Examination of the pyridine binding to the bifunctional lewis acid B,B′-diphenyldiboradiferrocene

Article abstract of DOI:10.1021/om8001608

The binding behavior of the bifunctional Lewis acid (η⁵- C₅H₅Fe)₂[μ-(C₅H ₃(BPh))₂] (3) toward different pyridine derivatives as the Lewis base has been investigated. The 1:1 and

Full text of DOI:10.1021/om8001608

3056
Organometallics 2008, 27, 3056–3064
Examination of the Pyridine Binding to the Bifunctional Lewis Acid
B,B′-Diphenyldiboradiferrocene
Thilagar Pakkirisamy,† Krishnan Venkatasubbaiah,† W. Scott Kassel,§,#
Arnold L. Rheingold,§ and Frieder Ja¨kle*,†
Department of Chemistry, Rutgers UniVersity-Newark, 73 Warren Street, Newark, New Jersey 07102, and
UniVersity of California, San Diego, La Jolla, California 92093
ReceiVed February 21, 2008
The binding behavior of the bifunctional Lewis acid (η5-C5H5Fe)2[µ-(C5H3(BPh))2] (3) toward different
pyridine derivatives as the Lewis base has been investigated. The 1:1 and 1:2 adducts, 3 · D and 3 · D2
(donor, D ) 4-tert-butylpyridine (tBupy) and 4-dimethylaminopyridine (DMAP)), were obtained by low-
temperature crystallization or slow solvent evaporation techniques. The complexes were fully characterized
by multinuclear NMR, IR, and elemental analysis. Single-crystal X-ray diffraction data were collected
for the 1:1 adduct with DMAP and for both 1:2 adducts, and the marked structural changes upon pyridine
binding are discussed in detail. In solution, the 1:2 adducts were found to readily undergo dissociation
with formation of the 1:1 adducts as the dominant species at room temperature. Variable-temperature
NMR and UV-visible titration studies demonstrate that the first binding process is more than 5 orders
of magnitude more favorable than the second binding process.
formation of reversible ansa-bridges through intramolecular
Lewis acid-base bonding and the assembly of coordination
polymers through intermolecular Lewis acid-base interactions
have been demonstrated by Wagner and co-workers for 1,1′-
diborylferrocenes (1, M ) Fe(II)).6–8 Jutzi and co-workers
introduced a trinuclear gallium-bridged ferrocenophane [{Fe(η5-
C5H4)2}3Ga2] and studied the electronic properties, coordination
behavior, and incorporation into polymeric materials.9 Aldridge
and co-workers demonstrated the use of 1,1′-diborylated fer-
rocenes for fluoride sensing applications.10 Related diborylco-
baltocenium complexes (1, M ) Co(III)) have been reported
by Herberich and co-workers to form an ansa-bridge between
the Cp rings upon anion binding.11 We have investigated the
binding properties of ferrocene-based heteronuclear bidentate
Introduction
Bifunctional organoboranes are widely studied as receptors
for neutral and anionic substrates and as Lewis acid activators
for various organic and organometallic reactions.1,2 They also
have attracted considerable interest as building blocks for the
preparation of new electronically interesting materials.3,4
The use of metallocenes as a redox-active backbone has
recently been investigated, and several examples of this intrigu-
ing class of bifunctional Lewis acids are worth noting.2,3,5 The
* To whom correspondence should be addressed. E-mail: fjaekle@
rutgers.edu.
† Rutgers University Newark.
§ University of California, San Diego.
# Current address: Department of Chemistry, Villanova University, 800
Lancaster Ave., 215D Mendel Science Center, Villanova, PA 19085.
(1) (a) Vaugeois, J.; Simard, M.; Wuest, J. D. Coord. Chem. ReV. 1995,
145, 55–73. (b) Piers, W. E.; Irvine, G. J.; Williams, V. C. Eur. J. Inorg.
Chem. 2000, 10, 2131–2142. (c) Hoefelmeyer, J. D.; Schulte, M.; Tschinkl,
M.; Gabbaï, F. P. Coord. Chem. ReV. 2002, 235, 93–103. (d) Dembitsky,
V. M.; Abu Ali, H.; Srebnik, M. Appl. Organomet. Chem. 2003, 17, 327–
345. (e) Wedge, T. J.; Hawthorne, M. F. Coord. Chem. ReV. 2003, 240,
111–128. (f) Melaïmi, M.; Gabbaï, F. P. AdV. Organomet. Chem. 2005,
53, 61–99.
(6) (a) Ja¨kle, F.; Priermeier, T.; Wagner, M. Organometallics 1996, 15,
2033–2040. (b) Herdtweck, E.; Ja¨kle, F.; Wagner, M. Organometallics 1997,
16, 4737–4745. (c) Grosche, M.; Herdtweck, E.; Peters, F.; Wagner, M.
Organometallics 1999, 18, 4669–4672. (d) Dinnebier, R. E.; Wagner, M.;
Peters, F.; Shankland, K.; David, W. I. F. Z. Anorg. Allg. Chem. 2000,
626, 1400–1405.
(7) Fontani, M.; Peters, F.; Scherer, W.; Wachter, W.; Wagner, M.;
Zanello, P. Eur. J. Inorg. Chem. 1998, 1453–1465.
(2) Ja¨kle, F. In Group 13 Chemistry: From Fundamentals to Applica-
tions; Shapiro, P. J., Atwood, D., Eds.; ACS: Washington, DC, 2002;
Chapter 7.
(3) Ma, K.; Scheibitz, M.; Scholz, S.; Wagner, M. J. Organomet. Chem.
2002, 652, 11–19.
(8) Scheibitz, M.; Bolte, M.; Lerner, H. W.; Wagner, M. Organometallics
2004, 23, 3556–3559.
(9) (a) Jutzi, P.; Lenze, N.; Neumann, B.; Stammler, H.-G. Angew.
Chem., Int. Ed. 2001, 40, 1424–1427. (b) Althoff, A.; Jutzi, P.; Lenze, N.;
Neumann, B.; Stammler, A.; Stammler, H. G. Organometallics 2003, 22,
2766–2774. (c) Althoff, A.; Eisner, D.; Jutzi, P.; Lenze; N.; Neumann, B.;
Schoeller, W. W.; Stammler, H. G. Chem.-Eur. J. 2006, 12, 5471–5480.
(10) (a) Bresner, C.; Aldridge, S.; Fallis, I. A.; Jones, C.; Ooi, L.-L.
Angew. Chem., Int. Ed. 2005, 44, 3606–3609. (b) Aldridge, S.; Bresner,
C.; Fallis, I. A.; Coles, S. J.; Hursthouse, M. B. Chem. Commun. 2002,
740–741. (c) Bresner, C.; Day, J. K.; Coombs, N. D.; Fallis, I. A.; Aldridge,
S.; Coles, S. J.; Hursthouse, M. B. Dalton Trans. 2006, 3660–3667. (d)
Day, J. K.; Bresner, C.; Coombs, N. D.; Fallis, I. A.; Ooi, L.-L.; Aldridge,
S. Inorg. Chem. 2008, 47, 793–804.
(4) (a) Ding, L.; Ma, K. B.; Durner, G.; Bolte, M.; de Biani, F. F.;
Zanello, P.; Wagner, M. J. Chem. Soc., Dalton Trans. 2002, 156, 6–1573.
