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SUPPLEMENTARY INFORMATIONDOI: 10.1038/NCHEM.1778

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Supplementary Information for:

Metal-Free Binding and Coupling of Carbon Monoxide at a Boron-Boron Triple Bond

Holger Braunschweig1*, Theresa Dellermann1, Rian D. Dewhurst1, William C. Ewing1, Kai

Hammond1, J. Oscar C. Jimenez-Halla1,2, Thomas Kramer1, Ivo Krummenacher1, Jan Mies1,

Ashwini K. Phukan1,3, Alfredo Vargas1

Affiliations

1 Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland,

97074 Würzburg, Germany

2 Facultad de Química, Universidad de Guanajuato, Mexico

3 Department of Chemical Sciences, Tezpur University, Napaam 784028, Assam, India

*Correspondence to: [email protected]

Website: http://www-anorganik.chemie.uni-wuerzburg.de/Braunschweig/

© 2013 Macmillan Publishers Limited. All rights reserved.



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Table of Contents

Synthetic Methods p3

Crystallographic Methods p5

Electrochemical Methods p7

Computational Methods p8

References p16




3

Synthetic methods

All manipulations were conducted under an atmosphere of dry argon or in vacuo using standard

Schlenk line or glove box techniques. Solvents (hexane, benzene) were purified by distillation

from appropriate drying agents (NaK2.8 alloy and sodium/benzophenone) under dry argon

immediately prior to use. C6D6 was degassed by three freeze-pump-thaw cycles and stored over

molecular sieves. NMR spectra were acquired on a Bruker Avance 500 NMR spectrometer (1H:

400.1 MHz; 11B: 160.4 MHz; 13C: 125.7 MHz). 1H and 13C{1H} NMR spectra were referenced to

external TMS via the residual protons of the solvent (1H) or the solvent itself (13C). 11B NMR

spectra were referenced to external BF3•OEt2. UV-vis spectra were measured on a JASCO V-660

UV-vis spectrometer. Elemental analyses were measured on an Elementar Vario MICRO cube

instrument. IR spectra were measured on a JASCO FT/IR 6200. The starting material 1 was

synthesized according to literature methods1.

Supplementary Figure S1. Synthesis of compounds 2 and 3 from diboryne 1.

Synthesis of 3 directly from 1. A solution of 1 (100 mg, 0.13 mmol) in benzene was treated

with carbon monoxide and stirred for 1 h at ambient temperature. The solvent was evaporated in




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vacuo and the crude product was washed with pentane (3 × 7 mL). Yield 83.0 mg (0.09 mmol,

70%, white solid). M.p.: 288 °C (decomp.). 1H NMR (500.1 MHz, C6D6, 293 K): δ = 7.24-7.21

(m, 4 H, CHaryl), 7.09-7.07 (m, 8 H, CHaryl), 6.19 (s, 4 H, CHNHC), 2.72-2.67 (m, 8 H, CHiPr),

1.31 (d, 24 H, CH3), 1.05 (d, 24 H, CH3). 13C{1H} NMR (125.8 MHz, C6D6, 293 K): δ = 189.0,

175.1, 161.3, 146.0, 134.5 (Cq), 130.6 (CHaryl), 124.3 (CHaryl), 123.0 (CHNHC), 29.2 (s, CHiPr),

25.0 (CH3), 24.0 (CH3). 11B NMR (160.4 MHz, C6D6, 293 K): δ = −3.6. IR: (solid) ν = 1670 (s),

1467 (s) cm-1. IR: (in benzene) ν = 1691 (s), 1428 (s), 1357 (s) cm-1. Elemental analysis calcd.

[%] for C58H72B2O4N4: C 76.31, H 7.92, N 6.15; found C 76.31 H 8.07, N 5.65.

Synthesis of 2. A solution of 1 (50 mg, 0.062 mmol) in hexane (3 mL) was treated under static

vacuum with carbon monoxide at –78 °C (2.8 mL, 1 bar, 0.12 mmol). The mixture was warmed

to ambient temperature within 3 h. The resulting brown suspension was filtered and washed with

hexane (2 × 3 mL) and dried in vacuo. Yield 35 mg (0.042 mmol, 68%). 1H NMR (400.1 MHz,

C6D6, 293 K): δ = 7.22-7.18 (m, 4 H, CHaryl), 7.06-7.04 (m, 8 H, CHaryl), 6.17 (s, 4 H, CHNHC),

3.07-3.00 (m, 8 H, CHiPr), 1.13 (vt, 48 H, CH3). 13C{1H} NMR (100.6 MHz, C6D6, 293 K): δ =

147.0, 135.5 (Cq), 129.6 (CHaryl), 123.8 (CHaryl), 121.2 (CHNHC), 28.7 (s, CHiPr), 24.9 (CH3),

24.2 (CH3) ppm. 11B NMR (128.4 MHz, C6D6, 293 K): δ = 18.4 ppm. IR: (in benzene) ν = 1926

(w), 1593 (m), 1568 (m), 1414 (m), 1353 cm-1 (s).




