Carbon-fluorine bond formation via a five-coordinate fluoro complex of ruthenium(II)

Article abstract of DOI:10.1002/(SICI)1522-2675(19991215)82:12<2448::AID-HLCA2448>3.0.CO;2-G

The 16-electron, five-coordinate fluoro complex [RuF(dppp)₂]PF₆ (1a; dppp = propane-1,3-diylbis[diphenylphosphine] smoothly reacts with 1,3- diphenylallyl bromide (=1,1'-(3-bromoprop-1-ene-1,3-diyl)bis[benzne]) in dry CDCl₃ to give 1,3-diphenylallyl fluoride and [RuBr(dppp)₂]⁺ in nearly quantitative yield. Under similar conditions, bromide (or chloride)/fluoride exchange also occurs with chlorotriphenylmethane, bromodiphenylmethane, and tert-butyl bromide. The crystal structure of 1a is reported.

Full text of DOI:10.1002/(SICI)1522-2675(19991215)82:12<2448::AID-HLCA2448>3.0.CO;2-G

2448
Helvetica Chimica Acta ± Vol. 82 (1999)
Carbon� Fluorine Bond Formation via a Five-Coordinate Fluoro Complex
of Ruthenium(II)
Preliminary Communication
by Peter Barthazy, Lukas Hintermann, Robert M. Stoop, Michael Wörle, Antonio Mezzetti*,
and Antonio Togni*
Laboratory of Inorganic Chemistry, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092 Zürich (Tel:
1 632 6121/2236; e-mail: mezzetti@inorg.chem.ethz.ch; togni@inorg.chem.ethz.ch)
0
The 16-electron, five-coordinate fluoro complex [RuF(dppp)
2
]PF
6
(1a; dppp ˆ propane-1,3-diylbis[diphen-
ylphosphine] smoothly reacts with 1,3-diphenylallyl bromide (ˆ1,1'-(3-bromoprop-1-ene-1,3-diyl)bis[ben-
‡
zene]) in dry CDCl
3
to give 1,3-diphenylallyl fluoride and [RuBr(dppp)
2
]
in nearly quantitative yield. Under
similar conditions, bromide (or chloride)/fluoride exchange also occurs with chlorotriphenylmethane,
bromodiphenylmethane, and tert-butyl bromide. The crystal structure of 1a is reported.
Introduction. ± F-Containing organic molecules find extensive use in biochemistry
and medicinal chemistry. Therefore, selective fluorination (C� F bond formation) has
to be regarded as one of the new frontiers in organic synthesis [1]. By contrast,
organometallic chemists have directed more efforts toward stoichiometric [2 ± 5] or
catalytic [6][7] C� F bond cleavage, and metal-assisted fluorination is essentially
restricted to the formation of fluoroacyl complexes [8] and to the use of metal fluoride
salts [9]. Driven by the interest in C� F activation, studies of the coordination
chemistry of fluoride have recently progressed from the stage of serendipity to
systematic investigation [10 ± 14].
Fluoride is known to stabilize early- or middle-transition-metal complexes with fewer
than six valence electrons by F ! M p-donation [10], whereas the four-electron
repulsion between the filled p-orbitals on the metal and the F p-orbitals is thought to
account for the paucity of fluoro complexes of the late-transition-metal ions [15].
Interestingly, this interaction can be exploited to stabilize ꢀoperationally unsaturatedꢁ
complexes with a formal 16-electron count [11][15]. However, five-coordinate fluoro
complexes with such an electron configuration are exceedingly rare [11], and most of
the reported complexes combining fluoride and phosphine ligands contain strong
p-acceptor co-ligands, generally as part of a trans-[F� M� CO] fragment featuring
push-pull interactions [11][14][15].
