Homeomorphic Isomerization as a Design Element in Container Molecules; Binding, Displacement, and Selective Transport of MCl2 Species (M = Pt, Pd, Ni).
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The dibridgehead diphosphine ((CH2)14)3 P (1) can rapidly turn inside-out (homeomorphic isomerization) to give a mixture of in,in and out,out isomers. The exo directed lone pairs in the latter are able to scavenge Lewis acidic MCl2; cagelike adducts of the in,in isomer, trans- Cl2(P((CH2)14)3 P) (M = 2/Pt, 3/Pd, 4/Ni), then form. The NiCl2 unit in 4 may be replaced by PtCl2 or PdCl2, but 2 and 3 do not give similar substitutions. U-tubes are charged with CH2Cl2 solutions of 1 (lower phase), an aqueous solution of K2MCl4 (charging arm; M = Pt, Pd), and an aqueous solution of excess KCl (receiving arm). The MCl2 units are then transported to the receiving arm until equilibrium is reached (up to 22 d). When the receiving arm is charged with KCN, transport is much faster (ca. 100 h) and higher K2MX4 equilibrium ratios are obtained (964). Analogous experiments with K2PtCl4/K2PdCl4 mixtures show PdCl2 transport to be more rapid. A similar diphosphine with longer methylene chains, P((CH2)18)3P, is equally effective. No transport occurs in the absence of 1, and other diphosphines or monophosphines assayed give only trace levels.