Reversible intramolecular coupling of the terminal borylene and a carbonyl ligand of [Cp(CO)2Mn=B-tBu]

Article abstract of DOI:10.1002/anie.201207017

Cyclic complex 2 with bridging carbonyl ligands was synthesized from a facile and reversible intermolecular carbonyl-borylene ligand coupling reaction at room temperature. Complex 2 exhibits an unprecedented coordination mode for boron-metal complexes, which is also reflected in its remarkable ¹¹B NMR chemical shift of -57.2 ppm. Findings from spectroscopic, X-ray, and computational studies are presented, along with a proposed mechanism. Copyright

Full text of DOI:10.1002/anie.201207017

Angewandte
Chemie
DOI: 10.1002/anie.201207017
Reversible Coupling of DCO and DBR
Reversible Intramolecular Coupling of the Terminal Borylene and
= À
a Carbonyl Ligand of [Cp(CO)2Mn B tBu]**
Holger Braunschweig,* Krzysztof Radacki, Rong Shang, and Christopher W. Tate
Since the discovery of complexes containing carbon–metal
multiple bonds, this area of chemistry has developed at an
increasing pace.[1,2] Metal carbyne complexes, following their
elder siblings metal carbenes,[3] play a versatile role in many
chemical processes, such as alkyne metathesis or polymeri-
zation of internal acetylenes.[3b,h,4] One particularly interesting
reaction of Group 6 metal carbyne complexes, first reported
by Kreissl et al.,[5] involves an intramolecular coupling
electronically
unique
borylene
derivative
[Cp-
(Mes*NC)(OC)Mn{C(O)B(tBu)(CNMes*)}] (2, Mes* =
2,4,6-tri(tert-butyl)phenyl). The formation of this complex is,
to the best of our knowledge, the first structurally charac-
terized example of a reversible, intramolecular CO–BR
coupling. Since Group 7 metal terminal borylene complexes
such as complex 1 are isoelectronic to Group 6 metal carbyne
À
complexes (Figure 1A,B), and the borylene ligand (DB R) is
ꢀ
between the carbido ligand ( CR) and a neighboring car-
bonyl ligand to form a ketenyl ligand (Scheme 1). This
Scheme 1. Metal carbyne–carbonyl coupling reactions.
fundamentally interesting and synthetically important
carbon–carbon bond formation by intramolecular CO–CR
coupling, owing to its similarities to the key step involved in
the Monsanto process, has been explored by many research
groups, mainly on Group 6 carbyne complexes.[1f,i,5b,6] It was
also extended to heavier thio- and seleno-carbonyl ana-
logues.[6c,7]
Metal borylene chemistry, although being much
younger,[8] has demonstrated intriguing reactivity.[9] In partic-
=
ular, the terminal Group 7 metal borylene complexes [LnM
À
B R] have been shown to undergo [2+2] cycloaddition with
Figure 1. A,B) Bonding descriptions of isoelectronic borylene and car-
byne complexes. C) Isolobal borylene and carbonyl ligands.
unsaturated polar organic substrates. In the case of
5
= À
[Cp(CO)2Mn B tBu] (1, Cp = h -C5H5) and, for example,
benzophenone, the resulting cycloaddition product is prone to
subsequent cycloreversion to afford otherwise synthetically
challenging carbene complexes.[10] This metathesis-type reac-
tivity mirrors that of carbyne complexes. Further exploration
of 1 led us to the results presented herein, which include the
successful generation and isolation of an unprecedented and
isolobal to a carbonyl ligand (Figure 1C), this observed
heterocoupling between a borylene and a carbonyl ligand is of
great significance as it not only relates closely to the carbyne–
CO coupling reactions described above, but also closes a gap
between the intramolecular CO–CO homocoupling reac-
tions[11] and the recently disclosed borylene–borylene homo-
coupling reaction.[12]
[*] Prof. Dr. H. Braunschweig, Dr. K. Radacki, Dr. R. Shang,
Dr. C. W. Tate
The formation of boron–carbon bonds has been a topic of
intense interest to synthetic chemists for its fundamental and
practical value in metal-assisted catalytic cross-coupling
reactions.[13] However, examples that demonstrate a reversible
Institut fꢀr Anorganische Chemie
Julius-Maximilians-Universitꢁt Wꢀrzburg
Am Hubland, 97074 Wꢀrzburg (Germany)
E-mail: h.braunschweig@uni-wuerzburg.de
formation of B C bonds remain rare.[14] Apart from this work,
À
there are only two structurally characterized metal complexes
[**] We are grateful to the European Research Council (Advanced
Investigator Grant to H.B.) for financial support.
