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Dalton Transactions Accepted Manuscript Dalton Transactions Accepted Manuscript This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/dalton Page 1 of 13 PleaseDalton do not Transactions adjust margins Dalton Transactions PERSPECTIVE Rare-Earth Metal πππ-Complexes of Reduced Arenes, Alkenes, and Alkynes: Bonding, Electronic Structure, and Comparison with Received 00th January 20xx, Accepted 00th January 20xx Actinides and Other Electropositive Metals a,b a,c DOI: 10.1039/x0xx00000x Wenliang Huang, and Paula L. Diaconescu www.rsc.org/ Rare-earth metal complexes of reduced π ligands are reviewed with an emphasis on their electronic structure and bonding interactions. This perspective discusses reduced carbocyclic and acyclic π ligands; in certain categories, when no example of a rare-earth metal complex is available, a closely related actinide analogue is discussed. In general, rare-earth metals have a lower tendency to form covalent interactions with π ligands compared to actinides, mainly uranium. Despite predominant ionic interactions in rare-earth chemistry, covalent bonds can be formed with reduced carbocyclic ligands, Manuscript especially multiply reduced arenes. electropositive metals, such as alkali, alkali earth, or early Introduction transition metals. The development of the organometallic chemistry of f The present perspective includes not only reduced six- elements has paralleled that of transition metal organometallic membered rings but also three-, four-, seven-, and eight- chemistry with early milestones including the isolation of membered carbon rings, alkenes, and alkynes (Chart 1a). For 1-3 reduced carbocyclic rings, negatively charged arenes, except mono-, bis-, or tris-cyclopentadienyl metal complexes and - the famous uranocene molecule. 4, 5 In the last decade, a for the cyclopentadienyl anion (Cp ) or its fused analogues renaissance featuring compounds containing metal-element such as indenyl, are surveyed. Neutral π ligands are also Accepted multiple bonds, metal arene complexes, molecular complexes omitted. Albeit the assignment of the negative charge on with the metal in a low oxidation state, and the display of an π ligands is somewhat arbitrary, we will use the following unprecedented reactivity for small molecule activation has qualitative criterion: the formal charge of the π ligands in a been observed. In addition to a showcase of beautiful complex is calculated taken into account that rare-earth synthetic methods, theoretical studies have contributed to the metals have a +3 oxidation state, except for ytterbium and europium, which are usually more stable in the +2 oxidation understanding of the electronic structure of these new 6 state in their organometallic chemistry. For instance, molecules. New concepts, such as inverse trans influence and * 2 2 * 5 18 7 (Cp Sm) ( : -(E)-stilbene) (Cp = -C Me ) qualifies to sterically induced reduction have been introduced. Recently, 2 2 µ−η η η 5 5 be included in our discussion, while Sm(AlCl ) (C Me )19 does following a long debate against the preponderance of ionic 4 3 6 6 not. Another important criterion is that the metal ion should interactions, covalent bonding has been accepted in actinide 8-10 coordinate to the face of the ligand, i.e., metallocycles of chemistry, especially for uranium, featuring the π π butadiene will not be included. Typical coordination modes contribution of both 5f and 6d orbitals. Several excellent include side-on, inverse sandwich, and sandwich (Chart 1b). reviews and articles have covered some of the latest 11-17 This perspective focuses on rare-earth metal (scandium, Transactions advancements in the field. Herein, we present a critical yttrium, and lanthanides) complexes; however, when such review of rare-earth metal complexes supported by reduced complexes are absent and structurally similar complexes of carbocyclic and acyclic π ligands, including cyclopropenyl, actinides are available and relevant, they will be included in cyclobutadienyl, arenes, cycloheptatrienyl, cyclooctatetraenyl, the corresponding section. In order to discuss electronic fused arenes, alkenes, dienes, and alkynes. Besides briefly structures comparatively, we only selected compounds that summarizing examples from the literature, the main purpose have been characterized by X-ray crystallography. However, of this perspective is to highlight the bonding and electronic the purpose of this perspective is not to summarize all existing