Rf-Complexes

Interaction of iron(II) chloride with the lithium salt of R4B2NJ (R = Me, Et) gives sandwiches 61 (R = Me, Et) (67ZAAC1, 96MI4), resembling in electronic properties those of ferrocene (99ICA(288)17). The n- rf-) complex stems from the further complex-formation of 61 (R = Me, Et) with mercury(II) salts via the unsubstituted nitrogen atom. 

The formation of the tricarbonylchromium-complexed fulvene 81 from the 3-dimethylamino-3-(2 -trimethylsilyloxy-2 -propyl)propenylidene complex 80 and 1-pentyne also constitutes a formal [3+2] cycloaddition, although the mechanism is still obscure (Scheme 17) [76]. The rf-complex 81 must arise after an initial alkyne insertion, followed by cyclization, 1,2-shift of the dimethylamino group, and subsequent elimination of the trimethylsilyloxy moiety. Particularly conspicuous here are the alkyne insertion with opposite regioselectivity as compared to that in the Dotz reaction, and the migration of the dimethylamino functionality, which must occur by an intra- or intermo-lecular process. The mode of formation of the cyclopenta[Z ]pyran by-product 82 will be discussed in the next section. 

Sect. 2.1.1) and [3C+2S] cyclopentene derivatives. The product distribution can be controlled by choosing the appropriate reaction conditions [72]. Moreover, the cyclopentene derivatives are the exclusive products from the coupling of fi-pyrrolyl-substituted carbene complexes [72b,c] (Scheme 25). The crucial intermediate chromacyclobutane is formed in an initial step by a [2+2] cycloaddition. This chromacyclobutane rearranges to give the rf-complex when non-coordinating solvents are used. Finally, a reductive elimination leads to the formal [3C+2S] cyclopentene derivatives. 

The proposed mechanism for the hydrosilylation of olefins catalyzed by chloroplatinic acid is outlined in Fig. 6. Catalysis by square-planar or trigonal bipyramidal rf complexes can be similarly described (54, 55, 105). 

A review9 with more than 37 references includes an examination of symmetry groups and chirality conditions for C60 and C70 bonded to one or two metals in rf and/or rf fashion. Palladium and platinum rf complexes of C6o and C70 are described (novel synthesis, NMR spectra, electrochemistry) as well as first optically active organometallic fullerene derivatives. 

Kiindig EP (2004) Synthesis of Transition Metal r 6-Arene Complexes. 7 3-20 Kiindig EP, Pape A (2004) Dearomatization via rf Complexes. 7 71-94... 

Both coordination modes, if and if, are combined in binuclear fj.-7jl rf complexes (C and C ) in which the heterocarbonyl coordinates to one metal in the rf and to the other in the if fashion. Complexes with (C ) and complexes without a metal-metal bond (C) are known. Complexes of type D (n-rf rf) have not been structurally characterized presumably due to the poorer donor properties of the second lone pair at the heteroatom E (o-CE) as compared to the zrCE orbital. The energy of the o-CE orbital is well below that of the zrcE orbital. In heteroformaldehydes, e.g., the energy separation between the o-CE and the zrCE orbital is 1.25 eV (E = S) and 1.85 eV (E = Se).33 However, complexes of type D might be intermediates in a possible isomerization process of complexes C (site exchange of LnM and LnM2). 

The major bonding interaction between the heterocarbonyl ligand and the metal in rf complexes is the back-bonding interaction (d — zr ce), whereas in 771 complexes the donation 771 (d — nE) dominates. Therefore, from the trends in orbital energies of the heterocarbonyl compounds (see Section II), one would expect that in mononuclear complexes rf coordination is increasingly favored within the series O, S, Se, Te. This expectation is conformed by the experimental results (see Section III,B). 

