Molybdenum carbonyl complexes with pyridylimino acidato ligands

Article abstract of DOI:10.1039/b511637f

Reactions of [Mo(CO)₄(pip)₂] with pyridine-2-carbaldehyde and the appropriate amino ester or amino acid produce complexes with chelated pyridylimino ligands bearing, respectively, an ester (1) or carboxylate (2a,b, 4, 5a,b) pendant arm, the structures of which have been determined by X-ray crystallography. In the case of α-amino acids, the resulting imino carboxylate complexes are unstable towards decarboxylation, this being complete for (R)-2-phenylglycine. The products of decarboxylation 3a-c were isolated and characterized, including X-ray structure determinations for 3b and 3c. In contrast, the derivatives of β-alanine (4) and 3- and 4-aminobenzoic acids (5a,b) are stable towards decarboxylation. The structure determinations show that the pyridyliminocarboxylate complexes crystallize as salts with piperidinium cations, forming hydrogen-bonded ion-pair dimers featuring twelve- or eight-membered rings. Protonation of carboxylate complexes with 2 M HCl in CH₂Cl₂/water yields the corresponding neutral complexes 6a,b containing a free carboxylic acid functionality. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b511637f

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Molybdenum carbonyl complexes with pyridylimino acidato ligands†
Rau´l Garc´ıa-Rodr´ıguez and Daniel Miguel*
Received 8th August 2005, Accepted 27th October 2005
First published as an Advance Article on the web 18th November 2005
DOI: 10.1039/b511637f
Reactions of [Mo(CO)4(pip)2] with pyridine-2-carbaldehyde and the appropriate amino ester or
amino acid produce complexes with chelated pyridylimino ligands bearing, respectively, an ester (1) or
carboxylate (2a,b, 4, 5a,b) pendant arm, the structures of which have been determined by X-ray
crystallography. In the case of a-amino acids, the resulting imino carboxylate complexes are unstable
towards decarboxylation, this being complete for (R)-2-phenylglycine. The products of decarboxylation
3a–c were isolated and characterized, including X-ray structure determinations for 3b and 3c. In
contrast, the derivatives of b-alanine (4) and 3- and 4-aminobenzoic acids (5a,b) are stable towards
decarboxylation. The structure determinations show that the pyridyliminocarboxylate complexes
crystallize as salts with piperidinium cations, forming hydrogen-bonded ion-pair dimers featuring
twelve- or eight-membered rings. Protonation of carboxylate complexes with 2 M HCl in CH2Cl2/
water yields the corresponding neutral complexes 6a,b containing a free carboxylic acid functionality.
Introduction
Results and discussion
The use of metallic complexes in the labeling of biological mole-
cules is an expanding field1 which benefits from the development
of increasingly accurate techniques for the observation of radio-
active,2 spectroscopic, and luminescence properties.3 Additionally,
metallic complexes are being used as models to gain understanding
of key steps of the metabolism such as dehydrations, dehydrogena-
tions or decarboxylations,4 or as mimics for biological catalysts.5
On the other hand, the continuing interest on amino acids as
ligands for metal complexes, or as building blocks in organometal-
lic chemistry is stimulated by the expectations of attaching the
resulting complexes to biological molecules.6 We have described
recently the use of amino acidate anions as bridging ligands in het-
erobinuclear complexes containing manganese and molybdenum,7
and explored the metalation of benzodiazepines in ruthenium
complexes.8 In the course of current studies focussed on pyridyl
imino ligands we became interested in using amino acids as
convenient blocks to build chelate N–N ligands containing a side
arm with a carboxylate functionality. This could be used to occupy
a third coordination position in the metal, as a hemilabile tripodal
ligand or, alternatively, it could serve as an anchoring point to a
biomolecule.
In recent years, Herrick et al. have made available a conve-
nient method for the preparation of iminoester complexes of
molybdenum9,10 and rhenium.11 We thought it would be interesting
to test the method for the preparation of complexes containing
a carboxylate or free carboxylic acid terminus which would be
more capable of establishing supramolecular interactions than
their ester analogues. In this paper we wish to report the use of
this methodology to prepare a family of molybdenum complexes
containing a pendant arm with a free acid or carboxylate
functionality, which exhibit interesting solid state structures.
Pyridylimino glycine ester complex
=
[Mo(CO)4(py-2-C(H) NCH2COOEt)] (1)
A mixture of [Mo(CO)4(pip)2] (pip = piperidine),12 pyridine-
2-carbaldehyde, and ethylglycine hydrochloride was heated in
◦
=
ethanol (50 C), to afford [Mo(CO)4(py-2-C(H) NCH2COOEt)]
(1) (Scheme 1), which was isolated as a crystalline solid, and
characterized by analytical and spectroscopic methods.
Scheme 1
IR spectra of 1 in CH2Cl2 solution display four metal carbonyl
bands in the region 2020–1820 cm−1 in a pattern consistent
with a cis-tetracarbonyl geometry. An additional weaker band
at 1734 cm−1 can be attributed to the ester carbonyl stretches. The
1
pyridine and imine resonances in the H NMR spectra appear
as expected. The imine proton resonance at d 8.49 ppm is most
informative since its position is very sensitive to the chemical
environment. The presence of the unreacted ester attached to
the diimine ligand was confirmed by the signals associated with
the ethyl group. No decarboxylation product was observed in
the1H NMR spectra, in contrast with the carboxylate complexes
described below.
The molecular structure of compound 1 was determined by X-
ray diffraction on a crystal grown by slow diffusion of a layer of
hexane into a solution of 1 in CH2Cl2 at −20 ◦C. Crystallographic
and refinement details are summarized in the Experimental
section, and a perspective view is presented in Fig. 1.
Qu´ımica Inorga´nica, Facultad de Ciencias, Universidad de Valladolid,
C/Prado de la Magdalena s/n., E-47071, Valladolid, Spain. E-mail: dmsj@
qi.uva.es
† Dedicated to Dr Jose´ A. Abad on occasion of his retirement.
