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

Abstract

The synthesis, structure, magnetic, and photophysical properties of two dinuclear, luminescent, mixed-ligand [CrIII2L(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(μ1,3-O2CR)CrN3 core structure with μ1,3-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 (λexc=405 nm). The absolute quantum yields (ΦL) for 1 and 2 were found to be strongly temperature dependent. At 77 K in frozen MeOH/CH2Cl2 glasses, ΦL=0.44±0.02 (for 1), ΦL=0.45±0.02 (for 2).