Three-Coordinate Nickel Carbene Complexes and Their One-Electron Oxidation Products

The synthesis and characterization of two new carbene complexes, (dtbpe)­NiCH­(dmp) (<b>1</b>; dtbpe = 1,2-bis­(di-<i>tert</i>-butylphosphino)­ethane; dmp = 2,6-dimesitylphenyl) and (dippn)­NiCH­(dmp) (<b>2</b>; dippn = 1,8-bis­(di-<i>iso</i>-propylphosphino)­naphthalene), are described. Complexes <b>1</b> and <b>2</b> were isolated by photolysis of the corresponding side-bound diazoalkane complexes, exemplified by (dtbpe)­Ni­{η²-N₂CH­(dmp)} (<b>3</b>). The carbene complexes feature Ni–C distances that are short and Ni–C–C angles at the carbene carbon that are intermediate between 120° and 180° (155.7(3)° and 152.3(3)°, respectively). The difference between the two carbenes became obvious when their reactivity toward 1-electron oxidizing agents was studied: the oxidation of <b>1</b> led to an internal rearrangement and the formation of a nickel­(I) alkyl [{κ²-P,C-di-<i>tert</i>-butylphosphino-di-<i>tert</i>-butyl-PCH­(dmp)­ethane}­Ni]­[BAr^F₄] (<b>4</b>), while the oxidation of <b>2</b> allowed the isolation of an unrearranged product, formulated as the cationic nickel­(III) carbene complex­[(dippn)­NiCH­(dmp)]­[BAr^F₄] (<b>6</b>). Both oxidations are chemically reversible and the respective reductions lead to the neutral carbene complexes, <b>1</b> and <b>2</b>

Synthesis and Structure of a Cu^I₃S Cluster Unsupported by Other Bridging Ligands

The facile synthesis of a {LCu^I}₃(μ₃-S) cluster supported by monodentate N-heterocyclic carbene ligands has been accomplished through an approach in which two-coordinate (NHC)­Cu⁺ centers are installed sequentially on a sulfido ligand. The reaction between (IPr)­CuCl (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene) and S­(SiMe₃)₂ yields (IPr)­Cu­(SSiMe₃) (<b>2</b>). Treatment of <b>2</b> with [(IPr)­Cu­(NCMe)]­[BF₄] produces the dicopper cluster [{(IPr)­Cu}₂(μ-SSiMe₃)]­[BF₄] (<b>3</b>[BF₄]), which undergoes subsequent reaction with (IPr)­CuF to afford [{(IPr)­Cu}₃(μ₃-S)]­[BF₄] (<b>1</b>[BF₄]) in high yield. The X-ray crystal structure of <b>1</b>[BF₄] establishes it as a rare example of a Cu^I_<i>n</i>(μ_<i>n</i>-S)­L_<i>m</i> cluster, and the first that is not also supported by other bridging ligands

Synthesis and Reactivity of NHC-Supported Ni₂(μ²‑η²,η²‑S₂)‑Bridging Disulfide and Ni₂(μ-S)₂‑Bridging Sulfide Complexes

The (IPr)Ni scaffold stabilizes low-coordinate, mononuclear and dinuclear complexes with a diverse range of sulfur ligands, including μ²-η²,η²-S₂, η²-S₂, μ-S, and μ-SH motifs. The reaction of {(IPr)­Ni}₂(μ-Cl)₂ (<b>1</b>, IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazolin-2-ylidene) with S₈ yields the bridging disulfide species {(IPr)­ClNi}₂(μ²-η²,η²-S₂) (<b>2</b>). Complex <b>2</b> reacts with 2 equiv of AdNC (Ad = adamantyl) to yield a 1:1 mixture of the terminal disulfide compound (IPr)­(AdNC)­Ni­(η²-S₂) (<b>3a</b>) and <i>trans</i>-(IPr)­(AdNC)­NiCl₂ (<b>4a</b>). <b>2</b> also reacts with KC₈ to produce the Ni–Ni-bonded bridging sulfide complex {(IPr)­Ni}₂(μ-S)₂ (<b>6</b>). Complex <b>6</b> reacts with H₂ to yield the bridging hydrosulfide compound {(IPr)­Ni}₂(μ-SH)₂ (<b>7</b>), which retains a Ni–Ni bond. <b>7</b> is converted back to <b>6</b> by hydrogen atom abstraction by 2,4,6-^tBu₃-phenoxy radical. The 2,6-diisopropylphenyl groups of the IPr ligand provide lateral steric protection of the (IPr)Ni unit but allow for the formation of Ni–Ni-bonded dinuclear species and electronically preferred rather than sterically preferred structures

