Control of iron(II) spin states in 2,2 ': 6 ',2 ''-terpyridine complexes through ligand substitution

A series of 6- and 6,6 "-arylsubstituted 2,2':6'.2 "-terpyridine ligands an d their iron(II) complexes have been prepared. The introduction of phenyl s ubstituents at both the 6- and 6 "-positions leads exclusively to the forma tion of orange high-spin iron(Ii) complexes whilst the presence of a single 6-phenyl substituent results in spin-crossover systems. The complexes [Fe( 2)(2)]X-2 (2 = 4,6-diphenyl-2,2':6',2 "-terpyridine; X = ClO4 or PF6) have been studied in detail, and the solid-state X-ray structures of both the lo w- and high-spin forms are reported. Mossbauer spectroscopic and magnetic s usceptibility measurements are reported, and the temperature and pressure d ependence of the high-spin/low-spin transition have been studied. An X-ray structural study of the complex [Fe(2)(2)](ClO4)(2) is also reported; this complex is highly distorted with two very long Fe N contacts of over 2.4 An gstrom and is best regarded as a four-coordinate iron complex.