Square Planar Cobalt(II) Hydride versus T-Shaped Cobalt(I): Structural Characterization and Dihydrogen Activation with PNP–Cobalt Pincer Complexes

Relationship between Transition-Metal Hydride Bond Lengths and Stretching Wavenumbers

Here it is demonstrated that there is a linear relationship between the terminal 3d metal hydride stretching wavenumber νMH and the metal hydride distance dMH reported to date: νMH ∼ (-1.05dMH + 3.35) × 103 cm-1. That is, upon moving from Group 3 to Group 10 metals in complexes and binary molecules, νMH (cm-1) increases as the metal-hydrogen distance dMH (Å) decreases as determined by X-ray diffraction, molecular spectroscopy, and, in one case, neutron diffraction. This trend contrasts with the relatively constant bond dissociation free energy (BDFE) of the diamagnetic complexes and the usually smaller BDFE (by as much as -20 kcal/mol) of paramagnetic ones, thus showing that there is no correlation between either νMH or its force constant with the BDFE. If a positive charge is added to the complex by metal-ion oxidation, νMH may increase or decrease depending on the coordination number and spin state of the complex.

T-shaped 14 Electron Rhodium Complexes: Potential Active Species in C-H Activation

Two T-shaped 14-electron rhodium complexes 2 a and 2 b, "framed" and thus stabilized by PNP pincer ligands have been synthesized. The bis(t-butyl)phosphine derived PNPtBu-rhodium complex 2 a was isolated from pentane as the more stable cyclometalated Rh(III) hydrido complex and found to be in equilibrium with the T-shaped 14e- Rh(I) complex 2 aT which itself could be directly crystallized upon change of the solvent. The cyclometallation is suppressed using an adamantyl substituted PNPAd ligand to give the analogous T-shaped Rh(I) species 2 b, stabilized through an agostic interaction with one of the adamantyl C-Hs. Depending on the solvent, complex 2 a reacted with ethylene either by π-coordination (4 a) or C-H activation giving a hydrido-vinyl Rh(III) species 4 b, both isomers being in equilibrium in solution. Complex 2 b was found to reversibly C-H activate arenes to form the hydrido-aryl Rh(III) complexes.

Carbon-Carbon Bond Activation at Chromium(I): An 11-Electron Complex Cleaving Dialkynes

An 11-electron T-shaped chromium(I) complex was obtained by reduction of a PNP-supported chromium(II) iodide complex. Its d5 high-spin electronic structure was characterized employing paramagnetic NMR, EPR, UV/Vis and magnetic measurements (SQUID). The complex readily reacts with conjugated dialkynes to cleave the internal C-C bond, forming the respective acetylide complexes. Varying the alkyne substituents enabled the isolation and characterization of dinuclear dialkynedichromium intermediates which thermally convert to the products.

Radicalizing CO by Mononuclear Palladium(I)

A mononuclear, T-shaped palladium(I) d9 metalloradical (3), stabilized by a bulky carbazole-based PNP-ligand, was obtained by reduction of palladium chloride or thermal Pd-C bond homolysis of the corresponding neopentyl complex. Pressurizing with CO gave the Pd(I) carbonyl complex, which was structurally characterized by X-ray diffraction. Delocalization of the unpaired electron to the carbonyl carbon was detected by EPR spectroscopy and theoretically modeled by DFT and ab initio methods. The partially reduced and radicalized CO slowly reacts with di(tert-butyl) disulfide under homolytic S-S cleavage and C-S bond formation to give the corresponding metallathioester.

