Felix Köhler

Coordination‐Induced Spin Crossover (CISCO) through Axial Bonding of Substituted Pyridines to Nickel–Porphyrins: σ‐Donor versus π‐Acceptor Effects

Nickel-porphyrins, with their rigid quadratic planar coordination framework, provide an excellent model to study the coordination-induced spin crossover (CISCO) effect because bonding of one or two axial ligands to the metal center leads to a spin transition from S=0 to S=1. Herein, both equilibrium constants K(1S) and K(2), and for the first time also the corresponding thermodynamic parameters DeltaH(1S), DeltaH(2), DeltaS(1S), and DeltaS(2), are determined for the reaction of a nickel-porphyrin (Ni-tetrakis(pentafluorophenyl)porphyrin) with different 4-substituted pyridines by temperature-dependent NMR spectroscopy. The association constants K(1S) and K(2) are correlated with the basicity of the 4-substituted pyridines (R: OMe>H>CO(2)Et>NO(2)) whereas the DeltaH(1S) values exhibit a completely different order (OMeCO(2)Et>NO(2)). 4-Nitropyridine exhibits the largest binding enthalpy, which, however, is overcompensated by a large negative binding entropy. We attribute the large association enthalpy of nitropyridine with porphyrin to the back donation of electrons from the Ni d(xz) and d(yz) orbitals into the pi orbitals of pyridine, and the negative association entropy to a decrease in vibrational and internal rotation entropy of the more rigid porphyrin-pyridine complex. Back donation for the nitro- and cyanopyridine complexes is also confirmed by IR spectroscopy, and shows a shift of the N-O and C-N vibrations, respectively, to lower wave numbers. X-ray structures of 2:1 complexes with nitro-, cyano-, and dimethylaminopyridine provide further indication of a back donation. A further trend has been observed: the more basic the pyridine the larger is K(1S) relative to K(2). For nitropyridine K(2) is 17 times larger than K(1S) and in the case of methoxypyridine K(2) and K(1S) are almost equal.

Design of a Neutral Macrocyclic Ionophore, Synthesis and Binding Properties for Nitrate and Bromide Anions

A macrocyclic neutral ionophore 8 (X = O) capable of binding weakly coordinating anions such as nitrate and bromide in DMSO solution has been designed by a stepwise, deductive approach. The optimum geometrical arrangement of the hydrogen bond donor sites in the target ionophore was determined by DFT calculations. From these data, a suitable macrocyclic molecular framework was constructed. The 24-membered macrocyclic ionophore was synthesized by standard macrocyclization methods. NMR titrations revealed molecular complexes with defined 1:1 stoichiometries in DMSO for 8 (X = O) with nitrate, hydrogensulfate, acetate, cyanide, iodide, and bromide ions, while dihydrogenphosphate, sulfate, and chloride ions yielded aggregates of higher stoichiometry. The nitrate binding constants of 8 (X = O) are substantial for a neutral ionophore with defined binding sites in pure DMSO solution. Bromide ions, which have a similar ion radius, are bound with an even higher affinity. Chloride is obviously too small, and iodine too large, to form 1:1 complexes. The binding motif of 8 (X = O) was compared with related molecules of similar structure, such as 8 (X = S) and 19. As predicted from calculations, the small structural variations give rise to a complete loss of nitrate and bromide ion binding ability in DMSO. This sensitivity to geometrical changes and the affinity of 8 (X = O) to nitrate and bromide ions, which are poor hydrogen bond acceptors, confirm the predicted complementarity of ionophore binding site and anion geometry. According to DFT and MD calculations the higher affinity of 8 (X = O) to bromide than to nitrate is mainly due to the greater flexibility of the bromide complex and thus to its higher entropy. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Light-Driven Coordination-Induced Spin-State Switching: Rational Design of Photodissociable Ligands

