Coordination modes and reaction of acetic anhydride with tantalum pentafluoride

Article abstract of DOI:10.1134/S0036023614020119

The composition and structure of compounds formed upon the reaction of equimolar amounts of TaF₅ and MeC(O)O(O)CMe in CH₂Cl ₂ and upon dissolution of the fluoride in an anhydride excess have been studied by ¹⁹F NMR (293-183 K). It has been found that, in both cases, the anhydride is fluorinated to form MeC(O)F (a quartet at 47.7 ppm, J _FH = 18 Hz) and the MeC(O)O^- anion. In the spectra of the reaction mixture in CH₂Cl₂, the signals of pentafluoro complexes are assigned to the TaF₅[OC(Me)OC(O)Me] adduct and the TaF₅[OC(O)Me]^- anion; on the basis of the chemical shifts of the singlets, they are assigned to the TaF₄[OC(Me)OC(O)Me] ⁺ and TaF₂[OCOC(O)Me] ₂ ³⁺ cations in which the anhydride acts as a chelating ligand. Upon the direct reaction of the reagents in an anhydride excess, the spectra at low temperatures show, in addition to the signal of the tetrafluoride cation, the signals of the hexafluorotantalate ion TaF ₆ ^- and four trifluoroacetate complexes. Close relative concentrations of the latter enable the suggestion that they form chain or cyclic structures. It is believed that, in these structures, the acetate group has monodentate and bridging coordination modes, while chelating coordination mode seems to be less probable.

Full text of DOI:10.1134/S0036023614020119

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2014, Vol. 59, No. 2, pp. 111–114. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © E.G. Il’in, V.V. Kovalev, 2014, published in Zhurnal Neorganicheskoi Khimii, 2014, Vol. 59, No. 2, pp. 243–246.
PHYSICAL METHODS
OF INVESTIGATION
Coordination Modes and Reaction of Acetic Anhydride
with Tantalum Pentafluoride
E. G. Il’in and V. V. Kovalev
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
Received July 31, 2013
Abstract—The composition and structure of compounds formed upon the reaction of equimolar amounts of
TaF5 and MeC(O)O(O)CMe in CH2Cl2 and upon dissolution of the fluoride in an anhydride excess have been
studied by 19F NMR (293–183 K). It has been found that, in both cases, the anhydride is fluorinated to form
MeC(O)F (a quartet at 47.7 ppm, JFH = 18 Hz) and the MeC(O)O– anion. In the spectra of the reaction mixꢀ
ture in CH2Cl2, the signals of pentafluoro complexes are assigned to the TaF5[OC(Me)OC(O)Me] adduct and
the TaF5[OC(O)Me]– anion; on the basis of the chemical shifts of the singlets, they are assigned to the
3+
TaF4[OC(Me)OC(O)Me]+ and TaF [OCOC(O)
cations in which the anhydride acts as a chelating
Me]2
2
ligand. Upon the direct reaction of the reagents in an anhydride excess, the spectra at low temperatures show,
−
in addition to the signal of the tetrafluoride cation, the signals of the hexafluorotantalate ion
and four
TaF6
trifluoroacetate complexes. Close relative concentrations of the latter enable the suggestion that they form
chain or cyclic structures. It is believed that, in these structures, the acetate group has monodentate and
bridging coordination modes, while chelating coordination mode seems to be less probable.
DOI: 10.1134/S0036023614020119
Carboxylic anhydrides RC(O)OC(O)R can be The reaction was carried out under different condiꢀ
considered analogues of bidentate bases, such as
RC(O)XC(O)R, R2P(O)XC(O)Y and
tions: by adding a solution of L in an inert polar solvent
CH2Cl2) to a weighed portion of pentafluoride under
,
(
R2P(O)XP(O)R2 (X = CH2, NH; Y = R, NR2). Upon
their reaction with highest fluorides of Group IV–VI
transition and mainꢀgroup elements, hard Lewis
acids, they can be coordinated monoꢀ or bidentately
and act as bridging or chelating ligands, depending on
the ratio of components; in addition, in the presence
of a mobile hydrogen atom, they can be substituted for
fluoride ions to produce HF [1–5]. Since
CH3C(O)OC(O)CH3 is known to be one of the most
active acylating agents, it was of interest to study how
this property would affect the character of reactions
with highest fluorides of d0 transition metals. To study
the interaction of carboxylic anhydrides with penꢀ
tafluorides, the products of the reaction of
CF3C(O)OC(O)CF3 with SbF5—two dimeric mixedꢀ
ligand complexes—have been isolated and studies by
Xꢀray crystallography [6, 7]. In one of them—
Sb2(O2CCF3)F9, fluoride ion and CF3C(O)O– anion
CH2Cl2 or by introducing TaF5 directly into cooled
acetic anhydride. All operations are carried out in a
dry glove box in a nitrogen atmosphere. The 19F NMR
spectra were recorded on a Bruker ACꢀ200 in the temꢀ
perature range 290–183 K. Chemical shifts were referꢀ
enced to CCl3F. Negative values corresponded to
upfield shifts of the signals.
The addition of a solution of L in CH2Cl2 to a TaF5
sample under CH2Cl2 at room temperature led to the
complete dissolution of the pentafluoride. The 19F
NMR spectrum of the resulting solution (Fig. 1)
showed three signals. Quartet D at 47.7 ppm and JFH
=
18 Hz reflected, in our opinion, the formation of an
organofluorine compound, acetic fluoride MeC(O)F,
and strong narrow line B at 94 ppm and a broad
exchange signal at 70 ppm were assigned to tantalum
fluoride complexes.
act as bridges; in the other—Sb2O(O2CCF3)2F6
,
With decreasing temperature, the exchange signal
was split, and the spectrum at 223 K (Fig. 1) showed
new resonance signals. Two strongest broad signals A
and A' at 106 and 66 ppm with an intensity ratio of 1 : 4
and two weak signals at 62 and 88 ppm (the latter had
a doublet fine structure) with the same intensity ratio
were indicative of the presence of two tantalum penꢀ
bridges are the oxygen ion and two CF3C(O)O– anions
[6, 7].
In the present work, the products of the reaction of
MeC(O)OC(O)Me with TaF5 have been studied by 19F
NMR. TaF5 was synthesized by fluorination of the
metal with elemental fluorine in a quartz system. The
anhydride was dried over P2O5, and distilled two times. tafluoride complexes in the solution. The formation of
111

