Expanding the Scope of Aluminum Chemistry with Noninnocent Ligands.

ConspectusAluminum is the most abundant metal in the earth's crust at 8%, and it is also widely available domestically in many countries worldwide, which ensures a stable supply chain. To further the applications of aluminum (Al), such as in catalysis and electronic and energy storage materials, there has been significant interest in the synthesis and characterization of new Al coordination compounds that can support electron transfer (ET) and proton transfer (PT) chemistry. This has been achieved using redox and chemically noninnocent ligands (NILs) combined with the highly stable M(III) oxidation state of Al and in some cases the heavier group 13 ions, Ga and In.When ligands participate in redox chemistry or facilitate the breaking or making of new bonds, they are often termed redox or chemically noninnocent, respectively. Al(III) in particular supports rich ligand-based redox chemistry because it is so redox inert and will support the ligand across many charge and protonation states without entering into the reaction chemistry. To a lesser extent, we have reported on the heavier group 13 elements Ga and In, and this chemistry will also be included in this Account, where available.This Account is arranged into two technical sections, which are (1) Structures of Al-NIL complexes and (2) Reactivity of Al-NIL complexes. Highlights of the research work include reversible redox chemistry that has been enabled by ligand design to shut down radical coupling pathways and to prevent loss of H2 from unsaturated ligand sites. These reversible redox properties have in turn enabled the characterization of Class III electron delocalization through Al when two NIL are bound to the Al(III) in different charge states. Characterization of the metalloaromatic character of square planar Al and Ga complexes has been achieved, and characterization of the delocalized electronic structures has provided a model within which to understand and predict the ET and PT chemistry of the NIL group 13 compounds. The capacity of Al-NIL complexes to perform ET and PT has been employed in reactions that use ET or PT reactivity only or in reactions where coupled ET/PT affords hydride transfer chemistry. As an example, ligand-based PT reactions initiate metal-ligand cooperative bond activation pathways for catalysis: this includes acceptorless dehydrogenation of formic acid and anilines and transfer hydrogenation chemistry. In a complementary approach, ligand based ET/PT chemistry has been used in the study of dihydropyridinate (DHP-) chemistry where it was shown that N-coordination of group 13 ions lowers kinetic barriers to DHP- formation. Taken together, the discussion presented herein illustrates that the NIL chemistry of Al(III), and also of Ga(III) and In(III) holds promise for further developments in catalysis and energy storage.

Group 13 ion coordination to pyridyl breaks the reduction potential vs. hydricity scaling relationship for dihydropyridinates.

The relationship Epvs. ΔGH- correlates the applied potential (Ep) needed to drive organohydride formation with the strength of the hydride donor that is formed: in the absence of kinetic effects Epvs. ΔGH- should be linear but it would be more energy efficient if Ep could be shifted anodically using kinetic effects. Biological hydride transfers (HT) performed by cofactors including NADH and lactate racemase do occur at low potentials and functional modeling of those processes could lead to low energy HT reactions in electrosynthesis and to accurate models for cofactor chemistry. Herein we probe the influence of N-alkylation or N-metallation on ΔGH- for dihydropyridinates (DHP-) and on Ep of the DHP- precursors. We synthesized a series of DHP- complexes of the form (pz2HP-)E via hydride transfer from their respective [(pz2P)E]+ forms where E = AlCl2+, GaCl2+ or Me+. Relative ΔGH- for the (pz2HP-)E series all fall within 1 kcal mol-1, and ΔGH- for (pz2HP)CH3 was approximated as 47.5 ± 2.5 kcal mol-1 in MeCN solution. Plots of Epvs. ΔGH- including [(pz2P)E]+ suggest kinetic effects shift Ep anodically by ∼215 mV.

Electrical-biological hybrid system for carbon efficient isobutanol production.

We have developed an electrical-biological hybrid system wherein an engineered microorganism consumes electrocatalytically produced formate from CO2 to supplement the bioproduction of isobutanol, a valuable fuel chemical. Biological CO2 sequestration is notoriously slow compared to electrochemical CO2 reduction, while electrochemical methods struggle to generate carbon-carbon bonds which readily form in biological systems. A hybrid system provides a promising method for combining the benefits of both biology and electrochemistry. Previously, Escherichia coli was engineered to assimilate formate and CO2 in central metabolism using the reductive glycine pathway. In this work, we have shown that chemical production in E. coli can benefit from single carbon substrates when equipped with the RGP. By installing the RGP and the isobutanol biosynthetic pathway into E. coli and by further genetic modifications, we have generated a strain of E. coli that can consume formate and produce isobutanol at a yield of >100% of theoretical maximum from glucose. Our results demonstrate that carbon produced from electrocatalytically reduced CO2 can bolster chemical production in E. coli. This study shows that E. coli can be engineered towards carbon efficient methods of chemical production.

