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]2·2MeCN 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⋯cation repulsions. In the solid state, the cobalt(II) complexes containing MeOtpy or EtOtpy ligands exhibit face-to-face ROtpy⋯tpyOR embraces supported by CHalkoxy⋯π interactions. These are the dominant packing interactions in [Co(MeOtpy)2][PF6]2 and [Co(EtOtpy)2][PF6]2·2MeCN. The transition from [Co(MeOtpy)2][PF6]2 to [Co(MeOtpy)2][PF6]2·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·MeCN and [Co(PrOtpy)2][PF6]2·MeCN·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·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·MeCN or [Co(PrOtpy)2][PF6]2·MeCN·Et2O to 2{[Co(PrOtpy)2][PF6]3}·5MeCN, oxidation of the metal tips the balance between tpy embraces and CHalkoxy⋯π interactions, and pairing of cations through the latter interactions is favoured. In [Co(EtOtpy)2]Br3·MeCN·H2O, the [Co(EtOtpy)2]3+ cations assemble into one-dimensional chains through CHalkoxy⋯π interactions.