User:Mcannos/Organolanthanide Chemistry

Organolanthanide Chemistry is

Introduction

An organolanthanide is a type of organometallic chemical compound in which the metal is of the lanthanide series in the periodic table. Presently, the term lanthanide is used to describe elements 57-71 (lanthanum to lutetium inclusively). "Ln" is widely used in chemical formulas to stand for any of these elements. Although both lanthanide and lanthanoid terminology has been used in the past, IUPAC currently recommends lanthanoid. An organolanthanoid consists of a lanthanioid bonded to at least one carbon atom through either single sigma-bonds and/or multiple pi-bonds. In contrast to d-block metals, organolanthanoids do not form complexes with CO under normal conditions and are also usually air- and moisture-sensitive. As with most organometallic compounds, organolanthanoids are able to act as effective catalysts for a variety of organic tranformations such as: hydrogenation, hydrosilylation, hydroboration, hydroamination reactions and the cyclization/polymerization of alkenes.[16]

1. History

In 1926, Aristid von Grosse postulated that it was possible for both transition metals and lanthanides to form stable organometallic compounds.[6] Gross based his assumption from main group elements obtaining 8 electrons in its valence shell, giving it a stable noble gas configuration. Transition metals require 18 electrons to fill their valence shell, while lanthanides need 32 electrons. Any compound that does not fulfill this noble gas configuration requirement is therefore reactive and may form bonds with carbon atoms. The first major breakthrough for organolanthanides followed the discovery of ferrocene and other sandwich compounds.[6] In 1954, Wilkinson and Birmingham managed to synthesize the first tricyclopentadienyl derivaties of yttrium, scandium, and almost all the other lanthanides.[7] The compounds were all crystalline solids, which were stable until 400°, but sensitive to both air and moisture.[6] LnCl3 + 3Na(C5H5) → (C5H5)3Ln + 3NaCl In 1968, the first indenyl derivative was synthesized by M.Tsutsui and H.J. Gysling.[8][9] LnCl3 + 3Na(C9H7) +THF → [(C9H7)3Ln(THF)] + 3NaCl In 1969, R.G. Hayes and J.L Thomas made the first lanthanide cyclooctatetraene complex, cyclooctatetraenylytterbium.[10] Following this, the first homoleptic compounds of lanthanides were synthesized: triphenylscandium (1968)[11], lithium tetraphenyllanthanate(III) (1970), and praseodymate(III) (1970).[6] ScCl3 + 3Li(C6H5) → Sc(C6H5)3 + 3LiCl [11] Cyclopentadienyllanthanoide chlorides, currently valuable synthetic precursors, are prepared by the transmetalation reaction of lanthanoid trichlorides with Na(C5H5) in THF. However, all attempts to make cyclopentadienyllanthanoide chlorides with light lanthanoids failed before 1980, which suggested that the lanthanoids constriction may play a role in the stability of the complex.[15] These complexes were synthesized in the 1980s with lighter lanthanoids using bulky substituted cyclopentadienyl ligands, such as; -C5Me5, -C5H4SiMe3, and bridged dicyclopentadienyl ligands.[15] Approximately 90% of organolanthanide complexes contain cyclopentadienyl ligands, but new research has shown that stable organolanthanide complexes can be made without the stabalizing effect of the cyclopentadienyl ligands.[17] In the last twenty years, organolanthanoid complexes have become known on the broad chemical front.[6] Currently, it is one of the most rapidly developing areas of organometallic chemistry, especially in the field of homogeneous catalysis. There are numerous scientific teams engaged in experiments to produce new catalytic organolanthanide complexes. 2. Organolanthanides as Catalysts

The availability of a range of different lanthanoid metals, coupled with a large amount of different ligands, provides an effective way to systematically alter the properties of the organometallic complex.[16] This helps to control their catalytic behaviour, such as chemoselectivity, enantioselectivity and diasterioselectivity. Reactions involving organolanthanoid catalysts involve mild inert reaction conditions. The presence of an (n5-C5R5) ligand on an organolanthanoid complex is a common feature. However, when R = H, the complex tends to be poorly soluble in hydrocarbon solvents and the catalytic activity is generally low.[16] Since hydrocarbon solvents are generally used for catalytic reactions and are able to bind to the Ln3+ center,  bulkier R groups are required to increase the catalytic value of the complex. This may affect the catalytic activity due to the steric demands of the ligands, which can hinder the association of the lanthanoid with the desired organic substrate, but also allows specificity to be built into the catalyst. One effective strategy to optimize a catalyst is to tilt the angle between two (n5-C5R5) groups by attaching them together.

