1,3-Diboraheterocycles as ligands in metal complexes involving endo-C–H bonds

Pentaalkyl-2,3-dihydro-1,3-diboroles possess unique properties in [LnM(C3B2HR5)] complexes because the neutral heterocycle functions as 4e donor toward metal complex fragments e.g. CpM (M = Co, Rh), (arene)Fe, and others. The specific feature of these complexes is its MeC–H group with methyl in the exo- and hydrogen in an endo-position, forming a bridging 3c,2e C–H–B or an axial M–C–H bonding. This reduces the strength of the C–H bond, and thus complexes of this type exhibit a high reactivity and synthetic potential. Various complexes with a MeC–H group between identical boron centers have been analyzed by X-ray diffraction and NMR studies regarding the bonding of the endo-C–H in bridging or in axial position. The bond lengths of endo-C–H and B–C, the NMR shifts as well as the coupling constant JC–H give information about the bonding situation. In the CpCo(C3B2HMeEt4) sandwich the endo-hydrogen could not be located, its 1H NMR spectrum shows a high-field quartet at −8.8 ppm, and a low JC–H = 81 Hz indicates a weakening of the bond strength. Deprotonation leads to the anion, used as building block for oligo-decker complexes. The complex Ni(C3B2H2Me4)2 having two endo-C–H bonds, exhibits a unique reactivity in loosing hydrogen at ambient temperature and forming 2,3,5-tricarba-hexaboranyl-nickel complexes. Only few complexes allowed to locate the endo-hydrogen in C–H–B position by X-ray diffraction studies, which is supported by calculations. The energy difference between bridging and axial positions is very small. The surprising formation of the slipped 34 VE triple-decker [(Cp∗Ru)2(μ-C6B4H2Me8)] as sideproduct was observed in the reaction of tetrameric (Cp∗RuCl) with C3B2HMe5 and zinc dust to improve the synthesis of the violet sandwich Cp∗Ru(C3B2Me5). The related chloro complex Cp∗Ru(C3B2ClMe4) is a postulated intermediate, however, its transformation into the dinuclear species must include the uptake of two hydrogen atoms, which is not yet clarified. The yellow bis(pentamethylcyclopentadienyl-ruthenium)-μ,η6:η6-hexahydro-tetraboranaphthalene has an unprecedented framework with a bridge-head diborane(4) unit, two additional boron atoms and two MeC–H groups each located between two different boron centers. The endo-hydrogen atoms were not found in the X-ray diffraction analysis, DFT calculations indicate their location in axial positions. 1H NMR data confirm the presence of two endo-H atoms (showing a quartet at – 4.6 ppm), of which only one could be deprotonated by potassium.