The elusive tripodal tris(2-pyridyl)borate ligand: A strongly coordinating tetraarylborate

Article abstract of DOI:10.1039/c2cc33059h

Tris(2-pyridyl)borates are introduced as a new robust and tunable scorpionate -type ligand family. A facile synthesis of this hitherto unknown ligand and its complexation to Fe(ii) are described; the optical and electrochemical properties of the resulting iron complex are compared to complexes derived from tris(pyrazolyl)borate, tris(2-pyridyl)aluminate, and corresponding charge-neutral ligands.

Full text of DOI:10.1039/c2cc33059h

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COMMUNICATION
The elusive tripodal tris(2-pyridyl)borate ligand: a strongly coordinating
tetraarylboratewz
Chengzhong Cui, Roger A. Lalancette and Frieder Jakle*
¨
Received 27th April 2012, Accepted 16th May 2012
DOI: 10.1039/c2cc33059h
Tris(2-pyridyl)borates are introduced as a new robust and
tunable ‘‘scorpionate’’-type ligand family. A facile synthesis of
this hitherto unknown ligand and its complexation to Fe(II) are
described; the optical and electrochemical properties of the
resulting iron complex are compared to complexes derived from
tris(pyrazolyl)borate, tris(2-pyridyl)aluminate, and corresponding
charge-neutral ligands.
polar bonds to carbon, an aspect that has enabled organoborane
chemistry as an important field in modern synthesis.4 Indeed,
tetraarylborates, and especially those containing electron-
withdrawing fluorine substituents as in B, are among the most
stable and weakly coordinating anions.5 They are widely
utilized in applications where reactive positively charged species
are encountered, for instance in Ziegler–Natta type olefin poly-
merization as well as in electrochemical processes.6
Based on these considerations, a promising ligand design is
to place 2-pyridyl (py) groups on boron to form tris(2-pyridyl)-
borate ligands C. While tris(2-pyridyl) tripod ligands have
been synthesized with a variety of different bridge atoms
(e.g. C, Si, Sn, Pb, N, P, As, Al, In),7 tris(2-pyridyl)borate
ligands have not been reported to date.8–12 In addition to the
expected enhanced stability, significant differences between the
coordination behavior of tris(pyrazolyl)borate and tris(2-pyridyl)-
borate ligands can be expected, considering that pyridine is a
better s donor than pyrazole. The successful preparation of
tris(2-pyridyl)borates is not only expected to lead to new catalytic
applications, but they could also prove highly versatile as building
blocks for functional supramolecular polymers. N-based tridentate
ligands such as terpyridines and bis(imidazolyl)pyridines have
had a dramatic impact on supramolecular chemistry and
polymer science.13
Since Trofimenko first introduced tris(2-pyrazolyl)borates
(Tp, A in Chart 1) in 1966, they have evolved into one of
the most useful classes of ligands in modern coordination
chemistry.1 They are commonly referred to as ‘‘scorpionates’’
to reflect their distinct coordination geometry and the potency
in forming complexes with essentially every metal in the
periodic table. Consequently, these and related ligands have
found widespread applications ranging from homogeneous
catalysis to bioinorganic chemistry and materials science.2
While the Tp ligands offer advantages in terms of their ease
of synthesis and tunability, the stability of the B–N bonds has
often been a concern.3 Ligand rearrangements illustrate the
relatively labile nature of the polar B–N bonds. As an example,
the HB(3-iPrpz)3 ligand was shown to undergo 1,2-borotropic
shifts in one of its pyrazolyl (pz) substituents to yield the
5-isopropylpyrazolyl isomer.3b Another concern is that decom-
position with formation of the parent pyrazoles is quite common
and often catalyzed by Lewis acids or Brønsted acids.3a,c
On the contrary, boron forms strong and significantly less