112
IL’IN, KOVALEV
B
223 K
D
C'
A
E
C
D
D
B
293 K
А + А' + B + B' + C + C' + E
48 ppm
110
100
90
80
70
60 50
40 ppm
19
1
2
Fig. 1. F NMR spectrum of an equimolar solution of MeC(O)OC(O)Me and TaF in CH Cl : A and A'—F and F in
5
2
2
–
2
+
1
2
4 5
[TaF OC(Me)О] , B—{[ ꢀOC(Me)OC(Me)O]TaF } , C and C'—F and F in TaF OCMeOC(O)Me, D—MeC(O)F, and
η
5
–
E—[TaF ]
.
6
CH3C(O)F as a result of the fluorination of acetic the spectra of the reaction products (Fig. 1), the latter
anhydride by the scheme
were assigned to complex
and C', to complex
The lack of fine structures of the 19F resonance
lines of complex at low temperatures points to a high
exchange rate, which can be caused by the presence of
the second donor site, the oxygen atom of the free
C=O group of the ligand capable of coordination to
the central tantalum atom to form an intermediate
sevenꢀcoordinate pentafluoride complex. In the latter
the acetate ion acts as a chelating ligand and forms an
unstable fourꢀmembered chelate ring.
1 and the weakest signals C
2.
−
OCMeOC(O)Me + F– = MeC(O)F +
MeCO2
allows us to suggest that one of these complexes can be
the tantalum pentafluoride complex with acetate ion
1
[TaF5OC(Me)О]– (
1)
and the other, the adduct of TaF5
with anhydride molecule TaF5OCMeOC(O)Me (
2).
The weak upfield signal at 38 ppm has been assigned to
the hexafluorotantalate ion [TaF6]– present in solution
in low concentration.
MeC(O)O−
OC(Me)OC(O)Me
F1
F1
F1
F1
F1
F1
F1
F1
As for the strong line at 94 ppm narrow at room
temperature, which had no fine structure down to the
freezing point of the solution, it was assigned on the basis
of its chemical shift to the tetrafluoride tantalum cation
Ta
F2
Ta
F2
{[ 3) with the bidentately
2ꢀOC(Me)OC(Me)O]TaF4}+ (
η
1
2
To confirm the formation of a fluoroacetate comꢀ coordinated ligand. This assignment is supported by the
plex, a suspension of TaF5 in CH2Cl2 was prepared to fact that the signals of the previously studied analogous
which finely ground crystals of anhydrous MeCOONa tantalum chelates [
or freshly distilled glacial acetic acid were added. After and [(
η
2ꢀ(MeC(O)CHC(O)Me)TaF4] [5]
η
2ꢀMeC(O)NC(O)Me)TaF4] [4] are observed
magnetic stirring of the suspension for 1 h, the in the same range of 19F NMR chemical shifts. Nonꢀ
19F NMR spectra showed signals at 70 and 118 ppm equivalence of the equatorial and axial fluorine atoms
with a 4 : 1 intensity ratio assigned by us to was observed only for the former complex with
[TaF5OC(Me)О]–. Since these shifts were close to decreasing temperature to 198 K, which is evidence of
those of the most intense signals A and A' observed in the fast intramolecular exchange of fluorine atoms
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 59 No. 2 2014

