Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 1355 149

Abboud, Taft and collaborators546 to define the ˛H2 and ˇ2H scales of HB acidities and basicities.

Quite generally, and for media ranging from the gas phase to CCl4, it is known547 that for thousands of (AH, B) systems, the bilinear equation 161b yields log10 Kc values with a satisfactory degree of precision:

wherein a and b are constants.

Experimental data on the HB basicity of the compounds most relevant to this chapter are scarce. Abboud and colleagues548 summarized the available data in 1988. They also determined new constants for the HB associations between 3,4-dinitrophenol and some thiocarbonyl compounds. Recently, Laurence and coworkers have published new results, particularly for thioamides and thioureas549. Some representative data are given in Table 19.

This table shows that: (i) CDS compounds are significantly weaker HB bases than their CDO homologs, a situation similar to that prevailing for S(sp3) and O(sp3) bases548. (ii) The dependence of ˇ2H on substitution needs more data to be fully understood. Thus, it seems that for weakly basic compounds the sensitivity of CDO compounds to substituent effects is more important than that of CDS while for the most basic compounds, sensitivities are nearly the same. (iii) 2,4-bisdimethylamino-4-methyl-1-thia-3-azabutadiene (154) is the strongest thiocarbonyl HB base ever reported.

In cases of intermolecular HB the situation is convoluted. On the basis of the available ˛H2 and ˇ2H parameters we know that, for homologous series of monofunctional compounds, ˛H2 OH > ˛H2 SH and ˇ2H CDS > ˇ2H CDO . The orders of magni-

tude are such, however, that the product ˛H2 OH Ð ˇ2H CDS is larger than the product ˛H2 SH Ð ˇ2H CDO . On the basis of equation 161b this fact, alone, would tend to shift the equilibrium towards the enol form of thioxoketones and thioxoaldehydes Of course, other factors come into play (see Section II).

4. Lithium cation affinities

A wealth of thermodynamic data are available for reaction 162 in the gas phase.

aFrom Reference 549.

bFrom Reference 546.

1466 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

Such information is not yet available for CDS compounds, although Alcami and coworkers116 and Speers and Laidig550 have reported results of theoretical studies, which include a number of G2(MP2) calculations on both families of compounds116. They show that, while the binding of LiC(g) involves essentially electrostatic interactions in both cases, the structures and energetics of the LiC adducts of CDO and of CDS are profoundly different. At the MP2/ 6-31CG(d, p) level, for example, LiC is found along the C2 symmetry axis of H2CO and close to the oxygen atom. In the case of thioformaldehyde, LiC is located near the sulfur atom, but now the LiSC angle amounts to 107.5°. This indicates that in the first case the ion is located between the lone pairs of the oxygen, while in the second, it is aligned with one of the sulfur lone pairs. A detailed discussion of various other carbonyl and thiocarbonyl compounds is given116. From the thermodynamic point of view, the lithium cation affinities of thiocarbonyls are appreciably smaller than those of carbonyls, at variance with the behavior towards the proton.

J. Synthesis and Reactivity in a Mass Spectrometer

Over these last few years, a powerful technique, neutralization reionization mass spectrometry (NRMS), has been developed that allows the generation and study of highly reactive species20 22,551. This technique has obviously not reached the ‘preparative’ scale of FVP but, nevertheless, it has led to some important discoveries. Typically, instruments of this sort are built around a multisector mass spectrometer552. Ions are generated by ionization of neutral species, ion dissociation or ion molecule reactions553. These primary ions can be characterized by appropriate techniques such as collision-induced dissociation (CID)554. Then, they are selected and separated under the influence of electric and magnetic fields. The selected ions are neutralized (e.g. with metal vapor or Xe) in the first collision chamber and the remaining ions are ejected. The fast neutral beam thus obtained undergoes collisions with a target neutral gas, such as O2, in the second collision chamber, this leading to a reionization process. The ions thus generated are mass-analyzed and identified.

An example is provided by thioxoethylenone, ODCDCDS. Its radical cation (155) can be generated555 by electron impact (70 eV) on its precursor (156) (equation 163).

In the first experiment, 155 thus generated was decomposed by collision. In the second one, the ion upon neutralization led to ODCDCDS which, in turn, was reionized and collision-decomposed. The MS obtained in both cases were essentially identical, showing the existence and stability (within the time scale of the experiment) of both ODCDCDS and 155.

