Tridentate pincer ligands based on a deprotonated carbazole core with pendant, alkyl-substituted tetrazoles (Fig. 1a, CzTR) provide a sterically tunable and electron-rich donor array. Lanthanide complexes of CzTR show an exceptional variety of ligand coordination modes including (a) the expected planar, mer geometry, (b) a canted, pseudo-fac geometry, (c) a tetrazole-bridging mode and (d) a bidentate mode with one weakly π-bonded tetrazole ring. Only the planar, tridentate bonding mode has been observed for transition metal CzTR complexes (Fe, Co, Ni, Pd, Pt). The chemistry of iron with CzTR is especially rich. Air stable Fe(CzTR)2 complexes show unusually sharp, two-step spin-crossover behaviour in the solid state from variable temperature SQUID magnetometer and Mössbauer data. In contrast, the air sensitive mono CzTR complexes, Fe(CzTR)(OAr’)(L) (Ar’ = C6H2-2,6-t-Bu2-4-Me), readily lose the 5th donor (L = THF, t-BuNC, CO, NO, OPPh3) to form a highly reactive 4-coordinate complex, Fe(CzTR)(OAr’), that adopts a trigonal bipyramidal geometry missing one equatorial ligand (Fig. 1b). The reactivity of Fe(CzTR)(OAr’) uncovered so far will be presented.

Carbazole-bis(tetrazoles): Versatile ligands for lanthanide and late transition metal chemistry

ACS Spring 2022

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