Reactions of Cyclopentadienyl-Amidinate Titanium Hydrazides with CO₂, CS₂, and Isocyanates: Ti=N_α Cycloaddition, Cycloaddition-Insertion, and Cycloaddition-NNR₂ Group Transfer Reactions

International audienceWe 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}(NtBu) with hydrazines R1R2NNH2 gave the structurally characterized series of hydrazides Cp*Ti{MeC(NiPr)2}(NNR1R2) (R1=Ph,R2=Ph (13), Me (14);R1=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 byDFT and compared with those of their methyl and phenyl imido counterparts. Reaction of 13-15 with CO2 orCS2 gaveTidNR cycloaddition products of the typeCp*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 TidNR. 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(NtBu)O} could be displaced by CO2 or TolNCO. In contrast, the reactions of 15 are dominated by cycloaddition- elimination reactions. Reaction with tBuNCO or Ar0NCO (2 equivs) gave tBuNCNNMe2 or the heterocycle 1,2,4-N(Me)2NC(NAr0)N(Ar0)C(O), respectively, along with [Cp*Ti{MeC- (NiPr)2}(μ-O)]2. With TolNCO (2 equivs), Cp*Ti{MeC(NiPr)2}{OC(NNMe2)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)