Crowded complexes

Mechanistic interpretation of activation volumes on square-planar complexes is complicated by the geometry. The sterically less crowded complexes may have loosely bound solvent molecules occupying the axial sites above and below the plane. Replacing them in the formation of a five-coordinate transition state or intermediate may result by compensation in relatively small volume effects. It is therefore difficult to distinguish between Ia and A mechanisms from the value of the activation volume. Nevertheless, the AV values are negative and together with the second-order rate laws observed, point to an a-activation for those solvent exchange reactions. 

These l-hydroxy-2-metalla (5,6,7)-allyl complexes result directly from the reactions of the less sterically crowded complexes [lr(Cn)(cod)](OTf) (Cn = 1,4,7-triazacyclononane) [82] and [Rh(/c -Py3S3)(cod)](BPh4) [81]. Slowing down the reactions by increasing the steric crowding around the metal, the kinetic isomers 2-irida oxetane and 6,7-oxarhoda tetracyclodecane can be isolated from the reactions of [lr(Cn )(cod)](OTf) [82] and [Rh(/c -L )(cod)](Pp6) (L = Cn, dpa-R ) [72,79] with H2O2, respectively. The... 

Another general method is based on oxygen insertion into metal-hydrogen bonds (50,72,79-81) by any of several known mechanisms. Hydrogen abstraction by superoxo complexes followed by oxygenation of the reduced metal, as in the catalytic reaction of Eqs. (3)-(4) (50,72), works well but is limited by the low availability of water-soluble transition metal hydrides and slow hydrogen transfer (equivalent of reaction (3)) for sterically crowded complexes. 

To relieve the strain of sterically demanding ligands, a metal often remains coordinatively unsaturated. Copper(I) halides and phosphines form cubane-like metal cluster compounds, Lm(CuX) ,12 With the bulky trimesitylphosphine, a monomeric two-coordinate [CuBr(Pmes)3] is formed, Br—Cu—P = 173.7°.252 The d(Cu—P) of 2.193 A is comparable to that in normal tet-rameric complexes, but d(Cu—Br) at 2.225 A is shorter, no doubt due to the reduced coordination number. Heating crowded complexes can also result in a reduction in coordination number (see equation 65). 

Reaction of crowded chromium alkenyl Fischer carbene (50) with bulky ketene acetals provides an interesting entry to 3-substituted pent-l-ynoate (53)45 Formation of the alkyne can be rationalized by a 1,4-nucleophilic addition of the ketene on the unsaturated carbene complex (crowded complexes will not undergo potential 1,2-addition), following by oxonium (51) formation and fragmentation to a vinylidene carbene complex (52), which undergoes a 1,3-shift to the alkynylchromium complex leading the alkyne after reductive elimination. 

The first tris(pentamethylcyclopentadienyl) lanthanide complex was isolated accidentally from the reaction of a divalent samarium complex (CsMes )2Sm with cyclooctatetraene [9]. Following this discovery, two more convenient methods were developed for the preparation of the sterically crowded complexes (C5Mes)3Ln [10],... 

RE yields ethane in each case. The most crowded complex, Pd(CH3)2(PPh3)2, undergoes reductive elimination the most rapidly. ... 

The complex [Cp 2Sm][BPh4] reacts with KCp in benzene to yield the sterically crowded complex Cp 3Sm. This reaction provides a convenient way to make Cp 3Ln complexes of lanthanide elements which do not have a readily... 

Oxidation of the sterically crowded complex Cp 3U provided access to [Cp 2U]2(/i-0) as the first molecular trivalent uranium oxide. The U-O-U angle in this molecule is 171.5(6)°.85 Reduction of the uranium(iv) thiolates... 

Proteins in the cell exist in a crowded complex mixture. Purification of individual proteins is achieved by a sequence of purification steps, including column chromatography. Protein samples may be analyzed by polyacrylamide gel electrophoresis. 

The synthesis of Cp 3U Cp = p -CsMes carved a new path for the researchers to go one step ahead in the electrochemical studies of organoac-tinide complexes. The reduction reaction involving more than two electrons are not common for metal complexes containing just one metal. However, the synthesis of complexes of the type Cp 3M led to the development of sterically induced reduction (SIR) chemistry in which sterically crowded complexes of redox inactive metals act as reductants [87,88]. Evans et al. showed that the sterically induced reduction couple, U(III)/U(IV), can act as a multielectron reductant [89]. As an example, Cp 3U reacts as a three-electron reductant with 1,3,5,7-C8H8, (Eq. 12) in which one electron arises from U(III) (Eq. 13) and two result from two CsMes /CsMes half reactions (Eq. 14) presumably via SIR. This phenomenon was further corroborated by the stepwise reduction of phenyl halide with Cp 3lJ (Eq. 15) ... 

