Theoretical Predictions of Redox Potentials of Fischer-Type Chromium Aminocarbene Complexes

Redox potentials of series of chromium aminocarbene complexes with general formulas [(CO)₅CrC­(R)­N­(CH₃)₂] and [(CO)₄CrC­(R)­N­(CH₂CHCH₂)₂] were calculated using DFT methods for both metal-localized oxidation and ligand-localized reduction processes. The electrostatic contribution of solvation was approximated by the polarizable continuum model (PCM); specific interactions of the complexes with counterions of supporting electrolyte were considered by explicitly including these ions in the model. The theoretical redox potentials were correlated with experimental values, and the qualities of the results of the approaches used were compared. It was shown that both sets of calculated redox potentials reproduce the experimental data well. The mean average error of the calculated redox potentials was 0.088 V with the counterions and 0.111 V without the counterions. The best results were obtained for oxidation processes, where the mean average error decreased from 0.110 to 0.059 V due to the inclusion of the counterions

Ruthenium Stilbenyl and Diruthenium Distyrylethene Complexes: Aspects of Electron Delocalization and Electrocatalyzed Isomerization of the <i>Z</i>‑Isomer

Regio- and stereoselective insertion of the terminal ethynyl functions of 4-ethynylstilbene, the <i>E</i> and <i>Z</i> isomers of 4,4′-bis­(ethynylphenyl)­ethene and a backbone-rigidified cyclohexenyl derivative of the <i>Z</i> isomer into the Ru–H bond of the complex RuClH­(CO)­(P^<i>i</i>Pr₃)₂ provides the corresponding vinyl ruthenium complexes, which have been characterized spectroscopically and by X-ray crystallography. Large red shifts of the UV/vis absorption bands evidence efficient incorporation of the vinyl metal subunit(s) into the conjugated π-system. All complexes oxidize at low potentials. The various oxidized forms of all complexes were generated and characterized by UV/vis/NIR, IR and EPR spectroscopies. These studies indicated electrocatalytic <i>Z</i>→<i>E</i> isomerization of the oxidized <i>Z</i>-distyrylethene complex <b>Ru-Z2</b>, which is prevented in its backbone-rigidified derivative <b>Ru-Z2fix</b>. The radical cations of the <i>E</i> and the configurationally stable cyclohexene-bridged <i>Z</i>-derivatives are spin-delocalized on the EPR time scale but charge-localized on the faster IR time scale. The degree of ground-state charge delocalization in the mixed-valent state has been quantified by the incremental shifts of the Ru–CO bands upon stepwise oxidation to the radical cations and the dications and was found to be remarkably large (19% and 9%) considering redox splittings Δ<i>E</i>_1/2 of just 49 or 74 mV. Quantum chemical studies with various levels of sophistication reproduce our experimental results including the electronic spectra of the neutral complexes and the intrinsically localized nature of the radical cations of the dinuclear complexes

Extreme Basicity of Biguanide Drugs in Aqueous Solutions: Ion Transfer Voltammetry and DFT Calculations

Ion transfer voltammetry is used to estimate the acid dissociation constants <i>K</i>_a1 and <i>K</i>_a2 of the mono- and diprotonated forms of the biguanide drugs metformin (MF), phenformin (PF), and 1-phenylbiguanide (PB) in an aqueous solution. Measurements gave the p<i>K</i>_a1 values for MFH⁺, PFH⁺, and PBH⁺ characterizing the basicity of MF, PF, and PB, which are significantly higher than those reported in the literature. As a result, the monoprotonated forms of these biguanides should prevail in a considerably broader range of pH 1–15 (MFH⁺, PFH⁺) and 2–13 (PBH⁺). DFT calculations with solvent correction were performed for possible tautomeric forms of neutral, monoprotonated, and diprotonated species. Extreme basicity of all drugs is confirmed by DFT calculations of p<i>K</i>_a1 for the most stable tautomers of the neutral and protonated forms with explicit water molecules in the first solvation sphere included

