Stable plumbylene dichalcogenolate monomers with large differences in their interligand angles and the synthesis and characterization of a monothiolato Pb(II) bromide and lithium trithiolato plumbate

Article abstract of DOI:10.1021/ic302513c

The complexes, Pb(ChArPri₄)₂ (Ch = O (1), S (2); ArPri₄ = C₆H₃-2,6-(C₆H ₃-2,6-Prⁱ₂)₂) were synthesized by alcoholysis and salt metathesis routes and represent the first fully characterized monomeric, two-coordinate, lead dichalcogenolates in the solid state. Structural studies showed that the S-Pb-S angle (77.21(4)) is about 22 narrower than the corresponding O-Pb-O angle. ²⁰⁷Pb NMR and electronic spectroscopy show that the separation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases from the bisaryloxo plumbylene to the bisthiolato derivative. Reaction of LiSArMe₆ with PbBr₂ in a 2:1 ratio led not to Pb(SArMe₆)₂, but to a mixture of the monothiolato lead(II) complex, {Pb(Br)(μ-SArMe₆)}₂ (3) and the lithium tristhiolato plumbate, LiPb(SArMe₆)₃ (4). 3 and 4 were isolated and purified by fractional recrystallization, and both were characterized by X-ray crystallography and spectroscopic studies.

Full text of DOI:10.1021/ic302513c

Article
pubs.acs.org/IC
Stable Plumbylene Dichalcogenolate Monomers with Large
Differences in Their Interligand Angles and the Synthesis and
Characterization of a Monothiolato Pb(II) Bromide and Lithium
Trithiolato Plumbate
Brian D. Rekken, Thomas M. Brown,† Marilyn M. Olmstead, James C. Fettinger, and Philip P. Power*
Department of Chemistry, University of CaliforniaDavis, Davis, California 95616, United States
S
* Supporting Information
ABSTRACT: The complexes, Pb(ChArPri )2 (Ch = O (1), S
4
(2); ArPri = C6H3-2,6-(C6H3-2,6-Pri2)2) were synthesized by
4
alcoholysis and salt metathesis routes and represent the first
fully characterized monomeric, two-coordinate, lead dichalco-
genolates in the solid state. Structural studies showed that the
S−Pb−S angle (77.21(4)°) is about 22° narrower than the
corresponding O−Pb−O angle. 207Pb NMR and electronic
spectroscopy show that the separation between the highest
occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) decreases from the
bisaryloxo plumbylene to the bisthiolato derivative. Reaction of
LiSArMe with PbBr2 in a 2:1 ratio led not to Pb(SArMe )2, but
6
6
to a mixture of the monothiolato lead(II) complex, {Pb(Br)(μ-SArMe )}2 (3) and the lithium tristhiolato plumbate,
6
LiPb(SArMe )3 (4). 3 and 4 were isolated and purified by fractional recrystallization, and both were characterized by X-ray
6
crystallography and spectroscopic studies.
(OBut)3}{μ-SSi(OBut)3}]2, have been isolated. Despite the
INTRODUCTION
■
large size of their thiolate substituents, they are dimeric and
feature a four-member Pb2S2 ring with three-coordinate lead
atoms12 Currently, the only fully structurally characterized, two-
coordinate plumbylene featuring a Pb−chalcogen bond is the
heteroleptic, aryl−arylthiolato species, Pb(Tbt)(STbt) (Tbt =
C6H2-2,4,6-{CH(SiMe3)2}3), which was obtained by Okazaki,
Tokitoh, and co-workers via a 1,2 aryl migration from the
corresponding plumbanethione, (Tbt)2PbS.13
The development and understanding of the bonding in two-
coordinate, lead dichalcogenolates is marked by the fact that all
fully characterized examples are either polymers, oligomers, or
base-stabilized adducts in the solid-state.14 Such species could
be of practical importance as precursors for important
semiconductors, such as PbS,15 and complexes of general
formula, [Pb(OR)2]n have proven useful in the preparation of
the piezoelectric ceramic, PZT, found in numerous electro-
mechanical applications such as ultrasound generators and
sonar detectors.16 The aryloxide, Pb(OC6H3-2,6-But2)2, (struc-
ture unknown) has been used to synthesize the three-
coordinate lead(II) complex, Pb(OC6H3-2,6-But2){N(C6H3-
2,6-Pri2)C(CH3)}2CH, in which the aryloxide ligand functions
as a pseudohalide and was shown to be resistant to
decomposition in contrast to the corresponding halide
Plumbylenes are the heaviest analogues of carbenes.1 Unlike
carbon, the lead 2+ oxidation state is favored over lead 4+ in
their inorganic derivatives because of the increasing stabilization
of the s-valence electrons.2 Nonetheless, the isolation of stable
monomeric, two-coordinate plumbylenes has proven more
difficult than their lighter group 14 congeners because of their
greater sensitivity to heat and light.3 The first stable
diorganoplumbylene, Pb{CH(SiMe3)2}2, was reported by
Lappert et al in 1973.4 Although it is a monomer in solution,
its crystal structure was not obtained until 1998 by Klink-
hammer which revealed the plumbylene to be a weakly
associated dimer featuring a long Pb−Pb bond of 4.129 Å.5 The
isoelectronic −N(SiMe3)2, substituted amido derivative, was
prepared in 1974;6 and the structure of Pb{N(SiMe3)2}2 was
determined in 1983 where it was shown to be a two-coordinate,
V-shaped monomer.7 For chalcogenolates, homoleptic lead(II)
8
9
dichalcogenolate derivatives, Pb(BHT)2 and Pb(SMes*)2
(BHTH = butylated hydroxytoluene, HOC6H2-2,6-But2-4-Me;
Mes* = C6H2-2,4,6-But3) were prepared in 1980 and 1983,
respectively. The former species was reported in a review to be
a monomer based upon weak X-ray data while the structure of
Pb(SMes*)2 currently remains unknown.10 The structure of the
related bisthiolato plumbylene, Pb(SC6H3-2,6-Pri2)2 was
determined to be a trimer in the solid-state.10 Subsequently,
several sterically encumbered bisthiolato derivatives of Pb(II,
Received: November 16, 2012
Published: February 26, 2013
11
including [Pb{SC(SiMe3)3}{μ-SC(SiMe3)3}]2 and [Pb{SSi-
© 2013 American Chemical Society
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Inorganic Chemistry
Article
161.53 (i-C6H3); 207Pb{1H}NMR (125.53 MHz, C6D6, 296 K): δ =
1070.3; UV−vis: [λ, nm (ε, M−1 cm−1)] 370 (850), 300 (2950), 293
(3200). IR (cm−1): (Pb−O) 490.
