Coordination schemes higher metal complexes

The transition metal complex-catalyzed formation of 1,3-dioxepanes from vinyl ethers has also been described. For example, reaction of allyl vinyl ether 157 with a nonhydridic ruthenium complex at higher temperatures and without any solvent produced 1,3-dioxepane 159 whereas, the use of a hydridic ruthenium complex resulted in the formation of vinyl ether 158 by double-bond isomerization (Scheme 43). It was suggested that cyclic acetal formation proceeds via a 7i-allyl-hydrido transient complex, which undergoes nucleophilic attack of the OH group at the coordinated Jt-allyl <2004SL1203>. 

Recently, Shirai and Hojo25) have reported the synthesis of the soluble coordinate polymers containing metal-phthalocyanines in the main chain according to Scheme 2 (Eq. 7). The polymers obtained by polycondensation of metal-phthalocyanines bearing peripheral carboxy groups attached to various aromatics, -QH4-, -CsHr-CHz-CgH, exhibit higher conductivities (10-11 10-6 Q cm-1) than the corresponding monomer complexes. 

They may be readily transformed to r] -aUcynyl and vinylidene complexes. The coupling of two ) -alkynyl ligands results in platinacyclopentadiene species." A platinum(II) disilyl( -alkyne) undergoes insertion (see Insertion) and reductive elimination (see Reductive Elimination) to disilacyclohexene (Scheme 31)." Metal-diaUcynyls can also play jr-donors to form the coordination complexes of various configurations, as shown in (10-13) and so on." Such a,tt-bridging modes may have interconversion. Multi- aUcynyl complexes can even constitute clusters or higher-order stractures. ... 

However, transition metal complexes with higher metal valency can also display oxidative addition in some cases. For example, plafinum(II) or ruthenium(II) complexes give tetra-valent organometallic complexes as shown in Scheme 3.2 and Eq. 3.1 [2-4]. In these reactions, prior coordination of a Lewis base is considered to increase nucleophilicity of the metal for oxidative addition. 

Mechanistic studies from Semmelhack s group showed that in the first step aryl halide undergoes oxidative addition to the low valent metal complex with formation of organometallic intermediate in a higher oxidation state [1,2]. For example, bis(l,5-cyclooctadienyl)nickel(0), Ni(COD)2, reacts with aryl halide (I) to give arylnickel(II) halide (VIII), which further reacts with another aryl halide molecule to form diarylnickel(IV) halide (IX). Each oxidative addition step includes substitution of ligands at metallic centre. 1,5-Cyclooctadiene (COD) dissociates from the nickel to form a coordinatively unsaturated metallic-centre, which does react with aryl halide. Biaryl II is formed by reductive elimination step from IX with liberation of nickel(II) halide [1,2], Scheme 1. 

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