Homobimetallics

A conjugated homobimetallic palladium(II) complex (see Figure 40) was formed by one-pot oxidative complexation of 1,4-phenylenediamine with a palladium(II) complex bearing the tri-dentate ligand shown. In the absence of 1,4-phenylenediamine this palladium(II) complex undergoes controlled formation of a macrocyclic tetramer via removal of a labile solvent ligand, as shown in Figure 41.514... 

Figure 40 A conjugated homobimetallic palladium(II) complex, formed by one-pot oxidative complexation of 1,4-phenylenediamine with a palladium(II) complex bearing the tridentate ligand.

Belokon et al.83 have investigated the formation of the homo-and bimetallic titanium complexes with di-Schiff base ligands, by means of FT NMR spectroscopy. The ligands have been shown to adapt the ds-p configuration in titanium (IV) complexes. Analysis of the 1H NMR spectra has allowed determination of the population of the homobimetallic complexes derived from two different Ti(IV) complexes [34],... 

The electronic structures of the hetero- and homobimetallic triple-deckers 11 and 12 were investigated by photoelectron spectroscopy and intermediate neglect of differential overlap (INDO) molecular orbital (MO) calculations, which revealed that the metal-borole interaction is stronger in the dimanganese system 12 than in 11, owing to better Mn-borole versus Co-borole orbital overlap.12... 

NHC ligands have been used to bridge different metals resulting in homo-bimetallic systems. Examples exist for palladium(II), rhodium(I), as well as chromium(O). " Homobimetallic ruthenium(II) systems have been shown to be superior catalysts in cyclopropanation reactions compared to their monometallic... 

A helical homobimetallic mercury(II) complex with a bridging bis(NHC) ligand serves as a starting point for a supramolecular assembly. Also tetrameric cyclic palladium(II) complexes have been obtained with bridging NHC-pyridine ligands. 

Starting from complex IX, Fiirstner developed a homobimetallic phenylindeny-lidene complex XXV (Equation 8.5), and both of these were used in the cyclization of medium-sized rings by RCM. A series of examples is presented which shows that indenylidene complexes are as good as or superior to the classical Grubbs first generation catalyst in terms of yield, reaction rate, and tolerance towards different functional groups (Scheme 8.17) [58]. 

Trost et al. [11] reported another impressive example of bimetallic catalysts in which a Zn-Zn homobimetallic complex (17, Scheme 7) serves as an effective catalyst for direct aldol reactions [11-13]. The proposed structure of the catalyst was verified by mass spectrometry and the best ratio of Et2Zn and the ligand. The chemical yield was moderate in the reaction of methyl ketones (1) (Scheme 7, top) [11,12], but a highly atom-economic system was achieved when a-hydroxylated ketones (10) were used as a substrate (Scheme 7, bottom) [13]. Excellent diastereo- and enantioselectivity were obtained under mild conditions. In contrast to the case of Shibasaki s heteropolymetallic catalyst, syn-1,2-diols (syn-11) were obtained as the major diastereomers. 

Metal(II) species, homobimetallic complexes - During a rhodium insertion into H2(TPP) using [Rh(CO)2Cl]2 in HOAc/NaOAc, a paramagnetic rhodium(II) porphyrin was observed [57] which could be transformed into a hydridorho-dium(III) porphyrin with dihydrogen (path — p) and subsequently into its corresponding base, the anion [Rh(TPP)], which may be likewise regarded as... 

Bimetallic alkenes, with platinum heterobimetallic complexes, 8, 637 homobimetallic complexes, 8, 635 Bimetallic complexes... 

Polymetallic alkynes, with platinum heterobimetallic derivatives, 8, 647 homobimetallic complexes, 8, 645 polymetallic derivatives, 8, 648 Polymetallic cyclopentadienyl iridium complexes, preparation and characteristics, 7, 370... 

Elhabiri, M., Hamacek, J., Bunzli, J. C. G., Albrecht-Gary, A. M., Lanthanide homobimetallic triple-stranded helicates Insight into the self-assembly mechanism. Eur. J. Inorg. Chem. 2004, 51-62. 

Despite the importance of isonitriles as assembling ligands and substrates in organometallic and coordination chemistry, reported studies concerning the structure and the reactivity of homobimetallics and heterobimetallics M(p-CNR)... 

FIGURE 29. Some examples of homobimetallic complexes with p-CNR ligands. 

The emission spectra of homobimetallic and heterobimetallic polymers 89 and 91 in PrCN at 77 K are shown in Figure 41 as examples. Details of the photophysical parameters are presented in Table 14. 

Reaction of the bis-tridentate receptor [L14-2H]2 with R(C104)3 in water produces the highly stable homobimetallic triple-stranded helicates [R2(L14-2H)3] (R = La-Tb except Pm and R = Er-Yb-Lu, log/S[S2(Li4 2H)3] = 51(4), Elhabiri et al. (1999)). NMR spectra point to rigid D3-symmetrical complexes for which exchange between the helical enantiomers is slow on the NMR time scale (i.e., the methylene protons of the ethyl residues are systematically diastereotopic, fig. 55). 

A homobimetallic cerium(IV)-calix[8]arene complex was used together with hydrogen peroxide in the regioselective hydroxylation of simple phenols (Eq. 38), Just 0.5 mol of the cerium complex was sufficient to convert 1 mol phenolic substrate [260],... 

Complexes of type [LyM (CH2)IIX ], where n > 1, have been shown to be useful precursors for hetero- and homobimetallic n(a.,a>) alkanediyl complexes, [LjcM(CH2)bM L, ] (where ML, is not necessarily the same as M Lj,). Such hydrocarbon-bridged binuclear compounds have been proposed as models for intermediates in the Fischer-Tropsch reaction (18,19) and other significant catalytic processes (20-23). Some [LyM (CH2)BX ] complexes are precursors to cyclic carbene complexes (Section III), whereas others have been shown to have synthetic utility in organic chemistry (24). 

On the other hand, an allyl ether can also be considered a protected alcohol that is liberated by Pd(0)-catalyzed allylation for further catalytic transformation. As an excellent example for sequential homobimetallic catalysis, Gabriele and coworkers [85] just recently disclosed a Pd(0)-catalyzed deprotection-Pd(II)-catalyzed heterocyclization of allyloxyaryl propargylic alcohols 97 to provide benzofurans 98 in excellent yields (Scheme 33). 

Mechanistically, this homobimetallic catalytic process can be described and rationalized as a Pd(0)-catalyzed deprotection of the phenyl allylether 97 furnishing phenolate 99 that now can enter the second Pd(II)-catalyzed cycle (Scheme 34). The destiny of the 7r-allyl-Pd complex is a carbonyl insertion to furnish, after nucleophilic displacement with methanol, but-3-enoic acid methyl ester and hydroiodic acid. The phenolate 99 cyclizes to give a vinyl-Pd species that inserts carbon monoxide followed by the attack of methanol. 

Scheme 34 Mechanistic rationale of the sequential homobimetallic deprotection-hetero-cyclization...

Homobimetallic iridium(I) complexes containing the binucleating PNNP ligand undergo oxidative addition and reductive elimination reactions with acetyl chloride and methyl iodide. Thus,... 

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