Triphenylphosphine-( N , N-dimethyldithiocarbamato )-gold ( I ) Methanol Solvate

A gold(I) complex with a triphenylphosphine and a monodentate N,N-dimethyldithiocarbamate ligand was synthesized and characterized by Raman spectroscopy and single crystal X-ray diffraction. DFT calculations (Gaussian 09, PBE1PBE/Lanl2dz) were undertaken for a single complex in the gas-phase. The DFT-optimized structure is in good agreement with the crystal structure and the DFT-calculated Raman spectrum also is in excellent agreement with the experimental spectrum. Assignments of the Raman peaks are based on these DFT calculations. Frontier molecular orbitals were calculated and their nature is discussed.


Introduction
Materials based on d 10 configured Au(I) centers, often as coordination polymers, are of interest to modern applications, e.g., as luminescent sensors [1,2].Dithiocarbamate (dtc) complexes are attractive due to their simple structures and wide variation of luminescence properties [3][4][5].In this note we describe a monometallic gold(I) compound with Au-dtc coordination, an example of a molecular structure without Au(I)-Au(I) interactions.Its frontier molecular orbitals characterize an electronic structure without aurophilic effects.

X-ray Structures
The complex crystallizes in the triclinic space group P1 along with a methanol solvent molecule (Figure 1).The crystal structure of the (Ph 3 P)Au(S 2 CNMe 2 )•0.5CH 3 OH system features two inequivalent complexes in the asymmetric unit with a linear coordination geometry (average S-Au-P angle of 174 • ) for the gold(I) center.It is noteworthy however that corresponding Au-S, Au-P, C-S or P-C bond lengths for the two inequivalent complexes are identical within three standard deviations.There are no substantial aurophilic interactions present in the crystal structure as the shortest Au-Au distance is ca.5.11 Å [6].
The average Au-S and Au-P bond lengths are ca.2.33 Å and 2.26 Å, respectively.These values are comparable within three standard deviations to ca. 2.34 and 2.25 Å measured for the ethyl-substituted analogue (Ph 3 P)Au(S 2 CNEt 2 ) [7].The dithiocarbamate ligand is coordinated in a monodentate fashion and the two C-S bond lengths for a single complex are nearly equivalent within three standard deviations.The average C-S bond length for the dithiocarbamate sulphur coordinated to the gold(I) center is ca.1.75 Å, whereas it is ca.1.70 Å for the uncoordinated sulphur.Similar values are obtained for the ethyl-substituted analogue [7] with ca.1.75 and 1.68 Å for the dithiocarbamate sulphurs coordinated to the gold(I) center and uncoordinated respectively.Additionally, P-C and C-N (dtc) Molbank 2017, 2017, M937 2 of 6 bond lengths (average value of ca.1.82 and 1.33 Å) compare well with the (Ph 3 P)Au(S 2 CNEt 2 ) [7] system with corresponding average values of ca.1.81 and 1.33 Å.
Interestingly, the Au-S bond lengths are longer than the corresponding ones for Au2(S2CNEt2)2 [8](average bond length of 2.29 Å ) where the dithiocarbamate (dtc) moiety acts as a Au(I)-Au(I)bridging bidentate ligand.This is in contrast to C-S and C-N (dtc) bond lengths which are undistinguishable within three standard deviations to Au2(S2CNEt2)2 [8] (average C-S and C-N (dtc) bond lengths of 1.72 and 1.34 Å respectively).
DFT calculations were performed on a single complex of the crystal structure in the gas phase and the DFT-optimized structure is in good agreement with the experimental structure (Table 1 and Table S1).
Figure 1.Asymmetric unit of the crystal structure of (Ph3P)Au(S2CNMe2)•0.5CH3OH with ellipsoids drawn at the 50% probability level.All hydrogen atoms were omitted for clarity, except the protic hydrogen from the solvent molecule (drawn as a fixed 0.30 Å radius sphere).
Table 1.Selected bond lengths or angles for the complex (Ph3P)Au(S2CNMe2) obtained from the crystal structure and from a DFT optimization in the gas phase (Gaussian 09, PBE1PBE/Lanl2dz).

Bonds or Angles
Crystal Interestingly, the Au-S bond lengths are longer than the corresponding ones for Au 2 (S 2 CNEt 2 ) 2 [8] (average bond length of 2.29 Å) where the dithiocarbamate (dtc) moiety acts as a Au(I)-Au(I)-bridging bidentate ligand.This is in contrast to C-S and C-N (dtc) bond lengths which are undistinguishable within three standard deviations to Au 2 (S 2 CNEt 2 ) 2 [8] (average C-S and C-N (dtc) bond lengths of 1.72 and 1.34 Å respectively).
DFT calculations were performed on a single complex of the crystal structure in the gas phase and the DFT-optimized structure is in good agreement with the experimental structure (Table 1 and Table S1).
Table 1.Selected bond lengths or angles for the complex (Ph 3 P)Au(S 2 CNMe 2 ) obtained from the crystal structure and from a DFT optimization in the gas phase (Gaussian 09, PBE1PBE/Lanl2dz).

