Corrigendum: Structural snapshots of concerted double E-H bond activation at a transition metal centre.

This corrects the article DOI: 10.1038/nchem.2792.

Structural snapshots of concerted double E-H bond activation at a transition metal centre.

Bond activation at a transition metal centre is a key fundamental step in numerous chemical transformations. The oxidative addition of element-hydrogen bonds, for example, represents a critical step in a range of widely applied catalytic processes. Despite this, experimental studies defining steps along the bond activation pathway are very rare. In this work, we report on fundamental studies defining a new oxidative activation pathway: combined experimental and computational approaches yield structural snapshots of the simultaneous activation of both bonds of a ß-diketiminate-stabilized GaH2 unit at a single metal centre. Systematic variation of the supporting phosphine ligands and single crystal X-ray/neutron diffraction are exploited in tandem to allow structural visualization of the activation process, from a η2-H,H σ-complex showing little Ga-H bond activation, through species of intermediate geometry featuring stretched Ga-H and compressed M-H/M-Ga bonds, to a fully activated metal dihydride featuring a neutral (carbene-type) N-heterocyclic GaI ligand.

A Gallium Hydride as an Oxidizing Agent: Direct Synthesis of IrV Complexes via Ga-H Bond Activation.

Reactions of the ß-diketiminate-stabilized gallium dihydride (Nacnac)Dipp GaH2 with chelating IrI bis(phosphine) precursors under an H2 atmosphere are shown to provide a simple route to IrV complexes stabilized by strongly σ-donating hydrides and the carbene-like (Nacnac)Dipp Ga donor. Characterization of these systems as seven-coordinate IrV tetrahydride species is supported by single crystal X-ray and neutron diffraction, and by T1 NMR measurements. By contrast related systems featuring more sterically demanding (non-chelating) ancillary ligands are better described in terms of a bis(hydride) dihydrogen [L3 Ir(H)2 (H2 )]+ formulation and a formal IrIII oxidation state.

Electronic Delocalization in Two and Three Dimensions: Differential Aggregation in Indium "Metalloid" Clusters.

Reduction of indium boryl precursors to give two- and three-dimensional M-M bonded networks is influenced by the choice of supporting ligand. While the unprecedented nanoscale cluster [In68 (boryl)12 ]- (with an In12 @In44 @In12 (boryl)12 concentric structure), can be isolated from the potassium reduction of a bis(boryl)indium(III) chloride precursor, analogous reduction of the corresponding (benzamidinate)InIII Br(boryl) system gives a near-planar (and weakly aromatic) tetranuclear [In4 (boryl)4 ]2- system.

A zwitterionic hydrocarbon-soluble borenium ion based on a ß-diketiminate backbone.

A versatile synthetic route has been developed to access strongly Lewis acidic borenium cations (and heavier group 13 analogues) featuring a pendant weakly-coordinating borate function. The hydrocarbon-soluble borenium/borate zwitterion is more strongly Lewis acidic than B(C6F5)3, despite featuring a pendant (non-fluorinated) aryl group and two flanking N-donors.

Stabilization of a two-coordinate, acyclic diaminosilylene (ADASi): completion of the series of isolable diaminotetrylenes, :E(NR(2))(2) (E = group 14 element).

An extremely bulky boryl-amide ligand, [N(SiMe3){B(DAB)}](-) (TBoN; DAB = (DipNCH)2, Dip = C6H3Pr(i)2-2,6), has been utilised in the preparation of the first isolable, two-coordinate acyclic diaminosilylene (ADASi), viz. :Si(TBoN)2. This is shown to have a frontier orbital energy separation, and presumed level of reactivity, intermediate between those of the two classes of previously reported isolable two-coordinate, acyclic silylenes.

Bifunctional Indenyl-Derived Receptors for Fluoride Chelation and Detection.

