Control of the stereochemical impact of the lone pair in lead(II) tris(pyrazolyl)methane complexes. Improved preparation of Na{B[3,5-(CF3)2C6H3]4}

Article abstract of DOI:10.1021/ic0100121

The reaction of Pb(acac)₂ with 2 equiv of [H(Et₂O)₂]{B[3,5- (CF₃)₂C₆H₃]₄} (HBAr_f) in CH₂Cl₂ followed by addition of 2 equiv of either HC(pz)₃ or HC(3,5-Me₂pz)₃ (pz = pyrazolyl ring) leads to the formation of {Pb[HC(pz)₃]₂}{B[3,5- (CF₃)₂C₆H₃] ₄}₂ (1) and {Pb[HC(3,5-Me₂pz)₃]₂}{B[3,5- (CF₃) ₂C₆H₃]₄}₂ (2), respectively. The cation in 1 has a distorted octahedral structure with a stereochemically active lone pair on lead(II). In contrast, the cation in 2 is trigonally distorted octahedral with the lone pair on the lead(II) clearly stereochemically inactive. The driving force for this cation to have a stereochemically inactive lone pair is that in this geometric arrangement the interligand distances between adjacent 3-position methyl groups are close to 4.0 A, the sum of the van der Waals radii of two methyl groups. To facilitate this chemistry, the synthesis of Na{B[3,5-(CF₃) ₂C₆H₃]₄}, needed to prepare HBAr_f, has been dramatically improved. The main change is to add NaBF₄ to the reaction mixture before forming the Grignard from the reaction of magnesium and 3,5-(CF₃)₂C₆H₃Br. The Grignard reacts with the NaBF₄ as it forms, reducing the danger of explosion and leading to a higher isolated yield of the product. Crystallographic information: 1 is triclinic, P1, a = 13.0133(6) A, b = 17.2210(7) A, c = 24.7634(11) A, α = 71.7300(10)°, β = 82.3630(10)°, γ = 70.5120(10)°, Z = 2; 2 is triclinic, P1, a = 12.756(4) A, b = 13.469(4) A, c = 17.160(5) A, α = 82.454(7)°, β = 89.904(8)°, γ = 72.995(7)°, Z = 1.

Full text of DOI:10.1021/ic0100121

3810
Inorg. Chem. 2001, 40, 3810-3814
Control of the Stereochemical Impact of the Lone Pair in Lead(II) Tris(pyrazolyl)methane
Complexes. Improved Preparation of Na{B[3,5-(CF3)2C6H3]4}
Daniel L. Reger,* Terri D. Wright, Christine A. Little, Jaydeep J. S. Lamba, and
Mark D. Smith
Department of Chemistry and Biochemistry, University of South Carolina,
Columbia, South Carolina 29208
ReceiVed January 3, 2001
The reaction of Pb(acac)2 with 2 equiv of [H(Et2O)2]{B[3,5-(CF3)2C6H3]4} (HBArf) in CH2Cl2 followed by addition
of 2 equiv of either HC(pz)3 or HC(3,5-Me2pz)3 (pz ) pyrazolyl ring) leads to the formation of {Pb[HC(pz)3]2}-
{B[3,5-(CF3)2C6H3]4}2 (1) and {Pb[HC(3,5-Me2pz)3]2}{B[3,5-(CF3)2C6H3]4}2 (2), respectively. The cation in 1
has a distorted octahedral structure with a stereochemically active lone pair on lead(II). In contrast, the cation in
2 is trigonally distorted octahedral with the lone pair on the lead(II) clearly stereochemically inactive. The driving
force for this cation to have a stereochemically inactive lone pair is that in this geometric arrangement the interligand
distances between adjacent 3-position methyl groups are close to 4.0 Å, the sum of the van der Waals radii of
two methyl groups. To facilitate this chemistry, the synthesis of Na{B[3,5-(CF3)2C6H3]4}, needed to prepare
HBArf, has been dramatically improved. The main change is to add NaBF4 to the reaction mixture before forming
the Grignard from the reaction of magnesium and 3,5-(CF3)2C6H3Br. The Grignard reacts with the NaBF4 as it
forms, reducing the danger of explosion and leading to a higher isolated yield of the product. Crystallographic
information: 1 is triclinic, P1h, a ) 13.0133(6) Å, b ) 17.2210(7) Å, c ) 24.7634(11) Å, R ) 71.7300(10)°,
â ) 82.3630(10)°, γ ) 70.5120(10)°, Z ) 2; 2 is triclinic, P1h, a ) 12.756(4) Å, b ) 13.469(4) Å, c ) 17.160(5)
Å, R ) 82.454(7)°, â ) 89.904(8)°, γ ) 72.995(7)°, Z ) 1.
