Synthesis and reactivity of semibridging borylene complexes

Article abstract of DOI:10.1021/om0605480

Reaction of the terminal borylene complexes [(OC)₅M=B= N(SiMe₃)₂] (M = Cr, W) with [M′(PCy₃)] (M′ = Pd, Pt) at room temperature led to spontaneous formation of the heterodinuclear complexes [(OC)₄M-(μ-CO){μ-BN(SiMe ₃)₂}M′(PCy₃)₂] (M = Cr, M′ = Pd; M = W, M′ = Pd; M = Cr, M′ = Pt) with concomitant liberation of PCy₃. X-ray diffraction studies revealed that the borylene ligand adopts a semibridging position between the two different metal centers, thus establishing a new coordination mode for borylenes and providing further experimental evidence for the theoretically predicted close relationship between BR and CO. After prolonged reaction times the liberated phosphine substitutes the carbonyl group in trans-position to the borylene moiety, forming complexes of the type [(Cy₃P)(OC)₃M(μ-CO)-{μ- BN(SiMe₃)₂}M′(PCy₃)] (M = Cr, M′ = Pt; M = W, M′ = Pt; M = W, M′ = Pd). Heating or photolytic activation of [(OC)₄M(μ-CO){μ-BN(SiMe₃) ₂}Pd(PCy₃)] afforded the terminal borylene complexes trans-[(Cy₃P)(OC)₄M=BN(SiMe₃)₂], which were fully characterized in the case of M = Cr. Structural data of trans-[(Cy₃P)(OC)₄Cr=BN(SiMe₃)₂] confirm the presence of an enhanced Cr-B π-back-donation imposed by the phosphine ligand in trans-position.

Full text of DOI:10.1021/om0605480

Organometallics 2006, 25, 5159-5164
5159
Synthesis and Reactivity of Semibridging Borylene Complexes
Holger Braunschweig,* Krzysztof Radacki, Daniela Rais, and Katharina Uttinger
Institut fu¨r Anorganische Chemie, Bayerische Julius-Maximilians-UniVersita¨t Wu¨rzburg, Am Hubland,
97074 Wu¨rzburg, Germany
ReceiVed June 23, 2006
Reaction of the terminal borylene complexes [(OC)5MdBdN(SiMe3)2] (M ) Cr, W) with [M′(PCy3)]
(M′ ) Pd, Pt) at room temperature led to spontaneous formation of the heterodinuclear complexes [(OC)4M-
(µ-CO){µ-BN(SiMe3)2}M′(PCy3)2] (M ) Cr, M′ ) Pd; M ) W, M′ ) Pd; M ) Cr, M′ ) Pt) with
concomitant liberation of PCy3. X-ray diffraction studies revealed that the borylene ligand adopts a
semibridging position between the two different metal centers, thus establishing a new coordination mode
for borylenes and providing further experimental evidence for the theoretically predicted close relationship
between BR and CO. After prolonged reaction times the liberated phosphine substitutes the carbonyl
group in trans-position to the borylene moiety, forming complexes of the type [(Cy3P)(OC)3M(µ-CO)-
{µ-BN(SiMe3)2}M′(PCy3)] (M ) Cr, M′ ) Pt; M ) W, M′ ) Pt; M ) W, M′ ) Pd). Heating or photolytic
activation of [(OC)4M(µ-CO){µ-BN(SiMe3)2}Pd(PCy3)] afforded the terminal borylene complexes trans-
[(Cy3P)(OC)4MdBN(SiMe3)2], which were fully characterized in the case of M ) Cr. Structural data of
trans-[(Cy3P)(OC)4CrdBN(SiMe3)2] confirm the presence of an enhanced Cr-B π-back-donation imposed
by the phosphine ligand in trans-position.
however, borylenes are highly reactive, transient species. They
can be generated and observed either at low temperatures in
matrices8 or at high temperatures in the gas phase,9 but cannot
be handled under ambient conditions. In 1995 we obtained
[{(η5-C5H5)Mn(CO)2}2{µ-BNMe2}] (1) as the first complex in
which a transient borylene was generated and stabilized in the
coordination sphere of a transition metal,10 and subsequent theor-
etical studies revealed that the σ-donating and π-accepting
properties of borylenes even exceed those of the isoelectronic
CO, thus rationalizing the thermodynamically very stable metal-
borylene linkage.11 In recent years we12 and others13 reported
on a variety of bridging and terminal borylene complexes,
Introduction
Carbonyl complexes constitute a pivotal class of organometal-
lics, which is characterized by an immense structural diversity
and potential for applications. Transition-metal carbonyl com-
plexes are common reagents in organometallic synthesis and
are widely used as homogeneous and heterogeneous catalysts,
for example in hydroformylations and the Monsanto and
Fischer-Tropsch processes.1 The [TM]-CO linkage is a text-
book example of the synergic bond between a ligand with good
σ-donor and π-acceptor abilities and a metal center. This favor-
able metal-ligand interaction gives rise to an ample range of
coordination modes for CO, including terminal, bridging, semi-
bridging, and triply bridging. While CO can be replaced by
many other ligands, the number of neutral isolobal substituents
is somewhat restricted to molecules such as N2,2 CNR,3 CS,4
or CCR2.5 Recent progress in the area of transition-metal
complexes of boron6 has put borylenes, B-R, in the focus as
another class of ligands closely related to the ubiquitous CO.7
In contrast to the metastable and readily available carbon oxide,
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* Corresponding author. Fax: (+49)931-888-4623. Tel: (+49)931-888-
5260. E-mail: h.braunschweig@mail.uni-wuerzburg.de.
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10.1021/om0605480 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 09/08/2006

5160 Organometallics, Vol. 25, No. 21, 2006
Braunschweig et al.
