Synthetic, structural and reaction chemistry of transition metal complexes containing the mesitylborylene ligand

Article abstract of DOI:10.1039/b206348b

The synthesis, spectroscopic and structural characterization of the bromo-boryl complexes (η⁵-C₅R₄R′)Fe(CO)₂- B(2,4,6-Me₃C₆H₂)Br (R = R′ = H, 2; R = H, R′ = Me, 3; R = R′ = Me, 4) are reported. These are shown to be versatile substrates for the synthesis of both asymmetric boryl complexes [e.g. (η⁵-C₅H₅)Fe(CO)₂B(2,4,6-Me ₃C₆H₂)OC₆H₄ ^tBu-4, 6], and bridging borylene complexes {e.g. [η⁵-C₅H₄R)Fe(CO)₂] ₂B(2,4,6-Me₃C₆H₂), R = H, 7; R = Me, 8} via substitution chemistry with retention of the metal-boron bond. Complexes 7 and 8 are thefirst reported examples of structurally characterized bridging borylene complexes without a supporting M-M bond. Photolytically induced CO loss from [η⁵-C₅H₅)Fe(CO)₂] ₂B(2,4,6-Me₃C₆H₂) yields the complex [η⁵-C₅H₅)Fe(CO)]₂(μ ₂-CO)[μ₂-B(2,4,6-Me₃C₆H ₂)] (11), which features a supported bridging borylene ligand.

Full text of DOI:10.1039/b206348b

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Synthetic, structural and reaction chemistry of transition metal
complexes containing the mesitylborylene ligand†
Deborah L. Coombs, Simon Aldridge* and Cameron Jones
Department of Chemistry, Cardiff University, PO Box 912, Park Place, Cardiff, UK CF10 3TB
Received 1st July 2002, Accepted 5th August 2002
First published as an Advance Article on the web 17th September 2002
The synthesis, spectroscopic and structural characterization of the bromo-boryl complexes (η5-C5R4RЈ)Fe(CO)2-
B(2,4,6-Me3C6H2)Br (R = RЈ = H, 2; R = H, RЈ = Me, 3; R = RЈ = Me, 4) are reported. These are shown to be versatile
t
substrates for the synthesis of both asymmetric boryl complexes [e.g. (η5-C5H5)Fe(CO)2B(2,4,6-Me3C6H2)OC6H4 Bu-
4, 6], and bridging borylene complexes {e.g. [(η5-C5H4R)Fe(CO)2]2B(2,4,6-Me3C6H2), R = H, 7; R = Me, 8} via
substitution chemistry with retention of the metal–boron bond. Complexes 7 and 8 are the first reported examples of
structurally characterized bridging borylene complexes without a supporting M–M bond. Photolytically induced CO
loss from [(η5-C5H5)Fe(CO)2]2B(2,4,6-Me3C6H2) yields the complex [(η5-C5H5)Fe(CO)]2(µ2-CO)[µ2-B(2,4,6-Me3C6H2)]
(11), which features a supported bridging borylene ligand.
reaction chemistry of boryl ligands BX2.14 Asymmetric boryl
1
Introduction
complexes [LnM–B(R)X, X = halide], although rare, are poten-
tial precursors to a range of borylene complexes of the type
LnM–B(R)–MЈLЈn via boron-centred substitution chemistry.
Choice of appropriately bulky aryl substituents (R) and labile
halide (e.g. X = Br) has allowed us to isolate a range of asym-
metric boryl complexes which prove to be versatile substrates
in the synthesis of hitherto unreported unsupported bridging
borylene complexes.15
Investigation of the bonding between transition metals and low
coordinate ligands of groups 13 and 14 continues to be the
driving force for considerable research effort.1–8 Whereas
multiple bonding (L M᎐ER ) involving alkylidene (ER
=
᎐
n
n
n
CR2)1 and more recently silylene ligands (ERn = SiR2)2 is now
well understood, the bonding in terminally bound group 13 diyl
systems (ERn = BR, AlR, GaR, InR) remains a matter of some
debate.4–8 The description of superficially similar complexes as
being bound via multiple bonds (e.g. L M᎐ER) or via donor/
᎐
n
acceptor interactions (LnM ER) reflects not only the funda-
mental questions of structure and bonding posed by such com-
plexes, but also the scarcity of structural data available.4,5,7 In
some respects the arguments are reminiscent of those related to
Fischer and Schrock carbenes.1b
Transition metal complexes containing diyls of the heavier
group 13 elements (particularly gallium and indium) are rela-
tively numerous with examples of both terminal and bridging
modes of coordination having been demonstrated crystallo-
graphically.7–10 Analogous compounds containing boranediyl
(or borylene, BR) or alanediyl (AlR) ligands are much less
common, with the first structurally characterized examples
having been reported as late as 1995.4,5,11–13 Since then, the
dozen or so compounds reported in the literature have generally
conformed to one of two structural types viz.: (i) terminally
bound BR (or AlR) ligands containing sterically bulky or π
2
Experimental
All manipulations were carried out under a nitrogen or argon
atmosphere using standard Schlenk line or dry box techniques.
Solvents were pre-dried over sodium wire and purged with
nitrogen prior to distillation. Hexanes were distilled from
potassium, and toluene was distilled from sodium prior to use.
C6D6 (Goss) was degassed and dried over potassium prior to
use. 2,4,6-Me3C6H2BBr2 (MesBBr2, 1), 2,4,6-tBu3C6H2BBr2,
NaOC6H4 Bu-4, Na[(η5-C5R4RЈ)Fe(CO)2] (R = RЈ = H; R = H,
t
RЈ = Me; R = RЈ = Me) and Na[Mn(CO)5] were prepared by
minor modification of literature methods.16–18
NMR spectra were measured on a Jeol 300 Eclipse Plus FT-
NMR spectrometer. Residual protons of solvent were used for
reference for 1H and 13C NMR, while a sealed tube containing
a solution of [(nBu4N)(B3H8)] in CDCl3 was used as an external
reference for 11B NMR measurements. Infrared spectra were
measured for each compound pressed into a disk with an excess
of dried KBr on a Nicolet 500 FT-IR spectrometer. Mass spec-
tra were measured by the EPSRC National Mass Spectrometry
Service Centre, University of Wales Swansea and by the
departmental service. Despite repeated attempts satisfactory
elemental microanalysis for new boryl and borylene complexes
was frustrated by their extreme air-, moisture- and (in some
cases) thermal-instability. Characterization of new compounds
is based upon multinuclear NMR, IR and mass spectrometry
data, supplemented by single crystal X-ray diffraction studies
for compounds 2, 3, 4, 6, 7, 8 and 11. In all cases the purity of
the bulk material was established by multinuclear NMR to be
>95%.
electron releasing R substituents [e.g. (OC) Cr᎐BSi(SiMe ) or
᎐
5
3 3
(OC)4Fe Al(η5-C5Me5)];4,6 and (ii) RE ligands adopting a
bridging mode of coordination between two (or more) metal
centres which are typically also linked by a metal–metal bond
or a second bridging ligand {e.g. [(η5-C5H4R)Mn(CO)2]2(µ2-
BX) (X = NMe2, R = H; X = OEt, R = Me; X = Cl, R = Me) or
(η5-C5Me5)Ir(PMe3)(µ-AlEt)2Ir(PMe3)(η5-C5Me5)}.11–13
We have recently been interested in the coordination chem-
istry of ligands containing trigonal boron centres and in par-
ticular the influence of substituents X on the structural and
† Dedicated to Professor Tom Fehlner on the occasion of his 65th
birthday.