1218 | Dalton Trans., 2006, 1218–1225
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
and no significant amount of the expected carboxylate compound
2c was observed during the reaction. For alanine and valine, the
decarboxylation seems to be slower, and it was possible to isolate
the carboxylate complexes 2a and 2b with moderate to good yields
(see Experimental section).
Suitable crystals could be grown for 2a, 3b and 3c, and the
results of the molecular structure determinations are presented in
Fig. 2 (compound 2a), Fig. 3 (3b) and Fig. 4 (3c). In the three
structures, the distorted coordination environment around Mo is
similar to that described above for complex 1.
The structure of 2a shows an interesting arrangement of
anion/cation pair dimers, which will be described in detail together
with those found in other iminocarboxylate complexes prepared
in this work (see below). On the other hand, the high value of the
Flack parameter for the structure [0.42(7)] suggests that a nearly
total racemization of the chiral a-carbon occurs in the formation
of 2a.
Fig. 1 Perspective view of compound◦1 showing the atom numbering.
˚
Selected bond lengths (A) and angles ( ): Mo(1)–C(1) 2.022(4), Mo(1)–
C(2) 1.934(4), Mo(1)–C(3) 1.939(4), Mo(1)–C(4) 2.023(5), Mo(1)–N(1)
2.238(3), Mo(1)–N(2) 2.231(3), C(18)–O(5) 1.179(4), C(18)–O(6) 1.329(4);
C(1)–Mo(1)–C(4) 169.81(13), C(2)–Mo(1)–N(2) 169.79(13), C(3)–Mo(1)–
N(1) 173.04(13), N(2)–Mo(1)–N(1) 71.93(10), O(5)–C(18)–O(6) 124.7(4).
The structures of 3b and 3c confirmed the decarboxylation
occurred to the parent acidato complexes 2b and 2c.
a-Amino acid decarboxylation is a very important step in the
synthesis of neurotransmitter amino compounds. The decarboxy-
lation process is thought to take advantage of p-electron delocal-
ization. Coenzyme PLP (pyridoxal-5ꢀ-phosphate) reacts with a a-
amino acid to form an imine bond between the amino group of the
amino acid and the aldehyde group of the PLP. Decarboxylation
is produced because PLP functionalities serve as an “electron
sink” that stabilizes the developing carbanion in the transition
structure attending the loss of CO2.13 It has been suggested that
the developing negative charge on the a-carbon of the amino acid
is delocalized via the p-system of the coenzyme bound to the
substrate. Scheme 3 offers a comparison of the electron flow in the
PLP-assisted enzymatic decarboxylation of amino acids, with that
tentatively proposed for complexes 2, in which the coordination
of the pyridinic and iminic N-atoms to the Mo(CO)4 fragment
The geometry around Mo is slightly distorted octahedral,
the main deviation being the small bite angle of the didentate
diimine: 71.9(1)◦. The pyridine and imine portions of the ligand
are essentially coplanar, with the metal atom also lying in the
plane. These (otherwise expectable) features are found, without
any significant difference, in the structures of the other complexes
described below, and will not be further commented.
Complexes derived from a-amino acids and their decarboxylation
products
◦
[Mo(CO)4(pip)2] was heated (50 C) with a mixture of pyridine-
2-carbaldehyde and a a-amino acid, in ethanol, and the reactions
were monitored by IR spectroscopy in solution. After completion,
the solvents were evaporated in vacuo, and the residue was washed
with hexane and then extracted with Et2O to separate decar-
boxylation products (see below). The resulting solid residue was
recrystallized from dichloromethane/hexane to afford the pi-
peridinium salts 2a and 2b (see Scheme 2) as microcrystalline
solids. The corresponding decarboxylation products 3a–c were
obtained from the ether extracts. The two series of products
can be easily distinguished by their IR and 1H NMR spectra
(see Experimental section). It is noteworthy that the extent of
decarboxylation depends strongly on the nature of the amino
acid as shown in Scheme 2 (percentages were obtained from the
1H NMR spectra of the crude reaction mixtures). In the case of
phenylglycine, only the decarboxylation product 3c was isolated,
Scheme 3
Scheme 2
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 1218–1225 | 1219
©

View Article Online
Fig. 3 Perspective view of compound 3b showing the atom numbering.
◦
˚
Selected bond lengths (A) and angles ( ): Mo(1)–C(1) 2.013(5), Mo(1)–
C(2) 1.962(5), Mo(1)–C(3) 1.956(5), Mo(1)–C(4) 2.029(5), Mo(1)–N(1)
2.247(3), Mo(1)–N(2) 2.224(3); C(1)–Mo(1)–C(4) 166.35(18), C(2)–
Mo(1)–N(2) 171.25(17), C(3)–Mo(1)–N(1) 170.38(16), N(2)–Mo(1)–N(1)
72.65(11).
Fig. 2 Perspective view of one of the two independent molecules in the
asymmetric unit of compound 2a showing the atom numbering (above)
and ion pair dimerization by hydrogen bonding (below). Selected bond
◦
˚
lengths (A) and angles ( ): Molecule 1: Mo(1)–C(1) 1.949(16), Mo(1)–C(2)
1.93(3), Mo(1)–C(3) 1.908(16), Mo(1)–C(4) 1.964(15), Mo(1)–N(1)
2.174(16), Mo(1)–N(2) 2.202(14), C(6)–O(5) 1.10(2), C(6)–O(6) 1.276(19);
C(1)–Mo(1)–C(4) 170.3(7), C(3)–Mo(1)–N(1) 173.7(6), C(2)–Mo(1)–N(2)
174.1(9), N(2)–Mo(1)–N(1) 74.0(5), O(5)–C(6)–O(6) 131.5(18). Molecule
2: Mo(51)–C(51) 2.06(2), Mo(51)–C(52) 1.934(13), Mo(51)–C(53) 2.04(2),
Mo(51)–C(54) 2.20(2), Mo(51)–N(51) 2.281(13), Mo(51)–N(52) 2.273(13),
C(56)–O(55) 1.211(18), C(56)–O(56) 1.291(19); C(51)–Mo(51)–C(54)
174.1(8), C(53)–Mo(51)–N(51) 171.5(8), C(52)–Mo(51)–N(52) 169.5(6),
N(52)–Mo(51)–N(51) 71.7(5), O(55)–C(56)–O(56) 119.5(15). Hydro-
gen bonds: H(31A) · · · O(6) 1.738(9), N(31)–H(31A) · · · O(6) 176.83(3),
H(31B) · · · O(56) 1.795(11), N(31)–H(31B) · · · O(56) 156.26(3), H(81A) · · ·
O(55) 1.835(10), N(81)–H(81A) · · · O(55) 174.43(3), H(81B) · · · O(5)
1.798(12), N(81)–H(81B) · · · O(5) 171.62(3).
