Synthesis and characterization of 6, 60-(2, 4, 6-triisopropylphenyl)-2, 20-bipyridine (tripbipy) and its complexes of the late first row transition metals

Article abstract of DOI:10.1021/ic9016382

The synthesis of tripbipy, a new substituted bipyridine ligand (6, 60-(2, 4, 6-triisopropylphenyl)-2, 20-bipyridine), and the syntheses, structures, and magnetic properties of the first coordination compounds based on this ligand are described. Tripbipy was synthesized by the Suzuki coupling of 2, 4, 6-triisopropylphenyl boronic acid and 6, 60-dibromo-2, 20-bipyridine. Reported here are the tripbipy complexes of five late first row transition metal chlorides (MCl₂; M=Fe, Co, Ni, Cu, Zn). Four of the complexes MCl₂tripbipy (M = Fe, Co, Ni, Zn) crystallize in the space group P2₁/c and are isomorphous with one solvent molecule of crystallization. The complex CuCl2tripbipy crystallizes in the space group P2₁2₁2₁ with two solvent molecules of crystallization. All MCl₂tripbipy complexes are four coordinate and contain distorted tetrahedral metal centers. CuCl2tripbipy shows a pseudo Jahn-Teller distortion, and X-band electron paramagnetic resonance (EPR) in a toluene glass gives approximate g ⊥ ||) values of 2.2 and 2.1. Magnetic measurements (M= Fe, Co, Ni, Cu) are consistent with high spin dⁿ configurations (n =6-9, S = 2, 3/2, 1, 1/2) tetrahedral complexes and give _xMT values at 300 K of 3.56, 2.10, 1.01, and 0.37 cm³ M^-1 K, respectively.

Full text of DOI:10.1021/ic9016382

1458 Inorg. Chem. 2010, 49, 1458–1464
DOI: 10.1021/ic9016382
Synthesis and Characterization of 6,60-(2,4,6-Triisopropylphenyl)-2,20-bipyridine
(tripbipy) and Its Complexes of the Late First Row Transition Metals
Eric E. Benson, Arnold L. Rheingold, and Clifford P. Kubiak*
Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive,
Mail Code 0358, La Jolla, California 92093-0358
Received August 15, 2009
The synthesis of tripbipy, a new substituted bipyridine ligand (6,60-(2,4,6-triisopropylphenyl)-2,20-bipyridine), and the
syntheses, structures, and magnetic properties of the first coordination compounds based on this ligand are described.
Tripbipy was synthesized by the Suzuki coupling of 2,4,6-triisopropylphenyl boronic acid and 6,60-dibromo-2,20-
bipyridine. Reported here are the tripbipy complexes of five late first row transition metal chlorides (MCl2; M=Fe, Co, Ni,
Cu, Zn). Four of the complexes MCl2tripbipy (M = Fe, Co, Ni, Zn) crystallize in the space group P21/c and are
isomorphous with one solvent molecule of crystallization. The complex CuCl2tripbipy crystallizes in the space group
P212121 with two solvent molecules of crystallization. All MCl2tripbipy complexes are four coordinate and contain
distorted tetrahedral metal centers. CuCl2tripbipy shows a pseudo Jahn-Teller distortion, and X-band electron
paramagnetic resonance (EPR) in a toluene glass gives approximate g^, values of 2.2 and 2.1. Magnetic
measurements (M = Fe, Co, Ni, Cu) are consistent with high spin dn configurations (n = 6-9, S = 2, 3/2, 1, 1/2)
tetrahedral complexes and give χMT values at 300 K of 3.56, 2.10, 1.01, and 0.37 cm3 M-1 K, respectively.
Introduction
complexes. In order to access a low coordinate active site, steric
protection of the metal site is often necessary. The use of large
substituents (mes=2,4,6-trimethylphenyl, dipp=2,6-diisopro-
pylphenyl, trip=2,4,6-triisopropylphenyl) in various blocking
positions of carbenes,6 arenes, and various other ligand frame-
works has shown interesting structural motifs and reactivity.
Inspired by the work of Robinson,7,8 Power,9-12 and
others using 2,4,6-triisopropylphenyl (trip) groups as large
blocking groups, we have developed a synthesis of a chelating
bipyridine with sterically encumbering trip groups in the 6,60
positions. Our first foray into the coordination chemistry of
tripbipy is described herein.
Bipyridines are one of the most ubiquitous classes of
ligands in coordination chemistry. Their ability to bind to a
wide range of metal ions and stabilize different oxidation
states has been extensively studied. The ability to tune both
the electronics and sterics of bipyridine ligands via manipula-
tion of substituents around the pyridyl rings has been a crucial
foundation for their widespread applicability. Besides their
abundant use in supramolecular,1 nanomaterial,2 macro-
molecular,3 and photophysical chemistry,4 bipyridine ligands
have been of interest because of their potential for ligand
centered redox chemistry and metal-to-ligand-charge-transfer
(MLCT) interactions.5 Herein, we report the synthesis of
tripbipy (6,60-(2,4,6-triisopropylphenyl)-2,20-bipyridine), a
new substituted bipyridine ligand, and the properties of the
first coordination compounds based on this ligand.
