A Redox-Active Ligand as a Reservoir for Protons and Electrons: O2 Reduction at Zirconium(IV)

Dioxygen Splitting by a Tantalum(V) Complex Ligated by a Rigid, Redox Non-Innocent Pincer Ligand

The reaction of TaMe₃ Cl₂ with the rigid acridane-derived trisamine H₃ NNN yields the tantalum(V) complex [TaCl₂ (NNN^cat )]. Subsequent reaction with dioxygen results in the full four-electron reduction of O₂ yielding the oxido-bridged bimetallic complex [{TaCl₂ (NNN^sq )}₂ O]. This dinuclear complex features an open-shell ground state due to partial ligand oxidation and was comprehensively characterized by single crystal X-ray diffraction, LIFDI mass spectrometry, NMR, EPR, IR and UV/VIS/NIR spectroscopy. The mechanism of O₂ activation was investigated by DFT calculations revealing initial binding of O₂ to the tantalum(V) center followed by complete O₂ scission to produce a terminal oxido-complex.

Exploring Ligand-Centered Hydride and H-Atom Transfer

The nickel(II) complex [ON(H)O]Ni(PPh₃) ([ON(H)O]^2- = bis(3,5-di-tert-butyl-2-phenoxy)amine), bearing a protonated redox-active ligand, was examined for its ability to serve as a hydrogen atom (H^•) and hydride (H^-) donor. Deprotonation of [ON(H)O]Ni(PPh₃) afforded the square-planar anion {[ONO^cat]Ni(PPh₃)}^1-, whereas hydrogen atom transfer from [ON(H)O]Ni(PPh₃) to TEMPO^• in the presence of added PPh₃ afforded five-coordinate [ONO]Ni(PPh₃)₂ that has been structurally characterized. In solution, this five-coordinate complex exists in equilibrium with four-coordinate [ONO]Ni(PPh₃), and this ligand exchange equilibrium correlates with a valence tautomerization between the redox-active ligand and the nickel center. Abstraction of a hydride from [ON(H)O]Ni(PPh₃) in the presence of PPh₃ afforded the octahedral complex, [ONO^q]Ni(OTf)(PPh₃)₂, which was characterized as an S = 1, nickel(II) complex. Bond dissociation free energy (BDFE) and hydricity (ΔG°_H-) measurements benchmark the thermodynamic propensity of this complex to participate in ligand-centered H^• and H^- transfer reactions.

Nonclassical oxygen atom transfer reactions of an eight-coordinate dioxomolybdenum(vi) complex

Eight-coordinate MoO₂(DOPO^Q)₂ can donate two oxygen atoms to substrates such as phosphines in a four-electron nonclassical oxygen atom transfer reaction.

O2 Activation by a Heterobimetallic Zr/Co Complex

Oxygen reduction is a critical half reaction in renewable fuel cell development and a key step in the development of aerobic oxidation reactions. Herein, we report rapid two-electron O₂ reduction by a d⁰ Zr^IV center with an appended redox-active Co^-I site serving as an electron reservoir. The early/late heterobimetallic Zr/Co complex (THF)Zr(MesNP ⁱPr₂)₃CoCN ^tBu (1) reacts readily with O₂ and O atom transfer reagents to generate reactive oxygenated species including terminal peroxo and oxo complexes, (O₂)Zr(MesNP ⁱPr₂)₃CoCN ^tBu (2) and O≡Zr(MesNP ⁱPr₂)₃CoCN ^tBu (3). The bimetallic Zr/Co complex provides a new cooperative synthetic pathway to promote multielectron redox processes such as oxygen reduction, with each metal playing a distinct role as a substrate binding site or redox mediator.

Radical-Type Reactivity and Catalysis by Single-Electron Transfer to or from Redox-Active Ligands

Controlled ligand-based redox-activity and chemical non-innocence are rapidly gaining importance for selective (catalytic) processes. This Concept aims to provide an overview of the progress regarding ligand-to-substrate single-electron transfer as a relatively new mode of operation to exploit ligand-centered reactivity and catalysis based thereon.

