β-Diketiminato Organolead Complexes: Structures, 207Pb NMR, and Hammett Correlations

Article abstract of DOI:10.1021/om501223a

The synthesis, structure, and spectroscopic details of a series of β-diketiminato lead(II) alkyl complexes are described. The Hammett correlation between the ²⁰⁷Pb NMR chemical shifts and σ_para Hammett constant was examined computationally and found to be due to the paramagnetic shielding contribution, whereas the diamagnetic and spin-orbit coupling contributions remained fairly constant across this series of compounds. (Graph Presented).

Full text of DOI:10.1021/om501223a

Article
pubs.acs.org/Organometallics
207
β‑Diketiminato Organolead Complexes: Structures, Pb NMR, and
Hammett Correlations
Morgan J. Taylor, Emma J. Coakley, Martyn P. Coles,† Hazel Cox,* and J. Robin Fulton*,†
Department of Chemistry, University of Sussex, Falmer, Brighton BN1 9RH, U.K.
S
* Supporting Information
ABSTRACT: The synthesis, structure, and spectroscopic details of a series of β-
diketiminato lead(II) alkyl complexes are described. The Hammett correlation
between the 207Pb NMR chemical shifts and σpara Hammett constant was examined
computationally and found to be due to the paramagnetic shielding contribution,
whereas the diamagnetic and spin−orbit coupling contributions remained fairly
constant across this series of compounds.
ligands such as methyl or ligands that are able to achieve a
planar geometry at the center bound to lead (e.g., N(SiMe3)2,
Ph) adopt an endo conformation, and complexes with sterically
encumbering ligands (e.g., OtBu, PCy2) possess an exo
conformation. Although the observation of the different
isomers is based purely on solid-state data, solution-phase
NMR studies on isostructural tin complexes, notably [(BDI)-
SnCl] and [(BDI)SnPCy2] (R = Cy), have revealed that these
isomers are maintained in solution.8 However, these solution-
INTRODUCTION
■
Several lead(II) β-diketiminate complexes, [(BDI)PbX] (BDI =
[{N(2,6-iPr2C6H3)C(Me)}2CH]−), have been reported over
the past decade, including lead halide, alkoxide, aryloxide,
amide, and alkyl complexes1−6 as well as more novel
compounds such as the lead phosphanide complexes [(BDI)-
PbPR2] (R = Ph, Cy, SiMe3) and the lead silylidenephospha-
nide [(BDI)Pb−PSi(R)Si(tBu)3] (R = 2,4,6-iPr3C6H2).7,8
The geometry of the ligands around the three-coordinate metal
center is always trigonal pyramidal, with the lone pair having
some directionality due to it possessing approximately 10% p
character.1,9−11 The metal center for these neutral molecules
lies outside the plane defined by the BDI backbone (NCCCN
plane), which is similar to four-coordinate metal complexes
bearing a BDI ligand12 and thus consistent with the presence of
a stereochemically active lone pair.13 The acute bond angles
observed at lead has also been attributed to a high 6p orbital
contribution to the lead−ligand bonds.14
Two different types of isomers have been observed in these
three-coordinate lead complexes (Figure 1): one in which the
metal center and X ligand are below the NCCCN plane and the
Pb−X bond is approximately perpendicular to the NCCCN
plane (“endo” conformation) and the other in which the lead
center lies above the NCCCN plane and the X ligand points
away from the “(BDI)Pb” core (“exo” conformation).15 β-
Diketiminato lead complexes with sterically unconstrained
phase studies relied on through-space coupling, JSnC and JSnH
,
between Sn and the isopropyl groups on the BDI aryl groups.
Unfortunately, through-space coupling between the lead and
any other nuclei has not been observed.
The 207Pb NMR chemical shift of β-diketiminato lead
complexes ranges from 800 to 3981 ppm, with one high-field
anomaly, [(BDI)Pb{P(SiMe3)2}], at −1737 ppm. Although this
is a small range inside the 207Pb window (∼17000 ppm), it is
relatively unpredictable. That is, the electron-withdrawing and
donating capacity of the “X” ligand does not govern the
chemical shift. The general explanation for the differing
chemical shifts is that the relative contribution to the isotropic
shielding (σ) of the 207Pb nuclei from the diamagnetic (σD),
paramagnetic (σP) and spin−orbit (σSO) shieldings (eq 1)
varies dramatically with small changes in the ligand.
σ = σD + σP + σSO
(1)
We were interested in examining the effect of the ligand on
the 207Pb chemical shift by focusing on β-diketiminato
organolead complexes, enabling us to ascertain differences in
Special Issue: Mike Lappert Memorial Issue
Received: November 30, 2014
Figure 1. Two different conformations of [(BDI)PbX].
© XXXX American Chemical Society
A
DOI: 10.1021/om501223a
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
chemical shift that are due to both a wide range of steric
environments and a full range of hybridization of the carbon
directly bound to the lead, thereby controlling the relative “s”
and “p” contributions to the Pb−X bond as the hybridization
changes from sp3 (alkyl) to sp2 (aryl) to sp (allynyl). Three
divalent organolead complexes, [(BDI)PbMe] (1), [(BDI)-
PbPh] (2), and [(BDI)PbCCPh] (3), were reported else-
where,6 all possessing an endo conformation. The 207Pb
chemical shifts for 1−3 are 3009 ppm (1), 2424 ppm (2),
and 1463 ppm (3). Although this is a broad range of chemical
shifts, this series lacks organolead complexes possessing an exo
conformation. We have expanded the range of known β-
diketiminato organolead complexes by synthesizing β-diketimi-
nato lead complexes bearing bulkier alkyl groups, including tBu
and neopentyl (Np) (Figure 2). This has not only provided
Figure 2. List of compounds used in this study and arranged by their
solid-state conformations. Complexes 1 − 3 have been previously
reported;6 complexes 4−8 are reported for the first time.
