Inorganic Chemistry, Vol.51, No.3, 1450-1461, 2012
Redox Non-innocence of Thioether Crowns: Elucidation of the Electronic Structure of the Mononuclear Pd(III) Complexes [Pd([9]aneS(3))(2)](3+) and [Pd([18]aneS(6))](3+)
The Pd(II) complexes [Pd([9]aneS(3))(2)](PF6)(2)center dot 2MeCN (1) ([9]aneS(3) = 1,4,7-trithiacyclononane) and [Pd([18]aneS(6))](PF6)(2) (2) ([18]aneS6 = 1,4,7,10,13,16-hexathiacyclooctadecane) can be oxidized electrochemically or chemically oxidized with 70% HClO4 to [Pd([9]aneS(3))(2)](3+) and [Pd([18]aneS(6))](3+), respectively. These centers have been characterized by single crystal X-ray diffraction, and by UV/vis and multifrequency electron paramagnetic resonance (EPR) spectroscopies. The single crystal X-ray structures of [Pd-III([9]aneS(3))(2)]-(ClO4)(6)center dot(H3O)(3)center dot(H2O)(4) (3) at 150 K and [Pd([18]aneS(6))]-(ClO4)(6)center dot(H5O2)(3) (4) at 90 K reveal distorted octahedral geometries with Pd S distances of 2.3695(8), 2.3692(8), 2.5356(9) and 2.3490(6), 2.3454(5), 2.5474(6) angstrom, respectively, consistent with Jahn-Teller distortion at a low-spin d(7) Pd(III) center. The Pd(II) compound [Pd([9)aneS(3))(2)}(PF6)(2) shows a one-electron oxidation process in MeCN (0.2 M NBu4PF6, 293 K) at E-1/2 = +0.57 V vs. Fc(+)/Fc assigned to a formal Pd(III)/Pd(II) couple. Multifrequency (Q-, X-, S-, and L-band) EPR spectroscopic analysis of [Pd([9]aneS(3))(2))(3+) and [Pd([18]aneS(6))](3+) gives g(iso) = 2.024, vertical bar A(iso(pd))vertical bar = 18.9x 10(-4) cm(-1); g(xx) = 2.046, g(yy) = 2.041, g(zz) = 2.004; vertical bar A(xx(pd))vertical bar = 24 X 10(-4) cm(-1), vertical bar A(yy(Pd))vertical bar = 22 x 10(-4) cm(-1), IA(zz(Pd))vertical bar = 14 x 10(-4) cm(-1), vertical bar a(xx(H))vertical bar = 4 X 10(-4) cm(-1), vertical bar a(yy(H))vertical bar = 5 X 10(-4) cm(-1), vertical bar a(zz(H))vertical bar= 5.5 X 10(-4) cm(-1) for [Pd([9]aneS(3))(2)](3+), and g(iso) = 2.015, broken vertical bar A(iso(pd))vertical bar = 18.8 x 10(-4) cm(-1); g(xx) = 2.048 g(yy) = 2.036, g(zz) = 1.998; vertical bar a(xx(H))vertical bar = 5, la(yy(H))vertical bar = 5, vertical bar a(zz(H))vertical bar = 6 x 10(-4) cm(-1); = vertical bar A(xx(Pd))vertical bar = 23 x 10(-4) cm(-1), vertical bar A(yy(pd))vertical bar = 22 x 10(-4) cm(-1), vertical bar A(zz(pd))vertical bar = 4 X 10(-4) cm(-1) for [Pd([18]aneS(6))](3+). Both [Pd([9]aneS(3))(2)](3+) and [Pd( [18]aneS(6))(3+) exhibit five-line superhyperfme splitting in the g(zz) region in their frozen solution EPR spectra. Double resonance spectroscopic measurements, supported by density functional theory (DFT) calculations, permit assignment of this superhyperfine to through-bond coupling involving four H-1 centers of the macrocyclic ring. Analysis of the spin Hamiltonian parameters for the singly occupied molecular orbital (SOMO) in these complexes gives about 20. 4% and 25% Pd character in [Pd([9]aneS(3))(2)](3+) and [Pd([18]aneS(6))](3+), respectively, consistent ith the compositions calculated from scalar relativistic DFT calculations.