New PtII diimine – dithiolate complexes containing a 1 , 2-dithiolate-1 , 2-closo-dicarbadodecarborane : an experimental and theoretical investigation †

Five new [Pt(N^N)(dtoc)] complexes (1–5; N^N = diimine: 2,2’-bipyridine and its 4,4’-alkyl/aryl-substituted derivatives or 1,10-phenanthroline; dtoc = 1,2-dithiolate-1,2-closo-dicarbadodecaborane) have been synthesized and characterized by spectroscopic and electrochemical methods, and by means of X-ray diffraction in the case of complexes 1 and 4. Hybrid DFT and time-dependent (TD) DFT calculations were performed on complexes 1–5 and the previously reported complex [Pt(Ph2phen)(dtoc)] (6; Ph2phen = 4,7-diphenyl-1,10-phenanthroline) both in the gas phase and in the presence of several solvents (CH2Cl2, CHCl3, CH3CN, acetone, THF, DMF, DMSO, and toluene) to gain an insight into the electronic structure of the complexes and explain their experimental features. Theoretical calculations allowed for the determination of structure–property relationships within the series of the six complexes considered, and the prediction of their second order nonlinear optical (SONLO) properties by evaluating their first static hyperpolarizabilities (βtot).


Introduction
Since the first reports by Miller and Dance in 1973, 1 diiminedithiolate metal complexes [M(N^N)(S^S)] have received a great deal of attention.These systems have been, in fact, applied to second order nonlinear optics (SONLO), 2,3 dye-sensitized solar cells, [4][5][6][7][8] light-to-chemical energy conversion, 9,10 DNA intercalation, 11,12 and as conducting 13 and magnetic 14 materials or sensors. 15Such a variety of potential applications arise from their peculiar electronic features.In fact, [M(N^N)(S^S)] com-plexes undergo reversible electrochemical processes 16 and show large molecular hyperpolarizabilities, 17 room-temperature luminescence in solution, 18,19 and a peculiar intense (molar extinction coefficient ε up to 10 4 M −1 cm −1 ) solvatochromic absorption in the vis-NIR region. 20,21This absorption, thoroughly investigated in the past by both experimental and theoretical means, 22 was assigned to a charge-transfer (CT) transition occurring from the HOMO, mainly localized on the S^S ligand and partly on the central metal ion, to the LUMO, centred on the diimine N^N. 23The nature of this transition, often described as a mixed-metal/ligand-to-ligand chargetransfer (MMLL'CT), 24 accounts for the negative solvatochromism featured by the corresponding absorption, since it generates an excited state, which is less polar or polarized in the opposite direction with respect to the ground state. 25tarting from the pioneering work carried out by Eisenberg and co-workers in the 1990s, 26,27 a large number of diiminedithiolate complexes have been reported, mainly featuring Pt II as a central metal ion.The possibility of tuning both the electrochemical and photophysical properties of these chromophores by systematically varying both the central metal ion 28 and the S^S and N^N ligand 29 was distinctly proved.In this context, some of the authors recently reported on a series of new Pt II diimine-dithiolate complexes featuring differently functionalized 1,2-dithiolato ligands belonging to the classes of R-edt 2− (monosubstituted ethylene-1,2-dithiolates; R = phenyl, 2-naphtyl, 1-pyrenyl) 30,31 and R-dmet 2− (N-substituted † Electronic supplementary information (ESI) available: Details on the characterization of complexes 2-5, crystallographic data for complexes 1 and 4, metric parameters optimized in the gas phase and in selected solvents, calculated eigenvalues and MO compositions, NPA charges, details on vertical transitions calculated at the TD-DFT level, correlations between UV-vis absorption wavelengths in selected solvents and empirical parameters, correlation between CV data and UV-vis absorption wavelengths, KS-MO scheme calculated for complex 2 in DMSO, correlation between experimental and calculated absorption wavelengths in DMSO, correlation between experimental and calculated solvatochromic absorption wavelengths.CCDC 1010434 and 1010435.For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/ c4dt01929f 2-thioxo-1,3-thiazoline-4,5-dithiolates; R = methyl, ethyl, phenyl), 32,33 along with variously alkyl-and aryl-substituted 2,2′-bipyridine and 1,10-phenanthroline ligands, thus showing the possibility of fine-tuning the linear and nonlinear optical properties of these systems. 34he twelve-vertex 1,2-closo-C 2 B 10 H 12 icosahedral carborane has been widely studied. 35The unique stabilities and geometrical properties of the isomeric closo C 2 B 10 H 12 carboranes suggested this species as a building block for stereo precise structural platforms with novel reaction centres having properties that cannot be achieved with organic hydrocarbon compounds. 36In addition, the 1,2-closo-C 2 B 10 H 12 cluster is electron-withdrawing through the carbon atoms 37 and electron donating through the boron atoms.Consequently, when sulphur atoms are bonded to the carbon cluster atoms, as in the dithiol 1,2-(SH) 2 -closo-C 2 B 10 H 10 (H 2 dtoc), the electron density will be depleted on them, thus varying the potentials required for the electrochemical reduction of the corresponding complexes. 38In particular, as far as diimine-dithiolate systems are concerned, it is reasonable to assume that more strong electron-deficient 39 moieties such as the 1,2-closo-C 2 B 10 H 12 cluster could affect the electrochemical and optical properties of these complexes with respect to those obtained with organic dithiolates.

