Time-resolved IR studies of [Re(LL)(CO)4]+

Understanding the Vibrational Structure and Ultrafast Dynamics of the Metal Carbonyl Precatalyst [Mn(ppy)(CO)4]

The solution phase structure, vibrational spectroscopy, and ultrafast relaxation dynamics of the precatalyst species [Mn(ppy)(CO)4] (1) in solution have been investigated using ultrafast two-dimensional infrared (2D-IR) spectroscopy. By comparing 2D-IR data with the results of anharmonic density functional theory (DFT) calculations, we establish an excellent agreement between measured and predicted inter-mode couplings of the carbonyl stretching vibrational modes of 1 that relates to the atomic displacements of axial and equatorial ligands in the modes and the nature of the molecular orbitals involved in M-CO bonding. Measurements of IR pump-probe spectra and 2D-IR spectra as a function of waiting time reveal the presence of ultrafast (few ps) intramolecular vibrational energy redistribution between carbonyl stretching modes prior to vibrational relaxation. The vibrational relaxation times of the CO-stretching modes of 1 are found to be relatively solvent-insensitive, suggestive of limited solvent-solute interactions in the ground electronic state. Overall, these data provide a detailed picture of the complex potential energy surface, bonding and vibrational dynamics of 1, establishing a fundamental basis for the next steps in understanding and modulating precatalyst behavior.

Ultrafast transient IR spectroscopy and DFT calculations of ruthenium(ii) polypyridyl complexes

Ultrafast time-resolved infrared spectroscopy of [Ru(bpy)₃]²⁺ (bpy = 2,2'-bipyridine), [Ru(mbpy)₃]²⁺ (mbpy = 6-methyl-2,2'-bipyridine) and [Ru(mphen)₃]²⁺ (mphen = 2-methyl-1,10'-phenanthroline) in deuterated acetonitrile serves to elucidate the evolution of the system following pulsed excitation into the ¹MLCT band at 400 nm. While for [Ru(bpy)₃]²⁺ no intermediate state can be evidenced for the relaxation of the corresponding ³MLCT state back to the ground state, for [Ru(mbpy)₃]²⁺ and [Ru(mphen)₃]²⁺ an intermediate state with a lifetime of about 400 ps is observed. The species associated IR difference spectra of this state are in good agreement with the calculated difference spectra of the lowest energy ³dd state using DFT. The calculated potential energy curves for all the complexes in the triplet manifold along the metal-ligand distance show that for [Ru(bpy)₃]²⁺ the ³dd state is at a higher energy than the ³MLCT state and that there is a substantial barrier between the two minima. For [Ru(mbpy)₃]²⁺ and [Ru(mphen)₃]²⁺, the ³dd state is at a lower energy than the ³MLCT state.

Kinetics and Mechanism of Formation of the Platinum−Thallium Bond: The [(CN)5Pt−Tl(CN)3]3- Complex

Formation kinetics of the metal-metal bonded [(CN)(5)PtTl(CN)(3)](3)(-) complex from Pt(CN)(4)(2)(-) and Tl(CN)(4)(-) has been studied in the pH range of 5-10, using standard mix-and-measure spectrophotometric technique at pH 5-8 and stopped-flow method at pH > 8. The overall order of the reaction, Pt(CN)(4)(2)(-) + Tl(CN)(4)(-) right harpoon over left harpoon [(CN)(5)PtTl(CN)(3)](3)(-), is 2 in the slightly acidic region and 3 in the alkaline region, which means first order for the two reactants in both cases and also for CN(-) at high pH. The two-term rate law corresponds to two different pathways via the Tl(CN)(3) and Tl(CN)(4)(-) complexes in acidic and alkaline solution, respectively. The two complexes are in fast equilibrium, and their actual concentration ratio is controlled by the concentration of free cyanide ion. The following expression was derived for the pseudo-first-order rate constant of the overall reaction: k(obs) = (k(1)(a)[Tl(CN)(4)(-) + (k(1)(a)/K(f)))(1/(1 + K(p)[H(+)]))[CN(-)](free) + k(1)(b)[Tl(CN)(4)(-)] + (k(1)(b)/K(f)), where k(1)(a) and k(1)(b) are the forward rate constants for the alkaline and slightly acidic paths, K(f) is the stability constant of [(CN)(5)PtTl(CN)(3)](3)(-), and K(p) is the protonation constant of cyanide ion. k(1)(a) = 143 +/- 13 M(-)(2) s(-)(1), k(1)(b) = 0.056 +/- 0.004 M(-)(1) s(-)(1), K(f) = 250 +/- 54 M(-)(1), and log K(p) = 9.15 +/- 0.05 (I = 1 M NaClO(4), T = 298 K). Two possible mechanisms were postulated for the overall reaction in both pH regions, which include a metal-metal bond formation step and the coordination of the axial cyanide ion to the platinum center. The alternative mechanisms are different in the sequence of these steps.

Involvement of a Binuclear Species with the Re−C(O)O−Re Moiety in CO2 Reduction Catalyzed by Tricarbonyl Rhenium(I) Complexes with Diimine Ligands: Strikingly Slow Formation of the Re−Re and Re−C(O)O−Re Species from Re(dmb)(CO)3S (dmb = 4,4‘-Dimethyl-2,2‘-bipyridine, S = Solvent)

Excited-state properties of fac-[Re(dmb)(CO)(3)(CH(3)CN)]PF(6), [Re(dmb)(CO)(3)](2) (where dmb = 4,4'-dimethyl-2,2'-bipyridine), and other tricarbonyl rhenium(I) complexes were investigated by transient FTIR and UV-vis spectroscopy in CH(3)CN or THF. The one-electron reduced monomer, Re(dmb)(CO)(3)S (S = CH(3)CN or THF), can be prepared either by reductive quenching of the excited states of fac-[Re(dmb)(CO)(3)(CH(3)CN)]PF(6) or by homolysis of [Re(dmb)(CO)(3)](2). In the reduced monomer's ground state, the odd electron resides on the dmb ligand rather than on the metal center. Re(dmb)(CO)(3)S dimerizes slowly in THF, k(d) = 40 +/- 5 M(-1) s(-1). This rate constant is much smaller than those of other organometallic radicals which are typically 10(9) M(-1) s(-1). The slower rate suggests that the equilibrium between the ligand-centered and metal-centered radicals is very unfavorable (K approximately 10(-4)). The reaction of Re(dmb)(CO)(3)S with CO(2) is slow and competes with the dimerization. Photolysis of [Re(dmb)(CO)(3)](2) in the presence of CO(2) produces CO with a 25-50% yield based on [Re]. A CO(2) bridged dimer, (CO)(3)(dmb)Re-CO(O)-Re(dmb)(CO)(3) is identified as an intermediate. Both [Re(dmb)(CO)(3)](2)(OCO(2)) and Re(dmb)(CO)(3)(OC(O)OH) are detected as oxidation products; however, the previously reported formato-rhenium species is not detected.