Cyclooctadiene Rh(I) Bis- and Tris(pyrazolyl)aluminate Complexes and Their Catalytic Activity on the Polymerization of Phenylacetylene.

In this work, we report the transfer of alkyl bis- and tris(pyrazolyl)aluminates metalloligands to an electron-rich organotransition metal center. The 16-electron heterobimetallic complexes of rhodium [Rh(COD){Al(Ph2pz)2Me2}] (3) and [Rh(COD){Al(Ph2pz)3Me}] (4) were obtained by metathesis reaction of the sodium bis- (1) and tris(pyrazolyl)aluminate (2) with [RhCl(COD)]2. For 3, 1H and 13C NMR in solution along with DFT calculations are consistent with a κ2-coordination mode of the bis(pyrazolyl)aluminate to a square-planar Rh(I) center. The X-ray structure of 4 shows a similar κ2-coordination mode of the tris(pyrazolyl)aluminate to Rh(I) with a pendant pyrazolyl moiety. The attempted synthesis of aluminate-rhodium complexes with R = CF3, tBu on the pyrazolate ring afforded [Rh(R2pz)(COD)]2 and [R2pzAlMe2]2. Complexes 3 and 4 were investigated as homogeneous catalysts in the polymerization of phenylacetylene (PA). Both complexes showed enhanced catalytic activity compared to analogous rhodium poly(pyrazolyl)borates. Optimized gas-phase DFT geometries of 3, 4, [Rh(COD){B(Ph2pz)2Me2}], and [Rh(COD){B(Ph2pz)3Me}] were used to compare bite angles, while DFT geometries of 3-CO, 4-CO, [Rh(CO)2{B(Ph2pz)2Me2}], and [Rh(CO)2{B(Ph2pz)3Me}] were employed to probe the electronic situation of the rhodium center through IR CO stretching modes. The wider bite angles and the less electron-rich rhodium center of the poly(pyrazolyl)aluminates compared with their borate analogues could be implicated in the better performance of the active catalytic species during polymerization of PA.

Frustrated Lewis Pair Chemistry Enables N2 Borylation by Formal 1,3-Addition of a B-H Bond in the Coordination Sphere of Tungsten.

The first example of a formal 1,3-B-H bond addition across the M-N≡N unit of an end-on dinitrogen complex has been achieved. The use of Piers' borane HB(C6 F5 )2 was essential to observe this reactivity and it plays a triple role in this transformation: 1) electrophilic N2 -borylation agent, 2) Lewis acid in a frustrated Lewis pair-type B-H bond activation, and 3) hydride shuttle to the metal center. This chemistry is supported by NMR spectroscopy and solid-state characterization of products and intermediates. The combination of chelate effect and strong σ donation in the diphosphine ligand 1,2-bis(diethylphosphino)ethane was mandatory to avoid phosphine dissociation that otherwise led to complexes where borylation of N2 occurred without hydride transfer.

Enhanced Activation of Coordinated Dinitrogen with p-Block Lewis Acids.

This Concept article highlights recent research on Lewis acid adducts of dinitrogen complexes, including our contributions. After a reminder of the early works, it is demonstrated that such kind of species offers a new platform for dinitrogen functionalization as well as valuable models for the understanding of elementary steps of (bio)catalytic cycles. When possible, parallels regarding this mode of activation from the orbital point of view are drawn between the different systems discussed herein.

Direct Synthesis of Heavy Grignard Reagents: Challenges, Limitations, and Derivatization.

The direct synthesis of organocalcium compounds (heavy Grignard reagents) by the reduction of organyl halides with activated calcium powder succeeded in a straightforward manner for organic bromides and iodides that are bound at sp2 -hybridized carbon atoms. Extension of this strategy to alkyl halides was very limited, and only the reduction of trialkylsilylmethyl bromides and iodides with activated calcium allowed the isolation of the corresponding heavy Grignard reagents. Substitution of only one hydrogen atom of the methylene moiety by a phenyl or methyl group directed this reduction toward the Wurtz-type coupling and the formation of calcium halide and the corresponding C-C coupling product. The stability of the methylcalcium and benzylcalcium derivatives in ethereal solvents suggests an unexpected reaction behavior of the intermediate organyl halide radical anions. Quantum chemical calculations verify a dependency between the ease of preparative access to organocalcium complexes and the C-I bond lengths of the organyl iodides. The bulkiness of the trialkylsilyl group is of minor importance. Chloromethyltrimethylsilane did not react with activated calcium; however, halogen-exchange reactions allowed the isolation of [Ca(CH2 SiMe3 )(thf)3 (µ-Cl)]2 . Furthermore, the metathetical approach of reacting [Ca(CH2 SiMe3 )I(thf)4 ] with KN(SiMe3 )2 and the addition of N,N,N',N'',N''-pentamethyldiethylenetriamine (pmdeta) allowed the isolation of heteroleptic [CaCH2 SiMe3 {N(SiMe3 )2 }(pmdeta)]. In the reaction of this derivative with phenylsilane, the trimethylsilylmethyl group proved to be more reactive than the bis(trimethylsilyl)amido substituent.