Results and Discussion
Syntheses. The synthesis of 6,60-(2,4,6-triisopropyl-
phenyl)-2,20-bipyridine (tripbipy) is summarized in
Low coordinate metal centers are often invoked as cata-
lytically active sites in a number of important catalytic systems,
from enzymes to small molecule activation in organometallic
(6) Wang, Y. Z.; Xie, Y. M.; Wei, P. R.; King, R. B.; Schaefer, H. F.;
Schleyer, P. V.; Robinson, G. H. J. Am. Chem. Soc. 2008, 130, 14970.
(7) Wang, Y.; Robinson, G. H. Organometallics 2007, 26, 2.
(8) Su, J.; Li, X.-W.; Crittendon, R. C.; Robinson, G. H. J. Am. Chem.
Soc. 1997, 119, 5471.
(9) Ni, C.; Ellis, B. D.; Fettinger, J. C.; Long, G. J.; Power, P. P. Chem.
Commun. 2008, 1014.
(10) Wolf, R.; Ni, C.; Nguyen, T.; Brynda, M.; Long, G. J.; Sutton, A. D.;
Fischer, R. C.; Fettinger, J. C.; Hellman, M.; Pu, L. H.; Power, P. P. Inorg.
Chem. 2007, 46, 11277.
(11) Hardman, N. J.; Wright, R. J.; Phillips, A. D.; Power, P. P. J. Am.
Chem. Soc. 2003, 125, 2667.
*To whom correspondence should be addressed. E-mail: ckubiak@ucsd.edu.
(1) Ye, B.-H.; Tong, M.-L.; Chen, X.-M. Coord. Chem. Rev. 2005, 249,
545.
(2) Fletcher, A. J.; Cussen, E. J.; Prior, T. J.; Rosseinsky, M. J.; Kepert,
C. J.; Thomas, K. M. J. Am. Chem. Soc. 2001, 123, 10001.
(3) Ulrich, S.; Schubert, C. E. Angew. Chem., Int. Ed. 2002, 41, 2892.
(4) Shan, B. Z.; Zhao, Q.; Goswami, N.; Eichhorn, D. M.; Rillema, D. P.
Coord. Chem. Rev. 2001, 211, 117.
(5) Sullivan, B. P.; Bolinger, C. M.; Conrad, D.; Vining, W. J.; Meyer, T.
J. J. Chem. Soc., Chem. Commun. 1985, 1414.
(12) Twamley, B.; Sofield, C. D.; Olmstead, M. M.; Power, P. P. J. Am.
Chem. Soc. 1999, 121, 3357.
r
pubs.acs.org/IC
Published on Web 01/07/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 4, 2010 1459
Scheme 1. Synthesis of Tripbipy
Scheme 2. Synthesis of MCl2tripbipy
Scheme 1. Tripbipy was readily synthesized by the Suzuki
coupling of 2,4,6-triisopropyphenyl (trip) boronic acid
and 6, 60-dibromo-2, 20-bipyridine in good yields
(>75%). The resulting white solid was characterized by
1H and 13C NMR, combustion analysis, and mass spectro-
metry. The coupling proceeds in high yield despite the
steric bulk of the isopropyl groups flanking the boronic
acid. Tripbipy is sparingly soluble in chlorinated solvents
(CH2CL2, CHCl3), toluene, and THF.
Tripbipy functions as a sterically encumbering ligand
for the late transition metal chlorides, (MCl2; M=Fe, Co,
Ni, Cu, Zn). The syntheses of all five complexes MCl2trip-
bipy (M=Fe, Co, Ni, Cu, Zn) are accomplished by slow
addition of ethanol solutions of the metal chloride hy-
drates to toluene solutions of tripbipy. The overall syn-
thetic plan is depicted in Scheme 2. The subsequent
heating and removal of water from the reaction resulted
in the complexes MCl2tripbipy in high yields. All five
MCl2tripbipy complexes are stable to the atmosphere;
however, in solution they are susceptible to attack from
nucleophiles (H2O, ACN, THF, pyridine), resulting in
varying degrees of ligand dissociation from the metal as
verified by 1H NMR, color change, and recovery of free
ligand.
