Synthesis, structure and spectral properties of dithiocarbamato bridged dirhenium(III,II) complexes: A combined experimental and theoretical study

Inorganica Chimica Acta xxx (2014) xxx–xxx

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Synthesis, structure and spectral properties of dithiocarbamato bridged dirhenium(III,II) complexes: A combined experimental and theoretical study Suman Mallick a, Mrinal Kanti Ghosh a, Manoj Mohapatra b, Sudip Mohapatra c, Swarup Chattopadhyay a,⇑ a b c

Department of Chemistry, University of Kalyani, Kalyani, Nadia 741235, WB, India Radiochemistry Division, RLG, Bhabha Atomic Research Centre, Mumbai 400085, India Department of Chemistry, Missouri University of S & T, Rolla, MO 65409, USA

a r t i c l e

i n f o

Article history: Received 19 May 2014 Received in revised form 14 July 2014 Accepted 20 July 2014 Available online xxxx SI: Metal-metal bonded compounds Keywords: Dirhenium(III,II) Dithiocarbamate Paramagnetic Edge-shared bioctahedra Density functional theory

a b s t r a c t Sodium salts of dimethyldithiocarbamate, diethyldithiocarbamate and pyrrolidinedithiocarbamate react with the multiply bonded paramagnetic dirhenium(III,II) complex Re2(l-O2CCH3)Cl4(l-dppm)2, 1 (dppm = Ph2PCH2PPh2) in refluxing ethanol to afford the paramagnetic substitution products of the type Re2(g2-S,S)2(l-S,S)(l-Cl)2(l-dppm), where S,S represents the dithiocarbamato ligands [S,S = S2CNMe2, 4(LMe); S2CNEt2, 4(LEt) and S2CN(CH2)4, 4(LPyr)]. These are the first examples of dirhenium complexes that contain bridging dithiocarbamato ligand along with the dppm ligand. These complexes have very similar spectral (UV–Vis, IR, EPR) and electrochemical properties which are also reported. The identity of 4(LEt) has been established by single-crystal X-ray structure determination (Re–Re distance 2.6385 (9) Å) and is shown to have edge-shared bioctahedral structure. The electronic structure and the absorption spectra of the complexes are scrutinized by the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) analyses. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction The multiply bonded paramagnetic dirhenium(III,II) complex Re2(l-O2CCH3)Cl4(l-dppm)2 1 (dppm = Ph2PCH2PPh2), with the r2p4d2d⁄1 electronic configuration has been prepared almost 25 years ago and its solid state structure was also determined in the year 1988 [1]. Since then efforts have been made to develop the reaction chemistry of this useful synthon 1. Although there has been a flurry of compounds prepared using the other two multiply bonded dirhenium(II,II) synthons Re2Cl4(l-dppm)2 2 [2] and cis-Re2(l-O2CCH3)2Cl2(l-dppm)2 3 [1] and an extensive chemistry which has now been developed involving 2 and 3 as the starting materials, there exist few examples of complexes in which 1 has been used as synthon. The substitutional lability of the l-O2CCH3 ligand in 1 has been demonstrated by its reaction with carboxylic acids in a fashion similar to that of cis-Re2(l-O2CCH3)2Cl2(l-dppm)2. The reaction of 1 with isonicotinic acid affords the expected dirhenium(III,II) complex Re2(l-O2CC5H4N)Cl4(l-dppm)2 [3] whereas with terephthalic acid the centrosymmetric ‘‘dimer-of-dimers’’

⇑ Corresponding author. Tel.: +91 33 25828750; fax: +91 33 25828282. E-mail address: [email protected] (S. Chattopadhyay).

