Structure and Properties of a Five-Coordinate Nickel(II) Porphyrin.

Axial coordination in nickel(II) porphyrins has been thoroughly investigated and is well understood. However, isolated five-coordinate nickel(II) porphyrins are still elusive after 50 years of intense research, even though they play a crucial role as intermediates in enzymes and catalysts. Herein we present the first fully stable, thoroughly characterized five-coordinate nickel(II) porphyrin in solution and in the solid state (crystal structure). The spectroscopic properties indicate pure high-spin behavior (S = 1). There are distinct differences in the NMR, UV-vis, and redox behavior compared to those of high-spin six-coordinate [with two axial ligands, such as NiTPPF10·(py)2] and low-spin four-coordinate (NiTPPF10) nickel(II) porphyrins. The title compound, a strapped nickel(II) porphyrin, allows a direct comparison of four-, five-, and six-coordinate nickel(II) porphyrins, depending on the environment. With this reference in hand, previous results were reevaluated, for example, the switching efficiencies and thermodynamic data of nickel(II) porphyrin-based spin switches in solution.

Photoswitchable Magnetic Resonance Imaging Contrast by Improved Light-Driven Coordination-Induced Spin State Switch.

We present a fully reversible and highly efficient on-off photoswitching of magnetic resonance imaging (MRI) contrast with green (500 nm) and violet-blue (435 nm) light. The contrast change is based on intramolecular light-driven coordination-induced spin state switch (LD-CISSS), performed with azopyridine-substituted Ni-porphyrins. The relaxation time of the solvent protons in 3 mM solutions of the azoporphyrins in DMSO was switched between 3.5 and 1.7 s. The relaxivity of the contrast agent changes by a factor of 6.7. No fatigue or side reaction was observed, even after >100,000 switching cycles in air at room temperature. Electron-donating substituents at the pyridine improve the LD-CISSS in two ways: better photostationary states are achieved, and intramolecular binding is enhanced.

Reversible coordination-induced spin-state switching in complexes on metal surfaces.

Molecular spin switches are attractive candidates for controlling the spin polarization developing at the interface between molecules and magnetic metal surfaces1,2, which is relevant for molecular spintronics devices3-5. However, so far, intrinsic spin switches such as spin-crossover complexes have suffered from fragmentation or loss of functionality following adsorption on metal surfaces, with rare exceptions6-9. Robust metal-organic platforms, on the other hand, rely on external axial ligands to induce spin switching10-14. Here we integrate a spin switching functionality into robust complexes, relying on the mechanical movement of an axial ligand strapped to the porphyrin ring. Reversible interlocked switching of spin and coordination, induced by electron injection, is demonstrated on Ag(111) for this class of compounds. The stability of the two spin and coordination states of the molecules exceeds days at 4 K. The potential applications of this switching concept go beyond the spin functionality, and may turn out to be useful for controlling the catalytic activity of surfaces15.

Crystal structure of (15,20-bis-(2,3,4,5,6-penta-fluoro-phen-yl)-5,10-{(pyridine-3,5-di-yl)bis-[(sulfane-diyl-methyl-ene)[1,1'-biphen-yl]-4',2-di-yl]}porph-yrin-ato)nickel(II) di-chloro-methane x-solvate (x > 1/2) showing a rare CN5 coordination.

The crystal structure of the title compound, [Ni(C63H31F10N5S2)]·xCH2Cl2 (x > 1/2), consists of Ni-porphyrin complexes that are located in general positions and di-chloro-methane solvent mol-ecules that are disordered around centers of inversion. The NiII ions are in a square-pyramidal (CN5) coordination, with four porphyrin N atoms in the equatorial and a pyridine N atom in the apical position and are shifted out of the porphyrine N4 plane towards the coordinating pyridine N atom. The pyridine substituent is not exactly perpendicular to the N4 plane with an angle of inter-section between the planes planes of 80.48 (6)°. The di-chloro-methane solvent mol-ecules are hydrogen bonded to one of the four porphyrine N atoms. Two complexes are linked into dimers by two symmetry-equivalent C-H⋯S hydrogen bonds. These dimers are closely packed, leading to cavities in which additional di-chloro-methane solvent mol-ecules are embedded. These solvent mol-ecules are disordered and because no reasonable split model was found, the data were corrected for disordered solvent using the PLATON SQUEEZE routine [Spek (2015 ▸). Acta Cryst. C71, 9-18].

Crystal structure of (15,20-bis-(2,3,4,5,6-penta-fluoro-phen-yl)-5,10-{(4-methyl-pyridine-3,5-di-yl)bis-[(sulfanediyl-methyl-ene)[1,1'-biphen-yl]-4',2-di-yl]}porphyrinato)nickel(II) di-chloro-methane x-solvate (x > 1/2).

The title compound, [Ni(C64H33F10N5S2)]·xCH2Cl2, consists of discrete NiII porphyrin complexes, in which the five-coordinate NiII cations are in a distorted square-pyramidal coordination geometry. The four porphyrin nitro-gen atoms are located in the basal plane of the pyramid, whereas the pyridine N atom is in the apical position. The porphyrin plane is strongly distorted and the NiII cation is located above this plane by 0.241 (3) Šand shifted in the direction of the coordinating pyridine nitro-gen atom. The pyridine ring is not perpendicular to the N4 plane of the porphyrin moiety, as observed for related compounds. In the crystal, the complexes are linked via weak C-H⋯F hydrogen bonds into zigzag chains propagating in the [001] direction. Within this arrangement cavities are formed, in which highly disordered di-chloro-methane solvate mol-ecules are located. No reasonable structural model could be found to describe this disorder and therefore the contribution of the solvent to the electron density was removed using the SQUEEZE option in PLATON [Spek (2015 ▸). Acta Cryst. C71, 9-18].

Coordination-induced spin-state-switch (CISSS) in water.

We present a non-ionic water-soluble porphyrin that does not exhibit measurable aggregation even at high concentrations in water. The spin state of the corresponding nickel(II) complex changes from completely diamagnetic (low-spin) to paramagnetic (high-spin) upon addition of a strong axial ligand. This leads to a strongly reduced NMR relaxation time of the water protons even at low concentrations of the complex.