P-Tolylimido rhenium(v) complexes-synthesis, X-ray studies, spectroscopic characterization, DFT calculations and catalytic activity

Article abstract of DOI:10.1039/c3dt52528g

Novel p-tolylimido rhenium(v) complexes trans-(Cl,Cl)-[Re(p-NC ₆H₄CH₃)Cl₂(py-2-COO)(PPh ₃)]·MeCN (1a), trans-(Cl,Cl)-[Re(p-NC₆H ₄CH₃)Cl₂(py-2-COO)(PPh₃)] (1b), trans-(Br,Br)-[Re(p-NC₆H₄CH₃)Br ₂(py-2-COO)(PPh₃)] (2), cis-(Cl,Cl)-[Re(p-NC ₆H₄CH₃)Cl₂(py-2-COO)(PPh ₃)] (3), cis-(Br,Br)-[Re(p-NC₆H₄CH ₃)Br₂(py-2-COO)(PPh₃)]·H₂O (4), trans-(Cl,Cl)-[Re(p-NC₆H₄CH₃)Cl ₂(OMe)] (5) and trans-(Br,Br)-[Re(p-NC₆H ₄CH₃)Br₂(OMe)(PPh₃) ₂]·1/2H₂O (6) were synthesized. The compounds were identified by elemental analysis, IR, NMR (¹H, ¹³C and ³¹P), UV-Vis spectroscopy and X-ray crystallography. Calculations at the DFT level were undertaken in order to elucidate the structural, spectroscopic and bonding properties of the trans-(Cl,Cl) and cis-(Cl,Cl) isomers of [Re(p-NC₆H₄CH₃)Cl ₂(py-2-COO)(PPh₃)]. In the first step, extended tests of the ability of different DFT methods (B3LYP, BP86, and PBE1PBE) were performed to find the best overall performer in describing the geometry of the imido Re(v) complexes; then, the electronic spectra of the trans and cis isomers of [Re(p-NC₆H₄CH₃)Cl₂(py-2-COO) (PPh₃)] were investigated at the TDDFT level employing the PBE1PBE functional. Additional information about bonding in the compounds [Re(p-NC ₆H₄CH₃)Cl₂(py-2-COO)(PPh ₃)] was obtained by NBO analysis. The catalytic activity of the complexes incorporating the picolinate ligand was studied and compared in the synthesis of N-substituted ethyl glycine esters from ethyl diazoacetate and amines.

Full text of DOI:10.1039/c3dt52528g

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p-Tolylimido rhenium(V) complexes – synthesis,
X-ray studies, spectroscopic characterization,
DFT calculations and catalytic activity†
Cite this: DOI: 10.1039/c3dt52528g
Barbara Machura,*a Izabela Gryca,a Jan Grzegorz Małecki,a Francisco Alonso*b and
Yanina Moglieb
Novel p-tolylimido rhenium(V) complexes trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)]·MeCN
(1a), trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)] (1b), trans-(Br,Br)-[Re(p-NC6H4CH3)Br2(py-2-
COO)(PPh3)] (2), cis-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)] (3), cis-(Br,Br)-[Re(p-NC6H4CH3)-
Br2(py-2-COO)(PPh3)]·H2O (4), trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(OMe)] (5) and trans-(Br,Br)-[Re-
(p-NC6H4CH3)Br2(OMe)(PPh3)2]·1/2H2O (6) were synthesized. The compounds were identified by elemental
analysis, IR, NMR (1H, 13C and 31P), UV-Vis spectroscopy and X-ray crystallography. Calculations at the DFT
level were undertaken in order to elucidate the structural, spectroscopic and bonding properties of the
trans-(Cl,Cl) and cis-(Cl,Cl) isomers of [Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)]. In the first step, extended
tests of the ability of different DFT methods (B3LYP, BP86, and PBE1PBE) were performed to find the best
overall performer in describing the geometry of the imido Re(V) complexes; then, the electronic spectra
of the trans and cis isomers of [Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)] were investigated at the TDDFT
level employing the PBE1PBE functional. Additional information about bonding in the compounds [Re-
(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)] was obtained by NBO analysis. The catalytic activity of the com-
plexes incorporating the picolinate ligand was studied and compared in the synthesis of N-substituted
ethyl glycine esters from ethyl diazoacetate and amines.
