MONOFLUORINATED, DIFLUORINATED, AND TETRAFLUORINATED CYCLAMS

A series of new mono-, di-, and tetrafluorinated dioxocyclams (2-4) and cyclams (6-8) has been synthesized and their complexing behaviors were investigated. The effects of the successive F substitutions were demonstrated by the drop in the amine basicities and the ligand field (LF) strengths in their Cu(II) and Ni(II) complexes. These are ascribable, though difficult to distinguish, to the strong electron-withdrawing effect and lipophilic effect characteristic of the F atom. Despite the weaker LF, the fluorinated dioxocyclams 2-4 generally form more stable Cu(II) complexes [CuII(H-2L)]0 19-21 than the nonfiuorinated 1. In their kinetics in acetate buffers (4.7 < pH < 5.7), the fastest overall complexation was observed with the tetrafluorinated 4, where the contribution of the unprotonated ligand species is most determining. The Ni(II) complex with the tetrafluorinated cyclam 8 is in almost 100% high-spin state (blue, λmax 517 nm, ϵ 7) in 0.1 M NaClO4 aqueous solution at 25 °C, while with nonfiuorinated 5 it is in a mixture of high-and low-spin states. The X-ray crystal structure of the NiII-8·Cl2 has been resolved by the heavy-atom method using 1550 unique reflections with I > 2σ(1). Final R was 0.056: monoclinic space group P21/n with a = 14.218 (8) Å, b = 8.524 (5) Å, c = 6.538 (5) Å, β = 98.46 (5)°, and V= 783.7 Å3; dcalcd = 1.703 g cm-3 for Z = 2, and mol wt 401.9. The purple 8 complex has a 6-coordinate octahedral structure with square-planar nitrogens and two axial chlorides. The bonding parameters are not appreciably different from those of the 5 complex. From the cyclic voltammetry of the series of CII(H-2L) (18-21) and NiIIL (22-25) in aqueous solution, as the number of F increases, both the oxidation processes for CuII ⥦ CuIII and NiII ⥦ NiIII and the reduction process for NiII ⥦ NiI show anodic shifts compared to the nonfiuorinated analogues. To explore useful applications, the Ni(II)-fiuorinated cyclam complexes 23-25 were tested as electrorcduction catalysts for CO2 ⟶ CO in H2O. The efficiency of (CO + H2) products yield and the CO/H2 selectivity depend upon the number of fluorine atoms. The tetrafluorinated complex 25 works more efficiently and selectively at a lower energy of -1.10 V vs SCE than the nonfiuorinated cyclam complex 22 does. © 1990, American Chemical Society. All rights reserved.