Synthesis of a Labile Sulfur-Centred Ligand, [S(H)C(PPh 2 S) 2 ] − : Structural Diversity in Lithium(I), Zinc(II) and Nickel(II) Complexes

: A high-yield synthesis of [Li{S(H)C(PPh 2 S) 2 }] 2 [Li 2 ∙( 3 ) 2 ] was developed and this reagent was used in metathesis with ZnCl 2 and NiCl 2 to produce homoleptic complexes 4 and 5b in 85 and 93 % yields, respectively. The solid-state structure of the octahedral complex [Zn{S(H)C(PPh 2 S) 2 } 2 ] ( 4 ) reveals notable inequivalence between the Zn–S(C) and Zn–S(P) contacts (2.274(1) Å vs. 2.842(1) and 2.884(1) Å, respectively). Two structural isomers of the homoleptic complex [Ni{S(H)C(PPh 2 S) 2 } 2 ] were isolated after prolonged crystallization processes. The octahedral green Ni(II) isomer 5a exhibits the two monoprotonated ligands bonded in a tridentate (S,S′,S′′) mode to the Ni(II) centre with three distinctly different Ni–S bond lengths (2.3487(8), 2.4500(9) and 2.5953(10) Å). By contrast, in the red-brown square-planar complex 5b the two ligands are S,S′-chelated to Ni(II) ( d (Ni‒S) = 2.165(2) and 2.195(2) Å) with one pendant PPh 2 S group. DFT calculations revealed that the energetic difference between singlet and triplet state octahedral and square-planar isomers of the Ni(II) complex is essentially indistinguishable. Consistently, VT and 31 P CP/MAS NMR spectroscopic investigations indicated that a mixture of isomers exists in solution at room temperature, while the singlet state square-planar isomer 5b becomes favoured at -40 o C.


Introduction
4][5] In most cases the dianion behaves as a tridentate ligand, which is S,C,S-coordinated to the metal centre and the primary interest in such complexes has been on the electronic structure and reactivity of the metal-carbon bond, as well as on the dichotomy with respect to their description as either Fischer carbenes or Schrock alkylidines.3a,6,7 In 2010 we described the use of Le Floch's dianion for the synthesis of the trichalcogenocentred PCP-bridged dianions [(E)C(PPh2S)2] 2-(1E; E = S, Se), 8 prompted by the unexpected discovery of the selenium analogue [(Se)C(PPh2Se)2] 2-(1Se) via an unprecedented Se-H + exchange at the carbon centre in reactions of the diseleno PCP-bridged monoanion [HC(PPh2Se)2] − with main group and transition metal halides (Hg, Sn, Te). 9 Early investigations of these trichalcogeno-centred PCP-bridged dianions have revealed a versatile coordination and redox behaviour. 10For example, while the dianionic ligand 1S displays the expected S,S′,S′′-coordination in octahedral In(III), Sn(IV), Pb(II) and Te(IV) complexes, 11,12 the diversity of these trichalcogeno ligands is evident in the group 11 metal compounds.In the mixed valent Au(I)-Au(III) complexes Au2•(1E)2 (E = S, Se) the dianions (1E) 2-act as bridging ligands that are E,E′-chelated to the square-planar Au(III) centre and S-monodentate towards the linear Au(I) atom. 12By contrast, in the reactions with CuCl2 the dianions in Li2•1E (E = S, Se) operate as reducing agents to give dinuclear Cu(I)-Cu(I) complexes in which the two Cu(I) centres are formally bridged by two radical anions [EC(PPh2S)2] •− that are 4 connected by a long E-E bond to form a dimeric dianion, [(SPh2P)2CEEC(PPh2S)2] 2− (2E; E = S, Se). 12 In earlier investigations we noticed the proclivity of the dimeric system 2S to undergo proton abstraction in the metathetical reaction with ZnCl2, which produces the tridentate monoanion [S(H)C(PPh2S)2] − (3) in the octahedral complex [Zn{S(H)C(PPh2S)2}2] (4). 10 In this context we note that the HOMOs of the monomeric and dimeric dianions 1E and 2E, respectively, display a significant contribution from the p-orbitals of the PCP-carbon atom, thereby contributing to the apparent tendency for proton abstraction in common organic solvents. 