Migratory aptitude of the Zr-C functionalities bonded to a macrocyclic structure : thermally- and solvent-assisted intra- and intermolecular migrations in dialkyl(dibenzotetramethyltetraazaannulene)zirconium(IV)

The Zr-C bond chem. and, particularly, its migration properties were studied where the metal is bonded to the dibenzotetramethyltetraaza[14]annulene (tmtaa) dianion in the model complexes cis-[Zr(tmtaa)R2]. In this respect the [Zr(tmtaa)] fragment was analyzed in comparison with the known [Cp2Zr] moiety, with the support of an extended Hueckel anal. The synthesis of [Zr(tmtaa)R2] (R = Me, 2; R = CH2Ph, 3) (shown as structure I) from [Zr(tmtaa)Cl2] (1) was 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 = CH2Ph, 5). The nature of such complexes was elucidated by an x-ray anal. 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(R2-tmtaa)] (R = CH2Ph, 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 gave [Zr(Me2-tmtaa)¬?¬?¬?Li(THF)2] (8) via the intermediacy of 4. Complex 8 is a bifunctional carrier of LiMe. The general consequences of two electrophilic sites on the ligand, eventually, in competition with the electrophilic metal were analyzed. The net charge and electrophilic index are significantly higher for the metal in [Cp2Zr]2+ than in [Zr(tmtaa)]2+. The relatively higher thermal inertness of 3 allowed the authors to study the migration of the alkyl group toward the unsatd. incoming nucleophiles, ButNC and CO. The reaction of 3 with ButNC gave the bis(??2-iminoacyl) compd. [Zr(tmtaa)(??2-C(Me):NBut)2]. The four low-lying d orbitals can accommodate the two ??2-iminoacyl groups. The reaction of 3 with Bu'NC led to the formation of the bis(y2-iminoacyl) compound [Zr(tmtaa)(v2-C(Me)=NBut)2T]h.e four low-lying d orbitals can accommodate the two y2-iminoacyl groups. The reaction of 3 with CO gave [Zr(tmtaa)(yz-OC(CH2Ph)~)(]1 1) via the intermediacy of a very reactive oxycarbene $-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) A, b = 20.666(4) A, c = 9.018(2) A, a = p = y = 90°, Z = 4, and R = 0.063. 6 is monoclinic, space group P2l/n, a = 19.859(5) A, b = 14.771(5) A, c = 9.072(4) A, a = y = 90°, /3 = 93.85(3)", Z = 4, and R = 0.044. 7 is monoclinic, space group C2/c, a = 23.608(6) A, b = 9.529(3) A, c = 19.050(4) A, a = y = 90", p = 99.30(2)", Z = 4, and R = 0.046. 10 is triclinic, space group P i , a = 12.800(4) A, b = 15.795(5) A, c = 11.758(1) A, a = 91.8(2)", p = 93.56(1)", y = 97.81(2)", Z = 2, and R = 0.042.