Synthesis and theoretical studies on rare three-coordinate lead complexes

Article abstract of DOI:10.1039/b702994b

A series of β-diketiminate lead halide complexes has been synthesised LPbCl (2), LPbBr (3) and LPbI (4) (L = {N(2,6-ⁱPr₂C ₆H₃)C(Me)}₂CH]), which includes a rare example of a three-coordinate lead iodide (4). The chloride and bromide complexes, 2 and 3, are relatively stable in both the solid and solution states, only slowly decomposing to elemental lead over the course of a month in solution, the lead iodide 4 appears to be less stable and decomposes after 3 d in the solid state at ambient temperatures. The lead chloride complex 2 was treated with KN(SiMe₃)₂ to yield an unusual terminal lead amide complex LPbN(SiMe₃)₂ (5). Unlike three-coordinate β-diketiminate transition metal-halide complexes, the ligands are present in a pyramidal arrangement around the lead centre, commonly attributed to the presence of a stereochemically active lone pair. We have investigated the influence of this lone pair on the geometry of the metal halide complexes 2-4, as well as the isostructural germanium and tin complexes (6 and 7, respectively) using DFT calculations. The lone pair in the lead complexes is significantly more diffuse than in the tin and germanium analogues and only a small amount of hybridisation between the 6s and 6p orbitals is observed. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b702994b

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www.rsc.org/dalton | Dalton Transactions
Synthesis and theoretical studies on rare three-coordinate lead complexes†
Matthew Chen, J. Robin Fulton,* Peter B. Hitchcock, Nick C. Johnstone, Michael F. Lappert* and
Andrey V. Protchenko
Received 27th February 2007, Accepted 20th April 2007
First published as an Advance Article on the web 17th May 2007
DOI: 10.1039/b702994b
A series of b-diketiminate lead halide complexes has been synthesised LPbCl (2), LPbBr (3) and LPbI
(4) (L = {N(2,6-iPr2C6H3)C(Me)}2CH]), which includes a rare example of a three-coordinate lead
iodide (4). The chloride and bromide complexes, 2 and 3, are relatively stable in both the solid and
solution states, only slowly decomposing to elemental lead over the course of a month in solution, the
lead iodide 4 appears to be less stable and decomposes after 3 d in the solid state at ambient
temperatures. The lead chloride complex 2 was treated with KN(SiMe3)2 to yield an unusual terminal
lead amide complex LPbN(SiMe3)2 (5). Unlike three-coordinate b-diketiminate transition metal–halide
complexes, the ligands are present in a pyramidal arrangement around the lead centre, commonly
attributed to the presence of a stereochemically active lone pair. We have investigated the influence of
this lone pair on the geometry of the metal halide complexes 2–4, as well as the isostructural
germanium and tin complexes (6 and 7, respectively) using DFT calculations. The lone pair in the lead
complexes is significantly more diffuse than in the tin and germanium analogues and only a small
amount of hybridisation between the 6s and 6p orbitals is observed.
which utilise sterically encumbered ligands to stabilise the metal
Introduction
centre. For instance, Power has popularised the use of bulky
Historically, lead has been one of the most versatile industrially
terphenyl ligands that can be used to stabilise tri-coordinate lead–
used metals since the Bronze era. Not only was it a key component
in metal piping due to its malleable properties and it resistance
to corrosion, but lead compounds have has also been used in
many other applications such as additives in paints and as an anti-
bromide complexes, notably the lead–bromide dimer (ArPbBr)2
(Ar = 2,6-Trip2–C6H3; Trip = 2,4,6-iPr3C6H2), and its analogues,
ArPbBr·pyridine and ArPbBr·NH3.7,8 This ligand system can also
be utilised to stablise one of the two reported three-coordinate lead
iodide complexes.9,10 A wider variety of lead chloride complexes
exist, both in the form of lead-dimers such as (RPbCl)2 (R =
C(SiMe2Ph)3),11 as well two monometallic complexes in which the
lead is bound to a chelating monoanionic ligand in addition to a
chloride anion.12,13
knock agent in petrol. However, due to its toxic properties, its use
as a key industrial metal has been minimised to very specialised
applications in the past 40 years, although its harmful effects are
still being felt today. For instance, lead is still responsible for the
greatest number of environmentally caused diseases in the United
States.1
Despite its historical significance, the chemistry of lead has
largely been ignored. With regards to divalent lead, most work has
focused on its diverse coordination chemistry with multidentate
ligands,1–3 with many studies focusing on how divalent lead might
bind to proteins in biological systems (a key pathway in lead
poisoning). However, outside gas-phase and theoretical studies,4–6
the chemistry of ligands bound to a divalent lead centre has
rarely been examined. We have been interested in synthesizing
low-coordinate lead complexes possessing a stable ancillary ligand
set in addition to a reactive functional group (such as a halide)
in order to explore lead-based reactivity. Only a handful of
three-coordinate, monometallic lead halides are known, most of
The hurdles with lead-based chemistry are the light sensitivity,
a tendency to form insoluble precipitates when the ligands possess
insufficient steric bulk in addition to its propensity to undergo
redox chemistry as it is readily reduced to form black lead(0). As
such, a ligand system that coordinates tightly to the lead metal cen-
tre and is also capable of providing enough steric bulk to prevent
the formation of oligomers must be employed. To this end we have
utilised the sterically encumbered b-diketiminate ligand [{N(2,6-
iPr2C6H3)C(Me)}2CH] (L, for brevity, this bulky b-diketiminate
ligand will be referred to as BDI) to help stabilise the desired
terminal lead halide complexes as precursors for novel, reactive
lead complexes. This bulky ligand has been successfully utilised to
stabilise a variety of different low-coordinate transition and main
group metal complexes.14–16 Although the isostructural divalent
germanium– and tin–halide complexes have been generated,17 to
our knowledge, the only reported b-diketiminate lead complex
is a bis-substituted b-diketiminate lead complex utilising the
N,Nꢀ-bis(trimethysilyl) b-diketiminate ligand.18 We reasoned that
ligand L would be bulky enough such that only one b-diketiminate
ligand could coordinate to the metal centre, but also allow for the
generation of our desired terminal-halide complexes.
