Cryo IR spectroscopy and cryo kinetics of dinitrogen activation and cleavage by small tantalum cluster cations.

We investigate small tantalum clusters Tan+, n = 2-4, for their capability to cleave N2 adsorption spontaneously. We utilize infrared photon dissociation (IR-PD) spectroscopy of isolated and size selected clusters under cryogenic conditions within a buffer gas filled ion trap, and we augment our experiments by quantum chemical simulations (at DFT level). All Tan+ clusters, n = 2-4, seem to cleave N2 efficiently. We confirm and extend a previous study under ambient conditions on Ta2+ cluster [Geng et al., Proc. Natl. Acad. Sci. U. S. A. 115, 11680-11687 (2018)]. Our cryo studies and the concomitant DFT simulations of the tantalum trimer Ta3+ suggest cleavage of the first and activation of the second and third N2 molecule across surmountable barriers and along much-involved multidimensional reaction paths. We unravel the underlying reaction processes and the intermediates involved. The study of the N2 adsorbate complexes of Ta4+ presented here extends our earlier study and previously published spectra from (4,m), m = 1-5 [Fries et al., Phys. Chem. Chem. Phys. 23(19), 11345-11354 (2021)], up to m = 12. We confirm the priory published double activation and nitride formation, succeeded by single side-on N2 coordination. Significant red shifts of IR-PD bands from these side-on coordinated µ2-κN:κN,N N2 ligands correlate with the degree of tilting towards the second coordinating Ta center. All subsequently attaching N2 adsorbates onto Ta4+ coordinate in an end-on fashion, and we find clear evidence for co-existence of end-on coordination isomers. The study of stepwise N2 adsorption revealed adsorption limits m(max) of [Tan(N2)m]+ which increase with n, and kinetic fits revealed significant N2 desorption rates upon higher N2 loads. The enhanced absolute rate constants of the very first adsorbate steps kabs(n,0) of the small Ta3+ and Ta4+ clusters independently suggest dissociative N2 adsorption and likely N2 cleavage into Ta nitrides.

Cryo-IR spectroscopy and cryo-kinetics of cluster N2 adsorbate complexes of tantalum cluster cations Ta5-8.

We present an IR-PD study of tantalum cluster adsorbate complexes [Tan(N2)m]+, abbreviated (n,m), n = 5-8. We utilize infrared spectroscopy of isolated and size selected clusters as prepared and characterized by a cryogenic tandem ion trap setup, and we augment our experiments with quantum chemical simulations at the level of density functional theory. The cluster adsorbate complexes (n,m) reveal vibrational bands above 2000 cm-1, which indicate end-on coordinated µ1-N2 oscillators, and bands below 2000 cm-1, which indicate side-on µ2-κN:κN,N coordinated ones. We observe a general increase in spectral complexity and an inhomogeneous broadening, mainly towards the red, at certain points of N2 loading m, which originates from an increasingly higher amount of double and triple N2 coordination at Ta sites, eventually at all of them. Other than the small tantalum clusters Tan+, n = 2-4, the IR-PD spectra of the initial N2 adsorbate species (n,1), n = 5-8, provide strong evidence for a lack of spontaneous N2 cleavage. Spontaneous N2 cleavage by Tan+, n = 5-8, seems suppressed. Therefore, the ability of a small Ta cluster to cleave dinitrogen disappears with one more tantalum core atom. The study of stepwise N2 adsorption on size selected Tan+, n = 5-8 clusters revealed adsorption limits m(max) of [Tan(N2)m]+ that are independent of cluster size within this size range. Cryo-adsorption kinetics at 26 K allowed for kinetic fits to consecutive N2 adsorption steps, and the fits revealed significant N2 desorption rates upon higher N2 loads, and the cluster adsorbate complexes eventually reached equilibrium. Some enhanced N2 desorption rates point towards likely adsorbate shell reorganization, and there is also some evidence for the coexistence of isomeric cluster adsorbate complexes.

