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The thermodynamics of ion pair symmetrization in a series of metallocenium species generated from Cp 2 ″ ZrMe 2 ( Cp ″ = 1 , 2 - Me 2 C 5 H 3 ) were studied using a variety of solution dynamic techniques including line broadening, 2D-EXSY, and 1D-DPFGSE-NOE. Ion pairs were generated by methide abstraction using the corresponding trityl salts [1-A] to yield [ Cp 2 ″ ZrMe ] + [ A ] - (A = {C6F4-1,2-[B(C6F5)2]2(μ-O(C6F5))}−, 2-O(C6F5); {C6F4-1,2-[B(C6F5)2]2(μ-OPh)}−, 2-OPh; {C6F4-1,2-[B(C6F5)2]2(μ-OMe)}−, 2-OMe; and [B(C6F5)4]−, 2-B(C6F5)4). The observed activation parameters were interpreted on the basis of a solvent-assisted mechanism of ion pair symmetrization.

Equimolar reactions of LScMe2 (L = (Ar)NC(tBu)CHC(tBu)N(Ar); Ar = 2,6-iPr2-C6H3) and HB(C6F5)2 proceed through an isolable ion pair 2 to a metalated scandium borate 3 with loss of methane. Multinuclear NMR experiments confirm the proposed structure, which was also verified by synthesis via alternative routes and derivatization. Deuterium labeling studies offer insight into the mechanism of methane loss, which occurs through C−H activation of an abstracted methide rather than the direct intramolecular metalation commonly observed for β-diketiminato scandium alkyl complexes. A large primary kinetic isotope effect of kH/kD = 8.7(6) was observed for the decomposition of 2, which corroborates the proposed mechanism while also implicating a highly reactive four-membered scandocycle intermediate, LSc((H)CH2B(C6F5)2). Reaction of 2 equiv of HB(C6F5)2 with LScR2 yields μ2-hydridoborate complexes [LScCH3][(μ-H)2B(C6F5)2] (5), while 4 equiv of borane react to afford the bis-μ2-hydridoborate complex [LSc][(μ-H)2B(C6F...

A family of isolable solvent separated organoscandium methyl cat- ions stabilized by b-diketiminato li- gands (Ar)NCA3)CHCA3)N(Ar) (Ar = 2,6-iPr-C6H3 ,L Me ) has been pre- pared by reaction of L Me ScR2 with (CPh3)(BA6F5)4) in the presence of an arene solvent. Arenes such as bromo- benzene, benzene, toluene, para-xylene and mesitylene bind the scandium center in an h 6 -bonding mode, yielding cations 1a -e. Their solution and solid- state structures have been explored using multinuclear NMR spectroscopy and X-ray crystallography. Mechanistic studies on arene exchange reactions and the insertion of diphenylacetylene indicate that these processes occur via arene intermediates of lower hapticity, followed by binding of the incoming re- agent. Which of the two steps is rate limiting depends on the arene being displaced and/or the nature of the in- coming substrate. The experiments present a unified view of these mecha- nisms, which have relevance to propa- gation processes in olefin polymeri- zations mediated by such cations.

[structure: see text]. Phenanthrene analogues with internalized B-N moieties were found to afford blue light emission with good quantum efficiencies, whereas the isomeric species with peripheral B-N moieties displayed only UV emission behavior, like the all-carbon framework.

The phosphonium alkylidene [(NHC)Cl2Ru=CH(PCy3)]+[B(C6F5)4]-, 1, (NHC = N-heterocyclic carbene, Cy = cyclohexyl, C6H11) reacts with 2.2 equiv of ethylene at -50 degrees C to form the 14-electron ruthenacyclobutane (NHC)Cl2Ru(CH2CH2CH2), 2. NMR spectroscopic data indicates that 2 has a C2v symmetric structure with a flat, kite shaped ruthenacyclobutane ring with significant Calpha-Cbeta agostic interactions with the Ru center. Intramolecular exchange of Calpha and Cbeta is fast (14(2) s-1 at 223 K) as measured by EXSY spectroscopy. Intermolecular exchange of Calpha and Cbeta with the methylene groups of free ethylene is much slower and first order in both [Ru] and [H2C=CH2] (4.8(3) x 10-4 M-1 s-1). Activation parameters for this process are DeltaH++ = 13.2(5) kcal mol-1 and DeltaS++ = -15(2) cal mol-1 K-1, also consistent with a rate limiting associative substitution as the key step in this exchange process. On the basis of this observation, mechanisms for the intermolecular exchange process are proposed and the implications for the mechanism of the propagation steps in catalytic olefin metathesis as mediated by Grubbs catalysts are discussed.