Figure 3. ORTEP diagrams of lead isopropyl complex 4 (a), tert-butyl
complex 5 (b), sec-butyl complex 6 (c), neopentyl complex 7 (d), and
benzyl complex 8 (e). H atoms are omitted for all diagrams, and BDI
C atoms are minimized in parts (b)−(e) for clarity. Ellipsoids are
shown at the 30% probability level.
examples of complexes with an exo conformation but also
increases the range of electron donors, allowing for a more
detailed study of the 207Pb chemical shift.
RESULTS AND DISCUSSION
■
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Isopropyl Complex 4 and tert-Butyl Complex 5 (L = BDI)
Synthesis of Lead Alkyl Complexes. Five new β-
diketiminato lead complexes were generated by treatment of
lead chloride, [(BDI)PbCl], with the appropriate Grignard (in
the case of isopropyl, sec-butyl, and benzyl lead complexes 4, 6,
and 8, respectively) or alkyllithium reagent (in the case of tert-
butyl and neopentyl lead complexes 5 and 7, respectively). The
1H NMR spectra of these complexes are similar, with two
multiplets observed between δ 3 and 4 ppm for the isopropyl
groups on the N-aryl substituent, consistent with a trigonal-
[LPbiPr] (4)
[LPbtBu] (5)
Pb−C(30)
Pb−N(1)
Pb−N(2)
N(1)−C(1)
C(1)−C(2)
C(2)−C(3)
C(3)−N(2)
2.305(4)
2.327(3)
2.330(3)
1.323(4)
1.404(5)
1.404(5)
1.332(5)
98.51(14)
97.67(14)
79.90(10)
120.6(2)
120.2(2)
276.08
2.333(6)
2.364(4)
2.362(4)
1.326(7)
1.402(8)
1.409(8)
1.315(7)
102.32(19)
104.22(18)
79.14(15)
114.8(3)
114.0(3)
285.68
pyramidal geometry at lead. Coupling between H and 207Pb
1
was only observed in the benzyl lead complex 8 (JHPb = 86 Hz).
The 207Pb NMR chemical shift ranged from δ 2872 ppm for
benzyl complex 8 to δ 3684 ppm for tert-butyl complex 5.
The solid-state structure was determined for compounds 4−
8 (Figure 3). See the Supporting Information for the data
collection parameters of complexes 4−8. The bond lengths and
bond angles for compounds 4 and 5 are given in Table 1, and
the bond lengths and bond angles for compounds 6−8 are
given in Table 2. As expected, consistent with other β-
diketiminato lead(II) complexes, a pyramidal geometry is
observed at lead, and the degree of pyramidalization (DOP)16
is consistent with that of other β-diketiminato group 14
complexes.3,17−20 However, in contrast to the previously
reported β-diketiminato organolead complexes 1−3, an exo
conformation is observed at the metal center of complexes 4−
8, with the lead atom significantly displaced from the NCCCN
plane (range 1.179−1.436 Å). This displacement is also
reflected in the torsion angle between the NCCCN plane
and a plane defined by N−Pb−N (NPbN plane). The exo
complexes 4−8 have a more acute torsion angle (range 131.3−
N(1)−Pb−C(30)
N(2)−Pb−C(30)
N(1)−Pb−N(2)
Pb−N(1)−C(1)
Pb−N(2)−C(2)
sum of angles
a
DOP
93.2
82.6
Pb−NCCCN plane
torsion angle
1.436
1.359
b
141.68
131.27
a
b
Degree of pyramidalization.16 Angle between the plane defined by
N(1)−C(1)−C(2)−C(3)−N(2) and the plane defined by N(1)−Pb−
N(2).
141.7°) than then endo complexes 1−3 (149.7° for acetylide
complex 3 to 164.0° for phenyl complex 2). The Pb−C bond
lengths range from 2.293(4) Å for neopentyl complex 7 to
2.333(6) for tert-butyl complex 5, a range similar to that of the
endo complexes (2.300 Å for methyl complex 1 and phenyl
complex 2, 2.276(3) Å for acetylide complex 3). There is no
B
DOI: 10.1021/om501223a
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
the Hammett σmeta constant (R2 = 0.84).22 The σpara correlation
increases when the chemical shift of the β-diketimato lead
chloride is included (R2 = 0.95); however, this correlation does
not hold with all β-diketimato lead complexes (see the
Supporting Information). If lead amido complexes [(BDI)-
PbNHPh] and [(BDI)PbNMe2] are included, the correlation
between σpara and the 207Pb chemical shift vanishes (R2 =
0.0082). Unfortunately, reliable σpara values could not be found
for all of the reported β-diketiminato lead complexes.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
sec-Butyl Complex 6, Neopentyl Complex 7, and Benzyl
Complex 8 (L = BDI)
[LPbsBu] (6)
[LPbNp] (7)
[LPbBn] (8)
Pb−C(16)*
Pb−N
N−C(1)
N−Pb−C(16)
N′−Pb−C(16)
N−Pb−N′
2.306(5)*
2.3287(19)
1.325(3)
97.5(6)
98.2(7)*
79.56(10)
118.73(15)
275.26
2.293(4)
2.3273(16)
1.324(2)
91.04(14)
97.54(15)
79.64(8)
119.41(12)
268.22
2.314(5)
2.321(2)
1.329(4)
93.38(11)
93.38(11)
80.22(12)
117.42(19)
266.98
a
a
A similar correlation between 103Rh NMR chemical shift and
σpara was observed by both the Leitner and Elsevier groups on a
series of rhodium complexes bearing bidentate aryl phosphine
ligands.23,24 The more electron rich the aryl phosphine, the
higher the chemical shift and less shielding of the rhodium
nuclei. In contrast, ring-substituted (η5-cyclopentadienyl)-
dicarbonylrhodium complexes, [(η5-C5H4R)Rh(CO)2], show
the opposite trend; electron-withdrawing groups decrease the
shielding of the rhodium nuclei. These contrasting results
reveal the sensitivity of nuclei to various perturbations,
presumably affecting the relative contributions from diamag-
netic, paramagnetic, and spin−orbit shieldings. A Hammett
correlation was also observed in the 183W NMR chemical shift
of a series of para-substituted pyridine tungsten complexes, cis-
[W(CO)4(PPh3)(4-RC5H4N)].25 Similar to the case for the
[(η5-C5H4R)Rh(CO)2] system, decreasing the electron density
on the pyridine resulted in greater deshielding of the nuclei.