Department of Chemistry, University of Sussex, Falmer, Brighton, BN1 9QJ,
UK. E-mail: j.r.fulton@sussex.ac.uk; m.f.lappert@sussex.ac.uk; Fax: +44
(0) 1273 677196; Tel: +44 (0) 1273 873170 (Fulton); +44 (0) 1273 678316
(Lappert)
† Electronic supplementary information (ESI) available: Thermal el-
lipsoid plot of 3; temperature dependent NMR spectra of 5; opti-
mised geometries for [{N(Me)C(Me)}2CH]PbX (X = Cl, Br, I) and
[{N(C6H5)C(Me)}2CH]PbX (X = Cl, Br, I); 1H and 13C NMR spectra
of compound 1. See DOI: 10.1039/b702994b
2770 | Dalton Trans., 2007, 2770–2778
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Table 1 Crystal structure and refinement data for 2, 3, 4 and 5
LPbCl (2)
LPbBr (3)
LPbI (4)
LPbN(SiMe3)2 (5)
Chemical formula
Formula weight
T/K
C29H41N2PbCl
660.28
173(2)
C29H41N2PbBr
704.74
173(2)
C29H41N2PbI
751.73
173(2)
C35H69N3Si2Pb·0.5(C6H6)
824.28
173(2)
˚
k/A
0.71073
0.71073
0.71073
0.71073
Crystal size/mm
Crystal system
Space group
0.2 × 0.2 × 0.2
0.20 × 0.15 × 0.15
0.25 × 0.25 × 0.10
Monoclinic
P21/c (No.14)
22.3656(5)
11.2144(3)
11.6021(3)
90
101.411(1)
90
2852.48(12)
4
0.2 × 0.2 × 0.1
Triclinic
Triclinic
Triclinic
¯
¯
¯
P1 (No.2)
P1 (No.2)
P1 (No.2)
˚
a/A
10.4070(1)
12.0343(1)
12.4700(2)
89.464(1)
72.864(1)
70.868(1)
1403.40(3)
2
1.56
10.4137(2)
12.0840(4)
12.4677(4)
89.224(2)
72.815(2)
70.638(2)
1407.89(7)
2
1.66
11.9346(1)
13.3033(1)
14.6879(2)
97.37
100.251(1)
115.650(1)
2012.49(4)
2
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
pc/Mg m−3
1.75
7.02
1.36
4.28
l/mm−1
6.13
7.43
h Range for data collection/◦
Measured/indep rflns/R(int)
Rflns with I > 2r(I)
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
3.72 to 27.48
27534/6372/0.046
6022
6372/0/300
1.071
R1 = 0.020, wR2 = 0.044
R1 = 0.022, wR2 = 0.045
0.79 and −1.21
3.44 to 26.07
19313/5529/0.043
5215
5529/0/300
1.260
R1 = 0.037, wR2 = 0.094
R1 = 0.040, wR2 = 0.095
2.40 and −1.18
3.51 to 25.99
37364/5590/0.060
5590
5590/0/300
1.166
R1 = 0.033, wR2 = 0.071
R1 = 0.041, wR2 = 0.074
2.22 and −1.26 (near Pb)
3.71 to 25.04
21146/6985/0.041
6494
6985/0/399
1.027
R1 = 0.025, wR2 = 0.056
R1 = 0.029, wR2 = 0.088
0.60 and −1.120
3
˚
Largest diff. peak and hole/e A
˚
Results and discussion
(vide infra). The lead–chloride bond length is 2.5653(7) A, slightly
shorter than other reported terminal lead chloride complexes.12,13
The N(1)–Pb–N(2) bond angle of 82.77◦ is smaller than that of
the isostructural LSnCl (85.2◦, DOP = 100%), which in turn is
smaller than the LGeCl complexes (90.89, DOP = 87.3%).17 The
diminishing trend in bond angles down the Group 14 metals is
consistent with the trend observed with Group 13 BDI complexes,
and has been attributed to the increase in size of the metal cation
of the heavier elements.20
Synthesis of lead halides
Treatment of a suspension of PbCl2 with a THF solution of
lithiated b-diketiminate (1) results in the formation of LPbCl (2)
in 64% yield after 1 d at room temperature (eqn (1)). The 1H NMR
spectrum revealed four doublets corresponding to the methyl
protons of the b-diketiminate isopropyl groups, suggesting a non-
planar environment around the metal centre. This was confirmed
by the X-ray crystal structure, which showed a pyramidal lead
comp◦lex with the sum of the bond angles around the metal centre
267.2 , or a degree of pyramidalization (DOP) of 103%,19 similar
to other Group 14 b-diketiminate metal halide complexes as well
as other tri-coordinate lead halides (Fig. 1, see Table 1 for crystal
structure and refinement data and Table 2 for selected bond lengths
and angles).11,17 This pyramidal geometry has previously been
attributed to the presence of a “stereochemically active” lone pair
(1)
◦
˚
Table 2 Selected bond lengths (A) and bond angles ( ) for 2, 3, and 4
LPbCl (2)
LPbBr (3)
LPbI (4)
Pb–N(1)
Pb–N(2)
Pb–X
N(1)–C(1)
C(1)–C(2)
C(2)–C(3)
C(3)–N(2)
2.290(2)
2.280(2)
2.5653(7)
1.320(3)
1.407(4)
1.392(4)
1.335(3)
82.77(7)
91.13(5)
93.27(5)
267.2
2.281(5)
2.292(5)
2.7149(8)
1.333(8)
1.392(10)
1.408(10)
1.324(9)
83.93(19)
93.28(14)
91.31(13)
268.5
2.311(4)
2.298(4)
2.9531(5)
1.334(7)
1.396(8)
1.413(8)
1.330(7)
83.26(16)
90.52(11)
93.02(11)
266.8
N(1)–Pb–N(2)
N(1)–Pb–X
N(2)–Pb–X
Sum of angles
Degree of
103
102
104
pyramidalization (%)a
Pb–C3N2-planeb
0.683
0.694
0.846
a Reference 19. b See text.
