Carbon dioxide activation by "Non-nucleophilic" lead alkoxides

Article abstract of DOI:10.1021/ic900134f

A series of terminal lead alkoxides have been synthesized utilizing the bulky β-diketiminate ligand [{N(2,6-ⁱPr₂C ₆H₃)-C(Me)}₂CH]^- (BDI). The nucleophilicities of these alkoxides have been examined, and unexpected trends were observed. For instance, (BDI)PbOR reacts with methyl iodide only under forcing conditions yet reacts readily, but reversibly, with carbon dioxide. The degree of reversibility is strongly dependent upon minor changes in the R group. For instance, when R = isopropyl, the reversibility is only observed when the resulting alkyl carbonate is treated with other heterocumulenes; however, when R = tert-butyl, the reversibility is apparent upon any application of reduced pressure to the corresponding alkyl carbonate. The differences in the reversibility of carbon dioxide insertion are attributed to the ground-state energy differences of lead alkoxides. The mechanism of carbon dioxide insertion is discussed.

Full text of DOI:10.1021/ic900134f

Inorg. Chem. 2009, 48, 8971–8976 8971
DOI: 10.1021/ic900134f
Carbon Dioxide Activation by “Non-nucleophilic” Lead Alkoxides
Eric C. Y. Tam, Nick C. Johnstone, Lorenzo Ferro, Peter B. Hitchcock, and J. Robin Fulton*
Department of Chemistry, University of Sussex, Falmer, Brighton BN1 9QJ, U.K.
Received January 21, 2009
A series of terminal lead alkoxides have been synthesized utilizing the bulky β-diketiminate ligand [{N(2,6-iPr2C6H3)-
C(Me)}2CH]- (BDI). The nucleophilicities of these alkoxides have been examined, and unexpected trends were
observed. For instance, (BDI)PbOR reacts with methyl iodide only under forcing conditions yet reacts readily, but
reversibly, with carbon dioxide. The degree of reversibility is strongly dependent upon minor changes in the R group.
For instance, when R = isopropyl, the reversibility is only observed when the resulting alkyl carbonate is treated with
other heterocumulenes; however, when R = tert-butyl, the reversibility is apparent upon any application of reduced
pressure to the corresponding alkyl carbonate. The differences in the reversibility of carbon dioxide insertion are
attributed to the ground-state energy differences of lead alkoxides. The mechanism of carbon dioxide insertion is
discussed.
Introduction
understand the degree of polarization of the bond. Herein,
the results of such studies are reported, and our findings
suggest that, although nucleophilic behavior with methyl
iodide is not observed, lead alkoxides readily insert carbon
dioxide into the Pb-O bond.
We have recently synthesized a series of monomeric
divalent lead halides utilizing the bulky β-diketiminate anion
[{N(2,6-iPr2C6H3)C(Me)}2CH]- (BDI) tostabilize the result-
ing three-coordinate lead complexes.10 The chloride complex
(BDI)PbCl (1) was used to synthesize monomeric lead aryl-
oxide complexes in good yield.11 Because metal aryloxide
complexes are generally not as reactive as their alkoxide
counterparts, for our reactivity studies, we turned our atten-
tion toward the synthesis of monomeric, terminal lead alk-
oxide complexes.
In contrast to the well-documented reactivity of transition-
metal alkoxides,1-3 the chemistry of lead alkoxides has
largely been ignored outside of gas-phase and theoretical
studies.4-6 Divalent lead has an aqueous acidity of 7.2, much
smaller than that predicted based upon electrostatic para-
meters.7 This acidity has been attributed to a more covalent
Pb-O bond, in contrast to the highly polarized transition
metal-oxygen bonds. In addition, lead’s aqueous acidity has
been used to justify its enhanced ability to cleave RNA. At
biologically relevant pHs, there will be a sufficient amount of
divalent lead hydroxide present to act as a base, deprotonat-
ing the 20-hydroxyl proton of the RNA backbone, and a
nucleophile in the cleavage of the resultant cyclic phosphate
intermediate.8,9 This implies that the Pb-O bond has some
degree of polarity to be able to act as both a base and a
nucleophile. However, no complementary chemical studies
have been performed to back this hypothesis. As such, we set
out to investigate the nature of the Pb-O bond in order to
Results and Discussion
The treatment of a toluene solution of chloride 1 with
KOiPr or KOtBu affords lead alkoxides (BDI)PbOiPr (2) and
(BDI)PbOtBu (3), respectively (eq 1). Although these lead
alkoxide complexes form a stable solid, they will slowly
decompose in solution at ambient temperatures. The X-ray
crystal structures were determined for both 2 and 3, showing
the expected pyramidal geometry of the ligands around the
lead center (Figure 1).10,11 Selected bond lengths and angles
for alkoxides 2 and 3 are listed in Table 1, and data collection
parameters are given in Table 2. Both alkyl groups of
complexes 2 and 3 lie away from the BDI-Pb core; this is
*To whom correspondence should be addressed. E-mail: j.r.fulton@
sussex.ac.uk.
(1) Bryndza, H. E.; Fong, L. K.; Paciello, R. A.; Tam, W.; Bercaw, J. E. J.
Am. Chem. Soc. 1987, 109, 1444.
(2) Bryndza, H. E.; Tam, W. Chem. Rev. 1988, 88, 1163.
(3) Fulton, J. R.; Holland, A. W.; Fox, D. J.; Bergman, R. G. Acc. Chem.
Res. 2002, 35, 44.
