Derivatives of the Dimethylbis(2-pyridyl)borate(1-) Ion: Synthesis and Structure

Article abstract of DOI:10.1021/ic950775m

The syntheses and structures of derivatives of a novel uninegative bidentate ligand, the dimethylbis(2-pyridyl)borate anion, (py)₂BMe₂^-, are reported. Lithium dimethylbis(2-pyridyl)borate, Li(py)₂BMe₂, was prepared by adding a solution of bromodimethylborane to a solution of 2-lithiopyridine in a 1:2 molar ratio, respectively; subsequent hydrolysis afforded hydrogen dimethylbis(2-pyridyl)borate, H(py)₂BMe₂, in 44% yield. Reaction of Li(py)₂BMe₂ with either bromodimethylborane or bromodiphenylborane afforded the corresponding tricyclic bis(organo)boronium dimethylbis(2-pyridyl)borate species R₂B(μ-py)₂BMe₂(R = CH₃, C₆H₅) in approximately 90% yields. Reaction of H(py)₂BMe₂ with anhydrous zinc(II) chloride or nickel(II) chloride hexahydrate in a 4:1 molar ratio gave the tetrahedral complex bis[dimethylbis(2-pyridyl)borato-N,N']zinc(II), Zn{(μ-py)₂BMe₂}₂, or the square planar complex bis[dimethylbis(2-pyridyl)borato-N,N']nickel(II), Ni{(μ-py)₂BMe₂}₂, respectively. H(py)₂BMe₂is monoclinic, P2₁/m, with a = 8.836(2) A?, b = 7.1825(5) A?, c = 9.2789(11) A?, β = 99.81(2)°, V = 580.29(13) A?³ and Z = 2. Ph₂B(μ-py)₂BMe₂ is monoclinic, P2₁/n, a = 8.0334(12) A?, b = 10.9169(9) A?, c = 22.981(2) A?, β = 94.653(9)°, V = 2008.8(4) A?³, and Z = 4. Zn{(μ-py)₂BMe₂}₂ is monoclinic, Cc, with a = 8.010(2) A?, b = 31.256(3) A?, c = 18.753(3) A?, β= 90.83(2)°, V = 4694.8(13) A?³, and Z = 8. Ni{(μ-py)₂BMe₂}₂is triclinic, P1?, with a = 7.675(2) A?, b = 8.819(2) A?, c = 9.459(2) A?, α = 115.72(3)°, β= 96.89(3)°, γ = 94.02(3)°, and Z = 1.

Full text of DOI:10.1021/ic950775m

2140
Inorg. Chem. 1996, 35, 2140-2148
Derivatives of the Dimethylbis(2-pyridyl)borate(1-) Ion: Synthesis and Structure
Thomas G. Hodgkins*,† and Douglas R. Powell‡
Departments of Chemistry, Pittsburg State University, Pittsburg, Kansas 66762, and
University of WisconsinsMadison, Madison, Wisconsin 53706
ReceiVed June 23, 1995X
The syntheses and structures of derivatives of a novel uninegative bidentate ligand, the dimethylbis(2-pyridyl)-
borate anion, (py)2BMe2-, are reported. Lithium dimethylbis(2-pyridyl)borate, Li(py)2BMe2, was prepared by
adding a solution of bromodimethylborane to a solution of 2-lithiopyridine in a 1:2 molar ratio, respectively;
subsequent hydrolysis afforded hydrogen dimethylbis(2-pyridyl)borate, H(py)2BMe2, in 44% yield. Reaction of
Li(py)2BMe2 with either bromodimethylborane or bromodiphenylborane afforded the corresponding tricyclic bis-
(organo)boronium dimethylbis(2-pyridyl)borate species R2B(µ-py)2BMe2 (R ) CH3, C6H5) in approximately 90%
yields. Reaction of H(py)2BMe2 with anhydrous zinc(II) chloride or nickel(II) chloride hexahydrate in a 4:1
molar ratio gave the tetrahedral complex bis[dimethylbis(2-pyridyl)borato-N,N ′]zinc(II), Zn{(µ-py)2BMe2}2, or
the square planar complex bis[dimethylbis(2-pyridyl)borato-N,N ′]nickel(II), Ni{(µ-py)2BMe2}2, respectively.
H(py)2BMe2 is monoclinic, P21/m, with a ) 8.836(2) Å, b ) 7.1825(5) Å, c ) 9.2789(11) Å, â ) 99.81(2)°, V
) 580.29(13) Å3, and Z ) 2. Ph2B(µ-py)2BMe2 is monoclinic, P21/n, a ) 8.0334(12) Å, b ) 10.9169(9) Å, c
) 22.981(2) Å, â ) 94.653(9)°, V ) 2008.8(4) Å3, and Z ) 4. Zn{(µ-py)2BMe2}2 is monoclinic, Cc, with a )
8.010(2) Å, b ) 31.256(3) Å, c ) 18.753(3) Å, â ) 90.83(2)°, V ) 4694.8(13) Å3, and Z ) 8. Ni{(µ-py)2-
BMe2}2 is triclinic, P1h, with a ) 7.675(2) Å, b ) 8.819(2) Å, c ) 9.459(2) Å, R ) 115.72(3)°, â ) 96.89(3)°,
γ ) 94.02(3)°, and Z ) 1.
Introduction
The first examples of the pyrazaboles, which possess the ring
system shown in Figure 1a, were reported in 1966.1 Subsequent
research involving this class of compounds is now quite
substantial.2 Pyrazaboles can be viewed as two BR2 moieties
bridged by two pyrazolyl groups. Since the pyrazolyl groups
are bonded to each of the boron atoms through the nitrogen
atoms, these groups may be referred to as “N1:N2-pyrazolyl”
groups, where “N1:N2” represents the two pyrazolyl atoms that
are directly bonded to boron. Recently, pyrazabole analogs
containing the bridging groups 1-methyl-C2:N3-imidazolyl,3
1-methyl-N4:C5-triazolyl,3 C2:N3-thiazolyl,3 and N1:C2-pyridyl4
were prepared.
Unlike symmetrically substituted pyrazolyl groups in a
pyrazabole, each of the latter bridging groups can, in theory,
give rise to an isomeric pair of ring systems, depending on their
orientation with respect to each other. In fact, the first
preparation and isolation of each member of a pair of this type
have only recently been reported4 (Figure 1b). As noted in this
report, the major product of the slow addition of bromodi-
methylborane, BrBMe2, to an equimolar quantity of 2-lithiopy-
ridine, 2-Li(py), is not dimeric dimethyl(2-pyridyl)borane,5 I,
but dimethylboronium dimethylbis(2-pyridyl)borate,6 II.
In order to explain why II is the major product, the formation
of the dimethylbis(2-pyridyl)borate ion, III, was proposed.4
Figure 1. (a) Structure of a pyrazabole. (b) Structure of I, dimeric
dimethyl(2-pyridyl)borane, and II, dimethylboronium bis(2-pyridyl)-
dimethylborate.
Herein we report the isolation and structure of a versatile
derivative of this ion, hydrogen dimethylbis(2-pyridyl)borate,
IV, and examples of the application of III as a new uninegative
bidentate ligand in coordination chemistry. The synthesis of
poly(2-pyridyl)borate ions such as III may be of interest to
coordination chemists. The electronic and steric properties of
these ligands may be similar to, but significantly different from,
those of their well-known poly(1-pyrazolyl)borate analogs.7 An
example may be the use of metal complexes of these ligands in
catalysis.
* To whom correspondence should be addressed.
† Pittsburg State University.
There is increasing interest in metal complexes containing
bulky nitrogen ligands in catalysis, such as in polymerization
reactions.8,9 Poly(1-pyrazolyl)borato ligands in which the
‡ University of WisconsinsMadison.
X Abstract published in AdVance ACS Abstracts, March 1, 1996.
(1) Trofimenko, S. J. Am. Chem. Soc. 1966, 88, 1842-1844.
(2) Niedenzu, K. Mol. Struct. Energ. 1988, 5, 357-372.
(3) Padilla-Martinez, I. I.; de Jesus Rosalez-Hoz, M.; Contreras, R.;
Kerschl, S.; Wrackmeyer, B. Chem. Ber. 1994, 127, 343-346.
(4) Hodgkins T. G. Inorg. Chem. 1993, 32, 6115-6116.
(5) Alternative name: 9,9,10,10-tetramethyl-4a,8a-diaza-9,10-dibora-9,-
10-dihydroanthracene.
(7) (a) Trofimenko, S. Chem. ReV. 1993, 93, 943-980. (b) Trofimenko,
S. Prog. Inorg. Chem. 1986, 34, 115-209. (c) Niedenzu, K.;
Trofimenko, S. Top. Curr. Chem. 1986, 131, 1-37.
(8) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995,
117, 6414-6415.