Metallated dihydropyridinates: prospects in hydride transfer and (electro)catalysis.

Hydride transfer (HT) is a fundamental step in a wide range of reaction pathways, including those mediated by dihydropyridinates (DHP-s). Coordination of ions directly to the pyridine ring or functional groups stemming therefrom, provides a powerful approach for influencing the electronic structure and in turn HT chemistry. Much of the work in this area is inspired by the chemistry of bioinorganic systems including NADH. Coordination of metal ions to pyridines lowers the electron density in the pyridine ring and lowers the reduction potential: lower-energy reactions and enhanced selectivity are two outcomes from these modifications. Herein, we discuss approaches for the preparation of DHP-metal complexes and selected examples of their reactivity. We suggest further areas in which these metallated DHP-s could be developed and applied in synthesis and catalysis.

Group 13 ion coordination to pyridyl models NAD+ reduction potentials.

N-alkylation and N-metallation of pyridine are explored herein to understand how metal-ligand complexes can model NAD+ redox chemistry. Syntheses of substituted dipyrazolylpyridine (pz2P) compounds (pz2P)Me+ (1+) and (pz2P)GaCl2+ (2+) are reported, and compared with (pz2P)AlCl2(THF)+ and transition element pz2P complexes from previous reports. Cyclic voltammetry measurements of cationic 1+ and 2+ show irreversible reduction events ∼900 mV anodic those for neutral pz2P complexes of divalent metals. We proposed that N-metallation using Group 13 ions of 3+ charge provides an electrochemical model for N-alkylated pyridyls like NAD+.

A Bifunctional Ionic Liquid for Capture and Electrochemical Conversion of CO2 to CO over Silver.

Electrochemical conversion of CO2 requires selective catalysts and high solubility of CO2 in the electrolyte to reduce the energy requirement and increase the current efficiency. In this study, the CO2 reduction reaction (CO2RR) over Ag electrodes in acetonitrile-based electrolytes containing 0.1 M [EMIM][2-CNpyr] (1-ethyl-3-methylimidazolium 2-cyanopyrolide), a reactive ionic liquid (IL), is shown to selectively (>94%) convert CO2 to CO with a stable current density (6 mA·cm-2) for at least 12 h. The linear sweep voltammetry experiments show the onset potential of CO2 reduction in acetonitrile shifts positively by 240 mV when [EMIM][2-CNpyr] is added. This is attributed to the pre-activation of CO2 through the carboxylate formation via the carbene intermediate of the [EMIM]+ cation and the carbamate formation via binding to the nucleophilic [2-CNpyr]- anion. The analysis of the electrode-electrolyte interface by surface-enhanced Raman spectroscopy (SERS) confirms the catalytic role of the functionalized IL where the accumulation of the IL-CO2 adduct between -1.7 and -2.3 V vs Ag/Ag+ and the simultaneous CO formation are captured. This study reveals the electrode surface species and the role of the functionalized ions in lowering the energy requirement of CO2RR for the design of multifunctional electrolytes for the integrated capture and conversion.

Pre-Equilibrium Reaction Mechanism as a Strategy to Enhance Rate and Lower Overpotential in Electrocatalysis.

Pre-equilibrium reaction kinetics enable the overall rate of a catalytic reaction to be orders of magnitude faster than the rate-determining step. Herein, we demonstrate how pre-equilibrium kinetics can be applied to breaking the linear free-energy relationship (LFER) for electrocatalysis, leading to rate enhancement 5 orders of magnitude and lowering of overpotential to approximately thermoneutral. This approach is applied to pre-equilibrium formation of a metal-hydride intermediate to achieve fast formate formation rates from CO2 reduction without loss of selectivity (i.e., H2 evolution). Fast pre-equilibrium metal-hydride formation, at 108 M-1 s-1, boosts the CO2 electroreduction to formate rate up to 296 s-1. Compared with molecular catalysts that have similar overpotential, this rate is enhanced by 5 orders of magnitude. As an alternative comparison, overpotential is lowered by ∼50 mV compared to catalysts with a similar rate. The principles elucidated here to obtain pre-equilibrium reaction kinetics via catalyst design are general. Design and development that builds on these principles should be possible in both molecular homogeneous and heterogeneous electrocatalysis.

