Symmetrical non-chelating poly-N-heterocyclic carbenes

Abstract

A facile synthetic protocol, which utilized dehydrative condensation of phthalic anhydride and primary amines, for symmetrical non-chelating poly-N-heterocyclic carbene ligands was established. With the designed bi-functional building block, poly-NHC ligands having various denticities and geometries could be readily achieved by varying the structure of poly-amino compounds. Examination of catalytic activity of a series of poly-NHC–Ni complexes revealed the lack of electronic interference among NHC–metal units.

---

Being one of the most versatile ligands in stabilizing transition metals and main group elements, N-heterocyclic carbene (NHC) has shown tremendous impacts on both applied and fundamental chemical researches.^1–9 Nowadays, part of the interests of NHC ligands has shifted towards developments in chelating- and bridging-type multi-topic NHC ligands.^10–21 One of the recent advances of poly-NHCs is its application in constructing the corresponding poly-nuclear metal complexes and organometallic suparmolecules.^22–28 To extend the metal–carbene chemistry to microporous organometallic polymers, non-chelating rigid poly-NHCs are indispensable. As shown in the works on porous NHC–metal polymers, the use of tris-NHCs featuring symmetrical and rigid molecular backbones like triptycene and triphenylene was imperative in realizing microporous polymers.^29,30 However, syntheses of symmetrical tris-NHCs such as type A³¹ and B^32–34 are not trivial (Scheme 1).³⁵ On the other hand, coupling of halobenzene and imidazoles has proved to be useful in expanding the ligand library of poly-NHCs (type C), which tend to form discrete molecular structures.^36–39 In order to broaden the application of poly-NHCs in material chemistry, we decided to develop a general synthetic method for highly symmetrical poly-benzimidazolium salts from assembly of simple molecular building parts. To show that these poly-benzimidazolium salts can serve as precursors for poly-NHCs, we have also prepared the corresponding poly-nuclear metal complexes and investigated their application in hydrothiolation reaction.

In our previous study on the spiroborate-linked anionic bis-NHC ligand (type D),⁴⁰ we have shown that a linear twisted bis-benzimidazolium salt could be readily prepared from reaction of benzimidazolium derivatives and arylboronic acid. Unfortunately, such assembly protocol was limited to the spiroborate linkage that restricted the development of the catechol-containing benzimidazolium molecule in poly-NHCs. To access a large ligand library of highly symmetrical poly-NHC ligands, we have designed a new bi-functional molecular building block, which features an imidazole moiety on one side of the benzene ring and a phthalic anhydride functionality on the other side. This molecular LEGO brick can be assembled with poly-amino compounds to yield the respective poly-phthalimide derivatives, which are stable under normal pH range or in the presence of bulky bases. The anticipated poly-benzimidazolium salts can be obtained after the second alkylation. With such method, the dimension, denticity, and symmetry of the poly-benzimidazolium salts can be readily controlled by the structure of poly-amino compounds. The corresponding poly-nuclear metal complexes can then be prepared after metalation. Since all ylidene–metal bonds lie on the symmetry axes of ligands, free rotation of chemical bonds has no impact on the relative metal–metal distances and orientations. Similar design of ligand has also been applied in the synthesis of dianionic Janus-type bis(maloNHC).⁴¹

Scheme 1

The synthesis of the molecular LEGO brick is straightforward. Starting from the commercially available 5,6-dimethyl benzimidazole, the N-ethyl-benzimidazole-5,6-dicarboxylic anhydride (1) could be prepared in multi-gram scale in four steps (Scheme 2). Synthesis of the corresponding benzimidazolium salt was found to be difficult, plausibly due to the presence of electron withdrawing acid anhydride group that reduced the nucleophilicity of the nitrogen atom. Therefore, the second alkylation was carried out after dehydrative condensation of 1 and poly-amino compounds to afford the anticipated poly-benzimidazolium salts.

Scheme 2 Synthesis of 1.

As shown in Scheme 3, linear bis-benzimidazolium salt ([2a][I]₂) was synthesized in good yield from condensation of 1 and para-phenylenediamine, followed by alkylation with ethyl iodide. The ¹H NMR spectroscopic characterization of [2a][I]₂ revealed that [2a]²⁺ could freely rotate in solution leading to detection of only one set of signals. The same assembly procedure could also be applied to ortho-phenylenediamine to afford the ortho-bis-benzimidazolium salt ([2b][I]₂), which has also been characterized by NMR and high-resolution mass spectroscopies. With two benzimidazolium groups in proximity, the yield of [2b][I]₂ was considerably lower than its para-isomer. Single crystal X-ray diffraction analysis of [2a][I]₂ further confirmed the identity of the compound. As shown in Fig. 1, the two benzimidazolium units are essentially co-planar with distance between the two C1 carbons of nearly 19 Å.

