Ir(III)-PC(sp³)P Bifunctional Catalysts for Production of H₂ by Dehydrogenation of Formic Acid: Experimental and Theoretical Study

Reversible storage of hydrogen in the form of stable and relatively harmless chemical substances such as formic acid (FA) is one of the cornerstones of a fossil-fuel-free economy. Recently, Ru­(III)-PC­(sp³)P (where PC­(sp³)P = modular dibenzobarrelene-based pincer ligand possessing a pendant functional group) complex <b>1</b> has been reported as a mild and <i>E</i>-selective catalyst in semihydrogenation of alkynes with stoichiometric neat formic acid. Discovery of the additive-free protocol for dehydrogenation of FA launched further studies aiming at the rational design of highly efficient catalysts for this reaction operating under neutral conditions. We now report the results of our investigation on a series of bifunctionl PC­(sp³)P complexes equipped with different outer-sphere auxiliaries, that allowed us to identify an amine-functionalized Ir­(III)-PC­(sp³)P complex <b>3</b>, as a clean and efficient catalyst for the FA dehydrogenation. The catalyst is suitable for fuel-cell applications demonstrating a TON up to 5 × 10⁵ and TOF up to 2 × 10⁴ h^–1 (3.8 × 10⁵ and 1.2 × 10⁴ h^–1 with no additives). In addition to the practical value of the catalyst, experimental and computational mechanistic studies provide rationale for the design of improved next-generation catalysts

Mechanism of Dimethylamine–Borane Dehydrogenation Catalyzed by an Iridium(III) PCP-Pincer Complex

The title complex (^<i>t</i>BuPCP)­IrH­(Cl) (<b>1</b>; ^<i>t</i>BuPCP = κ³-2,6-(CH₂P<i>t</i>Bu₂)₂C₆H₃) appeared to be moderately active in NHMe₂·BH₃ (DMAB) dehydrogenation, allowing the systematic spectroscopic (variable-temperature NMR and IR) investigation of the reaction intermediates and products, under both stoichiometric and catalytic regimes, combined with DFT/M06 calculations. The formation of the hexacoordinate complex (^tBuPCP)­IrHCl­(η¹-BH₃·NHMe₂) (<b>3</b>) stabilized by a NH···Cl hydrogen bond is shown experimentally at the first reaction step. This activates both B–H and Ir–Cl bonds, initiating the precatalyst activation and very first DMAB dehydrogenation cycle. The same geometry is suggested by the DFT calculations for the key intermediate of the catalytic cycle, (^tBuPCP)­IrH₂(η¹-BH₃·NHMe₂) complex (<b>6</b>). In these complexes, DMAB is coordinated trans to the ipso carbon, allowing the steric repulsion between the amine–borane and <i>tert</i>-butyl groups at the phosphorus atoms to be overcome. Under catalytic conditions (2–5 mol % of <b>1</b>) the hydride complex (^tBuPCP)­IrH­(μ²-H₂BH₂) (<b>5</b>) was identified, which is not a dormant catalytic species but the steady-state intermediate formed as a result of the B–N bond breaking. DMAB dehydrogenation yields the borazane monomer H₂BNMe₂ (detected by ¹¹B NMR); dimerization of this species gives the final product [H₂BNMe₂]₂ and (^tBuPCP)­IrH₄ as the catalyst resting state. The scenario of B–N bond cleavage in DMAB leading to byproducts of dehydrogenation such as bis­(dimethylamino)­hydroborane and (^tBuPCP)­IrH­(μ²-H₂BH₂) (<b>5</b>) is proposed. The results obtained allow us to suggest the mechanism of catalytic DMAB dehydrocoupling that could be generalized to other processes

Conformational Flexibility of Dibenzobarrelene-Based PC(sp³)P Pincer Iridium Hydride Complexes: The Role of Hemilabile Functional Groups and External Coordinating Solvents

Bifunctional dibenzobarrelene-based PC­(sp³)P pincer iridium complex <b>1</b> is known as an efficient catalyst in acceptorless dehydrogenation of alcohols and hydrogenation/hydroformylation of alkenes. In order to shed light on the mechanism of the hydrogen formation/activation, we performed variable-temperature IR and NMR (¹H, ³¹P) analysis of intra- and intermolecular interactions involving a hydride ligand and hydroxymethyl cooperating group in <b>1</b> and its analogues. The results of the spectroscopic measurements in different media (dichloromethane, toluene, DMSO, and mixed solvents) were compared with the quantum chemical (DFT/M06 and B3PW91) calculations. The obtained data imply flexibility of the dibenzobarrelene-based scaffold, unprecedented for conventional pincer ligands. Both the CH₂OH-substituted complex <b>1</b> and its COOEt analogue <b>3</b> prefer facial configuration of the PCP ligand with a P–Ir–P angle of ca. 100°. Such geometries are stabilized by Ir···O interaction with the dangling functional group and differ by the mutual arrangement of the H and Cl ligands. The complexes show dynamic equilibrium between the two most stable <i>fac</i>-isomers, which can be transformed into the meridional ones in the presence of coordinating additives (CH₃CN, DMSO, or CO, but not Et₃N). The process is reversible for CH₃CN but irreversible for DMSO and CO, in agreement with the Lewis basicity of these molecules

