Sandwich and half-sandwich metal complexes derived from cross-conjugated 3-methylene-penta-1,4-diynes

The cross-conjugated ethynyl-vinylidene (Ph2C=C(C CH){C(H)=CRu(PPh3)(2)Cp)IPF6 ([4a]PF6), and [FcC(H)=C(C CH){C(H)=CRu(PPh3)(2)Cp)]PF6 ([4b]PF6), and ethynyl-alkynyl (PPh3)(2)Cp) (5a), and FcC(H)=C(C CH){C=CRu(PPh3)(2)Cp) (5b) compounds (Cp = eta(5)-cyclopentadieny0 have been prepared from reactions of the known 3-methylene-penta-1,4-diynes Ph2C=C(C=CH)(2) (3a) and [FcCH=C(C CH)(2)] (3b) with [RuCl(PPh3)(2)Cp]. The compounds derived from 3b incorporating the more electron-rich alkene proved to be unstable during work-up, and attempts to prepare bis(ruthenium) complexes from 3a and 3b or from transmetallation reactions of the bis(alkynylgold) complex FcCH--C (C CAuPPh3)(2) (7) with RuCt(PPh3)(2)Cp were unsuccessful. The related bis- and tris(ferrocenyl) derivatives Ph2C=C(C CFc)(2) (6a) and FcCH=C(C CFc)(2) (6b) were more readily obtained from Pd(ii)/Cu(i) catalysed cross-coupling reactions of FcC CH with the 11-clibromo vinyl complexes PhC=CBr2 (la) and FcC(H)=CBrz (lb). Cyclic voltammetry of 6a and 6b using n-Bu4N(PF6] as the supporting electrolyte shows broad, overlapping waves arising from the sequential oxidation of the ferrocenyl moieties in electronically and chemically similar environments. Electrostatic effects between the ferrocenyl moieties are enhanced in solutions of the weakly ion-pairing electrolyte n-Bu4NIB{C6H3(CF3)(2)-3,5)(4)], leading to better resolution of the individual electrochemical processes. The comparative IR spectroelectrochemical response of 6a and 6b suggest the vinyl ferrocene moiety in 6b undergoes oxidation before the ethynyl ferrocene fragments. There is no evidence of electronic coupling between the metallocene moieties and [6a](+), [6b](+) (n = 1, 2) are best described as Class I mixed-valence compounds.