Influence of ligand steric bulk in the synthesis of transition-metal borylene complexes

Article abstract of DOI:10.1021/om030489f

The differing steric requirements of phenyl and mesityl substituents have been shown to influence reactivity in asymmetric transition-metal haloboryl complexes of the type (η⁵-C₅R₅)- Fe(CO)₂B(X)Ar (R = H, Me; X

Full text of DOI:10.1021/om030489f

Organometallics 2003, 22, 4213-4217
4213
In flu en ce of Liga n d Ster ic Bu lk in th e Syn th esis of
Tr a n sition -Meta l Bor ylen e Com p lexes
Deborah L. Coombs,† Simon Aldridge,*,† Simon J . Coles,‡ and
Michael B. Hursthouse‡
Departments of Chemistry, Cardiff University, P.O. Box 912, Park Place, Cardiff,
U.K. CF10 3TB, and University of Southampton, Highfield, Southampton, U.K. SO17 1BJ
Received J une 24, 2003
The differing steric requirements of phenyl and mesityl substituents have been shown to
influence reactivity in asymmetric transition-metal haloboryl complexes of the type (η5-C5R5)-
Fe(CO)2B(X)Ar (R ) H, Me; X ) Cl, Br; Ar ) Ph, Mes). Hence substitution of both halides
in PhBCl2 can be achieved by reaction with an excess of the bulky organometallic nucleophile
Na[(η5-C5Me5)Fe(CO)2], to generate [(η5-C5Me5)Fe(CO)]2(µ-CO)(µ-BPh) (4c), the first metal
complex containing the phenylborylene ligand.
their group 14 counterparts, however, 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. LnMdER or LnMtER)
or via donor/acceptor interactions (LnMrER), for ex-
ample, reflects not only the fundamental questions of
structure and bonding posed by such complexes but also
the scarcity of structural data available.4,5,7
In tr od u ction
A fuller understanding of the bonding between transi-
tion metals and low-coordinate ligands containing group
13 or 14 donor atoms continues to be the driving force
for significant research effort.1-10 In stark contrast to
* To whom correspondence should be addressed. E-mail: AldridgeS@
cardiff.ac.uk. Tel: (029) 20875495. Fax: (029) 20874030.
† Cardiff University.
‡ University of Southampton.
(1) See, for example: Nugent, W. A.; Mayer, J . M. Metal Ligand
Multiple Bonds; Wiley-Interscience: New York, 1988.
(2) See, for example: Mork, B. V.; Tilley, T. D. J . Am. Chem. Soc.
2001, 123, 9702 and references therein.
Within this area compounds containing boranediyl (or
borylene, BR) or allanediyl (AlR) ligands are a relatively
recent addition, 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) complexes containing terminally
bound ligands having sterically bulky or π-clectron-
releasing R substituents (e.g. (OC)5CrdBSi(SiMe3)3 and
(OC)4FerAl(η5-C5Me5))4-6 and (ii) complexes containing
bridging RE ligands spanning two (or more) metal
centers which are typically also linked by a metal-metal
bond or a second bridging ligand (e.g. [(η5-C5H4R)Mn-
(CO)2]2(µ-BX) (X ) NMe2, R ) H; X ) OEt, R ) Me; X
) Cl, R ) Me) and (η5-C5Me5)Ir(PMe3)(µ-AlEt)2Ir-
(PMe3)(η5-C5Me5)).11-13
(3) (a) Irvine, G. J .; Lesley, M. J . G.; Marder, T. B.; Norman, N. C.;
Rice, C. R.; Robins, E. G.; Roper, W. R.; Whittell, G.; Wright, L. J .
Chem. Rev. 1998, 98, 2685. (b) Smith, M. R., III. Prog. Inorg. Chem.
1999, 48, 505. (c) Braunschweig, H.; Colling, M. Coord. Chem. Rev.
2001, 223, 1.
(4) (a) Cowley, A. H.; Lomeli, V.; Voight, A. J . Am. Chem. Soc. 1998,
120, 6401. (b) Braunschweig, H.; Kollann, C.; Englert, U. Angew.
Chem., Int. Ed. 1998, 37, 3179. (c) Braunschweig, H.; Colling, M.;
Kollann, C.; Stammler, H. G.; Neumann, B. Angew. Chem., Int. Ed.
