Thermally stable lead(II) amidinates and guanidinates

Article abstract of DOI:10.1039/b718537e

The reactions of lithium amidinates and guanidinates [Li(Fiso)] (Fiso = N,N′-bis(2,6-diisopropylphenyl)formamidinate), [Li(Piso)] (Piso = N,N′-bis(2,6-diisopropylphenyl)(tert-butyl)amidinate), [Li(CyG)] (CyG = N,N″-bis(2,6-diisopropylphenyl)-N′,N′-dicyclohexylguanidinate) and [Li(HDG)] (HDG = N,N″-bis(2,6-diisopropylphenyl)-N′-(2,6- diisopropylphenyl)guanidinate) with anhydrous PbCl₂ in THF afforded the amidinate complexes [Pb(Fiso)₂] 1, and [Pb(Piso)Cl]₂2, and the guanidinate complexes [Pb(CyG)Cl]₂3, and [Pb(HDG)Cl] ₂4 in moderate yields. The complexes are thermally stable, 2, 3 and 4 decompose between 163-202°C whilst 1 melts with decomposition to lead metal at 310°C. 2, 3 and 4 are unsymmetrical dimers in the solid state but their NMR data suggested fluxional behaviour in solution. Storing the yellow crystals of [Pb(CyG)Cl]₂3 for one week in the perfluorinated crystallography oil under air resulted in oxygen incorporation into one isopropyl group of the CyG ligand giving a mixed amidinate-alkoxide dianion (CyGO) and afforded some new colourless crystals of [(CyGO)Pb]₂5 that were characterised by a X-ray single crystal structure determination. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b718537e

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Thermally stable lead(II) amidinates and guanidinateswz
Andreas Stasch,ab Craig M. Forsyth,a Cameron Jonesab and Peter C. Junk*a
Received (in Durham, UK) 30th November 2007, Accepted 6th February 2008
First published as an Advance Article on the web 3rd March 2008
DOI: 10.1039/b718537e
The reactions of lithium amidinates and guanidinates [Li(Fiso)] (Fiso = N,N0-bis(2,6-
diisopropylphenyl)formamidinate), [Li(Piso)] (Piso = N,N0-bis(2,6-diisopropylphenyl)-
(tert-butyl)amidinate), [Li(CyG)] (CyG = N,N00-bis(2,6-diisopropylphenyl)-N0,N0-
dicyclohexylguanidinate) and [Li(HDG)] (HDG = N,N00-bis(2,6-diisopropylphenyl)-N0-(2,6-
diisopropylphenyl)guanidinate) with anhydrous PbCl2 in THF afforded the amidinate complexes
[Pb(Fiso)2] 1, and [Pb(Piso)Cl]2 2, and the guanidinate complexes [Pb(CyG)Cl]2 3, and
[Pb(HDG)Cl]2 4 in moderate yields. The complexes are thermally stable, 2, 3 and 4 decompose
between 163–202 1C whilst 1 melts with decomposition to lead metal at 310 1C. 2, 3 and 4 are
unsymmetrical dimers in the solid state but their NMR data suggested fluxional behaviour in
solution. Storing the yellow crystals of [Pb(CyG)Cl]2 3 for one week in the perfluorinated
crystallography oil under air resulted in oxygen incorporation into one isopropyl group of the
CyG ligand giving a mixed amidinate-alkoxide dianion (CyGO) and afforded some new colourless
crystals of [(CyGO)Pb]2 5 that were characterised by a X-ray single crystal structure
determination.
example is the ‘nacnac’ ligand which has found applications
Introduction
in catalysis, bioinorganic molecules, organic-inorganic hy-
brids, multiply bonded M–X (X = C, N, P) species and low
coordinate, low valent complexes.4
Metal organoamides (i.e. compounds with a M–NR2 moiety)
are a significant and well populated compound class that have
been established for many years. Indeed, Lappert’s seminal
book Metal and Metalloid Amides, published in 1980, listed
100s of examples from across the periodic table. However,
only three complexes were given for lead(II), and one of these
was described as unstable.1 In the subsequent 25 years little
has changed and lead(II) organoamides remain underdeve-
loped.2 This is undoubtedly due to the diminishing industrial
utilization of lead compounds amid significant environmental
and health concerns3 as well as the inherent low stability of
these complexes with respect to precipitation of lead(0).
