Tutorial pubs.acs.org/Organometallics
Formation of High-Oxidation-State Metal−Carbon Double Bonds Richard R. Schrock*,† and Christophe Copéret‡ †
Department of Chemistry 6-331, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1-5, CH-8093 Zürich, Switzerland
‡
ABSTRACT: This tutorial explores the major pathways of forming metal−carbon double bonds in high-oxidation-state alkylidene complexes that began with the alkylidene chemistry of tantalum complexes in the 1970s and continued with the organometallic chemistry of Mo, W, and Re and the development of homogeneous catalysts for the metathesis of olefins. It also explores recent findings in surface organometallic chemistry and discusses the link between molecularly defined and heterogeneous catalysts. Recent results suggest that heterogeneous olefin metathesis catalysts that are activated toward metathesis upon exposure to olefins produce a d0 alkylidene through formation of a metallacyclopentane ring at d2 metal sites followed by “a ring contraction” to a metallacyclobutane, a reaction that was first observed in tantalum chemistry.
1. INTRODUCTION The heart of organotransition-metal chemistry lies in molecular species that have a metal−alkyl bond. But carbon, like nitrogen or oxygen, can also be bound to a transition metal through a double bond (MCRR′, an alkylidene or carbene) or a triple bond (MCR, an alkylidyne or carbyne), especially when the metal is in groups 5−7, where multiple bonds to nitrogen (imido or nitride) or oxygen (oxo) are also relatively common. An “alkylidene” or “alkylidyne” ligand is viewed as a dianion or trianion, respectively, when the transition metal thereby attains a d0 configuration. Alkylidene and alkylidyne complexes are required intermediates in catalytic alkene metathesis (by Mo,1 W,1 and Re1 or Ru2) or alkyne metathesis (by Mo or W),3 respectively. The mechanisms of alkene and alkyne metathesis reactions are similar in that a four-coordinate alkylidene or alkylidyne complex reacts with an alkene or alkyne, respectively, to give a trigonal-bipyramidal (TBP) intermediate in which the metallacyclobutane ring (as in olefin metathesis shown in eq 1)
bonds) are formed when the metal is Ta, Mo, W, or Re, especially in systems in which alkenes are metathesized. Alkylidenes are likely to be converted to alkylidyne complexes through loss of an α proton (vide infra); therefore, only alkylidenes are considered here. Virtually all of the information concerning high-oxidationstate alkylidenes has come from studies of isolable alkylidene complexes. Many of these reactions were carried out first with tantalum complexes.4 Tantalum metal−carbon bond chemistry helped us understand how alkylidene complexes are formed and how they react with olefins, but tantalum complexes themselves are not broadly active and useful in olefin metathesis reactions. Catalytic sites in classical heterogeneous catalyst systems5 that are based on Mo, W, or Re cannot be observed directly because the active site or sites correspond to only a small fraction of the total metal present and more than one type of active site might be present. In classical heterogeneous systems the reactions that yield a high-oxidation-state alkylidene complex must therefore have relatively poor yields in comparison to a variety of side reactions that do not produce an alkylidene and possibly lead to the formation of dormant species. The discovery and development of high-oxidation-state alkylidene complexes1,3,6 and their applications in olefin metathesis reactions7 have been reviewed multiple times. The aim here is to summarize our understanding as to what are the major pathways of forming high-oxidationstate alkylidene complexes in both homogeneous and heterogeneous catalysts, as far as is known at this point, including systems in which only olefins are present. For detailed mechanistic and kinetic studies of the formation of alkylidenes, alkylidynes, and bis-alkylidene complexes in tantalum and tungsten chemistry, the reader is referred to work by Xue.8
or metallacyclobutadiene ring (in alkyne metathesis) occupies two equatorial positions; the reversible formation of a metallacyclobutane ring is the key step in the olefin metathesis reaction. It should be noted that the metal is chiral when four different ligands are present (eq 1), and in that case the configuration at the metal inverts with each metathesis step (except precisely the reverse). Mo and W, primarily, and some Re “classical” catalysts for metathesis reactions that contain an M C or MC bond are formed in a variety of circumstances in both homogeneous and heterogeneous (e.g., metal oxo complexes supported on silica) systems. The subject of this tutorial is how high-oxidation-state metal−carbon double bonds (MC © 2017 American Chemical Society
Received: October 29, 2016 Published: March 2, 2017 1884
DOI: 10.1021/acs.organomet.6b00825 Organometallics 2017, 36, 1884−1892
Tutorial
Organometallics
2. SYNTHESIS OF ALKYLIDENES IN HOMOGENEOUS SYSTEMS 2.1. α-Hydrogen Abstraction. The first “d0” alkylidene complex was discovered in 19749 as the product of an attempted synthesis of Ta(CH2-t-Bu)5 from Ta(CH2-t-Bu)3Cl3 through addition of 2 equiv of LiCH2-t-Bu. Steric crowding in the intermediate Ta(CH2-t-Bu)5 is proposed to lead to an increase in the Ta−Cα−Cβ angle(s) in one or more of the neopentyl ligands, which encourages an agostic interaction10 between a CHα electron pair and the metal and leads to migration of a relatively acidic α proton to a neighboring strongly basic Cα center via a four-centered transition state (eq 2). Four-coordinate Ta(CH-t-
present in the complex mixture that Wilkinson prepared, but the yield was possibly low and its separation from other highly soluble products in that mixture impossible or impractical. Among other problems such as reduction of the metal (especially niobium), Me3CCH2MgCl would almost certainly also deprotonate a neopentylidene ligand, as noted above. Of the common alkyls that do not have a β proton (−CH 2 CMe 3 or −CH 2 CMe 2 Ph, −CH 2 SiMe 3 , −CH 2 Ph, −CH3), the first two are the most susceptible to intramolecular α hydrogen abstraction and the resulting neopentylidene or neophylidene is the least susceptible to decomposition through alkylidene coupling (vide infra). In stark contrast, the methyl group is the least susceptible to intramolecular α hydrogen abstraction and the resulting methylidene is the most susceptible to alkylidene coupling. This fact accounts for the relative rarity of synthesizing stable methylidene complexes through abstraction of an α hydrogen in a methyl ligand. Because one of the methods of preparing classical alkene metathesis catalysts involves alkylation by reagents such as AlEt2Cl, one must ask to what extent α hydrogen abstraction competes with a related, and much better known, “β hydrogen abstraction” via a five-centered transition state to give alkane and an olefin complex? In contrast to α hydrogen abstraction, “β hydrogen abstraction” leads to a formal reduction of a d0 metal, e.g., from Ta(V) to a Ta(III) olefin complex. In fact, Ta(III) olefin complexes such as Cp*Ta(olefin)Cl2 (Cp* = η5-C5Me5) are prepared readily through β hydrogen abstraction reactions.20 It also was shown that, in sterically crowded circumstances, α hydrogen abstraction competes with β hydrogen abstraction. For example,21 alkylation of [N3N*]TaCl2 ([N3N*]3− = [N(CH2CH2NSiEt3)3]3−) with 2 equiv of ethylmagnesium chloride yields [N3N*]Ta(C2H4) (from β abstraction) along with ∼10% [N3N*]TaCHMe (from α abstraction), while dialkylation of [N 3 N*]TaCl 2 with isopentylmagnesium bromide yields [N3N*]TaCHαCH2CHMe2 in 81% yield (eq 3) as the only
Bu)(CH2-t-Bu)3 reacts with air and water, but it is thermally stable toward intra- or intermolecular decomposition reactions. It melts around 72 °C and distills under a good vacuum (