Anion receptors based on a [CpFe(indenyl)] scaffold offer the possibility for the incorporation of adjacent Lewis acidic functions onto a six-membered carbocyclic framework, while at the same time retaining the colorimetric/electrochemical reporter mechanisms available to synthetically simpler ferrocene systems. Thus, [CpFe(indenyl)] systems featuring mutually ortho BMes2 and PPh2 Me(+) substituents (with either 4,5 or 5,6 regiochemistry) are accessible which are capable of cooperative fluoride ion fixation. Simultaneous binding at the borane and phosphonium centres can be established by spectroscopic, structural and computational approaches, and is responsible for the favourable thermodynamics associated with F(-) uptake. Thus, in contrast to simple BMes2 systems, the binding of fluoride is found to be more favourable than the uptake of cyanide (which interacts only with the borane Lewis acid). Moreover, in the case of a 4-(MePh2 P)-5-(Mes2 B)-7-Me-indenyl derivative, fluoride chelation is signalled not only by a large cathodic shift in the Fe(II) /Fe(III) potential (>500 mV in THF), but also by a distinct colour change from green (for the free receptor) to maroon for the adduct.

Extension of conjugation: probing anion binding strength and reporter mechanisms in (phenyl)cyclopentadienyl and indenyl receptors.

Two families of novel organo-iron functionalized boranes have been developed for anion binding/detection. Synthetically simple phenylferrocene systems, while stable to air and moisture, possess similar binding affinities to FcBMes2 and no workable discrimination between F(-) and CN(-). In addition, reduced electronic communication between receptor and reporter sites brought about by a lack of conformational rigidity means that photochemical and electrochemical responses to anion binding are intrinsically weak. Receptors based on a more rigid indenyl framework are available in two steps from organic precursors (in ca. 50% yield), and feature intrinsically higher binding constants for F(-) and especially CN(-), consistent with the lower LUMO energies implied by extended conjugation. One such system, (4-dimesitylboryl-indenyl)cyclopentadienyl-iron(ii), can be shown to bind cyanide in protic media with an accompanying green to red/pink colour change. Extremely high selectivity over fluoride and hydroxide and a detection limit of ca. 10 ppm represent a highly desirable combination among borane derived cyanide receptors.

Utilisation of a lithium boryl as a reducing agent in low oxidation state group 15 chemistry: synthesis and characterisation of an amido-distibene and a boryl-dibismuthene.

The first examples of an amido-distibene, L(†)Sb[double bond, length as m-dash]SbL(†) (L(†) = -N(Ar(†))(SiPr(i)3), Ar(†) = C6H2{C(H)Ph2}2Pr(i)-2,6,4), and a boryl dibismuthene, {(DAB)B}Bi[double bond, length as m-dash]Bi{B(DAB)} (DAB = {(C6H3Pr(i)2-2,6)NCH}2, have been prepared by reaction of a lithium boryl complex, (THF)2LiB(DAB), with extremely bulky amido-group 15 dihalide precursor compounds. In these reactions, the lithium boryl acts as a boryl transfer reagent and/or a strong reducing agent.

Cooperative bond activation and catalytic reduction of carbon dioxide at a group 13 metal center.

A single-component ambiphilic system capable of the cooperative activation of protic, hydridic and apolar HX bonds across a Group 13 metal/activated ß-diketiminato (Nacnac) ligand framework is reported. The hydride complex derived from the activation of H2 is shown to be a competent catalyst for the highly selective reduction of CO2 to a methanol derivative. To our knowledge, this process represents the first example of a reduction process of this type catalyzed by a molecular gallium complex.

Coordination and activation of Al-H and Ga-H bonds.

