Formation of a macrocycle from dichlorodimethylsilane and a pyridoxalimine Schiff base ligand

The reaction of dichlorodimethylsilane with a polydentate Schiff base ligand derived from pyridoxal and 2-ethanolamine yielded a macrocyclic silicon compound.

There are several potential coordination sites at the ligand molecule (I): the pyridine and the imino nitrogen atoms, two aliphatic and one phenolic hydroxyl groups. The presence of these functional groups makes it difficult to predict the structure of the reaction product with dichlorodimethylsilane. It was our initial goal to prepare a pentacoordinate silicon complex like (II). Surprisingly the macrocyclic silicon compound (III) was obtained from the reaction of (I) with Me 2 SiCl 2 . The reaction was performed in tetrahydrofuran in presence of triethylamine as supporting base to remove the hydrogen chloride, which is formed during the reaction. Recrystallization of the raw product from 1,2-dimethoxyethane and diethyl ether gave yellow crystals suitable for structure analysis.

Structural commentary
Compound (III) crystallizes in the monoclinic space group I2/c with the half macrocycle in the asymmetric unit. Fig. 1 shows the asymmetric unit and the atomic labelling scheme. The macrocycle is generated by a crystallographic C2 axis through the centre of the macrocycle (Fig. 2). The silicon atom is bound to the two methyl groups and to the aliphatic oxygen atoms O2 and O3, thus forming a macrocycle (Fig. 2). A quite similar macrocycle has been obtained from the reaction of a related pyridoxal-derived Schiff base and dichlorodiphenylsilane (Bö hme et al., 2008). The short Si-O bonds (see Table 1) are in the range for comparable Si-O bonds (Wagler et al., 2005;Bö hme et al., 2006Bö hme et al., , 2008Bö hme & Gü nther, 2007a;Bö hme & Foehn, 2007). The silicon atom is distorted tetrahedral with bond angles between 103.40 (5) and 113.16 (7) ( Table 1). The rather large bond angles at the oxygen atoms (see Table 1) have been explained by the ionic character of the Si-O bonds (Gillespie & Johnson, 1997). There is a strong intramolecular O-HÁ Á ÁN interaction (entry 1, Table 2) between the imine nitrogen atom N2 and the O1-H1 group in the neighbouring position at the pyridoxal ring. The formation of hydrogen bridges between the imine nitrogen atom and an ortho-hydroxyl group is a feature that is often observed in Schiff bases with o-hydroxy groups (Hö kelek et al., 2004;Filarowski et al., 1999). This strong intramolecular O-HÁ Á ÁN interaction leads to a six-membered pseudo ring consisting of H1-O1-C2-C3-C7-N2. This pseudo ring is planar with an r.m.s. deviation of 0.009 Å from the ring plane. According to the graph-set notation proposed by Etter et al. (1990) The molecular structure of (III), drawn with 50% probability displacement ellipsoids. Table 1 Selected geometric parameters (Å , ).

Figure 1
The asymmetric unit of (III), drawn with 50% probability displacement ellipsoids. The dashed line shows the intramolecular O1-H1Á Á ÁN2 hydrogen bond.
In summary, the crystal structure is dominated by C-HÁ Á ÁO and C-HÁ Á Á interactions, forming a highly ordered molecular network.
The potential bonding sites in combination with the cavity of the macrocycle makes (III) a suitable candidate for supramolecular recognition processes. The available pyridine N, azomethine N, and OH groups could be useful for the generation of nanostructures via complexation with transition metals (Leininger et al., 2000).

Database survey
A CSD search with ConQuest (Bruno et al., 2002) for macrocycles containing Schiff bases from pyridoxal and 2-aminoalcohols showed that only one comparable silicon compound exists (Bö hme et al., 2008, refcode MOKVEO). The main differences between these two structures of siliconcontaining macrocycles are as follows. First, (III) was found to crystallize without solvent while MOKVEO encloses chlorofom molecules. Probably as a result, the symmetry is lower in MOKVEO (triclinic, P1) than in (III) showing the monoclinic I2/c symmetry. On the basis of the structure of (III) presented here and the former investigation (Bö hme et al., 2008), it can be assumed that pyridoxalimine-derived Schiff bases prefer the formation of macrocycles with diorganosilane units. However, it seems to be possible that compound (I) can also act as a tridentate O,N,O-ligand, as was shown recently with a hexacoordinate titanium complex (Bö hme & Gü nther, 2020).

Synthesis and crystallization
The preparation of (III) was performed in Schlenk tubes under argon with dry and air-free solvents.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The hydrogen atom at O1 was refined freely. The methyl groups were refined as idealized rigid groups allowed to rotate but not tip (AFIX 137; C-H = 0.98 Å , H-C-H = 109.5 ). Other hydrogens were included using a riding model starting from calculated positions (C-H aromatic = 0.95, C-H methylene = 0.99 Å ). The U iso (H) values were fixed at 1.5 (for the methyl H) or 1.2 times the equivalent U eq value of the parent carbon atoms.  Table 2 Hydrogen-bond geometry (Å , ).

Funding information
Funding for this research was provided by: Open Access Funding by the Publication Fund of the TU Bergakademie Freiberg .

Computing details
Data collection: X-AREA (Stoe, 2009); cell refinement: X-AREA (Stoe, 2009); data reduction: X-RED (Stoe, 2009); program(s) used to solve structure: SHELXS (Sheldrick, 2008); program(s) used to refine structure: SHELXL2017/1 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: SHELXL2017/1 (Sheldrick, 2015). Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.