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 [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). The synthesis of mixed‐ligand chromium(III) complexes, [CrIII2L(O2CR)]3+, where L represents a macrocyclic binucleating hexaamine‐dithiophenolato ligand and R=Me or Ph, has been achieved. The complexes exhibit a relatively strong intramolecular ferromagnetic exchange interaction that leads to an S=3 spin ground state (J ∼24–34 cm−1). The complexes emit at 750 nm (ΦL=45 %) by excitation into the intense LMCT and d‐d* bands.