1,4-DIAZA-1,3-DIENE COMPOUNDS OF EARLY TR ANSITION-METALS - SIGMA-DONOR AND PI-ACCEPTOR COMPLEXES OF ZIRCONIUM AND HAFNIUM WITH PERPHENYL-SUBSTITUTED DAD LIGANDS - A COMPARISON OF STRUCTURES

Reaction of 1,4-Diaza-1,3-dienes (DAD) with the Lewis-acid ZrCl4 affor ds sigma-donor-complexes of composition ZrCl4 . DAD. The X-ray analysi s of the ZrCl4 . [N(C6H4-4-OMe) = C(Ph) = N(C6H4-4-OMe)] (2c) (space g roup P1 $($) over bar$$, triclinic; a = 11.339(2), b = 11.845(2), c = 12.415(3) Angstrom, alpha = 107.94(2), beta = 107.26(2), gamma = 104.7 3(2)degrees, Z = 2, R1 = 0.0266) shows that both N-atoms of the s-cis- configurated DAD-ligand occupy two corners of a distorted octahedron. There is only slight difference between the C = N bond lengths of the coordinated and noncoordinated ligand. In the homoleptic DAD-complexes of the type M(DAD)(3) M[N(R)- C(Ph)- C(Ph)- N(R)](3) [M = Zr (4b), Hf (5b), R = C6H4-4-Me] the DAD-ligands more act as pi-acceptor ligands. X-ray analysis shows that the complexes M[N(R)- C(Ph)- C(Ph)- N(R)](3 ) [M = Zr (4b), Hf (5b), R = C6H4-4-Me] have the identical structure m otive and crystallize in the triclinic space group P1 $($) over bar$$ (4b: a = 14.904(1), b = 15.451(2), c = 19.584(4) Angstrom, alpha = 112 .08(1), beta = 94.36(1), gamma = 97.60(1)degrees, Z = 2, R1 = 0.0911; 5 b a =14.798(2), b = 18.226(2), c = 22.902(2) Angstrom, alpha = 71.62 (1), beta 72.38(1), gamma = 87.27(1)degrees, Z = 2, R1 = 0.0644). The six N-atoms form a distorted octahedron in both complexes. The planari ty of the five-membered M-N-C-C-N rings and the almost similar C = N a nd C-C bond lengths are typical of the pi-acceptor function of the dia zadienes in 4 b and 5 b. The steric hindrance in 4 b and 5 b results i n a dynamical behavior and a asymmetrical distortion at low temperatur es as was observed by n.m.r..