Synthesis and Characterization of Aluminum-α-diimine Complexes over Multiple Redox States

The aluminum complexes (L_Mes^2–)­AlCl­(THF) (<b>3</b>) and (L_Dipp^–)­AlCl₂ (<b>4</b>) (L_Mes = <i>N</i>,<i>N</i>′-bis­[2,4,6-trimethylphenyl]-2,3-dimethyl-1,4-diazabutadiene, L_Dipp = <i>N</i>,<i>N</i>′-bis­[2,6-diisopropylphenyl]-2,3-dimethyl-1,4-diazabutadiene) were prepared by direct reduction of the ligands with sodium metal followed by salt metathesis with AlCl₃. The (L_Mes^–)­AlCl₂ (<b>5</b>) complex was prepared through one-electron oxidative functionalization of <b>3</b> with either AgCl or CuCl. Complex <b>3</b> was characterized using ¹H and ¹³C NMR spectoscopies. Single-crystal X-ray diffraction analysis of the complexes revealed that <b>3</b>–<b>5</b> are all four-coordinate, with <b>3</b> exhibiting a trigonal pyramidal geometry, while <b>4</b> and <b>5</b> exist between trigonal pyramidal and tetrahedral. Notable in the L_Mes complexes <b>3</b> and <b>5</b> is a systematic lengthening of the C–N_imido bonds and shortening of the C–C bond in the N–C–C–N backbone with increased electron density on the ligand. The geometries of the complexes <b>3</b> and <b>5</b> were optimized using DFT, which showed primarily ligand-based frontier orbitals, supporting the analysis of the solid-state structural data. The complexes <b>3</b>–<b>5</b> were also characterized by electrochemistry. The cyclic voltamogram of complex <b>3</b> showed an oxidation processes at −0.94 and −0.03 V versus ferrocene, while complexes <b>4</b> and <b>5</b> exhibit both reduction (−1.37 and −1.34 V, respectively) and oxidation (−0.62 and −0.73 V, respectively) features

Dinitrogen Reduction by a Chromium(0) Complex Supported by a 16-Membered Phosphorus Macrocycle

We report a rare example of a Cr–N₂ complex supported by a 16-membered phosphorus macrocycle containing pendant amine bases. Reactivity with acid afforded hydrazinium and ammonium, representing the first example of N₂ reduction by a Cr–N₂ complex. Computational analysis examined the thermodynamically favored protonation steps of N₂ reduction with Cr leading to the formation of hydrazine

Synthesis and Electrochemical Studies of Cobalt(III) Monohydride Complexes Containing Pendant Amines

Two new tetraphosphine ligands, P^{<i>n</i>C‑PPh₂}₂N^Ph₂ (1,5-diphenyl-3,7-bis­((diphenylphosphino)­alkyl)-1,5-diaza-3,7-diphosphacyclooctane; alkyl = (CH₂)₂, <i>n</i> = 2 (L2); (CH₂)₃, <i>n</i> = 3 (L3)), have been synthesized. Coordination of these ligands to cobalt affords the complexes [Co^II(L2)­(CH₃CN)]²⁺ and [Co^II(L3)­(CH₃CN)]²⁺, which are reduced by KC₈ to afford [Co^I(L2)­(CH₃CN)]⁺ and [Co^I(L3)­(CH₃CN)]⁺. Protonation of the Co^I complexes affords [HCo^III(L2)­(CH₃CN)]²⁺ and [HCo^III(L3)­(CH₃CN)]²⁺. The cyclic voltammetry of [HCo^III(L2)­(CH₃CN)]²⁺, analyzed using digital simulation, is consistent with an E_rC_rE_r reduction mechanism involving reversible acetonitrile dissociation from [HCo^II(L2)­(CH₃CN)]⁺ and resulting in formation of HCo^I(L2). Reduction of HCo^III also results in cleavage of the H–Co bond from HCo^II or HCo^I, leading to formation of the Co^I complex [Co^I(L2)­(CH₃CN)]⁺. Under voltammetric conditions, the reduced cobalt hydride reacts with a protic solvent impurity to generate H₂ in a monometallic process involving two electrons per cobalt. In contrast, under bulk electrolysis conditions, H₂ formation requires only one reducing equivalent per [HCo^III(L2)­(CH₃CN)]²⁺, indicating a bimetallic route wherein two cobalt hydride complexes react to form 2 equiv of [Co^I(L2)­(CH₃CN)]⁺ and 1 equiv of H₂. These results indicate that both HCo^II and HCo^I can be formed under electrocatalytic conditions and should be considered as potential catalytic intermediates

