Structural characterization of α-amino acid complexes of molybdates: A spectroscopic and DFT study

Article abstract of DOI:10.1039/c4ra14236e

The reactions of L-enantiopure α-amino acids (aaH) with Na₂MoO₄ led to the high-yield isolation of Mo(vi) complexes of general formula Mo₂O₄(OH)₄(aaH). A similar outcome was achieved by using (NH₄)₆Mo₇O₂₄ in the place of Na₂MoO₄. Solid-state IR and NMR spectroscopy indicated the presence of cis-MoO₂ units linked by a zwitterionic amino acid ligand, via a bidentate bridging coordination through the carboxylate group. Thus possible dinuclear and polynuclear structures are proposed on the basis of DFT calculations.

Full text of DOI:10.1039/c4ra14236e

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Structural characterization of a-amino acid
complexes of molybdates: a spectroscopic and DFT
Cite this: RSC Adv., 2015, 5, 9010
study†
Lorenzo Biancalana,a Marco Bortoluzzi,b Claudia Forte,c Fabio Marchettia
a
*
and Guido Pampaloni
The reactions of L-enantiopure a-amino acids (aaH) with Na2MoO4 led to the high-yield isolation of Mo(VI)
complexes of general formula Mo2O4(OH)4(aaH). A similar outcome was achieved by using (NH4)6Mo7O24
in the place of Na2MoO4. Solid-state IR and NMR spectroscopy indicated the presence of cis-MoO2 units
linked by a zwitterionic amino acid ligand, via a bidentate bridging coordination through the carboxylate
group. Thus possible dinuclear and polynuclear structures are proposed on the basis of DFT calculations.
Received 10th November 2014
Accepted 24th December 2014
DOI: 10.1039/c4ra14236e
www.rsc.org/advances
in mildly acidic conditions (4.5 < pH < 7), complex formation
was observed for those a-amino acids with an ionisable side-
Introduction
chain, however isolation in the solid state was limited to a
few cases.9,10 On the other hand, the products of the molyb-
date/a-amino acid interaction could be isolated more
favourably at lower pH values (pH ¼ 0–2).5a,9,11
Molybdenum is found in living organisms at the active site of
several enzymes involved in redox reactions, whose clear
understanding is still a big challenge for science.1 The main bio-
available form of molybdenum is the tetrahedral molybdate ion,
i.e. [MoO4]2ꢀ, which is stable and inert in non-acidic solutions,2
and presumed to have played a crucial role in biological
evolution.3
The study of the interaction between molybdenum compounds
and naturally-occurring molecules in aqueous solution is intended
to contribute to the coordination chemistry of Mo(VI) in water and
the related concerns, including the biological ones.4
On the other hand, a-amino acids are good candidates as
potential ligands, indeed their coordination to a variety of metal
ions has been extensively investigated.5 It is noteworthy that the
association of natural, chiral a-amino acids with water soluble,
biocompatible metal compounds may be a privileged way to
obtain potential catalysts for environmentally friendly asym-
metric synthesis.6
In this framework, the chemistry of a variety of Mo(VI)
species with a-amino acids in aqueous medium has aroused
huge interest.7 In particular, a series of studies were carried
out on complexation equilibria involving [MoO4]2ꢀ.8 Working
The exploration of this piece of chemistry bears an important
drawback: the products are insoluble materials whose crystallo-
graphic characterization still remains a hard, not achieved task.
As a matter of fact, the reported characterisations relied on
limited analytical and spectroscopic (IR) data, and for this reason
the structure of the Mo-oxido backbone and the amino acid
coordination mode have not been clearly elucidated hitherto.5a,9,11
In the light of this preface, we decided to deal with the reac-
tions of molybdates, i.e. Na2MoO4 and (NH4)6Mo7O24, with a
selection of a-amino acids (glycine, L-phenylalanine, L-leucine, L-
methionine, L-proline and N,N-dimethyl-L-phenylalanine), in
acidic aqueous solution. We present herein the results of our
investigation: a detailed structural characterization of the prod-
ucts has been possible for the rst time, based on solid-state
spectroscopic techniques (IR and NMR) and DFT calculations.
Results and discussion
Molybdenum(VI)/amino acid complexes (1–6, see Scheme 1)
were prepared from aqueous solutions of a-amino acids (aaH)
aUniversity of Pisa, Dipartimento di Chimica e Chimica Industriale, Via Moruzzi 3,
I-56124 Pisa, Italy. E-mail: guido.pampaloni@unipi.it; Web: http://www.dcci.unipi.
it/guido-pampaloni.html; fabmar@dcci.unipi.it; Tel: +39-050-2219219
bCa'Foscari University of Venice, Dipartimento di Scienze Molecolari e Nanosistemi,
Dorsoduro 2137, I-30123 Venezia, Italy
cIstituto di Chimica dei Composti OrganoMetallici, CNR, Area della Ricerca di Pisa, Via
Moruzzi 1, I-56124 Pisa, Italy
† Electronic supplementary information (ESI) available: Experimental details; 13C
CP-MAS NMR and IR spectra of 1–6 and related a-amino acids and a-ammonium
acid nitrates. The Cartesian coordinates of optimized geometries are collected in a
separated .xyz le. See DOI: 10.1039/c4ra14236e
Scheme 1 Synthesis of compounds 1–6.