Ruthenium Stilbenyl and Diruthenium Distyrylethene Complexes: Aspects of Electron Delocalization and Electrocatalyzed Isomerization of the <i>Z</i>‑Isomer

Regio- and stereoselective insertion of the terminal ethynyl functions of 4-ethynylstilbene, the <i>E</i> and <i>Z</i> isomers of 4,4′-bis­(ethynylphenyl)­ethene and a backbone-rigidified cyclohexenyl derivative of the <i>Z</i> isomer into the Ru–H bond of the complex RuClH­(CO)­(P^<i>i</i>Pr₃)₂ provides the corresponding vinyl ruthenium complexes, which have been characterized spectroscopically and by X-ray crystallography. Large red shifts of the UV/vis absorption bands evidence efficient incorporation of the vinyl metal subunit(s) into the conjugated π-system. All complexes oxidize at low potentials. The various oxidized forms of all complexes were generated and characterized by UV/vis/NIR, IR and EPR spectroscopies. These studies indicated electrocatalytic <i>Z</i>→<i>E</i> isomerization of the oxidized <i>Z</i>-distyrylethene complex <b>Ru-Z2</b>, which is prevented in its backbone-rigidified derivative <b>Ru-Z2fix</b>. The radical cations of the <i>E</i> and the configurationally stable cyclohexene-bridged <i>Z</i>-derivatives are spin-delocalized on the EPR time scale but charge-localized on the faster IR time scale. The degree of ground-state charge delocalization in the mixed-valent state has been quantified by the incremental shifts of the Ru–CO bands upon stepwise oxidation to the radical cations and the dications and was found to be remarkably large (19% and 9%) considering redox splittings Δ<i>E</i>_1/2 of just 49 or 74 mV. Quantum chemical studies with various levels of sophistication reproduce our experimental results including the electronic spectra of the neutral complexes and the intrinsically localized nature of the radical cations of the dinuclear complexes

Ruthenium Stilbenyl and Diruthenium Distyrylethene Complexes: Aspects of Electron Delocalization and Electrocatalyzed Isomerization of the <i>Z</i>‑Isomer

Regio- and stereoselective insertion of the terminal ethynyl functions of 4-ethynylstilbene, the <i>E</i> and <i>Z</i> isomers of 4,4′-bis­(ethynylphenyl)­ethene and a backbone-rigidified cyclohexenyl derivative of the <i>Z</i> isomer into the Ru–H bond of the complex RuClH­(CO)­(P^<i>i</i>Pr₃)₂ provides the corresponding vinyl ruthenium complexes, which have been characterized spectroscopically and by X-ray crystallography. Large red shifts of the UV/vis absorption bands evidence efficient incorporation of the vinyl metal subunit(s) into the conjugated π-system. All complexes oxidize at low potentials. The various oxidized forms of all complexes were generated and characterized by UV/vis/NIR, IR and EPR spectroscopies. These studies indicated electrocatalytic <i>Z</i>→<i>E</i> isomerization of the oxidized <i>Z</i>-distyrylethene complex <b>Ru-Z2</b>, which is prevented in its backbone-rigidified derivative <b>Ru-Z2fix</b>. The radical cations of the <i>E</i> and the configurationally stable cyclohexene-bridged <i>Z</i>-derivatives are spin-delocalized on the EPR time scale but charge-localized on the faster IR time scale. The degree of ground-state charge delocalization in the mixed-valent state has been quantified by the incremental shifts of the Ru–CO bands upon stepwise oxidation to the radical cations and the dications and was found to be remarkably large (19% and 9%) considering redox splittings Δ<i>E</i>_1/2 of just 49 or 74 mV. Quantum chemical studies with various levels of sophistication reproduce our experimental results including the electronic spectra of the neutral complexes and the intrinsically localized nature of the radical cations of the dinuclear complexes

Ruthenium Styryl Complexes with Ligands Derived from 2‑Hydroxy- and 2‑Mercaptopyridine and 2‑Hydroxy- and 2-Mercaptoquinoline

