Copper–Ligand Cooperativity in H2 Activation Enables the Synthesis of Copper Hydride Complexes

Visible-Light-Driven CO2 Reduction with Homobimetallic Complexes. Cooperativity between Metals and Activation of Different Pathways

Visible-light-driven reduction of CO2 to both CO and formate (HCOO-) was achieved in acetonitrile solutions using a homobimetallic Cu bisquaterpyridine complex. In the presence of a weak acid (water) as coreactant, the reaction rate was enhanced, and a total of ca. 766 TON (turnover number) was reached for the CO2 reduction, with 60% selectivity for formate and 28% selectivity for CO, using Ru(phen)32+ as a sensitizer and amines as sacrificial electron donors. Mechanistic studies revealed that with the help of cooperativity between two Cu centers, a bridging hydride is generated in the presence of a proton source (water) and further reacts with CO2 to give HCOO-. A second product, CO, was also produced in a parallel competitive pathway upon direct coordination of CO2 to the reduced complex. Mechanistic studies further allowed comparison of the observed reactivity to the monometallic Cu quaterpyridine complex, which only produced CO, and to the related homobimetallic Co bisquaterpyridine complex, that has been previously shown to generate formate following a mechanism not involving the formation of an intermediate hydride species.

CO2 reduction reaction on Sc-doped nanocages as catalysts

CONTEXT

The catalytic ability of Sc-doped C46 and Sc-doped Al23P23 as catalysts of CO2-RR to create the CH4 and CH3OH is investigated. The mechanisms of CO2-RR are examined by theoretical methods and ΔGreaction of reaction steps of CO2-RR mechanisms are calculated. The overpotential of CH4 and CH3OH production on Sc-doped C46 and Sc-doped Al23P23 is calculated. The Sc atoms of Sc-doped C46 and Sc-doped Al23P23 can adsorb the CO2 molecule as the first step of CO2-RR. The CH4 is produced from hydrogenation of *CH3O and the *CO → *CHO reaction step is the rate limiting step for CH4 production. The CH3OH can be formed on Sc-doped C46 and Sc-doped Al23P23 by *CO → *CHO → *CH2O → *CH3O → CH3OH mechanism and HCOOH → *CHO → *CH2O → *CH3O → CH3OH mechanism. The Sc-C46 and Sc-Al23P23 can catalyze the CO2-RR to produce the CH4 and CH3OH by acceptable mechanisms.

METHODS

Here, the structures are optimized by PW91PW91/6-311+G (2d, 2p) and M06-2X/cc-pVQZ methods in GAMESS software. The frequencies of nanocages and their complexes with species of CO2-RR are investigated by mentioned methods. The transition state of each reaction step of CO2-RR is searched by Berny method to find the CO2-RR intermediates. The ∆Eadsorption of intermediates of CO2-RR on surfaces of nanocages is calculated and the ∆Greaction of reaction steps of CO2-RR is calculated.

Reactivity of Ir(I)-aminophosphane platforms towards oxidants

The iridium(I)-aminophosphane complex [Ir{κ3C,P,P'-(SiNP-H)}(cod)] has been prepared by reaction of [IrCl(cod)(SiNP)] with KCH3COO. DFT calculations show that this reaction takes place through an unexpected outer sphere mechanism (SiNP = SiMe2{N(4-C6H4Me)PPh2}2; SiNP-H = CH2SiMe{N(4-C6H4Me)PPh2}2). The reaction of [IrCl(cod)(SiNP)] or [Ir{κ3C,P,P'-(SiNP-H)}(cod)] with diverse oxidants has been explored, yielding a range of iridium(III) derivatives. On one hand, [IrCl(cod)(SiNP)] reacts with allyl chloride rendering the octahedral iridium(III) derivative [IrCl2(η3-C3H5)(SiNP)], which, in turn, reacts with tert-butyl isocyanide yielding the substitution product [IrCl(η3-C3H5)(CNtBu)(SiNP)]Cl via the observed intermediate [IrCl2(η1-C3H5)(CNtBu)(SiNP)]. On the other hand, the reaction of [Ir{κ3C,P,P'-(SiNP-H)}(cod)] with [FeCp2]X (X = PF6, CF3SO3), I2 or CF3SO3CH3 results in the metal-centered two-electron oxidation rendering a varied assortment of iridium(III) compounds. [Ir{κ3C,P,P'-(SiNP-H)}(cod)] reacts with [FeCp2]+ (1 : 2) in acetonitrile affording [Ir{κ3C,P,P'-(SiNP-H)}(CH3CN)3]2+ isolated as both the triflato and the hexafluorophosphato derivatives. Also, the reaction of [Ir{κ3C,P,P'-(SiNP-H)}(cod)] with I2 (1 : 1) yields a mixture of iridium(III) derivatives, namely the mononuclear compound [IrI(κ2P,P'-SiNP)(η2,η3-C8H11)]I, containing the η2,η3-cycloocta-2,6-dien-1-yl ligand, and two isomers of the dinuclear derivative [Ir2{κ3C,P,P'-(SiNP-H)}2(μ-I)3]I, the first species being isolated in low yield. DFT calculations indicate that [IrI(κ2P,P'-SiNP)(η2,η3-C8H11)]I forms as the result of a bielectronic oxidation of iridium(I) followed by the deprotonation of the cod ligand by iodide and the protonation of the methylene moiety of the [Ir{κ3C,P,P'-(SiNP-H)}] platform by the newly formed HI. Finally, the oxidation of [Ir{κ3C,P,P'-(SiNP-H)}(cod)] by methyl triflate proceeds via a hydride abstraction from the cod ligand, with the elimination of methane and the formation of the η2,η3-cycloocta-2,6-dien-1-yl ligand with the concomitant two-electron oxidation of the iridium centre. The crystal structures of selected compounds have been determined.

