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

Article abstract of DOI:10.1021/ic700738w

The effect of subtle changes in the σ-electron donor ability of 4-substituted pyridine ligands on the lead(II) coordination environment of (2,6-Me₂C₆H₃S)₂Pb (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-Me₂C₆H₃S)₂Pb(pyCOH)] ₂ (3), [(2,6-Me₂C₆H₃S) ₂Pb(pyOMe)]₂^? (4), and (2,6-Me ₂C₆H₃S)²Pb(pyNMe₂) (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 ψ-trigonal bipyramidal S₃N 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 ψ-trigonal pyramidal S₂N bonding environment at lead(II). The observed structures may be rationalized in terms of a simple valence bond model and the σ-electron donor ability of the 4-pyridine ligands as derived from the analysis of proton affinity values. Solid-state ²⁰⁷Pb 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-Me₂C₆H ₃S)₂Pb(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.

Full text of DOI:10.1021/ic700738w

Inorg. Chem. 2007, 46, 8625−8637
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
Glen G. Briand,*,† Andrew D. Smith,† Gabriele Schatte,‡ Aaron J. Rossini,§ and Robert W. Schurko*,§
Department of Chemistry, Mount Allison UniVersity, SackVille, New Brunswick, Canada E4L 1G8,
Saskatchewan Structural Sciences Centre, UniVersity of Saskatchewan, Saskatoon, Saskatchewan
Canada S7N 5C9, and Department of Chemistry & Biochemistry, UniVersity of Windsor, Windsor,
Ontario, Canada N9B 3P4
Received April 18, 2007
The effect of subtle changes in the σ-electron donor ability of 4-substituted pyridine ligands on the lead(II) coordination
environment of (2,6-Me2C6H3S)2Pb (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-Me2C6H3S)2Pb(pyCOH)]2
(3), [(2,6-Me2C6H3S)2Pb(pyOMe)]2 (4), and (2,6-Me2C6H3S)2Pb(pyNMe2) (5) (pyCOH
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 -trigonal bipyramidal S3N 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 -trigonal pyramidal S2N
bonding environment at lead(II). The observed structures may be rationalized in terms of a simple valence bond
model and the -electron donor ability of the 4-pyridine ligands as derived from the analysis of proton affinity
) 4-pyridinecarboxaldehyde;
)
)
ψ
ψ
σ
values. Solid-state 207Pb 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-Me2C6H3S)2Pb(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.
Introduction
sively studied and has been achieved via a variety of syn-
thetic routes.7-13 It has recently been shown that lead(II)
thiolates represent possible organometallic precursors for the
There has been increasing interest in the preparation of
nanocrystals of narrow band gap semiconducting materials
for use in optoelectronic devices.1-6 In particular, the pro-
duction of lead sulfide (PbS) nanocrystals has been exten-
(3) McDonald, S. A.; Cyr, P. W.; Levina, L.; Sargent, E. H. Appl. Phys.
Lett. 2004, 85, 2089.
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E. J. D.; Levina, L.; Sargent, E. H. Nat. Mater. 2005, 4, 138.
(5) Sargent, E. H. J. Mod. Opt. 2004, 51, 2797.
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L.; Kumacheva, E.; Sargent, E. H. Opt. Lett. 2005, 30, 171.
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Kumacheva, E. AdV. Mater. 2004, 16, 926.
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394, 400.
* To whom correspondence should be addressed. E-mail: gbriand@mta.ca.
Tel: (506) 364-2346. Fax: (506) 364-2313 (G.G.B.), E-mail: rschurko@
uwindsor.ca. Tel: (519) 253-3000 x3548. Fax: (519) 973-7098 (R.W.S.).
† Mount Allison University.
‡ University of Saskatchewan.
§ University of Windsor.
(1) Chang, T. W. F.; Musikhin, S.; Bakueva, L.; Levina, L.; Hines, M.
A.; Cyr, P. W.; Sargent, E. H. Appl. Phys. Lett. 2004, 84, 4295.
(2) Kim, S.; Fisher, B.; Eisler, H. J.; Bawendi, M. J. Am. Chem. Soc.
2003, 125, 11466.
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Dunlop, H. Nanotechnology 2004, 15, 1328.
10.1021/ic700738w CCC: $37.00
Published on Web 09/15/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 21, 2007 8625

Briand et al.
Chart 1. Schematic Drawings of Lead Thiolate Structures, Including
(2,6-Me2C6H3S)2Pb(py)2 (2), [(2,6-Me2C6H3S)2Pb(pyCOH)]2 (3), [(2,6-
Me2C6H3S)2Pb(pyOMe)]2 (4), and (2,6-Me2C6H3S)2Pb(pyNMe2) (5) (py
) Pyridine; pyCOH ) 4-Pyridinecarboxaldehyde; pyOMe )
4-Methoxypyridine; pyNMe2 ) 4-Dimethylaminopyridine)
production of interesting thiolate-capped clusters and
nanocrystals.14-17 As a result of the well-known toxicity of
lead(II) complexes, there is also much interest in the synthe-
sis of simple coordination compounds to better understand
the binding of lead(II) by the cysteine residues of biomole-
cules.18-21
Although they have been known for a number of de-
cades,14,15 the inherent insolubility and nonvolatility associ-
ated with lead(II) thiolates has resulted in the delayed
development of their chemistry. The few structural studies
that have been undertaken show that simple lead(II) thiolates
(RS)2Pb (R ) C6H5, 4-MeC6H4, CH2) possess polymeric
structures in the solid state as a result of extensive intermo-
lecular Pb‚‚‚S interactions.22-24 The resulting high coordina-
tion numbers for lead(II) in these compounds, along with
its empty p orbitals and variable stereochemical activity of
the valence electron lone pair,25 suggest a potentially rich
coordination chemistry that has remained relatively
unexplored.18,26-28
In this context, we have recently reported the preparation
of the 2,6-substituted arylthiolate (2,6-Me2C6H3S)2Pb (1),
which has an increased solubility in comparison to the alkyl-
and unsubstituted arylthiolate [i.e., (PhS)2Pb] analogues.29
The reaction of 1 with pyridine (py) yielded the 1:2 adduct
(2,6-Me2C6H3S)2Pb(py)2 (2), which is monomeric and ex-
hibits no intermolecular Pb‚‚‚S contacts. Further, bridging
Lewis-base ligands, 4,4′-bipyridyl (bipy) and pyrazine (pyr),
afforded the 1:1 coordination polymers [(2,6-Me2C6H3S)2-
Pb(L)]∞ (L ) bipy, pyr). Interestingly, all of the complexes
show ψ-trigonal bipyramidal S2N2 coordination geometries,
with the thiolate sulfur atoms occupying equatorial positions
and the axial amine nitrogen atoms involved in dative
bonding interactions. To determine the effect of altering the
Lewis basicity of the amine ligand on the coordination
geometry of lead(II) in these systems, we have prepared
various 4-substituted pyridine adducts of 1. This has resulted
in the isolation of the complexes shown in Chart 1, which
have also been characterized with single-crystal X-ray
diffraction.
Whereas refined single-crystal X-ray data is used to
accurately portray molecular structure, solid-state 207Pb NMR
spectra can provide information on the electronic environ-
ment of the metal atom and the nature of bonding with
surrounding ligands. 207Pb (I ) 1/2) is the only NMR-active
isotope of lead and has a receptivity of 11.8 relative to 13C.30
The lead chemical shift range is very large, with a difference
of approximately 17 000 ppm between lead metal and
plumbocene.31-34 This large chemical shift range is a product
of the extremely polarizable lead valence orbitals, which give
rise to the large lead chemical shielding anisotropies (CSA)
observed in most solid-state 207Pb NMR spectra (with the
exception of complexes of high spherical or Platonic sym-
metry).31,35 Lead chemical shifts are extremely sensitive to
probes of the atomic/molecular environment and subtle
molecular changes induced by chemical (e.g., reactions,
solvent coordination) and physical changes (e.g., temperature,
stress).31,33,36,37
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V.; Sargent, E. H.; Kumacheva, E. Langmuir 2005, 21, 1086.
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1990, 94, 6390.
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Goldberg, D. P.; Godwin, H. A. Inorg. Chem. 2006, 45, 6574.
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Payne, J. C.; Bridgewater, B. M.; Mijovilovich, A.; Parkin, G.; Zaleski,
J. M.; Penner-Hahn, J. E.; Godwin, H. A. J. Am. Chem. Soc. 2005,
127, 9495.
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Granger, P. Pure Appl. Chem. 2001, 73, 1795.
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41, 153.
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W.; Kmetic, M. A.; Payne, N. C.; Vittal, J. J. Acta Crystallogr., Sect.
B 1997, 53, 457.
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37, 1853.
(26) The Encyclopedia of Inorganic Chemistry; Wiley & Sons: Chichester,
U.K., 1994; Vol. 4.
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K. E. J. Chem. Soc., Dalton Trans. 1997, 1675.
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J. J. Chem. Ber./Recl. 1988, 121, 1745.
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Met. Chem. 1999, 22, 373.
(34) Multinuclear NMR; Mason, J., Ed.; Plenum Publishing: New York,
1987; pp 305.
(35) Duncan, T., Michael A Compilation of Chemical Shift Anisotropies;
The Farragut Press: Chicago, 1990.
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8626 Inorganic Chemistry, Vol. 46, No. 21, 2007

