Migratory insertion of [B(C6F5)2] into C-H bonds: CO promoted transfer of the boryl fragment

Article abstract of DOI:10.1039/b604141h

The reaction of (η⁵-C₅H₅)Fe(CO) ₂B(C₆F₅)₂ with CO has been shown to proceed via ligand substitution at the metal with accompanying transfer of the boryl fragment (via C-H inserti

Full text of DOI:10.1039/b604141h

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Migratory insertion of [B(C6F5)2] into C–H bonds: CO promoted
transfer of the boryl fragment{
Simon Aldridge,*a Deborah L. Kays (ne´e Coombs),{a Amal Al-Fawaz,a Kevin M. Jones,a Peter N. Horton,b
Michael B. Hursthouse,b Ross W. Harringtonc and William Cleggc
Received (in Cambridge, UK) 21st March 2006, Accepted 21st April 2006
First published as an Advance Article on the web 10th May 2006
DOI: 10.1039/b604141h
1JBF = 66 Hz, dF 2188.7 for the latter) are consistent with species
of the type [Ar3BX]2 (X = H or F);8b,c additionally, monitoring of
photolytic or thermal reactions over a prolonged timeframe (ca.
36 h) implies that the initially formed BH-containing product is
converted into the latter BF species. Postulating that these two
compounds might result from the reaction of 1 with small
quantities of photolytically generated CO, we examined the
behaviour of the parent boryl complex to exposure to carbon
monoxide. Previous studies of the related complex (g5-
C5H5)Fe(CO)2B(C6H5)2 have been characterized by ‘immediate’
reaction with CO, although the iron and boron-containing
products in this case were not identified.9
The reaction of (g5-C5H5)Fe(CO)2B(C6F5)2 with CO has been
shown to proceed via ligand substitution at the metal with
accompanying transfer of the boryl fragment (via C–H
insertion) to the Cp ring, thereby generating the zwitterion
[g5-C5H4B(C6F5)2H]Fe(CO)3 in quantitative yield.
Reactions of transition metal boryl complexes with C–H bonds
and the exploitation of such reactivity in hydrocarbon function-
alization represent exciting recent steps towards the realization of
one of the ‘Holy Grails’ of organometallic chemistry.1–3 Initial
reports of stoichiometric alkane and arene functionalization by
group 6–8 metal boryl complexes1 have been followed by the
development of catalytic systems typically employing Rh or Ir
metal centres and either photolytic or thermal activation.2,3
Mechanistically, such reactivity has been shown to occur via
processes involving oxidative addition of B–X (X = H or B) and
C–H bonds, followed by facile reductive elimination of the B–C
bonded product.4
In the event, the reaction of 1 with CO at 1 atm pressure at
room temperature leads to quantitative conversion (by 1H and 11
B
NMR) to a single compound having the same spectroscopic
signals as the BH-containing species formed under photolytic
conditions. A combination of multinuclear (1H, 11B, 13C, 19F)
NMR, IR, mass spectrometry (including exact mass determina-
tion) and X-ray crystallography has been used to confirm that this
compound is the zwitterionic complex [g5-C5H4B(C6F5)2H]
Fe(CO)3 (2) featuring a hydroborate functionalized Cp ligand
(Scheme 1).§
The fundamental reaction steps which lead to attack by
transition metal boryl systems at C–H bonds have therefore been
investigated in some depth. By contrast, reactions which result in
the synthetic equivalent of boryl anion transfer are extremely rare.5
Unlike the corresponding gallium ligand systems for example,
reagents constituting the ‘free’ [BR2]2 anion are at best poorly
defined.6 In this paper we present preliminary results of the
reaction of (g5-C5H5)Fe(CO)2B(C6F5)2 with CO, which imply a
new mechanism for attack at C–H bonds by a transition metal
boryl complex. Migratory insertion of the [B(C6F5)2] fragment into
a C–H bond of the ancillary Cp ligand is driven by coordination of
CO at the metal centre.
