Synthesis, Structural Characterization, Reactivity, and Catalytic Properties of Copper(I) Complexes with a Series of Tetradentate Tripodal Tris(pyrazolylmethyl)amine Ligands

Novel
tris­(pyrazolylmethyl)­amine ligands TpaMe3, Tpa*,Br, and TpaBr3 have been synthesized and structurally characterized.
The coordination chemistries of these three new tetradentate tripodal
ligands and the already known Tpa and Tpa* have been explored using
different copper­(I) salts as starting materials. Cationic copper­(I)
complexes [TpaxCu]­PF6 (1–4) have been isolated from the reaction
of [Cu­(NCMe)4]­PF6 and 1 equiv of the ligand.
Complexes 2 (Tpax = Tpa*)
and 3 (Tpax = TpaMe3) have been characterized by X-ray studies. The former is a 1D helical
coordination polymer, and the latter is a tetranuclear helicate. In
both structures, the Tpax ligand adopts
a μ2:κ2:κ1-coordination
mode. However, in solution, all of the four complexes form fluxional
species. When CuI is used as the copper­(I) source, neutral compounds 5–8 have been obtained. Complexes 6–8 exhibit a 1:1 metal-to-ligand ratio,
whereas 5 presents 2:1 stoichiometry. Its solid-state
structure has been determined by X-ray diffraction, revealing its
3D polymeric nature. The polymer is composed by the assembly of [Tpa2Cu4I4] units, in which Cu4I4 presents a step-stair structure. The Tpa ligands bridge
the Cu4I4 clusters, adopting also a μ2:κ2:κ1-coordination mode.
As observed for the cationic derivatives, the NMR spectra of 5–8 show the equivalence of the three
pyrazolyl arms of the ligands in these complexes. The reactivities
of cationic copper­(I) derivatives 1–4 with PPh3 and CO have been explored. In all cases, 1:1
adducts [TpaxCuL]­PF6 [L = PPh3 (9–11), CO (12–15)] have been isolated. The crystal structure
of [Tpa*Cu­(PPh3)]­PF6 (9) has been
obtained, showing that the coordination geometry around copper­(I)
is trigonal-pyramidal with the apical position occupied by the tertiary
amine N atom. The Tpa* ligand binds the Cu center to three of its
four N atoms, with one pyrazolyl arm remaining uncoordinated. In solution,
the carbonyl adducts 13–15 exist
as a mixture of two isomers; the four- and five-coordinate species
can be distinguished by means of their IR νCO stretching
bands. Finally, the catalytic activities of complexes 1–4 have been demonstrated in carbene- and nitrene-transfer
reactions.