Stabilization of unusual metal–organic species has been a
major research focus and a common strategy to achieve this
goal has been the use of very bulky ligand systems. Mono-
anionic, bidentate organoamido frameworks with large groups
such as 2,6-diisopropylphenyl attached to the donor nitrogen
atoms have proven extremely useful in this context. Relevant
examples include the diketiminate4 and amidinate5,6 classes
(Fig. 1), where the metal, bound to the two nitrogen atoms, is
enveloped by the peripheral aryl groups leaving a restricted
reactivity opening. Further, the effective size of the ligand can
be increased by substitution of the backbone which pushes the
aryl groups closer to the metal binding sites. A popular
We have recently explored the use of very bulky amidinate
(and related guanidinate) ligands for development of low
coordinate, low oxidation state main group and rare earth
species.6 In this contribution, we report the synthesis of a
number of thermally stable and rare lead(II) complexes using
these bulky ligand systems and compare their synthetic and
structural outcomes. These studies complement recent
reports of the three coordinate lead(II) nacnac complexes
[Pb(nacnac)X] (X = Cl, Br, I).2a
Experimental
The lead complexes described herein are air and water sensi-
tive, hence all operations were carried out in an inert atmo-
sphere (purified N2 or Ar) involving conventional glove-box
and Schlenk techniques. Solvents were purified, dried and
deoxygenated by refluxing over and distillation from so-
dium-benzophenone, whilst C6D6 was dried over Na followed
by vacuum transfer to greaseless Schlenk tubes and stored
1
under purified argon or nitrogen. H and 13C NMR spectra
a School of Chemistry, Monash University, Clayton, VIC 3800,
Australia. E-mail: peter.junk@sci.monash.edu.au; Fax: +613 9905
4597; Tel: +613 9905 4570
b School of Chemistry, Cardiff University, Cardiff, UK CF10 3AT.
Fax: +4429 20874030; Tel: +4429 2087 4060
w Dedicated to Professor Jerry L. Atwood on the occasion of his 65th
birthday.
Fig. 1 Very bulky bidentate organoamido frameworks include dike-
timinate ligands (such as nacnac, R = CH3) or amidinate ligands
(in this study Fiso, R1 = H; Piso, R = t-Bu; CyG, R = NCy2; HDG,
R = NH(C6H3-2,6-i-Pr2).
z CCDC reference numbers 677774–677777. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b718537e
ꢀc
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were recorded on a Bruker AM 300 or DRX 400 spectrometer;
chemical shifts are referenced to internal solvent resonances
and reported relative to SiMe4. IR data (4000–650 cmꢁ1) were
obtained for Nujol mulls sandwiched between NaCl plates
with Perkin-Elmer 1600 FTIR. EI mass spectra and accurate
mass EI spectra were obtained from the EPSRC National
Mass Spectrometric Service at Swansea University. Lead(II)
chloride was obtained from Aldrich and sublimed before use.
The formamidines FisoH,7a PisoH,6g,7b and the guanidines
CyGH,6d HDGH7c were synthesised by the reported methods.
[Pb(CyG)Cl]2 3
n-BuLi (0.66 mL of a 1.6 M solution in hexanes, 1.06 mmol) was
added to a solution of [(CyG)H] (0.54 g, 1.00 mmol) in THF (20
mL) and stirred for 1 h at room temperature. The solution was
cooled to ꢁ80 1C and PbCl2 (0.32 g, 1.15 mmol) was added. The
reaction mixture was slowly warmed to room temperature and
stirred overnight. The solvent was removed under reduced
pressure and the residue was extracted into hexane (40 mL).
