A tris(pyrazolyl) η⁶-arene ligand that selects Cu(I) over Cu(II)

1,3,5-Tris{2′-[(pyrazol-1-yl)methyl]phenyl}benzene, 4, and its complexes with Cu(I) and Ag(I) have been prepared and characterized. Both Cu^I(4) and Ag^I(4) triflate crystallize in the rhombohedral space group R3̄, with the cations and anions each exhibiting crystallographically imposed 3-fold (C₃) symmetry. In both complexes, 4 behaves as a tris(pyrazolyl) η⁶-arene ligand whose arms act as three-pronged tweezers to form chiral, propeller-like cations with pyramidal MN(pyrazole)₃ coordination geometries. Centers of symmetry in the space group ensure that the crystals are racemates, with equal numbers of P,P,P and M,M,M enantiomers. In broad outline, each cation is shaped like a three-legged stool, with the metal ion centered at the top and pointed downward from a triangular N(pyrazole) plane toward the center of gravity (Cg) of the central benzene ring (a metal-endo conformation), which constitutes the bottom shelf of the stool. The Cu(I)⋯Cg and Ag(I)⋯Cg distances, 3.195(2) and 3.165(2) Å, respectively, support the existence of an η⁶ bonding interaction with Ag(I) and, to a lesser extent, with Cu(I). NMR data for Ag^I(4) suggest rapid interconversion of this cation in solution between P,P,P and M,M,M enantiomers. Our inability to prepare any Cu(II) complexes with 4 is consistent with cyclovoltammetric results, which suggest that the ligand is more easily oxidized than Cu(I).