Intramolecular N-coordination in ketiminoboranes

Abstract

Treatment of the imine PhC(=NSiMe₃)py with Et₂BOMe or BF₃·Et₂O afforded bicyclic ketiminoboranes 4a and 4bvia intramolecular N-coordination. The basicity of the imine N is evidenced by their reactivity towards Brønsted and Lewis acids and the structures of 4a·HCl and 4b·BF₃ are reported as well as the dipyridyl imine derivative 4c·HCl.

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The use of intramolecular coordination of a pyridyl group has been exploited in recent years as a route to novel main group heterocycles. For example Dyer and co-workers investigated the intramolecular N-coordination of 2-pyridyl-N-phosphino-imines (1) and found that an equilibrium existed between open and closed forms 1a and 1b.¹ The 2-coordinate nitrogen in 1b is found to be sufficiently basic to form the adduct 1c with Lewis acidic AlCl₃,¹ whilst oxidation with [(ⁱPr₂N)₂P][OTf] led to an unusual π-conjugated coupled products (1d and 1e) (Scheme 1) via an oxidative radical coupling process.²

Scheme 1

Studies on the chemistry of related group 16 compounds revealed similar behaviour between ring-open and ring-closed products (Scheme 2). For example, when X = Ar (E = S) the open-form 2a is favoured with a short intramolecular S⋯N contact whereas when X = Cl (E = S, Se) then the ring-closed form 2b was favoured.³ Work by Brusso and co-workers has revealed that at elevated temperatures ring-opening of these N-bridgehead thiadiazoles can occur.⁴ Similar intramolecular N-coordination has been implemented to generate hypervalent Si^IV (3).⁵

Scheme 2

In these compounds the group 14/15/16 heteroatoms are all formally electron precise centres and intramolecular N-coordination makes them hypervalent affording some degree of lability between open and closed forms. Conversely group 13 elements are Lewis acidic and ring closure is expected to be strongly favoured. Ketiminoboranes, R₂C=N-BR₂ were reported by Hawthorne, Wade and Lappert in the 1960's and are variously monomeric or dimeric depending upon substituents, with the monomeric ketimines R₂C=NBR₂ formally isoelectronic with allene.⁶ In the current manuscript we describe the synthesis of ketiminoboranes in which the R group is capable of intramolecular coordination forming novel C/N/B heterocycles (4a–c). These heterocycles are similar to N,N′-boron chelate complexes, particularly derivatives of BODIPY, which have attracted considerable attention for their fluorescent properties,⁷ as dyes in photodynamic therapy,⁸ as well as photo-induced electron and energy transfer⁹ and as optical switches¹⁰inter alia. In the current paper we describe the generation of 4a–4c and find that the 2-coordinate imine nitrogen is strongly basic, permitting us to isolate and structurally characterise 4a·HCl, and 4b·BF₃ and 4c·HCl.§

Compounds 4a–4c were prepared using a similar condensation reaction to that employed by Wade^6e to prepare Ph₂C=NBPh₂i.e. via the condensation of the imine Ar₂C=NSiMe₃ with either Et₂BOMe or BF₃ OEt₂. Crystals of 4a·HCl and 4c·HCl appeared over 3 days and were isolated by filtration (27–37% unoptimised isolated yield). The HCl presumably arises from adventitious hydrolysis of Me₃SiCl. Crystals of 4b·BF₃ were initially recovered in low yield from the reaction of PhC(=NSiMe₃)py with BF₃·Et₂O in a 1 : 1 ratio but substantially improved yields (61%) were achieved using a 1 : 2 ratio. This suggests that the low solubility of the adduct favours crystallisation of the 1 : 2 product.

