Bis-solvated dimer

The effect of solvation by Lewis base donor solvents is to reduce this complexity. THF, the most widely used solvent for these bases, forms bis-solvated dimeric complexes with most amidohthiums, since this smallest Li2N2 ring provides maximum room for the additional donor molecules. Chelating donors may even produce monomers, but ion separation is found in the sohd state only when the anion is stabihzed by electron-accepting heteroatoms. ... 

Since most of the synthetically useful enolate anions described in the previous section are prepared by the reactions of enolizable substrates with alkali metal amide bases, it is appropriate to note a few structures of these amide bases. The common bases in synthetic organic chemistry include LDA and LHMDS. The structures of both of these bases are known as the THF solvates.Both of these compounds form bis-solvated dimers corresponding to structure (201). The diethyl ether solvate of LHMDS also forms a bis-solvated dimer (202).Sodium hexamethyldisilazide crystallizes as an unaggregated monomer from benzene solution.Two different cryst line forms of KHMDS are known as the polymeric dioxane solvate (203), ° and the unsolvated dimer (204). ... 

A series of fundamental studies on the aggregation of LDA in solution revealed that, in monodentate etheric solvents, LDA without exception forms bis-solvated dimers 52 (Scheme 2.13) and, remarkably, the body of this structure is even maintained with bidentate ligands that do not lead to a chelates, as shown by dimeric structures 53 [62, 63]. 

Similar to the LDA dimers 52 in solution and in the crystal, the chiral amide 72a forms a bis-solvated dimer 82 as shown by the crystal structure [92] and NMR studies in THF [93]. The dimeric structure 83 was found in the case of Koga s base 75 (X = CH2) wherein hthium adopts a threefold coordination by chelation and not by coordination to THF [94] (Scheme 2.23). Similar dimeric structures were confirmed more recently by a variety of NMR techniques for chiral lithium amides derived from valinol [95]. 

In the monomeric structure of 10.21 the solvation of both Li" ions by two THF molecules prevents further aggregation. The four S-N bond lengths are equal at ca. 1.60 A indicating that the negative charge is delocalized over the S(N Bu)4 unit. In the dimer 10.22 one [McS(N Bu)3] anion is coordinated to both Li" ions, one of which is bis-solvated by THF, while the other is chelated only to the unsolvated Li" ion. 

In many cases the temperature dependence of the quadrupolar coupling constant is an indicator of dynamic processes, because the symmetry around the lithium cation is affected by motions which are fast on the NMR time scale. If the rate of these processes exceeds 1/x, the effective symmetry around the lithium cation increases and a decrease in x( Li) results. In Li MAS spectra, a broadening of the satellite transitions can be observed which eventually disappear completely if the rate of the dynamic process comes in the order of the quadrupole frequency. This behaviour was observed for the THF solvated dimer of bis(trimethylsilylamido)lithium, where the Li MAS spectrum at 353 K shows only the central transition and the sidebands caused by CSA and homonuclear Li- Li dipole coupling (Figure 27) . The simulation of the high-temperature spectrum yielded —20 ppm and 1300 Hz for these quantities, respectively. The dipole coupling agrees closely with the theoretical value of 1319 Hz calculated from the Li-Li distance of 2.4 A, which was determined by an X-ray study. 

The lithium enol derived from lV,lV-dimethylcycloheptatrienecarboxainide (172) crystallizes as the bis-THF-solvated dimer (173). Neither the amide nitrogens nor the extended ir-system participates in complexation to the lithium atoms in this complex. 

Complex 446 is a doubly chloro-bridged dimer in the solid state but is essentially monomeric in dichloromethane (or chloroform) solution (Scheme 101). Thus, coordination of an additional Lewis-basic solvent (S) molecule such as acetone, THF, and acetonitrile is feasible in a position tram to the Cp ligand to form a solvate, but no such stable solvates could be isolated. Methanol or water (X) displaces the ether side chain to form the bis(solvate) adducts however, the chelate is re-formed on removal of the solvent under vacuum to give stable methanol or water monoadducts. The THF-solvate of the water adduct has been structurally characterized. 

C5 gHs 6CI4CU4O2P4, Bis(diphenylphosphino)methane-bis(chlorocopper) dimer acetone solvate, 40B, 1064... 

Similar observations were made with bis[alkoxy(alkylamino)carbene]gold complexes, which are prepared by ring opening of trinuclear precursors with trifluoromethanesulfonic (triflic) acid. Through anion metathesis, several crystalline forms were obtained, the structures of which were determined. The triflate salt has a chain of cations in the crystal, while the chloroform solvate of a />-benzoquinolate contains only monomeric cations. In the solvent-free crystals, dimers are present (Scheme 59).251... 

Although the latter product is a solvated mononuclear [Rh(MeOH)2(diphos)]+ cation, in the solid state it is isolated as a binuclear complex of formula [Rh2 (diphos)2](BF4)2, in which each rhodium center is bonded to two phosphorus atoms of a chelating bis(diphenylphosphino)ethane ligand, and to a phenyl ring of the bis(diphenylphosphino)ethane ligand of the other rhodium atom. This dimer reverts to a mononuclear species on redissolving. The mechanism of hydrogenation of the prochiral alkene methyl(Z)-a-acetamidocinnamate, studied in detail by Halpern [31], is depicted in Scheme 1.7. 

A bis(cyclophane)ruthenium(II) complex has been prepared by using Bennett s procedure reaction of diene 265, obtained by Birch reduction of 4,5,7,8-tetramethyl[22](l,4)cyclophane 266, with ruthenium chloride gives the dimeric chloride complex 267 (Scheme 27, p. 223). Treatment of the solvated complex 268 with 266 in the presence of trifluoroacetic acid leads to 269 (163). The structures of complexes 267, 268, and 269 are based on ... 

A more general route to make bis(cyclophane)ruthenium(II) complexes involves a reduction of 263 (arene = benzene) with Red-Al to afford the [ 174-1,3-cyclohexadiene)(tf-cyclophane)]ruthenium(O) derivatives 271 (Scheme 28, p. 224). Treatment of 271 with hydrochloric acid gives the dimeric chloride complexes 272, which lead the desired bis(r)6-[2 ]cyclo-phane)ruthenium(II) complexes 274 via Bennett s procedure (145). Synthesis of the oligomer 275a is also achieved in quantitative yield by heating 274 with the solvated complex 7 (arene = C6Me6) in neat trifluoroacetic acid. 

Simple arene solvates of bismuth(III) halides and pseudohalides have been structurally characterized, as have adducts prepared in the presence of AlCls. " The sohd-state structures of these compounds show 7t-coordination of the arenes to the bismuth centers, as well as intermolecular Bi- X bonding that produces dimers or polymeric structures. Arene complexes that have been structurally characterized include those of benzene, o-, m- and jo-xylene, mesitylene, and hexamethylbenzene. tt-Coordination of arenes has also been seen in [Bi(OC6F5)3(toluene)]2 and BL4(/r4-0)(/u-OC6F5)6 /r3-OBi(/r-OC6F5)3 -2(C6H5CH3). 5... 

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