Tripyrrolylphosphine as a Unique Bridging Ligand in the Rh6(CO)14(μ2-P(NC4H4)3) Cluster:  Structure, Bonding, Fluxionality, Thermodynamics, and Kinetics Studies

• Published in: Journal of the American Chemical Society 2002-07, Vol.124 (30), p.8922-8931
• Main Authors: Babij, Claudia, Browning, C. Scott, Farrar, David H, Koshevoy, Igor O, Podkorytov, Ivan S, Poë, Anthony J, Tunik, Sergey P
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja012440b

Tripyrrolylphosphine reacts with the cluster Rh6(CO)15(NCMe) to afford the disubstituted Rh6(CO)14(μ2-P(NC4H4)3) derivative (2) via the Rh6(CO)15P(NC4H4)3 intermediate (1) with η1-P coordination. In the solid state, 2 has the phosphine occupying a bridging position where it is bonded to two neighboring Rh atoms in the Rh6 octahedron through the P-atom and an approximately tetrahedral α-carbon atom of one of the pyrrolyl rings. This can be described by the interaction of an electron pair, associated with a negative charge on one of the canonical forms of the NC4H4 ring, with the adjacent Rh center. 1H NMR spectra show that the solid-state structure is retained in solution, but the phosphine is not rigid, and three distinctive dynamic processes are observed. Each of these represents independent hindered rotation of inequivalent pyrrolyl rings about P−N bonds, the ring involved in the interaction with the Rh6 skeleton displaying the highest activation barrier with ΔH ⧧ = 15.8 ± 0.1 kcal mol-1 and ΔS ⧧ = 1.4 ± 0.3 cal K-1 mol-1. The assignment has been confirmed by 1H TOCSY and EXSY experiments, and a mechanism is proposed. The formation of 2 from 1 is reversible in the presence of CO, which is highly unusual for bridged clusters. The kinetics of the forward and reverse reactions have been studied, and the values of ΔH° and ΔS° for formation of 2 (+1.3 ± 0.5 kcal mol-1 and −9 ± 2 cal K-1 mol -1, respectively) show that the Rh−C bond in the bridge is comparable in strength with the Rh−CO bond it replaces. The intrinsic entropy of 2 is exceptionally unfavorable, overcoming the favorable entropy caused by CO release, and this allows the reversibility of bridge formation. The reactions proceed via a reactive intermediate that may involve agostic bonding of the ring. The reverse reaction has an exceedingly unfavorable activation entropy that emphasizes the unique nature of 2.