These results were attributed to differences in the polarizability
of the pyridine ligand affecting the paramagnetic shielding
contribution to the chemical shift. A correlation was
investigated for a series of tetrakis(phenyl)germanium com-
plexes;26 however, in this case, only a small correlation was
found, highlighting the limits of utilizing Hammett constants in
the investigation of trends in chemical shift. However, in all of
these examples, a substituent was changed on an aryl group that
was either directly bound to the metal or bound through a
donor ligand such as phosphine. In our system, we are
investigating the effect of the ligand directly bound to the metal
center.
Pb−N−C(1)
sum of angles
b
DOP
94.2
101.9
103.4
Pb−NCCCN plane
torsion angle
1.215
1.179
1.251
c
137.89
139.32
135.95
a
b
Atom bonded to lead is C(17) in structure 6. Degree of
pyramidalization.16 Angle between the plane defined by N(1)−
C(1)−C(2)−C(3)−N(2) and the plane defined by N(1)−Pb−N(2).
c
correlation between the Pb−C bond length and torsion angle.
However, the bond angles around lead are more acute for the
endo conformers (average 87.5°) than for the exo conformers
(91.5°). The differences between the endo and exo conformers
are presumably due to the differences in steric bulk at the alkyl
ligand; that is, in the absence of steric bulk, there is a preference
for the endo conformation, as the ligand is small enough to fit
between the bulky N-aryl substituents on the β-diketiminate
ligand. Preliminary density functional theory (DFT) inves-
tigations on the relative stabilities of these complexes revealed
that complexes 2, 5, and 7 all optimize to the observed
conformation, whereas for complexes 1, 4, 6, and 8 both endo
and exo structures are obtained as minima with the relative
energy difference between the conformers being small.21
207Pb NMR Spectroscopy. The electronic effect of the
alkyl group on the 207Pb NMR chemical shift was examined by
plotting the Hammett σpara constant versus the 207Pb chemical
shift.22 A good correlation (R2 = 0.94) was observed between
the chemical shift and σpara (Figure 4). Thus, the 207Pb NMR
chemical shifts of complexes 1−8 are directly related to the
electron donor capacity of the alkyl group; the more electron
donating the alkyl group, the higher the chemical shift. It is
notable that a Hammett correlation exists with complexes 1−8,
even though the data from both the endo and exo conformers
have been combined. A weaker correlation was observed with
Computational Studies: 207Pb NMR Chemical Shifts.
The 207Pb NMR chemical shifts were calculated using density
functional theory (DFT) to determine the factors contributing
to the excellent correlation between the experimental chemical
shifts and the Hammett σpara parameter. A handful of
computational studies have previously been performed on
207Pb NMR chemical shifts, including calculations on solid-state
207Pb NMR spectra27−30 and a solution-phase study investigat-
ing the importance of spin−orbit coupling in the chemical shift
calculations on a series of lead(IV) complexes.31 In this work, it
was found necessary to include relativistic effects at the spin−
orbit level, as neither scalar relativistic effects nor a relativistic
core potential were sufficient to model these systems (see the
Experimental Section for method details). These relativistic
calculations were performed on both the X-ray (solid-state) and
optimized (gas-phase) structures; we note that neither is the
perfect model for the solution-phase system in which the 207Pb
NMR was recorded. The 207Pb NMR data calculated using the
X-ray structures gave a more consistent set of errors and thus
will be the data discussed herein. Initially, the absolute isotropic
shielding of complexes 1−8 was calculated and converted into
chemical shifts by using the shielding of tetramethyllead,
[Pb(CH3)4] (TML), which was calculated to have an isotropic
shielding value of 3530 ppm. The NMR calculations were
performed on a geometry-optimized Td structure of TML at the
Figure 4. Hammett σpara constant versus 207Pb chemical shifts of
complexes 1−8.
C
DOI: 10.1021/om501223a
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Table 3. Experimental and Calculated 207Pb NMR Chemical Shifts (ppm) Relative to TML and Linear Regression Reference,
Hammett Constants (σpara), Absolute Isotropic Shielding (σcalcd), Spin−Orbit Shielding (σSO), Diamagnetic Shielding (σD), and
Paramagnetic Shielding (σP) for Complexes 1−8 using Eq 2
isotropic shielding σcalcd
σD + σP+ σSO
=
calcd 207Pb chemical shift
linear
exptl 207Pb chemical
Hammett
constant σpara
absolute isotropic
shielding σcalcd
entry
R
shift δexptl
TML δTML
regression δcalcd
σSO
σD
σP
1
2
3
4
5
6
7
8
Me
3002
2419
1463
3145
3684
3262
3506
2872
−0.17
−0.01
0.16
1731
2318
3142
1550
971
1799
1212
388
3008
2376
1509
3185
3794
3166
3445
2776
−2140
−2130
−2034
−2200
−2354
−2198
−2247
−2225
9955
9956
9956
9955
9955
9955
9954
9954
−6084
−5508
−4780
−6205
−6630
−6189
−6404
−5791
Ph
CCPh
iPr
−0.15
−0.20
−0.12
−0.17
−0.09
1980
2559
1962
2227
1592
tBu
sBu
Np
1568
1303
1938
Bn
same level of theory as that used for the lead complexes. This
resulted in significant error between the experimental and
calculated chemical shifts (Table 3); however, as the variation
in the error is relatively small, 225 ppm (1075−1300 ppm),
these errors can be considered systematic and a reflection of the
unsuitability of TML as a reference standard. This unsuitability
is not unusual and has been examined in detail in the
calculation of 13C NMR chemical shifts. When tetramethylsi-
lane (TMS) is used as a reference, errors can arise due to the
differences in the isotropic shielding values for carbon atoms
attached to other carbon atoms versus silicon atoms.32 To
reduce error, one technique is to choose a new reference
molecule that has properties similar to that of the molecules of
interest. Due to the limited number of known three-coordinate
divalent lead complexes, this is not possible.