that feature M C(A) B (A = O, S) connections,[3a,15] both of
which display coordination modes that differ greatly from this
study.
À
À
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207017.
Angew. Chem. Int. Ed. 2013, 52, 729 –733
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
729

.
Angewandte
Communications
The terminal borylene complex 1 reacted with two
equivalents of supermesityl isonitrile (CNMes*) in non-
coordinating solvents (n-pentane, n-hexane, benzene, or
toluene) at room temperature to form a dark brown solution,
from which the complex [Cp(Mes*NC)(OC)Mn{C(O)B-
(tBu)(CNMes*)}] (2) precipitated out as a brown crystalline
solid quantitatively within minutes. Subsequent treatment of
2 with two equivalents of tris(pentafluorophenyl)borane,
which serves as a strong Lewis acid, led to quantitative
reformation of the terminal borylene 1. The reaction of 1 with
one equivalent of the isonitrile yielded a mixture of the
starting material 1 and the product 2 in a 1:1 ratio. Similarly,
addition of one equivalent of tris(pentafluorophenyl)borane
to a solution of 2 led to a 50% conversion of 2 into
1 (Scheme 2). No intermediate was detected in either the
forward or reverse reactions by 11B or 1H NMR spectroscopy.
These observations implied that both isonitrile ligands are
essential for formation and stabilization of complex 2.
optimized geometry are consistent with the experimentally
observed values. Therefore, the structural features of 2 will be
discussed based on calculated values.[17]
The energy-minimized geometry of 2 revealed two differ-
ently bonded bridging carbonyl ligands, which was also
reflected by experimental IR data and natural bond order
(NBO) analysis of model complexes of 2. The [C2O2]
carbonyl ligand (Figure 2) exhibits strong covalent interac-
tions with both Mn1 and B1 atoms, which involve the p*
antibonding orbital of CO, a dz2 orbital of the manganese
center, and the p orbital of the boron center (Figure 3). The
Figure 3. A) Depiction of the HOMOÀ8 of the model complex [Cp-
(PhNC)(OC)Mn{C(O)B(Me)(CNPh)}] (2H), showing the p-type bond-
ing interactions of the bridging carbonyl with the metal and boron
centers. B) Calculated Wiberg bond indexes and natural charges of the
model complex.
Scheme 2. Reversible coupling of the CO and B(tBu) ligands.
À
Single-crystal X-ray crystallography revealed the unusual
molecular structure of 2 as a cyclic complex with one bridging
and one semi-bridging carbonyl ligand between the Mn and B
Mn1 C2 separation of 1.938 ꢀ in 1 is noticeably longer than
those observed for other related bridging carbonyl-containing
derivatives (1.806–1.844 ꢀ).[18] The B1 C2 bond length is
À
atoms (Figure 2).[16] The coordination mode of M C(O) E
1.586 ꢀ, which lies in the range of a normal B C single
À
À
À
bond.[19] These parameters imply a somewhat stronger
interaction between the carbonyl [C2O2] and the boron
moiety than the manganese metal fragment, which was also
reflected by the Wiberg bond index (WBI) analysis. The
À
Mn1 C2 interaction has a WBI of 0.57, which is significantly
À
lower than that of B1 C2 (0.98). On the other hand, the
carbonyl [C1O1] has a quite different bonding situation. The
À
À
bond lengths of Mn1 C1 (1.784 ꢀ) and C1 O1 (1.168 ꢀ) are
not significantly different to those observed for a conventional
terminal carbonyl ligand. However, the WBI analysis
revealed a small but non-negligible bond order of 0.21
between C1 and B1. An interaction of similar strength was
found between Mn1 and B1, with a WBI of 0.24.
