Colorimetric Metal‐Free Detection of Carbon Monoxide: Reversible CO Uptake by a BNB Frustrated Lewis Pair

Abstract We report two BNB‐type frustrated Lewis pairs which feature an acceptor‐donor‐acceptor functionalized cavity, and which differ in the nature of the B‐bound fluoroaryl group (C6F5 vs. C6H3(CF3)2‐3,5, Arf). These receptor systems are capable of capturing gaseous CO, and in the case of the ‐BArf 2 system this can be shown to occur in reversible fashion at/above room temperature. For both systems, the binding event is accompanied by migration of one of the aryl substituents to the electrophilic carbon of the CO guest. Experiments utilizing an additional equivalent of PtBu3 allow the initially formed (non‐migrated) CO adduct to be identified and trapped (via demethylation), while also establishing the reversibility of the B‐to‐C migration process. When partnered with the slightly less Lewis acidic ‐BArf 2 substituent, this reversibility allows for release of the captured carbon monoxide in the temperature range 40–70 °C, and the possibility for CO sensing, making use of the associated colourless to orange/red colour change.


Supporting Information
General considerations and starting material preparations s2 2.

General considerations and starting material preparations
All manipulations were carried out using standard Schlenk line or dry-box techniques under an atmosphere of argon or dinitrogen. Solvents were degassed by sparging with argon and dried by passing through a column of the appropriate drying agent. NMR spectra were measured in benzened 6 (which was dried over potassium), with the solvent then being distilled under reduced pressure and stored under argon in Teflon valve ampoules. NMR samples were prepared under argon in 5 mm Wilmad 507-PP tubes fitted with J. Young Teflon valves. 1 H, 31 P{ 1 H}, 13 C{ 1 H}, 11 B{ 1 H}, 19 F NMR spectra were measured on a Bruker Avance III HD nanobay 400 MHz or Bruker Avance 500 MHz spectrometer at ambient temperature and referenced internally to residual protio-solvent ( 1 H) or solvent ( 13 C) resonances and are reported relative to tetramethylsilane (δ = 0 ppm). Assignments were confirmed using two-dimensional 1 H-1 H and 13 C-1 H NMR correlation experiments. Chemical shifts are quoted in δ (ppm) and coupling constants in Hz. Elemental analyses were carried out by Elemental Microanalysis Ltd, Okehampton, Devon, UK. UV-Vis samples were prepared by dissolving the compound in chloroform (0.8 mM) and transferring the solution into a cuvette (l = 1.0 mm) fitted with a Teflon valve. UV-Vis spectra were measured on a Perkin Elmer Lambda 19 UV/vis/NIR spectrometer. Compound 1-H [s1] , KCH 2 Ph [s2] , ClBAr f 2 [s3] and ClB(C 6 F 5 ) 2 [s4] were synthesized according to literature. All other reagents were used as received. s3

Synthesis of 2
To a suspension of 1 (0.36 g, 0.88 mmol) in hexane (20 mL) at -78 o C was added sec-BuLi (1.29 mL of a 1.4 M solution in cyclohexane, 1.8 mmol). The reaction mixture was slowly warmed to room temperature and stirred for 4 h. After cooling back to 78 o C, a solution of ClBAr f 2 (0.67 g, 1.88 mmol) in toluene (10 mL) was then added dropwise, during which time the colour of the solution turned to red orange. After warming to room temperature and stirring overnight, the reaction mixture was allowed to settle and filtered by cannula. The resulting filtrate was dried under vacuum and deep red orange solid was obtained. Yield: 0.85 g, 86.0 %. 2 was further purified by heating the crystals of compound 4, giving a orange crystalline powder.

Synthesis of 3
3 was synthesized in a manner similar to 2 using ClB(C 6 F 5 ) 2 instead of ClBAr f 2 . To a suspension of 1 (0.39 g, 0.93 mmol) in hexane (20 mL) at -78 o C was added sec-BuLi (1.39 mL of a 1.4 M solution in cyclohexane, 1.86 mmol). The reaction mixture was slowly warmed to room temperature and stirred for 4 h. After cooling back to -78 o C a solution of ClB(C 6 F 5 ) 2 (0.71 g, 1.86 mmol) in hexane (15 mL) was then added, during which the colour of the solution turned to red purple. After warming to room temperature and stirring overnight, the reaction mixture was allowed to settle and filtered by cannula. The resulting filtrate was dried under vacuum and deep red orange solid was obtained. Yield: 0.80 g, 89.8 %.
Although we were unable to obtain crystalline samples of 3 suitable for microanalysis (samples are typically oily), we were able to exploit the synthesis of the derived ONNP t Bu 3 complex (see below) to isolate a crystalline bulk sample for proof of bulk composition (including microanalysis). UV-vis (chloroform, λ max ): 500 nm (ε = 43,525 L mol -1 cm -1 ).

