1,2-Bis(di-<i>tert</i>-butylphosphino)imidazole (<b>dtbpi</b>): A Versatile Imidazole-Based, Rigid, Bulky Bisphosphine Ligand for Transition Metals

A bulky bisphosphine, 1,2-bis­(di-tert-butylphosphino)­imidazole (dtbpi), featuring a rigid imidazole backbone, was synthesized in a simple two-step procedure. The ligand’s coordination chemistry and electron-donating abilities were investigated in the complexes (dtbpi)­Ni­(FDN) (3) and [(dtbpi)­Rh­(CO)2]­BF4 (4). Protonation of 4 at the ligand backbone’s nitrogen with strong Brønsted acids such as HBF4 and HBArF gave the dicationic dicarbonyl complexes [(H-dtbpi+)­Rh­(CO)2]­(BF4)­(BArF) (5a) and [(H-dtbpi+)­Rh­(CO)2]­(BF4)2 (5b), which served as model complexes of the ligand’s imidazolium-phosphine form and permitted characterization of its donating character. In order to evaluate solely the different electronic characters of the two isosteric P donors, the complexes (dtbpi)­PdCl­(Me) (6) and [(dtbpi)­RhCl]2 (7) were synthesized. An unexpected rearrangement of one of the formed stereoisomers of 6, with cleavage of an N–P bond, was found. A novel phosphine-phosphonium ylide ligand (8) has been made accessible via chemoselective alkylation of the carbon-bound P donor of dtbpi to yield the phosphonium salt [P-CH3-dtbpi]­BF4 (2). The ylide 8 was generated in situ by deprotonation of 2 with KHMDS. Its coordination to the PdCl­(Me) fragment led to the isolation of exclusively the Z isomer of [(8)­PdCl­(Me)] (Z-9). X-ray structure analysis of [(dtbpi)­Rh­(NBD)]­BF4 (10) and its dicationic analogue [(H-dtbpi+)­Rh­(NBD)]­(BF4)­(BArF) (11) revealed the structural changes provoked upon ligand backbone protonation.