Electronic supplementary information (ESI) available: logarithmic
plot for the photolytic conversion of 7 to 11. See http://www.rsc.org/
suppdata/dt/b2/b206348b/
Abbreviations: st = strong, md = medium, w = weak, sh =
shoulder, s = singlet, d = doublet, m = multiplet.
DOI: 10.1039/b206348b
J. Chem. Soc., Dalton Trans., 2002, 3851–3858
This journal is © The Royal Society of Chemistry 2002
3851

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Synthesis of (ꢀ5-C5R4RЈ)Fe(CO)2B(2,4,6-Me3C6H2)Br
(R ؍ RЈ ؍ H, 2; R ؍ H, RЈ ؍ Me, 3; R ؍ RЈ ؍ Me, 4)
CH), 133.7, 136.3 (mesityl aromatic quaternary), 145.5, 155.1
(OC6H4 aromatic quaternary), 215.9 (CO). 11B NMR (96 MHz,
C6D6) δ 80.3. IR (KBr disk, cmϪ1) ν(CO) 1997 st, 1933 st.
EI-MS: Mϩ = 456 (weak), isotopic pattern corresponding to 1
Fe, 1 B atoms, fragment ion peaks at m/z 428 [(M Ϫ CO)ϩ, 25%]
and 400 [(M Ϫ 2CO)ϩ, 85%].
The three complexes were synthesized in a similar manner.
Typically, to a suspension of Na[(η5-C5H5)Fe(CO)2] (0.32 g,
1.6 mmol) in toluene (20 cm3) was added a solution of 2,4,6-
Me3C6H2BBr2 (0.46 g, 1.6 mmol) in toluene (10 cm3) and the
reaction mixture was stirred at room temperature for 12 h.
Filtration, removal of volatiles in vacuo and recrystallization
from hexanes (ca. 20 cm3) at Ϫ30 ЊC yielded 2, 3 and 4 as pale
yellow crystals suitable for X-ray diffraction. Yields of 2–4 were
typically in the order of 50–60%. 2: 1H NMR (300 MHz, C6D6):
δ 2.17 (s, 6H, ortho-CH3), 2.20 (s, 3H, para-CH3), 4.00 (s, 5H,
η5-C5H5), 6.69 (s, 2H, aromatic CH). 13C NMR (76 MHz,
C6D6): δ 20.9 (para-CH3), 21.2 (ortho-CH3), 86.8 (η5-C5H5),
128.0 (aromatic CH), 131.1, 136.6 (aromatic quaternary), 213.8
Synthesis of [(ꢀ5-C5H4R)Fe(CO)2]2B(2,4,6-Me3C6H2)
(R ؍ H, 7; R ؍ Me, 8)
Both complexes were synthesized in
a similar manner.
Typically, a solution of 2,4,6-Me3C6H2BBr2 (0.29 g, 1.0 mmol)
in toluene (5 cm3) was added slowly to a suspension of Na-
[(η5-C5H5)Fe(CO)2] (1.00 g, 5.0 mmol) in toluene (20 cm3) and
the reaction mixture was heated to 40 ЊC for 4 d. Filtration,
removal of volatiles in vacuo and recrystallization from hexanes
(ca. 20 cm3) at Ϫ30 ЊC gave 7 and 8 as pale yellow crystals
suitable for X-ray diffraction in yields of up to 35%. 7: 1H
NMR (300 MHz, C6D6): δ 2.08 (s, 6H, ortho-CH3), 2.27 (s, 3H,
para-CH3), 4.03 (s, 10H, η5-C5H5), 6.74 (s, 2H, aromatic CH).
13C NMR (76 MHz, C6D6): δ 20.8 (para-CH3), 23.0 (ortho-
CH3), 86.6 (η5-C5H5), 128.1 (aromatic CH), 126.0, 128.5, 134.0
(aromatic quaternary), 217.3 (CO). 11B NMR (96 MHz, C6D6)
δ 158.0. IR (KBr disk, cmϪ1) ν(CO) 2010 st, 1997 md sh, 1949 st,
1931 w sh. EI-MS: Mϩ = 484 (weak), isotope pattern corre-
sponding to 2Fe, 1B atom, fragment ion peaks at m/z 456 [(M Ϫ
CO)ϩ, 20%], 428 [(M Ϫ 2CO)ϩ, weak], 400 [(M Ϫ 3CO)ϩ, 30%],
378 [(M Ϫ 4CO)ϩ, 35%]. 8: 1H NMR (300 MHz, C6D6): δ 1.49
(s, 6H, η5-C5H4CH3), 2.12 (s, 6H, ortho-CH3), 2.28 (s, 3H, para-
CH3), 3.94 (s, 2H, 2H of η5-C5H4CH3), 3.99 (d, 2H, 2H of
η5-C5H4CH3), 5.96 (s, 2H, aromatic CH). 13C NMR (76 MHz,
C6D6): δ 12.6 (η5-C5H4CH3), 20.8 (para-CH3), 23.0 (ortho-CH3),
86.5, 87.8 (CH of η5-C5H4CH3), 102.2 (quaternary in η5-C5H4-
CH3), 126.1 (aromatic CH), 128.5, 128.9, 133.8 (aromatic
quaternary), 217.9 (CO). 11B NMR (96 MHz, C6D6) δ 157.9.
IR (KBr disk, cmϪ1) ν(CO) 1999 st, 1967 st, 1942 st, 1923 st.
EI-MS: Mϩ = 512 (weak), isotope pattern corresponding to
2Fe, 1B atoms, fragment ion peaks at m/z 484 [(M Ϫ CO)ϩ,
10%], 456 [(M Ϫ 2CO)ϩ, 10%], 428 [(M Ϫ 3CO)ϩ, 5%], 400
[(M Ϫ 4CO)ϩ, weak].
