Fixation of atmospheric nitrogen and N-2 reduction to NH3 under mild conditions form an important and, at the same time, challenging area of modern fundamental science. This review considers studies of the reactivity of dinitrogen complexes of group 6 metals (mainly molybdenum and tungsten) with relatively simple phosphine ligands, which made it possible to identify the main stages of N-2 reduction in the coordination sphere of a transition metal and factors affecting the reaction efficiency. The attention is focused on the studies of the protonation of the coordinated N-2 molecule, which is a key step in the catalytic cycle of nitrogen reduction, proceeding as alternating proton and electron transfer steps.

The series of bimetallic complexes (tBuPXCYP)Pd-(mu-OC)M(CO)2L (X, Y = CH2, O; M = W, Mo; L = Cp, Tp) catalyzes formic acid decomposition into H2/CO2 in fairly mild conditions (25-50 degrees C, toluene) without any organic base additives. The catalytic activity of bimetallic complexes increases with CH2- substitution of the O-bridges in the (PXCYP)-frame as well as with the proton-donating ability of the Mo/W hydride. The best result was obtained with (tBuPCP)Pd(mu-OC)Mo(CO)2Cp (3), which gives complete conversion at 2 mol % loading in 30 min at 50 degrees C (TOF = 100 h-1). During the catalysis, LM(CO)3H and (tBuPXCYP)Pd(OCHO) form as visible intermediates, while the palladium hydride species are also involved in the catalytic cycle. The experimental data show that hydride abstraction/CO2 release from palladium formates (tBuPXCYP)Pd(OCHO) is assisted by-OCHO center dot center dot center dot H-A hydrogen bonding with excess formic acid or acidic hydride LM(CO)3H. These findings highlight the pivotal role of the formate interaction with Bro''nsted or Lewis acids at the hydride abstraction/CO2 release step and unify the mechanisms suggested for different catalytic systems.

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.

The mechanism of the consecutive halogenation of the tetrahydroborate anion [BH4]− by hydrogen halides (HX, X = F, Cl, Br) and hexahydro-closo-hexaborate dianion [B6H6]2− by HCl via electrophile-induced nucleophilic substitution (EINS) was established by ab initio DFT calculations [M06/6-311++G(d,p) and wB97XD/6-311++G(d,p)] in acetonitrile (MeCN), taking into account non-specific solvent effects (SMD model). Successive substitution of H− by X− resulted in increased electron deficiency of borohydrides and changes in the character of boron atoms from nucleophilic to highly electrophilic. This, in turn, increased the tendency of the B–H bond to transfer a proton rather than a hydride ion. Thus, the regularities established suggested that it should be possible to carry out halogenation more selectively with the targeted synthesis of halogen derivatives with a low degree of substitution, by stabilization of H2 complex, or by carrying out a nucleophilic substitution of B–H bonds activated by interaction with Lewis acids (BL3).

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-center dot), 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. (C) 2021 Elsevier B.V. All rights reserved.

The mechanism of the halogenation of decahydro-closo-decaborate dianion [B10H10]2− by HCl via the electrophile-induced nucleophilic substitution (EINS) was explored at M06/6-311++G(d,p) level of DFT theory in acetonitrile (MeCN) taking into account non-specific solvent effect (SMD model). The dihydrogen-bonded (DHB) complexes are the first and important reaction intermediates of the EINS process since they determine the principal direction of HCl attack and lead to the proton transfer to the most reactive and elongated B–H bond. Upon the successive replacement of H− with Cl− in the closo-borane structure, a gradual increase in the electron deficiency of closo-borane and the electrophilicity of boron atoms are observed. The lack of stabilization of the η2-H2 complexes for the subsequent stages of the reaction is associated with an increased electrophilicity of boron atoms in substituted closo-boranes. An increased electrophilicity of boron atoms in substituted closo-boranes and higher activation energy of each subsequent stage during the EINS process hamper the reaction and completely stop the chlorination on 3rd or 4th reactions steps. Thus, the data obtained indicate that for the selective synthesis of halogenated products [B10H(10 –x)Clx]2− (x = 5–7) it is necessary to use an approach alternative to the simple acid-initiated nucleophilic substitution. This could be the activation of the bond by Lewis superacids or transition metal catalysis.

