Apical substituents

Apical bonds are longer, and the substituents are less strongly bonded to phosphorus than the equatorial substituents. Apical substituents therefore tend to undergo exchange with nucleophiles or eliminate as leaving groups. 

The alkylidynetricobalt nonacarbonyl complexes all are highly colored, with colors ranging from red to purple to brown to black, depending on the apical substituent. Their thermal stability also depends on the apical substituent some survive heating to 100°-18o° and many may be sublimed in vacuum at 50°-80°C. Most, but not all, are air-stable (in contrast to their pyrophoric parent, dicobalt octacarbonyl) in the solid and in solution. 

Symbols of the apical substituents in the capping fragments of sarcophaginates and sepulchrates. 

In the synthesis of amidine-functionalized cobalt(III) NsSs-sarco-phaginates, malononitrile was used as a bifunctional carbon acid. The interaction of its pendant nitrile with the coordinated amino group of the [Co(ten)]3+ semiclathrochelate led to the formation of an amidine NsSs-cage with an amide apical substituent. 

Semiclathrochelate N3S3- and Nc-complexes with one of the tetrapodal units as apical substituent were prepared by the interaction of cobalt(III) ion with potentially octadentate tetrapodal N4S4- and Ns-ligands (Scheme 62). Cyclization of these complexes... 

The synthesized complex was easily reduced to the rhodium(II) sarcophaginate. The rhodium sarcophaginates 1 and 2 with other apical substituents were also prepared (Scheme 70). 

The most bulky fragmental ions resulted from heterolytic bond rupture and withdrawal of the apical substituent (OH-group) from a boron-containing capping fragment after addition of the proton ... 

It has been pointed out [52] that there is a correlation between the Hammet S (Tpara constants of the apical substituents and the redox potentials of the corresponding iron clathrochelates. Meanwhile, such a correlation is observed only inside the two groups of complexes with (a) OH, OCH3, F, Cl, and Br and (b) CH3, CsHs, and i-C4H9 substituents. No reasons are given for the existence of two groups of complexes. In spite of the fact that the cyclic voltammetry evidence indicates that the Fe +/ + couple is quasi-reversible, the electrolytic oxidation has failed to isolate stable iron(III) complexes [52]. 

The redox processes in macrobicyclic cobalt dioximates are reversible their redox potentials are higher than those of the [Co(sar)]3+/2+ couple, and this can be attributed to relative stabilization of the cobalt(II) oxidation state. As consequence, the cobalt dioximates have been isolated in both +2 and +3 oxidation states. The data on redox potentials of macrobicyclic cobalt dioximates are not sufficient to establish dependence on electronic characteristics of the apical substituents. Yet in this case, a distinct growth in the Eyz values was observed on increasing the ai value of... 

For amine cobalt complexes, the E values correlate with the calculated differences in strain energy for cobalt(III)/cobalt(II) complex pair. The structures of all the cobalt(III) complexes are more strained than those of the corresponding cobalt(II) complexes. The correlation does not hold when the number of protons in the coordinated amino groups is changed (for example, [Co(NH3)6], [Co(en)3] ", [Co(sep)]3+, and [Co(sar)]3+ cations). For a series of Ns-sarcophaginates with different apical substituents, no calculation of differences in strain energy changes has been done [344]. 

The E values depend on the electron density on the encapsulated metal ion and on donor atoms, and as a result, correlate a sum of the inductive constants of apical substituents. The slope of three linear correlation regressions for Ns-sarcophaginates (including a sepulchrate) and for sarta.cn and capten (higher potentials) and absar (lower potentials) ligand complexes is the same hence, in each group the E values are governed by an inductive effect. 

Fig. 53. Correlation of cte for apical substituents of the macrobicychc cobalt (III) complexes with Grob s cr values [352].

The redox potentials are determined by the differences in energy between two cobalt(III) and cobalt(II) states as a function of the apical substituents. Therefore, the electrochemical characteristics of the substituents should also correlate with other physical properties of the individual oxidation states provided these physical properties are affected by the substituents via the same mechanism [352]. 

Fig. 56. Plot of Log ku (1) and ai values (2) versus the Hammet s apara constants of apical substituents.

Macrobicyclic cobalt compounds satisfy all these requirements. Their additional advantage is that the redox potentials and electron-transfer rates may be varied on introduction of different apical substituents, by changing the charge of the complexes via protonation or deprotonation, or by altering steric factors. This allows one to select the most suitable complexes as ETAs. 

The structure of chlorophosphoranes - have been studied and a linear relationship was found between vp (asym) and the sum of the electronegativities of the apical substituents. The Raman spectra of the bis-trichloromethylphos-phorane (63) supported the presence of diapical trichloromethyl groups. Also the P-Cl force constant was lower than that for the monotrichloromethyl-phosphorane in accordance with a lower electronegativity for the apical groups. ... 

In place of two apical and three equatorial substituents, SP phosphorus intermediates possess one axial and four basal substituents. The axial substituent is similar to its equatorial counterpart, in that the bond with phosphorus is shorter and stronger. The four basal bonds, being longer and weaker, are akin to apical substituents, and so it is assumed that groups will enter and leave from the basal position in an SP intermediate. Holmes has stated that pseudorotation is not a required process for either retention or inversion. This is explained diagrammatically in Scheme 2(a), and derives... 

Table 1 Correlation of electronegativity with additive substituent effects for equatorial and apical substituents in phosphorus TBPs."...

As the Eqs. 2 and 3 indicate, electron transfer from the excited sensitizer (S ) to Co(lll) will be favored if the Co(III)/Co(ll) potential is less negative, whereas a more negative potential will favor H2 production. These two opposing influences are optimized in N3S3 donors with different apical substituents and they have been used for dihydrogen production from water at modest rates (61, 62). 

Stabilizing force for the anion. These hyperconjugative interactions are stronger for the equatorial OR groups where the donor lone pair and the acceptor o orbitals are brought closer as a result of the shorter P-O distances. As the P-0 distance between the donor and the acceptor decrease for the apical substituent, the stabilizing effect of anometic interaction greatly decreases as weU. As a consequence, the formation of an apical anion is less favorable. 

Apical — / substituents should have the converse effect, the apical carbon atom becoming more positive on forming the n complex. The tendency of alkyl groups to migrate ( migratory aptitude ) therefore increases in the order CH3 < primary alkyl < secondary alkyl < tertiary alkyl. 

To describe the polarization and transformations of a pentacoordinated transition state, given by an external field, two model reactions of a competitive interaction between the apical substituents were chosen. In the first one, with equal substituents, the polarization of the phosphorus atom arises from the conformation of the phosphoryl moiety itself and not from the apical substituents which are the same. In the second model, the asymmetry results from the different electrodonor ability of the axial substituents (Scheme 7). 

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