Reactivity of boryl complexes: Synthesis and structure of new neutral and cationic platinum boryls and borylenes

Article abstract of DOI:10.1021/om2012248

A reactivity study on a series of platinum boryl complexes was performed. The first stable base adducts of cationic haloboryl complexes of the form trans-[Pt{B(Br)(NMe₂)}(NCMe)(PCy₃)₂] ⁺ were isolated and fully characterized. The dianion [B ₁₂Cl₁₂]^2- was introduced as a weakly coordinating anion to complex chemistry forming a A₂X salt. Through the reaction of trans-[Pt{B(Br)(tBu)}Br(PCy₃)₂] with BBr₂tBu, the first highly soluble dinuclear platinum boryl complex, [Pt{B(Br)(tBu)}(μ-Br)(PCy₃)]₂, could be synthesized with concomitant buildup of the corresponding phosphine-borane adduct. In contrast to this observation, reaction of trans-[Pt{B(Br)(Mes)}Br(PCy ₃)₂] with BBr₃ leads to the formation of the cationic borylene complex trans-[Pt(BMes)Br(PCy₃)₂] ⁺ by abstraction of the bromo ligand bound mutually trans to the boryl ligand in the precursor and concomitant buildup of [BBr₄] ^-. Reaction of [Pt(PCy₃)₂] with BCl₃ and subsequent abstraction of the platinum-bound chloro ligand enabled the structural characterization of trans-[Pt(BCl₂)(PCy₃) ₂]⁺, which is isoelectronic with the metal-only Lewis pair trans-[Pt(BeCl₂)(PCy₃)₂]. The bonding situation in both systems was investigated in detail using quantum chemical calculations. A T-shaped cationic complex, trans-[Pt{B(Br)(Fc)}(PiPr ₃)₂]⁺, and its precursor trans-[Pt{B(Br)(Fc)} Br(PiPr₃)₂], both with reduced steric bulk at the phosphine ligands compared with their PCy₃ derivatives, were fully characterized.

Full text of DOI:10.1021/om2012248

Article
pubs.acs.org/Organometallics
Reactivity of Boryl Complexes: Synthesis and Structure of New
Neutral and Cationic Platinum Boryls and Borylenes
Nicole Arnold, Holger Braunschweig,* Peter Brenner, J. Oscar C. Jimenez-Halla, Thomas Kupfer,
and Krzysztof Radacki
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat Wurzburg, Am Hubland, D-97074 Wurzburg, Germany
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: A reactivity study on a series of platinum boryl
complexes was performed. The first stable base adducts of
cationic haloboryl complexes of the form trans-[Pt{B(Br)-
(NMe2)}(NCMe)(PCy3)2]+ were isolated and fully charac-
terized. The dianion [B12Cl12]2− was introduced as a weakly
coordinating anion to complex chemistry forming a A2X salt.
Through the reaction of trans-[Pt{B(Br)(tBu)}Br(PCy3)2]
with BBr2tBu, the first highly soluble dinuclear platinum boryl
complex, [Pt{B(Br)(tBu)}(μ-Br)(PCy3)]2, could be synthe-
sized with concomitant buildup of the corresponding
phosphine-borane adduct. In contrast to this observation,
reaction of trans-[Pt{B(Br)(Mes)}Br(PCy3)2] with BBr3 leads
to the formation of the cationic borylene complex trans-
[Pt(BMes)Br(PCy3)2]+ by abstraction of the bromo ligand bound mutually trans to the boryl ligand in the precursor and
concomitant buildup of [BBr4]−. Reaction of [Pt(PCy3)2] with BCl3 and subsequent abstraction of the platinum-bound chloro
ligand enabled the structural characterization of trans-[Pt(BCl2)(PCy3)2]+, which is isoelectronic with the metal-only Lewis pair
trans-[Pt(BeCl2)(PCy3)2]. The bonding situation in both systems was investigated in detail using quantum chemical calculations.
A T-shaped cationic complex, trans-[Pt{B(Br)(Fc)}(PiPr3)2]+, and its precursor trans-[Pt{B(Br)(Fc)}Br(PiPr3)2], both with
reduced steric bulk at the phosphine ligands compared with their PCy3 derivatives, were fully characterized.
C6H11), we were able to isolate a number of complexes with the
general formula trans-[Pt(BRR′)Hal(PCy3)2] with several
halide and R, R′ combinations.7 X-ray crystal structure analysis
of the bromide systems and comparison of Pt−Br and Pt−B
separations were used to measure the boryl ligands’ trans-
influence, confirming theoretical predictions that postulated
that electron donor substituents at the boron atom intensify the
ligand’s trans-influence.7c,8 The low-coordinate Pt(0) complex
1 plays a particularly important role as a substrate therein, as
this fragment has enabled the stabilization of unprecedented
boron-centered ligand systems, e.g., cationic T-shaped
boryls,8c,9 cationic and heterodinuclear borylenes,10 neutral
and cationic base-stabilized borylenes,8a,b,11 iminoboryls,12
oxoboryls,13 σ-borolyls,14 diboran(4)yls,15 and very recently
also (alkyl)(boryls).16
INTRODUCTION
■
Ever since the Nobel Prize in Chemistry in 1976 for Lipscomb’s
“studies on the structure of boranes illuminating problems of
chemical bonding” and three years later for H. C. Brown for
“the development of boron-containing compounds”, the
chemistry of boron has been a matter of particular interest in
both modern organic and organometallic chemical research.1
Recently this has been reaffirmed by the award of the 2010
Nobel Prize in Chemistry partly for the development of
palladium-catalyzed cross-coupling reactions with unsaturated
boranes, now known as the Suzuki−Miyaura protocol.2 For a
chemist new to the field, even a perfunctory glance over the
organometallic literature of the last 20 years will betray an
immense research interest in the isolation of postulated
intermediates in functionalization reactions of organic sub-
strates via late transition metals. In particular, boryl complexes3
have been investigated in great detail, as these act as
intermediates in hydroboration,4 diboration,5 and C−H
activation reactions.6 Platinum-centered complexes are of
particular interest, as these play a role as key intermediates
especially in the former functionalization reactions. In our
laboratories recent studies on platinum−boron chemistry have
enabled the isolation and structural characterization of a
number of new bonding motifs. Through the oxidative addition
of B−Hal bonds (Hal = F, Cl, Br, I) to [Pt(PCy3)2] (1) (Cy =
It should be mentioned also that E−Hal (E = Ga, Hal = Br, I;
E = Bi, Hal = Cl)17 bonds oxidatively add to the
aforementioned platinum bis(phosphine) system, whereas
Lewis-acidic substrates such as BeCl2 (s-block),18 AlHal3 (p-
block),19 and ZrCl4 (d-block)20 form T-shaped metal-only
Lewis pairs of the form trans-[Pt(EHaln)(PCy3)2].
Received: December 9, 2011
Published: February 8, 2012
© 2012 American Chemical Society
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Scheme 1. Reaction of trans-[Pt{B(Br)(NMe2)}(PCy3)2][BArf4] (2) in Dichloromethane: (a) MeCN Attacks the Platinum
Center; (b) 4-Picoline Results in the Formation of a Base-Stabilized Borylene
In this work, we present several examples that show a more
general applicability of our results concerning the choice of the
phosphine substituents that stabilize unsaturated cationic T-
shaped platinum boryl complexes, increasing the usability and
practicality of anions and the ease of abstraction of the bromo
ligand situated trans to the boryl ligand.
the case of related T-shaped cationic complexes in which at
least one halide is bound to the boron atom, the base 4-picoline
instead attacks the boron atom. This results in a formal halide
shift from the boron to the platinum center and the formation
of a so-called “base-stabilized borylene” of the general formula
trans-[Pt{BR(4-Me-C5H4N)}Br(PCy3)2]+, typically displaying
a
31P NMR signal 20 ppm upfield of that of their cationic
We present here (i) the first base adduct of a haloboryl
complex, in which a Lewis base (here, MeCN) does not attack
the Lewis-acidic boron center and (ii) the first organometallic
complex salt composed of two identical cationic complex
fragments with one dianionic dodecaborate [B12Cl12]2−
counterion. We show different reactivities of bis(phosphine)
platinum boryl complexes toward Lewis-acidic boranes that
form (iii) a new dinuclear platinum boryl complex through the
abstraction of a phosphine ligand and (iv) the synthesis of the
cationic platinum borylene trans-[Pt(BMes)Br(PCy3)2]+ ob-
tained through the abstraction of a bromide ligand using a
borane as the abstracting agent. In part (v) we report the
synthesis of the T-shaped cationic complex trans-[Pt(BCl2)-
(PCy3)2]+, which is isoelectronic to trans-[Pt(BeCl2)(PCy3)2].
