1,2-Palladasilacyclobutene: The Missing Link in the Pd-Catalyzed Annulation of Alkynes for the Silirene-to-Silole Transformation.

The palladium-catalyzed annulation reaction of alkynes enables an attractive approach to siloles. Their access from silirenes and terminal alkynes proved rather general, involving reactive intermediates that have remained elusive to date. Starting from 1,2-bis(3-thienyl)silirene as a source of photochromic siloles, the mechanism of the annulation reaction has been revisited, and palladasilacyclobutenes resulting from the activation of the silirene could be isolated and thoroughly characterized (NMR, X-ray, and DFT). Their role as reactive intermediates and their fate in the course of the reaction were also studied in situ. In combination with in-depth DFT calculations, a clearer picture of the mechanism and the reactive key species is disclosed.

An Experimental and Computational Investigation Rules Out Direct Nucleophilic Addition on the N2 Ligand in Manganese Dinitrogen Complex [Cp(CO)2 Mn(N2 )].

We have re-examined the reactivity of the manganese dinitrogen complex [Cp(CO)2 Mn(N2 )] (1, Cp=η5 -cyclopentadienyl, C5 H5 ) with phenylithium (PhLi). By combining experiment and density functional theory (DFT), we have found that, unlike previously reported, the direct nucleophilic attack of the carbanion onto coordinated dinitrogen does not occur. Instead, PhLi reacts with one of the CO ligands to provide an anionic acylcarbonyl dinitrogen metallate [Cp(CO)(N2 )MnCOPh]Li (3) that is stable only below -40 °C. Full characterization of 3 (including single crystal X-ray diffraction) was performed. This complex decomposes quickly above -20 °C with N2 loss to give a phenylate complex [Cp(CO)2 MnPh]Li (2). The latter compound was erroneously formulated as an anionic diazenido compound [Cp(CO)2 MnN(Ph)=N]Li in earlier reports, ruling out the claimed and so-far unique behavior of the N2 ligand in 1. DFT calculations were run to explore both the hypothesized and the experimentally verified reactivity of 1 with PhLi and are fully consistent with our results. Direct attack of a nucleophile on metal-coordinated N2 remains to be demonstrated.

Synthesis of Molecular Phenylcalcium Derivatives: Application to the Formation of Biaryls.

Hydrocarbon-soluble ß-diketiminato phenylcalcium derivatives, which display various modes of Ca-µ2 -Ph-Ca bridging, are accessible from reactions of Ph2 Hg and [(BDI)CaH]2 . Although the resultant compounds are inert toward the C-H bonds of benzene, they yield selective and uncatalyzed biaryl formation when reacted with readily available aryl bromides.

Synthesis, Characterization, and Comparative Theoretical Investigation of Dinitrogen-Bridged Group 6-Gold Heterobimetallic Complexes.

We have prepared and characterized a series of unprecedented group 6-group 11, N2-bridged, heterobimetallic [ML4(η1-N2)(µ-η1:η1-N2)Au(NHC)]+ complexes (M = Mo, W, L2 = diphosphine) by treatment of trans-[ML4(N2)2] with a cationic gold(I) complex [Au(NHC)]+. The adducts are very labile in solution and in the solid, especially in the case of molybdenum, and decomposition pathways are likely initiated by electron transfers from the zerovalent group 6 atom to gold. Spectroscopic and structural parameters point to the fact that the gold adducts are very similar to Lewis pairs formed out of strong main-group Lewis acids (LA) and low-valent, end-on dinitrogen complexes, with a bent M-N-N-Au motif. To verify how far the analogy goes, we computed the electronic structures of [W(depe)2(η1-N2)(µ-η1:η1-N2)AuNHC]+ (10W+) and [W(depe)2(η1-N2)(µ-η1:η1-N2)B(C6F5)3] (11W). A careful analysis of the frontier orbitals of both compounds shows that a filled orbital resulting from the combination of the π* orbital of the bridging N2 with a d orbital of the group 6 metal overlaps in 10W+ with an empty sd hybrid orbital at gold, whereas in 11W with an sp3 hybrid orbital at boron. The bent N-N-LA arrangement maximizes these interactions, providing a similar level of N2 "push-pull" activation in the two compounds. In the gold case, the HOMO-2 orbital is further delocalized to the empty carbenic p orbital, and an NBO analysis suggests an important electrostatic component in the µ-N2-[Au(NHC)]+ bond.

