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Crystallographic, spectroscopic, and computational studies on diiron(ii) hydride complexes reveal rapid hydride motions that cause major changes in the electronic structures of the iron sites.

Magneto-structural correlations for air-stable four-coordinate Co(ii) complexes reveal the dependence of magnetic properties on the dihedral twist angle and establish valuable design guidelines for achieving high magnetic anisotropy.

One-electron oxidation of [(Py5Me2)RuII(N3)]+ to [(Py5Me2)RuIII(N3)]2+ switches on metallo-Staudinger reactivity towards PPh3, forming [(Py5Me2)RuII(N(H)PPh3)]2+ through a putative [Ru–NNN–PPh3] intermediate.

Addition of HOTf to a mixture of CoIII(BDPP)(O2˙) (1, H2BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Cp*2Fe produced H2O2 in high yield implying formation of CoIII(BDPP)(OOH) (3), and reaction of Sc(OTf)3 with the same mixture gave a peroxo-bridged CoIII/ScIII5. These findings demonstrate the ambiphilic property of CoIII-superoxo 1.

Fixation and chemical reduction of CO2 are important for utilization of this abundant resource, and understanding the detailed mechanism of C–O cleavage is needed for rational development of CO2 reduction methods. Here, we describe a detailed analysis of the mechanism of the reaction of a masked two-coordinate cobalt(I) complex, LtBuCo (where LtBu = 2,2,6,6-tetramethyl-3,5-bis[(2,6-diisopropylphenyl)imino]hept-4-yl), with CO2, which yields two products of C–O cleavage, the cobalt(I) monocarbonyl complex LtBuCo(CO) and the dicobalt(II) carbonate complex (LtBuCo)2(μ-CO3). Kinetic studies and computations show that the κN,η6-arene isomer of LtBuCo rearranges to the κ2N,N′ binding mode prior to binding of CO2, which contrasts with the mechanism of binding of other substrates to LtBuCo. Density functional theory (DFT) studies show that the only low-energy pathways for cleavage of CO2 proceed through bimetallic mechanisms, and DFT and highly correlated domain-based local pair natural orbital coupled cluster (DLPNO-CCSD(T)) calculations reveal the cooperative effects of the two metal centers during facile C–O bond rupture. A plausible intermediate in the reaction of CO2 with LtBuCo is the oxodicobalt(II) complex LtBuCoOCoLtBu, which has been independently synthesized through the reaction of LtBuCo with N2O. The rapid reaction of LtBuCoOCoLtBu with CO2 to form the carbonate product indicates that the oxo species is kinetically competent to be an intermediate during CO2 cleavage by LtBuCo. LtBuCoOCoLtBu is a novel example of a thoroughly characterized molecular cobalt–oxo complex where the cobalt ions are clearly in the +2 oxidation state. Its nucleophilic reactivity is a consequence of high charge localization on the μ-oxo ligand between two antiferromagnetically coupled high-spin cobalt(II) centers, as characterized by DFT and multireference complete active space self-consistent field (CASSCF) calculations.

In addition to oxometal [M(n+)[double bond, length as m-dash]O] and imidometal [M(n+)[double bond, length as m-dash]NR] units, transient metal-iodosylarene [M((n-2)+)-O[double bond, length as m-dash]IPh] and metal-iminoiodane [M((n-2)+)-N(R)[double bond, length as m-dash]IPh] adducts are often invoked as a possible "second oxidant" responsible for the oxo and imido group transfer reactivity. Although a few metal-iodosylarene adducts have been recently isolated and/or spectroscopically characterized, metal-iminoiodane adducts have remained elusive. Herein, we provide UV-Vis, EPR, NMR, XAS and DFT evidence supporting the formation of a metal-iminoiodane complex 2 and its scandium adduct 2-Sc. 2 and 2-Sc are reactive toward substrates in the hydrogen-atom and nitrene transfer reactions, which confirm their potential as active oxidants in metal-catalyzed oxidative transformations. Oxidation of para-substituted 2,6-di-tert-butylphenols by 2 and 2-Sc can occur by both coupled and uncoupled proton and electron transfer mechanisms; the exact mechanism depends on the nature of the para substituent.

