Bis(boryl)metallocenes. 1. 1,1′-bis(diisopropylboryl)cobaltocenium cation: A novel anion-binding ligand

Article abstract of DOI:10.1021/om960237x

Three situations are observed when the new cobaltocene derivative Co(C₅H₄BPrⁱ₂)₂ (2) is oxidized. The cobaltocenium salt [2]PF₆ (3) is obtained with [FeCp₂]PF₆ as oxidant. With Cu(OH)₂ the inverse chelate [2]OH (4) with a μ-OH group bridging the two boron centers is formed. Oxidation with C₂Cl₆ gives a zwitterionic chloride (5) with a trigonal and a quaternized boron center; in solution 5 is fluxional.

Full text of DOI:10.1021/om960237x

3106
Organometallics 1996, 15, 3106-3108
Bis(bor yl)m eta llocen es. 1.
1,1′-Bis(d iisop r op ylbor yl)coba ltocen iu m Ca tion : A Novel
An ion -Bin d in g Liga n d
Gerhard E. Herberich,* Andreas Fischer, and Dag Wiebelhaus
Institut fu¨r Anorganische Chemie, Technische Hochschule Aachen, D-52056 Aachen, Germany
Received March 28, 1996X
Sch em e 1a
Summary: Three situations are observed when the new
cobaltocene derivative Co(C5H4BPri ) (2) is oxidized.
2 2
The cobaltocenium salt [2]PF6 (3) is obtained with
[FeCp2]PF6 as oxidant. With Cu(OH)2 the inverse che-
late [2]OH (4) with a µ-OH group bridging the two boron
centers is formed. Oxidation with C2Cl6 gives a zwitter-
ionic chloride (5) with a trigonal and a quaternized
boron center; in solution 5 is fluxional.
Anion binding or anion recognition may be effected
by hydrogen bonding, through polar bonds with Lewis-
acidic centers, or by the electrostatic attraction of
cationic centers; often these act in combination.1 Sev-
eral systems with two boron centers are known which
are able to act as molecular pincers, thereby binding
anions in inverse chelates.2 For instance, Katz et al.
have shown that 1,8-bis(boryl)naphthalenes bind H-,
F-, OH-, and Cl-; in each case the anion bridges the
two boron atoms.2c-e In this communication we give the
first account of a novel, cationic molecular pincer, a bis-
(boryl)cobaltocenium ion which displays a remarkable
variability in its interactions with anions.
We have recently described the alkali-metal borylcy-
clopentadienides M(C5H4BX2) (1; M ) Li, Na).3 These
a
1, M ) Li or Na
Legend: (a) [FeCp2] in CH2Cl2; (b) Cu(OH)2 in toluene; (c)
C2Cl6 in toluene.
a, X ) Me; b, X ) Pri; c, X ) NMe2; d, 2 X ) OCMe2CMe2O
new reagents allow the direct synthesis of boryl-
substituted cyclopentadienyl complexes. In many cases
boryl functionalities can be introduced by electrophilic
aromatic substitution with boron trihalides, and the
dibromoborylation of ferrocene is a pertinent example.4
In other cases, as, for example, Cp2TiCl2 or NiCp2,
where the borylation reaction does not work, our new
reagents offer a synthetic alternative to be tested.
In this vein, treatment of CoBr2‚DME with 1b af-
forded the purple, paramagnetic cobaltocene derivative
Co(C5H4BPri ) (2).5 Oxidation to give the cation 2+ is
2 2
seemingly trivial, but three different situations mate-
rialize experimentally (Scheme 1).
Oxidation with [FeCp2]PF6 in CH2Cl2 gives the yellow,
diamagnetic hexafluorophosphate 3.6 The crystal struc-
ture of 3 is that of a salt without specific interactions
between the B atom and the anion (Figure 1).7 In
solution the boron resonance for 3 is found at δ(11B) 79,
in the range expected for a dialkylarylborane.8 Fur-
X Abstract published in Advance ACS Abstracts, J une 1, 1996.
(1) (a) Izatt, R. M.; Pawlak, K.; Bradshaw, J . S.; Bruening, R. L.
Chem. Rev. 1995, 95, 2529. (b) Lehn, J .-M. Supramolecular Chemistry;
VCH: Weinheim, Germany, 1995; Chapter 3. (c) Kaufmann, D. E.;
Otten, A. Angew. Chem., Int. Ed. Engl. 1994, 33, 1832. (d) Yang, X.;
Knobler, C. B.; Zheng, Z.; Hawthorne, M. F. J . Am. Chem. Soc. 1994,
116, 7142 and literature quoted therein.