Fig. 1 Thermal ellipsoid plot (30%) of 2. H atoms omitted for clarity.
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Due to the heterogeneity of the reaction, yields of lead chloride
2 are sensitive to mixing rates and reaction times. If the reaction
mixture is stirred for longer than 24 h, a metallic precipitate is
observed, along with the formation of protonated ligand (LH).
However, due to the very low solubility of lead chloride in THF,
adequate mixing times are needed in order to obtain respectable
yields. The outcome of the reaction is not noticeably affected by
light, indicating the system to be photostable in the presence of
ambient light sources. Conveniently, lead chloride 2 is virtually
insoluble in pentane, and both lithiated ligand 1 and protonated
ligand LH can easily be washed away from complex 2 using this
solvent. Once isolated, this complex is stable indefinitely in the
solid state under inert atmospheres at ambient temperatures, but
toluene or benzene solutions will slowly degrade to an unidentified
insoluble white precipitate over the course of a month.
We attempted to generate the bis-substitued BDI–lead complex
(L2Pb) by treatment of PbCl2 with two equivalents (or more) of 1;
however, the only lead complex observed was the chloride 2.
b-Diketiminate lead bromide 3 was synthesized in an analogous
manner to that of chloride 2, resulting in an isolated yield of 65%.
Like the chloride, upon purification, the lead bromide 3 appears to
be fairly stable and can be stored in an inert environment at room
temperature with minimal decomposition. Crystals suitable for an
X-ray diffraction study were obtained by cooling a concentrated
toluene solution of lead bromide 3 to −30 ◦C. The X-ray structure
showed the complex to be isostructural with that of the lead
chloride complex 2, with a pyramidal arrangement of the ligands
around the metal and similar bond lengths and bond angles (see
ESI† for the thermal ellipsoid plot of 3). The crystral structure and
refinement data are shown in Table 1, and selected bond lengths
and bond angles are shown in Table 2. The Pb–Br bond length
Table 1, and selected bond lengths and bond angles are shown in
˚
Table 2. The Pb–I bond length (2.9531(5) A) is slightly longer than
the other known low valent Pb–I compounds such as Filippou’s
[Ar Pb-(l-I)]2 (Ar = 2,6-Trip2–C6H3),10 (2.9499(7) A for the shorter
˚
of the two Pb–I bonds) and lead–iodide moiety that has been
9
˚
stabilised by an iridium–arsenic metallomacrocycle (2.892(2) A).
Our lead iodide complex 4 appears to be less stable than the other
halides, and slowly decomposes at ambient temperatures, even in
the solid state.
Derivatisation of lead chloride 2
Treatment of lead chloride 2 with one equivalent of KN(SiMe3)2 in
THF results in the formation of b-diketiminate lead–amide 5 after
1 h at room temperature (Scheme 1). This compound can also be
made in a one-pot synthesis from protonated ligand LH by heating
a sealed vessel of a hexane mixture of LH and Pb[N(SiMe3)2]2
◦
1
to 90 C overnight. The H NMR spectrum of 5 revealed four
resonances corresponding to methyl protons of the b-diketiminate
isopropyl groups, similar to that noted for the halides 2–4. At room
temperature, the protons of the trimethylsilyl groups resonate as
two, very broad signals, becoming well resolved as singlets at d
◦
◦
0.46 and 0.12 ppm upon cooling to −40 C. Warming to 25 C
gave coalescence corresponding to a low energy (DG‡ = 14 kcal
mol−1) fluxional process. We attribute this to hindered rotation
about the Pb–N bond due to interaction of the silyl groups with
the isopropyl groups on the BDI ligand.
˚
is 2.7149(8) A, which is shorter than the Pb–Br bond length of
˚
Powers’ aryl lead bromide dimer, [ArPb(l-Br)]2 (2.7892(16) A
for the shorter of the two Pb–Br bonds),7 although slightly
longer than the lead–bromide bond of the pyridine complex of
Scheme 1
his monomeric aryl lead bromide complexes (PbBrAr–L, L =
7
˚
pyridine) at 2.7063(6) A.
The b-diketiminate lead iodide complex 4 was generated in 59%
isolated yield by treatment of lithium salt 1 with lead diiodide.
Crystals suitable for an X-ray diffraction study were obtained by
cooling a concentrated toluene solution to −30 ◦C. This complex
is a very rare example of a low-coordinate lead-iodide complex
(Fig. 2). The crystral structure and refinement data are shown in
This reactivity is in sharp contrast to that observed with the
isostructural LGeCl (6) complex where it was observed that
treatment of germanium chloride 6 with LiN(SiMe3)2 resulted in
the deprotonation of the ring (C3N2Ge) methyl group to give a
stable germylene complex.21 We were unable to find any evidence
of deprotonation of our ring (C3N2Pb) methyl group, even upon
heating to 60 ◦C.