Grafting of Lanthanum Complexes on a Functionalized Graphene Surface: Theoretical Investigation on Ethylene and 1,3-Butadiene Homo- and Co-Polymerization.

In this contribution, we describe the use of graphene as an efficient catalyst support and the role it plays in increasing the Lewis acidity of the supported metal complexes. By a density functional theory study, we show that the [La(N(SiMe3 )2 )3 ] complex can be easily grafted on graphene-OH and -COOH functionalized surfaces. Two stable mono-grafted compounds, (gO)-[La(N(SiMe3 )2 )2 ] and (gOO)-[La(N(SiMe3 )2 )2 ], are formed, behaving as stronger Lewis acids than the previously reported silica grafted analogues. To study the role of the graphene support in catalysis, we also computed the catalytic activity of the alkylated (gO)-[La(CH3 )2 ] and (gOO)-[La(CH3 )2 ] complexes in the ethylene and 1,3-butadiene homo- and co-polymerization reactions. Both compounds are efficient catalysts for the homo-polymerization of the ethylene and 1,3-butadiene. For the 1,3-butadiene homo-polymerization, the stereoselectivity outcome of the reaction differs according to the grafting site. The results computed for the co-polymerization reaction, finally, show that the high stability of the allylic products leads to the formation of block copolymers.

Calcium Hydride Reduction of Polycyclic Aromatic Hydrocarbons.

A molecular calcium hydride effects the two electron reduction of polyaromatic hydrocarbons, including naphthalene (E0 =-3.1 V).

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.

Heterolysis of Dihydrogen by Nucleophilic Calcium Alkyls.

ß-Diketiminato (BDI) calcium alkyl derivatives undergo hydrogenolysis with H2 to regenerate [(BDI)CaH]2 , allowing the catalytic hydrogenation of a wide range of 1-alkenes and norbornene under very mild conditions (2 bar H2 , 298 K). The reactions are deduced to take place with the retention of the dimeric structures of the calcium hydrido-alkyl and alkyl intermediates via a well-defined sequence of Ca-H/C=C insertion and Ca-C hydrogenation events. This latter deduction is strongly supported by DFT calculations (B3PW91) performed on the 1-hexene/H2 system, which also indicates that the hydrogenation transition states display features which discriminate them from a classical σ-bond metathesis mechanism. In particular, NBO analysis identifies a strong second order interaction between the filled α-methylene sp3 orbital of the n-hexyl chain and the σ* orbital of the H2 molecule, signifying that the H-H bond is broken by what is effectively the nucleophilic displacement of hydride by the organic substituent.

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.

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.

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.

Variable Ca-Caryl Hapticity and its Consequences in Arylcalcium Dimers.