(2) (a) Biallas, M. J . J . Am. Chem. Soc. 1969, 91, 7290. (b)
Saturnino, D. J .; Yamauchi, M.; Clayton, W. R.; Nelson, R. W.; Shore,
S. G. J . Am. Chem. Soc. 1975, 97, 6063. (c) Katz, H. E. J . Am. Chem.
Soc. 1985, 107, 1420. (d) Katz, H. E. J . Org. Chem. 1985, 50, 5027.
(e) Katz, H. E. Organometallics 1987, 6, 1134. (f) Ko¨ster, R.; Seidel,
G. Chem. Ber. 1992, 125, 627. (g) Ko¨ster, R.; Seidel, G.; Wagner, K.;
Wrackmeyer, B. Chem. Ber. 1993, 126, 305. (h) Cf. also: J a¨kle, F.;
Mattner, M.; Priermeier, T.; Wagner, M. J . Organomet. Chem. 1995,
502. 123.
1
thermore, in the H NMR spectrum, a single doublet is
seen for the methyl groups, which consequently are
chemically equivalent. Again, there is no specific
(5) Preparation of 2: Cyclopentadienyldiisopropylborane (29.7 g,
0.183 mol) was added with stirring to LiCp (11.4 g, 0.158 mol) in THF
(300 mL) at ambient temperature; stirring was continued for 70 h.3
CoBr2‚1.18DME (25.8 g, 0.080 mol, DME content determined by
elemental analysis) was added at -78 °C. The reaction mixture was
warmed to ambient temperature. Careful removal of all volatiles under
vacuum, suspension of the residue in hexane (150 mL), filtration with
the help of Kieselgur, and washing with hexane (4 × 20 mL) gave a
purple product solution. This was then concentrated to 20 mL and
kept at -30 °C to give 2 (29.78 g, 98%) as somewhat sticky crystals;
mp 68-70 °C. MS (EI): m/z (Irel) 381 (61, M+), 213 (100, M+ - 4 C3H6).
(3) Herberich, G. E.; Fischer, A. Organometallics 1996, 15, 58.
(4) (a) Ruf, W.; Renk, T.; Siebert, W. Z. Naturforsch. 1976, 31B,
1028. (b) Appel, A.; J a¨kle, F.; Priermeier, T.; Schmid, R.; Wagner, M.
Organometallics 1996, 15, 1188 and literature quoted therein.
S0276-7333(96)00237-3 CCC: $12.00 © 1996 American Chemical Society
+
+

Communications
Organometallics, Vol. 15, No. 14, 1996 3107
F igu r e 1. ORTEP plot of 3 (at the 30% probability level).
Bond distances (pm): Co-C(ring), 201.6(3)-204.8(3);
Co‚‚‚B1, 315.1(3); Co‚‚‚B2, 315.3(3); C-C(ring), 140.6(5)-
144.4(4); C1-B1, 156.1(5); C11-B1, 156.4(5); C21-B1,
157.7(5); C6-B2, 158.1(4); C31-B2, 156.0(5); C41-B2,
158.2(5). Distances from the ring plane (pm): 15.6(5) for
B1, 15.0(5) pm for B2, both towards the metal. Bond angles
(deg): C1-B1-C11, 121.7(2); C1-B1-C21, 116.3(2); C11-
B1-C21, 121.8(3); C6-B2-C31, 121.6(4); C6-B2-C41,
116.3(2); C31-B2-C41, 122.1(3).
F igu r e 2. ORTEP plot of 4 (at the 30% probability level).
The molecule possesses crystallographic C2 symmetry with
the Co and O atoms lying on the symmetry axis. Bond
distances (pm): Co-C1, 203.8(5); Co-C2, 201.2(5); Co-
C3, 204.0(5); Co-C4, 204.1(5); Co-C5, 201.1(5); Co‚‚‚B,
325.5(5); Co‚‚‚O, 335.8(5); C-C(ring), 140.7(6)-142.8(6);
C1-B, 162.8(7); B-C6, 164.4(7); B-C7, 163.2(8); B-O,
160.5(6). Distance from the ring plane for B: 5.5(7) pm
toward the metal. Bond angles (deg): B-O-B′, 144.8(5);
O-B-C1, 107.2(4); O-B-C6, 103.9(4); O-B-C7, 106.3(4);
C1-B-C6, 107.3(4); C1-B-C7, 111.1(4); C6-B-C7,
120.2(4).
interaction between the B atom and the anion. Never-
theless, cation 2+ is a strong Lewis acid. It forms 1/1
and 1/2 adducts with pyridine and polymerizes THF at
ambient temperature.