(4) Akibo-Betts, G.; Barran, P. E.; Puskar, L.; Duncombe, B.; Cox, H.;
Stace, A. J. J. Am. Chem. Soc. 2002, 124, 9257.
(5) Cox, H.; Stace, A. J. J. Am. Chem. Soc. 2004, 126, 3939.
(6) Stace, A. J. J. Phys. Chem. A 2002, 106, 7993.
(7) Burgess, J. Metal Ions in Solution; Ellis Horwood Ltd.: Chichester, U.K.,
1978.
(8) Barciszewska, M. Z.; Szymanski, M.; Wyszko, E.; Pas, J.; Rychlewski,
L.; Barciszewski, J. Mutat. Res. 2005, 589, 103.
(9) Winter, D.; Polacek, N.; Halama, E.; Streicher, B.; Barta, A. Nucleic
Acids Res. 1997, 25, 1817.
(10) Chen, M.; Fulton, J. R.; Hitchcock, P. B.; Johnstone, N. C.; Lappert,
M. F.; Protchenko, A. V. Dalton Trans. 2007, 2770.
(11) Fulton, J. R.; Hitchcock, P. B.; Johnstone, N. C.; Tam, E. C. Y.
Dalton Trans. 2007, 3360.
r
2009 American Chemical Society
Published on Web 08/21/2009
pubs.acs.org/IC

8972 Inorganic Chemistry, Vol. 48, No. 18, 2009
Tam et al.
Figure 1. ORTEP diagram of lead isopropoxide 2 (left) and lead tert-
butoxide 3 (right), with H atoms omitted and BDI C atoms minimized for
clarity. The ellipsoid probability is shown at 30%.
Table 1. Selected Bond Lengths and Angles for Compounds 2 and 3
LPbOiPr (2)
LPbOtBu (3)
Pb-O
2.135(3)
2.307(3)
2.311(3)
1.413(5)
80.56(9)
94.94(10)
93.49(10)
118.0(2)
268.99
2.126(3)
2.317(3)
2.299(3)
1.415(4)
81.04(10)
92.74(10)
92.26(10)
121.4(2)
266.04
Figure 2. ORTEP diagram of lead carbonate 5 showing both compo-
nents of the unit cell (major = black; minor = gray). H atoms are omitted
and BDI C atoms minimized for clarity. Atoms with a prime are at
equivalent positions (-x, -y, -z).
Pb-N1
Pb-N2
O-C30
N1-Pb-N2
N1-Pb-O
N2-Pb-O
Pb-O-C30
sum of the angles around Pb
DOPa (%)
13C NMR spectral resonance at 160.9 ppm. Crystals suitable
for an X-ray diffraction study were grown at -30 °C in
toluene. Two different binding modes for lead carbonate were
observed in the solid state (Figure 2; see Table 3 for selected
bond lengths and angles). In the major component, the
carbonate is bound in an η1 fashion and there is weak, long-
distance interaction between O2 and Pb0. In the minor com-
ponent, the carbonate is bound in an η2 fashion and there is a
shorter distance between O2a and Pb0, indicative of a stronger
interaction between the two lead carbonate molecules.
101
104
a Degree of pyramidalization (DOP) = [360 - (sum of the angles)/
0.9].20
in contrast to the isostructural tin system, in which the alkyl
group lies below the plane consisting of N1-Pb-N2.12
Two different types of lead alkoxide reactivities were
investigated: basicity and nucleophilicity. The treatment of
alkoxides 2 or 3 with 2,4-di-tert-butylphenol results in alcohol
exchange reactions to form the known (BDI)Pb-OAr
(Ar = 2,4-tBu2C6H3) complex 4 with elimination of the
corresponding aliphatic alcohols. This reactivity is similar to
that observed in late-transition-metal alkoxide systems. How-
ever, the presence of free alcohol results in decomposition of
aryloxide 4 to an insoluble white precipitate and protonated
BDI. Potentially because of their higher pKa, carbo acids are
not reactive toward 2 and 3; the treatment of isopropoxide 2
with fluorene did not yield the fluorenide anion, and the
addition of 1,4-cyclohexadiene did not result in dimerization
to an equilibrium mixture of 1,4- and 1,2-cyclohexadiene.
Both lead alkoxides 2 and 3 display seemingly contradictory
reactivities toward electrophiles. For instance, neither reacts
with benzyl bromide. When methyl iodide was added to either
2 or 3, the formation of lead iodide was only observed after 2
days at 60 °C.10 In sharp contrast, both alkoxides react readily
with CO2 (eq 2). The treatment of isopropoxide 2 with 1 equiv
of CO2 results in the clean and quantitative conversion to lead
alkyl carbonate 5 after 30 min at room temperature. The IR
spectrum shows a stretch at 1695 cm-1 (CCl4), indicative of a
carbonate carbonyl functionality; this is further supported by a
Because insertion of CO2 into transition-metal alkoxides
can be reversible,13 we investigatedwhether the same was true
for carbonate 5. Although the formation of isopropoxide 2 is
not observed when 5 is subjected to reduced pressure, the
addition of 13CO2 to the carbonate does result in 13C
incorporation into 5.
Lead tert-butoxide 3 also reacts readily with CO2 to form
lead carbonate 6 (IR: 1699 cm-1, CCl4). In contrast to the
isopropoxide system, the reaction is markedly reversible: the
application of reduced pressure results in the almost quanti-
tative formation of alkoxide 3, thwarting attempts to char-
acterize 6 in the solid state.
