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In this short review, we attempt to unfold various aspects of excited-state intramolecular proton transfer (ESIPT) from the studies that are available up to date. Since Weller's discovery of ESIPT in salicylic acid (SA) and its derivative methyl salicylate (MS), numerous studies have emerged on the topic and it has become an attractive field of research because of its manifold applications. Here, we discuss some critical aspects of ESIPT and tautomerization from the mechanistic viewpoint. We address excitation wavelength dependence, anti-Kasha ESIPT, fast and slow ESIPT, reversibility and irreversibility of ESIPT, hydrogen bonding and geometrical factors, excited-state double proton transfer (ESDPT), concerted and stepwise ESDPT. [ABSTRACT FROM AUTHOR]

The mononuclear zinc(II) complex cis-[ZnL2(H2O)2] (1; L = 4-(pyridin-3-ylcarbamoyl)benzoate) was synthesized and characterized. By soaking crystals of 1 in a mixture of DMF-H2O solution containing a slight excess of Cu(NO3)2 × 3H2O a transmetalation reaction occurred affording the related copper(II) complex trans-[CuL2(H2O)2] (2). The structures of the compounds were authenticated by single crystal X-ray diffraction revealing, apart from a change in the isomerism, an alteration in the relative orientation of the chelating carboxylate groups and of the pyridine moieties. H-bond interactions stabilize both geometries and expand them into two-dimensional (2D) networks. The transmetalation was confirmed by SEM–EDS analysis. Moreover, the thermodynamic feasibility of the transmetalation is demonstrated by density-functional theory (DFT) studies. The catalytic activities of 1 and 2 for the oxidation of styrene and for the nitroaldol (Henry) C-C coupling reaction were investigated. The copper(II) compound 2 acts as heterogeneous catalyst for the microwave-assisted oxidation of styrene with aqueous hydrogen peroxide, yielding selectively (>99%) benzaldehyde up to 66% of conversion and with a turnover frequency (TOF) of 132 h−1. The zinc(II) complex 1 is the most active catalyst (up to 87% yield) towards the nitroaldol (Henry) coupling reaction between benzaldehyde and nitro-methane or -ethane to afford the corresponding β-nitro alcohols. The reaction of benzaldehyde with nitroethane in the presence of 1 produced 2-nitro-1-phenylpropanol in the syn and the anti diastereoisomeric forms, with a considerable higher selectivity towards the former (66:34). [ABSTRACT FROM AUTHOR]

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.

The interaction of the mixed hydrido-tetrahydridoborate ruthenium(II) complex [(Triphos)RuH(η2-BH4)] [ 1; Triphos = κ3- P-CH3C(CH2CH2PPh2)3] with alcohols of variable acidic strength [MeOH, FCH2CH2OH (MFE), CF3CH2OH (TFE), (CF3)2CHOH (HFIP), and (CF3)3COH (PFTB)] was the subject of a combined computational (DFT) and spectroscopic (VT FTIR, NMR) study. The experimental spectra suggests that RuH ···HO bond formation precedes the protonation of 1, and H2 evolution leads to the loss of boron and the formation of the dimetallic [{(Triphos)RuH}2(µ,η2:η2-BH4)]+ cation. The experimentally determined basicity factor [ Ej(RuH)] of the Ru-bound hydrido ligand of 1.43 is among the highest determined for ruthenium hydrides. Such high basicity leads to very easy proton transfer to the RuH ligand for strong alcohols (HFIP and PFTB). An alternative reaction pathway involving the migration of the bridging hydride (BHbr) to the ruthenium center is suggested for weaker proton donors (MeOH and TFE). [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]

