“Swollen” Macrocycles: Palladium(II)-Directed Template Syntheses of Pendant-Arm 14-, 16-, and 18-Membered Macrocycles

The bis(diamine)palladium(II) cations (diamine =
ethane-1,2-diamine, propane-1,3-diamine, or
butane-1,4-diamine)
all undergo condensation reactions with formaldehyde and nitroethane to
produce macromonocycles where each
pair of cis-disposed primary amines has been converted to a
−NH−CH2−C(CH3)(NO2)−CH2−NH−
strap. The
14-membered-ring macrocycle has been previously prepared by
condensation around copper(II) and nickel(II),
but this does not permit synthesis of the larger ring macrocycles.
The macrocyclic complex (6,13-dimethyl-6,
13-dinitro-1,4,8,11-tetraazacyclotetradecane)palladium(II)
perchlorate crystallizes in the triclinic space group
P1̄,
a = 8.105(3) Å, b = 8.370(2) Å,
c = 9.437(4) Å, α = 69.04(3)°, β =
68.60(3), γ = 71.53(3)°. Complexes of
the 16- and 18-membered macrocycles
(3,11-dimethyl-3,11-dinitro-1,5,9,13-tetraazacyclohexadecane)palladium(II) perchlorate and
(3,12-dimethyl-3,12-dinitro-1,5,10,14-tetraaazacyclooctadecane)palladium(II)
perchlorate
crystallize in the monoclinic space group
P21/c, with a =
8.391(2) Å, b = 12.816(3) Å, c =
23.925(9) Å, and β
= 93.18(2)°, and the triclinic space group
P1̄, with a = 7.746(5) Å,
b = 9.912(5) Å, c = 18.96(2) Å,
α =
91.76(6)°, β = 101.73(7)°, and γ =
112.83(5)° respectively. The larger macrocycles are
“swollen” by incorporating
longer methylene chains, “swelling” leading to an increase in
Pd−N distance and in tetrahedral distortion, with
the dominant geometric isomer apparently changing with macrocycle size
from anti-disposed nitro pendants (14-membered) to the syn isomer (16-, 18-membered). An
irreversible PdII/IV oxidation occurs at ca +1 V (vs
Ag/AgCl), varying slightly with ring size, with a multi-electron nitro
group reduction observed near −0.8 V in each
case. Electronic spectra also vary slightly with ring
size.