Probing Lead(II) Bonding EnWironments
Preparation of (2,6-Me2C6H3S)2Pb(pyNMe2) (5). A solution
of pyNMe2 (0.20 g, 1.6 mmol) in toluene (4 mL) was added
dropwise to a solution of 1 (0.20 g, 0.40 mmol) in the same solvent
(5 mL) to give a greenish-yellow solution. After stirring for 10
min, the reaction was filtered and allowed to sit at 23 °C. The
precipitate was collected after 1 d to yield 5 as pale yellow crystals
(0.16 g, 0.26 mmol, 66%). Anal. Calcd for C23H28N2PbS2: C, 45.74;
H, 4.68; N, 4.64. Found: C, 45.65; H, 4.58; N, 4.42. Mp 90 °C
(d). FTIR (cm-1): 1612 vs, 1579 w, 1537 s, 1390 m, 1344 w, 1294
w, 1225 s, 1163 m, 1163 w, 1113 w, 1053 m, 1030 w, 1003 s, 949
w, 893 vw. Solution NMR data (thf-d8, ppm): 1H NMR, δ ) 2.42
Once relationships have been established between well-
characterized molecular structures and lead chemical shield-
ing (CS) tensors then solid-state NMR can be applied for
structural determination of complexes for which single-
crystal data are not readily available. Given the general
insolubility of homoleptic lead(II) thiolate complexes,22-24
the lability of their coordination complexes,29 and the well-
known sensitivity of lead CS tensors to subtle changes in
molecular structure,31,33,38 solid-state 207Pb NMR should serve
as a formidable probe of molecular structure in these systems.
To this end, solid-state 207Pb NMR experiments, in tandem
with density functional theory (DFT) calculations of CS
tensors, are used to measure and theoretically model lead
CS tensors and examine their relationships to structure,
symmetry, and bonding.
(s, 12 H, Me2C6H3S), 2.99 (s, 6 H, NC5H4NMe2), 6.55 (d, 3JH,H
)
3
7 Hz, 2 H, NC5H4NMe2), 6.69 (t, JH,H ) 7 Hz, 2 H, Me2C6H3S),
3
3
6.95 (d, JH,H ) 7 Hz, 4 H, Me2C6H3S), 8.21 (d, JH,H ) 7 Hz, 2
H, NC5H4NMe2). 13C{1H} NMR, δ ) 23.2 (Me2C6H3S), 38.1
(NC5H4NMe2), 106.7 (NC5H4NMe2), 125.1 (Me2C6H3S), 127.9
(Me2C6H3S), 143.2 (Me2C6H3S), 149.1 (NC5H4NMe2), 154.1
(NC5H4NMe2).
Experimental Section
X-ray Structural Analysis. Crystals of 3-5 were isolated from
the reaction mixtures as indicated above. Single crystals of each
compound were coated with Paratone-N oil, mounted using a
CryoLoop (Hampton Research), and frozen in the cold stream of
the goniometer. Data were measured on a Nonius KappaCCD
4-Circle Kappa FR540C diffractometer using monochromated
Mo KR radiation (λ ) 0.71073 Å) at -100 °C. Data were col-
lected using æ and/or ω scans.39 Data reduction was performed
with the HKL DENZO and SCALEPACK software, which corrects
for beam inhomogeneity, possible crystal decay, Lorentz, and
polarization effects. A multiscan absorption correction was applied
(SCALEPACK).40 Transmission coefficients were calculated using
SHELXL97-2.41,42 The structures were solved by direct methods
(SHELXS-97) and refined by full-matrix least-squares on F 2
(SHELXL97-2).42 The non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms were included at geometrically idealized
positions (C-H bond distances 0.95/0.99 Å) and not refined. The
isotropic thermal parameters of the hydrogen atoms were fixed at
1.2 times that of the preceding carbon atom.
General Considerations. 2,6-dimethylbenzenthiol (95%), 4-meth-
oxypyridine (97%), 4-pyridine carboxaldehyde (97%), and 4-di-
methylaminopyridine (99+%) were used as received from the
Aldrich Chemical Co. Lead(II) acetate trihydrate was used as
received from Fischer. (2,6-Me2C6H3S)2Pb (1) and (2,6-Me2C6H3S)2-
Pb(py)2 (2) were prepared as reported previously.29 General
characterization methods (NMR, IR, and elemental analysis) are
described in the Supporting Information.
Preparation of [(2,6-Me2C6H3S)2Pb(pyCOH)]2 (3). A solution
of pyCOH (0.17 g, 1.5 mmol) in toluene (2 mL) was added
dropwise to a solution of 1 (0.20 g, 0.40 mmol) in the same solvent
(11 mL) to give a clear orange solution. After stirring for 10 min,
the reaction was filtered and allowed to stand at 23 °C. After 1 h,
the precipitate was collected by filtration to yield 3 as orange
crystals (0.11 g, 0.090 mmol, 23%). Anal. Calcd for C44H46N2O2-
Pb2S4: C, 44.88; H, 3.95; N, 2.38. Found: C, 44.72; H, 4.00; N,
2.35. Mp 138 °C (d). FTIR (cm-1): 1707 s, 1668 w, 1560 m, 1415
m, 1316 w, 1228 m, 1211 m, 1167 vw, 1048 m, 1005 m. Solution
NMR data (thf-d8, ppm): 1H NMR, δ ) 2.55 (s, 12 H, Me2C6H3S),
6.69 (t, 3JH,H ) 7 Hz, 2 H, Me2C6H3S), 7.01 (d, 3JH,H ) 7 Hz, 4 H,
Powder X-ray diffraction patterns were collected using a Bruker
AXS HI-STAR system using a general area detector diffractions
system. Compounds 2, 4, and 5 were finely ground, packed into
1.0 mm glass capillary tubes, and flame sealed. The X-ray source
employed was Cu KR radiation (λ ) 1.540598 Å) with an area
detector using a 2θ range between 4.0 and 65.0°. Powder X-ray
diffraction patterns were simulated with the PowderCell software
package.43
Solution 207Pb and Solid-State 207Pb and 13C NMR. Solution
207Pb and solid-state 207Pb and 13C NMR spectra were acquired on
a Varian Infinity Plus spectrometer with an Oxford 9.4 T wide-
bore magnet [ν0(1H) ) 399.73, ν0(207Pb) ) 83.63, ν0(13C) ) 100.51
MHz]. Lead chemical shifts were referenced to Me4Pb (δiso ) 0.0
ppm) by setting the isotropic shift of a secondary standard of 0.5
M Pb(NO3)2(aq) to -2941.0 ppm.38 Carbon chemical shifts were
referenced to tetramethylsilane (δiso ) 0.0 ppm) by using the high-
3
Me2C6H3S), 7.72 (d, JH,H ) 6 Hz, NC5H4COH, 2 H), 8.84 (d,
3JH,H ) 6 Hz, 2 H, NC5H4COH), 10.03 (s, 1 H, COHC5H4N).
13C{1H} NMR, δ ) 23.8 (Me2C6H3S), 121.9 (NC5H4COH), 124.6
(Me2C6H3S), 126.6 (Me2C6H3S), 142.0 (NC5H4COH), 142.4
(Me2C6H3S), 151.1 (NC5H4COH), 191.4 (NC5H4COH).
Preparation of [(2,6-Me2C6H3S)2Pb(pyOMe)]2 (4). A solution
of pyOMe (0.18 g, 1.6 mmol) in dichloromethane (3 mL) was added
dropwise to a solution of 1 (0.20 g, 0.40 mmol) in the same solvent
(2 mL) to give a clear yellow solution. After stirring for 10 min,
the reaction sat for 1 h at 23 °C. The resulting precipitate was
collected by filtration to yield 4 as yellow crystals (0.075 g, 0.13
mmol, 32%). Anal. Calcd for C44H50N2O2Pb2S4: C, 44.73; H, 4.27;
N, 2.37. Found: C, 44.49; H, 4.60; N, 2.34. Mp 155 °C (d). FTIR
(cm-1): 1658 m, 1603 vs, 1566 s, 1504 s, 1331 m, 1292 vs, 1255
m, 1205 s, 1117 w, 1049 s, 1028 vs, 1003 s. Solution NMR data
(thf-d8, ppm): 1H NMR, δ ) 2.52 (s, 12 H, Me2C6H3S), 3.84 (s, 3
3
(39) Nonius, B. V. COLLECT data collection software, 1998.
(40) Otwinowski, Z.; Minor, W. In Methods Enzymology: Macromolecular
Crystallography; Carter, C. W., Sweet, R. M., Eds.; Academic Press:
San Diego, 1997; Vol. 276, pp 307.
H, NC5H4OMe), 6.70 (t, JH,H ) 7 Hz, 2 H, Me2C6H3S), 6.89 (d,
3
3JH,H ) 6 Hz, 2 H, NC5H4OMe), 7.00 (d, JH,H ) 7 Hz, 4 H,
3
Me2C6H3S), 8.42 (d, JH,H ) 6 Hz, 2 H, NC5H4OMe). 13C{1H}
NMR, δ ) 23.3 (Me2C6H3S), 54.5 (NC5H4OMe), 109.7 (NC5H4-
OMe), 124.5 (Me2C6H3S), 125.1 (NC5H4OMe), 126.5 (Me2C6H3S),
141.9 (Me2C6H3S), 150.8 (NC5H4OMe), 165.9 (NC5H4OMe).
(41) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115.
(42) Sheldrick, G. M. SHELXL97-2, Program for the Solution of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(43) Kraus, W.; Nolze, G. PowderCell for Windows, ver. 2.4; Federal
Institute for Materials Research and Testing: Berlin, Germany, 2000.
(38) Fayon, F.; Farnan, I.; Bessada, C.; Coutures, J.; Massiot, D.; Coutures,
J. P. J. Am. Chem. Soc. 1997, 119, 6837.
Inorganic Chemistry, Vol. 46, No. 21, 2007 8627