8,10 onsistently, during the current study a prolonged crystallization process of the reaction between Li2•1S and NiCl2 afforded green crystals of the octahedral Ni(II) complex [Ni{S(H)C(PPh2S)2}2] (5a) in which two monoanions [S(H)C(PPh2S)2] − (3) are S,S′,S′′-coordinated to Ni(II); the yield of 5a was only 12%, presumably as a result of the monoprotonation of 1S.Serendipitously, at the same time we also isolated small amounts of the red, square-planar isomer 5b in which the two [S(H)C(PPh2S)2] − (3) ligands are S,S′-chelated to the Ni(II) centre leaving a pendant Ph2P=S group.Although the conversion of four-coordinate to six-coordinate nickel(II) complexes upon coordination of solvent is well-known, 13 the identification of both four-and six-coordinate isomers incorporating the same ligand is, to our knowledge, unprecedented. 14 have now developed a high-yield synthesis for [Li{S

Synthesis and structural characterization of [Li{S(H)C(PPh2S)2}]2 [Li2•(3)2]
. Formation of the Zn(II) complex 4 from the dimeric system 2S and, especially, the intriguing structural dichotomy of the homoleptic Ni(II) complex 5 together with the potentially versatile coordination behaviour of the novel monoanion 3 provided strong incentives to develop a high-yield synthesis of an alkali-metal derivative of the monoanion.This was achieved via a straightforward two-step process in which the neutral dithio compound [H2C(PPh2S)2] was first monodeprotonated with n BuLi 15,16 and then treated with one equivalent of elemental sulfur in toluene to afford Li2•(3)2 as an analytically pure, pale yellow powder with toluene solvate in excellent yield (90 %) (Eq. 1 and 2).
The 7 Li and In contrast to the composition of the initial powdery product, the crystallization of Li2 Synthesis and structural characterization of the Zn(II) complex [Zn{S(H)C(PPh2S)2}2] (4).
Following our earlier report of the formation of the homoleptic Zn(II) complex 4 from treatment of the dimeric dianion 2S with ZnCl2, 10 the reaction of Li2•(3)2 with ZnCl2 in toluene was performed (Eq.3).The complex 4 was isolated as a white powder in excellent yield (85%). [ The 1 H NMR spectrum of 4 in CD2Cl2 at 23 o C exhibits a set of multiplets in the range of 7.12-8.02ppm and a triplet at 5.08 ppm with a coupling constant of 9.9 Hz consistent with two-bond  16,18 Similarly to the Li + salt Li2•(3)2, the Csp 3 −S bond length of 1.816(4) Å and the Csp 3 −P distances of ca.1.85 Å in 4 are 0.08 and 0.1 Å longer, respectively, than in the various complexes of the dianionic ligands 1S and 2S reflecting the four-coordinate carbon centre in the bridging P(S)C(H)P unit of the monoanion 3 vs. the corresponding three-coordinate carbon centre in 1S and 2S. 8,11,12The S−Zn−S bond angles in the range 83.8-96.2o show slightly more pronounced distortion compared to the analogous S−Ni−S angles in the octahedral Ni(II) complex 5a (vide infra).

Synthesis of the Ni(II) complex [Ni{S(H)C(PPh2S)2}2] (5) and crystal structures of the
octahedral and square-planar isomers 5a and 5b.As a further illustration of the synthetic potential of the monoanion 3, the homoleptic Ni(II) complex 5 was produced by utilizing the lithium complex Li2•(3)2 in a reaction with NiCl2 in toluene (Eq.4) to give complex 5 as an olive-green powder in excellent yield (93%). [

5
The   1).Predictably, the protonated ligand At this temperature the resonance at δ 42.8 appears as a doublet with a coupling constant of ca.20 Hz consistent with a two-bond 2 J( 31 P, 31 P) coupling, 19 however the broadness of the resonance at δ 68.0 obscures this small coupling.Thus the low temperature 31 P{ 1 H} NMR spectrum of the green product suggests the sole presence of the SP isomer 5b with two markedly different phosphorus environments.
