5 research outputs found
Redox-Controlled Interconversion between Trigonal Prismatic and Octahedral Geometries in a Monodithiolene Tetracarbonyl Complex of Tungsten
The tetracarbonyl compounds [WÂ(mdt)Â(CO)<sub>4</sub>]
(<b>1</b>) and [WÂ(Me<sub>2</sub>pipdt)Â(CO)<sub>4</sub>] (<b>2</b>) both
have dithiolene-type ligands (mdt<sup>2–</sup> = 1,2-dimethyl-1,2-dithiolate;
Me<sub>2</sub>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 <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 Me<sub>2</sub>pipdt ligand.
A survey of the Cambridge Structural Database identifies other WÂ(CO)<sub>4</sub> compounds with pseudo <i>C</i><sub>4<i>v</i></sub> disposition of CO ligands and suggests a d<sup>4</sup> 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 CH<sub>2</sub>Cl<sub>2</sub> reveals a reduction wave at −1.14 V (vs Fc<sup>+</sup>/Fc)
with an unusual offset between the cathodic and the anodic peaks (Δ<i>E</i><sub>p</sub>) of 0.130 V, which is followed by a second,
reversible reduction wave at −1.36 V with Δ<i>E</i><sub>p</sub> = 0.091 V. The larger Δ<i>E</i><sub>p</sub> observed for the first reduction is evidence of the trigonal
prism-to-octahedron geometry change attending this process. Tungsten
L<sub>1</sub>-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>]<sup>−</sup> and [<b>2</b>]<sup>−</sup> 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 [WÂ(mdt)Â(CO)<sub>4</sub>]
(<b>1</b>) and [WÂ(Me<sub>2</sub>pipdt)Â(CO)<sub>4</sub>] (<b>2</b>) both
have dithiolene-type ligands (mdt<sup>2–</sup> = 1,2-dimethyl-1,2-dithiolate;
Me<sub>2</sub>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 <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 Me<sub>2</sub>pipdt ligand.
A survey of the Cambridge Structural Database identifies other WÂ(CO)<sub>4</sub> compounds with pseudo <i>C</i><sub>4<i>v</i></sub> disposition of CO ligands and suggests a d<sup>4</sup> 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 CH<sub>2</sub>Cl<sub>2</sub> reveals a reduction wave at −1.14 V (vs Fc<sup>+</sup>/Fc)
with an unusual offset between the cathodic and the anodic peaks (Δ<i>E</i><sub>p</sub>) of 0.130 V, which is followed by a second,
reversible reduction wave at −1.36 V with Δ<i>E</i><sub>p</sub> = 0.091 V. The larger Δ<i>E</i><sub>p</sub> observed for the first reduction is evidence of the trigonal
prism-to-octahedron geometry change attending this process. Tungsten
L<sub>1</sub>-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>]<sup>−</sup> and [<b>2</b>]<sup>−</sup> 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 [WÂ(mdt)Â(CO)<sub>4</sub>]
(<b>1</b>) and [WÂ(Me<sub>2</sub>pipdt)Â(CO)<sub>4</sub>] (<b>2</b>) both
have dithiolene-type ligands (mdt<sup>2–</sup> = 1,2-dimethyl-1,2-dithiolate;
Me<sub>2</sub>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 <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 Me<sub>2</sub>pipdt ligand.
