The reactions of α-amino acids and α-amino acid esters with high valent transition metal halides: synthesis of coordination complexes, activation processes and stabilization of α-ammonium acylchloride cations

Article abstract of DOI:10.1039/c7ra00073a

Titanium tetrachloride smoothly reacted with a selection of α-amino acids (aaH) in CH₂Cl₂ affording yellow to orange solid coordination compounds, 1a-d, in 70-78% yields. The salts [NHEt₃][TiCl₄(aa)], 2a-b, were obtained from TiCl₄/aaH/NEt₃ (aa = l-phenylalanine, N,N-dimethylphenylalanine), in 60-65% yields. The complex , 3, was isolated from the reaction of l-proline with NbCl₅/NHⁱPr₂, performed in CH₂Cl₂ at room temperature. The X-ray structure of 3 features a bridging (E)-1,2-bis(3,4-dihydro-2H-pyrrol-5-yl)ethene-1,2-diolate ligand, resulting from the unprecedented C-C coupling between two proline units. Unusually stable α-ammonium acyl chlorides were prepared by the reactions of PCl₅/MCl_n (MCl_n = NbCl₅, WCl₆) with l-proline, N,N-dimethylphenylalanine, sarcosine and l-methionine. MX₅ (M = Nb, Ta; X = F, Cl) reacted with l-leucine methylester and l-proline ethylester to give ionic coordination compounds, [MX₄L₂][MX₆] (M = Nb, L = Me₂CHCH₂CH(NH₂)CO₂Me, X = F, 9; Cl, 11a; M = Nb, X = Cl, , 11c; Ta, 11d), in moderate to good yields. [NbCl₅(Me₂CHCH₂CHNH₃CO₂Me)][NbCl₆], 12, was isolated as a co-product of the reaction of NbCl₅ with l-leucine isopropylester, and crystallographically characterized. The reaction of NbCl₅ with l-serine isopropylester afforded NbCl₃(OCH₂CHNHCO₂ⁱPr), 13, in 66% yield. The activation of the ester O-R bond was observed in the reactions of l-leucine methyl ester with NbF₅ and l-proline ethyl ester with MBr₅ (M = Nb, Ta), these reactions proceeding with the release of EtF and EtBr, respectively. All the metal products were characterized by analytical and spectroscopic methods, while DFT calculations were carried out in order to provide insight into the structural and mechanistic aspects.

Full text of DOI:10.1039/c7ra00073a

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PAPER
The reactions of a-amino acids and a-amino acid
esters with high valent transition metal halides:
synthesis of coordination complexes, activation
processes and stabilization of a-ammonium
acylchloride cations†
Cite this: RSC Adv., 2017, 7, 10158
Lorenzo Biancalana,a Marco Bortoluzzi,b Eleonora Ferretti,‡a Mohammad Hayatifar,a
a
a
Fabio Marchetti, Guido Pampaloni and Stefano Zacchinic
*
Titanium tetrachloride smoothly reacted with a selection of a-amino acids (aaH) in CH2Cl2 affording yellow
to orange solid coordination compounds, 1a–d, in 70–78% yields. The salts [NHEt3][TiCl4(aa)], 2a–b, were
obtained from TiCl4/aaH/NEt3 (aa ¼ L-phenylalanine, N,N-dimethylphenylalanine), in 60–65% yields. The
complex
, 3, was isolated from the reaction of L-proline with
NbCl5/NHiPr2, performed in CH2Cl2 at room temperature. The X-ray structure of 3 features a bridging
(E)-1,2-bis(3,4-dihydro-2H-pyrrol-5-yl)ethene-1,2-diolate ligand, resulting from the unprecedented C–C
coupling between two proline units. Unusually stable a-ammonium acyl chlorides were prepared by the
reactions of PCl5/MCln (MCln ¼ NbCl5, WCl6) with L-proline, N,N-dimethylphenylalanine, sarcosine and L-
methionine. MX5 (M ¼ Nb, Ta; X ¼ F, Cl) reacted with L-leucine methylester and L-proline ethylester to
give ionic coordination compounds, [MX4L2][MX6] (M ¼ Nb, L ¼ Me2CHCH2CH(NH2)CO2Me, X ¼ F, 9; Cl,
11a;
M
¼
Nb,
X
¼
Cl,
, 11c; Ta, 11d), in moderate to good yields.
[NbCl5(Me2CHCH2CHNH3CO2Me)][NbCl6], 12, was isolated as a co-product of the reaction of NbCl5
with L-leucine isopropylester, and crystallographically characterized. The reaction of NbCl5 with L-serine
i
isopropylester afforded NbCl3(OCH2CHNHCO2 Pr), 13, in 66% yield. The activation of the ester O–R bond
Received 3rd January 2017
Accepted 30th January 2017
was observed in the reactions of L-leucine methyl ester with NbF5 and L-proline ethyl ester with MBr5 (M ¼
Nb, Ta), these reactions proceeding with the release of EtF and EtBr, respectively. All the metal products
were characterized by analytical and spectroscopic methods, while DFT calculations were carried out in
order to provide insight into the structural and mechanistic aspects.
DOI: 10.1039/c7ra00073a
rsc.li/rsc-advances
a stereogenic centre (making them suitable substrates for
asymmetric catalysis)2 and the possibility of rmly coordinating
metal ions.3
Introduction
a-Amino acids constitute a class of organic compounds
arousing great interest in synthetic chemistry, in view of their
easy availability and low toxicity,1 the typical presence of
The esterication of the carboxylic acid moiety is one of the
most viable modications of the a-amino acid skeleton, and
indeed a good number of a-amino acid esters have been
synthesized and employed with reference to several application
elds.4 Metal complexes containing a-amino acid esters as
ligands are especially relevant to bio-inorganic chemistry, being
useful to the synthesis of peptides,5,6 as biological models,7 and
as scaffolds for the development of new drugs.8 Furthermore, a-
amino acids and a-amino acid ester metal complexes, being
possible chiral sources,9 have found increasing attention as
privileged, potential catalysts for environmentally friendly
asymmetric syntheses.2h,10
aUniversity of Pisa, Dipartimento di Chimica e Chimica Industriale, Via G. Moruzzi 13,
I-56124 Pisa, Italy. E-mail: guido.pampaloni@unipi.it; Web: http://www.dcci.unipi.it/
guido-pampaloni.html; Tel: +39 050 2219221
bCa' Foscari University of Venice, Dipartimento di Scienze Molecolari e Nanosistemi,
Via Torino 155, I-30175 Mestre, Venezia, Italy
cUniversity of Bologna, Dipartimento di Chimica Industriale “Toso Montanari”, Viale
Risorgimento 4, I-40136 Bologna, Italy
† Electronic supplementary information (ESI) available: Bond distances and
angles and H-bond distances for compound 4 and for [12]ꢀ. The DFT-calculated
structures discussed in this paper. CCDC 1041071, 1041072, 1518993 and
1518994 For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c7ra00073a
It is noteworthy that the large majority of these studies refer
to middle to late transition metals, whereas very little is known
about the parallel chemistry with early transition metal
compounds. In particular, the homoleptic halides of high valent
¨
¨
‡ Present address: Institut fur Anorganische Chemie, Georg-August-Universitat
¨
¨
Gottingen, Tammannstrasse 4, D-37077 Gottingen, Germany.
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elements (oxidation state $ 4) belonging to groups 4, 5 and 6,
HVTMH, are strongly oxophilic species, usually very air sensitive
and incompatible with water. This characteristic has probably
discouraged the linking with an “opposite world”, i.e. the
exploration of the reactivity with a-amino acids, which in turn
exhibit high water affinity, and their simple derivatives such as
a-amino acid esters.
As a matter of fact, the coordination chemistry of HVTMH
with a-amino acid esters still remains an unexplored eld of
research, with the exception of a former synthetic study
regarding MoCl5.11 Similarly, the only information available up
to 2014 on the interaction of HVTMH with a-amino acids, in the
absence of further reactants, is a note dealing with the reactivity
of TiCl4 with glycine.12 In all of the cases, the structural char-
acterization of the products relied on limited data.
Scheme 1 Synthesis of titanium tetrachloride a-amino acid complexes.
Recently, in the framework of our interest in the chemistry of
HVTMH with naturally occurring compounds,13 we have found
that MoCl5 and WCl6 behave as chlorinating agents towards
natural a-amino acids, affording fairly stable a-ammonium
acylchloride salts.14
On the other hand, the interaction of MX5 (M ¼ Nb, Ta; X ¼
Cl, Br) with a-amino acids leads to dinuclear complexes con-
taining bridging a-amino acidato ligands via HX release.15
Subsequent activation of the coordinated a-amino acidato
moiety has been observed in mild conditions in some specic
cases, leading to iminium salts.
Herein, we will present an extension of our study on the
reactivity between a-amino acids and HVTMH, including the
synthesis of TiCl4 derivatives, the unprecedented metal medi-
ated C–C dimerization of a a-amino acid (L-proline) and the
stabilization of otherwise reactive a-ammonium acylchloride
cations. We will also describe some reactivity of a-amino acid
ester with niobium and tantalum pentahalides.16,17 All the
reactions were carried out in a weakly coordinating solvent
(CH2Cl2) by using enantiopure L-organic reactants in case. The
metal products were characterized by elemental analysis, IR and
NMR spectroscopy, while DFT calculations assisted the struc-
tural characterization. A DFT study was performed also in order
to shed some light on the NbCl5-directed unusual L-proline
pseudo dimerization.
the carboxylato group. In general, the wavenumber difference
(Dna–s ¼ na ꢀ ns) is considered as a useful parameter to
discriminate between monodentate, chelating, and bridging
bidentate carboxylato ligands. Dna–s values within the range 100
to 150 cmꢀ1 are typical of either chelating or bridging bidentate
carboxylates.18 In view of the IR data available for 1a–d, i.e. Dna–s
varies between 103 (1a) and 135 (1d) cmꢀ1, and the DFT results
(vide infra), we propose a bridging bidentate-coordination
fashion. This implies that the amino acid ligand should be
coordinated to titanium as a zwitterion. Accordingly, a broad IR
absorption is observed at 3091 cmꢀ1 in the IR spectrum of 1d,
assigned to ammonium N–H stretching vibration.
The geometry proposed on the basis of spectroscopic data
was supported by DFT calculations on the possible isomers of
1b. The dinuclear structure [TiCl4(m-k2O–O2CCH(CH2Ph)NH3)]2,
depicted in Fig. 1, resulted meaningfully more stable than
mononuclear structures (see Fig. S1 given as ESI†).
Compounds 1a–d display low solubility in common organic
solvents. The NMR spectra were recorded in CD3CN, displaying
single sets of resonances. The 1H NMR spectra exhibit broad
Results and discussion
Reactivity of MCln with a-amino acids
Titanium tetrachloride. Titanium tetrachloride smoothly
reacted with a series of a-amino acids in dichloromethane
affording moisture sensitive, yellow to orange solid materials
1a–d, in 70 to 78% yields (Scheme 1). Compounds 1 are coor-
dination adducts: in this respect, the reactivity of TiCl4 with
a-amino acids differs from that of MCl5 (M ¼ Nb, Ta), featured
by HCl release,15 and from those of MoCl5 and WCl6, leading to
Cl/O interchange products (see Introduction).14
Compounds 1a–d were characterized by analytical and
spectroscopic methods. The IR spectra (solid state) contain one
medium and one strong intensity absorption in the range 1600–
1400 cmꢀ1. These two absorptions are due, respectively, to the
asymmetric (na) and the symmetric (ns) stretching vibrations of
Fig. 1 DFT-optimized geometry of the most stable isomer of 1b (C-PCM/
M06 calculations). Selected computed bond lengths (A˚): Ti1–O 2.020,
2.081; Ti2–O 1.990, 2.080; Ti1–Cl (trans O) 2.295, 2.232; Ti2–Cl (trans O)
2.232, 2.304; Ti1–Cl (cis O) 2.266, 2.392; Ti2–Cl (cis O) 2.291, 2.394; C–O
1.244, 1.261, 1.251, 1.252; N–H 1.025, 1.028, 1.038, 1.027, 1.028, 1.037.
Selected computed angles (ꢁ): O–Ti1–O 86.1; O–Ti2–O 84.6; Ti1–O–C
149.8, 151.4; Ti2–O–C 139.2, 151.3; O–C–O 124.9, 126.4.
