Casey G. Davies

Dramatically Accelerated Selective Oxygen-Atom Transfer by a Nonheme Iron(IV)-Oxo Complex: Tuning of the First and Second Coordination Spheres

The new ligand N3PyamideSR and its FeII complex [FeII(N3PyamideSR)](BF4)2 (1) are described. Reaction of 1 with PhIO at −40 °C gives metastable [FeIV(O)(N3PyamideSR)]2+ (2), containing a sulfide ligand and a single amide H-bond donor in proximity to the terminal oxo group. Direct evidence for H-bonding is seen in a structural analogue, [FeII(Cl)(N3PyamideSR)](BF4)2 (3). Complex 2 exhibits rapid O-atom transfer (OAT) toward external sulfide substrates, but no intramolecular OAT. However, direct S-oxygenation does occur in the reaction of 1 with mCPBA, yielding sulfoxide-ligated [FeII(N3PyamideS(O)R)](BF4)2 (4). Catalytic OAT with 1 was also observed.

Direct Observation of a Nonheme Iron(IV)–Oxo Complex That Mediates Aromatic C–F Hydroxylation

The synthesis of a pentadentate ligand with strategically designed fluorinated arene groups in the second coordination sphere of a nonheme iron center is reported. The oxidatively resistant fluorine substituents allow for the trapping and characterization of an FeIV(O) complex at −20 °C. Upon warming of the FeIV(O) complex, an unprecedented arene C–F hydroxylation reaction occurs. Computational studies support the finding that substrate orientation is a critical factor in the observed reactivity. This work not only gives rare direct evidence for the participation of an FeIV(O) species in arene hydroxylation but also provides the first example of a high-valent iron–oxo complex that mediates aromatic C–F hydroxylation.

The Cys-Tyr cross-link of cysteine dioxygenase changes the optimal pH of the reaction without a structural change.

Cysteine dioxygenase (CDO) is a non-heme monoiron enzyme with an unusual posttranslational modification in the proximity of the ferrous iron active site. This modification, a cysteine to tyrosine thioether bond, cross-links two β-strands of the β-barrel. We have investigated its role in catalysis through a combined crystallographic and kinetic approach. The C93G variant lacks the cross-link and shows little change in structure from that of the wild type, suggesting that the cross-link does not stabilize an otherwise unfavorable conformation. A pH-dependent kinetic study shows that both cross-linked and un-cross-linked CDO are active but the optimal pH decreases with the presence of the cross-link. This result reflects the effect of the thioether bond on the pKa of Y157 and this residues role in catalysis. At higher pH values, kcat is also higher for the cross-linked form, extending the pH range of activity. We therefore propose that the cross-link also increases activity by controlling deleterious interactions involving the thiol/ate of C93.

Halogen Substitution Effects on N2O Schiff Base Ligands in Unprecedented Abrupt FeII Spin Crossover Complexes

A family of halogen-substituted Schiff base iron(II) complexes, [FeII (qsal-X)2 ], (qsal-X=5-X-N-(8-quinolyl)salicylaldimines)) in which X=F (1), Cl (2), Br (3) or I (4) has been investigated in detail. Compound 1 shows a temperature invariant high spin state, whereas the others all show abrupt spin transitions, at or above room temperature, namely, 295 K (X=I) up to 342 K (X=Br), these being some of the highest T1/2 values obtained, to date, for FeII N/O species. We have recently reported subtle symmetry breaking in [FeII (qsal-Cl)2 ] 2 with two spin transition steps occurring at 308 and 316 K. A photomagnetic study reveals almost full HS conversion of [FeII (qsal-I)2 ] 4 at low temperature (T(LIESST)=54 °K). The halogen substitution effects on the magnetic properties, as well as the crystal packing of the [FeII (qsal-X)2 ] compounds and theoretical calculations, are discussed in depth, giving important knowledge for the design of new spin crossover materials. In comparison to the well known iron(III) analogues, [FeIII (qsal-X)2 ]+ , the two extra π-π and P4AE interactions found in [FeII (qsal-X)2 ] compounds, are believed to be accountable for the spin transitions occurring at ambient temperatures.

Mixed Valency in a 3D Semiconducting Iron–Fluoranilate Coordination Polymer

A pair of coordination polymers of composition (NBu4)2[M2(fan)3] (fan = fluoranilate; M = Fe and Zn) were synthesized and structurally characterized. In each case the compound consists of a pair of interpenetrating three-dimensional, (10,3)-a networks in which metal centers are linked by chelating/bridging fluoranilate ligands. Tetrabutylammonium cations are located in the spaces between the two networks. Despite the structural similarity, significant differences exist between (NBu4)2[Fe2(fan)3] and (NBu4)2[Zn2(fan)3] with respect to the oxidation states of the metal centers and ligands. For (NBu4)2[Fe2(fan)3] the structure determination as well as Mössbauer spectroscopy indicate the oxidation state for the Fe is close to +3, which contrasts with the +2 state for the Zn analogue. The differences between the two compounds extends to the ligands, with the Zn network involving only fluoranilate dianions, whereas the average oxidation state for the fluoranilate in the Fe network lies somewhere between -2 and -3. Magnetic studies on the Fe compound indicate short-range ordering. Electrochemical and spectro-electrochemical investigations indicate that the fluoranilate ligand is redox-active in both complexes; a reduced form of (NBu4)2[Fe2(fan)3] was generated by chemical reduction. Conductivity measurements indicate that (NBu4)2[Fe2(fan)3] is a semiconductor, which is attributed to the mixed valency of the fluoranilate ligands.