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

Metalation of N -(2,6-dibenzhydryl-4-tolyl)adamantane-1-carboxamide ( 1 , Ar*N(H)-C(O)-Ad) with M(NMe 2 ) 4 (M = Ti, Zr, Hf) yields amidate complexes Ar*N=C(Ad)-O-Ti(NMe 2 ) 3 ( 2 ) as well as bis(amidate) compounds (Ar*N=C(Ad)-O) 2 M(NMe 2 ) 2 (M = Zr ( 3 ), Hf ( 4 )). In 2 , 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 3 and 4 , 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 2 is an unsuitable choice to catalyze hydrofunctionalization of alkynes with amines and phosphane oxides and despite the significantly smaller p K a value of the carboxylic amide, formation of carboxamide 1 is the dominant reaction upon addition of amines or phosphane oxides to release intramolecular steric strain introduced by the very bulky adamantylamidato ligand.