Stereochemistry of Boron-Functionalized (Dioximato)iron(II) Complexes. Control of Nonbonded Interactions on Soft Conformational Surfaces

Catalytic reduction of oxygen to water by non-heme iron complexes: exploring the effect of the secondary coordination sphere proton exchanging site

In this study, we prepared non-heme FeIII complexes (1, 2, and 3) of an N4 donor set of ligands (H2L, Me2L, and BPh2L). 1 is supported by a monoanionic bispyridine-dioxime ligand (HL). In 2 and 3, the primary coordination sphere of Fe remained similar to that in 1, except that the oxime protons of the ligand were replaced with two methyl groups and a bridging -BPh2 moiety, respectively. X-ray structures of the FeII complexes (1a and 3a) revealed similar Fe-N distances; however, they were slightly elongated in 2a. The FeIII/FeII potential of 1, 2, and 3 appeared at -0.31 V, -0.25 V, and 0.07 V vs. Fc+/Fc, respectively, implying that HL and Me2L have comparable donor properties. However, BPh2L is more electron deficient than HL or Me2L. 1 showed electrocatalytic oxygen reduction reaction (ORR) activity in acetonitrile in the presence of trifluoroacetic acid (TFAH) as the proton source at Ecat/2 = -0.45 V and revealed selective 4e-/4H+ reduction of O2 to H2O. 1 showed an effective overpotential (ηeff) of 0.98 V and turnover frequency (TOFmax) of 1.02 × 103 s-1. Kinetic studies revealed a kcat of 2.7 × 107 M-2 s-1. Strikingly, 2 and 3 remained inactive for electrocatalytic ORR, which established the essential role of the oxime scaffolds in the electrocatalytic ORR of 1. Furthermore, a chemical ORR of 1 has been investigated using decamethylferrocene as the electron source. For 1, a similar rate equation was noted to that of the electrocatalytic pathway. A kcat of 6.07 × 104 M-2 s-1 was found chemically. Complex 2, however, underwent a very slow chemical ORR. Complex 3 chemically enhances the 4e-/4H+ reduction of O2 and exhibits a TOF of 0.24 s-1 and a kcat value of 2.47 × 102 M-1 s-1. Based on the experimental observations, we demonstrate that the oxime backbone of the ligand in 1 works as a proton exchanging site in the 4e-/4H+ reduction of O2. The study describes how the ORR is affected by the tuning of the ligand scaffold in a family of non-heme Fe complexes.

N-methylpyrazinium ion derivatives of an iron (II) dimethylglyoximatoborate complex

New iron (II) bis(dimethylglyoximatoborate) complexes [Fe(dmgBR2)2(MePz)L]+, R = 4-methoxyphenyl and L = N-methylpyrazinium ion (MePz), tetracyanoethylene (tcne), carbon monoxide, or pyridine are described. The dicationic bis MePz derivative is the first example of a complex containing two small cationic ligands. All adopt the C2vconformation in solution, which serves to sandwich one axial ligand between 4-methoxyphenyl groups. Charge transfer spectra, conformational dynamics, and electrospray mass spectrometry studies are reported. Equilibrium and rate constants for ligand substitution reactions give a binding order trans to MePz: MePz ∼ CO < pyridine, CH3CN.

Hydrogen generation catalyzed by fluorinated diglyoxime-iron complexes at low overpotentials

