Structural trends in group 4 metal tetraaza macrocycle complexes. Molecular structures of (Me(4)taen)Zr(O(t)Bu)(2) and (Me(4)taen)Hf(NMe(2))(2)

The tetraaza macrocycle complexes (Me(4)taen)Zr(O(t)Bu)(2) (4) and (Me(4)taen)Hf(NMe(2))(2) (5) have been prepared and characterized by X-ray crystallography. Both species adopt cis structures with distorted octahedral metal geometries. These structures are similar to that observed earlier for (Me(4)taen)Zr(NMe(2))(2) (3) but quite different from the trigonal prismatic structures observed for (Me(4)taen)ZrCl2 (1) and (Me(4)taen)Zr(CH(2)Ph)(2) (2). The structures of these d(0) (Me(4)taen)MX(2) species are rationalized on electronic grounds. The conformational properties of the Me(4)taen(2-) macrocycle and the covalent character of and absence of a strong pi-component in the M-X bonds favor trigonal prismatic structures for 1 and 2, whereas M-X pi-bonding in 3-5 promotes distortion toward octahedral structures. Compound 4, C22H40N4O2Zr, crystallizes in the triclinic space group <P(1)over bar> with a = 9.6080(8) Angstrom, b = 9.7407(8) Angstrom, c = 14.5916(12) Angstrom, alpha = 92.714(1)degrees, beta = 107.808(1)degrees gamma = 99.810(1)degrees, and Z = 2. Compound 5, C18H34N6Hf . 0.5(C7H8), crystallizes in the triclinic space group <P(1)over bar> with a = 9.102(3) Angstrom, b = 11.242(4) Angstrom, c = 13.478(5) Angstrom, alpha = 108.672(5)degrees, beta = 96.310(5)degrees, gamma = 106.266(6)degrees, and Z = 2.