Syntheses and crystal structures of ethyltin complexes with ferrocenecarboxylic acid

: Three new ethyltin complexes containing ferro -cenecarboxylate, Et 2 Sn ( OC ( O ) Fc ) 2 ( 1 ) , [( Et 2 SnOC ( O ) Fc ) 2 O ] 2 ( 2 ) , and [ EtSn ( O ) OC ( O ) Fc ] 6 ( 3 ) ( Fc = C 5 H 5 FeC 5 H 4 ) , have been synthesized by the reaction of diethyltin dichloride with ferrocenecarboxylic acid in the presence of potassium hydroxide and characterized by means of elemental ana -lysis, FT - IR, NMR spectroscopy, and X - ray single crystal di ﬀ raction. In solid state, 1 is a weak dimer possessing a cyclic Sn 2 O 2 unit formed by the intermolecular Sn ⋯ O interaction, and the tin atom has a distorted pentagonal bipyramid geometry. Compound 2 is a four - tin nuclear diethyltin complex with a ladder framework, and each tin atom adopts a distorted trigonal bipyramidal con ﬁ gura tion in which two oxygen atoms occupy the axial posi tions. Compound 3 is a hexa - tin nuclear monoethyltin complex having a drum - shaped structure, and each of the tin atoms possesses a distorted octahedral geometry. The ferrocene units are attached to the tin atoms through the monodentate or bidentate coordinated carboxylates.


Introduction
Organotins are a kind of widely used main group metal compound, which have more applications than the organic derivatives of any other metal (Davies et al., 2008). They have been used as stabilizers for polyvinyl chloride, ionophores in sensors, insecticides, fungicides, organic synthesis reagents, reaction catalysts, and so on (Davies et al., 2008). Recent studies have shown that organotin carboxylates exhibit high cytotoxic activity and good prospects in cancer therapy (Bantia et al., 2019;Chen et al., 2020;Shang et al., 2011). Ferrocene is an organic transition metal compound, and its derivatives have been used as catalysts for asymmetric reactions and functional materials including photosensitizer, nonlinear optical material, membrane electrode, and smoke agent, and exhibited a variety of biological properties (Braga and Silva, 2013;Cunningham et al., 2020;Xiao et al., 2020). To synthesize organotin carboxylates containing ferrocenyl group is a valuable choice for broadening the use of ferrocene derivatives and searching for highly efficient and low toxic organotin anticancer drugs. Some organotin carboxylates containing ferrocenyl were synthesized in succession and also showed good in vitro anticancer activity. Some examples are as follows: R 3 SnOC  (Chandrasekhar et al., 2000(Chandrasekhar et al., , 2005aChandrasekhar and Thirumoorthi, 2008;Dong et al., 2014;Zhu et al., 2011). Up to now, the reaction of diethyltin dichloride with ferrocenecarboxylic acid (FcCOOH) has not been reported in the literature. In order to continue to expand the chemistry of the ferrocene-organotin compounds, we synthesized three ethyltin ferrocenecarboxylates, Et 2 Sn(OC(O)Fc) 2 (1), [(Et 2 SnOC(O)Fc) 2 O] 2 (2), and [EtSn(O)OC(O)Fc] 6 (3), and determined their crystal structures (Scheme 1).

Synthesis
In the presence of potassium hydroxide, diethyltin dichloride reacted with ferrocenecarboxylic acid (FcCOOH) to produce compounds 1-3 (Scheme 1). The nature of the product is related to the stoichiometry of the reaction as well as the reaction temperature (solvent reflux temperature). Under reflux conditions, diethyltin bis(ferrocenecarboxylate) (1) and bis[oxo-bis(diethyltin ferrocenecarboxylate)] (2) were formed in good yields (isolated yield >80%) in a methanolic solution, and hexakis(oxo-ethyltin ferrocenecarboxylate) (3) was isolated in a low yield (31%) in toluene. Compound 3 was formed by a Sn-C bond cleavage of diethyltin in the reaction process. A wide range of Sn-C bonds are cleaved by carboxylic acids leading to Sn-O bond formation (Chandrasekhar et al., 2005b). Although the Sn-C bond cleavage is a common reaction in phenyl-, benzyl-, allyl-, and methyl-tin compounds, very few examples of Sn-ethyl and Sn-butyl bonds cleavage are known. The cleavage of the alkyl-tin bond may involve an S E 2 mechanism (Chandrasekhar et al., 2005b;Davies, 2004;Gopal et al., 2014;Shankar et al., 2010). Compounds 1-3 are orange red crystals and stable in air. 1 and 2 are soluble in benzene, methanol, chloroform, and acetone, while 3 is not (Chandrasekhar and Thirumoorthi, 2008;Mairychova et al., 2014).

