Combining geometric constraint and redox non-innocence within an ambiphilic PBiP pincer ligand.

The synthesis of the first pincer ligand featuring a strictly T-shaped group 15 element and its coordination behaviour towards transition metals is described. The platform is itself derived from a trianionic redox non-innocent NNN scaffold. In addition to providing a rigid coordination environment to constrain a Bi centre in a T-shaped geometry to manipulate its frontier molecular orbital constitution, the NNN chelate displays highly covalent bonding towards the geometrically constrained Bi centre. The formation of intriguing ambiphilic Bi-M bonding interactions is demonstrated upon formation of a pincer complex as well as a multimetallic cluster. All compounds are comprehensively characterised by spectroscopic methods including X-ray Absorption Near Edge Structure (XANES) spectroscopy and complemented by DFT calculations.

Reactivity of a Strictly T-Shaped Phosphine Ligated by an Acridane Derived NNN Pincer Ligand.

The steric tuning of a tridentate acridane-derived NNN pincer ligand allows for the isolation of a strictly T-shaped phosphine that exhibits ambiphilic reactivity. Well-defined phosphorus-centered reactivity towards nucleophiles and electrophiles is reported, contrasting with prior reports on this class of compounds. Reactions towards oxidants are also described. The latter result in the two-electron oxidation of the phosphorus atom from +III to +V and are accompanied by a strong geometric distortion of the NNN pincer ligand. By contrast, cooperative activation of E-H (HCl, HBcat, HOMe) bonds proceeds with retention of the phosphorus redox state. When using H2 O as a substrate, the reaction results in the full disassembly of H2 O to its constituent atoms, highlighting the potential of this platform for small molecule activation reactions.

Dioxygen Splitting by a Tantalum(V) Complex Ligated by a Rigid, Redox Non-Innocent Pincer Ligand.

The reaction of TaMe3 Cl2 with the rigid acridane-derived trisamine H3 NNN yields the tantalum(V) complex [TaCl2 (NNNcat )]. Subsequent reaction with dioxygen results in the full four-electron reduction of O2 yielding the oxido-bridged bimetallic complex [{TaCl2 (NNNsq )}2 O]. This dinuclear complex features an open-shell ground state due to partial ligand oxidation and was comprehensively characterized by single crystal X-ray diffraction, LIFDI mass spectrometry, NMR, EPR, IR and UV/VIS/NIR spectroscopy. The mechanism of O2 activation was investigated by DFT calculations revealing initial binding of O2 to the tantalum(V) center followed by complete O2 scission to produce a terminal oxido-complex.

Halide Effects in Reductive Splitting of Dinitrogen with Rhenium Pincer Complexes.

Transition metal halide complexes are used as precursors for reductive N2 activation up to full splitting into nitride complexes. Distinct halide effects on the redox properties and yields are frequently observed yet not well understood. Here, an electrochemical and computational examination of reductive N2 splitting with the rhenium(III) complexes [ReX2(PNP)] (PNP = N(CH2CH2PtBu2)2 and X = Cl, Br, I) is presented. As previously reported for the chloride precursor ( J. Am. Chem. Soc.2018, 140, 7922), the heavier halides give rhenium(V) nitrides upon (electro-)chemical reduction in good yields yet with significantly anodically shifted electrolysis potentials along the halide series. Dinuclear, end-on N2-bridged complexes, [{ReX(PNP)}2(µ-N2)], were identified as key intermediates in all cases. However, while the chloride complex is exclusively formed via 2-electron reduction and ReIII/ReI comproportionation, the iodide system also reacts via an alternative ReII/ReII-dimerization mechanism at less negative potentials. This alternative pathway relies on the absence of the potential inversion after reduction and N2 activation that was observed for the chloride precursor. Computational analysis of the relevant ReIII/II and ReII/I redox couples by energy decomposition analysis attributes the halide-induced trends of the potentials to the dominating electrostatic Re-X bonding interactions over contributions from charge transfer.

Thermoneutral N-H Bond Activation of Ammonia by a Geometrically Constrained Phosphine.

A geometrically constrained phosphine bearing a tridentate NNS pincer ligand is reported. The effect of the geometric constraint on the electronic structure was probed by theoretical calculations and derivatization reactions. Reactions with N-H bonds result in formation of cooperative addition products. The thermochemistry of these transformations is strongly dependent on the substrate, with ammonia activation being thermoneutral. This represents the first example of a molecular compound that reversibly activates ammonia via N-H bond scission in solution upon mild heating.

The Metaphosphite (PO2 - ) Anion as a Ligand.

The utilization of monomeric, lower phosphorous oxides and oxoanions, such as metaphosphite (PO2 - ), which is the heavier homologue of the common nitrite anion but previously only observed in the gas phase and by matrix isolation, requires new synthetic strategies. Herein, a series of rhenium(I-III) complexes with PO2 - as ligand is reported. Synthetic access was enabled by selective oxygenation of a terminal phosphide complex. Spectroscopic and computational examination revealed slightly stronger σ-donor and comparable π-acceptor properties of PO2 - compared to homologous NO2 - , which is one of the archetypal ligands in coordination chemistry.

