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

The tetracarbonyl compounds [W­(mdt)­(CO)₄] (<b>1</b>) and [W­(Me₂pipdt)­(CO)₄] (<b>2</b>) both have dithiolene-type ligands (mdt^2– = 1,2-dimethyl-1,2-dithiolate; Me₂pipdt = 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 <b>1</b> and a dithioketone ligand, hence W(0) oxidation state, for <b>2</b>. Density functional theory (DFT) calculations on <b>1</b> 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 <b>2</b> 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 Me₂pipdt ligand. A survey of the Cambridge Structural Database identifies other W­(CO)₄ compounds with pseudo <i>C</i>_4<i>v</i> disposition of CO ligands and suggests a d⁴ electron count to be a probable common denominator. Reduction of <b>1</b> induces a geometry change to octahedral because the singly occupied molecular orbital (SOMO) is at lower energy in this geometry. The cyclic voltammogram of <b>1</b> in CH₂Cl₂ reveals a reduction wave at −1.14 V (vs Fc⁺/Fc) with an unusual offset between the cathodic and the anodic peaks (Δ<i>E</i>_p) of 0.130 V, which is followed by a second, reversible reduction wave at −1.36 V with Δ<i>E</i>_p = 0.091 V. The larger Δ<i>E</i>_p observed for the first reduction is evidence of the trigonal prism-to-octahedron geometry change attending this process. Tungsten L₁-edge X-ray absorption (XAS) data indicate a higher metal oxidation state in <b>1</b> than <b>2</b>. Electron paramagnetic resonance data for [<b>1</b>]⁻ and [<b>2</b>]⁻ are <i>both</i> diagnostic of dithiolene ligand-based sulfur radical, indicating that one-electron reduction of <b>1</b> <i>involves two-electron reduction of tungsten and one-electron oxidation of dithiolene ligand</i>

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

The tetracarbonyl compounds [W­(mdt)­(CO)₄] (<b>1</b>) and [W­(Me₂pipdt)­(CO)₄] (<b>2</b>) both have dithiolene-type ligands (mdt^2– = 1,2-dimethyl-1,2-dithiolate; Me₂pipdt = 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 <b>1</b> and a dithioketone ligand, hence W(0) oxidation state, for <b>2</b>. Density functional theory (DFT) calculations on <b>1</b> 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 <b>2</b> 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 Me₂pipdt ligand. A survey of the Cambridge Structural Database identifies other W­(CO)₄ compounds with pseudo <i>C</i>_4<i>v</i> disposition of CO ligands and suggests a d⁴ electron count to be a probable common denominator. Reduction of <b>1</b> induces a geometry change to octahedral because the singly occupied molecular orbital (SOMO) is at lower energy in this geometry. The cyclic voltammogram of <b>1</b> in CH₂Cl₂ reveals a reduction wave at −1.14 V (vs Fc⁺/Fc) with an unusual offset between the cathodic and the anodic peaks (Δ<i>E</i>_p) of 0.130 V, which is followed by a second, reversible reduction wave at −1.36 V with Δ<i>E</i>_p = 0.091 V. The larger Δ<i>E</i>_p observed for the first reduction is evidence of the trigonal prism-to-octahedron geometry change attending this process. Tungsten L₁-edge X-ray absorption (XAS) data indicate a higher metal oxidation state in <b>1</b> than <b>2</b>. Electron paramagnetic resonance data for [<b>1</b>]⁻ and [<b>2</b>]⁻ are <i>both</i> diagnostic of dithiolene ligand-based sulfur radical, indicating that one-electron reduction of <b>1</b> <i>involves two-electron reduction of tungsten and one-electron oxidation of dithiolene ligand</i>