A survey of the Cambridge Structural Database identifies other WÂ(CO)<sub>4</sub> compounds with pseudo <i>C</i><sub>4<i>v</i></sub> disposition of CO ligands and suggests a d<sup>4</sup> 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 CH<sub>2</sub>Cl<sub>2</sub> reveals a reduction wave at −1.14 V (vs Fc<sup>+</sup>/Fc)
with an unusual offset between the cathodic and the anodic peaks (Δ<i>E</i><sub>p</sub>) of 0.130 V, which is followed by a second,
reversible reduction wave at −1.36 V with Δ<i>E</i><sub>p</sub> = 0.091 V. The larger Δ<i>E</i><sub>p</sub> observed for the first reduction is evidence of the trigonal
prism-to-octahedron geometry change attending this process. Tungsten
L<sub>1</sub>-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>]<sup>−</sup> and [<b>2</b>]<sup>−</sup> 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 [(S<sub>2</sub>C<sub>2</sub>R<sub>2</sub>)ÂMÂ(μ-tpbz)ÂMÂ(S<sub>2</sub>C<sub>2</sub>R<sub>2</sub>)] (R =
CN, Me, Ph, <i>p</i>-anisyl; M = Ni, Pd, Pt; tpbz = 1,2,4,5-tetrakisÂ(diphenylphosphino)Âbenzene)
have been prepared by transmetalation with [(S<sub>2</sub>C<sub>2</sub>R<sub>2</sub>)ÂSnR′<sub>2</sub>] reagents, by direct displacement
of dithiolene ligand from [MÂ(S<sub>2</sub>C<sub>2</sub>R<sub>2</sub>)<sub>2</sub>] with 0.5 equiv of tpbz, or by salt metathesis using
Na<sub>2</sub>[S<sub>2</sub>C<sub>2</sub>(CN)<sub>2</sub>] in conjunction
with X<sub>2</sub>MÂ(μ-tpbz)ÂMX<sub>2</sub> (X = halide). X-ray
crystallography reveals a range of topologies (undulating, chair,
bowed) for the (S<sub>2</sub>C<sub>2</sub>)ÂMÂ(P<sub>2</sub>C<sub>6</sub>P<sub>2</sub>)ÂMÂ(S<sub>2</sub>C<sub>2</sub>) core. The [(S<sub>2</sub>C<sub>2</sub>R<sub>2</sub>)ÂMÂ(μ-tpbz)ÂMÂ(S<sub>2</sub>C<sub>2</sub>R<sub>2</sub>)] (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 [(<sup>−</sup>S<sup><b>•</b></sup>SC<sub>2</sub>R<sub>2</sub>)ÂMÂ(μ-tpbz)ÂMÂ(<sup>−</sup>S<sup><b>•</b></sup>SC<sub>2</sub>R<sub>2</sub>)]<sup>2+</sup>. 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, [(S<sub>2</sub>C<sub>2</sub>(CN)<sub>2</sub>)ÂNiÂ(tpbz)ÂNiÂ(S<sub>2</sub>C<sub>2</sub>(CN)<sub>2</sub>)] sustains
only reversible, metal-centered reductions. Spectroscopic examination
of [(<sup>−</sup>S<sup><b>•</b></sup>SC<sub>2</sub>(<i>p</i>-anisyl)<sub>2</sub>)ÂNiÂ(μ-tpbz)ÂNiÂ(<sup>−</sup>S<sup><b>•</b></sup>SC<sub>2</sub>(<i>p</i>-anisyl)<sub>2</sub>)]<sup>2+</sup> by EPR reveals a near degenerate
singlet–triplet ground state, with spectral simulation revealing
a remarkably small dipolar coupling constant of 18 × 10<sup>–4</sup> cm<sup>–1</sup> that is representative of an interspin distance
of 11.3 Ã…. 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 [WÂ(S<sub>2</sub>C<sub>2</sub>Me<sub>2</sub>)<sub>2</sub>L<sub>1</sub>L<sub>2</sub>]<sup><i>n</i></sup> (<i>n</i> = 0: L<sub>1</sub> = L<sub>2</sub> = CO, <b>1</b>; L<sub>1</sub> = L<sub>2</sub> = CN<sup><i>t</i></sup>Bu, <b>2</b>; L<sub>1</sub> = CO, L<sub>2</sub> = carbene, <b>3</b>; L<sub>1</sub> = CO, L<sub>2</sub> = phosphine, <b>4</b>; L<sub>1</sub> = L<sub>2</sub> = phosphine, <b>5</b>. <i>n</i> = 2–: L<sub>1</sub> = L<sub>2</sub> =
CN<sup>–</sup>, [<b>6</b>]<sup>2–</sup>) 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, Ï…<sub>CCchelate</sub>, 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>]<sup>2–</sup>, 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<sup>–</sup> 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)