Cobalt-Catalyzed Hydrogenation Reactions Enabled by Ligand-Based Storage of Dihydrogen

The use of supporting ligands that can store either protons or electrons has emerged as a powerful strategy in catalysis. While these strategies are potent individually, natural systems mediate remarkable transformations by combining the storage of both protons and electrons in the secondary coordination sphere. As such, there has been recent interest in using this strategy to enable fundamentally different transformations. Furthermore, outsourcing H-atom or hydrogen storage to ancillary ligands can also enable alternative mechanistic pathways and thereby selectivity. Here, we describe the application of this strategy to facilitate radical reactivity in Co-based hydrogenation catalysis. Metalation of previously reported dihydrazonopyrrole ligands with Co results in paramagnetic complexes, which are best described as having Co(II) oxidation states. These complexes catalytically hydrogenate olefins with low catalyst loadings under mild conditions (1 atm H₂, 23 °C). Mechanistic, spectroscopic, and computational investigations indicate that this system goes through a radical hydrogen-atom transfer (HAT) type pathway that is distinct from classic organometallic mechanisms and is supported by the ability of the ligand to store H₂. These results show how ancillary ligands can facilitate efficient catalysis, and furthermore how classic organometallic mechanisms for catalysis can be altered by the secondary coordination sphere.

3d Metal Complexes in T-shaped Geometry as a Gateway to Metalloradical Reactivity

ConspectusLow-valent, low-coordinate 3d metal complexes represent a class of extraordinarily reactive compounds that can act as reagents and catalysts for challenging bond-activation reactions. The pursuit of these electron-deficient metal complexes in low oxidation states demands ancillary ligands capable of providing not only energetic stabilization but also sufficiently high steric bulk at the metal center. From this perspective, pincer ligands are particularly advantageous, as their prearranged, meridional coordination mode scaffolds the active center while the substituents of the peripheral donor atoms provide effective steric shielding for the coordination sphere. In a T-shaped geometry, the transition metal complexes possess a precisely defined vacant coordination site, which, combined with the often observed high-spin electron configuration, exhibits unusually high selectivity of these compounds with respect to one-electron redox chemistry. In light of the intractable reaction pathways typically observed with related electronically unsaturated 3d transition metal complexes, the pincer coordination mode enables the isolation of low-valent compounds with more controlled and unique reactivity. We have thus investigated a series of T-shaped metal(I) complexes using three different types of pincer ligands, which may be regarded as "metalloradicals" due to their selectively exposed unpaired electrons.These compounds display remarkably high thermal stability and represent rarely observed "naked" monovalent metal species featuring both monomeric and dimeric structures. Extensive reactivity studies using various organic substrates highlight a strong tendency of these paramagnetic compounds to undergo one-electron oxidation, leading to the isolation of a plethora of metal(II) species with reduced organic ligands as unusual structural elements. The exploration of C₂ symmetric T-shaped Ni(I) complexes as asymmetric catalysts also shows success in enantioselective hydrodehalogenation of geminal dihalogenides. In addition, this specific class of low-valent, low-coordinate complexes can be further diversified by introducing redox-active pincer ligands or building homobimetallic systems with two T-shaped units.This Account focuses on the discussion of selected examples of iron, cobalt, and nickel pincer complexes bearing a [P,N,P] or [N,N,N] donor set; however, their electronic structure and radical-type reactivity can be broadly extended to other pincer systems. The availability of various types of pincer ligands should allow fine-tuning of the reactivity of the T-shaped complexes. Given the unprecedented reactivity observed with these compounds, we expect the studies of T-shaped 3d metal complexes to be a fertile field for advancing base metal catalysis.

An Unusual Mixed Valent Cobalt Dimer as a Catalyst for Anti-Markovnikov Hydrophophination of Alkynes

The reaction of [Co(PMe3)4] (1) with a redox-active NNN pincer ligand (L1) led us to isolate a unique binuclear cobalt complex ([(PMe3)2CoII(L13-)CoI(PMe3)3] (2)) anchored by a three electron reduced L1...

Photo-Initiated Cobalt-Catalyzed Radical Olefin Hydrogenation

Outer-sphere radical hydrogenation of olefins proceeds via stepwise hydrogen atom transfer (HAT) from transition metal hydride species to the substrate. Typical catalysts exhibit M-H bonds that are either too weak to efficiently activate H2 or too strong to reduce unactivated olefins. This contribution evaluates an alternative approach, that starts from a square-planar cobalt(II) hydride complex. Photoactivation results in Co-H bond homolysis. The three-coordinate cobalt(I) photoproduct binds H2 to give a dihydrogen complex, which is a strong hydrogen atom donor, enabling the stepwise hydrogenation of both styrenes and unactivated aliphatic olefins with H2 via HAT.