Reversible homolytic activation of water via metal-ligand cooperativity in a T-shaped Ni(ii) complex

A T-shaped Ni(ii) complex [Tol,PhDHPy]Ni has been prepared and characterized. EPR spectra and DFT calculations of this complex suggest that the electronic structure is best described as a high-spin Ni(ii) center antiferromagnetically coupled with a ligand-based radical. This complex reacts with water at room temperature to generate the dimeric complex [Tol,PhDHPy]Ni(μ-OH)Ni[Tol,PhDHPyH] which has been thoroughly characterized by SXRD, NMR, IR and deuterium-labeling experiments. Addition of simple ligands such as phosphines or pyridine displaces water and demonstrates the reversibility of water activation in this system. The water activation step has been examined by kinetic studies and DFT calculations which suggest an unusual homolytic reaction via a bimetallic mechanism. The ΔH ‡, ΔS ‡ and KIE (k H/k D) of the reaction are 5.5 kcal mol-1, -23.8 cal mol-1 K-1, and 2.4(1), respectively. In addition to the reversibility of water addition, this system is capable of activating water towards net O-atom transfer to substrates such as aromatic C-H bonds and phosphines. This reactivity is facilitated by the ability of the dihydrazonopyrrole ligand to accept H-atoms and illustrates the utility of metal ligand cooperation in activating O-H bonds with high bond dissociation energies.

A new bis-phenolate mesoionic carbene ligand for early transition metal chemistry

A new chelating mesoionic carbene ligand, derived from 1,2,3-triazoles, with two redox-active tert-butyl-phenolate linkers has been synthesized and explored towards its reactivity and electrochemical properties in early transition metal chemistry.

Six-coordinate [CoIII(L)2]z (z = 1-, 0, 1+) complexes of an azo-appended o-aminophenolate in amidate(2-) and iminosemiquinonate π-radical (1-) redox-levels: the existence of valence-tautomerism

Aerobic reaction of the ligand H2L1, 2-(2-phenylazo)-anilino-4,6-di-tert-butylphenol, CoCl2·6H2O and Et3N in MeOH under refluxing conditions produces, after work-up and recrystallization, black crystals of [Co(L1)2] (1). When examined by cyclic voltammetry, 1 displays in CH2Cl2 three one-electron redox responses: two oxidative, E11/2 = 0.30 V (peak-to-peak separation, ΔEp = 100 mV) and E21/2 = 1.04 V (ΔEp = 120 mV), and one reductive E1/2 = -0.27 V (ΔEp = 120 mV) vs. SCE. Consequently, 1 is chemically oxidized by 1 equiv. of [FeIII(η5-C5H5)2][PF6], affording the isolation of deep purple crystals of [Co(L1)2][PF6]·2CH2Cl2 (2), and one-electron reduction with [CoII(η5-C5H5)2] yielded bluish-black crystals of [CoIII(η5-C5H5)2][Co(L1)2]·MeCN (3). A solid sample of 1 exhibits temperature-independent (50-300 K) magnetism, revealing the presence of a free radical (S = 1/2), which exhibits an isotropic EPR signal (g = 2.003) at 298 K and at 77 K an eight-line feature characteristic of hyperfine-interaction of the radical with the Co (I = 7/2) nucleus. Based on X-ray structural parameters of 1-3 at 100 K, magnetic and EPR spectral behaviour of 1, and variable-temperature (233-313 K) 1H NMR spectral features of 1-3 and 13C NMR spectra at 298 K of 2 and 3 in CDCl3 point to the electronic structure of the complexes as either [CoIII{(LAP)2-}{(LISQ)}˙-] or [CoIII{(L1)2}˙3-] (delocalized nature favours the latter description) (1), [CoIII{(LISQ)˙-}2][PF6]·2CH2Cl2 (2) and [CoIII(η5-C5H5)2][CoIII{(LAP)2-}2]·MeCN (3) [(LAP)2- and (LISQ)˙- represent the redox-level of coordinated ligands o-amidophenolate(2-) ion and o-iminobenzosemiquinonate(1-) π-radical ion, respectively]. Notably, all the observed redox processes are ligand-centred. To the best of our knowledge, this is the first time that six-coordinate complexes of a common tridentate o-aminophenolate-based ligand have been structurally characterized for the parent 1, its monocation 2 and the monoanion 3 counterparts. Temperature-dependent 1H NMR spectra reveal the existence of valence-tautomeric equilibria in 1-3. Density Functional Theory (DFT) calculations at the B3LYP-level of theory corroborate the electronic structural assignment of 1-3 from experimental data. The origins of the observed UV-VIS-NIR absorptions for 1-3 have been assigned, based on time-dependent (TD)-DFT calculations.

