Square-Planar Cobalt(III) Pincer Complex

A series of square-planar cobalt­(II) complexes with pincer ligands {N­(CH₂CH₂P<i>t</i>Bu₂)₂}⁻ ({L₁^tBu}⁻), {N­(CH₂CH₂P<i>t</i>Bu₂)­(CHCHP<i>t</i>Bu₂)}⁻ ({L₂^tBu}⁻), and {N­(CHCHP<i>t</i>Bu₂)₂}⁻ ({L₃^tBu}⁻) was synthesized. Ligand dehydrogenation was accomplished with a new, high-yield protocol that employs the 2,4,6-tri<i>-tert</i>-butylphenoxy radical as hydrogen acceptor. [CoCl­{L_<i>n</i>^tBu}] (<i>n</i> = 1–3) were examined with respect to reduction, protonation, and oxidation, respectively. One-electron oxidations of [CoCl­(L₁^tBu)] and [CoCl­(L₂^tBu)] lead to ligand-centered radical reactivity, like amide disproportionation into cobalt­(II) amine and imine complexes. In contrast, oxidation of [CoCl­{L₃^tBu}] with Ag⁺ enabled the isolation of thermally stable, square-planar cobalt­(III) complex [CoCl­{L₃^tBu}]⁺, which adopts an intermediate-spin (<i>S</i> = 1) ground state with large magnetic anisotropy. Hence, pincer dehydrogenation gives access to a new platform for high-valent cobalt in square-planar geometry

Square-Planar Cobalt(III) Pincer Complex

A series of square-planar cobalt­(II) complexes with pincer ligands {N­(CH₂CH₂P<i>t</i>Bu₂)₂}⁻ ({L₁^tBu}⁻), {N­(CH₂CH₂P<i>t</i>Bu₂)­(CHCHP<i>t</i>Bu₂)}⁻ ({L₂^tBu}⁻), and {N­(CHCHP<i>t</i>Bu₂)₂}⁻ ({L₃^tBu}⁻) was synthesized. Ligand dehydrogenation was accomplished with a new, high-yield protocol that employs the 2,4,6-tri<i>-tert</i>-butylphenoxy radical as hydrogen acceptor. [CoCl­{L_<i>n</i>^tBu}] (<i>n</i> = 1–3) were examined with respect to reduction, protonation, and oxidation, respectively. One-electron oxidations of [CoCl­(L₁^tBu)] and [CoCl­(L₂^tBu)] lead to ligand-centered radical reactivity, like amide disproportionation into cobalt­(II) amine and imine complexes. In contrast, oxidation of [CoCl­{L₃^tBu}] with Ag⁺ enabled the isolation of thermally stable, square-planar cobalt­(III) complex [CoCl­{L₃^tBu}]⁺, which adopts an intermediate-spin (<i>S</i> = 1) ground state with large magnetic anisotropy. Hence, pincer dehydrogenation gives access to a new platform for high-valent cobalt in square-planar geometry

Square-Planar Cobalt(III) Pincer Complex

A series of square-planar cobalt­(II) complexes with pincer ligands {N­(CH₂CH₂P<i>t</i>Bu₂)₂}⁻ ({L₁^tBu}⁻), {N­(CH₂CH₂P<i>t</i>Bu₂)­(CHCHP<i>t</i>Bu₂)}⁻ ({L₂^tBu}⁻), and {N­(CHCHP<i>t</i>Bu₂)₂}⁻ ({L₃^tBu}⁻) was synthesized. Ligand dehydrogenation was accomplished with a new, high-yield protocol that employs the 2,4,6-tri<i>-tert</i>-butylphenoxy radical as hydrogen acceptor. [CoCl­{L_<i>n</i>^tBu}] (<i>n</i> = 1–3) were examined with respect to reduction, protonation, and oxidation, respectively. One-electron oxidations of [CoCl­(L₁^tBu)] and [CoCl­(L₂^tBu)] lead to ligand-centered radical reactivity, like amide disproportionation into cobalt­(II) amine and imine complexes. In contrast, oxidation of [CoCl­{L₃^tBu}] with Ag⁺ enabled the isolation of thermally stable, square-planar cobalt­(III) complex [CoCl­{L₃^tBu}]⁺, which adopts an intermediate-spin (<i>S</i> = 1) ground state with large magnetic anisotropy. Hence, pincer dehydrogenation gives access to a new platform for high-valent cobalt in square-planar geometry

