Probing the Factors That Stabilize Mononuclear Rhodium(II) Bis(phosphine), η⁶-Arene Complexes with Piano-Stool Geometries

Abstract

The synthesis, characterization, and electrochemistry of a series of Rh(I) bis(phosphine), η 6 -arene, pianostool complexes are reported. The study reported herein elucidates several of the important factors which lead to the stabilization of Rh(II) in this coordination environment. From the electrochemical data for a series of complexes of the type [Rh(η 2 -dppe)(η 6 -C 6 H 6-n X n )]BF 4 (X ) CH 3 , n ) 0-6) (1-7) it was shown that the addition of methyl groups to the arene ligand kinetically stabilize the Rh(II) center and thermodynamically stabilize the Rh(II) species by 16 mV per added methyl group. Furthermore, complexes which contain chelation to the arene ligand, such as [Rh(η 6 :η 1 -Ph(CH 2 ) 3 PPh 2 )(η 1 -Ph(CH 2 ) 3 PPh 2 )]BF 4 (12), kinetically stabilize the Rh(II) form, presumably from ligand substitution based decomposition reactions. The electrochemical studies of five isostructural and isoelectronic complexes, [Rh(η 2 -dppe)(η 6 -C 6 H 5 CH 3 )]BF 4 (2), [Rh(η 1 -n-BuPPh 2 ) 2 (η 6 -C 6 H 5 CH 3 )]BF 4 (8), [Rh(η 2 -dppp)(η 6 -C 6 H 5 -CH 3 )]BF 4 ( 9), [Rh(η 2 -dppb)(η 6 -C 6 H 5 CH 3 )]BF 4 (10), and 12 show that those complexes which contain bidentate, bis(phosphine) chelation with an ethyl or butyl bridge, 2 and 10, thermodynamically destabilize the Rh(II) form relative to those complexes which contain a less restricted bis(phosphine) chelate or no bis(phosphine) chelation. Using single-crystal X-ray data and extended Hu ¨ckel calculations, these counterintuitive electrochemical trends were explained in terms of not only the properties of the Rh(I) complex but also, in terms of the structural changes which are likely to occur upon oxidation of the metal center from Rh(I) to Rh(II).

The Organometallic Chemistry of Rh‐, Ir‐, Pd‐, and Pt‐Based Radicals: Higher Valent Species

“…In the case of ethyl or butyl linkers, irreversible oxidation waves are observed upon oxidation (Rh(I) / Rh(II) couple) at low scan rates. This is explained in terms of accommodation of the rhodium(II) state (19). The DFT calculations suggest that widening of the PÀ ÀRhÀ ÀP angle and lengthening of the RhÀ ÀP bonds occurs upon oxidation of rhodium(I) to rhodium(II).…”

“…9) has been described in detail (19)(20)(21)(22). The relative kinetic and thermodynamic stabilities of these complexes were investigated by electrochemical methods.…”

“…This trend is probably due to the fact that more electron-donating arenes coordinate stronger to the rhodium(II) center, thus preventing their substitution. Furthermore, the more substituted arene ligands protect the metal center from external attack through the increased steric bulk of the ligand (19). The Rh(I)/Rh(II) couple occurs at lower potentials upon increasing the arene substitution, which reflects the corresponding increase of electron density at the rhodium center.…”

Intramolecular dehydrofluorinative coupling of η6-arene and fluoroarylphosphine ligands in ruthenium complexes

“…The method provides a convenient route to complexes of tridentate tethered h 5 ,kP-cyclopentadieneide-phosphine ligands, which can be difficult to access by other routes [5]. Although tethered h 6 ,kParene-phosphine ligands are more readily accessible by other methods [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], intramolecular coupling of ligands is still attractive for reasons of enhanced regio-and stereo-selectivity. A small number of intramolecular coupling reactions that afforded complexes of h 6 ,kP-arene-phosphine ligands have been reported [21][22][23].…”

Rhodium(ii) dimers without metal–metal bonds

“…[1][2][3][4][5][6][7][8] Much more common are metal-metal bonded dimers, where the designation of the formal oxidation state +II is somewhat artificial and a consequence of the formal oxidation state counting rules. True paramagnetic mononuclear complexes of Rh II are relatively rare.…”