Laura Abad Galán

David B Cordes

Alexandra M Z Slawin

Denis Jacquemin
email: denis.jacquemin@univ-nantes.fr

Mark I Ogden
email: m.ogden@curtin.edu.au

Massimiliano Massi
email: m.massi@curtin.edu.au

Eli Zysman-Colman
email: eli.zysman-colman@st-andrews.ac.uk

A Laura

Prof Abad Galán

I Mark

Prof Asoc Ogden

Massi Max

B Laura

Prof David D Cordes

M Z Alexandra

Eli Slawin

Zysman-Colman

Prof C Denis Jacquemin

David B Cordes

Analysing the Relation Between Structure and Aggregation Induced Emission (AIE) Properties of Iridium(III) Complexes through Modification of Non-Chromophoric Ancillary Ligands

Unconventionally modified dibenzoylmethane (dbm) ligands have been synthesised and successfully utilised as ancillary ligands for neutral Ir(III) complexes of the formula [Ir(dFppy)2(LX)], where dFppyH is 2-(2,4-difluorophenyl)pyridine and LX is tribenzoylmethane (tbm) or 1-phenyl-3-(4-(pyridin-2yl)phenyl)propane-1,3-dione (pydbm). The modification of the ligands aims to prevent or enhance possible intermolecular interactions between the dFppy and/or the LX moiety in comparison with the previously reported [Ir(dFppy)2(dbm)] complex. The aggregation induced emission (AIE) properties of these complexes are significantly modulated, as a consequence of the different π-π interactions revealed by X-ray crystallography.

10.

Introduction

Since the observation of phosphorescence of tris(2,2'bipyridyl)ruthenium(II) dichloride [Ru(bpy)3]Cl2, [START_REF] Paris | [END_REF] phosphorescent transition metal complexes have garnered a great deal of attention [2][3][4][5][6] due to their use in a wide range of applications such as emitters for electroluminescent devices, 7 sensitizers for energy and electron transfer, [8][9][10][11] photocatalysis, 6,[12][13][14][15] dyes for solar cells, 16 imaging reagents as biological probes, [17][18][19] and sensors. 20,21 Aggregation-induced emission (AIE) is a photophysical phenomenon, first formulated in 2001, where non-emissive luminogens are induced to emit light after the formation of aggregates. 22 Its discovery provided a new platform of research that quickly motivated the science community because of its potential application particularly in life science and biomedical engineering. [23][24][25][26][27] Solid-state phosphorescence emission, sometimes described as AIE, has been often found in platinum (II) complexes. Owing to their planar structures, Pt-Pt interactions in the solid state are possible, allowing efficient metal-metal-to-ligand charge transfer (MMLCT) transitions. [28][29][30] In contrast, octahedral iridium complexes cannot interact in a similar manner, making MMLCT transitions impossible. In 2008, the first AIE iridium(III) complex was presented, [Ir(ppy)2(dbm)],

where ppyH = 2-phenylpyridine and dbm = dibenzoylmethane. 31 This complex showed strong π-π interactions between the offset pyridine rings of two adjacent ppy ligands with a distance of ~3.37 Å. This proximity modifies the nature of the excited states, presenting enhanced phosphorescence in the solid state. Since then numerous examples have been presented in the literature. [32][33][34][35][36][37] Moreover, several examples of dFppy-based iridium complexes have also shown AIE properties. 35,[38][39][40] The [Ir(dFppy)2(dbm)] complex [dFppyH = 2-(2,4difluorophenyl)pyridine], in particular, presented different π-π interactions between two neighbouring complexes in the X-ray structure that involved the dFppy and the ancillary dbm ligands. However, the lack of examples where structural organisation and AIE effects are directly compared inspired us to undertake this study. Two new complexes and the previously reported complex [Ir(dFppy)2(dbm)], which is used as reference, have been synthesised in an effort to better understand the relationship between intermolecular interactions, particularly involving the ancillary ligand, and the AIE phenomenon. The dFppy ligand was chosen as the cyclometalating moiety as it is known to favor π-π interactions in the solid state. Firstly, the previously reported dbm and tbm 41

Results and Discussion

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(Figure 2) in 65% yield. Both ligands were characterised by melting point, 1 H-NMR, 13 C-NMR spectroscopy, mass spectrometry and elemental analysis (See experimental section). Synthesis of the complexes. The chloro-bridged dimer, [Ir(dFppy)2(µ-Cl)]2, was synthesised in 67% yield by refluxing dFppyH in 2-ethoxyethanol with IrCl3 as the iridium source. For the synthesis of the previously reported complex [Ir(dFppy)2(dbm)] (1), sodium carbonate in 2-ethoxyethanol was used, affording the product in 90% yield, where the characterisation matched that previously reported. 42 When this methodology was followed for the synthesis of 2 using tbmH, the resulting retro-Claisen condensation product, 1, was obtained. However, when triethylamine in a mixture of DCM/EtOH (80:20 v/v) was employed, the desired complex 2 was isolated in 40% yield; the retro-Claisen product 1 corresponded to the remainder of the isolated material in a 30% yield (Figure 3).

The synthesis of 3' was first attempted without the presence of base in 2-ethoxyethanol. These conditions were chosen to favor the coordination of the pydbm in a C^N mode leaving thedione moiety free for further coordination. However, the crystal structure of the resulting purified product, obtained in 50% yield, showed coordination of the ligand in a ketonate mode within the complex [Ir(dFppy)2(pydbm)], 3 (Figure 4b). The conditions of the complexation reaction were then modified in an attempt to increase the yield of the reaction. When NEt3 and a mixture of DCM/EtOH (80:20 v/v) were used, complex 3 was synthesised in an excellent yield of 90%. Finally, the synthesis of complex 4, [Ir(dFppy)2(pytbm)], was attempted following the O^O coordination conditions. Unfortunately, the desired complex could not be isolated and only the retro-Claisen condensation complex, 3, was obtained as the main product of the reaction in 80% yield. Retro-Claisen condensation reactions of βtriketonates in solution has been previously noted 43,44 by us and the scope and mechanism of this transformation is currently under investigation. The three isolated complexes 1-3 were fully characterised by melting point, 1 H-NMR, 13 C-NMR, 19 F-NMR, mass spectrometry and elemental analysis (see experimental section). X-ray diffraction studies. The crystal structure of 1 had already been reported. The literature report stated that the [Ir(dFppy)2(dbm)] complex crystallised as two polymorphs: one in the space group C2/c and the other one in P . 38 The main difference between the two is the presence of different π-π interactions between two neighbouring complexes, involving a combination of two dFppy, one dFppy and one dbm or two dbm ligands. In the case of complexes 2 and 3, crystals were obtained from dichloromethane by slow diffusion of either ether (2) or hexane (3). Complex 3 crystallised in the triclinic space group P . The asymmetric unit contained two similar but symmetrically inequivalent Ir(III) units, with an Ir … Ir distance of 8.85 Å.

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Interestingly, these two independent Ir(III) complexes are connected by π-π interactions involving the offset pyridine group of a dFppy ligand and the phenyl ring of the pendant phenylpyridine moiety of pydbm, with interplanar distance and centroid-to-centroid distance of 3.315 Å and 3.808 Å, respectively (Error! Reference source not found.). In the case of 2, the complex crystallised in the monoclinic space group I2/a, with a single complex in the asymmetric unit. Only one set of π-π interaction are found (Error! Reference source not found.); between adjacent and offset dFppy ligands, with a centroid to centroid distance of 3.678(2) Å. This π-π interaction also has a pair of CH•••π interactions working in concert with it, involving hydrogen atoms on the π-stacking rings interacting with the π-system of further dFppy rings of the adjacent complex, at a distance of 2.62 Å. As predicted, the extra acyl group of the b-triketonate, in comparison to the dbm molecule, seems to be responsible of the lack of π-π interactions between the phenyl rings of two subsequent units. UV-VIS absorption. The absorption spectra of 1-3 measured in acetonitrile are presented in Error! Reference source not found. while Figure S13 provides the corresponding spectra obtained with Time-Dependent Density Functional Theory (TD-DFT). Complexes 2 and 3 present relatively unstructured spectra comparable to that of 1, which itself shows a profile equivalent to that reported in the literature. 38 Complexes 2 and 3 show higher molar absorptivities compared to 1, likely due to the presence of the extra acyl group and pyridine ring, respectively, in these complexes. The high energy absorption bands are assigned to π-π * transitions of the dFppy 45,46 and ancillary ligands. 31,47 (250 -350 nm). The low energy band at 380 nm is attributed to a metal-to-ligand charge transfer transition ( 1 MLCT), while bands above 400 nm can be assigned to mixed CT. 38 The onset of the CT bands is of comparable energy at ~465 nm for the three complexes, which suggests that these bands are based on the π orbitals of the dFppy ligands and similar metal orbitals. TD-DFT predicts the lowest vertical singlet transitions at 413 (f=0.02) and 406 nm (f=0.04), 409 (f=0.02) and 406 nm (f=0.03), and 426 (f=0.01) and 407 nm (f=0.04) for for 1, 2 and 3, respectively, all of which correspond to mixed HOMO to LUMO and HOMO to LUMO+1 transitions. As can be seen in Figure S14, the HOMO is localised on the metal and fluorinated phenyl rings, whereas the LUMO and LUMO+1 are localised on the ancillary ligand and C^N ligands, respectively. These bands therefore have a mixed MLCT and LLCT character. Electrochemistry. The electrochemical properties were studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in de-aerated MeCN at 298 K in order to estimate the energies of the frontier molecular orbitals of 1-3 A summary of the redox potentials of the complexes, referenced with respect to the saturated calomel electrode, SCE, (Fc/Fc + = 0.38 V in MeCN) 48 , is given in Table 1.

The electrochemistry of the related tris-cyclometalated Ir(III) complex, fac-Ir(dFppy)3, had been previously studied. 49 The reversible oxidation of this reference complex involves the FULL PAPER iridium ion and the π orbitals of the difluorophenyl (dFph) part of the dFppy ligands (Ir III/IV + dFph) whereas the reduction is localised on the pyridyl ring of the dFppy ligands. Thus, if oxidation or reduction processes involving the ancillary ligand do not take part in these systems, complexes 1-3 will present a comparable behaviour to that of fac-Ir(dFppy)3. The reported redox values for fac-Ir(dFppy)3 are Eox = 0.76 V; Ered = -2.49 V vs SCE in MeCN.

A second reference complex, [Ir(dFppy)2(acac)], 50 possesses comparable electrochemical properties (Eox = 0.76 V; Ered = -2.44 V vs SCE) to those of fac-Ir(dFppy)3.

The oxidation potentials of 1-3 were found, respectively, at 1.05 V, 1.30 V and 1.10 V (Figure 8). These numbers vary significantly from the oxidation potentials for the two reference complexes, suggesting some contribution from the ancillary ligands, likely via influence of the electron density on the iridium. Furthermore, while 1 and 3 show similar oxidation potentials, complex 2 presents a larger value, which is indicative of a stronger influence of tbm in comparison to dbm or pydbm on the oxidation process. When looking at the reduction potential values (Ered(1) = -1.85 V; Ered(2) = -1.67 V; Ered(3) = -1.60 V) a significant change with respect to the reduction of fac-Ir(dFppy)3 (Ered = -2.49 V) is observed that is indicative of an ancillary ligand-based LUMO, in agreement with previous assignments in the literature for the [Ir(dFppy)2(dbm)], 38,42 as well as with the present theoretical result (Figure S14). Surprisingly, significantly different values of the oxidation and reduction potentials as well as the redox gap were found in the literature for 2 (Eox 42 = 0.32 V, Ered 42 = -2.02 V and ΔE 42 = 2.34 eV; Eox 38 = 0.73 V, Ered 38 = -2.16 V and ΔE 38 =2.64 eV). 38,42 These literature values are also different to the experimental data observed in the present study under similar conditions (Eox(1) = 1.05 V, Ered(1) = -1.85 V and ΔE(1) =2.90 eV). This could be due to different values given for the internal standard ferrocene/ferrocenium (Fc/Fc + ). 48 While in the case reported here a value of 0.38 V vs SCE 48 was used, other numbers such as 0.40 V 51 or 0.45 V 52 can also be found in literature. Unfortunately, the reported literature for complex 2 does not include the number used for this referencing, making this comparison infeasible.

The LUMO of complex 3 is the most stabilised, which is a logical consequence of the presence of a pyridyl ring. The DFT calculation indeed also return a LUMO more stabilised in 3 than in both 1 and 2, by -0.16 eV. As can be seen in the ESI, the LUMO of 1 and 3 is localised almost exclusively on the ancillary ligand, whereas there is additionally a small contribution from the C^N ligand for 2. Photophysics. The photophysical properties of 1-3 were studied in MeCN and dichloromethane at room temperature, in a dichloromethane frozen matrix at 77 K, in doped films (10 wt% in PMMA) and in the solid state as powder samples. The photophysical data, including excited state lifetimes (tPL) and photoluminescence quantum yields (FPL) are reported in Error! Reference source not found. and 3.

