Benjamin J. Lear

Oxalate bridged MM (MM = Mo2, MoW, and W2) quadruply bonded complexes as test beds for current mixed valence theory: looking beyond the intervalence charge transfer transition.

The spectroscopic features of a series of oxalate bridged complexes [((t)BuCO(2))(3)MM](2)-mu(2)-O(2)CCO(2) (where MM = Mo(2), MoW, and W(2)) in their neutral and singly oxidized (mixed valence) states are examined as a function of temperature and solvent. A large degree of electronic coupling between the two MM centers is evident, principally involving the MM delta orbitals mediated by the oxalate bridge pi* orbital. In the oxidized states these mixed valence ions show solvent independent intervalence charge transfer (alternatively termed charge resonance) bands, consistent with assignment to Class III (or electronically delocalized) within the Robin-Day classification scheme. In both the neutral and oxidized states these complexes also show an intense metal-to-ligand charge-transfer (MLCT) transition, involving the lowest unoccupied molecular orbital (LUMO) of the bridge. The solvent and temperature dependence of this transition is also reported along with an inspection and simulation of the vibronic features, which are notably altered when switching between the neutral and the mixed valence states as well as with variation of the nature of the MM unit. Collectively, these observations allow us to comment on the validity and limitations of current theories dealing with mixed valence ions that have hitherto ignored the information that can be gained from MLCT transitions.

Extent of M2 δ to ligand π-conjugation in neutral and mixed valence states of bis(4-isonicotinate)-bis(2,4,6-triisopropylbenzoate) dimetal complexes (MM), where M = Mo or W, and their adducts with tris(pentafluorophenyl)boron.

The reaction between W(2)(T(i)PB)(4), where T(i)PB = 2,4,6-triisopropylbenzoate, and 2 equiv of 4-isonicotinic acid (nicH) yields the compound W(2)(T(i)PB)(2)(nic)(2), 2, and T(i)PBH. Compound 2 is related to the previously reported molybdenum analog, Mo(2)(T(i)PB)(2)(nic)(2), 1. Compounds 1 and 2 react with 2 equiv of B(C(6)F(5))(3) in THF to form the adducts M(2)(T(i)PB)(2)(nic-B(C(6)F(5))(3))(2), 1B (M = Mo) and 2B (M = W), which have been crystallographically characterized as solvates M(2)(T(i)PB)(2)(nic-B(C(6)F(5))(3))(2)·2THF n-hexane. Compounds 1 and 2 are intensely colored due to M(2) δ to π* MLCT transitions, and upon complexation with B(C(5)F(5))(3) to give 1B and 2B, these bands shift to lower energy and gain in intensity. Each compound shows two one-electron ligand-based reductions with a ΔE(1/2) = 120 (1), 300 (1B), 440 (2), and 650 mV (2B). The larger ΔE(1/2) values for the tungsten compounds reflect the greater orbital mixing of the metal 5d-based M(2) δ and the nic π* LUMO. Reduction of solutions of 1B and 2B with (C(5)Me(5))(2)Co leads to the anions 1B(-) and 2B(-), which have been characterized spectroscopically by electron paramagnetic resonance (EPR) and UV-vis-NIR absorption. The EPR spectra of 1B(-) and 2B(-) are consistent with ligand-based (i.e., organic) radicals. The electronic spectra contain low-energy narrow charge resonance (IVCT) bands at 3800 (1B(-)) and 4500 cm(-1) (2B(-)), consistent with fully delocalized mixed valence radical anions. The results are compared with electronic structure calculations and with the spectral features of the metal-centered delocalized mixed valence radical cations [(Bu(t)CO(2))(3)M(2)](2)-μ(2)-(O(2)C-CO(2))(+), to which they are remarkably similar, as well as with other organic-based mixed valence systems.

Effect of Protonation upon Electronic Coupling in the Mixed Valence and Mixed Protonated Complex, [Ni(2,3-pyrazinedithiol)2]

We demonstrate that protonation of a mixed valence molecule, generating a mixed valence mixed protonated (MVMP) state, results in a severe reduction in the electronic coupling intimately connected with electron transfer kinetics. This phenomenon is illustrated by synthesizing a mixed valence molecule, [Ni(2,3-pyrazinedithiol)2], that can be asymmetrically protonated, rendering the MVMP state. We characterize the structural, electronic, vibrational, and magnetic properties of this complex in five different states, including the mixed valence and MVMP states, and then analyze the intervalence charge transfer (IVCT) band to demonstrate a five-fold reduction in electronic coupling upon protonation. We conclude that the reduction in electronic coupling is a result of the asymmetry of the electronic orbitals of the redox sites that results from the asymmetric protonation. This conclusion suggests that many systems designed to link electron and proton transfer will also exhibit a decrease in electronic coupling upon protonation as the strength of the interaction between redox and protonation sites is increased.

