A base-stabilized lead(I) dimer and an aromatic plumbylidenide anion

Article abstract of DOI:10.1002/anie.201301954

The aromatic low-valent lead analogue of an indenyl anion (see scheme; 1) undergoes oxidation with SnCl₂ to form the base-stabilized lead(I) dimer 2. Reduction of 2 with lithium regenerates 1. These compounds were characterized by NMR spectroscopy and X-ray crystallography. Copyright

Full text of DOI:10.1002/anie.201301954

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Angewandte
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DOI: 10.1002/anie.201301954
Lead Chemistry
A Base-Stabilized Lead(I) Dimer and an Aromatic Plumbylidenide
Anion**
Siew-Peng Chia, Hong-Wei Xi, Yongxin Li, Kok Hwa Lim, and Cheuk-Wai So*
Heavier Group 14 alkyne analogues with the composition
REER (R = supporting ligand; E = Si, Ge, Sn, Pb) have
attracted much attention in the past decades owing to their
unique structures and reactivities.[1] A series of stable
disilynes, digermynes, and distannynes stabilized kinetically
by bulky aryl and silyl ligands was synthesized.[1] X-ray
crystallography showed that they have a trans-bent and planar
geometry in which the R-E-E angle decreases from silicon to
tin. Recent theoretical studies and UV/Vis spectroscopy
showed that they adopt a multiply bonded structure M in
solution (Scheme 1).[2] The reactivity of stable disilynes,
M in solution.[6] The singly bonded structure of [Ar*PbP-
bAr*] in the solid state is ascribed to packing effects.
Moreover, little is known about its reactivity,[7] and the
reduction of [Ar*PbPbAr*] has not yet been reported.
Recently, a series of novel base-stabilized Group 14
element(I) dimers [L⁄-⁄L] (E = Si, Ge, Sn, L = amidinate,
quanidinate, b-diketiminate, N-functionalized aryl, P-func-
tionalized amide) was synthesized.[8] They comprise an ⁄ ⁄
À
single bond and a lone pair of electrons on each E atom. Their
structures resemble the singly bonded structure S. Thus, they
are considered as base-stabilized heavier alkyne analogues.
Their reactivities showed that they are powerful reagents for
the activation of small molecules and unsaturated substra-
tes.[8b,c,g,9] In contrast, no examples of a base-stabilized lead(I)
dimer have yet been reported because of the synthetic
difficulties in preparing such molecules. For example, Jones
et al. reported that an attempt to isolate a b-diketiminate or
amidinate PbI dimer by the reduction of the corresponding
lead(II) chloride or triflate with the magnesium(I) dimer
failed, which led to the formation of homoleptic PbII complex
[HC(CtBuNMes)2PbD] (Mes = 2,4,6-Me3C6H2) or lead metal,
respectively.[8a,f] However, the lighter analogues were synthe-
sized in very high yield by a similar method. It seems that
a lead(I) dimer is unstable towards disproportionation.
Although lead(I) dimers are still unknown, they are
worthwhile synthetic targets both for fundamental reasons
and small-molecule activation. Moreover, they could show
different properties and reactivity compared with those of the
multiply bonded derivative [Ar*PbPbAr*], which has also
been little investigated compared with the lighter analogues.
Recently, a 2,6-diiminophenyl ligand has been shown to
stabilize a germanium(I) and tin(I) dimer.[8d,e] We anticipate
that the ligand is capable of stabilizing a lead(I) radical or its
dimeric derivative. Herein, we report the preparation of a 2,6-
diiminophenyllead(I) dimer [{LPbD}2] (L = 2,6-(CH =
NAr)2C6H3, Ar= 2,6-iPr2C6H3) by the oxidation of a 2,6-
diiminophenylplumbylidenide anion [LPb]À, which is the first
example of aromatic low-valent lead analogue of an indenyl
anion. The reduction of [{LPbD}2] with lithium regenerated
[LPb]À.
