Metallacycle complexes, structures

The a-osmiumdiazo compound 91 decomposes in a thermal reaction to yield the metallacyclic complex 93 (130). This resembles the electrophilic carbene insertion reaction forming OsCl(CO)2(PPh2C6H4CHCl) (PPh3) (77) (see Section V,D,2), and we suggest that a similar insertion reaction of an electrophilic, cationic osmium carbyne 92 is the key step in this transformation. An X-ray structure determination has confirmed the formulation of 93. 

Metallacyclic complexes containing two molecules of diyne per MCp2 group have also been isolated, that with titanium containing 2,4-alkynyl substituents (225) while with zirconium, the unusual seven-membered metallacumulene structure 226 is adopted, which has only one alkynyl substituent. The bi- and tricyclic 2/2 complexes 227 and 228 have so far been obtained only from reactions of TiCp2 with PhC=CC=CPh.30 ... 

Attempt to prepare Jt-complexes of triafulvenes and related methylene cyclopro-parenes285,427 428 directly by ligand exchange reaction with transition metal complexes resulted in metal insertion into the sigma bond, forming metallacyclic complexes. Thus reaction of the electron-poor triafulvene l,2-diphenyl-3-dicyanomethylenecyclopropene with (ethylene)bis(triphenylphosphine)platinum in refluxing benzene gave two crystalline products whose platinacyclobutene structure was confirmed by X-ray structure analysis (equation 364)429. 

Yamazaki s complex (Structure 5) contains two alkyne molecules linked together to form a five-membered metallacycle. Arene-solvated cobalt atoms, obtained by reacting cobalt vapor and arenes, have been used by Italian workers to promote the conversion of a,w-dialkynes and nitriles giving alkynyl-substituted pyridines [20]. -Tolueneiron(0) complexes have also been utilized for the co-cyclotrimerization of acetylene and alkyl cyanides or benzonitrile giving a-substituted pyridine derivatives. However, the catalytic transformation to the industrially important 2-vinylpyridine fails in this case acrylonitrile cannot be co-cyclotrimerized with acetylene at the iron catalyst [17]. 

The oxidative addition of iridium(I) and rhodium(I) complexes of the type trans-4 to 1,2,3-triphenylcyclopropenylium ion 3 gives the metallacyclic complexes The structures of... 

Erker et al. have shown that FAB adds to zirconocene (butadiene) complexes in toluene to yield zirconocene—(W-C4H6)—borate—betaine complexes (40, Scheme 14). A characteristic feature of the structure of 40 is the weak coordination of an ortho-fluoro substituent to the zirconium center to give a stable metallacyclic complex. This is a common structural... 

Metallacyclic complexes such as the reactant in Equation 10.7b are much more stable thermally than their acylic analogues. This metallacycle decomposes 10 times more slowly than the dibutyl compotmd, and it decomposes by pathways that do not include p-hydrogen elimination. Additional metal-alkyl complexes that are stable to p-hydrogen elimination because of their metallacyclic structure have been isolated as part of studies on complex molecule s mthesis. One example of such a complex is shown in Equation 10.9. ... 

The nitroso complex reacts with CO to give an insertion reaction into the N-Ru bond, generating a metallacyclic complex, the structure of which has been solved by X-ray crystallography in the case in which Ar = 4-chlorophenyl [146]. The CO insertion is fast and, when the reaction between Ru(CO)3(DPPE) and a nitro compound not containing strongly electron-withdrawing substituents is performed in a CO atmosphere, the intermediate nitroso complex is nor even observable. The same type of metallacyclic complexes are also formed by direct reaction of Ru(CO)3(DPPE) with aromatic nitroso compounds. The first steps of the reaction are summarised in Scheme 5 ... 

Since the first observable intermediate in Scheme 6 does not any longer contain the imido fragment, the way by which aniline is formed is not obvious. The initial metallacyclic complex does not react with CO in the absence of any other reagent, but, if an aniline is also added, a new metallacyclic complex is obtained, whose structure has been solved by X-ray crystallography in the case of Ar = Ph and shown to have the structure schematically represented below [109] ... 

Metallacyclic complexes having this stracture have been reported before (see the list of references in [1]). Note that the complex depicted above would be an isomer, as far as the metallacyclic part is considered, of a complex isolated in the case of the Ru(DPPE)-based catalytic system. In this case, however, the same structure would hardly account for the observed reactivity (see also later). If reaction 26 was performed in the presence of a large excess of methanol, only one equivalent of nitrosobenzene was needed to consume all the starting complex and aniline was the only organic product, suggesting a competitive trapping of the nitrene intermediate by either excess PhNO, to yield azoxybenzene, or methanol, to yield aniline. 

Some unsymmetrically substituted eomplexes, e.g. 26a (n = 1, 2, 4) possess the unique ability to metathesize their own ligands to form ehelated NHC-Ru complexes in which the NHC and die tegulaf earbene unit, Ru = CHR, are tethered by a variable eyelie structure. In one example, heating a solution of complex 26a (n = 2) in refluxing toluene afforded the metallacyclic complex 26 in 75% isolated yield (Scheme 16). ... 

In another DPT study, it was found that the incorporatiOTi of a second copper atom into the model resulted in the calculated reactivity of such dinuclear alkynylcopper(l) complexes being dramatically increased as compared to the calculated reactivity of mmionuclear alkynylcopper(l) complexes [97], Calculations were also carried out to locate the positions of copper atoms in dinuclear copper(l) acetylides, transition states, and a metallacycle for a copper(l)-catalysed reactimi of propyne with methyl azide two copper atoms were located, one of which carried as a spectator ligand, for example a prop-l-ynyl anion. A calculated activation energy of 54.0 kJ mol was found for the foimatimi of a metallacycle in a reaction of propyne with methyl azide. The diagrams in Scheme 14 show the calculated geometries and distances separating the two copper atoms in a dinuclear copper (1) acetylide in structure A, in a calculated transition state in structure B, and in a cuprated metallacycle in structure C. 

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],... 

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