Monomeric structures

Electron diffraction studies provide valuable information about structures in the gas phase. Consequently, this method is important for chalcogen-nitrogen compounds that are liquids or gases at room temperature. The application of this technique has provided evidence for the monomeric structures of the 1,2,3,5-dithiadiazolyl radical [CEsCNSSN] (3.3) and the 1,3,2-dithiazolyl [CEsCSNSCCEs] (3.4), a... 

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. 

The HIV-l protease is a remarkable viral imitation of mammalian aspartic proteases It is a dimer of identical subunits that mimics the two-lobed monomeric structure of pepsin and other aspartic proteases. The HIV-l protease subunits are 99-residue polypeptides that are homologous with the individual domains of the monomeric proteases. Structures determined by X-ray diffraction studies reveal that the active site of HIV-l protease is formed at the interface of the homodimer and consists of two aspartate residues, designated Asp and Asp one contributed by each subunit (Figure 16.29). In the homodimer, the active site is covered by two identical flaps, one from each subunit, in contrast to the monomeric aspartic proteases, which possess only a single active-site flap. 

Reagent 7 is easily prepared from commercially available diacetone-D-glucose and trichloro(cyclopentadienyl)titanium35 (Section 1.3.3.3.8.1.). The monomeric structure of reagent 7 was confirmed by an X-ray crystal structure analysis1 7. Complex 9 is obtained36 analogously from (7 .7 )-tartaric acid derived (R,7 )-2,3-CMsopropylidene-l,l,4,4-tetraphenyl-1,2,3,4-butanetetrol. 

It gives the parent ion in the mass spectrum and has a simple IR spectrum (i (Ru=0) 1040 cm-1 and (i/(Ru—F) 720 cm-1) similar to that of the vapour (1060, 710, 675 cm-1), implying a monomeric structure. Chlorides RuOC12 and Ru2OC1x (x = 5,6) have been claimed various oxo complexes Ru2OX o are well defined. 

In 1996, Hawker and Frechet83 discussed a comparison between linear hyperbranched and dendritic macromolecules (Fig. 5.17) obtained with the same monomeric structure, 3,5-dihydroxybenzoic. The thermal properties (glass transition and thermal decomposition) were not affected by the architecture. 

Consideration was given to effects, such as the acceptor properties of silicon and the silicon-halogen bond energies, which determined whether monomeric structures were retained. A mechanism for the exchange reactions was as formulated in which an intermediate, e.g. (34), was involved. 

T], rj ). A molecular weight determination for the complex supports a monomeric structure in solution. Reaction of ZnMe2 with (H0)2Si(0 Bu)2 leads to the formation of polymeric species, [Zn0Si(0 Bu)20] , that are soluble in organic solvents [107]. For comparison, the zinc sUoxane polymer [Zn0SiPh20] reported by Hornbraker and Conrad is an insoluble material contaminated with ZnO [108]. 

Spectroscopic investigations of the lithium derivatives of cyclohexanone (V-phenylimine indicate that it exists as a dimer in toluene and that as a better donor solvent, THF, is added, equilibrium with a monomeric structure is established. The monomer is favored at high THF concentrations.110 A crystal structure determination was done on the lithiated A-phenylimine of methyl r-butyl ketone, and it was found to be a dimeric structure with the lithium cation positioned above the nitrogen and closer to the phenyl ring than to the (3-carbon of the imine anion.111 The structure, which indicates substantial ionic character, is shown in Figure 1.6. 

The monomeric species can be isolated as a solid in the case of the A,A -dimesityl derivative. The crystal structures of both dimeric and monomeric structures are shown in Figure 10.6. 

M[pzTp](H20) (M = Na, K) have been determined by x-ray diffraction (30). In contrast to the discrete monomeric structures of the tris (pyrazolyl)hydroborato complexes Tl[TpRR ], K[TpBut2], and Cs[TpBut2] described earlier, the hydrated tetrakis(pyrazolyl)borato complexes M[PzTp](H20) (M = Na, K) exhibit an interesting polymeric-type structure, a portion of which is illustrated in Fig. 14. In each case, the cations occupy two different crystallographic sites, and the cation in one of the sites is coordinated to two pyrazolyl groups in a rr-type fashion. 

Examinations of the far-infrared spectra of solutions of Zr (allyl) 3CI and Zr (allyl) 2CI2 (9) suggest that the former exists in solution as the dimer (X), whereas the latter has the monomeric structure (XI). A broad intense peak at 244 cm-1 can be assigned to zirconium-bridging chlorine stretching mode. This band is completely absent from the spectrum of the dihalide and is replaced by a very strong band at 342 cm-1 due to the nonbridging chlorine. 

As documented in detail for organolithium species, ligand and donor play a key role in determining the degree of aggregation. Methyllithium adopts a hexameric structure in hydrocarbon solvents.13,15 In the presence of monodentate, donors such as THF or diethyl ether tetramers are observed, while the increase in donor denticity to 2 (1,1-Dimethoxyethane (DME), N,N,N, N -Tetramethylethylenediamine (TMEDA)) affords monomeric structures. Further documenting the differences between solution and solid states, [CH3Li]4 adopts a tetrameric structure in the latter.15,15a-15c... 

A family of recently published Cp complexes sheds a more detailed light on the role of the donors on the overall structural pattern. Demonstrated with a group of Cp derivatives in the presence of the crown ethers 15-crown-5 and 18-crown-6, monomeric complexes may be obtained in the form CpNa(15-crown-5) 84. In a parallel fashion, 18-crown-6 has been shown to be effective in supporting monomeric structures of the heavier alkali metals bound to Cp. Examples include CpM(18-crown-6) (M = K 85, Rb 86, Cs 87).100 101... 

There are two antigen binding sites on each of the basic Ig-type monomeric structures, formed by the heavy-light chain proximity in the N-terminal, hypervariable region at the tips of the y structure. The unique tertiary structure created by these subunit pairings produces... 

Monosulfoxide 48 where the transannular interaction of the sulfur atoms is minimal also reacts with triflic anhydride to give a white precipitate of highly strained disulfonium dication 49, which is extremely difficult to isolate.71 Dication 49 exhibits only one 1H NMR signal as a singlet at 3.80 ppm in agreement with a symmetric monomeric structure.71... 

Isopropyltriisopropoxystannane has monomeric structure in solution and the associated structure 48 in the solid phase. 

---