The synthesis of new weakly coordinating diborate anions: anion stability as a function of linker structure and steric bulk

Exploring the Reactivity of B-Connected Carboranylphosphines in Frustrated Lewis Pair Chemistry: A New Frame for a Classic System

The primary phosphines MesPH2 and tBuPH2 react with 9-iodo-m-carborane yielding B9-connected secondary carboranylphosphines 1,7-H2 C2 B10 H9 -9-PHR (R=2,4,6-Me3 C6 H2 (Mes; 1 a), tBu (1 b)). Addition of tris(pentafluorophenyl)borane (BCF) to 1 a, b resulted in the zwitterionic compounds 1,7-H2 C2 B10 H9 -9-PHR(p-C6 F4 )BF(C6 F5 )2 (2 a, b) through nucleophilic para substitution of a C6 F5 ring followed by fluoride transfer to boron. Further reaction with Me2 SiHCl prompted a H-F exchange yielding the zwitterionic compounds 1,7-H2 C2 B10 H9 -9-PHR(p-C6 F4 )BH(C6 F5 )2 (3 a, b). The reaction of 2 a, b with one equivalent of R'MgBr (R'=Me, Ph) gave the extremely water-sensitive frustrated Lewis pairs 1,7-H2 C2 B10 H9 -9-PR(p-C6 F4 )B(C6 F5 )2 (4 a, b). Hydrolysis of the B-C6 F4 bond in 4 a, b gave the first tertiary B-carboranyl phosphines with three distinct substituents, 1,7-H2 C2 B10 H9 -9-PR(p-C6 F4 H) (5 a, b). Deprotonation of the zwitterionic compounds 2 a, b and 3 a, b formed anionic phosphines [1,7-H2 C2 B10 H9 -9-PR(p-C6 F4 )BX(C6 F5 )2 ]- [DMSOH]+ (R=Mes, X=F (6 a), R=tBu, X=F (6 b); R=Mes, X=H (7 a), R=tBu, X=H (7 b)). Reaction of 2 a, b with an excess of Grignard reagents resulted in the addition of R' at the boron atom yielding the anions [1,7-H2 C2 B10 H9 -9-PR(p-C6 F4 )BR'(C6 F5 )2 ]- (R=Mes, R'=Me (8 a), R=tBu, R'=Me (8 b); R=Mes, R'=Ph (9 a), R=tBu, R'=Ph (9 b)) with [MgBr(Et2 O)n ]+ as counterion. The ability of the zwitterionic compounds 3 a, b to hydrogenate imines as well as the Brønsted acidity of 3 a were investigated.

Imidazolyl-phenyl (IMP) anions: a modular structure for tuning solubility and coordinating ability

The effect of counteranion upon a cation's solution-phase reactivity depends on a subtle interplay of weak interactions.

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

This Review gives a comprehensive overview of the most topical weakly coordinating anions (WCAs) and contains information on WCA design, stability, and applications. As an update to the 2004 review, developments in common classes of WCA are included. Methods for the incorporation of WCAs into a given system are discussed and advice given on how to best choose a method for the introduction of a particular WCA. A series of starting materials for a large number of WCA precursors and references are tabulated as a useful resource when looking for procedures to prepare WCAs. Furthermore, a collection of scales that allow the performance of a WCA, or its underlying Lewis acid, to be judged is collated with some advice on how to use them. The examples chosen to illustrate WCA developments are taken from a broad selection of topics where WCAs play a role. In addition a section focusing on transition metal and catalysis applications as well as supporting electrolytes is also included.

Exploiting directional long range secondary forces for regulating electrostatics-dominated noncovalent interactions

It has been well established that long range secondary electrostatic interactions (SEIs) have a significant effect on the stability of supramolecular complexes. However, general rules for exploiting SEIs in the rational design of diverse supramolecular complexes have been difficult to obtain. In this work, we outline a quantum chemical approach for understanding the strength of electrostatic interactions. This approach is seen to provide excellent correlation between the electrostatic force and the binding energy between two partners in hydrogen-bonded complexes, as well as that between two ions in ion-pair complexes. Furthermore, we illustrate how the understanding of the binding allows for the rational design of new complexes where the association constant between the two partners can be increased or decreased, as desired, by several orders of magnitude. Hence, the current work showcases a general, simple and powerful method of understanding and exploiting long range secondary electrostatic interactions.