Crystallographic studies
X-ray diffraction quality single crystals were successfully grown for complexes 1 and 4 (Fig. 1 and Table S1 in ESI †).The plati-num atom is coordinated in both complexes by the two diimine N atoms and the two S atoms of the dtoc 2− ligand in a distorted square planar cis-"S 2 N 2 " configuration.The Pt-N and Pt-S bond lengths closely resemble the values found in two similar bis-monothiolato o-carborane platinum complexes containing 2,2′-bipiridine 46 and substituted 1,10-phenantroline ligands, 41,43 and are in the range of bond lengths typical for  other Pt II diimine-dithiolate systems structurally characterized so far [2.045(16) and 2.239(16) Å, respectively]. 47nalogously, C-S and C-C bond lengths within the dtoc 2− ligand in complexes 1 and 4 are very close to the mean values found in the Cambridge Structural Database (CSD) for complexes featuring dtoc 2− as a bidentate ligand coordinated to a single transition metal ion [1.782(9) and 1.652(20) Å, respectively], 48 and only slightly different from the C carborane -thiol and the skeletal C-C bond distances found in the carboranyl sulphide anion [1,2-S(Ph)C 2 B 10 H 10 ] − . 49The dithiol-o-carborane forms in both cases a five membered chelate ring, with S-Pt-S/ N-Pt-N angles of 92.35(6)/79.8(2)and 92.19(7)°/78.6(2)°forcomplexes 1 and 4, respectively, which fall within the expected ranges. 50n complex 4 the bipyridyl ligand lies in the PtN 2 S 2 coordination plane, and a mirror plane bisects the molecule by passing through the bond joining the two pyridine rings, the platinum ion, and the C-C bond of carborane.Complex 1 displays a similar arrangement of the bipyridyl ligand, which however slightly deviates from planarity [dihedral angle N(1)-Pt-S(2)-C(2) = 173.92°;Fig. 1].Both complexes 1 and 4 are packed in a head-to-tail fashion to form columnar structures along axis a and b respectively, but they differ in the nature of the interactions involved.In complex 1 short intermolecular Pt⋯C contacts align the complex molecules in pillars developing along the direction of the a-axis with alternating Pt⋯C distances of 3.395(8) and 3.530(8) Å (Fig. 2). 51Similar short contacts have been explained with a weak axial interaction between the filled d z 2 orbital of platinum and the π* antibonding orbital of the involved C atoms in the bipyridyl ring. 52eak intermolecular interactions involving the B atoms of the dtoc 2− ligand and the H atoms of both ligands hold the stacks together [B(6)⋯H( 11 In complex 4 the molecules are aligned into mono-dimensional columns running along the crystallographic a-axis through extensive face-to-face π-π interactions involving both rings of bipy moieties with stacking distances of 3.89 Å, though no interactions involving the platinum ions can be evidenced.Weak interactions involving the H atoms of the tertbutyl substituents of the 2,2′-bipyridines and the B atoms of the dtoc 2− ligand [B(1)⋯H(14B) 3.034, B(3)⋯H(14B) 3.138, B(4) ⋯H(14B) 3.137 Å] are responsible for the 3D packing of the columns.It is noteworthy, probably due to the bulky substituents at the pyridine rings, that the structure of 4 contains solvent accessible channels running along the b-axis (376.16Å 3 , 12.6% of the cell volume, Fig. 3). 53ectrochemistry Cyclic voltammetry (CV) measurements (Table 1) performed in dry DMSO showed that complexes 1-6 feature monoelectronic reversible (i I pc /i I pa values in the range 0.8-1.3)reduction processes (E I 1/2 ) between −1.709 and −1.536 V vs. Fc + /Fc (Fig. 4 for complex 1), corresponding to the reduction of the neutral species to the corresponding monoanions.The potential values E I 1/2 determined for complexes 1-6 can be compared with the first reduction potentials determined for the [Pt(N^N)-(Me-dmet)] complexes featuring the same diimine ligands under the same experimental conditions. 34,54In fact, the two sets of data are very similar and linearly correlated (R 2 = 0.95).On passing from S^S = Me-dmet 2− to dtoc 2− , the potentials are only slightly shifted (on an average by 0.018 V), confirming that the potential of this reduction process depends on the nature of the diimine.Table 1 Electrochemical data recorded by CV for complexes 1-6 in DMSO solution at 298 K ( potentials  1 and Fig. 4 for complex 1), which leads from monoanionic to dianionic species.The second reduction process was previously reported for [Pt(Ph 2 Phen)(dtoc)] (6; Ph 2 Phen = 4,7diphenyl-1,10-phenanthroline; Scheme 1) in DMF (E 1/2 = −2.14V vs. Fc + /Fc). 