Theoretical Studies of Structures and Mechanisms in Organometallic and Bioinorganic Chemistry: Heck Reaction with Palladium Phosphines, Active Sites of Superoxide Reductase and Cytochrome P450 Monooxygenase, and Tetrairon Hexathiolate Hydrogenase Model

The electronic structures and reaction mechanisms of transition-metal complexes can be calculated accurately by density functional theory (DFT) in cooperation with the continuum solvation model. The palladium catalyzed Heck reaction, iron-model complexes for cytochrome P450 and superoxide reductase (SOR), and tetrairon hexathiolate hydrogenase model were investigated. The DFT calculations on the catalytic Heck reaction (between phenyl-bromide and ethylene to form the styrene product), catalyzed by palladium diphosphine indicate a four-step mechanism: oxidative addition of C6H5Br, migratory insertion of C6H5 to C2H4, b-hydride transfer/olefin elimination of styrene product, and catalyst regeneration by removal of HBr. For the oxidative addition, the rate-determining step, the reaction through monophosphinopalladium complex is more favorable than that through either the diphosphinopalladium or ethylene-bound monophosphinopalladium. In further study, for a steric phosphine, PtBu3, the oxidative-addition barrier is lower on monopalladium monophosphine than dipalladium diphosphine whereas for a small phosphine, PMe3, the oxidative addition proceeds more easily via dipalladium diphosphine. Of the phosphine-free palladium complexes examined: free-Pd, PdBr-, and Pd(h2-C2H4), the olefin-coordinated intermediate has the lowest barrier for the oxidativeaddition. P450 and SOR have the same first-coordination-sphere, Fe[N4S], at their active sites but proceed through different reaction paths. The different ground spin states of the intermediate FeIII(OOH)(SCH3)(L) model {L = porphyrin for P450 and four imidazoles for SOR} produce geometric and electronic structures that assist i) the protonation on distal oxygen for P450, which leads to O-O bond cleavage and formation of (FeIV=O)(SCH3)(L) H2O, and ii) the protonation on proximal oxygen for SOR, which leads to (FeIII-HOOH)(SCH3)(L) formation before the Fe-O bond cleavage and H2O2 production. The hydrogen bonding from explicit waters also stabilizes FeIII-HOOH over FeIV=O H2O products in SOR. The electrochemical hydrogen production by Fe4[MeC(CH2S)3]2(CO)8 (1) with 2,6-dimethylpyridinium (LutH ) were studied by the DFT calculations of proton-transfer free energies relative to LutH and reduction potentials (vs. Fc/Fc ) of possible intermediates. In hydrogen production by 1, the second, more highly reductive, applied potential (-1.58 V) has the advantage over the first applied potential (-1.22 V) in that the more highly reduced intermediates can more easily add protons to produce H2

Combining Sanford arylations on benzodiazepines with the nuisance effect

5-Phenyl-1,3-dihydro-2H-1,4-benzodiazepin-2-ones react under palladium- and visible light photoredox catalysis, in refluxing methanol, with aryldiazonium salts to afford the respective 5-(2- arylphenyl) analogues. With 2- or 4-fluorobenzenediazonium derivatives, both fluoroaryl- and methoxyaryl- products were obtained, the latter resulting from a SNAr on the fluorobenzenediazonium salt (“nuisance effect”). A computational DFT analysis of the palladium-catalysed and the palladium/ruthenium-photocalysed mechanism for the functionalization of benzodiazepines indicated that in the presence of the photocatalyst the reaction proceeds via a low-energy SET pathway avoiding the high-energy oxidative addition step in the palladium-only catalysed reaction pathway

Electronic structures and spectroscopy of the electron transfer series [Fe(NO)L2]z (z = 1+, 0, 1–, 2–, 3–; L = dithiolene)

The electronic structures and spectroscopic parameters for the electron transfer series [Fe(NO)(L)2]z (z = 1+, 0, 1−, and 2−; L = S2C2R2; R = p-tolyl, CN) were calculated using density functional theory and compared to experimental data. The tri-, di-, and monoanions are {FeNO}8,7,6 species, respectively. The neutral and monocationic members are {FeNO}6 ↔ {FeNO}7 complexes whose electronic structures are modulated by the dithiolene ligand substituents

