Reactivity of niobium(v) and tantalum(v) halides with carbonyl compounds: Synthesis of simple coordination adducts, C-H bond activation, CO protonation, and halide transfer

Article abstract of DOI:10.1039/b708531a

The metal halides of Group 5 MX₅ (M = Nb, Ta; X = F, Cl, Br) react with ketones and acetylacetones affording the octahedral complexes [MX₅(ketone)] (2a-j) and [TaX₄{κ²(O)- OC(Me)C(R)C(Me)O}] (R = H, Me, 5a-e), r

Full text of DOI:10.1039/b708531a

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Reactivity of niobium(V) and tantalum(V) halides with carbonyl compounds:
=
Synthesis of simple coordination adducts, C–H bond activation, C O
protonation, and halide transfer†
Fabio Marchetti,‡a Guido Pampaloni*a and Stefano Zacchinib
Received 7th June 2007, Accepted 17th July 2007
First published as an Advance Article on the web 6th August 2007
DOI: 10.1039/b708531a
The metal halides of Group 5 MX5 (M = Nb, Ta; X = F, Cl, Br) react with ketones and acetylacetones
affording the octahedral complexes [MX5(ketone)] (2a–j) and [TaX4{j2(O)-OC(Me)C(R)C(Me)O}]
(R = H, Me, 5a–e), respectively. The adducts [MX5(acetone)] are still reactive towards acetone,
acetophenone or benzophenone, giving the aldolate species [MX4{j2(O)-OC(Me)CH2C(R)(Rꢀ)O}]
(3a–e). The syntheses of 3b (M = Ta, X = F, R = Rꢀ = Ph) and 3d (M = Ta, X = Cl, R = Me, Rꢀ = Ph)
take place with concomitant formation of [(Ph2CO)2-H][TaF6], 4a, and [(MePhCO)2-H][TaCl6], 4b,
respectively. The compounds [acacH2][TaF6], 6, and [TaF{OC(Me)C(Me)C(Me)O}3][TaF6], 7, have
been isolated as by-products in the reactions of TaF5 with acacH and 3-methyl-2,4-pentanedione,
respectively. The molecular structures of 2i, 3c, 4a, 4b and 7 have been ascertained by single crystal
X-ray diffraction studies.
A few decades ago, some results were presented about the
Introduction
reactivity of MCl5 (M = Nb, Ta) with ketones,10 and a variety
The activation of C–E bonds (E = C, H, halogen) by electron-
of products was hypothesized on the basis of analytical and spec-
rich transition metal complexes, leading to C–C bond formation,
troscopic characterizations. It was supposed that the outcomes of
has opened the door to a huge variety of organic syntheses.1
Among the most interesting approaches, although not frequently
employed, is the use of acidic metal electrophiles, such as early
transition metal derivatives.2
these reactions could depend strictly on the stoichiometry used. In
particular, the formation of O-abstraction complexes, containing
=
the [Ta O] unit, was proposed for reactions involving an excess
of ketone.
In the light of these considerations, we became interested in
With the aim of shedding some light on this topic, and to verify
the reactivity of MX5 (M = Nb, X = Cl, 1a; Br, 1b; M = Ta,
the possibility of extending it to carbonyl compounds with the use
of Ta(V) and Nb(V) halides as potential activators for the organic
synthesis, we herein report the results of our investigations on the
chemistry of 1a–e with ketones and acetylacetones.
X = F, 1c; Cl, 1d; Br, 1e)3 with oxygen donor ligands. Indeed,
the coordination of O-donor ligands containing acidic hydro-
gens to 1a–e may potentially represent an effective way for
C–H bond activation and consequent organic transformations.
The fact that such coordination chemistry has been scarcely
developed,4 if we except alcohols or diols,5 makes the topic
rather intriguing. Significantly, only two compounds with the
general formula MX5(L), L = oxygen donor hydrocarbyl ligand,
namely [TaCl5(OEt2)] and [TaCl5{OC(tBu)(p-tolyl)}], have been
characterized by X-ray diffraction.6
In practice, the pentahalides of Group 5 can induce undesired
reactions when in contact with species containing Lewis basic
functionalities, as is the case for many potential O-donor ligands.
For instance, deoxygenation has been reported in reactions with
sulfoxides,7 phosphine oxides,8 and crown ethers,9 but the products
have been fully characterized including X-ray crystal structures in
the latter case only.
Results and discussion
Synthesis and spectral properties
The hexacoordinated complexes [MX5(L)] (Scheme 1) were ob-
tained in high yields (76–90%, with respect to the limiting reactant)
by treating suspensions of 1a–e in CH2Cl2 with the appropriate
carbonyl compound, in a 1 : 2 ketone (aldehyde) to metal molar
ratio.
MX5 + L → MX5(L)
All compounds 2a–j have been characterized by IR and NMR
spectroscopy, and elemental analysis. Moreover, the solid state
structure of 2i has been elucidated by X ray diffraction.
aUniversita` di Pisa, Dipartimento di Chimica e Chimica Industriale,
Via Risorgimento 35, I-56126, Pisa, Italy. E-mail: pampa@dcci.unipi.it;
Fax: +39 50 2219246; Tel: +39 50 2219219
The molecular structure of 2i is reported in Fig. 1 whereas
the main bond distances (A) and angles ( ) are listed in Table 1.
The molecule displays a slightly distorted octahedral geometry,
◦
˚
bUniversita` di Bologna, Dipartimento di Chimica Fisica e Inorganica, Viale
Risorgimento 4, I-40136, Bologna, Italy
˚
˚
with Ta–Cl [2.3059(6)–2.3466(7) A] and Ta–O [2.0108(18) A] bond
distances similar to those reported for [TaCl5{OC(tBu)(p-tolyl)}]
† CCDC reference numbers 649831–649835. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b708531a
‡ Born in Bologna (Italy) in 1974.
6
˚
˚
[Ta–Cl 2.290–2.337 A; Ta–O 2.040 A]. It is noticeable that in both
cases the Ta–Cl interactions trans to Ta–O bond are the shortest
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shows a strong absorption at 1663 cm−1 (in CH2Cl2 solution) due to
=
the C O stretching vibration (to be compared with the absorption
observed at 1713 cm−1 for the uncoordinated acetone). The most
salient feature of the NMR spectra of 2a–j, which contain single
sets of signals, is represented by the 13C NMR resonance of the
carbonyl carbon, which falls at typical low fields (e.g. 233.2 ppm
for 2g). In addition, the 19F NMR spectra of 2c–e show the pattern
typical of [TaF5(L)] adducts, consisting of two distinct resonances
(e.g., 80.2 and 39.0 ppm for 2d).12
We have observed that the carbonyl compound/metal halide
molar ratio is decisive for the success of the preparation, the
formation of complexes 2 occurring cleanly only when a two-
fold excess of metal halide is used with respect to the carbonyl
compound. As a matter of fact, we have found that either
a 1 : 1 molar ratio or an excess of the carbonyl compound
resulted in the formation of complicated mixtures, which could
be detected (but not identified) by NMR spectroscopy. This is
particularly true when adducts of a-hydrogen-containing carbonyl
compounds such as the derivatives of acetone and acetophenone
are concerned: the presence of acidic hydrogen atoms makes
these compounds susceptible to further reactions with carbonyl
substrates.
