Decarbonylation of phenylacetic acids by high valent transition metal halides

Article abstract of DOI:10.1039/c9dt00551j

Triphenylacetic acid underwent unusual decarbonylation when allowed to react with a series of halides of group 4-6 metals in their highest oxidation state, in dichloromethane at ambient temperature. Thus, the reaction of CPh₃COOH with MoCl₅, in 1:1 molar ratio, afforded the trityl salt [CPh₃][MoOCl₄], 1, in 79% yield, while the 1:2 reaction of CPh₃COOH with NbF₅ afforded [CPh₃][NbF₆], 2, in 70% yield, NbOF₃ being the metal co-product. CPh₃COOH reacted with NbCl₅, TiF₄ and WOCl₄ to give mixtures of compounds, however the cation [CPh₃]⁺ was NMR identified in each case. [CPh₃][NbCl₆], 3, was isolated from NbCl₅ and CPh₃COCl, prior to being generated from CPh₃COOH and PCl₅. The reaction of CPh₃COOH with TiCl₄ was non-selective, and the salt [CPh₃][Ti₂Cl₈(μ-κ²-O₂CCPh₃)], 4, was obtained in 18% yield. The decarbonylation reactions of CMePh₂COCl and CMe₂PhCOCl by means of NbCl₅ led to the indanes 5a-b, which were isolated in 79-97% yields after hydrolysis of the mixtures and subsequent alumina filtration of the organic phases. The reactions of CH(Ph)₂COOH with NbCl₅ and WCl₆ afforded NbCl₄(OOCCHPh₂), 6, and CHPh₂COCl, respectively, as the prevalent species. CPh₂(CH₂CH₂Br)COOH did not undergo CO release when allowed to interact with WCl₆, instead selective intramolecular condensation to C(Ph)₂C(O)OCH₂CH₂, 7, occurred. MeCCCOOH underwent hydrochlorination by WCl₆ to give MeC(Cl)CHCOOH, 8, in 72% yield. All the products were fully characterized by elemental analysis, IR and multinuclear NMR spectroscopy. In addition, the solid state structures of 1, 2, 4, 7, and 8 were elucidated by X-ray diffraction.

Full text of DOI:10.1039/c9dt00551j

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Decarbonylation of Phenylacetic Acids by High Valent Transition Metal
Halides
Niccolò Bartalucci,a Fabio Marchetti,a Stefano Zacchini,b,* Guido Pampaloni a,*
a Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via G. Moruzzi 13, I-56124 Pisa,
Italy.
b Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale Risorgimento 4,
I-40136 Bologna, Italy.
1

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Abstract
Triphenylacetic acid underwent unusual decarbonylation when allowed to react with a series of halides
of groups 4-6 metals in their highest oxidation state, in dichloromethane at ambient temperature. Thus,
the reaction of CPh3COOH with MoCl5, in 1:1 molar ratio, afforded the trityl salt [CPh3][MoOCl4], 1, in
79% yield, while the 1:2 reaction of CPh3COOH with NbF5 afforded [CPh3][NbF6], 2, in 70% yield
NbOF3 being the metal co-product. CPh3COOH reacted with NbCl5, TiF4 and WOCl4 to give mixtures
of compounds, however the cation [CPh3]+ was NMR identified in each case. [CPh3][NbCl6], 3, was
isolated from NbCl5 and CPh3COCl, prior generated from CPh3COOH and PCl5. The reaction of
CPh3COOH with TiCl4 was non-selective, and the salt [CPh3][Ti2Cl8(-2-O2CCPh3)], 4, was obtained
in 18% yield. The decarbonylation reactions of CMe(Ph)2COCl and CMe2(Ph)COCl by means of NbCl5
led to the indanes 5a-b, which were isolated in 79-97% yields after hydrolysis of the mixtures and
subsequent alumina filtration of the organic phases. The reactions of CH(Ph)2COOH with NbCl5 and
WCl6 afforded NbCl4(OOCCHPh2), 6, and CHPh2COCl, respectively, as the prevalent species.
CPh2(CH2CH2Br)COOH did not undergo CO release when allowed to interact with WCl6, instead
selective intramolecular condensation to
, 7, occurred. MeC≡CCOOH underwent
hydrochlorination by WCl6 to give MeC(Cl)=CHCOOH, 8, in 72% yield. All the products were fully
characterized by elemental analysis, IR and multinuclear NMR spectroscopy. In addition, the solid state
structures of 1, 2, 4, 7, 8 were elucidated by X-ray diffraction.
Introduction
Early transition metal halides, containing the metal in an oxidation state higher than +4, are featured
by a strong Lewis acidic character[1] and a quite positive oxidation potential.[2] These peculiar
characteristics give raise to several transformations on organic substrates, such as the formation of C-C
2

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bonds,[3] the dehydrogenative coupling of arenes,[4] the functionalization of alkenes,[5] the
polymerization of alkynes [6] and several cycloaddition reactions.[7]
In the recent years, we have studied the reactivity of high valent groups 4, 5 and 6 metal halides with
Lewis bases pointing out a marked difference of reactivity when moving from group 4 to group 6
elements.