Pronounced differences in the reactivity of isopropoxide 2
and tert-butoxide 3 were observed upon treatment with
phenyl isocyanate. With the former, insertion into the Pb-O
bond to generate lead carbamate 7 is observed after 1 day at
room temperature (eq 3). X-ray crystallography confirmed
the presence of a Pb-N carbamate bond (Figure 3), with the
N-phenyl group lying perpendicular to and below the plane
consisting of N1-Pb-N2. Selected bond lengths and angles
are listed in Table 4. In contrast, with tert-butoxide 3, an
(13) For example, see: Tsuda, T.; Saegusa, T. Inorg. Chem. 1972, 11, 2561.
Simpson, R. D.; Bergman, R. G. Organometallics 1992, 11, 4306. Mandal, S. K.;
Ho, D. M.; Orchin, M. Organometallics 1993, 12, 1714.
(12) Dove, A. P.; Gibson, V. C.; Marshall, E. L.; Rzepa, H. S.; White, A.
J. P.; Williams, D. J. J. Am. Chem. Soc. 2006, 128, 9834.

Article
Inorganic Chemistry, Vol. 48, No. 18, 2009 8973
Table 2. Crystallographic Data for Compounds 2, 3, 5, and 7
LPbOiPr (2)
LPbOtBu (3)
C33H50N2OPb
697.94
LPbOCO2 Pr (5)a
LPb[N(Ph)C(O)OiPr (7)b
i
chemical formula
fw
temperature (K)
C32H48N2OPb
683.91
172(2)
C33H48N2O3Pb 0.5(C7H8)
773.99
C44H65N3O2Pb
875.18
173(2)
3
173(2)
173(2)
0.710 73
˚
wavelength (A)
0.710 73
0.20 ꢀ 0.20 ꢀ 0.15
triclinic
0.710 70
0.25 ꢀ 0.20 ꢀ 0.10
monoclinic
P21/n (No. 14)
13.3743(1)
16.8363(2)
15.2434(2)
90
108.168(1)
90
3261.29(6)
4
1.42
0.710 73
0.15 ꢀ 0.15 ꢀ 0.1
cryst size (mm3)
cryst syst
0.15 ꢀ 0.10 ꢀ 0.05
triclinic
triclinic
space group
˚
˚
b (A)
˚
c (A)
P1 (No. 2)
8.6979(2)
12.1195(3)
15.1146(3)
92.885(1)
98.639(1)
97.444(1)
1559.71(6)
2
P1 (No. 2)
11.9673(4)
13.2339(5)
13.5828(3)
99.649(2)
106.996(2)
113.105(1)
1792.09(10)
2
P1 (No. 2)
11.5616(3)
11.6261(3)
18.2619(4)
102.146(1)
95.723(2)
112.814(1)
2166.78(9)
2
a (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
pc (Mg m-3
)
1.46
5.43
1.43
4.74
1.34
3.93
abs coeff (mm-1
)
5.20
θ range for data collection (deg) 3.40-26.02
3.43-25.82
41 847/6251/0.061
5497
6251/30/334
1.051
3.47-26.02
26 307/7017/0.066
5851
7017/34/380
0.962
3.44-26.01
23 224/8472/ 0.054
7159
8472/7/438
1.027
R1 = 0.041, wR2 = 0.083
measd/indep reflns/R(int)
reflns with I >2σ(I)
data/restraints/param
GOF on F2
final R indices [I>2σ(I)]
R indices (all data)
23 519/6112/0.044
5783
6112/0/337
0.826
R1 = 0.023, wR2 = 0.059 R1 = 0.027, wR2 = 0.064 R1 = 0.047, wR2 = 0.107
R1 = 0.026, wR2 = 0.061 R1 = 0.033, wR2 = 0.067 R1 = 0.0631, wR2 = 0.11571 R1 = 0.057, wR2 = 0.090
1.37 and -1.40 (near Pb) 2.45 and -0.72 (close to Pb) 0.96 and 1.40
3
˚
largst diff peak and hole (e A ) 0.78 and -1.37
a The OC(O)OiPr group is disordered unequally over two arrangements with only some of the atoms resolved. The positions for the major component could
be located, and for the minor component, approximate starting positions were estimated. The two orientations were then restrained to have similar geometry by
use of the same instruction. This refinement converged successfully. There is a molecule of toluene solvate disordered about an inversion center for which the H
atoms were omitted. All disordered atoms were left isotropic. b The poorly defined pentane solvate was included with isotropic C atoms and restrained geometry.
intractable reaction mixture is found. Even though the reaction
between isopropoxide 2 and CO2 is much faster than the
reaction between 2 and phenyl isocyanate, there is a thermo-
dynamic preference for the latter; the treatment of alkyl
carbonate 5 with phenyl isocyanate results in an exclusive
conversion to carbamate 7. Neither alkoxide reacts with
dicyclohexylcarbodiimide, even at elevated temperatures. Both
alkoxides 2 and 3, and their corresponding alkyl carbonates 5
and 6, react with CS2 to give intractable reaction mixtures.
