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

Abstract

The reaction of diazabutadienes of type R'N=C(R)-C(R)=NR', L (R = H, Me; R' = cycloalkyl, aryl) with ReVOCl₃(AsPh₃)₂ has furnished Re^VOCl₃(L), 1, from which Re^III(OPPh₃)Cl₃(L), 2, and Re^V(NAr)Cl₃(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)Cl₃(L'), 5, and Re^VI(NAr)Cl₃(L'), 4, respectively, where L' is the monoionized iminoacetamide ligand R'N=C(H)-C(=O)NR'^-. Finally, the reaction of Re^VO(OEt)X₂(PPh₃)₂ with L has furnished bivalent species of type Re^IIX₂(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)Cl₃(L') are paramagnetic (S = 1/2) and display sextet EPR spectra in solution. The g and A values of Re(NAr)Cl₃(L') are, respectively, lower and higher than those of ReX₂(L)₂. All the complexes are electroactive in acetonitrile solution. The Re(NAr)Cl₃(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)Cl₃(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 ~0.1 Å 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).