Alkaline earth metal complexes amides

Alkaline earth metal complexes, 6-10 amides, 9 anions, 10 cations... 

Barret and Hill have extensively studied the reactivity and catalytic activity of NHC-supported Group 2 amido complexes. Similar to several reports with the alkali metals, tricoordinate NHC-alkaline earth metal complexes 58 could be formed directly from the corresponding conjugate acid by addition of a metal amide salt (Scheme 5.8). Whereas carbene-Li complexes were found to be excellent carbene transfer reagents, 58 could function as a stable carbene equivalent. Indeed, in the presence of Lewis base donors such as triphenylphosphine oxide or protic substrates such as 2-methoxyethylamine, liberation of the free carbene was observed by and NMR. While the authors did not attempt to isolate this free carbene or investigate any additional reactivity, they claimed that the carbene was dissociated under catalyt-ically relevant conditions. 

It is also possible to isolate bis(carbene) complexes involving the heavier alkaline earth metals. Thus, the reaction of two equivalents of 4 (R = Me or (Bu, R = H) with calcium, strontium and barium bis(trimethylsilyl)amides [M N(SiMe3)2 2(thf)2] (M = Ca, Sr, Ba) resulted in the displacement of two thf molecules to afford the corresponding biscarbene species, 19 (19). The solubilities and stabilities of these complexes were found to decrease from calcium to barium. 

An important class of alkali and alkaline earth metal amides are Mulvey s inverse crown complexes (also discussed in Chapter 2, dealing with sodium and potassium amides), in which cationic homo- or heterometallic macrocycles are hosts to anionic guest moieties.The term inverse crown indicates that the Lewis acidic/Lewis basic sites are reversed or exchanged in comparison to conventional crown ether complexes. Scheme 3.9 illustrates the range of recently published alkali and alkaline earth metal amide inverse crown complexes (for related Zn species see Chapter 7 on group 12 amides). 

The first alkaline earth metal inverse crowns were discovered in the late 1990s via the fortuitous reaction of traces of O2 with a mixture of an alkali metal and a magnesium amide to afford complexes of e formulae (see Scheme 3.9) M2Mg2 N(SiMe3)2 4(O2)x(O)... 

The formation of alkaline earth metal bis[bis(tiialkylsilyl)amides] has been discussed in detail elsewhere." Like all heavier group 2 metal bis[bis(tiisalkylsilyl)amides], the complex [(Ca N(SiMe3)2 2)2] has a dimeric structure both in solution and the solid state (Figure 3.8), in which the calcium atoms are in a distorted trigonal planar environment. 

Earlier work in this field has been thoroughly reviewed [1,2]. However, to illustrate in a sensible and logical way the evolution from simple metal ion promotion of acyl transfer in supramolecular complexes to supramolecular catalysts capable of turnover catalysis, an account of earlier work is appropriate. The following sections present a brief overview of our earlier observations related to the influence of alkaline-earth metal ions and their complexes with crown ethers on the alcoholysis of esters and of activated amides under basic conditions. 

The findings that, both in ester and amide cleavage, an alkaline-earth metal ion is still catalytically active when complexed with a crown ether, and that a fraction of the binding energy made available by coordinative interactions with the polyether chain can be translated into catalysis, provide the basis for the construction of supramolecular catalysts capable of esterase and amidase activity. 

Our design of bimetallic catalysts based on crown-complexed alkaline-earth metal ions, for use in reactions of ester and activated amides endowed with a distal carboxylate anchoring group, is based on the mechanistic hypothesis outlined in Scheme 5.3. Such hypothesis critically rests on the finding that in EtOH solution... 

Led by the efforts of Westerhausen and coworkers, a variety of heavy alkaline earth metal zincates or aluminates under formation of alkaline earth metal-carbon contacts have been prepared. Examples include the reaction of trialkylalanes with donor-free calcium i)fr-amides to afford dimeric aluminates, shown in Figure 30. Noteworthy is the planar Ca2N2 center of this complex, in addition to close calcium-aluminum contacts, which may indicate the presence of three-center, two-electron bonding. 

Carbene complexes of alkaline earth metal amides and metallocenes have also been reported. Reaction of calcium, strontium, and barium bis(trimethylsilyl)amides [M(N(SiMe3)2 2(thf)2] (M = Ca, Sr, Ba) with two equivalents... 

Salt metathesis [Eq. (7)] is a well-established route in the synthesis of a variety of alkaline-earth metal alkyls [186,208-210], allyls [211-214], benzylates [149,178], cyclopentadienides [17, 108, 215-219], pentadienyls [220], fluorenyls [17, 221], indenyls [222], amides [112, 113, 223], p-diketiminates [112], guanidinates [15, 194], aUcoxides [224], aryloxides [224], silanides [210, 225-228], thiolates [229], phosphanides [230, 231], selenolates [229], and germanides [232, 233]. However, the route has been rarely used for the synthesis of more reactive alkyl and aryl metal complexes, largely due to issues pertaining to ether cleavage chemistry as metathesis typically requires the presence of an ethereal solvent. 

Transamination is used for alkaline earth elements and also for early transition metals when the amide complexes are available... 

Amide. — It has been pointed out before that europium behaves more or less like the alkaline earths and is closely related to strontium and barium. It is found to react with liquid ammonia at —78° C in much the same way as the alkali metals forming a characteristic deep blue solution. Eu(NH2)2 can be isolated [260] from the blue solution. Recent electron paramagnetic studies [261] of solutions of europium in liquid ammonia showed the presence of complex hyperfine lines arising from Eu2+ (8 7/2, g — 1.990 0.002) besides the characteristic single line of the solvated electron (g = 2.0014 0.0002) K The departure of the Eu2+ <7-value from the free electron value is explained as being due to spin-orbit coupling and a slight admixture (3.5%) of the 6P7/2 state. 

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