The activation of silanes in dehydrogenative coupling with alcohols under general base catalysis was studied experimentally (using multinuclear NMR, IR, and UV-visible spectroscopies) and computationally (at DFT M06/6-311++G(d,p) theory level) on the example of Ph4-nSiHn (n = 1-3) interaction with (CF3)2CHOH in the presence of Et3N. The effect of the phenyl groups' number and H- substitution by the electron-withdrawing (CF3)2CHO- group on Si-H bond hydricity (quantified as hydride-donating ability, HDA) and Lewis acidity of silicon atom (characterized by maxima of molecular electrostatic potential) was accessed. Our results show the coordination of Lewis base (Y = Me3N, ROH, OR-) leads to the increased hydricity of pentacoordinate hypervalent Ph4-nSi(Y)Hn complexes and a decrease of the reaction barrier for H2 release. The formation of tertiary complexes [Ph4-nSi(Y)Hn]···HOR is a critical prerequisite for the dehydrocoupling with alkoxides being ideal activators. The latter can be external or internal, generated by in situ HOR deprotonation. The mutual effect of tetrel interaction and dihydrogen bonding in tertiary complexes (RO-)Ph4-nSiHn···HOR leads to dichotomous activation of Si-H bond promoting the proton-hydride transfer and H2 release.

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.

Thermodynamic hydricity (HDAMeCN) determined as Gibbs free energy (ΔG°[H]−) of the H− detachment reaction in acetonitrile (MeCN) was assessed for 144 small borane clusters (up to 5 boron atoms), polyhedral closo-boranes dianions [BnHn]2−, and their lithium salts Li2[BnHn] (n = 5–17) by DFT method [M06/6-311++G(d,p)] taking into account non-specific solvent effect (SMD model). Thermodynamic hydricity values of diborane B2H6 (HDAMeCN = 82.1 kcal/mol) and its dianion [B2H6]2− (HDAMeCN = 40.9 kcal/mol for Li2[B2H6]) can be selected as border points for the range of borane clusters’ reactivity. Borane clusters with HDAMeCN below 41 kcal/mol are strong hydride donors capable of reducing CO2 (HDAMeCN = 44 kcal/mol for HCO2−), whereas those with HDAMeCN over 82 kcal/mol, predominately neutral boranes, are weak hydride donors and less prone to hydride transfer than to proton transfer (e.g., B2H6, B4H10, B5H11, etc.). The HDAMeCN values of closo-boranes are found to directly depend on the coordination number of the boron atom from which hydride detachment and stabilization of quasi-borinium cation takes place. In general, the larger the coordination number (CN) of a boron atom, the lower the value of HDAMeCN.

Two novel ruthenocene-based pincer palladium tetrahydroborates were characterized by XRD, NMR and FTIR. The alcoholysis of Pd(ii) tetrahydroborate LPd(BH4) (L = κ3-[{2,5-(tBu2PCH2)2C5H2}Ru(C5H5)]) yields the dinuclear cationic Pd(ii) tetrahydroborate with the bridging BH4- ligand [(LPd)2(μ,η1,2:η1,2-BH4)]+. The bifurcate dihydrogen-bonded complexes are the active intermediates of the first proton transfer in the step-wise alcoholysis of LPd(BH4), yielding eventually [(LPd)2(μ,η1,2:η1,2-BH4)]+. According to the X-ray and DFT/M06 geometry analysis, the BH4- ligand in both palladium tetrahydroborates has a mixed coordination mode η1,2. The possibility of BH3-group abstraction from LPd(BH4) by an excess of organic base (THF, Py) with the formation of hydride LPdIIH is shown. This Pd(ii) hydride is a very reactive compound able to rapidly capture CO2 (ca. 15 min) converting into the formate complex LPdII(η1-OC(O)H). The hydrolysis of LPdH with subsequent CO2 insertion yields a hydrocarbonate complex LPdII(η1-OC(O)OH). The hydrocarbonate complex forms hydrogen-bonded dimers in the crystal due to hydrogen bonds between the OC(O)OH fragments.

The tetranuclear silver(I) 3,5‐bis(trifluoromethyl)pyrazolate adducts with 2,2′‐bipyridine derivatives were prepared by the reaction of trinuclear metal complex with the diimines. It was shown that the product structure depends on the reagents ratio: mono‐ and bisdiimine complexes were synthesized. Compounds obtained possess green phosphorescence in the solid state. The short intramolecular Ag ··· Ag distances (≤ 3 Å) make possible electronic communication between the metal atoms which leads to the dual emissive behavior mainly due to the 3 LC phosphorescence with the impact of 3 MLCT. The behavior observed was rationalized by the TD‐DFT study.

