Structural variation in ethylenediamine and -diphosphine adducts of (2,6-Me2C6H3S)2Pb: A single crystal X-ray diffraction and 207Pb solid-state NMR spectroscopy study

Article abstract of DOI:10.1039/c3dt33070b

Coordination complexes of (2,6-Me₂C₆H ₃S)₂Pb (1) with flexible bidentate ligands have been prepared to explore new bonding environments for Pb(ii) thiolates. The reaction of 1 with a series of ethylenediamine and ethylenediphosphine ligands resulted in isolation of the adducts [(2,6-Me₂C₆H ₃S)₂Pb]₂(tmeda) (9), [(2,6-Me₂C ₆H₃S)₂Pb]₃(dmpe) (10) and [(2,6-Me₂C₆H₃S)₂Pb] ₂(dppe) (11) [tmeda = N,N,N′,N′- tetramethylethylenediamine; dmpe = bis(dimethylphosphino)ethane; dppe = bis(diphenylphosphino)ethane]. The X-ray crystal structure of 9 shows a dinuclear species in which tmeda is chelating a ψ-trigonal bipyramidal S₂N₂ Pb centre via axial and equatorial sites. The structure of 10 displays a trinuclear structural unit in which dmpe is chelating a ψ-trigonal bipyramidal S₂P₂ Pb centre via equatorial sites. Compounds 9 and 10 also contain a second unique metal centre with ψ-tetrahedral S₃Pb bonding motifs. The structure of 11 shows the dppe ligand bridging two Pb ψ-tetrahedral S₂P metal bonding environments. Static ²⁰⁷Pb solid-state NMR (SSNMR) spectra of 9-11 and [Ph₄As][(PhS)₃Pb] (12) were acquired with cross polarization (CP)-CPMG and frequency swept pulse (WURST)-CPMG pulse sequences, and the efficiencies of these pulse sequences are compared. The ²⁰⁷Pb SSNMR spectra reveal that the lead chemical shift anisotropies (CSA) vary greatly between the different Pb sites, and are generally large in magnitude. DFT calculations are utilized to relate the orientations of the ²⁰⁷Pb nuclear magnetic shielding tensors to the molecular structures, and to aid in spectral assignment where multiple Pb centres are present. The combination of X-ray diffraction, ²⁰⁷Pb SSNMR and DFT is shown to be invaluable for the structural characterization of these important structural motifs, and should find wide-ranging application to numerous lead coordination compounds. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt33070b

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Structural variation in ethylenediamine and
-diphosphine adducts of (2,6-Me2C6H3S)2Pb: a single
crystal X-ray diffraction and 207Pb solid-state NMR
spectroscopy study†
Cite this: Dalton Trans., 2013, 42, 9533
Aaron J. Rossini,a Alan W. Macgregor,a Anita S. Smith,b Gabriele Schatte,c
Robert W. Schurko*a and Glen G. Briand*b
Coordination complexes of (2,6-Me2C6H3S)2Pb (1) with flexible bidentate ligands have been prepared to
explore new bonding environments for Pb(II) thiolates. The reaction of 1 with a series of ethylenediamine
and ethylenediphosphine ligands resulted in isolation of the adducts [(2,6-Me2C6H3S)2Pb]2(tmeda) (9),
[(2,6-Me2C6H3S)2Pb]3(dmpe) (10) and [(2,6-Me2C6H3S)2Pb]2(dppe) (11) [tmeda = N,N,N’,N’-tetramethyl-
ethylenediamine; dmpe = bis(dimethylphosphino)ethane; dppe = bis(diphenylphosphino)ethane]. The
X-ray crystal structure of 9 shows a dinuclear species in which tmeda is chelating a ψ-trigonal bipyramidal
S2N2 Pb centre via axial and equatorial sites. The structure of 10 displays a trinuclear structural unit in
which dmpe is chelating a ψ-trigonal bipyramidal S2P2 Pb centre via equatorial sites. Compounds 9 and
10 also contain a second unique metal centre with ψ-tetrahedral S3Pb bonding motifs. The structure of
11 shows the dppe ligand bridging two Pb ψ-tetrahedral S2P metal bonding environments. Static 207Pb
solid-state NMR (SSNMR) spectra of 9–11 and [Ph4As][(PhS)3Pb] (12) were acquired with cross polariz-
ation (CP)-CPMG and frequency swept pulse (WURST)-CPMG pulse sequences, and the efficiencies of
these pulse sequences are compared. The 207Pb SSNMR spectra reveal that the lead chemical shift aniso-
tropies (CSA) vary greatly between the different Pb sites, and are generally large in magnitude. DFT calcu-
lations are utilized to relate the orientations of the 207Pb nuclear magnetic shielding tensors to the
molecular structures, and to aid in spectral assignment where multiple Pb centres are present. The combi-
nation of X-ray diffraction, 207Pb SSNMR and DFT is shown to be invaluable for the structural characteriz-
ation of these important structural motifs, and should find wide-ranging application to numerous lead
coordination compounds.
Received 21st December 2012,
Accepted 25th April 2013
DOI: 10.1039/c3dt33070b
www.rsc.org/dalton
study over the past number of years.1–6 This is a result of the
exploitation of these interactions for the self-assembly of
Introduction
The study of weak dative bonding interactions and metal supramolecular structures with useful physical properties.
bonding environments has emerged as an important field of Heavy p-block metals, such as Pb(II), are potentially diverse
coordination centers for such systems.7–13 For example, the
large atomic radius of Pb(II) allows high coordination numbers
for the element, while the number of possible bonding
environments is augmented by the variable stereochemical
activity of its valence lone pair.11,13–16 The preference for high
coordination numbers typically manifests itself in the for-
mation of polymeric species, as is observed in the solid-state
aDepartment of Chemistry and Biochemistry, University of Windsor, Windsor,
Ontario, Canada N9B 3P4. E-mail: rschurko@uwindsor.ca; Fax: +1 (519) 973-7098;
Tel: +1 (519) 253-3000 ext 3548
bSaskatchewan Structural Sciences Centre, University of Saskatchewan, Saskatoon,
Saskatchewan, Canada S7N 5C9. E-mail: gbriand@mta.ca; Fax: +1 (506) 364-2313;
Tel: +1 (506) 364-2346
cDepartment of Chemistry and Biochemistry, Mount Allison University, Sackville,
structures of the Pb(II) thiolates [(RS)2Pb]4 (R
= C6H5,
4-MeC6H4, CH2)17–20 via extensive intermolecular Pb⋯S
interactions.21–24 Unfortunately, this results in inherent insolu-
bility in these compounds,25 which has delayed the develop-
ment of their structural chemistry.