Fig. 4 Perspective view of compound ◦3c showing the atom numbering.
˚
Selected bond lengths (A) and angles ( ): Mo(1)–C(1) 2.041(4), Mo(1)–
C(2) 1.950(4), Mo(1)–C(3) 1.957(4), Mo(1)–C(4) 2.026(4), Mo(1)–N(1)
2.249(2), Mo(1)–N(2) 2.236(2); C(4)–Mo(1)–C(1) 168.72(12), C(3)–
Mo(1)–N(2) 169.87(12), C(2)–Mo(1)–N(1) 171.43(11), N(2)–Mo(1)–N(1)
72.50(9).
saturated carbon, or a phenyl group, would help to prevent
decarboxylation. To check this point, we used b-alanine and 3-,
and 4-amino benzoic acids.
helps to delocalize the charge towards the metal and, through
back-donation, to the good p-accepting carbonyl ligands.
The presence of the electron-withdrawing phenyl group in the
a-carbon of the phenylglycine derivative would explain its greater
tendency to decarboxylation. The electron withdrawing properties
of the coordination to Mo(CO)4 fragment help also to explain the
racemization observed for the structure of 2a.
=
b-Alanine derivative [Hpip][Mo(CO)4{py-2-C(H) NC2H4COO}]
(4)
[MoCO)4(pip)2] (pip = piperidine) was heated with a mixture
of pyridine-2-carbaldehyde and b−alanine in ethanol, to give
=
[Hpip][Mo(CO)4{py-2-C(H) NC2H4COO}] (4) (Scheme 4) in
good isolated yield. No significant amount of decarboxylation
product could be detected from the IR or 1H NMR spectra of the
crude reaction mixtures.
Within this context, it was expected that the use of amino acids
in which the two functionalities were separated by an additional
1220 | Dalton Trans., 2006, 1218–1225
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
Scheme 4
An X-ray determination (Fig. 5) showed the usual distorted
octahedral arrangement of ligands around Mo, as described above
for 1. More interestingly, it was also observed, as for 2a, dimeri-
zation of anion/cation pairs through hydrogen bond formation.
Complexes derived from amino benzoic acids 5a–b
[Mo(CO)4(pip)2] was stirred in ethanol at room temperature with
pyridine-2-carbaldehyde and the appropriate amino benzoic acid,
to afford, after workup, the piperidinium salts 5a and 5b (see
Scheme 5) as crystalline solids.
The X-ray determinations for 5a and 5b (Fig. 6 and 7) showed
again the ion-pair dimerization which is present in all the series
of piperidinium salts. In the structures of 2a, 4, and 5a the
piperidinium cation uses the two hydrogens at the nitrogen atom to
Scheme 5
bind oxygen atoms of two different carboxylate groups. These, in
turn, use both oxygen atoms, thus forming a puckered twelve-
membered ring. The distances (N–)H · · · O are in the range
˚
˚
1.7–2.0 A, and N · · · O in the range 2.7–2.8 A. The angles N–
H · · · O lie in the range 165–170◦. From these parameters, the
hydrogen bonds can be considered as “moderate”.14
Fig. 5 Perspective view of compound 4 showing the atom numbering (above) and ion pair dimerization by hydrogen bonding (below). Selected
◦
˚
bond lengths (A) and angles ( ): Mo(1)–C(1) 2.056(5), Mo(1)–C(2) 1.975(5), Mo(1)–C(3) 1.955(5), Mo(1)–C(4) 2.037(5), Mo(1)–N(1) 2.271(3),
Mo(1)–N(2) 2.235(3), C(5)–O(5) 1.215(5), C(5)–O(6) 1.253(5); C(1)–Mo(1)–C(4) 169.25(15), C(3)–Mo(1)–N(1) 170.16(15), C(2)–Mo(1)–N(2) 172.75(15),
N(2)–Mo(1)–N(1) 72.35(11), O(5)–C(5)–O(6) 123.1(4). Hydrogen bonds: H(3A) · · · O(5) 1.896(11), N(3)–H(3A) · · · O(5) 169.96(3), H(3B) · · · O(6)
1.808(10), N(3)–H(3B) · · · O(6) 179.38(3).
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 1218–1225 | 1221
©

View Article Online
Fig. 6 Perspective view of compound 5a showing the atom numbering
(above) and ion pair dimerization◦by hydrogen bonding (below). Selected
˚
bond lengths (A) and angles ( ): Mo(1)–C(1) 2.054(3), Mo(1)–C(2)
1.960(3), Mo(1)–C(3) 1.968(3), Mo(1)–C(4) 2.025(3), Mo(1)–N(1)
2.256(2), Mo(1)–N(2) 2.251(2), C(5)–O(5) 1.253(4), C(5)–O(6) 1.252(4);
C(1)–Mo(1)–C(4) 168.70(12), C(2)–Mo(1)–N(2) 170.89(12), C(3)–Mo(1)–
N(1) 172.70(10), N(2)–Mo(1)–N(1) 71.95(8), C(5)–O(6)–O(5) 125.0(3).
Hydrogen bonds: H(31A) · · · O(6) 1.865(12), N(31)–H(31A) · · · O(6)
172.97(3), H(31B) · · · O(5) 1.848(10), N(31)–H(31B) · · · O(5) 171.92(4).
