Molecular structure and cluster formation of a tert-butylborylene complex

Article abstract of DOI:10.1002/anie.200601237

B finds a new hive: The X-ray structure of [μ-BtBu{(η⁵- C₅H₅) Mn(CO)₂}₂] represents the first crystallographic characterization of an alkylborylene complex. Its substituted counterpart [μ-BtBu{(η⁵-C₅H ₄Me)-Mn(CO)₂}₂] reacts cleanly with [Pd(PCy₃)₂] (Cy = cyclohexyl) to form the first heterometallic μ₃-borylene complex 1, for which DFT and ELF computations indicate a localized bonding situation. (Figure Presented).

Full text of DOI:10.1002/anie.200601237

Communications
Borylene Complexes
DOI: 10.1002/anie.200601237
Molecular Structure and Cluster Formation of a
tert-Butylborylene Complex**
Holger Braunschweig,* Christian Burschka,
Michael Burzler, Stephanie Metz, and
Krzysztof Radacki
In 1995 we reported on the first spectroscopically character-
ized borylene complexes [m-BX{(h5-C5H4R)Mn(CO)2}2] (1:
X = NMe2, R = H; 2: X = tBu, R = Me)[1] and the molecular
structure of 1. During the past decade, intense research efforts
were focused on borylene complexes,[2] and many different
À
coordination modes for ligands of the type B R were
described and include terminal,[3] hetero-dinuclear,[4] and
semibridging[5] species, thus experimentally proving the
predicted close relationship between borylene ligands and
CO.[6,7] More recent studies focused on the reactivity of
borylene complexes and revealed some interesting character-
istics of these species, for example, i) the potential of terminal
borylene ligands to be transferred to metallic[8] and non-
metallic[9] substrates, ii) the strong tendency of some terminal
borylene ligands to add metal bases of the type [M(PR3)n]
(M = Pd, Pt),[5,10] and iii) the exceptional stability of the
central Mn2B framework in 1.[7] This stability allowed for
elusive substitution reactions at the borylene center without
cleavage of the metal–boron framework.[11] Given the pro-
nounced thermodynamic stability of these compounds with
respect to M–B dissociation,[6,7] the latter finding may appear
surprising; DFT calculations revealed a build-up of positive
charge at the boron center, and hence, the M–B bond is
kinetically labile and susceptible to nucleophilic cleavage.[7]
Despite significant progress in this area, the choice of
boron-bound substituents that sufficiently stabilize the metal-
coordinated borylene ligand is very much restricted to mainly
p-stabilizing heteroatoms such as N and O. Only very few
substituents with little or no p stabilization were reported,
such as sterically very demanding aryl[3b] or hypersilyl
groups.[3c] The tert-butylborylene complex 2 is in a way a
unique example, as it still represents the only alkylborylene
complex and, accordingly, has attracted much interest.[12]
Because of the highly unsaturated nature of the boron
center in 2, a reactivity pattern has to be expected that is
[*] Prof. Dr. H. Braunschweig,Dr. C. Burschka,M. Burzler,S. Metz,
Dr. K. Radacki
Institut für Anorganische Chemie
Bayerische Julius-Maximilians-Universität Würzburg
Am Hubland,97074 Würzburg (Germany)
Fax: (+49)931-888-4623
E-mail: h.braunschweig@mail.uni-wuerzburg.de
[**] This work was supported by the Deutsche Forschungsgemeinschaft.
We are grateful to BASF AG for a donation of chemicals. H.B. thanks
Prof. T. P. Fehlner (Notre Dame) for very helpful discussions.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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significantly different from that of its amino-stabilized
counterpart 1. Herein we report the synthesis and crystal
structure of [m-BtBu{(h5-C5H5)Mn(CO)2}2] (3), as well as the
unprecedented reactivity of its methyl-substituted analogue
[m-BtBu{(h5-C5H4Me)Mn(CO)2}2] (2) towards [Pd(PCy3)2]
(Cy = cyclohexyl) to yield a novel heterometallic boron
cluster.
