Amido precursor

The synthesis of the tellurium analogues of (24) and (25) requires a different approach, since it is not possible to prepare the necessary amido precursors in significant yields by telluration of [ BuN(H)P( U-N Bu)2PN(H) Bu]. However, the prior hthiation of this P(III)/P(III) system to give (7) followed by reaction with elemental tellurium generates the dianion (24, E = Te) as its dihthium salt (Eq. 5) [37] ... 

Typically, transition metal benzenedithiolates (and related derivatives) are prepared by the following methods salt elimination reactions using a metal halide and the dithiolate dianion, thiol exchange, and condensation of the free thiol with oxo, alkoxo, and amido precursors. In one example, the dithiol was... 

Derivatives of the chiral ligand binol have been introduced successfully to Ta chemistry by use of an amido precursor (Scheme 30).316... 

The synthetic procedure is very critical. In our case, we believe that the imido-lithium compound (Li2NR) is present in the solution of butyl lithium and para-toluidine, in diethyl ether, as reported for the dilithiated a-naphtylamine. This is a noticeable difference in comparison to typical preparations of ruthenium, osmium, and iridium imido complexes, " in which a dichlorometal complex and the monolithium salt (LiNHR) in a molar ratio 1 2, appropriate for a ftA-amido precursor, are used. In these cases a subsequent removal of amine, or a dehydro-halogenation step with LiNHR, is required to afford the products and free amine. Equation (1) summarizes our synthetic procedure ... 

The first example of group 4 complexes comprising an annulated derivative with a pendant NHC group was reported in 2006 by Downing and Danopoulos (Scheme 14.20) [14,68]. The titanium (III) complex 37 was obtained by reaction of the Ti(IV) bis-amido precursor TiQ2(NMc2)2 with the corresponding potassium fluorenyl NHC salt and proceeded with Ti(IV) to Ti(III) reduction. Dimethyl-functionalized N-heterocyclic carbene complexes of Ti(IV), Ti(III), and Zr(IV) were also prepared by salt metathesis reactions, and the bidentate coordination mode was confirmed by various X-ray diffraction studies (complexes 38) [69]. 

The first report outlining the synthesis of a homoleptic product was the synthesis by Bochkarev of the divalent compound (Yb(EBu)2 E = S, Se), prepared by a proton-transfer reaction of an amido precursor with HEBu. This was followed by the metatiietical synthesis of Ln(SR)6 and compounds of the heavier chalcogenolates (Ln(EC6H3R3)2 (E = Se, Te R = Me). " The structure of the thiolate compound was described, but stmctural characterization of selenolate and tellurolate compounds did not appear until later, when the E-Si(SiMes)3 ligand was introduced to the Ln field, and new synthetic approaches to complexes of the EPh ligand were developed. 

In order to avoid such surface contaminations and their drastic effects on magnetic properties, an organometallic approach could be advantageous since controlled decomposition under mild conditions can be achieved. Through intensive prospective work, we determined that amido precursors such as Fe[N(SiMe3)2]2(THF) (Me = CH3, THF = tetrahydrofurane) [43] or the dimer (Fe[N(SiMe3)2]2 2 [44] can yield unoxidized iron metal nanoparticles (MNPs) under mild conditions. These precursors exhibit a good compromise between stability (to be stored once prepared) and reactivity( to be decomposed under mild and reductive conditions). 

Aryloxo-NHC-containing complexes could also be produced from amido precursors where the ligands also acted as internal base as shown by the Shen group. Hence, a hydroxyaryl-imidazolium ligand reacted with [LiY N(z-Pr2) 4] and BuLi at — 78 °C to give the NHC-yttrium complex [(NHQ3Y] 21 (Scheme 6.2). " From [LiYb N(z -Pr2) 4], a bis-substituted ytterbium compound [(NHC)2Yb N(z-Pr2) ] 22 was prepared. Of note, all attempts to prepare the mono-substituted complex were unsueeessful. 

Aryloxo-NHC-containing complexes could also be produced from amido precursors acting as internal base. Shen presented a hydrox) ryl-imidazolium... 

Closely related mixed amido/imido/guanidinato tantalum complexes of the type Ta(NR R )[(R R2N)C(NR )2]( = NR ) (R R = Me, Et R = Cy, Pr R = Pr", BuO were synthesized by the insertion of carbodiimides into to tantalum-amide bonds in imidotantalum triamide precursors, and the effects of ligand substitution on thermal properties were studied by TGA/DTA measurements. In addition, selected compounds were pyrolyzed at 600 °C and the decomposition products were studied by GC-MS and NMR spectroscopy. ... 

