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Salen ligands (salen = N,N'-ethylenebis(salicylimine)) are well-known for their versatility and widespread utility in chelating metal complexes. However, installation of hydrogen-bonding units on the salen framework, particularly functional groups that require amine-based precursors such as (thio)ureas, is difficult to achieve without the use of protecting group strategies. In this report, we show that the phenylketone analog of salicyladehyde is a stable alternative that enables the facile installation of hydrogen bonding (thio)urea groups on the salen scaffold, thus imparting anion binding abilities to a metal salen complex. Synthesis of symmetric N-phenyl(thio)urea salen ligands functionalized at the 3,3'-position and an unsymmetric salen ligand with N-phenylurea at the 5-position was achieved. Subsequent metalation with nickel (II) acetate afforded the nickel(II) complexes that were investigated for their anion binding properties towards F-, Cl-, Br-, CH3COO-, and H2PO4 -. Solid-state structures of the nickel(II) complexes as well as the Cl- bound dimer of the symmetric urea complex were obtained. The unusual acidity of the (thio)urea groups is reflected in the pKa-dependent anion binding behavior of the nickel(II) complexes, as elucidated by 1H and 19F Nuclear Magnetic Resonance (NMR) spectroscopy and Diffusion Ordered Spectroscopy (DOSY) experiments. [ABSTRACT FROM AUTHOR]

Ionic liquids (ILs) and deep eutectic solvents (DESs) have tremendous potential for reactive capture and conversion (RCC) of CO2 due to their wide electrochemical stability window, low volatility, and high CO2 solubility. There is environmental and economic interest in the direct utilization of the captured CO2 using electrified and modular processes that forgo the thermal- or pressure-swing regeneration steps to concentrate CO2, eliminating the need to compress, transport, or store the gas. The conventional electrochemical conversion of CO2 with aqueous electrolytes presents limited CO2 solubility and high energy requirement to achieve industrially relevant products. Additionally, aqueous systems have competitive hydrogen evolution. In the past decade, there has been significant progress toward the design of ILs and DESs, and their composites to separate CO2 from dilute streams. In parallel, but not necessarily in synergy, there have been studies focused on a few select ILs and DESs for electrochemical reduction of CO2, often diluting them with aqueous or non-aqueous solvents. The resulting electrode–electrolyte interfaces present a complex speciation for RCC. In this review, we describe how the ILs and DESs are tuned for RCC and specifically address the CO2 chemisorption and electroreduction mechanisms. Critical bulk and interfacial properties of ILs and DESs are discussed in the context of RCC, and the potential of these electrolytes are presented through a techno-economic evaluation. [ABSTRACT FROM AUTHOR]

A molecular cobalt CO2 hydrogenation catalyst was explored for electrocatalytic CO2 reduction under aqueous conditions. The resulting pH-dependent selectivity between H2 and HCO2− is rationalized with thermodynamic analysis and stoichiometric experiments. [ABSTRACT FROM AUTHOR]

Developing improved methods for CO2 capture and concentration (CCC) is essential to mitigating the impact of our current emissions and can lead to carbon net negative technologies. Electrochemical approaches for CCC can achieve much higher theoretical efficiencies compared to the thermal methods that have been more commonly pursued. The use of redox carriers, or molecular species that can bind and release CO2 depending on their oxidation state, is an increasingly popular approach as carrier properties can be tailored for different applications. The key requirements for stable and efficient redox carriers are discussed in the context of chemical scaling relationships and operational conditions. Computational and experimental approaches towards developing redox carriers with optimal properties are also described. [ABSTRACT FROM AUTHOR]

