Cationic terminal borylene complexes: A synthetic and mechanistic investigation of m=b metathesis chemistry

Article abstract of DOI:10.1002/anie.200502343

(Chemical Equation Presented) Halide abstraction chemistry has been exploited to generate the cationic terminal borylene complex 1, which reacts with Ph₃P=S or Ph₃As=O to form 3 and 4. In the case of the corresponding reaction with Ph₃P=O, the intermediate 2 can be isolated, which provides evidence for a two-step addition/substitution mechanism for the overall metathesis reaction.

Full text of DOI:10.1002/anie.200502343

Angewandte
Chemie
=
M B Metathesis
DOI: 10.1002/anie.200502343
Cationic Terminal Borylene Complexes: A
Synthetic and Mechanistic Investigation of
=
M B Metathesis Chemistry**
Deborah L. Kays (nØe Coombs), Joanna K. Day,
Li-Ling Ooi, and Simon Aldridge*
Metathesis reactions constitute a key component of
modern synthetic chemistry; olefin metathesis, for
example, provides a versatile and widely exploited
carbon–carbon bond-forming methodology.[1] Such reac-
Scheme 1. Synthesis and reactions of 3. a) Na[CpFe(CO)2] (1 equiv), toluene, 208C,
20 h.; b) Na(BArf4), (1 equiv), dichloromethane, ꢀ78!208C, 30 min.; c) ppnCl
tions are typically catalyzed by organometallic com-
=
=
(1.67 equiv), dichloromethane, 208C, 30 min.; d) Ph3P S or Ph3As O (1 equiv),
dichloromethane, 208C, 30 min. ppn=bis(triphenylphosphoranylidene)ammonium.
[2]
=
plexes that contain M C bonds. The synthesis of
=
analogous complexes that contain M Si bonds, for
example, has led to an in-depth investigation of their
reactivity towards unsaturated substrates.[3]
product in the subsequent halide-abstraction step,[7b,8]
whereas [CpFe(CO)2{B(tmp)Cl}](tmp = tetramethylpipera-
mino) was inaccessible from tmpBCl2. Compound 2 is a pale
yellow sublimable crystalline solid, which was characterized
Synthetic approaches that lead to the isolation of related
=
systems with M B bonds have been developed only
recently:[4–7] for example, halide-abstraction chemistry gives
=
access to cationic terminal borylene complexes, [LnM
by multinuclear NMR and IR spectroscopy, mass spectrom-
etry, and X-ray crystallographic analysis (Figure 1).
BX]+.[7] Consequently, reports of the fundamental chemistry
=
of M B bonds are somewhat limited (predominantly to
metal–metal transfer reactions and addition/substitution
reactivity towards nucleophiles).[4,6,7] Thus, the chemistry of
[Cp*Fe(CO)2(BMes)]+
(Mes = mesityl = 2,4,6-Me3C6H2,
Cp* = pentamethylcyclopentadienyl), for example, is domi-
nated by the electrophilic character at both the Fe and B
centers.[7] In an attempt to tune the reactivity of these highly
unsaturated complexes we investigated the synthesis of
+ [5]
=
cationic aminoborylene systems, [LnM BNR2] . Chloride
abstraction from [CpFe(CO)2{B(NiPr2)Cl}]( 2) (Cp = cyclo-
pentadienyl) by Na(BArf4) (Arf = 3,5-(CF3)2C6H3) affords the
thermally robust cationic B/N vinylidene analogue
+
=
=
[CpFe(CO)2(BNiPr2)] , which undergoes, with E O and E
=
S bonds (E = P, As), the first reported examples of M B
metathesis chemistry.
Figure 1. Structure of 2 (50% ellipsoids; H atoms omitted for clarity).
Selected bond lengths [Š] and angles [8]: Fe(1)-B(1) 2.054(4), B(1)-
Cl(1) 1.841(4), B(1)-N(1) 1.389(5); centroid-Fe(1)-B(1)-N(1) 83.7(4).
