Photochemistry of Cr(CO)₄(bpy) (bpy = 2,2‘-Bipyridine) Studied by Time-Resolved Infrared Spectroscopy

Abstract

Employing time-resolved IR spectroscopy, we have demonstrated that, following photolysis of Cr(CO)4(bpy) (1) in CH2Cl2, the only detectable photoproduct on the 50 ns time scale is the solvated CO-loss intermediate fac-Cr(CO)3(bpy)(CH2Cl2) (2); this intermediate is formed following either MLCT (532 nm) or LF (355 nm) excitation. The rate of the back-reaction of 2 with CO has been measured (k 2 = 7.1(±0.5) × 106 M-1 s-1); in the presence of PPh3, the intermediate 2 reacts to produce fac-Cr(CO)3(bpy)(PPh3) (3) exclusively (k 2 = 1.0(±0.5) × 107 M-1 s-1). From UV/visible flash photolysis data, we conclude that this intermediate 2 is probably produced on a ps/sub-ps time scale.

Early photochemical dynamics of organometallic compounds studied by ultrafast time-resolved spectroscopic techniquesBased on the presentation given at Dalton Discussion No. 4, 10–13th January 2002, Kloster Banz, Germany.

“…Notably, the shape, number and positions of the photoproduct IR bands, measured on a ps time scale, are the same as those determined at 10 ns. As is explained in our previous nanosecond study, 65 the photoproduct IR spectrum belongs to fac-[Cr(L)(CO) 3 (bpy)] (L = CH 2 Cl 2 ). The ps TR-IR spectra thus show that the photoproduct has the facial geometry already at 1 ps.…”

“…This final band was attributed to the photoproduct fac-[Cr(L)(CO) 3 (bpy)] (L = py), by comparison with nanosecond TR-UV-Vis and IR spectra recorded earlier. 65 The rapidly decaying absorption seen at early times appears to be composed of the 640 nm band, which is due to fac-[Cr(L)(CO) 3 (bpy)], and another broad absorption, which extends deep into the red spectral region. This is typical of the bpy ؒϪ chromophore 66 which is present in Cr bpy MLCT excited states, which can be formally viewed as [Cr I (CO) 4 (bpy ؒϪ )].…”

“…It should also be noted that most of the mechanistic information was inferred from kinetics. The assignment of the 640 nm absorption band to the fac-[Cr(L)(CO) 3 (bpy)] photoproduct is only indirect, based on comparison with a previous nanosecond time-resolved study 65 and an assumption that no chemical reactions or structural changes occur between ca. 500 ps and 10 ns.…”

Structural and electronic changes accompanying reduction of Cr(CO)4(bpy) to its radical anion: a quantum chemical interpretation of spectroelectrochemical experiments

“…The Cr(CO) 4 (bpy) complex is widely studied as a prototype of an inert organometallic molecule which can be activated by an electron transfer or metal to ligand charge transfer (MLCT) excitation. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Both one-electron reduction and irradiation into the Cr→bpy MLCT absorption band labilise the axial Cr-CO bond, making the CO ligand susceptible to a facile substitution by a solvent molecule or by a Lewis base, L (e.g. phosphine) present in solution (Scheme 1).…”

Electrospray Mass Spectrometric Detection of Neutral Metal Bipyridine Complexes Using Sodium Ions and Its Application in the Analysis of a Photochemical Ligand Substitution Reaction

“…[5][6][7][8][9][20][21][22][23][24][25] Such reactions were traced using UV-vis spectra 20,22,23,25 , IR (including time-resolved techniques) 21 , and HPLC. 9,24 ES-mass spectrometry has also become increasingly important as a method to study such reactions since Arakawa and co-workers elucidated the photosubstitution reactions of ruthenium diimine complexes.…”