[(η5-C5H5)Fe(CO)2] 2B(2,4,6-Me3C6H2): Synthesis, spectroscopic and structural characterization of a transition metal complex containing an unsupported bridging borylen

Article abstract of DOI:10.1039/b201415g

The synthesis and characterisation of the dinuclear iron complex [(η⁵-C₅H₅)Fe(CO)₂] ₂B(2,4,6-Me₃C₆H₂) containing an unsupported bridging borylene ligand are reported.

Full text of DOI:10.1039/b201415g

[(h5-C5H5)Fe(CO)2]2B(2,4,6-Me3C6H2): synthesis, spectroscopic and
structural characterization of a transition metal complex containing an
unsupported bridging borylene ligand†
Simon Aldridge,* Deborah L. Coombs and Cameron Jones
Department of Chemistry, Cardiff University, PO Box 912, Park Place, Cardiff, UK CF10 3TB.
E-mail: aldridges@cardiff.ac.uk
Received (in Cambridge, UK) 7th February 2002, Accepted 7th March 2002
First published as an Advance Article on the web 20th March 2002
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The synthesis and characterisation of the dinuclear iron
complex [(h5-C5H5)Fe(CO)2]2B(2,4,6-Me3C6H2) containing
an unsupported bridging borylene ligand are reported.
shifts to low field expected for replacement of bromide by (h -
C5H5)Fe(CO)2 (dB 61.6, 111.4 and 158.0 for 1, 2 and 3,
respectively).
IR and mass spectrometric data for 3 are consistent with a
structure featuring four terminally bound carbonyl ligands.
Such a finding implies that complex 3 differs from other
reported borylene complexes in featuring an unsupported
bridging ligand, and is confirmed by the results of a single
crystal X-ray diffraction study (Fig. 1).¶ The asymmetric unit
contains two independent molecules of 3 each of which consists
The structural and reaction chemistry of transition metal
complexes containing low-coordinate ligands of group 13 and
14 have been the subject of much recent interest.1–4 Thus, for
example, the coordination chemistry of alkylidene,1 and more
recently silylene ligands have been well documented.2 Analo-
gous complexes containing group 13 diyl ligands (RE) are also
known, principally for the heavier elements, with examples of
both terminal and bridging modes of coordination having been
elucidated.4–6 The nature of the interaction between the ligand
and metal centre in certain diyl systems has been the subject of
much debate;5,7 the description of superficially similar com-
plexes as being bound via multiple bonds (e.g. RENMLn) or via
donor/acceptor interactions (RE?MLn) reflects not only the
fundamental questions of structure and bonding posed by such
complexes, but also the scarcity of structural data available.
Within this family of ligands, borylenes (RB) represent a very
recent (and considerably less numerous) addition,8–12 with the
work of Braunschweig, in particular, providing the basis for the
division of such complexes into two distinct structural types
viz.: (i) terminally bound RB ligands;9,10 and (ii) BR ligands
adopting a bridging mode of coordination across a M–M
linkage, thereby constituting a three-membered MBM metal-
lacycle.11,12 We have recently been interested in the coordina-
tion chemistry of ligands containing trigonal boron centres,13
and as an extension of this work, hereby report the synthesis of
a transition metal complex featuring a novel mode of coordina-
tion of the borylene ligand. Substitution at the boron centre
within an existing asymmetric boryl complex leads to the first
crystallographically characterized example of an unsupported
bridging borylene ligand.
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of two piano stool (h -C5H5)Fe(CO)2 fragments linked in m2,
1
1
h , h fashion by a single bridging mesitylborylene ligand
[(2,4,6-Me3C6H2)B]. The metal–metal distance is sufficiently
long to rule out any direct interaction between the metal centres
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{r(Fe–Fe) = 3.802(10) Å, compared to 2.548(1) Å for [(h -
C5H4Me)Fe(CO)]2(m2-CO)[m2-BN(SiMe3)2]11}. As such 3 rep-
resents to our knowledge the first structurally characterized
complex containing an unsupported bridging borylene li-
gand.14
Two structural features distinguish this mode of coordination
from that found in complexes containing a metal–metal bond.