Concentration of the solution under reduced pressure to ca. 12
mL and cooling to ꢁ30 1C gave yellow crystals of [Pb(CyG)Cl]2
1
3. Yield: 0.23 g (29%). Mp: 200–203 1C (decomp.). H NMR
(300 MHz, C6D6, 296 K): 0.73–0.94 (m, 16H, CH2), 1.34 (d, J
= 6.8 Hz, 24H, CH(CH3)2), 1.42 (d, J = 6.8 Hz, 24H,
CH(CH3)2), 1.36–1.90 (m, 24H, CH2), 3.47 (m, 4H, CHN),
3.91 (sept, J = 6.7 Hz, 8H, CH(CH3)2), 6.92–7.31 (m, 12H,
ArH). 13C NMR (75.5 MHz, C6D6, 296 K): 23.3 (CyC), 26.1
(CyC), 27.5 (CH(CH3)2), 28.4 (CH(CH3)2), 30.4 (CH(CH3)2),
35.5 (CyC), 59.1 (HCN), 123.9 (ArC), 125.8 (ArC), 140.5
(ArC), 145.3 (ArC), 167.7 (N3C). IR (v/cmꢁ1 Nujol): 1611
(m), 1583 (m), 1321 (m), 1278 (m), 1241 (m), 1164 (m), 1109
(m), 1022 (m), 796 (m), 752 (m). MS (EI): m/z (%) = 750 (3,
12M+ ꢁ Cl), 543 (22, CyGH+). Storing the yellow crystals of
[Pb(CyG)Cl]2 3 for one week in the perfluorinated crystal-
lography oil in air afforded some new colourless crystals of
[(CyGO)Pb]2 5 that were (only) characterised by a X-ray single
crystal determination.
[Pb(Fiso)2] 1
n-BuLi (2.90 mL of a 1.6 M solution in hexanes, 4.78 mmol)
was added at 0 1C to a solution of [(Fiso)H] (1.65 g, 4.53
mmol) in THF (40 mL) and stirred for 1 h at room tempera-
ture. The solution was cooled to ꢁ80 1C and PbCl2 (1.85 g,
6.65 mmol) was added. The reaction mixture was slowly
warmed to room temperature and stirred overnight. The
mixture was filtered and concentrated under reduced pressure
to ca. 15 mL. Cooling to ꢁ30 1C gave yellow crystals of
[Pb(Fiso)2] 1. Concentration of the supernatant solution and
cooling led to a further crop of 1. Yield: 0.85 g (40%). Mp: 310
C (melts with decomp. to Pb0). 1H NMR (300 MHz, C6D6, 296
K): 1.12 (d, J = 6.7 Hz, 24H, CH(CH3)2), 1.17 (d, J = 6.7 Hz,
24H, CH(CH3)2), 3.45 (sept, J = 6.7 Hz, 8H, CH(CH3)2),
6.94–7.27 (m, 12H, ArH), 10.81 (s, 2H, N2CH). 13C NMR
(75.5 MHz, C6D6, 296 K): 24.2 (CH(CH3)2), 25.1 (CH(CH3)2),
28.7 (CH(CH3)2), 29.0 (CH(CH3)2), 123.7 (ArC), 123.8 (ArC),
125.5 (ArC), 144.2 (ArC), 163.5 (N2CH). IR (v/cmꢁ1 Nujol):
1666 (s), 1592 (m), 1378 (m), 1319 (s), 1270 (s), 1183 (s), 1098
(s), 986 (m), 946 (m), 799 (m), 754 (s). MS (EI): m/z (%) = 891
(3, M+ ꢁ C3H7), 571 (100, FisoPb + H+), 529 (17, FisoPb ꢁ
C3H6+), 363 (41, FisoH+).
[Pb(HDG)Cl]2 4
n-BuLi (0.62 mL of a 1.6 M solution in hexanes, 0.99 mmol)
was added to a solution of [(HDG)H] (0.51 g, 0.94 mmol) in
THF (20 mL) and stirred 1 h at room temperature. The
solution was cooled to ꢁ80 1C and PbCl2 (0.35 g, 1.26 mmol)
was added. The reaction mixture was slowly allowed to warm
to room temperature and stirred overnight. The mixture was
filtered, the solvent was removed under reduced pressure and
the residue was extracted into toluene (20 mL). Concentration
of the solution under reduced pressure to ca. 6 mL and cooling
to ꢁ30 1C gave yellow crystals of [Pb(HDG)Cl] 4. Yield: 0.22 g
(30%). Mp: 163 1C (decomp.). 1H NMR (300 MHz, C6D6, 296
K): 0.88 (br d, J = 6.8 Hz, 24H, CH(CH3)2), 1.18 (d, J = 6.8
Hz, 24H, CH(CH3)2), 1.25 (br d, J = 6.8 Hz, 24H, CH(C-
H3)2), 3.19 (sept, J = 6.7 Hz, 4H, CH(CH3)2), 3.83 (m, broad,
8H, CH(CH3)2), 5.24 (s, 2H, NH), 6.87–7.21 (m, 18H, ArH).