Chart 1. Selected Terminal and Bridging Borylene
Complexes
Scheme 1. Synthesis of Semibridging Borylene Complexes
6, 7, and 10
including the species [{(η5-C5H4Me)Fe(CO)}2(µ-CO){µ-BN-
(SiMe3)2}](2),12a [(OC)5MdBdN(SiMe3)2] (M ) Cr,12b 3; W,12b
4), and [(η5-C5Me5)(OC)2Fe-BdCr(CO)5] (5).12eIntroductory
reactivity studies on bridging14 and terminal borylene complexes
revealed the propensity of the latter to transfer the B-R ligand
to organic15 substrates and transition metals, thus serving as a
unique source of the elusive borylenes under ambient conditions.
The successful intermetal-borylene transfer under thermal16 or,
more commonly, photochemical conditions17 and the pronounced
affinity of metal-coordinated boryls12g,18 and borylenes19 toward
low-valent species of the type [ML2] (M ) Pd, Pt; L ) PR3)
prompted us to investigate reactions of complexes of the type
[(OC)5MdBdN(SiMe3)2] (M ) Cr, 3; W, 4) with suitable Pd0
species. Recently, we communicated the formation and structure
of the first semibridging borylene complexes [(OC)4M(µ-CO)-
{µ-BN(SiMe3)2}Pd(PCy3)2] (M ) Cr, 6; W, 7) as an, at its time,
unprecedented example for the reaction of a terminal borylene
complex under thermal conditions.20
the photochemically induced borylene transfer to late transition
metals, we choose electron-rich, but coordinatively unsaturated
species [M′(PCy3)2] (M′ ) Pd,21a 8; Pt,21b 9) as potentially useful
acceptors for B-R. Interestingly, multinuclear NMR spectra
indicated a spontaneous reaction between [(OC)5MdBd
N(SiMe3)2] (M ) Cr,12b,17a 3; W,12b 4) and 8 and 9 in C6D6 at
ambient temperature, without irradiation, furnishing PCy3 and
the new complexes [(OC)4M(µ-CO){µ-BN(SiMe3)2}M′(PCy3)]
(M ) Cr, M′ ) Pd, 6; M ) W, M′ ) Pd, 7; M ) Cr, M′) Pt,
10) (Scheme 1).
The 1H NMR spectra show one singlet for the trimethylsilyl
group around δ ) 0.42, which is deshielded with respect to
that of the borylene precursors (δ ) 0.12-0.14).12b,17a A sharp
singlet at δ ) 10.0 in the 31P{1H} NMR spectra confirms the
liberation of one PCy3 ligand from the Pd and Pt precursors,
respectively,22 and in addition, one signal is found for the metal-
coordinated phosphine group (δ ) 34.2 (6), 41.9 (7), 70.0 (10)).
The new compounds display a broad singlet in the 11B{1H}
NMR spectra (δ ) 100 (6), 97 (7), 98 (10)), which is slightly
downfield-shifted with respect to that of the corresponding
precursors [(OC)5MdBdN(SiMe3)2] (δ ) 92.3 (3), 86.6
(4)),12b,17a thus indicating the presence of a bridging borylene
ligand. To obtain the pure compounds 6, 7, and 10, it is
necessary to restrict the time of reaction to a minimum, as the
presence of liberated PCy3 gives rise to subsequent transforma-
tions, which are discussed in detail in the following section.
The exact constitution of the new complexes could be
determined by performing a single-crystal X-ray diffraction
study of 6.20 Crystals of 6 were obtained by cooling a
concentrated toluene solution to -35 °C; the complex crystal-
lizes in the monoclinic space group P21/n.In the molecule, the
heteronuclear fragments [(OC)4Cr] and [PdPCy3] are linked by
a borylene and a CO group, which both adopt a semibridging
disposition (vide infra). The Cr1-B1 bond (2.0842(18) Å) is
approximately 9 pm longer than in the corresponding terminal
borylene complex (1.996(6) Å (3)),17a which is in agreement
with the increased coordination number of the boron center in
the former. Likewise, the Pd1-B1 bond distance (2.0425(18)
Å) corresponds to Pd-B bond distances commonly found in
complexes featuring three coordinated boron atoms linked to
palladium centers (2.006(9) to 2.077(6) Å).23 The B1-N1 bond
(1.377(2) Å) is only insignificantly longer than in the corre-
sponding borylene complex (1.353(6) Å (3)),17a thus proving
significant double-bond character of the boron-nitrogen linkage.
Here we report full details on the synthesis and structural
characterization of a representative range of novel complexes
featuring semibridging borylene ligands. Reactivity studies
carried out with these species gave access to the first terminal
borylene complexes that display a R3P ligand in trans-position
to the borylene, thus providing valuable experimental informa-
tion about the metal-borylene bonding.
Results and Discussion
Synthesis and Characterization of Heterodinuclear Bo-
rylene Complexes with One Phosphine Ligand. To extend
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Synthesis of Semibridging Borylene Complexes
Organometallics, Vol. 25, No. 21, 2006 5161
Figure 2. Molecular structure of 11.
