Cationic terminal borylenes by halide abstraction: Synthesis and spectroscopic and structural characterization of an Fe=B double bond

Article abstract of DOI:10.1021/ja0346936

The synthesis and the spectroscopic and structural characterization of the cationic terminal borylene complex [Cp*Fe(CO)₂(BMes)]⁺ are reported. Halide abstraction from the corresponding bromoboryl species using Na[BAr^f

Full text of DOI:10.1021/ja0346936

Published on Web 05/02/2003
Cationic Terminal Borylenes by Halide Abstraction: Synthesis and
Spectroscopic and Structural Characterization of an FedB Double Bond
Deborah L. Coombs, Simon Aldridge,* Cameron Jones, and David J. Willock
Department of Chemistry, Cardiff UniVersity, P.O. Box 912, Park Place, Cardiff, CF10 3TB U.K.
Received February 17, 2003; E-mail: AldridgeS@cardiff.ac.uk
Scheme 1 a
Compounds offering the potential for multiple bonding between
a transition metal and a main group element have excited much
recent interest, not only from a structure/bonding viewpoint but
also as potential intermediates in important catalytic processes.1-5
In comparison to group 14 systems [e.g., alkylidenes (R2C)1 and
silylenes (R2Si)2], however, the synthetic, structural, and reaction
chemistry of analogous compounds containing terminally bound
group 13 diyl ligands (RE) is less thoroughly understood.3-5 Within
a Reagents and conditions: (i) Na[BAr f4], -NaBr; CH2Cl2, -78 °C to
this field, terminal borylene systems (LnMBR) represent a very
recent and numerically small addition, with the highly Lewis acidic
room temperature. (ii) [Ph4P]Br, -[Ph4P][BAr f4]; CH2Cl2, room temper-
ature.
boron center typically being stabilized by sterically bulky or π-donor
R groups or by coordination of a tethered base.3
[CpFe(CO)2]2BMes7}. 11B shifts in the range δB 87-204 have been
A number of structural and computational studies have sought
reported by Braunschweig for terminal borylene complexes of
to probe the nature of the metal-boron bond in two-coordinate
groups 6 and 8, with lower field values being obtained for ligands
borylene complexes.4 To this end, systematic studies have been
containing weakly π-donor substituents.3b-f The measured 11B shift
severely hampered by the paucity of synthetic routes available and
for 2 is therefore consistent with a terminally bound BMes ligand,
by the narrow range of compatible metal fragments (predominantly
although categorical structural assignment could be made only on
metal carbonyls). Herein we report the development of a new route
the basis of X-ray crystallography.
to terminal borylene complexes, making use of halide abstraction
The solid-state structure of 2 (Figure 1) is based around an
to generate the first example of a cationic [LnMBR]+ species.6 The
asymmetric unit containing two (essentially identical) [Cp*Fe(CO)2-
compound so generated, [Cp*Fe(CO)2(BMes)]+[BAr f ]- [Ar f )
4
(BMes)]+ cations, two [BAr f ]- anions, and a molecule of CH2Cl2
4
C6H3(CF3)2-3,5], contains the shortest M-B distance yet reported,
a feature which is indicative of a novel FedB double bond.
Comparisons of experimental and computational data with those
solvent.10 There are no short cation-anion or cation-solvent
contacts. Of particular interest is the nearly linear Fe-B-C unit
[ Fe(1)-B(1)-C(1) ) 178.3(6)°] and the Fe-B distance [1.792-
reported for the donor/acceptor borylene Cp*BfFe(CO)4 are
(8) Å], which is significantly shorter than any transition metal-to-
consistent with a significantly different mode of attachment of the
boron linkage previously reported. It is also 11% shorter than that
found in Cp*BFe(CO)4 [2.010(3) Å] (a compound which contains
a BfFe donor/acceptor linkage 3a), 18% shorter than the σ-only
RB ligand.3a Additionally, non-heteroatom π-stabilized two-
coordinate borylenes find precedent only in (OC)5CrdBSi-
(SiMe3)3.3d
single bond found in the four-coordinate boryl complex (η5-
C5Me4Et)Fe(CO)2BH2‚PMe3 [2.195(14) Å], and 8.5% shorter than
the shortest Fe-B distance found for any three-coordinate boryl
complex [1.959(6) Å for CpFe(CO)2Bcat].11 By means of com-
parison, the Fe-C distance in [CpFe(CO)2(dCCl2)]+ [1.808(12)
Å] is ca. 12% shorter than that found in typical [CpFe(CO)2] alkyl
complexes.12 The Fe-B distance in 2 is therefore consistent with
the presence of an FedB double bond.
