Pre-catalyst resting states: a kinetic, thermodynamic and quantum mechanical analyses of [PdCl2(2-oxazoline)2] complexes
Gossage, R and Jenkins, HA and Jones, ND and Jones, Roderick and Yates, BF, Pre-catalyst resting states: a kinetic, thermodynamic and quantum mechanical analyses of [PdCl2(2-oxazoline)2] complexes, Dalton Transactions, 2008, (23) pp. 3115-3122. ISSN 1477-9226 (2008) [Refereed Article]
The treatment of cold (∼3 °C) methanolic solutions of Li 2PdCl4 with two equivalents of 2-phenyl-2-oxazoline (Phox) results in the isolation of [PdCl2(Phox)2] (3). This complex undergoes remarkably slow isomerisation (CHCl3-d) at room temperature to a corresponding thermodynamic form. In addition to a theoretical treatment (DFT), the isomerisation behaviour has been analysed both kinetically and thermodynamically. These investigations lead to the conclusion that the initially formed (i.e. kinetic) isomer of 3 is the cis-form which undergoes conversion to the corresponding thermodynamic trans-form via a dissociative (D) mechanism involving loss of a Phox ligand. The activation parameters ΔS‡ and ΔH‡ are found to be +304 (±3) J K-1 mol-1 and +176 (±1) kJ mol -1, respectively and indicate a high barrier to Pd-N bond cleavage under these conditions. The thermodynamic parameters show the expected endothermic nature of this process (+140 ± 17 kJ mol-1) and a slight positive overall entropy (ΔS° = +17 ± 2 J K-1 mol-1); this latter parameter is presumably due to the formation of the lower dipole moment trans-product when compared to the cis-isomer. Calculated (DFT) values of ΔG‡ and ΔH ‡ are in excellent agreement to those found experimentally. Further theoretical investigation suggests that two 14-electron three-coordinate T-shaped transition states (i.e., [PdCl2(Phox)]‡) are involved; the form pre-disposed to yield the thermodynamic trans-product following re-attachment of the released oxazoline is found to be energetically favoured. The analogous alkyloxazoline system [PdCl2(Meox) 2] (4: Meox = 2-methyl-2-oxazoline) has likewise been investigated. This material gives no indication of cis-trans isomerisation behaviour in solution (NMR) and is shown to exist (X-ray) in the trans-form in the solid-state (as do previously reported crystalline samples of 3). A DFT study of 4 reveals similar values of ΔS‡ and ΔH ‡ if a D type mechanism were operating to rapidly convert cis- to trans-4. However, a significantly higher thermodynamic stability of the trans-isomer relative to the cis-form is revealed versus similar calculations of the Phox derivative 3. This suggests the possibility that (i) reactions of Meox with Li2PdCl4 may lead directly to the trans-form of [PdCl2(Meox)2] or alternatively (ii) that alkyloxazoline complexes such as 4 may have a different, and presumably much more rapid, mechanism for isomerisation. The results are placed into the context that isomerisation behaviour, or lack thereof, could play a key preliminary role in later substrate modification. This is due to the fact that [PdX 2(oxazoline)2] compounds are well-known (pre-)catalysts for C-C bond forming chemistry.