COORDINATION MODES AND REACTION OF ACETIC ANHYDRIDE
113
2
D'
3
D'
D'
203 K
4
1
D'
3
D
2
D
4
D
1
D
B
C
A
D'
233 K
E
D
B
A
C
253 К
D
D'
E
B
А
Х
293 К
C
100
90
80
70
60
50
40
30 ppm
19
2
5 4
+
Fig. 2.
F NMR spectrum of a solution of TaF in a MeC(O)OC(O)Me excess: A—{[η ꢀOC(Me)OC(Me)O]TaF } ; B—
–
1
1
1
2
2
3
4
2
3
4
1a 1b 1c
2a 2b 2c
CH C(O)F; C—[TaF ] ; D and D' —F and F in dimer
6
; D , D , D and D' , D' , D' —F , F , F and F , F , F of
3
6
–
oligomer
5
, respectively; E—exchange signal of TaF OCMeOC(O)Me and [TaF OC(Me)О]
.
5
5
through the pseudorotation mechanism, i.e., though the tic anhydride excess) led to products of somewhat difꢀ
change of positions of nonequivalent fluorine atoms ferent composition. The 19F NMR spectrum of this
caused by rotation of the chelating ligand. The singlet in solution at 293 K (Fig. 2) showed, in addition to the
the lowest field at 111 ppm was assigned by us to the narrow signals of tetrafluoride cation
3 (A) and
trans ).
ꢀ
{TaF2[MeC(O)ОC(O)Me]2}3+ complex ( MeC(O)F (B), a weak and broad signal of the [TaF6]–
4
anion (C) and an exchange signal X, which split into a
number of separate signals with decreasing temperaꢀ
ture. With a decrease in temperature, new signals
appeared, and the hexafluorotantalate signal intensity
sharply increased; hence, the corresponding comꢀ
plexes are involved in intermolecular exchange of fluꢀ
Me
O C
Me
Me
F
F
F
F
C O
O C
O C
Ta
Ta
O
O
O
O C
C O
Me
F
F
Me
Me
oride ions with [TaF6]–
.
3
4
The reaction carried out under more severe conditions
With decreasing temperature to 203 K, broad sigꢀ
(without an inert solvent by dissolving TaF5 in an aceꢀ nals D and D' with the intensity ratio 1 : 2 (Fig. 2) were
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 59 No. 2 2014

114
IL’IN, KOVALEV
split and showed fine structure so that four triplets and and of the triplets enable the suggestion that the trifluꢀ
four doublets (Fig. 2, D1–4, D'1–4) with the 1 : 2 ratio
were observed, which pointed to the existence in soluꢀ
tion of four trifluorotantalum complexes. The formaꢀ
tion of MeC(O)F in the reaction of acetic anhydride
with TaF5 can be evidence of the presence of acetate
−
oro complexes corresponding to these signals have the
same composition of the coordination sphere and difꢀ
fer in the coordination modes of the ligand. For examꢀ
ple, acetate ion can be terminal or can form one or two
acetate bridges between tantalum cations. Close relaꢀ
tive intensities of the signals of the same type allow us
to assume that the corresponding complexes are
involved in oligomeric chains or rings with bridging
ions
in solution. In our opinion, these ions
MeCO2
are located in the inner coordination sphere of the tanꢀ
talum cations together with the fluorine ions. Similar
chemical shifts and relative intensities of the doublets and terminal acetate groups (
5, 6).
Me
F1b
Ta
O
F1a
Ta
F2b
F2b
O
O
C O
F2a
F2a
C O
Me
OC(O)Me
C Me
F1c
Ta
Me
O
F2c
F2c
F2c
Ta
O
O
C O
C O
F2c
F1c
O
C
O
Me
Me
Me
Me
F1a
F1
MeC(O)O
F2a
Ta
O
O
F2
F2
F2b
F2b
F2
F2
O C O
O C O
C O
C O
Me
Ta
Ta
Ta
F1
F2a
F1b
OC(O)Me
OC(O)Me
Me
6
5
The chelate coordination of the acetate ion seems
to be less probable.
REFERENCES
1. N. M. D. Brown and P. Bladon, Chem. Commun. 304
168 (1966).
2. J.ꢀY. Calves and J.ꢀE. Guerchais, J. Fluorine Chem.
47 (1974).
,
Thus, both in methylene chloride and in the
4,
absence of the solvent, in the reaction with ТаF5
,
acetic anhydride is not only monodentately or
bidentately coordinated to tantalum ion forming a
chelate ring and substituting for fluoride ions, but is
also fluorinated by TaF5 with cleavage of the oxygen
bridge and formation of MeC(O)F and acetate ion.
Upon the direct reaction of the components, there
are considerable amounts of trifluoro acetate comꢀ
plexes in solution constituting oligomeric chains or
rings with bridging and terminal coordination of
acetate ions.
3. Yu. A. Buslaev, E. G. Il’in, M. N. Shcherbakova, et al.,
Dokl. Akad. Nauk SSSR 271 (2), 373 (1983).
4. E. G. Il’in, E. Kherrman, V. V. Kovalev, et al., Dokl.
Akad. Nauk SSSR 320, 127 (1991).
5. E. G. Il’in, M. E. Ignatov, V. G. Khlebodarov, and
Yu. A. Buslaev, Dokl. Akad. Nauk SSSR 335, 463 (1994).
6. D. P. Bullivant, M. J. Haley, and M. F. A. Dove,
J. Chem. Soc., Chem. Commun., 584 (1961).
7. D. P. Bullivant, M. F. A. Dove, and M. J. Haley,
J. Chem. Soc., Dalton Trans., 109 (1980).
Translated by G. Kirakosyan
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 59 No. 2 2014