Silylamides lanthanide

Scheme 12.8 (a) Introduction of bulky ligands (L ) by direct grafting of heteroleptic lanthanide silylamide complexes on mesoporous MCM-41 (22-25) (cf route A in Scheme 12.3) (b) grafting of (homoleptic) lanthanide silylamide complexes on a mesoporous material (cf route A in Scheme 12.3) followed by a subsequent ligand exchange (cf route C in Scheme 12.3) via protonolysis of the Ln-N bond with HL (A-L). 

The first report on organolanthanide-promoted Tishchenko reactions, that is, the transformation of aldehydes or mixed aldehydes into the corresponding esters, including a mechanistic proposal appeared in 1996 [catalyst Cp2 LnCH(SiMe3)2 with Ln = La and Nd] [233]. Two years later, lanthanide silylamide complexes Ln[N(SiMe3)2]3 were found as easily accessible and even more active catalysts (Scheme 12.24) [234, 235]. 

Silica-grafted group 3 and lanthanide silylamides have been used as precursor to surface (3-diketonate complexes [(=SiO) Ln( Bu-COCHCO- C3F7)m(THF)i]... 

Scheme 45 Reactivity of lanthanide silylamide complexes toward trialkylaluminum R3AI (R = Me, Et, i-Bu CpR = substituted cyclopentadienyl ligand)...

Answer 4.15 (i) The chemicals are readily available. The chloride is fairly insoluble in organic solvents and it is possible that chlorine is retained in the product, (ii) There is no need to make the alkali metal alkoxide, as the alcohol is the starting material. The metal may need careful cleaning and there may be the need for heating and a catalyst. One product is a gas. (iii) The lanthanide silylamide has to be prepared first. Because of the bulky nature of the ligand, it may be inert to substimtion, but there are no problems with chloride retention and the reaction should be clean. 

JB The complexes LnM3 tris(binapthoxide) (M = alkali metal) have a good deal of utility as catalysts (Section 8.3). They are readily prepared from either the appropriate lanthanide silylamide, Ln[N(SiMc3)2]3, or chloride. Why choose the former, when the chloride can be bought off the shelf . 

Chemically [Ln CH(SiMe3)2 3] compounds behave as Lewis acids. They are of course attacked by moisture, but also react with nucleophiles such as amines and phenols to form the corresponding lanthanide silylamides and aryloxides ... 

The first metallasilsesquioxanes incorporating lanthanides were described in 1994 by Herrmann et alJ More recently, several Ln silsesquioxanes resulting from reactions of 1 with lanthanide silylamides and aryloxides have been reportedJ Our own efforts in this field resulted in the isolation of a novel Ce(lV) silsesquioxane complex as well as the structural characterization of a bimetallic Yb/Li derivative.Treatment of [Ce N(SiMe3)2 3] or anhydrous CeCls with two equivalents of Cy8Si80n(0H)2 (42) in diethyl ether in the presence of an excess of pyridine exclusively afforded the diamagnetic complex (Cy8Si80]3)2Ce(py)3 (43, Scheme 29.11). Quite surprisingly, in both cases cerium was oxidized to the tetravalent oxidation state. 

The MBL polymerization by non-lanthanocene(lll) silylamides, Ln[N (SiMe3)2]3 (Ln = La, Nd, Sm, Er), is much slower (>130 times) than the polymerization by Cp 2Sm(THF)2. The polymerization by these lanthanide silylamides is also Ul-controUed and can involve more than one silylamide ligand in chain initiation. 

More recently, Aspinah et al. reported the synthesis of a series of lanthanide silsesquioxanes resulting from reactions of 3 with lanthanide tris(silylamides) Ln[N(SiMc3)2]3 (Ln = Y, La, Pr, Eu, Yb). However, single crystals of these materials suitable for X-ray diffraction could not be obtained. The somewhat complicated situation is illustrated in Scheme 25. The lanthanide tris(silylamides) reacted with two-third equivalents of the trisilanol 3 in THF to give the lanthanide silsesquioxanes 85, which are dimeric in solution at 233 K. Reaction of Ln[N(SiMe3)2]3 with one equivalent of 3 in THF resulted in complete conversion of 3 to the trisilylated compound 14, as did the reaction of Ln[N(SiMe3)2]3 with two-third equivalents of 3 in toluene. 

As can be seen from Scheme III, lanthanide halides are suitable precursors for the synthesis of homoleptic derivatives such as silylamides [114], cyclopen-tadienyls [115] and aryloxides [116]. Such organometallies can be readily obtained in a pure form by sublimating them from the reaction mixture. They themselves are important precursors in organometallic transformations (vide infra). Heteroleptic complexes of the type CpxLn(halide)y (x + y = 2,3) are important synthetic precursors with respect to formation of various Ln-X bonds via simple metathesis reactions [2-29]. Fig. 4 indicates the lanthanide element bonds which are involved in these ubiquitous heteroleptic cyclopentadienyl systems. 

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],... 

A depending on the size of the lanthanide metals. Delocalization of electron density on four equivalent nitrogen atoms causes elongation of the Ln-N bonds at about 0.10-0.15 A compared to silylamides. The close proximity of the macrocyclic 7i-systems in sandwich complexes proved to be useful as structural and spectroscopic models for the bacteriochlorophyll [Mg(Bchl)]2, the special pair in the reaction center of bacterial photosynthesis [211,212]. The distance between the pyrrole rings in [Mg(Bchl)]2 is about 3 A. 

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]. 

According to this extended silylamide route, completely exchanged Ln(tritox)3(THF) are formed easily and in high yield for the smaller lanthanides. THF dissociation in solution probably induces the reaction of Ln(bdsa)3(THF)2... 

The silylamide route is also transferable to lanthanide(II) chemistry. However, now steric effects are of minor importance due to the enhanced steric flexibility of Ln(II) amides. Eu(II) and Yb(II) silylamides are accessible to exchange reactions for all reagents listed in Scheme 6. For example... 

Lanthanide(III) Pc doubledecker complexes, LnPc2, possess peculiar electronic properties which make them the first molecular semiconductors [287]. Applications in the field of high speed thin film electronics have been discussed (by Simon et al.). Like the silylamides, they are also sublimable. The radical... 

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. ... 

Synthetic strategies to alkoxide complexes have been covered in full by previous reviews [14]. The silylamide route proved to be an advantageous method of preparation, especially in the case of homoleptic derivatives [15]. The group (IIIA) elements - scandium, yttrium and lanthanum - are considered as lanthanides on the basis of their general chemical similarity to the true lanthanides. 

At this point the author would like to emphasize the importance and flexibility of the silylamide route. Ln(tritox)3 complexes are readily available from Ln[N(SiMe3)2]3 only for the larger lanthanide elements. However, the decreased steric bulk in Ln[N(SiHMe2)2]3(THF)2 amide precursors also allows the isolation of Ln(tritox)3 complexes of the smaller lanthanide elements [16, 40]. 

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