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(such as 6 in Fig. 1b). Copolymerizationo these unctional monomers producespolymers that can be crosslinked bydimerization o the pendant ketenes.

O course, the main motivation orproducing unctional polymers is theproduction o new technologies. One

example o how this method might be usedis demonstrated through the ormationo patterned thin lms (see Fig. 1c). Tepolymers proved to be stable and did notcrosslink under ambient conditions untilthe ketene was unmasked by pyrolysis othe Meldrums acid unctionality. Perhapssurprisingly given the reactivity oketenes residual ketene groups were

observed in the lms. It is proposed thatthese groups could be used to provideurther unctionality by reaction withnucleophiles as described above.

Tis research undoubtedly haspotential as a versatile way o introducingboth crosslinking and unctionality or

polymeric materials as part o a post-synthetic modication strategy, provided theapplication o interest can tolerate the hightemperature o the pyrolysis process and therelease o volatile by-products.

Steve Rimmer is in the Polymer Centre, Department

o Chemistry at the University o Sheeld, Sheeld,

S3 7HF, UK. email: s.rimmer@sheeld.ac.uk

Reerences

1. Marimuthu, M. & Kim, S. Curr. Nanosci. 5, 189203 (2009).

2. Place, E. S., George, J. H., Williams, C. K. & Stevens M. M.

Chem. Soc. Rev. 38, 11391151(2009).

3. Perlin, L., MacNeil, S. & Rimmer, S.Sof Matter

4, 23312349 (2008).

4. Jagur-Grodzinski, J. Polym. Adv. echnol.21, 2747 (2010).

5. Dhal, P. K., Polomoscanik, S. C., Avila, L. Z.,

Holmes-Farley, S. R. & Miller, R. J. Adv. Drug Deliv. Rev.61, 11211130 (2009).

6. Ghanbari, H. et al. rends Biotechnol.27, 359367 (2009).

7. Leibarth, F. A. et al. Nature Chem. 2, 207212 (2010).

8. Meldrum, A. N.J. Chem. Soc. 93, 598601 (1908).

9. Moad, G. & Solomon, D. H. Te Chemistry o Radical

Polymerization (Elsevier, 2005).

10. Kosravi, E. & Szymanska-Buzar, . (eds) Ring Opening Metathesis

Polymerisation and Related Chemistry: State o the Art and Visions

or the New Century (Kluwer Academic, 2002).

Isomerism is a central concept inchemistry and all who study this brancho science learn that molecules o the same

elemental composition may dier in the

way their constituent atoms are connectedby chemical bonds (constitutional isomers)or in the spatial arrangement o thechemical bonds (stereoisomers). Te latterclass can also be subdivided to distinguishdiastereomers (or example, cistrans- orEZ-isomers, conormers or rotamers) rommolecules that are mirror images o eachother (enantiomers).

Another, less amiliar class o isomersare electro-isomers (known also by theirshort name, electromers). Tese are pairso molecules that do not dier in theirconstitution, nor much in the spatial

disposition o the bonds between theatoms, but solely in the way the electronsare distributed amongst the atoms and thebonds that link them.

Te most obvious class o electromersare excited electronic states, but as thosehave usually only a very feeting existence,they cannot be put on an equal ooting withdierent stable ground-state structures. Inprinciple, states o dierent multiplicitysuch as those that occur in spin crossovercompounds or example, may be regardedas electromers, but in that research eld theterm spin isomerism is used to describesuch species.

Electromers o the same multiplicityare most oen encountered in transitionmetal complexes carrying ligands thatcan exist in dierent oxidation states,

such as the phenoxy/phenolate orthe quinone/semiquinone/catecholgroups1. In some such cases, equilibriabetween distinct electromers o the ormMz(Lny) Mze(Lnz+e) (where z,y and ereer to charges) can be observed to shion photolysis, or on changing externalparameters such as temperature orpressure (ie > 1, the e electrons may betranserred to n dierent ligand molecules).Unortunately, such electromers haverecently been named valence tautomers1,

which can lead to some conusion becausein organic chemistry the term tautomer

is reserved or isomers that dier in theirbonding pattern to hydrogen.Tere are, however, some cases where

electromerism is o a more subtle nature.Now writing inAngewandte ChemieInternational Edition, Grtzmacher, deBruin and colleagues present an intriguingorganometallic case in which the electromersdo not require redox-active ligands2.

Tey studied a rhodium complexcarrying bis(dibenzotropyl)phosphine(where the phosphorous atom is denotedP1) and triphenylphospine (P2) ligandswith the arrangement shown in Fig. 1a.Te complex was rst prepared in its

cationic orm, which is stable i pairedwith the very non-nucleophilic B[3,5-(CF3)2C6H3]4 (BARF) anion. On one-electron reduction using elemental

sodium, sodium naphthalenide or CoCp*2(Cp* = pentamethylcyclopentadienyl),two distinct radicals were ormed thatGrtzmacher, de Bruin and co-workersnamed L and M. Te two species wereully characterized by dierent orms oelectron paramagnetic resonance (EPR)spectroscopy, and the L M equilibriumcould be reversibly shied rom 66% o L at340 K to 88% o L at 190 K.

o understand the structure o theradicals, density unctional theory (DF)calculations were carried out that revealedtwo distinct potential energy minima

within approximately 2 kcal mol

1

, L

calcand Mcalc (in addition to a third one, Lcalc,which, however, diers rom Lcalc only byrotation around the RhP2Ph3 bond). Apartrom the RhP2Ph3 bond length which is0.047 longer in Lcalc the two isomershave very similar bond lengths, but Mcalcdistinguishes itsel by having a much smallerP1RhP2 angle, which gives it a structuresimilar to a trigonal bipyramidal complexsuch as is observed when the P1Ph moietyis replaced by the more electron-donatingSiMe group (Fig. 1a).

