XAFS Study of Gadolinium and Samarium Bisporphyrinate Complexes

The comparative X-ray absorption spectroscopy study of gadolinium and samarium bisporphyrinate complexes represented by the formulas GdIIIH(oep)(tpp), GdIII(oep)2, GdIIIH(tpp)2 and SmIIIH(oep)(tpp), SmIII(oep)2, SmIIIH(tpp)2 is reported. The XAFS spectra are recorded on the LURE-DCI storage ring (Orsay, France) in transmission mode on the microcrystalline samples at the Gd and Sm L3 edges. The local environment for Ln3+ ions has been reconstructed applying one-shell and two-shell XAFS analysis procedures. The protonated and nonprotonated bisporphyrinate complexes present different XAFS features. After our analysis on the title derivatives, the gadolinium ion (at 80 K) is found to be bonded:  (i) to eight nitrogen atoms at R(Gd−N) 2.50 Å, for GdIII(oep)2 [Debye−Waller (DW) factor 0.004 Å2]; (ii) to seven nitrogen atoms at R(Gd−N) 2.49 Å, for GdIIIH(oep)(tpp) [DW factor 0.005 Å2] and one nitrogen at long distance; and (iii) to six nitrogen atoms at R(Gd−N) 2.50 Å [DW factor 0.006 Å2] and two nitrogen atoms at long distance for GdIIIH(tpp)2. A similar coordination sphere has been detected for the corresponding Sm derivatives. So, the samarium ion (at room temperature) is bonded:  (i) to eight nitrogen atoms at R(Sm−N) 2.53 Å, for SmIII(oep)2 [DW factor 0.006 Å2]; (ii) to seven nitrogen atoms at R(Sm−N) 2.53 Å, for SmIIIH(oep)(tpp) [DW factor 0.006 Å2] and one nitrogen at long distance; and (iii) to six nitrogen atoms at R(Sm−N) 2.50 Å, for SmIIIH(tpp)2 [DW factor 0.006 Å2] and two nitrogen atoms at long distance. As far as concerns LnIII(oep)2 complexes, the increase of Ln−N distance in the series Gd3+ < Eu3+ < Sm3+ reflects an increase in the ionic radii, which are in good agreement with previously published XRD data on EuIII(oep)2. Moreover, the protonated LnIIIH(oep)(tpp) and LnIIIH(tpp)2 complexes possess systematically shorter distances of about 0.02 Å between the XAFS and XRD data. This difference is attributed to the asymmetry of the distribution concerning Ln−N distances.