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Crystallographic Methods

The crystal data of 2 and 3 were collected on a Bruker X8-Apex II diffractometer with a CCD

area detector and multi-layer mirror monochromated MoKα radiation. The structure was solved

using direct methods, refined with the Shelx software package and expanded using Fourier

techniques. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were

included in structure factor calculations. All hydrogen atoms were assigned to idealised

geometric positions.

Selected bond lengths (Å) and angles (°) for 2: B1-B2 1.549(3), C1-B1 1.565(3), C1-B2

1.485(3), O1-C1 1.249(2); O1-C1-B2 155.38(17), O1-C1-B1 143.62(16), B2-C1-B1 61.00(13).

Crystal data for 2: C55H72B2N4O, Mr = 826.79, brown block, 0.34×0.25×0.07 mm3, monoclinic

space group P21, a = 12.8198(10) Å, b = 14.7557(11) Å, c = 13.2979(10) Å, β = 105.229(2)°,

V = 2427.2(3) Å3, Z = 2, ρcalcd = 1.131 g·cm–3, µ = 0.066 mm–1, F(000) = 896, T = 100(2) K,

R1 = 0.0526, wR2 = 0.0956, 10380 independent reflections [2θ≤53.62°] and 575 parameters.

Selected bond lengths (Å) and angles (°) for 3: B1-C1 1.4507(18), B1-C2 1.5742(18), B1-C11

1.5471(18), C1-C1’ 1.426(2), C1-O1 1.3853(14), C2-O1 1.4270(15), C2-O2 1.2152(15); C1-B1-

C11 131.25(11), C1-B1-C2 101.60(10), C11-B1-C2 126.99(11). Sum of angles around B1:

359.84°. Angle between NHC plane and C3BO plane: 45.65. Deviations from C6B2O4 plane:

0.002 (C11) - 0.050 (O2) Å. Crystal data for 3: C64H78B2N4O4, Mr = 988.92, yellow block,

0.31×0.25×0.22 mm3, monoclinic space group P21/n, a = 13.6446(6) Å, b = 14.3161(7) Å,

c = 15.0434(7) Å, β = 102.678(2)°, V = 2866.9(2) Å3, Z = 2, σcalcd = 1.146 g·cm–3,




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µ = 0.070 mm–1, F(000) = 1064, T = 173(2) K, R1 = 0.0527, wR2 = 0.1055, 6085 independent

reflections [2θ≤53.52°] and 354 parameters.

Crystallographic data have been deposited with the Cambridge Crystallographic Data Center as

supplementary publication nos. CCDC 935712 (2) and 913268 (3). These data can be obtained

free of charge from The Cambridge Crystallographic Data Centre via

www.ccdc.cam.ac.uk/data_request/cif




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Electrochemical Methods

Cyclic voltammetry experiments were performed using a Gamry Instruments Reference 600

potentiostat. A standard three-electrode cell configuration was employed using a platinum disk

working electrode, a platinum wire counter electrode, and a silver wire, separated by a Vycor tip,

serving as the reference electrode. Formal redox potentials are referenced to the

ferrocene/ferrocenium redox couple. Tetra-n-butylammonium hexafluorophosphate ([n-

Bu4N][PF6]) was employed as the supporting electrolyte.

Supplementary Figure S2. Cyclic voltammograms of compound 1 in thf/0.1 M [n-Bu4N][PF6]

at room temperature. Scan rate = 250 mV/s-1. The voltammogram on the right depicts an

experiment with a smaller sweep-width to illustrate the reversibility of the first oxidation wave of

1.




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Computational Details

Modelling of the UV-vis and fluorescence spectra of 3

For the absorption and emission studies, calculations were conducted using the Gaussian09

program package2 at the ONIOM(M06-2X/6-311+G(d):PM6) level. In order to test the reliability

of our approach, we also carried out a geometry optimization (and harmonic frequency

calculations) on the product of the reaction at the M06-2X/6-31+G(d) level to compare the

performance of both methods with the experimental data (Table S1). The optimized geometrical

parameters of 3 are in good agreement with the crystallographic structure. Also, we have

computed the S-value test which is low (−0.038853) and therefore, our ONIOM2 partition

scheme is very suitable for this study. We performed TD-DFT calculations at the same combined

semi-empirical-DFT level of theory in order to provide some insight into the observed

fluorescence phenomenon in the bis(bora)lactone product. We computed the lowest single

excitation energies and oscillator strengths and later, all the fluorescence electronic transitions

that were calculated as vertical de-excitations based on the optimized geometry of S1 state. We

also checked for the energy difference between the T0 and S0 states, ΔH = 23.82 kcal/mol (ΔG =

22.65) so it is more likely that the photoluminescence proceeds from singlet states as discussed

below. We performed additional calculations in the basal and first singlet excited state geometry

of 3 (P1). We conducted geometry optimizations within the ONIOM methodology at the

CAS(4,3)/6-31+G(d) level over the bis(boralactone) core, i.e. using a multideterminantal

wavefunction method to incorporate all the possible configurations of HOMO-1, HOMO and

LUMO orbitals as they are the most relevant in the bonding picture along the B=C-C=B

fragment. Later, we refined the obtained orbital energies by applying the second-order

perturbational theory as CASPT2(4,3)/6-31+G(d) in a single-point calculation.




