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

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  Edder,C., Piguet,C., Bernardinelli,G., Mareda,J., Bochet,C.G.,   Bunzli,J.C.G., Hopfgartner,G. Inorganic Chemistry  2000 39 (22) 5059-5073

  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] (L-3) 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 L-1, the affinity of the bidentate binding unit of L-3   for H+ and for trivalent lanthanide ions (Ln(III) in   [Ln(L-3)(3)](3+) and [Ln(2)(L-3)(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 = Fe-II, Zn-II)   overcomes this effect and leads to homometallic complexes   [M-n(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 (Jc). The reaction of L3 With a   mixture of Ln(III) and M-II (M = Fe, Ni, Zn) in acetonitrile   gives the noncovalent podates [LnM(L-3)(3)](5+) in which Ln(III)   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(L-3)(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 Fen and Ni-II 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 a-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 Fe-II   spin-crossover processes occurring in [LnFe(L-3)(3)](5+) are   discussed, together with the origin of the entropic control of   the critical temperature in these thermal switches

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