13C NMR (75.5 MHz, C6D6, 296 K): 22.1 (CH(CH3)2), 24.1
(br, CH(CH3)2), 28.7 (CH(CH3)2), 28.8 (CH(CH3)2), 29.6
(CH(CH3)2), 123.4 (ArC), 123.7 (ArC), 126.5 (ArC), 128.9
(ArC), 129.7 (ArC), 133.4 (ArC), 146.7 (ArC), 148.6 (ArC),
164.4 (N3C). MS (EI): m/z (%) = 781 (1, 12M+), 539 (28,
HDGH+), 496 (62, HDGH+ ꢁ C3H7).
[Pb(Piso)Cl]2 2
n-BuLi (2.00 mL of a 1.6 M solution in hexanes, 3.20 mmol)
was added to a solution of [(Piso)H] (1.33 g, 3.16 mmol) in
THF (40 mL) and stirred for 1 h at room temperature. The
solution was cooled to ꢁ80 1C and PbCl2 (1.00 g, 3.60 mmol)
was added. The reaction mixture was slowly warmed to room
temperature and stirred overnight. The solvent was removed
under reduced pressure and the residue was extracted into
hexane (50 mL). Concentration of the solution under reduced
pressure to ca. 15 mL and cooling to ꢁ30 1C gave yellow
crystals of [Pb(Piso)Cl] 2. Yield: 0.48 g (23%). Mp: 198–202 1C
1
(decomp.). H NMR (400 MHz, C6D6, 296 K): 1.06 (s, 18H,
C(CH3)3), 1.35 (d, J = 6.7 Hz, 24H, CH(CH3)2), 1.42 (d, J =
6.7 Hz, 24H, CH(CH3)2), 3.96 (sept, J = 6.7 Hz, 8H, C-
H(CH3)2), 7.00–7.33 (m, 12H, ArH). 13C NMR (100.3 MHz,
C6D6, 296 K): 23.1 (C(CH3)3), 28.9 (CH(CH3)2), 29.1
(CH(CH3)2), 29.8 (CH(CH3)2), 53.0 (C(CH3)3), 123.7 (ArC),
126.3 (ArC), 141.1 (ArC), 144.8 (ArC), 176.2 (N2C). IR
(v/cmꢁ1 Nujol): 1614 (s), 1586 (m), 1377 (s), 1326 (m), 1255
(m), 1168 (m), 1100 (m), 1017 (m), 800 (m), 758 (m). MS (EI):
Structure determinations
Single crystals suitable for X-ray diffraction were grown from
hexane (1, 2) or toluene (3, 4). Full spheres or hemispheres
(2ymax 50–551) of CCD area-detector diffractometer data were
measured (Enraf Nonius KAPPA CCD, 1.01 frames, j- and
o-scans, monochromatic Mo-Ka radiation, l = 0.71073 A; T
ca. 123 K) yielding Nt reflections, these merging to N inde-
pendent data (Rint quoted) after empirical/multi-scan
+
+
+
1
2
1
2
1
2
m/z (%) = 662 (3, M ), 627 (2, M ꢁ Cl), 619 (5, M
ꢁ
C3H7), 420 (18, PisoH+). Accurate mass for C29H43N2Pb1Cl1;
Calcd: 662.2876; Found: 662.2875.
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absorption correction (SORTAV).8a Full matrix least squares
refinement on F2 (SHELX97)8b with anisotropic displacement
parameter forms for non-hydrogen atoms (unless otherwise
noted) and hydrogen atoms placed in calculated positions,
constrained to ride on the parent atoms, yielded final residuals
R (I 4 2s(I)) and wR2 (all data).
Crystal/refinement data
1. [Pb(Fiso)2], C50H70N4Pb, M = 934.29, monoclinic,
P21/c, a = 14.9163(3), b = 16.1789(3), c = 20.8178(5) A,
b = 110.090(1)1, V = 4718.26(17) A3. Z = 4. Dc = 1.315 g
cmꢁ3. mMo = 3.611 mmꢁ1; specimen: 0.25 ꢂ 0.20 ꢂ 0.10 mm;
T0min,max = 0.41, 0.69, Nt = 25834, N = 8289 (Rint = 0.072),
R = 0.058, wR2 = 0.146. Carbon atoms C(12) and C(13) were
modelled as disordered over two positions (0.75 : 0.25 after
trial refinement) and were refined with restrained geometry
and anisotropic thermal parameters. Large ADP for carbon
atoms C(24) and C(25) suggested similarly disordered i-Pr
groups but this could not be adequately modelled with the
current data. The largest residual electron density maximum
(3.34 e A3) was located ca. 0.5 A from Pb(1).