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

The tetracarbonyl compounds [W­(mdt)­(CO)₄] (<b>1</b>) and [W­(Me₂pipdt)­(CO)₄] (<b>2</b>) both have dithiolene-type ligands (mdt^2– = 1,2-dimethyl-1,2-dithiolate; Me₂pipdt = 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 <b>1</b> and a dithioketone ligand, hence W(0) oxidation state, for <b>2</b>. Density functional theory (DFT) calculations on <b>1</b> 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 <b>2</b> 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 Me₂pipdt ligand. A survey of the Cambridge Structural Database identifies other W­(CO)₄ compounds with pseudo <i>C</i>_4<i>v</i> disposition of CO ligands and suggests a d⁴ electron count to be a probable common denominator. Reduction of <b>1</b> induces a geometry change to octahedral because the singly occupied molecular orbital (SOMO) is at lower energy in this geometry. The cyclic voltammogram of <b>1</b> in CH₂Cl₂ reveals a reduction wave at −1.14 V (vs Fc⁺/Fc) with an unusual offset between the cathodic and the anodic peaks (Δ<i>E</i>_p) of 0.130 V, which is followed by a second, reversible reduction wave at −1.36 V with Δ<i>E</i>_p = 0.091 V. The larger Δ<i>E</i>_p observed for the first reduction is evidence of the trigonal prism-to-octahedron geometry change attending this process. Tungsten L₁-edge X-ray absorption (XAS) data indicate a higher metal oxidation state in <b>1</b> than <b>2</b>. Electron paramagnetic resonance data for [<b>1</b>]⁻ and [<b>2</b>]⁻ are <i>both</i> diagnostic of dithiolene ligand-based sulfur radical, indicating that one-electron reduction of <b>1</b> <i>involves two-electron reduction of tungsten and one-electron oxidation of dithiolene ligand</i>

Redox-Active Metallodithiolene Groups Separated by Insulating Tetraphosphinobenzene Spacers

Compounds of the type [(S₂C₂R₂)­M­(μ-tpbz)­M­(S₂C₂R₂)] (R = CN, Me, Ph, <i>p</i>-anisyl; M = Ni, Pd, Pt; tpbz = 1,2,4,5-tetrakis­(diphenylphosphino)­benzene) have been prepared by transmetalation with [(S₂C₂R₂)­SnR′₂] reagents, by direct displacement of dithiolene ligand from [M­(S₂C₂R₂)₂] with 0.5 equiv of tpbz, or by salt metathesis using Na₂[S₂C₂(CN)₂] in conjunction with X₂M­(μ-tpbz)­MX₂ (X = halide). X-ray crystallography reveals a range of topologies (undulating, chair, bowed) for the (S₂C₂)­M­(P₂C₆P₂)­M­(S₂C₂) core. The [(S₂C₂R₂)­M­(μ-tpbz)­M­(S₂C₂R₂)] (R = Me, Ph, <i>p</i>-anisyl) compounds support reversible or quasireversible oxidations corresponding to concurrent oxidation of the dithiolene terminal ligands from ene-1,2-dithiolates to radical monoanions, forming [(⁻S^<b>•</b>SC₂R₂)­M­(μ-tpbz)­M­(⁻S^<b>•</b>SC₂R₂)]²⁺. The R = Ph and <i>p</i>-anisyl compounds support a second, reversible oxidation of the dithiolene ligands to their α-dithione form. In contrast, [(S₂C₂(CN)₂)­Ni­(tpbz)­Ni­(S₂C₂(CN)₂)] sustains only reversible, metal-centered reductions. Spectroscopic examination of [(⁻S^<b>•</b>SC₂(<i>p</i>-anisyl)₂)­Ni­(μ-tpbz)­Ni­(⁻S^<b>•</b>SC₂(<i>p</i>-anisyl)₂)]²⁺ by EPR reveals a near degenerate singlet–triplet ground state, with spectral simulation revealing a remarkably small dipolar coupling constant of 18 × 10^–4 cm^–1 that is representative of an interspin distance of 11.3 Å. This weak interaction is mediated by the rigid tpbz ligand, whose capacity to electronically insulate is an essential quality in the development of molecular-based spintronic devices

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

An expanded set of compounds of the type [W­(S₂C₂Me₂)₂L₁L₂]^<i>n</i> (<i>n</i> = 0: L₁ = L₂ = CO, <b>1</b>; L₁ = L₂ = CN^<i>t</i>Bu, <b>2</b>; L₁ = CO, L₂ = carbene, <b>3</b>; L₁ = CO, L₂ = phosphine, <b>4</b>; L₁ = L₂ = phosphine, <b>5</b>. <i>n</i> = 2–: L₁ = L₂ = CN^–, [<b>6</b>]^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, <b>1</b> and [<b>6</b>]^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)