Scheme 1. Multiply bonded M and singly bonded structures S.
digermynes, and distannynes has been investigated extensi-
vely.[1d,3] One of the well-studied cases is reduction. Stable
disilynes, digermynes, and distannynes can undergo one-
electron and two-electron reduction to give the radical anions
[REER]CÀ and the doubly reduced species [REER]2À, respec-
tively.[1f,4] Comparison of their structural data with those of
À
the heavier alkyne analogues can provide insight into the E E
bonding.[2,5] In contrast, a diplumbyne is rare. Only [Ar*PbP-
bAr*] (Ar* = C6H3-2,6-(C6H2-2,4,6-iPr3)2) was synthesized
and structurally characterized by Power et al. in 2000.[1e] X-
ray crystallography showed that it has a strongly bent
À
structure, a long Pb Pb single bond, and a lone pair of
electrons on each lead atom (singly bonded structure S).
However, theoretical studies and UV/Vis spectroscopy
showed that [Ar*PbPbAr*] has a multiply bonded structure
[*] S.-P. Chia, Dr. Y. Li, Dr. C.-W. So
Division of Chemistry and Biological Chemistry
Nanyang Technological University
The reaction of [LBr][10] (1) with nBuLi in THF at À788C,
followed by the treatment with PbBr2, afforded [LPbBr] in
48.1% yield (2; Scheme 2).[11] It was isolated as a highly air-
and moisture-sensitive yellow crystalline solid. It is soluble in
hydrocarbon solvents and was characterized by NMR spec-
21 Nanyang Link, Singapore 637371 (Singapore)
E-mail: CWSo@ntu.edu.sg
Dr. H.-W. Xi, Prof. Dr. K. H. Lim
Division of Chemical and Biomolecular Engineering
Nanyang Technological University
1
troscopy. The H and 13C NMR spectra of 2 show one set of
62 Nanyang Drive, Singapore 637459 (Singapore)
signals owing to the 2,6-diiminophenyl ligand. No 207Pb NMR
resonance could be observed because the quadrupolar 79/81Br
nuclei may broaden the signal. The result is in accord with
reported problems in recording 207Pb NMR spectra of
[**] This work is supported by the Central Strategic Initiative for Inter-
disciplinary Competitive Fund.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301954.
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equivalent of Li in THF at À408C afforded a mixture of 2,6-
diiminophenyllithium (LLi)2 (major product), lead(I) dimer
[{LPbD}2], homoleptic plumbylene [L2PbD],[15] and unidentified
products, which was confirmed by NMR spectroscopy. Only
(LLi)2 can be isolated from the mixture by recrystallization
(Supporting Information, Figure S1). Similarly, the reaction
of 2 with 0.5 equivalent of [HC(CMeNMes)2Mg]2 in THF at
À408C afforded [L2PbD] and [HC(CMeNMes)2MgBr], which
was confirmed by NMR spectroscopy. The results are
consistent with the reduction of amidinate- or b-diketiminate
lead(II) halides.[8a,f]
In contrast, the reaction of 2 with excess Li in THF at
À408C afforded the plumbylidenide anion [Li(thf)4][LPb] (3),
in which the negative charge is aromatically delocalized in the
PbC3N five-membered ring.[11] Compound 3 is the first
aromatic low-valent lead analogue of an indenyl anion. The
results indicate that 2p orbitals of C/N atoms and a 6p orbital
