First paramagnetic zerovalent transition metal isocyanides. Syntheses, structural characterizations, and magnetic properties of novel low-valent isocyanide complexes of vanadium

Mikhail V. Barybin, Victor G Young, John E Ellis

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Abstract

The first homoleptic paramagnetic transition metal isocyanide, V(CNXyl)6 (2, Xyl = 2,6-dimethylphenyl), can be isolated in high yield by reacting bis(naphthalene)vanadium(0) (1a) or bis(1- methylnaphthalene)vanadium(0) (1b) with 6 equiv of CNXyl in tetrahydrofuran/heptane. Reduction of 2 with excess cesium graphite in THF affords excellent yields of [V(CNXyl)6]- (3) as an unsolvated Cs+ salt, the first homoleptic octahedral isocyanide metalate. Cs3 reacts with 2 equiv of 18-Crown-6 to give [Cs(18-Crown-6)2]3. Anion 3 can also be isolated as a practically insoluble [K(Crypt{2.2.2.})]+ salt by reducing 2 with potassium naphthalenide in the presence of Crypt{2.2.2.}. Complex 3 reduces [Et3NH]Cl to form paramagetic 2. Oxidation of 2 by ferricinium hexafluorophosphate in THF provides nearly quantitative yields of the 16- electron paramagnetic [V(CNXyl)6][PF6] (4[PF6]), analogous to the exceedingly unstable [V(CO)6]+. Interaction of V(CO)6 with excess CNXyl in heptane results in the efficient formation of trans-[V(CO)2(CNXyl)4] (5). Such a substitution reaction is highly unusual for V(CO)6. Oxidation of compound 5 by ferricinium hexafluorophosphate in THF affords homoleptic 4[PF6]. Complexes 2, 3, 4, and 5 were characterized by a variety of spectroscopic methods and X-ray crystallography. Spectroscopic, magnetic, and structural features of these novel electron rich vanadium isocyanides are discussed in detail. The average V-CN bond length increases in the series [V(CNXyl)6]- < V(CNXyl)6 < trans-V(CO)2(CNXyl)4 < [V(CNXyl)6]+. Well- resolved 1H and 13C NMR spectra were obtained for paramagnetic 2 and its chromium congener, [Cr(CNXyl)6]+. The importance of back-bonding in the mechanism of unpaired spin delocalization within 2 was demonstrated. Contrary to the previous prediction for dπ(M)-pπ*(L) unpaired spin delocalization in low-spin d5 octahedral complexes, negative spin appears to be induced on the CNXyl ligands of 2 by means of dπ(V)-pπ*(CNXyl) back-bonding.

Original languageEnglish (US)
Pages (from-to)4678-4691
Number of pages14
JournalJournal of the American Chemical Society
Volume122
Issue number19
DOIs
StatePublished - May 17 2000

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