Metal–Metal Interaction in Fischer Carbene Complexes: A Study of Ferrocenyl and Biferrocenyl Tungsten Alkylidene Complexes

Abstract

A series of ferrocenyl (Fc = ferrocenyl; fc = ferrocen-1,1'-diyl) and biferrocenyl (Bfc = 1',1''-biferrocenyl; bfc = 1',1''-biferrocen-1,1'''-diyl) mono-and biscarbene tungsten(0) complexes of the type [(CO) 5 W=C(OMe)R] (1, R = Fc; 3, R = Bfc) and [(CO) 5 W=C(OMe)-R'-(OMe)C=W(CO) 5 ] (2, R' = fc; 4, R' = bfc) were synthesized according to the classical synthetic methodology by reacting W(CO) 6 with LiR (R = Fc, fc, bfc), followed by a subsequent alkylation using methyl trifluoromethanesulfonate. Electrochemical investigations were carried out on these complexes to get a closer insight into the electronic properties of 1 -4. The ferrocenyl and biferrocenyl moieties in 1 -4 show reversible one electron redox events. It was further found that the Fischer carbene unit is reducible in an electrochemical one electron transfer process. For the tungsten carbonyl moieties, irreversible oxidation processes were found. In addition, charge transfer studies were performed on 1 -4 by the use of in situ UV-Vis-NIR and infrared spectroelectrochemical techniques. During the UV-Vis-NIR investigations typical low energy transitions for the mixed-valent biferrocenyl unit were found. A further observed high energy NIR absorption is attributed to a metal-metal charge transfer transition between the tungsten carbonyl fragment and the ferrocenyl/biferrocenyl group in the corresponding oxidized states, which can be described as class II systems according to Robin and Day. This assignment was verified by infrared spectroelectrochemical studies. The electrochemical investigations are supported by DFT calculations. The structural properties of 1 -4 in the solid state were investigated by single-crystal Xray diffraction studies showing no substituent effects on bond lengths and angles. The biferrocenyl derivatives exhibit synconformation of the ferrocenyl and carbene building blocks.

(Spectro)electrochemical investigations on (ferrocenyl)thiophenes modified by tungsten Fischer carbenes

“…This fact suggests only a very weak interaction between the Fischer carbene increments. Similar findings were made for ferrocenediyl and biferrocenediyl biscarbene complexes recently [5]. The tungsten carbonyl oxidation electrode reactions of 5 were detected in the same potential range as observed for ferrocenyl thiophene 4 (Table 2, Fig.…”

“…1). Similar observations were made for other Fischer carbene complexes previously [5][6][7]. Furthermore, a second tungsten carbene moiety on the thiophene system (2) results in a more anodic, irreversible tungsten oxidation process starting from E pa = 850 mV (Table 2 and Fig.…”

“…Further increasing the potential leads to a decrease of the latter bands and ν(CO) stretching frequencies at 2075 cm -1 and 1953 cm -1 were for 4 2+ . Within this regard, the shift in ν(CO) stretching frequencies upon generation of 4 2+ is comparable with the corresponding behavior of the ferrocenyl tungsten Fischer carbene complex, (CO) 5 W=C(OMe)Fc, itself during the Fc/Fc + redox process [5]. Thus, the interaction between the ferrocenyl unit and the Fischer carbene moiety should be very similar in both cases.…”

“…In case of the biscarbene 5, the first ferrocenyl based redox event was observed at E 0' = 230 mV, approximately 220 mV more anodic as detected for the corresponding 4/4 + redox event, but even slightly more cathodic as for the previously described ferrocenyl tungsten Fischer carbene system [5]. The redox potential of the 5 + /5 2+ process was determined to E 0' = 475 mV vs FcH/FcH + .…”

“…Combining the properties of Fischer carbenes with π-conjugated bridges could result in the design of new push-pull systems with novel non-linear optical (NLO) character. In our recent papers we paid attention to metal-metal interactions in Fischer carbenes on ferrocenyl and biferrocenyl tungsten alkylidene complexes [5]. UV-Vis-NIR spectroelectrochemical investigations revealed a high energy NIR absorption which was attributed to a metal-metal charge transfer transition (MMCT) between the tungsten and ferrocenyl/biferrocenyl moieties.…”