(CO). 11B NMR (96 MHz, C6D6) δ 111.4. IR (KBr disk, cmϪ1
)
ν(CO) 2016 st, 1962 st. EI-MS: Mϩ = 386 (weak), isotopic pat-
tern corresponding to 1 Fe, 1 B, 1 Br atoms, strong fragment ion
peaks at m/z 358 [(M Ϫ CO)ϩ, 25%] and 330 [(M Ϫ 2CO)ϩ,
1
100%]. 3: H NMR (300 MHz, C6D6): δ 1.32 (s, 3H, η5-C5H4-
CH3), 2.20 (s, 9H, overlap of ortho- and para-CH3), 3.97 (s,
4H, overlap of both sets of CH protons η5-C5H4CH3), 6.70
(s, 2H, aromatic CH). 13C NMR (76 MHz, C6D6): δ 12.4 (η5-
C5H4CH3), 20.9 (para-CH3), 21.2 (ortho-CH3), 86.7, 86.9 (CH
of η5-C5H4CH3), 104.2 (η5-C5H4CH3 quaternary), 131.2
(aromatic CH), 129.9, 136.5 (aromatic quaternary), 214.3 (CO).
11B NMR (96 MHz, C6D6) δ 111.3. IR (KBr disk, cmϪ1) ν(CO)
2009 st, 1961 st. EI-MS: Mϩ = 400 (weak), isotopic pattern
corresponding to 1 Fe, 1 B, 1 Br atoms, fragment ion peaks at
m/z 372 [(M Ϫ CO)ϩ, 30%] and 344 [(M Ϫ 2CO)ϩ, 70%]. 4:
1H NMR (300 MHz, C6D6): δ 1.31 (s, 15H, η5-C5Me5), 2.21 (s,
3H, para-CH3), 2.26 (s, 6H, ortho-CH3), 6.73 (s, 2H, aromatic
CH). 13C NMR (76 MHz, C6D6): δ 9.0 (η5-C5Me5), 20.9 (para-
CH3), 21.5 (ortho-CH3), 97.4 (η5-C5Me5 quaternary), 127.6
(aromatic CH), 132.3, 136.3 (aromatic quaternary), 216.0 (CO).
11B NMR (96 MHz, C6D6) δ 113.2. IR (KBr disk, cmϪ1) ν(CO)
2006 st, 1961 st. EI-MS: Mϩ = 456 (weak), isotopic pattern
corresponding to 1 Fe, 1 B, 1 Br atoms, fragment ion peaks at
m/z 428 [(M Ϫ CO)ϩ, 100%] and 400 [(M Ϫ 2CO)ϩ, 50%].
Synthesis of [(ꢀ5-C5H5)Fe(CO)2][(ꢀ5-C5H4Me)Fe(CO)2]B-
Synthesis of (OC)5MnB(2,4,6-Me3C6H2)Br 5
(2,4,6-Me3C6H2) 9
A solution of 2,4,6-Me3C6H2BBr2 (0.14 g, 0.5 mmol) in toluene
(5 cm3) was added to a suspension of Na[Mn(CO)5] (0.11 g,
0.5 mmol) in toluene (20 cm3) and stirred at room temperature
for 12 h. Filtration, removal of volatiles in vacuo and recrystal-
lization from hexanes (ca. 10 cm3) at Ϫ30 ЊC yielded yellow
A
solution of (η5-C5H4Me)Fe(CO)2B(2,4,6-Me3C6H2)Br
(0.21g, 0.5 mmol) in toluene (20 cm3) was added slowly to a
suspension of Na[(η5-C5H5)Fe(CO)2] (0.41 g, 2.1 mmol) in
toluene (10 cm3). The mixture was heated to 40 ЊC, for 6 d, at
which point the reaction was judged to be complete by 11B
NMR. Filtration, removal of volatiles in vacuo and recrystal-
lization from hexanes (ca. 20 cm3) at Ϫ30 ЊC yielded 9 as yellow
crystals (0.19 g, 76%). 1H NMR (300 MHz, C6D6): δ 1.47 (s, 3H,
η5-C5H4CH3), 2.11 (s, 6H, ortho-CH3), 2.28 (s, 3H, para-CH3),
3.93 (m, 2H, 2H of η5-C5H4CH3), 3.98 (m, 2H, 2H of η5-
1
crystals of 5 (0.06 g, 29%). H NMR (300 MHz, C6D6): δ 2.11
(s, 6H, ortho-CH3), 2.49 (s, 3H, para-CH3), 6.64 (s, 2H, aromatic
CH). 13C NMR (76 MHz, C6D6): δ 20.7 (para-CH3), 21.4 (ortho-
CH3), 128.4 (aromatic CH), 130.6, 137.7 (aromatic quaternary),
210.5 (CO). 11B NMR (96 MHz, C6D6) δ 119.4. IR (KBr disk,
cmϪ1) ν(CO) 2101 w, 2046 m, 2015 s, 1981 m sh, 1952 w.
C5H4CH3), 4.04 (s, 5H, η5-C5H5), 6.75 (s, 2H, aromatic CH). 13
C
NMR (76 MHz, C6D6): δ 12.6 (η5-C5H4CH3), 20.8 (para-CH3),
23.0 (ortho-CH3), 86.6 (CH of η5-C5H4CH3), 86.8 (η5-C5H5),
87.7 (CH of η5-C5H4CH3), 102.3 (quaternary in η5-C5H4CH3),
126.0, 133.9 (aromatic CH), 128.5 (aromatic quaternary), 217.4,
217.8 (CO). 11B NMR (96 MHz, C6D6) δ 157.6. IR (KBr disk,
cmϪ1) ν(CO) 2003 st, 1968 md, 1944 md, 1927 md. EI-MS: Mϩ =
498 (weak), isotope pattern corresponding to 2 Fe, 1 B atoms,
fragment ion peaks at m/z 470 [(M Ϫ CO)ϩ, weak], 442 [(M Ϫ
2CO)ϩ, weak], 414 [(M Ϫ 3CO)ϩ, 5%], 386 [(M Ϫ 4CO)ϩ,
weak].