The NMR data for 2 are consistent with a Cp-pendant
1
[B(C6F5)2H]- function {e.g. dB 224.8, JBH 90 Hz, cf. dB 223.4,
1JBH 86 Hz for Cp[g5-C5H4B(C6F5)2H]WH3}10c and the IR
stretching frequencies are also consistent with [(g5-C5H4R)
Fe(CO)3]+ and [ArB(C6F5)2H]2 fragments {n(CO) 2114, 2064,
2053 cm21, cf. 2120, 2068 cm21 for [CpFe(CO)3]+[PF6]2; n(BH)
2374 cm21 cf. 2365 cm21 for [(C6F5)3BH]2}.8a,b In addition, EI
mass spectra display the expected isotopic profiles for each of the
series of ions [M 2 H]+ and [M 2 nCO]+ (n = 1–3). The solid state
structure of 2 (Fig. 1) reveals the expected piano-stool geometry at
iron and pendant four-coordinate hydroborate fragment. There is
no evidence for any intramolecular interaction of the boron-bound
In the course of examining the photo-initiated reactivity of the
perfluoroaryl boryl complex (g5-C5H5)Fe(CO)2B(C6F5)2, 1,7
towards arenes we have observed the formation of small, but
reproducible quantities of two boron-containing products.
1
NMR data (dB 224.8, JBH = 90 Hz for the former, dB 22.2,
aCentre for Fundamental and Applied Main Group Chemistry, Cardiff
School of Chemistry, Main Building, Park Place, Cardiff, UK
CF10 3AT. E-mail: AldridgeS@cf.ac.uk; Fax: 02920 874030;
Tel: 02920 875495
bEPSRC National Crystallography Service, School of Chemistry,
University of Southampton, Highfield, Southampton, UK SO17 1BJ
cSchool of Natural Sciences (Chemistry), Bedson Building, University of
Newcastle, Newcatle-upon-Tyne, UK NE1 7RU
{ Electronic supplementary information (ESI) available: Synthetic,
spectroscopic and structural data for compound 3. See DOI: 10.1039/
b604141h
{ Present address: Central Research Laboratory, Mansfield Road,
Oxford, UK OX1 3TA.
Scheme 1 Synthesis of [g5-C5H4B(C6F5)2H]Fe(CO)3 (2) from 1 via CO-
promoted boryl migration. Reaction conditions: (i) CO (1 atm), C6H6 (or
C6D6), 20 uC, quantitative by NMR (1H, 11B), 74% isolated yield.
2578 | Chem. Commun., 2006, 2578–2580
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(although the precise mechanism for transfer of the boryl fragment
awaits more detailed kinetic and computational study).5 Precedent
for this type of migratory insertion chemistry can, however, be
found for the isoelectronic carbene fragment, [CR2]. Addition of
PF3 to the rhodium carbene complex CpRh(PiPr3)CPh2 leads to
the formation of [g5-C5H4(CPh2H)]Rh(PiPr3)(PF3) via insertion of
[CPh2] into one of the C–H bonds of the ancillary Cp ligand.15c An
intramolecular process involving PF3 coordination and carbene
ligand migration to the Cp ligand is proposed to operate in this
system and an analogous concerted intramolecular process
represents a plausible mechanism for the formation of 2.
Finally, in order to determine the identity of the second formed
(BF-containing) product in the initial photochemical experiments,
isolated samples of 1 were subjected to photolysis in benzene.
From these studies the identity of this second species was
confirmed crystallographically as [g5-C5H4B(C6F5)2F]Fe(CO)3 (3,
see electronic supplementary information, ESI),{ which presum-
ably originates from B–H/C–F exchange chemistry.
In conclusion, we have elucidated a new mode of reactivity for
transition metal boryl compounds, one which is more character-
istic of group 14 ligands,15 and which demonstrates an alternative
mechanism for attack on arene C–H bonds by metal boryl
systems. Further studies designed to probe the scope and
mechanism of this reactivity will be reported in a full account.
We thank the EPSRC for funding including that for the
National Crystallography and Mass Spectrometry Services and the
CCLRC for the award of SRS beamtime.
Fig. 1 Structure of [g5-C5H4B(C6F5)2H]Fe(CO)3 (2) with ORTEP
˚
ellipsoids set at the 50% probability level. Relevant bond lengths (A)
and angles (u) Fe–C(21) 1.799(2), Fe–centroid 1.721(2), B(1)–C(1) 1.615(3),
B(1)–C(6) 1.646(3), B(1)–H(1b) 1.16(2), C(21)–Fe(1)–C(22) 97.4(1).
