Perfluoroaryl boryl complexes: Synthesis, spectroscopic and structural characterisation of a complex containing the bis(pentafluorophenyl)boryl ligand

Article abstract of DOI:10.1039/b106881b

The synthesis, spectroscopic and structural characterisation of the bis(pentafluorophenyl)boryl derivative CpFe-(CO)₂B(C₆F₅)₂ are reported.

Full text of DOI:10.1039/b106881b

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Perfluoroaryl boryl complexes: synthesis, spectroscopic and structural
characterisation of a complex containing the
bis(pentafluorophenyl)boryl ligand†
S. Aldridge,*a A. Al-Fawaz,a R. J. Calder,a A. A. Dickinson,a D. J. Willock,a M. E. Lightb and
M. B. Hursthouseb
a Department of Chemistry, Cardiff University, PO Box 912, Park Place, Cardiff, UK CF10 3TB
b Department of Chemistry, University of Southampton, Highfield, Southampton, UK SO17 1BJ
Received (in Cambridge, UK) 30th July 2001, Accepted 10th August 2001
First published as an Advance Article on the web 4th September 2001
The synthesis, spectroscopic and structural characterisation
of the bis(pentafluorophenyl)boryl derivative CpFe-
(CO)2B(C6F5)2 are reported.
The formula of 2 was evident from multinuclear NMR data
and the structure was confirmed by the results of a single crystal
X-ray diffraction study. The asymmetric unit consists of two
very similar but independent molecules, each of which adopts
the expected half sandwich geometry at the iron centre with the
coordination sphere being completed by two carbonyls and one
bis(pentafluorophenyl)boryl ligand (one of the two molecules is
shown in Fig. 1). The geometry of the B(C6F5)2 ligand is such
that the planes of the C6F5 and BC2 units are inclined at an
average angle of 67.1(3)° to one another. This compares to an
average value of 34.3° for ClB(C6F5)2,10 and almost certainly
reflects the greater steric demands of CpFe(CO)2 compared to
Cl. 2 represents, to our knowledge, the first example of a
crystallographically characterized transition metal complex
containing the B(C6F5)2 ligand. Fe–B distances for the two
independent molecules are 1.965(5) and 1.964(4) Å, values
which are comparable to the distances found in CpFe(CO)2B-
(cat) 3 [1.959(6) Å],5 Cp*Fe(CO)2B(cat) [1.980(2) Å],5 and
CpFe(CO)2BO2C6H2O2BFe(CO)2Cp [1.971(2) Å].11 This com-
pares to an average value of 2.015(6) Å found for related boryl
complexes,∑ and the significantly longer bond found in the
analogous diphenylboryl complex, CpFe(CO)2BPh2 4 [2.034(3)
Å].5 The carbonyl stretching frequencies for 2 (2014 and 1968
cm21) are also comparable to those for 3 and 4 (2024, 1971 and
2021, 1951 cm21),5 and somewhat higher than the average
values for CpFe(CO)2 boryl complexes (2007 and 1949
cm21).∑
Transition metal boryl complexes (LnM–BX2) have been the
centre of intense recent research activity,1 partly because of
their involvement in the hydro- and diboration of carbon–
carbon multiple bonds,2 but also because of their implication in
highly selective stoichiometric and catalytic functionalization
of alkanes under either photolytic or thermal conditions.3–5
Such studies of reactivity have been complimented by numer-
ous structural investigations in which the nature of the metal–
boron bond has been probed by crystallographic, spectroscopic
and computational methods.1,6 One of the significant questions
investigated by such studies is the potential for the extremely
strongly s-donor boryl ligand also to act as a p acid by utilizing
the vacant boron-based orbital of p-symmetry. For complexes
bearing p donor boryl substituents (e.g. X2 = cat, ortho-
O2C6H4) research to date implies a relatively minor role for p
back bonding;1 to what extent the nature of the M–B bond can
be altered by variation in the electronic properties of X is the
subject of this and other studies.7 Ultimately a better under-
standing of the nature of the M–B bond may help to rationalize
the unusual reactivity of such complexes.
Pentafluorophenyl substituted boranes have received much
recent attention due to the robustness of the B–C bond, the
exploitation of their high Lewis acidity in a variety of catalytic
5
applications and reports that (h -C5Me5)Ir derivatives mediate
Despite the larger steric demands of the B(C6F5)2 ligand
compared to B(C6H5)2, the shorter Fe–B distance in 2
the activation of alkanes and arenes by [HB(C6F5)2]2.4 Given
that B(C6F5)3 is reported to have a Lewis acidity intermediate
between that of BCl3 and BF3,8 and that complexes containing
the BF2 ligand have only recently been reported,9 we have
sought to develop synthetic routes to metal complexes contain-
ing the B(C6F5)2 ligand. Such complexes would be expected to
contain a highly Lewis acidic boron centre and might therefore
act as useful probes of the potential for boryl ligands to act as p
acids.
10
The reaction between CpFe(CO)2Na and ClB(C6F5)2 1 in
toluene at 20 °C over a period of 3 h was monitored by 11B
NMR spectroscopy; the resonance due to the chloroborane
precursor 1 was replaced by a low-field signal at dB 121.5, in the
region characteristic of alkyl and aryl substituted transition
metal boryl complexes. After work up CpFe(CO)2B(C6F5)2 2
was obtained as air-, moisture- and photolytically-sensitive
golden yellow crystals in yields of up to 42%.‡ 2 has been
characterised§ by 1H, 13C, 11B and 19F NMR, IR, mass
spectrometry, elemental analysis and single crystal X-ray
diffraction.¶
Fig. 1 Molecular structure of one of the two independent molecules of 2.
Relevant bond lengths (Å) and angles (o): Fe(2)–B(2) 1.965(5), Fe(2)–
centroid 1.739(4), Fe(2)–C(25) 1.760(4), Fe(2)–C(26) 1.751(4), B(2)–
C(27) 1.595(5), B(2)–C(33) 1.598(5); C(25)–Fe(2)-C(26) 95.0(2), Fe(2)–
B(2)–C(27) 122.7(3), Fe(2)–B(2)–C(33) 125.8(2), C(27)–B(3)–C(33)
111.4(3), centroid–Fe(2)–B(2)–C(27) 28.4(3).
† Electronic supplementary information (ESI) available: computational
methodology, bond order calculations, calculated and crystallographically
determined structural parameters and experimentally determined CO
stretching frequencies for iron–boryl complexes. See http://www.rsc.org/
suppdata/cc/b1/b106881b/
1846
Chem. Commun., 2001, 1846–1847
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b106881b

