Model compounds that structurally mimic the hydrogen-producing active site of [FeFe]-hydrogenases have been studied to explore potential ground-state electronic structure effects on reaction mechanisms compared to hexacarbonyl derivatives. The time-dependent behavior of Fe2(μ-S2C3H6)(CO)4(PMe)2 (A) in room temperature n-heptane and acetonitrile solutions was examined using various ultrafast UV and visible excitation pulses with broadband IR-probe spectroscopy of the carbonyl (CO) stretching region. Ground- and excited-state electronic and CO-stretching mode vibrational properties of the possible isomers of A were also examined using density functional theory (DFT) computations. In n-heptane, 355 and 532 nm excitation resulted in short-lived (135 ± 74 ps) bands assigned to excited-state, CO-loss photoproducts. These bands decay away, forming new long-lived absorptions that are likely a mixture of isomers of both three-CO and four-CO ground-state isomers. These new bands grow in with a time scale of 214 ± 119 ps and persist for more than 100 ns. In acetonitrile, similar results are seen with a 532 nm pump, but the 355 nm data lack evidence of the longer-lived bands. In either solvent, the 266 nm pump data seem to also lack longer-lived bands, but the intensities are significantly lower in this data, making firm conclusions more difficult. We suggest that these wavelength-dependent excitation dynamics significantly alter potential mechanisms and efficiencies for light-driven catalysis.

The photosubstitution reactions of molybdenum hexacarbonyl with σ and π donor ligands were investigated using photoacoustic calorimetry and computational methods in a series of linear alkane solvents (pentane, hexane, heptane, octane, decane, and dodecane). The results show that reaction volumes make a significant contribution to the photoacoustic signal and must be considered during thermodynamic calculations based on photoacoustic measurements. The enthalpies of CO substitution by an alkane solvent and subsequent substitution by each Lewis base were determined. Corresponding Mo-L bond energies (kcal mol(-1)) were calculated: L = linear alkanes (13), triethylsilane (26), 1-hexyne (27), 1-hexene (27), and benzene (17). The relative energies are in agreement with computational results. The experimental reaction volume for CO substitution by alkane was positive (15 mL mol(-1)) and negative or close to zero for alkane substitution by a Lewis base (for example, -11 mL mol(-1) for triethylsilane and 3.6 mL mol(-1) for benzene). The errors in the experimental and computational reaction volumes are large and often comparable to the reaction volumes. An improved calibration of the methods as well as a better understanding of the underlying physics involved is needed. For the Lewis bases reported in this study, the second-order rate constants for the displacement of a coordinated alkane are less than diffusion control (5 × 10(6)-4 × 10(7) M(-1) s(-1)) and decrease monotonically with the alkane chain length. The rate constants correlate better with steric effects than with bond energies. An interchange mechanism is consistent with the results.

Chromium arene tricarbonyl complexes with tethered pyridinyl groups [Cr{η 6 -C 6 H 5 (CH 2 ) n (2-Py)}(CO) 3 ] (4−6) (2-Py = 2-pyridinyl, n = 1−3, respectively) were synthesized and irradiated to form the chelates [Cr{η 6 -C 6 H 5 (CH 2 ) n (2-Py)-κN}(CO) 2 ] (7−9). Studies examined the effect of ring size and structure on chromophore λ max , stability, and photosensitivity, which are factors important for photochromes based on linkage isomerization of tethered functional groups. The studies also include [Cr{η 6 -C 6 H 5 CH(2-Py)CH 2 CHCH 2 }(CO) 3 ] (3), which has a bifunctional tether of propenyl and pyridinyl groups, and irradiation produces the linkage isomers [Cr{η 6 -C 6 H 5 (CH(2-Py)CH 2 CHCH 2 )-κN}(CO) 2 ] (1) and [Cr{η 6 -C 6 H 5 (CH(2-Py)CH 2 CHCH 2 )(η 2 -CHCH 2 )}(CO) 2 ] (2). X-ray crystal structures for 7−9 show that the dihedral angle between the coordinated pyridinyl groups and the phenyl-chromium centroid increases from 1 to 73°(n = 1−3, respectively). The experimental and TDDFT computed optical changes accompanying an increase in the dihedral angle are modest and not monotonic for 7−9 due to structural changes inherent in the chelate rings. An increase in Cr−N bond lengths and decrease in their bond energies were observed experimentally and computationally for the series of 7−9. The quantum yields for formation of the five-, six-, and seven-membered chelate rings during the conversion of 4−6 to 7−9, respectively, were within experimental error for that observed for conversion of 10 [Cr{η 6 -C 6 H 6 }(CO) 3 ] with free pyridine to 11 [Cr{η 6 -C 6 H 6 }(C 5 H 5 N-κN)(CO) 2 ], indicating that the product-determining step precedes chelation. The enthalpies for chelation of 4−6 to 7−9 were determined independently by photoacoustic calorimetry and DFT computations. The computationally derived mechanism for thermal isomerization of 1 to 2 indicates that the transition state is a dissociative interchange with a free energy of activation of 27.9 kcal mol −1 (1 → 2), a result consistent with an experimentally bistable photochrome. The results indicate which tether properties are important for optimizing photochrome performance. ■ INTRODUCTIONRecent studies have examined organometallics that undergo a linkage isomerization as a photochromic mechanism. 1,2 This is a departure from other studies where the metal serves as a spectator and binds to a photochromic ligand but is not involved in bond-forming or -breaking processes. 3 Our group investigates the design of photochromes based on linkage isomerization: in particular, systems where functional groups tethered to a metal center undergo a photoinitiated exchange (Scheme 1). The isomerization in this case relies on the reversible interconversion of two chelates, each with a different coordinated functional group and ultimately a different ring system. The length of a tether and its composition (heteroatom, branching, or multiple bonds) are anticipated to affect the chelate ring properties, including the rate of chelate ring formation, ring strain, and ...

