Mu-Hyun Baik

Lewis acid stabilized methylidene and oxoscandium complexes.

The methylidene scandium complex (PNP)Sc(mu3-CH2)(mu2-CH3)2[Al(CH3)2]2 (PNP = N[2-P(CHMe2)2-4-methylphenyl]2-) can be prepared from the reaction of (PNP)Sc(CH3)2 and 2 equiv of Al(CH3)3. The Lewis acid stabilized methylidenes candium complex has been crystallographically characterized, and its bonding scheme analyzed by DFT. In addition, we report preliminary reactivity studies of the Sc-CH2 ligand with substrates such as H2NAr and OCPh2. While the former results in an Brønsted acid-base reaction, the latter reagent produces the olefin H2C CPh2 along with the novel oxoscandium complex (PNP)Sc(mu3-O)(mu2-CH3)2[Al(CH3)2]2, quantitatively.

Stereoselective rhodium-catalyzed [3 + 2 + 1] carbocyclization of alkenylidenecyclopropanes with carbon monoxide: theoretical evidence for a trimethylenemethane metallacycle intermediate.

The theoretically inspired development of a Rh-catalyzed [3 + 2 + 1] carbocyclization of carbon- and heteroatom-tethered alkenylidenecyclopropanes (ACPs) with CO for the stereoselective construction of cis-fused bicyclohexenones is described. This study demonstrates that the ring opening of alkylidenecyclopropane proceeds through a Rh(III)-trimethylenemethane complex, which undergoes rate-determining carbometalation through a transition state that accurately predicts the stereochemical outcome for this process. The experimental studies demonstrate the validity of this approach and include the first highly enantioselective reaction involving an ACP to highlight further the synthetic utility of this transformation.

How iron-containing proteins control dioxygen chemistry: a detailed atomic level description via accurate quantum chemical and mixed quantum mechanics/molecular mechanics calculations

Abstract Over the past several years, rapid advances in computational hardware, quantum chemical methods, and mixed quantum mechanics/molecular mechanics (QM/MM) techniques have made it possible to model accurately the interaction of ligands with metal-containing proteins at an atomic level of detail. In this paper, we describe the application of our computational methodology, based on density functional (DFT) quantum chemical methods, to two diiron-containing proteins that interact with dioxygen: methane monooxygenase (MMO) and hemerythrin (Hr). Although the active sites are structurally related, the biological function differs substantially. MMO is an enzyme found in methanotrophic bacteria and hydroxylates aliphatic C–H bonds, whereas Hr is a carrier protein for dioxygen used by a number of marine invertebrates. Quantitative descriptions of the structures and energetics of key intermediates and transition states involved in the reaction with dioxygen are provided, allowing their mechanisms to be compared and contrasted in detail. An in-depth understanding of how the chemical identity of the first ligand coordination shell, structural features, electrostatic and van der Waals interactions of more distant shells control ligand binding and reactive chemistry is provided, affording a systematic analysis of how iron-containing proteins process dioxygen. Extensive contact with experiment is made in both systems, and a remarkable degree of accuracy and robustness of the calculations is obtained from both a qualitative and quantitative perspective.

A computational study of the mechanism of the [(salen)Cr + DMAP]-catalyzed formation of cyclic carbonates from CO2 and epoxide.

Epoxide and CO2 coupling reactions catalyzed by (salen)Cr(III)Cl have been modeled computationally to contrast a monometallic vs. a bimetallic mechanism. A low-energy CO2 insertion step into the metal-alkoxide bond was located.

Intermolecular C-H bond activation of benzene and pyridines by a vanadium(III) alkylidene including a stepwise conversion of benzene to a vanadium-benzyne complex†

Breaking of the carbon–hydrogen bond of benzene and pyridine is observed with (PNP)V(CH2tBu)2 (1), and in the case of benzene, the formation of an intermediate benzyne complex (C) is proposed, and indirect proof of its intermediacy is provided by identification of (PNP)VO(η2-C6H4) in combination with DFT calculations.