Abstract: Compound trans-PtBr2(C2H4)(NHEt2) (1) has been synthesized by Et2NH addition to K[PtBr3(C2H4)] and structurally characterized. Its isomer cis-PtBr2(C2H4)(NHEt2) (3) has been obtained from 1 by photolytic dissociation of ethylene, generating the dinuclear trans-[PtBr2(NHEt2)]2 intermediate (2), followed by thermal re-addition of C2H4, but only in low yields. The addition of further Et2NH to 1 in either dichloromethane or acetone yields the zwitterionic complex trans-Pt(−)Br2(NHEt2)(CH2CH2N(+)HEt2) (4) within the time of mixing in an equilibrated process, which shifts toward the product at lower temperatures (ΔH° = −6.8 ± 0.5 kcal/mol, ΔS° = 14.0 ± 2.0 e.u., from a variable temperature IR study). 1H NMR shows that free Et2NH exchanges rapidly with H-bonded amine in a 4·NHEt2 adduct, slowly with the coordinated Et2NH in 1, and not at all (on the NMR time scale) with Pt-NHEt2 or –CH2CH2N(+)HEt2 in 4. No evidence was obtained for deprotonation of 4 to yield an aminoethyl derivative trans-[PtBr2(NHEt2)(CH2CH2NEt2)]− (5), except as an intermediate in the averaging of the diasteretopic methylene protons of the CH2CH2N(+)HEt2 ligand of 4 in the higher polarity acetone solvent. Computational work by DFT attributes this phenomenon to more facile ion pair dissociation of 5·Et2NH2 +, obtained from 4·Et2NH, facilitating inversion at the N atom. Complex 4 is the sole observable product initially but slow decomposition occurs in both solvents, though in different ways, without observable generation of NEt3. Addition of TfOH to equilibrated solutions of 4, 1 and excess Et2NH leads to partial protonolysis to yield NEt3 but also regenerates 1 through a shift of the equilibrium via protonation of free Et2NH. The DFT calculations reveal also a more favourable coordination (stronger Pt–N bond) of Et2NH relative to PhNH2 to the PtII center, but the barriers of the nucleophilic additions of Et2NH to the C2H4 ligand in 1 and of PhNH2 to trans-PtBr2(C2H4)(PhNH2) (1a) are predicted to be essentially identical for the two systems. [Copyright &y& Elsevier]

Metal-metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed via intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (M δ + ⋯M' δ - ) have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N-H and B-H bonds of the substrate via the synergetic action of Lewis acidic (M + ) and basic hydride (M-H) metal species derived from the same mononuclear complex (M-Br). It was also demonstrated that this system generated in situ from the air-stable Mn(i) complex fac -[(CO) 3 (bis(NHC))MnBr] and NaBPh 4 shows high activity for H 2 production from several substrates (Me 2 NHBH 3 , t BuNH 2 BH 3 , MeNH 2 BH 3 , NH 3 BH 3 ) at low catalyst loading (0.1% to 50 ppm), providing outstanding efficiency for Me 2 NHBH 3 (TON up to 18 200) that is largely superior to all known 3d-, s-, p-, f-block metal derivatives and frustrated Lewis pairs (FLPs). These results represent a step forward towards more extensive use of intermolecular bimetallic cooperation concepts in modern homogeneous catalysis., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

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···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 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 ν SiH range: the ν SiH⋅⋅⋅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 Et 3 SiH., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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-H δ- ⋅⋅⋅ 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-H δ+ ⋅⋅⋅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., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The thermodynamics of chloride dissociation from the 18e arene ruthenium iminophosphonamides [(η 6 -arene)RuCl{(R'N) 2 PR 2 }] (1 a-d) [previously known with arene=C 6 Me 6 , R=Ph, R'=p-Tol (a); R=Et, R'=p-Tol (b); R=Ph, R'=Me (c); and new with arene=p-cymene, R=Ph, R'=p-Tol (d)] was assessed in both polar and apolar solvents by variable-temperature UV/Vis, NMR, and 2D EXSY 1 H NMR methods, which highlighted the influence of the NPN ligand on the equilibrium parameters. The dissociation enthalpy ΔH d decreases with increasing electron-donating ability of the N- and P-substituents (1 a, 1 d>1 b>1 c) and solvent polarity, and this results in exothermic spontaneous dissociation of 1 c in polar solvents. The coordination of neutral ligands (MeCN, pyridine, CO) to the corresponding 16e complexes [(η 6 -arene)Ru{(R'N) 2 PR 2 }] + PF 6 - (2 a-d) is reversible; the stability of the 2⋅L adducts depends on the π-accepting ability of L. Carbonylation of 2 a and 2 d resulted in rare examples of cationic arene ruthenium carbonyl complexes (3 a, 3 d), while the monocarbonyl adduct derived from 2 c reacts further with a second equivalent of CO with conversion to carbonyl-carbamoyl complex 3 c, in which one CO molecule is inserted into the Ru-N bond. The new complexes 1 d, 2 d, 3 a, 3 c, and 3 d were isolated and structurally characterized., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Two Co(I) hydrides containing the tripodal polyphosphine ligand EP 3 , (κ 4 -EP 3 )Co(H) [E(CH 2 CH 2 PPh 2 ) 3 ; E = N (1), P (2)], have been exploited as ammonia borane (NH 3 BH 3 , AB) dehydrogenation catalysts in THF solution at T = 55 °C. The reaction has been analyzed experimentally through multinuclear ( 11 B, 31 P{ 1 H}, 1 H) NMR and IR spectroscopy, kinetic rate measurements, and kinetic isotope effect (KIE) determination with deuterated AB isotopologues. Both complexes are active in AB dehydrogenation, albeit with different rates and efficiency. While 1 releases 2 equiv of H 2 per equivalent of AB in ca. 48 h, with concomitant borazine formation as the final "spent fuel", 2 produces 1 equiv of H 2 only per equivalent of AB in the same reaction time, along with long-chain poly(aminoboranes) as insoluble byproducts. A DFT modeling of the first AB dehydrogenation step has been performed, at the M06//6-311++G** level of theory. The combination of the kinetic and computational data reveals that a simultaneous B-H/N-H activation occurs in the presence of 1, after a preliminary AB coordination to the metal center. In 2, no substrate coordination takes place, and the process is better defined as a sequential BH 3 /NH 3 insertion process on the initially formed [Co]-NH 2 BH 3 amidoborane complex. Finally, the reaction of 1 and 2 with NH-acids [AB and Me 2 NHBH 3 (DMAB)] has been followed via VT-FTIR spectroscopy (in the -80 to +50 °C temperature range), with the aim of gaining a deeper experimental understanding of the dihydrogen bonding interactions that are at the origin of the observed H 2 evolution.

