Catalytic Water Oxidation by Ruthenium(II) Quaterpyridine (qpy) Complexes: Evidence for Ruthenium(III) qpy‐N,N′′′‐dioxide as the Real Catalysts

Polypyridyl and related ligands have been widely used for the development of water oxidation catalysts. Supposedly these ligands are oxidation‐resistant and can stabilize high‐oxidation‐state intermediates. In this work a series of ruthenium(II) complexes [Ru(qpy)(L)₂]²⁺ (qpy=2,2′:6′,2′′:6′′,2′′′‐quaterpyridine; L=substituted pyridine) have been synthesized and found to catalyze Ce^IV‐driven water oxidation, with turnover numbers of up to 2100. However, these ruthenium complexes are found to function only as precatalysts; first, they have to be oxidized to the qpy‐N,N′′′‐dioxide (ONNO) complexes [Ru(ONNO)(L)₂]³⁺ which are the real catalysts for water oxidation.     

Spectroscopic Capture and Reactivity of a Low‐Spin Cobalt(IV)‐Oxo Complex Stabilized by Binding Redox‐Inactive Metal Ions

High‐valent cobalt‐oxo intermediates are proposed as reactive intermediates in a number of cobalt‐complex‐mediated oxidation reactions. Herein we report the spectroscopic capture of low‐spin (S=1/2) Co^IV‐oxo species in the presence of redox‐inactive metal ions, such as Sc³⁺, Ce³⁺, Y³⁺, and Zn²⁺, and the investigation of their reactivity in C? H bond activation and sulfoxidation reactions. Theoretical calculations predict that the binding of Lewis acidic metal ions to the cobalt‐oxo core increases the electrophilicity of the oxygen atom, resulting in the redox tautomerism of a highly unstable [(TAML)Co^III(O^.)]^2? species to a more stable [(TAML)Co^IV(O)(Mⁿ⁺)] core. The present report supports the proposed role of the redox‐inactive metal ions in facilitating the formation of high‐valent metal–oxo cores as a necessary step for oxygen evolution in chemistry and biology.     

M−O Bonding Beyond the Oxo Wall: Spectroscopy and Reactivity of Cobalt(III)‐Oxyl and Cobalt(III)‐Oxo Complexes

Terminal oxo complexes of late transition metals are frequently proposed reactive intermediates. However, they are scarcely known beyond Group 8. Using mass spectrometry, we prepared and characterized two such complexes: [(N4Py)Co^III(O)]⁺ ( 1 ) and [(N4Py)Co^IV(O)]²⁺ ( 2 ). Infrared photodissociation spectroscopy revealed that the Co?O bond in 1 is rather strong, in accordance with its lack of chemical reactivity. On the contrary, 2 has a very weak Co?O bond characterized by a stretching frequency of ≤659 cm^?1. Accordingly, 2 can abstract hydrogen atoms from non‐activated secondary alkanes. Previously, this reactivity has only been observed in the gas phase for small, coordinatively unsaturated metal complexes. Multireference ab‐initio calculations suggest that 2 , formally a cobalt(IV)‐oxo complex, is best described as cobalt(III)‐oxyl. Our results provide important data on changes to metal‐oxo bonding behind the oxo wall and show that cobalt‐oxo complexes are promising targets for developing highly active C?H oxidation catalysts.     

Water Oxidation by In Situ Generated [RuII(OH2)(NCNHCO)(pic)2]+

A dinuclear ruthenium complex [Ru^II(NC^NHC O)(pic)₂]₂²⁺ ( 2 ) was firstly prepared and characterized spectroscopically and electrochemically. Instead of the conventional ligand exchange, complex 2 dissociates in situ to afford two single‐site Ru aqua complexes, [Ru^II(OH₂)(NC^NHC O)(pic)₂]⁺, which mediates water oxidation through proton‐coupled electron transfer events. In electrokinetic studies, complex 2 demonstrated a TOF of 150.3 s⁻¹ comparable to those state‐of‐the‐art catalysts at neutral conditions. TONs of 2173 and 217 were attained in chemical and photochemical water oxidation when 2 was used as a catalyst, exhibiting good stability. Notably, a TOF of 1.3 s⁻¹ was achieved at CAN‐driven water oxidation, which outperformed most of the reported single‐site Ru complexes, indicating that complex 2 is one of most active water oxidation catalysts (WOCs) to date. The unique coordination configuration and outstanding catalytic performance of complex 2 might shed light on the design of novel molecular WOCs.     