Accepted version of article deposited April 11, 2016 as per http://www.rsc.org/journals-books-databases/journal-authors-reviewers/licences-copyright-permissions/#deposition-sharing, Polycyclic aromatic hydrocarbons in which one or more CC units have been replaced by isoelectronic BN units have attracted interest as potentially improved organic materials in various devices. This promise has been hampered by a lack of access to gram quantities of these materials. However, the exploitation of keystone reactions such as ring closing metathesis, borylative cyclization of amino styrenes and electrophilic borylation has lead to strategies for access to workable amounts of material. These strategies can be augmented by judicious postfunctionalization reactions to diversify the library of materials available. This Frontier article highlights some of the recent successes and shows that the long promised applications of BN-doped PAHs are beginning to be explored in a meaningful way.

A series of molecules capable of releasing of the strong organometallic Lewis acid B(C6F5)3 upon exposure to 254 nm light have been developed. These photo Lewis acid generators (PhLAGs) can now serve as photoinitiators for several important B(C6F5)3-catalyzed reactions. Herein is described the synthesis of the triphenylsulfonium and diphenyliodonium salts of carbamato- and hydridoborates, their establishment as PhLAGs, and studies aimed at defining the mechanism of borane release. Factors affecting these photolytic reactions and the application of this concept to photoinduced hydrosilylation reactions and construction of siloxane scaffolds are also discussed.

Welcome to this themed issue of Dalton Transactions in the area of earth abundant element (EAE) compounds in homogeneous catalysis.

Cationic tungsten(V) methylidynes [L4W(X)[triple bond]CH]+[B(C6F5)4]- [L = PMe3, 0.5dmpe (dmpe = Me2PCH2CH2PMe2), X = Cl, OSO2CF3] have been prepared in high yield by a one-electron oxidation of the neutral tungsten(IV) methylidynes L4W(X)[triple bond]CH with [Ph3C]+[B(C6F5)4]-. The ease and reversibility of the one-electron oxidation of L4W(X)[triple bond]CH were demonstrated by cyclic voltammetry in tetrahydrofuran (E1/2 is approximately -0.68 to -0.91 V vs Fc). The paramagnetic d1 (S = 1/2) complexes were characterized in solution by electron spin resonance (g = 2.023-2.048, quintets due to coupling to 31P) and NMR spectroscopy and Evans magnetic susceptibility measurements (mu = 2.0-2.1 muB). Single-crystal X-ray diffraction showed that the cationic methylidynes are structurally similar to the neutral precursor methylidynes. In addition, the neutral (PMe3)4W(Cl)[triple bond]CH was deprotonated with a strong base at the trimethylphosphine ligand to afford (PMe3)3(Me2PCH2)W[triple bond]CH, a tungsten(IV) methylidyne complex that features a (dimethylphosphino)methyl ligand.

The synthesis of four ruthenium phosphonium alkylidene complexes [(H2IMes)Cl2Ru CH(PCy3)]+[A]− (1, A = B(C6F5)4; 2, A = BF4; 3, A = OTf; 4, A = BPh4), differing only in the anion is described. The X-ray structures of 1, 3 and 4 show them to be isostructural in the cation, with no interaction between the Ru centers and the anion. Ring closing metathesis of a substrate to a six-membered methylcyclohexene at 0 °C in CD2Cl2 using 1 mol% catalyst, shows that catalysts 1–4 behave very similarly, and exhibit superior activity in comparison to Grubbs second generation and fast-initiating catalysts.

Treatment of the β-diketiminato-supported scandium amido methyl derivatives [ArNC(tBu)CHC(tBu)NAr]Sc(Me)(NHR) (Ar = 2,6-iPr2-C6H3; R = tBu, 2,6-iPr2-C6H3) with 1 equiv of tris(pentafluorophenyl)borane resulted in the formation of scandium amido ion pairs, 1a and 1b, respectively, in good yields. A single-crystal X-ray analysis of 1a was undertaken, and two crystallographically independent molecules were observed in which one contains an N−H···F−C hydrogen bond while the other does not. Several attempts to deprotonate the amido ligand to yield a neutral imido species resulted in either attack of the base at the metal center or deprotonation of the β-diketiminato ligand. For example, treatment of 1a with tert-butyllithium gave [ArNC(tBu)CHC(tBu)NAr]Sc(CMe3)(NHtBu) (2), while treatment with the phosphazene base [(Me2N)3PN]3PNtBu gave the previously characterized metalated complex 3.

Dianionic 2,2′-diboratabiphenyl derivatives were prepared as a new class of binucleating ligands. Preliminary investigations into their coordination chemistry involved Rh complexes. Their solution and solid state structures were studied and confirm the preference for η6–η6 coordination of the novel ligands, rendering them isoelectronic analogues of the ubiquitous bicyclopentadienyl system. Comparative studies suggest stronger donor properties for the fulvalene ligand as compared with the 2,2′-diboratabiphenyl system.Key words: dinucleating ligands, bimetallic complexes, rhodium complexes, boron heterocycles, X-ray structure.

