Toward the Understanding of Radical Reactions: Experimental and Computational Studies of Titanium(III) Diamine Bis(phenolate) Complexes

Radical reactions of titanium­(III) [Ti­(tBu2O2NN′)­Cl­(S)] (S = THF, 1; S = py, 2; tBu2O2NN′ = Me2N­(CH2)2N­(CH2-2-O-3,5-tBu2C6H2)2) are described. Reactions with neutral electron acceptors led to metal oxidation to Ti­(IV), [Ti­(tBu2O2NN′)­Cl­(TEMPO)] (4) being formed with the TEMPO radical and [Ti­(tBu2O2NN′)­Cl2] (9) with PhNNPh. [Ti­(tBu2O2NN′)­Cl2] was also formed when [Ti­(tBu2O2NN′)­Cl­(S)] was oxidized by [Cp2Fe]­[BPh4], but the [Cp2Fe]­[PF6] analogue yielded [Ti­(tBu2O2NN′)­ClF] (8). The reactions of [Ti­(tBu2O2NN′)­Cl­(S)] with O2 gave [Ti­(tBu2O2NN′)­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(tBu2O2NN′)­(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(tBu2O2NN′)­(CH2Ph)2] (7) and [Ti­(tBu2O2NN′)­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(tBu2O2NN′)­(κ2-{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.