Introduction
Chart 1
In 1984 Zuckerman published the structure of Sn(η5-C5Ph5)2
in which the two cyclopentadienyl rings were parallel, thus
showing that the lone pair on tin(II) was stereochemically
inactive, a violation of VSEPR theory.1 Subsequently, a number
of MCp2 (Cp ) substituted cyclopentadienyl ligand) structures
with M ) Ge, Sn, and Pb have been prepared in which the
rings are also parallel.2,3 Although in Zuckerman’s original paper
he argued that the reason for the parallel ring structure was
interligand repulsions, more recent work, including theoretical
calculations,3,4 indicate that this geometry can be observed in
complexes with no close contacts between the substituents on
the rings. For example, the complex Pb(1,2,4-(Pri)3C5H2)2 has
parallel rings, but there are no interligand methyl group contacts
less than 4.17 Å, where the van der Waals radii sum for two
methyl groups is 4.0 Å.2b From these results, Hanusa argued
against simple steric effects forcing the parallel arrangement
and favored crystal packing forces as the dominant factor.2b In
an earlier paper, he also compared group 2 and 14 metallocene
structures, and argued,2a on the basis of the similarity of the
structures of a large number of complexes, that “the confinement
of the metal valence electrons of the divalent Group 14 metals
to nondirectional orbitals of high s-character” limits their
influence on the structures of metallocene complexes.
Understanding the factors that control the stereoactivity of
the lone pair in lead(II) chemistry may be important in the design
of complexing agents that can remove this toxic metal selectively
from biological systems.5 We have launched a program aimed
at determining whether we can prepare pseudometallocene
complexes of the type Pb[tris(pyrazolyl)borate]2 in which the
stereoactivity of the lone pair on lead(II) is controlled by varying
the substituents, especially at the 3-position, on tris(pyrazolyl)-
borate ligands (Tp; Chart 1).6
Many groups have compared the donor properties of Cp and
Tp ligands, given that they are both formally six electron donor,
monoanionic ligands.7 Our initial results showed that we could
control the impact of the lone pair by changing the pyrazolyl-
(1) Heeg, M. J.; Janiak, C.; Zuckerman, J. J. J. Am. Chem. Soc. 1984,
106, 4259.
(2) (a) Burkey, D. J.; Hanusa, T. P. Comments Inorg. Chem. 1995, 17,
41. (b) Burkey, D. J.; Hanusa, T. P.; Huffman, J. C. Inorg. Chem.
2000, 39, 153 and references therein.
6
ring substituents. The complex Pb[HB(pz)3]2 (pz ) pyrazolyl
(3) Constantine, S. P.; Cox, H.; Hitchcock, P. B.; Lawless, G. A.
Organometallics 2000, 19, 317 and references therein.
(4) Armstrong, D. R.; Duer, M. J.; Davidson, M. G.; Moncrieff, D.;
Russell, C. A.; Stourton, C.; Steiner, A.; Stalke, D.; Wright, D. S.
Organometallics 1997, 16, 3340.
(5) Shimoni-Livny, L.; Glusker, J. P.; Bock, C. W. Inorg. Chem. 1998,
37, 1853 and reference therein.
(6) (a) Reger, D. L.; Huff, M. F.; Rheingold, A. L.; Haggerty, B. S. J.
Am. Chem. Soc. 1992, 114, 579. (b) Reger, D. L. Synlett 1992, 469.
10.1021/ic0100121 CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/20/2001

Impact of the Lone Pair in Pb(II) Complexes
Inorganic Chemistry, Vol. 40, No. 15, 2001 3811
0.50 mmol) was added to the solution. This solution was allowed to
stir for 24 h at room temperature. The solvent was removed under
reduced pressure, and an off-white solid remained (0.40 g, 0.16 mmol,
68%). Crystals suitable for X-ray structural analysis and the analytical
sample were grown from a CH2Cl2/hexane layered mixture. Mp: 144-
ring) has a highly distorted 6-coordinate structure. In this
structure, there is clearly a gap where the lone pair of electrons
is thought to be located and a pattern of long Pb-N bond
distances adjacent to the proposed site of the lone pair and short
distances opposite this site, the two main characteristics of lead-
(II) structures that contain a stereochemically active lone pair.6
In contrast, Pb[HB(3,5-Me2pz)3]2 has an octahedral structure
in which there is clearly a stereochemically inactive lone pair.6
We followed this research with analogous studies using tris-
(pyrazolyl)methane ligands (Chart 1), ligands that are isoelec-
tronic to tris(pyrazolyl)borate ligands, but are neutral rather than
anionic. Using these ligands, we showed that the cation in {Pb-
[HC(3,5-Me2pz)3]2}(BF4)2 was octahedral, but as observed with
the tris(pyrazolyl)borate ligands, the cation in {Pb[HC(pz)3]2}-
(BF4)2 has a highly distorted 6-coordinate structure.8
1
149 °C. H NMR (acetone-d6): δ 8.99 (2, s, HC(pz)3), 8.09, 7.69 (6,
6; d, d; J ) 2.2, 1.5 Hz; 3,5-H (pz)), 7.79, 7.68 (m, s; 16, 8; 2H, 4H
in C6H3(CF3)2), 6.45 (6, dd, J ) 2.0, 1.9 Hz; 4-H (pz)). Anal. Calcd
for C84H44 B2F48N12Pb: C, 41.97; H, 1.87. Found: C, 42.12; H, 1.14.
