Strong 1,4 P–O intramolecular interactions as a source of conformational preferences in α-stabilised phosphorus ylides. Part 2: metallic complexes

The complexes cis-[PdCl2{η2-[C(H)PH3]2CO}] (2) in two different stereochemical arrangements (cisoid–cisoid, 2cc; cisoid–transoid, 2ct) have been studied by DFT methods at the B3LYP level. The (2cc) structure is energetically more stable than the (2ct), being the main responsible of the energy difference between the two complexes the energetic gap between the cc and ct isomers of the free bis-ylide ligand [H3PC(H)–C(O)–C(H)PH3] (1). In (1) these differences arise from the presence of 1,4-intramolecular interactions between the phosphorus atoms and the carbonyl oxygen. That is, the conformational preferences observed in (1) due to the establishment of 1,4-P⋯O interactions are directly transferred to the metallic complexes (2) in such a way that the most stable structure for the free ligand gives the most stable complex. In the absence of the carbonyl group (e.g. [H3PC(H)–C(CH2)–C(H)PH3] (3) or [H3PC(H)–CH2–C(H)PH3] (5)) all isomers of a given bis-ylide (cc, ct and tt) become isoenergetic. The absence of discrimination in the free bis-ylides (3) and (5) gives isoenergetic cc and ct structures for the corresponding complexes cis-[PdCl2{η2-[C(H)PH3]2CCH2}] (4), cis-[PdCl2{η2-[C(H)PH3]2CH2}] (6) and [CpNi{η2-[C(H)PH3]2CH2}] (7), as stated by NMR spectroscopy for (7). The influence of other factors (change of the heteroatom at Cβ, change of the P substituents) in the energy of the different isomers of the bis-ylides and in the energy of the corresponding complexes has also been studied and discussed.