Abstract:
The series [(CO)5W=C(XR)Fc], 1 (XR = OEt) and 3 (XR = NHBu) as well as
[(CO)5W=C(XR)-Fc'-(XR)C=W(CO)5], 2 (XR = OEt) and 4 (XR = NHBu) of mono- and
biscarbene tungsten(0) complexes with Fc = FeII(C5H5)(C5H4) for monosubstituted derivatives
and Fc¢ = FeII(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•, at peak cathodic potentials smaller than -2 V, then a
ferrocenyl group oxidation in the range 0.206 < Eo' < 0.540 V and finally an electrochemically
irreversible three-electron W(0) oxidation at Epa > 0.540 V vs. FcH/FcH+ in CH2Cl2 /
[(nBu4)N][PF6]. This contrasts the sequence of oxidation events in ferrocenylcarbene complexes
of chromium where Cr(0) is first oxidised 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 threeelectron
W oxidation of 4 in CH2Cl2 or CH3CN / [(nBu4)N][PF6] are DEo' = Epa W(0) oxd 1b – Epa W(0)
oxd 1a = ca. 51 and 337 mV respectively. Tungsten oxidation was restricted to a W0/II couple in
CH2Cl2 / [(nBu4)N][B(C6F5)4]. From the computational results, the short-lived W(II) species
were observed to be stabilized by agostic CH···W interactions.
