Thomas M. Gilbert

B-H activation by frustrated Lewis pairs: borenium or boryl phosphonium cation?

Catechol borane reacts with the frustrated Lewis pairs tBu2RP (R = tBu, 2-C6H4(C6H5)) and B(C6F5)3 to give the species [(C6H4O2)BPtBu2R][HB(C6F5)3] that can formally be described as either borenium cation or boryl-phosphonium salts; the nature of these species was probed with DFT calculations.

From classical adducts to frustrated Lewis pairs: steric effects in the interactions of pyridines and B(C6F5)3.

The pyridine adducts of B(C(6)F(5))(3), (4-tBu)C(5)H(4)NB(C(6)F(5))(3) 1, ((2-Me)C(5)H(4)N)B(C(6)F(5))(3) 2, ((2-Et)C(5)H(4)N)B(C(6)F(5))(3) 3, ((2-Ph)C(5)H(4)N)B(C(6)F(5))(3) 4, ((2-C(5)H(4)N)C(5)H(4)N)B(C(6)F(5))(3) 5, (C(9)H(7)N)B(C(6)F(5))(3) 6, and ((2-C(5)H(4)N)NH(2-C(5)H(4)N))B(C(6)F(5))(3) 7, were prepared and characterized. The B-N bond lengths in 2-7 reflect the impact of ortho-substitution, increasing significantly with sterically larger and electron-withdrawing substituents. In the case of 2-amino-6-picoline, reaction with B(C(6)F(5))(3) affords the zwitterionic species (5-Me)C(5)H(3)NH(2-NH)B(C(6)F(5))(3) 8. In contrast, lutidine/B(C(6)F(5))(3) yields an equilibrium mixture containing both the free Lewis acid and base and the adduct (2,6-Me(2)C(5)H(3)N)B(C(6)F(5))(3) 9. This equilibrium has a DeltaH of -42(1) kJ/mol and DeltaS of -131(5) J/mol x K. Addition of H(2) shifts the equilibrium and yields [2,6-Me(2)C(5)H(3)NH][HB(C(6)F(5))(3)] 10. The corresponding reactions of 2,6-diphenylpyridine or 2-tert-butylpyridine with B(C(6)F(5))(3) showed no evidence of adduct formation and upon exposure to H(2) afforded [(2,6-Ph(2))C(5)H(3)NH][HB(C(6)F(5))(3)] 11 and [(2-tBu)C(5)H(4)NH][HB(C(6)F(5))(3)] 12, respectively. The energetics of adduct formation and the reactions with H(2) are probed computationally. Crystallographic data for compounds 1-10 are reported.

Heterolytic cleavage of disulfides by frustrated Lewis pairs.

The addition of diphenyl disulfide (PhSSPh) to tBu(2)P(C(6)F(4))B(C(6)F(5))(2) (1) affords the zwitterionic phosphonium borate [tBu(2)P(SPh)(C(6)F(4))B(SPh)(C(6)F(5))(2)] (2), while the addition of a base or donor solvent to 2 effected the liberation of disulfide and the formation of [tBu(2)P(C(6)F(4))B(donor)(C(6)F(5))(2)]. The reaction of 1 with S(8) gave tBu(2)P(S)(C(6)F(4))B(C(6)F(5))(2) (3). In a similar fashion, the frustrated Lewis pair of tBu(3)P/B(C(6)F(5))(3) reacts with RSSR to give [tBu(3)P(SR)][(RS)B(C(6)F(5))(3)] (R = Ph (4), p-tolyl (5), iPr (6)). In contrast, the corresponding reaction of BnSSBn yields a 1:1:1 mixture of tBu(3)P horizontal lineS, Bn(2)S, and B(C(6)F(5))(3). Species 4 reacts with p-tolylSSp-tolyl to give a mixture of 4, 5, PhSSPh, and p-tolylSS p-tolyl, while treatment of 5 with PhSSPh afforded a similar mixture. To probe this, a crossover experiment between [tBu(3)P(SPh)][B(C(6)F(5))(4)] (7) and [NBu(4)][(p-tolylS)B(C(6)F(5))(3)] (9) was performed. The former species was prepared by a reaction of 4 with [Ph(3)C][B(C(6)F(5)) (4)], while cation exchange of [(Et(2)O)(2)Li( p-tolylS)B(C(6)F(5))(3)] (8) with [NBu(4)]Br gave 9. The reaction of compounds 7 and 9 gave a statistical mixture of the cations [tBu(3)P(SR)](+) and anions [(RS)B(C(6)F(5))(3)](-), R = Ph, Sp-tolyl. The mechanism of this exchange process was probed and is proposed to be an equilibrium involving disulfide and the frustrated Lewis pair. Crystallographic data are reported for compounds 4-8, and the natures of the P-S cations are examined via DFT calculations.

Synthesis and reactivity of the phosphinoboranes R2PB(C6F5)2.

