A T-shape diphosphinoborane palladium(0) complex.

The reaction of CpPd(η (3) -C3H5) with the new diphosphinoborane ligand derivative (o-PCy2-C6H4)2BPh (Cy) DPB (Ph) affords the T-shape complex ( (Cy) DPB (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 DPB) 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 DPB. DPB 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 DPB we adapted the known reaction sequence for the production of Bourissou’s (o-PPh2-C6H4)2BPh ligand DPB (Scheme 1) [13-14].

Scheme 1

Synthesis of diphosphinoborane DPB 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 DBP 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].

DPB 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 DPB 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 DPB deviate from those observed for its aryl derivatives (DPB ((o-PPh2-C6H4)2BPh) and DPB ((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 (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.

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

CIF file of 9, CCDC 1471929.