The synthesis of [CpFe(CO)2(BNiPr2)]+(BArf4)ꢀ (3) is
outlined in Scheme 1. The key precursor 2 was synthesized by
the selective substitution of
a chloride substituent in
iPr2NBCl2 (1) by [CpFe(CO)2]ꢀ. The steric bulk of the
amino substituents is a key point: the use of the smaller NMe2
group results in the formation of a thermally fragile borylene
The reaction of 2 with Na(BArf4) in dichloromethane
results in a quantitative conversion (determined by H and
1
11B NMR spectroscopy) into 3. The latter product (along with
the C5H4Me analogue) is a colourless oil at (or close to) room
temperature, but its formulation can be definitively estab-
lished from spectroscopic and reactivity data. The measured
11B chemical shift for 3 (dB = 93.5 ppm) is very close to that
reported by Braunschweig et al. for neutral terminal-amino-
[*] Dr. D. L. Kays (nØe Coombs), J. K. Day, Dr. L.-L. Ooi, Dr. S. Aldridge
Centre for Fundamental and Applied Main Group Chemistry
School of Chemistry, Cardiff University
Main Building, Park Place, Cardiff, CF103AT (UK)
Fax: (+44)2920-874-030
E-mail: aldridges@cardiff.ac.uk
=
borylene systems of the type LnM BN(SiMe3)2 (dB = 86.6–
98.3 ppm).[5] The downfield shift upon chloride abstraction
(DdB = 38.1 between 2 and 3) mirrors that found for
[Cp*Fe(CO)2{B(Mes)Cl}]/[Cp*Fe(CO)2(BMes)]+ (dB = 112.1
and 145.0 ppm, respectively) and for [Cp*Fe(CO)2-
[**] We thank the EPSRC for funding and access to the National Mass
Spectrometry facility and Prof. C. Jones (Cardiff) for help in
modeling crystallographic disorder.
Supporting information for this article (crystal structure data for 2
and 6, and data for 4a, 4b, 5a, and 5b) are available on the WWW
under http://www.angewandte.org or from the author.
{B(NMe2)Cl}]/[Cp*Fe(CO)2(BNMe2)]+
(dB = 58.6
and
88.0 ppm, respectively).[7,8] The 1H and 13C NMR data are
Angew. Chem. Int. Ed. 2005, 44, 7457 –7460
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7457

Communications
consistent with the presence of Cp, NiPr2, and (BArf4)ꢀ
moieties in a 1:1:1 ratio, and the ES + mass spectrum shows
the presence of the [CpFe(CO)2(BNiPr2)]+ cation. The
observation of equivalent iPr substituents is consistent with
the observations of inequivalent iPr groups by 1H and
13C NMR and of significantly lower carbonyl stretching
frequencies (n˜ = 2004, 1949 cmꢀ1) are consistent with the
formation of a trigonal-planar boron center by coordination
of a Lewis base. Confirmation that the intermediate species is
indeed the B-bound Ph3PO adduct [CpFe(CO)2{B(NiPr2)-
(OPPh3)}]+(BArf4)ꢀ(6) was obtained by X-ray crystallo-
graphic analysis (Figure 2). Consistent with related complex-
=
the structure of the related neutral system [CpV(CO)3
BN(SiMe3)2][5c] and agrees with the results of DFT calcula-
tions for the model compounds [(h5-C5R5)Fe(CO)2(BNMe2)]+
(R = H, Me), for which a minimum-energy structure close to
Cs symmetry [ < centroid-Fe-N-C ꢁ 908 (84.68 for R = Me)]
and a low barrier to rotation about the Fe-B-N axis
( ꢁ 2.2 kcalmolꢀ1) have been calculated.[7b,c] Furthermore,
the IR spectrum of 3 shows carbonyl stretching frequencies
(n˜ = 2070, 2028 cmꢀ1) that are significantly blue shifted with
respect to
2
(n˜ = 2001, 1941 cmꢀ1) (Dn˜ ꢁ 50 cmꢀ1 for
[Cp*Fe(CO)2{B(Mes)Cl}]/[Cp*Fe(CO)2(BMes)]+) and are
very similar to those reported for the archetypal Fischer
+
ꢀ
=
carbene systems such as [CpFe(CO)2 CH(SPh)] (PF6) (n˜ =
2073, 2034 cmꢀ1).[9] Further evidence for the nature of 3 was
obtained: 1) from the reaction of 3 with ppnCl (see Support-
ing Information), which like the analogous reaction for
structurally characterized cationic derivatives,[6b,7] generates
a haloboryl complex (in this case 2) by the addition of a halide
=
at boron and 2) from the reaction of 3 with Ph3P O, which
proceeds via the structurally characterized adduct
[CpFe(CO)2{B(NiPr2)(OPPh3)}]+(BArf4)ꢀ (see below).