In order to provide support for this interpretation, the 31  Taken together the NMR and XRPD data indicate strongly that the olive-green product exists as the four-coordinate SP isomer 5b in both the solid-state and in solution at low temperatures.We note that the olive-green colour of the powder is in contrast to the red-brown colour of crystalline samples of 5b under microscope (vide supra).
The changes in the solution 31 P{ 1 H} NMR spectra of 5b in CD2Cl2 at various temperatures are reversible, i.e. the singlet at 51.3 ppm reappears when the solution is warmed up to room temperature.However, this singlet is off-centred compared to the average value of 55.5 ppm observed for the two resonances at low temperature, which rules out a simple two-site exchange process between the inequivalent phosphorus atoms of the SP isomer 5b.The possible involvement of other structural isomers must, therefore, be considered (cf.DFT calculations below).In that connection paramagnetically-shifted high-field resonances were not observed in the 31  Likewise, the geometric parameters optimized for the square-planar isomer 5b are in good agreement with the experimental values when optimized in a singlet state thus suggesting that in the solid-state 5a is in a triplet state and 5b in a singlet state in accordance with the crystal field theory.
If the square-planar geometry is relaxed into the triplet state, the optimization results in a severely distorted tetrahedral structure 5c.On the other hand, if the octahedral geometry is relaxed into the singlet state the resulting geometry, 5b', closely resembles the square-planar geometry with the noncoordinated terminal thiophosphinoyl functionalities rotated towards the Ni(II) centre (see Figure 5).
The free energies of the octahedral triplet (5a) and square-planar singlet (5b) are almost equal with 5a lying only 1.09 kJ mol -1 lower in energy.Geometries 5b' and 5c are located 15.27 kJ mol -1 and 18.09 kJ mol -1 above 5b on the free energy surface, respectively.For all geometries considered, the potential energy surface is extremely shallow and convergence criteria almost up to numerical precision had to be used to obtain fully converged geometries.Furthermore, the energy differences are fairly small and show considerable dependence on the choice of solvent model, exchangecorrelation functional and basis set.All this considered, the conclusions that can be drawn from the calculated energetics is that all four geometries lie very close to each other in energy and a relatively small perturbation can induce a non-negligible change in the energetics.
In order to obtain further insight into the temperature-dependent behaviour of the 31 P{ 1 H} NMR spectrum, the chemical shifts of the P atoms in the singlet state geometries 5b and 5b' were calculated using the gauge-independent atomic orbital (GIAO) formalism.Since no paramagnetic shift was experimentally observed, the triplet geometries 5a and 5c were excluded from the The experimental 31 P{ 1 H} NMR spectrum can be rationalized by assuming that the singlet geometries 5b and 5b' exist in a dynamic equilibrium at room temperature whereas only the lower energy singlet isomer 5b exists at lower temperatures.Consequently, there are four possible chemical environments at ambient temperatures for the P atomstwo in 5b and two in 5b'.A foursite dynamic exchange process would then average out the four calculated chemical shifts to 42.0 ppm, which is shifted slightly upfield compared to the average of the two calculated values for 5b (44.3 ppm).This is in good qualitative agreement with the experimental observation that the room temperature singlet is observed at 51.3 ppm, which is slightly upfield from the average value of the two signals observed at low temperature (55.5 ppm).