The room temperature emission spectra in deaerated MeCN show very weak emission at 629 nm, 498 nm and 625 nm for the 1, 2 and 3, respectively (Figure 9). In the case of complexes 1 and 3, the broad emission band is significantly red-shifted compared to the structured emission of fac-Ir(dFppy)3 at 469 nm, itself previously assigned to mixed MLCT/LC transitions. 49 The tPL were found to be on the order of nanoseconds (tPL (1) = 12 ns and tPL (3) = 7.5 ns) and the FPL values below 1%, indicative of large non-radiative decay. This photophysical profile strongly contrasts with the FPL and FPL of fac-Ir(dFppy)3 (tPL = 1.64 µs , FPL = 77%) 49 . Indeed, previous DFT calculations for 1 suggested that the emission had MLCT mixed with intraligand charge transfer (ILCT) character and the poor emissive properties in solution were caused by efficient non-radiative decay promoted by strong vibrations of the dbm in the low frequency region. 53 Therefore, comparable behaviour would be expected for complex 3. In the case of complex 2, although the emission maximum is not significantly shifted with respect to the 3 MLCT/LC transition of fac-Ir(dFppy), the tPL (30 ns), FPL (2%) and profile of the emission bands are indicative of significant changes and seem more characteristic of significantly quenched ligand-centred (LC) emission. For 1, 2 and 3, the DFT computed 0-0 phosphorescence wavelengths are 565, 518 and 576 nm, respectively, which should correspond to the intersection point between the absorption and phosphorescence bands. The ordering is in good agreement with the measurements of Figure

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9, although our value for 2 is too blue-shifted. Therefore, the triplet states of the three ancillary ligands were also estimated at the 0-0 transition of the phosphorescence spectra of their gadolinium complexes. Values of 487 nm, 183 nm and 504 nm for the dbm, tbm and pydbm were found, respectively (Figure S15). In the case of tbm, this experimental value matches with what it was observed for complex 2, which confirms the LC nature of this transition.

The evolution of the geometry of the structures in going from the lowest singlet to triplet states (coordinates in the ESI) is also markedly different. While for 2, the acac moiety is almost unaffected by the change of state, in both 1 and 3, one notices a shortening of the Ir-O bonds (from ca. 2.16 Å to 2.09 Å) and a significant change of dihedral angle of the acac moiety compared to the ideal orientation that goes from 2-3° in the singlet state to ca. 25° in the triplet state. The computed spin density difference plots can be found in Figure 10. For 1 and 3 the spin density of the triplet is mainly localised on the metal and the ancillary ligands, hence explaining the similarity between the two emission spectra. In contrast, in 2, the triplet state is mainly localised on one of the two more rigid C^N ligands according to DFT, consistent with the emission profile shown in Figure 9 as well as with the above described changes of structures. It is also interesting to note that the spin density on the Ir centre is estimated to be similar in all three complexes (ca. 0.5-0.6), i.e., it is the localisation of the triplet rather than the involvement of the metal that differs in the three structures. The photoluminescence spectra were also studied in deaerated dichloromethane at room temperature in order to assess the impact of using less polar solvents (Figure 9). CT states are frequently found to be stabilised upon increasing the solvent polarity. [54][55][56][57][58] This effect is commonly known as positive solvatochromism. However, solvatochromisim is a complex phenomenon as many different interactions and dynamic processes can take place. Indeed, the experimental data for complexes 1 and 3 showed an unusual but not unprecedented 59,60 negative solvatochromic shift. This indicates a decrease of the dipole moment in the excited state with respect to the ground state, i.e., DFT predicts that the total dipole moment of 1 (3) goes from 6.8 to 1.2 D (6.8 to 3.2 D) when going from the ground to the excited state. Such changes naturally translates into a higher stabilisation of the ground state in more polar solvents consistently with previous suggestions. 59 By contrast, the emission maximum undergoes a slight positive solvatochromism in complex 2, which is consistent with DFT (increase of dipole from 5.4 D to 8.9 D) and with the different nature of the triplet state (vide supra). 45 When the photoluminescence properties were studied in a frozen dichloromethane matrix, a clear blue-shift was identified in complexes 1 and 3, while the emission maximum of 2 remained unchanged (Figure 11). The fact that no rigidochromic shift was found for the emission maximum in 2 hints to a 3 LC nature emission for this complex in the solid-state. By contrast, the blue-shifted emission of complexes 1 and 3 suggests mixed CT character, in agreement with the literature and the RT PL studies. 42 The three complexes in the frozen matrix presented prolonged lifetime values of 800 ns, 3.77 µs and 630 ns for 1, 2 and 3, respectively. These longer lifetimes can be explained by a reduction of the vibrational motions at 77 K, which results in a decrease in the non-radiative decay rate. While 1 and 3 have similar ФPL values, that of 2 is much longer, which is another indication that in this case, the transition is LC in nature. 10.1002/ejic.201801118

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In contrast to the weak emission found for the three complexes in solution, intense luminescence was produced at ~550 nm when the complexes were doped into films (10 wt% in PMMA), with improved photoluminescence quantum yields of 8.2%, 8.9% and 6.3% for 1, 2 and 3, respectively (Figure 12). As previously observed for [Ir(dFppy)2(dbm)] (1), 38,42 the phosphorescence mechanism for 2 and 3 is modulated by intermolecular interactions. These interactions promote the stabilisation of the excited states and also induce reduced non-radiative decay. Interestingly, the maximum emission wavelength is similar for the three complexes at ~550 nm, which may indicate that the transitions occurring are based on similar delocalised states as previously suggested for 1. 31

Photoluminescence quantum yields in MeCN relative to

[Ru(bpy)3](PF6)2 in MeCN (FPLref = 9.5%) 61 When the photophysical behaviour of 1-3 were explored in the crystalline powder state, where there can be different sets of intermolecular interactions, a red-shift in the emission maximum was found for each of the here investigated complexes (Figure 12). These data suggest that the excited states change nature due to the different intermolecular interactions as a function of increased aggregation. Interestingly, the largest shift was found for complex 2, which happened to present the strongest ancillary ligand-based interactions as assessed by the relatively shorter distance of 3.2 Å, against 3.4 Å for the dbm and pydbm ligands in the crystal structure. Photoluminescence quantum yields and excited state lifetimes in the powder state were measured to be 21%, 3% and 6%, and 361 ns, 13 ns and 97 ns for 1, 2 and 3, respectively. These data reveal that the dbm-containing complexes are more efficient emitters in the powder. Indeed, while radiative decay rates (kr), calculated from FPL = kr/(kr + knr) were comparable for complexes 1 and 3 and significantly larger in the case of 2, non-radiative decay rates (knr) for 2 and 3 were found to be 30 and 4 times larger than that of 1 (22•10 6 s -1 ). It is important to note that the plane of the acac unit for complexes 1 and 3 in the excited state is significantly distorted with respect to the C-Ir-C plane while this distortion is almost neglible for complex 2. These data therefore suggest that the lack of distortion in the solid state may lead to possible deactivation pathways, which is in agreement with results previously found on acac and iminophenolate complexes. 62 Films were prepared by doping 10 wt% of the complex in PMMA.

As a function of this analysis, AIE studies were performed on 1-3. Indeed, addition of water into dilute acetonitrile solutions turned on the photoluminescence emission of the three complexes with a large enhancement in the emission intensity for 3, a modest enhancement for 1, and almost a negligible effect for the tbm analogue, 2 (Figures 13, S11 and S12, respectively). These results clearly suggest that the structural modifications of the ancillary ligands tune the intermolecular interactions, and hence directly affect the AIE behaviour. The PL intensity was a maximum when the water content reached 70%, 80% and 60% for 1, 2 and 3, respectively, and then decreased at higher percentages in every case. This is probably due to two different reasons: firstly, after aggregation, the molecules become covered with other aggregated particles and do not emit light; 63

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and secondly, because in the process of aggregation of crystallites, amorphous particles are simultaneously formed, which do not enhance the emission intensity. 64 Photophysical measurements. All samples were prepared in HPLC grade MeCN. Absorption spectra were recorded at room temperature using a Shimadzu UV-1800 double beam spectrophotometer. Molar absorptivity determination was verified by linear least squares fit of values obtained from at least four independent solutions at varying concentrations ranging from 5.22 × 10-5 to 8.41 × 10-6 M. The sample solutions for the emission spectra were prepared in HPLC grade MeCN and degassed via three freeze-pump-thaw cycles using an in-house designed quartz cuvette. Steady-state and time-resolved emission spectra were recorded at room temperature using a Gilden photonics Fluorimeter and Edinburgh Instruments FLS980 fluorimeter, respectively. For steady-state measurements at room temperature complexes 1-3 were excited at 420 nm. The excited state lifetimes of the complexes were obtained by time correlated single photon counting (TCSPC) at an excitation wavelength of 378 nm using a pulsed diode laser, and PL emission was detected at the corresponding steadystate emission maximum for each complex. Photoluminescence quantum yields in solution were determined using the optically dilute method. 65 A stock solution with absorbance of ca. 0.2 was prepared, and then four dilutions were prepared with dilution factors between 2 and 20 to obtain solutions with absorbances of ca. 0.103, 0.076, 0.052, and 0.026, respectively. The Beer-Lambert law was found to be respected (linear dependency) at the concentrations of the solutions. The emission spectra were then measured after the solutions were degassed by three freeze-pump-thaw cycles using an in-house designed quartz cuvette prior to spectrum acquisition. For each sample, linearity between absorption and emission intensity was verified through linear regression analysis, and additional measurements were acquired until the Pearson regression factor (R 2 ) for the linear fit of the data set surpassed 0.9. Individual relative quantum yield values were calculated for each solution, and the values reported represent the slope value. The Fs = Fr(Ar/As)(Is/Ir)(ns/nr) 2 equation was used to calculate the relative quantum yield of each of the sample, where Fr is the absolute quantum yield of the reference, n is the refractive index of the solvent, A is the absorbance at the excitation wavelength, and I is the integrated area under the corrected emission curve. The subscripts s and r refer to the sample and reference, respectively. A solution of [Ru(bpy)3](PF6)2 in deaerated MeCN at 298 K (FPL = 9.5%) was used as a reference. 61 Quantum yields in the solid state were measured with the use of an integrating sphere coated with BenFlect. 66 Electrochemical measurements. Cyclic voltammery (CV) and differential pulse voltammetry (DPV) measurements were

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performed on an Electrochemical Analyzer potentiostat model 600D from CH Instruments. Solutions were prepared in MeCN and degassed with solvent-saturated nitrogen by bubbling for ca. 10 min prior to scanning. Tetra(nbutyl)ammoniumhexafluorophosphate (TBAPF6; ca. 0.1 M in MeCN) was used as the supporting electrolyte. Two Ag/Ag + electrodes (silver wire in a solution of 0.1 M KCl in H2O) were used as the pseudoreference electrode and counter electrode, respectively; a platinum electrode was used for the working electrode. The redox potentials are reported relative to a saturated calomel electrode (SCE) electrode with a ferrocene/ferrocenium (Fc/Fc + ) redox couple as the internal reference (0.38 V vs SCE). 48 Theoretical methods. To perform DFT and TD-DFT calculations, we have used the Gaussian16 program. 67 Our calculations consisted in geometry optimisation vibrational spectra determinations and TD-DFT calculations of the different structures. We have applied default procedures, integration grids, algorithms and parameters, except for tighten energy (typically 10-10 a.u.) and internal forces (10-5 a.u.) convergence thresholds. The ground-state geometrical parameters have been determined with the M06 functional. 68 The vibrational spectrum has been subsequently determined analytically at the same level of theory and it has been checked that all structures correspond to true minima of the potential energy surface. At least, the first thirty low-lying excited-states have been determined within the vertical TD-DFT approximation using the same functional, that is suited for optical spectra 69,70 Phosphorescence was studied by optimizing the lowest triplet excited-state with unrestricted DFT (M06 functional). For the structural and vibrational calculations, we used the LanL2DZ atomic basis set and pseudo-potential for all atoms, augmented by d functions with α=0.587, 0.736, 0.961 and 1.577 for C, N, O and F, respectively and f functions with exponent of 0.938 for the Ir centres. During all steps, a modelling of bulk solvent effects (here CH3CN) through the Polarizable Continuum Model (PCM), 71 using the liner-response approach in its non-equilibrium limit for the TD-DFT part of the calculation. The contour threshold used to draw the MOs (spin densities) was set to 0.02 (0.002) au.

Synthesis of ligands pydbmH (1-phenyl-3-(4-(pyridin-2-yl)phenyl)propane-1,3-dione).