Scheme 3 Syntheses of poly-benzimidazolium salts. Reaction conditions: (i) DMF–CH₃CN (v/v = 1 : 1), reflux. (ii) EtI, DMF, reflux.

Fig. 1 Molecular structure of [2a][I]₂. Thermal ellipsoids were set at the 50% probability level. Hydrogen atoms, counter anions and solvent molecules were omitted for clarity.

Synthesis of tris-benzimidazolium salt ([3][I]₃) was achieved with the same procedure by utilizing 1,3,5-tri-(4-aminophenyl) benzene as the assembly platform. The symmetric structure of [3][I]₃ was confirmed with ¹H NMR spectrum, which contained only one set of signals for the benzimidazolium unit. The tricationic molecule was further characterized with HRTOF-MS spectrometry, where three signals were detected within one amu, as expected for molecules bearing three positive charges. Similar reaction was carried out using tetra-(4-aminophenyl) methane as starting material for the preparation of three-dimensional tetra-dentate ligand, the tetra-benzimidazolium salt ([4][I]₄). Once again, the existence of [4][I]₄ and its symmetric nature were confirmed with HRTOF-MS and ¹H NMR spectroscopy, respectively. In all cases, the denticity and geometry of the resulting poly-benzimidazolium salts were governed by the structure of poly-amino compounds. This result reemphasizes that this assembly procedure is an extreme versatile method for the synthesis of poly-benzimidazolium salts.

With compound [2a][I]₂, [3][I]₃, and [4][I]₄ in hand, we then began to examine whether these phthalimide-containing benzimidazolium salts could function as precursors for poly-NHCs. Reaction of these benzimidazolium salts with nickelocene resulted in isolation of poly-nuclear nickel carbene complexes in moderate to good yield. The reactions were carried out under nitrogen atmosphere in DMF and THF mixed solvent system at 100 °C. The brick-red compound [2a-(NiCpI)₂], [3-(NiCpI)₃], and [4-(NiCpI)₄] were characterized using multi-nuclear NMR spectroscopies and mass spectrometry. Two common ¹H NMR spectral features for compound [2a-(NiCpI)₂], [3-(NiCpI)₃], and [4-(NiCpI)₄] were observed. The first one was the disappearance of the characteristic ¹H resonance of the [N–CH–N]⁺ proton. The overall up-field shifted ¹H NMR spectrum also suggested the formation of nickel-NHC complexes. The second spectral feature was the simplicity of the signals, which was indicative of the presence of symmetrical molecule in solution. The formation of the Ni-ylidene bond was further verified with HMBC spectroscopy. The ylidene centre of compound [2a-(NiCpI)₂], [3-(NiCpI)₃], and [4-(NiCpI)₄] were detected at 191.7 ppm, 191.5 ppm, and 191.6 ppm, respectively. These values were essentially identical, suggesting the similarity in molecular electronics for all poly-nuclear nickel complexes in the series. In addition, the correlation between carbene carbon and Cp hydrogen in [3-(NiCpI)₃] was identified in the HMBC spectrum, confirming the formation of the C_(NHC)–Ni bonds. The identity of [2a-(NiCpI)₂] was further corroborated with single crystal X-ray diffraction analysis. The observed Ni-ylidene bond distance of 1.881 Å was typical,^42,43 and the two nickel centres were separated by 22.7 Å with the two Ni–I bonds pointing at opposite direction (Fig. 2).

Fig. 2 Molecular structure of [2a-(NiCpI)₂]. Thermal ellipsoids were set at the 50% probability level. Hydrogen atoms and solvent molecules were omitted for clarity.

In addition to the poly-NHC nickel complexes, we have also prepared the corresponding NHC–Rh(I) complexes. In all cases, the poly-benzimidazolium salt was deprotonated with KO^tBu in THF/DMF, and was allowed to react with [Rh(COD)Cl]₂ at ambient temperature. The formation of NHC–Rh(COD)I complexes was confirmed with NMR and mass spectroscopies. Once again, only one set of 1H resonance signals was observed for all poly-NHC rhodium complexes. The 13C NMR signals of the ylidene centre were detected at 204.2 ppm, 203.9 ppm, and 203.9 ppm for [2a-(Rh(COD)I)₂], [3-(Rh(COD)I)₃], and [4-(Rh(COD)I)₄], respectively.^44–46 Correlation between the ylidene centre and hydrogen atoms of COD ligand was also recognized in the HMBC spectrum of [2a-(Rh(COD)I)₂], supporting the formation of the anticipated complex.

