STRUCTURAL AND KINETIC-STUDIES OF SPIN-CROSSOVER IN AN IRON(II) COMPLEX WITH A NOVEL TRIPODAL LIGAND

Configurational and ligand conformational influences on the kinetics o f (1)A(1) reversible arrow T-5(2) spin crossover in the Fe(II) complex with the novel tripodal ligand, s((N-(2-pyridylmethyl)-N-methylamino) methyl)ethane (tptMetame), have been explored. Despite having six chel ate rings and three chiral nitrogen atoms, only one enantiomeric pair of isomers, Delta, SSS, and Lambda, RRR, of the complex ion is observe d. The conformation of the three rings forming the upper ''cap'' of th e complex structure can be assigned delta or lambda with respect to th e 3-fold molecular axis. X-ray data at 300 and 153 K, above and below the critical temperature for the spin transition, show that the confor mation of the ligand ''cap'' is the same as the absolute configuration of the complex, with the same Lambda lambda(CAP)(or Delta delta(CAP)) combination prevailing for both the LS ((1)A(1)) and HS (T-5(2)) isom ers. Molecular mechanics calculations further show that the ligand ene rgy remains lowest for this Lambda lambda(CAP) (or Delta delta(CAP)) c ombination at all Fe-N distances over the range spanning the LS and HS isomers. Measurements of the spin crossover relaxation time have been carried out in solution over the temperature range 293-170 K. The obs erved monophasic relaxation traces are also consistent with the absolu te configuration of the complex remaining unaltered during the spin cr ossover.