(9) Togni, A.; Venanzi, L. M. Angew. Chem., Int. Ed. Engl. 1994, 33,
497-526.
(6) Alternative name: 9,9,10,10-tetramethyl-4a,10a-diaza-9,10-dibora-9,-
10-dihydroanthracene.
0020-1669/96/1335-2140$12.00/0 © 1996 American Chemical Society

Derivatives of Dimethylbis(2-pyridyl)borate(1-)
Inorganic Chemistry, Vol. 35, No. 7, 1996 2141
Scheme 1
The above predictions have been confirmed in the present
work. Of the BrBMe2 used, ca. 45% was converted to III
(which was isolated as hydrogen dimethylbis(2-pyridyl)borate,
IV) while ca. 34% was converted to a mixture of I and II
consisting of 57% I, as compared to the 15% I that was formed
when the ratio of BrBMe2 to 2-Li(py) was 1:1.4
Synthesis of III and Its Hydrolysis To Form IV. The
reaction conditions used in the synthesis of III were similar to
those used in the earlier work4 for the synthesis of mixtures of
I and II, including the temperatures employed, the nature of
the solvents, and the concentrations of the reactants. As before,
the method of Gilman and Spatz was used for the generation
of solutions of 2-lithiopyridine.14 The chief difference between
the work described here and the earlier work4 was in the mole
ratio of BrBMe2 to 2-Li(py) employed (1:2, respectively, in the
present work but 1:1 in the earlier work). Copious tarry
byproducts are typically formed in reactions involving 2-Li-
(py), significantly lowering the yields of the desired products.
Extraction of the reaction mixture with water hydrolyzes III
to IV and also removes LiBr, LiOH, and some colored by-
products. Subsequent extraction of the organic layer with aque-
ous 1.00 M acetic acid produces protonated IV; a mixture of I,
II, and unidentified colored materials remains in the organic
layer. The product, IV, is precipitated as a yellowish solid when
the acetic acid solution is made strongly alkaline with aqueous
6 M NaOH solution. Final purification of IV was achieved by
repetition of the above, followed by recrystallization from hex-
ane. The reactions used to generate IV are illustrated in eq 1.
3-positions of the pyrazolyl rings are occupied by such groups
as isopropyl,10 tert-butyl,11,12 or neopentyl13 sometimes undergo
1,2-borotropic rearrangements at temperatures ranging from 2511
to 240 °C.13 The bulky substituent is thus effectively moved
from the 3- to the 5-position on some of the pyrazolyl rings in
these complexes, limiting their use as catalysts. However, this
type of rearrangement may not occur with the poly(2-pyridyl)-
borato ligands. Research on the synthesis and chemistry of
poly(2-pyridyl)borate ions, eventually leading to the preparation
of metal complexes of these ions that contain bulky groups in
the 6-positions of the pyridyl rings, may thus be quite fruitful.
The first results of research in this area are reported here.
2{2-Li(py)} + BrB(CH3)2 -LiBr8
Li salt of III +H O8 IV + LiOH (1)
2
Results and Discussion
1
The H NMR spectra of II4 and IV in CDCl3 feature the
General Remarks on the Addition of Solutions of Bro-
modimethylborane to Solutions of 2-Lithiopyridine. The
addition of bromodimethylborane, BrBMe2, to an equimolar
quantity of 2-lithiopyridine, 2-Li(py), results in the formation
of the isomeric analogs of a pyrazabole, dimeric dimethyl(2-
pyridyl)borane, I, and dimethylboronium dimethylbis(2-pyridyl)-
borate, II, in a ratio of ca. 15:85, respectively.4 A hypothetical
mechanism which may explain the observed ratio of these
isomers is shown in Scheme 1.
As indicated, the reaction of one BrBMe2 molecule with one
2-Li(py) molecule should give one molecule of monomeric
dimethyl(2-pyridyl)borane, V. This intermediate may either
dimerize to form I or react with a second molecule of 2-Li(py)
to form III. Once III appears, it can react with BrBMe2 to
form II, but not I. Therefore, the formation of I must occur
only in the early stages of the addition of solutions of BrBMe2
to solutions of 2-Li(py).
When BrBMe2 is added dropwise to 2-Li(py) in 1:2 molar
ratio, respectively, the major product should be III. Smaller
yields of I and II should also be expected. Furthermore, a much
higher ratio of I to II than that reported in the earlier work
(15:85) should be observed. This last prediction is justified on
the grounds that BrBMe2 should preferentially react with 2-Li-
(py), which is a stronger Lewis base than is III, and that,
according to this mechanism, only II can be formed from the
reaction of BrBMe2 with III.
expected 2-pyridyl ring pattern as well as a partially resolved
quartet in the high-field region (0.06 ppm for II and 0.07 ppm
for IV) that is attributable to the dimethylborate protons. The
spectrum of IV contains a signal at 18.70 ppm that is dependent
on both the nature of the solvent and the presence of moisture.
In fact, upfield shifts for this peak of as much as 8-9 ppm can
be ascribed to the presence of moisture in the sample. When
the solvent is CDCl3, the intensity of this peak decreases over
a period of hours as exchange with the deuterium atom in the
solvent occurs. These properties are consistent with the assign-
ment of this singlet with the nitrogen-bonded proton in IV.
The 11B and 13C NMR spectra of II and IV also have much
in common. The 11B NMR spectra of these compounds both
show a sharp singlet near -18 ppm; this signal in the spectrum
of II has previously been assigned to the dimethylborate boron
atom.4 The 13C NMR spectra of these two compounds each
exhibit quartets near 14 ppm (J ) 41.0 Hz) and near 190 ppm
(J ) 48.5 Hz), which are assigned to the dimethylborate carbon
atoms and the boron-bonded pyridyl carbon atoms, respectively.
The number and positions of the remaining peaks in the 13C
NMR spectrum of IV are consistent with their assignment to
the pyridyl ring carbon atoms.
Hydrogen dimethylbis(2-pyridyl)borate, IV, is a colorless
crystalline air-stable compound. It consists of long needles,
rods, or long, narrow platelets that melt at 80-81 °C. It is not
hygroscopic and is nearly odorless.
The molecular structure of IV is illustrated in Figure 2. An
X-ray crystal study of IV established that it is planar, with the
exception of the methyl groups. The boron atom lies in an
approximately tetrahedral environment, as expected.
(10) Trofimenko, S.; Calabrese, J. C.; Domaille, P. J.; Thompson, J. S.
Inorg. Chem. 1989, 28, 1091-1101.
(11) Caffyn, A. J. M.; Feng, S. G.; Dierdorf, A.; Gamble, A. S.; Eldredge,
P. A.; Vossen, M. R.; White, P. S.; Templeton, J. L. Organometallics
1991, 10, 2842-2848.
(12) Looney, A.; Parkin, G. Polyhedron 1990, 9, 265-276.
(13) Calabrese, J. C.; Trofimenko, S. Inorg. Chem. 1992, 31, 4810-4814.
(14) Gilman, H.; Spatz, S. M. J. Org. Chem. 1951, 16, 1485-1494.

2142 Inorganic Chemistry, Vol. 35, No. 7, 1996
Hodgkins and Powell
yields of II in the present work were 89%, in contrast to the
low yields of II reported previously.4
1
A H NMR spectrum of the crude reaction product of the
lithium salt of III with BrBMe2 showed no evidence for the
formation of I in this reaction. Therefore, the formation of I
upon the addition of BrBMe2 to an equimolar quantity of 2-Li-
(py) must occur only during the early stages of this addition.
Synthesis and Structure of Diphenylboronium Dimethyl-
bis(2-pyridyl)borate,16 VI. This compound was formed by
addition of bromodiphenylborane, BrBPh2, to a slurry of the
lithium salt of III, according to eq 2. The conditions were very
similar to those used in the synthesis of II from IV. However,
the product, VI, was much less soluble in the reaction mixture
at ambient temperature than was II, although the solvent
composition and the molar concentration of the reactants were
the same. The yield of crude VI was 94%.
Figure 2. Displacement ellipsoid diagram (50% thermal ellipsoids)
of IV, hydrogen dimethylbis(2-pyridyl)borate.