attributed to the ground-state energy differences of the
alkoxides. The stability of transition-metal alkoxides corre-
lates to the pKa of the corresponding alcohol; that is, the
higher the pKa, the more stable the metal alkoxide.1 If a
similar trend is assumed for lead alkoxides, then the lead
isopropoxide 2 is less stable than the lead tert-butoxide 3 (pKa
of isopropyl alcohol is 30.2, whereas the pKa of tert-butanol is
2 pKa units higher).14 The alkyl group will have a much
smaller effect on the pKa of the corresponding alkyl carbo-
nates. An attempt was made to determine the rate of 13CO2
exchange with each of the carbonates using 13C NMR
spectroscopy. Although exchange was observed, we were
unable to obtain reproducible rate data. As such, we inves-
tigated the relative ground-state differences between the
reactants and products using the B3LYP density functional
theory and LanL2DZ pseudopotentials (and basis set) im-
plemented in Gaussian 03.15 The ΔH° value for the insertion
of CO2 into lead isopropoxide 2 was slightly greater than
the analogous reaction for both the lead sec-butoxide 8 and
To further understand the differences observed in the
reactivity between isopropoxide 2 and tert-butoxide 3, we
synthesized the sec-butoxide complex 8. The treatment of sec-
butoxide 8 with CO2 resulted in the expected formation of the
corresponding carbonate 9 (IR: 1645 cm-1, CCl4). The
degree of reversibility of this reaction is intermediate between
the isopropoxide and tert-butoxide systems. For instance,
when a vacuum is applied for 10 min to an NMR-scale
solution of tert-butoxide carbonate 6, almost complete rever-
sion to alkoxide 3 is observed; however, when a similar
procedure is repeated for a solution of sec-butoxide carbo-
nate 9, only 20% reversion to alkoxide 8 is observed.
Unfortunately, as in the tert-butoxide case, this reversibility
has prevented solid-state characterization of sec-butoxide
carbonate 9. Interestingly, solutions of both the isopropoxide
carbonate 4 and sec-butoxide carbonate 9 are thermally
stable to 60 °C, whereas the tert-butoxide carbonate 5
decomposes after standing at room temperature overnight.
The noticeable difference in reactivity with respect to
deinsertion of CO2 from carbonates 5, 6, and 9 can be
(14) Olmstead, W. M.; Margolin, Z.; Bordwell, F. G. J. Org. Chem. 1980,
45, 3295.
(15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A.
D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cuiq, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Challacombe, M.; Pople, J. A. Gaussian 03, revision
C.02; Gaussian Inc.: Wallingford, CT, 2004.

8974 Inorganic Chemistry, Vol. 48, No. 18, 2009
Tam et al.
ether and pyrazolborate-zinc iodide complex, yet only
reacts with CO2 under forced conditions and not at all with
phenyl isocyanate. This implies that the lead alkoxides lack
nucleophilic characteristics. Space-filling models of alkoxide
2 reveal that the O atom lies inside a pocket surrounded by
the BDI isopropyl groups and the alkyl group on the O atom.
Thus, we cannot conclusively state that our lead alkoxides are
non-nucleophilic because of potential steric arguments.
However, we can conclude that a nucleophilic O atom is
not available for reactivity.
The mechanism of CO2 insertion into transition-metal
alkoxides is generally thought to proceed via a concerted
process in which the new carbon-oxygen and metal-
carbonate bonds form at the same time as the cleavage of
the metal-alkoxide bond.17 The symmetry of this transi-
tion state must vary depending upon the nucleophilicity of
the O atom and the propensity for CO2 to bind to the metal
center. In our examples, the lack of an available nucleophilic
O atom implies that coordination of CO2 to the metal center
is key, and it must be this coordination that induces the
alkoxide to react with CO2. This can occur either by changing
the steric environment around the alkoxide or by shifting the
electron density such that the alkoxide ligand becomes more
nucleophilic and CO2 becomes more electrophilic. However,
we have been unable to observe the binding of additional
ligands to the metal center, nor can we gain evidence for
binding using computational analysis. The addition of extra-
neous ligands such as CH3CN, THF, TMEDA, and N-
heterocyclic carbenes as well as softer ligands such as tri-
methyl- and triphenylphosphine did not result in either
measurable inhibition of CO2 insertion or strong evidence
of coordination. Indeed, crystals of the alkoxides 2, 3, or 8
grown in a 50:50 mixture of CH3CN and hexane did not
result in coordinated CH3CN in the solid state.
Figure 3. ORTEP diagram of lead carbamate 7 with H atoms omitted
and BDI C atoms minimized for clarity. The ellipsoid probability isshown
at 30%.
