Synthesis and Redox Behaviour of the Chalcogenocarbonyl Dianions, [(E)C(PPh 2 S) 2 ] 2− (E = S, Se): Formation and Structures of Chalcogen-Chalcogen Bonded Dimers and a Novel Selone

: The lithium salts of the chalcogenocarbonyl dianions, [(E)C(PPh 2 S) 2 ] 2− [E = S ( 4b ), Se ( 4c )], were produced by the reaction between Li 2 [C(PPh 2 S) 2 ] and elemental chalcogens in the presence of TMEDA,


Introduction
In contrast to the extensively studied N-bridged, monoanionic ligands [N(PPh2E)2] -(1, E = S, Se), which predominantly form E,E-chelated metal complexes without the participation of the N atom, [1,2] the isoelectronic C-bridged dianion [C(PPh2S)2] 2- (2) exhibits strong metal-carbon interactions in variety of coordination complexes. The first example, a binuclear Pt(II) complex of 2, involved bis-C,S-chelation and a quaternary carbon bridging the two metal centres. [3] The recent development of a synthesis of the dilithium derivative of 2 by Le Floch and co-workers opened the way for wide-ranging investigations of this intriguing dithio PCP-bridged ligand. [4,5] A variety of complexes with main group, [6] early [7] and late transition metals, [4,8] as well as lanthanides [9] and actinides, [10] have subsequently been prepared by metathesis of Li22 with metal halides and structurally characterised. The recurring theme in these complexes is the prevalence of strong interactions of the metal with the carbene centre of the ligand 2 (S,C,Schelation).
Unusual carbon-centred reactivity is also observed in the redox behaviour of 2. In contrast to the formation of chalcogen-chalcogen bonds upon oxidation of N-bridged ligands of the type 1, [11] treatment of Li22 with the mild oxidising agents C2Cl6 or I2 produces remarkably stable monomeric or dimeric carbenoids, respectively. [12,13] The nucleophilic reactivity of 2 is also illustrated by the reaction with CS2 to give the 1,1ethylenedithiolate [S2C=C(PPh2S)2] 2− . [4b] 4 The diseleno C-bridged monoanion [HC(PPh2Se)2] - (3), isoelectronic with 1 (E = Se), was first prepared from the neutral ligand [H2C(PPh2Se)2] and nBuLi and used as an in situ metathetical reagent for the preparation of homoleptic M(II) (M = Fe, Co, Ni) complexes. [14] Competition between the deprotonation process and cleavage of P=Se bonds by RLi reagents makes this synthesis of 3 inefficient, and also preempts the production of the Se analogue of the dianion 2. [15] Consequently, we developed an alternative, high-yield synthesis of Li3 in which the P-Se bonds are installed after the deprotonation step and we showed that this reagent can be used to prepare homoleptic Zn(II) and Hg(II) complexes of 3. [16] Interestingly, the reactions of MCl2 (M = Sn, Te) with Li3 in a 1:2 molar ratio produce homoleptic M(IV) complexes of the triseleno dianion [(Se)C(PPh2Se)2] 2-(4a) via a redox process that formally involves seleniumproton exchange. [17] A dinuclear Hg(II) complex of 4a is formed on mild heating of Hg(3)2. [17] Very recently, homoleptic Pb(II) complexes of the related trichalcogeno dianions 4b and 4c were obtained by chalcogen insertion into the Pb-C bond of dimeric, homoleptic Pb(II) complexes of 2. [6a] The first examples of metal complexes of ligands of the type 4 exhibit notably different structural features suggesting a rich coordination chemistry. [6a, 17, 18] With this in mind, we sought to develop an efficient synthesis of alkali-metal derivatives of these novel tridentate ligands that can be used as metathetical reagents. Concomitantly, we considered that investigations of the redox behaviour of these dianions represent a 5 worthwhile endeavour in view of the possible generation of chalcogenocarbonyl radical anions of the type 5 or the corresponding neutral chalcogenocarbonyls 6 upon oxidation.
In this context it is pertinent to note that Le Floch et al. have recently described the isolation of deep purple alkali-metal salts of the radical anion [Ph2C=C(PPh2S)2] −• in which the electron-accepting phosphine sulfide substituents exert a stabilising influence on the alkene radical. [19] Thioketyl radical anions [R2C=S] −• (e.g. R = tBu, Me) have been characterised in solution by EPR spectroscopy, but salts of these anions have not been isolated. [20] The effect of the PPh2S substituents on the stability of these radical anions of the type 5 was therefore of interest.
In this contribution we report the synthesis of dilithium derivatives of the dianions 4b and 4c and the X-ray structure of [Li(TMEDA)]24c. Investigations of the oxidation of the 4b and 4c with iodine revealed the formation of 7b and 7c, which are formally dimers of the corresponding anion radicals 5b and 5c, respectively, upon one-electron oxidation (Scheme 1). The X-ray structures of [Li(L)]27b [L = TMEDA, (THF)2] and [Li(TMEDA)]27c have been determined and the possible dissociation of these dimers into the paramagnetic species 5b and 5c has been investigated by EPR spectroscopy.
Interestingly, the two-electron oxidation of 4c with I2 produces the unusual selone 6c [21] as a LiI adduct, whereas the attempted two-electron oxidation of 4b with I2 generates the protonated monoanion 8b; the X-ray structures of [Li(L)]8b [L = TMEDA, (12-crown-4)2] were elucidated. The experimental work is supported by DFT calculations which provide insights into the molecular and electronic structures of the radical anions 5b and 5c, and the corresponding dimers 7b and 7c, as well as confirmation of the molecular structure of the protonated species 8b.  [17,22] Taken together, the 8 NMR spectroscopic data are consistent with the formation of the dianions 4b and 4c