Table 1. Selected Crystallographic Data
1
The H NMR spectrum of VI shows the expected 2-pyridyl
IV
VI
VII
VIII
ring pattern (doublets at 8.08 and 7.98 ppm, triplets at 7.74 and
7.08 ppm). The unresolved multiplets at ca. 7.25 and 7.02 ppm
are assigned to the phenyl group protons. The position of the
broad singlet at -0.01 ppm is that expected for the methyl
groups. The appearance of this signal is similar to that reported
formula
fw
space
group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
C12H15BN2 C24H24B2N2 (C12H14BN2)2Zn (C12H14BN2)2Ni
198.07
362.07
459.49
452.83
P21/m
(No. 11)
P21/n
(No. 14)
Cc (No. 9)
P1h (No. 2)
8.836(2)
7.1825(5) 10.9169(9) 31.256(3)
8.0334(12) 8.010(2)
7.675(2)
8.819(2)
9.459(2)
115.72(3)
96.89(3)
94.02(3)
567.3(2)
1
1
in the H NMR spectrum of Li(CH3)4 at -51 °C.17
9.2789(11) 22.981(2)
99.81(2) 94.653(9)
580.29(11) 2008.8(4)
18.753(3)
90.83(2)
The sharp singlet at -17.6 ppm in the 11B NMR spectrum
of VI is assigned to the dimethylborate moiety on the basis of
its position and width. The much broader singlet at +4.0 ppm
is assigned to the diphenylboronium moiety. The 11B NMR
spectrum of VI is thus similar to that of II reported earlier.4
4694.8(13)
2
4
8
Fcalcd
g cm-3
T, °C
λ, Å
,
1.134
1.197
1.300
1.326
The 13C NMR spectrum of VI bears some resemblance to
that of II. The quartet at 13.4 ppm (J ) 41.5 Hz) in the
spectrum of VI is assigned to the dimethylborate carbon atoms
on the basis of its position, multiplicity, and coupling constant;
these values are similar to those reported for the dimethylborate
carbon atoms in the spectrum of II (δ ) 14.7 ppm, q, J ) 41.0
Hz).4 The boron-bonded annular carbon atoms most likely give
rise to the quartets at 187.0 ppm (J ) 48.5 Hz)4 and 189.5 ppm
(J ) 47.5 Hz) in the spectra of II and VI, respectively. The
broad signal at 150.0 ppm in the spectrum of VI is assigned to
the boron-bonded phenyl carbon atoms. A comparison of the
spectrum of VI with that of II suggests that the peaks at 119.2,
130.3, 136.9, and 143.9 ppm in the spectrum of VI arise from
the pyridyl carbon atoms.
22
0.710 73
-160
-125
22
1.541 78
0.518
0.710 73
1.064
0.710 73
0.873
0.0308
0.0767
µ, mm-1 0.067
R(Fo)
Rw(Fo )
0.0380
0.1126
0.0460
0.1302
0.0667
0.1958
2
A comparison of the structure of IV with that of the related
compound hydrogen dihydrobis(1-pyrazolyl)borate, H[(pz)2BH2],15
shows some differences. The most obvious difference is that
the hydrogen bonding in IV is intramolecular, while that in
H[(pz)2BH2] is intermolecular. Therefore, at least in the solid
state, molecules of H[(pz)2BH2] exist as hydrogen-bonded
dimers, unlike IV, which is monomeric. The annular bond
lengths in these two compounds are typical of those within the
corresponding heteroaromatic ring systems. The NBN bond
angle in H[(pz)2BH2] (109.7(3)°) is very close to the tetrahedral
bond angle, while the C(pyridyl)BC(pyridyl) bond angle in IV
is slightly larger (112.8(2)°). The pyridyl rings in IV may be
forced apart by intramolecular hydrogen bonding, which appears
to explain both the coplanarity of the pyridyl rings and the fact
that this compound has few acidic properties.
The mass spectrum of VI, like that of II, does not exhibit a
parent ion cluster but does exhibit an intense cluster due to the
loss of CH3. It also shows clusters of peaks corresponding to
the loss from the parent ion of two methyl groups and the loss
of one methyl group and one phenyl group. However, no ion
clusters of significant intensity were observed corresponding
to the loss of phenyl groups without the simultaneous loss of a
methyl group. This observation implies that the boron-carbon
bonds involving phenyl groups are stronger than those involving
methyl groups.
Synthesis of II from IV. A suspension of the lithium salt
of III was generated by adding a solution of n-butyllithium to
a solution of IV at -78 °C. A solution of BrBMe2 was then
added to this slurry, according to eq 2. The reaction conditions
Although a crystal structure of II was attempted, the central
B2C2N2 ring in this compound was found to be disordered. To
obtain some information on this ring system, a crystal study of
VI was performed; the molecular structure of VI is illustrated
in Figure 3. The central ring of VI has a shallow boat
conformation. The annular boron-carbon bonds in VI (1.615-
(3) and 1.617(3) Å) are shorter than those in IV (1.631(3) and
1.632(3) Å). The B-N bonds (both 1.598(3) Å) in VI are
-C4H10
+BrBR2
IV + n-BuLi
8 Li salt of III
8
R2B(µ-py)2BMe2 + LiBr (2)
II (R ) Me)
VI (R ) Ph)
employed were very similar to those used earlier for the
synthesis of II in terms of reaction temperature, solvent
composition, and concentration of the reactants.4 However,
(16) Alternative name: 9,9-dimethyl-10,10-diphenyl-4a,10a-diaza-9,10-
dibora-9,10-dihydroanthracene.
(15) Jung, O.-S.; Jeong, J. H.; Sohn, Y. S. Organometallics 1991, 10, 2217-
(17) Williams, K. C.; Brown, T. L. J. Am. Chem. Soc. 1966, 88, 4134-
2221.
4140.

Derivatives of Dimethylbis(2-pyridyl)borate(1-)
Inorganic Chemistry, Vol. 35, No. 7, 1996 2143
Table 2. Atomic Coordinates (×104) and Equivalent Isotropic
Table 4. Atomic Coordinates (×104) and Equivalent Isotropic
Displacement Parameters (Å2 × 103) for IV
Displacement Parameters (Å2 × 103) for VII
x
y
z
U(eq)a
x
y
z
U(eq)a
C(1)
N(2)
C(3)
C(4)
C(5)
C(6)
C(7)
N(8)
C(9)
C(10)
C(11)
C(12)
B(13)
C(14)
10030(2)
11089(2)
12592(3)
13147(3)
12109(3)
10578(3)
7836(2)
8983(2)
8758(3)
7338(3)
6112(3)
6363(3)
8196(3)
7410(2)
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
4372(3)
2182(2)
1297(2)
1853(3)
3302(3)
4240(3)
56(1)
62(1)
78(1)
85(1)
86(1)
77(1)
55(1)
59(1)
72(1)
81(1)
86(1)
76(1)
57(1)
79(1)
Zn(1A)
N(1A)
C(2A)
C(3A)
C(4A)
C(5A)
C(6A)
B(7A)
C(8A)
1476(1)
1224(10)
725(15)
4377(1)
3744(2)
3548(3)
3120(4)
2868(3)
3066(4)
3517(3)
3753(4)
4112(3)
4155(4)
4467(4)
4748(4)
4701(4)
4398(2)
4914(3)
5089(3)
5483(3)
5727(3)
5558(4)
5141(3)
4920(4)
4794(3)
4924(3)
4805(3)
4535(4)
4422(3)
4543(2)
4003(3)
3404(4)
4471(4)
5234(4)
2022(1)
1484(2)
1328(3)
936(3)
7177(1)
7163(4)
6549(5)
6462(6)
7027(6)
7656(6)
7746(5)
8501(6)
8383(5)
8854(5)
8762(6)
8190(6)
7748(6)
7836(5)
7435(4)
8089(5)
8264(6)
7758(6)
7074(6)
6900(5)
6133(6)
5781(5)
5100(5)
4823(5)
5220(5)
5892(6)
6162(4)
8762(5)
9104(6)
6230(6)
5584(6)
7048(1)
6748(4)
6078(5)
5866(5)
6367(6)
7045(5)
7251(5)
8043(6)
8419(5)
9101(6)
9395(5)
9014(6)
8345(6)
8055(4)
7155(5)
7796(6)
7919(6)
7379(6)
6740(6)
6609(5)
5821(6)
5873(5)
5395(5)
5434(7)
5968(6)
6420(5)
6384(4)
7934(5)
8553(5)
5555(6)
5261(6)
23(1)
19(2)
30(3)
35(3)
32(2)
32(3)
25(2)
30(3)
24(2)
30(2)
34(3)
35(3)
33(3)
24(2)
21(2)
27(2)
30(2)
34(3)
34(3)
25(2)
24(3)
23(2)
27(2)
30(2)
33(2)
30(3)
18(2)
25(2)
48(3)
31(3)
41(3)
20(1)
22(2)
26(2)
27(2)
28(3)
27(2)
23(2)
26(3)
21(2)
27(2)
34(3)
33(2)
31(3)
17(2)
21(2)
33(3)
37(3)
35(3)
35(3)
24(2)
25(3)
24(2)
28(2)
36(3)
32(3)
21(2)
20(2)
27(3)
33(3)
27(2)
35(3)
742(16)
1236(16)
1703(14)
1642(13)
2099(18)
3626(13)
4995(14)
6181(15)
6008(14)
4652(14)
3528(12)
213(10)
3690(2)
-256(2)
-1040(2)
-2505(2)
-3297(3)
-2564(3)
-1075(3)
1529(3)
C(9A)
C(10A)
C(11A)
C(12A)
N(13A)
N(14A)
C(15A)
C(16A)
C(17A)
C(18A)
C(19A)
B(20A)
C(21A)
C(22A)
C(23A)
C(24A)
C(25A)
N(26A)
C(27A)
C(28A)
C(29A)
C(30A)
Zn(1B)
N(1B)
2086(2)
a U(eq) is defined as one-third of the trace of the orthogonalized Uij
tensor.