The Backbone of Success of P,N-Hybrid Ligands: Some Recent Developments

Organophosphorus ligands are an invaluable family of compounds that continue to underpin important roles in disciplines such as coordination chemistry and catalysis. Their success can routinely be traced back to facile tuneability thus enabling a high degree of control over, for example, electronic and steric properties. Diphosphines, phosphorus compounds bearing two separated P^III donor atoms, are also highly valued and impart their own unique features, for example excellent chelating properties upon metal complexation. In many classical ligands of this type, the backbone connectivity has been based on all carbon spacers only but there is growing interest in embedding other donor atoms such as additional nitrogen (–NH–, –NR–) sites. This review will collate some important examples of ligands in this field, illustrate their role as ligands in coordination chemistry and highlight some of their reactivities and applications. It will be shown that incorporation of a nitrogen-based group can impart unusual reactivities and important catalytic applications.

1H NMR global diatropicity in copper hydride complexes

Understanding the magnetic response of electrons in nanoclusters is essential to interpret their NMR spectra thereby providing guidelines for their synthesis towards various target applications. Here, we consider two copper hydride clusters that have applications in hydrogen storage and release under standard temperature and pressure. Through Born-Oppenheimer molecular dynamics simulations, we study dynamics effects and their contributions to the NMR peaks. Finally, we examine the electrons' magnetic response to an applied external magnetic field using the gauge-including magnetically induced currents theory. Local diatropic currents are generated in both clusters but an interesting global diatropic current also appears. This diatropic current has contributions from three μ₃-H hydrides and six Cu atoms that form a chain together with three S atoms from the closest ligands resulting in a higher shielding of these hydrides' ¹H NMR response. This explains the unusual upfield chemical shift compared to the common downfield shift in similarly coordinated hydrides both observed in previous experimental reports.

Distinct Reactivity Modes of a Copper Hydride Enabled by an Intramolecular Lewis Acid

We disclose a 1,4,7-triazacyclononane (TACN) ligand featuring an appended boron Lewis acid. Metalation with Cu(I) affords a series of tetrahedral complexes including a boron-capped cuprous hydride. We demonstrate distinct reactivity modes as a function of chemical oxidation: hydride transfer to CO2 in the copper(I) state and oxidant-induced H2 evolution as well as alkyne reduction.

Cyclometalated iridium complexes based on monodentate aminophosphanes

Monodentate aminophosphanes HNP [NH(4-tolyl)PPh₂] and SiMe₃NP [SiMe₃N(4-tolyl)PPh₂] react with [Ir(μ-Cl)(cod)]₂ affording tetra- or pentacoordinate complexes of formula [IrCl(L)_n(cod)] (L = HNP, n = 1, 2; L = SiMe₃NP, n = 1). The reaction of [IrCl(SiMe₃NP)(cod)] with carbon monoxide smoothly renders [Ir(CO)₃(SiMe₃NP)₂][IrCl₂(CO)₂]. The reaction of HNP or SiMe₃NP with [Ir(CH₃CN)₂(cod)][PF₆] yields the cyclometalated iridium(III)-hydride derivatives [IrH{κ²C,P-NR(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] (R = H, SiMe₃) as a result of the intramolecular oxidative addition of the tolyl C²-H bond to iridium. The straighforward formation of [IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)]⁺ was observed when the reaction was monitored by NMR spectroscopy at 233 K, whereas a more complex reaction sequence was observed in the formation of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)]⁺, including the formation of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)(cod)]⁺ and [Ir(cod)(HNP)₂]⁺. The "mixed" complex [IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(HNP)(cod)]⁺ was obtained upon reaction of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] with SiMe₃NP at 233 K. Finally, the reaction of [Ir(CH₃CN)₂(coe)₂][PF₆] with SiMe₃NP or HNP resulted in the formation of [Ir(CH₃CN)₂(SiMe₃NP)₂][PF₆] and [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)₂(CH₃CN)][PF₆], respectively. Both the OC-6-35 and the OC-6-52 isomers of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)₂(CH₃CN)]⁺ - featuring facial and meridional dispositions of the phosphorus atoms, respectively - were isolated depending on the reaction solvent. Several compounds described herein catalyse the dehydrogenation of formic acid in DMF, [IrCl(HNP)₂(cod)] being the most active, with TOF_{1 min} of about 2300 h^-1 (5 mol% catalyst, 50 mol% sodium formate, DMF, 80 °C).