Scheme 1 Synthesis of [Pb(Fiso)2] (1) (L = Fiso).
Results and discussion
The lithium salt of the bulky formamidinate ligand Fiso (=
N,N0-bis(2,6-diisopropylphenyl)formamidinate)9 prepared by
in situ reaction of the parent amine, reacted readily with PbCl2
in THF/hexane mixtures to afford crystalline, yellow, hexane
soluble [Pb(Fiso)2] (1) (Scheme 1). NMR characterisation of 1
showed four distinct 13C resonances for the i-Pr methyl groups
(but only two equal intensity doublets were resolved in the 1H
NMR spectrum) but only a single formamidinate NC(H)N
backbone resonance. This is consistent with observations for
[Pb(nacnac)Cl]2a and similarly suggests that 1 has a pyramidal
lead environment. Indeed, the crystal structure of 1 (Fig. 2)
shows a monomeric four coordinate complex with the lead
atom bound to two bidentate Fiso ligands with the amidinate
(NCN) coordination planes approximately cisoid (dihedral
angle of 76.2(2)1). Each Fiso ligand displays asymmetric
Pb–N distances with one ca. 0.1 A longer than the other.
Comparably dissimilar Pb–N distances (2.311(3)–2.669(3) A)
are observed in the bis(b-diketiminate)lead(II) complex
[Pb(L)2] (L = {N(SiMe3)C(Ph)}2CH),2b contrasting the near
symmetrical binding for the less crowded (b-diketiminate)-
chlorolead(II) complex [Pb(nacnac)Cl] (2.280(2), 2.290(2) A).2a
The cisoid coordination of the two Fiso ligands leaves exposed
a significant area of the lead coordination sphere. This space is
largely occupied by the pendant i-Pr groups which have the
methine C–H directed toward the metal centre with Pbꢃ ꢃ ꢃH
distances 2.65–2.95 A suggestive of agostic interactions. Even
so, there remains an unoccupied ‘hole’ in the molecule (Fig.
2(b)) plausibly indicating a stereochemically active lone pair
on lead. In a study on the role of the lone pair in lead
stereochemistry, a similar ‘‘hemidirected’’ coordination envir-
onment was typical for low coordination number (2–5) lead(II)
with hard donor ligands.10 However, DFT calculations on
[M(nacnac)X] complexes (M = Ge, Sn, Pb) have suggested
that the lone pair on lead is more diffuse than for the lighter
group 14 congeners.2a In the current structure, close approach
of a neighbouring molecule is prevented by the protruding Me
groups of the Fiso ligands (shortest intermolecular distance to
Pb(1) 4.18 A).
ꢀ
2. [Pb(Piso)Cl]2, C58H86Cl2Pb2, M = 1324.59, triclinic, P1,
a = 10.0568(3), b = 11.0744(2), c = 14.5525(4) A, a =
69.707(2), b = 73.897(1), g = 74.244(1)1, V = 1432.60(7) A3,
Z = 1 (dimer). Dc = 1.535 g cmꢁ0 3. mMo = 6.00 mmꢁ1
;
specimen: 0.25 ꢂ 0.20 ꢂ 0.12 mm; T min,max = 0.35, 0.54, Nt
= 18845, N = 5560 (Rint = 0.070), R = 0.037, wR2 = 0.080.
3ꢃ3PhMe. [Pb(CyG)Cl]2ꢃ3PhMe, C95H136Cl2Pb2,
M =
ꢀ
1847.38, triclinic, P1, a = 13.2064(3), b = 13.3847(3), c =
13.9337(4) A, a = 63.026(1), b = 80.217(1), g = 79.878(1)1, V
= 2149.88(9) A3, Z = 1. Dc = 1.427 g cmꢁ3. mMo = 4.021
mmꢁ1; specimen: 0.25 ꢂ 0.25 ꢂ 0.20 mm; T0min,max = 0.43,
0.50, Nt = 19 429, N = 9765 (Rint = 0.054), R = 0.041, wR2
= 0.099. The lattice PhMe molecules were modelled as
disordered over two positions, C(41)–C(47) (occupancies
0.54 : 0.46), the second component of C(48)–C(54) related
by inversion, and were refined with isotropic thermal para-
meters and restrained geometry. The largest residual electron
density maximum (2.21 e A3) was located ca. 0.9 A from Pb(1).
