Nmr Reagent

Chapter 24

.

  Organo Organometall metallic ic comple complexes xes of the lanth lanthanoid anoidss

751

APPLICATIONS Box 24.4 Lanth Lanthanoid anoid shift reagents reagents in NMR spectroscop spectroscopy y The magnetic field experienced by a proton is very different from fro m tha thatt of the applied applied field when a para paramag magnet netic ic met metal al centre is present, and results in the    range over which the  1 H NMR NM R sp spec ectr trosc oscopi opicc si signa gnals ls app appea earr be beinglarg inglarger er th than an in a spe specctrum of a related diamagnetic complex (see  Box 2.5). Signals for protons close  to the paramagnetic paramagnetic metal centre are significantly shifted an and d th this is ha hass th thee eff effec ectt of ‘s ‘spre pread adingout’ ingout’ th thee spe specctrum. Values of coupling constants are generally not much changed. 1 H NM NMR R sp spec ectr tra a of la larg rgee or orga ganic nic co comp mpou ound ndss or of  mixtures of diastereomers, for example, are often difficult to interpret and assign due to overlapping of signals. This is par partic ticula ularly rly true whe when n the spectrum spectrum is rec record orded ed on a lowfield lowfie ld (e.g (e.g.. 100 or 250 MHz) instrument. instrument. Paramagnetic Paramagnetic shif iftt lantha lan thanoi noid d com comple plexe xess hav havee app applic licat ation ion as   NMR sh reagents. The addition of a small amount of a shift reagent to a solution of an organic compound can lead to an equilibrium being established between the free and coordinated organic species. The result is that signals due to the organic species spec ies which origin originally ally overlapped, overlapped, spread out, and the

Lower coordination numbers can be stabilized by using aryloxy or amido ligands, for exam aryloxy example: ple: . .

5-coordinate: (24.2), (24.3);   3-coordinate:  ½ NdfNðSiMe3 Þ2 g3 .

In the solid state, ½NdfNðSiMe3 Þ2 g3   is trigonal pyramidal but this ma may y be a con consequ sequenc encee of cry crysta stall pac packin king g for forces ces (see   Section 19.7 ). ).

spectrum beco spectrum becomes mes easie easierr to inter interpret pret.. The europ europium(I ium(III) II) comple com plex x sho shown wn bel below ow is a com comme merci rciall ally y ava availa ilable ble shi shift ft reagent reag ent (Res (Resolveolve-Al AlTM ), us used ed,, fo forr ex exam ampl ple, e, to re reso solv lvee mixtures of diastereomers. t

Bu

t

t

Bu

O

Bu O

O Eu O

O t

Bu

O

t

Bu

t

Bu

See also:   Box 2.6  for application of Gd(III) complexes as MRI contrast agents.

and   23.9), lanthanoid metals do not form complexes with CO und with under er norm normal al con condit dition ions. s. Uns Unstab table le car carbon bonyls yls such as Nd(CO)6   have been prepared by matrix isolation. Organolant Orga nolanthanoi hanoids ds are usuall usually y air- and moist moisture-se ure-sensitiv nsitivee and some are pyrophoric; handling the compounds under inert atmospheres is essential.† 23.4

 -Bonded -Bonded complexes Reaction 24.16 shows a general method of forming Ln C  -bonds. -bonds.

O

O

LnCl3  þ  3LiR  LnR3  þ  3LiCl "

R2N

Nd

NR2

RO

Sm

NR2 O

OR OR

O

ð24:16Þ

In the presence of excess LiR and with R groups that are not too sterically demanding, reaction 24.16 may proceed further to give ½LnR4  or ½LnR6 3 (equations 24.17 and 24.18). THF;  21  218 8K

i

R = SiHMe2

R = 3,5- Pr2C6H3

(24.2)

(24.3)

YbCl3  þ 4t BuLi  ½LiðTHFÞ3 þ ½Ybt Bu4  þ 3LiCl "

ð24:17Þ DME;  19  195 5K

3 LuCl3  þ  6MeLi  ½LiðDMEÞþ þ 3LiCl 3  ½ LuMe6  "

DME=1,2-dimethoxyethane

24.8  Organometallic complexes of the lanthanoids Organolanthanoid chemistry is a rapidly expanding research area, and an exciting aspect of this area is the number of  efficien effic ientt cat cataly alysts sts for org organi anicc tra transf nsform ormati ations ons tha thatt hav havee been discovered (see  Box 24.5). In contrast to the extensive carbon car bonyl yl che chemis mistry try of the d -block -block met metals als (se (seee   Sections

ð24:18Þ

In the solid state, ½LuMe6 3 is octahedral (LuC ¼  253pm) and analogues for all the lanthanoids except Eu are known. In these reactions, a coordinating solvent such as DME or Me2 NCH2 CH2 NMe2   (TMED) (TMED) is nee needed ded to sta stabili bilize ze the

†

For details of inert atmosphere techniques, see: D.F. Shriver and M.A. Drezdon (1986)  The Manipulation of Air-sensitive Compounds , Wiley, New York.