N-Atom transfer via thermal or photolytic activation of a Co-azido complex with a PNP pincer ligand

Square-planar imido complexes of cobalt: synthesis, reactivity and computational study

Treatment of [Co(N2)(tBuPNP)] (tBuPNP = anion of 2,5-bis(di-tert-butylphosphinomethyl)pyrrole) with one equivalent of an aryl azide generates the four-coordinate imido complexes [Co(NAr)(tBuPNP)] (Ar = mesityl, phenyl, or 4-tBu-phenyl). X-ray crystallographic analysis of the compounds shows an unusual square-planar geometry about cobalt with nearly linear imido units. In the presence of the hydrogen atom donor, TEMPOH, [Co(NPh)(tBuPNP)] undergoes addition of the H atom to the imido nitrogen to generate the corresponding amido complex, [Co(NHPh)(tBuPNP)], whose structure and composition were verified by independent synthesis. Despite the observation of H atom transfer reactivity with TEMPOH, the imido complexes do not show catalytic activity for C-H amination or aziridination for several substrates examined. In the case of [Co(NPh)(tBuPNP)], addition of excess azide produced the tetrazido complex, [Co(N4Ph2)(tBuPNP)], whose bond metrics were most consistent with an anionic Ph2N4 ligand. Density Functional Theory (DFT) investigations of the imido and tetrazido species suggest that they adopt a ground state best described as possessing a low-spin cobalt(II) ion ferromagnetically coupled to an iminyl radical.

Ligand-Dominated Activation of CO2 and CS2 by the Putative Nickel Phosphiniminato Intermediates

Room-temperature photoactivation of the first- and second-generation PN3P-pincer nickel azido complexes 1a and 1b in the presence of CO2 or CS2 afforded N-bound carbamates, dithiocarbamates, and isothiocyanates, providing insights into CO2 and CS2 activation and demonstrating how a seemingly small difference in the ligand structure significantly influences the reactivity. Theoretical calculations disclosed that the charge of the phosphorus atom plays a critical role in determining the nitrogen atom transfer to form a plausible nickel phosphiniminato intermediate.

Magnesium complexes supported by a dianionic double layer nitrogen-phosphorus ligand: a synthesis and reactivity study

A heterobimetallic complex [MeN(CH2CH2NPiPr)2MgLiCl(THF)]2 (1) supported by a dianionic double layer nitrogen-phosphorus ligand was synthesized by the reaction of H2L1 (H2L1 = MeN(CH2CH2NHPiPr)2) with MgCl2 in the presence of n-BuLi. Reactions of complex 1 with 2 equivalents of CuI, AgI and AuCl·SMe2 led to the formation of heterobimetallic clusters [MeN(CH2CH2NPiPr)2MgCuI]2 (2), [MeN(CH2CH2NPiPr)2MgAgI]2 (3) and [MeN(CH2CH2NPiPr)2MgAuCl]2 (4), respectively. X-ray single-crystal diffraction analysis revealed that these complexes are dimers, which are composed of two [MeN(CH2CH2NPiPr)2Mg] units connected by coinage metals (i.e., Cu, Ag, and Au). The reactivity of 1 was further investigated and it was found that complex 1 could react with 4 equivalents of MeI, giving a complex [CH3N(CH2CH2NPiPr2Me)2MgI]+[I]- (5), which can be viewed as a magnesium complex supported by a neutral double layer nitrogen-phosphorus ligand (CH3N(CH2CH2NPiPr2Me)2). Complex 1 could also react with 2 equivalents of NaNH2, leading to the isolation of an amido anion bridged magnesium-sodium heterobimetallic cluster [MeN(CH2CH2NPiPr)2MgNaNH2]2 (6).

Unusual Hydrogenation Reactivities of a Thiolate-Bridged Dicobalt μ-Nitride Featuring a Bent {CoIII-N-CoIII} Core

