Pendant bases as proton transfer relays in diiron dithiolate complexes inspired by [Fe-Fe] hydrogenase active site

Three dinuclear iron complexes containing pendant nitrogen bases in phosphine ligands with general formular (μ-pdt) [Fe₂(CO)₅L] (where pdt is SCH₂CH₂CH₂S, L = PPh₂NH(CH₂)₂N(CH₃)₂ (5), PPh₂NH(2-NH₂C₆H₄) (6), PPh₂[2-N(CH₃)₂CH₂C₆H₄] (7)), were prepared as the models of the [Fe-Fe] hydrogenase active site. The molecular structures of 5-7 were characterized by X-ray crystallography. The secondary amine in 6 has weak intramolecular hydrogen bonding with both the terminal nitrogen and sulfur atom, which may suggest a proton transfer pathway from amine in phosphine ligand to the sulfur atom of active site. Protonation of complexes 5 and 6 only occurred at the terminal nitrogen atom. Electrochemical properties of the complexes were studied in the presence of triflic acid by cyclic voltammetry.     

Electrocatalytic oxygen reduction by iron tetra-arylporphyrins bearing pendant proton relays

Iron(III) meso-tetra(2-carboxyphenyl)porphine chloride (1) was investigated as a soluble electrocatalyst for the oxygen reduction reaction (ORR) in acetonitrile with [H(DMF)(+)]OTf(-). Rotating ring-disk voltammetry, spectroelectrochemistry, and independent reactions with hydrogen peroxide indicate that 1 has very high selectivity for reduction of O(2) to H(2)O, without forming significant amounts of H(2)O(2). Cyclic voltammetric measurements at high substrate/catalyst ratios (high oxygen pressure) allowed the estimation of a turnover frequency (TOF) of 200 s(-1) at -0.4 V vs Cp(2)Fe(+/0). This is, to our knowledge, the first reported TOF for a soluble ORR electrocatalyst under kinetically controlled conditions. The 4-carboxyphenyl isomer of 1, in which the carboxylic acids point away from the iron center, is a much less selective catalyst. This comparison shows that carboxylate groups positioned to act as proton delivery relays can substantially enhance the selectivity of ORR catalysis.     

Pendant amine bases speed up proton transfers to metals by splitting the barriers

By using density functional theory on [FeFe]-hydrogenase mimics we deconvolute the function of pendant amine bases in proton transfer to and from the metal center. By dividing the high free energy barrier into one high enthalpy-low entropy barrier and one with a low enthalpy-high entropy, a lower free energy barrier is reached.     

DFT study on the mechanism of water-assisted dihydrogen elimination in group 6 octahedral metal hydride complexes

The reaction of water with octahedral bis-, tris- and tetrakis-(phosphine)tungsten, (phosphine)molybdenum and (phosphine)chromium complexes has been studied using B3LYP/def2-TZVPP level of DFT to elucidate dissociative, associative and hydride migratory insertion mechanisms for hydrogen elimination. In the dissociative mechanism, phosphine dissociation requires 19.3-28.5 kcal mol(-1) of energy. The phosphine-water ligand exchange is endergonic due to poor coordination ability of water to group 6 metals (binding energy 8.8-15.5 kcal mol(-1)). The ligand exchange leads to intermolecular M-HH(2)O dihydrogen interaction and facilitates dihydrogen elimination (G(act) = 6.8-15.5 kcal mol(-1)). In the associative mechanism, a water molecule in the first solvation shell interacts with the M-H bond through a dihydrogen bond (interaction energy 2.7-4.0 kcal mol(-1)) and leads to the elimination of H(2) by forming a hydroxide complex. Compared to the dissociative mechanism, G(act) of associative mechanisms are ～22 kcal mol(-1) higher. In the hydride migratory insertion mechanism, the hydride ligand shifts to the CO ligand (G(act) = 25.4-30.4 kcal mol(-1)) to afford a formyl complex and subsequently the H-H bond coupling occurs between formyl and water ligand (G(act) = 2.8-4.4 kcal mol(-1)). In many cases, the migratory insertion mechanism can simultaneously operate with the dissociative mechanism as a minor pathway, whereas owing to high G(act) value, the associative mechanism can be described as inactive in the reaction. The general argument that dihydrogen elimination is preceded by the formation of a dihydrogen intermediate is not applicable for the systems studied herein as the most favoured dissociative mechanism does not pass through such an intermediate. On the other hand, irrespective of the mechanisms, dihydrogen elimination invariably occurs with the formation of a dihydrogen bonded transition state. Our results also suggest that group 6 octahedral metal hydride complexes are attractive targets for the design of water splitting reactions.     