473(13)
-107(14)
-956(15)
-1165(14)
-549(13)
-856(15)
928(14)
1467(13)
2970(14)
4037(14)
3494(15)
2012(10)
448(13)
Table 3. Atomic Coordinates (×104) and Equivalent Isotropic
Displacement Parameters (Å2 × 103) for VI
x
y
z
U(eq)a
C(1)
N(2)
C(3)
C(4)
C(5)
C(6)
C(7)
N(8)
2089(2)
1371(2)
1558(3)
2468(3)
3241(3)
3024(3)
188(3)
-474(2)
-1853(2)
-2651(3)
-2021(3)
-640(3)
1869(3)
1799(3)
3478(3)
471(3)
1883(2)
1663(3)
2829(3)
4277(3)
4515(3)
3338(3)
-891(2)
-856(3)
-2037(3)
-3299(3)
-3368(3)
-2186(3)
5801(2)
5995(1)
5172(2)
4126(2)
3901(2)
4729(2)
7559(2)
7725(1)
8429(2)
9007(2)
8868(2)
8145(2)
6767(2)
6017(2)
7708(2)
7262(2)
8256(2)
9510(2)
10373(2)
10015(2)
8784(2)
7926(2)
7060(2)
7766(2)
7637(2)
6785(2)
6064(2)
6203(2)
4333(1)
3781(1)
3349(1)
3434(1)
3985(1)
4421(1)
4712(1)
4150(1)
4031(1)
4455(1)
5029(1)
5149(1)
4855(1)
5470(1)
4910(1)
3605(1)
3472(1)
3567(1)
3429(1)
3184(1)
3076(1)
3219(1)
3050(1)
2544(1)
2073(1)
2093(1)
2588(1)
3058(1)
19(1)
18(1)
24(1)
27(1)
28(1)
26(1)
20(1)
18(1)
23(1)
27(1)
30(1)
28(1)
22(1)
28(1)
26(1)
18(1)
18(1)
23(1)
29(1)
28(1)
27(1)
23(1)
19(1)
22(1)
27(1)
31(1)
31(1)
25(1)
2711(18)
-1928(15)
-1867(15)
6082(1)
C(9)
C(10)
C(11)
C(12)
B(13)
C(14)
C(15)
B(16)
C(17)
C(18)
C(19)
C(20)
C(21)
C(22)
C(23)
C(24)
C(25)
C(26)
C(27)
C(28)
7281(10)
7031(13)
7560(13)
8462(14)
8695(13)
8098(12)
8411(15)
6613(12)
6233(13)
4716(14)
3655(13)
4078(15)
5593(10)
6336(11)
6836(15)
6920(15)
6446(15)
5984(15)
5935(13)
5479(15)
3997(12)
2614(12)
1428(15)
1538(14)
2858(13)
4075(10)
9564(13)
9521(15)
7139(14)
4944(15)
C(2B)
C(3B)
C(4B)
C(5B)
C(6B)
B(7B)
C(8B)
C(9B)
687(3)
849(3)
1247(3)
1449(4)
1577(3)
1450(3)
1571(4)
1826(4)
1951(3)
1831(3)
2655(2)
2822(3)
3254(4)
3525(4)
3359(3)
2912(3)
2711(4)
2352(3)
2330(4)
2019(4)
1700(4)
1733(3)
2032(2)
1882(4)
1124(4)
2470(3)
3084(3)
C(10B)
C(11B)
C(12B)
N(13B)
N(14B)
C(15B)
C(16B)
C(17B)
C(18B)
C(19B)
B(20B)
C(21B)
C(22B)
C(23B)
C(24B)
C(25B)
N(26B)
C(27B)
C(28B)
C(29B)
C(30B)
a U(eq) is defined as one-third of the trace of the orthogonalized Uij
tensor.
longer than the 1.55 Å average found in many pyrazaboles.18
However, B-N bond lengths associated with the Ph2B moiety
in the pyrazabole Ph2B(µ-pz)2BCl2 are only slightly shorter
(1.575(5) and 1.586(5) Å)19 than those found in VI.
The methyl carbon-boron bond lengths in VI are comparable
to those in IV. However, the phenyl carbon-boron bonds in
VI (1.622 Å average) are slightly longer than the phenyl
carbon-boron bonds in the pyrazabole Ph2B(µ-pz)2BCl2 (1.608-
(5) and 1.609(5) Å).19 Steric effects are probably responsible
for the fact that the PhBPh bond angle is wider than the MeBMe
bond angle in VI. In VI, the NBN bond angle (108.0(2)°) is
slightly narrower than the annular CBC bond angle; a “scis-
a U(eq) is defined as one-third of the trace of the orthogonalized Uij
tensor.
soring” effect caused by the bulky phenyl substituents may be
responsible for this difference.
(18) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A.
G; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1-S19.
(19) Clarke, C. M.; Das, M. K.; Hanecker, E.; Mariategui, J. F.; Niedenzu,
K.; Niedenzu, P. M.; No¨th, H.; Warner, K. R. Inorg. Chem. 1987, 26,
2310-2317.
Synthesis of the Divalent Metal Complexes of III. The
nickel(II) and zinc(II) complexes were synthesized by treating
a solution of the metal(II) chloride in 95% ethanol with an
ethanolic solution of IV at ambient temperature, according to

2144 Inorganic Chemistry, Vol. 35, No. 7, 1996
Hodgkins and Powell
Table 5. Atomic Coordinates (×104) and Equivalent Isotropic
Table 6. Selected Interatomic Distances (Å) and Angles (deg) for
IV, Hydrogen Dimethylbis(2-pyridyl)borate
Displacement Parameters (Å2 × 103) for VIII
x
y
z
U(eq)a
Bond Lengths
B(13)-C(1)
B(13)-C(7)
B(13)-C(14)
1.631(3)
1.632(3)
1.637(2)
N(8)-H(1a)
N(2)-H(1a)
N(2)‚‚‚N(8)
1.08(2)
1.59(2)
2.60(2)
Ni(1)
C(1)
N(2)
C(3)
C(4)
C(5)
C(6)
C(7)
N(8)
C(9)
C(10)
C(11)
C(12)
B(13)
C(14)
C(15)
0
0
0
28(1)
33(1)
30(1)
37(1)
45(1)
50(1)
46(1)
33(1)
29(1)
35(1)
45(1)
52(1)
47(1)
37(1)
44(1)
57(1)
2538(3)
2015(3)
2877(4)
4297(4)
4835(4)
3965(4)
1642(3)
1058(3)
1222(4)
1971(4)
2549(5)
2372(4)
1497(4)
-582(4)
2424(5)
-1941(3)
-1176(3)
-1268(3)
-2147(4)
-2972(4)
-2861(4)
399(3)
1267(3)
2973(3)
3931(4)
3106(4)
540(3)
-385(2)
-1577(3)
-1935(4)
-1053(4)
171(4)
2993(3)
2175(2)
2892(3)
4449(3)
5319(4)
4598(3)
2028(4)
1374(4)
3138(4)
Bond Angles
112.8(2) C(7)-N(8)-H(1a)
C(1)-B(13)-C(7)
108.5(6)
127.9(6)
C(14)-B(13)-C(14)a 110.4(2) C(9)-N(8)-H(1a)
C(1)-B(13)-C(14)
C(7)-B(13)-C(14)
109.3(2) N(8)-H(1a)‚‚‚N(2) 153.5(6)
107.5(2)
a Coordinates transformed by (x, 0.5 - y, z).
Table 7. Selected Bond Lengths (Å) and Angles (deg) for VI,
Diphenylboronium Dimethylbis(2-pyridyl)borate
1358(4)
-1672(4)
-2527(4)
-2469(4)
Bond Lengths
B(13)-C(1)
B(13)-C(7)
B(13)-C(14)
B(13)-C(15)
1.617(3)
1.615(3)
1.639(3)
1.649(3)
B(16)-N(2)
B(16)-N(8)
B(16)-C(17)
B(16)-C(23)
1.598(3)
1.598(3)
1.616(3)
1.627(3)
a U(eq) is defined as one-third of the trace of the orthogonalized Uij
tensor.
