Steric control in the reactions of 3-pyrazolecarboxylic acid with diorganotin dichlorides
Graphical abstract
The reaction of Bn2SnCl2 with 3-pyrazolecarboxylic acid in presence of KOH afforded the 2D-coordination polymer [{(Bn2Sn)2(μ-L)(μ-LH)(μ-OH)}2·2C2H5OH]n (ethanol molecules are shown by space filling model). In contrast, discrete product [{(t-Bu2Sn)2(μ-L)(LH)(μ-OH)}]·H2O was obtained when the reaction was carried out with t-Bu2SnCl2.
Research highlights
► Two-dimensional organotin coordination polymers. ► Hydroxide- and pyrazole-bridged dinuclear organotin compound. ► Multi-dentate coordination behavior of pyrazolecarboxylate. ► Organostannoxane macrocycles.
Introduction
In recent years, organotin compounds have been attracting interest due to a variety of reasons. Applications of organotin compounds in biology, coatings, catalysis and as additives to polymers have been a major impetus in this area [1], [2], [3], [4]. In addition the structural chemistry of organotin compounds in general and organostannoxanes in particular has been extremely rich and has proved to be a fertile area of research [5], [6], [7], [8], [9]. The reactions of appropriate organotin oxides, hydroxides and oxide-hydroxides with protic acids such as carboxylic acids, phosphorus-based acids or sulfonic acids have afforded a wide range of organostannoxanes with diverse structures [5], [6], [10], [11], [12], [13], [14], [15], [16]. Additionally, there has been a basic academic interest to study the nature of the organotin oxides/hydroxides and the mechanism of their formation [17], [18], [19], [20]. In this endeavor, we have, for example, isolated and characterized hydrated organotin cations and products formed from them under various conditions [21], [22], [23]. The knowledge that a given synthetic procedure can generate a specific type of organostannoxane has allowed the utilization of these synthetic methodologies to assemble dendrimer-like compounds containing a stannoxane core and a functional periphery [10], [11], [24], [25], [26].
The success of the synthetic methodologies to prepare diverse organostannoxanes has prompted many research groups to investigate the possibility of generating organotin-containing macrocycles and coordination polymers [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]. In view of this interest we have investigated the reactions of 3,5-pyrazoledicarboxylic acid (L′H3) with diorganotin dichlorides and have observed the formation of macrocycle-linked coordination polymers such as [(Bn2Sn)6(L′)4(μ-OH)2(Bn2 SnCl)2]n (Chart 1) [39], [40]. The macrocycles present in these coordination polymers are stitched together by the multitopic hexadentate pyrazoledicarboxylate ligand. These interesting results encouraged us to explore these reactions further. We have decided to replace L′H3 with 3-pyrazolecarboxylic acid (LH2) (Chart 2) with two objectives. One, the de-protonated form [L]2− of LH2, while still retaining a potential multitopic coordination behavior has only one carboxylate unit along with a pair of nitrogen atoms for coordination. It has been observed by us previously that while the two nitrogen atoms of the pyrazolyl group are effective in holding a pair of tin atoms together, the carboxylate ligands tend to generate the macrocycles as well as to propagate them in a two-dimensional manner [39], [40]. It was of interest to examine if the coordination environment of LH2 is sufficient to generate a macrocycle-containing coordination polymer. The second objective of utilizing LH2 was the possibility of realizing a molecular ditin motif particularly in the reactions of a sterically encumbered diorganotin dihalide. Accordingly, in the following, we describe the results of our investigations on the reactions of LH2 with dibenzyltin dichloride (Bn2SnCl2) and di-tert-butyltin dichloride (t-Bu2SnCl2).
Section snippets
Results and discussion
The reaction of 3-pyrazolecarboxylic acid (LH2) with Bn2SnCl2 leads to the formation of a 2D-coordination polymer, [{(Bn2Sn)2(μ-L)(μ-LH)(μ-OH)}2·2C2H5OH]n (1) (Scheme 1), whose structure was determined by X-ray crystallography (see below). Interestingly, the reaction of Bn2SnCl2 with 3,5-pyrazoledicarboxylic acid also leads to the formation of a coordination polymer. In the latter, the repeat motif of the coordination polymer consists of a hexatin ensemble as a result of the additional
X-ray crystal structures of 1 and 2
The structures of 1 and 2 are given in Fig. 1, Fig. 2. The bond parameters of these compounds are summarized in Table 1, Table 2. Compound 1 is a 2D-coordination polymer containing tetranuclear repeating units each of which is formed by the fusion of two symmetrically related ditin subunits (Fig. 1). The ditin motif itself is composed of two dibenzyltin units, two 3-pyrazolecarboxylates, and one hydroxide ligand. The two tin atoms of the ditin subunit are bridged by a hydroxide ligand as well
Conclusions
In summary, we have shown that the reactions of 3-pyrazolecarboxylic acid with diorganotin dihalides can be modulated by the type of substituents present on tin. With Bn2SnCl2, the multitopic nature of the ligand drives the reaction to a two-dimensional coordination polymer 1 where as with the more sterically hindered t-Bu2SnCl2 a molecular ditin compound 2 was isolated. Interestingly, the coordination polymer 1 contains interlinked tetranuclear macrocycles. Thus replacing
Reagents and general procedures
Solvents and other general reagents used in this work were purified according to standard procedures [47]. 3-Pyrazolecarboxylic acid [48], di(tert-butyl)tin dichloride [49] and dibenzyltin dichloride [50] were synthesized according to literature procedures.
Instrumentation
Melting points were measured using a JSGW melting point apparatus and were uncorrected. Elemental Analyses of the compounds were obtained using a Thermoquest CE instrument CHNS-O, EA/110 model. 1H and 119Sn NMR spectra were obtained on a JEOL
Acknowledgment
VC is thankful to the Department of Science and Technology for a J.C. Bose fellowship. RT thanks the Council of Scientific and Industrial Research for Senior Research Fellowship. RKM and BM thank the UGC and CSIR for Junior Research Fellowship.
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