Crystals suitable for an X-Ray diffraction study of amide 5
were obtained by slow evaporation of a concentrated benzene
solution of 5 at room temperature. Table 1 lists the crystal structure
and refinement data and Table 3 lists selected bond lengths and
angles. Similar to our lead–halide complexes, 2–4, a pyramidal
geometry is observed around the metal centre (Fig. 3), however,
the angles in this instance are more obtuse (sum of angles = 290◦,
DOP = 77.6%), with the angles between the N(1)–Pb–N(2) plane
and the amido ligand significantly larger at 102–105◦, even though
the bite angle made by the metallacycle [N(1)–Pb–N(2)] is still
only 83.54◦. However, the Pb–N(1) and Pb–N(2) bond lengths of
˚
(2.338 A) are longer than those of the halides 2–4. Amide complex
5 is an unusual example of a terminal lead amide complex and
exmplifies how the bulky bis-trimethylsilyl amide ligand can be
used to stabilise such species.22,23
Fig. 2 Thermal ellipsoid plot (30%) of 4. H atoms omitted for clarity.
2772 | Dalton Trans., 2007, 2770–2778
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◦
˚
halide complexes of Cu, Fe, Co and Zn are generally planar,
a shift from linearity occurs when bulky amides, phosphines or
thiolates are introduced into the coordination sphere. In contrast,
three-coordinate BDI metal halide complexes of divalent Group
14 metal centres all show this deviation. For instance, in LSnCl (7),
Table 3 Selected bond lengths (A) and bond angles ( ) for 5
Pb–N(1)
Pb–N(2)
Pb–N(3)
N(3)–Si(1)
2.338(3) N(1)–C(1)
2.338(3) C(1)–C(2)
2.281(3) C(2)–C(3)
1.718(3) C(3)–N(2)
1.721(3)
1.328(4)
1.399(5)
1.401(5)
1.318(4)
˚
N(3)–Si(2)
the tin metal centre lies outside the ligand plane by 0.658 A. This
N(1)–Pb–N(2)
N(1)–Pb–N(3)
N(2)–Pb–N(3)
Sum of angles
Degree of
83.54(9)
Pb–N(3)–Si(1)
104.91(10) Pb–N(3)–Si(2)
101.74(10) Si(1)–N(3)–Si(2)
107.35(14)
131.26(15)
120.73(16)
is less pronounced in the germanium complex (LGeCl, 6) in which
˚
the metal centre lies outside the ligand plane by only 0.564 A.
Interestingly, the deviation of the metal outside the C3N2 plane
that we have observed in our systems is commonly found with four-
coordinate BDI metal halides (although this phenomenon is not
observed with less sterically encumbered b-diketiminate ligands).
Presumably, as the angle between the N(1)–M–N(2) plane and the
halide becomes more acute, there is greater interaction between
the isopropyl groups and the halide, necessitating the metal move
outside of the plane of the b-diketiminate backbone.
290.2
77.6
pyramidalization (%)a
Pb–C3N2-planeb
0.407
a Reference 19. b See text.
The similarity of the non-planarity of our metallacycles (C3N2–
Pb) to four-coordinate BDI complexes, as well as the pyramidal
geometry around the metal centres, leaves open the possibility
that a fourth coordination site on our complexes is occupied by a
“stereochemically active” lone pair. Indeed, this lone pair has been
invoked to explain not only the pyramidal geometry of germanium
and tin BDI complexes, but also a “void” in the coordination
geometry of a multitude of divalent lead complexes.33 This void
is observed in almost all lead complexes with a coordination
number less than six, and hemidirected geometries have also been
observed in lead complexes with up to eight coordinated ligands.33
Interestingly, this lone pair on the lead metal centre has yet to
be conclusively shown to have any of the properties normally
attributed to a lone pair found in the lighter main group elements.
Indeed, one would predict that there should be minimal mixing
between the 6s and 6p orbitals due to the larger energy gap between
the two orbitals. Several groups have theoretically investigated the
mixing of the 6s and 6p orbitals, and have found that the harder
the ligands bound to divalent lead, the less mixing is observed
between the 6s and 6p orbitals (vide infra).
Fig. 3 Thermal ellipsoid plot (30%) of 5. H atoms omitted for clarity.
Structural studies of lead complexes
In all structures, the metal centre is significantly displaced from the
˚
N(1)–C(1)–C(2)–C(3)–N(2) (C3N2) plane by between 0.407 A (5)
˚
to 0.850 A (4). This deviation from the plane has been observed in
other b-diketiminate complexes and theories have been postulated
to explain this observation. One assumption is that the cavity
size of the ligand is not suitable for larger cations,24 however, the
largest cation to be coordinated to the b-diketiminate ligand is
Tl(I), which has one of the largest ionic radii of any metal.25 In
this example the metal deviates from the ligand plane by only
DFT Calculations
In order to investigate factors controlling the distance between the
metal centre and the C3N2 plane within the ligand system, as well
as to ascertain the importance of the lone pair in determining
the geometry of the ligands around the lead metal centre, we
performed calculations on our system using the B3LYP density
functional theory and LanL2DZ psuedo-potentials (and basis set)
implemented in Gaussian 03.34
Our initial investigations looked at geometric optimisation
of simpler ligand systems generated by replacing the sterically
encumbered N-(2,6-diisopropylpheny) groups with smaller N-
˚
0.076 A, too small to account for the deviation observed with
our smaller divalent lead cation.20 A second theory involves the
higher-order bonding of the BDI ligand to the metal centre, such
5
as an g -binding mode. This has been observed in b-diketiminate
complexes of heavier alkaline earth elements, rare-earth metals as
well as Group 4 metal centres.26–31 However, these examples are
extreme cases and involve obvious direct interaction of the ligand
backbone with the coordinatively unsaturated metal centre. In
most complexes, the out-of-plane ligand p-orbitals that could be
used to bind in an eta-fashion to the metal centre are considered
to be too low in energy and would not contribute to ligand–metal
binding.32
A third theory postulates that the BDI ligand around the metal
centre is too sterically encumbered, thus necessitating a deviation
from planarity. This is indeed the case with many transition metal
b-diketiminate complexes, whereas tri-coordinate b-diketiminate
methyl (LMe = [{N(Me)C(Me)}2CH]) and N-phenyl (LPh
=
[{N(C6H5)C(Me)}2CH]) groups. In all of the N-methyl cases
(PbCl, PbBr, and PbI), the optimised structure showed the lead
˚
essentially in the C3N2 plane (largest deviation of 0.078 A),
indicating that indeed, the deviation from planarity is most likely
caused by steric interactions between the halide and the aryl group.
