Reactions of Diiron μ-Aminocarbyne Complexes Containing Nitrile Ligands

O ligante acetonitrila nos complexos m-aminocarbinos [Fe2{m-CN(Me)R}(m-CO)(CO)(NCMe) (Cp)2][SO3CF3] (R = Me, 2a, CH2Ph, 2b, Xyl, 2c) (Xyl = 2,6-Me2C6H3) e facilmente deslocado por haletos e cianetos, resultando na formacao das especies neutras [Fe2{m-CN(Me)R}(m-CO)(CO)(X)(Cp)2 ] (X = Br, I, CN). Os complexos 2 sofrem desprotonacao e rearranjo da MeCN coordenada quando tratados com reagentes organolitio. A trimetilacetonitrila, a qual nao contem hidrogenios a acidos foi usada no lugar da MeCN para formar os complexos [Fe2{m-CN(Me)R}(m-CO)(CO)(NCCMe3 )(Cp)2][SO3CF3] (7a-c). As tentativas de substituicao do ligante nitrila no composto 3 por nucleofilos R- (atraves da reacao com RLi) foram frustradas e levaram a formacao de produtos de decomposicao. Entretanto, a reacao do composto 7c com LiCoCTol (Tol = C6H4Me), seguida pelo tratamento com HSO3CF3, resultou na formacao do complexo imino [Fe2{m-CN(Me)Xyl}(m-CO)(CO){N(H)C(Co CC6H4Me-4)CMe3}(Cp)2][SO 3CF3] (8), o qual foi obtido via adicao de acetileto a NCCMe3 coordenada.

Nitrilecontaining complexes have been often considered equivalent to the co-ordinatively unsaturated species because of the substitution lability that nitrile ligands usually exhibit. 9As expected, the complexes 2a-c undergo displacement of MeCN by a variety of ligands which include phosphines, isocyanides, halides and cyanides.Most of these substitution reactions have been previously described. 8,10In the experimental part are given details of the preparation and spectroscopic properties of the novel halide complexes [Fe 2 {µ-CN(Me)Xyl}(µ-CO)(CO)(X) (Cp) 2 ] (X= Br, 3a; I, 3b), Scheme 1.

