Hydrogen bonding and other noncovalent interactions accompany the reactions, both stoichiometric and catalytic, that involve polar substrates and functionalized metallocomplexes. Those involving metal hydrides are of particular interest to us for a long time. In this review we conceptualized our knowledge of hydrogen bonding involving metal hydrides and attempted to illustrate the role of these interactions in metal-ligand cooperation catalysis. Although it might add complexity to the reaction mechanism, taking into account these noncovalent interactions might be an important tool to affect a reaction rate and selectivity.

A cationic nickel(II) bis[2-(di-i-propylphosphino)-4-methylphenyl]amido complex has been prepared and characterized in solution (multinuclear 1H, 13C{1H}, 31P{1H} and 19F NMR spectroscopy) and in the solid state (single-crystal X-ray diffraction analysis). The complex was examined as electrocatalyst for proton reduction, using either acetic or trifluoroacetic acid as the proton source in acetonitrile. Electrochemical studies revealed that the complex exhibited a lower overpotential for proton reduction in the presence of acetic acid compared to other nickel pincer complexes. A possible catalytic pathway for the proton reduction process was proposed.

The electrocatalytic activity of PNP pincer complexes with general formula (PNP)MCl (PNP is bis(2-diisopropylphosphino-4-methylphenyl)amide; M = nickel, palladium and platinum) in the oxidative 2-aminoethanol (MEA, a recalcitrant pollutant) degradation process was studied using cyclic voltammetry method. Complexes fulfil the requirements typically expected of a redox mediator or catalyst. Their electrochemical oxidation yields the aminyl radical species which unpaired electron is localized on the ligand. The mechanism of interaction of these aminyl radical complexes with MEA was studied using ESR- and NMR-spectroscopy methods. The addition of MEA to the solution of model platinum derivative leads to the disappearance of the ESR signal responsible for the aminyl radical complex, whereas the 31P{1H} NMR spectrum revealed the regeneration of neutral (PNP)MCl. Palladium derivative was found to be the most effective catalyst of the series with an Icat/Ip value of 6.7. Controlled potential electrolysis has allowed us to identify the main products of the catalytic process and to propose the mechanism of the overall process.

The electrochemical properties of bis[2-(di-i-propylphosphino)-4-methylphenyl]amido complexes [(PNP)MCl] of Ni, Pd and Pt have been investigated by experimental electrochemical methods involving cyclic voltammetry (CV), in situ UV-vis- and EPR-spectroelectrochemistry, and density functional theory (DFT) calculations. A combination of spectroscopic and theoretical techniques suggests the formation of aminyl-radical complexes as a result of the electrooxidation process. The photophysical behavior of the electrochemically generated species was studied by in situ EPR measurements with photoexcitation, which revealed the formation of free aminyl radical through the cleavage of the ligand-metal bonds. Finally, the electrochemical generation and spectroelectrochemical studies of the hydride derivative [(PNP)PdH] allow the determination of the aminyl-radical hydride complex [(PNP)PdH][BF4], which has been generated with 33 % yield.

The interaction between (PNP)PdH (1); PNP = bis(2-diisopropylphosphino-4-methylphenyl)amide and different acids (CF3SO3H, HBF4∙Et2O, fluorinated alcohols and formic acid) was studied in benzene or toluene as well as in neat alcohols by IR and NMR spectroscopies. The structures of hydrogen-bonded complexes were also optimized at the DFT/ωB97-XD/def2-TZVP level. The nitrogen atom of the amidophosphine pincer ligand readily accepts proton not only from strong Brønsted acids but from relatively weak fluorinated alcohols. That suggests that binding to palladium(II) increases the diarylamine basicity, making it a strong base. Nevertheless, H+ can be taken from [(PN(H)P)PdH]+ (2) by pyridine or hexamethylphosphoramide (HMPA). These observations confirm the need for a shuttle base to form [(PN(H)P)PdH]+ (2) as the result of the heterolytic splitting of H2 by [(PNP)Pd]+. At that, a stoichiometric amount of formic acid protonates a hydride ligand yielding an unstable η2-H2 complex that rapidly converts into formate (PNP)Pd(OCHO), which loses CO2 to restore (PNP)PdH, whereas the relatively high acid excess hampers this reaction through competitive protonation at nitrogen atom.