2001, 40, 2299. (d) Braunschweig, H.; Colling, M.; Kollann, C.; Merz,
K.; Radacki, K. Angew. Chem., Int. Ed. 2001, 40, 4198. (e) Braunsch-
weig, H.; Colling, M.; Hu, C.; Radacki, K. Angew. Chem., Int. Ed. 2003,
42, 205. (f) Braunschweig, H.; Colling, M. Eur. J . Inorg. Chem. 2003,
383. (g) Irvine, G. J .; Rickard, C. E. F.; Roper, W. R.; Williamson, A.;
Wright, L. J . Angew. Chem., Int. Ed. 2000, 39, 948. (h) Rickard, C. E.
F.; Roper, W. R.; Williamson, A.; Wright, L. J . Organometallics 2002,
21, 4862.
(5) (a) Ehlers, W.; Baerends, E. J .; Bickelhaupt, F. M.; Radius, U.
Chem. Eur. J . 1998, 4, 210. (b) Radius, U.; Bickelhaupt, F. M.; Ehlers,
A. W.; Goldberg, N.; Hoffmann, R. Inorg. Chem. 1998, 37, 1080. (c)
Bickelhaupt, F. M.; Radius, U.; Ehlers, A. W.; Hoffmann, R.; Baerends,
E. J . New J . Chem. 1998, 1. (d) Macdonald, C. L. B.; Cowley, A. H. J .
Am. Chem. Soc. 1999, 121, 12113. (e) Uddin, J .; Boehme, C.; Frenking,
G. Organometallics 2000, 19, 571. (f) Frenking, G.; Fro¨hlich, N. Chem.
Rev. 2000, 100, 717. (g) Uddin, J .; Frenking, G. J . Am. Chem. Soc.
2001, 123, 1683. (h) Chen, Y.; Frenking, G. J . Chem. Soc., Dalton
Trans. 2001, 434.
We have recently been interested in developing new
routes to transition-metal complexes containing the
borylene ligand in a variety of novel coordination modes
(10) Cowley, A. H.; Decken, A.; Olazabal, C. A.; Norman, N. C. Z.
Anorg. Allg. Chem. 1995, 621, 1844.
(6) See, for example: (a) Dohmeier, C.; Loos, D.; Schno¨ckel, H.
Angew. Chem., Int., Ed. Engl. 1996, 35, 127. (b) Weiss, J .; Stetzkamp,
D.; Nuber, B.; Fischer, R. A.; Boehme, C.; Frenking, G. Angew. Chem.,
Int., Ed. Engl. 1997, 36, 70.
(7) See, for example: (a) Su, J .; Li, X.-W.; Crittendon, R. C.;
Campana, C. F.; Robinson, G. H. Organometallics 1997, 16, 4511. (b)
Cotton, F. A.; Feng, X. Organometallics 1998, 17, 128. (c) J utzi, P.;
Neumann, B.; Reumann, G.; Stammler, H.-G. Organometallics 1998,
17, 1305.
(8) See, for example: (a) Haubrich, S. T.; Power, P. P. J . Am. Chem.
Soc. 1998, 120, 2202. (b) J utzi, P.; Neumann, B.; Reumann, G.;
Schebaum, L. O.; Stammler, H.-G. Organometallics 1999, 18, 2550.
(9) See, for example: (a) He, X.; Bartlett, R. A.; Power, P. P.
Organometallics 1994, 13, 548. (b) Yamaguchi, T.; Ueno, K.; Ogino,
H. Organometallics 2001, 20, 501.
(11) (a) Braunschweig, H.; Wagner, T. Angew. Chem., Int. Ed. Engl.
1995, 34, 825. (b) Braunschweig, H.; Ganter, B. J . Organomet. Chem.
1997, 545, 163. (c) Braunschweig, H.; Mu¨ller, M. Chem. Ber. 1997,
130, 1295. (d) Braunschweig, H.; Kollann, C.; Englert, U. Eur. J . Inorg.
Chem. 1998, 465. (e) Shimoi, M.; Ikubo, S.; Kawano, Y. J . Am. Chem.
Soc. 1998, 120, 4222. (f) Braunschweig, H.; Colling, M. J . Organomet.
Chem. 2000, 614, 18. (g) Braunschweig, H.; Colling, M.; Hu, C.;
Radacki, K. Angew. Chem., Int. Ed. Engl. 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.: Okamura, R.; Tada, K.;
Matsubara, K.; Oshima, M.; Suzuki, H. Organometallics 2001, 20,
4772.
(13) Golden, J . T.; Peterson, T. H.; Holland, P. L.; Bergman, R. G.;
Andersen, R. A. J . Am. Chem. Soc. 1998, 120, 223.
