Metallacycles squares

Figure 3 represents the only dimer formed in this series of metallacycles. The coordination number of the indium atom is increased by interaction with the nitrogen atom of a second molecule. The dimer 41 has a center of inversion and three edge-fused four-membered rings. This type of structure is very common in metallacycles that are derived from the ligand 25 (5(5). The central N2In2 ring is almost square-planar [In-N = 2.27(1) A] and the In-C dis-... 

Square-pyramidal metallacycles are not observable at 25 °C upon adding 7-oxa-norbornadiene derivatives to Mo(=CHMe2R)(=NAr)(OCMe2CF3)2 except as a transient... 

Formation of the palladium(II) complexes can be achieved using standard protocols and resulting in square planar complexes with mer (pincer) chelate structure. The six-membered, very flexible metallacycles featuring alkyl linker chains display chiral puckering that would make the use of chiral analogues difficult in asymmetric catalytic applications. 

In some cases, a second product was also formed in the synthesis. This was identified as a CH activated or ortho-metalated complex (N,0)Ni-(o-Ph)PPh2= CH2, where an ortho C-H bond of a phosphorane phenyl group was deprotonated by the Ni-Me moiety with concomitant formation of methane. Fig. 3.10 shows the molecular structure of complex 132 obtained from ligand 114. The central square planar Ni atom is part of two five-membered metallacycles. 

Square-pyramidal metallacycles are not observable at 25°C upon adding 7-oxa-norbomadiene derivatives to Mo(=CHMe2R)(=NAr)(OCMc2CF3)2 except as a transient red colour, rapidly changing to the characteristic orange colour of the living carbene complexes. However, the metallacycles can be observed at low temperature (Bazan 1991c). 

Compounds of type 7 proved to be remarkably active catalysts for the metathesis of internal olefins. [44,68,69] The activity of such species for the metathesis of ordinary internal olefins (e.g., c 5 -2-pentene) appeared to maximize for the OCMe(CF3)2 species. New alkylidene complexes such as W(NAr)(CHPh)[OCMe(CF3)2]2 could be isolated, and in some cases trigonal bipyramidal (TBP) tungstacyclobutane intermediates were stable enough to be observed and isolated. On the basis of this work it was proposed that the rate of reaction of alkylidene complexes with olefins correlated directly with the electron-withdrawing ability of the alkoxide, as found in acetylene metathesis systems described earlier. In many circumstances trigonal bipyramidal or square pyramidal tungstacyclobutane intermediates could be observed. [44] In any system in which ethylene could be formed, unsubstituted metallacycles could... 

Bidimensional metaUacycles of porphyrins, also called molecular squares, were first obtained by treatment of meso-bis(4 pyridyl)porphyrins (i.e. 4 -czsDPyP and 4 -fransDPyP) with the appropriate linear or 90°-angular metal fragments. Reaction of 4 -cisDPyP with a linear connector such as trans-[PdCl2(PhCN)2] led to the 4 + 4 metallacycle in which the tra s-[PdCl2] imits are the sides and the porphyrins are the corners of the square (Fig. 8) [49,59]. 

Fig. 8 Example of a 4 + 4 metallacycle (molecular square) of porphyrins obtained by reaction of the angular 4 -cisDPyP unit with the linear connector tra s-[PdCl2] (see [49] and [59])...

---