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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. [ABSTRACT FROM AUTHOR]

The (carbonyl)dihydride complex [(triphos)Ru(CO)[H.sub.2]] (2) has been synthesized by reaction of the ruthenate [(triphos)Ru[H.sub.3]]K (triphos = MeC[(C[H.sub.2]P[Ph.sub.2]).sub.3]) with ethanol saturated with CO. A single crystal X-ray analysis and IR and NMR experiments have shown that 2 adopts in both the solid state and solution an octahedral coordination geometry with a facial triphos ligand, two cis terminal hydrides, and a terminal carbonyl. The reaction of hexafluoro-2-propanol (HFIP) with 2 has been studied in C[H.sub.2] [Cl.sub.2] solution by IR and NMR spectroscopy. The proton donor interacts with a terminal hydride of 2 forming a rather strong hydrogen bond. The resulting H-bonded adduct [{(triphos)Ru(CO)(H)H}***{HOCH [(C[F.sub.3]).sub.2]}] (2a) has fully been characterized by in situ NMR and IR techniques. Compound 2a is in equilibrium with the nonclassical [[eta].sup.2]-[H.sub.2] complex [[(triphos)Ru(CO)H([H.sub.2])].sup.+] (2b), which can independently be prepared by protonation of 2 with a strong protic acid at low temperature. Unequivocal characterization of the dihydrogen complex (2b) has been achieved by a multifaceted spectroscopic investigation ([T.sup.obs.sub.1min] = 0.005 s (200 MHz), [J.sub.H,D] [approximately equal to] 30 Hz, DQCC = 78.3 kHz). A combined IR and NMR study of the proton transfer reaction involving 2 and HFIP in C[H.sub.2][Cl.sub.2] to give, first, the H-bonded adduct (2a) and, then, the dihydrogen complex (2b) has demonstrated that all these species are in equilibrium in the temperature range from 190 to 260 K. The thermodynamic parameters for the formation of 2a have independently been determined by NMR and IR methods, while those for the formation of 2b have been obtained by IR spectroscopy. An energetic profile for the reaction sequence 2 [right arrow] 2a [right arrow] 2b is proposed and discussed. Key words: hydrides, hydrogen bonding, ruthenium, IR spectroscopy, NMR spectroscopy.

A series of arene ruthenium iminophosphonamide complexes, [(Arene)RuCl{R2P(NR′)2}] (1), bearing various arenes and R,R′ substituents on the NPN ligand have been investigated as precatalysts in acetophenone transfer hydrogenation in basic and base-free isopropanol. The results clearly demonstrate the presence of two distinct reaction mechanisms, which are controlled by the basicity of the N-atoms. Complexes 1 in which both R′ substituents are aryl groups are only active once the neutral hydride complex [(Arene)RuH{R2P(NR′)2}] (2) is generated in basic isopropanol, the latter being able to reduce a ketone via a stepwise hydride and proton transfer. On the other hand, complexes in which at least one R′ group is Me readily catalyze the reaction in the absence of base. In the latter case, the results of kinetic studies and DFT calculations support an outer-sphere concerted asynchronous hydride and proton transfer assisted by the basic N-atom of the NPN ligand, which promotes catalysis via precoordination of an alcohol molecule by hydrogen bonding. [ABSTRACT FROM AUTHOR]

The interaction of a set of mono‐, di‐ and trisubstituted silanes with OH proton donors of different strength was studied by variable temperature (VT) FTIR and NMR spectroscopies at 190–298 K. Two competing sites of proton donors coordination: SiH and π‐density of phenyl rings—are revealed for phenyl‐containing silanes. The hydrogen bonds SiH⋅⋅⋅HO and OH⋅⋅⋅π(Ph) are of similar strength, but can be distinguished in the νSiH range: the νSiH⋅⋅⋅HO vibrations appear at lower frequencies while OH⋅⋅⋅π(Ph) complexes give Si–H vibrations shifted to higher frequency. The calculations showed the manifold picture of the noncovalent interactions in hydrogen‐bonded complexes of phenylsilanes. As OH⋅⋅⋅HSi bonds are weak, the other noncovalent interactions compete in the stabilization of the intermolecular complexes. Still, the structural and electronic parameters of "pure" DHB complexes of phenylsilanes are similar to those of Et3SiH. Fierce competition: Two competing sites of proton‐donor coordination, SiH and π‐density of phenyl rings, are revealed for phenyl‐containing silanes. Thus, conventional FTIR spectroscopy can be used to detect and study the hydrogen bonding of silanes in solutions. The characteristics of dihydrogen bonds formed by silanes are also similar to those of DHBs formed by boranes and transition‐metal hydrides. [ABSTRACT FROM AUTHOR]