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Scheme 2 Synthesis of anionic a-aminoacidate titanium compounds.
resonances in the 7.7–7.0 ppm range, related to the uncoordi-
nated ammonium group. The 13C-NMR spectra of the more
soluble 1a–b show the resonance of the carboxylate carbon at
176.1 and 170.0 ppm, respectively. These values are similar to
those reported for O,O-coordinated a-amino acids in NbCl5
derivatives.15
The coordination of organic species to high valent transition
metal chlorides represents, in a number of cases, the prelimi-
nary step of some activation process.13c,19 The activation is fav-
oured by the strong Lewis acidity of the metal centre, and may
Fig. 2 DFT-optimized geometries of the most stable anions of 2a and
2b (C-PCM/M06 calculations). Selected computed bond lengths for 2a
(A˚): Ti–O 1.909; Ti–N 2.240; Ti–Cl (trans O) 2.321; Ti–Cl (trans N)
2.263; Ti–Cl (trans Cl) 2.340, 2.353; C–O 1.316; C]O 1.212; N–H
1.018, 1.021. Selected computed angles for 2a (ꢁ): O–Ti–N 76.0; O–Ti–
Cl 87.8, 88.8, 97.6, 160.9; C–O–Ti 127.5. Selected computed bond
lengths for 2b (A˚): Ti–O 1.893; Ti–N 2.434; Ti–Cl (trans O) 2.309; Ti–Cl
(trans N) 2.271; Ti–Cl (trans Cl) 2.333, 2.354; C–O 1.316; C]O 1.212.
Selected computed angles for 2b (ꢁ): O–Ti–N 73.2; O–Ti–Cl 88.6,
89.7, 98.5, 160.3; C–O–Ti 130.2.
¨
be triggered by the addition of a Bronsted base. For instance,
Peryshkov and coworkers recently described a C–H bond acti-
vation reaction of nitriles by means of NEt3 upon coordination
to TaCl5.20
Thus, the reaction of 1b with NEt3 proceeded with selective
deprotonation of the ammonium group; analogous result was
achieved by treatment of a TiCl4/L-N,N-dimethylphenylalanine
mixture with NEt3 (Scheme 2).21 The reactions of 1a,c,d with
NEt3 were not straightforward, leading to non identied
compounds; the solid isolated from 1a/NEt3 revealed to be
paramagnetic.
The CH2Cl2 soluble compounds 2a–b (Scheme 2) were iso-
lated by addition of hexane to the respective reaction mixtures.22
1
The H NMR spectra of 2a–b display a low eld resonance
accounting for the triethylammonium proton (e.g. at 9.09 ppm
in the case of 2a); the resonances of the N-bound protons within
the anion undergo signicant upeld shi on going from the
amino acid unit in 1b to the amino acidate one in 2b (Dd > 3
ppm). The IR spectra of 2a–b exhibit a strong absorption around
1700 cmꢀ1: this evidence suggests O,N-coordination of the
aminoacidate moiety, leaving a uncoordinated C]O bond. The
geometries of the 2a,b anions were DFT optimized, on consid-
ering either mononuclear and dinuclear structures as starting
points (Fig. S2†). Thus, mononuclear compounds bearing N,O-
chelating a-aminoacidates (Fig. 2) resulted much more stable
than dinuclear homologues (see ESI† for more details).
Scheme 3 Reaction of NbCl5 with N,N-dimethylphenylalanine.
Scheme 4 Unusual activation of L-proline by NbCl5/amine.
Niobium pentachloride. We reported that the 2 : 1 reactions
of NbX5 (X ¼ Cl, Br) with a variety of a-amino acids afforded
dinuclear a-aminoacidate complexes via HCl release. The addi-
tion of a further equivalent of organic reactant resulted in the
decarboxylation of one amino acidate moiety, with consequent
formation of iminium salts and Nb-formate species (see Scheme
3, showing the specic case of N,N-dimethylphenylalanine).15
With the aim of exploring the possibility of further activation
pathways, we investigated the reactions of Nb2Cl9(a-amino
acidate) complexes with NEt3. In general, the amino acidate
moiety did not undergo activation under these conditions, with an
exception provided by the Nb2Cl9(L-prolinate)/NEt3 system. This
latter evolved into a complicated mixture of products, including
minor amounts of
,
3. The use of NHiPr2 in the place of NEt3 allowed to isolate red
i
crystals of 3 (12% yield), and also yellow crystals of [NH2 Pr2]
[NbCl6], 4 (40% yield), Scheme 4.
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Scheme 5 NbCl5-promoted C–C pseudo-dimerization of L-proline
(A); peptide bond formation from two L-proline units (B).
Basically, the process leading to 3 is a C–C bond forming
condensation of two proline units (Scheme 5A). Differently,
typical a-amino acid condensation generates a peptide bond
(Scheme 5B).27
Fig. 3 ORTEP drawing of 3. Displacement ellipsoids are at the 50%
probability level.
In order to gain some insight into the mechanism of the low
yield formation of 3, a DFT study was undertaken (see Scheme S1
in the ESI†). It seems plausible that the C–C bond forming step
consists in the coupling of two acylchloride units, accompanied
by the release of HCl and assisted by the amine (see Scheme S1,†
E / F / G). Indeed the side reactions NHiPr2 + HCl /
Table 1 Selected bond lengths (A˚) and angles (ꢁ) for 3a
Nb(1)–Cl(1)
Nb(1)–Cl(3)
Nb(1)–O(1)
C(1)–O(1)
C(1)–C(2)
C(3)–C(4)
N(1)–C(2)
Cl(1)–Nb(1)–O(1)
Cl(3)–Nb(1)–Cl(4)
Nb(1)–O(1)–C(1)
Nb(1)–N(1)–C(5)
N(1)–C(2)–C(3)
C(1)–C(2)–C(3)
C(3)–C(4)–C(5)
O(1)–C(1)–C(2)
C(2)–C(1)–C(1_1)
2.3319(13)
2.3454(12)
1.927(3)
1.357(6)
1.471(7)
1.530(6)
1.276(6)
158.18(10)
170.32(5)
122.9(3)
132.0(3)
115.8(4)
133.5(4)
105.8(4)
114.6(4)
125.2(6)
Nb(1)–Cl(2)
Nb(1)–Cl(4)
Nb(1)–N(1)
C(1)–C(1_1)
C(2)–C(3)
C(4)–C(5)
N(1)–C(5)
Cl(2)–Nb(1)–N(1)
O(1)–Nb(1)–N(1)
Nb(1)–N(1)–C(2)
C(2)–N(1)–C(5)
N(1)–C(2)–C(1)
C(2)–C(3)–C(4)
C(4)–C(5)–N(1)
O(1)–C(1)–C(1_1)
2.2833(11)
2.3453(12)
2.235(4)
1.337(10)
1.488(7)
1.528(6)
1.477(6)
167.62(11)
73.03(14)
117.6(3)
110.4(4)
110.7(4)
102.3(4)
105.7(4)
120.2(6)
i
i
[NH2 Pr2]Cl and [NH2 Pr2]Cl + NbCl5 / 4 should contribute to
decrease the DG variation of the process leading to 3.
The crystals of 3 exhibited insufficient solubility in suitable
deuterated solvents, thus preventing the NMR characterization.
MCln/phosphorous pentachloride (MCln ¼ NbCl5, WCl6).
The carboxylic acid to acyl chloride conversion is an important
preliminary step for the subsequent functionalization of a-
amino acids.28 PCl5 has been traditionally employed as Cl-
source in order to obtain the relevant a-ammonium acyl-
chloride salts (Fischer procedure); the counterion is Clꢀ or
[PCl6]ꢀ depending on the employed PCl5/a-aminoacid molar
ratio.29 a-Ammonium acylchloride species stable at room
temperature have been obtained only with primary ammonium
groups and in the absence of donor atoms within the side chain.
On the other hand, in the case of the L-proline derivative
(secondary N), both Clꢀ and [PCl6]ꢀ salts undergo quick
degradation at room temperature, due to HCl release and
subsequent condensation reactions.29 Furthermore, the reac-
tions of PCl5 with L-N,N-dimethylphenylalanine (tertiary N),
sarcosine (secondary N) and L-methionine (thioether group), in
CH2Cl2, proceed with the formation of complicated mixtures of
products (1H and 31P NMR spectroscopy). Thus, the L-N,N-
dimethylphenylalanine and L-methionine acylchloride deriva-
tives have not been known heretofore, while the highly moisture
sensitive [NH(Me)CH2COCl][WOCl5] has been recently obtained
by ourselves from sarcosine/WCl6.14a
a
Symmetry transformation used to generate C(1_1): ꢀx + 1, ꢀy, ꢀz + 1.
The X-ray structure of 3 is shown in Fig. 3, with relevant
bonding parameters listed in Table 1; the X-ray structure of 4 is
given as ESI (Fig. S3; Tables S1A and S1B†).
ꢀ
Complex 3 displays crystallographic 1 (Ci) symmetry with the
inversion centre located on the middle of the C(1)–C(1_1) bond.
The complex is composed of an unprecedented anionic
ligand (E-1,2-bis(3,4-dihydro-2H-pyrrol-5-
yl)ethene-1,2-diolate) that is m-k2O,k 2N-coordinated over two
[NbCl4]+ cationic fragments. Such anionic ligand is almost
perfectly planar (mean deviation from the least squares plane
˚
˚
0.0387 A), being the two Nb atoms respectively 0.1879 A above
and below this plane. C(1), C(2) and N(1) [sum angles 360.0(9),
360.0(7) and 360.0(6)ꢁ, respectively] show a perfect sp2 hybrid-
Some of us recently reported13b,29 a straightforward and clean
route to unusually stable salts of the acylchloride derivative of L-
proline, by combination of the traditional PCl5-chlorinating
reaction with the considerable stability imparted by the [MCl6]ꢀ
(M ¼ Nb, Ta) anions, Scheme 6.13b–c,30
˚
ization, and the C(1)–C(1_1) [1.357(6) A] and C(2)–N(1) [1.276(6)
23
˚
A] distances are typical for double bonds.
The {C2O2} core of the (E)-1,2-bis(3,4-dihydro-2H-pyrrol-5-yl)
ethene-1,2-diolate ligand in 3 is a fully deprotonated 1,2-ene-
diol. In general, 1,2-enediols are quite unstable species,24 whose
stabilization may be supplied by hydrogen-bonded protons,25 or
by chelating (N,O) coordination to transition metals.26
We reckoned that the easily available [NbCl6]ꢀ anion could
provide stability also to other unstable/unknown a-ammonium
acylchloride cations (see above). Therefore, we tried to optimize
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and generalize the synthetic procedure shown in Scheme 6. crystal cell data with those reported in the literature,36 were
When a dichloromethane 1 : 1 molar mixture of PCl5 and NbCl5 obtained as minor products from NbCl5/L-N,N-dimethylpheny-
was treated with L-N,N-dimethylphenylalanine or sarcosine,31 lalanine and NbCl5/sarcosine, respectively.
the subsequent 31P NMR analysis on the reaction solution evi-
The synthetic approach leading to 5a–b exploits the M–Cl
denced the presence of POCl3 as prevalent phosphorous species (M ¼ P, Nb) bond energy scale,13b,37 making PCl5 a preferential
[singlet at 6.2 ppm (from PCl5/NbCl5/L-N,N-dimethylphenylala- chlorinating agent respect to NbCl5, and the stability of
nine) and 5.6 ppm (from PCl5/NbCl5/sarcosine), respective- the [NbCl6]ꢀ anion. Similar considerations led us to test the
ly].13b,29,32 The corresponding [NbCl6]ꢀ a-ammonium PCl5/WCl6 mixture; it should be noted that anionic simple
acylchloride salts, 5a–b, were isolated at room temperature in derivatives of WCl6 (i.e., WCl6, WOCl5ꢀ) have recently proposed
40–50% yields (Scheme 7). The presence of [NbCl6]ꢀ in 5a–b was as effective partners for the stabilization of otherwise reactive
unambiguously detected by a typical 93Nb NMR resonance cations.14a,38
around 0 ppm.13c,33 The structure of 5a was determined by X-ray
Hence, the reactions of PCl5/WCl6 (1 : 1 mixture) with
diffraction (Fig. 4, Table 2). It contains the [PhCH2CH(NHMe2) L-proline, L-N,N-dimethylphenylalanine, sarcosine and L-methi-
COCl]+ cation, which has never been reported heretofore. onine proceeded with PCl5 to POCl3 conversion (31P NMR), and
Within crystals of 5a, some intermolecular N–H/Cl hydrogen straightforwardly afforded the respective a-ammonium acyl-
bonds are present involving the ammonium group of the cation chloride cations (Scheme 7). According to elemental analyses
as donor and the chloride ligands of the anion as an acceptor. and magnetic measurements,38 the cations were isolated in
The bonding parameters of the cation are comparable to those good yields as [WCl6]ꢀ salts, 7a–b, respectively from PCl5/WCl6/
previously reported for other a-ammonium acylchloride L-proline and PCl5/WCl6/L-N,N-dimethylphenylalanine. Other-
salts.13b,14,29 Thus, the C(1)–O(1) distance [1.178(5) A] corre- wise, different anions were presumably associated with sarco-
˚
sponds to an almost pure double bond, whereas all the other sine and methionine derivatives, including [WOCl5]ꢀ (few
contacts are typical for single bonds.34 The C(2) atoms displays crystals of [MeNH2CH2C(]O)Cl][WOCl5] were isolated and X-
an absolute S conguration with rened Flack parameter ray characterized) and W(IV) species.
