The results of studies focussed on copper complexes of a variety of ligands with an NN₄ donor set are reported. The permethylated tetrapodal ligand 2 forms a complex with copper(I) which, upon reaction with dioxygen at –90 °C, yields a product having a bis(μ-oxido)dicopper(III) core (“O-type” product, 10), as inferred from UV/Vis and resonance-Raman spectroscopic data. The UV/Vis spectrum of 10 has two bands at 300 and 404 nm, with extinction coefficients of 9400 and 10400 L mol^–1 cm^–1, respectively. Resonance-Raman spectra display two ¹⁶O/¹⁸O-sensitive bands which, based on the isotopic shifts and the absolute frequencies, are attributed to the Cu–O stretching modes of the O-type product. Complex 10 shows tyrosinase-like activity, as its reaction with sodium p-tert-butylphenolate at –90 °C in THF yields p-tert-butylcatechol, in an ortho-hydroxylation reaction (yield: 30 %). Two new rigid tetrapodal pentadentate ligands (the “superpods” 3 and 4) can be synthesized by condensation of the primary polyamine 1 with paraformaldehyde. Their copper(II) complexes (5 and 6) have been spectroscopically characterized. As ascertained by X-ray crystallography, 5 has the Cu^II ion in a tetragonal-pyramidal environment, with almost uniform Cu–N bond lengths (basal bonds: 2.052 Å and 2.070 Å; apical bond: 2.077 Å). No significant Jahn–Teller distortion is observed here. In 6, the ligand acts as a multinucleating donor, which leads to the formation of a ladder-like cluster of [Cu(μ₃-OH)] units containing a total of two ligands, six copper(II) ions, four hydroxido ligands and eight trifluoroacetate ions. Two of the trifluoroacetate ions are non-coordinating. Variable-temperature magnetic susceptibility data are reported for this hexanuclear copper(II) cluster. Copper(I) complexes of 1 and 3 have been characterized and allowed to react with molecular oxygen, which caused the decomposition of the complexes. The IR spectra of the oxygenation products have bands at 1652 and 1632 cm^–1, respectively, which are absent in the spectra of 1 and 3, suggesting that amine functions have been oxidized to imines.

A mononuclear iron(II) complex is readily formed when combining equimolar amounts of iron(II) tetrafluoroborate hexahydrate, the pyridine-derived triphosphane C₅H₃N{2-[CMe(CH₂PMe₂)₂]}{6-[CMe₂(CH₂PMe₂)]} (2) and diethylphosphane (Et₂PH) in methanol at room temperature. The chelate ligand is in fact pentadentate, as one of the methyl groups of the neopentyl-like sidearm engages in a C–H-bonded contact (agostic interaction) with the metal centre, in addition to the expected NP₃ coordination. The remaining site of what is a distorted coordination octahedron is occupied by monodentate Et₂PH. The autoclave reaction of this purple complex with CO (10 bar, ethanol solvent, 65 °C) yields a yellow microcrystalline precipitate, whose analysis reveals a mixture of two products, in an approximate ratio of 40:1. One of the products is the cis-dicarbonyl complex[Fe(2)(CO)₂](BF₄)₂ (4), in which the chelate ligand acts as an NP₃ donor, and the other is the monocarbonyl complex of the tetrapodal pentadentate NP₄ ligand 1. The latter ligand is formed from 2 by incorporation of an additional PMe₂ donor, in what, in effect, is a metal-mediated phosphenium group transfer. A mechanism is suggested for this reaction. Other species in the reaction mixture have been identified on the basis of mass spectra, and the full NMR spectroscopic assignment of complex 4 (¹H, ³¹P, ¹³C) is reported.

The temperature-dependent and photodynamic spin behaviour of three iron(II) complexes with different 2,6-dipyridyl-4-phenyltriazine ligands L1–L3 was investigated in the solid state and in solution (DMSO). The ligands differ in the substituent R in the 4-position of the phenyl ring (L1: R = H; L2: R = OCH₃; L3: R = SAc), which allows the electronic properties of the ligands to be finetuned. The magnetic data for the complex [Fe(L3)₂](BF₄)₂ in the solid state indicate an incomplete spin transition to the high-spin form upon warming from liquid helium temperature, reaching about 30 % at 400 K. There is circumstantial evidence for paramagnetic contributions compatible with spin transitions, namely, temperature-dependent NMR spectroscopic line shifts and line broadening in solution (DMSO). However, an efficient thermally induced spin crossover in solution is hindered by the substitution lability of the complexes, as has been detected and analyzed in an extended temperature-dependent UV/Vis spectroscopic study. Essentially unaffected by thermally induced substitution lability, the transient dynamics of the iron(II) complexes after nanosecond laser flash excitation of their metal-to-ligand charge-transfer bands provide good evidence for efficient photoinduced spin transitions in solution in all cases. The range of measured lifetimes of the high-spin quintet states is in accord with previously published data. Importantly, in our series of iron(II) complexes, the lifetimes of the high-spin state reflect the electron-donating character of the ligands.

The cover picture shows the crystal structure of an iron(II) complex with a tridentate triazine-based ligand, one of three complexes whose thermal and photonic spin-dynamics are discussed in the article by G. Hörner, A. Grohmann et al. on p. 221 ff. While there is a competition between spin crossover and ligand exchange upon thermal excitation in solution, the complexes switch cleanly between their stable low-spin and metastable high-spin forms upon photoexcitation. Artwork: Holger Neumann, Gentura.

The ligand 2,6-bis(1H-pyrazol-1-yl)-4-(thiocyanatomethyl)pyridine (L) has been prepared from the hydroxymethyl precursor by OH/Br exchange and subsequent thiocyanate substitution. Two polymorphs of the unsolvated iron(II) complex [Fe(L)₂](BF₄)₂ crystallize at room temperature from the same solution (MeOH), and side by side. One (yellow) is high spin between 4 and 350 K, whereas the other (red-brown) shows spin crossover, with a thermal spin-transition centred at 272 K (width ca. 50 K). Both forms have been fully characterized by variable-temperature structure determination and magnetic susceptibility measurements. At a given temperature, the solid-state structures differ strongly with respect to the coordination geometry at iron(II), the orientation of the thiocyanatomethyl substituents, and cation-anion contacts. The implications of these structural differences for the observed spin behaviour are discussed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)