Unusual electronic effects of electron-withdrawing sulfonamide groups in optically and magnetically active self-assembled noncovalent heterodimetallic d-f podates

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-benzimi dazole] (L-3) is synthesized via a multistep strategy that allows the selec tive introduction of an electron-withdrawing sulfonamide group into the lig and backbone and its subsequent hydrolysis to the hydrophilic sulfonate gro up. Compared to that of the methylated analogue L-1, the affinity of the bi dentate binding unit of L-3 for H+ and for trivalent lanthanide ions (Ln(II I) in [Ln(L-3)(3)](3+) and [Ln(2)(L-3)(3)](6+) is reduced because the elect ron-withdrawing sulfonamide substituent weakens sigma -bonding,but improved retro-pi -bonding between the bidentate binding units of L3 and soft 3d-bl ock ions (M-II = Fe-II, Zn-II) overcomes this effect and leads to homometal lic complexes [M-n(L-i)(m)](2n+) (i = 1, 3) displaying similar stabilities. Theoretical ab initio calculations associate this dual effect with a globa l decrease in energy of pi and sigma orbitals when the sulfonamide group re places the methyl group, with an extra stabilization for the LUMO (Jc). The reaction of L3 With a mixture of Ln(III) and M-II (M = Fe, Ni, Zn) in acet onitrile gives the noncovalent podates [LnM(L-3)(3)](5+) in which Ln(III) i s nine-coordinated by the three wrapped tridentate segments, while the bide ntate binding units provide a facial pseudooctahedral site around M-II. The X-ray structure of [EuZn(L-3)(3)](ClO4)(4)(PF6) (CH3NO2)(3)(H2O) reveals t hat 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 compens ation effects. The introduction of spectroscopically and magnetically activ e Fen and Ni-II into the pseudooctahedral site allows the detailed investig ation of the electronic structure of the bidentate segment. Absorption spec tra, combined with electrochemical data, experimentally demonstrate the dua l effect associated with the attachment of the sulfonamide group (decrease of the a-donating ability of the pyridine lone pair and increase of the pi -accepting properties of the coordinated bidentate binding unit), The influ ences on the ligand field strength and on tunable room-temperature Fe-II sp in-crossover processes occurring in [LnFe(L-3)(3)](5+) are discussed, toget her with the origin of the entropic control of the critical temperature in these thermal switches.