New Brunswick, Canada E4L 1G8
†Electronic supplementary information (ESI) available: X-ray crystallographic
data for 9–11 in CIF format, complete details of 207Pb SSNMR experiments,
experimental and simulated MAS 207Pb SSNMR spectra of 9 and 12 and a
powder X-ray diffraction pattern of 10. CCDC 827117 (9), 827118 (10) and 827119
(11). For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt33070b
In light of this, we previously reported the preparation of
the 2,6-dimethylphenylthiolate (2,6-Me2C6H3S)2Pb (1), which
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shows an increased solubility in organic solvents over the single crystals isolated for characterization by X-ray diffraction
alkyl- and unsubstituted arylthiolate (RS)2Pb analogues.26 techniques. We have previously applied 207Pb SSNMR to
Reaction of 1 with pyridine (py) yielded the 1 : 2 adduct (2,6- characterize complexes 2, 7 and 8.29 With the aid of DFT calcu-
Me2C6H3S)2Pb(py)2 (2), which is monomeric and exhibits no lations of lead magnetic shielding (MS) tensors we have pre-
intermolecular Pb–S contacts.26 Further, bridging Lewis base viously demonstrated that the large lead CSAs arise from the
ligands 4,4′-bipyridyl (bipy) and pyrazine (pyr) afforded the presence of stereochemically active electron lone pairs, and
1 : 1 coordination polymers [(2,6-Me2C6H3S)2Pb(L)]4 [L = bipy that the lead MS tensor orientations are highly dependent
(3), pyr (4)].26 Interestingly, all complexes show a trigonal upon the orientation of the lone pair with respect to the mole-
bipyramidal S2N2 coordination geometry, with the thiolate cular frame. While this combined strategy represents a power-
sulfur atoms occupying equatorial positions (Chart 1). The ful probe of structure for lead complexes, acquisition of solid-
axial amine nitrogen atoms are involved in dative bonding state 207Pb NMR spectra is often very challenging because of
interactions with the [(2,6-Me2C6H3S)2Pb] unit, presumably the CSA induced broadening of the 207Pb powder patterns
through the empty valence p-orbital of the Pb(II) centre. A which often exceeds 2000 ppm. For example, previous spectral
similar bonding situation has also been described for acquisitions were often very time-consuming (>24 hours),
(H2NCH2CH2S)2Pb (5), which involves intramolecular S–N despite being acquired with a 1H–207Pb cross polarization/
bonding interactions via chelating aminoethane thiolate Carr-Purcell Meiboom Gill (CP/CPMG) pulse sequence for
ligands.27,28
signal enhancement. For complexes lacking protons these
To further manipulate the lead bonding environments, we issues could be exacerbated since it would not be possible to
have also previously investigated the influence of systemati- apply cross polarization and the longitudinal relaxation times
cally altering the Lewis basicity of the coordinating pyridine of lead (and other heavy element nuclides) are highly variable
ligands.29 This resulted in the isolation of dimeric [(2,6- (1 s < T1(207Pb) < 160 s).31,40–42 One possibility for accelerating
Me2C6H3S)2Pb(pyCHO)]2 (6) and [(2,6-Me2C6H3S)2Pb(pyOMe)]2 acquisition of 207Pb solid-state NMR spectra would be to apply
(7), which feature novel S3N ψ-trigonal bipyramidal coordi- the wideband uniform rate smooth truncation (WURST)-CPMG
nation environments, and monomeric [(2,6-Me2C6H3S)2Pb- pulse sequence, possibly in combination with frequency-
(pyNMe)] (8), which features a S2N ψ-tetrahedral bonding stepped acquisitions (i.e., variable-offset cumulative spectra,
environment (pyCHO = 4-pyridinecarboxaldehyde, pyOMe = VOCS).43–45
4-methoxypyridine, pyNMe2 = 4-dimethylaminopyridine). Like
In order to impose dative bonding at equatorial coordi-
2–5, dative Pb–N (and Pb–S) bonding interactions in 6 and 7 nation sites, adducts of 1 which incorporate flexible bidentate
also occur at axial coordination sites, with the thiolate sulfur ethylenediamine and diphosphine ligands have been pre-
atoms occupying equatorial positions.30
pared. This has resulted in isolation of the complexes [(2,6-
Owing to the insolubility of many of these lead thiolate Me2C6H3S)2Pb]2(tmeda) (9), [(2,6-Me2C6H3S)2Pb]3(dmpe) (10)
complexes, 207Pb solid-state NMR (SSNMR) has proven to be and [(2,6-Me2C6H3S)2Pb]2(dppe) (11) [tmeda = N,N,N′,N′-tetra-
useful for their structural characterization. It is well known methylethylenediamine; dmpe
=
bis(dimethylphosphino)-
that 207Pb SSNMR is a useful probe of molecular and electronic ethane; dppe = bis(diphenylphosphino)ethane]. The various
structure for lead containing materials,29,31–39 and is also very novel Pb(II) bonding environments and structural motifs of
useful for confirming that the bulk sample is representative of 9–11 have been probed by X-ray crystallographic analysis, 207Pb
Chart 1 Schematic drawings of the solid-state structures of complexes 2–11.
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solid-state NMR and DFT calculations. 207Pb SSNMR spectra of allowed to stand at 4 °C. After 1 d, the reaction was filtered to
9–11 and [Ph4As][(PhS)3Pb] (12) were acquired in a piecewise yield yellow crystals of 10 (0.077 g, 11%). Anal. Calcd for [(2,6-
fashion with the CP/CPMG pulse sequence. The Me2C6H3S)2Pb]3(dmpe): C, 40.63; H, 4.43; N, 0.00. Found: C,
WURST-CPMG pulse sequence was also used to obtain the 41.37; H, 4.66; N, <0.20%. Mp 152–155 °C (d). FT-IR (cm−1):
ultra-wideline (UW) 207Pb SSNMR spectra in a single experi- 584 vw, 762 s, 891 w, 945 s, 1049 m, 1157, w, 1302 w, 1558 vw,
ment. Total experiment times for spectra acquired with CP/ 2605 vw, 2665 w, 2725 m, 3190 w. NMR data (dmso-d6):
CPMG and WURST-QCPMG are compared in order to deter- 1H NMR, δ = 1.16 [s, 12H, (Me2P)2C2H4], 1.53 [m, 4H,
3
mine the most efficient method for acquiring UW 207Pb (Me2P)2C2H4], 2.42 (s, 36H, C6H3Me2), 6.69 (t, JHH = 7 Hz, 6H,
3
SSNMR spectra for these systems. DFT calculations were p-C6H3Me2), 6.95 (d, JHH = 7 Hz, 12H, m-C6H3Me2); 13C{1H}
carried out in order to determine the MS tensor orientations NMR,
δ = 13.24 [(Me2P)2C2H4], 24.37 (C6H3Me2), 27.34
and relate the experimentally observed CS parameters to the [(Me2P)2C2H4], 124.01 (C6H3Me2), 126.97 (C6H3Me2), 141.41
molecular structures.
(C6H3Me2), 142.34 (C6H3Me2); 31P{1H} NMR, δ = −34.47.
Preparation of [(2,6-Me2C6H3S)2Pb]2(dppe) (11). A solution
of dppe (0.167 g, 0.402 mmol) in toluene (2 mL) was added to
a slurry of (0.100 g, 0.208 mmol) in toluene (3 mL) to give a
cloudy orange solution. After 15 min, the red-orange solution
was filtered and left at 4 °C. After 3 d, the reaction mixture was
Experimental
General considerations
2,6-Dimethylbenzenethiol 95%, 1,2-bis(diphenylphosphino)- filtered to yield small yellow crystalline blocks of 11 (0.077 g,
ethane 97%, 1,2-bis(dimethylphosphino)ethane 97% and N,N, 31%). Anal. Calcd for [(2,6-Me2C6H3S)2Pb]2(dppe): C, 58.91; H,
N′,N′-tetramethylethylenediamine 99.5%+ (redistilled) were 5.13; N, 0.00. Found: C, 54.53; H, 4.54; N, <0.05%. Mp
used as received from Aldrich. Lead(II) acetate trihydrate was 150–152 °C (d). FT-IR (cm−1): 503 m, 570 w, 692 s, 733 s, 771 s,
used as received from Fisher. (2,6-Me2C6H3S)2Pb (1)22,23 and 847 vw, 1024 w, 1051 m, 1095 w, 1159 w, 1273 w, 1309 w,
[Ph4As][(PhS)3Pb] (12)46 were prepared according to previously 2669 w, 2723 w, 3043 w. NMR data (dmso-d6): 1H NMR, δ =
reported procedures. All reactions were carried out under an 2.03 [m, 4H, (Ph2P)2C2H4)], 2.43 (s, 24H, C6H3Me2), 6.69 (t,
3
atmosphere of dinitrogen using standard Schlenk techniques.