A practical synthetic route for the preparation of the known bifunctional perfluoroaryldiborane C6F4-1,2-[B(C6F5)2]2, 1, and the new diborane C6F4-1,2-[B(C12F8)]2, 2, that circumvents the use of toxic mercury reagents has been developed. Key intermediates and 2 have been fully characterized in solution and the solid state. Solution and structural investigations of their MeCN and THF adducts have shown that 2 is a superior Lewis acid to 1, primarily owing to the reduction in steric pressure afforded by the planar 9-borafluorene rings in 2 versus the freely rotating set of pentafluorophenyl rings in 1. The two compounds bind Lewis bases via differing coordination modes, as demonstrated by the solid-state structures of the Lewis base adducts 1·MeCN, 2·THF, and 2·MeCN. Solution studies suggest that bis Lewis base adducts are also accessible; the structures of two examples (1·(MeCN)2 and 2·(THF)2) have been determined, and their role in the dynamic solution behavior of these systems is discussed.

Perfluorinated 9-phenyl-9-borafluorene, 1, is an antiaromatic analog of the well-known tris(pentafluorophenyl)borane. Spectroscopic, structural, and electrochemical studies have been performed on 1 and its Lewis base adducts with MeCN, THF, and PMe3 with a view to assessing its comparative Lewis acid strength relative to B(C6F5)3. For the sterically undemanding Lewis base MeCN, 1 and B(C6F5)3 have comparable LA strengths, while for more sterically prominent THF, 1 is clearly the stronger Lewis acid (LA) based on competition experiments. We conclude that steric factors, rather than antiaromaticity, are the most important determinants in the LA strength differences between 1 and B(C6F5)3.Key words: boranes, Lewis acids, fluorinated compounds, heterocycles.

Calculations utilizing anionic substituted derivates of the cationic N(wedge)N--Ni(II) and Pd(II) diimine Brookhart complex have been carried out on the barriers of ethylene and acrylonitrile insertion into a M- methyl, propyl and CH(CN)Et bond for M = Ni, Pd. The possibility of side reactions such as chelate formation with the polar functionality and oligomerization of the active species after acrylonitrile insertion are explored. The diimine ring system N--N = -NR' 'CR(1)CR(2)NR' ' with R' ' = 2,6-C(6)H(3)(i-Pr)(2) and R(1),R(2) = Me was functionalized by adding one or two anionic groups (BF(3)(-), etc.) in place of i-Pr on the aryl rings or by replacing one Me diimine backbone group (R(1)) with BH(3)(-). The objective of this investigation is computationally to design catalysts for ethylene/acrylonitrile copolymerization that have activities that are comparable to that of the cationic Ni(II) diimine or at least the Pd(II) diimine Brookhart system for ethylene homopolymerization. Complexes that might meet this objective are discussed.

Kationische Borverbindungen sind instabile und hoch elektrophile Spezies, die eine Schlusselrolle in der Chemie des Bors einnehmen. Obwohl man sich schon sehr fruh fur die Chemie dieser Verbindungen interessierte, blieben die meisten bis in die jungste Zeit hinein chemische Kuriositaten. Allerdings summierten sich auch allmahlich die Hinweise auf ihr Potenzial als starke Elektrophile, und die Entwicklungen im Bereich der schwach koordinierenden Anionen liesen es lohnend erscheinen, dieses Teilgebiet der Borchemie genauer zu erforschen. Seit der letzten umfassenden Ubersicht uber kationische Borverbindungen sind fast 20 Jahre vergangen; die seit damals erzielten Fortschritte werden in diesem Aufsatz zusammengefasst.

Calculations have been carried out on the complexation energy of acrylonitrile and ethylene using derivatives of the cationic N∧N Ni(II)−diimine Brookhart complex. For acrylonitrile (CH2CHCN) considerations were given to both π binding through the olefinic CC functionality and σ binding through the NC group. The diimine ring system N∧N = −NR‘ ‘CR1CR2NR‘ ‘ with R‘ ‘ = 2,6-C6H3(i-Pr)2 and R1, R2 = CH3 was functionalized by substituting a carbon in the diimine ring by B- and R1 or R2 (or both) by anionic groups (BF3-, OBF3-, etc.). Substitutions were also carried out at the aryl rings by replacing H or i-Pr with anionic groups. The objective has been to find substitutions that would reduce or eliminate the preference for σ complexation of acrylonitrile, a prerequisite for ethylene/acrylonitrile copolymerization.

