Solvent-Induced Spin-State Change in Copper Corroles.

The electronic structure of copper corroles has been a topic of debate and revision since the advent of corrole chemistry. The ground state of these compounds is best described as an antiferromagnetically coupled Cu(II) corrole radical cation. In coordinating solvents, these molecules become paramagnetic, and this is often accompanied by a color change. The underlying chemistry of these solvent-induced properties is currently unknown. Here, we show that a coordinating solvent, such as pyridine, induces a change in the ground spin state from an antiferromagnetically coupled Cu(II) corrole radical cation to a ferromagnetically coupled triplet. Over time, the triplet reacts to produce a species with spectral signatures that are characteristic of the one-electron-reduced Cu(II) corrole. These observations account for the solvent-induced paramagnetism and the associated color changes that have been observed for copper corroles in coordinating solvents.

Structurally characterized terminal manganese(iv) oxo tris(alkoxide) complex.

A Mn(iv) complex featuring a terminal oxo ligand, [MnIV(O)(ditox)3][K(15-C-5)2] (3; ditox = t Bu2MeCO-, 15-C-5 = 15-crown-5-ether) has been isolated and structurally characterized. Treatment of the colorless precursor [MnII(ditox)3][K(15-C-5)2] (2) with iodosobenzene affords 3 as a green free-flowing powder in high yields. The X-ray crystal structure of 3 reveals a pseudotetrahedral geometry about the central Mn, which features a terminal oxo (d(Mn-Oterm = 1.628(2) Å)). EPR spectroscopy, SQUID magnetometry, and Evans method magnetic susceptibility indicate that 3 consists of a high-spin S = 3/2 Mn(iv) metal center. 3 promotes C-H bond activation by a hydrogen atom abstraction. The [MnIV(O)(ditox)3]- furnishes a model for the proposed terminal oxo of the unique manganese of the oxygen evolving complex of photosystem II.

Stereoelectronic Effects in Cl2 Elimination from Binuclear Pt(III) Complexes.

Halogen photoelimination is the critical energy-storing step of metal-catalyzed HX-splitting photocycles. Homo- and heterobimetallic Pt(III) complexes display among the highest quantum efficiencies for halogen elimination reactions. Herein, we examine in detail the mechanism and energetics of halogen elimination from a family of binuclear Pt(III) complexes featuring meridionally coordinated Pt(III) trichlorides. Transient absorption spectroscopy, steady-state photocrystallography, and far-infrared vibrational spectroscopy suggest a halogen elimination mechanism that proceeds via two sequential halogen-atom-extrusion steps. Solution-phase calorimetry experiments of the meridional complexes have defined the thermodynamics of halogen elimination, which show a decrease in the photoelimination quantum efficiency with an increase in the thermochemically defined Pt-X bond strength. Conversely, when compared to an isomeric facial Pt(III) trichloride, a much more efficient photoelimination is observed for the fac isomer than would be predicted based on thermochemistry. This difference in the fac vs mer isomer photochemistry highlights the importance of stereochemistry on halogen elimination efficiency and points to a mechanism-based strategy for achieving halogen elimination reactions that are both efficient and energy storing.

Electronic Structure of Copper Corroles.

The ground state electronic structure of copper corroles has been a topic of debate and revision since the advent of corrole chemistry. Computational studies formulate neutral Cu corroles with an antiferromagnetically coupled Cu(II) corrole radical cation ground state. X-ray photoelectron spectroscopy, EPR, and magnetometry support this assignment. For comparison, Cu(II) isocorrole and [TBA][Cu(CF3)4] were studied as authentic Cu(II) and Cu(III) samples, respectively. In addition, the one-electron reduction and one-electron oxidation processes are both ligand-based, demonstrating that the Cu(II) centre is retained in these derivatives. These observations underscore ligand non-innocence in copper corrole complexes.

Cyclometalated gold(III) trioxadiborrin complexes: studies of the bonding and excited states.

