Unusual electronic effects of electron-withdrawing sulfonamide groups in optically and magnetically active...

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 the selective introduction of an electron-withdrawing sulfonamide group into the ligand backbone and its subsequent hydrolysis to the hydrophilic sulfonate group. Compared to that of the methylated analogue L1, the affinity of the bidentate binding unit of L3 for H+ and for trivalent lanthanide ions (LnIII) in [Ln(L3)3]3+ and [Ln2(L3)3]6+ is reduced because the electron-withdrawing sulfonamide substituent weakens sigma-bonding, but improved retro-pi-bonding between the bidentate binding units of L3 and soft 3d-block ions (M(II) = FeII, ZnII) overcomes this effect and leads to homometallic complexes [Mn(L(i))m]2n+ (i = 1, 3) displaying similar stabilities. Theoretical ab initio calculations associate this dual effect with a global decrease in energy of pi and sigma orbitals when the sulfonamide group replaces the methyl group, with an extra stabilization for the LUMO (pi). The reaction of L3 with a mixture of LnIII and M(II) (M = Fe, Ni, Zn) in acetonitrile gives the noncovalent podates [LnM(L3)3]5+ in which LnIII is nine-coordinated by the three wrapped tridentate segments, while the bidentate binding units provide a facial pseudooctahedral site around M(II). The X-ray structure of [EuZn(L3)3](ClO4)4(PF6)(CH3NO2)3(H2O) reveals that the bulky sulfonamide group at the 5-position of the pyridine ring only slightly increases the Zn-N bond distances as a result of sigma/pi compensation effects. The introduction of spectroscopically and magnetically active FeII and NiII into the pseudooctahedral site allows the detailed investigation of the electronic structure of the bidentate segment. Absorption spectra, combined with electrochemical data, experimentally demonstrate the dual effect associated with the attachment of the sulfonamide group (decrease of the sigma-donating ability of the pyridine lone pair and increase of the pi-accepting properties of the coordinated bidentate binding unit). The influences on the ligand 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.