A series of cyclometalated Ni(II) complexes [Ni(PyPhPy)X] containing anionic N ∧ C ∧ N − tridentate ligand Py(Ph − )Py 2,6-di(2-pyridyl)benzene-1-ide, (Py(HPh)Py = 1,3-di(2-pyridyl)benzene) and X = Cl, Br, or I as coligands, were studied. All three complexes were obtained through direct C−H base-assisted nickelation from NiX 2 and Py(HPh)Py using KOAc/K 2 CO 3 in nonpolar high-boiling point solvents. While the overall molecular structures are quite similar to those of the previously studied [Ni(C ∧ N ∧ N)X] complexes with the anionic C ∧ N ∧ N tridentate − Phbpy (HPhbpy = 6-(phenyl)-2,2′-bipyridine) ligand, bond lengths in the molecular structures are slightly different. Large differences between these N ∧ C ∧ N and C ∧ N ∧ N Ni complexes which can be traced to the different orientation of the X coligand to the carbanionic phen-ide group, trans or cis, and the different ligand pattern, Py−Ph−Py versus Ph−Py−Py, were found for the UV−vis absorption spectra and the electrochemical reductions, while the oxidation potentials are very similar. Extended DFT calculations with the TPSSh functional associate the indifference of oxidation potentials with conserved energies of metal-borne HOMOs. By contrast, the diminished π-acceptor qualities of the N ∧ C ∧ N ligand translate into a destabilization of the LUMO by ca. 400 mV and a blue-shift of the leading visible transition by 80 nm in very good agreement with the experimental data.

The mechanism of the photodehydrodimerization of 2,5-dihydrofuran (2,5-DHF) by suspended zinc sulfide powders was investigated through emission, inhibition, adsorption, and quantum yield studies. Zinc and cadmium ions influenced the emission intensity only marginally but strongly inhibited the reaction, so the photoreactive surface sites were not identical with the emitting states, which had lifetimes in the 0.1 ± 24 ns range. Adsorption isotherms for these metal ions and 2,5-DHF in aqueous solution indicated the presence of mono-and multilayer adsorption. Zn 2 and Cd 2 were both adsorbed physically and by metal sulfide precipitatation, but in the case of Cd 2 a lattice exchange mechanism with ZnS afforded CdS and dissolved zinc ions also. 13C NMR spectra and the good agreement between the calculated number of zinc sites and the measured amount of 2,5-DHF in the saturated solvent ± solute surface monolayer indicated that 2,5-DHF was adsorbed perpendicular to the surface at all the available zinc sites. The true adsorption constant (170 AE 30 L mol À1 ) was consistent with the pseudo-constant (260 AE 50 L mol À1) obtained from the concentration dependence of the reaction rate. 13 C NMR signals of the a-carbon atoms of 2,5-DHF and THF adsorbed onto ZnS from the gas phase were downfield shifted by 1.5 and 0.7 ppm as compared to those of the free substrates, respectively. The downfield shift of the olefinic signals was about 0.4 ppm. It is postulated that a dissociative electron transfer from adsorbed 2,5-DHF to the reactive hole afforded a proton and the dihydrofuryl radical. The corresponding C ± H bond dissociation energies were calculated by ab initio methods for various substrates. As expected, the apparent quantum yield of various substrates increased linearly with decreasing C ± H bond dissociation energy. The intermediate dihydrofuryl radical dimerized to the products in the adsorbed state, as indicated by the linear increase of the square root of the reaction rate with increasing 2,5-DHF surface concentration and by competition experiments with THF/2,5-DHF mixtures. The reaction inhibition by Zn 2 and Cd 2 could be analyzed in terms of the Stern ± Volmer model only when the surface concentration was considered. Inhibition by Cd 2 is about three times faster than inhibition by Zn 2 , in which case no zinc metal was observed.