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and a molybdate salt, either Na2MoO4 or (NH4)6Mo7O24, with undergo negligible shi. As an example, Fig. 1 reports the
Mo/aaH ¼ 2 : 1 (eqn (1)). The resulting mixtures were acidied case of L-leucine.13f
to pH ¼ 2, then the precipitation of neutral colourless solids 1–6
followed in variable times (73–92% yields).12
The solid-state 13C cross polarization with magic angle
spinning (CP-MAS) spectra of 1–6 showed one, two or three sets
of isotropic resonances attributed to the aaH ligand (see ESI,
Table S2†). The appearance of more than one signal for a given
carbon is presumably due to the presence of different molecular
or crystallographic environments.14
(1)
In addition, we recorded 13C CP-MAS spectra of the a-amino
acids employed in this work and of their nitrate salts, [aaH2]
[NO3], aaH ¼ GlyH, PheH, LeuH, MetH.15 The latter compounds
were obtained as crystalline materials by slow evaporation of the
solvent from aqueous solutions containing aaH and HNO3. A
comparison of the isotropic 13C chemical shis related to aaH
with the corresponding ones detected for the respective [aaH2]+
species evidenced variations that are analogous to those
occurring in D2O (see above).
As reported in Table S2,† chemical shis for the carboxylic-,
a- and b-carbons of 1–4 are very close to those found for the
corresponding ammonium acid nitrate (i.e., they are deshielded
if compared to aaH). The spectrum of 5 shows three groups of
resonances: one of these follows the general trend observed for
1–4, while the other two are only slightly shied with respect to
ProH. Chemical shi values for 6 are deshielded if compared to
dmPheH, with the exception of the a-carbon.
The reactions were investigated on varying the reaction
conditions (pH, Mo/aaH ratio, temperature, nature of molyb-
date salt and acid; see ESI†). As a matter of fact, the formation of
1–6 was generally found to be selective and high yielded.
Compounds 1–6 were isolated in good to high yields even by
using a large excess of aaH. The synthesis of 1 was effectively
performed also at pH ca. 0 (HNO3, Na2MoO4$2H2O, room
ꢁ
temperature) and at pH ¼ 1 (HCl, (NH4)2MoO4, ca. 100 C), in
spite of the pH-dependent equilibria affecting the molybdate
ion.2 In other terms, the same products were isolated from
+
distinct solutions containing different cations (Na+ or NH4 )
and anions (Clꢀ or NO3ꢀ); in addition, chlorine was not incor-
porated in the isolated materials when using HCl. All of these
features agree with the idea that 1–6 are neutral species.
In the case of the reaction between Na2MoO4 and glycine, we
carefully monitored the pH of the reaction solution under
different initial conditions (see ESI†); our data indicated a
stoichiometric H+/Mo ratio of 4 (eqn (2)).
It may be concluded that, in general, the coordination of the
zwitterionic a-amino acid to the metal centre, aaH / Mo2O4
(OH)4(aaH), mimics the electronic effects of protonation, aaH /
[aaH2]+. This feature supports the hypothesis that, in 1–6, the
a-amino acid actually acts as an a-ammonium-carboxylate ligand.
4H+ + 2[MoO4]2ꢀ + GlyH / Mo2O4(OH)4(GlyH)
(2)
Complexes 1–6, once isolated in the solid state, resulted
almost insoluble, both in H2O and in 1 M HNO3 or HCl, while a
stronger acidic medium (4 M HCl or HNO3) was generally
needed to obtain colourless solutions. Concerning organic
solvents, complexes 1–6 did not dissolve in MeOH, EtOH,
Me2CO and MeCN; instead they dissolved into yellow solutions
when treated with DMSO.
The generally observed insolubility of 1–6 prompted us to
their characterization by means of solid-state techniques.
As described in the following sections, spectroscopic (IR,
NMR) and analytical data indicated the general basic structure
[(MoO2(OH))2(m-OH)2(m-aaH-kO,kO0)]. This outcome has been
supported also by DFT calculations and appears independent
on the nature of the ammonium group [i.e. primary (GlyH,
PheH, LeuH, MetH), secondary cyclic (ProH) or tertiary
(dmPheH)]. The general coordination mode proposed for aaH is
shared also by 4 despite the additional, potential donor atom (S)
contained in MetH.