Racemic monoperoxovanadium(V) complexes with achiral OO and ON donor set heteroligands: synthesis, crystal structure and stereochemistry of [NH3(CH2)2NH3][VO(O2)(ox)(pic)].2H2O and [NH3(CH2)2NH3][VO(O2)(ox)(pca)

Monoperoxovanadium(V) complexes, [NH3(CH2)2NH3][VO(O2)(ox)(pic)].2H2O (1) and [NH3(CH2)2NH3][VO(O2)(ox)(pca)] (2) [NH3(CH2)2NH3 = ethane-1,2-diammonium(2+), ox=oxalate(2-), pic=pyridine-2-carboxylate(1-), pca=pyrazine-2-carboxylate(1-)], were synthesized and characterized by X-ray analysis, IR and Raman spectroscopies. The five equatorial positions of the pentagonal bipyramid around the vanadium atoms are occupied by the eta2-peroxo ligand, two oxygen atoms of the ox, and the nitrogen atom of the pic or pca ligands, respectively. The oxo ligand and the oxygen atom of pic or pca are in the axial positions. Networks of X-HO (X=C, N or O) hydrogen bonds, and pi-pi interactions between aromatic rings in and anion-pi interactions in , determine the molecular packings and build up the supramolecular architecture. Three stereochemical rules for occupation of the donor sites in two-heteroligand [VO(O2)(L1)(L2)] complexes (L1, L2 are bidentate neutral or differently charged anionic heteroligands providing an OO, NN or ON donor set) are discussed. and crystallize as racemic compounds. The 51V NMR spectra proved that the parent complex anions of and partially decompose on dissolution in water to the monoperoxo-ox, -pic or -pca complexes.     

(18‐Crown‐6)‐μ‐oxo‐hexa­kis(tetra­hydro­borato)diuranium(IV): an unprecedented asymmetric dinuclear complex

In the title compound, (1,4,7,10,13,16‐hexa­oxacyclo­octa­decane‐1κ⁶O)‐μ‐oxo‐1:2κ²O:O‐hexa­kis(tetra­hydro­borato)‐1κ³H;2κ²H;2κ²H;2κ³H;2κ³H;2κ³H‐diuranium(IV), [U₂(BH₄)₆O(C₁₂H₂₄O₆)], one of the U atoms (U1), located at the centre of the crown ether moiety, is bound to the six ether O atoms, and also to a tridentate tetra­hydro­borate group and a μ‐oxo atom in axial positions. The other U atom (U2) is bound to the same oxo group and to five tetra­hydro­borate moieties, three of them tridentate and the other two bidentate. The two metal centres are bridged by the μ‐oxo atom in an asymmetric fashion, thus giving the species (18‐crown‐6)(κ³‐BH₄)U=(μ‐O)—U(κ³‐BH₄)₃(κ²‐BH₄)₂, in which the U1=O and U2—O bond lengths to the μ‐O atom [1.979 (5) and 2.187 (5) Å, respectively] are indicative of the presence of positive and negative partial charges on U1 and U2, respectively.     

Statistically disordered short hydrogen bonds in (pipzH2)(cdoH)2 and a comparison with (pipzH2)(cdo)·H2O (pipz is piperazine and cdoH2 is chelidonic acid)