Bond Angles
109.8(2) N(2)-B(16)-N(8)
C(1)-B(13)-C(7)
108.0(2)
C(14)-B(13)-C(15) 109.1(2) C(17)-B(16)-C(23) 112.4(2)
C(1)-B(13)-C(14)
C(1)-B(13)-C(15)
C(7)-B(13)-C(14)
C(7)-B(13)-C(15)
109.2(2) N(2)-B(16)-C(17)
109.3(2) N(2)-B(16)-C(23)
110.9(2) N(8)-B(16)-C(17)
108.6(2) N(8)-B(16)-C(23)
108.6(2)
110.1(2)
108.7(2)
109.0(2)
Table 8. Selected Bond Lengths (Å) and Angles (deg) for VII,
Bis[dimethylbis(2-pyridyl)borato-N,N ′]zinc(II)
a
b
Bond Lengths
Zn-N(1)
1.988(7)
2.018(8)
2.024(8)
2.019(7)
2.019(8)
1.63(2)
1.66(2)
1.65(2)
1.65(2)
1.64(2)
1.64(2)
1.61(2)
1.62(2)
Zn-N(13)
2.044(9)
2.024(8)
2.024(8)
1.63(2)
1.68(2)
1.62(2)
1.64(2)
1.61(2)
1.63(2)
1.65(2)
1.63(2)
Zn-N(14)
Zn-N(26)
B(7)-C(6)
B(7)-C(8)
B(7)-C(27)
B(7)-C(28)
B(20)-C(19)
B(20)-C(21)
B(20)-C(29)
B(20)-C(30)
Figure 3. Displacement ellipsoid diagram (50% thermal ellipsoids)
of VI, diphenylboronium dimethylbis(2-pyridyl)borate. The hydrogen
atoms have been omitted for clarity.
eq 3. It should be noted that IV is both the ligand source and
the proton acceptor in reactions of this type.
Bond Angles
N(1)-Zn-N(13)
97.0(3)
97.2(3)
96.5(3)
97.2(3)
4IV + MCl2 f 2IV‚HCl + M{(µ-py)2BMe2}2 (3)
M ) Zn, Ni
N(14)-Zn-N(26)
N(1)-Zn-N(26)
105.5(3)
141.1(3)
103.2(3)
112.3(3)
110.0(8)
110.5(9)
107.9(9)
111.1(9)
108.4(9)
108.9(10)
110.0(8)
109.0(9)
109.8(8)
111.9(9)
107.4(9)
108.6(8)
102.7(3)
143.5(4)
102.6(3)
114.6(3)
110.9(8)
106.2(9)
106.4(8)
111.3(9)
110.2(8)
111.5(9)
111.2(8)
110.2(9)
106.5(8)
111.0(9)
107.6(8)
110.2(9)
N(1)-Zn-N(14)
N(13)-Zn-N(14)
N(13)-Zn-N(26)
C(6)-B(7)-C(8)
The metal complexes precipitate from the reaction mixtures
over the course of several minutes; the reaction mixtures were
stirred for several hours to ensure maximum yields in each case
(>90%). Although the zinc complex was somewhat soluble in
the reaction medium, the nickel complex was not.
Both metal complexes of III are air-stable, high-melting solids
that are insoluble in both water and hexane. Bis[dimethylbis-
(2-pyridyl)borato-N,N ′]zinc(II), VII, is soluble in such solvents
as tetrahydrofuran, methylene chloride, chloroform, carbon
tetrachloride, acetone, ethyl acetate, and toluene at ambient
temperature. Bis[dimethylbis(2-pyridyl)borato-N,N ′]nickel(II),
VIII, is moderately soluble only in tetrahydrofuran at room
temperature and in benzene, dimethyl sulfoxide, and o-xylene
at higher temperatures.
Characterization of the Metal Complexes of III. NMR
spectra of the nickel complex could not be obtained due to its
insolubility in the available deuterated solvents. The 1H NMR
spectrum of the zinc complex in CDCl3 showed the expected
2-pyridyl ring pattern of two doublets and two triplets. The
methyl signal consisted of a broadened singlet at 0.34 ppm,
C(27)-B(7)-C(28)
C(6)-B(7)-C(27)
C(6)-B(7)-C(28)
C(8)-B(7)-C(27)
C(8)-B(7)-C(28)
C(19)-B(20)-C(21)
C(29)-B(20)-C(30)
C(19)-B(20)-C(29)
C(19)-B(20)-C(30)
C(21)-B(20)-C(29)
C(21)-B(20)-C(30)
somewhat farther downfield than those corresponding to the
boron-bonded methyl protons in other compounds described in
this work but not as far downfield as those in bis[dimethylbis-
(1-pyrazolyl)borato-N,N ′]zinc(II), Zn[(µ-pz)2BMe2]2, which are
located at 0.51 ppm.20
(20) Breakell, K. R.; Patmore, D. J.; Storr, A. J. Chem. Soc., Dalton Trans.
1975, 749-754.

Derivatives of Dimethylbis(2-pyridyl)borate(1-)
Inorganic Chemistry, Vol. 35, No. 7, 1996 2145
Table 9. Selected Bond Lengths (Å) and Angles (deg) for VIII,
Bis[dimethylbis(2-pyridyl)borato-N,N ′]nickel(II)
Bond Lengths
Ni-N(2)
B(13)-C(1)
B(13)-C(14)
1.906(2)
1.638(4)
1.639(4)
Ni-N(8)
B(13)-C(7)
B(13)-C(15)
1.902(2)
1.637(4)
1.623(4)
Bond Angles
89.34(9) N(2)-Ni-N(8)a
102.7(2)
N(2)-Ni-N(8)
90.66(9)
C(1)-B(13)-C(7)
C(14)-B(13)-C(15) 109.5(3)
C(1)-B(13)-C(14) 110.6(2)
C(1)-B(13)-C(15)
C(7)-B(13)-C(15)
111.9(3)
111.6(2)
C(7)-B(13)-C(14) 110.4(2)
a Coordinates transformed by (-x, -y, -z).
Figure 5. Displacement ellipsoid diagram (50% thermal ellipsoids)
of VIII, bis[dimethylbis(2-pyridyl)borato-N,N ′]nickel(II).
occupies a pseudotetrahedral environment. As previously noted,
the 1H NMR spectrum of this complex in CDCl3 exhibited only
one signal due to the methyl protons, implying that in solution
the Zn(NC)2B rings either are planar or undergo rapid boat-to-
boat interconversions. In the solid state, however, the Zn(NC)2B
rings in this complex exist in pronounced boat-shaped confor-
mations. As a result, this complex is chiral in the solid state;
the two enantiomers are present in equimolar amounts.
There is little variation in either the Zn-N bond lengths
(1.988(7)-2.044(9) Å, 2.018 Å average of eight) or the
intraannular NZnN bond angles (those in which the nitrogen
atoms belong to the same Zn(NC)2B ring) (96.5(3)-97.2(3)°,
97.0° average of four) in VII. The Zn-N bond lengths in this
complex are thus comparable to those in the only bis[bis(1-
pyrazolyl)borato]zinc(II) complex whose structure has been
determined, Zn((pz*)(pz**)BH2)2, where Hpz* ) 3,5-dimeth-
ylpyrazole and Hpz** ) 3,5-diphenylpyrazole (1.982(2)-2.019-
(2) Å, 1.998 average of four).22 However, the intraannular
NZnN bond angles in Zn((pz*)(pz**)BH2)2 (100.3(1), 100.9-
(1)°) are slightly wider than those in VII. Furthermore, one of
the interannular NZnN bond angles in each enantiomer of VII
is much wider than the others (141.1(3)° vs 103.2(3), 105.5(3),
112.3(3)°; 143.5(4)° vs 102.6(3), 102.7(3), 114.6(3)°). By
contrast, there is much less variation in the interannular NZnN
bond angles in Zn((pz*)(pz**)BH2)2 (106.4(1)-119.5(1)°,
113.9° average of four). Crystal packing forces, coupled with
the inherent polarizability of the zinc atoms, may be responsible
for these variations.
Figure 4. Displacement ellipsoid diagram (50% thermal ellipsoids)
of the two molecules of VII, bis[dimethylbis(2-pyridyl)borato-N,N ′]-
zinc(II).
The shape of the methyl signal in VII may be due to a boat-
to-boat interconversion of the Zn(NC)2B rings that is fairly rapid
on the NMR time scale. Such a boat-to-boat interconversion
has been observed in complexes such as Ni[(µ-pz)2GaMe2]2.21
However, the line shape of this signal is similar to that reported
for LiB(CH3)4 at -51 °C,17 and differences in transverse
relaxation times, T2, between the methyl protons in VII and
the methyl protons in other compounds described in this work
cannot be ruled out.
As expected, the 11B NMR spectrum of the zinc complex
displays one singlet at -16.6 ppm (h1/2 ) 15.0 Hz); its position
and line width are similar to those found for the borate boron
atoms in II,4 IV, and VI.