Square-Planar Cobalt(III) Pincer Complex

A series of square-planar cobalt­(II) complexes with pincer ligands {N­(CH₂CH₂P<i>t</i>Bu₂)₂}⁻ ({L₁^tBu}⁻), {N­(CH₂CH₂P<i>t</i>Bu₂)­(CHCHP<i>t</i>Bu₂)}⁻ ({L₂^tBu}⁻), and {N­(CHCHP<i>t</i>Bu₂)₂}⁻ ({L₃^tBu}⁻) was synthesized. Ligand dehydrogenation was accomplished with a new, high-yield protocol that employs the 2,4,6-tri<i>-tert</i>-butylphenoxy radical as hydrogen acceptor. [CoCl­{L_<i>n</i>^tBu}] (<i>n</i> = 1–3) were examined with respect to reduction, protonation, and oxidation, respectively. One-electron oxidations of [CoCl­(L₁^tBu)] and [CoCl­(L₂^tBu)] lead to ligand-centered radical reactivity, like amide disproportionation into cobalt­(II) amine and imine complexes. In contrast, oxidation of [CoCl­{L₃^tBu}] with Ag⁺ enabled the isolation of thermally stable, square-planar cobalt­(III) complex [CoCl­{L₃^tBu}]⁺, which adopts an intermediate-spin (<i>S</i> = 1) ground state with large magnetic anisotropy. Hence, pincer dehydrogenation gives access to a new platform for high-valent cobalt in square-planar geometry

Four- and Five-Coordinate Osmium(IV) Nitrides and Imides: Circumventing the “Nitrido Wall”

Osmium nitride chemistry is dominated by osmium­(VI) in octahedral or square-pyramidal coordination. The stability of the d² configuration and preference of the strong σ- and π-donor nitride for apical coordination is in line with the Gray–Ballhausen bonding model. In contrast, low-valent osmium­(IV) or other d⁴ nitrides are rare and have only been reported with lower coordination numbers (CN ≤ 4), thereby avoiding π-bonding conflicts of the nitride ligand with the electron-rich metal center. We here report the synthesis of the square-planar osmium­(IV) nitride [Os^IVN­(PNP)] (PNP = N­(CHCHP<i>t</i>Bu₂)₂). From there, a square-pyramidal isonitrile adduct could be isolated, which surprisingly features basal nitride coordination. Analysis of this five-coordinate d⁴ nitride shows an unusual binding mode of the isonitrile ligand, which explains the preference of the weakest σ-donor and strongest π-acceptor isonitrile for apical coordination

Dinitrogen Splitting and Functionalization in the Coordination Sphere of Rhenium

[ReCl₃(PPh₃)₂(NCMe)] reacts with pincer ligand HN­(CH₂CH₂P<i>t</i>Bu₂)₂ (<i>H</i>PNP) to five coordinate rhenium­(III) complex [ReCl₂(PNP)]. This compound cleaves N₂ upon reduction to give rhenium­(V) nitride [Re­(N)­Cl­(PNP)], as the first example in the coordination sphere of Re. Functionalization of the nitride ligand derived from N₂ is demonstrated by selective C–N bond formation with MeOTf

Lewis Acid-Assisted Formic Acid Dehydrogenation Using a Pincer-Supported Iron Catalyst

Formic acid (FA) is an attractive compound for H₂ storage. Currently, the most active catalysts for FA dehydrogenation use precious metals. Here, we report a homogeneous iron catalyst that, when used with a Lewis acid (LA) co-catalyst, gives approximately 1,000,000 turnovers for FA dehydrogenation. To date, this is the highest turnover number reported for a first-row transition metal catalyst. Preliminary studies suggest that the LA assists in the decarboxylation of a key iron formate intermediate and can also be used to enhance the reverse process of CO₂ hydrogenation

Mechanism of Chemical and Electrochemical N₂ Splitting by a Rhenium Pincer Complex

A comprehensive mechanistic study of N₂ activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl₂(PNP)] is presented (PNP^– = N­(CH₂CH₂P<i>t</i>Bu₂)₂^–). Low-temperature studies using chemical reductants enabled full characterization of the N₂-bridged intermediate [{(PNP)­ClRe}₂(N₂)] and kinetic analysis of the N–N bond scission process. Controlled potential electrolysis at room temperature also resulted in formation of the nitride product [Re­(N)­Cl­(PNP)]. This first example of molecular electrochemical N₂ splitting into nitride complexes enabled the use of cyclic voltammetry (CV) methods to establish the mechanism of reductive N₂ activation to form the N₂-bridged intermediate. CV data was acquired under Ar and N₂, and with varying chloride concentration, rhenium concentration, and N₂ pressure. A series of kinetic models was vetted against the CV data using digital simulations, leading to the assignment of an ECCEC mechanism (where “E” is an electrochemical step and “C” is a chemical step) for N₂ activation that proceeds via initial reduction to Re^II, N₂ binding, chloride dissociation, and further reduction to Re^I before formation of the N₂-bridged, dinuclear intermediate by comproportionation with the Re^III precursor. Experimental kinetic data for all individual steps could be obtained. The mechanism is supported by density functional theory computations, which provide further insight into the electronic structure requirements for N₂ splitting in the tetragonal frameworks enforced by rigid pincer ligands