In the field of CDS chemistry this method has proven to be quite useful to generate and identify species such as: CDCDS556, SDCDCDS176 and SDCDCDCDCDS556.

Later on, Schwarz and coworkers557 have described the generation of both evenand odd-numbered polycarbon disulfides S(Cn)S with n D 2 6.

Flammang, Wong, Wentrup and collaboration have reported the preparation and characterization of (methylimino)ethenethione, CH3NDCDCDS and iminoethenethione, HNDCDCDS558, and of their oxygen homologues559. These compounds are stable on the microsecond time scale. They were also studied theoretically, at the G2(MP2) level.

V. COORDINATION CHEMISTRY

The versatility of sulfur as the heteroatom in hetero-organic ligands has been the source of the diversity of organosulfur ligands known today30. Since Schaumann’s chapter1, there have been several studies about coordination complexes involving thioketenes and

thioaldehydes as ligands. For example, complexes of thioketenes with Mo560, Os561,562, W563 565, Co566, Fe566,567, Ru568, Ti569 and of thioaldehydes with Mo560, Re570, Ti571,572, W502,573,574 and Ta575 have been reported.

To obtain thioaldehyde complexes, Muraoka and coworkers573 have synthesized a new reagent [PPh4][W(CO)5SH] according reaction 164:

Reagents and conditions: (i) AgNO3 H2O, THF, room temp., 30 s (ca 100%); (ii) [PPh4]SH EtOH, THF,20 °C, 30 s 95%.

Aromatic thioaldehyde pentacarbonyltungsten(0) complexes were synthesized by treatment of N-phenyl or N-cyclohexyl imines of aryl aldehydes with [PPh4][W(CO)5SH] in CH2Cl2 or C6H6 in the presence of BF3 Ð OEt2 and MeCO2H (reaction 165):

Reagents and conditions: [PPh4][W(CO)5SH], BF3 Ð OEt2 MeCO2H, C6H6 or CH2Cl2,room temp.

Similar pentacarbonyltungsten(0) complexes of heteroaromatic thioaldehydes, such as 2-thioformylfuran 2-thioformylthiophene, thioarylenals or thioaryldienals, were also synthesized573.

Heterocyclic thiones have been extensively studied because of their facility to yield coordination complexes. All the ligands contain thione and occasionally thiol (mercapto) groups directly attached to the carbon atoms of heterocyclic molecules. They have been previously reviewed by Raper in 1985576, and very recently the same author has reviewed28 the copper complexes of heterocyclic thioamides and related ligands.

The combination of an exocyclic thione group and a heterocyclic molecule, which may contain nitrogen, oxygen, sulfur or a combination thereof, generates a group of molecules with considerable coordination potential.

An important factor in realizing such potential is that of prototropic tautomerism and, in particular, which tautomer is present in solution immediately prior to the formation of the metal ligand bond576. A common feature of all nitrogen-containing heterocyclic thiones is thione (157c) thiol (157b) tautomerism.

As already discussed above, the thione thiol equilibrium is dependent on environmental factors with the thiol form favored in the gas phase and nonpolar solvents, and the thione form favored in the solid state and polar solvents.

1468 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

Deprotonation of heterocyclic ‘thiones’ (157c) produces the corresponding ‘thionate’ ion (157d) in which an electron pair on the heterocyclic trigonal nitrogen and three electron pairs on the thionate sulfur generate considerable coordination potential (158).

Among the heterocyclic thiones, we present in Tables 20 25 the metal complexes of some of the thiocarbonyl ligands most widely used, which are represented in Figure 3. In these tables, data are given from 1988. In the case of copper, only complexes reported since Raper’s review28 are given.

FIGURE 3. Thiocarbonyl ligands involved in Tables 20 25: (a) pyridine-2-thione, (b) pyridine- 4-thione, (c) pyrimidine-2-thione, (d) 1,3-imidazoline-2-thione, (e) benz-1,3-imidazoline-2-thione,

(f) 1,3-imidazolidine-2-thione, (g) 1,3-thiazoline-2-thione, (h) benz-1,3-thiazoline-2-thione, (i) 1,3- thiazolidine-2-thione, (j) 1,2,4-triazoline-3(5)-thione, (k) 1,2,3,4-tetrazoline-5-thione

1470 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

TABLE 20. (continued)

1472 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

TABLE 22. (continued)

1474 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

TABLE 25. Metal complexes of tetrazoline-thiones