(b) Heilmann, J. B.; Scheibitz, M.; Qin, Y.; Sundararaman, A.; Ja¨kle, F.;
Kretz, T.; Bolte, M.; Lerner, H.-W.; Holthausen, M. C.; Wagner, M. Angew.
Chem., Int. Ed. 2006, 45, 920–925. (c) Jaska, C. A.; Emslie, D. J. H.; Bosdet,
M. J. D.; Piers, W. E.; Sorensen, T. S.; Parvez, M. J. Am. Chem. Soc. 2006,
128, 10885–10896. (d) Jaska, C. A.; Piers, W. E.; McDonald, R.; Parvez,
M. J. Org. Chem. 2007, 72, 5234–5243.
(5) (a) Examples of binding studies with organoboranes containing other
pendant transition metal complexes: Matsumi, N.; Chujo, Y.; Lavastre, O.;
Dixneuf, P. H. Organometallics 2001, 20, 2425–2427. (b) Lee, M. H.;
Gabbai, F. P. Inorg. Chem. 2007, 46, 8132–8138. (c) Sun, Y.; Ross, N.;
Zhao, S. B.; Huszarik, K.; Jia, W. L.; Wang, R. Y.; Macartney, D.; Wang,
S. N. J. Am. Chem. Soc. 2007, 129, 7510–7511. (d) Zhao, S. B.; McCormick,
T.; Wang, S. Inorg. Chem. 2007, 46, 10965–10967.
(11) (a) Herberich, G. E.; Fischer, A.; Wiebelhaus, D. Organometallics
1996, 15, 3106–3108. (b) Herberich, G. E.; Englert, U.; Fischer, A.;
Wiebelhaus, D. Organometallics 1998, 17, 4769–4775. (c) Herberich, G. E.;
Englert, U.; Fischer, A.; Wiebelhaus, D. Eur. J. Inorg. Chem. 2004, 4011–
4020.
10.1021/om8001608 CCC: $40.75
2008 American Chemical Society
Publication on Web 06/13/2008

Pyridine Binding to B,B′-Diphenyldiboradiferrocene
Organometallics, Vol. 27, No. 13, 2008 3057
Scheme 1. Formation of Adducts 4 and 5 (py ) pyridine; tBupy
) 4-tert-butylpyridine; DMAP ) 4-dimethylaminopyridine)
orange crystalline solid. The monocomplexed species 4b and
4c were obtained as red crystalline solids in 82% and 80% yield,
respectively, by layering of a 1:1 mixture of 3 and the respective
pyridine derivatives in CH2Cl2 at -35 °C with hexanes. The
1
stoichiometry of the individual adducts was confirmed by H
NMR integration, elemental analysis, and in the case of 4b,
5b, and 5c single-crystal X-ray diffraction analysis.
Solid State Structures. Orange block-like single crystals of
the bis-adduct 5b were grown by slow solvent evaporation of
a 1:2 mixture of 3/tBupy in chloroform, and needle-like orange
crystals of 5c suitable for X-ray diffraction analysis formed from
a mixture of 3 and 2 equiv of DMAP in CH2Cl2 at -35 °C. In
both cases the trans-isomer was obtained selectively upon
crystallization. The molecular structures of trans-5b and trans-
5c are displayed in Figure 1, and selected geometric parameters
are provided in Table 1. In both structures, the symmetry
equivalence of the ferrocene moieties, phenyl rings, and pyridine
ligands, respectively, allows the molecule to reside on a
crystallographic inversion center.
Lewis acids and, for example, successfully applied them as chiral
reagents in stereoselective allylation reactions.12
Upon pyridine binding, the geometric features of the dibo-
radiferrocene core change dramatically. While for the free acid
3 the central C4B2 ring of the bridging dibora-s-indacene moiety
is significantly tilted toward the CpFe fragments, for trans-5b
and trans-5c the opposite is true in that the boron centers are
bent away from the ferrocene moieties with interplanar angles
between the Cp and C4B2 rings of 5.5° and 4.2°, respectively
(see also Figure 2). This effect can be traced back to the
delocalized interaction between the electron-deficient boron
centers and the electron-rich iron atoms in 3,8,17,18 which is
absent in the complexes due to nucleophile binding to the
formerly empty p orbital of boron. In addition, steric congestion
at boron apparently forces the two ferrocene moieties to bend
away from the central C4B2 ring.18 Steric strain upon pyridine
binding is further reflected in significant Cp//Cp tilt angles of
5.4° (trans-5b) and 6.1° (trans-5c) between the substituted and
unsubstituted Cp rings of each of the ferrocene moieties. The
B-C bonds are also slightly longer than in 3 due to the hy-
bridization change upon expansion of the coordination sphere
to a tetrahedral environment.19 Consequently, compared to the
free acid, the B · · · B distance increases by 0.175 and 0.193 Å
for trans-5b and trans-5c, respectively. All these factors
ultimately lead to expansion of the molecule and considerably
longer Fe · · · Fe distances of 5.940 and 5.910 Å relative to that
of 5.123 Å in 3. An illustration of these effects is provided in
Figure 2.
Recently, we communicated the synthesis, redox chemistry,
and binding properties of an unusual ortho-diborylated ferrocene
dimer (3).13 A particularly interesting feature of the bifunctional
Lewis acid 3 is that the Lewis acidity of the boron centers can
be reversibly adjusted through oxidation of the iron centers.14
Another unusual aspect is that the three-dimensional structure
of 3 and consequently the separation of the Fe centers of the
ferrocene moieties can be reversibly modified through redox
chemistry.15 An alternative method of modulating the three-
dimensional geometry of 3 is through nucleophile binding. Here,
we provide full details of the binding properties of 3 toward
different pyridine derivatives16 and the effects on the molecular
geometry of the diboradiferrocene framework.
Results and Discussion
Synthetic Aspects. The bifunctional Lewis acid 3 was
allowed to react at RT in chlorinated solvents with 2 molar equiv
of 4-tert-butylpyridine (tBupy) and 4-dimethylaminopyridine
(DMAP), respectively (Scheme 1). Compound 5b was isolated
in 88% yield upon evaporation of the solvents and extraction
with hexanes, and 5c was obtained in 77% yield by crystal-
lization from CH2Cl2 at -35 °C. For the synthesis of 5a,
compound 3 was dissolved in pyridine and the mixture was kept
at -35 °C for 1 day, giving the bis-adduct in 80% yield as an
The B-N distances of 1.652(3) and 1.640(4) Å for trans-5b
and trans-5c, respectively, are similar to one another and in
the range of other B-N bond lengths reported for organoborane
pyridine complexes (mean B-N distance from CSD search for
R3B · Py: 1.650 Å). They are close to the B-N distances in
related ferrocenylborane pyridine adducts such as Fc3B · Py20
(1.658 Å) and FcBMe2 · DMAP17 (1.670 Å). However, they are
longer than for reported pyridine adducts of B(C6F5)3 such as
B(C6F5)3 · DMAP21 (1.604 Å).
(12) (a) Boshra, R.; Sundararaman, A.; Zakharov, L. N.; Incarvito, C. D.;
Rheingold, A. L.; Ja¨kle, F. Chem.-Eur. J. 2005, 11, 2810. (b) Boshra, R.;
Venkatasubbaiah, K.; Doshi, A.; Lalancette, R. A.; Kakalis, L.; Ja¨kle, F.