The complexes of [( tBuPCP)IrH(Cl)] { 1, tBuPCP = 1,3-bis[(di- tert-butylphosphanyl)methyl]benzene} with pyridine (py, 2), 4-aminopyridine (4apy, 3) and 2-aminopyridine (2apy, 4) have been synthesized and fully characterized by variable-temperature spectroscopy (NMR, IR, and UV/Vis). In each case, the reaction yields a mixture of cis- and trans-hydrido-chlorido isomers. The stereochemistry of the latter was confirmed by single-crystal X-ray analysis of 2a and 3a, which showed that the pyridine ligands coordinated opposite the aryl ring do not produce an additional twist of the pincer ligand but lead to the elongation of all bonds involving the iridium atom. The chlorido ligand becomes substantially labilized owing to the strong trans effect of the hydrido ligand, and that in 3a it is prone to intermolecular N-H ···Cl hydrogen bonding in the solid state. In the 2-aminopyridine complex 4, the intramolecular hydrogen bond to the chlorido ligand (isomer 4a′) is slightly more preferred over the N-H ···H bond (isomer 4a). The structures of the cis-hydrido-chlorido isomers 2b- 4b were established with the help of 15N-labeled pyridine. These isomers undergo fast ligand exchange on the NMR timescale that slows as the temperature decreases. The decoalescence temperature is the lowest for the 2-aminopyridine derivative 4b; therefore, it is the least stable. The repulsion between the tert-butyl groups of the pincer ligand and the o-NH2 group is the main reason for the lower stability of 4b. [ABSTRACT FROM AUTHOR]

Abstract: The proton transfer from substituted phenols (ArOH= p-nitrophenol, p-nitophenyl-aza-phenol) to the ruthenium dihydride complex [(PP3)RuH2], where PP3 =P(CH2CH2PPh2)3, was studied by variable temperature UV–visible spectroscopy in dichloromethane, THF, THF/CH3OH and THF/CH3CN mixtures, showing a significant influence of the polarity and/or the proton donating/proton accepting properties of the solvent on the position of the proton transfer equilibrium. The formation of ion pairs stabilized by hydrogen bonding between the non-classical cation [(PP3)RuH(η2-H2)]+ and the homoconjugated anion [ArOHOAr]− was determined in low-polar media. The increase of the media polarity has been proved to favor the proton transfer leading to the dissociation of hydrogen bonded ion pairs [{(PP3) RuH(η2-H2)}+⋯{ArOHOAr}−]. Under comparable media polarity, the presence of a protic solvent in the mixture greatly favors the proton transfer due to the additional H-bonding with solutes. The structural, energetic and electronic features of the dihydrogen bonded adducts and of the proton transfer products were investigated by means of DFT/B3LYP calculations on the model dihydride [{P(CH2CH2PH2)3}RuH2] using CH3OH or CF3OH as proton donors. The specific and non-specific influence of the media properties on the dihydrogen bonding and proton transfer was studied by introducing the second protic (proton donor or solvent) molecule in the model system or by CPCM calculations. The additional participation of external molecules, from either the solvent or the alcohol, strengthens remarkably the dihydrogen bonded adducts and eases the proton transfer process. [Copyright &y& Elsevier]