4-bromo benzoic acid (1 g, 4.90 mmol, 1.2 equiv.) was dissolved in MeOH in the presence of a catalytic amount of H2SO4 and refluxed for 16 h. The crude mixture was then neutralised with saturated Na2CO3 and extracted with ethyl acetate (3 x 15 mL). The organic layer was then dried under reduced pressure giving the correspondent methyl 4-bromobenzoate as a white solid in a quantitative yield. A suspension of NaH (60% in mineral oil, 468 mg, 11.70 mmol, 3 equiv.) was prepared in 15 mL of THF and stirred for 30 min. at 0 ºC. After this time, a solution of the acetophenone (465 mg, 3.90 mmol, 1 equiv.) in 7 mL of THF was added dropwise at 0 ºC. The same step was then performed with the previously prepared methyl 4bromobenzoate (1 g, 4.65 mml, 1.2 equiv.). The suspension was kept at room temperature for 1 h. and then heated at 40 ºC for 16 more h. The crude mixture was neutralised with an HCl solution (1 M) and extracted with ethyl acetate (3 x 15 mL). The compound was purified by recrystallisation from EtOH giving a white solid corresponding to 1-(4-bromophenyl)-3phenylpropane-1,3-dione (Br-dbmH) in 85% yield, whose characterisation matched with reported literature. 72 Finally, 2-(tributylstannyl)pyridine (607 mg, 1.65 mmol, 1 equiv.) was reacted with the previously formed Br-dbmH (500 mg, 1.65 mmol, 1 equiv.) using Stille coupling reaction conditions catalysed by Pd(PPh3)4 (92 mg, 0.08 mmol, 0.05 equiv.) in dry toluene. 73 The reaction mixture was refluxed for 32 h. at 120 ºC. The crude product was purified by silica gel chromatography (hexanes/ethyl acetate 90:10,), giving the pure compound as a light brown solid. Yield 40%. Rf = 0.55. M.p. 83-85 °C. [START_REF] Paris | [END_REF] pytbmH (2-benzoyl-1-phenyl-3-(4-(pyridin-2-yl)phenyl)propane-1,3-dione). NaH (60% in mineral oil, 40 mg, 0.99 mmol, 3 equiv.) and pydbmH (100 mg, 0.33 mmol, 1 equiv.) were combined in 10 mL of THF and the suspension was maintained at 0 ºC. To this suspension, the benzoyl chloride (93 mg, 0.66 mmol, 2 equiv.) in 5 mL of THF was added dropwise. After the addition, the mixture was stirred under a nitrogen atmosphere at 40 ºC for 24 h. The crude was neutralised with an acetic acid solution (1 M) and extracted with ethyl acetate (3 x 10 mL). The organic layers were dried under vacuum and the resulting solid washed with diethyl ether in order to remove the excess of benzoic acid formed in the reaction mixture. The pure compound resulted as a pale brown solid. Yield: 50%. M.p. 197-199 ºC. [START_REF] Paris | [END_REF]

Synthesis of complexes [Ir(dFppy)2(µ-Cl)]2 (Tetrakis[2-(4',6'-difluorophenyl)-pyridinato-N,C2']-bis(µ-chloro)diiridium(III))

. A modified version of the originally reported by Nonoyama was followed. 74 IrCl3 . 3H2O (1 equiv.) and the CˆN ligand (2.2 equiv.) were dissolved in 2ethoxyethanol and distilled water (3:1 v/v) to give a concentration of ca. 0.2 M. The mixture was degassed by three cycles of vacuum/ N2 and heated to reflux for 18 h. After 1 h. a yellow precipitate was observed. The reaction was cooled to room temperature follow by addition of water to favor the General procedure for the synthesis of [Ir(dFppy)2(L)]. To a round bottom flask containing [Ir(dFppy)2(µ-Cl]2 (100 mg, 0.08 mmol) and the LH (LH= dbmH, tbmH, pydbmH and pytbmH) (0.18 mmol, 2.2 equiv.) and NEt3 (0.18 mmol, 1 equiv.) were added in a mixture DCM/MeOH (80:20 v/v) to give a suspension with a concentration of ca. 0.02 M. The mixture was degassed via bubbling with N2 for 10 min, before the reaction vessel was sealed. The reaction mixture was heated to 40 0 C for 19 h. The solution was cooled to room temperature, and the solvent evaporated. The crude product was purified by silica gel chromatography in hexanes/DCM (70:30), giving in each case the pure compound as a yellow solid.

[Ir(dFppy)2(dbm)] (1). Yield: 90%. Rf = 0.43. M.p. 307-309 °C. [START_REF] Paris | [END_REF] H NMR (500 MHz, DMSO-d6) δ 8.50 (dd, J = 6.0, 1.5 Hz, 2H), 8.26 (d, J = 8.5 Hz, 2H), 8.06 -8.01 (m, 1H), 7.85 -7.77 (m, 4H), 7.54 -7.43 (m, 4H), 7.39 (t, J = 7.7 Hz, 4H), 6.76 (ddd, J = 12.2, 9.4, 2.4 Hz, 2H), 6.71 (s, 1H), 5.65 (dd, J = 8.7, 2.4 Hz, 2H). 19 F NMR (470 MHz, DMSO-d6) δ -108.88, -110.64 (d, J = 9.9 Hz). The characterisation matched with reported literature. 42 [Ir(dFppy)2(tbm)] (2). Yield: 40%. Rf = 0.67. M.p. 317-319 °C. [START_REF] Paris | [END_REF] H NMR (500 MHz, CDCl3) δ 8.52 (d, J = 5.8 Hz, 2H), 8.26 (d, J = 8.3 Hz, 2H), 7.76 (m, J = 17.1, 8.2 Hz, 6H), 7.43 (t, J = 7.3 Hz, 2H), 7.33 (t, J = 7.6 Hz, 4H), 7.10 (t, J = 7.3 Hz, 2H), 6.59 (s, 1H), 6.39 (ddd, J = 11.7, 9.4, 2.4 Hz, 2H), 5.77 (dd, J = 8.8, 2.3 Hz, 2H). 19 F NMR (471 MHz, CDCl3) δ -108.52 --108.68 (m), -110.69 --110.88 (m). 13 C NMR (126 MHz, CDCl3) δ 186. 24, 148.06, 138.46, 129.44, 129.11, 127.71, 127.04, 77.28, 77.03, 76.77, 65.89. HR-MS (TOFMS + ) [M+H] + : Calculated: 901.1668, Found: 901.1673. Elemental analysis calcd (%) for C44H27O3N2Ir: C, 58.73; H, 3.02; N, 3.11; found: C, 58.74; H, 3.12; N, 3.12.

[Ir(dFppy)2(pydbm)] (3). Yield: 90%. Rf = 0.25. M.p. 313-315 °C. [START_REF] Paris | [END_REF] H NMR (500 MHz, CDCl3) δ 8.72 (d, J = 4.3 Hz, 1H), 8.57 (t, J = 4.9 Hz, 2H), 8.29 (d, J = 8.2 Hz, 2H), 8.03 -7.86 (m, 4H), 7.86 -7.70 (m, 6H), 7.42 (dt, J = 51.5, 7.5 Hz, 3H), 7.14 (ddt, J = 7.5, 5.9, 1.6 Hz, 2H), 6.67 (s, 1H), 6.42 (ddt, J = 12.4, 9.3, 3.0 Hz, 2H), 5.80 (dt, J = 8.7, 2.9 Hz, 2H). 19 F NMR (377 MHz, CDCl3) δ -109.13 (dd, J = 9.9, 4.6 Hz), -111.33 (dd, J = 10.1, 3.2 Hz). 13

X-Ray Crystallography

X-ray diffraction data for both complexes were collected at 173 K using a Rigaku FR-X Ultrahigh Brilliance Microfocus RA generator/confocal optics with XtaLAB P200 diffractometer [Mo Kα radiation (λ = 0.71075 Å)]. Intensity data were collected using ω steps accumulating area detector images spanning at least a hemisphere of reciprocal space. Data for all compounds analysed were collected and processed (including correction for Lorentz, polarisation and absorption) using CrystalClear. 75 Structures were solved by Patterson methods (PATTY) [START_REF] Beurskens | The DIRDIF99 Program System[END_REF] and refined by full-matrix least-squares against F 2 (SHELXL-2018/3). [START_REF] Sheldrick | [END_REF] Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were refined using a riding model. All calculations were performed using the CrystalStructure interface. 78 Selected crystallographic data are presented in Table S1. CCDC 1849982-1849983 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.

FULL PAPER

  molecules were employed as ancillary ligands to probe if the extra carbonyl group on the α-CH position would inhibit the π-π interactions between the phenyl rings of the dbm moiety. Secondly, two new molecules, 1-phenyl-3-(4-(pyridin-2-yl)phenyl)propane-1,3-dione (pydbmH) and 2-benzoyl-1-phenyl-3-(4-(pyridin-2-yl)phenyl)propane-1,3dione (pytbmH) were designed to assess the effect of the extra pyridine ring, which could potentially allow intermolecular interactions in a similar manner to the dFppy moiety.

Figure 1 .

 1 Figure 1. Structures of the ancillary ligands used in this report.

  -3-(4-(pyridin-2yl)phenyl)propane-1,3-dione (pydbmH) was synthesised in two steps via a Claisen condensation to afford the intermediate 1-(4bromophenyl)-3-phenylpropane-1,3-dione (Br-dbmH) in 85% yield, followed by a Stille cross-coupling reaction in 40% yield to obtain the desired molecule. The 2-benzoyl-1-phenyl-3-(4-(pyridin-2-yl)phenyl)propane-1,3-dione molecule (pytbmH) was obtained by reaction of the pydbmH and benzoylchloride

Figure 2 .

 2 Figure 2. Reaction scheme for ligands pydbmH and pytbmH.

Figure 3 .

 3 Figure 3. Reaction scheme for complexes 1 and 2.

Figure 5 .

 5 Figure 5. Representation of the X-ray crystal structure of [Ir(dFppy)2(pydbm)] (3), emphasising the supramolecular composition formed by π-π stacking interactions. Hydrogen atoms and solvent molecules have been omitted for clarity.

  However, weak C-H•••O hydrogen bonds do occur, between a phenyl C-H of a coordinated ketone, and the non-coordinating ketone oxygen of an adjacent complex (CH•••O distance of 2.57 Å, corresponding C•••O separation of 3.234(4) Å). These interactions give rise to a weakly interacting zigzag chain running along the crystallographic b-axis.

Figure 6 .

 6 Figure 6. Representation of the X-ray crystal structure of [Ir(dFppy)2(tbm)] (2), emphasising the supramolecular structure formed by intermolecular interactions. Hydrogen atoms and molecules of solvent have been omitted for clarity.

Figure 7 .

 7 Figure 7. Absorption spectra of [Ir(dFppy)2(L)] for L = dbm (black trace), tbm (blue trace) and pydbm (red trace) in acetonitrile at room temperature (ca. 10 - 5 M).

Figure 8 .

 8 Figure 8. CV and DPV traces of 1 (black trace), 2 (red trace) and 3 (blue trace), versus SCE (Fc/Fc + = 0.38 V in MeCN). Scan rates of 100 mV s-1 with scans taken in the negative direction.

  10.1002/ejic.201801118

Figure 9 . 2 (

 92 Figure 9. Normalised emission (lexc = 420 nm) spectra for 1 (black trace), 2 (blue trace) and 3 (red trace) in MeCN (solid trace) and dichloromethane at room temperature (ca. 10 -5 M) (dashed trace).

Figure 10 .

 10 Figure 10. Spin density difference plots for the lowest triplet states of the three studied complexes, where the contour threshold for the spin density is 2 x 10 -3 au.

Figure 11 . 2 (

 112 Figure 11. Normalised emission (λexc = 420 nm) spectra for 1 (black trace), 2 (blue trace) and 3 (red trace) in MeCN (10 -5 M) at room temperature (dark trace) and in a dichloromethane matrix at 77 K (dashed trace).

FFigure 12 .

 12 Normalised emission spectra (lexc = 420 nm) for 1 (black trace), 2 (blue trace) and 3 (red trace) in doped films (10 wt% in PMMA) (solid trace) and as powder (dashed trace).

Figure 13 . 1 .

 131 Figure 13. a) Emission spectra (lexc= 420 nm) of 3 in dilute MeCN (1) MeCN-H2O (4:6 v/v) (2) and MeCN-H2O (1:9 v/v) (3). b) Variation of intensity according to water content.

  H NMR (500 MHz, CDCl3) δ 8.77 (dt, J = 4.7, 1.4 Hz, 1H), 8.15 (q, J = 8.6 Hz, 4H), 8.07 -8.02 (m, 2H), 7.86 -7.80 (m, 2H), 7.63 -7.57 (m, 1H), 7.56 -7.50 (m, 2H), 7.33 (ddd, J = 6.2, 4.8, 2.6 Hz, 1H), 6.95 (s, 1H).

  dimer. The isolated solid was washed with a mixture of water and ethanol (1:1 v/v) and then a mixture of hexanes and diethyl ether (1:1 v/v), before drying to give the title compound.Tetrakis[2-(4',6'-difluorophenyl)-pyridinato-N,C2']bis(µ-chloro)diiridium(III), [Ir(dFppy)2(µ-Cl)]2. Yield: 67%. 1 H-NMR (400 MHz, CDCl3) (ppm) δ 9.12 (d, J = 6.7 Hz, 1H), 8.31 (d, J = 9.3 Hz, 1H), 7.87 -7.78 (m, 1H), 6.83 (t, J = 7.2 Hz, 1H), 6.38 -6.31 (m, 1H), 5.29 (d, J = 9.1 Hz, 1H). 19 F-NMR (377 MHz, CDCl3) (ppm) δ -107.72 (d, J = 10.3 Hz), -110.32 (d, J = 10.3 Hz). The characterisation matched with reported literature.74

  Inorganic ChemistryThis article is protected by copyright. All rights reserved.

