Borylene-based functionalization of Pt-alkynyl complexes by photochemical borylene transfer from [(OC)5CrBN(SiMe3)2]

Article abstract of DOI:10.1039/b915926f

Photochemical borylene transfer from [(OC)₅CrBN(SiMe ₃)₂] to the platinum σ-alkynyl complex [Cl(PMe ₃)₂Pt-CCPh] gave facile access to the first transition metal-borirene complex. The title compound ha

Full text of DOI:10.1039/b915926f

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Borylene-based functionalization of Pt–alkynyl complexes by
photochemical borylene transfer from [(OC)5CrQBN(SiMe3)2]w
Holger Braunschweig,* Qing Ye and Krzystof Radacki
Received (in Cambridge, UK) 4th August 2009, Accepted 5th October 2009
First published as an Advance Article on the web 15th October 2009
DOI: 10.1039/b915926f
Photochemical borylene transfer from [(OC)5CrQBN(SiMe3)2]
to the platinum r-alkynyl complex [Cl(PMe3)2Pt–CRCPh]
gave facile access to the first transition metal–borirene complex.
The title compound has been fully characterized in solution and
by X-ray crystallography.
Since the first practical synthetic route to group 6 metal
Scheme
1 Synthesis of borirene 4 by photochemically-induced
borylene complexes [(OC)5MQBN(SiMe3)2] (M = Cr, 1;
Mo, 2) was developed in 1998,1 the reactivity of these species
has been the focus of intense research.2 Most notably, these
compounds have turned out to be convenient sources for the
borylene fragment ‘‘:B–X’’, a hypovalent, highly reactive
species that was only obtained under drastic conditions and
borylene transfer to Pt–alkynyl complex 3.
formation of a new boron- and phosphorous-containing
compound, as indicated by the presence of a new resonance
at d = 32.0 ppm in the 11B{1H} NMR spectrum, and at
d = ꢀ16.2 ppm (1JPt,P = 3377 Hz) in the 31P{1H} NMR
spectrum (see the ESIw). The former signal is in the expected
range for a borirene product.6a,b,10
which cannot be isolated as
a
free molecule.3 Thus,
BN(SiMe3)2 and related borylene moieties can be efficiently
transferred to other transition metals2g,4 or main-group-element
substrates, such as alkenes5 and alkynes.6 The latter provides a
facile access to borirenes, which are isoelectronic with cyclo-
propenium cations, and thus constitute the smallest boron
heterocycle that might exhibit 2p-aromatic stabilization.7
Moreover, boron-based p-conjugated systems have attracted
much attention due to their interesting photophysical properties
and their potential applications as electronic materials.8
Following these important results, we turned our attention
to the analogous borylene transfer to transition metal–alkynyl
complexes, to obtain such frameworks involving d-block
metals. Platinum–alkynyls were chosen as substrates for
borylene transfer reactions, as these complexes also represent
an important class of organometallics for optical and opto-
electronic applications.9 Herein, we report the preliminary
results of borylene transfer to the platinum–alkynyl
[Cl(PMe3)2Pt–CRCPh] (3), which leads to the first
platinum-substituted borirene.
The platinum-substituted amino borirene 4 was isolated by
filtration of the reaction mixture and subsequent crystallization
from hexanes at ꢀ60 1C as an analytically pure, colourless
solid in 53% yield. At ambient temperature, the 1H NMR
signal of the nitrogen-bound SiMe3 groups appears as a
singlet, implying rapid rotation around the B–N bond.6bz
However, at ꢀ75 1C the 1H NMR spectrum clearly shows
two signals for the nitrogen-bound trimethylsilyl groups,
1
which are separated by 40 Hz. From 500 MHz VT H NMR
spectroscopy, the barrier to rotation about the boron–nitrogen
bond was determined to have a value of DGaꢀ = 45.2 kJ molꢀ1
at the coalescence temperature of ꢀ55 1C, which is in good
agreement with previously reported data for borirenes with a
conjugated spacer [1,4-bis-{(m-(BNSiMe3)2(SiMe3CQC)}C6H4]
(38.7 kJ molꢀ1) and the bis(borirene) [{(m-(BNSiMe3)2-
(SiMe3CQC)}2] (43.1 kJ molꢀ1).6b These values are significantly
smaller than those commonly observed for aminoboranes of
the general formula R2NQBR2 (71–100 kJ molꢀ1),11 thus
supporting the presence of 2p-aromatic stabilization within
the boron-containing 3-membered ring with incorporation of
the pz-orbital at boron, which reduces the B–N p-contribution.
Single crystals of 4 were grown from hexane via evaporation
at room temperature. The result of the X-ray crystal structure
analysis is displayed in Fig. 1.w The molecule crystallizes in the
monoclinic space group P21/c, with two independent molecules in
the asymmetric unit.z Both independent molecules, denoted as
molecules A and B, feature very similar structures. The
distances within the BCC-ring (C2–B1 = 1.482(8) A (A),
1.487(8) A (B), C1–B1 = 1.511 (8) A (A), 1.502(8) A (B),
C1–C2 = 1.374(7) A (A), 1.372(7) A (B)) are comparable
to those of previously reported structurally characterized
aminoborirenes, which indicates the extensive delocalization
of the two electrons over a three-center bonding molecular
The title compound 4 was obtained upon photolysis of a
pale yellow solution of 1 in the presence of an equimolar
amount of platinum–alkynyl 3 for 7 h under dry argon at
room temperature (Scheme 1). The reaction was monitored by
multinuclear NMR spectroscopy, which revealed gradual
consumption of the starting materials, and quantitative
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat
¨
¨
Wurzburg, Am Hubland, D-97074 Wurzburg, Germany.
¨
¨
E-mail: h.braunschweig@mail.uni-wuerzburg.de;
Fax: +49 931 888 4623; Tel: +49 931 31 85260
w Electronic supplementary information (ESI) available: Experimental
details of all X-ray crystal structure determinations and a figure of the
molecular structure of 4. Experimental section including the syntheses,
full characterization, and spectroscopic data of all compounds. CCDC
reference number 742461. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b915926f
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Chem. Commun., 2009, 6979–6981 | 6979

