophysical properties of these isomers. The UV-vis spectra of complexes 1a-3a and 1b-3b recorded in dichloromethane shows an intense absorption band in the region of 236-250 nm that may be assigned to π-π* transitions of the phosphine and mercaptobenzothiazolyl ligands (Figure 6). Another electronic transition peak in the window of 317-421 nm is possibly due to the charge transfer transition(MLCT). The results indicate that the ligands orientation around the metal center does not significantly affect the general photo-physical properties of the complexes. Cyclic voltammetry. Lahiri, Liobet and coworkers have demonstrated that one may compare the electron density on the central metal atom provided by the ligand that is arranged in trans and cis fashion around the metal centre by using cyclic voltammetry as a tool. [25] These observations encourage us to conduct cyclic voltammetry experiment. The redox behaviour of trans-mer-1b, and mixture of cis-mer-2b, cis-fac-3b species has been studied by cyclic voltammetry (CV). The voltammograms of both compounds are almost similar; the main difference observed is the peak potentials. The cyclic voltammogram for trans-mer-1b in dichloromethane shows reversible waves at E1/2= 0.47 V, (Ipc/Ipa = 0.97) and for cis-fac-3b the reversible peaks appear at 0.59 V (Ipc/Ipa = 0.98), (Figure S22) respectively. These peaks have been assinged to Ru III /Ru II redox couple. In addition, the irreversible waves at 0.79 V (trans-mer-1b) and 0.93 V (cis-fac-3b) appears due to oxidation of the N,S-donor mercaptobenzothiazole redox centered couple. With reference to their respective positions, the phosphine and hydride ligands provided a higher electron density on the ruthenium metal enabling the cis-mer-2b, cis-fac-3b anodic peak to arise at a higher potential than the trans-mer-1b anodic peak. This makes trans isomer more capable of oxidising the metal centre than cis isomers. As a result, the oxidation potential emerges at a lower value for the trans isomer.

Conclusion

In summary, we have synthesized some interesting cismeridional and cis-facial isomers of ruthenium borate species which are supported by hemilabile N,S-chelated ruthenacycle. They are exciting stereoisomeric organometallic species, based on a cis-trans isomer they might open a new direction in catalysis. Further, we have studied the kinetic and thermodynamic stability of these stereoisomers of ruthenium borate complexes at different reaction conditions. In addition, we have synthesized cis-trans isomeric mixture of stereoisomers in which the octahedral Ru centre is coordinated with two units of chelate ring through 1,3-N,S-chelating fahsion along with two phosphine moities. Based on the J(H,P) coupling constant values obtained from boron decoupled proton NMR, the cistrans nomenclature for the molecules trans-mer-1a-b, cis-mer-2a-b, and cis-fac-3a-b has been defined. Comparative study of cis and trans isomer on the basis of diffrent in electron density

Chart 1 .

 1 (a) fac-and mer-rhodium DPEphos complexes, (b) Examples of facand mer-ruthenium complexes, (c) cis-mer and cis-fac complexes, our work.

Scheme 1 .

 1 Scheme 1. Synthesis of the Ru(II) borate of complexes cis-mer-2a-b, cis-fac-3a-b.

Figure 1 .

 1 Figure 1. Molecular structures of cis-mer-2a (a), cis-fac-3b (b), Selected bond lengths (Å) and angles (°). cis-mer-2a (a): Ru1-B1 2.544(6), Ru1-P1 2.3485(14), Ru1-S1 2.4618(15), Ru1-S3 2.3217(14), Ru1-S5 2.4396(14), N1-Ru1-S1 67.60(11), S3-Ru1-S5 87.43(5), S3-Ru1-P1 94.46(5). . cis-fac-3b (b): Ru1-B51 2.578(9), Ru1-P1 2.279(2), Ru1-N22 2.131(7), Ru1-S21 2.317(2), Ru1-S1 2.454(2), Ru1-S11 2.486(2), N22-Ru1-S1 67.6(2), S21-Ru1-S1 103.06(8), S21-Ru1-S11 87.37(]7), S1-Ru1-S11 84.11(8).

Scheme 3 .

 3 Scheme 3. Synthesis of trans-mer-1b from cis-4.

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Figure 2 .

 2 Figure 2. Molecular structures of cis-4 (a), selected bond lengths (Å) and angles (°). cis-4: Ru1-P1 2.300(3), Ru1-P2 2.301(3), Ru1-S1 2.433(3), Ru1-S3 2.450(3), Ru1-N42 2.177(10), N42-Ru1-S1 67.3(3) N52-Ru1-S3 67.1(3).

Figure 3 .

 3 Figure 3. 1 H and 31 P{ 1 H} NMR studies for interconversion of trans-mer-1b to cis-mer-2b and cis-fac-3b.

Figure 4 .

 4 Figure 4. Comparison of 1 H and 1 H{ 11 B} NMR spectrum for trans-mer-1b and cis-fac-3b.

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Figure 5 .

 5 Figure 5. (a), (b) and (c) represents the Ru-H-B interaction in trans-mer-1a, cis-mer-2a and cis-fac-3a respectively; (d) and (e) shows the Contour-line map of the Laplacian of the electron density in the Ru-SCN plane and Ru-H-B plane of cis-mer-2a. BCPs and RCPs correspond to blue and orange dots and red lines indicate bond paths.

Figure 6 .