Synthesis of (ꢀ5-C5H5)Fe(CO)2B(2,4,6-Me3C6H2)OC6H4tBu-4 6
A solution of (η5-C5H5)Fe(CO)2B(2,4,6-Me3C6H2)Br, 2, (0.14 g,
0.4 mmol) in toluene (10 cm3) was added to a suspension of
t
NaOC6H4 Bu-4 (0.07 g, 0.4 mmol) in toluene (2 cm3) at room
temperature. The resulting mixture was stirred at 65 ЊC for 21 d,
at which point the reaction was judged to be complete by 11B
NMR. Filtration, removal of volatiles in vacuo and recrystal-
lization from hexanes (ca. 10 cm3) at Ϫ30 ЊC yielded 6 as pale
1
yellow crystals suitable for X-ray diffraction (0.11 g, 60%). H
t
NMR (300 MHz, C6D6): δ 1.01 (s, 9H, Bu), 2.02 (s, 3H, para-
CH3), 2.31 (s, 6H, ortho-CH3), 4.11 (s, 5H, η5-C5H5), 6.55 (s,
2H, mesityl CH), 6.98 (s, 4H, overlap of both sets of aromatic
protons OC6H4). 13C NMR (76 MHz, C6D6): δ 20.9 (para-CH3),
21.9 (ortho-CH3), 31.1 (CCH3), 33.8 (CCH3), 85.6 (η5-C5H5),
119.0, 126.1 (OC6H4 aromatic CH), 127.7 (mesityl aromatic
Synthesis of [(ꢀ5-C5H5)Fe(CO)]2(ꢁ2-CO)[ꢁ2-B(2,4,6-Me3-
C6H2)] 11
A solution of [(η5-C5H5)Fe(CO)2]2B(2,4,6-Me3C6H2), 7, (0.04 g,
0.1 mmol) in C6D6 (2 cm3) was photolysed for 5 d using a
3852
J. Chem. Soc., Dalton Trans., 2002, 3851–3858

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standard 100 W light source, at which point the reaction was
formation of any products containing an Fe–B bond (as judged
1
judged to be complete by H NMR. Removal of volatiles in
by 11B NMR).
vacuo and recrystallization from hexanes (ca. 10 cm3) at Ϫ30 ЊC
yielded 11 as single red crystals suitable for X-ray diffraction.
The yield of 11 was typically in the order of 90%, based on the
General crystallographic method
1
Data for all compounds were collected on an Enraf Nonius
Kappa CCD diffractometer; data collection and cell refinement
were carried out using DENZO and COLLECT,19 and struc-
ture solution and refinement using SHELXS-97 and SHELXL-
97, respectively.20 Details of each data collection, structure
solution and refinement can be found in Table 1, relevant bond
lengths and angles are included in figure captions. The poor
quality of the data for compound 7 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
independent 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.
CCDC reference numbers 179292, 179293 and 187963–
187967.
amount of 7 used. H NMR (300 MHz, C6D6): δ 2.22 (s, 3H,
para-CH3), 2.52 (s, 6H, ortho-CH3), 4.31 (s, 10H, η5-C5H5), 6.86
(s, 2H, aromatic CH). 13C NMR (76 MHz, C6D6): δ 21.0 (para-
CH3), 24.4 (ortho-CH3), 86.9 (η5-C5H5), 129.0 (aromatic CH),
136.6, 138.7 (aromatic quaternary), 214.7 (terminal CO), 272.7
(bridging CO). 11B NMR (96 MHz, C6D6) δ 161.9. IR (KBr
disk, cmϪ1) ν(CO) 1947 md, 1924 st, 1780 md sh, 1773 st.
EI-MS: Mϩ = 455 (5%), isotope pattern corresponding to 2 Fe,
1 B atoms, fragment ion peaks at m/z 427 [(M Ϫ CO)ϩ, weak],
399 [(M Ϫ 2CO)ϩ, 10%], 371 [(M Ϫ 3CO)ϩ, 15%]. Kinetic
data for the conversion of 7 to 11 were obtained by periodic
measurement of the 1H NMR spectrum of the reaction mixture
and integration of the resonances due to the ortho methyl
groups of the mesityl substituents of each compound.
Reaction of (OC)5MnB(2,4,6-Me3C6H2)Br, 5, with Na[(ꢀ5-C5H5)-
Fe(CO)2]
A solution of (OC)5MnB(2,4,6-Me3C6H2)Br, 5, (0.186 g, 0.46
mmol) in toluene (15 cm3) was added to a suspension of
Na[(η5-C5H5)Fe(CO)2] (0.380 g, 1.90 mmol) in toluene
(10 cm3). The mixture was heated to 40 ЊC for 6 d, at which
point the 11B NMR spectrum of the reaction mixture showed
complete conversion of the signal due to 5 (δB 119.4) to a
single downfield resonance at δB 158.0. Filtration, removal of
volatiles in vacuo and recrystallization from hexanes (20 cm3)
at Ϫ30 ЊC yielded [(η5-C5H5)Fe(CO)2]2B(2,4,6-Me3C6H2), 7, as
the sole boron-containing product. The identity of 7 was con-
firmed by comparison of multinuclear NMR, IR and crystal-
lographic cell parameters with those of an authentic sample
prepared from 2,4,6-Me3C6H2BBr2, 1, and excess Na[(η5-C5-
H5)Fe(CO)2].
See http://www.rsc.org/suppdata/dt/b2/b206348b/ for crystal-
lographic data in CIF or other electronic format.
3
Results and discussion
(i) Synthesis and reactivity
The asymmetric mesityl(bromo)boryl complexes 2–5 can be syn-
thesised in moderate to good yields (29–60%) by the reaction
of mesitylboron dibromide (2,4,6-Me3C6H2BBr2, MesBBr2, 1)
with one equivalent of the appropriate organometallic anion
in toluene at room temperature (see Scheme 1). Under these
conditions substitution is selective for a single bromide,
with negligible quantities of the disubstituted species 7 and 8
being formed (as determined by 11B NMR spectroscopy).
Interestingly the analogous reaction with super-mesityl boron
dibromide (2,4,6-tBu3C6H2BBr2) fails to bring about substitu-
tion of either bromide under these or more forcing conditions.
Complexes 2–5, bearing a single remaining bromide at
boron, prove to be versatile substrates for further substitution
chemistry. Reaction with main group nucleophiles, such as the
phenoxide anion 4-tBuC6H4OϪ, results in high yielding
synthesis of further asymmetric mesitylboryl complexes.
Formation of the mesityl(aryloxo)boryl complex 6, for
example, proceeds cleanly, although the high steric demands
of the reagents ensure that reaction is slow (ca. 21 days for
completion). Not surprisingly, the corresponding reaction with
4-tert-butyl phenol, 4-tBuC6H4OH, yields no products con-
taining an Fe–B bond. Presumably the presence of reagents
containing a relatively acidic hydrogen brings about Fe–B bond
cleavage. Such reactivity would parallel that observed previ-
ously for Fe–B-containing systems.3,21a
Reaction of (ꢀ5-C5H5)Fe(CO)2B(2,4,6-Me3C6H2)Br,
2 with Na[Mn(CO)5]
A
solution of (η5-C5H5)Fe(CO)2B(2,4,6-Me3C6H2)Br, 2,
(0.125 g, 0.325 mmol) in toluene (10 cm3) was added to a
suspension of Na[Mn(CO)5] (0.272 g, 1.24 mmol) in toluene
(5 cm3). The mixture was heated to 40 ЊC for 3 d, at which point
the 11B NMR spectrum of the reaction mixture showed com-
plete conversion of the signal due to 2 (δB 111.4) to a single
downfield resonance at δB 160.7. Filtration, removal of volatiles
in vacuo and recrystallization from hexanes (10 cm3) at Ϫ30 ЊC
yielded a pale yellow powder in ca. 20% yield. Despite repeated
attempts it proved impossible to isolate the product of this
reaction as a crystalline material, or to obtain complete charac-
terization data due to its extreme thermal and moisture sensi-
tivity and its tendency to decompose with generation of Mn2-
(CO)10 and [(η5-C5H5)Fe(CO)2]2 even at Ϫ30 ЊC. The downfield
shift of ca. 50 ppm in the 11B NMR spectrum does suggest the
formation of a second metal–boron bond, although definitive
evidence for the formation of [(η5-C5H5)Fe(CO)2][(OC)5Mn]-
B(2,4,6-Me3C6H2) (10) is clearly lacking.