A rare family of base-free organoscandium alkyl cations stabilized by β-diketiminato ligands (Ar)NC(R)CHC(R)N(Ar) (Ar = 2,6-iPr-C6H3; R = CH3 (LMe), R = tBu (LtBu)) has been prepared by reaction of...

The reactions of a series of Pd(II) methyl compounds of general formula LPd(NCCH3)CH3, where L is a bulky phenoxydiazene or phenoxyaldimine ligand with the polar olefin acrylonitrile (AN), are reported. The compounds react with an excess of AN to give the products of 2,1 insertion into the Pd−Me bond, yielding dimers and/or trimers which feature bridging α-cyano groups. The reactions were studied by low temperature 1H NMR spectroscopy, revealing an initial formation of compounds featuring N-bound AN, which isomerized to an (unobserved) π-bound species that rapidly underwent 2,1 insertion into the Pd−Me bond. Intermediate oligomeric complexes retaining a Pd−Me function were observed at low [AN] in these reactions. Under pseudo first-order conditions, kobs values of 8.5 × 10-5 to 2.68 × 10-3 M-1 (−22 °C to 10 °C, 100 equiv of AN) and activation parameters of ΔH‡ = 14.4(5) kcal mol-1 and ΔS‡ = −19(5) eu were obtained in one case. Comparison of the overall rates of insertion between two LPd(NCCH3)CH3, differi...

The scandium-catalyzed intramolecular hydroamination of alkynes and alkenes is reported. Complex structure/catalyst activity investigations resulted in the identification of a highly catalytically active cationic, β-diketiminato scandium complex.

Treatment of the β-diketiminato-supported scandium dichlorides {[ArNC(R)CHC(R)NAr]-ScCl 2 } n (Ar = 2,6- i Pr 2 -C 6 H 3 ; R = CH 3 , la, n = 2; R = t Bu, 1b, n = 1) with 1 equiv of a lithium amide reagent LiN(H)R' (R' = t Bu, 2,6- i Pr 2 -C 6 H 3 ) gave scandium amido derivatives. For la, use of LiN(H) t Bu leads to the bis-amido derivative (6a) regardless of the equivalency of amide reagent employed, suggesting that facile ligand redistribution processes are operative when the ligand is the less bulky methyl-substituted example. For 1b, mono-amido chlorides 2b (R' = t Bu) and 3b (R' = 2,6- i Pr 2 -C 6 H 3 ) are obtained in good yields, and these compounds can be alkylated with MeLi to provide mono-amido methyl compounds 4b (R' = t Bu) and 5b (R' = 2,6- i Pr 2 -C 6 H 3 ). All four of these compounds were characterized crystallographically. The amido ligand occupies the exo coordination site exclusively, and there is no evidence in solution for a diastereomer with the amido group in the endo site. DFT calculations suggest that there is a strong steric preference and a slight electronic bias for the amido ligand to assume the exo position. Thermolysis of the amido methyl complex 4b leads to loss of CH 4 and production of a scandacylic product, 7, formed via metalation with one of the N-aryl isopropyl methyl groups. This compound was characterized crystallographically. Deuterium labeling experiments suggest that 7 is produced via direct metalation and does not form via a scandium imido intermediate.

A new chelating N,N ligand family incorporating an anilido−phosphinimine donor set has been designed and an example prepared in multigram quantities in three steps using a convenient, modular synthetic approach. The ligand 1-(NHAr)-2-(PPh2NAr‘)C6H4 (1·H; Ar = 2,6-iPr2-C6H3; Ar‘ = 2,4,5-Me3C6H2) was prepared in three steps in a 30−35% overall yield and was fully characterized in its protio form as well as in the form of its lithium salt, 1·Li. In both compounds, the donor array assumes a conformation in which the PN moiety dips out of the plane formed by the four atoms in the rigid ortho-disubstituted aryl ring of the ligand backbone. The coordinating properties of the new ligand were demonstrated by the synthesis of some organoaluminum derivatives. The monomeric compounds LAlMe2 (2) and LAlH2 (3) were prepared by the reaction of 1·H with AlMe3 and AlH3·NMe3, respectively, and were fully characterized by NMR spectroscopy and X-ray crystallography. Cationic derivatives were formed by activation of 2 and 3 w...