Using Substituted [Fe4N(CO)12]- as a Platform To Probe the Effect of Cation and Lewis Acid Location on Redox Potential.

The impact of cationic and Lewis acidic functional groups installed in the primary or secondary coordination sphere (PCS or SCS) of an (electro)catalyst is known to vary depending on the precise positioning of those groups. However, it is difficult to systematically probe the effect of that position. In this report, we probe the effect of the functional group position and identity on the observed reduction potentials (Ep,c) using substituted iron clusters, [Fe4N(CO)11R]n, where R = NO+, PPh2-CH2CH2-9BBN, (MePTA+)2, (MePTA+)4, and H+ and n = 0, -1, +1, or +3 (9-BBN is 9-borabicyclo(3.3.1)nonane; MePTA+ is 1-methyl-1-azonia-3,5-diaza-7-phosphaadamantane). The cationic NO+ and H+ ligands cause anodic shifts of 700 and 320 mV, respectively, in Ep,c relative to unsubstituted [Fe4N(CO)12]-. Infrared absorption band data, νCO, suggests that some of the 700 mV shift by NO+ results from electronic changes to the cluster core. This contrasts with the effects of cationic MePTA+ and H+ which cause primarily electrostatic effects on Ep,c. Lewis acidic 9-BBN in the SCS had almost no effect on Ep,c.

Metal carbonyl clusters of groups 8-10: synthesis and catalysis.

In this review article, we discuss advances in the chemistry of metal carbonyl clusters (MCCs) spanning the last three decades, with an emphasis on the more recent reports and those involving groups 8-10 elements. Synthetic methods have advanced and been refined, leading to higher-nuclearity clusters and a wider array of structures and nuclearities. Our understanding of the electronic structure in MCCs has advanced to a point where molecular chemistry tools and other advanced tools can probe their properties at a level of detail that surpasses that possible with other nanomaterials and solid-state materials. MCCs therefore advance our understanding of structure-property-reactivity correlations in other higher-nuclearity materials. With respect to catalysis, this article focuses only on homogeneous applications, but it includes both thermally and electrochemically driven catalysis. Applications in thermally driven catalysis have found success where the reaction conditions stabilise the compounds toward loss of CO. In more recent years, MCCs, which exhibit delocalised bonding and possess many electron-withdrawing CO ligands, have emerged as very stable and effective for reductive electrocatalysis reactions since reduction often strengthens M-C(O) bonds and since room-temperature reaction conditions are sufficient for driving the electrocatalysis.

Quantification of the Electrostatic Effect on Redox Potential by Positive Charges in a Catalyst Microenvironment.

Charged functional groups in the secondary coordination sphere (SCS) of a heterogeneous nanoparticle or homogeneous electrocatalyst are of growing interest due to enhancements in reactivity that derive from specific interactions that stabilize substrate binding or charged intermediates. At the same time, accurate benchmarking of electrocatalyst systems most often depends on the development of linear free-energy scaling relationships. However, the thermodynamic axis in those kinetic-thermodynamic correlations is most often obtained by a direct electrochemical measurement of the catalyst redox potential and might be influenced by electrostatic effects of a charged SCS. In this report, we systematically probe positive charges in a SCS and their electrostatic contributions to the electrocatalyst redox potential. A series of 11 iron carbonyl clusters modified with charged and uncharged ligands was probed, and a linear correlation between the νCO absorption band energy and electrochemical redox potentials is observed except where the SCS is positively charged.

Ligand Conjugation Directs the Formation of a 1,3-Dihydropyridinate Regioisomer.

The selective formation of the 1,4-dihydropyridine isomer of NAD(P)H is mirrored by the selective formation of 1,4-dihydropyridinate ligand-metal complexes in synthetic systems. Here we demonstrate that ligand conjugation can be used to promote selective 1,3-dihydropyridinate formation. This represents an advance toward controlling and tuning the selectivity in dihydropyridinate formation chemistry. The reaction of (I2P2-)Al(THF)Cl [1; I2P = bis(imino)pyridine; THF = tetrahydrofuran] with the one-electron oxidant (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) afforded (I2P-)Al(TEMPO)Cl (2), which can be reduced with sodium to the twice-reduced ligand complex (I2P2-)Al(TEMPO) (3). Compounds 2 and 3 serve as precursors for high-yielding and selective routes to an aluminum-supported 1,3-dihydropyridinate complex via the reaction of 2 with 3 equiv of potassium metal or the reaction of 3 with KH.

A Stable Organo-Aluminum Analyte Enables Multielectron Storage for a Nonaqueous Redox Flow Battery.

