RECOMPENSA Y RECONOCIMIENTO

K com plexes. The Cp2Ln com pounds are m ade by the reaction o f lanthanide dihalide w ith two equivalents o f sodium cyclopentadienide. (Eq. 1.3.6)

(Eq. 1.3.6) L nC lj + 2 N aC p > C p2Ln(THF)„ + 2 N aCl Ln = Sm, Eu and Yb

T he reduction o f cyclopentadienyl lanthanide(III) complexes is another route to the synthesis o f bis(cyclopentadienyl) lanthanide(II) derivatives.^"^ For example (Eq. 1.3.7,

1.3.8):

(Eq. 1.3.7) CpzYbCl + N a — C p2Y b(THF)„ + N aCl (Eq. 1.3.8) 3 CpzYbCI + Yb TH F Cp2Yb(THF)n + YbCl]

The divalent metallocenes are useful starting materials for the preparation o f a range o f C p2LnX c o m p o u n d s . F o r exam ple (Eq. 1.3.9, 1.3.10):

(Eq. 1.3.10) C p2L n (T H F )„+ A rO H * 1 C p2L n (0 A r)(T H F )„ + V2 H2

By using the more sterically demanding ligand Cp*, the first unsolvated samarocene (II) was prepared by the m etal vapour reaction o f samarium atoms and HCp* at -1 2 0 ° C /^ W hen the reaction mixture w as w orked up in TH F, the major product was the T H F solvated com plex [Cp*2Sm (TH F)2] (Fig. 1.3.3). This can also be prepared by the reaction o f sam arium diiodide and tw o equivalents o f K Cp* (Eq. 1.3.11).^^ The unsolvated complex (Fig. 1.3.4) can be obtained by the sublim ation o f the TH F adduct.^^ C p*2Eu and C p*2Yb are prepared in an analogous manner.

(Eq. 1.3.11) L n l2 ( 0 T f ) n + 2 KCp* — THF ^ [ Cp*2Ln(THF)^] + 2 K C l

Cp*2Ln sublim ation

)m

Fig. 1.3.3 The structure o f Cp*2Sm (T H F) 2 Fig. 1.3.4 The structure o f Cp*2Sm

A lthough theoretical calculations favour a linear structure, the energy difference betw een the bent and linear forms has been found to be very small. However, X -ray crystallography show s a bent structure w hich m ay arise from the w eak Van der Waals attractive forces betw een the cyclopentadienyl rings.^^

C yclooctatetraene reacts w ith dark blue solutions o f europium or ytterbium in liquid am m onia to give Ln(CgHg) (Eq. 1.3.12).

(Eq. 1.3.12) Ln + CgHg NHi [Ln(C;H ;)] Ln = Eu or Yb

The orange products are extremely air-sensitive and insoluble in organic solvents. This behaviour suggests a polym eric structure for the compound. Polym eric [Sm(CgHg)] is more easily prepared by reacting [S m l2(TH F)2] w ith K.2CgHg. The polym eric structure can be broken up by treatm ent w ith pyridine. This results in form ation o f the soluble monomeric species [(C sH ;)Ln(C5H ;N )3].«°

a com plexes. The metals Eu, Sm, and Yb react directly at -20°C w ith alkyl or aryl iodides in TH F to afford Grignard-like com pounds, R Lnl and A rL nl o f unknow n structure. H ow ever, they are very unstable and decom pose at low tem perature.^’ A well- characterised dialkyl lanthanide(II) com pound, [Y b{CH (SiM e3)2}2], w as prepared recently according to Eq. 1.3.13.^^

(Eq. 1.3.13) Y bl2 + 2 N a{C H (SiM e3)2} > [Y b{C H (SiM e3)2}2(Et2 0)]

B y using the more sterically demanding tris(trim ethylsilyl)m ethyl ligand, an unsolvated hom oleptic dialkyllanthanide(II), [Y b{C(SiM e3)3}2], w as isolated and structurally a n a l y s e d . A particularly surprising and interesting feature o f this com pound is the C-Yb- C angle o f 137’. All other metal species, for example M g{C (SiM e3)3}2^'’and N a{C (SiM e3)3}2' / ^ have been reported to be linear.

