Victor Sumerin

Facile Heterolytic H2 Activation by Amines and B(C6F5)3

In industrially important reactions, such as hydroformylation and hydrogenation, H2 gas serves as a reducing agent and/or a hydrogen-atom source. Even small improvements in the efficiency of these reactions translate into large monetary savings. The key step in these transformations is the activation of H2 at a transition metal. The nodal character of the energetically accessible d orbitals allows a transition-metal center to react directly with H2 in a concerted reaction with a low activation barrier. However, not only are transitionmetal complexes expensive, but the complete removal of metal impurities from the reaction product is generally required in the production of pharmaceutical intermediates owing to toxicity concerns. Although countless synthetic complexes and enzymes with transition metals at their reactive core are well known, there are significantly fewer examples of H H bond activation facilitated solely by a nonmetal. Several reactions of H2 with compounds containing main-group elements in lowtemperature matrices have been reported; however, H2 activation at nonmetals under mild conditions had only been observed by Power and co-workers in product mixtures of digermenes, digermanes, and primary germanes, until recently, when Stephan and co-workers reported the thermal liberation of H2 from a phosphonium borate salt. The resulting product, (C6H2Me3)2P(C6F4)B(C6F5)2, undergoes the addition of H2 at 25 8C to reform the original salt. [7] In an analogous fashion, mixtures of sterically demanding phosphanes and boranes (“frustrated Lewis pairs”) can also cleave H2 heterolytically to form phosphonium borates [R3PH][HBR’3]. [9] More recently, Bertrand and co-workers reported that selected organic carbenes are just nucleophilic enough to cleave H2 and NH3. [10] Herein we extend the family of “frustrated Lewis pairs” and demonstrate that not only bulky phosphanes and boranes or organic carbenes can cleave H2, but also inexpensive, stable amines in combination with B(C6F5)3. Solutions of stoichiometric mixtures of diisopropylethylamine, diisopropylamine, or 2,2,6,6-tetramethylpiperidine and B(C6F5)3 in toluene were investigated by H, B, and F NMR spectroscopy. The reactions of diisopropylethylamine and diisopropylamine with B(C6F5)3 gave mixtures of the salt 1a or 1b and the zwitterion 2a or 2b as expected (Scheme 1); however, no reaction was observed for 2,2,6,6tetramethylpiperidine, a bulky secondary amine with no a hydrogen atoms. Whereas the reaction between diisopropylamine and B(C6F5)3 is reversible at elevated temperature, the mixture of 1a and 2a from the reaction with diisopropylethylamine is thermally stable (Scheme 1).

Vanadium( iii ) phenoxyimine complexes for ethylene or ε-caprolactone polymerization : mononuclear versus binuclear pre-catalysts

The mononuclear {[C6H4NCH(ArO)]2VCl(THF)} (Ar = 2,4-t-Bu2C6H2 (1), Ar = C6H4 (2)), {O[C6H4NCH(ArO)]2}VCl(THF) (Ar = 2,4-t-Bu2C6H2 (3), Ar = C6H4 (4)) and the binuclear vanadium(III) complexes {[C6H4NCH(ArO)]VCl2(THF)2}2(μ-CH2CH2) (Ar = 2,4-t-Bu2C6H2 (5), Ar = C6H4 (6)), have been synthesized and fully characterized. The compounds [C6H5NCH(ArO)]VCl2(THF)2 (Ar = 2,4-t-Bu2C6H2 (7), Ar = C6H4 (8)), [2,4,6-Me3–C6H2NCH(ArO)]VCl2 (Ar = 2,4-t-Bu2C6H2 (9), Ar = C6H4 (10)) and [2,6-i-Pr2-C6H3NCH(ArO)]VCl2(THF)2 (Ar = 2,4-t-Bu2C6H2 (11), Ar = C6H4 (12)), {μ-CH2CH2[NCH(C6H4O)]2VCl(THF)} (14) and {C6H4[NCH(C6H4O)]2VCl(THF)} (15) were synthesized for comparative polymerization studies. The dizwitterionic compound [2,6-i-Pr2-C6H3N+(H)CH(C6H4O)]2VCl2O (13) was also isolated, and presumably formed via a fortuitous hydrolysis reaction. The complexes 2, 5 and 13 have been structurally characterized; the molecular structure of the parent ligand (L5) in 5 is also reported. All complexes have been screened for ethylene as well as e-caprolactone polymerization, and results are compared against those for known related mono- and bi-nuclear counterparts to evaluate for possible cooperative effects. The compounds 10 and 12 have been supported on modified SiO2, analysed by XPS and subjected to homo-polymerization (ethylene) and co-polymerization (1-hexene and ethylene) studies.

