N-tert-butoxycarbonyl (BOC) protected [V6O13{(OCH2)3CNH2}2]2?: synthesis, structural characterization, and solution behavior

Article abstract of DOI:10.1080/00958972.2020.1830074

Herein, we report the synthesis of N-tert-butoxycarbonyl (BOC) protected [V₆O₁₃{(OCH₂)₃C–NH₂}₂]^2? built from the Lindqvist-type hexavanadate. The reaction of di-tert-butyl dicarb

Full text of DOI:10.1080/00958972.2020.1830074

Journal of Coordination Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
N-tert-butoxycarbonyl (BOC) protected
2
−
[
V O {(OCH ) CNH } ] : synthesis, structural
6 13 2 3 2 2
characterization, and solution behavior
Ibrahima Fa Bamba , Clément Falaise , Gildas K. Gbassi , Patrick Atheba ,
Mohamed Haouas & Emmanuel Cadot
To cite this article: Ibrahima Fa Bamba , Clément Falaise , Gildas K. Gbassi , Patrick
Atheba , Mohamed Haouas & Emmanuel Cadot (2020): N-tert-butoxycarbonyl (BOC) protected
2
−
[
V O {(OCH ) CNH } ] : synthesis, structural characterization, and solution behavior, Journal of
6 13 2 3 2 2
Coordination Chemistry, DOI: 10.1080/00958972.2020.1830074
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2020.1830074
N-tert-butoxycarbonyl (BOC) protected
2
ꢀ
[
V O f(OCH ) CNH g ] : synthesis, structural
6
13
2 3
2 2
characterization, and solution behavior
a
a
b
b
Ibrahima Fa Bamba , Cl ꢀe ment Falaise , Gildas K. Gbassi , Patrick Atheba ,
a
a
Mohamed Haouas and Emmanuel Cadot
a
b
Institut Lavoisier de Versailles, CNRS, UVSQ, Universit ꢀe Paris-Saclay, Versailles, France; UFR Sciences
Pharmaceutiques et Biologiques (UFR SPB), Universit ꢀe F ꢀe lix Houphouet Boigny (UFHB), Abidjan,
C ^o te d’Ivoire
ABSTRACT
ARTICLE HISTORY
Herein, we report the synthesis of N-tert-butoxycarbonyl (BOC)
Received 10 July 2020
Accepted 25 September 2020
2ꢀ
protected [V O f(OCH ) C–NH g ] built from the Lindqvist-type
6
13
2 3
2 2
2
hexavanadate. The reaction of di-tert-butyl dicarbonate (BOC O)
KEYWORDS
with tris(hydroxymethyl)aminomethane (Tris) led to the organic
Polyoxovanadate; hybrid;
X-ray structure; NMR;
diffusion ordered
derivative [(OCH ) C–NH(BOC)] that reacts with the decavanadate
2
3
in dimethylacetamide (DMA) to form the [V
6
O
13f(OCH
2 3
)
2ꢀ
þ
C–NH(BOC)g ]
anion. The tetrabutylammonium (TBA ) salt of
2
spectroscopy; BOC
this hybrid polyoxovanadate, TBA
DMA, has been characterized in the solid-state by single-crystal
X-ray diffraction and infrared spectroscopy, and in solution by
2 6 2 3
[V O13f(OCH ) C–NH(BOC)g
2
]ꢁ
2
1
13
15
51
multinuclear ( H,
C,
N
and
V) and DOSY NMR, and
UV–visible spectroscopy.
CONTACT Emmanuel Cadot
emmanuel.cadot@uvsq.fr
Supplemental data for this article can be accessed online at https://doi.org/10.1080/00958972.2020.1830074.