of a low valent Pb atom can sufficiently overlap to form an
aromatic compound. In contrast, a similar tetravalent lead
analogue of a cyclopentadienyl anion, which is the lithiome-
sitylplumbole [C4Ph4Pb(Mes)Li],[16] does not show any aro-
matic character. Until now, only one tetravalent lead com-
pound, which is the dianionic plumbole [Li(dme)3]-
[(dme)Li(h5-PbC4Ph4)], shows considerable aromaticity.[16]
Compound 3 was isolated as a light-sensitive dark-red
crystalline solid in 12.6% yield. It is stable in ether solvents
and the solid state. It decomposes in toluene and C6D6 to form
LLi and lead metal, which was confirmed by NMR spectros-
copy. Thus, the spectroscopic analyses of 3 can only be
performed in [D8]THF. The 1H NMR spectrum at room
temperature shows a doublet and septet at d 1.12 and
3.35 ppm for the iPr substituents. It also displays signals for
phenyl protons at d 6.37–7.04 ppm. It is noteworthy that the
signal for HCNAr (d 7.92 ppm) shows an upfield shift
compared with that of 2 (d 9.15 ppm) and falls in the aromatic
proton region. Moreover, the 1H NMR spectrum of 3 was also
acquired at À608C, whereupon two sharp doublets (d 1.05,
1.10 ppm) and two septets (d 3.12, 3.24 ppm) for the non-
equivalent iPr protons were resolved. The results indicate that
compound 3 retains its solid-state structure at À608C in
solution, and the imino substituents are fluxional in solution
at room temperature. Moreover, the 207Pb{1H} NMR signal (d
3415 ppm) at room temperature shows a significant downfield
shift compared with that of the 2,6-diiminophenyllead(I)
dimer [{LPbD}2] (4, d 1684 ppm). The results indicate that the
negative charge at the Pb atom is stabilized by an aromatic
delocalization in the PbC3N five-membered ring. It is also
supported by DFT calculations of 3À (Supporting Informa-
tion, Figure S3, M06-2x/LanL08(d) level),[17] which reveal
pronounced aromaticity of the PbC3N ring, indicated by the
negative nucleus independent chemical shift value[18]
(NICS(1) = À6.33 ppm). Furthermore, the NICS(1) value of
the PbC3N ring is comparable with that of the dianionic
plumbole [Li(dme)3][(dme)Li(h5-PbC4Ph4)] (NICS(1) =
À6.28 ppm).[16]
Scheme 2. Synthesis of 2–4.
organolead(II) halides, such as [{Ar*PbBr}2],[12] [Ar*Pb-
(NC5H5)Br],[12] and [4-tBu-2,6-{P(OEt)2 O}2C6H2PbCl].
[13]
=
X-ray crystallography showed that the 2,6-diiminophenyl
ligand is bonded in tridentate fashion to the lead atom, which
adopts a seesaw geometry with the N atoms at the axial
positions and the C1 and Br1 atoms at the equatorial positions
(Figure 1). The C1-Pb1-Br1 (95.0(4)8) and N-Pb-Br (average
Figure 1. Molecular structure of compound 2 (ellipsoids set at 50%
probability). Hydrogen atoms and disorder in the iPr substituents are
omitted for clarity. Selected bond lengths [ꢀ] and angles [8]: Pb1–C1
2.289(19), Pb1–Br1 2.701(2), Pb1–N1 2.637(16), Pb1–N2 2.691(17),
N1–C7 1.27(2), N2–C20 1.25(3), C2–C20 1.46(3), C6–C7 1.48(3), C1–
C6 1.36(3), C1–C2 1.38(3); C1-Pb1-Br1 95.0(4), C1-Pb1-N1 68.6(6), C1-
Pb1-N2 68.0(6), N1-Pb1-N2 135.9(5), N1-Pb1-Br1 88.1(4), N2-Pb1-Br1
88.2(4).