Triangles and Squares for a Unique Molecular Crystal Structure: Unsupported Two-Coordinate Lithium Cations and CC Agostic Interactions in Cyclopropyllithium Derivatives.

Understanding and controlling the aggregation state is germane to alkyllithium chemistry. Lewis base-free alkyllithium compounds normally form tetrahedral tetramers or octahedral hexamers in the solid state with the lithium cations being three-coordinate. We report that the unsupported cyclopropyl derivative 1-(trimethylsilyl)cyclopropyllithium [{µ-c-C(SiMe3 )C2 H4 }Li]4 (1), synthesized by the reduction of 1-(phenylthio)-1-(trimethylsilyl)cyclopropane, crystallizes as a tetramer in the space group I-4 with the two-coordinate lithium atoms forming a square. CC agostic interactions complete the coordination sphere around each lithium. The aggregate is preserved in hydrocarbon solution. Furthermore, CC agostic interactions compete intra- and intermolecularly with Lewis base donation as in the unsaturated dimer of 1-(phenylthio)cyclopropyllithium [Li(thf)2 {µ-c-C(SPh)C2 H4 }2 Li (thf)] (3) which is also fully characterized.

Measuring the Relative Reactivity of the Carbon-Hydrogen Bonds of Alkanes as Nucleophiles.

We report quantitative measurements of the relative reactivities of a series of C-H bonds of gaseous or liquid Cn H2n+2 alkanes (n=1-8, 29 different C-H bonds) towards in situ generated electrophiles (copper, silver, and rhodium carbenes), with methane as the reference. This strategy surpasses the drawback of previous model reactions of alkanes with strong electrophiles suffering from C-C cleavage processes, which precluded direct comparison of the relative reactivities of alkane C-H bonds.

Coupling and Dearomatization of Pyridines at a Transient η2 -Cyclopropene/Bicyclobutane Zirconocene Complex.

This paper reports on stereospecific coupling reactions between an η2 -cyclopropene ligand and pyridine derivatives, which are preferred to alternative C-H bond activation reactions. The dicyclopropylzirconocene complex [Cp2 Zr(c-C3 H5 )2 ] (1) eliminates cyclopropane to generate the η2 -cyclopropene/bicyclobutane intermediate [Cp2 Zr(η2 -c-C3 H4 )] (A). A does not activate any pyridine C-H bonds, but rather pyridine inserts into a Zr-C bond of A, yielding an azazirconacycle with a dearomatized pyridyl group [Cp2 Zr{κ2 -N,C8 -(2-c-C3 H4 )-C5 H5 N}] (2). Kinetic data, isotopelabelling experiments, and DFT calculations indicate that the rate-determining step of this stereospecific reaction is cyclopropane elimination, and that the stability of the intermediate [Cp2 Zr(η2 -c-C3 H4 )(NC5 H5 )] (A-py) governs the selectivity of the reaction. Complex 2 tautomerizes to [Cp2 Zr{κ2 -N,C8 -(2-C3 H5 )-C5 H4 N}] (6) through a base-catalyzed proton migration accompanied by cyclopropyl opening and restoration of conjugation within the zirconacycle.

Supramolecular organization of perfluorinated 1H-indazoles in the solid state using X-ray crystallography, SSNMR and sensitive (VCD) and non sensitive (MIR, FIR and Raman) to chirality vibrational spectroscopies.

1H-Indazole derivatives exhibit a remarkable property since some of them form chiral supramolecular structures starting from achiral monomers. The present work deals with the study of three perfluorinated 1H-indazoles that resolve spontaneously as conglomerates. These conglomerates can contain either a pure enantiomer (one helix) or a mixture of both enantiomers (both helices) with an enantiomeric excess (e.e.) of one of them. The difficulty of the structural analysis of these types of compounds is thus clear. We outline a complete strategy to determine the structures and configurations (M or P helices) of the enantiomers (helices) forming the conglomerates of these perfluorinated 1H-indazoles based on X-ray crystallography, solid state NMR spectroscopy and different solid state vibrational spectroscopies that are either sensitive (VCD) or not (FarIR, IR and Raman) to chirality, together with quantum chemical calculations (DFT).