0.03(2).35
The characterization of the 1 : 1 mixture WCl6/PCl5 sug-
The iminium salt [PhCH2]NMe2][NbCl6] (see Scheme 3) and gested that both chlorides remained intact when mixed
the adduct NbCl5(O]PCl3), 6, identied by comparison of the together (see Eꢀxperimental for details). This implies that the
WCl6 to WCl6 reduction, as clearly observed in 7a–b, is
promoted by the a-amino acid. Analogous WCl6 reduction has
Scheme
6 Stable pyrrolidinium-2-carbonylchloride salts from L-
proline and MCl5 (M ¼ Nb, Ta).
Fig. 4 ORTEP drawing of 5a. Displacement ellipsoids are at the 50%
probability level.
Table 2 Selected bond lengths (A˚) and angles (ꢁ) for 5a
Nb(1)–Cl(1)
Nb(1)–Cl(3)
Nb(1)–Cl(5)
C(1)–Cl(7)
C(1)–C(2)
C(2)–N(1)
N(1)–C(5)
Cl(1)–Nb(1)–Cl(4)
Cl(3)–Nb(1)–Cl(6)
O(1)–C(1)–C(2)
C(1)–C(2)–C(3)
C(2)–C(3)–C(6)
C(2)–N(1)–C(5)
2.3305(11)
2.2921(11)
2.3844(11)
1.758(5)
1.516(7)
1.500(6)
1.501(6)
176.12(5)
179.14(4)
125.4(4)
115.1(4)
115.1(4)
114.9(3)
Nb(1)–Cl(2)
Nb(1)–Cl(4)
Nb(1)–Cl(6)
C(1)–O(1)
C(2)–C(3)
N(1)–C(4)
C(3)–C(6)
Cl(2)–Nb(1)–Cl(5)
O(1)–C(1)–Cl(7)
Cl(7)–C(1)–C(2)
C(1)–C(2)–N(1)
C(2)–N(1)–C(4)
C(4)–N(1)–C(5)
2.3344(11)
2.3475(11)
2.4206(11)
1.178(5)
1.553(6)
1.499(5)
1.516(6)
174.25(5)
120.6(4)
113.9(3)
107.3(4)
111.4(3)
111.4(4)
Scheme
7 Formation of otherwise unstable a-ammonium acyl-
chloride cations from a-amino acids and niobium and tungsten
chlorides.
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been previously observed in a number of cases by interaction adventitious presence of water.41 We were able to isolate satis-
with organic compounds.38,39
factory yields of well dened coordination compounds only in
All the a-ammonium acylchloride cations produced from two cases (Scheme 9).
PCl5/NbCl5 and PCl5/WCl6 were fully characterized by IR and
Compound 9 can be viewed as a coordination compound
NMR spectroscopy, and those cations derived from L-N,N- resulting from the unsymmetrical rupture of the structure of
dimethylphenylalanine and L-methionine are reported here for NbF5 (a tetramer in the solid state17a).13c,42 The IR spectrum
the rst time. The chloro-acyl moiety manifests itself by a strong shows a strong absorption at 1648 cmꢀ1, attributed to the
IR absorption in the region 1765–1783 cmꢀ1, other than the 13
NMR resonance in the range 169.0–171.7 ppm.
C
stretching vibration of the C]O bond belonging to the ester
function. The ca. 100 cmꢀ1 shi to lower wavenumbers is in
agreement with the coordination of the carbonyl moiety to
niobium. The shi of the absorptions due to the stretching of
the amino group from 3380 cmꢀ1 (in 8k) to 3232 cmꢀ1 (in 9)
suggests that also the nitrogen atom is involved in the coordi-
Reactivity of MCln with a-amino acid esters
Preparation of a-amino acid ester hydrochlorides and
a-amino acid esters. The a-amino acid ester derivatives, 8, were
prepared from the corresponding hydrochlorides, 8$HCl, which
were in general isolated (Scheme 8). Although most of the
compounds 8 and 8$HCl have been already appeared in the
literature,40 we decided to collect their preparations and IR and
NMR data in this paper, in view of possible modications to the
reported procedures or additional spectroscopic data.
1
nation to the metal centre. Accordingly, two low eld H NMR
resonances have been found for the NH2 group in 9 (d ¼ 8.6 and
7.0 ppm, CDCl3 solution). On the other hand, the same group
Reactions with niobium and tantalum pentahalides. The
reactions of a-amino acid esters with NbF5 are oen non
selective, affording in most cases mixtures of products where
the only recognizable compounds are the scarcely soluble
ammonium ester salts [RCH(NH3)COOR0][NbF6]. These might
Scheme 9 Coordination compounds from the reactions of NbF5 with
be formed as a consequence of some activation reaction or the L-proline ethylester and L-leucine methylester.
Scheme 8 Synthesis of a-amino acid esters.
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gives raise to a singlet at 1.65 ppm in the H NMR spectrum ligand were ruled out by DFT calculations. The optimized
of 8k.
geometry of the cation of 10 is depicted in Fig. 6 (see also Fig. S5
In addition, the 19F and 93Nb NMR spectra (decet at 103 ppm given as ESI†).
and septet at ꢀ1553 ppm, respectively) are unequivocal nger-
We extended the present study to the interaction of a-amino
acid esters with the heavier niobium pentahalides. These reac-
prints for the presence of the [NbF6]ꢀ anion in solution.13c,42,43
In conclusion, on considering the tendency of NbF5 to the tions led to complicated mixtures of metal products, with
unsymmetrical breaking of the Nb–F bridges, with formation of presumable activation of the organic substrates. Only in a few
[NbF4]+ cations and [NbF6]ꢀ anions,42 analytical and spectro- cases, all involving the metal pentachlorides, a clean reaction
scopic data suggest that 9 is a salt containing an octacoordinate pathway was observed (Scheme 10).
[NbF4(Me2CHCH2CHNH2CO2Me)2]+ cation, comprising two
All the identied products, 11a–d and 12, are colourless to
O,N-ligated a-amino acid esters, and a [NbF6]ꢀ anion. The pale yellow solids, being scarcely soluble in organic solvents.
coordination number of the cation was conrmed by DFT Spectroscopic considerations discussed for 10 are valid also for
calculations, being six-coordinate geometries less stable by 11a–d, thus suggesting the bidentate N,O coordination of two a-
more than 30 kcal molꢀ1. The optimized geometry is shown in amino acid ester ligands to the same metal centre within
Fig. 5. DFT calculations with dichloromethane as implicit a cation. The presence of the [NbCl6]ꢀ anion in 11a–c is the
solvent also indicated that the [NbF4(Me2CHCH2CHNH2CO2- consequence of unsymmetrical cleavage of the dinuclear NbCl5
Me)2][NbF6] salt is slightly more stable compared to its neutral structure,13c,33,46 and was unambiguously evidenced by a sharp
isomer NbF5(Me2CHCH2CHNH2CO2Me). The metal centre in 93Nb NMR resonance occurring in the interval 4–13 ppm.13c,33
this last species should be eight-coordinated, the a-amino acid
ester behaving as N,O-donor chelating ligand (Fig. S4†).
DFT calculations were carried out on the cation of 11a,
considering either one or two a-amino acid esters in the
It worth noting that the majority of coordination complexes niobium sphere. The coordination of another equivalent of the
containing a-amino acid ester ligands are based on late tran- a-amino acid ester to [NbCl4(Me2CHCH2CHNH2CO2Me)]+
sition metals (Ru, Os, Co, Rh, Pt, Zn).44 Only few examples are resulted a favourable process, being the associated DG variation
known with group 6 elements45 and also derivatives of group 4
and group 5 metals are very rare.
The reaction of NbF5 with L-proline ethyl ester, 8b, revealed
a different outcome, and 10 was obtained under the same
conditions employed for NbF5/8k. The use of Nb/8b molar ratio
¼ 2 afforded 10 with the best yield (Scheme 9). Ethyl uoride
was NMR identied as a co-product of the reaction performed in
CD2Cl2 in a closed tube, while L-proline was recovered aer
hydrolysis of the reaction mixture. These experimental facts
support the presence in 10 of a carboxylato moiety originated
from the cleavage of the ester function.
Compound 10 is a colourless solid whose salient spectro-
scopic features are two IR bands at 3381 cmꢀ1 (N–H) and 1636
cmꢀ1 (C]O), and 1H and 19F NMR resonances at 11.77 ppm
(NH) and 100.9 ppm ([NbF6]ꢀ), respectively. These data suggest
a bidentate N,O-coordination of the a-amino carboxylate ligand.
Dinuclear geometries with the a-amino acidate as bridging
Fig. 6 DFT-optimized geometry of the cation of 10 (C-PCM/M06
calculations). Selected computed bond lengths (A˚): Nb–O 1.902; Nb–
N 2.243; Nb–F 1.833, 1.835, 1.857; C–O 1.365; C]O 1.192; N–H 1.022.
Selected computed angles (ꢁ): O–Nb–N 73.5; O–Nb–F 97.7, 97.7,
147.3; C–O–Nb 131.0.
Fig. 5 DFT-optimized geometry of the cation of 9 (C-PCM/M06
calculations). Selected computed bond lengths (A˚): Nb–O 2.273,
2.273; Nb–N 2.364, 2.364; Nb–F 1.900, 1.900, 1.900, 1.900; C–O(Nb)
1.234, 1.234; C–O(Me) 1.303, 1.303; N–H 1.019, 1.019, 1.019, 1.020.
Selected computed angles (ꢁ): O–Nb–N 68.3, 68.3; O–Nb–O 135.0; Scheme 10 Synthesis of niobium pentachloride derivatives of a-
N–Nb–N 132.3; C–O–Nb 123.3, 123.3.
amino acid esters.
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around ꢀ25 kcal molꢀ1. The DFT-optimized geometry of promoted by the strongly acidic niobium chloride. Neverthe-
[NbCl4(Me2CHCH2CHNH2CO2Me)2]+ is represented in Fig. 7.
less, the occurrence of fortuitous hydrolysis might play some
A crop of X-ray quality crystals of 12 was obtained directly role and should not be ruled out.
from the reaction mixture aer separation from 11a.
12 represents the second crystallographically characterized
Compound 12 differs from the previous compounds as far as example where a cationic a-amino acid ester is coordinated to
the solubility is concerned. Once isolated in the solid state, it any metal centre, and the rst one where the coordination
does not dissolve again in common organic solvents.
occurs via oxygen. In fact, previous to this work, only the
Compound 12 consists of an ionic packing of [NbCl5(- structure of a Ru(II) complex containing a h6-bonded L-phenyl-
CH3)2CHCH2CHNH3CO2Me)]+ cations and [NbCl6]ꢀ anions. alaninium methyl ester was reported.47 More commonly, a-
The cation is represented in Fig. 8, and the related bonding amino acid esters act as ligands in the neutral form RCHNH2-
parameters are reported in Table 3. A view of the structure of the CO2R0, via the N-atom or both N and O.44,45
anion is given in Fig. S6,† the relevant bonding parameters
We moved to study the reaction of NbCl5 with L-serine iso-
being collected in Table S2A.† H-bonds between the NH3-group propylester, 8h, i.e. a a-amino acid ester bearing a peripheral
of the cation and the chlorides of [NbCl6]ꢀ are present within OH group and potentially acting as a pincer ligand. The 1 : 1
the crystals (see Table S2B† for details). Compound 12 crystal- reaction of NbCl5 and 8h in reuxing chloroform led to the
lizes in the chiral space group P21 and the C(2) atom of the a- formation of NbCl3(OCHCH2NHCOOiPr), 13, as a colourless
amino acid ester ligand displays S absolute conguration.
precipitate. The n(C]O) stretching band in 13 (1732 cmꢀ1) is
The source of protonation leading to 12 is not clear, being only slightly shied respect to 8h (1728 cmꢀ1), thus indicating
possibly the result of some activation of the organic reactant that the ester group is not involved in coordination.
