Protonation Studies of a Mono-Dinitrogen Complex of Chromium Supported by a 12-Membered Phosphorus Macrocycle Containing Pendant Amines
dataset
posted on 2015-05-18, 00:00 authored by Michael T. Mock, Aaron W. Pierpont, Jonathan D. Egbert, Molly O’Hagan, Shentan Chen, R. Morris Bullock, William G. Dougherty, W. Scott Kassel, Roger RousseauThe 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.