Y tterbium reacts w ith H g(C H =C H C l)2 to give the insufficiently characterised com pound, Y b(C H =C H C l)2.^^ It was reported that the blue solutions o f elemental europium or ytterbium in liquid am monia react with, terminal alkynes to give bis(alkynyl)lanthanide(II) com plexes. These are also available by transm etallation o f H g(C=CR)2 w ith the respective lanthanide as in Eq. 1.3.14:

(Eq. 1.3.14) Ln + H g(C ^C R)2 ™ » Ln(C=CR)2 + H g j,

R = Bu", Ln = Eu or Yb R = Ph, Ln = Yb

1.4.1 Pyrazole

Pyrazoles (Fig. 1.4.1) are five-m embered nitrogen-containing heterocycles which m ay bear non-coordinating substituents in the 1, 3, 4 or 5 positions. They are usually therm ally and hydrolytically stable. As ligands, they coordinate to metals and metalloids through N (2) 87>88.89,9o nucleophilicity and steric accessibility o f this nitrogen atom m ay be varied through appropriate ring substituents. W hen deprotonated, pyrazole becomes the pyrazolide ion (Fig. 1.4.2), w hich can coordinate through both nitrogen atoms as an exobidentate ligand o f C2v symmetry.^'

N — N N — N

Fig. 1.4.1 Pyrazole Fig. 1.4.2 The pyrazolide anion

Pyrazoles can be prepared by the Claisen condensation o f a deprotonated m ethyl ketone w ith an ester to give the (3-diketonate, follow ed by reaction w ith hydrazine. (Eq. 1.4.1)

(Eq. 1.4.1) R'

A N B a se N N I . N2H4 N --- N

c . •

There is an im portant class o f p oly p y razo ly l derivatives w hich can be expressed by the general structure, [RnE(pz)m]^ (Fig. 1.4.3),^^ w here E can be boron, carbon, or P = 0 , R„ are substituents such as H, alkyl or aryl groups and the num ber m o f pyrazole groups m ay be 2, 3, or 4 depending on E. The charge X depends on the nature o f R, E, and m and is usually 0 or -1.

R . N — N N — N R - E — N — N N — N N — N / N — E — N — N N - N Fig. 1.4.3 [R ,E (p z )J '

1.4.2 PoIy(pyrazolyI)borate ligands

T he [RnB(pz)4_n]* ligand system has been the m ost w idely applied o f the uninegative polypyrazolyl derivatives^^’^'^’^^’^^’^^for a num ber o f reasons: these ligands are fairly easy to prepare from readily available and relatively inexpensive starting materials; their salts are stable in air; and their electronic and steric param eters can be altered by a proper choice o f substituents w ithout affecting the sym m etry o f the original ligand.

Poly(pyrazolyl)borates fall into tw o broad categories : the bidentate, [R2B (pz)2]’, sy stem (Fig. 1.4.4) and the tridentate, [R B (p z)3]' system (Fig. 1.4.5). [B (pz)4]‘ acts as either a tridentate or a bidentate ligand.^^

m.

N — N

V

Fig. 1.4.4 Bidentate [RzBfpz);]' Fig. 1.4.5 Tridentate [R B (pz)d'

P oly(pyrazolyl)borate ligands are form ed by the substitution o f some or all o f the substituents o f tetrahydro or tetraalkylborane (BR4" , R = H or an alkyl group) anion w ith pyrazole groups. This is generally carried out by heating KBR4 or NaBR4 w ith the pyrazole above the m elting point o f pyrazole. The extent o f substitution is controlled by a com bination o f stoicheiom etry and reaction temperature. The first example o f this reaction

N — N R - B — N — N

show n below (Eq. 1.4.2-1.4.4). (Eq. 1.4.2) K B H4 + 2 H - N (Eq. 1.4.3) K B H4 + 3 H - N (Eq. 1.4.4) K B H4 + 4 H N T < 125'C T < 190'C T > 190”C K H2B- K HB N N N N + 2H , + 3H- + 4H,

It is notew o rthy that the boron-nitrogen bond is norm ally form ed w ith the nitrogen atom bearing the least sterically dem anding R group, leaving the more sterically hindered nitrogen atom to bind to the metal atom. Thus, reaction o f 3-t-butyl-5-m ethylpyrazole w ith potassium borohydride gives a good yield o f pure K H B (3-t-B u -5-M ep z)3.^* However, w hen 3,5-disubstituted pyrazoles are used, m ixtures m ay be form ed if there is insufficient difference betw een the bulk o f substituents. For example, if 3 -iso -p ro p y l-5 - m ethy lpy razole is used then tw o products K H B (3-i-P r-5-M epz)3 (80% ), K B H (3-i-Pr-5- M ep z)2(3-M e-5-i-P rpz) (20% ) are formed.^^ In special cases, the maintenance o f delocalization can be a m ore im portant factor to decide the bonding position. For example,

1-H -benzotriazole reacts w ith K B H4 to give the less sterically favoured product (Fig. ^4 Q 100.101.102

/

N — N H — B

Fig. 1.4.6 l-Hydro-/m (benzotriazoIyl)borate)

The bidentate ligands [R2B (p z)2]’ are, in a way, similar to (3-diketonates (for example acac)^^’^/ (Fig. 1.4.7) since they are both uninegative four electron donors. W hereas acac is planar, the B-N and M -N bond distances and bond angles in bis(pyrazolyl)borates make

the central ring non-planar, alm ost enforcing a boat structure (Fig. 1.4.8). Furtherm ore, for bis(pyrazolyl)borates, there is less tendency to form dim ers and other associated structures that are com m on with acac.

