Primary photochemical dynamics of metal carbonyl dimers and clusters in solution: Insights into the results of metal–metal bond cleavage from ultrafast spectroscopic studies

Abstract

Supporting Information PlaceholderABSTRACT: Metal carbonyl dimers and clusters constitute a diverse class of organometallic reagents and catalysts. The photochemistry of these complexes is a topic of significant and long-standing interest, as preparative-scale photolyses constitute many of the most synthetically powerful reactions in organometallic chemistry. The metal-metal bonding present in dimers and clusters is varied and significantly influences their overall reactivity. In this review we discuss the primary photochemical dynamics of transition metal carbonyl metal dimers and clusters, with a focus on the changes in metal-metal bonding that occur upon visible and/or ultraviolet photochemical excitation. The bulk of the results discussed here were obtained using ultrafast time-resolved infrared spectroscopy, a technique with high structural sensitivity afforded by the carbonyl reporter ligands of the complexes studied. Picosecond time resolution allows detailed monitoring of the photochemical reaction dynamics, including observation of initially excited complexes and short-lived transient metal-metal bond cleavage intermediates, as well as formation of the longer-lived, yet reactive, intermediates responsible for reactivity occurring on diffusion-limited time scales and beyond.

Photoinduced Electron Transfer from ZrOCo Binuclear Light Absorber to Pyridine Elucidated by Transient Optical and Infrared Spectroscopy

“…The origin of the unexpected long lifetime of the charge-separated pyr •– –Zr IV –O–Co III state is most likely crossing to a spin-forbidden state. , Our earlier study of TiOMn centers explored spin crossover in binuclear units; we proposed that the transient d 1 metal center was the most likely site of the spin crossover because it is the conserved feature of a variety of different metal combinations with long-lifetime MMCT states . In ZrOCo–pyr, there is no obvious barrier that would require the electron to remain at the zirconium center to undergo spin crossover before reducing the pyridine; as the spin–orbit coupling of cobalt is higher than that of pyridine, spin crossover at cobalt is considered more likely. − Cobalt is known to undergo spin crossover on the ultrafast time scale in a variety of oxidation states, geometries, and phases, ,− including in our previous works with organometallic systems. , In particular, this phenomenon has been characterized in great detail in photoinduced magnetization due to metal-to-metal charge transfer states in Prussian blue analogues. ,− As in those analogues, Co III is commonly found in an octahedral configuration, taking on a tetrahedral geometry when required by ligand steric constraints, as in the tungsten oxide matrices discussed earlier or with ligands like benzene dithiolates. , Tetrahedral cobalt centers are most frequently high-spin but display a low-spin ground state with strong-field norbornyl or phosphanido ligands. , Based on this information, we propose that the long lifetime of the pyr •– –Zr IV –O–Co III state is the result of spin crossover from the original optically formed quartet state to a doublet state. Our tetrahedral Co III forms in the high-spin state ( S = 1) alongside Zr III ( S = 1/2).…”

“…20 In ZrOCo-pyr, there is no obvious barrier that would require the electron to remain at the zirconium center to undergo spin crossover before reducing the pyridine; as the spin-orbit coupling of cobalt is higher than pyridine, spin crossover at cobalt is considered more likely. [55][56][57] Cobalt is known to undergo spin crossover on the ultrafast timescale in a variety of oxidation states, geometries, and phases, 54,[56][57][58][59][60][61][62] including in our previous works with organometallic systems. 63,64 In particular, this phenomenon has been characterized in great detail in photo-induced magnetization due to metal-to-metal charge transfer states in Prussian blue analogs.…”

Initial metal–metal bond breakage detected by fs X-ray scattering in the photolysis of Ru3(CO)12 in cyclohexane at 400 nm

“…The origin of the major photoproduct at 100 ps, Ru3(CO)10, also remained unclear. Successive single CO loss from Ru3(CO)12 has been proposed, 23 because simultaneous loss of two CO from Ru3(CO)12 in solution following single photon absorption at 400 nm is unlikely 23 and has not been experimentally proven. In the present study we used the fs time resolution of the XPP end station 24 at the LCLS X-ray Free Electron Laser 25 to determine the species formed about 100 fs after photoexcitation.…”

Primary Photochemical Dynamics of a Triply-Bonded Metal Carbonyl Dimer Probed Via Ultrafast Infrared Spectroscopy

“…[1][2][3] The majority of such complexes whose photochemistry has been characterized in detail contain a metal-metal (M-M) single bond, and photochemical excitation typically leads to a wavelength dependent mixture of M-M bond homolysis and CO-dissociation products. 4 The photochemical reactivity of homoleptic metal carbonyl dimers of Mn, [5][6][7][8] Re, [9][10][11] and Co, 12 have been characterized in detail, as has that of the cyclopentadienyl carbonyl-complexes of Mo, 13,14 W, 15,16 Fe, [17][18][19][20][21] and Ru. [22][23][24][25] Only a limited number of investigations have been made into the photochemistry of tetracarbonyl group VI dimers, [CpM(CO) 2 ] 2 .…”