‡
Results and Discussion. ± We report herein the 16-electron species [RuF(dppp)2]
(
1a; dppp ˆ propane-1,3-diylbis[diphenylphosphine]) and its use as a fluorinating
agent for selected organic bromides. Thus, we prepared the dark-red five-coordinate

Helvetica Chimica Acta ± Vol. 82 (1999)
2449
complex [RuF(dppp) ]PF (1a ´ PF ), containing the unprecedented FP donor set, by
2
6
6
4
1
the reaction of [RuCl(dppp) ]PF (1b ´ PF ) with TlF in CH Cl (Eqn. 1) ).
2
6
6
2
2
‡
‡
[
RuCl(dppp) ] ‡ TlF ! [RuF(dppp) ] ‡ TlCl
(1)
2
2
1
b
1a
The 31P-NMR spectrum of 1a consists of the AA'MM' part of a AA'MM'X spin system, with resolved
19
couplings between the F-atom and the two pairs of axial and equatorial P-atoms. The F-NMR spectrum
features the F-ligand as tt (X part of AA',MM'X) at d � 203.4. As already observed for a number of fluoro
3 2
complexes [4], the P,F coupling is not observed in CDCl that contains traces of H O.
The crystal structure of 1a displays a pseudo trigonal-bipyramidal structure (Y-
‡
2
shaped) for the cation [RuF(dppp) ] (Fig.) ), similar to that of the chloro analogues
2
‡
[
MCl(P� P) ] (MˆRu or Os, P� P ˆ diphosphine) [16]. This geometry is known to
2
optimize the X ! M p-donation in p-stabilized 16-electron complexes [17]. The F-
ligand is disordered between two positions (at 0.728(9) Š from each other)
approximately lying in the equatorial plane and with similar occupancies (46 and
1)
Fluorobis{propane-1,3-diylbis[diphenylphosphine-kP]}ruthenium(1‡) hexafluorophosphate ([RuF(dppp)
PF ; 1a ´ PF ): A suspension of [RuCl(dppp) ]PF (817 mg, 0.74 mmol) and TlF (200 mg, 0.90 mmol) in
CH Cl
2
)-
6
6
2
6
2
2
(20 ml) was stirred for 3 h at r.t. TlCl was filtered off, and a second portion of TlF (100 mg,
i
0
.45 mmol) was added. After 2 h, TlCl was filtered off, PrOH (50 ml) was added, and CH
2
Cl
2
was
1
evaporated: 725 mg (90%) of 1a ´ PF
6
. Red precipitate. H-NMR (CDCl
3
): 7.82 (m, 8 arom. H); 6.8 ± 7.5
31
(
2 2 2 2
m, 32 arom. H); 2.62 (m, 4 H, PCH ); 2.0 ± 2.5 (m, 2 PCH ); 1.58 (m, 2 H, CH ); 0.80 (m, 2 H, CH ). P-
NMR (CDCl
J(P,F) ˆ 710, PF
3
): 49.0 (dt, J(P,F) ˆ 47, J(P,P') ˆ 32); � 7.1 (td, J(P,F) ˆ 15.2, J(P,P') ˆ 32); � 143 (sept.,
19
6
). F-NMR (CDCl
3
, CFCl
3
reference): � 74.5 (d, J(P,F) ˆ 710, PF
6
); � 203.6 (tt, J(P,F) ˆ
‡
‡
4
7, J(P',F) ˆ 15, RuF). FAB-MS (pos.): 945 (100, M ), 511 (35, [M � dppp] ). Anal. calc. for
54 52 7 5 2 2
C H F P Ru ´ 0.5 CH Cl : C 57.81, H 4.72: found: C 57.81, H 4.82.