Received 13th September 2013,
Accepted 20th November 2013
DOI: 10.1039/c3dt52528g
www.rsc.org/dalton
oxidation states provide a large variety of stable complexes and
(b) most rhenium complexes are insensitive to air and moist-
Introduction
The widespread contemporary interest in the coordination ure, which makes their handling easier for synthetic purposes.
chemistry of rhenium arises mainly from the introduction of After the discovery by Toste that demonstrated the ability of
β− emitting isotopes 188Re and 186Re in radiotherapy. There is the high-valent oxo-rhenium complex [ReIO2(PPh3)2] to acti-
still a need for fundamental knowledge about the structural vate the Si–H bond of silanes and catalyze the hydrosilylation
and spectroscopic properties, redox activities, and the mechan- of carbonyl compounds, high-valent oxo-rhenium complexes
ism of ligand substitution reactions to develop new and were also investigated as catalysts for organic reductions,
improved Re radiopharmaceuticals.1
which represents a complete reversal from the traditional role
Another area of growing interest for rhenium(V) compounds of these compounds as oxidation catalysts.3 Espenson et al.
is their application as homogeneous catalysts in different found that methylrhenium trioxide (CH3ReO3) catalyzed the
organic chemistry transformations, especially oxidation reac- reaction of primary and secondary amines with ethyl diazo-
tions.2 Rhenium compounds offer some advantages with acetate to give N-substituted glycine ethyl esters.4
respect to other metal complexes because (a) their diverse
Previously, we reported the synthesis and properties of several
families of oxorhenium(V) complexes incorporating uninegative
bidentate N,O-chelating ligands. The studies revealed that
[ReOX3(PPh3)2] (X = Cl, Br) are excellent starting materials for the
syntheses of these compounds and that the reaction course of
[ReOX3(PPh3)2] with N,O-donor chelating ligands depends on
many factors, including the type of the ligand and experimental
conditions. The results also indicated that different regioisomers
of resulting [ReOX2(N–O)(PPh3)] and [ReOX(N–O)2] might have a
significant influence on their catalyst activity.5
aDepartment of Crystallography, Institute of Chemistry, University of Silesia,
9th Szkolna St., 40-006 Katowice, Poland. E-mail: basia@ich.us.edu.pl
bDepartamento de Química Orgánica, Facultad de Ciencias, and Instituto de Síntesis
Orgánica (ISO), Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain.
E-mail: falonso@ua.es
†Electronic supplementary information (ESI) available. CCDC 899354–899357
and 936243–936245. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt52528g
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In the current paper, we have turned our attention to N-substituted ethyl glycine esters from ethyl diazoacetate and
studies of imidorhenium(V) complexes and we examined the amines.
reactivity of pyridine-2-carboxylic acid (py-2-COOH) towards
[Re(p-NC6H4CH3)X3(PPh3)2] (X = Cl, Br). The complexes [Re(p-
NC6H4CH3)X3(PPh3)2] can be considered as analogues of
Results and discussion
Synthesis of the complexes
[ReOX3(PPh3)2]. The imido core NR2− is isoelectronic to O2−
and compounds [Re(p-NC6H4CH3)X3(PPh3)2] and [ReOX3-
(PPh3)2] showed similar ligation around the Re(V) atom with The complexes trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)-
halide ligands arranged in a meridional fashion and triphenyl- (PPh3)]·MeCN (1a), trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-
phosphine molecules trans located to each other.6
COO)(PPh3)] (1b), trans-(Br,Br)-[Re(p-NC6H4CH3)Br2(py-2-COO)-
Herein, several novel imidorhenium(V) complexes, trans- (PPh3)] (2), cis-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)]
(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)]·MeCN (1a), (3) and cis-(Br,Br)-[Re(p-NC6H4CH3)Br2(py-2-COO)(PPh3)]·H2O
trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)] (1b), trans- (4) were prepared by metathesis reactions between [Re(p-
(Br,Br)-[Re(p-NC6H4CH3)Br2(py-2-COO)(PPh3)] (2), cis-(Cl,Cl)- NC6H4CH3)X3(PPh3)2] and the carboxylic ligand (Scheme 1).
[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)] (3), cis-(Br,Br)-[Re(p- Interestingly, the reactions of [Re(p-NC6H4CH3)X3(PPh3)2] with
NC6H4CH3)Br2(py-2-COO)(PPh3)]·H2O (4), trans-(Cl,Cl)-[Re(p- py-2-COOH were independent of the molar ratio of the ligand
NC6H4CH3)Cl2(OMe)(PPh3)2] (5) and trans-(Br,Br)-[Re(p- to metal precursor. Only complexes with one chelating ligand
NC6H4CH3)Br2(OMe)(PPh3)2]·1/2H2O (6) are reported and were obtained even when an excess of picolinic acid was used.