Figure 2. Molecular structure of 2. Ellipsoids are set at 50% proba-
bility. The ellipsoids of supermesityl groups are omitted for clarity.
Selected bond lengths [ꢂ] and angles [8] from B3LYP/Def2-SVP
calculations: Mn1–B1 2.297, Mn1–C1 1.784, Mn1–C2 1.938, Mn1–C12
1.847, B1–C1 2.229, B1–C2 1.586, B1–C8 1.662, B1–C12A 1.511; Mn1-
C1-O1 169.4, Mn1-C2-O2 137.9.
The bonding situation of the boron atom in 2 is rather
difficult to elucidate in terms of interactions with five
À
À
neighboring atoms. The covalent bonds B1 C2 and B1 C8
À
and the dative bond B1 C12A are more conventional, which
(M = transition metals, E = Group 13 element) was, to the
best of our knowledge, unknown prior to this work. We were
able to obtain crystals that contained benzene, toluene, or no
solvent molecules in the unit cell. However, all three
structures show the same type of disorder: an inversion
center in the middle of a molecule leads to overlap of
a {CpMnCO} moiety with {tBuBCO} that causes a problem
with deconvolution of atom positions, especially for the Mn/B
pair and both carbonyl groups. For this reason, computational
studies using density functional theory methods were per-
formed. The IR and NMR calculations of 2 based on
is also evident from the NBO analysis described above. The
third valence electron of boron must be pairing with one from
the metal center and forming a more delocalized interaction
with both Mn1 and C1. This more diffused, three-center–two-
electron bonding mode is also reflected by a remarkable
11B NMR chemical shift of À57 ppm (Table 1), which is
shifted by more than 200 ppm upfield from its precursor
borylene complex (1, 145 ppm).[18b] It lies considerablyfurther
upfield than those observed for not only the isonitrile
coordinated boron species, BR3(CNR’),[20] but indeed most
commonly observed tetracoordinate boron compounds
730
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 729 –733

Angewandte
Chemie
Table 1: Selected experimental and calculated spectroscopic data of
complex 2.
together suggest that the boron in 2 is somewhat metallic in
nature, reflecting its high electropositivity.
À1 [a]
NMR (d, ppm)[b]
C1 C2
The mechanism of the formation of 2 calls for discussion.
The initial nucleophilic attack could happen either at the
manganese or the boron center. Indeed, both cases were
observed for the isoelectronic carbyne chemistry (Sche-
me 3).[6b,h,24] In this study, computational calculations support
the proposal that initially one isonitrile molecule approaches
˜
IR (n, cm
)
C1O1 C2O2 C12N2 C12AN2A
B1
C12
Expt. 1903 1704 2136
Calc. 1892 1745 2117
2056
2061
À57.2
260.8
193.1
À58.3 241.1 289.7 207.0
[a] Experimental values were obtained from a toluene solution of 2. The
calculated with B3LYP/Def2-SVP values were scaled with a factor of 0.96.
[b] Collected with a Bruker 500 FT-NMR spectrometer at À308C from
a [D8]toluene solution.
(BR3L, BR4À, and BH4 species, the 11B chemical shifts of
À
which lie in the range of 20 to À45 ppm);[21] it falls in the range
expected for boron cluster compounds. The natural chemical
shift (NCS) decomposition of shielding tensors computed
with GIAO method shows that in 1, the shielding of the boron
core (score = 166 ppm) is essentially reduced by anti-shielding
À
À
contributions from the Mn B bond (sMn-B = À137) and the B
CtBu bond (sB-C = À56 ppm), which lead to total shielding of
À44 ppm (re-calculated for the BF3·Et2O standard: d =
151 ppm). As known from earlier IGLO and NCS studies,
core contributions are transferable between different com-
pounds,[22] and in 2, score has the same value of 166 ppm.