2)
Crystals of 1a were obtained by slow evaporation from concentrated CH
Cl
2
/iPrOH solutions of the
2
3
complex. Crystal data: crystal size 0.50 Â 0.32 Â 0.22 mm , red platelets, C56
H
56Cl
4
F
7
P
5
Ru, M 1268.80, T 213
3
K; monoclinic, P2
1
/n, a ˆ 11.925(2) Š, b ˆ 14.798(2) Š, c ˆ 31.644(5) Š, b ˆ 99.59(2)8, Vˆ 5505.8(14) Š ,
�
3
� 1
F(000) ˆ 2604, Z ˆ 4, D
c
ˆ 1.531 Mg m ; m(MoKa) ˆ 0.686 mm , Siemens-Smart-Platform diffractometer
with CCD detector, normal focus molybdenum-target X-ray tube, graphite monochromator, w-scans, h � 16
to 15, k � 19 to 15, l � 41 to 44; 38 839 reflections for 1.318 < q < 29.948 (14 217 unique, Rint 0.0576). Unit cell
dimensions determination and data reduction were performed by standard procedures, and an empirical
absorption correction (SADABS) was applied. The structure was solved with SHELXS-96 by direct
methods, and refined by full-matrix least squares on F2 with anisotropic displacement parameters for all
non-H-atoms except disordered atoms, which were refined isotropically. Analysis of the electron-density
contour map showed that the F-ligand is disordered between two positions (F(1A) and F(1B)), and that a
small amount of Cl-atom (Cl) is present. Refinement of the occupancy factors f of these three atoms with
the restraints f(F(1A)) ‡ f(F(1B)) ‡ f(Cl) ˆ 1 and U(F(1A)) ˆ U(F(1B)) gave f(F(1A)) ˆ 0.458, f(F1B) ˆ
3
1
0
.396, and f(Cl) ˆ 0.146. The latter value was confirmed independently by integration of the P-NMR
spectrum of crystals taken from the same batch. Positional and thermal parameters, but not occupancies,
were refined in the last cycles. Two CH Cl molecules were found, one of which is disordered. H-Atoms
were introduced at calculated positions on non-disordered C-atoms of the cation and refined with the riding
model and individual isotropic thermal parameters. R 0.0598 and wR
ˆ 0.1259 (9196 unique reflections
with I > 2s(I)), R ˆ 0.1473 (all data), S ˆ 1.042. Max. and min. difference peaks ‡ 0.969
ˆ 0.1097 and wR
2
2
1
2
1
2
�
3
and � 0.894 eŠ , largest and mean were D/s ˆ 2.470 and 0.029. Crystallographic data (excluding structure
factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication no. CCDC 134839 (1a). Copies of the data can be obtained free
of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (‡44) 1223 336-033;
e-mail: deposit@ccdc.cam.ac.uk).

2450
Helvetica Chimica Acta ± Vol. 82 (1999)
‡
Figure. ORTEP View of [RuF(dppp)
2
] . 30% Probability ellipsoids. Only F(1A) is shown. Selected interatomic
distances [Š] and angles [deg]: Ru� F(1A) 2.030(7), Ru� F(1B) 2.033(9), Ru� P(1) 2.423(1), Ru� P(3)
2
.408(1), Ru� P(2) 2.261(1), Ru� P(4) 2.254(1), F(1A)� Ru� P(1) 88.8(2), F(1B)� Ru� P(1) 85.8(2),
F(1A)� Ru� P(2) 125.3(2), F(1B)� Ru� P(2) 145.5(2), F(1A)� Ru� P(3) 82.9(2), F(1B)� Ru� P(3) 85.5(2),
F(1A)� Ru� P(4) 139.6(2), F(1B)� Ru� P(4) 119.7(2), P(1)� Ru� P(2) 89.41(4), P(2)� Ru� P(3) 97.40(4),
P(1)� Ru� P(3) 171.26(4), P(2)� Ru� P(4) 94.85(4), P(1)� Ru� P(4) 96.51(4), P(3)� Ru� P(4) 88.39(4).