Backbone Dehydrogenation in Pyrrole-Based Pincer Ligands

Treatment of both [CoCl( ^tBuPNP)] and [NiCl( ^tBuPNP)] ( ^tBuPNP = anion of 2,5-bis((di- tert-butylphosphino)methyl)pyrrole) with one equivalent of benzoquinone affords the corresponding chloride complexes containing a dehydrogenated PNP ligand, ^tBudPNP ( ^tBudPNP = anion of 2,5-bis((di- tert-butylphosphino)methylene)-2,5-dihydropyrrole). Dehydrogenation of PNP to dPNP results in minimal change to steric profile of the ligand but has important consequences for the resulting redox potentials of the metal complexes, resulting in the ability to isolate both [CoH( ^tBudPNP)] and [CoEt( ^tBudPNP)], which are more challenging (hydride) or not possible (ethyl) to prepare with the parent PNP ligand. Electrochemical measurements with both the Co and Ni dPNP species demonstrate a substantial shift in redox potentials for both the M(II/III) and M(II/I) couples. In the case of the former, oxidation to trivalent Co was found to be reversible, and subsequent reaction with AgSbF₆ afforded a rare example of a square-planar Co(III) species. Corresponding reduction of [CoCl( ^tBudPNP)] with KC₈ produced the diamagnetic Co(I) species, [Co(N₂)( ^tBudPNP)]. Further reduction of the Co(I) complex was found to generate a pincer-based π-radical anion that demonstrated well-resolved EPR features to the four hydrogen atoms and lone nitrogen atom of the ligand with minor contributions from cobalt and coordinated N₂. Changes in the electronic character of the PNP ligand upon dehydrogenation are proposed to result from loss of aromaticity in the pyrrole ligand, resulting in a more reducing central amido donor. DFT calculations on the Co(II) complexes were performed to shed further insight into the electronic structure of the pincer complexes.

Coordination behavior of bis-phenolate saturated and unsaturated N-heterocyclic carbene ligands to zirconium: reactivity and activity in the copolymerization of cyclohexene oxide with CO2

Tetravalent zirconium complexes supported by tridentate bis-phenolate imidazolidin-2-ylidene (L1), imidazol-2-ylidene (L2) and benzimidazol-2-ylidene (L3) NHC ligands were synthesized and evaluated as precursors for the copolymerization of cyclohexene oxide (CHO) with CO₂. While the reactivity of the imidazolidinium [H₃L1] chloride salt with Zr(OiPr)₄(HOiPr), and subsequent ligand exchanges with either (CH₃)₃SiCl or LiOiPr lead to a series of heteroleptic compounds (κ³-O,C,O-L1)Zr(X)₂(THF) (X = Cl, OiPr), both imidazolium [H₃L2] and benzimidazolium [H₃L3] chloride salts give a mixture of homoleptic (κ³-O,C,O-NHC)₂Zr and zwitterionic (κ²-O,O-HL)ZrCl₂(OiPr) compounds along with traces or the absence of the heteroleptic intermediate (κ³-O,C,O-NHC)Zr(Cl)(OiPr)(THF). Such dissimilar reactivity between the unsaturated and saturated NHC ligands is predominantly ascribed to the increased acidity of azolium salts along with the π-donor strength of the C_carbene in L2 and L3-Zr moieties. The reactivity with the more acidic azolium salts (H₃L2/3) and the destabilized Zr-X_trans to NHC_carbene bond results in a significant increase in the amount of homoleptic compounds generating HCl. The released HCl reacts preferentially with the heteroleptic intermediates having non-planar NHC ligands (i.e. L2/3) promoting the formation of zwitterionic complexes. The in situ deprotonation of the isolated zwitterionic (κ²-O,O-HL3)ZrCl₂(OiPr) compound by using Ag₂O gives the homoleptic complex as the major component along with a bimetallic hydroxo-bridged [(κ³-O,C,O-L3)Zr(μ-OH)(OiPr)]₂ compound. Of particular interest is that only the heteroleptic NHC-Zr(iv) complexes were identified to be active and highly selective towards the copolymerization of CHO with CO₂ independently of the co-catalysts used (both anionic and neutral) under mild conditions (P_CO2 < 1 bar, T = 60 °C), and gave atactic and completely alternating copolymers in a controlled manner (M_w/M_n ≈ 1.3-1.8). In contrast, the isolated homoleptic, zwitterionic and bimetallic zirconium species were found to be inactive under similar reaction conditions. Although the activity found for NHC-Zr(iv) complexes is nearly of the same order of magnitude as that of the NHC-Ti(iv) analogues, these results are the first examples of tetravalent zirconium complexes achieving high selectivity (99% in PCHC) in the catalyzed copolymerization of CHO with CO₂.