Table 1 .

 1 Electrochemical data for complexes 1-3. a

	Complex	Eox / V b	HOMO / eV c	Ered / V b	LUMO/ eV c	DEH-L/ eV
	fac-Ir(dFppy)3 49	0.76	-5.56 -2.49	-2.31	3.25
	Ir(dFppy)2(acac) 50	0.76	-5.56 -2.44	-2.36	3.20
	1	1.05	-5.85 -1.85	-2.95	2.90
	2	1.30	-6.10 -1.67	-3.13	2.97
	3	1.10	-5.81 -1.60	-3.20	2.61

a. Measurements were performed in MeCN at 298 K at a scan rate of 100 mV s -1 , with Fc/Fc + used as an internal standard. b. Potential values were obtained from the DPV spectra and referenced with respect to SCE (Fc/Fc + = 0.38 V). 48 c. The energies of the HOMO and LUMO levels estimated using: EHOMO = -[Eox vs Fc/Fc+ + 4.8]eV and ELUMO = -[Ered vs Fc/Fc+ + 4.8]eV, respectively.

Table 2 .

 2 Photophysical data of 1-3 in MeCN and DCM (ca. 10 -5 M) at RT and

	77 K.
	Complex	Medium lPL / nm FPL / % a	tPL / ns
		MeCN-RT	629	1.02	12
	1	DCM-RT	660	-	18
		DCM-77K	595	-	800
		MeCN-RT	498	2.1	2 (35%), 30 (65%)
	2	DCM-RT	477	-	17
		DCM-77K	478	-	3770
		MeCN-RT	625	0.5	7.5
	3	DCM-RT	679	-	10
		DCM-77K	607	-	630
	a.

Table 3 .

 3 Photophysical

	Complex	Medium	lPL / nm	FPL / % a	tPL / ns	kr × 10 -6 / s -1	knr× 10 -6 / s -1
					3 (71%)
					140	-	-
	1	Film b	542	8.2	(17%) 425
		Powder	578	21	(12%)	5.78	21.94
					360
					1 (34%)	-	-
	2	Film b	542	8.9	3 (37%) 11 (28%)
		Powder	644	3.0	13	22.58	727.70
					4 (33%),
					116	-	-
	3	Film b	550	6.3	(42%), 358
		Powder	612	5.9	(26%)	6.20	97.21
					97

data of 1-3 in PMMA doped films and powder (lexc = 420 nm). a. Photoluminescece quantum yields measured with an integrating sphere. b.

  13 C NMR (126 MHz, CDCl3) δ 186.13, 184.88, 156.16, 149.93, 143.06, 135.71, 135.59, 132.54, 128.72, 127.65, 127.40, 127.12, 122.89, 120.97, 93.36, 77.28, 77.03, 76.77. HR-MS (FTMS + ) [M+H] + : Calculated: 302.1176, Found: 302.1209. Elemental analysis calcd (%) for C20H15NO2(H2O): C, 77.40; H, 5.20; N, 4.51; found: C, 77.80; H, 5.35 H, 4.27.

  C NMR (126 MHz, Chloroform-d) δ 207.07, 179.39, 178.63, 165.28, 163.59, 161.69, 159.75, 156.64, 151.56, 149.85, 148.02, 140.95, 138.00, 130.60, 128.77, 127.33, 126.89, 122.52, 121.76, 120.69, 115.18, 97.25, 95.20. HR-MS (FTMS + ) [M+H] + : Calculated: 874.1669, Found: 874.1664. Elemental analysis calcd (%) for C42H26O2N3Ir: C, 57.59; H, 3.00; N, 4.81; found: C, 57.78; H, 2.91; N, 5.00.

Acknowledgments

E.Z.-C. acknowledges the University of St. Andrews and EPSRC (EP/M02105X/1) for financial support. We thank Umicore AG for the gift of materials. We thank the EPSRC UK National Mass Spectrometry Facility at Swansea University for analytical services. This work is supported by a Royal Society International Exchanges Grant and by the Australian Research Council and. LAG thanks Curtin University for the postgraduate scholarship. This work used the computational resources of the CCIPL installed in Nantes.


Manisha Das

Gavin A Craig

Daniel Escudero

Mark Murrie

Antonio Frontera

Debashis Ray

Family of [Cu 2 ], [Cu 4 ] and [Cu 5 ] Aggregates: Alteration of Reaction Conditions, Ancillary Bridges and Capping Anions

Phenoxido-bridged [Cu 2 ] complex [Cu 2 (µ-H 4 L1)(µ-OH)(µ 1,3 -NO 3 )(NO 3 )(OH 2 )]•H 2 O (1), and its hierarchical [Cu 4 ] and [Cu 5 ] assemblies [Cu 4 (µ-

were obtained from the reactions of H 5 L1 (2,6-bis-{(1,3-dihydroxy-2-methylpropan-2-ylimino)methyl)}-4-methylphenol) with three copper(II) salts. Available NO 3 ¯, ClO 4 ¯ and CF 3 COO¯ ions have been trapped for 'spontaneous' anion directed 'self-assembly' reactions. All the synthesized complexes contain [Cu 2 (µ-H 4 L1)(µ-OH)] 2+ fragment, prone to assemble and crystallize [Cu 4 ] and [Cu 5 ] complexes in varying reaction conditions. They were characterized by UV-vis and IR spectroscopy, X-ray diffraction analysis and magnetic studies. A change from NO 3 ¯ to ClO 4 ¯ and CF 3 COO¯ result different course of reaction based on Cu 2 (µ-H 4 L1) fragments. Binding of NO 3 ¯ provided 1 as the isolated [Cu 2 ] complex by trapping the reactive fragment. In 2 a perchlorate ligand, in µ 1,3 -binding mode, has been realized as a solitary support for the condensation of two Cu 2 (µ-H 4 L1) fragments. The {Cu 5 (µ 3 -OH) 2 (µ 1,3 -O 2 CCF 3 ) 2 } 6+ constellation in 3 contains five Cu II centers with a unique Z-in distorted octahedral one in the central position. Binding of different anions to the copper(II) centers controls the nuclearity of the reaction products and tuning of the self-aggregate process within same ligand environment (µ-H 4 L1 -).

Magnetic properties of the compounds have been studied both experimentally and using DFT calculations, revealing moderate to strong antiferromagnetic coupling in all aggregates.

Introduction

In recent years the synthesis, spectroscopic and structural characterization, and catalytic evaluation of Cu 2 -compounds have received substantial attention from the view point of their accessibility, structures and their relevance to active sites in biology. 1 For example, the Cu 2 species in tyrosinase is present in a variety of plant and animal tissues. 2 Medicinal Cu II chemistry deals with opportunities in relation to metal ion coordination for the design and use as therapeutic agents that are not attainable through simple organic molecules which are pharmaceutically active. 3,4 In these class of copper(II) complexes the exploitation of the range of coordination numbers and geometries, formation of multinuclear aggregates, thermodynamic and kinetic control, and intrinsic properties of the complex species offer openings for their screening for reactivity patterns and properties. Synthetic endeavor for mimicking Cu enzymes is dependent on the nature and type of the donor atoms available on the ligand backbone. Schiff base ligands are one such type and their Cu II complexes are known to show in vitro anticancer potential against the HepG2 cell line. 5 Also Cu 2 compounds of the ligand system are known to promote the hydrolytic cleavage of doublestrand plasmid DNA and inhibit the growth of GLC4 and K562 cells without affecting macrophage viability. 6,7 . The presence of phenol unit and adjacent imine functions within ligand frame brings two Cu II (S = ½) centers into close proximity and allows interaction of their electron spins. Available oxygen donors lead to the formation of a Cu 2 O 2 type diamond core with modulation from HO -or O 2-bridges, required for further growth in aggregation.

The magnetic interaction is subjective to Cu•••Cu separation, the Cu-O distances, the Cu-O-Cu angles, the dihedral angle between the planes within the Cu 2 O 2 core, the out-ofplane displacement of substituents attached to the bridging groups, and the coordination geometry around each Cu II centre. 1 Design and synthesis of coordination aggregate complexes, formed by the incorporation of a group of Cu 2 -fragments, is dependent on the choice and selection of the ligand system giving ligand anion bound {Cu 2 L} species in solution. Collapse of two such {Cu 2 L} fragments provides structurally different [Cu 4 ] aggregates having cubane, 8 tetrahedron, 9 steppedcubane, 10 rhomboid, 11 and double-cubane 12 structures. Weak bridges like NO 3 ‾ and ClO 4 ‾ are not appropriate to show similar interactions with {Cu 2 L} moiety. Whereas carboxylate bridges can adopt µ 1,1 , µ 1,3 or µ 1,1,3 modes during such process. Synergistic aggregating potential of HO -and RCO 2 -groups have been shown to bind {Cu 2 L} fragments for Cu 6 aggregates. 13 4

From the functional behavior and application point of view the synthesized copper(II) complexes of phenol based multiple alcohol side arm bearing ligand system can be examined for their biomimetic catalytic oxidation efficiency 14 and biophysical studies. 15,16 The hydrogen bonding efficiency and hydrogen bonded network generation in solution from the coordinated and dangling alcohol arms of bound ligand could be useful during association of substrate molecules. In the present work, reactions of H 5 L1 (Chart 1, left) have been explored

with Cu(ClO 4 ) 2 , Cu(NO 3 ) 2 and Cu(O 2 CCF 3 ) to identify the role of anions in directing the assembly process. H 3 bemp, a close analog of H 5 L1, (Chart 1, 2,6-bis-[(2-hydroxyethylimino)-methyl]-4-methylphenol; right) is known to give other type of [Cu 4 ] aggregate. 16 Thus the primary coordinating ability of H 5 L1 and in situ generated HO -ions has been judged for aggregation in presence of nitrate, perchlorate and trifluoroacetate ions (Chart S1, ESI). Interestingly three complexes of varying nuclearity has been achieved in [Cu 2 (µ- Reactions of H 5 L1 were examined with different copper(II) salts, as summarized in Scheme 1, to identify the role of anionic groups in directing the self-aggregation processes. Reaction of Cu(NO 3 )

(µ-H 4 L1)(µ-OH)(µ 1,3 - NO 3 )(NO 3 )(OH 2 )]•H 2 O. O H NaNO COOH CH O H OH NO NO OH L H Cu COONa CH O H NO Cu L H CN CH OH CH 2 3 3 2 2 3 3 3 , 1 4 2 3 2 2 3 5 3 2 2 )] )( )( ( ) )( 1 ( [ 2 3 ) ( 2 1 3 3 + + + ⋅ - - -      →  + ⋅ + - µ µ µ •••(1) Reaction of Cu(ClO 4 )
+ + + ⋅ - - -    →  + ⋅ + µ µ µ •••(2)
Reaction of Cu(CF 3 COO) 2 •4H 2 O and H 5 L1 in 2.5:1 molar ratio in stirring MeOH and in absence of any added base, afforded a dark solution, from which 3 (eq 3) was subsequently isolated from the reaction medium as green block shaped single crystals suitable for X-ray analysis in 81% yield.

