Metallathiacrown Ethers: Synthesis and Characterization of Transition-Metal Complexes Containing α,ω-Bis(phosphite)-Polythioether Ligands and an Evaluation of Their Soft Metal Binding Capabilities

The
metallathiacrown ethers cis-Mo­(CO)4{2,2′-(C12H8O2)­PO­(CH2CH2S)nCH2CH2OP­(2,2′-(O2H8C12))} (n = 2, 3)
and cis-Mo­(CO)4{2,2′-(C12H8O2)­POCH2CH2S-1-(C6H4)-2-SCH2CH2OP­(2,2′-(O2H8C12))} have been prepared as soft
metal selective molecular receptors.
Multinuclear NMR spectroscopy and X-ray crystallography have been
used to show that byproducts formed during the syntheses of the metallathiacrown
ethers cis-Mo­(CO)4{2,2′-(C12H8O2)­PO­(CH2CH2S)nCH2CH2OP­(2,2′-(O2H8C12))} (n = 2, 3)
are homobinuclear complexes with cis-Mo­(CO)4(P-donor)­(S-donor) centers. The abilities of the metallathiacrown
ethers to bind PdCl2 and PtCl2 have been assessed
using 31P­{1H} NMR spectroscopy and X-ray crystallography.
The complexes showed null results with PdCl2; however,
the PtCl2 experiments showed that the complexes cis-Mo­(CO)4{2,2′-(C12H8O2)­PO­(CH2CH2S)nCH2CH2OP­(2,2′-(O2H8C12))} (n = 2, 3)
formed heterobinuclear cis,cis-{[Mo­(CO)4{2,2′-(C12H8O2)­PO­(CH2CH2S)nCH2CH2OP­(2,2′-(O2H8C12))}]­PtCl2} (n = 2, 3) complexes. The 31P­{1H} NMR spectra of these complexes suggest a cis-PtS2Cl2 coordination environment
in each, which leads to asymmetric binding in the latter complex.
Binding of HgCl2 by the complexes cis-Mo­(CO)4{2,2′-(C12H8O2)­PO­(CH2CH2S)nCH2CH2OP­(2,2′-(O2H8C12))} (n = 2, 3) has been studied using 31P­{1H} NMR spectroscopy. Each complex was titrated with
HgCl2, and the quantitative shifts in the 31P­{1H} NMR spectra were fit to a binding mechanism. The n = 2 metallathiacrown ether binds HgCl2 to form
a 1:1 complex with Ka = 12.0(0.2) M–1. In contrast, interaction of HgCl2 with
the n = 3 metallathiacrown ether results in isomerization
to the trans complex, and HgCl2 binds to both isomers.
A model has been adapted to fit the titration data of this complex
and to extract equilibrium constants for each step in the cycle: the
cis–trans equilibrium of the free, Kfree (0.570 (0.004)), and bound, Kbound (0.16
(0.03)), metallathiacrown ethers, as well as the association binding
constants for the cis, Kcis ([4.1(0.3)]
× 102 M–1) and trans, Ktrans ([1.1(0.2)] × 102 M–1), metallathiacrown ethers. The equilibrium constants demonstrate
that the cis metallathiacrown ether binds more strongly to the HgCl2 than does the trans metallathiacrown ether and that the cis–trans
equilibrium favors the cis metallathiacrown ether.