Reaction of Mes 2 BF with trimethylsilyl triflate (Me 3 SiOTf ) and [Ag(IMe) 2 ][Ag 2 I 3 ] (IMe=1,3-dimethylimidazol-2-ylidene) in refluxing chlorobenzene affords the carbene-stabilized borenium salt [(IMe)BMes 2 ]OTf ([1][OTf ]). As indicated by cyclic voltammetry, [1] þ is reversibly reduced to produce a radical that has been characterized by EPR spectroscopy.N-heterocyclic carbenes (NHC) have become ubiquitous ligands in main-group chemistry. 1 Because of their strong σ-donor properties, such ligands have served to stabilize a number of otherwise highly reactive species, including unusual cations 2 and low-coordinate species, 3,4 among others. Interestingly, however, such NHC ligands have never been introduced in diarylborenium cations ([(L)BAr 2 ] þ , with L=neutral ligand and Ar=aryl group). 5-8 Taking into account the unusual ligative properties of NHC ligands, we postulated that the resulting [(NHC)BAr 2 ] þ cations may possess a robust core and be amenable to reversible redox chemistry. Stimulated by this possibility, we have now decided to synthesize such a [(NHC)BAr 2 ] þ cation.The reaction of Mes 2 BF with trimethylsilyl triflate (Me 3 SiOTf ) and [Ag(IMe) 2 ][Ag 2 I 3 ] 9 (IMe=1,3-dimethylimidazol-2-ylidene) in refluxing chlorobenzene affords, after 12 h,[OTf ] was confirmed by 1 H NMR, which showed the presence of one IMe ligand bound to the boron center. The detection of two resonances for the o-methyl groups of the mesityl substituent is indicative of a C 2 geometry whose handedness does not change on the NMR time scale at room temperature. The presence of a tricoordinate boron atom was confirmed by the detection of a 11 B NMR signal at 66 ppm. This chemical shift is comparable to the value of 64 ppm reported for [(DMAP)BMes 2 )] þ (DMAP= p-(dimethylamino)pyridine), which also features a tricoordinate boron atom. 10 To our knowledge, NHCs have been incorporated in neutral borane 4,11 and boraanthracene 12 adducts but not in boreniums. 6 Hence, [1][OTf ] represents the first example of a borenium stabilized by a NHC ligand. The structure of [1][OTf ] has also been studied using single-crystal X-ray diffraction (Figure 1). 13 The boron center is trigonal planar, as indicated by the sum of the bond angles, which is equal to 359.9°. The B(1)-C(1) bond connecting the IMe ligand to the boron center (1.579(7) Å ) is comparable to the B(1)-C(6) (1.562(7) Å ) and B(1)-C(15) bonds (1.560(7) Å ), suggesting a strong coordination of the carbene ligand. The dihedral angle of 37.3°formed between the boron trigonal plane and the plane containing the IMe ligand is smaller than the average dihedral angle of 53.7°formed by the mesityl groups and the boron trigonal plane. This difference can be assigned to the lower steric hindrance of the IMe ligand, whose N-methyl groups adopt a more divergent orientation.The cyclic voltammogram of [1] þ in CH 2 Cl 2 displays a single reversible reduction wave at E 1/2 Red =-1.81 V vs Fc/Fc þ (Figure 2). The reduction potential of [1] þ is distinctively more positive than that of...

The adsorption state and thermal stability of V(benzene)2 sandwich clusters soft-landed onto a self-assembled monolayer of different chain-length n-alkanethiols (Cn-SAM, n = 8, 12, 16, 18, and 22) were studied by means of infrared reflection absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD). The IRAS measurement confirmed that V(benzene)2 clusters are molecularly adsorbed and maintain a sandwich structure on all of the SAM substrates. In addition, the clusters supported on the SAM substrates are oriented with their molecular axes tilted 70-80 degrees off the surface normal. An Arrhenius analysis of the TPD spectra reveals that the activation energy for the desorption of the supported clusters increases linearly with the chain length of the SAMs. For the longest chain C22-SAM, the activation energy reaches approximately 150 kJ/mol, and the thermal desorption of the supported clusters can be considerably suppressed near room temperature. The clear chain-length-dependent thermal stability of the supported clusters observed here can be explained well in terms of the cluster penetration into the SAM matrixes.

Gas-phase synthesized vanadium-benzene 1:2 (VBz(2)) sandwich clusters were size-selectively deposited onto bare gold and long-chain n-alkanethiolate [-S-(CH(2))(n-1)-CH(3); n = 16, 18, and 22] self-assembled monolayer (SAM)-coated gold substrates under ultrahigh vacuum (UHV) conditions. Investigation of the resulting deposited clusters was performed by infrared reflection absorption spectroscopy (IRAS) and thermal desorption spectroscopy (TDS). The IR frequencies of the soft-landed VBz(2) clusters show excellent agreement with the fundamentals reported in IR data of VBz(2) in an argon matrix. The analysis of IRAS spectra reveals that while there was no orientational preference of the VBz(2) clusters on a bare gold substrate, the VBz(2) clusters deposited onto the SAM substrates were highly oriented with the molecular axis 70-80 degrees tilted off the surface normal. In addition, analysis of TDS spectra revealed unusually large adsorption heats of the physisorbed VBz(2) clusters. The present results are explained by cluster penetration into the long-chain alkanethiolate SAM and for the first time demonstrate the matrix isolation of gas-phase organometallic clusters around room temperature.