Solid-state IR characterisation
The oxido-molybdenum backbone. The stretching vibrations
of Mo]O bonds usually give rise to strong and characteristic
infrared bands.16 In fact the IR spectra of 1–6 showed strong
absorptions around 944, 915 and 902 cmꢀ1 (see ESI, Table S3†),
which are diagnostic for the cis-MoO2 group.17 The presence of
three stretching absorptions instead of two [i.e., nsym(MoO2) and
nasym(MoO2)] might be ascribed to a solid-state effect17a,g or to
the fact that more than one {MoO2} unit is present within a
polynuclear complex.18 A medium-intensity band around 760
cmꢀ1 has been assigned to the antisymmetric stretching of a
bent Mo–O–Mo bridge,16,17g,18,19 while the absorption around 470
cmꢀ1 is presumably due to the corresponding nsym(Mo2O).19a,b,20
Solid-state NMR characterisation
In general, 13C chemical shis of a-amino acids in D2O vary
with pH.13 More precisely, an upeld shi of 13C resonances has
been observed on going from the amino acidate anion (aaꢀ) to
the zwitterion (aaH) and then to the ammonium acid (aaH2+).
This effect13d is signicant for the carboxylic unit and the
adjacent carbons; conversely the remaining nuclei, included
those belonging to the side-chain and the N-substituents, Fig. 1 13C chemical shifts at different pH values for L-leucine in D2O.13f
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Moreover, the strong and broad band around 500 cmꢀ1
The computational investigation was initially aimed to
may be related to other types of bridging oxido groups ascertain possible dinuclear structures for 1 and 6. The
(e.g. Mo2(m-O)2),21,22 however Mo–Oligand stretching vibrations calculated structures were subjected to DFT optimization
may also contribute.17g,22,23
both in the gas-phase (EDF2 functional) and in the presence
The carboxylate group. The wavenumber difference between of implicit solvation for water (PBE functional with Grimme's
the anti-symmetric and the symmetric stretching vibration of a corrections for dispersion), using valence-double-z quality
carboxylate ligand (Dnaꢀs) has proven to be a useful parameter basis sets. The lowest energy structures are shown in Fig. 2,
in order to discriminate between monodentate, chelating and while selected bond distances and angles are reported in
bridging-bidentate coordination modes.24 More specically, the Table 1. The Cartesian coordinates are given as ESI† as a
separation between carboxylate stretching wavenumbers rela- separated .xyz le.
rel
tive to a reference ionic carboxylate compound; hence Dn
¼
Both structures are built up of dinuclear molecules
aꢀs
0
Dnaꢀs(metal complex) ꢀ Dnaꢀs (ionic reference).
[(MoO2(OH))2(m-OH)2(m-O2CCHRNHR2 -kO,kO0)] (R ¼ H, R0 ¼ H
rel
aꢀs
We found a good correlation between Dn
and the coor- for 1, R ¼ CH2Ph, R0 ¼ Me for 6). Each molybdenum is bonded
dination mode of a carboxylate ligand by analyzing the IR data to two terminal oxido and one terminal hydroxido ligands. The
reported for a variety of crystallographically-characterized two MoO2(OH) units are bridged by two hydroxido ligands and
rel
dioxidomolybdenum(VI) compleꢀx1es.25 Thus Dn
fall within the carboxylate group of the a-amino acid. Moreover, one
aꢀs
the range +140 cmꢀ1 to +60 cm in the case of monodentate bridging OH ligand and one carboxylic oxygen are trans to a
ligands,26 ꢀ40 cmꢀ1 to ꢀ60 cmꢀ1 for chelating ligands,27 and terminal oxido ligand. The coordination spheres around the Mo
+30 cmꢀ1 to ꢀ30 cmꢀ1 for bidentate bridging ligands.28,7d,7e,21a centres can be described as distorted octahedra, as observable
rel
As shown in Table S3,† Dn
values calculated for 1–6 are from the O Obridging–Mo–
terminal–Mo–Oterminal (<85ꢁ) and
aꢀs
distributed within the range +33 cmꢀ1 to ꢀ38 cmꢀ1 (zwitter- Obridging (105ꢁ) angles. Both features (trans inuence of oxido
ionic amino acids in the solid state have been taken as ionic ligands and distortion from octahedral coordination) are
references). According to the aforementioned empirical rule, commonly observed in dioxidomolybdenum(VI) complexes.35 An
this evidence supports the bidentate-bridging coordination of intramolecular hydrogen bond involving the ammonium and
the carboxylate moiety in 1–6.
the nearest Mo]O group is present in the dinuclear model for 1
The ammonium group and other considerations. In the while is missing in the analogous for 6: this seems to indicate
1400–1600 cmꢀ1 spectral region, the antisymmetric dasym that intermolecular N–H/O interactions are not essential for
(NH3+) and symmetric dsym(NH3+) bands of the ammonium the stability of this type of structures.
group are recognizable for 1–4.29 One deformation band is
As discussed above, the DFT-optimised structures of 1 and
observed for 5, as expected for a secondary ammonium ion.30 6 are similar to each other: this agrees with the homogeneous
In the spectrum of 6, the n(N–CH3) has been recognized.31 All spectroscopic data collected, as discussed in the previous
of these absorptions are slightly shied with respect to the sections. Concerning other possible structures, the formal
corresponding, uncoordinated zwitterionic amino acid replacement of the bridging hydroxido groups with oxido
(see Table S3†). In addition, the IR spectra of 1–4 resemble ligands caused, on theoretical grounds, the dissociation of
the spectra of the corresponding [aaH2]NO3 species, for what the carboxylate moiety and/or the migration of one H+ from
regards the C–H and N–H stretching region (2800–3300 the ammonium to one oxido. Also the presence of a single
cmꢀ1),32 see Fig. S7.† All of these observations agree with the bridging ligand, besides the amino acid, was ruled out by
presence of a protonated ammonium group in compounds DFT calculations, because geometry optimizations of several
1–6.