Scheme 1
In order to investigate the point, we decided to test the
reactivity of the acetone adducts 2a, c, g with a series of
ketones, RRꢀCO. The reactions afforded the M(V) aldolate species
[MX4{OC(Me)CH2C(R)(Rꢀ)O}], 3a–e, see Scheme 2. The general
mechanism of formation of b-ketoalcohols from the correspond-
ing ketones catalyzed by Lewis acids is probably operative in the
conversion from 2 to 3.13,14
Fig. 1 Molecular structure of [TaCl5(Ph2CO)], 2i. Thermal ellipsoids are
drawn at 30% probability level.
◦
˚
Table 1 Selected bond distances (A) and angles ( ) for [TaCl5(OCPh2)],
2i
Ta(1)–Cl(1)
Ta(1)–Cl(2)
Ta(1)–Cl(3)
Ta(1)–Cl(4)
Ta(1)–Cl(5)
2.3417(7)
2.3304(7)
2.3059(6)
2.3466(7)
2.3408(7)
Ta(1)–O(1)
O(1)–C(1)
C(1)–C(2)
C(2)–C(8)
2.0108(18)
1.265(3)
1.459(4)
1.461(3)
O(1)–Ta(1)–Cl(2)
Cl(3)–Ta(1)–Cl(2)
O(1)–Ta(1)–Cl(5)
Cl(3)–Ta(1)–Cl(5)
O(1)–Ta(1)–Cl(1)
Cl(3)–Ta(1)–Cl(1)
Cl(2)–Ta(1)–Cl(1)
Cl(5)–Ta(1)–Cl(1)
O(1)–Ta(1)–Cl(4)
Cl(3)–Ta(1)–Cl(4)
91.10(6)
89.76(2)
89.40(6)
89.75(2)
85.80(5)
95.27(2)
90.09(3)
89.16(3)
85.08(6)
93.86(2)
Cl(2)–Ta(1)–Cl(4)
Cl(5)–Ta(1)–Cl(4)
O(1)–Ta(1)–Cl(3)
Cl(2)–Ta(1)–Cl(5)
Cl(1)–Ta(1)–Cl(4)
C(1)–O(1)–Ta(1)
O(1)–C(1)–C(2)
O(1)–C(1)–C(8)
C(2)–C(1)–C(8)
89.69(3)
91.14(2)
178.62(5)
179.06(3)
170.87(2)
172.22(18)
118.3(2)
118.9(2)
122.8(2)
Scheme 2
Complexes 3a–e, obtained in 55–65% yield (calculated with
respect to compounds 2) have been characterized by analytical
and spectroscopic methods, moreover the solid state structure
of 3c has been determined by X ray diffraction (Fig. 2 and
Table 2). Complexes 3a and 3c are formed (in admixture with other
unidentified products) also when the parent halide complexes,
NbCl5 and TaCl5, are treated with a two-fold molar excess (with
respect to the metal) of acetone. However, the two-step synthesis
shown in Scheme 2 is highly preferable, for it avoids the formation
of by-products.
˚
ones. The C(1)–O(1) interaction [1.265(3) A] is typical for a double
˚
bond whereas C(1)–C(2) and C(1)–C(8) [1.459(4) and 1.461(3) A,
respectively] are consistent with C(sp◦2)–C(sp3) single bonds.11 The
sum of the angles at C(1) [360.0(3) ] is consistent with the sp2
hybridisation of the carbon atom.
The tantalum atom in 3c is octahedrally coordinated with two
cis positions occupied by the chelating [OC(Me)CH2C(Me)2O]−
˚
The IR spectra of 2a–j show one absorption related to the
carbonylic stretching of the coordinated ketone. For instance, 2g
ligand. The Ta–Cl interactions [2.2913(9)–2.3722(9) A] display
values in the same range as found in 2i, with the shortest
4344 | Dalton Trans., 2007, 4343–4351
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◦
˚
Interestingly, the reaction of the acetophenone adduct 2h
with excess benzophenone (see Experimental) results in simple
substitution by the latter of the coordinated acetophenone, giving
2i in good yield (Scheme 3). In this case, the steric hindrance
due to the phenyl groups is probably responsible for inhibiting
the formation of the C–C coupling product, thus making the
substitution reaction the most accessible route. On the other
hand, the reactions of complex 2h with acetone and acetophenone
resulted in formation of mixtures of unidentifiable products.
Table 2 Selected bond distances (A) and angles ( ) for [TaCl4{OC(Me)-
CH2C(Me)2O}], 3c
Ta(1)–Cl(1)
Ta(1)–Cl(2)
Ta(1)–Cl(3)
Ta(1)–Cl(4)
Ta(1)–O(1)
2.3516(9)
2.2913(9)
2.3722(9)
2.3433(9)
2.192(2)
Ta(1)–O(2)
O(1)–C(1)
O(2)–C(3)
C(1)–C(2)
C(2)–C(3)
1.835(2)
1.244(4)
1.453(4)
1.495(5)
1.531(6)
O(2)–Ta(1)–O(1)
O(2)–Ta(1)–Cl(2)
O(2)–Ta(1)–Cl(4)
O(1)–Ta(1)–Cl(4)
Cl(2)–Ta(1)–Cl(4)
O(2)–Ta(1)–Cl(1)
O(1)–Ta(1)–Cl(1)
Cl(2)–Ta(1)–Cl(1)
O(1)–Ta(1)–Cl(3)
Cl(2)–Ta(1)–Cl(3)
Cl(4)–Ta(1)–Cl(3)
80.38(10)
98.88(8)
92.90(9)
83.50(7)
93.98(4)
90.08(8)
87.28(7)
95.29(4)
82.20(7)
98.66(4)
88.03(4)
Cl(1)–Ta(1)–Cl(3)
O(1)–Ta(1)–Cl(2)
Cl(4)–Ta(1)–Cl(1)
C(1)–O(1)–Ta(1)
C(3)–O(2)–Ta(1)
O(1)–C(1)–C(4)
O(1)–C(1)–C(2)
O(2)–C(3)–C(6)
O(2)–C(3)–C(5)
O(2)–C(3)–C(2)
O(2)–Ta(1)–Cl(3)
86.17(3)
177.32(7)
169.70(3)
128.7(2)
143.1(2)
119.4(3)
120.7(3)
106.8(3)
107.4(3)
107.4(3)
162.33(8)
Scheme 3
The benzophenone complex 2i reacts with a two-fold excess
of RRꢀCO (R = Rꢀ = Me; R = Me, Rꢀ = Ph), in CH2Cl2
solution, affording in good yields the substitution products 2g
and 2h respectively (Scheme 3). Coherently, the addition of large
amounts of benzophenone to 2i does not produce any apparent
change, apart from some decomposition (see Experimental). The
outcome of the reactions of 2i with ketones is probably related
to the lack of acidic hydrogens in the coordinated benzophenone,
which prevents the formation of aldol condensation products.
Based on the general mechanism proposed for the Lewis
acid-catalyzed aldol condensations,13 the reactions reported in
Scheme 2 should proceed with formation of H+. One attempt
aimed to crystallize 3d directly from the reaction mixture (see
Experimental) gave X-ray quality crystals of a minor product
instead. This compound was identified as [(MePhCO)2-H][TaCl6],
4b, which was characterized by X-ray diffraction, elemental
analysis and IR spectroscopy (vide infra). Analogously, [(Ph2CO)2-
H][TaF6], 4a, was isolated in ca. 30% yield from the mixture
obtained in the reaction of 2c with benzophenone. Compounds 4
are likely to result from combination of HX (formed according to
Scheme 2) with the free ketone and the still unreacted pentahalides,
see Scheme 4. Compound 4a is obtained in higher yield with
respect to 4b due to the higher stability of the [TaF6]− anion with
respect to [TaCl6]−.