However, in the N-phenyl examples, the lead was significantly out-
˚
˚
side the C3N2 plane (LPhPbCl, 0.221 A; LPhPbBr, 0.307 A; LPhPbI,
˚
0.473 A). These data are consistent with the isostructural N-phenyl
germanium and tin analogues in which the metal centres lie outside
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◦
˚
Table 4 Differences been key experimental and calculated bond lengths (A) and bond angles ( ) of compounds 2, 3 and 4
LPbCl (2)
LPbBr (3)
LPbI (4)
Obs.
Calc
2.256
2.259
2.5300
85.75
94.95
94.78
% Diff
Obs.
Calc.
% Diff
Obs.
Calc
% Diff
M–N(1)
M–N(2)
M–X
N(1)–M–N(2)
N(1)–M–X
N(2)–M–X
2.290(2)
2.280(2)
2.5653(7)
82.77(7)
91.13(5)
93.27(5)
−1.47
−0.92
−1.38
3.60
4.19
1.62
2.281(5)
2.292(5)
2.7149(8)
83.93(19)
93.28(14)
91.31(13)
2.258
2.258
2.8042
85.84
94.91
94.88
−1.03
−1.49
3.29
2.27
1.74
2.311(4)
2.298(4)
2.9531(5)
83.26(16)
90.52(11)
93.02(11)
2.262
2.258
3.0406
85.92
95.77
95.96
−2.14
−1.73
2.96
3.20
5.80
3.16
3.91
◦
˚
Table 5 Differences been key experimental and calculated bond lengths (A) and bond angles ( ) of compounds 6 and 7
LGeCl (6)
LSnCl (7)
Obs.
Calc
2.018
2.014
2.450
91.54
93.81
95.54
% Diff
Obs.
Calc.
% Diff
M–N(1)
M–N(2)
M–X
N(1)–M–N(2)
N(1)–M–X
N(2)–M–X
1.988(2)
1.997(3)
2.295(12)
90.89(10)
95.00(8)
95.60(8)
1.08
1.27
6.71
2.185(2)
2.180(2)
2.473(9)
85.2(8)
90.97(6)
93.47(6)
2.172
2.173
2.567
87.34
92.75
92.52
−0.59
−0.31
3.80
2.51
1.95
0.72
−1.85
0.60
1.02
the plane by 0.528 A and 0.495 A, respectively.35,36 However, these
˚
˚
p-character in the lone pair than the softer ligands (i.e. BiH3
or TeH2).33 Interestingly, the results showed that the less p-
character the lone pair possessed, the smaller the “void” in
the coordination sphere, indicating that the minor contribu-
tion the p-orbital gave to the lone pair significantly influenced
its directionality. Cox and co-workers found a similar trend
when they investigated the binding of alcohols to divalent lead,
the softer the ligand, the greater the p-orbtial population.4
Although our lead complexes are widely different from those
calculated in that study, we use their NBO analyses as a guideline
in our studies. As predicted, the amount of sp hybridisation of
the lone pair decreases with the heavier Group 14 complexes
(Table 6). For instance, the lone pair in germanium complex 6
has 81.58% s-character and 18.42% p-character, resulting in a
sp0.23 hybridized orbital. Predictably, this mixing diminishes in tin
complex 7, (86.09% s-character and 13.91% p-character, or sp0.16
hybridised orbital). However, all of the lead complexes 2–4 have
around 7–8% p-character in the lone pair, resulting in an sp0.08–0.09
hybridised orbital.
results are in contrast to b-diketiminate complexes of dichloro–
aluminium; the aluminium metal centre in both the N-methyl (LMe
)
and the (N-tolyl) (Ltol = [{N(4-MeC6H4)C(Me)}2CH]) derivatives
lies in the plane whereas the aluminium metal centre in the bulky
BDI–aluminium dichloride complex lies outside of the C3N2 plane
37–39
˚
by 0.525 A.
Thus, other factors are behind the non-planarity
of the lead metal with respect to the C3N2 plane in our systems,
such as potential interaction of the N-(2,6-diisopropylaryl) groups
with the lone pair.
To take into account the demands of the sterically encumbered
b-diketiminate ligand when understanding the geometry around
the metal centre and the influence of the lone pair, we performed
calculations on the complete molecules using input geometries
generated from the crystallographic analysis. In order to investi-
gate the hybridisation of the lone pair on lead, we included a full
natural bond orbital (NBO) analysis in these calculations. The
calculations were also performed on the isostructural germanium
and tin chloride complexes (6 and 7, respectively)17 to ascertain
any periodic trends with regard to the significance of the lone pair.
The bond lengths and bond angles of the optimised geometry are,
in general, within 2% of the solid state structures. However, the
metal–halide bond lengths tend to be significantly overestimated
for all complexes except for the lead chloride 2 (Tables 4 and 5). Al-
though the N(1)–Pb–X bond angle was considerably more obtuse
than our experimental values (most significantly for lead chloride 2
and lead iodide 4) the trend towards a tighter N(1)–M–N(2) angle
of Ge > Sn > Pb was reproduced in our calculations. The differ-
ences between the experimental and calculated results are probably
due to a combination of crystal packing effects not observed in
the gas phase as well as the limitations in the theory applied.
