The Direct Oxidative Addition of O2 to a Mononuclear Cr(I) Complex Is Spin Forbidden

Π-Character of Chromium Germylyne Complex in the Reactions with Enone, Butadiene, and Alkynes: Formation of Germacycles through [2+4] Cycloaddition with Conjugated Molecules

Germylyne complex Cp*(OC)2 Cr≡Ge{C(SiMe3 )3 } (1) reacted with methyl vinyl ketone to give an η3 -allyl complex 2 with an oxagermacyclopentenyl ring. An analogous η3 -allyl complex 3 with a germacyclopentenyl ring was obtained by the reaction with butadiene, a non-polar conjugated molecule, under photoirradiation. These reactions are accompanied by cleavage of the Cr≡Ge triple bond. On the other hand, the reactions of complex 1 with alkynes under photoirradiation resulted in clean substitution of a CO ligand of 1 to afford (η2 -alkyne)germylyne complexes, where the Cr≡Ge triple bond is intact.

Multielectron Bond Cleavage Processes Enabled by Redox-Responsive Phosphinimide Ligands

The activation of small molecules via multielectron redox processes offers promise in mediating difficult transformations related to energy conversion processes. While molecular systems that engage in one- and two-electron redox processes are widespread, those that participate in the direct transfer of four or more electrons to small molecules are very rare. To that end, we report a mononuclear CrII complex competent for the 4-electron reduction of dioxygen (O2) and nitrosoarenes. These systems additionally engage in facile two-electron group transfer reactivity, including O atom excision and nitrene transfer. Structural, spectroscopic, and computational studies support bond activation processes that intimately occur at a mononuclear chromium(phosphinimide) center and highlight the unusual structural responsiveness of the phosphinimides in stabilizing a range of metal redox states.

Four-Electron Oxidative Addition of an N=N Double Bond at a Chromium Metallocyclopropene

This report describes the synthesis of a pseudo-tetrahedral chromium alkyne complex supported by a bidentate phosphinimide ligand and its reactivity with an azobenzene derivative. Characterization of the former by structural and computational methods reveals an unprecedented extent of alkyne activation by a formal chromium(II) center, suggesting that this complex is best described as a chromium(IV)-metallocyclopropene. Exposure of this compound to 4,4'-difluoroazobenzene results in the formation of a chromium(VI) diimido complex, which constitutes a rare 4-electron oxidative addition of an N=N double bond. The isolation of a chromium(IV)-hydrazido intermediate enabled mechanistic investigations of this challenging bond cleavage process. This work substantiates the notion that terminal phosphinimide ligands can engender first-row transition metal ions with exceptional reactivity.

Expanding the Utility of β-Diketiminate Ligands in Heavy Group VI Chemistry of Molybdenum and Tungsten

We report the synthesis of 17 molybdenum and tungsten complexes supported by the ubiquitous BDI ligand framework (BDI = β-diketiminate). The focal entry point is the synthesis of four molybdenum and tungsten(V) BDI complexes of the general formula [MO(BDI^R)Cl₂] [M = Mo, R = Dipp (1); M = W, R = Dipp (2); M = Mo, R = Mes (3); M = W, R = Mes (4)] synthesized by the reaction between MoOCl₃(THF)₂ or WOCl₃(THF)₂ and LiBDI^R. Reactivity studies show that the BDI^Dipp complexes are excellent precursors toward adduct formation, reacting smoothly with dimethylaminopyridine (DMAP) and triethylphosphine oxide (OPEt₃). No reaction with small phosphines has been observed, strongly contrasting the chemistry of previously reported rhenium(V) complexes. Additionally, the complexes 1 and 2 are good precursors for salt metathesis reactions. While 1 can be chemically reduced to the first stable example of a Mo(IV) BDI complex 15, reduction of 2 resulted in degradation of the BDI ligand via a nitrene transfer reaction, leading to MAD (4-((2,6-diisopropylphenyl)imino)pent-2-enide) supported tungsten(V) and tungsten(VI) complexes 16 and 17. All reported complexes have been thoroughly studied by VT-NMR and (heteronuclear) NMR spectroscopy, as well as UV-vis and EPR spectroscopy, IR spectroscopy, and X-ray diffraction analysis.