The phosphinoboranes [R(2)PB(C(6)F(5))(2)](2) (R = Et 1, Ph 2) and R(2)PB(C(6)F(5))(2) (R = tBu 3, Cy 4, Mes 5) were synthesized from the reaction of (C(6)F(5))(2)BCl and the corresponding lithium phosphide. The relationships between B-P distance, P pyramidality, and the extent of BP multiple bonding were further explored computationally. Natural Bond Order (NBO) analyses of 3 and 4 showed that the π-bonding highest occupied molecular orbitals (HOMOs) were highly polarized. In addition the Lewis acid-base adducts, R(2)(H)P·B(H)(C(6)F(5))(2) (R = Et 6; Ph 7; tBu 8; Cy 9; Mes 10) were prepared via the reaction of the phosphines R(2)PH with the borane HB(C(6)F(5))(2). Compounds 1 and 2 showed no signs of reaction with H(2); however, reaction of compounds 3 and 4 with H(2) was observed to give 8 and 9. In a related set of reactions compounds 3 and 4 were reacted with H(3)NBH(3) or Me(2)(H)NBH(3) also led to the generation of 8 and 9, respectively. The reaction profile of the reaction of (CF(3))(2)BPR(2) with H(2) was examined computationally and shown to be exothermic. Efforts to effect the reverse reaction, that is, dehydrogenation of adducts 6-10 were unsuccessful. Compound 4 was also shown to react with 4-tert-butylpyridine to give Cy(2)PB(C(6)F(5))(2)(4-tBuC(5)H(4)N) 11 while reactions of 3 and 4 with the Lewis acid BCl(3) gave the dimers (R(2)PBCl(2))(2) (R = tBu 12, Cy 13) and the byproduct ClB(C(6)F(5))(2).

Preparation of aza-polycyclic aromatic compounds via superelectrophilic cyclizations.

Alkenyl-substituted N-heterocycles react in superacidic CF3SO3H (triflic acid) to produce dicationic intermediates. These superelectrophiles undergo cyclizations to give varied aza-polycyclic aromatic compounds in generally good yields (27-99%, 16 examples). Theoretical calculations indicate a preference for charge-separated dicationic intermediates.

Ab initio prediction of ring strain enthalpies of cyclic amine-boranes

Abstract Ring strain enthalpies for a series of cyclic amine-boranes (1-azonia-2-boratacycloalkanes) were calculated from MP4(SDTQ)/6-31G ∗ //MP2/6-31G ∗ energies by the group equivalent method. Small rings exhibit smaller RSEs than do the corresponding cycloalkanes; larger rings exhibit RSEs essentially identical to those of the corresponding cycloalkanes.

Donor-Acceptor Dissociation Energies of Group 13-15 Donor-Acceptor Complexes Containing Fluorinated Substituents: Approximate Lewis Acidities of (F3C)3M vs (F5C6)3M and the Effects of Phosphine Steric Bulk.

To study donor-acceptor complexes containing fluoroalkyl and -aryl substituents on the acceptors, ONIOM methods for optimizing large complexes and determining single point energies were tested. A two-layer ONIOM optimization procedure utilizing the MPW1K model followed by single point calculations using the composite three-layer ONIOM G2R3 method proved acceptable. The optimization model predicts M-X bond distances well when compared to experiment and shows that the distances increase discontinuously with the bulk of the phosphine. Unexpectedly, (RF)3B-XR3 and (RF)3Al-XR3 bond dissociation energies (ΔEDA) are comparable for several R substituents. For RF = CF3, both are predicted to exhibit M-X ΔEDA values in the range 55-80 kcal mol(-1), exceptionally strong for dative bond energies. For RF = C6F5, the ΔEDA values are predicted to lie in the range 30-45 kcal mol(-1). (F5C6)3BP(t-Bu)3, which does not contain a B-P bond, is predicted to display ΔEDA = 19 kcal mol(-1). The ΔEDA energies do not change smoothly as the steric bulk of the phosphine increases. However, intrinsic ΔEDA energies ΔEint show a regular increase as the donor ability of the phosphine increases, confirming that the reorganization energy of the individual moieties contributes sizably to the overall ΔEDA. The data indicate that PPh3 is approximately equivalent to PMe3 as a donor in terms of ΔEint.