Synthesis and reactivity of hydride and dihydrogen complexes of ruthenium with tris(pyrazolyl)borate and phosphite ligands

Chloro phosphite complexes RuClTpL(PPh₃) (1a, 1b) [L = P(OEt)₃, PPh(OEt)₂] and RuClTp[P(OEt)₃]₂ (1c) [Tp = hydridotris(pyrazolyl)borate] were prepared by allowing RuClTp(PPh₃)₂ to react with an excess of phosphite. Treatment of the chloro complexes 1 with NaBH₄ in ethanol yielded the hydride RuHTpL(PPh₃) (2a, 2b) and RuHTp[P(OEt)₃]₂ (2c) derivatives. Protonation reaction of 2 with Brønsted acids was studied and led to thermally unstable (above 10 °C) dihydrogen [Ru(η²- H₂)TpL(PPh₃)]⁺ (3a, 3b) and [Ru(η²-H₂)Tp{P(OEt)₃}₂]⁺ (3c) complexes. The presence of the η²-H₂ ligand is indicated by short T_1 min values and J_HD measurements of the partially deuterated derivatives. Aquo [RuTp(H₂O)L(PPh₃)]BPh₄ (4), carbonyl [RuTp(CO)L(PPh₃)]BPh₄ (5), and nitrile [RuTp(CH₃CN)L(PPh₃)]BPh₄ (6) derivatives [L = P(OEt)₃] were prepared by substituting H₂ in the η²-H₂ derivatives 3. Vinylidene [RuTp{CC(H)R}L(PPh₃)]BPh₄ (7, 8) (R = Ph, ^tBu) and allenylidene [RuTp(CCCR1R2)L(PPh₃)]BPh₄ (9-11) complexes (R1 = R2 = Ph, R1 = Ph R2 = Me) were also prepared by allowing dihydrogen complexes 3 to react with the appropriate HCCR and HCCC(OH)R1R2 alkynes. Deprotonation of vinylidene complexes 7, 8 with NEt₃ was studied and led to acetylide Ru(CCR)TpL(PPh₃) (12, 13) derivatives. The trichlorostannyl Ru(SnCl₃)TpL(PPh₃) (14) compound was also prepared by allowing the chloro complex RuClTpL(PPh₃) to react with SnCl₂ · 2H₂O in CH₂Cl₂.     

Theoretical description of dihydrogen/hydride and trihydride molybdocene complexes: An insight from static and molecular dynamics simulations

