Synthesis, Structures, Bonding, and Redox Chemistry of Ditungsten Butadiyne Complexes with W≡C—C≡W Backbones

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Complexes of the form XL 4 W≡C—C≡WL 4 X (L = 1/2 dmpe, 1/2 depe, P(OMe) 3; X = Cl, OTf) have been synthesized from (Bu t O) 3 WCCW(OBu t ) 3 in two steps via Cl 3 (dme)WCCW(dme)Cl 3, which undergoes facile four-electron reduction in the presence of L. The compounds possess formal d 2 −d 2 electron configurations. The molecular structures of Cl(dmpe) 2 WCCW(dmpe) 2 Cl and Cl{P(OMe) 3 } 4 WCCW{P(OMe) 3 } 4 Cl 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) 2 WCCW(dmpe) 2 Cl and the model compound Cl(PH 3 ) 4 WCCW(PH 3 ) 4 Cl, and on their monometallic analogs W(CH)(dmpe) 2 Cl and W(CH)(PH 3 ) 4 Cl, 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 d xy -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 d xy → π* transition(s), as the lowest-energy feature and electrochemical studies demonstrate that they undergo sequential one-electron oxidations to produce (d xy ) 2 −(d xy ) 1 and (d xy ) 1 −(d xy ) 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 d 2 −d 2 compounds. The first and second oxidation potentials of Cl(dmpe) 2 WCCW(dmpe) 2 Cl are separated by ≤0.4 V, corresponding to K com = 10 4 −10 6; 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) 2 WCCW(dmpe) 2 Cl and HCl (2 equiv) produces the d 0 −d 0 dihydride ion [Cl(H)(dmpe) 2 WCCW(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 L 4 WCCWL 4 unit into interior positions of metalloyne oligomers and polymers.