The (carbonyl)dihydride complex [(triphos)Ru(CO)H[sub 2] ] (2) has been synthesized by reaction of the ruthenate [(triphos)RuH[sub 3] ]K (triphos = MeC(CH[sub 2] PPh[sub 2] )[sub 3] ) with ethanol saturated with CO. A single crystal X-ray analysis and IR and NMR experiments have shown that 2 adopts in both the solid state and solution an octahedral coordination geometry with a facial triphos ligand, two cis terminal hydrides, and a terminal carbonyl. The reaction of hexafluoro-2-propanol (HFIP) with 2 has been studied in CH[sub 2] Cl[sub 2] solution by IR and NMR spectroscopy. The proton donor interacts with a terminal hydride of 2 forming a rather strong hydrogen bond. The resulting H-bonded adduct [{(triphos)Ru(CO)(H)H}···{HOCH(CF[sub 3] )[sub 2] }] (2a) has fully been characterized by in situ NMR and IR techniques. Compound 2a is in equilibrium with the nonclassical η[sup 2] -H[sub 2] complex [(triphos)Ru(CO)H(H[sub 2] )][sup +] (2b), which can independently be prepared by protonation of 2 with a strong protic acid at low temperature. Unequivocal characterization of the dihydrogen complex (2b) has been achieved by a multifaceted spectroscopic investigation (T[sup obs] [sub 1min] = 0.005 s (200 MHz), J[sub H,D] 30 Hz, DQCC = 78.3 kHz). A combined IR and NMR study of the proton transfer reaction involving 2 and HFIP in CH[sub 2] Cl[sub 2 ] to give, first, the H-bonded adduct (2a) and, then, the dihydrogen complex (2b) has demonstrated that all these species are in equilibrium in the temperature range from 190 to 260 K. The thermodynamic parameters for the formation of 2a have independently been determined by NMR and IR methods, while those for the formation of 2b have been obtained by IR spectroscopy. An energetic profile for the reaction sequence 22a2b is proposed and discussed.Key words: hydrides, hydrogen bonding, ruthenium, IR spectroscopy, NMR spectroscopy.On a synthétisé le complexe [(triphos)Ru(CO)H[sub 2] ] (2) (triphos = MeC(CH[sub 2] PPh[sub 2] )[sub 3] ), un (carbonyl)dihydrure, en faisant réagir le ruthénate [(triphos)RuH[sub 3] ]K avec de l'éthanol saturé de CO. La diffraction des rayons X sur un cristal unique et des expériences d'IR et de RMN ont permis de montrer que le composé 2, tant à l'état solide qu'en solution, adopte une géométrie de coordination octaédrique comportant un ligand triphos facial, deux hydrures terminaux cis et un carbonyle terminal. Faisant appel à la spectroscopie IR et RMN, on a étudié la réaction du composé 2 avec l'hexafluoropropan-2-ol (HFIP) en solution dans le CH[sub 2] Cl[sub 2] . Le donneur de proton interagit avec un hydrure terminal du composé 2 en formant une liaison hydrogène relativement forte. Faisant appel in situ d'IR et de RMN, on a fait un caractérisation complète de l'adduit à liaison hydrogène qui en résulte [{(triphos)Ru(CO)(H)H}···{HOCH(CF[sub 3] )[sub 2] }] (2a). Le composé 2a est en équilibre avec le complexe non classique η[sup 2] -H[sub 2] , [(triphos)Ru(CO)(H[sub 2] )][sup +] (2b) que l'on peut préparer de façon indépendante en procédant à la protonation du composé 2 à l'aide d'un acide protique fort, à basse température. On a effectué la caractérisation non ambiguë du complexe dihydrogéné, 2b, en faisant appel à une étude spectroscopique à multiples facettes (T[sup obs] [sub 1min] = 0,005 s (200 MHz), J[sub H,D] 30 Hz, DQCC = 78,3 kHz). Faisant appel à une combinaison de techniques d'IR et de RMN, on a étudié la réaction de transfert de proton du composé 2 avec l'HFIP, dans le CH[sub 2] Cl[sub 2] , qui donne premièrement l'adduit à liaison hydrogène 2a et ensuite le complexe dihydrogéné 2b; ces études ont démontré que toutes ces espèces sont en équilibre sur toute la plage de températures allant de 190-260 K. Faisant appel à l'IR et à la RMN, on a déterminé de façon indépendante les paramètres thermodynamiques pour la formation du composé 2a alors que ceux pour la formation de 2b ont été obtenus par spectroscopie IR. On propose et on discute d'un profil énergétique pour la séquence des réactions 2 2a2b.Mots clés : hydrures, liaison hydrogène, ruthénium, spectroscopie IR, spectroscopie RMN.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