43Weinstein and co-authors supposed this process to be localized on the diimine, 43 although in analogy to what was reported for different [Pt(bipy)L 2 ] (L = py, Me 2 Npy, Cl), [Pt(bipy)(en)], and [Pt( phen)( py) 2 ] complexes, 55 this reduction could be attributed to the formation of a dianionic Pt I radical species [Pt I (N^N • )(S^S)] 2− . 56n examination of the reduction potentials E I 1/2 and E II 1/2 listed in Table 1 shows that no significant effect is observed on passing from N^N = bipy to N^N = phen in complexes 1 and 2, respectively.The introduction of alkyl substituents at the diimine (3 and 4) induces a potential shift towards more negative values, while phenyl substituents (5 and 6) shift both E I 1/2 and E II 1/2 towards more positive values with respect to the complexes 1 and 2 featuring unsubstituted diimines.Given the predominant localization of the LUMO on the diimine in [Pt(N^N)(S^S)] systems, 23 this trend suggests that aryl substituents at the N^N ligands induce a stabilization of the LUMO, while alkyl substituents cause an opposite effect. 34This could also account for the absence of the reduction to dianionic forms in the case of complexes 3 and 4, whose E II 1/2 potential would be too negative to be electrochemically explored.
Although a further oxidation process at about 0.95 V vs. the Fc + /Fc couple was reported in DMF for 6, 43 this process was not observed in DMSO within the explored potential range.The lack of oxidation processes to cationic species suggests a remarkable stabilization of the HOMO in 1-6 as compared to different diimine-dithiolate complexes, possibly ascribed to the electron-withdrawing effect of the 1,2-closo-C 2 B 10 H 12 cluster. 38sorption UV-vis spectroscopy Diimine-dithiolate complexes show a peculiar low-energy solvatochromic absorption in the visible region. 20Accordingly, the UV-vis absorption spectra recorded in DMSO for complexes 1 and 3-5, bearing bipyridine ligands, feature a broad and asymmetric absorption band in the visible region, with absorption maxima in the range 420-440 nm, and molar extinction coefficients ε between 4700 and 8200 M −1 cm −1 (Table 2 and Fig. 5 for complex 1).In the case of complexes 2 and 6, bearing 1,10-phenanthroline ligands, this absorption is split into two bands, falling at 378/395 and 427/441 nm, respectively (ε values of about 6000-7000 M −1 cm −1 in DMSO; Table 2 and Fig. 5 for complex 2), in agreement with the data previously reported for complex 6. 43 An examination of the absorption spectra recorded for complexes 1-6 in eight different solvents (namely CH 2 Cl 2 , CHCl 3 , CH 3 CN, acetone, THF, DMF, DMSO, and toluene) shows a remarkable solvatochromism for the absorption bands in the visible region, with a shift in the λ max value of about 40-60 nm.
In particular, both the broad absorption featured by the bipyridine-containing complexes and the two bands shown in the case of complexes 2 and 6 display a negative solvatochromism, and a dependence of their ε values on the nature of the solvent (Table 2 and Fig. 6 for complex 2).
The observed solvatochromic behaviour is in agreement with the empirical scale previously proposed by Eisenberg and co-workers for platinum diimine-dithiolate complexes, 20 thus suggesting a CT character for the electronic transitions involved (Fig. S1 † for complex 2).
The λ max values recorded in DMSO for the solvatochromic absorption at the lowest energy of complexes 1-6 strongly depend on the nature of the diimine, analogously to what was previously observed for [Pt(N^N)(S^S)] complexes featuring  R-dmet 2− 1,2-dithiolato ligands. 34In particular on passing from complexes 1 and 2, bearing unsubstituted diimines, to complexes 5 and 6, where the N^N ligands are phenyl-substituted, a red-shift can be observed.On the other hand, alkyl substituents (complexes 3 and 4) induce a blue-shift of the λ max value of the corresponding complexes.Since the solvatochromic band is attributed to the HOMO-LUMO transition, its wavelength can be correlated to the LUMO energy, and thus spectroscopic and electrochemical CV data have been crosscompared.In fact, a linear correlation holds between the λ max values and the E 1/2 potentials (Fig. 7 and S2 † for E I 1/2 and E II 1/2 , respectively; R 2 = 0.82 and 0.98).This correlation suggests that the variation of the λ max values should be attributed mainly to a variation in the energies of the LUMO, localized in the diimine, while the uppermost filled MOs would not be affected, and should therefore be localized on the dtoc 2− ligand and/or the central metal ion, in agreement with what was reported for different [Pt(N^N)(S^S)] complexes. 25

