Halogen exchange by reaction of CpRu(Cl)(PPh3)2 with MeC(O)X (X = Br, I) and its mechanistic study

Article abstract of DOI:10.1016/j.jorganchem.2014.06.018

The treatment of CpRu(Cl)(PPh₃)₂ with MeC(O)X offers a very convenient procedure for the synthesis of CpRu(X)(PPh₃) ₂. The proposed mechanism involves an intermediate produced by the concerted liberation of PPh₃ by the incoming MeC(O)X and the subsequent subtraction of the X atom by the Ru atom to form a radical pair.

Full text of DOI:10.1016/j.jorganchem.2014.06.018

Journal of Organometallic Chemistry 769 (2014) 34e37
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Halogen exchange by reaction of CpRu(Cl)(PPh3)2 with MeC(O)X
(X ¼ Br, I) and its mechanistic study
,
Hitoshi Kuniyasu a *, Atsushi Sanagawa a, Takuya Nakajima a, Takanori Iwasaki a,
,
,
, c, **
Nobuaki Kambe a *, Karan Bobuatong b c, Masahiro Ehara b
a Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
b Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, 38 Nishigo-Naka, Myodaiji, Okazaki 444-8585, Japan
c Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto Daigaku-Katsura, Kyoto 615-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The treatment of CpRu(Cl)(PPh3)2 with MeC(O)X offers a very convenient procedure for the synthesis of
CpRu(X)(PPh3)2. The proposed mechanism involves an intermediate produced by the concerted libera-
tion of PPh3 by the incoming MeC(O)X and the subsequent subtraction of the X atom by the Ru atom to
form a radical pair.
Received 2 May 2014
Received in revised form
30 May 2014
Accepted 18 June 2014
Available online 19 July 2014
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Ruthenium
Acid halide
Halogen exchange
DFT calculation
Radical mechanism
Introduction
(Ln ¼ 2PPh3 or CH2]CH2), consistent with density functional the-
ory (DFT) calculations. Moreover, the theoretical mechanistic study
Few ligandeexchange reactions between MeX and ReX0 (Eq.
(1)), where X and X0 are halogen or pseudo-halogen atoms, M is a
metal, and R is a carbon functionality, have utilized in organome-
tallics compared to other types of ligand exchange, such as trans-
metalation [1,2].
suggested that the reaction proceeded via concerted s-bond
metathesis. It was demonstrated that the reactions of nickel triad
complexes such as cis-[Pt(Cl)2(PPh3)2], trans-[Pd(Cl)2(PPh3)2], and
Ni(Cl)2(dppe) with MeC(O)Br and MeC(O)I were quite convenient
for halogen exchanges. Similarly, the conversion of Au(L)Cl
(L ¼ PPh3, IPr) to the corresponding bromide and iodide was ach-
ieved, and a s-bond metathesis was also identified by theoretical
(1)
calculation [4]. This transformation benefits from its simple
handling. Analytically pure products were obtained successfully
just by mixing of reagents and the subsequent removal of the sol-
vent, by-product (MeC(O)Cl), and excess MeC(O)X by evaporation.
Procedures that are usually required in conventional syntheses
involving typical LiX, KX, and NaX metal salts, such as filtration,
extraction, drying in the presence of desiccant, and recrystalliza-
tion, were omitted. Herein, the Cl-to-X conversion of a ruthenium
chloride is reported along with its mechanistic study [5e7].
We have recently reported that the ligand exchange reaction
between MeX bonds of trans-M(X)[C(O)Ar](PPh3)2 and CeX0 bonds
of ArC(O)X0 (M ¼ Pt, Pd; X, X0 ¼ Cl, Br, I) successfully occurred to
give a clean equilibrium mixture of trans-M(X)[C(O)Ar](PPh3)2/
ArC(O)X0 and trans-M(X0)[C(O)Ar](PPh3)2/ArC(O)X [3]. The Gibbs
free energy (DG) of the reaction was equivalent to the DDG of
oxidative addition of ArC(O)X and ArC(O)X0 to M(PPh3)2Ln
* Corresponding authors. Fax: þ81 6 6879 7390.
Results and discussion
** Corresponding author. Department of Theoretical and Computational Molecular
Science, Institute for Molecular Science, 38 Nishigo-Naka, Myodaiji, Okazaki 444-
8585, Japan. Fax: þ81 6 6879 7390.
The reaction of CpRu(Cl)(PPh3)2 (1, 0.010 mmol) with MeC(O)Br
(2, 0.020 mmol) in C6D6 (0.75 mL) at room temperature was
monitored by 31P and 1H NMR spectroscopies (Eq. (2)). The gradual
E-mail addresses: kuni@chem.eng.osaka-u.ac.jp (H. Kuniyasu), kambe@chem.
eng.osaka-u.ac.jp (N. Kambe), ehara@ims.ac.jp (M. Ehara).
http://dx.doi.org/10.1016/j.jorganchem.2014.06.018
0022-328X/© 2014 Elsevier B.V. All rights reserved.

