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 [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 = −2 J S 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 ).