Raman Spectroscopy
The title compound is characterized by vibrational (Raman) spectroscopy (Figure 2).Some typical vibrations can be observed such as the C-N (dtc) stretching vibration (1488 cm −1 ) [9], the P-C stretching vibration (1098 cm −1 ) [10], the antisymmetric CS 2 stretching vibration (972 cm −1 ) [9] or the symmetric CS 2 stretching vibration (570 and 550 cm −1 ) [9].Moreover, the most intense Raman peak, located at 998 cm −1 , can be ascribed to a in-plane C-C-C angle deformation vibration for the phenyl rings of the triphenylphosphine moiety.Vibrational frequencies with detailed assignments obtained by DFT are given in Table S2 and a comparison of the experimental Raman spectrum with spectra of the precursors is also shown in Figure S1.The general agreement between the DFT-calculated Raman spectrum and the experimental one is good which confirms, along with the crystal structure, the identity of the synthesized complex.The title compound is characterized by vibrational (Raman) spectroscopy (Figure 2).Some typical vibrations can be observed such as the C-N (dtc) stretching vibration (1488 cm −1 ) [9], the P-C stretching vibration (1098 cm −1 ) [10], the antisymmetric CS2 stretching vibration (972 cm −1 ) [9] or the

Frontier Orbitals
The HOMO (Figure 3) is centered on the sulphur atoms of the dithiocarbamate moiety with some contribution from a gold d orbital.In contrast, the LUMO (Figure 3) is centered on the triphenylphosphine ligand and exhibits π* character.The nature of the lowest-energy excited state can thus be ascribed to a LL CT state, formed upon a transfer of electronic density from the dithiocarbamate ligand to the triphenylphosphine moiety.This is in stark contrast to the electronic structure in the presence of aurophilic interactions, where the frontier orbitals have strong M-M character [4,5].The calculated HOMO-LUMO gap at the PBE1PBE/Lanl2dz level of theory in the gas phase is 32,100 cm −1 (312 nm), suggesting a high-lying first excited state.located at 998 cm −1 , can be ascribed to a in-plane C-C-C angle deformation vibration for the phenyl rings of the triphenylphosphine moiety.Vibrational frequencies with detailed assignments obtained by DFT are given in Table S2 and a comparison of the experimental Raman spectrum with spectra of the precursors is also shown in Figure S1.The general agreement between the DFT-calculated Raman spectrum and the experimental one is good which confirms, along with the crystal structure, the identity of the synthesized complex.

Frontier Orbitals
The HOMO (Figure 3) is centered on the sulphur atoms of the dithiocarbamate moiety with some contribution from a gold d orbital.In contrast, the LUMO (Figure 3) is centered on the triphenylphosphine ligand and exhibits π* character.The nature of the lowest-energy excited state can thus be ascribed to a LL′CT state, formed upon a transfer of electronic density from the dithiocarbamate ligand to the triphenylphosphine moiety.This is in stark contrast to the electronic structure in the presence of aurophilic interactions, where the frontier orbitals have strong M-M character [4,5].The calculated HOMO-LUMO gap at the PBE1PBE/Lanl2dz level of theory in the gas phase is 32100 cm −1 (312 nm), suggesting a high-lying first excited state.

General Methods and Physical Measurements
Raman spectra were measured using an InVia spectrometer (Renishaw, Wotton-under-Edge, Gloucestershire, UK) coupled to an imaging microscope (Leica, Wetzlar, Hesse, Germany).The excitation wavelength used was 785 nm for all Raman spectra.The infrared spectrum of the studied compound was measured with an ALPHA FTIR spectrometer (Bruker, Billerica, MA, USA) with a resolution of 4 cm −1 .A single crystal of (Ph3P)Au(S2CNMe2)•0.5CH3OH was measured on a Venture Metaljet diffractometer (Bruker, Billerica, MA, USA).The crystal was kept at 105 K during data collection.Using Olex2 [11], the structure was solved with the SHELXT [12] structure solution program using Direct Methods and refined with the SHELXL [13] refinement package using least squares minimisation.

