Synthesis and Properties of Rhenium Carbonyl Complexes of α,α‘-Bis[(1-pyrenyl)pyrazol-1-yl]alkane Ligands

The reaction between (1-acetyl)pyrene and dimethylformamide dimethylacetal followed by condensation of the resulting product mixture with hydrazine affords 3(5)-(1-pyrenyl)pyrazole (2) in good yield. The easily separable bis[(1-pyrenyl)pyrazole]methane derivatives CH2(3-pzpyrene)2 (3a, pz = pyrazolyl ring) and CH2(3-pzpyrene)(5-pzpyrene) (3b) were prepared by metathetical reactions between pyrazole and CH2Cl2, while CH(nPr)(pzpyrene)2 (4) was prepared by transamination of 2 with butyraldehyde diethylacetal. Compounds 2−4 are luminescent under irradiation with UV light and have pyrenyl monomer-based emissions centered near 400 nm. Compounds 3a and 4 each react with Re(CO)5Br in a 1:1 molar ratio to form highly insoluble complexes Re(CO)3Br[(pzpyrene)2CH2] (5) and Re(CO)3Br[(pzpyrene)2CH(nPr)] (6). Complex Re(CO)3Br[(pz)2CMe2] (7) was also prepared. X-ray structural studies of 6 show extensive π-stacking of pyrenyl groups to form two-dimensional sheets. Pulsed field gradient spin−echo NMR (PGSE-NMR) experiments show that the complexes are monomeric in tetrachloroethane. Variable-temperature, difference NOE and 2-D NMR experiments demonstrate that isomers are present in solution that differ by restricted rotation about the pyrazolyl−pyrenyl bond. The pyrenyl-based emissions centered near 400 nm are quenched by complexation to the Re(CO)3Br moiety in 5 and 6.