Carbon-Doping Modulation of Dual Aromaticity and Magnetic Responses in Fe–Boron Heterocluster Anions

Abstract

Heteroatom doping is an effective strategy to tune the structural stability and physicochemical properties of clusters. To address the out-of-plane distortion of transition metal atoms induced by the relatively large cavities of pure boron ligands, we performed a systematic theoretical study on the structural evolution and stabilization mechanisms of the FeCxB(8–x)– (x = 1–4) cluster series. By combining global minimum searches via ABCluster with density functional theory (DFT) calculations at the M06-2X/def2-TZVP level, we identified the global minimum structures and systematically investigated their structural features, bonding nature, and magnetic responses as a function of carbon doping. The results indicate that the introduction of carbon atoms effectively shrinks the ligand ring size and enhances the system’s geometric rigidity, eventually converging to a highly symmetric (C4V) bowl-shaped half-sandwich geometry for FeC4B4–. Quantitative wave function analyses were performed using energy decomposition analysis (EDA-NOCV) and adaptive natural density partitioning (AdNDP). These analyses reveal that the high stability of FeC4B4– arises from cooperative stabilization, involving carbon-doping-induced geometric optimization of the Fe–ring framework, substantial electrostatic attraction, and significant orbital interactions associated with bidirectional donation/back-donation between the Fe 3d orbitals and the alternating B–C hybrid ring. Both the π and δ electron systems satisfy the 4n+2 Hückel rule, endowing the cluster with distinct π+δ dual aromaticity. Furthermore, magnetic response analyses (NICS, ICSS, and GIMIC) consistently demonstrate that the pronounced diatropic current within the cluster does not originate from a single orbital but is synergistically driven by a globally delocalized electronic system involving the metal center. This work elucidates the microscopic mechanisms underlying the stabilization of 3D heteroatomic coordination frameworks via carbon doping, providing a robust theoretical basis for the rational design of novel nanomaterials with tunable electron delocalization and tailored magnetic responses.