Pyridine complexes, lanthanide

It might be considered that the way in which to understand the water complex ion chemistry of the lanthanide ions in aqueous solution would be to start from their hydrates. In fact the hydrates prove to be most intransigent complexes. Their structures are still somewhat uncertain. I shall therefore start from a study of the tris-dipicolinates, i.e. tris 2,2 -carboxy-pyridine complexes, Ln(dipic)3, about which a great deal is known. 

Nuclear Magnetic Resonance Spectroscopy.—As noted above, conformational analysis of bicyclo[3.3.1]nonanes is still a topic of considerable interest. A variable-temperature n.m.r. analysis now provides the first case in which the boat-chair-chair-boat equilibrium is directly observed in the amines (17) and (18). In a related case, re-examination of the acetal (19) suggests that the preferred conformation involves a chair carbocyclic ring and a boat heterocyclic ring. This conclusion was made by n.m.r. analysis, using lanthanide shift reagents, by a study of nuclear Overhauser effects, and by measurement of relaxation times of protons. Details have been reported for other 3-azabicyclo[3.3.1]nonanes, and the non-additivity of substituent effects on chemical shifts in 9-thiabicyclo[3.3.1]non-2-enes has been analysed. Both and n.m.r. data have been reported for a series of 9-borabicyclo[3.3.1]non-anes and their pyridine complexes. 

Bis(benzimidazolyl)pyridine complexes alkyl chains, 80-84 dendrimeric ligands, 79/ design strategies, 83 dodecacatenar ligand, 78/ hexagonal columnar phase, 72-73 I-shaped lanthanide, 77/ liquid-crystalline lanthanide, 77/ melting point of LCs, 80-84 structural changes, 80-84 structures, 72-73, 75/... 

The coordination chemistry of ancillary amidinate ligands with a pyridine functionality has been described. Magnesium, aluminum, zirconium, and lanthanum complexes have been prepared in which the amidinate anions act as tridentate, six-electron-donor ligands Amidinate ligands containing quinolyl substituents were constructed in the coordination sphere of lanthanide... 

In addition to these systematic studies of lanthanide sulfoxide complexes, with variation in both sulfoxide and anion, other more isolated reports are available. Lanthanide isothiocyanate complexes of the cyclic sulfoxides thioxane oxide (490) and tetramethylene sulfoxide (493) have been synthesized and complexes of the unusual potentially chelating ligand 2-(ethylsulfinyl)pyridine-V-oxide (63) described. Detailed studies of the solvation of lanthanide-shift reagents by Me2SO have also appeared (178,179). 

Substitution at the 2-position of the pyridine ring in PyO introduces steric hindrance to coordination as is evident from the formation of Heptakis-2-MePyO complexes with lanthanide perchlorates (167) and pentakis-2-MePyO complexes with the corresponding bromides (168), iodides (162) and chlorides (169). The lanthanide nitrate complexes prepared by Ramakrishnan and Soundararajan (170) have the formula Ln(2-MePy0)3(N03)3 -xH20in which all the nitrate groups are bidentate. 

The importance of the carboxylate donors is underlined by a study of the lanthanide coordination chemistry of the similar terdentate ligand 2,6 -bis( 1 -pyrazol-3 -yl)pyridine, L24 (63). The complex structure of [Tb(L24)3][PF6]3, shown in Fig. 11, appears to be fairly robust in methanolic solution, with Horrocks analysis (q = 0.6) suggesting the 9-coordinate structure is retained the small quenching effect of outer sphere coordination explains the q-value. However, in aqueous solution, the lability of the ligands dramatically changes the luminescence. Whilst the emission decays are not exactly single exponential, approximate lifetimes in H20 and DoO suggest a solvation value of 4-5. 

Brunet, E. Juanes, 0. Sedano, R. Rodriguez-Ubis, J.-C. Lanthanide complexes of polycarboxylate-bearing dipyrazolylpyridine ligands with near-unity luminescence quantum yields the effect of pyridine substitution. Photochem. Photobiol. Sci. 2002, 1, 613-618. 

Renaud, F. Piguet, C. Bernardinelli, G. Biinzli, J.-C. G. Hopfgartner, G. In search for mononuclear helical lanthanide building blocks with predetermined properties lanthanide complexes with diethyl pyridine-2,6-dicarboxylate. Chem. Eur. J. 1997, 3(10), 1660-1667. 

Bardwell, D. A. Jeffery, J. C. Jones, P. L. McCleverty, J. A. Psillakis, E. Reeves, Z. R. Ward, M. D. Lanthanide complexes of the tetradentate N-donor ligand dihydrobis[3-(2-pyridyl)pyrazolyl]borate and the terdentate N-donor ligand 2,6-bis(lH-pyrazol-3-yl)pyridine syntheses, crystal structures and solution structures based on luminescence lifetime studies. J. Chem. Soc., Dalton Trans. 1997, 2079-2086. 

Bastida, R. de Bias, A. Castro, P. Fenton, D. E. Macias, A. Rial, R. Rodriguez, A. Rodriguez-Blas, T. Complexes of lanthanide(III) ions with macrocyclic ligands containing pyridine head units. J. Chem. Soc., Dalton Trans. 1996,1493-1497. 

The addition of a base, typically ammonia, to mixtures of transition metal halides and alcohols allows the synthesis of homoleptic alkoxides and phenoxides for a wide range of metals. Anhydrous ammonia was first used in the preparation of titanium alkoxides where the reaction is forced to completion by the precipitation of ammonium chloride.41 Although useful for the synthesis of simple alkoxides and phenoxides of Si, Ge, Ti, Zr, Hf, V, Nb, Ta and Fe, as well as a number of lanthanides,42-47 the method fails to produce pure /-butoxides of a number of metals.58 Presumably, secondary reactions between HC1 and Bu OH take place. However, mixing MC14(M = Ti, Zr) with the Bu OH in the presence of pyridine followed by addition of ammonia proves successful, giving excellent yields of the M(OBul)4 complexes.59... 

Trivalent yttrium and lanthanide metals, except for promethium, have been complexed to octaethylporphyrin by heating at 210 °C in an imidazole melt.17 The complexes obtained as hydroxides, Mm(OEP)(OH), are unstable in acidic media. As the charge radius ratio rule predicts, the early lanthanide metalloporphyrins, MIU(OEP)(OH) (M = La, Ce, PR, Nd), are demetallated during purification, and the middle series (M = Sm, Eu, Gd, Tb, Dy) in 1 % acetic acid in methanol, while the last five (M = Ho, Er, Tm, Yb, Lu) survive in 2% acetic acid in methanol but are dissociated in dilute hydrochloric acid. The Mnl(OEP)(OH) appears to coordinate more than one equivalent of pyridine and piperidine, and dimerizes in noncoordinating solvents such as benzene and dichloromethane at 10 4 M concentration. The dimer is considered to be a di-p-hydroxo-bridged species, different from the p-oxo dimer, Scin(OEP) 20 (Scheme 6). 

Numerous macrocyclic and macropolycyclic ligands featuring subheterocyclic rings such as pyridine, furan or thiophene have been investigated [2.70] among which one may, for instance, cite the cyclic hexapyridine torands (see 19) [2.39] and the cryptands containing pyridine, 2,2 -bipyridine (bipy), 9,10-phenanthroline (phen) etc. units [2.56,2.57,2.71-2.73]. The [Na+ c tris-bipy] cryptate 20 [2.71] and especially lanthanide complexes of the same class have been extensively studied [2.74, 2.75] (see also Sect. 8.2). 

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