Solution and Solid-State Ligand K-Edge XAS Studies of PdCl2 Diphosphine Complexes with Phenyl and Cyclohexyl Substituents

Abstract

Here we report P K-edge X-ray absorption spectroscopy (XAS), density functional theory (DFT), and time-dependent density functional theory (TDDFT) studies of [R2P(CH2)nPR2]PdCl2, where R = phenyl or cyclohexyl and n = 1–3. P K-edge XAS data were collected on room temperature CH2Cl2 solutions of [Ph2P(CH2)nPPh2]PdCl2, where n = 1 (dppm; 1), 2 (dppe; 2), or 3 (dppp; 3) to determine if solid-state variations in covalent Pd–P bonding reported previously (Inorg. Chem. 2015, 54, 5646) were present in solution. A flow cell was used to overcome challenges associated with photon-induced decomposition, which often occurs rapidly in solution during ligand K-edge XAS data collection at room temperature. While the solid-state data for 2 revealed a 10 % increase in P 1s→Pd-P σ* transition intensity compared to 1 and 3, no statistical difference in Pd–P covalency was observed for 1–3 in solution. In contrast, solid-state P K-edge XAS data collected on [Cy2P(CH2)nPCy2]PdCl2, where n = 1 (dcpm; 4), 2 (dcpe; 5), or 3 (dcpp; 6), matched the trend in P 1s→Pd–P σ* transition intensity for solids of 1–3 despite exchanging the phenyl substituents for cyclohexyl. DFT and TDDFT calculations conducted on 4–6 corroborated the experimental data. Collectively, the solid-state XAS and DFT results suggest that changes in the diphosphine backbone have the same relative effect on Pd–P σ covalency regardless of the substituents attached to phosphorus.