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
I. FA BAMBA ET AL.
1. Introduction
The need of redox-active molecular components to develop functional systems rele-
vant in the field of energy has led to a renewal of interest for the chemistry of the
polyoxovanadates (POVs). POVs, that represent a sub-class of the polyoxometalates
(
POMs), are anionic molecular oxides exhibiting a rich structural chemistry due to the
IV
V
ability of the vanadium to adopt different oxidation states (V and V ) and coordin-
ation geometries such as [VO ], [VO ], or [VO ] [1, 2]. The growing interest for POVs is
4
5
6
mainly due to their striking redox and catalytic properties making POVs particularly
appealing for both fundamental and applied studies related to redox-flow batteries
[3–6], lithium or sodium-ion batteries [7–10], water oxidation catalysis [11, 12], or
photocatalysis [13]. Recently, it was also demonstrated that POVs constitute excellent
models to study the intriguing properties of vanadium solid-state oxides [14–16].
Another attractive feature of POVs is their propensity to form a wide variety of
covalent organic-inorganic hybrids, offering a way to i) tune the POV properties and ii)
integrate POVs into functional systems or materials [17, 18]. The most studied hybrid
POVs are the bis(triol-ligands)-substituted Lindqvist-type hexavanadates, trans-
2
ꢀ
[
V O f(OCH ) CRg ] , that exhibit fascinating redox [19, 20], supramolecular [21–24],
6
13
2 3
2
and photochemical [25] properties. In the early 1990s, the pioneering work of Zubieta
demonstrated that reaction of various triol-ligands with decavanadate anion [V O ]
6ꢀ
10 28
2
ꢀ
in acetonitrile led to the formation of [V O f(OCH ) C–Rg ] with various functional
6
13
2 3
2
groups R ¼ NO , CH OH, CH [19, 26, 27]. Later, M u€ ller reported the pentaerythritol
2
2
3
derivative (R ¼ CH OH) can be also prepared in high yield from aqueous mixtures [28].
2
2
ꢀ
The [V O f(OCH ) C–CH OHg ] anion has been used as a platform for various post-
6
13
2 3
2
2
functionalizations [29, 30]. The use of –NH as functional group is also convenient for
2
post-functionalization of POMs [17, 21, 22, 31, 32], however, it is difficult to obtain
2
ꢀ
[
V O f(OCH ) C–NH g ]
by reacting tris(hydroxymethyl)aminomethane (Tris) with
6
13
2 3
2 2
vanadate precursors. Indeed the direct reaction of Tris with decavanadate in dry dime-
thylacetamide (90 C overnight) led to the desired compound with a very low yield
ꢂ
(
<6%) [21]. To tackle this issue, the group of Wei successfully isolated the
2
ꢀ
[V O f(OCH ) C–NH g ] anion in good yield (53%) employing a two-step process
6
13
2 3
2 2
strategy. First, the amino group is protected with 4-anisaldehyde prior to grafting reac-
tion and then hydrolysis of formed imine is achieved to recover the amine func-
tion [33].
Alternatively, N-tert-butoxycarbonyl group (BOC) is now ranked as “one of the most
commonly used protective groups for amines” [34]. Such BOC-based methodology
derives from that reported by Wei et al. but in this case, the organic precursor corre-
sponding to the BOC-protected Tris compound is synthesized straightforwardly in
methanol through a one-pot procedure leading to a 100% yield while the oxazolidine
derivatives were obtained from toluene with a lower yield (about 60%) after purifica-
tion [35]. Then, we show that synthesis methodology based on BOC protecting group
corresponds to an efficient and straightforward route to get the Tris-substituted
2
ꢀ
Lindqvist POV [V O f(OCH ) C–NH g ] in good yield (50%). The novel hybrid anion,
6
13
2 3
2 2
2
ꢀ
þ
[
V O f(OCH ) C–NH(BOC)g ] , isolated as tetra-n-butylammonium (TBA ) salt has
6
13
2 3
2
been characterized by single-crystal X-ray diffraction, infrared, UV–vis and multinuclear
1
13
51
NMR ( H, C, and V) spectroscopy.