derivatives.17 Subsequent reactions established that the
remaining aryloxide ligand could be replaced by the amido
(−N(SiMe3)2), phosphido (−P(SiMe3)2), or P-phosphasileno
(−PSi(R)Si(But)3, R = C6H2-2,4,6-Pri3) functional groups.17
Furthermore, variations of this complex demonstrated rever-
sible reactivity with carbon dioxide that may lead to new
methods for CO2 sequestration purposes.18 Bisthiolato
plumbylenes may also prove to have relevance for the
interaction of lead with disulfide bridges in cases of lead
poisonings.19 Where anemia-related symptoms for such
poisonings have been, studies have shown that the lead(II)
atom embeds itself into the zinc enzyme, aminolevulinic acid
dehydratase (ALAD),19,20 which possesses a trigonal pyramidal
Zn(II)(cystine)3 site (atypical geometry for zinc),19,21 for which
lead has a 500 times greater affinity.22 However, fine details
regarding the exact geometry of the trithiolato lead(II) site
remain ambiguous, and the number of model compounds is
limited.22−25 We now report the synthesis and characterization
of the monomeric two-coordinate plumbylene dichalcogeno-
Pb(SArPri
)
(2). A slurry of LiSArPri (1.266 g, 2.90 mmol) in
4
4
2
diethyl ether (60 mL) was added dropwise over 45 min to a diethyl
ether slurry (10 mL) of PbBr2 (0.532 g, 1.45 mmol) cooled in an ice
bath. The solution became yellow immediately and, after 1 h, the
solution was allowed to warm to ambient temperature and was stirred
overnight to give an orange solution. All volatile components were
removed under reduced pressure, and the residue was extracted with
toluene (60 mL) and filtered. The solution was concentrated to about
20 mL, and cooled overnight to about 6 °C to give 2 as a
microcrystalline, yellow powder. The solution was further concen-
trated to about 5 mL to give a second crop of the crystals. The
supernatant liquid was decanted, and a small amount of 2 (ca. 0.04 g)
was dissolved in a concentrated tetrahydrofuran (THF) solution and
stored at −17 °C for 4 days to yield X-ray quality, yellow crystals of 2
as a solvate with 2 THF molecules. Yield: 0.643 g (0.620 mmol, 43%).
1
mp 211−214 °C; H NMR (599.7 MHz, C6D6, 294 K): δ = 1.07 (d,
3
3
24H, CH(CH)2, JHH = 6.60 Hz), 1.23 (d, 24H, CH(CH)2, JHH
=
3
7.20 Hz), 2.84 (m, 8H, CH(CH)2, JHH = 6.60 Hz), 6.85 (t, 4H, p-
C6H3iPr2, 3JHH = 7.20 Hz), 7.06 (d, 8H, m-C6H3iPr2, 3JHH = 7.80 Hz),
Pri4
Pri4
lates, the isolobal Pb(OArPri )2, (1), and Pb(SAr )2, (2); Ar
4
3
3
= C6H3-2,6(C6H3-2,6-Pri2)2) which are stabilized by bulky
chalcogenolate ligands that carry the same terphenyl substituent
but feature Ch−Pb−Ch angles that differ by about 22°. In
7.15 (t, 2H, p-C6H3, JHH = 6.20 Hz), 7.16 (d, 4H, m-C6H3, JHH
=
5.40 Hz); 13C{1H} NMR (150.8 MHz, C6D6, 296 K): δ = 24.09 (o-
CH(CH3)2), 24.94 (o-CH(CH3)2), 31.05 (o-CH(CH3)2), 123.40 (m-
C6H2iPr3), 123.98 (p-C6H3), 128.59 (m-C6H3), 129.26 (o-C6H3),
140.70 (p-C6H2iPr2), 141.68 (o-C6H2iPr2), 144.32 (i-C6H2iPr2),
147.45 (i-C6H3); 207Pb{1H}NMR (125.53 MHz, C6D6, 296 K): δ =
4283. UV−vis: [λ, nm (ε, M−1 cm−1)] 422 (1300), 354 (1100), 292
(5800). IR: The Pb−S stretching band is tentatively assigned to an
absorption at 302 cm−1.
addition, the monothiolato lead(II) bromide, {Pb(μ-SArMe )-
6
Br}2 (3), and the lithium tristhiolato lead(II) complex, LiPb(μ-
SArMe )3 (4) were isolated during attempts to synthesize the
6
less sterically crowded lead dithiolate, Pb(SArMe )2.
