Synthesis and Characterisation of Luminescent [CrIII 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).

Keywords: crystal structures; dinuclear chromium(III) complexes; luminescence; magnetic properties; thiolato ligand.

Figure 1

Dinuclear chromium(III) complexes supported by macrocyclic (H₂L, this work) and open chain amino‐thiophenolato ligands (H₂L^N4S2, and HL^N2S, ^[52] ) respectively.

Scheme 1

Synthesis of 1 and 2.

Figure 2

Cyclic voltammograms of 1 (top) and 2 (bottom, blue curve) in acetonitrile at ambient temperature (Pt disk working electrode, Pt wire counter electrode, Ag/AgCl reference electrode, 0.1 M NBu₄PF₆ supporting electrolyte, scan rate 100 mV/s).

Figure 3

Structure of the [Cr^III ₂L(μ _1,3‐O₂CMe)]³⁺ cation in crystals of 1 ⋅ 2H₂O. Thermal ellipsoids are drawn at the 50 % probability level. Only one orientation of a disordered tert‐butyl group is shown. Hydrogen atoms are omitted for reasons of clarity.

Figure 4

Temperature dependence of μ _eff versus T for compounds 1 ⋅ 2H₂O (▪) and 2 ⋅ xMeCN (x ∼4.5) (▴) (B=0.5 T). The full lines represent the best theoretical fits to equation 3. Fit parameters are given in the text.

Figure 5

DFT calculated spin density plot for complex 1 (B3LYP/def2‐TZVPP level of theory, isosurface value 0.005 a.u., hydrogens omitted for clarity).

Figure 6

UV‐vis absorption spectra of 1 (black line) and 2 (blue dashed line) in fluid MeCN solution at 295 K (c=10⁻⁴  m). The inset shows the region of the spin‐forbidden ⁴A₂→²T₁, ²E transitions in the 650 to 750 nm range (note the different scales).

Figure 7

Qualitative energy level diagram of the ground and singly excited states for the chromium(III) complexes 1 and 2 showing electronic transitions and possible energy transfer pathways. Energy levels are not drawn to scale. The single ion energy levels (in the absence of exchange interactions) are shown on the left (levels assigned in O notation). Energy is absorbed by the LMCT and ligand field (ν₁, ν₂) transitions to populate the ⁴T₂ ⁴A₂ state. Intersystem crossing (ISC) then leads to population of the emissive ²T₁ ⁴A₂ and/or ²E⁴A₂ states. Non‐radiative (wavy lines) pathways lead to quenching of excited states. Right: The spin states (S) resulting from exchange interactions are only shown for the lowest excited ²E⁴A₂ and the ⁴A₂ ⁴A₂ ground state. The sign and size of the exchange interaction (J′) in the excited ²E⁴A₂ state could not yet be determined. Two of several allowed emission bands (expected to get intensity from the single‐ion mechanism and the exchange interactions) are drawn (Boltzmann population not considered). This level of approximation implies that the ²E and ²T₁ states are degenerate, which is clearly not the case in these complexes.

Figure 8

Excitation (λ_obs=750 nm, green curve) and emission spectra (λ_exc=405 nm, black line) of complex 1 at 77 K in a frozen CH₂Cl₂/MeOH 1 : 1 glassy matrix.