Two related proton‐transfer compounds, namely piperazine‐1,4‐diium 4‐oxo‐4H‐pyran‐2,6‐dicarboxylate monohydrate, C₄H₁₂N₂²⁺·C₇H₂O₆²⁻·H₂O or (pipzH₂)(cdo)·H₂O, (I), and piperazine‐1,4‐diium bis(6‐carboxy‐4‐oxo‐4H‐pyran‐2‐carboxylate), C₄H₁₂N₂²⁺·2C₇H₃O₆⁻ or (pipzH₂)(cdoH)₂, (II), were obtained by the reaction of 4‐oxo‐4H‐pyran‐2,6‐dicarboxylic acid (chelidonic acid, cdoH₂) and piperazine (pipz). In (I), both carboxyl H atoms of chelidonic acid have been transferred to piperazine to form the piperazine‐1,4‐diium ion. The structure is a monohydrate. All potential N—H donors are involved in N—H...O hydrogen bonds. The water molecule spans two anions via the 4‐oxo group of the pyranose ring and a carboxylate O atom. The hydrogen‐bonding motif is essentially two‐dimensional. The structure is a pseudomerohedral twin. In the asymmetric unit of (II), the anion consists of monodeprotonated chelidonic acid, while the piperazine‐1,4‐diium cation is located on an inversion centre. The single carboxyl H atom is disordered in two respects. Firstly, the disordered H atom is shared equally by both carboxylic acid groups. Secondly, the H atom is statistically disordered between two positions on either side of a centre of symmetry and is engaged in a very short hydrogen‐bonding interaction; the relevant O...O distances are 2.4549 (11) and 2.4395 (11) Å, and the O—H...O angles are 177 (6) and 177 (5)°, respectively. Further hydrogen bonding of the type N—H...O places the (pipzH₂)²⁺ cations in pockets formed by the chains of (cdoH)⁻ anions. In contrast with (I), the (pipzH₂)²⁺ cations form hydrogen‐bonding arrays that are perpendicular to the anions, yielding a three‐dimensional hydrogen‐bonding motif. The structures of both (I) and (II) also feature π–π stacking interactions between aromatic rings.     

Synthesis, Crystal Structure and Properties of Complex VO（C12H12N2O2） （C13H10NO2）

IntroductionAsthevanadate dependentenzyme ,ahaloperox idase,andvanadiumnitrogenasehavebeenfoundinsuccession ,somescientistsareinterestedinthecoor dinationchemistryandthebiochemistryofvanadi um[1] .Thereactivecenterofvanadiumbromoperoxi dasecontainstheoxovanadiumVO3+ coordinatedbyoxygenandnitrogenatoms[2 ] .However,thestudiesaboutoxovanadiumVO3+ coordinationcompoundscontainingbioactiveligandsarerarelyfound[3— 8] .Hydroximicacidthatcanbeconsideredtobethederivativeofthecarboxylicacid ,theamide…     

Alkylimido osmium(VI) and ruthenium(VI) porphyrins. Crystal and molecular structures of Os(TTP)(O)(NBut)- EtOH and OS(4-C1-TPP)(NBut)2

Oxo(tert-butylimido) or bis(tert-butylimido)osmium(VI) porphyrins Os(Por)(O)( NBu^t) and Os(Por)(NBu^t)2, [Por=meso-tetrakis(p-tolyl)porphyrinato (TTP) and meso-tetrakis(4-chlorophenyl)porphyrinato (4-CI-TPP)] were synthesized by air oxidation of bis(tert-butylamine)osmium(II) porphyrins [Os(Por)(H₂NBu^t)n (Por=TPP, 4-C1-TPP], depending on whether text-butylamine is present. The bis(tert-butylamine)ruthenium(II) porphyrins [Ru(Por)(H₂NBu^t)z, Por=TTP, 4-CI-TPP] can undergo bromine oxidation to give oxo(tert-butylimido)ruthenium(VI) complexes in quantitative yields. All these nem complexes were characterized by ¹H NMR, UV-Visible and IR spectroscopy. The X-ray crystal structures of Os(TTP)(O)(NBu^t).EtOH and Os(4-Cl-TPP)(NBu^t)₂ have been determined. Crystal data: for Os(TTP)(O)(NBu').EtOH: monoclinic, space group P2₁/c, a=1.3546(6) nm, b=2.3180(3) am, c=1.6817(3) nm, β=90.84(2)°, V=527.97(1) nm³, Z=4. The Os=O and Os=NBu^t distances in Os(TTP)(O)(NBu^t).EtOH are 0.1772(7) nm and 0.1759(9) nm, respectively. The average Os=NBu^t distance of Os(4-C1-TPP)(NBu^t)₂ is 0.1775 nm.     