According to spectroscopic (NMR, IR, UV/Vis) study, the interaction of pentaphosphaferrocene [Cp*Fe(η(5) -P5 )] with trimeric copper pyrazolate [(Cu{3,5-(CF3 )2 Pz})3 ] yields a new compound that is astonishingly stable in solution. Single-crystal X-ray analysis reveals unprecedented structural changes in the interacting molecules and the unique type of coordination [Cp*Fe(μ3 -η(5) :η(2) ,η(2) -P5 ){Cu(3,5-(CF3 )2 Pz)}3 ]. As a result of the 90° macrocycle folding, the copper atoms are able to behave both as a Lewis acid and as a Lewis base in the interaction with the cyclo-P5 ligand., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Combining variable-temperature infrared and NMR spectroscopic studies with quantum-chemical calculations (density functional theory (DFT) and natural bond orbital) allowed us to address the problem of competition between MH (M = transition metal) and BH hydrogens as proton-accepting sites in dihydrogen bond (DHB) and to unravel the mechanism of proton transfer to complex (PP3)RuH(η(1)-BH4) (1, PP3 = κ(4)-P(CH2CH2PPh2)3). Interaction of complex 1 with CH3OH, fluorinated alcohols of variable acid strength [CH2FCH2OH, CF3CH2OH, (CF3)2CHOH (HFIP), (CF3)3COH], and CF3COOH leads to the medium-strength DHB complexes involving BH bonds (3-5 kcal/mol), whereas DHB complexes with RuH were not observed experimentally. The two proton-transfer pathways were considered in DFT/M06 calculations. The first one goes via more favorable bifurcate complexes to BHterm and high activation barriers (38.2 and 28.4 kcal/mol in case of HFIP) and leads directly to the thermodynamic product [(PP3)RuHeq(H2)](+)[OR](-). The second pathway starts from the less-favorable complex with RuH ligand but shows a lower activation barrier (23.5 kcal/mol for HFIP) and eventually leads to the final product via the isomerization of intermediate [(PP3)RuHax(H2)](+)[OR](-). The B-Hbr bond breaking is the common key step of all pathways investigated.

Structural, spectroscopic, and electronic features of weak hydrogen-bonded complexes of CpM(CO)(3)H (M = Mo (1a), W (1b)) hydrides with organic bases (phosphine oxides R(3)PO (R = n-C(8)H(17), NMe(2)), amines NMe(3), NEt(3), 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 (DeltaH degrees , DeltaS degrees ) 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 CH(2)Cl(2), THF, and CH(3)CN 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 CH(3)CN (X = N) or between CH(2)Cl(2) 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 CH(2)Cl(2) or THF.

