Arnold Polly L, Kerr Ryan W F, Weetman Catherine, Docherty Scott R, Rieb Julia, Cruickshank Faye L, Wang Kai, Jandl Christian, McMullon Max W, Pöthig Alexander, Kühn Fritz E, Smith Andrew D
EaStCHEM School of Chemistry , University of Edinburgh , The King's Buildings , Edinburgh , EH9 3FJ , UK . Email:
EaStCHEM School of Chemistry , University of St. Andrews , North Haugh, St. Andrews , KY16 9ST , UK, E-mail:
Chem Sci. 2018 Sep 10;9(42):8035-8045. doi: 10.1039/c8sc03312a. eCollection 2018 Nov 14.
A series of rare earth complexes of the form Ln(L) supported by bidentate -aryloxide-NHC ligands are reported (L = 2-O-3,5-Bu-CH(1-C{N(CH)N(R)})); R = Pr, Bu, Mes; Ln = Ce, Sm, Eu). The cerium complexes cleanly and quantitatively insert carbon dioxide exclusively into all three cerium carbene bonds, forming Ce(L·CO). The insertion is reversible only for the mesityl-substituted complex Ce(L). Analysis of the capacity of Ce(L) to insert a range of heteroallenes that are isoelectronic with CO reveals the solvent and ligand size dependence of the selectivity. This is important because only the complexes capable of reversible CO-insertion are competent catalysts for catalytic conversions of CO. Preliminary studies show that only Ce(L·CO) catalyses the formation of propylene carbonate from propylene oxide under 1 atm of CO pressure. The mono-ligand complexes can be isolated from reactions using LiCe(NPr) as a starting material; LiBr adducts [Ce(L)(NPr)Br·LiBr(THF)] (R = Me, Pr) are reported, along with a hexanuclear N-heterocyclic dicarbene [LiCe(OArC-H)(NPr)(THF)] by-product. The analogous -aryloxide-NHC proligand (-L = 4-O-2,6-Bu-CH(1-C{N(CH)NMes}))) has been made for comparison, but the rare earth tris-ligand complexes Ln(-L)(THF) (Ln = Y, Ce) are too reactive for straightforward Lewis pair separated chemistry to be usefully carried out.
报道了一系列由双齿芳氧基-NHC配体支撑的Ln(L)型稀土配合物（L = 2-O-3,5- Bu - CH(1-C{N(CH)N(R)})); R = Pr、Bu、Mes; Ln = Ce、Sm、Eu）。铈配合物能干净且定量地将二氧化碳专一性地插入到所有三个铈卡宾键中，形成Ce(L·CO)。只有对甲苯基取代的配合物Ce(L)的插入反应是可逆的。对Ce(L)插入一系列与CO等电子的杂异烯能力的分析揭示了选择性对溶剂和配体大小的依赖性。这很重要，因为只有能够进行可逆CO插入的配合物才是CO催化转化的有效催化剂。初步研究表明，只有Ce(L·CO)在1个大气压的CO压力下催化环氧丙烷生成碳酸丙烯酯。使用LiCe(NPr)作为起始原料的反应可以分离出单配体配合物；报道了LiBr加合物[Ce(L)(NPr)Br·LiBr(THF)]（R = Me、Pr），以及六核N-杂环二卡宾[LiCe(OArC-H)(NPr)(THF)]副产物。制备了类似的芳氧基-NHC前体配体（-L = 4-O-2,6- Bu - CH(1-C{N(CH)NMes})))用于比较，但稀土三配体配合物Ln(-L)(THF)（Ln = Y、Ce）反应活性过高，无法有效地进行直接的路易斯对分离化学。