The bistability of spin states (e.g., spin crossover) in bulk materials is well investigated and understood. We recently extended spin-state switching to isolated molecules at room temperature (light-driven coordination-induced spin-state switching, or LD-CISSS). Whereas bistability and hysteresis in conventional spin-crossover materials are caused by cooperative effects in the crystal lattice, spin switching in LD-CISSS is achieved by reversibly changing the coordination number of a metal complex by means of a photochromic ligand that binds in one configuration but dissociates in the other form. We present mathematical proof that the maximum efficiency in property switching by such a photodissociable ligand (PDL) is only dependent on the ratio of the association constants of both configurations. Rational design by using DFT calculations was applied to develop a photoswitchable ligand with a high switching efficiency. The starting point was a nickel-porphyrin as the transition-metal complex and 3-phenylazopyridine as the photodissociable ligand. Calculations and experiments were performed in two iterative steps to find a substitution pattern at the phenylazopyridine ligand that provided optimum performance. Following this strategy, we synthesized an improved photodissociable ligand that binds to the Ni-porphyrin with an association constant that is 5.36 times higher in its trans form than in the cis form. The switching efficiency between the diamagnetic and paramagnetic state is efficient as well (72% paramagnetic Ni-porphyrin after irradiation at 365 nm, 32% paramagnetic species after irradiation at 440 nm). Potential applications arise from the fact that the LD-CISSS approach for the first time allows reversible switching of the magnetic susceptibility of a homogeneous solution. Photoswitchable contrast agents for magnetic resonance imaging and light-controlled magnetic levitation are conceivable applications.

Electronic decoupling of a cyclophane from a metal surface

Electronic self-decoupling of an organic chromophore from a metal substrate is achieved using a naphtalenediimide cyclophane to spatially separate one chromophore unit of the cyclophane from the substrate. Observations of vibronic excitations in scanning tunneling spectra demonstrate the success of this approach. These excitations contribute a significant part of the tunneling current and give rise to clear structure in scanning tunneling microscope images. We suggest that this approach may be extended to implement molecular functions at metal surfaces.

Controlled Formation of an Axially Bonded Co-Phthalocyanine Dimer

Isolated Co-phthalocyanine (CoPc) molecules were moved on a monolayer of CoPc on Cu(111) using an STM tip. If placed almost on top of another, the CoPc molecule in the second layer locks in place and the STM image at negative bias changes substantially. Density functional theory calculations explain the nature of the bonding mode and the change in STM.

Is the [9]Annulene Cation a Möbius Annulene?

Does it do the twist? High-level theoretical calculations show that the [9]annulene cation exists in two isomers: a twisted 4n electron Mobius aromatic compound, and a non-twisted boat-like Huckel conformation (see scheme). The latter form was detected by laser flash photolysis experiments; an example of a stable, charged, or uncharged parent Mobius annulene is still elusive.

Singly and Doubly Twisted [36]Annulenes: Synthesis and Calculations

A cyclophane with a [36]annulene periphery, including four anthrylene and two phenylene units, was synthesized in a four-step sequence using a McMurry cyclization. Upon crystallization from different solvents, four different conformations were determined by X-ray structure analysis. Two conformations exhibit a double twist (Hückel topology) and two structures include a single twist (Möbius topology). By using DFT calculations a conformation with a triple twist (Möbius) was located. However, our calculations and the NMR spectroscopy data do not provide evidence for aromaticity (for Möbius structures) or antiaromaticity (for Hückel structures).

Stability and Aromaticity of Charged Möbius[4n]Annulenes

A number of parent aromatic Möbius annulenes that violate the Hückel rule have been proposed theoretically. Unfortunately, these species are thermodynamically and kinetically unstable and probably impossible to synthesize. We therefore systematically screened a large number of annulene anions, dianions, and dications and predict that a Möbius [14]annulene dication is not only the most stable isomer among these monocyclic structures but also the global minimum on the (CH)(14)(2+) hypersurface including bicyclic and tricyclic isomers. Thus, under stable ion conditions the Möbius [14]annulene dication should be stable toward cis-trans isomerizations and electrocyclic reactions which are the most probable pathways limiting the lifetime of Möbius annulenes.

The [13]annulene cation is a stable Möbius annulene cation.

The isomers of the [n]annulene cation series (n = 13, 17, and 21) were systematically scrutinized by DFT, SCS-MP2, and coupled-cluster methods. Only in the case of the [13]annulene cation the global minimum is clearly a Mobius structure. According to our theoretical predictions, the [13]annulene cation should also be kinetically stable. Thus, the [13]annulene cation is the most promising candidate for the synthesis of a parent Mobius aromatic system among the neutral and cationic annulenes.

Photoswitching of azobenzene multilayers on a layered semiconductor

In situ photoelectron spectroscopy is used to study the adsorption and photoisomerization of azobenzene multilayers on the layered semiconductor HfS 2 at liquid nitrogen temperatures. The measured valence band spectra indicate weak molecule–substrate coupling and provide evidence for reversible switching of azobenzene multilayers by light with different wavelengths. The photoswitching manifests itself in spectral shifts due to changes in the electrical surface conductance and in modifications of the electronic structure consistent with the results of outer valence Green’s function calculations. The photoemission results appear to establish azobenzene as an optoelectrical molecular switch.