hydride (H1b) with the metal centre or with any of the co-ligands,
as has previously been observed for unsaturated group 4 systems
bearing pendant hydroborate functions.11 The closest intermole-
˚
cular contact involving H1b (2.32 A to the hydrogen attached to
Notes and references
Cp carbon C4) is within the sum of the conventional van der
˚
Waals radii (2.4 A) but is longer than distances typically associated
§ Synthesis of [g5-C5H4B(C6F5)2H]Fe(CO)3 (2): Room temperature reac-
tion of (g5-C5H5)Fe(CO)2B(C6F5)2 (1, 100 mg, 0.19 mmol) with CO (1 atm)
in benzene or benzene-d6 in the absence of light for 12 h leads to
quantitative conversion (by 1H and 11B NMR) to 2. Layering of the
reaction mixture with hexanes and cooling to 230 uC leads to the isolation
of 2 as pale yellow plates suitable for X-ray crystallography (isolated yield
78 mg, 74%). Spectroscopic data for 2: 1H NMR (300 MHz, C6D6) d 3.30
[b, 1H, BH], 3.71, 4.58 [m, both 2H, C5H4]. 13C NMR (126 MHz, C6D6)
d 83.5 [quaternary of C5H4], 85.0, 94.8 [CH of C5H4], 121.7 [ipso-C of
C6F5], 136.1, 138.0, 147.0 [CF of C6F5], 202.3 (CO). 11B (160 MHz, C6D6)
12
with unconventional hydrogen bonds (>2.2 A). A number of
˚
borate-functionalized cyclopentadienyl metal systems have been
reported previously from attack by a strongly Lewis acidic
hydrido- or halo-borane at an existing Cp ligand.10 In addition,
transfer of a metal-bound boron-containing fragment to a Cp
spectator ligand has been postulated to account for the observed
products in a number of systems. Thus, for example, the much
lower activity of CpFe(CO)2Bcat9 compared to Cp*Fe(CO)2Bcat9
(cat9 = 3,5-tBu2C6H2O2-1,2) in stoichiometric alkane borylation
chemistry has been ascribed to ready functionalization of Cp C–H
bonds, and the formation of [CpFe(CO)(m-CO)2Fe(CO)(g5-
C5H4BPh3)]2 from [CpFe(CO)2]2 and BPh3 is thought to occur
via the initially formed adduct [CpFe(CO)2(BPh3)]2.1d,13
d 224.8 [d, 1JBH = 90 Hz]. 19F NMR (283 MHz, C6D6) d 2130.8 [d, 3JFF
=
20.8 Hz, ortho-CF], 2160.3 [t, 3JFF = 20.8 Hz, para-CF], 2164.5 [m, meta-
CF]. IR (KBr, cm21) n(BH) 2374 w, n(CO) 2114 st, 2064 m sh, 2053 vs. EI-
MS m/z 548.9 [(M 2 H)+, 3%], correct isotope distribution for 1Fe and 1B
atoms, significant fragment ions at m/z 521.9 [(M 2 CO)+, 25%], 493.9
[(M 2 2CO)+, 85%] and 465.9 [(M 2 3CO)+, 100%]. Exact mass
(M 2 H)+: calc. 548.9438, meas. 548.9444. Crystallographic data for
¯
2: C20H5BF10FeO3, triclinic, P1, a = 7.4491(3), b = 8.4985(4), c =
˚
15.6646(5) A, a = 103.233(2), b = 90.730(2), c = 92.815(2)u, U =
To our knowledge the conversion of 1 to 2 represents the first
structurally authenticated example of migratory insertion into a
C–H bond for an isolated transition metal boryl complex.
Preliminary data for the reaction of 1 with the isonitrile (2,6-
Me2C6H3)NC are also consistent with an analogous migratory
insertion reaction. In both reactions, evidence that the boron-
bound hydrogen atom in 2 is initially derived from the Cp ligand
(rather than from the benzene solvent) is obtained from the
3
963.88(7) A , Z = 2, dc = 1.895 Mg m23, Mr = 549.90, T = 120(2) K. 19657
˚
reflections collected, 4417 independent [R(int) = 0.0369] which were used in
all calculations. R1 = 0.0319, wR2 = 0.0726 for observed unique reflections
[F2 > 2s(F2)]; R1 = 0.0393, wR2 = 0.0761 for all unique reflections. Max.
23
˚
A
and min. residual electron densities: 0.29 and 20.46 e
¯
Crystallographic data for 3: C20H4BF11FeO3, triclinic, P1, a = 7.5733(4),
.
