Reactions of the cis-diamminediaquaplatinum(II) cation with 2-aminomalonic acid and its homologs, aspartic and glutamic acids. Rearrangements of metastable complexes with carboxylate-bound ligands to N,O-chelates and formation of di- and trinuclear complexes

Abstract

Reactions of cis-Pt(NH 3) 2(H 2O) 2 2+ (1) with 2-aminomalonic acid (amalH 2), aspartic acid (aspH 2), and glutamic acid (gluH 2) have been studied by multinuclear ( 1H, 13C, 195Pt, 15N) NMR spectroscopy. With aspH 2 or gluH 2, 1 gives initially a complex in which the ligand is bound through only one carboxylate group, cis-Pt(NH 3) 2(LH 2-O)(H 2O) 2+. At pH <2, the α-carboxyl group is bound predominantly, but at pH 4-5, both carboxylate groups are involved to a similar extent. Over 2-3 days at pH 1.5, a complex with five-membered N,αO-chelate ring is formed, Pt(NH 3) 2(LH-N,αO) +. Reaction of aminomalonate with 1 gives initially a complex Pt(NH 3) 2(amalH-O,O′) +, with a six-membered chelate ring. Over 2-3 days at pH 1.5, this complex converts to Pt(NH 3) 2(amalH-N,O) +, with a five-membered ring. In solutions sufficiently acidic to protonate the uncoordinated carboxyl group, decarboxylation then occurs over several days to give the glycinate complex Pt(NH 3) 2(gly-N,O) +. The carboxylate group that is not part of the five-membered chelate ring in each of the three complexes Pt(NH 3) 2(L-N,O) is able to coordinate to platinum from excess 1. With aspartic and glutamic acids, one or two Pt(NH 3) 2(L) moieties can coordinate to one diammineplatinum(II) cation, to give [{Pt(NH 3) 2(L)}{Pt(NH 3) 2(H 2O)}] 2+ or [{Pt(NH 3) 2(L)} 2{Pt(NH 3) 2}] 2+, respectively. One Pt(NH 3) 2(amal) molecule can react with 1 to give [{Pt(NH 3) 2(amal)}{Pt(NH 3) 2(H 2O)}] 2+, which is in equilibrium with [{Pt(NH 3) 2} 2(amal)] 2+, in which the carboxylate group involved in a five-membered N,O-chelate ring with one Pt atom also bridges to the second Pt atom to complete a six-membered O,O′-chelate ring.