Results and Discussion
incorporating two TMEDA-chelated Li + cations symmetrically coordinated to the dianion.
As depicted in Figure 1a dimeric Hg(II) complex in which the carbon-bound selenium is also three-coordinate. [17] The PCP carbon in [Li(TMEDA)]24c exhibits a slight distortion from planarity with Σ  C = 354.8 o , which is comparable to that in the mercury complex Hg2(4a)2 (ca. 353 o ), while in the Pb4c, Sn(4a)2 and Te(4a)2 complexes the corresponding deviation from planarity is more pronounced (Σ  C is ca. 340 o ). [(CO)4Mn{(Ph2P)2CSSC(PPh2)2}Mn(CO)4)]. [24] The 7 Li and  (s). [27] Electronic  [19] Although the p-orbital at the carbon-bound chalcogen atom makes a significant contribution to the SOMO, the predicted hyperfine couplings (hfcs) to these nuclei are small due to the vanishing s-wave contribution; 4.6 and 20.6 G for 5b and 5c, respectively (cf. a 33 S ~ 2G for thioketyl anion radicals [R2C=S] −• ). [20a] Consequently, the largest hfcs in 5b (5c), - 19.4 (17.8) and -19.8 (18.1) G, are due to spin polarization and involve the two slightly inequivalent phosphorus centres. The interactions of the unpaired electron with the two sulfur atoms and with the protons at the four phenyl rings are considerably smaller and do not give rise to any significant splitting of the signal. Hence, the EPR spectra of 5b and 5c are expected to be dominated by a broadened binomial triplet with minor contributions from 33 S and 77 Se satellites.
When solutions of the [Li(TMEDA)] + salts of the dimeric dianions 7b and 7c in CH2Cl2 were monitored by EPR spectroscopy, two radical species were detected in both cases. A very broad binomial triplet with an hfc of ca. 18 G (g = 2.0622, Figure S5a) was detected for the selenium-containing system 7c that is consistent with coupling to two equivalent 31 P (I = ½) centres. Since the 77 Se satellites (I = ½, 7.6 %) are not observable due to the weakness of the broad signals, the assignment of this signal pattern to 5c is tentative. The all-sulfur dianion 7b, on the other hand, showed a broad singlet (g = 2.0260, Figure S5b) with a width of ca. 24 G, which cannot be attributed to radical anion 5b (cf. calculated a( 31 P) = 19 G for 5b). Significantly, the major radical species in the EPR spectra of CH2Cl2 solutions of both 7b and 7c exhibited a very strong 1:2:1 triplet with 33 S (I = 3/2, 0.75 %) satellites as judged by the relative intensities of the signals (g = 2.0132, Figure S5c). [28] This signal pattern can be simulated with hfcs of 19.2 and 5.70 G arising from coupling to two equivalent 31 P and two 33 S centres, respectively. Since this Selenocarbonyl compounds have attracted increasing attention as a result of their applications in conducting materials and biological systems. [21a, 30] The stability of selenoketones R2C=Se is enhanced if the substituents R (preferably both) contain a heteroatom, especially R2N groups. [21a] The thiophosphoryl derivative 6c is a rare example of a selone in which a phosphorus substituent is attached to the C=Se functionality. [18] We note, however, that the iodide ion in the adduct [LiI(TMEDA)]6c appears to provide a necessary stabilising influence on this unusual selone. [31,32] 14 The Se-I distance of 2.7218 (7) Å (Table 2) in [LiI(TMEDA)]6c represents a moderately strong interaction. This value is ca. 0.20 Å longer than the covalent Se-I bond lengths in the selenenyl iodides, ArSeI (Ar = 2,4,6-tBu3-C6H2, 2,4,6-Me3-C6H2); [33] cf.
sum of covalent radii for selenium and iodine is 2.50 Å [34] ), and it is at the higher end of the distances (2.56-2.73 Å) observed by Devillanova et al. for charge-transfer complexes of selones with I2 or IBr, which have been described as "strong" Se-I bonds. [35] The Se-I distance in [LiI(TMEDA)]6c also falls in the range of 2.74-2.78 Å reported for organoselenenyl iodides, which are stabilised by an intramolecular Se···N interaction that, paradoxically, weakens the Se-I bond through electron donation into the Se-I σ* orbital. [36] The C-Se distance of 1.815(4) Å falls within the range 1.77-1.84 Å reported for a wide variety of selenocarbonyl compounds, [21,30] with selenoketones falling in the lower end of that range. [37] It is comparable with the value of 1.817 (7) Å found for Nmethylbenzothiadiazole-2(3H)-selone. [38] We note, however, that the C-Se distance in the selenium donor is elongated by 0.04-0.06 Å in charge-transfer complexes that contain strong Se-I bonds. [35] Thus, it seems reasonable to infer that the C=Se distance in 6c, if it can be isolated, [31] will be significantly shorter than the value of 1.815(4) Å found for the LiI adduct. The C-Se distance for the π-bonded C=Se group in the copper complex of [(Se)C(PPh2O)2] is 1.846 (9) Ǻ. [18] The the product from the reaction between [Li(TMEDA)]27b and 12-crown-4 (Scheme 1). [26] Collectively, these NMR data suggest the formation of an unsymmetrical dimer that derivative of the dianion 7b. [39] The diamagnetic monoanion 8b is formally constructed from the radical anion 5b, [