Two principle strategies have been employed for the synthesis of siloxide-containing molecular precursors. The first involves a silanolysis, or condensation, reaction of the Si - OH groups with a metal amido, alkyl, hahde, or alkoxide complex. The second method involves salt metathesis reactions of an alkali metal siloxide with a metal hahde. Much of our work has been focused on formation of tris(tert-butoxy)siloxide derivatives of the early transition metals and main group elements. The largely imexplored regions of the periodic table include the lanthanides and later transition metals. 

The preparation of similar precursors suitable for the deposition of metal nitrides is analogous to the preparations of phosphorus and arsenic compounds. The initial reaction of metal trialkyls MR3 (M = A1, Ga, In) with amines (NHR 2) results in the formation of oligomeric amido compounds [R2MNR 2] (n = 2 or 3) which eliminate alkanes on thermolysis. The incorporation of a proton as a substituent on the pnictide bridging ligand has been examined, and many compounds of the type [R2MNHR ]2 have been synthesized. The presence of this proton may facilitate /3-elimination, allowing lower deposition temperatures to be used. 

The fact that complex 38 does not react further - that is, it does not oxidatively add the N—H bond - is due to the comparatively low electron density present on the Ir center. However, in the presence of more electron-rich phosphines an adduct similar to 38 may be observed in situ by NMR (see Section 6.5.3 see also below), but then readily activates N—H or C—H bonds. Amine coordination to an electron-rich Ir(I) center further augments its electron density and thus its propensity to oxidative addition reactions. Not only accessible N—H bonds are therefore readily activated but also C—H bonds [32] (cf. cyclo-metallations in Equation 6.14 and Scheme 6.10 below). This latter activation is a possible side reaction and mode of catalyst deactivation in OHA reactions that follow the CMM mechanism. Phosphine-free cationic Ir(I)-amine complexes were also shown to be quite reactive towards C—H bonds [30aj. The stable Ir-ammonia complex 39, which was isolated and structurally characterized by Hartwig and coworkers (Figure 6.7) [33], is accessible either by thermally induced reductive elimination of the corresponding Ir(III)-amido-hydrido precursor or by an acid-base reaction between the 14-electron Ir(I) intermediate 53 and ammonia (see Scheme 6.9). 

Among the polydentate carbene ligands, particular interest has recently been placed on cyclic polycarbenes. ImidazoUum precursors like 23 [89] or 24 [90, 91], which upon C2 deprotonation would lead to tetradentate or even hexadentate double-pincer NHC ligands, have been prepared. Their interesting coordination chemistry will be discussed in Sect. 4. Finally, Arnold et al. developed and reviewed NHC ligands which are functionalized with additional anionic (alkoxide or amido) donor groups [92]. 

However, either for aliphatic or aromatic amines, the corresponding S-phenylthio derivatives are adequate precursors in order to generate /i-amido organoUthium intermediates. Starting materials 157 were successively treated with n-butyllithium and lithinm in the presence of a catalytic amount of DTBB (15%) in THF at —78 °C giving the expected functionalized organolithium intermediates 158, which reacted with different electrophiles to afford, after hydrolysis, the corresponding products 159 (Scheme 56) " ". 

The acyclic precursor is an oc, 3-unsaturated amido aldehyde that was condensed with iV-methylhydroxylamine to generate the nitrone ( )-48, which then underwent a spontaneous cycloaddition with the alkene to afford the 5,5-ring system of the isoxazolidinyl lactam 47. The observed product arises via the ( )-nitrone transition state A [or the (Z)-nitrone equivalent] in which the position of the benzyl group ot to the nitrone effectively controls the two adjacent stereocenters while a third stereocenter is predicted from the alkene geometry. Both transition states maintain the benzyl auxiliary in an equatorial position and thus avoid the unfavorable 1,3-diaxial interaction with the nitrone methyl or oxygen found in transition state B. Semiempirical PM3 calculations confirm the extra stability, predicting exclusive formation of the observed product 47. Related cycloadducts from the intramolecular reaction of nitrones containing ester- rather than amide-tethered alkene functionality are also known (83-85). 

Fig. 3 Examples of precursors for ALD depositions of oxide films. Volatile a halides, b alkoxides, c -diketonates, d organometallics, e organometallic cyclopentadienyl-type compounds and f amido complexes have been exploited...

The volatile metal-containing precursors which satisfy the ALD criteria fall into four main categories (i) halides, (ii) y0-diketonate complexes, (iii) alkoxides, and (iv) true organometallics, viz. metal alkyls and cyclopentadienyl-type compounds (Fig. 3). Also amido complexes have recently gained attention as possible ALD precursors. Occasionally other compounds have been used as ALD precursors for thin films, for instance, metal nitrates, carboxy-lates and isocyanates [17,18]. 

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