Structural characterization and reactivity of a room-temperature-stable, antiaromatic cyclopentadienyl cation salt.

The singlet states of cyclopentadienyl (Cp) cations are considered as true prototypes of an antiaromatic system. Unfortunately, their high intrinsic reactivity inhibited their isolation in the solid state as a salt, and controlled reactions are also scarce. Here we present the synthesis and solid state structure of the room-temperature-stable Cp cation salt [Cp(C6F5)5]+[Sb3F16]-. Although the aromatic triplet state of the [Cp(C6F5)5]+ cation is energetically favoured in the gas phase according to quantum chemical calculations, coordination of the cation by either [Sb3F16]- or C6F6 in the crystal lattice stabilizes the antiaromatic singlet state, which is present in the solid state. The calculated hydride and fluoride ion affinities of the [Cp(C6F5)5]+ cation are higher than those of the perfluorinated tritylium cation [C(C6F5)3]+. Reactions of [Cp(C6F5)5]+[Sb3F16]- with CO, which probably yields the corresponding carbonyl complex, and of radical Cp(C6F5)5∙ with selected model substrates (Cp2Fe, (Ph3C∙)2 and Cp*Al) are also presented.

Metal-coordinated distibene and dibismuthene dications - isoelectronic analogues of butadiene dications.

We report the synthesis and solid-state structures of DMAP-coordinated ([L(DMAP)GaPn]2[OTf]2; Pn = Sb 3, Bi 4) and base-free dipnictene dications ([LGaPn]2[BArFx]2, Pn = Sb: x = 24, 5a; 20, 5b; Bi: x = 24, 6a; 20, 6b). Quantum chemical calculations indicate that the dications 52+ and 62+ represent isoelectronic analogues of the butadiene dication.

Comparing London dispersion pnictogen-π interactions in naphthyl-substituted dipnictanes.

Using a combination of NMR, single crystal X-ray diffraction (sc-XRD) and quantum chemistry, the structure-directing role of London Dispersion (LD) is demonstrated for dibismuthane Bi2Naph2 (1). 1 shows intermolecular Bi⋯π contacts in the solid-state, while π⋯π interactions as observed in the lighter homologues are missing. Comparison of the whole series of dipnictanes revealed the influence of the pnictogen atom on the strength of London dispersion and highlights its importance in heavy main group element chemistry.

Observation of Discrete Valence Tautomers in Crystalline Cyclopentadienyl Radicals.

Single crystal X-ray (sc XRD) analyses of three symmetrically substituted cyclopentadienyl radicals (1, 2, 5) containing sterically demanding aryl groups showed that they crystallize as discrete valence tautomers (Jahn-Teller distortion) in the solid state with the unpaired electron either located in the b1 orbital (type I, state 2B1), resulting in a localized radical with two adjacent double bonds, or the a2 orbital (type II, state 2A2), leading to an allyl-type radical. Their properties in solution were examined by EPR spectroscopy as well as cyclovoltammetry and UV/vis spectroscopy including two additional cyclopentadienyl radicals (1-5). The electronic nature of 1-5 was further investigated by quantum chemical calculations.

Synthesis of a Ga-Stabilized As-Centered Radical and a Gallastibene by Tailoring Group 15 Element-Carbon Bond Strengths.

A convenient synthetic route to Ga-stabilized pnictogen-centered radicals and gallapnictenes by manipulation of pnictogen-carbon bond strengths is presented. Two equivalents of LGa (L = HC[C(Me)N(Dip)]2, Dip = 2,6-i-Pr2C6H3) react with CpArAsCl2 (CpAr = C5(4-t-BuC6H4)5) with formation of the arsenic-centered radical [L(Cl)Ga]2As·1. In contrast, the analogous reaction with TerSbCl2 (Ter = 2,6-Mes2C6H3; Mes = 2,4,6-Me3C6H2) yields the gallastibene LGa═SbTer (2) containing a Ga-Sb double bond, whereas reactions of DipSbCl2 with one and two equivalents of LGa give the monoinsertion and bisinsertion products L(Cl)GaSb(ClDip) (3) and [L(Cl]Ga]2SbDip (4), respectively. 1-4 were structurally characterized by single crystal X-ray diffraction. The description of 1 as an arsenic-centered radical is consistent with results of electron paramagnetic resonance and density functional theory (DFT) studies. The π-bonding in LGa═SbTer (2) is estimated to 10.68 kcal mol-1 by variable-temperature (VT) NMR spectroscopy, and DFT studies reveal a significant π-bonding interaction between Sb and Ga.