The synthesis of the necessary bromoboryl precursors (η5-C5R5)-
Fe(CO)2B(Mes)Br [R ) Me (1) or H] has previously been reported.7
Treatment of 1 (46 mg, 0.1 mmol) with 1 equiv of Na[BAr f ] or
4
Ag[CB11H6Br6] in CH2Cl2 (20 mL) at -78 °C, followed by slow
warming to room temperature, leads to complete disappearance of
the 11B resonance due to 1 (δB 113) and (in addition to resonances
associated with the anion) to the appearance of a broad signal at
δB 145 (fwhm ca. 750 Hz). In the case of the reaction with
Na[BAr f ], filtration, layering with hexanes, and cooling to -50
Tilley has proposed that the RudSi double bond in the cationic
silylene complex [Cp*Ru(PMe3)2(dSiMe2)]+ is composed of a
SifRu donor/acceptor σ-component supplemented by RufSi
π-back-bonding into the vacant Si-based p orbital.13 A similar
bonding arrangement for 2 would imply a significantly greater
FefB back-bonding component than that found in Cp*BFe(CO)4.
Such an arrangement is conceivable, given the highly electrophilic
nature of the boron center in 2 and the significantly higher DFT-
predicted π-bonding capability of the BPh ligand compared to that
of BCp.4c The presence of an appreciable FefB back-bonding
component for 2 is implied by carbonyl stretching frequencies
(2055, 2013 cm-1) in excess of those of {Cp*Fe(CO)2[dCMe-
(OMe)]}+ (2045, 1999 cm-1). That these values are still short of
those reported for [Cp*Fe(CO)3]+ (2105, 2045 cm-1 9b), however,
4
°C for 1 week leads to the isolation of 2 as colorless crystals (70
mg, 56%) (Scheme 1).8
IR-measured carbonyl stretching frequencies for 2 (2055 and
2013 cm-1) are shifted to significantly higher wavenumber com-
pared to those of the starting boryl complex 1 (2006 and 1961
cm-1),7 consistent with the presence of a cationic complex
containing the Cp*Fe(CO)2 unit.9 1H and 13C NMR data for 2
confirm the presence of mesityl, Cp*, and carbonyl fragments, and
mass spectra display peaks consistent with [M - CO]+ and [M -
2CO]+ (EI) and [BAr f ]- ions (ES-). Additionally, the strongly
4
downfield 11B NMR shift (δB 145) is similar to that observed
previously for iron complexes containing mesitylborylene ligands,
albeit adopting a µ2-bridging mode of coordination {cf. δB 158 for
9
6356
J. AM. CHEM. SOC. 2003, 125, 6356-6357
10.1021/ja0346936 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. We thank the EPSRC, the Royal Society,
and the Nuffield Foundation for funding and Dr. A. S. Weller
(University of Bath) for providing a sample of Ag[CB11H6Br6]. The
EPSRC mass spectrometry service is also thanked.
Supporting Information Available: Details of the X-ray crystal-
lographic study of 2, the DFT calculations for [Cp*Fe(CO)2(BMes)]+,
and the reaction of 2 with [Ph4P]Br (PDF); X-ray crystallographic data
(CIF). This material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 1. Structure of one of the two crystallographically independent
cations in the asymmetric unit of 2. Relevant bond lengths (Å) and angles
(°): Fe(1)-B(1) 1.792(8), Fe(1)-C(20) 1.768(7), Fe(1)-Cp* centroid
1.733(7), B(1)-C(1) 1.491(10), Fe(1)-B(1)-C(1) 178.3(6), Cp* centroid-
Fe(1)-C(1)-C(2) 91.3(6). Hydrogen atoms omitted for clarity.
References
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(8) 1H NMR (300.53 MHz, 293 K, CD2Cl2): δ 2.01 (s, 15H, CH3 of Cp*),
2.34 (s, 3H, p-CH3 of Mes), 2.63 (s, 6H, o-CH3 of Mes), 6.92 (s, 2H,
m-H of Mes), 7.54 (s, 4H p-H of BAr f -), 7.70 (s, 8H, o-H of BAr f -).