An obstacle that possibly derailed earlier attempts at the
preparation of tris(2-pyridyl)borates is that highly Lewis
acidic haloboranes RBX2 (X = Br, Cl), which are commonly
reacted with arylating agents ArM (M = Li, Cu, MgX, SnR3,
HgR) to synthesize arylboranes and borates, readily form
Lewis acid–base complexes with pyridines.4 Moreover, ether
solvents tend to react with haloboranes to give dialkoxy-
substituted boranes, which in our hands proved to be not
sufficiently reactive to form the desired tris(2-pyridyl)borate
species. Hence, we decided to explore the use of an isolable
reagent that can be reacted in non-coordinating solvents. We
turned to 2-PyMgCl, which can be prepared on a 4100 g scale
by metal-halogen exchange of i-PrMgCl and 2-bromopyridine
in THF and isolated as a white crystalline solid. According to
a single crystal X-ray analysis,14 the latter consists of a dimeric
species with 3 coordinated molecules of THF and another
molecule of THF that is located in 1D-channels along the
crystallographic c-axis.w Importantly, this Grignard reagent
proved to be soluble and reasonably stable in toluene and
Chart 1 Comparison of strongly coordinating tris(pyrazolyl)borates,
weakly coordinating arylborates and the targeted tris(2-pyridyl)borates.
Department of Chemistry, Rutgers University-Newark,
73 Warren Street, Newark, NJ, USA. E-mail: fjaekle@rutgers.edu;
Fax: +1 973 353 1264; Tel: +1 973 353 5064
w This article is part of the ChemComm ‘Frontiers in Molecular Main
Group Chemistry’ web themed issue.
z Electronic supplementary information (ESI) available: Experimental
1
details. See DOI: 10.1039/c2cc33059h
CH2Cl2 based on H NMR analyses.
c
6930 Chem. Commun., 2012, 48, 6930–6932
This journal is The Royal Society of Chemistry 2012

Fig. 2 Ball-and-stick representation of the X-ray structure of the
Fe(II) complex 2 (hydrogen atoms are omitted or clarity).
Scheme 1 Synthesis of the tris(2-pyridyl)borate ligand 1.
mixture in the presence of NEt3 gave a red solid that was
purified by column chromatography and recrystallized from
toluene. The resulting complex Fe{(tBuPh)B(2-py)3}2 (2, see
Fig. 2) showed a sharp peak in the 11B NMR at ꢀ7.5 ppm.
While the 11B NMR shifts of 1 and 2 are fairly similar, the
To prepare the borate ligand 1 (Scheme 1), a solution of
t-butylphenyl dibromoborane15 in CH2Cl2 was slowly added
to 2-PyMgCl in CH2Cl2 at RT, and the mixture was stirred for
an additional 5 h. An aqueous Na2CO3 solution was then added
and the product extracted into CH2Cl2. Column chromatography
on NEt3-deactivated silica gel, followed by crystallization from
toluene gave the analytically pure ligand 1 in its protonated form
as a colorless solid in 54% yield.
1
pattern of the pyridyl rings in the H NMR changed drama-
tically as a result of mutual shielding effects of the six pyridyl
rings around the central Fe atom. Thus, the pyridyl protons in
the ortho and meta-positions to N experience large upfield shifts
from 8.49 and 7.10 to 7.11 and 6.44 ppm, respectively. Conver-
sely, the phenylene protons are strongly shifted downfield.
Single crystals of 2 were grown by slow evaporation of a
toluene solution. Coordination of the tris(2-pyridyl)borate
moieties as tridentate ligands to the metal center does not lead
to dramatic changes in the ligand architecture (Fig. 2). The
iron atom lies on a crystallographic inversion center and
adopts a slightly distorted octahedral geometry. Two of the
Fe–N bonds are of similar length (1.9902(13), 1.9880(13) A),
whereas the third is significantly shorter (1.9685(13) A). The
N–Fe–N angles are close to 901 with a maximum deviation of
0.271. The pyridine nitrogens of one ligand are arranged
coparallel to those of the second ligand at a distance of 2.29 A,
similar as in the aluminate complex Fe{MeAl(2-py)3}217 (2.19 A).