Reaction with organometallic nucleophiles proceeds in most
cases via displacement of bromide, resulting in the synthesis
of complexes 7–9 containing bridging borylene ligands. Yields
are of the order of 35% (overall from MesBBr2) and this
methodology gives access not only to symmetrically substituted
derivatives, but also to asymmetric borylene complexes such as
9. Two notable exceptions to this are (i) the inability to effect
Attempted reaction of 2,4,6-tBu3C6H2BBr2 with Na[(ꢀ5-C5H5)-
Fe(CO)2]
To a suspension of Na[(η5-C5H5)Fe(CO)2] (0.110 g, 0.55 mmol)
in toluene (10 cm3) was added a solution of 2,4,6-tBu3C6H2BBr2
(0.226 g, 0.54 mmol) in toluene (5 cm3) and the reaction mixture
was stirred at room temperature for 12 h. Measurement of the
11B NMR spectrum of the reaction mixture at this point
revealed a single resonance at δB 59.7 due to the starting
substitution of the second bromide with (η5-C5Me5)Fe(CO)2
Ϫ
as the nucleophile, presumably on steric grounds; and (ii) the
reaction of the manganese boryl complex (OC)5MnBMes(Br),
Ϫ
5, with an excess of (η5-C5H5)Fe(CO)2 which gives rise to the
symmetrical diiron complex [(η5-C5H5)Fe(CO)2]2BMes rather
than to [(η5-C5H5)Fe(CO)2][(OC)5Mn]BMes, 10. Presumably
displacement of the manganese carbonyl fragment reflects the
t
material Bu3C6H2BBr2. Extensive variation in reaction con-
Ϫ
ditions (temperature, time, ratio of reagents) did not lead to the
superior nucleophilicity of the (η5-C5H5)Fe(CO)2 anion and
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Scheme 1 Outline of the boron-centred substitution chemistry used to synthesize mesitylboryl and mesitylborylene complexes 2–11.
Ϫ
the better leaving group properties of Mn(CO)5
.
11B NMR
ligands. The shorter Fe–E bond lengths imposed for E = B lead
to a significantly compressed Fe–Fe distance [2.528(1) vs.
2.635(1) Å9b]. This in turn would bring about enhanced steric
interactions between the cyclopentadienyl ligands and the
mesityl substituent of the bridging borylene ligand. This factor
would be expected to be less important for the trans isomer, in
which rotation about the B–Cipso bond could place the plane of
the mesityl ligand such as to minimize unfavourable inter-
ligand contacts (vide infra). Consistent with this observation,
[(η5-C5H4Me)Fe(CO)]2(µ2-CO)[µ2-BN(SiMe3)2] synthesized by
Braunschweig et al. is also obtained exclusively as the trans
isomer.11d
data suggest the asymmetric complex 10 as a possible candidate
for the sole boron-containing product obtained from the
alternative reaction of excess NaMn(CO)5 with the iron
mesityl(bromo)boryl derivative 2. However, the extreme
thermal- and air-sensitivity of this compound prevented its
complete characterization. In general, complexes containing
the Mn(CO)5 fragment were found to be significantly more
susceptible to decomposition than their (η5-C5H5)Fe(CO)2 ana-
logues; all attempts, for example, to isolate the dimanganese
complex [(OC)5Mn]2BMes were unsuccessful.
The substitution chemistry at boron implicit in the synthesis
of complexes 6–10 generally proceeds in reasonable yield with-
out scission of the existing metal–boryl linkage. Such sub-
stitution chemistry is relatively rare for transition metal boryl
complexes, with these and most other reported examples
occurring for complexes containing B–Cl or B–Br bonds.4c,11f,14d
Presumably the relative lability of the B–halogen linkage and
the high leaving group ability of ClϪ or BrϪ facilitates halide
substitution under conditions which allow the existing M–B
bond to remain intact.
The course of the photolytic conversion of 7 to 11 can be
1
followed by NMR spectroscopy; integration of the H reson-
ances due the ortho methyl protons of each compound allows
the concentration/time dependence illustrated in Fig. 1 to be
Complexes 7–9 feature a novel unsupported bridging mode
of coordination of the borylene ligand. Previously reported
iron and manganese complexes containing a bridging BX
moiety typically also feature a M–M bond.11,12 Thus, for
example, the complex [(η5-C5H4Me)Fe(CO)]2(µ2-CO)[µ2-BN-
(SiMe3)2] features iron centres linked not only via a bridging
BN(SiMe3)2 ligand, but also by an Fe–Fe bond and a µ2
carbonyl ligand.11d The synthetic route to this compound is very
similar to that reported here for complexes 7 and 8, involving
Ϫ
the reaction of two equivalents of (η5-C5H4Me)Fe(CO)2 with
Cl2BN(SiMe3)2.11d In the case of [(η5-C5H4Me)Fe(CO)]2(µ2-CO)-
[µ2-BN(SiMe3)2], however, chloride displacement is also accom-
panied by loss of one CO ligand. Although spontaneous loss of
CO is not observed during the formation of 7 (or 8), photolysis
in benzene at room temperature does lead to the evolution of
one equivalent of CO. The exclusive organometallic product in
the case of 7 is the trans isomer (vide infra) of the supported
bridging borylene complex 11. This result contrasts with that
observed on photolysis of the analogous gallium-containing
complex [(η5-C5H5)Fe(CO)2]2GaMes, which is reported to yield
a mixture of cis- and trans-[(η5-C5H5)Fe(CO)]2(µ2-CO)(µ2-
GaMes).9b Conceivably this difference reflects the differing geo-
metric constraints imposed by bridging borylene and gallylene
Fig. 1 Plot of mole fraction against time in minutes for the photolytic
conversion of unsupported borylene 7 to Fe–Fe bonded species 11, as
1
determined by integration of the H NMR signals of the ortho methyl
groups of each compound (᭜, compound 7; ᭿, compound 11).
obtained. Complete conversion is obtained over a period of
ca. 80 h and a logarithmic plot for [7] confirms the expected first
order kinetics and yields a value of 1.8 × 10Ϫ5 sϪ1 for the corre-
sponding rate constant (logarithmic plot included with ESI†).
