High yield synthesis of a neutral and carbonyl-rich terminal arylborylene complex

Article abstract of DOI:10.1039/c2cc17445f

High yield synthesis of trans-[(Me₃P)(OC)₃Fe = BDur] (Dur, "Duryl" = 2,3,4,6-Me₄C₆H) is achieved by salt elimination and subsequent liberation of trimethylsilylbromide from K[Fe(CO)₃(PMe₃)S

Full text of DOI:10.1039/c2cc17445f

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COMMUNICATION
High yield synthesis of a neutral and carbonyl-rich terminal arylborylene
complexw
Holger Braunschweig,* Qing Ye and Krzysztof Radacki
Received 29th November 2011, Accepted 23rd January 2012
DOI: 10.1039/c2cc17445f
High yield synthesis of trans-[(Me3P)(OC)3Fe
=
BDur]
simple filtration and crystallization at low temperature.
The identity of 3 was confirmed by multinuclear NMR
spectroscopy, elemental analysis, IR spectroscopy and X-ray
structure analysis. The 11B NMR resonance of 3 was observed
as a broad signal at d = 146 ppm, which resembles that
(d = 145 ppm) of the cationic iron mesitylborylene complex
[Cp*Fe(CO)2(BMes)]+[BArf4]ꢀ reported by Aldridge et al.10a
1H, 13C and 31P NMR data for 3 confirm the presence of
PMe3, duryl and carbonyl fragments. IR spectra show one
band (1878 cmꢀ1) for the carbonyl groups, which is considerably
shifted to lower frequency in comparison to those of
[Fe(CO)4(PMe3)] (2051, 1977, 1935 cmꢀ1), but strongly resembles
those of trans-[Fe(CO)3LL0] (L, L0 = phosphine, ca. 1880 cmꢀ1),12
thus suggesting a trans-geometry for 3. Moreover, these data
are in line with the well known characteristics of borylene
ligands, i.e. a stronger ligand-to-metal s donation and a similar
degree of metal-to-ligand p backbonding in comparison to
carbonyl groups, which corresponds to previous observations9
and again confirms theoretical predictions.1
(Dur, ‘‘Duryl’’ = 2,3,4,6-Me4C6H) is achieved by salt elimina-
tion and subsequent liberation of trimethylsilylbromide from
K[Fe(CO)3(PMe3)SiMe3] and Br2BDur.
Terminal borylene complexes contain double bonds between
a transition metal and two-coordinated boron and are of
fundamental interest, in addition to their close relationship to
the isoelectronic carbonyls in terms of bonding and coordination
modes.1 More recently, a wide variety of catalytic2 as well as
stoichiometric synthetic applications3 have emerged, e.g. for the
generation of borirenes4 and new borylene complexes,5 for
metathesis reactions6 and borylation of alkenes (activation of
olefinic C–H bonds)7 and most recently for the synthesis of
unprecedented boracumulenes.8 The synthetic routes to terminal
borylene complexes include salt elimination protocols,9 halide
abstraction,10 borylene transfer5 and dehydrogenation.11 In
comparison to the family of aminoborylene complexes, which
benefit from a boron-bound substituent that provides both
steric and p-electron stabilization, the number of neutral
arylborylene complexes is severely limited. To date, the dehydro-
genation protocol reported by Sabo-Etienne and co-workers
constitutes the only synthetic approach to such species.11 Here,
we report a facile salt elimination route to a neutral and
carbonyl-rich terminal arylborylene complex of iron.
Single crystals suitable for X-ray crystallography were obtained
by cooling a saturated hexane solution of 3 to ꢀ30 1C (Fig. 1).
Compound 3 crystallizes in monoclinic space group P21/c, and
exhibits a trigonal bipyramidal geometry at iron with coordination
of trimethylphosphine and durylborylene ‘‘:BDur’’ at axial sites
(P–Fe–B = 172.20(4)1). The latter is coordinated to iron termi-
nally via the sp-hybridized boron atom (Fe–B–C = 176.12(10)1).
The Fe–B distance of 1.7929(13) A resembles that of the cationic
iron mesitylborylene complex [Cp*Fe(CO)2(BMes)]+[BArf4]ꢀ
(1.792(8) A),10a and thus indicates the presence of significant
The title complex 3 is synthesized (Scheme 1) in yields of
about 60% as an orange crystalline solid. The advantages of
this procedure are the facile work-up, mild reaction conditions
(RT) and scale-up capability. The product was isolated by
Scheme 1 Preparation of trans-[(Me3P)(OC)3Fe = BDur] (3).
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat,
¨
¨
Wurzburg, Am Hubland, D-97074, Wurzburg, Germany.
¨
¨
E-mail: h.braunschweig@mail.uni-wuerzburg.de;
Fax: +49 931 31 84623; Tel: +49 931 31 85260
w Electronic supplementary information (ESI) available: Experimental
details, analytical, X-ray diffraction analyses. CCDC 855614–855616.
