Aryl imido complexes of the group 4 metals: Structural aspects and mechanistic study of formation

Abstract The addition of aniline (PhNH2 ⩾ 2 equivalents) to the organometallic compounds [Ti(OC6H3Pri2-2,6)2(η2-ButNCCH2Ph)(CH2Ph)], [Ti(OC6H3Ph2-2,6)2(C4Et4)] and [Zr(OC6H3But2-2,6)2(CH3)2] in hydrocarbon solvents leads to the formation of the mononuclear bis(phenylamido) derivatives [M(OAr)2(NHPh)2] [M = Ti, OAr = OC6H3Pri2-2,6 (1); M = Ti, OAr = OC6H3Ph2-2,6 (2); M = Zr, OAr = OC6H3But2-2,6 (3)]. Treatment of [Hf(CH2Ph)4] first with PhNH2 (4 equivalents), followed by HOC6H3But2-2,6 (2 equivalents), leads to the related bis(phenylamido) compound 4 (M = Hf; OAr = OC6H3But2-2,6). The two homoleptic aryl amido compounds [M(NHC6H3Pri2-2,6)4] [M = Zr (5); Hf (6)] have also been obtained by addition of 2,6-diisopropylaniline to the tetra-benzyl compounds [M(CH2Ph)4] (M = Zr, Hf). The addition of 4-pyrrolidinopyridine (py′) to all of the aryl amido compounds except 4 leads to elimination of 1 equivalent of aryl amine and the formation of a series of five-coordinate aryl amido derivatives of the general formula [M(OAr)2(NPh)(py′)2] [M = Ti, OAr = OC6H3Pri2-2,6 (7); M = Ti, OAr = OC6H3Ph2-2,6 (8); M = Zr, OAr = OC6H3But2-2,6 (9); and [M(NHAr)2(NAr)(py′)2]; M = Zr, Ar = C6H3Pri2-2,6 (10); M = Hf, Ar = C6H3Pri2-2,6 (11)]. In the case of the hafnium bis(phenylamido) complex 4, addition of 4-pyrrolidinopyridine resulted in the formation of a simple adduct. [Hf(OC6H3But2-2,6)2(NHPh)2(py′)], [4·py′]. (A similar adduct, [4·py′], was detected in the conversion of 3–9.) No elimination of aniline from [4·py′] and formation of a phenylimido derivative were observed. Both 2,2′-bipyridine and 1,10-phenanthroline were found to eliminate aniline from compounds 1–3 to produce insoluble products. Addition of 3,4,7,8-tetramethyl-1,10-phenanthroline to 2, however, yielded a soluble phenylimido derivative (12). The four-coordinate aryl amido compounds 2, 3 and 5 were found to be pseudo-tetrahedral in the solid state, while the five-coordinate aryl imido compounds 7, 9, 10 and 11 are best described as distorted trigonal-bipyramidal with trans-axial pyridine ligands. In the phenanthroline derivative 12 a distorted trigonal-bipyramidal geometry exists about the titanium atom with an aryloxide oxygen atom trans to a phenanthroline nitrogen. The bonding of the aryl imido, aryl amido and pyridine groups is described. A detailed study of the reaction of a series of bis(aryl amido) complexes, [Zr(OAr)2(NHC6H4-4X)2] (3)X (X = H, F, CH3, OMe, Br), with a variety of pyridine ligands was undertaken. The mono-pyridine adduct [3X·py] is rapidly formed, followed by the slow formation of the corresponding aryl imido complex [Zr(OAr)2(NHC6H4-4X)(py)2] [9X] and an equivalent of substituted aniline. Equilibrium constants for the reaction [3Xpy] + py = [9X] + ArNH2 were measured. Formation of the aryl imido ligand was found to be favoured by a more basic pyridine ligand and by electron-withdrawing substituents on the aryl ring of the initial aryl amido group. The rate of attainment of the equilibrium situation from [3] and py was investigated and various pathways for the reaction are considered. X-ray crystal structures were determined for 2, 3, 5, 7, 9, 10, 11 and 12.