UV-Vis spectroscopy of the Ni(II) isomers 5.The electronic spectra of 5a, 5b, 5b' and 5c were calculated with time-dependent DFT (TDDFT) including the fifty lowest excitations.The UV-Vis spectrum simulated from the calculated excitation wavelengths and oscillator strengths along with the experimental spectrum of 5b is presented in Figure S2 and the numerical values are given in Table S2 (cf.ESI).Based on the calculations, the measured spectra cannot be unambiguously assigned to any specific isomer.Both of the singlet geometries (5b and 5b') show one major absorption peak at 280 nm and 278 nm, respectively, corresponding to a ligand-to-metal charge transfer excitation.It is also noteworthy that the absorption bands of these two isomers overlap almost perfectly.Therefore, the 5b-5b' equilibrium suggested by the low temperature 31 P{ 1 H} NMR measurements cannot be observed in the corresponding variable temperature UV-Vis spectra.
Indeed, the main absorption band in the experimental spectra is shifted only by 8 nm when the temperature is lowered from ambient conditions to -50 C° (Figure S3).

Conclusions
In order to pursue investigations of the chemistry of M(II) complexes 4 and 5 (M = Zn, Ni), a high- was developed.This important advancement opens the way for extensive studies of the coordination behaviour of this potentially versatile, sulfur-centred ligand in main group, transition-metal and lanthanide complexes for comparison with that of the corresponding dianion 1S. 10 The novel monoanionic ligand 3 adopts a distorted octahedral coordination around the metal centre in the homoleptic Zn(II) complex 4 with unusually weak Zn−S(P) contacts.Significantly, the first example of structurally characterized six-and four-coordinate isomers of a nickel(II) complex (5a and 5b) with the same ligand, viz.[S(H)C(PPh2S)2] -, was discovered.DFT calculations display virtually no difference between the energies of 5a and 5b in solution.Consistently, detailed NMR investigations indicate that a mixture of singlet state isomers persists in solution at room temperature, while the square-planar isomer 5b prevails at -40 o C as well as in the solid-state.In addition, the isomerization process was shown to be specific to the Ni(II) complex, since no splitting of the singlet in 31 P{ 1 H) NMR spectrum of the Zn(II) complex 4 was observable even at -90 o C.
Additional studies are in progress in order to elucidate the redox properties of the homoleptic Ni(II) complex 5 in the context of the similarity of the Ni−S coordination to ligand environments in the active sites of certain redox-active nickel enzymes.

Experimental Section
Reagents and general procedures: All reactions and the manipulations of products were performed under an argon atmosphere by using standard Schlenk techniques or an inert atmosphere glove box.
Computational details.All geometry optimizations, frequency calculations, stability analyses and TDDFT calculations were performed with Gaussian 09 revision D.01 26 using the LC-ωPBE rangeseparated hybrid functional 27 and def2-TZVP basis sets 28 .This functional was chosen due to its good ability to predict relative energies of different spin states 29 .The geometries of 5a, 5b, 5b' and 5c were fully optimized in both low-and high-spin states with no constraints placed on spatial or spin symmetries.Due to the flat nature of the potential energy surfaces the Gaussian "VeryTight" convergence criteria and "UltraFine" integration grid were used to ensure that the bottoms of potential energy minima were correctly located.Using looser convergence criteria leads to differences of several kJ mol -1 in the relative energies between the calculated geometries and can even affect the relative ordering of the geometries.The low-spin states show no spin-contamination and stability analysis was performed on the solutions to ensure that no lower energy broken symmetry states exist.It should be noted that if a conventional global hybrid (such as B3LYP 30 ) is used, the low-spin solutions break symmetry and show noticeable spin-contamination with partial delocalization of the spin density into the ligand sulfur atoms.Such an effect is also observed with the LC-ωPBE functional if smaller basis sets (such as 6-31G(d,p) 31 ) are used.Frequency calculations were carried out to ensure that the stationary geometries correspond to minima on the potential energy surface.Solvent effects were taken into account in the geometry optimizations, frequency calculations and TDDFT calculations with the polarizable continuum model (PCM) using the default parameters for CH2Cl2 as implemented in Gaussian 09.Same level of theory as for the geometry optimizations and frequency calculations was used for the TDDFT calculations.Fifty lowest roots were solved for all four isomers.

