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

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.