A series of Fe(2+) spin crossover (SCO) complexes [Fe(5/6)](2+) employing hexadentate ligands (5/6) with cis/trans-1,2-diamino cyclohexanes (4) as central building blocks were synthesised. The ligands were obtained by reductive amination of 4 with 2,2'-bipyridyl-6-carbaldehyde or 1,10-phenanthroline-2-carbaldehyde 3. The chelating effect and the rigid structure of the ligands 5/6 lead to exceptionally robust Fe(2+) and Zn(2+) complexes conserving their structure even in coordinating solvents like dmso at high temperatures. Their solution behavior was investigated using variable temperature (VT) (1)H NMR spectroscopy and VT Vis spectroscopy. SCO behavior was found for all Fe(2+) complexes in this series centred around and far above room temperature. For the first time we have demonstrated that the thermodynamics as well as kinetics for SCO can be deduced by using VT (1)H NMR spectroscopy. An alternative scheme using a linear correction term C(1) to model chemical shifts for Fe(2+) SCO complexes is presented. The rate constant for the SCO of [Fe(rac-trans-5)](2+) obtained by VT (1)H NMR was validated by Laser Flash Photolysis (LFP), with excellent agreement (1/(kHL + kLH) = 33.7/35.8 ns for NMR/LFP). The solvent dependence of the transition temperature T1/2 and the solvatochromism of complex [Fe(rac-trans-5)](2+) were ascribed to hydrogen bond formation of the secondary amine to the solvent. Enantiomerically pure complexes can be prepared starting with R,R- or S,S-1,2-diaminocyclohexane (R,R-trans-4 or S,S-trans-4). The high robustness of the complexes reduces a possible ligand scrambling and allows preparation of quasiracemic crystals of [Zn(R,R-5)][Fe(S,S-5)](ClO4)4·(CH3CN) composed of a 1 : 1 mixture of the Zn and Fe complexes with inverse chirality.

The response of the spin state to in situ variation of the coordination number (CISSS) is a promising and viable approach to smart sensor materials, yet it suffers to date from insensitive detection. Herein, we present the synthetic access to a family of planar nickel(II) complexes, whose CISSS is sensitively followed by means of fluorescence detection. For this purpose, nickel(II) complexes with four phenazine-based Schiff base-like ligands were synthesized and characterized through solution-phase spectroscopy (NMR and UV−vis), solid-state structure analysis (single-crystal XRD), and extended theoretical modeling. All of them reveal CISSS in solution through axial ligating a range of Nand O-donors. CISSS correlates nicely with the basicity of the axial ligand and the substitution-dependent acidity of the nickel(II) coordination site. Remarkably, three out of the four nickel(II) complexes are fluorescent in noncoordinating solvents but are fluorescence-silent in the presence of axial ligands such as pyridine. As these complexes are rare examples of fluorescent nickel(II) complexes, the photophysical properties with a coordination number of 4 were studied in detail, including temperature-dependent lifetime and quantum yield determinations. Most importantly, fluorescence quenching upon adding axial ligands allows a "black or white", i.e. digital, sensoring of spin state alternation. Our studies of fluorescence-detected CISSS (FD-CISSS) revealed that absorption-based CISSS and FD-CISSS are super proportional with respect to the pyridine concentration: FD-CISSS features a higher sensitivity. Overall, our findings indicate a favored ligation of these nickel(II) complexes in the excited state in comparison to the ground state.

A straightforward access is provided to iron(II) complexes showing exceedingly slow spin-state interconversion by utilizing trigonal-prismatic directing ligands (L(n)) of the extended-tripod type. A detailed analysis of the interrelations between complex structure (X-ray diffraction, density functional theory) and electronic character (SQUID magnetometry, Mössbauer spectroscopy, UV/vis spectroscopy) of the iron(II) center in mononuclear complexes [FeL(n)] reveals spin crossover to occur along a coupled breathing/torsion reaction coordinate, shuttling the complex between the octahedral low-spin state and the trigonal-prismatic high-spin state along Bailar's trigonal twist pathway. We associate both the long spin-state lifetimes in the millisecond domain close to room temperature and the substantial barriers against thermal scrambling (Ea ≈ 33 kJ mol(-1), from Arrhenius analysis) with stereochemical constraints. In particular, the topology of the κ(6)N ligands controls the temporary and structural dynamics during spin crossover.

