Platinum-Templated Coupling of B=N Units: Synthesis of BNBN Analogues of 1,3-Dienes and a Butatriene

Article abstract of DOI:10.1002/anie.202106161

The 1:2 reaction of [μ-(dmpm)Pt(nbe)]₂ (dmpm=bis(dimethylphosphino)methane, nbe=norbornene) with Cl₂BNR(SiMe₃) (R=tBu, SiMe₃) yields unsymmetrical (N-aminoboryl)aminoboryl Pt^I₂ complexes by B?N coupling via ClSiMe₃ elimination. A subsequent intramolecular ClSiMe₃ elimination from the tBu-derivative leads to cyclization of the BNBN unit, forming a unique 1,3,2,4-diazadiboretidin-2-yl ligand. In contrast, the analogous reaction with Br₂BN(SiMe₃)₂ leads, via a twofold BrSiMe₃ elimination, to a Pt^II₂ A-frame complex bridged by a linear BNBN isostere of butatriene. Structural and computational data confirm π electron delocalization over the entire BNBN unit.

Full text of DOI:10.1002/anie.202106161

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Accepted Article
Title: Platinum-templated Coupling of B=N Units: Synthesis of BNBN
Analogues of 1,3-Dienes and a Butatriene
Authors: Carina Brunecker, Merle Arrowsmith, Felipe Fantuzzi, and
Holger Braunschweig
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10.1002/anie.202106161
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Platinum-templated Coupling of B=N Units: Synthesis of BNBN
Analogues of 1,3-Dienes and a Butatriene
Carina Brunecker,[a,b] Merle Arrowsmith,[a,b] Felipe Fantuzzi,[a,b] Holger Braunschweig*[a,b]
Abstract: The 1:2 reaction of [μ-(dmpm)Pt(nbe)]2 (dmpm
bis(dimethylphosphino)methane, nbe norbornene) with
Cl2BNR(SiMe3) (R tBu, SiMe3) yields unsymmetrical (N-
=
access to an ever-increasing variety of B=N/C=C-isosteric
compounds and materials, including boron nitride[3] and
borocarbonitride (BxCyNz) nanomaterials,[4] hybrid organic-
inorganic BN-doped conjugated polymers,[5] (poly)aromatic
compounds,[6] and aromatic small molecules.[7] However, well-
=
=
aminoboryl)aminoboryl PtI2 complexes by B-N coupling via ClSiMe3
elimination. A subsequent intramolecular ClSiMe3 elimination from the
tBu-derivative leads to cyclization of the BNBN unit, forming a unique
1,3,2,4-diazadiboretidin-2-yl ligand. In contrast, the analogous
reaction with Br2BN(SiMe3)2 leads, via a twofold BrSiMe3 elimination,
defined
acyclic
conjugated
BN
chains,
such
as
poly(iminoboranes) (III) or BN-based cumulenes (IV), remain
difficult to access. The intuitive synthetic routes to III via the
polymerization of iminoborane (RB≡NR') precursors[8] or the
dehydrocoupling of amine borane (H2RB∙NH2R') precursors[9] are
in practice marred by the formation of cyclic oligomers such as I
and II. The most efficient access to higher oligo(iminoboranes) is
by B-N coupling of chloroborane and silylamine precursors via
ClSiMe3 elimination.[10] The group of Helten has used this
to a PtII A-frame complex bridged by a linear BNBN isostere of
2
butatriene. Structural and computational data confirm π electron
delocalization over the entire BNBN unit.
The replacement of C=C double bonds in organic molecules by
isosteric covalent B=N units is not only interesting from a
fundamental point of view, but also opens up the exploration of a
vast hybrid organic-inorganic chemical space. While the typical
B=N double bond (1.39 Å)[1] is only marginally longer than a C=C
double bond (1.34 Å, Figure 1), the intrinsic strong polarization of
B–N bonds imparts very different electronic properties and
stability to the resulting molecules and materials, which can be
exploited for new applications in materials science, catalysis, and
medicinal chemistry.
methodology
oligo(iminoboranes) (V) by polycondensation of 1,3-
bis(trimethylsilyl)-1,3,2-diazaborolidine precursors with
to
synthesize
the
first
well-defined
dichloro(organo)boranes (Scheme 1a).[11] Our group has also
reported the coupling of two Cl2BN(SiMe3)2 molecules with
[(C5H5)Ru(CO)2]Na with elimination of NaCl and ClSiMe3, yielding
the (N-aminoboryl)aminoboryl complex VI (Scheme 1b).[12]
Scheme 1. Examples of syntheses of oligo(iminoboranes) by B-N coupling via
ClSiMe3 elimination.