88.28) angles indicate little hybridization of the lead valence
orbitals as well as the presence of a lone pair with high
À
À
s character at the lead atom. The Pb C (2.289(19) ꢀ) and Pb
Br bonds (2.701(2) ꢀ) are comparable with those in the
pyridine-stabilized terphenyllead(II) bromide [Ar*Pb-
(NC5H5)Br] (Pb C 2.322(4), Pb Br 2.7063(6) ꢀ).[12] Further-
À
À
À
more, the Pb N bonds (2.637(16), 2.691(17) ꢀ) are longer
À
than the intermolecular Pb N bond in [Ar*Pb(NC5H5)Br]
(2.502(4) ꢀ).[12]
X-ray crystallography showed that compound 3 is mono-
meric (Figure 2). The 2,6-diiminophenyl ligand is bonded in
a bidentate fashion to the lead atom. The PbC3N ring is planar
and the sum of the internal bond angles is 540.08. The Li1
An attempt to isolate the 2,6-diiminophenyllead(I) dimer
[{LPbD}2] (4) by the reduction of 2 with alkali metals or the
magnesium(I) dimer[14] failed. The reaction of 2 with one
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NAr)2C6H3ED}2] (E = Ge, R = H: 438, 586 and 702 nm; E = Sn,
R = Me: 425, 457 and 561 nm).[8b,e] The shift is comparable
with that observed in the amidinate-stabilized Group 14
element(I) dimers [{[R1C(NAr)2]ED}2] (R1 = C6H4-4-tBu,
Ar= 2,6-iPr2C6H3; E = Si: 629, Ge: 502, Sn: 388 nm).[8a]
Moreover, this is opposite to the electronic spectra of
multiple-bonded heavier Group 14 alkyne analogues
[Ar*EEAr*] (E = Ge: 365, 490; Sn: 409, 593; Pb: 397,
719 nm) in which there is a bathochromic shift on descending
the group.[1e,g] The results imply that the Pb Pb bond in 4 has
À
little p character in solution, which is different from [Ar*PbP-
bAr*] comprising a multiply bonded structure M in solution.
X-ray crystallography showed that the 2,6-diiminophenyl
ligand is bonded in tridentate fashion to the lead atoms, which
adopt a seesaw geometry with the N atoms at the axial
positions and the C atoms at the equatorial positions
(Figure 3). The C-Pb-Pb angles (average 108.58) indicate
that there is a lone pair of electrons at the lead atoms.
Moreover, compound 4 has a gauche-bent structure (C-Pb-
Pb-C 102.28), which is different from [Ar*PbPbAr*] compris-
À
ing a trans-bent structure. The Pb Pb bond (3.1283(6) ꢀ) is
comparable with that in [Ar*PbPbAr*] (3.1881(1) ꢀ), which
À
À
indicates that the Pb Pb bond is a single bond. The Cimine
N
Figure 2. Molecular structure of compound 3 (ellipsoids set at 50%
probability). Hydrogen atoms and disorder in THF molecules omitted
for clarity. Selected bond lengths [ꢀ] and angles [8]: Pb1–N1 2.425(5),
N1–C7 1.347(10), C6–C7 1.380(10), C1–C6 1.444(9), Pb1–C1 2.180(5),
N2–C20 1.293(11), C20–C2 1.456(10); C1-Pb1-N1 73.2(2), Pb1-N1-C7
111.3(4), C1-C6-C7 118.8(7), C6-C7-N1 120.1(7), Pb1-C1-C6 116.6(5).
À
bonds (average 1.270 ꢀ) are shortened and the Pb N
À
(average 2.671 ꢀ) and Pb C (average 2.264 ꢀ) bonds are
lengthened compared with those of 3, which indicate that
À
atom is coordinated with four THF molecules. The Pb1 Li1
distance (8.041(8) ꢀ) is longer than the sum of van der Waals
radii, which suggests that there is no interaction between
these atoms. Comparing the bonding in the PbC3N five-
À
À
membered ring (Pb1 N1 2.425(5) ꢀ, N1 C7 1.347(10) ꢀ,
À
À
À
C6 C7 1.380(10) ꢀ, C1 C6 1.444(9) ꢀ, Pb1 C1 2.180(5) ꢀ)
À
À
with those in compound 2 (Pb N average 2.664 ꢀ, N1 C7
and N2 C20 average 1.26 ꢀ, C6 C7 and C2 C20 average
1.47 ꢀ, C1 C2 and C1 C6 average 1.37 ꢀ, Pb C
2.289(19) ꢀ), it is suggested that there is an aromatic
À
À
À
À
À
À
delocalization in the PbC3N five-membered ring. Moreover,
II
À
À
the Pb1 N1 bond is still longer than reported Pb Namide
single bonds (2.07(5)–2.35(5) ꢀ).[19]
Recently, Sekiguchi et al. reported that a stannylsodium
intermediate [tBu2MeSi]3SnNa underwent an oxidation with
SnCl2·dioxane to form a stable stannyl radical.[20] It is
anticipated that a lead(I) radical or its dimeric derivative
can be prepared by a similar strategy. The reaction of two
equivalents of 3 with SnCl2 in THFat À788C afforded the 2,6-
diiminophenyllead(I) dimer [{LPbD}2] (4).[11] The 1H and
13C NMR spectra of 4 show one set of signals for the 2,6-
diiminophenyl ligand. The 207Pb NMR signal of 4 (d
1684 ppm) shows an upfield shift compared with that of 3.