Theoretical calculations
With the aim of gaining an insight into the electronic structure of the title complexes and interpreting the electrochemical and spectroscopic features observed experimentally, a theoretical investigation was performed on complexes 1-6 at the density functional theory (DFT) level. 57Following the good results previously obtained with different [Pt(N^N)(S^S)] complexes, 34 the hybrid PBE0 functional was adopted, 58 and the all-electron double-ζ basis sets (BSs) with polarization functions by Schäfer, Horn, and Ahlrichs 59 were exploited for light atomic species.For the heavier Pt species, the CRENBL BS 60 with relativistic effective core potentials (RECPs) 61,62 was chosen to account for the relativistic effects occurring in the case of third-row transition metal ions. 63Given the role played by the solvent in the spectroscopic features of Pt II diiminedithiolate complexes, solvation calculations were also carried out at the same level of theory using the integral equation formalism of the polarizable continuous model (IEF-PCM) within the self-consistent reaction field (SCRF) approach. 64alculations were performed both in the gas phase and in the presence of DMSO for all the complexes 1-6, and in the case of complex 1, solvation calculations were performed by considering all of the eight solvents employed for the spectroscopic measurements (see above; Table S2 †).
The structural features optimized both in the gas phase and in DMSO are summarized in Table 3. Very similar sets of data were found for the optimized geometries of complexes 1-6 in the gas phase, the Pt-S and Pt-N distances falling in the ranges 2.284-2.289and 2.050-2.059Å, respectively.
angle, which is increased for all complexes by about 5°t owards planarity on passing from the gas phase to the implicitly solvated species.The calculations performed in the case of complex 1 taking into account eight different solvents confirm that the optimized parameters vary only slightly upon changing the solvent (Table S2 †). 65A very good agreement was found between the parameters optimized in the gas phase and the corresponding experimental structural data for complexes 1 and 4, with all of the bond lengths and angles differing by less than 0.03 Å and 2°, respectively.
In Fig. 8 and S3 † the ground-state (GS) bonding scheme is shown in terms of the frontier Kohn-Sham (KS) molecular orbital (MO) scheme for complexes 1 and 2, respectively.Analogously to what was reported for related complexes, 66,67 KS-HOMO and KS-HOMO−1 (MOs 102 and 101, respectively, in Fig. 8) are π-in-nature MOs mainly localized on the sulphur atoms of the dtoc 2− ligand.The stabilization effect induced by the electron-withdrawing dtoc 2− ligand allows for a larger extent of mixing between the ligand fragment molecular orbitals (FMOs) and the atomic orbitals (AOs) of the metal ion in the title complexes.In fact, KS-HOMOs show a remarkable contribution from 5d AOs of the Pt atom, which increases when the solvent is taken into account (17-18 and 24-25% in the gas phase and in DMSO, respectively; Fig. 9 for complex 1).Accordingly, KS-HOMO eigenvalues calculated for complexes 1-6 in the gas phase 68 are remarkably more negative than those calculated at the same level of theory for different [Pt(N^N)(S^S)] complexes reported previously. 34,69The stabilization of the eigenvalues calculated for HOMOs and HOMO−1s for the complexes featuring the dtoc 2− ligand is reflected in the absence of oxidation processes in the voltammograms recorded for complexes 1-6 in DMSO solution (see above). a Atom labelling scheme as reported in Fig. 1 for complex 1.The KS-LUMO is almost exclusively centred on the diimine ligand both in the gas phase and in DMSO (92-95%, Table S3 †).The localization of the KS-LUMO on the N^N ligand is in agreement with what was hypothesized based on the analysis of the electrochemical and spectroscopic data discussed above.Accordingly, the potentials of the two reduction processes E I 1/2 and E II 1/2 observed for the complexes are linearly correlated with the KS-eigenvalues calculated for LUMOs of 1-6 in DMSO solution (R 2 = 0.84 and 0.61 for the two reductions; Fig. 10).No significant changes were observed in the composition of the frontier MOs upon varying the solvents considered for the IEF-PCM SCRF calculations in the case of complex 1 (Table S4 †).
An analysis of the natural atomic charges 70 calculated on complexes 1-6 shows that a charge separation of about 1.0 and 1.2 e exists between the two ligands in the gas phase and in DMSO, respectively (Table S5 †).This polarization arises from the partial negative charge carried by the dtoc 2− ligand (ranging between −0.54 and −0.71 e in the gas phase and between −0.57 and −0.73 e in DMSO solution) and the partial positive charge shown by the N^N ligands (in the ranges 0.42-0.46e in the gas phase and 0.57-0.59e in DMSO), the remaining positive charge of the complex being carried by the platinum ion (0.13-0.14 and e in DMSO).The calculated charge separation, which confirms what was previously reported for Pt II diimine-dithiolate systems, 34 also shows a small but not negligible dependence on the nature of the solvent (Table S6 † for complex 1).
Time-dependent DFT (TD-DFT) calculations were carried out on complexes 1-6.Simulated UV-vis absorption spectra of the complexes are in very good agreement with the experimental ones, in particular as far as the solvation calculations are concerned (Tables 4 and S7, † and Fig. 11 for complex 2).For all complexes, the S0→S2 electronic transition was calculated to be responsible for the lowest energy solvatochromic absorption band observed experimentally in the 410-500 nm region (see above).This transition is mainly assigned to an almost pure (97% for complex 1, Table 4) monoelectronic excitation between the KS-HOMO−1 and KS-LUMO (KS-MOs 101 and 103, respectively, for complex 1; Table 4 and Fig. 8), in contrast with the previous DFT calculations reported on complex 6, where the transitions involving the frontier MOs were calculated to have negligible oscillator strengths. 67In all complexes, KS-HOMO−1, analogous to KS-HOMO, is a π-innature orbital predominantly localized on the sulfur atoms of the dtoc 2− ligand, with a significant contribution from the Pt atom (27-28% in DMSO; Fig. 8, 9 and S3 † and Table S3 †).Thus, given the localization of KS-LUMO described above for complexes 1-6, the transition could be considered a mixedmetal/ligand-to-ligand CT (MMLL'CT), analogous to most of the [Pt(N^N)(S^S)] complexes reported so far (where it arises from the HOMO→LUMO one-electron excitation). 25he CT character of this transition leads to a lowering in the charge separation in the excited state as compared to the GS that accounts for the negative solvatochromism observed.
The KS-LUMO composition also accounts for the linear correlation observed experimentally between the λ max of the corresponding absorption and the reduction potentials E I 1/2 and E II 1/2 of complexes 1-6, and for the dependence of the lowest energy solvatochromic band on the nature of the  1 and S3 †).N^N = unsubstituted (rhombs) 2,2'-bipyridine (empty) or 1,10-phenathroline (filled), and alkyl (circles) or phenyl (triangles) derivatives.
Table 4 Main singlet electronic transitions ( f > 0.005) calculated for complex 1 in DMSO (IEF-PCM SCRF Model) at the TD-DFT Level.For each transition, the excitation energy E (in eV), the absorption wavelength λ (in nm), the oscillator strength f, and the molecular orbital composition (%) of the excited state functions, along with the fragments where the involved KS-MOs are mainly localized, are reported a a MOs are labelled according to Fig. 8.