• Dihydrogen bonding determines reactivity of metal hydrides. • Role of (di)hydrogen bonding and proton transfer in (de)hydrogenation processes. • Steric and electronic effects of phosphines on the reactivity. • Reactivity of palladium, nickel, iridium and rutenium pincer hydrides. Transition-metal hydride complexes are plenteous in organometallic chemistry. The reactivity of metal hydrides is determined by three possible pathways of M–H bond cleavage: homolytic cleavage yielding a hydrogen radical (H), and heterolytic cleavage generating either a proton (H+) or hydride (H−). These steps, especially the proton and hydride transfer, are critical in a vast array of stoichiometric and catalytic transformations. Many late transition metal hydrides are supported by phosphine ligands, which allow tuning the electronic and steric properties at a metal center in a systematic and predictable way. In this non-exhaustive review we attempt to trace the influence of phosphines properties on the reactivity of transition metal hydrides in topical stoichiometric and catalytic processes highlighting the reactions' mechanisms and the role of hydrogen bonding interactions in the intermediates of hydrogen transfer reactions. [ABSTRACT FROM AUTHOR]

Reports on intermolecular hydrogen bonding of acidic alcohols with WH(CO)2(NO)L2 hydrides. Characterization by IR and NMR spectroscopy in hexane, toluene-d8 and CD2Cl2 solutions; Enthalpy; Relevance of hydrogen bonding to the mechanism of protonation of metal hydrides.

Studies the intermolecular hydrogen bonding of ReH2(CO)(NO)L2 hydrides with perfluoro-tert-butyl alcohol. Competition between M-H...H-OR and M-NO...H-OR interactions; Utility of the density functional calculations; Application of the infrared spectroscopy technique; Influence of the steric requirements of the phosphine ligands on the bonding site.

The article reports on the intermolecular hydrogen bonding between neutral transition metal hydrides and bases. It is indicated that such metal hydrides display two modes of reactivity depending on the substrates. Intermolecular dihydrogen bond was found between two neutral hydride complexes using H3BNEt3 as a base.

Abstract: Interaction of the salt (Ph3Ph name="dbnd6" />PPh3)BH3CN with the various OH and NH proton donors in low polar media was studied by variable temperature (200–290K) IR spectroscopy and theoretically by DFT calculations. The formation of two types of complexes containing non-classical dihydrogen bond to the hydride hydrogen (DHB) and classical hydrogen bond (HB) to nitrogen lone pair was shown in solution. The 1:1 complexes of both types (XH⋯H and XH⋯N) coexist in the presence of equimolar amount of proton donor. The addition of excess XH-acid leads to the increase of the classical HB content and appearance of the 1:2 complexes, where two basic sites work simultaneously. The structure, spectral characteristics, energy and electron redistribution were studied by DFT (B3LYP) method. The comparison DHB parameters of [BH3CN]− with those of the unsubstituted analogue [BH4]− allowed analyzing the electronic effects of the CN group on the basic properties of boron hydride moiety. The electronic influence of the BH3 group on CN−⋯HX hydrogen bond was also established by comparison with the corresponding classical HB to the CN− anion. [Copyright &y& Elsevier]

Graphical abstract Recent results in the chemistry of various pincer IrIII complexes are highlighted with the particular attention paid to the activation of Ir–Cl and Z–H bonds (Z–H = M–H, B–H, N–H, etc.) via low-energy (non-covalent) interactions (hydrogen bonded or Lewis complexes) and the role of such interactions in the proposed mechanisms of amine–borane dehydrogenation reaction. [ABSTRACT FROM AUTHOR]

The coordination of nitrogen bases (CH 3 CN, PhCN and 2-hydroxymetylpyridine (HMPy)) to ( t Bu PCP)IrH(Cl) ( 1 ) was investigated by variable temperature NMR, IR, UV–vis spectroscopies combined with DFT/M06 calculations. Hydrogen bonding of 1 and its ( t Bu PCP)IrH(Cl)(CO) with alcohols was also studied. At room temperature the formation constants K f of complex ( t Bu PCP)IrH(Cl)(PhCN) ( 2 ) is ca. 3.5 times higher than for complex ( t Bu PCP)IrH(Cl)(CH 3 CN) ( 3 ) (6.7 × 10 2 and 1.9 × 10 2 L mol −1 , respectively). At that quantitative analysis of variable temperature UV–vis spectra gave ΔH° = −7.0 kcal mol −1 and ΔS° = −13.7 cal mol −1 K −1 for 3 and ΔH° = −5.8 kcal mol −1 and ΔS° = −7.1 cal mol −1 K −1 for 2 . Relatively weak binding of 2-hydroxymethylpyridine to complex 1 yields two isomers, with HMPy ligand trans to hydride ( 4a ) and trans to arene ring ( 4b ) with formation constants K f ( 4a ) and K f ( 4b ) of only 28 and 18 L mol −1 at room temperature. Variable temperature measurements give ΔH°( 4a ) = −12.3 kcal mol −1 , ΔS°( 4a ) = −35.5 cal mol −1 K −1 and ΔH°( 4b ) = −11.7 kcal mol −1 , ΔS°( 4b ) = −34.5 cal mol −1 K −1 . DFT calculations on ( tBu PCP)IrH(Cl)(L) confirm preferential coordination of L = RCN, pyridines trans to the hydride ligand but show no alteration of Ir H bond properties. Instead, the elongation of Ir Cl bond is revealed that correlates with the strength of the Ir-L binding. The second isomers with L coordinated trans to arene ring of the pincer ligand are slightly less stable (≤2.8 kcal mol −1 according to DFT calculations) and are observed only for pyridines. In 1 and its ( tBu PCP)IrH(Cl)(L) derivatives, chloride ligand is able to serve as a proton accepting site forming Cl⋯H X hydrogen bonds in solution or in solid state. These interactions further activate the Ir Cl bond. [ABSTRACT FROM AUTHOR]