The bonding situation in both systems was analyzed using
quantum chemical methods, and the results are discussed.
Finally in (vi) we account for the isopropylphosphine derivative
of the first reported T-shaped cationic boryl complex, trans-
[Pt{B(Br)(Fc)}(PCy3)2]+.
starting materials, and a concomitant decrease in the 31P−195Pt
coupling constant of about 500 Hz.8c,11b,23 In contrast, addition
of MeCN to T-shaped cationic platinum haloboryls usually
leads to decomposition, as reported for trans-[Pt{B(Br)(Fc)}-
(PCy3)2]+.8c
(i). Adduct Formation of a T-Shaped Cationic
Haloboryl Complex. Contrasting these findings, we report
here the equimolar reaction of acetonitrile with the T-shaped
cationic amino(halo)boryl platinum complex trans-[Pt{B(Br)-
(NMe2)}(PCy3)2][BArf4] (2) (Arf = 3,5-C6H3(CF3)2). Addi-
tion of dissolved MeCN in CD2Cl2 to a yellowish solution of 2
led to the formation of a colorless solution. Monitoring the
reaction via multinuclear NMR spectroscopy revealed a new
31P NMR signal at 32.9 ppm with platinum satellites (1JP−Pt
=
2786 Hz). In the case of this reaction, the new resonance
showed a shift of 10 ppm to higher field, but only a small (ca.
20 Hz) decrease in the 31P−195Pt coupling constant compared
1
to the starting material (δ = 42.8, JP−Pt = 2803 Hz). This can
be attributed to the fact that the smaller base MeCN does not
attack the boron atom but occupies the vacant coordination site
at the platinum center due to steric reasons. While no 11B NMR
signal could be detected for the T-shaped cationic precursor,9
the spectrum of the new compound exhibits a very weak signal
at 25 ppm, which is comparable to that of trans-[Pt{B(Br)-
RESULTS AND DISCUSSION
■
T-shaped cationic complexes of the general formula trans-
[Pt(L)(PCy3)2]+ (L = BR2) are extraordinarily stable in
comparison to those in which the ligand L is not a boron-
centered ligand.8c,9 Due to the highly unsaturated, Lewis-acidic
nature of the metal center in cationic 14-electron d8 [ML3]
complexes, the reactivity of these species toward even weakly
coordinating nucleophiles is greatly enhanced, leading to
interactions with solvent molecules, weakly coordinating
anions,21 or agostic C−H bonds.22 In the Pt boryl complexes
investigated in our laboratories, trans-[Pt(BRR′)X(PCy3)2] (R
= e.g., NMe2, tBu, Me, Fc; RR′ = Cat′, X = Br; R = R′ = X = I,
Br; R = R′ = F, X = Br, Cl, BF4), the boryl ligand’s trans-
influence generally prevents such interactions with the vacant
site, an effect that can be moderated by altering the electron-
releasing ability of the boryl substituents. This phenomenon
validates the well-established exceptions in which the boron
atom bears electronegative substituents, namely, trans-[Pt-
(BCat)(PCy3)2][BArf4] (Cat = catecholato, C6H4O2) and the
cationic oxoboryl complex trans-[Pt(BO)(PCy3)2][pftb] (pftb
= OC(CF3)3). The former complex can alternatively form an
adduct with 4-picoline (= 4-Me-C5H4N) or interact with
solvent molecules (CD2Cl2) (as seen by low-temperature NMR
experiments).9 The latter complex forms an adduct with
acetonitrile (MeCN) upon halide abstraction, as an alternative
to the formation of a BO-cyclodimerization product in the
absence of donor solvents.13a It should be mentioned that, in
(NMe2)}Br(PCy3)2] (30 ppm).7c The H NMR signal of the
1
methyl group could be detected at 2.17 ppm (trans-[Pt(BO)-
(NCMe)(PCy3)2][BArf4] (4) = 2.46 ppm) and the 13C{1H}
signal at 2.9 ppm (4 = 4.0 ppm).
Layering the reaction mixture with hexane and cooling to
−35 °C led to the formation of colorless single crystals of trans-
[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2][BArf4] (3) suitable for
X-ray diffraction analysis (for crystallographic details see
Supporting Information). The compound crystallizes in the
triclinic space group P1. The molecular structure (Figure 1)
̅
confirms the formation of a square-planar cationic platinum
complex, in which MeCN is bound to the platinum center. The
B−Pt−N1 and the P1−Pt−P2 angles of 173.7(2)° and
170.5(1)°, respectively, lie in the typical range of slightly
distorted square-planar Pt complexes.
Compared to its precursor, the Pt−B bond is slightly
elongated (201.7(5) pm in 3 and 198.7(4) pm in 2)9 and
comparable to the isoelectronic, neutral parent compound
trans-[Pt{B(Br)(NMe2)}Br(PCy3)2] (Pt−B = 200.9(3) pm).7c
The Pt−N1 distance (217.7(4) pm) is distinctly longer than
that of trans-[Pt(BO)(NCMe)(PCy3)2][BArf4] (4) (209.4(3)
pm),13c indicative of the stronger trans-influence of the more
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Scheme 2. Synthesis of Derivatives of 3 Using Alternative
a
Counterions
Figure 1. Molecular structure of trans-[Pt{B(Br)(NMe2)}(NCMe)-
(PCy3)2][BArf4] (3) in the crystal. The thermal ellipsoids of the
phosphine substituents, counterion ([BArf4]), cocrystallized dichloro-
methane, and hydrogen atoms were omitted for clarity. Thermal
ellipsoids are displayed at the 50% probability level.
electron-releasing boryl moiety {B(Br)(NMe2)}. Still, the Pt−
N1 distance in trans-[PtH(NCMe)(PCy3)2][BPh4] (5) is
shorter (Pt−N 207.9(6) pm),24 indicating the strong trans-
influence of the boryl moiety. The N1−C1 triple bond
(114.1(5) pm) in contrast does not show a distinct elongation
compared to 4 (112.9(5) pm) and 5 (112(1) pm).
a
WCA = weakly coordinating anion.
(ii). Synthesis of Platinum Boryl Complexes by
Variation of the Weakly Coordinating Anion. Attempts
to vary the counterion using several main-group salts such as
Na2SO4 or Na2SiF6 in the T-shaped cationic systems were
unsuccessful and led to either no reaction or decomposition.
Exclusively weakly coordinating anions, i.e., sodium or silver
salts of tetraarylborates (e.g., [BArf4]−, [B(C6F5)4]−, [BPh4]−),
hexabromocarbaborate (e.g., [CB11Br6H6]−), or tetraalkoxides
of aluminum ([Al(pftb)4]−; pftb = OC(CF3)3) were suitable
alternatives to the routinely employed Na[BArf4] (Arf = 3,5-
C6H3(CF3)2) for the halide abstraction. In a typical reaction,
addition of equimolar amounts of the salt of the weakly
coordinating anion to trans-[Pt{B(Br)(NMe2)}Br(PCy3)2]
readily provided the corresponding T-shaped platinum boryl
complexes. It should be mentioned that the closo-dodecaper-
chloroborate salt Na2[B12Cl12],25 which shows very low
solubility in polar organic solvents such as CH2Cl2 or C6H5F,
could also be successfully used as a suitable abstracting reagent
after ultrasonic trituration of a 2:1 reaction mixture in these
solvents for two hours. The resulting A2X salt shows very good
solubility and yielded amorphous solids after filtering off the
sedimented NaBr salt and evaporation of the solvent.