The Mechanism of OO Bond Formation in Tanaka’s Water Oxidation Catalyst

The most appealing of the many strategies for meeting the ever-increasing demand for energy in a renewable fashion is to utilize solar energy. Artificial photosynthesis is a critical technology that may afford a permanent solution for both energy needs and to secure an inexhaustible supply of carbonbased chemical feedstocks. Solar energy is envisioned to drive the chemical reduction of carbon dioxide to ultimately give commodity chemicals that may be used as fuel. Inspired by natural photosynthesis, water oxidation is considered the ideal source for the electrons required to reduce one molecule of carbon dioxide. Recently, much progress was made on designing dinuclear, mononuclear, and tetranuclear homogeneous water oxidation catalysts. Our understanding of how to rationally design and systematically improve catalysts remains poor, however. The fundamental challenge is easy to identify: How can four electrons be removed efficiently from two oxo moieties to form molecular dioxygen and how do we promote O O coupling under mild conditions? The initial oxidation of water takes place commonly in a proton-coupled electron transfer process and is often accompanied by O O bond formation to give a peroxo intermediate. In many catalytic systems this step is rate-determining, which is plausible, because bringing two oxygen atoms that are formally in the oxidation state II in close proximity to each other is challenging. To enable rational strategies towards improving catalysts, we must better understand how currently known catalysts overcome this challenge in a conceptual sense. In previous work, we examined such a mechanism in Meyer s diruthenium-based blue dimer and found that the coupling of a metal-bound oxo with water, as first suggested by Hurst et al., is most viable. We proposed that the {(bpy)2Ru III OH2} fragment formally becomes a {(bpy)2Ru IV O} moiety in the catalytically competent intermediate, which engages in a radical recombination type of reaction with water to initially give an intermediate consisting of a {(bpy)2Ru IV OOH} fragment. This mechanism provided a plausible solution to the O O coupling challenge and has since been recognized as one general reactivity pattern in water oxidation catalysis. From a fundamental mechanistic perspective, Tanaka s complex, containing two quinone ligands attached to two ruthenium centers that are separated by a bis(terpyridine)substituted anthracene linker, is intriguing. This spatially extended linker is not likely to promote direct electronic communication between the metal centers, whereas the presence of redox non-innocent quinone ligands is suggestive of a non-classical M–L electronic structure. Interestingly, Tanaka initially proposed that once the hydroxo group on each ruthenium center is deprotonated, the O O bond may form in a non-rate-determining, spontaneous fashion. More recently, the O O bond is hypothesized to form after the removal of two protons and two electrons. Previously, we identified the catalytically competent intermediate for Tanaka s complex in water to be 1, which can be oxidized electrochemically in a single two-electron/two-proton coupled manner to afford intermediate 2 (Scheme 1). The redox active moiety is a Ru OH2 fragment, which becomes a Ru O moiety in the intermediate 2.

Cyanide — a Strong Field Ligand for Ferrohemes and Hemoproteins?

