Thermodynamic and Kinetic Data for Adduct Formation, cis–trans Isomerization and Redox Reactions of ML₄ Complexes: A Case study with Rhodium– and Iridium–tropp Complexes in d⁸, d⁹ and d¹⁰ Valence Electron Configurations (tropp= Dibenzotropylidene Phosphanes)

Abstract

The formation of adducts of the square-planar 16-electron complexes trans-[M(tropp(ph))(2)](+) and cis-[M(tropp(ph))(2)](+) (M=Rh, Ir; tropp(Ph)=5-diphenylphosphanyldibenzo[a,d]cycloheptene) with acetonitrile (acn) and Cl(-), and the redox chemistry of these complexes was investigated by various physical methods (NMR and UV-visible spectroscopy, square-wave voltammetry), in order to obtain some fundamental thermodynamic and kinetic data for these systems. A trans/cis isomerization cannot be detected for [M(tropp(ph))(2)](+) in non-coordinating solvents. However, both isomers are connected through equilibria of the type trans-[M(tropp(ph))(2)](+)+L<==>[ML(tropp(ph))(2)](n)<==>cis-[M(tropp(ph))(2)](+)+L, involving five-coordinate intermediates [ML(tropp(ph))(2)](n) (L=acn, n=+1; L=Cl(-), n=0). Values for K(d) (K(f)), that is, the dissociation (formation) equilibrium constant, and k(d) (k(f)), that is, the dissociation (formation) rate constant, were obtained. The formation reactions are fast, especially with the trans isomers (k(f)>1x10(5) m(-1) s(-1)). The reaction with the sterically more hindered cis isomers is at least one order of magnitude slower. The stability of the five-coordinate complexes [ML(tropp(ph))(2)](n) increases with Ir>Rh and Cl(-)>acn. The dissociation reaction has a pronounced influence on the square-wave (SW) voltammograms of trans/cis-[Ir(tropp(ph))(2)](+). With the help of the thermodynamic and kinetic data independently determined by other physical means, these reactions could be simulated and allowed the setting up of a reaction sequence. Examination of the data obtained showed that the trans/cis isomerization is a process with a low activation barrier for the four-coordinate 17-electron complexes [M(tropp(ph))(2)](0) and especially that a disproportionation reaction 2 trans/cis-[M(tropp(ph))(2)](0)-->[M(tropp(ph))(2)](+)+[M(tropp(ph))(2)](-) may be sufficiently fast to mask the true reactivity of the paramagnetic species, which are probably less reactive than their diamagnetic equilibrium partners.

From 0 to II in One‐Electron Steps: A Series of Ruthenium Complexes Supported by TropPPh2

“…Addition of one CO or pyridine to 1 yielded five‐coordinate complexes 2 or 3 of the type Ru 0 (L)(tropPPh 2 ) 2 in high yields (L=CO, pyridine), which were characterized by NMR and IR spectroscopy (see the Supporting Information). These reactions show that the Ru 0 center is Lewis acidic, as was also observed for related d 8 valence electron configured Rh I and Ir I tropPPh 2 complexes and Rh I amido olefin complex, [Rh(Ntrop 2 )(PPh 3 )], which is an excellent hydrogenation and dehydrogenation catalyst …”

“…Our previous work has shown that the bidentate phosphine tropPPh 2 in which the Ph 2 P group (σ‐donor) binds to the concavely shaped 5‐H‐dibenzo‐[a,d]cyclohepten‐5‐yl (trop) group with an olefin, C=C trop , as an additional π‐accepting binding site coordinates strongly to various transition metal centers, such as palladium, rhodium,, and iridium,, and especially stabilizes low‐valent oxidation states.…”

Intramolecular Alkyl Phosphine Dehydrogenation in Cationic Rhodium Complexes of Tris(cyclopentylphosphine)

“…[38] The NMR spectra are unchanged on cooling to 250 K. Dissolving crystalline material of 2 b (see below) results in NMR spectra that show major peaks for the trans isomer, that slowly approach the equilibrium concentrations observed previously.…”

“…A similar mixture of isomers, that do not interconvert on the NMR timescale, was observed for cis/trans- 6 ]. [38] Complexes 2 a and 2 b have similar NMR spectroscopic characteristics, but subtle differences make the identification of the two isomers possible. In their 6 ], [38] and is an indication of the relative trans influences of the phosphine and alkene ligands.…”

“…[38] Complexes 2 a and 2 b have similar NMR spectroscopic characteristics, but subtle differences make the identification of the two isomers possible. In their 6 ], [38] and is an indication of the relative trans influences of the phosphine and alkene ligands. The 31 P chemical shift of the phosphines have undergone a significant downfield change, as expected for a ligand which has become part of a strained five membered ring.…”