Group†6 Transition-Metal/Boron Frustrated Lewis Pair Templates Activate N2 and Allow its Facile Borylation and Silylation.

The reaction of trans-[M(N2 )2 (dppe)2 ] (M=Mo, 1Mo , M=W, 1W ) with B(C6 F5 )3 (2) provides the adducts [(dppe)2 M=N=N-B(C6 F5 )3 ] (3) which can be regarded as M/B transition-metal frustrated Lewis pair (TMFLP) templates activating dinitrogen. Easy borylation and silylation of the activated dinitrogen ligands in complexes 3 with a hydroborane and hydrosilane occur by splitting of the B-H and Si-H bonds between the N2 moiety and the perfluoroaryl borane. This reactivity of 3 is reminiscent of conventional frustrated Lewis pair chemistry and constitutes an unprecedented approach for the functionalization of dinitrogen.

Alkaline Earth-Olefin Complexes with Secondary Interactions.

Strontium and calcium (alkaline earth: Ae) olefin complexes stabilised by secondary Ae⋅⋅⋅F-C and ß-agostic Ae⋅⋅⋅H-Si interactions are presented. Olefin coordination onto the alkaline earths is plain in the solid state, and it is thermodynamically favoured over the coordination of THF. The existence of the Ae⋅⋅⋅olefin interactions is corroborated by solution NMR data and DFT computations. The coordination mode of the olefin varies with steric effects and, if enforced, olefin dissociation can be compensated by the other non-covalent interactions, as supported by DFT computations.

CH Bond Activation of Methane by a Transient η(2)-Cyclopropene/Metallabicyclobutane Complex of Niobium.

This study challenges the problem of the activation of a CH bond of methane by soluble transition metal complexes. High pressure solution NMR, isotopic labeling studies, and kinetic analyses of the degenerate exchange of methane in the methyl complex [Tp(Me2)NbCH3(c-C3H5)(MeCCMe)] (1) are reported. Stoichiometric methane activation by the mesitylene complex [Tp(Me2)Nb(CH2-3,5-C6H3Me2)(c-C3H5) (MeCCMe)] (2) giving 1 is also realized. Evidence is provided that these reactions proceed via an intramolecular abstraction of a ß-H of the cyclopropyl group to form either methane or mesitylene from 1 or 2, respectively, yielding the transient unsaturated η(2)-cyclopropene/metallabicyclobutane intermediate [Tp(Me2)Nb(η(2)-c-C3H4) (MeCCMe)] A. This is followed by its mechanistic reverse 1,3-CH bond addition of methane yielding the product.

Copper-Carbene Intermediates in the Copper-Catalyzed Functionalization of O-H Bonds.

Copper-carbene [Tp(x)Cu=C(Ph)(CO2Et)] and copper-diazo adducts [Tp(x)Cu{η(1)-N2C(Ph)(CO2Et)}] have been detected and characterized in the context of the catalytic functionalization of O-H bonds through carbene insertion by using N2=C(Ph)(CO2Et) as the carbene source. These are the first examples of these type of complexes in which the copper center bears a tridentate ligand and displays a tetrahedral geometry. The relevance of these complexes in the catalytic cycle has been assessed by NMR spectroscopy, and kinetic studies have demonstrated that the N-bound diazo adduct is a dormant species and is not en route to the formation of the copper-carbene intermediate.

Highly fluorinated tris(indazolyl)borate silylamido complexes of the heavier alkaline earth metals: synthesis, characterization, and efficient catalytic intramolecular hydroamination.

Heteroleptic silylamido complexes of the heavier alkaline earth elements calcium and strontium containing the highly fluorinated 3-phenyl hydrotris(indazolyl)borate {F12-Tp(4Bo, 3Ph)}(-) ligand have been synthesized by using salt metathesis reactions. The homoleptic precursors [Ae{N(SiMe3)2}2] (Ae = Ca, Sr) were treated with [Tl(F12-Tp(4Bo, 3Ph))] in pentane to form the corresponding heteroleptic complexes [(F12-Tp(4Bo, 3Ph))Ae{N(SiMe3)2}] (Ae = Ca (1); Sr (3)). Compounds 1 and 3 are inert towards intermolecular redistribution. The molecular structures of 1 and 3 have been determined by using X-ray diffraction. Compound 3 exhibits a Sr⋅⋅⋅MeSi agostic distortion. The synthesis of the homoleptic THF-free compound [Ca{N(SiMe2H)2}2] (4) by transamination reaction between [Ca{N(SiMe3)2}2] and HN(SiMe2H)2 is also reported. This precursor constitutes a convenient starting material for the subsequent preparation of the THF-free complex [(F12-Tp(4Bo, 3Ph))Ca{N(SiMe2H)2}] (5). Compound 5 is stabilized in the solid state by a Ca⋅⋅⋅ß-Si-H agostic interaction. Complexes 1 and 3 have been used as precatalysts for the intramolecular hydroamination of 2,2-dimethylpent-4-en-1-amine. Compound 1 is highly active, converting completely 200 equivalents of aminoalkene in 16 min with 0.50 mol % catalyst loading at 25 °C.