Synthesis and Characterization of Heterodinuclear Bo-
rylene Complexes with Two Phosphine Ligands. To complete
the series of new compounds with respect to combinations of
W with Pt, [(OC)5WdBdN(SiMe3)2] (5) was reacted with [Pt-
(PCy3)2] (9) in C6D6 under identical conditions. Surprisingly,
however, multinuclear NMR data proved the spontaneous
formation of a different type of product, namely, [(Cy3P)-
Figure 1. Molecular structure of 6.
The plane of the bis(trimethylsilyl)amino substituent is orientated
almost perpendicular to the plane of the metals and the boron
atom (Si1-N1-B1-Pd1: 91.12(15)° (6)). This particular
arrangement differs from other bridged borylene complexes, e.g.,
[{(η5-C5H5)Mn(CO)2}2{µ-BNMe2}]10a (1) or [{(η5-C5H4Me)-
Fe(CO)}2(µ-CO){µ-BN(SiMe3)2}] (2),12a which comprise for-
mally sp2-hybridized boron centers. In 1, the plane of the
dimethylamino substituent is almost coplanar (8(3)°) with
respect to the Mn2B plane, and a B-N bond length of 1.39(1)
Å indicates a strong B-N π-interaction.10a For 2, however, the
Si2B plane and the Fe2B plane are twisted by 53(1)°, and the
increased B1-N1 distance of 1.412(4) Å is in agreement with
a less effective π-interaction between the nitrogen and the boron
atom.12a Accordingly, in the new compound 6, the perpendicular
orientation of the B- and N-bonding planes should entirely
preclude any π-interaction and, thus, impose an even increased
B-N separation. Hence, a different bonding situation has to be
assumed for the bridged borylene complex 6, which can be
rationalized considering the theoretically predicted similarity
between CO and the borylene unit.24 The Lewis basic palladium
fragment donates electron density into the empty π*-orbitals
of CO and the π-orbitals of the borylene, and therefore, the
electronically saturated [(OC)5MdBdN(SiMe3)2] acts as a
π-acceptor. A partial rehybridization of the carbon and boron
centers is necessary to allow for efficient π-acceptance, which
results in the formation of two noncompensating, semibridging
ligands: CO and BN(SiMe3)2. In the case of the carbonyl group
the relevant bonding angles (Pd1-Cr1-C1: 54.21(6)°, Cr1-
C1-O1: 165.49(16)°) are consistent with the classification
scheme for semibridging carbonyl ligands developed by Crab-
tree.25 For borylenes, no literature data are accessible for
semibridging B-R moieties, but the angles (Pd1-Cr1-B1:
49.84(5)°, Cr1-B1-N1: 152.33(13)°) lie clearly between those
typically found for terminal and symmetrically bridging ligands.
Preliminary ELF calculations suggest the absence of a metal-
metal interaction, despite the relatively small chromium-
palladium distance of 2.6227(10) Å.
(OC)3W(µ-CO){µ-BN(SiMe3)2}Pt(PCy3)] (11). The 31P{1H}
4
NMR spectrum displayed two doublets (δ ) 76.8 (d, JP-P
)
1
4
1
10 Hz, JP-Pt ) 5001 Hz, PPt), 29.0 (d, JP-P ) 10 Hz, JP-W
) 243 Hz, PW)), indicating the presence of two nonequivalent
1
phosphine ligands. The singlet for the Me3Si groups in the H
NMR spectra (δ ) 0.64) and the broad resonance in the 11B-
{1H} NMR spectra (δ ) 99) were found downfield-shifted with
respect to the signals of the starting material.12b
The molecular constitution of the new compound could be
determined by an X-ray diffraction study of a single crystal of
11. Crystals were obtained be layering hexanes on a concentrated
solution of 11 and allowing the solvent to evaporate slowly.The
structural features of 11 in particular with respect to the
geometries of the CO (Pt1-W1-C1: 51.37(8)°, W1-C1-
O1: 165.7(2)°) and the borylene ligand (Pt1-W1-B1: 49.40-
(9)°, W1-B1-N1: 153.4(2)°) resemble those of the mono-
phosphine Pd complex 6. These data together with the orthogonal
disposition of the aminoborylene with respect to the M-B-Pt
plane (Si2-N1-B1-W1: 93.4(5)°) and the short B-N double
bond (B1-N1 ) 1.407(4) Å) unambiguously prove the presence
of semibridging borylene and CO ligands, respectively. Inspec-
tion of the W-B distance, however, reveals a distinct difference
between the mono- and diphosphine complexes. In the case of
11, the W1-B1 distance of 2.138(3) Å is shorter than that in
the terminal tungsten borylene complex 4 (W-B: 2.151(7)
Å),12b despite the greater coordination number of the boron atom
in the former. Apparently, the decreased W-B distance in 11
is due to the presence of the PCy3 group in trans-position and
its reduced π-acceptor abilities in comparison to CO, thus
inducing a stronger M-B dπ-pπ back-donation. A similar effect
is observed in the case of terminal borylene complexes with
Cy3P ligands in trans-position to the boron atom (vide infra).
Despite the relatively short tungsten-platinum distance of
2.70833(16) Å, preliminary ELF calculations provide no
evidence for a metal-metal interaction.
The formation of the Pt diphosphine complex 11 is obviously
due to the presence of free PCy3 in the reaction mixture, and
the selective substitution of a CO ligand trans to the BR2 group
(24) Barr, R. D.; Marder, T. B.; Orpen, A. G.; Williams, I. D. J. Chem.
Soc., Chem. Commun. 1984, 112-114, and references therein.
(25) Crabtree, R. H.; Lavin, M. Inorg. Chem. 1986, 25, 805-812.

5162 Organometallics, Vol. 25, No. 21, 2006
Braunschweig et al.
