EFFECT OF BASE ON THE SOLUTION BEHAVIOR OF XO(D-PENICILLAMINATO)(L-PENICILLAMINATO)RHENATE(V)

The effect of base on the solution behavior of oxo (D-penicillaminato) (L-penicillaminato)rhenate(V) (1) was investigated by H-1 NMR, resona nce Raman, and UV-visible spectroscopy. The H-1 NMR spectrum of 1 cons isted of two sets of sharp pen signals in DMSO-d(6) and D2O below pH 8 (penicillamine penH(4), the subscript on H indicating the number of d issociable protons present). The data were consistent with ReO(D-penH( 3))(L-penH(2)) and its enantiomer ReO(L-penH(3)) (D-penH(2)), with pen H(3) coordinated by N, S, and carboxyl O. These enantiomers have a cis -N-2,cis-S-2 coordination of the pen ligands, both CO2 groups anti to the oxo ligand and one CO2 coordinated trans to the oxo ligand (form I ). In D2O, between pH 8.6 and 10.1, the signals broadened, collapsed, and reemerged as one set of signals, but no accompanying changes were observed in either the resonance Raman or the UV-visible spectra. The NMR spectral changes were attributed to base-catalyzed interconversion between two enantiomers; both CO2 groups are deprotonated in D2O, and they alternate between ligated and deligated states. Near pH 11, a ne w form (II) in slow exchange with form I was detected by NMR spectrosc opy. The Re=O Raman band of I (963 cm(-1)) was replaced by a new midfr equency band (930 cm(-1)). The UV-visible bands (346 and 492 nm) decre ased in intensity. With increasing pH and near pH 12, the H-1 NMR sign als of II shifted. The midfrequency Re=O band was replaced by a low-fr equency band (845 cm(-1)), and both the UV and visible bands continued to decline as new shorter wavelength bands emerged. These spectral ch anges were consistent with deprotonation of II to give II'. The H-1 NM R spectra of 1 and ReO(D-penH(3))(D-penH(2)) were nearly identical at pH 12. At this pH, the latter complex is a trans-dioxo species with a Re=O band at 846 cm(-1); the similar NMR spectrum and Re=O band of 1 s uggest that II' is a trans-dioxo species also. This conclusion was sup ported by studies in methanol which showed that [ReO(OCH3)(D-penH(2))( L-penH(2))](2-) did not deprotonate.