High-valent iron(IV)-oxo species are key intermediates in the catalytic cycles of a range of O2-activating iron enzymes. This work presents a detailed study of the electronic structures of mononuclear ([FeIV(O)(L)(NCMe)]2+, 1, L = tris(3,5-dimethyl-4-methoxylpyridyl-2-methyl)amine) and dinuclear ([(L)FeIV(O)(μ-O)FeIV(OH)(L)]3+, 2) iron(IV) complexes using absorption (ABS), magnetic circular dichroism (MCD) spectroscopy and wave-function-based quantum chemical calculations. For complex 1, the experimental MCD spectra at 2–10 K are dominated by a broad positive band between 12 000 and 18 000 cm−1. As the temperature increases up to ∼20 K, this feature is gradually replaced by a derivative-shaped signal. The computed MCD spectra are in excellent agreement with experiment, which reproduce not only the excitation energies and the MCD signs of key transitions but also their temperature-dependent intensity variations. To further corroborate the assignments suggested by the calculations, the individual MCD sign for each transition is independently determined from the corresponding electron donating and accepting orbitals. Thus, unambiguous assignments can be made for the observed transitions in 1. The ABS/MCD data of complex 2 exhibit ten features that are assigned as ligand-field transitions or oxo- or hydroxo-to-metal charge transfer bands, based on MCD/ABS intensity ratios, calculated excitation energies, polarizations, and MCD signs. In comparison with complex 1, the electronic structure of the FeIVO site is not significantly perturbed by the binding to another iron(IV) center. This may explain the experimental finding that complexes 1 and 2 have similar reactivities toward C–H bond activation and O-atom transfer.

Two pro-ligands ((R)LH) comprised of an o,p-di-tert-butyl-substituted phenol covalently bonded to a benzimidazole ((Bz)LH) or a 4,5-di-p-methoxyphenyl substituted imidazole ((PhOMe)LH), have been structurally characterised. Each possesses an intramolecular O-H[dot dot dot]N hydrogen bond between the phenolic O-H group and an imidazole nitrogen atom and (1)H NMR studies show that this bond is retained in solution. Each (R)LH undergoes an electrochemically reversible, one-electron, oxidation to form the [(R)LH] (+) radical cation that is considered to be stabilised by an intramolecular O...H-N hydrogen bond. The (R)LH pro-ligands react with M(BF(4))(2).H(2)O (M = Cu or Zn) in the presence of Et(3)N to form the corresponding [M((R)L)(2)] compound. [Cu((Bz)L)(2)] (), [Cu((PhOMe)L)(2)] (), [Zn((Bz)L)(2)] and [Zn((PhOMe)L)(2)] have been isolated and the structures of .4MeCN, .2MeOH, .2MeCN and .2MeCN determined by X-ray crystallography. In each compound the metal possesses an N(2)O(2)-coordination sphere: in .4MeCN and .2MeOH the {CuN(2)O(2)} centre has a distorted square planar geometry; in .2MeCN and .2MeCN the {ZnN(2)O(2)} centre has a distorted tetrahedral geometry. The X-band EPR spectra of both and , in CH(2)Cl(2)-DMF (9 : 1) solution at 77 K, are consistent with the presence of a Cu(ii) complex having the structure identified by X-ray crystallography. Electrochemical studies have shown that each undergo two, one-electron, oxidations; the potentials of these processes and the UV/vis and EPR properties of the products indicate that each oxidation is ligand-based. The first oxidation produces [M(II)((R)L)((R)L )](+), comprising a M(ii) centre bound to a phenoxide ((R)L) and a phenoxyl radical ((R)L ) ligand; these cations have been generated electrochemically and, for R = PhOMe, chemically by oxidation with Ag[BF(4)]. The second oxidation produces [M(II)((R)L )(2)](2+). The information obtained from these investigations shows that a suitable pro-ligand design allows a relatively inert phenoxyl radical to be generated, stabilised by either a hydrogen bond, as in [(R)LH] (+) (R = Bz or PhOMe), or by coordination to a metal, as in [M(II)((R)L)((R)L )](+) (M = Cu or Zn; R = Bz or PhOMe). Coordination to a metal is more effective than hydrogen bonding in stabilising a phenoxyl radical and Cu(ii) is slightly more effective than Zn(II) in this respect.

The cobalt(II) anilinosalen complex [Co(II)(L)] was prepared and subsequently oxidized by one electron. The resulting cation comprises a square planar low spin Co(II) ion anti-ferromagnetically exchange coupled to an anilinyl radical.

The X-band EPR spectra of [(H2O)5CrIII–CHCl2]2+, [(H2O)5CrIII–CHCBr2]2+, [(H2O)5CrIII–CH3]2+, [(H2O)5CrIII–CH2CO2H]2+ and [(H2O)5CrIII–CH(CH3)OC2H5]2+ were measured at liquid helium temperatures. For all complexes the major derivative signal is observed at g′perp ≈ 4 with weaker signals at g′par ≈ 2 and g′ ≈ 5.9. The signals broaden with increasing temperature and can not be observed at T ≥ 20 K. the low-temperature spectra could readily be simulated by using the spin Hamiltonian formalism for S = 3/2 with axial and rhombic zero-field splitting parameters D = −1.6 cm−1 and E/D = 0.06, respectively. Based on these results the (H2O)5Cr–R2+ complexes studied here are best described as chromium(III) compounds with a covalent chromium–carbon bond. In accord with expectations, this bond has a considerably larger covalent nature than the Cr–X bond, where X = halide, etc.

