The addition of Et2O·BF3 or
Me2S·BCl3 to the BNBN-cumulene-bridged
Pt(II) A-frame
complexes [(μ-1,1-BNBN(TMS)2)(μ-dmpm)2Pt2X2] (TMS = SiMe3, dmpm = CH2(PMe2)2, X = Br 1
Br
, I 1
I
) resulted
in the oxidative addition of one B–F or B–Cl bond, respectively,
to the internal BN bond of the bridging, iminoborane-like B–NB–N
moiety, and coordination of one Pt(II) center to the resulting adjacent
BF2 (complex 2Br-F) or BCl2 (complexes 2Br-Cl and 2I-Cl) moiety, respectively. X-ray crystallographic and
multinuclear NMR-spectroscopic data show that the Pt→BF2 interaction in 2Br-F is very weak
and merely electrostatic, while the Pt→BCl2 interaction
in 2Br-Cl and 2I-Cl is a stronger donor–acceptor bond. In contrast, the reaction
of Me2S·BBr3 with 1
Br
yielded a ca. 3:2 mixture of the analogous B–Br
addition product to the iminoborane, 2Br-Br, and the product of a subsequent oxidative addition of one B–Br
bond of the chelating BBr2 moiety to the adjacent platinum
center, the mixed-valence boranediyl-bridged, Pt(II)–Pt(IV)-bromoboryl
complex 3-Br
5
. The analogous
reactions of Me2S·BI3 with 1
Br
and Me2S·BBr3 with 1
I
yielded complex product
mixtures of Pt(II)–Pt(II)-borane (2Br-I and 2I-Br, respectively) and Pt(II)–Pt(IV)-boryl
complexes (3-Br
n
I
5–n
, n = 1–3) analogous to 2X-Y and 3-Br
5
, respectively,
the proportion of the latter increasing with the proportion of iodide
in the precursor mixture. Both multinuclear NMR-spectroscopic and
X-ray crystallographic data show evidence of complex and extensive
inter- and intramolecular bromide–iodide exchanges between
the soft, iodide-affine platinum centers and the harder, more bromide-affine
boron centers. A clue to the mechanism of these halide exchanges is
provided by the reactions of BBr2Ar (Ar = 2,4,6-Me3C6H2 (Mes), 2,3,5,6-Me4C6H (Dur)) with 1
Br
, which
yielded the cationic Pt(II)–Pt(II)-borenium analogues of 2Br-Br, the complexes 4Br-Ar, generated by the sterics-induced displacement of the bromide substituent
from the chelating Pt→BBrAr moiety, and displaying a rare metal→borenium
donor–acceptor bond.