To get mechanistic insights, we carried out control experiments (Scheme 2). When iodonaphthalene (1h) was subject to the standard conditions in the absence of alkyl fluorides, the protonation product 4 was obtained in 50% yield after being quenched with 1.0 M aq. HCl, indicating the formation of aryl calcium species from 1h under mechanochemical conditions (Scheme 4a). In contrast, the reaction with alkyl fluoride 2a in the absence of 1h did not afford the protonation product 5, and low conversion of 2a was observed (5% conversion, Scheme 4a). Based on the results of pronation studies, we propose that this C(sp 3 )-F bond arylation most likely proceeds through the selective formation of aryl calcium nucleophiles by direct insertion of calcium metal into the C(sp 2 )-I bond, followed by their nucleophilic substitution of alkyl fluorides. To further support our hypothesis, we prepared an aryl calcium reagent using pre-activated Rieke calcium under solution-based conditions and subjected to the nucleophilic substitution (Scheme 4b). We found that the prepared aryl calcium reagent reacted with 1-fluorododecane (2a) to deliver the desired product 3ha in 21% yield (Scheme 4b), which supports our proposed mechanism. Furthermore, gain an insight into the mechanism of the nucleophilic substitution step, density functional theory (DFT) calculations were performed using iodobenzene (1a) and nbutyl fluoride as the model substrates. To reduce the calculation costs, we used THF as a ligand for calcium species instead of THP, which the optimal ligand. Preliminary calculations strongly suggest the mononuclear complex is highly unstable due to the sites for coordination on the calcium atom. Meanwhile, the artificial force-induced reaction (AFIR)-based sampling calculation focusing on the tetrameric model provided 3976 possible structures, of which the most stable one shows a highly symmetric cubic structure (Figure ). The coordination vacancies of each calcium atom in this cubic structure have been fulfilled, and the coordination number is six. Moreover, it was later confirmed by our DFT calculations that the tetramer is more stable than the dimer (Figure ). Therefore, it is suggested that the tetramer is the most plausible form of the structure with the composition formula of (PhCaI)(THF) 2 in the reaction system. Having confirmed the possible structure of the aryl calcium nucleophile, we then focused on rationalizing the detailed mechanism of the defluoarylation reaction. Shown in Figure are the three possible reaction pathways starting from the tetrameric species Ca-Tetra. To generate a vacant site for the coordination of alkyl fluoride, one of the THF ligands should leave at the first instance. The free energy is thus raised by 73.8 kJ•mol -1 for this step. The alkyl fluoride substrate then binds to the five-coordinate calcium atom via its fluorine atom, and the free energy is subsequently lowered by 53.1 kJ•mol -1 . The formation of Int2 is then followed by three possible reaction pathways, of which one is the arylation (TSA), one is the alpha-H abstraction reaction (TSB), and the other is the beta-H abstraction reaction (TSC). Among these three possible pathways, the arylation reaction has the lowest barrier, and the overall free energy barrier is found as +99.5 kJ•mol -1 . The calculation results are consistent with our experimental observations. In this transition state, the initial binding of the fluorine atom to the calcium atom significantly weakens the C(sp 3 )-F bond as calcium(II) is considered as a moderately strong Lewis acid for fluorine atom. At the transition state, the phenyl anion from another calcium center attacks the alphacarbon of the alkyl fluoride directly, while the abovementioned calcium atom takes the fluorine atom as a leaving group. The combination of these two effects realizes the cleavage of a chemically inert C(sp 3 )-F bond and leads to the formation of a new C(sp 3 )-C(sp 2 ) bond. The visualized geometries of intermediates and transition states shown in Figure are incorporated into the ESI.