Exc
diimine observed experimentally.Accordingly, the experimental λ max values of the lowest energy solvatochromic absorption can be directly correlated to the eigenvalues of the KS-LUMO calculated for complexes 1-6 (Fig. S4 †).
A very good agreement was also found between the experimental and calculated λ max values of the lowest energy absorption in different solvents in the case of complex 1 (R 2 = 0.94; Fig. S5 †). 71,72s regards complexes 2 and 6 bearing a 1,10-phenanthroline ligand, the solvatochromic absorption observed experimentally in the region 370-540 nm was calculated to derive mainly from the S0→S3 vertical transition (Table S7 †).This transition is mainly (73% for complex 2) due to the KS-HOMO→KS-LUMO+1 monoelectronic excitation (108→110 for complex 2, Table S7 and Fig. S3 †).Also in this case, the excitation can be considered a MMLL'CT, since KS-LUMO+1 is almost exclusively localized on the N^N ligand (99% for complex 2), thus accounting for its solvatochromism.
Finally, since complexes 1-6 could be exploited as SONLO materials, 41 in analogy with different Pt II complexes, 73 static dipole moments μ and static first hyperpolarizabilities β tot were calculated both in the gas phase and in DMSO (Table 5).The calculated values of β tot (75-136 × 10 −30 esu in the gas phase) are remarkably lower than those previously calculated for [Pt(N^N)(S^S)] complexes featuring different 1,2-dithiolato ligands (150-650 × 10 −30 esu in the gas phase), 34 thus confirming the experimental observations reported by Grinstaff and co-workers on the SONLO properties of [Pt(N^N)(S^S)] complexes with S^S = dithiolate o-carborane and different 1,2dithiolates. 42No significant differences were observed in this case between the hyperpolarizability values calculated in the gas phase and in DMSO, while a clear trend was observed upon varying the N^N ligand.The largest values of β tot were calculated for complexes 5 and 6, featuring phenyl-substituted diimines, thus confirming that aryl pendants at the N^N ligand could improve the SONLO properties of [Pt(N^N)(S^S)] systems. 34

