Oligonuclear Homoleptic Copper(I) Pyrazolates with Multinucleating Ligand Scaffolds: High Structural Diversity in Solid-State and Solution

The synthesis of three pyrazole-based, potentially binucleating ligands 3,5-bis(R1N(CH3)CH2)-4-R2pyrazole (L1H: R1 = pyridyl-2-methyl-, R2 = Ph; L2H: R1 = 8-quinolyl-, R2 = H; L3H: R1 = 8-quinolyl-, R2 = Ph) is described. Reaction of L1−3H with 1 equiv. of mesitylcopper affords oligonuclear homoleptic complexes of the type [CuL]n (1−3). The single crystal X-ray structure of 2 shows a tetranuclear assembly of linear coordinated copper(I)-centers bridged by pyrazolato ligands that alternate above and below the Cu4 plane, with additional weak interactions from some of the ligand side arms. As the single crystal X-ray structure of 3 reveals, phenyl substitution at the 4-position of the pyrazolato framework leads to significant structural modification of the Cu4 array, giving a rhombical tetranuclear complex with two linear coordinated copper(I) centers that exhibit a short intramolecular Cu···Cu contact (2.8212(10) Å) and two peripheral copper(I) centers in a distorted tetrahedral coordination mode. Thus, 3 represents a very rare example of an inorganic pyrazolato cuprate which can also be viewed as a partly rearranged structural isomer of 2. Furthermore, the crystal lattice of 3 shows an extended network of intra- and intermolecular π−π stacking interactions between the aromatic rings. In solution, 1−3 each form two types of oligomers a and b that slowly (<1 s−1) equilibrate at room temperature. Using Diffusion Ordered Spectroscopy (DOSY) and variable temperature 1H NMR spectroscopy it can be shown that a and b correspond to a tetrameric and a (planar) trimeric species. Coordination of the pyridyl/quinolyl side arms that is observed in the solid state seems to be only transient in solution.