Selectivity design using interligand contact: solvent extraction and structures of first-series-transition metal–bis(pyrazol-1-yl)borate complexes

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Abstract
The bis(pyrazol-1-yl)borates [H2B(pz)2]–, [H2B(pz)(dmpz)2]– and [H2B(dmpz)2]–(dmpz = 3,5-dimethylpyrazol-1-yl) formed 2:1 complexes with first-series-transition metals, which were extracted into chloroform. The anion [H2B(dmpz)2]– had unique characteristics: (i) extraction of the complexes was a slow process for CoII and NiII; (ii) extractability for NiII was low, and the selectivity pattern did not conform to the Irving–Williams order. The results suggested that its complex formation with small metal ions is kinetically and thermodynamically unfavourable. The crystal structures of [Ni{H2B(pz)(dmpz)2}2], [Ni{H2B(dmpz)2}2] and [Co{H2B(dmpz)2}2] have been determined and compared with those previously reported for [Ni{H2B(pz)2}2] and [Co{H2B(pz)2}2]. The complexes are similar in that the geometry at NiII is square planar and that at CoII is tetrahedral. However, [Co{H2B(dmpz)2}2] contains the highest interligand contact and is highly strained. This contact is the origin of the unusual selectivity of [H2B(dmpz)2]–.

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