Redox flow batteries (RFBs) operate by storing electrons on soluble molecular anolytes and catholytes, and large increases in the energy density of RFBs could be achieved if multiple electrons could be stored in each molecular analyte. Here, we report an organoaluminum analyte, [(I2P-)2Al]+, in which four electrons can be stored on organic ligands, and for which charging and discharging cycles performed in a symmetric nonaqueous RFB configuration remain stable for over 100 cycles at 70% state of charge and 97% Coulombic efficiency (I2P is a bis(imino)pyridine ligand). The stability of the analyte is promoted by the kinetic inertness of the anolyte to trace water in solvents and by the redox inertness of the Al(III) ion to the applied current. The solubility of the analyte was optimized by exchanging the counteranion for trifluoromethanesulfonate (triflate), and the cell was further optimized using graphite rods as electrodes which, in comparison with glassy carbon and reticulated vitreous carbon, eliminated deposition of analyte on the electrode. Proof-of-principle experiments performed with an asymmetric NRFB configuration further demonstrate that up to four electrons can be stored in the cell with no degradation of the analyte over multiple cycles that show 96% Coulombic efficiency.

Syntheses of Square Planar Gallium Complexes and a Proton NMR Correlation Probing Metalloaromaticity.

Syntheses of square planar (SP) coordination complexes of gallium(III) are reported herein. Using the pyridine diimine ligand (PDI), we prepared both (PDI2-)GaH (4) and (PDI2-)GaCl (5), which were spectroscopically and structurally characterized. Reduction of PDI using Na metal afforded "Na2PDI", which reacts with in situ-prepared "GaHCl2" or GaCl3 to afford the SP 4 and 5. The planar geometry of these and previously reported SP Al(III) complexes is attributed to energetic stabilization derived from a ring-current effect, or metalloaromaticity. Typically, aromaticity in metal-containing ring systems can be difficult to characterize or confirm experimentally. An experimental approach employing proton NMR spectroscopy and described here provided an estimate of a downfield chemical shift promoted by a small ring-current associated with metalloaromaticity. Near infrared spectroscopic analyses display ligand-metal charge transfer bands which support the assignment of aromaticity. The SP complexes (PDI2-)AlH (1), (PDI2-)AlCl (2), (PDI2-)AlI (3), 4, and 5 are all discussed in this report, using aromaticity as a model for their electronic structure and reactivity properties.

Fast Proton Transfer and Hydrogen Evolution Reactivity Mediated by [Co13C2(CO)24]4.

A common approach to speeding up proton transfer (PT) by molecular catalysts is manipulation of the secondary coordination sphere with proton relays and these enhance overall reaction rates by orders of magnitude. In contrast, heterogeneous electrocatalysts have band structures that promote facile PT concerted with electron transfer (ET), known as the Volmer mechanism. Here, we show that [Co13C2(CO)24]4-, containing multiple Co-Co bonds to statistically enhance observed rates of PT, promotes PT on the order of 2.3 × 109 M-1 s-1 which suggests a diffusion-limited rate. The fast ET and PT chemistry is attributed to the delocalized electronic structure of [Co13C2(CO)24]4-. Electrochemical characterization of [Co13C2(CO)24]4- in the presence and absence of protons reveals ET kinetics and diffusion behavior similar to other small clusters such as nanomaterials and fullerenes.

Delocalization tunable by ligand substitution in [L2Al] n- complexes highlights a mechanism for strong electronic coupling.

Ligand-based mixed valent (MV) complexes of Al(iii) incorporating electron donating (ED) and electron withdrawing (EW) substituents on bis(imino)pyridine ligands (I2P) have been prepared. The MV states containing EW groups are both assigned as Class II/III, and those with ED functional groups are Class III and Class II/III in the (I2P-)(I2P2-)Al and [(I2P2-)(I2P3-)Al]2- charge states, respectively. No abrupt changes in delocalization are observed with ED and EW groups and from this we infer that ligand and metal valence p-orbitals are well-matched in energy and the absence of LMCT and MLCT bands supports the delocalized electronic structures. The MV ligand charge states (I2P-)(I2P2-)Al and [(I2P2-)(I2P3-)Al]2- show intervalence charge transfer (IVCT) transitions in the regions 6850-7740 and 7410-9780 cm-1, respectively. Alkali metal cations in solution had no effect on the IVCT bands of [(I2P2-)(I2P3-)Al]2- complexes containing -PhNMe2 or -PhF5 substituents. Minor localization of charge in [(I2P2-)(I2P3-)Al]2- was observed when -PhOMe substituents are included.

Secondary Coordination Sphere Design to Modify Transport of Protons and CO2.

