Pre-catalyst resting states: a kinetic, thermodynamic and quantum mechanical analyses of [PdCl2(2-oxazoline)2] complexes

Abstract

The treatment of cold (∼3 °C) methanolic solutions of Li₂PdCl₄ 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–transisomerisation 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 [PdX₂(oxazoline)₂] compounds are well-known (pre-)catalysts for C–C bond forming chemistry.