Compound 4 was isolated as a light-sensitive dark-green
crystalline solid in 12.1% yield. It is stable in hydrocarbon
solvents and the solid state. The UV/Vis spectrum of 4 in THF
shows three absorption bands at 396, 473, and 526 nm in the
visible light region, which shows a hypsochromic shift
Figure 3. Molecular structure of compound 4 (ellipsoids set at 50%
probability). Hydrogen atoms and disorder in the Ar substituents are
omitted for clarity. Selected bond lengths [ꢀ] and angles [8]: Pb1–Pb2
3.1283(6), Pb1–N1 2.613(10), Pb1–N2 2.706(9), Pb2–N3 2.715(9),
Pb2–N4 2.650(8), Pb1–C1 2.287(11), Pb2–C33 2.240(11), N1–C3
1.276(14), N2–C8 1.287(14), C2–C3 1.455(17), C7–C8 1.463(16), C1–
C2 1.423(16), C1–C7 1.400(16); C1-Pb1-Pb2 111.3(3), C1-Pb1-N1
69.5(4), C1-Pb1-N2 68.6(4), N1-Pb1-Pb2 81.2(2), N2-Pb1-Pb2
119.42(19), C33-Pb2-Pb1 105.7(3), C33-Pb2-N3 68.5(3), C33-Pb2-N4
68.7(3).
=
compared with that of the lighter congeners [{2,6-(CR
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[4] a) C. Cui, M. M. Olmstead, J. C. Fettinger, G. H. Spikes, P. P.
there is no electronic delocalization in the PbC3N rings.
Moreover, the bond lengths of the PbC3N rings are compa-
rable with those in 2. It is concluded that compound 4 has
a singly bonded structure S in both solution and the solid
state.
To compare with the reactivity of multiply bonded heavier
Group 14 alkyne analogues, the reduction of compound 4
with two equivalents of lithium in THF was performed, which
quantitatively afforded compound 3.[11] It is in contrast to the
reduction of stable disilynes, digermynes, and distannynes to
give the radical anions [REER]CÀ or the doubly reduced
species [REER]2À. Moreover, the results imply that the
reaction of 2 with excess Li in THF proceeds through the
formation of a lead(I) radical intermediate, which then reacts
with lithium to form 4.
In conclusion, we have reported the synthesis of the first
base-stabilized lead(I) dimer by a simple procedure. X-ray
crystallography and NMR spectroscopy showed conclusively
that compound 4 has a singly bonded structure S in both
solution and the solid state. The reduction of 4 with lithium
afforded the aromatic plumbylidenide anion 3, which is in
contrast to the outcome by the reduction of multiply bonded
heavier Group 14 alkyne analogues. The small molecules
activation of 4 and the utilization of 3 as an electron-rich
ligand are currently under investigation.
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Received: March 8, 2013
Published online: April 29, 2013
[11] For the details of experimental procedures, see the Supporting
Information.
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[15] Although [L2PbD] was not reported, it can be synthesized by the
reaction of two equivalents of LLi with PbCl2 in THF. It has been
analyzed by X-ray crystallography. For details, see the Support-
ing Information. With the NMR signals at hand, we can illustrate
that [L2PbD] was formed by the reaction of 2 with one equivalent
of Li in THF.
Keywords: aromaticity · carbene homologues ·
diiminophenyl ligands · lead · low-valent compounds
.
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