Competing degenerate pathways for ring inversion in organometallic complexes are proposed to be ubiquitous examples that adhere to the principle of microscopic reversibility. The NMR spectra for ring inversion of two chromium arene dicarbonyl pyridyl chelates ([Cr{η6-C6H5(CH2) n (2-Py-κN)}(CO)2]; 2-Py = 2-pyridyl, n = 2 (1), and 3 (2)) and a manganese cyclopentadienyl dicarbonyl methyl sulfide chelate ([Mn{η5-C5H4COC(SCH3)2(SCH3-κS)}(CO)2] (3)) were characterized via variable-temperature NMR spectroscopy and DFT theoretical calculations.

Acquisition of highly accurate energetic data for chromium-containing molecules and various chromium carbonyl complexes is a major step toward calibrating bond energies and thermal isomerization energies from mechanisms for Cr-centered photochromic materials being developed in our laboratories. The performance of six density functionals in conjunction with seven basis sets, utilizing Gaussian-type orbitals, has been evaluated for the calculation of gas-phase enthalpies of formation and enthalpies of reaction at 298.15 K on various chromium-containing systems. Nineteen molecules were examined: Cr(CO), Cr(CO), Cr(CO)(CH), Cr(CO)(CClH), Cr(CO)(cis-(CClH)), Cr(CO)(gem-(CClH)), Cr(CO)(trans-(CClH)), Cr(CO)(CClH), Cr(CO)(CCl), CrO, CrF, CrCl, CrCl, CrBr, CrBr, CrOCl, CrOCl, CrOF, and CrOF. The performance of 69 density functionals in conjunction with a single basis set utilizing Slater-type orbitals (STO) and a zeroth-order relativistic approximation was also evaluated for the same test set. Values derived from density functional theory were compared to experimental values where available, or values derived from the correlation consistent composite approach (ccCA). When all reactions were considered, the functionals that exhibited the smallest mean absolute deviations (MADs, in kcal mol) from ccCA-derived values were B97-1 (6.9), VS98 (9.0), and KCIS (9.4) in conjunction with quadruple-ζ STO basis sets and B97-1 (9.3) in conjunction with cc-pVTZ basis sets. When considering only the set of gas-phase reaction enthalpies (ΔH°), the functional that exhibited the smallest MADs from ccCA-derived values were B97-1 in conjunction with cc-pVTZ basis sets (9.1) and PBEPBE in conjunction with polarized valence triple-ζ basis set/effective core potential combination for Cr and augmented and multiple polarized triple-ζ Pople style basis sets (9.5). Also of interest, certainly because of known cancellation of errors, PBEPBE with the least-computationally expensive basis set combination considered in the present study (valence double-ζ basis set/effective core potential combination for Cr and singly-polarized double-ζ Pople style basis sets) also provided reasonable accuracy (11.1). An increase in basis set size was found to have an improvement in accuracy for the best performing functional (B97-1).