The Ligand‐Based Quintuple Bond‐Shortening Concept and Some of Its Limitations

Herein, the ligand‐based concept of shortening quintuple bonds and some of its limitations are reported. In dichromium–diguanidinato complexes, the length of the quintuple bond can be influenced by the substituent at the central carbon atom of the used ligand. The guanidinato ligand with a 2,6‐dimethylpiperidine backbone was found to be the optimal ligand. The reduction of its chromium(II) chloride–ate complex gave a quintuply bonded bimetallic complex with a Cr? Cr distance of 1.7056 (12) Å. Its metal–metal distance, the shortest observed in any stable compound yet, is of essentially the same length as that of the longest alkane C? C bond (1.704 (4) Å). Both molecules, the alkane and the Cr complex, are of remarkable stability. Furthermore, an unsupported Cr^I dimer with an effective bond order (EBO) of 1.25 between the two metal atoms, indicated by CASSCF/CASPT2 calculations, was isolated as a by‐product. The formation of this by‐product indicates that with a certain bulk of the guanidinato ligand, other coordination isomers become relevant. Over‐reduction takes place, and a chromium–arene sandwich complex structurally related to the classic dibenzene chromium complex was observed, even when bulkier substituents are introduced at the central carbon atom of the used guanidinato ligand.     

Persistent Dialkylsilanone Generated by Dehydrobromination of Dialkylbromosilanol

A persistent dialkylsilanone was synthesized by the dehydrobromination of a dialkylbromosilanol with tris(trimethylsilyl)silyl potassium in solution at ?80 °C: It was characterized by NMR and IR spectroscopy, and was tested in several reactions. In ²⁹Si NMR spectrum in [D₈]toluene, the signal due to the unsaturated silicon nuclei was observed at 128.7 ppm. Reactions of the dialkylsilanone with water and mesitonitrile oxide gave a silanediol and a [2+3] cycloadduct, respectively. The silanone remains intact in [D₈]toluene below ?80 °C for at least two days, while it undergoes unprecedented isomerization to give a siloxysilene by means of 1,3‐silyl migration at higher temperatures.     

Greatly enhanced intermolecular π-dimer formation of a porphyrin trimer radical trications through multiple π bonds

A trefoil‐like porphyrin trimer linked by triphenylamine (TPA‐TPZn₃) was synthesized. A three‐electron oxidation of TPA‐TPZn₃ forms a radical trication (TPA‐TPZn₃³⁺), in which each porphyrin ring undergoes a one‐electron oxidation. The radical trication TPA‐TPZn₃³⁺ spontaneously dimerizes to afford (TPA‐TPZn₃)₂⁶⁺ in CH₂Cl₂. The characteristic charge‐resonance band due to the charge delocalization over the π system of (TPA‐TPZn₃)₂⁶⁺ was observed in the NIR region. The initial oxidation potential of TPA‐TPZn₃ is negatively shifted relative to that of the corresponding monomer porphyrin, which results from the stabilization of the oxidized state of TPA‐TPZn₃ associated with the dimerization. The thermodynamic parameters (i.e., ΔH, ΔS, and ΔG) for the formation of (TPA‐TPZn₃)₂⁶⁺ were determined by measuring Vis/NIR spectra at various temperatures. The formation constant of (TPA‐TPZn₃)₂⁶⁺ is significantly larger than that of the radical cation dimer of the corresponding monomer porphyrin (e.g., over 2000‐fold at 233 K). The electronic states were investigated using EPR spectroscopic analysis. The greatly enhanced dimerization of TPA‐TPZn₃³⁺ results from multiple π‐bond formation between the porphyrin radical cations.     

Non‐Pincer‐Type Mononuclear Scandium Alkylidene Complexes: Synthesis,Bonding, and Reactivity

The first non‐pincer‐type mononuclear scandium alkylidene complexes were synthesized and structurally characterized. These complexes exhibited short Sc?C bond lengths and even one of the shortest reported to date (2.1134(18) Å). The multiple character of the Sc?C bond was highlighted by a DFT calculation. This was confirmed by experimental reactivity study where the complex underwent [2+1] cycloaddition with elemental selenium and [2+2] cycloaddition with imine. DFT calculation also revealed a strong nucleophilic behavior of the alkylidene complex that was experimentally demonstrated by the C?H bond activation of phenylacetylene.     

Synthesis and Crystal Structure of a New Mixed-ligand Cluster [Mo3O4（C2O4）2bipy-（H2O）3]·EtOH·2H2O（bipy = 2,2′-Bipyridine）

The novel mixed-ligand neutral compound [Mo3O4（C2O4）2bipy-（H2O）3]·EtOH·2H2O（bipy = 2,2′-bipyridine） has been prepared by the reaction of oxalic acid elution of Mo（IV） and bipy, and characterized by single-crystal X-ray diffraction analysis and IR. The crystal is of triclinic, space groups P1^- with a = 9.5520（2）, b = 10.3730（1）, c = 13.5722（2）A, a = 74.940（12）,β = 80.772（14）,γ= 69.898（11）°, V = 1215.73（11）A^3, Z = 2, C16H24Mo3N2O18, Mr = 820.19, Dc = 2.241 g/cm^3,μ = 1.616 mm^-1, F（000） = 808, T = 293（2） K, the final R = 0.0424 and wR = 0.0939 for 4119 observed reflections with I 〉 2σ（I）. The trinuclear unit is coordinated by mixed ligands of oxalate and bipy. The intermolecular hydrogen bonding interactions among adjacent [Mo3O4（C2O4）2·bipy（H2O）3] extend the compound into a three-dimensional supramolecular framework. The uncoordinated water molecules and ethal molecules act as space-fillers and consolidate the whole architecture through hydrogen bonding interactions.