The tripodal amine chelate with two pyridyl groups and an α-hydroxy acid (AHA) group, Pyr-TPA-AHA, was synthesized. Different Fe(III) complexes form with this chelate depending upon the counterion of the Fe(III) source used in the synthesis. A dinuclear complex, Fe(III)2(Pyr-TPA-AHA)2(μ-O), 1, and mononuclear complexes Fe(III)(Pyr-TPA-AHA)X (X = Cl– or Br–, 2 and 3, respectively) were synthesized. 2 can be easily converted to 1 by addition of silver nitrate or a large excess of water. The structure of 1 was solved by X-ray crystallography (C32H34N6O7Fe2·13H2O, a = 14.1236(6) Å, b = 14.1236(6) Å, c = 21.7469(15) Å, α = β = γ = 90°, tetragonal, P42212, Z = 4). 2 and 3 each have simple quasireversible cyclic voltammograms with E1/2 (vs aqueous Ag/AgCl) = +135 mV for 2 and +470 for 3 in acetonitrile. The cyclic voltammogram for 1 in acetonitrile has a quasireversible feature at E1/2 = −285 mV and an irreversible cathodic feature at −1140 mV. All three complexes are photochemically active upon irradiation with UV light, resulting in cleavage of the AHA group and reduction of the iron to Fe(II). Photolysis of 1 results in reduction of both Fe(III) ions in the dinuclear complex for each AHA group that is cleaved, while photolysis of 2 and 3 results in reduction of a single Fe(III) for each AHA cleavage. The quantum yields for 2 and 3 are significantly higher than that of 1.

•Modifications to a α-hydroxy acid chelate produce photoactive Fe(III) complexes.•Monomeric or trimeric structures are determined by potential chelate ring size.•Number of irons and position of chelate hydroxy group affect quantum yields.The Sal-AHA chelates, which include a salicylidene moiety and an α-hydroxy acid moiety, have been shown to form trinuclear [Fe3(Sal-AHA)3(μ3-OR)]− clusters with Fe(III) that are photochemically active. Several minor modifications have been made to the chelate structure to evaluate the effects of different aspects of the chelate design on both the structure and the photochemistry of their Fe(III) complexes. Small steric changes have little effect on either overall structure or photochemistry. Shifting the position of the hydroxy group (α vs. β to the carboxylate), which bridges the irons in the cluster, also does not disrupt the trinuclear structure. However, shortening the length of the carbon chain between the salicylidene and the carboxylate results in loss of the trinuclear structure, forming FeL2 complexes instead. Factors that are important to the quantum yield in the photochemical reaction include both the position of the hydroxy group with respect to the carboxylate and the number of Fe(III) ions in the complex.Small changes in the structure of an α-hydroxy acid containing chelate determine whether a monomeric or trimeric Fe(III) complex is formed and modulate the quantum yield of the photochemical activity.Download high-res image (90KB)Download full-size image

The trimeric clusters [Fe(III)3(X-Sal-AHA)3(μ3-OCH3)]−, where X-Sal-AHA is a tetradentate chelate incorporating an α-hydroxy acid moiety (AHA) and a salicylidene moiety (X-Sal with X being 5-NO2, 3,5-diCl, all-H, 3-OCH3, or 3,5-di-t-Bu substituents on the phenolate ring), undergo a photochemical reaction resulting in reduction of two Fe(III) to Fe(II) for each AHA group that is oxidatively cleaved. However, photolysis of structurally analogous mixed Fe/Ga clusters demonstrate that a similar photolysis reaction will occur with only a single Fe(III) in the cluster. Quantum yields of iron reduction for the series of [Fe(III)3(X-Sal-AHA)3(μ3-OCH3)]− complexes measured by monitoring Fe(II) production are twice those for ligand oxidation, measured by loss of the CD signal for the complex due to cleavage of the chiral AHA group. These moderate quantum yields, around 1–2% in the UVA and UVB range, are higher for complexes with electron-withdrawing X groups than for electron-donating X groups. The observed final photolysis product of the chelate is different if irradiation is done in the air than if it is done under Ar. The first observed photochemical product is the aldehyde resulting from decarboxylation of the AHA. This is the final product under anaerobic conditions. In air, this is followed by an Fe- and O2-dependent reaction oxidizing the aldehyde to the corresponding carboxylate, then a second Fe- and light-dependent decarboxylation reaction giving a product that is two carbons smaller than the initial ligand. These reactivity studies have important biological implications for the photoactive marine siderophores. They suggest that different types of photochemical products for different siderophore structure types do not result from different initial photochemical steps, but rather from different susceptibility of the initial photochemical product to air oxidation.

