Sn II complexes

The reaction between Fe(IlI) and Sn(Il) in dilute perchloric acid in the presence of chloride ions is first-order in Fe(lll) concentration . The order is maintained when bromide or iodide is present. The kinetic data seem to point to a fourth-order dependence on chloride ion. A minimum of three Cl ions in the activated complex seems necessary for the reaction to proceed at a measurable rate. Bromide and iodide show third-order dependences. The reaction is retarded by Sn(II) (first-order dependence) due to removal of halide ions from solution by complex formation. Estimates are given for the formation constants of the monochloro and monobromo Sn(II) complexes. In terms of catalytic power 1 > Br > Cl and this is also the order of decreasing ease of oxidation of the halide ion by Fe(IlI). However, the state of complexing of Sn(ll)and Fe(III)is given by Cl > Br > I". Apparently, electrostatic effects are not effective in deciding the rate. For the case of chloride ions, the chief activated complex is likely to have the composition (FeSnC ). The kinetic data cannot resolve the way in which the Cl ions are distributed between Fe(IlI) and Sn(ll). 

In Sn(II) complexes, the tin atom was located above the di-Schiff base coordination plane, while in Sn(IV) complexes, it was coplanar with the imine coordination framework. The position of the metal was supported by X-ray data. For the compounds studied, the 119Sn chemical shift values varied from 501.4 to —1015.9 ppm. Increase in the coordination number from Sn(II) to Sn(IV) led to an increase in the tin shielding. The differences of up to 3.0 ppm between 5119Sn values for the complexes, being derivatives of R,R and S,S 1,2-diaminocyclohexane, were observed. 

The ligands are asymmetrically bound such that both a short and a long M-S bond arise from each ligand. The short bonds are positioned trans to each other in a PbS4 polyhedron that resembles a distorted square pyramid with Pb at the apex. This structure, which appears to result from lone pair-bonding pair repulsions, is quite similar for the Pb(II) and Sn(II) complexes (Table IV). In the Pb(i-PrXant)2 Py complex, the pyridine molecule is located above the lead atom with a Pb-N distance of 2.55(4) A (288). 

The Mossbauer spectra of the Sn(II) complexes (Table VI) show quadruple splittings that are consistent with a sterically active pair of electrons. However, the magnitude of these splittings appears more sensitive to the nature of the ligand rather than the stereochemistry of the Sn(II) ion (342). 

Catalysis with Bisoxazoline Complexes of Sn(II) and Cu(II). The bisoxazoline Cu(IT) and Sn(II) complexes 81-85 that have proven successful in the acetate additions with aldehydes 86,87, 88 also function as catalysts for the corresponding asymmetric propionate Mukaiyama aldol addition reactions (Scheme 8B2.8) [27]. It is worth noting that eithersyn or anti simple diastereoselectivity may be obtained by appropriate selection of either Sn(II) or Cu(II) complexes (Table 8B2.12). 

The Evans Cu(II)- and Sn(II)-catalyzed processes are unique in their ability to mediate aldol additions to pyruvate. Thus, the process provides convenient access to tertiary a-hydroxy esters, a class of chiral compounds not otherwise readily accessed with known methods in asymmetric catalysis. The process has been extended further to include a-dike-tone 101 (Eqs. 8B2.22 and 8B2.23). It is remarkable that the Cu(II) and Sn(II) complexes display enzyme-like group selectivity, as the complexes can differentiate between ethyl and methyl groups in the addition of thiopropionate-derived Z-silyl ketene acetal to 101. As discussed above, either syn or anti diastereomers may be prepared by selection of the Cu(II) or Sn(II) catalyst, respectively. 

The same bisoxazoline Cu(II) and Sn(II) complexes have been utilized successfully in the corresponding propionate aldol addition reactions (Scheme 8-7). A remarkable feature of these catalytic processes is that either syn or anti simple dia-stereoselectivity may be accessed by appropriate selection of either Sn(II) or Cu(II) complexes. The addition of either - or Z-thiopropionate-derived silyl ke-tene acetals catalyzed by the Cu(II) complexes afford adducts 78, 80, and 82 displaying 86 14-97 3 syn anti) simple diastereoselectivity. The optical purity of the major syn diastereomer isolated from the additions of both Z- and i -enol silanes were excellent (85-99% ee). The stereochemical outcome of the aldol addition reactions mediated by Sn(Il) are complementary to the Cu(U)-catalyzed process and furnish the corresponding anp -stereoisomers 79, 81, and 83 as mixtures of 10 90-1 99 syn/anti diastereomers in 92-99% ee. 