31
P NMR shifts of 5b and 5b' were calculated using the gauge-independent atomic orbital (GIAO) formalism 32 as implemented in the ADF 2014 program package 33 .Because the LC-ωPBE functional is not available in ADF the PBE0 global hybrid functional 34 was used instead.The LC-ωPBE/def2-TZVP-optimized geometries were used in the NMR calculations.Scalar relativistic effects were included using the zeroth order regular approximation (ZORA) 35 .Standard ZORA-TZ2P basis sets 36 as implemented in the ADF basis set library were used for atoms other than phosphorus.For P atoms special JCPL basis 37 was used, which includes the same functions as the corresponding TZ2P basis but is augmented with four additional steep s functions to offer more flexibility in the core region.The relative shifts were calculated using phosphoric acid as a reference.The reference geometry and shifts were calculated using an identical procedure as for 5b and 5b'.The resulting pale yellow solution was added dropwise to a solution of S8 (0.032 g, 1.00 mmol) in 10 mL of toluene at 23 o C and stirring was continued for 2 h resulting in a very pale yellow powder.
1 H NMR spectrum of the green product in CD2Cl2 at 23 o C exhibits a set of multiplets in the range of 7.28-8.08ppm for the C6H5 groups and, in contrast to the triplet observed for the Zn(II) complex 4, a broad singlet at 6.16 ppm attributed to the P(S)C(H)P hydrogen atom indicating a possible fluxionality in solution.The 31 P{ 1 H} NMR spectrum displays a singlet at 51.3 ppm suggesting a single environment for the phosphorus centres at room temperature.The crystal structures of the isomers 5a and 5b are shown in Figure 2. In both compounds, the asymmetric unit contains a one-half molecule of [Ni{S(H)C(PPh2S)2}2] and, in the case of 5a, a molecule of toluene solvate.In the octahedral isomer 5a the centrosymmetric molecule is comprised of two monoanionic ligands 3 coordinated to the nickel centre in a tridentate (S,S′,S′′) fashion with slightly distorted S−Ni−S bond angles in the range 87.1-92.9o (

3
displays significantly stronger bonds to the four-coordinate Ni(II) centre in the isomer 5b compared to the six-coordinate nickel atom in 5a; the almost equal Ni-S bond lengths of 2.165(2) and 2.195(2) Å in 5b are ca.0.29 Å shorter than the mean value in 5a.The Csp 3 −S bond length of 1.846(6) Å in 5b is significantly longer than the corresponding distance of 1.797(3) Å in 5a reflecting the elongated Ni−S(C) bond in the latter isomer.As expected, the P−S bond length of 1.952(2) Å for the dangling Ph2P=S group in 5b is shorter (by ca.0.05 Å) than the coordinated Ph2P−S functionality.The comparatively short distances for the coordinated P−S groups in 5a (1.970(1) and 1.962(1) Å) presumably reflect the relatively weak attachment of these sulfur atoms to the Ni(II) centre in the octahedral complex (vide supra).Structural dichotomy: spectroscopic and powder X-ray diffraction studies of the isomers 5a and 5b.Interestingly, the 31 P{ 1 H} NMR spectrum of the olive-green powder 5 in CD2Cl2 at 23 o C shows a singlet at 51.3 ppm.In an attempt to understand the significance of this observation variable temperature 31 P{ 1 H} NMR spectra were recorded (Figure 3).Upon cooling the solution, this singlet first broadens and then divides into two well-separated resonances at 42.8 and 68.0 ppm with equal intensity.Two broad signals are clearly evident at -40 o C and they become well-resolved at -95 o C.
P CP/MAS NMR spectrum and the powder X-ray diffraction pattern of the olive-green product were measured.The solid-state 31 P NMR spectrum displays two broad isotropic signals at 46.4 and 64.4 ppm accompanied by spinning side bands consistent with the crystal structure of 5b (Figure4).Significantly, the powder X-ray diffraction (XRPD) pattern of the olive-green product can be perfectly indexed and fitted by the Pawley refinement method (χ2= 1.40 with 413 reflections) with the unit cell and space group settings corresponding to that determined for the single-crystal structure of the square-planar isomer 5b without any residual unindexed diffraction profiles left in the pattern (see FigureS1in ESI).
