Group IV Complexes with Sterically Congested -Aryl-adamantylcarbamidate Ligands.

Metalation of -(2,6-dibenzhydryl-4-tolyl)adamantane-1-carboxamide (, ArN(H)-C(O)-Ad) with M(NMe) (M = Ti, Zr, Hf) yields amidate complexes ArN=C(Ad)-O-Ti(NMe) () as well as bis(amidate) compounds (Ar*N=C(Ad)-O)M(NMe) (M = Zr (), Hf ()). In , the amidate ligand acts as a monodentate base via the oxygen atom with the Ti center in a slightly distorted tetrahedral environment. The steric requirement of the amidate ligand stabilizes the small coordination number of four of the Ti atom. In congeners and , two bidentate amidate ligands exist in the coordination spheres, leading to hexacoordinate group IV metal atoms. The small bite angles of the Zr- and Hf-bound amidate ligands lead to severe distortion of the octahedral environments of the Zr and Hf centers. Titanium compound is an unsuitable choice to catalyze hydrofunctionalization of alkynes with amines and phosphane oxides and despite the significantly smaller p value of the carboxylic amide, formation of carboxamide is the dominant reaction upon addition of amines or phosphane oxides to release intramolecular steric strain introduced by the very bulky adamantylamidato ligand.