Mo–O–Mo and Mo–OH–Mo starting structures gave stationary
The deformation of the –SCH3 group gives rise to another points with two bridging OH ligands or structures having very
characteristic and intense band,33 falling at 1316 cmꢀ1 in the
spectrum of MetH and at 1334 cmꢀ1 in the spectrum of its
molybdenum complex, 4. A medium intensity absorption at
1333 cmꢀ1 was found in the IR spectra of two structurally
determined complexes of Nb(V) and Ta(V) bearing a L-methio-
nine ligand bonded to the metal centre through the amino and
the carboxylate groups.34 This suggests that the sulphur atom in
4 is not involved in coordination.
Theoretical calculations
In summary, analytical and spectroscopic characterization of 1–
6 highlighted: (i) an elemental composition coherent with the
empirical formula Mo2H4O8(aaH), (ii) the presence of terminal
Fig. 2 Lowest-energy DFT optimized dinuclear structures of 1 and 6
(cis-MoO2) and bridging oxido groups, (iii) a bidentate bridging
(gas-phase EDF2 calculations). White ¼ hydrogen, grey ¼ carbon, blue
coordination, via the carboxylate group, of the zwitterionic
¼ nitrogen, red ¼ oxygen, light blue ¼ molybdenum. Carbon-bonded
amino acid ligand.
hydrogen atoms have been omitted for clarity.
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Table 1 Selected bond distances (A˚) and angles (ꢁ) for the dinuclear
structures 1 and 6. Data computed using the Grimme-corrected PBE
functional in the presence of COSMO solvation model for water. Gas-
phase EDF2 data in parentheses
Atoms
1
6
Mo(1)–O(1)
Mo(2)–O(7)
Mo(1)–O(2)
Mo(2)–O(6)
Mo(1)–O(4)
Mo(1)–O(5)
Mo(2)–O(5)
Mo(2)–O(4)
Mo(1)–O(3)
Mo(2)–O(8)
Mo(1)–O(9)
Mo(2)–O(10)
O(1)–Mo(1)–O(2)
O(5)–Mo(1)–O(9)
O(4)–Mo(1)–O(5)
O(4)–Mo(2)–O(5)
1.735 (1.699)
1.731 (1.691)
1.743 (1.708)
1.760 (1.765)
2.221 (2.263)
2.096 (2.089)
2.217 (2.130)
2.067 (2.050)
1.942 (1.904)
1.936 (1.899)
2.422 (2.572)
2.385 (2.582)
105.5 (105.5)
85.9 (83.1)
1.742 (1.710)
1.740 (1.707)
1.740 (1.710)
1.741 (1.713)
2.312 (2.323)
2.072 (2.041)
2.304 (2.323)
2.085 (2.098)
1.954 (1.960)
1.949 (1.919)
2.269 (2.310)
2.298 (2.351)
106.3 (105.8)
77.8 (76.5)
68.1 (64.8)
68.6 (67.9)
75.5 (69.8)
75.4 (70.0)
high relative energies. Thus the Mo2(m-OH)2 core is strongly
suggested by theoretical calculations. Moreover, the coordi-
nation of water molecules to the Mo(VI) centres was very weak
in all of the cases considered here. The position of the
terminal OH ligands cis to the carboxylate groups resulted
favoured with respect to the trans conguration (DG ¼ ꢀ10.4
kcal molꢀ1 for 1, ꢀ4.3 kcal molꢀ1 for 6; see the ESI† .xyz le for
the 1-trans and 6-trans optimized geometries).
It should be mentioned that a series of X-ray determined
molybdates present a structural motif analogous to that
proposed for 1–6: cis-MoO2 groups bridged by two oxido and
one carboxylato unit derived from a zwitterionic ligand LH (a-,
b- or g-amino acids, or nicotinic acid).7d,7e,21,28 In such cases, the
resulting dinuclear frames, i.e. [Mo2O4(m-O)2(LH)], may
condense to three-units rings – in the presence of a central
heteroatom such as V(V), Se(IV) or As(III)7d,7e,28 – or to innite
oxido-bridged rail-like chains.21
Fig. 3 Lowest-energy DFT optimized polynuclear structures of 6 (6T,
6H and 6O) (PBE/Grimme/COSMO, in the presence of water as implicit
solvent). White ¼ hydrogen, grey ¼ carbon, blue ¼ nitrogen, red ¼
oxygen, light blue ¼ molybdenum. The substituents on the carboxyl-
ate-bonded carbons have been omitted for clarity.
Mo–O–Mo bridge is supported also by IR characterization (see
above).
