Cyclooctatetraenes lanthanides

Polydentate nitrogen ligands are very suited to stabilize mono(cyclooctatetraene) lanthanide(III) complexes. These include silylated benzamidinates [RC6H4C(NSiMe3)2]Ln-(COT)(THF) (Ln = Y, Ce, Pr, Nd, Sm, Tm, Lu R = H, MeO, CF3) (Figure 82a) and diiminophosphinates... 

COT ligands, due to their steric and electronic properties, can in principle stabilize the three possible formal lanthanide oxidation states (11, III, IV). The first cyclooctatetraene lanthanide complexes isolated were the Ln(COT) species [Ln = Eu", Yb COT=(C8Hji) Hayes and Thomas 1969], prepared by direct reaction of the metals and cyclooctatetraene in liquid ammonia. Deep red-black crystals of (COT)Yb(py)3-0.5py (Wayda et al. 1987) obtained by treatment of Yb(COT) with excess pyridine have a crystal structure of the shape of a three-legged piano stool for the coordinated ligands. The first monomeric Sm (COT) complex has been obtained with the highly substituted 1,3,6-trimethylsilylCOT ... 

Table 1. Summary of crystal and molecular data for actinide and lanthanide cyclooctatetraene complexes...

Several lanthanide cyclooctatetraene complexes have been synthesized, including both the divalent metal complexes Eu(CgHg) and Yb(CgH8) 44), and the trivalent complexes [M(CgH8)2] (M =Ce, Pr, Nd, SmorTb) 35). The former were preprired by direct reaction of the metal with cyclooctatetraene in liquid ammonia. No structural data exist for these divalent compounds, but they probably involve some kind of bridging interaction since, even with coordinated solvent, one COT... 

Alkynes can be selectively dimerized, cyclotrimerized, or polymerized with a large variety of transition metal and lanthanide catalysts nickel also catalyzes the cyclote-tramerization of HC=CH to cyclooctatetraene. Very electrophilic complexes such as Cp 2LnR and Group 4 compounds,137 as well as 18-electron species such as Cp RuH3(L) and Ru(Tp)Cl(PPh3)2, catalyze the linear dimerization of terminal alkynes 138... 

Intermolecular electron-transfer rates have been studied for uranocene and substituted derivatives of uranium, neptunium, and plutonium by examining the variable-temperature NMR spectra of mixtures of (CgH8)2An and [(C8Hg)2An]. In all cases, electron-transfer rates are rapid. Specific rates could not be derived for uranium and plutonium derivatives owing to the small chemical shift differences between analogous An(fV) and An(III) compounds, but in the case of (f-BuC8H7)2Np, the rate has been estimated to be of the same order of magnitude as comparable lanthanide cyclooctatetraene compounds ( 10 s ). ... 

Tris(cyclopentadienyl)lanthanide complexes with steri-cally more crowded Cp ligands such as C5Me4R (R = Me, Et, Tr, and SiMe3) are not assessable by simple metathesis between lanthanide trihalides and the respective alkali metal salt of the bulky Cp ligand. For instance, Cp 3Sm, obtainable from Cp 2Sm and cyclooctatetraene, reacts with THF with ring-opening forming Cp 2Sm[0(CH2)4Cp ](THF) (equation 14). 

The first tris(pentamethylcyclopentadienyl) lanthanide complex was isolated accidentally from the reaction of a divalent samarium complex (CsMes )2Sm with cyclooctatetraene [9]. Following this discovery, two more convenient methods were developed for the preparation of the sterically crowded complexes (C5Mes)3Ln [10],... 

This method of resolution of polyolefins has been extensively studied for cyclooctatetraene systems where excellent enantiomeric excesses are normally observed. Lanthanide-induced shifting can be used to determine the diastereoisomeric composition of the urazoles. Alternate means for the resolution of polyenes based on kinetic resolution using (+)-tetra-2-pinany Iborane have been described, but this reagent consumes valuable substrate. Chiral platinum complexes can also be used but at prohibitive cost on a large scale and with poor regioselec-tivity when several coordination sites are present. 

