Electron Dynamics and IR Peak Coalescence in Bridged Mixed Valence Dimers Studied by Ultrafast 2D-IR Spectroscopy.

Dynamic IR peak coalescence and simulations based on the optical Bloch equations have been used previously to predict the rates of intramolecular electron transfer in a group of bridged mixed valence dimers of the type [Ru3(O)(OAc)6(CO)L]-BL-[Ru3(O) (OAc)6(CO)L]. However, limitations of the Bloch equations for the analysis of dynamical coalescence in vibrational spectra have been described. We have used ultrafast 2D-IR spectroscopy to investigate the vibrational dynamics of the CO spectator ligands of several dimers in the group. These experiments reveal that no electron site exchange occurs on the time scale required to explain the observed peak coalescence. The high variability in FTIR peak shapes for these mixed valence systems is suggested to be the result of fluctuations in the charge distributions at each metal cluster within a single-well potential energy surface, rather than the previous model of two-site exchange.

Determining equilibrium fluctuations using temperature-dependent 2D-IR.

We demonstrate the capability of temperature-dependent 2D-IR to characterize sources of vibrational population transfer. In a model system of iron diene tricarbonyl "piano stool" complexes, this approach reveals symmetry breaking associated with equilibrium fluctuations and differentiates these from fluxional rearrangement. Tricarbonyl(1,3-butadiene)iron and tricarbonyl(1,5-cyclooctadiene)iron are shown to undergo intramolecular vibrational redistribution (IVR) coupled to the wagging motion of their carbonyl ligands. In the case of both molecules, these equilibrium fluctuations are distinguished from chemical exchange behaviors by their temperature dependence and arguments of molecular symmetry.

Ultrafast observation of a solvent dependent spin state equilibrium in CpCo(CO).

We report the observation of a solvent-dependent spin state equilibrium in the 16-electron photoproduct CpCo(CO). Time-resolved infrared spectroscopy has been used to observe the concurrent formation of two distinct solvated monocarbonyl photoproducts, both of which arise from the same triplet CpCo(CO) precursor. Experiments in different solvent environments, combined with electronic structure theory calculations, allow us to assign the two solvated photoproducts to singlet and triplet CpCo(CO)(solvent) complexes. These results add to our previous picture of triplet reactivity for 16-electron organometallic photoproducts, in which triplets were not believed to interact strongly with solvent molecules. In the case of this photoproduct, it appears that spin crossover does not present a significant barrier to reactivity, and relative thermodynamic stabilities determine the spin state of the CpCo(CO) photoproduct in solution on the picosecond time scale. While the existence of transition metal complexes with two thermally accessible spin states is well-known, this is, to our knowledge, the first observation of a transient photoproduct that exhibits an equilibrium between two stable spin states, and also the first observed case in which a solvent has been able to coordinate as a token ligand to two spin states of the same photoproduct.

Time-resolved IR studies on the mechanism for the functionalization of primary C-H bonds by photoactivated Cp*W(CO)3(Bpin).

Recently, transition-metal-boryl compounds have been reported that selectively functionalize primary C-H bonds in alkanes in high yield. We have investigated this process with one of the well-defined systems that reacts under photochemical conditions using both density functional theory calculations and pico- through microsecond time-resolved IR spectroscopy. UV irradiation of Cp*W(CO)(3)(Bpin) (Cp* = C(5)(CH(3))(5); pin = 1,2-O(2)C(2)-(CH(3))(4)) in neat pentane solution primarily results in dissociation of a single CO ligand and solvation of the metal by a pentane molecule from the bath within 2 ps. The spectroscopic data imply that the resulting complex, cis-Cp*W(CO)(2)(Bpin)(pentane), undergoes C-H bond activation by a sigma-bond metathesis mechanism--in 16 micros, a terminal hydrogen on pentane appears to migrate to the Bpin ligand to form a sigma-borane complex, Cp*W(CO)(2)(H-Bpin)(C(5)H(11)). Our data imply that the borane ligand rotates until the boron is directly adjacent to the C(5)H(11) ligand. In this configuration, the B-H sigma-bond is broken in favor of a B-C sigma-bond, forming Cp*W(CO)(2)(H)(C(5)H(11)-Bpin), a tungsten-hydride complex containing a weakly bound alkylboronate ester. The ester is then eliminated to form Cp*W(CO)(2)(H) in approximately 170 micros. We also identify two side reactions that limit the total yield of bond activation products and explain the 72% yield previously reported for this complex.