Protonation Studies of a Mono-Dinitrogen Complex of
Chromium Supported by a 12-Membered Phosphorus Macrocycle Containing
Pendant Amines
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Abstract
The reduction of <i>fac</i>-[CrCl<sub>3</sub>(P<sup>Ph</sup><sub>3</sub>N<sup>Bn</sup><sub>3</sub>)], (<b>1(Cl</b><sub><b>3</b></sub><b>)</b>), (P<sup>Ph</sup><sub>3</sub>N<sup>Bn</sup><sub>3</sub> =
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 Cr<sup>0</sup> complex,
Cr(N<sub>2</sub>)(dmpe)(P<sup>Ph</sup><sub>3</sub>N<sup>Bn</sup><sub>3</sub>), (<b>2(N</b><sub><b>2</b></sub><b>)</b>), containing a pentaphosphine coordination environment. <b>2(N</b><sub><b>2</b></sub><b>)</b> is supported by
a unique facially coordinating 12-membered phosphorus macrocycle containing
pendant amine groups in the second coordination sphere. Treatment
of <b>2(N<sub>2</sub>)</b> at −78 °C with 1 equiv
of [H(OEt<sub>2</sub>)<sub>2</sub>][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] results in protonation of the metal center, generating
the seven-coordinate Cr<sup>II</sup>–N<sub>2</sub> hydride
complex, [Cr(H)(N<sub>2</sub>)(dmpe)(P<sup>Ph</sup><sub>3</sub>N<sup>Bn</sup><sub>3</sub>)][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>], <b>[2(H)(N<sub>2</sub>)]<sup>+</sup></b>. Treatment
of <b>2(<sup>15</sup>N<sub>2</sub>) </b>with excess triflic
acid at −50 °C afforded a trace amount of <sup>15</sup>NH<sub>4</sub><sup>+</sup> from the reduction of the coordinated <sup>15</sup>N<sub>2</sub> ligand (electrons originate from Cr). Electronic
structure calculations were employed to evaluate the p<i>K</i><sub>a</sub> values of three protonated sites of <b>2(N</b><sub><b>2</b></sub><b>)</b> (metal center, pendant amine,
and N<sub>2</sub> ligand) and were used to predict the thermodynamically
preferred Cr-N<sub><i>x</i></sub>H<sub><i>y</i></sub> intermediates in the N<sub>2</sub> reduction pathway for <b>2(N</b><sub><b>2</b></sub><b>)</b> and the recently
published complex <i>trans</i>-[Cr(N<sub>2</sub>)<sub>2</sub>(P<sup>Ph</sup><sub>4</sub>N<sup>Bn</sup><sub>4</sub>)] upon
the addition of protons and electrons
