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 of (1)A(1) right harpoon over left harpoon (5)T(2) spin crossover in the Fe(II) complex with the novel tripodal ligand, 1,1,1-tris((N-(2-pyridylmethyl)-N-methylamino)methyl)ethane (tptMetame), have been explored. Despite having six chelate rings and three chiral nitrogen atoms, only one enantiomeric pair of isomers, Delta, SSS, and Lambda, RRR, of the complex ion is observed. The conformation of the three rings forming the upper "cap" of the complex structure can be assigned delta or lambda with respect to the 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 conformation of the ligand "cap" is the same as the absolute configuration of the complex, with the same Lambdalambda(CAP)(or Deltadelta(CAP)) combination prevailing for both the LS ((1)A(1)) and HS ((5)T(2)) isomers. Molecular mechanics calculations further show that the ligand energy remains lowest for this Lambdalambda(CAP) (or Deltadelta(CAP)) combination 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 observed monophasic relaxation traces are also consistent with the absolute configuration of the complex remaining unaltered during the spin crossover.