In this study ab initio Car–Parrinello molecular dynamics simulations, extended transition state (ETS)‐natural orbitals for chemical valence (NOCV) and QTAIM bonding analyses, were performed to characterize the ansa‐bridged molybdocene complexes [(C₅H₄)₂XMe₂MoH₃]⁺ for X = C, Si, Ge, Sn, Pb, and nonbridged Cp₂MoH system. The results have shown that the [(C₅H₄)₂CMe₂MoH(H₂)]⁺ complex exhibits nonclassical dihydrogen/hydride (H₂/H) conformation (97.6% of time of simulation), contrary to trihydride (H₃) structure noted for nonbridged Cp₂MoH (86.9%) and ansa‐bridged [(C₅H₄)₂SnMe₂MoH₃]⁺ (84.8%), [(C₅H₄)₂PbMe₂MoH₃]⁺ (84.9%) systems. Further, [(C₅H₄)₂SiMe₂MoH₃]⁺ and [(C₅H₄)₂GeMe₂MoH₃]⁺ complexes, appeared to exist in both conformations (H₂/H—55.4%, H₃—44.6% for Si‐based system and H₂/H—36.2%, H₃—63.8 % for germanium congener). It has been proven that the “steric availability” of the metal center, measured by the changes in the Cp? Mo? Cp angle (α), determines the existence of a given conformation—namely, the smaller value of the angle (molybdenum is sterically more accessible) the larger preference for the formation of dihydrogen/hydride structure. ETS‐NOCV method allowed to conclude that increase in the Cp? Mo? Cp angle (from α ca. 120° to α ca. 150°) leads to the enhancement of donation from H₂ and back‐donation from Mo to the σ*(H? H), what consequently leads to breaking of the H? H bond and formation of the trihydride structure. Systematical increase in the charge depletion from the σ‐bonding orbital of H₂ can be related to the reduction of the energy gap between the major orbitals involved in this contribution, namely highest occupied molecular orbital (HOMO) of H₂ with lowest unoccupied molecular orbital (LUMO) of [MoHL]⁺; ΔE = 0.0868 a.u. [for L =(C₅H₄)₂C], ΔE = 0.0827a.u. [for L = (C₅H₄)₂Si] ΔE = 0.0638 a.u. [for L = Cp₂]. Further, the relatively low energetic barrier to hydrogen exchange (ΔE^# = 3.3 kcal/mol) for carbon‐bridged complex, [(C₅H₄)₂CMe₂MoH_c(H_aH_b)]⁺ → [(C₅H₄)₂ CMe₂MoH_a(H_bH_c)]⁺, is related to strengthening of the Mo–H bonds when going from the substrate to the transition state (TS). Notably higher barrier to hydrogen rotation (ΔE^# = 10.1 kcal/mol) in [(C₅H₄)₂CMe₂MoH(H₂)]⁺ is due to lowering in the electrostatic stabilization as well as weakening of the donation (H₂ → Mo charge transfer) and practically lack‐of back‐donation (Mo → H₂) in the rotated TS. © 2012 Wiley Periodicals, Inc.     

Structure and dynamics of a dihydrogen/hydride ansa molybdenocene complex

In contrast to [Cp(2)MoH(3)](+), which is a thermally stable trihydride complex, the ansa-bridged analogue [(eta-C(5)H(4))(2)CMe(2)MoH(H(2))](+) (1) is a thermally labile dihydrogen/hydride complex. Partial deuteration of the hydride ligands allows observation of J(H)(-)(D) = 11.9 Hz in 1-d(1) and 9.9 Hz in 1-d(2) (245 K), indicative of a dihydrogen/hydride structure. There is a slight preference for deuterium to concentrate in the dihydrogen ligand. A rapid dynamic process interchanges the hydride and dihydrogen moieties in complex 1. Low temperature (1)H NMR spectra of 1 give a single hydride resonance, which broadens at very low temperature due to rapid dipole-dipole relaxation (T(1) = 23 ms (750 MHz, 175 K) for the hydride resonance in 1). Low temperature (1)H NMR spectra of 1-d(2) allow the observation of decoalescence at 180 K into two resonances. The bound dihydrogen ligand exhibits hindered rotation with DeltaG(150) = 7.4 kcal/mol, but H atom exchange is still rapid at all accessible temperatures (down to 130 K). Density functional calculations confirm the dihydrogen/hydride structure as the ground state for the molecule and give estimates for the energy of two hydrogen exchange processes in good agreement with experiment. The presence of the C ansa bridge is shown to decrease the ability of the metallocene fragment to donate to the hydrogens, thus stabilizing the (eta(2)-H(2)) unit and modulating the barrier to H(2) rotation.     