O H COOH CF COO CF CCF O CCF O OH L H Cu O H COO CF Cu L H OH CH 2 3 2 3 2 3 2 2 3 2 3 , 1 2 3 2 4 5 2 2 3 5 18 4 ) ]( ) ( ) ( ) ( ) 1 ( [ 4 ) ( 5 1 2 3 + + - - -    →  ⋅ + µ µ µ •••(3)
The elemental analysis and X-ray diffraction data established the products as [Cu 4 (µ- and 3348 cm -1 for 1, 2, and 3 respectively were found for alcohol arms and bound hydroxido groups. The coordination of nitrate group in 1 is verified by the presence of the ν 3 (E') mode of vibration at 1384 cm -1 . [START_REF] Drago | Physical Methods in Chemistry[END_REF][START_REF] Tiago | [END_REF] Peaks at 1433 cm -1 (ߥ̅ as ), 1329 cm -1 (ߥ̅ s ) are assigned to asymmetric and symmetric stretching modes of vibration of the bridging NO 3 ¯ group. 19,[START_REF] Nakamoto | Infrared and Raman Spectra of Inorganic and Coordination Compounds[END_REF] For complex 2, the presence of two different types of perchlorate anions have been detected by identifying the characteristic ߥ̅ as and ߥ̅ s stretching vibration modes. For anionic ClO 4 ¯ anions (T d symmetry), the ν 3 (T 2 ) (ν ClO ) band appeared at ~1088 cm -1 and ν 4 (T 2 ) (δ dOClO ) mode at 626 cm -1 . [START_REF] Nakamoto | Infrared and Raman Spectra of Inorganic and Coordination Compounds[END_REF] In case of bridging (C 2V ) ClO 4 ¯ anion the ν 3 mode split into three components at 1142, 1111 and 1088 cm -1 . 13,[START_REF] Lewis | [END_REF] In 3, two types of trifluroacetate groups were detected by the presence of broad asymmetric stretching vibrations ( as(COO) ) at 1670 cm -1 and medium intense symmetric stretching vibrations ( s(COO) ) at 1431 and 1329 cm -1 respectively. The characteristic ∆ value for free CF 3 CO 2 ¯group is higher (341 cm -1 ) compared to the bridged and chelated ones (∆ =239 cm -1 ). 22,23 The presence of electron withdrawing fluorine atoms moves the asymmetric stretching frequency to higher region. 24 ν ν ν ν ν ν

H 4 L1) 2 (µ-OH) 2 (µ 1,3 -ClO 4 )(OH 2 ) 2 ](ClO 4 ) 3 •2H 2 O and [Cu 5 (µ-H 4 L1) 2 (µ 3 -OH) 2 (µ 1,3 - O 2 CCF 3 ) 2 (O 2 CCF 3 ) 2 ](CF 3 COO) 2

Powder X-ray Diffraction Patterns

The powder XRD patterns of the bulk materials of 1-3 were collected using a Bruker AXS Xray diffractometer and compared with the simulations derived from the single crystal X-ray diffraction data. Figure S2 (ESI) shows that the powder patterns are in good agreement with the simulated data. The difference in intensity is due to the orientation of the powder samples during experiment. The similarity infers that the prepared powder samples are pure and have comparable composition to that of the single crystals. The molecular composition of single crystals and powder samples were same, thus corroborating the reproducibility of powder samples in multiple synthetic attempts.

Electronic Spectral Transitions

In MeOH solutions complexes 1, 2 and 3 register several characteristic bands within 800-200 nm range. The band maxima at 645 nm (ε = 67 L mol -1 cm -1 ), 666 nm (ε =129 L mol -1 cm -1 ), and 678 nm (ε =104 L mol showing corresponding band at 545 nm (ε = 64 L mol -1 cm -1 ). 25 Free ligand H 5 L1 in MeOH solution registers n→π* transition at 338 nm (ε = 3600 L mol -1 cm -1 ) and π→π* transition at 232 nm (ε = 18200 L mol -1 cm -1 ). The intense absorptions just below 400 nm are dominated by copper(II) bound ligand-based absorptions. The characteristic phenoxido O to Cu II charge transfer transition (LMCT) is observed at 368 nm (ε = 7600 L mol -1 cm -1 ) for 1, 374 nm (ε = 10500 L mol -1 cm -1 ) for 2, and 371 nm (ε max = 13700 L mol -1 cm -1 ) for 3. 26 Interestingly the other ligand based strong absorption bands were seen at 259 nm with ε values of 3510, 52400 and 64500 L mol -1 cm -1 for 1, 2 and 3, respectively. These are due to Cu II bound intra-ligand π → π* transitions mainly centered on Cu II -bound >C=N functions (Figure S3, ESI).

Description of Crystal

(O1W-H1WA•••O22, O2W-H2WA•••O30, O2W-H2WB•••O5, O1W- H1WB•••O26, O15-H15•••O2W
) producing a 2D sheet structure (Figure 5). These noncovalent and non-bonding interactions play an important role in the crystal lattice with entrapment of anions and lattice solvent molecules. fashion by two µ 3 -OH bridges (Figure 7b). Compared to 'paddlewheel' structures in case of copper(II)acetates 13 we take the advantage of perfluorinated carboxylate salt of copper(II) in the synthesis, which tend to form monomeric complexes. 31 Within the aggregate three different types of trifluroacetato groups were found (Figure 7a). These were µ the reverse scan a small peak at 0.115 V is found due to anodic stripping, which is absent when scanned between 0.0 V and 1.2 V. The irreversible oxidative response at E pa = 0.422 V is most probably due to Cu II Cu II → Cu III Cu III oxidation (Figure 9-ii-b inset). On scan reversal from 1.2 V, a small cathodic peak at E pc = 0.347 V is seen for the reduction of unstable Cu III Cu III fragment to Cu II Cu II species (i pc /i pa ≠1). The response for 2 is similar to that found for 1 with irreversible cathodic reductions at E pc = -0.503 V and-0.665 V respectively with anodic stripping at 0.155 V. The irreversible oxidative response for Cu II Cu II → Cu III Cu III occurs at E pa = 0.468 V (Figure 9-iii-b inset).

The nature of voltammogram for 3 is different from both 1 and 2. Now two well separated irreversible reductive responses were observed at E pc = -0.425 V and -0.822 V corresponding to Cu II Cu II → Cu I Cu I and Cu I Cu I → Cu 0 Cu 0 . Herein the anodic stripping potential for Cu 0 from the electrode surface was found at 0.275 V. Scanning in the positive direction resulted two irreversible oxidations at E pa = 0.557 V and E pa = 0.700 V, of which the latter was due to ligand centered oxidation (E pa for H 5 L1 = 0.730 V). EPR Spectra X-band EPR spectra for powdered samples of 2 and 3 were measured at 298 K and 75 K (Figure 10). An axial EPR signal was observed for 2 with g ║ = 2.264 and g ⊥ = 2.04 (g av = Simulation of the spectrum 36,37 considering a system with three electrons and three copper nuclei lead to a reasonable fit yielding the following gyromagnetic factors: g 1 ≈ g 2 = 2.04 (g ⊥ ) and g 3 = 2.20 (g ║ ) (Figure 11(ii)). The g av value of 2.09 obtained from the simulated data is in good agreement with that found experimentally. The g-values of axial spectrum are related by the expression 38 G = (g ║ -2.003)/(g ⊥ -2.003) and is a measure for exchange interactions between the Cu II centers as measured in the solid state. When G < 4.0, there is substantial exchange interaction in the polycrystalline sample. 38,39 In our case the G value for 3 is 5.324 demonstrating the presence of weak interactions which is in agreement with the magnetic and DFT studies. The absence of fine structures at the low temperatures ( < 75 K) is consistent with the presence of intercluster exchange interactions that tend to average out the fine structures arising from the multiplets. 40 The hyperfine features in the parallel component is observed at 75 K which is not distinguishable at room temperature due to signal broadening.

The A ║ value of 65.79 G is 2.7 times smaller than that obtained for isolated Cu II site and in case of complex 2 (A ║ ≈ 180 G).

small paramagnetic impurity of 1% (S = ½) was also included in the fit. The Hamiltonian used for the fitting procedure was: and the resulting curve is shown in Figure 12. The strength of the exchange interaction J 1 was found to be -42.9 cm -1 , and the g-value yielded was 2.16. It is difficult to offer a direct comparison of these values with other compounds in the literature, as there are very few examples of two hexa-coordinate Cu II centers bridged by one phenoxido, one hydroxido, and one nitrato group. For dinuclear Cu II complexes containing two hydroxido bridges, a linear correlation was found between the coupling constant J and the size of the Cu-O-Cu angle, with larger angles leading to stronger antiferromagnetic exchange. 42 However, this exchange interaction was also found to be dependent on the extent of displacement of the hydrogen atom of the hydroxido bridges away from the Cu 2 O 2 plane, with large displacements leading to much weaker antiferromagnetic exchange or even ferromagnetic exchange interactions. 43,44 We therefore used B3LYP/6-31+G* calculations (broken symmetry approach, see theoretical methods) to evaluate the magnetic coupling constant J in 1. The theoretical value of J (-36.9 cm -1 ) is in good agreement with the experimental one (-42.9 cm -1 ) and confirms the antiferromagnetic coupling. To investigate the mechanism for the magnetic exchange coupling, the spin density distribution was analyzed. The atomic spin population values on the Cu II centers and the donor atoms of the ligands are listed in Table 1. For the high-spin (HS) configuration, the Mulliken spin population data shows that some spin (ca. 0.62 e) is delocalized through the ligands, and the rest (1.38 e) is supported by the Cu II centers. The spin density plots corresponding to one of the 'broken-symmetry' wave function and the high-spin state for 1 are described in Figure 13, where α and β spin states are denoted by positive (blue) and negative (green) signs, respectively. The broken-symmetry spin population values at the magnetic centers are +0.73 on Cu(1) and -0.68 on Cu(2) and the spin delocalization is considerable (∼30% of the spin is delocalized to the ligand framework). The For 3, the fits were initially attempted using a model involving three distinct exchange interactions considering the topology of the complex as a pair of scalene triangles sharing a vertex (Cu3). However, this led to physically unreasonable exchange interactions, with the interaction between Cu1 and Cu3 or Cu2 and Cu3 found to be as strong as the interaction between Cu1 and Cu2, despite the significant differences in distance between the pairs of atoms and in their connectivity. Therefore, the data were fitted considering one exchange interaction J 1 between Cu1 and Cu2, (Figure S15 in ESI) and their symmetry equivalent counterparts, adopting an approach used previously for a compound containing a related ligand and the same topology. 29 The g values for the five Cu(II) ions were fixed at 2.16 based on the results for the fitting procedure for compound 1. The Hamiltonian was therefore:

ܪ ൌ െ2ܬ ଵ ൫ܵ መ ௨ଵ ܵ መ ௨ଶ ൯ ݃ߤ ܪ ܵ መ ଶ ୀଵ
ܪ ൌ െ2ܬ ଵ ൫ܵ መ ௨ଵ ܵ መ ௨ଶ ܵ መ ௨ଵ * ܵ መ ௨ଶ * ൯ ݃ߤ ܪ ܵ መ ହ ୀଵ
yielding a value for J 1 = -54.2 cm -1 (shown as the black curve in Figure 12). Simulations including weaker interactions between Cu1 and Cu2 and the central Cu3 ion did not lead to better agreement with the experimental data. DFT calculations indicated an antiferromagnetic interaction between Cu1 and Cu2 of -61 cm -1 , and ferromagnetic exchange between the Cu3 center and Cu1 and Cu2 centers of +2.5 cm -1 . The most notable aspect of all of the exchange interactions described is that they are relatively weak for coupling between the Cu II centers, and that they are significantly weaker than the interactions mediated by other related ligands. 29,10 As aforementioned, the B3LYP/6-31+G* calculation of the Cu1-Cu2 interaction in compound 3 reveals an antiferromagnetic coupling with J = -61.1 cm -1 , that is in good agreement with the experimental one (-54.2 cm -1 ). To investigate mechanism for the magnetic exchange coupling, the spin density distribution was studied. The atomic spin population values on the Cu1 and Cu2 metal centers and the O1 and O2 bridging donor atoms of the ligands are listed in Table 2. For the high-spin (HS) configuration, the Mulliken spin population data shows that a noticeable spin (ca. 0.63 e) is delocalized through the ligands, and the rest (1.37 e) is supported by the Cu magnetic centers (see Table 2). The spin density plots of the "brokensymmetry" wave function and the high-spin state for complex 3 are described in Figure 14, where α and β spin states are represented by blue and negative (green) signs, respectively.

The broken-symmetry spin population values at the magnetic centers are +0.71 on Cu(1) and -0.68 on Cu(2) and the spin delocalization is considerable (∼30% of the spin is delocalized to the ligand framework). The spin on the phenoxide and hydroxide O atoms (O1/O2) are 0.16 and 0.12 e in the HS state and negligible in the broken-symmetry state of complex 3, thus confirming that the bridging oxygen atoms mediate the magnetic exchange. Finally, we have also studied the magnetic behavior of compound 2 using DFT calculations.

We have initially analyzed three distinct exchange interactions (Cu1-Cu2, Cu3-Cu4 and Cu2-Cu4). The magnetic interaction between Cu2 and Cu4 (µ 1,3 -ClO 4 bridging, Cu•••Cu separation > 7Å) was found to be inexistent (J = 0). In contrast, the B3LYP/6-31+G* calculation of Js between Cu1 and Cu2 and between Cu3 and Cu4 showed the existence of antiferromagnetic coupling (-99.1 cm -1 and -90.0 cm -1 , respectively). Thus the J values predicted for this compound are slightly larger in absolute value than those observed for compounds 1 or 3. The atomic spin densities and spin density plots for compound 2 are given in the ESI (see Figure S14 and Tables S3 andS4). Unfortunately we were unable to obtain good magnetic data for 2, due to the presence of a paramagnetic impurity that dominated the data in the low temperature region, where a well-isolated S = 0 ground state can be expected based on the DFT calculations. The magnetic data for compound 2 together with a simulation based upon the results of the DFT calculations are shown in Figure S16 in ESI.

Experimental Section

Reagents and Materials

The chemicals used were obtained from the following sources: trifluroacetic acid (99%) from

Caution! Complexes of metal ions containing organic ligands along with perchlorate counter

anions are explosive in nature. Though we have not faced any problem while dealing with the title compounds, but it is safe to prepare small amount of sample and handle it with care.