From polygons to polyhedra through intermediate structures. A shape measures study of six-atom inorganic rings and clusters

Among the wealth of well-established molecular structures, inorganic rings and clusters present an overwhelming variety of geometries that chemists try to describe with a limited assortment of regular polygons and polyhedra. In the case of six-atom structures we usually employ the hexagon, the pentagonal pyramid, the trigonal prism and the octahedron. More often than not, however, real world structures deviate from those ideal geometries, and we try to cope with non-ideality by adding adjectives such as distorted, twisted, puckered or flattened, additionally nuanced by adverbs such as slightly, significantly or severely. This contribution presents a systematic structural perspective of six-atom groups in molecules by means of a continuous shape measures (CShM) analysis. The shape of a group of N points is defined by all the sets of 3 N Cartesian coordinates that can be generated by rigid translation, rotation, or isotropic scale change. Among all possible arrangements of N points in space, we select as reference shapes the corresponding regular N-vertex polygons and polyhedra, together with univocally defined combinations thereof (e.g., two coplanar or perpendicular edge-sharing squares). The present CShM study allows us to classify most of the structures not only by their closeness to a particular regular shape, but also by quantifying their position along minimal distortion interconversion pathways between two regular shapes.

Self-assembly of an organometallic Fe9O6 cluster from aerobic oxidation of (tmeda)Fe(CH2tBu)2

Aerobic oxidation of (tmeda)Fe(CH₂^tBu)₂ in toluene or THF solution leads to the self-assembly of a magic-sized all-ferrous oxide cluster containing the Fe₉O₆ subunit and bearing organometallic and diamine ligands. Mössbauer studies of the cluster are consistent with an all-ferrous assignment and magnetometry reveals complex intracluster and intercluster magnetic interactions.

The mechanism of copper-catalyzed oxytrifluoromethylation of allylamines with CO2: a computational study

We propose a novel Cu(i)–Cu(ii) catalytic cycle for the copper-catalyzed oxytrifluoromethylation of allylamines with CO₂, which proceeds through a radical–radical cross-coupling pathway.

Synthesis of a mononuclear, non-square-planar chromium(ii) bis(alkoxide) complex and its reactivity toward organic carbonyls and CO2

A rare non-square-planar mononuclear Cr(ii) bis(alkoxide) complex Cr(OR′)₂(THF)₂ is reported and its reactivity with organic carbonyls and CO₂ is investigated.

Dioxygen Activation by Non-Adiabatic Oxidative Addition to a Single Metal Center

A chromium(I) dinitrogen complex reacts rapidly with O2 to form the mononuclear dioxo complex [Tp(tBu,Me)Cr(V)(O)2] (Tp(tBu,Me) = hydrotris(3-tert-butyl-5-methylpyrazolyl)borate), whereas the analogous reaction with sulfur stops at the persulfido complex [Tp(tBu,Me)Cr(III)(S2)]. The transformation of the putative peroxo intermediate [Tp(tBu,Me)Cr(III)(O2)] (S = 3/2) into [Tp(tBu,Me)Cr(V)(O)2] (S = 1/2) is spin-forbidden. The minimum-energy crossing point for the two potential energy surfaces has been identified. Although the dinuclear complex [(Tp(tBu,Me)Cr)2(μ-O)2] exists, mechanistic experiments suggest that O2 activation occurs on a single metal center, by an oxidative addition on the quartet surface followed by crossover to the doublet surface.

Mechanism-based design of labile precursors for chromium(I) chemistry

Dinitrogen complexes of the type Tp(R,R)Cr-N2-CrTp(R,R) are not the most labile precursors for Cr(i) chemistry, as they are sterically protected from obligatory associative ligand substitution. A mononuclear alkyne complex - Tp(tBu,Me)Cr(η(2)-C2(SiMe3)2) - proved to be much more reactive.

Quasi-three-coordinate iron and cobalt terphenoxide complexes {Ar(iPr8)OM(μ-O)}2 (Ar(iPr8) = C6H-2,6-(C6H2-2,4,6-(i)Pr3)2-3,5-(i)Pr2; M = Fe or Co) with M(III)2(μ-O)2 core structures and the peroxide dimer of 2-oxepinoxy relevant to benzene oxidation

The bis(μ-oxo) dimeric complexes {Ar(iPr8)OM(μ-O)}2 (Ar(iPr8) = C6H-2,6-(C6H2-2,4,6-(i)Pr3)2-3,5-(i)Pr2; M = Fe (1), Co (2)) were prepared by oxidation of the M(I) half-sandwich complexes {Ar(iPr8)M(η(6)-arene)} (arene = benzene or toluene). Iron species 1 was prepared by reacting {Ar(iPr8)Fe(η(6)-benzene)} with N2O or O2, and cobalt species 2 was prepared by reacting {Ar(iPr8)Co(η(6)-toluene)} with O2. Both 1 and 2 were characterized by X-ray crystallography, UV-vis spectroscopy, magnetic measurements, and, in the case of 1, Mössbauer spectroscopy. The solid-state structures of both compounds reveal unique M2(μ-O)2 (M = Fe (1), Co(2)) cores with formally three-coordinate metal ions. The Fe···Fe separation in 1 bears a resemblance to that in the Fe2(μ-O)2 diamond core proposed for the methane monooxygenase intermediate Q. The structural differences between 1 and 2 are reflected in rather differing magnetic behavior. Compound 2 is thermally unstable, and its decomposition at room temperature resulted in the oxidation of the Ar(iPr8) ligand via oxygen insertion and addition to the central aryl ring of the terphenyl ligand to produce the 5,5'-peroxy-bis[4,6-(i)Pr2-3,7-bis(2,4,6-(i)Pr3-phenyl)oxepin-2(5H)-one] (3). The structure of the oxidized terphenyl species is closely related to that of a key intermediate proposed for the oxidation of benzene.