Table 3 Selected bond distances (A˚) and angles (ꢁ) for the [NbCl5(-
CH3)2CHCH2CHNH3CO2Me)]+ cation in 12. The data relative to the
[NbCl6]ꢀ anion are reported in Tables S2A and S2B
Nb(2)–Cl(7)
Nb(2)–Cl(9)
Nb(2)–Cl(11)
C(1)–O(1)
C(7)–O(2)
2.3555(9)
2.3318(10)
2.2918(9)
1.231(4)
1.473(3)
1.506(4)
174.26(3)
177.18(6)
125.5(3)
113.5(3)
109.4(2)
Nb(2)–Cl(8)
Nb(2)–Cl(10)
Nb(2)–O(1)
C(1)–O(2)
C(1)–C(2)
2.2641(10)
2.3250(10)
2.209(2)
1.291(4)
1.499(4)
1.534(4)
163.09(3)
145.2(2)
121.0(3)
112.3(3)
108.4(3)
C(2)–N(1)
C(2)–C(3)
Cl(7)–Nb(2)–Cl(10)
Cl(8)–Nb(2)–O(1)
O(1)–C(1)–O(2)
O(2)–C(1)–C(2)
C(1)–C(2)–N(1)
Cl(9)–Nb(2)–Cl(11)
Nb(2)–O(1)–C(1)
O(1)–C(1)–C(2)
C(1)–C(2)–C(3)
N(1)–C(2)–C(3)
Fig. 7 DFT-optimized geometry of the cation of 11a (C-PCM/M06
calculations). Selected computed bond lengths (A˚): Nb–O 2.261,
2.266; Nb–N 2.377, 2.381; Nb–Cl 2.380, 2.382, 2.413, 2.414; C–O(Nb)
1.236, 1.237; C–O(Me) 1.294, 1.294; N–H 1.020, 1.021, 1.020, 1.021.
Selected computed angles (ꢁ): O–Nb–N 68.4, 68.4; O–Nb–O 136.2;
N–Nb–N 132.9; C–O–Nb 123.7, 123.8.
Fig. 9 DFT-optimized geometry of 13 (C-PCM/M06 calculations).
Fig. 8 ORTEP drawing of the [NbCl5(CH3)2CHCH2CHNH3CO2Me)]+ Selected computed bond lengths (A˚): Nb–O 1.873; Nb–N 1.943; Nb–
cation in 12. The [NbCl6]ꢀ anion is reported in Fig. S6.† Displacement Cl 2.359, 2.368, 2.389; N–H 1.019. Selected computed angles (ꢁ): O–
ellipsoids are at the 50% probability level.
Nb–N 76.0; O–Nb–Cl 93.3, 96.7, 157.8; N–Nb–Cl 82.0, 109.3, 110.0.
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Compound 13 showed a single set of signals in H and 13C Solvents (Sigma-Aldrich) were distilled before use from appro-
NMR spectra (in CD3CN); 1H resonances are shied to priate drying agents. Chromatographic purication of organic
higher ppm values with respect to 8h [most notably d(NH) from products was carried out on columns of deactivated alumina
2.6 ppm in 8h to 7.0 ppm in 13], while 13C resonances are (4% w/w water). Infrared spectra were recorded at 298 K on a FT
shied to lower ppm values [e.g., d(CO) from 175 ppm in 8h to IR-Perkin Elmer Spectrometer, equipped with a UATR sampling
167 ppm in 13].
accessory. NMR spectra were recorded at 293 K on a Bruker
At variance to the other a-amino acid ester complexes of Avance II DRX400 instrument equipped with a BBFO broadband
niobium in this work, compound 13 is a neutral species in probe. The chemical shis for 1H and 13C were referenced to the
solution with a single 93Nb resonance at ꢀ493 ppm. DFT non-deuterated aliquot of the solvent; the chemical shis for
calculations suggest a mononuclear structure (Fig. 9) as the 93Nb were referenced to external [NEt4][NbCl6]; the chemical
most probable geometry. A positive Gibbs energy variation shis for 19F were referenced to external CFCl3. Conductivity
(about 3.5 kcal molꢀ1) is accompanied to the dimerization of measurement was carried out using an Eutech Con 700
this species to the dinuclear form (see Fig. S7 and S8† for more instrument (cell constant ¼ 1.0 cmꢀ1).49 Magnetic susceptibili-
details).
ties (reported per W atom) were measured on solid samples at
We could not cleanly isolate metal products from MBr5/a- 298 K with a Magway MSB Mk1 magnetic susceptibility balance
amino acid ester (M ¼ Nb, Ta). However, NMR investigations (Sherwood Scientic Ltd). Diamagnetic corrections were intro-
50
¨
outlined the release of ethyl bromide from L-proline ethylester, duced according to Konig. Carbon, hydrogen and nitrogen
in the presence of MBr5 (see Experimental for details).
analyses were performed on a Carlo Erba mod. 1106 instru-
ment. The chloride/bromide content was determined by the
Mohr method51 on solutions prepared by dissolution of the
solids in aqueous KOH and heated at boiling temperature for 72
Conclusions
The reactions of TiCl4 with a series of a-amino acids do not hours, followed by cooling to room temperature and addition of
proceed with HCl release, in spite of the Lewis acidic character HNO3 up to neutralization. Titanium, niobium and tantalum
of the metal centre, and afford dinuclear coordination were analyzed, respectively, as TiO2 and M2O5 (M ¼ Nb, Ta),
compounds containing zwitterionic ligands. Deprotonation of obtained by hydrolysis of the samples followed by calcination in
the ammonium function may be easily promoted by the addi- a platinum crucible.
tion of triethylamine, resulting in a modication of the coor-
dination fashion of the a-amino acidic frame. On the other
hand, the interaction of L-proline with NbCl5/NHEt2 has
provided the rst example of C–C bond forming self-
condensation of a a-amino acid, although in modest yield.
Reactions of TiCl4 with a-amino acids: synthesis of TiCl4(aa)
(aa ¼ L-proline, 1a; L-phenylalanine, 1b; sarcosine, 1c; N,N-
dimethylglycine, 1d)
The overall transformation may be regarded as a Lewis acid
General procedure. A suspension of the appropriate a-amino
induced intramolecular redox reaction, in which the carboxylate acid (1.50 mmol) in CH2Cl2 (ca. 15 mL) was treated with
group is reduced and the pyrrolidine ring is oxidized. The a solution (100 mg mLꢀ1) of TiCl4 (1.50 mmol) in heptane. The
combination of NbCl5 or WCl6 with the chlorinating power of mixture was stirred at room temperature overnight, then hexane
PCl5 has been exploited to develop a simple synthetic method (ca. 30 mL) was added. The precipitate was separated and dried
affording isolable salts of otherwise unstable a-ammonium in vacuo.
acylchloride cations. a-Amino acid esters usually behave as
TiCl4(L-proline), 1a. Yellow solid, yield 321 mg (70%). Anal.
bidentate O,N-ligands towards niobium and tantalum penta- calcd for C5H9Cl4NO2Ti: C, 19.70; H, 2.98; N, 4.60; Cl, 46.52; Ti,
halides, however activation of the ester function with release of 15.70. Found: C, 19.39; H, 3.09; N, 4.52; Cl, 45.88; Ti, 15.89. IR
alkyl halides has been observed in some cases.
(solid state): n ¼ 3219 mw, 2962 w, 1570 m, 1544 vs, 1441 vs,
1
1367 m, 1331 ms, 1260 m, 1081 m, 1031 ms, 798 s cmꢀ1. H
NMR (CD3CN): d ¼ 7.46, 7.15 (br, 2H, NH2); 4.53 (br, 1H, NCH);
3.54, 3.45, 2.42, 2.20, 2.06 (br, 6H, CH2) ppm. 13C NMR (CD3CN):
d ¼ 176.1 (OCO); 61.6 (CH); 47.6, 28.6, 23.7 (CH2) ppm.
TiCl4(L-phenylalanine), 1b. Light orange solid, yield 388 mg
Experimental
General
Warning: all the metal products reported in this paper are
highly moisture-sensitive, thus rigorously anhydrous condi- (73%). Anal. calcd for C9H11Cl4NO2Ti: C, 30.46; H, 3.12; N, 3.95;
tions were required for the reaction and crystallization proce- Cl, 39.96; Ti, 13.49. Found: C, 30.60; H, 3.02; N, 4.13; Cl, 39.40;
ꢁ
dures. The reaction vessels were oven dried at 140 C prior to Ti, 13.28. IR (solid state): n ¼ 3030 m-br, 1600 m, 1558 vs, 1445
use, evacuated (10ꢀ2 mmHg) and then lled with argon. TiCl4, vs-br, 1336 m, 1047 w, 744 m, 698 ms cmꢀ1. 1H NMR (CD3CN):
NbX5 (X ¼ F, Cl), PCl5 and WCl6 were purchased from Strem d ¼ 7.39–7.30, 6.98 (8H, Ph + NH3); 4.46 (m, 1H, CH); 3.28 (m,
(>98% purity) and stored in sealed tubes under argon atmo- 2H, CH2) ppm. 13C NMR (CD3CN): d ¼ 170.0 (OCO); 134.1 (ipso-
sphere. NbBr5 and TaBr5 were prepared according to literature Ph); 129.8, 129.2, 127.9 (C6H5); 61.8 (CH); 35.3 (CH2) ppm.
procedures and stored under argon atmosphere.48 Once iso-
TiCl4(sarcosine), 1c. Yellow solid, yield 448 mg (78%). Anal.
lated, the metal products were conserved in sealed glass tubes calcd for C3H7Cl4NO2Ti: C, 12.93; H, 2.53; N, 5.02; Cl, 50.87; Ti,
under argon. The organic reactants were commercial products 17.17. Found: C, 13.02; H, 2.43; N, 4.98; Cl, 36.23; Ti, 12.80. IR
(Sigma-Aldrich) stored under argon atmosphere as received. (solid state): n ¼ 3185 m, 2930 vw, 2810 vw, 1575 ms, 1561 vs,
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1454 s, 1429 w, 1409 ms, 937 w, 792 s, 689 m-w cmꢀ1. 1H NMR ltered off in order to remove a minor amount of solid, layered
(CD3CN): d ¼ 7.26, 7.00 (br, 2H, NH2); 4.03 (br, 2H, CH2); 2.81 with hexane and settled aside at ꢀ30 ꢁC. Red crystals of 3 were
(br, 3H, CH3) ppm.
recovered aer 48 h. Yield 56 mg, 12%. Anal. calcd for C10H12-
TiCl4(N,N-dimethylglycine), 1d. Yellow solid, yield 312 mg Cl8N2Nb2O2: C, 18.15; H, 1.83; N, 4.23; Cl, 42.86. Found: C,
(71%). Anal. calcd for C4H9Cl4NO2Ti: C, 16.41; H, 3.10; N, 4.78; 18.43; H, 1.63; N, 4.51; Cl, 43.12. IR (solid state): n ¼ 1580 m-s
Cl, 48.43; Ti, 16.35. Found: C, 16.29; H, 3.17; N, 4.91; Cl, 48.26; (nC]N), 1183 m-s (nC–O) cmꢀ1
.
Ti, 16.57. IR (solid state): n ¼ 3091 m-w, 2983 w, 1573 vs, 1438 s,
The mother liquors were dried in vacuo, hence the residue
was dissolved into CHCl3 (10 mL). The solution was layered with
1401 m, 1364 s, 1324 m, 1142 m-w, 932 m-w, 857 m, 716 m cmꢀ1
.
1H NMR (CD3CN): d ¼ 7.7 (br, 1H, NH); 4.09 (br, 2H, CH2); 3.01 pentane and settled aside at ꢀ30 ꢁC, thus 4 was isolated as
(br, 6H, CH3) ppm.
a yellow-orange crystals aer 48 h. Yield 238 mg, 40%. Anal.
calcd for C7H16Cl6NNb: C, 20.03; H, 3.84; N, 3.34; Cl, 50.67.