Inorg. Chem. 2007, 46, 10174–10186. (c) Boshra, R.; Doshi, A.; Ja¨kle, F.
Angew. Chem., Int. Ed. 2008, 47, 1134–1137. (d) Boshra, R.; Doshi, A.;
Ja¨kle, F. Organometallics 2008, 27, 1534–1541.
(17) Scheibitz, M.; Bolte, M.; Bats, J. W.; Lerner, H.-W.; Nowik, I.;
Herber, R. H.; Krapp, A.; Lein, M.; Holthausen, M.; Wagner, M. Chem.-
Eur. J. 2005, 11, 584–603.
(13) Venkatasubbaiah, K.; Zakharov, L. N.; Kassel, W. S.; Rheingold,
A. L.; Ja¨kle, F. Angew. Chem., Int. Ed. 2005, 44, 5428–5433.
(14) Venkatasubbaiah, K.; Nowik, I.; Herber, R. H.; Ja¨kle, F. Chem.
Commun. 2007, 2154–2156.
(18) Scheibitz, M.; Heilmann, J. B.; Winter, R. F.; Bolte, M.; Bats, J. W.;
Wagner, M. Dalton Trans. 2005, 159–170.
(19) Metz, M. V.; Schwartz, D. J.; Stern, C. L.; Marks, T. J.; Nickias,
P. N. Organometallics 2002, 21, 4159–4168.
(15) (a) Venkatasubbaiah, K.; Doshi, A.; Nowik, I.; Herber, R. H.;
Rheingold, A. L.; Ja¨kle, F. Chem.-Eur. J. 2008, 14, 444-458. (b)
Venkatasubbaiah, K.; Pakkirisamy, T.; Lalancette, R. A.; Ja¨kle, F. Dalton
Trans., in press.
(20) Soriano-Garcia, M.; Toscano, A.; Lopez, T.; Campero-Celis, A. J.
Crystallogr. Spectrosc. Res. 1987, 17, 719–728.
(21) Lesley, M. J. G.; Woodward, A.; Taylor, N. J.; Marder, T. B.;
Cazenobe, I.; Ledoux, I.; Zyss, J.; Thornton, A.; Bruce, D. W.; Kakkar,
A. K. Chem. Mater. 1998, 10, 1355.
(16) Acetonitrile does not significantly bind to 3 at RT; see ref 14.

3058 Organometallics, Vol. 27, No. 13, 2008
Pakkirisamy et al.
Figure 1. Molecular structures of trans-5b (left) and trans-5c (right) with thermal ellipsoids at the 50% probability level; cocrystallized
solvent molecules (CHCl3 for trans-5c) and hydrogen atoms are omitted for clarity.
Table 1. Selected Interatomic Distances (Å) and Angles (deg) for 4b,
trans-5b, and trans-5c in Comparison to Those of 3
4b
3
for B2
for B1
trans-5b trans-5c
B-CCp
1.546(2) 1.550(7) 1.610(7) 1.610(4) 1.619(4)
1.546(2) 1.553(7) 1.608(6) 1.602(4) 1.616(4)
1.564(2) 1.570(7) 1.609(7) 1.628(5) 1.628(4)
1.659(6) 1.652(3) 1.640(4)
B-CPh
B-N
Fe · · · B
3.032
2.957
3.103
5.123
2.920
3.076
3.298
3.414
3.198
5.445
3.408
3.376
3.278
5.940
3.371
3.396
3.296
5.910
Fe · · · B*
B · · · B*
Fe · · · Fe*
CCp-B-CCp
115.8(1) 115.6(4) 110.1(4) 110.1(2) 109.6(2)
CpCentroid-C-B 167.9
161.7
168.9
177.14a 174.8a
174.19a 176.3a
5.5
177.3a
175.9a
4.2
164.4
15.9
b
Cp//C4B2
staggering angle 11.4
23/24
33
36.1
Cpcent-Cpcent
Cp//Cp tilt
3.308
1.2
3.308/3.326
0.5/4.3
3.316
5.4
3.316
6.1
a The boron atom is above the plane of the respective substituted Cp
rings (exoside). b The dihedral angles, which were determined to be 8.3°
and 14.8°, are not meaningful because the C4B2 ring is strongly
puckered.
Figure 3. Molecular structure of 4b with thermal ellipsoids at the
50% probability level; hydrogen atoms are omitted for clarity.
Fe1 · · · B1 ) 3.298; Fe1 · · · B2 ) 2.920, Fe2 · · · B1 ) 3.414;
Fe2 · · · B2 ) 3.076 Å.
5b. As observed also in the free acid 3, both ferrocene moieties
are strongly tilted toward the free triarylborane group due to a
delocalized iron-boron interaction. However, they are not tilted
toward the boron that is ligated by the pyridine. As a
consequence, the Fe · · · B distances are considerable shorter for
the tricoordinate boron (Fe1 · · · B2 ) 2.920, Fe2 · · · B2 ) 3.076
Å) than for the tetracoordinate boron (Fe1 · · · B1 ) 3.298,
Fe2 · · · B1 ) 3.414 Å). Additional plots are provided in the
Supporting Information (Figure S8) to further illustrate these
unusual geometric features of 4b. Another result of these effects
is that the central C4B2 ring is strongly puckered, which stands
in contrast to the only slightly puckered structure of the 1:1
acetonitrile adduct of the perfluorodiboraanthracene derivative
[C6F4(µ-BC6F5)2C6F4] reported by Marks.19 In addition, the
dihedral angle between the two substituted Cp rings in 4b is
much larger at 19.7° than that for the two C6F4 rings in Marks’s
complex (9.9(3)°). The boron-carbon bond lengths at the
coordinated boron center (B1) are elongated compared to those
Figure 2. Illustration of the geometric changes to the diboradifer-
rocene framework upon pyridine binding to 3.
We obtained orange single crystals of the mono-adduct 4b
from a 1:1 mixture of 3/tBupy in CH2Cl2 that was layered with
hexanes and subsequently kept at -35 °C. An X-ray structure
plot of 4b is shown in Figure 3, and selected bond lengths and
angles are provided in Table 1. The molecular structure confirms
the formation of a 1:1 complex, which consists of one
tricoordinate Lewis acidic borane moiety and one coordinatively
saturated tetracoordinate boron center. Thus, 4b shows features
reminiscent of both the free Lewis acid 3 and the complex trans-

Pyridine Binding to B,B′-Diphenyldiboradiferrocene
Organometallics, Vol. 27, No. 13, 2008 3059
Figure 4. VT 11B NMR spectra for complexation of 3 with 1 equiv
of 4-tert-butylpyridine in CDCl3 (25 mM).
of the free Lewis acid site (B2) due to the change in the
hybridization resulting from expansion of the coordination
sphere to a tetrahedral environment for B1.19 The B-N distance
of 1.659(6) Å for 4b is similar to those of the bis-adducts trans-
5b and trans-5c (1.652(3) and 1.640(4) Å) and long in
comparison to that of the adduct B(C6F5)3 · DMAP.21
IR spectroscopy provides another potentially useful tool to
further study the complex formation through analysis of the
B-N stretching modes. Several bands are observed in the region
typical of B-N stretching vibrations (ca. 1150 to 1050 cm-1 7
)
at 1103, 1072, 1048, 1032 cm-1 for trans-5b and at 1105, 1078,
1034 cm-1 for trans-5c; similar modes are also found for the
mono-adducts 4b at 1105, 1072, 1046 cm-1 and 4c at 1104,
1081, 1050, 1032 cm-1. However, the free acid 3 also shows a
number of bands in this region (1107, 1082, 1041 cm-1). Thus,
a conclusive assignment as to which of the bands may represent
the B-N stretch was not possible. More definite evidence of
nucleophile binding is provided from inspection of the region
for the CdC/CdN ligand stretching modes around 1450 to 1650
cm-1, where the free acid 3 shows only very weak IR modes.
