| Abstract
| - The segmental ligand 2-(6-(N,N-diethylcarbamoyl)pyridin-2-yl)-1,1‘-dimethyl-2‘-(5-(N,N-diethylsulfonamido)pyridin-2-yl)-5,5‘-methylenebis[1H-benzimidazole] (L3) is synthesized via a multistep strategy that allows theselective introduction of an electron-withdrawing sulfonamide group into the ligand backbone and its subsequenthydrolysis to the hydrophilic sulfonate group. Compared to that of the methylated analogue L1, the affinity of thebidentate binding unit of L3 for H+ and for trivalent lanthanide ions (LnIII) in [Ln(L3)3]3+ and [Ln2(L3)3]6+ isreduced because the electron-withdrawing sulfonamide substituent weakens σ-bonding, but improved retro-π-bonding between the bidentate binding units of L3 and soft 3d-block ions (MII = FeII, ZnII) overcomes this effectand leads to homometallic complexes [Mn(Li)m]2n+ (i = 1, 3) displaying similar stabilities. Theoretical ab initiocalculations associate this dual effect with a global decrease in energy of π and σ orbitals when the sulfonamidegroup replaces the methyl group, with an extra stabilization for the LUMO (π). The reaction of L3 with a mixtureof LnIII and MII (M = Fe, Ni, Zn) in acetonitrile gives the noncovalent podates [LnM(L3)3]5+ in which LnIII isnine-coordinated by the three wrapped tridentate segments, while the bidentate binding units provide a facialpseudooctahedral site around MII. The X-ray structure of [EuZn(L3)3](ClO4)4(PF6)(CH3NO2)3(H2O) reveals thatthe bulky sulfonamide group at the 5-position of the pyridine ring only slightly increases the Zn−N bond distancesas a result of σ/π compensation effects. The introduction of spectroscopically and magnetically active FeII andNiII into the pseudooctahedral site allows the detailed investigation of the electronic structure of the bidentatesegment. Absorption spectra, combined with electrochemical data, experimentally demonstrate the dual effectassociated with the attachment of the sulfonamide group (decrease of the σ-donating ability of the pyridine lonepair and increase of the π-accepting properties of the coordinated bidentate binding unit). The influences on theligand field strength and on tunable room-temperature FeII spin-crossover processes occurring in [LnFe(L3)3]5+are discussed, together with the origin of the entropic control of the critical temperature in these thermal switches.
- Electron-withdrawing sulfonamide groups attached to the bidentate segment of the multidentate ligand L3 display dual σ/π effects which have been rationalized at the ab initio level. The resulting electronic structures control the binding of Fe(II) in the pseudooctahedral sites of the noncovalent podates [LnFe(L3)3]5+, leading to a fine and predictable tuning of spin-crossover and electrochemical properties.
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