A Variable‐Pressure 2D 1 H‐NMR Study of the Mechanism of Trimethyl Phosphate Intermolecular Exchange and cis / trains ‐Isomerisation of Tetrachlorobis(trimethyl phosphate)zirconium(IV)

In chloroform, [ZrCl 4 ·2(MeO) 3 PO] exists in both cis ‐ and trans ‐isomeric forms. Three reactions can be envisaged in the presence of excess (MeO) 3 PO = L: ( 1 ) cis ‐[ZrCl 4 ·2L] + *L⇆ cis ‐[ZrCl 4 ·L*L]+ L; ( 2 ) trans ‐[ZrCl 4 ·2L] + *L ⇆ trans ‐[ZrCl 4 ·L*L] + L; ( 3 ) cis ‐[ZrCl 4 ·2L]⇆ trans ‐[ZrCl 4 ·2L]. To distinguish between these possible reaction pathways, we have used 2D 1 H‐NMR spectroscopy. For the first time, variable‐pressure 2D exchange spectra were used for mechanistic assignments. cis / trans ‐Isomerisation was found to be the fastest reaction (in CHCl 3 /CDCl 3 ), with a small acceleration at higher pressure: it is concluded to be an intramolecular process with a slightly contracted six‐coordinate transition state. The intermolecular (MeO) 3 PO exchange on the cis ‐ and trans ‐isomer are second‐order processes and are strongly accelerated by increased pressure: I a mechanisms are suggested without ruling out limiting A mechanisms.