The molecule Cp 3UCl is considered to be a sterically highly crowded complex since the center is relatively small and is bonded to four ligands. Upon addition of another equivalent of PhCl, the complex Cp 3UCl subse-... 

The NMR of rrring carbons and eleven resonances were observed for the A -/ro/ij-l,3-dimethylindane ligand. Only a single carbonyl resonance was observed, 235.0 ppm, even down to — 80°C which suggests that the barrier to rotation about the metal-arene bond is quite low even in this relatively crowded complex. Alternatively the Cr(CO)3 moiety may be non rigid. 

Figure 2.17 shows oxidative addition of H2 only to the linear alkyl complex RRh(CO)L2. Hydrogenation via R Rh(CO)L2 could also occur, but must be very minor because of the high selectivity for hydrogenation of terminal alkenes. The selectivity could be due to an unfavorable oxidative addition of H2 to R Rh(CO)L2 because of steric crowding in the branched alkyl complex,t but a more Important factor may be a very small steady-state ratio of R Rh RRh due to the lower stability of the more crowded complex. 

Other titanium complexes supported by acetylacetonate 17-19, " showing similar structural analogies with 12-16, were reported to have comparable moderate catalytic activities, stereoselectivity and polydispersity for the bulk ROP of rac-LA (Scheme 6.2). In totality, the activity of these complexes seems to be limited, probably by a substantial 7t-donation interaction from the anionic 0,0-bidentate-type ligands to the Ti " Lewis acid that consequently decreases the rate of polymerisation in addition to the sterically crowded complexes limiting the incoming monomer approach. 

Dissociative mechanisms for square-planar substitutions are discussed in a review. A molecular orbital study of insertion of ethene into Pt—H bonds concludes that the reaction can be best described by a series of, preferably, dissociative steps. Rearrangements of three-co-ordinate ML3 T- or Y-shaped i -structures are discussed in this context. Three-co-ordinate intermediates are also suggested in the mechanisms for palladium(ii)-catalysed oxidations of olefins, and for electrophilic cleavage of platinum-carbon ff-bonds by protons. Parallel associative and dissociative processes have been proposed for a substitution reaction of a square-planar rhodium(i) complex in benzene solution. Especially, sterically crowded complexes have been thought to stabilize three-co-ordinate intermediates more easily. Recently determined activation volumes for sterically hindered square-planar complexes both of platinumand palladium are not compatible with dissociative activation, however. 

The pressure and temperature dependence for the substitution of bromide by iodide and thiourea in the sterically crowded complexes cis- and rra/w-[Pt(PEt3)-(2,4,6-Me3C6Ha)Br] have been determined by van Eldik, Palmer, and Kelm. For both the ki and k paths, the activation entropies are strongly negative and the activation volumes vary between —11 and -16 cm moLk This is consistent with an associative mechanism, contrary to previous suggestions. ... 

For example, the Crudden group showed that [(IMes)RhCl(PR3)2] complexes underwent dissociation of the carbene upon heating to 60-80 Treating this species with CO led to the less electron-rich and less sterically crowded complex [(IMes)RhCl(PR3)(CO)], which was stable for weeks at 80 °C. The latter complex also displayed significantly improved oxidative stability compared with the bis carbene analogue. 

Fe(arene)Cp] cations are important in metal-mediated organic synthesis, but the success of this approach depends on the efficient removal of the modified arene from the complex. Brown and co-workers have made a study of the relative merits of a range of decomplexation reagents in the demetallation of [Fe(arene)Cp]PF6 compounds. K BuO in pyridine or DMSO was found to be an excellent demetallating agent, even with sterically crowded complexes. 

In the solid state the complex is monomeric as is the less crowded complex (C5Me4H)3La (Schumann et al. 1993d). An improved synthesis (Evans et al. 1996) makes use of (C5Me5)2Sm(OEt2) and (CsMe5)2Pb for obtaining Cp Sm in >90% yield. This route allowed also the synthesis of the crowded (C5Me4Et)3Sm derivative... 

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