Reverse Catalase Reaction: Dioxygen Activation via Two-Electron Transfer from Hydroxide to Dioxygen Mediated By a Manganese(III) Salen Complex

Although atmospheric dioxygen is regarded as the most ideal oxidant, O2 activation for use in oxygenation reactions intrinsically requires a costly sacrificial reductant. The present study investigated the use of aqueous alkaline solution for O2 activation. A manganese(III) salen complex, Mn(III)(salen)(Cl), in toluene reacts with aqueous KOH solution under aerobic conditions, which yields a di-μ-oxo dimanganese(IV) salen complex, [Mn(IV)(salen)]2(μ-O)2. The (18)O isotope experiments show that (18)O2 is indeed activated to give [Mn(IV)(salen)]2(μ-(18)O)2 via a peroxide intermediate. Interestingly, the (18)OH(-) ion in H2(18)O was also incorporated to yield [Mn(IV)(salen)]2(μ-(18)O)2, which implies that a peroxide species is also generated from (18)OH(-). The addition of benzyl alcohol as a stoichiometric reductant selectively inhibits the (18)O incorporation from (18)OH(-), indicating that the reaction of Mn(III)(salen)(Cl) with OH(-) supplies the electrons for O2 reduction. The conversion of both O2 and OH(-) to a peroxide species is exactly the reverse of a catalase-like reaction, which has a great potential as the most efficient O2 activation. Mechanistic investigations revealed that the reaction of Mn(III)(salen)(Cl) with OH(-) generates a transient species with strong reducing ability, which effects the reduction of O2 by means of a manganese(II) intermediate.

Light-driven, proton-controlled, catalytic aerobic C-H oxidation mediated by a Mn(III) porphyrinoid complex

The visible light-driven, catalytic aerobic oxidation of benzylic C-H bonds was mediated by a Mn(III) corrolazine complex. To achieve catalytic turnovers, a strict selective requirement for the addition of protons was established. The resting state of the catalyst was unambiguously characterized by X-ray diffraction as [Mn(III)(H2O)(TBP8Cz(H))](+), in which a single, remote site on the ligand is protonated. If two remote sites are protonated, however, reactivity with O2 is shut down. Spectroscopic methods revealed that the related Mn(V)(O) complex is also protonated at the same remote site at -60 °C, but undergoes valence tautomerization upon warming.

Cr(i)Cl as well as Cr+ are stabilised between two cyclic alkyl amino carbenes

Complexes with two and three coordinate chromium(i).

Cr(i)Cl is a very unstable species. The present work describes the stabilisation of Cr(i)Cl in the low coordinate environment of cyclic alkyl(amino) carbene ligands and its synthetic application to yield an unprecedented cationic complex with a two coordinate Cr(i). One electron reduction of (cAAC)₂CrCl₂ (1) with equivalent amount of KC₈ results in the formation of (cAAC)₂CrCl (2), with a distorted trigonal planar configuration at the metal centre. SQUID, EPR and theoretical studies reveal a Cr(i) centre with S = 5/2 spin ground state for 2. It represents the first example of a mononuclear Cr complex showing slow relaxation of magnetisation under an applied magnetic field. The chlorine atom in 2 is expected to be prone to further reactions with appropriate reagents. This qualifies 2 as a promising precursor for the preparation of various interesting complexes with Cr(i) in a low coordinate environment. The first example of this metathesis reaction is observed when 2 is treated with Na[B(C₆H₃(CF₃)₂)₄] resulting in [(cAAC)₂Cr]⁺[B(C₆H₃(CF₃)₂)₄]^–, a linear cationic complex with two coordinate Cr(i) and an S = 5/2 spin ground state.