Physical Measurements

Magnetic Measurements

Direct current (dc) magnetic measurements were performed on polycrystalline samples of compounds 1 and 3 constrained in eicosane, using a Quantum Design SQUID magnetometer equipped with a 5 T magnet. The dc measurements were carried out in the temperature range 290 -2.0 K under applied fields of 1 T (1) and 0.1 T (2 and 3). Data were corrected for the diamagnetism of the compounds and for the diamagnetic contributions of the sample holder and eicosane through measurements.

Crystal Data Collection and Refinement

Single crystal X-ray diffraction data for 1-3 were collected on a Bruker SMART APEX-II CCD X-ray diffractometer furnished with a graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation by the ω scan (width of 0.3° frame -1 ) method at 293 K with a scan rate of 4 s per frame. SAINT and XPREP software 46 were used for data processing and space group determination. Direct method of SHELXS-2014 [START_REF] Sheldrick | SHELXS-2014, Program for Crystal Structure Solution[END_REF] were used to solve the structure and then refined with full-matrix least squares using the SHELXL (2014/7) [START_REF] Sheldrick | Crystal structure refinement with SHELXL[END_REF] program package included into WINGX system Version 2014.1. 49 Data were corrected for Lorentz and polarization effects; an empirical absorption correction was applied using the SADABS. [START_REF] Sheldrick | SADABS Software for Empirical Absorption Correction[END_REF] The locations of the heaviest atoms (Cu) were determined easily. The O, N, and C atoms were subsequently determined from the difference Fourier maps. These atoms are refined anisotropically. The H atoms on the bridging hydroxide were found in the difference Fourier maps and refined isotropically. The remaining H atoms were incorporated at calculated positions and refined with fixed geometry and riding thermal parameters with respect to their carrier atoms. Crystallographic diagrams were presented using DIAMOND software. [START_REF] Diamond | Visual Crystal Structure Information System, version 3.1, Crystal Impact[END_REF] A summary of the crystal data and relevant refinement parameters is summarized in Table 3.

Short distance of 0.96 Å between H14C and H16D is due to the disorder of C16.

Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC-1828020, 1828021, 1828022. These data can also be obtained free of cost at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre).

Theoretical Studies

The magnetic properties of compounds 1, 2 and 3 were calculated using the Gaussian 09 package [START_REF] Frisch | Gaussian 09[END_REF] by means of the density functional theory (DFT) and the broken symmetry approach. [START_REF] Noodleman | [END_REF]54,55 We have used the hybrid B3LYP 56 functional combined with the 6-31+G* basis set, 57 which is a good compromise between the size of the systems and the accuracy of the method. We have utilized the crystallographic coordinates and optimized the positions of the hydrogens to carry out the calculations. Moreover, the theoretical models were simplified using H atoms instead of methyl groups in the ligand.

For the calculations of the pentanuclear complex 3, we evaluated each individual coupling constant (J 1 and J 2 ) using the methodology proposed by Alvarez group 58,59 in mutinuclear systems. This methodology comprises the substitution of paramagnetic centers by diamagnetic ones to simplify the calculation of coupling constants to evaluations for magnetically coupled systems. This methodology is accurate and very convenient to estimate coupling constants individually.

Conclusions

We have demonstrated for the first time the use of same Schiff base H 5 L1 for the synthesis of

H 4

 4 L1)(µ-OH)(µ 1,3 -NO 3 )(NO 3 )(OH 2 )]•H 2 O (1), [Cu 4 (µ-H 4 L1) 2 (µ-OH) 2 (µ 1,3 -ClO 4 )(OH 2 ) 2 ](ClO 4 ) 3 •2H 2 O (2) and [Cu 5 (µ-H 4 L1) 2 (µ 3 -OH) 2 (µ 1,3 -O 2 CCF 3 ) 2 (O 2 CCF 3 ) 2 ](CF 3 COO) 2 (3). The reproducible synthesis, spectroscopic characterization, molecular structures determination and magnetic properties are described, discoursed and rationalized. Chart 1 H 5 L1 (used in this work) and related H 3 bemp Results and Discussion Synthetic Protocol [2,6-bis-{(1,3-dihydroxy-2-methylpropan-2-ylimino)methyl)}-4-methylphenol] (H 5 L1) was obtained in good yield (Scheme S1, ESI) from a reaction of 2,6-diformyl-4-methylphenol and 2-amino-2-methyl-1,3-propanediol in MeOH under stirring and refluxing condition.

for 2 and 3 (Scheme 1

 31 Scheme 1 Synthetic route for 1-3

Figure 2

 2 Figure 2 Hydrogen bonded 2D network in 1; inset: hydrogen bonded dimeric units

Figure 3 28 Figure 4 For 2

 32842 Figure 3 Molecular view of cationic part of 2 with partial atom numbering scheme. H-atoms and counter anions are omitted for clarity. Color code: Cu, green, N, blue, O, red, Cl, olive, C, black

Figure 5 2 [

 52 Figure 5 Supramolecular supports from hydrogen bonding in 2

Figure 6

 6 Figure 6 Molecular view of cationic part of 3 with partial atom numbering scheme. H atoms and counter anions are omitted for clarity. Color code: Cu, green; N, blue; O, red; F, lemon green; C, black Within the Cu 2 (µ-H 4 L1) fragments two square pyramidal Cu II centers are connected by the in plane phenoxido and hydroxido groups and bringing the Cu II centers at a distance of 2.909 Å. The square pyramidal natures of these Cu II centers are known from the calculated Addison parameters (τ) of 0.01 and 0.12. The Cu-O distances within this Cu 2 O 2 diamond core range

Figure 7 .

 7 Figure 7.(a) Core view of the [Cu 5 ] unit; (b and c) coordination environments of Cu 2 (µ-H 4 L1) fragment and central Cu II centre; (d) phenoxido and hydroxido bridges with 3

Figure 8

 8 Figure 8 Intermolecular hydrogen bonding network resulted a 2D structure; inset: enlarged view of hydrogen bonding interactions involving trifluroacetates.

  coming from all the compounds indicate their stability and aggregation behavior. A threeelectrode setup was used consisting of a Pt working electrode, an Ag/AgCl reference electrode and a platinum auxiliary electrode.

Figure 9

 9 Figure 9 Cyclic voltammograms for (i) H 5 L1, (ii) 1 [inset (a) from 0.0 V to -1.2 V, (b) from 0.0 V to 1.2 V], (iii) 2 [inset (a) from 0.0 V to -1.2 V, (b) from 0.0 V to 0.8 V] and (iv) 3 [inset (a) from 0.0 V to -1.2 V, (b) from 0.0 V to 1.2 V] in MeOH containing 0.1 M nBu 4 NClO 4 as supporting electrolyte at 298 K, at platinum working electrode, at a scan rate of 100 mV s -1 , using Ag/AgCl as the reference electrode

2 .

 2 114) (Figure 10-i). The average nuclear hyperfine constants are, A ║ = 180.6 G and A ⊥ = 41.66 G and no resonances below 1050 G and above 3850 G were detected. For 3 a different type of axial spectrum was obtained (Figure 10-ii). At 75 K hyperfine features of the parallel component were observed with a small A ║ value of 65.79 G. The g-tensors were found to be g ║ = 2.28 and g ⊥ = 2.10. The weak intensity, forbidden half field (∆M S = ±2) signals were observed at 1605 G (g = 4.298) and 1605.5 G (g = 4.296) for 2 and 3 respectively, clearly indicating the presence of spin-spin interactions. Unfortunately 1 gave poor quality spectra from which reliable A and g values could not be determined.

Figure 10 X

 10 Figure 10 X-band EPR spectra at 298 K and 75 K for microcrystalline samples of 2 (i) and 3 (ii) respectivelyThe X-band EPR spectrum for a polycrystalline sample of 3 in the temperature range 5 K -115 K is shown in Figure11 (i). At 5 K one broad symmetric transition was observed at 3312 G without any hyperfine feature. The intensity of the signal is temperature dependent and showed maxima at 10 K. Further rise in temperature led to continuous decrease in signal intensity. The rate of decrease of signal intensity is high upto 35 K then gradually decreasing at higher temperatures. The g value at the same time increases from 2.09 at 5 K to 2.12 at 115 K. This sort of change in signal intensity with rise in temperature is consistent with the presence of antiferromagnetic coupling interactions within the copper centers. The spectrum at 10 K shows the highest intensity band centered at g = 2.09 (3318 G for ν = 9.7196 GHz).

Figure 12 χ

 12 Figure 12 χ M T versus T plot for 1 and 3. The solid blue line represents a fit of the data for 1, while the solid black line is the fit of the data for 3 and the red line is a simulation based on the results of DFT calculations (see text for details)

  spin on the phenoxide and hydroxide O atoms (O1/O2) is ca. 0.37 e in the HS state and -0.05 e in the broken-symmetry state of 1, consequently, the bridging oxygen atoms mediate the magnetic exchange. Plots of the magnetic orbitals are given in the ESI (Figure S13) showing the contribution of d x 2 -y 2 atomic orbitals of the Cu II centers along with the p x and p y orbitals of the bridging O atoms.

Figure 13

 13 Figure 13 Representation of spin density (contour 0.004 e Å -3 ) at the high spin (a) and low spin (b) configurations of 1. Positive spin represented in blue and negative spin in green

Figure 14

 14 Figure 14 Representation of spin density (contour 0.004 e Å -3 ) at the high spin (a) and low spin (b) configurations of complex 3. Positive spin represented in blue and negative spin in green

  Elemental analyses (C, H and N) of the compounds were performed with a PerkinElmer model 240C elemental analyzer. A Shimadzu UV 3100 UV-vis-NIR spectrophotometer and a PerkinElmer RX1 spectrometer were used to record the solution electronic absorption spectra and FTIR spectra respectively. The purity of the powder compounds was determined by powder X-ray diffraction (PXRD) patterns using a Bruker AXS X-ray diffractometer (40 kV, 20 mA) using Cu-K α radiation (λ = 1.5418 Å) within 5-50° (2θ) angular range and a fixed-time counting of 4s at 25 °C. A Bruker esquire 3000 plus mass spectrometer was employed to collect the electrospray ionization (ESI) high resolution mass spectra of the three compounds. MALDI-TOF MS studies were performed with the DHBH (2,5dihydroxybenzoic acid) matrix by using a Bruker UltrafleXtreme instrument. CHI 1120A electrochemical analyzer was used for cyclic voltammetric measurements. Electrochemical measurements were performed at 25°C in MeOH in one compartment cell with platinum as working electrode, Pt wire as counter electrode and Ag/AgCl as reference electrode. Electron paramagnetic resonance (EPR) spectra were recorded at 9.13 GHz (X-band) in continuous wave mode with a Bruker ELEXSYS 580 X-band EPR spectrometer equipped with a standard accessory for room temperature operation (298 K).

[

  Cu 2 ], [Cu 4 ] and [Cu 5 ] complexes. Involvement of NO 3 ¯, ClO 4 ¯ and F 3 CCO 2 ¯ groups has been systematically examined for their specificity and anion binding dependent aggregation. The study also recognized the potential of H 5 L1 in phenoxido form to enforce the formation of the Cu II aggregates of varying nuclearity and structure. From the imine side arms one of the protonated alcohol end showed coordination whereas the other adjacent one remained dangling to contribute for hydrogen bonding interactions and stabilization of the entities in crystals. Structures in three dimension and bridging potential of NO3 ¯, ClO ¯ and F 3 CCO 2 duly acknowledged. M.D. is thankful to Mr. Sudipto Khamrui for his help with EPR measurements.

