A (.mu.-Oxo)bis(.mu.-carboxylato)diiron(III) Complex with a Tethered Phenoxyl Radical as a Model for the Active Site of the R2 protein of Ribonucleotide Reductase

Synthesis and characterization of a novel oxo-bridged binuclear iron(iii) complex: its catalytic application in the synthesis of benzoxazoles using benzyl alcohol in water

A novel oxo-bridged binuclear iron(iii) complex was found to be an efficient catalyst in the synthesis of benzoxazoles from benzyl alcohols.

Stereodynamic Quinone-Hydroquinone Molecules That Enantiomerize at sp3-Carbon via Redox-Interconversion

Since the discovery of molecular chirality, nonsuperimposable mirror-image organic molecules have been found to be essential across biological and chemical processes and increasingly in materials science. Generally, carbon centers containing four different substituents are configurationally stable, unless bonds to the stereogenic carbon atom are broken and re-formed. Herein, we describe sp³-stereogenic carbon-bearing molecules that dynamically isomerize, interconverting between enantiomers without cleavage of a constituent bond, nor through remote functional group migration. The stereodynamic molecules were designed to contain a pair of redox-active substituents, quinone and hydroquinone groups, which allow the enantiomerization to occur via redox-interconversion. In the presence of an enantiopure host, these molecules undergo a deracemization process that allows observation of enantiomerically enriched compounds. This work reveals a fundamentally distinct enantiomerization pathway available to chiral compounds, coupling redox-interconversion to chirality.

Structural, electrochemical, and spectroscopic investigation of acetate bridged dinuclear tetrakis-Schiff base macrocycles of Mn and Zn

The synthesis of Mn2LAc+, Zn2LAc+, and H4L2+ is described, where L is a tetrakis-Schiff base macrocycle formed using 4-tert-butyl-2,6-diformylphenol and 2,2′-diamino-N-methyldiethylamine resulting in an N6O2 coordination environment. In Mn2LAc+ and Zn2LAc+, the two metal centers are bridged by an acetate ligand. [Mn2LAc](ClO4)·(DMF)0.5, [Mn2LAc](ClO4)·(ACN)0.5, and [Zn2LAc](PF6) crystallized in the space group P2(1)/c, with nearly identical unit-cell dimensions and geometric structures. Electrochemical analysis of Zn2LAc+, and H4L2+ by cyclic voltammetry (CV) revealed two irreversible anodic waves that were assigned to oxidations of the phenolate ligands. CVs of Mn2LAc+ displayed two chemically reversible anodic waves corresponding to Mn(II/III) oxidations, followed by irreversible oxidations of the phenolate ligands. Interfacial electron transfer rates for the single electron oxidations from Mn2(II)LAc+ to Mn(II)Mn(III)LAc2+ to Mn2(III)LAc+ determined from digital simulation of the CVs were 0.6 and 1.1 × 10(–3) cm s(–1), respectively. The sluggish interfacial electron transfer rates observed in electrochemical scans of Mn2LAc+ are consistent with broken symmetry density functional theory electronic structure calculations (B3LYP/6-311G(2df)/6-311G(d,p)) that predict large structural rearrangements of the Mn coordination environment upon oxidation to Mn(III) with associated Jahn–Teller distortions. Titration of Mn2LAc+, Zn2LAc+, and H4L2+ with NOPF6 in acetonitrile allowed for the isolation and spectroscopic examination of higher oxidations and were consistent with electrochemical assignments. The electrochemical and spectroscopic analysis of these complexes will aid in future studies involving electrocatalytic processes with related dinuclear macrocycles.

Phenoxyl radical complexes of gallium, scandium, iron and manganese

The hexadentate macrocyclic ligands 1,4,7-tris(3,5-dimethyl-2-hydroxybenzyl)-1,4,7-triazacyclononane (L CH 3H3 ), 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane (L(Bu) H3 ) and 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane (L OCH 3-H3 ) form very stable octahedral neutral complexes LM(III) with trivalent (or tetravalent) metal ions (Ga(III) , Sc(III) , Fe(III) , Mn(III) , Mn(IV) ). The following complexes have been synthesized: [L(Bu) M], where M = Ga (1), Sc (2), Fe (3); [L(Bu) Mn(IV) ]PF6 (4'); [L OCH 3M], where M = Ga (1 a), Sc (2 a), Fe (3 a); [L OCH 3Mn(IV) ]PF6 (4 a'); [L CH 3M], where M = Sc (2 b), Fe (3 b), Mn(III) (4 b); [L CH 3Mn(IV) ]2 (ClO4 )3 (H3 O)(H2 O)3 (4 b'). An electrochemical study has shown that complexes 1, 2, 3, 1 a, 2 a and 3 a each display three reversible, ligand-centred, one-electron oxidation steps. The salts [L OCH 3Fe(III) ]ClO4 and [L OCH 3Ga(III) ]ClO4 , have been isolated as stable crystalline materials. Electronic and EPR spectra prove that these oxidations produce species containing one, two or three coordinated phenoxyl radicals. The Mössbauer spectra of 3 a and [3 a](+) show conclusively that both compounds contain high-spin iron(III) central ions. Temperature-dependent magnetic susceptibility measurements reveal that 3 a has an S = 5/2 and [3a](+) an S = 2 ground state. The latter is attained through intramolecular antiferromagnetic exchange coupling between a high-spin iron(III) (S1 = 5/2) and a phenoxyl radical (S2 = 1/2) (H = - 2JS1 S2 ; J = - 80 cm(-1) ). The manganese complexes undergo metal- and ligand-centred redox processes, which were elucidated by spectroelectrochemistry; a phenoxyl radical Mn(IV) complex [Mn(IV) L OCH 3](2+) is accessible.

Evolution of strategies to prepare synthetic mimics of carboxylate-bridged diiron protein active sites

We present a comprehensive review of research conducted in our laboratory in pursuit of the long-term goal of reproducing the structures and reactivity of carboxylate-bridged diiron centers used in biology to activate dioxygen for the conversion of hydrocarbons to alcohols and related products. This article describes the evolution of strategies devised to achieve these goals and illustrates the challenges in getting there. Particular emphasis is placed on controlling the geometry and coordination environment of the diiron core, preventing formation of polynuclear iron clusters, maintaining the structural integrity of model complexes during reactions with dioxygen, and tuning the ligand framework to stabilize desired oxygenated diiron species. Studies of the various model systems have improved our understanding of the electronic and physical characteristics of carboxylate-bridged diiron units and their reactivity toward molecular oxygen and organic moieties. The principles and lessons that have emerged from these investigations will guide future efforts to develop more sophisticated diiron protein model complexes.

Porphyrazines as Molecular Scaffolds: Flexible Syntheses of Novel Multimetallic Complexes

Reductive deselenation of selenodiazole-fused porphyrazines, followed by acylation of the resultant labile porphyrazinediamines, was used to prepare macrocycles bearing two Collins ligands, two oxamido residues, or two quinoline-2-carboxamido units. Peripheral coordination of copper(II) to the di-(quinoline-2-carboxamido)-porphyrazine gave a metal-linked face-to-face porphyrazine dimer array. Sequential derivatization of the two amino groups in the porphyrazinediamines was used to prepare mixed peripheral ligand systems including a dimetallic picolinamido-Schiff base porphyrazine. Such systems exhibit strong metal-metal spin coupling and are anticipated to be of value in the synthesis of novel electronic and magnetic materials.