Treatment of the oligomeric compound [Cp*ScCl2]n (Cp* = C5Me5) with tri-tert-butylphosphine oxide in THF leads to the monocyclopentadienyl scandium dichloride [Cp*(t-Bu3P=O)ScCl2] as a monomeric, THF-free solid in 72% yield. This compound can be alkylated with MeLi to give the corresponding dimethyl derivative, [Cp*(t-Bu3P=O)ScMe2], 1. The crystal structures of these two compounds have been done and reveal a three-legged piano stool geometry about the scandium center. The coordinated phosphine oxide ligand exchanges with free donor in both compounds via an associative mechanism, as indicated by the activation parameters (ΔH‡ = 6.4–8.4 kcal mol–1-; ΔS‡ = –22.5 to –26.6 eu). Thus, the Cp*(t-Bu3P=O) ligand set provides a stable platform for organoscandium chemistry. This was demonstrated by the activation of 1 by treatment with B(C6F5)3 to form the fully characterized contact ion pair [Cp*(t-Bu3P=O)ScMe]+[MeB(C6F5)3]–, 2, which is an active ethylene polymerization catalyst.Key words: organoscandium compounds, olefin polymerization, cationic complexes.

A practical and high yielding synthetic route was developed for the preparation of optically pure (R)-(+)-2-bis(pentafluorophenyl)boryl-, (R)-(+)-1a,b, and (R)-2-difluoroboryl-1,1'-binaphthyls, (R)-5a,b. Chiral boranes (R)-la and (R)-5a are active catalysts for the allylstannation of aromatic aldehydes although enantioselectivities for R products were low. A structural investigation of the adduct (′)-1a.o-anisaldehyde suggested that substituents in the 2'-position of the binaphthyl backbone could increase enantiofacial discrimination. Consistent with this structural study, the 2'-Me derivative (R)-1b gave slightly higher enantioselectivities and opposite configuration S of homoallylic alcohol products.

The perfluoroaryl borane B(C6F5)3 is an effective catalyst for a variety of organic transformations. In the hydrosilation of carbonyl functions, it activates the silane rather than the carbonyl substrate. In allylstannation reactions, two competing reaction pathways are observed, one involving tin cation catalysis, the other "true" borane catalysis. For B(C6F5)3, the former mechanism dominates, while for the weaker Lewis acid PhB(C6F5)2, the latter pathway is more prominent. Thus, chiral boranes of similar Lewis acid strength to PhB(C6F5)2 have the potential to mediate asymmetric allylstannation of aldehyde substrates.

The bright red title compound 1 was synthesized from (2-lithiophenyl)diphenylamine and bis(pentafluorophenyl)boron chloride. Its reactions with small acids like H2O and HCl proceeded easily giving zwitterionic compounds. For 1 and its water adduct 2 the crystal structures were determined, the latter featuring an ammonium borate structure containing a short intramolecular hydrogen bond bridge. Treatment of 1 with Jutzi's acid, [H(OEt2)2][B(C6F5)4], did not result in protonation of the nitrogen, but reaction of 1 with LiH in the presence of 12-crown-4, led to the isolation of the aminoborate [1-(Ph2N)-2-{B(H)(C6F5)2}C6H4][Li(12-crown-4)] (3). Borohydride 3 reacted with Jutzi's acid to regenerate 1 and liberate hydrogen.

The reactivity of highly Lewis acidic perfluorinated borafluorenes C12F8BR (R = C6F5, 1a; CH3, 1b) and the nonfluorinated 9-phenyl-9-borafluorene (2) toward [Cp*Al]4 was investigated. The reaction of 1 with [Cp*Al]4 leads to the formation of thermally robust η1 Lewis acid−base adducts 3a,b as the thermodynamically favored products. Use of the less Lewis acidic 2 does not alter the mode of reactivity, with the η1 Lewis acid−base 4 formed preferentially. Reduction of 2 to the 9-boratafluorene 2·Li2(THF)n is readily accomplished in THF solution. However, reaction of 2·Li2(THF)n with [Cp*AlCl2]2 or Cp*AlCl2(THF), 5, affords aluminum metal, 2·THF, and Cp*H as the main identifiable products. Compounds 3a, 3b, 4, and 5 were fully characterized including their X-ray structures. A DFT computational study was conducted to probe the reason for the strong preference for η1 bonding, which essentially stems from the localization of aromaticity in the flanking phenyl rings in the 9-borafluorene ring system.

The synthesis of copper and silver silyl complexes containing either N-heterocyclic carbenes or nitrogen donors is described. Alterations made to both the neutral donor ligands as well as the silyl group provided access to a number of different compounds. Many of the complexes synthesized were studied in the solid state and the effect of the donor ligand on the final structure of the complexes was examined. The thermal properties of the complexes were explored using thermogravimetric analysis, differential scanning calorimetry and sublimations. Some of the complexes synthesized were demonstrated to be promising volatile metal precursors.