10.1021/om030489f CCC: $25.00 © 2003 American Chemical Society
Publication on Web 09/10/2003

4214 Organometallics, Vol. 22, No. 21, 2003
Coombs et al.
Sch em e 1. Ha lid e Su bstitu tion a n d Abstr a ction Ch em istr y of Ir on Ha lobor yl Com p lexes
(Scheme 1).14,15 To this end asymmetric haloboryl
complexes (2) have proved to be valuable synthetic
intermediates, available in high yield from the corre-
CaH2). d6-Benzene and d8-toluene (both Goss) were degassed
and dried over potassium prior to use. PhBCl2 and bis-
(triphenylphosphoranyl)ammonium chloride, [PPN]Cl (both
Aldrich) were used without further purification; [(η5-C5-
sponding arylboron dihalide and displaying a range of
Me5)Fe(CO)2BMes]+[BArf ]- and Na[(η5-C5Me5)Fe(CO)2] were
4
useful further reactivity.14-16 Thus, halide abstraction
prepared by literature methods.17,18 NMR spectra were mea-
from the sterically hindered derivative 2c, containing
sured on a J EOL 300 Eclipse Plus FT-NMR spectrometer.
both η5-C5Me5 (Cp*) and mesityl (2,4,6-Me3C6H2, Mes)
1
Residual protons of solvent were used for reference for H and
13C NMR, while a sealed tube containing a solution of [(nBu4N)-
(B3H8)] in CDCl3 was used as an external reference for 11B
NMR. 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 spectra were measured by the
EPSRC National Mass Spectrometry Service Centre, Univer-
sity of Wales, Swansea, U.K. Perfluorotributylamine was used
as the standard for high-resolution EI mass spectra. 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 the new compounds 2e, 2f, and 4c is
therefore based upon multinuclear NMR, IR, and mass spec-
trometry data (including accurate mass measurement), supple-
mented by single-crystal X-ray diffraction studies in the case
of 2e and 4c. In all cases the purity of the bulk material was
established by multinuclear NMR to be >95%.
fragments, gives access to the cationic FedB double-
bonded Fischer carbene analogue 5.17 Furthermore, in
the case of the less congested η5-C5H5 (Cp) and η5-C5H4-
Me (Cp′) derivatives 2a and 2b, halide substitution is
possible, leading first to the novel unsupported bridged
borylene systems 3a and 3b and then (under photolytic
conditions) to the Fe-Fe-bonded species 4a and 4b.14,15
Very hindered systems such as Cp*Fe(CO)2B(Br)Mes
(2c) and CpFe(CO)2B(Br)(2,6-Ar2C6H3) (2d , Ar ) 2,4,6-
iPr3C6H2) have been shown to be inert toward boron-
centered substitution chemistry.15,16 Therefore, to fur-
ther investigate patterns of structural and reaction
chemistry in borylene systems, we have focused on less
sterically demanding RB ligands. Phenyl- and methyl-
borylene systems have previously been the subject of
extensive computational study,5e-g and herein we present
the first experimental realization of the PhB ligand.
Abbreviations: vs ) very strong, st ) strong, m ) medium,
b ) broad, s ) singlet, d ) doublet, t ) triplet.
Syn th esis of (η5-C5Me5)F e(CO)2B(Cl)P h (2e). To a sus-
pension of Na[(η5-C5Me5)Fe(CO)2] (185 mg, 0.69 mmol) in
toluene (15 cm3) at room temperature was added by syringe
PhBCl2 (105 mg, 0.66 mmol), and the reaction mixture was
stirred for 4 h. Filtration, removal of volatiles in vacuo, and
recrystallization from hexanes (ca. 20 cm3) at -30 °C yielded
2e as yellow crystals suitable for X-ray diffraction. Isolated
yields were typically on the order of 95 mg (39%). 1H NMR
Exp er im en ta l Section
All manipulations were carried out under a nitrogen or
argon atmosphere using standard Schlenk line or drybox
techniques. Solvents were predried over sodium wire (hexanes,
toluene) or molecular sieves (dichloromethane) and purged
with nitrogen prior to distillation from the appropriate drying
agent (hexanes, potassium; toluene, sodium; dichloromethane,
3
(300 MHz, C6D6): δ 1.46 (s, 15H, C5Me5), 7.22 (t, J H-H ) 7.2
(14) Aldridge, S.; Coombs, D. L.; J ones, C. Chem. Commun. 2002,
856.