Results and Discussion
The spectroscopic properties of 3a are similar to those of the chloride complex [Fe 2 {µ-CN(Me)Xyl}(µ-CO)(CO) (Cl)(Cp) 2 ] previously reported. 8Both chloride and bromide complexes show the presence, in CDCl 3 solution, of only the cis isomer as usually found in di-iron µ-aminocarbyne complexes 3,11 (cis and trans are referred to the mutual position of the Cp rings with respect to the Fe-Fe bond).By contrast, the iodide complex 3b consists of both cis and trans isomers, possibly due to bulkiness of the iodide ligand.
The reactions of 2a-c with cyanide illustrate well the change in the reactivity pattern induced by the presence of the acetonitrile ligand.In fact, whereas the addition of cyanide occurs at the carbyne carbon of 1a-c to form the alkylidene compounds 8 The MeCN ligand in 2a-c seems therefore able to direct selectively the nuclephilic attack at the metal atom.In order to determine whether this would apply to carbon nucleophiles such as organolithium or organocopper reagents, the reactions of 2a-c with LiR were studied.The results reported in Scheme 3 show that organolithium reagents (including less basic lithium acetylides) behave towards 2a-c like strong bases rather than nucleophiles, leading to the deprotonation of the coordinated MeCN instead of giving its displacement, and yielding the cyano methyl complexes [Fe 2 {µ-CN(Me)R}(µ-CO)(CO)(CH 2 CN) (Cp) 2 ] (6a-c). 12Proton abstraction from coordinated acetonitrile is not surprising, and its enhanced acidity has been exploited to catalyse MeCN condensation with carbonyl compounds. 13However the rearrangement to cyanomethyl ligand is rather unusual and has been previously observed only in the case of [RhOs(CH 2 CN) (CO) 3 (dppm) 2 ]. 14 The IR spectra of 7a-c, in CH 2 Cl 2 solution, exhibit one terminal and one bridging carbonyl absorption (e.g. at 1982 and 1815 cm -1 for 7a).The 1 H and 13 C NMR spectra show two signals of the same intensity for the nonequivalent Cp groups (for 7a at δ 5.01, 4.85 and 89.0 87.5, respectively).Likewise each of the N-bonded methyl groups in 7a gives rise to a singlet resonance.The NMR spectra of 7b, which contain the asymmetrically substituted µ-C=N(Me)(CH 2 Ph) show the presence of two isomers in solution.These isomeric forms, which are usually found in complexes of the type [Fe 2 {µ-CN(Me)R}(µ-CO)(CO) (L)(Cp) 2 ] and indicated as α and β isomers, 3,10,11,15 are due to the different orientations of R and Me with respect to the non equivalent Fe atoms (Figure 1).Compound 7c consists of only one of the two possible α and β isomers.This is presumably due to the steric demand of the Xyl group, compared to Me, which inhibits the formation of more hindered isomer.It should be reminded that interconvertion of the α and β isomers via rotation around the µ-C-N bond is not possible due to the double bond character of this interaction.
The 13 C NMR spectra of 7a-c, exhibit the characteristic low field signal of the bridging carbyne carbon (e.g. at δ 339.0 for 7c).
The expected displacement of the nitrile ligand by the carbon nucleophile does not take place and the reaction mixtures contain unidentified decomposition products.On the other hand organo-lithium reagents are also strong reducing agent and since it is known 16 that the aminocarbyne complexes 1b can be reduced to the unstable radical [Fe 2 {µ-CN(Me)CH 2 Ph}(µ-CO)(CO) 2 (Cp) 2 ] • , it is likely that the above reactions of 7c proceed through a radical mechanism.A remarkable exception to this general trend is represented by the reaction with LiCCtol: treatment of 7c with tolylacetylide, followed by addition of HSO 3 CF 3 results in the formation of the imino complex [Fe 2 {µ-CN(Me)Xyl}(µ-CO)(CO){N(H)C(C≡CC 6 H 4 Me-4)CMe 3 } (Cp) 2 ] [CF 3 SO 3 ] (8), Scheme 4.
Compound 8 has been characterized by elemental analyses and IR and NMR spectroscopy.
The IR spectrum, in CH 2 Cl 2 solution, exhibits one terminal and one bridging carbonyl at 1977 and 1817 cm -1 , respectively.Evidence of the imine coordination is given by the IR spectrum, ν(N-H) at 3314 (in KBr pellets) and by 1 H NMR resonance at δ 6.12 due to the N-H proton.Major features, in the 13 C NMR spectrum of 8 include the expected low-field resonance of the µ-aminocarbyne carbon (at δ 340.1), which indicates that the carbyne ligand has been unaffected by the reaction, and the signal attributable to the imine carbon at δ 186.7.
The formation of complex 8 is the result of a nucleophilic attack at the coordinated NCCMe 3 , which presumably forms the azavinylidene intermediate [Fe 2 {µ-CN(Me)Xyl}(µ-CO)(CO){N=C(C≡CC 6 H 4 Me-4)CMe 3 } (Cp 2 )] ( 9).In spite of the fact that several azavinylidene complexes are known, 17 the intermediate 9 appears too unstable in our hands to be characterized even by  spectroscopy.However nitrogen protonation transforms the azavinyldene into the more stable imino complex 8.
Nucleophilic addition at coordinated nitriles is not a rare occurrence, in spite of the fact that they usually behave as labile ligands. 18However additions generally involve water, alcohols, amines and only at very low extent, carbon nucleophiles with formation of imine ligands. 19inally it should be underlined that the formation of 9 from the nitrile precursor 7c, as well as the synthesis of the cyanomethyl 6a-c, indicate that nitrile ligands are strongly activated by coordination to the diiron µ-aminocarbyne frame, and that, far from behaving exclusively as labile ligands, they show a remarkable reactivity.

General
All reactions were carried out routinely under nitrogen using standard Schlenk techniques.Solvents were distilled immediately before use under nitrogen from appropriate drying agents.Glassware was oven-dried before use.Infrared spectra were recorded on a Perkin-Elmer 983-G spectrophotometer, 1 H and 13 C NMR spectra on a Varian Gemini 300.Unless otherwise stated, NMR signals due to trace amounts of second isomeric form are italicized.All the reagents were commercial products (Aldrich) of the highest purity available and used as received.[Fe 2 (CO) 4 (Cp) 2 ] was from Strem and used as received.Compounds [Fe 2 {µ-CN(Me)R}(µ-CO)(CO) 2 (Cp) 2 ] [SO 3 CF 3 ] (R = Me 2a R=CH 2 Ph 2b, Me 2 C 6 H 3 2c) 3,11 were prepared as described in the literature.

Synthesis of [Fe
Compound 1a (320 mg, 0.60 mmol) and trimethylacetonitrile (100 mg, 1.20 mmol) in THF (10 mL) was treated with anhydrous Me 3 NO (45 mg, 0.60 mmol) and the mixture was stirred for 120 min.Filtration on a Celite pad and removal of the solvent gave a brown residue that was washed with petroleum ether (bp.40-60 °C) and crystallized from CH 2 Cl 2 layered with n-pentane at -20 °C yielding 7a as a brown microcrystalline solid (253 mg, 72%