Synthesis and Electrochemical Studies of Cobalt(III) Monohydride Complexes Containing Pendant Amines

Two new tetraphosphine ligands, P^{<i>n</i>C‑PPh₂}₂N^Ph₂ (1,5-diphenyl-3,7-bis­((diphenylphosphino)­alkyl)-1,5-diaza-3,7-diphosphacyclooctane; alkyl = (CH₂)₂, <i>n</i> = 2 (L2); (CH₂)₃, <i>n</i> = 3 (L3)), have been synthesized. Coordination of these ligands to cobalt affords the complexes [Co^II(L2)­(CH₃CN)]²⁺ and [Co^II(L3)­(CH₃CN)]²⁺, which are reduced by KC₈ to afford [Co^I(L2)­(CH₃CN)]⁺ and [Co^I(L3)­(CH₃CN)]⁺. Protonation of the Co^I complexes affords [HCo^III(L2)­(CH₃CN)]²⁺ and [HCo^III(L3)­(CH₃CN)]²⁺. The cyclic voltammetry of [HCo^III(L2)­(CH₃CN)]²⁺, analyzed using digital simulation, is consistent with an E_rC_rE_r reduction mechanism involving reversible acetonitrile dissociation from [HCo^II(L2)­(CH₃CN)]⁺ and resulting in formation of HCo^I(L2). Reduction of HCo^III also results in cleavage of the H–Co bond from HCo^II or HCo^I, leading to formation of the Co^I complex [Co^I(L2)­(CH₃CN)]⁺. Under voltammetric conditions, the reduced cobalt hydride reacts with a protic solvent impurity to generate H₂ in a monometallic process involving two electrons per cobalt. In contrast, under bulk electrolysis conditions, H₂ formation requires only one reducing equivalent per [HCo^III(L2)­(CH₃CN)]²⁺, indicating a bimetallic route wherein two cobalt hydride complexes react to form 2 equiv of [Co^I(L2)­(CH₃CN)]⁺ and 1 equiv of H₂. These results indicate that both HCo^II and HCo^I can be formed under electrocatalytic conditions and should be considered as potential catalytic intermediates

Dinitrogen Reduction by a Chromium(0) Complex Supported by a 16-Membered Phosphorus Macrocycle

We report a rare example of a Cr–N₂ complex supported by a 16-membered phosphorus macrocycle containing pendant amine bases. Reactivity with acid afforded hydrazinium and ammonium, representing the first example of N₂ reduction by a Cr–N₂ complex. Computational analysis examined the thermodynamically favored protonation steps of N₂ reduction with Cr leading to the formation of hydrazine

Control of Interpenetration in Two-Dimensional Metal–Organic Frameworks by Modification of Hydrogen Bonding Capability of the Organic Bridging Subunits

