Metallacycles

Metallacycle fomiation has also been observed in bis-Cp complexes. Heating Cp 2UR[P(Si(CH2)2)2] (R = Cl [146840-37-17, CH [146840-39-3]) results in the metaHation of the phosphido ligand. These complexes are stmcturaHy similar to the group 4 and 6 transition-metal metallacycle complexes, but show a dramatically reduced reactivity. 

Coordination as the motif in the rational design of supramolecular metallacyclic polygons and polyhedra 97ACR502. 

Metallacycles as intermediates in redox rearrangements of cyclopropane and cyclobutane derivatives 96MI10. 

Metallacycles containing metal-heteroatom bond in their ring system 98YGK171. 

On heating, the neopentyl Pt(PEt3)2(CH2CMe3)2 undergoes an intramolecular metallation elimination [108a] (Figure 3.59), which appears to involve initial phosphine loss affording a platinum(IV) metallacycle. 

A careful investigation of the reaction kinetics and detailed trapping experiments allow the conclusion that in this case a a-bond metathesis reaction mechanism applies. The polymerization reaction of PhSiH3 by CpCp Hf(SiH2Ph)Cl has been monitored by H-NMR spectroscopy. The data k(75 °C) = 1.1(1) x 10-4 M 1 s AH = 19.5(2) kcal mol" AS = -21(l)euandkH/fcD = 2.9(2) (75 °C) are in good agreement with the proposed mechanism with a metallacycle as transition state [164],... 

The diazaphosphane or aminoiminophosphane ligands with a NPN framework are another subclass of cyclophosphazenes. These compounds with both phosphorus in oxidation state (111) [104-110] and (V) [111-112] have been employed in the synthesis of four membered heterocycles and coordination chemistry with group 13 derivatives. Several complexes of trivalent phosphorus derivatives with both aluminum halide and alkyls are known as illustrated for 48 in Scheme 21 [113-119]. The structure determination of 48 confirms the formation of a four membered metallacycle [116, 117],... 

R=Me the connectivity is TiCp2(/U-S3)(/U-S4)NMe. This different behavior between the SyNH and SyNMe systems cannot be attributed to either recognizable steric or electronic reasons. By contrast, the reaction with RN(/t-S2)2NR (R=Me, n-Oct) in hexane at 20 °C gave not the expected seven-membered ring compound but a six-membered metallacycle, TiCpyl/f-SylNR [36] (Scheme 5). 

Keywords Valence electron rule, Metal ring, Metal cluster, AN +2 valence electron rule, 8/V +6 valence electron rule, 6N +14 valence electron rule, Pentagon stability, Cyclopentaphosphane, Hydronitrogen, Polynitrogen, Triazene, 2-Tetrazene, Tetrazadiene, Pentazole, Hexazine, Nitrogen Oxide, Disiloxane, Disilaoxirane, 1,3-Cyclodisiloxane, Metallacycle, Inorganic heterocycle... 

There are many four-membered metallacycles containing short metal—metal nonbonded distances. Cyclodisilazanes (Scheme 12a) isoelectronic to 1,3-cyclodis-iloxanes also have short Si—Si distances [136, 137]. 

Short nonbonded Si—Si distances have been observed in four membered metallacycles (Scheme 12b) with a Pt, Ir, W, or Nb atom [138-142] in place of one of the oxygen (nitrogen) atoms of 1,3-cyclodisilazanes (1,3-cyclodisilazanes) and in U-silylene-bridged dinuclear platinum complexes (Scheme 12c) [143, 144]. Electron donating occupied orbitals are expected to be on the platinum atoms like lone pair orbitals on the oxygen atoms in cyclodisiloxanes. 

A novel chiral dissymmetric chelating Hgand, the non-stabiUzed phosphonium ylide of (R)-BINAP 44, allowed in presence of [Rh(cod)Cl]2 the synthesis of a new type of eight-membered metallacycle, the stable rhodium(I) complex 45, interesting for its potential catalytic properties (Scheme 19) [81]. In contrast to the reactions of stabihzed ylides with cyclooctadienyl palladium or platinum complexes (see Scheme 20), the cyclooctadiene is not attacked by the carbanionic center. Notice that the reactions of ester-stabilized phosphonium ylides of BINAP with rhodium(I) (and also with palladium(II)) complexes lead to the formation of the corresponding chelated compounds but this time with an equilibrium be-... 

Scheme 5 Metallacycle ring contraction mediated by NCA addition...

The catalytic cycle, which is supported by stoichiometric and labeling experiments, is shown in Scheme 38. Loss of 2 equiv. of N2 from 5 affords the active species a. Reaction of a with the 1,6-enyne gives the metallacycle complex b. Subsequently, b reacts with H2 to give the alkenyl hydride complex c or the alkyl hydride complex d. Finally, reductive elimination constructs the C-H bond in the cyclization product and regenerates intermediate a to complete the catalytic cycle. 

With regard to the mechanism of the cycloisomerization, Fiirstner et al. found strong evidence of a metallacyclic intermediate. By labeling the allylic position of enynes 46 and 48, they showed that reactions yielding traws-annulated rings 47 transferred the deuterium atom to the exocychc double bond (eq. 1 in Scheme 10), whereas c -annulated rings 49 formed with complete preservation of the position of the deuterium atom (eq. 2 in Scheme 10). This corresponds well to a metallacycUc... 

Scheme 10 Deuterium-labeling experiment providing evidence for a metallacyclic reaction mechanism [17]...

As complex 40 proved to be active in cycloaddition reactions and is isoelectronic to Rh(+1), which is a potent catalyst for [2 + 2 + 2] cycloadditions [20, 21], it was expected that 40 might also be active in those reactions, which is indeed the case. Triyne 54 could be converted to the [2 + 2 + 2]-cycloaddition product 55 in good yield (eq. 3 in Scheme 11). Mechanistically, this reaction is also assumed to proceed via a metallacyclic intermediate. 

A supramolecular assembly of macromolecules bearing antenna dendron has been reported. Pyrazole-anchored PBE dendrons were synthesized to examine the coordination behavior to transition-metal cations (Cu, Au, Ag) [31]. Self-assembled metallacycles were found. The Cu-metallacycle further formed luminescent fibers about 1 pm in diameter. The luminescence (605 nm) occurred by the excitation of the dendron (280 nm) and the excitation spectrum was coincident with the absorption spectrum of the dendron, suggesting the antenna effect. Interestingly, the luminescence of the Cu-metallacycle fiber disappeared when the fiber was dissociated into the individual metallacycles in C2H2. 

From the results discussed so far, it is evident that only CH2 groups have been observed in the very early stages of the ethylene polymerization reaction. Of course, this could be due to formation of metallacycles, but can be also a consequence of the high TOP which makes the observation of the first species troublesome. To better focalize the problem it is useful to present a concise review of the models proposed in the literature for ethylene coordination, initiation, and propagation reactions. 

Experimental results supporting the metallacycles model for initiation and propagation have also been proposed [77,99,135,136]. As already discussed, Ghiotti et al. [53] and Zecchina et al. [77] did not obtain IR spectral evidence indicating the presence of vinyl or methyl groups in the firsts stages... 

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