Synthesis, Structures, Bonding, and Redox Chemistry of Ditungsten Butadiyne Complexes with WCCW Backbones

Complexes of the form XL4WCCWL4X (L = 1/2 dmpe, 1/2 depe, P(OMe)3; X = Cl, OTf) have been synthesized from (ButO)3WCCW(OBut)3 in two steps via Cl3(dme)WCCW(dme)Cl3, which undergoes facile four-electron reduction in the presence of L. The compounds possess formal d2−d2 electron configurations. The molecular structures of Cl(dmpe)2WCCW(dmpe)2Cl and Cl{P(OMe)3}4WCCW{P(OMe)3}4Cl were determined by X-ray crystallography; bond distances within the backbone are consistent with a WCCW canonical structure. Density-functional-theory calculations on Cl(dmpe)2WCCW(dmpe)2Cl and the model compound Cl(PH3)4WCCW(PH3)4Cl, and on their monometallic analogs W(CH)(dmpe)2Cl and W(CH)(PH3)4Cl, indicate that the WCCW backbone is strongly π-conjugated; this is supported by the observation of low-energy π → π* transitions for the compounds. The calculations predict that δ symmetry dxy-derived orbitals should be (or lie near) the highest occupied molecular orbital. Consistent with this prediction, the electronic spectra of the compounds exhibit a band attributable to dxy → π* transition(s), as the lowest-energy feature and electrochemical studies demonstrate that they undergo sequential one-electron oxidations to produce (dxy)2−(dxy)1 and (dxy)1−(dxy)1 congeners. Due to the δ symmetry of the redox orbitals, the oxidized congeners maintain the WCCW canonical structure of the parent d2−d2 compounds. The first and second oxidation potentials of Cl(dmpe)2WCCW(dmpe)2Cl are separated by ≤0.4 V, corresponding to Kcom = 104−106; the interaction between redox orbitals is largely electrostatic in origin and not the result of significant direct δ orbital overlap. The reaction between Cl(dmpe)2WCCW(dmpe)2Cl and HCl (2 equiv) produces the d0−d0 dihydride ion [Cl(H)(dmpe)2WCCW(dmpe)2(H)Cl]2+, which is formulated as maintaining the WCCW backbone on the basis of its X-ray crystal structure and NMR spectra. This family of WCCW derivatives expands the relatively small class of MCCM compounds and is distinctive among them because their ancillary ligands should allow incorporation of the L4WCCWL4 unit into interior positions of metalloyne oligomers and polymers.