characterized structurally and spectroscopically. Calculations On the other hand, the reactions were found to be solvent
at the DFT level were undertaken to elucidate the structural, dependent. They resulted in the formation of trans-(Cl,Cl)-
spectroscopic and bonding properties of the trans-(Cl,Cl) and [Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)]·MeCN (1a) and trans-
cis-(Cl,Cl) isomers of [Re(p-NC6H4CH3)X2(py-2-COO)(PPh3)]. (Br,Br)-[Re(p-NC6H4CH3)Br2(py-2-COO)(PPh3)] (2) when the
The catalytic activity of the complexes incorporating the picoli- reactions were carried out in acetonitrile. However, refluxing of
nate ligand was studied and compared in the synthesis of [Re(p-NC6H4CH3)X3(PPh3)2] with py-2-COOH in methanol led
Scheme 1 Formation of complexes 1–6.
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to a mixture of the compounds trans-(Cl,Cl)-[Re(p-NC6H4CH3)- 1278 cm−1 for 3 and 1314 and 1280 cm−1 for 4. High values of
Cl2(py-2-COO)(PPh3)] (1b) and cis-(Cl,Cl)-[Re(p-NC6H4CH3)- Δν(CO2) = νas(CO2) − νs(CO2) (equal to ≈350 cm−1) reflect a
Cl2(py-2-COO)(PPh3)] (3) for [Re(p-NC6H4CH3)Cl3(PPh3)2], and unidentate coordination mode of the carboxylate group in the
trans-(Br,Br)-[Re(p-NC6H4CH3)Br2(py-2-COO)(PPh3)] (2) and [Re(p-NC6H4CH3)X2(py-2-COO)(PPh3)] compounds.12 For trans-
cis-(Br,Br)-[Re(p-NC6H4CH3)Br2(py-2-COO)(PPh3)]·H2O (4) for (X,X) and cis-(X,X) isomers of [Re(p-NC6H4CH3)X2(py-2-COO)-
[Re(p-NC6H4CH3)Br3(PPh3)2]. We anticipated that compounds (PPh3)], these absorptions are very similar and they are not dis-
[Re(p-NC6H4CH3)X3(PPh3)2] might react with methanol, exhi- tinctive for molecular structure determination. This con-
biting a different reactivity pattern from that in acetonitrile. As clusion was confirmed by calculations. The calculated νas(CO2)
expected, the reactions of [Re(p-NC6H4CH3)X3(PPh3)2] with and νs(CO2) are almost identical for both trans-(X,X) and cis-
methanol afforded methoxy imido complexes trans-(Cl,Cl)- (X,X), despite the different molecular symmetry of these
[Re(p-NC6H4CH3)Cl2(OMe)(PPh3)2] (5) and trans-(Br,Br)-[Re(p- isomers (1820, 1370 and 1333 cm−1 for 1 and 1814, 1363 and
NC6H4CH3)Br2(OMe)(PPh3)2]·1/2H2O (6) as a result of the 1336 cm−1 for 3). In comparison with the experimental data,
replacement of the labile X ligand trans to the imido group however, these values are shifted to higher frequencies by
with the methoxy ion coming from methanol. This behaviour ∼8%, which is a usual feature for this approach (Fig. S1†).
is related to that observed for oxocomplexes [ReOX3(PPh3)2] in
The calculated ν(Re–NC6H4CH3) frequencies appear at
which the X ligand trans to the oxo group is readily exchanged 1555, 1447 and 1063 cm−1 for 1, and 1555, 1453 and
by the methoxy group giving [ReOX2(OMe)(PPh3)2].7 The com- 1065 cm−1 for 3. Experimentally, these vibrations are difficult
plexes 5 and 6, treated with py-2-COOH, gave compounds 3 to identify as the ν(Re–NC6H4CH3) stretches are mixed with
and 4 with the halide ligands in cis relative dispositions νCvN and νP–C modes of PPh3 and py-2-COO− ligands.11
(Scheme 1).
1H NMR spectra made it possible to distinguish between cis
All complexes 1–6 show high stability toward air and moist- and trans stereoisomers of [Re(p-NC6H4CH3)X2(py-2-COO)-
ure both in the solid state and in solution for several weeks at (PPh3)] (Fig. S2†). Distinctive signals attributed to the methyl
ambient temperature. The configurational stability of cis and protons of the p-tolylimido group of 1a, 1b, 2, 3 and 4 appear
trans isomers of [Re(p-NC6H4CH3)X2(py-2-COO)(PPh3)] in solu- in the range 2.30–2.20 ppm. The aromatic region is dominated
tion at room temperature was confirmed by NMR spectroscopy. by signals of triphenylphosphine protons, which obscure some
No cis–trans isomerism in solution was evidenced by NMR protons of the arylimido group. For both isomers, the signals
studies.
