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A T-shape diphosphinoborane palladium(0) complex

  1. and
Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, D-52074 Aachen, Germany
  1. Corresponding author email
This article is part of the Thematic Series "Organometallic chemistry".
Guest Editor: B. F. Straub
Beilstein J. Org. Chem. 2016, 12, 1573–1576. https://doi.org/10.3762/bjoc.12.152
Received 18 Apr 2016, Accepted 04 Jul 2016, Published 22 Jul 2016

Abstract

The reaction of CpPd(η3-C3H5) with the new diphosphinoborane ligand derivative (o-PCy2-C6H4)2BPh CyDPBPh affords the T-shape complex (CyDPBPh)Pd(0) 9, which was characterized by X-ray analysis.

Keywords: ambiphilic ligand; coordination chemistry; diphosphinoborane; organometallics; palladium

Introduction

The amplification of traditional bidentate chelating L2-type ligands with a tethered borane functionality (e.g., Bourissou’s diphospinoborane (o-PR2-C6H4)2BR’ ligand RDPBR’) has received considerable attention [1-3], with first catalytic applications emerging [4]. The acyclic boron group in these ligands can adopt a variety of coordination modes (Figure 1) [5].

[1860-5397-12-152-1]

Figure 1: Selected M→B coordination modes 1–5 [6-10] and Hofmann’s Rucaphos complex 6 [11].

The borane can act as a σ-acceptor ligand in case of η1-B coordination (e.g., 1 [6] and 2 [7]), or as a boron containing π-ligand adopting η2-B,C (3) [8] or η3-B,C,C coordination (4 and 5) [5,9,10]. Changes of the hapticity appear to have significant influence onto the reactivity of the coordinated transition metal towards substrates [8]. For zerovalent palladium complexes only few examples featuring a η1-type Pd→B interaction have been reported [6,7]. However, these complexes require phosphines or pyridines as a stabilizing co-ligand, which can act as an inhibitor in catalytic transformations [7]. Similarly, monometallic 14 VE palladium complexes featuring a chelating diphosphine, such as in Hofmanns Rucaphos complexes 6, are very scarce [11]. While the dative Pd→B bond is strong in zerovalent Pd(0) DPB complexes such as 2, only weak Pd→B interactions have been observed for the respective Pd(II) complexes [7,12]. Discrimination by the borane functionality between the oxidations states Pd(0)/Pd(II) is of potential interest for organometallic transformations involved in homogeneous catalysis, such as the reductive elimination. Here we report the synthesis of the diphosphinoborane (o-PCy2-C6H4)2BPh ligand CyDPBPh. CyDPBPh reacts with CpPd(η3-C3H5) yielding monometallic zerovalent palladium complex 9 featuring a distinct η1-B coordination mode, without the need of a stabilizing co-ligand.

Findings

For the synthesis of CyDPBPh we adapted the known reaction sequence for the production of Bourissou’s (o-PPh2-C6H4)2BPh ligand PhDPBPh (Scheme 1) [13,14].

[1860-5397-12-152-i1]

Scheme 1: Synthesis of diphosphinoborane CyDPBPh and complex 9.

Starting material (2-bromophenyl)dicyclohexylphosphine (7) was produced by palladium catalyzed coupling of dicyclohexylphosphine with 1-iodo-2-bromobenzene [15]. Phosphine 7 was lithiated in diethyl ether with n-BuLi [16,17], affording the diethyl ether adduct 8. Reaction of 8 with 0.5 equiv of PhBCl2 in toluene at −78 °C produced the desired ligand CyDBPPh in 86% isolated yield. Typical resonances for a DPB ligand were observed in the 31P NMR spectrum at δ 1.70 and in the 11B NMR spectrum at δ 41 (w1/2 = 1300 ± 120 Hz), which are indicative for a dynamic P→B bond in solution [18].

