Students working with Dr. Hoefelmeyer will investigate the topic of heterogeneous frustrated Lewis pairs. According to the Lewis definition of acids and bases, a Lewis acid is an electron acceptor, and a Lewis base is an electron donor. Naturally, Lewis acids are electron deficient and Lewis bases are electron rich and typically have lone pairs of electrons. Upon combining Lewis acids and bases, a neutralization reaction takes place with formation of a dative bond.
Our group has an interest in the chemistry of Frustrated Lewis Pairs (FLPs). FLPs are bulky Lewis acid, Lewis base combinations that are unable to form a dative bond to each other. FLPs exhibit unusual reactivity, such as heterolytic bond dissociation in small guest molecules, catalysis, , and can participate as ambiphilic ligands in coordination complexes. Most FLPs are homogeneous as unimolecular or bimolecular Lewis acid/Lewis base combinations. Examples include (C6F5)3B/PR3, (C6F5)B-C6F4-PR2 (R = t-butyl or mesityl), and (C6F5)3B/2,6-dimethylpyridine. While most FLP studies are in homogeneous solution, there are reasons to expect advantages in heterogeneous FLP systems, such as increased stability. Additionally, there are indications that phenomena reported in the heterogeneous catalysis literature may partially arise from the influence of surface Lewis acid/base pairs. Some examples of heterogeneous FLPs in which the Lewis acid, Lewis base, or both exist as insoluble solid state materials are reported. Reaction of B(C6F5)3 or HB(C6F5)2 with silica leads to =Si-O-B(C6F5)2 groups on the surface. Subsequent addition of (t-Bu)3P leads to a heterogeneous FLP that is capable of H-H bond cleavage. Addition of B(C6F5)3/Et2NPh to silica leads to heterolytic dissociation of the silanol O-H bond and formation of =Si-O-B(C6F5)3-[PhEt2NH+] ion pairs on the silica surface.
We propose to expand upon the concept of heterogeneous FLPs using two broad approaches. One set of FLP materials will consist of a solid that serves as one member of the FLP in combination with a molecule that provides the complimentary Lewis site. This type of material could be formed from a solid with Lewis acid sites in combination with a bulky molecular base, or the reverse analog in which the solid has Lewis base sites and is in combination with a bulky molecular Lewis acid. The solid supports should be anhydrous to avoid immediate heterolysis of water to Lewis base bound protons and Lewis acid bound hydroxide. Anhydrous materials with Lewis acid sites include Al2O3 and ZrO2; whereas, MgO is an example of a basic material. We note that surface metal hydroxides (M-OH), prevalent on metal oxides, can rapidly react with Lewis acids/Lewis bases. Therefore, materials such as metal phosphates (AlPO4, FePO4) may be attractive candidates for providing heterogeneous Lewis sites. Candidates for molecular Lewis acids include Mes3B, Ph3B, and (C6F5)3B; while examples of molecular Lewis acids include trialkylamines and 2,6-disubstituted pyridines.
The other set of FLP materials will be prepared by grafting Lewis sites on high surface area silica followed by addition of a complimentary Lewis acid/base. An excellent example of this protocol was just reported by the Erasmus group. Aminopropyltrialkoxysilanes are commercially available and are effective reagents for attaching aminopropyl groups to silica surfaces. We propose to modify the aminopropyl group, a non-bulky primary amine, upon reaction with alkylhalides to increase its steric bulk. This will allow some tuning of the steric and hydrophobic properties of the amine. This will be followed by addition of molecular Lewis acids, such as Mes3B, Ph3B, and (C6F5)3B, to prepare the heterogeneous FLPs. The reverse analog may be prepared upon reaction of B(C6F5)3 or HB(C6F5)2 with silica to produce Lewis acidic =Si-O-B(C6F5)2 groups followed by addition of a bulky base. The doubly grafted heterogeneous FLP in which silica contains both alkylamine and -B(C6F5)2 on the surface can prepared as well.