To examine whether the assembly of multiple metal centres affects the catalytic performance of each individual NHC-metal unit, nickel catalysed hydrothiolation of alkyne was carried out using the aforementioned Ni–NHC complexes.⁴⁷ In order to evaluate the catalytic activity of Ni complex supported by the phthalimide-containing NHC ligand, mononuclear nickel complex, [5-(NiCpI)], was also prepared. As shown in Fig. 3, the reaction time profile of the hydrothiolation showed insignificant correlation of the product yield to the molecular backbone of the poly-nuclear nickel complexes. In other word, the catalytic activity of every [NiCpI] moiety in the poly-NHC nickel complexes was independent of each other. This result suggests that poly-NHC ligands reported in this work are great candidates for construction of organometallic catalysts embedded extended networks, in which the interference between catalytic centres is minimized.⁴⁸

Fig. 3 Reaction time profile of the NHC–Ni catalysed hydrothiolation. Reaction condition: 1-heptyne 1 eq., thiophenol 2.5 eq., Ni atom 3 mol%, Et₃N 6 mol%, CDCl₃, 60 °C.

In conclusion, we have designed and synthesized a bi-functional molecular building block for the assembly of poly-benzimidazolium salts. The assembly method has proved to be adaptable for all aniline derivatives we have examined thus far, even for the othro-di-substituted one. These poly-benzimidazolium salts could also be converted into the corresponding poly-nuclear nickel and rhodium complexes. With this assembly approach, the size, denticity, geometry, and dimension of symmetrical non-chelating poly-NHC ligands for application in organometallic supramolecules can be readily prepared from simple molecular building units. Extension of this synthetic approach to amino acid containing NHC–metal complexes and application of the reported poly-NHC in organometallic polymers are currently under investigation.

Acknowledgements

This work is supported by the Ministry of Science and Technology of Taiwan (NSC 101- 2113-M-002-013-MY2). C.-W. Chiu is grateful to Kenda Foundation for the Golden-Jade Fellowship.

