1 MSc: f-Elements, Prof. J.-C. Bünzli, 2008 U Chapter 4 Organometallics 4.1The first compounds...

1MSc: f-Elements, Prof. J.-C. Bünzli, 2008

U

Chapter 4 OrganometallicsChapter 4 Organometallics

4.1 The first compounds4.2 Alkyl derivatives4.3 Cyclopentadienyl derivatives4.4 Carbonyl and related complexes4.5 Arene complexes4.6. Cyclooctatetraene derivatives4.7 Divalent lanthanide chemistry

USm

Sm


4.1 The first compounds

cyclopentadienyl Cp-

cyclooctatetraenyl Cot2-pentamethylcyclopen-tadienyl Cp*

Fe

[Fe(5-C5H5)2] 1952

Ferrocene:

points to the bindingof the 5 carbon atoms

Chapter 4 Organometallics


d-Transition metals: 18-electron rulepoints to stable compounds.

ΔΟ

1a1g

π

p

s

d

6 LM2a1g

1eg

2eg

2t2g

1t2g

1t2u1t1g

3t1u

2t1u

1t1u

σ

What about f-elements?Much energy is neededto “transform” f-elementsinto d-elements: cf.the energies of the[Xe]4fN-15d16s2 configura-tion (Ch. 1) versus[Xe]4fN6s2.


Slide 24


UCl

[U(5-C5H5)3Cl]dark red

UCl4 + 3 NaCp [U(Cp)3Cl] + 3 NaCl (in thf)UCl4 + 3 NaCp [U(Cp)3Cl] + 3 NaCl (in thf)

L. T. Reynolds, G. Wilkinson,J. Inorg. Nucl. Chem. 1956,2, 246

LnCl3(anh) + 3 NaCp [Ln(Cp)3] + 3 NaCl (in thf)LnCl3(anh) + 3 NaCp [Ln(Cp)3] + 3 NaCl (in thf)

J.M. Birmingham, G. Wilkinson,J. Am. Chem. Soc. 1954,76, 6210

Ln

Sm Ce, Sm: orangePr: pale greenNd: pale blue



U 1968UCl4 + 2 K2(Cot) [U(Cot)2] + 4 KCL (in thf)UCl4 + 2 K2(Cot) [U(Cot)2] + 4 KCL (in thf)

A. Streitwieser Jr. et al.J. Am. Chem. Soc. 1968, 90, 7368

d and f organometallic compounds are particularly sensitiveto oxygen and water and this problem is more importantfor f-elements in view of their larger ionic radii and theirlower propensity to form covalent bonds.

For f-elements, relativistic effects (see Ch. 1) increasethe complexity of theoretical models aiming at explainingthe chemical bonds in organometallic compounds.



4.2 Alkyl derivatives (-bonds)

Most difficult to synthesize. Bulky ligands should be used.Two main paths:

a) With the help of a bis(trimethylsilyl)methyl ligand

Li[CH(SiMe3)2] + UL3 [U({CH(SiMe3)2}3] (in hexane)Li[CH(SiMe3)2] + UL3 [U({CH(SiMe3)2}3] (in hexane)

C3-symmetry

U-C: 2.48 ÅC-U-C: 107.7o

O-

C

H

Si

Me

Me

MeSi

Me

Me

MeL =



LuCl3 + 4 Li(t-but) + Me2N-(CH2)2-NMe2

[Lu(t-but)4]-[Li(tmed)2]+ (in Et2O/pentane)LuCl3 + 4 Li(t-but) + Me2N-(CH2)2-NMe2

[Lu(t-but)4]-[Li(tmed)2]+ (in Et2O/pentane)

H. Schumann et al., J. Organomet.Chem. 1986, 306, 215

CS symmetryLu-C 2.32-2.43 ÅC-Lu-C 107-109 o

Allows alkylation reactions:

O

O

tBut

tBut

HO

b) Smaller alkyl groups may also be used, often leading to negatively charged species



H. Schumann et al., J. Organomet. Chem.1984, 263, 29.

LuCl3 + 6 LiMe + 3 dme [Li(dme)]+3 [Lu(Me)6]3- + 3 LiClLuCl3 + 6 LiMe + 3 dme [Li(dme)]+3 [Lu(Me)6]3- + 3 LiCl