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Supplementary Figure S3. The selected two-layered ONIOM is shown for compound 3 (or P1),

where the inner layer is depicted using the ball & stick type and the outer layer is depicted by the

wireframe style. Boron atoms are colored in green, nitrogens are light blue, oxygens are red,

carbons are grey and hydrogens are white.




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Table S1. Comparison of the optimized geometries of 3 with our experimental data. Bond

distances are given in Angstrøms and angles in degrees.

Geometrical

Parameter

Experimental

value

ONIOM(M06-

2X:PM6)

M06-2X/6-

31+G(d)

B1-C1 1.4314 1.4524 1.4598

B1-C2 1.5720 1.5875 1.5986

B1-C3(NHC) 1.5417 1.5439 1.5491

C1-C4 central bond 1.4326 1.4438 1.4428

C2=O1 1.2127 1.2112 1.2095

C2-O2-C4 107.9 108.9 109.6

C2-B1-C3-N3 145.9 152.9 163.6




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Supplementary Figure S4. Relevant frontier molecular orbitals of compound 3 (P1), isosurface

value = 0.03 a.u.




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Supplementary Figure S5. Comparison of both optimized geometries of ground state (left) and

singlet first excited state (right) involved in the observed fluorescence process. Selected

geometrical parameters are shown: bond distances in Å, angles in degrees. Our CASPT2(4,3)

calculations in both optimized geometries also support the bonding situation of the ground state

and the biradical character of the first singlet excited state.

Supplementary Table S2. Vertical excitation energies, maximum wavelength and oscillator

strengths of both electronic transitions calculated at the specified level of theory.

Transition Excitation energy (eV) λmax (nm) Oscillator strength

S0→S1 2.80 542.2 0.90

S1→S0 2.49 567.6 0.89

Compound 3 is red-orange in solution at room temperature, but appears yellow at −78 °C. Strong

absorption bands in the green-blue-violet region of the UV-vis spectrum of 3 explain this red-

orange color (Figure S6a). Pulvinic acid derivatives are partly responsible for the fluorescence of

lichens, including pulvinic acid lactone - which fluoresces yellow under UV light3,4. The

structural similarities of bis(boralactone) 3 to pulvinic acid lactone are striking - they differ only




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in the replacement of a [Ph-C] fragment with an isoelectronic [NHC→B] fragment. Likewise,

compound 3 fluoresces yellow under UV light; the fluorescence spectrum of 3 showed two major

bands at 549 and 590 nm (Figure S6b). Time-dependent DFT calculations reproduced the major

absorption (exp: 540 nm; calc: 542 nm) and emission bands (exp: 549 nm; calc: 568 nm) of 3

very well (see Figure S6a). Further calculation at the CASPT2(4,3) level indicates that upon UV

irradiation, compound 3 effectively forms a bis(base-stabilized boraalkene) with a B=C-C=B

core, is excited to a singlet diradical state with a B(�)-C=C-B(�) core. A small contraction in the

B-C(carbene) distances (1.5305 vs. 1.5440 Å), and an increase in the coplanarity of the NHC and

bis(boralactone) rings (torsion angle: 18.9 vs 27.4°), both indicate that a non-negligible amount

of B=C double bonding is present in the excited state molecule.




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Supplementary Figure S6. Experimental and computed photophysical properties of 3. a,

Absorbance (blue) and fluorescence (red) spectra of 3. The insets show the room-temperature

color of 3 (left) and its fluorescence (right). b, Simulated UV-Vis and fluorescence spectra

according to TD-DFT calculations, corresponding to the S0-S1 observed transitions.




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Geometry optimisations of 1, 2 and 3 for energy evaluations

Geometry optimizations were carried out in the gas phase using the Amsterdam Density

Functional (ADF)5,6 program at the OLYP/DZP7–11 level. To obtain the singlet state, spin-

restricted calculations were performed constraining the projection of the total electronic spin

along a reference axis to zero. The Jmol11 program was used for vizualisation purposes.

Supplementary Figure S7. Calculated and experimental (in italics) geometrical parameters for

1, 2 and 3. Bond distances in Å, calculations were performed at the OLYP/DZP level of theory.




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References

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