˚
b = 8.4751(4), c = 15.5751(7) A, a = 103.050(2), b = 90.431(2), c =
3
92.131(2)u, U = 937.06(8) A , Z = 2, dc = 1.938 g cm23, Mr = 567.9, T =
˚
˚
120 K, l = 0.6751 A. 10183 reflections collected, 5501 independent
[Rint = 0.0198] which were used in all calculations. R = 0.0358, Rw = 0.0992
for observed unique reflections [F2 > 2s(F2)]; R = 0.0378, Rw = 0.1011 for
all unique reflections. Max. and min. residual electron density: 0.37 and
1
formation of the same H isotopomer in benzene-d6. Reactivity
towards CO or (2,6-Me2C6H3)NC which occurs via boryl ligand
displacement has been observed previously, albeit with the boron
containing product being formed via B–B reductive elimination
rather than insertion chemistry.14 For the chemistry presented
herein, the net chemical transformation of 1 into 2 involves a very
rare example of overall transfer of the boryl anion, [BR2]2
20.45 e A23. CCDC 602510 and 602511. For crystallographic data in CIF
˚
or other electronic format see DOI: 10.1039/b604141h
1 (a) K. M. Waltz, X. He, C. N. Muhuro and J. F. Hartwig, J. Am. Chem.
Soc., 1995, 117, 11357; (b) K. M. Waltz and J. F. Hartwig, Science,
1997, 277, 211; (c) K. M. Waltz, C. N. Muhuro and J. F. Hartwig,
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 2578–2580 | 2579

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Organometallics, 1999, 18, 3383; (d) K. M. Waltz and J. F. Hartwig,
J. Am. Chem. Soc., 2000, 122, 11358; (e) K. Kawamura and
J. F. Hartwig, J. Am. Chem. Soc., 2001, 123, 8422; (f) T. Ishiyama,
J. Takagi, J. F. Hartwig and N. Miyaura, Angew. Chem., Int. Ed., 2002,
41, 3056.
and S. Sasaki, J. Am. Chem. Soc., 2003, 125, 16114; (d) C. E. Webster,
Y. Fan, M. B. Hall, D. Kunz and J. F. Hartwig, J. Am. Chem. Soc.,
2003, 125, 858; (e) W. H. Lam, K. C. Lam, Z. Y. Lin, S. Shimada,
R. N. Perutz and T. B. Marder, Dalton Trans., 2004, 1556.
5 X. He and J. F. Hartwig, Organometallics, 1996, 15, 400.
2 (a) P. Nguyen, H. P. Blom, S. A. Westcott, N. J. Taylor and
T. B. Marder, J. Am. Chem. Soc., 1993, 115, 9329; (b) H. Chen and
J. F. Hartwig, Angew. Chem., Int. Ed., 1999, 38, 3391; (c) C. N. Iverson
and M. R. Smith, III, J. Am. Chem. Soc., 1999, 121, 7696; (d) H. Chen,
S. Schlecht, T. C. Semple and J. F. Hartwig, Science, 2000, 287, 1995; (e)
J.-Y. Cho, C. N. Iverson and M. R. Smith, III, J. Am. Chem. Soc., 2000,
122, 12868; (f) S. Shimada, A. S. Batsanov, J. A. K. Howard and
T. B. Marder, Angew. Chem., Int. Ed., 2001, 40, 2168; (g) J. Y. Cho,
M. K. Tse, D. Holmes, R. E. Maleczka, Jr. and M. R. Smith, III,
Science, 2002, 295, 305; (h) T. Ishiyama, J. Tagaki, K. Ishida,
N. Miyaura, N. R. Anastasi and J. F. Hartwig, J. Am. Chem. Soc.,
2002, 124, 390.