The double H‐atom acceptability of the P=O group in new XP(O)(NHCH2C6H4‐2‐Cl)2 phosphoramidates [X = C6H5O– and CF3C(O)NH–]: a database analysis of compounds having a P(O)(NHR) group

In the hydrogen‐bond patterns of phenyl bis(2‐chlorobenzylamido)phosphinate, C₂₀H₁₉Cl₂N₂O₂P, (I), and N,N′‐bis(2‐chlorobenzyl)‐N′′‐(2,2,2‐trifluoroacetyl)phosphoric triamide, C₁₆H₁₅Cl₂F₃N₃O₂P, (II), the O atoms of the related phosphoryl groups act as double H‐atom acceptors, so that the P=O...(H—N)₂ hydrogen bond in (I) and the P=O...(H—N_amide)₂ and C=O...H—N_C(O)NHP(O) hydrogen bonds in (II) are responsible for the aggregation of the molecules in the crystal packing. The presence of a double H‐atom acceptor centre is a result of the involvement of a greater number of H‐atom donor sites with a smaller number of H‐atom acceptor sites in the hydrogen‐bonding interactions. This article also reviews structures having a P(O)NH group, with the aim of finding similar three‐centre hydrogen bonds in the packing of phosphoramidate compounds. This analysis shows that the factors affecting the preference of the above‐mentioned O atom to act as a double H‐atom acceptor are: (i) a higher number of H‐atom donor sites relative to H‐atom acceptor centres in molecules with P(=O)(NH)₃, (N)P(=O)(NH)₂, C(=O)NHP(=O)(NH)₂ and (NH)₂P(=O)OP(=O)(NH)₂ groups, and (ii) the remarkable H‐atom acceptability of this atom relative to the other acceptor centre(s) in molecules containing an OP(=O)(NH)₂ group, with the explanation that the N atom bound to the P atom in almost all of the structures found does not take part in hydrogen bonding as an acceptor. Moreover, the differences in the H‐atom acceptability of the phosphoryl O atom relative to the O atom of the alkoxy or phenoxy groups in amidophosphoric acid esters may be illustrated by considering the molecular packing of compounds having (O)₂P(=O)(NH) and (O)P(=O)(NH)(N)groups, in which the unique N—H unit in the above‐mentioned molecules almost always selects the phosphoryl O atom as a partner in forming hydrogen‐bond interactions. The P atoms in (I) and (II) are in tetrahedral coordination environments, and the phosphoryl and carbonyl groups in (II) are anti with respect to each other (the P and C groups are separated by one N atom). In the crystal structures of (I) and (II), adjacent molecules are linked via the above‐mentioned hydrogen bonds into a linear arrangement parallel to [100] in both cases, in (I) by forming R₂²(8) rings and in (II) through a combination of R₂²(10) and R₂¹(6) rings.     

Facile Conversion of syn‐[FeIV(O)(TMC)]2+ into the anti Isomer via Meunier's Oxo–Hydroxo Tautomerism Mechanism

The syn and anti isomers of [Fe^IV(O)(TMC)]²⁺ (TMC=tetramethylcyclam) represent the first isolated pair of synthetic non‐heme oxoiron(IV) complexes with identical ligand topology, differing only in the position of the oxo unit bound to the iron center. Both isomers have previously been characterized. Reported here is that the syn isomer [Fe^IV(O_syn)(TMC)(NCMe)]²⁺ ( 2 ) converts into its anti form [Fe^IV(O_anti)(TMC)(NCMe)]²⁺ ( 1 ) in MeCN, an isomerization facilitated by water and monitored most readily by ¹H NMR and Raman spectroscopy. Indeed, when H₂¹⁸O is introduced to 2 , the nascent 1 becomes ¹⁸O‐labeled. These results provide compelling evidence for a mechanism involving direct binding of a water molecule trans to the oxo atom in 2 with subsequent oxo–hydroxo tautomerism for its incorporation as the oxo atom of 1 . The nonplanar nature of the TMC supporting ligand makes this isomerization an irreversible transformation, unlike for their planar heme counterparts.     

(OXO)(Phosphine)Ruthenium(IV) Complexes: Oxygen Atom Transfer in the Oxidation of Olefins,Sulfides, Sulfoxides,and Free Phosphines

