Reactivity of Tetrahalo- and Difluorodiboranes(4) toward Lewis Basic Platinum(0): Bis(boryl), Borylborato, and Doubly Boryl-Bridged Platinum Complexes

Article abstract of DOI:10.1021/jacs.8b08428

The reaction of the tetrahalodiboranes(4) B₂F₄, B₂Cl₄, and B₂Br₄ with a Lewis basic platinum(0) complex led to the isolation of the cis-bis(difluoroboryl) complex cis-[(Cy₃P)₂Pt(BF₂)₂] (1) and the novel borylborato complexes trans-[(Cy₃P)₂Pt{B(X)-BX₃}] (2, X = Cl; 3, X = Br), respectively. The trans influence of the borylborato group was found to be one of the strongest ever observed experimentally. Furthermore, the reactivity of little-explored diaryldifluorodiboranes(4) F₂B-BMes₂ and the new derivative F₂B-BAn₂ (An = 9-anthryl) toward a range of platinum(0) complexes was investigated. Reactions with relatively nonbulky platinum(0) complexes led to the formation of unsymmetrical cis-bis(boryl) complexes cis-[(R₃P)₂Pt(BF₂)(BMes₂)] (6, R = Me; 7, R = Et) as well as the first example of a fourfold-unsymmetrical bis(boryl) complex, [(Me₃P)(Cy₃P)Pt(BF₂)(BMes₂)] (12). The use of a more bulky Pt complex provided access to the unprecedented dinuclear bis(boryl) complexes [{(iPr₃P)Pt}₂(μ-BF₂)(μ-BAr₂)] (8, Ar = Mes; 9, Ar = An), which feature two different μ₂-bridging boryl ligands.

Full text of DOI:10.1021/jacs.8b08428

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Article
Reactivity of Tetrahalo- and Difluorodiboranes(4)
Towards Lewis-Basic Platinum(0): Bis(boryl), Borylborato
and Doubly Boryl-Bridged Platinum Complexes
Jonas H. Muessig, Dominic Prieschl, Andrea Deißenberger, Rian D. Dewhurst,
Maximilian Dietz, J. Oscar C. Jiménez-Halla, Alexandra Trumpp, Sunewang R. Wang,
Carina Brunecker, Alena Haefner, Annalena Gärtner, Torsten Thiess, Julian Böhnke,
Krzysztof Radacki, Rüdiger Bertermann, Todd B. Marder, and Holger Braunschweig
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08428 • Publication Date (Web): 17 Sep 2018
Downloaded from http://pubs.acs.org on September 17, 2018
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Journal of the American Chemical Society
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Reactivity of Tetrahalo- and Difluorodiboranes(4) Towards Lewis-
Basic Platinum(0): Bis(boryl), Borylborato and Doubly Boryl-
Bridged Platinum Complexes
+
,†,‡
+,†,‡
†,‡
†,‡
Jonas H. Muessig,
Dominic Prieschl,
Andrea Deißenberger, Rian D. Dewhurst, Maximilian
†
,‡
§
†,‡
†,‡
†,‡
Dietz, J. Oscar C. Jiménez-Halla, Alexandra Trumpp, Sunewang R. Wang, Carina Brunecker,
Alena Haefner, Annalena Gärtner, Torsten Thiess, Julian Böhnke, Krzysztof Radacki,
Rüdiger Bertermann, Todd B. Marder, Holger Braunschweig,*
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†
,‡
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†,‡
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,‡
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,†,‡
+
These authors contributed equally to this work
†
Institute for Inorganic Chemistry, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074
Würzburg, Germany
‡
Institute for Sustainable Chemistry & Catalysis with Boron, Am Hubland, 97074 Würzburg, Germany
Departamento de Química, División de Ciencias Naturales y Exactas, Universidad de Guanajuato, No-
§
ria Alta S/N, Col. Noria Alta, Guanajuato, C.P. 36050, Gto., Mexico
ABSTRACT: The reaction of tetrahalodiboranes(4) B F , B Cl and B Br with a Lewis basic platinum(0) complex led to isolation
2
4
2
4
2
4
of cis-bisdifluoroboryl complex cis-[(Cy P) Pt(BF ) ] (1) and novel borylborato complexes trans-[(Cy P) Pt{B(X)−BX }] (2: X =
3
2
2 2
3
2
3
Cl; 3: X = Br), respectively. The trans-influence of the borylborato group was found to be one of the strongest ever observed exper-
imentally. Furthermore, the reactivity of little-explored diaryldifluorodiboranes(4) F B–BMes and the new derivative F B–BAn
2
2
2
2
(An = 9-anthryl) towards a range of platinum(0) complexes was investigated. Reactions with relatively non-bulky platinum(0)
complexes led to the formation of unsymmetrical cis-bis(boryl) complexes cis-[(R P) Pt(BF )(BMes )] (6: R = Me; 7: R = Et), as
3
2
2
2
well as the first example of a fourfold unsymmetrical bis(boryl) complex [(Me P)(Cy P)Pt(BF )(BMes )] (12). Use of a more bulky
3
3
2
2
Pt complex provided access to unprecedented dinuclear bis(boryl) complexes [{(iPr P)Pt} (µ-BF )(µ-BAr )] (8: Ar = Mes; 9: Ar =
3
2
2
2
An), which feature two different bridging (ꢀ ) boryl ligands.
