Synthesis of some very bulky N,N′-disubstituted amidines and initial studies of their coordination chemistry

\N,N'-Bis(2,6-diisopropylphenyl)-4-toluamidine, -4-anisylamidine and -acetamidine have been prepared for the first time from 2,6-diisopropylaniline and the acid chlorides via the: corresponding imidoyl chlorides. The crystal structures of all three amidines were determined, indicating that the first is a disordered mixture of Z-anti and E-syn tautomeric forms, the second Z-anti, and the third E-anti in the solid state. Despite these differences, all three form identical coordination complexes with Mo(CO), in which the ligand is in the Z-anti geometry, and the metal is pi-coordinated to the imino 2,6-diisopropylphenyl ring, and the amino N-H unit is directed towards metal. The coordination mode was confirmed by single-crystal X-ray structure determination of the toluamidine and methylamidine complexes with Mo(CO),. In the case only of the methylamidine, an isolable intermediate is first formed in which the neutral amidine is coordinated in a monodentate fashion to an Mo(CO)(5) unit. The crystal structure of this complex shows that the ligand is in the E-anti geometry, with the imino nitrogen coordinated to Mo, d(Mo-N) = 2.352(2) Angstrom. The structures are closely related in that the initial Mo(CO)(5) N-coordination sets up the metal for conversion to the more thermodynamically stable pi-coordinated Mo(CO)(3) complex. The high steric bulk of these superamidine ligands is seen in the failure of any of them to form the metal-metal bonded Mo-2(amidinate)(4) complexes typically prepared using common, less bulky, amidines.