Which Type of Pincer Complex Is Thermodynamically More Stable? Understanding the Structures and Relative Bond Strengths of Group 10 Metal Complexes Supported by Benzene-Based PYCYP Pincer Ligands.

The influence of the pincer platform composition and substitution on the reactivity and physical properties of pincer complexes can be easily explored through different experimental techniques. However, the influence of these factors on the molecular structures and thermodynamic stability of pincer complexes is usually very subtle and cannot always be unambiguously established. To rationalize this subtle influence, a survey of crystallographic data from 130 group 10 metal pincer complexes supported by benzene-based PYCYP pincer ligands, [2,6-(R2PY)2C6H3-nR'n]MX (Y = CH2, NH, O, S; M = Ni, Pd, Pt; R = tBu, iPr, Ph, Cy, Me; R' = CO2Me, tBu, CF3, Ac; n = 0-2; X = F, Cl, Br, I, H, SH, SPh, SBn, Ph, Me, N3, NCS), was carried out. Theoretical calculations for some selected complexes were performed to evaluate the relative bond strength. It was found that the M-Cipso bond length decreases following the linker series of CH2 > NH > O and that the relative M-Cipso bond strength increases following the linker series of CH2 < NH < O. In most cases, the M-P bond length decreases following the linker series of NH > CH2 > O. The relative M-P bond strength increases following the linker series of CH2 < NH < O. A comparison of the thermochemical balance for the isodesmic displacement of the side-arm interactions with PH3 as a probe ligand indicated that the Ni-P bond in a PCCCP-type pincer complex is far less difficult to break compared with that in a POCOP-type complex. As a result, with the same donor substituents and the same auxiliary ligand, the POCOP-type pincer complexes are thermodynamically more stable than the PCCCP complexes. The influence of other backbone and donor substitutions as well as the pincer platform composition on the M-Cipso, M-P, and M-X bond lengths, relative bond strengths, and P-M-P bite angles was also discussed.