S=3/2 <=> S=1/2 Spin Crossover Behavior in Five-Coordinate Halido-and Pseudohalido-bis(omicron-iminobenzosemiquinonato)iron(lll)Complexes

摘要

Five-coordinate halido-and pseudohalido-bis(omicron-iminobenzosemiquinonato)iron(lll)complexes[Fe~IIIX(I~(ISQ))_2](X=Cl~-(1),Br~-(2a,2b),I~-(3),N_3~-(4),and NCS~-(5))have been synthesized where(I~(ISQ))~(1 centre dot-)represents the pi radical anion N-phenyl-omicron-imino(4,6-di-tert-butyl)benzosemiquinonate(1-).The molecular structures of the two polymorphs 2a and 2b have been determined at 100,220,and 295 K,respectively,by single crystal X-ray crystallography.Variable temperature magnetic susceptibility data reveal the following electronic ground states,S_t:For 1,it is 3/2.Polymorph 2a contains a 1:1 mixture of 3/2 and 1/2 forms in the range 4.2 to ~150 K;above 150 K the latter form undergoes a spin crossover 1/2 - 3/2.Polymorph 2b contains only the S_t=3/2 form(4-300 K).Complex 3 contains the S_t=1/2 form in the range 4-130 K,but above 130 K,a spin crossover to the 3/2 form is observed which is confirmed by three crystal structure determinations at 100,220,and 295 K.Complex 4 possesses an S_t=1/2 ground state at 80 K and undergoes a spin crossover at higher temperatures.Complex 5 has a temperature-independent S_t=3/2 ground state.All crystal structures of 1,2a,2b,3,4,and 5,regardless at which temperature the data sets have been measured,show that two omicron-iminobenzosemiquinonate(l-)pi radical anions are N,0-coordinated in all of these neutral iron complexes.The Fe-N and Fe-0 bond distances are longer in the S_t=3/2 and shorter in the S_t=1/2 forms.The S_t=3/2 ground state is attained via intramolecular antiferromagnetic coupling between a high spin ferric ion(S_(Fe)=5/2)and two ligand n radicals whereas the St=1/2 form is generated from exchange coupling between an intermediate spin ferric ion(S_(Fe)=3/2)and two ligand radicals.