Oxygen Reduction by Homogeneous Molecular Catalysts and Electrocatalysts

The oxygen reduction reaction (ORR) is a key component of biological processes and energy technologies. This Review provides a comprehensive report of soluble molecular catalysts and electrocatalysts for the ORR. The precise synthetic control and relative ease of mechanistic study for homogeneous molecular catalysts, as compared to heterogeneous materials or surface-adsorbed species, enables a detailed understanding of the individual steps of ORR catalysis. Thus, the Review places particular emphasis on ORR mechanism and thermodynamics. First, the thermochemistry of oxygen reduction and the factors influencing ORR efficiency are described to contextualize the discussion of catalytic studies that follows. Reports of ORR catalysis are presented in terms of their mechanism, with separate sections for catalysis proceeding via initial outer- and inner-sphere electron transfer to O₂. The rates and selectivities (for production of H₂O₂ vs H₂O) of these catalysts are provided, along with suggested methods for accurately comparing catalysts of different metals and ligand scaffolds that were examined under different experimental conditions.

Syntheses and characterization of hepta-coordinated Group 4 amidinate complexes

Hepta-coordinated Group 4 amidinate complexes have been synthesized and characterized by ¹⁵N chemical shifts through ¹H–¹⁵N gHMBC NMR.

Structural and Electronic Noninnocence of α-Diimine Ligands on Niobium for Reductive C-Cl Bond Activation and Catalytic Radical Addition Reactions

A d⁰ niobium(V) complex, NbCl₃(α-diimine) (1a), supported by a dianionic redox-active N,N'-bis(2,6-diisopropylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene (α-diimine) ligand (ene-diamido ligand) served as a catalyst for radical addition reactions of CCl₄ to α-olefins and cyclic alkenes, selectively affording 1:1 radical addition products in a regioselective manner. During the catalytic reaction, the α-diimine ligand smoothly released and stored an electron to control the oxidation state of the niobium center by changing between an η⁴-(σ²,π) coordination mode with a folded MN₂C₂ metallacycle and a κ²-(N,N') coordination mode with a planar MN₂C₂ metallacycle. Kinetic studies of the catalytic reaction elucidated the reaction order in the catalytic cycle: the radical addition reaction rate obeyed first-order kinetics that were dependent on the concentrations of the catalyst, styrene, and CCl₄, while a saturation effect was observed at a high CCl₄ concentration. In the presence of excess amounts of styrene, styrene coordinated in an η²-olefinic manner to the niobium center to decrease the reaction rate. No observation of oligomers or polymers of styrene and high stereoselectivity for the radical addition reaction of CCl₄ to cyclopentene suggested that the C-C bond formation proceeded inside the coordination sphere of niobium, which was in good accordance with the negative entropy value of the radical addition reaction. Furthermore, reaction of 1a with (bromomethyl)cyclopropane confirmed that both the C-Br bond activation and formation proceeded on the α-diimine-coordinated niobium center during transformation of the cyclopropylmethyl radical to a homoallyl radical. With regard to the reaction mechanism, we detected and isolated NbCl₄(α-diimine) (6a) as a transient one-electron oxidized species of 1a during reductive cleavage of the C-X bonds; in addition, the monoanionic α-diimine ligand of 6a adopted a monoanionic canonical form with selective one-electron oxidation of the dianionic ene-diamido form of the ligand in 1a.

