Turning {M(tpy)2}n+ embraces and CH•••π interactions on and off in homoleptic cobalt(II) and cobalt(III) bis(2,2′:6′,2"-terpyridine) complexes

We present a structural investigation of thirteen homoleptic complexes containing {Co(tpy)(2)}(2+) or {Co(tpy)(2)}(3+) cores and tpy, Cltpy, MeOtpy, EtOtpy or PrOtpy ligands (Cltpy = 4'-chloro-2,2':6',2 ''-terpyridine, MeOtpy = 4'-methoxy-2,2':6',2 ''-terpyridine, EtOtpy = 4'-ethoxy-2,2':6',2 ''-terpyridine, PrOtpy = 4'-propoxy-2,2':6',2 ''-terpyridine). For the twelve hexafluoridophosphate salts, several trends in the ways in which the cations pack in the solid state emerge. In [Co(tpy)(2)][PF6](2)center dot 2MeCN and [Co(Cltpy)(2)][PF6](2), two-dimensional grids of cations assemble through tpy embraces. Oxidation of cobalt(II) to cobalt(III) results in a loss of the grid, consistent with the presence of an increased number of anions and the need to minimize cation center dot center dot center dot cation repulsions. In the solid state, the cobalt(II) complexes containing MeOtpy or EtOtpy ligands exhibit face-to-face ROtpy center dot center dot center dot tpyOR embraces supported by CHalkoxy center dot center dot center dot pi interactions. These are the dominant packing interactions in [Co(MeOtpy)(2)][PF6](2) and [Co(EtOtpy)(2)][PF6](2)center dot 2MeCN. The transition from [Co(MeOtpy)(2)][PF6](2) to [Co(MeOtpy)(2)][PF6](2)center dot MeCN causes the packing to revert to a two-dimensional grid with tpy embraces; the MeCN solvent molecules produce periodic dislocations in the grid-like array. On going from ethoxy to propoxy derivatives, we observe a reversion to assemblies dependent upon tpy embraces. One-dimensional chains are favoured in [Co(PrOtpy)(2)][PF6](2)center dot MeCN and [Co(PrOtpy)(2)][PF6](2)center dot MeCN center dot Et2O, with the degree of solvation influencing the packing motifs. Oxidation of each of [Co(ROtpy)(2)][PF6](2) (R - Me, Et, Pr) results in increased separation of the cations. The crystal packing in [Co(Xtpy)(2)][PF6](3)center dot MeCN for X Cl and MeO is virtually superimposable, and the [Co(Xtpy)(2)](3+) cation only has to undergo a slight twist within its lattice cavity to accommodate the Cl or MeO substituent. On going from [Co(PrOtpy)(2)][PF6](2)center dot MeCN or [Co(PrOtpy)(2)][PF6](2)center dot MeCN center dot Et2O to 2{[Co(PrOtpy)(2)][PF6](3)}center dot 5MeCN, oxidation of the metal tips the balance between tpy embraces and CHalkoxy center dot pi interactions, and pairing of cations through the latter interactions is favoured. In [Co(EtOtpy)(2)]Br-3 center dot MeCN center dot H2O, the [Co(EtOtpy)(2)](3+) cations assemble into one-dimensional chains through CHalkoxy center dot pi interactions.