Synthesis and Electronic Structure of Neutral Square-Planar High-Spin Iron(II) Complexes Supported by a Dianionic Pincer Ligand

Iron Olefin Metathesis: Unlocking Reactivity and Mechanistic Insights

Olefin metathesis catalyzed by iron complexes has garnered substantial interest due to iron's abundance and nontoxicity relative to ruthenium, yet its full potential remains untapped, largely because of the propensity of iron carbenes to undergo cyclopropanation instead of cycloreversion from a metallacycle intermediate. In this report, we elucidate the reactions of [{PC(sp2)P}Fe(L)(N2)], ([PC(sp2)P] = bis[2-(diisopropylphosphino)phenyl]methylene) with strained olefins, unveiling their capability to yield metathesis-related products. Our investigations led to the isolation of a structurally characterized metallacyclobutane during the reaction with norbornadiene derivatives, ultimately leading to a ring-opened iron alkylidene. These findings provide compelling evidence that iron complexes adhere to the Chauvin olefin metathesis mechanism.

Synthesis and Characterization of Isostructural Th(IV) and U(IV) Pyridine Dipyrrolide Complexes

A series of pyridine dipyrrolide actinide(IV) complexes, (MesPDPPh)AnCl2(THF) and An(MesPDPPh)2 (An = U, Th, where (MesPDPPh) is the doubly deprotonated form of 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine), have been prepared. Characterization of all four complexes has been performed through a combination of solid- and solution-state methods, including elemental analysis, single crystal X-ray diffraction, and electronic absorption and nuclear magnetic resonance spectroscopies. Collectively, these data confirm the formation of the mono- and bis-ligated species. Time-dependent density functional theory has been performed on all four An(IV) complexes, providing insight into the nature of electronic transitions that are observed in the electronic absorption spectra of these compounds. Room temperature, solution-state luminescence of the actinide complexes is presented. Both Th(IV) derivatives exhibit strong photoluminescence; in contrast, the U(IV) species are nonemissive.

Tunable Self-Referenced Molecular Thermometers via Manipulation of Dual Emission in Platinum(II) Pyridinedipyrrolide Complexes

Optical temperature sensors based on self-referenced readout schemes such as the emission ratio and the decay time are crucial for a wide range of applications, with the former often preferred due to simplicity of instrumentation. This work describes a new group of dually emitting dyes, platinum(II) pincer complexes, that can be used directly for ratiometric temperature sensing without an additional reference material. They consist of Pt(II) metal center surrounded by a pyridinedipyrrolide ligand (PDP) and a terminal ligand (benzonitrile, pyridine, 1-butylimidazol or carbon monoxide). Upon excitation with blue light, these complexes exhibit green to orange emission, with quantum yields in anoxic toluene at 25 °C ranging from 13% to 86% and decay times spanning from 8.5 to 97 μs. The emission is attributed to simultaneous thermally activated delayed fluorescence (TADF) and phosphorescence processes on the basis of photophysical investigations and DFT calculations. Rather uniquely, simple manipulations in substituents of the PDP ligand and alteration of the terminal ligand allow fine-tuning of the ratio between TADF and phosphorescence from almost 100% TADF emission (Pt(MesPDPC6F5(BN)) to over 80% of phosphorescence (Pt(PhPDPPh(BuIm)). Apart from ratiometric capabilities, the complexes also are useful as decay time-based temperature indicators with temperature coefficients exceeding 1.5% K-1 in most cases. Immobilization of the dyes into oxygen-impermeable polyacrylonitrile produces temperature sensing materials that can be read out with an ordinary RGB camera or a smartphone. In addition, Pt(PhPDPPh)Py can be incorporated into biocompatible RL100 nanoparticles suitable for cellular nanothermometry, as we demonstrate with temperature measurements in multicellular colon cancer spheroids.

Four- and three-coordinate planar iron(II) complexes supported by bulky organosilyl ligands

The ligand exchange reaction of (THF)2Fe[Si(SiMe3)3]2 with 2 equivalents of an N-heterocyclic carbene (NHC) led to the formation of a square-planar iron(II) complex with trans-oriented -Si(SiMe3)3 ligands. Conversely, the introduction of a cis-coordinate bidentate organosilyl ligand instead of -Si(SiMe3)3 resulted in the formation of a square planar iron(II) complex supported by a cis-coordinate bidentate organosilyl ligand. A three-coordinate planar iron(II) bis(silyl) complex was also synthesized using a cis-coordinate bidentate organosilyl ligand and a cyclic (alkyl)(amino)carbene auxiliary ligand. Investigation of the catalytic performance of these complexes in the hydrosilylation of acetophenone revealed that the square-planar iron(II) complex with trans-oriented -Si(SiMe3)3 ligands exhibits superior reactivity relative to its tetrahedral precursor.