Therefore, a theoretical approach was attempted by consid-
ering the polymerization of the dinuclear structures (Fig. 2).
Preliminary DFT calculations indicated the condensation of two
terminal OH groups as the only way to the formation of polymer
chains maintaining the dinuclear Mo2O4(m-OH)2(m-aaH-kO,kO0)
core.36 In order to restrict the inuence of N–H/O hydrogen
bonds during the research of the most stable minima, the
dinuclear structure optimized for 6 was selected as the starting
point. Fig. 3 shows the lowest-energy structures for condensa-
tion products containing four (6T), six (6H) and eight (6O) metal
centres. Selected ranges of bond distances and angles are
reported in Table 2, while the Cartesian coordinates are
collected in a separated .xyz le.†
On theoretical grounds, the condensation reactions leading
to polynuclear structures (see eqn (3) for aaH ¼ dmPheH and
n ¼ 1–3) appear thermodynamically favoured, as observable
from the DG variations (including solvation) reported in Table 3
for each condensation. This point suggests that 1–6 probably
possess, to some extent, a polymeric structure, in accordance
with their insolubility in water and in organic solvents. By
assuming further condensation reactions between Mo2
subunits (n > 3), an oxido-bridged 2D coordination polymer of
general formula {Mo2O4(OH)2(aaH)(m-O)}n$H2O would be
obtained.
Mo2 subunits in the polynuclear structures are held together
not only by a covalent oxygen bridge, but also by an intra-
molecular hydrogen bond between two OH groups, acting as
bridging ligands within distinct Mo2 frames. These interactions
are sketched in Fig. 4 for clarity. The presence of a single bent
HO–{Mo2O4(OH)2(aaH)(m-O)}n–H + Mo2O4(OH)4(aaH) /
HO–{Mo2O4(OH)2(aaH)(m-O)}(n+1)–H + H2O
(3)
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Table 2 Selected ranges of bond distances (A˚) and angles (ꢁ) for the
computed polynuclear structures of 6. PBE/COSMO calculations,
EDF2 data in parenthesesa
combination of experimental results with DFT calculations
has allowed the unprecedented structural identication of
the dinuclear core and the coordination mode of the a-amino
acid. Moreover, the presumable formation of coordination
polymers by condensation reactions has been discussed.
This paper contributes to clarify the structural features of a
series of compounds obtained by a simple reaction, arousing
great interest for the implications associated with the nature
of the reactants and the use of water as medium (see
Introduction).
Atoms
6T
6H
6O
Mo–O(a)
Mo–O(b)
Mo–O(c)
1.737–1.747
(1.708–1.723)
1.942–1.959
(1.923–1.950)
2.236–2.339
(2.237–2.401)
2.028–2.104
(2.013–2.099)
1.920–1.925
(1.887–1.915)
2.247–2.329
(2.311–2.358)
2.571
1.738–1.764
(1.703–1.741)
1.941–1.952
(1.923–1.956)
2.257–2.347
(2.268–2.427)
2.017–2.107
(1.979–2.099)
1.921–1.937
(1.883–1.955)
2.228–2.327
(2.285–2.345)
2.548–2.828
(2.598–2.724)
103.5–105.5
(104.1–105.6)
71.6–74.4
1.738–1.763
1.949–1.951
2.250–2.358
2.014–2.125
1.912–1.953
2.243–2.321
2.579–2.719
103.5–106.7
71.4–74.1
Mo–O(d)
Experimental
General
Mo–O(e)
O(c)/O(c) (H-bond)
O(a)–Mo–O(a)
O(c)–Mo–O(c)
Mo–O(d)–Mo
All manipulations were performed in air with common labo-
ratory glassware. Reactions were carried out using deionised
water. The following reagents and solvents were used as
received: Na2MoO4$2H2O, (NH4)6Mo7O24$4H2O, CaH2 (Alfa
Aesar), glycine (GlyH), L-phenylalanine (PheH), L-leucine
(LeuH), L-proline (ProH), L-methionine (MetH), N,N-dimethyl-
L-phenylalanine (dmPheH) (Apollo Scientic), NaOH, DMSO
(Carlo Erba), potassium hydrogen phthalate (Fluka), MoO2
(acac)2, 65% HNO3, 37% HCl, methanol, 96% ethanol,
acetone, diethyl ether (Sigma-Aldrich). (NH4)2MoO4 was
prepared from (NH4)6Mo7O24$4H2O according to the litera-
ture.37 An Orion pH-meter equipped with a Hamilton glass pH-
electrode was used for pH measurements. The instrument was
routinely calibrated with standard pH ¼ 4.0 and pH ¼ 7.0
buffer solutions (Fluka). Infrared spectra were recorded on a
Perkin Elmer FT-IR spectrometer. The UATR sampling acces-
sory was used in the 650–4000 cmꢀ1 range. Spectra in the 360–
650 cmꢀ1 region were recorded in the transmission mode on
Nujol mulls. NMR solution spectra were recorded on a Bruker
Avance DRX-400 instrument equipped with a BBFO broad-
(2.601)
103.5–106.5
(104.6–105.5)
73.1–74.0
(69.8–71.3)
151.6
(69.9–71.2)
138.6–152.8
(140.9–160.6)
136.4–153.9
(157.8)
a
Mo–O(a) ¼ Mo]O(terminal); Mo–O(b) ¼ Mo–OH(terminal); Mo–O(c)
¼ Mo–OH(bridging); Mo–O(d) ¼ Mo–O(bridging); Mo–O(e) ¼ Mo–
O(COO).