Figure 2. Molecular structure of trans-[Pt{B(Br)(NMe2)}(NCMe)-
(PCy3)2]2[B12Cl12] (6) in the crystal. The cations of the A2X salt are
crystallographically identical. The thermal ellipsoids of the phosphine
substituents, cocrystallized dichloromethane, toluene, and hydrogen
atoms were omitted for clarity. Thermal ellipsoids are displayed at the
50% probability level.
similar to those of 3 and need no further discussion (Table 1),
except that the MeCN ligand is no longer linearly bound to the
In order to enhance the crystallization properties of the
synthesized compounds, we attempted to saturate the 14-
electron complex trans-[Pt{B(Br)(NMe2)}(PCy3)2]2[B12Cl12]
(6) both electronically and coordinatively through the addition
of an excess of MeCN to a light yellow solution of the
compound. Decoloration of the solution, monitoring of the
reaction via multinuclear spectroscopy (δ 31P{1H} 6 = 28.8,
1JP−Pt = 2778 Hz; δ 11B{1H} = 24.4), and comparison of the
spectroscopic data with those of trans-[Pt{B(Br)(NMe2)}-
(NCMe)(PCy3)2][BArf4] (3) indicated the formation of similar
platinum-phosphine environments. Layering the reaction
mixture with hexanes and slow evaporation of the solvents
yielded colorless crystals suitable for X-ray diffraction (Figure
2). The compound crystallizes in the monoclinic space group
Table 1. Bond Distances (pm) and Angles (deg) in
trans-[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2][BArf4] (3) and
trans-[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2]2[B12Cl12] (6)
3
6
N1−C1
Pt−B
Pt−N1
average Pt−P
P1−Pt−P2
B−Pt−N1
Pt−N1−C1
N1−C1−C2
114.1(5)
201.7(5)
217.7(4)
233.5
114.5(5)
200.5(4)
216.8(3)
234.3
170.5(1)
173.7(2)
170.9(4)
179.1(5)
168.0(1)
164.5(1)
149.5(3)
175.1(4)
P
21/n, and the observed bond distances are unobtrusive and very
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platinum (Pt−N1−C1 = 149.5(3)°). This might be explained
by packing effects. However, the geometrical arrangement of
the counterion in the molecular structure shows no deviation
from perfect icosahedral symmetry, and therefore only a very
weak interaction between the two ions is suggested. The weak
interaction is also accompanied in solution by slight differences
and crystallization from toluene led to the formation of
colorless crystals of 8, which were found to be suitable for X-ray
diffraction (Figure 3).
1
in the positions of the 13C{1H} and H NMR signals (δ
1
13C{1H} = 4.2; δ H = 2.46) compared with 3 (δ 13C{1H} =
1
2.9; δ H = 2.17).,
Reactivity of Platinum Boryl Complexes toward Lewis-
Acidic Boranes. The platinum boryls investigated in our
laboratories display a specific reactivity toward Lewis-acidic
boranes. As we reported earlier, reaction of trans-[Pt(BI2)I-
(PCy3)2] with BI3 or trans-[Pt{B(Br)(Fc)}Br(PCy3)2] with
BBr2Fc (Fc = ferrocenyl) led to the abstraction of one
phosphine ligand with concomitant buildup of the correspond-
ing phosphine-borane adduct and formation of halide-bridged
dinuclear complexes [Pt{B(X)(R)}(μ-X)(PCy3)]2 (X = Br,
I).7e,11a
(iii). Phosphine Abstraction from a Platinum Bis-
(phosphine) Boryl Complex via a Lewis-Acidic Borane.
In a similar reaction of [Pt(PCy3)2] with two equivalents of
BBr2tBu in benzene, the formation of the known complex trans-
[Pt{B(Br)(tBu)}Br(PCy3)2] (7), characterized by its character-
Figure 3. Molecular structure of [Pt{B(Br)(tBu)}(μ-Br)(PCy3)]2 (8)
in the crystal. Cocrystallized toluene, hydrogen atoms, and thermal
ellipsoids of the phosphine substituents were omitted for clarity.
Thermal ellipsoids are displayed at the 50% probability level.
istic 31P{1H} NMR shift (23.7 ppm, JP−Pt = 2962 Hz),7c the
1
phosphine-borane adduct Cy3P−BBr2tBu (δ 31P{1H} = 0.0, δ
11B = 3.1), and a new platinum-containing compound (δ
1
31P{1H} = 28.2, JP−Pt = 4740 Hz) was observed, of which the
The molecular structure of 8 displays a virtually centrosym-
metric dinuclear moiety in which two platinum centers are
bridged by two bromo ligands. Only one platinum environment
will be discussed here. The metal center shows a slightly
distorted square-planar coordination sphere (P1−Pt1−Br1′ =
171.7(1)°; B1−Pt1−Br1 = 164.3(1)°), and the Pt2Br2 fragment
adopts the geometric arrangement of a slight butterfly (torsion
angle Br1−Pt−Br1′−Pt′ = 17.0(1)°). The Pt−Br1 bond trans to
the boryl moiety (269.2(1) pm) is about 1% longer than that in
9 (267.9(6) pm) and 8% longer than the Pt−Br1′ bond trans to
the phosphine (249.4(1) pm), indicative of the strong trans-
influence of the boryl ligand. The {BBrtBu} group, with its
strong σ-donating substituent, was found to exert the strongest
trans-influence of all boryl groups examined since 2007.7c The
Pt−B distance (197.3(4) pm) is in the expected range for these
compounds (Table 2).
latter data were assigned to the dinuclear complex [Pt{B(Br)-
(tBu)}(μ-Br)(PCy3)]2 (8) (cf. [Pt{B(Br)(Fc)}(μ-Br)(PCy3)]2
(9), δ = 23.6, JP−Pt = 4739 Hz).11a
1
Scheme 3. Synthesis of [Pt{B(Br)(tBu)}(μ-Br)(PCy3)]2 (8)
via Phosphine Abstraction
Table 2. Bond Distances (pm) and Angles (deg) in
[Pt{B(Br)(tBu)}(μ-Br)(PCy3)]2 (8) and
[Pt{B(Br)(Fc)}(μ-Br)(PCy3)]2 (9)
8
9
Pt−Br1
Pt−Br1′
Pt−B
Pt−P
P−Pt−Br1′
B−Pt−Br1
Br1−Pt−Br1′−Pt′
269.2(1)
249.4(1)
197.3(4)
225.0(1)
171.7(1)
164.3(1)
17.0(1)
267.9(1)
249.2(1)
198.3(1)
224.2(1)
170.1(1)
167.3(1)
34.1(1)
It should be emphasized that in contrast to known
monophosphine platinum boryl complexes,7e,11a which show
poor solubility even in polar solvents such as dichloromethane
or fluorobenzene, the new compound 8 shows very good
solubility even in hexanes, a feature that might be useful for
The mixture was heated to 50 °C for five hours, after which
the consumption of 7 was complete. Evaporation of the solvent
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investigating the subsequent reactivity of this class of
compounds.
(iv). Formation of a Cationic Platinum Borylene
Complex by Halide Abstraction via a Strongly Lewis-
Acidic Borane. Contrasting these observations, the addition of
an excess of BBr2Mes to a solution of trans-[Pt{B(Br)(Mes)}-
Br(PCy3)2] (10) did not lead to a reaction, perhaps due to the
steric hindrance of the mesityl substituent at the boron and the
reduced Lewis acidity of the parent borane, due to the π−p
overlap from the aromatic substituent to the vacant orbital of
boron. However, addition of the more Lewis acidic BBr3 to a
solution of 10 in benzene led to the precipitation of a colorless
solid. Whereas the investigation of the solution via multinuclear
NMR spectroscopy did not give information about the reaction
products, dissolution of the solid in dichloromethane enabled
the detection of new signals. A high-field-shifted (with respect
to its precursor 10) singlet at −24.5 ppm could be detected in
the 11B NMR spectrum, which suggests the formation of
tetrabromoborate.26 In analogy, the formation of the unusual
dinuclear cationic complex [Pt2(BCl2)2(PMe3)4(μ-Cl)][BCl4]
linked by a single bridging chloro ligand was observed by the
formal reaction of two equivalents of trans-[Pt(BCl2)Cl-
(PMe3)2] with one equivalent of BCl3.27 A doublet
corresponding to a phosphine-borane adduct could not be
detected. The 31P{1H} NMR spectrum shows a singlet at 45
ppm with platinum satellites (1JP−Pt = 2073 Hz). Comparison
with the 31P{1H} NMR signal of trans-[Pt(BMes)Br(PCy3)2]-
[B(C6F5)4]10b (11) suggested the formation of the cationic
platinum borylene complex fragment with concomitant buildup
of [BBr4]−.