l-terephthalate complex [(l-dppm)2Cl4Re2]2(l-O2CC6H4CO2) [4] is formed. Similar dicarboxylate-bridged complexes are isolated with the use of adipic acid, fumaric acid and 4,40 -biphenyldicarboxylic acid [3]. When trans-1,4-cyclohexanedicarboxylic acid is used the reduced Re4+ 2 complex cis-Re2(l-O2CC6H10CO2Et)2Cl2(ldppm)2 was obtained [3]. On the other hand the reactions of 1 with tetrafluoro terephthalic acid and 1,10 -ferrocene dicarboxylic acid afford the known Re2Cl4(l-dppm)3 complex [3]. When 1 is made to react with acetylene dicarboxylic acid the paramagnetic lalkyne complex Re2(l-Cl)(l-g2-HCCH)Cl4(l-dppm)2 [5] is formed whereas when 2-butynoic acid is used the diamagnetic l-carbyne complex Re2(l-Cl)(l-CCH2CH3)Cl4(l-dppm)2 [5] is isolated by decarboxylation method. All the reactions cited above show that 1 contains labile l-O2CCH3 group in combination with substitutionally inert dppm and Cl ligands, but none has yet been reported in which the dppm and the Cl ligands have been replaced by other suitable chelating or bridging ligands. Of the diverse array of dirhenium complexes that are known [6], few have been isolated that contain dithiocarbamate ligand in the coordination sphere. The only compounds that we are aware of are the Re(IV)–Re(IV) dimer, Re2(l-S)2(S2CNR2)4 and the Re(III)–Re(III) dimer [Re2(l-SS2CNR2)2(S2CNR2)3]+ where R = Me and iBu [7]. No other dithiocarbamato complexes of dirhenium are known where there is at least one Re–Re bond

http://dx.doi.org/10.1016/j.ica.2014.07.047 0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

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present. This has prompted us to examine the reactions of Re2(lO2CCH3)Cl4(l-dppm)2 1 towards various dithiocarbamato ligands in order to assess the stability of the Re5+ 2 core to ligands of this type. In the course of this work we have discovered that under refluxing condition in ethanol, 1 reacts with the sodium salts of LMe, LEt and LPyr ligands (Chart 1) to afford the paramagnetic Re5+ 2 complexes Re2(l-Cl)2(l-dppm)(l-LR)(g2-LR)2 4 [R = Me, Et, Pyr], where substitution of two chloride ligands, one dppm ligand and the acetate ligand occurs and the dithiocarbamato ligands are involved in both the bridging and chelating coordination modes to the dirhenium core. This is, to our knowledge, the first example of bridging dithiocarbamato ligand occurring in a dimetal unit containing dppm ligand. The synthetic procedures, structures and properties of the resulting complexes are reported. To get better insight into the geometry, electronic structure and optical properties of these complexes, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) studies have also been presented. These combined experimental and theoretical studies provide the first detailed investigation of the electronic structure of the complexes of type 4. 2. Experimental

2.3. Synthesis of Re2(l-Cl)2(l-dppm)(l-S2CNMe2)(g2-S2CNMe2)2, 4(LMe) This complex was prepared by the same procedure as above using Re2(l-O2CCH3)Cl4(l-dppm)2 (100 mg, 0.0745 mmol) and sodium dimethyldithiocarbamate hydrate (48 mg, 0.3352 mmol); yield 72 mg (81%). Anal. Calc. for C34H40N3P2S6Cl2Re2: C, 34.36; H, 3.39; N, 3.54. Found: C, 34.25; H, 3.46; N, 3.62%. IR (KBr, cm1): 1434 (mC–N), 739 (mCS), 1092 (mas(SCS)) and 689 (ms(SCS)); UV–Vis [kmax, nm (e, dm3 mol1 cm1)]: 710 (1660), 470 (6340), 430 (6240); E1/2 (versus Ag/AgCl, CH2Cl2, scan rate 100 mV s1): 0.23 V (DEp = 150 mV). 2.4. Synthesis of Re2(l-Cl)2(l-dppm)(l-S2CN(CH2)4)(g2-S2CN(CH2)4)2, 4(LPyr) The procedure was the same as for compound 4(LEt) but by using sodium pyrrolidinedithiocarbamate (56.7 mg, 0.3352 mmol); yield 75 mg (79%). Anal. Calc. for C40H46N3P2S6Cl2Re2: C, 37.93; H, 3.66; N, 3.32. Found: C, 37.83; H, 3.58; N, 3.38%. IR (KBr, cm1): 1446 (mC–N), 738 (mCS), 1091 (mas(SCS)) and 690 (ms(SCS)); UV–Vis [kmax, nm (e, dm3 mol1 cm1)]: 660 (2060), 460 (6300); E1/2 (versus Ag/AgCl, CH2Cl2, scan rate 100 mV s1): 0.26 V (DEp = 150 mV).

