Electronic Structures of the [Fe(N₂)(SiP^iPr₃)]^+1/0/–1 Electron Transfer Series: A Counterintuitive Correlation between Isomer Shifts and Oxidation States

The electronic structure analysis of the low-spin iron­(II/I/0) complexes [Fe­(N₂)­(SiP^iPr₃)]^+/0/– (SiP^iPr₃ = [Si­(<i>o</i>-C₆H₄PⁱPr₂)₃]⁻) recently published by J. Peters et al. (<i>Nature Chem.</i> <b>2010</b>, <i>2</i>, 558–565) reveals that the redox processes stringing this electron transfer series are best viewed as metal-centered reductions, i.e. Fe^IIN₂⁰ → Fe^IN₂⁰ → Fe⁰N₂⁰. Superficially, the interpretation seems to be incompatible with the Mössbauer measurement, because the observed isomer shifts are more negative for the lower oxidation states, whereas typically iron-based reduction tends to increase the isomer shift. To rationalize the experimental findings, we analyzed the contributions from the 1s to 4s orbitals to the charge density at the Mössbauer nucleus and found that the positive correlation between the isomer shift and the oxidation state results from an unusual shrinking of the Fe–N₂ bond upon reduction due to enhanced N₂ to Fe π-backbonding. The other effects of reduction arising from shielding of the nuclear potential, decreasing covalency, and changes in the 4s population would induce the usual negative correlation. The structure distortion dictates the radial distribution of the 4s orbital and the charge density at the nucleus such that a virtually linear relationship between the isomer shift and the Fe–N₂ distance could be identified for this series

Electronic Structure Contributions of Non-Heme Oxo-Iron(V) Complexes to the Reactivity

Oxo-iron­(V) species have been implicated in the catalytic cycle of the Rieske dioxygenase. Their synthetic analog, [Fe^V(O)­(OC­(O)­CH₃)­(PyNMe₃)]²⁺ (<b>1</b>, PyNMe₃ = 3,6,9,15-tetraazabicyclo[9.3.1]­pentadeca-1(15),11,13-triene-3,6,9-trimethyl), derived from the O–O bond cleavage of its acetylperoxo iron­(III) precursor, has been shown experimentally to perform regio- and stereoselective C–H and CC bond functionalization. However, its structure–activity relation is poorly understood. Herein we present a detailed electronic-structure and spectroscopic analysis of complex <b>1</b> along with well-characterized oxo-iron­(V) complexes, [Fe^V(O)­(TAML)]⁻ (<b>2</b>, TAML = tetraamido macrocyclic ligand), [Fe^V(O)­(TMC)­(NC­(O)­CH₃)]⁺ (<b>4</b>, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), and [Fe^V(O)­(TMC)­(NC­(OH)­CH₃)]²⁺ (<b>4-H</b>^<b>+</b>), using wave function-based multireference complete active-space self-consistent field calculations. Our results reveal that the <i>x</i>/<i>y</i> anisotropy of the ⁵⁷Fe <i>A</i>-matrix is not a reliable spectroscopic marker to identify oxo-iron­(V) species and that the drastically different <i>A</i>_<i>x</i> and <i>A</i>_<i>y</i> values determined for complexes <b>1</b>, <b>4</b>, and <b>4-H</b>⁺ have distinctive origins compared to complex <b>2</b>, a genuine oxo-iron­(V) species. Complex <b>1</b>, in fact, has a dominant character of [Fe^IV(O···OC­(O)­CH₃)^2–•]²⁺, i.e., an <i>S</i>_Fe = 1 iron­(IV) center antiferromagnetically coupled to an O–O σ* radical, where the O–O bond has not been completely broken. Complex <b>4</b> is best described as a triplet ferryl unit that strongly interacts with the <i>trans</i> acetylimidyl radical in an antiferromagnetic fashion, [Fe^IV(O)­(^•NC­(O^–)­CH₃)]⁺. Complex <b>4-H</b>⁺ features a similar electronic structure, [Fe^IV(O)­(^•NC­(OH)­CH₃)]²⁺. Owing to the remaining approximate half σ-bond in the O–O moiety, complex <b>1</b> can arrange two electron-accepting orbitals (α σ*_O–O and β Fe-d_<i>xz</i>) in such a way that both orbitals can simultaneously interact with the doubly occupied electron-donating orbitals (σ_C–H or π_C–C). Hence, complex <b>1</b> can promote a concerted yet asynchronous two-electron oxidation of the C–H and CC bonds, which nicely explains the stereospecificity observed for complex <b>1</b> and the related species