Spectroscopic analysis
Ferrocenecarboxylic acid displays a broad band at 3,200-2,500 cm −1 and a strong band at 1,658 cm −1 , which are assigned to the ν(OH) and ν as (COO) stretching vibrations, respectively. In 1-3, the absorption band at 3,200-2,500 cm −1 disappears, and the ν s (COO) and ν as (COO) bands shift considerably, indicating the formation of the ethyltin complexes by the carboxyl (COO) oxygen atom coordination to tin (Shang et al., 2011;Zhu et al., 2011). The difference between the ν as (COO) and ν s (COO) of carboxylate moiety, Δν(COO), has been used to determine its bidentate or monodentate coordination mode (Chen et al., 2020;Deacon and Phillips, 1980;Shang et al., 2011;Tian et al., 2020). In 1-3, FcCOO is bidentate, which is evidenced by the small Δν(COO) value (179 cm −1 for 1, 160 cm −1 for 2, 192 cm −1 for 3). In 2, there is also a carboxyl group of monodentate coordination to tin because of a large Δν(COO) value of 285 cm −1  Wang et al., 2010), which is in agreement with the below X-ray structure of 2.
The 1 H NMR spectra of 1-3 exhibit the expected integration and multiplicities of the resonance absorption peaks. The chemical shifts of the protons of Cp-rings (C 5 H 5 and C 5 H 4 ) appear at ∼4.20, 4.40, and 4.80 ppm, respectively. The 13 C chemical shift of carboxyl carbon in 1 and 2 is 182.1 and 177.6 ppm, respectively, and the resonance absorptions of carbon atoms of Cp-rings appear in the region of 69-75 ppm. In 2, the (CH 3 CH 2 ) 2 Sn moiety displays two sets of 1 H and 13 C NMR signals (see Section 4), which is consistent with the NMR spectra of the other diorganooxotin carboxylates [(R 2 SnOOCR′) 2 O] 2 , such as [(n-Bu 2 SnOOCFc) 2 O] 2 (Tao and Xiao, 1996) and [(Et 2 SnOOCC 6 H 3 N 2 S) 2 O] 2 (Wang et al., 2010). The 119 Sn chemical shifts primarily depend on the coordination number and the type of the donor atoms bonded to tin atom (Davies, 2004;Holecek et al., 1986). Compound 1 displays a single 119 Sn resonance at −149.5 ppm, suggesting that the tin atom in 1 is five-coordinated in the CDCl 3 solution (Holecek et al., 1986). In solution, there may be the following situations: one of the two carboxyl groups of the ligands is monodentate coordination, and the other is bidentate chelation coordination (Chandrasekhar and Thirumoorthi, 2007). In compound 2, a pair of 119 Sn resonances is observed at −189.0 and −199.2 ppm due to the presence of endo-and exo-cyclic tin atoms. These values are comparable with those reported in other dimeric distannoxanes (Chandrasekhar et al., 2005a;Wang et al., 2010;Zhu et al., 2011) and correspond to a five-coordinated or weakly six-coordinated tin atom (Holecek et al., 1986). Due to the poor solubility of 3, the 13 C and 119 Sn NMR cannot be obtained for identification.
Complex 2 exists as a centrosymmetric dimer ( Figure 2) and possesses a ladder framework built up around the planar cyclic Sn 2 O 2 unit (Sn (2) (2)   structural features of 2 are consistent with Type I among the structures of [(R 2 SnOOCR′) 2 O] 2 summarized by Tiekink (1991). In the molecular structure of 2, the central Sn 2 O 2 ring, carboxylates, and substituted cyclopentadiene rings are essentially in the same plane, and the maximum deviation (O(3)) from the mean plane is 0.248(3) Å. The ferrocenyls are located in the upper and lower sides of this plane, respectively.
Compound 3 crystallizes in a trigonal space group R3ā nd is a hexa-tin nuclear organotin complex possessing the drum-shaped structure. Although many of such drum compounds have been reported (Basu Baul et al., 2017;Shang et al., 2011;Tiekink, 1991;Xiao et al., 2019), to our knowledge, 3 is the first  (Chandrasekhar et al., 2000;Chandrasekhar and Thirumoorthi, 2008), indicating that R bound to tin has little effect on the drum structure.

Experimental
All chemical reagents and solvents were purchased from Sinopharm Chemical Reagents Company (Shanghai, China) and used directly without further purification. The instruments used for the characterization of compounds are as follows: a Perkin Elmer 2400 Series II elemental analyzer (carbon and hydrogen analyses), a Nicolet 470 FT-IR spectrophotometer (IR spectra), and a Bruker Avance III HD500 NMR spectrometer ( 1 H and 13 C NMR spectra).

Synthesis of diethyltin bis (ferrocenecarboxylate) (1)
Ferrocenecarboxylic acid (0.460 g, 2 mmol), potassium hydroxide (0.112 g, 2 mmol), and methanol (25 mL) were added into a 50 mL round bottom flask and stirred for 10 min. Diethyltin dichloride (0.248 g, 1 mmol) was added, and then the reaction mixture was refluxed for 4 h. The orange red solution was cooled to room temperature and filtered. The solvent was removed from the filtrate by a rotary evaporator. The orange yellow solid obtained was recrystallized from the mixed solvents of n-hexane and trichloromethane

X-ray crystallography
The orange-red single crystals of 1 and 2 were obtained from cyclohexane-benzene (1:1, v/v), and compound 3 was obtained by slow evaporation of toluene solution, respectively. Diffraction data were collected at room temperature on a Bruker Smart Apex imaging-plate area detector fitted with graphite monochromatized Mo-Kα radiation (0.71073 Å). Structure solution and refinement were completed using SHELXS-97 (Sheldrick, 2008) and SHELXL-2018(Sheldrick, 2015, respectively. The nonhydrogen atoms were refined anisotropically, and hydrogen atoms were placed at calculated positions. In 2, the ethyl group was disordered over two positions, and the site occupancy was refined to 0.736(16):0.264(16). Crystal data and refinement parameters are summarized in Table 2. Crystallographic data have been deposited in the Cambridge Crystallographic Data Centre with supplementary publication numbers CCDC 2094054-2094056.
Funding information: Authors state that no funding is involved.