(15) Coombs, D. L.; Aldridge, S.; J ones, C. J . Chem. Soc., Dalton
Trans. 2002, 3851.
(16) Coombs, D. L.; Aldridge, S.; J ones, C. Appl. Organomet. Chem.
2003, 17, 356.
3
Hz, 1H, aromatic p-CH), 7.31 (virtual t, J H-H ) 7.5 Hz, 2H,
aromatic m-CH), 8.17 (d, 3J H-H ) 7.2 Hz, 2H, aromatic o-CH).
13C NMR (76 MHz, C6D6): δ 9.4 (η5-C5Me5), 95.7 (η5-C5Me5
(17) Coombs, D. L.; Aldridge, S.; J ones, C.; Willock, D. J . J . Am.
Chem. Soc. 2003, 125, 6356.
(18) (a) King, R. B.; Bisnette, M. B. J . Organomet. Chem. 1967, 8,
287. (b) King, R. B. Acc. Chem. Res. 1970, 3, 417.

Transition-Metal Borylene Complexes
Organometallics, Vol. 22, No. 21, 2003 4215
Ta ble 1. Cr ysta llogr a p h ic Da ta for 2e a n d 4c
quaternary), 127.7, 131.0, 134.1 (aromatic CH), 216.3 (CO).
11B NMR (96 MHz, C6D6): δ 111 (b, fwhm ) 400 Hz). IR (KBr
disk, cm-1): ν(CO) 1995 st, 1929 st. MS (EI): 342 [(M - CO)+,
30%], isotope pattern corresponding to 1 Fe, 1 B, 1 Cl atoms.
Syn th esis of [(η5-C5Me5)F e(CO)]2(µ-CO)(µ-BP h ) (4c). To
a suspension of Na[(η5-C5Me5)Fe(CO)2] (297 mg, 1.1 mmol) in
toluene (15 cm3) at room temperature was added by syringe
PhBCl2 (60 mg, 0.4 mmol) in toluene (3 cm3), and the reaction
mixture was stirred for 72 h. Filtration, removal of volatiles
in vacuo, and recrystallization from hexanes (ca. 20 cm3) at
-50 °C yielded 4c as red crystals suitable for X-ray diffraction.
Isolated yields were typically on the order of 145 mg (65%).
1H NMR (300 MHz, C6D6): δ 1.53 (s, 30H, C5Me5), 7.23 (t, 3J H-H
) 7.4 Hz, 1H, aromatic p-CH), 7.37 (virtual t, 3J H-H ) 7.4 Hz,
2e
4c
empirical formula
fw
C
18H20BClFeO2
C
29H35BFe2O3
370.45
120(2)
0.71073
monoclinic
P21
554.08
120(2)
0.71073
triclinic
P1h
temp (K)
wavelength (Å)
cryst syst
space group
unit cell dimens
a (Å)
7.6864(4)
16.4353(8)
3.9744(6)
90
102.373(3)
90
9.2733(4)
9.2964(5)
17.1859(9)
104.092(2)
92.461(3)
114.425(3)
1290.96(11)
2
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
3
2H, aromatic m-CH), 7.76 (d, J H-H ) 7.2 Hz, 2H, aromatic
V (Å3)
1724.36(13)
4
o-CH). 13C NMR (76 MHz, C6D6): δ 8.9 (η5-C5Me5), 96.8 (η5-
C5Me5 quaternary), 127.4, 128.0, 129.1 (aromatic CH), 215.2
(terminal CO), 280.9 (bridging CO). 11B NMR (96 MHz,
C6D6): δ 159 (b, fwhm ) 840 Hz). IR (KBr disk, cm-1): ν(CO)
1917 vs, 1889 m, 1754 vs. MS (EI): M+ ) 554 (40%), isotope
pattern corresponding to 2 Fe, 1 B atoms, fragment ion peaks
at m/z 526 [(M - CO)+, 12%], 498 [(M - 2CO)+, 15%], 470 [(M
- 3CO)+, 30%]. Exact mass: calculated 554.1373, measured
554.1370.