(S)C(PPh2S)2] −• , and a neutral radical [H(S)C(PPh2S)2] •
connected through an S-S bond, which is significantly shorter than that in the dianionic precursor 7b (by ca. 0.09 Å, Table 5). The four-coordinate C (2)  weak C-H···S interaction in the solid state. [24] In summary, the dianionic all-sulfur system 7b exhibits a pronounced tendency for proton abstraction which preempts the formation of the expected two-electron oxidation product SC(PPh2S)2. In this context we note that the HOMO of 7b exhibits a significant contribution from the p-orbitals of the two backbone carbon atoms. By contrast, we were unable to detect the formation of the analogous protonated species from 7c despite the observation that H2C(PPh2S)2 is the final decomposition product from both 7b and 7c (vide supra).

Conclusions
We have developed a straightforward and efficient synthesis of the trichalcogeno species  were calculated at the optimized geometries using the same basis set-density functional combination. All calculations were performed with Gaussian 03 [42] and Turbomole 6.1 [43] program packages. Visualizations for Figure 3 were done with gOpenMol. [44] X-ray crystallography.  4c, 6c, 7b, 7c and 8b). The crystals of all compounds were coated with Paratone 8277 oil and mounted on a glass fibre. Diffraction data were collected on a Nonius KappaCCD diffractometer using monochromated MoK radiation ( = 0.71073 Å) at -100 °C. The data sets were corrected for Lorentz and polarization effects, and empirical absorption correction was applied to the net intensities. The structures were solved by direct methods using SHELXS-97 and refined using SHELXL-97. [45,46] After full-matrix least-squares refinement of the non-hydrogen atoms with anisotropic thermal parameters, the hydrogen atoms were placed in calculated positions (C-H = 0.95 Å for -CH, 0.99 Å for -CH2 and 20 0.98 Å for -CH3 hydrogens). The isotropic thermal parameters of the hydrogen atoms were fixed at 1.2 times that of the corresponding carbon for -CH and -CH2 hydrogens, and 1.5 times for -CH3 hydrogens. In the structures of 8b the hydrogen atom bonded to the PCP carbon was located from the Fourier density map and it was refined as isotropic.
In the final refinement the remaining hydrogen atoms were riding on their respective carbon atoms.
In   22 C 57.58, H 6.79, N 7.26; found: C 57.36, H 6.45, N 7.19. X-ray quality crystals were obtained by layering n-hexane onto the toluene solution of [Li(TMEDA) 6, H 5.11, N 3.23; found: C 52.17, H 5.17, N 3.33. 13 C and 77 Se NMR spectra were not obtained due to decomposition of the adduct in solution. X-ray quality crystals were obtained by layering  (9) as proven by the NMR spectroscopy. [27] X-ray quality crystals of )2]8b were obtained by layering n-hexane onto the CH2Cl2 solution of the yellow powder after 5 h at 5 o C.        )2]8b with thermal ellipsoids drawn at 50% probability