JOURNAL OF COORDINATION CHEMISTRY
3
2
. Experimental
.1. Materials
All chemicals were used as received without any further treatment. The precursors
TBA) [H V O ] and tert-butyl N-[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]carba-
2
(
3
3 10 28
mate were synthesized and characterized following published procedures [36, 37].
2
.2. Preparation of TBA [V O f(OCH ) C–NH(BOC)g ]ꢁ2DMA
2
6
13
2 3
2
TBA [H V O ] (6.24 g, 3.7 mmol) and tert-butyl N-[1,3-dihydroxy-2-(hydroxymethyl)-
3
3 10 28
propan-2-yl]carbamate (2.49 g, 11.2 mmol) were dissolved in 75 ml of N,N-dimethylace-
ꢂ
tamide (DMA). The resulting orange solution was heated at 80 C for 48 h. After
cooling to room temperature, slow diffusion of the solution (red-orange) in ether led
to the formation of 5 g of red needles of TBA [V O f(OCH ) C–NH(BOC)g ]ꢁ2DMA
2
6
13
2 3
2
(
notated V -Tris-BOC) which are then isolated by filtration and dried in air. Yield: 50%
6
based on vanadium. Anal. Calcd. (found) for C H N O V : C, 42.4(42.3); H, 7.7(7.4);
56 122 6 25 6
N, 5.3(5.1); V, 19.3(18.9).
2
.3. Preparation of Na [V O f(OCH ) C–NH(BOC)g ]ꢁ6H O
2
6
13
2 3
2
2
Five hundred milligrams of TBA [V O f(OCH ) C–NH(BOC)g ]ꢁ2DMA (0.31 mmol) were
2
6
13
2 3
2
dissolved in 10 ml of acetonitrile. Then this reddish solution was dropped into 5 mL of
acetonitrile containing 2 g of sodium perchlorate hydrate (16 mmol) under stirring.
Immediately after the addition of the first drops, a dark orange precipitate formed.
The
resulting
mixture
is
stirred
for
2 h.
350 mg
of
Na [V O f(OCH ) C–NH(BOC)g ]ꢁ6H O (0.27 mmol) is recovered by centrifugation
2
6
13
2 3
2
2
(5000 rpm) and washed with acetonitrile and diethyl ether. Yield: 87% based on van-
adium. Anal. Calcd. (found) for C H N Na O V : C, 19.7(18.6); H, 4.0(4.25); N, 2.5(2.6);
18
44
2
2 29 6
Na, 4.16(3.97); V, 27.7(25.7).
2
.4. Preparation of [V O f(OCH ) C–NH )g ]ꢁ2H O
6
13
2 3
3
2
2
Three hundred and fifty milligrams of Na [V O f(OCH ) C–NH(BOC)g ]ꢁ6H O was dis-
2
6
13
2 3
2
2
solved in 15 mL of water, then 5.5 mL of concentrated HCl (37%) were slowly added,
provoking formation of a reddish precipitate. After one day of stirring, the resulting
solid is recovered by centrifugation and washed twice with acetonitrile. The dried sol-
ids were added to about 10 mL of DI water and ammonia aqueous solution (25%) was
added to obtain a clear solution (pH ꢃ 9). The evaporation of this solution led to the
crystallization of [V O f(OCH ) C–NH )g ]ꢁ2H O. Single-crystal X-ray diffraction reveals
6
13
2 3
3
2
2
that the crystal exhibits the same unit cell as the compound reported previously by
Wei and co-workers [33].
2.5. Single-crystal X-ray diffraction
A crystal of V -Tris-BOC was selected under polarizing optical microscope and glued
6
in paratone oil. X-ray intensity data were collected at low temperature (T ¼ 220(2) K)

4
I. FA BAMBA ET AL.
Table 1. Crystal data and structure refinement for V -Tris-BOC.
