Electron-donating Ligands Research Articles

Palladium‐Catalyzed Cascade Reaction for the Synthesis of Substituted Isoindolines

Isoindoline heterocycles have demonstrated potential in medicinal chemistry as they exhibit activity across diverse biological targets. They are present in molecules which act as bronchodilaters, N-methyl-d-aspartate agonists, multidrug resistance reversal agents, and fibrinogen receptor antagonists. While several approaches to the synthesis of unsubstituted or monosubstituted isoindolines have been reported, few methods exist to produce disubstituted isoindolines with high diastereoselectivity. In addition, there are no diastereoselective methods for the synthesis of 1,3-disubstituted isoindolines that allow for incorporation of readily available boronic acids, which are practical building blocks in medicinal chemistry. Synthetic methods for isoindoline synthesis that provide straightforward introduction of substitutents on the heterocycle would enable preparation of families of biologically significant compounds. We designed a cascade sequence for isoindoline synthesis that we anticipated could be catalyzed by a palladium(II) complex and would utilize boronic acids as a starting material (Scheme 1). The cascade reaction would initiate with arylation of imine 1. The resultant sulfonamide would engage the pendant allylic acetate by aminopalladation; b-acetoxy elimination would release the isoindoline product. A major challenge was identification of a catalyst with the appropriate electronic balance to facilitate all steps in the catalytic cycle. While nucleophilic arylation of imines requires electron-donating ligands, migratory insertion is generally promoted by palladium(II) catalysts with electrophilic character. We selected phosphinite palladacycle 3, which is a catalyst with demonstrated activity for arylation of imines, with the thought that the p-accepting phosphonite would balance s donation from the aryl group. In practice, we have found this complex to be an effective catalyst for our cascade sequence (see below). We began our investigation with reaction conditions similar to those employed for imine arylation. At room temperature, in the presence of catalyst 3, effective arylation of 1 with phenyl boronic acid occurs (Table 1, entry 1). Elevated reaction temperatures promote the cyclization reaction (Table 1, entry 2; Method A). Notably, isoindoline 2 is generated as a single diastereomer under these reaction conditions. Scheme 1. Proposed synthesis of isoindoline derivatives. L= ligand, Ts=4-toluenesulfonyl.

The Proximal Hydrogen Bond Network Modulates Bacillus subtilis Nitric-oxide Synthase Electronic and Structural Properties

Bacterial nitric-oxide synthase (NOS)-like proteins are believed to be genuine NOSs. As for cytochromes P450 (CYPs), NOS-proximal ligand is a thiolate that exerts a push effect crucial for the process of dioxygen activation. Unlike CYPs, this catalytic electron donation seems controlled by a hydrogen bond (H-bond) interaction between the thiolate ligand and a vicinal tryptophan. Variations of the strength of this H-bond could provide a direct way to tune the stability along with the electronic and structural properties of NOS. We generated five different mutations of bsNOS Trp66, which can modulate this proximal H-bond. We investigated the effects of these mutations on different NOS complexes (FeIII, FeIICO, and FeIINO), using a combination of UV-visible absorption, EPR, FTIR, and resonance Raman spectroscopies. Our results indicate that (i) the proximal H-bond modulation can selectively decrease or increase the electron donating properties of the proximal thiolate, (ii) this modulation controls the σ-competition between distal and proximal ligands, (iii) this H-bond controls the stability of various NOS intermediates, and (iv) a fine tuning of the electron donation by the proximal ligand is required to allow at the same time oxygen activation and to prevent uncoupling reactions.

Efficient Charge Separation in TiO2 Films Sensitized with Ruthenium(II)-Polypyridyl Complexes: Hole Stabilization by Ligand-Localized Charge-Transfer States