Te dierences in the electronic structureo L and M express themselves most clearly

ISOMERISM

The same but diferentElectromerism is an unamiliar concept to many chemists and reers to molecules that are not conventional isomersbut instead difer in how the electrons are distributed across their structure. A novel example o such electromers

has now been demonstrated.

Thomas Bally

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in their EPR spectra, especially the dierencein the hyperne coupling constants to the 31P1atom in Fig. 1a (Aiso = 53 MHz or L versus602 MHz or M) and, to a lesser degree, inthat to the103Rh atom (Aiso = 10 MHz or Lversus 3 MHz or M). Te DF calculationsshow, however, that the changes inAiso donot translate directly into changes in spindensity on these atoms, presumably due tothe changes in hybridization at the rhodiumatom: although the calculated spin densityon the P1 atom increases rom 12% to 28%

on going rom L

to M

, so does that onrhodium (rom 33 to 58%).More importantly, spin density shis

rom the hydrocarbon parts o the ligands which carry 46% o the spin in L tothe central P1RhP2 moiety, which carriesnearly 90% o the spin in M (with more

than hal o that on the rhodium atom) see Fig. 1b. Tis leads Grtzmacher, deBruin and co-workers to describe L as astrongly delocalized organometallic radicalwhereas they consider M as a metalloradical with a high localization o spin in theRhP1 bond.

In a molecular orbital (MO) picture,the odd electron in L occupies anMO that is localized mostly on the twoP1-(dibenzotropyl) ligands, whereas inM it resides in an MO that is localized

mostly on the Rh- and the P

1

-atom. Withthis perspective, M can be regarded asa low-lying excited state o L that gainsa spectroscopically palpable existenceby virtue o a barrier (calculated asapproximately 3 kcal mol1) that separates itrom its electronic isomer L.

As electronic reorganizations areinherently barrierless, the barrier thatseparates M rom L must arise becausethe ground state o M correlates with anexcited state o L along the deormationthat leads rom the structure o M to thestructure o L, and vice versa. Indeed, it

would be interesting to know where theelectronic state o L, which correspondsto the electronic structure o M, lies andvice versa, but Grtzmacher, de Bruin andco-workers do not give any inormation onthat. Te rhodium complex described does,however, demonstrate that electromerism inorganometallic chemisty is not restricted tocomplexes carrying redox-active ligands. Itcan occur in cases where an unpaired electronoccupies one or another orbital o a complex,provided that there is a suitable distortionthat stabilizes the excited state to the extentthat it becomes an isomeric ground state.

Tis situation is reminiscent o the onethat prevails in the open-shell compoundsthat are subjected to Jahneller distortion,except that in those cases, only one othe electromers usually corresponds toa potential energy minimum, whereasthe other is a transition state or itsautomerization3. In recent research4 we haveobserved a case o true electromerism in apair o organic radical cations containingtwo allylic moieties. Te two electromersdistinguish themselves by the nature o thesingly occupied MO (SOMO), which inone case corresponds to the positive, and

in the other to the negative combinationo the constituent allylic SOMOs. Tisexample shows that the phenomenon oelectromerism is not bound to the presenceo transition metals and, as Grtzmacher,de Bruin and co-workers have shown, toredox-active ligands and thus it may bemore prevalent throughout chemistry thanwe might think.

Tomas Bally is in the Department o Chemistry,

University o Fribourg, Prolles, CH1700 Fribourg,

Switzerland. email: Tomas.Bally@unir.ch

Reerences1. Evangelista, E. & Ruiz-Molina, D. Eur. J. Inorg. Chem. 2005,

29572971 (2005).

2. Puschmann, F. F. et al.Angew. Chem. Int. Ed. 49, 385389 (2010).

3. Herzberg, G. Electronic Spectra and Electric Structure o

Polyatomic Molecules (Krieger, 1991).

4. Mller, B., Bally, ., Gerson, F., de Meijere, A. & von Seebach, M.

J. Am. Chem. Soc. 125, 1337613783 (2003).

a

b

=

P1

Ph

Rh

P2Ph3

BARF

P1Ph

Rh

P2Ph3

M

122.0

P1Ph

Rh

P2Ph3

L

165.5

Reduction +

P1Ph

P1Ph

ML

Ph

PhPh

Ph

Ph

Ph

Fg 1 | Electromerism in an organometallic compound. a. Formation and structures o radicals L and

M. The benzo rings that are annellated to the tropyl ligands are omitted or clarity and replaced with red

double bonds. b. The diferent spin densities (shown in blue) o the electromeric radicals L and M. Part b

reproduced with permission rom re. 2, Wiley 2010.

20 Macmillan Publishers Limited. All rights reserved10