Structural and Spectroscopic Characterization of Rhenium Complexes Containing Neutral, Monoanionic, and Dianionic Ligands of 2,2′-Bipyridines and 2,2′:6,2″-Terpyridines: An Experimental and Density Functional Theory (DFT)-Computational Study

The molecular and electronic structures of the members of the following electron transfer series have been determined by single crystal X-ray crystallography, temperature dependent magnetic susceptibility measurements, and UV–vis–NIR and electron paramagnetic resonance spectroscopy and verified by density functional theory calculations (DFT B3LYP): [Re­(^Mebpy)₃]^<i>n</i>, [Re­(tpy)₂]^<i>n</i>, [Re­(Tp)­(bpy)­Cl]^<i>n</i> (<i>n</i> = 2+, 1+, 0, 1−), and [Re­(bpy)­(CO)₃]^1+,0,1– (^Mebpy = 4, 4′-dimethyl-2,2′-bipyridine; Tp^– = tris-pyrazolylborate, tpy = 2, 2′:6, 2″-terpyridine). For each series we show that the average C_py–C_py bond length and the average C–N_chel bond distance vary in a <i>linear</i> fashion with the charge <i>n</i> of the N,N′-coordinated (bpy)^<i>n</i> and N,N′,N″-coordinated (tpy)^<i>n</i> ligand. Consequently, the difference Δ between these two bond lengths varies also linearly with <i>n</i>. Δ is shown to be a useful single marker for the oxidation level of these two heterocyclic ligands (neutral, π-radical anion, and dianion). In addition, we have synthesized and structurally as well as spectroscopically characterized the following complexes: [(^cyDAB^•)­Re^IVCl₃(PPh₃)]⁰ <b>1</b>, [Re^III(tpy^•)­Cl­(PPh₃)₂]Cl <b>2</b>, [Re^III(tpy⁰)₂Cl]­(OTf)₂·2Et₂O <b>8</b>. There are no structurally significant (experimentally detectable) π-back-bond effects of the neutral bpy⁰ or tpy⁰ ligands irrespective of the d^<i>N</i> configuration (<i>N</i> = 0–7) of the central Re atom

Structural and Spectroscopic Characterization of Rhenium Complexes Containing Neutral, Monoanionic, and Dianionic Ligands of 2,2′-Bipyridines and 2,2′:6,2″-Terpyridines: An Experimental and Density Functional Theory (DFT)-Computational Study

The molecular and electronic structures of the members of the following electron transfer series have been determined by single crystal X-ray crystallography, temperature dependent magnetic susceptibility measurements, and UV–vis–NIR and electron paramagnetic resonance spectroscopy and verified by density functional theory calculations (DFT B3LYP): [Re­(^Mebpy)₃]^<i>n</i>, [Re­(tpy)₂]^<i>n</i>, [Re­(Tp)­(bpy)­Cl]^<i>n</i> (<i>n</i> = 2+, 1+, 0, 1−), and [Re­(bpy)­(CO)₃]^1+,0,1– (^Mebpy = 4, 4′-dimethyl-2,2′-bipyridine; Tp^– = tris-pyrazolylborate, tpy = 2, 2′:6, 2″-terpyridine). For each series we show that the average C_py–C_py bond length and the average C–N_chel bond distance vary in a <i>linear</i> fashion with the charge <i>n</i> of the N,N′-coordinated (bpy)^<i>n</i> and N,N′,N″-coordinated (tpy)^<i>n</i> ligand. Consequently, the difference Δ between these two bond lengths varies also linearly with <i>n</i>. Δ is shown to be a useful single marker for the oxidation level of these two heterocyclic ligands (neutral, π-radical anion, and dianion). In addition, we have synthesized and structurally as well as spectroscopically characterized the following complexes: [(^cyDAB^•)­Re^IVCl₃(PPh₃)]⁰ <b>1</b>, [Re^III(tpy^•)­Cl­(PPh₃)₂]Cl <b>2</b>, [Re^III(tpy⁰)₂Cl]­(OTf)₂·2Et₂O <b>8</b>. There are no structurally significant (experimentally detectable) π-back-bond effects of the neutral bpy⁰ or tpy⁰ ligands irrespective of the d^<i>N</i> configuration (<i>N</i> = 0–7) of the central Re atom

Investigations of the Magnetic and Spectroscopic Properties of V(III) and V(IV) Complexes