Aqueous direct air capture (DAC) is a key technology toward a carbon negative infrastructure. Developing sorbent molecules with water and oxygen tolerance and high CO2 binding capacity is therefore highly desired. We analyze the CO2 absorption chemistries on amines, alkoxides, and phenoxides with density functional theory calculations, and perform inverse molecular design of the optimal sorbent. The alkoxides and phenoxides are found to be more suitable for aqueous DAC than amines thanks to their water tolerance (lower pKa prevents protonation by water) and capture stoichiometry of 1:1 (2:1 for amines). All three molecular systems are found to generally obey the same linear scaling relationship (LSR) between pKCO2 and pKa, since both CO2 and proton are bonded to the nucleophilic (alkoxy or amine) binding site through a majorly s bonding orbital. Several high-performance alkoxides are proposed from the computational screening. Phenoxides have comparatively poorer correlation between pKCO2 and pKa, showing promise for optimization. We apply a genetic algorithm to search the chemical space of substituted phenoxides for the optimal sorbent. Several promising off-LSR candidates are discovered. The most promising one features bulky ortho substituents forcing the CO2 adduct into a perpendicular configuration with respect to the aromatic ring. In this configuration, the phenoxide binds CO2 and a proton using different molecular orbitals, thereby decoupling the pKCO2 and pKa. The pKCO2-pKa trend and off-LSR behaviors are then confirmed by experiments, validating the inverse molecular design framework. This work not only extensively studies the chemistry of the aqueous DAC, but also presents a transferrable computational workflow for understanding and optimization of other functional molecules. [ABSTRACT FROM AUTHOR]

[Pt(depe)2](PF6)2 electrocatalyzes the reversible conversion between CO2 and HCO2− with high selectivity and low overpotential but low rates. A comprehensive kinetic analysis indicates the rate determining step for CO2 reduction is the reactivity of a Pt hydride intermediate to produce HCO2−. To accelerate catalysis, the use of cationic and hydrogen-bond donor additives are explored. [ABSTRACT FROM AUTHOR]

We report the first example of field-induced single molecule magnet (SMM) behaviour in a square-planar S = 1/2 Co(II) pincer complex [(PNNNP)CoBr]Br (2). The related five-coordinate complexes [(PCNCP)CoBr2] (1) and [(PONOP)CoBr2] (3) also exhibit SMM properties. Partial spin crossover displayed by 3 allows for assignment of distinct relaxation modes to each spin state. [ABSTRACT FROM AUTHOR]

Reversible catalysis is a hallmark of energy‐efficient chemical transformations, but can only be achieved if the changes in free energy of intermediate steps are minimized and the catalytic cycle is devoid of high transition‐state barriers. Using these criteria, we demonstrate reversible CO2/HCO2− conversion catalyzed by [Pt(depe)2]2+ (depe=1,2‐bis(diethylphosphino)ethane). Direct measurement of the free energies associated with each catalytic step correctly predicts a slight bias towards CO2 reduction. We demonstrate how the experimentally measured free energy of each step directly contributes to the <50 mV overpotential. We also find that for CO2 reduction, H2 evolution is negligible and the Faradaic efficiency for HCO2− production is nearly quantitative. A free‐energy analysis reveals H2 evolution is endergonic, providing a thermodynamic basis for highly selective CO2 reduction. [ABSTRACT FROM AUTHOR]

Most bond-forming and -breaking redox reactions require the concomitant transfer of protons. Unassisted proton movement can result in kinetic and thermodynamic barriers that inhibit the rate of these reactions, leading to slow and/or inefficient catalysis. These barriers can be circumvented by effective proton management through molecular design. Different strategies for managing proton movement are discussed with examples from biological and synthetic systems. As proton management is particularly important in redox reactions for chemical fuel generation and utilization, the focus will be on catalysts for H–H and O–O bond formation and cleavage. However, we expect the approaches discussed herein will be general to most multi-electron, multi-proton reactions. [ABSTRACT FROM AUTHOR]