Z
density (calcd) (Mg/m3) 1.427
1.425
1.153
abs coeff (mm-1
)
1.034
F(000)
768
580
cryst color, habit
cryst size (mm3)
θ range for data
colln (deg)
pale yellow plate
red plate
0.16 × 0.16 × 0.04 0.20 × 0.14 × 0.02
1.94-24.12
2.92-27.47
index ranges (h, k, l)
-8 to +8,
-11 to +12;
-12 to +11;
-22 to +22
21 173
-17 to +18,
-16 to +16
16 024
Syn th esis of (η5-C5Me5)F e(CO)2B(Cl)Mes (2f). To a solu-
tion of [PPN]Cl (74 mg, 0.13 mmol) in dichloromethane (2 cm3)
was added a solution of [(η5-C5Me5)Fe(CO)2BMes]+[BArf4]- (160
mg, 0.13 mmol) in dichloromethane (8 cm3) at room temper-
ature, and the reaction mixture was stirred for 1.5 h. Filtra-
tion, removal of volatiles in vacuo, and recrystallization from
hexanes (ca. 10 cm3) at -50 °C yielded 2f as a pale yellow
microcrystalline material. Isolated yields were typically on the
order of 45 mg (85%). 1H NMR (300 MHz, C6D6): δ 1.35 (s,
15H, C5Me5), 2.20 (s, 3H, p-CH3), 2.28 (s, 6H, o-CH3), 6.74 (s,
2H, aromatic CH). 13C NMR (76 MHz, C6D6): δ 9.0 (η5-C5Me5),
21.0 (p-CH3), 21.6 (o-CH3), 97.1 (η5-C5Me5), 127.7 (aromatic
CH), 133.2, 136.2 (aromatic quaternary), 216.2 (CO). 11B NMR
(96 MHz, C6D6): δ 112 (b, fwhm ) 580 Hz). IR (KBr disk,
cm-1): ν(CO) 1996 m, 1936 m. MS (EI): M+ 412 (5%), isotopic
pattern corresponding to 1 Fe, 1 B, 1 Cl atoms, strong fragment
ion peaks at m/z 384 [(M - CO)+, 60%] and 356 [(M - 2CO)+,
100%]. Exact mass: calculated 412.1058, measured 412.1055.
Rea ction of (η5-C5Me5)F e(CO)2B(Cl)Mes (2f) w ith Ex-
cess Na [(η5-C5Me5)F e(CO)2]. To a suspension of Na[(η5-C5-
Me5)Fe(CO)2] (70 mg, 0.3 mmol) in a Young NMR tube was
added a solution of 2f (15 mg, 0.04 mmol) in d8-toluene (ca.
0.8 cm3). The reaction mixture was monitored by 11B NMR
after 2.5 h at room temperature and then periodically over 1
week at 40 °C. At room temperature only the resonance due
to 2f was observed (δB 112); on heating no conversion to any
other Fe-B-containing species was observed. Ultimately, after
prolonged heating a reduction in intensity of the resonance
due to 2f was observed, although no corresponding new
features grew in.
no. of rflns collected
no. of indep rflns/Rint
5177 (0.0928)
5789 (0.0592)
98.3
completeness to θmax (%) 99.8
abs cor
semiempirical from Sortav
equivalents
max and min transmissn 0.9598, 0.8520
0.9773, 0.8022
full-matrix least
squares on F2
5789/0/327
refinement method
full-matrix least
squares on F2
5177/13/426
no. of data/restraints/
params
goodness of fit on F2
1.088
0.984
final R indices (I > 2σ(I)) R1 ) 0.0786,
R1 ) 0.0675,
wR2 ) 0.1638
R1 ) 0.0840,
wR2 ) 0.1719
wR2 ) 0.1895
R1 ) 0.0962,
wR2 ) 0.1989
R indices (all data)
largest diff peak and
2.052 and -0.504 1.466 and -0.540
hole (e Å-3
)
Resu lts a n d Discu ssion
Reaction of PhBCl2 with 1 equiv of Na[(η5-C5Me5)Fe-
(CO)2] in toluene at room temperature leads to the
formation of a single boron-containing species (by 11B
NMR) with the observed chemical shift (δB 111) being
consistent with the selective replacement of one halide
by an (η5-C5Me5)Fe(CO)2 fragment (cf. δB 61.6, 56.0, and
113.2 for 1a , 1c, and 2c, respectively15). 1H and 13C
NMR, IR, and mass spectral data are also consistent
with a formulation as the asymmetric phenyl(chloro)-
boryl complex (η5-C5Me5)Fe(CO)2B(Cl)Ph (2e), and these
inferences were confirmed by the results of a single-
crystal X-ray diffraction study (see Figure 1 and Table
1).
