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)

Abstract

To distinguish between these possible reaction pathways, we have used 2D 'H-NMR spectroscopy. For the first time, variable-pressure 2D exchange spectra were used for mechanistic assignments. cisltrans-Isomerisation was found to be the fastest reaction (in CHCl,/CDCI,), 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),PO exchange on the cis-and trans-isomer are second-order processes and are strongly accelerated by increased pressure: I, mechanisms are suggested without ruling out limiting A mechanisms.

Inorganic and Bioinorganic Solvent Exchange Mechanisms

“…The intermolecular exchanges are now second order with very negative Δ S ⧧ values and, when measured, a negative Δ V ⧧ value too (−11.5 cm 3 mol -1 for trans -ZrCl 4 ·2(CH 3 O) 3 PO). The two 2D 1 H NMR exchange correlation spectra (Figure ) obtained at 0.1 and 198 MPa exemplify this behavior for ZrCl 4 ·2(CH 3 O) 3 PO . The intermolecular processes show an important increase in rate with pressure (large negative Δ V ⧧ values), whereas for the intramolecular isomerization the effect is much smaller (small Δ V ⧧ value).…”

“…The two 2D 1 H NMR exchange correlation spectra (Figure 8) obtained at 0.1 and 198 MPa exemplify this behavior for ZrCl 4 ‚2(CH 3 O) 3 PO. 231 The intermolecular processes show an important increase in rate with pressure (large negative ∆V q values), whereas for the intramolecular isomerization the effect is much smaller (small ∆V q value). This is typical of I a /A behaviors and an intramolecular process.…”

Activation and Reaction Volumes in Solution. 3

“…The cis-trans isomerization of ZrCl 4 ‚2(MeO) 3 PO (entry 867) in CHCl 3 82,83 exemplifies the difference in mechanism caused by change in the ionic radius of the metal ion, since in this case the mechanism is associative (∆V q ) -1.6 cm 3 mol -1 ), whereas the analogous titanium compound undergoes isomerization by a dissociative mechanism (∆V q ) + 6.2 cm 3 mol -1 ). 64 The first report on results of high-pressure measurements on metal cluster compound isomerizations illustrated the difficulty in mechanistic diagnosis, as the process is a multistep one.…”

“…Both axial and equatorial exchange of DMSO from TiO(DMSO) 5 2+ occur with an expanded transition state; 62 the faster axial exchange is proposed to proceed by a D mechanism, while the loss of an equatorial DMSO is in concert with an axial to equatorial coordinated solvent migration. Exchange of solvent (S) on cis-TiCl 4 ‚2S complexes (entries [13][14][15] proceeds by a D mechanism 64 (a striking contrast to similar reactions with zirconium complexes 82,83 (vide infra) in which bond making is occurring as the transition state is formed).…”

“…The water ex-change on the tren complex ion (entry 45) proceeds by an associative interchange mechanism. 80 Ligand exchange on tetrachloro zirconium complexes (entry 48) in a noncoordinating solvent is characterized by a strongly associative mechanism reflecting the accessibility to the incoming ligand with increasing radius 82,83 (cf. titanium complexes, 64 entries 13-15).…”