SYNTHESIS AND OXIDATIVE DEMETALLATION OF 2 NEW TUNGSTEN PORPHYRINS

Syntheses and spectral characteristics are reported for two new tungsten porphyrins, (5, 10, 15, 20-tetraphenylporphina-to)tungstyl(V) methoxide, W(TPP), and [5, 10, 15, 20-tetrakis(p-sulfonatophenyl)porphinato]aquooxotungstate(V), W(TPPS). These two porphyrins are shown to undergo slow demetalation in the presence of air, preceded by oxidation to W(VI) by molecular oxygen. The oxidative demetalation of W(TPPS) exhibits kinetics second order in [W(TPPS)] at pH ≿2 and µ= 0.10 M, with the pseudo-second-order rate constant k = 4.09 X 10-2M-1s-1in a buffered solution of pH 3.97 saturated with air. A medium effect on the rate constant causes only a slight change in kobsiover a >104-fold change in [H+], A mechanism is proposed for the demetalation, involving replacement of H20 by O2coordinated to W(TPPS), then subsequent oxidation of W(V) to W(VI) within this O2complex and loss of the porphyrin ligand from W(VI). At pH <2, the kinetics anomalously, but clearly, appear to be -1 order in [W(TPPS)], with fc ≈ 3.2 X 10-13M2s-1at pH 1.00 (HClO4). © 1979, American Chemical Society. All rights reserved.