Differing reactivities of zirconium and titanium alkoxides with phenyl isocyanate: an experimental and computational study

Abstract

The insertion reaction of 1 and 2 equiv of PhNCO (2) into Ti(OⁿBu)₄ (1aTi) and Zr(OⁿBu)₄ (1aZr) results in the formation of a carbamate ligand (PhNCOOⁿBu) referred to as a monoinsertion ligand (MIL). Reaction of metal n-butoxide (1a) with more than 2 equiv of PhNCO leads to the competitive insertion of PhNCO into the M-N bond of metal carbamate or into the M-O bond of the alkoxide. The ligand formed by the insertion of PhNCO into a metal carbamate is an allophanate ligand (PhNCONPhCOOⁿBu), referred to as a double insertion ligand (DIL). The complexes containing these ligands were hydrolyzed, and the organic products 5 (from MIL) and 6 (from DIL) were spectroscopically characterized. The reactivity of Ti(OⁿBu)₄ is very similar to the reactivity of Ti(OⁱPr)₄ reported earlier by us. The formation of DIL over MIL is clearly favored in the case of Ti(OnBu)4, whereas the formation of DIL is less favored with Zr(OⁿBu)₄. At -80 °C, Zr(OⁿBu)₄ gives only MIL, and so only 5 is isolated on hydrolysis. In contrast, Ti(OⁿBu)₄ initially produces DIL and then it slowly decays to MIL, and both 5 and 6 are isolated on hydrolysis. The unusual preference for monoinsertion or double insertion has been probed computationally using model complexes Ti(OMe)₄ (1bTi) and Zr(OMe)₄ (1bZr) at the B3LYP/LANL2DZ level of theory. The relative thermodynamic stabilities of the intermediate model metal complexes (10b, 11b, 12b, and 13b) are indicative of the diverse behavior shown by Ti and Zr. The barrier for the formation of 11bZr is assigned to a high-energy intermediate 10bZr, which results in the reduced preference for DIL formation. The smaller size of Ti precludes higher coordination numbers; thus the heptacoordinated intermediate 12bTi is higher in energy compared to 12bZr. This facilitates the formation of MIL in the case of Zr(OⁿBu)₄.