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Table 1 Calculated bonding parameters for complexes 2–4
21 °C), d 3.91 (s, C5H5). 13C NMR ([2H6]benzene, 21 °C), d 86.3 (C5H5),
137.3, 139.9, 140.7 (aromatic CF), 211.0 (CO). 11B NMR (toluene, 21 °C),
d 121.5 (br). 19F NMR (toluene, 21 °C), d 2132.9 (virtual dq, ortho-CF, J
21.2, 6.1 Hz), 2153.7 (tt, para-CF, J 20.8, 6.1 Hz), 2161.5 (m, meta-CF).
IR (KBr disk, cm21) n(CO) 2014s, 1968s. Elemental analysis: calc. for
C19H5BF10FeO2, C, 43.72; H, 0.97%. Found: C, 43.55; H, 0.97%. No
significant changes in the 19F NMR spectrum were observed on cooling to
2110 °C.
Breakdown of
covalent contribution Electrostatic
Mayer
to bondb (%)
contribution to
BDE (D0)a/ bond
BDEc/kJ
Compound kJ mol21
order
s
p
mol21
¯
2
3
4
224.2
274.6
231.2
0.999
0.957
0.888
81.9
89.2
90.4
18.0
10.7
9.5
66.7
12.0
40.4
¶ Crystallographic data for C19H5BF10FeO2, 2: triclinic, space group P1, a
= 10.6895(2), b = 13.4060(2), c = 13.6885(2) Å, a = 93.628(3), b =
93.588(3), g = 113.060(3)°, U = 1793.08(5) Å3, Z = 4, Dc = 1.933 Mg
m23, M = 521.89, T = 120(2) K. 12458 reflections collected, 6094
a Bond dissociation energy (D0) associated with homolytic cleavage of the
Fe–B bond. b Decomposition of bonding density according to s and p
symmetry calculated using the method described in ref. 7 and also in the
ESI.† c Electrostatic contribution to BDE calculated on an atom pair
interaction basis, taking atomic charges from the Mulliken analysis.
independent (Rint = 0.0589) which were used in all calculations. R1
=
0.0431, wR2 = 0.0741 for observed unique reflections [F2 > 2s(F2)] and
R1 = 0.0829, wR2 = 0.0858 for all 6094 unique reflections. The max. and
min. residual electron densities on the final difference Fourier map were
0.356 and 20.431 e Å23, respectively. Bond lengths (Å) and angles (°) not
included in caption: Fe(1)–B(1) 1.964(4), Fe(1)–centroid 1.729(3), Fe(1)–
C(6) 1.760(4), B(1)–C(8) 1.589(5); C(6)–Fe(1)–C(7) 93.98(17), centroid–
Fe(1)–B(1)–C(14) 27.9(3). CCDC reference number 168238. See http:
//www.rsc.org/suppdata/cc/b1/b106881b/ for crystallographic data in CIF
or other electronic format.
(compared to 4) testifies to the greater Lewis acidity of the
boron centre. In addition, the torsion angle (q) between the Cp
centroid–Fe–B and BC2 planes is significantly smaller in 2
[average value of 28.2(3)° for 2 vs. 75° for 4]. Hoffmann and
coworkers have concluded that a value of q = 0° provides for
best overlap between metal and ligand p orbitals in the pseudo-
∑ Average Fe–B bond length measured for crystallographically charac-
5
terized complexes of the type (h -C5R5)Fe(CO)2BX2; average symmetric
and antisymmetric CO stretching frequencies measured for complexes of
isoelectronic CpFe(CO)2CH2 system.12 The smaller angle
+
5
the type (h -C5H5)Fe(CO)2BX2.5,11,13–15
measured for 2 may therefore also be indicative of a stronger p
interaction than in 4, although almost certainly the high steric
demands of the B(C6F5)2 ligand prevent the attainment of a
virtually coplanar arrangement such as that found in 3 (7.9°).
Further investigation of the bonding in CpFe(CO)2B(C6F5)2
and comparison with the related species 3 and 4 was carried out
by the use of density functional theory,** the preliminary
results of which are reported here. Calculated structural
parameters for 2–4 are in good agreement with those measured
crystallographically and are included in the ESI.† The bonding
interaction between the Fe and B atom was calculated using an
approach based on Mulliken analysis but with an additional sub-
division of the bonding density into s and p contributions. This
was achieved by aligning the Fe–B bond with the z-axis and
then calculating the bonding density for atomic orbital pairs of
s and p symmetry separately. The results of analysis of the
calculated density are reproduced in Table 1. In comparison