Highly Active, Thermally Stable, Ethylene-Polymerisation Pre-Catalysts Based on Niobium/TantalumImine Systems

The reactions of MCl5 or MOCl3 with imidazole-based pro-ligand L(1)H, 3,5-tBu2-2-OH-C6H2-(4,5-Ph2-1H-)imidazole, or oxazole-based ligand L(2)H, 3,5-tBu2-2-OH-C6H2 (1H-phenanthro[9,10-d])oxazole, following work-up, afforded octahedral complexes [MX(L(1,2))], where MX=NbCl4 (L(1), 1a; L(2), 2a), [NbOCl2(NCMe)] (L(1), 1b; L(2), 2b), TaCl4 (L(1), 1c; L(2), 2c), or [TaOCl2(NCMe)] (L(1), 1d). The treatment of α-diimine ligand L(3), (2,6-iPr2C6H3N=CH)2, with [MCl4(thf)2] (M=Nb, Ta) afforded [MCl4(L(3))] (M=Nb, 3a; Ta, 3b). The reaction of [MCl3(dme)] (dme=1,2-dimethoxyethane; M=Nb, Ta) with bis(imino)pyridine ligand L(4), 2,6-[2,6-iPr2C6H3N=(Me)C]2C5H3N, afforded known complexes of the type [MCl3(L(4))] (M=Nb, 4a; Ta, 4b), whereas the reaction of 2-acetyl-6-iminopyridine ligand L(5), 2-[2,6-iPr2C6H3N=(Me)C]-6-Ac-C5H3N, with the niobium precursor afforded the coupled product [({2-Ac-6-(2,6-iPr2C6H3N=(Me)C)C5H3N}NbOCl2)2] (5). The reaction of MCl5 with Schiff-base pro-ligands L(6)H-L(10)H, 3,5-(R(1))2-2-OH-C6H2CH=N(2-OR(2)-C6H4), (L(6)H: R(1)=tBu, R(2)=Ph; L(7)H: R(1)=tBu, R(2)=Me; L(8)H: R(1)=Cl, R(2)=Ph; L(9)H: R(1)=Cl, R(2)=Me; L(10)H: R(1)=Cl, R(2)=CF3) afforded [MCl4(L(6-10))] complexes (M=Nb, 6a-10a; M=Ta, 6b-9b). In the case of compound 8b, the corresponding zwitterion was also synthesised, namely [Ta(-)Cl5(L(8)H)(+)]·MeCN (8c). Unexpectedly, the reaction of L(7)H with TaCl5 at reflux in toluene led to the removal of the methyl group and the formation of trichloride 7c [TaCl3(L(7-Me))]; conducting the reaction at room temperature led to the formation of the expected methoxy compound (7b). Upon activation with methylaluminoxane (MAO), these complexes displayed poor activities for the homogeneous polymerisation of ethylene. However, the use of chloroalkylaluminium reagents, such as dimethylaluminium chloride (DMAC) and methylaluminium dichloride (MADC), as co-catalysts in the presence of the reactivator ethyl trichloroacetate (ETA) generated thermally stable catalysts with, in the case of niobium, catalytic activities that were two orders of magnitude higher than those previously observed. The effects of steric hindrance and electronic configuration on the polymerisation activity of these tantalum and niobium pre-catalysts were investigated. Spectroscopic studies ((1)H NMR, (13)C NMR and (1)H-(1)H and (1)H-(13)C correlations) on the reactions of compounds 4a/4b with either MAO(50) or AlMe3/[CPh3](+)[B(C6F5)4](-) were consistent with the formation of a diamagnetic cation of the form [L(4)AlMe2](+) (MAO(50) is the product of the vacuum distillation of commercial MAO at +50 °C and contains only 1 mol% of Al in the form of free AlMe3). In the presence of MAO, this cationic aluminium complex was not capable of initiating the ROMP (ring opening metathesis polymerisation) of norbornene, whereas the 4a/4b systems with MAO(50) were active. A parallel pressure reactor (PPR)-based homogeneous polymerisation screening by using pre-catalysts 1b, 1c, 2a, 3a and 6a, in combination with MAO, revealed only moderate-to-good activities for the homo-polymerisation of ethylene and the co-polymerisation of ethylene/1-hexene. The molecular structures are reported for complexes 1a-1c, 2b, 5, 6a, 6b, 7a, 8a and 8c.

Tweezers for Parahydrogen: A Metal-Free Probe of Nonequilibrium Nuclear Spin States of H2 Molecules

To date, only metal-containing hydrogenation catalysts have been utilized for producing substantial NMR signal enhancements by means of parahydrogen-induced polarization (PHIP). Herein, we show that metal-free compounds known as molecular tweezers are useful in this respect. It is shown that ansa-aminoborane tweezers QCAT provided (20-30)-fold signal enhancements of parahydrogen-originating hydrogens in (1)H NMR spectra. Nuclear polarization transfer from the polarized hydrogens to (11)B nuclei leads to a 10-fold enhancement in the (11)B NMR spectrum. Moreover, our results indicate that dihydrogen activation by QCAT and CAT tweezers is carried out in a pairwise manner, and PHIP can be used for understanding the activation mechanism in metal-free catalytic systems in general.

Amine-Borane Mediated Metal-Free Hydrogen Activation and Catalytic Hydrogenation

The use of frustrated Lewis pairs (FLPs) as hydrogenation catalysts is attracting increasing attention as one of the most modern and rapidly growing areas of organic chemistry, with many research groups around the world working on this subject. Since the pioneering studies of the groups of Stephan and Piers on the Lewis acid-base pairs, which do not react irreversibly with each other and act as a trap for small molecules, numerous FLPs for hydrogen activation have been reported. Among others, intra- and intermolecular systems based on phosphines, organic carbenes, amines as Lewis bases, and boranes or alanes as Lewis acids were studied. This review presents a progression from the first observation of the facile heterolytical cleavage of hydrogen gas by amines and B(C6F5)3 to highly active non-metal catalysts for both enantioselective and racemic hydrogenation of unsaturated nitrogen-containing compounds and also internal alkynes.