DIG3tren (DIG = diisopropylguanidyl) is a four-coordinate ligand readily synthesized from diisopropylcarbodiimide and tren. It acts as a tripodal ligand with divalent late first row transition metal ions, as exemplified by its cobalt(II) complexes. Synthesis of various [Co(DIG3tren)X]Y (X = F, Cl, Br, N3, SCN; Y = X or BPh4) complexes has exploited salt metathesis reactions starting from previously published [Co(DIG3tren)OAc]OAc and [Co(DIG3tren)](BPh4)2 complexes. While the [Co(DIG3tren)X]X were obtained by “brute force” (i.e. using multiple cycles and large excesses) salt metatheses with of the acetate complex with NaX or NaX salts, a more efficient synthesis of [Co(DIG3tren)X]Y from [Co(DIG3tren)](BPh4)2 and (Bu4N)X exploits the relative insolubility of (Bu4N)(BPh4) in, for instance, isopropanol solvent. X-ray crystallographic studies reveal how the conformation of the DIG3tren ligand responds to the presence of various X auxiliary ligands. In the absence of additional Lewis bases (i.e. in [Co(DIG3tren)](BPh4)2) the Co2+ is trigonal monopyramidal coordinated with a non-coordinating hemisphere covered completely by three isopropyl groups – one from each DIG arm. In the presence of pseudohalides (X = N3–, SCN–), the isopropyl groups spread apart slightly to allow monodentate coordination of a nitrogen atom of X–. Halide binding results in a large change in the guanidine N-C-N-H dihedral angles; this moves the isopropyl groups away and allows formation of two or three intracomplex NH…X hydrogen bonds. UV-vis spectrophotometric titrations and ESI MS have been used to compare the strength of different halide and pseudohalide anion binding with the [CoDIG3tren]2+ unit; fluoride binds more strongly than other halides, and binds stronger than pseudohalides in non-protic solvents, but weaker than pseudohalides in polar protic solvents such as methanol.

Syntheses, structural and stability comparisons of [Co(DIG3tren)X]Y, (X = halide or pseudohalide; Y = X or BPh4)

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.