Zerovalent Iron Isocyanide Complexes

In this contribution we describe facile, high-yield syntheses of the series of zerovalent iron isocyanide complexes Fe(CO)5 (CNC6H3Me2-l,3)n ( n = 1-5). The starting material is iron pentacarbonyl, and cobalt(II) chloride is used as a catalyst to achieve the stepwise replacement of carbonyl groups by 2-isocyano-l,3-dimethylbenzene.4,9 

Although the synthetic procedure is exemplified by the use of 2-isocyano-1,3-dimethylbenzene, the series of complexes Fe(CO)5 (CNR) (n = 1-5) can be obtained for other aromatic isocyanides, and for some alkyl isocyanides (e.g., n = 1-4, R = t-Bu n = 1-3, R = Me, C6HU, C6H5CH2) using a similar technique.4 

The reaction mixture is cooled to room temperature and the solvent and excess Fe(CO)5 are removed under vacuum. The product is obtained as a yellow, crystalline solid. Yield 8.6 g (96%). This material is sufficiently pure for most purposes, but it may be recrystallized from pentane. 

Submitted by MICHEL O. ALBERS, ERIC SINGLETON, and NEIL J. 

Checked by ARTHUR Y.J. CHEN, RUSTY BLANSKI, and HERBERT D. 

A recent review has highlighted the extensive and interesting chemistry of metal isocyanide complexes. Although synthetic procedures are varied, a vast number are based on substitution in metal carbonyl complexes by isocyanides. Such procedures are, however, not always successful. This is especially so in cases where multiple substitution of CO is required, as in the syntheses of homoleptic isocyanide complexes. Many of the inherent difficulties are iUustrated by the reaction of iron pentacarbonyl with isocyanides. 

Pentacarbonyl iron is fairly inert to substitution reactions, and attempts to prepare Fe(CO)5, (CNR) (n = 1-5) by the direct reaction of F CO)s with isocyanides in Carius tubes has produced only the complexes Fe(CO)5 (CNR) (n = 1 and 2). The products were obtained as mixtures that required separation. Other syntheses, including photochemical and trimethyl-amine M-oxide promoted displacement of carbonyl groups, or other means, give the same products in variable yield. Procedures based on diiron nonacarbonyl and triiron dodecacarbonyP have produced similar results. The only zerovalent iron complex Fe(CO)5 (CNR), where 2 is the complex Fe(CNR)5 prepared either by metal vapor synthesis techniques or by sodium amalgam reduction of iron(II) bromide in the presence of isocyanide.  

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The same reaction sequence was monitored by uv-visible spectroscopy and despite the lack of optical data for the zerovalent binary iron-isocyanide complexes, comparison with the spectrum for FeCCO) (18) reveals a close similarity. After the photolysis the original spectrum [an intense absorption at 232 nm with two shoulders at 297 and 340 nm] had decayed to be replaced by a new spectrum consisting of two bands at 220 and 382 nm. 

Isocyanide groups may bind to metals in a variety of ways. For 1 1 adducts (Figure 4.19), the isocyanide group is approximately linear, although some flexibility seems to exist in a bis(t-butylisocyanide)iron(Il)tetraphenylporphyrinato complex. For zerovalent metals with much electron density available for do-... 

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