For example, TiCl4 generally reacts with oxygen donors affording hexacoordinate complexes, and
activation of the ligands is observed at high temperature only.[8] On the other hand, in view of the lower
M-Cl bond energy[9] and the higher oxidation state, MoCl5 and WCl6 make possible stoichiometric
modifications of simple molecules through unconventional pathways even at room temperature or
below. In particular, tungsten hexachloride shows examples which include the conversion of imines into
[10]
aza-2-allenium and nitrilium cations,
the former being generated via unusual N2 elimination, and of
α-diimines into quinoxalinium salts.[11]
Unique reactivity patterns have been observed with niobium and tantalum pentahalides[1c,7,12] and,
considering also the relative cheapness and the substantial non-toxicity of the related metal elements,[13]
a significant advance in the use of these systems as promoters for organic transformations has been
experienced in the last years.[7,14] When compared to the heavier halides of Group 6, NbX5 (X = F, Cl)
are more inert towards reduction processes, and are characterized by relatively strong niobium-halide
bonds,[9] thus disfavouring halide/oxygen exchange reactions.[15] As a matter of fact, NbF5 forms adducts
when allowed to react with alcohols and carboxylic acids,[16] while NbCl5 in the same conditions affords
metal alcoholates/carboxylates through HCl release.[14e, 17] The formation of metal carboxylates is also
the typical reactivity displayed by low- or medium valent transition metal chlorides with carboxylic
acids,[18] including CPh3COOH.[19] On the other hand, the reactivity of simple carboxylic acids
(RCOOH) with MoCl5 and WCl6 shows similitudes with the behaviour of PCl5,[20] thus Cl/O exchange
takes place with formation of the corresponding acyl chlorides RC(O)Cl. This outcome, however, is
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often accompanied by side reactions,[21] possibly giving an explanation why the interaction of Group 6
high valent metal halides with carboxylic acids (and their acyl chloride counterparts), especially the
ones bearing an additional functional group, has been sparingly explored heretofore.
In the framework of our project, aimed to develop the chemistry of homoleptic halides of early
transition metals, and following our recent communication on the unusual reactivity exhibited by WCl6
with CPh3COOH and related carboxylic acids,[22] herein we report an extension of this study to other
halides of high valent groups 4-6 metals.
Results and discussion
1. Decarbonylation reactions of triphenylacetic acid.
The 1:1 molar reactions of triphenylacetic acid with a series of high valent metal chlorides/fluorides, i.e.
MoCl5, WOCl4, NbCl5, NbF5 or TiF4, were carried out in dichloromethane at ambient temperature and
cleanly proceeded with CO extrusion (detected by gas-chromatography), to give the trityl cation
associated to various counteranions (Scheme 1). Similarly to what found for the CPh3COOH/WCl6
reaction system,[22] the preliminary formation of CPh3COX (X = Cl or F), resulting from halide/oxygen
exchange between the organic substrate and the metal halide, is likely to precede the CO release step.
Indeed, CPh3COCl was NMR detected along the pathways to 1 and 3. Olah observed a very unstable
coordination complex between CPh3COF and SbF5, decarbonylating to [CPh3]+.[23]
The reaction involving MoCl5 straightforwardly afforded 1, isolated in 78% yield, wherein the anion is
the Cl/O exchange metal product (Scheme 1a). On the other hand, the cation [CPh3]+ was clearly
recognized by 13C NMR in the reaction mixtures obtained from CPh3COOH/WOCl4 and
CPh3COOH/TiF4 (diagnostic resonance at ca. 210 ppm), but the clean isolation of products failed
(Scheme 1b). The 19F NMR spectrum of CPh3COOH/TiF4 displayed a complicated set of signals.
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a)
[CPh3][MoOCl5]
1
b)
O
O
[CPh3]+
MXn
HX
CO
c)
Ph3C
X
Ph3C
OH
[CPh3][NbF6] + NbOF3
2
d)
[CPh3][NbCl6] + [CPh3][NbOCl4]
PCl5
e)
3
POCl3
HCl
NbCl5
CO
Scheme 1. Decarbonylation of triphenylacetic acid and triphenylacetic acid chloride by reactions with high valent halides.
MXn = MoCl5 (a); WOCl4 or TiF4 (b); NbF5 (c); NbCl5 (d).
The combination of CPh3COOH with niobium pentahalides yielded 2-3 in admixture with niobium oxo-
halide co-products (Scheme 1c-d), and the optimal stoichiometry for these reactions resulted to be
Nb/acid 2:1 (Eqn. 1). In other terms, it appears that half of the metal reactant serves the halide/O
exchange, whereas the remaining aliquot is employed to form the anion within 2-3. NbOF3 was easily
separated by precipitation and characterized by IR spectroscopy (typical band at 995 cm1),[24] thus
allowing the isolation of 2.
2 NbX5 + CPh3COOH  [CPh3][NbX6] + CO + HX + NbOX3
(1)
X = F, Cl
Both anions [NbCl6] ( = 8.8 ppm)[25] and [NbOCl4] (δ = 343 ppm)[26] were identified by 93Nb NMR
in the reaction system CPh3COOH/NbCl5, but the isolation of a well-defined product was not possible in
this case. The IR spectrum of the solid residue suggested the presence of NbOCl3 (broad absorption at
780 cm).[27]
In order to cleanly generate 3, CPh3COCl was prior generated from CPh3COOH and PCl5, and then
allowed to react with NbCl5 (Scheme 1e).
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Compounds 1-3 were characterized by elemental analysis, IR and multinuclear NMR spectroscopy.
19
Salient data are represented by IR band due to the Mo=O moiety in 1 (v = 993 cm)[28] and F NMR
resonance related to the [NbF6] anion in 2 (decet centred at 102 ppm).[29] Furthermore, the solid state
structures of 1 and 2 were elucidated by single crystal X-ray diffraction studies. Views of these
structures are shown in Figures 1 and 2, while relevant bonding parameters are reported in Tables 1-2.
Figure 1. Molecular structure of [CPh3][MoOCl4], 1, with key atoms labelled. Thermal ellipsoids are at the 50% probability
level.
Table 1. Selected bond distances (Å) and angles (°) for 1.
Mo(1)–O(1)
Mo(1)–Cl(3)
Mo(1)–Cl(5)
C(1)–C(8)
1.6549(14)
2.3484(5)
2.3320(6)
1.455(2)
Mo(1)–Cl(2)
Mo(1)–Cl(4)
C(1)–C(2)
2.3527(5)
2.3427(5)
1.444(2)
1.442(2)
C(1)–C(14)
O(1)–Mo(1)–Cl(2)
O(1)–Mo(1)–Cl(4)
Cl(2)–Mo(1)–Cl(3)
C(2)–C(1)–C(8)
105.27(5)
105.66(5)
150.226(17)
119.34(15)
119.31(15)
O(1)–Mo(1)–Cl(3)
O(1)–Mo(1)–Cl(5)
Cl(4)–Mo(1)–Cl(5)
C(2)–C(1)–C(14)
104.49(5)
105.51(6)
148.82(2)
121.35(15)
C(8)–C(1)–C(14)
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Figure 2. Molecular structure of [CPh3][NbF6], 2, with key atoms labelled. Thermal ellipsoids are at the 50% probability
level.