Figure 4. Molecular structure of trans-[Pt(BMes)Br(PCy3)2][BBr4]
(12) in the crystal. Cocrystallized dichloromethane, hydrogen atoms,
and thermal ellipsoids of the phosphine substituents were omitted for
clarity. Thermal ellipsoids are displayed at the 50% probability level.
Table 3. Bond Distances (pm) and Angles (deg) in trans-
[Pt(BMes)Br(PCy3)2][B(C6F5)4] (11) and trans-
[Pt(BMes)Br(PCy3)2][BBr4] (12)
11
12
Pt−Br1
Pt−B
average Pt−P
P−Pt−P
B−Pt−Br
254.1(1)
185.9(3)
236.0
251.9(1)
186.3(4)
236.5
Scheme 4. Reactivity of 10 toward Lewis-Acidic Boranes
171.7(1)
178.1(1)
168.7(1)
167.3(1)
equimolar reaction of BCl3 with 1 we could isolate trans-
[Pt(BCl2)Cl(PCy3)2] (15) as a colorless, moderately air- and
moisture-stable solid, which appears to be a suitable precursor
for the formation of 13. The 31P{1H} NMR spectrum shows a
singlet at 24.2 ppm with 195Pt satellites (1JP−Pt = 2675 Hz) that
lie in the range of other derivatives of this type, e.g., trans-
[Pt{B(Br)(NMe2)}Br(PCy3)2] (26.8 ppm, 1JP−Pt = 2845 Hz).7d
Due to the unresolved coupling to the 31P and 195Pt nuclei, the
11B NMR shows a very broad singlet at 59.4 ppm (ω1/2 ≈ 1500
Hz).
The isolation of 15 completes the series of dihaloboryl
complexes of the general formula trans-[Pt(BHal2)X(PCy3)2]
(Hal = X = I, Br, Cl; Hal = F, X = Br, Cl, BF4).7a,c,e X-ray
diffraction experiments performed on suitable single crystals
(Figure 5) show a platinum center in a slightly distorted square-
planar coordination sphere (P−Pt−P = 174.11(3)° and B−Pt−
Cl = 178.7(1)°). The Pt−Cl bond (244.1(2) pm) is about 10%
longer than those in trans-[PtCl2(PCy3)2] (231.7(2) pm),29 but
lies in the range of other chloroboryl complexes of the general
formula trans-[Pt(BCl2)Cl(PR3)2] (R3 = Ph3, MePh2,
Me2Ph),27 which can be assigned to the strong trans-influence
of the boryl moiety.
X-ray diffraction experiments confirm the formation of the
suggested complex salt trans-[Pt(BMes)Br(PCy3)2][BBr4] (12)
(Figure 4).
The molecular structure of 12 reveals a completely separated
pair of ions. The cation is essentially identical to that of the
related borylene 11 and needs no further discussion (Table 3).
(v). Synthesis of trans-[Pt(BCl2)(PCy3)2]+: A Cationic
Boryl Complex Isoelectronic to the Metal-Only Lewis
Pair trans-[Pt(BeCl2)(PCy3)2]. Due to the fact that the late
transition metal base [Pt(PCy3)2] (1) forms dative bonds to
several s-, p-, and d-block elements,18−20,28 we were especially
interested in the structural characterization of the cationic
system trans-[Pt(BCl2)(PCy3)2]+ (13), which is isoelectronic to
the metal-only Lewis pair trans-[Pt(BeCl2)(PCy3)2)]18 (14). By
Addition of equimolar amounts of Na[BArf4] to a colorless
solution of 15 in CD2Cl2 led to the formation of a light yellow
solution and the precipitation of a colorless solid. NMR
experiments performed on the reaction mixture revealed a
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shorter than in the precursor 15, which lies in the range of
other cationic platinum boryl complexes in which one bromo
ligand has been abstracted.8c,9,14 The solid-state molecular
structures of the isoelectronic complexes 13 and 14 appear very
similar, although the bond distances around the metal center
and the element atom differ (Table 4). This is most likely due
Table 4. Bond Distances (pm) and Angles (deg) in trans-
[Pt(BCl2)(PCy3)2]+ (13) and trans-[Pt(BeCl2)(PCy3)2] (14)
E = B (13)
E = Be (14)
Pt−E
194.2(3)
232.5
216.8(4)
228.7
average Pt−P
average E−Cl
closest Pt−C
P−Pt−P
175.3
192.2
302.4(2)
170.1(1)
102.1(1)
324.2(3)
172.6(1)
107.6(1)
Pt−P−C
Figure 5. Molecular structure of trans-[Pt(BCl2)Cl(PCy3)2] (15) in
the solid state. Hydrogen atoms and thermal ellipsoids of the
phosphine substituents were omitted for clarity. Thermal ellipsoids are
displayed at the 50% probability level.
to the different nature of the bonding situations prevailing in
these systems. Frenking et al.28 showed via DFT calculations
that the metal fragment in 14 acts as both a σ and a π donor,
while it was demonstrated that in transition metal boryl
complexes the boryl group acts as a σ donor,7b,8a−c and the π
back-donation into the vacant orbital at the boron atom is weak
to negligible.8d−r Also the bond distances of neutral and
cationic compounds cannot be compared directly because of
the ionic character of 13 and crystal-packing effects.
31P{1H} NMR signal at 43.5 ppm (1JP−Pt = 2657 Hz) that is low
field shifted with respect to that of the precursor.
Scheme 5. Synthesis of trans-[Pt(BCl2)Cl(PCy3)2] (15) and
trans-[Pt(BCl2)(PCy3)2]+ (13)
In order to gain deeper insights into the nature of Pt−B and
Pt−Be bonds in the isoelectronic systems, we performed
quantum chemical calculations at the ZORA BP86/TZ2P level.
By structural optimization of both complexes restricted with a
Cs symmetry plane (the maximum symmetry allowed within the
particular substitution pattern in these species), we were able to
separate σ and π orbital contributions by an energy
decomposition analysis (EDA, Table 5), which is discussed
Filtering off the precipitate and slow evaporation of the
solvent enabled the isolation of yellow, rhombic single crystals.
The molecular structure of [13][BArf4] (Figure 6) shows a
T-shaped platinum center, with the phosphine ligands situated
Table 5. EDA Results for the Bonding Analysis in Both
a
Species Calculated at the ZORA BP86/TZ2P Level
13
14
symmetry
ΔEint
Cs
Cs
−153.73
−40.63
ΔEPauli
155.24
80.31
b
ΔVelstat
−118.69 (38.4%)
−190.27 (61.6%)
−160.67 (84.4%)
−29.61 (15.6%)
58.60
−66.75 (55.2%)
−54.18 (44.8%)
−44.42 (82.0%)
−9.76 (18.0%)
21.64
b
ΔEoi
ΔEσ
ΔEπ
c
c
ΔEprep
ΔE (= −De)
r(Pt−E)
Energy values are in kcal mol−1
−95.13
196.2
−18.99
220.7
d
a
b
.
