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The use of iron as a replacement for noble metals in photochemical and photophysical applications is challenging due to the typically fast deactivation of short-lived catalytically active states. Recent success of a cyclometalated iron(III) complex utilizing a bis-tridentate ligand motif inspired the use of phenyl-1H-pyrazole as a bidentate ligand. Five complexes using the tris(1-phenylpyrazolato-N,C2)iron(III) complex scaffold are presented. In addition to the parent complex, four derivatives with functionalization in the meta-position of the phenyl ring are thoroughly investigated by single crystal diffractometry, UV-Vis-spectroscopy, and cyclic voltammetry. Advanced X-ray spectroscopy in the form of X-ray absorption and emission spectroscopy allows unique insights into the electronic structure as well as DFT calculations. The ligand design leads to overlapping MLCT and LMCT absorption bands, and emissive behavior is suppressed by low-lying MC states. [ABSTRACT FROM AUTHOR]

Increasing the metal‐to‐ligand charge transfer (MLCT) excited state lifetime of polypyridine iron(II) complexes can be achieved by lowering the ligand's π* orbital energy and by increasing the ligand field splitting. In the homo‐ and heteroleptic complexes [Fe(cpmp)2]2+ (12+) and [Fe(cpmp)(ddpd)]2+ (22+) with the tridentate ligands 6,2"‐carboxypyridyl‐2,2'‐methylamine‐pyridyl‐pyridine (cpmp) and N,N'‐dimethyl‐N,N'‐di‐pyridin‐2‐ylpyridine‐2,6‐diamine (ddpd) two or one dipyridyl ketone moieties provide low energy π* acceptor orbitals. A good metal‐ligand orbital overlap to increase the ligand field splitting is achieved by optimizing the octahedricity through CO and NMe units between the coordinating pyridines which enable the formation of six‐membered chelate rings. The push‐pull ligand cpmp provides intra‐ligand and ligand‐to‐ligand charge transfer (ILCT, LL'CT) excited states in addition to MLCT excited states. Ground and excited state properties of 12+ and 22+ were accessed by X‐ray diffraction analyses, resonance Raman spectroscopy, (spectro)electrochemistry, EPR spectroscopy, X‐ray emission spectroscopy, static and time‐resolved IR and UV/Vis/NIR absorption spectroscopy as well as quantum chemical calculations. [ABSTRACT FROM AUTHOR]

Combining strong σ-donating N-heterocyclic carbene ligands and π-accepting pyridine ligands with a high octahedricity in rigid iron(II) complexes increases the 3MLCT lifetime from 0.15 ps in the prototypical [Fe(tpy)2]2+ complex to 9.2 ps in [Fe(dpmi)2]2+12+. The tripodal CNN ligand dpmi (di(pyridine-2-yl)(3-methylimidazol-2-yl)methane) forms six-membered chelate rings with the iron(II) centre leading to close to 90° bite angles and enhanced iron-ligand orbital overlap. [ABSTRACT FROM AUTHOR]

Two closely related FeII complexes with 2,6-bis(1-ethyl-1H-1,2,3-triazol-4yl)pyridine and 2,6-bis(1,2,3-triazol-5-ylidene)pyridine ligands are presented to gain new insights into the photophysics of bis(tridentate) iron(II) complexes. The [Fe(N^N^N)2]2+ pseudoisomer sensitizes singlet oxygen through a MC state with nanosecond lifetime after MLCT excitation, while the bis(tridentate) [Fe(C^N^C)2]2+ pseudoisomer possesses a similar 3MLCT lifetime as the tris(bidentate) [Fe(C^C)2(N^N)]2+ complexes with four mesoionic carbenes. [ABSTRACT FROM AUTHOR]