6
Me6
Me6
{Pb(μ-SArMe )Br}2 (3) and LiPb(μ-SAr )3 (4). LiSAr (1.282 g,
3.64 mmol) was dissolved in about 60 mL of diethyl ether and added
dropwise over 30 min to a diethyl ether (10 mL) slurry of PbBr2
(0.668 g, 1.82 mmol) cooled to about −78 °C. After addition, the
solution was stirred at about −78 °C for a further 1 h and then allowed
to warm slowly to room temperature. The solution was stirred for 2 d
to give a pale yellow solution and a white precipitate. All the volatile
components were removed under reduced pressure. Toluene (80 mL)
was added, and filtration afforded a clear, faint yellow solution. The
filtrate was concentrated under reduced pressure to about 60 mL and
placed in an about −17 °C freezer to afford X-ray quality crystals of 4
as colorless blocks after 3 days (0.732 g). The supernatant liquid was
decanted, and concentrated to about 30 mL and placed in an about
−17 freezer to afford a second fraction of 4 (0.369 g) after 2 days. The
supernatant liquid was decanted, and concentrated to about 15 mL
and placed in an about −17 °C freezer to afford X-ray quality crystals
of 3 as colorless prisms after 4 days (0.387 g). The supernatant liquid
was decanted into a second flask, and concentrated to about 8 mL and
placed in an about −17 °C freezer to afford a second fraction of 3
(0.114 g) after 5 days. Attempts to isolate any further fractions were
6
EXPERIMENTAL SECTION
■
All manipulations were performed with the use of modified Schlenk
techniques under an N2 atmosphere or in a Vacuum Atmospheres
drybox under N2. Solvents were distilled over a potassium mirror and
degassed immediately prior to use via freeze−pump−thaw cycles.
Unless otherwise noted, all chemicals were obtained from commercial
sources and used without further purification. HOArPri , HSAr
,
Pri4
4
4 27a
6 26
4 27b
6
HSArMe LiOArPri
,
LiSArMe
,
LiSArPri
,
and Pb{N(SiMe3)2}2
6
were prepared according to literature procedures.26,27 1H and 13C
NMR spectra were recorded on a Varian 600 MHz instrument and
referenced internally to either protio benzene or trace silicone grease
(δ = 0.29 in C6D6). 207Pb NMR (operating at 125.53 MHz) spectra
were acquired on a Varian 600 MHz instrument and were referenced
to PbMe4 in C6D6 (δ = 0). IR spectra were recorded as Nujol mulls
between CsI plates on a Perkin-Elmer 1430 spectrophotometer. UV−
visible spectra were recorded as dilute hexanes solutions in 3.5 mL
quartz cuvettes using a HP 8452 diode-array spectrophotometer.
Melting points were determined on a Meltemp II apparatus using glass
capillaries sealed with vacuum grease and are uncorrected.
Pb(OArPri )2 (1). A solution of 0.951 g (2.30 mmol) of HOAr
Pri4
4
unsuccessful. The total yield of {Pb(μ-SArMe )Br}2 (3): 0.501 g (0.382
6
mmol, 42.0%) mp 268−270 °C. 1H NMR (599.7 MHz, C6D6, 294 K):
δ = 2.20 (s, 24H, o-CH3), 2.27 (s, 12H, p-CH3), 6.82 (s, 8H,
dissolved in diethyl ether (45 mL) and cooled to about 0 °C was
added over 20 min to a diethyl ether solution (10 mL) of 0.548 g (1.15
mmol) of Pb{N(SiMe3)2}2 cooled to 0 °C wrapped in aluminum foil.
The solution was stirred for 1 h and then allowed to warm to ambient
temperatures and was stirred for an additional 8 h to give a clear, pale
yellow solution. The solution was concentrated to about 5 mL and
cooled to about 6 °C for 2 days to produce X-ray quality, pale yellow
crystalline blocks of 1 as a solvate with 0.5 molecules of diethyl ether.
A small amount of lead metal was also deposited. Yield: 0.664 g (0.640
3
C6H2Me3), 6.84 (d, 4H, m-C6H3, JH,H = 7.34 Hz), 7.13 (t, 2H, p-
3
C6H3, JH,H = 7.34 Hz); 13C{1H} NMR (C6D6, 150.8 MHz, 296 K): δ
= 21.56 (p-CH3), 21.69 (o-CH3), 125.75 (p-C6H3), 129.69 (o-
C6H2Me3), 130.08 (m-C6H3), 135.27 (o-C6H3), 137.84 (m-C6H2Me3),
138.26 (p-C6H2Me3), 138.91 (i-C6H2Me3), 144.37 (i-C6H3); 207Pb
NMR (125.5 MHz, C6D6, 296 K): δ = 2223.0 ppm; IR: The Pb−S
stretching band is tentatively assigned to an absorption at 340 cm−1.
1
Total yield of LiPb(μ-SArMe )3 (4): 1.101 g (0.77 mmol, 42.3%) mp
6
mmol, 56%) mp: 295 °C. H NMR (599.7 MHz, C6D6, 303 K): δ =
3
305−307 °C. 1H NMR (599.7 MHz, C6D6, 294 K): δ = 2.10 (s
(broad), 54H, o-CH3), 7.01 (multi, 21H). IR (cm−1): The Pb−S
stretching band tentatively assigned to 242.