The phenyl rings are also positioned in a coparallel arrangement
at a distance of 0.97 A from one another.
The ligand was analyzed by multinuclear NMR and the
assignments of the pyridine signals were confirmed by COSY,
NOESY, and HMQC techniques. The 11B NMR shows a
sharp singlet at ꢀ10.8 ppm, which is in the region expected for
1
tetraarylborates, while a singlet in the H NMR at 19.5 ppm
confirms the presence of a nitrogen-bound proton. The 13C NMR
exhibits characteristic quartets (J = 50 Hz) near 155 and
185 ppm, which are due to coupling of the 11B nucleus to the
quaternary phenyl and pyridyl carbons, respectively. The structure
of the ligand was further confirmed by single-crystal X-ray
diffraction analysis. One of two crystallographically independent,
but otherwise very similar molecules in the unit cell is displayed
in Fig. 1. For each molecule, two of the pyridyl rings are almost
in a coplanar arrangement with small dihedral angles of 18.95
and 21.191, respectively, as a result of hydrogen bonding between
the pyridyl-bound proton of one of the pyridyl rings and the
nitrogen of the other.16 The B–C distances to the phenyl rings
(1.628, 1.638 A) are similar to those to the pyridyl substituents
(1.635–1.643 A) and in the range typically observed for
arylborates, as are the C–B–C angles (106.2–113.51). This
confirms the absence of any significant steric strain.
The Fe–N bond lengths are indicative of the ligand field
strength, which determines the spin state and hence the optical
and magnetic properties of Fe(II) complexes. The Fe–N bonds of
1.9685(13)–1.9902(13) A in 2 are comparable to the ones in neutral
tripodal pyridyl ligands, e.g. Fe–N = 1.970(5)–1.995(5) A for
[Fe{N(2-py)3}2]2+18 and Fe–N = 1.947(2)–1.954(2) A for
[Fe{HC(2-py)3}2]2+19 Both of these complexes were reported
to be diamagnetic even at room temperature.19 In contrast, the
We next explored the ability of 1 to form complexes with
transition metal ions. Treatment with FeCl2 in a THF/MeOH
17
Fe–N bond lengths in the complex Fe{MeAl(2-py)3}2 are
significantly longer at 2.054(3) A, and this complex proved to
be paramagnetic.
The comparatively short Fe–N bonds in 2 suggest that the
complex is diamagnetic in the solid state. Based on the NMR
data, 2 is diamagnetic also in solution, consistent with a low
spin configuration. UV-vis measurements were performed in
CH2Cl2 to further explore this aspect. The absorption maxima
at ca. 480 and 425 (sh) nm (Fig. 3a) can be assigned to M - L
charge transfer (CT). The corresponding cationic Fe(II) complexes
[Fe{HC(2-py)3}2][NO3]2 (439, 370 nm) and [Fe{PO(2-py)3}2]-
[NO3]2 (465, 385 nm) show similar absorption bands, but at
comparatively higher energy.19 Importantly, the optical spectra
are quite different from those reported for the paramagnetic
complex Fe{MeAl(2-py)3}217 (510, 431, 366 nm in THF).
Fig. 1 Ball-and-stick representation of the X-ray structure of the
ligand 1 (hydrogen atoms are omitted except for the acidic proton H1).
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6930–6932 6931

This material is based upon work supported by the National
Science Foundation under Grant No. CHE-0956655 and CRIF-
0443538. We thank Dr Kakalis and Dr Shipman for acquisition
of 2D NMR data, and Prof. Sheridan for helpful discussions.
Notes and references
1 (a) S. Trofimenko, J. Am. Chem. Soc., 1966, 88, 1842;
(b) S. Trofimenko, Polyhedron, 2004, 23, 197.