Although the reactivity of cationic aminoborylene com-
plex 3 towards Clꢀ is indicative of electrophilic character, its
reactivity towards unsaturated substrates suggests a broader
=
scope for its chemistry. Thus, the reaction of 3 with Ph3P S in
dichloromethane at room temperature leads to the formation
of [iPr2NB(m-S)2BNiPr2] (4a) and [CpFe(CO)2(PPh3)]+-
(BArf4)ꢀ (5a) with a conversion of > 95% (as determined
Figure 2. Structure of the cationic component of 6 (50% ellipsoids; H
atoms omitted for clarity). Selected bond lengths [Š] and angles [8]:
Fe(1)-B(1) 2.057(4), B(1)-O(3) 1.469(4), B(1)-N(1) 1.397(5), P(1)-O(3)
1.540(2); P(1)-O(1)-B(1) 148.0(2), centroid-Fe(1)-B(1)-N(1) 72.5(5).
1
by H, 11B and 31P NMR spectroscopy). The identities of the
isolated products 4a and 5a were confirmed by comparison of
NMR (1H, 11B, 13C, 19F, and 31P) and IR spectra and mass
spectrometry data with those reported for authentic sam-
ples.[10a–c] The similar reactivity of 3 towards Ph3As O led to
es,[4b,c,14] the Fe B bond length for 6 [2.057(4) Š]is more akin
=
ꢀ
the isolation of (iPr2NBO)3 (4b) and [CpFe(CO)2(AsPh3)]+-
to that expected for a single bond than a double bond
(2.054(4) and 1.792(8) Š for 2 and [Cp*Fe(CO)2(BMes)]+,
respectively).[7] Such a phenomenon has previously been
ascribed to significant contributions from resonance forms
(BArf4)ꢀ (5b).[10c,d]
=
=
The reactions of 3 with Ph3P S and Ph3As O therefore
represent, to our knowledge, the first examples of net
metathesis reactions for a terminal borylene complex.[11]
Although metathesis chemistry has been reported for iso-
[4b,c]
ꢀ
that incorporate a formal M B bond (Scheme 2),
and a
description of 6 as an amino(oxy)boryl species that features a
= =
electronic vinylidene systems [CpM(CO)2 C CH2](M = Mn,
pendant cationic phosphorus centre is probably most apt.
Re),[12] no such reactivity has been reported for neutral
aminoborylene complexes.[5,6] The origins of the different
reactivity of 3 and an idea of the likely mechanism can be
Consistent with this, the P O distance within 6 is significantly
ꢀ
longer than that found in free Ph3PO (1.540(2) vs. 1.493 Š
(mean)).[15]
=
gauged by examining of the analogous reaction with Ph3P O.
Given the isolation of 6, it is plausible that the first step in
=
This reaction proceeds at a significantly slower rate than those
the reaction mechanism involves coordination of Ph3E X
=
=
of Ph3P S or Ph3As O which presumably reflects the greater
=
strength of the P O bond. In this case, however, it is possible
to identify a reaction intermediate, which was characterized
by NMR signals at dB = 48.9 ppm and dP = 48.3 ppm. The
former resonance is consistent with values previously
reported for base-stabilized terminal borylene complexes
(e.g. dB = 51.7–53.2 ppm for N-donor adducts of osmium
aminoborylenes),[4b,c] whereas the 31P chemical shift is as
expected for donor/acceptor adducts of Ph3PO with boron-
centered Lewis acids (dP = 43.6–46.7 ppm).[13] Furthermore,
Scheme 2. Proposed addition/substitution pathway for the metathesis
=
reaction of 3 (exemplified by Ph3P S).