Upon oxidation of 2 with Cu(OH)2 in toluene the
soluble, deep yellow complex 4 is formed.9 The X-ray
crystal structure determination10 (Figure 2) reveals a
molecular structure with a bridging µ-OH group be-
tween two B atoms; in other words, 2+ forms an inverse
chelate with OH-. The boron resonance at δ(11B) 0 lies
in the range expected for tetracovalent boron, and a pair
1
of diastereotopic methyl groups is seen in the H NMR
spectrum. Thus, the same inverse chelate structure
prevails in solution.
Oxidation of 2 with C2Cl6 (or less smoothly with CuCl)
affords the deep yellow chloride 5.11 In the crystal one
B atom is trigonal and the other is tetracoordinate with
a rather long B-Cl bond (Figure 3).12 In solution one
would expect to see two boron resonances and two
different C5H4BPri2 ligands with three different methyl
groups (4/2/2) in the respective NMR spectra. Experi-
mentally a single boron resonance is observed at δ(11B)
40, corresponding to a mean chemical shift for tricoor-
(6) Preparation of 3: [FeCp2]PF6 (2.95 g, 8.92 mmol) in CH2Cl2 was
added to a solution of 2 (3.41 g, 8.96 mmol) in CH2Cl2 (50 mL). After
careful removal of the solvent, the residue was suspended in hexane
to dissolve the ferrocene formed, collected on a frit, and washed several
times with hexane to give 3 (3.94 g, 84%) as a highly sensitive, greenish
yellow powder. Recrystallization from CH2Cl2 at -30 °C gave yellow
crystals; mp 137-139 °C. MS (SIMS): m/z (Irel) 381 (100, M+) from
cation spectrum, 145 (100, PF6-) from anion spectrum. 1H NMR (500
MHz, CD3NO2) for C5H4 δ 6.00 (m, 8H), for BPri δ 2.03 (sept, J ) 7.3
2
Hz, 4 BCH), 1.16 (d, J ) 7.3 Hz, 8 Me). 11B NMR (160 MHz, CD3-
NO2): δ 79. 13C{1H} NMR (63 MHz, CD3NO2): for C5H4 δ 91.4, 89.7,
signal of C-1 not observed; for BPri δ 24.3 (br, BCH) and 18.7 (Me).
19F NMR (470 MHz, CD3NO2): δ -273.90 (d, J ) 707 Hz). 31P NMR
(202 MHz, CD3NO2): δ -145.05 (sept, J ) 707 Hz).
(9) Preparation of 4: A solution of 2 (440 mg, 1.16 mmol) in toluene
(6 mL) was stirred with Cu(OH)2 (57 mg, 0.58 mmol) for 10 min.
Filtration and crystallization from toluene gave 4 (355 mg, 77%) as
deep yellow microcrystals mp 131 °C dec. MS (SIMS): m/z (Irel) 398
(100, M+). 1H NMR (300 MHz, C6D6): for C5H4 δ 4.79 (m, 4H), 4.18
(7) Crystal data for 3: yellow crystals, 0.6 × 0.6 × 0.4 mm,
(m, 4H); for BPri δ 1.09 (sept, J ) 7.0 Hz, 4 BCH), 1.40 (d, J ) 7.0
2
monoclinic, a ) 840.0(2) pm, b ) 2189.9(7) pm, c ) 1404.2(4) pm, â )
Hz, 4 Me), 1.20 (d, J ) 7.0 Hz, 4 Me); for OH δ 2.61. 11B NMR (29
96.67(2)°, V ) 2.565(2) nm3, Z ) 4, space group P21/c (No. 14), dcalc
)
MHz, CDCl3): δ 0. 13C{1H} NMR (67.9 MHz, C6D6): for C5H4 δ 111
1.359 g cm-3, µ ) 7.76 cm-1, F(000) ) 1096.0. Data collection: ENRAF-
Nonius CAD4, Mo KR radiation, graphite monochromator, ω scan (3
< θ < 25°) at 243 K; 5055 reflections measured, 3595 unique reflections
with I > 1.5σ(I), no absorption correction applied. Solution and
refinement:15 434 parameters, R ) 0.054, Rw ) 0.070, w-1 ) σ2(Fo),
GOF ) 2.09; non-hydrogen atoms were refined anisotropically and all
(s, br, C-1), 87.8, 82.1; for BPri δ 22.6 (Me) and 21.8 (Me), signal for
2
BCH very broad, partially hidden.