Table 2. Selected bond distances (Å) and angles (°) for 2.
Nb(1)–F(1)
Nb(1)–F(3)
Nb(1)–F(5)
C(1)–C(2)
C(1)–C(14)
1.8698(13)
1.8885(13)
1.8919(14)
1.447(3)
Nb(1)–F(2)
Nb(1)–F(4)
Nb(1)–F(6)
C(1)–C(8)
1.8963(13)
1.8774(13)
1.8892(13)
1.440(3)
1.452(3)
F(1)–Nb(1)–F(4)
F(3)–Nb(1)–F(6)
C(2)–C(1)–C(14) 118.87(18)
178.25(6)
178.39(6)
F(2)–Nb(1)-F(5) 179.20(5)
C(2)–C(1)–C(8) 120.79(18)
C(8)–C(1)–C(14) 120.29(18)
The solid state structure of 1 is an ionic packing of [CPh3]+ cations and [MoOCl4]– anions. The former
cation has been previously characterized with miscellaneous anions showing very similar bonding
parameters.[30] The [MoOCl4]– anion displays a distorted square-pyramidal structure as previously found
in other [MoOCl4]– salts.[17a, 28f, 31] The Mo(1)O(1) bond [1.6549(14) Å] reveals a strong -character, as
expected for a Mo(V)=O unit.[32] The [MoOCl4]– anions may be found in the solid state in different
forms: (A) isolated anions; (B) weak intermolecular adducts with a donor atom of the cation or the co-
crystallized solvent molecule; (C) dimeric {[MoOCl4]–}2 units; (D) infinite {[MoOCl4]–} chains.[31] In
the case of 1, the [MoOCl4]– anions are isolated (A). The solid state structure of 2 consists of an ionic
packing of [CPh3]+ cations and [NbF6]– anions,[15b, 25c, 33] which have been previously described in the
literature.
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The study of the reactivity of triphenylacetic acid with metal halides was extended to titanium
tetrachloride. The 1:1 molar reaction afforded a complicated mixture of products. Fractional
crystallization of the raw material afforded a 18% yield of the crystalline salt [CPh3][Ti2Cl8(O2CCPh3)],
4. This outcome indicates that TiCl4 exhibits a hybrid behaviour towards triphenylacetic acid, the latter
being partially converted into carboxylate and partially decarbonylated, thus determining the non-
selectivity of the reaction. The scarcely soluble compound 4 was characterized by elemental analysis, IR
spectroscopy and single crystal X-ray diffraction. The solid state structure of 4 consists of an ionic
packing of [CPh3]+ cations and [Ti2Cl8(OOCCPh3)]– anions. The latter anion is unprecedented and
closely related to the previously reported neutral species Ti2Cl7(O2CR)(RCOOH) (R = p-C6H4Cl,
CH=CHMe),[34] wherein a terminal chloride ligand has been replaced by a O-bonded CPh3COO
fragment. Thus, [Ti2Cl8(OOCCPh3)]– is a dimeric anion, composed by two distorted octahedral Ti(IV)
centres, each bonded to three terminal and two -bridging chlorides as well as the O-atom of a bridging
[Ph3CCOO]– carboxylate ligand. All bonding parameters closely resemble those reported in the case of
Ti2Cl7(O2CR)(RCOOH).[34] The carboxylate ligand gives rise to a strong IR absorption at 1644 cm, to
be compared with the value 1693 cm, characteristic of CPh3COOH.
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Figure 3. Molecular structure of [CPh3][Ti2Cl8(OOCCPh3)], 4, with key atoms labelled. Thermal ellipsoids are at the 50%
probability level.
Table 3. Selected bond distances (Å) and angles (°) for 4.
Ti(1)–O(2)
Ti(1)–Cl(1)
Ti(1)–Cl(2)
Ti(1)–Cl(3)
Ti(1)–Cl(7)
Ti(1)–Cl(8)
C(1)–O(1)
C(1)–C(2)
2.008(4)
Ti(2)–O(1)
Ti(2)–Cl(4)
Ti(2)–Cl(5)
Ti(2)–Cl(6)
Ti(2)–Cl(7)
Ti(2)–Cl(8)
C(1)–O(2)
C(2)–C(3)
2.000(3)
2.2406(16)
2.2165(16)
2.2368(16)
2.4790(15)
2.5054(15)
1.262(6)
2.2402(16)
2.2205(16)
2.2272(16)
2.4835(15)
2.5051(15)
1.249(6)
1.534(7)
1.550(7)
C(2)–C(9)
C(31)–C(32)
C(31)–C(44)
1.536(7)
1.447(7)
1.454(7)
C(2)–C(15)
C(31)–C(38)
1.560(7)
1.433(7)
O(2)–Ti(1)–Cl(1)
Cl(2)–Ti(1)–Cl(8)
Cl(3)–Ti(1)–Cl(7)
Cl(2)–Ti(1)–Cl(3))
Cl(7)–Ti(1)–Cl(8)
Ti(1)–Cl(7)–Ti(2)
Ti(1)–O(2)–C(1)
O(1)–C(1)–O(2)
O(2)–C(1)–C(2)
C(1)–C(2)–C(9)
170.42(11)
166.45(6)
168.12(6)
99.84(6)
79.15(5)
97.73(5)
141.4(3)
123.2(5)
120.1(4)
105.4(4)
O(1)–Ti(2)–Cl(4)
Cl(5)–Ti(2)–Cl(8))
Cl(6)–Ti(2)–Cl(7)
Cl(5)–Ti(2)–Cl(6)
Cl(7)–Ti(2)–Cl(8)
Ti(1)–Cl(8)–Ti(2)
Ti(2)–O(1)–C(1)
O(1)–C(1)–C(2)
C(1)–C(2)–C(3)
C(1)–C(2)–C(15)
C(32)–C(31)–C(44)
171.96(11)
165.76(6)
168.41(6)
101.52(6)
79.07(5)
96.48(5)
140.1(3)
116.7(4)
109.8(4)
110.3(4)
118.7(5)
C(32)–C(31)–C(38) 119.8(5)
C(38)–C(31)–C(44) 119.8(5)
