Structure and Dynamic Exchange in Rhodium η2-Naphthalene and Rhodium η2-Phenanthrene Complexes: Quantitative NOESY and EXSY Studies

Abstract

Complexes of naphthalene and phenanthrene with rhodium(η5-cyclopentadienyl)(trimethylphosphine) have been studied by quantitative two-dimensional nuclear Overhauser (NOESY) and exchange spectroscopy (EXSY). Naphthalene coordinates in the η2-1,2-mode as (η5-C5H5)Rh(PMe3)(η2-C10H8). At 260 K, NOESY peaks establish that the solution conformer has the hydrogen atoms on the coordinated double bond bent out of the arene plane toward the PMe3 ligand. The effective average distance, reff, of these hydrogen atoms from those in the PMe3 ligand is calculated as 3.52 Å by matrix analysis of the NOESY spectrum. At room temperature, (η5-C5H5)Rh(PMe3)(η2-C10H8) undergoes an intramolecular [1,3]-metallotropic shift within the coordinated ring with ΔG⧧300 of 74.4 kJ mol-1 detected by EXSY spectroscopy. This species is in equilibrium (a) with the C−H activated isomer (η5-C5H5)Rh(PMe3)(C10H7)H and (b) with the dinuclear complex [(η5-C5H5)Rh(PMe3)]2(μ-1,2-η2-3,4-η2-C10H8) and free naphthalene. The free energy change at 300 K for conversion of (η5-C5H5)Rh(PMe3)(η2-C10H8) to (η5-C5H5)Rh(PMe3)(C10H7)H is +11.5 kJ mol-1 compared to +2.2 kJ mol-1 for the (η5-C5Me5) analogue. The crystal structure of the dinuclear complex, [(η5-C5H5)Rh(PMe3)]2(μ-1,2-η2-3,4-η2-C10H8), shows that this molecule adopts the structure with the two rhodium centers coordinated antifacially to the same ring of the naphthalene ligand. The C−C bond lengths of the coordinated ring show conspicuous alternation, while those of the uncoordinated ring differ less than those in free naphthalene. The mean separation of the hydrogen atoms attached to the coordinated CC bond from the PMe3 hydrogen atoms, averaged from the crystal structure as 1/〈r-3〉1/3 for each independent rhodium center, is 3.56 Å, compared to 3.48 Å for reff measured in solution by NOESY spectroscopy. The phenanthrene complex (η5-C5H5)Rh(PMe3)(η2-9,10-C14H10) adopts a conformation similar to the naphthalene complex; the value of reff is estimated to be reduced to 3.43 Å.