Alkylidyne ligand synthesis

Synthesis from pre-existing alkylidyne complexes L M( = CR) can also occur via the modification of the alkylidyne ligand, involving the elaboration of R or the replacement of R by R. These modifications typically involve alkylidyne complexes which already incorporate the poly(pyrazolyl)borate ligand, and consequently there is overlap between this section and Section IV, and such syntheses will be expanded upon therein. 

Two commonly used synthetic methodologies for the synthesis of transition metal complexes with substituted cyclopentadienyl ligands are important. One is based on the functionalization at the ring periphery of Cp or Cp metal complexes and the other consists of the classical reaction of a suitable substituted cyclopentadienyl anion equivalent and a transition metal halide or carbonyl complex. However, a third strategy of creating a specifically substituted cyclopentadienyl ligand from smaller carbon units such as alkylidynes and alkynes within the coordination sphere is emerging and will probably find wider application [22]. 

Byers, P.K, Carr, N. and Stone, F.G.A. (1990) Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 106. Synthesis and reactions of the alkylidyne complexes [M ( CR)(CO)2 (C6F5)AuC(pz)3 j (M = W or Mo, R — alkyl or aryl, pz — pyrazol-l-yl) crystal structure of pjC PtAu(C6F5)( l3-CMe)(CO)2(PMe2Ph)2 (C6F5)AuC(pz)3 ]. Journal of the Chemical Society, Dalton Transactions, (12), 3701—3708. 

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. 

The synthesis of alkylidyne complexes by y -addition of electrophiles to vinylidene and acetylide ligands is now well established (5,6). Pombeiro and co-workers synthesized several new rhenium alkylidyne complexes by protonation of the electron-rich vinylidene complexes 13 [Eq. (18)] (55). The mechanism of formation of the benzylcarbyne complex 14 (R = Ph)... 

Protonation of vinylidene and acetylide ligands was also found to be useful for the synthesis of high-oxidation state molybdenum alkylidyne complexes. Green and co-workers demonstrated protonation of the vinylidene complex 20 as shown in Eq. (21) (64). Selegue and co-workers... 

Bis-nitrogen donor ligand-substituted alkylidyne complexes such as 78 were found to be good alternatives as starting materials to the thermally labile tetracarbonyl complexes 44). Substitution of one pyridine ligand in the bispyridine-substituted complexes 78 occurs in the presence of excess chloride [Eq. (64)] 103). The anionic complexes 79 are stable in the presence of excess chloride but are too labile to be isolated in pure form. These complexes were used in the synthesis of stable alkylidyne alkene tungsten complexes [Eq. (180), Section IV,G]. With phosphines both pyridine ligands in 78 are substituted [Eq. (65)] 44). For PMej the reaction of the tetracarbonyl complex W(CPh)Br(CO)4 had previously been shown to... 

The transfer of a proton from an imido ligand in ISO to the alkylidyne carbon was an essential step in the synthesis of the active olefin metathesis catalysts 153 [Eq. (134)] (153-156). Triethylamine serves as a catalyst for... 

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. 

The a-hydride elimination reactions of Ta and Nb have been investigated most extensively. The driving force for the reaction is provided by the interaction of an empty metal orbital with the a C—H bonding electrons. Steric crowding on the metal as well as the absence of p hydrogens on the ligands are important for successful synthesis. The same principles governing the alkyl-to-alkylidene transformation apply to the alkyli-dene-to-alkylidyne transformation and a number of alkylidyne complexes have been made by the same methods described here for aikyiidene synthesis. ... 

Boron atoms in transition metal clusters Denuding the boron atom of B-H interactions in transition metal boron clusters Transition metal boride clusters at the molecular level Recent advances in the chemistry of carborane metal complexes incorporating d and /block elements The interplay of alkylidyne and carbaborane ligands at metal centres. I. Synthesis of electronically unsaturated mixed-metal complexes. II. Proton-mediated reactions... 

The first alkylidyne-metal complexes were prepared by Fischer et al. more than 30 years ago. Since this pioneering event, the chemistry of the transition metal-carbon triple bond present in such complexes has developed into a major field of research and though the poly(pyrazolyl)borate ligands were discovered 7 years prior to the synthesis of the first alkylidyne complexes, their importance and significance in this field has only more recently been truly appreciated. 

A typical synthesis of poly(pyrazolyl)borate alkylidyne complexes involves displacement of labile ligands from pre-existing alkylidynes prepared according to the classic oxide-abstraction Fischer preparation (Scheme 2), which has been developed by Mayr and co-workers into an efficient approach to Group 6 alkylidynes. 

Scheme 8. High-valent Tp -alkylidyne synthesis from ligand substitution.

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