Examination of Protonation-Induced Dinitrogen Splitting by in Situ EXAFS Spectroscopy.

The splitting of dinitrogen into nitride complexes emerged as a key reaction for nitrogen fixation strategies at ambient conditions. However, the impact of auxiliary ligands or accessible spin states on the thermodynamics and kinetics of N-N cleavage is yet to be examined in detail. We recently reported N-N bond splitting of a {Mo(µ2:η1:η1-N2)Mo}-complex upon protonation of the diphosphinoamide auxiliary ligands. The reactivity was associated with a low-spin to high-spin transition that was induced by the protonation reaction in the coordination periphery, mainly based on computational results. Here, this proposal is evaluated by an XAS study of a series of linearly N2 bridged Mo pincer complexes. Structural characterization of the transient protonation product by EXAFS spectroscopy confirms the proposed spin transition prior to N-N bond cleavage.

A platinum(II) metallonitrene with a triplet ground state.

Metallonitrenes (M-N) are complexes with a subvalent atomic nitrogen ligand that have been proposed as key reactive intermediates in nitrogen atom transfer reactions. However, in contrast to the common classes of nitride complexes (M≡N) and organic nitrenes (R-N), structurally and spectroscopically well defined 'authentic' metallonitrenes with a monovalent atomic nitrogen ligand remain elusive. Here we report that the photolysis of a platinum(II) pincer azide complex enabled the crystallographic, spectroscopic, magnetic and computational characterization of a metallonitrene that is best described as a singly bonded atomic nitrogen diradical ligand bound to platinum(II). The photoproduct exhibits selective C-H, B-H and B-C nitrogen atom insertion reactivity. Despite the subvalent metallonitrene character, mechanistic analysis for aldehyde C-H amidation shows nucleophilic reactivity of the N-diradical ligand. Ambiphilic reactivity of the metallonitrene is indicated by reactions with CO and PMe3 to form isocyanate and phosphoraneiminato platinum(II) complexes, respectively.

A Terminal Chlorophosphinidene Complex.

Terminal, electrophilic phosphinidene complexes (M=PR) are attractive platforms for PR-transfer to organic substrates. In contrast to aryl- or alkylphosphinidene complexes terminal chlorophosphinidenes (M=PCl) have only been proposed as transient intermediates but isolable example remain elusive. Here we present the transfer of PCl from chloro-substituted dibenzo-7λ3-phosphanorbornadiene to a square-planar osmium(II) PNP pincer complex to give the first isolable, terminal chlorophosphinidene complex with remarkable thermal stability. Os=P bonding was examined computationally giving rise to highly covalent {OsII=PICl} double bonding.

Combined experimental and theoretical studies towards mutual osmium-bismuth donor/acceptor bonding.

Osmium(ii) PNP pincer complexes bearing a hemilabile pyridyl-pyrazolide (PyrPz) ligand have been synthesised, and their reactivity towards Lewis acidic bismuth compounds has been examined. Reactions with BiCl3 resulted in chlorine-atom-transfer to give an osmium(iii) species. Reactions with cationic bismuth species led to adduct formation through N → Bi bond formation via the PyrPz ligand. Theoretical analyses revealed that steric interactions hamper Os → Bi bond formation and indicate that such interactions are possible upon reducing the steric profile around the osmium atom. Analytical techniques include NMR, IR, and EPR spectroscopy, cyclic voltammetry, elemental analysis and DFT calculations.

Recent developments in the chemistry of non-trigonal pnictogen pincer compounds: from bonding to catalysis.

The combination of well-established meridionally coordinating, tridentate pincer ligands with group 15 elements affords geometrically constrained non-trigonal pnictogen pincer compounds. These species show remarkable activity in challenging element-hydrogen bond scission reactions, such as the activation of ammonia. The electronic structures of these compounds and the implications they have on their electrochemical properties and transition metal coordination are described. Furthermore, stoichiometric and catalytic bond forming reactions involving B-H, N-H and O-H bonds as well as carbon nucleophiles are presented.

Oxidative Coupling of Terminal Rhenium Pnictide Complexes.

The isolation of rhenium(V) complexes with terminal phosphide and arsenide ligands was achieved upon decarbonylation of rhenium(III) pnictaethynolates. One-electron oxidation of the pnictide complexes yielded Pn-Pn (Pn=P, As) coupling products, which were spectroscopically and crystallographically characterized. Computational bond analysis suggests that these complexes are best described as {Pn2 }0 complexes that are stabilized by donor-acceptor interactions with the metal and a pyrazole ligand.

Interconversion of Phosphinyl Radical and Phosphinidene Complexes by Proton Coupled Electron Transfer.

The isolable complex [Os(PHMes*)H(PNP)] (Mes*=2,4,6-t Bu3 C6 H3 ; PNP=N{CHCHPt Bu2 }2 ) exhibits high phosphinyl radical character. This compound offers access to the phosphinidene complex [Os(PMes*)H(PNP)] by P-H proton coupled electron transfer (PCET). The P-H bond dissociation energy (BDE) was determined by isothermal titration calorimetry and supporting DFT computations. The phosphinidene product exhibits electrophilic reactivity as demonstrated by intramolecular C-H activation.