6
Empirical formula
58 122 6 25 6
C H N O V
Formula weight
Temperature/K
Crystal system
Space group
a/Å
1607.23
220
Monoclinic
P2 /c
1
14.8061(3)
10.8895(3)
24.3085(6)
90
b/Å
c/Å
ꢂ
a/
ꢂ
b/
101.0590(10)
ꢂ
c/
90
3846.50(16)
2
3
Volume/Å
Z
q
3
calcg/cm
1.388
F(000)
Crystal size/mm
Radiation
1696
3
0.16 ꢄ 0.14 ꢄ 0.12
MoKa (k ¼ 0.71073)
4.674 to 55.77
ꢂ
2H range for data collection/
Index ranges
ꢀ19 ꢅ h ꢅ 19
ꢀ
14 ꢅ k ꢅ 14
ꢀ
31 ꢅ l ꢅ 31
Reflections collected
81909
Independent reflections
9131
R
R
int ¼ 0.0399
sigma ¼ 0.0192
9131/0/440
1.083
Data/restraints/parameters
2
Goodness-of-fit on F
Final R indexes [I ꢆ 2r (I)]
Final R indexes [all data]
Largest diff. peak/hole /e Å
R
wR
R
wR
1
¼ 0.0501
¼ 0.1364
¼ 0.0653
¼ 0.1541
2
1
2
-3
0.69/ꢀ0.45
on a Bruker D8 VENTURE diffractometer equipped with a PHOTON III C14 using a high
brilliance IlS microfocus X-ray Mo Ka monochromated radiation (k ¼ 0.71073 Å). Data
reduction was accomplished using SAINT V7.53a. The substantial redundancy in data
allowed a semi-empirical absorption correction (SADABS V2.10) to be applied, on the
basis of multiple measurements of equivalent reflections. Using Olex2 [38], the struc-
ture was solved with the ShelXT [39] structure solution program using Intrinsic
Phasing and refined with the ShelXL [40] refinement package using least squares mini-
mization. The remaining non-hydrogen atoms were located from Fourier differences
and were refined with anisotropic thermal parameters. Positions of the hydrogens
2
ꢀ
þ
belonging to [V O f(OCH ) C–NH(BOC)g ] , TBA or DMA were calculated and
6
13
2 3
2
refined isotropically using the gliding mode. Crystallographic data for single-crystal X-
ray diffraction studies are summarized in Table 1. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.
uk/structures-beta/. Deposit number: 2006943.
2.6. NMR studies
1
13
1
15
1
51
The H, Cf Hg, Nf Hg, and V NMR solution spectra were recorded in 5 mm o.d.
tubes with a Bruker Avance 400 spectrometer equipped with a BBI probehead.
Chemical shifts are referenced to external 1% Me Si in CDCl ( H and C), MeNO2
1
13
4
3
15
51
(
N) and 90% VOCl3 in C D6 ( V). Translational diffusion measurements were
6

JOURNAL OF COORDINATION CHEMISTRY
5
2ꢀ
Figure 1. Synthetic pathway leading to [V O f(OCH ) C–NH(BOC)g ]
.
6
13
2 3
2
performed using Bruker’s “ledbpgs2s” stimulated echo DOSY pulse sequence including
bipolar and spoil gradients. Apparent diffusion coefficients were obtained using an
adapted algorithm based on the inverse Laplace transform stabilized by maximum
entropy [41].
2.7. Fourier transform infrared
Fourier transform infrared (FT-IR) spectra were recorded on a 6700 FT-IR Nicolet spec-
trophotometer using diamond ATR technique. The spectra were recorded on non-
ꢀ
1
diluted compounds from 400 to 4000 cm . ATR correction was applied.
2.8. UV–vis spectra
The UV–vis spectra of solutions were recorded on a Perkin-Elmer Lambda-750 using
calibrated 0.2 cm Quartz-cell.
