3458 Organometallics, Vol. 29, No. 15, 2010
Braunschweig et al.
complexes stimulated great efforts in accessing and under-
standing the corresponding compounds. In this field, term-
inal borylene complexes [LnMdBR] exhibiting a consider-
able metal-boron multibond character account for the
largest proportion.2,3 Preparation of the previously unpre-
cedented oxoboryl complexes [LnM-BtO]10 as well as a
variety of iminoboryl complexes [LnM-BtN-R]11,12 de-
monstrated that the electron deficiency at boron can also be
compensated almost exclusively by a doubly π-donating
main group substituent. The latter compounds constitute
isoelectronic BN analogues of the well-established class of
σ-alkynyl complexes, which have proven their relevance as
rigid-rod polymers and optoelectronic materials.13 Despite
the polarity of the BtN triple bond, the iminoboryl com-
plexes display a remarkable stability. However, in particular
the platinum derivatives undergo highly selective reactions
resembling their main group-substituted analogues.14,15
Thus, coordination of Lewis-acidic aluminum trichloride to
the imino nitrogen center of trans-[(Cy3P)2BrPt(BtNSiMe3)]
(1) yielded the first neutral borylene complex of platinum,
trans-[(Cy3P)2BrPtdBdN(SiMe3)(AlCl3)],16 whereas sodium-
(phenyl)acetylide leaves the iminoboryl moiety intact. In-
stead it reacts under substitution of the bromide, forming
trans-[(Cy3P)2Pt(BtNSiMe3)(CtCPh)] (2),12 in which the
two isoelectronic ligands are coordinated to the same metal
center. Furthermore, different moderately Brønsted-acidic
substrates react via 1,2-dipolar addition, yielding unsymme-
trically substituted boryl ligands,12,16 the generation of
Scheme 1. Synthesisoftrans-[(Cy3P)2BrPt{B(Br)N(H)SiMe3}] (4)
which otherwise would demand considerable preparative
efforts. Herein, we report the extension of this chemistry
to the reversible addition of hydrogen bromide to trans-
[(Cy3P)2BrPt(BtNSiMe3)] (1) and its behavior toward hydro-
boration reagents.
Results and Discussion
As was published previously, trans-[(Cy3P)2BrPt(BtNSiMe3)]
(1) turned out to be surprisingly stable toward pure water.
Only the presence of acidic reagents catalyzed 1,2-dipolar
addition of H2O to the BtN triple bond. Thus, reaction with
hydrobromic acid yielded exclusively the platinaborinic acid
trans-[(Cy3P)2BrPt{B(OH)N(H)SiMe3}] (3).12 In the ab-
sence of protic solvents, hydrogen bromide itself undergoes
instant reaction with the iminoboryl complex. Hence, addi-
tion of an ethereal hydrogen bromide solution to a toluene
solution of 1 at -70 ꢀC causes immediate formation of a
colorless precipitate, which dissolves upon warming to room
temperature. Analysis of the reaction mixture by means of
nuclear magnetic resonance (NMR) spectroscopy reveals
formation of one new compound. The boron (11B) NMR
resonance (38 ppm) and phosphorus (31P) NMR data (20.2
(7) (a) Brown, H. C. Hydroboration; Wiley-Interscience: New York,
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1
ppm, JPt-P = 2805 Hz) are indicative of a boryl complex
featuring a tricoordinate boron atom.1,9,17 However, the
values markedly differ from those observed for 3 (11B: 33 ppm,
31P: 25.7 ppm, 1JPt-P = 2894 Hz),12 and the differences in the
11B NMR chemical shifts are in the typical range observed
for formal substitution of an oxygen-centered substituent by
a bromide.18 The proton (1H) NMR spectrum of the product
displays a sharp signal at 4.55 ppm with an integration of 1
with respect to the cyclohexyl (66) and trimethylsilyl (9)
resonances. On the basis of 15N,1H NMR correlation spectros-
copy this signal is assigned to a hydrogen directly bound to
nitrogen. Thus, the spectroscopic data are consistent with the
formation of trans-[(Cy3P)2BrPt{B(Br)N(H)SiMe3}] (4) ob-
tained by 1,2-dipolar addition of hydrogen bromide to the
BtN triple bond (Scheme 1). The product was isolated in
almost quantitative yields (91%), and colorless crystals were
obtained by recrystallization from a mixture of benzene and
hexane.
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The results of X-ray diffraction studies confirm the pro-
posed constitution of 4 (Figure 1). The geometrical para-
meters around platinum are not exceptional and are in full
agreement with those of other boryl complexes.1,9,17 As was
previously observed for the adducts of 1 with methanol16
and aniline,12 4 exhibits a configuration at the BN moiety
that corresponds to formal trans addition of the element
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