Fig. 2 Molecular structure of [TaCl4{OC(Me)CH2C(Me)2O}], 3c. Ther-
mal ellipsoids are drawn at 30% probability level.
˚
distance [Ta(1)–Cl(2) 2.2913(9) A] trans to ketonic oxygen of the
[OC(Me)CH2C(Me)2O]− ligand. As in 2i, this has to be ascribed
to a minor trans competition between a neutral and an anionic
ligand compared to two anionic ligands. For what concerns
the [OC(Me)CH2C(Me)2O]− ligand, the short bond between Ta
˚
and the alkoxide oxygen [Ta(1)–O(2) 1.835(2) A] and the long
˚
bond to the coordinated ketone [Ta(1)–O(1) 2.192(2) A] are
˚
noteworthy. Distinguishable C–O single [O(2)–C(3) 1.453(4) A]
˚
=
and C O double bonds [O(1)–C(1) 1.244(4) A] are also ev-
ident. A compound containing the same ligand coordinated
to tantalum has been previously reported, i.e. TaCl3[C4H4B-
NH(CHMe2)2][Me2C(O)CH2C(O)Me],14 and the bonding param-
eters are similar.
The IR spectra of 3a–e (dichloromethane solutions) exhibit
absorption due to the coordinated carbonylic function, in the
1
range 1635 (3a)–1609 (3e) cm−1. The H NMR spectra contain
one resonance for the methylenic protons [3.43 (3c) 2.83 (3e) ppm];
major features of the 13C NMR spectra are given by the signals
attributed to the O-bound carbon atoms, which are observed at
Scheme 4
The molecular structures of 4a and 4b were ascertained by
X-ray diffraction (Fig. 3 and 4; Tables 3 and 4). They both contain
octahedral [TaX6]− anions (X = F, 4a; Cl, 4b) and an organic
cation composed by two ketones linked by a bridging proton,
i.e. [(RR C O)2H] . A search on the Cambridge Crystallographic
Data Centre has shown only another case of such a kind of cation,
=
ca. 225 ppm (C O–M) and ca. 85 ppm (C–O–M) ppm. The
=
C O bond order decreasing, as a consequence of formation of
the aldolate, is clearly evidenced by the upfield shifted resonance
of the carbonyl carbon atom, with respect to that of the free ketone
(207.1 ppm).15
ꢀ
+
=
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◦
˚
Table 4 Selected bond distances (A) and angles ( ) for [(MePhCO)2H]-
[TaCl6], 4b
Ta(1)–Cl(1)
Ta(1)–Cl(2)
Ta(1)–Cl(3)
C(1)–C(2)
2.3344(10) C(2)–O(1)
2.3586(11) C(2)–C(3)
2.3466(10) O(1)–H(1)
1.246(5)
1.464(6)
1.221(3)
2.443(6)
1.493(6)
O(1)–O(1)#2
Cl(1)–Ta(1)–Cl(3)
Cl(1)–Ta(1)–Cl(3)#1
Cl(1)#1–Ta(1)–Cl(2)
Cl(1)–Ta(1)–Cl(2)
Cl(3)#1–Ta(1)–Cl(2)
Cl(3)–Ta(1)–Cl(2)
90.02(4)
89.98(4)
90.30(4)
89.70(4)
90.49(4)
89.51(4)
Cl(1)#1–Ta(1)–Cl(1) 180
Cl(3)#1–Ta(1)–Cl(3) 180
Cl(2)–Ta(1)–Cl(2)#1 180
O(1)–C(2)–C(3)
O(1)–C(2)–C(1)
C(3)–C(2)–C(1)
118.0(4)
120.7(4)
121.3(4)
Symmetry transformations used to generate equivalent atoms: #1 −x + 2,
−y + 1, −z #2 −x, −y + 1, −z + 1.
The IR spectra of 4a–b, recorded in the solid state, display a
strong absorption due to the carbonylic stretching vibration, at
1591 (4a) and 1648 (4b) cm−1. In addition, a weak absorption is
present at ca. 3100 cm−1, which has been attributed to the O–
H–O hydrogen bond interaction. The NMR spectra of 4a, in
CDCl3 solution, show the resonances typical of benzophenone,
thus suggesting that the protonated species [(Ph2CO)2-H][TaF6] is
stable in the solid state only.
Fig. 3 Molecular structure of [(Ph2CO)2H][TaF6], 4a. Thermal ellipsoids
are drawn at 30% probability level. Only independent atoms are labeled.
◦
˚
Table 3 Selected bond distances (A) and angles ( ) for [(Ph2CO)2H]-
[TaF6], 4a
In the framework of the present study, and in view of the paucity
of data concerning halo-acetylacetonate derivatives of niobium(V)
and tantalum(V),19 we decided to investigate the reactivity of
tantalum(V) halides with 2,4-pentanediones. Indeed the reactions
of TaX5 with a slight excess of 2,4-pentanedione or 3-methyl-2,4-
pentanedione (mixtures with the respective tautomers) result in
formation of the acetylacetonato complexes TaX4(LL) (5a–e), in
up to 82% yields, Scheme 5
Ta(1)–F(1)
Ta(1)–F(2)
Ta(1)–F(3)
C(1)–O(1)
1.8867(18) C(1)–C(8)
1.8849(15) C(1)–C(2)
1.8897(17) O(1)–H(1)
1.463(3)
1.465(3)
1.225 (2)
2.450(3)
1.259(3)
O(1)–O(1)#2
F(2)#1–Ta(1)–F(2) 91.18(11)
F(2)#1–Ta(1)–F(1) 89.56(8)
F(2)–Ta(1)–F(1)
F(1)–Ta(1)–F(3)#1 90.18(9)
F(2)–Ta(1)–F(3)
F(1)–Ta(1)–F(3)
F(3)#1–Ta(1)–F(3)
89.60(13)
F(1)–Ta(1)–F(1)#1 179.0(1)
F(2)–Ta(1)–F(3)#1 179.21(7)
89.74(8)
O(1)–C(1)–C(2)
C(8)–C(1)–C(2)
C(7)–C(2)–C(3)
121.2(2)
121.4(2)
119.6(2)
89.61(10)
90.52(9)
Symmetry transformations used to generate equivalent atoms: #1 −x + 1,
y, −z + 3/2 #2 −x + 1, −y + 1, −z + 2.
namely [(C12H8D2O)2H][SbCl6].16 In both structures 4a and 4b,
the bridging proton is located on an inversion center and only one
of the two ketones composing the dimer is crystallographically
independent. The result is a perfectly symmetrical hydrogen bond
˚
˚
[O(1)–H(1) 1.23 A, O(1)–O(1)#2 2.450(3) A, O(1)–H(1)–O(1)#2
180◦ for 4a; O(1)–H(1) 1.22 A, O(1)–O(1)#2 2.443(6) A, O(1)–
˚
˚
H(1)–O(1)#2 180◦ for 4b], and a slight elongation of the C O
bond compared to the free ketone [C(1)–O(1) 1.259(3) A for
=
˚
Scheme 5
˚
˚
4a; C(2)–O(1) 1.246(5) A for 4b with respect to 1.216(2) A for
17
˚
Compounds 5a–e have been characterized by IR and NMR
spectroscopy, and elemental analysis. The IR spectra exhibit a
strong absorption in the range 1521–1540 cm−1, attributable to the
carbonylic stretching vibration. These values are in accord with
a symmetrically coordinated acetylacetonate bidentate ligand.20
Coherently, the NMR spectra, which consists of single sets of
signals, show one resonance due to the R unit (e.g. at 6.37 ppm
for the CH proton in 5c) and another resonance accounting for
the two equivalent methyl groups (at 2.33 ppm for 5c). The most
salient aspect of the 13C NMR spectra is given by the resonance
ascribable to the two equivalent carbonyl carbons, which results
in an upfield shift with respect to the uncoordinated acetylacetone
(e.g. at 191.8 ppm in the case of 5c to be compared to the signal
of free acetylacetone at 203.5 ppm). Furthermore, the 19F NMR
acetophenone (at 154 K ) and 1.211(7) A for benzophenone (at
293 K.18)
Fig. 4 Molecular structure of [(MePhCO)2H][TaCl6], 4b. Thermal ellip-
soids are drawn at 30% probability level. Only independent atoms are
labeled.