2
INTRODUCTION
ples derived from diaryl- (B
2
X
2
Ar
2
; Ar = Mes, Dur; X = Cl,
5
Br) or diamino (B X (NMe ) ; X = Cl, Br, I) diboranes.
2
2
2 2
1
Since the introduction of B Cl in 1925 by Stock, dibo-
2
4
ranes(4) have become an immensely useful class of com-
pounds in organic and inorganic chemistry and have already
proven their potential as synthetic precursors in many applica-
tions, e.g. functional materials and pharmaceuticals. In par-
ticular, the installation of boryl functional groups into organic
substrates has been of major interest over the last 20 years,
leading to the preparation of a diverse range of substrates for
Suzuki-Miyaura coupling reactions and the installation of a
2
wide range of functional groups. Of major importance for this
transition-metal (TM)-catalyzed functionalization is research
on low-valent transition metal compounds, which are key
3
intermediates in B–C bond formation reactions. Similarly, the
formation of a cis-bis(boryl) species by oxidative addition of a
B–B bond to a metal center is a key step of the catalytic dibo-
ration of organic substrates, and insertion of low-valent transi-
tion metal fragments into boron-halide bonds is used for the
4
preparation of transition metal boryl species. The known
synthetic routes to boryl complexes are oxidative addition of
B–H, B–B and B–X bonds, salt elimination from haloboranes
4
or rearrangement of dinuclear boryloxycarbyne complexes.
While these reactions have been used to prepare a wide range
of transition metal boryl complexes, the homologated variant
of boryl complexes – diboran(4)yl complexes (A, Figure 1) of
the type [L M–B(X)–BX ] – are limited to only a few exam-
Figure 1. Selected transition metal complexes synthesized using
diboranes(4).
x
2
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Over the last 35 years, a large variety of transition metal
complexes with distinct bonding modes have been prepared
starting from diborane(4) precursors. In addition to symmet-6
rical and unsymmetrical bis(boryl) complexes (B, Figure 1),
rane(4) easily accessible from commercially available start-
ing materials via solution phase syntheses with standard
Schlenk apparatus.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
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2
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6
5c
double oxidative addition of B (NMe ) I at platinum, com-
2
2 2 2
plexes with bridging boryl groups between two metal centers
7
(
C, Figure 1) and the coordination of a metal in a H-B-B-H
3
3
8
cavity of sp -sp diboranes have been published. Kleeberg
and coworkers also very recently isolated bis-µ-boryl-bridged
complexes [(iPr P)Cu-BDmab] (Dmab = 1,2-(NMe) C H )
3
2
2
6
4
(
D, Figure 1) and [(1,2-C H (Ph P) )Cu-BPin] (Pin = (OC-
6 4 2 2 2
9
0
1
2
3
4
5
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9
0
1
2
3
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0
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9
0
1
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9
0
Me ) ), starting from unsymmetrical diboranes(4).
2
2
Our interest in the discovery of new bonding modes of dibo-
ranes at TM centers led us to consider the little-explored fami-
lies of diaryldifluorodiboranes(4), which show an unusual
1
0
1
,1’-substitution pattern, and tetrahalodiboranes (B X ; X =
2 4
F, Cl, Br, I). Tetrahalodiboranes feature historically difficult
1
1
preparation methods. Our continuing interest in the use of
tetrahalodiboranes for the preparation of boron–boron multiple
12
bonds led us to develop simple solution-phase syntheses of
B X (X = F, Cl, I) starting from B Br , making these species
2
4
2
4
13
more synthetically accessible. In contrast to well-studied
diboranes(4), such as bis(pinacolato)-,
bis(neopentaneglycolato)- and bis(catecholato)diborane, the
only known attempts to react tetrahalodiboranes with TM
14
fragments were made by Norman, Marder and coworkers
around the turn of the millennium, and very recently by our
15
group. The former work led to isolation of two products of
B–B bond oxidative addition (cis-[Pt(BF ) L ], L = 2 PPh ,
2
2
2
2
3
Ph P(CH ) PPh ) as well as an oxidative addition / dispropor-
2
2 4
2
tionation product with Vaska’s complex: (fac,trans-
14
[
Ir(BF ) (CO)L ]). Earlier this year we reported the isolation
Scheme 1. Top: Formation of cis-[(Cy P) Pt(BF ) ] (1) and
2
3
2
3
2
2 2
3
3
of the diplatinum(II) complex [{(Cy P)(I B)Pt} (ꢀ :η :η -
B I )] (E, Figure 1), supported by the bridging diboranyl dian-
ion ligand [B I ] , which was prepared by addition of B I to
borylborato complexes trans-[(Cy
3
P)
Pt{B(X)−BX
2
3
}] (2: X = Cl;
3
2
2
2
3: X = Br). Bottom: Crystallographically-determined solid-state
structures of 1 and 2. Atomic displacement ellipsoids depicted at
the 50% probability level and omitted for the ligand periphery.
