Unusual Structure and Reactivity of the Photogenerated Intermediate Cp*Cr(μ-CO)₃CrCp* (Cp* = η⁵-C₅Me₅)

Abstract

Employing time-resolved IR spectroscopy, we have demonstrated that, following photolysis of [Cp*Cr(CO)2]2 (1) in n-heptane, the only initial photoproduct is the triply bridged CO-loss intermediate Cp*Cr(μ-CO)3CrCp* (4). Two bridging ν(CO) bands are observed, resulting from the splitting of the e‘ mode of 4, which leads to the conclusion that the CO bridging groups are asymmetric. The rate of the back reaction of 4 with CO has been measured (k 2 = 5.2 (±0.5) × 107 M-1 s-1), and this is 1000 times faster than the rate of the analogous back reaction of CO with CpFe(μ-CO)3FeCp. Moreover, unlike many CO-loss intermediates, there is no reaction of 4 with PPh3. The rate of reaction of 4 with THF, to produce Cp*2Cr2(CO)3(THF) (6), has been estimated (k 2 = 1.6 (±0.2) × 108 M-1 s-1).

Unsaturation in binuclear cyclopentadienylchromium carbonyl thiocarbonyls: Viability of (η5-C5H5)2Cr2(CS)2(CO)3 in contrast to (η5-C5H5)2Cr2(CO)5

“…However, if spin is conserved upon disproportionation of Cp 2 Cr 2 (CS) 2 (CO) 2 to give Cp 2 Cr 2 (CS) 2 (CO) 3 plus the higher energy singlet Cp 2 Cr 2 (CS) 2 (CO) structure 21-3, then the disproportionation process is endothermic requiring an energy of 14.1 kcal/mol (B3LYP) or 20.3 kcal/mol (BP86). The conservation of spin in reactions of such organometallic compounds is suggested by the experimental observation [55,56] that photolysis of Cp 2 Cr 2 (CO) 4 gives the higher energy Cp 2 Cr 2 (CO) 3 singlet spin state rather than a lower energy triplet state.…”

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

“…32 The reaction of 2 with THF to form 3 has also been observed, 30,31 although 2 does not appear to react with PPh 3 . 29,31 The structure of the molybdenum congener of 1, [Cp*Mo(CO) 2 ] 2 ( 6) is generally similar to that of 1, with differences in the bonding angles. 33 Existing knowledge on the photochemistry of 6 is limited, and this can likely be at least partially attributed to its lower solution-phase stability (vide infra).…”

“…21 Turner and coworkers then characterized the solution-phase photochemistry of 1 using nanosecond to microsecond TRIR spectroscopy and made the surprising observation that the bridging CO band of 2 is actually two separate bands split by 11 cm -1 in n-heptane at room temperature. 31 On this basis it was determined the structure of 2 must involve a distortion from D 3h symmetry, either due to a particularly strong interaction of the (µ-CO) 3 group with the Cp* rings, effectively lowering the symmetry to C 2v or to a nonlinear Cp*-Cr-Cr-Cp* arrangement, lowering the symmetry of the (µ-CO) 3 group to C s (at most). Studies into the reactivity of 2 with CO demonstrate a bimolecular rate constant ca.…”

“…1000 times greater than that observed for the reaction of the triplet Cp 2 Fe 2 (µ-CO) 3 with CO, suggesting that 2 may be a singlet. 31 A subsequent DFT study concluded that 2 is a singlet on the basis of comparison of calculated and experimental infrared frequencies, though the triplet was predicted to be energetically favored by more than 10 kcal/mol. 32 The reaction of 2 with THF to form 3 has also been observed, 30,31 although 2 does not appear to react with PPh 3 .…”

“…31 A subsequent DFT study concluded that 2 is a singlet on the basis of comparison of calculated and experimental infrared frequencies, though the triplet was predicted to be energetically favored by more than 10 kcal/mol. 32 The reaction of 2 with THF to form 3 has also been observed, 30,31 although 2 does not appear to react with PPh 3 . 29,31 The structure of the molybdenum congener of 1, [Cp*Mo(CO) 2 ] 2 ( 6) is generally similar to that of 1, with differences in the bonding angles.…”