{Pb[HC(3,5-Me2pz)3]2}{B[3,5-(CF3)2C6H3]4}2 (2). Pb(acac)2 (0.10
g, 0.25 mmol) was charged into a round-bottom Schlenk flask, and
CH2Cl2 (10 mL) was added. A 2 molar equiv sample of HBArf (0.50
g, 0.50 mmol) was placed in a Schlenk flask and dissolved in CH2Cl2
(20 mL). This solution was added to the lead suspension and the
resulting homogeneous solution allowed to stir for 0.5 h. Hexane (40
mL) was added to this solution, and a white precipitate formed. The
mixture was filtered and the white solid dried under reduced pressure.
The white solid was redissolved in CH2Cl2 (20 mL), and 2 molar equiv
of HC(3,5-Me2pz)3 (0.15 g, 0.50 mmol) was added to the solution. This
solution was allowed to stir for 24 h at room temperature. The solvent
was removed under reduced pressure, and an off-white solid remained
(0.43 g, 0.17 mmol, 69%). Crystals suitable for X-ray structural analysis
and the analytical sample were grown from a CH2Cl2/hexane layered
Given Hanusa’s suggestion that crystal packing is important
in determining the solid-state geometries of Group 14 MCp2
complexes, we found it interesting that both Pb[HB(3,5-Me2-
pz)3]2 and {Pb[HC(3,5-Me2pz)3]2}(BF4)2 crystallize in the highly
symmetric R3h and R3hc space groups. We decided to pre-
pare complexes of the {Pb[HC(3,5-Me2pz)3]2}2+ and {Pb[HC-
1
(pz)3]2}2+ cations with a very different anion other than BF4
-
mixture. Mp: 176-181 °C. H NMR (acetone-d6): δ 8.28 (2, s, HC-
(3,5-Me2pz)3), 7.79, 7.68 (m, s; 16, 8; 2H, 4H in C6H3(CF3)2), 6.07 (s,
6, 4-H (pz)), 2.80, 2.29 (18, 18; br, s; 3,5-Me (pz)). Anal. Calcd for
C96H68B2F48N12Pb: C, 43.55; H, 2.68. Found: C, 43.65; H, 2.89.
to dramatically change the crystal packing forces. We report
here the syntheses and solid-state structures of {Pb[HC(pz)3]2}-
{B[3,5-(CF3)2C6H3]4}2 (1) and {Pb[HC(3,5-Me2pz)3]2}{B[3,5-
(CF3)2C6H3]4}2 (2). We also describe a modification of the
preparation of Na{B[3,5-(CF3)2C6H3]4} that is safer and leads
to pure product in very high yield.
Na{B[3,5-(CF3)2C6H3]4}. A 500 mL three-necked flask fitted with
a reflux condenser and an addition funnel was charged with Mg (1.01
g, 41.7 mmol), NaBF4 (0.70 g, 6.4 mmol), and 150 mL of Et2O. (Note:
All glassware was flame dried prior to use.) Dibromoethane (0.49 mL,
5.7 mmol) was added, and the flask was heated for several minutes
with a heat gun to initiate the reaction. The heat was removed, and
3,5-bis(trifluoromethyl)bromobenzene (6.2 mL, 36 mmol), diluted with
50 mL Et2O, was added dropwise over ca. 30 min. The addition causes
the solution to gently reflux, and once all of the 3,5-bis(trifluoromethyl)-
bromobenzene had been added, the reaction mixture was heated with
a heating mantel to continue the reflux for an additional 30 min. The
heat was then removed and the reaction mixture left to stir overnight
at room temperature. The reaction mixture was added to Na2CO3 (16
g) in water (200 mL), stirred for 30 min, and filtered. The aqueous
layer was extracted with ether (3 × 50 mL), and the combined organic
layer was dried over sodium sulfate and treated with decolorizing
charcoal. The mixture was filtered and the ether removed under vacuum.
The remaining oily-solid was dissolved in 200 mL of benzene, and
water was removed with a Stark trap by azeotropic distillation for 2 h.
The solvent volume was reduced to 50 mL and the solution cooled to
room temperature and filtered via cannula filtration to remove unreacted
starting material and other soluble impurities. The remaining white solid
was dried under vacuum (5.04 g, 5.6 mmol, 90%). Mp: 300-306 °C.
1H NMR (acetone d6): δ 7.79 (8, br, 2,6-HB[3,5-(CF3)2C6H3]4), 7.67
(4, s, 4-HB[3,5-(CF3)2C6H3]4). Anal. Calcd for C32H12F24BNa (sample
protected from air): C, 43.37; H, 1.36. Found: C, 43.48; H, 1.61. Anal.