An exploration of secondary coordination sphere (SCS) functional groups is presented with a focus on proton transport to a metal hydride active site for H2 formation and transport of CO2 so that formate can be obtained. In MeCN-H2O, pKa(AH) and steric bulk of the SCS groups are discussed along with their influence on each step in the mechanism for CO2 to formate catalysis and along with the influence of the proton source, which is MeCN-H2O or (MeCN)2H2O in MeCN-H2O (95:5) under N2 atmosphere. Under CO2, carbonic acid is also available. Catalysts containing various SCS groups were synthesized from [Fe4N(CO)12]- and have the form [Fe4N(CO)11L]- where L is Ph2P-SCS. Hydride formation rates are distinct under N2 versus CO2, and that variation is dependent on the size of the SCS group. Under CO2, larger SCS groups inhibit access of the MeCN-H2O adducts to the active site and formate formation is observed, whereas smaller SCS groups allow transport of these adducts. This is best illustrated by catalysts containing the small SCS group pyridyl and the large SCS group N,N-dimethylaniline which both have the same pKa(AH) value. The smaller pyridyl group promotes selective H2 evolution, whereas larger N,N-dimethylaniline supports selective formate formation by slowing the transport of large MeCN-H2O adducts, allowing hydride transfer to the smaller substrate CO2.

Organic Electron Delocalization Modulated by Ligand Charge States in [L2M]n- Complexes of Group 13 Ions.

Water-stable organic mixed valence (MV) compounds have been prepared by the reaction of reduced bis(imino)pyridine ligands (I2P) with the trichloride salts of Al, Ga, and In. The coordination of two tridentate ligands to each ion affords octahedral complexes that are accessible with five ligand charge states: [(I2P0)(I2P-)M]2+, [(I2P-)2M]+, (I2P-)(I2P2-)M, [(I2P2-)2M]-, and [(I2P2-)(I2P3-)M]2-, and for M = Al only, [(I2P3-)2M]3-. In solid-state structures, the anionic members of the redox series are stabilized by the intercalation of Na+ cations within the ligands. The MV members of the redox series, (I2P-)(I2P2-)M and [(I2P2-)(I2P3-)M]2-, show characteristic intervalence transitions, in the near-infrared regions between 6800-7400 and 7800-9000 cm-1, respectively. Cyclic voltammetry (CV), NIR spectroscopic, and X-ray structural studies support the assignment of class II for compounds [(I2P2-)(I2P3-)M]2- and class III for M = Al and Ga in (I2P-)(I2P2-)M. All compounds containing a singly reduced I2P- ligand exhibit a sharp, low-energy transition in the 5100-5600 cm-1 region that corresponds to a π*-π* transition. CV studies demonstrate that the electron-transfer events in each of the redox series, Al, Ga, and In, span 2.2, 1.4, and 1.2 V, respectively.

New Characterization of V{N(SiMe3)2}3: Reductions of Tris[bis(trimethylsilyl)amido]vanadium(III) and -chromium(III) To Afford the Reduced Metal(II) Anions [M{N(SiMe3)2}3]- (M = V and Cr).

During the preparation of V{N(SiMe3)2}3 (1), a discrepancy between the violet color that we observed and the brown color previously reported prompted further investigation of this compound. As a result, a new spectroscopic study and a full structural characterization are presented. The synthesis, spectroscopy, and structural characteristics of its reduced salt, [K(18-crown-6)(Et2O)2][V{N(SiMe3)2}3] (2), and its chromium congener, [K(18-crown-6)(Et2O)2][Cr{N(SiMe3)2}3] (3), are also described. The 1H NMR spectra for 1-3 and Cr{N(SiMe3)2}3 as well as their cyclic voltammograms are also reported.

Electrochemical Reduction of N2 to NH3 at Low Potential by a Molecular Aluminum Complex.

Electrochemical generation of ammonia (NH3 ) from nitrogen (N2 ) using renewable electricity is a desirable alternative to current NH3 production methods, which consume roughly 1 % of the world's total energy use. The use of catalysts to manipulate the required electron and proton transfer reactions with low energy input is also a chemical challenge that requires development of fundamental reaction pathways. This work presents an approach to the electrochemical reduction of N2 into NH3 using a coordination complex of aluminum(III), which facilitates NH3 production at -1.16 V vs. SCE. Reactions performed under 15 N2 liberate 15 NH3 . Electron paramagnetic resonance spectroscopic characterization of a reduced intermediate and investigations of product inhibition, which limit the reaction to sub-stoichiometric, are also presented.