CyDPBPh was reacted with 1 equiv of CpPd(η3-C3H5) in benzene. Complete conversion towards complex 9 with equimolar formation of 5-allylcyclopenta-1,3-diene was reached within 18 h at 50 °C. Complex 9 showed a singlet resonance at δ 41.0 in the 31P NMR spectrum and a broad resonance at δ 22 (w1/2 = 800 ± 50 Hz) in the 11B NMR spectrum. High field shift and narrowing of the 11B NMR with respect to the free CyDPBPh ligand indicated the presence of a strong dative Pd(0)→B bond [7]. Despite the absence of a stabilizing co-ligand, we found complex 9 to be very stable in solution. The coordinating properties of CyDPBPh deviate from those observed for its aryl derivatives (PhDPBPh ((o-PPh2-C6H4)2BPh) and PhDPBMes ((o-PPh2-C6H4)2B(Mes))). For these ligands the reaction with one equivalent of CpPd(η3-C3H5) leads to 50% consumption of CpPd(η3-C3H5) with simultaneous formation of 5-allylcyclopenta-1,3-diene, but complete conversion of the ligand pointing towards the formation of a bisligand complex (DPB)2Pd [7]. Unlike complex 2 we were unable to form a pyridine adduct complex by treatment of 9 with 10 equiv of pyridine. Single crystals of complex 9 suitable for X-ray diffraction analysis were grown from hexane (Figure 2).

[1860-5397-12-152-2]

Figure 2: Thermal ellipsoid plots of complex 9 at the 50% probability level. H atoms and one molecule of hexane have been omitted for clarity. Selected interatomic distances (Å) and angles (°): Pd1–B1 2.243(2), Pd1–P1 2.2761(6), Pd1–P2 2.3084(6), B1–Pd1–P1 85.82(6), B1–Pd1–P2 82.49(6), P1–Pd1–P2 157.72(2), C15–B1–C20 110.94(18), C15–B1–C35 116.58(18), C20–B1–C35 112.56(18).

The solid-state structure of 9 displayed a slightly distorted T-shape geometry around the palladium center. A short Pd1–B1 distance of 2.243(2) Å (cf. complex 2: 2.194(3) Å) and a significant pyramidalization at the boron center (ΣBα = 341°) is observed, indicating a strong Pd(0)→B bond. The distance between C20 and Pd1 was found to be 3.0805(22) Å. The η1-B coordination mode was well reproduced by DFT calculations (Supporting Information File 1). DFT calculations predict T-shape complexes with an almost linear P–Pd–P angle for model complexes (PMe3)2Pd → EX3 (E = B; X = H, F, Cl, Br, I) [17]. In complex 9 the trans-coordinated palladium center featured an obtuse P1–Pd1–P2 angle of 157.72(2)°.

Conclusion

In conclusion we synthesized the zerovalent palladium complex [{(o-PCy2-C6H4)2BPh}Pd(0)] 9. Complex 9 supplements the few known examples (e.g., 6 [11]) of 14 VE palladium complexes bearing a chelating diphosphine ligand by introduction of a borane acceptor functionality.

Supporting Information

Supporting Information File 1: Experimental procedures and characterization data; crystallographic information for 9; 1H, 11B, 13C and 31P NMR spectra.
Format: PDF Size: 1.2 MB Download
Supporting Information File 2: CIF file of 9, CCDC 1471929.
Format: CIF Size: 22.5 KB Download

Acknowledgements

We are grateful for financial support of this research by the Funds of the Chemical Industry (fellowships to PS and MET). We thank Q. Guo for collection of X-ray diffraction data and J. Wiesenthal for experimental assistance. We thank Prof. Dr. J. Okuda for his continuous and generous support.