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Angewandte
Chemie
yellow crystals. Yield of isolated product: 0.105 g, 43%. 1H NMR
(E = P, As ; X = O, S) at boron, and that the overall metathesis
chemistry of 3 therefore occurs through a combined addition/
substitution pathway (Scheme 2). Further studies aimed at
better understanding the reactivity of M = B bonds towards
unsaturated substrates (including C = E multiple bonds) will
be reported in a full account.[17]
(400 MHz, CD2Cl2): d = 1.06 (d, J = 6 Hz, 6H; CH(CH3)2), 1.20 (d,
J = 6 Hz, 6H; CH(CH3)2), 3.25 (sept, J = 6 Hz, 1H; CH(CH3)2), 4.10
(sept, J = 6 Hz; CH(CH3)2), 4.50 (s, 5H; Cp-H), 7.38–7.48 (m, 9H;
Ph3PO-CHo and Ph3PO-CHp), 7.55 (s, 4H; BArf4ꢀ-Hp), 7.60–7.78 (m,
6H; Ph3PO-Hm), 7.73 ppm (s, 8H; BArf4ꢀ-Ho); 13C NMR (76 MHz,
C6D6): d = 22.4, 23.9 (CH(CH3)2), 47.2 (CH(CH3)2), 84.4 (C(Cp)),
117.5 (BArf4ꢀ-CHp), 122.6 (Ph3PO-Cipso), 124.6 (q, 1JCF = 272 Hz;
BArf4ꢀ-CF3), 128.9 (q, 2JCF = 34 Hz; BArf4ꢀ-Cm), 130.1 (Ph3PO-CHm),
133.5 (Ph3PO-Co), 134.9 (BArf4ꢀ-CHo), 135.9 (Ph3PO-Cp), 161.8 (q,
1JCB = 49 Hz; BArf4ꢀ-Cioso), 214.7 ppm (CO); 11B NMR (96 MHz,
CD2Cl2): d = ꢀ7.7 (BArf4ꢀ), 48.9 ppm (b, fwhm ꢁ 480 Hz, B-
(OPPh3)NiPr2); 19F NMR (283 MHz, CD2Cl2): d = ꢀ62.7 ppm (CF3);
31P NMR (121 MHz, CD2Cl2): d = 48.3 ppm (Ph3PO); IR (CD2Cl2
soln): n˜ = 2004, 1949 cmꢀ1 n(CO). Crystal data: C63H46B2F24FeNO3P,
Experimental Section
2: Reaction of 1 (1.199 g, 6.6 mmol) with Na[CpFe(CO)2](1.318 g,
6.6 mmol) in toluene (40 cm3) at room temperature for 20 h, followed
by filtration, removal of volatile compounds in vacuo, and extraction
with hexanes ( ꢁ 40 cm3) yielded crude 2 as an oily brown solid.
Yellow crystals suitable for X-ray diffraction were obtained by
sublimation under high vacuum (408C at 10ꢀ4 Torr). Yield (of
sublimed material): 0.259 g, 12%. 1H NMR (400 MHz, C6D6): d =
1.11 (d, J = 6.7 Hz, 6H; CH(CH3)2 of iPr), 1.39 (d, J = 6.7 Hz, 6H;
CH(CH3)2), 3.40 (sept, J = 6.7 Hz, 1H; CH(CH3)2), 4.44 (sept, J =
6.7 Hz, 1H; CH(CH3)2), 4.69 ppm (s, 5H; Cp-H); 13C NMR (76 MHz,
C6D6): d = 21.2 (CH(CH3)2), 23.9 (CH(CH3)2), 47.8 (CH(CH3)2), 55.2
(CH(CH3)2), 88.2 (Cp), 215.6 ppm (CO); 11B NMR (96 MHz, C6D6):
d = 55.4 ppm. IR (C6D6 soln); n˜ = 2001, 1941 cmꢀ1 nCO; MS (EI): m/z
(%): 295 (65) [MꢀCO]+, 288 (weak) [MꢀCl], 267 (10) [Mꢀ2CO]+,
223 (100) [MꢀNiPr2]+, M+ not observed; MS: calcd for [MꢀCO]+:
295.0592; found: 295.0595; calcd for [MꢀCl]+: 288.0853; found
288.0856. Crystal data: C13H19BClFeNO2, orthorhombic, Pbca, a =
11.7410(4), b = 13.9170(4), c = 19.0830(7) Š, V= 3118.15(18) Š3, Z =
8, 1calcd = 1.378 Mgmꢀ3, Mr = 323.40, T= 150(2) K. 22231 reflections
collected, 3175 independent (R(int) = 0.1524), which were used in all
calculations. R1 = 0.0572, wR2 = 0.1157 for observed unique reflec-
tions (F2 > 2s(F2)) and R1 = 0.1048, wR2 = 0.1338 for all unique
reflections.ꢀ3 [1M6] ax./min. residual electron densities 0.505/
¯
triclinic, P1, a = 13.0324(2), b = 13.9949(2), c = 19.1002(3) Š, a =
68.7080(10), b = 83.7430(10), g = 87.4800(10)o, V= 3226.47(8) Š3,
Z = 2, 1calcd = 1.471 Mgmꢀ3, Mr = 1429.45, T= 150(2) K. 51247 reflec-
tions collected, 14704 independent (R(int) = 0.1118) which were used
in all calculations. R1 = 0.0718, wR2 = 1791 for observed unique
reflections (F2 > 2s(F2)) and R1 = 0.1223, wR2 = 0.1992 for all unique
reflections.ꢀ3 [1M6] ax./min. residual electron densities 1.086/
ꢀ0.608 eŠ
.