3JHH = 7 Hz, 4H, p-C6H3Me2), 6.98 (d, JHH = 7 Hz, 8H,
Solution 1H, 13C{1H} and 31P{1H} NMR spectra were m-C6H3Me2), 7.29 [m, 20H, (Ph2P)2C2H4]; 13C{1H} NMR, δ =
recorded on a JEOL GMX 270 MHz spectrometer (270.2, 67.9 23.78 [(Ph2P)2C2H4], 24.34 (C6H3Me2), 123.99 (C6H3Me2),
and 109 MHz, respectively) or a Varian MERCURY 200 MHz + 126.92 (C6H3Me2), 129.31 [(Ph2P)2C2H4], 133.01 [(Ph2P)2C2H4],
spectrometer (200.0, 50.3 and 81.0 MHz, respectively), and 138.36 [(Ph2P)2C2H4], 141.33 (C6H3Me2), 142.35 (C6H3Me2); 31P{1H}
chemical shifts are calibrated to the residual solvent signals. NMR, δ = −13.40.
FT-IR spectra were recorded as Nujol mulls on a Mattson
X-ray structural analysis
Genesis II FT-IR spectrometer in the range 4000–400 cm−1
.
Melting points were recorded on an Electrothermal Crystals of 9–11 were isolated from the reaction mixtures as
MEL-TEMP melting point apparatus and are uncorrected. indicated above. Single crystals of each compound were coated
Elemental analyses were performed by Chemisar Laboratories with Paratone-N oil, mounted using a CryoLoop (Hampton
Inc., Guelph, Ontario.
Research) and frozen in the cold stream of the goniometer.
Preparation of [(2,6-Me2C6H3S)2Pb]2(tmeda) (9). Tmeda Data were measured on a Bruker AXS/Nonius KappaCCD
(0.099 g, 0.83 mmol) was added dropwise to a yellow slurry of 4-Circle Kappa FR540C diffractometer using monochromated
1 (0.200 g, 0.415 mmol) in toluene (3 mL). After 10 min, the Mo Kα radiation (l = 0.71073 Å) at −100 °C. Data were collected
reaction mixture was filtered, layered with hexane (1 mL) and using φ and/or ω scans.47 Data reduction was performed with
allowed to stand at 4 °C. After 3 d, the reaction was filtered to the HKL DENZO and SCALEPACK software, which corrects for
yield yellow crystals of 9 (0.188 g, 39%). Anal. Calcd for [(2,6- beam inhomogeneity, possible crystal decay, Lorentz and
Me2C6H3S)2Pb]2(tmeda): C, 46.13; H, 5.17; N, 2.39. Found: C, polarisation effects. A multi-scan absorption correction was
41.55; H, 5.05; N, 2.59%. Mp 144–146 °C (d). FT-IR (cm−1): applied (SCALEPACK).48 Transmission coefficients were calcu-
586 w, 669 m, 758 s, 787 m, 947 m, 1022 m, 1051 s, 1160 w, lated using SHELXL97-2.49 The structures were solved by direct
1244 vw, 1286 w, 1527 m, 1579 w, 2723 w, 3045 m. NMR data methods (9, 11) or the heavy atom method (10) (SHELXS-97)50
(dmso-d6): 1H NMR, δ = 2.27 [s, 12H, (Me2N)2C2H4], 2.40 (s, and refined by full-matrix least squares on F2
3
24H, C6H3Me2), 2.51 [s, 4H, (Me2N)2C2H4], 6.68 (t, JHH = 7 Hz, (SHELXL-2012).49 The non-hydrogen atoms were refined aniso-
4H, p-C6H3Me2), 6.95 (d, 3JHH = 7 Hz, 8H, m-C6H3Me2); 13C{1H} tropically. Hydrogen atoms were included at geometrically
NMR,
[(Me2N)2C2H4], 123.78 (C6H3Me2), 126.81 (C6H3Me2), 141.43 not refined. The isotropic thermal parameters of the hydrogen
(C6H3Me2), 142.19 (C6H3Me2). atoms were fixed at 1.2 times that of the preceding carbon
δ = 24.19 (C6H3Me2), 46.09 [(Me2N)2C2H4], 57.74 idealized positions (C–H bond distances 0.95/0.99 Å) and were
Preparation of [(2,6-Me2C6H3S)2Pb]3(dmpe) (10). Dmpe atom.
(0.128 g, 0.852 mmol) was added dropwise to a slurry of 1
In the structure of 9 the carbon atoms of the CH2-groups
(0.194 g, 0.123 mmol) in toluene (3.5 mL). After 10 min, the [labelled C(1A), C(2A), C(1B) and C(2B)] in the TMEDA ligand
reaction mixture was filtered, layered with hexane (2 mL) and were disordered over two positions with refined site-occupancy
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factors of 0.620(14) and 0.380(14), respectively. In the structure structure for which a quadruple-ζ doubly polarized basis set
of (10) approximately 23% of the unit cell volume (1576 Å3) was employed. All calculations were carried out with complete
comprises a region of disordered solvent which could not be molecular units (except for those mentioned below) using
modeled as discrete atomic sites. The program PLATON/ atomic coordinates from single-crystal X-ray experiments,
SQUEEZE51 was used to calculate the contribution from the however, the position of H atoms were corrected by setting
solvent region to the diffraction. A total electron count of 148 C–H bond lengths to more realistic values of 1.09 Å. For 12, the
electrons was found in the total solvent-accessible void volume Ph4As+ cation was excluded and calculations were performed on
within the unit cell. The electron count of 74 e−/void corres- a single negatively charged (PhS)3Pb unit. For 11, only one Pb
ponds to 1.5 molecules of toluene in each void. Toluene was unit was employed for calculations. This was accomplished by
used as a solvent for growing the crystals. The data were then replacing the second phosphine atom with a hydrogen atom
corrected for disordered electron density leading to a set of and removing the other lead and thiolate ligand atoms.
solvent-free diffraction intensities. The modified data
improved the R-factors (before SQUEEZE: R1 = 0.0361, wR2
=
0.1009; after SQUEEZE: R1 = 0.0316, wR2 = 0.0743). Derived
values (formula weight, density, absorption coefficient) do not
contain the contribution of the disordered solvent. The carbon
atoms [labelled C(41A), C(42A), C(43B), C(41B), C(42B) and
C(43B)] in the dmpe ligand of 10 were disordered over two
sites. The site-occupancy factors were allowed to refine and
converged to 0.65 and 0.35, respectively.
Results and discussion
The persistence of a ψ-trigonal bipyramidal S2N2 coordination
geometry for Pb(II) in which py (2), bipy (3) and pyr (4) adducts
of (2,6-Me2C6H3S)2Pb occupy trans axial coordination sites26–28
has prompted us to study the effect of tethering the donor
atoms via a flexible backbone. This allows for the possibility of
chelation of the metal centre via cis-coordination sites and the
isolation of novel Pb(II) bonding environments. Ethylenedi-
amine and diphosphine ligands were chosen as they allow for
the formation of favourable five-membered chelate rings.27,28
Further, structurally characterized adducts of neutral dipho-
sphine ligands with Pb(II) centers have not been previously
reported.
Solid-state NMR
Solid-state 207Pb spectra were acquired on a Varian Infinity
Plus spectrometer with an Oxford 9.4 T wide-bore magnet
(v0 (1H) = 400 MHz, v0 207Pb) = 83.74 MHz). Lead chemical
(
shifts were referenced to tetramethyllead (Pb(CH3)4, δiso
0.0 ppm) by using a 0.5 M aqueous solution of Pb(NO3)2 (δiso
−2941 ppm) as a secondary standard.38
=
=
Syntheses
Solid-state NMR experiments were carried out using either a
4 mm HX double resonance Varian/Chemagnetics probe or a
4 mm HXY triple resonance Varian/Chemagnetics probe. Lead
acetate hydrate [Pb(OAc)2·xH2O] was used to optimize para-
meters for cross polarization/Carr-Purcell-Meiboom-Gill (CP/
CPMG)32,52 and WURST-CPMG53,54 experiments. The total
spectrum for each CP/CPMG experiment was obtained by co-
adding individual sub-spectra that were collected at evenly
spaced transmitter offsets of 20 or 24 kHz. Other experimental
details regarding CP/CPMG experiments can be found in the
ESI (Table S1†). TPPM 1H decoupling was used for all
experiments.55 WURST-CPMG experiments were able to be
completed without piecewise acquisition and employed 50 μs
WURST pulses with a pulse power of ca. 47 kHz. Further
experimental details regarding WURST-CPMG experiments
also may be found in the ESI.† All 207Pb spectra were simulated
using the WSolids NMR simulation program,56 and processed
with the NUTS computer program (Acorn NMR).