  2

  •3H 2 O and H 5 L1 in presence of CH 3 CO

2

Na as base in a 2:1:2 ratio in MeOH-MeCN medium at room temperature stirring condition afforded a greenish solution, from which dark-green crystals of 1 were subsequently isolated in 54% yield. Use of other 5 carboxylate salts (propionate, trifluoroacetate) as base did not alter the formation and yield of 1 by incorporating these anions in the molecules. The elemental analysis and single-crystal X-ray diffraction studies confirm the composition of 1 as [Cu 2

  2

  •6H 2 O with H 5 L1 in presence of CH 3 CO 2 Na in MeOH solution provided a green reaction mixture from which green block shaped crystals of 2 were isolated in 69% yield. Unique bridging coordination of ClO 4 ¯ in presence of available CH 3 CO 2 ¯ ions result [Cu 4 ] complex 2 (eq 2) through a different self-aggregation route without any abound

	acetate group.
	2	H	5	L 1	Cu 4	ClO (	4	)	2	6	H	2	O		CH 4	COONa 3	CH	3	OH	Cu [	4	(	H	4	L 1 )	2	(	OH	)	2
	(	, 1	3	ClO	4	OH )(	2	)	2	ClO ](	4	)	3	2	H	2	O	CH 4	COOH 3	4	NaClO	4	18	H	2	O

Structures [Cu 2 (µ-H 4 L1)(µ-OH)(µ 1,3 -NO 3 )(NO 3 )(OH 2 )]•H 2 O (1).

  diamond core the Cu-O distances range from 1.910 to 1.958 Å, where the shortest distances are established from phenoxido group. One nitrate group in µ 1,3 -bridging mode also binds the two copper (II) centers. The O-N-O bridging part of nitrate group connects two almost coplanar CuO 3 N planes from two apical sides and register a dihedral angle of 5.68°. Molecular view of 1 with partial atom numbering scheme. H-atoms are omitted for clarity. Color code: Cu, green, N, blue, O, red, C, black The N 2 O 5 donors of H 5 L1 provided two phenoxido-bridged meridional O 2 N halves to bind two Cu II centers. The second -CH 2 OH arms on each side remain pendant and participate in hydrogen bonding interactions within the crystal lattice. For NO 5 octahedral geometry, the Cu-N and Cu-O distances range from 1.927 to 2.475 Å for Cu1 and 1.910 to 2.574 Å for

	Cu2 centers respectively. Cu-O bond from bridging nitrate provides the longest (2.574 Å)
	separation above the CuO 3 N plane (for Cu1 center). The other Cu-O bond distance (for Cu2
	center) from bridging nitrate is 2.432 Å, confirming the asymmetric bridging by NO 3 ¯ group.
	The water O atom is found at 2.549 Å from Cu2, whereas the O atom of the NO 3 ¯ group is at
	2.475 Å from Cu1.
	Two discrete molecular units are engaged in intermolecular hydrogen bonding interactions
	involving the coordinated hydroxyl group of a ligand arm with a hydroxide bridge (O4-
	H4•••O2) and dangling hydroxide group (O3-H3•••O12), producing a hydrogen bonded
	dimeric assembly of two Cu 2 units. The coordinated water molecule functions both as a
	hydrogen bond donor and acceptor and form hydrogen bonding interactions with the nitrate X-ray structural analysis shows that ion (O11-H11B•••O7, O11-H11A•••O9) and lattice trapped water molecule (O1W-1 crystallized in the monoclinic P2 1 /n space group. The interatomic connectivity of the H1WA•••O11). The lattice water molecule also involved in hydrogen bonding interaction with asymmetric unit of 1 is shown in Figure 1, and selected bond distances and angles are given available nitrate (O1W-H1WB•••O7), bridging hydroxide (O2-H2•••O1W) and dangling in Table S1 (ESI). The neutral complex is formed from one coordinating Schiff base ligand, hydroxyl group of ligand (O12-H12•••O1W). Such hydrogen-bonding interactions extend the two nitrate ions and a lattice water molecule. Two octahedral Cu II centers in the dinuclear hydrogen bonded dimeric assembly in two dimensions producing a two dimensional unit are bridged by the in plane phenoxido and hydroxido groups resulting in short Cu•••Cu separation of 2.896 Å in two adjacent octahedral coordination geometry. Within the Cu 2 O 2 hydrogen bonded network (Figure 2).

Figure 1

. MALDI-TOF Analysis.

  geometry and environment, e.g., square pyramidal O 4 N, and octahedral O 5 N and O 6 . Nine different Cu-O bonds are found from coordination of water, alcohol hydroxyl, bridging nitrate and perchlorate, bridging and chelated trifluoroacetate, bridging phenoxido and hydroxido groups. The longest ones in this set are from bridging perchlorato O atoms and Cu 5 ] aggregate. In all the complexes, due to involvement in hydrogen bonding interactions, the H atom of the bridging hydroxide group remains out of plane with respect to the Cu 2 O 2 diamond core. The geometrical features with respect the Cu-O(Ph)-Cu angle (α) (97.95 for 1, 99.59 and 99.33 for 2, and 97.74 for 3), out-of-plane angle deviation (φ) (12.83 for 1, 4.77 and 5.20 for 2 and 5.57 for 3) and Cu-O-Cu-O torsion angle (τ) for the positioning of phenyl ring of bridging phenoxido group with respect to the molecular plane bearing the two Cu II ions can modify the nature of magnetic interactions in the solid state (vide supra) (see Figure S4 in ESI). in MeOH solutions (Figures S5-S7 in ESI). Anion dependent charge neutralization of Cu 2 (µ-H 4 L1) unit formed in solution resulted in the [Cu 2 ], [Cu 4 ] and [Cu 5 ] aggregates as compounds 1-3. The analysis establishes the populations of key fragments in solution. All three compounds exhibited a common base peak at m/z = 240.02 which can be attributed to [Cu 2 (µ-H 4 L1)(µ-OH)] 2+ (C 17 H 26 Cu 2 N 2 O 6 ; Calcd 240.02) fragment in the solution phase. For 1, the nitrato-bridged structure found in the solid state is not survived in MeOH solution medium and no other peaks of even smaller m/z value corresponding to any rational fragment is found in this analysis. The mass spectrum of 2 in MeOH medium records two peaks at m/z = 562.97 and 580.98 corresponding to the fragments [Cu 2 (µ-H 4 L1)(ClO 4 )]H + (C 17 H 26 ClCu 2 N 2 O 9 ; Calcd 562.99) and [Cu 2 (µ-H 4 L1)(µ-OH)(ClO 4 )] + (C 17 H 26 ClCu 2 N 2 O 10 ; Calcd 580.98). Compared to the base peak of m/z = 240.02 for [Cu 2 (µ-H 4 L1)(µ-OH)] 2+ the relative intensity of these two peaks are less than 10% indicating poor abundance in solution. Clearly this may be attributed to better coordinating potential of CF 3 COO¯ as compared to both NO 3 ¯ and ClO 4 ¯. A low intensity peak at m/z = 596.99 corresponding to [Cu 2 (µ-H 2 L1)(CF 3 COO)]Na + (C 19 H 23 Cu 2 F 3 N 2 NaO 7 ; Calcd 596.99) was also present in the spectrum of 3We have used this technique to create ions from all three compounds with nominal fragmentation. In search of higher nuclearity fragments in solution which might not have survived in the ESI process the compounds were subjected to MALDI-TOF analysis using 2,5-dihydroxybenzoic acid (DHBH) as the matrix. The spectrum of all the complexes exhibit a base peak at m/z = 339.61 corresponding to the protonated ligand = 678.61, 802.96, 864.94 and 1387.36 corresponding to the species [Cu 2 (µ-H 4 L1)(µ-OH)(CF 3 COO)(CH 3 OH) 2 ]Na + (C 21 H 33 Cu 2 F 3 N 2 NaO 10 ; Calcd 679.05), [Cu 3 (µ-H 4 L1)(µ-OH)(CF 3 COO) 2 (CH 3 OH)] + (C 22 H 30 Cu 3 F 6 N 2 O 11 ; Calcd 802.96), [Cu 3 (µ-H 4 L1)(µ-OH)(CF 3 COO) 2 (CH 3 OH) 3 ] + (C 24 H 38 Cu 3 F 6 N 2 O 13 ; Calcd 865.01) and [Cu 5 (µ-H 4 L1) 2 (µ-

	next to these are from bridging nitrato O atoms. Water coordinated to the copper(II) center OH) 2 (CF 3 COO) 2 (CH 3 OH) 3 (H 2 O)]Na + (C 41 H 67 Cu 5 F 6 N 4 NaO 20 ; Calcd 1387.06). This study
	makes a large (2.549 Å) Cu-O separation in complex 1 compared to those in complex 2 thus clearly indicates the power of relatively soft technique responsible for low fragmentation
	(2.357 and 2.354 Å). The Cu-O bonds from bridging trifluoroacetate groups are longer than while obtaining ions of aggregate compounds 2 and 3. Thus in the gas phase it is possible to
	the chelated ones. The Cu-O bonds from bridging phenoxido group within Cu 2 (µ-H 4 L1) part detect the formation of tetranuclear [Cu 4 (µ-H 4 L1) 2 (µ-OH) 2 (µ 1,3 -ClO 4 )] 3+ and pentanuclear
	in all the compounds are in 1.910-1.939 Å range. In 1 two types of coordination from nitrato [Cu 5 (µ-H 4 L1) 2 (µ-OH) 2 (CF 3 COO) 2 (CH 3 OH) 3 (H 2 O)]Na + species which ultimately crystallize
	anions stabilize the Cu 2 (µ-H 4 L1) in a particular manner different from a single perchlorato in the solid state.
	anion for clipping two Cu 2 (µ-H 4 L1) units in 2. The µ 3 -OH connection in 3, connecting three
	Cu(CF 3 CO 2 ) 2 •4H 2 O on the other hand resulted in the trapping of trifluoroacetate chelated copper(II) within the resulting [Cu 5 ] complex. The trapping of [Cu(CF 3 CO 2 ) 2 ] unit by two [Cu 2 (H 4 L1)(CF 3 CO 2 )] units, through µ 3 -OH bridging, is responsible for the generation of Cu [H 5 L1]H + (C 17 H 27 N 2 O 5 ; Calcd 339.19) along with a peak at m/z = 616.65 arising from [Cu 2 (µ-H 4 L1)(DHB)] + (C 24 H 30 Cu 2 N 2 O 9 ; Calcd 616.05) (Figures S9-S11 in ESI). This later new [Cu 5 ] structural motif in Cu II -carboxylate chemistry.
	species is formed from the bridging coordination of carboxy end of the anionic matrix with

1.939 Å range. All together a variety of Cu-O bonds were found which differ from each with respect to the II centers, provide one short (1.919 Å) and two long (1.975 and 1.995 Å) Cu-O bonds. Interestingly the coordination of Cu II center to alcoholic OH arms has no effect with regard to C-O separation compared to the free and dangling arms. The trifluoroacetato-bridge clips the two Cu II centers in 3 and also in bidentate chelating mode to the central Cu II ion. In both the two cases the Cu-O separations are different, 2.201 and 2.301 Å in case of bridging mode and 1.976 and 2.048 Å for chelating mode. These two different modes of binding are responsible for varying the C-O bond distances in 3. For bridging trifluoroacetate groups the C-O distances are 1.236 and 1.246 Å, whereas for chelating mode the same are 1.138 and 1.137 Å. The Cu-O-Cu angles within the {Cu 2 (µ-H 4 L1)(OH)} fragments is in 97.73-99.59° range for phenoxido bridges vs. 94.24-97.08° for hydroxido bridges. In 3, the Cu-O(H)-Cu angle is further shortened due to the involvement of the µ 3 -OH connector required for the growth of [33 Mass Spectroscopic Evidence of Fragments High Resolution ESI-MS Analysis. The existence of Cu 2 (µ-H 4 L1) based fragments in solution were established from mass spectral (ESI-MS positive) analysis of compounds 1-3 This is due to the fact that monodentate coordination of perchlorato anion is less stable than the bridging version as identified in 2 in the solid state (vide supra). In these two cases the monocationic charge states correspond to the species where one perchlorato anion is associated with the complex fragments. In case of complex 3 the mass spectrum displays a peak at m/z =575.01 corresponding to [Cu 2 (µ-H 2 L1)(CF 3 COO)]H + (C 19 H 24 Cu 2 F 3 N 2 O 7 ; Calcd 575.01). The abundance of this species was found to be close to 40% of the base peak implying the presence of this species in considerable amount in solution compared to those obtained for compound 2. dinuclear fragments observed in ESI-MS analysis ([Cu 2 (µ-H 4 L1)(µ-OH)] 2+ ). For 2 a peak at m/z = 371.42 is observed due to the presence of [Cu 4 (µ-H 4 L1) 2 (µ-OH) 2 (µ 1,3 -ClO 4 )(OH 2 ) 3 ] 3+ (C 34 H 58 ClCu 4 N 4 O 19 ; Calcd 371.68) species in solution. The spectrum of 3 shows peaks at m/z Reasoning behind the Isolated Products The tuning of reaction conditions under which the [Cu 2 ], [Cu 4 ] and [Cu 5 ] family formed, were analyzed in terms of anion dependence and synthetic environments. Competitive coordination of auxiliary anions to the metal ion centers influences the coordination aggregation process. Multinuclear bridging ability to connect more than two metal ions by NO -ions is not frequently documented in the literature. Herein use of nitrate salt in the reaction medium trapped the product as [Cu 2 L] (1). Isolation of this dinuclear precursor from the reaction medium thus tempted us to examine the role of other anions for spontaneous selfaggregation reaction. Formation of [Cu 4 ] species as complex 2 is dependent on the nature of the anion and the presence of perchlorate anion being a factor in its isolation as X-ray quality crystals. The highest possible bridging capacity of twelve metal ions by single ClO 4 ‾ anion is known in [Cu 12 La 6 ] complex. 34 Connection of two Cu 2 L fragments by a single ClO 4 -anion to

provide the [Cu 4 ] complex is not common in the literature.