Six coordination polymers were prepared by linking Mn­(SCN)₂ units by three different bis­(4-pyridyl) substituited hydrazone derivatives (<b>L</b>) in three different solvents (methanol, ethanol, and acetonitrile) in order to study the effect of the hydrogen bonding ability of <b>L</b> on the formation of solvates rather than interpenetrated solvent-free interpenetrated structures. When the ligand <b>L</b> which cannot act as a hydrogen donor was used, in all three solvents the same product was obtained. This was a [Mn­(SCN)₂<b>L</b>₂]_<i>n</i> metal–organic framework, consisting of two-dimensional (2D) networks, each interpenetrating two neighboring ones. When the bridging ligands <b>L</b> have additional functional groups capable of acting as hydrogen donors or acceptors, synthesis from acetonitrile yields non-interpenetrating 2D [Mn­(SCN)₂<b>L</b>₂]_<i>n</i> networks with solvent molecules occupying the voids of the network. Other solvents were found to yield interpenetrated solvent free networks, or they replaced some of the <b>L</b> ligands, forming one-dimensional coordination polymers

Control of Interpenetration in Two-Dimensional Metal–Organic Frameworks by Modification of Hydrogen Bonding Capability of the Organic Bridging Subunits

Six coordination polymers were prepared by linking Mn­(SCN)₂ units by three different bis­(4-pyridyl) substituited hydrazone derivatives (<b>L</b>) in three different solvents (methanol, ethanol, and acetonitrile) in order to study the effect of the hydrogen bonding ability of <b>L</b> on the formation of solvates rather than interpenetrated solvent-free interpenetrated structures. When the ligand <b>L</b> which cannot act as a hydrogen donor was used, in all three solvents the same product was obtained. This was a [Mn­(SCN)₂<b>L</b>₂]_<i>n</i> metal–organic framework, consisting of two-dimensional (2D) networks, each interpenetrating two neighboring ones. When the bridging ligands <b>L</b> have additional functional groups capable of acting as hydrogen donors or acceptors, synthesis from acetonitrile yields non-interpenetrating 2D [Mn­(SCN)₂<b>L</b>₂]_<i>n</i> networks with solvent molecules occupying the voids of the network. Other solvents were found to yield interpenetrated solvent free networks, or they replaced some of the <b>L</b> ligands, forming one-dimensional coordination polymers

Studies of a Series of [Ni(P^R₂N^Ph₂)₂(CH₃CN)]²⁺ Complexes as Electrocatalysts for H₂ Production: Substituent Variation at the Phosphorus Atom of the P₂N₂ Ligand

A series of [Ni(P^R₂N^Ph₂)₂(CH₃CN)](BF₄)₂ complexes containing the cyclic diphosphine ligands [P^R₂N^Ph₂ = 1,5-diaza-3,7-diphosphacyclooctane; R = benzyl (Bn), <i>n</i>-butyl (<i>n-</i>Bu), 2-phenylethyl (PE), 2,4,4-trimethylpentyl (TP), and cyclohexyl (Cy)] have been synthesized and characterized. X-ray diffraction studies reveal that the cations of [Ni(P^Bn₂N^Ph₂)₂(CH₃CN)](BF₄)₂ and [Ni(P^{<i>n</i>‑Bu}₂N^Ph₂)₂(CH₃CN)](BF₄)₂ have distorted trigonal bipyramidal geometries. The Ni(0) complex [Ni(P^Bn₂N^Ph₂)₂] was also synthesized and characterized by X-ray diffraction studies and shown to have a distorted tetrahedral structure. These complexes, with the exception of [Ni(P^Cy₂N^Ph₂)₂(CH₃CN)](BF₄)₂, all exhibit reversible electron transfer processes for both the Ni(II/I) and Ni(I/0) couples and are electrocatalysts for the production of H₂ in acidic acetonitrile solutions. The heterolytic cleavage of H₂ by [Ni(P^R₂N^Ph₂)₂(CH₃CN)](BF₄)₂ complexes in the presence of <i>p</i>-anisidine or <i>p</i>-bromoaniline was used to determine the hydride donor abilities of the corresponding [HNi(P^R₂N^Ph₂)₂](BF₄) complexes. However, for the catalysts with the most bulky R groups, the turnover frequencies do not parallel the driving force for elimination of H₂, suggesting that steric interactions between the alkyl substituents on phosphorus and the nitrogen atom of the pendant amines play an important role in determining the overall catalytic rate

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