Hydrogen atoms omitted for clarity. Selected bond lengths (Å)
and angles (°) for 1: Pt1–P1 2.3902(10), Pt–B1 2.049(4), B1–F1
2
4
2–
2
4
2 4
1
5
trans-[PtI(BI )(PCy ) ].
2
3 2
The work presented herein details our reactivity studies of0
these two little-studied families of diboranes(4) towards Pt
complexes. While in some cases we observed formation of the
somewhat expected oxidative addition of the B–B bond, other
reactions led to the discovery of a number of novel coordina-
tion modes of boron-containing ligands, including neutral and
1
7
.344(4), B1–F2 1.344(4), P1-Pt-P1’ 108.82(5), B1-Pt-B1’
1.8(2), P1-Pt-B1 90.00(11), F1-B1-F2 110.3(3). The molecular
structure of 2 exhibited disorder of the B(Cl)−BCl unit in a 64:36
3
ratio. The major orientation is shown; however, detailed discus-
sion of the bond lengths and angles is not possible.
1
anionic complexes with η -borylborato ligands, and dinuclear,
doubly-boryl-bridged complexes.
B F was subsequently condensed on the transition metal
2
4
Lewis base [Pt(PCy ) ] at −78 °C and the reaction was moni-
3
2
31
1
11
19
RESULTS AND DISCUSSION
tored by P{ H}, B and F NMR spectroscopy (δ = 33.8
P
1 2
(
JP-Pt = 1615 Hz) ppm; δ = 47 (s) ppm; δ = −12.5 (br t, J
B F F-Pt
Reactivity of [(Cy P) Pt] towards B X . As shown by Nor-
= 1044 Hz) ppm). The NMR spectroscopic data of the main
product in the reaction mixture was consistent with those of
Norman, Marder and coworkers and clearly indicated for-
mation of the bis(boryl) complex cis-[(Cy P) Pt(BF ) ] (1,
3
2
2
4
man, Marder and coworkers, B F reacts with platinum and
2
4
14
iridium complexes with activation of the B–B bond. Due to
1
1f
the difficult preparation of B F , which involves gas-phase
2
4
3
2
2 2
14
syntheses and requires specialized apparatus, the coordination
chemistry of this reagent was not explored further. We thus set
out to develop a more convenient synthetic protocol for the
generation of B F . Our simple synthesis of the double dime-
Scheme 1). The complex decomposes upon crystallization,
precluding its full spectroscopic characterization. However,
the structure of 1 was confirmed by multinuclear NMR tech-
niques and X-ray diffraction analysis. Single crystals of 1
suitable for X-ray diffraction were grown by crystallization
from dichloromethane. The platinum center is coordinated in a
square-planar fashion and the Pt−B bond distances (2.049(4)
Å) are nearly identical to those of the cis-bis(boryl)platinum
complexes synthesized by Norman, Marder and coworkers
2
4
1
3
thylsulfide adduct of B Br , reported recently, led us to at-
2
4
tempt its halide exchange with fluoride sources LiF and SbF3.
Treatment of B Br (SMe ) with either LiF or SbF led to
2
4
2 2
3
bromide/fluoride exchange along with dissociation of the
dimethyl sulfide donors, providing B F (Scheme 1). This
2
4
synthesis makes the industrially-relevant tetrafluorodibo-
2
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1
1
(
cis-[Pt(BF ) L ], L2
=
2PPh3 (Pt−B: 2.058(6) Å),
P{ H} NMR spectra indicated the formation of these com-
2
2 2
1
4
1
2
3
4
5
6
7
8
9
Ph P(CH ) PPh (Pt−B: 2.044(6) Å).
plexes.
2
2 4
2
1
3
Having both B Cl and B Br in hand, we were also eager
2
4
2
4
to test their reactions with [Pt(PCy ) ]. Due to the increasing
3
2
lability of B–X bonds upon going from B−F to B−Cl and
B−Br, we expected that oxidative addition of the B–X bond
might be favored over that of the B–B bond. Mixing hexane
solutions of [Pt(PCy ) ] and B X (X = Cl, Br) at −78 °C led to
3
2
2
4
precipitation of yellow solids 2 and 3 (Scheme 1). Complexes
and 3 were found to decompose at ambient temperatures in
2
solution; therefore, NMR spectra were obtained at approxi-
mately −40 °C. These compounds showed single resonances in
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
1
1
their P NMR spectra (2: δ = 44.8 ( J = 3175 Hz); 3: δ =
P
P-Pt
P
1
42.0 ( J = 3160 Hz)) reminiscent of the signals of cationic
P-Pt
T-shaped
boryl-diphosphine
Pt
complexes
(e.g.