Protonolysis and Amide Exchange Reactions of a Three-Coordinate Cobalt Amide Complex Supported by an N‑Heterocyclic Carbene Ligand

A three-coordinate cobalt species, IPrCoCl­{N­(SiMe₃)₂} [<b>1</b>; IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazolin-2-ylidene], was synthesized by the reaction of {IPrCoCl₂}₂ with NaN­(SiMe₃)₂. Compound <b>1</b> is a useful starting material for low-coordinate (IPr)Co species. <b>1</b> reacts with 2,6-di-<i>tert</i>-butyl-4-methylphenol (BHT-H) via aminolysis of the Co–N bond to generate a three-coordinate phenoxide complex, IPrCoCl­(O-2,6-^tBu₂-4-MeC₆H₂) (<b>2</b>). The reaction of <b>1</b> with 2,6-diisopropylaniline (NH₂DIPP) generates IPrCoCl­(NHDIPP) (<b>4</b>), which undergoes disproportionation to form a mixture of <b>4</b>, {IPrCoCl₂}₂, and IPrCo­(NHDIPP)₂ (<b>3</b>). The same product mixture is formed by the reaction of <b>1</b> with Li­[NH­(DIPP)], which unexpectedly proceeds by amide exchange. Compound <b>3</b> was synthesized independently by the reaction of {IPrCoCl₂}₂ with 4 equiv of Li­[NH­(DIPP)]. The reaction of <b>1</b> with the bulkier lithium 2,6-dimesitylanilide (LiNHDMP) also proceeds by amide exchange to generate IPrCoCl­(NHDMP) (<b>5</b>), which is stable toward disproportionation. Compounds <b>1</b> and <b>2</b> exhibit trigonal-planar geometries at cobalt in the solid state. The solid-state structure of <b>3</b> also contains a trigonal-planar cobalt center and exhibits close Co---H contacts involving the methine hydrogen atoms of the NH­(DIPP) groups in the axial positions. The solid-state structure of <b>5</b> features an interaction between cobalt and a flanking aryl group of the anilide ligand, resulting in pyramidalization of the cobalt center

Functionalization of Complexed N₂O in Bis(pentamethylcyclopentadienyl) Systems of Zirconium and Titanium

Methyl triflate reacts with the metastable azoxymetallacyclopentene complex Cp*₂Zr­(N­(O)­NCPhCPh), generated <i>in situ</i> from nitrous oxide insertion into the Zr–C bond of Cp*₂Zr­(η²-PhCCPh) at −78 °C, to afford the salt [Cp*₂Zr­(N­(O)­N­(Me)­CPhCPh)]­[O₃SCF₃] (<b>1</b>) in 48% isolated yield. A single-crystal X-ray structure of <b>1</b> features a planar azoxymetallacycle with methyl alkylation taking place only at the β-nitrogen position of the former Zr­(N­(O)­NCPhCPh) scaffold. In addition to <b>1</b>, the methoxy-triflato complex Cp*₂Zr­(OMe)­(O₃SCF₃) (<b>2</b>) was also isolated from the reaction mixture in 26% yield and fully characterized, including its independent synthesis from the alkylation of Cp*₂ZrO­(NC₅H₅) with MeO₃SCF₃. Complex <b>2</b> could also be observed, spectroscopically, from the thermolysis of <b>1</b> (80 °C, 2 days). In contrast to Cp*₂Zr­(N­(O)­NPhCCPh), the more stable titanium N₂O-inserted analogue, Cp*₂Ti­(N­(O)­NCPhCPh), reacts with MeO₃SCF₃ to afford a 1:1 mixture of regioisomeric salts, [Cp*₂Ti­(N­(O)­N­(Me)­CPhCPh)]­[O₃SCF₃] (<b>3</b>) and [Cp*₂Ti­(N­(OMe)­NCPhCPh)]­[O₃SCF₃] (<b>4</b>), in a combined 65% isolated yield. Single-crystal X-ray diffraction studies of a cocrystal of <b>3</b> and <b>4</b> show a 1:1 mixture of azoxymetallacyle salts resulting from methyl alkylation at both the β-nitrogen and the β-oxygen of the former Ti­(N­(O)­NCPhCPh ring. As opposed to alkylation reactions, the one-electron reduction of Cp*₂Ti­(N­(O)­NCPhCPh) with KC₈, followed by encapsulation with the cryptand 2,2,2-Kryptofix, resulted in the isolation of the discrete radical anion [K­(2,2,2-Kryptofix)]­[Cp*₂Ti­(N­(O)­NCPhCPh)] (<b>5</b>) in 68% yield. Complex <b>5</b> was studied by single-crystal X-ray diffraction, and its solution X-band EPR spectrum suggested a nonbonding σ-type wedge hybrid orbital on titanium, d­(<i>z</i>²)/d­(<i>x</i>²–<i>y</i>²), houses the unpaired electron, without perturbing the azoxymetallacycle core in Cp*₂Ti­(N­(O)­NCPhCPh). Theoretical studies of Ti and the Zr analogue are also presented and discussed