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spectra of 5a–b are as expected for cis-TaF4L2 species, and exhibit
two equally intense resonances at ca. 97 and 40 ppm.12
Interestingly, at variance to compounds 5c–e which are the only
products of the respective reactions, the formation of 5a and 5b is
accompanied by secondary products.
A careful reading of the NMR spectra, carried out on the
mixture derived from the reaction of acacH with TaF5, allowed
us to identify a second set of resonances, which was assigned
to the adduct [acacH2][TaF6], 6. This latter was separated from
5a by crystallization (see Experimental), and its identity was
corroborated by X-ray data.21 The IR spectrum in the solid state
shows an absorption at 3124 cm−1, attributable to the presence
of O–H units. The 1H NMR spectrum exhibits two resonances at
6.04 and 2.33 ppm (1 : 6 integral values ratio) ascribable to a –CH
fragment and to two equivalent methyl groups respectively. These
data are in agreement with those reported by Brouwer (6.19 and
2.58 ppm, in H2SO4 or FSO3H solutions)22 for the cation derived
Fig. 5 Molecular structure of the cation in [TaF{OC(Me)C(Me)C-
(Me)O}3][TaF6], 7. Thermal ellipsoids are drawn at 30% probability level.
Only independent atoms are labeled.
+
from O-protonation of acetylacetone in its enolic form (acacH2 ).
1
Indeed, a H NMR resonance observed at 11.97 ppm accounts
for two protons and has been attributed to two oxygen-bonded
protons, while the presence of the [TaF6]− anion is confirmed by
the resonance at −131.7 ppm in the 19F NMR spectrum.12,23
We therefore conclude that 6 exists in CDCl3 solution as the
hexafluorotantalate salt of the protonated enolic form of acety-
lacetone, (see Chart 1). It is worth mentioning that protonated
acetylacetones have been observed only in strong acid solutions22,24
and, more recently, in the gas phase.25
◦
˚
Table 5 Selected bond distances (A) and angles ( ) for [TaF{OC(Me)-
C(Me)O}3] [TaF6], 7
Ta(1)–F(1)
Ta(1)–O(1)
Ta(1)–O(2)
C(1)–C(2)
C(2)–O(1)
C(2)–C(3)
1.909(5)
2.005(3)
2.056(4)
1.490(7)
1.305(6)
1.403(7)
C(3)–C(4)
C(3)–C(6)
C(4)–O(2)
C(4)–C(5)
Ta(2)–F(3)
Ta(2)–F(2)
1.398(7)
1.511(7)
1.290(6)
1.504(7)
1.893(3)
1.897(3)
O(1)–Ta(1)–O(2)
C(2)–O(1)–Ta(1)
C(4)–O(2)–Ta(1)
O(1)–C(2)–C(3)
O(1)–C(2)–C(1)
C(3)–C(2)–C(1)
77.53(14)
133.9(3)
134.0(3)
123.3(5)
113.6(4)
123.1(5)
C(4)–C(3)–C(2)
C(4)–C(3)–C(6)
C(2)–C(3)–C(6)
O(2)–C(4)–C(3)
O(2)–C(4)–C(5)
C(3)–C(4)–C(5)
118.3(5)
121.9(5)
119.7(5)
123.9(5)
113.7(4)
122.3(5))
fold symmetry; the latter possesses the expected octahedral
geometry and does not deserve any further comment. Conversely,
the [TaF{OC(Me)C(Me)C(Me)O}3]+ cation shows a seven-
coordinated tantalum centre with a capped trigonal prismatic
Chart 1
˚
coordination geometry. The Ta(1)–F(1) interaction [1.909(5) A] is
The formation of 6 appears to be a consequence of the
synthesis of 5a, analogously to that discussed about 4a–b. Thus,
since formation of 5a is accompanied by production of HF, the
combination of this with still unreacted acacH and TaF5 affords
slightly longer than the ones present in the [TaF6]− anion [1.893(3)–
˚
1.897(3) A], probably because of the higher steric hindrance due
to the presence of the three bulky acetylacetonates.
Both the Ta(1)–O(1) and Ta(1)–O(2) [2.005(3) and 2.056(4) A]
˚
acacH2 , with [TaF6]− as anionic counterpart.
distances are in the range observed for the Ta–Oketone in-
teractions [see structures 2i and 3c] but significantly shorter
than those observed in other halo-b-diketonates such as
+
As far as the reaction of TaF5 with 3-methyl-2,4-pentanedione
is concerned, several secondary products were formed and the
identification was possible only for the most abundant of them,
7, obtained in a reproducible way in ca. 1 : 5 molar ratio
with respect to 5b (by 1H NMR analysis). We noticed that
the yield of 7 increased significantly (up to 1 : 3 molar ratio,
with respect to 5b) when a two-fold excess of 3-methyl-2,4-
pentanedione was used. Under these conditions, compound 7 was
isolated and characterized by spectroscopic, analytical, and X-ray
analyses as the ionic complex [TaF{OC(Me)C(Me)C(Me)O}3]
[TaF6] (Chart 1)
5
19b
˚
Ta(g -C5Me5)F3(dibenzoylmethanate) [2.049(4)–2.120(4) A],
19c
˚
trans,cis-TaCl2(OMe)2(acac) [2.026(10)–2.083(12) A],
trans,
cis-TaCl2(OMe)2(tmhd), tmhdH = 2,2,6,6-tetramethylheptane-
3,5-dione, [2.051(10)–2.055(9) A],19c] and in [Ta(tmhd)4][TaCl6]
˚
19d
˚
[2.018(8)–2.134(8) A, mean values]. Complete delocalisation is
present within the [OC(Me)C(Me)C(Me)O]− ligand, as demon-
strated by the fact that the C(2)–O(1), C(2)–C(3), C(3)–C(4)
and C(4)–O(2) bond lengths [1.305(6), 1.403(7), 1.398(7) and
˚
1.290(6) A, respectively] are intermediate between those of single
The solid state structure of 7 (Fig. 5 and Table 5) con-
sists of separated [TaF{OC(Me)C-(Me)C(Me)O}3]+ cations and
[TaF6]− anions, both having crystallographically imposed three-
and double bonds, and the sum of the angles at C(2), C(3) and
C(4) carbon atoms is 360.0(8), 359.9(9) and 359.9(8)◦, respectively,
thus suggesting their sp2 hybridisation.
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The IR spectrum of 7 (in solid state) presents one band
attributed to the bidentate acetylacetonate ligands, at 1552 cm−1.
Salient spectroscopic data for 7 are given by the 19F NMR
spectrum, which shows two resonances, one attributed to the
unique fluorine within the cation (86.4 ppm) and the other one
(−131.8 ppm) accounting for the [TaF6]− unit.
procedures.27 Solvents and liquid reagents were distilled before
use under an argon atmosphere from appropriate drying agents.
