P. Chandrasekaran

Synthesis, structures, and properties of mixed dithiolene-carbonyl and dithiolene-phosphine complexes of tungsten.

A new, high yield synthesis of [Ni(S(2)C(2)Me(2))(2)] (3) is described using 4,5-dimethyl-1,3-dithiol-2-one, Me(2)C(2)S(2)CO (1), as dithiolene ligand precursor. Reaction of (Me(2)C(2)S(2))Sn(n)Bu(2), 2, with WCl(6) produces tris(dithiolene) [W(S(2)C(2)Me(2))(3)] (6) and demonstrates the potential synthetic utility of this compound in metallodithiolene synthesis. The series of compounds [W(S(2)C(2)Me(2))(x)(CO)(6-2x)] (x = 1-3), obtained as a mixture via the reaction of [Ni(S(2)C(2)Me(2))(2)] with [W(MeCN)(3)(CO)(3)], has been characterized structurally. A trigonal prismatic geometry is observed for [W(S(2)C(2)Me(2))(CO)(4)] (4) and confirmed by a DFT geometry optimization to be lower in energy than an octahedron by 5.1 kcal/mol. The tris(dithiolene) compound [W(S(2)C(2)Me(2))(3)] crystallizes in disordered fashion upon a 2-fold axis in C2/c, a different space group than that observed for its molybdenum homologue (P1), which is attributed to a slightly smaller chelate fold angle, alpha, in the former. The reactivity of 4 and [W(S(2)C(2)Me(2))(2)(CO)(2)] (5) toward PMe(3) has been examined. Compound 4 yields only [W(S(2)C(2)Me(2))(CO)(2)(PMe(3))(2)] (7), while 5 produces either [W(S(2)C(2)Me(2))(2)(CO)(PMe(3))] (8) or [W(S(2)C(2)Me(2))(2)(PMe(3))(2)] (9) depending upon reaction conditions. Crystallographic characterization of 5, 8, and 9 reveals a trend toward greater reduction of the dithiolene ligand (i.e., more ene-1,2-dithiolate character) across the series, as manifested by C-C and C-S bond lengths. These structural data indicate a profound effect exerted by the pi-acidic CO ligands upon the apparent state of reduction of the dithiolene ligand in compounds with ostensibly the same oxidation state.

Synthesis, Structures, and Properties of 1,2,4,5-Benzenetetrathiolate Linked Group 10 Metal Complexes

Dimetallic compounds [(P-P)M(S(2)C(6)H(2)S(2))M(P-P)] (M = Ni, Pd; P-P = chelating bis(phosphine), 3a-3f) are prepared from O=CS(2)C(6)H(2)S(2)C=O or (n)Bu(2)SnS(2)C(6)H(2)S(2)Sn(n)Bu(2), which are protected forms of 1,2,4,5-benzenetetrathiolate. Selective monodeprotections of O=CS(2)C(6)H(2)S(2)C=O or (n)Bu(2)SnS(2)C(6)H(2)S(2)Sn(n)Bu(2) lead to [(P-P)Ni(S(2)C(6)H(2)S(2)C=O)] or [(P-P)Ni(S(2)C(6)H(2)S(2)Sn(n)Bu(2))]; the former is used to prepare trimetallic compounds [(dcpe)Ni(S(2)C(6)H(2)S(2))M(S(2)C(6)H(2)S(2))Ni(dcpe)] (M = Ni (6a) or Pt (6b); dcpe = 1,2-bis(dicyclohexylphosphino)ethane). Compounds 3a-3f are redox active and display two oxidation processes, of which the first is generally reversible. Dinickel compound [(dcpe)Ni(S(2)C(6)H(2)S(2))Ni(dcpe)] (3d) reveals two reversible oxidation waves with DeltaE(1/2) = 0.66 V, corresponding to K(c) of 1.6 x 10(11) for the mixed valence species. Electrochemical behavior is unstable to repeated scanning in the presence of [Bu(4)N][PF(6)] electrolyte but indefinitely stable with Na[BArF(24)] (BArF(24) = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate), suggesting that the radical cation generated by oxidation is vulnerable to reaction with PF(6)(-). Chemical oxidation of 3d with [Cp(2)Fe][BArF(24)] leads to formation of [3d][BArF(24)]. Structural identification of [3d][BArF(24)] reveals appreciable shortening and lengthening of C-S and C-C bond distances, respectively, within the tetrathioarene fragment compared to charge-neutral 3d, indicating this to be the redox active moiety. Attempted oxidation of [(dppb)Ni(S(2)C(6)H(2)S(2))Ni(dppb)] (3c) (dppb = 1,2-bis(diphenylphosphino)benzene) with AgBArF(24) produces [[(dppb)Ni(S(2)C(6)H(2)S(2))Ni(dppb)](2)(mu-Ag(2))][BArF(24)](2), [4c][BArF(24)](2), in which no redox chemistry has occurred. Crystal structures of bis(disulfide)-linked compounds [(P-P)Ni(S(2)C(6)H(2)(mu-S(2))(2)C(6)H(2)S(2))Ni(P-P)] are reported. Near IR spectroscopy upon cationic [3d](+) and neutral 6a reveals multiple intense absorptions in the 950-1400 nm region. Time-dependent density functional theory (DFT) calculations on a 6a model compound indicate that these absorptions are transitions between ligand-based pi-type orbitals that have significant contributions from the sulfur p orbitals.