2. Decarbonylation reactions and activation reactions of mono- and diphenylacetic acids.
The reactions of CMePh2COCl and CMe2PhCOCl with NbCl5 proceed as reported in Scheme 2
affording the indanes 5a-b, which were isolated in good yields (after hydrolysis of the mixtures and
subsequent alumina filtration of the organic phases) and characterized analytically and
spectroscopically. Compound 5b was also obtained, although in a lower yield, by using NbOCl3 in the
place of NbCl5.
Scheme 2. Synthesis of indanes by reaction of phenylacetic acids with NbCl5.
Indanes were previously obtained by elaborated protocols consisting in dimerization of cyclobutenes
constructed by coupling of C2 blocks.[35]
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The reactivity of diphenylacetic acid was investigated towards NbCl5 and WCl6. Both reactions failed to
produce decarbonylation products, and instead evidenced the typical reactivity of the two chlorides
towards carboxylic acids.[17b, c, 21a, 36] Thus, NbCl4(O2CCHPh2), 6, was isolated in good yield and fully
characterized (Eqn. 2), while CHPh2COCl was the only species NMR detected in the WCl6/acid reaction
solution (Eqn. 3).
NbCl5 + CHPh2COOH  NbCl4(O2CCHPh2)
(2)
(3)
6
WCl6 + CHPh2COOH  CHPh2COCl + WOCl4
[37]
It is presumable that the relative instability of the carbocation [CHPh2]+
observing decarbonylation from diphenylacetic acid.[22]
is responsible for not
The interaction of 4-bromo-2,2-diphenylbutyric acid with WCl6 led to cyclization of the organic
substrate via HBr release, affording 7 in high yield. In this case, the presence of bromine as a -
substituent is the key for inhibiting the decarbonylation pathway.
The product 7 was purified by alumina chromatography and characterized by analytical and
spectroscopic techniques, and its identity confirmed by X-ray diffraction (Figure S1 and Table S1). The
ester group manifests itself with a strong IR absorption at 1762 cm and a 13C NMR resonance at 177.8
ppm.
Finally, the reaction of WCl6 with an alkynyl carboxylic acid, i.e. MeC≡CCOOH, was investigated in
dichloromethane. This reaction resulted in the prevalent hydrochlorination of the triple carbon-carbon
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bond, affording MeC(Cl)=CHCOOH, 8, which was identified by IR and NMR spectroscopy and X-ray
diffraction (Figure S2 and Table S2).
Conclusions
The carboxylic acid function, in organic chemistry, can act as a leaving one, with elimination of either
carbon dioxide or, less frequently, carbon monoxide. The decarbonylation reactions, leading to alkenes
or C−C coupling products, are usually promoted by low valent transition metal compounds and take
place under harsh conditions (typically, temperatures above 150 °C).[22]
We have previously found that the decarbonylation pathway is straightforward for α-phenylacetic
acids at room temperature in the presence of WCl6, through the preliminary formation of the related
carboxylic acid chlorides. Herein, we have demonstrated that this non-conventional mode of reactivity
has some general character, thus working with a series of homoleptic halides of high valent transition
metals belonging to groups 4-6. However, a minimum oxidation state of +5 for the metal centre seems
necessary to make the reaction selective. It should be remarked that triphenylacetic acid typical reacts
with low- or medium valent transition metal chlorides affording metal triphenyl-acetates via HCl
elimination.[19] Our results indicate that the α-carbon in the carboxylic acid must be fully substituted in
order to permit the CO release and the relative stabilization of the descending radical cation, and that the
substitution at the -carbon may also play an important role. The radical cations may then be converted
in situ into indanes depending on the specific stability/chemistry of the former.
This paper confirms the potential of high valent transition metal halides as promoters of valuable
organic transformations, in view of their possible dual role as halogenating agents and oxidants.
Experimental Section.
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Methods and materials. Warning: all the metal products reported in this paper are highly moisture-
sensitive, thus rigorously anhydrous conditions were required for the reaction and crystallization
procedures. The reaction vessels were oven dried at 140 °C prior to use, evacuated (10–2 mmHg) and
then filled with argon. MoCl5 (99.6% purity), NbCl5 (99+%), NbF5 (99.5%) and TiF4 (98%) were
purchased from Strem and stored in sealed tubes under argon atmosphere. TiCl4 (99.0+%) and
carboxylic acids (>97%) were purchased from Sigma-Aldrich or TCI Europe and stored under argon
[38]
[27]
atmosphere as received. WOCl4
and NbOCl3
were prepared according to the literature. Once
isolated, the metal products were conserved in sealed glass tubes under nitrogen. Solvents (Sigma-
Aldrich) were distilled before use from appropriate drying agents. Infrared spectra were recorded at 298
K on a FT IR-Perkin Elmer Spectrometer, equipped with a UATR sampling accessory. NMR spectra
were recorded at 293 K on a Bruker Avance II DRX400 instrument equipped with a BBFO broadband
1
13
probe. The chemical shifts for H and C were referenced to the non-deuterated aliquot of the solvent;
the chemical shifts for 19F and 93Nb were referenced, respectively, to CFCl3 and [NEt4][NbCl6] as
external references. Gas-chromatographic analyses were carried out on a Dani 3200 instrument,
equipped with capillary column with molecular sieves (2m; 0.25in ID), using Ar at p = 1.5 atm as a gas
carrier, at 50 °C. Carbon and hydrogen analyses were performed on a Vario MICRO cube instrument
(Elementar). The chloride content was determined by the Mohr method[39] on solutions prepared by
dissolution of the solids in aqueous KOH at boiling temperature, followed by cooling down to ambient
temperature and addition of HNO3 up to neutralization. Titanium content was determined
spectrophotometrically by measuring the 410 nm absorbance of an aqueous solution after treatment with
few drops of 30% aqueous H2O2 (UV-VIS spectrum was recorded with Perkin-Elmer Lambda EZ201
spectrophotometer at 293 K).