Coupling of CO2 and epoxides catalysed by novel N-fused mesoionic carbene complexes of nickel(II)

We report the syntheses of two rigid mesoionic carbene (MIC) ligands with a carbazole backbone via an intramolecular Finkelstein-cyclisation cascade and investigate their coordination behavior towards nickel(II) acetate. Despite the nickel(II) carbene complexes 4a,b showing only minor differences in their chemical composition, they display curious differences in their chemical properties, e.g. solubility. Furthermore, the potential of these novel MIC complexes in the coupling of carbon dioxide and epoxides as well as the differences in reactivity compared to classical NHC-derived complexes are evaluated.

Selectivity Reverse of Hydrosilylation of Aryl Alkenes Realized by Pyridine N-Oxide with [PSiP] Pincer Cobalt(III) Hydride as Catalyst

Six silyl cobalt(III) hydrides 1-6 with [PSiP] pincer ligands having different substituents at the P and Si atoms ([(2-Ph₂PC₆H₄)₂MeSiCo(H)(Cl)(PMe₃)] (1), [(2-Ph₂PC₆H₄)₂HSiCo(H)(Cl)(PMe₃)] (2), [(2-Ph₂PC₆H₄)₂PhSiCo(H)(Cl)(PMe₃)] (3), [(2-ⁱPr₂PC₆H₄)₂HSiCo(H)(Cl)(PMe₃)] (4), [(2-ⁱPr₂PC₆H₄)₂MeSiCo(H)(Cl)(PMe₃)] (5), and [(2-ⁱPr₂PC₆H₄)₂PhSiCo(H)(Cl)(PMe₃)] (6)) were synthesized through the reactions of the ligands (L1-L6) with CoCl(PMe₃)₃ via Si-H bond cleavage. Compounds 1-6 have catalytic activity for alkene hydrosilylation, and among them, complex 3 is the best catalyst with excellent anti-Markovnikov regioselectivity. A silyl dihydrido cobalt(III) complex 7 from the reaction of 3 with Ph₂SiH₂ was isolated, and its catalytic activity is equivalent to that of complex 3. Complex 7 and its derivatives 10-12 could also be obtained through the reactions of complexes 3, 1, 4, and 5 with NaBHEt₃. The molecular structure of 7 was indirectly verified by the structures of 10-12. To our delight, the addition of pyridine N-oxide reversed the selectivity of the reaction, from anti-Markovnikov to Markovnikov addition. At the same time, the reaction temperature was reduced from 70 to 30 °C on the premise of high yield and excellent selectivity. However, this catalytic system is only applicable to aromatic alkenes. On the basis of the experimental information, two reaction mechanisms are proposed. The molecular structures of cobalt(III) complexes 3-6 and 10-12 were determined by single crystal X-ray diffraction analysis.

Synthesis and Catalytic Reactivity of Cobalt Pincer Nitrosyl Hydride Complexes

The synthesis, characterization, and catalytic activity of low-spin {CoNO}⁸ pincer complexes of the type [Co(PCP)(NO)(H)] are described. These compounds are obtained either by reacting [Co(PCP)(κ²-BH₄)] with NO and Et₃N or, alternatively, by reacting [Co(PCP)(NO)]⁺ with boranes, such as NH₃·BH₃ in solution. The five-coordinate, diamagnetic Co(III) complex [Co(PCP^NMe-iPr)(NO)(H)] was found to be the active species in the hydroboration of alkenes with anti-Markovnikov selectivity. A range of aromatic and aliphatic alkenes were efficiently converted with pinacolborane (HBpin) under mild conditions in good to excellent yield. Mechanistic insight into the catalytic reaction is provided by means of isotope labeling, NMR spectroscopy, and APCI/ESI-MS as well as DFT calculations.

