Cobalt Complexes with “Click”-Derived Functional Tripodal Ligands: Spin Crossover and Coordination Ambivalence

We demonstrate the use of a Cu(I) catalyzed “Click” reaction in the synthesis of novel ligands for spin crossover complexes. The reaction between azides and alkynes was used to synthesize the reported tripodal ligand tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine, TBTA, and the new ligands tris[(1-cyclohexyl-1H-1,2,3-triazol-4-yl)methyl]amine, TCTA, and tris[(1-n-butyl-1H-1,2,3-triazol-4-yl)methyl]amine, TBuTA. Reactions of TBTA with Co(ClO4)2 lead to complexes of the form [Co(TBTA)(CH3CN)3](ClO4)2, 1, and [Co(TBTA)2](ClO4)2, 2, where complex formation can be controlled by the metal/ligand ratio and the complexes 1 and 2 can be chemically and reversibly switched from one form to another in solution resulting in coordination ambivalence. The benzyl substituents of TBTA in 2 show intramolecular C–H---π T-stacking that generates a chemical pressure to stabilize the low spin (LS) state at lower temperatures. The structural parameters of 2 are consistent with a Jahn–Teller active LS Co(II) (elongation) ion showing four short and two long bonds. 2 shows spin-crossover (SCO) behavior in the solid state and in solution with a high T0 close to room temperature which is driven by the T-stacking. 1 remains high spin (HS) between 2 and 400 K. Reversible chemical switching is observed between 1 and 2 at room temperature, with an accompanying change in the spin state from HS to LS. The importance of the intramolecular T-stacking in driving the SCO behavior is proven by comparison with two analogous compounds that lack an aromatic substituent and remain HS down to very low temperatures.