Measurement of Magnitude and Sign of Heteronuclear Coupling Constants in Transition Metal Complexes

Abstract

Sets of specifically tailored E.COSY-type correlation experiments and double-quantum/zero-quantum (DQ/ZQ) experiments are presented which enable the determination of sign and size of small heteronuclear coupling constants across the metal center of transition metal complexes. For the octahedrally coordinated complexes, [Ru(TPM)(H)(CO)(PPh3)]+[BF4]−(1) and [Ir(TPM)(H)(CO)(CO2CH3)]+[BF4]−(2), 14 of 15 and 15 of 15 possible two-bond scalar coupling constants across the metal center were measured, respectively, using15N and15N/13C enriched samples (TPM = tris(1-pyrazolyl)methane)). The reduced coupling constants2KX-M-Y= 4π22J/(hγXγY) were found to be positive when the coupled nuclei X and Y weretranswith respect to the metal center, and negative when the coupled nuclei were incisposition. The validity of this sign rule was verified forJCC,JNN,JPN,JPC,JCN,JHP,JHC, andJHNcouplings. Idiosyncracies associated with 2D NMR spectra for the sign determination of coupling constants with15N which lead to corrections for the signs ofJHN,JPN, andJCNcouplings reported previously are discussed.