References

  1. Fontaine, F.-G.; Boudreau, J.; Thibault, M.-H. Eur. J. Inorg. Chem. 2008, 5439–5454. doi:10.1002/ejic.200800784
    Return to citation in text: [1]
  2. Amgoune, A.; Bourissou, D. Chem. Commun. 2011, 47, 859–871. doi:10.1039/C0CC04109B
    Return to citation in text: [1]
  3. Kameo, H.; Nakazawa, H. Chem. – Asian J. 2013, 8, 1720–1734. doi:10.1002/asia.201300184
    Return to citation in text: [1]
  4. Bouhadir, G.; Bourissou, D. Chem. Soc. Rev. 2016, 45, 1065–1079. doi:10.1039/C5CS00697J
    Return to citation in text: [1]
  5. Emslie, D. J. H.; Cowie, B. E.; Kolpin, K. B. Dalton Trans. 2012, 41, 1101–1117. doi:10.1039/C1DT11271F
    Return to citation in text: [1] [2]
  6. Zech, A.; Haddow, M. F.; Othman, H.; Owen, G. R. Organometallics 2012, 31, 6753–6760. doi:10.1021/om300482m
    Return to citation in text: [1] [2] [3]
  7. Schindler, T.; Lux, M.; Peters, M.; Scharf, L. T.; Osseili, H.; Maron, L.; Tauchert, M. E. Organometallics 2015, 34, 1978–1984. doi:10.1021/acs.organomet.5b00217
    Return to citation in text: [1] [2] [3] [4] [5] [6] [7]
  8. Harman, W. H.; Peters, J. C. J. Am. Chem. Soc. 2012, 134, 5080–5082. doi:10.1021/ja211419t
    Return to citation in text: [1] [2] [3]
  9. Emslie, D. J. H.; Harrington, L. E.; Jenkins, H. A.; Robertson, C. M.; Britten, J. F. Organometallics 2008, 27, 5317–5325. doi:10.1021/om800670e
    Return to citation in text: [1] [2]
  10. Cowie, B. E.; Emslie, D. J. H. Organometallics 2015, 34, 4093–4101. doi:10.1021/acs.organomet.5b00539
    Return to citation in text: [1] [2]
  11. Schnetz, T.; Röder, M.; Rominger, F.; Hofmann, P. Dalton Trans. 2008, 2238–2240. doi:10.1039/b802684j
    Return to citation in text: [1] [2] [3]
  12. Bontemps, S.; Sircoglou, M.; Bouhadir, G.; Puschmann, H.; Howard, J. A. K.; Dyer, P. W.; Miqueu, K.; Bourissou, D. Chem. – Eur. J. 2008, 14, 731–740. doi:10.1002/chem.200701027
    Return to citation in text: [1]
  13. Sircoglou, M.; Bontemps, S.; Mercy, M.; Saffon, N.; Takahashi, M.; Bouhadir, G.; Maron, L.; Bourissou, D. Angew. Chem., Int. Ed. 2007, 46, 8583–8586. doi:10.1002/anie.200703518
    Return to citation in text: [1]
  14. Conifer, C. M.; Law, D. J.; Sunley, G. J.; White, A. J. P.; Britovsek, G. J. P. Organometallics 2011, 30, 4060–4066. doi:10.1021/om200341t
    Return to citation in text: [1]
  15. Murata, M.; Buchwald, S. L. Tetrahedron 2004, 60, 7397–7403. doi:10.1016/j.tet.2004.05.044
    Return to citation in text: [1]
  16. Harder, S.; Brandsma, L.; Kanters, J. A.; Duisenberg, A.; van Lenthe, J. H. J. Organomet. Chem. 1991, 420, 143–154. doi:10.1016/0022-328X(91)80257-K
    Return to citation in text: [1]
  17. Goedecke, C.; Hillebrecht, P.; Uhlemann, T.; Haunschild, R.; Frenking, G. Can. J. Chem. 2009, 87, 1470–1479. doi:10.1139/V09-099
    Return to citation in text: [1] [2]
  18. Bontemps, S.; Bouhadir, G.; Dyer, P. W.; Miqueu, K.; Bourissou, D. Inorg. Chem. 2007, 46, 5149–5151. doi:10.1021/ic7006556
    Return to citation in text: [1]

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