Modulation of Proton-Coupled Electron Transfer through Molybdenum-Quinonoid Interactions

An expanded series of π-bound molybdenum-quinonoid complexes supported by pendant phosphines has been synthesized. These compounds formally span three protonation-oxidation states of the quinonoid fragment (catechol, semiquinone, quinone) and two different oxidation states of the metal (Mo(0), Mo(II)), notably demonstrating a total of two protons and four electrons accessible in the system. Previously, the reduced Mo(0)-catechol complex 1 and its reaction with dioxygen to yield the two-proton/two-electron oxidized Mo(0)-quinone compound 4 was explored, while, herein, the expansion of the series to include the two-electron oxidized Mo(II)-catechol complex 2, the one-proton/two-electron oxidized Mo-semiquinone complex 3, and the two-proton/four-electron oxidized Mo(II)-quinone complexes 5 and 6 is reported. Transfer of multiple equivalents of protons and electrons from the Mo(0) and Mo(II) catechol complexes, 1 and 2, to H atom acceptor TEMPO suggests the presence of weak O-H bonds. Although thermochemical analyses are hindered by the irreversibility of the electrochemistry of the present compounds, the reactivity observed suggests weaker O-H bonds compared to the free catechol, indicating that proton-coupled electron transfer can be facilitated significantly by the π-bound metal center.

Synthesis and Characterization of Paramagnetic Tungsten Imido Complexes Bearing α-Diimine Ligands

Tungsten imido complexes bearing a redox-active ligand, such as N,N'-bis(2,6-diisopropylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene (L1), N,N'-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (L2), and 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (L3), were prepared by salt-free reduction of W(═NC6H3-2,6-(i)Pr2)Cl4 (1) using 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (MBTCD) followed by addition of the corresponding redox-active ligands. In the initial stage, reaction of W(═NC6H3-2,6-(i)Pr2)Cl4 with MBTCD afforded a tetranuclear W(V) imido cluster, [W(═NC6H3-2,6-(i)Pr2)Cl3]4 (2), which served as a unique precursor for introducing redox-active ligands to the tungsten center to give the corresponding mononuclear complexes with a general formula of W(═NC6H3-2,6-(i)Pr2)Cl3(L) (3, L = L1; 4, L = L2; and 6, L = L3). X-ray analyses of complexes 3 and 6 revealed a neutral coordination mode of L1 and L3 to the tungsten in solid state, while the electron paramagnetic resonance (EPR) spectra of 3 and 4 clarified that a radical was predominantly located on the tungsten center supported by neutral L1 or L2, and the EPR spectra of complex 6 indicated that a radical was delocalized over both the tungsten center and the monoanionic redox-active ligand L3.

RhCl(PPh3)3-mediated C–H oxyfunctionalization of pyrrolido-functionalized bisazoaromatic pincers: a combined experimental and theoretical scrutiny of redox-active and spectroscopic properties

Non-trivial coordination mode of symmetrical NNN ligands with Rh(iii) leads to redox-active NNO-scaffolds via C(sp²)–H oxyfunctionalization at rt, opening an opportunity to juxtapose different redox-active domains.

New avenues for ligand-mediated processes--expanding metal reactivity by the use of redox-active catechol, o-aminophenol and o-phenylenediamine ligands

Redox-active ligands have evolved from being considered spectroscopic curiosities - creating ambiguity about formal oxidation states in metal complexes - to versatile and useful tools to expand on the reactivity of (transition) metals or to even go beyond what is generally perceived possible. This review focusses on metal complexes containing either catechol, o-aminophenol or o-phenylenediamine type ligands. These ligands have opened up a new area of chemistry for metals across the periodic table. The portfolio of ligand-based reactivity invoked by these redox-active entities will be discussed. This ranges from facilitating oxidative additions upon d(0) metals or cross coupling reactions with cobalt(iii) without metal oxidation state changes - by functioning as an electron reservoir - to intramolecular ligand-to-substrate single-electron transfer to create a reactive substrate-centered radical on a Pd(ii) platform. Although the current state-of-art research primarily consists of stoichiometric and exploratory reactions, several notable reports of catalysis facilitated by the redox-activity of the ligand will also be discussed. In conclusion, redox-active ligands containing catechol, o-aminophenol or o-phenylenediamine moieties show great potential to be exploited as reversible electron reservoirs, donating or accepting electrons to activate substrates and metal centers and to enable new reactivity with both early and late transition as well as main group metals.

