Molecular Assemblies of Metal Complexes via Base-Pairing of Nucleic Acids in the Crystalline State

The nature of the molecular assemblies formed in the crystalline state by cobalt(II) terpyridine (terpy) complexes incorporating appended adenine (A) or thymine (T) bases was found to be controlled by which bases are present. Single-crystals of the cobalt(II) complexes [Co(A-C6-terpy) (2)](BF4)(2) (1) and [Co(T-C6-terpy)(2)](BF4)(2) (2) have needle and block habits, respectively. Subsequent mixing of 1 and 2 in MeOH resulted in isolation of [Co(A-C6-terpy)(1.5)(T-C6-terpy) 0.5](BF4)(2) (3) as plate-like crystals. A 3D network structure is present in 1 that incorporates 1D chains, whereas 2 adopts a 2D stacked structure constructed from ladder-type assemblies. For 3, dimer-rings consisting of [Co(A-C6terpy)(2)](2+) and [Co(A-C6-terpy)(T-C6-terpy)](2+) units are generated by means of base-pairing between A and T. Notably, 3 displays the first crystal structure of a heteroleptic cobalt(II) complex of [Co(A-C6-terpy)(T-C6-terpy)](BF4)(2). These assembly differences involving the terpyridine cobalt(II) complex units in 1-3 affect the cooperativities influencing their spin crossover (SCO) behavior. The influence of the terminal nucleobases on the resulting assembly has been probed by investigating the co-crystallization of [Co(terpy) 2](BF4)(2) (4) with [Co(C6-terpy)(2)](BF4)(2) (5) and 1 with 5.