Calcd for C32H12F24BNa‚2H2O (sample not protected from air): C,
41.68; H, 1.74. Found: C, 41.43; H, 1.75.
Experimental Section
General Procedures. All operations were carried out under a
nitrogen atmosphere using either standard Schlenk techniques or a
Vacuum Atmospheres HE-493 drybox. All solvents were dried,
degassed, and distilled prior to use. Proton chemical shifts are reported
in parts per million versus TMS. Lead(II) acetylacetonate (Pb(acac)2)
and 3,5-bis(trifluoromethyl)bromobenzene were purchased from Aldrich
Chemicals. HC(pz)3 and HC(3,5-Me2pz)3 were prepared according to
our recently reported procedures.9 Na{B[3,5-(CF3)2C6H3]4} (NaBArf)
was prepared as outlined below and converted to [H(Et2O)2]{B[3,5-
(CF3)2C6H3]4} (HBArf) by the published procedures.10 Elemental
analyses were performed by Robertson Microlit Laboratories, Inc. and
Desert Analytics Laboratories. Warning: The Grignard reagent
prepared in situ in the preparation of Na{B[3,5-(CF3)2C6H3]4} is
potentially explosiVe.11 Lead(II) compounds are extremely toxic, and
care should be used when handling them.
{Pb[HC(pz)3]2}{B[3,5-(CF3)2C6H3]4}2 (1). Pb(acac)2 (0.10 g, 0.25
mmol) was charged into a round-bottom Schlenk flask and CH2Cl2 (10
mL) added. A 2 molar equiv sample of HBArf (0.50 g, 0.50 mmol)
was placed in a Schlenk flask and dissolved in CH2Cl2 (20 mL). This
solution was added to the lead suspension and the resulting homoge-
neous solution allowed to stir for 0.5 h. Hexane (40 mL) was added to
this solution, and a white precipitate formed. The mixture was filtered
and the white solid dried under reduced pressure. The white solid was
redissolved in CH2Cl2 (20 mL), and 2 molar equiv of HC(pz)3 (0.11 g,
X-ray Structure Determination. Crystal, data collection, and
refinement parameters are given in Table 1. A colorless parallelepiped
crystal of 1 of dimensions 0.34 mm × 0.10 mm × 0.08 mm and a
colorless plate crystal 2 of dimensions 0.32 mm × 0.22 mm × 0.08
mm were epoxied onto the end of a thin glass fiber. The X-ray intensity
data were measured at 293 K using a Bruker SMART APEX CCD-
based diffractometer system equipped with a Mo target X-ray tube
(λ ) 0.71073 Å).
The unit cell was initially determined on the basis of reflections
harvested from a set of three scans measured in orthogonal wedges of
reciprocal space. Subsequently, 2060 data frames for 1 and 1405 data
frames for 2 were collected with a scan width of 0.3° in ω and an
exposure time of 20 s/frame for 1 and 10 s/frame for 2. The first 50
frames were recollected at the end of the data set to monitor crystal
decay. The raw data frames were integrated with the Bruker SAINT
(7) (a) Trofimenko, S. Scorpionates - The Coordination Chemistry of Poly-
(pyrazolyl)borate Ligands; Imperial College Press: London, 1999; p
31. (b) Tellers, D. M.; Skoog, S. J.; Bergman, R. G.; Gunnoe, T. B.;
Harman, W. D. Organometallics 2000, 19, 2428.
(8) Reger, D. L. Collins, J. E.; Rheingold, A. L.; Liable-Sands, L. M.;
Yap, G. P. A. Inorg. Chem. 1997, 36, 345.
(9) Reger, D. L.; Grattan, T. C.: Brown, K. J.; Little, C. A.; Lamba, J. J.
S.; Rheingold, A L.; Sommer, R D. J. Organomet. Chem. 2000, 607,
120.
(10) (a) Brookhart, M.; Grant, B.; Vople, J. A. F. Organometallics 1992,
11, 3920. (b) Hughes, R. P.; Lindner, D. C.; Rheingold, A. L.; Yap,
G. P. A. Inorg. Chem. 1997, 36, 1726.
(11) Moore, E. J.; Waymouth, R. Chem. Eng. News 1997, March 17, 6.

3812 Inorganic Chemistry, Vol. 40, No. 15, 2001
Reger et al.