Another method used in 13C NMR chemical shift
calculations32 and used by Buhl for calculating transition-
̈
metal NMR chemical shifts33 is linear regression analysis, in
which the calculated isotropic shielding is plotted against the
experimental chemical shift, δexptl. The resulting linear
correlation allows for the determination of a new reference,
taken as the intercept at δexptl = 0. This can then be used to
convert absolute shielding (σcalcd) to calculated chemical shift
(δcalcd) by eq 2, thus avoiding the use of an unsuitable reference
point.
Figure 5. Correlation plot for calculated absolute isotropic shielding,
σcalcd, and experimental chemical shift, δexptl, for the 207Pb NMR of
compounds 1−8.
spin−orbit (σSO) shielding contributions (Table 3).31 The
diamagnetic contribution, σD, remained relatively constant
between all complexes (2 ppm difference). Diamagnetic
shielding is a result of electrons moving in orbitals determined
by the ground-state wave function and is thereby dependent
upon the electron density at the nucleus. Thus, for a series of
structurally similar compounds, this term is not anticipated to
vary significantly.
δ
= (intercept − σcalcd)/slope
(2)
calcd
A plot of absolute shielding versus experimental 207Pb
chemical shifts gives a very good correlation (R2 = 0.99) with a
slope of −0.95, indicative of a well-performing method, and an
intercept of 4576 ppm (Figure 5).32 Using this linear regression
reference point for calculating the chemical shifts reduced the
errors between the experimental and calculated chemical shifts
by 1 order of magnitude or more. The errors ranged from −110
to 95 ppm (Table 3), which are less than 10% of the chemical
shift span reported in this study.
The spin−orbit contribution, σSO, showed a variation of 320
ppm among complexes 1−8, which is relatively small in
comparison to the variation in calculated chemical shift (over
2000 ppm). In previous studies, it was found that spin−orbit
coupling is very sensitive to the atomic number of the atoms
directly coordinated to the Pb center.31 Thus, only minor
variations would be expected between these complexes. In
contrast, the paramagnetic term, σP, increases significantly (by
up to 1850 ppm) as the chemical shift decreases. This term is
due to the circulation of electrons within the ground- and
excited-state orbitals. Its magnitude depends on the asymmetric
electron distribution close to the nucleus; therefore, for s
orbitals it is 0 but can be quite large for an asymmetric
distribution of p and d electrons when these electrons have low-
lying excited states.34 A strong correlation was found between
the Hammett constant and paramagnetic shielding (Figure 6).
That is, the paramagnetic shielding alone correlates with the
Hammett constant and thus the electron-withdrawing and
There is a very good correlation (R2 = 0.93) between the
calculated chemical shift (δcalcd) and the Hammett parameter,
enabling the theory to elucidate the reasons for it. The
calculated chemical shifts did not correlate directly to the Pb−C
bond length. This indicates that electronic rather than
structural factors are primarily responsible for the correlation.
It was noted earlier that spin−orbit coupling was very
important; therefore, the contributions to the absolute shielding
were evaluated. The absolute shielding can be deconstructed
into the relative diamagnetic (σD), paramagnetic (σP), and
D
DOI: 10.1021/om501223a
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
CH), 3.59 (sept, J = 6.9 Hz, 2H, CHMe2), 3.48 (sept, J = 6.8 Hz, 2H,
CHMe2), 1.75 (s, 6H, NCMe), 1.26 (d, J = 6.9 Hz, 12H, CHMe2), 1.24
(d, J = 6.8 Hz, 12H, CHMe2), 0.59 (s, JMe‑Pb = 89 Hz, 3H, PbMe).
UV−vis: λmax 396.0 nm, λsecondary 353.0 nm.
[CH{(CH3)CN-2,6-iPr2C6H3}2PbPh] (2): Modified Procedure.6
[(BDI)PbCl] (300 mg, 0.45 mmol) was dissolved in ∼10 mL of
toluene in a Schlenk tube and cooled to −78 °C. PhMgBr (3.0 M, 150
μL, 0.45 mmol) was mixed with ∼5 mL of toluene in another Schlenk
tube and was added dropwise to the LPbCl solution. The mixture was
left to react for 2 h, after which the toluene was removed in vacuo,
pentane was added, and the yellow solution was filtered through
Celite. The solution was concentrated and the product crystallized at
1
−30 °C, giving cubic yellow crystals. Yield: 80 mg (25.4%). H NMR
(500 MHz, C6D6, 30 °C): δ 8.52 (d, J = 6.9 Hz, 2H, o-HPh), 7.58 (t, J
= 7.6 Hz, 2H, m-HPh), 7.12 (m, 6H, Haryl), 7.05 (m, 1H, p-HPh), 4.80
(s, 1H, middle CH), 3.58 (sept, J = 6.9 Hz, 2H, CHMe2), 3.22 (sept, J
= 6.8 Hz, 2H, CHMe2), 1.77 (s, 6H, NCMe), 1.38 (d, J = 6.9 Hz, 6H,
CHMe2), 1.25 (d, J = 6.9 Hz, 6H, CHMe2), 1.07 (d, J = 6.9 Hz, 6H,
CHMe2), 0.52 (d, J = 6.7 Hz, 6H, CHMe2). UV−vis: λmax 397.0 nm,
λsecondary 360.0 nm.
Figure 6. Hammett constant σpara versus calculated paramagnetic
shielding, σP, of complexes 1−8.