Reaction of (2,6-Me2C6H3S)2Pb (1) with an excess of the appro-
priate ethylenediamine or diphosphine ligand in toluene
yielded products which exhibit varying metal–ligand ratios.
Reaction with two equivalents of tmeda yielded the 2 : 1
adduct [(2,6-Me2C6H3S)2Pb]2(tmeda) (9), while reaction with
seven equivalents of the diphosphine analogue dmpe resulted
in isolation of the 3 : 1 complex [(2,6-Me2C6H3S)2Pb]3(dmpe)
(10). Finally, reaction with two equivalents of the bis(diphenyl-
phosphine) analogue yielded the 2 : 1 adduct [(2,6-
Me2C6H3S)2Pb]2(dppe) (11). All products were isolated in low
to moderate yields, and were the only crystalline materials iso-
lated from a number of reactions involving various (2,6-
Me2C6H3S)2Pb : ligand ratios. Although satisfactory elemental
analysis could not be obtained for compounds 9 and 11, the
products were spectroscopically and structurally characterized
(vide infra).
Solution NMR studies
DFT calculations
Solution 1H and 13C{1H} NMR spectra of adducts 9–11 in
The NMR and EPR modules57–59 of the Amsterdam Density dmso-d6 generally show similar resonances to those observed
Functional (ADF) program package60–62 running on the Shared for 126 and the corresponding ethylenediamine/-diphosphine
Hierarchical Academic Research Computing Network (SHARC- ligands in the same solvent. Interestingly, the tmeda (9: δ 2.27
1
NET) were used to carry out all theoretical calculations. Relati- and 2.51) and dmpe (10: δ 1.16 and 1.53) H NMR signals in
vistic effects were accounted for using the zeroth-order regular these complexes show slight shifts from those of the respective
approximation (ZORA).63–67 All-electron gauge including uncomplexed ligands (tmeda: δ 2.11 and 2.27; dmpe: δ 0.96
atomic orbitals (GIAO)68 triple-ζ doubly-polarized basis sets and 1.36). The 31P{1H} NMR spectrum of 10 also shows a shift
were used for all atoms, except for the Pb atom(s) in each in the phosphorus signal (δ −33.88) from that of the free dmpe
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ligand (δ −47.55). This suggests weak Pb–N and Pb–P inter-
actions in dmso-d6 solution for 9 and 10, respectively. Interest-
ingly, the solution 1H, 13C{1H } and 31P{1H } NMR spectra of 11
show no shifts from those of the free dppe ligand in dmso-d6.
These data are consistent with previous observations for 2–8,
in which solution 1H, 13C and 207Pb NMR data showed the com-
plexes to be quite labile in solution.26,29 The structures, there-
fore, do not persist and can be studied only in the solid state.
Table 3 Selected bond distances (Å) and angles (°) for 10
10
Pb1–P1
Pb1–S1
Pb2–S1
Pb2–S2
Pb2–S3
2.8389(10)
2.9869(9)
2.7385(9)
2.6430(12)
2.6839(10)
P1–Pb1–P1*
S1–Pb1–S1*
S1–Pb1–P1
S1–Pb1–P1*
S1–Pb2–S2
S1–Pb2–S3
S2–Pb2–S3
73.49(5)
154.15(4)
77.91(3)
81.42(3)
84.99(4)
84.38(3)
95.43(4)
X-ray structural analyses
Crystals suitable for X-ray crystallographic analysis were iso-
lated for 9–11 by the slow evaporation of reaction mixtures at
23 °C. Crystallographic data is given in Table 1. Selected bond
distances and angles are given in Tables 2–4.
The X-ray crystal structure of 9 (Fig. 1) shows a dinuclear
unit containing two unique Pb centers. Pb1 is four coordinate,
Table 4 Selected bond distances (Å) and angles (°) for 11
11
Table 1 Crystallographic data for 9–11
Pb1–S1
Pb1–S2
Pb1–P1
2.6079(13)
2.6171(11)
2.8161(11)
9·C7H8
10
11
Formula
FW
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C45H60N2Pb2S4
1171.57
C54H70P2Pb3S6
1594.97
Monoclinic
C2/c [No. 15]
20.2754(2)
20.1016(2)
18.9169(2)
90
114.6800(6)
90
7005.65(13)
4
C58H60P2Pb2S4
1181.48
S1–Pb1–S2
S2–Pb1–P1
S1–Pb1–P1
96.86(4)
86.84(3)
85.43(4)
Triclinic
Triclinic
ˉ
ˉ
P1 [No. 2]
P1 [No. 2]
11.9125(1)
14.0434(2)
14.9513(2)
99.7056(8)
109.47205(8)
90.8606(8)
2317.58(5)
2
10.0551(2)
11.3774(2)
12.8875(3)
87.0455(9)
71.5348(10)
81.6016(7)
1383.42(5)
1
α (°)
β (°)
γ (°)
V (Å3)
Z
F(000)
1144
1.679
7.467
173(2)
3064
1.512
7.444
173(2)
666
1.634
6.321
173(2)
ρcalcd (g cm−3
)
μ (mm−1
T (K)
)
Λ (Å)
0.71073
0.0236
0.0506
0.71073
0.0316
0.0743
0.71073
0.0381
0.0766
a
R1
b
wR2
a R1 = [Σ||Fo| − |Fc||]/[Σ|Fo|] for [Fo2 > 2σ(Fo2)]. b wR2 = {[Σw(Fo2 − Fc2)2]/
[Σw(Fo4)]}2.
Table 2 Selected bond distances (Å) and angles (°) for 9·C7H8
9·C7H8
Pb1–N1
Pb1–N2
Pb1–S1
Pb1–S2
Pb2–S2
Pb2–S3
Pb2–S4
2.455(2)
2.632(3)
2.5880(8)
3.0276(8)
2.7027(8)
2.6172(8)
2.6257(8)
Fig. 1 X-ray structure of 9 (30% probability ellipsoids). Hydrogen atoms are
removed for clarity.
and is bonded to two nitrogen atoms of a chelating tmeda
ligand [Pb1–N1 2.455(2) and Pb1–N2 2.632(3) Å] and two thio-
late sulfur atoms [Pb1–S1 2.5880(8) and Pb1–S2 3.0276(8) Å]. If
the stereochemically active lone pair is considered, the result-
ing coordination geometry at Pb1 is distorted ψ-trigonal bipyr-
amidal. Here, both N1 and S1 occupy equatorial positions [S1–
Pb1–N1 = 89.57°], while N2 and S2 occupy the axial positions
[S2–Pb1–N2, 154.91(7)°]. The Pb1–N2 bond distance is signifi-
cantly larger than that of Pb1–N1, which is presumably a result
N1–Pb1–N2
S1–Pb1–S2
S1–Pb1–N1
S1–Pb1– N2
S2–Pb1–N1
S2–Pb1–N2
S2–Pb2–S3
S4–Pb2–S2
S3–Pb2–S4
71.94(9)
97.12(2)
89.57(7)
93.16(7)
85.28(6)
154.91(7)
99.62(2)
89.42(2)
96.07(3)
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of the trans influence imposed by the S2 thiolate sulfur atoms [Pb1–S1 2.9870(9) Å)]. The Pb1–P1 bond distance is in
atom.29,69 The Pb1–N1 bond distance is shorter than those the range of those previously reported for complexes involving
observed in previously reported tmeda adducts of Pb(II) [Pb–N monodentate phosphine ligands [2.758(2) and 2.884(3) Å].74 If
2.608(44)–2.879(3) Å)],70–72 while the Pb1–N2 bond distance is a stereochemically active lone pair is considered, the resulting
similar. Pb2 is bonded to two terminal thiolate sulfur atoms geometry at Pb1 is distorted ψ-trigonal bipyramidal, with the
via short Pb–S bonds [2.6172(8) and 2.6257(8) Å], which are two phosphorus atoms occupying equatorial positions [P1–
comparable to those Pb–S distances observed in the Pb1–P1* 73.48(5)°] and the two sulfur atoms occupying axial
[(PhS)3Pb]− anion (12, Chart 2) [2.619(1)–2.696(3) Å]46,73 sites [S1–Pb1–S1* 154.14(4)°]. As in compound 9, Pb2 is co-
(vide infra). A third thiolate sulfur atom is more weakly bonded ordinated by three thiolate sulfur atoms, two of which are
[Pb2–S2 2.7027(8) Å], and bridges the two unique Pb centres. terminal [Pb2–S2 2.6428(12) and Pb2–S3 2.6840(10) Å] and one
This results in a pyramidal coordination geometry for Pb2, of which bridges Pb1 and Pb2 [Pb2–S1 2.7387(9) Å]. The bond
which may be described as distorted ψ-tetrahedral if a stereo- angles at Pb2 support a distorted ψ-tetrahedral geometry with
chemically active lone pair is considered. Finally, 9 crystallizes a stereochemically active lone pair.