Firstly the Fe–B–Fe angle is significantly wider in 3 [130.8(5)°]
than is typically found in bridging borylenes which form part of
a three-membered MBM metallacycle (which fall in the range
75–90°11,12). The opening out of the M–B–M angle in the
absence of a constraining metal–metal bond mirrors the
behaviour of analogous gallium and indium systems.6
Secondly, the increased steric crowding at boron is reflected
in Fe–B distances [2.091(10) and 2.090(10) Å] which are
considerably longer than those found (i) in supported bridging
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borylene systems {e.g. 2.007(3) and 2.002(3) Å for [(h -
Reaction of mesityl boron dibromide (1) with one equivalent
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of (h -C5H5)Fe(CO)2Na leads to the isolation of the asymmetric
mesityl(bromo)boryl complex 2 (Scheme 1).‡ Forcing condi-
tions (96 h at 40 °C, five equivalents of anion) are required
before complete substitution of the bromide ligands is achieved
yielding 3. Spectroscopic data for 2 and 3 are in accordance with
the proposed formulation.§ In particular, the measured 11B
NMR resonances for the two compounds display the successive
Scheme 1 Synthesis of boryl complexes 2 and 2A, and borylene complex 3.
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(i) (h -C5H4R)Fe(CO)2Na (1 equiv.), toluene, room temperature, 12 h. (ii)
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Fig. 1 Molecular structure of one of the two independent molecules of [(h -
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(h -C5H5)Fe(CO)2Na (4 equiv.), toluene, 40 °C, 96 h, R = H.
C5H5)Fe(CO)2]2B(2,4,6-Me3C6H2) 3. Relevant bond lengths (Å) and angles
(°): Fe(1)–B(1) 2.090(10), Fe(2)–B(1) 2.091(10), Fe(1)–C(15) 1.730(10),
Fe(1)–Cp 1.745(10), Fe…Fe 3.802(10), B(1)–C(1) 1.571(14); Fe(1)–B(1)–
Fe(2) 130.8(5), C(1)–B(1)–Fe(1)–Cp 18.7(6), Fe(1)–B(1)–C(1)–C(2)
† Electronic supplementary information (ESI) available: spectroscopic data
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for (h -C5H4Me)Fe(CO)2B(2,4,6-Me3C6H2)Br 2A.See http://www.rsc.org/
5
suppdata/cc/b2/b201415g/
83.6(6). Cp = h -C5H5 centroid.
856
CHEM. COMMUN., 2002, 856–857
This journal is © The Royal Society of Chemistry 2002

g = 90.83(3)°, U = 2104.1(7) Å3, Z = 4, Dc = 1.528 Mg m23, M =
483.91, T = 150(2) K. 29336 reflections collected, 7309 independent
[R(int) = 0.0953] which were used in all calculations. R1 = 0.0932, wR2
=
0.2638 for observed unique reflections [F2 > 2s(F2)] and R1 = 0.1110,
wR2 = 0.2712 for all unique reflections. Max. and min. residual electron
densities: 2.578 (near Fe3) and 20.997 e Å23, respectively. The poor
quality of the data is due to the fact that the crystal was found to be a
multiple. It was not possible to deconvolute the diffraction patterns of each
component. This problem was found to be inherent in several crystals
selected. The asymmetric unit contains two crystallographically independ-
ent molecules. All attempts to refine the structure in higher symmetry space
groups failed. Despite the poor quality of the data the gross molecular
framework of the molecule is unambiguous and fully supported by the
spectroscopic data.
For C17H18BBrFeO2 2A: monoclinic, P21/c, a = 8.315(2), b = 15.283(3),
c = 14.113(3), b = 104.89(3)°, U = 1733.2(6) Å3, Z = 4, Dc = 1.536 Mg
m23, M = 400.88, T = 150(2) K. 31913 reflections collected, 3963
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Fig. 2 Molecular structure of (h -C5H4Me)Fe(CO)2B(2,4,6-Me3C6H2)Br,
independent [R(int) = 0.0588] which were used in all calculations. R1
=
0.0448, wR2 = 0.0923 for observed unique reflections [F2 > 2s(F2)] and
R1 = 0.0555, wR2 = 0.0974 for all unique reflections. Max. and min.
residual electron densities: 1.941 (near Br1) and 21.813 e Å23, re-
spectively.
2A. Relevant bond lengths (Å) and angles (°): Fe(1)–B(1) 1.962(4), B(1)–
Br(1) 2.005(3), B(1)–C(3) 1.571(4), Fe(1)–Cp 1.735(4); C(3)–B(1)–Fe(1)–
Cp 3.8(2).
C5H4Me)Fe(CO)]2(m2-CO)[m2-BN(SiMe3)2]11}and (ii) in the
asymmetric boryl precursor 2A [1.962(4) Å] (Fig. 2).∑ This
lengthening of the Fe–B distance, together with carbonyl
stretching frequencies (2010 and 1949 cm21) which differ little
CCDC reference numbers 179292 and 179293. See http://www.rsc.org/
suppdata/cc/b2/b201415g/ for crystallographic data in CIF or other
electronic format.