Briand et al.
frequency peak of adamantane as a secondary reference (δiso
)
nonrelativistic calculations and ZORA calculations, respectively.
However, the largest basis set available for lead for the nonrela-
tivistic calculations was a frozen-core basis set, where the core was
extended to the 4d shell for lead (Pb‚4d). All of the calculations
were performed using atomic coordinates from single-crystal X-ray
structures. All of the calculations employed a discrete molecular
unit, except for those upon 4, for which a dimer was used.
Calculations on Me4Pb (207Pb NMR standard) were performed upon
atomic coordinates from the previously published low-temperature
(150 K) crystal structure.68
38.56 ppm).44
Solution 207Pb NMR spectra were acquired on a 5 mm HXY
triple resonance Varian/Chemagnetics probe. All of the solid-state
NMR experiments were performed on a double resonance 4 mm
HX Varian/Chemagnetics probe. 1H-207Pb variable-amplitude cross-
polarization/MAS (VACP/MAS)45,46 and cross-polarization/Carr-
Purcell-Meiboom-Gill (CP/CPMG)47,48 experiments were opti-
mized on a sample of lead acetate hydrate [Pb(OAc)2‚xH2O]. All
of the 1H-207Pb CP experiments employed 2.0 µs π/2 proton pulses
and Hartman-Hahn matching fields of approximately 55 kHz.
Individual CP/CPMG subspectra were co-added to form the total
spectrum (Figure S1). Further details on CP/CPMG experiments
are given in the Supporting Information (Table S1 and Figure S2).
Static CP/CPMG 207Pb NMR patterns were simulated using the
WSolids program.49 1H-207Pb VACP/MAS spectra were simulated
with the SIMPSON program.50 1H-13C VACP/MAS spectra of
complexes 2, 4, and 5 are shown in Figure S3. The TPPM
decoupling scheme was employed in all of the experiments.51
DFT Calculations. Theoretical calculations were performed with
the EPR and NMR modules52-54 of the Amsterdam Density
Functional (ADF) program suite55-57 on a dual-2.0 GHz Xeon Dell
Precision 650 workstation. The VWN-BP functional was used for
electron exchange and correlation for all of the calculations.58-61
Relativistic effects (including spin-orbit) were taken into account
with the zeroth-order regular approximation (ZORA).62-66 In the
current version of ADF, analysis of the contributions to CS from
the mixing of molecular orbitals can only be performed for
nonrelativistic calculations. All-electron gauge including atomic
orbitals (GIAO)67 triple-ú singly polarized and triple-ú doubly
polarized basis sets were employed on all of the atoms for the
Results and Discussion
The persistence of a ψ-trigonal bipyramidal S2N2 coordi-
nation geometry for lead(II) in pyridine, bipyridine, and
pyrazine adducts of (2,6-Me2C6H3S)2Pb has prompted us to
study the effect of systematically altering the donor ability
of pyridine on the coordination chemistry at lead(II) in these
systems. Pyridine ligands were chosen because they allow
for the fine-tuning of the basicity of the coordinating nitrogen
atom through the electron withdrawing/donating effects of
the para substituent. Further, their relative electron donor
abilities may be determined from gas-phase and aqueous
proton affinity (PA) data. All of the prior studies yield the
following trend for proton affinity (Lewis basicity) values:
pyNMe2 > pyOMe > py > pyCHO.69-74
Syntheses and Solution NMR. The reaction of 1 with 4
equivs of the appropriate 4-substituted pyridine in toluene
or dichloromethane yielded only 1:1 adducts, namely, 3, 4,
and 5, as crystalline solids in moderate yields. These results
are in contrast to the isolation of the 1:2 adduct (2,6-
Me2C6H3S)2Pb(py)2 (2) for the unsubstituted pyridine ligand.29
In the preparation of 2, however, reaction conditions were
significantly different, with the product being isolated from
neat pyridine. Attempts to obtain any crystalline material
through the reaction of 1 with 4 equivs of pyridine in various
organic solvents were unsuccessful. As was previously
(44) Earl, W. L.; Vanderhart, D. L. J. Magn. Reson. 1982, 48, 35.
(45) Peersen, O. B.; Wu, X. L.; Kustanovich, I.; Smith, S. O. J. Magn.
Reson. Ser. A 1993, 104, 334.
(46) Peersen, O. B.; Wu, X. L.; Smith, S. O. J. Magn. Reson. Ser. A 1994,
106, 127.
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7112.
(48) Siegel, R.; Nakashima, T. T.; Wasylishen, R. E. J. Phys. Chem. B
2004, 108, 2218.
(49) Eichele, K.; Wasylishen, R. E. WSolids: Solid-State NMR Spectrum
Simulation, ver. 1.17.30, 2001.
(50) Bak, M.; Rasmussen, J. T.; Nielsen, N. C. J. Magn. Reson. 2000, 147,
296.
(51) Bennett, A. E.; Rienstra, C. M.; Auger, M.; Lakshmi, K. V.; Griffin,
R. G. J. Chem. Phys. 1995, 103, 6951.
(52) Schreckenbach, G.; Ziegler, T. J. Phys. Chem. 1995, 99, 606.
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Chem. Acc. 1998, 99, 391.
1
observed for 2, solution H and 13C{1H} NMR spectra for
3, 4, and 5 dissolved in thf-d8 have similar resonances to
those of 1 and the corresponding uncomplexed 4-substituted
pyridine ligands. Solution 207Pb NMR spectra of 2, 4, and 5
dissolved in thf confirm this hypothesis (Figure S4). Similar
207Pb NMR resonances suggest that the complexes are labile
in solution and that their structures only persist in the solid
state (Supporting Information).
X-ray Structural Analyses. Crystallographic data is given
in Table 1, and selected bond distances and angles are given
in Table 2. The structure of 3 has a single unique four-
coordinate lead center bonded to two thiolate sulfur atoms
[Pb1-S1 ) 2.6636(16); Pb1-S2 ) 2.6256(17) Å] and a
(56) Velde, G. T.; Bickelhaupt, F. M.; Baerends, E. J.; Guerra, C. F.; Van,
Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001,
22, 931.
(57) ADF2005.01 SCM, Theoretical Chemistry, Vrije Universiteit Am-
sterdam: The Netherlands.
(58) Becke, A. D. Phys. ReV. A 1988, 38, 3098.
(59) Perdew, J. P. Phys. ReV. B 1986, 34, 7406.
(60) Perdew, J. P. Phys. ReV. B 1986, 33, 8822.
(61) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.
(62) Autschbach, J.; Ziegler, T. In Calculation of NMR and EPR Param-
eters; Wiley-VCH: Weinheim, Germany, 2004; p 249.
(63) Autschbach, J. In Calculation of NMR and EPR Parameters; Wiley-
VCH: Weinheim, Germany, 2004; p 227.
(68) Fleischer, H.; Parsons, S.; Pulham, C. R. Acta Crystallogr., Sect. E:
Struct. Rep. Online 2003, 59, M11.
(69) Arnett, E. M.; Chawla, B.; Bell, L.; Taagepera, M.; Hehre, W. J.;
Taft, R. W. J. Am. Chem. Soc. 1977, 99, 5729.
(70) Aue, D. H.; Bowers, M. T. Gas Phase Ion Chem. 1979, 2, 1.
(71) Hopkins, H. P.; Alexander, C. J.; Ali, S. Z. J. Phys. Chem. 1978, 82,
1268.
(64) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993,
99, 4597.
(65) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1994,
101, 9783.
(66) van Lenthe, E.; van Leeuwen, R.; Baerends, E. J.; Snijders, J. G. Int.
J. Quantum Chem. 1996, 57, 281.
(72) Kebarle, P. Annu. ReV. Phys. Chem. 1977, 28, 445.
(73) Taageper, M; Taft, R. W.; Brownlee, R. T.; Beauchamp, J. L.; Holtz,
D.; Henderso,Wg. J. Am. Chem. Soc. 1972, 94, 1369.
(74) Voets, R.; Francois, J. P.; Martin, J. M. L.; Mullens, J.; Yperman, J.;
Vanpoucke, L. C. J. Comput. Chem. 1989, 10, 449.
(67) Ditchfield, R. Mol. Phys. 1974, 27, 789.
8628 Inorganic Chemistry, Vol. 46, No. 21, 2007