H. Kuniyasu et al. / Journal of Organometallic Chemistry 769 (2014) 34e37
35
clean conversion of 1 (
d
40.1) to CpRu(Br)(PPh3)2 (3) (
d
39.0) and
energy of TS2, which leads to the formation of 9, was 101.6 kJ/mol
higher than the reactants. On the other hand, the formation of 7
required 147.5 kJ/mol, which was clearly energetically more
demanding. Similarly, the energy of 8 (128.3 kJ/mol) was much
higher than that of TS5 (95.6 kJ/mol), which generates 3 by elimi-
nation 4. Therefore, we concluded that Mechanism 1 is unlikely. In
intermediate 9 (79.2 kJ/mol), one hydrogen of Me of the incoming
acetyl bromide is found in close proximity to the Cl atom, hydrogen
bond-like interaction as suggested by the Cl/H (2.53 Å). Two
MeC(O)Cl (4) was observed (6.3% of 3 after 0.5 h) [5c,8]. After 9 h, 3
was produced quantitatively. No intermediate was detected during
the course of the reaction. A similar treatment of 1 with MeC(O)I (5)
produced CpRu(I)(PPh3)2 (6) (d 38.0) after 2.5 h at room tempera-
ture. Next, preparative scale reactions were executed to demon-
strate the utility of the present reaction as a synthetic method.
Compounds 1 (1.0 mmol) and 2 (5.0 mmol) were added to C6H6
(50 mL) in a 100 mL flask fitted with a stirring bar in a glove box.
After the reaction mixture was stirred at 25 ꢀC for 3 h, the solvent,
excess 2 (b.p. 75e77 ꢀC), and 4 (b.p. 52 ꢀC) were removed in vacuo.
NMR spectra and elemental analysis showed that analytically pure
3 was obtained. A similar large scale reaction was conducted with 5
at 25 ꢀC for 4 h to quantitatively afford 6 [9].
pathways were considered from 9: The
s-bond metathesis that
provides 11 via the transition state TS1 (Mechanism 2), and the
oxidative addition of BreC bond, which yields Ru(IV) complex 10
via TS3 followed by the CleC bond-forming reductive elimination
to afford 11 via TS4 (Mechanism 3). The energies of TS1 and 11
amounted to 117.9 and 72.3 kJ/mol, respectively. The energies of
TS3, 10, and TS4 equaled 124.7, 93.6 and 117.5 kJ/mol, respectively.
The energy of TS1 was 6.8 kJ/mol lower than that for TS3, sug-
gesting that Mechanism 2 was more favorable than Mechanism 3.
After the formation of 11, in which the Br/H distance is 2.53 Å, the
associative elimination of 4 via TS5 (95.6 kJ/mol) produced 3 and 4.
A total energy change of ꢁ15.5 kJ/mol was calculated for the entire
reaction
(D
G
¼
ꢁ16.0 kJ/mol). In TS2, dihedral angles
:RueCleCeO and :RueBreCeO equaled ꢁ179.4ꢀ and 179.9ꢀ,
respectively. Likewise, in TS5, the dihedral angles :RueCleCeO
and :RueBreCeO equaled ꢁ179.9ꢀ and 177.8ꢀ, respectively. This
shows that Ru, Cl, C, O of the carbonyl group, and Br atoms are
nearly coplanar in TS2 and TS5. On the other hand, the dihedral
angles :RueCleBreC of TS2, 9, TS1, 11, and TS5 amounted to
179.8ꢀ, 116.0ꢀ, 137.9ꢀ, 104.0ꢀ, and 179.9ꢀ, respectively. Therefore, the
RueCleCeBr quadrangle bends downward from flat TS2 to 9, and
flips back to TS1 before bending downward again to 11 and flat-
tening again to TS5. The dihedral angle :RueCleBreC of TS3, 10,
and TS4 equaled 102.1ꢀ, 104.8ꢀ, and 93.6ꢀ in TS3, 10, and TS4,
respectively, suggesting that this quadrangle folded similarly to the
RueCleCeBr quadrangle. However, this movement in Mechanism
(2)
The mechanism of the reaction between 1 and 2 was theoreti-
cally investigated DFT using the M06 functional [10,11].
This computational method was shown to give reliable geom-
etries and energies in previous studies on the ligand exchange re-
actions of trans-M(Cl)[C(O)Ph](PPh3)2 (M ¼ Pt, Pd) and Au(Cl)(PPh3)
with RC(O)Br (R ¼ Ph, Me) [3,4]. Three possible reaction pathways,
Mechanism 1e3, initially were investigated (Scheme 1). In mech-
anism 1, the liberation of PPh3 produces the coordinately unsatu-
rated CpRu(Cl)(PPh3) (7). Next,
s-bond metathesis between the
RueCl bond of 7 and the CeBr bond of 2 affords TS1, and the sub-
sequent elimination of 4 yields CpRu(Br)(PPh3) (8), which can un-
dergo recoordination by PPh3 to form 3. The associative elimination
of PPh3 to form 9 before TS1 formation is considered in Mechanism
2. Mechanism 3 involves the oxidative addition of the BreC bond to
9 that produces Ru(IV) complex 10 [8d,12]. In addition to the spe-
cies shown in Scheme 1, the study identified transition states TS2,
TS3, TS4 and TS5. The energy diagram and optimized transition
state and intermediate structures are shown in Fig. 1 [13,14]. The
3 was less dynamic than in the of s-bond metathesis mechanism.
The C]O group gradually changed direction from right to left in
both mechanisms during this folding process (Fig. 1).
Besides those shown in Scheme 1, another possible reaction
mechanism involves the formation of the 18-electron cationic
complex 12 (L0 ¼ PPh3 or solvent), which reacts with 2 to provide 13,
and the subsequentelimination of L0 toform3 (Scheme 2)[5b,c,15]. If
the reaction proceeded via 12, the addition of free PPh3 or acetoni-
trile would accelerate the reaction [16a]. However, the trans-
formation was actually rather suppressed by the addition of PPh3
(0.1 equiv) or hardly affected by the addition of CD3CN (20 equiv)
under the same conditions described in Eq. (2), excluding the for-
mation of 12 and this mechanism. This retardation is consistent with
Mechanism 2, which involves the elimination of PPh3.
In agreement with the concerted
s-bond metathesis mecha-
nism, no significant effect was observed by the addition of 2,2,6,6-
tetramethylpiperidine 1-oxyl (TEMPO) and galvinoxyl during the
transformation of trans-Pt(Cl)[C(O)Ph](PPh3)2 to the corresponding
bromide [3]. In stark contrast, the radical and radical inhibitor
remarkably affected the present Ru-system. The reaction of 1 with 2
in the presence of 0.1 equiv of TEMPO under otherwise similar
conditions only gave 19% of 3 after 9 h (Eq. (3), compare this with
the result of Eq. (2)). On the other hand, the same reaction in the
presence of 1.0 equiv of 9,10-dihydroanthracene (DHA) produced 3
in 75% after 0.5 h and quantitatively within 2 h [16]. These results
clearly show the participation of a radical species in the reaction
mechanism. Reports have shown that Cp*Ru(Cl)(PPh3)2, an analog
of 1, subtracted a halogen from halogenated compounds during the
process of the atom transfer radical addition (ATRA) to alkenes [17].
A revised mechanism, Mechanism 4, is therefore proposed in
Scheme 1. Possible reaction pathways of the reaction between 1 and 2.