13C NMR (75.57 MHz, 293 K, CD2Cl24): δ 8.7 (CH3 of Cp*), 20.84(o-
CH3 of Mes), 21.4 (p-CH3 of Mes), 97.1 (quaternary C of Cp*), 116.7
Figure 2. DFT (BLYP/TZP)-calculated HOMO-3 for [Cp*Fe(CO)2-
(BMes)]+ showing Fe-B π-bonding character.
almost certainly reflects the stronger σ-donor nature of BR over
CO, as predicted by Hoffmann and Baerends.4a
The orientation of the mesityl fragment in 2 [torsion, Cp*
centroid-Fe(1)-C(1)-C(2) ) 91.3(6)°] is such that an FefB
π-interaction involving the HOMO of the [Cp*Fe(CO)2]+ frag-
ment14 could populate one of the two perpendicular vacant p orbitals
at boron, with the other being stabilized by π-interaction with the
mesityl ring. Consistent with this, the distance B(1)-C(1) [1.491-
(10) Å] is significantly shorter than that found in 1 [1.569(3) Å].
To explore more rigorously the bonding situation in 2, DFT
calculations were carried out for the cation [Cp*Fe(CO)2(BMes)]+
at the BLYP/TZP level of theory, using methods previously
described.15c The agreement between (fully optimized) calculated
[Fe(1)-B(1) 1.843 Å, B(1)-C(1) 1.495 Å, Fe(1)-B(1)-C(1) )
177.8°, torsion ) 93.7°] and measured geometric parameters is
generally very good, with a 2-3% overestimate in the Fe-B bond
length, mirroring previous studies.15 Based on a population analysis
of the molecular orbitals at the DFT relaxed geometry, the Fe-B
bond has a 62:38 σ:π breakdown of the covalent contribution to
bonding {cf. 64:36 for [CpFe(CO)2(dCH2)]+ 15c}. The occupied
MO (HOMO-3) shown in Figure 2 shows the Fe-B π-bond formed
by the overlap of B px and Fe dxz type orbitals. Similarly, HOMO-9
shows distinct C-B π-bonding character utilizing the perpendicular
B py orbital. These calculations therefore support a bonding model
in which boron engages in π-bonding to both [Cp*Fe(CO)2]+ and
Mes moieties.
Previous computational studies have predicted significant B-
centered reactivity toward nucleophiles for terminal borylene
complexes due to the high positive charge and LUMO amplitude
at boron.4 On the other hand, the current study shows that, in the
presence of suitable steric shielding, even complexes bearing a net
positive charge can be isolated. Indeed, it seems likely that the
combination of charge and steric shielding in 2 retards the
dimerization process observed for the putative isoelectronic man-
ganese system [(η5-C5H4Me)Mn(CO)2(BCl)].16 Not unexpectedly,
the cationic nature of 2 appears (on first investigation) to dominate
its chemistry. 2 is extremely moisture sensitive and reacts rapidly
with [Ph4P]Br in CH2Cl2 to regenerate bromoboryl complex 1.
1
(p-CH of BAr f -), 123.8 (q, JCF ) 271 Hz, CF3 of BAr f -), 125.1 (m-
4
4
CH of Mes), 128.6 (m-C of BAr f -), 134.0 (o-CH of BAr f -), 147.9,
4
4
150.1 (quaternary C’s of Mes), 160.9 (q, 1JCB ) 49 Hz, ipso-C of BAr f -),
4
211.4 (CO) (the ipso-C of mesityl ligand was not observed). 19F NMR
(282.78 MHz, 293 K, CD2Cl2): δ -62.8 (CF3). 11B NMR (96.42 MHz,
293 K, CD2Cl2): δ -7.6 (BAr f -), 145 (BMes). IR (CH2Cl2 solution):
4
νCO ) 2055, 2013 cm-1. EI-MS: m/z (relative intensity) [M - CO]+
)
349 (15), [M - 2CO]+ 321 (5), correct isotope distribution for 1Fe, 1B.
ES-MS (negative): m/z (relative intensity) [BAr f
]
) 863 (100).
-
4
(9) See, for example: Nlate, S.; Guerchais, V.; Lapinte, C. J. Organomet.
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(10) Crystal data for 2‚1/2CH2Cl2: C53.5H39B2ClF24FeO2, monoclinic, C2/c, a
) 36.301(7), b ) 25.675(5), and c ) 23.956(5) Å, â ) 93.81(3)°, V )
22278(8) Å3, Z ) 16, Dcalc ) 1.530 g cm-3, µ(MoKR) ) 0.440 mm-1. A
suitable crystal was covered in predried, precooled mineral oil and mounted
on a Enraf-Nonius Kappa CCD diffractometer at 150(2) K. A total of
19 345 unique reflections were collected (2.9 < θ < 25.0°). Final
R-factor: R1 ) 0.088.
(11) (a) Yasue, T.; Kawano, Y.; Shimoi, M. Chem. Lett. 2000, 58. (b) Hartwig,
J. F.; Huber, S. J. Am. Chem. Soc. 1993, 115, 4908.
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