We have studied the interfacial electron-transfer dynamics on TiO(2) film sensitized with synthesized ruthenium(II)-polypyridyl complexes--[Ru(II)(bpy)(2)(L(1))] (1) and [Ru(II)(bpy)(L(1))(L(2))] (2), in which bpy=2,2'-bipyridyl, L(1)=4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol, and L(2)=4-(N,N-dimethylaminophenyl)-2,2'-bipyridine-by using femtosecond transient absorption spectroscopy. The presence of electron-donor L(2) and electron-acceptor L(1) ligands in complex 2 introduces lower energetic ligand-to-ligand charge-transfer (LLCT) excited states in addition to metal-to-ligand (ML) CT manifolds of complex 2. On photoexcitation, a pulse-width-limited (<100 fs) electron injection from populating LLCT and MLCT states are observed on account of strong catecholate binding on the TiO(2) surface. The hole is transferred directly or stepwise to the electron-donor ligand (L(2)) as a consequence of electron injection from LLCT and MLCT states, respectively. This results an increased spatial charge separation between the hole residing at the electron-donor (L(2)) ligand and the electron injected in TiO(2) nanoparticles (NPs). Thus, we observed a significant slow back-electron-transfer (BET) process in the 2/TiO(2) system relative to the 1/TiO(2) system. Our results suggest that Ru(II) -polypyridyl complexes comprising LLCT states can be a better photosensitizer for improved electron injection yield and slow BET processes in comparison with Ru(II)-polypyridyl complexes comprising MLCT states only.

Molecular and Electronic Structures of Dinuclear Iron Complexes Incorporating Strongly Electron-Donating Ligands: Implications for the Generation of the One- and Two-Electron Oxidized Forms

The ligands (L(t-Bu(2)))(2-), (L(Me(2)))(2-), and (L(Cl(2)))(2-) have been employed for the synthesis of the dinuclear Fe(III) complexes [L(t-Bu(2))Fe(μ-O)FeL(t-Bu(2))], [L(Me(2))Fe(μ-O)FeL(Me(2))], and [L(Cl(2))Fe(μ-O)FeL(Cl(2))]. The strongly electron-donating groups (tert-amines and phenolates) were chosen to increase the electron density at the coordinated ferric ions and thus to facilitate the oxidation of the complexes, with the possibility of fine-tuning the electronic structures by variation of the remote substituents. Molecular structures established in the solid (by single-crystal X-ray diffraction) and in solution (by X-ray absorption spectroscopy) show that the Fe ions are five-coordinate in a square-pyramidal coordination environment with the ligand adopting a trans-conformation. Spectroscopic and magnetic characterization establishes the highly covalent nature of the Fe(III)-O(oxo) and Fe(III)-O(Ph) bonds. The variations in the donor capabilities of the phenolates (due to changes in the remote substituents) are compensated for by a flexible electron donation of the Fe(III)-O(oxo) bonding. Spectroelectrochemical characterization demonstrates that [L(t-Bu(2))Fe(μ-O)FeL(t-Bu(2))] can be oxidized reversibly at +0.27 and +0.44 V versus Fc(+)/Fc, whereas [L(Me(2))Fe(μ-O)FeL(Me(2))] and [L(Cl(2))Fe(μ-O)FeL(Cl(2))] exhibit irreversible oxidations at +0.29 and +0.87 V versus Fc(+)/Fc, respectively. UV-vis, electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS), and Mössbauer spectroscopy show that the successive oxidations of [L(t-Bu(2))Fe(μ-O)FeL(t-Bu(2))] are ligand-centered leading to the monophenoxyl radical complex [(•)L(t-Bu(2))Fe(III)(μ-O)Fe(III)L(t-Bu(2))](+) (with the oxidation primarily localized on one-half of the molecule) and the diphenoxyl radical complex [(•)L(t-Bu(2))Fe(III)(μ-O)Fe(III•)L(t-Bu(2))](2+). Both products are unstable in solution and decay by cleavage of an Fe(III)-O(oxo) bond. The two-electron oxidized species is more stable because of two equally strong Fe(III)-O(oxo) bonds, whereas in the singly oxidized species the Fe(III)-O(oxo) bond of the non-oxidized half is weakened. The decay of the monocation results in the formation of [L(t-Bu(2))Fe(III)](+) and [L(t-Bu(2))Fe(IV)=O], while the decay of the dication yields [(•)L(t-Bu(2))Fe(III)](2+) and [L(t-Bu(2))Fe(IV)=O]. Follow-up reactions of the oxidized fragments with the counteranion of the oxidant, [SbCl(6)](-), leads to the formation of [Fe(III)Cl(4)](-).