Iron-Catalyzed Intermolecular C-H Amination Assisted by an Isolated Iron-Imido Radical Intermediate

Here we report the use of a base metal complex [(tBu pyrpyrr2 )Fe(OEt2 )] (1-OEt2 ) (tBu pyrpyrr2 2- =3,5-tBu2 -bis(pyrrolyl)pyridine) as a catalyst for intermolecular amination of Csp3 -H bonds of 9,10-dihydroanthracene (2 a) using 2,4,6-trimethyl phenyl azide (3 a) as the nitrene source. The reaction is complete within one hour at 80 °C using as low as 2 mol % 1-OEt2 with control in selectivity for single C-H amination versus double C-H amination. Catalytic C-H amination reactions can be extended to other substrates such as cyclohexadiene and xanthene derivatives and can tolerate a variety of aryl azides having methyl groups in both ortho positions. Under stoichiometric conditions the imido radical species [(tBu pyrpyrr2 )Fe{=N(2,6-Me2 -4-tBu-C6 H2 )] (1-imido) can be isolated in 56 % yield, and spectroscopic, magnetometric, and computational studies confirmed it to be an S = 1 FeIV complex. Complex 1-imido reacts with 2 a to produce the ferrous aniline adduct [(tBu pyrpyrr2 )Fe{NH(2,6-Me2 -4-tBu-C6 H2 )(C14 H11 )}] (1-aniline) in 45 % yield. Lastly, it was found that complexes 1-imido and 1-aniline are both competent intermediates in catalytic intermolecular C-H amination.

Organometallic Intermediates in the Synthesis of Photoluminescent Zirconium and Hafnium Complexes with Pyridine Dipyrrolide Ligands

The two commercially available zirconium complexes tetrakis(dimethylamido)zirconium, Zr(NMe2)4, and tetrabenzylzirconium, ZrBn4, were investigated for their utility as starting materials in the synthesis of bis(pyridine dipyrrolide)zirconium photosensitizers, Zr(PDP)2. Reaction with one equivalent of the ligand precursor 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine, H2MePDPPh, resulted in the isolation and structural characterization of the complexes (MePDPPh)Zr(NMe2)2thf and (MePDPPh)ZrBn2, which could be converted to the desired photosensitizer Zr(MePDPPh)2 upon addition of a second equivalent of H2MePDPPh. Using the more sterically encumbered ligand precursor 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine, H2MesPDPPh, only ZrBn4 yielded the desired bis-ligand complex Zr(MesPDPPh)2. Careful monitoring of the reaction at different temperatures revealed the importance of the organometallic intermediate (cyclo-MesPDPPh)ZrBn, which was characterized by X-ray diffraction analysis and 1H NMR spectroscopy and shown to contain a cyclometalated MesPDPPh unit. Taking inspiration from the results for zirconium, syntheses for two hafnium photosensitizers, Hf(MePDPPh)2 and Hf(MesPDPPh)2, were established and shown to proceed through similar intermediates starting from tetrabenzylhafnium, HfBn4. Initial studies of the photophysical properties of the photoluminescent hafnium complexes indicate similar optical properties compared to their zirconium analogues.

Dynamic effects on ligand field from rapid hydride motion in an iron(ii) dimer with an S = 3 ground state

Hydride complexes are important in catalysis and in iron-sulfur enzymes like nitrogenase, but the impact of hydride mobility on local iron spin states has been underexplored. We describe studies of a dimeric diiron(ii) hydride complex using X-ray and neutron crystallography, Mössbauer spectroscopy, magnetism, DFT, and ab initio calculations, which give insight into the dynamics and the electronic structure brought about by the hydrides. The two iron sites in the dimer have differing square-planar (intermediate-spin) and tetrahedral (high-spin) iron geometries, which are distinguished only by the hydride positions. These are strongly coupled to give an S total = 3 ground state with substantial magnetic anisotropy, and the merits of both localized and delocalized spin models are discussed. The dynamic nature of the sites is dependent on crystal packing, as shown by changes during a phase transformation that occurs near 160 K. The change in dynamics of the hydride motion leads to insight into its influence on the electronic structure. The accumulated data indicate that the two sites can trade geometries by rotating the hydrides, at a rate that is rapid above the phase transition temperature but slow below it. This small movement of the hydrides causes large changes in the ligand field because they are strong-field ligands. This suggests that hydrides could be useful in catalysis not only due to their reactivity, but also due to their ability to rapidly modulate the local electronic structure and spin states at metal sites.

