4-Pyridinylboronic acid

The reaction of CH(3)Co(DH)(2)H(2)O with 4-pyridinyl boronic acid in methanol or water affords the dinuclear complexes [MeCo(DH)(DB(OR)(4-Py))](2), with R = Me (2) or H (3), respectively, through reaction of boron with the oxime oxygens of the alkylcobaloxime and coordination of the pyridinyl N to cobalt. The reaction is strongly pH dependent, and the formation of the complexes requires a neutral medium. The complexes have been fully characterized by (1)H and (13)C NMR spectroscopy, ESI-MS spectrometry, and elemental analysis. The X-ray structure shows that in 2, the pyridinyl groups are facing each other and nearly perpendicular both to the plane of the Co B Co1 B1 atoms and to the mean equatorial plane, so that the complex may be considered a molecular box. A dimeric arrangement has already been found in the related [MeCo(DH)(DB(OMe)(3-Py))](2) (1) complex, which forms a distorted molecular rectangle [Dreos, R.; Nardin, G.; Randaccio, L.; Tauzher, G.; Vuano, S. Inorg. Chem. 1997, 36, 2463]. The dimerization is possible in both cases, as the conformational freedom of the B bridge compensates for the different position (3- or 4-) of the pyridinyl N donor.

In an effort to describe π-hole interactions, we undertook accurate high-resolution X-ray diffraction analyses of single crystals of 1,4-dinitrobenzene, a co-crystal of cis-tartaric acid and bis-pyridine N-oxide and the hydrochloride of B-4-pyridinylboronic acid. We selected these three compounds owing to the π-hole accessibility features that the sp2 hybridized B, C and N atoms provide, thus allowing us to compare the fundamental characteristics of π-hole interactions using Bader's Atom in Molecules (AIM) theory. This particular study required extremely accurate experimental diffraction data, because the interaction of interest is weak. As shown by the experimental charge density maps of the -YO2 (Y = B, C, N) units, we assign the depletion of electron-density present in the central boron, carbon and nitrogen atoms (electrophilic π-holes) as the main origin for the establishment of intermolecular Lewis acid-Lewis base attractive interaction with complementary electron-rich regions. Unexpectedly, the Bader's analyses of both experimentally and theoretically calculated charge distribution maps for the solid involving the - BO2H2 group do not show the presence of bond paths, neither of the bond critical points, between the interacting electron rich sites and the boron or carbon atoms featuring the electron hole. In contrast, these topological descriptors of chemical interactions for the AIM theory were easily located in the solid-state structures of the compounds involving the carboxylic and the nitro groups.