Found: C, 19.84; H, 3.75; N, 3.20; Cl, 49.95. IR (solid state): n ¼
Reactions of TiCl4 with a-amino acid/NEt3: synthesis of
[NHEt3][TiCl4(aa)] (aa ¼ L-phenylalanine, 2a; N,N-
dimethylphenylalanine, 2b)
3130 s-br, 3080 s-br, 2987 w-m, 1606 m, 1565 m, 1424 m cmꢀ1
.
1H NMR (CD3CN): d ¼ 6.30 (br, 1H, NH); 3.48 (m, 2H, CH); 1.31
(m, 12H, Me) ppm. 13C NMR (CD3CN): d ¼ 47.7 (CH); 18.2
General procedure. A suspension of the appropriate a-amino
acid (1.00 mmol) in CH2Cl2 (ca. 10 mL) was treated with
a solution (100 mg mLꢀ1) of TiCl4 (1.00 mmol) in heptane. The
mixture was stirred at room temperature overnight, then hexane
(ca. 30 mL) was added. The liquors were eliminated with
a syringe, then CH2Cl2 (20 mL) and NEt3 (1.00 mmol) were
added in the order given. The mixture was allowed to stir for 5 h,
then hexane (30 mL) was added. The resulting precipitate was
separated and dried in vacuo.
93
(Me) ppm. Nb NMR (CD3CN): d ¼ ꢀ0.2 (Dn12 ¼ 95 Hz) ppm.
Reactions of a-amino acids with PCl5/NbCl5: synthesis of
[(R)(Me)NHCH(R0)C(O)Cl][NbCl6] (R ¼ Me, R0 ¼ CH2Ph, 5a;
R ¼ R0 ¼ H, 5b)
General procedure. A suspension of PCl5 (169 mg, 0.81
mmol) and NbCl5 (220 mg, 0.81 mmol) in CH2Cl2 (10 mL) was
stirred at room temperature for 2–3 h. Then the appropriate
a-amino acid (0.81 mmol) was added. The resulting mixture was
stirred at room temperature for 2 h. The solution was concen-
trated to 3–5 mL, then it was layered with pentane and stored in
the freezer (ꢀ30 ꢁC) for one week. A crop of crystalline material
was collected and then stored at ꢀ30 ꢁC. By slow evaporation of
the crystallization solutions under inert atmosphere, few crys-
tals of [PhCH2 ¼ NMe2][NbCl6] and NbCl5(O]PCl3), 6, were
obtained from PCl5/NbCl5/L-N,N-dimethylphenylalanine and
PCl5/NbCl5/sarcosine, respectively.
[NHEt3][TiCl4(L-phenylalanine)], 2a. Light brown solid, yield
297 mg (65%). Anal. calcd for C15H26Cl4N2O2Ti: C, 39.50; H,
5.75; N, 6.14; Cl, 39.50; Ti, 10.50. Found: C, 39.33; H, 5.87; N,
6.16; Cl, 39.19; Ti, 10.61. IR (solid state): n ¼ 3306 w, 3240 w-br,
2984 w-br, 2675 w-br, 2488 w-br, 1691 vs, 1652 vs, 1568 s, 1454 s,
1228 m, 1099 m-s, 1070 m-s, 749 vs, 702 s cmꢀ1 1H NMR
.
(CD2Cl2): d ¼ 9.09 (s, 1H, NH); 7.39–7.30 (5H, Ph); 4.38 (m, 1H,
CH); 4.13, 3.62 (br, 2H, NH2); 3.26 (m, 6H, NCH2); 3.40, 3.17 (dd,
3
2H, CH2Ph); 1.40 (t, JHH ¼ 7.34 Hz, 9H, NCH2CH3) ppm. 13C
NMR (CD2Cl2): d ¼ 179.5 (OCO); 135.9 (ipso-Ph); 129.4, 129.2,
127.5 (Ph); 61.7 (CH); 47.1 (NCH2); 38.6 (CH2Ph); 9.0
(NCH2CH3) ppm.
[Me2NHCH(CH2Ph)C(O)Cl][NbCl6], 5a. Orange solid, yield
214 mg (47%) from PCl5/NbCl5/L-N,N-dimethylphenylalanine.
Anal. calcd for C11H15Cl7NNbO: C, 25.49; H, 2.92; N, 2.70; Cl,
47.88; Nb, 17.92. Found: C, 25.23; H, 2.80; N, 2.67; Cl, 47.60; Nb,
18.10. IR (solid state): n ¼ 3087 w-m, 3029 w, 2939 w, 1767 vs
(nC]O), 1460 s, 1440 m, 1411 w, 1374 w, 1340 w, 1287 w, 1198 w,
1172 w, 1138 w, 1078 w, 1033 m, 1016 w, 991 w-m, 936 m, 917 w,
[NHEt3][TiCl4(L-N,N-dimethylphenylalanine)], 2b. Yellow solid,
yield 290 mg (60%). Anal. calcd for C17H30Cl4N2O2Ti: C, 42.18;
H, 6.25; N, 5.79; Cl, 29.29; Ti, 9.89. Found: C, 42.36; H, 6.08; N,
5.65; Cl, 29.41; Ti, 9.72. IR (solid state): n ¼ 3260 w-br, 2963 w-br,
2679 w-br, 1702 s, 1660 vs, 1581 vs, 1454 vs, 1260 m, 1012 s,
741 s, 699 s cmꢀ1. 1H NMR (CD2Cl2): d ¼ 8.0 (br, 1H, NH); 7.36–
7.26 (5H, Ph); 4.47 (m, 1H, CH); 3.32 (m, 6H, NCH2); 3.19, 3.07
(m, 2H, CH2Ph); 2.95, 2.73 (s, 6H, NMe2); 1.42 (t, 3JHH ¼ 7.34 Hz,
9H, NCH2CH3) ppm. 13C NMR (CD2Cl2): d ¼ 178.8 (OCO); 138.9
(ipso-Ph); 129.9, 129.2, 128.6, 126.6 (Ph); 75.6 (CH); 50.9, 47.6
(NMe2); 47.5 (NCH2); 30.8 (CH2Ph); 8.9 (NCH2CH3) ppm.
896 vs, 839 m, 820 w, 745 m, 726 vs, 693 vs cmꢀ1 1H NMR
.
(CD2Cl2): d ¼ 7.68 (br, 1H, NH); 7.47–7.35 (m, 5H, Ph); 4.83 (s,
1H, CH); 3.62 (m, 2H, CH2); 3.34, 3.22 (s, 6H, NMe2) ppm. 13C
NMR (CD2Cl2): d ¼ 170.6 (C]O); 130.7, 130.0, 129.2 (Ph); 127.7
(ipso-Ph); 76.5 (CH); 44.4, 43.4 (NMe2); 34.6 (CH2) ppm. 93Nb
NMR (CD2Cl2): d ¼ 8.1 (Dn12 ¼ 3·102 Hz) ppm.
[MeNH2CH2C(O)Cl][NbCl6], 5b. Yellow solid, yield 144 mg
(43%) from PCl5/NbCl5/sarcosine. Anal. calcd for C3H7Cl7-
NNbO: C, 8.70; H, 1.70; N, 3.38; Cl, 59.92; Nb, 22.43. Found: C,
8.80; H, 1.59; N, 3.31; Cl, 59.60; Nb, 22.55. IR (solid state): n ¼
3138 m-s (nN–H), 2983 w, 2943 w, 1766 vs (nC]O), 1546 w-m, 1454
Reaction of NbCl5 with L-proline/NHiPr2: synthesis and
i
isolation of
[NbCl6], 4
, 3, and [NH2 Pr2]
NbCl5 (0.385 g, 1.42 mmol) and L-proline (0.163 g, 1.42 mmol) m, 1430 m, 1414 m, 1397 vs, 1341 m, 1160 m, 1129 w, 1036 m,
were allowed to react in CH2Cl2 (20 mL). The solution was 999 vs, 926 vs, 789 vs, 767 vs cmꢀ1. 1H NMR (CD3CN): d ¼ 7.42
repetitiously purged with nitrogen gas in order to remove (br, 2H, NH2); 4.67 (t, 2H, 4JHH ¼ 5.38 Hz, CH2); 3.21 (q, 3H, 4JHH
released HCl. Aer six hours, the yellowish mixture was treated ¼ 5.38 Hz, Me) ppm. 13C NMR (CD3CN): d ¼ 169.0 (C]O); 58.2
93
with NHiPr2 (0.203 mL, 1.45 mmol), then the stirring was pro- (CH2); 35.9 (Me) ppm. Nb NMR (CD3CN): d ¼ 0.1 (Dn1 ¼ 31
2
longed for additional 20 min. The nal dark-red mixture was Hz) ppm.
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Reactions of a-amino acids with PCl5/WCl6: synthesis of
[NH2(CH2)3CHC(O)Cl][WCl6], 7a, [Me2NHCH(CH2Ph)C(O)Cl]
[WCl6], 7b, and [(R)NH2CH(R0)C(O)Cl]+ (R ¼ Me, R0 ¼ H; R ¼
H, R0 ¼ CH2CH2SMe)
[NH3CH(CH2CH2SMe)C(O)Cl]+. Brown solid, from PCl5/WCl6/
L-methionine. IR (solid state): n ¼ 3350 w-m (nN–H), 3008 w, 2921
w, 1779 s (nC]O), 1569 w, 1480 w, 1449 w-m, 1415 m, 1367 w-m,
1309 w, 1263 w, 1176 w, 1144 w, 1094 w, 1025 m, 964 vs, 897 s,
759 s, 733 m, 699 w, 661 w cmꢀ1. Magnetic measurement:
ccMorr ¼ 7.08 ꢂ 10ꢀ4 cgsu.
General procedure. A suspension of PCl5 (163 mg, 0.78
mmol) and WCl6 (310 mg, 0.78 mmol) in CD2Cl2 (4 mL) was
stirred at room temperature overnight. Then the appropriate a-
amino acid (0.78 mmol) was added. The resulting mixture was
stirred at room temperature for 3 h. Thus 31P NMR analyses
revealed the presence of POCl3 as unique phosphorous species.
In addition, 1H and 13C NMR analyses on PCl5/WCl6/L-N,N-
dimethylphenylalanine and PCl5/WCl6/sarcosine solutions
pointed out the clean formation of 7a and [MeNH2CH2C(O)Cl]+.
These solutions were layered with hexane and stored at ꢀ30 ꢁC
for a few days, thus resulting in the isolation of 7a and a mixture
of [MeNH2CH2C(O)Cl]+ salts. In the cases of PCl5/WCl6/L-
methionine and PCl5/WCl6/L-proline, dark solid materials
precipitated, which were isolated from the respective yellow
solutions and dried in vacuo.
Synthesis of a-amino acid ester hydrochlorides
These compounds were obtained by a slight modication of the
literature procedures.
Procedure A (compounds 8a–d·HCl). A 250 mL ask was
charged with the appropriate alcohol (120 mL)/a-amino acid
(ca. 35 mmol) combination. SOCl2 (12 mL, 170 mmol) was
slowly added (3 h) to the suspension under vigorous stirring at
room temperature. Aer 24 h stirring, volatiles were removed in
vacuo at room temperature. The residue was suspended in Et2O
(50 mL) for 4 h. The suspension was ltered and the resulting
ꢁ
solid was dried in vacuo at 40 C.
Procedure B (compounds 8e–i·HCl). SOCl2 (10 mL, 138
mmol) was slowly added (30 minutes) at 0 ꢁC to the alcohol (80
mL) in a 500 mL Schlenk tube. The solution was then allowed to
reach room temperature and the a-amino acid (24 mmol) was
introduced. The mixture was reuxed for 8 h and a pale yellow
solution was obtained. Aerwards, the volatiles were removed in
vacuo and the residue was suspended in Et2O (50 mL) for 2 h.
The suspension was ltered and the resulting solid was dried in
In a different experiment, a 1 : 1 PCl5/WCl6 mixture obtained
in CD2Cl2 (3 mL) was analyzed. LM (CD2Cl2) ¼ 0.3 S cm2 molꢀ1
.
31P NMR (CD2Cl2): d ¼ ꢀ81.1 (PCl5) ppm. Cl analysis was carried
out on the solid residue obtained by removal of the volatiles in
vacuo. Anal. calcd for Cl11PW: Cl, 64.48. Found: Cl, 63.91.
Magnetic measurement: diamagnetic.