Fig. 7 Perspective view of compound 5b showing the atom numbering
(above) and ion pair dimerization by hydrogen bonding (below). Selected
◦
˚
bond lengths (A) and angles ( ): Mo(1)–C(1) 2.038(10), Mo(1)–C(2)
1.988(11), Mo(1)–C(3) 1.943(10), Mo(1)–C(4) 2.017(11), Mo(1)–N(1)
2.236(7), Mo(1)–N(2) 2.245(7), C(5)–O(5) 1.261(16), C(5)–O(6) 1.195(17),
C(4)–Mo(1)–C(1) 168.3(4), C(3)–Mo(1)–N(2) 172.0(3), C(2)–Mo(1)–N(1)
172.0(3), N(2)–Mo(1)–N(1) 72.4(3), O(5)–C(5)–O(6) 124.1(12). Hydro-
gen bonds: H(90A) · · · O(5) 1.965(13), O(90)–H(90A) · · · O(5) 144.89(6),
H(31A) · · · O(5) 1.897(12), N(31)–H(31A) · · · O(5) 177.21(3), H(31B) · · ·
O(6) 1.898(10), N(31)–H(31B) · · · O(6) 162.60(5), H(31B) · · · O(5)
2.481(12), N(31)–H(31B) · · · O(5) 138.64(5).
The structure of 5b is an exception in the series: the dimers
are built by using only one of the oxygens of the carboxylate
groups, thus forming smaller rings of only eight members. Apart
from this, there is a disordered molecule of water (occupancy 0.3),
hydrogen bonded to the carboxylate oxygen, and significant C–
˚
H(piperidine) · · · O(carboxyl) interactions (CH · · · O 2.41 A, angle
C–H · · · O 160.8◦),15 which could account for the adoption of a
different geometry for the dimer in this case.
Protonation of imino acidate complexes
As a test for the stability of the iminocarboxylate complexes
against water, and in order to obtain new complexes containing
a free carboxylic acid functionality, we carried out a two-phase
protonation of 5a and 2b. Thus, a stirred solution of compound
5a or compound 2b in CH2Cl2 was treated with 2 M HCl to afford
the compounds 6a,b (see Scheme 6), which could be isolated in
good yields.
An X-ray determination was carried out on a crystal of 6a
(Fig. 8). A remarkable feature of the structure of 6a is that
the dimer ring lies at the twofold axis, rather than at the
inversion center, which is considered the “normal” arrangement
for carboxylic acids.16 Such an anomalous case had been found
earlier in the structure of 3,5-dinitrocinnamic acid,17 and it had
been attributed to the existence of additional intermolecular C–
H · · · O bonds, which apparently are absent in 6a.
Scheme 6
Experimental
instrumentation and experimental procedures have been given
elsewhere.18 Literature procedures for the preparation of starting
materials are quoted in each case. Ligands and other reagents
were purchased and used without purification unless otherwise
stated.
Materials and general methods
All operations were performed under an atmosphere of dry
dinitrogen using Schlenk and vacuum techniques. Details of the
1222 | Dalton Trans., 2006, 1218–1225
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
tered through kieselguhr. Addition of hexane and slow evap-
oration at reduced pressure gave compounds 2 as red–black
microcrystals.
2a. Yield 0.22 g (59%). Anal. Calc. for C18H21MoN3O6: C 45.87,
H 4.49, N 8.92. Found: C 46.07, H 4.65, N 8.89%. IR (CH2Cl2),
m(CO): 2014 s, 1906 vs, 1880 (sh), 1830 s, cm−1. 1H NMR (CDCl3):
6
=
d 9.09 [d (5 Hz), 1H, H of py], 8.63 [s, 1H, py-C(H) N], 7.88 [m,
1H, H4 of py], 7.73 [d (8 Hz), 1H, H3 of py], 7.37 [m, 1H, H5 of
py], 4.64 [q (7 Hz), 1H, NCHCOO], 3.10–1.26 [m (br), 10H, CH2
of Hpip], 1.73 [d (7 Hz), 3H, CHCH3] ppm.
2b. Yield 0.29 g (73%). Anal. Calc. for C20H25MoN3O6: C 48.10,
H 5.05, N 8.41. Found: C 48.39, H 4.80, N 8.62%. IR (CH2Cl2),
m(CO): 2013 s, 1904 vs, 1881 (sh), 1832 s, cm−1. 1H NMR (CDCl3):
6
=
d 9.05 [d (5 Hz), 1H, H of py], 8.84 [s, 1H, py-C(H) N], 7.87 [m,
1H, H4 of py], 7.73 [d (8 Hz), 1H, H3 of py], 7.36 [m, 1H, H5 of
py], 4.29 [d (10 Hz), 1H, NCHCOO], 3.10–1.26 [m (br), 10H, CH2
of Hpip], 2.48 [m, 1H, CH(CH3)2], 1.09 [d (6 Hz), 3H, CHCH3],
0.98 [d (6 Hz), 3H, CHCH3] ppm.
Decarboxylated compounds 3a–c. The ether extracts obtained
in the preparation of compounds 2 (see above) were filtered
through kieselguhr. Addition of hexane to the filtrate, and slow
evaporation gave compounds 3 as red–purple microcrystals.
Fig. 8 Perspective view of compound 6a showing the atom number-
ing (above) and ion pair dimerization◦by hydrogen bonding (below).
˚
Selected bond lengths (A) and angles ( ): Mo(1)–C(1) 1.80(3), Mo(1)–
C(2) 1.89(2), Mo(1)–C(3) 1.729(19), Mo(1)–C(4) 1.96(2), Mo(1)–N(1)
2.180(15), Mo(1)–N(2) 2.250(14), C(5)–O(5) 1.25(2), C(5)–O(6) 1.22(2);
C(1)–Mo(1)–C(4) 169.0(9), C(2)–Mo(1)–N(2) 170.8(8), C(3)–Mo(1)–N(1)
172.0(6), N(2)–Mo(1)–N(1) 71.1(5), O(6)–C(5)–O(5) 121.9(15). Hy-
drogen bonds: H(5) · · · O(5) 1.296(9), O(5) · · · H(5) · · · O(5) 172.86(10),
H(6) · · · O(6) 1.256(10), O(6) · · · H(6) · · · O(6) 158.63(10).