A Different Perspective on Tuning the Photophysical and Photochemical Properties: The Influence of Constitutional Isomers in Group 6 Carbonyl Complexes with Pyridyl-Mesoionic Carbenes.

Exploring novel and existing design principles to tune the photochemical and photophysical properties of transition-metal complexes is an important goal in contemporary research. Here, we highlight the influence of constitutional isomers of pyridyl-1,2,3-triazolylidene mesoionic carbene (MIC) ligands on the photophysical and photochemical properties of the corresponding tetracarbonyl group 6 metal complexes (M = Cr, Mo, W). All new complexes [M(C-C)] presented herein incorporate a C-C linked pyridyl-MIC ligand and were fully characterized by X-ray diffraction analysis, elemental analysis, and 1H NMR and IR spectroscopy. Detailed photophysical investigations reveal a single emission in the VIS region, which extends into the NIR with lifetimes of up to 3.5 µs in the solid state at lower temperatures. The quantum yields were determined for all three complexes, and, in particular, the W0 complex shows an unusually high quantum yield of 29% compared to the values of 0.02% obtained for the [M(C-N)] isomers investigated in earlier works. Beyond this, the investigated W0 complex also exhibits an emission at 717 nm in a fluid solution. The combination of luminescence and FTIR-step scan spectroscopy with theoretical calculations reveals an emissive 3MLCT state. Irradiation of the presented complexes leads to a clean cleavage of one axial CO ligand. A metastable 16 VE species with a vacant axial coordination site was detected in the solid state at low temperatures. In solution, the respective solvato complexes are formed. A dark reverse reaction is observed, as previously described for the [M(C-N)] analogues. The increased electron density induced by the C-C linked pyridyl-MIC ligand leads to an increased kinetic rate constant for the reformation of the starting species and is also reflected in the lower photodissociation quantum yields.

Glutathione kinetically outcompetes reactions between dimedone and a cyclic sulfenamide or physiological sulfenic acids.

Dimedone and its derivates are used as selective probes for the nucleophilic detection of sulfenic acids in biological samples. Qualitative analyses suggested that dimedone also reacts with cyclic sulfenamides. Furthermore, under physiological conditions, dimedone must compete with the highly concentrated nucleophile glutathione. We therefore quantified the reaction kinetics for a cyclic sulfenamide model peptide and the sulfenic acids of glutathione and a model peroxiredoxin in the presence or absence of dimedone and glutathione. We show that the cyclic sulfenamide is stabilized at lower pH and that it reacts with dimedone. While reactions between dimedone and sulfenic acids or the cyclic sulfenamide have similar rate constants, glutathione kinetically outcompetes dimedone as a nucleophile by several orders of magnitude. Our comparative in vitro and intracellular analyses challenge the selectivity of dimedone. Consequently, the dimedone labeling of cysteinyl residues inside living cells points towards unidentified reaction pathways or unknown, kinetically competitive redox species.

Stable Molybdenum(0) Carbonyl Complex for Upconversion and Photoredox Catalysis.

Photoactive complexes with earth-abundant metals have attracted increasing interest in the recent years fueled by the promise of sustainable photochemistry. However, sophisticated ligands with complicated syntheses are oftentimes required to enable photoactivity with nonprecious metals. Here, we combine a cheap metal with simple ligands to easily access a photoactive complex. Specifically, we synthesize the molybdenum(0) carbonyl complex Mo(CO)3(tpe) featuring the tripodal ligand 1,1,1-tris(pyrid-2-yl)ethane (tpe) in two steps with a high overall yield. The complex shows intense deep-red phosphorescence with excited state lifetimes of several hundred nanoseconds. Time-resolved infrared spectroscopy and laser flash photolysis reveal a triplet metal-to-ligand charge-transfer (3MLCT) state as the lowest excited state. Temperature-dependent luminescence complemented by density functional theory (DFT) calculations suggest thermal deactivation of the 3MLCT state via higher lying metal-centered states in analogy to the well-known photophysics of [Ru(bpy)3]2+. Importantly, we found that the title compound is very photostable due to the lack of labilized Mo-CO bonds (as caused by trans-coordinated CO) in the facial configuration of the ligands. Finally, we show the versatility of the molybdenum(0) complex in two applications: (1) green-to-blue photon upconversion via a triplet-triplet annihilation mechanism and (2) photoredox catalysis for a green-light-driven dehalogenation reaction. Overall, our results establish tripodal carbonyl complexes as a promising design strategy to access stable photoactive complexes of nonprecious metals avoiding tedious multistep syntheses.

