Theodore J. Burkey

Development of Ultrafast Photochromic Organometallics and Photoinduced Linkage Isomerization of Arene Chromium Carbonyl Derivatives

We review recent studies of processes relevant to photoinduced linkage isomerization of organometallic systems with the goal of preparing organometallics with an efficient and ultrafast photochromic response. The organometallic system thus corresponds to two linkage isomers with different electronic environments that are responsible for different optical properties. Much of this work has focused on examining processes following irradiation of cyclopentadienyl manganese tricarbonyl derivatives (compounds 3-21) including solvent coordination, thermal relaxation, solvent displacement by tethered functional groups (chelation), dissociation of tethered functional groups, and linkage isomerization. A new platform is investigated for obtaining a photochromic response in new experiments with arene chromium dicarbonyl complexes. A photochromic response is observed for arene chromium dicarbonyl complexes with tethered pyridine and olefin functional groups based on light-driven linkage isomerization on the nanosecond time scale. Irradiation at 532 nm of 23 ([Cr{eta(6)-C(6)H(5)CH(2-Py-kappaN)CH(2)CH=CH(2)}(CO)(2)]) (Py = pyridine) results in the isomerization to 22 ([Cr{eta(6)-C(6)H(5)CH(2-Py)CH(2)-eta(2)-CH=CH(2)}(CO)(2)]), and 355 nm irradiation isomerizes 22 to 23. The ultrafast linkage isomerization has been investigated at room temperature in n-heptane solution on the picosecond to microsecond time scale with UV- or visible-pump and IR-probe transient absorption spectroscopy by comparing the dynamics with model compounds containing only a tethered pyridine. Irradiation of 24 ([Cr{eta(6)-C(6)H(5)(CH(2))(3)(2-Py)}(CO)(3)]) and 25 ([Cr{eta(6)-C(6)H(5)(CH(2))(2)(2-Py)}(CO)(3)]) at 289 nm induces CO loss to immediately yield a Cr-heptane solvent coordinated intermediate of the unsaturated Cr fragment, which then converts to the kappaN(1)-pyridine chelate within 200 and 100 ns, respectively. Irradiation of 26 ([Cr{eta(6)-C(6)H(5)CH(2)(2-Py)}(CO)(3)]) also induces CO loss to immediately yield three species: the Cr-heptane solvent coordinated intermediate, a kappaN(1)-Py nitrogen chelate, and an agostic eta(2)-chelate in which the pyridine is coordinated to the metal center via a C-H agostic bond as opposed to the nitrogen lone pair. Both the transient Cr-heptane coordinated intermediate and the agostic pyridine chelate convert to the stable kappaN(1)-pyridine chelate within 50 ns. Similar reaction dynamics and transient species are observed for the chelate 33 ([Cr{eta(6)-C(6)H(5)CH(2)(2-Py)-kappaN}(CO)(2)]) where a Cr-Py bond, not a Cr-CO bond, initially cleaves.

ALKANE COORDINATION BY MOLYBDENUM AND CHROMIUM PENTACARBONYLS: AN EXAMINATION OF THE ENERGETICS OF INTERMOLECULAR AGOSTIC BONDING

Abstract The photochemistry of Mo(CO)6 in heptane has been investigated using actinometry and photoacoustic calorimetry. The quantum yield for CO substitution on Mo(CO)6 by piperidine in heptane was found to be 0.93 for 337 nm irradiation at 25°C This was clearly different than the quantum yield found for CO substitution on Cr(CO)6 by piperidine (0.75). Two heat decays were observed following flash photolysis of Mo(CO)6 in the presence of piperidine. The first heat decay is independent of piperidine concentration (8–80 mM), while the lifetime (τ 2 ) of the second heat decay decreases with increasing piperidine concentration. A plot of 1/τ2 shows a first-order dependence on piperidine concentration. The first heat decay has been assigned to the displacement of CO on Mo(CO)6 by heptane and the second, to the displacement of heptane on Mo(CO)5(heptane) by piperidine. The second-order rate constant for heptane displacement is 4.8 × 107 M−1S−1. The enthalpies of CO displacement by heptane (ΔH 1) and heptane di...

High-pressure photoacoustic calorimetry

A high-pressure photoacoustic calorimeter has been developed to operate up to 200 MPa. Photoacoustic calorimetry can be used to study the microsecond kinetics and thermodynamics of reactions in solution. Both thermal expansion and volume of reaction contribute to the generation of the photoacoustic signal, and the separation of these contributions has been achieved by the variation of pressure. The construction of a stainless-steel flow cell, bellows, and a high-pressure manifold are described. These components enable rapid sample change and sample flow at high pressure. As a test, the enthalpy and volume of reaction were determined for diphenylcyclopropenone decomposition to diphenylethyne and carbon monoxide.

Equilibrium models of Cr3+ and Cu2+ with glutamate

Speciation diagrams and stability constants for glutamate (Glu) with (Cr3+) and (Cu2+) in aqueous solutions are presented. The current study covers a larger pH-range affording accurate results, and reveal a different set of species for Cu2+ and species not previously reported for Cr3+. For the Cu2+ Glu system, the most successful model that refined the potentiometric data contains the simple one-to-one complex, the bis-complex and the mono-hydroxo complex. The overall stability constants for Cu2+–Glu complexes have respective values of log β110 = 7.6 ± 0.2, log β11-1 = 1.3 ± 0.7, log β120 = 13.6 ± 0.2. Attempts to refine the stability constant for the mono-protonated metal complex (log β111) that was reported in the literature indicated that this mono-protonated species did not form to an appreciable amount to be important for the model presented here. For the Cr3+ Glu system, the overall stability constants for the complexes formed have the values of log β110 = 8.34 ± 0.03, log β11-1 = 1.9 ± 0.1 and log β11-2 = −4.6 ± 0.1. These results for Cr3+ system covers wider pH-range and have more accuracy than those reported previously. The NMR experiments for Glu revealed downfield shifts of all protons as pH values decrease from 11.21 to 2.85.