This article has reviewed the synthesis and reactivity towards small molecules of a range of U(lll) cyclooctatetraene and pentalene complexes. It is evident that in many cases the uranium centre is capable of tt back-bonding through the 5/ orbitals additionally, a C - C agostic interaction between a bound substrate and a U(IV) centre has been observed. Clearly uranium is capable of bonding with a degree of covalency , and this is perhaps why the reduction chemistry of U(III) is so rich and diverse, and not simply an iteration of low-valent lanthanide chemistry. 

The reaction of bis(cyclooctatetraene)uranium with air occurs explosively however, the derivatives of U(COT)2 are considerably more stable, for example, [U(l,3,5,7-C8H4Ph4)2] is stable in air for several weeks. This happens because of steric hindrance which prevent the attack of oxygen on the uranium atom (Table 11.1). Other cyclooctatetraene complexes of the actinides and lanthanides may be oxidized in a similar way. 

A single lanthanide (IV) metallocene, i.e. bis(cyclooctatetraene)cerium(IV) Ce(C8Hg)2 or cerocene, has been studied theoretically. Rosch and Streitwieser (1978,1983) reported the results of nonrelativistic and quasirelativistic scattered-wave Xa calculations taking into account the scalar relativistic effects for cerocene, thorocene and uranocene at fixed idealized Dgh geometries (fig. 13). The authors discuss the participation of metal d and f... 

This reaction is also of limited use ytterbium and europium metals are the only lanthanides which dissolve in liquid ammonia solution to give divalent metal ions and solvated electrons.52 in the above reaction, the initial product formed is probably an [8]annuleneytterbium(II) complex which then reacts with additional cyclooctatetraene to form the ytter-bium(III) complex. 

The bis([8]annulene)lanthanate(III) anions which are complexed by an [8]annulenelanthanate(III) cation are the only other class of complexes which contain two [8]annulene dianions in a sandwich arrangement about a lanthanide(III) ion. This class was first reported for the cerium(III) ion, from the reduction of a cerium(IV) alkoxide with triethylaluminum in the presence of cyclooctatetraene.3 ... 

A more general synthetic procedure for the synthesis of Ln2(CQHg)3 is by the reaction of neutral cyclooctatetraene with the lanthanide atom vapor at -198 C.S3... 

Controversy exists in naming sandwich organometallic actinide and lanthanide compounds of cyclooctatetraene dianion [8]annulene dianion and [8]annulene-metal(X) complex are chosen here because these names properly describe the delocalization of charge in the ligand and also emphasize the formal oxidation state of the metal in the complex. 

Bis([8]annulene) complexes, lanthanide 84 Bis(cyclooctatetraene) complexes, cerium 84,88 Bis([8]annulene) complexes, cerium 84,88 Bis(cyclooctatetraene) cerate anion 85,88 Bis([8]annulene) cerate anion 85,88 Molecular orbital calculations 86 Xa Scattered-wave calculations 86 Quasi-relativistic Xa Scattered-wave calculations 86... 

Cyclooctatetraene forms complexes with both lanthanide and actinide elements. They are prepared by reacting the anhydrous metal halides with K2(CgHg) in tetrahydrofuran. The anion CgHJ" is a planar ten-electron aromatic system and this planar structure is retained by the CgHg ligand in the complexes. 

Streitwieser noticed that the HOMOs of C Hg have the correct symmetry to interact with the 5/orbitals of an actinide (or 4/of a lanthanide). This led to the preparation of the sandwich complex bis(cyclooctatetraene)uranium (uran-ocene). Uranocene is pyrophoric in air, but it thermally very robust. It forms deep green crystals which can be sublimed under vacuum. X-ray diffraction shows that the rings are parallel and eclipsed (Dg ). 

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