Evidence for a Carbonyl-Containing Intermediate in the 308-nm Photolysis of trans-RhCl(CO)(PMe₃)₂

Abstract

Time-resolved infrared absorption spectroscopy is used to monitor the events during the first approximately 100 mus following 308-nm photolysis of trans-RhCl(CO)(PMe3)2 (I) in a C6D6 or Decalin solution. Upon photolysis, the intensity of the carbonyl stretching absorption of I decreases and a new C-O stretching absorption (A) appears at 2006 cm(-1) in C6D6 (2013 cm(-1) in Decalin). Within approximately 100 mus, A disappears concurrently with partial regeneration of I and oxidative addition of a solvent C-D (C-H) bond. Our observation of the prompt formation of a carbonyl-containing intermediate contrasts with prior observations that photolysis at longer wavelengths leads solely to dissociation of CO from I. The implications of the observation of A for the understanding of the mechanisms of photocatalytic reactions of I are discussed.

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).…”

“…[20] Interestingly, if the same experiment is performed in the presence of 4,4'-bipyridine, new bands at ñ = 1986, 2065, and 2034 cm À1 are not observed, but the fast intensity decay of the n(Rh I ÀCO) band at ñ = 1963 cm À1 is seen (cf. It was evidenced clearly that the carbonyl ligand is released gradually during the overall reaction.…”

“…However, such a carbonyl-containing intermediate(s) was also evidenced in photolytic reactions, as reported by Krishnan and Schultz. [20] Interestingly, if the same experiment is performed in the presence of 4,4'-bipyridine, new bands at ñ = 1986, 2065, and 2034 cm À1 are not observed, but the fast intensity decay of the n(Rh I ÀCO) band at ñ = 1963 cm À1 is seen (cf. Figure S1).…”