The percentage values in
parentheses give the contribution to the total attractive interactions
c
ΔEelstat + ΔEoi. The percentage values in parentheses give the
d
Figure 6. Molecular structure of trans-[Pt(BCl2)(PCy3)2]+ (13) in the
solid state. The counterion ([BArf4]), cocrystallized dichloromethane,
hydrogen atoms, and the thermal ellipsoids of the phosphine
substituents were omitted for clarity. Thermal ellipsoids are displayed
at the 50% probability level.
contribution to the total orbital interactions, ΔEoi. Calculated length
of the analyzed bond in pm (E = B, Be).
below (see also the Computational Methodology section in the
Supporting Information). However, it should be indicated that
the true minima are 12.0 kcal/mol (13) and 12.2 kcal/mol (14)
lower in energy and possess C1 symmetry. In all cases, the
optimized molecular structures are in good agreement with
experimental ones (Table 5). The values obtained for trans-
[Pt(BeCl2)(PCy3)2] are very close to those reported by
mutually trans. This discrete mononuclear arrangement
strongly contrasts the aforementioned dinuclear cationic
complex trans-[Pt2(BCl2)2(PMe3)4(μ-Cl)][BCl4].27 The
{BCl2} fragment is staggered by 72.5(2)° with respect to the
P−Pt−P axis, and the Pt−B bond (194.2(3) pm) is about 1.5%
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Frenking et al., who previously performed an analysis on a
simplified model.28
molecular plane for maximizing the number of these
interactions. Moreover, we located a curved bond path
connecting the Pt center and one proximal hydrogen in both
complexes. More specifically, there are two interactions
between the metal center and the phosphine substituents:
one between the dxz orbital of Pt and the π* orbital of one
vicinal CH group and one between the σCH* orbital and the
According to our calculations, the Pt−B bond dissociation
energy of 13 is five times that of the electron-neutral trans-
[Pt(BeCl2)(PCy3)2]. The different strength of the stabilization
of the Pt−B bond in 13 is substantiated by orbital interactions
(ΔEoi), which show a value of 190.3 kcal per mole, representing
nearly 62% of the total attractive interactions in the system. In
contrast, the Pt−Be interaction has a much smaller value of
54.2 kcal per mole, and this represents only 45% of the total
stabilizing interactions in 14. Moreover, σ interactions in trans-
[Pt(BCl2)(PCy3)2]+ contribute more to bond stabilization
(84%) than in trans-[Pt(BeCl2)(PCy3)2] (82%). Steric
repulsions display the same scaled trend in both complexes.
Further (de)stabilization results from the deformation energies
(ΔEprep), whereas the {Pt(PCy3)2} fragment in 13 is more
distorted than that in 14.
2
2
dx −y orbital of Pt. As mentioned above, this agostic interaction
was not observed experimentally.
(vi). Stabilization of a T-Shaped Cationic Complex
with Reduced Steric Bulk at the Phosphine Substitu-
ents. The platinum boryl systems investigated in our
laboratories comprise the sterically extremely demanding
PCy3 ligand, which kinetically stabilizes the boryl complexes.
In contrast to d8 ML3 systems in which agostic interactions
were found to activate ligand C−H bonds (e.g., [PtMe-
(PiPr3)2][1-H-closo-CB11Me11]: Pt−C = 285.9 pm),22g such an
interaction could be excluded in [Pt{B(Br)(Fc)}(PCy3)2]-
[BArf4] (16) via the structural characterization, low-temper-
ature NMR experiments, and ELF calculations.8c In order to
investigate whether trialkylphosphines other than PCy3 have
the potential to stabilize T-shaped cationic boryl complexes, we
attempted to prepare the PiPr3 derivative of trans-[Pt{B(Br)-
(Fc)}(PCy3)2][BArf4] (16). The compound trans-[Pt{B(Br)-
(Fc)}Br(PiPr3)2] (17), which is the iPr3P derivative of trans-
[Pt{B(Br)(Fc)}Br(PCy3)2] (18),7b appeared to be a suitable
precursor for trans-[Pt{B(Br)(Fc)}(PiPr3)2][BArf4] (19).
Because the tris(phosphine) precursor [Pt(PiPr3)3] 20 is
known to dissociate to form a bis(phosphine) complex and
one equivalent of free PiPr3 when dissolved in organic
solvents,30 the formation of phosphine-borane Lewis adducts
in the course of the reaction was assumed. For this reason two
equivalents of FcBBr2 were added to a yellowish solution of 20
in C6D6, which led to the formation of a light red solution and
concomitant precipitation of a red solid. Monitoring the
reaction via multinuclear NMR spectroscopy revealed a new
signal in the 31P{1H} NMR spectrum at 30.7 ppm with 195Pt
satellites (1JP−Pt = 2936 Hz), which is shifted to high field with
Furthermore, we computed the degree of donation and back-
donation in these compounds as well as atomic charges derived
from our analysis (Table 6). As expected, a large electron
Table 6. Results of Structural Analyses in trans-
[Pt(ECl2)(PCy3)2]n (13, E = B, n = +; 14, E = Be, n = 0)
Calculated at the ZORA BP86/TZ2P Level
13
1.12
14
0.41
donation (d)
back-donation (b)
ratio d/b
0.30
3.73
0.18
2.28
VDD atomic charges (in au) and dipole moment (in D)
Q(Pt)
Q(E)
Q(P)
Q(Cl)
μ
−0.086
0.009
−0.281
0.043
0.127
0.100
−0.014
2.024
−0.220
6.069
donation from the metal center toward boron can be observed.
As a result, the platinum fragment in the cationic compound 13
displays an electron donation nearly 1.5 times that of the
platinum fragment to the BeCl2 fragment in 14. The ratios of
donation/back-donation in the particular systems suggest that
this effect is more important for the stabilization in 13 rather
than in 14, which is in good agreement with the
aforementioned EDA results.
Voronoi-like atomic charges show that the Pt center in the
cationic boron complex displays only a small negative value and
the B and Cl atoms are nearly electron neutral. Compared to
these values, larger negative values for both chlorine and
platinum atoms and a larger positive value for the Be atom in
14 are calculated. Also, the P atoms in 13 show positive values
larger than those in 14 due to the strong donation from both
phosphines to the metal center. This indicates a compensation
of the global positive charge in 13 by electron density flows
from Pt and Cl atoms to the boron atom (increasing donation
from phosphorus to Pt as well) and back-donation from the
boron to the Pt center, which strengthens the Pt−B bond in
this complex. The dipole moment value in 14 is three times
that of complex 13, which is in line with the results of the EDA.
Analysis of the topology of the electron density from the fully
relaxed geometry optimizations results in expected bond and
ring critical points along with bond paths between chlorines
and hydrogens of cyclohexyl groups (Figure S1 in Supporting
Information). Therefore, {ECl2} fragments deviate from the Cs
respect to its precursor [Pt(PiPr3)2] (21) (δ 31P = 72.8, 1JP−Pt
=
4202 Hz) and a broad singlet at −1 ppm, which was assigned to
the phosphine-borane adduct iPr3P−B(Fc)Br2.30 A broad
singlet was detected in the 11B NMR spectrum at 72.9 ppm
(w1/2 ≈ 1680 Hz), which is in good agreement with that of 18
(δ 11B = 82, ω1/2 ≈ 1980 Hz) and a broad singlet at −4.4 ppm,
which was assigned to the phosphine-borane adduct. Filtering
off the precipitate, evaporation of the solvent, and recrystalliza-
tion from toluene led to the formation of single crystals suitable
for X-ray diffraction.
Using freshly prepared 21 from trans-[PtCl2(PiPr3)2] and
Na[C10H8] in tetrahydrofuran, in situ reaction with BBr2Fc
improved the yield of 17 to 71%.
The molecular structure of 17 (Figure 7) shows a platinum
center in a slightly distorted square-planar coordination
geometry. The Pt−B bond (197.4(3) pm) is about 1% shorter
than that in 18 (199.6(4) pm), which is most likely due to the
difference in the Tolman cone angle (PiPr3 = 160 10°; PCy3
= 179
10°)31 and the decreased steric demand at the
periphery of the PiPr3 ligand compared to PCy3, thus relieving
the interaction of the ligands with the Fc group.31 The other
bond lengths and angles are unremarkable and very similar to
those in 18 and require no further discussion (Table 7).
Addition of Na[BArf4] to a solution of 17 in CD2Cl2
intensified the red color of the solution and furthermore led
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Figure 7. Molecular structure of trans-[Pt{B(Br)(Fc)}Br(PiPr3)2]
(17) in the crystal. Cocrystallized toluene, disorder of one phosphine
group, and thermal ellipsoids of the phosphine substituents were
omitted for clarity. Thermal ellipsoids are displayed at the 50%
probability level.
Figure 8. Solid-state molecular structure of trans-[Pt{B(Br)(Fc)}-
(PiPr3)2][BArf4] (19). Counterion ([BArf4]−), hydrogen atoms, and
the thermal ellipsoids of the phosphine substituents are omitted for
clarity. Thermal ellipsoids are displayed at the 50% probability level.
other cationic 14-electron platinum compounds, for which
agostic interactions were reported.22 The disorder of the
particular propyl group might render an agostic interaction
unlikely, as such an interaction would fix the geometric
arrangement of the isopropyl group. The bond distances of 16
and 19 are similar and need no further discussion (Table 8).
