Enhanced π-Back-Donation as a Way to Higher Coordination Numbers in d¹⁰ [M(NHC)_n] Complexes: A DFT Study

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• 作者：Dr. Jö ; rn Nitsch ; Dr. Lando P. Wolters ; Dr. Cé ; lia Fonseca Guerra ; Prof. Dr. F. Matthias Bickelhaupt and Dr. Andreas Steffen
• 刊名：Chemistry - A European Journal
• 出版年：2017
• 出版时间：January 12, 2017
• 年：2017
• 卷：23
• 期：3
• 页码：614-622
• 全文大小：1129K
• ISSN：1521-3765

文摘

We aim to understand the electronic factors determining the stability and coordination number of d¹⁰ transition-metal complexes bearing N-heterocyclic carbene (NHC) ligands, with a particular emphasis on higher coordinated species. In this DFT study on the formation and bonding of Group 9–12 d¹⁰ [M(NHC)_n] (n=1–4) complexes, we found that all metals form very stable [M(NHC)₂] complexes, but further coordination depends on the specific interplay of 1) the interaction energy (ΔE_int) between the [M(NHC)_n−1] (n=2–4) fragment and the incoming NHC ligand, and 2) the strain energy (ΔE_strain) associated with bending of the linear NHC-M-NHC arrangement. The key observation is that ΔE_strain, which is an antagonist for higher coordination numbers, can significantly be lowered by M→NHC π*-back-donation. This leads to favorable thermodynamics for n=3–4 for highly electrophilic metals in our study, and thus presents a general design motif to achieve coordination numbers beyond two. The scope of our findings extends beyond the NHC model systems and has wider implications for the synthesis of d¹⁰ [ML_n] complexes and their catalytic activity.