CC Coupling by Thermolysis of Alkynyl Phosphonium Borates

Compounds containing both phosphine and borane fragments have attracted increasing attention over the past several years. Perhaps the most widely studied use of such molecules has been as ligands for transition metals. Indeed systems that incorporate both P and B centers have been shown to be quite versatile for the coordination chemistry of transition metals. Moreover the resulting complexes give rise to unusual reactivity and have furnished new organometallic catalysts. While the impact on the reactivity of such ancillary P/B ligands is interesting, materials containing P and B centers also exhibit interesting properties. For example, phosphorus ylide–borane adducts have been employed as latent catalysts for thermosetting resins. In addition, more elaborate molecules containing P and B centers have been prepared to exploit the combination of such electron-donor and -acceptor capabilities. Together with highly conjugated organic p systems, such materials exhibit intriguing electronic and optical properties and have found applications as fluorescent and nonlinear optical materials. In another application, Gabbai et al have pioneered combinations of the Lewis acidity of boranes with the electron-withdrawing abilities of phosphonium groups to effect enhanced Lewis acidity at the B center for use as fluoride sensors. In a somewhat more fundamental context, we have recently described “frustrated Lewis pairs” comprising P and B centers in which steric demands preclude classical Lewis acid–base adduct formation. Such systems retain Lewis acidity and basicity that can be exploited to activate a variety of small molecules including olefins, dienes, THF, BH bonds, terminal alkynes, CO2, [12] and N2O. [13] Most notably these systems effect the heterolytic activation of H2 and can be subsequently employed in metal-free hydrogenation catalysis of imines, aziridines, nitriles, enamines, and silylACHTUNGTRENNUNGenolethers.[10,14,19] More recently, Bercaw et al have used related phosphine-borane systems to effect H2 activation and reduce metal-bound carbonyl fragments, while Bourissou and co-workers have exploited donor/acceptor properties of phosphine-boranes to effectively trap reactive intermediates such as singlet O2. [16] The nature of the organic links between the P and B centers in the above systems are frequently arene-based although two-carbon alkyl or alkenyl chains have also been examined. Alkynyl-bridged systems bearing P and B centers were prepared some years ago by Marder and others. and shown to exhibit nonlinear optical properties. More recently, Yamaguchi and co-workers have described an ingenious synthetic pathway for phosphonium borate linked stilbene derivatives formed by intramolecular nucleophilic addition of phosphine and borane across a C C unit in efforts to develop new organic materials. Despite the interest in the electronic properties, little attention has focused on the reactivity of such alkynyl-bridged P/B systems. In a recent report, we described the unconventional binding mode of the alkynyl phosphine-borane, tBu2PC CBACHTUNGTRENNUNG(C6F5)2, to Ni. Herein, the thermal reactivity of alkynyl phosphonium borates is examined. In two such systems the strongly polarized alkynyl group is shown to react via group migration and C C coupling to give rise to unusual C4 olefinic and cumulenic fragments. The mechanism of these unusual rearrangements is probed computationally. Thermolysis of the phosphonium borate tBu2PHC CBHACHTUNGTRENNUNG(C6F5)2 (1) was performed in [D5]bromobenzene at 150 8C for 12 h. The solution became yellow and subsequently orange after about 5 min. Following completion of the thermolysis, the solution was allowed to cool and a product (2) crystallized from the solution (Scheme 1). The crystals were isolated and washed with benzene affording 2 in 40 % yield. The H NMR spectrum of 2 showed broad multiplets at d= 5.90 and 3.01 ppm as well as doublets at d=1.46 and [a] X. Zhao, Prof. Dr. D. W. Stephan Department of Chemistry, University of Toronto 80 St. George St., Toronto, ON, M5S 3H6 (Canada) E-mail : dstephan@chem.utoronto.ca Homepage: http://www.chem.utoronto.ca/staff/DSTEPHAN [b] Prof. Dr. T. M. Gilbert Department of Chemistry & Biochemistry Northern Illinois University DeKalb, IL 60115 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201001575.

New measurements of the thermochemistry of SF5− and SF6−

Abstract Energy-resolved collision-induced dissociation (CID) experiments using a flowing afterglow-tandem mass spectrometer (MS) have been performed on SF 5 − and SF 6 − , two key components of SF 6 -based plasmas. G3/B3LYP and G3(MP2) calculations have also been performed on these systems. The results are compared to other experimental and computational determinations of the thermochemistry of these species. CID of SF 5 − gives a 0xa0K bond energy of D (SF 4 ue5f8F − )=2.38±0.10xa0eV (230±10xa0kJxa0mol −1 ). This is in good agreement with the high-level computational results. The threshold for dissociation of SF 6 − (into SF 5 − +F) of 1.85±0.12xa0eV (178±12xa0kJxa0mol −1 ). This dissociation appears to have a barrier greater than the endothermicity, although the barrier is much smaller than that for photodetachment of the same ion.

Structures of homoleptic triply bonded M2(OR)6 compounds where the alkoxide is tertiary: the effect of steric bulk and alkoxide conformation on structural parameters

Abstract The structures of three homoleptic metal–metal triply-bonded M2(OR)6 compounds {Mo2(OCMe2CMe2O)3, 1; Mo2(OCMe2Ph)6, 2; and W2[OCMe(CF3)2]6, 3} containing tertiary alkoxides are reported. The data demonstrate that, overall, the M2O6 cores are isomorphous regardless of the substitution pattern of the alkoxide. Dimer 3 is the first example of a W2(OR)6 compound containing terminal tertiary alkoxides to be structurally characterized; it is of interest as a close structural analogue of the chemically remarkable W2(OCMe3)6. The hexa(hexafluoro-t-butoxide) dimer, though slightly disordered, displays distances and angles similar to other W2(OR)6 species despite the poor electron-donor capacity of the fluoroalkoxide group. The data from these structures, combined with literature data, show that proximal alkoxide ligands generally exhibit shorter M–O bond distances and larger M–M–O and M–O–C angles than do distal alkoxides regardless of metal type, alkoxide steric bulk, and alkoxide π donor ability in a range of compounds. This suggests electrostatic and conformational considerations are of greater importance to the bonding in these dimers than are M–O π interactions.