However here, in contrast to its precursor 1, score is little
Scheme 3. Selected examples of nucleophilic addition to metal carbyne
complexes. A) Fischer’s reaction of Group 7 metal carbynes with
isonitrile. B) Filippou’s selective addition of the trimethylphosphine
5
5
ꢀ
ligand to [L(OC)2Cr CPh] (L=h -C5H5, h -C5Me5).
affected by negligible contributions from sMn-B* = 7 and sB-C
=
À8 ppm, thus resulting in a total shielding of 165 ppm (re-
calculated for the BF3·Et2O standard: d = À58 ppm).
The 1H and 13C{1H} NMR spectra of 2 are largely
unremarkable, showing two sets of signals for the two
chemically different isonitrile ligands. The observed proton
integrals are consistent with the confirmed structure of 2. One
sharp 13C NMR signal was observed at 260 ppm, attributed to
the carbonyl carbon nucleus, which is shifted noticeably
downfield from the borylene precursor 1 (224 ppm). The
observation of one single CO environment suggests a fluxional
process between the two CO ligands that bridge the metal and
boron centers. The low-lying transition state of Cs symmetry
with DE = 2 kJmolÀ1 supports that hypothesis. Another sharp
singlet was observed at 193 ppm, which was assigned to the
metal-coordinated terminal isonitrile carbon nucleus. The
boron-coordinated terminal isonitrile carbon nucleus was not
observed.
the boron atom in 1. The low-lying transition state (DE =
19.8 kJmolÀ1) regroups to the marginally bent trigonal
geometry intermediate (Int; ]Mn-B-C 1428). This first step
is slightly exothermic (DE = À77.7 kJmolÀ1). Subsequently,
a second isonitrile molecule attacks the manganese atom. The
higher-lying transition state (DE = 74.0 JmolÀ1) rearranges
then to 2H (DE = À204.5 kJmolÀ1). The total enthalpy change
for this reaction is À188.3 kJmolÀ1 (Figure 4).
In conclusion, a cyclic complex 2 with bridging carbonyl
ligands was synthesized from an intermolecular carbonyl–
borylene ligand coupling reaction involving the borylene
complex 1 and two equivalents of supermesitylisonitrile.
Upon treatment with a strong Lewis acid, complex 2 loses
The IR spectrum of 2 showed two absorption bands at
2135 (medium) and 2056 cmÀ1 (strong) corresponding to the
CN stretches from the two isonitrile ligands. Two less-intense
bands were observed at 1903 and 1704 cmÀ1, corresponding to
À
À
the semi-bridging (C1 O1) and the bridging (C2 O2) car-
bonyl ligands between the Mn and B atoms, respectively.
These values are significantly lower than those observed for
the precursor borylene complex 1 (1968 and 1912 cmÀ1) and
its bridging CO-containing derivatives,[18] which again implies
an enhanced p-back-donation from the MnÀB core. The
À
bridging carbonyl (C2 O2) may be viewed as analogous to
those bridging between two transition metals. Furthermore,
the low energetic barrier to the fluxional process between the
two CO ligands of 2 resembles the exchange of bridging and
semi-bridging CO ligands via low energy concerted pathways
of complexes containing metal–metal bonds.[23] These points
Figure 4. Energy profile for the reaction of 1 with two equivalents of
PhNC calculated at the B3LYP/Def2-SVP level.
Angew. Chem. Int. Ed. 2013, 52, 729 –733
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
731

.
Angewandte
Communications
metallics 1986, 5, 2481 – 2488; l) J. B. Sheridan, G. L. Geoffroy,
A. L. Rheingold, Organometallics 1986, 5, 1514 – 1515; m) S. J.
Holmes, R. R. Schrock, M. R. Churchill, H. J. Wasserman,
Organometallics 1984, 3, 476 – 484; n) E. O. Fischer, A. C.
