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

Abstract

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) 2 PyrNN] and [Cu(hfac) 2 PyrIN], 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 NO 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 metalradical exchange in 1 and 2, respectively in agreement with experimental results.

Multifunctional Dithiadiazolyl Radicals: Fluorescence, Electroluminescence, and Photoconducting Behavior in Pyren-1′-yl-dithiadiazolyl

“…These operate via a non-emissive radical/fluorophore system whose emission can be “switched on” when the radical character is quenched. This has been used in the detection of antioxidants at sub-μM concentrations. , Alternatively Cu 2+ ions have been shown to switch on emission through metal coordination to the radical fluorophore in which strong antiferromagnetic metal-radical exchange coupling likely gives rise to a singlet ground state . In the current context, it is noteworthy that attachment of a NN radical directly to the pyrene fluorophore in the 1-position (radical 2 ) or via a heterocyclic or non-conjugated spacer have been shown to lead to complete quenching of emission. , It was therefore surprising to observe incomplete quenching of fluorescence in 1 , despite the radical being directly bonded to the fluorophore.…”

“…Pyrene dimers are known to display relatively efficient fluorescence at longer wavelengths (usually 480–500 nm) . As described earlier, in many cases radicals efficiently quench emission entirely. − In a smaller cross-section of systems such as triaryl methyl radicals, the fluorescence arises from excitation and relaxation of the unpaired electron . In such cases these radicals have been reported to fluoresce in solution or when doped into host matrices but the emission vanishes completely in neat solid materials. , In 1 the process of π*−π* dimerization between radicals effectively quenches the sample paramagnetism, which augured well for continued emission in the solid state.…”

Synthesis, Crystal Structure and Magnetic Properties of a Trinuclear Copper(II) Complex Based on P-Cresol-Substituted Bis(α-Nitronyl Nitroxide) Biradical

“…Bond lengths of the N−O group within the four NN moieties are comprised in the range 1.254(6)–1.300(5) Å ( Table 2 ) with small elongation (0.03 Å) of the coordinated ones and are characteristic of free and coordinated nitroxide radicals [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ]. The O-N-C-N-O moieties are strictly planar ( Table 3 ), as expected due to electron delocalization.…”

“…Therefore, an almost unlimited number of open-shell molecules may be synthesized. Thus, nitronyl nitroxide have been grafted on alcanes [ 4 ], pyridine [ 5 , 6 , 7 , 8 , 9 , 10 ], imidazole [ 11 ], triazole [ 12 ], bipyridine [ 13 ], phenol [ 14 , 15 , 16 , 17 ], pyrene [ 18 ], azulene [ 19 ], phthalocyanine [ 20 ], porphyrin [ 21 ], cyclotriphosphazene [ 22 ], or even on graphene [ 23 ]. Most importantly, nitronyl nitroxide has one of the best stabilities among the free radicals, and this is retained in most of its derivative so that they can generally be handled under mild but normal conditions, even when coordinated with most metal ions.…”

Coordination entities of a pyrene-based iminopyridine ligand: Structural and photophysical properties

“…12 The pyrene unit has been used in combination with nitronylnitroxide and iminonitroxide radicals and their corresponding copper(II) coordination entities in which strong intramolecular copper(II)-radical AFM and FM exchange interaction was observed. 13 Pyridine containing Schiff bases, such as 2-iminopyridyls form stable coordination entities with various transition metals. For instance, they have been used in metallo-organic self-assembling media to produce discrete metallo-supramolecular helicates, 14 cages 15 and capsules.…”