3a. Yield 0.014 g (5%). Anal. Calc. for C12H10MoN2O4: C 42.12,
H 2.94, N 8.19. Found: C 42.29, H 2.99, N 7.82%. IR (CH2Cl2),
m(CO): 2015 s, 1906 vs, 1886 (sh), 1835 s, cm−1. 1H NMR (CDCl3):
6
=
d 9.12 [d (5 Hz), 1H, H of py], 8.45 [s, 1H, py-C(H) N], 7.90 [td
(7 and 2 Hz), 1H, H4 of py], 7.72 [d (7 Hz), 1H, H3 of py], 7.40
[m, 1H, H5 of py], 4.10 [q (7 Hz), 2H, NCH2], 1.59 [t (7 Hz), 3H,
NCH2CH3] ppm.
Complexes
3b. Yield 0.018 g (6%). Anal. Calc. for C14H14MoN2O4: C 45.42,
H 3.81, N 7.57. Found: C 45.18, H 3.63, N 7.87%. IR (CH2Cl2),
m(CO): 2015 s, 1907 vs, 1886 (sh), 1836 s, cm−1. 1H NMR (CDCl3):
=
[Mo(CO)4(py-2-C(H) NCH2COOEt)] (1). To a solution of
pyridine-2-carbaldehyde (0.085 g, 0794 mmol) and [H3NCH2-
COOEt]Cl (0.111 g, 0.794 mmol) in ethanol (20 cm3) was added
[Mo(CO)4(pip)2] (0.3 g, 0.794 mmol), and the mixture was heated
at 50 ◦C, with stirring, for 30 min. The color changed quickly
from yellow to dark purple. The solvents were evaporated in vacuo,
and the residue was washed with hexane (2 × 15 cm3), and then
extracted repeatedly with Et2O. The collected extracts were filtered
through kieselguhr. Addition of hexane and slow evaporation at
reduced pressure gave compound 1 as black microcrystals. Yield
0.196 g (62%). Anal. Calc. for C14H12MoN2O6: C 42.02, H 3.02,
N 7.00. Found: C 42.37, H 2.80, N 6.82%. IR (CH2Cl2), m(CO):
6
=
d 9.11 [d (6 Hz), 1H, H of py], 8.35 [s, 1H, py-C(H) N], 7.91 [td
(7 and 1 Hz), 1H, H4 of py], 7.73 [d (7 Hz), 1H, H3 of py], 7.41 [m,
1H, H5 of py], 3.82 [m, 2H, NCH2], 2.65 [m, 1H, CH(CH3)2], 0.98
[d (7 Hz), 6H, CH(CH3)2] ppm.
3c. Yield 0.20 g (62%). Anal. Calc. for C17H12MoN2O4: C 50.51,
H 2.99, N 6.93. Found: C 50.38, H 2.90, N 6.81%. IR (CH2Cl2),
m(CO): 2016 s, 1910 vs, 1886 (sh), 1837 s, cm−1. 1H NMR (CDCl3):
6
=
d 9.09 [d (5 Hz), 1H, H of py], 8.46 [s, 1H, py-C(H) N], 7.88 [m,
1H, H4 of py], 7.73 [d (7 Hz), 1H, H3 of py], 7.40 [m, 6H, H5 of py
and C6H5], 5.24 [s, 2H, NCH2] ppm.
2017 s, 1912 vs, 1888 (sh), 1841 s, cm−1. 1H NMR (CDCl3): d 9.12
6
=
[d (5 Hz), 1H, H of py], 8.49 [s, 1H, py-C(H) N], 7.92 [m, 1H,
=
[Hpip][Mo(CO)4{py-2-C(H) NCH2CH2COO}] (4). A sus-
H4 of py], 7.80 [d (7 Hz), 1H, H3 of py], 7.44 [m, 1H, H5 of py],
4.85 [s, 2H, NCH2COOEt], 4.34 [q (7 Hz), 2H, OCH2CH3], 1.37
[t (7 Hz), 3H, OCH2CH3] ppm.
pension of b-alanine (0.071 g, 0.79 mmol) and pyridine-2-
carbaldehyde (0.085 g, 0794 mmol), in ethanol (20 cm3) was stirred
at 50 ◦C for 10 min. Then [Mo(CO)4(pip)2] (0.3 g, 0.794 mmol)
was added, and the mixture was stirred at 50 ◦C for 35 min.
The color changed quickly from yellow to dark purple. The
solvents were evaporated in vacuo, and the residue was washed
with hexane and Et2O. The resulting solid residue was dissolved
in CH2Cl2 and filtered through kieselguhr. Addition of hexane
and slow evaporation at reduced pressure gave compound 4 as red
microcrystals. Yield 0.26 g (71%). Anal. Calc. for C18H21MoN3O6:
C 45.87, H 4.49, N 8.92. Found: C 45.66, H 4.26, N 7.71%. IR
General preparation of complexes derived from a-amino acids
=
[Hpip][Mo(CO)4{py-2-C(H) NCH(R)COO}] (2a,b). A suspen-
sion of the amino acid (0.794 mmol) and pyridine-2-carbaldehyde
◦
(0.085 g, 0794 mmol) in EtOH (20 cm3) was stirred at 50 C for
10 min. To this was added [Mo(CO)4(pip)2] (0.3 g, 0.794 mmol),
and the mixture was heated at 50 ◦C, with stirring, for 15 min.
The color changed quickly from yellow to dark purple. The
solvents were evaporated in vacuo, and the residue was washed
with hexane (2 × 15 cm3), and then extracted with Et2O to
separate the neutral decarboxylation products 3 (see below).