Since attempts to obtain suitable single crystals of 2 failed,
its unsubstituted counterpart [m-BtBu{(h5-C5H5)Mn(CO)2}2]
(3) was prepared by an analogous procedure[13] from K[{(h5-
C5H5)Mn(CO)2}H] and B2tBu2Cl2 and recrystallized from
hexanes at À358C.
is 20 ppm highfield-shifted with respect to that of 3, thus
indicating the formation of 4. The 31P NMR spectrum
revealed signals at d = 10 (PCy3), 92
([(h5-
C5H4Me)Mn(CO)2PCy3][17] (5)), and 27 ppm (4), as well as
the signal from [Pd(PCy3)2] at d = 39 ppm. After three weeks
the reaction was complete, and no additional signals indica-
tive of further byproducts were detected. The new complex 4
was isolated as red crystals in 25% yield after repeated
recrystallization from hexane. The formation of 4 requires the
extrusion (and trapping by PCy3) of one {(h5-
C5H4Me)Mn(CO)2} fragment from 2, thereby formally
Compound 3 crystallizes in the space group P21/n and
displays the expected central three-membered Mn2B ring
with an exocyclic B1 C5 bond length of 1.610(2) Š,[14] which
À
is in the normal range for tert-butylboranes (Figure 1).[15]
giving
a
terminal
=
borylene
which is subsequently stabi-
complex
“[(h5-
[18]
C5H4Me)(OC)2Mn BtBu]”,
lized by two {Pd(PCy3)} moieties. Neither the controlled
degradation of the thermodynamically very stable Mn2B core
under thermal conditions nor the multiple addition of metal
bases to a boryl or borylene center has any precedence in
metal–boron chemistry, and the occurrence of these trans-
formations is here ascribed to the presence of the boron-
bound alkyl substituent. Complex 4 crystallizes in the space
¯
group P1 and consists of an MnPd2 isosceles triangle (bond
À
À
lengths: Mn1 Pd1 2.6426(10) Š, Mn1 Pd2 2.6458(10) Š,
À
Pd1 Pd2 2.8423(7) Š) which is capped in a m3 fashion by
the tert-butylborylene ligand (Figure 2).[14] The B1 Mn1 bond
length of 1.987(7) Š is close to that of 1, while the B Pd bond
À
À
À
À
lengths (B1 Pd1 2.144(7) Š, B1 Pd2 2.128(8) Š) are com-
parable to those of [(h5-C5Me5)Fe(m-CO)2(m-BCl2)Pd(PCy3)]
Figure 1. Molecular structure of 3 (thermal ellipsoids set at 50%
probability). Bond lengths [Š] and angles [8]: B1–Mn1 2.0288(16),B1–
Mn2 2.0309(16),Mn1–Mn2 2.7952(5),B1–C5 1.610(2); Mn1-B1-Mn2
87.03(7),B1-Mn2-Mn1 46.45(5).
(2.062(4) Š),[19] thus indicating M B single bonds in 4.
À
Compound 4 can be viewed as the first example of a
heterometallic m3-borylene complex. The class of m3-borylene
Despite the absence of p stabilization from the boron-
bound substituent, the geometry of the central Mn2B
À
moiety, with two nearly identical B Mn bond lengths of
À
À
2.0288(16) Š (B1 Mn1) and 2.0309(16) Š (B1 Mn2) and a
À
Mn Mn bond length of 2.7952(5) Š, closely resembles that of
the corresponding amino-[1] or chloroborylene[16] complexes.
In contrast to the similarities with respect to their
molecular structures, 1 and 3 show markedly different
reactivity, as already indicated by the extreme sensitivity of
3 in stark contrast to the stability of 1 even towards air and
water. The aminoborylene species also proved unreactive
towards [Pd(PCy3)2], whereas 2 underwent a clean, albeit
slow, reaction with [Pd(PCy3)2] to form the unprecedented
trimetallic species [(m3-BtBu){[(h5-C5H4Me)Mn(CO)2][Pd-
(PCy3)]2}] (4).
A 1:2 mixture of 2 and [Pd(PCy3)2] in C6D6 was monitored
by multinuclear NMR spectroscopy at ambient temperature.
After a few hours, a slowly increasing new 11B NMR
spectroscopic resonance at d = 150 ppm was detected, which
Figure 2. Molecular structure of 4 (thermal ellipsoids set at 50%
probability). Bond lengths [Š] and angles [8]: Mn1–Pd1 2.6426(10),
Mn1–Pd2 2.6458(10),Pd1–Pd2 2.8423(7),B1–Mn1 1.987(7),B1–Pd1
2.144(7),B1–Pd2 2.128(8); O1-C1-Mn1 158.9(5),O2-C2-Mn1 160.1(5).