with a non-coordinated toluene molecule in the lattice.
The X-ray structure of 11 (Fig. 3) depicts a dinuclear unit
The X-ray structure of compound 10 (Fig. 2) depicts a tri- with one crystallographically unique Pb center. Pb1 is bonded
nuclear unit with two unique Pb centers. Pb1 is four coordi- to two thiolate sulfur atoms [Pb–S1 2.6079(13) and Pb–S2
nate, and is bonded by two phosphorus atoms of the chelating 2.6171(11) Å] and one phosphorus atom of the bridging dppe
dmpe ligand [Pb1–P1 2.8386(10) Å] and two thiolate sulfur ligand [Pb1–P1 2.8161(11) Å]. This results in a three-coordinate
pyramidal geometry at Pb1, which may be described as dis-
torted ψ-tetrahedral if the stereochemically active lone pair is
considered.
Although a comparison of 9 and 10 shows their overall
dinuclear and trinuclear structures to be quite different, both
exhibit two crystallographically unique Pb centers. Pb1 of both
complexes shows a distorted ψ-trigonal bipyramidal coordi-
nation geometry, where the metal center is complexed by
either a chelating tmeda (9) or dmpe (10) ligand and two thio-
Chart 2 Schematic drawing of the lead coordination environment of
complex 12.
late sulfur atoms. In 9, however, the atoms of the chelating
Fig. 2 X-ray structure of 10 (30% probability ellipsoids). Hydrogen atoms are
removed for clarity. Symmetry transformations used to generate equivalent
atoms: (*) −x + 1, y, −z + 0.5.
Fig. 3 X-ray structure of 11 (30% probability ellipsoids). Hydrogen atoms are
removed for clarity. Symmetry transformations used to generate equivalent
atoms: (*) −x + 1, −y + 1, −z.
9538 | Dalton Trans., 2013, 42, 9533–9546
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sideband intensities which strongly influence the outcomes of
Herzfeld–Berger analyses. Hence, all 207Pb SSNMR spectra
were acquired from stationary samples in a piecewise fashion
using the CP/CPMG and WURST-CPMG pulse sequences.
We have recently demonstrated that the WURST-CPMG
sequence can be utilized to acquire UW SSNMR spectra of
spin-half nuclei with fewer sub-spectra, or in some cases, with
a single experiment.75 In line with these previous observations,
it is possible to acquire UW SSNMR spectra of complexes 9–12
in a single WURST-CPMG experiment, although long recycle
delays are required (90 s) compared to corresponding CP
experiments (20–45 s). For comparison, all of the 207Pb SSNMR
spectra are pictured in Fig. 5 and total times for WURST-CPMG
and CP/CPMG experiments are listed in Table 5. The
207Pb SSNMR static spectra of 11 and 12 were acquired
more quickly with the WURST-CPMG pulse sequence
than with the CP/CPMG sequence. Although the CP/CPMG
pulse sequence yields sub-spectra with superior signal to noise
and requires shorter recycle delays, the WURST-CPMG
sequence permits the acquisition of the total SSNMR spectrum
in a single experiment. In contrast, for 9 and 10 it was found
that CP/CPMG experiments were faster than WURST-CPMG
experiments. This is due to a combination of increased
1H–207Pb CP efficiency, decreased 1H longitudinal relaxation
times [T1(1H)] and/or increased 207Pb longitudinal relaxation
times [T1(207Pb)] for 9 and 10 as compared to 11 and 12.
However, we note that even for cases where WURST-CPMG is
less efficient than CP/CPMG, the former eliminates the need
for piecewise experiments, and the requirement that personnel
be present to set the transmitter and retune the probe between
acquisitions of sub-spectra.
It is possible to extract the CS tensor parameters, which
include the isotropic shift (δiso), the span (Ω) and the skew (κ)
(see Table 6 for definitions of these parameters in terms of
principal components of the CS tensor), by simulating the
static 207Pb SSNMR powder patterns. δiso represents
the average chemical shift value that would be observed if the
molecule was rapidly tumbling in an isotropic fashion (δiso is
normally what is measured in solution NMR experiments). Ω
indicates the breadth of the chemical shift (CS) pattern in
ppm, and reports the magnitude of the CSA. Finally, κ is a
dimensionless parameter which indicates the degree of axial
symmetry of the CS tensor (−1 < κ < +1, where κ = +1 and −1
represent the two extremes of axial symmetry). In general, δiso
of all of the compounds are similar, in spite of the large
Fig. 4 Novel bonding environments for Pb(II) exhibited by compounds (a) 9,
(b) 10 and (c) 11.
ligand occupy an axial and an equatorial site, whereas in 10
they occupy two equatorial positions. In our previous work
with Pb(II) thiolate amine adducts (2–4), the amine ligands
occupied axial position of the ψ-trigonal bipyramidal bonding
environment in each case.26 Therefore, use of bidentate
ligands with flexible ethylene backbones has allowed for equa-
torial dative bonding interactions and isolation of novel
coordination environments at the Pb(II) thiolate center (Fig. 4a
and b). Interestingly, the Pb2 center in 9 and 10 shows a three
coordinate pyramidal geometry similar to that previously
observed in the [Ph4As]+ and [n-Pr4N]+ salts of the monomeric
tris-thiolate anion [(PhS)3Pb]−.46,73 Unlike the monomeric
anion [Ph4As][(PhS)3Pb] (12), however, both 9 and 10 exhibit
one longer Pb–Sbridging bonding interaction, although the
Pb2–Sbridging bond is significantly shorter than the
Pb1–Sbridging interaction in both structures.
In order to further our investigation of the dative phos-
phine–Pb(II) interaction, dppe was employed to determine the
effect of changing the size and electron donor abilities of
the phosphine substituents. Although dppe and dmpe share
the same PCH2CH2P backbone, the dppe ligand in 11 bridges
two metal centers, while dmpe chelates a single Pb atom in 10.
This is possibly a result of steric crowding by the bulkier
phenyl substituents on the phosphorus atoms in 11, as indi-
cated by the orientation of the bulky 2,6-dimethylphenylthio-
late ligands away from these groups. The Pb1–P1 distance in
10 [2.8386(10) Å] was found to be significantly longer than that
in 11 [2.8161(11) Å], while a similar observation was made
regarding Pb1–S bond distances [10: 2.9870(9) Å; 11:
2.6079(13) and 2.6171(11) Å]. This is presumed to be mainly a
result of the four- versus three-coordinate Pb bonding environ-
ments in 10 and 11, respectively. As in 9 and 10, the distorted
ψ-tetrahedral S2P motif in 11 represents a novel bonding
environment for Pb(II) (Fig. 4c).
