Dinitrogen or dichloromethane?

Abstract

A recent article in this journal reports the crystal structure of (octaethylazaporphyrinato)iron(III) chloride, (OEP)Fe^(III)-Cl·CH_2Cl_3·N_2, at 130 K, in which the N_2 molecule is referred to as "unusual, but no other explanation for two equal-sized atoms separated by 1.21 (2) Å. could be found". Even more unusual is a "close contact" of 1.86 Å between two such molecules (related by a center of symmetry), forming an angle N-N≡N of 109° and thus leading to a sort of perazabuta-1,3-diene molecule. If this were a genuine new tetranitrogen molecules so gently produced, it would be a remarkable discovery, worth celebrating as a milestone in synthetic and structural chemistry and in nitrogen fixation. Since we were skeptical of the ability of these crystals, which were grown at room temperature, to capture and contain a nitrogen molecule (and to cause it to dimerize as well), we have undertaken a re-examination of the structure. Perhaps unfortunately, we have concluded that the earlier interpretation as dinitrogen is erroneous.

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