Copper(II) coordination compounds with sterically constraining pyrenyl nitronyl nitroxide and imino nitroxide

2-(1′-Pyrenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (PyrNN) and 2-(1′-pyrenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl (PyrIN) form pentacoordinate metal–radical complexes with Cu(hfac)2: [Cu(hfac)2PyrNN] and [Cu(hfac)2PyrIN], 1 and 2, respectively. In 1, one aminoxyl unit of PyrNN coordinates to Cu(II) in a distorted square pyramidal environment. In 2, an unusual example of monodentate Cu–N coordination of the imino nitroxide, PyrIN, gives an environment intermediate between square pyramidal and trigonal bipyramid. Both complexes show novel close contacts of the pyrene π-electron cloud with the Cu(II) ion. Compound 1 has a particularly short Cu–O(N) bond of 1.948 Å, giving strong intramolecular antiferromagnetic (AFM) spin-pairing and an absence of significant paramagnetic susceptibility response. The magnetic behavior of compound 2 indicates copper–radical intramolecular ferromagnetic exchange of 2J = 151 cm−1, and intermolecular AFM spin pairing of 2J = −220 cm−1 due to close antiparallel contacts between non-coordinated N–O units. EPR spectra for both complexes are silent at 293 and 77 K. UB97D/6-31G(d) computations find strong antiferromagnetic (2J = −213 cm−1) and ferromagnetic (2J = 610 cm−1) intramolecular metal–radical exchange in 1 and 2, respectively in agreement with experimental results.

Two copper(II) 1:1 complexes of 2-pyrenyl substituted nitronyl nitroxide [Cu(hfac)2(PyrNN)] (1) and imino nitroxide [Cu(hfac)2(PyrIN)] (2) are pentacoordinated with strong anti- and ferromagnetic intramolecular metal-to-radical magnetic exchange, respectively. Both exhibit novel close contacts of copper(II) with the pyrene pi-cloud folded beneath rest of the ligand sphere.Figure optionsDownload as PowerPoint slide