1.07 (d, 24H, CH(CH3)2, JHH = 7.20 Hz), 1.09 (d, 24H, CH(CH3)2,
3JHH = 6.60 Hz), 2.97 (m, 8H, CH(CH3)2, JHH = 6.60 Hz), 6.72 (t,
3
3
3
2H, p-C6H3, JHH = 6.90 Hz), 7.08 (d, 4H, m-C6H3, JHH = 7.05 Hz),
3
7.19 (d, 8H, m-C6H3iPr2, JHH = 7.04 Hz), 7.21 (t, 4H, p-C6H3iPr2,
X-ray Crystallography. Crystals 1−4 were removed from a
Schlenk tube under N2 and covered with a layer of hydrocarbon oil. A
suitable crystal was selected, attached to a glass fiber and quickly
placed in low temperature N2 stream. The data for 1 was collected at
about 90 K on a Bruker DUO APEX II CCD diffractometer with Mo
3JHH = 7.04 Hz); 13C{1H} NMR (150.8 MHz, C6D6, 296 K): δ = 23.87
(o-CH(CH3)2), 24.86 (o-CH(CH3)2), 30.75 (o-CH(CH3)2), 116.24,
(m-C6H2iPr3), 124.47 (p-C6H3), 128.60 (m-C6H3), 130.56 (o-C6H3),
130.64 (p-C6H2iPr2), 140.06 (o-C6H2iPr2), 148.36 (i-C6H2iPr2),
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Table 1. Crystal Data and Structure Refinement for 1−4
compound number
empirical formula
formula weight
wavelength (Å)
crystal system
space group
a (Å)
1
2
3
4
C
248H316O10Pb4
C72H98O3PbS2
1282.81
C51.50H54Br2Pb2S2
1311.26
C86H91LiPbS3
1434.89
0.710 73
Monoclinic
P21/m
4285.75
0.710 73
Triclinic
0.710 73
0.710 73
Monoclinic
C2/c
Monoclinic
P21/n
P1
̅
16.8740(14)
19.5017(17)
33.268(3)
24.531(4)
16.631(2)
16.071(2)
90
8.8927(8)
40.868(4)
13.0063(12)
90
13.2474(5)
20.2689(7)
13.9860(5)
90
b (Å)
c (Å)
α (deg)
96.326(4)
β (deg)
92.024(5)
100.779(3)
90
92.323(5)
90
108.5190(10)
90
γ (deg)
91.391(5)
volume (Å)
10869.6(16)
2
6440.8(16)
4
4723.0(8)
4
3560.9(2)
2
Z
density (calculated) (Mg/m3)
1.309
1.323
1.844
1.338
absorption coefficient (mm−1
)
3.146
2.73
8.934
2.502
F(000)
4424
2672
2516
1.480
crystal size (mm)
0.366 × 0.265 × 0.197
pale yellow block
97776
0.48 × 0.33 × 0.16
yellow block
7395
0.447 × 0.434 × 0.301
colorless prism
40295
0.434 × 0.271 × 0.150
colorless block
31827
crystal color and habit
reflections collected
independent reflections
observed reflections (I > 2σ(I))
data/restraints/parameters
final R indices (I > 2σ(I))
49485 [R(int) = 0.0221]
42395
7395 [R(int) = 0.0000]
6398
10796 [R(int) = 0.0294]
9881
8385 [R(int) = 0.0182]
7859
49485/85/2479
R1 = 0.0247
wR2 = 0.0567
R1 = 0.0331
wR2 = 0.0596
7395/30/376
R1 = 0.0330
wR2 = 0.0759
R1 =0.0418
wR2= 0.0775
10796/2/550
R1 = 0.0249
wR2 = 0.0491
R1 = 0.0289
wR2 = 0.0500
8385/0/485
R1 =0.0217
wR2 = 0.0494
R1 =0.0243
R indices (all data)
wR2 = 0 0504
Scheme 1. Synthesis of Plumbylenes 1 and 2
Kα (λ = 0.71073 Å) radiation. As an example, a crystal 2, a yellow
block with approximate dimensions 0.16 × 0.33 × 0.48 mm was placed
and optically centered on the Bruker APEX CCD system at −183 °C
(90 K).29 Indexing of the unit cell was attempted using a random set of
reflections collected from three series of 0.3° wide ω-scans, 10 s per
frame, and 30 frames per series that were well distributed in reciprocal
space. Four ω-scan data frame series were collected [MoKα] with 0.3°
wide scans, 35 s per frame and 606, 455, 606, 455 frames collected per
series at varying φ angles (φ = 0°, 90°, 180°, 270°). The crystal to
detector distance was 5.23 cm, thus providing a complete sphere of
data to 2θmax = 55.06°. The data for 3 and 4 were collected on a
Bruker SMART APEX II CCD diffractometer with Mo Kα (λ =
0.71073 Å) radiation. The crystal structures (Table 1) were solved by
direct methods using the SHELX version 6.1 program package.28,29 All
non-hydrogen atoms were refined anisotropically. Absorption
corrections were applied using SADABS program (SADABS, an
empirical absorption correction programs, part of the SAINTPlus NT
version 5.0 package; Bruker AXS: Madison, WI, 1998). Thermal
ellipsoid plots drawn using OLEX2 software.30 A summary of crystal
data, collection parameters, and some refinement details is given in
Table 1.
RESULTS AND DISCUSSION
■
Synthesis. The bisthiolato plumbylene, 2, was prepared by
27b
the addition of (LiSArPri
)
to PbBr2. However, the aryloxo
4
2
derivative 1 could not be prepared by this route because it led
to decomposition with deposition of lead metal. Instead, an
alcoholysis route involving the bisamido plumbylene, Pb{N-
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(SiMe3)2}2 as the lead source, was used.8 An overview of the
synthesis of 1 and 2 is shown in Scheme 1.
decomposed upon melting at 295 °C. Plumbylene 2 was
isolated as a microcrystalline yellow solid in about 43% yield
and is thermally and photolytically stable, even in ethereal
The mono- and tristhiolato lead(II) complexes {Pb(μ-
Me6
SArMe )Br}2 (3) and LiPb(μ-SAr )3 (4) were prepared by
solutions. Pb(SArPri )2 becomes orange upon heating and melts
6
4
the same method as 2 and isolated through multiple
recrystallizations to remove the impurities as indicated by
solution color. Overall, about 91.7% of the initial lead used in
the reaction was accounted for in roughly equal molar amounts
of 3 (43.5%) and 4 (48.4%).
The two-coordinate, V-shaped plumbylene 1 (Figure 1), was
isolated as pale yellow crystals in about 56% yield and is both
at about 211 °C.