Figure 1. Idealized angles for steric protection of metal centers.
and crystallizes in the space group P21/c with one solvent
molecule (CHCl3). The structure shows a distorted tetra-
hedral coordination environment around the iron center
(Figure 2). Calculation of Houser’s τ4 four-coordinate
geometry index13 gives a value of 0.83 which suggests that
the four coordinate geometry present is closely related
to a distorted trigonal pyramidal structure (idealized
trigonal pyramid=0.85). One of the triisopropylphenyl
groups is nearly orthogonal to the bipyridine plane, with a
torsion angle of 86.69(4)ꢀ. The other trip group is inclined
at an angle of 69.49(4)ꢀ relative to the plane of the bipy
ring. This is most likely caused by crystal packing effects,
and a C-H-π interaction between the solvent of crystal-
lization and three of the carbons in the benzene ring of the
X-ray Crystallography. The MCl2tripbipy complexes
(M = Fe, Co, Ni, Cu, Zn) were readily crystallized by
vapor diffusion of pentane or diethyl ether into solutions
of the complex in chloroform.
FeCl2tripbipy. The molecular structure of FeCl2trip-
bipy was determined by single crystal X-ray diffraction
˚
trip group (C15-H41, 2.772 A). The bite angle of the
bipyridine is 77.24(6)ꢀ which is similar to the reported
(13) Yang, L.; Powell, D. R.; Houser, R. P. Dalton Trans. 2007, 955.

1460 Inorganic Chemistry, Vol. 49, No. 4, 2010
Benson et al.
Table 1. Selected Bond Distances (A), Angles (deg), and τ4 Geometry Index13 for
˚
MCl2tripbipy Complexes
Fe
Co
Ni
Cu
Zn
M-N1
M-N2
M-Cl1
M-Cl2
2.1318(14) 2.0558(25) 2.0173(21) 1.9811(23) 2.0798(26)
2.1217(15) 2.0638(25) 2.0287(21) 2.0590(24) 2.0932(27)
2.2526(5) 2.2348(9) 2.2419(11) 2.2380(9) 2.2298(9)
2.2154(5) 2.1992(9) 2.1836(11) 2.1738(9) 2.1833(9)
N1-M-N2 77.24(6)
Cl1-M-N1 112.31(4) 105.55(7) 98.34(7)
80.64(10) 81.60(9)
81.73(10) 79.72(10)
96.36(7) 104.10(8)
Cl1-M-N2 103.13(4) 113.28(7) 107.17(7) 128.66(7) 111.54(8)
Cl1-M-Cl2 119.19(2) 114.55(3) 121.31(5) 100.22(4) 117.71(4)
Cl2-M-N1 114.23(4) 123.47(8) 126.76(7) 146.60(7) 122.92(8)
Cl2-M-N2 123.41(4) 114.95(7) 114.06(7) 109.02(7) 114.79(8)
τ4
0.83
0.86
0.79
0.60
0.85
Figure 2. Molecular structure of FeCl2tripbipy.
Figure 4. Molecular structure of NiCl2tripbipy.
NiCl2tripbipy. The nickel complex crystallizes in the
same space group, P21/c, as the homologous iron(II) and
cobalt(II) complexes, in an isomorphous unit cell that
contains one solvent molecule (CHCl3) (see Figure 4). As
with the previous two compounds, one of the triispropyl-
phenyl groups is nearly orthogonal to the bipyridine
plane, and the other significantly distorted (86.96(7)ꢀ vs
71.20(7)ꢀ). The bite angle of the bipyridine is 81.60(9)ꢀ,
which is smaller than most four-coordinate bipyridine
structures found in the Cambridge database. This may
arise from the torsion of the complex toward a square
planar geometry or a pseudo Jahn-Teller distortion. The
τ4 value of 0.79 shows a stronger distortion from the
tetrahedral geometry than is seen in FeCl2tripbipy,
CoCl2tripBipy, and ZnCl2tripbipy (Table 1).
CuCl2tripbipy. The copper complex is the only com-
pound in this series that did not crystallize in the space
group P21/c. Rather it crystallized in the space group
P212121 with two independent solvent molecules (CH-
Cl3). The geometry around the metal center is severely
distorted, τ4=0.60. We attribute this distortion away from
tetrahedral geometry to a strong pseudo Jahn-Teller
distortion which is not uncommon with Cu2þ d9 com-
plexes.16 We still see the orthogonal and canted disposi-
tions of the two trip groupsthatare seenin the structures of
the iron, cobalt, and nickel complexes; however, there is no
solvent molecule within close contact of the trip group to
be the sole cause of the distortion. This can be attributed
to crystal packing effects, and the steric repulsion of
the chloride due to an elongation of the Cu-Cl bond
arising from the pseudo Jahn-Teller distortion. A Houser
Figure 3. Molecular structure of CoCl2tripbipy.
structures of FeCl2(6,6’dimethyl-2,2’bipyidine) which are
77.50(10)ꢀ and 78.12(8)ꢀ for the two polymorphs.14 The
˚
difference in Fe-Cl distances (Fe1-Cl1 = 2.2526(5) A,
˚
Fe1-Cl2 = 2.2154(5) A) and Fe-N (Fe1-N1 =
˚
2.1217(15), Fe1-N2 = 2.1318(14) A) are also believed
to be indicative of crystal packing effects rather than a
Jahn-Teller distortion, as comparable distortions are
seen with the structurally similar diamagnetic ZnCl2trip-
bipy complex (vide infra).