The compounds are extremelyair- and water-sensitive.They were obtained withseveral Ln.The structure of [Er(Me)6]3-

is also known, with[Li(Me2N(CH2)2NMe2]+ ascounterion

Lu-C 2.48-2.57 ÅC-Lu-C 86-96 o

C-Lu-C eq 176 o



Decomposition of alkyl complexes via -elimination

LnR

H

R

Ln

H

LnH + RLnR

H

R

Ln

H

LnH + R

f-elements do not form strong bonds with alkenes, sothat decomposition proceeds easily

Conditions for -elimination:- the -carbon must bear a H atom- the Ln-C-C-H fragment must be able to adopt a planar conformation- the metal ion must have an appropriate empty orbital for binding the H-atom



f-Elements have a large number of empty orbitals, sothat they are extremely susceptible to -elimination.The strategy for producing alkyl derivatives with lowcoordination numbers therefore involves bulky ligands forwhich -elimination is not possible, such as CH(SiMe3)2.

Agostic interaction

SiMe

Me

CH H

H

Me3Si

U

L

L

U-H-C bridges are formed withH-atoms in position with respectto U: this is called a -agosticinteraction.Therefore, the coordination numberis larger than 3 in this compoundand this is often the case for low-CN organometallic compounds



4.3 Cyclopentadienyl derivatives (-bonds)

Cp and its derivatives are among the most popular ligandsin organometallic chemistry of d- and f-transition metals.

Cp forms both covalent and ionic complexes with 1-5C-atoms coordinated. The 5-mode is usually consideredto occupy 3 coordination sites (this is somewhat arbitrary).

Simple LnCp3 cyclopentadienyls

• LnCp3 features polar Ln-Cp bonds as shown by:

2 LnCp3 + 3 FeCl2 3 FeCp2 + 2 LnCl3

• Rapid exchange of Cp ligands occurs

• LnClCp2 compounds can be isolated as dimers or as

Lewis acid adducts



LnCl

OLn

Cl

N

Ln Ln

Cl

Cl

Most dimersfeature Cl- asbridging ligands



Structural changes along the series

La-Pr “11-coordinate” [Ln(5Cp)3(2Cp)](under the form of a coordination polymer)

Ln

Ln

Ln

Y, Sm-Yb “9-coordinate” [LnCp3]

G.Laubereau & J.H. Burns Inorg.Chem.1970, 9, 1091,

Sm-C 2.75 Å


Cp is countedas tridentate!


Sc, Lu “8-coordinate” [Lu(5-Cp)2(1-Cp)2

(coordination polymer)

Ln

Ln

Ln

AnCp3 compounds behave similarly to LnCp3. Most ofthe Cp chemistry of Th, U, Pa and Np elementshowever involves +4 oxidation state:

AnCl4 + n NaCp [AnCpnCl4-n] + n NaClAnCl4 + n NaCp [AnCpnCl4-n] + n NaCl

Simple AnCpn cyclopentadienyls



[An(Cp)4] are the only complexeswith four 5-Cp coordinated ina tetrahedral arrangement.As a comparison, the ZrIV

analogue is [Zr(5-Cp)3(1-Cp)]

An

Th-C 2.87 ÅTh-Cp 2.61 ÅU-Cp 2.59 Å

R. Maier e al., J. Alloys & Cmpnds1993, 190, 269.



[UCp3Cl] + FeCl2 ferrocene

indicating a more covalent U-Cp bond than in theanalogue Ln compounds. On the other hand, the Clligand in [AnCp3Cl] can easily be substituted, makingthese complexes important synthons in An organo-metallic chemistry.

Substituted Cp’s are also used, asin this ThIV derivative:[Th(Me3SiCp)3Cl]

R. C. Blake et al., J. Organomet.Chem. 1998, 551, 261.



AnCl

AnOEt

AnBH4

AnCH3

AnCl4 TlCp NaOEt

NaBH4LiMe



Substituted cyclopentadienyls

SiSi Si

SiSi

MeMe

Me

Me

Me

Me Me

MeMe

MeMe

Me

MeMe Me

The substitution givesmore stable, more solublecompounds which areeasier to crystallize.Cp

Cp*

Cp’’

However, Ln(Cp*)3 andAn(Cp*)3 are not verystable, because theligand is too bulky.

On the other hand, [Ln(Cp*)2Cl], [An(Cp*)2Cl] and[An(Cp*)2Cl2] are versatile and have an extensivechemistry.