[NH2(CH2)3CHC(O)Cl][WCl6], 7a. Green solid, yield 302 mg
(73%) from PCl5/WCl6/L-proline. Anal. calcd for C5H9Cl7NOW:
C, 11.31; H, 1.71; N, 2.64; Cl, 46.72. Found: C, 11.20; H, 1.78; N,
2.50; Cl, 46.33. IR (solid state): n ¼ 3133 m (nN–H), 3073 m, 2955
w, 1768 vs (nC]O), 1563 m, 1455 w, 1375 w-m, 1343 w-m, 1079 w,
ꢁ
vacuo at 40 C.
L-Proline methylester hydrochloride, 8a·HCl.40a,b Colourless
solid, yield 97%. IR (liquid lm): n ¼ 3115 w (nN–H), 3021 w (nN–
H), 2805 m, 1755 vs (nC]O), 1634 m, 1442 m, 1391 m-s, 1089 m,
1
1043 m, 996 vs, 942 m, 906 w, 864 m cmꢀ1. H NMR (CD3CN):
1
1015 m cmꢀ1. H NMR (CDCl3): d ¼ 10.68, 9.04 (br, 2H, NH2);
d ¼ 7.36 (t, 2H, 3JHH ¼ 50 Hz, NH2); 4.80 (m, 1H, CH); 3.45 (m,
2H, NCH2); 2.60, 2.36 (m, 2H, CH2); 2.14–2.06 ppm (m, 2H,
CH2) ppm. 13C NMR (CD3CN): d ¼ 171.7 (C]O); 68.3 (CH); 48.4
(NCH2); 29.1, 23.7 ppm (CH2) ppm. Magnetic measurement:
ccMorr ¼ 3.60 ꢂ 10ꢀ4 cgsu, meff ¼ 0.93 BM.
4.51 (m, 1H, CH); 3.86 (s, 3H, OMe); 3.54 (m, 2H, NCH2); 2.48,
2.20, 2.10 (m, 4H, CH2) ppm. 13C{1H} NMR (CDCl3): d ¼ 169.3
(C]O); 59.2 (CH); 53.6 (OMe); 46.0 (NCH2); 28.7, 23.6
(CH2) ppm.
L-Proline ethylester hydrochloride, 8b·HCl.40a Colourless solid,
yield 79%. 1H NMR (CDCl3): d ¼ 10.57, 8.92 (br, 2H, NH2); 4.42
[Me2NHCH(CH2Ph)C(O)Cl][WCl6], 7b. Dark yellow – brown
solid, yield 285 mg (60%) from PCl5/WCl6/L-N,N-dimethylpheny-
lalanine. Anal. calcd for C11H15Cl7NOW: C, 21.69; H, 2.48; N,
2.30; Cl, 40.73. Found: C, 21.78; H, 2.35; N, 2.20; Cl, 40.32. IR
(solid state): n ¼ 3122 m-br, 3073 m, 2987 w-m, 2946 w, 2789 w,
1783 vs (nC]O), 1564 w, 1455 m, 1433 w-m, 1400 m, 1347 w, 1260
w, 1164 w-m, 1041 w, 1001 s, 952 m, 930 m, 857 s, 802 s, 774 s
cmꢀ1. 1H NMR (CD2Cl2): d ¼ 7.74 (br, 1H, NH); 7.49–7.39 (m, 5H,
Ph); 4.66 (br, 1H, CH); 3.62 (m-br, 2H, CH2); 3.31, 3.19 (s, 6H,
NMe2) ppm. 13C NMR (CD2Cl2): d ¼ 170.8 (C]O); 131.3, 130.0,
129.9, 129.2 (Ph); 78.3 (CH); 50.2, 48.1 (NMe2); 36.7 (CH2) ppm.
Magnetic measurement: ccMorr ¼ 4.02 ꢂ 10ꢀ4 cgsu, meff ¼ 0.98 BM.
[MeNH2CH2C(O)Cl]+. Green solid, from PCl5/WCl6/sarcosine.
IR (solid state): n ¼ 3104 m-br (nN–H), 2980 w, 2938 w, 2796 w,
1765 vs (nC]O), 1562 w-m, 1455 m, 1431 m, 1406 m, 1345 w-m,
1165 w-m, 1136 w, 1037 w, 1009 s, 991 vs, 951 m, 929 vs, 854 s,
806 vs, 773 vs, 728 s cmꢀ1. 1H NMR (CD3CN): d ¼ 7.29 (br, 2H,
NH2); 4.58 (s, 2H, CH2); 3.21 (s, 3H, Me) ppm. 13C NMR
(CD3CN): d ¼ 169.0 (C]O); 63.2 (CH2); 40.4 (Me) ppm. Magnetic
measurement: ccMorr ¼ 1.13 ꢂ 10ꢀ3 cgsu.
3
(m, 1H, CH); 4.24 (q, JHH ¼ 5.87 Hz, 2H, OCH2); 3.50 (m, 2H,
3
NCH2); 2.40, 2.10 (m, 4H, CH2); 1.27 (t, JHH ¼ 5.87 Hz, 3H,
OCH2CH3) ppm. 13C{1H} NMR (CDCl3): d ¼ 168.7 (C]O); 62.9
(OCH2); 59.2 (CH); 45.9 (NCH2); 28.7, 23.6 (CH2); 14.0
(OCH2CH3) ppm.
L-Proline isopropylester hydrochloride, 8c·HCl.40a Colourless
solid, yield 97%. 1H NMR (CDCl3): d ¼ 10.70, 8.81 (br, 2H, NH2);
5.11 (sept, 3JHH ¼ 6.2 Hz, 1H, OCH); 4.44 (m, 1H, CH); 3.57 (m,
2H, NCH2); 2.43, 2.12, 2.03 (m, 4H, CH2); 1.29 (pseudo-t, 3JHH
¼
6.2 Hz, 6H, OCHMe2) ppm. 13C{1H} NMR (CDCl3): d ¼ 168.3 (C]
O); 71.2 (OCH); 59.3 (CH); 46.1 (NCH2); 29.0, 23.6 (CH2); 21.6
(OCHMe2) ppm.
L-Phenylalanine methylester hydrochloride, 8d·HCl.40b Colour-
less crystalline solid, yield 83%. IR (solid state): n ¼ 3091 m-br
(nN–H), 2944 w-sh, 2906 w-sh, 2838 w, 2625 w, 1743 vs (nC]O),
1583 w-m, 1495 m, 1447 w-m, 1436 w-m, 1400 w, 1358 w, 1327 w,
1291 w-m, 1238 vs, 1214 vs, 1146 m, 1119 m, 1084 s, 1061 m,
1
1033 w, 990 m, 934 m, 865 w, 760 n, 741 vs, 701 vs cmꢀ1. H
NMR (CDCl3): d ¼ 8.7 (br, 3H, NH3); 7.31, 7.28 (m, 5H, Ph); 4.38
10168 | RSC Adv., 2017, 7, 10158–10174
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(br, 1H, CH); 3.72 (s, 3H, OMe); 3.42 (m-br, 2H, CH2) ppm. 13
C
61.0 (OCH2); 59.7 (CH); 46.9 (NCH2); 30.2, 25.4 (CH2); 14.2
{1H} NMR (CDCl3): d ¼ 169.2 (C]O); 133.9 (ipso-Ph); 129.6, (OCH2CH3) ppm.
129.0, 127.7 (Ph); 54.4 (CH); 53.0 (OMe); 36.3 (CH2) ppm.
L-Proline isopropylester, 8c.40i Pale orange liquid, yield 60%.
L-Leucine ethylester hydrochloride, 8e·HCl.40d Colourless solid, 1H NMR (CDCl3): d ¼ 5.03 (sept, 3JHH ¼ 6.3 Hz, 1H, OCH); 3.71
yield 80%. 1H NMR (CDCl3): d ¼ 8.5 (br, 3H, NH3); 4.26 (m, 2H, (dd, JHH ¼ 8.3, 5.6 Hz, 1H, CH); 3.12–3.04 (m, 1H, NCHH0);
3
OCH2); 4.07 (m, 1H, CH); 1.92 (m, 1H, CHMe2); 1.83 (m, 2H, 2.94–2.85 (m, 1H, NCHH0); 2.28 (br, 2H, NH2); 2.18–2.06 (m, 1H)
3
3
CH2); 1.31 (m, 6H, CHMe2); 1.23 (t, JHH ¼ 6.85 Hz, 3H, OCH2- and 1.86–1.67 (m, 3H, CH2CH2); 1.25 (d, JHH ¼ 3.5 Hz, 3H,
CH3) ppm. 13C{1H} NMR (CDCl3): d ¼ 169.9 (C]O); 62.6 (OCH2); CHMeMe0); 1.23 (d, 3JHH ¼ 3.6 Hz, 3H, CHMeMe0) ppm. 13C{1H}
51.9 (CH); 39.5 (CH2); 24.5 (CHMe2); 22.2, 22.1 (CHMe2); 14.0 NMR (CDCl3): d ¼ 174.6 (C]O); 68.0 (OCH); 59.6 (CH); 46.8
(OCH2CH3) ppm.
L-Leucine isopropylester hydrochloride, 8f·HCl.40e Colourless
(NCH2); 30.1, 25.2 (CH2); 21.5 (OCHMe2) ppm.
L-Phenylalanine methylester, 8d.40c Pale orange liquid, yield
solid, yield 81%. 1H NMR (CDCl3): d ¼ 8.21, 6.64 (br, 3H, NH3); 63%. IR (liquid lm): n ¼ 3381 w (nN–H), 3062 w, 3028 w, 2951 w,
5.11 (m, 1H, OCH); 4.01 (m, 1H, CH); 1.89 (m, 1H, CHMe2); 1.81 1732 vs (nC]O), 1603 w-m, 1496 m, 1454 m, 1436 m, 1266 m,
(m, 2H, CH2); 1.28 (m, 6H, CHMe2); 0.98 (m, 6H, OCHMe2) ppm. 1195 s, 1172 s, 1111 m, 1076 m, 1009 m, 826 m, 812 m, 744 m-s,
13C{1H} NMR (CDCl3): d ¼ 169.3 (C]O); 70.8 (OCH); 52.0 (CH); 699 vs cmꢀ1. 1H NMR (CDCl3): d ¼ 7.24–7.12 (5H, Ph); 3.66 (m,
39.5 (CH2); 24.8 (CHMe2); 22.2 (CHMe2); 21.6 (OCHMe2) ppm.
1H, CH); 3.63 (s, 3H, OMe); 3.01, 2.80 (m, 2H, CH2) ppm. 13C
Glycine isopropylester hydrochloride, 8g·HCl.40f Colourless {1H} NMR (CDCl3): d ¼ 175.3 (C]O); 137.3 (ipso-Ph); 129.3,
1
solid, 97% yield. H NMR (CDCl3): d ¼ 8.50 (br, 3H, NH3); 5.10 128.5, 126.7 (Ph); 55.8 (OMe); 51.8 (CH); 41.0 (CH2) ppm.
(sept, 3JHH ¼ 6.1 Hz, 1H, OCH); 3.96 (q, 3JHH ¼ 4.2 Hz, 2H, CH2);
L-Leucine ethylester, 8e.40d Pale yellow liquid, yield 74%. H
NMR (CDCl3): d ¼ 4.16 (m, 2H, OCH2); 3.44 (m, 1H, CH); 1.77
1
3
1.26 (d, JHH ¼ 6.1 Hz, 6H, CHMe2) ppm.
L-Serine isopropylester hydrochloride, 8h·HCl.40g Colourless (m, 1H, CHMe2); 1.65 (s, 2H, NH2); 1.55, 1.43 (m, 2H, CH2); 1.27
1
3
solid, 98% yield. H NMR (DMSO-d6): d ¼ 8.51 (br, 3H, NH3); (t, JHH ¼ 6.85 Hz, 3H, OCH2CH3); 0.93 (m, 6H, CHMe2) ppm.
5.71–5.52 (m, 1H, OH); 5.05–4.94 (m, 1H, CHMe2); 4.04–3.98 (m, 13C{1H} NMR (CDCl3): d ¼ 176.5 (C]O); 60.7 (OCH2); 52.9 (CH);
1H, CHH0); 3.83–3.78 (s, 2H, CHH0 + CHN); 1.26–1.21 (m, 6H, 44.0 (CH2); 24.7 (CHMe2); 22.9, 21.8 (CHMe2); 14.2
CHMe2) ppm. 13C{1H} NMR (DMSO-d6): d ¼ 167.6 (CO); 69.6 (OCH2CH3) ppm.