4ꢃ2PhMe. [Pb(HDG)Cl]2ꢃ2PhMe, C88H120Cl2N6Pb2, M =
ꢀ
1747.20, triclinic, P1, a = 10.9449(4), b = 13.1345(4), c =
15.3776(7) A, a = 109.593(2), b = 98.675(2), g = 95.682(3)1,
V = 2032.07(14) A3, Z = 2. Dc = 1.428 g cmꢁ3. mMo = 4.250
mmꢁ1; specimen: 0.20 ꢂ 0.15 ꢂ 0.10 mm; T0min,max = 0.43,
0.63, Nt = 21071, N = 7131 (Rint = 0.104), R = 0.056, wR2
= 0.087.
5ꢃ3PhMe. [Pb(CyGO)]2ꢃ3PhMe, C95H134N6O2Pb2, M =
1806.46, monoclinic, P21/c, a = 10.0458(2), b = 24.5646(6),
c = 17.3771(4) A, b = 97.058(1)1, V = 4255.67(17) A3, Z =
4. Dc = 1.410 g cmꢁ3. mMo = 4.002 mmꢁ1; specimen: 0.20 ꢂ
0.15 ꢂ 0.08 mm; T0min,max = 0.48, 0.72, Nt = 35104, N =
8328 (Rint = 0.086), R = 0.055, wR2 = 0.083. The lattice
PhMe molecules were modelled as disordered over two posi-
tions, C(38)–C(44) (occupancies 0.51 : 0.49), the second com-
ponent of C(45)–C(51) related by inversion, and were refined
with restrained geometry and anisotropic thermal parameters.
Analogous reactions of the lithium salts of the more bulky
amidinate Piso = N,N0-bis(2,6-diisopropylphenyl)-tert-butyl-
amidinate) and related guanidinate (CyG = N,N00-bis(2,6-
diisopropylphenyl)-N0,N0-dicyclohexylguanidinate, HDG
=
(HDG = N,N00-bis(2,6-diisopropylphenyl)-N0-2,6-diisopropyl-
phenylguanidinate) ligands with PbCl2, followed by removal
of the THF and extraction into hexane, yielded the chloride
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were satisfyingly predictable on the basis of the relative sizes of
the respective ligands. Thus, whilst the smaller Fiso gave the
Pb(L)2 derivative (see above), the larger Piso, CyG and HDG
gave only the Pb(L)Cl, despite variation of the ratio of the
reagents. These are in agreement with our findings for
germanium(II)6g and aluminium(III)6i and demonstrate the tune-
able nature of the amidinate ligand framework and that these
effects can be transferred to divergent (and larger) metal systems.
Mass spectra of the three [Pb(L)Cl]2 complexes (L = Piso 2,
CyG 3, HDG 4) showed Pb(L)Cl+ ions as the highest ob-
served mass species suggesting monomeric structures as found
for [Pb(nacnac)X] (X = Cl, Br, I). However, the crystal
structures of all three complexes (Fig. 3–5) showed each to
be a loosely associated chloride bridged dimer. In 2–4 the lead
centres are bound to a single bidentate amidinate or guanidi-
nate ligand and one chloride ligand having normal terminal
Pb–Cl distances. For example, the value for three coordinate
[Pb(nacnac)Cl] is 2.5653(7) A,2a whilst those for four coordi-
nate [Pb(L)(m-Cl)]2 (L = C(SiMe3)2(SiMe2OMe)) are 2.680(5)
and 2.868(5) A.11 The Pb–N distances for 2 and 3 are virtually
identical, indicating symmetrical binding of the amidinate or
guanidinate ligand. For 4 there is a slight degree asymmetry
with a difference of ca. 0.06 A between the two Pb–N
distances. Additional to the primary amidinate/guanidinate
and chloride ligation, in each complex there is a longer Pbꢃ ꢃ ꢃCl
contact to the Cl ligand of a neighbouring molecule (related by
inversion symmetry) with bond distances (A) 3.037(2)
4–3.260(2) 2. The shorter two contacts are comparable to
those observed in polymeric lead(II) chloride complexes, e.g.