The dimeric ß-diketiminato calcium hydride, [(Dipp BDI)CaH]2 (Dipp BDI = HC{(Me)CN-2,6-i-Pr2 C6 H3 }2 ), reacts with ortho-, meta- or para-tolyl mercuric compounds to afford hydridoarylcalcium compounds, [(Dipp BDI)2 Ca2 (µ-H)(µ-o-,m-,p-tolyl)], in which dimer propagation occurs either via µ2 -η1 -η1 or µ2 -η1 -η6 bridging between the calcium centers. In each case, the orientation and hapticity of the aryl units is dependent upon the position of the methyl substituent. While wholly organometallic meta- and para-tolyl dimers, [(Dipp BDI)Ca(m-tolyl)]2 and [(Dipp BDI)Ca(p-tolyl)]2 , can be prepared and are stable, the ortho-tolyl isomer is prone to isomerization to a calcium benzyl analog. Computational analysis of this latter process with density functional theory (DFT) highlights an unusual mechanism invoking the generation of an intermediate dicalcium species in which the group 2 centers are bridged by a toluene dianion formed by the formal attachment of the original hydride anion to the initially generated ortho-tolyl substituent. Use of a more sterically encumbered aryl substituent, {3,5-t-Bu2 C6 H3 }, facilitates the selective formation of [(Dipp BDI)Ca(µ-H)(µ-3,5-t-Bu2 C6 H3 )Ca(Dipp BDI)], which can be converted into the unsymmetrically-substituted σ-aryl calcium complexes, [(Dipp BDI)Ca(µ-Ph)(µ-3,5-t-Bu2 C6 H3 )Ca(Dipp BDI)] and [(Dipp BDI)Ca(µ-p-tolyl)(µ-3,5-t-Bu2 C6 H3 )Ca(Dipp BDI)] by reaction with the appropriate mercuric diaryl. Conversion of [(Dipp BDI)Ca(H)(Ph)Ca(Dipp BDI)] to afford [{{(Dipp BDI)Ca}2 (µ2 -Cl)}2 (C6 H5 -C6 H5 )], comprising a biphenyl dianion, is also reported. Although this latter transformation is serendipitous, AIM analysis highlights that, in a related manner to the ortho-tolyl to benzyl isomerization, the requisite C-C coupling may be facilitated in an "across dimer" fashion by the experimentally-observed polyhapto engagement of the aryl substituents with each calcium.

Interplay between cationic and neutral species in the rhodium-catalyzed hydroaminomethylation reaction.

The reactivity of [Rh(CO)(2){(R,R)-Ph-BPE}]BF(4) (2) toward amine, CO and/or H(2) was examined by high-pressure NMR and IR spectroscopy. The two cationic pentacoordinated species [Rh(CO)(3) {(R,R)-Ph-BPE}]BF(4) (4) and [Rh(CO)(2)(NHC(5)H(10)){(R,R)-Ph-BPE}]BF(4) (8) were identified. The transformation of 2 into the neutral complex [RhH(CO)(2){(R,R)-Ph-BPE}] (3) under hydroaminomethylation conditions (CO/H(2), amine) was investigated. The full mechanisms related to the formation of 3, 4 and 8 starting from 2 are supported by DFT calculations. In particular, the pathway from 2 to 3 revealed the deprotonation by the amine of the dihydride species [Rh(H)(2)(CO)(2){(R,R)-Ph-BPE}]BF(4) (6), resulting from the oxidative addition of H(2) on 2.

Hydroboration of vinyl halides with mesitylborane: a direct access to (mesityl)(alkyl)haloboranes.

The direct access to (mesityl)(alkyl)haloboranes (Mes(Alk)BX) (X = Br, Cl) from mesitylborane dimer and vinyl halides is presented. The involved hydroboration reaction results in the transfer of the halogen atom from the carbon of the starting material to the boron in the final product. The reactivity of the obtained Mes(Alk)BX has been evaluated for the synthesis of the bipyridyl boronium cations and 2-arylpyridine derived boron N^C-chelates. The formation mechanism of Mes(Alk)BX is apprended by DFT-calculations which shows that their formation involves two concomitant pathways derived from the regioslectivity of the hydroboration reaction.

Mechanistic investigations via DFT support the cooperative heterobimetallic C-H and O-H bond activation across Ta[double bond, length as m-dash]Ir multiple bonds.

A rare heterobimetallic oxidative addition of X-H (X = C, O) bonds is reported. DFT suggests that steric constraints around the bimetallic core play a critical role to synergistically activate C-H bonds across the two metals and thus explains the exceptional H/D exchange catalytic activity of unhindered surface organometallic Ta/Ir species observed experimentally.

Olefin epoxidation by H2O2/MeCN catalysed by cyclopentadienyloxidotungsten(VI) and molybdenum(VI) complexes: experiments and computations.