The mass spectra of these two complexes have much in
common. Neither exhibits parent ion clusters, but ion clusters
were observed that correspond to the loss of one methyl group
in each case. An ion cluster corresponding to the species
M(py)3BMe+ was also observed in each complex. The mass
spectra of both complexes also contain ion clusters correspond-
ing to the ions (py)2BMe2+, (py)2BMe+, and (py)BMe+; these
arise from ligand fragmentation. An ion cluster was also
observed in the spectra of both complexes that may arise from
an ion with the structure MeB(µ-py)3BMe+. If this structure is
correct, a significant rearrangement of the parent ion is
implicated.
The molecular structure of the nickel complex, VIII, is shown
in Figure 5. An X-ray crystal study of this compound showed
that the nickel atom is surrounded by the ligand nitrogen atoms
in a square planar arrangement. As in the case of the zinc
complex, the Ni(NC)2B rings exist in a pronounced boat-shaped
conformation. This conformation places one of the methyl
group hydrogen atoms almost directly over the nickel atom. The
21
complexes Ni[(µ-pz)2BGaMe2]2 and Ni[(µ-pz)2BR2]2 (R )
C2H5,23,24 C4H924) have a similar shape. The Ni-N bond lengths
in VIII are 0.116 Å shorter than the Zn-N bond lengths in
VII. This difference may be ascribed to a limited degree of
overlap of the empty ligand π* orbitals with filled dxz and dyz
orbitals on the metal atom in the nickel complex.
+
An ion cluster corresponding to the species M(py)2BMe2
(loss of one ligand per molecule) was observed only for the
zinc complex. On the other hand, a significant ion cluster
corresponding to the loss of two methyl groups from the parent
ion as well as an ion cluster corresponding to the species
Some interesting comparisons can be made between the
dimethylbis(2-pyridyl)borato ligand and bis(1-pyrazolyl)borato
+
M(py)BCH2 was observed only for the nickel complex.
The molecular structure of VII is shown in Figure 4. An
X-ray crystal study of this compound showed that the zinc atom
(22) Capparelli, M. V.; Agrifoglio, G. J. Crystallogr. Spectrosc. Res. 1992,
22, 651-657.
(23) Echols, H. M.; Dennis, D. Acta Crystallogr., Sect. B 1974, B30, 2173-
2176.
(21) Herring, F. G.; Patmore, D. J.; Storr, A. J. Chem. Soc., Dalton Trans.
1975, 711-717.
(24) Trofimenko, S. J Am. Chem. Soc. 1967, 89, 6288-6294.

2146 Inorganic Chemistry, Vol. 35, No. 7, 1996
Hodgkins and Powell
calcd from data
Table 10. Shape of the Ligands in Ni((py)2BMe2)2, VIII, and [Bis(pyrazolyl)borato]nickel(II) Complexes
dihedral angle
between heteroaromatic
rings in ligand, deg
dihedral angle
between MN2 plane
dist from boron to
N2E2 plane,a Å
dist from nickel to
N2E2 plane,a Å
and heteroaromatic ring, deg
given in ref
Ni((py)2BMe2)2
Ni((pz)2BH2)2
Ni((pz)2BEt2)2
Ni((pz)2BPh2)2
66.3(1)
68.3
66.6
0.728(5)
0.707
0.711
0.946(1)
0.700
0.714
52.9(1), 52.3(1)
47.8, 28.9
44.7, 42.0
44.9
this work
25a
23
63.0
0.695
0.788
26
a See Figure 6.
complex. X-ray crystal studies of these complexes showed that
the environment around the metal atom is pseudotetrahedral for
the zinc complex and square planar for the nickel complex. The
M(NC)2B ring adopts a boat-shaped conformation in each
complex. This conformation leads to the zinc complex existing
as an enantiomeric pair in the solid state.
Figure 6. The N2E2 plane in bis(2-pyridyl)borato complexes (E ) C,
n ) 4) and bis(1-pyrazolyl)borato complexes (E ) N, n ) 3).
Experimental Section
General Considerations. Bromodimethylborane, 2-bromopyridine,
and hexane were purchased from Aldrich and used as received. Diethyl
ether (Mallinckrodt) was stirred over sodium metal and then filtered
before use. A solution of n-butyllithium in hexane, ca. 1.6 M, was
purchased from Aldrich; its precise concentration was determined by
titration.27 A sample of bromodiphenylborane was kindly donated by
Professor Emeritus Dr. Kurt Niedenzu of the University of Kentucky.
Reagent grade anhydrous zinc(II) chloride (J. T. Baker, Inc.) was dried
for 30 min at 110 °C immediately before use. Temperatures of -78
°C were attained using a dry ice-acetone bath. Transfer of air-sensitive
solutions was performed using needle and cannula techniques under a
nitrogen cover.28
NMR spectra were recorded on a Bruker 500-MHz NMR spectrom-
eter. Chemical shift data are given in ppm with positive values
indicating downfield shifts from the reference (external (CH3)4Si for
1H and 13C NMR, external (C2H5)2O·BF3 for 11B NMR); h1/2 ) peak
width at half-height. Mass spectral data were obtained on a Kratos
MS-80 spectrometer operating in the electron impact mode and are
listed to m/z 100 (referenced to the species with 11B, 58Ni, and 64Zn)
with relative abundances in parentheses (abbreviations: M ) parent
ion, Me ) CH3, Ph ) C6H5, py ) C5H4N); the observed isotope pattern
matches the calculated isotope pattern for M - CH3. Infrared spectra
were obtained using a BOMEM Michelson MB-100 FTIR spectrometer
having 4 cm-1 resolution; samples were prepared as KBr pellets.
Melting points (uncorrected) were determined on a Mel Temp block.
Elemental analyses for IV and VI were performed by Galbraith
Laboratories, Knoxville, TN; elemental analyses for VII and VIII were
performed by Desert Analytics Laboratory, Tucson, AZ.
X-ray Crystal Data. The cell parameter and intensity data were
collected on Siemens P4 diffractometers equipped with graphite
monochromators. Selected crystallographic data are reported in Table
1. Atomic coordinates and thermal parameters for non-hydrogen atoms
for IV, VI, VII, and VIII are listed in Tables 2-5, respectively.
Selected bond lengths and bond angles for IV, VI, VII, and VIII are
listed in Tables 6-9, respectively. Data for compounds VI and VII
were collected at low temperatures using locally modified chilled N2
gas devices. The space group for VII was assigned on the basis of
systematic absences and the fact that the metals could not be related to
one another by a center of symmetry because of the existing symmetry.
All structures were solved by direct methods and refined by full-matrix
least-squares methods on F 2. All non-hydrogen atoms were refined
anisotropically. Hydrogen atom positions were initially calculated and
then refined using a riding model. The XSCANS program29 was used
to collect intensity data. The SHELXL-93 program30 was used for atom
parameter refinement. All other calculations were performed with the
ligands. Data regarding the general shape and orientation of
the ligands in the square planar complexes VIII and Ni-
26
((pz)2BR2)2 (pz ) pyrazolyl, R ) H,25 C2H5,23 and C6H5
)
are given in Table 10. A comparison of the data in the first
two columns of this table reveals that the two types of ligands
have similar shapes. However, the data in the third column
indicate that the ligands in VIII are “tilted” to a greater extent,
relative to the NiN4 plane, than are the ligands in the bis(1-
pyrazolyl)borato complexes. Therefore, the acute dihedral
angles between the NiN4 plane and the planes defined by each
of the heteroaromatic rings are significantly wider in VIII than
in the bis(1-pyrazolyl)borato complexes. This implies that the
degree of overlap between the filled nickel dxz and dyz orbitals
and empty π* orbitals in the heteroaromatic rings should be
somewhat less in VIII than in the bis(1-pyrazolyl)borato
complexes. This may explain the slightly longer Ni-N bond
lengths in VIII (1.902(2), 1.906(2) Å) as compared to those in
Ni((pz)2BH2)2 (1.889(2), 1.895(2) Å),25b Ni((pz)2BEt2)2 (1.875(8),
1.880(4) Å),23,25b or Ni((pz)2BPh2)2 (1.891(3) Å).26
Summary
The reaction of 2-lithiopyridine with bromodimethylborane,
BrBMe2, in a 2:1 mole ratio, respectively, and the subsequent
hydrolysis of the crude reaction product lead to the formation
of hydrogen dimethylbis(2-pyridyl)borate, IV, which was
isolated in 44% yield. Strong intramolecular hydrogen bonding
makes molecules of this compound planar (except for the methyl
group atoms) and appears to account for the lack of acidic
properties of this compound in most of its reactions. Derivatives
of the dimethylbis(2-pyridyl)borate anion can be synthesized
by two general methods:
(1) The lithium salt of this anion can be regenerated by
allowing IV to react with an equimolar quantity of n-butyl-
lithium; subsequent reaction of this salt with equimolar quantities
of either bromodimethylborane or bromodiphenylborane yields
dimethylboronium dimethylbis(2-pyridyl)borate, II, or diphe-
nylboronium dimethylbis(2-pyridyl)borate, VI, respectively, in
ca. 90% yield. An X-ray crystal study of VI showed that the
central B(NC)2B ring is boat-shaped.
