A T-shape diphosphinoborane palladium(0) complex

Abstract

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

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 ^RDPB^R’) 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].

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 ^CyDPB^Ph. ^CyDPB^Ph 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 ^CyDPB^Ph we adapted the known reaction sequence for the production of Bourissou’s (o-PPh₂-C₆H₄)₂BPh ligand ^PhDPB^Ph (Scheme 1) [13–14].

Scheme 1.

Synthesis of diphosphinoborane ^CyDPB^Ph 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 ^CyDBP^Ph 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 (w_1/2 = 1300 ± 120 Hz), which are indicative for a dynamic P→B bond in solution [18].

^CyDPB^Ph 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 (w_1/2 = 800 ± 50 Hz) in the ¹¹B NMR spectrum. High field shift and narrowing of the ¹¹B NMR with respect to the free ^CyDPB^Ph 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 ^CyDPB^Ph deviate from those observed for its aryl derivatives (^PhDPB^Ph ((o-PPh₂-C₆H₄)₂BPh) and ^PhDPB^Mes ((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).

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 (ΣB_α = 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

File 1

Experimental procedures and characterization data; crystallographic information for 9; ¹H, ¹¹B, ¹³C and ³¹P NMR spectra.

File 2

CIF file of 9, CCDC 1471929.

This article is part of the Thematic Series "Organometallic chemistry".