The novel iridium(III) hydride [(kappa(3)-P,P,P-NP(3))IrH(3)] [NP(3) = N(CH(2)CH(2)PPh(2))(3)] was synthesized and characterized by spectroscopic methods and X-ray crystallography. Its reactivity with strong (HBF(4)) and medium-strength [the fluorinated alcohols 1,1,1-trifluoroethanol (TFE) 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 acid TFE, the only species observed in the 190-298 K temperature range was the dihydrogen-bonded adduct between the hydride and the alcohol, while with the stronger acid HBF(4), the proton transfer was complete, giving rise to a new intermediate [(kappa(3)-P,P,P-NP(3))IrH(4)](+). With a medium-strength acid like HFIP, 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 [(kappa(4)-NP(3))IrH(2)](+), after slow 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.

Low-temperature (200 K) protonation of [Mo(CO)(Cp*)H(PMe(3))(2)] (1) by Et(2)OHBF(4) gives a different result depending on a subtle solvent change: The dihydrogen complex [Mo(CO)(Cp*)(eta(2)-H(2))(PMe(3))(2)](+) (2) is obtained in THF, whereas the tautomeric classical dihydride [Mo(CO)(Cp*)(H)(2)(PMe(3))(2)](+) (3) is the only observable product in dichloromethane. Both products were fully characterised (nu(CO) IR; (1)H, (31)P, (13)C NMR spectroscopies) at low temperature; they lose H(2) upon warming to 230 K at approximately the same rate (ca. 10(-3) s(-1)), with no detection of the non-classical form in CD(2)Cl(2), to generate [Mo(CO)(Cp*)(FBF(3))(PMe(3))(2)] (4). The latter also slowly decomposes at ambient temperature. One of the decomposition products was crystallised and identified by X-ray crystallography as [Mo(CO)(Cp*)(FHFBF(3))(PMe(3))(2)] (5), which features a neutral HF ligand coordinated to the transition metal through the F atom and to the BF(4) (-) anion through a hydrogen bond. The reason for the switch in relative stability between 2 and 3 was probed by DFT calculations based on the B3LYP and M05-2X functionals, with inclusion of anion and solvent effects by the conductor-like polarisable continuum model and by explicit consideration of the solvent molecules. Calculations at the MP4(SDQ) and CCSD(T) levels were also carried out for calibration. The calculations reveal the key role of non-covalent anion-solvent interactions, which modulate the anion-cation interaction ultimately altering the energetic balance between the two isomeric forms.

Compounds ((n)Bu(4)P)[PtBr(3)(C(2)H(4))] (1), trans-[PtBr(2)(NH(2)Ph)(C(2)H(4))] (2), cis-[PtBr(2)(NH(2)Ph)(C(2)H(4))] (3), ((n)PBu(4))(2)[PtBr(4)] (4), ((n)PBu(4))[PtBr(3)(NH(2)Ph)] (5), and cis-[PtBr(2)(NH(2)Ph)(2)] (6), as well as the trichlorido analogue of 1, ((n)Bu(4)P)[PtCl(3)(C(2)H(4))] (1(Cl)), have been investigated experimentally by both IR and Raman spectroscopy, and theoretically by geometry optimization and normal-mode analysis by the DFT approach. An analysis of the normal modes of coordinated ethylene in compounds 1, 1(Cl), 2, and 3 followed by a potential energy distribution investigation shows extensive vibrational coupling between the nu(C=C) and delta(s)(CH(2)) A(1) modes in two bands at around 1510-1520 and 1230-1250 cm(-1), the latter one having greater nu(C=C) contribution. The rho(w)(CH(2)) A(1) mode, the contribution of which the above two bands is negligible, is responsible for a lower-frequency band at 995-1005 cm(-1). A complete vibrational analysis, backed up by the DFT calculations, has also been carried out on the Pt-Br stretching vibrations of the tetrabromido complex 4, the tribromido complexes 1 and 5, and the dibromido complexes 2, 3, and 6, and on the Pt-Cl vibrations of the analogous complex 1(Cl). The study illustrates the advantages of coupling high-level computations to the vibrational analyses to make unambiguous band assignments in IR and Raman spectroscopy.

