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

The reduction of fac-[CrCl₃(P^Ph₃N^Bn₃)], (1(Cl₃)), (P^Ph₃N^Bn₃ = 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)(P^Ph₃N^Bn₃), (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 Cr^II-N₂ hydride complex, [Cr(H)(N₂)(dmpe)(P^Ph₃N^Bn₃)][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 pK_a values of three protonated sites of 2(N₂) (metal center, pendant amine, and N₂ ligand) and were used to predict the thermodynamically preferred Cr-N_xH_y intermediates in the N₂ reduction pathway for 2(N₂) and the recently published complex trans-[Cr(N₂)₂(P^Ph₄N^Bn₄)] upon the addition of protons and electrons. (Chemical Equation Presented).