We have recently reported the synthesis of the boranediyl A-
frame complexes 2-XY from the twofold oxidative addition of
dihaloborane precursors (X2BY, X = Cl, Br, I; Y = X, alkyl, aryl,
amino) to the bis(dimethylphosphino)methane (dmpm)-bridged
Figure 1. Conjugated organic systems and their all-BN isosteres.
Pt0 complex 1 (Scheme 2a).[13] Inspired also by the metal-
2
Since the landmark synthesis of borazine by Stock and Pohland
in 1926 (Figure 1, I),[2] new synthetic methodologies have enabled
templated coupling of two BN units at ruthenium in complex VI
(Scheme 1b),[12] we now report the use of the Pt2(dmpm)2 scaffold
as a template for the coupling of B=N units derived from the
coupling of dihalo(silylamino)boranes (X2BNR(SiMe3), X = Cl, Br;
R = tBu, SiMe3) by elimination of XSiMe3, ultimately leading to the
isolation of the first BNBN-cumulene, isosteric with butatriene.
[a]
[b]
C. Brunecker, Dr. M. Arrowsmith, Dr. F. Fantuzzi, Prof. Dr. H.
Braunschweig
Institute for Inorganic Chemistry, Julius-Maximilians-Universität
Würzburg, Am Hubland, 97074 Würzburg (Germany).
E-mail: h.braunschweig@uni-wuerzburg.de
C. Brunecker, Dr. M. Arrowsmith, Dr. F. Fantuzzi, Prof. Dr. H.
Braunschweig
Institute for Sustainable Chemistry & Catalysis with Boron, Julius-
Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg
(Germany).
Scheme 2. Synthesis of boranediyl-bridged diplatinum A-frame complexes.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.202106161
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Scheme 3. Reactions of complex 1 with Cl2BNR(SiMe3) (R = tBu, SiMe3). Isolated yields in parentheses.
Whereas the reaction of complex 1 with Cl2BNMe2 yields the
aminoboranediyl-bridged A-frame complex 2-ClNMe2 (Scheme 2a),
the reactions of 1 with Cl2BNR(SiMe3) (R = tBu, SiMe3) always
than in 3tBu. Crystallization attempts of 4tBu always yielded
pseudo-merohedrally twinned crystals (see solid-state structure in
Figure 2), in which the BNBN heterocycle presents a twofold
disorder by rotation of about the Pt2‒B1 bond, thus precluding
any discussion of bond lengths and angles in this unit. Despite the
well-established chemistry of 1,3,2,4-diazadiboretidines η4
ligands for transition metals,[17] 3tBu represents a hitherto unknown
binding mode of this type of ligand as an anionic η1-ligand via
coordination at boron. In solution at room temperature, compound
4tBu decomposed very slowly but selectively over a period of
several weeks to complex 5-Cl and an unidentified intractable
colorless solid, by formal loss of “[BN(tBu)]2” (Scheme 3d).[15]
proceeded in a 1:2 ratio. The resulting products 3tBu and 3SiMe3
,
which precipitated as pale yellow solids, both display two broad
11B NMR resonances, at 53 (fwmh ≈ 1280 Hz, PtB) and 32
(fwmh ≈ 880 Hz, N2BCl) ppm for 3tBu, and 57 (fwmh ≈ 1990 Hz,
PtB) and 33 (fwmh ≈ 750 Hz, N2BCl) ppm for 3SiMe3 (Scheme 3a).