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Reaction of MoCl5 with triphenylacetic acid: synthesis of [CPh3][MoOCl4], 1. A suspension of
MoCl5 (0.250 g, 0.915 mmol) in CH2Cl2 (15 mL) was treated with CPh3COOH (0.270 g, 0.936 mmol).
The mixture was stirred at ambient temperature for 18 hours, then the final brown solution was
concentrated to ca. 3 mL and added of hexane (30 mL). The resulting precipitate was washed with
pentane (20 mL) and dried under vacuum: compound 1 was isolated as a dark-red powder. Yield 0.359
g, 79%. Anal. Calcd for C19H15Cl4MoO: C, 45.91; H, 3.04; Cl, 28.53. Found: C, 46.06; H, 3.02; Cl,
28.31. IR (solid state): ν/cm = 3062vw, 1579vs, 1479m, 1446s, 1385w, 1352vs, 1292s, 1180m,
1169m, 1098w, 1029w, 993vs (Mo=O), 976w, 949w, 913w, 841w-m, 803m, 768m-sh, 762m, 697vs,
1
13
684s. H NMR (CD2Cl2): δ/ppm = 8.5, 8.1 (br, Ph). C{1H} NMR (CD2Cl2): δ/ppm = 211.6 (CPh3),
148.3, 147.5, 141.3, 135.1 (Ph). Crystals of 1 suitable for X-ray analysis were collected from a
dichloromethane reaction solution layered with hexane and settled aside at 30°C for one week.
Reaction of NbF5 with triphenylacetic acid: synthesis of [CPh3][NbF6], 2, and NbOF3. The reaction
of NbF5 (0.200 g, 1.06 mmol) with CPh3COOH (0.154 g, 0.534 mmol) was carried out with a procedure
analogous to that described for MoCl5/CPh3COOH. The final yellow solution was separated from a
pale-yellow precipitate by filtration. IR analysis on the precipitate clearly evidenced the presence of a
strong band at 995 cm (Nb=O, NbOF3). Compound 2 was recovered from the solution as a yellow solid
after work-up. Yield 0.167 g, 70%. Crystals of 2 suitable for X-ray analysis were collected from a
dichloromethane reaction solution layered with hexane and stored at 30°C for one week. Anal. Calcd
for C19H15F6Nb: C, 50.69; H, 3.36; Nb, 20.64. Found: C, 50.64; H, 3.41; Nb, 20.43. IR (solid state):
ν/cm = 2958w, 2925w, 1581vs, 1483m, 1449s, 1355vs, 1294s, 1186m, 1169w-m, 1030w, 995w-m,
982w-br, 914w, 846m, 837m, 806m, 765m, 733w, 699vs. 1H NMR (CD3CN): δ/ppm = 8.30, 7.88, 7.76
(m, CPh3). 13C{1H} NMR (CD3CN): δ/ppm = 212.0 (CPh3), 143.1, 140.2, 130.1 (Ph). 19F NMR
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(CD3CN): δ/ppm = 102 (decet, 1JNbF = 3.4∙102 Hz, NbF6 ). 93Nb NMR (CD3CN): δ/ppm = 1556 (hept,

1JNbF = 3.4∙102 Hz, NbF6 ).

Reaction of NbCl5 with triphenylacetic acid: identification of [CPh3]+, [NbCl6], [NbOCl4]- and
NbOCl3. The reaction of NbCl5 (0.110 g, 0.407 mmol) with CPh3COOH (0.125 g, 0.433 mmol) was
carried out with in CD2Cl2 (5 mL). The reaction solution was analyzed by NMR spectroscopy after 72
hours. 1H NMR (CD2Cl2): δ/ppm = 8.31 (t, 3JHH = 7.4 Hz, 3 H, [CPh3]+para); 7.94 (t, 3JHH = 7.6 Hz, 6 H,
13
[CPh3]+meta); 7.73 (d, J = 7.5 Hz, 6 H, [CPh3]+ortho); 7.32-7.27 (m, CPh3Cl). C{1H} NMR (CD2Cl2):
δ/ppm = 210.7 (CPh3), 143.5, 142.8, 140.0, 130.7 (Ph). 93Nb NMR (CD2Cl2): δ/ppm = 6.6 (½ = 138
Hz, NbCl6 ); 107.2 (½ = 142 Hz, minor); 358 (½ = 2∙103 Hz, NbOCl4 ); 588 (½ = 6∙104 Hz,




minor). Ratio NbCl6 /NbOCl4  2 : 5. The IR spectrum of the solid residue obtained by elimination of
volatile materials appeared as follows. IR (solid state): ν/cm = 3059vw, 1579vs, 1481s, 1447m-s,
1353vs, 1293s, 1183m, 1165w, 1083w, 994m, 835m-sh, 804m, 780br (Nb=O, NbOCl3), 763s, 696vs.