Solid-state 207Pb NMR spectroscopy
In this section, we examine the 207Pb SSNMR spectra of 9, 10, chemical shift range of lead, which is on the order of
11 and 12. Before the spectra of the individual complexes and 18 000 ppm.31 The exception is the δiso for the PbP2S2 site of
relations to molecular structure are discussed, strategies for 10, which is distinct from those of the other complexes. Posi-
acquiring the UW 207Pb SSNMR spectra are briefly summar- tive κ values and large values of Ω are observed for all of the
ized. 1H–207Pb CP/MAS experiments conducted on 12 and 9 compounds (Ω > 3400 ppm, except for the PbP2S2 site of 10),
yielded incompletely excited spectra, similar to those pre- consistent with those previously observed by our research
sented in our previous work on analogous systems (Fig. S1†).29 group for analogous lead(II) thiolate complexes (2, 7 and 8).29
CP/MAS spectra are useful for obtaining accurate measure-
The 207Pb SSNMR spectra of 12 are discussed first, as this
ments of 207Pb isotropic chemical shifts (δiso); however, they complex contains a single Pb site in a ψ-tetrahedral PbS3
are unsuitable for extracting anisotropic CS tensor parameters, environment. Similar PbS3 sites are also observed in binuclear
due to non-uniform excitation and variation in spinning 9 and trinuclear 10. The static 207Pb spectra of 12 have a
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Fig. 5 1H–207Pb CP/CPMG (top spectra: A, C, E and G) and 207Pb WURST-QCPMG (bottom spectra: B, D, F and H) SSNMR spectra of 12, 11, 9 and 10. Analytical
simulations (solid red traces) are overlaid on the experimental spectra (black traces). For 9 simulations of the two overlapping individual sites are also shown (inlaid,
blue and green traces). 207Pb CS tensor parameters obtained from the simulations are listed in Table 6.
The positive value of κ indicates that δ33 is the distinct com-
Table 5 Comparison of experimental times between CP/CPMG and
ponent of the CS tensor (i.e., δ11 and δ22 are similar in magni-
tude). Based on the site symmetry, and previous studies of
lead coordination compounds by our group and others,29,35 it
is predicted that δ33, which corresponds to the direction of
highest magnetic shielding, is aligned along or near the direc-
tion of the electron lone pair. The CS tensor parameters, as
well as the orientation of the CS tensor in the molecular
frame, are discussed in detail in the theoretical section below.
Compound 11 features a single Pb site which corresponds
to a distorted ψ-tetrahedral PbS2P environment. The 207Pb
NMR spectra of 11 reveal a span of 4400 ppm and skew of
0.48. This span is larger than that measured for the PbS3 sites
herein, and similar in magnitude to spans measured for other
PbS2En (E = P or N, n = 1 or 2) environments (i.e., 2, 8 and site
1 of 9).29 As in the case of 12, the skew again indicates that δ33
is the distinct principal component, which is likely oriented
along or near the direction of the Pb lone pair.
WURST-CPMG
9
10
11
12
Compound
Pulse sequence
CP/CPMG
Total experiment time (h)
Number of sub-spectra
Transients per sub-spectrum
Pulse delay (s)
20.3
19
192
20
22
22
120
30
23
23
120
30
32
20
128
45
Pulse sequence
WURST-CPMG
Total experiment time (h)
Number of sub-spectra
Transients per sub-spectrum
Recycle delaya (s)
41.6
1
1664
90
60
1
2400
90
20.2
1
808
90
19
1
759
90
a Recycle delays were not optimized for each complex because of the
lengthy experimental times required to obtain spectra of reasonable
signal to noise.
31P MAS SSNMR spectra were also acquired for 11 (Fig. S2†),
breadth of ca. 280 kHz (Fig. 5). Simulation of a CSA powder in the hope of observing indirect spin–spin coupling between
pattern which fits the outer manifold of the spikelet spectrum 207Pb and neighboring 31P nuclei. A single resonance with
yields the 207Pb chemical shift tensor parameters, δiso
=
δiso = 17(1) ppm is observed. Since the natural abundance of
2750 ppm, Ω = 3400 ppm and κ = 0.37. The value of Ω observed 207Pb is 22.6%, the coupling is manifested in the form of two
for 12 is smaller than those previously observed for the ana- satellite peaks, comprising a doublet of ca. 22.6% of the total
logous thiolate complexes 2, 7 and 8 which feature PbS2Nn (n = integrated intensity, with 1J (207Pb, 31P) = 1895(100) Hz. This
1 or 2) coordination environments for which Ω > 3900 ppm.29 coupling is not visible in the 207Pb CP/CPMG NMR spectrum,
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Table 6 Experimental 207Pb chemical shift tensor parametersa
Compound (lead environment)
δisoc (ppm)
Ωd (ppm)
κe
0.35(7)
0.88(10)
0.78(15)
0.37(5)
0.48(5)
−0.06(10)
0.60(5)
δ11 (ppm)
δ22 (ppm)
δ33 (ppm)
2 (PbS2N2)b
2733(5)f
2873(5)
2852(5)
2750(20)
2610(40)
2706(20)
3120(20)
−167(50)
3040(100)
4700(100)
3900(100)
4500(100)
3400(70)
4400(100)
3600(300)
3650(200)
1450(100)
3580(200)
4814
4256
4519
4262
4458
4542
4580
473
3286
4022
4024
3176
3314
2634
3850
2
114
356
19
812
58
942
930
−977
773
7 (PbS3N)b
8 (PbS2N)b
12 (PbS3)
11 (PbS2P)
9 Site 1 (PbS2N2)
9 Site 2 (PbS3)
10 Site 1 (PbP2S2)
10 Site 2 (PbS3)
0.35(5)
0.80(5)
4353
3995
a The principal components of the chemical shift tensor are defined as δ11 > δ22 > δ33, where δ11 and δ33 are oriented along the directions of
c
lowest and highest shielding, respectively. b CS tensor parameters were previously measured in ref. 29. δiso = (δ11 + δ22 + δ33)/3. Values are with
respect to neat (CH3)4Pb (δiso = 0.0 ppm). d Ω = (δ11 − δ33), based upon simulations of static CP/CPMG spectra. e κ = 3(δ22 − δiso)/Ω, −1.0 < κ < +1.0.
f Values in parentheses denote the uncertainty in the last digit of each parameter where indicated.
due to its small magnitude relative to the lead CSA (Ω = PbS3 environments, respectively, in a 1 : 2 ratio. The CS tensor
4400 ppm or ca. 370 kHz) and the inherently low resolution of parameters of site 2 (ψ-tetrahedral, PbS3) are Ω = 3580 ppm
the CPMG spectra. It is possible that this coupling might be and κ = 0.80, and are similar to the PbS3 sites of 12 and 9. As
observed in 207Pb MAS or CP/MAS NMR spectra; unfortunately, in the cases above, δ33 is the distinct component of the CS
these experiments were unsuccessful (refer to ESI†) due to tensor, and is likely oriented along the pseudo three-fold
lengthy 207Pb T1 constants and inefficient CP at even very slow rotational symmetry axis of the PbS3 unit. The CS tensor para-
spinning rates.
meters of site 1 (PbP2S2) are Ω = 1470 ppm, which is much
Two distinct but overlapping powder patterns are observed smaller than those of any of the other systems, and κ = 0.35,
in the 1H–207Pb CP/CPMG and 207Pb WURST-CPMG NMR which again indicates that δ33 is the distinct component. For
spectra of compound 9 (Fig. 5), which is consistent with its site 1, δiso = −167 ppm, which is much lower than δiso values
crystal structure. DFT calculations (vide infra) were used to observed for all of the other complexes. The disparate CS
assign the experimental CS tensors to a crystallographic site. tensor parameters are most likely observed for site 1 due to the
Site 1 of 9 has a four-coordinate PbS2N2 environment, with a absence of short Pb–S contacts (Pb–S distances of 2.99 Å) and
distorted ψ-trigonal bipyramidal geometry (n.b., the bidentate the presence of short Pb–P contacts (Pb–P distances of 2.84 Å).