CH{(CH3)CN-2,6-iPr2C6H3}2PbiPr] (4). [(BDI)PbCl] (300 mg,
0.45 mmol) was dissolved in ∼10 mL of toluene in a Schlenk tube and
-donating properties of the ligand. This confirms that the
paramagnetic shielding alone is trend setting and that the more
electron donating the substituent, R, the more negative the
paramagnetic shielding contribution, resulting in a smaller
isotropic shielding and a more downfield (larger) chemical
shift.
i
cooled to −78 °C. PrMgCl (2.0 M, 230 μL, 0.45 mmol) was mixed
with ∼5 mL of toluene in another Schlenk tube and was added
dropwise to the cold LPbCl solution. The mixture was stirred for 2 h,
after which the toluene was removed in vacuo. Pentane was then
added and the orange solution filtered through Celite. The solution
was concentrated and placed at −30 °C for crystallization. Orange
crystals suitable for X-ray diffraction were grown from a heptane
CONCLUSIONS
■
1
solution, also held at −30 °C. Yield: 161 mg (53.1%). H NMR (400
The 207Pb NMR chemical shift of β-diketiminato lead
complexes, [(BDI)PbX], is largely unpredictable on compar-
ison of different classes of terminal X ligands. This is
presumably due to differences in the contributions to the
nuclear shielding from the diamagnetic, paramagnetic, and
spin−orbit terms. However, across a class of terminal X ligands,
notably the organolead derivatives, the relative contributions of
the diamagnetic and spin−orbit terms to the overall shielding
are constant, allowing for trends in 207Pb NMR chemical shifts
to appear. The observed correlation with the Hammet σpara
constant highlights influence of subtle electronic effects well
beyond organic molecules.
MHz, C6D6, 30 °C): δ 7.07 (m, 6H, Haryl), 4.62 (s, 1H, middle CH),
3.79 (sept, J = 6.9 Hz, 2H, CHMe2), 3.32 (sept, J = 6.9 Hz, 2H,
CHMe2), 3.31 (sept, J = 6.9 Hz, 1H, PbCHMe2) 1.72 (s, 6H, NCMe),
1.54 (d, J = 4.0 Hz, 6H, PbCHMe2), 1.42 (d, J = 6.9 Hz, 6H, CHMe2),
1.25 (d, J = 6.9 Hz, 6H, CHMe2), 1.23 (d, J = 6.9 Hz, 6H, CHMe2),
1.19 (d, J = 6.8 Hz, 6H, CHMe2). 13C NMR (400 MHz, C6D6, 30 °C):
δ 165.9 (NCMe), 143.7, 143.3, 142.5, 126.6, 125.4, 124.0, 123.6, 123.2
(Caryl), 101.7 (Pb-CHMe2), 97.5 (middle CH), 28.2 (CHMe2), 27.6
(CHMe2), 24.7 (CHMe2), 24.3 (CHMe2), 24.2 (CHMe2), 24.1
(CHMe2), 23.0 (NCMe), 17.2 (PbCMe2). 207Pb NMR (600 MHz,
C6D6, 30 °C): 3145 ppm. Anal. Calcd for C32H48N2Pb: C, 57.55; H,
7.19; N, 4.20. Found: C, 57.49; H, 7.27; N, 4.15. UV−vis: λmax 354.0
nm, λsecondary 448.9 nm.
[CH{(CH3)CN-2,6-iPr2C6H3}2PbtBu] (5). [(BDI)PbCl] (300 mg,
0.45 mmol) was dissolved in ∼10 mL of toluene in a Schlenk tube and
cooled to −78 °C. tBuLi (1.89 M, 240 μL, 0.45 mmol) was mixed with
∼5 mL of toluene in another Schlenk tube and was added dropwise to
the cold LPbCl solution. The mixture was stirred at −78 °C overnight,
after which the toluene was removed in vacuo and replaced with
pentane and the bright orange solution was filtered through Celite.
Large red crystals, suitable for X-ray analysis, were obtained from a
EXPERIMENTAL SECTION
■
All manipulations were carried out under an atmosphere of dry
nitrogen or argon using standard Schlenk techniques or in an inert-
atmosphere glovebox. Solvents were dried from the appropriate drying
agent, distilled, degassed, and stored over 4 Å sieves. [(BDI)PbCl] (1)
and [(BDI)PbCCPh] were prepared according to the literature.1,6 The
1H and 13C NMR spectra were recorded on a Varian 400, 500, or 600
1
1
MHz spectrometer. The H and 13C NMR spectroscopic chemical
concentrated pentane solution at −30 °C. Yield: 174 mg (56.0%). H
shifts are given relative to residual solvent peaks, and the 207Pb shifts
were externally referenced to PbMe4. Reagents were purchased from
Acros Organics or Sigma-Aldrich. Neopentyllithium was donated by
Prof. Geoff Cloke’s research group. UV−vis spectra were recorded on
a Varian Cary 50 instrument. The data for the X-ray structure were
collected at 173 K on a Nonius Kappa CCD diffractometer (λ(Mo
Kα) = 0.71073 Å) and refined using the SHELXL-97 software
package.35
NMR (400 MHz, C6D6, 30 °C): δ 7.08 (m, 6H, Haryl), 4.67 (s, 1H,
middle CH), 3.85 (sept, J = 6.9 Hz, 2H, CHMe2), 3.22 (sept, J = 6.8
Hz, 2H, CHMe2), 2.20 (s, 9H, CMe3), 1.75 (s, 6H, NCMe), 1.44 (d, J
= 6.9 Hz, 6H, CHMe2), 1.26 (d, J = 6.9 Hz, 6H, CHMe2), 1.21 (d, J =
6.8 Hz, 6H, CHMe2), 1.18 (d, J = 6.8 Hz, 6H, CHMe2). 13C NMR
(400 MHz, C6D6, 30 °C): δ 167.9 (NCMe), 144.6, 143.3, 142.3, 125.3,
123.8, 123.6 (Caryl), 123.2 (PbCMe3), 97.7 (middle CH), 28.6
(CHMe2), 28.0 (CHMe2), 26.9 (CHMe2), 26.6 (CHMe2), 25.0
(CHMe2), 24.6 (CHMe2), 24.1 (PbCMe3), 23.5 (NCMe), 23.0
(PbCMe3). 207Pb NMR (600 MHz, C6D6, 30 °C): 3684 ppm. Anal.