2.1 Organolanthanides as Catalysts for the Polymerization of α-olefins Group 3 organometallic and organolanthanide compounds are among the most active known catalysts for the Ziegler-Natta type polymerization processes. [1] The absense of a rigid stereo-chemical arrangement around the lanthanide allows for the ligands to determine their stereochemistry; thus, many catalytic processes involving organolanthanides are highly stereospecific.[12] The most active of these catalysts are those containing cyclopentadienyl ligands (Cp2MX2). These complexes are extremely active homogeneous ethylene polymerization catalysts with turnover frequencies exceeding 1800 S-1 (25 °C, 1 atm of ethylene pressure) for M = La.[4] Lanthanide cyclopentadienyl sandwich complexes can be modified by substituting different lanthanide metals (due to their varied atomic size) or modifying the ligands to allow olefins better access to the catalytic center. Ligand modification can involve changing the substituents on the Cp, bridging the Cp’s, or tethering a Cp to create a constrained geometry ligand system.[1]

Despite their high activity and promise on a laboratory scale, there have been no industrial applications of these catalysts to date. The main issues preventing commercialization of lanthanide polymer catalysts is the high cost of rare earth metals. 2.2 Organolanthanides as Catalysts for other for other reactions

2.1.1 Selective Hydrogenation

Organolanthanides have been shown to be selective hydrogenation catalysts. Catalysts with the formula Cp2L2Ln are selective towards the hydrogenation of terminal alkenes.[5] Recent work with these catalysts have been able to achieve enantioselective olefin hydrogenation[19]

Despite their activity and selectivity, these catalysts can be costly and complicated to prepare and handle, making them less attractive than many alternative transition-metal-based catalysts.

The mechanism for the hydrogenation of olefins begins with a sigma-bond metathesis of R' group of the precursor with H2, releasing R'H and the active catalyst. The olefin then coordinates to the catalyst through an agostic interaction, followed by the 1,2-insertion of hydrogen onto the olefin. The product is then released from the catalyst after undergoing another sigma-bond methathesis with hydrogen, regenerating the active catalyst.

2.1.2 Hydroamination/Cyclization

A reaction which has generated much interest is the bonding of an amine onto an unsaturated carbon-carbon bond. This reaction is very useful in organic synthesis as it is atom economical, can be done inter- and/or intra-molecularly and, depending on the choice of amine and unsaturated bond, can infer chirality into the molecule[13]. After many attempts at producing a transition metal catalyst for this reaction, success was found with lanthanide and actinide catalysts of the form Cp2LnR ( where R= H, CH2SiMe3; Ln = La, Sm, YNd, Lu).[13] These catalysts were found to be very kinetically labile, having high electrophilicity, high turnover frequencies, high stereoselectivtity and diastereoselectivity.[13][14] Since these metal centers only have one stable oxidation state (3+), they usually only undergo two types of reactions: olefin insertions and sigma-bond metathesis. The single oxidation state prevents other reaction types such as reductive elimination and oxidative addition. The catalyst precursor undergoes a sigma-bond metathesis with H2NR', transfering a hydrogen from the amine to R' while simultaneously forming a L-N sigma bond, the product of this substitution is the active catalyst in the hydroamination/cyclization cycle.[14] The rate limiting formation of an agostic bond between the unsaturated bond and the metal center, is then followed by a 1,2-insertion of the amine across the unsaturated bond. In the case of intramolecular olefin insertion, this is the cyclization step. The product is then released via another sigma-bond metathesis with the starting substrate and the cycle repeats.[13]

In the case where R' is an alkene, the molecule can undergo an intramolecular amination reaction; further studies have found that when R' is an diene, allene of alkyne the reaction proceeds becomes more exergonic with a quicker with a higher turnover number.[14] 2.1.3 Other Catalytic Activity

3. Organolanthanides in Materials Chemistry 3.1 Semi-conductors 3.2 Magnetic Semi-conductors 4. References== References ==