Time-Resolved Optical and Infrared Spectral Studies of Intermediates Generated by Photolysis of trans-RhCl(CO)(PR₃)₂. Roles Played in the Photocatalytic Activation of Hydrocarbons¹

Abstract

Described are picosecond and nanosecond time-resolved optical (TRO) spectral and nanosecond time-resolved infrared (TRIR) spectral studies of intermediates generated when the rhodium(I) complexes trans-RhCl(CO)L2 (L = PPh3 (I), P(p-tolyl)3 (II), or PMe3 (III)) are subjected to photoexcitation. Each of these species, which are precursors in the photocatalytic activation of hydrocarbons, undergoes CO labilization to form an intermediate concluded to be the solvated complex RhCl(Sol)L2 (A(i)). The picosecond studies demonstrate that an initial transient is formed promptly (<30 ps), which decays to A(i) with lifetimes ranging from 40 to 560 ps depending upon L and the medium. This is proposed on the basis of ab initio calculations to be a metal-to-ligand charge transfer (MLCT) excited state. Second-order rate constants (kCO) for reaction of the A(i) with CO were determined, and these depend on the nature of L and the solvent, the slowest rate being for A(I) in tetrahydrofuran (kCO = 7.1 x 10(6) M(-1) x s(-1)), the fastest being for A(III) in dichloromethane (1.3 x 10(9) M(-1) x s(-1)). Each A(i) also undergoes competitive unimolecular reaction with solvent to form long-lived transients with TRIR properties suggesting these to be Rh(III) products of oxidative addition. Although this was mostly suppressed by the presence of higher concentrations of CO (which trapped A(i) to re-form the starting complexes in each case), both TRO and TRIR experiments indicate that a fraction of the oxidative addition could not be quenched. Thus, the short-lived MLCT state or a vibrationally hot species formed during the decay of this excited state appears to participate directly in C-H activation.

Understanding the reactivity of transition metal complexes involving multiple spin states

“…Both cis and trans isomers for the 14-electron RhCl(PR3)2 intermediate may adopt either a singlet or a triplet configuration and either spin state is predicted to be lower in energy depending on the computational method used. [55][56][57][58][59] Correlated methods, however, favor a singlet ground state in better agreement with the experimental results. Clearly, great care must be exercised in the selection of the computational technique for the investigation of two-state reactivity.…”

Photocatalytic Acceptorless Alkane Dehydrogenation: Scope, Mechanism, and Conquering Deactivation with Carbon Dioxide

“…[16][17][18][19][20] This assignment can be confirmed by a similar experiment with deuterated n-[D 18 ]octane in which a new band at ñ = 1466 cm À1 was observed that results from n(Rh III ÀD) vibrations in addition to the bands at ñ = 1963 and 1985 cm À1 , whereas the band at ñ = 2065 cm À1 was absent ( Figure S2). [16][17][18][19][20] This assignment can be confirmed by a similar experiment with deuterated n-[D 18 ]octane in which a new band at ñ = 1466 cm À1 was observed that results from n(Rh III ÀD) vibrations in addition to the bands at ñ = 1963 and 1985 cm À1 , whereas the band at ñ = 2065 cm À1 was absent ( Figure S2).…”

“…After 10 min, two new bands appear simultaneously at ñ = 1986 and 2065 cm À1 , which can be assigned to the n(Rh III ÀCO) and n(Rh III ÀH) modes, respectively. [16][17][18][19][20] This assignment can be confirmed by a similar experiment with deuterated n-[D 18 ]octane in which a new band at ñ = 1466 cm À1 was observed that results from n(Rh III ÀD) vibrations in addition to the bands at ñ = 1963 and 1985 cm À1 , whereas the band at ñ = 2065 cm À1 was absent ( Figure S2). Alternatively, these bands might stem from two different intermediate Rh complexes that contain either CO or H as the ligand (vide infra).…”

From curiosity to applications. A personal perspective on inorganic photochemistry

“…We studied this reaction first with the conventional flash system, 52 then later using laser flash photolysis. 53 In benzene solution, this substrate undergoes reversible photo-dissociation of CO ( Scheme 4 , k CO = 7.8 × 10 8 M –1 s –1 ). By carrying out the experiment in the presence of reactants such as PPh 3 , H 2 and ethylene, it was possible to determine other second order rate constants for trapping the Rh(PPh 3 ) 2 Cl intermediate.…”

“…For example, the k L for the reaction with PPh 3 was determined to be ∼6 × 10 7 M –1 s –1 in 296 K benzene while that with H 2 was nearly a factor of 30 slower. In addition, k CO values in cyclohexane and dichloromethane were ∼2-fold and 5-fold lower, respectively, suggesting that the intermediate is not three-coordinate, but is weakly bound by solvent in the position vacated by CO. 53 …”