The reaction of Ni(TPAP)(COD) {where TPAP = [(NC5H4)CH2]3P(NC2H4)3N} with Fe(CO)5 resulted in the isolation of the title heterobimetallic NiFe(TPAP)(CO)5 complex di-μ-carbonyl-tricarbon­yl[2,8,9-tris­(pyridin-2-yl­meth­yl)-2,5,8,9-tetra­aza-1-phosphabi­cyclo­[3.3.3]undeca­ne]ironnickel, [FeNi(C24H30N7P)(CO)5]. Characterization of the complex by ¹H and 31P NMR as well as IR spectroscopy are presented. The structure of NiFe(TPAP)(CO)5 reveals three terminally bound CO mol­ecules on Fe0, two bridging CO mol­ecules between Ni0 and Fe0, and TPAP coordinated to Ni0. The Ni—Fe bond length is 2.4828 (4) Å, similar to that of the reduced form of the active site of NiFe hydrogenase (∼2.5 Å). Additionally, a proximal pendant base from one of the non-coordinating pyridine groups of TPAP is also present. Although involvement of a pendant base has been cited in the mechanism of NiFe hydrogenase, this moiety has yet to be incorporated in a structurally characterized synthetic mimic with key structural motifs (terminally bound CO or CN ligands on Fe). Thus, the title complex NiFe(TPAP)(CO)5 is an unique synthetic model for NiFe hydrogenase. In the crystal, the complex mol­ecules are linked by C—H...O hydrogen bonds, forming undulating layers parallel to (100). Within the layers, there are offset π–π [inter­centroid distance = 3.2739 (5) Å] and C—H...π inter­actions present. The layers are linked by further C—H...π inter­actions, forming a supra­molecular framework. [ABSTRACT FROM AUTHOR]

Summary of main observation and conclusion: Selectively electrochemical conversion of CO2 into organic fuel using renewable electricity is one of the most sought‐after processes. In this paper, we report the electrochemical reduction of CO2 (CO2RR) on the nanoporous Ag electrodes made of compacted Ag nanoparticles (AgNPs), which were prepared by one‐step reduction in the water phase with or without the surfactant sodium dodecyl sulfate (SDS). The scanning electron microscope (SEM) characterizations show that the compacted Ag electrodes have the nanoporous morphology formed by stacking AgNPs. Compared with the nanoporous Ag electrode without SDS modification (C‐AgNPs), the SDS‐modified AgNPs electrode (C‐AgNPs‐SDS) is highly effective in improving selective CO production in a wide range of potentials (–0.69 V — –1.19 V, vs. RHE), with a Faradaic efficiency of 92.2% and a current density of –8.23 mA·cm–2 for CO production at –0.79 V (vs. RHE). C‐AgNPs‐SDS is also catalytically stable with only less than 7% deactivation after 8 h of continuous electrolysis. Nanoporous Ag electrodes were prepared through mechanically compressing AgNPs which were synthesized by one‐step reduction in the water phase without and with the surfactant sodium dodecyl sulfate (SDS). The SDS‐modified AgNPs electrode (C‐AgNPs‐SDS) exhibits the best catalytic performance for selectively converting CO2 to CO. [ABSTRACT FROM AUTHOR]

A critical challenge in electrocatalytic CO2 reduction to renewable fuels is product selectivity. Desirable products of CO2 reduction require proton equivalents, but key catalytic intermediates can also be competent for direct proton reduction to H2. Understanding how to manage divergent reaction pathways at these shared intermediates is essential to achieving high selectivity. Both proton reduction to hydrogen and CO2 reduction to formate generally proceed through a metal hydride intermediate. We apply thermodynamic relationships that describe the reactivity of metal hydrides with H+ and CO2 to generate a thermodynamic product diagram, which outlines the free energy of product formation as a function of proton activity and hydricity (ΔGH-), or hydride donor strength. The diagram outlines a region of metal hydricity and proton activity in which CO2 reduction is favorable and H+ reduction is suppressed. We apply our diagram to inform our selection of [Pt(dmpe)2](PF6)2 as a potential catalyst, because the corresponding hydride [HPt(dmpe)2]+ has the correct hydricity to access the region where selective CO2 reduction is possible. We validate our choice experimentally; [Pt(dmpe)2](PF6)2 is a highly selective electrocatalyst for CO2 reduction to formate (>90% Faradaic efficiency) at an overpotential of less than 100 mV in acetonitrile with no evidence of catalyst degradation after electrolysis. Our report of a selective catalyst for CO2 reduction illustrates how our thermodynamic diagrams can guide selective and efficient catalyst discovery. [ABSTRACT FROM AUTHOR]

