NHC–manganese(i) complexes as carbene transfer agents

Abstract

Tautomerization of coordinated azoles to their corresponding N-heterocyclic carbenes (NHCs) has been carried out by reaction of complexes fac-[Mn(L)(CO)₃(dppe)]⁺ (L = N-phenylimidazole) and fac-[Mn(L)(CO)₃(bipy)]⁺ (L = N-methylbenzimidazole, benzoxazole, benzothiazole) with KO^tBu and subsequent protonation of the azolyl intermediates with NH₄PF₆. Several NHC–manganese(I) complexes bearing an N–H residue of general formula fac-[Mn(NHC)(CO)₃(dppe)]⁺ and fac-[Mn(NHC)(CO)₃(bipy)]⁺ have been tested as carbene transfer agents to the gold fragments [Au(L)]⁺ (L = PPh₃, CNPh, CNXylyl), allowing isolation or spectroscopic detection of various Mn(I)/Au(I) heterometallic intermediates containing azolyl bridging ligands, which liberate the gold(I) carbene complexes [Au(NHC)(L)]⁺ by means of acid hydrolysis. By contrast, when using the silver(I) fragment [Ag(PPh₃)]⁺ as carbene acceptor no transmetallation process occurred but instead inverse tautomerization of the NHC to the corresponding imidazole ligand was observed.