Table 1. Crystallographic Data for the Structural Analyses of
{Pb[HC(pz)3]2}{B[3,5-(CF3)2C6H3]4}2·0.7CH2Cl2 (1) and
{Pb[HC(3,5-Me2pz)3]2}{B[3,5-(CF3)2C6H3]4}2·2CH2Cl2 (2)
1
2
empirical formula C84.70H45.40B2Cl1.40F48N12Pb C98H72B2Cl4F48N12Pb
fw
2421.57
P1h
triclinic
2700.29
P1h
space group
cryst syst
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
triclinic
12.756(4)
13.469(4)
17.160(5)
82.454(7)
89.904(8)
72.995(7)
2792.7(15)
1
13.0133(6)
17.2210(7)
24.7634(11)
71.7300(10)
82.3630(10)
70.5120(10)
4965.4(4)
2
Z
cryst color
colorless
colorless
1.606
1.737
avoid heating large amounts of the potentially explosive
Grignard reagent prepared from 3,5-(CF3)2C6H3Br.11 Specifi-
cally, at the start of the reaction NaBF4 is added to the reaction
flask along with ether and the magnesium turnings. The
magnesium turnings are activated with C2H4Br2 to ensure that
the Grignard reaction initiates immediately upon dropwise
addition of 3,5-(CF3)2C6H3Br to the ether mixture. Presumably,
most of the potentially explosive Grignard reagent formed during
this addition reacts with the NaBF4 and does not build up in
solution. Residual water and diethyl ether, introduced in the
initial workup, are removed by a brief benzene azeotrope
procedure.10b The benzene is removed by filtration, yielding an
off-white solid in 90% yield that is pure by NMR and elemental
analysis (the literature yield is 70% obtained on a solid not
purified as indicated above10). Monitoring each stage of the
workup by NMR shows that prior to the benzene filtration step
impurities containing the 3,5-(CF3)2C6H3 group are present,
impurities that are removed by this procedure. Thus, our new
reaction conditions not only increase the safety of the procedure,
but improve the yield of pure product. Three other important
points were determined in our efforts to improve this prepara-
tion: (1) The excess of 3,5-(CF3)2C6H3Br (25.6 mmol required
based on NaBF4, 36 mmol used) reported in the literature
preparation10 appears to be necessary to obtain the high yields
reported here. (2) The Na{B[3,5-(CF3)2C6H3]4} solid that has
been dried by the benzene azeotrope procedure is much less
soluble in CH2Cl2 than the solid taken into this procedure. (3)
We have shown by elemental analysis that pure Na{B[3,5-
(CF3)2C6H3]4} as prepared here picks up 2 equiv of water when
handled in air.
Structure of {Pb[HC(pz)3]2}{B[3,5-(CF3)2C6H3]4}2 (1).
Figure 1 shows the ORTEP diagram of the cation in {Pb[HC-
(pz)3]2}{B[3,5-(CF3)2C6H3]4}2 (1), and selected bond distances
and angles are shown in Table 2. The lead(II) is 6-coordinate
with a highly asymmetric arrangement of the nitrogen donor
atoms. As is typical of distorted lead(II) structures, the three
Pb-N bond distances adjacent to the “open space” in the
structure are longer (average 2.70 Å) than those away from it
(average 2.53 Å). The intraligand bond angles are fairly similar,
ranging from 68.23(16)° to 72.71(17)° (average 69.8°); however,
the interligand bond angles vary from 74.25(18)° to 146.60-
(16)°.
The structure of 1 is fairly similar to that of {Pb[HC(pz)3]2}-
(BF4)2 (3; Table 2),6a although the structure of 3 is centrosym-
metric. The average Pb-N bond distance in 1 is 2.62 Å with a
range of values from 2.499(5) to 2.790(5) Å, and for 3 the
average is 2.69 Å with a much smaller range of 2.609(5)-2.789-
(5) Å. The average intraligand bond angle for 1 (69.8°) is 2.1°
greater than for 3 (67.7°). The biggest difference between the
two structures is in the range of the interligand bond angles,
F(calcd), g cm-3 1.620
µ(Mo KR), mm-1 1.876
R1(F),a wR2(F2)a 0.0611, 0.1631
0.0661, 0.1495
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2
;
w ) 1/[σ2(Fo ) + (aP)2 + bP], where P is [2Fc2 + max(Fo , 0)]/3.
2
2
software package (SAINT+ Version 6.02a, Bruker Analytical X-ray
Systems, Inc., Madison, WI, 1998) using a narrow-frame integration
algorithm, yielding for 1 a total of 51219 reflections to a maximum 2θ
angle of 52.9° (0.80 Å resolution), of which 20274 were independent
(completeness 99.2%, Rint ) 0.0317) and 13186 were greater than
2σ(I). For 2, a total of 23838 reflections were measured to a maximum
2θ angle of 50.0° (0.84 Å resolution), of which 9802 were independent
(completeness 98.0%, Rint ) 0.0880) and 4828 were greater than
2σ(I). Corrections for Lorentz and polarization effects were also applied
by SAINT. The final triclinic cell constants are based upon the
refinement of the XYZ centroids of 5950 reflections for 1 and 2010
reflections for 2 above 10σ(I). Analysis of the data showed negligible
crystal decay during data collection for 1. Comparison of identical
reflections measured at the beginning and at the end of the data
collection for 2 showed an average loss of intensity of 5.7%; therefore,
a time decay correction was applied. An empirical absorption correction
based on the multiple measurement of equivalent reflections was applied
to both structures with the program SADABS.