MnVN Schiff‐base complexes incorporating a Na+ (1Na), K+ (1K), Ba2+ (1Ba), or Sr2+ (1Sr) ion in the ligand framework are reported. The MnVI/V reduction potentials for 1Na, 1K, 1Ba, and 1Sr are 0.591, 0.616, 0.805, and 0.880 V vs. Fe(C5H5)2+/0, respectively, exhibiting significant positive shifts compared to a MnN Schiff‐base complex in the same primary coordination environment but with no associated alkali or alkaline earth metal ion (A, E1/2=0.427 V vs. Fe(C5H5)2+/0). One‐electron oxidation of the MnVN complexes results in bimolecular coupling to form N2 with rates (k2) at 20 °C of 2166, 684, 857, and 99.7, an 87 m−1 s−1 for A, 1Na, 1K, 1Ba, and 1Sr respectively, following an inverse linear free energy relationship. Thus, increasing charge through proximal cations results in MnVIN complexes that are both more oxidizing and more stable to bimolecular coupling, a trend diametrically opposed to when complexes were modified by ligand substituents through inductive effects. [ABSTRACT FROM AUTHOR]

MnVN Schiff‐base complexes incorporating a Na+ (1Na), K+ (1K), Ba2+ (1Ba), or Sr2+ (1Sr) ion in the ligand framework are reported. The MnVI/V reduction potentials for 1Na, 1K, 1Ba, and 1Sr are 0.591, 0.616, 0.805, and 0.880 V vs. Fe(C5H5)2+/0, respectively, exhibiting significant positive shifts compared to a MnN Schiff‐base complex in the same primary coordination environment but with no associated alkali or alkaline earth metal ion (A, E1/2=0.427 V vs. Fe(C5H5)2+/0). One‐electron oxidation of the MnVN complexes results in bimolecular coupling to form N2 with rates (k2) at 20 °C of 2166, 684, 857, and 99.7, an 87 m−1 s−1 for A, 1Na, 1K, 1Ba, and 1Sr respectively, following an inverse linear free energy relationship. Thus, increasing charge through proximal cations results in MnVIN complexes that are both more oxidizing and more stable to bimolecular coupling, a trend diametrically opposed to when complexes were modified by ligand substituents through inductive effects. An electrostatic barrier: The MnVI/V reduction potentials for nitride complexes containing K+, Na+, Ca2+, or Sr2+ are positively shifted compared to a MnN Schiff‐base complex in the same primary coordination environment but with no associated cation. One‐electron oxidation of the MnVN complexes results in bimolecular coupling rates to form N2 following an inverse linear free energy relationship. [ABSTRACT FROM AUTHOR]

Flexible ligands that can adapt their donor strength have enabled unique reactivity in a wide range of inorganic complexes. Most examples are composed of flexible multi-dentate ligands containing a donor that can vary its interaction through its distance to the metal center. We describe an alternative type of adaptable ligand interaction in the neutral multi-dentate ligand tris(2-pyridylmethyl)-azaphosphatrane (TPAP), which contains a proazaphosphatrane unit. Prozaphosphatranes are intrinsically strong phosphorus donors; upon coordination to more Lewis acidic atoms the phosphorus can accept additional electron density from a tertiary nitrogen to form a transannular bond, increasing its donor strength. An experimental and computational investigation of the varying degree of transannular interaction in TPAP when coordinated to late transition metals is reported. The synthesis and characterization of the complexes M(TPAP), where M = Co(i)Cl, Ni(0)(1,5-cyclooctadiene), Ni(ii)(CH3CN)(BF4)2, Pd(ii)(CH3CN)(BF4)2, or Pt(ii)Cl(PF6) is described. Structural characterization and density functional theory calculation of these complexes, along with the previously reported [Co(ii)(TPAP)(CH3CN)](BF4)2 establish significant increases in the degree of transannular interaction of the proazaphosphatrane unit when coordinated to more electron deficient metal ions. [ABSTRACT FROM AUTHOR]