N - N

/

N - N

Fig. 1.4.7 The bis(pyrazolyl)borate analogy with acac

\ ' ' N —

Fig. 1.4.8 The steric structure of bis(pyrazolyl)borate complexes.

It is interesting that agostic interactions are quite com m on in bis p y razo ly lb o rate com plexes, principally because the preferred boat conform ation that the chelate ring adopts tends to push one o f the substituents on the boron tow ard the metal.^"^ This will be discussed further below.

W lien bis(pyrazolyl)borates react w ith divalent cations they usually fonu neutral com plexes L2M, containing tw o six m em bered rings. For example, the divalent first row transition m etal ions (Mn, Fe, Co, Ni, Co, Zn) can coordinate to tw o equivalents o f Bp to give com pounds w ith the form ula [H2B (pz)2]2M, w hich are generally very stable (those o f iron and m anganese are oxygen sensitive) and are readily soluble in organic solvents. These were all found to have a tetrahedral geom etry around the m etal centre apart from the Ni"^

2+

and C u“ com plexes which were square planar (Eq. 1.4.5).103

(Eq. 1.4.5) N— N M = Mi a n f l C u N — N I.', I W n , F e . C o a n a 2 n 27

predom inantly octahedral "full sandw ich" com plexes (Eq. 1.4.6). It is also possible to m ake the environm ent around M m ore crow ded by the substitution o f carbon(3) in the p y razo le, thus N (2) is unavailable for further coordination and a "half sandw ich" com plex results (Eq.

1.4.7)/^-""' (Eq. 1.4.6) (Eq. 1.4.7) N - N / 2 K H - B -n- N N - N N - N / + K H - n — N — N \ N - N

w

M

1

N N U J ' jX ) N / ) N N " ^ 7 | I N N . / I \

Tp ligand are often considered analogues o f Cp" (Fig. 1.4.9). Their key sim ilarities and dissim ilarities are tabulated in Table. 1.4.1^^’^^

O '

_{N - ^} / , A n a lo g o f Cp T p C o m m o n F e a tu r e s e le c t r o n s d o n a t e d 6 6 c o o r d in a tio n s it e s o c c u p ie d 3 3 c h a r g e - 1 - 1 D if fe r e n t ia t in g F e a tu r e s s y m m e t r y o f L M F r a g m e n t Cs.. C v s u b s t it u t a b le p o s itio n s 5 10 n u m b e r o f p o s s ib le R - s u b s lilu t e d 1 4 (m o n o ) lig a n d s w i i h r e t e n t i o n o f t h e 6 (b is) o n 'g in o f s y m m e t r y ( i s n s y m m e t r i c ) 4 (tr is) 1 (te tr a k is ) 15 to ta l m o n o m e r ic L M X a v a ila b le ? no. y e s ( X = h a lid e ) (e x c e p t for B e) u n c h a r g e d is o s t e r ic , is o s y m m e tr ic . no y e s , k n o w n a n d i s o e le c t r o n ic a n a lo g ? (C -b a s e d ) - 2 c h a r g e , is o e le c t r o n ic , and no y e s , u n k n o w n i s o s y m m e t r ic a n a lo g ? (B e -b a s e d )

Fig. 1.4.9 Cp' ligand Table 1.4.1 The key similarties and dissimilarties o f Cp and Tp ligand T etrakis(pyrazolyl)borates behave sim ilarly to Tp to form octahedral com plexes w ith m any divalent cations. There is a big difference betw een them and tris(pyrazolyl)borates in that the additional pyrazole group in tetrak is(pyrazolyl)borate can be protonated at the uncoordinated nitrogen. This allow s them to be dissolved in dilute m ineral acid.94