1
band probe. Chemical shis for H and 13C were referenced
to tetramethylsilane. CP-MAS NMR spectra were recorded on a
Bruker AMX300WB spectrometer working at 75.47 MHz for
carbon, using a CP pulse sequence with a 90ꢁ pulse of 3.4 ms, a
contact time of 2 ms and a recycle delay of 4 s. 4 mm zirconia
rotors were used. A 9 kHz spinning speed was used except
when, due to the consistency of the sample, homogeneous
packing within the rotor was difficult and a stable rotation was
achieved at a lower spinning speed (5 kHz). Carbonyl and
aromatic spinning sidebands were detected at ꢂ119 ppm and
ꢂ66 ppm for 9 kHz and 5 kHz rotation, respectively. Spectral
editing techniques38 were used to aid the spectral assignment
in the case of dmPheH and its molybdenum complex. Carbon,
hydrogen and nitrogen analyses were performed on a Carlo
Erba mod. 1106 instrument. Molybdenum content was
determined with the spectrophotometric method proposed by
Crouthamel e Johnson39 aer dissolution of a weighted
amount of sample (30–60 mg) in 100 mL of 4 M HCl. A cali-
bration curve was obtained using (NH4)6Mo7O24$4H2O as
standard (R2 ¼ 0.999), while MoO2(acac)2 was used as a
reference compound (anal. found: Mo 30.2%. C10H14MoO4
requires Mo 29.8%).
Fig.
4 Sketch of the covalent and hydrogen-bond interactions
between Mo2 units.
Table 3 Computed DG variations (including implicit solvation, water
as solvent, 298 K) for the condensation steps leading from 6 to 6O
(PBE/Grimme/COSMO calculations)
Step
DG, kcal molꢀ1
6 + 6 / 6T + H2O
6T + 6 / 6H + H2O
6H + 6 / 6O + H2O
ꢀ10.0
ꢀ7.1
ꢀ4.1
Conclusions
We have carried out a detailed solid state characterization of
Mo(VI) compounds isolated by reactions of molybdate salts
with a-amino acids in acidic aqueous solutions. The
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3.25%. C5H13Mo2NO10 requires C, 13.68; H, 2.98; Mo, 43.71; N,
3.19%.40 IR (ATR and Nujol): ~nmax/cmꢀ1 3545w-br, 3157w-br,
3040w-br, 2776w, 1603s, 1558m-sh, 1472m, 1432s, 1394m,
1372m, 1338m, 1301w, 1258w, 1175w, 944vs, 914vs, 901vs,
767m, 543vs-br, 463m-sh, 389w. 13C NMR (75.47 MHz, CP-MAS):
d/ppm 176.8 (CO), 175.2 (CO), 173.5 (CO), 65.0 (CH), 61.0 (CH),
49.7 (CH2N), 47.3 (CH2N), 46.9 (CH2N), 30.2 (CH2CH), 29.4
(CH2CH), 26.2 (CH2(CH2)2), 25.8 (CH2(CH2)2), 23.7 (CH2(CH2)2).
Mo2O4(OH)4(dmPheH), 6. A precipitate formed immediately
(862 mg, 83%). Anal. found: C, 26.05; H, 3.55; Mo, 37.5; N,
2.73%. C11H19Mo2NO10 requires C, 25.55; H, 3.70; Mo, 37.11; N,
2.71%.40 IR (ATR and Nujol): ~nmax/cmꢀ1 3563w-br, 3410w,
3055w, 3040w, 3025w, 2965w, 2943w, 2734w, 1626s, 1471m,
1455m, 1432m-sh, 1495m, 1412s, 1358m, 1260w, 1185w, 1170w,
1081w, 1046w, 946vs, 916vs, 901vs-sh, 769m, 742m, 705m,
547vs-br, 484m-sh, 438w, 385w.13C NMR (75.47 MHz, CP-MAS):
d/ppm 171.6 (CO), 140.9 (Ph), 130.8 (Ph), 128.8 (Ph), 127.8 (Ph),
69.9 (CH), 42.0 (NCH3), 36.6 (NCH3), 30.4 (CH2).
Synthesis of Mo2O4(OH)4(aaH), 1–6
Compound Na2MoO4$2H2O or (NH4)6Mo7O24$4H2O (4.00 mmol
of Mo) and the a-amino acid (aaH, 2.00 mmol) were dissolved in
water (30 mL), then 1 M HNO3 was added until pH ¼ 1.6–2.0.
