The synthesis and structure of bis ( pyridine-2-carboxy ) difluoro ( λ 6 )-and bis ( pyridine-2-carboxy ) fluorophenyl ( λ 5 ) siliconium

Reactions of silicon fluorides with picolinic acid (PicH) and its O-trimethylsilyl derivative afforded neutral hexaor pentacoordinate complexes F2SiPic2 (5) and PhFSiPic2 (6). According to NMR data, complex 5 with SiO2N2F2 fragment exists as a mixture of two isomers with equivalent 5a and non-equivalent 5b fluorine atoms in a 1.5:1 ratio. Quantum chemical calculations at the B3LYP/6-311G** level confirm the stability of the lowest-energy transstructure of 5a.


Introduction
Hypercoordinate organosilicon compounds have attracted much attention due to their unusual bonding properties and enhanced reactivities.They are considered as models of intermediate or transition state of S N 2 reactions and as synthons in preparing new silicon compounds.A large variety of neutral and ionic penta-or hexacoordinate complexes have been prepared where silicon is bonded to O,O-, O,N-or N,N-bidentate ligands. 1 Previously it had been shown that a Ph-Si bond cleavage reaction provides a convenient approach that was used to obtain hypervalent fluoro complexes with N→Si or O→Si coordination bond. 2 For example, the reaction of phenyltrifluorosilane with ethanolamine and its N-methyl or N,N'-dimethyl derivatives and with diethanolamine and its N-methyl derivative afforded pentacoordinate silicon complexes A and B, respectively (Chart 1).

Chart 2
In this study our interest was focused on using a new ligand derived from pyridine-2carboxylic (picolinic) acid for coordination expansion at silicon.It is known that picolinic acid can act as a bidentate ligand for a wide range of elements (Ag, Cu, Zn, Cd, Co, Ni, Sn) to form metallacycles upon coordination to a metal atom. 4Thus, in the solid state X-ray analysis reveals that the tin atom in complex Me 2 ClSnPic is located in the center of a distorted octahedron (Chart 3). 5

Discussion
We found that reaction of phenyltrifluorosilane 1 with pyridine-2-carboxylic acid 3 proceeds via cleavage the Si-C and Si-F bonds to give a mixture of two isomeric bicyclic compounds bis(pyridine-2-carboxy)difluoro(λ 6 )siliconium 5a and 5b in a ratio of 1.5:1 (58% overall yield) (Scheme 1a).Alternatively, the same complexes with a SiO 2 N 2 F 2 skeleton were obtained by passing an excess of SiF 4 gas 2 through the hexane solution of O-trimethylsilyl derivative of picolinic acid 4 in 45% yield (Scheme 1b).
Complexes 5 and 6 are stable under an inert atmosphere.They are soluble in common organic solvents and can be crystallized as colorless powders from chloroform solutions.
The 1 H and 13 C NMR spectra of F 2 SiPic 2 5 display two different sets of resonance signals of the pyridine ring indicating the existence of two isomers with the N→Si coordination bond.In the 1 H NMR spectrum of complex 5 the signals from the proton of the pyridine ring are shifted to lower field relatively to those for PicH 3, especially in positions 4 and 5 (more than 0.5 ppm).Besides, the shielding the α-carbon atoms (by 3-7 ppm) and deshielding the β-and γ-carbons (by 2-6 ppm) of pyridine ring in the 13 C NMR spectrum of 5 as compared with those for 3 is observed.These data are indicative of increasing positive charge on nitrogen because of involving heteroatom in the coordination N-Si bonding.Similar behavior has been observed for protonated pyridine derivatives. 7he fluorine atoms of major hexacoordinate complex 5a appear as a sharp singlet in the 19 F NMR spectrum at δ -119.77ppm with silicon satellite (J Si-F = 148 Hz) showing fluxional behavior due to a rapid exchange process on the NMR time scale.Minor 5b exhibits a 19 F NMR resonance as two doublets at δ -139.19 ppm (J F-F = 24.4Hz, J Si-F = 135.8Hz) and δ -141.04 ppm (J Si-F = 122.1 Hz) corresponding to two non-equivalent atoms of fluorine.
The 29 Si NMR spectrum of complex 5 in DMSO-d 6 at room temperature shows unresolved multiplet resonances of two isomers (from δ -175 ppm to δ -182 ppm) in the range typical for hexacoordinate silicon compounds. 1wo 15 N resonances 5a at δ -105.8 ppm (major) and 5b δ -117.1 ppm (minor) are observed in the 15 N NMR of 5 which are greatly shielded (by 40−60 ppm) a higher field compared to that for pyridine.
The geometry of complex 5 in gas phase was optimized by using density functional theory (DFT) method.DFT calculations were carried out with B3LYP at the 6-311G** level of theory.
The optimized structures of isomers 5a, 5a' and 5b correspond to minima on the potential energy surface (Figure 1).It can be assumed that complex 5a is a mixture of two hexacoordinate isomers with N-transand N-cis-orientation rapid fluxional exchange (on the NMR time scale) (Chart 4).