Understanding protected area resilience: a multi-scale, social-ecological approach.

Protected areas (PAs) remain central to the conservation of biodiversity. Classical PAs were conceived as areas that would be set aside to maintain a natural state with minimal human influence. However, global environmental change and growing cross-scale anthropogenic influences mean that PAs can no longer be thought of as ecological islands that function independently of the broader social-ecological system in which they are located. For PAs to be resilient (and to contribute to broader social-ecological resilience), they must be able to adapt to changing social and ecological conditions over time in a way that supports the long-term persistence of populations, communities, and ecosystems of conservation concern. We extend Ostrom's social-ecological systems framework to consider the long-term persistence of PAs, as a form of land use embedded in social-ecological systems, with important cross-scale feedbacks. Most notably, we highlight the cross-scale influences and feedbacks on PAs that exist from the local to the global scale, contextualizing PAs within multi-scale social-ecological functional landscapes. Such functional landscapes are integral to understand and manage individual PAs for long-term sustainability. We illustrate our conceptual contribution with three case studies that highlight cross-scale feedbacks and social-ecological interactions in the functioning of PAs and in relation to regional resilience. Our analysis suggests that while ecological, economic, and social processes are often directly relevant to PAs at finer scales, at broader scales, the dominant processes that shape and alter PA resilience are primarily social and economic.

Intramolecular C-C agostic complexes: C-C sigma interactions by another name.

In this comprehensive review the developments in the synthesis, characterization and reactivity of complexes (s-, d- and f-block) in which a C-C single bond interacts with a metal centre are discussed: so called C-CM agostic complexes. Such species are of significant interest with regard to structure and bonding, the activation of C-C single bonds and, thus, catalytic methods of C-C bond formation (or activation). Examples of C-C agostic complexes of early and later transition metals, actinides and group 1 metals are discussed, along with C-C agostic interactions in metallacyclobutanes. Examples of Si-Si∙∙∙M, B-C∙∙∙M and B-B∙∙∙M agostic interactions are also presented. Throughout, the structural, spectroscopic and computational markers that indicate the likely presence of a C-C∙∙∙M agostic interaction in a complex are highlighted.

Catalytic functionalization of methane and light alkanes in supercritical carbon dioxide.

The development of catalytic methods for the effective functionalization of methane yet remains a challenge. The best system known to date is the so-called Catalytica Process based on the use of platinum catalysts to convert methane into methyl bisulfate with a TOF rate of 10(-3) s. In this contribution, we report a series of silver complexes containing perfluorinated tris(indazolyl)borate ligands that catalyze the functionalization of methane into ethyl propionate upon reaction with ethyl diazoacetate (EDA) by using supercritical carbon dioxide (scCO2) as the reaction medium. The employment of this reaction medium has also allowed the functionalization of ethane, propane, butane, and isobutane.

Functionalization of non-activated C-H bonds of alkanes: an effective and recyclable catalytic system based on fluorinated silver catalysts and solvents.

The complexes F(n)-Tp(4Bo,3Rf)Ag(L) (F(n)-Tp(4Bo,3Rf)=a perfluorinated hydrotris(indazolyl) borate ligand; L=acetone or tetrahydrofuran) efficiently catalyze the functionalization of non-activated alkanes such as hexane, 2,3-dimethylbutane, or 2-methylpentane by insertion of CHCO(2)Et units (from N(2)CHCO(2)Et, ethyl diazoacetate, EDA) into their C-H bonds. The reactions are quantitative (EDA-based), with no byproducts derived from diazo coupling being formed. In the case of hexane, the functionalization of the methyl C-H bonds has been achieved with the highest regioselectivity known to date with this diazo compound. This catalytic system also operates under biphasic conditions by using fluorous solvents such as Fomblin or perfluorophenanthrene. Several cycles of catalyst recovery and reuse have been performed, with identical chemo- and regioselectivities.