Two heterometallic complexes [{CuII(HLOX)NiII(N3)}2] and [(tmtacn)CoIII(μ-OH)CuII(LOX)NiII(OH2)2][ClO4]2 (tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) containing the same [CuIINiII] core embedded in an unsymmetrical dicompartmental imine-oxime ligand H4LOX have been synthesized and characterized. Their crystal structures show that the CuII resides at the N(oxime)2O(phenolate)2 site and assumes a planar geometry. The NiII is six-co-ordinated and bound to an N2O4 donor array comprising two iminonitrogens, two phenolate oxygens and two axially co-ordinated H2O molecules. The cobalt(III) is low spin and six-co-ordinated. In DMF solution at 10 K the EPR spectra of the complexes exhibit a spin-doublet ground state with “inverted” g values which demonstrate the delocalization of the unpaired electron over the CuNi core. Magnetic susceptibility measurements over the range 2–290 K confirm that the paramagnetic nickel(II) and copper(II) centres are antiferromagnetically coupled, with values for the exchange coupling constant J through the phenolate oxygens of −115 cm−1 and −130 cm−1 respectively. Considering the dimer as a single tetranuclear unit, the coupling constant J′ through the path Cu–N–O–Ni is very small (≈1 cm−1) but positive (ferromagnetic coupling).

The reduction of the bulky 1,2-diketone bis(2,6-diisopropylphenyl)glyoxal () and FeBr(2) with 1.5 equivalents of Na results in a Class 2 mixed valent H.S. Fe(II) L.S. Fe(III) complex (2) with two five-coordinate Fe centres which are antiferromagnetically exchange-coupled to give a total spin S(t) = 3/2 ground state and an S(t) = 5/2 excited state that are separated by about 25 cm(-1) (for Delta(J) approximately 5J).

The reaction of LFeIIICl3 with Ni2(Hdfmp)2(L = 1,4,7-trimethyl-1,4,7-triazacyclononane, H3dfmp = 2-hydroxy-5-methylbenzene-1,3-dicarbaldehyde dioxime) produces in the presence of triethylamine, acetate and tetrafluoroborate ions, the dark brown complex [L2Fe2(µ2-O2CMe)2(dfmp)2(MeOH)2Ni2](BF4)2 which is characterized by X-ray crystallography and magnetic susceptibility measurements and found to have a paramagnetic ground state.

Ligating properties of a tripodal, potentially tetradentate aminetris(phenol) ligand, tris(2-hydroxy-3,5-di-tert-butylbenzyl)amine, H(3)L, containing [N,O,O,O] donor atoms toward the vanadium ions in +III and IV oxidation states have been studied. The structures of complexes 1 [LV(III)(CH(3)OH)](0), 2 [LV(IV)(OCH(3))](0) and 3 [LV(IV)(acac)](0) were determined by X-ray diffraction methods as having five-coordinate V(III), 1, five-coordinate non-oxo-vanadium(IV), 2, and six-coordinate non-oxo-vanadium(iv) 3, respectively. Compounds 1-3 were also studied with electrochemical methods, variable-temperature (2-295 K) magnetic susceptibility measurements and X-band electron paramagnetic resonance (EPR) spectroscopy. The electrochemical results of 2 and 3 suggest metal-centered oxidation, i.e. the generation of a V(V)-phenolate species. EPR investigations indicate a (d(xy))(1) ground state showing a considerable increase in the in-plane π-bonding, as is expected for a phenolate ligand.

The tetrafunctional N-donor ligand 1 is able to form homoleptic complex cations with dodecahedral ([Fe{C12H6N2(CHNC6H4 SMe)2}2]2+2) or tetrahedral ([Cu2{C12H6N2(CHNC6 H4SMe)2}2]2+3) binding domains.

Oxidation of the diferric complex [(LFeOFeL)-O-III-L-III] to the monoradical complex [(LFeOFeL center dot)-O-III-L-III](+) and the diradical complex [(LFeOFeL center dot)-Fe-center dot-O-III-L-III](2+) is followed by a decay into monomeric complexes including a highly reactive putative [LFeIV=O] complex.