Heterolytic H–H and H–B Bond Cleavage Reactions of {(IPr)Ni(μ-S)}₂

Kinetic and DFT computational studies reveal that the reaction of {(IPr)­Ni­(μ-S)}₂ (<b>1</b>, IPr = 1,3-bis­(2,6-diisopropyl-phenyl)­imidazolin-2-ylidene) with dihydrogen to produce {(IPr)­Ni­(μ-SH)}₂ (<b>2</b>) proceeds by rate-limiting heterolytic addition of H₂ across a Ni–S bond of intact dinuclear <b>1</b>, followed by <i>cis</i>/<i>trans</i> isomerization at Ni and subsequent H migration from Ni to S, to produce the bis-hydrosulfide product <b>2</b>. Complex <b>1</b> reacts in a similar manner with pinacolborane to produce {(IPr)­Ni}₂­(μ-SH)­(μ-SBPin) (<b>3</b>), showing that heterolytic activation by this nickel μ-sulfide complex can be generalized to other H–E bonds

Functionalization of Complexed N₂O in Bis(pentamethylcyclopentadienyl) Systems of Zirconium and Titanium

Methyl triflate reacts with the metastable azoxymetallacyclopentene complex Cp*₂Zr­(N­(O)­NCPhCPh), generated <i>in situ</i> from nitrous oxide insertion into the Zr–C bond of Cp*₂Zr­(η²-PhCCPh) at −78 °C, to afford the salt [Cp*₂Zr­(N­(O)­N­(Me)­CPhCPh)]­[O₃SCF₃] (<b>1</b>) in 48% isolated yield. A single-crystal X-ray structure of <b>1</b> features a planar azoxymetallacycle with methyl alkylation taking place only at the β-nitrogen position of the former Zr­(N­(O)­NCPhCPh) scaffold. In addition to <b>1</b>, the methoxy-triflato complex Cp*₂Zr­(OMe)­(O₃SCF₃) (<b>2</b>) was also isolated from the reaction mixture in 26% yield and fully characterized, including its independent synthesis from the alkylation of Cp*₂ZrO­(NC₅H₅) with MeO₃SCF₃. Complex <b>2</b> could also be observed, spectroscopically, from the thermolysis of <b>1</b> (80 °C, 2 days). In contrast to Cp*₂Zr­(N­(O)­NPhCCPh), the more stable titanium N₂O-inserted analogue, Cp*₂Ti­(N­(O)­NCPhCPh), reacts with MeO₃SCF₃ to afford a 1:1 mixture of regioisomeric salts, [Cp*₂Ti­(N­(O)­N­(Me)­CPhCPh)]­[O₃SCF₃] (<b>3</b>) and [Cp*₂Ti­(N­(OMe)­NCPhCPh)]­[O₃SCF₃] (<b>4</b>), in a combined 65% isolated yield. Single-crystal X-ray diffraction studies of a cocrystal of <b>3</b> and <b>4</b> show a 1:1 mixture of azoxymetallacyle salts resulting from methyl alkylation at both the β-nitrogen and the β-oxygen of the former Ti­(N­(O)­NCPhCPh ring. As opposed to alkylation reactions, the one-electron reduction of Cp*₂Ti­(N­(O)­NCPhCPh) with KC₈, followed by encapsulation with the cryptand 2,2,2-Kryptofix, resulted in the isolation of the discrete radical anion [K­(2,2,2-Kryptofix)]­[Cp*₂Ti­(N­(O)­NCPhCPh)] (<b>5</b>) in 68% yield. Complex <b>5</b> was studied by single-crystal X-ray diffraction, and its solution X-band EPR spectrum suggested a nonbonding σ-type wedge hybrid orbital on titanium, d­(<i>z</i>²)/d­(<i>x</i>²–<i>y</i>²), houses the unpaired electron, without perturbing the azoxymetallacycle core in Cp*₂Ti­(N­(O)­NCPhCPh). Theoretical studies of Ti and the Zr analogue are also presented and discussed