Complexes 3R are reminiscent of complex VI (Scheme 1b), which
shows similar 11B NMR resonances at 60.3 and 35.0 ppm.[12] The
31P{1H} spectra of 3R show two multiplets with higher order
satellites in a 1:1 ratio, at −14.3 (1JP-Pt = 3195 Hz, P2PtCl) and
−29.9 (1JP-Pt = 2733 Hz, P2PtB) ppm for 3tBu, and −14.3 (1JP-Pt
=
3150 Hz, P2PtCl) and −29.6 (1JP-Pt = 2708 Hz, P2PtB) ppm for
3SiMe3. X-ray crystallographic analyses of single crystals of 3tBu
confirmed the coupling of the two BN units at one platinum center
(Figure 2). Due to systematic rotational disorder of the terminal
B(Cl)NtBu(SiMe3) moiety, structural parameters cannot be fully
discussed. The Pt–Pt distance of 2.7067(6) Å, however, is clearly
indicative of Pt–Pt bonding. The Pt2–B1 bond length of 2.039(6)
Å is within the typical range for square planar platinum
amino(chloro)boryl complexes (2.00–2.85 Å), while the B1–N1
bond length of 1.421(8) Å is slightly longer than in these
complexes (ca. 1.39 Å)[14] due to the additional π electron
Scheme 4. Reaction of complex 1 with BBr2N(SiMe3)2. Isolated yield in
parentheses.
delocalization over the entire BNBN unit in 3tBu
.
To our surprise the reaction of 1 with Br2BN(SiMe3)2 resulted
instead in the formation of the A-frame complex 6, isolated as a
yellow solid in 46% yield (Scheme 4).[18] The 11B NMR spectrum
of 6 displays two broad resonances at ca. 57 (fwmh ≈ 1510 Hz)
and 26 (fwmh ≈ 690 Hz) ppm, the former being attributed to the
platinum-bound boron nucleus by analogy with the 11B NMR shift
of the related dimethylaminoboranediyl-bridged A-frame complex
2-BrNMe2 (δ(11B) = 52 ppm),[13] the latter to the dicoordinate NBN
boron nucleus. The 31P{1H} NMR spectrum showed a singlet at ‒
7.1 ppm, close to that of 2-BrNMe2 (δ(31P) = ‒5.6 ppm), with a
higher-order satellite splitting pattern typical for A-frame
Complex 3SiMe3 could not be fully characterized as it
decomposed rapidly in solution into ClSiMe3 and a number of
dmpm-containing platinum complexes, the known complex [μ-
1
(dmpm)PtCl]2 (5-Cl: δ(31P) = –19.3 ppm, JP–Pt = 2650 Hz)[13a]
being the major decomposition product (Scheme 3b, see Figure
S18 in the SI). The fate of the remaining [BNSiMe3]2 fragment
could not be determined as the 11B NMR spectrum of the final
product mixture was silent, and a colorless by-product, insoluble
in all common organic solvents, was formed.[15] In contrast, 3tBu
was stable in solution at room temperature but selectively
converted to 4tBu at 80 °C by intramolecular cyclization of the
BNBN moiety under ClSiMe3 elimination (Scheme 3c). This
reaction is analogous to the cyclization of RClB-N(tBu)-B(Cl)-
3
1
complexes (1JP-Pt = 3568 Hz, JP-Pt = 272 Hz, JPt-Pt = 1826 Hz).
11B and 31P{1H} NMR-spectroscopic monitoring of the reaction
showed no sign of formation the bromide analogue of 3SiMe3
.