Transition metal nitrides have received considerable attention owing to their crucial roles in nitrogen fixation and nitrogen atom transfer reactions. Compared to the early and middle transition metals, it is much more challenging to access late transition metal nitrides, especially cobalt in group 9. So far, only a handful of cobalt nitrides have been reported; consequently, their hydrogenation reactivity is largely unexplored. Herein, we present a structurally and spectroscopically well-characterized thiolate-bridged dicobalt μ-nitride [Cp*CoIII(μ-SAd)(μ-N)CoIIICp*] (2) featuring a bent {CoIII(μ-N)CoIII} core. Remarkably, complex 2 can realize not only direct hydrogenation of nitride to amide but also stepwise N-H bond formation from nitride to ammonia. Specifically, 2 can facilely activate dihydrogen (H2) at mild conditions to generate a dicobalt μ-amide [Cp*CoII(μ-SAd)(μ-NH2)CoIICp*] (4) via an unusual mechanism of two-electron oxidation of H2 as proposed by computational studies; in the presence of protons (H+) and electrons, nitride 2 can convert to dicobalt μ-imide [Cp*CoIII(μ-SAd)(μ-NH)CoIIICp*][BPh4] (3[BPh4]) and to CoIICoII μ-amide 4, and finally release ammonia. In contrast to 2, the only other structurally characterized dicobalt μ-nitride Na(THF)4{[(ketguan)CoIII(N3)]2(μ-N)} (ketguan = [(tBu2CN)C(NDipp)2]-, Dipp = 2,6-diisopropylphenyl) (e) that possesses a linear {CoIII(μ-N)CoIII} moiety cannot directly react with H2 or H+. Further in-depth electronic structure analyses shed light on how the varying geometries of the {CoIII(μ-N)CoIII} moieties in 2 and e, bent vs linear, impart their disparate reactivities.

Electronic insights into aminoquinoline-based PNHN ligands: protonation state dictates geometry while coordination environment dictates N-H acidity and bond strength

A variety of transition metal complexes bearing aminoquinoline PN^HH'-R ligands R = Ph (L1H), Cy (L2H) and their amido analogues are reported for rhodium(I) ([Rh(L1H)(PPh₃)]⁺1 and Rh(L1)(PPh₃) 2), cobalt(II) (Co(L2)(Cl) 3), and iron(II) ([Fe(L1H)₂]²⁺5, Fe(L1)₂6, and [Fe(C₅Me₅)(L1H)]PF₆7). The acid-base and redox properties of the amido complexes 2, 6, and their protio parent complexes 1, and 5 permit the determination of the pK_a and bond dissociation free energy (BDFE) of their N-H bonds while the ligand scaffold is coordinated to metal centres of square planar and octahedral geometry, respectively. From relative concentrations obtained by the use of ³¹P{¹H} NMR spectroscopy, a pK_a^THF value of 14 is calculated for rhodium complex 1, 6.4 for iron complex 5, and 24 for iron complex 7. These data, when combined with elecrochemical potentials obtained via cyclic voltammetry, allow the calculations of BDFE values for the N-H bond of 69 kcal mol^-1 for 1, and of 55 kcal mol^-1 for 5.

Mapping the Reactivity of Dicobalt Bridging Nitrides in Constrained Geometries

Low-nuclearity nitrides of the late transition metals are rare and reactive molecular species, with little experimental precedent. The first putative examples of dicobalt bridging nitrides, [(nPDI2)Co2(μ-N)(PMe3)2][OTf]3 (n[Co2N]3+; PDI = pyridyldiimine; n = 2 or 3, representing the length of the aliphatic chain linking PDI imino groups), were reported recently and shown to undergo a range of intramolecular reaction pathways, including N-H bond formation, C-H bond insertion, and P═N bond formation at the bridging nitride. The specific mode of reactivity changed with the phase of the reaction and the size of the macrocycle used to support the transient species. The present contribution offers a computational investigation into both the geometric and electronic structures of these nitrides as well as the factors governing their reaction selectivity. The compounds n[Co2N]3+ exhibit μ-N-based lowest unoccupied molecular orbitals (LUMOs) that are consistent with subvalent, electrophilic nitrides. The specific orientations of the LUMOs induce ring-size-dependent stereoelectronic effects, thereby causing the product selectivity observed experimentally. Notably, the nitrides also exhibit a degree of nucleophilicity at μ-N by way of a high-energy, μ-N-based lone pair. This ambiphilic character appears to be a direct result of the constrained environment imposed by the folded-ligand geometries of n[Co2N]3+. When combined with the experimental findings, these data led to the conclusion that the folded-ligand isomers are the reactive species and that the constrained geometry imposed by the macrocyclic ligand plays an important role in controlling the reaction outcome.

