Abstract
The syntheses of 4-mercapto-2,6-di(pyrazol-1-yl)pyridine (bppSH) and bis[2,6-di(pyrazol-1-yl)pyrid-4-yl]disulfide (bppSSbpp) are reported. In contrast to previously published "back-to-back" bis-[2,6-di(pyrazol-1-yl)pyridine] derivatives, which form coordination polymers with transition ions that are usually insoluble, bppSSbpp yields soluble oligomeric complexes with iron(ii) and zinc(ii). Mass spectrometry and DOSY data show that [{Fe(μ-bppSSbpp)}n](2n+) and [{Zn(μ-bppSSbpp)}n](2n+) form tetranuclear metallacycles in nitromethane solution (n = 4), although (1)H NMR and conductivity measurements imply the iron compound may undergo more fragmentation than its zinc congener. Both [{Fe(bppSH)2](2+) and [{Fe(μ-bppSSbpp)}n](2n+) exhibit thermal spin-crossover in CD3NO2 solution, with midpoint temperatures near 245 K. The similarity of these equilibria implies there is little cooperativity between the iron centres in the metallacyclic structures.
Highlights
One of the most useful series of compounds in the field of spin-crossover research[1,2,3,4,5] has proven to be [Fe(bpp)2]2+, where bpp is 2,6-di( pyrazol-1-yl)pyridine or a derivative of it (Scheme 1).[6,7,8] Two particular properties of [Fe(bpp)2]2+ centres account for their ubiquity
The identity of bppSSbpp was confirmed by mass spectrometry (m/z = 507.1, corresponding to [Na(bppSSbpp)]+) and by a crystal structure determination (Fig. 2)
In each case the interacting bpp fragments are coplanar by symmetry, and separated by 3.508(4)–3.510(4) Å (ESI†)
Summary
Scheme 1 The ligands discussed in this work. through their pyridyl groups, have yielded 1D coordination polymers with spin-crossover functionality.[14,18,19]. A group of compounds from the bpp family that have not been investigated until now are derivatives bearing sulfur substituents at their pyridyl ring, of which 4-mercapto-2,6di(pyrazol-1-yl)pyridine (bppSH) is the parent (Scheme 1). While such compounds could be used in self-assembled monolayers‡, bppSH is of interest as a precursor to larger assembly structures through disulfide formation, or alkylation or ligation of the peripheral thiyl group.
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