Structural Evidence of Spin State Selection and Spin Crossover Behavior of Tripodal Schiff Base Complexes of tris(2-aminoethyl)amine and Related Tripodal Amines

Abstract

A review of the tripodal Schiff base (SB) complexes of tris(2-aminoethyl)amine, Nap(CH2CH 2NH2)3 (tren), and a few closely related tripodal amines with Cr(II), Mn(III) (d4), Mn(II), Fe(III) (d5), Fe(II) (d6), and Co(II) (d7) is provided. Attention is focused on examination of key structural features, the M-Nimine, M-Namine, or M-O and M-Nap bond distances and Nimine-M-N(O) bite and C-Nap-C angles and how these values correlate with spin state selection and spin crossover (SCO) behavior. A comparison of these experimental values with density functional theory calculated values is also given. The greatest number, 132, of complexes is observed with cationic mononuclear iron(II) in a N6 donor set, Fe(II)N6. The dominance of two spin states, high spin (HS) and low spin (LS), in these systems is indicated by the bimodal distribution of histogram plots of Fe(II)-Nimine and Fe(II)-Nazole/pyridine bond distances and of Nimine–Fe(II)-Nazole/pyridine and C-Nap-C bond angles. The values of the two maxima, corresponding to LS and HS states, in each of these histograms agree closely with the theoretical values. The iron(II)-Nimine and iron(II)-Nazole/pyridine bond distances correlate well for these complexes. Examples of SCO complexes of this type are tabulated and a few of the 20 examples are discussed that exhibit interesting features. There are only a few mononuclear iron(III) cationic complexes and one is SCO. In addition, a significant number of supramolecular complexes of these ligands that exhibit SCO, intervalence, and chiral recognition are discussed. A summary is made regarding the current state of this area of research and possible new avenues to explore based on analysis of the present data.