Bis(diisopropylphosphino)pyridine iron dicarbonyl, dihydride, and silyl hydride complexes

Treatment of the bis(diisopropylphosphino)pyridine iron dichloride, ((iPr)PNP)FeCl2 ((iPr)PNP = 2,6-(iPr2PCH2)2(C5H3N)), with 2 equiv of NaBEt3H under an atmosphere of dinitrogen furnished the diamagnetic iron(II) dihydride dinitrogen complex, ((iPr)PNP)FeH2(N2). Addition of 1 equiv of PhSiH3 to ((iPr)PNP)FeH2(N2) resulted in exclusive substitution of the hydride trans to the pyridine to yield the silyl hydride dinitrogen compound, ((iPr)PNP)FeH(SiH2Ph)N2, which has been characterized by X-ray diffraction. The solid-state structure established a distorted octahedral geometry where the hydride ligand distorts toward the iron silyl. Both ((iPr)PNP)FeH2(N2) and ((iPr)PNP)FeH(SiH2Ph)N2 form eta2-dihydrogen complexes upon exposure to H2. The iron hydrides and the eta2-H2 ligands are in rapid exchange in solution, consistent with the previously reported "cis" effect, arising from a dipole/induced dipole interaction between the two ligands. Taken together, the spectroscopic, structural, and reactivity studies highlight the relative electron-donating ability of this pincer ligand as compared to the redox-active aryl-substituted bis(imino)pyridines.     

Zero-point corrections and temperature dependence of HD spin-spin coupling constants of heavy metal hydride and dihydrogen complexes calculated by vibrational averaging

Vibrational corrections (zero-point and temperature dependent) of the H-D spin-spin coupling constant J(HD) for six transition metal hydride and dihydrogen complexes have been computed from a vibrational average of J(HD) as a function of temperature. Effective (vibrationally averaged) H-D distances have also been determined. The very strong temperature dependence of J(HD) for one of the complexes, [Ir(dmpm)Cp*H2]2 + (dmpm = bis(dimethylphosphino)methane) can be modeled simply by the Boltzmann average of the zero-point vibrationally averaged JHD of two isomers. For this complex and four others, the vibrational corrections to JHD are shown to be highly significant and lead to improved agreement between theory and experiment in most cases. The zero-point vibrational correction is important for all complexes. Depending on the shape of the potential energy and J-coupling surfaces, for some of the complexes higher vibrationally excited states can also contribute to the vibrational corrections at temperatures above 0 K and lead to a temperature dependence. We identify different classes of complexes where a significant temperature dependence of J(HD) may or may not occur for different reasons. A method is outlined by which the temperature dependence of the HD spin-spin coupling constant can be determined with standard quantum chemistry software. Comparisons are made with experimental data and previously calculated values where applicable. We also discuss an example where a low-order expansion around the minimum of a complicated potential energy surface appears not to be sufficient for reproducing the experimentally observed temperature dependence.     

A dihydrogen bond between a bridging hydride and the NH proton of a coordinated dimethylamine: solid state, solution and theoretical characterization

The addition of one equivalent of dimethylamine (DMA) to the 44 valence-electron triangular cluster anion [Re₃(μ₃-H)(μ-H)₃(CO)₉]⁻ (1) affords the novel unsaturated derivative [Re₃(μ-H)₄(CO)₉(DMA)]⁻ (2, 46 valence electrons) which contains a dimethylamine molecule terminally coordinated to a cluster vertex. Theoretical calculations (DFT) reveal that in the more stable conformation the dimethylamine NH proton is directed towards the hydride bridging the opposite cluster edge in syn position, the close proximity of the ligands bound to the cluster surface allowing the formation of an unconventional N-H ? (μ-H)Re₂ hydrogen bond. The presence of this conformation in the solid state has been proven by an X-ray structural analysis of crystalline [PPh₄]2. Spectroscopic evidences (IR and NMR) indicate that the dihydrogen bond is maintained also in solution and, by the evaluation of the proton spin-lattice relaxation rates at variable temperature, a good estimate of the H ? H distance in solution has been determined.     