(2) Reaction of IV with either anhydrous ZnCl2 or NiCl2·6H2O
in a 4:1 molar ratio, respectively, gives 1 mole of the
corresponding bis[dimethylbis(2-pyridyl)borato-N,N ′]metal(II)
(27) Gilman, H.; Haubein, A. H. J. Am. Chem. Soc. 1944, 66, 1515-1516.
(28) Shriver, D. F.; Drezdon, M. A. The Manipulation of Air-SensitiVe
Compounds; John Wiley & Sons: New York, 1969; pp 13-24.
(29) XSCANS Software Users Guide; Siemens Analytical X-ray Instru-
ments: Madison, WI, 1993.
(30) Sheldrick, G. M. SHELXL 93: Program for the Refinement of Crystal
Structures. University of Goettigen, Germany, 1993.
(25) (a) Clemente, D. A.; Cingi-Biagini, M. Inorg. Chem. 1987, 26, 2350-
2359. (b) Echols, H. M.; Dennis, D. Acta Crystallogr., Sect. B 1976,
B32, 1627-1630.
(26) Cotton, F. A.; Murillo, C. A. Inorg. Chim. Acta 1976, 17, 121-124.

Derivatives of Dimethylbis(2-pyridyl)borate(1-)
Inorganic Chemistry, Vol. 35, No. 7, 1996 2147
SHELXL+ package.31 The complex, neutral-atom scattering factors
were taken from ref 32.
Spectral Data for Dimeric Dimethyl(2-pyridyl)borane, I. IR
(cm-1): 2933 (vs), 2921 (vs), 1619 (s), 1485 (s), 1283 (s), 1191 (m),
1133 (m), 1095 (vs), 1007 (vs), 970 (vs), 758 (vs), 742 (vs).
((py)BMe). IR (cm-1): 2909 (vs), 2854 (s), 2508 (m, broad, N-H···N),
2145 (m, broad, N-H···N), 1626 (vs), 1584 (vs), 1509 (s), 975 (vs),
937 (s), 740 (vs), 606 (m, broad).
Synthesis of Dimethylboronium Dimethylbis(2-pyridyl)borate, II,
from IV. A sample of 1.69 M n-butyllithium in hexane (1.9 mL, 3.2
mmol) was added to a stirred solution of 0.628 g (3.17 mmol) of IV in
a mixture of 7.1 mL of hexane and 20.1 mL of anhydrous diethyl ether
at -78 °C under a nitrogen cover. A large amount of white solid
appeared. The resulting mixture was stirred for 10 min, and then a
chilled (-78 °C) solution of 0.388 g (3.21 mmol) of bromodimeth-
ylborane in 5.2 mL of hexane was added over a 2 min period. Stirring
was continued while the reaction mixture warmed to ambient temper-
ature overnight; most of the solid dissolved during this time.
Synthesis of Hydrogen Dimethylbis(2-pyridyl)borate, IV.
A
solution of 2-bromopyridine (10.516 g, 66.6 mmol) in 52 mL of
anhydrous diethyl ether was chilled to -78 °C. It was then added
over a 10 min period to a stirred solution of 1.60 M n-butyllithium in
hexane (41.6 mL, 66.6 mmol) and 150 mL of diethyl ether at -78 °C
under a nitrogen cover. The solution became red-brown to burgundy
red. After 10 min, a solution of bromodimethylborane (4.018 g, 33.3
mmol) in 52 mL of hexane was added over a 20 min period. Stirring
was continued while the reaction mixture warmed to room temperature
over a 17 h period.
Subsequently, 0.81 g of a brownish solid was removed by filtration.
The filtrate was stirred for 75 min with activated charcoal and was
filtered again. The tea-colored filtrate was shaken with 100 mL of
water, and the lower (aqueous) layer was discarded. The organic layer
was shaken with three 75 mL portions of aqueous 1.00 M acetic acid
solution.
The organic layer was subsequently evaporated to dryness; the
residue, which consisted of a mixture of dark oily materials and
crystalline solids, weighed 1.34 g. A 1H NMR spectrum of this residue
in CDCl3 indicated that it was primarily a mixture of I and II.4
Integration of this spectrum revealed that the area ratio of the 0.36
ppm peak to the 0.22 ppm peak was ca. 1:2.65, respectively.
The combined acetic acid layers were made alkaline with 75 mL of
aqueous 6.0 M NaOH solution. A yellowish solid, mixed with a little
darker oil, appeared. The oil was removed mechanically. After 30
min, the mixture was filtered, and the crude IV was washed thoroughly
with water and air-dried for 30 min. The combined aqueous filtrate
and washings also gave a little solid which, after being filtered off,
washed, and dried, brought the total mass of crude IV to 3.061 g (46%
yield).
A colorless, gelatinous solid (0.084 g) was then removed by filtration,
and the filtrate was evaporated to dryness under a nitrogen flow. The
residue (1.042 g) was treated with three 10 mL portions of aqueous
1.00 M acetic acid solution. The undissolved solid was dried and
identified as II by its melting point, 175-177 °C (lit.4 178-179 °C),
and 1H NMR spectrum in CDCl3. The 1H NMR spectrum of this solid
showed no evidence that it contained either I (no signals near 0.22
ppm) or IV (no signals in the 8.4-18.8 ppm region). This solid
weighed 0.678 g (89% yield). Neither the mass nor the melting point
of this solid changed after it was treated with three additional 10 mL
portions of aqueous 1.00 M acetic acid solution and recovered in the
manner described above. IR (cm-1): 2923 (vs), 1620 (vs), 1482 (vs),
1299 (s), 1108 (vs), 978 (s), 958 (s), 744 (vs).
Synthesis of Diphenylboronium Dimethylbis(2-pyridyl)borate,
VI. A sample of 1.69 M n-butyllithium in hexane (2.21 mL, 3.73
mmol) was added to a stirred solution of 0.735 g (3.71 mmol) of IV in
a mixture of 8.3 mL of hexane and 23.3 mL of diethyl ether at -78 °C
under a nitrogen cover. The resulting slurry was stirred for ca. 15 min,
and then a solution of 0.916 g (3.74 mmol) of bromodiphenylborane
in 5.8 mL of hexane was added over a 1 min period. Stirring was
continued while the reaction mixture warmed to room temperature over
a 15 h period.
The combined solids were stirred with 150 mL of hexane, and a
small amount of tarry material was removed by decantation. An
extraction and precipitation procedure similar to that used for the filtered
reaction mixture was employed for the resulting yellowish solution in
which two 40 mL portions of aqueous 1.00 M acetic acid and 40 mL
of aqueous 6.0 M NaOH were used. The resulting cream-colored solid
was washed with three 30 mL portions of water, and the solid was
air-dried (mp 76-79 °C). The 1H NMR spectrum of this solid in CDCl3
solution was consistent with that expected for IV. Yield: 2.872 g,
44%. Repeated recrystallization from hexane gave colorless needles
(mp 80-81 °C). Part of the latter sample (51 mg) was sublimed in an
evacuated ampule at bath temperatures of 60-65 °C (the solid was
deposited at a temperature of 30-35 °C) over a 3 month period to
give an analytical sample (mp 80-81 °C).
This compound is very soluble in polar organic solvents such as
acetone, chloroform, methylene chloride, 95% ethanol, diethyl ether,
or tetrahydrofuran. It is less soluble in aliphatic hydrocarbons such as
hexane. It is soluble in aqueous acidic solutions, only very sparingly
soluble in water (ca. 4 mg/100 mL at 25 °C), and insoluble in aqueous
alkaline solutions.
Anal. Calcd for C12H15BN2 (Mr ) 198.08): C, 72.77; H, 7.63; B,
5.46; N, 14.14. Found: C, 72.95; H, 7.78; B, 5.49; N, 14.11. 1H NMR
(δ, CDCl3): 18.70 (1H, s), 8.34 (2H, d, J ) 5.4 Hz), 7.81 (2H, d, J )
8.0 Hz), 7.68 (2H, td, J ) 7.7, 1.3 Hz), 7.10 (2H, td, J ) 6.7, 1.3 Hz),
0.07 (6H, partially resolved q, J ) 4.4 Hz). 1H NMR (δ, CD2Cl2):
17.92 (1H, s), 8.39 (2H, d, J ) 5.5 Hz), 7.80 (2H, d, J ) 8.0 Hz), 7.71
(2H, td, J ) 7.8, 1.5 Hz), 7.14 (2H, td, J ) 6.4, 1.5 Hz), 0.04 (6H,
partially resolved q, J ) 4.5 Hz). 11B NMR (δ, CDCl3): -18.4 (s,
h1/2 ) 5.6 Hz). 13C{1H} NMR (δ, CDCl3): 191.8 (q, J ) 48.5 Hz),
140.1, 136.7, 129.3, 118.8, 13.6 (q, J ) 41.0 Hz). Mass spectrum,
m/z(+) (relative intensity): 198.1343 (1.7) (M, calc 198.1328), 197
(2.2) (M - H), 183 (100) (M - Me), 120 (20) (M - (py)), 104 (99)
The reaction mixture was filtered, and the recovered solid was air-
dried. It was treated with a total of 30 mL of aqueous 1.00 M acetic
acid solution. The mixture was filtered, and the crude product was
washed with a total of 20 mL of water and air-dried (0.959 g). It
exhibited a H NMR spectrum that was consistent with the desired
product.