P{ 1 H} NMR spectra (up to -1100 ppm) in the temperature range +23 o C to -50 o C, which excludes the presence of the tetrahedral (Td) isomer.For comparison,31 P chemical shifts in the range -437 to -725 ppm have been reported for Td [Ni{N(PR2S)2}2] (R = i Pr, Ph) complexes which incorporate a comparable, PNP-bridged sulfur-centred monoanion.20This observation also suggests that the Oh isomer, if it is present in solution at room temperature, is in the singlet rather than triplet ground state.DFT calculations of the Ni(II) isomers 5a and 5b.In view of the possible octahedralsquareplanar equilibrium, a series of DFT calculations were carried out.Singlet and triplet state geometries were optimized starting from both the octahedral and square-planar crystal structures and are summarized in Figure5.In contrast to the absence of triplet state geometries in solution as suggested by the NMR experiments, the solid-state structural parameters of the octahedral geometry 5a are reasonably well reproduced by the calculation when confining the spin-state to a triplet.
calculations.The chemical shifts of the two equivalent phosphorus atoms were averaged in both systems.The calculated chemical shifts (relative to phosphoric acid) show values of δ 36.5 and 52.0 for 5b and δ 31.6 and 47.9 for 5b' in qualitative agreement with the low-temperature experimental chemical shifts of 42.8 and 68.0 ppm.It should be noted that NMR shifts are a challenging property to calculate especially for heavier elements and moderate errors in the absolute values are typically seen − especially when an external reference is used.The relative values of the chemical shifts should, however, be reliable.

Table 1
).Most notably, three distinctly different Ni−S bond lengths are observed for the complex 5a; analogously to the Zn(II) complex 4, the sulfur atom bonded to the carbon centre forms the strongest contact to the Ni(II) centre (2.349(1) Å), whereas the two weakly coordinated thiophosphinoyl functionalities in 5a are markedly inequivalent with Ni−S(P) distances of 2.450(1) and 2.595(1) Å.In contrast to the octahedral geometry in 4 and 5a, a distorted square-planar geometry is apparent around the nickel atom in 5b with S−Ni−S bond angles of 85.3 and 94.7 o (Table 31e 1 H,7Li and31P{ 1 H) NMR spectra were obtained in CD2Cl2 at 23 ºC on a Bruker Avance III 300 spectrometer operating at 300.15, 116.65 and 121.52 MHz, respectively.The solid-state and variable temperature NMR measurements were performed on Bruker Avance 400 and Bruker Avance DRX 500 spectrometers, respectively. 1H NMR spectra are referenced to the solvent signal and the chemical shifts are reported relative to (CH3)4Si.The 7 Li and 31 P{ 1 H} NMR spectra are referenced externally and the chemical shifts are reported relative to a 1.0 M solution of Chemistry, University of Jyväskylä [compounds Li2•(3)2, 4 and 5b].Spectroscopic methods: X-ray crystallography: Crystallographic data for compounds Li2•(3)2, 4, 5a•(C7H8)2 and 5b are summarized in Table 2. Crystals of [Li{S(H C and stirred for 15 min.The reaction mixture was allowed to reach room temperature and stirring was continued for an additional 12 h.The LiCl precipitate was removed by filtration and the greenish brown filtrate was kept in a glove box at room temperature for slow evaporation of solvent. o Dark green crystals were isolated, washed with benzene and dried by prolonged pumping under vacuum.Yield: 0.042 g (12%).Characterization data for 5a•C7H8: Anal.Calcd (%) for C57H50NiP4S6: C, 61.68; H, 4.54.Found: C, 61.48; H, 4.70. 31P{ 1 H} NMR (CD2Cl2, 23 °C): δ 51.2