Fe(II) complexes containing the fluorinated ligand 1,2-bis(perfluorophenyl)ethane-1,2-dionedioxime (dAr(F)gH(2); H = dissociable proton) exhibit relatively positive Fe(II/I) reduction potentials. The air-stable difluoroborated species [(dAr(F)gBF(2))(2)Fe(py)(2)] (2) electrocatalyzes H(2) generation at -0.9 V vs SCE with i(cat)/i(p) ≈ 4, corresponding to a turnover frequency (TOF) of ∼20 s(-1) [Faradaic yield (FY) = 82 ± 13%]. The corresponding monofluoroborated, proton-bridged complex [(dAr(F)g(2)H-BF(2))Fe(py)(2)] (3) exhibits an improved TOF of ∼200 s(-1) (i(cat)/i(p) ≈ 8; FY = 68 ± 14%) at -0.8 V with an overpotential of 300 mV. Simulations of the electrocatalytic cyclic voltammograms of 2 suggest rate-limiting protonation of an Fe("0") intermediate (k(RLS) ≈ 200 M(-1) s(-1)) that undergoes hydride protonation to form H(2). Complex 3 likely reacts via protonation of an Fe(I) intermediate that subsequently forms H(2) via a bimetallic mechanism (k(RLS) ≈ 2000 M(-1) s(-1)). 3 catalyzes production at relatively positive potentials compared with other iron complexes.

Tetracyanoethylene (TCNE): The Characteristic Geometries and Vibrational Absorptions of Its Numerous Structures

Tetracyanoethylene (TCNE) undergoes numerous reactions and is reported to exist in many structural motifs. Identification of these forms and motifs can be challenging. Nonetheless, the number of nu(CN) absorptions and their frequencies provide insight with respect to the specific forms and charge on the TCNE fragment. Particularly informative is the average of the fundamental nu(CN) bands, as well as the length of the central C-C bond. This Review discusses the assignment of structure and formal charge for TCNE-containing compounds. Scrutiny of previous assignments reveals some discrepancies which are discussed, and provides a basis for further study of TCNE structure-function relationships. Several multimetal complexes with bridging [TCNE](z) units exhibit mixed valency and extensive delocalization. The scarcity of suitable model compounds, especially those with M(d(pi-pi*)) backbonding to the CN groups, have thwarted the detailed description of these valence ambiguous compounds; thus, new well-characterized polynuclear compounds are needed.

Heterotrinuclear Oxo-Bridged Fe(III)ORu(IV)OFe(III) Complexes of Ruthenium Porphyrin and Borylated Low-Spin Iron Dioximes(1)

The synthesis, structure, spectra, and reactivity of diamagnetic linear chain heterotrinuclear oxo-bridged complexes L-Fe(III) N(4)-O-Ru(IV)(TPP')-O-Fe(III)N(4)-L are described where TPP' is tetrakis(4-methoxyphenylporphyrinate), N(4) is one of the borylated bis(dioximate) macrocycles, N(4) = ((DMG)BF(2))(2) in 1, ((DMG)BPh(2))(2) in 2, and ((DPG)BF(2))(2) in 3, and L is a monodentate terminal ligand, L = BuNH(2), CH(3)CN, Py, 1-MeIm, (BuO)(3)P, etc. Crystal data for 3-(BuNH(2))(2) = [(BuNH(2))Fe((DPG)BF(2))(2)-O](2)Ru(TPP') monoclinic space group P2(1)/n, a = 16.845 Å, b = 27.184 Å, c = 24.593 Å, beta = 90.41 degrees, Fe-O = 1.79(1) Å, Ru-O = 1.80(1) Å, Fe-O-Ru = 175 degrees. Large porphyrin and phenyl ring current shifts permit ready characterization of the materials by (1)H NMR. A red-shifted Soret band at 433 nm and two charge transfer transitions in the 650-850 nm region are observed in the visible spectra. Clean reduction of 1, 2, and 3-(CH(3)CN)(2) with 4-tert-butylcatechol is observed giving Fe(II) and Ru(II) products. The kinetics of reduction parallel those of low-spin (&mgr;-oxo)diiron FeN(4) systems showing a 1/[CH(3)CN] dependence, but the rates are 10(3)-10(5) times slower. The Ru(IV) is proposed to stabilize the &mgr;-oxo-Fe bonds toward reductive cleavage via delocalization of the oxo pi electrons onto the ruthenium. Electrochemical data for the heterotrinuclear Fe-O-Ru-O-Fe systems are compared with those for the binuclear Fe-O-Fe systems. The HOMO is stabilized by 120 mV over the corresponding Fe-O-Fe system.