The two crystalline products obtained from the 2:1 reaction
of LiSArMe and PbBr2 were {Pb(μ-SArMe )Br}2, (3), and
6
6
LiPb(SArMe )3, (4). The plumbate LiPb(μ-SAr
)
was
Me6
6
3
obtained as the first crystalline fraction as colorless blocks, in
about 48% yield. These crystals were difficult to redissolve in
the toluene solvent. The second crystalline fractions afforded an
about 44% total yield of {Pb(μ-SArMe )Br}2 (3). A THF
6
solution of {Pb(μ-SArMe )Br}2 has a pale yellow color at
6
ambient temperature; however, upon warming to 65 °C the
color intensifies. Cooling to ambient temperature restored the
original pale yellow color. In the solid-state, heating 3 to about
246 °C, resulted in a red color which intensified until the
complex melted to afford a dark red liquid at about 269 °C.
Like Pb(OArPri )2, 3 is reversibly thermochromic; however,
4
unlike the plumbylene, 3 appears to be stable (at least
temporarily) in the liquid phase.
The isolation of 3 and 4 from the reaction of 2 equiv of
LiSArMe with PbBr2 contrasts with the synthesis of homoleptic
6
2, which was obtained from the corresponding reaction of
LiSArPri with PbBr2. Seemingly, the reaction with LiSArMe of
4
6
any Pb(SArMe )2 produced during the synthesis of 3 and 4 is
6
competitive with its reaction with PbBr2 as {Pb(μ-SArMe )Br}2.
6
Apparently, the reaction equilibria are such that roughly equal
amounts of 3 and 4 (stabilized by complexation of the Li+ ion)
are observed.
Structures. A selection of important distances and angles
for 1−4 is given in Table 2.
A single crystal X-ray diffraction study shows that 1
crystallizes with four monomeric molecules and two solvent
ethers in the asymmetric unit with an average Pb···Pb
separation of 10.920(1) Å. The average O−Pb−O and Pb−
O−C angles are 99(1)° and 127(2)°. The O−Pb−O angle is
significantly wider than that reported for Pb(BHT)2 (86.2(4)°)
whereas the Pb−O−C angle is similar to the 124(2)° in
Pb(BHT)2.10 The average Pb−O bond length is 2.216(8) Å
which is at the upper end of the known range (2.17(1)−
2.240(5) Å) for Pb(II)−O bond lengths found in aggregated
complexes of formulas {Pb(OR)2}n.31−34 Using the molecule
containing Pb(1) as a representative example, the closest
approach between other ligand-atoms and lead involves
Pb(1)−C(37) (2.991(2) Å), and the Pb−centroid distance is
3.38(4) Åboth of which are within the sum of Pb−C van der
Waals radii (3.72 Å)35 but more than 0.7 Å longer than a typical
Pb(II)−C distance of 2.19 Å.36
Figure 1. Thermal ellipsoid (30%) drawing of one of the
crystallographically independent molecules of 1. Hydrogen atoms
and cocrystallized diethyl ether are not shown. Selected bond lengths
(Å) and angles (deg): Pb(1)−O(1) 2.228(2), Pb(1)−O(2) 2.211(2),
O(1)−C(1) 1.347(3), O(2)−C(31) 1.350(3), Pb(1)−C(7) 2.997(2),
Pb(1)−C(37) 2.991(2), Pb(1)−Centroid(1) 2.978, Pb(1)−Cent-
roid(2) 2.963, O(1)−Pb(1)−O(2) 100.62(6), Pb(1)−O(1)−C(1)
127.0(1), Pb(1)−O(2)−C(31) 130.1(1), O(1)−C(1)−C(2)
122.7(2), O(1)−C(1)−C(6) 119.2(2), O(2)−C(31)−C(32)
122.5(2), O(2)−C(31)−C(36) 119.2(2), Centroid(1)−Pb(1)−Cent-
roid(2) 146.49.
thermally and photolytically stable under ambient conditions in
the solid state. Nonetheless, it was found that 1 decomposes in
ethereal solutions at room temperature under ambient light.
The reversibly, thermochromic plumbylene becomes an intense
The plumbylene 2 (Figure 2) crystallizes as a two-coordinate,
V-shaped monomer with a crystallographically required 2-fold
red color upon heating and quickly returns to its original color
on cooling to ambient temperature. Pb(OArPri
) rapidly
2
4
Table 2. Selected Distances (Å) and Angles (deg) for Pb(II) Chalcogenolate Complexes (Pb(ChAr)2), Ch = O (1), S (2);
Me6
{Pb(μ-SArMe )Br}2 (3) [Pb(1) and Pb(2) sites], and LiPb(μ-SAr )3 (4)
6
compound
Pb−Ch
Ch−Pb−Ch
Pb−Ch−C
Pb−Ch−C−C
Ch−C
Pb−Centroid
Ch···Ch
a
a
a
a
a
a
a
(1) Pb(OArPri
)
2.216(8)
99(1)
127(2)
26(5)
1.35(2)
2.95(3)
3.38(4)
4
2
(2) Pb(SArPri
)
2.5656(9)
77.21(4)
74.46(2)
77.02(3)
113.42(11)
31.0(1)
1.771(3)
1.784(1)
1.784(1)
1.780(1)
3.046
3.202(1)
3.396(1)
3.396(1)
4
2
a
a
a
a
a
a
a
(3) {Pb(μ-SArMe )Br}2[Pb(1)]
2.806(3)
113.5(3)
34(10)
3.2(3)
6
a
a
a
a
(3) {Pb(μ-SArMe )Br}2[Pb(2)]
2.727(3)
110.9(3)
50(11)
3.5(3)
6
a
a
a
a
a
a
(4) LiPb(μ-SArMe
)
3
2.675(4)
81.1(5)
118.9(5)
42.8(3)
3.702(9)
3.1(5)
6
a
Average value.