A new base metal iron‐cobalt dyad has been obtained by connection between a heteroleptic tetra‐NHC iron(II) photosensitizer combining a 2,6‐bis[3‐(2,6‐diisopropylphenyl)imidazol‐2‐ylidene]pyridine with 2,6‐bis(3‐methyl‐imidazol‐2‐ylidene)‐4,4′‐bipyridine ligand, and a cobaloxime catalyst. This novel iron(II)‐cobalt(III) assembly has been extensively characterized by ground‐ and excited‐state methods like X‐ray crystallography, X‐ray absorption spectroscopy, (spectro‐)electrochemistry, and steady‐state and time‐resolved optical absorption spectroscopy, with a particular focus on the stability of the molecular assembly in solution and determination of the excited‐state landscape. NMR and UV/Vis spectroscopy reveal dissociation of the dyad in acetonitrile at concentrations below 1 mM and high photostability. Transient absorption spectroscopy after excitation into the metal‐to‐ligand charge transfer absorption band suggests a relaxation cascade originating from hot singlet and triplet MLCT states, leading to the population of the 3MLCT state that exhibits the longest lifetime. Finally, decay into the ground state involves a 3MC state. Attachment of cobaloxime to the iron photosensitizer increases the 3MLCT lifetime at the iron centre. Together with the directing effect of the linker, this potentially makes the dyad more active in photocatalytic proton reduction experiments than the analogous two‐component system, consisting of the iron photosensitizer and Co(dmgH)2(py)Cl. This work thus sheds new light on the functionality of base metal dyads, which are important for more efficient and sustainable future proton reduction systems. [ABSTRACT FROM AUTHOR]

Abstract: Metal‐ versus ligand‐centered redox processes and the effects of substituents on the ligands on the spectroscopic properties of the metal complexes are at the heart of research on metal complexes with non‐innocent ligands. This work presents three examples of chromium complexes that contain both oxido and corrolato ligands, with the substituents on the corrolato ligands being different in the three cases. Combined X‐ray crystallographic, electrochemical, UV/Vis/NIR/EPR spectroelectrochemical, and EXAFS/XANES measurements, together with DFT calculations, have been used to probe the complexes in three different redox forms. This combined approach makes it possible to address questions related to chromium‐ versus corrolato‐centered redox processes, and the accessibility (or not) of CrIV, CrV, and CrVI in these complexes, as well as their spin states. To the best of our knowledge, these are the first EXAFS/XANES investigations on Cr‐corrolato complexes in different redox forms, and hence these data should set benchmarks for future investigations on such complexes by this method. [ABSTRACT FROM AUTHOR]

Tyrosinase model systems pinpoint pathways to translating Nature's synthetic abilities for useful synthetic catalysts. Mostly, they use N-donor ligands which mimic the histidine residues coordinating the two copper centres. Copper complexes with bis(pyrazolyl)methanes with pyridinyl or imidazolyl moieties are already reported as excellent tyrosinase models. Substitution of the pyridinyl donor results in the new ligand HC(3- tBuPz)2(4-CO2MePy) which stabilises a room-temperature stable μ-η2:η2-peroxide dicopper(II) species upon oxygenation. It reveals highly efficient catalytic activity as it hydroxylates 8-hydroxyquinoline in high yields (TONs of up to 20) and much faster than all other model systems (max. conversion within 7.5 min). Stoichiometric reactions with para-substituted sodium phenolates show saturation kinetics which are nearly linear for electron-rich substrates. The resulting Hammett correlation proves the electrophilic aromatic substitution mechanism. Furthermore, density functional theory (DFT) calculations elucidate the influence of the substituent at the pyridinyl donor: the carboxymethyl group adjusts the basicity and nucleophilicity without additional steric demand. This substitution opens up new pathways in reactivity tuning. [ABSTRACT FROM AUTHOR]

Bis(pyrazolyl)methane ligands are excellent components of model complexes used to investigate the activity of the enzyme tyrosinase. Combining the N donors 3- tert-butylpyrazole and 1-methylimidazole results in a ligand that is capable of stabilising a (μ-η2:η2)-dicopper(II) core that resembles the active centre of tyrosinase. UV/Vis spectroscopy shows blueshifted UV bands in comparison to other known peroxo complexes, due to donor competition from different ligand substituents. This effect was investigated with the help of theoretical calculations, including DFT and natural transition orbital analysis. The peroxo complex acts as a catalyst capable of hydroxylating a variety of phenols by using oxygen. Catalytic conversion with the non-biological phenolic substrate 8-hydroxyquinoline resulted in remarkable turnover numbers. In stoichiometric reactions, substrate-binding kinetics was observed and the intrinsic hydroxylation constant, kox, was determined for five phenolates. It was found to be the fastest hydroxylation model system determined so far, reaching almost biological activity. Furthermore, Hammett analysis proved the electrophilic character of the reaction. This sheds light on the subtle role of donor strength and its influence on hydroxylation activity. [ABSTRACT FROM AUTHOR]