Cooperativity-Driven Reactivity of a Dinuclear Copper Dimethylglyoxime Complex.

In this report, we present the dinuclear copper(II) dimethylglyoxime (H2 dmg) complex [Cu2 (H2 dmg)(Hdmg)(dmg)]+ (1), which, in contrast to its mononuclear analogue [Cu(Hdmg)2 ] (2), is subject to a cooperativity-driven hydrolysis. The combined Lewis acidity of both copper centers increases the electrophilicity of the carbon atom in the bridging µ2 -O-N=C-group of H2 dmg and thus, facilitates the nucleophilic attack of H2 O. This hydrolysis yields butane-2,3-dione monoxime (3) and NH2 OH that, depending on the solvent, is then either oxidized or reduced. In ethanol, NH2 OH is reduced to NH4 + , yielding acetaldehyde as the oxidation product. In contrast, in CH3 CN, NH2 OH is oxidized by CuII to form N2 O and [Cu(CH3 CN)4 ]+ . Herein are presented the combined synthetic, theoretical, spectroscopic and spectrometric methods that indicate and establish the reaction pathway of this solvent-dependent reaction.

Mechanistic and Kinetic Investigations of ON/OFF (Photo)Switchable Binding of Carbon Monoxide by Chromium(0), Molybdenum(0) and Tungsten(0) Carbonyl Complexes with a Pyridyl-Mesoionic Carbene Ligand.

Invited for the cover of this issue are Gereon Niedner-Schatteburg, Biprajit Sarkar and co-worker at TU Kaiserslautern and the University of Stuttgart. The image depicts the selective dissociation of an axial CO from a metal complex. Read the full text of the article at 10.1002/chem.202201038.

Unexpected Boost in Activity of a Cu(I) Photosensitizer by Stabilizing a Transient Excited State.

In this study, we present a slight but surprisingly successful structural modification of the previously reported heteroleptic Cu(I) photosensitizer Cubiipo ([(xantphos)Cu(biipo)]PF6; biipo = 16H-benzo-[4',5']-isoquinolino-[2',1':1,2]-imidazo-[4,5-f]-[1,10]-phenanthrolin-16-one). As a key feature, biipo bears a naphthalimide unit at the back, which is directly fused to a phenanthroline moiety to extend the conjugated π-system. This ligand was now altered to include two additional methyl groups at the 2,9-positions at the phenanthroline scaffold. Comparing the novel Cudmbiipo complex to its predecessor, ultrafast transient absorption spectroscopy reveals the efficient suppression of a major deactivation pathway by stabilization of a transient triplet state. Furthermore, quantitative measurements of singlet oxygen evolution in solution confirmed that a larger fraction of the excited-state population is transferred to the photocatalytically active ligand-centered triplet 3LC state with a much longer lifetime of ∼30 µs compared to Cubiipo (2.6 µs). In addition, Cudmbiipo was compared with the well-established reference complex Cubcp ([(xantphos)Cu(bathocuproine)]PF6) in terms of its photophysical and photocatalytic properties by applying time-resolved femto- and nanosecond absorption, step-scan Fourier transform infrared (FTIR), and emission spectroscopies. Superior light-harvesting properties and a greatly enhanced excited-state lifetime with respect to Cubcp enable Cudmbiipo to be more active in exemplary photocatalytic applications, i.e., in the formation of singlet oxygen and the isomerization of (E)-stilbene.