A series of ruthenium styryl complexes with potentially noninnocent κ²[N,O]⁻ or κ²[N,S]⁻ ligands have been prepared by treatment of 5-coordinated 16-valence-electron ruthenium styryl complexes Ru­(CO)­Cl­(P^<i>i</i>Pr₃)₂(CHCH-C₆H₄-4R) with deprotonated bidentate 2-hydroxy- or 2-mercaptopyridines or 2-hydroxy- or 2-mercaptoquinolines. These 6-coordinated complexes have been characterized by NMR and IR spectroscopy and by cyclic voltammetry. Moreover, the structures of complexes <b>1d</b>, <b>2a</b>, <b>3c</b>, <b>5b</b>, and <b>6b</b> have been established by X-ray crystallography. Our results indicate that the pyridine-derived complexes exist as two isomers that differ with respect to the orientation of the κ²[N,O]⁻ or κ²[N,S]⁻ donor ligands relative to the CO and alkenyl ligands in the equatorial plane. The equilibrium between the two isomers is thermodynamically controlled. Thus, the relative amount of the minor isomer increases at higher temperatures. With the 2-hydroxyquinoline- or 2-mercaptoquinoline-derived ligands only one isomer is observed. Electrochemical studies show that these complexes undergo one or two reversible consecutive one-electron oxidations, the potentials of which respond to the electronic properties of the 4-substituent at the styryl ligand and those of the ancillary chelate ligand. Strong ligand contributions to the first oxidation of the complexes were experimentally verified by IR and EPR spectroelectrochemistry. Quantum chemical calculations reproduce our experimental results, including the positions of the Ru­(CO) vibrational bands of the neutral complexes and of their corresponding radical cations. Our combined results indicate that the oxidation of all complexes is dominated by the styryl ligand, irrespective of the electronic nature of the 4-substituent and of the [N,O]⁻ or [N,S]⁻ chelate ligand

Ruthenium Styryl Complexes with Ligands Derived from 2‑Hydroxy- and 2‑Mercaptopyridine and 2‑Hydroxy- and 2-Mercaptoquinoline

A series of ruthenium styryl complexes with potentially noninnocent κ²[N,O]⁻ or κ²[N,S]⁻ ligands have been prepared by treatment of 5-coordinated 16-valence-electron ruthenium styryl complexes Ru­(CO)­Cl­(P^<i>i</i>Pr₃)₂(CHCH-C₆H₄-4R) with deprotonated bidentate 2-hydroxy- or 2-mercaptopyridines or 2-hydroxy- or 2-mercaptoquinolines. These 6-coordinated complexes have been characterized by NMR and IR spectroscopy and by cyclic voltammetry. Moreover, the structures of complexes <b>1d</b>, <b>2a</b>, <b>3c</b>, <b>5b</b>, and <b>6b</b> have been established by X-ray crystallography. Our results indicate that the pyridine-derived complexes exist as two isomers that differ with respect to the orientation of the κ²[N,O]⁻ or κ²[N,S]⁻ donor ligands relative to the CO and alkenyl ligands in the equatorial plane. The equilibrium between the two isomers is thermodynamically controlled. Thus, the relative amount of the minor isomer increases at higher temperatures. With the 2-hydroxyquinoline- or 2-mercaptoquinoline-derived ligands only one isomer is observed. Electrochemical studies show that these complexes undergo one or two reversible consecutive one-electron oxidations, the potentials of which respond to the electronic properties of the 4-substituent at the styryl ligand and those of the ancillary chelate ligand. Strong ligand contributions to the first oxidation of the complexes were experimentally verified by IR and EPR spectroelectrochemistry. Quantum chemical calculations reproduce our experimental results, including the positions of the Ru­(CO) vibrational bands of the neutral complexes and of their corresponding radical cations. Our combined results indicate that the oxidation of all complexes is dominated by the styryl ligand, irrespective of the electronic nature of the 4-substituent and of the [N,O]⁻ or [N,S]⁻ chelate ligand

