We report the synthesis and structural characterization of the new monomeric borane complex H3B·hppH (hppH = 1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a]pyrimidine), which represents the first example of a structurally characterized 1:1 complex between hppH and a group 13 element hydride. Significant intramolecular and, in the crystalline phase, intermolecular H···H contacts are established in this complex. It is shown that the complex can be used as a precursor to new dinuclear boron(II) compounds featuring a B–B single bond. Thus H2 elimination followed by dimerization of H3B·hppH leads to [(hpp)BH]2 with two bridging hpp units. The structural details derived from X‐ray diffraction measurements are reported.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

In this work we report on the syntheses and properties of several new Ni complexes featuring the chelating bisguanidines bis(tetramethylguanidino)benzene (btmgb), bis(tetramethylguanidino)naphthalene (btmgn), and bis(tetramethylguanidino)biphenyl (btmgbp) as ligands. All complexes were structurally characterized by single-crystal X-ray diffraction and quantum chemical calculations. A detailed inspection of the magnetic susceptibility of [(btmgb)NiX(2)] and [(btmgbp)NiX(2)] (X=Cl, Br) revealed a linear temperature dependence of chi(-1)(T) above 50 K, which was in agreement with a Curie-Weiss-type behavior and a triplet ground state. Below approximately 25 K, however, magnetic susceptibility studies of the paramagnetic d(8) Ni complexes revealed the presence of a significant zero-field splitting (ZFS) that results from spin-orbit mixing of excited states into the triplet ground state. The electronic consequences that might arise from the mixing of states as well as from a possible non-innocent behavior of the ligand have been explored by an experimental charge density study of [(btmgb)NiCl(2)] at low temperatures (7 K). Here, the presence of ZFS was identified as one potential reason for the flat angle-spherical Cl-Ni-Cl deformation potential and the distinct differences between the angle-spherical X-Ni-X valence angles observed by experiment and predicted by DFT. An analysis of the topology of the experimentally and theoretically derived electron-density distributions of [(btmgb)NiCl(2)] confirmed the strong donor character of the bisguanidine ligand but clearly ruled out any significant non-innocent ligand (NIL) behavior. Hence, [(btmgb)NiCl(2)] provides an experimental reference system to study the mixing of certain excited states into the ground state unbiased from any competing NIL behavior.

Several new mono-and dinuclear complexes of Co II and Ni II with chelating guanidine ligands were synthesized. The molecular structures for most of the complexes in the crystalline state were derived from single-crystal X-ray diffraction, and some characteristic structural details are discussed. The molecular magnetism was further studied by superconducting

A detailed nuclear magnetic resonance (NMR) study was carried out on a series of paramagnetic, tetrahedrally coordinated nickel(II) dihalide complexes featuring chelating guanidine ligands. A complete assignment of the NMR signals for all complexes was achieved by sophisticated NMR experiments, including correlation spectra. The effects of halide exchange, as well as the variation in the guanidine-metal bite angles on the paramagnetic shifts, were assessed. The paramagnetic shift was derived with the aid of the diamagnetic NMR spectra of the analogous Zn complexes, which were synthesized for this purpose. The experimentally derived paramagnetic shift was then compared with the values obtained from quantum chemical (DFT) calculations. Furthermore, variable-temperature NMR studies were recorded for all complexes. It is demonstrated that NMR spectroscopy can be applied to evaluate the rate constants of fast fluxional processes within paramagnetic and catalytically active metal complexes.

Herein thermal and catalytic dehydrogenation of the guanidine–borane adducts H3B·hppH (hppH = 1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a]pyrimidine) and H3B·N(H)C(NMe2)2 are analysed. Thermal decomposition of H3B·hppH at 80 °C leads to [HB(μ‐hpp)]2 and a second boron hydride, which is tentatively identified as [(κ2N‐hpp)BH2]. Decomposition in boiling toluene (110 °C) leads to a mixture of [H2B(μ‐hpp)]2 and [HB(μ‐hpp)]2, from which [H2B(μ‐hpp)]2 can be separated and crystallised. In the presence of a catalyst (with Cp2TiCl2/nBuLi or [Rh(1,5‐cod)Cl]2 as precatalysts) dehydrogenation at 80 °C leads predominantly to [H2B(μ‐hpp)]2. In the case of H3B·N(H)C(NMe2)2 uncatalysed dehydrogenation turns out to be a very slow process even at 110 °C. Interestingly, the ultimate product of this process is oligomeric methylimino borane, [HBNMe]n. This pathway can be modelled and understood with the aid of quantum chemical calculations. Faster dehydrogenation can be initiated by addition of a catalyst. Finally, the possible mechanisms for thermal and Cp2Ti‐catalysed dehydrogenation are analysed for the model compound H3B·N(H)C(NH2)2 by means of quantum chemical (DFT) calculations.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)