36
H. Kuniyasu et al. / Journal of Organometallic Chemistry 769 (2014) 34e37
Fig. 1. Energy diagram for the reaction of 1 with 2. Phenyl of PPh3 and hydrogen atoms of Cp are omitted for clarity. Atoms in blue, orange, green, dark red, and red represent Ru, P,
Cl, Br, and O, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Scheme 1. After the formation of 9 via TS2, the Br atom is subtracted
by the Ru atom to give radical pair 14 consisting of a Ru(III) frag-
ment and an acetyl radical. Subsequent Cl atom subtraction by the
acetyl radical would afford 11. This process could be affected by the
radical inhibitor or promoter. The energy of 14 would be lower than
TS1. In fact, the combined energy of CpRu(Cl)(Br)(PPh3) and
CH3C(O)ꢂ is 71.4 kJ/mol, implicating the presence of radical process
with lower energy (Fig. 1).
1: 31P NMR (160 MHz, C6D6)
3: 31P NMR (160 MHz, C6D6)
6: 31P NMR (160 MHz, C6D6)
d
d
d
40.10.
38.97.
37.97.
A ligand exchange reaction between Cl of CpRu(Cl)(PPh3)2 (1) and
Br of MeC(O)Br (2) (Eq. (2))
Into
CpRu(Cl)(PPh3)2 (1, 0.010 mmol, 500
solution of S]P(C6H4OMe-p)3 (0.0050 mmol, 50
benzene-d6), and benzene-d6 (180 L). After the sensitivity of 1 to
the internal standard was measured by 31P NMR spectroscopy, a
a
dry Pyrex NMR tube were added
L/20 mM in benzene-d6), a
L/100 mM in
a solution of
m
m
m
(3)
solution of MeC(O)Br (2, 0.020 mmol, 20 mL/1.0 M in benzene-d6)
was added and the reaction was monitored by 31P NMR spectros-
copy. The gradual conversion of 1 to 3 was confirmed. The reaction
time and yield are as follows: 0.5 h, 6.3%; 1 h, 15%; 2 h, 38%; 3 h,
62%; 4 h, 80%; 5 h, 90%; 6 h, 95%; 7 h, 98%; 8 h, 99%; 9 h, 100%.
Conclusion
Preparative scale ligand exchange reactions
This paper clearly demonstrates acetyl bromide (2) and iodide
(5) as convenient Cl-to-Br and Cl-to-I conversion reagents in
CpRu(Cl)(PPh3)2 reaction. Moreover, DFT calculations suggest that
the associative liberation of PPh3 before the generation of associ-
ated complex intermediate. The significant influence by the radical
inhibitor and promoter suggests the participation of the radical
species during the process. Further efforts are underway to validate
the mechanistic involvement of the radical pair species.
Into a 100 mL flask were added CpRu(Cl)(PPh3)2 (1, 726.2 mg,
1.0 mmol), MeC(O)Br (2, 614.8 mg, 5.0 mmol), and C6H6 (50 mL) in a
glove box at room temperature. After the solution was stirred for
3 h, the solvent, excess 2 (b.p. 75e77 ꢀC) and MeC(O)Cl (4, b.p.
52 ꢀC) were removed in vacuo to give analytically pure 1 quanti-
tatively (764.8 mg, 99%). Similarly, CpRu(I)(PPh3)2 (6) was isolated
by the treatment of 1 with MeC(O)I (5, b.p. 108 ꢀC) (812.5 mg, 99%).
Computational details
Experimental
All the calculations in this study were performed using the
GAUSSIAN 09 suite of programs [10]. We applied the M06 func-
General comments
The 31P and 1H NMR spectra in benzene-d6 were measured with
a ECS400 (400 MHz) spectrometer. The chemical shifts of the 31P
NMR spectra in benzene-d6 were recorded relative to 85% H3PO4
(aq) as an external standard, and S]P(C6H4OMe-p)3 was used as an
internal standard to calculate the yields of products (The sensitiv-
ities of ruthenium complexes to the internal standard were
measured individually). Acetyl halides and benzoyl bromide were
commercially obtained. Benzene-d6 was purified by distillation
from sodium benzophenone ketyl before use. CpRu(Cl)(PPh3)2 (1)
was prepared according to a literature [18]. Registry No. of 1:
32993-05-8; 3: 32993-06-9; 6: 34692-10-9.
tional, which has demonstrated as
a useful functional for
Scheme 2. Another possible mechanism via cationic complexes.

