Triaminoborane-bridged diphosphine complexes with Ni and Pd: coordination chemistry, structures, and ligand-centered reactivity

Abstract

The synthesis, coordination chemistry, and reactivity of two diphosphines containing the cyclic triaminoborane 1,8,10,9-triazaboradecalin (TBD) are described. To evaluate the ligand-centered reactivity of ^PhTBDPhos and ^iPrTBDPhos, the complexes (^PhTBDPhos)MCl₂ and (^iPrTBDPhos)MCl₂, where M = Ni and Pd, were prepared and characterized by elemental analysis, multinuclear NMR spectroscopy (¹H, ¹³C, ³¹P, and ¹¹B), and single-crystal X-ray diffraction (XRD). Despite very low boron Lewis acidity in the TBD backbone, (^PhTBDPhos)NiCl₂ (1) and (^PhTBDPhos)PdCl₂ (3) react with H₂O, alcohols, and hydrated fluoride reagents in the presence of NEt₃ to yield trans H–O or H–F addition across the bridgehead N–B bond. In contrast, ^iPrTBDPhos shows no appreciable reactivity when bound to NiCl₂ (2) and PdCl₂ (4), which is attributed to the sterically-bulky isopropyl substituents blocking substrate access to boron in the TBD backbone. The new complexes {[(^PhTBDPhos-H₂O)Ni]₂(μ-OH)₂}Cl₂ (5), {[(^PhTBDPhos-H₂O)Pd]₂(μ-OH)₂}Cl₂ (6), (^PhTBDPhos-MeOH)NiCl₂ (7), (^PhTBDPhos-MeOH)PdCl₂ (8), (^PhTBDPhos-C₃H₅OH)PdCl₂ (9), and {[(^PhTBDPhos-HF)Ni]₂(μ-OH)₂}Cl₂ (10) were isolated, and all but 6 were structurally characterized by single-crystal XRD. Multinuclear NMR studies revealed that isolated, crystallographically-authenticated samples of 5–9 lose ligand-bound water or alcohol with reappearance of starting materials 1 and 3 when dissolved in NMR solvents. Addition of NEt₃ attenuated the water and alcohol loss from 5–9 to allow ¹H, ¹³C, ³¹P, and ¹¹B NMR data to be collected for all the compounds, confirming the determined structures. Additional reactivity experiments with NaOMe and fluoride reagents suggested that participation of the bridgehead nitrogen in the TBD backbone is important for promoting reactivity at boron when ^PhTBDPhos is bound to Ni and Pd. The term “cooperative ligand-centered reactivity” (CLR) is proposed to define chemical reactions that appear to require participation of more than one atom on the ligand, such as those reported here.