Herein, we utilize a variety of physical methods including magnetometry (SQUID), electron paramagnetic resonance (EPR), and magnetic circular dichroism (MCD), in conjunction with high-level ab initio theory to probe both the ground and ligand-field excited electronic states of a series of V­(IV) (<i>S</i> = ¹/₂) and V­(III) (<i>S</i> = 1) molecular complexes. The ligand fields of the central metal ions are analyzed with the aid of ab initio ligand-field theory (AILFT), which allows for a chemically meaningful interpretation of multireference electronic structure calculations at the level of the complete-active-space self-consistent field with second-order N-electron valence perturbation theory. Our calculations are in good agreement with all experimentally investigated observables (magnetic properties, EPR, and MCD), making our extracted ligand-field theory parameters realistic. The ligand fields predicted by AILFT are further analyzed with conventional angular overlap parametrization, allowing the ligand field to be decomposed into individual σ- and π-donor contributions from individual ligands. The results demonstrate in VO²⁺ complexes that while the axial vanadium–oxo interaction dominates both the ground- and excited-state properties of vanadyl complexes, proximal coordination can significantly modulate the vanadyl bond covalency. Similarly, the electronic properties of V­(III) complexes are particularly sensitive to the available σ and π interactions with the surrounding ligands. The results of this study demonstrate the power of AILFT-based analysis and provide the groundwork for the future analysis of vanadium centers in homogeneous and heterogeneous catalysts

A Cubic Fe₄Mo₄ Oxo Framework and Its Reversible Four-Electron Redox Chemistry

The potential of iron molybdates as catalysts in the Formox process stimulates research on aggregated but molecular iron–molybdenum oxo compounds. In this context, [(Me₃TACN)­Fe]­(OTf)₂ was reacted with (<i>n</i>Bu₄N)₂[MoO₄], which led to an oxo cluster, [[(Me₃TACN)­Fe]­[μ-(MoO₄-κ³<i>O</i>,<i>O</i>′,<i>O</i>″)]]₄ (<b>1</b>, Fe₄Mo₄) with a distorted cubic structure, where the corners are occupied by (Me₃TACN)­Fe²⁺ and [MoO]⁴⁺ units in an alternating fashion, being bridged by oxido ligands. The cyclic voltammogram revealed four reversible oxidation waves that are assigned to four consecutive Fe^II → Fe^III transfers and motivated attempts to isolate compounds containing the respective cations. Indeed, a salt with a Fe^II₂Fe^III₂Mo^VI₄ constellation, [Fe₄Mo₄]­(TCNQ)₂ (<b>2</b>), could be isolated after treatment with TCNQ. The Fe^IIFe^III₃Mo^VI₄ stage could be reached via oxidation with DDQ or 3 equiv of thianthrenium hexafluorophosphate (ThPF₆), giving [Fe₄Mo₄]­(DDQ)₃ (<b>4</b>) or [Fe₄Mo₄]­(PF₆)₃ (<b>5</b>), respectively. The fully oxidized Fe^III₄Mo^VI₄ state was generated through oxidation with 4 equiv of ThPF₆, leading to [Fe₄Mo₄]­(PF₆)₄, which showed a unique behavior: upon storage, one of the [MoO]⁴⁺ corners inverts, so that the terminal oxido ligand is located in the interior of the cage, leading to the formation of [[(Me₃TACN)­Fe]₄[μ-([MoO₄]₃[MoO₄(MeCN-κ<i>N</i>)])-κ³<i>O</i>,<i>O</i>′,<i>O</i>″)]­(PF₆)₄ (<b>7</b>). In this form, the compound could no longer be employed to enter the cyclic voltammogram recorded for <b>1</b>, <b>3</b>, and <b>5</b> from the oxidized side; no discrete redox events were observed. Compounds <b>1</b>–<b>3</b> and <b>7</b> were characterized structurally and <b>1</b>, <b>3</b>, and <b>7</b> additionally by SQUID measurements and Mössbauer spectroscopy. The data reveal a high degree of charge delocalization. ¹⁶O/¹⁸O exchange experiments with labeled water performed with <b>1</b> revealed an interesting parallel with the Formox catalyst: water−¹⁸O exchanges its label with all of the oxido ligands (bridging and terminal). This property relates to the ion mobility being held responsible for the activity of iron molybdate catalysts compared to neat MoO₃ or Fe₂O₃

Investigations of the Magnetic and Spectroscopic Properties of V(III) and V(IV) Complexes