Compounds [Cp*(2)M(2)O(5)] (M = Mo, 1; W, 2) are efficient pre-catalysts for cyclooctene (COE) epoxidation by aqueous H(2)O(2) in acetonitrile/toluene. The reaction is quantitative, selective and takes place approximately 50 times faster for the W system (k(obs) = 4.32(9)x10(-4) s(-1) at 55 degrees C and 3x10(-3) M concentration for the dinuclear complex, vs. 1.06(7)x10(-5) s(-1) for the Mo system). The rate law is first order in catalyst and COE substrate (k = 0.138(7) M(-1) s(-1) for the W system at 55 degrees C), whereas increasing the concentration of H(2)O(2) slows down the reaction because of an inhibiting effect of the greater amount of water. The activation parameters for the more active W systems (DeltaH(double dagger) = 10.2(6) kcal mol(-1); DeltaS(double dagger) = -32(2) cal mol(-1) K(-1)) were obtained from an Eyring study in the 25-55 degrees C temperature range. The H(2)O(2)urea adduct was less efficient as an oxidant than the aqueous H(2)O(2) solution. Replacement of toluene with diethyl ether did not significantly affect the catalyst efficiency, whereas replacement with THF slowed down the process. The epoxidation of ethylene as a model olefin, catalysed by the [Cp*MO(2)Cl] systems (M = W, Mo) in the presence of H(2)O(2) as oxidant and acetonitrile as solvent, has been investigated by DFT calculations with the use of the conductor-like polarisable continuum model (CPCM). For both metal systems, the rate-limiting step is the transfer of the hydroperoxido O(alpha) atom to the olefin, in accordance with the first-order dependence on the substrate and the zero-order dependence on H(2)O(2) found experimentally in the catalytic data. The activation barrier corresponding to the rate-limiting step is 4 kcal lower for the W complex than for the corresponding Mo analogue (32.3 vs. 28.3 kcal mol(-1)). This result reproduces well the higher catalytic activity of the W species. The different catalytic behaviour between the two systems is rationalised by a natural bond orbital (NBO) study and natural population analyses (NPA). Compared to Mo, the W(VI) centre withdraws more electron density from the sigma bonding [O-O] orbital and favours, as a consequence, the nucleophilic attack of the external olefin on the sigma*[O-O] orbital.

Probing the stereo-electronic properties of cationic rhodium complexes bearing chiral diphosphine ligands by 103Rh NMR.

(103)Rh NMR represents a powerful tool to assess the global electronic and steric contribution of diphosphine ligands on [Rh(COD)(diphosphine)](+) complexes. In the case of DIOP, BINAP and MeDUPHOS, this approach proved to be more informative than classical CO-stretching frequency measurements. After validation, this method has been extended to a set of seven diphosphines. (103)Rh NMR measurements on [Rh(COD)(diphosphine)]PF(6) lead to the following order of donor properties: dppe > MeBPE > MeDUPHOS > dppb > DIOP > BINAP > Tol-BINAP. This trend has been validated by DFT in the case of DIOP, BINAP and MeDUPHOS. In conjunction, (31)P NMR chemical shift has been shown to reflect the ring constraints of the Rh-diphosphine scaffold. This contribution is a step towards a mechanistic investigation of the catalytic hydrogenation of unsaturated substrates by (103)Rh NMR and DFT.

Which Methodological Practice(s) for Psychotherapy Science? A Systematic Review and a Proposal.

Three main methodological practices have been employed in psychotherapy science: the empirical-quantitative, empirical-qualitative, and theoretical one. Some scholars have argued for a monopoly of the empirical-quantitative methodological practice, sustained by scientific monism. This systematic review aims at exploring the contribution of each methodological practice to the field. Fifteen journals were searched from 2003 to 2013. A total of 9796 publications and 9915 studies met inclusion criteria and were coded for the methodological practice employed. Empirical-quantitative studies were the most published overall, over time, and in most of the journals considered. This overwhelming prevalence increased when considering the citation rates of the publications. We argue that these results are indicative of a quantitative monopoly, which in turn is due to a lack of critical thinking sustaining scientific monism. We call for disciplinary critical thinking and the consequent scientific pluralism valuing the plurality, diversity, and multiplicity of all the existing methodological practices.

Correction to: Which Methodological Practice(s) for Psychotherapy Science? A Systematic Review and a Proposal.

We would like to note the following corrections to the original version of Zhang, Sun, Lin and Ren.