Conclusions
Five new [Pt(N^N)(S^S)] complexes featuring the 1,2-dithiolato-1,2-closo-dicarbadodecaborane ligand (dtoc 2− ) in combination with differently substituted 2,2′-bipyridine and 1,10-phenanthroline ligands were synthesized and characterized by both experimental and theoretical means, and their features were compared to those of the only previously reported complex of the same type (6).The very first structural information on [Pt(N^N)(dtoc)] complexes were also obtained by single crystal X-ray diffraction.
As already observed for different Pt II diimine-dithiolate complexes, solvatochromic absorptions are displayed in the visible region of the complexes' spectra, and in particular a single broad absorption was observed in the case of the complexes featuring a 2,2′-bipyridine derivative (λ max = 420-440 nm in DMSO).Based on TD-DFT calculations, the broad absorption was attributed to the HOMO−1/LUMO electronic excitation, whose MMLL'CT character accounts for its solvatochromism.On the other hand, two different bands were shown in the case of complexes bearing 1,10-phenanthroline derivatives (λ max = 427-441 nm in DMSO), both assigned to excitations having a MMLL'CT character as well.The DFT calculations reported in this work thus allowed for the first persuasive interpretation of the peculiar spectroscopic features of [Pt(N^N)(dtoc)] complexes, in particular those containing the 1,10-phenanthroline moiety.
In the title complexes, the electron-withdrawing effect of the dtoc 2− ligand determines a remarkable stabilization of the HOMOs and HOMO−1s with respect to [Pt(N^N)(S^S)] systems bearing different sulfur ligands.Thus, no oxidation processes were observed by CV in DMSO solution within the explored range of potentials (−2.5-1.5 V vs. Fc + /Fc).As a consequence, the energy gap between HOMO/HOMO−1 and LUMO is significantly increased with respect to other [Pt(N^N)(S^S)] complexes, thus shifting the typical solvatochromic absorption to higher energies.4).It is worth underlining that the peculiar electronic structure of [Pt(N^N)(S^S)] complexes allows the fine-tuning of the energy of their frontier orbitals by a proper choice of the ligands, and hence the tailoring of both the electrochemical stability of the neutral complexes and the energy of the solvatochromic visible absorption.In this context: (1) Donor S^S ligands raise HOMOs eigenvalues, while electron-withdrawing dithiolates, such as dtoc 2− , exert the opposite effect.
(2) Diimines with donor (alkyl) substituents feature the LUMO at higher energy as compared to phenyl-substituted ones. (