The compound [Cp*Mo(PMe3)3H] (1) is reversibly oxidized at E1/2 = -1.40 V vs ferrocene in MeCN. Its oxidation with Cp2FePF6 yields thermally stable [Cp*Mo(PMe3)3H]PF6 (2), which has been isolated and characterized by IR and EPR spectroscopy and by single-crystal X-ray diffraction. The 1H and 31P NMR spectra of 1 show two types of PMe3 ligands in a 1:2 ratio at low temperature, but only one average signal at room temperature, with activation parameters of DeltaH++ = 11.7(3) kcal mol-1 and DeltaS++ = -3(1) eu for the exchange process. Although only one species is evidenced by NMR for 1 and by EPR for 2, the solution IR spectra of each complex show two bands in the v(Mo-H) region (1, major at 1794 cm-1 and minor at ca. 1730 cm-1; 2, ca. 1800 and 1770 cm-1 with approximately equal intensity), the position and relative intensity being little dependent on the solvent. A thorough DFT investigation suggests that these are different rotamers involving different relative orientations of the Cp* ring and the PMe3 ligands in these complexes. This ring rotation process is very rapid on the NMR and EPR time scale but slow on the IR time scale. The X-ray data and the theoretical calculations suggest the presence of weak Mo-H...F interactions in compound 2. The possibility of PMe3 dissociation, as well as other intramolecular rearrangements, for 1 and 2 is excluded by experimental and computational studies. Protonation of 1 yields [Cp*Mo(PMe3)3H2]+ (3), which also reveals a dynamic process interconverting the two inequivalent H ligands and the three PMe3 ligands (two sets in a 1:2 ratio in the frozen structure) on the NMR time scale (activation parameters of DeltaH++ = 9.3(1) kcal/mol and DeltaS++ = -4.1(4) eu). A DFT study suggests that this exchange process occurs via a low-energy symmetric dihydride intermediate and not through a dihydrogen complex.

The hydrogen-bonding and proton-transfer pathway to complex [Cp*W(dppe)H(3)] (Cp*=eta(5)-C(5)Me(5); dppe=Ph(2)PCH(2)CH(2)PPh(2)) was investigated experimentally by IR, NMR, UV/Vis spectroscopy in the presence of fluorinated alcohols, p-nitrophenol, and HBF(4), and by using DFT calculations for the [CpW(dhpe)H(3)] model (Cp=eta(5)-C(5)H(5); dhpe=H(2)PCH(2)CH(2)PH(2)) and for the real system. A study of the interaction with weak acids (CH(2)FCH(2)OH, CF(3)CH(2)OH, (CF(3))(2)CHOH) allowed the determination of the basicity factor, E(j)=1.73+/-0.01, making this compound the most basic hydride complex reported to date. A computational investigation revealed several minima for the [CpW(dhpe)H(3)] adducts with CF(3)CH(2)OH, (CF(3))(2)CHOH, and 2(CF(3))(2)CHOH and confirms that these interactions are stronger than those established by the Mo analogue. Their geometries and relative energies are closely related to those of the homologous Mo systems, with the most stable adducts corresponding to H bonding with M-H sites, however, the geometric and electronic parameters reveal that the metal center plays a greater role in the tungsten systems. Proton-transfer equilibria are observed with the weaker proton donors, the proton-transfer step for the system [Cp*W(dppe)H(3)]/HOCH(CF(3))(2) in toluene having DeltaH=(-3.9+/-0.3) kcal mol(-1) and DeltaS=(-17+/-2) cal mol(-1) K(-1). The thermodynamic stability of the proton-transfer product is greater for W than for Mo. Contrary to the Mo system, the protonation of the [Cp*W(dppe)H(3)] appears to involve a direct proton transfer to the metal center without a nonclassical intermediate, although assistance is provided by a hydride ligand in the transition state.

The isomerization of complex [Cp*Fe(dppe)(eta2-H2)]+, generated in situ by low-temperature protonation of Cp*Fe(dppe)H with either HBF4 or CF3COOH, to the dihydride tautomer trans-[Cp*Fe(dppe)(H)2]+ is irreversible and follows first-order kinetics in the -10 to +15 degrees C range with Delta H double dagger = 21.6 +/- 0.8 kcal mol(-1) and DeltaS double dagger = 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)(eta2-HD)]+ complex establishes a very rapid isotope redistribution equilibrium with the eta2-H2 and eta2-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)(eta2-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.