Mechanistic and Kinetic Investigations of ON/OFF (Photo)Switchable Binding of Carbon Monoxide by Chromium(0), Molybdenum(0) and Tungsten(0) Carbonyl Complexes with a Pyridyl-Mesoionic Carbene Ligand.

This work tackles the photochemistry of a series of mononuclear Cr0 , Mo0 and W0 carbonyl complexes containing a bidentate mesoionic carbene ligand of the 1,2,3-triazol-5-ylidene type. FTIR spectroscopy, combined with density functional theory calculations, revealed a clean photo-induced reaction in organic solvents (acetonitrile, pyridine, valeronitrile) to give mainly one photoproduct with monosubstitution of a carbonyl ligand for a solvent molecule. The highest photodissociation quantum yields were reached for the Cr0 complex under UV irradiation (266 nm). Based on previous investigations, the kinetics of the dark reverse reactions have now been determined, with reaction times of up to several hours in pyridine. Photochemical studies in the solid state (KBr matrix, frozen solution) also showed light-induced reactivity with stabilization of the metastable intermediate with a free coordination site at very low temperature. The identified reactive species emphasizes a mechanism without ligand-sphere reorganization.

The overlooked NIR luminescence of Cr(ppy)3.

Cr(ppy)3, a structural analog of the green phosphorescent Ir(ppy)3, emits even in solution at room temperature from a weakly distorted spin-flip state at 910 nm (Hppy = 2-phenylpyridine). The low energy arises from an enhanced covalence of the Cr-C bonds as compared to Cr-N bonds. Lower temperature reduces thermally activated decay increasing the emission intensity.

Cryo kinetics of N2 adsorption onto bimetallic rhodium-iron clusters in isolation.

We report the N2 cryo adsorption kinetics of selected gas phase mixed rhodium-iron clusters [RhiFej]+ in the range of i = 3-8 and j = 3-8 in 26 K He buffer gas by the use of a cryo tandem RF-hexapole trap-Fourier transform ion cyclotron resonance mass spectrometer. From kinetic data and fits, we extract relative rate constants for each N2 adsorption step and possible desorption steps. We find significant trends in adsorption behavior, which reveal adsorption limits, intermittent adsorption limits, and equilibrium reactions. For those steps, which are in equilibrium, we determine the Gibbs free energies. We conclude on likely ligand shell reorganization and some weakly bound N2 ligands for clusters where multiple N2 adsorbates are in equilibrium. The relative rate constants are transferred to absolute rate constants, which are slightly smaller than the collision rate constants calculated by the average dipole orientation (Langevin) theory. The calculated sticking probabilities increase, in general, with the size of the clusters and decrease with the level of N2 adsorption, in particular, when reaching an adsorption/desorption equilibrium. We receive further evidence on cluster size dependent properties, such as cluster geometries and metal atom distributions within the clusters through the accompanying spectroscopic and computational study on the equiatomic i = j clusters [Klein et al., J. Chem. Phys. 156, 014302 (2022)].

Cryo infrared spectroscopy of N2 adsorption onto bimetallic rhodium-iron clusters in isolation.

We investigated the N2 adsorption behavior of bimetallic rhodium-iron cluster cations [RhiFej(N2)m]+ by means of InfraRed MultiplePhotoDissociation (IR-MPD) spectroscopy in comparison with density functional theory (DFT) modeling. This approach allows us to refine our kinetic results [Ehrhard et al., J. Chem. Phys. (in press)] to enhance our conclusions. We focus on a selection of cluster adsorbate complexes within the ranges of i = j = 3-8 and m = 1-10. For i = j = 3, 4, DFT suggests alloy structures in the case of i = j = 4 of high (D2d) symmetry: Rh-Fe bonds are preferred instead of Fe-Fe bonds or Rh-Rh bonds. N2 adsorption and IR-MPD studies reveal strong evidence for preferential adsorption to Rh sites and mere secondary adsorption to Fe. In some cases, we observe adsorption isomers. With the help of modeling the cluster adsorbate complex [Rh3Fe3(N2)7]+, we find clear evidence that the position of IR bands allows for an element specific assignment of an adsorption site. We transfer these findings to the [Rh4Fe4(N2)m]+ cluster adsorbate complex where the first four N2 molecules are exclusively adsorbed to the Rh atoms. The spectra of the larger adsorbates reveal N2 adsorption onto the Fe atoms. Thus, the spectroscopic findings are well interpreted for the smaller clusters in terms of computed structures, and both compare well to those of our accompanying kinetic study [Ehrhard et al., J. Chem. Phys. (in press)]. In contrast to our previous studies of bare rhodium clusters, the present investigations do not provide any indication for a spin quench in [RhiFej(N2)m]+ upon stepwise N2 adsorption.