Reaction of NbCl5 with triphenylacetic acid chloride: synthesis of [CPh3][NbCl6], 3. A solution of
CPh3COOH (172 mg, 0.60 mmol) in CD2Cl2 (5 mL) was treated with PCl5 (123 mg, 0.59 mmol). The
mixture was left stirred at ambient temperature for 3 hours. Then NbCl5 (155 mg, 0.57 mmol) was
added, and the resulting mixture was stirred at ambient temperature for 48 hours. The volatiles were
removed under vacuum, the residue was repeatedly washed with hexane (3 x 20 mL) and then dried
under vacuum. Anal. Calcd for C19H15Cl6Nb: C, 41.57; H, 2.75; Cl, 38.75. Found: C, 41.03; H, 2.88; Cl,
1
13
38.66. H NMR (CD2Cl2): δ/ppm = 8.27, 7.90, 7.71 (m, 15 H, Ph). C{1H} NMR (CD2Cl2): δ/ppm =
211.5 (CPh3), 143.6, 140.8, 131.4, 128.5 (Ph). 93Nb NMR (CD2Cl2): δ/ppm = 7.0 (NbCl6 ).
‒
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Reactions of WOCl4/TiF4 with triphenylacetic acid: identification of [CPh3]+. The reactions of TiF4
or WOCl4 (ca. 1 mmol) with 0.5 eq. of CPh3COOH was carried out with a procedure analogous to that
described for MoCl5/CPh3COOH. The final mixture was eliminated of the volatile materials, then the
resulting residue was dried under vacuum.
From TiF4. IR (yellow solid): ν/cm = 3061vw, 1581s, 1485m, 1446m, 1357s, 1295w, 1187w, 866m,
796vs, 758s, 731s, 694vs. 1H NMR (CD3CN): δ/ppm = 8.30, 7.89, 7.72 (m, CPh3).
From WOCl4. IR (orange solid): ν/cm = 3060vw, 1579vs, 1481s, 1447m-s, 1353vs, 1293s, 1183m,
1
1165w-m, 1083w, 974m, 832m, 804m, 760s, 696vs. H NMR (CD2Cl2): δ/ppm = 8.20, 7.89, 7.67 ppm
(m, CPh3). 13C{1H} NMR (CD2Cl2): δ/ppm = 210.2 (CPh3), 143.4, 142.9, 140.1, 130.9 (Ph).
Reaction of TiCl4 with triphenylacetic acid: isolation of [CPh3][Ti2Cl8(-2-O2CCPh3)], 4. The
reaction of CPh3COOH (0.280 g, 0.971 mmol) with TiCl4 (1.0 M solution in heptane, 0.97 mL) was
carried out with a procedure analogous to that described for MoCl5/CPh3COOH. IR and NMR analyses
on the obtained orange solid residue suggested the formation of a complicated mixture of products.
Yellow crystals of 3 suitable for X-ray analysis were isolated from a CH2Cl2 reaction mixture layered
with hexane and settled aside at 30 °C for 72 h. Yield 0.080 g, 18%. Anal. Calcd for C39H30Cl8O2Ti2:
C, 51.47; H, 3.32; Cl, 31.17; Ti, 10.52. Found: C, 51.35; H, 3.28; Cl, 31.05; Ti, 10.64. IR (solid state):
ν/cm = 3059w, 3023w, 1644s(COO), 1580w, 1537s, 1493s, 1446m, 1365s, 1334m, 1264w, 1191w,
1160w, 1085w, 1034w, 1002w, 986w, 940w, 904w, 753s-sh, 740vs, 697vs, 669s, 658.
Reaction of NbCl5 with 2,2-diphenylpropionic acid chloride: synthesis of 1-methyl-1,3,3-triphenyl-
2,3-dihydro-1H-indene, 5a, (Chart 1).[22,40]
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Chart 1. Structure of 5a.
A solution of CMePh2COOH (229 mg, 1.01 mmol) in dichloromethane (10 mL) was treated with PCl5
(209 mg, 1.00 mmol), and the resulting mixture was left stirring at ambient temperature for 2 hours.
Then NbCl5 (91 mg, 0.34 mmol) was added, and the mixture was stirred for additional 48 hours. An
excess of H2O (ca. 5 mmol) was added to the final solution, and the so obtained mixture was stirred for
12 hours. Afterwards the organic phase was separated and filtered through an alumina column using
diethyl ether as eluent. The product was isolated as a colourless solid upon removal of the volatiles
under vacuum. Yield 176 mg, 97%. 1H NMR (CD2Cl2): δ/ppm = 7.49-7.14 (19 H, arom CH); 3.60, 3.28
2
13
(d, JHH = 13.52 Hz, 2 H, CH2); 1.71 (s, 3 H, Me). C{1H} NMR (CD2Cl2): δ/ppm = 150.6, 149.4,
148.9, 148.6, 147.5 (arom + CPh2); 128.8, 128.7, 128.0, 127.9, 127.6, 127.4, 126.9, 126.0, 125.7, 125.6,
125.1 (arom); 61.4 (CH2); 51.2 (Me).
Reaction of niobium chlorides with 2-methyl-2-phenylpropanoic acid chloride: synthesis of 1,1,3-
trimethyl-3-phenyl-2,3-dihydro-1H-indene, 5b, (Chart 2).[22,41]
Ph
Chart 2. Structure of 5b.
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A solution of CMe2PhCOOH (154 mg, 0.94 mmol) in dichloromethane (10 mL) was treated with PCl5
(200 mg, 0.96 mmol). The resulting solution was left stirring at ambient temperature for ca. 2 hours.
Then NbCl5 (85 mg, 0.31 mmol) was added, and the mixture was stirred for additional 48 hours. An
excess of H2O (ca. 5 mmol) was added to the final solution, and the so obtained mixture was stirred for
12 hours. Afterwards the organic phase was separated and filtered through an alumina column using
diethyl ether as eluent. The product was isolated as a colourless solid upon removal of the volatiles
under vacuum and characterized by elemental analysis and 1H NMR. Yield 88 mg, 79%.
In a different experiment, a dichloromethane solution of freshly prepared 2-methyl-2-phenylpropanoyl
chloride was added of 1 eq. of NbOCl3, and the mixture was left stirring at ambient temperature for 5
days. Treatment with water and successive work-up were performed as described above. In this case. the
title product was isolated in 70% yield.
Reactions of CHPh2COOH with metal chlorides.
