Inspection of the Duality of a Verdazyl-Based Radical in Transition Metal Complexes: A π* Donor Ligand and a Magnetic Partner

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• 作者：Jean-Baptiste Rota ; Lucie Norel ; Cyrille Train ; Nadia Ben Amor ; Daniel Maynau ; Vincent Robert
• 刊名：Journal of the American Chemical Society
• 出版年：2008
• 出版时间：August 6, 2008
• 年：2008
• 卷：130
• 期：31
• 页码：10380-10385
• 全文大小：223K
• 年卷期：v.130,no.31(August 6, 2008)
• ISSN：1520-5126

文摘

The behavior of a verdazyl-based radical bound to open-shell transition metal ions in the structurally and magnetically characterized [M(hfac)₂imvd°] (M = Mn, Ni; hfac = (1,1,1,5,5,5)hexafluoroacetylacetonate; imvd° = 3-(2′-imidazolyl)-1,5-dimethyl-6-oxoverdazyl) complexes is rationalized using ab initio wave-function-based calculations analysis. The calculated exchange coupling constants J (H = −Js_M.s_imvd°; J_Mn^calcd = −63 cm⁻¹, J_Ni^calcd = 205 cm⁻¹) are in excellent agreement with the experimental ones (J_Mn^exp = −63 cm⁻¹, J_Ni^exp = 193 cm⁻¹). Even though both rings are involved through the binding mode of the imvd° radical, the spin density remains essentially localized on the nitrogen-rich ring. The singularity stems from its bidentate coordinating character. The analysis of the correlated wave function suggests that the verdazyl-based radical acts as a π* donor ligand which allows ligand-to-metal charge transfer and excludes metal-to-ligand charge transfer. This reflects the weak covalent character of the M-imvd° π coordination bond. From a magnetic point of view, the through-space exchange governs the ferromagnetic character in the Ni derivative up to 153 cm⁻¹ as expected from a description limited to the magnetic orbitals. Nevertheless, the CI expansion displays the participation of excited doublet and quartet states (spin polarization) on the verdazyl moiety which leads to a significant additional ferromagnetic contribution (52 cm⁻¹). In the [Mn(hfac)₂imvd°] analogue, the antiferromagnetic contribution arising from kinetic exchange is only one-third of the observed exchange coupling constant. It is necessary to introduce dynamical correlation effects to quantitatively recover the exchange interaction in this compound. Since the π* donor and spin-polarized characters of the verdazyl moiety dominate over the negligible polarizability of the imidazole part, it is concluded that the noninnocent nature of the imvd° radical is held by the verdazyl ring part.