2 Recent examples: (a) D. L. Reger, J. R. Gardinier, W. R. Gemmill,
M. D. Smith, A. M. Shahin, G. J. Long, L. Rebbouh and
F. Grandjean, J. Am. Chem. Soc., 2005, 127, 2303; (b) F. Zhang,
T. Morawitz, S. Bieller, M. Bolte, H.-W. Lerner and M. Wagner,
Dalton Trans., 2007, 4594; (c) P. Hamon, J.-Y. Thepot,
M. L. Floch, M.-E. Boulon, O. Cador, S. Golhen, L. Ouahab,
L. Fadel, J.-Y. Saillard and J.-R. Hamon, Angew. Chem., Int. Ed.,
2008, 47, 8687; (d) Y. Qin, C. Cui and F. Jakle, Macromolecules,
2008, 41, 2972; (e) F. A. Jov, C. Pariya, M. Scoblete, G. P. A. Yap
and K. H. Theopold, Chem.–Eur. J., 2011, 17, 1310.
3 (a) S. Trofimenko, Scorpionates: The Coordination Chemistry of
Polypyrazolylborate Ligands, Imperial College Press, London,
1999; (b) J. M. White, V. W. L. Ng, D. C. Clarke, P. D. Smith,
M. K. Taylor and C. G. Young, Inorg. Chim. Acta, 2009,
362, 4570; (c) T. F. S. Silva, K. V. Luzyanin, M. V. Kirillova,
M. F. G. da Silva, L. M. D. R. S. Martins and A. J. L. Pombeiro,
Adv. Synth. Catal., 2010, 352, 171.
4 F. Jakle, in Encyclopedia of Inorganic Chemistry, ed. R. B. King,
Wiley, Chichester, 2005, pp. 560–598.
5 I. Krossing and I. Raabe, Angew. Chem., Int. Ed., 2004, 43, 2066.
6 (a) E. Y.-X. Chen and T. J. Marks, Chem. Rev., 2000, 100, 1391;
(b) R. J. LeSuer and W. E. Geiger, Angew. Chem., Int. Ed., 2000,
39, 248.
Fig. 3 (a) UV-vis spectra of complexes 2 (red) and [2+][FeCl4]
(purple). (b) Cyclic voltammogram for complex 2 (1 ꢁ 10ꢀ3 M,
0.1 M [Bu4N]PF6 in CH2Cl2, scan rate 100 mV sꢀ1; vs. Fc/Fc+ (*)).
The redox properties of 2 were studied by cyclic voltammetry
(Fig. 3b). A reversible redox process at ꢀ350 mV vs. Fc/Fc+
indicates that 2 is far more electron-rich than ferrocene and
even more easily oxidized than the Tp complex Fe{HB(pz)3}2
(ꢀ270 mV vs. Fc/Fc+),20 consistent with the anticipated
strong s-donor character of the ligand. Preparative oxidation
of 2 with aqueous FeCl3 resulted in a dark purple solid.
UV-Vis analysis of the product ([2+]FeCl4) in CH2Cl2
7 Review: L. F. Szczepura, L. M. Witham and K. J. Takeuchi,
Coord. Chem. Rev., 1998, 174, 5.
8 A methyltris(2-pyridyl)borate structure has been claimed in a
patent, but neither a synthetic procedure nor any characterization
details were provided, putting in doubt whether the compound was
indeed obtained: N. Ito, T. Umeda, S. Hobara and S. Maehara,
JP 2009137936, Hokko Chemical Industry Co, Ltd, Japan, 2009.
9 Bis(2-pyridylborate)s are more readily accessible. In combination
with Pt as the metal, they have been successfully applied in C–H
activation. See, for example: (a) T. G. Hodgkins and D. R. Powell,
Inorg. Chem., 1996, 35, 2140; (b) E. Khaskin, P. Y. Zavalij and
A. N. Vedernikov, J. Am. Chem. Soc., 2006, 128, 13054.
10 Tris(2-oxazolinyl)borates have been applied in homogeneous
catalysis. See: J. F. Dunne, S. R. Neal, J. Engelkemier, A. Ellern
and A. D. Sadow, J. Am. Chem. Soc., 2011, 133, 16782.