Probing Lead(II) Bonding EnWironments
Table 1. Crystallographic Data for 3-5
3
4
5
formula
fw
cryst syst
space group
a (Å)
C44H46N2O2Pb2S4 C44H50N2O2Pb2S4 C23H28N2PbS2
1177.45
orthorhombic
Pbca
1181.48
orthorhombic
Pbca
603.87
monoclinic
C2/c
13.7810(3)
14.5170(4)
21.2790(6)
90
13.5750(1)
14.6870(1)
21.9220(2)
90
28.0430(4)
9.8580(2)
18.8720(4)
90
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
90
90
90
90
116.9971(6)
90
4257.05(19)
4
4370.72(6)
4
4648.61(15)
8
F(000)
2272
2288
1.795
7.923
173(2)
0.71073
0.0325
0.0555
2352
1.725
7.449
173(2)
0.71073
0.0335
0.0620
F
calcd, g cm-3 1.837
µ, mm-1
T, K
λ, Å
R1a
8.134
173(2)
0.71073
0.0406
0.0696
wR2b
a R1 ) [∑||Fo| - |Fc||]/[∑|Fo|] for [Fo2 > 2σ(Fo )]. b wR2 ) {[∑w(Fo
2
2
- Fc )2]/[∑w(Fo )]}1/2
.
2
4
Figure 1. X-ray structure of 3 (30% probability ellipsoids). Hydrogen
atoms are removed for clarity. Symmetry transformations used to generate
equivalent atoms: (*) -x + 1, -y + 1, -z + 1.
Table 2. Selected Bond Distances (Angstroms) and Angles (Degrees)
for 3-5
3
4
5
Pb1-S1
2.6636(16)
2.6256(17)
3.1742(16)
2.571(5)
101.32(5)
87.37(11)
87.86(12)
74.57(5)
2.6481(6)
2.6179(7)
3.2671(6)
2.497(2)
99.66(2)
89.00(5)
88.89(5)
74.957(19)
95.16(2)
163.89(5)
105.043(19)
48.75(9)
2.6211(12)
2.6070(11)
Pb1-S2
Pb1-S1*
Pb1-N1
2.432(3)
87.55(4)
86.69(9)
90.87(9)
S1-Pb1-S2
S1-Pb1-N1
S2-Pb1-N1
S1-Pb1-S1*
S2-Pb1-S1*
N1-Pb1-S1*
Pb1-S1-Pb1*
S1-Pb1-S2-C21
S2-Pb1-S1-C11
94.20(5)
161.89(11)
105.43(5)
-49.2(2)
-156.6(2)
172.91(14)
-121.18(16)
159.09(9)
pyCHO nitrogen atom [Pb1-N1 ) 2.571(5) Å] (Figure 1).
The metal center is also bound to a thiolate sulfur atom of
a neighboring moiety via a long intermolecular Pb‚‚‚S1*
contact [3.1742(16) Å]. The result is a dimeric structure in
which each lead center is in a ψ-trigonal bipyramidal S3N
bonding environment. Here, the datively bonded pyCHO
nitrogen atom and the weakly coordinated thiolate sulfur
atom are in the axial positions, and the two closely bonded
thiolate sulfur atoms are in the equatorial positions. The
remaining equatorial site is presumably occupied by a
sterochemically active lone pair of electrons (vide infra). The
structure is significantly distorted from an idealized geometry
[N1-Pb1-S1* ) 161.89(11); S1-Pb1-S2 ) 101.32(5)°].
Comparison of the equatorial Pb-S bond distances shows
only a small difference between those to the bridging
[Pb1-S1 ) 2.6636(16)] and terminal [Pb1-S2 ) 2.6256-
(17) Å] thiolate sulfur atoms.
The 4-methoxypyridine adduct, 4, crystallizes in the same
space group as 3 (Pbca) and possesses a similar dimeric
structure (Figure 2). As with 3, the single unique lead center
displays a four-coordinate ψ-trigonal bipyramidal S3N bond-
ing environment. The most significant structural changes
include a decrease in the lead-pyridine bond distance
[Pb1-N1 ) 2.497(2) Å] and an increase in the inter-
molecular Pb1‚‚‚S1* bond distance [3.2671(6) Å]. The
Figure 2. X-ray structure of 4 (30% probability ellipsoids). Hydrogen
atoms are removed for clarity. Symmetry transformations used to generate
equivalent atoms: (*) -x + 1, -y + 1, -z + 1.
latter is accompanied by a slight decrease in the Pb1-S1
(Pb1*-S1*) bond distance [2.6481(6) Å]. Bond angles at
the lead(II) center are only slightly different than those in 3
[N1-Pb1-S1* ) 163.89(5); S1-Pb1-S2 ) 99.66(2)°].
The 4-dimethylaminopyridine adduct, 5, has a three-
coordinate lead center, with a pyridine nitrogen atom
[Pb1-N1 ) 2.432(3) Å] and two thiolate sulfur atoms
[Pb1-S1 ) 2.6211(12) and Pb1-S2 ) 2.6070(11) Å]
(Figure 3). The N-Pb-S [86.69(9) and 90.87(9)°] and
S-Pb-S [87.55(4)°] bond angles support a pyramidal
geometry, which may be described as ψ-trigonal pyramidal,
if a basal stereochemically active lone pair is considered (vide
infra). Unlike 3 and 4, compound 5 is monomeric with no
intermolecular Pb‚‚‚S contacts and crystallizes in a different
space group (C2/c). The shortest intermolecular distance is
Inorganic Chemistry, Vol. 46, No. 21, 2007 8629

