Inorganic Chemistry
Article
Scheme 1. Reaction of [(bipy)Pt(Ar)2], 1, with Electrophilic Fluorinating Reagents at RT
To study the Ar−F bond formation in more detail, we
decided to study the reactivity of bisaryl-platinum(II)
complexes with electrophilic fluorinating reagents to obtain
bisaryl-fluoro-platinum(IV) complexes which are in a dilemma
between Ar−Ar and Ar−F bond formation reaction and
investigate the impact of various parameters such as the type
and amount of fluorinating source, steric hindrance of aryl
groups, time and temperature of reactions, on these two
competitive pathways. Also, the milder electrophilic fluorinat-
ing reagents, NFSI (N-fluorobenzenesulfonimide) and Select-
fluor (N-chloromethyl-N′-fluorotriethylenediammonium bis-
(tetrafluoroborate)) were chosen instead of XeF2. Both of
them do not attack glass vessels like most fluoride-containing
reagents, and their use does not involve any special techniques
or equipment in storing and handling. So, they have simpler
reaction conditions and provide more suitable results for future
practical fluorination transformations. However, in all our
experiments, the fluorinating reagents were used under air, and
dry solvents were not used.
Figure 1. X-ray crystal structure of [(bipy)Pt(4-CH3−C6H4)2F-
(NCCH3)]BF4, 2a. The H atoms were omitted for clarity.
2. RESULTS AND DISCUSSION
As represented in Scheme 1, the bis(para-Tolyl)platinum(II)
complex [(bipy)Pt(4-CH3−C6H4)2], 1a, was reacted with 1
equiv of Selectfluor in acetonitrile to generate the fluoro-
Pt(IV) complex, 2a. The signal of coordinated fluoro ligand
appears at δ = −295.3 ppm with 1JPt−F = 1592 Hz in 19F NMR.
The crystal 2a was obtained by slow diffusion of n-hexane into
its dichloromethane solution. The X-ray crystal structure 2a
(Figure 1) shows the platinum center is located in an almost
perfect octahedral environment with coordinated acetonitrile
solvent trans to fluoro ligand. The N(2)−Pt(1)−F(1) bond
angle being 175.9° and the Pt(1)−F(1) bond length of 1.94 Å,
is within the typical range of fluoro-Pt(IV) complexes.14,15
This complex was stable at RT and did not have any tendency
for either Ar−Ar or Ar−F bond formation even after about 1
month in solution and solid state. Also, by the reaction of 1a
with 2 equiv of Selectfluor, 2a was stable after a week at RT.
After heating at 80°C for 12 days, 4,4′-dimethylbiphenyl
signals are observed in ≈35% yield (based on 1H NMR). No 4-
fluorotoluene was observed by GC-MS, 1H NMR, or 19F
NMR, indicating that no Ar−F bond formation occurred
(Figures S2−S6).
However, the NFSI could be used in various organic solvents
especially inert and noncoordinating solvents such as CHCl3,
CH2Cl2, and toluene.19 Moreover, the NFSI contains a bulky
anionic residue [dibenzenesulfonimide]− having a coordinating
ability that could be another distinct parameter to have a
different reactivity in comparison with Selectfluor.
To modify the rate and yield of the Ar−Ar bond formation,
the fluorinating reagent was changed to NFSI. By the reaction
of 1a with 1 equiv of NFSI in chloroform, the biaryl product
1
signals were observed with ≈35% yield (based on H NMR)
after about 1 month at RT. The signal of plausible resulted
fluoro-Pt(IV), 3a, was observed as a singlet at δ = −263.7 ppm
with 1JPt−F = 1111 Hz in 19F NMR. Increasing the temperature
of this reaction to 60 °C was successful in acquiring even more
yield (>95%) of this product. Also, the amount of biaryl
formation increased to 45% by the reaction of 1a with 2 equiv
of NFSI in 2 weeks (Figures S7−S10). Thus, it is obvious that
NFSI has a higher reactivity to induce the Ar−Ar formation,
but none of them was successful to form the Ar−F bond from
1a.
While the Selectfluor has a higher reduction potential,16 it
has a greater N−F bond dissociation energy than NFSI17 does.
In addition, the suitable solvents for the reactions with
Selectfluor are limited to high polar solvents which have a high
coordinating ability such as MeCN, DMF, and H2O.18
It is worth noting that the reaction of XeF2 with a complex
similar to 1a in dichloromethane, as a usual solvent for
reactions with XeF2, did not lead to any Ar−Ar formation, and
only [Pt(bipy)F(Ar)2Cl] was formed and decomposed to
unknown products after a day at RT (Figure S11). XeF2 has an
1017
Inorg. Chem. 2021, 60, 1016−1020