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 [(Cr2L)-L-III(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(mu-SR)(2)(mu(1,3)-O2CR)CrN3 core structure with mu(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=-2JS(1)S(2)) are significantly larger than in related bis-mu-alkoxido-mu-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 (LMCT)-L-4 state on the (4)A(2) -> T-2(1) (nu(2)) bands (lambda(exc)=405 nm). The absolute quantum yields (phi(L)) for 1 and 2 were found to be strongly temperature dependent. At 77 K in frozen MeOH/CH2Cl2 glasses, phi(L)=0.44 +/- 0.02 (for 1), phi(L)=0.45 +/- 0.02 (for 2).