Structural and Electronic Differences of Copper(I) Complexes with Tris(pyrazolyl)methane and Hydrotris(pyrazolyl)borate Ligands

Copper(I) complexes with tripodal nitrogen-containing neutral ligands such as tris(3,5-diisopropyl-1-pyrazolyl)methane
(L1‘) and tris(3-tertiary-butyl-5-isopropyl-1-pyrazolyl)methane (L3‘), and with corresponding anionic ligands such as
hydrotris(3,5-diisopropyl-1-pyrazolyl)borate (L1-) and hydrotris(3-tertiary-butyl-5-isopropyl-1-pyrazolyl)borate (L3-)
were synthesized and structurally characterized. Copper(I) complexes [Cu(L1‘)Cl] (1), [Cu(L1‘)(OClO3)] (2),
[Cu(L1‘)(NCMe)](PF6) (3a), [Cu(L1‘)(NCMe)](ClO4) (3b), [Cu(L1‘)(CO)](PF6) (4a), and [Cu(L1‘)(CO)](ClO4) (4b) were
prepared using the ligand L1‘. Copper(I) complexes [Cu(L3‘)Cl] (5) and [Cu(L3‘)(NCMe)](PF6) (6) with the ligand
L3‘ were also synthesized. Copper(I) complexes [Cu(L1)(NCMe)] (7) and [Cu(L1)(CO)] (8) were prepared using the
anionic ligand L1-. Finally, copper(I) complexes with anionic ligand L3- and acetonitrile (9) and carbon monoxide
(10) were synthesized. The complexes obtained were fully characterized by IR, far-IR, 1H NMR, and 13C NMR
spectroscopy. The structures of both ligands, L1‘ and L3‘, and of complexes 1, 2, 3a, 3b, 4a, 4b, 5, 6, 7, and 10
were determined by X-ray crystallography. The effects of the differences in (a) the fourth ligand and the counteranion,
(b) the steric hindrance at the third position of the pyrazolyl rings, and most importantly, (c) the charge of the N3
type ligands, on the structures, spectroscopic properties, and reactivities of the copper(I) complexes are discussed.
The observed differences in the reactivities toward O2 of the copper(I) acetonitrile complexes are traced back to
differences in the oxidation potentials determined by cyclic voltammetry. A special focus is set on the carbonyl
complexes, where the 13C NMR and vibrational data are presented. Density functional theory (DFT) calculations
are used to shed light on the differences in CO bonding in the compounds with neutral and anionic N3 ligands. In
correlation with the vibrational and electrochemical data of these complexes, it is demonstrated that the C−O
stretching vibration is a sensitive probe for the “electron richness” of copper(I) in these compounds.