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(C(3)B(2)HR(5))] complexes because the neutral heterocycle functions as 4e donor toward metal complex fragments e.g. pM (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 J(C-H) give information about the bonding situation. In the CpCo(C(3)B(2)HMeEt(4)) sandwich the endo-hydrogen could not be located, its (1)H NMR spectrum shows a high-field quartet at -8.8 ppm, and a low J(C-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(C(3)B(2)H(2)Me(4))(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)(mu-C(6)B(4)H(2)Me(8))] as sideproduct was observed in the reaction of tetrameric (Cp*RuCl) with C(3)B(2)HMe(5) and zinc dust to improve the synthesis of the violet sandwich Cp*Ru(C(3)B(2)Me(5)). The related chloro complex Cp*Ru(C(3)B(2)ClMe(4)) 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)- mu,eta(6):eta(6)-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. (C) 008 Published by Elsevier B.V.