Sensitivity of X-ray core spectroscopy to changes in metal ligation: a systematic study of low-coordinate, high-spin ferrous complexes.

In order to assess the sensitivity and complementarity of X-ray absorption and emission spectroscopies for determining changes in the metal ligation sphere, a systematic experimental and theoretical study of iron model complexes has been carried out. A series of high-spin ferrous complexes, in which the ligation sphere has been varied from a three-coordinate complex, [L(tBu)Fe(SPh)] (1) (where L(tBu) = bulky β-diketiminate ligand; SPh = phenyl thiolate) to four-coordinate complexes [L(tBu)Fe(SPh)(X)] (where X = CN(t)Bu (2); 1-methylimidazole (3); or N,N-dimethylformamide (DMF) (4)), has been investigated using a combination of Fe K-edge X-ray absorption (XAS) and Kβ X-ray emission (XES) spectroscopies. The Fe K XAS pre-edge and edge of all four complexes are consistent with a high-spin ferrous assignment, with the largest differences in the pre-edge intensities attributed to changes in covalency of the fourth coordination site. The X-ray emission spectra show pronounced changes in the valence to core region (V2C) as the identity of the coordinated ligand is varied. The experimental results have been correlated to density functional theory (DFT) calculations, to understand key molecular orbital contributions to the observed absorption and emission features. The calculations also have been extended to a series of hypothetical high-spin iron complexes to understand the sensitivity of XAS and XES techniques to different ligand protonation states ([L(tBu)Fe(II)(SPh)(NHn)](3-n) (n = 3, 2, 1, 0)), metal oxidation states [L(tBu)Fe(SPh)(N)](n-) (n = 3, 2, 1), and changes in the ligand identity [L(tBu)Fe(IV)(SPh)(X)](n-) (X = C(4-), N(3-), O(2-); n = 2, 1, 0). This study demonstrates that XAS pre-edge data have greater sensitivity to changes in oxidation state, while valence to core (V2C) XES data provide a more sensitive probe of ligand identity and protonation state. The combination of multiple X-ray spectroscopic methods with DFT results thus has the potential to provide for detailed characterization of complex inorganic systems in both chemical and biological catalysis.

Ancillary Ligand Effects upon Dithiolene Redox Noninnocence in Tungsten Bis(dithiolene) Complexes

An expanded set of compounds of the type [W(S2C2Me2)2L1L2](n) (n = 0: L1 = L2 = CO, 1; L1 = L2 = CN(t)Bu, 2; L1 = CO, L2 = carbene, 3; L1 = CO, L2 = phosphine, 4; L1 = L2 = phosphine, 5. n = 2-: L1 = L2 = CN(-), [6](2-)) has been synthesized and characterized. Despite isoelectronic formulations, the compound set reveals gradations in the dithiolene ligand redox level as revealed by intraligand bond lengths, υ(CCchelate), and rising edge energies in the sulfur K-edge X-ray absorption spectra (XAS). Differences among the terminal series members, 1 and [6](2-), are comparable to differences seen in homoleptic dithiolene complexes related by full electron transfer to/from a dithiolene-based MO. The key feature governing these differences is the favorable energy of the CO π* orbitals, which are suitably positioned to overlap with tungsten d orbitals and exert an oxidizing effect on both metal and dithiolene ligand via π-backbonding. The CN(-) π* orbitals are too high in energy to mix effectively with tungsten and thus leave the filled dithiolene π* orbitals unperturbed. This work shows how, and the degree to which, the redox level of a noninnocent ligand can be modulated by the choice of ancillary ligands(s).