(ii) Structural and spectroscopic studies
11B NMR spectroscopy proves to be an informative probe of
the substitution chemistry outlined in Scheme 1. Replacement
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of a single bromide by an organometallic fragment results in
the 50–60 ppm downfield shift typically observed on formation
of a M–B single bond. Further substitution chemistry results in
an upfield shift on replacement of bromide by a better π donor
aryloxo substituent (δB 111.4 for 2, 80.3 for 6) or in a further
downfield shift of ca. 50 ppm on formation of a second M–B
linkage. Interestingly, although the coordination environment
at the boron centre is significantly altered on Fe–Fe bond
formation (e.g. 11 from 7) there is little change in the measured
11B resonance (δB 162 and 158, respectively).
Single crystals of the precursor mesityl(bromo)boryl com-
plexes 2–4, the mesityl(aryloxo)boryl complex 6, the unsup-
ported bridging borylenes 7 and 8 and the photolytically
generated Fe–Fe bonded borylene 11 proved to be accessible
by controlled cooling of concentrated solutions in hexanes.
Molecular structures are depicted in Figs. 2–8 and details of
Fig. 4 Molecular structure of (η5-C5Me5)Fe(CO)2BMes(Br), 4. Rele-
vant bond lengths (Å) and angles (Њ): Fe(1)–B(1) 1.972(2), B(1)–Br(1)
2.018(2), B(1)–C(1) 1.569(3), Fe(1)–(η5-C5Me5) centroid 1.746(2); C(1)–
B(1)–Fe(1)–(η5-C5Me5) centroid 2.3(2), C(2)–C(1)–B(1)–Fe(1) 89.5(2).
Fig. 2 Molecular structure of (η5-C5H5)Fe(CO)2BMes(Br), 2. Rele-
vant bond lengths (Å) and angles (Њ): Fe(1)–B(1) 1.964(5), B(1)–Br(1)
2.013(4), B(1)–C(3) 1.563(6), Fe(1)–(η5-C5H5) centroid 1.729(4); C(3)–
B(1)–Fe(1)–(η5-C5H5) centroid 10.2(2), C(4)–C(3)–B(1)–Fe(1) 91.9(3).
t
Fig. 5 Molecular structure of (η5-C5H5)Fe(CO)2BMes(OC6H4 Bu-4),
6. Relevant bond lengths (Å) and angles (Њ): Fe(1)–B(1) 2.040(2), B(1)–
O(3) 1.377(3), B(1)–C(11) 1.577(3), Fe(1)–(η5-C5H5) centroid 1.739(3);
C(11)–B(1)–Fe(1)–(η5-C5H5) centroid 4.8(2), C(16)–C(11)–B(1)–Fe(1)
89.1(2).
Fig. 3 Molecular structure of (η5-C5H4Me)Fe(CO)2BMes(Br), 3.
Relevant bond lengths (Å) and angles (Њ): Fe(1)–B(1) 1.962(4), B(1)–
Br(1) 2.005(3), B(1)–C(3) 1.571(4), Fe(1)–(η5-C5H4Me) centroid
1.735(4); C(3)–B(1)–Fe(1)–(η5-C5H4Me) centroid 3.8(2), C(4)–C(3)–
B(1)–Fe(1) 88.3(2).
data collection, structure solution and refinement are included
in Table 1. The structures of compounds 3 and 7 have previ-
ously been communicated.15
Fig. 6 Molecular structure of one of the two crystallographically
independent molecules of [(η5-C5H5)Fe(CO)2]2BMes, 7. Relevant bond
lengths (Å) and angles (Њ): Fe(1)–B(1) 2.090(10), Fe(2)–B(1) 2.091(10),
The structures of boryl complexes 2–4 (Figs. 2–4) display the
expected half sandwich geometry at the iron centre with the
coordination sphere being completed by two carbonyls and one
mesityl(bromo)boryl ligand. Iron–boron distances for the three
compounds are short [1.964(5), 1.962(4) and 1.972(2) Å,
respectively],21 and this together with Cp centroid–Fe–B–Cipso
angles close to 0Њ (10.2, 3.8 and 2.3Њ, respectively) and relatively
high carbonyl stretching frequencies (2016, 1962; 2009, 1962;
and 2006, 1961 cmϪ1) indicates a modest Fe–B π back bonding
interaction from the metal-based HOMO.21a,22 In each case
Fe(1)–C(15)
1.730(10),
Fe(1)–(η5-C5H5)
centroid
1.745(10),
Fe(1) ؒ ؒ ؒ Fe(2) 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) 115.7(6), C(1)–B(1)–Fe(2) 113.5(6), C(1)–
B(1)–Fe(1)–(η5-C5H5) centroid 18.7(6), Fe(1)–B(1)–C(1)–C(2) 83.6(6).
there is clearly little π interaction between the boron centre and
the mesityl fragment, as manifested by Fe–B–Cipso–Cortho angles
of 91.9, 88.3 and 89.5Њ, respectively. This almost certainly
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X = OEt, R = Me; X = Cl, R = Me) each of which contains a
single Mn–Mn bond and in [(η5-C5H4Me)Fe(CO)]2(µ2-CO)[µ2-
BN(SiMe3)2], which contains both an Fe–Fe linkage and a
bridging carbonyl, are 2.790(2), 2.817(1), 2.824(2) and 2.548(1)
Å, respectively.11a–d,g As such 7 and 8 represent to our know-
ledge the first reported structurally characterized transition
metal complexes containing an unsupported bridging borylene
ligand.26 Indeed, this particular structural motif is extremely
rare among dinuclear complexes of the group 8 metals con-
taining bridging ligands of the lighter main group elements;
the overwhelming majority of complexes containing bridging
alkylidene (CR2), silylene (SiR2) or alanediyl (AlR) ligands also
feature a metal–metal bond or additional bridging ligands.