Glusker et al. used NBO analysis to investigate factors con-
trolling the geometry of homoleptic, 4-coordinate lead complexes
in the gas phase, and found that the amount of hybridisation
was dependent upon the hardness of the bound ligand, with
harder ligands (i.e. NH3 or OH2) resulting in a slightly higher
The molecular orbtial schemes for the lead halides (2–4) as well
as the isostructural germanium and tin complexes (6 and 7, respec-
tively) are shown in Fig. 4 and 5. The LUMO in all complexes lies
mainly on the b-diketiminate backbone. Whereas the HOMO also
Table 6 NBO analysis of M(II) for compounds 2, 3, 4, 6, and 7
Natural electron
configuration
Lone pair NBO
on M
Lone pair
occupancy
LPbCl (2)
LPbBr (3)
LPbI (4)
Pb: 6s(1.85) 6p(0.86)
7p(0.01)
Pb: 6s(1.86) 6p(0.88)
7p(0.01)
Pb: 6s(1.87) 6p(0.93)
7p(0.01)
Ge: 4s(1.68)
s[91.79%] p 0.09
[8.21%]
s(92.62%) p 0.08
(7.38%)
s(92.96%) p 0.08
(7.04%)
s(81.58%) p 0.23
(18.42%)
1.98295
1.98358
1.98362
1.97071
1.97938
LGeCl (6)
LSnCl (7)
4p(1.23) 5p(0.01)
Sn: 5s(1.75) 5p(0.98)
s(86.09%) p 0.16
(13.91%)
2774 | Dalton Trans., 2007, 2770–2778
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Fig. 4 Calculated frontier MO scheme for (a) LPbCl 2, (b) LPbBr 3 and (c) LPbI 4.
has contribution from the b-diketiminate backbone, contribution
from the metal lone pair can be observed from the lighter Group
14 chloride complexes. However, a smaller lone pair contribution
to the HOMO −1 is observed for the BDI lead chloride 2 and
lead bromide 3 complexes with the predominant feature of the
HOMO − 1 for 3 being the contribution from the pz orbital on the
bromide ligand. This feature is amplified in the BDI lead iodide 4 in
which the major freature of the HOMO − 1 and HOMO − 2 are the
pz and px orbitals of the halogen (−133.7 and −134.3 kcal mol−1,
respectively). With this latter compound, significant contribution
from the lone pair is not observed until the HOMO −3 orbital
(−148.1 kcal mol−1), although there is considerable mixing with
the py orbital of the iodide ligand. A similar molecular orbital
is found with the other lead halide complexes at a much lower
energy, HOMO − 6 for lead bromide 3 (−153.1 kcal mol−1) and
HOMO − 7 for BDI lead chloride 2 (−161.9 kcal mol−1).
the nature of the bonding of three-coordinate lead complexes to
gain an appreciation of the nature of the lone pair. To this end, we
have generated some rare examples of three-coordinate substituted
lead halides. Both the lead chloride 2 and lead bromide 3 appear
to be relatively stable whereas the lead iodide 4 slowly decomposes
in both solution and the solid state. In addition, we have shown
that lead chloride is reactive and can readily be converted to an
unusual terminal bulky b-diketiminate lead amide 5.
The solid state structures of these complexes were determined,
showing that each exhibits a pyramidal geometry around the metal
centre with the sum of the bond angles around 270◦ (DOP =
103%). The metal centre was significantly outside the C3N2 plane
˚
by at least 0.408 A. DFT calculations showed that this deviation
from planarity can be attributed to a combination of steric
and electronic factors. The NBO analysis of the lead complexes
revealed little mixing between the 6s and 6p orbitals in the lone
pair, especially compared to the isotructural tin and germanium
systems.
In contrast to the lead complexes where the lone pair appears
to contribute to more than one molecular orbital, the lone pair in
the germanium and tin complexes are more distinct and can be
found at HOMO −1 for both complexes (−143.1 kcal mol−1 for
LGeCl 6 and −133.7 kcal mol−1 for LSnCl 7). This trend is akin
to those observed with the Group 13 M(I) BDI complexes.20
Experimental
General methods
All manipulations were carried out in an atmosphere of dry
nitrogen or argon using standard Schlenk tube techniques or
in a glovebox. Solvents were dried from the appropriate drying
agent, distilled, degassed and stored over a potassium or sodium
Conclusions
This purpose of this study was to generate a new class of lead
complexes. As a key part of the study, we also wanted to explore
˚
mirror or 4A sieves. The NMR spectra were recorded on a
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28.0 (CHMe2), 27.7 (NCMe), 25.2, 24.7, 24.6 and 24.3 (CHMe2).
IR (Nujol, cm−1) 1621 (s), 1550 (s), 1517 (s), 1317 (s), 1169 (s),
1118 (s), 1100 (s), 933 (s), 840 (s), 790 (s), 760 (s). Anal. calc. for
C29H41N2PbCl: C, 52.75; H, 6.26; N, 4.24. Found: C, 52.84; H, 6.39;
N, 4.16%.
[CH{(CH3)2CN-2,6-iPr2C6H3}2PbBr], 3. Lithium b-dike-
timinate LiL (0.364 g, 0.857 mmol) was added to a stirred
suspension of PbBr2 (0.315 g, 0.857 mmol) in 5 mL THF at room
temperature. The reaction vessel was wrapped in foil and the
mixture was stirred overnight. The solvent was removed under
vacuum and the residue was extracted with toluene (2 × 5 mL).