tmeda ligand binds to Pb in both equatorial and axial posi- Prediction of the orientation of the CS tensor for site 1 is
tions). Accordingly, this unique lead coordination environ- difficult because of the unique coordination environment of
ment possesses markedly different CS tensor parameters, with the Pb centre; however, theoretically calculated tensor orien-
Ω = 4050 ppm and κ = 0.07 (though the isotropic shift is still in tations are discussed below.
the same region as those of all of the other sites). The value of
Ω observed for site 1 of 9 is slightly less than that previously
observed for the analogous complex, 2, which features a
Calculation of lead magnetic shielding (MS) tensors with
density functional theory
similar PbS2N2 environment where pyridine ligands bind
exclusively in axial positions (Ω = 4700 ppm).29 The value of κ
for site 1 indicates that all three principal components of the
CS tensor are significantly different from one another, making
a prediction of the CS tensor orientation based on molecular
symmetry alone more difficult. We also note that the poorly
resolved low-frequency edge of the spectrum leads to a signifi-
cant error range in the reported values of the CS tensor
element corresponding to the direction of highest magnetic
shielding, δ33. Site 2 (distorted ψ-tetrahedral, PbS3) has Ω =
3600 ppm, similar to the PbS3 sites of 10 and 12, and κ = 0.63,
which is intermediate to the κ values of the PbS3 sites of 10
and 12. δ33 is again the distinct component of the CS tensor,
and is likely oriented in a similar manner to that described for
10 and 12.
Calculations of NMR parameters using density functional
theory (DFT) allow for the correlation of observed NMR para-
meters to molecular structure, as well as for the exploration of
the origins of nuclear magnetic shielding interactions. It is
crucial to develop such understandings for unique coordi-
nation environments such as the ones described herein, since
this will afford rapid structural interpretation for many current
and future systems for which crystallographic data is unavail-
able (i.e., sub-microcrystalline and disordered systems). Fur-
thermore, in compounds containing multiple sites, NMR
parameters gleaned from DFT calculations can aid in the
assignment of the spectra. In cases where MAS NMR experi-
ments are impractical, these calculations are an invaluable
tool for making spectral assignments. Since the current work
deals with Pb chemical shifts, the zeroth-order relativistic
approximation (ZORA)65–67 was employed for all DFT calcu-
lations. This methodology is essential for systems containing
heavy atoms, as relativistic effects on the MS tensor become
pronounced for heavy nuclei.63,64,76–79
The 1H–207Pb CP/CPMG and 207Pb WURST-CPMG NMR
spectra of 10 show the presence of two well separated and
resolved CSA patterns. There are two crystallographically
unique Pb sites, sites 1 and 2, which correspond to PbP2S2 and
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Table 7 Theoretical and experimental MS and CS tensor parameters
σ11 (ppm)
σ22 (ppm)
σ33 (ppm)
σiso (ppm)
δisoa (ppm)
Ω (ppm)
κ
12 (PbS3)
Paramagnetic
Diamagnetic
Spin–orbit
Total
−7494
9956
1029
3785
−6071
9959
1211
4931
−5545
9964
2788
7082
−6370
9960
1676
5266
1948
8
1760
3297
3400
−0.46
0.27
0.79
0.31
0.37
2799
2750
Experimental
11 (PbS2P)
Paramagnetic
Diamagnetic
Spin–orbit
Total
−7969
9957
1070
3140
−5990
9961
1255
5239
−4455
9963
3146
8560
−6138
9960
1824
5646
3514
6
2076
5419
4400
−0.13
−0.21
0.82
0.23
0.48
2419
2610
Experimental
9 Site 1 (PbS2N2)
Paramagnetic
Diamagnetic
Spin–orbit
Total
Experimental
−8199
9950
838
−5995
9956
1204
5170
−4126
9959
3233
9027
−6107
9955
1758
5806
4073
8
2396
6406
3600
−0.08
−0.32
0.69
0.20
−0.06
2621
2459
2706
9 Site 2 (PbS3)
Paramagnetic
Diamagnetic
Spin–orbit
Total
Experimental
−7050
9956
764
−6660
9961
1129
4452
−5092
9962
2855
7556
−6267
9960
1583
5275
1958
6
2091
3740
3650
0.60
−0.76
0.65
0.66
0.60
3816
2790
3120
10 Site 1 (PbP2S2)
Paramagnetic
Diamagnetic
Spin–orbit
Total
Experimental
−4537
9952
3013
8564
−4400
9953
3172
8636
−3756
9964
3322
9482
−4231
9956
3169
8894
781
12
309
919
1450
0.65
0.87
−0.03
0.84
−829
−167
0.35
10 Site 2 (PbS3)
Paramagnetic
Diamagnetic
Spin–orbit
Total
Experimental
−7825
9957
610
−6637
9961
873
−5117
9964
2736
7501
−6527
9960
1406
4840
2708
7
2125
4489
3580
0.12
−0.05
0.75
0.56
0.80
3012
4006
3225
3040
a The principal magnetic shielding tensor components are related to the principal chemical shift tensor components by the equation δii = σref
−
σii, where σref corresponds to the calculated 207Pb isotropic shielding value of tetramethyllead (PbMe4, σref = 8064.8). All calculated chemical shift
tensor parameters were determined using the formulas provided in Table 6.
The theoretical 207Pb MS tensor parameters for 12 and site shielded) principal component, is aligned along the direction
1 of 9 (Table 7) agree well with experimentally determined of the lone pair, similar to results reported previously.29,35
results, which is gratifying, given the challenging nature of σ11 and σ22 are directed into unique environments, with σ11
quantum chemical calculations involving heavy atoms such as aligned near a local pseudo-mirror plane, which explains why
lead. For the remaining Pb sites, Ω is generally overestimated, the MS tensor is not axially symmetric (i.e., the skew is not
except for site 1 of 10 where it is slightly underestimated. close to +1). Compound 11 also contains a single, three coordi-
Calculated values of κ generally agree well with experimental nate Pb site (PbS2P), which is in an environment of lower sym-
results, with the exception of 11. Despite the vast chemical metry than that of compound 12, due to the distinct Pb–P
shift range of 207Pb, predicted values of δiso agree well (i.e., bond which is significantly longer than the Pb–S bonds. In
deviate by ca. 250 ppm or less) with those observed experimen- this case, σ33 is aligned close to the plane of the two Pb–S
tally, with the exception of site 1 of 10 and, to a lesser extent, bonds (Table 7) in the presumed direction of the electron lone
site 2 of 9 (conversion of the MS parameters to CS parameters pair.
is described in the footnotes of Table 7).
Two distinct Pb sites are present in 9: one four-coordinate
The theoretical lead MS tensor orientations are depicted in environment (PbS2N2, site 1), and one three-coordinate
Fig. 6. 12 possesses a distorted PbS3 ψ-tetrahedral coordi- environment (PbS3, site 2). Site 2, much like compound 12,
nation environment. As predicted from our experimental data, has a distorted ψ-tetrahedral coordination geometry; however,
the MS tensor is oriented such that σ33, the distinct (and most one of the three S atoms has a longer Pb–S bond than the
9542 | Dalton Trans., 2013, 42, 9533–9546
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Fig. 6 Lead MS tensor orientations obtained from ZORA-DFT calculations for complexes 9–12. Angles describing the orientation of principal tensor components
with respect to atoms and bond angles are provided in Table S4.†
other two, due to interactions with an adjacent Pb atom. As a
In comparing the Pb MS tensor parameters and orien-
result, there is a reduced local symmetry compared to 12, and tations, some interesting trends emerge. First, consider the
σ33 is not clearly aligned along a particular symmetry element. PbS3 environments in compounds 12, 9 and 10. Close examin-
The coordination sphere of site 1 of 9 contains two nitrogen ation of the Pb–S bond lengths and S–Pb–S bond angles indi-
atoms and two sulfur atoms, with one of the sulfur atoms posi- cate that 12 is the most symmetric, possessing a local pseudo
tioned between the two distinct Pb sites. σ33 is oriented ca. 15° mirror plane, and 10 is the least symmetric. The orientation of
from the shorter, covalent Pb–S bond, near the position of the the σ33 component of the MS tensor seems to move from the
lone pair and σ11 is in the same plane as the N–Pb–N bonding S–Pb–S plane in 9 and 10 and becomes aligned with a pseudo-
arrangement, bisecting the N–Pb–N angle.