Mechanism of Chemical and Electrochemical N₂ Splitting by a Rhenium Pincer Complex

A comprehensive mechanistic study of N₂ activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl₂(PNP)] is presented (PNP^– = N­(CH₂CH₂P<i>t</i>Bu₂)₂^–). Low-temperature studies using chemical reductants enabled full characterization of the N₂-bridged intermediate [{(PNP)­ClRe}₂(N₂)] and kinetic analysis of the N–N bond scission process. Controlled potential electrolysis at room temperature also resulted in formation of the nitride product [Re­(N)­Cl­(PNP)]. This first example of molecular electrochemical N₂ splitting into nitride complexes enabled the use of cyclic voltammetry (CV) methods to establish the mechanism of reductive N₂ activation to form the N₂-bridged intermediate. CV data was acquired under Ar and N₂, and with varying chloride concentration, rhenium concentration, and N₂ pressure. A series of kinetic models was vetted against the CV data using digital simulations, leading to the assignment of an ECCEC mechanism (where “E” is an electrochemical step and “C” is a chemical step) for N₂ activation that proceeds via initial reduction to Re^II, N₂ binding, chloride dissociation, and further reduction to Re^I before formation of the N₂-bridged, dinuclear intermediate by comproportionation with the Re^III precursor. Experimental kinetic data for all individual steps could be obtained. The mechanism is supported by density functional theory computations, which provide further insight into the electronic structure requirements for N₂ splitting in the tetragonal frameworks enforced by rigid pincer ligands

Synthesis and Structure of Six-Coordinate Iron Borohydride Complexes Supported by PNP Ligands

The preparation of a number of iron complexes supported by ligands of the type HN­{CH₂CH₂(PR₂)}₂ [R = isopropyl (^ⁱPrPNP) or cyclohexyl (^CyPNP)] is reported. This is the first time this important bifunctional ligand has been coordinated to iron. The iron­(II) complexes (^ⁱPrPNP)­FeCl₂(CO) (<b>1a</b>) and (^CyPNP)­FeCl₂(CO) (<b>1b</b>) were synthesized through the reaction of the appropriate free ligand and FeCl₂ in the presence of CO. The iron(0) complex (^ⁱPrPNP)­Fe­(CO)₂ (<b>2a</b>) was prepared through the reaction of Fe­(CO)₅ with ^ⁱPrPNP, while irradiating with UV light. Compound <b>2a</b> is unstable in CH₂Cl₂ and is oxidized to <b>1a</b> via the intermediate iron­(II) complex [(^ⁱPrPNP)­FeCl­(CO)₂]Cl (<b>3a</b>). The reaction of <b>2a</b> with HCl generated the related complex [(^ⁱPrPNP)­FeH­(CO)₂]Cl (<b>4a</b>), while the neutral iron hydrides (^ⁱPrPNP)­FeHCl­(CO) (<b>5a</b>) and (^CyPNP)­FeHCl­(CO) (<b>5b</b>) were synthesized through the reaction of <b>1a</b> or <b>1b</b> with 1 equiv of ⁿBu₄NBH₄. The related reaction between <b>1a</b> and excess NaBH₄ generated the unusual η¹-HBH₃ complex (^ⁱPrPNP)­FeH­(η¹-HBH₃)­(CO) (<b>6a</b>). This complex features a bifurcated intramolecular dihydrogen bond between two of the hydrogen atoms associated with the η¹-HBH₃ ligand and the N–H proton of the pincer ligand, as well as intermolecular dihydrogen bonding. The protonation of <b>6a</b> with 2,6-lutidinium tetraphenylborate resulted in the formation of the dimeric complex [{(^ⁱPrPNP)­FeH­(CO)}₂(μ₂,η¹:η¹-H₂BH₂)]­[BPh₄] (<b>7a</b>), which features a rare example of a μ₂,η¹:η¹-H₂BH₂ ligand. Unlike all previous examples of complexes with a μ₂,η¹:η¹-H₂BH₂ ligand, there is no metal–metal bond and additional bridging ligand supporting the borohydride ligand in <b>7a</b>; however, it is proposed that two dihydrogen-bonding interactions stabilize the complex. Complexes <b>1a</b>, <b>2a</b>, <b>3a</b>, <b>4a</b>, <b>5a</b>, <b>6a</b>, and <b>7a</b> were characterized by X-ray crystallography