Scheme 2. Synthesis of Semibridging Borylene Complexes
with Two Phosphine Ligands
Figure 3. Molecular structure of 14.
can be explained by the strong trans effect of the latter.26 The
isolation and full spectroscopic characterization of the complex
[(Cy3P)(OC)3Cr(µ-CO){µ-BN(SiMe3)2}Pt(PCy3)] (12) provide
further evidence for this assumed course of the reaction. After
separation of deposited, crystalline [(OC)4Cr(µ-CO){µ-BN-
(SiMe3)2}Pt(PCy3)] (10) from its reaction mixture, the resulting
mother liquor contained dissolved 10 and free PCy3 according
to 1H and 31P{1H} NMR spectroscopy. Over a period of 2 days
these species were gradually consumed and the diphospine
complex 12 was formed as indicated by corresponding reso-
nances in the 1H (δ ) 0.63), 11B{1H} (δ ) 97), and 31P{1H} (δ
) 63.9, 3JP-Pt ) 127 Hz; 70.1, 1JP-Pt ) 4972 Hz) NMR spectra.
Likewise, 1:1 mixtures of [(OC)5WdBdN(SiMe3)2] (4) and
[Pd(PCy3)] (8), from which the monophosphine complex
[(OC)4W(µ-CO){µ-BN(SiMe3)2}Pd(PCy3)] (7) was isolated after
1 day (vide supra), afforded the diphosphine species [(Cy3P)-
(OC)3W(µ-CO){µ-BN(SiMe3)2}Pd(PCy3)] (13) after prolonged
time at ambient temperature. 13 was isolated as orange crystals
in analytically pure form and displays a broad signal at δ ) 98
in the 11B{1H} NMR spectrum and two doublets at δ ) 39.7
(4JP-P ) 11 Hz) and 30.8 (4JP-P ) 11 Hz) in the 31P{1H} NMR
spectra.
monoclinic space group P21/c, and the P-Cr-B and the Cr-
B-N moieties are almost linearly arranged (P1-Cr1-B1:
177.87(6)°, Cr1-B1-N1: 175.91(16)°). The B1-N1 bond
(1.364(3) Å) is negligibly longer than the B1-N1 bond of the
unsubstituted borylene 3 (1.353(6) Å),17a but the Cr1-B1 bond
(1.915(2) Å) is 8.1 pm shorter (1.996(6) Å).17a The latter can
be rationalized, considering the different electronic situation
imposed by the phosphine ligand in trans-position. Being a much
less effective π-acceptor with respect to CO, the Cy3P ligand
allows for a stronger dπ-pπ back-bonding to boron, which leads
to a significant decrease of the Cr-B separation. Consequently,27
a more pronounced umbrella effect is detectible for 14 with
respect to the unsubstituted borylene complex 3, as expressed
by the mean values of B-M-Ceq and M-Ceq-O angles for
the C1-O1 and C3-O3 carbonyls (14: B1-Cr1-C(1,3): 79°,
Cr1-C(1,3)-O(1,3): 174°; 3: B-Cr-Ceq: 88°, Cr-Ceq-O:
179°).17a It should be noted that the Si2NB plane in 14 is almost
coplanar with respect to the C2-Cr1-C4 plane (Si1-N1-Cr1-
C2: 11.46°, Si2-N1-Cr1-C4: -1.99°), thus precluding a
corresponding bending of the C2-O2 and C4-O4 carbonyl
ligands toward the borylene for sterical reasons (B1-Cr1-
C(2,4): 92°, Cr1-C(2,4)-O(2,4): 178°). All relevant structural
data indicate a significantly enhanced Cr-B π-back-bonding
in 14, thus emphasizing its importance for the synergic metal-
boron bonding even in the case of aminoborylene complexes,
in which the electron deficiency of the boron center is already
partially saturated by the presence of a B-N double bond.
Likewise, warming or irradiation of the tungsten species 7
led to the mononuclear phosphine complex trans-[(Cy3P)-
(OC)4WdBN(SiMe3)2] (15) as indicated by its characteristic
1H NMR (δ ) 0.38), 11B{1H} NMR (δ ) 90), and 31P{1H}
NMR (δ ) 35.1) data. In contrast to the clean synthesis of 14
according to Scheme 3, however, the formation of compound
15 is accompanied by various, mostly unidentified side products,
thus precluding its isolation in pure form. Among the latter,
the boron-free species [(Cy3P)W(CO)5] (31P{1H} δ ) 32.3)28
and the aforementioned diphosphine complex [(Cy3P)(OC)3W-
(µ-CO){µ-BN(SiMe3)2}Pd(PCy3)] (13: 31P{1H} δ ) 39.8 (d),
30.8 (d)) were unambiguously identified by their characteristic
31P{1H} NMR resonances. It should be noted that the synthesis
of the dinuclear Pt species [(OC)4Cr(µ-CO){µ-BN(SiMe3)2}-
Pt(PCy3)] (10) and [(Cy3P)(OC)3M(µ-CO){µ-BN(SiMe3)2}Pt-
Reactivity. The color of solid samples of the bridged borylene
complexes 6, 7, and 10-13 ranges from yellow to red, with
the chromium complexes displaying the least intense color with
respect to their tungsten congeners. All new compounds proved
to be air and moisture sensitive and thermolabile in solution.
