Programa de Educación Emocional

Aspects o f thiaether chemistry have been the subject o f several reviews. 7,•76•77•7,•79 It is generally observed that the simple thiaethers (R2S) are poor ligands; a problem surmounted by the use o f polythiaether macrocycles. Cyclic thiaethers have now been shown to bind a wide range o f transition metal ions to form stable complexes. An appraisal o f the nature of

1) Non-bonded lone p a ir effects

C o o r d in a te power decreases along the series R ,P > RjS > RC1. This can, in some part, be ascribed to the stereoelectronic repulsion resulting from the increasing number o f non­ bonding electron pairs on the donor atom and metal-based electrons.

2) x-bonding

Historically, the x-acidity o f second-row donor atoms was attributed to metal do n o r-d-

orbital back-bonding. Subsequent calculations have shown the 3d orbitals to be energetically unattainable. Recent studies have reconciled experiment with theory for phosphine donor complexes. The M -* P x-bonding employs P-X a* orbitals ( X « phosphine substituent) for electron acceptance.*0 It is likely that a similar rationalisation can be invoked for thiaether x-bonding. A further factor, not applicable to group 15 donors, is the existence o f a second non-bonded electron pair. In consequence, x-donation to empty metal d orbitals o f suitable symmetry, warrants consideration. Currently, there is little experimental evidence to support this notion.79

Evidence for the existence o f x-acidity in thiaether donor bonds has been exhaustively reviewed.*1 Properties examined include vibrational spectroscopy, stabilisation of low-spin states, nephelauxetic effects, thermal stability, photoelectron spectroscopy and redox potentials.

thiaether donor- bonding merits consideration o f 1) non-bonded lone pair effects, 2) x- bonding, 3) charge neutralisation.

The ff-donating ability o f a Lewis base is affected by its electronegativity, its size (for matching o f orbital energies and orbital overlap with the acceptor), dipole moment, and polarisability. The poor o-donor ability o f the thiaethers is manifest in the widespread coordination by "non-coordinating" species to metal centres whose coordination sphere is primarily thiaether, e.g. [Pb(9S3)2(C104)4] where two monodentate perchlorate anions remain bound to the lead(II) centre.*2 * The crown thiaethers provide an excellent means o f imposing a homoleptic thiaether environm ent around metal ions in which the c o o r d in a te properties o f the ligand have often been found not to match the typical stereochemical preferences o f the cation. This has permitted the study o f metal ion properties when bound in unusual coordination geometries. In part, this can be ascribed to the enthalpic cost imposed by exo to endo reorganisation required for metal ion encapsulation. In the brief summary o f the properties o f some sulfur macrocycles that follows, the examples included demonstrate the great structural variety of their complexes. The polythiaether macrocycles and their complexes have been the subject o f recent reviews.»j.«4.«s.»i

3) C h a rg e neutralisation

,4,7-trithiacyclononane

A [333] conformation with C , point symmetry was confirmed by the crystal structure reported in 1980.“ The endodentate geometry adopted by the ligand contrasts with other polythiaether macrocycles where the sulfur lone pairs are directed out o f the ring cavity. Bond lengths are C-S, 1-820(5), 1.823(5) À; C-C, 1-510(6) À; with a non-bonded S -S contact o f 3-451(2) À some 0-25 À less than the sum o f the van der Waals radii o f sulfur. 9S3 is atypical o f sulfur m acrocycles, being preorganised for coordination to the face o f a metal ion. Photoelectron spectroscopy confirms that the [333] conformation is retained in the gas phase.17 Oxidation o f 9S3 leads to bicyclic sulfonium cation formed via transannular C-S bond formation and C- H bond cleavage. Interestingly, a recent publication showed 9S3 coordinated to Re(VII), challenging the view that thiaether coordination is the preserve o f low substrate oxidation states.'* Despite a predominance o f facial, tridentate coordination for this ligand, other modes have been characterised, for example:

1) [C u « 9 S 3 )Jl * »

2) [Cu^SS),)]1* »

3) iIrH (9S3)J24 *'

- bis(tridentate).

- tridentate (x2), tridentate ligand bridging binuclear. - bidentate, tridentate.

1 ,4 ,7 ,1 0-tet ra t h ¡acyclododecane

T he [3333] conformation o f 12S4 was confirmed with the crystal and molecular structure determination by Robinson e t a l in 1988.” All sulfur atoms are located at the com er o f a square and all eight C-S bonds adopt gauche placements. An approximate D« symmetry is apparent. Semi-empirical molecular-mechanics calculations show the crystal coordinates to represent an essentially strain-free structure.” The range o f coordinative modes exhibited by for this ligand is illustrated by:

1) [A1(CH,),(12S4>] 94 - monodentate. 2) [Re(CO),( 12S4)]♦ ** - tridentate. 3) [Cu( 12S4)(OH j)]2 * » - tetradentate.

1,4,8,11 -tet rat h iucy clot et radecane

T he structure, reported in 1976 by DeSimone e t al, revealed that 14S4 crystallises in three conformations; all with sim ilar exo conformations and the sulfur atoms occupying the comers o f quadrilateral m olecules.97 This is entirely in accord with the tendency for S4 macrocycles to partake in exodentate co-ordination; endodentate ion encapsulation requires extensive ligand reorganisation. The a form contains one, and the fl form two independent molecules. The c o o r d i n a te versatility o f this molecule is best demonstrated by example: 1 2 3

1) [(HgClj),(14S4)] 99 - tetradentate, ligand bridging, binuclear. 2) [H g ( H ,0 )( 1 4 S 4 ) f 99 - tetradentate.

1,4,7,10,13-pentathiacyclopentadecane

In common with other macrocyclic polythiaethers the crystal structure o f this compound shows that an exo conformation is adopted with all sulfur atoms pointing out o f the ring cavity.101 The ring adopts an irregular C, symmetry. The structural chemistry o f this ligand has been less fully explored than for many other common thiamacrocycles. Reports include:

1) (ReBr(CO),(15S5)) ' • - bidentate. 2) [Cu(15S5)l* 1(0 - tetradentate. 3) [(15S5)Cd(C104)(0Hj)J * ,0* - pentadentate.

4) lAg2(15S5)J2* 105 - tetradentate / pentadentate, asymmetric bridging ligands, binuclear.

1,4,7,10,13,16-hexathiaeyclooctadecane

In contrast to the solid state structures o f the sulfur macrocycles with a smaller ring size (except 9S3), the single-crystal X-ray structure o f this compound shows that whilst four o f the six sulfur atom s adopt the expected exodentate geometry, the remaining two are directed into the macrocyclic cavity. All C-S bonds in 18S6 adopt gauche configurations. Examples o f structurally characterised complexes incorporating this ligand cover:

1) ICu2(CH,CN )2(1 8S6)]2+ - hexadentate (2x3) bridging ligand, binuclear. 2) [Co(18S6)]2+ 107 - hexadentate (meso isomer).

3) [ CuCl ( 18S6) ] n - bidentate bridging ligand, tetradentate bridging ligand, polymeric.