Gen er a l Cr ysta llogr a p h ic Meth od . Data for compounds
2e and 4c were collected on an Enraf-Nonius Kappa CCD
diffractometer; data collection and cell refinement were carried
out using DENZO and COLLECT19,20 and structure solution
and refinement using SHELXS-97 and SHELXL-97, respec-
tively.21 Details of each data collection, structure solution, and
refinement can be found in Table 1, relevant bond lengths and
angles are included in the figure captions, and complete details
of each structure have been deposited with the CCDC (Nos.
211359 (2e) and 209269 (4c)).
The crystal structure of 2e features two (essentially
identical) molecules within the asymmetric unit with
the half-sandwich coordination geometry at iron being
completed by two carbonyls and a novel phenylchlo-
roboryl ligand. 2e therefore represents a rare example
of a chloroboryl complex and is the first to contain the
-B(Cl)Ph unit. Interestingly, the orientation of this
ligand differs considerably from that of the mesitylbro-
moboryl ligand found in complexes 2a -c. Whereas the
-B(Br)Mes ligand is essentially coplanar with that
defined by the Cp centroid-Fe-B unit in all cases
( centroid-Fe-B-Cipso ) 10.2(2), 3.8(2), and 2.3(2)° for
(19) Otwinowski, Z.; Minor, W. Methods in Enzymology; Carter, C.
W., Sweet, R. M., Eds.; Academic Press: New York, 1996, Vol. 276, p
307.
(20) COLLECT: Data Collection Software; Nonius B.V., 1999.
(21) Sheldrick, G. M. SHELX97: Programs for Crystal Structure
Analysis (Release 97-2); University of Go¨ttingen, Go¨ttingen, Germany,
1998.

4216 Organometallics, Vol. 22, No. 21, 2003
Coombs et al.
F igu r e 1. Structure of one of the two independent
molecules in the asymmetric unit of (η5-C5Me5)Fe(CO)2B-
(Cl)Ph (2e). Relevant bond lengths (Å), bond angles (deg),
and torsion angles (deg): Fe(1)-B(1) ) 2.006(10), B(1)-
Cl(1) ) 1.812(12), B(1)-C(13) ) 1.561(15), Fe(1)-(η5-C5-
Me5) centroid ) 1.716(8); C(13)-B(1)-Fe(1)-(η5-C5Me5)
centroid ) 78.6(7), C(14)-C(13)-B(1)-Fe(1) ) 31.3(9).
Hydrogen atoms are omitted for clarity.
F igu r e 2. Structure of [(η5-C5Me5)Fe(CO)]2(µ-CO)(µ-BPh)
(4c). Relevant bond lengths (Å), bond angles (deg), and
torsion angles (deg): Fe(1)-B(1) ) 1.934(6), Fe(2)-B(1) )
1.936(6), Fe(1)-C(12) ) 1.924(5), Fe(2)-C(12) ) 1.930(5),
Fe(1)-(η5-C5Me5) centroid ) 1.757(5), Fe-Fe ) 2.573(3),
B(1)-C(13) ) 1.542(8); Fe(1)-B(1)-Fe(2) ) 83.3(2), Fe(1)-
B(1)-C(4)-C(5) ) 59.4(3). Hydrogen atoms are omitted for
clarity.
2a , 2b, and 2c, respectively), the -B(Cl)Ph unit in 2e
adopts a near-perpendicular orientation ( centroid-
Fe-B-Cipso ) 77.6(7)° (mean)). Furthermore, there is
a marginal (at the 3σ level) lengthening of the Fe-B
bond on going from mesityl(bromo)boryl complex 2c
(1.972(2) Å) to the phenyl(chloro)boryl species 2e (2.005-
(10) Å (mean)), consistent with the boryl ligand becom-
ing a slightly poorer σ-donor and/or π-acceptor. Addi-
tionally, it is worth noting that the near-perpendicular
orientation of aryl and boryl planes enforced in mesityl
complexes 2a -c (presumably on steric grounds) is not
in evidence for the smaller phenyl substituent ( Fe-
B-Cipso-Cortho ) 29.5(9)° (mean) for 2e, cf. 89.5(2)° for
2c).
