Probing Lead(II) Bonding Environments in 4-Substituted Pyridine Adducts of (2,6-Me₂C₆H₃S)₂Pb:  An X-ray Structural and Solid-State ²⁰⁷Pb NMR Study

Abstract

The effect of subtle changes in the sigma-electron donor ability of 4-substituted pyridine ligands on the lead(II) coordination environment of (2,6-Me(2)C(6)H(3)S)(2)Pb (1) adducts has been examined. The reaction of 1 with a series of 4-substituted pyridines in toluene or dichloromethane results in the formation of 1:1 complexes [(2,6-Me(2)C(6)H(3)S)(2)Pb(pyCOH)](2) (3), [(2,6-Me(2)C(6)H(3)S)(2)Pb(pyOMe)](2) (4), and (2,6-Me(2)C(6)H(3)S)(2)Pb(pyNMe(2)) (5) (pyCOH = 4-pyridinecarboxaldehyde; pyOMe = 4-methoxypyridine; pyNMe2 = 4-dimethylaminopyridine), all of which have been structurally characterized by X-ray crystallography. The structures of 3 and 4 are dimeric and have psi-trigonal bipyramidal S(3)N bonding environments, with the 4-substituted pyridine nitrogen and bridging sulfur atoms in axial positions and two thiolate sulfur atoms in equatorial sites. Conversely, compound 5 is monomeric and exhibits a psi-trigonal pyramidal S(2)N bonding environment at lead(II). The observed structures may be rationalized in terms of a simple valence bond model and the sigma-electron donor ability of the 4-pyridine ligands as derived from the analysis of proton affinity values. Solid-state (207)Pb NMR experiments are applied in combination with density functional theory (DFT) calculations to provide further insight into the nature of bonding in 4, 5, and (2,6-Me(2)C(6)H(3)S)(2)Pb(py)(2) (2). The lead chemical shielding (CS) tensor parameters of 2, 4, and 5 reveal some of the largest chemical shielding anisotropies (CSA) observed in lead coordination complexes to date. DFT calculations using the Amsterdam Density Functional (ADF) program, which take into account relativistic effects using the zeroth-order regular approximation (ZORA), yield lead CS tensor components and orientations. Paramagnetic contributions to the lead CS tensor from individual pairs of occupied and virtual molecular orbitals (MOs) are examined to gain insight into the origin of the large CSA. The CS tensor is primarily influenced by mixing of the occupied MOs localized on the sulfur and lead atoms with virtual MOs largely comprised of lead 6p orbitals.

Effect of Co-Ordination Chemistry and Oxidation State on the 207Pb Magnetic-Shielding Tensor: A DFT/ZORA Investigation

“…[24][25][26]41 Such calculations provide a strong linear correlation between experimental and calculated magnetic-shielding values. However, with this procedure, the span (Ω) of the shielding tensor is systematically underestimated for hemidirected lead sites, by as much as several hundred ppm.…”

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

“…[15,20] The span (W = 1208 AE 160 ppm) is larger than that of 1, which is expected given the greater variation in the coordination sphere, although it is still relatively small compared with other organolead compounds. [14,15,32,33] À ). [5,7] [8] for…”

New methods for the acquisition of ultra-wideline solid-state NMR spectra of spin-1/2 nuclides

“…MAS techniques have also been used to acquire broad 119 Sn [39,40] and 199 Hg spectra [41]. However, we note that in two recent papers on UW 207 Pb and 195 Pt SSNMR [42,43] that MAS experiments on nuclei with extremely broad patterns are often unreliable, due to slight mis-sets of the magic-angle [44] and/or incomplete excitation of the manifold of spinning sidebands, both of which yield patterns which do not allow for accurate assessments of the chemical shift tensor parameters.…”

“…These experiments are especially useful when cross-polarization (CP)/MAS NMR experiments are limited by weak dipolar couplings and poor excitation bandwidths, or other complications arising from acquisition of broad powder patterns using MAS NMR [44,47,48]. 1 H-X (where X is a heavy spin-1/2 nucleus) CP-CPMG experiments are capable of yielding high quality UW NMR spectra from which chemical shift tensors can be readily extracted; however, the acquisitions of broad patterns are labour intensive, requiring the collection of many sub-spectra due to the limited excitation bandwidth associated with CP [42]. CPMG can also be used to directly excite the X nuclei with modest improvements in excitation bandwidth with respect to the CP-CPMG experiments; however, such experiments still require numerous sub-spectra to be acquired and are often hampered by large longitudinal (T 1 ) relaxation time constants.…”

“…In numerous cases where the CP efficiency is greatly reduced under conditions of MAS (even under slow spinning), static piecewise 1 H-207 Pb CP-CPMG NMR experiments are useful for the acquisition of 207 Pb NMR spectra. However, many 207 Pb NMR spectra are far too broad to be obtained in a single experiment, even under conditions of MAS[38,42]. Pb(OAc) 2 Á3H 2 O is often used as a setup standard for stationary 1 H-207 Pb CP-CPMG NMR experiments, due to its relatively narrow pattern and short 1 H T 1 (ca.…”