Catalytic Activity of Phosphine−Copper Complexes for Hydroboration of Styrene with Pinacolborane: Experiment and Theory

We have carried out density functional theory calculations as well as experiments to rationalize the catalytic activity of various phosphine-copper complexes for the hydroboration of styrene with pinacolborane. The experimentally obtained catalytic efficiency was explained on the basis of activation barriers for consecutive reaction mechanism steps as well as by molecular orbitals and charges in the transition state. Bidentate ligands were found to be more efficient than monodentate ligands for catalytic activity. Bidentate ligands make the reactant complexes less stable than monodentate ligands due to steric hindrance. This information could be usefully utilized for new catalysts design. The calculated kinetic data were consistent with the experimental conversion efficiency in a process that was hypothesizd to undergo the addition of Cu-H to styrene as the rate-limiting step. From the electronic distribution of the HOMO and the charge of the copper atom in the transition state, it was found that styrenes substituted with electron withdrawing groups would give higher conversions, and the catalytic efficiency could be increased with properly designed electron-donating ligands for the copper catalyst complex.

A simple synthesis of functionalized 3-methyl-1-pyridinyl-1 H-imidazolium salts as bidentate N-heterocyclic-carbene precursors and their application in Ir-catalyzed arene borylation

The selective alkylation of functionalized 2-(1 H-imidazol-1-yl)pyridines 1 furnishes 3-methyl-1-pyridinyl-1 H-imidazolium salts 2, which can be deprotonated to deliver strongly electron-donating bidentate N-heterocyclic carbene ligands (NHC). The synthesis of these ligands and their application in the iridium-catalyzed C–H activated borylation of arenes with its current scope and limitations are reported.

Protonolysis of Platinum(II) and Palladium(II) Methyl Complexes: A Combined Experimental and Theoretical Investigation

The protonolysis of platinum(II) and palladium(II) methyl complexes has been investigated by both experiment and computation. Previously the protonolysis of (COD)PtII(CH3)2 by CF3COOY or (dppe)PdII(CH3)2 by CF3CY2OY (Y = H, D) was found to be accompanied by abnormally large and highly temperature-dependent kinetic isotope effects (KIEs), suggesting the involvement of tunneling. Here we find normal KIEs and no evidence of tunneling for protonolysis of (tmeda)PtII(CH3)Cl by CF3COOY (Y = H, D). Density functional theory (DFT) calculations indicate that protonation at the metal center followed by reductive coupling to the methane σ adduct (stepwise pathway) is favored for Pt complexes with good electron donor ligands, whereas direct protonation of the M−CH3 bond to generate the methane σ adduct (concerted pathway) is favored for Pt with electron-withdrawing ligands as well as for Pd. We suggest that KIE behavior consistent with tunneling may be an experimental indicator of the concerted pathway.

Developing chiral phosphorus ligands for asymmetric hydrogenations

Chiral rigid and electron-donating phosphocyclic phosphine ligands and modular atropisomeric biaryl phosphine ligands were designed and synthesized. The performance of these chiral ligands in the asymmetric hydrogenations of prochiral dehydroamino acid derivatives, enamides, imines, keto esters, and ketones has been demonstrated to be excellent.

Intermolecular Insertion of Ethylene and Octene into a Palladium−Amide Bond. Spectroscopic Evidence for an Ethylene Amido Intermediate

We report a series of palladium diarylamido complexes containing a cyclometalated phosphine ligand and a coordinated THF that form enamine products from migratory insertion of ethylene and octene into the metal-nitrogen bond. The reactions of these complexes were sensitive to the electronic properties of the diarylamido group. The complex with the more electron-donating amido ligand reacted nearly 2 orders of magnitude faster than the complex with the least electron-donating amido group. The mechanism of the reactions with alkenes involves exchange of olefin for THF, as revealed by the positive order of the reaction in alkene and inverse order in the concentration of THF. A syn-aminopalladation that would result from migratory insertion was revealed by the stereochemistry of the enamine product resulting from reaction with cis-ethylene-d(2). Finally, a three-coordinate THF-free species was isolated from a synthesis of the amido complex in aromatic solvents, and spectroscopic evidence for an ethylene amido species was gained, in part by the addition of H(2)(13)C=(13)CH(2) to the three-coordinate amido complex at -100 degrees C.