F
17
F
[
(Cy P) Pt(BBrFc)][BAr ] (Fc = ferrocenyl; Ar = 3,5-
3
2
4
1
C H (CF ) ); δ = 41.7 ( J = 2914 Hz)). Furthermore, the
6
3
3 2
P
P-Pt
1
1
B NMR spectrum of each complex showed two strongly
separated resonances (2: δ = 67 (s), 5.4 (s); 3: δ = 66 (s), –
B
B
5
.0 (s)) indicating the presence of both three- and four-
31
coordinate boron atoms. Solid-state P VACP/MAS NMR
VACP = variable amplitude cross polarization; MAS = magic
(
1
11
angle spinning) (δ = 48 ( J
= 3160 Hz)) and
B
P-Pt
VACP/MAS NMR (δ = 67, 3) were performed on solid 2,
indicating that the solution structure of 2 is preserved in the
solid state.
Single crystals of 2 suitable for X-ray diffraction were
grown from toluene, allowing us to describe the structures of 2
and 3 as those shown in Scheme 1, namely as zwitterionic Pt
complexes bearing highly unusual borylborato ligands
Scheme 2. Top: Formation of 4 and 5. Bottom: Crystallograph-
ically-determined solid-state structure of 4. Atomic displacement
ellipsoids depicted at the 50% probability level and omitted for
the ligand periphery. Hydrogen atoms and the [N(nBu) ] cation
4
are omitted for clarity. Selected bond lengths (Å) and angles (°)
for 4: Pt1–P1 2.3310(14), Pt–B1 1.984(7), Pt–Cl5 2.5415(14),
B1–B2 1.768(10), B1–Cl1 1.850(7), B2–Cl 1.884(7)-1.903(6),
Cl5-Pt-B1 178.6(2), P1-Pt-P2 175.18(5).
(
-B(X)BX ; X = Cl, Br) with a trigonal-planar boron atom
3
bound to both the platinum center and a second, tetrahedral
boron atom. The molecular structure of 2 was found to have a
disorder of the B Cl unit in a 64:36 ratio and the major orien-
Single crystals of 4 suitable for X-ray diffraction were
grown by crystallization from diffusion of hexane into a satu-
rated dichloromethane solution at −30 °C. Upon halide addi-
tion, the geometry around the platinum center changes from
distorted T-shaped (2) to square-planar (4) (P1-Pt-P2
2
4
tation is shown in Scheme 1.
Compound 2 can also be prepared using B Cl (SMe ) ;
2
4
2 2
however, the reactions of B Br (SMe ) or B I (SMe ) with
2
4
2 2
2 4
2 2
2
0
[
Pt(PCy ) ] did not lead to the analogous borylborato com-
175.18(5)°). The B−B distances of 4 (1.768(10) Å) and B Cl
2 4
3 2
plexes. Tetrachlorodiborane(4) bis-adducts with stronger main
(1.75(5) Å) are identical within experimental uncertainty;
however, this is likely just a reflection of the high uncertainty
group Lewis bases (e.g. PMe ) were found to be inert towards
3
the platinum complex (see Supporting Information for the
synthesis of B Cl (PMe ) ).
in the latter distance. The Pt−B bond length of 4 is in the ex-
19,21-23
pected range (4: 1.984(7) Å).
As judged by comparison
2
4
3 2
Marder, Lin and Braunschweig have demonstrated that the
trans-influence of boryl ligands can be gauged by comparison
of the X−Pt bond length trans to the boryl group in complexes
of the Pt−Cl distance, the trans influence of the -B(Cl)BCl
3
ligand (4: Pt–Cl 2.5415(14) Å) is significantly stronger than
that of both mono- and diboryl ligands, e.g. those of trans-
4
c,19
21
of the type trans-[(Cy P) PtX{BX(R)}].
