Synthesis, Structure, and Catalytic Applications for <i>ortho</i>- and <i>meta</i>-Carboranyl Based NBN Pincer-Pd Complexes

Abstract

o- and m-Carborane-based NBN pincer palladium complexes (oCB-L1)Pd, (oCB-L2)Pd, and (mCB-L1)Pd are synthesized in two steps from commercially available starting materials. The pincer complexes were prepared by the reaction of bis-[R(hydroxy)methyl]-1,2-dicarba-closo-dodecaborane (R = 2-pyridyl oCB-L1, 6-methyl-2-pyridyl oCB-L2) or bis-[2-pyridyl (hydroxy)methyl]-1,2-dicarba-meta-dodecaborane (mCB-L1) with [PdCl2(MeCN)2] under mild conditions. The X-ray structure determination of all carboranyl pincer complexes shows unambiguously B-H activation of the carborane cages. The results agree with the Pd-B bonds in all complexes exhibiting strong σ-electron donation. Theoretical calculations reveal the importance of considering the solid state intermolecular hydrogen bonding when investigating the trans influence in organometallic chemistry. A localized orbitals approach has also been applied to analyze the metal oxidation state in the carboranyl pincer complexes. Catalytic applications of (oCB-L1)Pd and (mCB-L1)Pd have shown the complexes are good catalyst precursors in Suzuki coupling in water and with very low amounts of catalyst loadings.