Calcd for C33H50N2Pb: C, 58.13; H, 7.34; N, 4.11. Found: C, 58.06; H,
7.28; N, 4.03. UV−vis: λmax 375.0 nm, λsecondary 451.1 nm.
[CH{(CH3)CN-2,6-iPr2C6H3}2PbMe] (1): Modified Procedure.6
[(BDI)PbCl] (300 mg, 0.45 mmol) was dissolved in ∼10 mL of
toluene in a Schlenk tube and cooled to −78 °C. MeMgBr (1.6 M, 330
μL, 0.45 mmol) was mixed with ∼5 mL of toluene in another Schlenk
tube and was added dropwise to the LPbCl solution. The mixture was
stirred for 1 h, after which the toluene was removed in vacuo, pentane
was added, and the lemon yellow solution was filtered through Celite.
The solution was concentrated and the product left to crystallize at
−30 °C, giving bright yellow crystals. Yield: 115 mg (39.6%). 1H NMR
(400 MHz, C6D6, 30 °C): δ 7.06 (m, 6H, Haryl), 4.81 (s, 1H, middle
[CH{(CH3)CN-2,6-iPr2C6H3}2PbsBu] (6). [(BDI)PbCl] (300 mg,
0.45 mmol) was dissolved in ∼10 mL of toluene in a Schlenk tube and
s
cooled to −78 °C. BuMgCl (1.9 M, 260 μL, 0.5 mmol) was mixed
with ∼5 mL of toluene in another Schlenk tube and was added
dropwise to the cold LPbCl solution. The mixture was stirred at −78
°C for 2 h, after which the toluene was evaporated and replaced with
E
DOI: 10.1021/om501223a
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
pentane and the orange solution was filtered through Celite. The
solution was placed at −30 °C, and small, dark orange crystals were
on all other atoms. Frequency analysis was used to confirm minima
obtained via geometry optimization; however, only the single-point
energies calculated using the X-ray crystal structure are discussed in
the main text (coordinates are provided in the CIF files of the
Supporting Information). Magnetic shielding tensors were calculated
using the gauge-including atomic orbitals (GIAO) method.43,44 NMR
calculations were performed on both the X-ray crystal structure and
optimized geometry for each complex 1−8. The maximum chemical
shift errors using the geometry-optimized structures were larger (∼475
ppm) than those using the X-ray structure (∼250 ppm), and thus the
geometry-optimized results are not presented. Furthermore, it was
found that the spin−orbit coupling was extremely important in order
to provide quantitative agreement with experimental chemical shifts;
therefore, only the calculations which include spin−orbit relativistic
effects are presented in the main text.
1
observed after a few days. Yield: 186 mg (60.7%). H NMR (400
MHz, C6D6, 30 °C): δ 7.06 (m, 6H, Haryl), 4.63 (s, 1H, middle CH),
3.80 (sept, J = 6.9 Hz, 2H, CHMe2), 3.32 (sept, J = 6.6 Hz, 2H,
CHMe2), 2.46 (m, 1H, PbCH(Me)CH2Me), 1.73 (s, 6H, NCMe),
1.61 (d, J = 7.3 Hz, 3H, PbCH(Me)CH2Me), 1.45 (d, J = 6.7 Hz, 6H,
CHMe2), 1.25 (m, 2H, PbCH(Me)CH2Me), 1.18 (d, J = 6.7 Hz, 6H,
CHMe2), 0.80 (t, J = 7.3 Hz, 3H, PbCH(Me)CH2Me). 13C NMR (400
MHz, C6D6, 30 °C): δ 169.5, 166.1 (NCMe), 143.9, 143.3, 142.5,
131.8, 125.4, 124.0, 123.6 (Caryl), 108.7 (PbCH(Me)CH2Me), 97.5
(middle CH), 28.3, 28.2 (CHMe2), 27.7 (PbCH(Me)CH2Me), 26.9,
26.8 (CHMe2), 25.0, 24.7, 24.3, 24.2, 24.1 (CHMe2), 23.1, 23.0
(NCMe), 15.6 (PbCH(Me)CH2Me), 13.0 (PbCH(Me)CH2Me). 207Pb
NMR (600 MHz, C6D6, 30 °C): 3262 ppm. Anal. Calcd for
C33H50N2Pb: C, 58.13; H, 7.34; N, 4.11. Found: C, 57.03; H, 7.29;
N, 4.02. UV−vis: λmax 360.1 nm, λsecondary 452.0 nm.
ASSOCIATED CONTENT
* Supporting Information
■
[CH{(CH3)CN-2,6-iPr2C6H3}2PbNp] (7). [(BDI)PbCl] (300 mg,
0.45 mmol) was dissolved in ∼10 mL of toluene in a Schlenk tube and
cooled to 0 °C. NpLi (35 mg, 0.45 mmol) was mixed with ∼5 mL of
toluene in another Schlenk tube and was added dropwise to the cold
LPbCl solution. The mixture was stirred for 2 h, after which the
toluene was removed in vacuo. Pentane was then added, and the
orange solution was filtered through Celite. The solution was
concentrated and placed at −30 °C. After 1 week, orange crystals
S
Figures, a table, and CIF files giving crystallographic data and
complete ORTEP diagrams for complexes 4−8. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for H.C.: H.Cox@sussex.ac.uk.
*E-mail for J.R.F.: J.Robin.Fulton@vuw.ac.nz.