 6 Figure 6. UV-vis spectra of trans-mer-1a-b, cis-mer-2a-b and cis-fac-3a-b.

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Acknowledgements

This work was supported by Council of Scientific & Industrial Research (CSIR) (Scheme No. 01(3055)/21/EMR-II), New Delhi, India. A. A thanks Council of Scientific & Industrial Research (CSIR) and S. S. thanks INSPIRE for research fellowship. M. Z. thank IIT Madras for research fellowship. IIT Madras is gratefully acknowledged for computational facilities. Keywords: borate • facial • isomerism • meridional • ruthenium

Entry for the Table of Contents

The cis-mer-and cis-fac-κ 2 -N,S-chelated ruthenium borate complexes have been synthesised from the thermolysis of trans-mer isomers. The hemilability of the flexible borate ligand is one of the key factors for the formation of these isomers.

Experimental Section

General Procedures and Instrumentation. All operations were done by using glove box techniques and standard Schlenk line under an atmosphere of dry argon. Solvents such as tetrahydrofuran, hexane and toluene were distilled through Na/benzo-phenoneketyl and dichloromethane was dried over calcium hydride prior to use under argon. CDCl3 dried over calcium hydride for 12 h, and degassed by three freezepump-thaw cycles, stored over 4 Ǻ molecular sieves in a Young's ampoule under argon. The complexes [R3P(κ 2 -N,S-(C7H4NS2) Ru{κ 3 -H,S,S′-H2B(C7H4NS2)2}] [16b] (1a: R = Cy; 1b: R = Ph), [Ru(PPh3)3Cl2] [26] was prepared according to literature. The external reference for the 11 B{ 1 H} NMR spectroscopy, [Bu4N][(B3H8)] [27] was synthesized according to the literature method. The 1 H, 1 H{ 11 B}, 11 B{ 1 H}, 13 C{ 1 H}, and 31 P{ 1 H} were recorded on Bruker 400 and 500 MHz instruments. The residual solvent protons were used as reference (δ, ppm, Benzene-d6, 7.16, CDCl3, 7.26, Toluene-d8, 7.09 ppm), while as an external reference for 11 B NMR spectra, a sealed tube containing [Bu4N][(B3H8)] in Benzene-d6 (δB, ppm, -30.07) was used. To remove the broad 11 B background signal of the NMR tube, 1 H decoupled 11 B spectra of all compounds were administered with a backward linear prediction algorithm. Aluminum supported silica gel TLC plates with 250 µm diameter (Merck TLC plates) were utilized in thin layer chromatographic separation of the reaction mixture. High-resolution ESI-MS were recorded with an Agilent 6545A Q-TOF mass spectrometer. Infrared spectra were obtained on a Jasco FT/IR-1400 spectrometer.

Please see synthetic procedures and experimental details in Supporting information.

X-ray structure analysis. Suitable X-ray quality crystals of cis-fac-3b and cis-4 were grown by slow diffusion of a hexane/dichloromethane solution at -5 ˚C, respectively. Whereas the crystals of cis-mer-2a were grown by slow diffusion of a chloroform/hexane at -33 ˚C. Crystal data of cis-mer-2a, cis-fac-3b and cis-4 were obtained using a D8 VENTURE Bruker SC diffractometer with graphite monochromated Mo-Kα (λ = 0.71073 Å) radiation at 150 (2) K. All the structures were solved using SIR97 [28] and refined with SHELXL-2018, SHELXL-2014, and SHELXL-2017. [START_REF] Sheldrick | [END_REF] Using Olex2 [30] all the molecular structures were drawn. The non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogens could be located in the difference Fourier map. However, the hydrogen atoms bonded to carbon and boron atoms were fixed at chemically meaningful positions and were allowed to ride with the parent atom during the refinement. Crystallographic data have been deposited with the Cambridge Crystallographic Data Center as supplementary publication no CCDC-2052114 (for cis-mer-2a), CCDC-2052098 (for cis-fac-3b), CCDC-1588903 (for cis-4). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Crystal data for cis-mer-2a: C41H49BN3PRuS6, Mr = 1131.74 triclinic, space group P-1, a = 9.6800( 9 6)°, γ = 90°, V = 9344(3) Å3, Z = 8, ρcalcd = 1.362 g/cm3, μ = 0.619 mm-1, F(000) = 3920.0, R1 = 0.1595, wR2 = 0.3902, 21342 independent reflections [2θ ≤ 54.968°] and 977 parameters. [17] Computational Details. Density functional theory (DFT) calculations employing BP86 [23] functional were performed with the Gaussian 16 [31] program. Full geometry optimizations were conducted in gaseous state (no solvent effect) without any symmetry constraints using a def2-tzvp basis set from Basis Set Exchange Library. [START_REF]Basis Set Exchange Library[END_REF] The optimized geometries were characterized as true minima by conducting frequency calculations. We have computed 11 B NMR chemical shifts using gauge-including atomic orbitals (GIAOs) [33] method at the same basis set by B3LYP. The 11 B NMR chemical shifts were calculated relative to B2H6 and converted to the usual [BF3.OEt2] scale by using experimental δ ( 11 B) value of B2H6 ( δ = 16.6 ppm). [34] Natural bonding analyses were carried out with the natural bond orbital (NBO) 6.0 version of program. [35] Wiberg bond indices (WBI) [36] were obtained from natural bond orbital analysis. The electron density and Laplacian electronic distribution plots were generated using Multiwfn package. [START_REF] Lu | Multiwfn: A multifunctional wavefunction analyzer[END_REF] All the optimized structures and orbital plots were gerenated by using Chemcraft. [START_REF]Chemcraft -graphical software for visualization of quantum chemistry computations[END_REF]