Dalton structure of these rare-earth metal π complexes; in addition, literature examples, but rather to extract useful information we will discuss the similarities and differences in bonding and from isolated examples in order to discuss the bonding and electronic structure between rare-earth metal complexes and electronic structure of rare-earth metal complexes. those of closely related actinides and, in some cases, other The review is organized as follows: first, carbocyclic π ligands except arenes will be discussed in the order of their ring size from the smallest, three-membered ring, to the This journal is © The Royal Society of Chemistry 20xx Dalton Trans ., 2015, 00 , 1-11| 1 Please do not adjust margins PleaseDalton do not Transactions adjust margins Page 2 of 13 ARTICLE Journal Name largest, eight-membered ring; then, arenes and aryl [2+2] cycloaddition: first, two equivalents of diphenylacetylene substituted arene complexes of rare-earth metals will be are reductively coupled to form a metallocycle of 1,3- surveyed; after that, fused arenes, alkenes, and alkynes will be butadieneyl, which then undergoes ring closure to form the included. cyclobutadienyl dianion (Scheme 1). The ability of uranium to induce this ring closure echoes that of transition metals. Indeed, some transition metals, such as cobalt, can readily promote this transformation under microwave conditions to 23 generate various C 4R4 rings in high yield. Xy Xy Xy Xy Ph Me 2Si N N SiMe 2 Me 2Si N N SiMe 2 Ph Me 4 PhCCPh Me 2 U U Me 2 2 U Me 2 HC Si CH HC Si U CH Si 43 d, 20% Si Ph N N N N Me 2Si N N SiMe 2 Me 2Si N N SiMe 2 Xy Xy Xy Ph Xy Xy Xy Xy Xy +2 PhCCPh Xy Xy Ph Ring closure -toluene Me 2Si N Ph N SiMe 2 Chart 1. (a) π Ligands discussed in this perspective; (b) Typical Me Me HC 2 U U 2 CH coordination modes for metals and π ligands: side-on, inverse Si Si N Ph N sandwich, and sandwich. Me 2Si N Ph N SiMe 2 Xy Xy Xy Xy Manuscript Part 1. Survey of Rare-Earth Metal πππ Scheme 1. Formation of an inverse sandwich cyclobutadienyl Xy 5 5 Complexes dianion complex of uranium, [U(Ts )] 2(µ−η :η -C4Ph 4), and proposed reaction intermediate. 1. Carbocylic π ligands with a ring size of three, four, seven, and eight 1.1 C 3 rings The simplest example of a reduced three-membered ring is - the cyclopropenyl anion, C 3H3 , which is considered to be either antiaromatic 20 or nonaromatic. 21 The substituted cyclopropenyl, such as triphenylcyclopropenyl and Accepted tributylcyclopropenyl, forms complexes with late transition + - metals by reacting C 3R3 X (X = halogen) and metal carbonyls. However, in those complexes, the cyclopropenyl is best described as an aromatic cation and not as an anion. No cyclopropenyl lanthanide complexes have been reported, likely due to two reasons: first, Ln(III) ions are highly Lewis acidic and lack the necessary π basicity to stabilize an interaction with the cyclopropenyl cation; second, although Ln(III) ions could stabilize the negative charge of the cyclopropenyl anion by an Xy 5 5 electrostatic interaction, its antiaromaticity or nonaromaticity Figure 1. Solid state molecular structure of [U(Ts )] 2(µ−η :η - makes its isolation elusive. To date, there is no example of a C4Ph 4) with hydrogen atoms omitted for clarity. Adapted by cyclopropenyl anion metal complex reported. permission from Macmillan Publishers Ltd: Nature 22 Communications, copyright (2013). 1.2 C 4 rings Transactions Xy 5 5 Cyclobutadiene has been long sought after by synthetic The solid state molecular structure of [U(Ts )] 2(µ−η :η - chemists. Despite the fact that free cyclobutadiene C 4H4 has C4Ph 4) (Figure 1) showed an asymmetric coordination pattern not been isolated, the cyclobutadienyl dianion can be with a relatively wide range of U-Cring distances of 2.655(5)- stabilized by metal complexes and it fulfills the Hückel rule for 2.871(5) Å and a U-Cipso distance of 2.864(5) Å. The electronic Xy 5 5 aromaticity. Although lanthanide dianionic cyclobutadienyl structure of [U(Ts )] 2(µ−η :η -C4Ph 4) has been studied by complexes are still unknown, a uranium analogue has recently density functional theory (DFT) calculations. The calculated been synthesized by reacting the inverse sandwich uranium charges on uranium (+3.08) and on the C 4 ring (-2.36) are Dalton Xy 6 6 Xy toluene complex [U(Ts )] 2(µ−η :η -C7H8) (Ts = consistent with the formulation of two uranium(IV) ions and a HC(SiMe 2NAr) 3; Ar = 3,5-Me 2C6H3) with excess dianion of C 4Ph 4.
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