Electronic Excitations in Fischer-Type Cr and W Aminocarbene Complexes: A Combined ab Initio and Experimental Study

The influence of the substitution on the carbene ligand in the series of Fischer-type Cr and W aminocarbene complexes was studied experimentally by UV–vis spectroscopy and theoretically by comparative ab initio SA-CASSCF/MS-CASPT2 and TD-DFT methods. Both calculations interpreted the experimental UV–vis spectra and their variations caused by substitution effects well. TD-DFT analysis of individual transitions using electron density redistributions indicated that the variation of the absorption spectra due to substitution is accompanied by a change in the character of the low-lying excited states participating in the visible bands. Correlated MS-CASPT2 calculations confirmed the TD-DFT assignments of the lowest-lying transitions in the visible region almost quantitatively

Solar Cell Sensitizer Models [Ru(bpy-R)₂(NCS)₂] Probed by Spectroelectrochemistry

Complexes [Ru­(bpy-R)₂(NCS)₂], where R = H (<b>1</b>), 4,4′-(CO₂Et)₂ (<b>2</b>), 4,4′-(OMe)₂ (<b>3</b>), and 4,4′-Me₂ (<b>4</b>), were studied by spectroelectrochemistry in the UV–vis and IR regions and by in situ electron paramagnetic resonance (EPR). The experimental information obtained for the frontier orbitals as supported and ascertained by density functional theory (DFT) calculations for <b>1</b> is relevant for the productive excited state. In addition to the parent <b>1</b>, the ester complex <b>2</b> was chosen for its relationship to the carboxylate species involved for binding to TiO₂ in solar cells; the donor-substituted <b>3</b> and <b>4</b> allowed for better access to oxidized forms. Reflecting the metal-to-ligand (Ru → bpy) charge-transfer characteristics of the compounds, the electrochemical and EPR results for compounds <b>1</b>–<b>4</b> agree with previous notions of one metal-centered oxidation and several (bpy-R) ligand-centered reductions. The first one-electron reduction produces extensive IR absorption, including intraligand transitions and broad ligand-to-ligand intervalence charge-transfer transitions between the one-electron-reduced and unreduced bpy-R ligands. The electron addition to one remote bpy-R ligand does not significantly affect the N–C stretching frequency of the Ru^IINCS unit. Upon oxidation of Ru^II to Ru^III, however, the single N–C stretching band exhibits a splitting and a shift to lower energies. The DFT calculations serve to reproduce and understand these effects; they also suggest significant spin density on S for the oxidized form

Anharmonicity Effects in IR Spectra of [Re(X)(CO)₃(α-diimine)] (α-diimine = 2,2′-bipyridine or pyridylimidazo[1,5‑<i>a</i>]pyridine; X = Cl or NCS) Complexes in Ground and Excited Electronic States

Infrared spectra of [Re­(X)­(CO)_<b>3</b>(α-diimine)] (α-diimine = 2,2′-bipyridine, X = Cl, NCS, or pyridylimidazo­[1,5-<i>a</i>]­pyridine, X = Cl) in the ground and the lowest triplet electronic states were calculated by a global hybrid density functional going beyond the harmonic level by means of second-order vibrational perturbation theory (VPT2) and including bulk solvent effects by the polarizable continuum model (PCM). The full-dimensionality (FD) VPT2 is compared with the reduced-dimensionality (RD) model, where only selected vibrational modes are calculated anharmonically. The simulated difference IR spectra (excited state minus ground state) in the ν­(CO) region closely match experimental time-resolved infrared (TRIR) spectra. Very good agreement was also obtained for ground-state spectra in the fingerprint region. In comparison with the harmonic simulated spectra, the calculated anharmonic frequencies are closer to experimental values and do not require scaling when the B3LYP functional is used. Several spectral features due to combination bands have been identified by VPT2 simulations in the ν­(CO) spectral region, which are of importance for a correct interpretation of TRIR experiments