Received: July 5, 2005
Published online: October 17, 2005
Keywords: boron · borylenes · halide abstraction · metathesis ·
.
vinylidene ligands
[1]T. M. Trnka, R. H. Grubbs, Acc. Chem. Res. 2002, 35, 18 – 29.
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ꢀ0.537 eŠ
.
3: Reaction of 2 (0.259 g, 0.80 mmol) and Na(BArf4) (0.710 g,
0.80 mmol) in dichloromethane (15 cm3) from ꢀ78!208C over
30 min leads to a quantitative conversion (determined by 11B NMR)
of 2 (dB = 55.4 ppm) to 3 (dB = 93.5 ppm). Filtration, and recrystalli-
zation from dichloromethane/hexanes (or fluorobenzene/hexanes) at
ꢀ308C leads to the isolation of 3 as a spectroscopically pure
colourless oil. Yield of isolated material: 0.465 g, 50%. 1H NMR
(400 MHz, CD2Cl2): d = 1.39 (d, J = 6.6 Hz, 12H; CH(CH3)2), 3.32
(sept, J = 6.6 Hz, 2H; CH(CH3)2), 5.33 (s, 5H, Cp-H), 7.55 (s, 4H;
BArf4ꢀ-Hp), 7.70 ppm (s, 8H; BArf4ꢀ-Ho); 13C NMR (76 MHz,
CD2Cl2): d = 24.4 (CH(CH3)2), 51.0 (CH(CH3)2), 87.1 (C(Cp)),
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117.6 (BArf4ꢀ-CHp), 124.6 (q, 1JCF = 272 Hz, BArf4ꢀ-CF3), 128.8 (q,
1
2JCF = 34 Hz, BArf4ꢀ-CHm), 134.8 (BArf4ꢀ-CHo), 161.8 (q, JCB
=
49 Hz, BArf4ꢀ-Cipso), 205.6 ppm (CO); 11B NMR (96 MHz, CD2Cl2):
d = ꢀ7.7 (BArf4ꢀ), 93.5 ppm (b, fwhm ꢁ 615 Hz, BNiPr2); 19F NMR
(283 MHz, CD2Cl2): d = ꢀ62.6 ppm (CF3); IR (CD2Cl2 soln): n˜ =
2070, 2028 cmꢀ1 nCO; MS (ES): m/z (%): M+ 288.1 (10).
=
Typical reaction: 3 (0.068 g, 0.06 mmol) and Ph3P S (1.0 equiv)
were stirred together in dichloromethane for 30 min, after which the
reaction was judged to be complete by 11B and 31P NMR (conversion
of signals at dB = 93.5 ppm and dP = 43.7 ppm to dB = 35.6 ppm and
dP = 60.8 ppm, respectively). Removal of volatile components in va-
cuo and extraction into hexanes gave iPr2NB(m-S)2BNiPr2 (4a), which
was identified by comparison of 1H, 13C, and 11B NMR spectroscopic
and mass spectrometry data with those reported previously.[10a,b]1 H,
13C, 11B, 19F, and 31P NMR and IR spectra of the hexane-insoluble
product confirmed it to be [CpFe(CO)2(PPh3)]+(BArf4)ꢀ (5a).[10c]
A
similar procedure was adopted for the reaction of 3 with Ph3As = O.
[10c,d]
6: Reaction of 3 (0.199 g, 0.17 mmol) and Ph3PO (0.048 g,
0.17 mmol) in dichloromethane (5 cm3) at room temperature over a
period of 30 min, followed by filtration and recrystallization from
dichloromethane/hexanes at ꢀ308C led to the isolation of 6 as pale
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Angew. Chem. Int. Ed. 2005, 44, 7457 –7460
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www.angewandte.org
7459

Communications
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[16]Further details of the crystal structures investigation may be
obtained from the Fachinformationszentrum, Karlsruhe, D-
76344 Eggenstein-Leopoldshafen, Germany (fax: (+ 49)7247-
808-666; e-mail: crysdata@fiz-karlsruhe.de) on quoting the
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=
[11]Metathesis chemistry has been reported for C B bonds, for
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[17]For a related article, see the following Communication in this
issue: H. Braunschweig, T. Herbst, D. Rais, F. Seeler, Angew.
Chem. 2005, 117, 7627 – 7629; Angew. Chem. Int. Ed. 2005, 44,
7461 – 7463 (DOI: 10-1002/anie.200502524).
7460 www.angewandte.org
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