Toward the understanding of radical reactions: experimental and computational studies of titanium(III) diamine...

Radical reactions of titanium(III) [Ti((tBu2)O2NN')Cl(S)] (S = THF, 1; S = py, 2; (tBu2)O2NN' = Me2N(CH2)2N(CH2-2-O-3,5-(t)Bu2C6H2)2) are described. Reactions with neutral electron acceptors led to metal oxidation to Ti(IV), [Ti((tBu2)O2NN')Cl(TEMPO)] (4) being formed with the TEMPO radical and [Ti((tBu2)O2NN')Cl2] (9) with PhN═NPh. [Ti((tBu2)O2NN')Cl2] was also formed when [Ti((tBu2)O2NN')Cl(S)] was oxidized by [Cp2Fe][BPh4], but the [Cp2Fe][PF6] analogue yielded [Ti((tBu2)O2NN')ClF] (8). The reactions of [Ti((tBu2)O2NN')Cl(S)] with O2 gave [Ti((tBu2)O2NN')Cl]2(μ-O) (3). The DFT calculated Gibbs energy for the above reaction showed it to be exergonic (ΔG298 = -123.6 kcal·mol(-1)). [Ti((tBu2)O2NN')(CH2Ph)(S)] (S = THF, 5; py, 6) are not stable in solution for long periods and in diethyl ether gave 1:1 cocrystals of [Ti((tBu2)O2NN')(CH2Ph)2] (7) and [Ti((tBu2)O2NN')Cl]2(μ-O) (3), most probably resulting from a disproportionation process of titanium(III) followed by oxygen abstraction by the resulting Ti(II) species. The oxidation of [Ti((tBu2)O2NN')(κ(2)-{CH2-2-(NMe2)-C6H4})] (10), which is a Ti(III) benzyl stabilized by the intramolecular coordination of the NMe2 moiety, led to a complex mixture. Recrystallization of this mixture under air led to a 1:1 cocrystal of two coordination isomers of the titanium oxo dimer (3). In one of these isomers, one metal is pentacoordinate and the dimethylamine moiety of the diamine bis(phenolate) ligand is not bonded to the metal, displaying a coordination mode of the ligand never observed before. The other titanium center is distorted octahedral with two cis-phenolate moieties. In the second unit, the coordination of the two ancillary ligands to the titanium centers reveals mutually cis-phenolate groups in one-half of the molecule and trans-coordinated in the other titanium center, keeping a distorted octahedral environment around each titanium.