Compression Produces a Square-Planar Iron Tetracarbonyl

Iron carbonyls are known to form 18-electron complexes like Fe(CO)₅, Fe₂(CO)₉, and Fe₃(CO)₁₂ having terminal or bridged Fe-CO bonding. Based on genetic algorithm-assisted density functional theory (DFT) calculations, it is predicted that at pressures above 2 GPa, iron tetracarbonyl, Fe(CO)₄, attains a square-planar geometry with a 16-electron count. Compression overcomes the [Ar]4s²3d⁶ (S = 2) → [Ar]4s⁰3d⁸ (S = 0) excitation energy to stabilize a closed-shell Fe(CO)₄ with a d⁸-configuration. Strong σ(4CO) → Fe (d_x²_-y²) bonding along with Fe(d_xz, d_yz) and Fe(d_xy) → π (CO)₄* back-bonding assists the formation of square-planar Fe(CO)₄ under pressure. Compression progressively flattens and destabilizes the ambient pressure C_2v structure of Fe(CO)₄, and beyond 2 GPa, it undergoes a sharp C_2v → D_4h transition with ΔV_unit-cell = 2.1% and trans-θ(OC-Fe-CO) = 180°. Realizing a square-planar geometry in a four-coordinated Fe-carbonyl complex shows the rich prospects of the new chemistry under pressure.

Synthesis and Characterization of Pyridine Dipyrrolide Uranyl Complexes

The first actinide complexes of the pyridine dipyrrolide (PDP) ligand class, (^MesPDP^Ph)UO₂(THF) and (^Cl2PhPDP^Ph)UO₂(THF), are reported as the U^VI uranyl adducts of the bulky aryl substituted pincers (^MesPDP^Ph)^2- and (^Cl2PhPDP^Ph)^2- (derived from 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine (H₂^MesPDP^Ph, Mes = 2,4,6-trimethylphenyl), and 2,6-bis(5-(2,6-dichlorophenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine (H₂^Cl2PhPDP^Ph, Cl₂Ph = 2,6-dichlorophenyl), respectively). Following the in situ deprotonation of the proligand with lithium hexamethyldisilazide to generate the corresponding dilithium salts (e.g., Li₂^ArPDP^Ph, Ar = Mes of Cl₂Ph), salt metathesis with [UO₂Cl₂(THF)₂]₂ afforded both compounds in moderate yields. The characterization of each species has been undertaken by a combination of solid- and solution-state methods, including combustion analysis, infrared, electronic absorption, and NMR spectroscopies. In both complexes, single-crystal X-ray diffraction has revealed a distorted octahedral geometry in the solid state, enforced by the bite angle of the rigid meridional (^ArPDP^Ph)^2- pincer ligand. The electrochemical analysis of both compounds by cyclic voltammetry in tetrahydrofuran (THF) reveals rich redox profiles, including events assigned as U^VI/U^V redox couples. A time-dependent density functional theory study has been performed on (^MesPDP^Ph)UO₂(THF) and provides insight into the nature of the transitions that comprise its electronic absorption spectrum.

High Magnetic Anisotropy of a Square-Planar Iron-Carbene Complex

Among Earth-abundant catalyst systems, iron-carbene intermediates that perform C-C bond forming reactions such as cyclopropanation of olefins and C-H functionalization via carbene insertion are rare. Detailed descriptions of the possible electronic structures for iron-carbene bonds are imperative to obtain better mechanistic insights and enable rational catalyst design. Here, we report the first square-planar iron-carbene complex (MesPDPPh)Fe(CPh2), where [MesPDPPh]2- is the doubly deprotonated form of [2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine]. The compound was prepared via reaction of the disubstituted diazoalkane N2CPh2 with (MesPDPPh)Fe(thf) and represents a rare example of a structurally characterized, paramagnetic iron-carbene complex. Temperature-dependent magnetic susceptibility measurements and applied-field Mössbauer spectroscopic studies revealed an orbitally near-degenerate S = 1 ground state with large unquenched orbital angular momentum resulting in high magnetic anisotropy. Spin-Hamiltonian analysis indicated that this S = 1 spin system has uniaxial magnetic properties arising from a ground MS = ±1 non-Kramers doublet that is well-separated from the MS = 0 sublevel due to very large axial zero-field splitting (D = -195 cm-1, E/D = 0.02 estimated from magnetic susceptibility data). This remarkable electronic structure gives rise to a very large, positive magnetic hyperfine field of more than +60 T for the 57Fe nucleus along the easy magnetization axis observed by Mössbauer spectroscopy. Computational analysis with complete active space self-consistent field (CASSCF) calculations provides a detailed electronic structure analysis and confirms that (MesPDPPh)Fe(CPh2) exhibits a multiconfigurational ground state. The majority contribution originates from a configuration best described as a singlet carbene coordinated to an intermediate-spin FeII center with a (dxy)2{(dxz),(dz2)}3(dyz)1(dx2-y2)0 configuration featuring near-degenerate dxz and dz2 orbitals.