Cyclometalated Platinum(II) 6-Phenyl-4-(9,9-dihexylfluoren-2-yl)-2,2′-bipyridine Complexes: Synthesis, Photophysics, and Nonlinear Absorption

A series of mononuclear and dinuclear cyclometalated platinum(II) 6-phenyl-4-(9,9-dihexylfluoren-2-yl)-2,2'-bipyridine complexes (F-1-F-5) were synthesized and their photophysical properties were systematically investigated. All complexes exhibit strong (1)pi,pi* absorption bands in the UV region, and a broad, structureless charge transfer band in the visible region. The charge-transfer band is broadened and red-shifted for F-3-F-5 compared to those for F-1 and F-2 because of the electron-donating acetylide ligand and the involvement of the ligand-to-ligand charge transfer character. The molar extinction coefficients for the dinuclear complex F-5 are much higher than those for the mononuclear complexes F-1-F-4, indicating the electronic coupling through the bridge ligand. All complexes are emissive in solution at room temperature and in glassy matrix at 77 K. When excited at the charge transfer absorption band, the complexes exhibit a long-lived red/orange emission around 600 nm, which is attributed to a triplet metal-to-ligand charge transfer/intraligand charge transfer emission ((3)MLCT/(3)ILCT). For emission at 77 K, the emitting state is tentatively assigned as (3)MLCT for F-2-F-4, and (3)MLCT/(3)pi,pi* for F-1 and F-5 taking into account the emission energy, the shape of the spectrum, the lifetime, and the thermally induced Stokes shift. F-1-F-4 exhibit broad triplet transient difference absorption in the visible to the near-IR region, with a lifetime comparable to those measured from the decay of the (3)MLCT/(3)ILCT emission. Therefore, F-1-F-4 give rise to a strong reverse saturable absorption for ns laser pulses at 532 nm. Z-scan experiments were carried out at 532 nm using both ns and ps laser pulses, and the experimental data was fitted by a five-band model to extract the singlet and triplet excited-state absorption cross sections. The degree of reverse saturable absorption follows this trend: F-1 = F-2 > F-3 > F-4 > F-5, which is mainly determined by the ratio of the triplet excited-state absorption cross-section to that of the ground-state and the triplet excited-state quantum yield. Comparison of the photophysics of F-1, F-2, and F-3 to those of their corresponding Pt complexes without the fluorenyl substituent discovers that F-1-F-3 exhibit larger molar extinction coefficients for their low-energy charge transfer absorption band, longer triplet excited-state lifetimes, higher emission quantum yields, and increased ratios of the excited-state absorption cross-section to that of the ground-state.

Iridium-Catalyzed Asymmetric Hydrogenation of N-Arylimines

Chiral amines and their derivatives are of great significance in pharmaceutical, biological, and synthetic chemistry. Examples can be found in many drugs or drug candidates, such as Cinacalcet and NPS R-568. Catalytic asymmetric hydrogenation of imines is a powerful method for obtaining enantiomerically pure amine products due to its high efficiency, low catalyst loading, and atom economy. The authors report that high enantiomeric excesses were obtained in the asymmetric hydrogenation of various N-arylimines catalyzed by an iridium complex containing a highly rigid, electron-donating P-chiral bisphospholane ligand, DuanPhos. High turnover numbers (TONs) were achieved under mild reaction conditions.

Contrasting Effects of Axial Ligands on Electron‐Transfer Versus Proton‐Coupled Electron‐Transfer Reactions of Nonheme Oxoiron(IV) Complexes

The effects of axial ligands on electron-transfer and proton-coupled electron-transfer reactions of mononuclear nonheme oxoiron(IV) complexes were investigated by using [Fe(IV)(O)(tmc)(X)](n+) (1-X) with various axial ligands, in which tmc is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane and X is CH(3)CN (1-NCCH(3)), CF(3)COO(-) (1-OOCCF(3)), or N(3) (-) (1-N(3)), and ferrocene derivatives as electron donors. As the binding strength of the axial ligands increases, the one-electron reduction potentials of 1-X (E(red), V vs. saturated calomel electrode (SCE)) are more negatively shifted by the binding of the more electron-donating axial ligands in the order of 1-NCCH(3) (0.39) > 1-OOCCF(3) (0.13) > 1-N(3) (-0.05 V). Rate constants of electron transfer from ferrocene derivatives to 1-X were analyzed in light of the Marcus theory of electron transfer to determine reorganization energies (lambda) of electron transfer. The lambda values decrease in the order of 1-NCCH(3) (2.37) > 1-OOCCF(3) (2.12) > 1-N(3) (1.97 eV). Thus, the electron-transfer reduction becomes less favorable thermodynamically but more favorable kinetically with increasing donor ability of the axial ligands. The net effect of the axial ligands is the deceleration of the electron-transfer rate in the order of 1-NCCH(3) > 1-OOCCF(3) > 1-N(3). In sharp contrast to this, the rates of the proton-coupled electron-transfer reactions of 1-X are markedly accelerated in the presence of an acid in the opposite order: 1-NCCH(3) < 1-OOCCF(3) < 1-N(3). Such contrasting effects of the axial ligands on the electron-transfer and proton-coupled electron-transfer reactions of nonheme oxoiron(IV) complexes are discussed in light of the counterintuitive reactivity patterns observed in the oxo transfer and hydrogen-atom abstraction reactions by nonheme oxoiron(IV) complexes (Sastri et al. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 19 181-19 186).