The hydride ion transfer and proton transfer are the key steps in the reactions of (de)hydrogenation, dehydrocoupling, production of H 2 , and reduction of CO 2 with the participation of transition metal hydrides; complexes with bifunctional ligands often act as catalysts for these transformations. The aim of this work was to study the hydride-donating properties of pincer palladium(II) hydride (PNP)PdH ( 1 ; PNP is bis(2-diisopropylphosphino-4-methylphenyl)amide). For this purpose, its reaction with Lewis acids (BF 3 ·Et 2 O, B(C 6 F 5 ) 3 ) was studied using IR and NMR spectroscopies combined with quantum chemical calculations (DFT/M06/def2-TZVP). Correlations between electrochemical reduction potentials of the corresponding cations and thermodynamic hydridity of the metal hydrides proposed in the literature were also applied. [(PNP)Pd(MeCN)][BF 4 ] undergoes an irreversible two-electron reduction in acetonitrile ( E_p^ = –1.82 V). The use of the obtained potential in correlations gives an overestimated value of the hydride donating ability Δ G_H^-^^∘. It was found that the reaction of 1 with boron-containing Lewis acids unexpectedly leads to the protonation of the nitrogen atom of the pincer ligand with an impurity of water, rather than the reaction with the hydride ligand. According to DFT calculations, the proton affinity of the nitrogen atom is much higher than that of PdH, which determines its higher activity in protonation processes.

Ключевыми стадиями реакций (де)гидрирования, дегидросочетания, получения H 2 , восстановления CO 2 с участием гидридов переходных металлов являются перенос гидрид-иона и перенос протона, а катализаторами данных превращений часто выступают комплексы с бифункциональными лигандами. Целью настоящей работы было исследование гидридодонорных свойств пинцетного гидрида палладия(II) (PNP)PdH ( 1 ; PNP = бис (2-диизопропилфосфино-4-метилфенил)амид). Для этого методами ИК- и ЯМР-спектроскопии исследовано его взаимодействие с кислотами Льюиса (BF 3 · Et 2 O, B(C 6 F 5 ) 3 ) с привлечением квантово-химических расчетов (DFT/M06/def2-TZVP), а также использованы предложенные в литературе корреляции потенциалов электрохимического восстановления соответствующих катионов с термодинамической гидридностью. [(PNP)Pd(MeCN)][BF 4 ] претерпевает необратимое двухэлектронное восстановление в ацетонитриле ( \(E_{p}^{{{\text{red}}}}\) = –1.82 В). Для полученного потенциала корреляции дают завышенное значение гидридодонорной способности \(\Delta G_{{{{{\text{H}}}^{--}}}}^{^\circ }.\) Установлено, что реакция 1 с борсодержащими кислотами Льюиса неожиданно приводит к протонированию атома азота пинцетного лиганда примесью воды, а не к взаимодействию с гидридным лигандом. По данным DFT-расчетов, сродство к протону атома азота значительно выше, чем PdH, что обусловливает его более высокую активность в процессах протонирования.

The catalytic activity in amine-borane dehydrogenation is shown for the first time for Ln(II) species using complexes [{(p-tBu-C6H4)2CH}2M·L] (M = Yb, Sm, L = (DME)2, TMEDA). The protonation of M(II)-C bonds with HNR1R2BH3 affords amidoborane complexes [M(NR1R2BH3)2L], which under excess HNMe2BH3 transform to [NMe2BH2NMe2BH3]- derivatives, both serving as the dehydrocoupling intermediates.

Федеральное государственное бюджетное учреждение наукиИнститут неорганической химии им.А. В. Николаева Сибирского отделения Российской академии наук

Bimetallic complexes [LW(CO)2(μ-CO)⋯Pd(PCP)] cooperatively activate amine-boranes for their dehydrogenation via N–H proton tunneling at RDS and H2 evolution from two neutral hydrides.

Dibenzobarrelene-based PCsp3P pincer iridium complex bearing dangling CH2OH groups, ((PCsp3PCH2OH)IrH(Cl), catalyzes the dehydrogenation of amine–boranes at the reaction rate changing counter intuitively in the order: Me2NH·BH3 > > ButNH2·BH3 > NH3·BH3. The spectral (IR and NMR) data and DFT/M06 calculations have revealed that the binding of amine–boranes to the dangling OH group leads to an additional stabilization of the Ir···OH bond, thus hampering the dehydrogenation reaction, whereas the amine–borane coordination to iridium entails a fac- to mer-transformation of the complex and initiates the catalytic H2 evolution.