In the case of 6, multinuclear NMR spectra of a solution in
C6D6 indicated the formation of a new compound with
concomitant darkening of the NMR sample, which proceeds
very slowly at ambient temperature, but is completed after 1
day at 80 °C. Photolysis of 6 furnishes the same product within
4 h with concomitant formation of a black deposit. The observed
1
singlets in the H NMR (δ ) 0.38), 11B{1H} NMR (δ ) 94),
and 31P{1H} NMR spectra (δ ) 64.77) clearly indicate that 6
is converted into trans-[(Cy3P)(OC)4CrdBN(SiMe3)2] (14) with
migration of the PCy3 ligand from Pd to Cr, formation of Pd0,
and release of one CO group. Compound 14 could be isolated
after repeated recrystallization steps as yellow crystals. The fact
that the substitution occurs selectively in position trans to the
borylene ligand is ascribed to the strong trans effect of the latter.
The constitution of the new compound 14 could be deter-
mined by performing a single-crystal X-ray diffraction study.
Suitable crystals were obtained by cooling a concentrated hexane
solution of 14 to -35 °C.The complex crystallizes in the
(27) Uddin, J.; Boehme, C.; Frenking, G. Organometallics 2000, 19,
571-582.
(28) Demircan, O.; O¨ zkar, S.; U¨ lku¨, D.; Yildirim, L. T. J. Organomet.
Chem. 2003, 688, 68-74.
(26) (a) Zhu, J.; Lin, Z.; Marder, T. B. Inorg. Chem. 2005, 44, 9384-
9390. (b) Sakaki, S.; Kai, S.; Sugimoto, M. Organometallics 1999, 18,
4825-4837.

Synthesis of Semibridging Borylene Complexes
Organometallics, Vol. 25, No. 21, 2006 5163
Scheme 3. Formation of a Phosphine-Substituted Borylene
Complex
standard methods and were distilled from appropriate drying agents
(sodium and sodium wire) under dry argon and stored over
molecular sieves prior to use. Deuterated solvents (C6D6) were
degassed by three freeze-pump-thaw cycles and stored over
molecular sieves in the glovebox. The irradiation experiments were
carried out through use of a Heraeus TQ 150 high-pressure Hg
lamp. IR spectra for compounds were recorded as toluene solutions
between KBr plates on a Bruker Vector 22 FT-IR spectrometer.
NMR spectra were recorded on Bruker DRX 300, Bruker AMX
400, and Bruker Avance 500 NMR spectrometers. Reaction
1
mixtures were controlled on a Varian 200 NMR spectrometer. H
and 13C{1H} NMR spectra were referenced to external TMS via
the residual protio solvent (1H) or the solvent itself (13C). 11B{1H}
and 31P{1H} NMR spectra were referenced to external BF3‚OEt2
and 85% H3PO4, respectively. Microanalyses for C, H, and N were
performed by Mr. C. P. Kneis (University of Wu¨rzburg) on a Leco
CHNS-932 instrument.
(PCy3)] (M ) Cr, 12; W, 11) according to Schemes 1 and 2
under moderate thermal conditions (i.e., ambient temperature)
is also accompanied by the formation of similar side products,
thus accounting for the observed moderate yields of the isolated
pure products. In the case of the reaction of 4 with 9 for
example, NMR data recorded 1 h after mixing of the reactants
indicate 11 as the major product. After 1 day a 3:2:1:1 mixture
of the subsequently isolated product 11, the borylene complex
15, [(Cy3P)W(CO)5], and trans-[(Cy3P)2W(CO)4] (31P{1H} δ
) 31.4 ppm) was observed.28
Interestingly, the formation of trans-[(Cy3P)(OC)4CrdBN-
(SiMe3)2] (14) from the dinuclear species 6 represents the only
access to substituted terminal borylene complexes, as earlier
attempts to obtain mixed phosphine borylene complexes by
direct reaction of 3 with PR3 met with no success.29
Synthesis of [(OC)4Cr(µ-CO)(µ-BN(SiMe3)2)Pd(PCy3)] (6). A
pale yellow solution of 8 (0.276 g, 0.413 mmol) in toluene (4 mL)
was added to solid 3 (0.150 g, 0.413 mmol). Yellow crystals started
to form from the bright yellow solution at room temperature. The
crystallization was completed at -35 °C, yielding 0.249 g of 6
1
(81%). H NMR (400 MHz, C6D6, 25 °C, TMS): δ 1.94-1.13
(m, 33 H, Cy), 0.42 (s, 18 H, SiMe3). 13C{1H} NMR (101 MHz,
C6D6, 25 °C): δ 223.5 (d, JC-P ) 2 Hz, CO), 223.0 (d, JC-P ) 3
Hz, CO), 33.7 (d, 1JC-P ) 14 Hz, C1, Cy), 31.4 (d, 3JC-P ) 6 Hz,
2
C
3,5, Cy), 27.7 (d, JC-P ) 11 Hz, C2,6, Cy), 26.4 (s, C4, Cy), 3.7
(s, SiMe3). 11B{1H} (64 MHz, C6D6, 25 °C): δ 100 (s, ω1/2 ) 752
Hz). 31P{1H} NMR (162 MHz, C6D6, 25 °C): δ 34.2 (s). IR
(toluene): 2030, 1957, 1932, 1858 cm-1, ν (CdO). Anal. Calcd
for C29H51NBCrO5PPdSi2: C 46.44, H 6.85, N 1.87. Found: C
46.43, H 6.80, N 1.86.