Tetravalent cerium pseudohalide complexes supported by the Kläui tripodal ligand [Co(η5-C5H5){P(O)(OEt)2}3]−

Cerium(iv) pseudohalide complexes supported by the Kläui tripodal ligand [Co(η⁵-C₅H₅){P(O)(OEt)₂}₃]⁻ (L_OEt⁻) have been synthesized and structurally characterized.

Influence of Organoboron Compounds on Ethylene Polymerization Using Cp2ZrCl2/MAO as Catalyst System

Organoboron compounds of nonionic and ionic nature, tris(pentafluorophenyl)borane, and N,N-dimethylanilinium tetra(pentafluorophenyl)borate were evaluated to act in conjunction with MAO as activators on ethylene polymerization by using the catalyst Cp2ZrCl2. A decrease on the catalytic activity was observed in both cases in relation with a reference polyethylene which was synthesized in absence of any organoboron compound. An increase on the crystallinity degree and molecular weight, as well as an improvement in thermal and dynamic-mechanical properties, was observed in polyethylenes synthetized in presence of tris(pentafluorophenyl)borane. A low density polyethylene with improved thermal stability was obtained when N,N-dimethylanilinium tetra(pentafluorophenyl)borate was employed as activator.

Discrete, solvent-free alkaline-earth metal cations: metal···fluorine interactions and ROP catalytic activity

Efficient protocols for the syntheses of well-defined, solvent-free cations of the large alkaline-earth (Ae) metals (Ca, Sr, Ba) and their smaller Zn and Mg analogues have been designed. The reaction of 2,4-di-tert-butyl-6-(morpholinomethyl)phenol ({LO(1)}H), 2-{[bis(2-methoxyethyl)amino]methyl}-4,6-di-tert-butylphenol ({LO(2)}H), 2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-4,6-di-tert-butylphenol ({LO(3)}H), and 2-[(1,4,7,10-tetraoxa-13-azacyclo-pentadecan-13-yl)methyl]-1,1,1,3,3,3-hexafluoropropan-2-ol ({RO(3)}H) with [H(OEt(2))(2)](+)[H(2)N{B(C(6)F(5))(3)}(2)](-) readily afforded the doubly acidic pro-ligands [{LO(1)}HH](+)[X](-) (1), [{LO(2)}HH](+)[X](-) (2), [{LO(3)}HH](+)[X](-) (3), and [{RO(3)}HH](+)[X](-) (4) ([X](-) = [H(2)N{B(C(6)F(5))(3)}(2)](-)). The addition of 2 to Ca[N(SiMe(3))(2)](2)(THF)(2) and Sr[N(SiMe(3))(2)](2)(THF)(2) yielded [{LO(2)}Ca(THF)(0.5)](+)[X](-) (5) and [{LO(2)}Sr(THF)](+)[X](-) (6), respectively. Alternatively, 5 could also be prepared upon treatment of {LO(2)}CaN(SiMe(3))(2) (7) with [H(OEt(2))(2)](+)[X](-). Complexes [{LO(3)}M](+)[X](-) (M = Zn, 8; Mg, 9; Ca, 10; Sr, 11; Ba, 12) and [{RO(3)}M](+)[X](-) (M = Zn, 13; Mg, 14; Ca, 15; Sr, 16; Ba, 17) were synthesized in high yields (70-90%) by reaction of 3 or 4 with the neutral precursors M[N(SiMe(3))(2)](2)(THF)(x) (M = Zn, Mg, x = 0; M = Ca, Sr, Ba, x = 2). All compounds were fully characterized by spectroscopic methods, and the solid-sate structures of compounds 1, 3, 7, 8, 13, 14, {15}(4)·3CD(2)Cl(2), {16}(4)·3CD(2)Cl(2), and {{17}(4)·EtOH}·3CD(2)Cl(2) were determined by X-ray diffraction crystallography. Whereas the complexes are monomeric in the case of Zn and Mg, they form bimetallic cations in the case of Ca, Sr and Ba; there is no contact between the metal and the weakly coordinating anion. In all metal complexes, the multidentate ligand is κ(6)-coordinated to the metal. Strong intramolecular M···F secondary interactions between the metal and F atoms from the ancillary ligands are observed in the structures of {15}(4)·3CD(2)Cl(2), {16}(4)·3CD(2)Cl(2), and {{17}(4)·EtOH}·3CD(2)Cl(2). VT (19)F{(1)H} NMR provided no direct evidence that these interactions are maintained in solution; nevertheless, significant Ae···F energies of stabilization of 25-26 (Ca, Ba) and 40 kcal·mol(-1) (Sr) were calculated by NBO analysis on DFT-optimized structures. The identity and integrity of the cationic complexes are preserved in solution in the presence of an excess of alcohol (BnOH, (i)PrOH) or L-lactide (L-LA). Efficient binary catalytic systems for the immortal ring-opening polymerization of L-LA (up to 3,000 equiv) are produced upon addition of an excess (5-50 equiv) of external protic nucleophilic agents (BnOH, (i)PrOH) to 8-12 or 13-17. PLLAs with M(n) up to 35,000 g·mol(-1) were produced in a very controlled fashion (M(w)/M(n) ≈ 1.10-1.20) and without epimerization. In each series of catalysts, the following order of catalytic activity was established: Mg ≪ Zn < Ca < Sr ≈ Ba; also, Ae complexes supported by the aryloxide ligand are more active than their parents supported by the fluorinated alkoxide ancillary, possibly owing to the presence of Ae···F interactions in the latter case. The rate law -d[L-LA]/dt = k(p)·[L-LA](1.0)·[16](1.0)·[BnOH](1.0) was established by NMR kinetic investigations, with the corresponding activation parameters ΔH(++) = 14.8(5) kcal·mol(-1) and ΔS(++) = -7.6(2.0) cal·K(-1)·mol(-1). DFT calculations indicated that the observed order of catalytic activity matches an increase of the L-LA coordination energy onto the cationic metal centers with parallel decrease of the positive metal charge.