Table 3. Selected Bond Lengths and Angles for Compound 5
Pb-O1
2.217(10)
2.292(5)
2.291(5)
N1-Pb-N2
83.23(18)
83.5(3)
82.2(3)
91.7(3)
90.2(3)
145.7(4)
130.5(3)
96.3(3)
137.8(4)
113.5(8)
127.9(12)
107.9(12)
124.1(13)
106.2(9)
148.4(11)
125.1(13)
114.9(16)
Pb-N1
Pb-N2
Pb-O1A
Pb-O2A0
N1-Pb-O1
N2-Pb-O1
2.399(13)
2.777(13)
3.7842(5)
1.309(16)
1.188(17)
1.372(14)
1.451(15)
1.314(18)
1.18(2)
N1-Pb-O1A
N2-Pb-O1A
O1-Pb-O2A0
N1-Pb-O2A0
N2-Pb-O2A0
O1A-Pb-O2A0
Pb-O1-C30
O1-C30-O2
O1-C30-O3
O2-C30-O3
Pb-O1A-C30A
Pb0-O2A-C30A
O1A-C30-O2A
O1A-C30-O3A
Pb Pb0
3 3 3
O1-C30
O2-C30
O3-C30
O3-C31
O1A-C30A
O2A-C30A
O3A-C30A
O3A-C31A
1.355(15)
1.453(16)
sum of the angles
around Pb
266.04
104
DOPa (%)
O2A-C30-O3A
119.8(16)
a Degree of pyramidalization (DOP) = [360 - (sum of the angles)/
0.9].20
The differing reactivity between our lead alkoxides and
transition-metal alkoxides is intriguing both from a funda-
mental viewpoint and from implications in the mechanism of
lead-mediated RNA cleavage, and we are testing whether our
apparent non-nucleophilic behavior can be reproduced with
less sterically hindered lead alkoxides. In addition, we are
currently investigating the mechanism of CO2 insertion using
kinetic studies on the slower isostructural tin system.12
Table 4. Selected Bond Lengths and Angles for Compound 7
Pb-N1
2.335(4)
2.321(4)
2.340(4)
1.420(6)
1.340(6)
1.236(6)
1.348(6)
1.466(7)
N1-Pb-N2
82.47(14)
95.94(14)
96.84(14)
132.5(3)
103.2(3)
121.8(5)
116.4(5)
121.7(5)
116.9(4)
275.25
Pb-N2
N1-Pb-N3
Pb-N3
N2-Pb-N3
N3-C30
N3-C36
C36-O1
C36-O2
O2-C37
Pb-N3-C30
Pb-N3-C36
N3-C36-O1
N3-C36-O2
O1-C36-O2
C36-O2-C37
sum of the angles around Pb
DOPa
Experimental Section
All manipulationswerecarriedout inan atmosphereofdry
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
94%
˚
a Degree of pyramidalization (DOP) = [360 - (sum of the angles)/
0.9].20
4 A sieves. (BDI)PbCl (1) was prepared according to the
literature.10 Potassium alkoxide salts were prepared by the
slow addition of the relevant alcohol (dried and distilled) to a
suspension of potassium hydride. Phenyl isocyanate and
carbon disulfide were freshly dried and distilled before use.
Carbon dioxide was used as received (Union Carbide,
tert-butoxide 3 cases (-9.4 kcal mol-1 vs -8.9 and -8.4 kcal
mol-1, respectively). Although these are gas-phase calcula-
tions, it is reassuring that the trends observed are similar to
our experimental results.
1
99.999%), and 13CO2 was 99 atom %. The H and 13C
NMR spectra were recorded on a Bruker DPX 300 MHz
spectrometer, a Varian 400 MHz spectrometer, or a Varian
500 MHz spectrometer. The 1H and 13C NMR spectroscopy
chemical shifts are given relative to residual solvent peaks,
The slow to nonexistent reactivity of our lead alkoxides
with aliphatic electrophiles is in sharp contrast to the reac-
tivity observed with transition-metal alkoxides. The most
striking difference is with Vahrenkamp’s pyrazolborate-zinc
methoxide complex (TpPh,MeZn-OMe),16 which reacts read-
ily with methyl iodide to generate the corresponding dimethyl
(17) For a few examples, see ref 15 as well as the following: Chisholm, M.
H.; Cotton, F. A.; Extine, M. W.; Reichert, W. W. J. Am. Chem. Soc. 1978,
100, 1727. Darensbourg, D. J.; Sanchez, K. M.; Rheingold, A. L. J. Am. Chem.
Soc. 1987, 109, 290. Campora, J.; Matas, I.; Palma, P.; Alvarez, E.; Graiff, C.;
Tiripicchio, A. Organometallics 2007, 26, 3840.
(16) Brombacher, H.; Vahrenkamp, H. Inorg. Chem. 2004, 43, 6042.

Article
Inorganic Chemistry, Vol. 48, No. 18, 2009 8975
and the 207Pb elements were externally referenced to PbMe4.
The data for the X-ray structures were collected at 173 K on a
(CHMe2), 24.7 (NCCH3), 24.5 (CH(CH3)2), 22.8 (CH(CH3)2),
22.7 (CH(CH3)2), 14.3 (CH(CH3)2). 207Pb NMR (400 MHz,
toluene-d8, 303 K): δ 808.7. IR (Nujol, ν/cm-1): 3056 (s), 1611
(s), 1583 (s), 1551 (s), 1519 (s), 1317 (s), 1294 (s), 1171 (s), 1108
(s), 1055 (s), 1020 (s). IR (CCl4, ν/cm-1): 3060 (s), 2963(s), 2928
(s), 2871 (s), 2335 (br), 1699 (s), 1463 (s), 1438 (s), 1387 (br), 1319
(s), 1292 (s), 1173 (s), 1115 (s). Anal. Calcd for C33H48N2O3Pb:
C, 54.45; H, 6.65; N, 3.85. Found: C, 54.49; H, 6.71; N, 3.75.
[CH{(CH3)2CN-2,6-iPr2C6H3}2Pb(CO2)OtBu] (6). 3 (0.050 g,
0.072 mmol) was dissolved in toluene-d8 in an NMR tube
sealed with a Young’s tap. The gas inside the NMR tube was
evacuated. CO2 (0.0047 mg, 0.109 mmol) was then added. A
yellow solution mixture was observed, and the reaction mixture
was kept at room temperature for 24 h and was monitored by 1H
NMR spectroscopy. 1H NMR (400 MHz, toluene-d8, 203 K): δ
7.15 (s, 2H, ArH), 7.07 (d, 2H, J = 4 Hz, ArH), 6.95 (d, 2H, J =
8 Hz, ArH), 4.79 (s, 1H, middle CH), 3.64 (m, 2H, CHMe2), 2.94
(m, 2H, CHMe2), 1.62 (s, 6H, CCH3), 1.58 (br, 6H, J=4 Hz,
C(CH3)2), 1.50 (d, 6H, J = 4 Hz, C(CH3)2), 1.17 (d, 6H, J =
4 Hz, C(CH3)2), 1.08 (s, 9H, C(CH3)3). 13C{1H} NMR
(400 MHz, toluene-d8, 203 K): δ 163.5 (NCMe), 160.3 (OCO),
144.2 (ipso-C), 142.3 (o-C), 141.6 (o-C), 103.5 (middle CH), 76.6
(OCHMe2), 28.5 (OCH(CH3)2), 28.1 (NCCH3), 27.6 (CHMe2),
27.4 (CHMe2), 26.8 (CHMe2), 26.5 (CHMe2), 25.0 (C(CH3)2),
24.7 (C(CH3)2), 24.3 (C(CH3)2), 24.1 (C(CH3)2). 207Pb NMR
(400 MHz, toluene-d8, 303 K): δ 817.4. IR (CCl4, ν/cm-1): 3060
(s), 2965 (s), 2928 (s), 2869 (s), 1699 (s), 1463 (s), 1438 (s), 1389
(b), 1366 (b), 1172 (s), 1102 (s).