The modes of interaction of donor-stabilized Group 13 hydrides (E=Al, Ga) were investigated towards 14- and 16-electron transition-metal fragments. More electron-rich N-heterocyclic carbene-stabilized alanes/gallanes of the type NHC⋅EH3 (E=Al or Ga) exclusively generate κ(2) complexes of the type [M(CO)4 (κ(2)-H3 E⋅NHC)] with [M(CO)4 (COD)] (M=Cr, Mo), including the first κ(2) σ-gallane complexes. ß-Diketiminato ('nacnac')-stabilized systems, {HC(MeCNDipp)2 }EH2 , show more diverse reactivity towards Group 6 carbonyl reagents. For {HC(MeCNDipp)2 }AlH2, both κ(1) and κ(2) complexes were isolated, while [Cr(CO)4 (κ(2)-H2 Ga{(NDippCMe)2 CH})] is the only simple κ(2) adduct of the nacnac-stabilized gallane which can be trapped, albeit as a co-crystallite with the (dehydrogenated) gallylene system [Cr(CO)5 (Ga{(NDippCMe)2 CH})]. Reaction of [Co2 (CO)8] with {HC(MeCDippN)2 }AlH2 generates [(OC)3 Co(µ-H)2 Al{(NdippCme)2 CH}][Co(CO)4] (12), which while retaining direct AlH interactions, features a hitherto unprecedented degree of bond activation in a σ-alane complex.

Al-H σ-bond coordination: expanded ring carbene adducts of AlH3 as neutral bi- and tri-functional donor ligands.

Thermally robust expanded ring carbene adducts of AlH3 have been synthesized with a view to probing their ligating abilities via Al-H σ-bond coordination. While κ(2) binding to the 14-electron [Mo(CO)4] fragment is readily demonstrated, interaction with [Mo(CO)3] results in µ:κ(1),κ(1) and µ:κ(2),κ(2) bridging linkages rather than terminal κ(3) binding.

Characterisation of the paramagnetic [2Fe-2S]+ centre in palustrisredoxin-B (PuxB) from Rhodopseudomonas palustris CGA009: g-matrix determination and spin coupling analysis.

Palustrisredoxin-B (PuxB) from Rhodopseudomonas palustris (CGA009) is a [2Fe-2S] ferredoxin which is able to accept electrons from NADH via the flavin-dependent palustrisredoxin reductase (PuR); these electrons can then be transferred to the P450 enzyme (CYP199A2). This work reports on the paramagnetic state of the [2Fe-2S](+) cluster in PuxB, both alone and in the PuR-PuxB complex. Aided by the X-ray crystal structure of PuxB, the protons nearest to the reduced [2Fe-2S](+) cluster were used as magnetic probes to quantify the g-matrix orientation and anisotropic magnetic moment of the paramagnetic centre. (1)H hyperfine couplings were measured with W-band Davies ENDOR and X-band HYSCORE spectroscopy and fitted to a model in which (1)H dipolar couplings were calculated assuming point magnetic moments located at the Fe ions, and bridging and coordinating cysteine sulfur atoms. The absolute sign of a (1)H hyperfine coupling was measured using a variable mixing time ENDOR experiment to confirm the assignment of the Fe(3+) and Fe(2+) ions. For the anti-ferromagnetically coupled cluster the magnetic moment is described in terms of spin projection factors, and our analysis yields values of K(exp)(A) = +2.33 to +1.85 (ferric site), and K(exp)(B) = -1.33 to -0.85 (ferrous site). These values are discussed in terms of the delocalisation of the spin density and hence the limitations of applying a local site spin coupling model to calculate the spin projection factors in a complex with considerable overlap of the α- and ß-spin magnetic oribitals. The accurate description of the g-matrix orientation and magnetic moment of this [2Fe-2S](+) cluster enable it to be utilised as a paramagnetic spin probe, for example, to measure electron-electron distances. In the pdb reference frame of PuxB (code ) the g(∥) axis vector is g(∥) = [-0.6524 ± 0.0248, -0.6269 ± 0.0115, 0.4259 ± 0.0405], with the principal g-values of g(⊥) = 1.9328 ± 0.0003, g(∥) = 2.0233 ± 0.0003.

Reversible C-H activation of a P(t)Bu(i)Bu2 ligand to reveal a masked 12 electron [Rh(PR3)2]+ cation.

[Rh(P(t)Bu(i)Bu(2))(2)][BAr(F)(4)], formed by removal of H(2) from [RhH(2)(P(t)Bu(i)Bu(2))(2)][BAr(F)(4)], is in rapid equilibrium between C-H activated Rh(III) isomers, but reacts as a masked 12-electron [Rh(P(t)Bu(i)Bu(2))(2)](+) Rh(I) cation.