Photolysis of 7 results in loss of CO and formation of com-
pound 11 (Fig. 8). Crystallographic analysis of 11 confirms the
more conventional bridging geometry implied by infrared spec-
troscopy, with an Fe–Fe distance of 2.528(1) Å being similar to
that found in [(η5-C5H4Me)Fe(CO)]2(µ2-CO)[µ2-BN(SiMe3)2]
[2.548(1) Å11d] and therefore entirely consistent with an Fe–Fe
single bond. 11 is formed in solution exclusively as a single
isomer (as shown by multinuclear NMR), with the results of
the crystallographic study implying that this features a trans
arrangement of cyclopentadienyl ligands. The underlying steric
reasons for the formation of the trans isomer can be understood
by reference to Fig. 9 which shows the view along the B–Cipso
Fig. 7 Molecular structure of [(η5-C5H4Me)Fe(CO)2]2BMes, 8. Rele-
vant bond lengths (Å) and angles (Њ): Fe(1)–B(1) 2.093(2), Fe(1_2)–B(1)
2.093(2), Fe(1)–C(1) 1.745(3), Fe(1)–(η5-C5H5) centroid 1.749(3),
Fe(1) ؒ ؒ ؒ Fe(1_2) 3.800(2), B(1)–C(9) 1.580(5); Fe(1)–B(1)–Fe(1_2)
130.4(2), C(9)–B(1)–Fe(1)–(η5-C5H5) centroid 21.3(3), Fe(1)–B(1)–
C(9)–C(10) 83.8(2).
Fig. 8 Molecular structure of [(η5-C5H5)Fe(CO)]2(µ-CO)(µ-BMes),
11. Relevant bond lengths (Å) and angles (Њ): Fe(1)–B(1) 1.956(5),
Fe(2)–B(1) 1.966(5), Fe(1)–C(1) 1.919(5), Fe(2)–C(1) 1.920(5), Fe(1)–
(η5-C5H5) centroid 1.739(3), Fe(1) ؒ ؒ ؒ Fe(2) 2.528(1), B(1)–C(4)
1.556(7); Fe(1)–B(1)–Fe(2) 80.3(2), Fe(1)–B(1)–C(4)–C(5) 60.4(2).
Fig. 9 Molecular structure of [(η5-C5H5)Fe(CO)]2(µ-CO)(µ-BMes), 11
viewed along the axis of the B(1)–C(4) bond.
axis. The sterically favourable coplanar orientation of cyclo-
pentadienyl and mesityl groups would not be possible for the
cis isomer.
reflects the high steric requirements of the (η5-C5R5)Fe(CO)2
moiety, as the corresponding torsion angle in (MesBO)3, for
example, is 35.3Њ.23 To our knowledge complexes 2–4 represent
the first crystallographically characterized transition metal
complexes containing a bromide-substituted boryl ligand –
B(Br)X,24 and feature B–Br distances [2.103(4), 2.005(3) and
2.018(2) Å, respectively] which are towards the longer end of
those reported for trigonal boron.25
Replacement of the bromide substituent in 2 by a 4-tert-butyl
phenoxide group (giving 6; Fig. 5) leads to significant elongation
of the Fe–B distance [from 1.964(5) to 2.040(2) Å], as expected
for a boryl ligand which is both a poorer π acceptor and of
greater steric bulk. The orientation of mesityl and (η5-C5H5)-
Fe(CO)2 fragments with respect to the boryl ligand remain
essentially unaltered.
The crystal structures of the disubstituted species 7 (Fig. 6)
and 8 (Fig. 7) each consists of two piano stool (η5-C5H4R)-
Fe(CO)2 (R = H, Me) fragments linked in µ2,η1,η1 fashion by
a single bridging mesitylborylene ligand [(2,4,6-Me3C6H2)B].
Metal–metal distances in both compounds [3.802(10) and
3.800(2) Å, respectively] are long enough to preclude any direct
interaction between the metal centres. The corresponding dis-
tances for [(η5-C5H4R)Mn(CO)2]2(µ2-BX) (X = NMe2, R = H;
There are two major differences between the different types
of bridging mesitylborylene complex. Firstly the Fe–B–Fe
angle is significantly wider in 7 or 8 [132.7(5) and 130.4(2)Њ,
respectively] than is typically found in bridging borylenes which
form part of a three-membered MBM metallacycle {e.g. 80.3(2)
and 78.9(1)Њ for 11 and [(η5-C5H4Me)Fe(CO)]2(µ2-CO)[µ2-BN-
(SiMe3)2], respectively11d}. 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. The slightly wider angle found in 7 and 8 than in [(η5-
t
C5R5)Fe(CO)2]2GaX (X = Mes, R = Me, θ = 124.4Њ; X = Bu,
R = H, θ = 122.4Њ) or in [(OC)4Co]2In(2,4,6-tBu3C6H2) (θ =
122.2Њ),9,10 for example, almost certainly reflects the smaller
covalent radius of boron. The shorter M–E distances found for
E = B necessitate greater opening out of the M–E–M angle to
minimize steric interaction between the bulky organometallic
fragments.
Secondly, the Fe–B distances in the unsupported complexes 7
and 8 [2.089(9) and 2.093(2) Å] are significantly longer than
those found in similar complexes containing a metal–metal
bond {cf. mean values of 1.961(5) and 2.003(3)Å for 11 and [(η5-
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G. Frenking, J. Am. Chem. Soc., 2001, 123, 1683; (h) Y. Chen
and G. Frenking, J. Chem. Soc., Dalton Trans., 2001, 434.
6 See, for example: C. Dohmeier, D. Loos and H. Schnöckel, Angew.
Chem., Int. Ed. Engl., 1996, 35, 127; J. Weiss, D. Stetzkamp,
B. Nuber, R. A. Fischer, C. Boehme and G. Frenking, Angew.
Chem., Int. Ed. Engl., 1997, 36, 70.
7 See, for example: J. Su, X.-W. Li, R. C. Crittendon, C. F. Campana
and G. H. Robinson, Organometallics, 1997, 16, 4511; F. A. Cotton
and X. Feng, Organometallics, 1998, 17, 128; P. Jutzi, B. Neumann,
G. Reumann and H.-G. Stammler, Organometallics, 1998, 17, 1305.
8 See, for example: S. T. Haubrich and P. P. Power, J. Am. Chem.
Soc., 1998, 120, 2202; P. Jutzi, B. Neumann, G. Reumann, L. O.
Schebaum and H.-G. Stammler, Organometallics, 1999, 18, 2550.
9 See, for example: (a) X. He, R. A. Bartlett and P. P. Power,
Organometallics, 1994, 13, 548; (b) T. Yamaguchi, K. Ueno and
H. Ogino, Organometallics, 2001, 20, 501.
C5H4Me)Fe(CO)]2(µ2-CO)[µ2-BN(SiMe3)2],
respectively11d}.