Briand et al.
Table 3. Experimental 207Pb Chemical Shift Parameters
δ11 δ22 δ33 δiso
Ω
compound (ppm)a (ppm) (ppm) (ppm)b
(ppm)c
κd
2
4
5
4814
4256
4519
3286
4022
4024
114
356
19
2733(5)e 4700(100) 0.35(7)
2873(5) 3900(100) 0.88(10)
2852(5) 4500(100) 0.78(15)
a The chemical shift tensor is described by three principal components
ordered such that δ11 g δ22 g δ33. Shift values for the individual components
are calculated from the values of Ω, κ, and δiso
3. δiso is given relative to (CH3)4Pb [δiso
.
b δiso ) (δ11 + δ22 + δ33)/
(
207Pb) ) 0.0 ppm]. All of the
values are taken from VACP/MAS NMR spectra acquired at a spinning
speed of 5000 Hz. c Ω ) δ11 - δ33. On the basis of simulations of the
static CP/CPMG spectra. d κ ) 3(δ22 - δiso)/Ω, -1.0 e κ e +1.0. On the
basis of simulations of the static CP/CPMG spectra. e The uncertainty in
the last digit of each value is denoted in brackets. Error bounds in the
principle CS tensor components (eg., δ11) are of a similar magnitude to
those of Ω.
Figure 3. X-ray structure of 5 (30% probability ellipsoids). Hydrogen
atoms are removed for clarity.
Chart 2. Schematic Drawing of the Bicyclic Aminoethanethiolate
Complex (H2NCH2CH2S)2Pb
Figure 4. Valence bond descriptions for a) compounds 3 and 4, b)
compound 5, and c) compound 2.
the relative Lewis basicity of the unsubstituted pyridine (vide
supra). This observation is likely a result of the strong trans
influence of the two axial pyridine ligands. The structure
also exhibits shorter equatorial Pb-S bond distances [2.6078-
(9) and 2.6079(9) Å] than observed in compounds 3, 4, and
5.
between neighboring lead atoms [Pb1‚‚‚Pb1* ) 4.249(1) Å]
and is outside the sum of the van der Waals radii (4.0 Å).75,76
The relative Lewis basicities of the pyridine ligands and
a simple valence bond model (Figure 4) may be used to
rationalize the observed structures of 3, 4, and 5. First, it
should be noted that the Pb1-N1 bond distances in the
structures of 3 [2.571(5)], 4 [2.497(2)], and 5 [2.432(3) Å]
decrease as the Lewis basicity of the corresponding 4-sub-
stituted pyridine ligands is increased: pyNMe2 > pyOMe
> pyCHO. Second, the pyridine nitrogen and trans-thiolate
sulfur atoms (N1 and S1*, respectively) in compounds 3 and
4 may be viewed as axially coordinating to the (2,6-
Me2C6H3S)2Pb unit via the empty p orbital on the lead(II)
atom, yielding a three-center four-electron bond and a
ψ-trigonal bipyramidal S3N bonding environment (part a of
Figure 4). As a consequence of the resulting trans influ-
ence, the intermolecular Pb1‚‚‚S1* bond distance increases
[3, 3.1742(16); 4, 3.2671(6) Å] as the Pb1-N1 bond dis-
tance decreases. In compound 5, the Pb1-N1 bond appears
to be strong enough to preclude the formation of a trans
Pb‚‚‚S interaction to the Lewis-acidic lead center, resulting
in a S2N bonding environment and a monomeric structure
(part b of Figure 4). A similar bonding model may also be
used to describe the ψ-trigonal bipyramidal S2N2 structure
of the bis-pyridine adduct 2, which involves the coordina-
tion of two pyridine ligands via an empty 6p orbital of
lead(II) (part c of Figure 4).29 The Pb-N bond distances
[2.695(3) and 2.689(3) Å] are longer than expected, given
Ab initio calculations for the bicyclic aminoethanethiolate
complex (H2NCH2CH2S)2Pb (6), which exhibits a S2N2
ψ-trigonal bipyramidal bonding environment via intramo-
lecular Pb-N interactions (Chart 2), suggest that np(S) and
n(N) compete for interaction with the 6p(Pb) orbital.18
Although the S-Pb-S-C torsion angles in 3 and 4 deviate
significantly from 0 or 180° (Table 2), indicating minimal
π-type np(S)-6p(Pb) interaction, the S1-Pb1-S2-C21 tor-
sion angle of compound 5 [172.91(14)°] is very close to 180°.
This may indicate that such an interaction is significant in
precluding the Pb‚‚‚S* bond formation.
Solid-State 207Pb NMR Spectroscopy. In this section, we
discuss the acquisition and interpretation of solid-state
207Pb NMR spectra of complexes 2, 4, and 5, to examine
the electronic structure, bonding environment at the lead
center, and presence and/or influence of the lone pair of
electrons on the lead CSA. In many cases, it has been
observed that the direct detection of 207Pb NMR spectra in
solids is challenging, because the 207Pb longitudinal relaxation
times (T1) may be very long.38,77-79 To circumvent the long
(77) Grutzner, J. B.; Stewart, K. W.; Wasylishen, R. E.; Lumsden, M. D.;
Dybowski, C.; Beckmann, P. A. J. Am. Chem. Soc. 2001, 123, 7094.
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Phys. Chem. B 1999, 103, 10617.
(75) Bondi, A. J. Phys. Chem. 1964, 68, 441.
(76) Brown, I. D. Chem. Soc. ReV. 1978, 7, 359.
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Perry, D. L. Magn. Reson. Chem. 2006, 44, 357.
8630 Inorganic Chemistry, Vol. 46, No. 21, 2007

Probing Lead(II) Bonding EnWironments
Figure 5. 1H-207Pb VACP/MAS NMR spectra of 2, 4, and 5, each acquired at three spinning speeds (νrot). Dashed lines indicate δiso. A recycle delay of
10 s was used for all of the spectra and 2004 to 8428 scans were acquired. The relative vertical scaling of the spectra is given on the left.
207Pb T1 constants, 1H-207Pb variable-amplitude cross-polari-
zation (VACP/MAS) NMR experiments45,46 were performed
on compounds 2, 4, and 5 (Figure 5). Normally, only two
spinning speeds are required to differentiate peaks as a result
of spinning sidebands from that of the isotropic chemical
at three different spinning speeds (expanded views of the
spectra are given in Figure S5). The isotropic shifts in this
series of complexes are similar to those reported from
solution 207Pb NMR experiments on [(PhS)3Pb][Ph4As], δiso
) 2868 ppm,80 which has an S3 lead coordination environ-
1
shift in the MAS NMR spectra of a spin /2 nucleus. How-
ever, the overlap of spinning sidebands due to large spans
(80) Dean, P. A. W.; Vittal, J. J.; Payne, N. C. Inorg. Chem. 1984, 23,
(Ω, defined in Table 3) requires that the spectrum be acquired
4232.
Inorganic Chemistry, Vol. 46, No. 21, 2007 8631