Pursuit of an Electron Deficient Titanium Nitride

The nitride salt [(PN)₂Ti≡N{μ₂-K(OEt₂)}]₂ (1) (PN^- = (N-(2-PⁱPr₂-4-methylphenyl)-2,4,6-Me₃C₆H₂) can be oxidized with two equiv of I₂ or four equiv of ClCPh₃ to produce the phosphinimide-halide complexes (NPN')(PN)Ti(X) (X^- = I (2), Cl (3); NPN' = N-(2-NPⁱPr₂-4-methylphenyl)-2,4,6-Me₃C₆H₂^2-), respectively. In the case of 2, H₂ was found to be one of the other products; whereas, HCPh₃ and Gomberg's dimer were observed upon the formation of 3. Independent studies suggest that the oxidation of 1 could imply the formation of the transient nitridyl species [(PN)₂Ti(≡N•)] (A), which can either oxidize the proximal phosphine atom to produce the Ti(III) intermediate [(NPN')(PN)Ti] (B) or, alternatively, engage in H atom abstraction to form the parent imido (PN)₂Ti≡NH (4). The latter was independently prepared and was found to photochemically convert to the titanium-hydride, (NPN')(PN)Ti(H) (5). Isotopic labeling studies using (PN)₂Ti≡ND (4-d₁) as well as reactivity studies of 5 with a hydride abstractor demonstrate the presence of the hydride ligand in 5. An alternative route to putative A was observed via a photochemically promoted incomplete reduction of the azide ligand in (PN)₂Ti(N₃) (6) to 4. This process was accompanied by some formation of 5. Frozen matrix X-band EPR studies of 6, performed under photolytic conditions, were consistent with species B being formed under these reaction conditions, originating from a low barrier N-insertion into the phosphine group in the putative nitridyl species A. Computational studies were also undertaken to discover the mechanism and plausibility of the divergent pathways (via intermediates A and B) in the formation of 2 and 3, and to characterize the bonding and electronic structure of the elusive nitrogen-centered radical in A.

Metal-ligand cooperative transformation of alkyl azide to isocyanate occurring at a Co-Si moiety

A cobalt-silyl moiety reveals metal-ligand cooperative group transfer to generate isocyanate from the reaction of alkyl azide and CO. This reaction involves the reversible insertion of a nitrene group into a Co-Si bond. Photolysis leads to ligand substitution of a Co(CO)₂ species, allowing the successful catalytic conversion of AdN₃ to AdNCO under CO(g).

Synthesis of Chromium(II) Complexes with Chelating Bis(alkoxide) Ligand and Their Reactions with Organoazides and Diazoalkanes

Synthesis of new chromium(II) complexes with chelating bis(alkoxide) ligand [OO]^Ph (H₂[OO]^Ph = [1,1′:4′,1′’-terphenyl]-2,2′’-diylbis(diphenylmethanol)) and their subsequent reactivity in the context of catalytic production of carbodiimides from azides and isocyanides are described. Two different Cr(II) complexes are obtained, as a function of the crystallization solvent: mononuclear Cr[OO]^Ph(THF)₂ (in toluene/THF, THF = tetrahydrofuran) and dinuclear Cr₂([OO]^Ph)₂ (in CH₂Cl₂/THF). The electronic structure and bonding in Cr[OO]^Ph(THF)₂ were probed by density functional theory calculations. Isolated Cr₂([OO]^Ph)₂ undergoes facile reaction with 4-MeC₆H₄N₃, 4-MeOC₆H₄N₃, or 3,5-Me₂C₆H₃N₃ to yield diamagnetic Cr(VI) bis(imido) complexes; a structure of Cr[OO]^Ph(N(4-MeC₆H₄))₂ was confirmed by X-ray crystallography. The reaction of Cr₂([OO]^Ph)₂ with bulkier azides N₃R (MesN₃, AdN₃) forms paramagnetic products, formulated as Cr[OO]^Ph(NR). The attempted formation of a Cr–alkylidene complex (using N₂CPh₂) instead forms chromium(VI) bis(diphenylmethylenehydrazido) complex Cr[OO]^Ph(NNCPh₂)₂. Catalytic formation of carbodiimides was investigated for the azide/isocyanide mixtures containing various aryl azides and isocyanides. The formation of carbodiimides was found to depend on the nature of organoazide: whereas bulky mesitylazide led to the formation of carbodiimides with all isocyanides, no carbodiimide formation was observed for 3,5-dimethylphenylazide or 4-methylphenylazide. Treatment of Cr₂([OO]^Ph)₂ or H₂[OO]^Ph with NO⁺ leads to the formation of [1,2-b]-dihydroindenofluorene, likely obtained via carbocation-mediated cyclization of the ligand.