Redox-Controlled Interconversion between Trigonal Prismatic and Octahedral Geometries in a Monodithiolene Tetracarbonyl Complex of Tungsten

The tetracarbonyl compounds [W(mdt)(CO)(4)] (1) and [W(Me(2)pipdt)(CO)(4)] (2) both have dithiolene-type ligands (mdt(2-) = 1,2-dimethyl-1,2-dithiolate; Me(2)pipdt = 1,4-dimethylpiperazine-2,3-dithione) but different geometries, trigonal prismatic (TP) and octahedral, respectively. Structural data suggest an ene-1,2-dithiolate ligand description, hence a divalent tungsten ion, for 1 and a dithioketone ligand, hence W(0) oxidation state, for 2. Density functional theory (DFT) calculations on 1 show the highest occupied molecular orbital (HOMO) to be a strong W-dithiolene π bonding interaction and the lowest unoccupied molecular orbital (LUMO) its antibonding counterpart. The TP geometry is preferred because symmetry allowed mixing of these orbitals via a configuration interaction (CI) stabilizes this geometry over an octahedron. The TP geometry for 2 is disfavored because W-dithiolene π overlap is attenuated because of a lowering of the sulfur content and a raising of the energy of this ligand π orbital by the conjugated piperazine nitrogen atoms in the Me(2)pipdt ligand. A survey of the Cambridge Structural Database identifies other W(CO)(4) compounds with pseudo C(4v) disposition of CO ligands and suggests a d(4) electron count to be a probable common denominator. Reduction of 1 induces a geometry change to octahedral because the singly occupied molecular orbital (SOMO) is at lower energy in this geometry. The cyclic voltammogram of 1 in CH(2)Cl(2) reveals a reduction wave at -1.14 V (vs Fc(+)/Fc) with an unusual offset between the cathodic and the anodic peaks (ΔE(p)) of 0.130 V, which is followed by a second, reversible reduction wave at -1.36 V with ΔE(p) = 0.091 V. The larger ΔE(p) observed for the first reduction is evidence of the trigonal prism-to-octahedron geometry change attending this process. Tungsten L(1)-edge X-ray absorption (XAS) data indicate a higher metal oxidation state in 1 than 2. Electron paramagnetic resonance data for [1](-) and [2](-) are both diagnostic of dithiolene ligand-based sulfur radical, indicating that one-electron reduction of 1 involves two-electron reduction of tungsten and one-electron oxidation of dithiolene ligand.

X-ray absorption spectroscopy systematics at the tungsten L-edge.

A series of mononuclear six-coordinate tungsten compounds spanning formal oxidation states from 0 to +VI, largely in a ligand environment of inert chloride and/or phosphine, was interrogated by tungsten L-edge X-ray absorption spectroscopy. The L-edge spectra of this compound set, comprised of [W0(PMe3)6], [WIICl2(PMePh2)4], [WIIICl2(dppe)2][PF6] (dppe = 1,2-bis(diphenylphosphino)ethane), [WIVCl4(PMePh2)2], [WV(NPh)Cl3(PMe3)2], and [WVICl6], correlate with formal oxidation state and have usefulness as references for the interpretation of the L-edge spectra of tungsten compounds with redox-active ligands and ambiguous electronic structure descriptions. The utility of these spectra arises from the combined correlation of the estimated branching ratio of the L3,2-edges and the L1 rising-edge energy with metal Zeff, thereby permitting an assessment of effective metal oxidation state. An application of these reference spectra is illustrated by their use as backdrop for the L-edge X-ray absorption spectra of [WIV(mdt)2(CO)2] and [WIV(mdt)2(CN)2]2– (mdt2– = 1,2-dimethylethene-1,2-dithiolate), which shows that both compounds are effectively WIV species even though the mdt ligands exist at different redox levels in the two compounds. Use of metal L-edge XAS to assess a compound of uncertain formulation requires: (1) Placement of that data within the context of spectra offered by unambiguous calibrant compounds, preferably with the same coordination number and similar metal ligand distances. Such spectra assist in defining upper and/or lower limits for metal Zeff in the species of interest. (2) Evaluation of that data in conjunction with information from other physical methods, especially ligand K-edge XAS. (3) Increased care in interpretation if strong π-acceptor ligands, particularly CO, or π-donor ligands are present. The electron-withdrawing/donating nature of these ligand types, combined with relatively short metal–ligand distances, exaggerate the difference between formal oxidation state and metal Zeff or, as in the case of [WIV(mdt)2(CO)2], exert the subtle effect of modulating the redox level of other ligands in the coordination sphere.

Crystal structure of unsymmetrical α-di­imine palladium(II) complex cis-[{ArN=C(Me)–(Et)C=NAr}PdCl2] [Ar = 2,6-(iPr)2C6H3]

The synthesis and crystal structure of palladium(II) complex, cis-[{ArN=C(Me)-(Et)C=NAr}PdCl2] (Ar = 2,6-iPr2C6H3), containing unsymmetrical α-diimine ligand, is reported.