Briand et al.
ment, but are clearly distinct from the S2N environment in
[PATH-Pb][ClO4], for which δiso ) 2358 ppm.19
breadth of anisotropic chemical shifts (i.e., the size of the
CSA) and the degree of axial symmetry of the CS tensor.
Though the CS tensor parameters are distinct (Table 3), they
do not vary widely in this series of complexes. This suggests
that the orientations of the CS tensors in the molecular frames
are related among the three complexes and that the general
modes of bonding and electronic environments at the lead
centers are very similar. However, there are some interesting
trends in these parameters which can be correlated to
molecular structure and symmetry.
A temperature-dependent chemical shift is observed for
compounds 2 and 4 (Figure S5 and Table S2). This behavior
is well-known and was previously observed in the 207Pb
NMR spectra of Pb(NO3)2,81-85 among other lead-containing
systems. Minor impurity peaks are visible in the VACP/MAS
NMR spectra and are attributed to hydration of the samples
because the MAS spectra were acquired after the static NMR
and powder X-ray diffraction experiments (Figure S6).
Relatively accurate values for δiso (Table 3) can be obtained
from the VACP/MAS spectra; however, the large number
of spinning sidebands and the poor signal-to-noise of the
spectra make extraction of the CS tensor parameters by the
Herzfeld-Berger analysis difficult and potentially inaccurate.
With this in mind, static (i.e., stationary sample) 1H-207Pb
cross-polarization/Carr-Purcell-Meiboom-Gill (CP/CP-
MG) experiments were performed.47,48,86-88 Because of the
large breadth of the lead powder patterns, the entire spectrum
must be acquired in a piecewise frequency offset fashion
(Figure S1), which requires both constant resetting of the
transmitter frequency and probe retuning. The appearance
of spikelets in the frequency domain spectra arise from the
Fourier transform of the CPMG echo train (alternatively and
completely equivalently, one may co-add the echoes and
process the summed spin echo to produce a static pattern).
Acquisition of static CP/CPMG spectra is not hindered by a
First, all three patterns have extremely large spans (Table
3). The Ω values observed for complexes 2 and 5 are several
hundred ppm larger than those observed for yellow lead
oxide (Ω ) 3917 to 3995 ppm) and lead(II) dialkyldithio-
phosphates (Ω ) 3840 ppm), which, to the best of our
knowledge, are the largest spans previously reported for lead
complexes.33,38,78,89 Second, all of the patterns possess positive
skews, denoting δ33 as the distinct component in each case,
and implying that it must be oriented in a distinct/unique
environment within the molecule. Positive skews have
previously been observed in the 207Pb NMR spectra of several
lead(II) complexes, which are also thought to have an
electron lone pair molecular orbital (MO) localized at the
lead centers.33,37,38,78 Because it is well-known that CS tensor
components orient along/near symmetry elements, and by
virtue of the fact that the δ33 is the distinct component in
each complex, it is very likely that δ33 is oriented along/
near the direction of the presumed lone pair of electrons
(contained within the PbS2 plane, Figure 4). Finally, despite
the similarity of the isotropic shifts of 4 and 5, the spectra
of their respective S3N and S2N metal bonding environments
are readily distinguished by the differences in Ω and κ (this
is visually apparent in comparing these spectra as well, Figure
6). With this limited set of structural parameters and NMR
data, two observations can be made: (i) decreased Pb-S
bond lengths are correlated with larger spans and (ii) the
unique four-coordinate S2N2 environment of 2 leads to a less-
axial CS tensor.
Theoretical Calculations of Lead CS Tensors. Ab initio
and/or pure DFT computational methods can be used to
calculate the CS tensor parameters, their orientations within
the molecular frames, and contributions from individual
MOs. Ramsey’s theory of magnetic shielding arbitrarily
decomposes the total magnetic shielding (σ) at a nucleus into
diamagnetic and paramagnetic terms, such that σ ) σd +
σp.90-95 The diamagnetic and paramagnetic terms arise from
the circulation of electrons in ground state MOs and the
mixing of separate MOs, respectively, both of which are
induced by the presence of an external magnetic field. Ziegler
and co-workers have developed a DFT-GIAO formalism for
the calculation of magnetic shielding, which further parti-
1
partial averaging of the H-207Pb dipolar interactions that
would occur under MAS conditions; hence, high signal-to-
noise static patterns can be acquired very rapidly with respect
to VACP/MAS NMR spectra of the same quality. Another
clear advantage of the CP/CPMG experiments over their
VACP/MAS counterparts is the overall excitation of the
207Pb NMR powder patterns. Simulations of the VACP/MAS
spectra that employ the CS tensor parameters extracted from
the CP/CPMG spectra (vide infra) clearly demonstrate that
the former are not uniformly excited and are unsuitable
candidates for the accurate measurement of CS tensor
parameters via Herzfeld-Berger methods (Figure S7). This
is in contrast to direct excitation and detection of broad
207Pb NMR powder patterns. For instance, Antzutkin et al.
recently measured and simulated 207Pb MAS NMR spectra
of lead patterns with spans close to 4000 ppm and breadths
of ca. 300 kHz (acquisition times of ca. 24-48 h).89
Co-addition of the subspectra and simulation of the
observed patterns (Figure 6) yield the lead CS tensor
parameters (Table 3). The δiso is located at the center of
gravity of the powder pattern. The Ω and κ describe the
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8632 Inorganic Chemistry, Vol. 46, No. 21, 2007

Probing Lead(II) Bonding EnWironments
Figure 6. 1H-207Pb CP/CPMG static NMR spectra and WSolids simulations of compounds 2, 4, and 5. The CS tensor parameters obtained from the
simulations can be found in Table 3.
tions the paramagnetic terms into contributions from the
mixing of occupied MOs (σpocc-occ) and the mixing of
occupied and virtual MOs (σpocc-vir),52-54 the details of which
are neatly summarized elsewhere.96 Furthermore, for heavy
nuclei (207Pb, 195Pt, etc.), it is often necessary to include the
relativistic contributions to the total shielding as described
by the spin-orbit coupling term (σso) to improve agreement
between experiment and theory.53,62,63,90,97-103
ZORA64-66 are shown in Table 4. In each case, the ZORA
calculations yield values of κ, which are in good agreement
with experimental values. Theoretical values of Ω are lower
than experimental values in each case, which arise from
consistent underestimation of deshielding along the directions
of σ11 and σ22. The errors in the calculated σ11 and σ22
parameters are unsurprising, given the difficulties associated
with accurately calculating the excited electronic states of
larger molecules containing lead atoms. Because the chemical
shift range of lead is approximately 17 000 ppm, and the
spans of the CS tensors are so large, these errors are relatively
small (ranging from 15 to 30%). The calculations also
qualitatively predict the relative values of δiso (the 207Pb
nucleus in 2 is predicted to be the most shielded and that in
4 is predicted to be the least shielded). While the theoretical
tensors are not identical to the experimental measurements,
they are still extremely useful in understanding the origin
of the lead CS in these systems.
Lead chemical shielding parameters from calculations
performed with the ADF program suite incorporating
(96) Harris, K. J.; Bernard, G. M.; McDonald, C.; McDonald, R.; Ferguson,
M. J.; Wasylishen, R. E. Inorg. Chem. 2006, 45, 2461.
(97) Willans, M. J.; Demko, B. A.; Wasylishen, R. E. Phys. Chem. Chem.
Phys. 2006, 8, 2733.
(98) Bagno, A.; Casella, G.; Saielli, G. J. Chem. Theory Comput. 2006, 2,
37.
(99) Jokisaari, J.; Jarvinen, S.; Autschbach, J.; Ziegler, T. J. Phys. Chem.
A 2002, 106, 9313.
(100) Bryce, D. L.; Wasylishen, R. E.; Autschbach, J.; Ziegler, T. J. Am.
Chem. Soc. 2002, 124, 4894.
(101) Rodriguez-Fortea, A.; Alemany, P.; Ziegler, T. J. Phys. Chem. A
1999, 103, 8288.
(102) Autschbach, J.; Ziegler, T. J. Chem. Phys. 2000, 113, 9410.
(103) Autschbach, J.; Ziegler, T. J. Chem. Phys. 2000, 113, 936.
Contributions to the principal shielding components of the
total shielding tensor have been tabulated for the diamagnetic,
Inorganic Chemistry, Vol. 46, No. 21, 2007 8633