Two stereoisomers of pentacoordinate iridium(III) hydridochloride with triptycene-based PC(sp(3))P pincer ligand (1,8-bis(diisopropylphosphino)triptycene), 1 and 2, differ by the orientation of hydride ligand relative to the bridgehead ring of triptycene. According to DFT/B3PW91/def2-TZVP calculations performed, an equatorial Cl ligand can relatively easily change its position in 1, whereas that is not the case in 2. Both complexes 1 and 2 readily bind the sixth ligand to protect the empty coordination site. Variable temperature spectroscopic (NMR, IR, and UV-visible) studies show the existence of two isomers of hexacoordinate complexes 1.MeCN, 2.MeCN, and 2.Py with acetonitrile or pyridine coordinated trans to hydride or trans to metalated C(sp(3)), whereas only the equatorial isomer is found for 1.Py. These complexes are stabilized by various intramolecular noncovalent C-H center dot center dot center dot Cl interactions that are affected by the rotation of isopropyls or pyridine. The substitution of MeCN by pyridine is slow yielding axial Py complexes as kinetic products and the equatorial Py complexes as thermodynamic products with faster reactions of 1.L. Ultimately, that explains the higher activity of 1 in the catalytic alkenes isomerization observed for allylbenzene, 1-octene, and pent-4-enenitrile, which proceeds as an insertion/elimination sequence rather than through the allylic mechanism.

The coordination of pyridine (Py) and benzonitrile (PhCN) to benzene based pincer hydridochlorides [(PCP)IrH(Cl)] (1), [(POCOP)IrH(Cl)] (2), [{EtO(O)C‐POCOP}IrH(Cl)] (3), and [(PCN)IrH(Cl)] (4) was studied spectroscopically and computationally to deduce the ligand influence. The variable temperature NMR (1H, 31P, 15N) and UV/Visible spectroscopic measurements revealed preferential coordination of these N‐donor ligands in the apical position of 1–4 and gave the formation enthalpies for the hexacoordinate complexes, which follow the order: 1 ≈ 4 > 3 > 2. This order nicely agrees with the order of Lewis acidity obtained for these complexes in DFT calculations. The orbital and electron density distribution analysis were performed at the DFT/M06 theory level for 1–4 and a series of p‐substituted PCP‐based hydridochlorides. The increasing Lewis acidity of iridium [as a maximum energy, VS,max, of molecular electrostatic potential (MEP)] correlates with decreasing basicity of Cl‐ligand (as MEP minimum, VS,min) as well as with Hammett σp parameters of p‐substituents. Importantly, these properties are conserved upon conversion into the corresponding dihydrides, as shown on the example of 1 and 4.

The interactions of HA acids {indole, fluorinated alcohols, phenols, and [CpW(CO)(3)H] (2)]} with the title hydrides [((PCP)-P-tBu)MH] [M = Ni (1a), Pd (1b)] have been studied by a combination of spectroscopic (variable-temperature IR, NMR, UV/Vis) and computational (DFT/M06, AIM) methods in THF and toluene. The formation of the dihydrogen bond (DHB) 1HA is the first step in a process leading to proton transfer and H-2 evolution. The DHBs of 1 with 2 are much weaker than those of 1 with NH or OH acids, but the former complexes are more reactive. Kinetic studies of the reactions of 4-(4-nitrophenylazo)phenol and [CpWH(CO)(3)] with 1b in THF gave activation enthalpies H = 9.2 +/- 0.4 and 6.5 +/- 1.5 kcalmol(-1) and activation entropies S = -30 +/- 1 and -35 +/- 6 calmol(-1)K(-1), respectively. Calculations revealed the ((2)-H-2)-like TS (E = 9.3 kcalmol(-1)) for the reaction of 1b with p-nitrophenol and the [Pd((2)-H-2)]+OAr- complex as a local minimum at around 5 kcalmol(-1) above the DHB adduct. In the reaction of 1b with 2, both the acidic WH and hydridic NiH or PdH bonds undergo heterolytic cleavage (E = 7.4 kcalmol(-1)) to yield the unusual mu,(1:1)-H-2 end-on complex. The mu,(1:1)-H-2 molecule transforms easily into the more stable (2)-H-2 side-on tautomer, which eventually gives the bimetallic product after H-2 evolution.

