Reactions of cyclopentadienyl-amidinate titanium hydrazides with CO _2, CS_2, and isocyanates: Ti=N_α cycloaddition, cycloaddition-insertion, and cycloaddition-NNR2 group transfer reactions

摘要

We report a comprehensive combined experimental and DFT investigation of the synthesis, molecular and electronic structures, and reactivity of terminal hydrazido complexes with CO_2, CS_2, isocyanates, and isothiocyanates. Reaction of Cp*Ti{MeC(N~iPr) _2}(N~tBu) with hydrazines R~1R ~2NNH_2 gave the structurally characterized series of hydrazides Cp*Ti{MeC(NiPr)_2}(NNR~1R ~2) (R~1 = Ph, R~2 = Ph (13), Me (14); R ~1 = R~2 = 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 CO_2 or CS_2 gave Ti=N_α cycloaddition products of the type Cp* Ti{MeC(N~iPr)_2}{N(NR_2)C(E)E} (R = Ph, Me; E = O, S) and, in the case of CO_2, cycloaddition-insertion products Cp*Ti{MeC(N~iPr)_2}{OC(O)N(NR_2)C(O)O}, in which 2 equivs of CO_2 had effectively inserted into Ti=N _α. Reaction of 13 with isocyanates and isothiocyanates also gave stable cycloaddition products. Cp*Ti{MeC(N~iPr) _2}{N(NPh_2)C(NTol)O} reacted further with CO_2 to give a mixed cycloaddition-insertion product. The ~tBuNCO moiety in Cp*Ti{MeC(N~iPr)_2}{N(NPh_2)C(N ~tBu)O} could be displaced by CO_2 or TolNCO. In contrast, the reactions of 15 are dominated by cycloaddition-elimination reactions. Reaction with tBuNCO or Ar′NCO (2 equivs) gave ~tBuNCNNMe_2 or the heterocycle 1,2,4-N(Me) _2NC(NAr′)N(Ar′)C(O), respectively, along with [Cp*Ti{MeC(N~iPr)_2}(μ-O)]_2. With TolNCO (2 equivs), Cp*Ti{MeC(N~iPr)_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 NNR_2 for the metal center or the organic fragment (CO, CS, or RNC).