To gain infor-
mation about the trans-influence of the -B(X)BX3 ligand,
compound 2 was reacted with [N(nBu) ]X (X = Cl, Br) to
[(Cy P) PtCl(BCl )]
(Pt−Cl:
2.441(2)
Å),
trans-
trans-
3
2
3
2
2
2
2
[(Cy P) PtCl(BClMes)] (Pt−Cl: 2.5019(4) Å)
3
2
5
4
[(Cy P) PtCl{B(Mes)BXMes)}] (Pt−Cl: 2.5016(5) Å), and to
3
2
11
transfer a halide to the platinum center (Scheme 2). The
B
our knowledge represents the longest Pt−Cl bond ever ob-
served in trans-boryl complexes. Comparison with other boryl
ligands is difficult as their trans-influence was evaluated by
measurement of a Pt−Br distance, and chloro-substituted dibo-
ranes(4) do not always react in the same manner as the bromo
derivatives. In other cases, there is no solid-state structure
available for the chloro derivatives. According to these results,
the borylborato group has one of the strongest trans-influences
determined at square planar Pt(II) centers and therefore is
likely one of the strongest σ-donor boryl ligands reported to
date. The formation of complexes 2 and 3 is likely driven by a
combination of the strong trans-influence of the diboryl (-
NMR spectra of the isolated compounds (4: X = Cl; 5: X = Br)
showed two broad singlets slightly shifted to low field com-
pared to the resonances of 2 (4: δ = 78, 10; 5: δ = 69, 6)
B
B
31
1
while their P{ H} NMR spectra show similar signals (4: δ =
P
1
1
4
2.7 ( J = 3215 Hz; 5: δ = 43.5 ( J = 3216 Hz)) with
P-Pt P P-Pt
1
slightly larger J coupling constants, consistent with a small
P-Pt
increase in electron density at the Pt center. Complexes 4 and
5
are temperature sensitive and decompose in solution at am-
bient temperature. Likely due to the higher lability of its B−Br
bonds, we were unable to isolate corresponding halide-
11
addition complexes derived from 3. Nevertheless, B and
B(Cl)BCl ) moiety, which labilizes the Cl ligand trans to it in
2
3
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the initially formed BCl oxidative addition product, combined
To extend the structural variety of the family of difluorodibo-
ranes(4) we synthesized the corresponding bis(9-anthryl)
derivative (for details see Supporting Information).
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
with the strong Lewis acidity of the B–BCl unit which ac-
2
cepts the chloride anion. It should be noted that an analogous
main-group example of a chloride migration at a B Cl frag-
The reaction of F B–BMes with equimolar amounts of ste-
2
4
2
2
ment was observed by Siebert upon reacting B Cl with deca-
rically undemanding, bisphosphine platinum(0) complexes
[Pt(PMe ) ] (generated in situ from [Pt(nbe) ] and two equiva-
2
4
I/III
methylsilicocene, which led to the mixed-valent B species
3
2
3
18
25
26
(
Me C )B−BCl . Due to the fact that the initially formed BCl
lents of PMe3) and [Pt(PEt ) ] (generated from [Pt(PEt ) ])
5
5
3
3 3 3 4
oxidative addition product could not be detected by NMR
spectroscopy even in the solid state, a direct halide migration
from boron to boron cannot be completely ruled out.
results in oxidative addition of the B–B bond to afford un-
symmetrical bis(boryl) complexes cis-[(R P) Pt(BF )(BMes )]
3
2
2
2
(6: R = Me; 7: R = Et) (Scheme 3). After workup, 6 and 7
were isolated as yellow crystals in low yields of 28% and
23%, due predominantly to the extreme solubility of the com-
0
Reactivity of Pt complexes towards unsymmetrical
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
31
1
difluorodiboranes(4)
We
pounds even in nonpolar solvents. Each P{ H} NMR spec-
1
have
recently
demonstrated
that
1,1’-
trum shows one multiplet signal (6: δ = –19.4, JP–
P
1
difluorodiboranes(4) of the type F B–BAr (Ar = Mes, Dur),
= 1648 Hz; 7: δ = 11.2, J = 1672 Hz) as well as one
2
2
Pt
P
P-Pt
1
0
1
2
first synthesized by Berndt and coworkers, show interesting
reactivity and can be used to access species that are not avail-
able with symmetrical 1,2-diaryldihalodiboranes(4). While
symmetric diboranes(4) react with Lewis bases to form either
3e
doublet signal (6: δ = –20.8, J = 812 Hz, J = 29 Hz; 7:
P P–Pt P–P
1 2
δ = 12.2, J = 841 Hz, J = 27 Hz), all with Pt satellites.
P
P-Pt
P-P
1
The detected J coupling constants are, to the best of our
P-Pt
knowledge, the smallest ever observed in platinum cis-
2
3
halide-bridged adducts or rearranged sp -sp diboranes,
(bisboryl) complexes. As expected, two signals were observed
11
difluorodiboranes(4) react with small Lewis bases to either
in the B NMR spectrum of both complexes (6: δ = 105.5
P
form bis-adducts or monoadducts wherein the BAr unit bears
and 43.4; 7: δ = 104.9 and 42.4), which are both low-field
2
P
2
4
the Lewis base donor. With this in mind, we sought to de-
shifted with respect to the corresponding resonances of F B–
2
1
0
termine if the reactivity of unsymmetrical diboranes F B–BAr2
BMes2
(δ
=
28,
84).