(10) Crystal data for 4: yellow crystals, 0.25 × 0.45 × 0.6 mm,
orthorhombic, a ) 785.8(2) pm, b ) 1451.2(1) pm, c ) 1877.4(2) pm, V
) 2.1410(9) nm3, Z ) 4, space group Pccn (No. 56), dcalc ) 1.235 g cm-3
,
µ ) 8.06 cm-1, F(000) ) 856. Data collection: ENRAF-Nonius CAD4,
Mo KR radiation, graphite monochromator, ω scan (3 < θ < 28°) at
253 K; 2851 reflections measured, 1344 unique reflections with I >
σ(I), no absorption correction applied. Solution and refinement:15 119
parameters, R ) 0.082, Rw ) 0.079, w-1 ) σ2(Fo), GOF ) 1.71; non-
hydrogen atoms were refined anisotropically and all hydrogen atoms
were treated as riding.16
hydrogen atoms were refined isotropically; PF6 disordered.16
-
(8) For reference data see: (a) No¨th, H.; Wrackmeyer, B. In NMR
Basic Principles and Progress; Diehl, P., Fluck, E., Kosfeld, R., Eds.;
Springer-Verlag: Berlin, 1978; Vol. 14. (b) Wrackmeyer, B. Annu. Rep.
NMR Spectrosc. 1988, 20, 61. (c) Siedle, A. R. Annu. Rep. NMR
Spectrosc. 1988, 20, 205.
+
+

3108 Organometallics, Vol. 15, No. 14, 1996
Communications
to 4 in toluene, a common proton resonance is observed
for the protons of the acid and the µ-OH group. Thus,
we see fast exchange of the protic hydrogens but no
decomplexation of the µ-OH group. Taken together,
these observations demonstrate high stability of the
inverse chelate 4. At longer reaction times (3-4 days,
20 °C) slow protolytic cleavage of one of the two BPri
2
groups takes place.
The molecular pincer 2+ differs from the 1,8-bis-
(boryl)naphthalenes by its charge and by the flexibility
of its molecular skeleton. In any case, it does interact
with anions by the field effect14 of the cobaltocenium
moiety. However, the flexibility of 2+ allows for more
varied anion binding. Thus, we have the three situa-
tions exemplified above: simple salt formation with
-
PF6 as in 3 and coordination to one or two boron
centers as in 5 or 4. In the inverse chelate 4 the internal
flexibility of 2+ is decreased to allow the formation of
the energetically favorable B-O-B′ group. The Cl-
bridge formation is obviously less favorable than the
open zwitterionic structure 5.
It will be of interest to replace the bulky isopropyl
substituents with the sterically less demanding methyl
groups or with more electronegative substituents such
as chlorine. In addition to these aspects the selectivity
of 2+ in binding different anions and the dynamics of
the anion coordination are currently under investiga-
tion.
F igu r e 3. ORTEP plot of 5 (at the 30% probability level).
Bond distances (pm): Co-C(ring), 200.5(4)-209.6(2);
Co‚‚‚B1, 346.1(3); Co‚‚‚B2, 317.2(3); Co‚‚‚Cl, 397.79(9);
C-C(ring), 139.7(5)-144.1(5); C1-B1, 162.1(5); C11-B1,
163.2(5); C21-B1, 164.4(5); C6-B2, 157.5(4); C31-B2,
156.6(5); C41-B2, 156.5(5); Cl-B1, 198.2(3). Distances
from the ring plane (pm): for B1 24.7(3) away from the
metal, for B2 13.9(4) pm toward the metal. Bond angles
(deg): Cl-B1-C1, 105.9(2); Cl-B1-C11, 108.5(2); Cl-B1-
C21, 106.9(2); C1-B1-C11, 115.3(3); C1-B1-C21, 107.7(3);
C11-B1-C21, 112.2(2); C6-B2-C31, 122.6(3); C6-B2-
C41, 117.4(3); C31-B2-C41, 119.8(3).
Ack n ow led gm en t. We thank Dr. U. Englert for
advice with the structure determinations. This work
was generously supported by the Deutsche Forschungs-
gemeinschaft and the Fonds der Chemischen Industrie.
dinate boron as in 3 and tetracoordinate boron as in 4.