Table 7. Bond Distances (pm) and Angles (deg) in trans-
[Pt{B(Br)(Fc)}Br(PiPr3)2] (17) and trans-
[Pt{B(Br)(Fc)}Br(PCy3)2] (18)
17
18
Pt−Br1
Pt−B
average P−Pt
P−Pt−P
Br−Pt−B
261.1(1)
197.4(3)
233.0
161.4(1)
199.6(3)
234.2
Table 8. Bond Distances and Angles in
trans-[Pt{B(Br)(Fc)}(PiPr3)2][BArf4] (19) and
trans-[Pt{B(Br)(Fc)}(PCy3)2][BArf4]
172.4(1)
169.1(1)
170.0(1)
172.7(1)
to precipitation of a colorless solid within minutes. A 31P{1H}
NMR spectrum performed on the reaction mixture shows a
singlet at 56.7 ppm (1JP−Pt = 2936 Hz), which is low-field
shifted by about 26 ppm with respect to that of the starting
material 17. Besides the sharp 11B NMR resonance at −7.6
ppm for the borate anion, a broad resonance at 43.3 ppm (ω1/2
≈ 1180 Hz) could be observed. This value is high-field shifted
by about 30 ppm compared with that of its precursor, which lies
in the expected range for T-shaped cationic boryl complexes,9
and constitutes the first detectable 11B NMR shift of a cationic
platinum ferrocenylboryl complex.8c
19
16
Pt−B
average Pt−P
P−Pt−P
195.6(2)
231.8
196.6(4)
232.0
164.76(2)
163.0(1)
CONCLUSION
■
The reactivity studies performed on a series of platinum boryl
complexes reported in this article include the full character-
ization of the first base adducts of cationic haloboryl complexes
in which the base attacks the metal center and not the boron-
based ligand. The first highly soluble tBu-substituted dinuclear
platinum boryl complex was obtained by abstraction of one
phosphine ligand from the initially formed mononuclear boryl
complex by reaction of the low-valent platinum precursor
bis(tricyclohexylphosphine)platinum(0) with two equivalents
of borane. In contrast, the mesityl-substituted boryl complex
reacts with a Lewis-acidic borane with formation of the cationic
borylene complex by abstraction of the bromo ligand and
concomitant buildup of the corresponding borate.
The bonding situation in structurally optimized full geo-
metries restricted to Cs symmetry was analyzed via DFT
calculations of the fully characterized T-shaped cationic
dichloro boryl complex reported herein and the isoelectronic
beryllium dichloride adduct of the 14-electron platinum
precursor. We could show that the dissociation energy of the
cationic boryl complex is five times that of the metal-only Lewis
pair and also the metal−element bond has a more covalent
character than in the beryllium dichloride adduct. Although
agostic interactions could not be observed experimentally, weak
orbital interactions between the metal center and one proximal
CH group of a phosphine substituent were found in both
Scheme 6. Synthesis of the T-Shaped Cationic Boryl
Complex trans-[Pt{B(Br)(Fc)}(PiPr3)2][BArf4] (19)
Slow evaporation of the solvent yielded single crystals of
trans-[Pt{B(Br)(Fc)}(PiPr3)2][BArf4] (19), from which an X-
ray diffraction study confirmed that the Pt-bound bromide is
abstracted in the course of the reaction (Figure 8).
The compound crystallizes in the orthorhombic space group
P2(1)2(1)2(1) and shows the appearance of the expected, slightly
distorted, T-shaped cationic three-coordinate platinum center
(P−Pt−P 164.8(1)°; 16 163.0(1)°) in which one propyl group
is disordered. Also, the shortest Pt−C distance is that to a
methine-carbon of the disordered propyl group (320.2 and
338.0 pm) and to the corresponding disordered methyl group
(324.5 and 326.0 pm), which is considerably longer than in
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days, colorless crystals were formed, which were washed gently with a
small amount of cold hexane. Crystallization from toluene yielded
trans-[Pt{B(Br)(tBu)}(μ-Br)(PCy3)]2 (126 mg, 89.8 μmol, 73%).
1H NMR (500.13 MHz, CD2Cl2, 297.6 K): δ 2.14−2.07 (m, 6H,
Cy), 1.85−1.82 (m, 6H, Cy), 1.72−1.65 (m, 9H, Cy), 1.34−1.24 (m,
12H, Cy), 1.21 (s, 9H, tBu). 11B{1H} NMR (160.46 MHz, CD2Cl2,
298.1 K): δ 68.7 (ω ≈ 1700 Hz). 13C{1H} NMR (100.61 MHz,
CD2Cl2, 296.2 K): δ 35.9 (d, 1JC−P = 31 Hz, C1 Cy); 30.3 (br s, Ci tBu,
systems by analysis of the topology of the electron density in
the fully relaxed geometry optimizations.
A T-shaped cationic complex with reduced steric bulk at the
phosphine substituents, compared to its tricyclohexylphosphine
derivative, was fully characterized.
EXPERIMENTAL SECTION
■
3
C3,5 Cy), 29.6 (d, JC−P = 11 Hz, C3,5 Cy), 26.6 (s, C4 Cy). 31P{1H}
All manipulations were conducted either under an atmosphere of dry
argon or in vacuo using standard Schlenk line or glovebox techniques.
[Pt(PCy3)2],32 [Pt(PiPr3)3],33 Na[BArf4],34 BBr2tBu,35 BBr2Mes,36
and BBr2Fc37 were prepared according to published procedures. BCl3
was purchased from GHC Gehrling, Holz + Co and used as received.
BBr3 was purchased from Acros Organics and stirred over Hg prior to
use.
NMR (202.46 MHz, CD2Cl2, 298.0 K): δ 28.2 (1JP−Pt = 4740 Hz).
Anal. Calcd (%) for C44H84B2Br2P2Pt2: C 37.57, H, 6.02. Found: C
38.15, H 5.98.
Synthesis of trans-[Pt(BMes)Br(PCy3)2][BBr4] (12). A solution
of freshly prepared trans-[Pt(BBrMes)Br(PCy3)2] (27.7 mg, 26.1
μmol) was mixed with a solution of BBr3 in C6D6 (0.3 mL, 0.1 M, 30.0
μmol). Within a few minutes, colorless crystals of trans-[Pt(BMes)-
Br(PCy3)2][BBr4] were formed (22.0 mg, 17.0 μmol, 65%).
1H NMR (400.13 MHz, CD2Cl2, 296.1 K): δ 7.05 (s, 2H, Mes),
2.79 (m, 6H, Cy), 2.74 (s, 6H, Meo, Mes), 2.39 (s, 3H, Mep, Mes),
2.05−2.00 (m, 12H, Cy), 1.79−1.61 (m, 30H, Cy), 1.10−1.07 (m,
18H, Cy). 11B{1H} NMR (128.39 MHz, CD2Cl2, 296.1 K): δ −24.5
(s, BBr4). 13C{1H} NMR (100.61 MHz, CD2Cl2, 296.5 K): δ 151.0 (s,
Cp, Mes), 147.1 (s, Co, Mes), 132.0 (br s, Ci, Mes), 130.2 (m, Cm,
Solvents were dried according to standard procedures, degassed,
and stored under argon over activated molecular sieves. C6D6 and
CD2Cl2 were degassed by three freeze−pump−thaw cycles and stored
over molecular sieves. Reagents were dried and purified by standard
procedures. NMR spectra were acquired on a Bruker AMX 400 or a
Bruker Avance 500 NMR spectrometer. Chemical shifts are given in
ppm and are referenced against external Me4Si (1H, 13C), BF3·Et2O
(11B), and H3PO4 (85%, 31P). Assignments were made from the
analysis of 1H,13C-HMQC and 1H,13C-HMBC NMR spectroscopic
experiments. Microanalyses (C, H, N) were performed on a Leco
Instruments elemental analyzer, type CHNS 932.