The resulting deep yellow solution was slowly evaporated in vacuo
until crystallisation began and was stored at −30 ◦C overnight
yielding [LPbBr] as a yellow precipitate which was isolated by
washing with pentane (0.171 g, 65%). 1H NMR (C6D6, 293 K): d
7.24 (d, 2 H, J = 7.5 Hz, m-H), 7.19 (t, 2 H, J = 7.5 Hz, p-H),
7.10 (d, 2 H, J = 7.5 Hz, m-H), 4.93 (s, 1 H, middle CH), 4.00
(septet, 2 H, J = 6.8 Hz, CHMe2), 3.07 (septet, 2 H, J = 6.8 Hz,
CHMe2), 1.70 (s, 6 H, NCMe), 1.53 (d, 6 H, J = 6.5 Hz, CHMe2),
1.28 (d, 6 H, J = 6.8 Hz, CHMe2), 1.21 (d, 6 H, J = 6.9 Hz,
1
CHMe2), 1.10 (d, 6 H, J = 6.7 Hz, CHMe2). 13C{ H} NMR
(C6D6, 293 K): d 164.4 (NCMe), 145.8 and 142.5 (ipso- and o-C
of Ar), 125.9, 124.8 and 124.5 (m- and p-CH of Ar), 106.1 (middle
CH), 29.3 and 28.6 (CHMe2), 27.8 (NCMe), 25.1, 24.8, 24.7 and
24.3 (CHMe2). IR (Nujol, cm−1) 1623 (s), 1552 (s), 1520 (s), 1316
(s), 1248 (s), 1171 (s), 1097 (s), 1015 (s), 933 (s), 840 (s), 793 (s),
755 (s). Anal. calc. for C29H41N2PbBr: C, 49.42; H, 5.86; N, 3.97.
Found: C, 49.44; H, 5.93; N, 3.89%.
[CH{(CH3)2CN-2,6-iPr2C6H3}2PbI], 4. Lithium b-dike-
timinate LiL (0.162 g, 0.354 mmol) was added to a stirred
suspension of PbI2 (0.164 g, 0.354 mmol) in 5 mL THF at room
temperature. The reaction vessel was wrapped in foil and the
mixture was stirred overnight. The solvent was removed under
vacuum and the residue was extracted with toluene (2 × 5 mL).
The resulting deep yellow solution was slowly evaporated in vacuo
until crystallisation began and was stored at −30 ◦C overnight
yielding [LPbI] as a yellow precipitate which was isolated by
washing with pentane (0.152 g, 59%). 1H NMR (C6D6, 293 K): d
7.11 (d, 2 H, J = 7.5 Hz, m-H), 7.09 (t, 2 H, J = 7.5 Hz, p-H),
7.06 (d, 2 H, J = 7.5 Hz, m-H), 4.93 (s, 1 H, middle CH), 4.00
(septet, 2 H, J = 6.8 Hz, CHMe2), 3.07 (septet, 2 H, J = 6.8 Hz,
CHMe2), 1.70 (s, 6 H, NCMe), 1.53 (d, 6 H, J = 6.5 Hz, CHMe2),
1.28 (d, 6 H, J = 6.8 Hz, CHMe2), 1.21 (d, 6 H, J = 6.9 Hz,
Fig. 5 Calculated frontier MO scheme for (a) LGeCl 6 and (b) LSnCl 7.
DPX 300 or AM-500 Bruker spectrometer with residual solvent
1
signals as internal reference (1H and 13C{ H}) or with an external
reference (SiMe4 for 29Si). Elemental analyses were carried out at
the University of North London. LH and LiL (1) were prepared
according to literature procedures.40 K{N(SiMe3)2} (Aldrich) was
used as received.
Syntheses
[CH{(CH3)2CN-2,6-iPr2C6H3}2PbCl], 2. Lithium b-dike-
timinate LiL (2.83 g, 6.67 mmol) was added to a stirred
suspension of PbCl2 (1.86 g, 6.68 mmol) in 10 mL THF at
room temperature and the mixture was stirred overnight. The
solvent was removed under vacuum and the residue was extracted
with toluene (2 × 10 mL). The resulting deep yellow solution
was slowly evaporated in vacuo until crystallisation began and
was stored at −30 ◦C overnight yielding [LPbCl] as a yellow
precipitate which was isolated by washing with pentane (2.455 g,
56%). 1H NMR (C6D6, 293 K): d 7.23 (d, 2 H, J = 7.5 Hz,
m-H), 7.18 (t, 2 H, J = 7.5 Hz, p-H), 7.10 (d, 2 H, J = 7.5 Hz,
m-H), 4.91 (s, 1 H, middle CH), 3.99 (septet, 2 H, J = 6.8 Hz,
CHMe2), 3.08 (septet, 2 H, J = 6.8 Hz, CHMe2), 1.71 (s, 6 H,
NCMe), 1.54 (d, 6 H, J = 6.7 Hz, CHMe2), 1.27 (d, 6 H, J =
6.8 Hz, CHMe2), 1.19 (d, 6 H, J = 6.9 Hz, CHMe2), 1.12 (d, 6
1
CHMe2), 1.10 (d, 6 H, J = 6.7 Hz, CHMe2). 13C{ H} NMR
(C6D6, 293 K): d 165.2 (NCMe), 146.0 and 142.6 (ipso- and o-C
of Ar), 127.0, 125.9 and 123.7 (m- and p-CH of Ar), 106.4 (middle
CH), 29.3 and 28.7 (CHMe2), 27.8 (NCMe), 25.0, 24.5, 23.5 and
20.8 (CHMe2). IR (Nujol, cm−1) 1517 (s), 1314 (s), 1268 (s), 1169
(s), 1097 (s), 1018 (s), 933 (s), 840 (s), 793 (s), 755 (s). Despite
repeated attempts, no suitable combustion analysis was obtained
due to the unstable nature of this compound.
[CH{(CH3)2CN-2,6-iPr2C6H3}2PbN(SiMe3)2], 5.