C3 axis in 12 as the local symmetry is increased. Clearly,
Finally, compound 10 has two distinct Pb centres: one four- simple symmetry arguments cannot be made to describe these
coordinate (PbP2S2) site (site 1), and two symmetry related, changes in MS tensor parameters and orientations; rather, it is
three coordinate (PbS3) sites (site 2). Site 1 has a unique four a delicate interplay between paramagnetic shielding contri-
coordinate environment, with bonds to a bidentate DMPE butions involving the Pb lone pair which is a major factor in
ligand, and two long range Pb–S contacts (2.99 Å) to two sulfur determining the orientation of these tensors, as we have noted
atoms from each of the neighboring PbS3 sites. σ33 is aligned in previous work.29
in the general direction of the long Pb–S bonds, while σ11 and
For the PbS2P environment of 11 the bond angles are
σ22 are nearly aligned with the two Pb–P bonds. This tensor similar to those of the PbS3 site of 10; in essence, the differ-
orientation indicates that paramagnetic deshielding at the ence between the two sites is a phosphorus atom in place of a
lead center is primarily due to mixing of low-lying virtual Pb–S sulfur atom within the first coordination sphere of the Pb
anti-bonding MOs with high energy occupied Pb–P bonding center in 11. The similar ligand arrangement of the PbS3 site
MOs. For site 2 of 10, one coordinating sulfur atom possesses of 10 and the PbS2P site of 11 is reflected in the comparability
a long range contact to the adjacent Pb centre of site 1. These of the calculated MS tensor orientations. However, 11 pos-
lengthened Pb–S bonds reduce the local symmetry at the Pb sesses a much larger Ω and intermediate κ relative to the other
centre of site 2 of 10. Accordingly, σ33 is shifted away from the three coordinate Pb compounds. The PbP2S2 site of 10 is dis-
pseudo three-fold axis of rotation.
tinct from the PbS2P site of 11, due to its higher coordination
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number and significant lengthening of the Pb–S bonds. involving of other flexible multidentate ligands may yield
Accordingly, the Ω and κ values of the PbP2S2 site of 10 are dis- other new and interesting structural motifs and bonding
tinct from those of 11. The reduced span in the PbP2S2 site of environments for Pb(II) thiolates.
10 compared to that in 11 results from a difference in elec-
207Pb SSNMR experiments have been successfully carried
tronic structure at the lead sites (resulting from differing out on compounds 9–12. The sensitivity of 207Pb SSNMR as a
arrangements of the Pb–P bonds, the lengthening of the Pb–S probe of molecular structure is evident, as the resulting
bonds in 10, etc.); notably, the paramagnetic contributions to spectra are sensitive to subtle differences in the coordination
the CS/MS tensors are greatly reduced in the former (Table 5), environment of the Pb centre. The CP/CPMG pulse sequence
consistent with its unique orientation.
allows for these spectra to be readily acquired in a piecewise
Finally, the PbS2N2 site of 9, exhibits the second largest Ω fashion, though employment of the WURST-CPMG sequence
in this series of complexes. The PbS2N2 environment has an allows for each of the spectra to be acquired in a single experi-
unusual geometry, and undoubtedly has closely spaced occu- ment with comparable total experiment times.75 Based on our
pied and virtual molecular orbitals which are induced to mix success with these challenging systems, the WURST-CPMG
by the external magnetic field, thereby giving rise to the large pulse sequence should be very useful for acquiring wideline
span (a full discussion of this is beyond the scope of the SSNMR spectra of spin-half nuclei commonly found in in-
current work). We note that in our previous investigation, the organic systems (e.g., 77Se, 89Y, 109Ag, 119Sn, 125Te, 183W, 195Pt,
PbS2N2 environment of 2 possess a very large Ω value (Ω = 199Hg, 205Tl, 207Pb, etc.), and is necessary for acquiring wide-
4700 ppm), which we demonstrated was attributable to the line SSNMR spectra from solids where CP experiments can not
presence of a stereochemically active lone pair.29 The value of be applied (such as oxides and other extended solids devoid of
κ is near zero (the only such case in this series); this is easier 1H and 19F nuclei).
to rationalize: each of the CS tensor components are comple-
Nuclear magnetic shielding tensors calculated utilizing
tely different from one another, meaning they arise from ZORA-DFT methods agree reasonably well with experimental
mixing of MOs oriented in distinct electronic environments, data, and aid in spectral assignment for cases where multiple
consistent with the asymmetric nature of this site. Though Pb sites exist. These calculations also allow for the determi-
there are no clear general trends correlating the Ω values and nation of MS tensor orientations, which correlate spectral para-
simple structural features, PbS3-type environments tend to meters to the molecular structure. MS tensor orientations and
have CS tensors which are close to being axially symmetric, parameters are highly dependent upon the coordination
whereas the PbS2P, PbP2S2 and PbS2N2 sites each have CS environment of the Pb centre (i.e., the variation in the number
tensors with three distinct principal components. Nonetheless, and nature of coordinating ligands). Consequently, we believe
an understanding of the lead CS tensors and orientation that the use of these two pulse sequences for signal enhance-
dependence and origins in the local lead environments are ment, in combination with DFT calculations of magnetic
crucial for future structural interpretation of analogous shielding properties, will allow for many Pb-containing coordi-
systems, as well as for developing a fundamental understand nation complex materials to be studied using 207Pb SSNMR.
of nuclear magnetic shielding of heavy nuclei such as lead.
We are hopeful that this work inspires further research in this
area, and aids others in exploring the nature of lead-contain-
ing materials.
Conclusions
Coordination complexes incorporating flexible bidentate
ligands have been prepared in order to identify new bonding
Acknowledgements
motifs for Pb(II). The reaction of 1 with excess tmeda, dmpe GGB thanks Dan Durant for assistance in collecting solution
and dppe resulted in isolation of the adducts 9–11, respecti- NMR data, and the Natural Sciences and Engineering Research
vely, each of which exhibits a novel bonding environment for Council (NSERC) of Canada, the New Brunswick Innovation
Pb(II). Compound 9 shows a dinuclear species in which a Foundation and Mount Allison University for financial
tmeda ligand is chelating a distorted ψ-trigonal bipyramidal support. AJR thanks NSERC for a doctoral scholarship. AWM
S2N2 Pb(II) center via axial and equatorial sites, while 10 shows thanks the University of Windsor for a tuition scholarship.
a trinuclear species in which the dmpe ligand is chelating the RWS thanks the Ontario Ministry of Research and Innovation
distorted ψ-trigonal bipyramidal S2P2 Pb(II) center via equa- for an Early Researcher Award, and the Centre for Catalysis
torial sites. Compound 11 shows the dppe ligand bridging two and Materials Research at the University of Windsor for provid-
Pb centers, and a distorted ψ-tetrahedral S2P metal bonding ing additional funding. NSERC, the Canadian Foundation for
environment. Further, compounds 9 and 10 also exhibit a dis- Innovation, the Ontario Innovation Trust and the University of
torted ψ-tetrahedral (RS)3Pb arrangement, similar to that pre- Windsor are thanked for funding the solid state NMR facility
viously observed for 12. Compounds 10 and 11 represent the at the University of Windsor. RWS is also grateful to NSERC
first structurally characterized neutral diphosphine ligand for Discovery and Accelerator Supplement grants. The compu-
adducts of Pb(II). The observation of both chelating and brid- tational portions of this work were made possible by the facili-
ging coordination modes in 9–11 suggests that studies ties of the Shared Hierarchical Academic Research Computing
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