2.1. Materials and methods 2.5. X-ray crystallography The compound Re2(l-O2CCH3)Cl4(l-dppm)2 1 was prepared by the literature method [1]. The ligands sodium dimethyldithiocarbamate hydrate, sodium diethyldithiocarbamate trihydrate and sodium pyrrolidinedithiocarbamate were purchased from Sigma Aldrich, India. All other reagents were obtained from commercial sources and were used as received, except for drying of solvents by routine techniques. All manipulations were carried out under an inert atmosphere using Schlenk techniques. Infrared spectra were recorded on a Perkin-Elmer L120-00A FT-IR spectrometer as a KBr pellet. Electronic spectra were recorded on a Shimadzu UV1800 PC spectrophotometer. Microanalyses were performed using a Perkin-Elmer 2400 series-II elemental analyser. EPR spectra were recorded on a Bruker EMX series (EMM1843) spectrometer. All electrochemical measurements were performed under a nitrogen atmosphere using CHI 600D electrochemistry system. The supporting electrolyte was tetrabutylammonium perchlorate and potentials are referenced to Ag/AgCl electrode. 2.2. Synthesis of Re2(l-Cl)2(l-dppm)(l-S2CNEt2)(g2-S2CNEt2)2, 4(LEt) A mixture of Re2(l-O2CCH3)Cl4(l-dppm)2 (100 mg, 0.07 45 mmol) and sodium diethyldithiocarbamate trihydrate (75.4 mg, 0.3352 mmol) in 30 mL of ethanol was refluxed overnight and then cooled to room temperature. The brown crystalline solid thus obtained was filtered off and washed with ethanol (3  5 mL) followed by diethyl ether (2  5 mL) and dried in vacuo; yield 70 mg (71%). Anal. Calc. for C40H52N3P2S6Cl2Re2C2H5OH: C, 38.26; H, 4.43; N, 3.19. Found: C, 38.45; H, 4.40; N, 3.26%. IR (KBr, cm1): 1432 (mC–N), 737 (mCS), 1089 (mas(SCS)) and 690 (ms(SCS)); UV–Vis [kmax, nm (e, dm3 mol1 cm1)]: 725 (2400), 470 (8180), 430 (8200); E1/2 (versus Ag/AgCl, CH2Cl2, scan rate 100 mV s1): 0.29 V (DEp = 170 mV). S

S C S

C

NEt2 LEt

S NMe2

S LMe Chart 1.

N

C S

LPyr

Single crystals of composition Re2(l-Cl)2(l-dppm)(l-S2CNEt2)(g2-S2CNEt2)2C2H5OH, 4(LEt) C2H5OH were harvested directly from the reaction medium. The crystal was mounted on a Bruker AXS SMART APEX CCD diffractometer (Mo Ka, 0 k = 0.71073 Å A). The data were reduced in SAINTPLUS [8] and empirical absorption corrections were applied using the SADABS [8] package. The metal atoms were located by Patterson method and the rest of the non-hydrogen atoms were emerged from successive Fourier synthesis. The isotropic refinement of all the atoms revealed high thermal parameters for the ethyl groups attached to N1 atom. Careful inspection indicated that the ethyl groups are disordered in two different orientations and they were modeled accordingly. Refinement with two different orientations of ethyl groups resulted in better R-factor and gave a ratio of 65:35 for the occupancies of the two orientations. Hydrogen atoms were placed in idealized positions. The structures were refined by a full matrix least-squares procedure on F2. All non-hydrogen atoms were refined anisotropically. All calculations were performed using the SHELXTL V6.14 program package [9]. Molecular structure plots were drawn using the Oak Ridge thermal ellipsoid plot ORTEP-32 [10]. The key crystallographic data for 4(LEt)C2H5OH is given in Table 1. 2.6. Computational study The ORCA 2.9.1 software package was used for all DFT computations [11]. The geometry of the complexes was fully optimized in the gas phase without imposing any symmetry constraints. The single crystal X-ray coordinates have been used as the initial input in the calculation. Geometry optimizations for the complexes were converged to the S = 1/2 spin and employed the Becke–Perdew (BP86) functional [12] and the SV(P) (Ahlrichs split valence polarized) basis with the SV/C auxiliary basis for all atoms except for sulfur, nitrogen, phosphorus and chlorine, where the larger TZVP (Ahlrichs triple-valence polarized) basis in conjunction with the TZV/J auxiliary basis were used [13]. For rhenium atoms the scaled-ZORA (zeroth-order regular approximation) Hamiltonian was used to take account of the relativistic effect in the calculations [14]. These calculations employed the resolution of identity (RI) approximation developed by Neese [15]. The coordinates of all DFT energy minimized model of the complexes presented in