Infrared spectra were recorded at 298 K either on a FTIR-Perkin
Elmer Spectrometer equipped with a UATR sampling accessory
(powdery solid samples) or on a FTIR Perkin Elmer Paragon 500
Spectrometer (solid dispersed in Nujol samples, or solutions inside
CaF2 0.1 mm cells). NMR measurements were performed on a
Varian Gemini 200BB spectrometer, at 298 K. The chemical shifts
Conclusions
1
for H and 13C were referenced to internal TMS. The chemical
This paper represents an attempt to rationalize the reactivity of
pentahalides of niobium and tantalum with oxygenated ligands,
namely ketones and b-diketones.
shifts for 19F were referenced to CFCl3
Synthesis
We have shown that octahedral complexes [MX5(ketone)] (M =
Nb, Ta; X = F, Cl, Br) can be obtained cleanly only by reacting
the corresponding metal halide with a defect of the ketone.
The addition of b-diketonates to TaX5, in nearly 1 : 1 molar
ratio, represents the most convenient route to the octahedral
species [MX4(b-diketonate)]. In every case, no products derived
from oxygen abstraction from the carbonyl reactants have been
observed, at variance to that reported previously.10 The solid
state structure of [TaCl5(Ph2CO)] represents one of the very few
examples of crystallographically characterized Lewis acid–base
adducts of neutral oxygen-donor ligands with Group 5 metal
halides in the highest oxidation state.
Heterolytic C–H bond activation occurs when the acetone
adducts [MX5(Me2CO)] are treated with ketones: the reactions
proceed via C–C bond formation and cleanly affords deriva-
tives containing O,Oꢀ-coordinated aldolate ligands. The same
C–H bond activation process could not be extended to other
[MX5(ketone)] complexes, mainly because of the absence of
enolizable protons or of the steric demand of the ketones involved.
We have isolated significant amounts of salts of the [TaX6]−
anions (X = F, Cl), when HX is formed in the reactions.
Under these circumstances, uncommon ionic compounds can be
produced, where the cationic part consists of either two ketones
linked by a bridging proton or of the protonated enolic form of
acetylacetone. This has been confirmed by X-ray structural studies
carried out on the [(MePhCO)2 − H]+ and [(Ph2CO)2 − H]+ salts,
and by the isolation and the spectroscopic characterization of the
hexafluorotantalate salt of protonated acetylacetone. Noteworthy
is the fact that, by contrast with the [(MePhCO)2 − H]+ and
[(Ph2CO)2 − H]+ salts, which are stable only in the solid state,
[acacH2][TaF6] is stable in CH2Cl2 or CDCl3. The formation of
[TaF{OC(Me)C(Me)C(Me)O}3][TaF6] is observed when TaF5 is
reacted with 3-methyl-2,4-pentanedione. By contrast, the reactions
of TaCl5 or TaBr5 with acetylacetones proceed without formation
of by-products. These findings are likely to be associated with the
higher stability of the [TaF6]− ion with respect to the [TaCl6]− and
[TaBr6]− ones.26
Preparation of [MX5(L)] [M = Nb, X = Cl, L = Me2CO, 2a; M =
Nb, X = Br, L = Me2CO, 2b; M = Ta, X = F, L = Me2CO,
2c; M = Ta, X = F, L = MePhCO, 2d; M = Ta, X = F, L =
Ph2CO, 2e; M = Ta, X = Cl, L = HPhCO, 2f; M = Ta, X =
Cl, L = Me2CO, 2g; M = Ta, X = Cl, L = MePhCO, 2h; M = Ta,
X = Cl, L = Ph2CO, 2i; M = Ta, X = Br, L = Me2CO, 2j]. The
preparation of 2g is described in detail, the others being performed
in a similar way. A suspension of TaCl5 (0.350 g, 0.977 mmol) in
CH2Cl2 (10 mL) was treated with anhydrous acetone (0.036 mL,
0.490 mmol). The mixture was stirred for 30 min, then it was
filtered in order to remove unreacted TaCl5. The resulting solution
was dried in vacuo, thus compound 2g was obtained as a white
microcrystalline powder (0.179 g, 88% yield). Anal. Calc. for
C3H6Cl5OTa C, 8.7; H, 1.5; Cl, 42.6; Ta, 43.5. Found C, 9.1; H,
−1
=
1.4; Cl, 42.1; Ta, 43.0. IR (CH2Cl2 solution, cm ): 1663 (C O).
1H NMR d: 2.85 (s, Me). 13C NMR d: 233.2 (CO); 32.9 (Me).
2a (yellow, 81% yield). Anal. Calc. for C3H6Cl5NbO: C, 11.0;
H, 1.8; Cl, 54.0; Nb, 28.3. Found: C, 11.4; H, 2.1; Cl, 53.1; Nb,
−1
1
=
27.6. IR (CH2Cl2 solution, cm ): 1663 (C O). H NMR d: 2.81
(s, Me). 13C NMR d: 230.5 (CO); 32.4 (Me).
2b (yellow, 79% yield). Anal. Calc. for C3H6Br5NbO: C, 6.5; H,
1.1; Br, 72.6; Nb, 16.9. Found: C, 5.9; H, 1.1; Br, 71.8; Nb, 15.9.
−1
1
=
IR (CH2Cl2 solution, cm ): 1683 (C O). H NMR d: 2.82 (s, 6 H,
Me). 13C NMR d: 236.6 (CO); 33.0 (Me).
2c (light yellow, 78% yield). Anal. Calc. for C3H6F5OTa: C,
10.8; H, 1.8; Ta, 54.2. Found: C, 11.3; H, 1.7; Ta, 53.2. IR (CH2Cl2
−1
1
solution, cm ): 1661 (C O). H NMR d: 2.78 (s, Me). 13C NMR
d: 235.6 (CO); 32.2 (Me). 19F NMR d: 80.2 (br, 4 F, cis-F); 41.5
(br, 1 F, trans-F).
=
2d (pale yellow, 76% yield). Anal. Calc. for C8H8F5OTa: C,
24.3; H, 2.0; Ta, 45.7. Found: C, 24.5; H, 2.2; Ta, 43.9. IR (solid
state, cm−1): 3069 vw, 1593 m, 1557 s, 1497 m, 1470 s, 1450 m,
1426 m, 1360 ms, 1311 s, 1292 vs, 1234 vs, 1193 m, 1165 wm
1098 m, 1019 m, 1006 ms, 979 s, 875 vs, 817 s, 765 vs, 735 vs. 1H
NMR d: 7.99–7.42 (5 H, Ph); 2.62 (s, 3 H, Me). 13C NMR d: 199.1
(CO); 136.6, 133.3, 128.5 (Ph); 26.4 (Me). 19F NMR d: 80.2 (br,
4 H, cis-F); 39.0 (br, 1 H, trans-F).
2e (orange, 89% yield). Anal. Calc. for C13H10F5OTa: C, 34.1;
Experimental
1
H, 2.2; Ta, 39.5. Found: C, 34.8; H, 2.5; Ta, 40.3. H NMR d:
7.90–7.49 (Ph). 13C NMR d: 205.0 (CO); 136.0–128.8 (Ph). 19F
NMR d: 72.8 (br, 4 F, cis-F); 55.1 (br, 1 F, trans-F).