Mn12 -Acetate Complexes Studied as Single Molecules.

The phenomenon of single molecule magnet (SMM) behavior of mixed valent Mn12 coordination clusters of general formula [MnIII 8 MnIV 4 O12 (RCOO)16 (H2 O)4 ] had been exemplified by bulk samples of the archetypal [MnIII 8 MnIV 4 O12 (CH3 COO)16 (H2 O)4 ] (4) molecule, and the molecular origin of the observed magnetic behavior has found support from extensive studies on the Mn12 system within crystalline material or on molecules attached to a variety of surfaces. Here we report the magnetic signature of the isolated cationic species [Mn12 O12 (CH3 COO)15 (CH3 CN)]+ (1) by gas phase X-ray Magnetic Circular Dichroism (XMCD) spectroscopy, and we find it closely resembling that of the corresponding bulk samples. Furthermore, we report broken symmetry DFT calculations of spin densities and single ion tensors of the isolated, optimized complexes [Mn12 O12 (CH3 COO)15 (CH3 CN)]+ (1), [Mn12 O12 (CH3 COO)16 ] (2), [Mn12 O12 (CH3 COO)16 (H2 O)4 ] (3), and the complex in bulk geometry [MnIII 8 MnIV 4 O12 (CH3 COO)16 (H2 O)4 ] (5). The found magnetic fingerprints - experiment and theory alike - are of a remarkable robustness: The MnIV 4 core bears almost no magnetic anisotropy while the surrounding MnIII 8 ring is highly anisotropic. These signatures are truly intrinsic properties of the Mn12 core scaffold within all of these complexes and largely void of the environment. This likely holds irrespective of bulk packing effects.

On the Hydrogen Oxalate Binding Motifs onto Dinuclear Cu and Ag Metal Phosphine Complexes.

We report the binding geometries of the isomers that are formed when the hydrogen oxalate ((CO2 )2 H=HOx) anion attaches to dinuclear coinage metal phosphine complexes of the form [M1 M2 dcpm2 (HOx)]+ with M=Cu, Ag and dcpm=bis(dicyclohexylphosphino)methane, abbreviated [MM]+ . These structures are established by comparison of isomer-selective experimental vibrational band patterns displayed by the cryogenically cooled and N2 -tagged cations with DFT calculations of the predicted spectra for various local minima. Two isomeric classes are identified that feature either attachment of the carboxylate oxygen atoms to the two metal centers (end-on docking) or attachment of oxygen atoms on different carbon atoms asymmetrically to the metal ions (side-on docking). Within each class, there are additional isomeric variations according to the orientation of the OH group. This behavior indicates that HOx undergoes strong and directional coordination to [CuCu]+ but adopts a more flexible coordination to [AgAg]+ . Infrared spectra of the bare ions, fragmentation thresholds and ion mobility measurements are reported to explore the behaviors of the complexes at ambient temperature.

Advancing Inorganic Coordination Chemistry by Spectroscopy of Isolated Molecules: Methods and Applications.

A unique feature of the work carried out in the Collaborative Research Center 3MET continues to be its emphasis on innovative, advanced experimental methods which hyphenate mass-selection with further analytical tools such as laser spectroscopy for the study of isolated molecular ions. This allows to probe the intrinsic properties of the species of interest free of perturbing solvent or matrix effects. This review explains these methods and uses examples from past and ongoing 3MET studies of specific classes of multicenter metal complexes to illustrate how coordination chemistry can be advanced by applying them. As a corollary, we will show how the challenges involved in providing well-defined, for example monoisomeric, samples of the molecular ions have helped to further improve the methods themselves thus also making them applicable to many other areas of chemistry.