Chart 4
Similar type of conformational exchange was reported previously for hexacoordinate complexes with SiO 2 N 2 X 2 fragment. 6elected angles for 5a, 5a' and 5b are listed in Table 1.These values are consistent well with those found for hexacoordinate silicon compounds. 9he Si-N distance for 5a (1.937 Å) is shorter by 0.1 Å than distance between atoms of nitrogen and silicon in complex 5a' (2.062 Å).The Si-N and Si-N' distances for 5b are 2.085 and 2.009 Å respectively.The Si-F distances of fragments O-Si-F in the structure 5a (1.662 Å) and 5b (1.659 Å) and N→Si-F in the structure 5a' (1.631 Å) and 5b (1.632 Å) are similar.The existence of two stable isomeric structures 5a and 5b which is evident from the above calculations is consistent with the NMR data.
In IR spectra of pyridine metal complexes, a shift of the stretching vibrational bands of С=С and С=N bonds to higher frequency is commonly used as a criteria for metal coordination 10 .A comparison of the intense bands ν(С=С) and ν(С=N) in the IR spectra of complex 5 (1615 сm −1 ) with those of compounds PicH 3 (1598 сm −1 ) and Me 3 SiPic 4 (1590, 1570 сm −1 ) showed a major shift to higher frequencies.These data indicate coordination of the nitrogen atom to silicon.Similarly, a shift to a higher frequency is observed on going from compound 4 to its hydrochloride (1624 cm −1 ) due an increase in electron density on the nitrogen atom upon protonation.
In IR spectra of hexacoordinated silicon compounds with N→Si bond absorption bands in the region 900-500 сm −1 are attributed to vibrations ν(Si-F) that are largely mixed with deformational vibrations δ(C-H) bonds of aliphatic and aromatic fragments of molecules. 11The experimental frequencies ν(Si-F) were assigned by their comparison with those obtained from the calculated spectra.The vibrations ν as (Si-F) and ν s (Si-F) for 5a' (865, 822 сm −1 ) and 5b (847, 795 сm −1 ) has been calculated to be intense.In contrary, calculated for 5a vibrations ν(Si-F) (891, 799, 724 and 519 сm −1 ) are low intense and mixed with vibrations δ(C-H) of the pyridine ring.These values agree closely with the ν(Si-F) observed in the IR spectrum of complex 5 which shows a weak band at 547 сm −1 and a doublet band of a small intensity (904, 886 сm −1 ), an intense band out plane deformational vibrations of CH bond at 759 сm −1 having two shoulders at 806 and 743 cm −1 .The minor component 5b has weak bands in the region of 900-800 сm -1 .In the IR spectrum of complex 6, an intense band of vibrations νSi-F is observed at 892 сm −1 which is close to calculated value of 857 cm −1 .

Conclusion
In summary, hexacoordinate complex 5 with SiO 2 N 2 F 2 fragment exists as a mixture of two isomers 5a and 5b with equivalent and non-equivalent fluorine atoms.In five-membered complex 6 the silicon atom is pentacoordinate.

Experimental Section
General.Picolinic acid was purchased from commercial sources (Acros) and was used as received.IR spectra of compounds were recorded on a FT-IR Spectrometer Bruker Vertex 70 (thin films or KBr pellets). 1H, 13 C, 15 N, 19 F, and 29 Si NMR spectra were recorded on a Bruker DPX-400 spectrometer ( 1 H, 400.13 MHz, 13 C, 100.61МHz, 15 N, 40.56 МHz, 19 F, 376.50 МHz, 29 Si 79.49МHz) at room temperature.Compounds 1, 3−6 were recorded as solutions in CDCl 3 and DMCO-d 6 (Me 4 Si as internal standard) at room temperature. 29Si NMR spectra were obtained by using the INEPT pulse sequence.The precision of measurements of the 1 Н-and 13 Сchemical shifts was 0.01 and 0.02 ppm, respectively and 0.1 ppm 29 Si.Analysis and assignment of the 1 H NMR data were supported by 1 H, 1 H COSY, 13 C, 1 H HSQC experiments.Assignment of the 13 C NMR data was supported by 13 C, 1 H HSQC experiments.Assignment of the 15 N NMR data was supported by HMBC( 15 N-1 H) experiments.
The calculations were performed by the DFT method using the B3LYP exchange correlation potential and the 6-311G** basis set as implemented in the Gaussian03 program package. 12All calculated structures correspond to minima on the potential energy surface (PES) as proved by positive eigenvalues of the corresponding Hessian matrices.All energies were calculated with the ZPE correction.
Microelemental analysis was carried out in Analytical Group of the Physical Chemistry Laboratory of our Institute.