Conclusion
Reaction of the terminal borylene complexes [(OC)5MdBd
N(SiMe3)2] (M ) Cr, 3; W, 4) with [M′(PCy3)2] (M′ ) Pd, 8;
Pt, 9) led to a variety of heterodinuclear, semibridging borylene
complexes of the type [(OC)4M(µ-CO){µ-BN(SiMe3)2}M′-
(PCy3)] (M ) Cr, M′ ) Pd, 6; M ) W, M′ ) Pd, 7; M ) Cr,
M′ ) Pt, 10) with liberation of free PCy3. The trans effect of
the bridging borylene facilitates the subsequent substitution of
a Cr- or W-bound CO by PCy3, which gave access to several
diphosphine complexes, [(Cy3P)(OC)3M(µ-CO){µ-BN(SiMe3)2}-
M′(PCy3)] (M ) Cr, M′ ) Pt, 12; M ) W, M′ ) Pd, 13; M )
W, M′ ) Pt, 11). Structural characterization of [(Cy3P)(OC)3W-
(µ-CO){µ-BN(SiMe3)2}Pt(PCy3)] (11) has confirmed the pres-
ence of not only a semibridging borylene but also a decreased
W-B distance, imposed by the weak π-acceptor PCy3 in trans-
position to the borylene. Likewise, in trans-[(Cy3P)(OC)4Crd
BN(SiMe3)2] (14), which was obtained by photolysis from
[(OC)4Cr(µ-CO){µ-BN(SiMe3)2}Pd(PCy3)] (6) as the first ter-
minal borylene complex with a heteroleptic set of neutral co-
ligands, the phosphine imposes a significantly enhanced Cr-B
dπ-pπ back-bonding. The assembled experimental data establish
the semibridging coordination mode as an alternative for the
binding of borylenes to transition metals, thus emphasizing the
close relationship between BR and CO. Furthermore, the first
structural data on bridging and terminal borylene complexes
with a PR3 co-ligand in trans-position stress the significance of
the π-contribution to the net synergic M-B bonding even in
the case of aminoborylenes, for which the electron deficiency
of the boron center is already partially saturated by the presence
of a B-N double bond.
Synthesis of [(OC)4W(µ-CO)(µ-BN(SiMe3)2)Pd(PCy3)] (7). A
pale yellow solution of 8 (0.135 g, 0.202 mmol) in toluene (4 mL)
was added to solid 4 (0.100 g, 0.202 mmol). After 24 h the volume
of the red solution was reduced to approximately half, and the
mixture was cooled to -80 °C. An orange microcrystalline solid
of 7 deposited after several days, which was isolated and dried in
1
vacuo (0.120 g, 67%). H NMR (300 MHz, C6D6, 25 °C, TMS):
δ 1.96-1.13 (m, 33 H, Cy), 0.42 (s, 18 H, SiMe3). 13C{1H} NMR
1
(75 MHz, C6D6, 25 °C): δ 203.1 (d, JC-W ) 121 Hz, JC-P ) 2
Hz, CO), 200.5 (d, JC-P ) 3 Hz, CO), 34.0 (d, 1JC-P ) 12 Hz, C1,
Cy), 31.4 (d, 3JC-P ) 5 Hz, C3,5, Cy), 27.8 (d, 2JC-P ) 11 Hz, C2,6
,
Cy), 26.4 (s, JC-P ) 1 Hz, C4, Cy), 3.9 (s, SiMe3). 11B{1H} (96
MHz, C6D6, 25 °C): δ 97 (s, ω1/2 ) 884 Hz). 31P{1H} NMR (121
MHz, C6D6, 25 °C): δ 41.9 (s). IR (toluene): 2043, 1955, 1932,
1873 cm-1, ν (CdO). Anal. Calcd for C29H51NBO5PPdSi2W: C
39.49, H 5.83, N 1.59. Found: C 39.48, H 5.75, N 1.65.
4
Synthesis of [(OC)4Cr(µ-CO){µ-BN(SiMe3)2}Pt(PCy3)] (10).
Solid 3 (0.045 g, 0.124 mmol) was added to a pale yellow solution
of 9 (0.094 g, 0.124 mmol) in C6D6 (0.5 mL). The solution turned
orange, and after 5 min orange crystals formed. These were
separated from the solution and recrystallized from toluene/hexane
by slow evaporation in the glovebox. After 4 days orange crystals
of 10 could be isolated (yield 0.025 g, 25%). 1H NMR (500 MHz,
C6D6, 27 °C): δ 2.00-1.09 (m, 33 H, Cy), 0.43 (s, 18 H, SiMe3).
13C{1H} NMR (126 MHz, C6D6, 27 °C): δ 223.5 (s, CO), 223.3
1
(d, JC-P ) 2 Hz, CO), 35.8 (d, JC-P ) 24 Hz, C1, Cy), 31.0 (s,
2
C
3,5, Cy), 27.6 (d, JC-P ) 11 Hz, C2,6, Cy), 26.4 (s, C4, Cy), 3.5
(s, SiMe3). 11B{1H} NMR (160 MHz, C6D6, 27 °C): δ 98 (s, ω1/2
) 1230 Hz). 31P{1H} NMR (202 MHz, C6D6, 27 °C): δ 70.0 (s,
1JP-Pt ) 5134 Hz). IR (toluene): 2029, 1956, 1933, 1858 cm-1, ν
(CdO). Anal. Calcd for C29H51NBCrO5PPtSi2: C 41.53, H 6.13,
N 1.67. Found: C 41.22, H 6.05, N 1.64.
