Organolanthanide chemistry
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]
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Contents
1. History
2. Organolanthanides as Catalysts
2.1 Organolanthanides as Catalysts for the Polymerization of α-olefins
2.2 Organolanthanides as Catalysts for other for other reactions
2.1.1 Selective Hydrogenation
2.1.2 Hydroamination/Cyclization
2.1.3 Other Catalytic Activity
3. Organolanthanides in Materials Chemistry
3.1 Semi-conductors
3.2 Magnetic Semi-conductors
4. References
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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.
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.
Example of increase the catalytic activity of an organolanthanide complex by tilting the cyclopentapentadienyl ligands.[16]
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]
Examples of homogeneous catalysts for polymerization[18]
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.
Organolanthanides as Catalysts for other for other reactions
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]
Examples of Cp*YMe-THF catalysts, reagents and products in Selective Hydrogenation[18]
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.
Catalytic cycle of olefin hydrogenation [18]
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]
Catalytic Cycle for Hydroamination/Cyclization of an aminodiene[14]
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]
Other Catalytic Activity
There are other less thoroughly investigated instances of lanthanoid catalysis. These include the hydrosilylation of alkynes[20], catalysis in the Mukaiyama reaction[21], the hydroboration[22] of olefins, and hydrogenation of imines [23]. The mechanisms for these reactions follow the same general principles as previously observed. Sigma-bond metathesis of the precursor to the activated form Cp*2LnH followed by the coordination of the unsaturated bond . This is proceeded by a 1,2-insertion of H, then release of product through another sigma-bond metathesis to regnerate the active catalyst.
Typical Catalytic Cycle for Hydrosilylation[20]
Hydroboration also undergoes similar process.
Organolanthanides in Materials Chemistry
Semi-conductors
Lanthanide metals are suitable doping agents for III-V semiconductors such as semi-conductor lasers, optical amplifiers and light emitting diodes.[2] Since all metals in the lanthanide series typically carry a +3 charge, this offers tunability with regards to metal choice, since all of the lanthanides will be suitable +3 doping agents. Each lanthanide metal varies in atomic size which tunes for incorporation in solid semi-conductor lattices. Each lanthanide also varies in its optical transmission spectrum which tunes for the optical properties of absorption or emission.
Incorporating lanthanides into semi-conductors presents an issue. The synthetic routes require the starting materials to be liquids or gases for a homogeneous distribution throughout the product material. Organolanthanide complexes are known to be highly air-sensitive, thermally very robust high-melting materials. High melting points in the range of 150-350°C are common, and the design of volatile, low-melting precursors is a synthetic challenge.
The solution was found using organolanthanide complexes such as tris(cyclopentadienyl) lanthanides. These complexes are very volatile, allowing gas-phase incorporation into the semi-conductor. These compounds were first synthesized[3] from LnCl3 and Na[C5H5] via a salt metathesis but other methods and derivatives are also possible. Other appropriate precursor compounds include mixed-sandwich lanthanide complexes containing n5 cyclopentadienyl and n8 cyclooctatetraenyl rings. These compounds also exhibit high volatility.
Magnetic Semi-conductors
Rare earth monochalcogenides LnE (Ln = Yb, Eu, Sm; E = S, Se, Te) have interesting magnetic properties.[2] These materials are normally obtained by high-temperature syntheses from the correspondent elements, but in this case the materials often contain impurities which are readily incorporated due to the high oxophilicity of the rare earth center. The use of organolanthanides could enable us to obtain the LnE materials with required purity by a relative low temperature synthesis.
References
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2) Gun'ko, Y. K. and F. T. Edelmann (1997). "Organolanthanides in Materials Science." Comments on Inorganic Chemistry: A Journal of Critical Discussion of the Current Literature 19(3): 153 - 184.
3) J. M. Birmingham and G. Wilkinson, J. Am. Chem. Soc. 78,42 (1956).
4) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schumann, H.; Marks, T. J. Journal of the American Chemical Society 2002, 107, 8091-8103.
5) Jeske, G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8111.
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8) Tsutsui, M.; Gysling, H.J., J. Am. Chem. Soc. 91, 3175 (1968).
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11) Hart, F.A.; Massey, A.G.; Saran, M.S. J. Organomet. Chem. 21, 147 (1970).
12) "A Lanthanide Lanthology; Part II, M-Z", p.16 (MolyCorp, Inc: USA) 1994.
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16) Housecroft, C.E.; Sharp, A.G., "Inorganic Chemistry" 2nd ed. p.752 (Pearson Education Limited: England) 2005.
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21) Gong, L.; Streitwieser, A.; J. Org. Chem. 1990, 55, 6235-6236
22) Harrison, K. N.; Marks, T.J.; J. Am. Chem. Soc., 1992, 114 (23), 9220-9221.
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Authors:
1. Kevin Weinreich (U)
2. Derek Mandel (U)
3. Mike Hamilton (U)
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