Synthesis and structural characterization of dialkyltin complexes of N-salicylidene-L-valine

The synthesis and characterization of five new chiral dialkyltin complexes of N-salicylidene-L-valine, [2-O-3-R-5-R'C6H2C(H)=NCH(CH(CH3)2)C(O)O]SnR''2 (R, R', R'' = H, H, Me (1); H, Br, Me (2); OMe, H, Et (3); Br, Br, n-Bu (4); CH(OMe)2, Me, n-Bu (5)), have been reported. Compounds 1-5 are all (S)-enantiomers, and their crystal structures have been studied. Compound 1 displays a trimeric macrocyclic structure in which the coordination environment of each tin atom is a distorted [SnC2NO3] octahedron. In complexes 2-5, the tin atom has an intermediate geometry between trigonal bipyramidal and square pyramidal, and 3 is closer to a square pyramid. In crystals, a zigzag supramolecular chain is formed by the intermolecular C-H O, O-H O or Sn O interactions.


Introduction
L-Valine is one of the 20 amino acids that make up protein, and is also essential amino acid and glycogen amino acid for mammalian. N-Salicylidene-L-valine derived from L-valine and salicylaldehyde, a chiral Schiff base carboxylate ligand, has excellent coordination ability and various coordination modes to metal ions. The structures and properties of metal complexes with this ligand have been extensively studied (Belokon et al., 2009;Chen et al., 2004Chen et al., , 2007Ucar et al., 2017;Yu et al., 2015). These chiral metal complexes have been used as catalysts of the enantioselective reactions, as efficient reagents for DNA cleavage, and as chiral fluorescent molecular sensors. Organotin compounds have been widely used in organic synthesis, catalysis, materials, and medicinal/biocidal aspects (Davies et al., 2008). Organotin complexes with carboxylate ligands have attracted a lot of attention because they display higher catalytic and cytotoxic activity and the diversified structures (Arjmand et al., 2014;Bantia et al. 2019;Davies et al., 2008;Eng, 2017;Tian et al., 2019). Some organotin complexes of N-salicylidene-Lvaline have been synthesized, and displayed good optical and biological properties (Beltran et al., 2003;Rivera et al., 2006;Tian et al., 2005Tian et al., , 2016Tian et al., , 2018Yao et al. 2017). They are synthesized by treating the corresponding oxide or hydroxide with N-salicylidene-L-valine or from the reaction of the corresponding chloride with a sodium (or potassium) N-salicylidene-L-valinate. Here, we report one-step synthesis and structural assignment of five new chiral dialkyltin complexes of N-salicylidene-L-valine derived from salicylaldehyde and L-valine, [2-O-3-R-5-R'C 6 H 2 C(H)=NCH(CH(CH 3 ) 2 )C(O)O]SnR'' 2 (R, R', R'' = H, H, Me (1); H, Br, Me (2); OMe, H, Et (3); Br, Br, n-Bu (4); CH(OMe) 2 , Me, n-Bu (5)) (see Scheme 1).

Results and discussion
The title complexes 1-5 were obtained from the reaction of dialkyltin dichloride, salicylaldehyde, L-valine and DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) in 1:1:1:2 molar ratios with the yields of 69-87% (see Scheme 1). In the reaction, the Schiff base ligand, DBUsalt N-salicylidene-L-valinate, was formed in situ. The rotation data of 1-5 showed that they have optical activity. The enantiomer was known to be S configuration, which can be confirm by the crystal structures shown below. The method of one-pot reaction is simpler and more effective than the two-step method that requires isolation of the Schiff base carboxylate ligand (Baul et al., 2005(Baul et al., , 2017Tian et al., 2005). These yellow chiral complexes are air stable and can be further explored as potential chiral Lewis acid catalysts.

