Synthesis and characterization of bis[dicarboxylatotetraorganodistannoxane] units involving 5-[(E)-2-(aryl)-1-diazenyl]-2-hydroxybenzoic acids: An investigation of structures by X-ray diffraction, NMR, electrospray ionisation MS and assessment of in vitro cytotoxicity

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Abstract

Reactions of 5-[(E)-2-(aryl)-1-diazenyl]-2-hydroxybenzoic acids (LHH′, where the aryl group is an R-substituted phenyl ring such that for L1HH′: R = H; L2HH′: R = 2′-CH3; L3HH′: R = 3′-CH3; L4HH′: R = 4′-CH3; L5HH′: R = 4′-Cl; L6HH′: R = 4′-Br) with nBu2SnO in a 1:1 molar ratio yielded complexes of composition {[nBu2Sn(LH)]2O}2. The complexes have been characterized by 1H, 13C, 119Sn NMR, ESI-MS, IR and 119mSn Mössbauer spectroscopic techniques in combination with elemental analyses. The crystal structures of {[nBu2Sn(L1H)]2O}2 (1), {[nBu2Sn(L4H)]2O}2 (4), {[nBu2Sn(L5H)]2O}2 (5) and {[nBu2Sn(L6H)]2O}2 (6) were determined. The compounds are centrosymmetric tetranuclear bis(dicarboxylatotetrabutyldistannoxane) complexes containing a planar Sn4O2 core in which two μ3-oxo O-atoms connect an Sn2O2 ring to two exocyclic Sn-atoms. The four carboxylate ligands display two different modes of coordination where both modes involve bridging of two structurally distinct Sn-atoms. The solution structures were confirmed by 119Sn NMR spectroscopy by observing two tin resonances in compounds 1, and 46. The observed difference between the two tin resonances was about 3 ppm while the differences in 13C resonances were even smaller. Compounds {[nBu2Sn(L2H)]2O}2 (2) and {[nBu2Sn(L3H)]2O}2 (3) undergo a very complex exchange processes in deuteriochloroform solution. The in vitro cytotoxic activity of compounds 1 and 4 against WIDR, M19 MEL, A498, IGROV, H226, MCF7 and EVSA-T human tumour cell lines is reported.

Graphical abstract

Di-n-butyltin(IV) complexes of the type {[nBu2Sn(LH)]2O}2 (LH = 5-[(E)-2-(aryl)-1-diazenyl]-2-hydroxybenzoate) have been prepared and characterized by NMR, ESI-MS, IR and 119mSn Mössbauer spectroscopy. The crystal structures of {[nBu2Sn(L1H)]2O}2 (1), {[nBu2Sn(L4H)]2O}2 (4), {[nBu2Sn(L5H)]2O}2 (5) and {[nBu2Sn(L6H)]2O}2 (6) were determined. The in vitro cytotoxicity of 1 and 4 against human tumor cell lines is reported.

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Introduction

The design and synthesis of coordination polymers with unusual and tailored structures are fundamental steps in the discovery and fabrication of various technologically useful materials [1], [2], [3], [4]. The majority of such species have been constructed from organic ligands and metal ions by spontaneous self-assembly and such assemblies have resulted in a host of intriguing structural topologies and has also extensive application in material science [5]. It is well known that bis[dicarboxylatotetraorganodistannoxanes] {[R2Sn(O2CR′)]2O}2 have been studied extensively because they have useful applications in biology and catalysis [6], [7], [8], [9]. In the solid state, four basic structural motifs have been observed for {[R2Sn(O2CR′)]2O}2 complexes when potential coordination of the carboxylate ligand to tin originates only from the carboxylate oxygen atoms; other motifs arise when other potential donor atoms reside in R′ [10] (Scheme 1). The dominant form is (I), but a significant number of structures adopt motifs IIV. There is a wide representation of both carboxylate and alkyl ligands among the structures, but there is no apparent correlation between the nature of the ligands and the observed structural motif. Most of the compounds with general formula {[R2Sn(O2CR′)]2O}2 contain a centrosymmetric Sn2O2 core which is connected to two exocyclic tin atoms via μ3-oxo O-atoms to give a R8Sn4O2 central unit. The structural motifs differ from each other in the mode of coordination of the four carboxylate ligands to the R8Sn4O2 framework. The coordination geometry around the endo and exocyclic tin atoms varies from distorted trigonal bipyramidal to distorted octahedral, depending on the participating donor atoms (see Scheme 1). In addition to these solid-state studies, 119Sn NMR studies are important for the structure elucidation in solution for an understanding of the chemical properties of {[R2Sn(O2CR′)]2O}2 complexes, such as their unusual catalytic activity for urethane formation [11]. In this context, several compounds of this class were investigated by 119Sn NMR spectroscopy in order to confirm the geometries around endo- and exocyclic tin atoms, Sn–Sn coupling and thereby the dimeric formulation [12]. The complex structural nature of these {[R2Sn(O2CR′)]2O}2 compounds provides a useful platform for the investigation of the scope and limitations of 119Sn resonance assignment strategies for structural characterization by 2D gradient-assisted 1H–119Sn HMQC NMR in solution and MAS 119Sn NMR in the solid state [7], [13].