Theoretical analysis of proton relays in electrochemical proton-coupled electron transfer

The coupling of long-range electron transfer to proton transport over multiple sites plays a vital role in many biological and chemical processes. Recently the concerted proton-coupled electron transfer (PCET) reaction in a molecule with a hydrogen-bond relay inserted between the proton donor and acceptor sites was studied electrochemically. The standard rate constants and kinetic isotope effects (KIEs) were measured experimentally for this double proton transfer system and a related single proton transfer system. In the present paper, these systems are studied theoretically using vibronically nonadiabatic rate constant expressions for electrochemical PCET. Application of this approach to proton relays requires the calculation of multidimensional proton vibrational wave functions and the incorporation of multiple proton donor-acceptor motions. The decrease in proton donor-acceptor distances due to thermal fluctuations and the contributions from excited electron-proton vibronic states play important roles in these systems. The calculated KIEs and the ratio of the standard rate constants for the single and double proton transfer systems are in agreement with the experimental data. The calculations indicate that the standard PCET rate constant is lower for the double proton transfer system because of the smaller overlap integral between the ground state reduced and oxidized proton vibrational wave functions, resulting in greater contributions from excited electron-proton vibronic states with higher free energy barriers. The theory predicts that this rate constant may be increased by modifying the molecule in a manner that decreases the equilibrium proton donor-acceptor distances or alters the molecular thermal motions to facilitate the concurrent decrease of these distances. These insights may guide the design of more efficient catalysts for energy conversion devices.     

Inclusion complexes of Schiff bases as phytogrowth inhibitors

This article details the preparation, characterization and phytotoxic evaluation of several Schiff base inclusion complexes obtained from β-cyclodextrin and p-sulfonic acid calix[6]arene. The inclusion complexes (1:1 molar ratio) were prepared by mixing a 5 mmol L^?1 aqueous solution (containing 1 % DMSO) of Schiff bases (guests) with aqueous solution (containing 1 % DMSO) of 5 mmol L^?1 of β-cyclodextrin or p-sulfonic acid calix[6]arene (hosts). The host–guest systems were characterized via a series of NMR experiments. The ability of the complexes to interfere with the radicle elongation of Sorghum bicolor (dicotyledonous species) and Cucumis sativus (monocotyledonous species) was evaluated. After 48 h, the inclusion complexes inhibited the radicle elongation of both species from 11 to 56 %. The formation of inclusion complexes was also investigated theoretically by molecular dynamics simulations in aqueous solution through implicit approach. Based on the experimental observation, the phytotoxic activity evaluated can be attributed to the formation of host–guest systems. This was supported by the theoretical findings based on stable interaction energy analyses for all the studied supramolecular systems.     

Preparation and reactivity of mixed-ligand iron(II) hydride complexes with phosphites and polypyridyls

Hydride complexes [FeH(N-N)P3]BPh4 (1, 2) [N-N = 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen); P = P(OEt)4, PPh(OEt)2, and PPh2OEt] were prepared by allowing FeCl2(N-N) to react with phosphite in the presence of NaBH4. The hydrides [FeH(bpy)2P]BPh4 (3) [P = P(OEt)3 and PPh(OEt)2] were prepared by reacting the tris(2,2'-bipyridine) [Fe(bpy)3]Cl2.5H2O complex with the appropriate phosphite in the presence of NaBH4. The protonation reaction of 1 and 2 with acid was studied and led to thermally unstable (above -20 degrees C) dihydrogen [Fe(eta2-H2)(N-N)P3]2+ (4, 5) derivatives. The presence of the H2 ligand is indicated by short T(1 min) values (3.1-3.6 ms) and by J(HD) measurements (31.2-32.5 Hz) of the partially deuterated derivatives. Carbonyl [Fe(CO)(bpy)[P(OEt)3]3](BPh4)2 (6) and nitrile [Fe(CH3CN)(N-N)P3](BPh4)2 (7, 8) [N-N = bpy, phen; P = P(OEt)3 and PPh(OEt)2] complexes were prepared by substituting the H2 ligand in the eta2-H2 4, 5 derivatives. Aryldiazene complexes [Fe(ArN=NH)(N-N)P3](BPh4)2 (9, 10, 11) (Ar = C6H5, 4-CH3C6H4) were also obtained by allowing hydride [FeH(N-N)P3]BPh4 derivatives to react with aryldiazonium cations in CH2Cl2 at low temperature.     