The resulting solid residue was dissolved in CH2Cl2 and fil-
(CH2Cl2), m(CO): 2015 s, 1907 vs, 1883 (sh), 1836 s, cm−1. 1H NMR
6
=
(CDCl3): d 9.07 [d (5 Hz), 1H, H of py], 8.58 [s, 1H, py-C(H) N],
7.89 [m, 1H, H4 of py], 7.72 [d (8 Hz), 1H, H3 of py], 7.39 [m,
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 1218–1225 | 1223
©

View Article Online
1H, H5 of py], 4.25 [t (7 Hz), 2H, NCH2CH2], 2.90 [t (7 Hz), 2H,
CH2CH2COO], 3.10–1.26 [m (br), 10H, CH2 of Hpip] ppm.
X-Ray diffraction study of 1, 2a, 3b, 3c, 4, 5a, 5b and 6a
Crystals were grown by slow diffusion of hexane into concentrated
solutions of the complexes in CH2Cl2 at −20 ◦C. Intensity
measurement was made with a Bruker AXS SMART 1000 diffrac-
tometer with graphite-monochromated Mo-Ka X-radiation and
a CCD area detector. A hemisphere of the reciprocal space (full
sphere for 2a) was collected up to 2h = 48.6◦. Raw frame data
were integrated with the SAINT19 program. A semi-empirical
absorption correction was applied with the program SADABS.20
The structures were solved by direct methods with SIR2002,21
under WINGX,22 and refined against F2 with SHELXTL.23 All
non-hydrogen atoms were refined anisotropically unless otherwise
stated. Hydrogen atoms were set in calculated positions and refined
as riding atoms, with a common thermal parameter. Calculations
and graphics were made with SHELXTL and PARST,24 and
graphics were made with SHELXTL. Crystal data, particular
details and CCDC reference numbers are given below for each
structure.
=
[Hpip][Mo(CO)4{py-2-C(H) NC6H4(COO)-4}] (5a) and [Hpip]-
=
[Mo(CO)4{py-2-C(H) NC6H4(COO)-3}] (5b). A suspension of
the appropriate aminobenzoic acid (0.794 mmol), pyridine-2-
carbaldehyde (0.085 g, 0794 mmol) and [Mo(CO)4(pip)2] (0.3 g,
0.794 mmol) in EtOH (20 cm3) was stirred at room temperature
for 3 h (for 5a) or 1 h (for 5b). Then the solution was warmed
gently to ensure complete reaction. The color changed from
yellow to dark violet. The solvents were evaporated in vacuo,
and the residue was washed with hexane (2 × 15 cm3) and Et2O
(10 cm3). The resulting solid residue was dissolved in CH2Cl2
and filtered through kieselguhr. Addition of hexane and slow
evaporation at reduced pressure gave compounds 5a,b as dark
purple microcrystals.
5a. Yield 0.313 g (76%). Anal. Calc. for C22H21MoN3O6: C
50.88, H 4.08, N 8.09. Found: C 50.46, H 4.26, N 7.65%. IR
(CH2Cl2), m(CO): 2016 s, 1911 vs, 1890 (sh), 1842 s, cm−1. 1H NMR
Compound 1. C14H12MoN2O6, M = 400.20, monoclinic, space
6
=
(CDCl3): d 9.21 [d (6 Hz), 1H, H of py], 8.57 [s, 1H, py-C(H) N],
˚
group = P21/c, a = 7.311(5), b = 16.795(10), c = 12.823(8) A,
8.16 [d (8 Hz), 2H, H2 and H6 of Ph], 7.90 [m, 2H, H3 of py and H4
of py], 7.48 [m, 3H, H5 of py, H3 of Ph and H5 of Ph], 3.10–1.26
[m (br), 10H, CH2 of Hpip] ppm.
◦
3
˚
b = 91.026(10) , U = 1574(2) A , T = 296(2) K, Z = 4, l(Mo-
Ka) = 0.865 mm−1, 7056 reflections collected, 2259 unique (Rint
=
0.0378), R1 = 0.0271, wR2 = 0.0522. CCDC 281340.
5b. Yield 0.334 g (81%). Anal. Calc. for C22H21MoN3O6: C
Compound 2a. C18H21MoN3O6, M = 471.32, triclinic, space
50.88, H 4.08, N 8.09. Found: C 51.19, H 4.38, N 7.95%. IR
˚
group = P1, a = 9.094(3), b = 10.836(3), c = 12.111(4) A, a =
1
(CH2Cl2), m(CO): 2016 s, 1911 vs, 1890 (sh), 1841 s, cm−1. H
◦
3
˚
91.259(5), b = 109.767(5), c = 109.143(4) , U = 1049.0(5) A ,
NMR (CDCl3): d 9.19 [d (5 Hz), 1H, H6 of py], 8.62 [s, 1H, py-
T = 296(2) K, Z = 2, l(Mo-Ka) = 0.662 mm−1, 6870 reflections
collected, 5785 unique (Rint = 0.0204), R1 = 0.0509, wR2 = 0.1427.
The choice of the chiral space group P1 was suggested by the
XPREP subroutine of SHELXTL on the basis of the value of
the mean |EE*−1| = 0.795. The Flack parameter was refined
with the instruction TWIN, and converged to 0.42(7), indicating
the presence of two enantiomers in the lattice. Alternatively, the
structure was solved and refined on the centrosymmetric space
2
6
=
C(H) N], 8.07 [m, 2H, H and H of Ph, signals overlap], 7.95 [td
(8 and 1 Hz), 1H, H4 of py], 7.85 [d (8 Hz), 1H, H3 of py], 7.69 [d
(8 Hz), 1H, H4 of Ph], 7.49 [m, 2H, H5 of Ph and H5 of py, signals
overlap], 3.10 to 1.26 [m (br), 10H, CH2 of Hpip] ppm.