The ¹H NMR spectrum of 4a·HCl clearly reveals a broad singlet at 16.3 ppm consistent with N-protonation whilst the ¹¹B NMR spectrum revealed a singlet at +8 ppm consistent with a tetrahedral B centre and a molecular ion peak at m/z = 251 with an isotope distribution pattern consistent with 4a·H⁺. The structure of 4a·HCl was determined by X-ray diffraction (Fig. 1) and found to crystallise as a THF solvate. The B–C bonds are unexceptional but the B–N bond lengths are slightly different (within 3 esd's) with the B1–N1 bond (1.561(5) Å) somewhat shorter than the formally dative pyridyl B–N bond (1.595(5) Å). Both are consistent with B–N single bond character (1.57–1.60 Å).¹¹ The C10–N1 bond at 1.285(4) is short, consistent with significant imine character. At 3.058(3) Å the N1⋯Cl1 distance is consistent with a conventional N–H⋯Cl hydrogen-bonded contact.¹²

Fig. 1 Molecular structure of 4a·HCl (THF solvent omitted for clarity) with thermal ellipsoids drawn at the 50% probability level. Selected bond lengths: B1–C31 1.598(6), B1–C33 1.598(5), B1–N1 1.561(5), B1–N2 1.595(5), N1–C10 1.285(4), N2–C11 1.355(4), C10–C11 1.476(4) Å.

The ¹¹B and ¹⁹F spectra of 4b·BF₃ revealed four¹⁹F and four¹¹B NMR resonances, the intensities of which varied depending upon solvent. In the ¹¹B NMR in MeCN two triplet resonances are observed in the 4–8 ppm range corresponding to two chemically distinct BF₂ environments, comparable with other 4-coordinate BN₂F₂ centres.^10,13 In addition, a quartet at 0 ppm and a singlet at −1 ppm are observed (see ESI†). The quartet we tentatively assign to the N-coordinated BF₃ in 4b·BF₃ and the singlet as BF₃ MeCN, based on chemical shift. These observations suggest a dynamic equilibrium (eqn (1)) in which the coordinated BF₃ is labile in the presence of coordinating solvents. In the ¹⁹F NMR three resonances exhibit ¹¹B hyperfine coupling (see ESI†) and the HMQC 2D NMR spectrum (Fig. 2) along with coupling constants confirms the assignments of the corresponding BF₂ and BF₃ groups. In the ¹⁹F NMR spectrum in MeCN the BF₂ fluorine atoms in both 4b and 4b·BF₃ appear around −159 ppm, reflecting very similar chemical environments whereas the BF₃ resonances appear at −152 ppm (BF₃·MeCN) and −141 ppm (4b·BF₃). The resonance at −152 ppm appears as two signals in an approximate 4 : 1 ratio separated by 0.3 ppm and reflects the ¹¹B and ¹⁰B isotopomers (∼80 : 20 natural abundance). In non-coordinating solvents such as benzene just two ¹¹B resonances are detected suggesting displacement of BF₃ in non-coordinating solvents is unfavourable and the structure of 4b·BF₃ appears fully retained in solution.

Fig. 2 ¹¹B–¹⁹F HMQC NMR spectra of 4b·BF₃ in MeCN.

Crystals of 4b·BF₃ were grown from the mother liquor on standing for 24–48 h. Single crystal structure determination revealed one molecule per asymmetric unit (Fig. 3). The heterocyclic C₂N₂B ring exhibits a similar geometry to the ethyl derivative with a longer B–N bond to the pyridyl nitrogen (1.600(2) Å) than to the imine nitrogen (1.574(2) Å) and a short imine-like C=N bond (1.286(2) Å). These distances fall at the extremes of those reported previously for other C₂N₂B heterocycles with a pyridyl nitrogen atom coordinated to a BF₂ group in which the dative bonds fall in the range 1.60–1.63 Å and the covalent B–N bonds fall in the range 1.50–1.57 Å.^9,12 The exo B–N dative bond length to the BF₃ group, at 1.600(2) Å, is identical to the dative pyridyl-N–B bond.

Fig. 3 Molecular structure of 4b·BF₃ with ellipsoids drawn at the 50% probability level. Selected bond lengths: B1–F11 1.366(2), B1–F12 1.364(2), B1–N11 1.600(2), B1–N12 1.574(2), N11–C11 1.349(2), C10–N12 1.286(2), C10–C11 1.490(2), N12–B2 1.600(2) B2–F21 1.377(2), B2–F22 1.377(2), B2–F23 1.366(2) Å.

