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

Abstract

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)₃ in which the ligand is in the Z-anti geometry, and the metal is π-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)₃. 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) Å. 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 π-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 Mo₂(amidinate)₄ complexes typically prepared using common, less bulky, amidines.