Method A. K{N(SiMe3)2} (0.1592 g, 0.80 mmol) was added
to a solution of 2 (0.5159 g, 0.78 mmol) in THF (10 mL) and
the mixture was stirred for 1 h at room temperature. The solvent
was removed under vacuum and the residue was extracted with
hexane (2 × 10 mL). The combined extracts were filtered, the
1
H, J = 6.8 Hz, CHMe2). 13C{ H} NMR (C6D6, 293 K): d 164.2
(NCMe), 145.7 and 142.5 (ipso- and o-C of Ar), 127.0, 125.4
and 123.7 (m- and p-CH of Ar), 104.4 (middle CH), 28.7 and
2776 | Dalton Trans., 2007, 2770–2778
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filtrate was concentrated in vacuo and stored at −27 ◦C for 2 d
yielding [Pb(L){N(SiMe3)2}]·0.5C6H14 (0.291 g, 45%, first crop) as
bright yellow crystals, mp 135–140 ◦C (decomposition). 1H NMR
(C6D5CD3, 253 K): d 7.17 (dd, 2 H, J = 7.7 and 1.8 Hz, m-H), 7.09
(t, 2 H, J = 7.7 Hz, p-H), 7.02 (dd, 2 H, J = 7.5 and 1.8 Hz, m-H),
4.91 (s, 1 H, middle CH), 3.75 (septet, 2 H, J = 6.8 Hz, CHMe2),
3.07 (septet, 2 H, J = 6.8 Hz, CHMe2), 1.58 (s, 6 H, NCMe),
1.46 (d, 6 H, J = 6.8 Hz, CHMe2), 1.27 (d, 6 H, J = 6.8 Hz,
CHMe2), 1.23 (m, CH2 of hexane), 1.15 (d, 12 H, J = 6.8 Hz,
CHMe2), 0.92 (t, CH3 of hexane), 0.46 (s, 9 H, SiMe3), 0.14 (s, 9
7 L. Pu, B. Twamley and P. P. Power, Organometallics, 2000, 19, 2874–
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8 S. Hino, M. M. Olmstead, A. D. Phillips, R. J. Wright and P. P. Power,
Inorg. Chem., 2004, 43, 7346–7352.
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11 C. Eaborn, K. Izod, P. B. Hitchcock, S. E. Sozerli and J. D. Smith,
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12 W. P. Leung, C. W. So, Y. S. Wu, H. W. Li and T. C. W. Mak, Eur. J. Inorg.
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1
H, SiMe3). 13C{ H} NMR (C6D5CD3, 253 K): d 165.28 (NCMe),
144.63, 143.76 and 143.18 (ipso- and o-C of Ar), 126.77, 124.84 and
124.49 (m- and p-CH of Ar), 105.10 (middle CH), 32.27 (CH2 of
hexane), 28.23 and 27.88 (CHMe2), 26.78 (NCMe), 25.75, 25.67,
25.31 and 24.54 (CHMe2), 23.34 (CH2 of hexane), 14.65 (CH3 of
14 L. Bourget-Merle, M. F. Lappert and J. R. Severn, Chem. Rev., 2002,
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15 J. M. Smith, A. R. Sadique, T. R. Cundari, K. R. Rodgers, G. Lukat-
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1
hexane), 7.14 and 7.05 (SiMe3). 29Si{ H} NMR (C6D5CD3, 253
K): d −3.24 and −3.35. Anal. calc. for C35H62N3PbSi2: C, 53.33;
16 D. J. E. Spencer, N. W. Aboelella, A. M. Reynolds, P. L. Holland and
W. B. Tolman, J. Am. Chem. Soc., 2002, 124, 2108–2109.
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18 P. B. Hitchcock, M. F. Lappert and A. V. Protchenko, Chem. Commun.,
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H, 7.93; N, 5.33. Found: C, 53.19; H, 8.05; N, 5.43%.
Method B. LH (0.9220 g, 2.24 mmol) was added to a so-
lution of [Pb{N(SiMe3)2}2] (1.172 g, 2.22 mmol) in hexane
(20 mL). The resulting solution was degassed, vacuum-sealed
and stored at 90 ◦C overnight (in an NMR-controlled ex-
periment, the reaction in C6D6 was 95% complete after 3 h
heating at 90 ◦C). Removing volatiles under vacuum produced
[Pb(L){N(SiMe3)2}]·0.5C6H14 in nearly quantitative yield. X-Ray
diffraction study on the hexane-solvated crystals revealed large
residual density on the final difference map, thus, to improve
the data quality, [Pb(L){N(SiMe3)2}]·0.5C6H14 was desolvated by
heating in dynamic vacuum at 70 ◦C for 1 h, dissolved in benzene
and crystallised by slow evaporation at room temperature to give
large clear crystals of [Pb(L){N(SiMe3)2}]·0.5C6H6.
19 Z. B. Maksic and B. Kovacevic, J. Chem. Soc., Perkin Trans. 2, 1999,
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23 A. Murso, M. Straka, M. Kaupp, R. Bertermann and D. Stalke,
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224–234.
Crystal structure determinations
27 F. Bonnet, M. Visseaux, D. Barbier-Baudry, E. Vigier and M. M.
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Data were collected at 173 K on a Nonius Kappa CCD diffrac-
˚
tometer, k(Mo-Ka) = 0.71073 A. Details of the crystal data,
intensity collection and refinment are listed in Table 1. The
structures were refined with SHELXL-97.41
CCDC reference numbers 638198–638201.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b702994b
31 M. Rahim, N. J. Taylor, S. Xin and S. Collins, Organometallics, 1998,
17, 1316–1323.
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Acknowledgements
33 L. Shimoni-Livny, J. P. Glusker and C. W. Bock, Inorg. Chem., 1998,
We would like to thank M. Coles and J. Weston for helpful
discussions regarding the DFT calculations. We also wish to
acknowledge the use of the EPSRC’s Chemical Databse Service at
Daresbury. JRF would like to thank the Leverhulme Foundation
for funding.
37, 1853–1857.
34 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
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