Rhenium chemistry of diazabutadienes and derived iminoacetamides spanning the valence domain II-VI. Synthesis, characterization, and metal-promoted regiospecific imine oxidation

The reaction of diazabutadienes of type R'N=C(R)-C(R)=NR', L (R = H, Me; R' = cycloalkyl, aryl) with Re-V-OCl3(AsPh3)(2) has furnished (ReOCl3)-O-V(L), 1, from which Re-III(OPPh3)Cl-3(L), 2, and Re-V(NAr)Cl-3(L), 3, have been synthesized. Chemical oxidation of 2(R = H) by aqueous H2O2 and of 3(R = H) by dilute HNO3 has yielded Re-IV(OPPh3)Cl-3(L'), 5, and Re-VI(NAr)Cl-3(L'), 4, respectively, where L' is the monoionized iminoacetamide ligand R'N=C(H)-C(=O)-NR'-. Finally, the reaction of (ReO)-O-V(OEt)X-2(PPh3)(2) with L has furnished bivalent species of type (ReX2)-X-II(L)(2), 6(X = Cl, Br). The X-ray structures of 1 (R = Me, R' = Ph), 3 (R = H, R' = Ph, Ar = Ph), and 4 (R = H, R' = cycloheptyl, Ar = C6H4Cl) are reported revealing meridional geometry for the ReCl3 fragment and triple bonding in the ReO (in 1) and ReNAr (in 3 and 4) fragments. The cis geometry (two Re-X stretches) of ReX2(L)(2) is consistent with maximized Re-II-L back-bonding. Both ReX2(L)(2) and Re(NAr)Cl-3(L') are paramagnetic (S = 1/2) and display sextet EPR spectra in solution. The g and A values of Re(NAr)Cl-3(L') are, respectively, lower and higher than those of ReX2(L)(2). All the complexes are electroactive in acetonitrile solution. The Re(NAr)Cl-3(L) species display the Re-VI/Re-V couple near 1.0 V versus SCE, and coulometric studies have revealed that, in the oxidative transformation of 3 to 4, the reactive intermediate is Re-VI(NAr)Cl-3(L)(+) which undergoes nucleophilic addition of water at an imine site followed by induced electron transfer finally affording 4. In the structure of 3 (R = H, R' = Ph, Ar = Ph), the Re-N bond lying trans to the chloride ligand is similar to0.1 Angstrom shorter than that lying trans to NPh. It is thus logical that the imine function incorporating the former bond is more polarized and therefore subject to more facile nucleophilic attack by water. This is consistent with the regiospecificity of the imine oxidation as revealed by structure determination of 4 (R = H, R' = cycloheptyl, Ar = C6H4Cl).