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∑ 2A [containing the (h -C5H4Me) ligand] gave easier access to single
crystals; spectroscopic data for 2A are included in the ESI.†
5
from those reported for (h -C5H5)Fe(CO)2CH3 (2010 and 1958
cm21 15) implies that there is little or no p stabilization of the
boron centre through Fe–B back bonding. Additionally, there is
little or no p interaction between the boron centre in 3 and the
mesityl ring system, with the angle between the Fe2B and
BCipsoCortho planes being of the order of 83.6(4)°. Clearly the
orientation of the plane of the mesityl ligand is largely
determined by efforts to minimize steric interaction with the
1 W. A. Herrmann, Adv. Organomet. Chem., 1982, 20, 159; W. A. Nugent
and J. M. Mayer, Metal Ligand Multiple Bonds, Wiley Interscience,
New York, 1988.
2 See, for exampleB. V. Mork and T. D. Tilley, J. Am. Chem. Soc., 2001,
123, 9702.
3 G. J. Irvine, M. J. G. Lesley, T. B. Marder, N. C. Norman, C. R. Rice,
E. G. Robins, W. R. Roper, G. Whittell and L. J. Wright, Chem. Rev.,
1998, 98, 2685; H. Braunschweig and M. Colling, Coord. Chem. Rev.,
2001, 223, 1.
4 See, for example: J. Weiss, D. Stetzkamp, B. Nuber, R. A. Fischer, C.
Boehme and G. Frenking, Angew. Chem., Int. Ed. Engl., 1997, 36, 70;
S. T. Haubrich and P. P. Power, J. Am. Chem. Soc., 1998, 120, 2002; P.
Jutzi, B. Neumann, G. Reumann, L. O. Schebaum and H.-G. Stammler,
Organometallics, 1999, 18, 2550.
5 J. Su, X.-W. Li, R. C. Crittendon, C. F. Campane and G. H. Robinson,
Organometallics, 1997, 16, 4511; F. A. Cotton and X. Feng,
Organometallics, 1998, 17, 128.
6 X. He, R. A. Bartlett and P. P. Power, Organometallics, 1994, 13, 548;
T. Yamaguchi, K. Ueno and H. Ogino, ibid, 2001, 20, 501; A. H.
Cowley, A. Decken, C. A. Olazabal and N. C. Norman, Z. Anorg. Allg.
Chem., 1995, 621, 1844.
5
(h -C5H5)Fe(CO)2 moieties.
Recent theoretical studies have indicated that borylene
ligands bind strongly to transition metal centres, but that such
complexes are likely to be kinetically labile due to the build up
of positive charge at boron.16 The synthesis of 3 indicates that
in the presence of suitable steric shielding it is possible to isolate
complexes containing highly Lewis acidic boron centres and
novel modes of coordination of the borylene ligand.
We would like to thank the EPSRC, the Royal Society and the
Nuffield Foundation for funding.
Notes and references
† Electronic supplementary information (ESI) available: spectroscopic data
for (h -C5H4Me)Fe(CO)2B(2,4,6-Me3C6H2)Br 2A.See http://www.rsc.org/
suppdata/cc/b2/b201415g/
7 C. L. B Macdonald and A. H. Cowley, J. Am. Chem. Soc., 1999, 121,
12113; G. Frenking and N. Fröhlich, Chem. Rev., 2000, 100, 717.
8 Several examples of boron-containing clusters featuring facing capping
BR units which may alternatively be described as triply bridging (m3)
borylenes are known, e.g., R. Okamura, K. Tada, K. Matsubara, M.
Oshima and H. Suzuki, Organometallics, 2001, 20, 4772.
9 A. H. Cowley, V. Lomeli and A. Voight, J. Am. Chem. Soc., 1998, 120;
G. J. Irvine, C. E. F. Rickard, W. R. Roper, A. Williamson and L. J.
Wright, Angew. Chem., Int. Ed., 2000, 39, 948.
10 H. Braunschweig, C. Kollann and U. Englert, Angew. Chem. Int. Ed.
Engl., 1998, 37, 3179; H. Braunschweig, M. Colling, C. Kollann, H. G.
Stammler and B. Neumann, ibid, 2001, 40, 2298; H. Braunschweig, M.
Colling, C. Kollann, K. Merz and K. Radacki, ibid, 2001, 40, 4198.