3
‡
4
0% for F(1A) and F(1B), respectively) ). [RuF(dppp) ] is nearly isostructural with
2
the chloro analogue 1b, the main differences being due to the smaller size of the
F-ligand. Thus, six Ph rings form a pocket around the halide ligand in both 1a and 1b,
but in the fluoro complex, a twist of the axial Ph groups C(7)� C(12) and C(31)� C(36)
reduces the nonbonded distances between F and the H atoms (calculated positions)
o
‡
from values in the range 2.99 ± 2.49 Š in [RuCl(dppp) ] (1b) to 2.17 ± 2.59 Š in 1a,
2
clearly shorter than the sum of the Van der Waals radii (2.67 Š). Short nonbonded F ´´´
H� C distances have been observed in other fluoro complexes [14]. Interestingly, the
positional disorder in 1a appears to minimize the F ´´´ H nonbonded distances. The
Ru� F distances (2.030(7) and 2.033(9) Š) in 1a are very close to 2.02 Š, the value
calculated from the Ru� Cl distance of 2.371(5) Š in 1b by subtracting the difference
of the atomic radii of Fand Cl (0.35 Š), suggesting that the p-effects on the bonding are
similar in 1a and 1b.
With the aim of assessing the reactivity of complex 1a, we found that the F-ligand is
smoothly transferred to selected organic electrophiles. Thus, when equivalent amounts
of the five-coordinate 1a and 1,3-diphenylallyl bromide (2) [19] were mixed in an NMR
tube in dry CDCl , an immediate color change from red to brown was observed. The
3
3)
2 2
The crystal contains 14% of 1b as impurity due to partial reaction with the CH Cl solvent upon
crystallization, as confirmed by P-NMR. Attempts to grow suitable crystals in other solvents were
3
1
unsuccessful. The related Ru� Cl distance (2.315(11) Š) is close to the value found in 1b (2.371(5) Š); see
[
18].

Helvetica Chimica Acta ± Vol. 82 (1999)
2451
1
19
H- and F-NMR spectra of the resulting solution indicated that the starting material 2
was converted to 1,3-diphenylallyl fluoride 3 in nearly quantitative yield (> 80%).
Although the reaction between 1a and 2 was carried out in a glove-box in dry solvents, a
small amount (>10%) of bis(1,3-diphenylallyl) ether was also formed. This side
reaction was completely suppressed when the experiment was carried out in a Teflon
3
1
tube. The P-NMR spectrum of the reaction solution shows the quantitative formation
‡
of [RuBr(dppp) ] (1c) (Scheme), which may be recovered from the reaction
2
4
mixture ). The synthesis of an authentic sample of the allyl fluoride derivative 3
proved to be far from trivial. When 1,3-diphenylprop-2-en-1-ol was reacted with
(diethylamino)sulfur trifluoride (DAST) [20] or Olahꢁs reagent (HF/pyridine) [21],
bis(1,3-diphenylallyl) ether was the only product isolated. Thus, rigorous exclusion of
O and H O is imperative, and, indeed, the reaction of 2 with soluble, dry fluoride
2
2
5
sources such as TBAT ) [22] or KF in DMF [23] yielded samples of 3 sufficiently pure
6
for unambiguous identification ). Under similar conditions, bromide (or chloride)/
fluoride exchange also occurred with chlorotriphenylmethane (ˆ1,1',1''-(chlorome-
thylidyne)tris[benzene], bromodiphenylmethane (ˆ1,1'-(bromomethylene)bis[ben-
zene], and tert-butyl bromide (ˆ2-bromo-2-methylpropane), and the corresponding
7
fluorinated products were identified spectroscopically by NMR ) [24]. The reaction of
the latter substrate was quite sluggish. However, no elimination of HBr to afford 2-
methylprop-1-ene could be observed.
Conclusions. ± The C� F bond-forming process observed is still a rare example of a
reaction of an organic elecrophile with a fluoro complex. Derivative 1a acts as a donor
of ꢀnakedꢁ fluoride [25] as well as a Lewis acid and bromide scavenger. Although similar
III
reactions have been reported previously by Bergman and co-workers with an Ir 18-
electron system [12], we believe that a coordinatively unsaturated complex such as 1a
offers advantages in terms of reactivity. Although the use of a fluoro complex is not
strictly necessary for the above type of reaction to occur, the use of well-defined fluoro
4)
[RuBr(dppp)
1
]
‡
(1c): 31P-NMR (CDCl
): � 1.0 (t, J(P,P') ˆ 31.3); 38.6 (t, J(P,P') ˆ 31.3). FAB-MS (pos.)