[Sm(Cp*)3]

W. J. Evans et al., J. Am. Chem. Soc. 1991, 113, 7423

Trigonal co-ordination

Sm-C 2.82 Å

Cp*-Sm-Cp* 120o

2 [Sm(Cp*)2] + Cot [Sm(Cp*)3] + [Sm(Cp*)(Cot)]2 [Sm(Cp*)2] + Cot [Sm(Cp*)3] + [Sm(Cp*)(Cot)]



An active Th catalyst (A) for ethene polymerization:

It also inserts CO:

2H2

Th

Me

Me

Th

H

HTh

Th

Cl

Cl

2 LiMe

Th

H

O

H

2 CO

2

A



ThTh

H

H

+ 2 [Et3NH][BPh4]dmpe

KCp*(18C6)… [Th(Cp*)3H]

W.J. Evans et al., Organometallics2001, 20, 5489.

Th-C 2.87 ÅTh-H 2.33 Å

1,2-bis(dimethylphosphinoethane)



tBut substituents are also commonly grafted onto Cp,yielding a wealth of interesting compounds

[Yb{(tbut)2Cp}3]

A.V. Khvostov et al.J. Organomet. Chem.1998, 568, 113.

[{YbCl(tButCp)2}2]

J.S. Ren et al., Jiegou Huaxue1997, 16, 380



4.4 Carbonyl and related complexes

d-Transition metal chemistry:

-donation

M C O M C O

-retrodonation

4f-Transition elements: the filled metal orbitals are notoutside orbitals so that -overlap is not effective.

Ln(g) + n CO (g) Ln(CO)n (g), n = 1-6, in Ar at <40 K

Identified by IR spectra. Decompose upon increasing T.




Laser-ablated Gd atoms and Gd2 dimers co-depositedwith CO onto a CsI window (under Ar):

Gd(CO)x x = 1-3

Gd Gd

CO

Studied by vibrationalspectroscopy with 12C/13Csubstitution + DFT calculations

Gd Gd

O

C

C

O

Activation of CO

Cleavage of COGd Gd

C

O

Xi Jin et al. J. Phys. Chem. A 2006, 12585


5f-Transition elementsMore covalent so that AnCp3CO complexes are known

SiMe3

SiMe3

Me3Si

U CO

(CO) = 1976 cm-1 (CO) = 1900 cm-1

Since (CO) = 2146 cm-1 for free CO, these valuespoint to a significant amount of back bonding.

UCO



Dinitrogen, isocyanide, alkene, and alkyne compounds

Dinitrogen is isoelectronic with CO, but less efficient for-bond formation.In the case of f-elements, is forms complexes onlywith the low oxidation states, e.g. SmII, YbII, UIII.

U

NR

N

NN

RR

U

NR

N

NN

RR

N

N

Isocyanides CN-R:

[LnCp3(CN-Et)] have (CN)values higher than in free CN-Rpointing to negligible back-

bonding

Alkene and alkyne complexes are also very difficult tomake and only occur with low oxidation states



4.5 Arene complexes6-arene ligands (such as benzene) are -acceptors and-donors.

Note andrefer to the symmetry of the bondinginteraction with respect to the metal-ligand axis:they correspond to 0, 1, 2, and 3 nodal planes.

Arene ligands usually form complexes when the metal isin a low oxidation state.

Ln = Pr, Nd,Gd, Tb, Dy,Ho, Er, Sc, Y

tBut

tBut

tBut

tBut

tBut

tBut

Ln

Ln(g) + tBut3Ph-196oC

0-valent compoundssublime in vacuo at100 oC



A SmIII compound is also known:

Sm ClClCl Cl

ClClAlAl

Cl

Cl

Cl

Cl

AlClCl

SmCl3 + 3 AlCl3 + PhMe6

The 0-valent compounds form only for Ln for whichthe promotion energy [Xe]4fN6s2 [Xe]4fN-15d16s2 isrelatively small (cf. Ch. 1).Therefore, involvement of the 5d electron is holding thebis(arene) Ln0 compounds together.



4.6 Cyclooctatetraene complexes

Cot2- is ideally suited for f-element organometallicchemistry because it has a high valency and is stericallybulky (as much as Cp*).

Uranocene, D8h symmetryThe two rings are eclipsed in conformation, therefore theD8h symmetry

The major source of binding is the interaction betweenthe ring e2g orbitals and the U(6d) orbitals (-bonds).

The second source of binding is between the ring e2u

orbitals and the U(5f) orbitals -bonds)These are onlyavailable with f-elements.



