HIGH-PRESSURE NMR KINETICS .45. A VARIABLE-PRESSURE 2D H-1-NMR STUDY OF THE MECHANISM OF TRIMETHYL PHOSPHATE INTERMOLECULAR EXCHANGE AND CIS TRANS-ISOMERIZATION OF TETRACHLOROBIS(TRIMETHYL PHOSPHATE)ZIRCONIUM(IV)

In chloroform, [ZrCl4·2(MeO)3PO] exists in both cis‐ and trans‐isomeric forms. Three reactions can be envisaged in the presence of excess (MeO)3PO = L: (1) cis‐[ZrCl4·2L] + *L⇆cis‐[ZrCl4·L*L]+ L; (2) trans‐[ZrCl4·2L] + *L ⇆ trans‐[ZrCl4·L*L] + L; (3) cis‐[ZrCl4·2L]⇆ trans‐[ZrCl4·2L]. To distinguish between these possible reaction pathways, we have used 2D 1H‐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 CHCl3/CDCl3), 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)3PO exchange on the cis‐ and trans‐isomer are second‐order processes and are strongly accelerated by increased pressure: Ia mechanisms are suggested without ruling out limiting A mechanisms. Copyright © 1990 Verlag GmbH & Co. KGaA, Weinheim