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0.000004

Empirical Formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z l (Mo Ka) (mm1) Total reflections Independent reflections (Rint) R1, wR2 [I > 2r(I)] Goodness-of-fit (GOF) on F2 Largest difference in peak and hole (e Å3)

C40H52N3P2S6 Cl2Re2C2H5OH 1318.51 triclinic  P1 11.441(5) 12.816(6) 19.954(9) 94.244(6) 105.724(5) 116.293(5) 2460.4(18) 2 5.380 28 677 11 701 (0.0378) 0.0321, 0.0790 1.131 1.851 and 2.234

this study are included in Tables S1–S3  (Supporting information). Electronic transition energies and intensities were computed for 4(LEt) using the time-dependent (TD)-DFT method within the Tamm–Dancoff approximation [16]. These calculations employed the B3LYP functional [17] and ZORA (Re), TZVP (P, S, N, and Cl) and SVP (C and H) basis sets. Forty excited states were calculated by including all one-electron excitations within an energy window of ±3 hartree with respect to the HOMO/LUMO energies. Isosurface plots of molecular orbitals and electron density difference maps (EDDMs) were generated using the gOpenMol program using isodensity values of 0.05 and 0.003 b2, respectively.

3. Results and discussion The reactions of the dirhenium(III,II) synthon Re2(l-O2CCH3)Cl4(l-dppm)2 with the sodium salt of three different dithiocarbamato ligands (LMe, LEt and LPyr) give products in which one dppm, two chloride and the acetato groups are displaced in refluxing ethanol and the resulting complexes contain two bridging chloride ligands, one bridging dppm ligand and the dithiocarbamato ligands are involved both in the chelating and bridging modes. The complexes were isolated as brown crystalline solid in excellent yield. Specific compounds will be identified by putting LR in parenthesis e.g. 4(LMe) stands for Re2(l-Cl)2(l-dppm)(l-S2CNMe2)(g2-S2CNMe2)2. All the complexes behave as non electrolyte in acetone and other common solvents and they have very similar spectroscopic and cyclic voltammetric properties (vide infra) which implies a close similarity between their electronic and molecular structures. All the complexes of type 4 are electroactive in dichloromethane solution and display one quasi-reversible one-electron reduc4+ tion process which is assignable to the Re5+/ couple. The 2 Re2 cyclic voltammogram of 4(LEt) is shown in Fig. 1 whereas for 4(LMe) and 4(LPyr) the same have been included in Figs. S1 and S2  (Supporting information). The marginal shift in the potentials presumably reflects the effect of different substituent on nitrogen atom of the dithiocarbamato ligand. The spin states of the Re25+ complexes in CDCl3 solution were probed using the 1H NMR method of Evans [18]. Analysis of the 1 H NMR data revealed room temperature effective magnetic moments of 1.66–1.98 lB (Table S4 , Supporting information). This value is consistent with the presence of one unpaired electron per dirhenium unit. The paramagnetic nature of the complexes of type 4 has also been probed by EPR experiment, run at a frequency of 9.41 GHz in the solid state at 298 K. The spectra are so similar to one another that the complexes must possess the same electronic

Current/1e-5A

Table 1 Summary of X-ray crystallography for 4(LEt)C2H5OH.

