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The hydrodefluorination reaction of perfluorinated arenesusing[Ni2(iPr2Im)4(COD)] (1; iPr2Im = 1,3-bis(isopropyl)imidazolin-2-ylidene) as a catalystas well as stoichiometric transformations to elucidate the decisivesteps for this reaction are reported. The reaction of hexafluorobenzenewith 5 equiv of triphenylsilane in the presence of 5 mol % of 1affords 1,2,4,5-tetrafluorobenzene after 48 h at 60 °Cand 1,4-difluorobenzene after 96 h at 80 °C; the reaction ofperfluorotoluene and 5 equiv of Et3SiH for 4 days at 80°C results in the selective formation of 1-(CF3)-2,3,5,6-C6F4H. Stoichiometric transformations of the complexes cis-[Ni(iPr2Im)2(H)(SiPh3)] and cis-[Ni(iPr2Im)2(H)(SiMePh2)] with hexafluorobenzene at room temperature lead to the formationof trans-[Ni(iPr2Im)2(F)(C6F5)] (2) and trans-[Ni(iPr2Im)2(H)(C6F5)] (4) with elimination of the corresponding silane or fluorosilane. Thereactions of the C–F activation products trans-[Ni(iPr2Im)2(F)(C6F5)] (2) and trans-[Ni(iPr2Im)2(F)(4-(CF3)C6F4)] (3) with PhSiH3and Ph2SiH2afford the hydride complexes trans-[Ni(iPr2Im)2(H)(C6F5)] (4) and trans-[Ni(iPr2Im)2(H)(4-(CF3)C6F4)] (5), which convert into the compounds trans-[Ni(iPr2Im)2(F)(2,3,5,6-C6F4H)] (7), trans-[Ni(iPr2Im)2(F)(3-(CF3)-2,4,5-C6F3H)] (9a), and trans-[Ni(iPr2Im)2(F)(2-(CF3)-3,4,6-C6F3H)](9b), respectively. In the case of the rearrangementof trans-[Ni(iPr2Im)2(H)(4-(CF3)C6F4)] (5) the intermediate [Ni(iPr2Im)2(η2-C,C-(CF3)C6F4H)](8) was detected. Reaction of 8with perfluorotoluenegave the C–F activation product trans-[Ni(iPr2Im)2(F)(4-(CF3)C6F4)] (3). All theseexperimental findings point to a mechanism for the HDF by [Ni(iPr2Im)2] via the “fluorideroute” involving C–F activation of the polyfluoroarene,H/F exchange of the resulting nickel fluoride, reductive eliminationof the polyfluoroaryl nickel hydride to an intermediate with a η2-C,C-coordinated arene ligand, subsequent ligand exchangewith the higher fluorinated polyfluoroarene, and renewed C–Factivation of the polyfluoroarene. Without additional reagents, [Ni(iPr2Im)2(η2-C,C-(CF3)C6F4H)] (8) rearranges to the isomers trans-[Ni(iPr2Im)2(F)(3-(CF3)-2,4,5-C6F3H)](9a; major) and trans-[Ni(iPr2Im)2(F)(2-(CF3)-3,4,6-C6F3H)] (9b; minor) ina ratio of 80:20. DFT calculations performed on conversion of trans-[Ni(iPr2Im)2(H)(4-(CF3)C6F4)] 5into the two products trans-[Ni(iPr2Im)2(F)(3-(CF3)-2,4,5-C6F3H)] 9aand trans-[Ni(iPr2Im)2(F)(2-(CF3)-3,4,6-C6F3H)] (9b) usingthe commonly accepted intramolecular concerted pathway via η2-C,F-σ-bound transition states predict 9bto be the major product. We thus propose that this reaction mechanismis not valid for the [Ni(NHC)2]-mediated C–F activationof partially fluorinated arenes with special substitution patterns. [ABSTRACT FROM AUTHOR]