All manipulations of air and/or moisture sensitive compounds
were performed under an atmosphere of pre-purified argon using
standard Schlenk techniques. The reaction vessels were oven dried
at 150 ◦C prior to use, evacuated (10−2 mmHg) and then filled with
argon. All the reagents, including MX5 (M = Nb, Ta, X = Cl; M =
Ta, X = F), were commercial products (Aldrich) of the highest
purity available. TaBr5 was prepared according to published
2f (yellow, 83% yield). Anal. Calc. for C7H6Cl5OTa: C, 18.1; H,
1.3; Cl, 38.2; Ta, 39.0. Found: C, 17.6; H, 1.4; Cl, 37.5; Ta, 38.6.
IR (solid state, cm−1): 1684 w, 1601 m, 1586 vs, 1562 vs, 1450 s,
1388 s, 1318 m, 1228 ms, 1170 m, 1095 w, 1068 w, 1020 w, 996 w,
1
840 m, 816 m, 758 vs, 672 vs, 659 vs. H NMR d: 10.00 (s, 1 H,
4348 | Dalton Trans., 2007, 4343–4351
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CH); 8.28–6.73 (5 H, Ph). 13C NMR d: 192.3 (CO); 136.2, 134.4,
129.6, 128.9, 128.6, 125.9 (Ph).
(10 H, Ph); 2.83 (s, 2 H, CH2); 2.40 (s, 3 H, Me). 13C NMR d: 225.3
=
(C O); 138.6, 136.0, 134.2, 131.0, 128.7 (Ph); 82.1 (CPh2); 53.7
2h (light yellow, 86% yield). Anal. Calc. for C8H8Cl5OTa: C,
20.1; H, 1.7; Cl, 37.1; Ta, 37.9. Found: C, 20.6; H, 1.5; Cl, 36.9;
Ta, 37.2. IR (solid state, cm−1): 1598 w, 1580 m, 1538 vs, 1489 m,
1455 s, 1400 m, 1348 m, 1315 s, 1302 s, 1184 wm, 1168 w, 1103 w,
1069 w, 1011 w, 995 w, 977 s, 815 m, 770 vs, 750 s, 676 vs. 1H NMR
d: 8.47–7.29 (5 H, Ph); 3.29 (s, 3 H, Me). 13C NMR d: 202.9 (CO);
140.2, 133.3, 130.1 (Ph); 27.8 (Me).
2i (light yellow, 90% yield). Anal. Calc. for C13H10Cl5OTa: C,
28.9; H, 1.9; Cl, 32.8; Ta, 33.5. Found: C, 29.1; H, 2.2; Cl, 32.0;
Ta, 33.1. IR (solid state, cm−1): 1586 s, 1574 ms, 1481 s, 1460 vs,
1446 s, 1415 vs, 1339 vs, 1314 s, 1296 vs, 1182 s, 1162 ms, 1109 wm,
996 m, 947 m, 925 m, 839 m, 806 m, 772 vs, 739 vs, 700 vs, 670 s.
1H NMR d: 8.26–7.70 (Ph). 13C NMR d: 208.6 (CO); 138.5, 136.0,
132.9, 129.4 (Ph).
(CH2); 31.4 (Me).
Substitution reactions on ketone-adducts.
A solution of
[TaCl5(MePhCO)] 2h (0.093 g, 0.190 mmol) in CH2Cl2 (10 mL) was
treated with Ph2CO (70 mg, 0.38 mmol) and stirred for 3 h. Hence,
the solvent was removed in vacuo and the resulting residue was
washed with pentane (2 × 5 mL). A yellow solid was obtained and
identified as [TaCl5(Ph2CO)], 2i, by 1H and 13C NMR spectroscopy.
Yield: 80 mg, 76%.
In a similar way, three distinct CH2Cl2 solutions of complex
[TaCl5(Ph2CO)] (2i) were treated respectively with two equivalents
of Me2CO and MePhCO and stirred for 24 h. Compounds
[TaCl5(L)] (L = Me2CO, 2g; MePhCO, 2h) were finally recovered
in 70–75% yields.
2j (orange, 81% yield). Anal. Calc. for C3H6Br5OTa: C, 5.6; H,
0.9; Br, 62.6; Ta, 28.3. Found: C, 5.8; H, 1.1; Br, 61.7; Ta, 28.0. IR
Preparation of [L2H][TaX6] [L = Ph2CO, X = F, 4a; L =
MePhCO, X = Cl, 4b]. Compound 4a was obtained in admixture
with 3b in the reaction of 2c with Ph2CO (see above), and
was isolated as white crystalline needles, by layering directly
the reaction mixture (dichloromethane solution) with pentane at
298 K (yield: ca. 30%). Anal. Calc. for C26H21F6O2Ta: C, 47.3; H,
3.2; Ta, 27.4. Found: C, 47.8; H, 3.6; Ta, 26.1. IR (solid state, cm−1):
3162 m (O–H–O), 1591 ms (CO), 1576 m, 1490 s, 1447 ms, 1435 ms,
1341 s, 1297 ms, 1260 m, 1183 wm, 1162 w, 996 w, 924 w, 795 s,
761 vs, 727 vs, 699 vs, 671 vs.
Compound 4b was isolated in the course of preparation of
compound 3d. The final reaction mixture was layered with
pentane, at ca. 253 K; hence, yellow crystals, corresponding to
4b, were obtained after 4 days and separated mechanically from
green microcrystalline powder of 3d (yield: ca. 8%). Anal. Calc.
for C16H17Cl6O2Ta: C, 30.3; H, 2.7; Cl, 33.5; Ta, 28.5. Found: C,
29.8; H, 3.0; Cl, 33.0; Ta, 28.0. IR (solid state, cm−1): 3108 br-s
−1
1
=
(CH2Cl2 solution, cm ): 1665 (C O). H NMR d: 2.92 (s, 6 H,
Me). 13C NMR d: 238.3 (CO); 33.0 (Me).
Preparation of [MX4{OC(Me)CH2C(R)(Rꢀ)O}] [M = Nb, X =
Cl, R = Rꢀ = Me, 3a; M = Ta, X = F, R = Rꢀ = Ph, 3b; M = Ta,
X = Cl, R = Rꢀ = Me, 3c; M = Ta, X = Cl, R = Me, Rꢀ =
Ph, 3d; M = Ta, X = Cl, R = Rꢀ = Ph, 3e]. The preparation
of 3c is described in detail, the others being performed in a
similar way, by reacting compounds 2 with the appropriate ketone.
Complex [TaCl5(Me2CO)] 2g (0.200 g, 0.480 mmol) was dissolved
in CH2Cl2 (10 mL) and treated with anhydrous acetone (0.039 mL,
0.53 mmol). The solution was stirred for 30 min. Hence, pentane
was layered over the solution, and massive crystallization of white
3c occurred in a few hours (0.134 g, 64% yield). Alternatively,
compound 3c was prepared in admixture with minor products
by reacting TaCl5 (0.300 g, 0.837 mmol) with acetone (0.16 mL,
2.15 mmol). Anal. Calc. for C6H11Cl4O2Ta: C, 16.4; H, 2.5; Cl,
32.4; Ta, 41.3. Found:C, 16.6;H, 2.3;Cl, 32.1;Ta, 40.3. IR (CH2Cl2
=
(O–H–O), 1648 m (C O), 1598 m, 1572 wm, 1446 wm, 1357 wm,
1279 w, 1264 m.