(CHMe2); 59.5 (CH2); 54.4 (CHN); 21.5 and 21.4 (CHMe2) ppm.
L-Leucine isopropylester, 8f.40e Pale yellow liquid, yield 55%.
L-Tyrosine isopropylester hydrochloride, 8i·HCl.40h Colourless 1H NMR (CDCl3): d ¼ 5.05 (sept, 1H, OCH); 3.49 (m, 1H, CH);
solid, 71% yield. 1H NMR (DMSO-d6): d ¼ 9.49 (s, 1H, OH); 8.49 2.90 (s, 2H, NH2); 1.80 (m, 1H, CHMe2); 1.57, 1.50 (m, 2H, CH2);
3
3
(br, 3H, NH3); 7.01 (d, JHH ¼ 8.2 Hz, 2H, Ar); 6.71 (d, JHH
¼
1.26 (m, 6H, OCHMe2); 0.94 (m, 6H, CHMe2) ppm. 13C{1H} NMR
8.2 Hz, 2H, Ar); 4.88 (sept, 3JHH ¼ 5.9 Hz, 1H, OCH); 4.07 (t, 3JHH (CDCl3): d ¼ 175.1 (C]O); 68.5 (OCH); 52.8 (CH); 43.5 (CH2);
3
¼ 6.6 Hz, 1H, NCH); 3.17–2.83 (m, 2H, CH2); 1.16 (d, JHH
¼
24.7 (CHMe2); 22.9, 21.9 (CHMe2); 21.7 (OCHMe2) ppm.
Glycine isopropylester, 8g.40f Pale yellow viscous liquid, yield
3
6.2 Hz, 3H) and 1.06 (d, JHH ¼ 6.2 Hz, 3H, CHMe2) ppm.
Synthesis of a-amino acid esters. Three different procedures 60%. 1H NMR (CDCl3): d ¼ 5.05 (sept, 3JHH ¼ 6.3 Hz, 1H, OCH);
were adopted. Compounds 8a–g were prepared by treating the 3.40 (s, 2H, CH2); 1.89 (br, 2H, NH2); 1.24 (d, 3JHH ¼ 6.2 Hz, 6H,
appropriate a-amino acid ester hydrochloride with NH3(aq) as CHMe2) ppm.
described in detail for 8a. Compounds 8h–j were prepared by
L-Serine isopropylester, 8h.40f Compound 8h·HCl (1.61 g, 8.75
treating the appropriate a-amino acid ester hydrochloride with mmol) was dissolved in NaOH 1.0 M (8.75 mL, 8.75 mmol) in
NaOH(aq) as described in detail for 8h. Compounds 8k–l were a 25 mL round bottom ask. The solution was stirred at room
obtained directly from the appropriate alcohol/a-amino acid temperature for 10 minutes then extracted with CH2Cl2 (3 ꢂ 20
(ca. 50 mmol) combination, followed by treatment with NH3(aq)
attempts to isolate (8k–l)·HCl led to mixtures of products.
;
mL). The organic phase was dried with Na2SO4, ltered and the
solvent was removed by distillation at 40 ꢁC and p ¼ 700 mbar.
L-Proline methylester, 8a.40a,c Compound 8a·HCl (10.2 g, 60.4 The product was obtained as a colourless viscous liquid, 1.11 g,
mmol) was dissolved into CH2Cl2 (100 mL) and the solution was yield 87%. IR(liquid lm): n ¼ 3500–3100 br, 3362 w, 3308 w,
treated with a 28% w/w NH3 aqueous solution until neutrality. 2981 w, 2938 w, 2879 w, 1728 s (nC]O), 1595 w, 1468 w, 1455 w,
The mixture was le stirring at room temperature for 24 h. The 1384 w-sh, 1375 m, 1326 w, 1209 m, 1179 m, 1145 m, 1105 s,
phases were separated and the aqueous layer was extracted with 1039 m, 933 m, 903 w, 849 w, 822 m cmꢀ1. 1H NMR (CDCl3): d ¼
3
2
CH2Cl2 (2 ꢂ 50 mL). The organic phase was ltered through an 5.07 (sept, JHH ¼ 6.2 Hz, 1H, OCH); 3.85 (dd, JHH ¼ 10.2 Hz,
alumina pad and solvent was removed by distillation at 40 C 3JHH ¼ 2.5 Hz, 1H, CHH0); 3.71 (dd, JHH ¼ 10.7 Hz, JHH
¼
2
3
ꢁ
and p ¼ 700 mbar. The product was obtained as a pale orange 5.8 Hz, 1H, CHH0); 3.66–3.60 (m, 1H, CHN); 2.59 (br, 3H, NH2 +
1
liquid, which was stored under argon. Yield 4.71 g (60%). H OH); 1.29–1.25 (m, 6H, CHMe2) ppm. 13C{1H} NMR (CDCl3): d ¼
NMR (CDCl3): d ¼ 3.54 (s, 3H, CH3); 2.92 (m, 1H, CH); 2.48 (m, 173.3 (CO); 68.8 (OCH); 63.9 (CH2); 56.1 (CHN); 21.7 (Me2) ppm.
2H, CH2N); 1.99, 1.64 (m, 4H, CH2) ppm.
1H NMR (CD3CN): d ¼ 4.97 (sept, 3JHH ¼ 6.3 Hz, 1H, OCH); 3.62–
L-Proline ethylester, 8b.40a Pale yellow liquid, yield 65%. IR 3.58 (m, 2H, CH2); 3.38 (t, 3JHH ¼ 4.8 Hz, 1H, CHN); 2.19 (br, 3H,
(liquid lm): n ¼ 1730 vs (nC]O) cmꢀ1. 1H NMR (CDCl3): d ¼ 4.12 NH2 + OH); 1.23–1.20 (m, 6H, Me2) ppm. 13C{1H} NMR (CD3CN):
(q, 3JHH ¼ 6.85 Hz, 2H, OCH2); 3.73 (m, 1H, CH); 3.03, 2.90 (m, d ¼ 174.6 (CO); 68.9 (OCH); 65.1 (CH2); 57.3 (CHN); 22.0 and
2H, NCH2); 2.07, 1.81, 1.73 (m, 4H, CH2); 1.22 (t, 3JHH ¼ 6.85 Hz, 21.9 (Me2) ppm. The compound was dissolved in CDCl3 and
3H, OCH2CH3) ppm. 13C{1H} NMR (CDCl3): d ¼ 173.9 (C]O); stored in a graduated Schlenk tube under nitrogen. The
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concentration of the solution was determined by 1H NMR, using 3.84, 3.70 (m, 2H, NCH2); 2.60, 2.28, 2.20 (m, 4H, CH2) ppm. 19
CH2Br2 as internal standard.
F
1
NMR (CD3CN): d ¼ 100.9 (decet, JNbF ¼ 343 Hz, NbF6ꢀ) ppm.
L-Tyrosine isopropylester, 8i.40j Colourless solid, yield 83%.
The reaction of NbF5 (0.50 mmol) with 8b (0.50 mmol) was
3
1H NMR (CDCl3): d ¼ 7.02 (d, JHH ¼ 8.3 Hz, 2H, Ph); 6.68 (d, performed also inside a sealed NMR tube (CD2Cl2, 0.70 mL). A
3
3JHH ¼ 8.3 Hz, 2H, Ph); 5.05 (sept, JHH ¼ 6.5 Hz, 1H, OCH); complicated mixture of compounds were detected by
3.73–3.63 (m, 1H, NCH); 3.23 (br, 3H, NH2 + OH); 3.04 (dd, 2JHH NMR, including ethyl uoride [d(1H) ¼ 4.38 (m), 1.35 (t) ppm;52
¼ 13.8 Hz, 3JHH ¼ 5.0 Hz, 1H, CHH0); 2.81 (dd, 2JHH ¼ 13.7 Hz, d(13C) ¼ 77.6, 13.7 ppm;52 d(19F) ¼ ꢀ209.9 (br) ppm]. Addition of
3JHH ¼ 7.7 Hz, 1H, CHH0); 1.29–1.22 (m, 6H, CHMe2) ppm.
D2O (ca. 0.1 mL) to a solution of 10 in CD3CN (0.6 mL)53 resulted
L-Alanine ethylester, 8j.40k Prepared from commercial L- in the formation of an abundant precipitate; a solution was
alanine ethylester hydrochloride (Fluka). Pale yellow viscous separated whose 13C NMR analysis evidenced the presence of L-
3
liquid, yield 60%. 1H NMR (CDCl3): d ¼ 4.01 (q, JHH ¼ 6.6 Hz, proline only.
2H, OCH2); 3.37 (q, 3JHH ¼ 6.6 Hz, 1H, NCH); 1.54 (br, 2H, NH2);
3
1.17 (d, JHH ¼ 6.9 Hz, 3H, CH3CH); 1.13 (t, 3JHH ¼ 7.1 Hz, 3H,
Reactions of NbCl5 with a-amino acid esters
CH3CH2) ppm.
L-Leucine methylester, 8k.40l Pale yellow liquid, yield 42%. IR
Synthesis and isolation of [NbCl4(Me2CHCH2CHNH2CO2-
(liquid lm): n ¼ 3380 w-m (nN–H), 2956 m, 2932 w, 2869 w, 2851 Me)2][NbCl6], 11a, and [NbCl5(Me2CHCH2CHNH3CO2Me)]
w, 1740 vs (nC]O), 1610 w, 1468 m, 1436 m, 1384 w, 1368 w, 1316 [NbCl6], 12. A suspension of NbCl5 (199 mg, 0.736 mmol) in
w, 1270 m, 1198 s, 1145 m, 1010 m, 971 w, 921 w, 869 w, 837 w, CH2Cl2 (15 mL) was treated with 8k (97 mg, 0.74 mmol). Aer
1
822 w cmꢀ1. H NMR (CDCl3): d ¼ 3.64 (s, 3H, OMe); 3.41 (m, 24 h stirring at room temperature, a pale orange solution was
1H, CH); 1.70 (m, 1H, CHMe2); 1.65 (s, 2H, NH2); 1.4 (m-br, 2H, obtained. By addition of hexane, 11a was obtained as a colour-
CH2); 0.86 (m, 6H, CHMe2) ppm. 13C{1H} NMR (CDCl3): d ¼ less solid. Yield 130 mg (44% based on Nb). Anal. calcd for
177.1 (C]O); 53.0 (OMe); 52.0 (CH); 44.2 (CH2); 24.8 (CHMe2);
23.2, 22.0 (CHMe2) ppm.
C14H30Cl10N2Nb2O4: C, 20.24; H, 3.64; N, 3.37; Cl, 42.68; Nb,
22.37. Found: C, 20.05; H, 3.18; N, 3.27; Cl, 42.98; Nb, 23.13. IR
L-Phenylalanine isopropylester, 8l.40m Colourless solid, yield (solid state): n ¼ 3292 w (nN–H), 3243 w, 2959 w-m, 1633 m (nC]
1
36%. H NMR (CDCl3): d: M7.32–7.22 (5H, Ph); 5.03 (sept, 1H, O), 1569 m, 1467 m, 1387 m, 1327 m, 1290 w-m, 1152 w-m, 1123
OCH); 3.71 (m, 1H, CH); 3.08, 2.90 (m, 2H, CH2); 2.10 (s, 2H, m, 1015 w-m, 934 w-m, 856 vs, 746 m-s cmꢀ1. 1H NMR (CDCl3):
NH2); 1.24, 1.20 (m, 6H, OCHMe2) ppm. 13C{1H} NMR (CDCl3): R d ¼ 7.17 (br, 2H, NH2); 4.44 (s, 3H, OMe); 4.40 (m, 1H, CH); 1.96
(174.2 (C]O); 137.2 (ipso-Ph); 129.4, 128.5, 126.8 (Ph); 68.6 (m, 1H, CHMe2); 1.51 (m, 2H, CH2); 1.10 (m, 6H, CHMe2) ppm.
ꢀ
(OCH); 55.8 (CH); 40.9 (CH2); 21.7 (OCHMe2) ppm.
93Nb NMR (CDCl3): d ¼ 7.8 (Dn1/2 ¼ 4 ꢂ 102 Hz, NbCl6 ) ppm.42a
Hydrolysis53 of a CD3CN solution (0.6 mL) of 11a resulted in the
formation of an abundant precipitate; a solution was separated
whose 13C NMR analysis evidenced the presence of 8k only.
