Cyano-protonated complexes

Fig. 7. Optimized geometries of the most stable cyano-protonated complexes of [Fe(CN)sNO]3 and [Fe(CN)6HNO]3-.

Furthermore, protonation results in a significant distortion of the coordination polyhedron, i.e., the metal ion is displaced from the plane formed by the four cyano ligand carbon atoms toward the oxo along the M = 0 axis by as much as 0.34 A, which represents about 20% of the total metal-oxo bond length. In spite of this distortion stronger metal-cyano bonds are observed crystallographically, suggesting a better n back-donation by the metal center to the cyano carbons since d-ff overlap is increased. This observation is in line with both the 13C and 15N chemical shift and kinetic data (Section V) for the protonated complexes (8). 

The proton transfer processes described above induce interesting effects on the geometry of these metal complexes upon protonation (see also Section II). If it is assumed that the equatorial cyano ligands form a reference plane and are stationary for any of these distorted octahedral cyano oxo complexes, the protonation/deprotonation process as illustrated in Scheme 3 is responsible for the oxygen exchange at the oxo sites. This process effectively induces a dynamic oscillation of the metal center along the O-M-O axis at a rate defined by kmv, illustrated in Fig. 15. This rate of inversion is determined by the rate at which the proton is transferred via the bulk water from the one... 

Oxo cyano—MoIV complexes are well known and their chemistry has been extensively reviewed.652,724,725 Recent work has included the measurement of the proton, oxygen, and cyanide exchange rates for archetypal (all trans) [Mo02(CN)4]4, [MoO(OH)(CN)4]3, and... 

Finally, a good correlation also exists between the distortion induced by protonation, as evident from both the apical displacement of the Re(V) center from the equatorial cyano-carbon plane as well as the bond angle (O-Re-CN), deviating significantly from the ideal 90° as in the symmetrical dioxo complex, see Table I. 

Protonation of an oxo ligand in any of these four metal systems results in electron density changes on the metal centers in the proton-ated complexes. The decrease in electron density on any one oxo ligand upon protonation of the other oxo ligand trans thereof is clear from the 170 chemical shifts for all four metal systems (see Table II and III and Fig. 11). On the other hand, protonation in these systems results in an increase in the trans M = 0 bond strength as was shown crystallographically (see Table I), which in turn also results in an increase in electron density on the cyano carbon atoms as observed from the 13C chemical shifts. 

The different exchange processes described in Sections III-V can be combined to illustrate the reactivity of the different sites in these oxo cyano complexes as a function of pH and the possible interdependence thereof. The three processes that are compared are the inversion along the O-M-O axis (illustrated in Fig. 15 related to proton exchange), the oxygen exchange, and the cyanide exchange. 

The protons released are presumably available to compensate for the loss of the charge balancing cations within the zeolite. In conventional syntheses, the phtha-lonitrile condensation normally requires the nucleophilic attack of a strong base on the phthalonitrile cyano group [176, 177]. This function is presumably accommodated by the Si-O-Al (cation) basic sites within the ion-exchanged faujasite zeolites [178, 179]. The importance of this role is perhaps emphasized by the widespread use of alkali metal exchanged faujasites, particularly the more basic NaX materials of higher aluminium content [180, 181] as hosts for encapsulated phthalocyanine complexes. 

The H NMR spectra of the biscyano complex in micelles and in the absence of micelles do not show any significant difference in their general pattern though a small but distinct upfield shift in the heme methyl proton resonances is observed upon incorporation of the heme inside the micelle [22] (Fig. 8). The heme methyl signals for cyano (pyridinato) hemin in aqueous SDS appear further downfield than those for biscyano hemin, which has been attributed to increased asymmetry in the cyano (pyridinato) hemin complex [22]. Further, the upfield shift increases from SDS to CTAB, perhaps due to the change in... 

Fig. 14a-e. Proton NMR spectra of 5% aqueous sodium dodecyl sulphate (a) in aqueous solution in absence of any paramagnetic complex, (b) in presence of biscyano hemin (0.1 mM (c) in presence of aqua (hydroxo) hemin, (d) in presence of hydroxo (pyridinato) hemin and (e) in presence of cyano (pyridinato) hemin (Taken from Ref. 67)... 

Depending on the substituents, either elimination reactions or solvolysis leads to the product 36 or 37, respectively. The formation of addition product 37 can be explained by heterolytic cleavage under formation of a Co(III) complex and a carbanion, which is then protonated by the solvent. Owing to the carbanion-stabilizing ability of the cyano group this pathway is pronounced in the reaction with acrylonitrile. The product 39 is formed... 

Intermolecular addition of carbon nucleophiles to the ri2-pyrrolium complexes has shown limited success because of the decreased reactivity of the iminium moiety coupled with the acidity (pKa 18-20) of the ammine ligands on the osmium, the latter of which prohibits the use of robust nucleophiles. Addition of cyanide ion to the l-methyl-2//-pyr-rolium complex 32 occurs to give the 2-cyano-substituted 3-pyrroline complex 75 as one diastereomer (Figure 15). In contrast, the 1-methyl-3//-pyrrolium species 28, which possesses an acidic C-3-proton in an anti orientation, results in a significant (-30%) amount of deprotonation in addition to the 2-pyrroline complex 78 under the same reaction conditions. Uncharacteristically, 78 is isolated as a 3 2 ratio of isomers, presumably via epimerization at C-2.17 Other potential nucleophiles such as the conjugate base of malononitrile, potassium acetoacetate, and the silyl ketene acetal 2-methoxy-l-methyl-2-(trimethylsiloxy)-l-propene either do not react or result in deprotonation under ambient conditions. 

---