Metal alkylidyne fragments

W.L. Gladfelter, University of Minnesota What is the possibility of forming carbon-carbon bonds using two metal alkylidyne fragments ... 

F.G.A. Stone With increasing frequency, as our work in this area develops, we are observing reactions in which the metal alkylidyne fragments link to form C-C bonds Please refer to compounds (39) and (40), and the relevant references to these species. 

Metal alkylidyne fragments are frequently invoked as intermediates in the transformation of hydrocarbons on metal surfaces. These species are usually formulated as triply bridging alkylidynes however, terminal surface alkylidynes may be considered as reactive surface intermediates (30). Evidence for metal carbyne intermediates on Pt—Co bimetallic surfaces was found in a study of the isomerization and hydrogenolysis of alkanes (3]). 

It has already been seen that the reactivity of alkylidene and alkylidyne complexes may be dictated by the nature of both the metal and the carbon substitutents. These principles apply equally to alkene coordination (Figure 6.8). Two metal-ligand fragments serve to illustrate these principles in the activation of alkenes to electrophilic and nucleophilic... 

Bocarsly and Mayr first reported the luminescence properties of transition metal alkylidyne see Alkylidyne) complexes [XW(CO)2(L2)(CPh)] (X = hahdes, L2 = TMEDA, (py)2 or dppe). The complexes exhibited broad and stmctureless emission bands at ca. 625-640mn, which was attributable to an excited state of MLCT (dxy(W) - 7r (W=C)) character. With an extension of the alkylidyne unit, the complexes [XW(CO)2 (L2) (C-C6H4-/ -(C=C-C6H4-/ -) -N=C)] (L2 = TMEDA, dppe n = 0, 1) containing an isocyanide moiety have been shown to possess rich luminescence properties. Perturbation of the absorption and emission properties of the complexes by coordination of the isocyanide moiety to metal-containing fragments such as/ac-ReCl(CO)3, cw-PdCb, trans-Vdh, and trans-Vth was also studied. 

Because the allenyl fragment is constructed from C (carbide), CH (alky-lidyne) and CH2 (alkylidene) units, several possible routes to allenyl complexes via C-C coupling reactions can be envisaged for instance, reaction of a metal C fragment (alkylidene, alkylidyne) with a C2 substrate... 

FIGURE 13. Dimetallacyclopropene complexes obtained in the addition of gold(I) fragments to metal alkylidyne complexes... 

The chemistry of CR fragments ligating metal centers has been a topic of considerable interest since the discovery of the first alkylidyne-metal complexes in E.O. Fischer s Laboratory in 1973 (1). These ligands have been implicated in Fischer-Tropsch reactions (2) and in alkyne metathesis (5). Moreover, the isolation of stable compounds containing carbon-metal triple bonds completed the matrix of bond types represented here ... 

Further developments are likely as the chemistry of the compounds described above is explored. Moreover, entirely new dimensions may be added. For example, the synthesis of tungsten-alkylidyne complexes with carba-borane ligands with cage structures smaller than the icosahedral C2B9 fragment should result in the isolation of new electronically unsaturated metal cluster and electron-deficient molecules of types as yet unknown. 

A number of elegant syntheses of mixed-metal clusters have been accomplished through the condensation of unsaturated metal complexes with metal donors. For example, condensation of the unsaturated cluster H20s3(CO)io with the alkylidyne see Alkylidyne) complex CpW(CO)2(CR) produces H20s3W(/u.3-CR)(C0)i2. Metal-metal, metal-carbon, or carbon-carbon multiple bonds may serve as the sites for metal fragment condensation. Other examples are shown in equations (80 and (9). 

The generation and interconversion of hydrocarbon fragments on metal surfaces is an important aspect of transition metal catalysis. In an effort to model and understand these transformations, much attention has been focused on the synthesis and reactivity of organic species coordinated at polynuclear transition metal centers. Organodiruthenium complexes have provided a particularly rich area of study. The availability of a variety of organometallic derivatives of the bis(T) -cyclopentadienyl)diruthenium carbonyl system has allowed extensive examination of the reactivity of bridging alkylidene, alkylidyne, and ethenylidene ligands. 

Trivalent fragments such as RC= (carbyne or alkylidyne group) and N= (nitride) that are triply bound to a metal are three-electron donors. 

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