Selectivity of tungsten mediated dinitrogen splitting vs. proton reduction.

Mo complexes are currently the most active catalysts for nitrogen fixation under ambient conditions. In comparison, tungsten platforms are scarcely examined. For active catalysts, the control of N2 vs. proton reduction selectivities remains a difficult task. We here present N2 splitting using a tungsten pincer platform, which has been proposed as the key reaction for catalytic nitrogen fixation. Starting from [WCl3(PNP)] (PNP = N(CH2CH2PtBu2)2), the activation of N2 enabled the isolation of the dinitrogen bridged redox series [(N2){WCl(PNP)}2]0/+/2+. Protonation of the neutral complex results either in the formation of a nitride [W(N)Cl(HPNP)]+ or H2 evolution and oxidation of the W2N2 core, respectively, depending on the acid and reaction conditions. Examination of the nitrogen splitting vs. proton reduction selectivity emphasizes the role of hydrogen bonding of the conjugate base with the protonated intermediates and provides guidelines for nitrogen fixation.

Metal-Ligand Cooperative Synthesis of Benzonitrile by Electrochemical Reduction and Photolytic Splitting of Dinitrogen.

Thermal nitrogen fixation relies on strong reductants to overcome the extraordinarily large N-N bond energy. Photochemical strategies that drive N2 fixation are scarcely developed. Here, the synthesis of a dinuclear N2 -bridged complex is presented upon reduction of a rhenium(III) pincer platform. Photochemical splitting into terminal nitride complexes is triggered by visible light. Clean nitrogen transfer with benzoyl chloride to free benzamide and benzonitrile is enabled by cooperative 2 H+ /2 e- transfer of the pincer ligand. A three-step cycle is demonstrated for N2 to nitrile fixation that relies on electrochemical reduction, photochemical N2 -splitting and thermal nitrogen transfer.

Dinitrogen Splitting Coupled to Protonation.

The coupling of electron- and proton-transfer steps provides a general concept to control the driving force of redox reactions. N2 splitting of a molybdenum dinitrogen complex into nitrides coupled to a reaction with Brønsted acid is reported. Remarkably, our spectroscopic, kinetic, and computational mechanistic analysis attributes N-N bond cleavage to protonation in the periphery of an amide pincer ligands rather than the {Mo-N2 -Mo} core. The strong effect on electronic structure and ultimately the thermochemistry and kinetic barrier of N-N bond cleavage is an unusual case of a proton-coupled metal-to-ligand charge transfer process, highlighting the use of proton-responsive ligands for nitrogen fixation.

A square-planar osmium(ii) complex.

Reduction of the pincer complex [OsIIICl2(PNP)] (PNP = N(CHCHPtBu2)2) affords the isolation and full characterization of an osmium(ii) complex with square-planar coordination geometry, i.e. [OsIICl(PNP)]. Spectroscopic, structural and magnetic data in combination with multireference computations indicate strong temperature independent paramagnetism, which arises from an energetically well separated ground state that mixes with excited states through spin-orbit coupling.

Homolytic N-H activation of ammonia: hydrogen transfer of parent iridium ammine, amide, imide, and nitride species.

The redox series [Ir(n)(NHx)(PNP)] (n = II-IV, x = 3-0; PNP = N(CHCHPtBu2)2) was examined with respect to electron, proton, and hydrogen atom transfer steps. The experimental and computational results suggest that the Ir(III) imido species [Ir(NH)(PNP)] is not stable but undergoes disproportionation to the respective Ir(II) amido and Ir(IV) nitrido species. N-H bond strengths are estimated upon reaction with hydrogen atom transfer reagents to rationalize this observation and are used to discuss the reactivity of these compounds toward E-H bond activation.

[IrCl{N(CHCHPtBu2)2}]-: a versatile source of the Ir(I)(PNP) pincer platform.

The iridium(II) complex [IrCl{N(CHCHPtBu2)2}] is reduced by KC8 to give the anionic iridium(I) pincer complex [IrCl{N(CHCHPtBu2)2}](-) which was isolated and fully characterized upon stabilization of the counter cation with crown ether as [K(15-cr-5)2][IrCl{N(CHCHPtBu2)2}]. This unprecedented anionic iridium(I) pincer complex completes the unusual, structurally characterized Ir(I)/Ir(II)/Ir(III) redox series [IrCl{N(CHCHPtBu2)2}](-/0/+), all in a square-planar coordination geometry, emphasizing the versatility of this PNP pincer ligand in stabilizing a broad range of oxidation states. The anionic chloro complex is a versatile source of the Ir(PNP) platform. Its reactivity was examined towards chloride ligand substitution against CO and N2, and oxidative addition of C-electrophiles, C-H bonds and dioxygen, allowing for the isolation of iridium(I) and iridium(III) (PNP) carbonyl, hydrocarbyl and peroxo complexes which were spectroscopically and crystallographically characterized.