Intense bands are found for trans-5c at 1629 and 1542 cm-1
and for 4c at 1632 and 1543 cm-1, which are similar to those
reported for FcBMe2(DMAP)7 (1636, 1544 cm-1) and strongly
shifted to higher energy in comparison to the free ligand
vibrations (1603, 1537, 1519 cm-1). The spectra of trans-5b
and 4b feature prominent bands at 1628, 1504 cm-1 and 1627,
1503 cm-1, respectively, which are similar to those for
FcBMe2(picoline)7 (1628, 1507 cm-1).
Solution Binding Studies: Observation of Mono-
adducts in Solution. While the bis-adducts 5b and 5c are readily
isolated as solid materials, their formation in solution is not
favorable. Therefore, we will first discuss the properties of the
mono-adducts 4b and 4c, which are more readily observed in
solution. They were generated by mixing a CDCl3 solution of
3 in a 1:1 molar ratio with tBupy and DMAP, respectively, and
the resulting solutions were subjected to VT NMR spectroscopic
analysis.
The 11B NMR spectrum of 4b in CDCl3 at -50 °C shows
two distinct resonances of approximately equal intensity at ca.
54 and -0.2 ppm, consistent with one boron center that is
coordinated by pyridine and one tricoordinated borane moiety
(Figure 4). As expected, the resonance at -0.2 ppm for the
tetracoordinate boron is considerably sharper than that for the
tricoordinate borane moiety. The 11B NMR spectra undergo
distinct changes with temperature as shown for 4b in Figure 4.
The signal due to the tetracoordinate boron moiety becomes
smaller as the temperature is raised and almost disappears at
+50 °C. This indicates that at higher temperature the equilib-
Figure 5. Aromatic/Cp region of the NOESY NMR spectrum of
4b in CDCl3 at -50 °C (20 mM); a small signal due to a trace
amount of 3 is marked with an asterisk.
rium is shifted toward the free acid 3. A separate peak for the
precursor 3 is not observed, but this is not surprising since the
signal is very broad and the peaks for 3 and the tricoordinate
boron atom of 4b are expected to appear in the same region
and hence to overlap. Similar results are obtained for 4c, except
that the resonance for the tetracoordinate boron, which is found
at -1.9 ppm, is relatively more intense, indicating stronger
binding of DMAP. At -50 °C only one signal could be
identified at -1.9 ppm, presumably due to extreme broadening
of the second resonance.
1
The H NMR spectra are strongly broadened at RT, and
especially for 4b the signals are close to those of the free acid
3. However, at -50 °C well-resolved signals are observed,
which can be assigned to the mono-adducts 4b and 4c. The 1H
NMR data are discussed here for 4b; very similar data were
obtained also for 4c. The chemically nonequivalent Ph groups
give rise to two separate sets of signals. For 4b, one set with a
distinct doublet for the ortho-protons at δ 7.98 is attributed to
the Ph group (A) attached to the tricoordinate boron, while an
upfield shift of the ortho-protons of the Ph ring (B) to δ 7.25 is
observed upon pyridine coordination (see F1 dimension in
Figure 5). This assignment is further confirmed by the observa-
tion of NOE peaks between the pyridyl moiety and the Ph group
B, but not the Ph group A (not shown). Two sets of signals are
also found for the ferrocene moieties, because one of the
ferrocene moieties (A) is positioned beside the tBuPy ring and
the other one (B) next to the Ph group B. The individual
resonances in the Cp region at δ 4.69, 4.65, 4.43, 3.21 are
assigned to the ferrocene moiety (A) and the signals at δ 4.71,
4.57, 4.45, 3.35 to the ferrocene moiety (B) based on H,H-COSY

3060 Organometallics, Vol. 27, No. 13, 2008
Pakkirisamy et al.
Figure 7. UV-visible titration for the complexation of 3 with
4-dimethylaminopyridine in CH2Cl2 (for 3, c ) 2.99 × 10-4 M).
the conditions employed, only the mono-adduct 4c is formed
without any significant amount of 5c. Fitting of the spectral
data to a 1:1 binding process by analysis of 248 different
wavelengths using the program Hyperquad gave a binding
constant of log K1 ) 4.28(1). A similar UV-visible titration
with tBupy gave log K1 ) 2.49(1) (see Supporting Information),
which is in reasonably good agreement with the NMR results
described above. Thus, binding of the first molecule of DMAP
to 3 is about 2 orders of magnitude stronger than that of tBupy.
Analysis of the spectral titration curves also allows the
prediction of the UV-visible spectra of the mono-adducts 4b
and 4c. The calculated data are shown in comparison to the
spectrum of the free acid in Figure S4 of the Supporting
Information. The intense low-energy absorption of 3 at 498 nm
(ε ) 5340 M-1 cm-1) experiences a hypsochromic shift upon
pyridine binding, and the intensity is strongly diminished.
Absorption maxima of 475 nm (ε ) 2230 M-1 cm-1) and 476
nm (ε ) 1820 M-1 cm-1) are determined for 4b and 4c,
respectively.
Figure 6. Cp region of the VT 1H NMR spectra for the complex-
ation of 3 with 1 equiv of tert-butylpyridine in CDCl3 (20 mM). A
trace amount of ferrocene is marked with an astersisk. For detailed
assignments of the resonances of complex 4b see Figure 5.
correlation and NOE spectroscopy (Figure 5). The ortho-protons
of the Ph ring B display NOE peaks with Cp-5A and Cp-5B,
whereas those of the Ph ring A show cross-peaks with Cp-3A
and Cp-3B. Most importantly, a distinct cross-peak of the ortho-
tBuPy protons at δ 8.58 with the signal for the free Cp ring
C5H5-A at 3.21 ppm allows the assignment of that ferrocene
moiety as being adjacent to the pyridine group. A cross-peak
of the ortho-protons of the phenyl ring B with the resonance
for the free Cp ring B at δ 3.35 is then consistent with the
ferrocenyl group B in a position next to the phenyl ring B. The
assignment of the free Cp rings is further confirmed by a weak
cross-peak between the Cp-5B resonance at δ 4.71 and the signal
for the free Cp ring B at δ 3.35 (not shown).
Observation of the Bis-adducts in Solution. Formation of
the bis-adducts 5 from 3 and 2 equiv of base was not observed
in CDCl3 at room temperature, and only small amounts of the
DMAP adduct 5c were formed at -50 °C. The bis-adduct 5a
was, however, detected at low temperature in neat d5-pyridine,
and 5c was readily observed in CDCl3 in the presence of a 5-fold
excess of DMAP as described in the following.