Preparation and reactivity with azo-species of hydride and dihydrogen complexes of osmium stabilised by tris(pyrazolyl)borate and phosphite ligands

Mixed-ligand OsCl(Tp)L(PPh₃) complexes 1 [Tp = hydridotris(pyrazolyl)borate; L = P(OMe)₃, P(OEt)₃ and PPh(OEt)₂] were prepared by allowing OsCl(Tp)(PPh₃)₂ to react with an excess of phosphite. Treatment of chlorocomplexes 1 with NaBH₄ in ethanol afforded hydride OsH(Tp)L(PPh₃) derivatives 2. Stable dihydrogen [Os(η²-H₂)(Tp)L(PPh₃)]BPh₄ derivatives 3 were prepared by protonation of hydrides 2 with HBF₄ · Et₂O at −80 °C. The presence of the η²-H₂ ligand is supported by short T_1 min values and J_HD measurements on the partially deuterated derivatives. Treatment of the hydride OsH(Tp)[P(OEt)₃](PPh₃) complex with the aryldiazonium salt [4-CH₃C₆H₄N₂]BF₄ afforded aryldiazene [Os(4-CH₃C₆H₄NNH)(Tp){P(OEt)₃}(PPh₃)]BPh₄ derivative 4. Instead, aryldiazenido [Os(4-CH₃C₆H₄N₂)(Tp)[P(OEt)₃](PPh₃)](BF₄)₂ derivative 5 was obtained by reacting the hydride OsH(Tp)[P(OEt)₃](PPh₃) first with methyltriflate and then with aryldiazonium [4-CH₃C₆H₄N₂]BF₄ salt. Spectroscopic characterisation (IR, ¹⁵N NMR) by the ¹⁵N-labelled derivative strongly supports the presence of a near-linear Os-NN-Ar aryldiazenido group. Imine [Os{η¹-NHC(H)Ar}(Tp){P(OEt)₃}(PPh₃)]BPh₄ complexes 6 and 7 (Ar = C₆H₅, 4-CH₃C₆H₄) were also prepared by allowing the hydride OsH(Tp)[P(OEt)₃](PPh₃) to react first with methyltriflate and then with alkylazides.     

Naphthalene proton sponges as hydride donors: diverse appearances of the tert-amino-effect

It has been shown that the 1-NMe(2) group in the 2-substituted 1,8-bis(dimethylamino)naphthalenes (proton sponges) can intramolecularly donate a hydride ion to an appropriate electron-accepting ortho-substituent such as diarylcarbenium ion, β,β'-dicyanovinyl or methyleneiminium group. This produces the 1-N(+)(Me)=CH(2) functionality and triggers a number of further transformations (tert-amino effect) including peri-cyclization, ortho-cyclization or hydrolytic demethylation. In each particular case, the course of the reaction is determined by the nature of the ortho-substituent and the most potent nucleophile presenting in the reaction mixture. For 2,7-disubstituted 1,8-bis(dimethylamino)naphthalenes, two types of tandem tert-amino effect with the involvement of both peri-NMe(2) groups have been registered. The conclusion was made that proton sponges are generally more active in the tert-amino reactions than the corresponding monodimethylaminoarenes. This is ascribed both to higher electron donor ability of proton sponges and markedly shortened distance between electrophilic C(α)-atom in the ortho-substituent and hydrogen atoms of the nearest NMe(2) group. Most conversions observed proceed in good to high yields and are useful for the preparation of derivatives of benzo[h]quinoline, quino[7,8:7',8']quinoline, 2,3-dihydroperimidine, N,N,N'-trimethyl-1,8-diaminonaphthalene and proton sponge itself.