The two C5H4BPri2 ligands appear equivalent in the 1H
and 13C NMR spectra down to -80 °C. Obviously, the
chloride 5 is highly fluxional in solution.
Su p p or tin g In for m a tion Ava ila ble: Tables of bond
distances and angles, anisotropic thermal parameters, and
atom coordinates for 3-5 (17 pages). Ordering information
is given on any current masthead page.
When MeOH is added to a solution of the µ-OH
complex 4 in toluene, a small fraction is transformed
OM960237X
into the µ-OMe complex Co(C5H4BPri ) (µ-OMe) (6).13
2 2
(13) Observation of 6: MeOH (7.9 mg, 246 µmol) was added to a
solution of 4 (9.7 mg, 24.4 µmol) in C6D6 (0.6 mL) at 20 °C. The
equilibrium constant was calculated from initial weights of the
reactants and relative intensities in the 1H NMR spectrum; K ) [3.2-
Equilibrium (K ) [3.2(5)] × 10-2 at 20 °C) is reached
within minutes. When CF3CO2H (1-4 equiv) is added
(5)] × 10-2, from five runs. 1H NMR (250 MHz, C6D6): for C5H4
δ
(11) Preparation of 5: A solution of 2 (1.28 g, 3.36 mmol) in toluene
(30 mL) was treated with hexachloroethane (0.61 g, 2.58 mmol) for 10
min. Careful removal of all volatiles and crystallization from toluene
gave 5 (1.195 g, 85%) as deep yellow microcrystals; mp 104-106 °C.
1H NMR (80 MHz, C6D6): for C5H4 δ 5.05 (m, 4H), 4.83 (m, 4H); for
5.04 (m, 4H), 4.29 (m, 4H); δ 3.32 (s, 3H, OMe); for BPri δ 1.43 (m, 8
2
Me), 1.2-1.1 (m, hidden, 4 BCH). 11B NMR (29 MHz, C6D6): δ 2. 13C-
{1H} NMR (63 MHz, C6D6): for C5H4 δ 87.5, 81.3, signal of C-1 not
observed; for OCH3 δ 51.4; for BPri δ 21.2 (Me) and 18.4 (br, BHC).
2
(14) For some recent examples of anion binding by the field effect,
see: Weiss, R.; Pomrehn, B.; Hampel, F.; Bauer, W. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1319. Worm, K.; Schmidtchen, F. P. Angew.
Chem., Int. Ed. Engl. 1995, 34, 65. Steed, J . W.; J uneja, R. K.; Atwood,
J . L. Angew. Chem., Int. Ed. Engl. 1994, 33, 2456.
BPri 1.43 (sept, J ) 6.0 Hz, 4 BCH), 1.13 (d, J ) 6.0 Hz, 4 Me). 11B
2
NMR (29 MHz, C6D6): δ 40. 13C{1H} NMR (126 MHz, C6D6): for C5H4
δ 88.3, 84.9, signal of C-1 not observed; for BPri δ 22.3 (br, BCH) and
2
20.6 (Me).
(12) Crystal data for 5: yellow crystals, 0.5 × 0.45 × 0.2 mm,
(15) Frenz, B. A. ENRAF-Nonius SDP Version 5.0, Delft, The
Netherlands, 1989, and local programs.
monoclinic, a ) 1115.1(2) pm, b ) 819.6(3) pm, c ) 1247.7(2) pm, â )
96.25(1)°, V ) 1.1335(8) nm3, Z ) 2, space group P21 (No. 4), dcalc
)
(16) Further details of the crystal structure analyses are available
on request from the Fachinformationszentrum Karlsruhe, Gesellschaft
fu¨r wissenschaftlich-technische Information mbH, D-76344 Eggenstein-
Leopoldshafen, Germany, on quoting the depository numbers CSD-
391010 for 3, CSD-391011 for 4, and CSD-391012 for 5, the names of
the authors, and this journal citation.
1.220 g cm-3, µ ) 8.77 cm-1, F(000) ) 444. Data collection: ENRAF-
Nonius CAD4, Mo KR radiation, graphite monochromator, ω scan (3
< θ < 27°) at 253 K; 2770 reflections measured, 2358 unique reflections
with I > σ(I), empirical absorption correction (PSI17). Solution and
refinement:15 234 parameters, R ) 0.031, Rw ) 0.037, w-1 ) σ2(Fo),
GOF ) 1.31; non-hydrogen atoms were refined anisotropically and all
hydrogen atoms were treated as riding.16
(17) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.
1968, A24, 351.
+
+