Over the past decade, several groups have targeted well-defined organometallic Group 3 metal complexes with ancillary ligand supporting environments alternative to the ubiquitous bis-Cp donor set. In addition to a desire to develop the fundamental organometallic chemistry of Group 3 bis-alkyl derivatives, these compounds are of interest as catalyst precursors for olefin and lactide polymerization processes, as well as olefin hydrosilation, amination and hydrogenation cycles. This review surveys the non-Cp organometallic chemistry of the ligands which have so far been explored for this purpose, commenting on the fine balance of steric and electronic properties necessary to stabilize monomeric, base-free organometallic compounds of these metals. While bona fide organometallic compounds have not been explicitly prepared in all cases, promising ligands in this regard are also included. The review covers the literature from about 1994 forward, encompassing 35 new ligand systems.

Reaction of McConville's chelating amido titanium complex [(Ar)NCH2CH2CH2N(Ar)]Ti(CH3)2 (Ar = 2,6-i-Pr2C6H3) with either elemental Se or the tellurium atom source Te=PBu3 resulted in the formation of bis-µ-chalcogenido dimers [(Ar)NCH2CH2CH2N(Ar)]2Ti(µ-E)2 (E = Se, 2; Te, 3) with concommitant loss of EMe2. The dimers 2 and 3 were characterized spectroscopically and via X-ray crystallography. The two compounds are isostructural in the solid state. The tellurido dimer 3 may also be synthesized by reduction of the diamido dichloride [(Ar)NCH2CH2CH2N(Ar)]2TiCl2 with Na–Hg amalgam followed by treatment with Te=PBu3. This dimer is unreactive toward further Te=PBu3 or stannanes such as HSnBu3. Unlike decamethyltitanocene derivatives, the diamido complex is not an effective catalyst precursor for the heterohydrodecoupling of Te=PBu3 and HSnBu3.Key words: diamido titanium complexes, selenides, tellurides.

The 1,4 hydrosilation of a variety of simple α,β-unsaturated enones as catalyzed by B(C 6 F 5 ) 3 (1–2%) is described. For substrates with no steric hindrance near the β-carbon, 1,4 addition of silane is very clean; in other instances, 1,2 hydrosilation is competitive. The reaction is facile with five commercially available silane reagents. For two examples, a novel hydrosilation of the resulting silylenol ethers was also observed. The net trans stereochemistry of H–Si addition to the silylenol ether C C bond was established and points to a stepwise mechanism for this reaction. This was supported by the observation and full spectroscopic characterization of a silylcarboxonium ion intermediate with an [HB(C 6 F 5 ) 3 ] − counteranion in the hydrosilation of the silylenol ether derived from 4,4-dimethyl-2-cyclohexen-1-one and PhMe 2 SiH.

Organometallic derivatives of scandium and yttrium supported by a bulky salicylaldiminato ligand were prepared from the tris(alkyl) precursors [M(CH2SiMe2R)3(THF)2] (M = Sc, 1-ScR; M = Y, 1-YR; R = CH3, Ph) via alkane elimination. The new precursors 1-MPh are convenient alternatives to the 1-MMe derivatives, due to their higher thermal stability and crystallinity; 1-ScPh has been characterized crystallographically. Reaction of these compounds with 1 equiv of protio ligand gives isolable mono(ligand) bis(alkyl) derivatives only for 1-Y; for other derivatives 1-Sc, mixtures were obtained. The products of the former reaction retain either two (2-YPh) or one (3-YPh) THF ligands; both of these compounds have been characterized crystallographically. In solution, both exhibit fluxional exchange between two geometric isomers which were characterized by variable-temperature NMR spectroscopy. Heating solutions of either compound leads to facile ligand redistribution. Reactions of tris(alkyls) 1 with 2 equiv of prot...

Trapping of the tantalocene borataalkene complex Cp2Ta[CH2B(C6F5)2] with 2 equiv of cyclohexyl or benzyl isocyanide gives Ta(III) products where the metallocene fragment is ligated by an η3-1-azaal...

Adducts of the N-benzyl imines Ph(R)CNBn (R = Ph, CH3, H) and tBu(CH3)CNBn and the N-phenyl imine Ph(H)CNPh with the Lewis acid tris(pentafluorophenyl)borane, B(C6F5)3, have been prepared and characterized in solution and in the solid state. For each imine, the Lewis acid is N-bound, with the exception of the sterically demanding base tBu(CH3)CNBn, which reacts with B(C6F5)3 through its enamine tautomer to form an α-C bound adduct, 5. In the N-bound imine−borane adducts 1−4, steric crowding and π-stacking between C6F5 and C6H5 rings results in restricted rotation about the B−N and B−C bonds; the dynamic behavior which results can be studied using variable-temperature 19F and 1H NMR spectroscopy. For the unsymmetrical ketimines and aldimines, kinetic adducts of the thermodynamically dominant free imine are observed to form initially upon treatment with B(C6F5)3; these adducts slowly convert thermally to the thermodynamic adducts of the less stable imine. For the ketimine Ph(CH3)CNBn, both the kinetic adduc...

