Cyclic Tetranuclear and Hexanuclear Palladium(II) Complexes and Their Host−guest Chemistry

Cyclic tetrameric complexes have been prepared by the reaction of Pd(en)Cl2 or Pd(dapol)Cl2, or their nitrato
analogues, with Na2(5‘GMP) in aqueous solution, where en = 1,2-diaminoethane, dapol = 1,3-diamino-2-propanol,
and 5‘GMP = guanosine 5‘-monophosphate. Addition of certain small molecules containing hydrophobic groups
resulted in the expansion of the tetramer to a cyclic hexamer with strong bonding of one guest per hexameric host.
At pH 5−6, the guest molecule can be a cation, anion, or neutral, and those species containing trimetylsilyl and
t-butyl groups bonded the most strongly. The size of the central cavity of the [Pd(en)(5‘GMP)]6 host has been
estimated to be 5.2 Å. Formation of the host−guest complex caused a large upfield shift (Δδ) of 2.5−2.9 ppm in
the 1H NMR spectrum of the most highly affected guest protons, which were those in closest proximity to the
guanine nucleobases. NOESY spectra were used to determine the interaction sites between the host and the
guest. Apparent association constants determined at 26 °C and pD 5.4 for the [Pd(en)(5‘GMP)]6−DSS and [Pd(en)(5‘GMP)]6−t-butanol systems, where DSS is 3-(trimethylsilyl)-1-propanesulfonate anion, were 1.36 ± 0.11 ×
104 and 2.74 ± 0.95 × 104 M-3/2, respectively. The Pd(dapol)−5‘GMP system forms hexameric host−guest complexes,
similar in nature to those of the Pd(en)−5‘GMP system. The molecular and crystal structures of Pd(dapol)Cl2 are
also reported.