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.

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.