Kinetic and mechanistic studies of the oxidation of olefins, sulfides, and sulfoxides by [Ru^IV(bpy)₂(O)- (PR₃)](ClO₄)₂ (bpy = 2,2′-bipyridine; R = ethyl or phenyl) complexes have been conducted in both methylene chloride and acetonitrile. In all cases, the rate law shows a first-order dependence on both the concentration of (oxo)ruthenium(IV) species and the target substrate. In addition, product distributions of substrate oxidation exhibit a strong dependence on both the particular phosphine ligand and the solvent utilized in the experiment. On the basis of labelling experimcnts and kinetic evidence, a mechanism is proposed involving a two-electron, oxygen atom insertion into the target substrate. Notably, an (oxidized substrate)ruthenium(II) complex has been isolated and characterized for the oxidation of styrene by the (oxo)(triethylphosphine)ruthenium(IV) complex, where a cyclic voltammogram of this complex displays one quasi-reversible Ru(III)/Ru(II) couple with an E_1/2 = 1.24 V vs SSCE. Kinetic analysis of styrene oxidation indicates that the formation of benzaldehyde from styrene does not occur simply by two sequential two-electron steps. In this regard, alternative mechanisms are discussed.     

Cerium(IV)-driven oxidation of water catalyzed by mononuclear ruthenium complexes

This paper reviews results from study of mononuclear ruthenium complexes capable of catalyzing the oxidation of water to molecular oxygen. These catalysts may be classified into three groups, with different rate laws associated with O₂ evolution. In one class, O₂ evolution proceeds via radical coupling of the oxygen atom of an Ru^V=O species with a hydroxocerium(IV) ion. O₂ evolution catalyzed by the second class occurs via acid–base reaction of the oxygen atom of an Ru^V=O species with a water molecule. In the third group, the dominant mechanism is oxo–oxo radical coupling between two Ru^V=O species. Several significant properties of the oxidant Ce(IV) are also discussed, including the singlet biradical character of the hydroxocerium(IV) ion.     

Catalysis of Mononuclear Aquaruthenium Complexes in Oxygen Evolution from Water: A New Radical Coupling Path using Hydroxocerium(IV) Species

The mechanism of O₂ evolution from water catalyzed by a series of mononuclear aquaruthenium complexes, [Ru(terpy)(bpy)(OH₂)]²⁺, [Ru(tmtacn)(R₂bpy)(OH₂)]²⁺ (R=H, Me, and OMe; R₂bpy=4,4′‐disubstituted‐2,2′‐bipyridines), and [Ru(tpzm)(R₂bpy)(OH₂)]²⁺ (R=H, Me, and OMe), is investigated, where terpy=2,2′:6′,2′′‐terpyridine, bpy=2,2′‐bipyridine, tmtacn=1,4,7‐trimethyl‐1,4,7‐triazacyclononane, and tpzm=tris(1‐pyrazolyl)methane. The kinetics of O₂ evolution is investigated as a function of either the catalyst concentration or the oxidant concentration by employing Ce(NH₄)₂(NO₃)₆ as an oxidant; these catalysts can be classified into two groups that have different rate laws for O₂ evolution. In one class, the rate of O₂ evolution is linear to both the catalyst and Ce⁴⁺ concentrations, as briefly reported for [Ru(terpy)(bpy)(OH₂)]²⁺ (S. Masaoka, K. Sakai, Chem. Lett. 2009 , 38, 182). For the other class, [Ru(tmtacn)(R₂bpy)(OH₂)]²⁺, the rate of O₂ evolution is quadratic to the catalyst concentration and independent of the Ce⁴⁺ concentration. Moreover, the singlet biradical character of the hydroxocerium(IV) ion was realized by experimental and DFT investigations. These results indicate that the radical coupling between the oxygen atoms of a Ru^V?O species and a hydroxocerium(IV) ion is the key step for the catalysis of [Ru(terpy)(bpy)(OH₂)]²⁺ and [Ru(tpzm)(R₂bpy)(OH₂)]²⁺, while the well‐known oxo‐oxo radical coupling among two Ru^V?O species proceeds in the catalysis of [Ru(tmtacn)(R₂bpy)(OH₂)]²⁺. This is the first report demonstrating that the radical character provided by the hydroxocerium(IV) ion plays a crucial role in the catalysis of such ruthenium complexes in the evolution of O₂ from water.     