3 For recent developments in the area of CH borylation chemistry, see for
example: (a) R. E. Maleczka, F. Shi, D. Holmes and M. R. Smith, III,
J. Am. Chem. Soc., 2003, 125, 7792; (b) T. Ishiyama and N. Miyaura,
J. Organomet. Chem., 2003, 680, 3; (c) T. Ishiyama, Y. Nobuta,
J. F. Hartwig and N. Miyaura, Chem. Commun., 2003, 2924; (d)
J. D. Lawrence, M. Takahashi, C. Bae and J. F. Hartwig, J. Am. Chem.
Soc., 2004, 126, 15334; (e) D. N. Coventry, A. S. Batsanov, A. E. Goeta,
J. A. K. Howard, T. B. Marder and R. N. Perutz, Chem. Commun.,
2005, 2172; (f) J. F. Hartwig, K. S. Cook, M. Hapke, C. D. Incarvito,
Y. Fan, C. E. Webster and M. B. Hall, J. Am. Chem. Soc., 2005, 127,
2358; (g) H. Hata, H. Shinokubo and A. Osuka, J. Am. Chem. Soc.,
2005, 127, 8264; (h) G. A. Chotana, M. A. Rak and M. R. Smith, III,
J. Am. Chem. Soc., 2005, 127, 10539; (i) T. M. Boller, J. C. Murphy,
M. Hapke, T. Ishiyama, N. Miyaura and J. F. Hartwig, J. Am. Chem.
Soc., 2005, 127, 14263; (j) I. A. I. Mkhalid, D. N. Coventry, D. Albesa-
Jove, A. S. Batsanov, J. A. K. Howard, R. N. Perutz and T. B. Marder,
Angew. Chem., Int. Ed., 2006, 45, 489.
6 (a) J. J. Eisch, M. P. Boleslawski and K. Tamao, J. Org. Chem., 1989,
54, 1627; (b) M. Wagner, N. J. R. v. E. Hommes, H. No¨th and
P. v. R. Schleyer, Inorg. Chem., 1995, 34, 607; (c) R. J. Baker and
C. Jones, Coord. Chem. Rev., 2005, 249, 1857.
7 (a) S. Aldridge, A. Al-Fawaz, R. J. Calder, A. A. Dickinson,
D. J. Willock, M. E. Light and M. B. Hursthouse, Chem. Commun.,
2001, 1846; (b) A. Al-Fawaz, S. Aldridge, D. L. Coombs,
A. A. Dickinson, D. J. Willock, L. Ooi, M. E. Light, S. J. Coles and
M. B. Hursthouse, Dalton Trans., 2004, 4030.
8 (a) L. Busetto and R. J. Angelici, Inorg. Chim. Acta, 1968, 2, 291; (b)
R. Taube, S. Wache and J. Sieler, J. Organomet. Chem., 1993, 459, 335;
ˇ
(c) X. Chen, F.-C. Liu, C. E. Plecnik, S. Liu, B. Du, E. A. Meyers and
S. G. Shore, Organometallics, 2004, 23, 2100.
9 J. F. Hartwig and S. Huber, J. Am. Chem. Soc., 1993, 115, 4908.
10 (a) J. F. Hartwig and X. He, Organometallics, 1996, 16, 5350; (b)
H. Braunschweig and C. Kollann, Z. Naturforsch., B: Chem. Sci., 1999,
54, 839; (c) L. H. Doerrer, A. J. Graham, D. Haussinger and M. L. H.
Green, J. Chem. Soc., Dalton Trans., 2000, 813; (d) S. Aldridge,
R. J. Calder, S. J. Coles and M. B. Hursthouse, J. Chem. Crystallogr.,
2003, 33, 809.
11 P. Arndt, W. Baumann, A. Spannenberg, U. Rosenthal, V. V. Burlakov
and V. B. Shur, Angew. Chem., Int. Ed., 2003, 42, 1414.
12 R. H. Crabtree, R. E. M. Siegbahn, O. Eisenstein, A. L. Rheingold and
T. F. Koetzle, Acc. Chem. Res., 1996, 29, 348.
13 J. M. Burlitch, J. H. Burk, M. E. Leonowicz and R. E. Hughes, Inorg.
Chem., 1979, 18, 1702.
14 G. J. Irvine, M. J. G. Lesley, T. B. Marder, N. C. Norman, C. R. Rice,
E. G. Robbins, W. R. Roper, G. R. Whittell and L. J. Wright, Chem.
Rev., 1998, 98, 2685.
4 For recent computational studies of CH borylation pathways see: (a)
X. Wan, X. Wang, Y. Luo, S. Takami, M. Kubo and A. Miyamoto,
Organometallics, 2002, 21, 3703; (b) W. H. Lam and Z. Lin,
Organometallics, 2003, 22, 473; (c) H. Tamura, H. Yamazaki, H. Sato
15 (a) G. Fachinetti and C. Floriani, J. Chem. Soc., Chem. Commun., 1974,
516; (b) S. R. Berryhill, G. L. Clevenger and F. Y. Burdurlu,
Organometallics, 1985, 4, 1509; (c) U. Herber, E. Bleuel, O. Gevert,
M. Laubender and H. Werner, Organometallics, 1998, 17, 10.
2580 | Chem. Commun., 2006, 2578–2580
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