1,10-phenanthroline, also known as o-phenanthroline, is widely used as a chelating agent of choice for transition metals. Mono-, bis-, and tris- chelates are used as catalysts for water splitting and organic synthesis, hybrid materials, electron-transfer reagents, magnetic materials, DNA intercalation agents, and therapeutic materials among a host of other applications. While 1,10-phenanthroline is ubiquitous in its chelating abilities, 4,7-phenanthroline (m-phenanthroline) is often used as a bridging agent with transition metals, leading to complex supramolecular architectures, especially with Cu(I). Much less studied is 1,7-phenanthroline (abbreviated 1,7-phen), the para isomer. One of the reasons is undoubtedly due to the distal nature of nitrogen placement. Although one might anticipate that 1,7-phenanthroline would form bridging compounds in a fashion analogous to the 4,7 isomer, there is a decided lack of ability of 1,7-phenanthroline to bridge. In attempting to synthesize Cu(I) bridging 1,7-phenanthroline complexes for use as polymeric catalysts for atom-transfer radical reactions (ATRA), we have instead synthesized the first example of a three-coordinate Cu(I) complex containing 1,7-phenanthroline: Cu(1,7-phen)(PPH3)2PF6. It is a three-coordinate Cu(I) compound with the hexafluorophosphate anion in the coordination sphere. A three-coordinate geometry is somewhat unusual for Cu(I) complexes, which prefer tetrahedral or pseudo-tetrahedral geometries. Although it might be expected that the ligand 1,7-phenanthroline (also known as p-phenanthroline) will form bridges between Cu(I) centers to complete the tetrahedral arrangement, in fact once the nitrogen at position 7 on the heterocycle binds to the copper, the remaining nitrogen at position 1 is rendered inert. Several nitrogen heterocyclic ligands containing multiple nitrogen atoms display this property. We are referring to ligand of this sort as “inhibited” ligands. The structural and photophysical properties of the compound are described here, as well as DFT studies that confirm what has been observed experimentally regarding the inhibition of donation by the second nitrogen after the first nitrogen has been coordinated.

Synthesis, photophysical properties, and DFT studies of Cu(1,7-phen)(PPH3)2PF6: An unusual three-coordinate Cu(I) compound and the concept of “inhibited” ligands

255th National Meeting (2018)

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