with 3 and 4, the Fe–B bond in 2 shows significantly increased
contributions not only from p symmetry covalent interactions
but also from electrostatic attraction between the organome-
tallic and boryl fragments. On the other hand it is worth
mentioning that even in this system the barrier to rotation about
the Fe–B bond is such that there is no sign of restricted rotation
from VT-NMR experiments even at 2110 °C.
** Details of DFT calculations: gradient corrected DFT calculations were
carried out using the ADF2000.01 code,16 with functionals for exchange
and correlation due to Becke17 and Lee, Yang and Parr,18 respectively. A
basis set was constructed from Slater type orbitals with triple zeta valence
shell and a single polarization function per atom (ADF IV). All structures
were fully optimised at the BLYP level of theory with no symmetry
restrictions. Convergence criteria: (i) energy change on next step < 1 3
1023 Eh; (ii) gradient < 1 3 1023 Eh Å21; and (iii) uncertainty in Cartesian
coordinates < 1 3 1022 Å.
1 H. Wadepohl, Angew. Chem., Int. Ed. Engl., 1997, 36, 2441; G. J. Irvine,
M. J. G. Lesley, T. B. Marder, N. C. Norman, C. R. Rice, E. G. Robins,
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H. Braunschweig, Angew. Chem., Int. Ed., 1998, 37, 1787; M. R. Smith,
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In conclusion the synthesis, spectroscopic and structural
characterization of the first transition metal complex containing
the B(C6F5)2 ligand are reported. As expected the electron
withdrawing nature of the pentafluorophenyl substituents leads
to an enhanced p interaction with the metal centre, although
even here this still represents only a small contribution to the
overall metal ligand bond. Further studies aimed at extending
the known chemistry of this ligand are currently underway.
The support of the Royal Society and EPSRC are gratefully
acknowledged. Calculations were carried out with the help of
support from the EPSRC, Synetix and OCF.
10 D. J. Parks, W. E. Piers and G. P. A. Yap, Organometallics, 1998, 17,
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11 S. Aldridge, R. J. Calder, A. A. Dickinson, D. J. Willock and J. W.
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13 H. Braunschweig, C. Kollann and M. Müller, Eur. J. Inorg. Chem.,
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Notes and references
‡ Dropwise addition of a solution of ClB(C6F5)210 (0.4 g, 1.05 mmol) in 12
cm3 toluene to 1 equivalent of CpFe(CO)2Na suspended in toluene at 20 °C
led to the evolution of an orange–red solution and an off-white precipitate.
Examination of the 11B NMR spectrum of the solution after 3 h revealed that
the signal at dB 59.1 characteristic of 1 had disappeared, the sole resonance
observed being at dB 121.5. Filtration of the reaction mixture at this point,
removal of volatiles in vacuo and recrystallization from hexanes at 230 °C
afforded golden yellow crystals of 2 in yields of ca. 42%. Crystals suitable
for X-ray crystallography could be grown by careful cooling of concen-
trated solutions in either toluene or hexanes.
14 H. Braunschweig, B. Ganter, M. Koster and T. Wagner, Chem. Ber.,
1996, 129, 1099.
15 H. Braunschweig, C. Kollann and U. Englert, Eur. J. Inorg. Chem.,
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16 E. J. Baerends, D. E. Ellis and P. Ros, Chem. Phys., 1973, 2, 41; L.
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17 A. D. Becke, Phys. Rev. A, 1988, 38, 3098.
18 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785.
§ Spectroscopic data for 2: MS(EI): M+ = 522 (weak), isotopic pattern
corresponding to 1 B and 1 Fe atom, strong fragment ion peaks at m/z 494
[(M 2 CO)+, 60%] and 466 [(M 2 2CO)+, 18%]. 1H NMR ([2H6]benzene,
Chem. Commun., 2001, 1846–1847
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