Unidentate coordination of 2,2?-bipyridine and 1,10-phenanthroline in a cyclometallated rhodium(III) complex. Evidence from1H and13C NMR spectra

Abstract

The binuclear cyclometallated complex $[RhCl (\mu -Cl)(bBzlH_2bz)]_2 (bBzlH_2bzH D 1,3-bis(benzimidazolyl)(benzene))$ undergoes a dichloro bridge cleavage reaction with 2,20-bipyridine (bipy) or 1,10-phenanthroline (phen) in the presence of perchlorate to yield a mononuclear complex of the type $RhCl(OClO_3)(bBzlH_2bz)(N-N)(N-N D bipy or phen)$. Surprisingly, the N-heterocycle, bipy or phen, is neither chelating nor bridging bidentate in the complex. Such a monodentate coordination of bipy or phen was detected using two-dimensional 1H–1H correlated and NOE experiments (DQF-COSY and ROESY), $^1H -^{13}C$ single- and multiple-bond correlated two-dimensional NMR experiments (PFG-HSQC and PFG-HMBC) and $^1H$, $^{13}C$ spin-lattice relaxation time measurements. The noncoordination of the pendant nitrogen of the heterocycle bipy or phen is evidenced by the observation of two sets of signals together with the presence of interligand NOEs only between the coordinated part of the heterocycle and the bisbenzimidazole as seen in the corresponding ROESY spectrum. Further, the $^1H$ and $^{13}C$ spin–lattice relaxation times show lower values for the nuclei in the coordinated part of the heterocycle, bipy or phen, than for the uncoordinated parts, supporting the fact that only one of the two nitrogens of the heterocycle has coordinated to the metal and thus behaves as monodentate ligand.