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of the 2.554(5)−2.981(1) Å range observed in the aggregated
and base stabilized lead(II) thiolates mentioned above.9,14,38,39
It should be noted that the Pb−S length in the plumbylene,
Pb(Tbt)(STbt) (2.498(10) Å), is distinctly shorter than that in
2.13 The difference could be a result of decreased interligand
steric repulsion within Pb(Tbt)(STbt). The Pb−S−C angle is
113.42(11)° with C−S distances of 1.771(3) Å which is wider
than the Pb−S−C angle (107.4(6)°) and shorter than the C−S
distance (1.88(1) Å) found in Pb(Tbt)(STbt).13 The closest
nonbonded ligand approach to lead involves C(19) at 3.170(3)
Å which is within the sum of Pb−C van der Waals radii (3.72
Å).35 The closest approach between Pb(1) and a THF oxygen
is 5.420(7) Å and is well above the sum of the Pb−O van der
Waals radii (3.54 Å).35
The S−Pb−S angle in 2 is about 22° narrower than the
corresponding O−Pb−O angle in 1. One of the reasons for the
difference in the angles may be the shorter Pb−O bonds36 and
greater EN character of the OArPri ligand which afford greater
4
bond pair-bond pair repulsions and produce a less diffuse metal
lone pair leading to the wider angle in 1.3 However, such a large
disparity in the interligand angle is not observed between
Sn(BHT)2 (O−Sn−O = 88.8(2)°),8 Sn(OC6H3-2,6-But2)2,
(O−Sn−O = 88.8(1)°),40 and Sn(SMes*)2 (S−Sn−S =
85.4(1)°)9 which are the only closely related pairs of divalent
group 14 element aryloxide and thiolates for which data are
available. Thus, it seems probable that the acute S−Pb−S angle
of 2 may result from the peculiar steric characteristics of the
terphenyl substituent which produce both front side (side of
the lead lone pair) and back side (the side of the sulfurs)
interactions that involve a combination of imperfectly under-
stood steric, dispersion, and packing forces that afford the
unusual angles.41 Geminal bonding interactions between the
two Pb−S bonds are also plausible,42 suggesting that the
complex is effectively an intermediate “snapshot” of disulfide
elimination.43
Figure 2. Thermal ellipsoid (30%) drawing of 2. Hydrogen atoms and
two cocrystallized THF molecules are not shown. Selected bond
lengths (Å) and angles (deg): Pb(1)−S(1) 2.5656(9), S(1)−C(1)
1.771(3), Pb(1)···C(19) 3.170(3), Pb(1)···Centroid 3.046, S(1)−
Pb(1)−S(1A) 77.21(4), Pb(1)−S(1)−C(1) 113.42(11), S(1)−C(1)−
C(2) 116.0(3), S(1)−C(1)−C(6) 123.8(2), Centroid−Pb(1)−Cent-
roid 144.21.
axis and 9.5139(9) Å intermolecular Pb···Pb separation. It has
an acute S−Pb−S interligand angle of 77.21(4)° which can be
compared to the angles in cyclic dimers and trimers (73.5(1)−
92.8(1)°)9,12,14 or in the Lewis base complexed monomer,
Pb(SRf)2(THF) (Rf = C6H2-2,4,6-(CF3)3) (84.4(1)°);37
although it is wider than some angles in polymeric, base
stabilized species such as {Pb(SPh)2(Py)}n (70.05(5)°).38 The
Pb−S bond length is 2.5656(9) Å which is near the shorter end
The monothiolato lead(II) bromide complex 3, (Figure 3)
crystallizes as a thiolate bridged dimer featuring an asymmetric,
puckered, four-membered Pb2S2 ring core. The structure may
be contrasted with those of the centrosymmetric, aryl lead(II)
Figure 3. Thermal ellipsoid (30%) drawings of 3. Hydrogen atoms and one molecule of cocrystallized toluene are not shown for clarity. Selected
bond lengths (Å) and angles (deg): Pb(1)−S(1) 2.8096(9), Pb(1)−S(2) 2.8029(9), Pb(1)−Br(1) 2.693(1), Pb(1)···Br(2) 3.792(1), Pb(2)−S(1)
2.7299(9), Pb(2)−S(2) 2.7239(9), S(1)−Pb(1)−S(2) 74.46(2), Pb(2)−Br(2) 2.7108(5), S(1)−Pb(2)−S(2) 77.02(3).
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Figure 4. Thermal ellipsoid (30%) drawing of 4. Hydrogen atoms are not shown. Selected bond lengths (Å) and angles (deg): Pb−S(1) 2.6783(4),
Pb−S(2) 2.6781(4), Pb−S(3) 2.6693(4), S(1)−Pb−S(2) 80.46(1), S(1)−Pb−S(3) 81.56(1), and S(2)−Pb−S(3) 80.38(1).
Pri4
Figure 5. 207Pb NMR spectra of (a) Pb(OArPri )2 (1) and (b) Pb(SAr )2 (2).
4
halide dimers (ArPri PbBr)2 and (ArPri PbBr)2 which are
associated through halogen bridging.44,45 The intramolecular
Pb···Pb distance is 4.0011(9) Å and the closest intermolecular
Pb···Pb distance is 10.144(1) Å. Pb(1) features a distorted saw-
horse geometry with a stereochemically active lone pair
directed perpendicular to the planes of the central arenes of
the terphenyl ligands.23 The Pb(1) atom is bonded to two
bridging sulfur atoms, and to Br(1) which is exo to the Pb2S2
ring. It also possesses a long interaction to Br(2) (3.792(1) Å)
that is endo to the Pb2S2 moiety (cf. sum of the van der Waals
radii for Pb−Br = 3.87 Å).35 The bridging Pb(1)−S(1) and
Pb(1)−S(2) distances, 2.8096(9) and 2.8029(9) Å, are
significantly longer (ca. 0.25 Å) than the Pb−S distance
observed in 2, although they are within the range of distances
discussed for aggregated bisthiolato lead complexes.9,14,38,39
elongated (by ca. 0.02 Å) in comparison to Pb(1)−Br(1),
perhaps as a result of Br(2)···Pb(1) interaction. The distances
between Pb(2) and the centroids of the two closest arene rings
are 3.715 and 3.200 Å, respectively, and the centroid−Pb(2)−
centroid angle is 112.00°. Thus, the structural data show that
the distances to Pb(1) are about 0.1 Å longer then the Pb−S
distances to Pb(2). The longer distances arise from the higher
effective coordination number of Pb(1) due to its interaction
with Br(2) in addition to Br(1), S(1), and S(2).