In this work, two iron(II) coordination compounds with a N 2 O 2 coordinating Schiff base-like ligand bearing a redox active tetrathiafulvalene (TTF) unit and pyridine or trans-1,2-bis(4-pyridylethylene) as an axial ligand are synthesized. Crystals suitable for single X-ray structure analysis were obtained for the new ligand. The complexes were characterized by magnetic susceptibility measurements, T-dependent UV−vis spectroscopy, and cyclic voltammetry. Both complexes display spin transition behavior below room temperature with T 1/2 values of 146 and 156 K. The mononuclear iron(II) complex [Fe TTF L(py) 2 ] is relatively stable up to 400 K compared to similar complexes, showing no loss of axial ligands upon heating. Temperature dependent Mossbauer spectroscopy was conducted for the coordination polymer {[Fe TTF L(bpee)]} n to get more information regarding the origin of the stepwise spin crossover (SCO) behavior observed in the magnetic measurements. The change of the spin state is accompanied by a change of the optical properties, which can be monitored by VT−UV−vis spectroscopy for the mononuclear complex and has been analyzed in theoretical studies. The redox behavior of the iron(II) complexes reveals three reversible redox steps which are located at the iron center and at the TTF unit of the ligand. Oxidation of the TTF unit induces characteristic changes in the UV−vis spectrum that can be followed by spectroelectrochemical UV−vis spectroscopy. Addressing the potential of the ironcentered redox process results in similar changes in the UV−vis spectrum, which indicates an electronic coupling of the redox active unit with the metal center under certain circumstances.

New organonickel complexes [Ni­((R)­Ph­(R′)­bpy)­Br] carrying various substituted derivatives of the tridentate –C∧N∧N ligand 6-(phenyl-2-ide)-2,2′-bipyridine (−Phbpy) were synthesized from the precursor [Ni­(COD)2] (COD = 1,5-cyclooctadiene) and the protoligands (ligand precursors) Br–(R)­Ph­(R′)­bpy. Several synthetic routes for the protoligands were studied and compared. All new compounds have been analyzed and spectroscopically characterized. From several complexes crystal and molecular structures were obtained from XRD experiments. UV–vis absorption spectroscopy and detailed electrochemical measurements reveal the impact of the various substituents on the electronic structure of the complexes. Quantum chemical DFT calculations illustrate the composition of highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) and support the assignment of the single-electron reduction and oxidation products as bpy-localized ligand radical species and transient nickel­(III) intermediates, respectively.

Fe spin crossover complexes [Fe(L)] (L = 2-(6-R-pyridin-2-yl)-1,10-phenanthroline with R = H, methoxy, bromo, -(1H-pyrazol-1-yl) or L = 2-(3-methoxy-pyridin-2-yl)-1,10-phenanthroline) were prepared. These air stable and durable complexes show SCO behaviour with very different transition temperatures T ranging from 130 K to 600 K depending on the substitution pattern. The use of H NMR spectroscopy to elucidate the thermodynamics and kinetics of SCO in a solution of this series is described in detail. By introduction of an additional pyrazole donor (R) in the ortho-position to the pyridine, the N6 octahedral coordination sphere is expanded to N8 coordination with a trigonal dodecahedral structure. This leads to a strong stabilization of the high spin state and an increased longitudinal relaxation R of the proton spins. The larger R values were ascribed to different electronic structures with non-orbital degenerate quintet ground states and a larger energetic separation from the first excited state. These results are also supported by Mössbauer spectroscopy. The N8 coordination sphere stabilizes the complex in the high spin state and no indication for SCO was found. DFT calculations confirmed the experimentally obtained order of T and allowed the calculation of the complex structure in experimentally non-accessible spin states. Complexes of this series can be oxidized to the Fe complexes in a chemically reversible fashion. Interestingly, the lowest oxidation potential was observed for the N8 coordinated complex.