Trioxadiborrins are chelating ligands that assemble in dehydration reactions of boronic acids. They are structurally related to ß-diketonate ligands, but have a 2-charge. Little is known of the bonding properties of trioxadiborrin ligands. Presented here are density-functional theory (DFT) studies of cyclometalated gold(III) trioxadiborrins. Substituent effects are evaluated, and comparison is made to the cyclometalating 2-(4-tolyl)pyridine (tpy) ligand on gold. The tpy ligand binds more strongly than any trioxadiborrin ligand considered here, and the two ligands bind competitively to gold. The 1,3-diphenyl trioxadiborrin ligand of 1 has a larger absolute binding enthalpy to gold than its ß-diketonate analogue. Conjugation between boron and aryl substituents delocalizes charge and attenuates the trioxadiborrin's binding capacity. Steric effects that disrupt conjugation between boron and aryl substituents cause the trioxadiborrin to chelate more tightly. Fragment bond orders are divided into in-plane and out-of-plane contributions for square planar 1. In-plane bonding accounts for 88% of bond order between (tpy)Au2+ and the trioxadiborrin ligand. Cyclometalated gold(III) trioxadiborrin complexes were previously shown to be phosphorescent. Spin-unrestricted triplet-state geometry optimizations find that the ten largest excited-state distortions all occur on the tpy ligand. A plot of spin density in triplet 1 shows spin to reside predominantly on tpy. The 77 K luminescence spectrum of 1 is reported here. Time-dependent DFT and configuration interaction singles calculations (corrected for doubles excitations) overestimate the emission energy by ∼ 0.12 eV.

Cyclometalated (boroxinato)gold(III) complexes from arrested transmetalation.

Organic boroxines are ubiquitous, but metallaboroxine analogues remain rare. A new class of (boroxinato)gold species are demonstrated here, as are observations of phosphorescence from boroxinato complexes. Four new compounds are crystallographically characterized.

Trap-Free Halogen Photoelimination from Mononuclear Ni(III) Complexes.

Halogen photoelimination reactions constitute the oxidative half-reaction of closed HX-splitting energy storage cycles. Here, we report high-yielding, endothermic Cl2 photoelimination chemistry from mononuclear Ni(III) complexes. On the basis of time-resolved spectroscopy and steady-state photocrystallography experiments, a mechanism involving ligand-assisted halogen elimination is proposed. Employing ancillary ligands to promote elimination offers a strategy to circumvent the inherently short-lived excited states of 3d metal complexes for the activation of thermodynamically challenging bonds.

Ultrafast Photoinduced Electron Transfer from Peroxide Dianion.

The encapsulation of peroxide dianion by hexacarboxamide cryptand provides a platform for the study of electron transfer of isolated peroxide anion. Photoinitiated electron transfer (ET) between freely diffusing Ru(bpy)3(2+) and the peroxide dianion occurs with a rate constant of 2.0 × 10(10) M(-1) s(-1). A competing electron transfer quenching pathway is observed within an ion pair. Picosecond transient spectroscopy furnishes a rate constant of 1.1 × 10(10) s(-1) for this first-order process. A driving force dependence for the ET rate within the ion pair using a series of Ru(bpy)3(2+) derivatives allows for the electronic coupling and reorganization energies to be assessed. The ET reaction is nonadiabatic and dominated by a large inner-sphere reorganization energy, in accordance with that expected for the change in bond distance accompanying the conversion of peroxide dianion to superoxide anion.

Water oxidation catalysis by Co(II) impurities in Co(III)4O4 cubanes.

The observed water oxidation activity of the compound class Co4O4(OAc)4(Py-X)4 emanates from a Co(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co(II) impurity as the major source of water oxidation activity that has been reported for Co4O4 molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis.

Geminally diaurated aryls bridged by semirigid phosphine pillars: syntheses and electronic structure.

Geminally diaurated µ2 -aryl complexes have been prepared where gold(I) centers were bridged by the semirigid diphosphine ligands bis(2-diphenylphosphinophenyl)ether (DPEphos) and 4,6-bis(diphenylphosphanyl)dibenzo[b,d]furan (DBFphos). Diaurated complexes were synthesized in ligand redistribution reactions of the corresponding di-gold dichlorides with di-gold diaryls (six of them new) and silver(I) salts. Diaurated complexes were isolated as salts of the minimally coordinating anions SbF6 (-) and ReO4 (-). Efforts to prepare salts of the tetraarylborate [B(3,5-(CF3)2 C6 H3)4](-) led to transmetalation from boron, with crystallization of the fluorinated aryl complex. The new complexes were characterized by multinuclear NMR, absorption and emission spectroscopies, 77 K emission lifetimes, and by combustion analysis; three are crystallographically characterized. Structures of geminally diaurated aryl ligands are compared to those of mono-aurated analogues. Both crystal structures and density-functional theory calculations indicate slight but observable disruptions of aryl ligand aromaticity by geminal di-gold binding. An intermolecular aurophilic interaction in one structurally authenticated complex was examined computationally.