11 An interesting class of tris(pyridyl)boronates has been introduced:
P. J. Bailey, N. L. Bell, L. L. Gim, T. Yucheng, N. Funnell,
F. White and S. Parsons, Chem. Commun., 2011, 47, 11659.
12 Tris(2-pyridylaluminates) are known, but the higher Al–C bond
polarity makes these ligands more susceptible to hydrolytic degra-
dation. See ref. 17 and: T. H. Bullock, W. T. K. Chan and
D. S. Wright, Dalton Trans., 2009, 6709.
revealed a weak, broad band at 574 nm (e = 370 cmꢀ1
M
ꢀ1),
in addition to a stronger absorption at 340 nm; the lower energy
absorption at 574 nm is comparable in energy to those reported
for [Fe{HB(pz)3}2]PF6 (556 nm)20 and [Fe{HC(pz)3}2][ClO4]3
(466, 560 (sh) nm19) and assigned to a dd transition. A single
crystal X-ray analysis (Fig. 4) revealed a structure that is quite
similar to that of the neutral complex 2, except for that the
Fe–N bond lengths in [2+] are less evenly distributed from
1.965(3) A to 2.011(3) A, reflecting significant distortion of the
octahedral geometry.
In conclusion, we have introduced the first examples of
tris(2-pyridyl)borate ligands and their metal complexes. Given
the high stability, the strongly donating ability toward main
group and transition metals, and the possibly for modular
synthesis and ligand fine-tuning using different pyridyl deri-
vatives, we anticipate broad applications of this new ligand
class in catalysis, bioinorganic chemistry, and in the field of
supramolecular polymer chemistry.
13 (a) C.-A. Fustin, P. Guillet, U. S. Schubert and J.-F. Gohy, Adv.
Mater., 2007, 19, 1665; (b) M. Burnworth, D. Knapton, S. J. Rowan
and C. Weder, J. Inorg. Organomet. Polym. Mater., 2007, 17, 91.
14 For a similar structure with Br instead of Cl, see: A. V. Churakov,
D. P. Krutko, M. V. Borzov, R. S. Kirsanov, S. A. Belov and
J. A. K. Howard, Acta Crystallogr., Sect. E: Struct. Rep. Online,
2006, 62, m1094.
15 Y. Qin, I. Kiburu, S. Shah and F. Jakle, Org. Lett., 2006, 8, 5227.
16 The two hydrogen-bonded pyridine rings in protonated
dimethylbis(2-pyridyl)borate are perfectly coplanar; see ref. 9a.
17 C. S. Alvarez, F. Garcia, S. M. Humphrey, A. D. Hopkins, R. A.
Kowenicki, M. McPartlin, R. A. Layfield, R. Raja, M. C. Rogers,
A. D. Woods and D. S. Wright, Chem. Commun., 2005, 198.
18 E. S. Kucharski, W. R. McWhinnie and A. H. White, Aust. J.
Chem., 1978, 31, 53.
19 P. A. Anderson, T. Astley, M. A. Hitchman, F. R. Keene,
B. Moubaraki, K. S. Murray, B. W. Skelton, E. R. T. Tiekink,
H. Toftlund and A. H. White, J. Chem. Soc., Dalton Trans., 2000, 3505.
20 D. Chanaka, L. De Alwis and F. A. Schultz, Inorg. Chem., 2003,
42, 3616.
Fig. 4 Ball-and-stick representation of the X-ray structures of
[2+]FeCl4 (hydrogen atoms are omitted for clarity).
c
6932 Chem. Commun., 2012, 48, 6930–6932
This journal is The Royal Society of Chemistry 2012