Multinuclear NMR is not only a potential and unique technique for the structural and stereochemical characterization of homometallic Sn(II) complexes, but also for oligometallic derivatives having more than one metallic element, provided that the other metal(s) is(are) NMR active. There are several NMR... 

Pioneering studies of stoichiometric Sn(II)-promoted additions of enol silanes to aldehydes by Mukaiyama and Kobayashi are valuable resources in understanding the catalytic versions of the reaction. Stoichiometric quantities of optically active Sn(II) complexes prepared from diamines mediate a collection of aldol addition reactions (Eqs. 7and 8) [7,75a, 75b, 75c]. Thus, the addition of the... 

The electrochemical behaviour of the Sn(OEC)Cl2 complexes [85] was distinctly different from that of the other OEC complexes. The first oxidation potential was shifted to some 450 mV positive of the potential of the free bases due to highly electropositive Sn,v ion [89,95]. The second oxidation was irreversible with a peak height greater than that of one electron process. Three reduction processes were observed for frans-Sn(OEC)Cl2. The first at — 0.98 v was a reversible one electron process and second reduction at — 1.37 V was a two electron process while the third reduction at — 1.78 V was a reversible one electron process. cis-Sn(OEC)Cl2 behaved identically except for the lesser stability of the reduced species formed in these processes. The first reduction was attributed to the formation of Snlv(OEC)Cl2 anion radical and subsequent reductions could involve the formation of ring protonated or Sn(II) complexes. 

On the other hand, there are cases in which the amount of reductant must be strictly controlled. This is indicated when more than one oxidation state is favored with a certain ligand, or when hydrolysis products interfere with complex stability. This precaution is possible with Sn(II) complexes (Sn-tartrate, Sn-gluconate, Sn-citrate, Sn-EDTA, etc.), which release small amounts of stannous ion into solution. In addition, another reducing agent, including the ligand itself, might be considered. 

Munze R (1980) Electrochemical investigation on the reduction of TcOJ with tin(II) in the presence of citrate. Radiochem Radioanal Lett 43 219-224 Nakayama M, Terahara T, Wada M, Ginoza Y, Harada K, Sugii A, Nakayama H (1995) Insoluble macromolecular Sn(II) complex for the " Tc labelling of protein-bearing mercapto groups. In Nicolini M, Mazzi U (eds) Tc, Re and other metals in chemistry and nuclear medicine 4. SG Editoriali, Padova, Italy, pp 299-320... 

In the Sn(ii) complex [Li2(/Li3-OBu )2(Ar-OBu )4Sn2] the lithiums are four-coordinated but the tin atoms are three-coordinated (pyramidal) with a Li206Su2 cage built from two eco-norcubane Sn2Li203 units sharing a Li202 four-membered ring." In the Sn(iv)... 

In the Sn(ii) complex [K(/Li3-OBu )2(/r-OBu )Sn] both metals are in distorted five-coordinated configurations in a polymeric structure of alternating potassium and tin atoms linked by bridging alkoxo groups in a structure different from the Fi and Na analogues." The binuclear potassium tin(ii) triphenylsiloxide exists in two forms both containing the K(/Lt-OSiPh3)3Sn unit with three-coordinated tin. In... 

Somewhat different situation arises, when the reduction of Sn(II) complexes is irreversible. To construct cathodic NTP, the anodic component of the total current density should be ignored as negligible. In so doing, the products of cathodic process, whose concentrations are multiplies at the anodic exponent, are excluded from consideration. Thus, prewave-containing cathodic voltammo-grams can be transformed to linear NTP, if special requirements on the possible EAC are imposed. The main of them consists in the occurrence of a sharp fall in EAC surface concentration within the narrow range of current density. SnLH" complexes satisfy this condition and linear NTPs are obtained for this... 

Other species is desirable first. To obtain such data, reliable stability constants of complexes and protonated ligands are required. Unfortunately, the data concerning Sn(II) complexes are few in number we were able to find only two articles containing quantitative characteristics of Sn(II) gluconate [94] and sulfate [95] complexes. The latter compounds can be also formed, because 0.5 M Na2S04 was used as a supporting electrolyte. Finally, the Sn(Il) hydrolysis processes should be also considered. 

CI2CH)2Sn.xTHF results from LiCHCl2 and SnCl2 and has been characterised as a stannylene. However SnClj and Me Si(Me2P)2 Li give not the stannylene but the non-fluxional spiro-Sn(II) complex... 

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