Mechanistic experiments suggest that the sequestration of free tris(pentafluorophenyl)borane by the hemilabile carboxylate contributes to the selectivity observed and prevents over reduction to methane.

Perfluorarylborane Lewis acids catalyse the addition of silicon-hydrogen bonds across C=C, C=N and C=O double bonds. This 'metal-free' hydrosilylation has been proposed to occur via borane activation of the silane Si-H bond, rather than through classical Lewis acid/base adducts with the substrate. However, the key borane/silane adduct had not been observed experimentally. Here it is shown that the strongly Lewis acidic, antiaromatic 1,2,3-tris(pentafluorophenyl)-4,5,6,7-tetrafluoro-1-boraindene forms an observable, isolable adduct with triethylsilane. The equilibrium for adduct formation was studied quantitatively through variable-temperature NMR spectroscopic investigations. The interaction of the silane with the borane occurs through the Si-H bond, as evidenced by trends in the Si-H coupling constant and the infrared stretching frequency of the Si-H bond, as well as by X-ray crystallography and theoretical calculations. The adduct's reactivity with nucleophiles demonstrates conclusively the role of this species in metal-free 'frustrated-Lewis-pair' hydrosilylation reactions.

Use of pentamethylcyclopentadienyl ligands (C_5Me_5) in place of cyclopentadienyl (Cp) provides reactive, but thermally robust monomelic 14-electron scandocene derivatives. A useful starting material for the synthesis of a variety of derivatives of permethylscandocene is the chloride complex. All experiments discussed in this chapter are conducted under argon in a glove box or a high-vacuum line. The chapter describes the procedures for the preparation and properties of bis(η^5-pentamethylcyclopentadienyl) chloroscandium, bis(η^5-pentamethylcyclopentadienyl) methylscandium, bis(η^5-pentamethylcyclopentadienyl) phenylscandium, and bis(η^5-pentamethylcyclopentadienyl)(o-tolyl) scandium.

Reaction of [Ph3C]+[C6F4-1,2-{B(C6F5)2}2(μ-OCH3)]-, 1, with Bu3SnH gives the solvated stannylium ion [Bu3Sn(arene)]+[C6F4-1,2-{B(C6F5)2}2(μ-OCH3)]-, 2, as a light brown oil (δ 119Sn = 434 ppm). This material is thermodynamically stable toward transfer of the chelated OMe- anion to “Bu3Sn+” as evidenced by the reaction of free diborane C6F4-1,2-[B(C6F5)2]2 with 2 equiv of Bu3SnOMe. This reaction produces the stannyloxonium complex [(Bu3Sn)2OCH3]+[C6F4-1,2-{B(C6F5)2}2(μ-OCH3)]-, 3 (δ 119Sn = 277 ppm), which is stable at −60 °C. Upon warming, the cation in 3 undergoes decomposition to unidentified products, while the anion remains intact. Ion pair 2 reacts rapidly with ethereal HCl in CH2Cl2 to generate Bu3SnCl and the oxonium acid [(Et2O)2H]+[C6F4-1,2-{B(C6F5)2}2(μ-OCH3)]-, 4, isolated in 76% yield as a white, crystalline solid. Acid 4 is thermally stable in solution and was characterized crystallographically. The C2B2OMe core of the anion in 4 deviates from planarity due to intermolecular interactions in t...

Reaction of Cp2Ta(CH2)3 with 2 equiv of HB(C6F5)2 results in production of the dihydride Cp2Ta(CH2B(C6F5)2)(μ-H)(H), 1, plus 1 equiv of H3CB(C6F5)2. The pathway to 1 involves stepwise attack of bor...

Several diorganoscandium complexes stabilized by the β-diketiminato ligands (Ar)NC(R)CHC(R)N(Ar) (Ar = 2,6-iPr-C6H3; R = CH3 (ligand a), R = tBu (ligand b)) have been synthesized. Reaction of the lithium salts of the ligands with ScCl3·3THF leads to the complexes LScCl2(THF)n, which may be readily alkylated to form the dialkyl derivatives. Most are isolated as base-free, four-coordinate complexes. Several have been characterized via X-ray crystallography, and a detailed discussion of their structures is presented. Steric interactions between Ar and the Sc−alkyl groups force the scandium to adopt an out-of-plane bonding mode. In solution, this is manifested via a fluxional process which equilibrates the two diastereotopic alkyl groups and ligand groups as well. The barriers to this process roughly correlate with the steric bulk of the alkyl substituents. At elevated temperatures, the dialkyl derivatives LScR2 undergo a metalation process whereby one of the alkyl groups is eliminated as RH, and a ligand iPr...