Cyanide ion, a versatile diatomic ligand, has been extensively investigated as both a classic inhibitor and as a ligand for exploring properties of hemes and hemoproteins. Unlike CO and O2 which bind only to iron(II) species, CN− can bind to both iron(II) and -(III) hemo-proteins. Stable low-spin (LS) iron(III) proteins can be straightforwardly prepared.[1–3] In contrast, (cyano)iron(II) hemoproteins are usually indirectly formed by reduction of (cyano)iron(III) proteins. Cyanide bound, iron(II) forms of myoglobin,[4] hemoglobin,[5] horseradish peroxidase[6] and a number of cytochrome oxidase derivatives[7] are known. Many, but not all, of the iron(II) species, have lower binding constants than the iron(III) analogues. The equilibrium constant for cyanide binding for iron(III) hemoproteins is often ≥105 M−1 compared to ≤102 M−1 for iron(II) species.[3] Since the first reported isolation of a (cyano)heme was reported by us in 1980,[8] a number of electronic and geometric structure issues have been brought forward.[9] All of the known species are LS iron(III) derivatives, either bis(cyano) [FeIII(Por)(CN)2]− or mixed-ligand [FeIII(Por)(CN)(L)] complexes.[9] However, there are currently no (cyano)iron(II) porphyrinate derivatives reported, presumably because of its known lower stability/affinity compared to iron(III). It might be thought that (cyano)iron(II) species would be preferred since a filled d6 shell should strongly π-bond to the π-accepting cyanide ligand. We now report the first (cyano)iron(II) porphyrinate species, five-coordinate [K(222)][Fe(TPP)(CN)] (Figure 1). The average equatorial Fe–Np bond distance (1.986 (7) A) and the axial Fe–C distance (1.8783 (10) A) are consistent with a LS state.[10] However, T-dependent Mossbauer spectra reveal a more complicated picture of the iron spin state. A single quadrupole doublet is observed, whose value decreases from 1.827 mm/s at 25 K to 0.85 mm/s at 300 K; the isomer shift varies between 0.37 to 0.47 mm/s. The most probable explanation is that a thermally induced spin crossover is occurring, whose interconversion is rapid on the Mossbauer time scale (< 10−8 s).[11]a This interpretation has been confirmed by both DFT calculations and magnetic susceptibility measurements. Figure 1 100 K ORTEP diagram of [Fe(TPP)(CN)]−. Thermal ellipsoids are contoured at the 50% probability level. Hydrogens omitted for clarity. The magnetic susceptibility of [K(222)][Fe(TPP)(CN)] was investigated over the temperature range of 2–400 K. Figure 2 shows the product of the molar susceptibility (χm) (corrected for paramagnetism (TIP)) and temperature (T) in an external magnetic field of 2 T, which provides direct evidence for an S = 0 (LS) ↔ S = 2 (HS) spin crossover. AT 400 K, the value of χmT (2.96 cm3 K mol−1) is close to that expected for the HS state, but the lack of significant plateau suggests that the transition is not quite complete at this temperature. The spin-state transition occurs over a large temperature range (~175–400 K) and is reversible; both ascending and descending temperature measurements are shown in Figure 2 and no hysteresis was observed. The transition temperature T1/2 (defined as temperature at which complexes shows a population of 50% in the HS state) of this gradually proceeding spin transition is about 265 K. Figure 2 also plots the observed time-averaged quadrupole splitting value against temperature; the strong correlation between the quadrupole splitting and the susceptibility is clear. Figure 2 χmT versus T plot for [K(222)][Fe(TPP)(CN)] at 2T applied field. The Mossbauer quadrupole splitting values are also presented for comparison. To gain a better understanding of the thermodynamics regarding the spin-states, density functional theory was employed (see Supporting Information).[12] At low temperature only the low-spin S = 0 state was thermodynamically accessible. With increasing temperature the S = 2 state became significant, and a spin-crossover event is predicted to occur near 325 K (Figure S1) in good agreement with the value of 265 K from experiment. The intermediate-spin S = 1 state was disfavored over the entire temperature range explored. We have also investigated the T-dependent structures of the iron complex, since changes in metal–donor atom distances, along with changes in magnetic properties, are the two hallmarks of spin-state transitions. Structures have been determined at 100 K (two crystals) and 296, 325 and 400 K.[13] A change from a LS to a HS state in the five-coordinate complex is expected to lead to increases in the axial Fe–C distance, the equatorial Fe–Np bond distances, and the displacement of the iron atom from the mean porphinato plane. The results are summarized in the ORTEP drawings given in Figure 3, for simplicity only the cyanide group and FeN4 porphyrin core are shown. The Fe–C distance elongates by 0.23 A (Figure 3), which is amongst the largest changes in bond lengths that have been observed for iron(II) spin crossover compounds.[11b]b This is in part because the axial and equatorial bond distance increases must be asymmetric owing to the macrocyclic constraints of the porphyrin ring; note that Fe–Np has increased by 0.103 A over the same temperature range. The 100 K Fe–Np average bond length of 1.986 (7) A is that for a pure LS state whereas the 400 K value of 2.089 (8) A is slightly less than expected for anionic HS iron(II) complex, consistent with the idea that the spin state transition is not quite complete. Also completely consonant with expectation are the increases in the displacement of the iron from the mean plane of the four nitrogen plane. Figure 3 Four ORTEP diagrams of [K(222)][Fe(TPP)(CN)] displaying the cyanide groups and the core atoms of porphyrin (Fe and four pyrrole N atoms). Values of axial ligand and average equatorial bond distances are given as well as the iron displacement from the ... The anisotropic thermal parameters also show evidence of the spin crossover. As expected, the magnitude of all atomic anisotropic displacement parameters increase upon increasing temperature. However, the cyanide carbon atom shows different behavior over the temperature range. The thermal parameters at 100 and 400 K are close to isotropic, consistent with a single carbon atom site, whereas at intermediate temperatures with substantial populations of two spin states and differing carbon sites, the thermal parameters are much more prolate with elongation along the Fe–C bond direction. Importantly, the C–N bond distance in all structures remains nearly constant, as expected if only CN− atoms occupy two sites. Additional evidence for the spin crossover comes from T-dependent infrared spectra, which has the advantage of a shorter time scale (10−13 s) and thus can detect both spin isomers. Measurements at 296 K, as either Nujol mulls or KBr pellets, show two distinct ν(C–N) frequencies at 2070 and 2105 cm−1, with the first being the stronger. (S.I.) On cooling, the 2105 cm−1 peak gradually decreases while the 2070 cm−1 peak increases. At 150 to 160 K, the stretch at 2105 cm−1 disappears and thus corresponds to the HS stretch. A similar pattern of T-dependent azide stretches was observed in a 5/2, 3/2 spin crossover complex.[14] In coordination chemistry, cyanide and CO have been deeply entrenched as strong field ligands.[15, 16] Recently, Miller et al. showed that [(NEt4)3][Cr(II)(CN)5][17] is a distorted trigonal bipyramidal complex that was not low spin. Two different theoretical calculations[18] have suggested that the HS state results from the buildup of electrostatic (ligand–ligand) repulsions and not the ligand field of cyanide per se; the cyanide ligand is behaving as a strong field ligand in this Cr complex. However, [K(222)][Fe(TPP)(CN)] represents a case where the CN− should unequivocally lead to LS species. That it does not, strongly demonstrates the weaker field nature of cyanide, even in a case where π-back bonding should be maximized. In summary, the synthesis and characterization of the first cyanoiron(II) porphyrinate, [K(222)][Fe(TPP)(CN)], is presented. It forms a LS to HS crossover complex; coordination of a single axial cyanide ligand does not generate a sufficiently strong ligand field to ensure a low-spin complex under all conditions.[19] This is in distinct contrast to the five-coordinate CO complex, that is low spin under all known conditions.