Novel copper(II) heterochelate: synthesis,structural features and fluorescence studies

Fluorescence properties of four based derivatives [Aⁿ] (where n = 1–4) and their Cu(II) heterochelates of the type [Cu(Aⁿ)(CQ)(OH)]^?xH₂O {where A¹ = 3‐(2‐oxo‐2H‐chromen‐3‐yl)‐4H‐furo[3,2‐c]chromen‐4‐one, A² = 8‐methyl‐3‐(2‐oxo‐2H‐chromen‐3‐yl)‐4H‐furo[3,2‐c]chromen‐4‐one, A³ = 6‐methyl‐3‐(2‐oxo‐2H‐chromen‐3‐yl)‐4H‐furo[3,2‐c]chromen‐4‐one, A⁴ = 8‐chloro‐3‐(2‐oxo‐2H‐chromen‐3‐yl)‐4H‐furo[3,2‐c]chromen‐4‐one and x = 3, 2, 4, 1} were studied at room temperature. The fluorescence spectra of heterochelates show red shift, which may be due to the chelation by the ligands to the metal ion. It enhances ligand ability to accept electrons and decreases the electron transition energy. The kinetic parameters such as order of reaction (n), energy of activation (E_a), entropy (ΔS^#), pre‐exponential factor (A), enthalpy (ΔH^#) and Gibbs free energy (ΔG^#) have been reported. The antimicrobial activity of Clioquinol and Cu(II) heterochelates have been determined and described. All the heterochelates showed a more effective antimicrobial activity than the free ligand. Copyright © 2010 John Wiley & Sons, Ltd.     

O−O Bond Formation and Liberation of Dioxygen Mediated by N5‐Coordinate Non‐Heme Iron(IV) Complexes

Formation of the O?O bond is considered the critical step in oxidative water cleavage to produce dioxygen. High‐valent metal complexes with terminal oxo (oxido) ligands are commonly regarded as instrumental for oxygen evolution, but direct experimental evidence is lacking. Herein, we describe the formation of the O?O bond in solution, from non‐heme, N₅‐coordinate oxoiron(IV) species. Oxygen evolution from oxoiron(IV) is instantaneous once meta‐chloroperbenzoic acid is administered in excess. Oxygen‐isotope labeling reveals two sources of dioxygen, pointing to mechanistic branching between HAT (hydrogen atom transfer)‐initiated free‐radical pathways of the peroxides, which are typical of catalase‐like reactivity, and iron‐borne O?O coupling, which is unprecedented for non‐heme/peroxide systems. Interpretation in terms of [Fe^IV(O)] and [Fe^V(O)] being the resting and active principles of the O?O coupling, respectively, concurs with fundamental mechanistic ideas of (electro‐) chemical O?O coupling in water oxidation catalysis (WOC), indicating that central mechanistic motifs of WOC can be mimicked in a catalase/peroxidase setting.     

Structural, Spectroscopic, and Magnetochemical Characterization of the Trinuclear Vanadium(III) Carboxylates [V(3)O(O(2)CR)(6)L(3)](ClO(4)) (R = Various Groups; L = Pyridine, 4-Picoline, 3,5-Lutidine)

Synthetic procedures are described that allow access to the [V(3)O(O(2)CR)(6)L(3)](ClO(4)) (R = various groups; L = pyridine (py), 4-picoline (pic) or 3,5-lutidine (lut)) family of complexes. Treatment of VCl(3)(THF)(3) with NaO(2)CR (R = Me, Et) in RCO(2)H/py, pic/MeCN, or CH(2)Cl(2) solution followed by addition of NBu(n)(4)ClO(4) leads to isolation of [V(3)O(O(2)CR)(6)L(3)](ClO(4)) salts in 47-95% yields. A similar procedure for R = C(6)H(5), C(6)H(4)-p-OMe, C(6)H(3)-m-Me(2), and C(6)H(4)-p-Cl but omitting addition of NaO(2)CR provides the corresponding benzoate or substituted-benzoate derivatives in 24-56% yields. The X-ray structure of [V(3)O(O(2)CEt)(6)(pic)(3)](ClO(4)) (4) shows the anion to consist of a [V(3)O](7+) triangular fragment with a &mgr;(3)-O(2)(-) ion in the V(3) plane; each triangular edge is bridged by two EtCO(2)(-) groups in their familiar syn,syn modes, and there is a terminal pic group on each V(III) completing distorted octahedral geometries at the metal atoms. The cation has imposed C(2) symmetry (isosceles V(3) triangle), the C(2) axis passing through one V atom and the central &mgr;(3)-O atom, but has D(3)(h)() virtual symmetry (equilateral V(3) triangle). Complex 4 crystallizes in monoclinic space group C2/c with the following unit cell dimensions at -171 degrees C: a = 13.935(2) ?, b = 18.323(2) ?, c = 17.470(2) ?, beta = 95.55(1) degrees, V = 4439.7 ?(3), Z = 4. The structure was solved using 2657 unique reflections with F > 3sigma(F) and refined on F to conventional R (R(w)) values of 0.058 (0.066). Variable-temperature, solid-state magnetic susceptibility measurements were made on complex 1 in the 5.01-280 K region in a 1 kG magnetic field. The effective magnetic moment (&mgr;(eff)) per V(3) unit decreases gradually from 4.64 &mgr;(B) at 280 K to 1.76 &mgr;(B) at 5.01 K. The data were fit to the theoretical expression for an isosceles V(III)(3) complex, and the fitting parameters were J = -18.0(7) cm(-)(1), J' = -10.4(4) cm(-)(1), and g = 1.985, with TIP held constant at 600 x 10(-)(6) cm(3) mol(-)(1); J' refers to the unique exchange interaction within the isosceles triangle. The ground state of complex 1 thus has S = 0.     