2.9. Elemental analysis
C, H, and N elemental analyses were carried out by BioCIS-UMR 8076 Service
Chromato-Masse Microanalyse Facult ꢀe de Pharmacie Tour D5-5 ꢁe me ꢀe tage 5 avenue JB
^
Cl ꢀe ment 92290 CHATENAY-MALABRY. Na and V contents were determined by
Inductively Coupled Plasma optical emission spectroscopy (ICP-AES) using an Agilent
720 Series with axially-viewed plasma and Na/V calibration curves.
3
. Results and discussion
.1. Synthesis
The hybrid hexavanadate [V O f(OCH ) C–NH(BOC)g ] was prepared in two steps
3
2
ꢀ
6
13
2 3
2
(Figure 1). The first one consists in the preparation of the tert-butyl N-[1,3-dihydroxy-
2
-(hydroxymethyl)propan-2-yl]carbamate that can be easily prepared in very good
yield (100%) from the reaction of tert-butyloxycarbonyl with di-tert-butyl dicarbonate
in methanol [37]. The second synthetic step is inspired by the procedures allowing the
formation of the various hybrid Lindqvist-type POV involving the reaction of the deca-
vanadate (TBA) [H V O ] with the desired Triol-ligands in hot N,N-dimethylacetamide.
3
3 10 28

6
I. FA BAMBA ET AL.
2
ꢀ
Figure 2. Structural view of [V O f(OCH ) C–NH(BOC)g ] . Orange polyhedron: VO ; red sphere:
6
13
2 3
2
6
oxygen; black sphere: carbon; blue sphere: nitrogen; light pink sphere: hydrogen.
Adapting this synthetic route, we prepared in good yield (50%) the desired com-
pound, as demonstrated by structural characterization, FT-IR spectroscopy, and multi-
nuclear NMR in solution.
3.2. Crystal structure
(
TBA) [V O f(OCH ) C–NH(BOC)g ]ꢁ2DMA, notated V -Tris-BOC, crystallizes in the cen-
2
6
13
2 3
2
6
trosymmetric space group P2 /c and the asymmetric unit contains half of the hybrid
1
2
ꢀ
þ
hexavanadate [V O f(OCH ) C–NH(BOC)g ] , one TBA cation, and one dimethylace-
6
13
2 3
2
tamide molecule of crystallization. The structure of the [V O f(OCH ) C–N
6
13
2 3
2
ꢀ
H(BOC)g ] anion, shown in Figure 2, corresponds to the archetypical Lindqvist-type
2
polyanion in which the six vanadium centers form an octahedron and two trialkoxo
ligands occupy the opposite faces of the octahedron in trans-mode. An inversion cen-
ter is located at the central m -oxygen atoms of the POV. Within the inorganic core
6
8
ꢀ
fV O g , each vanadium has a distorted octahedral geometry in which the shortest
6
19
t
and longest V–O distances are observed with the terminal oxygen
O
(
1.600(2)–1.606(2) Å) and the central m -oxygen atom (2.2251(4)–2.2492(4) Å), respect-
6
ively. The bond length between the vanadium ions and bridging m -oxygen is
2
1
.802(2)–1.854(2) Å, while the V–O distances observed with the alkoxy bridging m -oxy-
2
gen is slightly longer (2.000(2)–2.038(2) Å). The geometric parameters of the inorganic
core in V -Tris-BOC are similar to those previously observed in equivalent Tris-deriva-
6
2
ꢀ
tive hexavanadate [V O f(OCH ) CNH ] [33].