A) Synthesis of NbCl4(O2CCHPh2), 6. A solution of CH(Ph)2COOH (230 mg, 1.08 mmol) in CH2Cl2
(10 mL) was added of NbCl5 (292 mg, 1.08 mmol). The mixture was stirred at room temperature for 18
hours. The resulting red solution was eliminated of the volatiles, then the residue was washed with
hexane (2 x 10 mL) and dried under vacuum. A red solid was obtained. Yield 338 mg, 70%. Anal. Calcd
for C14H11Cl4NbO2: C, 37.71; H, 2.49; Cl, 31.80. Found: C, 37.25; H, 2.50; Cl, 31.70. IR (solid state):
ν/cm = 3060w, 3027w, 1579s, 1481m, 1447m, 1416m, 1354s, 1295m, 1261w-m, 1180m, 1079w,
1000w, 995w, 978w, 920w, 800vs-br, 733vs, 694vs. 1H NMR (CD2Cl2): δ/ppm = 7.44-7.12 (10 H, Ph);
13
93
5.37 (s, 1 H, CH). C{1H} NMR (CD2Cl2): δ/ppm = 167.2 (OCO); 143.5-126.1 (Ph); 59.1 (CH). Nb
NMR (CDCl3): δ/ppm = 234 (½ = 2.7∙103 Hz).
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B) Formation of CHPh2COCl. A solution of CH(Ph)2COOH (269 mg, 1.27 mmol) in CD2Cl2 (2 mL)
was added of WCl6 (251 mg, 0.633 mmol). A light red solution formed after 18 hours, which was
analyzed by NMR spectroscopy. The analysis clearly evidenced the formation of CH(Ph)2C(O)Cl,[42] in
13
admixture with a secondary, non-identified species [1H: δ/ppm = 7.50-7.38 (Ph), 5.40 (CH); C{1H}:
δ/ppm 182.3 (C=O), 136.4 (ipso-Ph), 129.1, 128.8, 128.2 (Ph), 57.2 (CH)]. The solution was dried under
vacuum, then the residue was characterized by IR spectroscopy. IR (solid state): ν/cm = 1702s, 1659s,
1599s, 1581s.
Reactions of WCl6 with 4-bromo-2,2-diphenylbutyric acid: synthesis of 3,3-diphenyldihydrofuran-
2(3H)-one, 7, (Chart 3).[43]
Chart 3. Structure of 7.
The title compound was obtained by a procedure similar to that described for 5a, from WCl6 (188 mg,
0.474 mmol) and CPh2(CH2CH2Br)COOH (303 mg, 0.949 mmol), reaction time: 26 h. The product was
isolated as colourless crystals upon crystallization from a diethyl ether/pentane mixture, at 30 °C.
Yield 197 mg, 72%. Anal. Calcd for C16H14O2: C, 80.65; H, 5.92. Found: C, 79.75; H, 5.80. IR (solid
state): ν/cm = 3056w, 2988w, 2912w, 1762vs (C=O), 1596w, 1493m, 1446m, 1369m, 1340w, 1322w,
1226w, 1207w, 1160s, 1090w-m, 1021s, 1000w-m, 954m, 916w, 897w, 834w, 772m, 753s, 725w,
698vs, 675m. 1H NMR (CDCl3): δ/ppm = 7.44-7.29 (10 H, Ph); 4.25 (t, 2 H, J = 6.36 Hz, Ph2CCH2);
2.98 (t, 2 H, J = 6.36, 6.85 Hz, CH2Br). 13C{1H} NMR (CDCl3): δ/ppm = 177.8 (C=O); 140.8 (ipso-Ph);
128.8, 127.7, 127.6 (Ph); 65.3 (Ph2CCH2); 56.5 (CPh2); 37.5 (CH2Br).
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Reaction of MeC≡CCOOH with WCl6: synthesis of MeC(Cl)=CHCOOH, 8. MeC≡CCOOH (85 mg,
1.01 mmol) and WCl6 (415 mg, 1.05 mmol) were allowed to react in dichloromethane (30 mL) at room
temperature for 7 days. During this period, the progress of the reaction was evaluated by NMR analysis
performed on aliquots of the reaction mixture. The final dark red/brown solution was treated with water.
The organic phase was then recovered by filtration, and after removal of the volatiles, 8 was recovered
as a dark brown solid. Yield 88 mg, 72%. Anal. Calcd. for C4H5ClO2: C, 39.86; H, 4.18; Cl, 29.41; O,
26.55. Found: C, 39.71; H, 4.23; Cl, 29.11; O, 26.38. 1H NMR (CDCl3): δ/ppm = 9.2 (br); 6.14, 6.09 (s,
1H, CH cis + trans); 2.62, 2.33 (s, 3 H, Me cis + trans). 13C{1H} NMR (CDCl3): δ/ppm = 170.0, 169.2
(C=O cis + trans); 155.79, 149.50 (=CCl cis + trans); 118.74, 116.35 (=CH cis + trans); 28.73, 24.17
(Me cis + trans).
X-ray quality crystals of 8 were collected from a concentrated solution of the crude product in hexane,
stored at 30°C.
Gas chromatographic analyses.
A mixture of metal compound (ca. 1 mmol) and the appropriate carboxylic acid (1 eq.) in CH2Cl2 (10
mL) was stirred at ambient temperature for 72 hours in a Schlenk tube tapped with a silicon stopper. An
aliquot of the reaction atmosphere was withdrawn by a 1 mL syringe through the stopper and injected
into the GC instrument. The yields of CO formation were estimated based on analyses of gaseous
standard mixtures containing known amounts of CO. From MoCl5/CPh3COOH: 45%; from
NbF5/CPh3COOH: 38%; from NbCl5/CPh3COOH: 39%; from TiF4/CPh3COOH: 9.4%;
WCl6/CPh2(CH2CH2Br)COOH: 3.1%; from WCl6/CHPh2COOH: 0%.
