Exploring the Reaction Space of a Tricobalt Nitride

Abstract

Reaction chemistry was explored from the tricobalt platform [(tbsL)Co3] where [tbsL]6– = [1,3,5-C6H9(NC6H4-o-NSitBuMe2)3]6–. Treatment of the polynucleating ligand tbsLH6 with three equivalents of metalating agent Co(N(SiMe3)2)2(py)2 results in the formation of the tricobalt solvento adduct: (tbsL)Co3(py). The cyclohexyl backbone of the ligand enforces metal atom proximity, while the bulky tert-butyldimethylsilyl (tbs) capping units precludes close metal-metal contacts resulting in an intermediate spin state in (tbsL)Co3(py) (S = 3/2). (tbsL)Co3(py) was competent for imido and nitride transfer chemistry from organic azides, inorganic azides, and diazoalkanes to form tricobalt m3-imides (tbsL)Co3(m3–NR) and nitrides (tbsL)Co3(m3–N). The aforementioned (tbsL)Co3(m3–NR) (R = Ph) can also be synthesized via N–N bond cleavage upon treatment of (tbsL)Co3(py) with hydrazines (PhHNNHPh) and diazenes (PhNNPh). The tricobalt m3–nitride (tbsL)Co3(m3–N) was chemically accessed across four different oxidation states ([(tbsL)Co3(m3–N)]0/–1/–2/–3) without any loss of structural integrity. X-ray absorption spectroscopy data of the electron transfer series indicates that the redox is borne out largely on the cobalt centers which feature shifting of the Co K-edge energies with each successive reduction in addition to a diminishing pre-edge absorption intensity. The latter effect is reflective of the diminishing Co–N covalency. The changes in the cobalt oxidation state traversing the redox series are further reflected in changes in the Co–Co and Co–N bond metrics observed by single crystal X-ray crystallography. Changing the oxidation state of (tbsL)Co3(m3–N) results in divergent reactivity pathways at the bridging (m3–N) moiety. The neutral (tbsL)Co3(m3–N) possesses an electrophilic reactivity profile akin to mononuclear late transition metal terminal nitrides; whereas the anionic [(tbsL)Co3(m3–N)]–1 possesses a nucleophilic reactivity profile akin to mononuclear, anionic early transition metal terminal nitrides. Treatment of both nitrides with ambiphilic CO results in nitride carbonylation to generate [(tbsL)Co3(m3–NCO)]0/–1. The resultant cyanate bound cluster exhibits ligand lability and treatment of the cyanate bound cluster with inorganic azide results in regeneration of the tricobalt nitride, closing a reaction cycle, and allowing for the possibility of catalysis.