HYDROLYTIC POLYMERIZATION OF RHODIUM(III) - CHARACTERIZATION OF VARIOUS FORMS OF A TRINUCLEAR AQUA ION

Solution studies of a trinuclear rhodium(III) aqua ion; using a variet y of solution techniques (NMR, UV/VIS spectroscopy, charge per Rh dete rmination and elution behaviour) have shown that this oligomer can exi st in several structural forms. The form isolated; by ion-exchange chr omatography following aging of Rh3+ solutions in base,does not have a linear (1) or bent (2) arrangement of rhodium(III) centres but consist s of a triangular arrangement of such centres linked to each other by one mu-OH bridge (4). The fact that 4 exhibits two Rh-103 NMR signals at delta 9964 and 10000 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 mu- OH bridging group. On heating this form is transformed into a new ion (3) which has a different UV/VIS spectrum and two Rh-103 NMR signals a t delta 9671 and 9841 in a 1:2 ratio. The structure of this form is pr oposed to consist of a triangular arrangement of metal atoms, two magn etically equivalent rhodium(III) centres linked together by two mu-OH bridges and each linked to the third Rh-III via a single mu-OH bridge. The low pK(a1) for this ion of 0.5 +/- 0.1 is attributed to the forma tion of a strong hydrogen-bond between one of the mu-OH groups bridgin g the two equivalent rhodium(III) centres and the terminal OH group ge nerated by deprotonation. The existence of a hydrogen-bond interaction in both 3 and 4 has literature precedent. At high pH, 3 is transforme d into a new aqua ion, 5, which exhibits a single Rh-103 NMR signal at delta 10049 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 rh odium(III) centres are linked by a single mu(3)-OH group and pairs of Rh atoms are further linked by a mu-OH bridge. A variable-pH and -temp erature kinetic study revealed that 4 undergoes an irreversible intram olecular condensation process which produces 3 or 5, depending on the pH. The increase in rate with pH is consistent with the existence of t wo pathways attributable to the reaction of singly and doubly deproton ated forms of 4. The rate increase of ca. 100 fold matches those obser ved for related chromium(III) systems.