Linkage Isomerism in a Homoleptic Fe(II) Complex with BODIPY-1H-Tetrazole Ligands

Abstract

We examined the coordination behavior of the sterically demanding 4,4-difluoro-1,3,5,7-tetramethyl-8-[(1H-tetrazol-1-yl)methyl]-4-bora-3a,4a-diaza-s-indacene ligand L in octahedral Fe(II) coordination compounds. Using four different solvents – ClCH₂CN, BrCH₂CN, CH₃OH, and (CH₃)₂CO – five unique crystal structures were obtained and characterized by single-crystal X-ray diffraction. In ClCH₂CN and BrCH₂CN, L coordinated equatorially through N4 and apically through its less basic N3 atom, yielding a pseudo-homoleptic [Fe(L^N4)₄(L^N3)₂]²⁺ architecture. This represents the first documented example of monodentate 1H-tetrazole N3/N4 linkage isomerism in an Fe(II) complex. Conversely, O-donor solvents (CH₃OH and (CH₃)₂CO) resulted in the formation of a heteroleptic [Fe(L)₄(solvent)₂]²⁺ motif, in which the solvent molecules coordinated through their O-atoms and occupied two coordination sites. The CH₃OH adduct underwent a topotactic single-crystal-to-single-crystal transformation upon air exposure, exchanging CH₃OH for H₂O, and was stabilized by extensive hydrogen bonding. Unlike classical [Fe(1-alkyl-1H-tetrazole)₆]²⁺ systems, none of the investigated coordination compounds exhibited SCO properties, instead remained locked in a single spin state. These findings demonstrate that steric bulk can override the ligand’s N4 coordination preference to Fe(II), either by enforcing N3 coordination or by permitting coordination of co-ligands, and thus provide guidance for the design of 1H-tetrazole-based Fe(II) materials.