In contrast to 771 complexes the 13C-NMR heterocarbonyl resonance signal in rf complexes is observed at a much higher field in the range between 8 = 27 and 75. In [W(CO)5 Se = C(Aryl)H ] complexes the resonance of the C=Se carbon is temperature dependent due to the temperature dependence of the rj lrf equilibrium and rapid interconversion of the isomers. The position of the C = Se resonance of binuclear ix-yf. rf complexes usually resembles that of the corresponding t complexes. Two sets of signals for the MLn fragments in binuclear /a-tj1 7 complexes indicate that the isomerization G/H is slow on the 13C-NMR time scale [Eq. (2)]. 

In solution [M(CO)5 Se—C(Aryl)H ] complexes show a thermochromic behavior. The color of the compounds is caused by a MLCT of d electrons into the LUMO localized mainly in the Se = C(Aryl)H ligand. In rf complexes this transition is at considerably lower energy than in if complexes. The observed color of solutions of [M(CO)5 Se = C(Aryl)H ] therefore depends on the rflif equilibrium, which is temprature dependent. Thus, solutions of, e.g., [W(CO)5 Se = C(Ph)H ] are blue at room temperature and green at -78°C.45... 

As expected, the C = E distance and the M-E-C angle vary considerably depending on the rf- or -bonding mode. In rf complexes the E-C distance is short and the M-E-C angle is large, whereas in if complexes the E-C distance is long and the M-E-C angle is small. 

Analogous ligand exchange reaction of cis- and Pww-Feist s esters with the dirhodium complex [/i-ClRh(ethylene)2]2 in pentane gave the corresponding Feist s esters rf-complexes [/(-ClRhL, (L = cF, tF) (equation 310). Further reaction of the latter complex [p-ClRhL2]2 (L = tF) with cyclopentadienylthallium (CpTl) in CH2C12 afforded the monorhodium complex CpRh(tF)2, and reaction with a mixture of both CpTl and dirhodium... 

IR data of MS02 6 complexes are diagnostic for both >7 -planar and if geometries for / complexes the S02 asymmetric and symmetric stretches, respectively, are in the ranges 1300-1225 and 1140-1065 cm-1, while for rf complexes they occur at 1160-1100 and 950-850 cm-. 23 The / -pyramidal geometry is not expected for MS02 6 complexes and it appears that no examples are known. 

Figure 1. (Bottom) Diagram of the electrostatic potential adjacent to a membrane bearing a positive charge. The zeta potential is the potential at the hydrodynamic plane of shear, which should be about 2 A from the surface of the membrane. (Top) Schematic of the location of the probe molecules used to detect the potential produced by the adsorption of calcium and other alkaline earth cations to membranes formed from PC. The divalent cation cobalt and the amphipathic, anionic, fluorescent probe TNS will sense the potential at the interface. The non-actin-Rf complex will sense the potential in the center of the membrane.

RHF 3-21G optimized in-plane jj1 complex HF 6-31G optimized out-of-plane rf complex... 

The two extremes of the Dewar-Chatt-Duncanson model for olefin coordination can also be applied to describe aldehyde, ketone, and imine complexes. Resonance structure A is an rf complex of Zr(II), while its resonance structure B is a zirconaaziridine containing Zr-C and Zr-N bonds (Fig. 6). X-ray structural studies of zirconaaziridines and their observed reactivity suggest that resonance structure B is more important. 

The interconversion of zirconaaziridine enantiomers is slower when there is not a primary or secondary alkyl on the zirconaaziridine carbon and isomerization to an azaallyl hydride is not possible. A mechanism that remains available involves the isomerization of each enantiomer to a planar rf complex (Eq. 43), such as that known to interconvert the enantiomers of aromatic aldehydes [16]. For the chelated zirconaaziridine 2d, high-level density functional theory (DFT) methods and a continuum solvation model have shown that enantiomer interconversion occurs through an rj1-imine intermediate (A) rather than through homolysis (B) or heterolysis (C) of the Zr-C bond [71] (Fig. 13). 

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