The interaction of CpM(CO)3H (M = Mo, W) hydrides as proton donors with different bases (B = pyridine, (n-Oc)3PO, ((CH3)2N)3PO, H3BNEt3) was studied by variable temperature IR spectroscopy and theoretically by DFT/B3LYP calculations. The data obtained show for the first time the formation of intermolecular hydrogen bonds between the neutral transition metal hydrides and bases in solutions of low polarity. These M-H...B hydrogen bonds are shown to precede the hydrides' deprotonation.

This Account presents our view of unconventional intermolecular hydrogen bonds (HBs) for organometallic complexes and transition-metal or main-group hydrides. Over the past decade, low-temperature spectroscopic (IR, UV, and NMR) studies combined with theoretical calculations have disclosed the static and dynamic features of different HBs. Their guiding role in the proton-transfer processes was determined, as well as the energetic characteristics of HB intermediates and the activation barriers. Nevertheless, there is still much to explore in terms of the prediction of HB properties and control of protonation/deprotonation processes.

The present contribution reports experimental and computational investigations of the interaction between [Cp*Fe(dppe)H] and different proton donors (HA). The focus is on the structure of the proton transfer intermediates and on the potential energy surface of the proton transfer leading to the dihydrogen complex [Cp*Fe(dppe)(H2)]+. With p-nitrophenol (PNP) a UV/Visible study provides evidence of the formation of the ion-pair stabilized by a hydrogen bond between the nonclassical cation [Cp*Fe(dppe)(H2)]+ and the homoconjugated anion ([AHA]-). With trifluoroacetic acid (TFA), the hydrogen-bonded ion pair containing the simple conjugate base (A-) in equilibrium with the free ions is observed by IR spectroscopy when using a deficit of the proton donor. An excess leads to the formation of the homoconjugated anion. The interaction with hexafluoroisopropanol (HFIP) was investigated quantitatively by IR spectroscopy and by 1H and 31P NMR spectroscopy at low temperatures (200-260 K) and by stopped-flow kinetics at about room temperature (288-308 K). The hydrogen bond formation to give [Cp*Fe(dppe)H]HA is characterized by DeltaH degrees =-6.5+/-0.4 kcal mol(-1) and DeltaS degrees = -18.6+/-1.7 cal mol(-1) K(-1). The activation barrier for the proton transfer step, which occurs only upon intervention of a second HFIP molecule, is DeltaH(not equal) = 2.6+/-0.3 kcal mol(-1) and DeltaS(not equal) = -44.5+/-1.1 cal mol(-1) K(-1). The computational investigation (at the DFT/B3 LYP level with inclusion of solvent effects by the polarizable continuum model) reproduces all the qualitative findings, provided the correct number of proton donor molecules are used in the model. The proton transfer process is, however, computed to be less exothermic than observed in the experiment.

The interaction of [NbCp(2)H(3)] with fluorinated alcohols to give dihydrogen-bonded complexes was studied by a combination of IR, NMR and DFT methods. IR spectra were examined in the range from 200-295 K, affording a clear picture of dihydrogen-bond formation when [NbCp(2)H(3)]/HOR(f) mixtures (HOR(f) = hexafluoroisopropanol (HFIP) or perfluoro-tert-butanol (PFTB)) were quickly cooled to 200 K. Through examination of the OH region, the dihydrogen-bond energetics were determined to be 4.5+/-0.3 kcal mol(-1) for TFE (TFE = trifluoroethanol) and 5.7+/-0.3 kcal mol(-1) for HFIP. (1)H NMR studies of solutions of [NbCp(2)H(2)(B)H(A)] and HFIP in [D(8)]toluene revealed high-field shifts of the hydrides H(A) and H(B), characteristic of dihydrogen-bond formation, upon addition of alcohol. The magnitude of signal shifts and T(1) relaxation time measurements show preferential coordination of the alcohol to the central hydride H(A), but are also consistent with a bifurcated character of the dihydrogen bonding. Estimations of hydride-proton distances based on T(1) data are in good accord with the results of DFT calculations. DFT calculations for the interaction of [NbCp(2)H(3)] with a series of non-fluorinated (MeOH, CH(3)COOH) and fluorinated (CF(3)OH, TFE, HFIP, PFTB and CF(3)COOH) proton donors of different strengths showed dihydrogen-bond formation, with binding energies ranging from -5.7 to -12.3 kcal mol(-1), depending on the proton donor strength. Coordination of proton donors occurs both to the central and to the lateral hydrides of [NbCp(2)H(3)], the former interaction being of bifurcated type and energetically slightly more favourable. In the case of the strong acid H(3)O(+), the proton transfer occurs without any barrier, and no dihydrogen-bonded intermediates are found. Proton transfer to [NbCp(2)H(3)] gives bis(dihydrogen) [NbCp(2)(eta(2)-H(2))(2)](+) and dihydride(dihydrogen) complexes [NbCp(2)(H)(2)(eta(2)-H(2))](+) (with lateral hydrides and central dihydrogen), the former product being slightly more stable. When two molecules of TFA were included in the calculations, in addition to the dihydrogen-bonded adduct, an ionic pair formed by the cationic bis(dihydrogen) complex [NbCp(2)(eta(2)-H(2))(2)](+) and the homoconjugated anion pair (CF(3)COO...H...OOCCF(3))(-) was found as a minimum. It is very likely that these ionic pairs may be intermediates in the H/D exchange between the hydride ligands and the OD group observed with the more acidic alcohols in the NMR studies.

