SOLUTION STUDIES OF -PENICILLAMINATO-N,S,O)(SYN-D-PENICILLAMINATO-N,S)

The known Re(V)=O complex of D-penicillamine (D-penH(4); the subscript indicates the number of dissociable protons) is isolated as a neutral form, ReO(D-penH(3))(D-penH(2)) (1). The complex is six-coordinate wi th one CO2- bound trans to the oxo ligand, resulting in a cis-N-2, cis -S-2, trans-O-2 geometry. One carboxyl group is anti and the other is syn to the oxo ligand; only the anti-pen CO2- can coordinate. For 1, i n high-pH solutions, we have found unusual H-1 NMR spectral properties that have led to the discovery of a facile oxo ligand exchange proces s. 1 was also studied by acid-base titration, which revealed that the neutral-pH form (I) was [ReO(syn-D-penH(2)-N,S)(anti-D-penH(2)-N,S,O)] (-). Significant spectroscopic changes occurred at pH similar to 11 an d above. The two sets of pen H-1 NMR signals observed at neutral pH de creased in intensity as the pH was raised, and several original signal s shifted. Simultaneously, one new set of pen signals emerged. These n ew signals also shifted with increasing pH. These results indicate tha t (a) form I is in fast equilibration with a deprotonated form (I'), ( b) this mixture is in slow equilibrium with forms having different cha racteristics (II and II'), and (c) II and II' rapidly interconvert. Th e II/II' component has one set of pen signals. Acid-base titrations in dicated that the conversions from form I to forms I' and II each invol ve consumption of 1 mol of OH-. The nature of the forms present at dif ferent pH's was also examined by resonance Raman, UV-visible, and circ ular dichroism (CD) spectroscopy. These methods indicate OH- converts I into an equilibrium mixture: I' (by deprotonation of one NH) and II (by displacement of the axial carboxyl by axial hydroxo). An additiona l equivalent of OH- converted these two forms to II', which is probabl y a trans-dioxo species. Under conditions in which I', II, and II' coe xisted, only the last two interconverted rapidly on the NMR time scale , since they interconverted by addition or loss of a proton. I' can co nvert to II only by a slower deligation process. II and II' were readi ly distinguished by the Raman experiment but not the longer time scale NMR experiments. Results in methanol supported this interpretation an d only I, I', and IIMeOH were formed. IIMeOH differs from II in having an axial methoxo ligand. The methoxo ligand cannot be converted to an oxo ligand; thus, no II' was observed in methanol, and the NMR spectr um of IIMeOH has two sets of pen signals. The one set of pen signals f or II in water can be explained by facile proton exchange interconvert ing the axial hydroxo/oxo and oxo/hydroxo sites.