−1
1
=
solution, cm ): 1635 (C O). H NMR d: 3.43 (s, 2 H, CH2); 2.80
Reaction of TaF5 with acacH: synthesis and isolation
of compounds [TaF4{j2(O)–OC(Me)C(H)C(Me)O}], 5a, and
[acacH2][TaF6], 6. Compound TaF5 (0.450 g, 1.63 mmol) was
introduced into a Schlenk tube containing 12 mL of CH2Cl2. The
mixture was treated with 2,4-pentanedione (0.18 mL, 1.76 mmol)
and stirred for 1 h. The volatile materials were removed and a pale
yellow oily residue was obtained. A 1H NMR spectrum run on the
mixture (dissolved in CDCl3) revealed the presence of compounds
5a and 6 in 5 : 2 ratio. The products were separated as follows:
the oil was dissolved in CH2Cl2, the resulting solution was layered
with pentane and left at room temperature for 48 h. Formation of
a yellow precipitate occurred, which was isolated from the mother-
liquor as a yellow powder and identified as 5a (0.280 g, 48%). Anal.
Calc. for C5H7F4O2Ta: C, 16.9; H, 2.0; Ta, 50.8. Found: C, 17.0;
H, 2.3; Ta, 49.9. IR (solid state, cm−1): 2926 w, 1527 vs (CO), 1424
m, 1332 m, 1287 s, 1037 m, 941 s, 819 s. 1H NMR d: 6.27 (s, 1 H,
CH); 2.33 (s, 6 H, Me). 13C NMR d: 194.7 (CO); 110.2 (CH); 26.1
(Me). 19F NMR d: 98.9 (s br, 2 F, TaF4); 41.2 (s br, 2 F, TaF4).
The mother-liquor was dried in vacuo giving a oily yellow residue.
Crystallization from a CH2Cl2 solution layered with pentane, at
13
=
=
(s, 3 H, MeC O); 1.58 (s, 6 H, Me2CO). C NMR d: 226.4 (C O);
=
84.5 (Me2CO); 52.9 (CH2); 34.9 (MeC O); 27.9 (Me2CO).
3a (yellow, 63% yield). Anal. Calc. for C6H11Cl4NbO2: C, 20.6;
H, 3.2; Cl, 40.5; Nb, 26.6. Found: C, 21.0; H, 2.6; Cl, 39.3; Nb,
−1
1
=
26.2. IR (CH2Cl2 solution, cm ): 1631 (C O). H NMR d: 2.97
(s, 2 H, CH2); 2.80 (s, 3 H, MeC O); 1.63 (s, 6 H, Me2CO). 13C
NMR d: 220.7 (C O); 86.0 (Me2CO); 57.0 (CH2); 32.2 (MeC O);
27.9 (Me2CO).
=
=
=
3b (yellow, 55% yield). Anal. Calc. for C16H15F4O2Ta C, 38.7;
H, 3.0; Ta, 36.5. Found C, 37.9; H, 2.6; Ta, 37.0. IR (CH2Cl2
−1
1
=
solution, cm ): 1623 (C O). H NMR d: 3.21 (s, 2 H, CH2); 2.84
13
=
=
(s, 3 H, MeC O); 1.41 (s, 6 H, Me2CO). C NMR d: 221.7 (C O);
=
80.0 (Me2CO); 55.3 (CH2); 33.6 (MeC O); 28.6 (Me2CO).
3d (green, 66% yield). Anal. Calc. for C11H13Cl4O2Ta: C, 26.4;
H, 2.6; Cl, 28.4; Ta, 36.2. Found: C, 26.0; H, 2.3; Cl, 28.0; Ta,
−1
1
=
36.9. IR (CH2Cl2 solution, cm ): 1625 (C O). H NMR d: 8.31–
=
7.54 (5 H, Ph); 3.23 (s, 2 H, CH2); 2.89 (s, 3 H, MeC O); 2.44 (s,
3 H, MeCO). C NMR d: 224.7 (C O); 138.2–129.0 (Ph); 84.9
(PhCO); 52.0 (CH2); 31.6 (MeC O); 25.5 (MeCO).
3e (yellow, 59% yield). Anal. Calc. for C16H15Cl4O2Ta: C, 34.2;
13
=
=
◦
–20 C, afforded a microcrystalline white solid corresponding to
H, 2.7; Cl, 25.2; Ta, 32.2. Found: C, 34.0; H, 2.8; Cl, 24.8; Ta, 31.7.
compound 6 (0.092 g, 14% yield). Anal. Calc. for C5H9F6O2Ta C,
15.2; H, 2.3; Ta, 45.7. Found C, 14.8; H, 2.1; Ta, 45.1. IR (solid
−1
1
=
IR (CH2Cl2 solution, cm ): 1609 (C O). H NMR d: 8.25–7.57
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state, cm−1): 3124 w (O-H), 1560 ms, 1424 ms, 1332 s, 1287 vs, 1037
Preparation of [TaX4{j2(O)–OC(Me)C(R)C(Me)O}] [X = Cl,
R = H, 5c; X = Cl, R = Me, 5d; X = Br, R = Me, 5e]. The
preparation of 5c is described in detail, those of 5d-e being
performed in a similar way. To a suspension of TaCl5 (0.225 g,
0.628 mmol) in CH2Cl2 (10 mL), anhydrous 2,4-pentanedione
(0.070 mL, 0.69 mmol) was added. The mixture was stirred for 1 h,
during which the solid progressively dissolved affording a yellow
solution. Hence, the final solution was dried in vacuo affording
an ochre–yellow residue. This latter was washed with pentane
(2 × 10 mL), thus a microcrystalline powder corresponding to 5c
(0.213 g, 80% yield) was obtained. Anal. Calc. for C5H7Cl4O2Ta
C, 14.2; H, 1.7; Cl, 33.6; Ta, 42.9. Found C, 14.9; H, 1.5; Cl, 33.8;
Ta, 42.1. IR (solid state, cm−1): 2967 w, 2918 w, 1531 vs (CO),
1426 m, 1410 ms, 1356 m, 1288 vs, 1261s-sh, 1031 s, 1012 s, 948
m-sh, 934 ms, 817 s, 676 s. 1H NMR d: 6.37 (s, 1 H, CH); 2.33 (s,
6 H, Me). 13C NMR d: 191.8 (CO); 113.1 (CH); 26.2 (Me).
5d (orange, 75% yield). Anal. Calc. for C6H9Cl4O2Ta C, 16.5;
H, 2.1; Cl, 32.5; Ta, 41.5. Found C, 16.9; H, 2.3; Cl, 32.1; Ta, 41.4.
IR (solid state, cm−1): 2939 w, 2920 wm, 1540 vs (CO), 1455 vs,
1406 s, 1359 m, 1250 vs, 1181 vs, 1089 m, 980 ms, 815 s, 705 s. 1H
NMR d: 2.37 (s, 6 H, OCMe); 2.27 (s, 3 H, OCCMe). 13C NMR
d: 188.1 (CO); 116.4 (OCCMe); 25.6 (OCMe); 14.3 (OCCMe).
5e (yellow, 82% yield). Anal. Calc. for C6H9Br4O2Ta C, 11.7; H,
1.5; Br, 52.1; Ta, 29.5. Found C, 12.1; H, 1.8; Br, 51.8; Ta, 29.0.
IR (solid state, cm−1): 2950 w, 2918 wm, 1721 w, 1699 w, 1531 vs
(CO), 1480 vs, 1412 s, 1371 m, 1321 m, 1255 vs, 1181 s, 1116 w,
978 s, 905 m, 823 s, 718 vs. 1H NMR d: 2.35 (s, 6 H, OCMe); 2.17
(s, 3 H, OCCMe).
