A Structure Comparison of Ni(II) Complexes Supported by PNCNP and POCOP Pincer Ligands

Coordination mode and stability of the tetrahydroborate ligand in group 10 metal pincer complexes

The coordination mode of the BH₄^- ligand in transition metal tetrahydroborate complexes is mainly dominated by the nature of the metal centres. However, other factors can also play important roles sometimes. In order to rationalize the coordination modes and the stability of the BH₄^- ligand in group 10 metal tetrahydroborate pincer complexes, [2,6-(^tBu₂PO)₂C₆H₃]Pt(η¹-HBH₃) and [C₆H₄-o-(NCH₂P^tBu₂)₂B]M(η²-H₂BH₂) (M = Ni, Pt) were prepared and characterized. A structural comparison of [2,6-(^tBu₂PCH₂)₂C₆H₃]Ni(BH₄), [2,6-(^tBu₂PO)₂C₆H₃]M(BH₄) and [C₆H₄-o-(NCH₂P^tBu₂)₂B]M(BH₄) (M = Ni, Pd, and Pt) indicates that the M-P bond length, the P-M-P bite angle and the trans-influence of the central atom in the pincer platform also affect the coordination mode of the BH₄^- ligand. The nickel complexes tend to adopt a monodentate coordination mode while the palladium and platinum complexes can adopt either the monodentate or the bidentate mode depending on the structural features of the pincer platforms. Longer M-P bonds and smaller P-M-P bite angles favour the bidentate mode. The stability of the BH₄^- ligand is influenced by both the coordination mode and the nature of the metal centre. The BH₃ species is released more easily from complexes with less electron rich metal centres. Following the series of Ni, Pd, and Pt, complexes with the same pincer ligand more easily lose a BH₃ moiety.

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)₂C₆H_3-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-C_ipso bond length decreases following the linker series of CH₂ > NH > O and that the relative M-C_ipso 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-C_ipso, M-P, and M-X bond lengths, relative bond strengths, and P-M-P bite angles was also discussed.