Practical Aspects of Carbon−Carbon Cross-Coupling Reactions Using Heteroarenes

The use of cross-coupling reactions for the preparation of alkylated and arylated heteroaromatic compounds has increased tremendously over the past two decades. This has been driven on the one hand by the increasingly complex structures of new drugs, most of which contain one or more heterocyclic motifs. On the other hand, the development of new catalysts and reaction conditions for these reactions has rendered even the most unreactive of heteroarenes amenable to cross-coupling chemistry. Not only have new bulky electron-donating ligands been created that allow the coupling of aryl chlorides under mild conditions, but also the use of ligand-free palladium, in particular at very low doses, sometimes called homeopathic palladium, has served to bring down the cost of these reactions. More recent and enabling developments are the use of catalysts based on cheap metals such as nickel, copper, and iron. Scale-up issues are availability and cost of starting materials, cost of the catalysts (related to cost of the metal and the ligand, intrinsic activity and stability of the catalyst), solvent choice, and removal of the metal to <10 ppm from the final product. This latter point is aggravated with heteroaromatics as they tend to be good ligands for the transition metal. For the same reason substrate and product inhibition are quite common.

Synthesis of 1,4-Dichloro-1,3-butadienes by Rhodium Complex Catalyzed Reaction of Terminal Alkynes with Trichloroacetyl Chloride

Chlorinative dimerization of terminal alkynes with trichloroacetyl chloride as chlorine donor proceeds in the presence of rhodium catalysts to give (Z,Z)-1,4-dichloro-1,3-butadienes stereoselectively. Ligand screening has revealed that reactions using sterically bulky and electron-donating ligands like trimesitylphosphine are high yielding. The reaction is compatible with a range of functional groups to give the title compounds nearly quantitatively in most cases. A mechanistic possibility involving coupling of beta-chloroalken-1-yl intermediate has been discussed.

Solventless migratory-insertion reactions of substituted cyclopentadienyl iron complexes induced by electron donor ligands

Reaction between solid (C5H5)Fe(CO)2CH3 and a range of solid phosphine ligands, L (L = PPh3, P(m-CH3C6H4)3, P(p-CH3C6H4)3, P(p-FC6H4)3, P(p-ClC6H4)3, PCy3) occurred in the absence of solvent in the meltphase to give the migratory-insertion products, (C5H5)Fe(CO)(COCH3)(L). The reaction was more rapid with small electron withdrawing ligands. Insertion reaction between (RC5H4)Fe(CO)2R’ (R = H, CH3; R’ = CH3,CH2Ph) and gaseous ligands such as SO2 and CO were also studied. The insertion of SO2 occurred readily for all the substrates investigated, but CO insertion did not occur (< 1 %) using the solventless reaction condition.

Spectroscopic and Mechanistic Studies on Oxidation Reactions Catalyzed by the Functional Model SR Complex for Cytochrome P450: Influence of Oxidant, Substrate, and Solvent

