Electrochemical and Computational Study of Tungsten(0) Ferrocene Complexes: Observation of the Mono-Oxidized Tungsten(0) Ferrocenium Species and Intramolecular Electronic Interactions

The series [(CO)(5)W=C(XR)Fc] (1, XR = OEt; 3, XR = NHBu) as well as [(CO)(5)W=C(XR)-Fc-(XR)C:=W(CO)(5)] (2, XR = OEt; 4, XR = NHBu) of monoand biscarbene tungsten(0) complexes with Fc = Fe-II(C-5-H-5)-(C-5,H-4) for monosubstituted derivatives and Fc' = Fe-II(C5H4)(2) for disubstituted derivatives were synthesized and characterized spectroscopically. The oxidized ferrocenium complex [1(+)]-PF6 was also synthesized and characterized. Electrochemical and computational studies were mutually consistent in confirming the sequence of redox events for the carbene derivatives 1-4 as first a carbene double-bond reduction to a radical anion, W--C-center dot, at peak cathodic potentials less than -2 V, then a ferrocenyl group oxidation in the range 0.206 < E degrees' <0.540 V, and finally an electrochemically irreversible three-electron W(0) oxidation at E-pa > 0.540 V vs FcH/FcH(+) in CH2Cl2/[(Bu-n(4))N][PF6]. This is in contrast with the sequence of oxidation events in ferrocenylcarbene complexes of chromium, where Cr(0) is first oxidized in a one-electron-transfer process, then the ferrocenyl group, and finally formation of a Cr(II) species. The unpaired electron of the reductively formed radical anion is mainly located on the carbene carbon atom. Electronic interactions between two carbene double bonds (for biscarbenes 2 and 4) as well as between two W centers (for 4) were evident. Differences in redox potentials between the "a" and "b" components of the three-electron W oxidation of 4 in CH2Cl2 or CH3CN/ [(Bu-n(4))N][PF6] are Delta E degrees' = E-pa(W(0) oxd 1(b)) -E-pa(W(0) oxd 1a) = ca. 51 and 337 mV, respectively. Tungsten oxidation was restricted to a W-0/II couple in CH2Cl2/[(Bu-n(4))N][B(C6F5)(4)]. From the computational results, the short-lived W(II) species were observed to be stabilized by agostic CH center dot center dot center dot W interactions.