4
6
The lithium trithioplumbate 4 (Figure 4) crystallizes as
monomeric units with a trigonal pyramidal geometry at lead
and about 10.4307(3) Å intermolecular Pb···Pb separations.
The lithium and lead atoms are disordered over two sites. The
Li−S distances, (Li−S(1) 3.584(16) Å, Li−S(2) 3.560(16) Å,
and Li−S(3) 3.539(15) Å), are much longer than the range of
Li−S distances (2.33(2)−2.65(2) Å) in the precursor, LiSArMe
,
6
The Pb(1)−Br(1) distance, 2.693(1) Å, is shorter than those in
of (ArPri PbBr)2 and (Ar PbBr)2 which have average Pb−Br
which is a trimer in the solid state, wherein the lithiums interact
with two thiolato sulfurs.26,46 The lithium ions also display
contacts to the centroids of the flanking mesityl substituents
with Li−centroid distances of 3.23(1), 3.56(2), and 3.18(1) to
the C(5), C(18), and C(31) rings, such that the Li+ ion is
encapsulated in a cavity composed of the thiolate sulfurs and
flanking mesityls. The Pb−S bond lengths are 2.6783(4),
2.6781(4), and 2.6693(4) Å and are longer than those in
Payne’s complex, [As(C6H5)4][Pb(SPh)3] which has Pb−S
bond lengths of 2.619(1), 2.623(1), and 2.647(1) Å, but are
Pri6
4
distances of about 2.92 Å.44,45 The S(1)−Pb(1)−S(2) angle of
74.46(2)° is slightly narrower than the S−Pb−S angle in 2
(77.21(4)°) but is similar to angles observed for the central,
tetra-coordinate lead atom in {Pb(SC6H3-2,6-Pri2)2}3
(73.5(1)°).9 The distances between Pb(1) and the centroids
of the two closest arene rings are 2.984 and 3.494 Å while the
centroid−Pb(1)−centroid angle is 116.87°. The Pb(2) atom
has similar Pb(2)−S(1) and Pb(2)−S(2) distances of
2.7299(9) and 2.7239(9) Å as well as a S−Pb(2)−S angle of
77.02(3)°. The Pb(2)−Br(2) distance, 2.7108(5) Å, is slightly
similar to those in [NPrn ][Pb(SPh)3]25 which was prepared by
4
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Figure 6. 207Pb NMR spectrum of {PbBr(μ-SArMe )}2 (3).
6
Christou and co-workers and has Pb−S distances of 2.696(3),
2.665(3), and 2.633(3) Å. The S−Pb−S angles of 4 are
80.46(1), 81.56(1), and 80.38(1)° and are about 10° narrower
than those in the Payne (90.32(4)°, 91.08(5)°, 96.14(5)°) and
Christou (89.7(1)°, 93.7(1)°, 102.7(1)°) complexes. The Pb−
S−C angles of 4 are 119.40(5), 118.34(4), and 118.95(5)° and
are considerably wider than the Pb−S−C angles in complexes
of Payne (101.4(1), 98.8(2), and 97.1(2)°) and Christou
(98.1(3), 94.9(3), and 93.7(3)°). The distances between lead
and the centroids of the three closest arene rings are 3.703,
3.690, and 3.713 Å which are just within the sum of the van der
Waals radii for lead and carbon (Pb−C = 3.72 Å).35 The closest
other atoms to lead are the ortho-carbons, of the flanking arene
rings (C(6), C(19, and C(32)) that have Pb−C distances of
3.402(2), 3.414(1), and 3.373(2) Å. A list of selected distances
and angles for compounds 1−4 are given in Table 2.
upfield of 2 as well as the range of three-coordinate lead(II)
thiolates observed by Briand and Dean (δ = 2518−2999);52,53
though, it is downfield of 1. The most probable explanation
may arise from the fact that bromine is more electronegative
(2.96) than sulfur (2.58), which also stabilizes the lone-pair
orbitals on the lead atoms. The broadness of the signal may be
due to an equilibrium between the two lead sites as it has been
shown that line broadening can occur when multiple lead sites
rapidly exchange ligands.53 Possibilities include an equilibrium
between the trigonal pyramidal and saw-horse geometries,
which sees the bromines alternate between the endo and exo
positions, or a simple equilibrium where the site of dimer
bridging alternates between the thiolates and the bromides.