In a different experiment, the reaction solution was set aside
Reactions of NbF5 with a-amino acid esters
Synthesis of [NbF4(Me2CHCH2CHNH2CO2Me)2][NbF6], 9. A
suspension of NbF5 (167 mg, 1.15 mmol) in CH2Cl2 (15 mL) was at ca. ꢀ30 ꢁC for two weeks. Thus pale yellow crystals of 12 were
treated with 8k (167 mg, 1.15 mmol). Aer 24 h stirring at room collected. Yield 65 mg (25% based on Nb). Anal. calcd for C7-
temperature, a colourless solution was obtained. By addition of H16Cl11NNb2O2: C, 11.64; H, 2.23; N, 1.94; Cl, 54.01; Nb, 25.74.
hexane (10 mL), 9 was obtained as a colourless solid, which was Found: C, 11.75; H, 2.16; N, 1.98; Cl, 53.70; Nb, 25.55. IR (solid
recovered by ltration and dried in vacuo at room temperature. state): n ¼ 3282 w-br (nN–H), 3241 w, 2959 w-m, 1641 m (nC]O),
Yield 185 mg (48%). Anal. calcd for C14H30F10N2Nb2O4: C, 25.24; 1559 m, 1457 m-w, 1377 m, 1327 m, 1281 w-m, 1158 w, 1128 m,
H, 4.54; N, 4.20; Nb, 27.89. Found: C, 25.01; H, 4.1; N, 4.01; Nb, 1011 w-m, 922 w-m, 860 vs, 744 m-s cmꢀ1
.
25.71. IR (solid state): n ¼ 3232 w (nN–H), 3064 m, 2966 m, 2877
Synthesis of [NbCl4(Me2CHCH2CHNH2CO2Et)2][NbCl6], 11b,
m, 1648 vs (nC]O), 1499 m, 1456 m, 1374 w, 1328 w, 1289 w, 1262
, 11c. These products were
w, 1170 w, 1129 w, 1067 w, 834 br-s cmꢀ1. 1H NMR (CDCl3): d ¼ prepared by a procedure analogous to that described for 11a,
8.6, 7.0 (br, 2H, NH2); 4.66 (s, 3H, OMe); 4.19 (m, 1H, CH); 2.02– from the appropriate niobium pentahalide (ca. 0.70 mmol)/a-
1.52 (m, 3H, CHMe2 + CH2); 1.03 (m, 6H, CHMe2) ppm. 19F NMR amino acid ester combination.
(CDCl3): d ¼ 140 (Dn1/2 ¼ 2.5 ꢂ 103 Hz, NbF4+); 103 (decet, 1JNbF
11b. Colourless viscous solid, yield 73%. Anal. calcd for C16-
¼ 345 Hz, NbF6ꢀ) ppm. 93Nb NMR (CDCl3): d ¼ ꢀ1553 (septet, H34Cl10N2Nb2O4: C, 22.38; H, 3.99; N, 3.26; Cl, 41.28; Nb, 21.64.
1JNbF ¼ 366 Hz, NbF6ꢀ) ppm.
Found: C, 22.25; H, 4.06; N, 3.16; Cl, 41.70; Nb, 21.29. 1H NMR
Synthesis of
, 10. Compound (CDCl3): d ¼ 7.00 (br, 2H, NH2); 4.50 (m, 2H, OCH2); 4.31 (br,
10 was prepared by a procedure analogous to that described for 1H, CH); 1.96 (m, 1H, CHMe2); 1.47 (m, 2H, CH2); 1.42 (m, 3H,
the synthesis of 9, from NbF5 (222 mg, 1.18 mmol) and 8b OCH2CH3); 1.07 (m, 6H, CHMe2) ppm. 13C NMR (CDCl3): d ¼
(85 mg, 0.59 mmol). Yield 167 mg (60%). Anal. calcd for C5H8- 170.2 (C]O); 65.5 (OCH2); 54.1 (CH); 39.4 (CH2); 25.0 (CHMe2);
F9NNb2O2: C, 12.75; H, 1.71; N, 2.97; Nb, 36.46. Found: C, 12.70; 22.3, 21.9 (CHMe2); 14.1 (OCH2CH3) ppm. 93Nb NMR (CDCl3):
H, 1.61; N, 3.03; Nb, 36.27. IR (solid state): n ¼ 3381 vw, 2965 w, d ¼ 13 (Dn1/2 ¼ 4 ꢂ 102 Hz, NbCl6ꢀ); ꢀ540 (Dn1/2 ¼ 8 ꢂ 103 Hz,
1636 vs (nC]O), 1481 m, 1346 w, 1296 w, 1281 w, 1234 w, 1138 w, NbCl4+) ppm.
1073 w, 1010 w, 983 m, 925 w, 910 w, 873 m, 765 m, 690 vs cmꢀ1
.
11c. Colourless solid, yield 70%. Anal. calcd for C14H26Cl10-
1H NMR (CD3CN): d ¼ 11.77 (br, 1H, NH); 4.83 (m, 1H, CH); N2Nb2O4: C, 20.34; H, 3.17; N, 3.39; Cl, 42.88; Nb, 22.48. Found:
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Table 4 Crystal data and experimental details for 3, 4, 5a and 12
3
4
5a
12
Formula
Fw
C
10H12Cl8N2Nb2O2
C6H16Cl6Nb
407.81
C10H12Cl8N2Nb2O2
518.30
C7H16Cl11NNb2O2
721.98
661.64
T, K
100(2)
100(2)
100(2)
100(2)
Crystal system
Space group
Monoclinic
P21/c
Monoclinic
P21/c
Orthorhombic
P21212
6.9864(4)
12.7821(7)
20.9934(11)
90
1874.73(18)
4
1.836
Monoclinic
P21
˚
a, A
6.7734(9)
13.4440(18)
11.9844(16)
103.854(2)
1059.6(2)
2
2.074
2.097
640
8.065(3)
18.492(7)
10.184(4)
94.036(5)
1515.1(10)
4
1.788
1.821
808
10.045(2)
6.9628(17)
16.994(4)
106.428(3)
1140.2(5)
2
2.103
2.297
700
˚
b, A
˚
c, A
ꢁ
b,
3
˚
Cell volume, A
Z
Dc, g cmꢀ3
m, mmꢀ1
F(000)
1.635
Independent reections
Data/restraints/parameters
Goodness on t on F2
R1 (I > 2s(I))
2315 [Rint ¼ 0.0261]
2315/0/109
1.065
3507 [Rint ¼ 0.0656]
3507/2/137
1.073
3338 [Rint ¼ 0.0312]
3338/0/195
1.214
4492 [Rint ¼ 0.0312]
4492/4/217
1.023
0.0406
0.0442
0.0249
0.0217
wR2 (all data)
Largest diff. peak and hole, e A
0.0913
2.092/ꢀ1.392
0.0870
0.769/ꢀ0.891
0.0575
0.372/ꢀ0.692
0.0428
0.286/ꢀ0.360
ꢀ3
˚
C, 19.98; H, 3.54; N, 3.47; Cl, 42.09; Nb, 21.79. IR (solid state): n ¼ experiment, the mixture obtained from NbBr5/8b was elimi-
2961 w, 2880 w, 1609 vs (nC]O), 1456 s, 1342 w, 1271 m, 1233 w-m, nated of the volatile materials; the residue was dissolved into
1052 w, 974 m, 922 w-m, 869 m, 802 m, 756 m, 656 w-m cmꢀ1. 1H CD3CN (0.6 mL) and then treated with H2O (ca. 0.2 mL).53
A
NMR (CD3CN): 7.2 (br, 2H, NH2); 4.73 (m, 1H, CH); 4.31 (m, 2H, yellow solution was separated from an abundant precipitate,
OCH2); 3.79 (br, 2H, NCH2); 2.58, 2.18 (m, 4H, CH2); 1.30 (m, 3H, and 13C NMR analysis of the solution evidenced the presence of
OCH2CH3) ppm. 93Nb NMR (CD3CN): d ¼ 4 (Dn1/2 ¼ 3 ꢂ 102 Hz, L-proline only.
NbCl6ꢀ); ꢀ569 (Dn1/2 ¼ 2 ꢂ 103 Hz, NbCl4 ) ppm.
+
i
Synthesis of NbCl3(OCH2CHNHCO2 Pr), 13. A suspension of
NbCl5 (258 mg, 0.954 mmol) and 8h (1.32 mL of a 0.72 M
solution in CDCl3, 0.95 mmol) in CHCl3 (8 mL) was reuxed for
31 h. Then, the mixture was allowed to cool to room tempera-
ture and the colourless solution was separated from the col-
ourless precipitate. The solid was washed with CH2Cl2 (2 ꢂ 5
mL) and dried in vacuo. Yield 239 mg (66% based on Nb). Anal.
calcd for C6H11Cl3NNbO3: Cl, 30.9. Found: Cl, 30.8.54 IR (solid
state): n ¼ 3580–3200 br, 3120 m-br, 2985 m, 2965 m, 2940 m,
1732 s (nC]O), 1675 w, 1576 m, 1486 m, 1471 m, 1387 m, 1377 m,
1304 w, 1258 s, 1184 w, 1146 m, 1098 s, 1080 s, 1033 s, 937 m,
895 m, 860 w, 800 s, 757 m cmꢀ1. 1H NMR (CD3CN): d ¼ 6.98 (br,
X-ray crystallographic studies
Crystal data and collection details for 3, 4, 5a and 12 are re-
ported in Table 4. The diffraction experiments were carried out
on a Bruker APEX II diffractometer equipped with a CCD
detector and using Mo-Ka radiation (l ¼ 0.71073 A). Data were
˚
corrected for Lorentz polarization and absorption effects
(empirical absorption correction SADABS).55 Structures were
solved by direct methods and rened by full-matrix least-
squares based on all data using F2.56 All non-hydrogen atoms
were rened with anisotropic displacement parameters. All
hydrogen atoms were xed at calculated positions and rened
by a riding model, except hydrogens bonded to N(1) in 4, 5a and
12 which were located in the Fourier map and rened iso-
tropically using the 1.2 fold (for 4 and 5a) and 1.5 fold (for 12)
Uiso value of the parent N-atom. The N(1)–H distances were
3
1H, NH); 5.09 (sept, JHH ¼ 6.2 Hz, 1H, CHMe2); 4.18–4.07
3
(m,1H, NCH), 4.01–3.92 (m, 2H, CH2); 1.28 (d, JHH ¼ 6.1 Hz,
6H, CHMe2) ppm. 13C{1H} NMR (CD3CN): d ¼ 167.3 (C]O); 72.2
(CHMe2); 63.4 (CH2); 59.9 (br, NCH); 21.7, 21.6 (CHMe2) ppm.
93Nb{1H} NMR (CD3CN): d ¼ ꢀ493.7 (Dn1/2 ¼ 2.3 ꢂ 103 Hz) ppm.
A sample of 13 was suspended in D2O (1 mL) with vigorous
stirring for 14 h.53 NMR analysis of the solution revealed the
presence of 8h only (pH ¼ 1.8).
˚
restrained to 0.93 A (s.u. 0.02).
Computational studies
The computational geometry optimizations were carried out
without symmetry constrains using the hybrid-GGA EDF2
functional,57 in combination with the 6-31G** basis set (ECP-
based LANL2DZ basis set for elements beyond Kr).58 The
“restricted” formalism was applied in all cases. The soware
MBr5 (M ¼ Nb, Ta) mediated formation of EtBr from L-proline
ethylester
The reactions of MBr5 (ca. 0.50 mmol) with 8b (0.50 mmol) were used was Spartan 08.59 Further computational geometry opti-
carried out in CD2Cl2 (ca. 1 mL). Subsequent NMR analysis mizations were carried out without symmetry constrains, using
pointed out the formation of a complicated mixture of the hyper-GGA functional M06,60 in combination with a polar-
compounds, including ethyl bromide.52 In
a
different ized basis set composed by the 6-31G(d,p) set on the light atoms
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and the ECP-based LANL2TZ(f) on the metal centre.61 The C-
PCM implicit solvation model (3 ¼ 9.08) was added to M06
calculations.62 Gaussian ’09 was used as soware.63 All the
stationary points were characterized by IR simulations
(harmonic approximation), from which zero-point vibrational
energies and thermal corrections (T ¼ 298.15 K) were ob-
tained.64 Vibrational simulation supported the interpretation of
experimental IR data. Cartesian coordinates of the optimized
geometries are collected in a separated. xyz le.
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Acknowledgements
The University of Pisa is gratefully acknowledged for nancial
support.
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