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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 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. [ABSTRACT FROM AUTHOR]

Abstract: The ability of neutral transition‐metal hydrides to serve as a source of hydride ion H− or proton H+ is well appreciated. The hydride ligands possessing a partly negative charge are proton accepting sites, forming a dihydrogen bond, M−Hδ−⋅⋅⋅δ+HX (M=transition metal or metalloid). On the other hand, some metal hydrides are able to serve as a proton source and give hydrogen bond of M−Hδ+⋅⋅⋅X type (X=organic base). In this paper we analyse recent works on transition‐metal and boron hydrides showing i) how formation of an intermolecular complex between the reactants changes the Z−H (M−H and X−H) bond polarity and ii) what is the implication of such activation in the mechanisms of hydrides reactions. [ABSTRACT FROM AUTHOR]

The interactions of HA acids {indole, fluorinated alcohols, phenols, and [CpW(CO)3H] ( 2)]} with the title hydrides [( tBuPCP)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) 1···HA is the first step in a process leading to proton transfer and H2 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 Δ H≠ = 9.2 ± 0.4 and 6.5 ± 1.5 kcal mol-1 and activation entropies Δ S≠ = -30 ± 1 and -35 ± 6 cal mol-1 K-1, respectively. Calculations revealed the (η2-H2)-like TS (Δ E≠ = 9.3 kcal mol-1) for the reaction of 1b with p-nitrophenol and the [Pd(η2-H2)]+OAr- complex as a local minimum at around 5 kcal mol-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 (Δ E≠ = 7.4 kcal mol-1) to yield the unusual µ,η1:1-H2 end-on complex. The µ,η1:1-H2 molecule transforms easily into the more stable η2-H2 side-on tautomer, which eventually gives the bimetallic product after H2 evolution. [ABSTRACT FROM AUTHOR]

Abstract: Proton transfer is shown being an important step in many transformations involving main group element hydrides and dihydrogen bonded (E–H⋯HX) complexes are key intermediates. This review covers recent developments in the studies of boron hydrides reactivity orchestrated by dihydrogen bonds, focusing on their role in such reactions as proton transfer and H2 evolution, dimerization and hydrogen–deuterium exchange. The purpose of our review is to show what the discovery of dihydrogen bond gave for the development of boron hydride chemistry and what the theoretical and experimental studies of dihydrogen bonds with boron hydrides gave for a hydrogen bond concept. Spectral, structural and electronic features of these hydrogen bonds are summarized and analyzed in this review paying particular attention to the mechanistic details. [Copyright &y& Elsevier]

The interaction of the ruthenium hydride complex CpRuH(CO)(PCy[sub 3]) (1) with proton donors HOR of different strength was studied in hexane and compared with data in dichloromethane. The formation of dihydrogen-bonded complexes (2) and ion pairs stabilized by hydrogen bonds between the dihydrogen ligand and the anion (3) was observed. Kinetics of the interconversion from 2 to 3 was followed at different (CF[sub 3])[sub 3]COH concentrations between 200 and 240 K. The activation enthalpy and entropy values for proton transfer from the dihydrogen-bonded complex 2 to the (η²-H[sub 2])-complex 3 (ΔH[sup ‡] = 11.0 ± 0.5 kcal/mol and ΔS[sup ‡] = -19 ± 3 eu) were obtained for the first time. The results of the DFT study of the proton transfer process, taking CF[sub 3]COOH and (CF[sub 3])[sub 3]COH as a proton donors and introducing solvent effects in the calculation with the PCM method, are presented. The role of homoconjugate pairs [ROHOR][sup -] in the protonation is analyzed by means of the inclusion of an additional ROH molecule in the calculations. The formation of the free cationic complex [CpRu(CO)(PCy[sub 3])(η²-H[sub 2])][sup +] is driven by the formation of the homoconjugated anionic complex [ROHOR][sup -]. Solvent polarity plays a significant role stabilizing the charged species formed in the process. The theoretical study also accounts for the dihydrogen release and production of CpRu(OR)(CO)(PCy[sub 3]), observed at temperatures above 250 K. [ABSTRACT FROM AUTHOR]

The novel iddium(lll) hydride [(K3-P,P,P-NP3)trH3] [NP3 = N(CH2CH2PPh2)3] was synthesized and characterized by spectroscopic methods and X-ray crystallography. Its reactivily with strong (HBF4) and medium-strength [the fluorinated alcohols 11,1-trifluoroethanol (TEE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP)] proton donors was investigated through low-temperature IR and multinuclear NMR spectroscopy. In the case of the weak add TFE, the only spedes observed in the 190-298 K temperature range was the dihydrogen-bonded adduct between the hydride and the alcohol, while with the stronger add HBF4, the proton transfer was complete, gMng rise to a new intermediate [(K3-P,P,P- NP3)IrH4]+. With a medium-strength add like HFlP, two different sets of signals for the intermediate species were observed besides dihydrogen bond formation. In all cases, the final reaction product at ambient temperature was found to be the stable dihydride [(K4-NP3)lrH2]+, after stow molecular dihydrogen release. The nature of the short-living species was investigated with the help of density functional theory calculations at the M05-2X//6-31 ++G(df,pd) level of theory. [ABSTRACT FROM AUTHOR]

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]

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]