˚
NoniusKappaCCD diffractometer, k(Mo KR) = 0.710 73A
and refined using the SHELXL-97 software package.18
[CH{(CH3)2CN-2,6-iPr2C6H3}2PbOiPr] (2).
1 (1.08 g,
1.64 mmol) was added to a stirred suspension of KOiPr
(0.127 g, 1.64 mmol) in 10 mL of toluene at room temperature.
The reaction vessel was wrapped in foil, and the mixture was
stirred overnight. The mixture was filtered through a pad of
Celite, and the solvent was removed in a vacuum. The resulting
brown oil was dissolved in a minimum of pentane and 2 crystal-
1
lized upon standing at -30 °C overnight (0.686 g, 62%). H
NMR (C6D6, 293 K): δ 7.26 (d, 2H, J = 7.5 Hz, m-H), 7.20
(t, 2H, J=7.5 Hz, p-H), 7.09 (d, 2H, J=7.5 Hz, m-H), 4.96
(septet, 1H, J = 6.8 Hz, CHMe2), 4.71 (s, 1H, middle CH), 3.95
(septet, 2H, J = 6.8 Hz, CHMe2), 3.18 (septet, 2H, J = 6.8 Hz,
CHMe2), 1.69 (s, 6H, NCMe), 1.57 (d, 6H, J = 6.7 Hz, CHMe2),
1.29 (d, 6H, J = 6.8 Hz, CHMe2), 1.20 (d, 6H, J = 6.9 Hz,
CHMe2), 1.17 (d, 6H, J= 6.8 Hz, CHMe2), 1.07 (d, 6H, J =
5.9 Hz, CHMe2). 13C{1H} NMR (C6D6, 293 K): δ 163.9
(NCMe), 145.1 and 143.3 (ipso- and o-C of Ar), 126.4, 124.8,
and 123.9 (m- and p-CH of Ar), 100.2 (middle CH), 66.0
(OCHMe2), 30.6 (NCMe), 28.2 (OCHMe2), 26.3, 25.7, 24.9,
and 24.6 (CHMe2). IR (Nujol, ν/cm-1): 1553 (s), 1512 (s), 1317
(s), 1265 (s), 1172 (s), 1117 (s), 1012 (s), 960 (s),790 (s), 752 (s),
722 (s). Anal. Calcd for C32H48ON2Pb: C, 56.20; H, 7.07; N,
4.10. Found: C, 56.30; H, 7.17; N, 4.13.
[CH{(CH3)2CN-2,6-iPr2C6H3}2PbOtBu] (3). 1 (0.908 g, 1.38
mmol) was added to a stirred suspension of KOtBu (0.154 g, 1.38
mmol) in toluene (15 mL) at room temperature, and the mixture
was stirred overnight. The reaction mixture was filtered through
Celite. The solvent was removed under vacuum, producing an
orange solid as the crude product. The crude product was
washed with pentane and recrystallized from toluene overnight,
[CH{(CH3)2CN-2,6-iPr2C6H3}2Pb(N(Ph)CO)OiPr]
(7).19
Phenyl isocyanate (0.032 mL, 0.294 mmol) was added to a
solution of 2 (0.196 g, 0.287 mmol) intoluene(3mL). Thereaction
vessel was wrapped in foil andstirred at roomtemperature for3 h.
The volatiles were removed, and the resulting orange residue
was dissolved in a minimum of pentane. 7 crystallized upon
standing at -30 °C overnight (0.100 g, 44%). 1H NMR
(500 MHz, toluene-d8, 303 K): δ 7.59 (br, 2H, ArH), 7.27
(t, 2H, J=15 Hz, ArH), 6.85 (t, 1H, J=15 Hz, ArH), 5.03 (s, 1H,
middle CH), 4.91 (br, 1H, OCHMe2), 3.13 (br, 4H, CHMe2),
1.67 (s, 6H, CCH3), 1.28 (d, 1H, J = 5 Hz, C(CH3)2), 1.25 (d, 1H,
J = 5 Hz, C(CH3)2), 1.15 (2H, br d, J = 10 Hz, C(CH3)2),
0.98 (6H, br, C(CH3)2), 0.88 (d, 2H, J = 10 Hz, C(CH3)2),
0.87 (d, 2H, J=5 Hz, OC(CH3)2). 13C{1H} NMR (500 MHz,
toluene-d8, 303 K): δ 165.0, 143.3, 129.2, 128.3, 127.9, 126.8,
126.0, 121.7, 103.2, 67.2, 34.6, 28.2, 25.5, 25.2, 24.6, 22.8, 22.4,
14.3. 13C{1H} NMR (400 MHz, toluene-d8, 198 K): δ 164.7,
164.0, 163.7, 161.4, 158.9, 147.7, 146.6, 144.6, 144.1, 142.9,
142.4, 141.7, 141.4, 129.7, 127.3, 126.5, 126.0, 125.4, 68.0,
67.1, 66.8, 66.4, 65.8, 65.3, 34.5, 27.2, 26.7, 25.8, 25.0, 24.9,
24.4, 24.3, 24.2, 23.1, 22.3, 22.2, 21.9, 14.6. IR (Nujol, ν/cm-1):
1745 (s), 1688 (s), 1578 (s), 1546 (s), 1515 (s), 1483 (s), 1315 (s),
1231 (s), 1168 (s), 1109 (s), 1054 (s), 1035 (s), 1021 (s). Anal.