H. Kuniyasu et al. / Journal of Organometallic Chemistry 769 (2014) 34e37
37
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The effective core potentials including relativistic effects (RECP)
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outer shell (4p4d5s) of the Ru atom are described through the
corresponding LanL2DZ basis set. The 6-31G(d,p) basis sets were
employed for cyclopentadienyl ligand, phosphorus atom of PPh3, Cl,
Br, and carbon and oxygen atoms of carbonyl group of acetyl bro-
mide and acetyl chloride. The STO-6G basis sets were applied for
phenyl group of PPh3 and methyl group of acetyl bromide and
acetyl chloride. During the optimization, all the molecular struc-
tures were fully relaxed without any symmetry constraints. All the
ground state structures optimized are local minima; vibrational
analyses performed at the optimized structures contained no
imaginary frequencies. All the optimized transition state structures
possessed only one imaginary frequency.
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Acknowledgments
[10] (a) M.J. Frisch, et al., GAUSSIAN09, Revision B.01, Gaussian, Inc., Wallingford,
CT, 2010;
This study was partly supported by a grant from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
(b) J. Tao, J.P. Perdew, V.N. Staroerov, G.E. Scuseria, Phys. Rev. Lett. 91 (2003)
146401.
Masahiro Ehara acknowledges
a Grant-in-Aid for Scientific
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(b) Y. Zhao, D.G. Truhlar, Chem. Phys. Lett. 502 (2011) 1e13;
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(d) M. Mantina, R. Valero, D.G. Truhlar, J. Chem. Phys. 131 (2011) 064706-
1e064706-5;
Research from the Japan Society for the Promotion of Science (JSPS,
No. 22000009) and JENESYS Programs. Computations were partly
performed at the Research Center for Computational Science,
Okazaki, Japan. This work was partly supported by the JSPS
Japanese-German Graduate Externship.
(e) R. Kang, J. Jiannian Yao, H. Hui Chen, J. Chem. Theory Comput. 9 (2013)
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[13] The structures, energy diagrams, selected angles and distances between two
elements, Cartesian coordinates of the compounds, and IRC energy diagrams
described in this manuscript are shown in the Supplementary data.
[14] The reaction of 1 with PhC(O)Br also occurs to give 3 quantitatively and the
reaction mechanism has been examined theoretically. See Supplementary
data.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2014.06.018.
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