Synthesis and Characterisation of Luminescent [CrIII2L(μ-carboxylato)]3+ Complexes with High-Spin S=3 Ground States (L=N6S2 donor ligand)

The synthesis, structure, magnetic, and photophysical properties of two dinuclear, luminescent, mixed-ligand Cr^III₂L(O₂CR)]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 N₃Cr(μ-SR)₂(μ_1,3-O₂CR)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=−2JS₁S₂) 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 ⁴LMCT state on the ⁴A₂ → ²T₁ (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/CH₂Cl₂ glasses, Φ_L=0.44±0.02 (for 1 ), Φ_L=0.45±0.02 (for 2 ).