Carbon Dioxide Promoted H+ Reduction Using a Bis(imino)pyridine Manganese Electrocatalyst

Heating a 1:1 mixture of (CO)5MnBr and the phosphine-substituted pyridine diimine ligand, (Ph2PPr)PDI, in THF at 65 °C for 24 h afforded the diamagnetic complex [((Ph2PPr)PDI)Mn(CO)][Br] (1). Higher temperatures and longer reaction times resulted in bromide displacement of the remaining carbonyl ligand and the formation of paramagnetic ((Ph2PPr)PDI)MnBr (2). The molecular structure of 1 was determined by single crystal X-ray diffraction, and density functional theory (DFT) calculations indicate that this complex is best described as low-spin Mn(I) bound to a neutral (Ph2PPr)PDI chelating ligand. The redox properties of 1 and 2 were investigated by cyclic voltammetry (CV), and each complex was tested for electrocatalytic activity in the presence of both CO2 and Brønsted acids. Although electrocatalytic response was not observed when CO2, H2O, or MeOH was added to 1 individually, the addition of H2O or MeOH to CO2-saturated acetonitrile solutions of 1 afforded voltammetric responses featuring increased current density as a function of proton source concentration (icat/ip up to 2.4 for H2O or 4.2 for MeOH at scan rates of 0.1 V/s). Bulk electrolysis using 5 mM 1 and 1.05 M MeOH in acetonitrile at -2.2 V vs Fc(+/0) over the course of 47 min gave H2 as the only detectable product with a Faradaic efficiency of 96.7%. Electrochemical experiments indicate that CO2 promotes 1-mediated H2 production by lowering apparent pH. While evaluating 2 for electrocatalytic activity, this complex was found to decompose rapidly in the presence of acid. Although modest H(+) reduction activity was realized, the experiments described herein indicate that care must be taken when evaluating Mn complexes for electrocatalytic CO2 reduction.

Synthesis and structural characterization of hexacoordinate silicon, germanium, and titanium complexes of the E. coli siderophore enterobactin.

The E. coli siderophore enterobactin, one of the strongest Fe(III) chelators known to date, is also capable of binding Si(IV) under physiological conditions. We report on the synthesis and structural characterization of the tris(catecholate) Si(IV) -enterobactin complex and its Ge(IV) and Ti(IV) analogues. Comparative structural analysis, supported by quantum-chemical calculations, reveals the correlation between the ionic radius and the structural changes in enterobactin upon complexation.

How a [CoIV

The mechanism of water oxidation performed by a recently discovered cobalt complex [Co(Py5)(OH2)](ClO4)2 (1; Py5=2,6-(bis(bis-2-pyridyl)-methoxymethane)pyridine) was examined using quantum chemical models based on density functional theory. The computer models were first benchmarked against the experimental cyclic voltammetry data to identify the catalytically competent resting state of the catalyst, which was thought to contain a Co(IV) -oxyl complex. The electronic structure calculations suggest that the low-spin doublet state is energetically most favorable, but the catalytically most active species is the intermediate-spin quartet complex that is almost isoenergetic with the doublet state. The electronic structure of the quartet state shows significant spin polarization on the terminal oxygen atom, which is consistent with an intramolecular electron transfer from the oxygen to the metal. Based on the calculated spin densities, the formally [Co(IV) a bond and a half O] can be viewed as a biradicaloid [Co(II)-(⋅O⋅)](2+), that is, a cobalt-oxene moiety. This electronic structure is reminiscent of many other systems where similar electronic patterns were proposed to be responsible for the oxidative reactivity. In this context, this first-row transition-metal system constitutes a logical extension, because the oxyl-radical character is maximized by using the more easily accessible high-spin configurations in which two half-filled Co-dπ orbitals can work in concert to maximize the oxyl-radical character to ultimately afford a new reactive intermediate that can be characterized as carrying a biradicaloid oxene moiety with a formal oxidation state of zero. This conceptual proposal for the catalytically active species provides a plausible rationale for the remarkable oxidative reactivity.