The coordination chemistry of CrCl3 with three pentadentate ligands having the [1,4,7]-triazacyclononane-1,4-diacetato motif have been investigated. The new resulting six-coordinate Cr(III)-chloro species react cleanly with sodium azide to form the corresponding azido species, which undergo photolysis under irradiation at 419 nm in acetonitrile–water solution to form Cr(V)-nitrido species that are partially hydrolyzed to their corresponding Cr(III)-hydroxo counterparts. Five of these Cr complexes have been characterized by X-ray crystallography. The hydroxo and nitrido species co-crystallize complicating crystal structural refinement. What at first appeared to be the longest CrN triple bond distance yet observed (1.66 A) was found after re-refinement to be an artifact of positional disorder of the Cr atoms of 73% nitrido and 27% hydroxo species. The re-refined CrN bond distance is estimated as 1.58 A, which agrees well with the observed CrN stretching frequency of 971 cm−1 found by IR spectroscopy. UV-vis data for the “nitrido” complexes suggest that they are all three partially hydrolyzed. These results emphasize the care that must be taken in the characterization of compounds that may co-crystallize with structurally similar analogs.

Owing to their topology, three iminosemiquinone radical ligands couple in a ferromagnetic fashion, mediated by a low-spin Co(III) ion.

A general approach for the rational synthesis of linear trinuclear complexes containing three different metals MAMBMC is described and a member of the series, FeIIICuIINiII, has been characterized by X-ray crystallography and magnetic susceptibility measurements.

One-electron reduction of [{LFe III (Ph 2 acac)} 2 (µ-O)] [BPh 4 ] 2 1[BPh 4 ] 2 (L = 1,4,7-trimethyl-1,4,7-triazacyclononane, Ph 2 acac - = 1,3-diphenylpropane-1,3-dionate) in CH 2 Cl 2 –toluene by 1 equiv. of cobaltocene produces the mixed-valent complex [{LFe(Ph 2 acac)} 2 (µ-O)][BPh 4 ]·C 6 H 5 Me 2[BPh 4 ]· C 6 H 5 Me which is characterized by X-ray crystallography; 2 has an S = ½ ground state (antiferromagnetically coupled high-spin Fe III and high-spin Fe II with J = -65 cm -1 ) and exhibits class II behaviour according to the Robin and Day classification.

The hexadentate ligand 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane, H3L, forms a very stable neutral complex [CrIIIL]·2MeCN 2 with chromium(III) which undergoes electrochemically three one-electron oxidations to yield [CrIIIL]+, [CrIIIL]2+ and [CrIIIL]3+, respectively; these contain one, two and three coordinated phenoxyl radical ligands; [CrIIIL]·2MeCN and [CrIIIL]ClO4·3MeCN are characterized by X-ray crystallography.

The new phenol-imidazole pro-ligands (R)LH react with Co(BF(4))(2).6H(2)O in the presence of Et(3)N to form the corresponding [Co(II)((R)L)(2)] compound (R = Ph (1), PhOMe (2), or Bz (3)). Also, (Bz)LH, reacts with Co(ii) in the presence of Et(3)N and H(2)O(2) to form [Co(III)((Bz)L)(3)](4). The structures of 1.2.5MeCN, 2.2DMF, 3.4MeOH, and 4.4DMF have been determined by X-ray crystallography. 1, 2, and 3 each involve Co(II) bound to two N,O-bidentate ligands with a distorted tetrahedral coordination sphere; 4 involves Co(III) bound to three N,O-bidentate ligands in a mer-N(3)O(3) distorted octahedral geometry. [Co(II)((R)L)(2)](R = Ph or PhOMe) undergo two, one-electron, oxidations. The products of the first oxidation, [1](+) and [2](+), have been synthesised by the chemical oxidation of 1 and 2, respectively; these cations, formulated as [Co(II)((R)L*)((R)L)(2)](+), comprise one phenoxyl radical and one phenolate ligand bound to Co(II) and are the first phenoxyl radical ligand complexes of tetra-coordinated Co(II). 4 undergoes two, one-electron, ligand-based oxidations, the first of which produces [4](+), [Co(III)((Bz)L*)((Bz)L)(2)](+). Unlike [1](+) and [2](+), product of the one-electron oxidation of [Co(II)((Bz)L)(2)], [3](+), is unstable and decomposes to produce [4](+). These studies have demonstrated that the chemical properties of [M(II)((R)L*)((R)L)(2)](+)(M = Co, Cu, Zn) are highly dependent on the nature of both the ligand and the metal centre.

The reaction of Cu(MeCO2)2·H2O with 1,4,7-trimethyl-1,4,7-triazacyclononane (L) and imidazole (ImH) in methanol affords, in the presence of a small amount of triethylamine and NaClO4, the blue trinuclear complex [L3Cu3(Im)3](ClO4)3 which has been characterized by X-ray crystallography, EPR and magnetic susceptibility measurements to be a spin-frustrated triangular imidazolate-bridged copper(II) complex without any µ3-X ligand.