Experimental Section
General Procedures. All manipulations were conducted either
under an atmosphere of dry argon or in vacuo using standard
Schlenk line or glovebox techniques. Solvents were dried by
Synthesis of [(Cy3P)(OC)3W(µ-CO){µ-BN(SiMe3)2}Pt(PCy3)]
(11). Solid 4 (0.050 g, 0.101 mmol) was added to a pale yellow
solution of 9 (0.077 g, 0.101 mmol) in toluene (0.5 mL). The
(29) Kollann, C. Ph.D. Thesis, RWTH Aachen, 1999.

5164 Organometallics, Vol. 25, No. 21, 2006
Braunschweig et al.
mixture turned brown-red and was layered with hexane (2 mL).
The solvent was allowed to evaporate from the reaction mixture in
the glovebox. First some side products precipitated, which were
removed. The residual red solution was left in the glovebox, and
the solvent was allowed to evaporate slowly. After 7 days bright
orange crystals of 11 formed, which were recrystallized from
toluene/hexane, yielding 30 mg (24%). 1H NMR (400 MHz, C6D6
, 27 °C): δ 2.20-1.20 (m, 66H, Cy), 0.64 (s, 18H, SiMe3). 13C-
{1H} NMR (101 MHz, C6D6 , 27 °C): δ 212.2 (s, CO), 212.1 (s,
CO), 37.8 (d, 1JC-P ) 17 Hz, C1, CW), 36.4 (d, 1JC-P ) 23 Hz, C1,
-80 °C. The first crystals were removed, because they were
unreacted starting material. Then the solution was concentrated,
and after 2 days at -80 °C crystals were isolated. They were
recrystallized several times from hexane at -35 °C to remove
colloidal palladium, yielding pale yellow crystals of 14 (0.021 g,
1
28%). H NMR (400 MHz, C6D6 , 21 °C): δ 2.04-1.53 (m, 33H,
Cy), 0.38 (s, 18H, SiMe3). 13C{1H} NMR (101 MHz, C6D6 , 21
°C): δ 224.3 (d, 2JC-P ) 13 Hz, CO), 37.3 (d, 1JC-P ) 13 Hz, C1,
2
Cy), 29.9 (s, C3,5, Cy), 28.1 (d, JC-P ) 10 Hz, C2,6, Cy), 26.8 (s,
C4, Cy), 3.0 (s, SiMe3). 11B{1H} NMR (96 MHz C6D6 , 23 °C): δ
94 (s, ω1/2 ) 1100 Hz). 31P{1H} NMR (162 MHz, C6D6 , 21 °C):
δ 64.7 (s). IR (toluene): 1898, 1868 cm-1, ν (CdO). Anal. Calcd
for C28H51NBCrO4PSi2: C 54.62, H 8.35, N 2.28. Found: C 54.65,
H 8.04, N 2.30.
3
CyPt), 31.1 (d, JC-P ) 2 Hz, C3,5, CyPt), 30.5 (s, C3,5, CyW), 28.0
(m, 2 overlapping d, C2,6, CyW+Pt), 26.8 (s, C4, CyW), 26.7 (s, C4,
CyPt), 4.2 (s, SiMe3). 11B{1H} NMR (160 MHz, C6D6, 27 °C): δ
99 (br s). 31P{1H} NMR (202 MHz, C6D6 , 27 °C): δ 76.8 (d, 4JP-P
) 10 Hz, 1JP-Pt ) 5001 Hz, PPt), 29.0 (d, 4JP-P ) 10 Hz, 1JP-W
243 Hz, JP-Pt ) 106 Hz PW). IR (toluene): 1982, 1882, 1835,
1786 cm-1, ν (CdO). Anal. Calcd for C46H84NBO4P2PtSi2W: C
45.17, H 6.92, N 1.15. Found: C 44.97, H 6.78, N 1.13.
)
Crystal Structure Determination. The crystal data of 6 and
14 were collected at a Bruker Apex diffractometer with CCD area
detector and graphite-monochromated Mo KR radiation, and the
data of 11 were collected at a Bruker X8Apex diffractometer with
CCD area detector and multilayer mirror monochromated Mo KR
radiation. The structures were solved using direct methods, refined
with the Shelx software package (G. Sheldrick, University of
Go¨ttingen, 1997) and expanded using Fourier techniques. All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
assigned to idealized positions and were included in structure factor
calculations.
3
Synthesis of [(Cy3P)(OC)3Cr(µ-CO){µ-BN(SiMe3)2}Pt(PCy3)]
(12). Solid 3 (0.045 g, 0.124 mmol) was added to a pale yellow
solution of 9 (0.094 g, 0.124 mmol) in C6D6 (0.5 mL). The solution
turned orange, and after 5 min orange crystals of 10 formed and
were removed. The residual orange solution was layered with
hexane and left in the glovebox for 4 days to let the solvent
evaporate slowly. Orange crystals of 12 formed, which were
recrystallized from toluene/hexane, yielding 0.020 g (15%). 1H
NMR (400 MHz, C6D6 , 23 °C): δ 2.30-1.20 (m, 66H, Cy), 0.63
(s, 18H, SiMe3). 13C{1H} NMR (101 MHz, C6D6 , 23 °C): δ 228.7
Crystal data for 6: C29H51BCrNO5PPdSi2, Mr ) 750.07, yellow
block, 0.30 × 0.25 × 0.15, monoclinic space group P21/n, a )
10.293(4) Å, b ) 16.264(7) Å, c ) 21.535(9) Å, â ) 90.285(8)°,
2
4
1
(dd, JC-P ) 14 Hz, JC-P ) 2 Hz, CO), 37.5 (d, JC-P ) 14 Hz,
V ) 3605(3) Å3, Z ) 4, Fcalcd ) 1.382 g‚cm-3, µ ) 0.946 cm-2
,
1
3
C1, CyCr), 36.0 (d, JC-P ) 23 Hz, C1, CyPt), 31.1 (d, JC-P ) 2
F(000) ) 1560, T ) 193(2) K, GOF ) 1.040, R1 ) 0.0304, wR2 )
0.0686, 8944 independent reflections [2θ e 56.56°] and 425
parameters.