of the picolinate ligand are shifted upfield in comparison with
Since the imido core NR2− is isoelectronic to O2− and tends the free ligand (doublet at 8.70 and multiplet at 8.08–7.94) due
to form analogous complexes with rhenium, a short compari- to coordination to the rhenium atom. The stronger upfield
son with Re(V) oxocompounds is worthwhile. The majority of shift of these signals in cis-(X,X) isomers is consistent with an
the reported oxorhenium(V) complexes [ReOX2(N–O)(PPh3)] of electron transfer occurring from the halide ion into the trans
uninegative bidentate N,O-chelating ligands have cis-arranged pyridine ring of the picolinate ligand.
halide ions.8 A rare trans-X,X conformation has been found in
The coordination of the phosphine in [Re(p-NC6H4CH3)-
[ReCl2(APO)(PPh3)], [ReOCl2(DPO)(PPh3)],9 [ReOX2(hbt)(PPh3)]4 X2(py-2-COO)(PPh3)] and [Re(p-NC6H4CH3)X2(OMe)(PPh3)2]
and [ReOX2(hpb)(PPh3)]4 (APOH = 4-anilino-3-penten-2-one; was additionally confirmed by 31P NMR spectroscopy. As
DPOH = 4-[2,6-dimethylanilino]-3-penten-2-one, Hhbt = 2-(2′- expected, the single phosphorus signals, observed for 1–6 in
hydroxyphenyl)-2-benzothiazole and Hhpb
= 2-(2′-hydroxy- the range 25.57–25.65 ppm, are downfield from uncoordinated
phenyl)-1H-benzimidazole. As far as [ReOX2(py-2-COO)(PPh3)] triphenylphosphine (−6 ppm).
is concerned, only the cis-(X,X) isomers have been synthesized
Molecular structures
and characterized so far.10
For compounds [Re(NAr)X2(N-O)(PPh3)], the trans-X,X A definite proof for the structures of 1–6 was provided by the
arrangement of halide ions seems to be favoured, although the X-ray diffraction results (Tables 1 and S1;† Fig. 1 and 2).
imido rhenium complexes incorporating uninegative N,O-
The complexes 1a, 1b, 2, 3 and 4 show octahedral geometry
donor chelate ligands are distinctly less elaborated.11 Calcu- about the central rhenium atom defined by the p-methylphenyl-
lations at the DFT level were undertaken for trans-(Cl,Cl)- and imido group, two halide ions, the phosphorus atom of the
cis-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)] to get
a
PPh3 molecule, and the carboxylate chelate ligand. X-ray
deeper insight into the structural, spectroscopic and bonding studies revealed two different ligand arrangements in these
properties of isomeric forms.
compounds. The halide ions of 1a, 1b and 2 are in trans rela-
tive dispositions, whereas the complexes 3 and 4 were found to
be cis-(X,X) isomers. In all these compounds the oxygen atom
IR and 1H NMR spectra
The IR spectra of 1a, 1b, 2, 3 and 4 are dominated by very of carboxylate ligand occupies the trans position to the
strong bands (at 1679 cm−1 for 1a, 1679 cm−1 for 1b, p-methylphenylimido ion. This occupancy is justified by the
1677 cm−1 for 2, 1690 cm−1 for 3 and 1682 cm−1 for 4) trans-influence of the imido group forcing harder the oxygen
assigned to νas(CO2) stretching mode. Strong νs(CO2) bands are atom of the N–O ligand into the trans position. Triphenyl-
found at 1313 and 1282 cm−1 for 1a·MeCN, 1314 and phosphine molecule with its π-acidity adopts a cis disposition
1282 cm−1 for 1b, 1312 and 1287 cm−1 for 2, 1314 and with respect to the linear RNuRe–O core and stabilizes it due
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Fig. 1 The molecular structure of 1a, 1b, 2, 3 and 4. Displacement ellipsoids are drawn at 50% probability.
Fig. 2 The molecular structure of 5 and 6. Displacement ellipsoids are drawn at 50% probability.