Dimerization Mechanism of Bis(triphenylphosphine)copper(I) Tetrahydroborate: Proton Transfer via a Dihydrogen Bond

The mechanism of transition-metal tetrahydroborate dimerization was established for the first time on the example of (Ph₃P)₂Cu­(η²-BH₄) interaction with different proton donors [MeOH, CH₂FCH₂OH, CF₃CH₂OH, (CF₃)₂CHOH, (CF₃)₃CHOH, <i>p</i>-NO₂C₆H₄OH, <i>p</i>-NO₂C₆H₄NNC₆H₄OH, <i>p</i>-NO₂C₆H₄NH₂] using the combination of experimental (IR, 190–300 K) and quantum-chemical (DFT/M06) methods. The formation of dihydrogen-bonded complexes as the first reaction step was established experimentally. Their structural, electronic, energetic, and spectroscopic features were thoroughly 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(η²-BH₄) dimerization, yielding [{(Ph₃P)₂Cu}₂(μ,η⁴-BH₄)]⁺

Steric and Acidity Control in Hydrogen Bonding and Proton Transfer to <i>trans-</i>W(N₂)₂(dppe)₂

The interaction of <i>trans-</i>W­(N₂)₂(dppe)₂ (<b>1</b>; dppe = 1,2-bis­(diphenylphosphino)­ethane) with relatively weak acids (<i>p</i>-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

Dihydrogen Bond Intermediated Alcoholysis of Dimethylamine–Borane in Nonaqueous Media

Dimethylamine–borane (DMAB) acid/base properties, its dihydrogen-bonded (DHB) complexes and proton transfer reaction in nonaqueous media were investigated both experimentally (IR, UV/vis, NMR, and X-ray) and theoretically (DFT, NBO, QTAIM, and NCI). The effects of DMAB concentration, solvents polarity and temperature on the degree of DMAB self-association are shown and the enthalpy of association is determined experimentally for the first time (−Δ<i>H</i>°_assoc = 1.5–2.3 kcal/mol). The first case of “improper” (blue-shifting) NH···F hydrogen bonds was observed in fluorobenzene and perfluorobenzene solutions. It was shown that hydrogen-bonded complexes are the intermediates of proton transfer from alcohols and phenols to DMAB. The reaction mechanism was examined computationally taking into account the coordinating properties of the reaction media. The values of the rate constants of proton transfer from HFIP to DMAB in acetone were determined experimentally [(7.9 ± 0.1) × 10^–4 to (1.6 ± 0.1) × 10^–3 mol^–1·s^–1] at 270–310 K. Computed activation barrier of this reaction Δ<i>G</i>^‡theor_298 K(acetone) = 23.8 kcal/mol is in good agreement with the experimental value of the activation free energy Δ<i>G</i>^‡exp_270 K = 21.1 kcal/mol

The Role of Weak Interactions in Strong Intermolecular M···Cl Complexes of Coinage Metal Pyrazolates: Spectroscopic and DFT Study

The nondestructive reversible complexation of the macrocyclic group 11 metal pyrazolates {[3,5-(CF₃)₂Pz]­M}₃ (M = Cu­(I), Ag­(I)) to the halogen atom X = Cl, Br of η³-allyliron tricarbonyl halides (η³-2-R-C₃H₄)­Fe­(CO)₃X is revealed by the variable-temperature spectroscopic (IR, NMR) study combined with density functional theory calculations. The composition of all complexes at room temperature is determined as 1:1. In the case of the [AgL]₃ macrocycle, complexes 1:2 are observed at low temperature (<260 K). The complex’s stability depends on the substituents in the allyl fragment and halide ligand as well as on the metal atom (Ag­(I), Cu­(I)) in the macrocycle. For bulky substituents (Me and Ph) the endo/exo equilibrium of the parent (η³-2-R-C₃H₄)­Fe­(CO)₃X shifts upon the complex formation in favor of the exo isomer due to additional noncovalent interactions of the substituent with macrocycle

Dihydrogen Bonding in Complex (PP₃)RuH(η¹‑BH₄) Featuring Two Proton-Accepting Hydride Sites: Experimental and Theoretical Studies

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(η¹-BH₄) (<b>1</b>, PP₃ = κ⁴-P­(CH₂CH₂PPh₂)₃). Interaction of complex <b>1</b> 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 BH_term and high activation barriers (38.2 and 28.4 kcal/mol in case of HFIP) and leads directly to the thermodynamic product [(PP₃)­RuH_eq(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)­RuH_ax(H₂)]⁺[OR]⁻. The B–H_br bond breaking is the common key step of all pathways investigated