2
Hz, C3,5, CyPt), 30.0 (s, C3,5, CyCr), 28.2 (d, JC-P ) 10 Hz, C2,6
,
2
CyCr), 27.9 (d, JC-P ) 11 Hz, C2,6, CyPt), 26.8 (s, C4, CyCr), 26.6
(d, C4, JC-P ) 1 Hz, CyPt), 4.0 (s, SiMe3). 11B{1H} NMR (160
Crystal data for 11: C46H84BNO4P2PtSi2W, Mr ) 1223.01,
orange block, 0.19 × 0.13 × 0.03, triclinic space group P1h, a )
12.1893(3) Å, b ) 13.4380(4) Å, c ) 16.1074(4) Å, R ) 96.5790-
(10)°, â ) 100.9850(10)°, γ ) 96.4730(10)°, V ) 2548.38(12) Å3,
Z ) 2, Fcalcd ) 1.594 g‚cm-3, µ ) 5.147 cm-2, F(000) ) 1228, T
) 100(2) K, GOF ) 1.038, R1 ) 0.0247, wR2 ) 0.0584, 10 337
independent reflections [2θ e 52.76°] and 523 parameters.
Crystal data for 14: C28H51BCrNO4PSi2, Mr ) 615.66, yellow
block, 0.25 × 0.16 × 0.10, monoclinic space group P21/c, a )
20.8563(14) Å, b ) 9.1712(6) Å, c ) 18.3901(12) Å, â )
100.7070(10)°, V ) 3456.4(4) Å3, Z ) 4, Fcalcd ) 1.183 g‚cm-3, µ
) 0.477 cm-2, F(000) ) 1320, T ) 193(2) K, GOF ) 1.057, R1
) 0.0417, wR2 ) 0.1074, 6830 independent reflections [2θ e
52.12°] and 343 parameters.
Crystallographic data for 6, 11, and 14 have been deposited with
the Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC-258168, CCDC-609382, and CCDC-609381,
respectively. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
4
MHz, C6D6, 27 °C): δ 97 (br s, ω1/2 ) 2110 Hz). 31P{1H} NMR
1
(162 MHz, C6D6 , 23 °C): δ 70.1 (s, JP-Pt ) 4972 Hz, PPt), 63.9
3
(s, JP-Pt ) 127 Hz, PCr). IR (toluene): 1967, 1886, 1856, 1793
cm-1, ν (CdO). Anal. Calcd for C46H84NBCrO4P2PtSi2: C 50.63,
H 7.75, N 1.28. Found: C 51.48, H 7.65, N 1.22.
Synthesis of [(Cy3P)(OC)4W{µ-BN(SiMe3)2}Pd(PCy3)] (13).
Solid 4 (0.030 g, 0.061 mmol) was added to a pale yellow solution
of 8 (0.040 g, 0.061 mmol) in C6D6 (0.5 mL). The solvent was
allowed to evaporate from the red reaction mixture in the glovebox.
The solid residue was dissolved in 1.5 mL of hexane the next day
and stored at -35 °C. After 3 days an orange solid of 13 was
1
isolated (0.018 g, 26%). H NMR (400 MHz, C6D6 , 21 °C): δ
2.17-1.60 (m, 66H, Cy), 0.63 (s, 18H, SiMe3). 13C{1H} NMR (101
2
4
MHz, C6D6 , 21 °C): δ 212.0 (dd, JC-P ) 7 Hz, JC-P ) 2 Hz,
1
1
CO), 37.5 (d, JC-P ) 17 Hz, C1, CyW), 34.2 (d, JC-P ) 11 Hz,
3
C1, CyPd), 31.5 (d, JC-P ) 5 Hz, C3,5, CyW), 30.5 (s, C3,5, CyPd),
2
28.0 (d, JC-P ) 11 Hz, C2,6, CyPd+W), 26.8 (s, C4, CyW), 26.8 (s,
C4, CyPd), 4.4 (s, SiMe3). 11B{1H} NMR (96 MHz, C6D6, 23 °C):
δ 98 (br s). 31P{1H} NMR (162 MHz, C6D6 , 21 °C): δ 39.7 (d,
4JP-P ) 11 Hz, PPd), 30.8 (d, 4JP-P ) 11 Hz, 1JP-W ) 367 Hz, PW).
IR (toluene): 1980, 1928, 1880, 1814 cm-1, ν (CdO). Anal. Calcd
for C36H84NBO4PdPSi2W: C 48.71, H 7.46, N 1.23. Found: C
49.31, H 7.21, N 1.10.
Acknowledgment. This work was supported by the DFG.
D.R. thanks the Alexander von Humboldt-Foundation for a
postdoctoral fellowship.
Synthesis of trans-[(Cy3P)(OC)4CrdBN(SiMe3)2] (14). A yel-
low solution of 6 (0.090 g, 0.120 mmol) in C6D6 (1 mL) was
photolyzed at 0 °C in a NMR quartz tube for 3 h. After that, the
solvent of the dark brown reaction mixture was evaporated in vacuo.
The solid residue was dissolved in 5 mL of hexane and stored at
Supporting Information Available: Crystal data in cif format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM0605480