6
13
2 3
2
The three-dimensional cohesion is insured mainly by a network of short contacts
2ꢀ
and hydrogen bonds (Figure 3). The inorganic core of [V O f(OCH ) C–NH(BOC)g ]
6
13
2 3
2
is surrounded by four tetrabutylammonium cations interacting through numerous
short contacts (d
¼ 2.38–2.80 Å) between the hydrogens of alkyl tails and the
C-HꢁꢁꢁO–V
8
ꢀ
oxygens located at the surface of the fV O g motif (Figure 3(A)). Such an organiza-
6
19
tion involves formation of a neutral chain fTBA [V O f(OCH ) C–NH(BOC)g ]g along
2
6
13
2 3
2
8
ꢀ
the b axis where the fV O g motifs are separated by a pair of TBA cations. Such
6
19
chains are connected in the ab plane by intercalated solvent molecules (DMA) of crys-
tallization (two DMA per POV unit). Each DMA molecule interacts with one POV by
one hydrogen bond between the oxygen atom of the DMA and the N–H group of
POV (d
¼ 2.221(4) Å) and with another POV by short contact (d
¼
N–HꢁꢁꢁO–C
C–HꢁꢁꢁO–V
2.561(2) Å) (Figure 3(B)). From this organization, the formation of layers is stacked

JOURNAL OF COORDINATION CHEMISTRY
7
þ
Figure 3. A) Illustration of the short contacts occurring between POVs and TBA showing a pair of
þ
TBA connecting two POVs along the b axis. Dotted blue lines: short contacts between the hydro-
þ
gens of the alkyl chains of TBA and the oxygens located at the surface of the POV. The BOC
chain is omitted for clarity. B) Illustration of the supramolecular interaction between the POV and
the DMA. Dotted green lines: hydrogen bonds between the NH groups of the POV-Tris and the
oxygen atom of the DMA. Dotted blue lines: short contacts between hydrogens of DMA and ter-
minal oxygens of the POV. C) Illustration of the packing along the a axis, showing the stacking of
the layer along the c axis. To highlight the two different layers, two colors have been used for the
POVs. DMA molecules have been omitted for clarity.
ꢂ
along the c axis, with 180 rotation and a shift of b/2 from one to the other. The
2
ꢀ
organic tails of the [V O f(OCH ) C–NH(BOC)g ] anion point upward and downward
6
13
2 3
2
to two adjacent layer planes, generating van der Waals interactions between the layers
that ensure the 3D cohesion of the crystal structure (Figure 3(C)).
3.3. FT-IR spectroscopy
Figure S8 shows the FT-IR spectrum of (TBA) [V O f(OCH ) C–NH(BOC)g ]ꢁ2DMA. The
2
6
13
2 3
2
8
ꢀ
three characteristic features of the inorganic core fV O g are located at 949, 805,
6
19
ꢀ
1
ꢀ1
t
and 720 cm . The strong peak at 949 cm corresponds to the V ¼ O (terminal oxy-
b
gen atoms) vibration. The two other peaks are attributed to vibrations of V–O –V that
involves bridging oxygens of the metal-oxo framework. Numerous strong IR absorp-
tions of the organic part of the hybrid hexavanadate, the DMA and the TBA cations

8
I. FA BAMBA ET AL.
1
Figure 4. H NMR spectra of A) Na [V O f(OCH ) C–NH(BOC)g ] in D2O and B)
2
6
13
2 3
2
ꢇ
[
V O f(OCH ) C–NH g ] in D O/NaOD. ( ) denotes residual BOC (7%).
6
13
2 3
3
2
2
ꢀ
1
are observed at 1000–1650 cm , however, the identification of all peaks is challeng-
ꢀ
1
ing. The broad C ¼ O stretching peaks at 1706 and 1642 cm can be attributed to the
amide groups of the ligand f(OCH ) C–NH(BOC)g and the DMA, respectively. The
2
3
ꢀ
1
peaks at 1472, 1400, and 1370 cm are assigned to asymmetric and symmetric defor-
mations of the C–H of the methyl and methylene groups of the TBA cation, the ligand,
and the DMA. Asymmetric and symmetric vibrations of C–H are observed at 2873
ꢀ
1
and 2964 cm
.