For crystallographic data in CIF or other electronic format see DOI:
10.1039/c2cc17445f
Fig. 1 Molecular structure of trans-[(Me3P)(OC)3Fe = BDur] (3).
Ellipsoids drawn at the 50% probability level; hydrogen atoms have
been omitted for clarity. Relevant bond lengths [A] and angles [1]:
Fe–B 1.7929(13), Fe–P 2.2719(3), B–C 1.5255(16); Fe–B–C 176.12(10),
P–Fe–B 172.20(4).
c
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Fe–B multiple bond character. In addition, the Fe–B bond is 3.7%
shorter than that of the corresponding cationic iron amino-
borylene complex [CpFe(CO)2(BNCy2)]+ (1.859(6) A)13
and even 10.8% shorter than in the neutral iron complex
[(Z5-C5Me5)BFe(CO)4] (2.101(3) A),9c in which the boron
atom resides at the apex of a nido-tetracarbapentaborane unit
and binds via an exohedral lone pair to the iron center. This
distinctively shortened Fe–B bond can be explained by an
enhanced p acidity of the boron center in 3 due to the absence
of a p donating amino substituent or its incorporation into a
cluster cage and the presence of the weakly p-acidic phosphine
ligand trans to the borylene ligand, and thus an increased
degree of Fe-B p backbonding.9b
Fig. 2 Molecular structure of [(OC)2(Me3P)Fe(m-CO)(m-BDur)Pt(PCy3)]
(5). Ellipsoids drawn at the 50% probability level; ellipsoids of the ligands,
hydrogen atoms and co-crystallized solvent molecules have been omitted
for clarity. Relevant bond lengths [A] and angles [1]: Fe–Pt 2.5710(5), Fe–B
1.970(4), Pt–B 1.976(4), Fe–P1 2.2037(10), Pt–P2 2.2621(9), Fe–C1 1.789(4),
Pt–C1 2.185(4); Fe–B–Pt 81.32(14), Fe–C1–O1 165.2(3), Fe–C1–Pt
79.93(13), Pt–C1–O1 114.8(3), Fe–B–C2 142.0(3), Pt–B–C2 136.2(3).
Subsequently, we investigated typical borylene reactivity
patterns of the title complex 3, to compare its chemical
behaviour with that of neutral complexes with B–R and
B = NR2 (R = alkyl, silyl) ligands. During the last few years,
[Pt(PCy3)2] (4), which possesses a electron-rich and highly
unsaturated platinum center, was intensively investigated in
our laboratory as a powerful transition metal Lewis base toward
metal-coordinated boron-based ligands,14 thereby establishing,
e.g. the concept of transition metal stabilized terminal borylene
complexes. As the corresponding examples are restricted to
alkylborylene and aminoborylene species, we sought to contribute
a hitherto unknown arylborylene metal-adduct. To this end, 3 was
treated with 4 (Scheme 2) affording the heterodinuclear species
[(OC)2(Me3P)Fe(m-CO)(m-BDur)Pt(PCy3)] (5), as indicated by a
slightly upfield-shifted broad resonance at d = 126 ppm in the
11B NMR spectrum and two signals at d = 66.8 ppm
the previously reported heterodinuclear borylene species.14a,16
Moreover, the almost orthogonal orientation of the duryl
substituent at boron with respect to the Fe–B–Pt–C1 four
membered ring is presumably due to steric congestion imposed
by the duryl group and the PCy3 ligand.
Another particularly intriguing area of terminal borylene
chemistry is borylene metathesis through [2 + 2] cycloaddition.
While Aldridge and co-workers reported first examples of
reaction products of a formal borylene metathesis, they
proposed a non-concerted mechanism for the formation of
the latter.17 Subsequently, we studied the terminal alkylborylene
complex [(Z5-C5H5)(OC)2Mn = BtBu] and its propensity to
react with polar, unsaturated substrates such as benzophenone
to afford structurally characterized cyclic intermediates,
e.g. [(Z5-C5H5)(OC)2Mn{B(tBu)OC(Ph)2}] (8), that underwent
spontaneous cycloreversion with clean formation of the meta-
thesis products.6 Although these results established a concerted
mechanism for borylene metathesis, its application remained
restricted to the aforementioned tert-butyl borylene species. Given
the fundamental importance of metal complexes [LxM = ERn]
in metathesis reactions,18 we attempted to extend this important
reactivity pattern. Thus, 3 was treated with benzophenone (6)
as a representative carbonyl substrate, furnishing a new cyclic
species 7 as indicated by a significantly up-field shifted 11B
NMR resonance at d = 75 ppm, which corresponds to that of
8 at d = 72 ppm. Accordingly, a new singlet at d = 12.4 ppm
was observed in the 31P NMR spectrum.