Ligand Control over the Electronic Properties within the Metallic Core of Gold Nanoparticles

The behavior of electrons within the metallic core of gold nanoparticles (AuNPs) can be controlled by the nature of the surface chemistry of the AuNPs. Specifically, the conduction electron spin resonance (CESR) spectra of AuNPs of diameter 1.8-1.9 nm are sensitive to ligand exchange of hexanethiol for 4-bromothiophenol on the surface of the nanoparticle. Chemisorption of the aromatic ligand leads to a shift in the metallic electrons g-factor toward the value expected for pure gold systems, suggesting an increase in metallic character for the electrons within the gold core. Analysis by UV/Vis absorption spectroscopy reveals a concomitant bathochromic shift of the surface plasmon resonance band of the AuNP, indicating that other electronic properties of AuNPs are also affected by the ligand exchange. In total, our results demonstrate that the chemical nature of the ligand controls the valence band structure of AuNPs.

Direct Test of the Equivalency of Dynamic IR and Dynamic Raman Spectroscopies As Techniques for Observing Ultrafast Molecular Dynamics

We report the temperature-dependent infrared (IR) and Raman spectra of Fe(CO)3(η(4)-norbornadiene). This molecule undergoes carbonyl ligand site exchange on the vibrational time scale, and the effect of this exchange is observable as coalescence of the carbonyl bands in both the IR and Raman spectra. We outline a theory that we used to account for these effects and report simulations of the experimental spectra. We used these simulations to extract the carbonyl ligand exchange rates at various temperatures from the IR and Raman data. This data was used to calculate the activation energy for carbonyl exchange, yielding activation energies of 1.2 ± 0.2 and 1.4 ± 0.1 kcal/mol from the IR and Raman data, respectively. These activation energies are statistically identical and are consistent with previously reported values. This constitutes the first direct comparison between dynamic IR and Raman spectroscopies, and we find them to give identical results.

Comparing the energetic and dynamic contributions of solvent to very low barrier isomerization using dynamic steady-state vibrational spectroscopy.

We report the solvent-dependent dynamics of carbonyl site exchange for Fe(CO)3(η(4)-norbornadiene) (FeNBD) in a series of linear and nonlinear alkanes. The barrier to exchange is very low (∼1.5 kcal/mol), and the resulting carbonyl dynamics are rapid enough to lead to a change in the vibrational spectra, which we use to extract the ultrafast rates of exchange from linear Raman spectra of FeNBD. The dynamics of the carbonyl exchange has a weak dependence upon the solvent, and we analyze this dependence in terms of energetic (reaction field) and dynamic (Kramers theory) models of solvent effects. We find that both models can reproduce the observed solvent dependence but that the dynamic model provides a more physically satisfying picture for the solvent effects than does the energetic model. Finally, we find that cyclohexane is more strongly coupled to the dynamics of FeNBD than are the noncyclic alkanes.

Solvent versus Temperature Control over the Infrared Band Shape and Position in Fe(CO)3(η4-Ligand) Complexes

The solute-solvent interactions between Fe(CO)3(η(4)-cyclooctatetraene) (FeCOT) and 27 solvents were examined by infrared (IR) spectroscopy. The observed change in band shape and position of the carbonyl bands as a function of solvent was found to be very similar to that previously observed in temperature-dependent IR experiments of Fe(CO)3(η(4)-norborndiene) (FeNBD). While for FeNBD the change in band shape results from dynamic exchange of carbonyl ligands, temperature-dependent IR experiments in ethyl acetate show that the observed changes are not a result of carbonyl ligand site exchange for FeCOT. We therefore concluded that the solvent dependence of the IR spectra must be a consequence of a static solute-solvent interaction. We find that the linear solvation energy model (J. Am. Chem. Soc. 1977, 99, 6027-6038; Chem. Soc. Rev. 1993, 22, 409-416) provides a satisfactory account for the spectral changes due to the solvent. From this model, we are able to conclude that the solute-solvent interactions of this system are influenced by the solvents polarizability and hydrogen bonding acidity. We also observed interdependence between the change in fwhm and band positions for all three carbonyl bands, which brings us to the conclusion that the observed changes in the IR carbonyl band shape of FeCOT are a consequence of the solute-solvent interactions, rather than any solvent friction effects. This implies that care must be taken to separate the effects of chemical dynamics and solvatochromism when examining IR spectra of molecules suspected of exhibiting dynamically broadened vibrational spectra.