Reactivity of a T-shaped cobalt(I) pincer-complex

The reactivity of a paramagnetic T-shaped cobalt(i) complex, [(iPrboxmi)Co], stabilised by a monoanionic bis(oxazolinylmethylidene)-isoindolate (boxmi) NNN pincer ligand is described. The exposure to carbon monoxide as an additional neutral ligand resulted in the square-planar species [(iPrboxmi)Co(CO)], accompanied by a change in the electronic spin state from S = 1 to S = 0. In contrast, upon treatment with trimethylphosphine the formation of the distorted tetrahedral complex [(iPrboxmi)Co(PMe3)] was observed (S = 1). Reacting [(iPrboxmi)Co] with iodine (I2), organic peroxides (tBu2O2, (SiMe3)2O2) and diphenyldisulphide (Ph2S2) yielded the tetracoordinated complexes [(iPrboxmi)CoI], [(iPrboxmi)Co(OtBu)], [(iPrboxmi)Co(OSiMe3)] and [(iPrboxmi)Co(SPh)], respectively, demonstrating the capability of the boxmi-supported cobalt(i) complex to homolytically cleave bonds and thus its distinct one-electron reactivity. Furthermore, a square-planar cobalt(ii) alkynyl complex [(iPrboxmi)Co(CCArF)] was identified as the main product in the reaction between [(iPrboxmi)Co] and a terminal alkyne, 4-fluoro-1-ethynylbenzene. Putting such species in the context of the previously investigated hydroboration catalysis, its stoichiometric reaction with pinacolborane revealed its potential conversion into a cobalt(ii) hydride complex, thus confirming its original attribution as off-cycle species.

Electrocatalytic Proton Reduction by a Cobalt(III) Hydride Complex with Phosphinopyridine PN Ligands

Cobalt complexes with 2-(diisopropylphosphinomethyl)pyridine (PN) ligands have been synthesized with the aim of demonstrating electrocatalytic proton reduction to dihydrogen with a well-defined hydride complex of an Earth-abundant metal. Reactions of simple cobalt precursors with 2-(diisopropylphosphino-methyl)pyridine (PN) yield [Co^II(PN)₂(MeCN)][BF₄]₂ 1, [Co^III(PN)₂(H)(MeCN)][PF₆]₂ 2, and [Co^III(PN)₂(H)(Cl)][PF₆] 3. Complexes 1 and 3 have been characterized crystallographically. Unusually for a bidentate PN ligand, all three exhibit geometries with mutually trans phosphorus and nitrogen ligands. Complex 1 exhibits a distorted square-pyramidal geometry with an axial MeCN ligand in a low-spin electronic state. In complexes 2 and 3, the PN ligands lie in a plane leaving the hydride trans to MeCN or chloride, respectively. The redox behavior of the three complexes has been studied by cyclic voltammetry at variable scan rates and by spectroelectrochemistry. A catalytic wave is observed in the presence of trifluoroacetic acid (TFA) at an applied potential close to the Co(II/I) couple of 1. Bulk electrolysis of 1, 2, or 3 at a potential of ca. -1.4 V vs E(Fc⁺/Fc) in the presence of TFA yields H₂ with Faradaic yields close to 100%. A catalytic mechanism is proposed in which the pyridine moiety of a PN ligand acts as a pendant proton donor following opening of the chelate ring. Additional mechanisms may also operate, especially in the presence of high acid concentration where speciation changes.

Reaction Pathways and Redox States in α-Selective Cobalt-Catalyzed Hydroborations of Alkynes

Cobalt(II) alkyl complexes supported by a monoanionic NNN pincer ligand are pre‐catalysts for the regioselective hydroboration of terminal alkynes, yielding the Markovnikov products with α:β‐(E) ratios of up to 97:3. A cobalt(II) hydride and a cobalt(II) vinyl complex appear to determine the main reaction pathway. In a background reaction the highly reactive hydrido species specifically converts to a coordinatively unsaturated cobalt(I) complex which was found to re‐enter the main catalytic cycle.