3.4. NMR spectroscopy
1
13
1
(
TBA) [V O f(OCH ) C–NH(BOC)g ]ꢁ2DMA has been characterized by H, Cf Hg, and
2
6
13
2 3
2
5
1
51
V NMR in acetonitrile (ACN). The V NMR spectrum (Supporting Information, Figure
S1) exhibits a single resonance at ꢀ497 ppm (half-height line width d m1/2 ca. 400 Hz)
indicating a highly symmetric POV, consistent with a trans-coordination mode. Similar
2
ꢀ
result was observed with [V O f(OCH ) C–N(4-CONHC H N)g ]
(ꢀ495 ppm/DMSO-
6
13
2 3
5
4
2
1
d ) [20]. The H NMR spectrum (Supporting Information, Figure S1) confirms the graft-
6
ing of organic moieties on the surface of the POV showing the signals of methylenic
protons V -(OCH ) and methyl groups of Tris-BOC at 5.16 and 1.38 ppm, respectively.
6
2 3
Signals of TBA cations at 0.99, 1.45, 1.60, and 3.16 ppm, and of DMA crystallization
solvent at 1.98, 2.84, and 2.97 ppm are also identified. Integration of these signals
reveals molar ratios Tris-BOC/TBA/DMA ¼ 1/1/1, consistent with a global POV charge
of 2- and co-crystallization of 2 DMAC molecules with one POV unit as determined by
1
3
XRD analysis. C NMR spectrum (Supporting Information, Figure S2) shows the

JOURNAL OF COORDINATION CHEMISTRY
9
5
1
Figure 5. V NMR spectra of A) Na [V O f(OCH ) C–NH(BOC)g ] in D O and B) [V O f(OCH )
2
6
13
2 3
2
2
6
13
2 3
C–NH g ] in D O/NaOD.
3
2
2
resonances of Tris-BOC moieties at 28.3, 52.1, 83.7, 87.2, and 155.5 ppm. The overall
NMR data are in agreement with pure and stable compound in acetonitrile.
(
TBA) [V O f(OCH ) C–NH(BOC)g ]ꢁ2DMA is not soluble in aqueous solution.
2
6
13
2 3
2
þ
However, the exchange of counter cations TBA by sodium allows solubilizing the
hybrid POV Na [V O f(OCH ) C–NH(BOC)g ]ꢁ6H O in water. Hydrolysis of the peptidic
2
6
13
2 3
2
2
function is therefore achieved in acidic medium using HCl, which provokes an immedi-
ate precipitation. The resulting solid consists of the neutral protonated form,
1
[
V O f(OCH ) C–NH g ]. Figure 4(A) shows the H NMR spectra of Na [V O f(OCH )
6
13
2 3
3 2
2
6
13
2 3
C–NH(BOC)g ] and [V O f(OCH ) C–NH g ] in D O. The spectrum of
2
6
13
2 3
3 2
2
Na [V O f(OCH ) C–NH(BOC)g ] exhibits the signals of CH of Tris moieties at
2
6
13
2 3
2
2
5
.32 ppm and of the methyl groups of the BOC at 1.32 ppm. To verify that the BOC is
still attached to the POV upon its dissolution in aqueous solution, DOSY NMR is car-
ried out. The spectrum shown in Supporting Information (Figure S3) reveals that the
2
two organic parts (Tris and BOC) diffuse with the same rate at 295 mm /s. This proves
that the Tris–BOC is robust and remains intact in aqueous solution, and then acidic
protons are needed to hydrolyze the amide function. The neutral solid product
[
V O f(OCH ) C–NH g ] is insoluble in water and deprotonation of ammonium groups
6
13
2 3
3 2
is needed to solubilize it. Therefore, NMR measurements were conducted in basic
medium (NaOD/D O). The H NMR spectrum (Figure 4(B)) shows the signal of CH of
1
2
2
the Tris at 5.00 ppm and a small signal at ca. 1.2 ppm accounting for residual BOC.
