Synthesis and hydroformylation evaluation of Fréchet-type organometallic dendrons with N,O-salicylaldimine Rh(i) complexes at the focal point

Schiff Base in Ketoamine Form and Rh(η4-cod)-Schiff Base Complex with Z' = 2 Structure from Pairwise C-H···Metallochelate-π Contacts

Condensation of 2-hydroxybenzaldehyde (salicylaldehyde) or 2-hydroxy-1-naphthaldehyde with 2-ethylaniline yields the Schiff base compound of (E)-2-(((2-ethylphenyl)imino)methyl)phenol (HL1) or (E)-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-ol (HL2), which in turn react with the dinuclear complex of [Rh(η4-cod)(µ-O2CCH3)]2 (cod = cycloocta-1,5-diene) to afford the mononuclear (η4-cod){(E)-2-(((2-ethylphenyl)imino)methyl)phenolato-κ2N,O}rhodium(I), [Rh(η4-cod)(L1)] (1) or (η4-cod){(E)-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-olato-κ2N,O}rhodium(I), [Rh(η4-cod)(L2)] (2) (L1 or L2 = deprotonated Schiff base ligand). The X-ray structure determination revealed that the HL2 exists in the solid state not as the usual (imine)N···H-O(phenol) form (enolamine form) but as the zwitterionic (imine)N-H+···-O(phenol) form (ketoamine form). 1H NMR spectra for HL2 in different solvents demonstrated the existence of keto-enol tautomerism (i.e., keto ⇆ enol equilibrium) in solution. The structure for 1 and 2 showed that the deprotonated Schiff base ligand coordinates to the Rh(η4-cod)-fragment as a six-membered N^O-chelate around the rhodium atom with a close-to-square-planar geometry. Two symmetry-independent molecules (with Rh1 and Rh2) were found in the asymmetric unit in 1 in a structure with Z' = 2. The supramolecular packing in HL2 was organized by π-π and C-H···π contacts, while only two recognized C-H···π contacts were revealed in 1 and 2. Remarkably, there were reciprocal or pairwise C-H···π contacts between a pair of each of the symmetry-independent molecules in 1. This pairwise C-H contact to the Rh-N^O chelate (metalloaromatic) ring may be a reason for the two symmetry-independent molecules in 1. Differential scanning calorimetry (DSC) analyses revealed an irreversible phase transformation from the crystalline-solid to the isotropic-liquid phase and subsequently confirmed the thermal stability of the compounds. Absorption spectra in solution were explained by excited state properties from DFT/TD-DFT calculations.

Comparison of an Offline SPE-GC-MS and Online HS-SPME-GC-MS Method for the Analysis of Volatile Terpenoids in Wine

The aroma profile is an important marker for wine quality. Various classes of compounds are responsible for the aroma of wine, and one such class is terpenoids. In the context of this work, a validated gas chromatography-mass spectrometry (GC-MS) method for the quantitation of terpenoids in red and white wine using headspace solid-phase microextraction (HS-SPME) and solid-phase extraction (SPE) was established. Calibrations were performed in the respective base wine using both sample preparation methods. The linearity, precision and accuracy evaluated for the respective matrices were excellent for both sample preparations. However, the HS-SPME approach was more sensitive and more accurate. For both sample preparations, the quantification limits were lower than the odor thresholds in wine. The terpenoid concentrations (µg/L) were evaluated for 13 white wines using both sample preparation methods. Importantly, the online HS-SPME approach was more sensitive than the offline SPE method. The major terpenoids identified in the white wines evaluated were linalool (0.2-63 µg/L), geraniol (nd-66 µg/L) and α-terpineol (nd-85 µg/L).