Low-Coordinate Iron Chalcogenolates and Their Complexes with Diethyl Ether and Ammonia

Treatment of Fe{N(SiMe₃)₂}₂ with 2 equiv of the appropriate phenol or thiol affords the dimers {Fe(OC₆H₂-2,6-Bu^t₂-4-Me)₂}₂ (1) and {Fe(OC₆H₃-2,6-Bu^t₂)₂}₂ (2) or the monomeric Fe{SC₆H₃-2,6-(C₆H₃-2,6-Prⁱ₂)₂}₂ (3) in moderate to excellent yields. Recrystallization of 1 and 2 from diethyl ether gives the corresponding three-coordinate ether complexes Fe(OC₆H₃-2,6-Bu^t₂-4-Me)₂(OEt₂) (4) and Fe(OC₆H₃-2,6-Bu^t₂)₂(OEt₂) (5). In contrast, no diethyl ether complex is formed by the dithiolate 3. The ¹H NMR spectra of 4 and 5 show equilibria between the ether complexes and the base-free dimers. A comparison of these spectra with those of the dimeric 1 and 2 allows an unambiguous assignment of the paramagnetically shifted signals. Treatment of 1 with excess ammonia gives the tetrahedral diammine Fe(OC₆H₂-2,6-Bu^t₂-4-Me)₂(NH₃)₂ (6). Ammonia is strongly coordinated in 6, with no apparent loss of ammine ligand either in solution or upon heating under low pressure. In contrast, significantly weaker ammonia coordination is observed when dithiolate 3 is treated with excess ammonia, which gives the diammine Fe{SC₆H₃-2,6-(2,6-Prⁱ₂-C₆H₃)₂}₂(NH₃)₂ (7). Complex 7 readily loses ammonia either in solution or under reduced pressure to give the monoammine complex Fe{SC₆H₃-2,6-(2,6-Prⁱ₂-C₆H₃)₂}₂(NH₃) (8). The weak binding of ammonia by iron thiolate 7 reflects the likely behavior of the proposed iron-sulfur active site in nitrogenases, where release of ammonia is required to close the catalytic cycle.

Nickel(II) carbonyl, ammonia, and aceto-nitrile complexes supported by a pyridine dipyrrolide pincer ligand

The synthesis, isolation and crystal structures of nickel(II) carbonyl, aceto-nitrile and ammonia complexes supported by a dianionic, pyridine dipyrrolide pincer ligand [pyrr2py]2-, namely, carbonyl[2,2'-(pyridine-2,6-di-yl)bis-(3,5-di-p-tolyl-pyrrolido-κN)]-nickel(II), [Ni(C41H33N3)(CO)], ammine[2,2'-(pyridine-2,6-di-yl)bis-(3,5-di-p-tolyl-pyrrolido-κN)]nickel(II), [Ni(C41H33N3)(NH3)], and (aceto-nitrile-κN)[2,2'-(pyridine-2,6-di-yl)bis-(3,5-di-p-tolyl-pyrrolido-κN)]nickel(II), [Ni(C41H33N3)(CH3CN)], as well as the free ligand 2,6-bis-(3,5-di-p-tolyl-pyrrol-2-yl)pyridine, C41H35N3 or [pyrr2py]H2 are reported. The nickel complexes are four-coordinate and adopt a square-planar geometry. The CO stretch of the nickel-bound carbon monoxide ligand of [pyrr2py]Ni(CO) has been observed at 2101 cm-1. The ammonia and aceto-nitrile complexes, [pyrr2py]Ni(NH3) and [pyrr2py]Ni(NCMe) feature all-nitro-gen coordination spheres around nickel consisting of different N-donor ligand types.