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blue-shifted with respect to those of previously reported
main-group borirene compounds (257–276 nm).6b
In summary, exploiting the ability of compounds 1 and 2 as
convenient sources of the borylene moiety under standard
conditions, we have reported the first successful borylene-
based functionalization of metal–alkynyl complexes by photo-
chemical borylene transfer. Structural and spectroscopic data
of the Pt–borirene complex 4 indicate extensive p-delocalisation
within the boron heterocycle. Studies targeting the transfer of
the borylene unit to alkynyl complexes with different metals
and to metal–alkynyl polymers are currently under way.
This work was supported by DFG and FCI, and carried out
within the GRK 1221.
Notes and references
z Crystal data for 4: C20H41BClNP2PtSi2, Mr = 655.01, colourless
needle, 0.21 ꢂ 0.09 ꢂ 0.09 mm3, monoclinic space group P21/c,
a = 13.670(4), b = 19.510(6), c = 21.795(6) A, b = 92.514(12)1,
Fig. 1 The molecular structure of ClPt(PMe3)2{m-(BNSiMe3)2CQC}Ph
(4) in the solid state.
V = 5807(3) A3, Z = 8, rcalcd = 1.498 g cmꢀ3, m = 5.125 mmꢀ1
,
F(000) = 2608, T = 100(2) K, R1 = 0.0360, wR2 = 0.0912, 14 305
independent reflections [2y r 56.661] and 529 parameters.
orbital comprised of the pz-atomic orbitals of boron and
carbon.6 Consistent with the result from VT 1H NMR spectro-
scopy, a slightly elongated B–N separation of 1.428(7) A (A),
1.421(7) A (B), which matches those of previously reported
aminoborirenes, indicates a weaker p-interaction between the
boron and nitrogen centers. The distance between Pt1, which
is in square-planar arrangement, and the sp2-hybridized C1
(1.974(5) A (A), 1.973(5) A (B)) is slightly elongated in
comparison to that between Pt and the sp-hybridized carbon in
trans,trans-[(Ph3P)2(Cl)Pt–CRC–Pt(PPh3)2(Cl)] (1.958(4) A).12
UV-vis spectra of the Pt–alkynyl precursor 3 and the
borirene 4 were recorded in hexane solution (Fig. 2).w The
optical properties and electronic structures of Pt–alkynyl
complexes have been well studied throughout the literature.13
In comparison, the UV-vis spectra of 3 exhibit similar absorption
bands between 250 and 300 nm, with vibronic contributions
particularly from the CRCPh ligand.13b In contrast, the
spectra of the Pt-substituted borirene 4 display broad, feature-
less absorptions with maxima occurring at higher energies.
Furthermore, most likely due to the organometallic substituent,
the absorption maximum (lmax = 247 nm) of 4 is somewhat
Spectroscopic data for 4: 1H NMR: d = 0.46 (s, 18H, Si(CH3)3),
1.08 (m, 18 H, P(CH3)3), 8.39 (m, 2H, CH-o of C6H5), 7.36 (m, 2H,
CH-m of C6H5), 7.18 (m, 1H, CH-p of C6H5); 13C{1H} NMR:
(C bonded to boron not detected), 3.78 (s, Si(CH3)3), 13.60 (m, P(CH3)3),
129.28 (s, C-i of C6H5), 125.39 (s, CH-p of C6H5), 128.08 (s, CH-p or
CH-m of C6H5), 128.23 (s, CH-p or CH-m of C6H5); 11B{1H} NMR:
d = 32.01(s); 31P{1H} NMR: d = ꢀ16.23 (1JPt,P = 3377 Hz).
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