The resulting pale-yellow solution (cMo ¼ 0.11 M) was stirred at
room temperature. A colourless solid (1–6) was obtained in a
variable time, depending on the amino acid employed. Then the
reaction mixture was ltered (ltrated solution pH ¼ 2.1–2.5).
The solid was washed with acetone, diethyl ether and then
dried in vacuo over CaH2. Details for single preparations with
Na2MoO4$2H2O are given below; analogous outcomes were
obtained by using (NH4)6Mo7O24$4H2O.
Mo2O4(OH)4(GlyH), 1. A precipitate formed in 25 minutes
(715 mg, 90%). Anal. found: C, 6.03; H, 2.13; Mo, 49.1; N, 3.67%.
C2H9Mo2NO10 requires C, 6.02; H, 2.27; Mo, 48.1; N, 3.51%.40 IR
(ATR and Nujol): ~nmax/cmꢀ1 3565w, 3424m, 3362m, 3225m,
3045m, 3011m, 2926m, 2757m, 2650m, 1672w, 1623s, 1590s,
1512s, 1456s, 1412s, 1340s, 1148w, 1118w, 1100w-sh, 946vs,
918vs, 903vs, 761m, 732w-sh, 531vs-br, 473m-sh, 384w. 13C
NMR (75.47 MHz, CP-MAS): d/ppm 172.0 (CO), 41.3 (CH2).
Mo2O4(OH)4(PheH), 2. A precipitate formed in 2.5 hours (805
mg, 90%). Anal. found: C, 22.05; H, 2.85; Mo, 40.1; N, 2.75%.
C9H15Mo2NO10 requires C, 22.10; H, 3.09; Mo, 39.2; N, 2.86%.40
IR (ATR and Nujol): ~nmax/cmꢀ1 3605w-sh, 3561w, 3136w-br,
3033w, 2945w, 2908w, 2831w, 2764w, 1607s, 1525m, 1498m-
sh, 1454m, 1426s, 1346m, 1300w, 1248w, 1143w, 1105w,
1084w, 942vs, 913vs-sh, 904vs, 856w-sh, 756m, 738w-sh, 705w,
534vs-br. 13C NMR (75.47 MHz, CP-MAS): d/ppm 172.6 (CO),
134.5 (Ph), 133.6 (Ph), 132.5 (Ph), 129.6 (Ph), 57.3 (CH), 35.6
(CH2).
Preparation of [aaH2]NO3, aaH ¼ GlyH, PheH, LeuH, MetH
The a-amino acid (2–4 mmol) was dissolved in 1 M HNO3 (10
mL for GlyH, PheH and LeuH; a stoichiometric amount of HNO3
for MetH) and the resulting colourless solution was allowed to
evaporate slowly at room temperature. Colourless crystals of
[aaH2]NO3 were formed in a period of 5–12 days. These were
collected and dried in vacuo over CaH2.
[GlyH2]NO3. Anal. found: C, 17.23; H, 4.11; N, 20.01%.
C2H6N2O5 requires C, 17.40; H, 4.38; N, 20.29%. IR (ATR): ~nmax
/
cmꢀ1 3205m, 3150w, 3032m, 2966m, 2909m, 2837m, 2748w,
2719w, 2640m, 2541w, 2442w, 2332w, 2262w, 2236w, 2221w,
1986w, 1724s, 1627w, 1596w, 1517m, 1442m, 1416s, 1355s,
1331s, 1215s, 1124s, 1043s, 914s, 890s, 868s, 813s, 738m, 712w,
662w.13C NMR (75.47 MHz, CP-MAS): d/ppm 171.7 (CO), 41.1
(CH2).
Mo2O4(OH)4(LeuH), 3. No precipitate was observed aer
18 hours. Thus the solvent was completely removed in vacuo at
50 ꢁC affording a pale-yellow residue. This residue was washed
with water, acetone, diethyl ether and dried in vacuo over CaH2;
a colourless solid was obtained (704 mg, 77%). Anal. found: C,
16.15; H, 3.70; Mo, 42.7; N, 3.01%. C6H17Mo2NO10 requires C,
[PheH2]NO3. Anal. found: C, 47.01; H, 5.11; N, 12.01%.
C9H12N2O5 requires C, 47.37; H, 5.30; N, 12.28%. IR (ATR): ~nmax
/
15.83; H, 3.76; Mo, 42.17; N, 3.08%.40 IR (ATR and Nujol): ~nmax
/
cmꢀ1 3148m-br, 3026m-br, 2972m-br, 1717s, 1667m, 1606w,
1584w, 1505m, 1496m, 1471w, 1453m, 1445m, 1408m, 1357s,
1335s, 1304m, 1200s, 1150m, 1119m, 1082w, 1045w, 1033w,
986w, 972w, 943w, 922w, 904w, 873w, 851w, 814m, 796w, 759w,
743w-sh, 737m, 693s.13C NMR (75.47 MHz, CP-MAS): d/ppm
173.9 (CO), 173.2 (CO), 135.2 (Ph), 132.8 (Ph), 132.3 (Ph), 129.8
(Ph), 129.3 (Ph), 127.6 (Ph), 127.3 (Ph), 56.3 (CH), 55.5 (CH), 37.0
(CH2), 36.3 (CH2).
cmꢀ1 3487m-br, 3133m-sh, 3055m, 2956m, 2931m-sh, 2874m,
1620s-sh, 1599s, 1513m, 1506m, 1494m, 1468m, 1428s, 1389m,
1369m, 1350m, 1338m, 1318m, 1293w, 1172w, 1133w, 1123w,
939vs, 909vs-sh, 896vs, 768w, 750w-sh, 537vs-br, 479s-sh, 380w.