CoCl2tripbipy. The cobalt complex was prepared in
similar manner to FeCl2tripbipy and crystallizes in the
same space group P21/c and an isomorphous unit cell
with one solvent molecule of chloroform. The complex is
structurally similar to FeCl2tripbipy (Figure 3), in that
there is one trip group close to orthogonal (87.01(7)ꢀ) to
the plane of the bipyridine, and the other that is distorted
(71.65(8)ꢀ) again, presumably from a C-H-π interaction
˚
from the chloroform (C15-H41 = 2.771 A). The bite
angle of the chelating pyridine is (N1-Co1-N2)
80.64(10)ꢀ and is smaller than most Co2þ structures
reported15 but is larger than typical Co3þ bipyridine
complexes. Again, due to crystal packing effects, we see
a slight difference in bond lengths for the Co-Cl and
Co-N bonds (Table 1), similar to the trend observed for
FeCl2tripbipy. Using the Houser τ4 parameter, we obtain
a value of 0.86 again suggesting a distorted trigonal
pyramidal geometry in the solid state.
(14) Chan, B. C. K.; Baird, M. C. Inorg. Chim. Acta 2004, 357, 2776.
(15) Carsten, B.; Margareta, Z.; Daniel, B. Angew. Chem., Int. Ed. Engl.
1990, 29, 205.
(16) Reinen, D.; Atanasov, M.; Nikolov, G.; Steffens, F. Inorg. Chem.
1988, 27, 1678.

Article
Inorganic Chemistry, Vol. 49, No. 4, 2010 1461
Figure 5. Molecular structure of CuCl2tripbipy.
Figure 7. Near-infrared visible spectra of MCl2tripbipy complexes.
structure is quite different from the iron, cobalt, nickel,
and zinc members of the series. The bulk of the trip groups
has effectively controlled the coordination environment
around the metal center in both coordination number,
and geometry. This is in contrast to the more prevalent
metal halide complexes in which there is coordination of
two or three bipyridines to the metal center. Previously
the MCl2bipy (M = Fe, Co, Ni) complexes have
been obtained through thermal decomposition of the
parent tris-bipyridine complex and have been shown to
consist of polymeric chains containing six-coordinate
metal centers.18,19
In our case, the tripbipy ligand appears to be an
enforcer of tetrahedral coordination geometries, preclud-
ing six-coordination in the cases of iron(II) and cobalt(II),
and a square planar geometry in the cases of nickel(II)
and copper(II). By far, the majority of nickel(II) bipy
four-coordinate complexes are square planar.20 By using
bipyridine as a backbone to rigidly hold the trip groups
near the metal center, we are able to narrow the open
space between the trip groups. This can be quantified as
the angle defined between the two trip groups and the
metal center. In the ZnCl2tripbipy complex, this angle is
42.72(12)ꢀ. In comparison, substituted 2,6-bisphenylben-
zene (m-terphenyl) complexes present an angle between
the two flanking phenyl group planes of nearly 120ꢀ.7,9-12
A comparison of the angles between the substituents of a
2,6-phenyl type ligand vs a 6,60-bipy ligand is presented in
Figure 1. This protection of the metal center by tripbipy
may allow for the stabilization of geometries and coordi-
nation environments unseen with substituted terphenyl
groups.
Figure 6. Molecular structure of ZnCl2tripbipy.
τ4 value of 0.60 is severely distorted from the traditional
four-coordinate geometries, and its value lies closer to that
of a seesaw (0.65). However, close inspection of the
geometry suggests that the structure most closely resem-
bles an intermediate geometry between tetrahedral and
square planar, where the steric bulk of the trip groups
impedes the transition to square planar (Figure 5).
ZnCl2tripbipy. The zinc complex crystallizes in same
space group P21/c and isomorphous unit cell with one
solvent of crystallization as the iron, cobalt, and nickel
complexes (Figure 6). As with the previous metals, we see
a distortion in one of the trip groups while the other is
nearly normal to the bipyridine ligand plane (70.96(8)ꢀ vs
86.94(7)ꢀ). As in the related complexes, this is attributed
˚
to the solvent C-H bond being within 2.777 A
(C15-H41) of the π system of the distorted trip group,
which is within the range for a C-H-π interaction.17
However, in solution, the molecule shows C2v symmetry
1
from the 13C and H NMR, suggesting that the distor-
tion from tetrahedral geometry arises from crystal pack-
ing effects rather than an intrinsic electronic or steric
property.