Probing ligand-induced modulation of metallic states in small gold nanoparticles using conduction electron spin resonance

Thiolate-protected gold nanoparticles have a rich history as model systems for understanding the physical and chemical properties of metallic nanoscale materials that, in turn, form the basis for applications in areas such as molecular electronics, photocatalytic systems, and plasmonic solar cells. It is well known that the electronic properties of gold nanoparticles can be tuned by modifying the geometry, size and dielectric surrounding of the particle. However, much less is known of how modifications to the surface chemistry modulates the electronic properties of gold nanoparticles. In part, this stems from the fact that there are few good tools for measuring the electronic properties with the sensitivity required for following the response to subtle changes in surface chemistry. In this work, we demonstrate conduction spin electron resonance (CESR) to be a sensitive and selective probe to determine how changes in surface chemistry of gold nanoparticles affect the metallic states near the Fermi energy. Using a series of para-substituted aromatic thiolate ligands, we find that the g-factor, as measured using CESR, correlates well with experimental and computational parameters often used to understand ligand effects in classical inorganic complexes. This suggests classical inorganic reasoning can function as a framework for understanding how to control the electronic properties of gold nanoparticles using their surface chemistry.

Steady-State Spectroscopic Analysis of Proton-Dependent Electron Transfer on Pyrazine-Appended Metal Dithiolenes [Ni(pdt)2], [Pd(pdt)2], and [Pt(pdt)2] (pdt = 2,3-Pyrazinedithiol)

We report the structural, electronic, and acid/base properties of a series of ML2 metal dithiolene complexes, where M = Ni, Pd, Pt and L = 2,3-pyrazinedithiol. These complexes are non-innocent and possess strong electronic coupling between ligands across the metal center. The electronic coupling can be readily quantified in the monoanionic mixed valence state using Marcus-Hush theory. Analysis of the intervalence charge transfer (IVCT) band reveals that that electronic coupling in the mixed valence state is 5800, 4500, and 5700 cm(-1) for the Ni, Pd, and Pt complexes, respectively. We then focus on their response to acid titration in the mixed valence state, which generates the asymmetrically protonated mixed valence mixed protonated state. For all three complexes, protonation results in severe attenuation of the electronic coupling, as measured by the IVCT band. We find nearly 5-fold decreases in electronic coupling for both Ni and Pt, while, for the Pd complex, the electronic coupling is reduced to the point that the IVCT band is no longer observable. We ascribe the reduction in electronic coupling to charge pinning induced by asymmetric protonation. The more severe reduction in coupling for the Pd complex is a result of greater energetic mismatch between ligand and metal orbitals, reflected in the smaller electronic coupling for the pure mixed valence state. This work demonstrates that the bridging metal center can be used to tune the electronic coupling in both the mixed valence and mixed valence mixed protonated states, as well as the magnitude of change of the electronic coupling that accompanies changes in protonation state.

Photothermal Effectiveness of Magnetite Nanoparticles: Dependence upon Particle Size Probed by Experiment and Simulation

The photothermal effect of nanoparticles has proven efficient for driving diverse physical and chemical processes; however, we know of no study addressing the dependence of efficacy on nanoparticle size. Herein, we report on the photothermal effect of three different sizes (5.5 nm, 10 nm and 15 nm in diameter) of magnetite nanoparticles (MNP) driving the decomposition of poly(propylene carbonate) (PPC). We find that the chemical effectiveness of the photothermal effect is positively correlated with particle volume. Numerical simulations of the photothermal heating of PPC supports this observation, showing that larger particles are able to heat larger volumes of PPC for longer periods of time. The increased heating duration is likely due to increased heat capacity, which is why the volume of the particle functions as a ready guide for the photothermal efficacy.