A series of five new α-hydroxy acid-containing chelates inspired by photoactive marine siderophores, along with their Fe(III) complexes, have been synthesized and characterized. These chelates, designated X-Sal-AHA, each contributes a bidentate salicylidene moiety (X-Sal, X = 5-NO2, 3,5-diCl, H, 3,5-di-tert-butyl, or 3-OCH3 on the phenolate ring) and a bidentate α-hydroxy acid moiety (AHA). The X-ray crystal structure of Na[Fe3(3,5-diCl-Sal-AHA)3(μ3-OCH3)] shows an Fe(III) trimer with the triply deprotonated, trianionic ligands each spanning two Fe(III)’s that are bridged by the hydroxyl group of the ligand. Additionally, a μ3-methoxy anion caps the Fe(III)3 face. Electrospray ionization mass spectra demonstrate that this structure is representative of the Fe(III) complexes of all five derivatives in methanol solution, with the exception of the X = 3,5-di-t-Bu derivative having a μ3-OH bridge rather than a methoxy bridge. Stability constants determined from reduction potentials range from 1034 for the 5-NO2 derivative to >1040 for the 3,5-di-tBu derivative. All five complexes are photoactive when irradiated by sunlight, with the relative rate of photolysis as monitored by Fe(II) transfer correlating with the Hammett σ+ parameter for the phenolate ring substituents.

In the aerobic oxidation of methanol catalyzed by a Ni(II)(TRISOX) complex [H3TRISOX = tris(1-propan-2-onyl oxime)amine], an intermediate is observed spectroscopically. The intensities of both the UV–Vis absorption and electron paramagnetic resonance (EPR) spectra associated with this intermediate maximize during the time period of maximum formaldehyde production, and decrease as the methanol oxidation activity decreases. The UV–Vis spectrum has prominent features at 350, 420, and 535 nm. The EPR spectrum is centered at g = 2.00 and shows splittings of 28 ± 5 G. Both of these spectra are consistent with characterization of the intermediate as including one or more iminoxyl radicals derived from the oximate groups of the TRISOX ligand. Spectroscopic features very similar to those in the air-oxidized intermediate are observed in electrochemically oxidized samples, suggesting that the electrochemically generated complex will be a useful model for the intermediate observed during catalytic turnover. The crystal structure of a Ni(II) complex with an intermediate protonation state of the ligand, [Ni(II)2(H2TRISOX)2(μ2:η1-ONO2)](NO3) · (CH3CN) · 5(H2O), 4, has been structurally characterized. Comparison to the previously reported [Ni(II)(H2TRISOX)(CH3CN)]2(ClO4)2, 3, shows that bis(μ-oximate) dimers can form either with or without an additional bridging ligand. Addition of the nitrato bridge decreases the Ni–Ni distance from 3.5752(13) Å in 3 to 3.2014(4) Å in 4. It is intriguing to note that the reactions catalyzed by the Ni(II)(TRISOX) complex, the net transfer of two hydrogen atoms from an alcohol or amine substrate to O2, are the same reactions catalyzed by several different metalloenzymes that also incorporate both a redox active metal and a redox active organic component in their active sites.Spectroscopic characterization of an intermediate in the aerobic oxidation of methanol catalyzed by a Ni(II) complex of a polyoximate ligand demonstrates that the ligand oximates are involved as redox-active components of the reaction, being oxidized to iminoxyl radicals during catalytic turnover.

An unusual oxygen-activating Ni(II)-oximate complex oxidizes two-hydrogen atom donating substrates, including the traditionally inert alcohol, methanol, as well as ethanol, benzyl alcohol, benzylamine, and N-methylbenzylamine.