Metal-Metal Cooperation in the Oxidation of a Flapping Platinum Butterfly by Haloforms: Experimental and Theoretical Evidence

摘要

The model 1-DFT for the butterfly complex [{Pt(C<^>C*)(mu-pz)}2] (1; HC<^>C* = 1-(4-(ethoxycarbonyl)phenyl)-3-methyl-1H-imidazol-2-ylidene) shows two minima in the potential energy surface of the ground state in acetone solution: the butterfly-wing-spreading molecules 1-s, (d(PtPt) <= 3.20 angstrom) and the wing-folding molecules 1-f (d(Pt-Pt) = 3.00 angstrom). Both minima are very close in energy (Delta G degrees = 1.7 kcal/mol) and are connected through a transition state, which lies only 1.9 kcal/mol above 1-s and 0.2 kcal/mol above 1-f. These very low barriers support a fast interconversion process, resembling a butterfly flapping, and the presence of both conformers in acetone solution. However, the 1-f ratio is so low that it is undetectable in the excitation and emission spectra of 1 in 2-MeTHF of diluted solutions (105 M) at 77 K, while it is seen in more concentrated solutions (103 M). In acetone solution, 1 undergoes a [2c, 2e] oxidation by CHX3 (X = Cl, Br) in the sunlight to render the Pt-2(III,III) compounds [{Pt(C<^>C*)(mu-pz)X)}(2)] (X = Cl (2-Cl), Br (2-Br)). In concentrated solutions, 1 can react with CHCl3 under blue light to give 2-Cl and with CHBr3 in the dark, the latter rendering the compound [BrPt(C<^>C*)(mu-pz)(2)Pt(C<^>C*)CHBr2] (3-Br) or mixtures of 2-Br and 3-Br if the reaction is performed under an argon atmosphere or in the air, respectively. Mechanistic studies showed that in concentrated solutions the oxidation processes follow a radical mechanism being the MMLCT-based species 1-f, those which trigger the reaction of 1 with CHBr3 and CHCl3. In the ground state (S0f), it promotes the thermal oxidation of 1 by CHBr3 and in the first singlet excited state (S1f) the blue-light-driven photooxidation of 1 by CHCl3. Complexes, 2-Cl, 2-Br, and 3-Br were selectively obtained and fully characterized, showing PtPt distances (ca. 2.6 angstrom) shorter than that of the starting complex, 1. They are, together with the analogous [{Pt(C<^>C*)(mu-pz)I)}(2)] and [IPt(C<^>C*)(mu-pz)(2)Pt(C<^>C*)CHI2], the only dinuclear metalmetal-bonded Pt-III(mu-pz)(2)Pt-III compounds reported to date.