0.000002 0.000000 -0.000002 -0.000004 0.0

-0.1

-0.2

-0.3

-0.4

-0.5

Potential/V Fig. 1. Cyclic voltammogram of 4(LEt) in dichloromethane solution recorded at a scan rate of 100 mV s1.

ground state. The EPR spectrum of 4(LEt) is shown in Fig. 2 whereas for 4(LMe) and 4(LPyr) the same have been included in Figs. S3 and S4  (Supporting information). The structural identity of 4(LEt) as an edge-shared bioctahedral species in which the bridging ligands are the two chlorine atoms was established by a single crystal X-ray structure analysis of 4(LEt)C2H5OH. An ORTEP representation of the structure is shown in Fig. 3 and selected bond parameters are listed in Table 2. The lattice ethanol molecule was refined with full occupancy about a general position. A disorder in the NEt2 group of the bridging dithiocarbamato ligand complicated the structure refinement but was modeled satisfactorily. The two rhenium atoms are bound to same ligand atom types and the ReS3Cl2P coordination sphere has distorted octahedral geometry as can be seen from the angles at the metal centre. The Re1, Cl1, Cl2, S5 and S6 atoms define an equatorial plane with mean deviation of 0.0691 Å whereas for Re2, Cl1, Cl2, S3 and S4 atoms this value is 0.0779 Å. The P2– Re1–S1 and P1–Re2–S2 angles are 170.26(5)° and 171.21(4)°, respectively. The two dithiocarbamato ligands are bound in a conventional bidentate chelating fashion to the two rhenium atoms whereas the third dithiocarbamato and the dppm ligands behave as bridging ligand and bridge the Re–Re bond. The Re1–Re2 distance is 2.6385(9) Å which accords with the formal Re–Re bond order of 1.5 [5]. The Re–Re distance in 4(LEt)C2H5OH falls within the range of the corresponding distances in other structurally characterized edge-shared bioctahedral dirhenium complexes [19]. The two Re–P bond lengths (2.41 Å) in 4(LEt)C2H5OH are slightly shorter than that observed in other structurally characterized dppm containing Re5+ 2 complexes [3–5]. The average Re–S distance is 2.48 Å, this value corresponds well with that observed in other structurally characterized dithiocarbamato con-

Fig. 2. Solid state X-band EPR spectrum of 4(LEt) at 298 K.

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S. Mallick et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx Table 3 Selected optimized geometrical parameters (bond lengths (Å) and angles (°)) of 4 in the ground state.

Table 2 Selected bond lengths (Å) and angles (°) for 4(LEt)C2H5OH. Re1–Re2 Re1–Cl1 Re1–Cl2 Re1–P2 Re1–S5 Re1–S6 Re1–S1 Re2–Cl1 Re2–Cl2 Re2–P1 Re2–S4 Re2–S3 Re2–S2

2.6385(9) 2.2988(16) 2.2952(16) 2.4074(16) 2.4697(17) 2.4707(17) 2.5048(17) 2.3027(16) 2.2984(16) 2.4082(16) 2.4711(17) 2.4732(17) 2.4979(16)

S5–Re1–S6 S4–Re2–S3 Re1–Cl1–Re2 Re1–Cl2–Re2 P2–Re1–S1 P1–Re2–S2 Cl2–Re1–Cl1 Cl2–Re2–Cl1 S5–Re1–Cl2 S6–Re1–Cl1 S3–Re2–Cl1 S4–Re2–Cl2

70.60(4) 70.41(4) 69.97(4) 70.11(4) 170.26(5) 171.21(4) 110.08(4) 109.83(4) 160.61(5) 158.77(5) 160.71(5) 158.57(5)

4(LEt)

4(LMe)

4(LPyr)

2.695 2.442 2.440 2.362 2.491 2.479 2.502 2.442 2.440 2.362 2.494 2.477 2.503 71.05 71.02 66.99 67.06 171.63 171.72 112.67 112.66 157.39 157.59 157.57 157.23

2.693 2.441 2.440 2.365 2.493 2.478 2.499 2.443 2.439 2.365 2.493 2.478 2.502 71.08 71.07 66.92 66.99 171.61 171.54 112.69 112.68 157.32 157.86 157.42 157.52

2.693 2.444 2.440 2.363 2.500 2.483 2.503 2.443 2.439 2.363 2.497 2.485 2.503 71.24 71.24 66.88 67.01 171.42 171.51 112.62 112.67 157.19 158.01 157.78 157.45

4 (L Et ) 4 (L Me ) 4 (L Pyr)

7000 6000

ε ((M-1cm-1)

Fig. 3. ORTEP representation of 4(LEt)C2H5OH with thermal ellipsoids at the 30% probability level. Hydrogen atoms and ethanol molecule have been omitted for clarity.