DOSY NMR (Supporting Information, Figure S4) confirms the assignments where two
2
distinct diffusion rates are measured at 422 and 758 mm /s for the POV–Tris and BOC,
1
3
respectively. C NMR (Supporting Information, Figures S5 and S6) provides further
1
support
of
H
assignments
of
Na [V O f(OCH ) C–NH(BOC)g ]
and
2
6
13
2 3
2
[
V O f(OCH ) C–NH g ]. To provide definitive proof of hydrolysis of the amide func-
6
13
1
2 3
3 2
5
15
1
tion, N NMR has been carried out. The Nf Hg HMBC (Supporting Information,

1
0
I. FA BAMBA ET AL.
Figure 6. UV–vis spectra of TBA [V O f(OCH ) C–NH(BOC)g ]ꢁ2DMA in MeCN (black line) and
2
6
13
2 3
2
Na [V O f(OCH ) C–NH(BOC)g ]ꢁ6H O in water (red line).
2
6
13
2 3
2
2
Figure S7) reveals that the initial signal of the amide function in
2
ꢀ
[
V O f(OCH ) C–NH(BOC)g ] at ꢀ290.7 ppm shifts to ꢀ352.8 ppm, corresponding to
6
13
2 3
2
51
typical chemical shift ranges for amines. Finally, the V NMR spectra (Figure 5) of
Na [V O f(OCH ) C–NH(BOC)g ] and [V O f(OCH ) C–NH g ] are comparable, exhibit-
2
6
13
2 3
2
6
13
2 3
3 2
ing a broad resonance at ca. ꢀ507 ppm (half-height line widths d m ꢃ 500–1000 Hz).
1/2
This signature is typical of Lindqvist-type structure.
3.5. UV–vis spectroscopy
2
ꢀ
The UV–vis spectrum of [V O f(OCH ) C–NH(BOC)g ] has been measured in aceto-
6
13
2 3
2
þ
nitrile using the TBA salt. This spectrum exhibits two strong absorption bands at 262
and 365 nm (Figure 6), and the corresponding molar extinction coefficients are 20,300
and 8465 M cm , respectively. These values are in the same range as those
observed by Zubieta on various hexavanadates [19]. These absorption bands are
attributed to the oxygen to metal charge transfer transition. The UV–vis spectrum of
ꢀ
1
ꢀ1
2ꢀ
[
V O f(OCH ) C–NH(BOC)g ] has also been measured in aqueous media using the
6
13
2 3
2
sodium salt. The spectrum exhibits similar features even if a slight red-shift (þ10 nm)
is observed, indicating a solvent dependence of the absorptions position.
4. Conclusion
We have synthesized a new tris(hydroxymethyl)aminomethane (Tris) based functional-
ized Lindqvist-type hexavanadate, TBA [V O f(OCH ) C–NH(BOC)g ], using N-tert-
2
6
13
2 3
2
butoxycarbonyl (BOC) as protecting group of the Tris amine function. The compound
has been characterized by single-crystal XRD and FT-IR spectroscopy in the solid state

JOURNAL OF COORDINATION CHEMISTRY
11
as well as UV–vis and multinuclear NMR spectroscopies in solution. The sodium salt
obtained by cationic exchange is found highly soluble and stable in aqueous solution,
which allows deprotection of the amine group to obtain the dianion
2
ꢀ
[
V O f(OCH ) C–NH g ] . This new synthetic method offers a simplified procedure
6
13
2 3
2 2
for obtaining Tris-Lindqvist vanadate anion with good yield.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
Authors gratefully acknowledge financial support from LabEx CHARMMMAT (grant number ANR-
1
1-LBX-0039). This work was also supported by i) University of Versailles Saint Quentin, ii) CNRS,
iii) R ꢀe gion Ile de France through DIM Nano K. Ibrahima Fa Bamba thanks the Embassy of Ivory
Coast in France for his scholarship. M. Frigoli (ILV) is acknowledged for fruitful discussions.
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