Unsaturated metallacycles such as (η5-C5H5)Co(R—N=N—N=N—R), where R = alkyl or aryl, offer the possibility of π-delocalized bonding. Xα calculations predict that two dπ electrons on the (η5-C6H5)Co fragment should strongly interact with an empty low-lying π* orbital of the tetraazadiene ligand to yield metallacycle π and π* orbitals. A single-crystal X-ray diffraction study at −150 °C of the R = C6F5 derivative provides direct evidence of the π-acceptor abilities of the RN4R ligand. The [formula omitted] ring is planar and exhibits unusually short Co—N(1) and Co—N(4) distances of 1.802 (2) and 1.819 (2) Å, respectively, indicative of multiple-bond character. The short N(2)—N(3) bond [1.279 (2) Å] and long N(1)—N(2) [1.360 (2) Å] and N(3)—N(4) [1.355 (2) Å] bonds demonstrate that π back-bonding has altered the pattern of unsaturated bonds in the tetraazadiene chelate. Electronic absorption spectra of (η5-C6H5)Co(RN4R) [R = CH3, C6H6, C6F5, 2,4-F2C6H3, 2,6-(CH3)2C6H3] display three bands characteristic of the cobalt-tetraazadiene ring: λmax1 (ϵ), 600–670 nm (300–700); λmax2 (ϵ), 425–470 nm (5000–9000); λmax3 (ϵ), 335–390 nm (<4000). These absorptions are attributed to one-electron transitions that terminate in the low-lying metallacycle π* orbital. Crystals of C17H5CoF10N4· 1/2C6H6 at −150 °C are monoclinic, space group P21/c, with four formula units in a cell of dimensions a = 8.612 (4) Å, b = 22.687 (12) Å, c = 9.820 (6) Å, and β = 91.58 (2)∘. Least-squares refinement of 315 variables has led to a final value of the R index on F2 of 0.047 for 5367 observations; the conventional R index on F is 0.033 for 4313 observations having Fo2 > 3σ(Fo2).
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry