12-(N-Methylnitrilium)monocarba-closo-dodecaborate Ylide†

The syntheses, spectral characterization, and crystal structures of 12-(N-methylnitrilium)monocarba-closo-dodecaborate and 12-(N-methylamidinium)-monocarba-closo-dodecaborate ylides are reported. The carborate anion behaves as an inert and non-conjugating negative charge.

Croat.Chem.Acta 87 (2014) 357.methylation reactions are clean and no substitution at the BH vertices in 2 occurs, illustrating the deactivating effect of the cyano group.Although the resulting Nmethylnitrilium ylide (3) reacts readily with nucleophiles, it can be easily isolated.Reaction of 3 with aqueous NH 3 can be performed in situ or after isolation and affords the N-methylamidinium ylide 4. Single crystals of 3 and 4 suitable for X-ray diffraction analysis were obtained by slow diffusion of n-pentane into a concentrated solution in CH 2 Cl 2 (Figures 1 and 2).The quality of the single crystal of 3 and consequently the precision of structure determination were markedly affected by severe cracking of the crystal as it was cooled.An attempt to determine the structure at room temperature was unsuccessful due to overall disorder of the carborane cage.In spite of this drawback, the reported low-temperature structural parameters of 3, like those of 4, lie within the common range.For example, they compare well with parameters reported for cesium 12-(2-phenylethynyl)-closo-1-carbadodecaborate. 18able 1 lists the most important bond lengths and stretching frequencies of the new ylide 3 and several related compounds and compares them with the results of DFT calculations (BP86/SV(P)).Natural atomic charges calculated by the NBO method 19 are also shown.

DISCUSSION
−29 However, only a limited number of stable nitrilium ylides have been isolated and thoroughly characterized. 25,26,30ost nitrilium ylides carry their negatively charged group at the nitrogen atom and allow this group to conjugate with the CN group.They display a characteristic IR stretch band at 2150−2200 cm −1 , strongly redshifted from an ordinary cyano group of nitriles, suggesting a description of bonding in terms of cumulated double bonds.Only a few known nitrilium ylides carry a non-conjugating negatively charged group on the CN carbon 12−16 and in these, the IR stretch occurs near 2300 cm −1 and is thus similar to that in free nitrilium cations 31 and the related ylides of the type R 3 BC=NMe, which are best described as Lewis acid-base adducts of RB 3 with an isonitrile. 32,33he new ylide 3 carries a negative charge adjacent to the CN carbon atom and thus belongs to the rarer category.Its CN stretch occurs at 2347 cm −1 , in a region characteristic of free nitrilium cations. 31Among the known nitrilium ylides of this type, (CF 3 ) 3 B−CN−Me offers the closest analogy, in that it has also been prepared from the anionic nitrile by methylation on nitrogen.Both compounds carry the negative charge delocalized at a weakly coordinating anionic moiety adjacent to the nitrilium carbon, and the similarity is reflected in the calculated charge distribution (Table 1).The CB 11 anion thus clearly acts as a non-conjugating substituent, and its negative charge is not available for donation of electron density to stabilize a cumulene resonance structure, which would weaken the CN bond and red-shift of the CN stretching band.These unconventional nitrilium ylides resemble their cationic analogs much more closely than the more usual ylide systems and are thus best described as carborane-nitrilium zwitterions.
The differences in the frequencies of the CN stretching vibration in 3 and in a series of more conventional nitrilium ylides have been reproduced by DFT calculations.While these did not reproduce the absolute magnitude of the experimentally observed frequencies very well, the tendency towards significantly higher wave numbers for the boron based nitrilium ylides after methylation is clearly reflected.These calculations also revealed that the charge distribution in 3 closely resembles that in free nitrilium cations 31 and differs strongly from that in the more ordinary nitrilium ylides.

Materials
Unless otherwise noted, all reactions were carried out under argon atmosphere with dry solvents, freshly distilled under anhydrous conditions.Standard Schlenk and vacuum line techniques were employed for all manipulations of air-or moisture-sensitive compounds.Yields refer to isolated, spectroscopically homogenous materials.NHMe 3 + CB 11 H 12 −1 (1) was purchased from Katchem Ltd. (Elišky Krásnohorské 6, Prague 110 00, Czech Republic).Sulfolane, CH 3 NO 2 , Me 3 OBF 4 , and MeOTf were purchased from Sigma Aldrich.

Equipment and Measurements
NMR spectra were measured in acetone-d 6 and chloroform-d. 1H NMR chemical shifts were referenced with respect to the chemical shift of the residual protons present in the deuterated solvents: acetone-d 6 (2.05 ppm) and methylene chloride-d 2 (5.32 ppm).For 11 B NMR signal BF 3 .Et 2 O in a coaxial capillary was used as an external standard.For 13 C NMR, the signal of the acetone-d 6 trideuteriomethyl group (δ = 29.80 ppm) and methylene chloride-d 2 (54.00 ppm) were used. 1 H and 13 C spectra were recorded with Bruker Avance 400 and 500 spectrometers working at 400.1 and 499.8 MHz for 1 H NMR, and at 100.6 MHz and 125.7 MHz for 13 C NMR. 1 H{ 11 B}, 11 B and 11 B { 1 H} NMR spectra were accumulated with Bruker Avance 400 and 500 spectrometers working at 400.1 MHz and 499.8 MHz for 1 H NMR and 128.3 MHz and 160.4 MHz for 11 B NMR.The carbon signal of the C-1 carborane vertex is usually not easily detectable directly because of 13 C-11 B coupling, and therefore HSQC and HMBC were used for the assignment of 13 C NMR resonances.Assignments of boron signals were done by 11 B and 11 B COSY NMR.Electrospray ionization mass spectrometry (ESI-) spectra were recorded with a Waters Micromass ZQ spectrometer.IR spectra were recorded in KBr pellets with a Bruker EQUINOX 55 (IFS 55) spectrometer.Elemental analyses were obtained using a Perkin-Elmer PE 2400 Series II analyzer.Singlecrystal X-ray diffraction data were obtained from Nonius KappaCCD difractometer equipped with Bruker ApexII-CCD detector using monochromatized MoKα radiation (λ = 0.71073 Å) at 150(2) K.The structures were solved by direct methods and refined by fullmatrix least squares based on F 2 (SHELXS; SHELXL97). 34The hydrogen atoms were found on difference Fourier map and recalculated into idealized positions (riding model) with assigned temperature factors, either H iso (H) = 1.2 U eq (pivot atom) or 1.5 U eq for methyl moiety.
Computed optimized geometry of CpCNMe