Alkylidyne complexes chemistry

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. 

The chemistry of alkylidene and alkylidyne complexes of early transition metals was developed by Schrock and co-workers and these complexes turned out to be of crucial importance to alkene and alkyne metathesis. Initially their research focused on tantalum complexes of the type CpTaCEIE, which after a-elimination (Figure 16.6) led to alkylidene complexes Cp(R)Cl2Ta=CHR [11]. 

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. Mayr, Comments on the Chemistry of Low-Valent Alkylidyne Complexes of the Group 6 Transition Metals, Comments Inorg. Chem. 10, 227-266 (1990). 

The chemistry of metal-carbon triple bonds has developed considerably during the late 1980s. The synthetic basis was broadened, the utility of high-valent metal alkylidynes in metathesis reactions was further developed and refined, and the potential of low-valent carbyne complexes for applications in organic synthesis has become more apparent. The discovery of novel iridium alkylidyne complexes indicates that the full range of metal-carbon triple bonds is not yet known. We can therefore expect that future work in this area of organometallic chemistry will lead to new discoveries with fundamental implications and practical applications. 

A binuclear calixarene-supported niobium-alkylidyne complex ((42) Scheme 32) was prepared by deprotonation of a mononuclear alkylidene species (14).36 The chemistry of calixarene-supported species containing metal-carbon bonds has been reviewed.37... 

The alkylidyne complexes, L Mo(CR) (Lra = Tp(CO)2, (bpy)Br(CO)2 R, e.g., Ph derivatives), react with one equivalent of propylene sulfide to form thioacyl complexes, L Mo(r/2-SCR). In the case of Tp(CO)2Mo(r/2-SCCf,114Me-4) conversion into dithiocarboxylate, mixed thioselenocarboxylate/thio-latocarbene, cc-thiolatoalkyl, and a,a-bis(thiolato)alkyl species has been demonstrated 897 More recently, related thiocarboxamide and alkoxythiocarbonyl chemistry has been reported 898 Moreover, Mo(CO)L2 (L = depe, dppe) react with HC(S)NMe2 to produce thiocarboxamide hydrides (389) that convert at 125 °C to aminomethylidyne hydrosulfides (390).8"... 

Other (neutral) tantalum alkylidyne complexes formed by a hydrogen abstraction reactions followed, [33,34] although the alkylidyne ligand became much more prominent in the chemistry of Mo, W, and Re. 

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. 

There are a number of reviews that detail the various facets of alkylidyne chemistry, but the majority of these pre-date the enormous growth in the field that has been possible with the appreciation of the kinetic and thermodynamic stability conferred upon alkylidyne complexes by the inclusion of the bulky pyrazolylborate ligands. Furthermore, though a general overview of the chemistry of pyrazolylborate derivatives has appeared, there currently exists no comprehensive source of information specific to the chemistry of alkylidyne complexes co-ligated by poly(pyrazolyl)borates, a situation which this review attempts to... 

Reactivity modes of the poly(pyrazolyl)borate alkylidyne complexes follow a number of recognised routes for transition metal complexes containing metal-carbon triple bonds, including ligand substitution or redox reactions at the transition metal centre, insertion of a molecule into the metal-carbon triple bond, and electrophilic or nucleophilic attack at the alkylidyne carbon, C. Cationic alkylidyne complexes generally react with nucleophiles at the alkylidyne carbon, whereas neutral alkylidyne complexes can react at either the metal centre or the alkylidyne carbon. Substantive work has been devoted to neutral and cationic alkylidyne complexes bearing heteroatom substituents. Differences between the chemistry of the various Tp complexes have previously been rationalised largely on the basis of steric effects. 

Treatment of TpMo( = CR)(CO)2 (R = C6H4Me-4) initially gives a mixture of TpMoFe( U-CR)(CO) (k = 5, 6), but the saturated molybdenum complex readily loses CO under a nitrogen atmosphere." Parallel reaction between bis(pyrazolyl) borate alkylidyne complexes and Fe2(CO)g similarly provides the 32-valence electron dimetal species BpWFe( -CR)(CO)6." These heteronuclear bimetallic complexes have a rich further chemistry, as summarised in Scheme 78. 

In Section 24.12, we introduced alkene (olefin) metathesis, i.e. metal-catalysed reactions in which C=C bonds are redistributed. The importance of alkene and alkyne metathesis was recognized by the award of the 2005 Nobel Prize in Chemistry to Yves Chauvin, Robert H. Grubbs and Richard R. Schrock for the development of the metathesis method in organic synthesis . Examples of alkene metathesis are shown in Figure 27.3. The Chauvin mechanism for metal-catalysed alkene metathesis involves a metal alkyli-dene species and a series of [2 + 2]-cycloadditions and cycloreversions (Figure 27.4). Scheme 27.6 shows the mechanism for alkyne metathesis which involves a high oxidation state metal alkylidyne complex, L M=CR. 

Five papers detail the chemistry of tricobalt alkylidyne complexes of the type [ 03(00)9(113-CR)] " . These include the preparation of [Mo2( l-02CR )4] (R = 03(00)9(113-0)) , and a study of the redox properties of compounds containing two of these cluster units linked via various carbon fragments. The scrambling of carbonyls between [0o2(0O)J and [Oo((X))4] is proposed to occur through an intermediate tricobalt complex [Oo3(CX))io], and not via ion... 

Despite their obvious similarity to alkenes and alkynes, transition-metal alkylidene and alkylidyne complexes have not been used as building blocks for the synthesis of lowdimensional materials analogous to polyenes and polyynes. We have begun to explore the syntheses, structures, and properties of conjugated complexes and polymers derived from metal-alkylidyne complexes as part of our effort to develop the chemistry of transition-metal analogues of conjugated organic compounds. 

Given the isoelectronic relationship between [CR] and [NO] and the ubiquity of this latter ligand in the coordination chemistry of later transition metals, the scarcity of mononuclear alkylidyne complexes of metals from groups 8-10 is surprising [1-4]. Isolated examples have been reported for iron [5], cobalt [6], ruthenium [4,7], osmium [4,8-9] and iridium [10]. Most of the examples known employ routes with extensive precedent in early transition metal systems, i.e., either electrophilic attack at the p-atom of a hetero carbonyl (CS [5], CTe [4], or C=CH2 [10]) or the Lewis-acid assisted abstraction of an alkoxide group from a carbene precursor [5] (Scheme 1). The one approach which is, too date, peculiar to group 8 metals involves reduction of a divalent dichlorocarbene complex by lithium aryls [4]. The limitation of this procedure to ruthenium and osmium is presumably not a feature of these metals but rather a result of the present lack of synthetic routes to suitable dihalocarbene precursor complexes of earlier metals. 

The present study has not led to stable alkylidyne complexes of iron, however the problems encountered here represent diversions encountered in applying synthetic strategies developed for group 6 metals. The complex [Fe(=CN Pr2)(CO)3(PPh3)]+ has been reported by Fischer et al [5] and indicates that given suitable synthetic methodology the chemistry of this class of compounds may yet be developed. 

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