The combination of variable temperature (190-297 K) IR and NMR spectroscopy studies with quantum-chemical calculations at the DFT/B3PW91 and AIM level had the aim to determine the mechanism of proton transfer to CpRuH(dppe) (1, dppe = Ph2P(CH2)2PPh2) and the structures of intermediates. Dihydrogen bond (DHB) formation was established in the case of interaction with weak proton donors like CF3CH2OH. Low-temperature protonation (at about 200 K) by stronger proton donors leads via DHB complex to the cationic nonclassical complex [CpRu(ŋ²-H2)(dppe)]+ (2). Thermodynamic parameters of DHB formation (for CF3CH2OH: ΔH°HB = -4.9 ± 0.2 kcal mol-1, ΔS°HB = -17.8 ± 0.7 cal-mol-1K-1 ) and proton transfer (for (CF3)2CHOH: ΔH°PT = -5.2 ± 0.3 kcal mol-1, = -23 ± 1 cal mol-1K-1) were determined. Above 240 K 2 transforms into trans-[CpRu(H)2(dppe)]+ (3) yielding a mixture of 2 and 3 in 1:2 ratio. Kinetic analysis and activation parameters for the "[Ru(ŋ²-H2)]+ trans- [RU(H)2]+" transformation indicate reversibility of this process in contrast to irreversible intramolecular isomerization of the Cp* analogue. Calculations show that the driving force of this process is greater stability (by 1.5 kcal-mol-1 in ΔE scale) of the dihydride cation in comparison with the dihydrogen complex. The calculations of the potential energy profile indicate the low barrier for deprotonation of 2 suggesting that the formation of trans-[CpRu(H)2(dppe)]+ proceeds via deprotonation of [Ru(ŋ²-H2)]+ to DHB complex, formation of hydrogen bond with Ru atom and subsequent proton transfer to the metal site. [ABSTRACT FROM AUTHOR]

Structural, spectroscopic, and electronic features of weak hydrogen-bonded complexes of CpM(CO)3H (M = Mo (1a), W (1b)) hydrides with organic bases (phosphine oxides R3PO (R = n-C8H17, NMe2), amines NMe3, NEt3, and pyridine) are determined experimentally (variable temperature IR) and computationally (DFT/M05). The intermediacy of these complexes in reversible proton transfer is shown, and the thermodynamic parameters (ΔH°, ΔS°) of each reaction step are determined in hexane. Assignment of the product ion pair structure is made with the help of the frequency calculations. The solvent effects were studied experimentally using IR spectroscopy in CH2Cl2, THF, and CH3CN and computationally using conductor-like polarizable continuum model (CPCM) calculations. This complementary approach reveals the particular importance of specific solvation for the hydrogen-bond formation step. The strength of the hydrogen bond between hydrides 1 and the model bases is similar to that of the M-H⋯X hydrogen bond between 1 and THF (X = O) or CH3CN (X = N) or between CH2Cl2 and the same bases. The latter competitive weak interactions lower the activities of both the hydrides and the bases in the proton transfer reaction. In this way, these secondary effects shift the proton transfer equilibrium and lead to the counterintuitive hampering of proton transfer upon solvent change from hexane to moderately polar CH2Cl2 or THF. [ABSTRACT FROM AUTHOR]