Synthesis, Characterization, and Reactivity of 2,2′-Oxydiethanol Complexes of Silicon

GRAPHICAL ABSTRACT Abstract A series of 2,5,8-trioxasilocanes–1,1-diethoxy-2,5,8-trioxasilocane (5), 1,1-diisothiocyanto-2,5,8-trioxasilocane (6), 1,1-diisocyanto-2,5,8-trioxasilocane (7), 1-phenyl-1-chloro-2,5,8-trioxasilocane (9), and 1-phenyl-1-isothiocyanato-2,5,8-trioxasilocane (10) are prepared starting from diethylene glycol, which acts as a dianionic tridentate ligand. The composition and structures of all novel compounds are established by IR spectroscopy, 1H and 13C NMR spectroscopy, elemental analysis, and thermogravimetric analysis. The structural data obtained from geometry optimizations by DFT calculations correlate with the experimental results.


INTRODUCTION
Heterosilocanes, commonly represented as RR Si(OCH 2 CH 2 ) 2 X (X = O, S, N), are a group of compounds containing nitrogen, oxygen, or sulphur as the donor atoms in which a transannular interaction is possible between the silicon and donor atom containing an 124 G. SINGH AND S. GIRDHAR unshared electron pair. [1][2][3] Donor-functionalized 2,2 -oxydiethanol ligands (Digol) [4][5] can perfectly surround metals by three coordination sites: an alcoholate double function for charge neutralization and a donor function to compensate the electronic demand of the metal. The intramolecular coordination is possible with this ligand for which there exists a favorable conformation aided by the rigid geometry of the ligand allowing interaction between the silicon and the oxygen atom.
The first representative of this class of compounds, Me 2 Si(OCH 2 CH 2 ) 2 О, was prepared by the reaction of diethylene glycol with dimethyldichlorosilane, dimethyldiethoxysilane, or dimethyldibutoxysilane. [6][7][8] The only heterosilocane studied by singlecrystal X-ray diffraction is 1,1-diphenyl-2,5,8-trioxasilocane Ph 2 Si(OCH 2 CH 2 ) 2 O. 9 Surprisingly, the chemistry of diethylene glycol complexes is relatively unexplored; some compounds with lanthanides, alkali, and alkaline-earth metals as well as with yttrium and titanium have been reported. [10][11] The tri-and tetraethylene glycol complexes of hydrated lanthanides and the triethylene glycol complex of neodymium nitrate have been reported in the literature. [12][13] The literature also contains reports on some monocyclopentadienyl tantalum complexes with the tridentate diethylene glycolate ligand. 14 The 2,2 -oxydiethanol complex of molybdenum with mer-coordination of the tridentate ligand has also been described. 15 It is well known that diethanolamine and its N-substituted derivatives (O,N,Odonor ligands, structural analogues of diethyleneglycol) act as tridentate ligands toward silicon. The resulting complexes can be viewed as the corresponding silatranes RSi(OCH 2 CH 2 ) 3 N [16][17][18][19][20] in which one OCH 2 CH 2 unit has been removed from the heterocyclic framework and replaced by another substituent. Hypercoordinated group 14 element derivatives display a wide range of biological activity, which makes them interesting for medicinal chemistry and pharmacology. Both silatranes RSi(OCH 2 CH 2 ) 3 N and heterosilocanes RR Si(OCH 2 CH 2 ) 2 X (X = O, S, N) are physiologically active. [21][22][23] The aim of the investigations, the results of which are described in this paper, was to synthesize 2,5,8-trioxasilocanes by reaction of diethylene glycol, a tridentate dianionic ligand, with different silanes via silicon-oxygen donor action. The complexes were characterized by elemental analysis, IR, 1 H, and 13 C NMR spectroscopy and thermogravimetric analysis. The geometries of all complexes reported herein were optimized by density functional theory (DFT) using Becke's three-parameter exchange functional with Lee, Yang, and Parr's correlation functional (B3LYP); the total energies, dipole moments, and bond lengths were calculated.

Synthesis
The complexes were synthesized by reaction of diethylene glycol 1 with various silanes (2)(3)(4)8). Diethylene glycol 1 was allowed to react with diethoxydichlorosilane 2 in the presence of triethylamine as a base scavenger to yield complex 5 (Scheme 1). Complexes 6 and 7 were prepared from the transesterification reaction of diethylene glycol with diethoxydiisothiocyanatosilane 3 and diethoxydiisocyanatosilane 4, respectively, following Scheme 2. Similarly, the reaction of diethylene glycol with phenyltrichlorosilane 8 resulted in the formation of compound 9, which further reacted with potassium isothiocyanate to give complex 10 as shown in Scheme 3.

IR Spectroscopy
The IR spectra of the new compounds were recorded in the range of 4000-400 cm −1 . All bands observed were consistent with expected structures. The main absorption peaks of interest were those of Si-O and O→Si. The stretching vibration ν(Si-O) was assigned to the bands present in the region 1070-1097 cm −1 . In addition, the symmetric deformational vibration of the 2,5,8-trioxasilocane skeleton with a predominant contribution from the bond O→Si was observed in the region 547-622 cm −1 . The IR spectra of complexes 6 and 10 showed strong NCS bands at 2160 and 2066 cm −1 , respectively. The absorption band corresponding to NCO group was observed at 2288 cm −1 .

