Abstract
Copper(I) halides react quantitatively with pyrrolidine (Pyr) in dioxygen-free methylene chloride or nitrobenzene to form tetranuclear copper(I) complexes [(Pyr)
n
CuX]
4
O
2
; n = 1 or 2, X = Cl, Br or I. These complexes are very soluble and completely reduce dioxygen to a dioxo bridging ligand with stoichiometry, Δ[Cu(I)]/Δ[O
2
] = 4.0. Analytical and cryoscopic data establish the formation of discrete tetranuclear products. The stable oxo solids [(Pyr)
n
CuX]
4
O
2
mimic the tyrosinase copper protein and are homogeneous oxidative coupling catalysts for phenols. Electronic transition spectra in the near infrared for [(Pyr)
n
CuX]
4
O
2
are explained as charge transfer from a minimum of three halo ligands to a copper(II) site, (LMCT). Therefore, one can expect a higher molar absorptivity when X = I, as in the [(Pip)
n
CuX]
4
O
2
system. However, molar absorptivity is about half of expected. The lower absorptivity explains the step structure of [(Pyr)
n
CuI]
4
O
2
, in which only two copper(II) sites, instead of four as in [(Pyr)
n
CuX]
4
O
2
, X = Cl or Br, have three iodo ligands per copper center. In addition to infrared and electronic spectra for [(Pyr)
n
CuI]
4
O
2
, EPR bands are observed, and it is concluded that two different types of copper(II) sites are present as represented in
scheme 1
.