[PbCl2(L)]n (L = phen, bpy; Pb–Cl 2.858(3), 3.023(3) A, in
which far infrared data suggested only weak Pb–Cl binding.12
For 2 and 3, the N–Pb(1)–Cl(1) angles are all approximately
equal as was observed for monomeric [Pb(nacnac)Cl]
(although the current values are larger due to the smaller bite
angle of the amidinate NCN backbone), suggesting that the
molecular association is sufficiently weak as to not perturb the
lead coordination environment. However, for 4, which has a
Pbꢃ ꢃ ꢃCl distance only marginally shorter than for 3, there is
one acute and one obtuse N–Pb(1)–Cl(1) angle, which, along
Fig. 2 Molecular diagram of [Pb(Fiso)2] (1): (a) with 30% thermal
ellipsoids and hydrogen atoms omitted for clarity; (b) reverse angle
view with atoms as space filling representations showing the vacancy in
the lead coordination sphere. Selected bond distances: Pb(1)–N(1)
2.465(7), Pb(1)–N(2) 2.332(6), Pb(1)–N(3) 2.463(6), Pb(1)–N(4)
2.359(6) A.
complexes [Pb(L)Cl]2 (L = Piso 2, CyG 3, HDG 4) in moderate
yields (Scheme 2). There was no spectroscopic evidence of other
species in solution, and whilst the residual material after hexane
extraction was coloured (suggesting retention of a lead species),
the material was not soluble in common solvents (e.g. THF) and
thus unlikely to be a putative PbL2 complex. The products were
highly soluble in hexane (plausibly accounting for the moderate
isolated yields) and appeared stable in solution for several weeks
with respect to precipitation of lead(0). These synthetic results
Fig. 3 Molecular diagram of [Pb(Piso)Cl]2 shown with 30% thermal
ellipsoids and hydrogen atoms omitted for clarity. Selected bond
distances (A) and angles (1): Pb(1)–N(1) 2.330(4), Pb(1)–N(2) 2.326(4),
Pb(1)–Cl(1) 2.571(2), Pb(1)–Cl(1i) 3.260(2); Cl(1)–Pb(1)–Cl(1i) 78.49(5),
N(1)–Pb(1)–Cl(1) 93.6(1), N(1)–Pb(1)–Cl(1i) 160.1(1), N(2)–Pb(1)–Cl(1)
Scheme 2 Synthesis of [Pb(L)Cl]2 complexes (L = Piso, R = t-Bu;
CyG, R = NCy2; HDG, R = N(H)C6H3-2,6-i-Pr2.
98.2(1), N(2)–Pb(1)–Cl(1i) 106.1(1). Symmetry operator: ꢁx, ꢁy, ꢁz.
i
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Fig. 4 Molecular diagram of [Pb(CyG)Cl]2 shown with 50% thermal
ellipsoids and hydrogen atoms and lattice toluene omitted for clarity.
Selected bond distances (A) and angles (1): Pb(1)–N(3) 2.311(4), Pb(1)–
N(4) 2.360(3), Pb(1)–Cl(1) 2.598(1), Pb(1)–Cl(1i) 3.093(1); Cl(1)–Pb(1)–
Cl(1i) 78.43(3), Cl(1)–Pb(1)–N(3) 101.99(9), Cl(1)–Pb(1)–N(4) 96.10(8),
N(3)–Pb(1)–Cl(1i) 93.64(8), Symmetry operator: i2 ꢁ x, 2 ꢁ y, 1 ꢁ z.
Fig. 6 Molecular diagram of [Pb(CyGO)]2 shown with 50% thermal
ellipsoids and hydrogen atoms and lattice toluene omitted for clarity.
