Lanthanides nitrates

Various crown ethers (p. 96) with differing cavity diameters provide a range of coordination numbers and stoichiometries, although crystallographic data are sparse. An interesting series, illustrating the dependence of coordination number on cationic radius and ligand cavity diameter, is provided by the complexes formed by the lanthanide nitrates and the 18-crown-6 ether (i.e. 1,4,7,10,13,16-... 

Phenols are easily mononitrated by sodium nitrate in a two-phase system fwater-etheri in the presence of HCl and a catalydc amoimt of l.arNO i. Various lanthanide nitrates have been used in the nitradon of 3-subsdnited phenols to give regioselecdvely the 3-subsdnited 5-nitrophenols. 

At this stage, the influence of several lanthanide salts was evaluated (Figure 35.2) (19). In the presence of lanthanide nitrates, the conversion and the selectivity towards 1,3-PDO decreased compared to the reference, i.e., the reaction run without any additives, except H2WO4. In contrast, the addition of lanthanide chlorides had a positive effect on the selectivity to 1,3-PDO. However, low conversions were observed in all cases. Under these conditions, the highest ratio l,3-PDO/l,2-PDO reached 0.9. 

Dehydrating agents have commonly been employed in the preparation of lanthanide sulfoxide complexes from hydrated lanthanide salts. For example, dimethoxypropane has been used to prepare both (CH2)4SO (65) and rePr2SO (56) complexes of the lanthanide nitrates. An alternative dehydrating agent is ethyl orthoformate [Eq. (14)]. 

Complexes of the lanthanides with a few cyclic amides are known. Miller and Madan have reported the complexes of 7-butyrolactam with lanthanide nitrates (60) and perchlorates (61). Complexes of lanthanide perchlorates and lighter lanthanide nitrates with BuL have a L M of 8 1. However, complexes of heavier lanthanide nitrates have a L M of only 3 1. By changing the solvent used for the crystallization of the abovementioned complexes, complexes of the formula [La(BuL)4(N03)3] and [Gd(BuL)3(N03)3] could be prepared (60). Complexes of NMBuL (61, 62) and CLM (63-66) have also been reported. 

Probably, the first series of lanthanide complexes with neutral oxygen donor ligands is that of AP with the lanthanide nitrates. In 1913, Kolb (79) reported tris-AP complexes with lighter lanthanide nitrates and tetrakis-AP complexes with heavier lanthanide nitrates. Subsequently, complexes of lanthanide nitrates with AP which have a L M of 6 1 and 3 1 have also been prepared (80-82). Bhandary et al. (83) have recently shown through an X-ray crystal and molecular structure study of Nd(AP)3(N03)3 that all the nitrates are bidentate and hence the coordination number for Nd(III) is nine in this complex. Complexes of AP with lanthanide perchlorates (81, 84), iodides (81, 85), and isothiocyanates (66, 86, 87) are known. While the perchlorates and iodides in the respective complexes remain ionic, two of the isothiocyanates are coordinated in the corresponding complexes of AP with lanthanide isothiocyanates. 

Complexes of alcohols like methanol, ethanol, 2-propanol and n-butanol (116-122), and ethers like Diox (47,120,123-125) and THF (126-128) have been prepared. The bonding between these ligands and the metal ions is considered to be very weak. In recent years, complexes of the lanthanides with a few macrocyclic polyethers have been reported. Cassol et al. (129) have prepared the complexes of benzo-15-crown-5 and dibenzo-18-crown-6 with lanthanide nitrates and isothiocyanates. King and Heckley (130) have also reported the complexes of these ligands with lanthanide nitrates. The heavier lanthanide nitrate complexes of dibenzo-18-crown-6... 

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. 

Complexes of lanthanide chlorides 156,173), bromides (256), and iodides 174) with 2,6-DMePyO have also been prepared and characterized. The presence of bridging 2,6- DMePyO molecules has been suggested in the complexes of lanthanide iodides. Vicentini and De Oliveira (2 73) have reported tetrakis-2,6-DMePyO complexes with lanthanide nitrates. However, by changing the method of synthesis, tris-2,6-DMePyO complexes with the lanthanide nitrates could be prepared in this laboratory (252). All the nitrate groups in the tris-2,6-DMePyO complexes are bidentate. In the 2,4,6-TMePyO complexes (252) also the nitrate groups are coordinated to the lanthanide ion in a bidentate fashion. 

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. 

Fig. 1. Reactions of lanthanide nitrates with TPPO (redrawn and reproduced with permission from Pergamon Press Limited, from Ref. 203)...

Sylvanovich and Madan (234) have isolated the complexes of OMPA with lanthanide nitrates. With the lighter lanthanide nitrates, bis-OMPA complexes were obtained and the heavier lanthanide nitrates yielded complexes of the type Ln2(OMPA)3-(N03)6. In the latter complexes both bridging and chelating ligands are present. Complexes of OMPA with lanthanide perchlorates are also known (235). Airoldi et al. 

