Lanthanide-assisted self-assembly of an inert, metal-containing nonadentate tripodal receptor

An inert nine-coordinate metal-contg. tripodal receptor is selectively formed by means of a self-assembly process with Ln(III) ions as template, and the subsequent modification of the resulting helicate is discussed. Thus, bis(pyridylbenzimidazolyl)methane deriv. I (L2) was reacted with Co(ClO4)2 and Ln(ClO4)3 to give HHH-[LnCo(L2)3]5+ (Ln = La, Eu, Lu) which adopt the expected triple-helix structure in a C3-sym. head-to-head arrangement of the three strands. The Co(II) occupies the facial pseudooctahedral site produced by the three wrapped bidentate binding units, and Ln(III) the remaining nine-coordinate sites. Reaction of Co(ClO4)2 with L2 in the absence of Ln(III) affords [Co(L2)3]2+ which exists in soln. as a 4:1 mixt. of mer- and fac-isomers, resp. Oxidn. of HHH-[LnCo(L2)3]5+ (Ln = La, Lu) with Br2 in MeCN gives quant. the poorly sol. HHH-[LnCoIII(L2)3]6+. The structure of [LaCoIII(L2)3 (ClO4)5.5(OH)0.5.cntdot.4MeCN.cntdot.2H2O was detd. by single-crystal x-ray crystallog. Addn. of D2O to HHH-[LnCoIII(L2)3]6+ (Ln = La, Lu) in CD3CN results in decomplexation of Ln(III), which can be monitored by electrospray MS and 1H NMR spectroscopy. Treatment of HHH-[LnCoIII(L2)3]6+ with a stoichiometric amt. of (Bu4N)4EDTA forms solvated inert metal-contg. tripodal receptor fac-[Co(L2)3](ClO4)3