Insertion reactions of alkynes and organic isocyanides into the palladium–carbon bond of dimetallic Fe–Pd alkoxysilyl complexes

Abstract

Insertion of MeO₂C–CC–CO₂Me (DMAD) into the Pd–C bond of the heterodimetallic complex [(OC)₃Fe{μ-Si(OMe)₂(OMe)}(μ-dppm)Pd(dmba-C)] (2) (dppm = Ph₂PCH₂PPh₂, dmba-C = metallated dimethylbenzylamine) and [(OC)₃{(MeO)₃Si}Fe(μ-dppm)Pd(8-mq-C,N)] (3) (8-mq-C,N = cyclometallated 8-methylquinoline) yielded the σ-alkenyl complexes [(OC)₃Fe{μ-Si(OMe)₂(OMe)}(μ-dppm)Pd{C(CO₂Me)C(CO₂Me)(o-C₆H₄CH₂NMe₂)}] (7) and [(OC)₃Fe{μ-Si(OMe)₂(OMe)}(μ-dppm)Pd{C(CO₂Me)C(CO₂Me)(CH₂C₉H₆N)}] (8), respectively. The latter afforded the adduct [(OC)₃{(MeO)₃Si}Fe(μ-dppm)Pd{C(CO₂Me)C(CO₂Me)(CH₂C₉H₆N)}(CNBu^t)] (9) upon reaction with 1 equiv. of Bu^tNC. The heterodinuclear σ-butadienyl complexes [(OC)₃Fe{μ-Si(OMe)₂(OMe)}(μ-dppm)Pd{C(Ph)C(Ph)C(CO₂Me)(CO₂Me)(o-C₆H₄CH₂NMe₂)}] (11) and [(OC)₃Fe{μ-Si(OMe)₂(OMe)}(μ-dppm)Pd{C(Ph)C(CO₂Et)C(Ph)C(CO₂Et)(CH₂C₉H₆N)}] (13) have been obtained by reaction of the metallate K[Fe{Si(OMe)₃}(CO)₃(dppm-P)] (dppm = Ph₂PCH₂PPh₂) with [PdCl{C(Ph)C(Ph)C(CO₂Me)C(CO₂Me)(o-C₆H₄CH₂NMe₂)}] or [PdCl{C(Ph)C(CO₂Et)C(Ph)(CO₂Et)}(CH₂C₉H₆N)], respectively. Monoinsertion of various organic isocyanides RNC into the Pd–C bond of 2 and 3 afforded the corresponding heterometallic iminoacyl complexes. In the case of complexes [(OC)₃{(MeO)₃Si}Fe(μ-dppm)Pd{C(NR)(CH₂C₉H₆N)}] (15a R = Ph, 15b R = xylyl), a static six-membered C,N chelate is formed at the Pd centre, in contrast to the situation in [(OC)₃Fe{μ-Si(OMe)₂(OMe)}(μ-dppm)Pd{C(NR)(o-C₆H₄CH₂NMe₂)}] (14a R = o-anisyl, 14b R = 2,6-xylyl) where formation of a µ-η²-Si–O bridge is preferred over NMe₂ coordination. The outcome of the reaction of the dimetallic alkyl complex [(OC)₃Fe{μ-Si(OMe)₂(OMe)}(μ-dppm)PdMe] with RNC depends both on the stoichiometry and the electronic donor properties of the isocyanide employed for the migratory insertion process. In the case of o-anisylisocyanide, the iminoacyl complex [(OC)₃Fe{μ-Si(OMe)₂(OMe)}(μ-dppm)Pd{C(N-o-anisyl)Me}] (16) results from the reaction in a 1 : 1 ratio. Addition of three equiv. of o-anisylisocyanide affords the tris(insertion) product [(OC)₃Fe{μ-Si(OMe)₂(OMe)}(μ-dppm)Pd{[C(N-o-anisyl)]₃Me}] (18). After addition of a fourth equivalent of o-anisylNC, exclusive formation of the isocyanide adduct [(OC)₃{(MeO)₃Si}Fe(μ-dppm)Pd{[C(N-o-anisyl)]₃Me}(CN-o-anisyl)] (19) was spectroscopically evidenced. In the complex [(OC)₃Fe{μ-Si(OMe)₂(OMe)}(μ-dppm)Pd{[C(N-o-C₆H₄COCH₂)]₂Me}] (20), the σ-bound diazabutadienyl unit is part of a 12-membered organic macrocyle which results from bis(insertion) of 1,2-bis(2-isocyanophenoxy)ethane into the Pd–Me bond of the precursor complex [(OC)₃Fe{μ-Si(OMe)₂(OMe)}(μ-dppm)PdMe]. In contrast, addition of two equivalents of tert-butylisocyanide to a solution of the latter afforded [(OC)₃{(MeO)₃Si}Fe(μ-dppm)Pd{C(NBu^t)Me}(CNBu^t)] (21) in which both a terminal and an inserted isocyanide ligand are coordinated to the Pd centre. In all cases, there was no evidence for competing CO substitution at the Fe(CO)₃ fragment by RNC. The molecular structures of the insertion products 8·CH₂Cl₂ and 16·CH₂Cl₂ have been determined by X-ray diffraction.