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X-ray Crystallographic studies. Crystal data and collection details for 1, 2, 4ꞏ2CH2Cl2, 6, 8, A1 and
A2 are reported in Table 4. The diffraction experiments were carried out on a Bruker APEX II
diffractometer equipped with a CCD detector (1, 2, 4ꞏ2CH2Cl2) or a PHOTON100 detector (6, 8, A1,
A2) using Mo-K radiation. Data were corrected for Lorentz polarization and absorption effects
(empirical absorption correction SADABS).[44] Structures were solved by direct methods and refined by
full-matrix least-squares based on all data using F2.[45] Hydrogen atoms were fixed at calculated
positions and refined by a riding model. One CH2Cl2 molecule of 4ꞏ2CH2Cl2 is disordered and, thus, it
has been split into two positions and refined isotropically using one occupancy factor per disordered
group. The CH2Cl2 molecules of 4ꞏ2CH2Cl2 have been restrained to have similar geometries (SAME
line in SHELXL; s.u. 0.02) and their C−Cl distances have been restrained to 1.75 Å (s.u. 0.02).
Table 4. Crystal data and experimental details for [CPh3][MoOCl4], 1, [CPh3][NbF6], 2, [CPh3][Ti2Cl8(2-O2CCPh3)],
4ꞏ2CH2Cl2,
A2.
, 6, MeC(Cl)=CHCOOH, 8, CPh2(CH2CH2Br)COOH, A1 and CPh2(CH2CH3)COOH,
1
C19H15Cl4MoO
497.05
2
C19H15F6Nb
450.22
4ꞏ2CH2Cl2
C41H34Cl12O2Ti2
1079.88
6
C16H14O2
238.27
A1
C16H15BrO2
319.19
A2
C16H16O2
240.29
8
C4H5ClO2
120.53
Formula
Fw
0.71073
100(2)
Monoclinic
P21/n
8.9934(15)
19.335(3)
11.3379(19)

1949.6(6)
4
0.71073
100(2)
Monoclinic
P21/c
8.5095(15)
12.681(2)
16.036(3)

1725.9(5)
4
0.71073
100(2)
Monoclinic
P21/n
17.897(3)
10.5524(18)
25.825(5)

4666.8(14)
4
0.71073
100(2)
Monoclinic
Pc
7.3476(6)
15.2022(11)
10.9707(9)
91.960(3)
1224.71(17)
4
0.71073
100(2)
Orthorhombic
Pccn
14.500(3)
26.776(6)
7.1309(15)
90
2768.6(10)
8
0.71073
100(2)
Monoclinic
P21/n
9.062(9)
14.647(14)
9.918(10)
96.13(2)
1309(2)
4
0.71073
292(2)
Monoclinic
P21/n
5.387(15)
9.02(3)
11.16(3)
101.87(5)
531(3)
, Å
Temperature, K
Crystal system
Space group
a, Å
b, Å
c, Å


Cell volume, Å3
Z
4
Dc, g cm
3
1.693
1.225
1.733
0.757
1.537
1.064
1.292
0.084
1.532
2.964
1.219
0.079
1.509
0.597
, mm
1
F(000)
988
896
2176
504
1296
512
248
2.100 - 26.996
2.050 - 27.000
1.234 - 25.026
1.339 - 25.998
1.597 - 24.997
2.490 - 27.990
2.929 - 24.999
 limits, 
Reflections
collected
21352
18372
43028
14249
17881
14653
3721
Independent
reflections
4254
[Rint = 0.0254]
3763
[Rint = 0.0335]
8248
[Rint = 0.0717]
4780
[Rint = 0.0446]
2442
[Rint = 0.0841]
3146
[Rint = 0.0291]
925
[Rint = 0.2230]
Data / restraints /
parameters
Goodness on fit
on F2
4254 / 0 /226
1.104
3763 / 0 / 235
1.044
8248 / 27 / 512
1.045
4780 / 2 / 325
1.070
2442 / 24 / 173
1.247
3146 / 0 / 165
1.081
925 / 0 / 66
1.081
0.0227
0.0560
0.0266
0.0647
0.0617
0.1896
0.0489
0.0995
0.0590
0.1285
0.0458
0.1114
0.1338
0.3275
R1 (I> 2(I))
wR2 (all data)
Largest diff. peak
and hole, e.Å-3
0.528 / -0.587
0.365 / -0.458
2.805 / -1.361
0.192 / -0.272
0.867 / -0.805
0.382 / -0.233
0.550 / -0.511
20

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Acknowledgement
We thank the University of Pisa (Fondi di Ateneo) for funding. We are grateful to Mr. Francesco del
Cima and Dr. Debora Preti for the execution of GC analyses.
Supporting Information. Spectroscopic data of carboxylic acids, figures of the IR and NMR spectra of
products. X-ray structures of 6, 8, A1 and A2. CCDC reference numbers 1891866 (1), 1891867 (2),
1891868 (4ꞏ2CH2Cl2), 1891869 (6), 1891870 (8), 189186671 (A1) and 1891872 (A2) contain the
supplementary crystallographic data for the X-ray studies reported in this paper. These data can be
obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12, Union Road, CambridgeCB2 1EZ, UK; fax: (internat.) +44-1223/336-
033; e-mail: deposit@ccdc.cam.ac.uk).
References and Notes
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Afewerki and A. Còrdova, Chem. Rev., 2016, 116, 13512-13570. (c) M. M. Rahman, M. D. Smith,
J. A. Amaya, T. M. Makris and D. V. Peryshkov, Inorg. Chem., 2017, 56, 11798-11803. (d) M.
Bortoluzzi, E. Ferretti, M. Hayatifar, F. Marchetti, G. Pampaloni and S. Zacchini, Eur. J. Inorg.
Chem., 2016, 3838-3845. (e) H. Tsurugi, H. Tanahashi, H. Nishiyama, W. Fegler, T. Saito, A.
Sauer, J. Okuda and K. Mashima, J. Am. Chem. Soc., 2013, 135, 5986-5989.
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