Protonation Studies of a Mono-Dinitrogen Complex of Chromium Supported by a 12-Membered Phosphorus Macrocycle Containing Pendant Amines

Michael T. Mock, Aaron W. Pierpont, Jonathan D. Egbert, Molly O'Hagan, Shentan Chen, R. Morris Bullock, William G. Dougherty, W. Scott Kassel, Roger Rousseau

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Abstract

The reduction of fac-[CrCl3(PPh3NBn3)], (1(Cl3)), (PPh3NBn3 = 1,5,9-tribenzyl-3,7,11-triphenyl-1,5,9-triaza-3,7,11-triphosphacyclododecane) with Mg in the presence of dmpe (dmpe = 1,2-bis(dimethylphosphino)ethane) affords the first example of a monodinitrogen Cr0 complex, Cr(N2)(dmpe)(PPh3NBn3), (2(N2)), containing a pentaphosphine coordination environment. 2(N2) is supported by a unique facially coordinating 12-membered phosphorus macrocycle containing pendant amine groups in the second coordination sphere. Treatment of 2(N2) at -78°C with 1 equiv of [H(OEt2)2][B(C6F5)4] results in protonation of the metal center, generating the seven-coordinate CrII-N2 hydride complex, [Cr(H)(N2)(dmpe)(PPh3NBn3)][B(C6F5)4], [2(H)(N2)]+. Treatment of 2(15N2) with excess triflic acid at -50°C afforded a trace amount of 15NH4+ from the reduction of the coordinated 15N2 ligand (electrons originate from Cr). Electronic structure calculations were employed to evaluate the pKa values of three protonated sites of 2(N2) (metal center, pendant amine, and N2 ligand) and were used to predict the thermodynamically preferred Cr-NxHy intermediates in the N2 reduction pathway for 2(N2) and the recently published complex trans-[Cr(N2)2(PPh4NBn4)] upon the addition of protons and electrons. (Chemical Equation Presented).

Original languageEnglish
Pages (from-to)4827-4839
Number of pages13
JournalInorganic Chemistry
Volume54
Issue number10
DOIs
Publication statusPublished - May 18 2015

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ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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