Besides the alteration o f the substituents on the pyrazole ring, the properties o f pyrazolylborates can be changed som ew hat by using a substituted borane instead o f KBH4 to react w ith pyrazole. For example, the square planar Ni^^ complexes [H2B (pz)2]2N i can readily form an adduct w ith pyridine o f the type [H2B (pz)2]2N i(p y )2. In this complex the pyridines occupy axial coordination sites giving a pseudo-octahedral coordination. B ut the analogous com pound w ith the diethyl-bis(pyrazolyl)borate ligand leads to the blocking o f the axial sites via an agostic interaction w ith the m ethylene proton o f the alkyl group. The agostic bond strength o f this ligand w ith m olybdenum in form ally 16-electron com plexes, (E t2B (pz)2)M o(C O )2('n^-CH2C (R )C H2) (R = H or Ph), has been extensively investigated and shown to be surprisingly strong (17-19 kcal It has also been show n that there is substantial interaction o f the B-H bond in an unsub stitu ted analogue o f this com pound (Fig. 1.4.10).’°^

Mo— CO

Me

Me Me

Fig 1.4.10 A gostic interaction o f [(Et2B(pz)2)Mo(CO)2(r|^-CH2CHCH2)].

To conclude, the introduction o f substituents onto the pyrazole ring to alter the properties o f these ligands can be sum m arised as follows: the substituent on C(3) has the m ost direct im pact on the accessibility o f the coordinated metal to other reactants; the substituent on C(4) can be used to increase or decrease the electron density o f the ligand by its electron- donating or w ithdraw ing p ro p erty and to alter the solubility o f the ligand; the C(5) substituents tend to stabilised the ligand through steric protection o f B-H bond, and can tighten the ligand "bite" at the metal through non-bonding repulsion at the boron end. Stabilization o f the ligand can also be achieved by the introduction o f an alkyl or aryl group at boron.^^

Because the pyrazolylborate ligands are anionic, have fairly hard donor groups, and their steric properties can be m odified, they should be ideal ligands for lanthanide ions for which the bonding is generally agreed to be ionic and for which the steric saturation o f the metal coordination sphere is critically im portant.

Fig. 1.4.11 The structure of Yb(Tph

The reaction o f lanthanide trichlorides w ith three equivalents o f K Tp in aqueous solution gave com pounds o f fom iula L n (T p )3. ” ° The X -ray crystal structure o f the y tterb iu m com pound was determ ined and show s the metal atom to be eight-coordinate w ith one rib­ and tw o i]^-pyrazolylborate lig an d s.'” (Fig. 1.4.11) 'h , ” C, and ” b N M R studies o f y tterbium and lutetium com plexes have show n that they are stereochem ically rigid on the N M R tim escale at room tem perature."^

A ttem pts to prepare com plexes o f the type L nT p2X, analogous to m etallocenes, have been rep o rted to be unsuccessful owing to ligand redistribution reactions. This was believed to result from the insufficient steric saturation o f the metal centre. Hence, in the presence o f p o ten tia lly bidentate ligands, tractable com plexes could be isolated. It was reported th at the reaction o f Ln(IlI) ions w ith tw o equivalents o f K Tp in the presence o f bidentate ligands such as tropolonate and p-diketonate give stable eight-coordinate com plexes. It w as show n by X -ray crystallography that the com plexes [Tp2L n(tropolonate)] (Ln = Y, La, Ce, Pr, N d, Sm, Eu, Tb, Yb, Lu) (Fig. 1.4.12),"^ [Tp2L n(P-diketonate)] (Fig. 1.4.13),"" and [Tp2L n(carboxylate)],"^’"^ ’"^ assum e a bent sandw ich configuration.

o

• ^ i

^ N

i 2 O ' - L n ' ^ ' 0 /

V i ^ l 'r. ^

Fig. 1.4.12 The structure of Tp2Ln(tropolonate) Fig. 1.4.13 The structure of Tp2Ln(P-diketonate)

Similar reactions w ith tw o equivalents o f K T p and half an equivalent o f sodium oxalate give the binuclear oxalato com plexes [(T p2L n )2C2 0 4] (Ln = Y, Sm, Dy, Yb, L u).” ^ T hese com plexes are bridged by an oxalato unit that form s two bidentate five m em bered rings (Fig. 1.4.14).

To CO

L n

a //

' o ' ( / , / n N

\ f / / i

\ \ \ /

Fig. 1.4.14 A possible structure of (TpLn)2C20^

An interesting h a lf sandw ich com pound has been made by E delm ann’s group according to Eq. 1.4.8:

(Eq. 1.4.8)

oo

+ 2 _K+ R = H, M e L n = Ce, Pr, N d , S m T H F k

D epending on the ionic radius o f the lanthanide element, these com pounds can either be unsolvated or may contain a coordinated THF m olecule.’

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