Synthesis and Structure of Novel Iridium(I) Complexes Containing η4-1,6-Diene and Diphosphine Ligands: Remarkable Effect of Ligand Bite Angle upon Ligand Dissociation

Abstract

The treatment of [IrCl(cod)]2 with a bidentate diphosphine ligand, bis(diphenylphosphino)methane (dppm), and 1,6-dienes such as dimethyl 2,2-diallylmalonate (1) and diallyl ether (2) produces a ligand replacement reaction to afford novel iridium(I) complexes bearing η4-1,6-diene ligands, IrCl(dppm)[η4-CH2CHCH2C(CO2Me)2CH2CHCH2] (3) and IrCl(dppm)(η4-CH2CHCH2OCH2CHCH2) (4), respectively, in excellent yields. The molecular structure of the diallyl ether complex 4, which has been unequivocally determined by a single-crystal X-ray diffraction study, discloses that 2 coordinates to the iridium metal in an η4 fashion with a distorted-trigonal-bipyramidal geometry. In a similar manner, 1,2-bis(diphenylphosphino)ethane (dppe) is also a tolerant diphosphine ligand, and the corresponding η4-1,6-diene complexes IrCl(dppe)[η4-CH2CHCH2C(CO2Me)2CH2CHCH2] (5) and IrCl(dppe)(η4-CH2CHCH2OCH2CHCH2) (6) have been isolated in high yields. On the other hand, diphosphine ligands with a carbon linkage longer than that in dppe, such as 1,3-bis(diphenylphosphino)propane (dppp) and 1,4-bis(diphenylphosphino)butane (dppb), never activate the cod ligand, resulting in no exchange with the 1,6-dienes. Instead of the expected η4-1,6-diene complexes, IrCl(cod)(dppp) (7) and IrCl(cod)(dppb) (8) are exclusively formed. The molecular structure of 8 is also confirmed by an X-ray crystallographic analysis. The coordination geometry of 8 is consistent with a distorted square pyramid. The reactivity toward the ligand displacement depends on the natural bite angle of the bidentate phosphine ligands.