Re1–Re2 Re1–Cl1 Re1–Cl2 Re1–P2 Re1–S5 Re1–S6 Re1–S1 Re2–Cl1 Re2–Cl2 Re2–P1 Re2–S4 Re2–S3 Re2–S2 S5–Re1–S6 S4–Re2–S3 Re1–Cl1–Re2 Re1–Cl2–Re2 P2–Re1–S1 P1–Re2–S2 Cl2–Re1–Cl1 Cl2–Re2–Cl1 S5–Re1–Cl2 S6–Re1–Cl1 S3–Re2–Cl1 S4–Re2–Cl2

5000 4000 3000 2000 1000

taining rhenium complexes [7,20]. The H(O) atom of the ethanol molecule is involved in hydrogen bonding with the S1 atom of the bridging dithiocarbamato ligand. The O  S distance is 3.311 Å and
0

400

600

800

1000 1200 1400 1600

Wavelength (nm) Fig. 5. UV–Vis–NIR spectra of 4 in dichloromethane solutions.

were performed in gas phase assuming an S = ½ spin state. The geometry used for the ground state optimization is based on the crystal structure parameters of complex 4(LEt) without any ligand

Fig. 4. DFT optimized molecular structures of 4 [Re: Dark blue, Cl: Light green, S: Yellow, P: Orange, N: Sky blue, C: Dark grey]. H-atoms are omitted for clarity.

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simplification. The optimized geometries of the synthesized complexes are shown in Fig. 4 and the significant metrical parameters are listed in Table 3. The optimized geometries of all the three complexes do not show significant differences in the coordination sphere around the rhenium centers which means that the ligands bind in a similar fashion in the complexes. The optimized structural parameters of the complexes are in general agreement with the experimental values (Table 2) and the slight discrepancy arises due to the crystal lattice distortion existing in the real molecules. The absorption spectra of the complexes were recorded in dichloromethane solution at room temperature and display well resolved peaks in the region 1600–380 nm which is depicted in Fig. 5. All of the three complexes show a very low intensity broad peak 1130 nm. For 4(LMe) and 4(LEt) we observed one medium intensity broad peak in the region 710–725 nm and two well resolved sharp peaks at 430 nm and 470 nm. For 4(LPyr) the medium intensity broad peak appears at 660 nm and one sharp peak appears at 460 nm.

The isodensity plots for spin-up MOs from HOMO5 (H5) to LUMO+5 (L+5) for 4(LEt) is shown in Fig. 6. For spin-down MOs the isodensity plots from HOMO5 (H5) to LUMO+5 (L+5) for 4(LEt) are included in Fig. S5  (Supporting information). The partial spin-up frontier molecular orbital compositions and energy levels of 4(LEt) are listed in Table 4. For spin-down MOs the same are included in Table S5  (Supporting information). HOMO is mainly composed of rhenium d-orbitals and ligand p-orbitals while LUMO is mainly composed of rhenium vacant d-orbitals and ligand p⁄orbitals. The compositions of HOMO and LUMO are useful in understanding the nature of transition as well as the absorption spectra of the complexes (vide infra). Table 5 represents the Mulliken atomic charges for 4(LEt). The calculated charges on dirhenium unit are considerably lower than the formal charge +5. This difference is a result of significant charge donation from the Cl, S and P donors. The most relevant calculated absorption energies associated with their oscillator strengths, the main configuration and their

HOMO -5

HOMO - 4

HOMO -2

HOMO -1

LUMO

LUMO +1

LUMO +3

LUMO +4

HOMO -3

HOMO

LUMO +2

LUMO +5

Fig. 6. Isodensity plot of spin-up frontier molecular orbitals of 4(LEt).