Selected bond distances (A) and angles (1): Pb(1)–N(1) 2.633(4),
Pb(1)–N(2) 2.282(4), Pb(1)–O(1) 2.345(4), Pb(1)–O(1i) 2.278(3);
O(1)–Pb(1)–O(1i) 77.8(1), N(1)–Pb(1)–O(1) 129.5(1), N(1)–Pb(1)–O(1i)
106.9(1), N(2)–Pb(1)–O(1) 76.7(1), N(2)–Pb(1)–O(1i) 90.2(1). Symmetry
operator: i ꢁx, 1 ꢁ y, 1 ꢁ z.
with the asymmetric ligand binding, clearly indicates a distor-
tion of the coordination environment caused by the dimerisa-
tion. The structures of 2 and 3 can be compared with those of
their tin(II) analogues, [Sn(Piso)Cl]13 and [Sn(CyG)Cl],6h
which show very similar pyramidal three coordinate metal
environments, but are isolated monomers in the crystal lattice
and do not form the loose association observed in the current
lead(II) structures. With the bridging chloride interactions
included, the lead coordination environments in complexes
2–4 are distinctly pyramidal.
which is also bound to Pb(1) and this may cause the distortion of
the amidinate binding. The Pb(1)–N(2) distance is shorter than
those observed in 1–4 above. Four coordinate lead(II) complexes
with bridging alkoxide ligands have been observed previously,
viz.
[Pb(OCH(CF3)2)(m-OCH(CF3)2)(py-4-NMe2)]2
and
[Pb(OCH(CF3)2)2(m-OCH(CF3)2)]2[Me2NH2]2, and have similar
Pb–O distances (2.247(6)–2.716(6) A and 2.243(6)–2.659(5) A).14
The origin of 5 is speculative given the uncontrolled nature of its
formation. However, recent work by Roesky and co-workers
have shown that deliberate oxidation of aluminium nacnac
complexes by either t-BuOOH or molecular oxygen leads to
an analogous modification of the nacnac ligands and the
formation of a ArCMe2Oꢁ appended to the ketiminate back-
bone.15 Thus, it seems likely that 5 results from a reaction of 3
with molecular oxygen, the protective covering of the hydro-
phobic fluorocarbon oil allowing only slow ingress of oxygen
and preventing further hydrolysis and/or total degradation of
the complex. Overall, O2 has been added to the molecule whilst
two equivalents of HCl have been eliminated and presumably
this is a radical process induced by the di-radical O2 species.
A deliberate synthesis of 5 was not attempted (Fig. 6).
A small sample of yellow crystals of [Pb(CyG)Cl], covered in a
viscous fluorocarbon oil (used to mount crystals for X-ray
analysis) was exposed to air. Surprisingly, new colourless crys-
tals formed which were analysed by X-ray structure determina-
tion. The product, [Pb(CyGO)]2 (5) (Fig. 5) was shown to
contain a modified CyG ligand in which one of the substituent
i-Pr groups on the aryl rings had converted to an alkoxide
moiety. The structure is dimeric, bridged by the alkoxide oxygen
atoms and both lead atoms each bound to the amidinate
fragment by both nitrogen atoms. The Pb–N distances are
asymmetric with that to N(1) ca. 0.3 A longer than for N(2).
The latter is attached to the alkoxide substituted aryl group
Conclusions
The results presented herein show that bulky bis(2,6-diisopro-
pyl)-amidinate and -guanidinate ligands are suitable for the
isolation of thermally stable lead(II) organoamide complexes.
As we have found for other metal systems, the number of
ligands that can fit around a single metal centre is controlled
by the steric size of the ligand. Thus, the smaller Fiso allows
two ligands per metal (i.e. PbL2) whereas the larger Piso, CyG
and HDG allow only one (i.e. PbLX). The retention of one
reactive halide functionality in the latter class is significant and
enables either formation of further derivatives or as a leaving
group for reduction reactions aimed at sub-valent species.16
Fig. 5 Molecular diagram of [Pb(HDG)Cl]2, shown with 50% thermal
ellipsoids and hydrogen atoms and lattice PhMe omitted for clarity.
Selected bond distances (A) and angles (1): Pb(1)–N(1) 2.405(5),
Pb(1)–N(2) 2.346(5), Pb(1)–Cl(1) 2.619(2), Pb(1)–Cl(1i) 3.037(2);
Cl(1)–Pb(1)–Cl(1i) 78.14(6), Cl(1)–Pb(1)–N(1) 89.3(1), Cl(1)–Pb(1)–N(2)
106.5(1), N(1)–Pb(1)–Cl(1i) 151.2(1), N(2)–Pb(1)–Cl(1i) 102.2(1). Sym-
metry operator: i1 ꢁ x, 1 ꢁ y, 1 ꢁ z.
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834 | New J. Chem., 2008, 32, 829–834