3
Mes), 37.5 (vt, N = |1JP−C + JP−C| = 28 Hz, C1, Cy); 30.6 (m, C3,5,
Cy), 27.8 (vt, N = |1JP−C + 3JP−C| = 11 Hz, C2,6, Cy), 26.5 (s, C4, Cy),
23.3 (s, Meo, Mes), 22.7 (s, Mep, Mes). 31P{1H} NMR (161.98 MHz,
CD2Cl2, 296.5 K): δ 45.0 (1JP−Pt = 2073 Hz). Anal. Calcd (%) for
C45H77B2Br5P2Pt: C 41.70, H 5.99. Found: C 41.10, H 6.21.
Synthesis of trans-[Pt(BCl2)Cl(PCy3)2] (15). Solid [Pt(PCy3)2]
(201 mg, 265 μmol) was dissolved in 2 mL of toluene and cooled to
−78 °C. Addition of a BCl3 solution in toluene (2 M, 0.14 mL, 280
μmol) led to the formation of a colorless suspension. The mixture was
heated until all solids were dissolved. Slow cooling to ambient
temperature yields trans-[Pt(BCl2)Cl(PCy3)2] as a colorless solid (168
mg, 192 μmol, 72%).
Synthesis of trans-[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2][BArf4] (3).
Addition of MeCN (1.0 mg, 24.4 μmol) to a light yellow solution of
trans-[Pt{B(Br)(NMe2)}(PCy3)2][BArf4] (30.0 mg, 17.1 μmol) in
CD2Cl2 (0.5 mL) led to the formation of a nearly colorless solution of
trans-[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2][BArf4]. Layering the sol-
ution with a few drops of hexane and cooling to −35 °C yielded
colorless crystals (25.0 mg, 13.9 μmol, 82%).
1H NMR (400.13 MHz, CD2Cl2, 296.3 K): δ 7.73 (m, 8H, BArf4),
7.56 (br s, 4H, BArf4), 3.18 (s, 3H, NMe2), 2.89 (s, 3H, NMe2), 2.33−
2.27 (m, 6H, Cy), 2.17 (s, 3H, NCMe), 2.02−1.99 (m, 6H Cy), 1.90−
1.85 (m, 18H, Cy), 1.78−1.75 (m, 6H, Cy), 1.67−1.51 (m, 12H, Cy),
1.31−1.21 (m, 20H, Cy). 11B{1H} NMR (128.38 MHz, CD2Cl2, 296.4
K): δ 25.0 (br s), −6.7. 13C{1H} NMR (100.61 MHz, CD2Cl2, 196.2
1H NMR (400.13 MHz, CD2Cl2, 297.2 K): δ 2.49 (m, 6H, Cy),
2.03 (m, 12H, Cy), 1.81 (m, 12H, Cy), 1.70 (m, 18H, Cy), 1.25 (m,
18H, Cy). 11B{1H} NMR (160.46 MHz, CD2Cl2, 297.2 K): δ 59.0 (br
s, ω1/2 ≈ 1500 Hz). 13C{1H} NMR (100.61 MHz, CD2Cl2, 297.2 K): δ
35.4 (m, C1, Cy), 30.4 (m,C3,5, Cy), 28.0 (m, C2,6, Cy), 26.9 (m, C4,
Cy). 31P{1H} NMR (161.98 MHz, CD2Cl2, 297.2 K): δ 24.2 (s, 1JP−Pt
= 2675 Hz). Anal. Calcd (%) for C36H66BCl3P2Pt: C 49.52, H 7.62.
Found: C 49.42, H 7.64.
1
K): δ 162.1 (q, Ci, JC−B = 50 Hz), 135.2 (s, Co, BArf4), 129.2 (br q,
2JC−F = 32 Hz Cm, BArf4), 125.0 (q, 1JC−F = 272 Hz, CF3, BArf4), 117.7
(br s, Cp, BArf4). 47.2 (s, NMe2), 41.1 (s, NMe2), 35.5 (vt, N = |1JP−C
+ 3JP−C| = 27 Hz, C1, Cy), 30.8 (m, C3,5, Cy), 28.0 (m, C2,6, Cy), 26.5
(s, C4, Cy), 2.9 (s, MeCN). 19F{1H} NMR (376.50 MHz, CD2Cl2,
296.3 K): δ −62.9. 31P{1H} NMR (161.98 MHz, CD2Cl2, 296.1 K): δ
32.9 (s, 1JP−Pt = 2786 Hz). Anal. Calcd (%) for C72H87B2BrF24N2P2Pt:
C 48.18, H 4.98, N 1.56. Found: C 48.68, H 4.93, N 1.51.
Synthesis of trans-[Pt(BCl2)(PCy3)2][BArf4] (13). Solid trans-
[Pt(BCl2)Cl(PCy3)2] (15) (50.0 mg, 0.057 mmol) and Na[BArf4]
(50.8 mg, 57.3 μmol) are mixed in CD2Cl2. The reaction mixture
immediately turns yellow, and a colorless solid precipitates, which is
filtered off. Layering the solution with hexane and cooling to −35 °C
yields colorless crystals (52.0 mg, 30.6 μmol, 53%).
Synthesis of trans-[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2]2[B12Cl12]
(6). Addition of Na2[B12Cl12] (31.0 mg, 51.6 μmol) to a solution of
trans-[Pt{B(Br)(NMe2)}Br(PCy3)2] (100 mg, 103 μmol) in (0.5 mL)
CD2Cl2 and one drop of MeCN, sonication, and filtering off the
precipitated colorless solid led to the formation of a nearly colorless
solution of trans-[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2]2[B12Cl12].
Layering the solution with a few drops of hexane and cooling to
−35 °C led to the formation of colorless crystals (88.0 mg, 72.8 μmol,
73%).
1H NMR (500.13 MHz, CD2Cl2, 297.2 K): δ 7.73 (m, 8H, BArf4),
7.57 (br s, 4H, BArf4), 2.34 (m, 6H, Cy), 2.91 (m, 24H, Cy), 1.80 (m,
6H, Cy), 1.61 (m, 12H, Cy), 1.33 (m, 18H, Cy). 11B{1H} NMR
(160.46 MHz, CD2Cl2, 297.2 K): δ −7.6 (s, BArf4). 13C{1H} NMR
1
(125.76 MHz, CD2Cl2, 297.2 K): δ 162.2 (q, JC−B = 49.9 Hz, Ci,
2
3
BArf4), 135.2 (s, Co, BArf4), 129.3 (qq, JC−F = 31.4 Hz, JC−B = 3.17
Hz, Cm, BArf4), 125.0 (q, JC−F = 272 Hz, CF3, BArf4), 117.9 (sep,
1
3JC−F = 4.2 Hz, Cp, BArf4), 35.1 (m, C1, Cy), 30.4 (s, C3,5, Cy), 27.4
1H NMR (400.13 MHz, CD2Cl2, 296.3 K): δ 3.17 (s, 3H, NMe2),
2.90 (s, 3H, NMe2), 2.44 (s, 3H, NCMe), 2.32−2.26 (m, 6H, Cy),
2.00−1.78 (m, 30H, Cy), 1.70−1.62 (m, 6H, Cy), 1.57−1.49 (m, 6H,
Cy), 1.34−1.21 (m, 18H, Cy). 11B{1H} NMR (128.38 MHz, CD2Cl2,
296.2 K): δ 24.4 (br s), −12.9 (s, B12Cl12). 13C{1H} NMR (100.61
MHz, CD2Cl2, 296.2 K): 124.1 (s, MeCN), 47.5 (s, NMe2), 41.2 (s,
(m, C2,6, Cy), 26.1 (s, C4, Cy). 31P{1H} NMR (202.46 MHz, CD2Cl2,
1
297.2 K): δ 43.5 (br s, JP−Pt = 2657 Hz). Anal. Calcd (%) for
C68H78B2Cl2F24P2Pt: C 48.02, H 4.62. Found: C 48.21, H 4.79.
Synthesis of trans-[Pt{B(Br)(Fc)}Br(PiPr3)2] (17). Addition of a
solution of BBr2Fc (280 mg, 767 μmol) in toluene (2 mL) to a freshly
prepared solution of [Pt(PiPr3)2] (396 mg, 767 μmol) in toluene (3
mL) results in a light red solution. Cooling of the reaction mixture to
−65 °C overnight led to the formation of a red solid, which was
separated and washed gently with 2 mL of cold hexane (474 mg, 545
μmol, 71%). Single crystals were obtained by crystallization from
dichloromethane at −35 °C.
