Templated Formation of Binuclear Macrocycles via Hemilabile Ligands

A new approach to synthesizing binuclear metallomacrocycles is reported. This approach utilizes flexible bisphosphine alkyl−aryl ether hemilabile ligands which complex Rh(I) to form “condensed” macrocycles held together by a series of both strong and weak links of the following type: [(κ2:μ2:κ2-(1,4-(Ph2PCH2CH2O)2-2,3,5,6-((CH3)4C6)))2Rh2][BF4]2 (5a) or [(μ2,η1:η6:η1-(1,4-(Ph2P(CH2)nO)2-X))2Rh2][BF4]2 (5b, n = 2, X = 2,3,5,6-((CH3)4C6); 6, n = 3, X = 2,3,5,6-((CH3)4C6); 7, n = 2, X = C6H4; 8, n = 2, X = C6H4−C6H4). Introduction of ligands that will bind to Rh(I) more strongly than either the Rh−η6-aryl or Rh−η1-ether weak links of 5a−8 results in the breaking of the weak links to form a series of 26−34-membered macrocyclic ring structures such as [(μ2-(1,4-(Ph2P(CH2)nO)2X))2(L1)m(L2)oRh2][BF4]2 (9, n = 2, X = 2,3,5,6-((CH3)4C6)), L1 = CO, m = 6, L2 = none; 10, n = 2, X = C6H4, L1 = CO, m = 6, L2 = none; 11, n = 2, X= C6H4−C6H4; L1 = CO, m = 6, L2 = none; 12, n = 2, X = 2,3,5,6-((CH3)4C6)), L1 = CO, m = 2, L2 = CH3CN, o = 2; 13, n = 3, X = 2,3,5,6-((CH3)4C6)), L1 = CO, m = 2, L2 = CH3CN, o = 2; 14, n = 2, X = C6H4, L1 = CO, m = 2, L2 = CH3CN, o = 2; 15, n = 2, X = C6H4−C6H4, L1 = CO, m = 2, L2 = CH3CN, o = 2). These homobimetallic macrocycles can be used to sequester bifunctional aromatic molecules to form host−guest structures such as [(μ2-(1,4-(Ph2PCH2CH2O)2-2,3,5,6-((CH3)4C6)))2(CO)2(μ2-1,4-(−NC)2C6H4)Rh2][BF4]2 (16) and [(μ2-(1,4-(Ph2PCH2CH2O)2-2,3,5,6-((CH3)4C6)))2(CH3CN)2(μ2-1,4-(−CN)2C6H4)Rh2][BF4]2 (17). The synthetic methods described within this article represent a powerful new way of making binuclear macrocycles from flexible ligands in nearly quantitative yields. Moreover, the macrocycles are highly tailorable with respect to cavity size and hydrophobicity and the metals' steric and electronic environments. Solid-state structures as determined by single-crystal X-ray diffraction studies are presented for compounds 5a, 6, 7, 9, 10, 12, and 17.