2
‡
3
‡
007 (100, M ), 926 (6, [M � Br] ).
5)
6)
TBATˆ tetrabutylammonium difluorotriphenylsilicate.
2
1,1'-[(1E)-3-Fluoroprop-1-ene-1,3-diyl]bis[benzene] (3): In freshly distilled (CaH ) DMF (10 ml), 1,1'-
[
(1E)-3-bromoprop-1-ene-1,3-diyl]bis[benzene] (206 mg, 0.85 mmol) and KF (90 mg, 1.55 mmol) were
stirred for 2 d at r.t. in the dark. Filtration and evaporation of the solvent yielded a brownish oil containing
some DMF and bis(1,3-diphenylallyl) ether (less than 3% by 1H-NMR). 1H-NMR (CDCl
): 7.45 ± 7.26
m, 10 arom. H); 6.72 (ddd, J(F,H) ˆ 15.9, 4.0, 0.9, 1 H, H� C(1'')); 6.38 (ddd, J(F,H) ˆ 15.9, 11.7, 6.7,
3
(
1
3
H� C(2'')); 6.02 (ddd, J(F,H) ˆ 47.5, 6.7, 0.9, H� C(3'')). C-NMR (CDCl
3
): 138.9 (d, J(F,C) ˆ 22, 1 C);
35.7 (d, J(F,C) ˆ 2, 1 C); 133.0 (d, J(F,C) ˆ 12, 1 CH); 128.5 ± 128.4 (several CH); 128.3 (d, J(F,C) ˆ 15,
1
1
CH); 127.0 (d, J(F,C) ˆ 22, 1 CH); 126.7 (d, J(F,C) ˆ 1.5, 1 CH); 126.1 (d, J(F,C) ˆ 6, 1 CH); 93.8
1
9
‡
(
1
d, J(F,C) ˆ 169, 1 CH). F-NMR (CDCl
3
): � 165.4 (ddd,J(F,C) ˆ 47.5, 11.7, 4.0). EI-MS: 212 (100, M ),
‡
92 (20, [M � F] ), 133 (48).
7)
2 6 6
Reactions of 1a with other substrates: [RuF(dppp) ]PF (1a´ PF ; 22 mg, 20 mmol) and the appropriate sub-
strate (20 mmol) were dissolved in CDCl
Yields were determined by F-NMR and H-NMR. Triphenylchloromethane gave triphenylfluoromethane
3
(2 ml) in an Young-valve NMR tube equipped with a Teflon liner.
1
9
1
1
9
(
90%) after 1 min at r.t. ( F-NMR: � 126.4 (s) ([24a]: � 126.5)). tert-Butyl bromide gave tert-butyl fluoride
19
(
20%) after 1 d at 508 ( F-NMR: � 131 (10 lines, J(H,F) ˆ 21) ([24b]: � 132 (10 lines, J(H,F) ˆ 21));
1
H-NMR: 1.39 (d, J(H,F) ˆ 21) ([24b]: 1.30 (d, J(H,F) ˆ 20))). Diphenylmethyl bromide gave diphenyl-
methyl fluoride (85%) after 1 d at r.t. 19F-NMR: � 167 (10, J(H,F) ˆ 49) ([24a]: � 169 (10, J(H,F) ˆ 48)).

2452
Helvetica Chimica Acta ± Vol. 82 (1999)
Scheme
complexes in fluorination reactions clearly offers advantages in terms of transport of F-
ions in organic solvents and may open new avenues for asymmetric C� F bond
formation.
In conclusion, we have shown that fluoride can be used to stabilize 16-electron
II
species of relatively soft metal ions such as Ru , and that the resulting complexes react
with activated organic halides to form a new C� F bond. We are currently extending our
investigation to less reactive organic substrates, as well as toward the development of a
catalytic fluorination process.
Novartis Services Ltd., Basel, is gratefully acknowledged for financial support to L. H.
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[
Received September 22, 1999