Highly fluorinated aryl-substituted tris(indazolyl)borate thallium complexes: diverse regiochemistry at the B-N bond.

The synthesis and characterization (mainly by (19)F NMR and X-ray diffraction) of highly fluorinated aryl-4,5,6,7-tetrafluoroindazoles and their corresponding thallium hydrotris(indazolyl)borate complexes are reported [aryl = phenyl, pentafluorophenyl, 3,5-dimethylphenyl, 3,5-bis(trifluoromethyl)phenyl]. Thanks to N-H···N hydrogen bonds, the indazoles crystallize as dimers that pack differently depending on the nature of the aryl group. The thallium hydrotris(indazolyl)borate complexes Tl[Fn-Tp(4Bo,3aryl)] resulting from the reaction of aryl-4,5,6,7-tetrafluoroindazoles [aryl = phenyl, 3,5-dimethylphenyl, 3,5-bis(trifluoromethyl)phenyl] with thallium borohydride adopt overall C(3v) symmetry with the indazolyl groups bound to boron via their N-1 nitrogen in a conventional manner. When the perfluorinated pentaphenyl-4,5,6,7-tetrafluoroindazole is reacted with thallium borohydride, a single regioisomer of C(s) symmetry having one indazolyl ring bound to boron via its N-2 nitrogen, TlHB(3-pentafluorophenyl-4,5,6,7-tetrafluoroindazol-1-yl)(2)(3-pentafluorophenyl-4,5,6,7-tetrafluoroindazol-2-yl) Tl[F27-Tp((4Bo,3C6F5)*)], is obtained for the first time. Surprisingly, the perfluorinated dihydrobis(indazolyl)borate complex Tl[F(18)-Bp(3Bo,3C6F5)], an intermediate on the way to the hydrotris(indazolyl)borate complex, has C(s) symmetry with two indazolyl rings bound to boron via N-2. The distortion of the coordination sphere around Tl and the arrangement of the complexes in the crystal are discussed.

C-H bond activation of benzene by unsaturated η2-cyclopropene and η2-benzyne complexes of niobium.

We report the synthesis of a niobium cyclopropyl complex, Tp(Me2)NbMe(c-C(3)H(5))(MeCCMe), and show that thermal loss of methane from this compound generates an intermediate that is capable of activating both aliphatic and aromatic C-H bonds. Isotopic labeling, trapping studies, a detailed kinetic analysis, and density functional theory all suggest that the active intermediate is an η(2)-cyclopropene complex formed via ß-hydrogen abstraction rather than an isomeric cyclopropylidene species. C-H activation chemistry of this type represents a rather unusual reactivity pattern for η(2)-alkene complexes but is favored in this case by the strain in the C(3) ring which prevents the decomposition of the key intermediate via loss of cyclopropene.

Tryptophan metabolism and related B vitamins in the multiparous sow fed ad libitum after farrowing.

Although dietary content of tryptophan has been related to variations of feed intake in lactating sows, the mechanisms remain to be elucidated. Twenty multiparous crossbred Landrace × Large White sows were used to assess variations of tryptophan metabolism around farrowing. Sows were fed 3 kg/d of a standard gestation diet from insemination until farrowing. They were then fed ad libitum a standard lactation diet until weaning. Sows were catheterised on day 70 of gestation and blood samples were drawn on day 37 before parturition, daily during the week before and the week after farrowing, and on days 14 and 21 of lactation. Plasma concentrations of amino acids, kynurenine, niacin, haptoglobin, urea, and vitamin B6 concentration in red blood cells were determined. During the week following parturition, plasma tryptophan and niacin decreased while plasma kynurenine increased (p < 0.05). On the 2nd and 3rd weeks of lactation, plasma tryptophan and kynurenine returned to pre-farrowing concentrations, while niacin increased throughout lactation (p < 0.05). Vitamin B6 increased progressively during the week after farrowing (p < 0.05) and remained constant at a high concentration thereafter. The average feed intake of the sow during lactation was positively correlated with the mean concentrations of niacin (r(2) = 0.25; p < 0.001) and kynurenine (r(2) = 0.31; p < 0.001) in plasma and with vitamin B6 in red blood cells (r(2) = 0.68; p < 0.001). This study suggests that tryptophan catabolism presumably through the kynurenine pathway is high during the 1st week after farrowing, and that dietary supply of niacin and vitamin B6 could be transiently suboptimal in early lactation.