2,2':6',2''-terpyridine substituted in the 4'-position by the solubilizing and sterically demanding tert-butyl group : a surprisingly new ligand

We describe the synthesis and characterization of 4′-tert-butyl-2,2′:6′,2′′-terpyridine (4′-tButpy, 1), a convergent tpy ligand that exhibits both a sterically demanding and solubilizing 4′-substituent. In the solid state, molecules of 1 pack with alternating tpy and tert-butyl domains, and the bulky alkyl substituents prevent the molecules from engaging in the face-to-face π-interactions which are typical of simple tpy ligands. Instead, the predominant packing forces involve CH⋯N hydrogen bonds and weak CH⋯π contacts. The syntheses of the homoleptic complexes [M(1)2][PF6]2 (M = Fe, Co, Zn and Ru) and the heteroleptic [Ru(tpy)(1)][PF6]2 are described. The complexes have been fully characterized in solution, including the 1H NMR spectroscopic characterization of the paramagnetic [Co(1)2][PF6]2. Cyclic voltammetric data are consistent with the tert-butyl substituent being slightly electron releasing. The single crystal structures of [Zn(1)2][PF6]2 and [Ru(1)2][PF6]2 have been determined; the compounds are essentially isomorphous. The packing of the cations is such that the tert-butyl substituents are accommodated in pockets between the tpy domains of adjacent cations, and as a consequence, the {M(tpy)2}-embrace that is a ubiquitous feature of many related structures is not observed.