Iron and chromium complexes containing tridentate chelates based on nacnac and imino- and methyl-pyridine components: triggering C-X bond formation

Nacnac-based tridentate ligands containing a pyridyl-methyl and a 2,6-dialkyl-phenylamine (i.e., (2,6-R2-C6H3N═C(Me)CH═C(Me)NH(CH2py); R = Et, {Et(nn)PM}H; R = (i)Pr, {(i)Pr(nn)PM}H) were synthesized by condensation routes. Treatment of M{N(TMS)2}THFn (M = Cr, n = 2; M = Fe, Co, n = 1; TMS = trimethylsilane; THF = tetrahydrofuran) with {(i)Pr(nn)PM}H) afforded {(i)Pr(nn)PM}MN(TMS)2 (1-M(iPr); M = Cr, Fe); {Et(nn)PM}MN(TMS)2 (1-M(Et); M = Fe, Co) was similarly obtained. {R(nn)PM}FeBr (R = (i)Pr, Et; 2-Fe(R)) were prepared from FeBr2 and {R(nn)PM}Li, and alkylated to generate {R(nn)PM}Fe(neo)Pe (R = (i)Pr, Et; 3-Fe(R)). Carbonylation of 3-Fe(R) provided {(i)Pr(nn)PM}Fe(CO(neo)Pe)CO (4-Fe(iPr)), and carbonylations of 1-Fe(R) (R = Et, (i)Pr) and 1-Cr(iPr) induced deamination to afford {R(nn)PI}Fe(CO)2 (R = (i)Pr, 5-Fe(iPr); Et, 5-Fe(Et)), where PI is pyridine-imine, and {κ(2)-N,N-pyrim-pyr}Cr(CO)4 (6-Cr(iPr)), in which the aryl-amide side of the nacnac attacked the incipient PI group. Carbon-carbon bonds were formed at the imine carbon of the {R(nn)PI} ligand. Addition of [{(i)Pr(nn)PI}(2-)](K(+)(THF)x)2 to FeCl3 generated {(i)Pr(nn)CHpy}2Fe2Cl2 (7-Fe(iPr)), and TMSN3 induced the deamination of 1-Fe(Et), but with disproportionation to provide {[Et(nn)CHpy]2}Fe (8-Fe(Et)). Ph2CN2 induced C-C bond formation with 1-Fe(iPr) via its thermal degradation to ultimately afford {(i)Pr(nn)CHpy}2(FeN═CPh2)2 (9-Fe(iPr)). The compounds were examined by X-ray crystallography (1-M(iPr), M = Cr, Fe; 1-Co(Et); 2-Fe(iPr); 4-Fe(iPr); 5-Fe(iPr); 6-Cr(iPr); 7-Fe(iPr); 8-Fe(Et); 9-Fe(iPr)), Mössbauer spectroscopy, and NMR spectroscopy. Structural parameters assessing redox noninnocence are discussed, as are structural and mechanistic consequences of the various electronic environments.

Iron complexes derived from {nacnac-(CH2py)2}- and {nacnac-(CH2py)(CHpy)}n ligands: stabilization of iron(II) via redox noninnocence

Nacnac-based tetradentate chelates, {nacnac-(CH2py)2}(-) ({nn(PM)2}(-)) and {nacnac-(CH2py)(CHpy)}(n) ({nn(PM)(PI)}(n)) have been investigated in iron complexes. Treatment of Fe{N(TMS)2}2(THF) with {nn(PM)2}H afforded {nn(PM)2}FeN(TMS)2 [1-N(TMS)2], which led to {nn(PM)2}FeCl (1-Cl) from HCl and to {nn(PM)2}FeN3 (1-N3) upon salt metathesis. Dehydroamination of 1-N(TMS)2 was induced by L (L = PMe3, CO) to afford {nn(PM)(PI)}Fe(PMe3)2 [2-(PMe3)2] and {nn(PM)(PI)}FeCO (3-CO). Substitution of 2-(PMe3)2 led to {nn(PM)(PI)}Fe(PMe3)CO [2-(PMe3)CO], and exposure to a vacuum provided {nn(PM)(PI)}Fe(PMe3) (3-PMe3). Metathesis routes to {nn(PM)(PI)}FeL2 (2-L2; L = PMe3, PMe2Ph) and {nn(PM)(PI)}FeL (3-L; L = PMePh2, PPh3) from [{nn(PM)(PI)}(2-)]Li2 and FeBr2(THF)2 in the presence of L proved feasible, and 1e(-) and 2e(-) oxidation of 2-(PMe3)2 afforded 2(+)-(PMe3)2 and 2(2+)-(PMe3)2 salts. Mössbauer spectroscopy, structural studies, and calculational assessments revealed the dominance of iron(II) in both high-spin (1-X) and low-spin (2-L2 and 3-L) environments, and the redox noninnocence (RNI) of {nn(PM)(PI)}(n) [2-L2, 3-L, n = 2-; 2(+)-(PMe3)2, n = 1-; 2(2+)-(PMe3)2, n = 0]. A discussion regarding the utility of RNI in chemical reactivity is proffered.