The mechanism of transition-metal tetrahydroborate dimerization was established for the first time on the example of (Ph3P)2Cu(&eegr;2-BH4) interaction with different proton donors [MeOH, CH2FCH2OH, CF3CH2OH, (CF3)2CHOH, (CF3)2CHOH, p-N02C6H40H, p-N02CsH2N=NC2H,OH, p-NO2C6H4NH2] using the combination of experimental (IR, 190--300 K) and quantum-diemical (DFT/M02) methods. The formation of dihydrogen-bonded complexes as the first reaction step was established experimentally. Their structural, electronic, energetic, and spectroscopic features were thorouglily analyzed by means of quantum-chemical calculations. Bifurcate complexes involving both bridging and terminal hydride hydrogen atoms become thermodynamically preferred for strong proton donors. Their formation was found to be a prerequisite for the subsequent proton transfer and dimerization to occur. Reaction kinetics was studied at variable temperature, showing that proton transfer is the rate-determining step. This result is in agreement with the computed potential energy profile of (Ph3P)2Cu(&eegr;2-BH4) dimerization, yielding [{(Ph3P)2Cu}2(&eegr;2-BH2)]&eegr;+. [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]

The isomerization of complex [Cp*Fe(dppe)(ρ²-H2)]+, generated in situ by low-temperature protonation of Cp*Fe(dppe)H with either HBF4 or CFsCOOH, to the dihydride tautomer trans-[Cp*Fe(dppe)(H)2]+ is irreversible and follows first-order kinetics in the -10 to +15 °C range with ΔH‡ = 21.6 ± 0.8 kcal mol-1 and ΔS‡ = 5 ± 3 eu. The isomerization rate constant is essentially independent of the nature and quantity of a strong acid. Density functional theory (DFT) calculations on various models, including the complete system at both the quantum mechanics/ molecular mechanics (QM/MM) and full QM levels, probe the relative importance of steric and electronic effects for the relative stability of the nonclassical and classical isomers and identify two likely isomerization mechanisms: a ‘direct’ pathway involving simultaneous H-H bond breaking and cis-trans isomerization and a ‘via Cp’ pathway involving agostic C5Me5H intermediates. Both pathways are characterized by activation energies in close correspondence with the experimental value (21.3 and 22.2 kcal mol-1, respectively). Further kinetic studies were carried out for the Cp*Fe(dppe)H + CF3COOD and Cp*Fe(dppe)D + CF3COOD systems at 273 K. The [Cp*Fe(dppe)(ρ²-HD)]+ complex establishes a very rapid isotope redistribution equilibrium with the ρ²-H2 and ρ²-D2 analogues. The equilibrium constant value (K = 3.3 ± 0.3) indicates a significant equilibrium isotope effect. Simulation of the rate data provides access to the individual isomerization rate constants kHH, kHD, and kDD, for the three isotopomers, yielding kinetic isotope effects: kHH/kHD = 1.24 ± 0.01 and kHD/kDD = 1.58 ± 0.01 (and, consequently, kHH/kDD = 1.96 ± 0.02). The analysis of the DFT-calculated frequencies, using the [Cp*Fe(dhpe)H2]+ model system, for the [Cp*Fe(dhpe)(ρ²-Xy)]+ isotopomers as well as transition states for the ‘direct’ (TSdir) and ‘via Cp’ (TSrot) pathways (X = H, D) allowed the computation of the expected isotope effects. A comparison with the experiment strongly suggests that the mechanism occurs via the ‘direct’ pathway for the present system, although the small difference in the calculated energy barriers suggests that the ‘via Cp’ pathway may be preferred in other cases. [ABSTRACT FROM AUTHOR]

The reaction of the isostructural anions of group 13 hydrides EH4- (E = B, Al, Ga) with proton donors of different strength (CH3OH, CF3CH2OH, and CF3OH) was studied with different theoretical methods [DFT/B3LYP and second-order Meller-Plesset (MP2) using the 6-311++G(d,p) basis set]. The results show the general mechanism of the reaction: the dihydrogen-bonded (DHB) adduct (EH⋯HO) formation leads through the activation barrier to the next concerted step of H2 elimination and alkoxo product formation. The structures, interaction energies (calculated by different approaches including the energy decomposition analysis), vibrational E-H modes, and electron-density distributions were analyzed for all of the DHB adducts. The transition state (TS) is the dihydrogen complex stabilized by a hydrogen bond with the anion [EH3(η²-H2)⋯OR-]. The single exception is the reaction of BH4- with CF3OH exhibiting two TSs separated by a shallow minimum of the BH3(η²-H2)⋯OR- intermediate. The structures and energies of all of the species were calculated, leading to the establishment of the potential energy profiles for the reaction. A comparison is made with the mechanism of the proton-transfer reaction to transition-metal hydrides. The solvent influence on the stability of all of the species along the reaction pathway was accounted for by means of polarizable conductor calculation model calculations in tetrahydrofuran (THF). Although in THF the DHB intermediates, the TSs, and the products are destabilized with respect to the separated reactants, the energy barriers for the proton transfer are only slightly affected by the solvent. The dependence of the energies of the DHB complexes, TSs, and products as well as the energy barriers for the H2 release on the central atom and the proton donor strength is also discussed. [ABSTRACT FROM AUTHOR]