NtBu(SiMe3) (R
=
NMe2, NEt2, Et, iBu) to 1,3,2,4-
We propose that the formation of complexes 3R and 6
proceeds via
same intermediate η1-(silylamino)haloboryl
diazadiboretidines by ClSiMe3 elimination, reported by Paetzold
in 1988.[16] The 11B NMR spectrum of 4tBu is nearly identical to that
of 3tBu, displaying two broad resonances at 54 (fwmh ≈ 1480 Hz,
PtB) and 32 (fwmh ≈ 470 Hz, N2BCl) ppm. The conversion of 3tBu
to 4tBu is evidenced more clearly by changes in the 31P{1H}
spectrum, which shows two new 1:1 multiplets with higher-order
satellites, both shifted ca. 2 ppm downfield from 3tBu, at −12.8 (1JP-
a
complex Int-XR formed by the oxidative addition of X2BNR(SiMe3)
to 1 (Scheme 5).[18] This step can be followed either by B-N
coupling with a second equivalent X2BNR(SiMe3) via XSiMe3
elimination (reaction rate constant ka) to form an η1-(N-
aminoboryl)aminoboryl complex analogous to 3R, or by the
oxidative addition of the second B–X bond of the
= 3198 Hz, P2PtCl) and −27.6 (1JP-Pt = 2632 Hz, P2PtB) ppm,
Pt
the 1JP-Pt coupling constant of the latter being ca. 100 Hz smaller
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Figure 2. Crystallographically derived molecular structures of (from left to right) 3tBu (least disordered one of the two molecules of 3tBu in the asymmetric unit), 4tBu
and 6. Thermal ellipsoids at 50% probability. Thermal ellipsoids of ligand periphery and hydrogen atoms omitted for clarity. Only the major part of the disorders in
3tBu (terminal B(Cl)NtBu(SiMe3) moiety) and 4tBu (entire (BNtBu)2Cl moiety and one dmpm ligand) are shown. Due to the restraints applied to these disorders during
refinement, the structural parameters of 3tBu and 4tBu may not be fully discussed. Selected bond lengths (Å) and angles (°) for 3tBu: Cl1–Pt1 2.4939(13), Pt1–Pt2
2.7067(6), Pt‒P 2.2446(14)–2.2651(14), Pt2–B1 2.039(6), B1–N1 1.421(8), Cl1-Pt1-Pt2 172.32(3), P1-Pt2-B1 174.14(16), Σ(∠B1) 360.0(4), torsion angles P1-
Pt1-Pt2-P2 –47.8(4), P3-Pt1-Pt2-P4 ‒54.32(5); for 4tBu: Cl1–Pt1 2.535(3), Pt1–Pt2 2.7214(7): for 6: Pt1⋅⋅⋅Pt2 3.2397(3), Pt1−B1 2.028(6), Pt2−B1 2.021(6), Pt1−Br1
2.6098(6), Pt2−Br2 2.6363(6), Pt‒P 2.2679(15)‒ 2.2913(14), B1−N1 1.396(7), N1‒B2 1.237(8), B2‒N2 1.388(8), Pt1-B1-Pt2 106.3(3), B1-N1-B2 173.8(6), N1-B2-
N2 171.3(7), torsion angles P1-Pt1-Pt2-P2 ‒12.29(5), P3-Pt1-Pt2-P4 ‒23.83(5).
24°, which could result from the steric repulsion between the
SiMe3 groups and the dmpm ligands.
Scheme 5. Proposed mechanism of formation of 3R and 6 via the common
intermediate Int-XR.
silylamino(halo)boryl ligand to platinum to form the
(silylamino)boranediyl A-frame complex 2-XNR(SiMe3) (reaction rate
constant kb). For R = SiMe3, the latter then undergoes twofold
XSiMe3 elimination with a second equivalent of X2BN(SiMe3)2 to
form complex 6. The selectivity of the reaction is therefore
determined by the relative values of the reaction rate constants ka
and kb: for X = Cl the rate of B-N coupling supersedes that of
oxidative addition of B–Cl to Pt, leading to the exclusive formation
of 3R, the opposite being the case for X = Br, leading to the
exclusive formation of 6.
The solid-state structure of 6 (Figure 2) confirmed the
Figure 3. a) Selected IBOs of 6. b) The fully π-delocalized MOs of 6 (left,
formation of the near-linear BNBN unit bridging the two platinum
centers (B1-N1-B2 173.8(6), N1-B2-N2 171.3(7)°). While the Pt–
B bond lengths of 2.028(6) and 2.021(6) Å are similar to those in
complex 2-BrNMe2 (2.028(10), 2.042(9) Å), the A-frame structure
itself is more strongly distorted from the ideal A-frame than in 2-
BrNMe2, as evident in the much shorter Pt∙∙∙Pt distance (6
3.2397(3); 2-BrNMe2 3.3003(4) Å) and larger P1/3-Pt1-Pt2-P2/4
torsion angles (6 ‒12.29(5), ‒23.83(5); 2-BrNMe2 4.96(7),
15.62(8)°).[13] Furthermore, the B1–N1 and B2–N2 bond lengths
of 1.396(7) and 1.388(8) Å are within the range of partial double
bonds, whereas the central N1–B2 bond is significantly shorter
(1.237(8) Å), corresponding to a partial triple bond.[1] While the
linear BNBN motif can be viewed formally as a 1-boryl-2-
(amino)iminoborane, the delocalization of the π electron density
apparent in the B–N bond lengths makes it structurally more akin
to an all-BN isostere of a butatriene. Unlike butatriene, however,
which is fully planar, the B1 and N2 planes form an angle of ca.