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Table 4 Spin-up frontier molecular orbital composition (%) in the ground state for 4(LEt). Orbital

Energy (eV)

LUMO+5 LUMO+4 LUMO+3 LUMO+2 LUMO+1 LUMO HOMO HOMO1 HOMO2 HOMO3 HOMO4 HOMO5

0.4794 0.6348 0.7445 0.7636 0.8358 1.3679 3.9899 4.1152 4.1870 5.1664 5.3283 5.5501

Contribution (%) Re

Cl

LEt

dppm

6.1 4.2 2.4 2.0 1.8 72.3 75.4 64.1 60.5 8.5 49.5 4.3

0.2 0.0 0.1 0.2 0.0 20.1 0.1 14.1 0.3 0.4 11.0 0.8

7.9 2.9 81.4 86.3 0.5 5.2 16.2 10.7 30.8 87.7 26.8 89.2

85.8 92.9 16.1 11.5 97.7 2.4 8.3 8.4 8.4 3.4 7.3 5.7

Table 5 Mulliken atomic charges for 4(LEt). Re1 Re2 Cl1 Cl2 P1 P2

0.245992 0.286627 0.108012 0.109337 0.514090 0.523471

S1 S2 S3 S4 S5 S6

0.066487 0.065313 0.014519 0.066887 0.071683 0.021473

assignments as well as the experimental result of 4(LEt) is given in Table 6. The corresponding simulated UV–Vis absorption spectrum of 4(LEt) is shown in Fig. S6  (Supporting information). Several transitions can be envisaged going from the lower to the higher energy region of the spectrum. The low energy transition in the experimental spectrum is observed near 1130 nm as a very weak and broad band. This feature in the NIR region is analyzed and it is in agreement with the absorptions at 1051.5 nm (1.179 eV, f = 0.0000065) and 1018.1 nm (1.218 eV, f = 0.000094). Among these two calculated transitions, the first one can be described in terms of a linear combination of the H1(a) ? L(a) and H(b) ? L+1(b) character and it is assigned primarily to the [d(Re) + p(Cl) + p(LEt)] ? [d(Re) + p⁄(Cl)] transition while the second appears due to H(a) ? L(a) character and it is assigned to [d(Re) + p(LEt)] ? [d(Re) + p⁄(Cl)] transition. Theoretical analysis of these transitions revealed that they are

mainly of d–d in nature with an admixture of MLCT and LMCT. The medium intensity broad band at 725 nm can be correlated to the theoretical transitions at 775.5 nm (1.599 eV, f = 0.0718) and 760.9 nm (1.630 eV, f = 0.000234). The former excitation appears due to H2(b) ? L(b) character and it is assigned to [d(Re) + p(Cl) + p(LEt ) + p(dppm)] ? [d(Re) + p⁄(LEt ) + p⁄(dppm)] transition. The latter transition can be ascribed to the linear combination of the H 2(a ) ? L(a) and H1(b) ? L+1(b) character and it is assigned to the [d(Re) + p(LEt)] ? [d(Re) + p⁄(Cl)] transition. They are essentially of d–d in nature with admixture of MLCT/LMCT character. The absorption bands at 470 and 430 nm consist of several transitions and these are due to H ? L+1(a), H ? L+2(a), H ? L+3(a), H1 ? L+1(a), H1 ? L+2(a), H1 ? L+3(a), H2 ? L+1(a), H2 ? L+2(a), H2 ? L+3(a), H2 ? L+4(a), H ? L+3(b), H ? L+5(b), H1 ? L+2(b) and H1 ? L+3(b) transitions and arise from [d(Re) + p(LEt) + p(dppm)] ? [p⁄(dppm)], [d(Re) + p(LEt) + p(dppm)] ? [p⁄(LEt) + p⁄(dppm)] and [d(Re) + p(Cl) + p(LEt) + p(dppm)] ? [p⁄ (LEt) + p⁄(dppm)] transitions with primarily MLCT in character. Electron density difference maps (EDDMs) showing surface contour plots of loss and gain of electron density for a given electronic transition have been included in the Fig. S7  (Supporting information). 4. Conclusions The results of this study demonstrate that the reaction of the multiply bonded paramagnetic dirhenium(III,II) complex Re2(l-O2CCH3)Cl4(l-dppm)2 with various dithiocarbamato ligands are noteworthy because we have the first example of bridging dithiocarbamato ligand occurring at a paramagnetic dimetal unit with the symmetrical diphenylphosphino methane bridging ligand. The identity of the products have been established by single-crystal X-ray structure determination of 4(LEt) and is shown to have an edge-shared bioctahedral geometry in which two chlorine atoms bridge the metal centers. To gain better insight into the geometry and electronic structures of the complexes density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were also performed. This provides for the first time, a detailed assignment of the significant spectral features of the investigated complexes. For 4(LEt) the calculations predict that the very weak and broad band near 1130 nm and the medium intensity broad peak at 725 nm in the experimental spectrum are

