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Synthesis and Characterisation of Luminescent [Cr^III ₂ L(μ-carboxylato)]³⁺ Complexes with High-Spin S=3 Ground States (L=N₆ S₂ donor ligand).

ABSTRACT: The synthesis, structure, magnetic, and photophysical properties of two dinuclear, luminescent, mixed-ligand [Cr^III ₂ L(O₂ CR)]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 N₃ Cr(μ-SR)₂ (μ_1,3 -O₂ CR)CrN₃ core structure with μ_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=-2JS₁ S₂ ) 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 A₂ → ² T₁ (ν₂ ) 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/CH₂ Cl₂ glasses, Φ_L =0.44±0.02 (for 1), Φ_L =0.45±0.02 (for 2).

SUBMITTER: Borner M

PROVIDER: S-EPMC8596867 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The molecular wheel [Cr10(OMe)20(O2CCMe3)10], abbreviated {Cr10}, with an unusual intermediate total spin S = 9 and non-negligible cluster anisotropy, D/kB = -0.045(2) K, is a rare case among wheels based on an even number of 3d-metals, which usually present an antiferromagnetic (AF) ground state (S = 0). Herein, we unveil the origin of such a behavior. Angular magnetometry measurements performed on a single crystal confirmed the axial anisotropic behavior of {Cr10}. For powder samples, the temperature dependence of the susceptibility plotted as χT(T) showed an overall ferromagnetic (FM) behavior down to 1.8 K, whereas the magnetization curve M(H) did not saturate at the expected 30 μB/fu for 10 FM coupled 3/2 spin Cr3+ ions, but to a much lower value, corresponding to S = 9. In addition, the X-ray magnetic circular dichroism (XMCD) measured at high magnetic field (170 kOe) and 7.5 K showed the polarization of the cluster moment up to 23 μB/fu. The magnetic results can be rationalized within a model, including the cluster anisotropy, in which the {Cr10} wheel is formed by two semiwheels, each with four Cr3+ spins FM coupled (JFM/kB = 2.0 K), separated by two Cr3+ ions AF coupled asymmetrically (J23/kB = J78/kB = -2.0 K; J34/kB = J89/kB = -0.25 K). Inelastic neutron scattering and heat capacity allowed us to confirm this model leading to the S = 9 ground state and first excited S = 8. Single-molecule magnet behavior with an activation energy of U/kB = 4.0(5) K in the absence of applied field was observed through ac susceptibility measurements down to 0.1 K. The intriguing magnetic behavior of {Cr10} arises from the detailed asymmetry in the molecule interactions produced by small-angle distortions in the angles of the Cr-O-Cr alkoxy bridges coupling the Cr3+ ions, as demonstrated by ab initio and density functional theory calculations, while the cluster anisotropy can be correlated to the single-ion anisotropies calculated for each Cr3+ ion in the wheel.

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Synthesis and Characterisation of Luminescent [Cr^III ₂ L(μ-carboxylato)]³⁺ Complexes with High-Spin S=3 Ground States (L=N₆ S₂ donor ligand).

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Synthesis and Characterisation of Luminescent [CrIII 2 L(μ-carboxylato)]3+ Complexes with High-Spin S=3 Ground States (L=N6 S2 donor ligand).

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Synthesis and Characterisation of Luminescent [Cr^III ₂ L(μ-carboxylato)]³⁺ Complexes with High-Spin S=3 Ground States (L=N₆ S₂ donor ligand).