A comparison of the steric and electronic properties
expected for the -B(Cl)Ph ligand with those of related
species might have been expected to result in an Fe-B
distance for 2e shorter than those found in (η5-
C5H5)Fe(CO)2BPh2 (2.034(3) Å) and (η5-C5Me5)Fe(CO)2B-
(Cl)NMe2 (2.027(5) Å).11d,22 Although the relatively
poorly determined Fe-B distance for 2e means that any
differences are not statistically significant, a comparison
of carbonyl stretching frequencies with those for (η5-C5-
Me5)Fe(CO)2B(Cl)NMe2 (1988, 1928 cm-1 vs 1995, 1929
cm-1 for 2e) is consistent with -B(Cl)Ph being a slightly
better π-acceptor ligand than -B(Cl)NMe2.11d
Selective replacement of a single chloride in the
phenyldichloroborane precursor 1c with 1 equiv of Na-
[(η5-C5Me5)Fe(CO)2] therefore mirrors the reactivity
observed for aryldibromoboranes 1a and 1b.14-16 The
reaction of PhBCl2 with 5 equiv of Na[(η5-C5Me5)Fe-
(CO)2] in toluene over a period of 12 h at room temper-
ature also results in the formation of a single boron-
containing species. In this case, the observed chemical
shift (δB 159) is consistent with replacement of both
B-Cl bonds by B-Fe linkages.15 13C NMR and IR data
are consistent with a compound containing both bridg-
ing and terminal carbonyls, and mass spectroscopic data
imply the structure 4c, containing three CO ligands.
These spectroscopic inferences were confirmed by the
results of a single-crystal X-ray diffraction study (see
Figure 2).
Spectroscopic data are consistent with the presence
of a single isomer in solution, and the results of the
diffraction study imply that this is the trans isomer.
This result is in line with that found previously for the
mesitylborylene species [(η5-C5H5)Fe(CO)]2(µ-CO)(µ-
BMes) (4a )15 but contrasts with the gallium complex
[(η5-C5H5)Fe(CO)]2(µ-CO)(µ-GaMes), which is reported
to be formed as a mixture of cis and trans isomers by
the photolysis of [(η5-C5H5)Fe(CO)2]2GaMes.9b Conceiv-
ably this difference reflects the differing geometric
constraints imposed by bridging borylene and gallylene
ligands. The shorter Fe-E bond lengths imposed for E
) B lead to a significantly compressed Fe-Fe distance
(2.573(3) (4c) and 2.528(1) Å (4a ) vs 2.653(1) Å for trans-
[(η5-C5H5)Fe(CO)]2(µ-CO)(µ-GaMes)). This in turn brings
about enhanced steric interactions between the cyclo-
pentadienyl ligands and the aryl substituent of the
bridging borylene ligand. This factor would be expected
to be less important for the trans isomer, in which rota-
tion about the B-Cipso bond could place the plane of the
aryl group parallel to those of the cyclopentadienyl
ligands so as to minimize unfavorable interligand con-
tacts (see Figure 2). Consistent with this, increasing the
steric bulk of the cyclopentadienyl ligand in the gallium
system has been reported to alter the product distribu-
tion; photolysis of [(η5-C5Me5)Fe(CO)2]2GaMes yields
exclusively trans-[(η5-C5Me5)Fe(CO)]2(µ-CO)(µ-GaMes).9b
The Fe-B distances for 4c (1.936(6) and 1.934(6) Å)
are, to our knowledge, the shortest reported for a bond
between iron and any three-coordinate boron ligand
system. Hence, for example, these distances are signifi-
cantly shorter than those found in 4a (1.956(5) and
1.966(5) Å15) or in [(η5-C5H4Me)Fe(CO)]2(µ-CO)[µ-BN-
(22) Hartwig, J . F.; Huber, S. J . Am. Chem. Soc. 1993, 115, 4908.

Transition-Metal Borylene Complexes
Organometallics, Vol. 22, No. 21, 2003 4217
(SiMe3)2] (2.007(3) and 2.002(3) Å11d), despite the in-
creased bulk of the Cp* ligand at iron. Clearly, the
smaller steric constraints of the BPh ligand allow for a
noticeable contraction of the Fe2B unit. The smaller
steric bulk of the phenyl unit compared to mesityl would
also appear to influence the course of the synthetic
chemistry outlined in Scheme 1. Thus, mesityl com-
pound 2c has been found to be inert to further substitu-
tion chemistry with organometallic nucleophiles,15
whereas replacement of both chloride groups with bulky
organometallic fragments is possible for PhBCl2. Con-
ceivably this might also reflect the differing labilities
of the B-Cl and B-Br bonds in (η5-C5Me5)Fe(CO)2B-
(Cl)Ph (2e) and (η5-C5Me5)Fe(CO)2B(Br)Mes (2c), al-
though B-Br bonds would, if anything, be expected to
be more labile than analogous B-Cl linkages.23 That
these differences in reactivity are due to steric rather
than electronic factors has been confirmed by indepen-
dent synthesis of the complex (η5-C5Me5)Fe(CO)2B(Cl)-
Mes (2f) and examination of its reactivity toward further
B-Fe bond formation.