NMR Spectroscopy
In the 1 H NMR spectra of the new complexes, the resonance at 5.08 ppm was absent suggesting deprotonation of aliphatic OH groups of diethylene glycol and indicating involvement of these oxygen atoms in coordination. In the 1 H NMR spectrum of complex 5 signals for OCH 2 and CH 3 protons appeared at 3.82 and 1.21 ppm, respectively. The complexes 9 and 10 showed multiplets in the aromatic region (6.62-9.04 ppm) corresponding to phenyl group. The resonances in the 1 H spectrum at 3.43-3.93 ppm were assigned to the protons of methylene (-CH 2 -) groups in diethylene glycol.
In the 13 C NMR spectra of all complexes, the signal ascribed to the methylene groups of diethylene glycol carbon atoms appeared at 60.8-72.9 ppm. In complex 5, the

Thermogravimetric Analysis
The thermal stability of the new complexes was studied by thermogravimetric analysis (Figure 1). The complexes were heated from 25 to 1000 • C under nitrogen atmosphere. In the TGA curve of complex 5, initially the loss of both the ethoxy groups occurred (calcd. 41.10%, exp. 40.54%). The next step involved the decomposition of the rest of the compound in the temperature range above 320 • C, which continued until the formation of SiO 2 residue (calcd. 26.11%, exp. 27.02%).
In complex 6, the first step corresponded to the loss of NCS group in the temperature range below 246 • C (calcd. 25.38%, exp. 27.57%), while the second step involved the decomposition of rest of the complex; the residue corresponded to SiO 2 (calcd. 24.19%, exp. 25.27%). In the TGA curve of compound 7, the first step observed was the loss of NCO group (calcd. 19.43%, exp. 19.44%) below 235 • C, followed by the complete decomposition of the rest of the complex; also in this case, the residue corresponded to the formation of oxides of silicon. The TGA curve of complex 9 revealed the loss of the chlorine atom and of the phenyl ring (calcd. 45.14%, exp. 46.10%) in the first step, while the second step showed the decomposition of the ligand and the formation of a residue corresponding to oxides of silicon. Compound 10 showed the loss of phenyl and NCS groups (calcd. 50.74%,

Computational Study
The geometries of all structures reported herein ( Figure 2) were optimized by Density Functional Theory (DFT) 24 using B3LYP 25 along with the 3-21G basis set which has proven its utility for describing accurate geometrical features. All DFT optimizations were preformed with the Gaussian 03 set of programs.
Perhaps the most intriguing aspect of the higher-coordinate 2,5,8-trioxasilocanes is the nature of the silicon-oxygen (O→Si). The intramolecular coordination from a donor group is aided by the rigid geometry of the ligand in which the donor atom is always held in close proximity of the Si center. The ethereal oxygen atom in diethyleneglycol is more likely to coordinate with the silicon atom as there is no steric consideration which needs to be accounted for as in the case of the analogous diethanolamine derivatives. The optimized geometrical parameters of the synthesized compounds are summarized in Table 1.
The silicon-oxygen internuclear distance (O→Si) in all the optimized structures was found in the range of 2.29-2.81 Å. It is important to mention that in the heterosilocane 1,1-diphenyl-2,5,8-trioxasilocane, the distance between oxygen and silicon atoms is 2.98 Å as determined by single-crystal X-ray diffraction studies. These distances are considerably shorter than the sum of the van der Waals radii of the O and Si atoms of 3.60 Å, which indicates a weak O→Si transannular coordination interaction in these molecules. The O→Si transannular interaction in these molecules was substantially weaker than the quasisilatranes, which corresponds to a higher donor ability of the nitrogen atom relative to oxygen. The geometry of 2,5,8-trioxasilocanes could be called as crown shaped and is stabilized by the O-Si interaction.

CONCLUSION
Bicyclic organosilicon compounds with penta-coordinate silicon atom, 2,5,8trioxasilocanes, have been synthesized from the reaction of diethylene glycol with diethoxysilanes and phenyltrichlorosilane. The complexes have been characterized by various spectroscopic techniques and thermogravimetric analysis. The experimental observations have been complemented by computational studies, which indicate the existence of O→Si transannular coordination interaction. Systematic studies on compounds of this type could further offer new perspectives for the chemistry of 2,5,8-trioxasilocanes, known as close structural analogues of silatranes.

Materials and Methods
All manipulations were carried out in nitrogen atmosphere using vacuum glass line. The solvents were freshly distilled according to standard procedures before use. Silicon tetrachloride (Merck), absolute ethanol (CYC China), phenyltrichlorosilane (Aldrich), triethylamine (SDFCL), potassium thiocyanate (Aldrich), and potassium isothiocyanate (Aldrich) were used as supplied. Diethylene glycol (CDH) was distilled under reduced pressure prior to use. Dichlorodiethoxysilane, diethoxydiisothiocyanatosilane, and diethoxydiisocyantosilane were synthesized according to a reported procedure. 26 Infrared spectra were routinely obtained as thin films or Nujol mulls and KBr pellet on a Perkin-Elmer RX-I FT IR spectrophotometer. The 1 H (300 MHz) and 13 C NMR (75.45 MHz) NMR spectra were recorded with a Jeol and Bruker FT NMR (AL 300 MHz) spectrometer. Chemical shifts are given in ppm relative to internal DMSO-d 6 /CDCl 3 and external tetramethylsilane (TMS). The elemental analyses were obtained with a FLASH-2000 Organic Elemental Analyzer. The quantum mechanical calculations were carried out using the GAUSSIAN 03 series of programs. Geometries were fully optimized at Density Functional Theory level (DFT), using Becke's three parameter hybrid exchange functional and the correlation functional of Lee, Yang, and Parr (B3LYP) with 3-21G basis set.

FUNDING
Financial support from the University Grants Commission (UGC), New Delhi, is gratefully acknowledged.

SUPPLEMENTAL MATERIAL
Supplemental data for this article can be accessed on the publisher's website at http://dx.doi.org/10.1080/10426507.2014.931400