Notes and references

1. W. A. Herrmann and C. Köcher, Angew. Chem., Int. Ed., 1997, 36, 2162–2187.
2. D. Bourissou, O. Guerret, F. P. Gabbaï and G. Bertrand, Chem. Rev., 2000, 100, 39–91.
3. D. Enders, O. Niemeier and A. Henseler, Chem. Rev., 2007, 107, 5606–5655.
4. F. E. Hahn and M. C. Jahnke, Angew. Chem., Int. Ed., 2008, 47, 3122–3172.
5. B. Alcaide, P. Almendros and A. Luna, Chem. Rev., 2009, 109, 3817–3858.
6. S. Díez-González, N. Marion and S. P. Nolan, Chem. Rev., 2009, 109, 3612–3676.
7. C. Samojlowicz, M. Bieniek and K. Grela, Chem. Rev., 2009, 109, 3708–3742.
8. M. Fevre, J. Pinaud, Y. Gnanou, J. Vignolle and D. Taton, Chem. Soc. Rev., 2013, 42, 2142–2172.
9. M. N. Hopkinson, C. Richter, M. Schedler and F. Glorius, Nature, 2014, 510, 485–496.
10. M. Poyatos, J. A. Mata and E. Peris, Chem. Rev., 2009, 109, 3677–3707.
11. U. Kernbach, M. Ramm, P. Luger and W. P. Fehlhammer, Angew. Chem., Int. Ed., 1996, 35, 310–312.
12. R. E. Douthwaite, J. Houghton and B. M. Kariuki, Chem. Commun., 2004, 698–699.
13. I. Nieto, F. Cervantes-Lee and J. M. Smith, Chem. Commun., 2005, 3811–3813.
14. S. B. Muñoz, W. K. Foster, H.-J. Lin, C. G. Margarit, D. A. Dickie and J. M. Smith, Inorg. Chem., 2012, 51, 12660–12668.
15. S. Gonell, M. Poyatos, J. A. Mata and E. Peris, Organometallics, 2012, 31, 5606–5614.
16. S. C. Seitz, F. Rominger and B. F. Straub, Organometallics, 2013, 32, 2427–2434.
17. B. Blom, G. Tan, S. Enthaler, S. Inoue, J. D. Epping and M. Driess, J. Am. Chem. Soc., 2013, 135, 18108–18120.
18. N. Darmawan, C.-H. Yang, M. Mauro, M. Raynal, S. Heun, J. Pan, H. Buchholz, P. Braunstein and L. De Cola, Inorg. Chem., 2013, 52, 10756–10765.
19. T. O. Howell, A. J. Huckaba and T. K. Hollis, Org. Lett., 2014, 16, 2570–2572.
20. S. Schick, T. Pape and F. E. Hahn, Organometallics, 2014, 33, 4035–4041.
21. Y. Li, J. Tang, J. Gu, Q. Wang, P. Sun and D. Zhang, Organometallics, 2014, 33, 876–884.
22. A. J. Boydston, K. A. Williams and C. W. Bielawski, J. Am. Chem. Soc., 2005, 127, 12496–12497.
23. A. J. Boydston and C. W. Bielawski, Dalton Trans., 2006, 4073–4077.
24. A. J. Boydston, J. D. Rice, M. D. Sanderson, O. L. Dykhno and C. W. Bielawski, Organometallics, 2006, 25, 6087–6098.
25. K. A. Williams, A. J. Boydston and C. W. Bielawski, Chem. Soc. Rev., 2007, 36, 729–744.
26. J. Choi, H. Y. Yang, H. J. Kim and S. U. Son, Angew. Chem., Int. Ed., 2010, 49, 7718–7722.
27. M. Schmidtendorf, T. Pape and F. E. Hahn, Angew. Chem., Int. Ed., 2012, 51, 2195–2198.
28. C. Mejuto, G. Guisado-Barrios and E. Peris, Organometallics, 2014, 33, 3205–3211.
29. C. Zhang, J.-J. Wang, Y. Liu, H. Ma, X.-L. Yang and H.-B. Xu, Chem.–Eur. J., 2013, 19, 5004–5008.
30. S. Gonell, M. Poyatos and E. Peris, Chem.–Eur. J., 2014, 20, 5746–5751.
31. K. A. Williams and C. W. Bielawski, Chem. Commun., 2010, 5166–5168.
32. S. Gonell, M. Poyatos and E. Peris, Angew. Chem., Int. Ed., 2013, 52, 7009–7013.
33. S. Gonell, R. G. Alabau, M. Poyatos and E. Peris, Chem. Commun., 2013, 49, 7126–7128.
34. Y.-T. Wang, M.-T. Chang, G.-H. Lee, S.-M. Peng and C.-W. Chiu, Chem. Commun., 2013, 49, 7258–7260.
35. C. Segarra, J. Linke, E. Mas-Marza, D. Kuck and E. Peris, Chem. Commun., 2013, 49, 10572–10574.
36. A. Rit, T. Pape and F. E. Hahn, J. Am. Chem. Soc., 2010, 132, 4572–4573.
37. A. Rit, T. Pape, A. Hepp and F. E. Hahn, Organometallics, 2011, 30, 334–347.
38. R. Maity, A. Rit, C. Schulte to Brinke, J. Kösters and F. E. Hahn, Organometallics, 2013, 32, 6174–6177.
39. R. Maity, H. Koppetz, A. Hepp and F. E. Hahn, J. Am. Chem. Soc., 2013, 135, 4966–4969.
40. J.-H. Su, G.-H. Lee, S.-M. Peng and C.-W. Chiu, Dalton Trans., 2014, 43, 3059–3062.
41. D. Tapu, Z. McCarty and C. McMillen, Chem. Commun., 2014, 50, 4725–4728.
42. C. D. Abernethy, J. A. C. Clyburne, A. H. Cowley and R. A. Jones, J. Am. Chem. Soc., 1999, 121, 2329–2330.
43. R. A. Kelly, N. M. Scott, S. Díez-González, E. D. Stevens and S. P. Nolan, Organometallics, 2005, 24, 3442–3447.
44. A. P. Blum, T. Ritter and R. H. Grubbs, Organometallics, 2007, 26, 2122–2124.
45. P. Bazinet, T.-G. Ong, J. S. O'Brien, N. Lavoie, E. Bell, G. P. A. Yap, I. Korobkov and D. S. Richeson, Organometallics, 2007, 26, 2885–2895.
46. J. Li, I. C. Stewart and R. H. Grubbs, Organometallics, 2010, 29, 3765–3768.
47. D. A. Malyshev, N. M. Scott, N. Marion, E. D. Stevens, V. P. Ananikov, I. P. Beletskaya and S. P. Nolan, Organometallics, 2006, 25, 4462–4470.
48. J. A. Mata, F. E. Hahn and E. Peris, Chem. Sci., 2014, 5, 1723–1732.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental details for all reported compounds and X-ray crystallographic data of [2a][I]₂, [2a-(NiCpI)₂], and [5][I]. CCDC 1020160–1020162. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra12597e

---