Synthesis of complexes
Platinum dichlorodiimine complexes [PtCl 2 (N^N)] 44,45 and 1,2dithiol-1,2-closo-dicarbadodecarborane 40 were synthesized according to published procedures.The complex [Pt(Ph 2 phen)-(dtoc)] (6) was synthesized through a modification of the procedure reported in the literature. 41Compounds 1-5 were all synthesized according to a similar procedure, described here in detail for complex 1.The details of the characterization of complexes 2-5 are available as ESI.† [Pt(bipy)(dtoc)] (1).To a solution of 1,2-dithiolato-1,2-closodicarbadodecarborane (0.10 g, 0.48 mmol) in 5 mL of dry THF a two-fold excess (0.96 mmol) of n-BuLi in hexane solution was added dropwise under a nitrogen atmosphere at 0°C, and the reaction mixture was stirred for 2 hours at the same temperature.An equimolar suspension of [PtCl 2 (bipy)] (0.20 g, 0.48 mmol) in 20 mL of the same solvent was then added dropwise, and the system was left under magnetic stirring and under N 2 at room temperature for one week.The resulting orange precipitate was collected by filtration and washed with water.X-ray quality crystals were obtained by slow infusion of petroleum ether into a dichloromethane solution of the complex.Yield: 0.

Characterization
Microanalyses were performed by using a Perkin-Elmer 240 B microanalyser.Infrared spectra were recorded on a Thermo-Nicolet 5700 spectrometer at room temperature: KBr pellets with a KBr beam-splitter and KBr windows (4000-400 cm −1 , resolution 4 cm −1 ) were used.The 1 H-NMR (300.13MHz), 13 C { 1 H}-NMR (75.47 MHz) and 11 B-NMR (96.29 MHz) spectra were recorded on a Bruker ARX 300WB spectrometer.Chemical shift values for 1 H-NMR spectra were referenced to an internal standard of SiMe 4 in deuterated solvents.The 13 C{ 1 H}-NMR spectra of slightly soluble compounds were dissolved in DMF and spectra run in a double tube containing acetone-d 6 .Chemical shift values for 11 B-NMR spectra were referenced relative to external BF 3 •OEt 2 .Chemical shifts are reported in units of parts per million downfield from the reference, and all coupling constants are reported in hertz.Absorption spectra were recorded on solutions of the investigated complexes in spectrophotometric grade solvents at 298 K in a quartz cell of 10.00 mm optical path using a Thermo Evolution 300 (190-1100 nm) spectrophotometer.Cyclic voltammetry measurements (scan rate 10-1000 mV s −1 ) were performed in dry DMSO in a Metrohm voltammetric cell, with a combined working and counter platinum electrode and a standard Ag/ AgCl reference electrode using a Metrohm Autolab PGSTAT 10 potentiostat (C ≈ 1.5 × 10 −3 M; supporting the electrolyte [NBu 4 ][PF 6 ] 0.10 M): reported data are referred to the Fc + /Fc reversible couple.The DMSO solutions were deaerated with analytical grade nitrogen at the start of each experiment to prevent oxygen interference.All experiments were performed at room temperature.

X-ray crystallography
Single crystal X-ray diffraction data were collected at 120(2) K by combined Φ and Ω scans on a Bruker Nonius KappaCCD area detector situated at the window of a rotating anode (graphite Mo Kα radiation, λ = 0.71073 Å).The structures were solved by direct methods with SHELXS-97 and refined on F 2 by using SHELXL-97. 74Complete crystallographic data have been deposited at the Cambridge Crystallographic Data Centre, CCDC 1010434 (complex 1) and 1010435 (complex 4).