35

The µ 1,3 mode of one non-planar perchlorate anion functioning as bridging ligand led to the isolation of 2 while other three perchlorate anions were required for charge neutralization only. In case of 3 the reaction with

Electrochemistry

Further characterization in solution was achieved from cyclic voltammetric measurements in MeOH for 1-3. The characteristic voltammetric responses for {Cu 2 (µ-H 4 L1)} fragments

  The former response is due to Cu II Cu II → Cu III Cu III oxidation and the corresponding reduction of Cu III Cu III → Cu II Cu II is found at E pc = 0.296 V. The ∆E p value found to be 261 mV (E 1/2 = 0.426 V) and i pc /i pa ratio ≈ 1 indicate some stability of Cu III Cu III species within cyclic voltammetric time

scale. The similar nature of the voltammograms for 1 and 2 suggest the existence of [Cu 2 (µ-H 4 L1)(µ-OH)] 2+ fragments in MeOH solutions in both the two cases as also evidenced from the ESI-MS analysis (vide supra). For 3 the distinct responses were found from species like [Cu 2 (µ-H 2 L1)(CF 3 COO)]H + having 40% abundance to that of the base peak in ESI-MS analysis.

Table 1

 1 Atomic spin densities (e) computed for the high and low spin configuration of 1 at

	the B3LYP/6-31+G*
	Atom Label	High spin	Low Spin
	Cu1	0.71	0.73
	Cu2	0.67	-0.68
	N1	0.11	0.11
	N2	0.10	-0.10
	O1	0.15	-0.03
	O2	0.22	-0.02
	O3	0.02	0.02
	O4	0.03	-0.03

Table 2

 2 Atomic spin densities (e) computed for the High and Low spin configuration of compound 3 at the B3LYP/6-31+G*

	Atom Label	High spin	Low Spin
	Cu1	0.68	-0.68
	Cu2	0.69	0.71
	N1	0.13	-0.14

6-bis-{(1,3-dihydroxy-2-methylpropan-2-ylimino)methyl)}-4- methylphenol]. The

  The as-prepared solution of H 5 L1 (25 mL), containing 1 mmol of H 5 L1 (approx. 0.33 g), was diluted to 30 mL by MeOH. To this, a MeOH solution (15 mL) of copper(II) nitrate trihydrate (0.485 g, 2 mmol) was added slowly and the resulting green solution was stirred for ca. 15 min. Following this, a MeOH solution (10 mL) of sodium acetate (0.164 g, 2 mmol) was added during which the solution color changed to dark green and the stirring was continued for 1 h. The solution was filtered, mixed with equal volume of MeCN and kept in air at room temperature for slow evaporation.

	[Cu 2 (µ-H 4 L1)(µ-OH)(µ 1,3 -NO 3 )(NO 3 )(OH 2 )]•H 2 O (1). Green block shaped single crystals suitable for X-ray analysis were obtained after 8 days
	from this MeOH-MeCN (1:1) solvent mixture. Yield: 0.35 g (54%). Anal.calc. for
	C 17 H 30 Cu 2 N 4 O 14 (641.55 g mol -1 ): C, 31.83; H, 4.71; N, 8.73. Found: C, 31.91; H, 4.74; N,
	8.74. Selected FT-IR bands: (KBr, cm -1 , vs=very strong, br=broad, s=strong, m=medium, SRL, India, copper(II) carbonate hydroxide, copper(II) nitrate trihydrate and triethylamine w=weak): 3400 (br), 1635 (vs), 1384(vs). UV-vis spectra [λ max , nm (ε, L mol -1 cm -1 )]: from Merck, India and 2-amino-2-methyl-1,3-propanediol from Alfa Aesar. Copper(II) perchlorate hexahydrate was freshly prepared by treating hydrated copper(II) carbonate (MeOH solution) 645 (67), 368 (7600).
	(22.12 g, 0.1 mol) with 1:1 aqueous solution of perchloric acid. Similarly copper(II)
	trifluroacetate was obtained by reacting copper(II) carbonate (11.06 g, 0.05 mol) with
	trifluroacetic acid (11.40 g, 0.1 mol) in water. 2,6-diformyl-4-methylphenol was prepared
	following a modified literature procedure providing better yield. 45 All the chemicals and
	solvents used in this work were reagent-grade materials and were used as received without
	further purification.
	Synthesis
	Ligand	H 5 L1	[2,Schiff base ligand H 5 L1 was prepared by reacting 4-methyl-2,6-
	diformylphenol (0.820 g, 5 mmol) and 2-amino-2-methyl-1,3-propanediol (1.05 g, 10 mmol)
	under stirring condition for 1 h followed by refluxing for another 2 h. The solvent was
	removed under vacuum to yield a yellow oily mass, which was characterized by FTIR, NMR
	spectroscopy and used directly without further purification. FT-IR (cm -1 , KBr pellet): 3341
	(br), 1638 (m), 1219 (w), 1052 (w), 772 (vs). 1 H NMR (600 MHz, (CD 3 ) 2 SO, δ ppm): 8.61
	(2H, imine-H), 7.55 (2H, aromatic-H), 3.42 (8H, -OCH 2 ), 2.50 (3H, -CH 3 substituent on
	phenyl ring), 1.18 (6H, -CH 3 ). 13 C NMR (150 MHz, (CD 3 ) 2 SO, δ ppm): 163.58 (imine C),
	124.78-160.01 (aromatic C), 66.06 (methylene C attached with O), 63.72 (tertiary C attached
	to imine N), 22.65 (methyl C attached with benzene ring), 18.63 (methyl C).

[Cu

4 (µ-H 4 L1) 2 (µ-OH) 2 (µ 1,3 -ClO 4 )(OH 2 ) 2 ](ClO 4 ) 3 •2H 2

	molecular formula [Cu 4 (µ-H 4 L1) 2 (µ-OH) 2 (µ 1,3 -ClO 4 )(OH 2 ) 2 ](ClO 4 ) 3 •2H 2 O. Anal calc. for
	C 34 H 60 Cl 4 Cu 4 N 4 O 32 (1432.85 g mol -1 ): C, 28.50; H, 4.22; N, 3.91. Found: C, 28.08; H, 4.13;
	N, 3.80. Selected FT-IR bands: (KBr, cm -1 , vs=very strong, br=broad, s=strong, m=medium,
	w=weak): 3338 (br), 1636 (s), 1142 (vs), 1111 (vs), 1088 (vs). UV-vis spectra [λ max , nm (ε, L
	mol -1 cm -1 )]: (MeOH solution) 666 (129), 374 (10500).

O (2). Complex 2 was prepared following a similar procedure as described above for 1 using Cu(ClO 4 ) 2 •6H 2 O (0.74 g, 2 mmol) instead of Cu(NO 3 ) 2 •3H 2 O. Green block shaped crystals suitable for X-ray analysis were obtained after 1 week. Yield: 0.494 g (69%). The elemental analysis data confirms the

[Cu 5 (µ-H 4 L1) 2 (µ 3 -OH) 2 (µ 1,3 -O 2 CCF 3 ) 2 (O 2 CCF 3 ) 2 ](CF 3 COO) 2 (3). To

			a stirred 'solution
	of H 5 L1' (approx. 0.33 g, 1 mmol) in MeOH (20 mL), a MeOH solution (10 mL) of
	Cu(CF 3 COO) 2 •4H 2 O (0.904 g, 2.5 mmol) was added. The resulting green solution was stirred
	for 2 h. Then the solution was filtered and kept for slow evaporation in air. Green block
	shaped single crystals suitable for X-ray analysis were obtained after 1 week from the MeOH
	reaction medium. Yield: 0.69 g (81%). Microanalytical data are consistent with the formula
	[Cu 5 (µ-H 4 L1) 2 (µ 3 -OH) 2 (µ 1,3 -O 2 CCF 3 ) 2 (O 2 CCF 3 ) 2 ]•(CF 3 COO) 2 .	Anal.	calc.	for
	C 46 H 52 Cu 5 F 18 N 4 O 24 (1704.67g mol -1 ) C, 32.41; H, 3.07; N, 3.29. Found: C, 32.47; H, 3.10;
	N, 3.31. Selected FT-IR bands: (KBr, cm -1 , vs=very strong, br=broad, s=strong, m=medium,

w=weak) 3348 (br), 1670 (vs), 1644 (s) 1431 (m), 1329 (m). UV-vis spectra [λ max , nm (ε, L mol -1 cm -1 )]: (MeOH solution) 678 (104), 371 (13700).

Table 3

 3 Crystal data and structure refinement details for 1 to 3 Formula C 17 H 30 Cu 2 N 4 O 14 C 34 H 60 Cl 4 Cu 4 N 4 O 32 C 46 H 52 Cu 5 F 18 N 4 O 24 Σ( ||F o |-|F c || )/Σ|F o |. wR 2 = [ Σw(|F o |-|F c |) 2 /Σw(F o ) 2 ] 1/2 . w = 0.75/(σ 2 (F o ) +0.0010F o 2 )

	parameters	1	2		3
	F.W. (g mol -1 )	641.55	1432.85	1704.67
	crystal system	Monoclinic	Triclinic	Monoclinic
	space group	P 2 1 /n	P	1	P 2 1 /c
	Crystal color	Green	Green	Green
	Crystal size/mm 3	0.37×0.27×0.18	0.38×0.28×0.17	0.36×0.25×0.17
	a/ Å	9.4758(14)	13.616(3)	14.1130(14)
	b/ Å	8.9390(13)	14.129(3	17.8567(18)
	c/ Å	28.633(4)	15.755(4)	12.4740(12)
	α/ deg	90.00	105.525(6)	90.00
	β/ deg	95.851(4)	94.758(6)	107.507(3)
	γ / deg	90.00	112.669(6)	90.00
	V/ Å 3	2412.7(6)	2635.1(11)	2998.0(5)
	Z	4	2		2
	D c /g cm -3	1.766	1.803	1.888
	µ (mm -1 )	1.842	1.896	1.884
	F(000)	1320	1460	1710
	T/K	293(2)	293(2)	293(2)

Published on 17 July 2018. Downloaded by University of Glasgow Library on 7/18/2018 8:25:08 AM. View Article Online DOI: 10.1039/C8NJ02131G

New Journal of Chemistry

Acknowledgements

M.D. is grateful to IIT Kharagpur for her research fellowship. We are also thankful to DST, New Delhi, for providing the Single Crystal X-ray Diffractometer facility in the Department of Chemistry, IIT Kharagpur under FIST program. EPR facility by CRF, IIT Kharagpur is

Magnetic Measurements

The temperature dependence of the molar magnetic susceptibility, χ M , of 1-3 was measured under a magnetic field of 1 T for 1 and 0.1 T for 2 and 3 over the temperature range 290 -2 K. Plots of χ M T versus temperature, T, for compounds 1 and 3 is shown in Figure 12. At 290 K, the values of χ M T are 0.78 (1) and 1.96 cm 3 mol -1 K (3), which is consistent with the expected values for two Cu II sites in case of 1 (0.75 cm 3 mol -1 K, for g = 2.0) and five Cu II centers for 3 (1.875 cm 3 mol -1 K, for g = 2.0). For both the compounds, a decrease in χ M T is observed upon lowering the temperature, with the data entering a plateau at low temperatures.

For 1, the value of χ M T is very close to zero below 20 K, reaching a minimum value of 0.002 cm 3 mol -1 K measured at 2 K. In the case of 3, the χ M T data also enter a plateau below 20 K, where χ M T = 0.51 cm 3 mol -1 K, with a further slight decrease to χ M T = 0.44 cm 3 mol -1 K at 2 K. These data indicate overall antiferromagnetic exchange interactions in both compounds.

To determine the size of the exchange, the data were fitted using the program Phi 2.0. 41 The data for 1 were fitted considering one coupling constant arising from the interaction between the two Cu II ions, J 1 (Figure S15 Only NO 3 ¯ as ancillary ligand is appropriate for the isolation of 1. Whereas ClO 4 ¯ ion, as a very rare example, could bridge two {Cu 2 (µ-H 4 L1)} fragments in 2. We have shown that {Cu 2 (µ-H 4 L1)(µ-OH)} 2+ units, resulting from H 4 L1 -and water derived HO -bridges, and characterized by HRMS analysis, were accountable for the trapping of {Cu(O 2 CCF 3 ) 2 } units to give 3. The magnetic exchange interactions found for the complexes were weaker than those normally found for related Cu II base multinuclear complexes. The coupling is determined by the geometry in the plane of the Cu II ions, with a strong influence from the proton on the bridging hydroxido ion. As assemblies show significant hydrogen bonding interactions, it could be that by altering the nature of these supramolecular interactions that we can tune the magnetic interaction.

Electronic Supplementary information X-ray crystallographic data in CIF format, Chart S1, Scheme S1, Figures S1-S16, Tables S1-S4. CCDC 1828020, 1828021 and 1828022 contain the supplementary crystallographic data in CIF format for complexes 1-3.

Author Information

Corresponding Author *E-mail: dray@chem.iitkgp.ernet.in. Tel: (+91) 3222-283324. Fax: (+91) 3222-82252.

ORCID

Debashis Ray: 0000-0002-4174-6445

Conflict of interest

The authors declare no competing financial interest