Soft Lewis acids

Donor strengths, taken from ref. 207b, based upon the solvent effect on the symmetric stretching frequency of the soft Lewis acid HgBr2. Gutmann s donor number taken from ref 207b, based upon AHr for the process of coordination of an isolated solvent molecule to the moderately hard SbCL molecule in dichioroethane. ° Bulk donor number calculated as described in ref 209 from the solvent effect on the adsorption spectrum of VO(acac)2. Taken from ref 58, based on the NMR chemical shift of triethylphosphine oxide in the respective pure solvent. Taken from ref 61, based on the solvatochromic shift of a pyridinium-A-phenoxide betaine dye. 

With soft Lewis acids, phenotellurazines 43 form molecular complexes in which the tellurium center serves as the ligating atom [82DOK(266)1164 85KGS757]. The 1 1 complexes 49 are formed with salts of Hg(II) and Ag(I). PdCb reacts with 43 to give 1 2 complexes 50. Whereas the Hg(II) and Pd(II) complexes are stable on exposure to air, the Ag(I) complexes rapidly decompose when isolated from their solutions. 

In recent studies, we found that primary and secondary amines, alone or somehow faster in the presence of the soft Lewis acid Ag+ (refs. 7-9) substitute the bromine in (S)-2-bromopropanamides, in an organic solvent, at room temperature, and yield N-alkyl-, and N,N -dialky 1-aminopropanamides, with inversion of configuration and high enantiomeric excess. Conversely, in the presence of silver oxide (Ag20), much faster reactions occur with retention of configuration, giving... 

Once acids and bases have been classified as hard or soft, a simple rule can be given hard acids prefer to bond to hard bases, and soft acids prefer to bond to soft bases (the HSAB principle)P The rule has nothing to do with acid or base strength but merely says that the product A—B will have extra stability if both A and B are hard or if both are soft. Another rule is that a soft Lewis acid and a soft Lewis base... 

A soft Lewis acid has a relatively high polarizability. Large atoms and low oxidation states often convey softness. Contrast with Hg , a typical soft acid. The ionic radius of Hg is 116 pm, almost twice the size of... 

Soft Lewis acids tend to combine with soft Lewis bases. 

Metals that are soft Lewis acids, for example cadmium, mercury, and lead, are extremely hazardous to living organisms. Tin, in contrast, is not. One reason is that tin oxide is highly insoluble, so tin seldom is found at measurable levels in aqueous solution. Perhaps more important, the toxic metals generally act by binding to sulfur in essential enz Tnes. Tin is a harder Lewis acid than the other heavy metals, so it has a lower affinity for sulfur, a relatively soft Lewis base. 

C21-0010. Indium is a relatively soft Lewis acid. Use this fact to predict properties of indium compared with its horizontal neighbor (tin) and its diagonal neighbor (lead) in the periodic table. 

Cu2+(aq) to 0.9 in [Cu(tetb)]2+. There is also a modest increase in softness due to changes in EA and CA parameters, which can be interpreted in terms of the symbiosis proposed by Jorgensen (33). The low Da parameter for [Cu(tetb)]2+ suggests that the main effect in transforming Cu(II) to a soft Lewis acid in [Cu(tetb)]2+ is the altered steric situation in coordination of bulky donor atoms to the metal ion. 

Metal cations can lend electrophilic assistance to weaken the Pd—X bonds in the intermediate R-Pd —X. Either full fission of this bond, leading to the realization of a polar mechanism, or partial polarization, might take place. Soft Lewis acids (the cations of Cu, Ag, Tl) are used most often (see Chapter 9.8 for a discussion of how metal ions act as Lewis-acid catalysts). 

Earlier it was described how PH3 is a much weaker base than NH3. That is certainly true when the interaction of these molecules with H+ is considered. However, if the electron pair acceptor is Pt2+, the situation is quite different. In this case, the Pt2+ ion is large and has a low charge, so it is considered to be a soft (polarizable) Lewis acid. Interaction between Pt2+ and PH3 provides a more stable bond that when NH3 bonds to Pt2+. In other words, the soft electron acceptor, Pt2+, bonds better to the softer electron donor, PH3, than it does to NH3. The hard-soft interaction principle does not say that soft Lewis acids will not interact with hard Lewis bases. In fact, they will interact, but this is not the most favored type of interaction. 

There are numerous similarities between ammonia and phosphine, but the latter is a much weaker base (see Chapter 9). In fact, phosphonium salts are stabilized by large anions that are also the conjugates of strong acids. Accordingly, the most common phosphonium salts are the iodides, bromides, tetrafluoroborates, and so forth. Phosphine and substituted phosphines are good Lewis bases toward soft Lewis acids, and many coordination compounds of this type are known. 

This class of donor is activated by soft Lewis acids, such as copper triflate at room temperature, and despite their hydrolytic instability, they appear inert to conditions of sulfoxide activation, TMSOTf or Tf20 (Scheme 4.53). Activation is achieved with stoichiometric promoter in the presence of the acceptor alcohol, and although the mechanism has not been investigated, presumably it proceeds via coordination followed by collapse to a stabilized oxacarbenium ion. The method is compatible with standard glycosidation solvents such as dichloromethane, acetonitrile and diethyl ether, and ester-directed couplings do not lead to orthoesters, perhaps as a result of the presence of the Lewis acid promoter [303,304]. 

The Lewis acidic properties of chiral Cu(I) complexes have recently been exploited to augment the chemistry of the more developed Cu(II) Lewis acids. As a corollary, much less is known about the chemistry of Cu(I) Lewis acids. They have traditionally been categorized as soft Lewis acids, compared to the borderline-hard Cu(II) Lewis acids (1). 

The principle of hard and soft Lewis acids and bases, proposed by Pearson (1963), is useful to describe these reactions. A Lewis acid is any chemical species that employs an empty electronic orbital available for reaction, while a Lewis base is any chemical species that employs a doubly occupied electronic orbital in a reaction. Lewis acids and bases can be neutral molecules, simple or complex ions, or neutral or charged macromolecules. The proton and all metal cations of interest in subsurface aqueous solutions are Lewis acids. Lewis bases include H, O, oxyanions, and organic N, S, and P electron donors. A list of selected hard and soft Lewis acids and bases found in soil solutions is presented in Table 6.1. 

An inner-sphere complex is formed between Lewis acids and bases, while an outer-sphere complex involves a water molecule interposed between the acid and the base. A hard Lewis acid is a molecular unit of small size, high oxidation state, high electronegativity, and low polarizability whereas a soft Lewis acid is a molecular unit of relatively large size, characterized by low oxidation state, low electronegativity, and high polarizability. Based on this characterization, hard bases prefer to complex hard acids, and soft bases prefer to complex soft acids, under similar conditions of acid-base strength. 

Table 6.1 Hard and soft Lewis acids and bases in solution (Sposito 1981)...

Scheme 1 (a) An antibonding o molecular orbital is thought to be the origin of the soft Lewis acidity of BiX3. ED electron deficiency, (b) Alternative mode of action in situ release of TfOH as the catalytic active species... 

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