Reactivity cooperative ligand-centered

Strategy III. Cooperative ligand-centered reactivity based on redox active ligands... 

The synthesis of multimetallic transition metal complexes where the metals are held at specific distances from each other is an important objective because of their potential role in multimetal-centered catalysis in both biological and industrial reactions (1). Moreover, such systems, through cooperative electronic and/or steric effects between metal centers, might give rise to distinct reactivity patterns for both their stoichiometric and catalytic reactions, which are not available to their monometallic analogues (2). Of the ligands that are able to maintain the metal centers in close proximity, the pyrazolate ion (pz ) appears to be a particularly suitable candidate. Pyrazoles (Hpz ) are weak bases (3, 4) and behave as 2-monohapto ligands. 

Catalytic HPNP-tranesterification reactivity (Fig. 40, bottom has been reported for zinc complexes assembled using calix[4]arene ligands (Fig. 65).172,248 251 Complex 9 induces a 23,000-fold rate enhancement over the uncatalyzed intramolecular cyclization reaction of HPNP.248 Kinetic and mechanistic studies indicate strong binding of the HPNP substrate to the complex (K— 5.5 x 10-4M-1) and a kc ll value of 7.7 x 10 4s 1 at pH = 7.0 in acetonitrile/20 mM HEPES (1 1) at 25 °C. As 9 is 50 times more active for HPNP transesterification than 8, which contains only one zinc center, cooperativity between metal centers occurs in the former complex. Notably, the monozinc calix[4]arene complex 8 is 6 times more reactive than 7. This result provides evidence that the hydrophobic calix[4]arene moiety plays an important role in the reaction. 

L(37)0)Zn2]3 +, which exhibits a second-order rate constant that is 120-fold larger than that found for [(L360H)Zn]2+. This dramatic difference in reactivity for the binuclear complexes is attributed to cooperative interactions in [(L(37)0)Zn2]3 + that are not achievable using the binuclear ligands (L38-L40) shown in Fig. 71. For example, the two centers in [(L(37)0)Zn2]3+ may be an optimal distance to provide... 

Transmissions of ligand /ra r-effects and influences along the backbone of the dinuclear species have been documented and the occurrence of cooperative reactivity of the two iridium centers in these systems has been... 

The catalytic performance of NHC-Pd complexes in this reaction is however usually less satisfactory than that of catalytic systems based on other ligands, such as, for example, phosphane hgands, and poly-NHC Pd complexes make no exception to this general trend. Consequently, although reports on the catalysis of the Heck reactions by these complexes have been numerous, they are often hmited to the reaction of aryl iodide or electron-poor, activated aryl bromide substrates, whereas electron-neutral or electron-rich aryl bromides require high temperatures for satisfactory yields. Reactivity of aryl chlorides is confined to electron-poor substrates and is observed only under very drastic conditions (T> 150 °C with tetra-alkylammonium salts as promoters). " An exception is represented by the work of Huynh and Guo on dipalladium(II) complexes with the Janus-type ditz ligand of structure 68." In this case, Heck reactions of activated aryl chlorides were possible already at 120 °C without addition of promoters the authors ascribe this result to the cooperative effect of the two metal centers. Poly-NHC complexes of type 69 have been found to catalyze also the less common diarylation (i.e., double Heck reaction) of ethyl acrylate with aryl bromides, albeit at 120 °C and with tetrabutylammonium bromide as additive. " Simple Pd species such as Pd(OAc)2 and PdCl2 were found to be quite ineffective under these conditions. 

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