The interaction of trans-W(N2)2(dppe)2 (1; dppe = 1,2-bis(diphenylphosphino)ethane) with relatively weak acids (p-nitrophenol, fluorinated alcohols, CF3COOH) was studied by means of variable temperature IR and NMR spectroscopy and complemented by DFT/B3PW91-D3 calculations. The results show, for the first time, the formation of a hydrogen bond to the coordinated dinitrogen, W-N≡N···H-O, that is preferred over H-bonding to the metal atom, W···H-O, despite the higher proton affinity of the latter. Protonation of the core metal-the undesirable side step in the conversion of N2 to NH3-can be avoided by using weaker and, more importantly, bulkier acids.

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 Et3 SiH.

•Trimeric Cu(I) and Ag(I) pyrazolates forms complexes with the wide range of bases.•The majority of complexes in solution have the 1:1 composition.•The complex formation constants are highly sensitive to the solvent.•The 1:1 complex is the primary building block in the solid state.•The driving force of crystal packing is an interaction with the primary basic site.

The hydride donating ability (HDA), determined as Gibbs free energy (Delta G degrees H-) for the reaction of H dissociation, was assessed via the DFT/M06/6-311++G (d,p) calculations for 90 tetracoordinated borohydrides Li [L3B-H] taking into account the solvent effects via the optimization in MeCN and CH2Cl2 under SMD model. Obtained this way, the HDAmecN values vary from 118.2 to 13.4 kcal/mol and correlate well with the Lewis acidity parameters (AN, HA and FA) of parent trigonal boranes (L36). These data show numerically how the variation of the substituents at the boron atom allows the fine-tuning the B H bond reactivity (reduction power) in the reactions involving hydride transfer as well as the selectivity of the reduction processes. The analysis of the data obtained shows that by varying the number of substituents and their nature, it is possible not only to change the properties of neutral trisubstituted boranes from highly electrophilic (represented by halogenide- and pseudohalogenide-boranes) to highly nucleophilic (exemplified by alkoxy-an amidoboranes), but also to repolarize the boron-bound hydrogen and make the proton transfer process more favourable than the hydride transfer. (C) 2018 Elsevier B.V. All rights reserved.

The interaction of copper(I) and silver(I) macrocyclic pyrazolates with aromatic ligands of ruthenium sandwiches (Cp*Rulnd, CpRulnd, and Ind(2)Ru) in solution is shown for the first time. The similar mode of coordination of macrocycles to the C-6 fragment of indenyl ligand was found both in the solution and in the solid state. Complexation of macrocycles with the nonencumbered sandwiches (CpRulnd, Ind2Ru) leads to the formation of infinite stacks via alternating molecules of macrocycles and sandwich compounds as one-dimensional coordination polymers with a regular structure. Coordination mode of the indenyl ligand is independent of the second part of the ruthenium sandwich as well as of the aromatic ligand coordinated to another face of the macrocycle. The general principle of macrocycle supramolecular packing suggests coordination of two ligands on both faces of the macrocycle.

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).

Bis(diphenylphosphino)methane copper(I) tetrahydroborate was synthesized by ligands exchange in bis(triphenylphosphine) copper(I) tetrahydroborate, and characterized by XRD, FTIR, NMR spectroscopy. According to XRD the title compound has dimeric structure, [(μ2-dppm)2Cu2(η2-BH4)2], and crystallizes as CH2Cl2 solvate in two polymorphic forms (orthorhombic, 1, and monoclinic, 2) The details of molecular geometry and the crystal-packing pattern in polymorphs were studied. The rare Twisted Boat-Boat conformation of the core Cu2P4C2 cycle in 1 is found being more stable than Boat-Boat conformation in 2.

The dihydrogen bond-an interaction between a transition-metal or main-group hydride (M-H) and a protic hydrogen moiety (H-X)-is arguably the most intriguing type of hydrogen bond. It was discovered in the mid-1990s and has been intensively explored since then. Herein, we collate up-to-date experimental and computational studies of the structural, energetic, and spectroscopic parameters and natures of dihydrogen-bonded complexes of the form M-H···H-X, as such species are now known for a wide variety of hydrido compounds. Being a weak interaction, dihydrogen bonding entails the lengthening of the participating bonds as well as their polarization (repolarization) as a result of electron density redistribution. Thus, the formation of a dihydrogen bond allows for the activation of both the MH and XH bonds in one step, facilitating proton transfer and preparing these bonds for further transformations. The implications of dihydrogen bonding in different stoichiometric and catalytic reactions, such as hydrogen exchange, alcoholysis and aminolysis, hydrogen evolution, hydrogenation, and dehydrogenation, are discussed.