Mixed cryptates

Low oxidation state complexes are usually stabilised by r-acid ligands such as CO, PR3 (phosphines), olefins etc. The use of the word acid here refers to Lewis acidity, i.e. electron acceptance. Binding to r-acid ligands occurs in a synergic fashion that comprises two components  

forward donation from the olefin r-bonding orbital to an empty metalp-orbital  

back donation from filled metal rf-orbital into olefin antibonding orbital. 

This has the following observable effects in alkenes such as ethene  

---

Reinen D, Atanasov M (2004) The Angular Overlap Model and Vibronic Coupling in Treating s-p and d-s Mixing - a DFT Study 107 159-178 Reisfeld R (2003) Rare Earth Ions Their Spectroscopy of Cryptates and Related Complexes in Glasses 106 209-237 Renz F, see Gutlich P (2004) 107 27-76 Reyes M, see Contreras RR (2003) 106 71-79 Ricciardi G, see Rosa A (2004) 112 49-116... 

We began this feasibility project with cryptand [2.2.2] and the monovalent Ag - 110m and divalent Sr-85 cations. These two metal ions have diameters of approximately 0.250 and 0.254 nm, respectively (14). The internal diameter of [2.2.2] has been estimated by CPK space-filling models to be 0.28 nm (3,15). Cryptate complexes form readily upon mixing a solution containing the tracer cation with a solution of cryptand. In order to insure complete complexation we used a molar ratio of ligand to ion of three. 

The subject matter of this chapter will be subdivided into sections concerning template synthesis of the complexes structural and thermodynamic properties of the complexes with synthetic cyclic polyamines complexes with mixed-donor macrocycles reactivity of the complexes cryptates and complexes with phthalocyanines and porphyrins. 

Lateral macrobicycles are dissymmetric by design thus, monoelectronic reduction of the Cu(ll) ion bound to the [12]-N2S2 macrocyclic subunit in the bis-Cu(ll) cryptate 45, gives a mixed valence Cu(i)-Cu(ll) complex [4.6]. Macrotricycle 46 forms a dinuclear Cu(ll) cryptate that acts as a dielectronic receptor and exchanges two electrons in a single electrochemical wave [4.7]. Complexes of type 47 combine a redox centre and a Lewis acid centre for the potential activation of a bound substrate [4.8]. 

In the analogous tetrahedral dinegative oxoanion cryptates a similar mix of direct and indirect water-bridged H-bonds links the encapsulated oxoanion with the NH1 donors. The direct NH+ 0" contacts are (Table 1) on average, slightly shorter than those... 

Cryptand 193 forms a cryptate containing two copper(II) ions. The reduction potentials of the two ions would be expected to be vastly different, because the Cu2 + complex of 194 is reduced at a potential 500 mV less positive than the Cu2 + complex of 195. This is indeed found to be the case. The dinuclear Cu2+ complex of 193 undergoes a one-electron reduction at +550 mV, The second Cu2+ ion is reduced at +70 mV. Therefore, the first reduction must be that of the Cu2+ ion complexed by the [12]-N2S2 subunit, resulting in the facile formation of a Cu(I)-Cu(II) mixed valence dinuclear cryptate (196). This system suggests the possibility of the formation of heterometallic dinuclear complexes120). 

A mixed-valence silver(I)-silver(III) cryptate complex has been synthesized by condensation of tris(3-aminopropyl)amme and terephthaldehyde in the presence of AgNOs. Other pyridine silver(III) complexes have been obtained by oxidation of silver nitrate and ammonia with ammonimn peroxydisulfate in aqueous ammonia solution. An air-stable diamagnetic silver(III) complex of a N-confused tetraphenylporphyrin, 5,10,15,20-tetraphenyl-2-aza-21-carboporphyrin argentate(III), has been described. ... 

Other lipophilic cations that have been approved as myocardial perfusion agents include a Tc(III) Tc-teboroxime cryptate, (Cardiotec ), the trans-dioxo technetium(V) bis-phosphine complex, "Tc-tetrofosmin (Myoview ) and the mixed ligand Tc(III) Schiff-base bisphosphine Tc-Q12 complex, Tc-furifosmin (TechneScan) (Figure 3). 

The dissociation reactions of TE cryptates in water (and other solvents) have been followed conductimetrically in stopped-flow experiments (67). An equilibrium mixture of TE cryptate, with a concentration ratio of TE to cryptate > 1, was mixed with an excess of acid (HCIO4, CF3SO3H). The pseudo-first-order rate constant, k, was found to be linearly dependent on the acid concentration ... 

The X-ray crystal structures of the lO-Na" " and 10 Ag" " complexes were solved 1 Professor Atwood[24]. Both complexes are typical cryptates as the cation resides in the internal cavity of the ligand. We noted that the Fe-Na" " distance in the lO Na" " complex is 4.39 A while the Fe-Ag+ distance in 10 Ag is 3.37 A. The latter distance is very short and allows us to postulate that some degree of d orbital mixing may take place between the metal centers in this complex. This postulate was further supported by UV-VIS spectral evidence [24]. 

The Crowns were found in the U.S.A. The Cryptates , a more recent French discovery, are mixed ether-amine macrocycles that not merely encircle the salts of inorganic cations, as Crowns do, but actually surround them in all directions, leading to somewhat tighter binding (Lehn, Sauvage, and Dietrich, 1970), and to very strong binding of strontium vts d vis calcium. Of the alkali metals, potassium is often preferred to sodium and lithium. 

---