Iron alkylidyne complexes

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 work described herein is directed towards the synthesis of alkylidyne complexes of the later transition metals, specifically iron. Two approaches present themselves for the synthesis of alkylidyne complexes which might otherwise be unstable, viz steric or electronic stabilisation. The first approach involves the accumulation of steric bulk in the vicinity of the metal-carbon multiple bond, an effect easily acheived for... 

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. 

DIVERSIONS EN ROUTE TO ALKYLIDYNE COMPLEXES OF IRON A. F. Hill... 

Knox and co-workers synthesized a triruthenium cluster by the reaction of the coordinatively unsaturatcd diruthenium alkyne complex (CpRu)2(/r-CO)(/r-RCCR) 39 with a monometallic carbonyl complex M(CO)4(L) (M = Fe, Ru). Two isomers, 40 and 41, were formed in the reaction of 39 with Ru(CO)4(CH2 = CH2) (Equation (13)). The ratio between 40 and 41 was shown to be dependent on the nature of the substituents of the alkyne. In the case of diphenylacetylene complex, coalescence of the H signals of these isomers, 40a and 41a, was observed. This shows that isomerization between the two isomers took place at considerable rate. In contrast, the reaction of 39 with Fe(CO)4(thf) exclusively afforded a 3- ( )-alkyne complex, in which the alkyne moiety was 7r-coordinated to an iron center. Knox and co-workers also reported the syntheses of triruthenium /i3-alkylidyne complexes by the photolysis of a bimetallic /r-alkylidene complex and a bimetallic diruthenacy-clopentenone complex. In these reactions, formation of the triruthenium frameworks was rationalized by the coupling reaction of the monometallic coordinatively unsaturated species generated by the photolysis with the starting bimetallic complexes. 

With the failure of the diisopropylamino group to favour formation of alkylidyne iron complexes we next turned our attention to the possibility of using steric factors to favour alkylidyne formation. The reaction of [Fe(CO)5] with simple aryl or alkyl lithium reagents (LiR R = Me, tBu, C6H4Me-4, C6H40Me-4) followed by trifluoroacetic anhydride and triphenylphosphine lead in all cases to the exclusive formation of [Fe(CO)3(PPh3)2] in high yield (Scheme 9). 

A series of heterotrimetallie alkyne clusters [CpNiCoM(C0)g(u3-RC2R)] (M=Fe, Ru, Os) have been synthesised and characterised crystallographically. In the iron complex (141) the alkyne is orientated parallel to the Ni-Co edge but in (CpNiCoM(C0)g(u3-PhC2Ph)] (M=Ru, Os) the alkyne orientation is parallel to the Ni-M edge. The reversible transformation alkyne (142) vinylidene (143) = alkylidyne (144) (R=H, Bu ) has been reported and the... 

The Iron Triad.—Mild protonation of the anion [Fe4(CO)u(/t-CO)(/e3-COMe)] yields [Fe4H(CO)i2(i -COMe)], which in turn reacts with strong acid to generate methane from the iy -COMe liquid X-ray crystal structures are reported for both iron complexes. Methods for the s mthesis of triangular metal-cluster complexes containing iUg-alkylidyne ligands have been reported and structures of this tsrpe have been established by X-ray diffraction methods for... 

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