Dimer chemistry

It is well-known that an extensive chemistry of mononuclear chromium(II) exists in addition to the dimeric chemistry relevant to this review. Furthermore, oxidation of chromous acetate with a variety of oxidants has been shown to produce monomeric chromium(III) species. The rate law for disappearance of the chromous dimer has been interpreted as indicating that dissociation of the acetate dimer precedes the oxidation step (45). 

Confirmation of the bridging behavior of dppm in rhenium dimer chemistry has been obtained by x-ray analysis of the minor product of the reaction of dppm with Re2Cll in acetone (111). The brown, air-stable solid recrystallized from toluene and having the stoichiometry Re2 Gs (dppm)j 2C6H5CH3 has the eclipsed structure with two bridging dppm ligands and one terminal chloride as shown in Fig. 17. This dimer thus differs from the Re2Cls... 

Manufacture. For the commercial production of DPXN (di-/)-xylylene) (3), two principal synthetic routes have been used the direct pyrolysis of -xylene (4, X = Y = H) and the 1,6-Hofmaim elimination of ammonium (HNR3 ) from a quaternary ammonium hydroxide (4, X = H, Y = NR3 ). Most of the routes to DPX share a common strategy PX is generated at a controlled rate in a dilute medium, so that its conversion to dimer is favored over the conversion to polymer. The polymer by-product is of no value because it can neither be recycled nor processed into a commercially useful form. Its formation is minimised by careful attention to process engineering. The chemistry of the direct pyrolysis route is shown in equation 1 ... 

The dimer of phosphonic acid, diphosphonic acid [36465-90-4] (pyrophosphoms acid), H4P2O3, is formed by the reaction of phosphoms trichloride and phosphonic acid in the ratio of 1 5. It is also formed by the thermal decomposition of phosphonic acid. Unlike the chemistry of phosphoric acid, thermal dehydration does not lead to polymers beyond the dimer extended dehydration leads to a disproportionation to condensed forms of phosphoric acid, such as [2466-09-3] and phosphine. 

The monomer, CPD, obtained via cracking of the dimer, DCPD, and the dimer both have extensive uses. Cyclopentadiene is probably the most widely studied conjugated, cycHc diolefin system. Eleven review articles dealing with the chemistry of cyclopentadiene have been pubHshed (1—11). An article dealing specifically with European uses of DCPD has also been pubHshed (12). The discovery ia 1951 of stable metal derivatives has given additional impetus to the study of the chemistry of cyclopentadiene. Eive review articles have been pubHshed on this subject (13—17). 

The chemistry of benzazetidin-2-ones (251) can also be explained in terms of facile ring opening to the iminoketenes (252) which dimerize, rearrange or can be intercepted by nucleophiles or in cycloadditions depending on the conditions. Indeed, this ring opening precludes their isolation in all but exceptional cases (Section 5.09.4.3.5) (76AHC(19)215). 

The accessibility of the +4 and +6 oxidation states for sulfur and, to a lesser extent, selenium gives rise to both acyclic and cyclic molecules that have no parallels in N-O chemistry. Thus there is an extensive chemistry of chalcogen diimides RN=E=NR (E = S, Se, Te) (Section 10.4). In the case of Te these unsaturated molecules form dimeric structures reflecting the increasing reluctance for the heavier chalcogens to form multiple bonds to nitrogen. The acyclic molecule N=Sp3,... 

Compare your results to the experimental values of -34.0 2 kcal mol for the lithium reaction and -3.6 .5 kcal mol" for the water dimer reaction. Use the same model chemistry as in Example 8.2 B3LYP/6-311+G(2df,2p) // B3LYP/6-31G(d). 

The structure of the UQ-cyt c reductase, also known as the cytochrome bc complex, has been determined by Johann Deisenhofer and his colleagues. (Deisenhofer was a co-recipient of the Nobel Prize in Chemistry for his work on the structure of a photosynthetic reaction center [see Chapter 22]). The complex is a dimer, with each monomer consisting of 11 protein subunits and 2165 amino acid residues (monomer mass, 248 kD). The dimeric structure is pear-shaped and consists of a large domain that extends 75 A into the mito-... 

The M(05H5)201 compounds, which are actually 01-bridged dimers, [(05H5)M(/l-01)2-M(05H5)], provide an extensive substitution chemistry in which /r-Ol can be replaced by a variety of ligands including H, ON, NH2, MeO and alkyl groups. 

Because of its significance, some basic principles of the Ni-catalyzed dimerization of propene in chloroaluminate ionic liquids should be presented here. Table 5.2-2 displays some reported examples, selected to explain the most important aspects of oligomerization chemistry in chloroaluminate ionic liquids [97]. 

The activation of silylene complexes is induced both photochemically or by addition of a base, e.g. pyridine. A similar base-induced cleavage is known from the chemistry of carbene complexes however, in this case the carbenes so formed dimerize to give alkenes. Finally, a silylene cleavage can also be achieved thermally. Melting of the compounds 4-7 in high vacuum yields the dimeric complexes 48-51 with loss of HMPA. The dimers, on the other hand, can be transformed into polysilanes and iron carbonyl clusters above 120 °C. In all cases, the resulting polymers have been identified by spectroscopic methods. 

---