3
NMe2), 35.5 (vt, N = |1JP−C + JP−C| = 27 Hz, C1, Cy), 30.9 (m, C3,5,
Cy), 28.1 (m, C2,6, Cy), 26.7 (s, C4, Cy), 4.2 (s, MeCN). 31P{1H}
1
NMR (161.98 MHz, CD2Cl2, 296.1 K): δ 28.8 (br s, JP−Pt = 2778
Hz). Anal. Calcd (%) for C80H150B14Br2Cl12N4P4Pt2: C 39.73, H 6.25,
N 2.32. Found: C 40.19, H 6.18, N 2.27.
Synthesis of trans-[Pt{B(Br)(tBu)}(μ-Br)(PCy3)]2 (8). A yellowish
mixture of BBr2tBu (140 mg, 615 μmol) and [Pt(PCy3)2] (186 mg,
246 μmol) in C6D6 (0.5 mL) was heated to 50 °C for 5 h. After 11
1H NMR (500.13 MHz, C6D6, 297.2 K): δ 4.74 (m, 2H, Cp), 4.26
(m, 2H, Cp), 4.09 (s, 5H, Cp), 2.94 (m, 6H, CH), 1.35 (m, 18H, CH3,
1905
dx.doi.org/10.1021/om2012248 | Organometallics 2012, 31, 1897−1907

Organometallics
Article
iPr), 1.28 (m, 18H, CH3, iPr). 11B{1H} NMR (160.46 MHz, C6D6,
297.2 K): δ 72.9 (br s; ω1/2 ≈ 1680 Hz). 13C{1H} NMR (125.76 MHz,
C6D6, 297.2 K): δ 77.2 (br s, C5H5FeC5H4B), 71.8 (br s,
C5H5FeC5H4B), 69.6 (br s, C5H5FeC5H4B), 26.2 (m, CH, iPr), 20.8
(br s, CH3, iPr), 20.6 (br s, CH3, iPr). 31P{1H} NMR (202.46 MHz,
Semple, T. C.; Hartwig, J. F. Science 2000, 287, 1995. (c) Hartwig, J. F.
Chem. Soc. Rev. 2011, 40, 1992.
(7) (a) Bauer, J.; Braunschweig, H.; Kraft, K.; Radacki, K. Angew.
Chem. 2011, 123, 10641; Angew. Chem., Int. Ed. 2011, 50, 10457.
(b) Braunschweig, H.; Radacki, K.; Rais, D.; Seeler, F. Organometallics
2004, 23, 5545. (c) Braunschweig, H.; Brenner, P.; Muller, A.;
Radacki, K.; Rais, D.; Uttinger, K. Chem.Eur. J. 2007, 13, 7171.
(d) Braunschweig, H.; Fuss, M.; Radacki, K.; Uttinger, K. Z. Anorg.
Allg. Chem. 2009, 635, 208. (e) Braunschweig, H.; Radacki, K.;
Uttinger, K. Inorg. Chem. 2007, 46, 8796.
1
C6D6, 297.2 K): δ 30.7 (br s, JP−Pt = 2936 Hz). Anal. Calcd (%) for
C28H51BBr2FeP2Pt: C 38.60, H 5.90. Found: C 39.52, H 5.99.
Synthesis of trans-[Pt{B(Br)(Fc)}(PiPr3)2][BArf4] (20). Solid
trans-[Pt(BBrFc)Br(PiPr3)2] (50.0 mg, 57.4 μmol) and Na[BArf4]
(50.8 mg, 57.3 μmol) were mixed in CD2Cl2 (2 mL), and a colorless
solid precipitated. The mixture was layered with hexane and cooled to
−35 °C, which led to the formation of a deep red solid (47.7 mg, 28.7
μmol, 50%). Dark red single crystals were obtained by crystallization
from dichloromethane at −35 °C.
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Onno, J. M.; Volatron, F. Inorg. Chem. 2002, 41, 6656. (m) Hartwig, J.
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1H NMR (500.13 MHz, C6D6, 296.7 K): δ 7.73 (m, 8H, BArf4),
7.57 (br s, 4H, BArf4), 4.67 (m, 2H, Cp), 4.52 (m, 2H, Cp), 4.22 (s,
5H, Cp), 2.44 (m, 6H, CH), 1.37 (m, 18H, CH3, iPr), 1.34 (m, 18H,
CH3, iPr). 13C{1H} NMR (125.76 MHz, C6D6, 279.2 K): δ 162.2 (q,
2
1JC−B = 49.7 Hz, Ci, BArf4), 135.2 (s, Co, BArf4), 129.6 (qq, JC−F
=
30.9 Hz, JC−B = 3.02 Hz, Cm, BArf4), 125.0 (q, JC−F = 273 Hz, CF3,
3
1
−
BArf4), 117.9 (sep, 3JC
= 4.4 Hz, Cp, BArf4), 76.1 (br s,
F
C5H5FeC5H4B), 73.9 (br s, C5H5FeC5H4B), 70.6 (br s,
C5H5FeC5H4B), 25.0 (m, Ci, iPr), 20.2 (br s, CH3, iPr), 20.0 (br s,
CH3, iPr). 11B{1H} NMR (160.46 MHz, C6D6, 297.2 K): δ 43.3 (br s,
w1/2 ≈ 1180 Hz), −7.6 (s, BArf4). 31P{1H} NMR (202.46 MHz, C6D6,
1
297.2 K): δ 56.7 (s, JP−Pt = 2969 Hz). Anal. Calcd (%) for
C60H63B2BrF24FeP2Pt: C 43.56, H 3.84. Found: C 43.85, H 3.75.
ASSOCIATED CONTENT
■
S
* Supporting Information
Details on the X-ray diffraction and theoretical studies are
provided. CCDC-857112 (3), 857113 (6), 857114 (8), 857115
(12), 857116 (15), 857117 (13), 857118 (17), and 857119
(19) 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. This material is available free of charge via
the Internet at http://pubs.acs.org.
(9) Braunschweig, H.; Radacki, K.; Uttinger, K. Chem.Eur. J. 2008,
14, 7858.
(10) (a) Braunschweig, H.; Radacki, K.; Rais, D.; Seeler, F.; Uttinger,
K. J. Am. Chem. Soc. 2005, 127, 1386. (b) Braunschweig, H.; Radacki,
K.; Uttinger, K. Angew. Chem. 2007, 119, 4054; Angew. Chem., Int. Ed.
2007, 46, 3979.
(11) (a) Braunschweig, H.; Brenner, P.; Radacki, K. Z. Anorg. Allg.
Chem. 2009, 635, 2089. (b) Braunschweig, H.; Radacki, K.; Uttinger,
K. Organometallics 2008, 27, 6005.
AUTHOR INFORMATION
■
Corresponding Author
*Tel: (+49) 931-31-85260. Fax: (+49) 931-31-84623. E-mail:
h.braunschweig@uni-wuerzburg.de.
(12) (a) Braunschweig, H.; Radacki, K.; Rais, D.; Uttinger, K. Angew.
Chem. 2006, 118, 169; Angew. Chem., Int. Ed. 2006, 45, 162.
(b) Braunschweig, H.; Matz, F.; Radacki, K.; Schneider, A.
Organometallics 2010, 29, 3457. (c) Braunschweig, H.; Kupfer, T.;
Radacki, K.; Schneider, A.; Seeler, F.; Uttinger, K.; Wu, H. J. Am.
Chem. Soc. 2008, 130, 7974. (d) Braunschweig, H.; Radacki, K.; Rais,
D.; Schneider, A.; Seeler, F. J. Am. Chem. Soc. 2007, 129, 10350.
(13) (a) Braunschweig, H.; Radacki, K.; Schneider, A. Angew. Chem.
2010, 122, 6130; Angew. Chem., Int. Ed. 2010, 49, 5993.
(b) Braunschweig, H.; Radacki, K.; Schneider, A. Science 2010, 328,
345. (c) Braunschweig, H.; Radacki, K.; Schneider, A. Chem. Commun.
2010, 46, 6473.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Dr. Carsten Knapp and his co-workers for the
donation of Na2[B12Cl12] and the Deutsche Forschungsge-
meinschaft (DFG) for financial support.
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