Table 6 Main calculated optical transitions for 4(LEt) with composition in terms of molecular orbital contribution of the transition, computed vertical excitation energies and oscillator strength. Excitation

Composition

E (eV)

Oscillator strength (f)

ktheo (nm)

Assign

kexp (nm)

3

H1 ? L(a) (0.484) H ? L+1(b) (0.479) H ? L(a) (0.969) H2 ? L(b) (0.767) H2 ? L(a) (0.473) H1 ? L+1(b) (0.474) H ? L+2(a) (0.469) H ? L+3(a) (0.183) H1 ? L+2(a) (0.105) H ? L+1(a) (0.149) H1 ? L+2(b) (0.119) H2 ? L+3(a) (0.120) H1 ? L+2(a) (0.111) H ? L+3(b) (0.161) H1 ? L+1(a) (0.754) H ? L+5(b) (0.113) H2 ? L+1(a) (0.195) H2 ? L+4(a) (0.196) H1 ? L+3(b) (0.150) H2 ? L+2(a) (0.455) H1 ? L+3(a) (0.250)

1.179

0.000006478

1051.5

1130

1.218 1.599 1.630

0.000093662 0.071840506 0.000233611

1018.1 775.5 760.9

2.487

0.020035811

498.6

2.561

0.003013351

484.2

2.594

0.003441399

477.9

2.660

0.004241374

466.1

2.820

0.001239144

439.6

2.866

0.000329138

432.6

d–d/MLCT/LMCT d–d/MLCT/LMCT d–d/MLCT/LMCT d–d/MLCT/LMCT d–d/MLCT/LMCT d–d/MLCT/LMCT MLCT MLCT MLCT MLCT MLCT MLCT MLCT MLCT MLCT MLCT MLCT MLCT MLCT MLCT MLCT

4 7 8 18 23

26

28 37

39

Please cite this article in press as: S. Mallick et al., Inorg. Chim. Acta (2014), http://dx.doi.org/10.1016/j.ica.2014.07.047

725

470

430

S. Mallick et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx

mainly of d–d in nature with some MLCT/LMCT character whereas the absorption bands at 470 and 430 nm are primarily of MLCT in character. Our search for new dirhenium complexes with different dithiocarbamato ligands and their characterization are continuing. Acknowledgments Financial support from the Department of Science and Technology, Government of India (No. SR/S1/IC-65/2010) is gratefully acknowledged. University of Kalyani for infrastructural facilities. The support of DST under FIST program to the Department of Chemistry and PURSE to the University of Kalyani is also acknowledged. S.M. thanks UGC, India and M.K.G. thanks University of Kalyani for the pre-doctoral fellowships. Appendix A. Supplementary material CCDC 1003337 for 4(LEt) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.ica.2014.07.047. References [1] A.R. Cutler, D.R. Derringer, P.E. Fanwick, R.A. Walton, J. Am. Chem. Soc. 110 (1988) 5024. [2] J.K. Bera, P.E. Fanwick, R.A. Walton, Inorg. Chim. Acta 311 (2000) 138. [3] J.K. Bera, R. Clérac, P.E. Fanwick, R.A. Walton, J. Chem. Soc., Dalton Trans. (2002) 2168.

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