Revealing the thermodynamic driving force for ligand-based reductions in quinoids; conceptual rules for designing redox active and non-innocent ligands

Metal and ligand-based reductions have been modeled in octahedral ruthenium complexes revealing metal-ligand interactions as the profound driving force for the redox-active behaviour of orthoquinoid-type ligands. Through an extensive investigation of redox-active ligands we revealed the most critical factors that facilitate or suppress redox-activity of ligands in metal complexes, from which basic rules for designing non-innocent/redox-active ligands can be put forward. These rules also allow rational redox-leveling, i.e. the moderation of redox potentials of ligand-centred electron transfer processes, potentially leading to catalysts with low overpotential in multielectron activation processes.

Combination of redox-active ligand and lewis acid for dioxygen reduction with π-bound molybdenum-quinonoid complexes

A series of π-bound Mo-quinonoid complexes supported by pendant phosphines have been synthesized. Structural characterization revealed strong metal-arene interactions between Mo and the π system of the quinonoid fragment. The Mo-catechol complex (2a) was found to react within minutes with 0.5 equiv of O(2) to yield a Mo-quinone complex (3), H(2)O, and CO. Si- and B-protected Mo-catecholate complexes also react with O(2) to yield 3 along with (R(2)SiO)n and (ArBO)(3) byproducts, respectively. Formally, the Mo-catecholate fragment provides two electrons, while the elements bound to the catecholate moiety act as acceptors for the O(2) oxygens. Unreactive by itself, the Mo-dimethyl catecholate analogue reduces O(2) in the presence of added Lewis acid, B(C(6)F(5))(3), to generate a Mo(I) species and a bis(borane)-supported peroxide dianion, [[(F(5)C(6))(3)B](2)O(2)(2-)], demonstrating single-electron-transfer chemistry from Mo to the O(2) moiety. The intramolecular combination of a molybdenum center, redox-active ligand, and Lewis acid reduces O(2) with pendant acids weaker than B(C(6)F(5))(3). Overall, the π-bound catecholate moiety acts as a two-electron donor. A mechanism is proposed in which O(2) is reduced through an initial one-electron transfer, coupled with transfer of the Lewis acidic moiety bound to the quinonoid oxygen atoms to the reduced O(2) species.

Homoleptic nickel(II) complexes of redox-tunable pincer-type ligands

Different synthetic methods have been developed to prepare eight new redox-active pincer-type ligands, H(X,Y), that have pyrazol-1-yl flanking donors attached to an ortho-position of each ring of a diarylamine anchor and that have different groups, X and Y, at the para-aryl positions. Together with four previously known H(X,Y) ligands, a series of 12 Ni(X,Y)2 complexes were prepared in high yields by a simple one-pot reaction. Six of the 12 derivatives were characterized by single-crystal X-ray diffraction, which showed tetragonally distorted hexacoordinate nickel(II) centers. The nickel(II) complexes exhibit two quasi-reversible one-electron oxidation waves in their cyclic voltammograms, with half-wave potentials that varied over a remarkable 700 mV range with the average of the Hammett σ(p) parameters of the para-aryl X, Y groups. The one- and two-electron oxidized derivatives [Ni(Me,Me)2](BF4)n (n = 1, 2) were prepared synthetically, were characterized by X-band EPR, electronic spectroscopy, and single-crystal X-ray diffraction (for n = 2), and were studied computationally by DFT methods. The dioxidized complex, [Ni(Me,Me)2](BF4)2, is an S = 2 species, with nickel(II) bound to two ligand radicals. The mono-oxidized complex [Ni(Me,Me)2](BF4), prepared by comproportionation, is best described as nickel(II) with one ligand centered radical. Neither the mono- nor the dioxidized derivative shows any substantial electronic coupling between the metal and their bound ligand radicals because of the orthogonal nature of their magnetic orbitals. On the other hand, weak electronic communication occurs between ligands in the mono-oxidized complex as evident from the intervalence charge transfer (IVCT) transition found in the near-IR absorption spectrum. Band shape analysis of the IVCT transition allowed comparisons of the strength of the electronic interaction with that in the related, previously known, Robin-Day class II mixed valence complex, [Ga(Me,Me)2](2+).

