Syntheses and Crystal Structures of Copper(II) Bis(pyrazolyl)acetic Acid Complexes

Abstract: The reaction of bis(pyrazol-1-yl)acetic acid (L) with copper(II) bromide provides two new complexes: [Cu(L-H)Br(µ-Br)]₂ (1) and [Cu(L)₂]⋅4H₂O (2). Complex 1 is prepared by the stoichiometric reaction of L-H with copper(II) bromide in tetrahydrofuran. Dissolution of 1 in methanol causes its conversion into the bis-ligated complex (2), a highly hygroscopic material. Complex 2 can also be synthesized directly by the stoichiometric reaction of L-H with CuBr₂ in methanol. Complex 1 is dimeric, interlinked by two bridging bromide ligands, and possesses terminal bromide ligands on each copper atom. The two pyrazolyl ligands in 1 coordinate with the nitrogen atoms to complete the coordination sphere of copper, resulting in a highly distorted geometry that lies between a trigonal bipyramidal or square pyramidal geometry with wide bond angles between the trans-oriented pyrazolyl nitrogen atoms and bromide ligands. Complex 2 assumes a pseudo-trigonal prismatic geometry with the deprotonated carboxylic acid that interacts weakly with copper. Graphical Abstract: Reaction of bis(pyrazol-1-yl)acetic acid (L-H) with copper(II) bromide provides two new complexes: [Cu(L-H)Br(µ-Br)]₂ (1) and [Cu(L)₂]⋅4H₂O (2), these compounds represent the first, well-defined, independent copper(II) compounds supported by the base bis(pyrazolyl)acetic acid ligand. [Figure not available: see fulltext.].