=
[Mo(CO)4{py-2-C(H) NC6H4-COOH-4}] (6a) and [Mo(CO)4-
=
{py-2-C(H) NCH(CH(CH3)2)COOH}] (6b). A stirred solution
of compound 2b (0.1 g, 0.200 mmol) or compound 5a (0.1 g,
0.192 mmol) in CH2Cl2 was washed with 2 M HCl (3 × 10 cm3)
and then water (2 × 15 cm3). The solution was dried with
magnesium sulfate and filtered through kieselguhr. The solvents
were evaporated in vacuo, and the residue was washed with hexane
(2 × 15 cm3) and Et2O (2 × 5 cm3). The resulting solid residue was
dissolved in CH2Cl2. Addition of hexane and slow evaporation at
reduced pressure gave compounds 6 as dark red (6a) or black (6b)
microcrystals.
¯
group P1. However this lead to poorer results. Apart from the
expectable disorder in the a-carbon of the amino acid, it was found
that all the atoms of the piperidinium cation were also heavily
disordered, with very high temperature factors. Several attempts
to model the disorder led to no better results than those obtained
with the non-centrosymmetric space group. CCDC 281341.
Compound 3b. C14H14MoN2O4, M = 370.21, monoclinic,
space group = P21/c, a = 6.655(1), b = 15.676(3), c = 15.454
◦
3
˚
˚
(3) A, b = 99.916(4) , U = 1588.2(5) A , T = 296(2) K, Z = 4,
l(Mo-Ka) = 0.841 mm−1, 6971 reflections collected, 2288 unique
(Rint = 0.0325), R1 = 0.0336, wR2 = 0.0876. CCDC 281342.
6a. Yield 0.070 g (84%). Anal. Calc. for C17H10MoN2O6: C
47.02, H 2.32, N 6.45. Found: C 47.25, H 2.63, N 6.21%. IR
(CH2Cl2), m(CO): 2017 s, 1914 vs, 1893 (sh), 1845 s, cm−1. 1H NMR
6
=
(CDCl3): d 9.23 [d (6 Hz), 1H, H of py], 8.60 [s, 1H, py-C(H) N],
Compound 3c. C17H12MoN2O4, M = 404.23, monoclinic,
space group = P21/n, a = 10.401(1), b = 15.754(2), c = 10.818(1)
8.24 [d (8 Hz), 2H, H2 and H6 of Ph], 7.94 [m, 2H, H3 of py and
H4 of py], 7.54 [m, 3H, H5 of py, H3 of Ph and H5 of Ph] ppm.
◦
3
˚
˚
A, b = 108.019(2) , U = 1685.8(4) A , T = 299(2) K, Z = 4,
l(Mo-Ka) = 0.800 mm−1, 10821 reflections collected, 2418 unique
(Rint = 0.0370), R1 = 0.0251, wR2 = 0.0560. CCDC 281343.
6b. Yield 0.065 g (79%). Anal. Calc. for C15H14MoN2O6: C
43.49, H 3.41, N 6.76. Found: C 43.89, H 3.89, N 6.65%. IR
(CH2Cl2), m(CO): 2016 s, 1909 vs, 1888 (sh), 1838 s, cm−1. 1H NMR
Compound 4. C18H21MoN3O6, M = 471.32, monoclinic, space
6
˚
=
(CDCl3): d 9.12 [d (5 Hz), 1H, H of py], 8.65 [s, 1H, py-C(H) N],
group = C2/c, a = 17.481(8), b = 11.553(6), c = 20.934(10) A,
◦
7.94 [m, 1H, H4 of py], 7.82 [d (8 Hz), 1H, H3 of py], 7.44 [m, 1H,
H5 of py], 4.36 [d (7 Hz), 1H, NCH], 2.71 [m, 1H, CH(CH3)2], 1.16
[d (6 Hz), 3H, CHCH3], 1.00 [d (6 Hz), 3H, CHCH3] ppm.
b = 98.40(2) , U = 4182(4) A , T = 293(2) K, Z = 8, l(Mo-
3
˚
Ka) = 0.664 mm−1, 9331 reflections collected, 3065 unique (Rint
=
0.0270), R1 = 0.0349, wR2 = 0.0984. CCDC 281344.
1224 | Dalton Trans., 2006, 1218–1225
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
Compound 5a. C22H21MoN3O6, M = 519.36, triclinic, space
Vittal, W. Henderson, J. R. Wheaton, I. H. Hall, T. S. A. Hor and Y. K.
Yan, J. Organomet. Chem., 2002, 650, 123.
¯
˚
group = P1, a = 6.654(1), b = 8.374(2), c = 21.538(4) A, a =
3 (a) L. Pu, Chem. Rev., 2004, 104, 1687; (b) J. Wald, R. Alberto, K.
Ortner and L. Candreia, Angew. Chem., Int. Ed., 2001, 40, 3062.
4 (a) R. N. Patel, N. Singh and K. K. Shukla, J. Inorg. Biochem., 2005,
99, 651; (b) M. H. Schifeld and D. Y. Chung, J. Inorg. Biochem., 2004,
96, 226; (c) M. Merkel and D. Schiedens, Eur. J. Inorg. Chem., 2004,
783; (d) T. Humphry, M. Forconi, N. H. Williams and A. C. Hengge,
J. Am. Chem. Soc., 2002, 124, 14860.
5 (a) R. Mej´ıa-Rodr´ıguez, D. Chong, J. H. Reibenspies, M. P. Soriaga
and M. Y. Darensbourg, J. Am. Chem. Soc., 2004, 126, 12004; (b) F.
Gloaguen, J. D. Lawrence, M. Schmidt, S. R. Wilson and T. B.
Rauchfuss, J. Am. Chem. Soc., 2001, 123, 12518; (c) S. J. George, Z.
Cui, M. Razavet and C. J. Pickett, Chem. Eur. J., 2002, 8, 4037.
6 (a) D. R. Van Staveren, E. Bothe and N. Metzler-Nolte, Organo-
metallics, 2003, 22, 3102; (b) D. R. Van Staveren, E. Bill, E. Bothe,
M. Bu¨hl, T. Weyhermu¨ller and N. Metzler-Nolte, Chem. Eur. J., 2002,
8, 1649; (c) D. R. Van Staveren, T. Weyhermu¨ller and N. Metzler-Nolte,
Organometallics, 2000, 19, 3730.