For both structures, the space group P1h was assumed and confirmed
by successful solution and refinement of the structure. The structures
were solved by a combination of direct methods and difference Fourier
syntheses, and refined by full-matrix least-squares against F2. For 1,
significant disorder of some of the CF3 groups of the anion was
observed; these were modeled as two half-occupied CF3 groups rotated
by 60° relative to one another. A disordered methylene chloride
molecule is present which refined to an occupancy of ca. 70%; C-Cl
distance restraints were applied to this molecule. For 2, significant
rotational disorder of some of the CF3 groups of the anion was observed;
these were modeled as two partially occupied CF3 groups rotated by
60° relative to one another. All non-hydrogen atoms were refined with
anisotropic displacement parameters; hydrogen atoms were refined using
a riding model. All software and sources of the scattering factors are
contained in the SHELXTL (5.10) program library (G. Sheldrick, Bruker
AXS, Madison, WI).
Results and Discussion
The reaction of Pb(acac)2 with 2 equiv of [H(Et2O)2]{B[3,5-
(CF3)2C6H3]4} (HBArf) in CH2Cl2 followed by addition of 2
equiv of either HC(pz)3 or HC(3,5-Me2pz)3 leads to the for-
mation of the desired two complexes {Pb[HC(pz)3]2}{B[3,5-
(CF3)2C6H3]4}2 (1) and {Pb[HC(3,5-Me2pz)3]2}{B[3,5-(CF3)2-
C6H3]4}2 (2), respectively. These compounds are air-stable, high-
melting solids that are soluble in THF, acetone, and CH2Cl2.
As part of our synthetic studies, we have modified the
literature preparation10 of Na{B[3,5-(CF3)2C6H3]4} (NaBArf) to

Impact of the Lone Pair in Pb(II) Complexes
Inorganic Chemistry, Vol. 40, No. 15, 2001 3813
Table 2. Selected Bond Distances and Bond Angles for {Pb[HC(pz)3]2}{B[3,5-(CF3)2C6H3]4}2 (1) and {Pb[HC(pz)3]2}(BF4)2 (3)
1
3
1
3
Bond Distances (Å)
Pb-N(11)
Pb-N(21)
Pb-N(31)
Pb-N(41)
2.499(5)
2.790(5)
2.514(5)
2.692(5)
2.789(5)
2.660(5)
2.609(5)
Pb-N(51)
2.586(5)
2.619(5)
3.00
Pb-N(61)
intraligand donor N‚‚‚N av distance
Pb distance out of N3 donor plane
3.00
2.05
1.96
Selected Bond Angles (deg)
N(11)-Pb-N(21)
N(11)-Pb-N(31)
N(21)-Pb-N(31)
N(41)-Pb-N(51)
N(41)-Pb-N(61)
N(51)-Pb-N(61)
N(11)-Pb-N(41,31A)
N(11)-Pb-N(51,11A)
68.81(16)
72.71(17)
68.23(16)
69.14(17)
70.50(15)
69.39(17)
133.36(16)
146.60(16)
67.8(2)
65.5(2)
69.8(2)
N(11)-Pb-N(61,21A)
N(21)-Pb-N(41,31A)
N(21)-Pb-N(51,11A)
N(21)-Pb-N(61,21A)
N(31)-Pb-N(41,31A)
N(31)-Pb-N(51,11A)
N(31)-Pb-N(61,21A)
av Fe-N(1)-N(2)-C(1) torsion
134.95(16)
74.43(19)
104.93(17)
142.73(19)
74.25(18)
142.56(17)
88.86(17)
174
119.0(2)
78.7(2)
119.0(2)
140.9(2)
72.0(2)
132.5(2)
78.7(2)
167
132.5(2)
161.4(2)
N-Pb-N angles are 180°, as required by the symmetry, making
the structure a trigonally distorted octahedron. The lone pair
on the lead(II) is clearly stereochemically inactiVe.
Although 2 is not in the highly symmetric R3hc space group
as is {Pb[HC(3,5-Me2pz)3]2}(BF4)2 (4),6a the bond distances and
angles about the lead(II) center in the two structures are
generally similar. The average Pb-N distance in 2 of 2.59 Å is
0.05 Å shorter than that found in 4. The range of values in the
lower symmetry 2 is only 0.02 Å. The cis intraligand N-Pb-N
bond angles in 2 have a range of 2.4° and average 72.1°, just
0.3° less than in 4.
