Stereochemical Analysis of Palladium(II) Complexes of the Selenium Coronands 1,5,9,13-Tetraselenacyclohexadecane and 1,5,9,13,17,21-Hexaselenacyclotetracosane

Abstract

Crystal structures for [Pd(16Se4)][BF4]2 (1) (16Se4 = 1,5,9,13-tetraselenacyclohexadecane) and [Pd(16Se4)][BF4]Cl (2) both contain the [Pd(16Se4)]2+ ion which has square-planar coordination about Pd and a stereochemistry (denoted c,c,c) in which all of the selenium nonbonding valence electron pairs are mutually cis with respect to the coordination plane. In both structures the conformation adopted by the cation deviates slightly but significantly from molecular mm (C2v) symmetry. The asymmetry is most clearly resolved for 2, in which the Pd−Se bond lengths vary significantly (2.444(1)−2.463(1) Å). For 1: T = 295 K; fw = 764.2; space group Pnma; Z = 4; a = 12.356(1) Å; b = 10.397(1) Å; c = 16.233(2) Å; V = 2193.7 Å3; RF = 0.036 for 997 data (Io ≥ 2.5σ(Io)) and 131 variables. For 2: T = 295 K; fw = 712.8; space group P1̄; Z = 2; a = 7.2377(4) Å; b = 10.7784(7) Å; c = 12.9523(9) Å; α = 76.607(6)°; β = 86.020(6)°; γ = 85.186(5)°; V = 978.17 Å3; RF = 0.023 for 2134 data (Io ≥ 2.5σ(Io)) and 214 variables. The 77Se, 1H, 13C, and 1H−13C correlated NMR spectra for 1 in D2O solution were fully assigned. At room temperature, [Pd(16Se4)]2+ occurs in D2O solution predominantly as the c,t,c and c,c,c stereoisomers. Variable temperature NMR spectroscopic results demonstrate that at an elevated temperature (389 K) configurational interconversion is rapid on the 1H-NMR time scale. Cyclic voltammetry of 1 and 2, in MeCN solution with Pt electrodes, shows that the redox reaction of the pair Pd(II)/Pd(I) is irreversible with cathodic peak potentials at −0.56 and −0.47 V vs SCE, respectively. The crystal structure of [(PdCl)2(24Se6)][BF4]2·2CH2Cl2 (3·2CH2Cl2) reveals a complex dication in which two individual palladium(II) centers are each bound by three adjacent selenium atoms. Both palladium centers display square planar coordination with a chlorine atom occupying the fourth site. The complex exhibits trans relationships between adjacent pyramidal selenium centers and adopts a conformation which has only approximate m (Cs) symmetry. The mutually trans Pd−Se bond lengths range from 2.421(1) to 2.428(1) Å. The Pd−Se bond lengths trans to the chlorine atom are 2.372(1) and 2.364(1) Å, and the corresponding Pd−Cl bond lengths are 2.343(2) and 2.351(3) Å. For 3·2CH2Cl2: T = 195 K; monoclinic, space group P21/c; Z = 4; a = 16.421(2) Å; b = 19.353(2) Å; c = 12.397(3) Å; β = 91.16(2)°; V = 3938.9 Å2; RF = 0.037 for 3793 data (Io ≥ 2.5σ(Io)) and 409 variables. Cyclic voltammetry of 3 in MeCN solution with Pt electrodes shows an irreversible cathodic peak potential at −0.58 V vs SCE corresponding to a Pd(II)/Pd(I) pair. Electrolysis in MeCN confirms that this is a two-electron reduction for the complex.