Synthesis of 4
2 (30 mg, 0.027 mmol) was dissolved in hexane (2 mL) and the solution degassed by the freezepump-thaw method, before CO was admitted (ca. 1 atm), at which point the product precipitated from the solution. The reaction mixture was kept stand for 10 min before the solvent was decanted. The obtained white crystalline solid was then washed twice with cold hexane (2 x 1 ml) and dried under vacuum. Crystals which were qualified for X-ray diffraction was obtained by dissolving 2 in toluene in NMR tube and charged with 1 atm CO. Yield: 27 mg, 87.8 %.

Synthesis of 5
3 (45mg, 0.048 mmol) was dissolved in 2 ml of hexane, the resulting solution was processed with three times freeze-pump-thaw and backfilled with 1 atm CO at -40 o C. The supernatant was decanted and the white powder was further washed with cold hexane (2 x 2 ml) and dried under vacuum. Yield: 10 mg, 22 %.

Synthesis of 6
3 (50 mg, 0.053 mmol) was dissolved in pentane (2 mL) and the solution degassed by the freezepump-thaw method, before CO was admitted (ca. 1 atm). The reaction mixture was heated at 60 o C for 10 min, cooled to room temperature and filtered by cannula. The resulting filtrate was concentrated to 5 ml and cooled to 4 o C for crystallization. 6 was obtained as a colourless crystalline material, which was washed twice with cold pentane (2 x 1 mL) and dried under vacuum. Crystals which were qualified for XRD was obtained by slow-evaporating a pentane solution of 6

Synthesis of 7
4 (124 mg, 0.11 mmol) and DMAP (13 mg, 0.11 mmol) were dissolved in chloroform (1 mL) and layered with pentane. The crystals of 7 could be obtained after few days. The crystals was then isolated, washed with benzene (2 x 1 ml) and pentane (2 x

Synthesis of 8
Compound 8 can be obtained by two methods. Method A: 2 (61.0 mg, 0.054 mmol) and P t Bu 3 (10.7 mg, 0.054 mmol) were dissolved in benzene, and the solution degassed by the freeze-pump-thaw before CO was admitted (ca. 1 atm). The reaction mixture was then stirred until the colour faded to almost colourless, upon which crystalline product precipitated from the solution which were qualified for XRD. The supernatent was then decanted off, and the product washed with cold hexane twice (2 x 5 mL) and dried. Yield: 57.1 mg, 79.6 %. Method B: 4 (99.5 mg, 0.086 mmol) and P t Bu 3 (17.5 mg, 0.086 mmol) were suspended in benzene (2 mL), and the mixture stirred for 10 min. The solvent was then decanted, affording the product as a colourless crystalline material, which was washed with cold hexane (2 x 5 mL) and dried. Yield: 85.0 mg, 72.6 %).  s9

Synthesis of 9
The synthesis of 9 was achieved via a similar method to the synthesis of 8 (via method A). 3 (16.6 mg, 0.018 mmol) and P t Bu 3 (3.6 mg, 0.018 mmol) were dissolved in benzene (1 mL), and the solution degassed by the freeze-pump-thaw method before CO was admitted (ca. 1 atm). The reaction mixture was shaken until the colour faded, at which point crystalline 9 precipitated from the solution. The supernatent was then decanted off, and the product washed with cold hexane (2 x 5 mL) and dried. Yield: 11.5 mg, 57.0%.
s11 Figure S3. 1 H NMR spectrum of 2 in C 6 D 6 at 298 K. Figure S4. 19 F NMR spectrum of 2 in C 6 D 6 at 298 K.
Figs S6 and S7 here -1 H and 19 F NMR spectra of 3 s14 Figure S9. 1 H NMR spectrum of 4 in CDCl 3 at 298 K. Figure S10. 19 F NMR spectrum of 4 in CDCl 3 at 298 K.
s25 Figure S30. 1 H NMR spectrum of 9 in CD 2 Cl 2 at 298 K. Figure S31. 19 F NMR spectrum of 9 in CD 2 Cl 2 at 298 K.

Computational studies
The computational work was performed using DFT within the Gaussian09 (Revision D.01) program package. [5] Geometry optimizations of the monoanionic ligand systems were performed with the PBE1PBE hybrid exchange-correlation functional [6] using a TZVP basis set. [7] Grimme's empirical dispersion correction (DFT-D3) was included in all geometry optimizations. [8] Unless otherwise stated, geometry optimizations were carried out for the full system, and frequency calculations were performed to confirm the nature of the stationary points found (minimum).