The μ-η2:η2-peroxodicopper(II) core found in the oxy forms of the active sites of type III dicopper proteins have been a key target for bioinorganic model studies. Here, it is shown that simple bis(oxazoline)s (BOXs), which are classified among the so-called 'privileged ligands', provide a suitable scaffold for supporting such biomimetic copper/dioxygen chemistry. Three derivatives R,HBOX-Me2 (R = H, Me, tBu) with different backbone substituents have been used. Their bis(oxazoline)-copper(I) complexes bind dioxygen to yield biomimetic μ-η2:η2-peroxodicopper(II) species. O2 can be reversibly released upon an increase in temperature. Their formation kinetics have been studied under cryo-stopped-flow conditions for the tBu derivative, giving activation parameters Δ H‡on = (2.27 ± 0.18) kcal mol-1, Δ S‡on = (-46.3 ± 0.8) cal K-1 mol-1 for the binding event and Δ H‡off = (11.7 ± 1.9) kcal mol-1, Δ S‡off = (-16.1 ± 8.2) cal K-1 mol-1 for the release of O2, as well as thermodynamic parameters Δ H° = (-10.0 ± 1.7) kcal mol-1 and Δ S° = (-32.7 ± 7.4) cal K-1 mol-1 for this equilibrium. The μ-η2:η2-peroxodicopper(II) complexes have been isolated as surprisingly stable solids and investigated in depth by a variety of methods, both in solution and in the solid state. Resonance Raman spectroscopy revealed a characteristic isotope-sensitive stretch $\tilde {\nu}$O-O = 731-742 cm-1 (Δ[18O2] ≈ -39 cm-1) and an intense feature around 280 cm-1 diagnostic for the fundamental symmetric Cu2O2 core vibration. A slight butterfly-shape of the Cu2O2 core has been derived from EXAFS data and DFT calculations. SQUID magnetic data evidenced strong antiferromagnetic coupled CuII2 (-2 J ≥ 1000 cm-1). Thermal degradation in solution yields bis(hydroxo)-bridged [(tBu,HBOX-Me2)(L)Cu(OH)]2(PF6)2 (L = H2O, MeCN or THF); whereas in the case of H,HBOX-Me2, ligand oxygenation has been detected. Preliminary reactivity studies with the substrate 2,4-di- tert-butylphenol indicate the formation of the C-C coupling product 3,3′,5,5′-tetra- tert-butyl-2,2′-biphenol, whereas ortho-hydroxylation was not observed. The copper(I) complex [(tBu,HBOX-Me2)Cu(MeCN)]PF6 as well as two dicopper(II) complexes [(L)(tBu,HBOX-Me2)Cu(OH)]2(PF6)2 have been characterised by single-crystal X-ray diffraction. Considering the vast number of known BOX derivatives, a rich and versatile Cu/O2 chemistry based on this platform is anticipated. [ABSTRACT FROM AUTHOR]

Orange-red trinuclear nickel(II) complexes are obtained by reacting Ni(ClO4)2(H2O)6 with 3(5)-arylpyrazoles in the presence of a base. Herein, the diamagnetic nickel centers are coordinated in a square-pyramidal manner by four pyrazolato ligands, each of which bridges two nickel sites. The structurally unique complexes were investigated by NMR, IR, fluorescence, and X-ray spectroscopy. All seven compounds described here were structurally elucidated by single-crystal X-ray diffraction. With 3(5)-ferrocenylpyrazole as the substrate, a nonametallic system that possesses six redox-active ferrocenyl moieties was obtained. [ABSTRACT FROM AUTHOR]

The exchange of the monocarboxylate ligand of the zirconium methacrylate oxocluster Zr6O4(OH)4(OMc)12(OMc = CH2CH(CH3)COO) with dicarboxylic acids (trans,transmuconic acid and terephthalic acid) leads to porous zirconium dicarboxylates exhibiting the UiO-66 architecture; the mild reaction conditions allow the control of their particle size. [ABSTRACT FROM AUTHOR]

