Louise A. Berben

A Redox Series of Aluminum Complexes: Characterization of Four Oxidation States Including a Ligand Biradical State Stabilized via Exchange Coupling

Electrophilic activation and subsequent reduction of substrates is in general not possible because highly Lewis acidic metals lack access to multiple redox states. Herein, we demonstrate that transition metal-like redox processes and electronic structure and magnetic properties can be imparted to aluminum(III). Bis(iminopyridine) complexes containing neutral, monoanionic, and dianionic iminopyridine ligands (IP) have been characterized structurally and electronically; yellow (IP)AlCl(3) (1), deep green (IP(-))(2)AlCl (2) and (IP(-))(2)Al(CF(3)SO(3)) (3), and deep purple [(IP(2-))Al](-) (5) are presented. The mixed-valent, monoradical complex (IP(-))(IP(2-))Al is unstable toward C-C coupling, and [(IP(2-))Al](2-)(μ-IP-IP)(2-) (4) has been isolated. Variable-temperature magnetic susceptibility and EPR spectroscopy measurements indicate that the biradical character of the ligand-based triplet in 2 is stabilized by strong antiferromagnetic exchange coupling mediated by aluminum(III): J = -230 cm(-1) for Ĥ = -2J(Ŝ(L(1))·Ŝ(L(2))). Coordination geometry-dependent (IP(-))-(IP(-)) communication through aluminum(III) is observed electrochemically. The cyclic voltammogram of trigonal bipyramidal 2 displays successive ligand-based oxidation events for the two IP(1-/0) processes, at -0.86 and -1.20 V vs SCE. The 0.34 V spacing between redox couples corresponds to a conproportionation constant of K(c) = 10(5.8) for the process (IP(-))(2)AlCl + (IP)(2)AlCl → 2(IP(-))(IP)AlCl consistent with Robin and Day Class II mixed-valent behavior. Tetrahedral 5 displays localized, Class I behavior as indicated by closely spaced redox couples. Furthermore, CVs of 2 and 5 indicate that changes in the coordination environment of the aluminum center shift the potentials for the IP(1-/0) and IP(2-/1-) redox couples by up to 0.9 V.

Directing the Reactivity of [HFe4N(CO)12]− toward H+ or CO2 Reduction by Understanding the Electrocatalytic Mechanism

Selective reactivity of an electrocatalytically generated catalyst-hydride intermediate toward the hydrogen evolution reaction (HER) or reduction of CO(2) is key for a CO(2) reduction electrocatalyst. Under appropriate conditions, Et(4)N[Fe(4)N(CO)(12)] (Et(4)N-1) is a catalyst for the HER or for CO(2) conversion at -1.25 V vs SCE using a glassy carbon electrode.

Aluminum–Ligand Cooperative N–H Bond Activation and an Example of Dehydrogenative Coupling

Activation of N-H bonds by a molecular aluminum complex via metal-ligand cooperation is described. ((Ph)I2P(2-))AlH (1b), in which (Ph)I2P(2-) is a tridentate bis(imino)pyridine ligand, reacts with anilines to give the N-H-activated products ((Ph)HI2P(-))AlH(NHAr) (2). When heated, 2 releases H2 and affords ((Ph)I2P(-))Al(NHAr) (3). Complex 1b catalyzes the dehydrogenative coupling of benzylamine to afford H2, NH3, and N-(phenylmethylene)benzenemethanamine.

Aluminium–ligand cooperation promotes selective dehydrogenation of formic acid to H2 and CO2

Herein, we report that molecular aluminium complexes of the bis(imino)pyridine ligand, (PhI2P2−)Al(THF)X, X = H (1), CH3 (2), promote selective dehydrogenation of formic acid to H2 and CO2 with an initial turnover frequency of 5200 turnovers per hour. Low-temperature reactions show that reaction of 1 with HCOOH affords a complex that is protonated three times: twice on the PhI2P2− ligand and once to liberate H2 or CH4 from the Al-hydride or Al-methyl, respectively. We demonstrate that in the absence of protons, insertion of CO2 into the Al-hydride bond of 1 is facile and produces an Al-formate. Upon addition of protons, liberation of CO2 from the Al-formate complex affords an Al-hydride. Deuterium labelling studies and the solvent dependence of the reaction indicate that outer sphere β-hydride abstraction supported by metal–ligand cooperative hydrogen bonding is a likely mechanism for the C–H bond cleavage.

