Ambiphilic Diphosphine–Borane Ligands: Metal→Borane Interactions within Isoelectronic Complexes of Rhodium, Platinum and Palladium

Abstract

Coordination of an ambiphilic diphosphine-borane (DPB) ligand to the RhCl(CO) fragment affords two isomeric complexes. According to X-ray diffraction analysis, each complex adopts a square-pyramidal geometry with trans coordination of the two phosphine buttresses and axial RhB contacts, but the two differ in the relative orientations around the rhodium and boron centres. DFT calculations on the actual complexes provide insight into the influence of the pi-accepting CO co-ligand, compared with previously reported complexes [Rh(mu-Cl)(dpb)]2 and [RhCl(dmap)(dpb)]. In addition, comparison of the nu(CO) frequency of [RhCl(CO)(dpb)] with that of the related borane-free complex [RhCl(CO)(iPr2PPh)2] substantiates the significant electron-withdrawing effect that the sigma-accepting borane moiety exerts on the metal. Valence isoelectronic [PtCl2(dpb)] and [PdCl2(dpb)] complexes have also been prepared and characterized spectroscopically and structurally. The pronounced influence of the transition metal on the magnitude of the M-->B interaction is highlighted by geometric considerations and NBO analyses.

σ-Acceptor, Z-type ligands for transition metals

“…The reaction of DPB with [Rh(m-Cl)(nbd)] 2 (nbd = 2,5-norbornadiene) afforded the chloro-bridged dimeric Rh complex [(DPB)Rh (m-Cl)] 2 . 84 Similarly the borane moiety of [(DPB)Rh(m-Cl)] 2 is not affected by CO bubbling and the monomeric [(DPB)RhCl(CO)] complex is selectively obtained, as a 8 : 2 mixture of two diastereomers. The 11 B NMR resonance signal is shifted upfield by about 30 ppm upon coordination, the RhB distance is short (2.306(3) Å , r = 1.02) and the boron environment is strongly pyramidalized (SB a = 338.81).…”

Crystal field arguments to explain the trans labilisation within transition metal–borane complexes

“…Other examples include metalborane coordination bridged to other transition metal centres [43][44][45][46]. More recently, Bourissou has provided a number of further examples that are supported by C2-dialkylphosphine-bridging units [22][23][24][25][26], and we have recently reported the first nitrogen-based examples based on 7-azaindole [27,28]. Although there are now a fair number of examples of such compounds, little is known concerning their reactivity.…”

Coordination Chemistry of Neutral (Ln)–Z Amphoteric and Ambiphilic Ligands

“…Quite surprisingly, the boranobis(phosphane) ligand can also adopt a trans geometry when coordinated to precursor [Rh(µ-Cl)(CO) 2 ] 2 , as observed in the solid-state structure of 14. [28] The carbonyl stretching frequency in the latter complex at 2001.8 cm -1 , relative to 1966.7 cm -1 for trans-[RhCl(CO)(iPr 2 PPh) 2 ], is an indication that the MǞB dative interaction withdraws significant electron density from the metal centre. Compound 15, synthesized from precursor AuCl(SMe 2 ), has a square-planar geometry, which is confirmed by the fact that the sum of the angles around Au is approximately 360°(ΣAu α = 362.2°for R = iPr and 364.2°for R = Ph).…”

“…Ligand exchange from precursors M(cod)Cl 2 (M = Pt and Pd) and [RhCl(nbd)] 2 gave compounds 10, 11 [28] and 12, [29] respectively. As observed with boranotris(phosphane) complexes 7 and 8, the phosphane buttresses of the amphoteric ligand can induce the formation of a MǞB dative interaction.…”