Briand et al.
Table 4. ZORA and Nonrelativistic (NR) ADF Calculations of 207Pb CS Tensor Parameters
shielding
term
σ11
σ22
(ppm)
σ33
(ppm)
σiso
(ppm)
δiso
Ω
(ppm)
complex
method
(ppm)a
(ppm)a
κ
(CH3)4Pb
ZORA
NR
Exptl
ZORA
Total
Total
Totalb
Total
σd
7523.7
5447.1
2717
4252.6
9949.2
-6448.7
752.1
5131.6
10 062.3
-4930.7
3275
4217.4
9948.8
-6501.1
769.7
5146.9
10 058.7
-4911.7
3012
3998.3
9948.5
-6729.2
779.0
7523.8
5453.0
4245
5301.0
9946.1
-5440.1
795.1
5771.9
10 059.5
-4287.6
3509
4341.2
9947.6
-6099.9
493.6
5231.3
10 062.6
-4831.3
3507
4341.1
9947.0
-6339.1
733.2
7546.5
5453.0
7417
7531.0
5451.1
4793
0.0
0.0
2733
1803.1
22.8
5.9
4700
3378.6
-8.8
1680.6
1706.8
1322.3
-10.1
1332.5
3900
3164.9
-7.4
1742.6
1429.7
1240.6
-5.8
1246.4
4500
3780.0
-5.3
1.00
-0.99
0.35
2
4
5
7631.2
9940.4
-4768.1
2458.9
6453.9
10 052.2
-3598.2
7175
7382.2
9941.4
-4758.5
2199.4
6387.5
10 052.9
-3665.3
7512
7778.2
9943.2
-4559.0
2394.0
6473.9
10 056.5
-3582.6
5728.27
9945.2
-5552.3
1335.3
5785.8
10 058.0
-4272.2
4653
5313.6
9945.9
-5786.5
1154.2
5588.6
10 058.0
-4469.5
4677
5372.5
9946.2
-5875.8
1302.1
5492.4
10 059.3
-4566.9
0.38
σp
σso
NR
Total
σd
-334.7
0.03
σp
Exptl
ZORA
Total
Total
σd
2873
2217.8
0.88
0.92
σp
σso
Total
σd
σp
Total
Total
σd
σp
σso
NR
-137.5
0.87
Exptl
ZORA
2852
2158.8
0.78
0.82
2170.2
1615.0
1726.7
-4.3
NR
Total
σd
σp
4747.2
10 060.8
-5313.6
5256.2
10 060.7
-4804.5
-41.3
0.41
1731.0
a The CS tensor is described by three principal components ordered such that σ11 e σ 22 e σ 33 (highest to lowest chemical shift). The principal chemical
shift components (δjj) are related to the principal chemical shielding components (σjj) by the equation δjj ) (σiso, ref - σjj)(106)/(1 - σiso, ref) ≈ σiso,ref - σjj,
where jj ) 11, 22, or 33 and σiso,ref refers to the isotropic shielding value of the reference compound (σiso,ref ) σiso(Me4Pb)). b The experimental shift values
have been converted to shielding values by subtracting them from the ZORA-calculated value of σiso(Me4Pb); σjj ) 7531.0 - δjj.
Figure 7. Lead CS tensor orientations for complexes 2, 4, and 5. The tensor orientations have been generated from the ZORA calculations.
paramagnetic, and spin-orbit terms for each compound
(Table 4 and Supporting Information). The diamagnetic terms
for each compound are highly isotropic (small Ω values)
and do not contribute to the large shielding anisotropies. The
paramagnetic term makes much larger contributions to the
isotropic shielding values than the spin-orbit term, but both
terms contribute relatively equally to the spans of the CS
tensors. Comparison of the ZORA and nonrelativistic (NR)
calculations makes it very clear that the inclusion of
relativistic effects for the calculations of CS tensors for heavy
nuclei is absolutely necessary.
can be rationalized by considering the orientation of the CS
tensor in the molecular frame (Table S3). The lead CS tensor
orientations generated from the ZORA calculations are
presented in Figure 7 for complexes 2, 4, and 5. In all three
systems, the σ33 component is oriented in the presumed
direction of the stereochemically active lone pair and close
to the plane of the S-Pb-S bonding arrangement. Con-
versely, the σ11 and σ22 components are not directed along
Pb-S or Pb-N bonds and are oriented as such because of
large paramagnetic deshielding contributions arising from
the mixing of occupied MOs localized on lead, sulfur,
and nitrogen atoms with an assortment of low-lying virtual
MOs.
The nature of the CS tensor (i.e., the large span and the
positive skews) and its relationship to molecular structure
8634 Inorganic Chemistry, Vol. 46, No. 21, 2007

Probing Lead(II) Bonding EnWironments
Table 5. Contributions to Paramagnetic Shielding from Mixing of occ-vir MOs in 2
σ
iso from occ
Sum of σiso
(ppm)
major occ-
vir Pairs
σ
iso of MO
pair (ppm)
σ11
(ppm)
σ22
(ppm)
σ33
(ppm)
occupied MO
MO (ppm)a
1 to 120
121
122
-332.7
-399.0
-323.8
-332.7
-731.7
-1055.5
122-138
122-143
Sum
-81.6
-76.4
-158.0
-11.5
-27.6
-39.1
-219.6
-19.8
-239.4
-13.6
-181.8
-195.4
123 and 124
125
126
127 to 129
130
-6.3
-428.2
-467.5
-3.1
-1061.7
-1489.9
-1957.4
-1960.6
-2854.3
125-143
126-143
-135.6
-124.8
-10.8
-6.7
-389.4
-318.0
-65.2
8.8
-894.7
130-138
130-139
130-144
Sum
131-138
131-139
131-140
131-143
Sum
132-138
132-142
132-143
Sum
133-139
133-143
Sum
-451.4
-78.3
-468.9
12.9
-54.4
-510.4
54.1
78.5
-3.2
-314.5
-185.1
-105.5
-86.1
172.6
-19.0
45.5
-1210.8
-1165.3
-2247.4
-816.9
-16.2
-156.4
-989.5
-281.7
546.9
-68.3
-231.5
-41.4
-341.2
-38.2
-27.8
63.8
-582.5
-584.7
-31.5
-169.2
376.6
175.9
-25.8
66.9
41.1
-1284.9
-84.0
-613.7
-88.6
131
-461.8
-3316.0
199.2
-84.8
-315.0
126.2
-256.9
-231.1
-451.2
-78.9
76.4
132
-240.6
-308.5
-3556.6
-3865.1
-1216.4
18.3
231.7
-298.4
430.4
-611.8
-181.4
-1743.0
145.8
-1052.3
1271.4
-691.5
579.9
133 (HOMO)
Total
-1696.8
a This column corresponds to the sum of the isotropic shieldings of the occ-vir MO pairs that contain the specified occ MO.
Our hypotheses regarding both the large spans and the high
positive skews and their relation to molecular structure can
be confirmed by analyzing the contributions made by
individual MOs to the paramagnetic shielding terms. Analy-
ses of the contributions of particular MOs to paramagnetic
shielding in simple molecular systems have previously been
demonstrated using both the Gaussian software package104-106
and the ADF software.52,96,107,108 The NMR/EPR module of
the ADF program suite outputs the paramagnetic shielding
contributions arising from mixing of individual occupied-
virtual (occ-vir) and occupied-occupied (occ-occ) MO
pairs. However, this analysis is only available for NR
calculations in the current implementation of ADF. Despite
the fact that the NR calculations show poor quantitative
agreement with experiment, the paramagnetic contributions
and CS tensor orientations are very similar to those generated
from ZORA calculations (Figure S8). Hence, it is reasonable
to examine individual contributions from MO pairs to the
paramagnetic terms of the lead CS tensor.
Figure 8. The occupied and virtual MOs of 2 that make significant
contributions to the paramagnetic shielding term. A partial MO diagram
indicating the magnetic dipole allowed transitions between the occupied
and virtual orbitals that are pictured, is also shown. The MOs are visualized
at the 97% electron density level.
As a result of the relatively high local symmetry at the
lead atom in complex 2, we have chosen this as the focus of
our discussion of MO contributions to magnetic shielding
(a full MO analysis for all of the systems is beyond the scope
of the current article). The contributions from the mixing of
occupied (occ) and virtual (vir) MOs in complex 2 are
summarized in Table 5 (these MOs are depicted in Figures
8 and S9) and account for upward of 90% of the total
isotropic shielding contribution from the paramagnetic term.
Therefore, occ-occ mixing will not be considered. As a
result of the enormous number of MO pairs, only those
contributing greater than 2% (∼77 ppm) of the total isotropic
shielding contribution of occ-vir MO mixing (-3865.1
ppm) are listed. In this way, analysis of MO contributions
to paramagnetic shielding can be focused upon a select few
MOs that are near the HOMO and/or involved in metal-
(104) Wiberg, K. B.; Hammer, J. D.; Keith, T. A.; Zilm, K. J. Phys. Chem.
A 1999, 103, 21.
(105) Wiberg, K. B.; Hammer, J. D.; Zilm, K. W.; Cheeseman, J. R. J.
Org. Chem. 1999, 64, 6394.
(106) Wiberg, K. B.; Hammer, J. D.; Zilm, K. W.; Cheeseman, J. R.; Keith,
T. A. J. Phys. Chem. A 1998, 102, 8766.
(107) Forgeron, M. A. M.; Wasylishen, R. E. J. Am. Chem. Soc. 2006,
128, 7817.
(108) Feindel, K. W.; Ooms, K. J.; Wasylishen, R. E. Phys. Chem. Chem.
Phys. 2007, 9, 1226.
Inorganic Chemistry, Vol. 46, No. 21, 2007 8635