Undergraduate students will perform synthesis and characterization of the heterogeneous FLPs. Undergraduates will prepare the anhydrous solids followed by addition of a solution of the complimentary Lewis functional group or grafting reagents. The department has FTIR, DSC-TGA, 400 MHz NMR with solid state probe, and electron microscopy (SEM, TEM) for materials characterization. The materials will be evaluated for hydrogen uptake and catalytic hydrogenation of imines, alkenes, aromatics, and carbon dioxide. Students will use GC-MS and NMR for catalysis studies.
1. J.-H. Son, M.A. Pudenz, J.D. Hoefelmeyer ‘Reactivity of the Bifunctional Ambiphilic Molecule 8-(dimesitylboryl)quinoline: Hydrolysis and Coordination to Cu(I), Ag(I) and Pd(II)’ Dalton Trans. 2010, 39, 11081-11090.
2. J.-H. Son, J.D. Hoefelmeyer ‘1,2-nucleophilic addition of 2-(picolyl)boranes to nitrile, aldehyde, ketone, and amide’ Org. Biomol. Chem. 2012, 10, 6656-6664.
3. S.R. Tamang, J.-H. Son, J.D. Hoefelmeyer ‘Preparation of RHgCl via Transmetalation of (8-quinolyl)SnMe3 and Redistribution to R2Hg (R = 8-quinolyl): A Highly Distorted Diorganomercury(II) with 84 Degree C-Hg-C Angle’ Dalton Trans. 2014, 43, 7139-7145.
4. J.-H. Son, S.R. Tamang, J.I. Fostvedt, J.D. Hoefelmeyer ‘Dehydrodechlorination of Methylene Chloride, Chloroform, and Chlorodiphenylmethane in the Presence of Ga/N Lewis Pairs’ Organometallics 2017, 36, 474-479.
5. D.W. Stephan ‘The broadening reach of frustrated Lewis pair chemistry’ Science 2016, 354, aaf7229.
6. D.W. Stephan ‘Frustrated Lewis Pairs: From Concept to Catalysis’ Acc. Chem. Res. 2015, 48, 306-316.
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9. L. Wang, G. Kehr, C.G. Daniliuc, M. Brinkkotter, T. Weigand, A.L. Wubker, H. Eckert, L. Liu, J.G. Brandenburg, S. Grimme, G. Erker ‘Solid state frustrated Lewis pair chemistry’ Chem. Sci. 2018, 9, 4859-4865.
10. J. Zhao, H. Chen, J. Xu, J. Shen ‘Effect of Surface Acidic and Basic Properties of the Supported Nickel Catalysts on the Hydrogenation of Pyridine to Piperidine’ J. Phys. Chem. C 2013, 117, 10573-10580.
11. Y. Ma, S. Zhang, C. Chang, Z. Huang, J.C. Ho, Y. Qu ‘Semi-solid and solid frustrated Lewis pair catalysts’ Chem. Soc. Rev. 2018, 47, 5541-5553.
12. Y.J. Wanglee, J. Hu, R.E. White, M.Y. Lee, S.M. Stewart, P. Perrotin, S.L. Scott ‘Borane-Induced Dehydration of Silica and the Ensuing Water-Catalyzed Grafting of B(C6F¬5)3 To Give a Supported, Single-Site Lewis Acid, ≡SiOB(C6F5)2’ J. Am. Chem. Soc. 2012, 134, 355-366.
13. J.Y. Xing, J.C. Buffet, N.H. Rees, P. Nrby, D. O’Hare ‘Hydrogen cleavage by solid-phase frustrated Lewis pairs’ Chem. Commun. 2016, 52, 10478-10481.
14. N. Millot, A. Cox, C.C. Santini, Y. Molard, J.M. Basset ‘Surface Organometallic Chemistry of Main Group Elements: Selective Synthesis of Silica Supported [SiOB(C6F5)3]-[HNEt2Ph]+’ Chem. Eur. J. 2002, 8, 1438-1442.
15. K. Mentoor, L. Twigge, J.W. Hans Niemantsverdriet, J.C. Swarts, E. Erasmus ‘Silica Nanopowder Supported Frustrated Lewis Pairs for CO2 Capture and Conversion to Formic Acid’ Inorg. Chem. 2021, 60, 55-69.