Structure and dynamics in metal phosphine complexes using advanced NMR studies with para-hydrogen induced polarisation

Abstract

The iridium and rhodium phosphine complexes IrCl(CO)(PPh₃)₂ 1 (Vaska’s complex), Rh(PMe₃)₄Cl 2, and Rh(PMe₃)₃Cl 3, add H₂ to form the corresponding dihydrides. Exchange with para-hydrogen (p-H₂) provides a means of observing ¹H NMR signals due to the metal bound hydrides at significantly enhanced levels of sensitivity. We show that monitoring these metal hydride complexes can be achieved by a range of 2D NMR methods, based on standard experiments, which have been modified to achieve optimum signal. The assignment of heteronuclei, including low sensitivity nuclei such as ¹⁰³Rh, determination of heteronuclear coupling constants and measurement of their relative signs, is described for these systems using p-H₂ derived starting magnetisation. In the case of Vaska’s complex the dihydride addition product contains a trans labilised carbonyl ligand, and substitution with appropriate phosphines brings about the formation of metal phosphine complexes with new ligand spheres. Appropriately modified NOESY experiments are demonstrated to rapidly probe structural arrangements, and monitor dihydride exchange. For Ir(H)₂Cl(PPh₃)₃ dihydride exchange is shown to proceed mainly via Ir(H)₂Cl(PPh₃)₂, which is shown to contain inequivalent hydrides. The reactivity of the arsine complex IrCl(AsPh₃)₃ 9 towards H₂ is examined, and the NOESY approach used to make structural assignments in the reaction product.