A series of ruthenium(II) complexes have been prepared by using bidentate chelating N-heterocyclic carbene (NHC) ligands that feature different donor groups E (E=olefin, thioether, carboxylate, and NHC). Rigid coordination of all donor sites was concluded from NMR spectroscopy, and the electronic impact of the donor group was evaluated by electrochemical analyses. The chelating donor group had a strong influence on the activity of the metal center in catalyzing direct hydrogenation of styrene. A thioether group or a second NHC donor site essentially deactivates the metal center. Complexes comprising a NHC tethered with an olefin or a carboxylate group showed appreciable activity, though only the carboxylate-functionalized system proved to be a precursor for homogeneous hydrogenation. According to in situ high-pressure NMR analyses, complexes featuring a carboxylate chelating group are remarkably resistant toward reductive elimination even under strongly reducing conditions and may, therefore, be used repeatedly.

Publication informationChemical Communications, 46 (35)

We report on the markedly increased spin crossover activity of materials when self-assembled in solution. Spin crossover is directly related to long-range elastic interactions, typically observed in the solid state, and the transition may be gradual over a large temperature range. 1 Effects such as solvent cocrystallization often modify the transition in an unpredictable way. This is predominantly because spin transitions are crucially affected by intermolecular interactions beyond crystal packing effects, such as elastic strain. 2 Such intermolecular interactions are the classical domain to be engineered by supramolecular chemistry. 3 Specifically, the promotion of close metal-metal interactions may stimulate cooperative behavior, which is expected to result in more abrupt spin transitions. This concept has been implemented for engineering of spin crossover active materials in the solid state 4 and recently also in soluble nanoparticles. 5 Here, we have used a hydrophilic/lipophilic bias 6 for inducing a favorable arrangement of potentially spin-labile iron(III) centers. Supramolecular organization in solution leads, most remarkably, to a new type of magnetically active material for liquidphase processing.The Fe(sal 2 trien) complex 2a is known to undergo a gradual spin transition in the solid state. 7 Ligand functionalization and introduction of a lipophilic side chain was accomplished by selective alkylation of 4-hydroxysalicylaldehyde. Condensation of the aldehyde 1 with triethylenetetramine and subsequent metalation with iron(III) nitrate, either in situ or after isolation of the hexadentate ligand precursor, afforded the alkyl-tailed complexes 2b-d (Scheme 1). Metathesis of the noncoordinating anion was straightforward because of the different solubility properties of KPF 6 and the alkyltailed iron(III) complexes and gave, after extraction, complexes 3. A single crystal structure determination of complex 3c unambiguously confirmed the connectivity pattern ( Figure 1a). The bond lengths around the two crystallographically independent iron centers are diagnostic for high spin Fe III at 173 K. 1c The mutual cis coordination of the phenolate moieties arranges the alkyl chains in the same direction and maximizes intermolecular van der Waals interactions. As a consequence, the crystal packing displays apolar and polar layers consisting of the lipophilic side chains, and the charged metal coordination site and the PF 6 -anions, respectively ( Figure 1b).Despite the layered crystal packing, complex 3c does not exhibit thermochromic behavior in the solid state. Magnetic measurements indicate the absence of thermally induced spin crossover in the 30-300 K range and antiferromagnetism or zero-field splitting below 30 K. 1c Similar results have been obtained for complexes 2b-d. This suggests that alkyl functionalization in 2 generates spinstable Fe III centers. The spin stability of 2d in the solid state may also originate from the absence of favorable metal-metal interactions, as powder diffraction showed this compl...