Carbon–Hydrogen Bond Activation, C–N Bond Coupling, and Cycloaddition Reactivity of a Three-Coordinate Nickel Complex Featuring a Terminal Imido Ligand

The three-coordinate imidos (dtbpe)­NiNR (dtbpe = ^<i>t</i>Bu₂PCH₂CH₂P^<i>t</i>Bu_2, R = 2,6-^<i>i</i>Pr₂C₆H₃, 2,4,6-Me₃C₆H₂ (Mes), and 1-adamantyl (Ad)), which contain a legitimate Ni–N double bond as well as basic imido nitrogen based on theoretical analysis, readily deprotonate HCCPh to form the amide acetylide species (dtbpe)­Ni­{NH­(Ar)}­(CCPh). In the case of R = 2,6-^<i>i</i>Pr₂C₆H₃, reductive carbonylation results in formation of the (dtbpe)­Ni­(CO)₂ along with the N–C coupled product keteneimine PhCHCN­(2,6- ^<i>i</i>Pr₂C₆H₃). Given the ability of the NiN bond to have biradical character as suggested by theoretical analysis, H atom abstraction can also occur in (dtbpe)­NiN­{2,6-^<i>i</i>Pr₂C₆H₃} when this species is treated with HSn­(^<i>n</i>Bu)₃. Likewise, the microscopic reverse reactionconversion of the Ni­(I) anilide (dtbpe)­Ni­{NH­(2,6-^<i>i</i>Pr₂C₆H₃)} to the imido (dtbpe)­NiN­{2,6-^<i>i</i>Pr₂C₆H₃}is promoted when using the radical Mes*O^• (Mes* = 2,4,6-^<i>t</i>Bu₃C₆H₂). Reactivity studies involving the imido complexes, in particular (dtbpe)­NiN­{2,6-^<i>i</i>Pr₂C₆H₃}, are also reported with small, unsaturated molecules such as diphenylketene, benzylisocyanate, benzaldehyde, and carbon dioxide, including the formation of C–N and N–N bonds by coupling reactions. In addition to NMR spectroscopic data and combustion analysis, we also report structural studies for all the cycloaddition reactions involving the imido (dtbpe)­NiN­{2,6-^<i>i</i>Pr₂C₆H₃}

Synthesis and Reactivity of Two-Coordinate Ni(I) Alkyl and Aryl Complexes

Reaction of [(IPr)­Ni­(μ-Cl)]₂ (<b>1-Cl</b>; IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazolin-2-ylidene) with ClMg­{CH­(SiMe₃)₂}·Et₂O affords (IPr)­Ni­{CH­(SiMe₃)₂} (<b>2</b>), a two-coordinate Ni­(I) alkyl complex. An analogous two-coordinate aryl derivative, (IPr)­Ni­(dmp) (dmp = 2,6-dimesitylphenyl), can be similarly prepared from Li­(dmp) and <b>1-Cl</b>. Reaction of <b>2</b> with alkyl bromides gives the three-coordinate Ni­(II) alkyl halide complex (IPr)­Ni­{CH­(SiMe₃)₂}­Br. Evidence for a radical mechanism is presented to explain the reaction of <b>2</b> with alkyl halides

Synthesis and Reactivity of Two-Coordinate Ni(I) Alkyl and Aryl Complexes

Reaction of [(IPr)­Ni­(μ-Cl)]₂ (<b>1-Cl</b>; IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazolin-2-ylidene) with ClMg­{CH­(SiMe₃)₂}·Et₂O affords (IPr)­Ni­{CH­(SiMe₃)₂} (<b>2</b>), a two-coordinate Ni­(I) alkyl complex. An analogous two-coordinate aryl derivative, (IPr)­Ni­(dmp) (dmp = 2,6-dimesitylphenyl), can be similarly prepared from Li­(dmp) and <b>1-Cl</b>. Reaction of <b>2</b> with alkyl bromides gives the three-coordinate Ni­(II) alkyl halide complex (IPr)­Ni­{CH­(SiMe₃)₂}­Br. Evidence for a radical mechanism is presented to explain the reaction of <b>2</b> with alkyl halides