1
The variable-temperature H NMR spectra over the range
from -40 to +60 °C are displayed in Figure 6. At low
temperature the mono-adduct 4b is the dominant species, as
already noted above, but the signals for the free acid 3 become
relatively more intense as the temperature is gradually increased.
At room temperature, the signals are strongly broadened, and
at +60 °C only one set of broad resonances is found that is
very close to the shifts for the free acid 3. In this temperature
range ligand dissociation becomes very prominent and well-
defined coalescence temperatures could not be determined.
However, analysis of the signal intensities over the range from
-40 to +25 °C allows for construction of a van’t Hoff plot for
the equilibrium between 3 and 4b. A linear plot of the
equilibrium constant ln K1 versus 1/T with good correlation of
R2 ) 0.9978 is obtained (see Figure S1 in the Supporting
Information). The enthalpy change for the tert-butylpyridine
binding was determined to be ∆Hass ) -63.8 ( 1.7 kJ mol-1
and the entropy change to ∆Sass ) -174 ( 7 J mol-1 K-1. A
binding constant of log K1(298) ) 2.1 ( 0.1 is deduced.
Initial NMR screening indicated that DMAP is much more
strongly bound than tBupy and the 1:1 complex with DMAP is
the dominant species even at ambient temperature. A set of small
signals for 3 indicates a certain degree of ligand dissociation
also for 4c, but the signals are strongly broadened and accurate
signal integration in the temperature range from +20 to +60 °C
was not possible. Hence, we decided to use UV-visible spectral
titrations, which provide good sensitivity at far lower concentra-
The 11B NMR of 3 in neat d5-pyridine revealed two distinct
signals of similar intensity at RT, consistent with the expected
structure of monocomplexed 4a described above (Figure 8).
When the temperature is gradually lowered to -50 °C, the signal
at ca. 55 ppm further broadens and can eventually not be
resolved. This suggests that the equilibrium is shifted toward
the doubly complexed species 5a. However, the observation that
the 11B NMR signal at ca. 0 ppm remains very broad indicates
that 5a is still in dynamic exchange with a smaller amount of
4a, which is not detectable. Similar 11B NMR data were obtained
when 3 was examined in the presence of a 5-fold excess of
DMAP in CDCl3 solution.
The low-temperature 1H NMR spectrum of 5a in d5-pyridine
at -50 °C (Figure 9) shows two sets of resonances in the Cp
region that can be assigned to the cis- and trans-isomers of 5a.
The individual assignments were further confirmed by 2D NMR
spectroscopy, and the ratio of cis-5a:trans-5a was determined
by 1H NMR integration to be 1:1.5. Similar patterns were
observed for 5c, which was detected at -50 °C upon dissolution
of 3 in the presence of a 5-fold excess of DMAP in CDCl3
(0.023 M in 3). The ratio of cis-5c:trans-5c was determined to
be 1:4, and the ratio of 5c:4c at that temperature was 2.7:1 on
1
tions than those typically used for H NMR analysis. A plot
1
for the titration of 3 with DMAP is shown in Figure 7. Under
the basis of H NMR integration.

Pyridine Binding to B,B′-Diphenyldiboradiferrocene
Organometallics, Vol. 27, No. 13, 2008 3061
Figure 10. Variable-temperature 1H NMR spectra of a 1:5 mixture
of 3/4-dimethylaminopyridine (DMAP) in CDCl3 (0.023 M in 3).
Only the region featuring the Me groups of the DMAP and the
unsubstituted Cp signals is shown. Labeling: “c” corresponds to
cis-5c; “t” corresponds to trans-5c; “free” denotes the signal due
to excess DMAP.
Figure 8. VT 11B NMR spectra of 4a/5a in d5-pyridine (20 mM).
and Cp-5 protons equivalent. However, the individual ferrocene
moieties are not equivalent, because one ferrocene moiety (A)
is flanked by two pyridine rings, while the other one (B) is close
to two phenyl groups. Consequently, two sets of signals are
observed for the free Cp rings and also for the Cp-4 and Cp-
3,5 protons. In contrast, the ferrocene moieties are equivalent
in the trans-isomer, which features an inversion center. Interest-
ing to point out is the unusually strong upfield shift of the
resonances for the free Cp rings (cis-5a, δ 3.08, 2.63/trans-5a,
δ 2.82; cis-5c, δ 2.73, 2.52/trans-5c, δ 2.62), which is attributed
to the ring current effect of the adjacent phenyl and pyridyl
substituents on boron.
Thermodynamic data for the binding of the second molecule
1
of DMAP to 3 were obtained by H NMR analysis of a 1:5
mixture of 3/DMAP at variable temperatures (Figure 10). Under
the conditions applied the concentration of 3 is negligible so
that only the equilibrium between 4c and 5c needs to be
considered. Van’t Hoff analysis using integration of the signals
for the free Cp rings of 4c and 5c at variable temperatures gave
a linear plot with a correlation of R2 ) 0.9912. The binding
enthalpy was determined from the slope to be ∆Hass ) -44.5
( 3.4 kJ mol-1, and the entropy term was determined to be
∆Sass ) -171 ( 14 J mol-1 K-1. The stepwise binding constant
at 298 K was calculated to be log K2(298) ) -1.1 ( 0.1 (log
K2(223) ) +1.5 ( 1), which shows that binding of the second
DMAP molecule is only favorable at low temperature and by
more than 5 orders of magnitude weaker than the first binding
process.
Conclusions
With pyridine derivatives as the Lewis base the bifunctional
Lewis acid 3 forms isolable complexes, in which either one or
both Lewis acidic boron centers are coordinated. Single-crystal
X-ray diffraction data of the 1:1 and 1:2 adducts reveal
pronounced structural changes of the molecular framework upon
pyridine binding. The delocalized interaction with the electron-
rich ferrocene moieties, which is responsible for tilting of the
ferrocene moieties toward boron in the free acid, is intercepted
by the tetracoordination of boron. In the case of the mono-adduct
a strongly puckered central C4B2 ring results because one of
the boron atoms is able to interact with the ferrocene moieties
while the empty p orbital of the other one is engaging in a
donor-acceptor interaction with the pyridine nitrogen. Binding
1
Figure 9. Cp region of the H NMR spectrum of 3 in d5-pyridine
at -50 °C (20 mM). Labeling: c corresponds to cis-5a; t
corresponds to trans-5a; * corresponds to a small amount of the
mono-adduct 4a.
For both compounds, the cis-isomer features a mirror plane
that intersects the two ferrocene moieties and renders the Cp-3

3062 Organometallics, Vol. 27, No. 13, 2008
Pakkirisamy et al.
CA) using the VNMR software package (Varian Inc., Palo Alto,
CA). In order to decrease t1 ridges arising from incorrect treatment
of the first data point in the discreet Fourier transform (FT)
algorithm, the spectrum corresponding to the first t1 value was
divided by 2 prior to FT along t1.31 Shifted (COSY) or unshifted
(NOESY, HMQC) Gaussian window functions were used in both
dimensions. Data sets were zero-filled in the t1 dimension yielding
1000 × 1000 final matrices that have not been symmetrized.