2
with transition metals would yield similarly unusual structures.
Scheme 3. Top: Formation of cis-bis(boryl) complexes 6 and 7 and bis-µ-boryl-bridged complexes 8, 9, 10 and 11. Bottom: Crystallo-
graphically-determined solid-state structures of 6, 7 and 9. Atomic displacement ellipsoids depicted at the 50% probability level and omit-
ted for the ligand periphery. Hydrogen atoms omitted for clarity. Selected bond lengths (Å) and angles (°) for 6: Pt–P1 2.3476(8), Pt–P2
2.3394(11), Pt–B1 2.036(4), B1–B2 2.621(4), Pt–B2 2.098(3), B1–F1 1.343(4), B1–F2 1.348(3), P1–Pt–P2 97.62(3), P1–Pt–B1 86.47(9),
B1–Pt–B2 78.66(13), B2–Pt–P2 97.51(8), P2–Pt–B1 175.57(9), P1–Pt–B2 163.07(8) F1-B1-F2 110.1(3); for 7: Pt1-B1 2.041(3), Pt1-B2
2
9
2
1
.096(3), Pt1- B1–B2 2.519(3), P1 2.3494(7), Pt1-P2 2.3591(7), B1-F1 1.341(3), B1-F2 1.350(3), B1-Pt1-B2 75.03(10), P1-Pt1-P2
7.28(2), B1-Pt1-P1 89.53(8), B2-Pt1-P2 98.96(7), B1-Pt1-P2 169.94(8), B2-Pt1-P1 163.07(6), F1-B1-F2 110.5(2); for 9: Pt1–Pt2
.6630(7), Pt1–P1 2.2741(12), Pt2–P2 2.2778(12), B1–Pt1 2.113(5), B1–Pt2 2.111(5), B2–Pt1 2.122(4), B2–Pt2 2.140(5), B1-Pt1-B2
02.35(18),
B1-Pt2-B2
101.81(18),
Pt1-B2-Pt2
77.33(15),
B1-Pt1-Pt2
50.87(13),
B2-Pt1-Pt2
51.64(12)
4
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1
9
The F NMR spectra of compounds 6 and 7 show typical
signals for BF ligands bound to Pt(II) centers (6: δ = –28.2
noted that the difluorodiboranes used are not thermally stable,
which partly explains the substantial amount of decomposition
products observed. This is particularly problematic when the
reaction is performed at 80 °C, which is required in order to
1
2
3
4
5
6
7
8
9
2
F
2
2
(t, J = 975 Hz); 7: δ = –28.7 (t, J = 966 Hz)) and are
F–Pt
F
F-Pt
1
9
consistent with the F NMR data of cis-[Pt(BF ) L ] (L =
2
2
2
2
1
4
2
PPh , Ph P(CH ) PPh ).
induce reaction of [Pt(PCy ) ] with diboranes. However, the
3
2
2 4
2
3 2
Single crystals of 6 and 7 suitable for X-ray diffraction were
grown from pentane at −35 °C. The X-ray structures of 6 and
confirmed the selective cis oxidative addition of the B–B
bond and the nearly square-planar platinum(II) centres (6: ΣPt
360.3°; 7: Σ = 360.8°) (Scheme 3). The Pt–B1 bond lengths
similarity of the multinuclear NMR data obtained from 10 to
those of 8 and 9 provide confidence about the constitution of
7
the former. Reaction of F
a stronger π-accepting BAr
[Pt(PCy ] showed no reaction even at higher temperatures
2
B–BAn
2
, which is presumed to have
group than F
B–BMes , with
2
2
2
=
)
3 2
Pt
are slightly shorter (6: 2.036(4) Å; 7: 2.041(3) Å) than the
Pt–B2 bond lengths (6: 2.098(3) Å; 7: 2.096(3) Å). The
P1-Pt-P2 angles (6: 97.62(3)°; 7: 97.82(2)°) are significantly
larger than the B1-Pt-B2 angles (6: 78.66(13)°; 7: 75.03(10)°).
Nevertheless, the B1–B2 distance (6: 2.621(4) Å, 7: 2.519(3)
Å) does not indicate a B···B interaction as observed in
(up to 80 °C). These results led us to propose the synthesis of
0
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
doubly-boryl-bridged complexes like 10 by employing Pt
complexes bearing one phosphine and labile co-ligands. Thus,
3
1
upon treating [(Cy
P)Pt(nbe) ] (nbe = norbornene) with
3
2
F
2
B–BMes , NMR spectroscopy of the reaction mixture indi-
2
cated formation of compound 10 after heating (60 °C). How-
ever, the isolation problems observed earlier, due to incom-
plete conversion and decomposition, remained. In an attempt
to use a more activated diborane precursor to promote this
27
[(Me P) Co(Bcat) ] (2.185 Å). All other structural parame-
3
3
2
ters of complexes 6 and 7 are in good agreement with previous
findings; for example, the B1–F bond lengths (6: 1.342(4),
2
3
reaction, [Pt(PCy ) ] was combined with the sp -sp diborane
1
1
.348(4) Å; 7: 1.341(3), 1.350(3) Å) and F1-B1-F2 angles (6:
3 2
24a
F B–BMes (PMe )
at room temperature; however, this
10.1(3)°; 7: 110.5(2)°) correspond to those reported for
2
2
3
14
reaction did not result in formation of 10. Instead, a yellow
solid was isolated (12, Scheme 4) that showed two multiplets
cis-[(Ph P) Pt(BF ) ] (1.327(6), 1.33(7) Å; 110.8(5)°).