Although no structure of alkali metal complexed trithiolato
plumbates such as 4 have been previously determined, 207Pb
NMR studies have shown that excess LiSPh can react with
polymeric [Pb(SPh)2]n to give a lithium trithioplumbate
complex that possesses a labile thiolate ligand and produces
chemical shifts in the region of δ = 2518−2999.52,53 Despite
this information, several attempts to acquire a signal for 4
proved fruitless. It is unclear if this is due to the low solubility
of 4 or other effects associated with solvent or temperature.53
In the electronic spectra, the most noticeable difference
between the plumbylenes is a red shift of the two lowest energy
absorptions between 1 to 2. Based on data for other
plumbylenes, the lowest energy transition likely originates
from a lead 6n → 6p transition for both 1 (370 nm) and 2 (422
nm).1,54 The higher transition energy of 1 is likely due to
electron-withdrawing effects from the more electronegative
oxygen which stabilizes the lead lone pair.54,55 The amount of
π-bonding from the chalcogen lone-pair to the lead p orbital is
Spectroscopic Analysis. The 207Pb NMR signal for 1 was
identified at δ = 1070.3 (Figure 5a). For 2, the 207Pb NMR
signal appears significantly further downfield at δ = 4283.2
(Figure 5b). Thus, the chemical shifts of the signals of 1 and 2
are the opposite of what is expected on the basis of the σ-
inductive effects of oxygen and sulfur. The difference is likely
due to greater contributions from the paramagnetic term, σp, in
2 where the energy separation between the highest occupied
molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) is less than that of 1 (see
below).47 This results in a deshielding of the nucleus and
consequently a more downfield chemical shift than that in 1.47
Both signals are well upfield of that observed or the diaryl
plumbylene, Pb(ArPri )2, which has a 207Pb NMR chemical shift
4
of δ = 9430.48,49 The downfield shift of the diaryl plumbylenes
bearing the more electropositive substituents is consistent with
greater paramagnetic interactions involving the excited state
which acts to deshield the 207Pb NMR nucleus.1a,48−50 The shift
of 2 is comparable to bisamido plumbylenes such as
Pb{N(SiMe3)2}2 (δ = 4916).50 However, it contrasts with the
chemical shift observed for the previously reported bis-
most likely small as it was recently shown that a silicon
Pri4
complex, Si(SArMe )2, related to Pb(SAr )2 has only a minor
6
degree of Si−S π-bonding.56 In the absence of data for a series
of compounds, it is unclear how much the Ch−Pb−Ch angle
affects the energy of the HOMO−LUMO gap as well as other
transitions.
Me6
(terphenyl)amido plumbylene, Pb({N(H)ArMe }2, (Ar
=
6
CONCLUSIONS
C6H3-2,6-(C6H2-2,4,6- Me3)2) (δ = 2871) which probably has
Pb···C interactions with the terphenyl substituents.51 The 207Pb
NMR signal for 2 is significantly further downfield than the
monomeric and dimeric, three- and four-coordinate pyridine
adducts of lead dithiolates (δ = 2733−2873) reported by
Briand et al.52
■
Two isolobal examples of monomeric, two-coordinate
dichalcogenolate plumbylenes have been isolated and fully
characterized. The structural data reveal a large disparity in the
Ch−Pb−Ch bond angle with the dithiolate plumbylene being
about 22° narrower. In addition, it has been demonstrated that
the steric congestion of the aryl group can greatly affect the
metathesis reaction pathway as attempts to produce the less
The 207Pb NMR signal for 3 (Figure 6) is broad (ν1/2 = 351
Hz) with a chemical shift of δ = 2223. This is significantly
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hindered bisthiolato plumbylene, Pb(SArMe )2, by the same
method as 2 resulted in the synthesis of both the mono- and
trithiolato lead(II) complexes 3 and 4. As a whole, complexes
1−4 demonstrate that the steric loading at the lead(II) ion can
have dramatic consequences on the geometry. Furthermore,
these results suggest that steric effects should be strongly
considered when attempting to understand how lead and other
heavy metals distort the geometry of metalloenzymes, since it
has been shown the electronic factors only account for part of
the story. Future work will investigate the effects that govern
the contraction of the Ch−M−Ch bond angle and its possible
origin in dispersion forces between the bulky terphenyl
substituents.
6
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ASSOCIATED CONTENT
* Supporting Information
■
S
1H NMR spectra of 1 and 2, CIF files, and further thermal
ellipsoid plots for the X-ray structures of 1, 2, 3, and 4.
Expanded table of bond lengths and angles for the structures of
1, 2, 3, and 4. This material is available free of charge via the
Internet at http://pubs.acs.org.
(16) (a) Noheda, B. Curr. Opin. Solid State Mater. Sci. 2002, 6, 27−34
, and references contained within. (b) Turova, N. Y.; Turevskaya, E. P.;
Kessler, V. G.; Yanovskaya, M. I. The Chemistry of Metal Alkoxides;
Kluwer Academic Publishers: Norwell, MA, 2002; pp 141−144.
(17) Yao, S.; Block, S.; Brym, M.; Driess, M. Chem. Commun. 2007,
3844−2846.
AUTHOR INFORMATION
Corresponding Author
*E-mail: power@chem.ucdavis.edu.
■
Present Address
(18) Tam, E. C. Y.; Johnstone, N. C.; Ferro, L.; Hitchcock, P. B.;
Fulton, J. R. Inorg. Chem. 2009, 48, 8971−8976.
†Department of Chemistry, University of Nevada, Reno, 1664
N Virginia St., Reno, NV 89512.
(19) Magyar, J. S.; Weng, T. -C.; Stern, C. M.; Dye, D. F.; Rous, B.
W.; 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, 9495−9505 , and references therein.
Author Contributions
The manuscript was written through contributions of all
authors.
(20) (a) Warren, M. J.; Cooper, J. B.; Wood, S. P.; Shoolingin-
Jordan, P. M. Trends. Biochem. Sci. 1998, 23, 217−221. (b) Godwin, H.
A. Curr. Opin. Chem. Biol. 2001, 5, 223−227.
Notes
The authors declare no competing financial interest.
(21) (a) Erskine, P. T.; Senior, N.; Awan, S.; Lambert, R.; Lewis, G.;
Tickle, I. J.; Sarwar, M.; Spencer, P.; Thomas, P.; Warren, M. J.;
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Lambert, R.; Coker, A.; Lewis, G.; Spencer, P.; Sarwar, M.; Wood, S.
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4266−4276. (c) Jaffe, E. K.; Martins, J.; Li, J.; Kervinen, J.; Dunbrack,
R. L., Jr. J. Biol. Chem. 2001, 276, 1531−1537.
ACKNOWLEDGMENTS
■
We are grateful to the U.S. National Science Foundation for
research funding (Grant CHE-09848417) and for the dual
source X-ray diffractometer (Grant 0840444).
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