Reactivity of Copper(I) Complexes with Tripodal Ligands towards O₂: Structures of a Precursor [L³Cu^I(NCCH₃)](BF₄), L³ = Tris(3‐isopropyl‐4,5‐trimethylenepyrazolyl)methane and of its Oxidation Product [L³Cu^II(μ‐OH)₂Cu^IIL³](BF₄)₂ with Strong Antiferromagnetic Spin‐Spin Coupling

Abstract

The molecular structure of the highly oxygen-sensitive complex [L 3 Cu I (NCCH 3 )](BF 4 ) (1) reveals approximately symmetrical coordination by the fac-tridentate (tripodal) ligand L 3 ϭ tris(3-isopropyl-4,5-trimethylenepyrazolyl)methane and a rather short Cu I -N(acetonitrile) distance of 1.865(5) Å . In CH 2 Cl 2 at Ϫ78°C the colourless compound reacts with O 2 to yield a labile purple intermediate (λ max 517 nm) Ϫ presumably a peroxodicopper(II) complex Ϫ which decomposes at Ϫ30°C. No such intermediate was observed on reaction of the Cu I complex of bis(2-pyridylmethyl)benzylamine with O 2 at Ϫ80°C. However, an EPR spectrum with g ʈ ϭ 2.17 and gϭ 2.03 without 63,65 Cu hyperfine splitting was observed at low temperatures. Exposure of the precursor 1 to air under ambient conditions yields dinuclear [L 3 Cu II (µ-OH) 2-2 solution. The structure of the hexakis-(dichloromethane) solvate of 2 with CuϪCu and Cu-O distances of 3.055 and 1.94 Å , respectively, is typical for dihydroxo-bridged dicopper compounds with square-pyramidal Cu II configuration (τ ϭ 0.03), adopting an anti arrangement. In agreement with the relatively wide Cu-O-Cu angles of 103.5°an analysis of the temperature dependence of the magnetic susceptibility revealed a rather strong (J ϭ Ϫ633 cm Ϫ1 ) antiparallel spin-spin coupling. The effect is ascribed to the steric bulk of the ligand L 3 .

Solvent switchable CuII complexes

“…Therefore, strong antiferromagnetic exchange interactions require both good s-bonding orientation of the magnetic orbitals (i.e., the orbitals that contain the unpaired electrons) and good superexchange pathways provided by the bridging atom orbitals. [43][44][45][46] In the reported complex, the d x 2 Ày 2 corresponds to the magnetic orbital, which is localized in the same plane of the bridging ligands, presenting both phenoxido and hydroxido ligands in an equatorial-equatorial coordination mode. This structural feature promotes the observed strong antiferromagnetic interaction between the Cu II ions.…”

Chemical Fixation of Atmospheric CO2 by Copper(II) Complexes of a Tridentate N‐donor Ligand

“…Such green dihydroxo‐bridged dicopper(II) species (see below) are frequently encountered as end‐products from the reaction of copper(I) complexes with dioxygen [Equation ]. [11e], , From such a solution we isolated a light‐blue solid [{(L 3 )Cu II (μ‐OH)} 2 ](ClO 4 ) 2 ( 2 ) in 55 % yield. When dissolved in CH 2 Cl 2 /MeCN the light‐blue solid generates green solutions.…”

“…[12a], In DMF‐MeOH (1:1, v/v) solution at 100 K, complexes 2 – 4 dissociates to monomers (Figure S16, Supporting Information) displaying axial behavior. [13a],…”

“…We observed only irreversible oxidation, generating a green solution. [11e], , On exposure to air of such green solutions containing {(L 3 ) 2 Cu II 2 (μ‐OH) 2 } 2+ led to the isolation of 4 . Notably, 4 is formed without addition of any external base.…”

“…Subsequent reaction of such highly reactive intermediates with trace amount of moisture present in organic solvents/externally‐added water produce {Cu II 2 (μ‐OH) 2 } 2+ species as the end‐product. [11e], , The dihydroxo–bridged dicopper(II) complexes are generally inactive to give any further reaction and hence are not usually investigated any further. However, we and others have investigated nucleophilic properties of Cu II –OH species on the hydrolysis of unactivated esters and conversion of CO 2 to CO 3 2– .…”