Spectroscopic analysis
Spectral identification of 1-5 is based on IR and NMR ( 1 H, 13 C and 119 Sn) experiments. In IR spectra of 1-5, the ν(C=N) band appears in the range of 1609-1620 cm -1 . Compared with the free Schiff bases (∼1640 cm -1 ), this band shifts towards lower wave-number, confirming there is the coordination of C=N to tin atom in the complexes (Beltran et al., 2003). In 2-5, the strong absorptions, occurring in the ranges of 1653-1672 and 1344-1360 cm -1 , are assigned to the asymmetric stretching vibration [ν(C(O)O) asym ] and symmetric stretching vibration [ν(C(O)O) sym ] of the carboxylate group, respectively. In 1, the ν(C(O)O) asym band moves to a low wave-number and appears at 1583 cm -1 .
is usually used to judge the bonding mode of carboxylate to metal atom (Baul et al., 2017;Deacon and Phillips, 1980). For the bidentate carboxylate group, the Δν(C(O)O) value is generally less than 200 cm -1 , while the Δν(C(O)O) value of the monodentate carboxylate group is generally greater than 200 cm -1 . The Δν(C(O)O) values of 1-5 are 190, 315, 293, 327, and 312 cm -1 , respectively, indicating that in 1 the carboxylate is bidentate coordination to tin, and in 2-5 there is a monodentate carboxylate moiety.
The 1 H and 13 C chemical shift values of 1-5 are consistent with the predicted structures. The azomethine proton (CH=N, H-7) resonance displays signal at 8.19-8.26 ppm as singlet with the 3 J( 119 Sn -1 H) of 44-52 Hz. The chiral carbon proton (N-CH, H-8) exhibits a doublet with center at ~3.85 ppm, and the coupling constant, 3 J( 119 Sn -1 H), lies in the rang of 35-40 Hz. The appearance of 119 Sn -1 H coupling proves that there is coordination of azomethine nitrogen to tin in solution for these complexes. The resonance signals of benzene ring protons in salicylidene appear in the range of 6.71-7.85 ppm. The signals of the carboxylate carbon (C-11), azomethine carbon (C-7), and chiral carbon (C-8) appear in the range of 172. 45-173.24, 170.89-173.00, and 74.11-74.62 ppm, respectively. In complexes 1-5, the chiral carbon and two rigid chelate rings around tin make the two organic groups bound to tin atom (R'' 2 Sn) have different chemical environments, and give rise to different signals (Baul et al., 2017;Singh et al., 2018;Tian et al., 2005). For example, in 1 the Sn-Me groups display two sets of 1 H and 13 C NMR signals, and appear at 0.60, 0.97 ( 1 H) and -0.28, 1.76 ( 13 C) ppm, respectively.

Conclusions
Five chiral dialkyltin complexes of N-salicylidene-L-valine have been prepared by one-pot reaction in the presence of organic strong base DBU. In non-coordinated solvent, each complex is monomer containing a five-coordinated tin atom. In crystal state, complex 1 displays a trimeric structure with 12-membered macrocycle, and each tin atom possesses a distorted octahedron geometry. Complexes 2-5 are all mononuclear tin complexes and their tin atoms have the geometries between the trigonal bipyramid and square pyramid. These chiral complexes can be further explored as potential chiral Lewis acid catalysts.

Experimental
All chemicals were commercial grade and had not been further purified before use. Dialkyltin dichlorides (Me 2 SnCl 2 , Et 2 SnCl 2 , and n-Bu 2 SnCl 2 ) and 3-methoxysalicyaldehyde were purchased from Tianjin Heowns Biochemical Technology Company Limited (Tianjin, China), and other materials were from Energy Chemical Reagent Company Limited (Shanghai, China). Physical measurements including elecmental analyses and IR and NMR spectra were the same as our previous report (Yao et al., 2017).

X-ray crystallography
The yellow crystals of 1-5 suitable for X-ray investigation were obtained from the dichloromethane-methanol solution of respective complex by slow evaporation. The intensity data for 1-5 were collected at 295(2) K on a Bruker Smart Apex area-detector fitted with graphite monochromatized Mo-Kα radiation (0.71073 Å) using an ω-φ scan mode. The structures were solved by direct methods using SHELXS-97 (Sheldrick, 2008) and refined by a full-matrix least squares procedure based on F 2 using the SHELXL2014 program (Sheldrick, 2015). The non-hydrogen atoms were refined anisotropically and hydrogen atoms were placed at calculated positions in the riding model approximation. For compound 1, the residual electron density was difficult to model and therefore, the SQUEEZE routine in PLATON (Spek, 2015) was used to remove the contribution of the electron density in the solvent region from the intensity data and the solvent-free model was employed for the final refinement. The solvent formula mass was not taken into account during refinement. In the unit cell, the six major_platon_squeeze_void_volume add up to 1127 Å 3 and electrons of 299, and they roughly match 15 water and 9 methanol molecules according to the solvent of crystallization. The details (SQUEEZE RESULTS) were documented in the CIF. In addition, one reflection (0 1 5) with intensities seriously effected by the beamstop was omitted during the refinement. In compounds 4 and 5, the n-butyl groups attached to Sn(1) were refined by using DFIX, SIMU and DELU instructions. Crystallographic data and refinement details for 1-5 are listed in Table 3. Crystallographic information of 1-5 has been deposited with the Cambridge Crystallographic Data Centre, and the CCDC numbers are 1975365-1975369.