HOMO–30) and H2BNBNH2 (right, HOMO–3), highlighting the cumulenic
character of their BNBN motifs.
The electronic structure of 6 was further investigated using DFT
and intrinsic bond orbital (IBO)[19] calculations. The BNBN motif in
the optimized structure of 6, obtained at the M06[21]-D3[22]/cc-
pVDZ[23],aug-cc-pVDZ-PP{Pt}[24] level of theory, shows a larger
deviation from linearity (B1-N1-B2 161.3°, N1-B2-N2 176.2°) than
that of the solid-state structure. Similar results were obtained with
other density functionals (see details in the SI). In order to
investigate the origin of this deviation, we performed
computations on four truncated model systems, in which the PMe2
and SiMe3 groups were successively replaced with PH2 and SiH3
or H, respectively (see Figure S19 in the SI). In all of these cases,
the BNBN moiety was found to be linear (B1-N1-B2 and N1-B2-
N2 178.8–180.0°). The distortion from linearity therefore seems
to arise from the steric repulsion between the PMe2 and SiMe3
substituents, although the additional influence of crystal packing
forces in the solid-state structure cannot be discounted.
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Furthermore, the calculated Mayer bond orders (MBOs)[25] of the
BNBN motif in 4 (B1-N1: 1.38, N1-B2: 2.11, B2-N2: 1.32) are very
similar to those obtained for the parent H2BNBNH2 system (B1-
N1: 1.51, N1-B2: 2.13, B2-N2: 1.43), these values suggesting
strong cumulenic character in both cases. Indeed, inspection of
the IBOs of 6 (Figure 3a) reveals that IBO-1 and IBO-3, which are
orthogonal to the (Pt1-B1-Pt2) plane, are partially delocalized to
the neighboring B2 and B1 atoms, evidencing deviation from the
1-boryl-2-(amino)iminoborane picture. This view is also supported
by inspection of the canonical Kohn-Sham molecular orbitals
(MOs) of 6 and H2BNBNH2 (Figure 3b and S20 in the SI), where
π electron delocalization over the entire BNBN unit is observed.
The description of 6 as a BNBN analogue of butatriene is,
therefore, fully supported by quantum chemical investigations.
To conclude, we have shown that the [μ-(dmpm)Pt]2
framework acts as an effective template for the coupling of B=N
units obtained by the intermolecular B-N coupling of
dihalo(silylamino)boranes via halosilane elimination. For
Cl2BNR(SiMe3) precursors BN chain growth occurs at a side-on
PtI2 complex, whereas for Br2BN(SiMe3)2 an A-frame PtII2 complex
bridged by a linear BNBN unit is formed. Structural and
computational analyses confirm a cumulenic motif isosteric with
butatriene.
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We thank the Deutsche Forschungsgemeinschaft for financial
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Alexander von Humboldt (AvH) Foundation for a CAPES-
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slowly in solution to [μ-(dmpm)PtBr]2 (5-Br, see reference [13]) and
intractable polymeric “[BN(SiMe3)]n”.
Keywords: isosterism • butatriene analogue • B-N coupling •
1,3,2,4-diazadiboretidin-2-yl ligand • A-frame complex
[19] Since the homocoupling of Cl2BNR(SiMe3) by ClSiMe3 elimination does
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10.1002/anie.202106161
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The [μ-(dmpm)Pt]2 template promotes
the coupling of B=N units derived from
dihalo(silylamino)borane precursors
by B-N bond formation through
Carina Brunecker, Merle Arrowsmith,
Felipe Fantuzzi, Holger Braunschweig*
Page No. – Page No.
intermolecular halosilane elimination.
For Cl2BNR(SiMe3) η1-(N-
Platinum-templated Coupling of B=N
Units: Synthesis of BNBN Analogues
of 1,3-Dienes and a Butatriene
aminoboryl)aminoboryl and η1-1,3,2,4-
diazaboretidin-2-yl PtI2 complexes are
obtained, whereas for Br2BN(SiMe3)2
an A-frame complex bridged by the
first BNBN analogue of a butatriene is
formed, as confirmed by structural and
theoretical analyses.
This article is protected by copyright. All rights reserved.