Single or Paired? Structure and Reactivity of PNP-Chromium(II) Hydrides

The preparation and reactivity of a range of novel paramagnetic chromium(II) complexes supported by a carbazole-based PNP pincer ligand is reported. Deprotonation of the ligand precursors ^R(PNP)H (1^R) and subsequent reaction with chromium(II) chloride led to the formation of square-planar chlorido complexes ^R(PNP)CrCl (2^R). Further reaction with various alkylating agents resulted in the isolation of chromium alkyl complexes ^R(PNP)CrR' (3^R-R') which were then hydrogenated to yield two rare examples of paramagnetic chromium(II) hydrides 4^iPr and 4^tBu. Both compounds were characterized by X-ray diffraction and paramagnetic NMR spectroscopy supported by a comprehensive DFT-supported assignment of the resonances. While the di(tert-butyl)phosphino PNP substituted complex 4^tBu was found to exhibit a monomeric square-planar molecular structure, its isopropyl-substituted analog 4^iPr forms a dimer, also indicated by a strong antiferromagnetic coupling of the chromium centers. The pronounced reactivity of these compounds toward C═X double bonds was demonstrated by reaction with benzophenone, N,N'-dicyclohexylcarbodiimide, and carbon dioxide, which gave the corresponding insertion products. The alkoxido complex 5^iPr, the amidinato complex 6^iPr, and the formato compound 7^tBu were also characterized by X-ray diffraction.

Concerted aryl-sulfur reductive elimination from PNP pincer-supported Co(iii) and subsequent Co(i)/Co(iii) comproportionation

This report discloses a combined experimental and computational study aimed at understanding C-S reductive elimination from Co(iii) supported by a diarylamido/bis(phosphine) PNP pincer ligand. Divalent (PNP)Co-aryl complexes could be easily oxidized to five-coordinate Co(iii) derivatives, and anion metathesis provided five-coordinate (PNP)Co(Ar)(SAr') complexes of Co(iii). In contrast to their previously described (POCOP)Co(Ar)(SAr') analogs, but similarly to the (PNP)Rh(Ar)(SAr') and (POCOP)Rh(Ar)(SAr') analogs, (PNP)Co(Ar)(SAr') undergo C-S reductive elimination with the formation of the desired diarylsulfide product ArSAr'. DFT studies and experimental observations are consistent with a concerted process. However, in contrast to the Rh analogs, the immediate product of such reductive elimination, the unobserved Co(i) complex (PNP)Co, un-dergoes rapid comproportionation with the (PNP)Co(Ar)(SAr') starting material to give Co(ii) compounds (PNP)Co-Ar and (PNP)Co-SAr'.

Opening up the Valence Shell: A T-Shaped Iron(I) Metalloradical and Its Potential for Atom Abstraction

A thermally stable, T-shaped, d7 high-spin iron(I) complex was obtained by reduction of a PNP-supported ferrous chloride. Paramagnetic NMR spectroscopy combined with DFT modeling was used to analyze the electronic structure of the coordinatively highly unsaturated complex. The metalloradical character of the compound was demonstrated by the formation of a benzophenone ketyl radical complex upon addition of benzophenone. Furthermore, the compound displays a rich chemistry as an oxygen-atom abstractor from epoxides, yielding a dinuclear, diferrous [Fe2 O] complex.

Effect of the distant substituent on the slow magnetic relaxation of the mononuclear Co(ii) complex with pincer-type ligands

A hexacoordinated complex [Co(pydm)2](mdnbz)2 from the family of pincer complexes was prepared and structurally characterized. The complex behaves as an S = 3/2 spin system with a considerable zero-field splitting parameter D/hc ∼ +50 cm-1. The AC susceptibility measurements show a slow magnetic relaxation with three relaxation channels: at the low-frequency (LF), intermediate-frequency (IF) and high-frequency (HF) domains. At T = 2.0 K and an external field BDC = 0.25 T, the relaxation times of the individual modes are τ(LF) = 282 ms, τ(IF) = 3.1 ms, and τ(HF) = 0.16 ms, and the mole fractions of the slowly relaxing species are x(LF) = 0.19, x(IF) = 0.45, and x(HF) = 0.37. A comparison with the analogous complex [Co(pydm)2](dnbz)2 possessing a demethylated counter anion and identical metal cation shows that even small modifications in the composition of SIMs are no longer underestimated for the slow magnetic relaxation.