Calcd for C39H53N3O2Pb: C, 58.31; H, 6.66; N, 5.23. Found: C,
58.39; H, 6.54; N, 5.24.
[CH{(CH3)2CN-2,6-iPr2C6H3}2PbOsBu]. A suspension of
KOsBu (0.085 g, 757 mmol in 3 mL of toluene) was added
dropwise to a stirred solution of 1 (0.500 g, 757 mmol) in toluene
(10 mL) at room temperature. The reaction mixture was stirred
overnight. The deep-yellow solution was filtered through Celite,
and the solvent was removed under vacuum. The resulting
yellow solid was dissolved in the minimum amount of pentane
(∼7 mL) and stored at -30 °C overnight, yielding yellow crystals
of (BDI)PbOsBu (0.441 g, 83%). 1H NMR (toluene-d8, 303 K):
δ 7.17 (d, 2H, J = 7.5 Hz, m-H), 7.03 (t, 2H, J = 16.9 Hz, p-H),
1
yielding yellow light-sensitive crystals of 3 (0.720 g, 75%). H
NMR (300 MHz, C6D6, 293 K): δ 7.26 (dd, 2H, J=7.5 Hz,
ArH), 7.10 (d, 2H, J = 7.5 Hz, ArH), 7.04 (dd, 2H, J=7.5 Hz,
ArH), 4.57 (s, 1H, middle CH), 3.83 (septet, 2H, J=6.8 Hz,
CHMe2), 3.12 (septet, 2H, J = 6.8 Hz, CHMe2), 1.65 (s, 6H,
CCH3), 1.63 (d, 6H, J = 6.8 Hz, C(CH3)2), 1.25 (d, 6H, J =
6.9 Hz, C(CH3)2), 1.19 (d, 6H, J = 6.9 Hz, C(CH3)2), 1.14
(d, 6H, J=6.8 Hz, C(CH3)2), 0.88 (s, 9H, C(CH3)3). 13C{1H}
NMR (300 MHz, C6D6, 293 K): 164.0 (NCMe), 145.1 (ipso-C),
142.8 (o-C), 141.5 (o-C), 126.0 (p-C), 123.7 (m-C), 123.6 (m-C),
98.0 (middle CH), 69.1 (OCMe3), 36.7 (OC(CH3)3), 28.4
(CHMe2), 28.0 (CHMe2), 26.1 (NCCH3), 25.2 (C(CH3)2), 24.8
(C(CH3)2), 24.6 (C(CH3)2), 23.9 (C(CH3)2). IR (Nujol, ν/cm-1):
1556 (s), 1511 (s), 1318 (s), 1262 (s), 1227 (s), 1015 (s), 941 (s), 838
(s), 791 (s), 751 (s). Anal. Calcd for C33H50N2OPb: C, 56.79; H,
7.22; N, 4.01. Found: C, 56.70; H, 7.15; N, 3.97.
[CH{(CH3)2CN-2,6-iPr2C6H3}2Pb(CO2)OiPr] (5). 2 was dis-
solved in toluene (2 mL) and loaded into an ampule wrapped in
aluminum foil. The reaction vessel was connected to a Schlenk
line and a cylinder of high-purity CO2. The vessel was sub-
merged in a dry ice/acetone bath, and after three pump/refill
cycles, CO2 was introduced at a pressure of 1.5 bar. Thawing
followed by removal of the solvent gave 5 as a yellow solid in
quantitative yield. 1H NMR (C6D6, 300 K): δ 7.19 (s, 6H, ArH),
4.97 (septet, 1H, J = 6.3 Hz, OCHMe2), 4.86 (s, 1H, middle
CH), 3.39-3.32 (br multiplet, 4H), 3.02 (septet, 2H, J=6.9 Hz,
CHMe2), 1.69 (s, 6H, NCMe), 1.30 (d, 12H, J = 6.3 Hz,
CHMe2), 1.20 (d, 12H, J=6.3 Hz, CHMe2), 1.24 (d, 6H, J=
6.8 Hz, OCHMe2). 13C{1H} NMR (500 MHz, toluene-d8, 303
K): δ 164.1 (NCMe), 160.9 (OCdO), 142.8 (ipso-C), 129.2 (o-C),
128.3 (o-C), 127.0 (p-C), 125.4 (m-C), 124.5 (m-C), 103.6 (middle
CH), 68.7 (OC(CH3)2), 34.6 (OC(CH3)2), 28.3 (CHMe2), 26.0
(19) The solution-phase chemistry of this compound is complex and is
undergoing further studies to understand the variable-temperature NMR
spectroscopic behavior.
(18) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal
Structures; University of Gottingen: Gottingen, Germany, 1997.