Synthesis,Characterization, and Reactivity of a Uranium(VI) Carbene Imido Oxo Complex

We report the uranium(VI) carbene imido oxo complex [U(BIPM^TMS)(NMes)(O)(DMAP)₂] ( 5 , BIPM^TMS=C(PPh₂NSiMe₃)₂; Mes=2,4,6‐Me₃C₆H₂; DMAP=4‐(dimethylamino)pyridine) which exhibits the unprecedented arrangement of three formal multiply bonded ligands to one metal center where the coordinated heteroatoms derive from different element groups. This complex was prepared by incorporation of carbene, imido, and then oxo groups at the uranium center by salt elimination, protonolysis, and two‐electron oxidation, respectively. The oxo and imido groups adopt axial positions in a T‐shaped motif with respect to the carbene, which is consistent with an inverse trans‐influence. Complex 5 reacts with tert‐butylisocyanate at the imido rather than carbene group to afford the uranyl(VI) carbene complex [U(BIPM^TMS)(O)₂(DMAP)₂] ( 6 ).     

Synthesis,Characterization, and Reactivity of a Uranium(VI) Carbene Imido Oxo Complex

We report the uranium(VI) carbene imido oxo complex [U(BIPM^TMS)(NMes)(O)(DMAP)₂] ( 5 , BIPM^TMS=C(PPh₂NSiMe₃)₂; Mes=2,4,6‐Me₃C₆H₂; DMAP=4‐(dimethylamino)pyridine) which exhibits the unprecedented arrangement of three formal multiply bonded ligands to one metal center where the coordinated heteroatoms derive from different element groups. This complex was prepared by incorporation of carbene, imido, and then oxo groups at the uranium center by salt elimination, protonolysis, and two‐electron oxidation, respectively. The oxo and imido groups adopt axial positions in a T‐shaped motif with respect to the carbene, which is consistent with an inverse trans‐influence. Complex 5 reacts with tert‐butylisocyanate at the imido rather than carbene group to afford the uranyl(VI) carbene complex [U(BIPM^TMS)(O)₂(DMAP)₂] ( 6 ).     

A Highly Reactive Oxoiron(IV) Complex Supported by a Bioinspired N3O Macrocyclic Ligand

The sluggish oxidants [Fe^IV(O)(TMC)(CH₃CN)]²⁺ (TMC=1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane) and [Fe^IV(O)(TMCN‐d₁₂)(OTf)]⁺ (TMCN‐d₁₂=1,4,7,11‐tetra(methyl‐d₃)‐1,4,7,11‐tetraazacyclotetradecane) are transformed into the highly reactive oxidant [Fe^IV(O)(TMCO)(OTf)]⁺ ( 1 ; TMCO=4,8,12‐trimethyl‐1‐oxa‐4,8,12‐triazacyclotetradecane) upon replacement of an NMe donor in the TMC and TMCN ligands by an O atom. A rate enhancement of five to six orders of magnitude in both H atom and O atom transfer reactions was observed upon oxygen incorporation into the macrocyclic ligand. This finding was explained in terms of the higher electrophilicity of the iron center and the higher availability of the more reactive S=2 state in 1 . This rationalizes nature's preference for using O‐rich ligand environments for the hydroxylation of strong C−H bonds in enzymatic reactions.