UV-visible absorption data were acquired on a Varian Cary 500
UV-vis/NIR spectrophotometer. Solutions were prepared using a
microbalance ((0.1 mg) and volumetric glassware and then charged
into quartz cuvettes with sealing screw caps (Starna) inside the
glovebox. The titrations of 3 with tBupy and DMAP were per-
formed by sequential addition of a stock solution of the pyridine
derivative in CH2Cl2 to a solution of 3 in CH2Cl2 in a Quartz cuvette
using a microliter syringe. The binding constants were determined
with the Hyperquad program by assuming a 1:1 binding process
(justified on the basis of the NMR studies) and using 32 titration
points and 282 different wavelengths spanning a range from ca.
315 to 609 nm.
IR spectra were obtained on a Nicolet IR-200 spectrometer. All
samples were prepared as KBr pellets. Elemental analyses were
performed by Quantitative Technologies Inc., Whitehouse, NJ.
Details of the X-ray diffraction experiments and crystal structure
refinements for 4b, trans-5b, and trans-5c are provided in Table
2. Data were collected on Bruker SMART APEX CCD diffrac-
trometers using Cu KR (1.54178 Å) or Mo KR (0.71073 Å)
radiation. A SADABS32 absorption correction was applied. The
structures were solved using direct methods, completed by subse-
quent difference Fourier syntheses and refined by full matrix least-
squares procedures on F2. All non-hydrogen atoms were refined
with anisotropic displacement coefficients. The H atoms were placed
at calculated positions and were refined as riding atoms. All software
and source scattering factors are contained in the SHELXTL
program package.33 Crystallographic data for the structures of 4b,
trans-5b, and trans-5c have been deposited with the Cambridge
Crystallographic Data Center as supplementary publication nos.
CCDC-678401, CCDC-275646, and CCDC-678402, respectively.
Copies of the data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: (+44) 1223-
336-033; e-mail: deposit@ccdc.cam.ac.uk).
of the second pyridine leads to the more symmetric structure
of compounds 5. However, the latter show features indica-
tive of increased steric crowding. For instance, the ferrocene
moieties are bent away from the boron centers, as evident from
the position of the boron atoms above the planes of the
substituted Cp rings. Moreover, the Cp rings of the individual
ferrocene moieties are tilted by about 5-6°, opening up the
ferrocenes toward the dibora-bridge. Consistent with increased
steric strain in 5 is the facile pyridine dissociation from the bis-
adducts in solution. This results in a very small binding constant
determined for the coordination of the second relative to the
first DMAP molecule, and hence an unusually pronounced
negative binding cooperativity effect.
Experimental Section
Materials and General Methods. Pyridine, 4-tert-butylpyridine
(tBupy), and 4-dimethylaminopyridine (DMAP) were purchased
from Acros. All reactions and manipulations were carried out under
an atmosphere of prepurified nitrogen using either Schlenk tech-
niques or an inert-atmosphere glovebox (Innovative Technologies).
Hydrocarbon and chlorinated solvents were purified using a solvent
purification system (Innovative Technologies), and the chlorinated
solvents were subsequently degassed via several freeze-pump-thaw
cycles.
The 499.9 MHz 1H NMR, 125.7 MHz 13C NMR, and 160.3
11B NMR spectra were recorded on a Varian INOVA NMR
spectrometer (Varian Inc., Palo Alto, CA) equipped with a boron-
free 5 mm dual broadband gradient probe (Nalorac, Varian Inc.,
Martinez, CA). All solution 1H and 13C NMR spectra were
referenced internally to the solvent signals, and 11B NMR spectra
were referenced externally to BF3 · Et2O in C6D6 (δ ) 0).
Two-dimensional proton correlation spectroscopy, COSY,22
proton NOE spectroscopy, NOESY,23 and one-bond proton-carbon
correlation spectroscopy, HMQC,24 measurements were recorded
in the absolute value mode (COSY) or the phase-sensitive mode
(NOESY, HMQC) by employing the TPPI improvement25 of the
States-Haberkorn-Ruben hypercomplex method.26 Selection of
desirable coherences and artifact suppression were accomplished
by z-gradients (COSY, echo N-type coherence selection27) or phase
cycles of 32 (NOESY) or 16 (HMQC) steps. The NOESY data set
was acquired using a 400 ms mixing time. The HMQC pulse
sequence included the BIRD filter to suppress signals from C12-
attached protons28 and C13-decoupling during proton acquisition,
using a 3.6 kHz field strength and the GARP decoupling scheme.29
Typically, 256 t1 increments of 2000 complex data points over 5.5
kHz (proton) and 22.5 kHz (carbon) spectral widths were collected
with 1 (COSY), 32 (NOESY), or 16 (HMQC) scans per t1
increment, preceded by 16 or 32 dummy scans, and a relaxation
delay of 2 s. Baseline distortion was addressed by properly adjusting
the sampling delay and the signal phase.30 Data sets were processed
on a Sun Blade 100 workstation (Sun Microsystems Inc., Palo Alto,
Complexation of 3 with 1 equiv of 4-tert-Butylpyridine:
Synthesis of 4b. To a solution of 3 (13.9 mg, 25 µmol) in CH2Cl2
(3 mL) was added tBupy (5.2 mg, 38 µmol) in CH2Cl2 (2 mL) at
room temperature, and the mixture was stirred for 1 h. The reaction
solution was layered with hexanes and kept for crystallization for
2 days at -35 °C. The product was isolated as orange block-like
crystals. Yield: 14.1 mg (82%). IR (cm-1): 3061, 2967, 1627, 1503,
1428, 1406, 1286, 1192, 1104, 1072, 1045, 812, 736, 707, 570,
482. Anal. Calcd for (C41H39B2Fe2N): C 72.51 H 5.78 N 2.06.
Found: C 72.14 H 5.82 N 2.02.
Low-Temperature NMR Characterization of 4b. Complex 4b
was generated by mixing 3 with tBupy in CDCl3 in a 1:1 molar
ratio and studied by VT NMR spectroscopy. At room temperature
the equilibrium was found to be far on the side of the free acid (3):
1H NMR (500 MHz, CDCl3, 25 °C) δ 8.56 (br, 2H, o-tBupy), 7.96
(br, 4H, o-Ph), 7.49 (b m, 8H, m-tBupy, m,p-Ph), 5.04 (b, 2H, 4-Cp),
(22) (a) Aue, W. P.; Bartholdi, E.; Ernst, R. R. J. Chem. Phys. 1976,
64, 2229. (b) Bax, A.; Freeman, R. J. Magn. Reson. 1981, 44, 542.
(23) (a) Muller, L. J. Am. Chem. Soc. 1979, 101, 4481. (b) Jeener, J.;
Meier, B. H.; Bachmann, P.; Ernst, R. R. J. Chem. Phys. 1979, 71, 4546.
(24) (a) Macura, S.; Ernst, R. R. Mol. Phys. 1980, 41, 95–117. (b) Bax,
A.; Subramanian, S. J. Magn. Reson. 1986, 67, 565.
(25) (a) Redfield, A. G.; Kunz, S. D. J. Magn. Reson. 1975, 19, 250–
254. (b) Marion, D.; Wu¨thrich, K. Biochem. Biophys. Res. Commun. 1983,
113, 967.
4.92 (b, 4H, 3,5-Cp), 3.90 (b, 10H, free Cp), 1.37 (s, 9H, Me); 11
B
NMR (160 MHz, CDCl3, 25 °C) δ 55.5 (w1/2 ) 1300 Hz) and -0.7
(w1/2 ) 320 Hz) (ratio 27:1). At low temperature, the 1:1 adduct
(26) States, D. J.; Haberkorn, R. A.; Ruben, D. J. J. Magn. Reson. 1982,
48, 286–292.