Iron Dihydride Complex Stabilized by an All-Phosphorus-Based Pincer Ligand and Carbon Monoxide

PNP-pincer-stabilized iron carbonyl dihydride complexes are key intermediates in catalytic hydrogenation and dehydrogenation reactions; however, decomposition through these intermediates has been observed. This inspires the development of a PPP-pincer system that may show improved catalyst stability. In this work, bis[2-(diisopropylphosphino)phenyl]phosphine (or ^iPrPP^HP) is used to react with FeCl₂ under a carbon monoxide (CO) atmosphere to yield trans-(^iPrPP^HP)Fe(CO)Cl₂. A subsequent reaction with NaBH₄ produces syn/anti-(^iPrPP^HP)FeH(CO)Cl or cis,anti-(^iPrPP^HP)Fe(CO)H₂, depending on the amount of NaBH₄ employed. The cis-dihydride complex shows catalytic activity for the conversion of PhCHO to PhCH₂OH (under H₂) or PhCO₂CH₂Ph (under Ar). It also catalyzes the dehydrogenation of PhCH₂OH to PhCHO and PhCO₂CH₂Ph, albeit with limited turnover numbers. A more efficient catalytic process is the dehydrogenation of formic acid to carbon dioxide (CO₂), which can operate under additive-free conditions. Mechanistic investigation suggests that the cis-dihydride complex undergoes protonation with formic acid to release H₂ while forming anti-(^iPrPP^HP)FeH(CO)(OCHO)·HCO₂H, in which the CO ligand has shifted and the formate is hydrogen-bonded to formic acid. The hydrido formate complex loses CO₂ under ambient conditions, completing the catalytic cycle by reforming the cis-dihydride complex.

Synthesis and reactivity of an iridium complex based on a tridentate aminophosphano ligand

The iridium(III) hydride compound [IrH{κ³C,P,P'-(SiNP-H)}(CN^tBu)₂][PF₆] (1PF₆) was obtained by reaction of [Ir(SiNP)(cod)][PF₆] with CN^tBu as the result of the intramolecular oxidative addition of the SiCH₂-H bond to iridium(I) [SiNP = Si(CH₃)₂{N(4-tolyl)PPh₂}₂, SiNP-H = CH₂Si(CH₃){N(4-tolyl)PPh₂}₂]. The mechanism of the reaction was investigated by NMR spectroscopy and DFT calculations showing that the pentacoordinated intermediate [Ir(SiNP)(cod)(CN^tBu)][PF₆] (2PF₆) forms in the first place and that further reacts with CN^tBu, affording the square planar intermediate [Ir(SiNP)(CN^tBu)₂][PF₆] (3PF₆) that finally undergoes the intramolecular oxidative addition of the SiCH₂-H bond. The reactivity of 1PF₆ was investigated. On one hand, the reaction of 1PF₆ with N-chlorosuccinimide or N-bromosuccinimide provides the haloderivatives [IrX{κ³C,P,P'-(SiNP-H)}(CN^tBu)₂][PF₆] (X = Cl, 4PF₆; Br, 5PF₆), and the reaction of 5PF₆ with AgPF₆ in the presence of acetonitrile affords the solvato species [Ir{κ³C,P,P'-(SiNP-H)}(CH₃CN)(CN^tBu)₂]²⁺ (6²⁺) isolated as the hexafluorophosphate salt. On the other hand, the reaction of 1PF₆ with HBF₄ gives the iridium(III) compound [IrH(CH₂SiF₂CH₃)(HNP)₂(CN^tBu)₂][BF₄] (7BF₄) as the result of the formal addition of hydrogen fluoride to the Si-N bonds of 1⁺ [HNP = HN(4-tolyl)PPh₂]. A similar outcome was observed in the reaction of 1PF₆ with CF₃COOH rendering 7PO₂F₂. In this case the intermediate [IrH{κ²C,P-CH₂SiMeFN(4-tolyl)PPh₂}(HNP)(CN^tBu)₂]⁺ (8⁺) was observed and characterised in situ by NMR spectroscopy. DFT calculations suggests that the reaction goes through the sequential protonation of the nitrogen atom of the Si-N-P moiety followed by the formal addition of fluoride ion to silicon. Also, the crystal structures of SiNP, 1PF₆, 4PF₆ and 7BF₄ have been determined by X-ray diffraction measurements.