Strong antiferromagnetism in the dinuclear 2-pyridone complex with N–C–O bridges: A paddle-wheel analogue of the dinuclear tetracarboxylates

A novel dinuclear complex [Cu2(μ-L)4(HL)2] (1) was isolated from starting 2-pyridone (HL) via a resonance and a tautomeric transformation. Each copper centre is in a square-pyramidal coordination sphere, defined by two oxygen atoms (Cu–O4 1.978(5), Cu–O11 1.964(4) Å) and two nitrogen atoms (Cu–N2 2.003(5), Cu–N3 2.007(5) Å) of four bridging deprotonated pyridin-2-olates and an oxygen atom on the top from a neutral 2-pyridone (Cu–O2 2.227(5) Å), analogous to tetracarboxylate paddle-wheel complexes. Compound 1 was compared with mixed pyridin-2-olato/methanoato analogues [Cu2(μ-HCO2)2(μ-L)2(HL)2] · 2CH3CN (2) and [Cu2(μ-HCO2)2(μ-L)2(HL)2] (2a) (2a is an air stable form obtained from 2 outside mother-liquid). The EPR spectra of air stable 1 and 2a show three signals Hz1, H⊥2 and Hz2, typical for the binuclear systems with spin S = 1, both revealing strong antiferromagnetism 2J = −334 (1) and −324 cm−1 (2a). Interestingly, only for 1 additional H⊥1 signal at 100 mT is noticed (D(1) = 0.293 cm−1 < hν = 0.320 cm−1 < D(2a) = 0.347 cm−1). On the other hand, several broad signals in the 100–450 mT region, only in the high temperature spectrum for 2a are observed. These results are in agreement with the magnetic susceptibility analysis.

Compound [Cu2(μ-L)4(HL)2] (1) (HL = 2-pyridone) shows the dinuclear cage structure with four triatomic N–C–O bridges (paddle wheel) and monodentate apical 2-pyridones. The EPR spectra show six signals, as suggested for S = 1 spin state and D = 0.293 cm−1 < hν = 0.320 cm−1. Compound 1 is compared with related mixed pyridin-2-olato-methanoato complex [Cu2(μ-HCO2)2(μ-L)2(HL)2] (2a), showing similarly strong antiferromagnetism (2J = −334 (1), −324 cm−1 (2a)).Figure optionsDownload as PowerPoint slide