Synthesis and Characterisation of Luminescent [CrIII 2 L(μ-carboxylato)]3+ Complexes with High-Spin S=3 Ground States (L=N6 S2 donor ligand)

Abstract

The synthesis, structure, magnetic, and photophysical properties of two dinuclear, luminescent, mixed-ligand [Cr^III ₂ L(O₂ CR)]³⁺ complexes (R=CH₃ (1), Ph (2)) of a 24-membered binucleating hexa-aza-dithiophenolate macrocycle (L)^2- are presented. X-ray crystallographic analysis reveals an edge-sharing bioctahedral N₃ Cr(μ-SR)₂ (μ_1,3 -O₂ CR)CrN₃ core structure with μ_1,3 -bridging carboxylate groups. A ferromagnetic superexchange interaction between the electron spins of the Cr³⁺ ions leads to a high-spin (S=3) ground state. The coupling constants (J=+24.2(1) cm^-1 (1), +34.8(4) cm^-1 (2), H=-2JS₁ S₂ ) are significantly larger than in related bis-μ-alkoxido-μ-carboxylato structures. DFT calculations performed on both complexes reproduce both the sign and strength of the exchange interactions found experimentally. Frozen methanol-dichloromethane 1 : 1 solutions of 1 and 2 luminesce at 750 nm when excited into the ⁴ LMCT state on the ⁴ A₂ → ² T₁ (ν₂ ) bands (λ_exc =405 nm). The absolute quantum yields (Φ_L ) for 1 and 2 were found to be strongly temperature dependent. At 77 K in frozen MeOH/CH₂ Cl₂ glasses, Φ_L =0.44±0.02 (for 1), Φ_L =0.45±0.02 (for 2).

Keywords: crystal structures; dinuclear chromium(III) complexes; luminescence; magnetic properties; thiolato ligand.