Exploring the Limits of Catalytic Ammonia–Borane Dehydrogenation Using a Bis(<i>N</i>‑heterocyclic carbene) Iridium(III) Complex

Ammonia borane dehydrogenation can be catalyzed by a number of organometallic species. [Ir­(κ²-I^<i>t</i>Bu)₂]­[PF₆] (I^<i>t</i>Bu = 1,3-bis­(<i>tert</i>-butyl)­imidazol-2-ylidene) is the most active catalyst for this process that has been reported to date. We explore herein the absolute limits of the use of this and related complexes, including [Ir­(κ²-I^<i>t</i>Bu)₂]­[BAr^F₄], [Ir­(H)₂(I^<i>t</i>Bu)₂]­[BAr^F₄], and [Ir­(κ²-I^<i>t</i>Bu)₂(NH₃)]­[BAr^F₄] (BAr^F₄ = tetrakis­(3,5-bis­(trifluoromethyl)­phenyl)­borate)

Protonation Studies of a Tungsten Dinitrogen Complex Supported by a Diphosphine Ligand Containing a Pendant Amine

Treatment of <i>trans</i>-[W­(N₂)₂(dppe)­(P^EtN^MeP^Et)] (dppe = Ph₂PCH₂CH₂PPh₂; P^EtN^MeP^Et = Et₂PCH₂N­(Me)­CH₂PEt₂) with 3 equiv of tetrafluoroboric acid (HBF₄·Et₂O) at −78 °C generated the seven-coordinate tungsten hydride <i>trans</i>-[W­(N₂)₂(H)­(dppe)­(P^EtN^MeP^Et)]­[BF₄]. At higher temperatures, protonation of a pendant amine is also observed, affording <i>trans</i>-[W­(N₂)₂(H)­(dppe)­(P^EtN^Me(H)­P^Et)]­[BF₄]₂, with formation of the hydrazido complex [W­(NNH₂)­(dppe)­(P^EtN^Me(H)­P^Et)]­[BF₄]₂ as a minor product. A similar product mixture was obtained using triflic acid (HOTf). The protonated products are thermally sensitive and do not persist at ambient temperature. Upon acid addition to the carbonyl analogue <i>cis</i>-[W­(CO)₂(dppe)­(P^EtN^MeP^Et)], the seven-coordinate carbonyl hydride complex <i>trans</i>-[W­(CO)₂(H)­(dppe)­(P^EtN^Me(H)­P^Et)]­[OTf]₂ was generated. A mixed diphosphine complex without the pendant amine in the ligand backbone, <i>trans</i>-[W­(N₂)₂(dppe)­(depp)] (depp = Et₂P­(CH₂)₃PEt₂), was synthesized and treated with HOTf, selectively generating a hydrazido complex, [W­(NNH₂)­(OTf)­(dppe)­(depp)]­[OTf]. Computational analysis probed the proton affinity of three sites of protonation in these complexes: the metal, pendant amine, and N₂ ligand. Room-temperature reactions with 100 equiv of HOTf produced NH₄⁺ from reduction of the N₂ ligand (electrons come from W). The addition of 100 equiv of HOTf to <i>trans</i>-[W­(N₂)₂(dppe)­(P^EtN^MeP^Et)] afforded 0.81 equiv of NH₄⁺, while 0.40 equiv of NH₄⁺ was formed upon treatment of <i>trans</i>-[W­(N₂)₂(dppe)­(depp)] with HOTf, showing that the complexes containing proton relays produce more products of reduction of N₂

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

The reduction of <i>fac</i>-[CrCl₃­(P^Ph₃N^Bn₃)], (<b>1­(Cl</b>_<b>3</b><b>)</b>), (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), (<b>2­(N</b>_<b>2</b><b>)</b>), containing a pentaphosphine coordination environment. <b>2­(N</b>_<b>2</b><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(N₂)</b> 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], <b>[2­(H)­(N₂)]⁺</b>. Treatment of <b>2­(¹⁵N₂) </b>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 p<i>K</i>_a values of three protonated sites of <b>2­(N</b>_<b>2</b><b>)</b> (metal center, pendant amine, and N₂ ligand) and were used to predict the thermodynamically preferred Cr-N_<i>x</i>H_<i>y</i> intermediates in the N₂ reduction pathway for <b>2­(N</b>_<b>2</b><b>)</b> and the recently published complex <i>trans</i>-[Cr­(N₂)₂­(P^Ph₄N^Bn₄)] upon the addition of protons and electrons

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

The reduction of <i>fac</i>-[CrCl₃­(P^Ph₃N^Bn₃)], (<b>1­(Cl</b>_<b>3</b><b>)</b>), (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), (<b>2­(N</b>_<b>2</b><b>)</b>), containing a pentaphosphine coordination environment. <b>2­(N</b>_<b>2</b><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(N₂)</b> 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], <b>[2­(H)­(N₂)]⁺</b>. Treatment of <b>2­(¹⁵N₂) </b>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 p<i>K</i>_a values of three protonated sites of <b>2­(N</b>_<b>2</b><b>)</b> (metal center, pendant amine, and N₂ ligand) and were used to predict the thermodynamically preferred Cr-N_<i>x</i>H_<i>y</i> intermediates in the N₂ reduction pathway for <b>2­(N</b>_<b>2</b><b>)</b> and the recently published complex <i>trans</i>-[Cr­(N₂)₂­(P^Ph₄N^Bn₄)] upon the addition of protons and electrons