13C NMR (75.47 MHz, CP-MAS): d/ppm 173.2 (CO), 53.2 (CHN),
39.2 (CH2), 25.1 (CHMe2), 21.3 (CH3), 20.1 (CH3).
Mo2O4(OH)4(MetH), 4. A precipitate formed in 40 minutes
(781 mg, 82%). Anal. found: C, 12.38; H, 2.99; Mo, 41.5; N,
3.10%. C5H15Mo2NO10S requires C 12.69; H 3.20; Mo 40.56; N
2.96%.40 IR (ATR and Nujol): ~nmax/cmꢀ1 3600w-sh, 3457w-br,
3365w-br, 3143m, 3066m, 2992w, 2922m, 2766–2443w, 1603vs,
1575s, 1558m-sh, 1504s, 1440m-sh, 1427s, 1358m, 1334s,
1317m-sh, 1280w, 1252w, 1195w, 1148m, 1103w, 1073w, 968w-
sh, 942vs, 913vs, 894vs, 762m, 748w-sh, 540vs-br, 486m-sh.
13C NMR (75.47 MHz, CP-MAS): d/ppm 174.4 (CO), 171.8 (CO),
54.7 (CH), 31.5 (CH2CH), 30.4 (CH2CH), 28.7 (SCH2), 15.3
(SCH3), 14.3 (SCH3), 12.4 (SCH3).
[LeuH2]NO3. Anal. found: C, 37.21; H, 6.99; N, 14.25%.
C6H14N2O5 requires C, 37.11; H, 7.27; N, 14.43%.IR (ATR): ~nmax
/
cmꢀ1 3221w, 3197w, 3149w, 2973m, 2958m, 2918m, 2875m,
2768w, 2701w, 2649w, 2583w, 2537w, 1717s, 1624m, 1579w,
1507s, 1473w, 1454m, 1413s, 1386s, 1344s, 1323s, 1297m,
1212s, 1171s, 1142s, 1075m, 1035s, 995m, 939m, 912s, 807s,
752w, 730w, 711m. 13C NMR (75.47 MHz, CP-MAS): d/ppm 173.9
(CO), 173.1 (CO), 52.9 (CHN), 39.2 (CH2), 24.7 (CHMe2), 23.3
(CH3), 22.3 (CH3), 21.7 (CH3), 20.0 (CH3).
[MetH2]NO3. Anal. found: C, 27.99; H, 5.46; N, 13.01%.
C5H12N2O5S requires C, 28.30; H, 5.70; N, 13.20%. IR (ATR):
~nmax/cmꢀ1 3182m, 3044m, 2964m, 2921m, 2853m, 2713w,
Mo2O4(OH)4(ProH), 5. A precipitate formed in 25 minutes
(642 mg, 73%). Anal. found: C, 13.83; H, 2.85; Mo, 44.8; N,
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2640w, 2604w, 1724s, 1620w, 1573w, 1506m, 1407s, 1381s,
1340s, 1287s, 1238m, 1225m, 1181s, 1135s, 1100m, 1073m,
1049m, 1035m, 984m, 948m, 913m, 864s, 817m, 773m, 756m,
731m, 700m, 655w.13C NMR (75.47 MHz, CP-MAS): d/ppm 173.7
(CO), 172.2 (CO), 54.6 (CH), 54.4 (CH), 32.9 (CH2CH), 30.2
(SCH2), 16.0 (SCH3), 15.5 (SCH3).
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Computational details
The geometry optimization of the complexes was carried out
without symmetry constrains, using the hybrid DFT functional
EDF2,41 in combination with the split-valence double-z polar-
ized basis set LACVP** (6-31G** on the light atoms, LANL2DZ
effective core potential on molybdenum centres),42 and with the
GGA PBE functional combined with an atom-centred doubly
numerical polarized basis set.43 Dispersion correction was
added through the Grimme's approach44 and the COSMO
implicit solvation model for water was added.45 On the basis of
the experimentally observed diamagnetism, in all cases the
molecules were considered in the singlet state and the
“restricted” approach was adopted. IR simulations on the
equilibrium geometries were carried out using the harmonic
approximation, from which zero-point vibrational energies and
thermal corrections to enthalpy and entropy were obtained
using standard formulae.46 The Cartesian coordinates of the
most stable geometries are collected in a separated .xyz le.†
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Acknowledgements
CINECA is gratefully acknowledged for computational support.
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