A series of P-phospholyl-substituted N-heterocyclic phosphines was prepared and characterized by single-crystal X-ray diffraction and solution and solid-state 31P NMR spectroscopy. The molecular structures are distinguished by the presence of P-P bonds of exceptionally variable lengths (2.35-2.70 Å) that are all well beyond the standard distance of 2.21 Å. The unique flexibility is best illustrated by a specimen 4f where minor conformational changes of remote substituents induce a deviation in P-P bond lengths of some 5 pm between crystallographically independent molecules in the same unit cell. Computational studies suggest to rationalize the bond elasticity as the consequence of a very flat potential energy basin that allows even weak forces to have large impact on bond lengths. Solid-state 31P NMR studies show that the bond distance variation coincides with substantial changes in the magnitude and sign of ¹Jpp, which is explained in the context of a dominant Fermi contact contribution. A relation between increasing internuclear distance and decreasing magnitude of ¹Jpp was experimentally proven by determination of effective dipolar coupling constants by the double-quantum dephasing experiment (DoDe) for the crystallographically independent conformers of 4f and further supported by comparison with calculated coupling tensors with inclusion of the anisotropic J-coupling. NMR studies revealed large discrepancies in the values of ¹Jpp measured in solution and the solid state and a substantial temperature dependence of the former. Interpretation of this behavior was feasible by taking into account that the value of ¹Jpp in solution is affected by both temperature-dependent equilibria between trans and gauche conformers and additional bond length relaxation that accompanies the dissolution process. Consideration of experimental observations and population analysis of computed electron densities suggested to classify the P-P bonds in the molecules under study as "dative" rather than "normal" covalent bonds and to address the compounds 4 as hybrids between covalent diphosphines and phosphenium-phospholide contact ion pairs. [ABSTRACT FROM AUTHOR]

The neutral hexacoordinate silicon(IV) complex 6 (SiO(2)N(4) skeleton) and the neutral pentacoordinate silicon(IV) complexes 7-11 (SiO(2)N(2)C skeletons) were synthesized from Si(NCO)(4) and RSi(NCO)(3) (R = Me, Ph), respectively. The compounds were structurally characterized by solid-state NMR spectroscopy (6-11), solution NMR spectroscopy (6 and 10), and single-crystal X-ray diffraction (8 and 11 were studied as the solvates 8 x CH(3)CN and 11 x C(5)H(12) x 0.5 CH(3)CN, respectively). The silicon(IV) complexes 6 (octahedral Si-coordination polyhedron) and 7-11 (trigonal-bipyramidal Si-coordination polyhedra) each contain two bidentate ligands derived from an alpha-amino acid: (S)-alanine, (S)-phenylalanine, or (S)-tert-leucine. The deprotonated amino acids act as monoanionic (6) or as mono- and dianionic ligands (7-11). The experimental investigations were complemented by computational studies of the stereoisomers of 6 and 7.

Herein we report for the first time full details on the synthesis and structural characterization of novel homodinuclear bridging cobalt and nickel borylene complexes containing bridging carbonyl ligands, an unusual coordination motif rarely before observed for homodinuclear borylene complexes. Furthermore, the homodinuclear nickel complex represents the first instance of a nickel borylene complex. Quantum chemical analyses of charge-density topology, electron localization function (ELF) and natural charges indicate the absence of direct metal-metal bonds in both the cobalt and nickel systems, in contradiction with electron counting. The topology of the Laplacian of the electron density and of the ELF around the bridging boron atom is consistent with a bis-metallo-substituted borane situation for the dicobalt system, but with a three-center-bonding borylene for the dinickel complex.

Bonding in borylene-, carbene-, and vinylidene-bridged dinuclear manganese complexes [MnCp(CO)(2)](2)X (X = B-tBu, B = NMe(2), CH(2), C=CH(2)) has been compared by analyses based on quantum theory of atoms in molecules (QTAIM), on the electron-localization function (ELF), and by natural-population analyses. All of the density functional theory based analyses agree on the absence of a significant direct Mn-Mn bond in these complexes and confirm a dominance of delocalized bonding via the bridging ligand. Interestingly, however, the topology of both charge density and ELF related to the Mn-bridge-Mn bonding depend qualitatively on the chosen density functional (except for the methylene-bridged complex, which exhibits only one three-center-bonding attractor both in -nabla(2)rho and in ELF). While gradient-corrected functionals provide a picture with localized two-center X-Mn bonding, increasing exact-exchange admixture in hybrid functionals concentrates charge below the bridging atom and suggests a three-center bonding situation. For example, the bridging boron ligands may be described either as substituted boranes (e.g., at BLYP or BP86 levels) or as true bridging borylenes (e.g., at BHLYP level). This dependence on the theoretical level appears to derive from a bifurcation between two different bonding situations and is discussed in terms of charge transfer between X and Mn, and in the context of self-interaction errors exhibited by popular functionals.