1
4
(Pt–PCy3, JPt–P = 4967 Hz, JP–P = 14.6 Hz) and 36.1 ppm
4
(Fe–PMe3, JP–P = 14.6 Hz) in the 31P{1H} NMR spectrum.
The concomitant formation of free PCy3 was indicated by a
31P NMR resonance at d = 9.8 ppm.
Complex 5 was isolated as deep red crystals by crystallization
from toluene/hexane solutions at ꢀ35 1C and subjected to X-ray
diffraction analysis (Fig. 2).
%
Complex 5 crystallizes in the triclinic space group P1, and
the overall geometry of the central Fe–B–Pt–C1 fragment,
which is approximately planar as indicated by the sum (359.8)
of angles within the four-membered ring, revealed the semi-
bridging coordination mode for both arylborylene (:BDur)
and carbonyl ligands.14a,15 The Fe–B separation (1.970(4) A) is
lengthened by more than 10% in comparison to precursor 3 as
a result of the increased coordination number at boron. The
Pt–B bond length of 1.976(4) A falls in the expected range for
Complex 7 was isolated in high yield (80%) as colorless
crystals upon cooling a hexane/toluene solution to ꢀ35 1C
(Fig. 3). X-Ray diffraction analysis confirmed the constitution
%
of 7, which crystallizes in the triclinic space group P1 with two
independent molecules in the asymmetric unit, both featuring
very similar structural parameters. The overall geometry, in
particular the Fe–B–O–C four-membered ring resembles that
of the previously reported manganese analogue 8. That is
(a) the planarity of the ring as indicated by the sum of the
internal angles (360.01), (b) significant elongation of the Fe–B
separation (2.0882(16) A, falling in the expected range for iron
boryl complexes5d) as a result of the increased coordination
Scheme 2 Synthesis of [(OC)2(Me3P)Fe(m-CO)(m-BDur)Pt(PCy3)]
(5) and [(OC)2(Me3P)Fe{B(Dur)OC(Ph)2}] (7).
c
2702 Chem. Commun., 2012, 48, 2701–2703
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3 For selected reviews on borylene complex reactivity see:
(a) H. Braunschweig and D. Rais, Heteroat. Chem., 2005, 16, 566;
(b) H. Braunschweig, C. Kollann and F. Seeler, Struct. Bonding, 2008,
130, 1; (c) C. E. Anderson, H. Braunschweig and R. D. Dewhurst,
Organometallics, 2008, 27, 6381; (d) D. Vidovic, G. A. Pierce and
S. Aldridge, Chem. Commun., 2009, 1157; (e) H. Braunschweig,
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T. Marder, I. Ferna
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2009, 131, 8989; (c) H. Braunschweig, I. Ferna
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K. Radacki and F. Seeler, Angew. Chem., Int. Ed., 2007, 46, 5215;
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G. Frenking and S. De, Inorg. Chem., 2011, 50, 62.
Fig. 3 Molecular structure of [(OC)3(Me3P)Fe{B(Dur)OC(Ph)2}] (7).
Ellipsoids drawn at the 50% probability level; ellipsoids of the ligands,
hydrogen atoms and the second independent molecule in the asymmetric
unit have been omitted for clarity. Relevant bond lengths [A] and
angles [1]: Fe–B 2.0882(16), B–O 1.3503(18), O–C 1.4690(15), C–Fe
2.1116(13); B–Fe–C 62.56(5), Fe–C–O 95.59(7), C–O–B 101.25(10),
O–B–Fe 100.60(9).
5 (a) H. Braunschweig, M. Colling, C. Kollann, B. Neumann and
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number at the boron atom and loss of Fe = B double bond
character, and (c) very similar B–O and C–O bond lengths.
However, in stark contrast to the manganese analogue 8,
which spontaneously undergoes a cycloreversion in solution
within a couple of hours, leading to a corresponding manganese
carbene complex and boroxine as a trimerisation product of
R–B = O, 7 undergoes merely slow decomposition (t1/2 = 4 d
in benzene). As indicated by 1H, 11B and 31P NMR spectra, no
clean formation of the expected boroxine and iron carbene
complexes was observed.
In conclusion, we have presented the high-yield synthesis of the
carbonyl-rich durylborylene complex trans-[(Me3P)(OC)3Fe =
BDur] (3) as a rare example of an arylborylene species. Preliminary
reactivity studies furnished the first example of a metal base-
stabilized arylborylene complex and the product of a selective
[2 + 2] cycloaddition with benzophenone as a representative
unsaturated, polar substrate. The latter result in particular
conveys a tentative impression of the behaviour of arylborylenes,
suggesting that their degree of reactivity is somewhere between
that of amino- and alkylborylene complexes.
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 2701–2703 2703