Radical-Type Reactivity and Catalysis by Single-Electron Transfer to or from Redox-Active Ligands

Controlled ligand-based redox-activity and chemical non-innocence are rapidly gaining importance for selective (catalytic) processes. This Concept aims to provide an overview of the progress regarding ligand-to-substrate single-electron transfer as a relatively new mode of operation to exploit ligand-centered reactivity and catalysis based thereon.

Nickel-Alkyl Complexes with a Reactive PNC-Pincer Ligand

Based on previous work related to the design and application of rigid tridentate phosphine-pyridine-phenyl coordination offered by a PNC-pincer ligand upon cyclometalation to nickel, the synthesis, spectroscopic and solid state characterization and redox-reactivity of two NiII(PNC) complexes featuring either a methyl (2CH3 ) or CF3 co-ligand (2CF3 ) are described. One-electron oxidation is proposed to furnish C-C reductive elimination, as deduced from a combined chemical, electrochemical, spectroscopic and computational study. One-electron reduction results in a ligand-centered radical anion, as supported by electrochemistry, UV spectroelectrochemistry, EPR spectroscopy, and DFT calculations. This further attenuates the breadth of chemical reactivity offered by such PNC-pincer ligands.

Nitrogen atom transfer mediated by a new PN3P-pincer nickel core via a putative nitrido nickel intermediate

A synthetic cycle for a complete nitrogen atom transfer reaction is achieved by irradiating the (PN³P)Ni(N₃)/RNC mixture and subsequent treatment of the resultant products with alkyl halides.

Redox-active diaminoazobenzene complexes of rhodium(iii): synthesis, structure and spectroscopic characterization

Coordination diversity of an aromatic diamine with Rh(iii) is presented together with the elucidation of the molecular and electronic structures, electron transfer, and electronic transitions.

Electrocatalytic Azide Oxidation Mediated by a Rh(PNP) Pincer Complex

One-electron oxidation of the rhodium(I) azido complex [Rh(N3 )(PNP)] (5), bearing the neutral, pyridine-based PNP ligand 2,6-bis(di-tert-butylphosphinomethyl)pyridine, leads to instantaneous and selective formation of the mononuclear rhodium(I) dinitrogen complex [Rh(N2 )(PNP)]+ (9+ ). Interestingly, complex 5 also acts as a catalyst for electrochemical N3- oxidation (Ep ≈-0.23 V vs. Fc+/0 ) in the presence of excess azide. This is of potential relevance for the design of azide-based and direct ammonia fuel cells, expelling only harmless dinitrogen as an exhaust gas.

Ring-Size-Modulated Reactivity of Putative Dicobalt-Bridging Nitrides: C-H Activation versus Phosphinimide Formation

Dicobalt complexes supported by flexible macrocyclic ligands were used to target the generation of the bridging nitrido species [(ⁿ PDI₂ )Co₂ (μ-N)(PMe₃ )₂ ]³⁺ (PDI=2,6-pyridyldiimine; n=2, 3, corresponding to the number of catenated methylene units between imino nitrogen atoms). Depending on the size of the macrocycle and the reaction conditions (solution versus solid-state), the thermolysis of azide precursors yielded bridging phosphinimido [(² PDI₂ )Co₂ (μ-NPMe₃ )(PMe₃ )₂ ]³⁺ , amido [(ⁿ PDI₂ )Co₂ (μ-NH₂ )(PMe₃ )₂ ]³⁺ (n=2, 3), and C-H amination [(³ PDI₂ *-μ-NH)Co₂ (PMe₃ )₂ ]³⁺ products. All results are consistent with the initial formation of [(ⁿ PDI₂ )Co₂ (μ-N)(PMe₃ )₂ ]³⁺ , followed by 1) PMe₃ attack on the nitride, 2) net hydrogen-atom transfer to form N-H bonds, or 3) C-H amination of the alkyl linker of the ⁿ PDI₂ ligand.

Migratory insertion and hydrogenation of a bridging azide in a thiolate-bridged dicobalt reaction platform

A novel well-defined thiolate-bridged dicobalt azido complex is converted to a rare sulfilimide-bridged dicobalt complex via nitrogen atom migratory insertion into the Co-S bond upon thermolysis. Intriguingly, the homolytic cleavage of hydrogen is achieved by this azide under mild conditions to furnish a partially hydrogenated azido complex.