General Methods and Physical Measurements
Raman spectra were measured using an InVia spectrometer (Renishaw, Wotton-under-Edge, Gloucestershire, UK) coupled to an imaging microscope (Leica, Wetzlar, Hesse, Germany).The excitation wavelength used was 785 nm for all Raman spectra.The infrared spectrum of the studied compound was measured with an ALPHA FTIR spectrometer (Bruker, Billerica, MA, USA) with a resolution of 4 cm −1 .A single crystal of (Ph 3 P)Au(S 2 CNMe 2 )•0.5CH 3 OH was measured on a Venture Metaljet diffractometer (Bruker, Billerica, MA, USA).The crystal was kept at 105 K during data collection.Using Olex2 [11], the structure was solved with the SHELXT [12] structure solution program using Direct Methods and refined with the SHELXL [13] refinement package using least squares minimisation.

Synthesis of (Ph
All reagents were purchased from commercial sources and used as received without further purification.Sodium dimethyldithiocarbamate monohydrate (NaS 2 CNMe 2 •H 2 O) was purchased from Sigma-Aldrich (St. Louis, MO, USA) and Chloro(triphenylphosphine)gold(I) (Ph 3 PAuCl, 99.99% metals basis) was purchased from Alfa Aesar (Ward Hill, MA, USA).Dichloromethane and methanol were of certified ACS grade and were purchased from Fisher Scientific (Hampton, NH, USA).
The procedure used was inspired by a literature procedure [3] and was initially applied to obtain X-ray quality single crystals of the Au(I) dithiocarbamate Au 2 (S 2 CNMe 2 ) 2 .A first solution of Ph 3 PAuCl (0.20 mmol, 0.1005 g) in ca. 2 mL of dichloromethane was prepared at room temperature.A second solution of NaS 2 CNMe 2 •H 2 O (0.20 mmol, 0.0329 g) in ca. 2 mL of methanol was prepared at room temperature.The first solution was added to the second solution at ambient temperature.Upon mixing, the resulting solution became of a pale, faint yellow color.The resulting solution was then left to stir for ca.19 h.After 19 h of stirring, the cloudy yellow solution was filtered off on a Büchner funnel to remove colorless PPh 3 or (Ph 3 P)AuCl crystals.The filtered solution was collected and was left to evaporate slowly at room temperature.A suitable X-ray quality yellow crystal was then picked and used for X-ray diffraction.  , 48401 reflections measured (5.302 • ≤ 2θ ≤ 110.428 • ), 8169 unique (R int = 0.0588, R sigma = 0.0360) which were used in all calculations.The final R 1 was 0.0394 (I > 2σ(I)) and wR 2 was 0.1064 (all data).CCDC 1533239 contains the supplementary crystallographic data for this paper.These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.

DFT Calculations
All DFT calculations were undertaken using the Gaussian 09 (Gaussian Inc., Wallingford, CT, USA) [14] software package with methods as implemented in the software.All results of calculations were visualized with Gaussview (5.0.9 release, Gaussian Inc., Wallingford, CT, USA) [14].A ground-state geometry optimization was performed on a single molecule of (Ph 3 P)Au(S 2 CNMe 2 ) in the gas phase using one of the two inequivalent complexes in the asymmetric unit of the crystal structure presented in this report as a starting point.This optimization was carried out with the hybrid exchange-correlation functional PBE1PBE [15] along with the relativistic basis set Lanl2dz [16] with effective core potentials for all atoms using the default convergence criterion.A subsequent frequency calculation was performed on the optimized structure to obtain the calculated Raman spectrum presented in this work and revealed no imaginary frequencies.Additionally, molecular orbitals were also calculated for this optimized structure.

Supplementary Materials:
The following are available online: Figure S1.Raman spectra of the synthesized compound (Ph 3 P)Au(S 2 CNMe 2 )•0.5CH 3 OH and of its precursors at room temperature.Raman spectra were measured with an excitation wavelength of 785 nm and are offset along the vertical axis for clarity.Figure S2.Comparison of the experimental infrared spectrum at room temperature of (Ph 3 P)Au(S 2 CNMe 2 )•0.5CH 3 OH with the DFT-calculated one in the gas phase for a single (Ph 3 P)Au(S 2 CNMe 2 ) molecule (Gaussian 09, PBE1PBE/Lanl2dz).Table S1.Atoms coordinates of the DFT-optimized ground-state structure of (Ph 3 P)Au(S 2 CNMe 2 ) in the gas phase (Gaussian 09, PBE1PBE/Lanl2dz).Table S2.Comparison between experimental and DFT-calculated (Gaussian 09, PBE1PBE/Lanl2dz) Raman shifts for the main Raman peaks.