Ultrafast and long-time excited state kinetics of an NIR-emissive vanadium(iii) complex I: synthesis, spectroscopy and static quantum chemistry.

In spite of intense, recent research efforts, luminescent transition metal complexes with Earth-abundant metals are still very rare owing to the small ligand field splitting of 3d transition metal complexes and the resulting non-emissive low-energy metal-centered states. Low-energy excited states decay efficiently non-radiatively, so that near-infrared emissive transition metal complexes with 3d transition metals are even more challenging. We report that the heteroleptic pseudo-octahedral d2-vanadium(iii) complex VCl3(ddpd) (ddpd = N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine) shows near-infrared singlet → triplet spin-flip phosphorescence maxima at 1102, 1219 and 1256 nm with a lifetime of 0.5 µs at room temperature. Band splitting, ligand deuteration, excitation energy and temperature effects on the excited state dynamics will be discussed on slow and fast timescales using Raman, static and time-resolved photoluminescence, step-scan FTIR and fs-UV pump-vis probe spectroscopy as well as photolysis experiments in combination with static quantum chemical calculations. These results inform future design strategies for molecular materials of Earth-abundant metal ions exhibiting spin-flip luminescence and photoinduced metal-ligand bond homolysis.

Novel Cofacial Porphyrin-Based Homo- and Heterotrimetallic Complexes of Transition Metals.

We present a straightforward and generally applicable synthesis route for cofacially linked homo- and heterotrimetallic trisporphyin complexes. The protocol encompasses synthesising the first aryl-based, trans-o-phenylene trisporphyrin starting from pyrrole and benzaldehyde with an overall yield of 3.6 %. It also allows investigating the respective cis-isomer as the first conformationally restricted planar-chiral trisporphyrin. The free-base ligand was used in subsequent metalation reactions to afford the corresponding homotrimetallic Mn(III)-, Fe(III)-, Ni(II)-, Cu(II)-, Zn(II)- and Pd(II) complexes - additionally, a small adaptation of the protocol resulted in the defined Ni(II)Fe(III)Ni(II) complex in a total yield of 2.3 %. By monitoring Ni(II) insertion into the empty trimeric ligands, we affirmed that the outer porphyrin rings are filled before the internal ring. The molecular species were characterised by 1 H NMR, UV-Vis, photoluminescence, IR, MS, CID, and high-resolution IMS measurements.

A Cyclometalated NHC Iridium Complex Bearing a Cationic (η5 -Cyclopentadienyl)(η6 -phenyl)iron Backbone*.

Nucleophilic substitution of [(η5 -cyclopentadienyl)(η6 -chlorobenzene)iron(II)] hexafluorophosphate with sodium imidazolate resulted in the formation of [(η5 -cyclopentadienyl)(η6 -phenyl)iron(II)]imidazole hexafluorophosphate. The corresponding dicationic imidazolium salt, which was obtained by treating this imidazole precursor with methyl iodide, underwent cyclometallation with bis[dichlorido(η5 -1,2,3,4,5-pentamethylcyclopentadienyl]iridium(III) in the presence of triethyl amine. The resulting bimetallic iridium(III) complex is the first example of an NHC complex bearing a cationic and cyclometallated [(η5 -cyclopentadienyl)(η6 -phenyl)iron(II)]+ substituent. As its iron(II) precursors, the bimetallic iridium(III) complex was fully characterized by means of spectroscopy, elemental analysis and single crystal X-ray diffraction. In addition, it was investigated in a catalytic study, wherein it showed high activity in transfer hydrogenation compared to its neutral analogue having a simple phenyl instead of a cationic [(η5 -cyclopentadienyl)(η6 -phenyl)iron(II)]+ unit at the NHC ligand.