3
2
2 2
Upon reacting F B–BMes or F B–BAn (An = 9-anthryl)
2
2
2
2
3
1
1
1
with bis-phosphine platinum(0) complexes bearing bulkier and
in its P{ H} NMR spectrum (δ
P
= 37.7 ( JP-Pt = 971 Hz,
2
1
2
stronger σ-donor phosphine ligands (PiPr or PCy ), we ob-
J
P-F = 28.5 Hz), −22.0 ( JP-Pt = 1545 Hz,
J
P-F = 127 Hz))
), 42.4
= −28.3 (br s)), indicated
formation of the heteroleptic bis(boryl) complex
[(Me P)(Cy P)Pt(BF )(BMes )] 12 (Scheme 4).
3
3
3
1
1
11
served color changes to dark red solutions and P{ H} NMR
which, together with its B NMR (δ = 106.2 (s, BMes
B
2
19
spectra of higher order (δ = 77.7 (8), 78.2 (9) and 64.9 (10)),
(s, BF )) and the F NMR data (δ
2 F
P
indicating formation of dinuclear complexes (Scheme 3). The
1
1
B NMR spectra of 8-10 showed broad singlets in the range
3
3
2
2
of 85-90 ppm for the BAr ligand and broad multiplets be-
2
tween
1
9
8
-21 ppm for the BF ligand. The F NMR spectra of 8-10
2
showed multiplets from –40.0 to –45.8 ppm.
Single crystals of 9 suitable for X-ray diffraction were
grown by crystallization from benzene. The core of the dinu-
clear bis(boryl) complex 9 resembles a planar rhombus with
four sides of nearly identical lengths. The Pt–B distances (Pt1–
B1 2.112(4) Å, Pt1–B2 2.122(4) Å, Pt2–B1 2.111(5) Å, Pt2–
B2 2.140(5) Å) of complex 9 are longer than those of
bis(boryl) complexes 1, 6, and 7 (avg. Pt–B distance: 2.072 Å)
and the
previously reported borylene-bridged complex
28
[
(OC) (Me P)Fe(µ-CO)(µ-BDur)Pt(PCy )] (Pt–B 1.976(4)
2
3
3
Å, Fe–B 1.970(4) Å, Fe-B-Pt 81.32(14)°). While the B1–F
distances (1.365(5), 1.373(6) Å) are also slightly longer than
those of 6 and 7, the F1-B1-F2 angle of 9 (110.2(4)°) lies in
the expected range. Both Pt–P distances (2.274(1), 2.278(1) Å)
are shorter than those of complexes 6 and 7, while the P-Pt-B
angles (B1-Pt1-P1 104.7(1)°, B2-Pt1-P1 151.7(1)°) indicate a
dramatic bending of the phosphine ligands towards the less
bulky difluoroboryl ligand. The Pt–Pt bond (2.6630(7) Å) is
Scheme 4. Formation of unsymmetrical bis(boryl) complex
(Me P)(Cy P)Pt(BF )(BMes )] 12.
[
3
3
2
2
To gain more information about the constitution of 12, 2D
31
1
NMR experiments were performed. P- H HMQC measure-
29
3
1
comparable to that in [Pt Cl (dppm) ] (Pt–Pt 2.651(2) and
2
2
2
ments show correlation of the P NMR resonance at δ = –
P
1
shorter than in the A-frame complex [Pt (µ-HgCl )Cl (dppm) ]
2
2
2
2
22.0 ppm with only one signal in the H NMR spectrum at
.98 ppm (d, 9H, J = 7.1 Hz), indicating that this P NMR
3
0
2
31
(Pt–Pt 2.7119(8) Å).
0
P–H
While the reactions with [Pt(PiPr ) ] were performed at am-
3
2
resonance corresponds to the PMe ligand. Furthermore, both
3
19
bient temperatures and showed full conversion to the corre-
sponding product within 15 h, reaction of F B–BMes with
phosphine resonances sharpen significantly upon F decou-
2
2
pling, indicating a strong phosphorus-fluorine interaction.
