Structural Trends in Group 4 Metal Tetraaza Macrocycle Complexes. Molecular Structures of (Me₄taen)Zr(OtBu)₂ and (Me₄taen)Hf(NMe₂)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)ZrCl 2 ( 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 π-component in the M−X bonds favor trigonal prismatic structures for 1 and 2, whereas M−X π-bonding in 3 − 5 promotes distortion toward octahedral structures. Compound 4, C 22 H 40 N 4 O 2 Zr, crystallizes in the triclinic space group P1̄ with a = 9.6080(8) Å, b = 9.7407(8) Å, c = 14.5916(12) Å, α = 92.714(1)°, β = 107.808(1)° γ = 99.810(1)°, and Z = 2. Compound 5, C 18 H 34 N 6 Hf·0.5(C 7 H 8 ), crystallizes in the triclinic space group P1̄ with a = 9.102(3) Å, b = 11.242(4) Å, c = 13.478(5) Å, α = 108.672(5)°, β = 96.310(5)°, γ = 106.266(6)°, and Z = 2.