Characterising Lone‐Pair Activity of Lead(II) by ²⁰⁷Pb Solid‐State NMR Spectroscopy: Coordination Polymers of [N(CN)₂]⁻ and [Au(CN)₂]⁻ with Terpyridine Ancillary Ligands

Abstract

A series of lead(II) coordination polymers containing [N(CN)(2)](-) (DCA) or [Au(CN)(2)](-) bridging ligands and substituted terpyridine (terpy) ancillary ligands ([Pb(DCA)(2)] (1), [Pb(terpy)(DCA)(2)] (2), [Pb(terpy){Au(CN)(2)}(2)] (3), [Pb(4'-chloro-terpy){Au(CN)(2)}(2)] (4) and [Pb(4'-bromo-terpy)(μ-OH(2))(0.5){Au(CN)(2)}(2)] (5)) was spectroscopically examined by solid-state (207)Pb MAS NMR spectroscopy in order to characterise the structural and electronic changes associated with lead(II) lone-pair activity. Two new compounds, 2 and [Pb(4'-hydroxy-terpy){Au(CN)(2)}(2)] (6), were prepared and structurally characterised. The series displays contrasting coordination environments, bridging ligands with differing basicities and structural and electronic effects that occur with various substitutions on the terpyridine ligand (for the [Au(CN)(2)](-) polymers). (207)Pb NMR spectra show an increase in both isotropic chemical shift and span (Ω) with increasing ligand basicity (from δ(iso) = -3090 ppm and Ω = 389 ppm for 1 (the least basic) to δ(iso) = -1553 ppm and Ω = 2238 ppm for 3 (the most basic)). The trends observed in (207)Pb NMR data correlate with the coordination sphere anisotropy through comparison and quantification of the Pb-N bond lengths about the lead centre. Density functional theory calculations confirm that the more basic ligands result in greater p-orbital character and show a strong correlation to the (207)Pb NMR chemical shift parameters. Preliminary trends suggest that (207)Pb NMR chemical shift anisotropy relates to the measured birefringence, given the established correlations with structure and lone-pair activity.

Correlating Structural Features and 207Pb NMR Parameters with the Stereochemical Activity of PbII Lone Pairs in Birefringent Pb[2,6‐bis(benzimidazol‐2‐yl)pyridine] Complexes

“…The birefringence values for a group of materials containing terpy derivatives correlate with 207 Pb shielding anisotropies, which suggests that the birefringence is closely related to stereochemically active lone pairs (and therefore more polarizable Pb II centres) . In this study we find no correlation between 207 Pb parameters and the birefringence (see Figure S5 in the Supporting Information), which is consistent with our recent findings that the relative orientation of highly anisotropic components and crystal‐growth direction have a larger effect on the measureable birefringence.…”

“…Note that Pb(bipy) 2 [Au(CN) 2 ] 2 was excluded as there is no defined axial angle. Because 207 Pb NMR shielding spans are known to correlate with lone‐pair stereochemical activity,, these suggestive trends against chemically sensible measures of local distortions can assist in qualitatively determining the degree of Pb II lone‐pair stereoactivity.…”

“…Solid‐state NMR spectroscopy probes the local structure around an NMR‐active nucleus (spin quantum number, I ≥ 1/2) and is sensitive to coordination number, bond lengths and local asymmetry. As such, it can be used to measure lone‐pair activity . We have previously reported the 207 Pb magic‐angle spinning (MAS) NMR spectroscopic behaviour of Pb[Au(CN) 2 ] – coordination polymers containing various nitrogen‐donor capping ligands, for which it was demonstrated that the anisotropic nature of the chemical shift could be correlated with structural distortion parameters that are associated with lone‐pair activity , …”

Determination of the Full 207Pb Chemical Shift Tensor of Anglesite, PbSO4, and Correlation of the Isotropic Shift to Lead–Oxygen Distance in Natural Minerals

“…In unrivalled accuracy, however, the principal components of the CS 34 tensor may be extracted from orientation-dependent measurements of single crystals [9]. Furthermore, 35 the orientation of the CS tensor in the crystal structure, expressed by the corresponding eigenvectors, 36 is unequivocally only available from single-crystal NMR spectroscopy. In spite of these advantages, 37 single-crystal NMR is comparatively seldom employed for tensor determination.…”

Determination of the 31P and 207Pb Chemical Shift Tensors in Pyromorphite, Pb5(PO4)3Cl, by Single‐Crystal NMR Measurements and DFT Calculations

“…The anisotropy of the chemical shift tensors, measured by Δδ, is smaller for pyromorphite, but still relatively large, reflecting the presence of an electron lone pair at the 207 Pb. [26] Table 2. Chemical shift tensor of 31 P at position 6h in the pyromorphite structure, as determined from single-crystal NMR experiments and DFT calculations.…”