1H-Detected Multinuclear NMR Experiments for the Measurement of Small Heteronuclear Coupling Constants in Transition Metal Complexes

The scalar coupling constants between protons, nitrogens, and phosphorus in the metal complex chloro(triphenylphosphine)bis[bis(1-pyrazolyl)methane]ruthenium(II) chloride, [RuCl(PPh3)(BPM)2]Cl (1), were measured with a set of specially adapted NMR experiments. The absolute sign of the coupling constants was determined by relating the signs of the measured couplings to that of a one-bond proton-carbon coupling constant. A complete set of coupling constants >|0.4| Hz was obtained with use of a single sample with 99% 15N-labeled bis-pyrazolyl ligands. The data show that the two-bond 15N-15N and 31P-15N couplings across the metal center are significantly larger, if the two metal-ligating atoms are trans rather than cis with respect to one another. Furthermore, all trans couplings JPN and JNN are positive, while the corresponding cis couplings are negative or too small to be measured. The conformation dependence of the scalar coupling constants supports the rapid structural characterization of catalytically active organometallic complexes by NMR spectroscopy. The proposed set of NMR experiments includes HSQC experiments with small flip angles, a quantitative long-range 15N-15N correlation experiment, and DQ/ZQ experiments for the determination of the sign and size of JNN and JPN coupling constants in linear spin systems.