Migratory Aptitude of the Zr-C Functionalities Bonded to a Macrocyclic Structure: Thermally- and Solvent- Assisted Intra and Intermolecular Migrations in Dialkyl(bibenzotetramethyltetraazaannulene)zirconium(IV)

The Zr-C bond chemistry and, particularly, its migration properties have been investigated where the metal is bonded to the dibenzotetramethyltetraaza[14]annulene (tmtaa) dianion in the model complexes cis-[Zr(tmtaa)R(2)]. In this respect the [Zr(tmtaa)] fragment has been analyzed in comparison with the well-known [cp(2)Zr] moiety, with the support of an extended Huckel analysis. The synthesis of [Zr(tmtaa)R(2)] (R = Me, 2; R = CH(2)Ph, 3) from [Zr(tmtaa)Cl-2] (1) has been achieved under highly controlled reaction conditions. Complexes 2 and 3 are thermally labile, and one of the alkyl groups undergoes a thermally-induced migration to one of the imino groups, leading to a trianionic ligand [Zr(R-tmtaa)R] (R = Me, 4; R = CH(2)Ph, 5). The nature of such complexes has been elucidated by an X-ray analysis on the THF-solvated form of 4, [Zr(Me-tmtaa)(Me)(THF)] (6). Nucleophiles are even more effective in inducing the alkyl migration to the ligand. In the presence of THF or pyridine 5 may be easily converted to [Zr(R(2)-tmtaa)] (R = CH(2)Ph, 7), in which both alkyl groups have migrated to the imino groups of the ligand. The use of an excess of LiMe during the alkylation of 1 led to the formation of [Zr(Me(2)-tmtaa)...Li(THF)(2)] (8) via the intermediacy of 4. Complex 8 is a bifunctional carrier of Lih le. The general consequences of two electrophilic sites on the ligand, eventually, in competition with the electrophilic metal have been analyzed. The net charge and electrophilic index are significantly higher for the metal in [Cp(2)Zr](2+) than in [Zr(tmtaa)](2+). The relatively higher thermal inertness of 3 allowed us to study the migration of the alkyl group toward the unsaturated incoming nucleophiles, Bu(t)NC and CO. The reaction of 3 with Bu(t)NC led to the formation of the bis(eta(2)-iminoacyl) compound [Zr(tmtaa)(eta(2)-C(Me)=NBu(t))(2)]. The four low-lying d orbitals can accommodate the two eta(2)-iminoacyl groups. The reaction of 3 with CO gave [Zr(tmtaa)(eta(2)-OC(CH(2)Ph)(2))] (11) via the intermediacy of a very reactive oxycarbene eta(2)-acyl compound and ultimately gave 12 and 13, illustrating the reactivity of both zirconium and the iminato carbons in the [Zr(tmtaa)] fragment. The crystallographic details are as follows: 3 is orthorhombic, space group Pnma, a = 16.028(3) Angstrom, b = 20.666(4) Angstrom, c = 9.018(2) Angstrom, alpha = beta = gamma = 90 degrees, Z = 4, and R = 0.063. 6 is monoclinic, space group P2(1)/n, a = 19.859(5) Angstrom, b = 14.771(5) Angstrom, c = 9.072(4) Angstrom, alpha = gamma = 90 degrees, beta = 93.85(3)degrees, Z = 4, and R = 0.034. 7 is monoclinic, space group C2/c, a = 23.608(6) Angstrom, b = 9.529(3) Angstrom, c = 19.050(4) Angstrom, alpha = gamma = 90 degrees, beta = 99.30(2)degrees, Z = 4, and R = 0.046. 10 is triclinic, space group P (1) over bar, a = 12.800(4) A, b = 15.795(5)Angstrom, c = 11.758(1) Angstrom, alpha = 91.8(2)degrees, beta = 93.56(1)degrees, gamma = 97.81(2)degrees, Z = 2, and R = 0.042.