Dioxygen reactivity of biomimetic Fe(II) complexes with noninnocent catecholate, o-aminophenolate, and o-phenylenediamine ligands

This study describes the O2 reactivity of a series of high-spin mononuclear Fe(II) complexes each containing the facially coordinating tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine ((Ph2)TIP) ligand and one of the following bidentate, redox-active ligands: 4-tert-butylcatecholate ((tBu)CatH(-)), 4,6-di-tert-butyl-2-aminophenolate ((tBu2)APH(-)), or 4-tert-butyl-1,2-phenylenediamine ((tBu)PDA). The preparation and X-ray structural characterization of [Fe(2+)((Ph2)TIP)((tBu)CatH)]OTf, [3]OTf and [Fe(2+)((Ph2)TIP)((tBu)PDA)](OTf)2, [4](OTf)2 are described here, whereas [Fe(2+)((Ph2)TIP)((tBu2)APH)]OTf, [2]OTf was reported in our previous paper [Bittner et al., Chem.-Eur. J. 2013, 19, 9686-9698]. These complexes mimic the substrate-bound active sites of nonheme iron dioxygenases, which catalyze the oxidative ring-cleavage of aromatic substrates like catechols and aminophenols. Each complex is oxidized in the presence of O2, and the geometric and electronic structures of the resulting complexes were examined with spectroscopic (absorption, EPR, Mössbauer, resonance Raman) and density functional theory (DFT) methods. Complex [3]OTf reacts rapidly with O2 to yield the ferric-catecholate species [Fe(3+)((Ph2)TIP)((tBu)Cat)](+) (3(ox)), which undergoes further oxidation to generate an extradiol cleavage product. In contrast, complex [4](2+) experiences a two-electron (2e(-)), ligand-based oxidation to give [Fe(2+)((Ph2)TIP)((tBu)DIBQ)](2+) (4(ox)), where DIBQ is o-diiminobenzoquinone. The reaction of [2](+) with O2 is also a 2e(-) process, yet in this case both the Fe center and (tBu2)AP ligand are oxidized; the resulting complex (2(ox)) is best described as [Fe(3+)((Ph2)TIP)((tBu2)ISQ)](+), where ISQ is o-iminobenzosemiquinone. Thus, the oxidized complexes display a remarkable continuum of electronic structures ranging from [Fe(3+)(L(2-))](+) (3(ox)) to [Fe(3+)(L(•-))](2+) (2(ox)) to [Fe(2+)(L(0))](2+) (4(ox)). Notably, the O2 reaction rates vary by a factor of 10(5) across the series, following the order [3](+) > [2](+) > [4](2+), even though the complexes have similar structures and Fe(3+/2+) redox potentials. To account for the kinetic data, we examined the relative abilities of the title complexes to bind O2 and participate in H-atom transfer reactions. We conclude that the trend in O2 reactivity can be rationalized by accounting for the role of proton transfer(s) in the overall reaction.

Trianionic pincer and pincer-type metal complexes and catalysts

This review provides a comprehensive examination of the synthesis, characterization, properties, and catalytic applications of trianionic pincer metal complexes.

Reactions of d(0) tungsten alkylidyne complexes with O2 or H2O. Formation of an oxo siloxy complex through unusual silyl migrations

(Me3SiCH2)3(Me3SiC≡)W←O=PMe3 (1), an adduct between (Me3SiCH2)3W≡CSiMe3 (2) and O=PMe3, reacts with O2 to give O=W(OSiMe3)(CH2SiMe3)3 (3) and CO2. Reaction of 2 with H2O yields 3 and the trimer [(μ-O)W(CH2SiMe3)2(=O)(THF)]3 (4). In the reaction of D2O with 2, 3-d(n) and methane isotopologues CH2D2, CHD3 and CD4 have been observed.