(20) Maksic, Z. B.; Kovacevic, B. J. Chem. Soc., Perkin Trans. 2 1999,
2623.
::
::

8976 Inorganic Chemistry, Vol. 48, No. 18, 2009
Tam et al.
7.02 (d, 2H, J=18.4 Hz, m-H), 4.60 (s, 1H, middle CH), 4.56
(m, 1H, OCH(CH3)CH2CH3), 3.79 (hept, 1H, J = 6.9 Hz,
CHMe2), 3.77 (hept, 1H, J = 7.1 Hz, CHMe2), 3.10 (septet,
2H, J = 6.8 Hz, CHMe2), 1.64 (s, 3H, NCMe), 1.63 (s, 3H,
NCMe), 1.47 (dd, 6H, J1 = 6.5 Hz, J2 = 4.8 Hz, CHMe2), 1.21
(dd, 6H, J1=6.8 Hz, J2=1.7 Hz, CHMe2), 1.15 (d, 6H, J =
6.8 Hz, CHMe2), 1.12 (d, 6H, J = 6.8 Hz, CHMe2), 0.78 (d, 3H,
J = 6.0 Hz, OCH(CH3)CH2CH3), 0.48 (t, 3H, J = 7.4 Hz,
OCH(CH3)CH2CH3). 13C{1H} NMR (C6D6, 303 K): δ 163.4
(NCMe), 144.7 (ipso-C), 142.8 and 141.6 (o-C), 125.8 (p-C),
123.4 and 123.3 (m-C), 99.1 (middle CH), 70.8 (OC(CH3)CH2-
CH3), 36.3 (OC(CH3)CH2CH3), 27.9 (NCMe), 27.8 and 27.7
(CHMe2), 26.0, 25.0, 24.2, and 23.7 (CHMe2), 24.4 (OC(CH3)-
CH2CH3), 10.5 (OC(CH3)CH2CH3). 207Pb NMR (400 MHz,
C6D6): δ 1542.8. IR (Nujol, ν/cm-1): 3057, 1556 (s), 1516 (s),
1437 (s), 1251, 1171, 1100 (s), 1023 (s), 986, 961, 936, 917, 840,
791 (s), 750. IR (CCl4, ν/cm-1): 3059 (w), 2962, 2927, 2989, 2291
(w), 2004 (w), 1857 (w), 1550 (s), 1463, 1437, 1359, 1320, 1252 (s),
1217 (s), 1172. Anal. Calcd for C33H48N2O3Pb: C, 56.79; H,
7.22; N, 4.01. Found: C, 56.66; H, 7.08; N, 3.89.
3H, J = 6.9 Hz, OCH(CH3)CH2CH3), 1.11 (d, 12H, CHMe2),
0.90 (t, 3H, J = 7.4 Hz, OCH(CH3)CH2CH3). 13C{1H} NMR
(C6D6, 303 K): δ 163.7 (NCMe), 160.7 (OCO2), 142.4 (ipso-C),
127.7 and 127.5 (o-C), 126.5 (p-C), 124.4 and 124.0 (m-C), 103.2
(middle CH), 73.2 (OC(CH3)CH2CH3), 29.3 (OC(CH3)CH2-
CH3), 27.8 (NCMe), 24.2 (CHMe2), 24.1 (CHMe2), 19.7
(OC(CH3)CH2CH3), 9.8 (OC(CH3)CH2CH3). 207Pb NMR
(400 MHz, C6D6): δ 810.3. IR (Nujol, ν/cm-1): 3027, 2336
(w), 1940 (w), 1855 (w), 1800 (w), 1604, 1549 (s), 1260 (s), 1081
(s, br), 804 (s), 694 (s). IR (CCl4, ν/cm-1): 3723, 2693, 3621,
3590, 3063 (w), 2963, 2928, 2870, 2337 (s), 2005 (w), 1857 (w),
1645, 1550 (s), 1459, 1437, 1383, 1364, 1319, 1254 (s), 1217 (s),
1174. Note: traces of 8 were found in isolated solid-state samples
of 9 (even crystallized samples), preventing acceptable elemental
analysis.
Computational Details. All calculations were performed using
the Gaussian 03, revision C.02, suite of programs using the
B3LYP density functional theory and LanL2DZ pseudopoten-
tials (and basis set).15
[CH{(CH3)2CN-2,6-iPr2C6H3}2Pb(CO2)OsPr] (9). 8 (101 mg,
0.145 mmol) was dissolved in toluene (8 mL) in a sealable
ampule. The gas was evacuated, and CO2 was added (0.220
mmol). The reaction mixture was cooled to -80 °C to give a
yellow powder. 1H NMR (C6D6, 303 K): δ 7.09 (s, br, 6H, ArH),
4.78 (s, 1H, middle CH), 4.71 (sext, 1H, J = 6.21 Hz, OCH-
(CH3)CH2CH3), 3.30 (br, 4H, CHMe2), 1.67 (m, 1H, OCH-
(CH3)CH2CH3), 1.62 (s, 6H, NCMe), 1.48 (m, 1H, OCH-
(CH3)CH2CH3), 1.26 (d, 12H, J1 = 6.2 Hz, CHMe2), 1.18 (d,
Acknowledgment. The authors are grateful for financial
support from the EPSRC (Grant EP/E032575/1 to L.F.) and
the University of Sussex.
Supporting Information Available: ORTEP diagram, crystal-
lographic data, and selected bond lengths and angles for com-
plex 8 and crystallographic data in CIF format for complexes 2,
3, 5, 7, and 8. This material is available free of charge via the
Internet at http://pubs.acs.org.