ligands in a cyclic framework would favor
selective reactivity of the unrestrained phenyl
group.
With these considerations in mind, we syn-
thesized bismine ( 1 ) and attempted its oxida-
tion to Bi(V) from the 6s^2 orbital, through
reaction with XeF 2. A smooth conversion to
high-valent Bi(V) species was achieved (>95%
yield), and x-ray crystallographic analysis re-
vealed a symmetric Bi(V) dimeric structure ( 2 )
(Fig.2A).EachBiatomin 2 adopts a distorted
octahedral geometry, with two fluorine atoms
positioned trans to each other [F1–Bi1–F2 and
F3–Bi2–F4, 164.2(2)°], one of them bridging to
the other metal center. The bridging Bi–Fbonds
are 0.47 Å longer than the terminal bonds.
Moreover, the O atoms of the sulfone moiety
interact with the Bi centers [Bi1–O1 and Bi2–
O2, 3.430(6) Å], thereby forcing the F atom
away from linearity and engaging in binding
with another Bi, forming a dimer in the solid
state. When 2 was heated to 110°C in CDCl 3 ,
an unexpected 45% yield of fluorobenzene ( 3 )
was obtained. This reductive elimination of
C–F bond stands in stark contrast to previous
reports, in which attempts to thermally in-
duce C–F bond formation from Bi(V) fluoride
compounds resulted in decomposition or traces
of fluorinated arenes ( 40 , 41 ). On the basis of
the crystallographic information, we antici-
pated that tuning the electronic properties of
the sulfone could influence the Bi(V) center,
thus affecting the C–F bond-formation step.
Indeed, with a NMe (Me, methyl) group in
place of one of the O atoms in the sulfone
group ( 4 ), a notable reduction in yield was
observed ( 3 ,28%,Fig.2B).However,when
the methyl group in 4 was replaced with CF 3
( 5 ), a 94% yield of fluorobenzene ( 3 ) was ob-
tained after thermal decomposition of the cor-
responding Bi(V) intermediate. Both 4 and
5 were characterized by x-ray crystallography
(Fig. 2B). Despite their comparable Bi–Ndis-
tances [3.055(2) Å in 4 and 3.038(3) Å in 5 ],
the electronic nature of the CF 3 group clearly
has a notable promotional effect on the re-
ductive elimination process. The elimination
of fluorobenzene ( 3 ) was accompanied by the
smooth formation of the corresponding fluo-
robismine ( 6 ), which was confirmed by x-ray
crystallography. At this point, we presumed
that the productive elimination of fluoroben-
zene from 5 could be ascribed to the low pro-
pensity of the NCF 3 group to coordinate to Bi
after oxidation, which results in a monomeric
Bi(V) difluoride complex. Although attempts
to crystallize Bi(V) difluoride compounds de-
rived from 4 and 5 were unsuccessful, the
installation of a methyl residue at the ortho po-
sition, with respect to the Bi center ( 7 ), permitted
the crystallization of difluorobismine ( 8 ) (Fig.
2C). X-ray analysis of 8 revealed a monomeric
pentavalent complex with trigonal bipyramid
geometry (TBP), in which both F atoms occupy
apical positions. This geometry is in agree-
ment with the polarity rule for TBP complexes,
which predicts that the most electronegative
and least sterically demanding substituentsare always placed in apical positions—in this
case, the fluorides ( 42 ). As anticipated, coordi-
nation of the pending group in 8 is weaker
[Bi–N 3.566(4) Å] than in the dimeric sulfone
counterpart 2 [Bi–O3.430(6)Å].
We then turned our attention to the mech-
anism of this unusual C–Fbondreductive
elimination. Several para-substituted aryl-
bismine complexes were synthesized ( 5 and 9
to 13 , Fig. 3A) and oxidized with XeF 2 to the
corresponding pentavalent bismine species
( 14 to 19 ,Fig.3A).Thermaldecomposition
of 14 at 90°C exhibited a first-order kinetic
profile (kobs=1.3×10−^4 s−^1 ,wherekobsis the
apparent reaction rateconstant), in which
fluorobenzene ( 3 ) was produced at the same
rate as fluorobismine ( 6 )(kobs= 1.0 × 10−^4 s−^1
andkobs=1.1×10−^4 s−^1 ,respectively), reach-
ing 94% yield after 7 hours (Fig. 3A1). An
Eyring analysis over a 25°C range revealed a
small enthalpy barrier (DH‡=15.5±0.7kcal
mol−^1 ,whereDH‡is the change in enthalpy
between reactants and transition state) but a
surprisingly high entropic contribution [DS‡=- 34.7 ± 1.9 entropy unit (1 e.u. = 1 cal K− 1 mol− 1 ),
whereDS‡is the change in entropy between
reactants and transition state]. A large and
negative value of the entropy parameter is
consistent with an associative process in the
transition state, although cationic pathways
with high degrees of solvent reorganization
have also been postulated ( 43 , 44 ). Hammett
kinetic analysis of the thermal decomposition
of 14 to 19 to aryl fluorides ( 3 and 20 to 24 )
Planaset al.,Science 367 , 313–317 (2020) 17 January 2020 2of5
Fig. 2. Design principle and proof of concept.(A) Design of the bismine complexes to enable C–F reductive elimination. (B) Oxidative addition and reductive
elimination sequence to forge C–F bonds. (C) Trigonal bipyramidal monomeric Bi(V) difluoride bearing NCF 3. Unless specified, yields were calculated by^19 F NMR.
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