◦
3
˚
86.669(3), b = 87.253(3), c = 69.782(4) , U = 1123.8(4) A ,
T = 293(2) K, Z = 2, l(Mo-Ka) = 0.626 mm−1, 4981 reflections
collected, 3207 unique (Rint = 0.0134), R1 = 0.0253, wR2 = 0.0802.
CCDC 281345.
Compound 5b. C22H21MoN3O6·0.3H2O, M = 524.76, mono-
clinic, space group = C2/c, a = 15.710(6), b = 6.746(3), c =
◦
3
˚
˚
43.915(17) A, b = 90.260(6) , U = 4654(3) A , T = 293(2) K,
Z = 8, l(Mo-Ka) = 0.607 mm−1, 9975 reflections collected, 3357
unique (Rint = 0.0505), R1 = 0.0808, wR2 = 0.1896. A peak lying on
a twofold axis was found to correspond to a disordered molecule
of water. The occupancy was refined to a value of 0.3. CCDC
281346.
7 B. Gala´n, D. Miguel, J. Pe´rez and V. Riera, Organometallics, 2002, 21,
2979.
8 J. Pe´rez, V. Riera, A. Rodr´ıguez and D. Miguel, Organometallics, 2002,
21, 5437.
9 R. S. Herrick, K. L. Houde, J. S. McDowell, L. P. Kizeck and G.
Bonavia, J. Organomet. Chem., 1999, 589, 29.
10 R. S. Herrick, C. J. Ziegler, H. Bohan, M. Corey, M. Eskander, J.
Giguere, N. McMicken and I. Wrona, J. Organomet. Chem., 2003, 687,
178.
11 R. S. Herrick, I. Wrona, N. McMicken, G. Jones, C. J. Ziegler and J.
Shaw, J. Organomet. Chem., 2004, 689, 4848.
Compound 6a. C17H10MoN2O6·0.1CH2Cl2, M = 442.70, mon-
oclinic, space group = C2/c, a = 24.17(3), b = 10.648(11), c =
◦
3
˚
˚
16.561(17) A, b = 102.79(2) , U = 4156(8) A , T = 296(2) K,
Z = 8, l(Mo-Ka) = 0.688 mm−1, 9235 reflections collected, 3007
unique (Rint = 0.0902), R1 = 0.0871, wR2 = 0.2065. Several peaks
were found near to an inversion centre, and were modelled as a low
occupancy (0.1) molecule of dichloromethane which was refined
as a rigid group with a common isotropic temperature factor for
Cl and C atoms. CCDC 281347.
12 D. J. Darensbourg and R. L. Kump, Inorg. Chem., 1978, 17, 2680.
13 R. D. Bach and C. Canepa, J. Am. Chem. Soc., 1997, 119, 11725.
14 G. R. Desiraju, Angew. Chem., Int. Ed. Engl., 1995, 34, 2311; D. Braga,
F. Grepioni and G. R. Desiraju, J. Organomet. Chem., 1997, 548, 33.
15 G. R. Desiraju, Acc. Chem. Res., 1991, 24, 290.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b511637f
16 G. R. Desiraju, Chem. Commun., 1997, 1475.
17 G. R. Desiraju and C. V. K. Sharma, J. Chem. Soc., Chem. Commun.,
1991, 1239.
Acknowledgements
18 G. Barrado, M. M. Hricko, D. Miguel, V. Riera, H. Wally and S.
Garc´ıa-Granda, Organometallics, 1998, 17, 820.
We thank the Spanish MCYT (BQU2002-03414) and JCyL
(VA052/03 and VA012C05) for financial support and the
University of Valladolid for a grant to R. G.-R.
19 SAINT+: SAX area detector integration program. Version 6.02. Bruker
AXS, Inc., Madison, WI, 1999.
20 G. M. Sheldrick, SADABS, Empirical Absorption Correction Program,
University of Go¨ttingen: Go¨ttingen, Germany, 1997.
21 M. C. Burla, M. Camalli, B. Carrozzini, G. L. Cascarano, C.
Giacovazzo, G. Polidory and R. Spagna, SIR2002, A program for
automatic solution and refinement of crystal structures; A. Altomare,
M. C. Burla, M. Camalli, G. L. Cascarano, C. Giacovazzo, A.
Guagliardi, A. G. G. Moliterni, G. Polidori and R. Spagna, J. Appl.
Crystallogr., 1999, 32, 115.
References
1 (a) K. K. W. Lo, W. K. Hui and D. C. M. Ng, J. Am. Chem. Soc.,
2002, 124, 9344; (b) M. K. Brennaman, J. H. Alstrum-Acevedo, C. N.
Fleming, P. Jang, T. J. Meyer and J. M. Papanikolas, J. Am. Chem. Soc.,
2002, 124, 15094.
22 L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837.
23 G. M. Sheldrick, SHELXTL, An integrated system for solving, refining,
and displaying crystal structures from diffraction data. Version 5.1.
Bruker AXS, Inc. Madison, WI, 1998.
2 (a) K. A. Stephenson, J. Zubieta, S. R. Bannerjee, M. K. Levadala, L.
Taggart, L. Ryan, N. McFarlane, D. R. Boreham, K. P. Maresca, J. W.
Babich and J. F. Valliant, Bioconjugate Chem., 2004, 15, 128; (b) M. B.
Skaddan, F. R. Wu¨st, S. Jonson, R. Syhre, M. J. Welch, H. Spies and
J. Katzenellenbogen, Nucl. Med. Biol., 2000, 27, 269; (c) J. Zhang, J. J.
24 (a) M. Nardelli, Comput. Chem., 1983, 7, 95–97; (b) M. Nardelli, J. Appl.
Crystallogr., 1995, 28, 659.
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 1218–1225 | 1225
©