(30) Hoult, D. I.; Chen, C. N.; Eden, H.; Eden, M. J. Magn. Reson.
1983, 51, 110.
(31) Otting, G.; Widmer, H.; Wagner, G.; Wu¨thrich, K. J. Magn. Reson.
1986, 66, 187–193.
(32) Sheldrick, G. M. SADABS, Version 2. Multi-Scan Absorption
Correction Program; University of Go¨ttingen: Germany, 2001.
(33) Sheldrick, G. M. SHELXTL, Version 6.14; Bruker AXS Inc.:
Madison, WI, 2004.
(27) von Kienlin, M.; Moonen, C. T. W.; van der Toorn, A.; van Zijl,
P. C. M. J. Magn. Reson. 1991, 93, 423.
(28) Garbow, J. R.; Weitekamp, D. P.; Pines, A. Chem. Phys. Lett. 1982,
93, 504.
(29) Shaka, A. J.; Barker, P. B.; Freeman, R. J. Magn. Reson. 1985,
64, 547.

Pyridine Binding to B,B′-Diphenyldiboradiferrocene
Organometallics, Vol. 27, No. 13, 2008 3063
Table 2. Details of X-ray Structure Determinations
4b
trans-5b
trans-5c
empirical formula
MW
C41H39B2Fe2N
679.05
C50H52B2Fe2N2
814.26
C48H48B2Fe2N4Cl6
1026.92
T, K
100(2)
213(2)
100(2)
wavelength, Å
cryst syst
space group
a, Å
b, Å
c, Å
1.54178
orthorhombic
Pna2(1)
9.5763(6)
19.3314(12)
17.4565(9)
90
90
90
3231.6(3)
4
1.396
0.71073
monoclinic
C2/c
28.2223(18)
8.3188(5)
21.8954(14)
90
127.6860(10)
90
4068.1(4)
1.54178
triclinic
j
P1
9.9484(4)
10.4634(4)
11.6449(4)
70.117(2)
78.568(3)
84.032(3)
1116.41(7)
1
1.527
8.828
528
R, deg
ꢀ, deg
γ, deg
V, Å3
Z
F
4
calc, g cm-3
1.527
0.751
1712
µ(Cu/MoKR), mm-1
F(000)
7.423
1416
cryst size, mm
θ range, deg
0.24 × 0.19 × 0.17
3.41 - 67.31
-11 e h e 11
-21 e k e 22
-20 e l e 20
16 928
0.30 × 0.20 × 0.15
1.82 - 28.22
-34 e h e 36
-11 e k e 10
-28 e 1 e 27
14 480
0.25 × 0.18 × 0.12
4.10 - 67.82
-8 e h e 11
-12 e k e 12
-13 e l e 13
10 152
limiting indices
no. of reflns collected
no. of indep reflns
absorp corr
refinement method
no. of data/restraints/params
GOF on F2
5309 [R(int) ) 0.1105]
4785 [R(int) ) 0.0362]
semiempirical from equivalents
full-matrix least-squares on F2
4785/0/253
1.150
3656 [R(int) ) 0.0275]
5309/1/419
0.935
3656/0/282
1.078
final R indices
R1 ) 0.0458
wR2 ) 0.0828
R1 ) 0.0810
wR2 ) 0.0953
0.003(5)
R1 ) 0.0527
wR2 ) 0.1257
R1 ) 0.0610
wR2 ) 0.1303
R1 ) 0.0459
wR2 ) 0.1159
R1 ) 0.0459
wR2 ) 0.1181
[I > 2σ(I)]a
R indices (all data)a
Flack param
peakmax/holemin (e Å-3
)
0.656/-0.382
0.517/-0.423
0.744/-0.383
2
2 2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) {∑[w(Fo - Fc ) ]/∑[w(Fo2)2]}1/2
.
was observed. Assignments are based on 2D NMR correlations
(COSY, HMQC, NOESY). For 4b (20 mM solution in CDCl3):
1H NMR (500 MHz, CDCl3, -50 °C) δ 8.58 (d, J ) 6.5 Hz, 2H,
o-tBupy), 7.99-7.97 (m, 2H, o-PhA), 7.53-7.49 (m, 5H, p-PhA,
m-PhA and m-tBupy), 7.25-7.23 (m, 4H, o,m-PhB), 7.18 (m, 1H,
p-PhB), 4.71 (pst, J ) 2 Hz, 1H, 4-CpB), 4.69 (pst, J ) 2 Hz, 1H,
4-CpA), 4.65 (nr, 1H, 5-CpA), 4.57 (nr, 1H, 3-CpB), 4.45 (nr, 1H,
5-CpB), 4.43 (nr, 1H, 3-CpA), 3.35 (s, 5H, CpB), 3.21 (s, 5H, CpA),
1.35 (s, 9H, tBu); 13C NMR (125.69 MHz, CDCl3, -50 °C) δ
164.8 (i-tBupy), 146.3 (o-tBupy), 144.8 (i-PhA), 133.2 (o-PhB),
133.0 (o-PhA), 128.0 (p-PhA), 127.4 (m-PhA), 126.7 (m-PhB), 124.8
(m-PhB), 121.6 (m-tBupy), 80.2 (i-CpA, i-CpB), 78.9 (3-CpA), 78.1
(5-CpA, 5-CpB), 76.5 (5-CpB), 76.2 (4-CpA), 75.8 (5-CpA), 74.8 (4-
CpB, 3-CpA), 69.7 (CpB), 68.4 (CpA), 35.6 (-CMe3), 30.2 (-CMe3);
11B NMR (160 MHz, CDCl3, -50 °C) δ 54 (w1/2 ) 4800) and
-0.2 (w1/2 ) 1700).
assignments are based on an H,H-COSY spectrum and comparison
with those of 4b. For 4c: 1H NMR (500 MHz, CDCl3, -50 °C) δ
8.15 (d, J ) 7.0 Hz, 2H, o-DMAP), 8.00 (d, J ) 7.0 Hz, 2H, o-PhA),
7.52-7.51 (m, 3H, p-PhA, and m-PhA), 7.27-7.24 (m, 4H, o,m-
PhB), 7.15 (m, 1H, p-PhB), 6.53 (d, J ) 6.5 Hz, 2H, m-DMAP),
4.71 (nr, 1H, 4-CpB), 4.66 (nr, 1H, 4-CpA), 4.61 (nr, 1H, 5-CpB),
4.60 (nr, 1H, 3-CpA), 4.50 (nr, 1H, 5-CpB), 4.45 (nr, 1H, 3-CpA),
3.37, 3.34 (s, 5H, CpA and s, 5H, CpB), 3.14 (s, 6H, NMe2); 11B
NMR (160 MHz, CDCl3, -50 °C) δ -1.9 (w1/2 ) 1000 Hz), the
signal for the tricoordinate B is broadened beyond detection.
