Non-monotonic Structure-Property Relationships in Small Iron-Doped Boron Clusters: A DFT Study of B n Fe (n = 3-6)

Small iron-doped boron clusters are useful model systems for examining how structure, excited-state behavior, and chemical reactivity evolve at low nuclearity. Here, the neutral B_nFe series (n = 3-6) was studied to determine whether increasing boron content yields a simple size-dependent progression or instead leads to distinct mechanistic regimes. The results reveal a clearly non-monotonic evolution, with B₄Fe emerging as the most mechanistically transparent molecular-activation prototype, B₃Fe defining a dissociative activation limit, and B₆Fe representing the global softness/electrophilicity limit.

The B_nFe clusters were investigated using an integrated density functional theory workflow combining ground-state optimization, harmonic vibrational analysis, selected-state TDDFT calculations, natural transition orbital analysis, global conceptual-DFT descriptors, condensed Fukui functions, and a prototype H₂-activation test. The optimized PBE0/def2-TZVP structures confirmed stable minima and showed a non-monotonic structural trend: B₃Fe is compact and planar, B₄Fe is non-planar but relatively rigid, B₅Fe is the most distorted and vibrationally soft member, and B₆Fe regains partial compactness while remaining non-planar. Optical analysis showed that B₄Fe and B₅Fe absorb at similar low energies, but B₄Fe is much brighter and more coherent. Global descriptors identified B₆Fe as the softest and most electrophilic cluster, whereas local Fukui analysis showed that B₄Fe provides the clearest Fe/B reactivity differentiation. In the H₂-interaction analysis, B₃Fe yielded a dissociative or near-dissociative product, whereas B₄Fe-B₆Fe retained molecular H₂ adducts, with B₄Fe showing the strongest geometric activation of the H-H bond.