-bonds (major contribution to binding)

Two verticalnodal planes,but no nodalplane betweenthe rings,hence e2g


6d


-bonds

Nodal planebetween therings, hencee2u



6d

5fe2u+e2g

e1u+e1g

a2u+a1g

e2g

e1g

a1g

e2u

a2u

e1u

e3u

U

UIV

anti-bonding

20 e- 2 e-



Lanthanide Cot2- complexes[Ce(8-Cot)2] is known for a long time. Recent calculationstend to prove that it should however be formulated as[Ce3+(Cot2)3-]. It can be reduced by potassium:

LnCl3 + 2 K2Cot K[Ln(8-Cot)2] + 3 KClLnCl3 + 2 K2Cot K[Ln(8-Cot)2] + 3 KCl

Other Ln compounds are obtained as follows:

[Ce(8-Cot)2] + K K[Ce(8-Cot)2][Ce(8-Cot)2] + K K[Ce(8-Cot)2]

These compounds are essentially ionic. They react readilywith UCl4 to give uranocene.



Example: synthesis of themixed species [Sm(Cp*)(Cot)]

[Sm(Cp*)3] + Cot [SmCp*Cot] +(Cp*)2[Sm(Cp*)3] + Cot [SmCp*Cot] +(Cp*)2

In sterically crowded complexes such as [Ln(Cp*)3], (Cp*)-

functions as one-electron reductive species.

[Ln(Cp*)3] 1e- + ½ (Cp*)2 + [Ln(Cp*)2]+ [Ln(Cp*)3] 1e- + ½ (Cp*)2 + [Ln(Cp*)2]+

W. J. Evans et al., Dalton Trans. 2000,1609



purple

4.7 Divalent lanthanide chemistry

Initial studies involved SmII, EuII, YbII

SmI2 + 2 NaCp* [Sm(Cp*)2(thf)2] (in thf)

[SmII(Cp*)2(thf)2] [SmII(Cp*)2] + 2 thf (upon heating)SmI2 + 2 NaCp* [Sm(Cp*)2(thf)2] (in thf)

[SmII(Cp*)2(thf)2] [SmII(Cp*)2] + 2 thf (upon heating)

W. J. Evans et al., J. Am. Chem. Soc.1981, 103, 6507 and 1984, 106, 4270

Sm-C:

2.79 Å

2.86 Å



These species are highly reactive and generate interestingredox chemistry. For instance, they can add dinitrogenupon crystallization:

W. J. Evans et al., J. Am. Chem. Soc.1988, 110, 6877

First N22- complex of a

Ln ion

2 [Sm(Cp*)2] + N2 [Sm2(Cp*)4N2]2 [Sm(Cp*)2] + N2 [Sm2(Cp*)4N2]

Sm-C 2.73 ÅSm-N 2.36 ÅN-N 1.09 Åsimilar to N2 ?



2 [SmII(Cp*)2(thf)2] + N2O [SmIII2(Cp*)4-O)] + N2 2 [SmII(Cp*)2(thf)2] + N2O [SmIII2(Cp*)4-O)] + N2

An O2- bridging ligand: [SmIII(Cp*)2-O-SmIII(Cp*)2


In fact, many O-containing substrates produce thiscompound (NO, thf, a.s.o)

Sm-O 2.09 ÅSm-C 2.74 Å



Insertion of trans-azobenzene: [SmII2(Cp*)4N2Ph2]


2 [SmII(Cp*)2(thf)2] +N2Ph [SmIII2(Cp*)41:1-N2Ph2)]2 [SmII(Cp*)2(thf)2] +N2Ph [SmIII2(Cp*)41:1-N2Ph2)]

blue

Magnetic data indicate thepresence of SmIII (1.9 M.B.)

Sm-C 2.74 ÅSm-N 2.40 Å N-N 1.25 Å, identical to neutral N2Ph2

expected distance: 1.44 Å

Sm-H distances (orthoposition) are close toagostic interaction



W. J. Evans et al., J. Am. Chem. Soc.1988, 110, 4983

Insertion of CO:

[SmII2(Cp*)4Phe2N2] + 2 CO

[SmII2(Cp*)4Phe2N2]

(in thf)

green

C2h



Chalcogenolate complexes:

W.J. Evans et al., Inorg. Chem. 2005, Published on the web, May 13

[SmII(Cp*)2(thf)2] + PhEEPh [SmIII2(Cp*)4(PhE)2] + 2 thf

E = S, Se, Te[SmII(Cp*)2(thf)2] + PhEEPh [SmIII

2(Cp*)4(PhE)2] + 2 thf E = S, Se, Te

[SmIII2(Cp*)4(PhS)2]

orange

Sm-S 2.76 Å



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1 MSc: f-Elements, Prof. J.-C. Bünzli, 2008 U Chapter 4 Organometallics 4.1The first compounds...

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