Complexation of 3 with 2 equiv of Pyridine: Synthesis of
5a. Compound 3 (17.1 mg, 31 µmol) was dissolved in pyridine
(49 mg, 630 µmol) at RT. After 1 day at -35 °C a crystalline solid
was obtained, which was washed with hexanes and dried under
high vacuum. Yield for 5a: 17.5 mg (80%). Anal. Calcd for 5a
(C
42H36B2Fe2N2): C 71.85 H 5.17 N 3.99. Found: C 71.70 H 5.15
N 3.47. 11B NMR (160 MHz, CDCl3, 25 °C): δ 54.4 (w1/2 ) 1400
Hz), 0.1 (w1/2 ) 900 Hz).
Complexation of 3 with 1 equiv of 4-Dimethylaminopyri-
dine: Synthesis of 4c. To a solution of 3 (40 mg, 74 µmol) in
CH2Cl2 (4 mL) was added DMAP (9 mg, 74 µmol) in CH2Cl2 (2
mL) at room temperature, and the mixture was stirred for 1 h. The
reaction solution was layered with hexanes and kept for crystal-
lization for 2 days at -35 °C. The product was isolated as a reddish
crystalline solid. Yield: 39.5 mg (80%). IR (cm-1): 3057, 2923,
1632, 1543, 1393, 128, 1226, 1189, 1104, 1081, 1050, 1032, 816,
717. Anal. Calcd for (C39H36B2Fe2N2) · (CH2Cl2)0.2: C 68.93 H 5.37
N 4.10. Found: C 68.88 H 5.35 N 4.12 (the sample contained a
Low-Temperature NMR Characterization of 5a. The cis- and
trans-isomers of 5a were identified at -50 °C in neat d5-pyridine
by 2D NMR spectroscopy (COSY). The ratio of cis/trans-isomer
was found to be 40/60 under these conditions. 1H NMR (500 MHz,
C5D5N, -50 °C): NMR data for cis-5a: δ 7.42-7.20 (nr, 10H, Ph),
4.62 (nr, 1H, Cp-4B), 4.56 (nr, 1H, Cp-4A), 4.49 (nr, 2H, Cp-3A/
5A), 4.45 (nr, 2H, Cp-3B/5B), 3.08 (s, 5H, C5H5-B), 2.63 (s, 5H,
C5H5-A); NMR data for trans-5a: δ 7.42-7.20 (m, 10H, Ph), 4.56
(nr, 2H, Cp-4), 4.43, 4.41 (nr, nr, 2H, 2H, Cp-3 and Cp-5), 2.82 (s,
10H, C5H5). An additional set of small and very broad signals is
assigned to 4a (δ 8.6, 8.3-7.5, 6.4, 4.9-4.6, 3.55, 3.21).
1
small amount of residual CH2Cl2 according to H NMR despite
evacuation under high vacuum at 40 °C for 12 h).
Low-Temperature NMR Characterization of 4c. Complex 4c
was generated by mixing 3 with DMAP in CDCl3 in a 1:1 molar
ratio and studied by VT NMR spectroscopy. The complex was
detected at RT as well as -50 °C; at RT the resonances are
broadened and a small amount of free 3 is observed. The
Complexation of 3 with 2 equiv of 4-tert-Butylpyridine:
Synthesis of 5b. Neat tBupy (9.9 mg, 73 µmol) was added to a
solution of 3 (20.1 mg, 36 µmol) in CHCl3 (10 mL) at RT. After
stirring for 3 h the solvent was removed under reduced pressure

3064 Organometallics, Vol. 27, No. 13, 2008
Pakkirisamy et al.
and the remaining solid was washed with hexanes. Orange crystals
of trans-5b were obtained by slow evaporation of a solution in
CHCl3. Yield: 26.4 mg (88%). 1H NMR (500 MHz, CDCl3, 25 °C):
δ 8.56, 7.97, 7.48 and 7.32 (b, 18H, aromatic), 4.75, 4.65, 3.83,
3.41, and 3.24 (b, 16H, Cp), 1.34 (s, 18H, Me). 11B NMR (160
integration. Both the cis- and trans-isomers of 5c (20:80 ratio) were
identified. For 5c: 1H NMR (500 MHz, CDCl3, -50 °C) the signals
in the aromatic region could not be assigned; for cis-5c: δ 4.31
(nr, 1H, Cp-4B), 4.21 (nr, 2H, Cp-3B/5B), 4.16 (nr, 1H, Cp-4A),
4.04 (nr, 2H, Cp-3A/5A), 3.12 (s, 12H, NMe2), 2.73, 2.52 (s, 5H,
C5H5-B and s, 5H, C5H5-A); for trans-5c: δ 4.22, 4.12 (nr, nr, 2H,
2H, Cp-3 and Cp-5), 4.08 (nr, 2H, Cp-4), 3.10 (s, 12H, NMe2),
2.62 (s, 10H, C5H5).
MHz, CDCl3, 20 °C): δ 55.4 (w1/2 ) 1100 Hz) and -0.4 (w1/2
)
550 Hz). IR (cm-1): 3059, 2965, 1628, 1504, 1428, 1277, 1222,
1157, 1103, 1072, 1048, 1035, 999, 834, 814, 773, 738, 713, 572,
501, 479, 463. Anal. Calcd for (C50H52B2Fe2N2): C 73.75 H 6.43
N 3.44. Found: C 73.99 H 6.64 N 3.68.
Acknowledgment. Acknowledgment is made to the
donors of the Petroleum Research Fund, administered by the
American Chemical Society, and to the Rutgers University
Research Council for support of this research. We thank the
National Science Foundation for partial funding of an X-ray
diffractometer (NSF-CRIF 0443538). F.J. thanks the Alfred
P. Sloan Foundation for a research fellowship and the
National Science Foundation for a CAREER award. We are
grateful to Dr. Kakalis for acquisition of 2D NMR data.
Complexation of 3 with 2 equiv of 4-Dimethylaminopyri-
dine: Synthesis of 5c. To a suspension of 3 (17.5 mg, 32 µmol) in
CH2Cl2 (3 mL) was added DMAP (8.6 mg, 71 µmol) in CH2Cl2 (2
mL) at room temperature, and the mixture was stirred for 2 h. The
reaction solution was kept for crystallization for 2 days at -35 °C,
and the product was isolated as orange needle-like crystals. Yield:
19.4 mg (77%). A sample of trans-5c for X-ray diffraction and
elemental analysis was obtained by recrystallization from CHCl3
at -35 °C. 11B NMR (160 MHz, CDCl3, 25 °C): δ 54.1 (w1/2
)
1600 Hz), -1.8 (w1/2 ) 480 Hz) (ratio 0.5:1). IR (KBr, cm-1):
3056, 2921, 1629, 1542, 1225, 1148, 1105, 1078, 1034, 808, 719,
708, 478. Anal. Calcd for (C46H46B2Fe2N4)(CHCl3)2.3: C 54.59 H
4.58 N 5.27. Found: C 54.44 H 4.14 N 5.20.
Supporting Information Available: Crystallographic data for
compounds 4b and trans-5c, including tables of crystal data, atomic
coordinates, bond lengths and angles, and anisotropic thermal
parameters; details of the VT NMR studies; details of the
UV-visible titration of 3 with tert-butylpyridine. This material is
available free of charge via the Internet at http://pubs.acs.org.
Low-Temperature NMR Characterization of 5c. The low-
temperature NMR of a 1:5 mixture of 3/DMAP in CDCl3 showed
signals corresponding to complexes 4c, 5c, and free DMAP. The
ratio of 5c:4c was determined to be 73:27 based on 1H NMR
OM8001608