to the unsupported mesitylborylene complexes 3a and
3b.14,15 A similar situation was reported by Braunsch-
weig and co-workers, who isolated only [(η5-C5H4R)Fe-
(CO)]2(µ-CO)[µ-BN(SiMe3)2] from the reaction of Na[(η5-
C5H4R)Fe(CO)2] (R ) H, Me) with Cl2BN(SiMe3)2 in
benzene.11d In the case of the arylborylene ligands, it
would appear that only by utilizing the less bulky Cp
or Cp′ ligands at iron is it possible to isolate the
unsupported bimetallic systems. Previous work has
shown that the degree of steric crowding at boron is
maximized in unsupported bridged systems such as 3a
and 3b,14,15 and it is therefore logical that this mode of
coordination is only accessible in conjunction with the
less sterically bulky iron fragments.
Con clu sion s
The differing steric requirements of phenyl and mesi-
tyl substituents have been shown to influence reactivity
in asymmetric transition metal haloboryl complexes.
Hence, substitution of both halides in PhBCl2 can be
achieved by reaction with an excess of the bulky organo-
metallic nucleophile Na[(η5-C5Me5)Fe(CO)2], whereas
the mesityl species (η5-C5Me5)Fe(CO)2B(X)Mes (X ) Cl,
Br) are seemingly inert to further Fe-B bond formation.
Use of phenyl-substituted precursors allows the syn-
thesis of 4c, the first metal complex containing a
phenylborylene ligandsa model system which has pre-
viously been the subject of several computational
studies.5e-g Fe-B distances in 4c are the shortest yet
reported for any three-coordinate boron ligand system
containing iron and are significantly shorter than those
found in the analogous mesityl complexes.
Reaction between the highly electrophilic terminal
borylene complex [(η5-C5Me5)Fe(CO)2BMes]+[BArf ]- (5)
4
and 1 equiv of [PPN]Cl in dichloromethane at room
temperature leads to the generation of (η5-C5Me5)Fe-
(CO)2B(Cl)Mes (2f) in good (85%) isolated yield. Boron-
centered reactivity toward anionic nucleophiles for 5 is
consistent with the predictions of several theoretical
studies5 and proves to be one of the dominant features
of its chemistry.24 Spectroscopic data for 2f are entirely
consistent with the proposed structure, and further
support for this assignment comes from the observation
of similar spectroscopic features for the analogous
(structurally characterized) phenyl compound (η5-C5-
Me5)Fe(CO)2B(Cl)Ph (2e) (vide supra). In this respect,
similarities in 11B NMR (δB 111 and 112 for 2e and 2f,
respectively) and IR data (1995, 1929 cm-1 and 1996,
1936 cm-1 for 2e and 2f, respectively) are particularly
revealing.
In common with its bromo-substituted counterpart 2c,
2f does not undergo any further Fe-B bond formation
on heating with excess Na[(η5-C5Me5)Fe(CO)2] in tolu-
ene. It therefore seems likely that the contrasting
reactivity of phenyl-substituted 2e is steric in its origins.
Finally, it is interesting to note that in our investigation
of the PhBCl2/Na[(η5-C5Me5)Fe(CO)2] system we were
unable to identify any intermediate species analogous
Ack n ow led gm en t. We wish to acknowledge the
support of the EPSRC, the Royal Society, and Cardiff
University for the funding of this project. The assistance
of the EPSRC National Mass Spectrometry Service
Centre, University of Wales, Swansea, U.K., is also
gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: Tables and struc-
tures giving crystallographic data for the structural analyses
of compounds 2e and 4c and figures giving 1H, 13C and 11B
NMR spectra for compounds 2e, 2f, and 4c. This material is
available free of charge via the Internet at http://pubs.acs.org.
Crystallographic data for 2e and 4c have also been deposited
with the Cambridge Crystallographic Data Centre (CCDC Nos.
211359 and 209269, respectively).
(23) See, for example: Greenwood, N. N.; Earnshaw, A. Chemistry
of the Elements; Pergamon Press: Oxford, U.K., 1990.
(24) Aldridge, S.; Coombs, D. L. Unpublished results.
OM030489F