A Variable-Pressure 2D 1H-NMR Study of the Mechanism of Trimethyl Phosphate Intermolecular Exchange and cis/trains-Isomerisation of Tetrachlorobis(trimethyl phosphate)zirconium(IV)
✍ Scribed by Urban Frey; Lothar Helm; André E. Merbach
- Publisher
- John Wiley and Sons
- Year
- 1990
- Tongue
- German
- Weight
- 255 KB
- Volume
- 73
- Category
- Article
- ISSN
- 0018-019X
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✦ Synopsis
Abstract
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.