Angew. Chem. Int. Ed. 2006, 45, 4352 –4355
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83.4, 79.4 (C5H4Me), 35.2 (d, 1JCP = 10.5 Hz, Cy), 32.1, 31.8 (brs, Cy),
30.7 (s, tBu), 28.6, 28.5, 27.3 (s, Cy), 12.7 ppm (s, C5H4CH3), the signal
for the semibridging CO ligand was not observed; 11B{1H} NMR
(64 MHz, C6D6, 258C): 150 ppm (brs); 31P{1H} NMR (202 MHz,
C6D6, 258C): 27.1 ppm; IR (benzene): n˜ = 1811, 1770 cmÀ1
compounds is restricted to very few examples that exclusively
consist of homometallic frameworks of either Co[20] or, more
commonly, Ru centers.[21] A formal electron count (each of
the
fragments
{tBuB},
{Cy3PPd},[22]
and
{(h5-
C5H4Me)Mn(CO)2} contributes two electrons to the cluster
framework) is in agreement with a metallaborane that is two
electrons short of a closo species (that is, hypercloso). DFT
studies, however, suggest a more localized bonding picture.[23]
Natural bond orbital (NBO) calculations indicate a rather
Received: March 29, 2006
Published online: June 1, 2006
Keywords: boron · borylene complexes · cluster compounds ·
.
À
strong covalent Mn B bond (Wiberg bond index, WBI = 0.8)
manganese · palladium
À
and somewhat weaker Pd B interactions (WBI = 0.42 and
À
0.48). The Pd Pd interaction (WBI = 0.05), however, was
[1] H. Braunschweig, T. Wagner, Angew. Chem. 1995, 107, 904 – 905;
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calculated to be almost nonexistent. This description is
reflected by electron localization function (ELF) computa-
tions (Figure 3), which revealed two fused basins between the
[2] a) H. Braunschweig, D. Rais, Heteroat. Chem. 2005, 16, 566 –
571; b) S. Aldridge, D. Coombs, Coord. Chem. Rev. 2004, 248,
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Figure 3. ELF=0.6 plot for the model of complex 4. The ELF contribu-
tions of ligand atoms are omitted for clarity.
[4] H. Braunschweig, K. Radacki, D. Rais, F. Seeler, K. Uttinger, J.
Am. Chem. Soc. 2005, 127, 1386 – 1387.
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boron and manganese atoms that also affect both palladium
centers, thus representing two (3c, 2e) Mn-B-Pd bonds.
Furthermore, no basins indicative of a typical metallaborane
cluster were found over any triangular face of the {BMnPd2}
core.
This description is reminiscent of the bonding situation
found in metal-base-stabilized amino-[5] and metallobory-
lene[10] complexes. In sharp contrast to these {(Me3Si)2N}- and
{Cp*Fe(CO)2}-substituted borylenes, however, the presence
of the non-p-stabilizing tert-butyl group in 4 allows for the
unprecedented addition of two metal bases to the boron
center.
[10] H. Braunschweig, K. Radacki, D. Rais, F. Seeler, Angew. Chem.
2006, 118, 1087 – 1090; Angew. Chem. Int. Ed. 2006, 45, 1066 –
1069.
Experimental Section
All manipulations were conducted in an atmosphere of dry argon by
employing either standard Schlenk techniques or a glovebox.
4: [Pd(PCy3)2] (0.200 g, 0.300 mmol) was added to a solution of 2
(0.060 g, 0.134 mmol) in C6D6 (0.8 mL) at room temperature. The
course of the reaction was monitored by multinuclear NMR
spectroscopy. After three weeks the reaction was judged to be
complete. The solvent was removed in vacuo, and hexane was added
to the residue. Compound 4 was separated by fractional crystalliza-
tion and recrystallization from hexanes at À358C as red crystals
(0.035 g, 25% yield). 1HNMR (500 MHz, C 6D6, 258C, TMS): d =
4.59(m, 2H, C5H4CH3), 4.54 (m, 2H, C5H4CH3), 2.04 (s, 3H,
C5H4CH3), 1.65 (s, 9H, tBu), 2.15–1.15 ppm (m, 66H, Cy);
13C{1H} NMR (126 MHz, C6D6, 258C): d = 98.3 (ipso-C, C5H4CH3),
[11] H. Braunschweig, M. Müller, Chem. Ber. 1997, 130, 1295 – 1298.