Photocrystallographic observation of halide-bridged intermediates in halogen photoeliminations.

Polynuclear transition metal complexes, which frequently constitute the active sites of both biological and chemical catalysts, provide access to unique chemical transformations that are derived from metal-metal cooperation. Reductive elimination via ligand-bridged binuclear intermediates from bimetallic cores is one mechanism by which metals may cooperate during catalysis. We have established families of Rh2 complexes that participate in HX-splitting photocatalysis in which metal-metal cooperation is credited with the ability to achieve multielectron photochemical reactions in preference to single-electron transformations. Nanosecond-resolved transient absorption spectroscopy, steady-state photocrystallography, and computational modeling have allowed direct observation and characterization of Cl-bridged intermediates (intramolecular analogues of classical ligand-bridged intermediates in binuclear eliminations) in halogen elimination reactions. On the basis of these observations, a new class of Rh2 complexes, supported by CO ligands, has been prepared, allowing for the isolation and independent characterization of the proposed halide-bridged intermediates. Direct observation of halide-bridged structures establishes binuclear reductive elimination as a viable mechanism for photogenerating energetic bonds.

Two-electron HCl to H2 photocycle promoted by Ni(II) polypyridyl halide complexes.

Photochemical HX splitting requires the management of two protons and the execution of multielectron photoreactions. Herein, we report a photoinduced two-electron reduction of a polypyridyl Ni(II) chloride complex that provides a route to H2 evolution from HCl. The excited states of Ni complexes are too short to participate directly in HX activation, and hence, the excited state of a photoredox mediator is exploited for the activation of HX at the Ni(II) center. Nanosecond transient absorption (TA) spectroscopy has revealed that the excited state of the polypyridine results in a photoreduced radical that is capable of mediating HX activation by producing a Ni(I) center by halogen-atom abstraction. Disproportionation of the photogenerated Ni(I) intermediate affords Ni(II) and Ni(0) complexes. The Ni(0) center is capable of reacting with HX to produce H2 and the polypyridyl Ni(II) dichloride, closing the photocycle for H2 generation from HCl.

Halogen Photoelimination from Dirhodium Phosphazane Complexes via Chloride-Bridged Intermediates.

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.

Photo-active cobalt cubane model of an oxygen-evolving catalyst.

A dyad complex has been constructed as a soluble molecular model of a heterogeneous cobalt-based oxygen-evolving catalyst (Co-OEC). To this end, the Co(4)O(4) core of a cobalt-oxo cubane was covalently appended to Re(I) photosensitisers. The resulting adduct was characterised both in the solid state (by X-ray diffraction) and in solution using a variety of techniques. In particular, the covalent attachment of the Re(I) moieties to the Co(4)O(4) core promotes emission quenching of the Re(I) photocentres, with implications for the energy and electron transduction process of Co-OEC-like catalysts.

Deciphering radical transport in the large subunit of class I ribonucleotide reductase.

Incorporation of 2,3,6-trifluorotyrosine (F(3)Y) and a rhenium bipyridine ([Re]) photooxidant into a peptide corresponding to the C-terminus of the ß protein (ßC19) of Escherichia coli ribonucleotide reductase (RNR) allows for the temporal monitoring of radical transport into the α2 subunit of RNR. Injection of the photogenerated F(3)Y radical from the [Re]-F(3)Y-ßC19 peptide into the surface accessible Y731 of the α2 subunit is only possible when the second Y730 is present. With the Y-Y established, radical transport occurs with a rate constant of 3 × 10(5) s(-1). Point mutations that disrupt the Y-Y dyad shut down radical transport. The ability to obviate radical transport by disrupting the hydrogen bonding network of the amino acids composing the colinear proton-coupled electron transfer pathway in α2 suggests a finely tuned evolutionary adaptation of RNR to control the transport of radicals in this enzyme.