■
1
suitable for X-ray analysis were observed. Yield: 209 mg (66.8%). H
NMR (400 MHz, C6D6, 30 °C): δ 7.27 (m, 6H, Haryl), 4.73 (s, 1H,
middle CH), 3.89 (sept, J = 6.9 Hz, 2H, CHMe2), 3.41 (sept, J = 6.8
Hz, 2H, CHMe2), 1.84 (s, 6H, NCMe), 1.62 (d, J = 6.9 Hz, 6H,
CHMe2), 1.36 (d, J = 6.9 Hz, 6H, CHMe2), 1.34 (d, J = 6.9 Hz, 6H,
Present Address
†School of Chemical and Physical Sciences, Victoria University
of Wellington, P.O. Box 600, Wellington, New Zealand.
t
CHMe2), 1.29 (d, J = 6.8 Hz, 6H, CHMe2), 1.01 (s, 2H, PbCH2 Bu),
t
0.79 (s, 9H, PbCH2 Bu). 13C NMR (400 MHz, C6D6, 30 °C): δ 166.2
Notes
(NCMe), 143.4, 143.3, 142.8, 125.4, 123.9 (Caryl), 114.0
(PbCH2CMe3), 97.3 (middle CH), 36.6 (PbCH2CMe3), 28.3
(CHMe2), 27.7 (CHMe2), 26.6 (CHMe2), 24.8 (CHMe2), 24.6
(CHMe2), 24.4 (CHMe2), 22.9 (NCMe). 207Pb NMR (600 MHz,
C6D6, 30 °C): 3506 ppm. Anal. Calcd for C34H52N2Pb: C, 58.69; H,
7.48; N, 4.03. Found: C, 58.64; H, 7.56; N, 3.98. UV−vis: λmax 344.0
nm, λsecondary 451.0 nm.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.R.F. and M.J.T. are grateful for financial support from the
EPSRC (Grant No. EP/E-32575/1). H.C. and E.J.C. acknowl-
edge the use of the EPSRC UK National Service for
Computational Chemistry Software (NSCCS) at Imperial
College London in carrying out this work.
[CH{(CH3)CN-2,6-iPr2C6H3}2PbBn] (8). [(BDI)PbCl] (300 mg,
0.45 mmol) was dissolved in ∼10 mL of toluene in a Schlenk tube and
cooled to −78 °C. BnMgCl (20 wt %, 343 mg, 0.45 mmol) was mixed
with ∼5 mL of toluene in another Schlenk tube and was added
dropwise to the LPbCl solution. The mixture was stirred for 2 h, after
which the toluene was removed in vacuo, pentane was added, and the
orange solution was filtered through Celite. The solution was
concentrated and the product left to crystallize at −30 °C, giving
DEDICATION
■
We dedicate this article to Professor Mike Lappert, an
inspiration and a true hero of Group 14.
1
pale orange crystals. Yield: 126 mg (39.2%). H NMR (400 MHz,
REFERENCES
■
C6D6, 30 °C): δ 7.13 (m, 6H, Haryl), 6.98 (t, J = 7.6 Hz, 2H, m-HPh),
6.57 (t, J = 7.4 Hz, 1H, p-HPh), 5.94 (d, J = 7.4 Hz, 2H, o-HPh), 4.64
(s, 1H, middle CH), 3.74 (sept, J = 6.9 Hz, 2H, CHMe2), 3.19 (sept, J
= 6.8 Hz, 2H, CHMe2), 1.75 (s, JCH2‑Pb = 87 Hz, 2H, PbCH2), 1.68 (s,
6H, NCMe), 1.44 (d, J = 6.9 Hz, 6H, CHMe2), 1.24 (d, J = 6.8 Hz, 6H,
CHMe2), 1.15 (d, J = 6.8 Hz, 6H, CHMe2), 1.05 (d, J = 6.9 Hz, 6H,
CHMe2). 13C NMR (400 MHz, C6D6, 30 °C): δ 165.5 (NCMe),
143.4, 143.2, 142.7, 141.9, 138.4, 134.9, 127.2, 125.7, 124.2, 124.0,
122.4 (Caryl), 98.2 (middle CH), 94.6 (Pb-CH2), 28.3 (CHMe2), 27.6
(CHMe2), 26.5 (CHMe2), 24.7 (CHMe2), 24.6 (CHMe2), 24.2
(CHMe2), 23.0 (NCMe). 207Pb NMR (400 MHz, C6D6, 30 °C): δ
2871. Anal. Calcd for C36H46N2Pb: C, 60.40; H, 6.71; N, 3.91. Found:
C, 60.45; H, 6.79; N, 3.87. UV−vis: λmax 345.0 nm, λsecondary 446.0 nm.
Computational Details. DFT NMR calculations were performed
on the lead complexes 1−8 and tetramethyllead (TML) using the
Amsterdam Density Functional (ADF) package,36−38 which allows for
a detailed treatment of relativistic effects. Relativistic effects were
accounted for using the zeroth-order regular approximation
(ZORA).39 All calculations were performed using the BP86 func-
tional,40−42 with a Slater-type orbital all-electron relativistic quadruple-
ζ basis set, with four sets of polarization functions (ZORA-QZ4P) on
the Pb center, and a triple-ζ basis set with polarization (ZORA-TZP)
(1) Chen, M.; Fulton, J. R.; Hitchcock, P. B.; Johnstone, N. C.;
Lappert, M. F.; Protchenko, A. V. Dalton Trans. 2007, 2770−2778.
(2) Fulton, J. R.; Hitchcock, P. B.; Johnstone, N. C.; Tam, E. C. Y.
Dalton Trans. 2007, 3360−3362.
(3) Tam, E. C. Y.; Johnstone, N. C.; Ferro, L.; Hitchcock, P. B.;
Fulton, J. R. Inorg. Chem. 2009, 48, 8971−8976.
(4) Taylor, M. J.; Saunders, A. J.; Coles, M. P.; Fulton, J. R.
Organometallics 2011, 30, 1334−1339.
(5) Harris, L. A. M.; Tam, E. C. Y.; Coles, M. P.; Fulton, J. R. Dalton
Trans. 2014, 43, 13803−13814.