[12] a) H. Braunschweig in Inorganic Chemical Highlights (Eds.: G.
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4th ed., Teubner, Wiesbaden, 2002.
[13] H. Braunschweig, B. Ganter, J. Organomet. Chem. 1997, 545,
163 – 167.
[14] The X-ray diffraction data for 3 and 4 were collected on a Stoe-
IPDS diffractometer with an image plate by using graphite-
monochromated MoKa radiation. The structures were solved by
using direct methods, expanded by Fourier techniques, and
refined with the SHELX software package (G. M. Sheldrick,
SHELX-97 Universität Göttingen, 1997) All non-hydrogen
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Chemie
atoms were refined anisotropically. Hydrogen atoms were placed
in idealized positions and included in the structure-factor
calculations. CCDC-602961 (3) and CCDC-602990 (4) contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_-
request/cif. Crystal data for 3: C18H19BMn2O4, Mr = 420.02, dark
red polyhedra, 0.5 ” 0.5 ” 0.4 mm3, monoclinic, space group P21/
n, a = 8.5604(9), b = 13.3225(10), c = 15.7316(17) Š, b =
95.861(13)8, V= 1784.7(3) Š3, Z = 4, 1calcd = 1.563 gcmÀ3, m =
1.433 mmÀ1
, F(000) = 856, T= 173(2) K, R(all data): R1 =
0.0267, wR2 = 0.0641, 2.608 < q < 28.018, 20339/4062 reflections
collected/unique, Rint = 0.0340, 229 parameters. Crystal data for
4: C48H82BMnO2P2Pd2, Mr = 1031.63, red prisms, 0.2 ” 0.2 ”
0.1 mm3, triclinic, space group P1, a = 10.7449(9), b =
¯
21.7062(16), c = 33.376(3) Š, a = 72.686(9), b = 82.471(10), g =
77.668(9)8, V= 7241.2(10) Š3, Z = 6, 1calcd = 1.419 gcmÀ3, m =
1.097 mmÀ1
, F(000) = 3228, T= 173(2) K, R(I>2s(I)): R1 =
0.0535, wR2 = 0.1218, 2.488 < q < 26.558, 58283/27614 reflections
collected/unique, Rint = 0.0741, 1536 parameters. Three mole-
cules were found in the independent part of the cell and differ
essentially only by rotation of the PCy3 groups; all bond lengths
and angles given in the text are for the same molecule.
[15] R. Littger, H. Nöth, M. Suter, Eur. J. Inorg. Chem. 2000, 1571 –
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[18] The analogous chloroborylene complex [(h5-C5H4Me)-
=
(OC)2Mn BCl] is believed to be a key intermediate in the
photochemically induced formation of the cluster [B2Cl2{(h5-
C5H4Me)Mn(CO)2}2].[16]
[19] H. Braunschweig, K. Radacki, D. Rais, G. R. Whittell, Angew.
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[21] R. Okamura, K. Tada, K. Matsubara, M. Oshima, H. Suzuki,
Organometallics 2001, 20, 4772 – 4774.
[22] Pd and Pt complexes are often stable with 16 electrons, and
corresponding fragments can release two more electrons to
bonding cluster MOs; see for example, C. E. Housecroft,
Boranes and Metallaboranes, 2nd ed., Ellis Horwood, New
York, 1994, p. 120.
[23] Electronic-structure calculations were performed with Gaus-
sian03 Rev.B.04 (M. J. Frisch et al., Gaussian, Inc., Wallingford
CT, 2004). The electron localization function (ELF) was
computed with the TopMoD program package (S. Noury, X.
Krokidis, F. Fuster, B. Silvi, TopMoD, Universitꢀ Pierre et Marie
Curie, 1997). The natural bond orbital (NBO) analysis was
performed with the NBO5.0G program (E. D. Glendening, J. K.
Badenhoop, A. E. Reed, J. E. Carpenter, J. A. Bohmann, C. M.
Morales, and F. Weinhold, Theoretical Institute, Univeristy of
Wisconsin, Madison, 2001). See the Supporting Information for
details.
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