Electrocatalytic hydrogen evolution from water by a series of iron carbonyl clusters.

The development of efficient hydrogen evolving electrocatalysts that operate near neutral pH in aqueous solution remains of significant interest. A series of low-valent iron clusters have been investigated to provide insight into the structure-function relationships affecting their ability to promote formation of cluster-hydride intermediates and to promote electrocatalytic hydrogen evolution from water. Each of the metal carbonyl anions, [Fe4N(CO)12](-) (1(-)), [Fe4C(CO)12](2-) (2(2-)), [Fe5C(CO)15](2-) (3(2-)), and [Fe6C(CO)18](2-) (4(2-)) were isolated as their sodium salt to provide the necessary solubility in water. At pH 5 and -1.25 V vs SCE the clusters afford hydrogen with Faradaic efficiencies ranging from 53-98%. pH dependent cyclic voltammetry measurements provide insight into catalytic intermediates. Both of the butterfly shaped clusters, 1(-) and 2(2-), stabilize protonated adducts and are effective catalysts. Initial reduction of butterfly shaped 1(-) is pH-independent and subsequently, successive protonation events afford H1(-), and then hydrogen. In contrast, butterfly shaped 2(2-) undergoes two successive proton coupled electron transfer events to form H22(2-) which then liberates hydrogen. The higher nuclearity clusters, 3(2-) and 4(2-), do not display the same ability to associate with protons, and accordingly, they produce hydrogen less efficiently.

Catalysis by Aluminum(III) Complexes of Non-Innocent Ligands

Non-Innocent ligand complexes of aluminum are described in this Concept article, beginning with a discussion of their synthesis, and then structural and electronic characterization. The main focus concerns the ability of the ligands in these complexes to mediate proton transfer reactions. As examples, aluminum-ligand cooperation in the activation of polar bonds is described, as is the importance of hydrogen bonding to stabilization of a transition state for β-hydride abstraction. Taken together these reactions enable catalytic processes such as the dehydrogenation of formic acid.

Countercations Direct One- or Two-Electron Oxidation of an Al(III) Complex and Al(III)–Oxo Intermediates Activate C–H Bonds

Hydrogen abstraction by aluminum(III)-oxo intermediates via reaction pathways reminiscent of late transition metal chemistry has been observed. Oxidation of M(+)[(IP(2-))(2)Al](-) (IP = iminopyridine, M = Na, Bu(4)N) yielded [Na(THF)(DME)][(IP(-))(IP(2-))Al(OH)] (3) or [(IP(-))(2)Al(OH)] (4), via O-atom transfer and subsequent C-H activation or proton abstraction, respectively.

Enhancing the magnetic anisotropy of cyano-ligated chromium(II) and chromium(III) complexes via heavy halide ligand effects.

A method of increasing the axial zero-field splitting parameter for transition metal complexes of utility in the assembly of magnetic clusters is demonstrated through the use of heavy atoms as auxiliary ligands. The octahedral complexes [Cr(dmpe)(2)(CN)X](+) (dmpe = 1,2-bis(dimethylphosphino)ethane, X = Cl, Br, I) and Cr(dmpe)(2)(CN)X (X = Cl, I) are synthesized and structurally characterized. Variable-field magnetization measurements show the magnitude of D for these complexes to increase significantly as the halide ligand varies from chloride to iodide, ranging from 0.11 cm(-1) for [Cr(dmpe)(2)(CN)Cl](+) to 6.26 cm(-1) for Cr(dmpe)(2)(CN)I.

Redox rich dicobalt macrocycles as templates for multi-electron transformations

Pyridazine-templated dicobalt macrocycles reversibly support five oxidation states with unusually positive Co(II)/Co(I) redox couples, and are also active proton reduction electrocatalysts.

Electrocatalytic Hydrogen Production by an Aluminum(III) Complex: Ligand-Based Proton and Electron Transfer.

Environmentally sustainable hydrogen-evolving electrocatalysts are key in a renewable fuel economy, and ligand-based proton and electron transfer could circumvent the need for precious metal ions in electrocatalytic H2 production. Herein, we show that electrocatalytic generation of H2 by a redox-active ligand complex of Al(3+) occurs at -1.16 V vs. SCE (500 mV overpotential).