Steric considerations have long been known as important forces controlling ligand coordination to metal ions in the solution and crystalline phase.  Additionally, the presence of coordinating anions, like halides, or weakly coordinating anions, like nitrate, can influence the way ligands coordinate to the metal ion in question.  With a coordinatively flexible metal ion such as CuII, the interplay between these two competing forces can allow for fine-tuning of coordination complex structures with minimal effort.  By simply changing the starting CuII salt from CuCl2�2 H2O to Cu(NO3)2�2.5 H2O, complexes with 2:1 and 4:1 ligand to metal (L:M) ratios and varying coordination numbers have been easily and rapidly synthesized from 2:1 stoichiometric ratios of L:M in water solutions at room temperature using 3-methylpyrazole, 4-methylpyrazole, 3,5-dimethylpyrazole, and pyrazole.  Each reaction formed single crystals which were analyzed by X-ray diffraction.  For pyrazole, 4:1 L:M complexes were formed with both CuCl2�2 H2O and Cu(NO3)2�2.5 H2O, with CuII adopting an octahedral ligand environment and the anions coordinated in a trans fashion as the pyrazole ligands did not sterically interfere with anion coordination. With 3-methylpyrazole, an extended structure with a 2:1 L:M ratio was formed using CuCl2�2H2O, containing two bridging Cl- ions with CuII in an octahedral environment. With Cu(NO3)2�2.5 H2O, however, an isolated 4:1 L:M complex was formed with CuII in an octahedral environment with the anions coordinated in a trans fashion. Interestingly, with 3,5-dimethylpyrazole, an extended structure containing a 5-coordinate CuII center with one bridging and one pendant Cl- ion is formed with the CuCl2�2 H2O salt.  With Cu(NO3)2�2 H2O, 3,5-dimethylpyrazole also forms a 5-coordinate complex, though it is an isolated structure with the fifth coordination site occupied by a water molecule, not nitrate ions, which are relegated to outer sphere coordination.  Significant coordination control is shown simply through varying the counterion and ligand sterics in CuII complexes of methylated pyrazoles.

Crystallographic evidence of coordination environment control through the interplay of anion coordination strength and ligand sterics in Cu<sup>II</sup> complexes of methylated pyrazoles

247th National Meeting (2014)

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Crystallographic evidence of coordination environment control through the interplay of anion coordination strength and ligand sterics in CuII complexes of methylated pyrazoles

Crystallographic evidence of coordination environment control through the interplay of anion coordination strength and ligand sterics in Cu^II complexes of methylated pyrazoles