Bond energies, reaction volumes, and kinetics for σ- and π-complexes of MoCO5L.

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.

Volume of reaction, energetics, and kinetics of tetrahydrothiophene displacement of solvent from Mo(CO)5(alkane)

Laser-induced optoacoustic spectroscopy (LIOAS) was used to determine the enthalpies and volumes of reactions for each step in ligand substitution of molybdenum hexacarbonyl with tetrahydrothiophene (THT). The enthalpy for substitution of CO on Mo(CO)6 by an alkane and of coordinated alkane on Mo(CO)5(alkane) by THT are 114 and -99 kJ mol(-1), respectively. Likewise, the volumes of reaction are 13 and -16 ml mol(-1). These results allow the calculation of the bond energies for Mo-alkane (56 kJ mol(-1)) and Mo-THT (155 kJ mol(-1)). Furthers studies should reveal that, for certain cases, the volume of reaction can be neglected in the determination of enthalpies of ligand substitution by LIOAS.

Calibrating Reaction Enthalpies: Use of Density Functional Theory and the Correlation Consistent Composite Approach in the Design of Photochromic Materials

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)6, Cr(CO)5, Cr(CO)5(C2H4), Cr(CO)5(C2ClH3), Cr(CO)5(cis-(C2Cl2H2)), Cr(CO)5(gem-(C2Cl2H2)), Cr(CO)5(trans-(C2Cl2H2)), Cr(CO)5(C2Cl3H), Cr(CO)5(C2Cl4), CrO2, CrF2, CrCl2, CrCl4, CrBr2, CrBr4, CrOCl2, CrO2Cl2, CrOF2, and CrO2F2. 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-1) 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 (ΔrH°gas), 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).

Reactions of Zn2+, Cd2+ and Hg2+ with Free Adenine

We are reporting the fluorescence quenching, IR, Raman, 1 H-NMR, and potentiometric studies for the Zn 2+ :adenine and the Cd 2+ :adenine systems under ambient conditions. IR and Raman spectra suggest that Zn 2+ and Cd 2+ interact with adenine but the modes of interaction differ. Fluorescence spectra indicate that the interaction involving Zn 2+ is more favorable than that of Cd 2+ , and this effect is due to the difference in ionic radii. 1 H-NMR, potentiometry, and speciation diagrams indicate the formation of strong metal ion adenine complexes. Potentiometric titrations of the heavier member of group 12 metals (Hg 2+ ) show similar results to that of Zn 2+ and Cd 2+ . Some differences in the 1 H-NMR experiments appeared between both (Zn 2+ and Cd 2+ ) compared to that of Hg 2+ . Due to the fluorescence quenching of adenine, adenine can be used as a sensor of Zn 2+ and Cd 2+ .

Linkage Isomerization via Geminate Cage or Bimolecular Mechanisms: Time-Resolved Investigations of an Organometallic Photochrome

The extent of the photoinitiated linkage isomerization of dicarbonyl(3-cyanomethylpyridine-κN)(η(5)-methylcyclopentadienyl)manganese (4) to dicarbonyl(3-cyano-κN-methylpyridine)(η(5)-methylcyclopentadienyl)manganese (5) was examined by time-resolved infrared spectroscopy on picosecond to microsecond time scales in room temperature isooctane to determine the extent the isomerization occurs as a geminate cage rearrangement. We previously reported that a substantial part of the conversion between 4 and 5 must be a bimolecular reaction between a solvent coordinated dicarbonyl(η(5)-methylcyclopentadienyl)manganese (3) and uncoordinated 3-cyanomethylpyridine. For the purpose of designing a molecular device, it would be desirable for the photoisomerization to occur in a geminate cage reaction, because the faster the isomerization, the less opportunity for side reactions to occur. In this study, assignments of transients are identified by comparison with transients observed for model reactions. Within 100 μs after photolysis of 4 in isooctane, no 5 is observed. Instead, the solvent coordinated 3 is observed within 25 ps after irradiation. The formation of 5 is observed only in the presence of 9 mM 3-cyanomethylpyridine but not until 10-50 μs after irradiation of 4. Within the limits of detection, these results indicate the conversion of 4 to 5 occurs exclusively via a bimolecular reaction of 3-cyanomethylpyridine with solvent coordinated 3 and not a geminate cage reaction between 3-cyanomethylpyridine and the dicarbonyl(η(5)-methylcyclopentadienyl)manganese fragment.

Tricarbon­yl(2-methyl-2-η6-phenyl-1,3-dioxolane)chromium(0)

The structure of the title compound, [Cr(C10H12O2)(CO)3], is presented. The distorted piano-stool geometry features an off-center Cr(CO)3 fragment which reduces contact with the dioxolane ring. The dioxolane ring, in twisted conformation, is syn-oriented towards the Cr(CO)3 moiety.