(6) Jana, A.; Sarish, S. P.; Roesky, H. W.; Schulzke, C.; Doring, A.;
John, M. Organometallics 2009, 28, 2563−2567.
(7) Yao, S.; Block, S.; Brym, M.; Driess, M. Chem. Commun. 2007,
3844−3846.
(8) Tam, E. C. Y.; Maynard, N. A.; Apperley, D. C.; Smith, J. D.;
Coles, M. P.; Fulton, J. R. Inorg. Chem. 2012, 51, 9403−9415.
(9) Shimoni-Livny, L.; Glusker, J. P.; Bock, C. W. Inorg. Chem. 1998,
37, 1853−1867.
(10) Cox, H.; Stace, A. J. J. Am. Chem. Soc. 2004, 126, 3939−3947.
(11) Aitken, G. D. C.; Cox, H.; Stace, A. J. J. Phys. Chem. A 2012,
116, 3035−3041.
F
DOI: 10.1021/om501223a
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(12) Taylor, M. J.; Coles, M. P.; Fulton, J. R. Aust. J. Chem. 2015,
DOI: 10.1071/CH14526.
(13) Davidovich, R. L.; Stavila, V.; Marinin, D. V.; Voit, E. I.;
Whitmire, K. H. Coord. Chem. Rev. 2009, 253, 1316−1352.
(14) Kaupp, M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1993, 115,
1061−1073.
(15) Harris, L. A. M.; Coles, M. P.; Fulton, J. R. Inorg. Chim. Acta
2011, 369, 97−102.
(16) Maksic, Z. B.; Kovacevic, B. J. Chem. Soc. Perk. T. 1999, 1,
2623−2629.
(17) Ferro, L.; Hitchcock, P. B.; Coles, M. P.; Cox, H.; Fulton, J. R.
Inorg. Chem. 2011, 50, 1879−1888.
(18) Ferro, L.; Hitchcock, P. B.; Coles, M. P.; Fulton, J. R. Inorg.
Chem. 2012, 51, 1544−1551.
(19) Tam, E. C. Y.; Coles, M. P.; Smith, J. D.; Fulton, J. R. Polyhedron
2015, 85, 284−294.
(20) Tam, E. C. Y.; Harris, L. M.; Borren, E. S.; Smith, J. D.; Lein,
M.; Coles, M. P.; Fulton, J. R. Chem. Commun. 2013, 49, 10278−
10280.
(21) Taylor, M. J. Ph.D. Thesis, University of Sussex, 2012.
(22) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165−195.
(23) Bregman, F. R.; Ernsting, J. M.; Muller, F.; Boele, M. D. K.; van
der Veen, L. A.; Elsevier, C. J. J. Organomet. Chem. 1999, 592, 306−
311.
(24) Leitner, W.; Buhl, M.; Fornika, R.; Six, C.; Baumann, W.;
Dinjus, E.; Kessler, M.; Kruger, C.; Rufinska, A. Organometallics 1999,
18, 1196−1206.
(25) Carlton, L.; Mokoena, L. V.; Fernandes, M. A. Magn. Reson.
Chem. 2013, 51, 199−202.
(26) Yoder, C. H.; Agee, T. M.; Griffith, A. K.; Schaeffer, C. D.;
Carroll, M. J.; DeToma, A. S.; Fleisher, A. J.; Gettel, C. J.; Rheingold,
A. L. Organometallics 2010, 29, 582−590.
(27) Dmitrenko, O.; Bai, S.; Dybowski, C. Solid State Nucl. Magn.
Reson. 2008, 34, 186−190.
(28) Greer, B. J.; Michaelis, V. K.; Katz, M. J.; Leznoff, D. B.;
Schreckenbach, G.; Kroeker, S. Chem. - Eur. J. 2011, 17, 3609−3618.
(29) Rossini, A. J.; Macgregor, A. W.; Smith, A. S.; Schatte, G.;
Schurko, R. W.; Briand, G. G. Dalton Trans. 2013, 42, 9533−9546.
(30) Briand, G. G.; Smith, A. D.; Schatte, G.; Rossini, A. J.; Schurko,
R. W. Inorg. Chem. 2007, 46, 8625−8637.
(31) Rodriguez-Fortea, A.; Alemany, P.; Ziegler, T. J. Phys. Chem. A
1999, 103, 8288−8294.
(32) Lodewyk, M. W.; Siebert, M. R.; Tantillo, D. J. Chem. Rev. 2012,
112, 1839−1862.
(33) Buhl, M. Chem. Phys. Lett. 1997, 267, 251−257.
(34) Carrington, A.; McLachlan, A. D. Introduction to Magnetic
Resonance: With Applications to Chemistry and Chemical Physics; Harper
& Row: New York, 1967.
(35) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112−122.
(36) Guerra, C. F.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor.
Chem. Acc. 1998, 99, 391−403.
(37) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Guerra, C. F.;
Van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem.
2001, 22, 931−967.
(38) In ADF2013; SCM, Theoretical Chemistry, Vrije Universiteit,
Amsterdam, The Netherlands; http://www.scm.com.
(39) van Lenthe, E.; Ehlers, A.; Baerends, E. J. J. Chem. Phys. 1999,
110, 8943−8953.
(40) Becke, A. D. Phys. Rev. A 1988, 38, 3098−3100.
(41) Perdew, J. P. Phys. Rev. B 1986, 34, 7406−7406.
(42) Perdew, J. P. Phys. Rev. B 1986, 33, 8822−8824.
(43) Cheeseman, J. R.; Trucks, G. W.; Keith, T. A.; Frisch, M. J. J.
Chem. Phys. 1996, 104, 5497−5509.
(44) Schreckenbach, G.; Ziegler, T. J. Phys. Chem. 1995, 99, 606−
611.
G
DOI: 10.1021/om501223a
Organometallics XXXX, XXX, XXX−XXX