1
The solvent was removed from the filtrate from the original reaction
mixture, and the residue was treated with a total of 20 mL of 1.00 M
aqueous acetic acid solution. The resulting mixture was filtered, the
insoluble portion was air-dried and then stirred with 80 mL of hot
hexane, and 0.022 g of a brownish, tarry residue was removed by
filtration. The resulting hexane solution was evaporated to dryness to
1
give 0.303 g of a yellowish solid. A H NMR spectrum of this solid
in CDCl3 indicated that it consisted mostly of the desired product (total
yield of crude VI: 94%). When recrystallized from a hexane-THF
mixture, VI melts at 213-215 °C; the melt was light yellow. When
this sample was reheated, it melted at ca. 200-213.5 °C; this melt
was orange.
Diphenylboronium dimethylbis(2-pyridyl)borate, VI, is air-stable.
It is soluble in acetone, diethyl ether, benzene, tetrahydrofuran,
chloroform, and methylene chloride at ambient temperature. It is also
soluble in hot 1-propanol and hot hexane and sparingly soluble in hot
ethanol. It is insoluble in aqueous solvents as well as hot methanol.
Experiments involving heating evacuated ampules containing this
compound indicate that the volatility of this compound is very low.
Anal. Calcd for C24H24B2N2 (Mr ) 362.09): C, 79.61; H, 6.68; B,
5.97; N, 7.74. Found: C, 79.27; H, 6.50; B, 5.91; N, 7.93. 1H NMR
(δ, CDCl3): 8.08 (1H, d, J ) 6.0 Hz), 7.98 (1H, d, J ) 8.5 Hz), 7.74
(1H, t, J ) 7.7 Hz), 7.26-7.23 (3H, m), 7.08 (1H, t, J ) 7.2 Hz),
7.03-7.01 (2H, m), -0.01 (3H, s, h1/2 ) 9 Hz). 11B NMR (δ,
CDCl3): 4.0 (s, h1/2 ) 174 Hz), -17.6 (s, h1/2 ) 11.6 Hz). 13C{1H}
NMR (δ, CDCl3): 189.5 (q, J ) 47.5 Hz), 150.0 (broad, low intensity),
143.9, 136.9, 133.3, 130.3, 127.7, 126.6, 119.2, 13.4 (q, J ) 41.5 Hz).
Mass spectrum, m/z(+) (relative intensity): 347.1909 (100) (M - Me,
calc 347.1891), 332 (6) (M - 2Me), 270 (11) (M - Me, Ph), 269 (10)
(M - Me, (py)), 166 (15) ((py)BPh).
(31) SHELXTL+, Version 4.0; Siemens Analytical X-ray Instruments:
Madison, WI, 1990.
(32) International Tables for Crystallography; Kluwer: Boston, 1993; Vol.
C, Tables 6.1.1.4, 4.2.6.8, and 4.2.4.2.

2148 Inorganic Chemistry, Vol. 35, No. 7, 1996
Hodgkins and Powell
Synthesis of Bis[dimethylbis(2-pyridyl)borato-N,N ′]zinc(II). A
solution of 68.1 mg of anhydrous ZnCl2 was dissolved in 22 mL of
95% ethanol. The resulting solution was added to a stirred solution of
0.4221 g of IV in 28 mL of 95% ethanol, and a colorless precipitate
formed slowly. The resulting mixture was stirred at room temperature
overnight and then filtered. The colorless solid was washed with five
5 mL portions of 95% ethanol and then air-dried. It weighed 0.1977
g and began to melt at 237 °C (in vacuo) with extensive decomposition.
A sample of this solid that was recrystallized from a hexane-THF
mixture began to melt at 244 °C (in vacuo) with extensive decomposi-
tion.
powder weighed 86.8 mg (91% yield) after it was washed with five 5
mL portions of 95% ethanol and air-dried. A sample of this solid that
was recrystallized from o-xylene was obtained as small yellow-orange
blocks. A sample of this solid slowly darkened at temperatures above
250 °C but did not melt below 400 °C; darkening was most rapid near
340 °C.
Anal. Calcd for C24H28B2N4Ni (Mr ) 452.85): C, 63.66; H, 6.23;
N, 12.37. Found: C, 63.61; H, 6.24; N, 12.29. Mass spectrum, m/z(+)
(relative intensity): 437.1608 (96) (M - Me, calc 437.1619), 422 (49)
(M - 2Me), 318 (96) (Ni(py)3BMe), 286 (34) ((py)3B2Me2), 239 (35)
(Ni(py)2BCH2), 197 (18) ((py)2BMe2), 182 (100) ((py)2BMe), 161 (25)
(Ni(py)BCH2), 104 (23) ((py)BMe). IR (cm-1): 1595 (s), 1460 (s),
1421 (vs), 1218 (s), 990 (s), 962 (m), 764 (vs), 755 (vs), 745 (s), 490
(m).
The filtrate from the original reaction mixture was combined with
the washings, and the solvent was removed under a nitrogen flow. The
residue was treated with three 10 mL portions of aqueous 1.00 M acetic
acid solution. The resulting mixture was filtered, and the insoluble
material was dried (0.0250 g) and found to be identical to the original
reaction precipitate by its melting point in vacuo (which began at 236
Acknowledgment. This work was supported in part by a
University of Wisconsin Academic Staff Development Grant.
Most of the synthetic work described in the present study was
performed at UWsWhitewater. T.G.H. thanks the present and
past Chairs of the Department of Chemistry at UWsWhitewater,
Dr. Steven Anderson and Dr. Kirk Romary, respectively, for
their generous support. This author also thanks the Department
of Chemistry at UWsOshkosh for the funds needed to obtain
elemental analyses. Funds from an NSF grant (CHE-9105497)
and from the University of WisconsinsMadison were used to
purchase X-ray diffractometers and computers. The authors also
thank the following at UWsMadison: David Snyder and Dr.
Cornelis Hop for obtaining mass spectra, Dr. Charles Fry for
obtaining NMR spectra, and Dr. Donald Gaines for many useful
comments. Comments by the reviewers were also very helpful.
1
°C with decomposition) and its H NMR spectrum. Total yield: ca.
97%.
Anal. Calcd for C24H28B2N4Zn (Mr ) 459.51): C, 62.73; H, 6.14;
N, 12.19. Found: C, 62.67; H, 6.12; N, 12.00. 1H NMR (δ, CDCl3):
7.91 and 7.90 (2H, overlapping d, J ) ca. 7 and 4 Hz, respectively),
7.62 (1H, td, J ) 7.6, ca. 1.5 Hz), 6.94 (1H, td, J ) 6.4, ca. 1.5 Hz),
0.34 (3H, s, h1/2 ) 6.8 Hz). 11B NMR (δ, CDCl3): -16.6 (s, h1/2
)
15.0 Hz). Mass spectrum, m/z(+) (relative intensity): 443.1570 (100)
(M - Me, calc 443.1557), 324 (22) (Zn(py)3BMe), 286 (40) ((py)3B2-
Me2), 261 (49) (Zn(py)2BMe2), 197 (8) ((py)2BMe2), 182 (42) ((py)2-
BMe), 104 (33) ((py)BMe). IR (cm-1): 2851 (m), 1598 (vs), 1463
(vs), 1422 (vs), 1212 (m), 983 (s), 952 (m), 749 (vs, doublet?), 657
(s), 416 (s).
Synthesis of Bis[dimethylbis(2-pyridyl)borato-N,N ′]nickel(II). A
solution of 50.1 mg of NiCl2·6H2O in 15 mL of 95% ethanol was added
to a stirred solution of 0.253 g of IV in 25 mL of 95% ethanol at
ambient temperature. A greenish, gelatinous precipitate formed almost
immediately but was gradually replaced by a bright yellow solid over
the next 30 min. The resulting mixture was stirred for another 4 h and
was then filtered; the mother liquor did not produce a precipitate when
treated with 6.0 M NH3 and dimethylglyoxime solution. The yellow
Supporting Information Available: Tables of complete crystal and
intensity collection data, hydrogen atomic coordinates and isotropic
temperature factors, bond distances and angles, and anisotropic
displacement parameters for IV, VI, VII, VIII and extensive infrared
data for I, II, IV, VII, and VIII (33 pages). Ordering information is
given on any current masthead page.
IC950775M