Kinetic and mechanistic studies on the formation of an oxoiron(IV) porphyrin cation radical bearing a thiolate group as proximal ligand are reported. The SR complex, a functional enzyme mimic of P450, was oxidized in peroxo-shunt reactions under different experimental conditions with variation of solvent, temperature, and identity and excess of oxidant in the presence of different organic substrates. Through the application of a low-temperature rapid-scan stopped-flow technique, the reactive intermediates in the SR catalytic cycle, such as the initially formed SR acylperoxoiron(III) complex and the SR high-valent iron(IV) porphyrin cation radical complex [(SR(*+))Fe(IV)=O], were successfully identified and kinetically characterized. The oxidation of the SR complex under catalytic conditions provided direct spectroscopic information on the reactivity of [(SR(.+))Fe(IV)=O] towards the oxidation of selected organic substrates. Because the catalytically active species is a synthetic oxoiron(IV) porphyrin cation radical bearing a thiolate proximal group, the effect of the strong electron donor ligand on the formation and reactivity/stability of the SR high-valent iron species is addressed and discussed in the light of the reactivity pattern observed in substrate oxygenation reactions catalyzed by native P450 enzyme systems.

Enhanced Reactivities of Iron(IV)‐Oxo Porphyrin π‐Cation Radicals in Oxygenation Reactions by Electron‐Donating Axial Ligands

The proximal axial ligand in heme iron enzymes plays an important role in tuning the reactivities of iron(IV)-oxo porphyrin pi-cation radicals in oxidation reactions. The present study reports the effects of axial ligands in olefin epoxidation, aromatic hydroxylation, alcohol oxidation, and alkane hydroxylation, by [(tmp)(+*) Fe(IV)(O)(p-Y-PyO)](+) (1-Y) (tmp = meso-tetramesitylporphyrin, p-Y-PyO = para-substituted pyridine N-oxides, and Y = OCH(3), CH(3), H, Cl). In all of the oxidation reactions, the reactivities of 1-Y are found to follow the order 1-OCH(3) > 1-CH(3) > 1-H > 1-Cl; negative Hammett rho values of -1.4 to -2.7 were obtained by plotting the reaction rates against the sigma(p) values of the substituents of p-Y-PyO. These results, as well as previous ones on the effect of anionic nucleophiles, show that iron(IV)-oxo porphyrin pi-cation radicals bearing electron-donating axial ligands are more reactive in oxo-transfer and hydrogen-atom abstraction reactions. These results are counterintuitive since iron(IV)-oxo porphyrin pi-cation radicals are electrophilic species. Theoretical calculations of anionic and neutral ligands reproduced the counterintuitive experimental findings and elucidated the root cause of the axial ligand effects. Thus, in the case of anionic ligands, as the ligand becomes a better electron donor, it strengthens the FeO-H bond and thereby enhances its H-abstraction activity. In addition, it weakens the Fe=O bond and encourages oxo-transfer reactivity. Both are Bell-Evans-Polanyi effects, however, in a series of neutral ligands like p-Y-PyO, there is a relatively weak trend that appears to originate in two-state reactivity (TSR). This combination of experiment and theory enabled us to elucidate the factors that control the reactivity patterns of iron(IV)-oxo porphyrin pi-cation radicals in oxidation reactions and to resolve an enigmatic and fundamental problem.

Anionic Mixed Thiolate−Sulfide-Bridged Roussin’s Red Esters [(NO)2Fe(μ-SR)(μ-S)Fe(NO)2]− (R = Et, Me, Ph): A Key Intermediate for Transformation of Dinitrosyl Iron Complexes (DNICs) to [2Fe-2S] Clusters

