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

Abstract

The cross-conjugated ethynyl–vinylidene [Ph₂CC(CCH){C(H)CRu(PPh₃)₂Cp}]PF₆ ([4a]PF₆), and [FcC(H)C(CCH){C(H)CRu(PPh₃)₂Cp}]PF₆ ([4b]PF₆), and ethynyl–alkynyl Ph₂CC(CCH){CCRu(PPh₃)₂Cp} (5a), and FcC(H)C(CCH){CCRu(PPh₃)₂Cp} (5b) compounds (Cp = η⁵-cyclopentadienyl) have been prepared from reactions of the known 3-methylene-penta-1,4-diynes Ph₂CC(CCH)₂ (3a) and [FcCHC(CCH)₂] (3b) with [RuCl(PPh₃)₂Cp]. 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 FcCHC(CCAuPPh₃)₂ (7) with RuCl(PPh₃)₂Cp were unsuccessful. The related bis- and tris(ferrocenyl) derivatives Ph₂CC(CCFc)₂ (6a) and FcCHC(CCFc)₂ (6b) were more readily obtained from Pd(II)/Cu(I) catalysed cross-coupling reactions of FcCCH with the 1,1-dibromo vinyl complexes PhCCBr₂ (1a) and FcC(H)CBr₂ (1b). 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-Bu₄N[B{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⁺ (n = 1, 2) are best described as Class I mixed-valence compounds.