Summary of Structures. The tripbipy complexes of iron,
cobalt, nickel, and zinc all crystallize in the same space
group (P21/c) and in an almost identical unit cell (a =
8.96(5), b=28.14(7), c=16.78(8), R=90, β = 99.8(3), γ=
90), where the standard deviations are those for the mean
cell constants for the four structures. These four com-
plexes are nearly isostructural, with only minimal changes
around the metal core. The CuCl2tripbipy complex, on
the other hand, crystallized in the space group (P212121)
with two solvent molecules of crystallization, and its
Electronic Spectra. The electronic spectra of the title
compounds in dichloromethane are shown in Figure 7. All
compounds show a ligand based π f π* transition centered
near 310 nm (ε=11000-17000 M-1 cm-1). TheFecomplex
shows a single d-d transition at 505 nm (ε=260 M-1 cm-1
)
(18) Reiff, W. M.; Dockum, B.; Weber, M. A.; Frankel, R. B. Inorg.
Chem. 1975, 14, 800.
(19) Lee, R. H.; Griswold, E.; Kleinberg, J. Inorg. Chem. 1964, 3, 1278.
(20) Matsunaga, P. T.; Hillhouse, G. L.; Rheingold, A. L. J. Am. Chem.
Soc. 2002, 115, 2075.
(17) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond In Structural
Chemistry and Biology; Oxford University Press: New York, 1999.

1462 Inorganic Chemistry, Vol. 49, No. 4, 2010
Benson et al.
mol-1 K, and then decreases below 18 K. This behavior
suggests weak intermolecular coupling at lower tempera-
tures, with the decrease of χMT below 18 K being attrib-
uted to zero-field splitting.
The χMT of CoCl2tripbipy at 300 K (2.10 cm3 mol-1 K)
supports the assignment of a high spin tetrahedral Co2þ
center (S=3/2). Unlike the FeCl2tripbipy complex, we do
not see any long-range coupling at lower temperatures.
The χMT stays nearly constant from 300 to 40 K and then
below 40 K starts decreasing presumably from the zero-
field splitting. The χMT at 300 K for NiCl2tripbipy at 4 T
is 1.01 cm3 mol-1 K and is consistent with a Ni2þ center
with 2 unpaired electrons giving a S=1 ground state. As
with the CoCl2tripbipy system, we do not see magnetic
coupling at lower temperatures, and similarly, we see χMT
staying relatively constant until the temperature is below
40 K. The magnetic response of CuCl2tripbipy gives a
χMT at 300 K of 0.37 cm3 mol-1 K and stays relatively
constant over the entire temperature range; it is not until
below 5 K that we begin to see the value decrease.
The observed μeff values of the MCl2tripbipy complexes
compare favorably with the spin only values. The C2v
symmetry of the complexes sufficiently lifts orbital de-
generacies to quench spin-orbit coupling. The magnetic
behavior of MCl2tripbipy differs from reported MCl2bi-
py complexes. FeCl2bipy has been shown to be a ferro-
magnet with a Curie temperature of ∼4 K,18 while in the
case of FeCl2tripbipy, the metal centers are magnetically
dilute from the large separation in the lattice (d(Fe-Fe)
Figure 8. Plot of μeff vs T for MCl2tripbipy, M = Fe, Co, Ni, Cu.
which we assign to the 5E2 f 5T2 transition, consistent with
a monomeric tetrahedral iron center. Here, the irreducible
representations and states are referred to Td symmetry, not
the structurally more precise C2v symmetry for comparison
to related tetrahedral complexes. The electronic spectrum of
FeCl2tripbipy is quite different from that of FeCl2bipy
which shows transitions corresponding to polymeric chains
in C2v symmetry.18
˚
8.6 A) imposed by the bulky tripbipy ligand. With the
The CoCl2tripbipy complex shows three distinct bands
at 690, 640, 580 nm (ε=310, 210, 230 M-1 cm-1), and
these are assigned to the nominally 4A2 f 4T1 transition.
The fine structure of this transition is attributed to
reduced symmetry of the T1 state. NiCl2tripbipy shows
a number of peaks in the regions of 480-610 nm and
845-1280 nm (Figure 7). These are attributed to the
nickel and cobalt MCl2bipy analogs, a high μeff value has
been reported (R-CoCl2bipy 5.10 μB, NiCl2bipy 3.38 μB),
consistent with octahedral chloro-bridged chains.19 In a
similar system NiI2(PPh3) shows values close to the spin
only value (2.9 μB) arising from distortion of the molecule
away from tetrahedral symmetry.25
Electrochemistry. The electrochemistry of FeCl2trip-
bipy in CH2Cl2 shows a reversible 1e- oxidation at 0.48 V
vs Fc/Fcþ corresponding to the FeII/FeIII couple and an
irreversible 2e- reduction at -1.87 V vs Fc/Fcþ that is
tentatively assigned to the FeII/Fe0 couple. The number of
electrons transferred was determined by differential pulse
voltametry with a known concentration of an internal
standard (Fc). Controlled potential electrolysis experi-
ments were inconclusive due to fouling of the electrode
presumably from decomposition of the complex. Sample
voltamograms can be found in the Supporting Informa-
tion. CoCl2tripbipy shows an irreversible 2e- reduction at
-1.68 V vs Fc/Fcþ. No oxidation to CoIII was observed
out to þ1.2 V vs Fc/Fcþ in CH2CL2. As with FeCl2-
tripbipy, we assign this as a metal-based reduction.