Filippou, H. G. Alt, K. Ackermann, J. Organomet. Chem. 1983,
254, C21 – C23.
both isonitrile ligands and reverts back to 1 quantitatively (as
observed by NMR spectroscopy). Complex 2 exhibits unique
structural and electronic properties, which may result in new
reactivity that is of both fundamental and synthetic impor-
tance.
[7] a) I. Baxter, A. F. Hill, J. M. Malget, A. J. P. White, D. J.
Williams, Chem. Commun. 1997, 2049 – 2050; b) a. F. Hill, J. M.
Malget, Chem. Commun. 1996, 1177 – 1178.
Received: August 30, 2012
Published online: November 20, 2012
[8] H. Braunschweig, T. Wagner, Angew. Chem. 1995, 107, 904 – 905;
Angew. Chem. Int. Ed. Engl. 1995, 34, 825 – 826.
À
Keywords: B C bond formation · carbonyl–borylene coupling ·
heterocoupling · reversible intramolecular ligand coupling ·
borylene complexes
.
[9] a) S. Aldridge, D. L. Coombs, Coord. Chem. Rev. 2004, 248, 535 –
559; b) C. E. Anderson, H. Braunschweig, R. D. Dewhurst,
Organometallics 2008, 27, 6381 – 6389; c) H. Braunschweig, C.
Kollann, F. Seeler, Struct. Bonding (Berlin) 2008, 130, 1 – 27;
d) H. Braunschweig, R. D. Dewhurst, A. Schneider, Chem. Rev.
2010, 110, 3924 – 3957.
[10] H. Braunschweig, M. Burzler, K. Radacki, F. Seeler, Angew.
Chem. 2007, 119, 8217 – 8219; Angew. Chem. Int. Ed. 2007, 46,
8071 – 8073.
[11] a) D. H. Berry, J. E. Bercaw, A. J. Jircitano, K. B. Mertes, J. Am.
Chem. Soc. 1982, 104, 4712 – 4715; b) E. M. Carnahan, J. D.
Protasiewicz, S. J. Lippard, Acc. Chem. Res. 1993, 26, 90 – 97;
c) R. N. Vrtis, S. G. Bott, S. J. Lippard, Organometallics 1993, 12,
592; d) R. N. Vrtis, S. G. Bott, S. J. Lippard, Organometallics
1992, 11, 270 – 277; e) R. N. Vrtis, S. G. Bott, R. L. Rardin, S. J.
Lippard, Organometallics 1991, 10, 1364 – 1373; f) P. A. Bian-
coni, S. J. Lippard, C. P. Rao, R. N. Vrtis (Massachusetts Institute
of Technology, USA). Application: US, 1991, pp. 17; g) R. N.
Vrtis, S. J. Lippard, Isr. J. Chem. 1990, 30, 331 – 341; h) R. N.
Vrtis, S. Liu, C. P. Rao, S. G. Bott, S. J. Lippard, Organometallics
1991, 10, 275 – 285; i) R. N. Vrtis, C. P. Rao, S. G. Bott, S. J.
Lippard, J. Am. Chem. Soc. 1988, 110, 7564 – 7566; j) R. N. Vrtis,
C. P. Rao, S. Warner, S. J. Lippard, J. Am. Chem. Soc. 1988, 110,
2669 – 2670; k) P. A. Bianconi, R. N. Vrtis, C. P. Rao, I. D.
Williams, M. P. Engeler, S. J. Lippard, Organometallics 1987, 6,
1968 – 1977.
[1] a) L. M. Caldwell, Adv. Organomet. Chem. 2008, 56, 1 – 94;
b) T. D. McGrath, F. G. A. Stone, Adv. Organomet. Chem. 2005,
53, 1 – 40; c) M. I. Bruce, P. J. Low, Adv. Organomet. Chem. 2004,
50, 179 – 444; d) P. J. Brothers, Adv. Organomet. Chem. 2000, 46,
223 – 321; e) S. A. Brew, F. G. A. Stone, Adv. Organomet. Chem.
