Synthesis, coordination and catalytic application of hybrid phosphinoferrocene ligands bearing imidazole N-donor groups

Abstract

Hybrid P,N-donors are versatile ligands in coordination chemistry and catalysis, capable of forming hemilabile coordination bonds. This article describes the preparation of three structurally related hybrid phosphinoferrocene donors bearing 1-methyl-1H-imidazol-2-yl pendants attached via CH(OH), C=O, and CH2 linkers at position 1' of the ferrocene unit, Ph2PfcYImMe (fc = ferrocene-1,1'-diyl, ImMe = 1-methyl-1H-imidazol-2-yl, and Y = the linker). Coordination behaviour of these compounds towards soft Pd(II) strongly depends on the nature of the linker: the flexible nonconjugated linkers (CH(OH) and CH2) enable P,N-chelate coordination, whereas a similar coordination of the ketone appears to be hindered by the linker’s tendency to remain conjugated and by its ability to decrease electron density at the imidazole unit. As a result, complexes of types [PdCl2(L-κ2P,N)] and [Pd(L-κ2P,N)2][BF4]2 could be obtained from the former compounds, whereas the C=O-bridged ligand afforded the bis-phosphine complex [PdCl2(L-κP)2] as the only isolable product. The chelate complexes [PdCl2(Ph2PfcCH(X)ImMe-κ2P,N)] (X = H and OH) were evaluated in Pd-catalysed Suzuki-Miyaura cross-coupling of 2-bromopyridine with 4-tolylboronic acid, and their catalytic performance was compared with the related compounds, including the widely studied complex [PdCl2(dppf-κ2P,P')] (dppf = 1,1ʹ-bis(diphenylphosphino)ferrocene).