Dennis Sheberla

High Electrical Conductivity in Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2, a Semiconducting Metal–Organic Graphene Analogue

Reaction of 2,3,6,7,10,11-hexaaminotriphenylene with Ni(2+) in aqueous NH3 solution under aerobic conditions produces Ni3(HITP)2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene), a new two-dimensional metal-organic framework (MOF). The new material can be isolated as a highly conductive black powder or dark blue-violet films. Two-probe and van der Pauw electrical measurements reveal bulk (pellet) and surface (film) conductivity values of 2 and 40 S·cm(-1), respectively, both records for MOFs and among the best for any coordination polymer.

Electrochemical oxygen reduction catalysed by Ni3(hexaiminotriphenylene)2.

Control over the architectural and electronic properties of heterogeneous catalysts poses a major obstacle in the targeted design of active and stable non-platinum group metal electrocatalysts for the oxygen reduction reaction. Here we introduce Ni3(HITP)2 (HITP=2, 3, 6, 7, 10, 11-hexaiminotriphenylene) as an intrinsically conductive metal-organic framework which functions as a well-defined, tunable oxygen reduction electrocatalyst in alkaline solution. Ni3(HITP)2 exhibits oxygen reduction activity competitive with the most active non-platinum group metal electrocatalysts and stability during extended polarization. The square planar Ni-N4 sites are structurally reminiscent of the highly active and widely studied non-platinum group metal electrocatalysts containing M-N4 units. Ni3(HITP)2 and analogues thereof combine the high crystallinity of metal-organic frameworks, the physical durability and electrical conductivity of graphitic materials, and the diverse yet well-controlled synthetic accessibility of molecular species. Such properties may enable the targeted synthesis and systematic optimization of oxygen reduction electrocatalysts as components of fuel cells and electrolysers for renewable energy applications.

Different electronic structure of phosphonyl radical adducts of N-heterocyclic carbenes, silylenes and germylenes: EPR spectroscopic study and DFT calculations

Stable N-heterocyclic carbenes and germylenes were allowed to react with a phosphonyl radical, (i-PrO)2(O)P˙ (7), generated by photolysis of [(i-PrO)2(O)P]2Hg. The products were identified by EPR spectroscopy. An unsaturated carbene (1) and germylene (3) react with 7 at the divalent atom to give unstable radical products (τ½ = 0.2 s). A benzo-annulated carbene (4) and a saturated germylene (6) react with 7 to give more active radicals. An unsaturated (2) and a saturated silylene (5) undergo rapid reaction (in the dark) with [(i-PrO)2(O)P]2Hg to yield unusual silyl phosphites. In these cases only secondary radicals were observed. DFT (PBE0/TZVP//B3LYP/6-31+G(d)) calculations of the radical adducts of the different (C, Si, Ge) unsaturated N-heterocyclic divalent species with the phosphonyl radical show that the unpaired electron is delocalized over the five-membered ring; the spin density on the central atoms decreases in the order C, 39% > Si, 14% > Ge, 2%. These trends can be understood in terms of a zwitterionic structure of the radical adducts. The calculations of the radical adducts of 4, 5 and 6 with 7 indicate larger spin density on the central atom, 47%, 58% and 42% on C, Si, Ge, respectively.

Isolation of Silenolates (R3Si)2SiC(OLi)Ad with a Doubly Bonded Silicon Atom

Metal enolates are an important class of reactive intermediates widely employed in organic synthesis. In contrast, little is known about silenolates, the silicon analogues of enolates. Enolates exist in two tautomeric forms, the enol form and the keto form, and their reactions reflect the coexistence of these two forms. The dominant structure of alkali metal enolates is the enol form both in nonsolvating media and in various solvating media such as THF, N,N,N’,N’-tetramethylethylenediamine, and [18]crown-6. Silenolates also exist in two tautomeric forms: the keto form a (acyl silyl anion) and the enol form b (silene) [Eq. (1)], and they also show ambident reactivity. The first silenolate (solvated), recently isolated and characterized by X-ray crystallography, has the keto form a. An enol-form silenolate bwas not yet reported. Isolation of an enol-form silenolate is challenging, because it has a Si=C p bond which is thermodynamically and kinetically less stable than a C=C bond. In addition, enol-form silenolates can be regarded as functional silenes, which are reagents of growing importance in silicon chemistry. Here we report the synthesis, isolation, and X-ray molecular structure of the first enol-form silenolates (tBuMe2Si)2Si=C(OLi)Ad (1) and (tBu2MeSi)2Si=C(OLi)Ad (2). We show by DFT quantum-mechanical calculations that, in contrast to organic enolates, which exist predominantly in the enol form regardless of solvation, the structure of silenolates 1 and 2 is strongly dependent on the solvent. Silenolate 1 was synthesized by metal–halogen exchange between tBuMe2SiLi (in excess) and bromo acyl silane Br(tBuMe2Si)2SiC(O)Ad (3) in hexane at 78 8C. Upon warming to room temperature pale yellow crystals of silenolate 1 precipitated (10% yield). The major product is substitution product 4 [Eq. (2)].

Isolable Photoreactive Polysilyl Radicals

Reaction of silyl substituted dichlorosilanes with lithiosilanes in hexane leads exclusively to the corresponding stable silyl radicals. Two radicals, the new (t-Bu(2)MeSi)(2)HSi(t-Bu(2)MeSi)(2)Si* (1) and the previously isolated (t-Bu(2)MeSi)(3)Si* (2), were isolated and fully characterized including by X-ray crystallography. This one-step method is general and was applied for the synthesis of other silyl radicals. Upon irradiation radical 1 (yellow solution in hexane) decays to yield the corresponding disproportionation products, silane and disilene (blue colored). In contrast, radical 2 is photostable in the absence of additives, but it abstracts hydrogen from triethylsilane and 2-propanol upon irradiation. DFT calculations and irradiation experiments with lambda > 400 nm suggest that SOMO-1 --> SOMO excitation, which provides better electron accepting properties to the radical, is responsible for the photoreactivity of 1 and 2.

Activation of Homolytic Si-Zn and Si-Hg Bond Cleavage, Mediated by a Pt(0) Complex, via Novel Pt-Zn and Pt-Hg Compounds.

The thermally stable [(tBuMe2 Si)2 M] (M=Zn, Hg) generate R3 Si(.) radicals in the presence of [(dmpe)Pt(PEt3 )2 ] at 60-80 °C. The reaction proceeds via hexacoordinate Pt complexes, (M=Zn (2 a and 2 b), M=Hg (3 a and 3 b)) which were isolated and characterized. Mild warming or photolysis of 2 or 3 lead to homolytic dissociation of the Pt-MSiR3 bond generating silyl radicals and novel unstable pentacoordinate platinum paramagnetic complexes (M=Zn (5), Hg (6)) whose structures were determined by EPR spectroscopy and DFT calculations.