Lanthanide complexes thiocyanates

We have noted that N-bonded thiocyanate complexes dissociate readily in solution. Mixed ligand complexes of lanthanides with thiocyanates and L = ethanol, pyridine, triphenyl phosphorous oxide, phenanthroline, etc. have been synthesized. 

Lanthanide complexes of 1,10-phenanthroline (phen) have been obtained as chlorides, nitrates, sulfates, perchlorates acetates, thiocyanates and selenocyanates from various solvents. 

A number of chloride complexes include Pr(terpy)Cl3.8H2O X-ray diffraction established the presence of a [Pr(terpy)C1(H20)5] + ion. Lanthanide nitrates react with 1 mole of terpy in MeCN to give [La(terpy)(N03)3(H20) ] (Ln = La, = 2 Ln = Ce-Ho, n = 1 Ln = Er-Lu, n = 0) with coordination numbers decreasing from 11 (La) to 9 (Er-Lu) if these are crystallized from water [Ln(terpy)(N03)2(H20)4] NO3 (Ln = La-Gd) and [Ln(terpy)(N03)2(H20)3]N03.2H20 (Ln = Tb-Lu) are formed these have both biden-tate and ionic nitrate and nine- and eight-coordinate lanthanides, respectively. Thiocyanate complexes Ln(terpy)2(NCS)3(Ln = Pr, Nd) are also nine coordinate. 

Another possibility is to apply the double-double effect to the differentiation of outer-from inner-sphere complexes of lanthanides and actinides because both the magnitude and direction of the effect depend on the difference in the electron-donor ability between water in the aquoion and the ligand in the complex. Therefore, a substantial difference between outer- and inner-sphere complexes with respect to the double-double effect should exist. Research in this field, using lanthanide complexes with thiocyanates as an example, is being made in our laboratory. 

The vast majority of the macrocyclic complexes of the lanthanide(III), yttrium(III) and uranyl ions obtained so far by metal-templated synthesis are of the Schiff-base type even the few known examples of simple polyamine complexes actually result from the cyclic condensation of a diamine with a (modified) carbonyl precursor. In general, the metal-templated synthesis is facilitated by the presence of oxygen-donor anions, such as nitrate, acetate, or trifluoromethylsulfonate lanthanide(III) thiocyanates have also been successfully used. With a few exceptions, the outcome of the synthesis appears to be independent of the order of addition of the reactants. No deliberate attempts have been made to investigate the detailed mechanism of these metal-templated cyclic condensation reactions. 

The reactions of lanthanide thiocyanates, nitrates, and chlorides with TPPO have been studied by Cousins and Hart (202, 203, 205). The reactions of lanthanide nitrates with TPPO in ethanol, acetone, ethylacetate and tetrahydrofuran are given in Fig. 1. The nature of the complexes isolated depends on the concentrations of the ligand and the metal ion, temperature of mixing, presence or absence of the seed of the desired complex, size of the cation, and the nature of the solvent. Tetrakis-TPPO complexes of Ce(III) and Nd(III) perchlorates have been reported (206, 207). Two of the perchlorates are coordinated to the metal ion in these complexes. 

Association between lanthanide ions and azide or thiocyanate ions has been studied in solution by electronic, Raman and NMR spectroscopy.186,187 The complexation constant in water between Nd3+ and N3 is approximately 2.5. Longitudinal relaxation time studies for Gd-Dy indicate that the M—N—NN angle is bent (135° approximately). 

Only a few structures of thiocyanate complexes are known for the lanthanides and actinides, i.e. for Er,341 Th342 and U.343 They all contain terminal N-bonded NCS and have as an interesting aspect a high coordination number, e.g. as in [NEt4]4[Th(NCS)8].342... 

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). 

The ligand 1,2-bis(pyridine-a-aldimino) ethane has four nitrogen atoms for coordination and forms 1 1 solid complexes with lanthanide [246] chlorides, nitrates, thiocyanates or salicylates. Infrared spectra show evidence for nitrate coordination and pyridyl coordination. Attempts to isolate bis complexes were futile although there is spectral evidence for their existence in solution. 

Eermium coprecipitates with lanthanide fluorides and hydroxides, showing it to form lanthanide-like Em + ions. These elute from cation-exchange resins slightly before Es +, whilst its chloride and thiocyanate complexes are eluted from anionic exchange resins just... 

Unusually, chloride is foimd in the first coordination sphere of lanthanum as well as nitrate in the serendipitously discovered mixed anion complexes [EaCl2(N03)(12-crown-4)]2 and [LaCl2(N03)(18-crown-6)]. A few complexes have been reported with other halides, such as [Sml3(dibenzo-18-crown-6)] (tricapped trigonal prismatic) and [LaBr3(12-crown-4)(acetone)] (distorted square antiprismatic). Lanthanide thiocyanate complexes of crown ethers are now starting to be studied. Several thiocyanate complexes of the... 

The thiocyanate complexes Ln(phen)3(NCS)3 (Ln = Pr, Nd) and Pr(bipy)3(NCS)3 are all nine-coordinate monomers. A considerable number of bipy (and phen) adducts of lanthanide dithiocarbamates [Ln(S2CNR2)3(L)]... 

All the examples reported so far appear to contain N-bonded thiocyanate groups and to dissociate readily in solution. Thus, [ScL3](NCS)3 (L = bipy or phen) has been prepared and characterized 223). Treatment with warm EtOH gives [ScL2(NCS)2](NCS) where the formulation is supported by conductivity measurements (in CHgCN or CH3NO2) and infrared mull spectra in aqueous solutions the coordinated NCS is displaced 223, 225, 464). Other mixed liquid complexes of these metals with NCS are listed in Table XX. A 1 1 adduct is formed between 1,3,5-trinitrobenzene and several lanthanide thiocyanates. No... 

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