Features of the electronic structure of adducts of transition metal hydride complexes (Cp*M(dppe)H, dppe is the 1,2-(diphenylphosphino)ethane, M = Fe, Ru, Os; CpM(CO) 3 H, M = Mo, W) with acids and bases were analyzed with the ADF2014 program using energy decomposition analysis (EDA) by the Ziegler-Rauk method combined with the natural orbitals for chemical valence theory (ETS-NOCV). The nature of orbital interactions in the complex determines the reaction pathway: σ MH → σ* OH interaction leads to the proton transfer to hydride ligand, n M → σ* OH leads to the metal atom protonation, n N → σ* MH implies the metal hydride deprotonation, and σ MH → n* B corresponds to the hydride transfer to Lewis acid. It was shown that M-H bond polarization change has the similar character upon the formation of complexes with Brønsted and Lewis acids. The ease of polarization of M-H bonds in complexes CpM(CO) 3 H determines their reactivity as proton and hydride ion donors.

The interaction between various proton donors (indole, CF3CH2OH, (CF3)(2)CHOH, (CF3)(3)COH) and (NNC)PtH hydrido complex (NNC-H = 6-(1,1'-dimethylbenzyl)-2,2'-bipyridine) was investigated through low-temperature IR and NMR spectroscopy in combination with density functional theory calculations at the M06 level of theory. The experiment shows formation of very weak hydrogen bonded complexes (Delta H-HB degrees, ca. -1.0 kcal mol(-1)), which undergo subsequent proton transfer surprisingly easy. Computational analysis of the hydrogen bonded complexes geometry, electronic parameters (obtained by NBO and AIM analysis), and orbital interaction energies shows that all the complexes are better described as bonded to the metal atom. At that in case of weak alcohols (CH3OH, TFE) there is also the additional interaction with the hydride ligand. These computational results allow explaining the observed experimental trends and give the first example of hydrogen bonding to a metal atom in the presence of hydride ligand.

The RhCl(3)·3H(2)O/PPh(3)/nBu(4)PI catalytic system for the hydroamination of ethylene by aniline is shown to be thermally stable by a recycle experiment and by a kinetic profile study. The hypothesis of the reduction under catalytic conditions to a Rh(I) species is supported by the observation of a high catalytic activity for complex [RhI(PPh(3))(2)](2). New solution equilibrium studies on [RhX(PPh(3))(2)](2) (X = Cl, I) in the presence of ligands of relevance to the catalytic reaction (PPh(3), C(2)H(4), PhNH(2), X(-), and the model Et(2)NH amine) are reported. Complex [RhCl(PPh(3))(2)](2) shows broadening of the (31)P NMR signal upon addition of PhNH(2), indicating rapid equilibrium with a less thermodynamically stable adduct. The reaction with Et(2)NH gives extensive conversion into cis-RhCl(PPh(3))(2)(NHEt(2)), which is however in equilibrium with the starting material and free Et(2)NH. Excess NHEt(2) yields a H-bonded adduct cis-RhCl(PPh(3))(2)(Et(2)NH)···NHEt(2), in equilibrium with the precursors, as shown by IR spectroscopy. The iodide analogue [RhI(PPh(3))(2)](2) shows less pronounced reactions (no change with PhNH(2), less extensive addition of Et(2)NH with formation of cis-RhI(PPh(3))(2)(NHEt(2)), less extensive reaction of the latter with additional Et(2)NH to yield cis-RhI(PPh(3))(2)(Et(2)NH)···NHEt(2). The two [RhX(PPh(3))(2)](2) compounds do not show any evidence for addition of the corresponding X(-) to yield a putative [RhX(2)(PPh(3))(2)](-) adduct. The product of C(2)H(4) addition to [RhI(PPh(3))(2)](2), trans-RhI(PPh(3))(2)(C(2)H(4)), has been characterized in solution. Treatment of the RhCl(3)·3H(2)O/PPh(3)/nBu(4)PI/PhNH(2) mixture under catalytic conditions yields mostly [RhCl(PPh(3))(2)](2), and no significant halide exchange, demonstrating that the promoting effect of iodide must take place at the level of high energy catalytic intermediates. The equilibria have also been investigated at the computational level by DFT with treatment at the full QM level including solvation effects. The calculations confirm that the bridge splitting reaction is slightly less favorable for the iodido derivative. Overall, the study confirms the active role of rhodium(I) species in ethylene hydroamination catalyzed by RhCl(3)·3H(2)O/PPh(3)/nBu(4)PI and suggest that the catalyst resting state is [RhCl(PPh(3))(2)](2) or its C(2)H(4) adduct, RhCl(PPh(3))(2)(C(2)H(4)), under high ethylene pressure.