Briand et al.
ligand bonding (e.g., MO pairs involving occ MOs from 1
to 120 contribute to only 8.6% of the occ-vir paramagnetic
contribution).
Let us consider a specific example to clarify the data in
Table 5. The mixing of the occ MO 130 (HOMO -3) with
virtual orbitals contributes -894.7 to the total paramagnetic
shielding (σp) of -4272.2 ppm. Mixing of occ MO 130 with
vir MO 138 (denoted as 130-138) accounts for -451.4 of
the -894.7 ppm contributed by occ MO 130. The remaining
contribution to shielding from occ MO 130 results from its
mixing with many other virtual MOs, most notably vir MOs
139 and 144 (an extended data analysis including all of the
occ-vir contributions can be found in Table S4).
The largest contributions to paramagnetic shielding arise
from the mixing of a variety of occ MOs with vir MOs 138,
139, and 143 (Table 5). These virtual MOs are of high lead
p orbital character, as demonstrated by the gross populations
contributed by individual AOs (Table S5). MO 138 has
relatively high lead 6py and slight 6s characters but also has
significant contributions from carbon 2p orbitals on the
neighboring aromatic rings. MO 139 and MO 143 are
comprised of mixtures of large lead 6px and 6pz AO
contributions. The occupied MOs which mix with these three
virtual MOs vary in their relative amounts of lead, sulfur,
nitrogen, or carbon AO character. Large deshielding contri-
butions involve occ MOs 130 to 133 (HOMO -3 to HOMO),
and occ MOs 121, 122, 125, and 126. MO 133 corresponds
to the stereochemically active lead lone pair, whereas MOs
130, 131, and 132 are largely localized on the sulfur atoms.
MO 121 describes Pb-N bonding, MO 122 describes S-C
σ bonding, MO 125 describes Pb-S bonding and S-C π
bonding, and MO 126 describes S-C π bonding and some
degree of localized nitrogen 2p character. Hence, the origin
of the lead CS tensor is relatively complex and highly
dependent upon the characteristics of the lead lone pair, 3p
AOs localized on sulfur atoms, and variable types of σ and
π bonding.
Understanding why certain MO pairs make large para-
magnetic deshielding contributions can be accomplished
through relatively simple visualization. MOs that contribute
to shielding along the direction of a particular principal
component must be relatively close in energy but also have
the appropriate symmetries to interfere or overlap with one
another when induced to mix by a magnetic field. This type
of mixing can be visualized by rotating the appropriate MOs
about the axes that define the principal components of the
CS tensor. This concept has previously been demonstrated
for several relatively simple systems.52,105,106,108,109 We will
use the arbitrary convention of right-handed 90 degree
rotations of the virtual MOs about their gauge origin (i.e.,
contributing AOs are rotated in a right-handed fashion at
the nuclear site). Rotations resulting in the constructive or
destructive overlap of MO lobes generate deshielding and
shielding contributions, respectively.106 The simplest ex-
amples to consider are the 133-143 and 133-139 MO pairs
Figure 9. A representation of the rotation of vir MOs 139 and 143 of
complex 2. The axes of rotation correspond to the directions of σ11 and σ22
in the molecular frame. Only lead, sulfur, and nitrogen atoms are shown.
(Figure 9). The 133-143 MO pair makes significant
deshielding contributions along σ11 and σ22. A right-handed
rotation of 143 about the σ11 axis results in a constructive
overlap with 133, and similarly, the same rotation about σ22
leads to a constructive overlap, but to a lesser extent (and
hence the smaller deshielding contribution). In the case of
the 133-139 MO pair, rotation about σ11 does not generate
any significant overlap, and the deshielding contribution is
minimal. On the other hand, right-handed rotation about σ22
causes a large destructive overlap (i.e., lobes of opposite
phases are overlapped), leading to a large shielding contribu-
tion. All of the other shielding and deshielding contributions
can be rationalized and visualized in this manner, by simple
rotations of the virtual orbitals.
Conclusions
We have studied the effects of varying the σ-electron donor
ability of pyridine Lewis-base ligands on the coordination
environment of lead(II) in bis-thiolate complexes. The
reaction of (2,6-Me2C6H3S)2Pb with a series of 4-substituted
pyridines in toluene or dichloromethane led to the isolation
1
of a series of 1:1 complexes. Solution H, 13C{1H}, and
207Pb NMR spectra suggest that the coordination complexes
are unstable in thf solutions and may only be studied in the
solid state. X-ray crystallographic analysis shows compounds
3 and 4 to be dimeric, with a ψ-trigonal bipyramidal S3N
bonding environment for lead(II), whereas compound 5 is
monomeric and exhibits a ψ-trigonal pyramidal S2N bonding
environment at lead(II). The structures may be rationalized
using a simple bonding description, in which the axial lead-
nitrogen and long intermolecular lead-sulfur dative bonding
interactions occur via the empty 6p orbitals on lead(II). The
lead-nitrogen bond distances were found to shorten with
increasing σ-electron donor ability of the 4-substituted
pyridine ligand. This affect was accompanied by a lengthen-
ing of the trans intermolecular lead-sulfur contact, which
was absent for the strongest pyridine donor pyNMe2 in
compound 5. This study demonstrates that novel bonding
environments for lead(II) may be accessed through subtle
alterations of Lewis-base donor ligands.
(109) Grutzner, J. B. In Recent AdVances in Organic NMR Spectroscopy;
Lead CS tensor parameters have been measured from
207Pb CP/CPMG solid-state NMR spectra for complexes 2,
Lambert, J. B., Rittner, R., Eds.; Norell Press: Landisville, 1987, p
17.
8636 Inorganic Chemistry, Vol. 46, No. 21, 2007

Probing Lead(II) Bonding EnWironments
4, and 5. The CP/CPMG sequence has been demonstrated
to be much more efficient and accurate than MAS methods
for the acquisition of the 207Pb spectra of extremely broad,
CSA-dominated powder patterns. Similar lead isotropic
chemical shifts are observed within this series of compounds;
however, CS tensor parameters are useful for differentiating
the three types of lead coordination environments. DFT
calculations that account for relativistic effects adequately
reproduce the experimental CS tensor parameters and trends.
Examination of the individual MOs generated by nonrela-
tivistic calculations provides insight into the molecular
origins of lead CS and evidence for the existence and position
of the stereochemically active lone electron pair. Experi-
mental measurement and theoretical calculations of lead CS
tensor parameters are clearly useful for the elucidation of
molecular and electronic structure, bonding, and symmetry.
data. R.W.S. thanks NSERC, the Canadian Foundation for
Innovation (CFI), the Ontario Innovation Trust (OIT), and
the Centre for Catalysis and Materials Research (CCMR) at
the University of Windsor for funding. A.J.R. thanks the
Ontario Ministry of Training, Colleges and Universities for
an Ontario Graduate Scholarship. Mr. Cory M. Widdifield
and Mr. Joel A. Tang are thanked for assistance with ADF
calculations.
Supporting Information Available: X-ray crystallographic data
for 3-5 in CIF format, powder X-ray diffraction patterns for
compounds 2, 4, and 5, additional 207Pb and 13C NMR data, and
coordinates and energies for ab initio/DFT calculations where
appropriate. This material is available free of charge via the Internet
at http://pubs.acs.org. Crystallographic data for all of the structures
in this article have been deposited with the Cambridge Crystal-
lographic Data Centre, under CCDC 643476 (3), 643477 (4), and
643478 (5). Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, U.K. (fax, +44-1223-336033; e-mail, deposit@ccdc.cam.ac.uk;
web, http://www.ccdc.cam.ac.uk).
Acknowledgment. G.G.B. thanks the Natural Sciences
and Engineering Research Council (NSERC) of Canada, the
New Brunswick Innovation Foundation (NBIF), and Mount
Allison University for financial support. G.G.B. also thanks
Mr. Dan Durant for assistance in collecting solution NMR
IC700738W
Inorganic Chemistry, Vol. 46, No. 21, 2007 8637