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

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.