Theoretical calculations (DFT B3LYP/6-311G*) on the reaction of 4b with BF₃ indicate adduct formation in the gas phase is favoured by 75 kJ mol⁻¹ (see ESI†). Additional calculations along the B⋯N bond forming pathway reveal no significant activation energy barrier to formation of 4b·BF₃. However stabilisation of the ‘free’ BF₃ in coordinating solvents through adduct formation such as MeCN·BF₃ or THF·BF₃ is expected to destabilise 4b·BF₃ with respect to loss of BF₃. An NBO analysis revealed a bonding pattern best represented by the figure shown for 4b·BF₃ (eqn (1)) (see ESI†). Notably the reaction of py₂C=O with Li[N(SiMe₃)₂]/Me₃SiCl, followed by 1 equivalent of Et₂BOMe afforded the pyridyl analogue, 4c·HCl in which the diazaborole nitrogen is protonated rather than the pyridyl nitrogen atom, reflecting the strongly basic nature of the diazaborole nitrogen atom (pK_b = 5.6, calculated using DFT methods), cf. pyridine (pK_b = 8.8).¹⁴ Synthetic details and crystallographic data for 4c·HCl are available as ESI.†

The current studies reflect the diversity of heterocyclic ring systems accessible by intramolecular N-coordination. Unlike the later p-block elements in which intramolecular coordination generates a hypervalent multi-centre bonding interaction, the electron poor boron centre adopts a 4-coordinate electron-precise centre upon intramolecular coordination. The resultant heterocycle offers a strongly basic nitrogen atom which affords similar acid–base chemistry to the N-pyridyl phosphine-imines.

Acknowledgements

We would like to thank EPSRC (C.E.B.), NSERC and the Canada Research Chairs program (J.J.H.) and the University of Windsor (L.M.) for financial support. In addition we are indebted to Dr M. Revington for assistance with the 2D multinuclear NMR studies.

Notes and references

1. D. A. Smith, A. S. Batsanov, K. Miqueu, J.-M. Sotiropoulos, D. C. Apperley, J. A. K. Howard and P. Dyer, Angew. Chem., Int. Ed., 2008, 47, 8674.
2. D. A. Smith, A. S. Batsanov, M. A. Fox, A. Beeby, D. C. Apperley, J. A. K. Howard and P. Dyer, Angew. Chem., Int. Ed., 2009, 48, 9109.
3. C. E. Bacon, D. J. Eisler, R. L. Melen and J. M. Rawson, Chem. Commun., 2008, 4924.
4. A. A. Leitch, I. Korobkov, A. Assoud and J. L. Brusso, Chem. Commun., 2014, 50, 4934.
5. See for example: (a) G. Klebe, J. W. Bats and K. Hensen, J. Chem. Soc., Dalton Trans., 1985, 1; (b) G. Klebe, J. W. Bats and H. Fuess, J. Am. Chem. Soc., 1984, 106, 5203.
6. (a) M. F. Hawthorne, Tetrahedron, 1962, 17, 117; (b) J. E. Lloyd and K. Wade, J. Chem. Soc., 1964, 1649; (c) I. Pattison and K. Wade, J. Chem. Soc. A, 1967, 1098; (d) V. A. Dorokhov and M. F. Lappert, Chem. Commun., 1968, 250; (e) J. R. Jennings, I. Pattison, C. Summerford, K. Wade and B. K. Wyatt, Chem. Commun., 1968, 250; (f) C. Summerford and K. Wade, J. Chem. Soc. A, 1969, 1487; (g) C. Summerford and K. Wade, J. Chem. Soc. A, 1970, 2010.
7. N. Boens, V. Leen and W. Dehaen, Chem. Soc. Rev., 2012, 41, 1130; G. Ulrich, R. Ziessel and A. Harriman, Angew. Chem., Int. Ed., 2008, 47, 1184.
8. A. Kamkaew, S. H. Lim, H. B. Lee, L. V. Kiew, L. Y. Chung and K. Burgess, Chem. Soc. Rev., 2013, 42, 77.
9. (a) Y. Nagata and Y. Chujo, Macromol., 2008, 41, 3488; (b) C. C. Cheng, W.-S. Yu, P.-T. Chou, S.-M. Peng, G.-H. Lee, P.-C. Wu, Y.-H. Song and Y. Chi, Chem. Commun., 2003, 2628.
10. Y. Yang, R. P. Hughes and I. Aprahamian, J. Am. Chem. Soc., 2012, 134, 15221.
11. C. E. Housecroft and A. G. Sharpe, Inorganic Chemistry 3rd Edition, Pearson Ed, Harlow, England, 2008.
12. A search of the CSD (2013) afforded intermolecular hydrogen-bonded N⋯Cl distances in the range 2.939–3.298 Å (mean 3.12 Å) between three-coordinate N (R₂NH) and Cl⁻ ions.
13. (a) K. Hensen, G. Klebe, J. W. Bats and H. Fuess, Fresenius’ J. Anal. Chem., 1982, 312, 24; (b) G. Klebe, K. Hensen and H. Fuess, Chem. Ber., 1983, 116, 3125; (c) J. J. Klappa, S. A. Geers, S. J. Schmidtke, L. A. MacManus-Spencer and K. McNeill, Dalton Trans., 2004, 883; (d) M. Mao, S. Xiao, J. Li, Y. Zou, R. Zhang, J. Pan, F. Dan, K. Zou and T. Yi, Tetrahedron, 2012, 68, 5037; (e) M. Mao, R. Zhang, S. Xiao and J. Zou, J. Coord. Chem., 2011, 64, 3303; (f) Q. Cao, S. Xiao, M. Mao, X. Chen, S. Wang, L. Li and K. Zou, J. Organomet. Chem., 2012, 717, 147.
14. N. Tro, T. D. Frigden and L. E. Shaw, Chemistry: A Molecular Approach, (Canadian Edition), Pearson Ed., 2013.

