TEORÍA DE LA EQUIDAD

K com plexes. The first organom etallic com pounds o f lanthanides to be prepared were the cyclopentadienyl derivatives. Tricyclopentadienyl lanthanide was obtained by the reaction o f three equivalents o f sodium cyclopentadienide w ith the anhydrous chloride o f the lanthanide in (Eq. 1.2.4, Fig. 1.2.7)

(Eq. 1.2.4) LnC lj + 3 N aC p ™'' » C pjL n + 3 N aC l Ln = Y, La, Ce, Pr, Nd, Sm, Gd or Dy

The solid state structures o f all o f these complexes appear to be polym eric w ith a cyclopentadienyl ligand bridging the metal centres. The m ethylcyclopentadienyl analogues have also been isolated.'^’ CpgLn reacts w ith neutral ligands such as THF, R3P =0 , and RN=C, to give discrete Cp3LnL molecules, w hich have three rings and L arranged in a quasi-tetrahedral fashion.

O

Fig. 1.2.7 The structure o f Cp)Ce(THF)

The bis-cyclopentadienyl halides can be obtained in a variety o f w ays, for example (Eq. 1.2.5-1.2.7):

(Eq. 1.2.5) L nC lj + 2 M C p > Cp^LnCl + 2 MCI

(Eq. 1.2.6) 2 Cp3L n + L nC l3 > 3 Cp2LnCl

fNH4Cl ... > (NH) + CpH

(Eq. 1.2.7) Cp3L n + j c p H

A s m entioned in Section 1.2, due to coordinative unsaturation, Cp2LnCl derivatives are often not possible for the lighter lanthanides (from La to Nd). In general, the com pounds C p2LnCl are unstable as monom ers. They tend to form chloro-bridged dimers o f the ty p e [Cp2L n(p-C l)]2. The bridge is easily cleaved by donor solvents such as THF. N evertheless these com pounds have been successfully used as precursors to the corresponding alkyl com plexes (Eq. 1.2.8, 1.2.9).

THF (Eq. 1.2.8) CpzLnCl.THF + L i R --- ^ CpzLnR.TH F + LiCl -78"C Toluene ^R-» Pvridine (Eq. 1.2.9) [Cp2L nC l]2 + LiA lR4 --- ^ Cp2L n ( > 1 , ^ [Cp2LnR]2 0 C R R Toluene

Ziegler-N atta c a t a l y s t s , p r o b a b l y due to the steric bulk o f the pentam ethy l­ cyclopentadienyl ring w hich ensures very rapid monom er-dimer equilibration, which gives rise to vacant coordination sites on the metal. These com pounds are also found to activate benzene and m ethane (Equation 1.2.10, 1.2.11).'^^’'^^

(Eq. 1.2.10) Cp*2Lu(CH3)

-CH,

/ = \ Cp*2Lu(CH3)

Cp*2Lu— ^ ^ ^ Cp*2Lu-(\ ^— LuCp*2

Cyclohexane

. (Eq. 1.2.11) C p*2L u (‘^CH3) + CH4 . ~ Cp*2L u(C H ;) + '^CH , A nother organic ligand w hich has been used successfully to stabilise the trivaient lanthanides is the cyclooctatetraen dianion (CgHg^). The reaction o f lanthanide trichlorides with two equivalents o f K2CgHg gives K[Ln(CgHg)2] (Eq. 1.2.12).'^^

(Eq. 1.2.12) LnCls + 2 K2CgHg TH F K[Ln(CgHg)2] + 3 KCl Ln = Y, La, Ce, Pr, Nd, Sm, Gd or Tb

The structural chem istry o f the cerium(IH) anion [Ce(CgHg)2]' has been investigated in detail/^ The X -ray structure analysis o f [Li(TH F)4][Ce(CgHg)2] indicates a clear separation o f the [Li(TH F)4]'^ cation and [Ce(CgHg)2]' anion. (Fig. 1.2.8)

0

0

Fig. 1.2.8 T he structure o f [L i(T H F)d[C e(C gH g)2]

U nsolvated L n2(CgHg)3 com pounds w hich are made by atom vapour reaction, i.e. by the co-condensation o f lanthanide atom s w ith CgHg on a cold surface, have not been structurally characterised.'^^

The m onocyclooctatetraenyl lanthanide chlorides, m ade by the reaction o f LnCl] w ith one equivalent o f K2CsHg, are know n for La, Ce, Pr, Nd, Sm, Er, and Lu. They have been used to synthesise the m ixed sandw ich com pounds (CsH5)Ln(CgHg) w hich are believed to have polym eric rather than sim ple sandw ich structures.^®

T he lanthanides are know n to form I'l^-allyl com pounds o f tw o m ain sorts: L nC p2(C3H$) and L iL n(C3H$)4.^' A nionic allyl complexes o f the lanthanides show prom ising catalytic properties for the polym erization o f som e dienes.

Because o f the tendency o f lanthanides to foina highly ionic com pounds, the m ajority o f their organom etallic com pounds are complexed w ith anions. H ow ever, it has been show n that a neutral arene ligand can be bonded to a lanthanide. The com pound is (r|^- C6M e6)Sm (r|^-A lC l4)3,^^ w hose structure is show n in Fig. 1.2.9:

C I O, Cll Cl3 Cl2 ClI Cl9 A u l ClI I Cl5 ClI O Al3 C l 5 / Cl8

Fig. 1.2.9 The structure of (r|CC(,Me6)Sm(n^-AlCh)

a com plexes. A num ber o f com pounds are know n in w hich all the metal to carbon bonds are a bonds. These include hom oleptic species such as L i[L nPh4] (Ln = La or Pr),^^ [L i(D M E )3]3[L nM c6] (Ln = Ce, Pr, Gd, Tb, Dy, Er or Lu),^^ and L n [(C H2)2PM e2]3 ((Ln = La, Pr, N d, Sm, Gd, Ho, Er or Lu),^^ or heteroleptic com plexes such as L n (C H2S iM e3)3(T H F)3 (Ln = Y or Yb)^®. In addition, com plexes such as Ln(o- C6H4C H2N M c2)3 (Ln = Y, La, N d, Er, Yb or Lu)^^ (Fig. 1.2.10) in w hich a chelate ring is form ed m ay show enhanced stability.

Me2N/i Mc2N

Fig. 1.2.10 The structure o f LnO-C^H^CH^NMe;);

H ow ever the synthesis o f sim ple neutral organolanthanide complexes containing only a Ln-C bonds is quite problem atic due to the tendency o f the “large” lanthanide ion to prefer high coordination num bers and this leads to the form ation o f solvent adducts or oligomeric species.

The first unsolvated hom oleptic trialkyllanthanides, L n{C H (SiM e3)2}3 (Ln = La or Sm) w ere successfully prepared using a very highly sterically dem anding ligand.