Recently, the complexation chemistry of 5-[(E)-2-(aryl)-1-diazenyl]-2-hydroxybenzoic acids (Fig. 1) was investigated with (i) triorganotin(IV) complexes of the type R3SnLH (R = Me, nBu, Ph) [14], [15], [16] and the reactivities of the products were studied using α-diimine to ascertain the coordination ability to the Sn-complex [16], [17] and (ii) diorganotin(IV) complexes of the types nBu2Sn(LH)2 [18], [19], [20] and nBu2Sn(LaH · LbH)2 (mixed ligands) [21]. Furthermore, organotin(IV) complexes of this family of carboxylate ligands were investigated for toxicity effects on Aedes aegypti mosquito larvae [19] and sea urchin development [17] and in vitro cytotoxicity in human cancer cell lines [20]. In view of the importance of organotin(IV) complexes of this class of ligands and the possible applications of {[R2Sn(O2CR′)]2O}2 compounds in biology and catalysis, we report further results concerning structural motifs in bis[dicarboxylatotetrabutyldistannoxanes] and their cytotoxic activity in human tumour cell linesin vitro. The complexes have been characterized in the solid state by means of 119mSn Mössbauer and IR spectroscopic techniques and in solution by 1H, 13C, 119Sn NMR and ESI mass spectrometric studies. The crystal and molecular structures of {[nBu2Sn(L1H)]2O}2 (1), {[nBu2Sn(L4H)]2O}2 (4), {[nBu2Sn(L5H)]2O}2 (5), {[nBu2Sn(L6H)]2O}2 (6) were also determined in order to elucidate the Sn-coordination modes and environments.

Section snippets

Materials

Di-n-butyltin oxide (Fluka) was used as received. The solvents used in the reactions were of AR grade and dried using standard literature procedures.

Physical measurements

Carbon, hydrogen and nitrogen analyses were performed with a Perkin Elmer 2400 series II instrument. IR spectra in the range 4000–400 cm−1 were obtained on a BOMEM DA-8 FT-IR spectrophotometer as KBr discs. The 1H, 13C and 119Sn NMR spectra were recorded on a Bruker ACF 300 spectrometer and measured at 300.13, 75.47 and 111.92 MHz, respectively, or

Synthesis

The 5-[(E)-2-(aryl)-1-diazenyl]-2-hydroxybenzoic acids (LHH′) react with di-n-butyltin(IV) oxide in anhydrous toluene in a 1:1 ratio to give compounds of the formulation {[nBu2Sn(LH)]2O}2 (16). The complexes were isolated as orange crystalline solids in good yield and purity. They are stable in air and are soluble in common organic solvents.

Spectroscopy

Diagnostically important infrared absorption frequencies for the carboxylate antisymmetric [νasym(OCO)] stretching vibration of the complexes appears at

Acknowledgements

The financial support of the Department of Science and Technology, New Delhi, India (Grant No. SP/S1/IC-03/2005, TSBB), the Czech Science Foundation (Grant No. 203/03/1118, AL) and the Ministry of Education, Youth and Sports of the Czech Republic (Project No. MSM0021627502, MH) are gratefully acknowledged. R.J.B. acknowledges the DoD-ONR for funds to upgrade the diffractometer and also the ONR/ASEE program for Summer Faculty Fellowships at the Naval Research Laboratory. E.R. is indebted to the

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