The influence of pentaamminerhodium(III) on the1H n.m.r. spectra and pyrrole pKas of coordinated imidazoles and pyrazoles

Abstract

The following [(NH3)5RhLH]Cl3 salts were preparedvia the [(NH3)5Rh(O3SCF3)](O3SCF3)2 synthetic route; LH=1-methylimidazole (1CH3imH), 2-methylimidazole (2CH3imH), 4-methylimidazole (4CH3imH), 5-methylimidazole(5CH3imH), and pyrazole (pyzH). pKa's at 25.0°C were determined for [(NH3)5RhLH]3+ complexes as follows: 2CH3imH, 10.4±0.1; 5CH3imH/4CH3imH isomer mixture, 10.3±0.1; pyzH, 6.54±0.05. The influence on the pKa's of imidazoles is dominated by σ withdrawal of the rhodium(III) centre and may be compensated by the presence of ring methylation by only 0.5log units for cobalt(III) and rhodium(III) derivatives, compared to 1.3 units for the π-withdrawing ruthenium(III) centre. In the case of the π-acceptor pyrazole ring, [(NH3)5Rh]3+ is observed to serve as a slight π-donor and raises the pKa above the cobalt(III) analogue. The1H n.m.r. spectra of [(NH3)5RhLH]3+ complexes of the substituted imidazoles and pyrazole exhibit a deshielding order. C−2H>C−5H>C−4H for imidazoles and C−3H>C−5H>C−4H for pyrazole, as do their cobalt(III) analogues. The magnitude of Δδ values (Δδ=δfree L-δcomplex) are virtually the same as in the cobalt(III) systems which shows that TIP influences are unimportant compared to ring rehybridization in estabilishing chemical shifts for both the cobalt(III) and rhodium(III) complexes. The imidazolato and pyrazolato complexes exhibit resonances upfield of the respective substituted imidazole or pyrazole complex in keeping with more negative charge on the rings; the influence is largest at C−2H of imidazolates and C−3H of pyrazolate.