Lanthanides chemistry

One of the distinctive aspects of transition-metal and lanthanide chemistry is cluster formation via metal-metal bonding that is characteristic of many of these elements in low oxidation states [1]. The unique structural, chemical, and... 

The metal vapor method also has played an important role in the development of organometallic lanthanide chemistry (6-10). This high energy technique demonstrated that the lanthanide metals had a much greater range of organometallic chemistry than had been assumed previously. The metal vapor technique applied to lanthanides identified reasonable new research goals which could subsequently be pursued by solution techniques. Not only the metal vapor reactions. 

Lanthanides Chemistry and Use in Organic Volume Editor S. Kobayashi... 

Quantum chemists have developed considerable experience over the years in inventing new molecules by quantum chemical methods, which in some cases have been subsequently characterized by experimentalists (see, for example, Refs. 3 and 4). The general philosophy is to explore the Periodic Table and to attempt to understand the analogies between the behavior of different elements. It is known that for first row atoms chemical bonding usually follows the octet rule. In transition metals, this rule is replaced by the 18-electron rule. Upon going to lanthanides and actinides, the valence f shells are expected to play a role. In lanthanide chemistry, the 4f shell is contracted and usually does not directly participate in the chemical bonding. In actinide chemistry, on the other hand, the 5f shell is more diffuse and participates actively in the bonding. 

Kobayashi S (1999) Lanthanide triflate-catalyzed carbon-carbon bond-forming reactions in organic synthesis. In Kobayashi S (ed) Topics in organometallic chemistry, vol 2, Lanthanides chemistry and use in organic synthesis. Springer, Heidelberg... 

Kagan, H. B. Namy, J.-L. Influence of Solvents or Additives on the Organic Chemistry Mediated by Diiodosamarium. In Lanthanides Chemistry and Use in Organic Synthesis, Kobayashi, S., Ed. Springer Berlin, 1999 pp 155-198. 

The study of coordination compounds of the lanthanides dates in any practical sense from around 1950, the period when ion-exchange methods were successfully applied to the problem of the separation of the individual lanthanides,131-133 a problem which had existed since 1794 when J. Gadolin prepared mixed rare earths from gadolinite, a lanthanide iron beryllium silicate. Until 1950, separation of the pure lanthanides had depended on tedious and inefficient multiple crystallizations or precipitations, which effectively prevented research on the chemical properties of the individual elements through lack of availability. However, well before 1950, many principal features of lanthanide chemistry were clearly recognized, such as the predominant trivalent state with some examples of divalency and tetravalency, ready formation of hydrated ions and their oxy salts, formation of complex halides,134 and the line-like nature of lanthanide spectra.135... 

This is arguably the most prolific area of tetravalent lanthanide chemistry, and Ce, Pr, Nd, Tb and Dy are all represented. 

Lanthanide chemistry with Schiff bases is quite extensive and numerous acyclic, cyclic, monometallic and polymetallic (4f-4f, 4f-5f, 4f-nd) complexes have been synthesized and studied, in particular with compartmental ligands, owing to the ability of the latter to bind two or more metal ions in close proximity. Asymmetrization of these ligands also provides important diversification of the coordinating sites (Vigato and Tamburini, 2004). On the other... 

The historical sketch outlines the class of lanthanide amides this article is to deal with and which is further manifested in Scheme 1. Organometallic amides which can be classified as dialkyl (-aryl, -silyl) amides and amides derived from unsaturated heterocyclic ligands are seen with respect to N-unsubstituted (primary, inorganic) amides. The consideration of more classic coordination compounds like acid amides or sulfonamides, often ascribed as wet chemistry , is excluded. The historical data also demonstrate the relatively late start of lanthanide amide chemistry reflecting the late industrial establishment of the lanthanide elements (separation, purification, etc.) [9], However, lanthanide amides are still the youngest class in conjunction with the most important pillars of organometallic lanthanide chemistry, namely the alkyls/cyclopentadienyls (LnCp3, 1954, [10]) and the alkoxides (Ce(OR)4 1956 [11a] La(OR)3 , 1958 [lib]). Indeed most of the work has been conducted in the last ten years. 

The present article will focus in particular on structurally characterized complexes and will refer to current trends and potential applications, simultaneously aiming at a coherent picture of the entire family of organometallic lanthanide amides including the inorganic derivatives. The elements Sc, Y, La will be treated as lanthanide elements Ln. Previous reviews cover this subject mostly as an aspect wrapped up under a comprehensive depiction of both metal amides [19] and lanthanide chemistry [20]. Other articles focus on special topics in this field, e.g., inorganic amides [21], silylamides [22], phthalocyanines [23] or porphyrins [24],... 

Unlike their O-counterparts, namely the aryloxides [92], the anilides have gained entry into lanthanide chemistry only very recently [93]. Their suitability as ligands in /-element chemistry was demonstrated in [K(THF)2][U(NHC6H3iPr2-2,6)5] [94],... 

The introduction of the btsa ligand into lanthanide chemistry by Bradley led to the isolation of the homoleptic compounds Ln(btsa)3 [7], interpreted as the first 3-coordinate lanthanide complexes. This result was indeed spectacular and exploration of the chemical and physical properties of this simple system is continuing [104-106]. 

Meso-porphyrinogens formally represent the structural transition between pyrroles and porphyrins. Stimulated by the work of Floriani et al. [251], porphyrinogens were considered as alternative ligand sets in lanthanide chemistry by Jubb and Teuben [68,252]. The tetraamide ligand offers a coordination environment which is obviously sterically and electronically more flexible than that provided by the standard Cp2Ln- fragment. The conformational flexibility... 

The resurgence of organometallic lanthanide amide chemistry and in particular that of the silylamides is certainly connected with their use as key synthetic-precursors. The so-called silylamide route is standard procedure in synthetic lanthanide chemistry. Suitable substrates are generally more Broensted acidic compounds like alcohols, phenols, cyclopentadienyls, acetylenes, phosphanes, thiols as listed in Scheme 12 [133,140,254-263]. 

Supposedly, lanthanide chemistry is lagging behind main group and d-transition metal chemistry. The concluding statement which Bradley made with respect to transition metal dialkylamides and silylamides almost 20 years ago [19b] is a current topic in lanthanide amide chemistry To date homogenous transition-metal catalysis has been restricted to hydrocarbon systems involving the facile formation and rupture of M-H, M-C, C-H and C-C bonds. An extension to include M-N, C-N, M-O, and C-O bonds seems plausible and could lead to substantial advances in transition-metal catalysis. ... 

These anionic ligands, especially those containing trimethylsilyl substituents [25,26], combine several unique features which make them extremely useful for lanthanide chemistry. 

Scheme 5. Siloxide ligands employed in lanthanide chemistry...

The importance of the chelate effect combined with the construction of multidentate ligands is well known in lanthanide chemistry. This is expressed in the rich coordination chemistry of / -diketonates [88] or complexes with Schiff bases [89] and macrocyclic polyethers [90] where lanthanide cations achieve steric saturation by high coordination numbers. Entrapment of the cation in a macrocyclic cavity results in greater complex stability. However, simply functionalized ligands such as ethanolamines can also supply a suitable ligand sphere [91-93],... 

Although the +3 oxidation state dominates lanthanide chemistry, other oxidation states are accessible, especially if a 4f°, 4f7 or 4f14 configuration is generated. The most common 2+ ions are Eu2+ (4f7) and Yb2+ (4f14), and the most common 4+ ions are Ce4+ (4f°) and Tb4+ (4f7). Of the five lanthanides that exhibit tetravalent chemistry, Nd4+ and Dy4+ are confined to solid-state... 

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