Transformations of dinitrosyl iron complex (DNIC) [(NO)(2)Fe(SEt)(2)](-) and the anionic Roussin's red ester (RRE) [(NO)(2)Fe(mu-SEt)(2)Fe(NO)(2)](-) into [2Fe-2S] clusters facilitated by HSCPh(3)/Me(2)S(3) and [Fe(SEt)(4)](-), respectively, via an intermediate anionic mixed thiolate-sulfide-bridged RRE [(NO)(2)Fe(mu-SEt)(mu-S)Fe(NO)(2)](-) through the reassembling process ([(NO)(2)Fe(SEt)(2)](-) (2)/[(NO)(2)Fe(mu-SEt)(2)Fe(NO)(2)](-) (4) --> [(NO)(2)Fe(mu-SEt)(mu-S)Fe(NO)(2)](-) (3-Et) --> [(NO)(2)Fe(mu-S)(2)Fe(NO)(2)](2-) (5) --> [(SEt)(2)Fe(mu-S)(2)Fe(SEt)(2)](2-) (1)) were demonstrated. The anionic mixed thiolate-sulfide-bridged RRE 3-Et was characterized by IR, UV-vis, electron paramagnetic resonance, (1)H NMR, cyclic voltammetry, and single-crystal X-ray diffraction. In contrast to the nucleophilicity displayed by complex 2, the inertness of [(NO)(2)Fe(SPh)(2)](-) toward HSCPh(3) implicates how the reducing ability of the coordinated thiolates of DNICs modulate the release of sulfide from HSCPh(3) via reduction and the conversion of DNICs into the anionic mixed sulfide-thiolate-bridged complex. The reversible interconversion between complex 3-Et and complex [(NO)(2)Fe(mu-SPh)(mu-S)Fe(NO)(2)](-) (3-Ph) via protonation and a bridged-thiolate exchange reaction, respectively, demonstrates that the [{Fe(NO)(2)}(9)-{Fe(NO)(2)}(9)] motif displays a preference for the stronger electron-donating alkylthiolate-bridged ligand over the phenylthiolate-bridged ligand. This study may signify that the anionic mixed thiolate-sulfide-bridged RREs act as a key intermediate in the transformation of DNICs into [2Fe-2S] clusters. Also, the thiolate-coordinate DNICs serve as not only the thiolate/electron carrier activating the incorporation of sulfide of HSCPh(3) (Me(2)S(3)) to assemble the [Fe(mu-S)(2)Fe] core but also the Fe source in the biosynthesis of the [2Fe-2S] and [4Fe-4S] iron-sulfur clusters.

Unveiling How Stereoelectronic Factors Affect Kinetics and Thermodynamics of Protonation Regiochemistry in [FeFe] Hydrogenase Synthetic Models: A DFT Investigation

The DFT investigation of protonation regiochemistry for a series of [Fe(I)](2)(edt)(PR)(x)(CO)(6-x)] complexes differing for steric and electronic properties of ligands has allowed the disclosure of several key relations between the structure of the complexes and reactivity toward acids, from both a thermodynamics and kinetics perspective. The phosphine/CO ratio strongly affects both the thermodynamics and kinetics of protonation. In particular, with the exception of dppv complexes, in which steric factors become more important, the presence of phosphines, which are better electron donors than CO ligands, leads to lower reaction barriers. The presence of bulky phosphine ligands, which severely hinder the accessibility to the Fe-Fe bond, is a crucial factor responsible for kinetic preference of terminal- versus mu-protonation in symmetric complexes. The investigation of asymmetric models allowed us to rationalize why protonation takes place preferentially on the less electron-rich iron atom, i. e., the iron atom coordinated by the largest number of CO ligands. Importantly, the presence of at least one electron-donor ligand on the protonating Fe atom is fundamental to allow facile terminal protonation, suggesting that one of the reasons for the presence of CN(-) ligands in the enzyme might be related to the facile formation of catalytically relevant terminally protonated species. Finally, it was found that poorly reacting mu-H Fe(II)Fe(II) species are always thermodynamically more stable than corresponding terminal-hydride forms, indicating that one of the main challenges for the development of efficient synthetic catalysts inspired to the [FeFe] hydrogenase active site will be the design of complexes that undergo terminal protonation but cannot interconvert to the corresponding mu-H forms.

Modified cobalt(II) acetylacetonate complexes as catalysts for Negishi-type coupling reactions: influence of ligand electronic properties on catalyst activity

Four known electronically diverse cobalt(II) acetylacetonate derivatives were synthesized by replacement of the acetylacetonate methyl groups with combinations of tert-butyl, ethoxy, and trifluoromethyl groups in order to study the effect of catalyst electronic properties on the reaction rate and product yield of the cobalt-catalyzed reaction between haloalkenes and butylzinc iodide. Infrared spectroscopy of these compounds showed an increase in the CO stretching frequency as the ligand substituents became more electron withdrawing. These compounds, in addition to cobalt(II) acetylacetonate itself, were evaluated as catalysts for the coupling reaction between (E)-1-iodo-1-octene and butylzinc iodide to form (E)-5-dodecene. Faster reaction rates were observed and higher yields of 5-dodecene were produced when catalysts containing electron-donating ligands were employed. Side reactions, including the homocoupling of 1-iodo-1-octene to produce 7,9-hexadecadiene, were also observed under the reported reaction conditions. The rate of side-product formation was more competitive with the rate of the cross-coupling reaction when slower, electron-deficient catalysts were employed.