FeIII and FeII Phosphasalen Complexes: Synthesis, Characterization, and Catalytic Application for 2-Naphthol Oxidative Coupling

We report on the synthesis and characterization of three iron(III) phosphasalen complexes, [Fe^III (Psalen)(X)] differing in the nature of the counter-anion/exogenous ligand (X^- =Cl^- , NO₃ ^- , OTf^- ), as well as the neutral iron(II) analogue, [Fe^II (Psalen)]. Phosphasalen (Psalen) differs from salen by the presence of iminophosphorane (P=N) functions in place of the imines. All the complexes were characterized by single-crystal X-ray diffraction, UV/Vis, EPR, and cyclic voltammetry. The [Fe^II (Psalen)] complex was shown to remain tetracoordinated even in coordinating solvent but surprisingly exhibits a magnetic moment in line with a Fe^II high-spin ground state. For the Fe^III complexes, the higher lability of triflate anion compared to nitrate was demonstrated. As they exhibit lower reduction potentials compared to their salen analogues, these complexes were tested for the coupling of 2-naphthol using O₂ from air as oxidant. In order to shed light on this reaction, the interaction between 2-naphthol and the Fe^III (Psalen) complexes was studied by cyclic voltammetry as well as UV/Vis spectroscopy.

Effects of Ligand Substitution on the Optical and Electrochemical Properties of (Pyridinedipyrrolide)zirconium Photosensitizers

A series of seven bis(pyridinedipyrrolide)zirconium complexes, Zr(^R₁PDP^R₂)₂, where [^R₁PDP^R₂]^2- is the doubly deprotonated form of [2,6-bis(5-R₁-3-R₂-1H-pyrrol-2-yl)pyridine], were prepared and characterized in solution by NMR, UV/vis absorption, and emission spectroscopy and cyclic voltammetry. The molecular structures were determined by single-crystal X-ray crystallography. All complexes exhibit remarkably long emission lifetimes (τ = 190-576 μs) with high quantum efficiencies (Φ_PL = 0.10-0.38) upon excitation with visible light in a benzene solution. The substituents on the pyrrolide rings were shown to have significant effects on the photoluminescence and electrochemical properties of these compounds. The R₂ substituents (R₂ = H, Me, Ph, or C₆F₅) show only limited effects on the absorption and emission profiles of the complexes but allow systematic tuning of the ground- and excited-state redox potentials over a range of almost 600 mV. The R₁ substituents (R₁ = H, Me, Ph, or 2,4,6-Me₃Ph) influence both the optical and electrochemical properties through electronic effects. Additionally, the R₁ substituents have profound consequences for the structural flexibility and overall stability of the compounds. Distortions of the Zr(PDP)₂ core from idealized D_2d symmetry in the solid state can be traced to the steric profiles of the R¹ substituents and correlate with the observed Stokes shifts for each compound. The complex with the smallest ligand system, Zr(^HPDP^H)₂, coordinates two additional solvent molecules in a tetrahydrofuran (THF) solution, which allowed the isolation of photoluminescent, eight-coordinate Zr(^HPDP^H)₂(THF)₂. The photoredox catalytic dehalogenation of aryl iodides and aryl chlorides using the most reducing derivative, Zr(^MePDP^Me)₂, highlights the potential of Zr(PDP)₂ photosensitizers to promote challenging reductive transformations under mild conditions upon excitation with green light.

Delayed fluorescence from a zirconium(IV) photosensitizer with ligand-to-metal charge-transfer excited states

Advances in chemical control of the photophysical properties of transition-metal complexes are revolutionizing a wide range of technologies, particularly photocatalysis and light-emitting diodes, but they rely heavily on molecules containing precious metals such as ruthenium and iridium. Although the application of earth-abundant 'early' transition metals in photosensitizers is clearly advantageous, a detailed understanding of excited states with ligand-to-metal charge transfer (LMCT) character is paramount to account for their distinct electron configurations. Here we report an air- and moisture-stable, visible light-absorbing Zr(IV) photosensitizer, Zr(^MesPDP^Ph)₂, where [^MesPDP^Ph]^2- is the doubly deprotonated form of [2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine]. This molecule has an exceptionally long-lived triplet LMCT excited state (τ = 350 μs), featuring highly efficient photoluminescence emission (Ф = 0.45) due to thermally activated delayed fluorescence emanating from the higher-lying singlet configuration with significant LMCT contributions. Zr(^MesPDP^Ph)₂ engages in numerous photoredox catalytic processes and triplet energy transfer. Our investigation provides a blueprint for future photosensitizer development featuring early transition metals and excited states with significant LMCT contributions.

Manipulating metal spin states for biomimetic, catalytic and molecular materials chemistry

The relationship between ligand design and spin state in base metal compounds is surveyed. Implications and applications of these principles for light-harvesting dyes, catalysis and materials chemistry are summarised.