DFT calculations
Theoretical calculations were performed at the DFT level 57 with the Gaussian 09 suite of programs (rev.A.02) 75 on a E4 workstation equipped with four quad-core AMD Opteron processors and 16 Gb of RAM, running the 64 bit version of the Ubuntu 12.04 Linux operating system.The PBE0 (PBE1PBE) hybrid functional was adopted, 58 and Schäfer, Horn, and Ahlrichs double-ζ plus polarization all-electron basis sets 59 were used for C, H, N, B, and S, whereas CRENBL BS 60 with relativistic effective core potentials (RECPs) 61,62 was chosen for the heavier Pt species.The tight SCF convergence criterion (SCF = tight keyword) and fine numerical integration grids [Integral-(FineGrid) keyword] were used.The nature of the minima of each optimized structure was verified by harmonic frequency calculations ( freq = raman keyword).Natural 70 and Mulliken 76 atomic charges were calculated at the optimized geometries at the same level of theory, and electronic transition energies and oscillator strength values were calculated at the TD-DFT level (100 states).The electronic spectra were simulated by a convolution of Gaussian functions centered at the calculated excitation energies.Solvation calculations were also carried out in the presence of a set of different solvents (CH 2 Cl 2 , CHCl 3 , CH 3 CN, acetone, THF, DMF, DMSO, and toluene), implicitly taken into account by the polarizable continuum model in its integral equation formalism variant (IEF-PCM). 64The total static second-order (quadratic) hyperpolarizability (the first hyperpolarizability) 77 β tot was calculated as described by Mendes et al. 78 from the relation: where each static component β i (i = x, y, z) is obtained from the equation: By using Kleinman symmetry relationships, 79 the final equation was obtained: The programs Gaussview 5.0 80 and Molden 5.0 81 were used to investigate the charge distributions and molecular orbital shapes.The software GaussSum 2.1 82 was used to calculate the molecular orbital contributions (MOC) from groups of atoms, along with the contribution of singly excited configurations to each electronic transition, and to generate all the necessary data to simulate absorption spectra.

Fig. 3
Fig. 3 Space fill style view evidencing the void channels in the packing of complex 4.

Fig. 8
Fig. 8 KS-MO scheme and isosurface drawings calculated for complex 1 in DMSO (IEF-PCM SCRF model; C s point group; contour value = 0.05 e).
) As a consequence, on passing from complexes featuring a donor dithiolate and a phenyl-substituted diimine, such as [Pt(Ph 2 bipy)(Me-dmet)], to complexes featuring electron-withdrawing dithiolates and alkyl-substituted diimines, such as [Pt(Me 2 bipy)(dtoc)], the wavelength of the solvatochromic band can be varied by more than 150 nm in DMSO (λ max = 437 and 602 nm for [Pt(Me 2 bipy)(dtoc)] and [Pt(Ph 2 bipy)-(Me-dmet)], respectively).Finally, the calculated β tot values show that [Pt(N^N)(dtoc)] complexes have a less pronounced SONLO behaviour in comparison with [Pt(N^N)(S^S)] systems bearing 1,2-dithiolene ligands.The static β tot values reported in this work are in agreement with the trend previously observed with regard to the influence of the N^N ligand on the hyperpolarizability values, which showed that the presence of phenyl substituents increases β tot values.The insight into the electrochemical and linear and nonlinear spectroscopic features of [Pt(N^N)(dtoc)] complexes given in the present work represents therefore a significant step forward in the understanding of the electronic features of this class of compounds, and opens the way to their employment in devices of interest in optical processing.Experimental section Chemicals 2,2′-Bipyridine, 1,10-phenanthroline and derivatives, K 2 [PtCl 4 ], n-butyllithium (n-BuLi) were obtained from commercial sources.The solvents for the syntheses were of reagent grade.THF was freshly distilled on LiAlH 4 before use.

Table 2
Absorption maxima λ max (in nm) and molar extinction coefficients (in M −1 cm −1 , in parentheses) a of the lowest energy solvatochromic absorption band in the Vis region for complexes 1-6 in different solvents a If not reported, the molar extinction coefficients were not determined for solubility reasons.

Table 3
Selected optimized bond lengths (in Å) and angles (in °) for complexes 1-6 in the gas phase and in DMSO (IEF-PCM SCRF model, in parentheses) a

Table 5
Static first hyperpolarizability β tot (in a.u. and esu) and static dipole moment μ (in D) calculated for complexes 1-6 in the gas phase and in DMSO (IEF-PCM SCRF model)