Synthesis and Characterisation of Luminescent [Cr(III) (2)L(μ‐carboxylato)](3+) Complexes with High‐Spin S=3 Ground States (L=N(6)S(2) donor ligand)

The synthesis, structure, magnetic, and photophysical properties of two dinuclear, luminescent, mixed‐ligand [Cr(III) (2)L(O(2)CR)](3+) complexes (R=CH(3) (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 N(3)Cr(μ‐SR)(2)(μ (1,3)‐O(2)CR)CrN(3) core structure with μ (1,3)‐bridging carboxylate groups. A ferromagnetic superexchange interaction between the electron spins of the Cr(3+) 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=−2JS(1)S(2)) 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)A(2) → (2)T(1) (ν(2)) 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/CH(2)Cl(2) glasses, Φ (L)=0.44±0.02 (for 1), Φ (L)=0.45±0.02 (for 2).