Formation of Dimetal Compounds

Protonation of Mixtures Containing Two Distinct Alkylidyne-Metal Complexes 

Dimetal complexes in which an RC=ML moiety formally acts as a four-electron donor to a metal (M ) as in the products 45 are electronically unsaturated species with a cluster-valence-electron (c.v.e.) count of 32. The three-membered rings may be depicted in a number of ways, the most relevant of which are shown below. 

Representation A most clearly indicates the origin of the bridging alkyli-dyne ligand, while the schematic B may be considered a more faithful indication of the observed bond lengths in certain X-ray structures (45). The fully delocalized structure C may be preferred in cases where the 

When a THF solution of 45a is heated at reflux temperature for 2 hr, carbon monoxide is liberated and complex 46a, with a three-center-two-electron (3c-2e) B-H- W bridge bond, is formed, thereby maintaining the valence electron count at the metal center bearing the cyclopentadienyl group (44). Complex 46a is thus the thermodynamic product of protonat-ing a mixture of la and 4g with HBF4 Et20. 

Contrasting with these results are protonation studies of the WC2B9H9Me2 cage systems of la and lb or the MC2B,0 polyhedra of the 

Protonation of Mixtures Corrtaining Two DIstM Alkylidyne -Meta/ Complexes 

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Two different methodologies are available for the proton-mediated formation of dimetal complexes from the compounds 1 or 2. In the first, mixtures containing one of the reagents 1 or 2 together with one of the neutral alkylidyne(cyclopentadienyl)metal complexes 3 are protonated. Protonation of the salts 1 or 2 generates an electronically and coordinatively unsaturated alkylidene-metal species, which can bind a molecule of type 3 to generate a metal-metal bond. This process is formally similar to the... 

The interaction of butadiynediyl dimetal complexes [Fp -C -CsC-M, Fp =FeCp (CO)2, M= Fp, Rp, SiMea, Rp= RuCp(CO)2] with diiron nonacarbonyl, Fe2(CO)9, results in the formation of a mixture of products, as is also observed in the case of their interaction with organic acetylenes. Interesting polymetallic complexes, propargylidene-ketene compounds, zwitterionic cluster compounds, and pa-p -propargylidene-cyclobutene compoimds were isolated from the reaction mixtures and successfully characterized. The product distributions were found to be dependent on the metal fragment (M) at the other end of the C4 rod. The results of the reaction are described... 

Sometimes the Ag - NHC reagent has a twofold effect (i) transmetallation of the carbene and (ii) oxidation of the metal. The reaction of the dimetal-lic Ag biscarbene 43 with [ (p-cymene)RuCl2 h yields the Ru(II) complex 44 (Scheme 36). However, when the same complex 43 reacts with RuCl2(PPh3)3, a Ru(III) complex is obtained (45) with a CCO tripod coordination of the ligand [140]. In this latter case, the reduction of Ag(I) to Ag(0) is confirmed by the formation of a silver mirror in the reaction vessel. Compound 43 can also react with Cul to afford a square planar NHC - Cu(I) complex [141]. 

Since the W(C0)2(O-CsHs) fragment is isolobal with RC5 (23), addition of [w=CC6HifMe-4(C0)2(ti-CsHs)] across the C02 and Rh2 dimetal centres of [Co2(C0)s] and [Rh2(y-CO)2(n-CsMes)2] to give compounds (40) and (41), respectively, has also been successfully accomplished(21). Formation of (40) and (41) in this manner is analogous to the reaction of alkynes with these and other dimetal complexes affording tetrahedrane-type structures with a dicarbadimetalla core. 

The metallation of a-olefins with sodium alkyls results after acidification in the formation of unsaturated.acids [28]. This reaction is accompanied by isomerization of the double bond (Figure 12) and by the formation of a small quantity of dicarboxylic acids via dimetal lation of the starting compound. For example, the carboxylation of the sodium derivatives of a Cg-n olefin fraction in the presence of amylchloride gives up to 37 % acids which were almost completely (93-100 %) unsaturated [29]. 

Martin-Matute and coworkers described Cp Ir(III) complexes having hydroxyl-, ether-, and alkoxide-functionahzed NHC ligands, and their application in the N-alkylation of amines with primary and secondary alcohob [66]. In particular, the hydroxyl-functionalized complex dbplayed excellent catalytic outcomes, a broad substrate scope, and allowed amines to be alkylated with alcohob at temperatures as low as 50 °C. Indeed, thb hydroxyl-functionalized complex b one of the best catalysts known to date. The authors proposed a metal-ligand bifunctional mechanism for the iV-alkylation of amines with alcohob using this complex, which involves the formation of alcohol/alkoxide intermediates. Complex 38 (Figure 10.10) proved to be excellent for the A/,N -dialkylation of p-, m-, and 0-phenylenediamine with primary alcohob. The authors observed that the dimetal-lic compound 38 performs better than the monoiridium one, suggesting that a cooperative effect between the two metab may be at play [94]. 

Two synthetic approaches have been prevalent in the preparation of these dinuclear coupled complexes. The first involves the reaction between a dimetal tetracarboxy-late and a bridging ligand that contains two active protons such as a dicarboxylic acid. This has been extensively used by the Chisholm group in the synthesis of dimers of dimers, compounds of type I [7,8], In reactions of this type the formation of the dimer of dimers is achieved by its preferential precipitation from solution. The most simple example of this type of reaction is seen in the reactions of oxalic acid with the dimetal pivalates shown in (2) (M = Mo or W). 

Reactions of (HBrSi—CH2)3 with KFe(CO)2cp or KMn(CO)5 generate dimetal substituted derivatives, and only in the case of KCo(CO) is the formation of the trimetallo species 3 achieved, along with a tetrasubstituted 1,3,5-trisilacyclohexane species. The compounds are presented in Table 65. The thermal stability of these Co(CO)4-substituted 1,3,5-trisilacyclohexanes decreases as the number of Co(CO)4 substituents coordinated to the ring increases. 

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