Herein, we utilize a variety of physical methods including magnetometry (SQUID), electron paramagnetic resonance (EPR), and magnetic circular dichroism (MCD), in conjunction with high-level ab initio theory to probe both the ground and ligand-field excited electronic states of a series of V­(IV) (<i>S</i> = ¹/₂) and V­(III) (<i>S</i> = 1) molecular complexes. The ligand fields of the central metal ions are analyzed with the aid of ab initio ligand-field theory (AILFT), which allows for a chemically meaningful interpretation of multireference electronic structure calculations at the level of the complete-active-space self-consistent field with second-order N-electron valence perturbation theory. Our calculations are in good agreement with all experimentally investigated observables (magnetic properties, EPR, and MCD), making our extracted ligand-field theory parameters realistic. The ligand fields predicted by AILFT are further analyzed with conventional angular overlap parametrization, allowing the ligand field to be decomposed into individual σ- and π-donor contributions from individual ligands. The results demonstrate in VO²⁺ complexes that while the axial vanadium–oxo interaction dominates both the ground- and excited-state properties of vanadyl complexes, proximal coordination can significantly modulate the vanadyl bond covalency. Similarly, the electronic properties of V­(III) complexes are particularly sensitive to the available σ and π interactions with the surrounding ligands. The results of this study demonstrate the power of AILFT-based analysis and provide the groundwork for the future analysis of vanadium centers in homogeneous and heterogeneous catalysts

Design and Characterization of Phosphine Iron Hydrides: Toward Hydrogen-Producing Catalysts

Diamagnetic iron chloro compounds [(P^Ph₂N^Ph₂)­FeCp*Cl] [<b>1Cl</b>] and [(P^Cy₂N^Ph₂)­FeCp*Cl] [<b>2Cl</b>] and the corresponding hydrido complexes [(P^Ph₂N^Ph₂)­FeCp*H] [<b>1H</b>] and [(P^Cy₂N^Ph₂)­FeCp*H] [<b>2H</b>] have been synthesized and characterized by NMR spectroscopy, electrochemical studies, electronic absorption, and ⁵⁷Fe Mössbauer spectroscopy (P^Ph₂N^Ph₂ = 1,3,5,7-tetraphenyl-1,5-diphospha-3,7-diazacyclooctane, P^Cy₂N^Ph₂ = 1,5-dicyclohexyl-3,7-diphenyl-1,5-diphospha-3,7-diazacyclooctane, Cp* = pentamethylcyclopentadienyl). Molecular structures of [<b>2Cl</b>], [<b>1H</b>], and [<b>2H</b>], derived from single-crystal X-ray diffraction, revealed that these compounds have a typical piano-stool geometry. The results show that the electronic properties of the hydrido complexes are strongly influenced by the substituents at the phosphorus donor atoms of the P^R₂N^Ph₂ ligand, whereas those of the chloro complexes are less affected. These results illustrate that the hydride is a strong-field ligand, as compared to chloride, and thus leads to a significant degree of covalent character of the iron hydride bonds. This is important in the context of possible catalytic intermediates of iron hydrido species, as proposed for the catalytic cycle of [FeFe] hydrogenases and other synthetic catalysts. Both hydrido compounds [<b>1H</b>] and [<b>2H</b>] show enhanced catalytic currents in cyclic voltammetry upon addition of the strong acid trifluoromethanesulfonimide [NHTf₂] (p<i>K</i>_a^MeCN = 1.0). In contrast to the related complex [(P^<i>t</i>BuN^Bn)₂FeCp^C6F5H], which was reported by Liu et al. (<i>Nat. Chem.</i> <b>2013</b>, <i>5</i>, 228–233) to be an electrocatalyst for hydrogen splitting, the here presented hydride complexes [<b>1H</b>] and [<b>2H</b>] show the tendency for electrocatalytic hydrogen production. Hence, the catalytic direction of this class of monoiron compounds can be reversed by specific ligand modifications

Modeling the Active Site of [NiFe] Hydrogenases and the [NiFe_u] Subsite of the C-Cluster of Carbon Monoxide Dehydrogenases: Low-Spin Iron(II) Versus High-Spin Iron(II)

A series of four [S₂Ni­(μ-S)₂FeCp*Cl] compounds with different tetradentate thiolate/thioether ligands bound to the Ni­(II) ion is reported (Cp* = C₅Me₅). The {S₂Ni­(μ-S)₂Fe} core of these compounds resembles structural features of the active site of [NiFe] hydrogenases. Detailed analyses of the electronic structures of these compounds by Mössbauer and electron paramagnetic resonance spectroscopy, magnetic measurements, and density functional theory calculations reveal the oxidation states Ni­(II) low spin and Fe­(II) high spin for the metal ions. The same electronic configurations have been suggested for the C_red1 state of the C-cluster [NiFe_u] subsite in carbon monoxide dehydrogenases (CODH). The Ni–Fe distance of ∼3 Å excludes a metal–metal bond between nickel and iron, which is in agreement with the computational results. Electrochemical experiments show that iron is the redox active site in these complexes, performing a reversible one-electron oxidation. The four complexes are discussed with regard to their similarities and differences both to the [NiFe] hydrogenases and the C-cluster of Ni-containing CODH