Similarly, NiCl2tripbipy shows an irreversible reduction
at -1.43 vs Fc/Fcþ. This is tentatively assigned to the
NiII/Ni0 couple.
3
3
3
3
various components of the T1 f A2 and T1 f T2
transitions, respectively. Again these are the transitions
are referred to Td symmetry and compare favorably to the
2- 21
spectra of tetrahedral NiCl4
.
The copper complex shows a MLCT band at 450 nm (ε=
860 M-1 cm-1) and a broad d-d transition centered at 890
nm (ε = 100 M-1 cm-1) and contains the transitions
expected for 2E2
f
2T2. This matches well with the
literaturefor monomeric CuII complexes.22-24 As expected
ZnCl2tripbipy shows no d-d transitions, only a ligand
based π -π* transition at 310 nm (ε=17000 M-1 cm-1
)
Magnetic Studies. To further characterize the proper-
ties of the new tripbipy complexes, magnetic data were
used to confirm the number of unpaired spins within each
complex. Magnetic data of the MCl2tripbipy complexes
at 4 T is shown in Figure 8. For FeCl2tripbipy, the χMT at
300 K (3.56 cm3 mol-1 K) is consistent with a tetrahedral
Fe2þ center with an S=2 ground state. This value is nearly
constant through the range of 300-100 K and increases
upon cooling to reach a maximum value of 4.27 cm3
The electrochemistry of CuCl2tripbipy in CH2Cl2 at
slow scan rates (0.025 V/s) shows a single reversible
reduction at 0.05 V vs Fc/Fcþ. At faster scan rates
(>0.05 V/s), the voltamograms become more convo-
luted, which may be due to a coupled chemical reaction
with the solvent and or electrolyte. CuCl2tripbipy will
(21) Donoghue, J. T.; Drago, R. S. Inorg. Chem. 1962, 1, 866.
(22) Davis, W. M.; Zask, A.; Nakanishi, K.; Lippard, S. J. Inorg. Chem.
1985, 24, 3737.
(23) Hathaway, B. J.; Billing, D. E. Coord. Chem. Rev. 1970, 5, 143.
(24) Kwong, H. L.; Lee, W. S.; Ng, H. F.; Chiu, W. H.; Wong, W. T. J.
Chem. Soc. Dalton Trans. 1998, 1043.
(25) Venanzi, L. M. J. Chem. Soc. 1958, 719.

Article
Inorganic Chemistry, Vol. 49, No. 4, 2010 1463
Table 2. Crystallographic Data and Refinement Information
compound
empirical formula
formula weight
FeCl2tripBipy
CoCl2tripBipy
NiCl2tripBipy
CuCl2tripBipy
ZnCl2tripBipy
FeCl2C40H52 CHCl3
CoCl2C40H52 CHCl3
NiCl2C40H52 CHCl3
CuCl2C40H52 2CHCl3
ZnCl2C40H52 CHCl3
3
3
3
3
3
806.98
monoclinic
8.9257(5)
28.1409(17)
16.8549(10)
90
810.07
monoclinic
8.9196(7)
28.052(2)
16.8310(14)
90
809.83
monoclinic
9.036(6)
28.218(18)
16.666(11)
90
934.06
orthorhombic
13.785(5)
16.968(6)
19.525(7)
90
816.55
monoclinic
8.9639(8)
28.157(2)
16.7861(14)
90
crystal system lattice parameters
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
100.1670(10)
90
100.069(2)
90
99.386(9)
90
90
90
99.866(1)
90
3
˚
V (A )
space group
Z value
4167.1(4)
P21/c
4146.4(6)
P21/c
4192(5)
P21/c
4567.17(27)
P212121
4
4174.0(6)
P21/c
4
1.286
4
1.298
4
1.283
4
1.300
F
calc (g cm-3
)
1.360
μ (Mo KR) (mm-1
temperature (K)
2θ max (deg)
)
0.71073
150(2)
50.9
0.71073
150(2)
50.82
0.71073
150(2)
51.42
0.71073
150(2)
50.98
0.71073
150(2)
50.78
no. obs (I > 2R(I))
no. parameters
goodness of fit
max shift in cycle
residuals: R1; wR2
largest peak
7713
454
7645
454
7858
454
8485
490
7721
454
1.041
0.002
1.064
0.002
1.064
0.002
0.955
0.001
1.013
0.001
0.0309; 0.0751
0.435
0.0497; 0.1371
0.595
0.0423; 0.1020
1.016
0.0317; 0.0753
0.303
0.0491; 0.1135
0.732
deepest hole
-0.456
-0.893
-0.672
-0.345
-0.467
oxidize Fc to Fcþ as evident in the CV’s and the color
change of the solution upon addition of ferrocene.