1993, 35, 135 – 186; f) A. Mayr, H. Hoffmeister, Adv. Organomet.
Chem. 1991, 32, 227 – 324; g) F. G. A. Stone, Adv. Organomet.
Chem. 1990, 31, 53 – 89; h) W. E. Buhro, M. H. Chisholm, Adv.
Organomet. Chem. 1987, 27, 311 – 369; i) H. P. Kim, R. J.
Angelici, Adv. Organomet. Chem. 1987, 27, 51 – 111; j) M. A.
Gallop, W. R. Roper, Adv. Organomet. Chem. 1986, 25, 121 –
198; k) E. O. Fischer, Adv. Organomet. Chem. 1976, 14, 1 – 32.
[2] See the Supporting Information for computational details.
[3] a) R. Choukroun, C. Lorber, C. Lepetit, B. Donnadieu, Organo-
metallics 2003, 22, 1995 – 1997; b) R. R. Schrock, C. Czekelius,
Adv. Synth. Catal. 2007, 349, 55 – 77; c) R. H. Grubbs, T. M.
Trnka, Ruthenium Org. Synth. 2004, 153 – 177; d) R. H. Grubbs,
D. M. Lynn, Aqueous-Phase Organomet. Catal., 2nd ed., 2004,
550 – 566; e) R. H. Grubbs, T. M. Trnka, M. S. Sanford, Curr.
Methods Inorg. Chem. 2003, 3, 187 – 231; f) B. C. Gates, Angew.
Chem. 1993, 105, 240 – 241; Angew. Chem. Int. Ed. Engl. 1993,
32, 228 – 229; g) T. M. Trnka, R. H. Grubbs, Acc. Chem. Res.
2001, 34, 18 – 29; h) R. R. Schrock, Acc. Chem. Res. 1986, 19,
342 – 348; i) P. Jean-Louis Hꢁrisson, Y. Chauvin, Makromol.
Chem. 1971, 141, 161 – 176.
[4] a) X. Wu, M. Tamm, Beilstein J. Org. Chem. 2011, 7, 82 – 93;
b) J. M. Blackwell, J. S. Figueroa, F. H. Stephens, C. C. Cummins,
Organometallics 2003, 22, 3351 – 3353.
[5] a) F. R. Kreissl, K. Eberl, W. ꢂdelhoven, Chem. Ber. 1977, 110,
3782 – 3791; b) W. J. Sieber, D. Neugebauer, F. R. Kreissl, Z.
Naturforsch. B 1985, 40, 1500 – 1503; c) W. J. Sieber, K. Eberl, M.
Wolfgruber, F. R. Kreissl, Z. Naturforsch. B 1983, 38, 1159 –
1160; d) F. R. Kreissl, H. Keller, W. Schꢃtt, J. Organomet.
Chem. 1992, 441, 75 – 80.
[6] a) A. C. Filippou, Org. Synth. Organomet. Proc. Symp. 5th 1997,
97 – 119; b) A. C. Filippou, D. Wossner, G. Kociok-Kohn, I. Hinz,
J. Organomet. Chem. 1997, 541, 333 – 343; c) A. F. Hill, J. M.
Malget, A. J. P. White, D. J. Williams, Chem. Commun. 1996,
721 – 722; d) E. O. Fischer, C. Apostolidis, E. Dornberger, A. C.
Filippou, B. Kanellakopulos, B. Lungwitz, J. Mꢃller, B.
Powietzka, J. Rebizant, W. Roth, Z. Naturforsch. B 1995, 50,
1382 – 1395; e) T.-Y. Lee, A. Mayr, J. Am. Chem. Soc. 1994, 116,
10300 – 10301; f) A. Mayr, R. T. Chang, T.-Y. Lee, O. K. Cheung,
M. A. Kjelsberg, G. A. McDermott, E. D. Van, J. Organomet.
Chem. 1994, 479, 47 – 53; g) K. A. Belsky, M. F. Asaro, S. Y.