1
m, 941 m, 819 ms. H NMR d: 11.97 (s, 2 H, OH); 6.04 (s, 1 H,
CH); 2.33 (s, 6 H, Me). 13C NMR d: 109.8 (CH); 102.3 (COH);
23.8 (Me). 19F NMR d: −131.7 (m, TaF6−).
Reaction of TaF5 with MeC(O)C(Me)(H)C(O)Me: synthesis and
isolation of compounds [TaF4{j2(O)–OC(Me)C(Me)C(Me)O}],
5b, and [TaF{OC(Me)C(Me)C-(Me)O}3][TaF6], 7. A suspension
of TaF5 (0.320 g, 1.16 mmol) in CH2Cl2 (15 mL) was treated
with 3-methyl-2,4-pentane-1,3-dione (0.140 mL, 1.20 mmol). The
mixture was stirred for 1 h, then the solvent was removed in
1
vacuo obtaining an orange oily residue. H NMR spectrum run
on the mixture (dissolved in CDCl3), revealed the presence of
5b and 7 in 5 : 1 ratio. This residue was dissolved in CH2Cl2
(7 mL), the resulting solution was layered with pentane and left
at ca. –20 ◦C for 24 h. Formation of a yellow orange precipitate,
corresponding to complex 5b, occurred (0.301 g, 70%). Anal. Calc.
for C6H9F4O2Ta C, 19.5; H, 2.5; Ta, 48.9. Found C, 18.9; H, 2.8;
Ta, 48.4. IR (solid state, cm−1): 1521 vs (CO). 1H NMR d: 2.28 (s, 3
H, OCCMe); 2.20 (s, 6 H, OCMe). 13C NMR d: 195.9 (CO); 115.2
(OCCMe); 25.2 (OCMe); 23.5 (OCCMe). 19F NMR d: 96.3 (s br, 2
F, TaF4); 39.4 (s br, 2 F, TaF4). When the reaction was performed by
using a two-fold excess of 3-methyl-2,4-pentanedione, an orange
residue was obtained upon removal of the volatile materials. This
residue was dissolved in CH2Cl2 (10 mL), the solution was layered
with pentane and left at room temperature. Orange X-ray quality
crystals of 7 were collected after 48 h (Yield: 0.083 g, 17%). Anal.
Calc. for C18H27F7O6Ta2 C, 25.9; H, 3.3; Ta, 43.4. Found C, 25.3;
H, 3.0; Ta, 43.0. IR (solid state, cm−1): 1552 vs (CO). 1H NMR d:
2.22 (s, 3 H, OCCMe); 2.00 (s, 6 H, OCMe). 13C NMR d: 184.8
(CO); 111.8 (OCCMe); 21.1 (OCMe); 12.6 (OCCMe). 19F NMR
d: 86.4 (br, 1 F, LnTaF); −131.8 (m, 6 F, TaF6−).
X-Ray crystallographic studies. Crystal data and collection
details for [TaCl5(Ph2CO)], 2i, [TaCl4{OC(Me)CH2C(Me)2O}],
3c, [(Ph2CO)2H][TaF6], 4a [(MePhCO)2H][TaCl6], 4b, and
Table 6 Crystal data and structure refinement for [TaCl5(OCPh2)], 2i, [TaCl4{OC(Me)CH2C(Me)2O}], 3c, [(Ph2CO)2H][TaF6], 4a, [(Me(Ph)CO)2H]-
[TaCl6], 4b and [TaF{OC(Me)C(Me)C(Me)O}3][TaF6], 7
Complex
2i
3c
4a
4b
7
Formula
Fw
C13H10Cl5Ta
540.41
100(2)
0.71073
Monoclinic
C2/c
29.442(3)
8.8967(10)
12.8833(14)
90
103.032(2)
90
3287.6(6)
8
2.184
7.489
C6H11Cl4O2Ta
437.90
100(2)
0.71073
Monoclinic
P21/c
6.7058(12)
15.462(3)
12.189(2)
90
102.753(2)
90
1232.7(4)
4
2.360
9.752
C26H21F6O2Ta
660.38
100(2)
0.71073
Monoclinic
C2/c
14.504(3)
9.6665(18)
17.500(5)
90
107.741(2)
90
2336.8(9)
4
1.877
4.773
C16H17Cl6O2Ta
634.95
100(2)
0.71073
Monoclinic
P21/c
8.9858(11)
11.0670(14)
11.0162(14)
90
91.807(2)
90
1095.0(2)
2
1.926
5.759
C18H27F7O6Ta2
834.30
100(2)
0.71073
Rhombohedral
¯
R3
11.3387(15)
11.3387(15)
32.002(8)
90
90
120
3563.2(11)
6
2.333
9.292
2352
T/K
˚
k/A
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
Cell volume/A
Z
Dc/g cm−3
l/mm−1
F(000)
2032
816
1280
608
Crystal size/mm
0.22 × 0.18 × 0.14
1.42–27.09
17756
0.24 × 0.18 × 0.16
2.16–27.00
13011
0.22 × 0.18 × 0.15
2.44–26.37
11565
0.20 × 0.16 × 0.12
2.27–27.98
11768
0.18 × 0.14 × 0.12
1.91–27.99
12926
h limits/◦
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness on fit on F2
R1 [I > 2r(I))
wR2 (all data)
Largest diff. peak and hole, e A
3633 (Rint = 0.0269) 2700 (Rint = 0.0375) 2383 (Rint = 0.0317) 2579 (Rint = 0.0318) 1887 (Rint = 0.0718)
363/0/181
1.179
0.0172
0.0392
0.420/–1.065
2700/0/121
1.116
0.0221
0.0583
1.764/–1.203
2383/0/159
1.079
0.0168
0.0443
1.398/–0.360
2579/0/116
1.155
0.0241
0.0633
1.736/–0.886
1887/0/103
1.030
0.0343
0.0931
1.907/–2.345
−3
˚
4350 | Dalton Trans., 2007, 4343–4351
This journal is
The Royal Society of Chemistry 2007
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7 D. B. Copley, F. Fairbrother, K. H. Grundy and A. Thompson, J. Less-
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8 D. B. Copley, F. Fairbrother and A. Thompson, J. Less-Common Met.,
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[TaF{OC(Me)C(Me)C(Me)O}3][TaF6], 7, are reported in Table 6.
The diffraction experiments were carried out on a Bruker APEX
II diffractometer equipped with a CCD detector using Mo-
Ka radiation. Data were corrected for Lorentz polarization and
absorption effects (empirical absorption correction SADABS28).
Structures were solved by direct methods and refined by full-matrix
least-squares based on all data using F2.29 Hydrogen atoms bonded
to C-atoms were fixed at calculated positions and refined by a
riding model. Oxygen bonded hydrogen atoms in 4a and 4b were
located in the Fourier map and found to be on an inversion centre
for both structures; they were refined isotropically using the 1.5
fold Uiso value of the parent O-atom. All non-hydrogen atoms were
refined with anisotropic displacement parameters.
11 J. March, Advanced Organic Chemistry, 3rd edn, J. Wiley, New York,
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12 Yu. A. Buslaev and E. G. Ilyin, J. Fluorine Chem., 1974, 4, 271.
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Acknowledgements
The authors wish to thank the Ministero dell’Istruzione,
dell’Universita` e della Ricerca (MIUR, Roma), Programmi di
Ricerca Scientifica di Notevole Interesse Nazionale 2004–2005,
for financial support, and Ms Veronika Wiescholek, on leave
from University of Mainz (Germany), for assistance with the
experimental work.
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This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 4343–4351 | 4351
©