High-Spin Iron(II) as a Semitransparent Partner for Tuning Europium(III) Luminescence in Heterodimetallic d–f Complexes

Abstract

The segmental ligand 2-{6-(N,N-diethylcarbamoyl)pyridin-2-yl}-1,1'dimethyl-5,5'-methylene-2'-(6-methylpyridine-2-yl)bis[1H-benzimidazole] (L 3) reacts with a stoichiometric mixture of Ln III (Ln La, Eu, Gd) and M II (M Zn, Fe) in acetonitrile to produce selectively the heterodimetallic triple-stranded helicates (HHH)-[LnM(L 3) 3 ] 5. In these complexes, M II is pseudooctahedrally coordinated by the three wrapped bidentate binding units, thus forming a noncovalent tripod which organizes the three unsymmetrical tridentate segments to give ninefold coordination to Ln III. The introduction of a methyl group at the 6 position of the terminal pyridine in L 3 sterically reduces the complexing ability of the bidentate segment for M II. Spectroscopic (ESI-MS, UV/Vis/NIR, NMR), magnetic and electrochemical measurements show that 1) the head-tohead-to-head triple helical complexes (HHH)-[LnM(L 3) 3 ] 5 are quantitatively formed in solution only for ligand concentrations larger than 0.01m, 2) Fe II adopts a pure high-spin electronic configuration in (HHH)-[LnFe(L 3) 3 ] 5 , and 3) the Fe II/ Fe III oxidation process is prevented by steric constraints. Detailed photophysical studies of (HHH)-[Eu-Zn(L 3) 3 ] 5 confirm that the pseudotri-capped trigonal-prismatic lanthanide coordination site is not affected by the methyl groups bound to the terminal pyridine, thus leading to significant Eucentered emission upon UV irradiation. In (HHH)-[EuFe(L 3) 3 ] 5 , a resonant intramolecular Eu 3 Fe II hs energy transfer partially quenches the Eu-centered luminescence; however, the residual red emission demonstrates that high-spin iron(ii) is compatible with the sensitization of Eu III in heterodimetallic d ± f complexes. The influence of the electronic configuration of Fe II on the efficiency of Eu III 3 Fe II energy-transfer processes is discussed together with its consequence for the design of optically active spin-crossover supramolecular devices.

Monitoring Fe(II) Spin-State Equilibria via Eu(III) Luminescence in Molecular Complexes: Dream or Reality?

“…Multistep Synthesis of the Segmental Ligand L6 2, left and Table S4). 15,26 Monitoring the magnetic susceptibilities with the help of a SQUID magnetometer reveals, as previously reported for the perchlorate analogue [LaFe(L4) 3 ](ClO 4 ) 5 , 15 that the magnetic Fe(II) center in [LaFe(L4) 3 ](CF 3 SO 3 ) 5 adopts exclusively a high-spin configuration (S = 5/2) over the whole accessible temperature range (red trace in Figure 3). The Curie constant C hs = 3.97(1) cm 3 •K•mol −1 and paramagnetic independent paramagnetism TIP hs ≈ 0 cm 3 •mol −1 were fitted using eq 3 within the 150−400 K range and fell within the range for Fe II high-spin complexes (Table S5).…”

“…However, the relatively high energies of their emissive levels (17 277 cm −1 for Eu( 5 D 0 ) and 20 462 cm −1 for Tb( 5 D 4 ) in the aquo ions) 13 also make them good candidates for the alternative sensing mechanism, where they act as donors. In this case, their luminescence is quenched by energy transfers toward neighboring d-block acceptors in Eu-M and Tb-M molecular pairs where M = Cr(III), 14 Fe(II), 15 Fe(III), 12 Cu(II), 16 and Ru(II). 17 From a quantitative point of view, the probability W D,A that a resonant energy-transfer process occurs from a donor D (Eu or Tb) to an acceptor A (d-block cation) is given by Fermi's golden rule.…”

“…The characteristic lifetime τ obs = 1/k Eu relax = 2.43(1) ms measured at 293 K is coherent with the previously reported value of τ obs = 2.89(2) ms at 295 K for a 3.7 × 10 −3 M acetonitrile solution of the analogous [EuZn(L4) 3 ](ClO 4 ) 5 helicate. 15 As predicted by eq 14, the temperature dependence of the relaxation rate constant k Eu relax = 1/τ obs monitored in [EuZn(L5) 3 ] 5+ obeys an Arrhenius-type law with k Eu rad = 339(2) s −1 , k Eu nonrad,0 = 8(2) × 10 8 s −1 and E Eu nonrad = 39.4(6) kJ•mol −1 (Figure 5a) leading to almost quantitative intrinsic quantum yields at low temperature (eq 15 and Table S7).…”

Trinuclear Cobalt(II) and Zinc(II) Salamo‐type Complexes: Syntheses, Crystal Structures, and Fluorescent Properties

“…Transition metal complexes with Salen‐type ligands or its derivatives [ R –CH=N–(CH) n –N=CH– R ] have been extensively investigated as nonlinear optical materials,1–3 catalysts,4,5 magnetic materials,6–8 models of reaction centers of metalloenzymes, and biological models in understanding the structures of biomolecules and biological processes 9. The composition of asymmetric Salen‐type bisoxime ligands is rather unusual than that of symmetric Salen‐type bisoxime ligands.…”

Synthesis and Crystal Structure of Supramolecular Trinuclear Nickel(II) Complex with 5-Methoxy-4'-chloro-2,2'-[ethylenedioxybis(nitrilomethylidyne)]diphenol

“…Transition metal complexes having Salen-type ligands or its analogues are extensively investigated, because not only these series of complexes played an important role in the development of modern coordination chemistry 1 , but they can also be found at key points in the development of their unique catalytic activity 2 , buliding blocks for cyclic supramolecular structures 3 , interesting magnetic properties [4][5][6] , nonlinear optical materials 7,8 , thin films 9 and excellent biological activity 10,11 , such as anticancer and anticoagulant activity 12 . Although some advance has been made in the studies of Ni(II) complexes with Salen-type and Salamo-type ligands 13-16b , it still seems there could be new and specific applications for such unique complexes, which can introduce a variety of solvent effect.…”