Chen, A. Mayr, Organometallics 1992, 11, 1926 – 1936; h) A.
Mayr, S. M. Holmes, C. M. Bastos, Organometallics 1992, 11,
4358 – 4362; i) P. K. Baker, G. K. Barker, D. S. Gill, M. Green,
A. G. Orpen, I. D. Williams, A. J. Welch, J. Chem. Soc. Dalton
Trans. 1989, 1321 – 1323; j) J. B. Sheridan, D. B. Pourreau, G. L.
Geoffroy, A. L. Rheingold, Organometallics 1988, 7, 289 – 294;
k) H. P. Kim, S. Kim, R. A. Jacobson, R. J. Angelici, Organo-
[12] H. Braunschweig, Q. Ye, A. Vargas, R. D. Dewhurst, K. Radacki,
A. Damme, Nat. Chem. 2012, 4, 563 – 567.
[13] N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457 – 2483.
[14] G. Schmid, W. Meyer-Zaika, R. Bçse, N. Augart, Angew. Chem.
1988, 100, 956 – 957; Angew. Chem. Int. Ed. Engl. 1988, 27, 952 –
953.
[15] I. R. Crossley, A. F. Hill, A. C. Willis, Organometallics 2007, 26,
3891 – 3895.
[16] CCDC 898073, 898074, and 898075 contain the supplementary
crystallographic data for Compound 2. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[17] Geometry optimization and spectroscopic data were calculated
for complex 2. Calculations on orbital analyses and mechanistic
studies were carried out using smaller model compounds. See the
Supporting information for details on computations.
[18] a) H. Braunschweig, M. Burzler, R. D. Dewhurst, K. Radacki, F.
Seeler, Z. Anorg. Allg. Chem. 2008, 634, 1875 – 1879; b) H.
Braunschweig, M. Burzler, T. Kupfer, K. Radacki, F. Seeler,
Angew. Chem. 2007, 119, 7932 – 7934; Angew. Chem. Int. Ed.
2007, 46, 7785 – 7787.
[19] Searches were conducted on The Cambridge Structural Data-
base (CSD).
[20] a) C. Klçfkorn, M. Schmidt, T. Spaniol, T. Wagner, O. Costisor, P.
Paetzold, Chem. Ber. 1995, 128, 1037 – 1043; b) H. Jacobsen, H.
Berke, S. Dçring, G. Kehr, G. Erker, R. Frçhlich, O. Meyer,
Organometallics 1999, 18, 1724 – 1735; c) C. M. Mçmming, G.
Kehr, B. Wibbeling, R. Frçhlich, G. Erker, Dalton Trans. 2010,
39, 7556 – 7564; d) M. Tamm, T. Lꢃgger, F. E. Hahn, Organo-
metallics 1996, 15, 1251 – 1256.
732
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 729 –733

Angewandte
Chemie
[21] a) S. Hermanek, Chem. Rev. 1992, 92, 325 – 362; b) H. Nçth,
NMR, 14: Nuclear Magnetic Resonance Spectroscopy of Boron
Compounds, Springer-Verlag, Berlin, Heidelberg, New York,
1978.
[23] F. A. Cotton, Prog. Inorg. Chem. 1976, 21, 1 – 28.
[24] a) E. O. Fischer, W. Schambeck, F. R. Kreissl, J. Organomet.
Chem. 1979, 169, C27 – C30; b) E. O. Fischer, W. Schambeck, J.
Organomet. Chem. 1980, 201, 311 – 318.
[22] a) J. A. Bohmann, F. Weinhold, T. C. Farrar, J. Chem. Phys. 1997,
107, 1173 – 1184; b) U. Fleischer, Deuterium and Shift Calcula-
tion, Springer, Berlin, 1991.
Angew. Chem. Int. Ed. 2013, 52, 729 –733
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
733