Electrochemistry of ZnCl2tripbipy in CH2CL2 shows a
reversible ligand based reduction at -1.94 V vs Fc/Fcþ.
Due to the filled d orbitals of zinc and the reversibility of
the reduction, this reduction is assigned as ligand-based.
This also lends credence to the assignment of the reduc-
tions seen in the previous Fe, Co, and Ni complexes as
metal, not ligand based.
EPR. In order to further probe the electronic structure
of CuCl2tripbipy, EPR was performed in a toluene solu-
tion at room temperature and in a frozen glass at X band
frequencies. The isotropic g value in fluid solution is 2.044
with an apparent A value of 222 MHz, which is consistent
with coupling to 65,63Cu nuclei with I=3/2. In a frozen
glass at 110 K, the spectrum becomes more convoluted
and gives apparent g^ and g values near 2.2 and 2.1,
respectively. The EPR data suggest a tetrahedral com-
pression toward square planar resulting in the free elec-
tron residing in a dxy orbital. This is consistent with the
shorter than average Cu-Cl bonds and strongly distorted
tetrahedral molecular geometry seen in the X-ray diffrac-
tion study. The hyperfine coupling to the Cu nucleus was
unresolved due to overlap of the g^ and g transitions.
EPR spectra and simulations can be found in the
Supporting Information.
the presence of other nucleophilic ligands. We anticipate that
tripbipy will find important applications in stabilizing co-
ordinativly unsaturated species in low oxidation states.
Experimental Section
General Considerations. 6,60-Dibromo-2,20-bipyridine was
synthesized according to literature procedures;26 all other che-
micals were purchased from commercial sources and used as
received. Toluene and methylene chloride were sparged with
argon and dried over basic alumina with a custom dry solvent
system. CHCl3 was filtered through activated basic alumina and
distilled over P2O5. Magnetic measurements were collected on
a Quantum Design MPMS 5 SQUID magnetometer. The
susceptibility measurements were performed in the 1.8-300 K
temperature range with an applied field of 4 T. Data were
corrected for diamagnetic contributions from both the sample
holder and the tripbipy ligand using Pascal’s constants. Electro-
chemistry was carried out using a BAS epsilon workstation
using a glassy carbon working electrode, Pt wire counter, and
Ag/AgCl pseudoreference electrode using tetrabutylammonium
hexafluorophosphate (TBAH) as the supporting electrolyte. All
solutions were referenced to Fc/Fcþ using an internal standard.
1H NMR were obtained using a Jeol ECA-500 spectrometer. 13
C
NMR were obtained using Varian Mercury 400 and 500 spectro-
meters. All NMR spectra were referenced to internal solvent
peaks. UV/vis/NIR spectra were collected on a Shimadzu UV-
3600 in dichloromethane. Combustion analysis was performed
by Midwest MicroLab, LLC, Indianapolis, IN.
Crystallographic Structure Determinations. Single-crystal
X-ray structure determinations were carried out at 150(2) K
on either a Bruker P4 or platform diffractometer using Mo KR
Conclusions
˚
radiation (λ=0.71073 A) in conjunction with a Bruker APEX
Tripbipy has been shown to be an effective ligand for
control of the coordination number and geometry of five late
first row transition metals. Coordination to metal chlorides
(M=Fe, Co, Ni, Cu, Zn) gives pseudotetrahedral coordina-
tion environments. Tripbipy can beregarded as an “enforcer”
of tetrahedral geometries when alternate octahedral or
square planar geometries are available. Tripbipy, through
its large flanking arms and narrow coordination region
between them, is able to protect the metal centers to which
it is attached. Tripbipy does show a tendency to be labilized in
detector. All structures were solved by direct methods using
SHELXS-97 and refined with full-matrix least-squares proce-
dures using SHELXL-97.27 Crystallographic data collection
and refinement information can be found in Table 2.
Synthesis of Trip2Bipy (1). To a toluene (250 mL) solution
of 6,60-dibromo-2,20-bipyridine (2 g, 6.37 mmol) an excess of
(26) Parks, J. E.; Wagner, B. E.; Holm, R. H. J. Organomet. Chem. 1973,
56, 53.
(27) Sheldrick, G. Acta Cryst. A 2008, 64, 112.

1464 Inorganic Chemistry, Vol. 49, No. 4, 2010
Benson et al.