Bis-(pentafluorophenyl)boryl ferrocene, 1, was prepared via borylation of ferrocene with HB(C6F5)2 or via a transmetallation reaction involving FcHgCl and ClB(C6F5)2 in 87–91% yield. The compound is characterized by a deep maroon colour. A significant intramolecular iron–boron interaction is manifested in the solution spectroscopic (Fe [Formula: see text] B charge transfer band at ~230 nm, ε = 1.33 × 104) and solid-state crystallographic data (Fe-B = 2.924 Å) This interaction has an impact on the Lewis acidity of the boron center which, unlike the related compound B(C6F5)3, does not strongly bind Lewis bases such as acetone, THF, or acetonitrile. However, an adduct between the stronger base PMe3 and 1 forms readily and this complex (2) was fully characterized. The electron withdrawing -B(C6F5)2 group causes 1 to be oxidized at +450 mV relative to ferrocene. Oxidation of 1 with [NO][BF4], AgOSO2CF3, or AgC6F5 leads to the zwitterionic ferrocenium borates 3-F, 3-OTf, and 3-C6F5, respectively. Each of these compounds was characterized spectroscopically and via X-ray crystallography. The properties of these compounds relative to 1 suggest that oxidation of the iron center significantly enhances the Lewis acidity of the boron center. Due to the σ-donating ability of the borate substituents, zwitterions 3 are weaker oxidizing agents than unsubstituted ferrocenium salts.Key words: organoboranes, ferrocene derivatives, Lewis acids.

The reactions of the monocyclopentadienyl titanium dimethyl compounds Cp(L)TiMe2 (L = tBu2CN; Cp = C5H5, 1a; C5Me5, 1b; C5Me4SiMe3, 1c) with the trityl borate activator [Ph3C]+[B(C6F5)4]- are described. Formation of μ-methyl dimers of formula {[Cp(L)TiMe]2(μ-Me)}+[B(C6F5)4]- as a 1:1 mixture of rac and meso diastereomers is observed when 0.5 equiv of [Ph3C]+[B(C6F5)4]- is employed (−25 °C, C6D5Br; Cp = C5H5, rac/meso 2a; C5Me5, rac/meso 2b; C5Me4SiMe3, rac/meso 2c). Dynamic NMR and crossover experiments suggest that the dimers 2 are relatively nonlabile with respect to dissociation, intramolecular methyl group exchange, or diastereomer interconversion. Dimers 2 are observed to undergo methane loss in solution at room temperature, affording the new dimeric compounds 3a−c, {[Cp(L)Ti]2(μ-CH2)(μ-CH3)}+[B(C6F5)4]-. For the less sterically demanding C5H5 ligand, 3a is formed as a mixture of rac/meso diastereomers (7:3), but for the bulkier C5Me5 and C5Me4SiMe3 ligands, the rac isomers of 3b and 3c are formed ex...

The diborylated ferrocene compound 1,1'-Fc[B(C6F5)2]2 (2) was prepared via a transmetallation reaction involving 1,1'-Fc(HgCl)2 and two equivalents of ClB(C6F5)2. Although this reaction is very clean spectroscopically, isolated yields of 2 were never greater than 40% owing to the high solubility of the compound in non-donor solvents. Compound 2 is similar in its properties to the related monoborylated complex FcB(C6F5)2 (1) reported previously, particularly the UV–vis spectrum, which exhibits an intense charge transfer absorption (λmax = 233 nm, ε = 1.64 × 104) assignable to an Fe dz 2 / dx 2 – y 2 [Formula: see text] Bp transition. However, 2, is significantly more Lewis acidic than 1, reacting with weak donors such as acetonitrile at room temperature, and effecting methide abstraction from Cp2ZrMe2 under mild conditions. Complex 2 also forms a bis(trimethylphosphine) adduct (3), which was structurally characterized via X-ray crystallography.Key words: organoboranes, ferrocene derivatives, multidentate Lewis acids.

Facile metal-free splitting of molecular hydrogen (H(2)) is crucial for the utilization of H(2) without the need for toxic transition-metal-based catalysts. Frustrated Lewis pairs (FLPs) are a new class of hydrogen activators wherein interactions with both a Lewis acid and a Lewis base heterolytically disrupt the hydrogen-hydrogen bond. Here we describe the activation of hydrogen exclusively by a boron-based Lewis acid, perfluoropentaphenylborole. This antiaromatic compound reacts extremely rapidly with H(2) in both solution and the solid state to yield boracyclopent-3-ene products resulting from addition of hydrogen atoms to the carbons alpha to boron in the starting borole. The disruption of antiaromaticity upon reaction of the borole with H(2) provides a significant thermodynamic driving force for this new metal-free hydrogen-splitting reaction.