Reactions of cyclopentadienyl-amidinate titanium hydrazides with CO 2, CS2, and isocyanates: Ti=Nα cycloaddition, cycloaddition-insertion, and cycloaddition-NNR2 group transfer reactions

Article abstract of DOI:10.1021/om101167p

We report a comprehensive combined experimental and DFT investigation of the synthesis, molecular and electronic structures, and reactivity of terminal hydrazido complexes with CO₂, CS₂, isocyanates, and isothiocyanates. Reaction of Cp*Ti{MeC(NⁱPr) ₂}(N^tBu) with hydrazines R¹R ²NNH₂ gave the structurally characterized series of hydrazides Cp*Ti{MeC(NⁱPr)₂}(NNR¹R ²) (R¹ = Ph, R² = Ph (13), Me (14); R ¹ = R² = Me (15)). The energetics of this imide/hydrazine exchange reaction, as well as the electronic and molecular structures of 13-15, have been evaluated by DFT and compared with those of their methyl and phenyl imido counterparts. Reaction of 13-15 with CO₂ or CS₂ gave Ti=N_α cycloaddition products of the type Cp* Ti{MeC(NⁱPr)₂}{N(NR₂)C(E)E} (R = Ph, Me; E = O, S) and, in the case of CO₂, cycloaddition-insertion products Cp*Ti{MeC(NⁱPr)₂}{OC(O)N(NR₂)C(O)O}, in which 2 equivs of CO₂ had effectively inserted into Ti=N _α. Reaction of 13 with isocyanates and isothiocyanates also gave stable cycloaddition products. Cp*Ti{MeC(NⁱPr) ₂}{N(NPh₂)C(NTol)O} reacted further with CO₂ to give a mixed cycloaddition-insertion product. The ^tBuNCO moiety in Cp*Ti{MeC(NⁱPr)₂}{N(NPh₂)C(N ^tBu)O} could be displaced by CO₂ or TolNCO. In contrast, the reactions of 15 are dominated by cycloaddition-elimination reactions. Reaction with ^tBuNCO or Ar′NCO (2 equivs) gave ^tBuNCNNMe₂ or the heterocycle 1,2,4-N(Me) ₂NC(NAr′)N(Ar′)C(O), respectively, along with [Cp*Ti{MeC(NⁱPr)₂}(μ-O)]₂. With TolNCO (2 equivs), Cp*Ti{MeC(NⁱPr)₂}{OC(NNMe ₂)N(Tol)C(NTol)O} was formed via a series of cycloaddition- elimination and cycloaddition-insertion steps. The energetics and mechanisms of the cycloaddition, cycloaddition-insertion, and cycloaddition-extrusion processes of various model imido and hydrazido complexes have been investigated using DFT. In the latter (metathesis) reactions, the reaction outcomes depend on a delicate balance of the relative affinities of NR or NNR₂ for the metal center or the organic fragment (CO, CS, or RNC).