The reaction between CpFe(dppe)H and a number of different proton donors (2-fluoroethanol, MFE; 2,2,2-trifluoroethanol, TFE; hexafluoro-2-propanol, HFIP; perfluoro-tert-butyl alcohol, PFTB; and trifluoroacetic acid, TFA) has been investigated spectroscopically by variable-temperature infrared, UV-visible, and NMR spectroscopy, and has been measured kinetically by the stopped-flow technique with UV-visible detection. The low-temperature IR study shows the establishment of hydrogen-bonding interactions which involve the hydride ligand as the proton accepting site. This investigation quantifies the thermodynamics of the hydrogen-bonding interaction and the basicity factor (E(j)) of the hydride complex. All techniques agree in indicating an equilibration process, after the immediate hydrogen-bond formation, between the hydride complex and an intermediate dihydrogen complex, [CpFe(dppe)(H(2))](+). The equilibrium is shifted toward the dihydrogen complex to a greater extent for the stronger alcohols and for higher alcohol/Fe ratios. The observed equilibration rate constant is linearly dependent on the alcohol concentration, in agreement with the involvement of two alcohol molecules and the formation of a homoconjugate pair. The rate constant increases with the acidity of the proton donor (TFE < HFIP < PFTB < TFA). The rate of the subsequent irreversible isomerization leading to the classical dihydride complex, [CpFe(dppe)H(2)](+), is first order, and the rate constant does not depend on the proton donor nature. The reaction continues, if conducted in CH(2)Cl(2), with a third, slower step leading to the paramagnetic [CpFe(dppe)Cl](+) product. The kinetic data are in accord with an isomerization mechanism consisting of an intramolecular reorganization, leading in one step from the dihydrogen complex to the classical dihydride species, and disagree with the occurrence of a proton-transfer process at the metal site.

The interaction of the ruthenium hydride complex CpRuH(CO)(PCy(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(3))(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 (eta(2)-H(2))-complex 3 (DeltaH() = 11.0 +/- 0.5 kcal/mol and DeltaS() = -19 +/- 3 eu) were obtained for the first time. The results of the DFT study of the proton transfer process, taking CF(3)COOH and (CF(3))(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](-) 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(3))(eta(2)-H(2))](+) is driven by the formation of the homoconjugated anionic complex [ROHOR](-). 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(3)), observed at temperatures above 250 K.

The reactions of the octahedral dihydrido complexes [MH(2)(PP(3))] [M=Fe, Ru, Os; PP(3)=P(CH(2)CH(2)PPh(2))(3)] with a variety of weak ROH acids have been studied by IR and NMR methods in either CH(2)Cl(2) or THF in the temperature range from 190 to 290 K. This study has allowed the determination of the spectral and thermodynamic properties associated with the formation of dihydrogen bonds (DHB) between the terminal hydrides and the OH group. Both the DHB enthalpy values and the hydride basicity factors (E(j)) have been found to increase in the order Fe < Ru < Os. The proton transfer process, leading to the DHB complexes, and eventually to eta(2)-H(2) products, has been found to depend on the acidic strength of the alcohol as well as the nature of the solvent. Low temperature IR and NMR techniques have been used to trace the complete energy profile of the proton transfer process involving the osmium complex [OsH(2)(PP(3))] with trifluoroethanol.