That the Fe–B distances in 7 and 8 are also significantly greater
than those found in the mesityl(bromo)boryl precursors 2 and 3
implies that steric repulsion between (η5-C5H5)Fe(CO)2 frag-
ments contributes to bond lengthening. π-Stabilisation of the
boron centre in 7 and 8 is therefore clearly minimal, with Fe–B
distances among the longest measured for trigonal boron,21 and
the orientation of the mesityl fragment reflecting little overlap
with the aromatic π system [torsion angle, Fe–B–Cipso–Cortho
=
83.6(4)Њ]. Steric shielding would therefore appear to be a crucial
factor in the isolation of complexes 7 and 8. The use of
sterically demanding substituents in the isolation of labile
borylene complexes has previously been suggested in theor-
etical studies by Baerends and co-workers.5a From a synthetic
viewpoint, however, choice of the appropriate level of steric
shielding is clearly important. In our hands the complexes [(η5-
C5H5)Fe(CO)2]2BX [X = Mes (7), Ph and Cl] are accessible
via metathesis chemistry,27 but the latter two compounds are
significantly more sensitive to decomposition; on the other
hand (η5-C5H5)Fe(CO)2BR(Br) [R = 2,6-(2,4,6-iPr3C6H2)2C6H3]
is inert to further substitution chemistry at boron,28 and 2,4,6-
tBu3C6H2BBr2 is unreactive towards substitution of either
bromide by organometallic fragments.
10 A. H. Cowley, A. Decken, C. A. Olazabal and N. C. Norman,
Z. Anorg. Allg. Chem., 1995, 621, 1844.
11 (a) H. Braunschweig and T. Wagner, Angew. Chem., Int. Ed. Engl.,
1995, 34, 825; (b) H. Braunschweig and B. Ganter, J. Organomet.
Chem., 1997, 545, 163; (c) H. Braunschweig and M. Müller,
Chem. Ber., 1997, 130, 1295; (d ) H. Braunschweig, C. Kollann
and U. Englert, Eur. J. Inorg. Chem., 1998, 465; (e) M. Shimoi,
S. Ikubo and Y. Kawano, J. Am. Chem. Soc., 1998, 120, 4222;
( f ) H. Braunschweig and M. Colling, J. Organomet. Chem., 2000,
614, 18; (g) H. Braunschweig, M. Colling, C. Hu and K. Radacki,
Angew. Chem., Int. Ed., 2002, 41, 1359.
12 Several examples of boron-containing clusters featuring facing
capping BR units which may alternatively be described as triply
bridging (µ3) borylenes are known, e.g. R. Okamura, K. Tada,
K. Matsubara, M. Oshima and H. Suzuki, Organometallics, 2001,
20, 4772.
13 J. T. Golden, T. H. Peterson, P. L. Holland, R. G. Bergman and
R. A. Andersen, J. Am. Chem. Soc., 1998, 120, 223.
14 (a) S. Aldridge, R. J. Calder, A. A. Dickinson, D. J. Willock and
J. W. Steed, Chem. Commun., 2000, 1377; (b) S. Aldridge,
A. Al-Fawaz, R. J. Calder, A. A. Dickinson, D. J. Willock, M. Light
and M. B. Hursthouse, Chem. Commun., 2001, 1846; (c) A. A.
Dickinson, D. J. Willock, R. J. Calder and S. Aldridge,
Organometallics, 2002, 21, 1146; (d ) S. Aldridge, R. J. Calder,
R. E. Baghurst, M. E. Light and M. B. Hursthouse, J. Organomet.
Chem., 2002, 649, 9.
4
Conclusions
The chemistry undertaken during this study reveals (i) that
bromo-substituted boryl complexes prove to be versatile sub-
strates for the synthesis of transition metal compounds con-
taining novel boryl or borylene ligands; (ii) that boron-centred
substitution chemistry in these boryl systems can generally be
carried out with retention of the M–B linkage; (iii) that the
steric properties of the mesityl substituent are ideal for the
synthesis of novel unsupported bridged borylene systems;
and (iv) that photolytic conversion to a more conventional
bridging system can easily be accomplished. Attempts to
further broaden the synthetic scope of mesityl(bromo)boryl
complexes are ongoing and will be reported in due course.
15 A preliminary account of some of this work has previously been
communicated: S. Aldridge, D. L. Coombs and C. Jones, Chem.
Commun., 2002, 856.
16 W. Gerrard, M. Howarth, E. F. Mooney and D. E. Pratt, J. Chem.
Soc., 1963, 1583.
17 R. J. Wehmschulte and P. P. Power, Inorg. Chem., 1996, 35, 3262.
18 R. B. King and M. B. Bisnette, J. Organomet. Chem., 1967, 8, 287;
R. B. King, Acc. Chem. Res., 1970, 3, 417.
19 Z. Otwinowski, W. Minor, in Methods in Enzymology, C. W. Carter
and R. M. Sweet, ed., Academic Press, New York, 1996, vol. 276,
p. 307.
Acknowledgements
We would like to thank the EPSRC, the Royal Society and the
Nuffield Foundation for funding. We are also grateful to
the EPSRC National Crystallography Service for the data
collection for compounds 2 and 11.
20 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46, 467.
21 (a) J. F. Hartwig and S. Huber, J. Am. Chem. Soc., 1993, 115, 4908;
(b) X. He and J. F. Hartwig, Organometallics, 1996, 15, 400;
(c) H. Braunschweig, B. Ganter, M. Koster and T. Wagner, Chem.
Ber., 1996, 129, 1099; (d ) H. Braunschweig, C. Kollann and
M. Müller, Eur. J. Inorg. Chem., 1998, 291; (e) H. Braunschweig
and M. Koster, J. Organomet. Chem., 1999, 588, 231; ( f ) H.
Braunschweig, C. Kollann and K. W. Klinkhammer, Eur. J. Inorg.
Chem., 1999, 1523; (g) K. M. Waltz and J. F. Hartwig,
Organometallics, 1999, 18, 3383; (h) T. Yasue, Y. Kawano and
M. Shimoi, Chem. Lett., 2000, 58; (i) K. M. Waltz and J. F. Hartwig,
J. Am. Chem. Soc., 2000, 122, 11358; (j) H. Braunschweig,
M. Colling, C. Kollann and U. Englert, J. Chem. Soc., Dalton Trans.,
2002, 2289.
22 B. E. R. Schilling, R. Hoffmann and D. Lichtenberger, J. Am. Chem.
Soc., 1979, 101, 585.
23 B. Neumuller and F. Gahlmann, J. Organomet. Chem., 1991, 414,
271.
24 Related boryl complexes containing bromide-substituted
diborane(4) ligands [i.e. LnM–B(R)–B(R)Br] have been reported:
H. Braunschweig, M. Koster and R. Wang, Inorg. Chem., 1999, 38,
415.
25 A survey of the Cambridge Structural Database reveals values in the
range 1.865–2.002.
References and notes
1 (a) W. A. Herrmann, Adv. Organomet. Chem., 1982, 20, 159;
(b) W. A. Nugent and J. M. Mayer, Metal Ligand Multiple Bonds,
Wiley Interscience, New York, 1988.
2 See, for example: B. V. Mork and T. D. Tilley, J. Am. Chem. Soc.,
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27 R. J. Calder, Ph.D. thesis, Cardiff University, 2002.
28 D. L. Coombs and S. Aldridge, unpublished results.
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