There is a surprisingly large difference in the two structures
in the degree of tilting the pyrazolyl rings make away from an
ideal C3V type arrangement. In the absence of tilting, the Pb-
N(n1)-N(n2)-C(n1) torsion angles (where n denotes the ring
number) would be 180°, and the metal atom would reside in
the planes defined by the pyrazolyl rings, planes that also contain
the lone pair on the donor nitrogen atom. In complex 2 this
angle averages 159°, whereas in 4 it is 130°. A consequence of
the difference in tilting is that the average N‚‚‚N intraligand
nonbonding distance increases from 3.04 Å in 2 to 3.12 Å in 4.
This increased tilting is likely responsible for the longer Pb-N
bond distances in 4. Another change caused by the tilting is in
the nonbonding distances between adjacent interligand 3-methyl
groups. For 2 these distances average 4.02 Å, nearly exactly
the van der Waals radii of two methyl groups, but decrease to
3.93 Å in 4.
Figure 1. ORTEP diagram of {Pb[HC(pz)3]2}2+ with thermal ellipsoids
drawn at the 30% probability level.
We have previously commented on the fact that in octahedral
{M[HC(3,5-Me2pz)3]2}n+ and M[HB(3,5-Me2pz)3]2 complexes
this ring tilting increases with increasing size of the metal.6,8,12,16
Table 4 shows data for cations in complexes of the HC(3,5-
Me2pz)3 ligand that we have characterized, to date. The bite
size of this ligand is largely fixed by the size of the pyrazolyl
ring, the C-N bond distances, and the N-C-N bond angles,
but is increased by this tilting to accommodate large metal ions.
It is clear from these results with 2 and 4 that factors other
than metal size are important in determining the degree of tilting.
The difference in the tilting and, to a lesser extent, the Pb-N
bond distances between 2 and 4 are a surprisingly large
consequence of changes in the crystal packing forces. The
Figure 2. ORTEP diagram of {Pb[HC(3,5-Me2pz)3]2}2+ with thermal
ellipsoids drawn at the 30% probability level.
which at 74.25(18)-146.60(16)° for 1 is much lower than that
for 3, 72.0(2)-161.4(2)°. Clearly, crystal lattice forces in these
compounds impact the structures of the cations, but both have
highly distorted arrangements about the lead(II).
Structure of {Pb[HC(3,5-Me2pz)3]2}{B[3,5-(CF3)2C6H3]4}2
(2). Figure 2 shows the ORTEP diagram of the cation in {Pb-
[HC(3,5-Me2pz)3]2}{B[3,5-(CF3)2C6H3]4}2 (2), and selected
bond distances and angles are shown in Table 3. The lead(II) is
6-coordinate with a highly symmetric pseudooctahedral ar-
rangement of the nitrogen donor atoms. The three Pb-N
distances are nearly equal. The chelate rings restrict the
intraligand N-Pb-N bond angles to an average of 72.1° with
cis interligand N-Pb-N bond angles averaging 107.9°. All trans
(12) Reger, D. L.; Collins, J. E.; Layland, R.; Adams, R. D. Inorg. Chem.
1996, 35, 1372.
(13) Reger, D. L.; Little, C. A.; Rheingold, A. L.; Liable-Sands, L. M.;
Smith, M. D. Unpublished results.
(14) Reger, D. L.; Collins, J. E.; Yap, G. P. A. Organometallics 1997, 16,
349.
(15) Reger, D. L. Collins, J. E.; Myers, S. M.; Rheingold, A. L.; Liable-
Sands, L. M. Inorg. Chem. 1996, 35, 4904.
(16) Reger, D. L.; Little, C. A.; Young V.; Pink, M. Inorg. Chem. 2001,
40, 2870.

3814 Inorganic Chemistry, Vol. 40, No. 15, 2001
Reger et al.