Bis(pyrazolyl)methane copper peroxide complexes display tyrosinase‐like catalytic phenol hydroxylation activity. An exchange of the pyridinyl unit (background) by a methylimidazolyl unit (middle) enhances the catalytic activity. In turn, substituting the pyridinyl with a carboxymethyl group (foreground) enhances the stability and the catalytic activity in catalytic phenol hydroxylation. This makes the presented tyrosinase model the fastest catalytic model. Kinetic measurements reveal substrate‐binding behaviour and a biomimetic electrophilic reaction mechanism. DFT calculations show that the carboxymethyl substituent weakens the pyridinyl donor, which enhances the activity. For more details, see the Full Paper by S. Herres‐Pawlis et al. on page 12171 ff. [ABSTRACT FROM AUTHOR]

Polynuclear spincrossover (SCO) complexes prepared by the combination of [Fe(DMF)6]2+ and NH2trz (NH2trz = 4-amino-1,2,4-triazole) were studied (2ns- = counterion 2-naphthalenesulfonate). It is demonstrated that these [Fe(NH2trz)3](2ns)2 complexes can be dissolved-contrary to common reported experience-in N,N-dimethyl-formamide (DMF) and, therefore, can be conveniently processed by simple means. The resulting solutions were examined with UV/vis and X-ray absorption spectroscopy (XANES and EXAFS) as well as with small-angle X-ray scattering (SAXS). At a molar NH2trz/Fe2+ ratio of 3/1, corresponding to the stoichiometric ratio of the ideal coordination compound, [Fe(NH2trz)3]2+ in the low-spin state was found to be in equilibrium with polynuclear species in the high-spin state. The equilibrium can be shifted virtually completely to the side of low-spin Fe2+ by an excess of the ligand. The polymer therewith formed contains 100 or more Fe2+ ions and is of a pronounced rigid-rod structure, with Fe-Fe distances around 3.32 Å (in comparison to 3-94 Å of the polynuclear species in the high-spin state). Reversible spin crossover takes place in solution upon a temperature increase to around 60 °C; this process is associated with a shift in equilibrium toward species shorter than the initial polynuclear species. [ABSTRACT FROM AUTHOR]

Abstract: Three new chiral ligands bearing an O,O′,N donor set (OmethoxyOhydroxyNpyridine) were synthesised and coordinated to FeIII, FeII, NiII, CuII and ZnII to yield complexes with the general formula [M(OON)Cl x ] y . While the pyridine N and the hydroxy O atoms coordinate strongly to all applied metal ions, the methoxy donor seems not to be involved in coordination, although some evidence for a weak interaction between OMe and the ZnII were found in NMR spectra. In the bidentate O′,N coordination mode the new ligands exhibit several coordination geometries as analysed in the solid compounds by XRD, EXAFS and EPR and in solution by UV–Vis absorption, cyclic voltammetry, EXAFS, EPR or NMR spectroscopy. [Copyright &y& Elsevier]

Three different zirconium thio and oxothio clusters, characterized by different coordination modes of dithioacetate and/or monothioacetate ligands, were obtained by the reaction of monothioacetic acid with zirconium n-butoxide, Zr(OnBu)4, in different experimental conditions. In particular, we isolated the three polynuclear Zr3(μ3-SSSCCH3)2(SSCCH3)6·2nBuOH (Zr3), Zr4(μ3-O)2(μ-η1-SOCCH3)2(SOCCH3)8(OnBu)2 (Zr4), and Zr6(μ3-O)5(μ-SOCCH3)2(μ-OOCCH3)(SOCCH3)11(nBuOH) (Zr6) derivatives, presenting some peculiar characteristics. Zr6 has an unusual star-shaped structure. Only sulfur-based ligands, viz., chelating dithioacetate monoanions and an unusual ethane-1,1,1-trithiolate group μ3 coordinating the Zr ions, were observed in the case of Zr3. 1D and 2D NMR analyses confirmed the presence of differently coordinated ligands. Raman spectroscopy was further used to characterize the new polynuclear complexes. Time-resolved extended X-ray absorption fine structure measurements, devoted to unraveling the cluster formation mechanisms, evidenced a fast coordination of sulfur ligands and subsequent relatively rapid rearrangements. [ABSTRACT FROM AUTHOR]