Iron-Catalyzed Activation of Carbon–Halogen Bonds

We have studied the cross-coupling reaction of C(spn)–X bonds (n = 1–3; X = F, Cl, Br, I) mediated by the model iron-d8 catalyst Fe(CO)4 with the archetypal model substrates H3C–CH2–X, H2CCH–X, and HCC–X, utilizing relativistic density functional theory at ZORA-OPBE/TZ2P. The barrier of the oxidative-addition step decreases as the C(spn)–X bond varies from X = F to Cl to Br to I and from C(sp3) to C(sp2) to C(sp). Activation strain and energy decomposition analyses uncover that the lowering of the reaction barrier from X = F to I is caused by (i) a weaker C(spn)–X bond that needs to be broken, (ii) enhanced HOMO–LUMO interactions, and (iii) a stronger electrostatic attraction between the catalyst and the substrate due to the more diffuse electron density and higher nuclear charge of the X atom when varying from X = F to I.

本研究针对由模型d⁸铁催化剂Fe(CO)₄介导的C(spⁿ)-X键（n=1~3；X=F、Cl、Br、I）交叉偶联反应展开了系统探究，反应底物选用典型模型底物H₃C-CH₂-X、H₂C=CH-X及HC≡C-X，并采用ZORA-OPBE/TZ2P水平下的相对论密度泛函理论开展计算。氧化加成步骤的反应能垒，会随着C(spⁿ)-X键的X从F依次变为Cl、Br、I，以及C原子的杂化态从sp³转变为sp²再到sp而逐渐降低。活化应变与能量分解分析结果表明，X从F到I时反应能垒降低的原因主要有三点：其一，需断裂的C(spⁿ)-X键键能更弱；其二，最高占据分子轨道-最低未占据分子轨道（HOMO-LUMO）相互作用得到增强；其三，随着X从F到I，其电子云愈发弥散且核电荷更高，使得催化剂与底物间的静电吸引作用更强。