2,4,6-triisopropylbenzene boronic acid (4 g, 16.1 mmol) sus-
pended in 30 mL of methanol was added. A 30 mL portion of
2 M sodium carbonate and 175 mg of Pd(PPh3)4 (2.3% mol cat)
were added and refluxed for 72 h in air. After cooling, the layers
were separated, the organic layer was washed with brine (2 ꢀ
100 mL), and the aqueous layer was washed with chloroform
(2 ꢀ 100 mL). The organic fractions were combined and dried
under rotary evaporation. The crude solid was then dissolved in
minimal amount of hot chloroform, filtered, and crashed out
with methanol. The white precipitate was filtered and dried in
vacuo at 80 ꢀC. Yield: 2.78 g, 4.97 mmol, 78.0%. 1H NMR (500
MHz, CH2Cl2, 20 ꢀC): δ=8.41 (d, 2H, J=8 Hz), 7.81 (t, 2H, J=8
Hz), 7.27 (d, 2H, J=8 Hz), 7.13 (s, 4H), 2.97 (sep., 2H, J=7 Hz),
2.60 (sep., 4H, J=7 Hz), 1.32 (d, 12H, J=7 Hz), 1.14 (d, 12H, J=
6 Hz), 1.12 (d, 12H, J=6 Hz). 13C NMR (125.1 MHz, CHCl3, 20
ꢀC): δ=159.3, 156.0, 148.7, 146.5, 136.9, 136.4, 125.1, 120.9,
119.2, 34.6, 30.5, 24.5, 24.3, 24.2 ppm. Anal. calcd for C40H52N2:
C, 85.66; H, 9.35; N, 4.99. Found: C, 85.41; H, 9.44; N, 5.04.
Synthesis of MCl2tripbipy. To a solution of tripbipy in toluene
(200 mg, 0.357 mmol, 25 mL), 1 equiv of the corresponding
metal chloride or chloride hydrate in 3 mL EtOH was added
slowly and refluxed for 3 h. The condenser was then removed,
and the solution was reduced to 10 mL under heat and N2. After
cooling, 50 mL of pentane was added to the solution and cooled
to -20 ꢀC overnight. Crystals were filtered and washed with
pentane and dried in vacuo.
suitable for X-ray diffraction were grown from the vapor
diffusion of Et2O into chloroform.
NiCl2tripbipy. 84.8 mg NiCl2 6H2O, 0.357 mmol. Yield 198
3
mg, 0.287 mmol, 80.4%. Recrystallized from CHCl3/pentane as
the chloroform solvate. Anal. calcd for C40H52N2Cl2Ni CHCl3:
3
C, 60.81; H, 6.60; N, 3.46. Found: C, 61.23; H, 6.76; N, 3.48.
Crystals suitable for X-ray diffraction were grown from the
vapor diffusion of pentane into chloroform.
CuCl2tripbipy. 60.8 mg CuCl2 2H2O, 0.357 mmol. Yield 247
3
mg, 0.348 mmol, 97.5%. Orange-red crystals, Anal. calcd for
C40H52N2Cl2Cu: C, 69.10; H, 7.54; N, 4.03. Found: C, 68.19; H,
7.52; N, 3.99. Crystals suitable for X-ray diffraction were grown
from the vapor diffusion of Et2O into chloroform.
ZnCl2tripbipy. 48.6 mg ZnCl2, 0.357 mmol. Yield 235 mg,
0.337 mmol, 94.5%. 1H NMR (500.2 MHz, CH2Cl2, 20 ꢀC): δ=
8.31 (d, 2H, J=7 Hz), 8.16 (t, 2H, J=8 Hz), 7.66 (d, 2H, J=7 Hz),
7.03 (s, 4H), 2.86 (sep., 2H, J=7 Hz), 2.39 (sep., 4H, J=7 Hz),
1.29 (d, 12H, J=7 Hz), 1.22 (d, 12H, J=7 Hz), 0.95 (d, 12H, J=7
Hz). 13C NMR (100.6 MHz, CHCl3, 20 ꢀC): δ=162.0, 150.7,
149.6, 146.7, 139.8, 131.6, 129.7, 121.0, 120.5, 34.5, 26.1, 24.1,
22.7 ppm. Anal. calcd for C40H52N2Cl2Zn: C, 68.91; H, 7.52; N,
4.02. Found: C, 68.80; H, 7.49; N, 4.07. Crystals suitable for X-
ray diffraction were grown from the vapor diffusion of Et2O into
chloroform.
Acknowledgment. This work was funded by the Helios Solar
Energy Research Center, which is supported by the Director,
Office of Science, Office of Basic Energy Sciences of the U.S.
Department of Energy, under Contract No. DE-AC02-
05CH11231.
FeCl2tripbipy. 70.9 mg FeCl2 4H2O, 0.357 mmol. Yield 218
3
mg, 0.318 mmol, 89.1%. Anal. calcd for C40H52N2Cl2Fe: C,
69.87; H, 7.62; N, 4.07. Found: C, 68.68; H, 7.75; N, 3.82.
Crystals suitable for X-ray diffraction were grown from vapor
diffusion of pentane into chloroform.
CoCl2tripbipy. 46.3 mg CoCl2, 0.357 mmol. Yield 202 mg,
0.292 mmol, 82.0%. Anal. calcd for C40H52N2Cl2Co: C, 69.56;
H, 7.59; N, 4.06. Found: C, 68.73; H, 7.53; N, 4.04. Crystals
Supporting Information Available: Figures S1-S7 and a cif
file. This material is available free of charge via the Internet at
http://pubs.acs.org.