Lanthanide cyanide

Iodine cleaves one erbium-cyclopentadienyl bond in tricyclopentadienyl erbium with formation of pink dicyclopentadienyl erbium iodide (Maginn et al., 1963), and the tricyclopentadienyl complexes of neodymium and ytterbium are cleaved by hydrogen cyanide with formation of the corresponding dicyclopentadienyl lanthanide cyanide (Kanellakopulos et al., 1974). The colors and some physical data of the cyclopentadienyl rare earth halides and cyanides are given in table 4. 

The major composition of the Gd catalyst prepared from Gd(0-iPr)3 and ligand (7a) in a ratio of 1 2 was determined to be Gd/ligand = 2/3 by ESI-MS (electronspray ionization mass spectrometry) analysis [84a]. The mass value and the isotope distribution pattern matched well with the calculated values. NMR studies supported the formation of lanthanide cyanide. Free ligand (7a) was disilylated when treated with TMSCN. The relationship between the enantiomeric excess of the product and the ratio of Gd/ligand (Figure 13.6) was also consistent with these observations. The postulated mechanism is shown in Scheme 13.29. One of the Gd in the Gd/ligand (7a) =2 3 complex is speculated to work as Lewis acid to activate the ketone, and the other Gd center would work as a nucleophile. The two Gd centers work cooperatively to promote the reaction smoothly. 

The affinity of cyanide groups for lanthanide ions has motivated the use of [M(CN)6]3 tectons (with M = Cr3+, Mn3+, Fe2+, Fe3+, Co3+) that can give rise to a wide variety of one-dimensional cyanide-bridged structures ((E) topologic mode in Scheme 4.2) [90]. Some noticeable compounds are [Ln(DMF)4(H20)2Mn(CN)6]-H20 K chains (DMF, dimethylformamide), where antiferromagnetic coupling was observed between Mn3+ tecton and Sm3+, Tb3+,... 

Most lanthanide compounds are sparingly soluble. Among those that are analytically important are the hydroxides, oxides, fluorides, oxalates, phosphates, complex cyanides, 8-hydroxyquinolates, and cup-ferrates. The solubility of the lanthanide hydroxides, their solubility products, and the pH at which they precipitate, are given in Table 2. As the atomic number increases (and ionic radius decreases), the lanthanide hydroxides become progressively less soluble and precipitate from more acidic solutions. The most common water-soluble salts are the lanthanide chlorides, nitrates, acetates, and sulfates. The solubilities of some of the chlorides and sulfates are also given in Table 2. 

The complications which result from the hydrolysis of alkali metal cyanides in aqueous media may be avoided by the use of non-aqueous solvents. The one most often employed is liquid ammonia, in which derivatives of some of the lanthanides and of titanium(III) may be obtained from the metal halides and cyanide.13 By addition of potassium as reductant, complexes of cobalt(O), nickel(O), titanium(II) and titanium(III) may be prepared and a complex of zirconium(0) has been obtained in a remarkable disproportion of zirconium(III) into zirconium(IV) and zirconium(0).14 Other solvents which have been shown to be suitable for halide-cyanide exchange reactions include ethanol, methanol, tetrahydrofuran, dimethyl sulfoxide and dimethylformamide. With their aid, species of different stoichiometry from those isolated from aqueous media can sometimes be made [Hg(CN)3], for example, is obtained as its cesium salt form CsF, KCN and Hg(CN)2 in ethanol.15... 

Anion sensing using visible-emitting lanthanide probes has proven successful (Tsukube et al., 2006) and this work is now being extended to Ybm probes, particularly for the detection of thiocyanate. The latter is the principal metabolite of cyanide anion and exists in human serum, saliva, and urine. The luminescent probe is a complex of hexadentate tetrakis(2-pyridylmethyl)ethylenediamine (tpen, see fig. 119) which bears two water molecules, [Yb(tpen)(H20)2](0tf)3. In absence of anion coordination, the 980-nm luminescence is quenched, but the replacement of the water molecules with thiocyanate or other anions such as acetate, nitrate or halogenides removes the quenching, which makes the complex a responsive probe. The largest effect (a six-fold increase in luminescence) is obtained for thiocyanate, followed by acetate and nitrate (3.5-fold) and chloride (two-fold). 

Complex formation is useful for metal speciation and also for the separation of diverse metal ions. Among a variety of complexing reagents [20-22] cyanide is probably the most important. IPC separation of metal ions as metallocyanide complexes with a suitable cationic IPR is a reliable technique [23]. Complexation of trace level lanthanides with a-hydroxy isobutyric acid and separation under IPC condition shortened analysis time from days to minutes [24]. Flow injection was successfully coupled to IPC to simplify batch precomplexation detection limits were at sub-microgram per liter levels [2]. 

To overcome the lack of sensitization of the lanthanide emissions for the complex of dendrimers 5 (see above), a supra-molecular approach has been pursued (41). It was known that complexes of Ru containing 2,2 -bipyridine (bpy) and cyanide ligands, that is, [Ru(bpy)2(CN)2] and [Ru(bpy)(CN)4] , are liuni-nescent and can play the role of ligands giving rise to supercomplexes (42,43). Titration of an acetonitrileidichloromethane 1 1 (v/v) solution of [Ru(bpy)2(CN)2] with Nd causes... 

The high sensitivity of lanthanide reagents to steric factors is also observed in the cyanosilylation reaction of ketones catalyzed by ytterbium cyanide, Yb(CN)3 (Eq. 7) [10], Other reactions, for example epoxide and the aziridine opening by tri-methylsilyl cyanide, TMSCN, are also efficiently catalyzed by Yb(CN)3 [11]. This Yb reagent is not regarded as a Lewis acid but as the active species in these reactions. 

In catalytic processes with enzymes such as D-oxynitrilase and (R) xynitrilase (mandelonitrilase) or synthetic peptides such as cyclo[(5)-phenylalanyl-(5)-histidyl], or in reaction with TMS-CN pro-mot by chiral titanium(IV) reagents or with lanthanide trichlorides, hydrogen cyanide adds to numerous aldehydes to form optically active cyanohydrins. The optically active Lewis acids (8) can also be used as a catalyst. Cyanation of chiral cyclic acetals with TMS-CN in the presence of titanium(IV) chloride gives cyanohydrin ethers, which on hydrolysis lead to optically active cyanohydrins. An optically active cyanohyrMn can also be prepared from racemic RR C(OH)CN by complexation with bru-... 

Silylated nucleophiles such as trimethylsilyl cyanide are assumed to preferentially attack on the less hindered side of styrene oxide, due to the bulky r-butoxide group of a mixed Sml2(Of-Bu) precatalyst (eq. (14)) [136]. Unique stereocontrol (chelation versus nonchelation) in the Eu(fod)3-catalyzed cyanosilylation (fod = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate) of chiral alkoxy and a-amino aldehydes could be explained by lanthanide-induced shift NMR analysis... 

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