The Synthesis, Structure and Reactivity of B(C6F5)3-Stabilised Amide (MNH2) Complexes of the Group 4 Metals

Treatment of the homoleptic titanium amides [Ti(NR(2))(4)] (R=Me or Et) with the Brønsted acidic reagent H(3)NB(C(6)F(5))(3) results in the elimination of one molecule of amine and the formation of the four-coordinate amidoborate complexes [Ti(NR(2))(3){NH(2)B(C(6)F(5))(3)}], the identity of which was confirmed by X-ray crystallography. The reaction with [Zr(NMe(2))(4)] proceeds similarly but with retention of the amine ligand to give the trigonal-bipyramidal complex [Zr(NMe(2))(3){NH(2)B(C(6)F(5))(3)}(NMe(2)H)]. Cyclopentadienyl (Cp) amidoborate complexes, [MCp(NR(2))(2){NH(2)B(C(6)F(5))(3)}] (M=Ti, R=Me or Et; M=Zr, R=Me) can be prepared from [MCp(NR(2))(3)] and H(3)NB(C(6)F(5))(3), and exhibit greater thermal stability than the cyclopentadienyl-free compounds. H(3)NB(C(6)F(5))(3) reacts with nBuLi or LiN(SiMe(3))(2) to give LiNH(2)B(C(6)F(5))(3), which complexes with strong Lewis acids to form ion pairs that contain weakly coordinating anions. The attempted synthesis of metallocene amidoborate complexes from dialkyl or diamide precursors and H(3)NB(C(6)F(5))(3) was unsuccessful. However, LiNH(2)B(C(6)F(5))(3) does react with the highly electrophilic reagents [MCp(2)Me(mu-Me)B(C(6)F(5))(3)] to give [MCp(2)Me(mu-NH(2))B(C(6)F(5))(3)] (M=Zr or Hf). Comparison of the molecular structures of the Group 4 amidoborate complexes reveals very similar B--N, Ti--N and Zr--N bond lengths, which are consistent with a description of the bonding as a dative interaction between an {M(L)(n)(NH(2))} fragment and the Lewis acid B(C(6)F(5))(3). Each of the structures has an intramolecular hydrogen-bonding arrangement in which one of the nitrogen-bonded hydrogen atoms participates in a bifurcated FHF interaction to ortho-F atoms.