The L M in the complexes of lanthanide nitrates with TMSO decreases along the lanthanide series (264, 265). All these complexes contain both bidentate and mono-dentate nitrate groups (264), the monodentate nitrates giving way to bidentate nitrates as the cationic radius decreases. 

Complexes of TSO with lanthanide perchlorates which have the formula Ln(TS0)9(C104)3 have been reported by Edwards et al. (266) (Ln = Ce or Y). Later, Vicentini and Perrier (267) have prepared the whole series of complexes of TSO with lanthanide perchlorates and have shown that the L M in these complexes gradually decreases from 9 1 to 7 1 as the cationic size decreases. These authors could not prepare Y(TS0)g(C104)3 reported by Edwards et al. (266). Instead, they obtained the complex of the composition Y(TS0)7(C104)3. Two series of complexes of TSO with lanthanide hexafluorophosphates are known (268, 269). While the L M in one of the series is 7.5 1, in the other series it is found to be 8 1. The change in the stoichiometry of the two series of compounds is attributed to the preparative procedures adopted. In both the series of complexes, the PFg ion remains ionic. Lanthanide nitrates (270), chlorides (270), and isothiocyanates (271) also yield complexes with TSO. In all these complexes, changes in the stoichiometry could be observed when the lanthanide series was traversed. In all these complexes the anions are coordinated to the metal ion. 

The spectra of the adducts of lanthanide nitrates (123) and perchlorates (124, 125) with dioxane and lanthanide acetates with DMF (44) show very little change from the corresponding ligand spectra. In these cases, it has been suggested that the ligands occupy only lattice positions and that there is no strong interaction of the ligand with the metal. 

In the case of complexes of lanthanide nitrates with CP (239), the infrared spectra exhibit two uCz=Q vibrations apart from a single pP=0- The J>p=o and one of the c=o vibrations occur at frequencies lower than the corresponding ligand frequencies But the second r>c=0 1S not much shifted from the free ligand value. These observations have been interpreted in terms of Structure VIII containing both coordinated... 

In all other complexes of lanthanide nitrates, the mode of coordination has been identified, with some ambiguity, from IR data alone. In most of the cases, either the criterion suggested by Addison et al. (287), Curtis and Curtis (288) or Lever et al. (289) has been used for this purpose. Ionic nitrate groups have been identified in the complexes of BuL (60), HMPA (223), and O-PhenNO (178) with lanthanide nitrates. Both unidentate and bidentate nitrate groups are present in the complexes of lanthanide nitrates with MP (232), OMPA (234), CMP (240), TMSO (264), DMF (42), and BuL (60). Complexes of PyO, 2-MePyO, 2,6-DMePyO, and 2,4,6-TMePyO which have the general formula Ln(L)3(N03)3 -x H20 contain only bidentate nitrate groups (152,170). 

As one traverses through the lanthanide series, there is a reduction in the cation size as the atomic number increases. This results in small differences in the strength of interactions of the ligand with the lanthanide ions. These trends are reflected in the IR spectra of these complexes in a few cases. Cousins and Hart (203) have observed an increase in Pp Q with decreasing lanthanide ion radius for the complexes of TPPO with lanthanide nitrates. This observation has been attributed to an increase in the Ln—O bond strength with an increase in the atomic number of the lanthanide ion. 

The increase in i>p 0 is due to a progressive increase in the coupling of M-0 and P-0 vibrations with an increasing atomic number of the lanthanide ion. Similarly, McRae and Karraker (201) have found that the Pp 0 increases with decreasing ionic radius in the complexes of TPP with lanthanide nitrates. This trend has, however, been explained by them in terms of relative influence of attractive and repulsive forces in these complexes. As the size of the lanthanide ion decreases, the repulsive forces in-... 

The vibrations of the anions are also affected by the change in the strength of interaction between the anion and the metal ion. For example, in the complexes of TPP with lanthanide nitrates (201), the band around 1300 cm-1, assigned to of the bidentate nitrate group (C2v), increases in frequency from 1282 cm-1 for the bis-... 

In several cases it has proved advantageous to remove water from hydrated salts. In the preparation of PrJ SO complexes of lanthanide nitrates" and Bu"SO complexes of lanthanide perchlorates,12 2,2 -dimethoxypropane and triethyl orthoformate respectively were used as reaction media as 50% solutions in ethanol. 

Metal complexes also can show contradictive behavior with benzo-15-crown-5 the complexation of lanthanide perchlorates is entropy-driven in acetonitrile, while the complexation of lanthanide nitrates with corresponding disubstituted derivatives is primarily enthalpy-driven, with small entropic differences.44... 

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