2
more sterically demanding [Pt(PCy ) ] did not lead to full
3
2
Analysis of the J coupling constants led us to determine
P-F
2
conversion to 10 even at higher temperatures (80 °C), presum-
ably due to steric hindrance. The persistence of starting mate-
rial and the large amount of decomposition products in the
reaction mixture made isolation of 10 impossible. It should be
that the PMe ligand ( J = 127 Hz) is mutually trans to the
BF ligand, while the PCy ligand ( J = 28.5 Hz) is cis to the
3
P-F
2
2
3
P-F
BF ligand. This was also found to be the case in the solid
2
state, as determined by crystallographic analysis of 12 (see
5
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Supporting Information for details). This allowed us to assign makes it easily accessible via solution phase synthesis. While
Page 6 of 9
3
1
0
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
the P NMR spectra of 6 and 7 with confidence. The very
with B F and Pt a cis-bis(boryl) complex is formed, B Cl
2
4
2
4
1
small J
coupling constants of 812 and 841 Hz, respective-
and B Br led to the synthesis of zwitterionic platinum com-
2 4
P–Pt
ly, can be assigned to PR ligands trans to the BMes group,
plexes bearing novel borylborato (-B(X)BX ) ligands, which
3
2
3
consistent with the much larger trans-influence of the latter
were found to be exceedingly strong σ-donor ligands. A new
unsymmetrical 1,1-diaryl-2,2-difluorodiborane(4) compound
was prepared bearing 9-anthryl substituents, and the reactivity
than the BF group. It should also be noted that 12 can also be
2
prepared
by
combination
of
0
F B–BMes , PMe and [Pt(PCy ) ] at –55 °C.
of this family of diboranes was studied with Pt precursors.
2
2
3
3 2
F B–BMes forms oxidative addition products when treated
2
2
0
with Pt complexes bearing small phosphines, including the
first example of a fourfold unsymmetrical cis-bis(boryl) com-
plex, whereas increasing the size and the donor strength of the
phosphine ligands leads to unprecedented doubly boryl-
bridged diplatinum complexes. The presented results highlight
the sensitivity of these reactions to steric hindrance and donor
strength at the TM center.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2. Left: HOMO; middle: LUMO; right: IBO of com-
pound 9.
ASSOCIATED CONTENT
Supporting Information including full experimental details, spec-
tra, crystallographic files (CIF) and computational details. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Finally, we computed the structure of complex 9 (see Sup-
porting Information for further details) in order to visualize the
frontier molecular orbitals (Fig. 2). The HOMO is mainly
located at the two platinum centers and the BAn boron atom,
2
AUTHOR INFORMATION
Corresponding Author
which describes the nucleophilic part of the compound. In
contrast, the LUMO is the antibonding orbital of the same
Pt–Pt–B ring, which comprises the electrophilic portion of 9.
These findings are in contrast with Kleeberg’s bis-µ-boryl-
bridged complex [(iPr P)Cu-BDmab] (E, Fig. 1), which was
*h.braunschweig@uni-wuerzburg.de
3
2
ACKNOWLEDGMENTS
found to have multicenter bonding character within the B Cu2
2
core involving the HOMO, HOMO-5, HOMO-11 and
HOMO-16 orbitals. All these molecular orbitals showing
substantial boron and copper contributions are symmetrical
Financial support from the Deutsche Forschungsgemeinschaft is
gratefully acknowledged.
9
NOTES AND REFERENCES
and involve both bridging boryl groups. However, by visual-
izing intrinsic bond orbitals (IBOs), we found one showing
multicenter contributions over the Pt B core (for a complete
(1) Stock, A.; Brandt, A.; Fischer, H. Ber. Dtsch. Chem. Ges. B
1925, 58B, 643–657.
2
2
(2) Boronic Acids: Preparation and Applications in Organic
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2
Synthesis and Medicine, Hall, D. G. (Ed.), Wiley‐VCH Verlag
GmbH & Co. KgaA, 2015.
(3) Selected reviews on diboranes and their synthetic uses: (a)
BF , this MO gets more contribution from the BAn boron
2
2
atom (52%) than the BF boron (25%), so the multicenter
2
bonding character is slipped from the four-membered ring
center, in contrast to symmetrical complexes like that of
Kleeberg. The dramatic bending of the phosphine ligands
towards the less bulky difluoroboryl ligand in 9 itself indicates
a different bonding situation than in [(iPr P)Cu-BDmab] (see
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Mkhalid, I. A. I.; Westcott, S. A.; Marder, T. B. Chem. Rev
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2
,
.
,
,
3
2
solid-state structure in Scheme 3).
,
, 7
,
Moreover, calculated Wiberg bond indices reveal the
bis-µ-boryl-bridged bonding mode of 9 (mean value of 0.65
among the Pt–B interactions), whereas the Pt–Pt interaction
shows a very low value of 0.29. Both platinum centers hold an
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2
,
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bis(boryl)-bridged compound.
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