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

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 FeC _x B_(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 (C_4V) 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.