The hydricity (ΔGH−) of a newly synthesized nickel hydride was experimentally determined in acetonitrile (50.6 kcal mol−1), dimethyl sulfoxide (47.1 kcal mol−1), and water (22.8 kcal mol−1). The hydricity values indicate hydride transfer from [HNi(TMEPE)2][BF4] (TMEPE = 1,2-bis[di(methoxyethyl)phosphino]ethane) to CO2 is exergonic in water and endergonic in the organic solvents. [ABSTRACT FROM AUTHOR]

We describe the structural and electronic impacts of modifying the bridging atom in a family of Co(ii) pincer complexes with the formula Co(t-Bu)2PEPyEP(t-Bu)2Br2 (Py = pyridine, E = CH2, NH, and O for compounds 1–3, respectively). Structural characterization by single crystal X-ray diffraction indicates that compounds 1 and 3 are 5-coordinate complexes with both bromides bound to the Co(ii) ion, while compound 2 is square planar with one bromide in the outer coordination sphere. The reduction potentials of 1–3, characterized by cyclic voltammetry, are consistent with the increasing electron-withdrawing character of the pincer ligand as the linker (E) between the pyridine and phosphine arms becomes more electronegative. Magnetic property studies of compounds 1 and 2 confirm high- and low-spin behavior, respectively, through a broad temperature range. However, complex 3 features an unusual combination of high spin S = 3/2 Co(ii) and temperature dependent spin-crossover between S = 3/2 and S = 1/2 states. The different magnetic behaviors observed among the three CoBr2 pincer complexes reflects the importance of small ligand perturbations on overall coordination geometry and resulting spin state properties. [ABSTRACT FROM AUTHOR]

The Tolman electronic parameters (TEP) and cone angles were experimentally measured for a series of substituted proazaphosphatrane ligands by synthesizing their respective Ni(LR)(CO)3 complexes, where L = P(RNCH2CH2)3N and R = Me, iPr, iBu and Bz. The complexes Ni(LMe)(CO)3 (1), Ni(LiPr)(CO)3 (2), Ni(LiBu)(CO)3 (3) and Ni(LBz)(CO)3 (4) display CO vibrational frequencies (A1 mode) at 2057.0, 2054.6, 2054.9 and 2059.1 cm−1, respectively. The TEPs for the phosphine ligands in 1–3 are among the lowest measured, with values close to P(tBu)3 the most donating phosphine measured by Tolman. The cone angles of LR measured in 1–4 are 152, 179, 200 and 207° for R = Me, iPr, iBu and Bz, respectively. The substituted proazaphosphatranes have larger cone angles compared to the analogous trialkyl subsituted monophosphines. Our study demonstrates that while the cone angles have a significant dependence on R, all of the substituted proazaphosphatranes are strong electron donors. Additionally, in order to determine the electronic donor strength of our previously reported multidentate ligand, TPAP, Ni(TPAP)(CO)2 (5) (TPAP = tris(2-pyridylmethyl)azaphosphatrane) and Ni(LMe)2(CO)2 (6) were also synthesized and evaluated in a similar fashion. [ABSTRACT FROM AUTHOR]

An improved preparation for a tetradentate neutral N2Py2ligand with two dimethylamine functionalities in the secondary coordination sphere (LDMA) is reported. This synthetic route uses cheaper and more readily available precursors with fewer steps and greater overall yield. The synthesis and characterization of the corresponding Cu(II) and Cu(I) complexes of this ligand and its analog lacking pendant bases (LH) are also described. The cyclic voltammograms indicate that the Cu(II/I) couple for theLDMAcomplex is over 400 mV positive of the quasi-reversible Cu(II/I) couple for the LHcomplex. Structural characterization indicates this is likely a result of steric effects in [Cu(LDMA)]2+that are not present in the [Cu(LH)]2+complex. Protonation of both diamagnetic Cu(I) complexes is cleanly reversible.1H NMR spectroscopic studies of the protonated complexes reveals that protonation occurs on the tertiary amines in the ligand backbone for both complexes. [ABSTRACT FROM AUTHOR]