Table 3. Selected Bond Distances and Bond Angles for {Pb[HC(3,5-Me2pz)3]2}{B[3,5-(CF3)2C6H3]4}2 (2) and {Pb[HC(3,5-Me2pz)3]2}(BF4)2 (4)
2
4
2
4
Bond Distances (Å)
Pb-N(11)
Pb-N(21)
Pb-N(31)
2.600(6)
2.581(6)
2.577(7)
2.635(7)
intraligand donor N‚‚‚N av distance
Pb distance out of N3 donor plane
3.04
1.90
3.12
1.93
Selected Bond Angles (deg)
N(11)-Pb-N(21)
N(11)-Pb-N(31)
N(21)-Pb-N(31)
N(11)-Pb-N(11A)
N(11)-Pb-N(21A)
N(11)-Pb-N(31A)
73.32(19)
70.9(2)
72.00(19)
180.0
106.68(19)
109.1(2)
72.4(3)
N(21)-Pb-N(21A)
180.0
108.00(19)
180.0
166.4
156.0
N(21)-Pb-N(31A)
N(31)-Pb-N(31A)
180.0
Pb-N(11)-N(12)-C(11) torsion
Pb-N(21)-N(22)-C(21) torsion
Pb-N(31)-N(32)-C(31) torsion
130.1
107.6(3)
155.2
Table 4. Average M-N-N-C Torsion Angles (deg), Average M-N Bond Distances (Å), and Average Intraligand N...N Distances (Å) of
Selected {M[HC(3,5-Me2pz)3]2}n+ Complexes
compound
M-N-N-C torsion angle
M-N distance
N···N distance
ref
{Tl[HC(3,5-Me2pz)3]2}(PF6)
{K[HC(3,5-Me2pz)3]2}(PF6)
123
127
130
159
137
149
141
169
178
2.92
2.82
2.635
2.59
2.51
2.49
2.32
2.18
1.98
3.24
3.21
3.12
3.04
3.12
3.10
2.98
2.91
2.72
12
13
8
{Pb[HC(3,5-Me2pz)3]2}(BF4)2
{Pb[HC(3,5-Me2pz)3]2}(BArf)2
{Ag[HC(3,5-Me2pz)3]2}(O3SCF3)
{Na[HC(3,5-Me2pz)3]2}(I)
{Cd[HC(3,5-Me2pz)3]2}(BF4)2
{Fe[HC(3,5-Me2pz)3]2}(BF4)2 (high spin)
{Fe[HC(3,5-Me2pz)3]2}(BF4)2 (low spin)
this work
14
13
15
16
16
Table 5. Average Pb-N Distances (Å)
Conclusion
{Pb[HC(pz)3]2}{B[3,5-(CF3)2C6H3]4}2 (1)
{Pb[HC(pz)3]2}(BF4)2 (3)
Pb[HB(pz)3]2 (5)
{Pb[HC(3,5-Me2pz)3]2}{B[3,5-(CF3)2C6H3]4}2 (2)
{Pb[HC(3,5-Me2pz)3]2}(BF4)2 (4)
Pb[HB(3,5-Me2pz)3]2 (6)
2.62
2.69
2.61
Drastically changing the characteristics of the counterion from
[BF4]- to {B[3,5-(CF3)2C6H3]4}-, thus dramatically changing
the crystal lattice forces, changes the details of the structures
of the two lead(II) cations, but the change does not impact
whether the lone pair is stereochemically active or not. Most
importantly, {Pb[HC(3,5-Me2pz)3]2}2+ has a trigonally distorted
octahedral structure in both cases. The change from a distorted
structure in unsubstituted 1 and 3 to a structure with a clearly
stereochemically inactive lone pair in 2 and 4 is dominated by
forces within the cations. Specifically, in the octahedral
structures adopted by 2 and 4 the interligand distances between
adjacent 3-position methyl groups are close to 4.0 Å, the sum
of the van der Waals radii of two methyl groups. It appears
that the driving forces for these 6-coordinate complexes to have
a stereochemically inactive lone pair in 2 and 4 are these
interligand interactions within each cation and not crystal
packing forces. A similar conclusion was also made in a recent
survey of structures of known lead(II) coordination complexes,5
a conclusion now verified by our two additional structures. In
the absence of these forces in 1 and 3 the lone pair becomes
stereochemically active, producing the distorted structure gener-
ally found for PbN6 central cores with chelating ligands lacking
bulky substituents.5
2.59
2.635(7)a
2.610(5)a
a Exact distance given as all Pb-N distances are equivalent.
potential energy surface for these changes must be quite soft,
with these large changes in tilting and modest change in bond
distances not impacting importantly on the stability of the
coordination complex. A recent paper has drawn a somewhat
similar conclusion in a study of samarium complexes of the
[HB(3,5-Me2pz)3]- ligand.17
Table 5 compares the average Pb-N distances in the six
structures discussed here. Given that the lone pair is in an orbital
containing some 6p character in distorted lead(II) structures, it
is expected that the Pb-N bond distances will be shorter in
these structures (more s character, an orbital with a lesser radial
extension, in the bonds) compared to symmetrical structures in
which the lone pair is in a pure 6s orbital.5,18 In addition, the
methyl-substituted complexes, the complexes that are sym-
metrical, have greater interligand repulsions from the 3-position
methyl groups, possibly also leading to longer Pb-N bonds.
There is no indication from the data in Table 5 that either factor
causes the Pb-N bonds to be longer in the symmetrical
structures. These results support calculations that indicate that
in distorted structures the amount of mixing of the p orbital
into the lone pair orbital is small.5
Acknowledgment. We thank the National Science Founda-
tion (Grant CHE-9727325) for support. The Bruker CCD single-
crystal diffractometer was purchased using funds provided by
the NSF Instrumentation for Materials Research Program
through Grant DMR:9975623.
Supporting Information Available: Two X-ray crystallographic
files in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
(17) Hillier, A. C.; Liu, S.-Y.; Sella, A.; Elsegood, M. R. J. Inorg. Chem.
2000, 39, 2635.
(18) Hancock, R. D.; Shaikjee, M. S.; Dobson, S. M.; Boeyens, J. C. A.
Inorg. Chim. Acta 1988, 154, 229.
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