Electronic Structures and Spectroscopy of the Electron Transfer Series [Fe(NO)L₂]^<i>z</i> (<i>z</i> = 1+, 0, 1–, 2–,3–; L = Dithiolene)

The electronic structures and spectroscopic parameters for the electron transfer series of [Fe(NO)(L)₂]^<i>z</i> (<i>z</i> = 1+, 0, 1–, 2–, 3–; L = S₂C₂R₂; R = <i>p</i>-tolyl (<b>1</b>) and CN (<b>2</b>)) were calculated and compared to experiment. Some compounds in the series were isolated and characterized by spectroscopy. The calculations support the notion that all the monocation (<i>S</i>_t = 0), neutral (<i>S</i>_t = ¹/₂), and monoanion (<i>S</i>_t = 0) complexes contain NO⁺ (<i>S</i>_NO = 0), in which the redox active fragment is either the bis-dithiolene (2 L) or the central iron. The calculated electronic structures give insight into how <i>p</i>-tolyl and CN substituents and the redox states of the 2 L ligand impact the spin density on the iron in the monocation and neutral species. The electronic structure of [<b>1</b>]⁰ has some [Fe^I(NO⁺)(L₂^2–)]⁰ character in resonance with [Fe^II(NO⁺)(L₂^2–)]⁰ whereas [<b>2</b>]⁰ has a smaller amount of a [Fe^I(NO⁺)(L₂^2–)]⁰ description in its ground state wavefunction. Similarly, the electronic structure of [<b>1</b>]¹⁺ also has some [Fe^I(NO⁺)(L₂^1–)]¹⁺ character in resonance with [Fe^II(NO⁺)(L₂^2–)]¹⁺ whereas [<b>2</b>]¹⁺ is best described as [Fe^II(NO⁺)(L^•)₂]¹⁺. For the monoanion, the bis-dithiolene fragment is fully reduced and both [<b>1</b>]⁻ and [<b>2</b>]⁻ are best formulated as [Fe^II(NO⁺)(L₂^4–)]⁻. The reduction of the monoanion to give dianions [<b>1</b>]^2– and [<b>2</b>]^2– results in {FeNO}⁷ species. The calculated ⁵⁷Fe isomer shift and hyperfine couplings are in line with the experiment and support a description of the form [Fe^III(NO^–)(L₂^4–)]^2–, in which Fe(III) <i>S</i>_Fe = ³/₂ is antiferromagnetically coupled to NO^– (<i>S</i>_NO = 1). Finally, the calculated redox potential and ν(NO) frequency for the {FeNO}⁸ trianionic species [<b>2</b>]^3– is in agreement with experiment and consistent with a triplet ground state [Fe^II(NO^–)(L₂^4–)]^3–, in which Fe(II) (<i>S</i>_Fe = 2) is involved in antiferromagnetic coupling with NO^– (<i>S</i>_NO = 1)

Dealing with Complexity in Open-Shell Transition Metal Chemistry from a Theoretical Perspective: Reaction Pathways, Bonding, Spectroscopy, And Magnetic Properties

This chapter illustrate the challenges that are met in theoretical transition metal chemistry: (1) reactivity of high-valent iron-oxo sites and the challenge of multiple spin-state channels; (2) The treatment of magnetic spectroscopic observables in the case of (near) orbital degeneracy; (3) The experimentally validated description of transition metal complexes with coordinated ligand radicals; (4) The calculation of the magnetic properties of oligonuclear transition metal clusters with applications to Photosystem II. The subjects treated in the chapter are related to the fact that open-shell transition metals display a high degree of electronic complexity. This shows up in their reaction pathway that will frequently show multistate reactivity. Likewise, the magnetic and electronic properties of open-shell transition metals can be very complicated, as the case of Jahn Teller systems, and special techniques need to be employed to successfully model them. The intricate bonding situations that are created by exchange coupling (in essence, nothing but a very weak chemical bond) in metal radical systems and oligonuclear metal clusters are another area that is highly challenging to theory