Modifying the Chemistry of a Macrocyclic Cobalt Complex by Remote Site Manipulation

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A macrocyclic tetraamido ligand 5,6:(3-pyrido)-12,12-diethyl-2,2,9,9-tetramethyl-3,8,11,13-tetraoxo-1,4,7,10-tetraazacyclotridecane, H 4 1, has been designed to incorporate Co II and Co III in the primary coordination site and has a secondary bidentate site, comprising pyridine- N and amido- O donor atoms, that coordinates Lewis acids. The complex PPh 4 [2] was obtained by deprotonation of H 4 1 with lithium diisopropylamide in the presence of CoCl 2, followed by air oxidation and precipitation from water by addition of PPh 4 Cl. The X-ray crystal structure of PPh 4 [2] was obtained. The compound crystallizes in the space group P1̄ (No. 2) with unit cell dimensions a = 12.936(4) Å, b = 13.253(5) Å, c = 13.288(2) Å, α = 93.75(2)°, β = 90.91(2)°, γ = 91.53(3)°, V = 2272.1(1.2) Å 3, and Z = 2. Lewis acids can be bound either reversibly as in the case of alkali and alkaline earth cations or irreversibly as found for [(bpy) 2 Ru] 2+ (bpy is 2,2‘-bipyridine) and [(Me 2 bpy) 2 Ru] 2+ (Me 2 bpy is 4,4‘-(CH 3 ) 2 -2,2‘-bipyridine). The Lewis acid in the secondary coordination site broadly modifies the chemistry at Co III and Co II in [2] - and [2] 2-, respectively. The axial binding affinity of the Co III center for 1-CH 3 -imidazole was 1 to 2 orders of magnitude larger when divalent Lewis acids were in the secondary coordination site than that for [2] - . The reactivities of [2] 2-, Ca 2+ /[2] 2-, and [(bpy) 2 Ru [2] ] toward O 2 were significantly different. The reaction of [2] 2- with O 2 rapidly generated [2] -, apparently by an outer-sphere electron-transfer pathway. In contrast, the oxidation of Ca 2+ /[2] 2- by O 2 was a complex reaction in which some of the Ca 2+ was consumed and the final species depended on the amount of Ca 2+ present at the start of the reaction. Finally, in the case of [(bpy) 2 Ru [2] ], it appeared as though the dinuclear complex [(bpy) 2 Ru [2] ] 2 (μ-O 2 ) formed upon reaction with O 2 .