Molecular and Electronic Structures of Homoleptic Six-Coordinate Cobalt(I) Complexes of 2,2′:6′,2″-Terpyridine, 2,2′-Bipyridine, and 1,10-Phenanthroline. An Experimental and Computational Study

The crystal structures of nine homoleptic, pseudooctahedral cobalt complexes, <b>1</b>–<b>9</b>, containing either 2,2′:6′,2″-terpyridine (tpy), 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine (^tbpy), or 1,10-phenanthroline (phen) ligands have been determined in three oxidation levels, namely, cobalt­(III), cobalt­(II), and, for the first time, the corresponding presumed cobalt­(I) species. The intraligand bond distances in the complexes [Co^I(tpy⁰)₂]⁺, [Co^I(^tbpy⁰)₃]⁺, and [Co^I(phen⁰)₃]⁺ are identical, within experimental error, not only with those in the corresponding trications and dications but also with the uncoordinated neutral ligands tpy⁰, bpy⁰, and phen⁰. On this basis, a cobalt­(I) oxidation state assignment can be inferred for the monocationic complexes. The trications are clearly low-spin Co^III (<i>S</i> = 0) species, and the dicationic species [Co^II(tpy⁰)₂]²⁺, [Co^II(^tbpy⁰)₃]²⁺, and [Co^II(phen⁰)₃]²⁺ contain high-spin (<i>S</i> = ³/₂) Co^II. Notably, the cobalt­(I) complexes do not display any structural indication of significant metal-to-ligand (t_2g → π*) π-back-donation effects. Consistent with this proposal, the temperature-dependent molar magnetic susceptibilities of the three cobalt­(I) species have been recorded (3–300 K) and a common <i>S</i> = 1 ground state confirmed. In contrast to the corresponding electronic spectra of isoelectronic (and isostructural) [Ni^II(tpy⁰)₂]²⁺, [Ni^II(bpy⁰)₃]²⁺, and [Ni^II(phen⁰)₃]²⁺, which display d → d bands with very small molar extinction coefficients (ε < 60 M^–1 cm^–1), the spectra of the cobalt­(I) species exhibit intense bands (ε > 10³ M^–1 cm^–1) in the visible and near-IR regions. Density functional theory (DFT) calculations using the B3LYP functional have validated the experimentally derived electronic structure assignments of the monocations as cobalt­(I) complexes with minimal cobalt-to-ligand π-back-bonding. Similar calculations for the six-coordinate neutral complexes [Co^II(tpy^•)₂]⁰ and [Co^II(bpy^•)₂(bpy⁰)]⁰ point to a common <i>S</i> = ³/₂ ground state, each possessing a central high-spin Co^II ion and two π-radical anion ligands. In addition, the excited-states and ground state magnetic properties of [Co^I(tpy⁰)₂]­[Co^I−(CO)₄] have been explored by variable-temperature variable-magnetic-field magnetic circular dichroism (MCD) spectroscopy. A series of strong signals associated with the paramagnetic monocation exhibit pronounced <i>C</i>-term behavior indicative of the presence of metal-to-ligand charge-transfer bands [in contrast to d–d transitions of the nickel­(II) analogue]. Time-dependent DFT calculations have allowed assignment of these transitions as Co­(3d) → π*­(tpy) excitations. Metal-to-ligand charge-transfer states intermixing with the Co­(d⁸) multiplets explain the remarkably large (and negative) zero-field-splitting parameter <i>D</i> obtained from SQUID and MCD measurements. Ground-state electron- and spin-density distributions of [Co^I(tpy⁰)₂]⁺ have been investigated by multireference electronic structure methods: complete active-space self-consistent field (CASSCF) and N-electron perturbation theory to second order (NEVPT2). Both correlated CASSCF/NEVPT2 and spin-unrestricted B3LYP-based DFT calculations show a significant delocalization of the spin density from the Co^I d_{<i>xz</i>,<i>yz</i>} orbitals toward the empty π* orbitals located on the two central pyridine fragments in the trans position. This spin density is of an alternating α,β-spin polarization type (McConnel mechanism I) and is definitely not due to magnetic metal-to-radical coupling. A comparison of these results with those for [Ni^II(tpy⁰)₂]²⁺ (<i>S</i> = 1) is presented