Hydrolytic polymerization of rhodium(III). Characterization of various forms of a trinuclear aqua ion

Abstract

Solution studies of a trinuclear rhodium(III) aqua ion, using a variety of solution techniques (NMR, UV/VIS spectroscopy, charge per Rh determination and elution behaviour) have shown that this oligomer can exist in several structural forms. The form isolated, by ion-exchange chromatography following aging of Rh³⁺ solutions in base, does not have a linear (1) or bent (2) arrangement of rhodium(III) centres but consists of a triangular arrangement of such centres linked to each other by one µ-OH bridge (4). The fact that 4 exhibits two ¹⁰³Rh NMR signals at δ 9964 and 10 000 in a 2∶1 ratio is rationalized in terms of the stabilization of a conformation with two rhodium environments through strong hydrogen-bonding between a terminal H₂O or OH ligand and a µ-OH bridging group. On heating this form is transformed into a new ion (3) which has a different UV/VIS spectrum and two ¹⁰³Rh NMR signals at δ 9671 and 9841 in a 1∶2 ratio. The structure of this form is proposed to consist of a triangular arrangement of metal atoms, two magnetically equivalent rhodium(III) centres linked together by two µ-OH bridges and each linked to the third Rh^IIIvia a single µ-OH bridge. The low pK_a1 for this ion of 0.5 ± 0.1 is attributed to the formation of a strong hydrogen-bond between one of the µ-OH groups bridging the two equivalent rhodium(III) centres and the terminal OH group generated by deprotonation. The existence of a hydrogen-bond interaction in both 3 and 4 has literature precedent. At high pH, 3 is transformed into a new aqua ion, 5, which exhibits a single ¹⁰³Rh NMR signal at δ 10 049 and a different UV/VIS spectrum to that of 3 and 4. This is interpreted in terms of a symmetric structure in which the three rhodium(III) centres are linked by a single µ₃-OH group and pairs of Rh atoms are further linked by a µ-OH bridge. A variable-pH and -temperature kinetic study revealed that 4 undergoes an irreversible intramolecular condensation process which produces 3 or 5, depending on the pH. The increase in rate with pH is consistent with the existence of two pathways, attributable to the reaction of singly and doubly deprotonated forms of 4. The rate increase of ca. 100 fold matches those observed for related chromium(III) systems.