11 H. Braunschweig and T. Wagner, Angew. Chem. Int. Ed. Engl., 1995,
34, 825; H. Braunschweig and M. Müller, Chem. Ber., 1997, 130, 1295;
H. Braunschweig, C. Kollann and U. Englert, Eur. J, Inorg. Chem.,
1998, 465.
5
‡ Reaction of a toluene solution of (2,4,6-Me3C6H2)BBr2 (1) (0.312 g, 1.08
5
mmol) with 1 equiv. of (h -C5H5)Fe(CO)2Na at 20 °C over 12 h, followed
by filtration, removal of volatiles in vacuo and recrystallization from
hexanes (ca. 20 cm3) leads to the isolation of 2 as a pale yellow crystalline
5
solid in yields of up to 60%. Use of a five-fold excess of (h -
C5H5)Fe(CO)2Na over 96 h at 40 °C, subsequent filtration, removal of
volatiles in vacuo and recrystallization from hexanes leads to the isolation
of 3 in yields of up to 35%.
§ Spectroscopic data for 2 and 3. 2: MS(EI): M+ = 386 (weak), isotopic
pattern corresponding to 1 B, 1 Fe, 1 Br atoms, strong fragment ion peaks at
m/z 358 [(M 2 CO)+, 25%] and 330 [(M 2 2CO)+, 100%]. 1H NMR
([2H6]benzene, 21 °C), d 2.17 [s, 6H, ortho-CH3], 2.20 [s, 3H, para-CH3],
5
4.00 [s, 5H, h -C5H5], 6.69 [s, 2H, aromatic CH]. 13C NMR ([2H6]benzene,
5
21 °C), d 20.9 [para-CH3], 21.2 [ortho-CH3], 86.8 [h -C5H5], 128.0
[aromatic CH], 131.1, 136.6 [aromatic quaternary], 213.8 [CO]. 11B NMR
([2H6]benzene, 21 °C), d 111.4. IR (KBr disk, cm21) n(CO) 2016s, 1962s.
3: MS(EI): M+ = 484 (weak), fragment ion peaks at m/z 456 [(M 2 CO)+,
20%], 428 [(M 2 2CO)+, weak], 400 [(M 2 3CO)+, 30%], 372 [(M 2
4CO)+, 35%]. 1H NMR ([2H6]benzene, 21 °C), d 2.08 [s, 6H, ortho-CH3],
12 M. Shimoi, S. Ikubo and Y. Kawano, J. Am. Chem. Soc., 1998, 120,
4222.
13 S. Aldridge, R. J. Calder, A. A. Dickinson, D. J. Willock and J. W.
Steed, Chem. Commun, 2000, 1377; S. Aldridge, A. Al-Fawaz, R. J.
Calder, A. A. Dickinson, D. J. Willock, M. Light and M. B. Hursthouse,
Chem. Commun., 2001, 1846; A. A. Dickinson, D. J. Willock, R. J.
Calder and S. Aldridge, Organometallics, 2002, 21, 1146.
14 A precedent of sorts for this mode of coordination of the borylene ligand
is found in the unstable boron sub-fluoride B3F5 [(F2B)BF(BF2)]:P. L.
Timms, Acc. Chem. Res., 1973, 6, 118.
15 K. Pannell, C. C. Wu and G. J. Long, J. Organomet., Chem., 1980, 193,
359.
16 A. W. Ehlers, E. J. Baerends, F. M. Bickelhaupt and U. Radius, Chem.
Eur. J., 1998, 4, 210.
5
2.27 [s, 3H, para-CH3], 4.03 [s, 10H, h -C5H5], 6.74 [s, 2H, aromatic CH].
13C NMR ([2H6]benzene, 21 °C), d 20.8 [para-CH3], 23.0 [ortho-CH3],
5
86.6 [h -C5H5], 128.1 [aromatic CH], 126.0, 128.5, 134.0 [aromatic
quaternary], 217.3 [CO]. 11B NMR ([2H6]benzene, 21 °C), d 158.0. IR (KBr
disk, cm21) n(CO) 2010s, 1997m sh, 1949s, 1931w sh. Satisfactory
elemental analyses for 2 and 3 were frustrated by the extremely ready
decomposition of the two compounds.
¯
¶
Crystallographic data: for C23H21BFe2O4 3: triclinic, P1, a
=
11.938(2), b = 12.404(3), c = 15.431(3) Å, a = 111.56(3), b = 97.15(3),
CHEM. COMMUN., 2002, 856–857
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