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Footnotes

† In memory of Ken Wade: Teacher, oft-times mentor, colleague and friend. His contributions in the field of structure and bonding in main group chemistry will continue into the future, but his guidance, encouragement and support for so many of the young academics he came in contact with will be sorely missed.

‡ Electronic supplementary information (ESI) available: Full experimental details, details of the computational and crystallographic studies and crystallographic data in cif format. CCDC 1043411–1043413. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt00196j

§ Crystal data for 4a·HCl·THF monoclinic space group P2₁/c, M = 358.70. T = 240(2) K, a = 9.2150(2), b = 13.4773(3), c = 17.1221(4) Å, b = 97.697(2)°, V = 2107.29(8) ų, Z = 4, D_c = 1.131 g cm⁻³, μ(Mo-Kα) = 0.191 mm⁻¹. 21 422 reflections measured (3.76 ≤ 2θ ≤ 29.98°) of which 6052 unique (R_int = 0.055). Final R₁ (I > 2σ(I)) = 0.093, wR₂ (all data) = 0.179 for 214 parameters. Max/min electron density +0.50/−0.48 e⁻ Å⁻³.

Crystal data for 4b·BF₃ orthorhombic space group Pbca, M = 297.83. T = 173(2) K, a = 9.934(3), b = 12.290(4), c = 21.001(7) Å, V = 2564.0(14) ų, Z = 8, D_c = 1.543 g cm⁻³, μ(Mo-Kα) = 0.142 mm⁻¹. 25 423 reflections measured (1.94 ≤ 2θ ≤ 27.88°) of which 2957 unique (R_int = 0.052). Final R₁ (I > 2σ(I)) = 0.048, wR₂ (all data) = 0.139 for 197 parameters. Max/min electron density +0.49/−0.20 e⁻ Å⁻³.

Crystal data for 4c·HCl·THF monoclinic space group P2₁, M = 359.71. T = 180(2) K, a = 9.38350(10), b = 12.8596(3), c = 16.6196(3) Å, β = 99.1807(12)°, V = 1979.77(6) ų, Z = 4, D_c = 1.207 g cm⁻³, μ(Mo-Kα) = 0.204 mm⁻¹. 25 481 reflections measured (2.69 ≤ 2θ ≤ 26.37°) of which 7194 unique (R_int = 0.051). Final R₁ (I > 2σ(I)) = 0.045, wR₂ (all data) = 0.106 for 453 parameters. Max/min electron density +0.30/−0.34 e⁻ Å⁻³.

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