to accessible π-donation from rhenium to the PPh3 molecule. ligands in high oxidation state complexes, in which the metal
Interestingly, no extraordinary differences in terms of bond is relatively electron-deficient and some π-bonding between
lengths and angles can be noticed between cis-(X,X) and trans- the imido nitrogen atom and the metal exist. The Re–Nimido
(X,X) isomers of [Re(p-NC6H4CH3)X2(py-2-COO)(PPh3)]. The bond lengths of 1.708(6) Å in 1a, 1.650(17) Å in 1b, 1.708(11) Å
Re–Nimido–Cimido bond angles of 167.9(6)° in 1a, 171.9(10)° in in 2, 1.708(4) Å in 3 and 1.717(3) Å in 4 fall in the range typical
1b, 173.1(10)° in 2, 174.4(3)° in 3 and 175.1(3)° in 4 agree with of mononuclear complexes of rhenium(V) having [ReuNR]3+
a
linear coordination mode of the arylimido ligands core, and reflect the expected triple ReuN bond.6,11 The
(167–176°), and these values are typical of phenyl imido interatomic distance between the rhenium atom and the
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carboxylate oxygen atom is almost equal to an ideal single Re– functional (Table S2†). Further calculations were performed
O bond length (ca. 2.04 Å),13 indicating lack of delocalization using the PBE1PBE method in combination with the basis sets
in the RNuRe–O unit.
presented in Table 2 as LTZ631+.
The complexes 5 and 6 were found to be trans-(X,X) isomers
For both compounds, the calculated charge on the rhenium
with the methoxy ligand trans to the ReuNR moiety. The atom is considerably lower than the formal charge +5 as a
elongation of the Re–Nimido (in comparison with the corres- product of significant charge donation from ligands to dRe
ponding values for 1–4) is accompanied by a shortening of the orbitals in the examined structures (Table 3). Less positive
Re–O distance trans to the ReuNR moiety. It is a consequence charge on the rhenium atom in cis-(Cl,Cl) isomer can be corre-
of the competition of the methoxy ion with the imido group in lated with higher electron density delocalization from Cl
the interaction with the dπ orbitals of the metal. The Re–OCH3 ligands towards the rhenium center occurring for this ligand
distances in 5 [1.915(3) Å] and 6 [1.898(3) Å] indicate a signifi- arrangement. The Cl ligands of trans-(Cl,Cl) isomer are more
cant double-bond character.14 The larger delocalization of elec- negatively charged as compared to cis-(Cl,Cl) one. For the
tron density in the linear RNuRe–OCH3 core results in a less other donor atoms of coordination sphere no significant vari-
pronounced electron transfer by π donation to the equatorial P ations in the calculated atomic charges are observed between 1
atoms. In compounds 5 and 6, phosphorus atoms form con- and 3.
siderably longer Re–P bond lengths as compared to complexes
1–4.
For further understanding of the bond characteristics in
reported isomers of [Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)],
natural bond orbital (NBO) studies have been performed. For
the cis-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)] complex,
the results of NBO point to the existence of one σ and two π
natural Re–Nimido orbitals. Both σ and π bonds are strongly
polarized towards the nitrogen atom. The hybridization at
rhenium shows clearly that d contribution dominates the
metal bonding in σ and π Re–Nimido bonds. For π bonds the
nitrogen atom of the imido ligand employs pure or quasi pure
p atomic orbitals (Fig. S4b†).
For the trans-(Cl,Cl) isomer, two Re–Nimido natural bond
orbitals were confirmed by NBO analysis. Both orbitals result
from overlapping of the empty dxy and dyz rhenium orbitals
with the occupied px and pz orbitals of the deprotonated nitro-
gen of the imido ligand, and they are of π character (Fig. S4a†).
Lack of σRe–NR natural orbital indicates a conceivable predomi-
nant Coulomb-type Re–ligand interaction.16 The nature of Re–
Nimido bonds is in agreement with the results obtained in pre-
vious studies regarding the analysis of the bonding nature of
rhenium imido complexes.5,11
DFT calculations
In the first step, extended tests of the ability of different DFT
methods (B3LYP, BP86, and PBE1PBE) to find the best overall
performer in describing the geometry of the imido Re(V) com-
plexes were performed. The geometries of trans-(Cl,Cl)-[Re(p-
NC6H4CH3)Cl2(py-2-COO)(PPh3)]
and
cis-(Cl,Cl)-[Re(p-
NC6H4CH3)Cl2(py-2-COO)(PPh3)] were optimized in singlet
states using three DFT methods (B3LYP, BP86, and PBE1PBE)
in combination with several basis sets given in Table 2.
All the methods give acceptable deviations between the
experimental and theoretical geometric data. The largest devi-
ations were found for the BP86 procedure, whereas both geo-
metries were best reproduced by employing the PBE1PBE
method (Table S2†). It gives an excellent estimation of Re–
Nimido bond lengths with deviations smaller than 0.003 Å.
Deviations of the distance Re–Nimido for the methods B3LYP,
and BP86 are larger, from 0.009 to 0.014 Å for B3LYP, and
from 0.030 to 0.033 Å for BP86 (Fig. S3†). The Re–Npy and Re–
O bond lengths are also well reproduced with errors ranging
from −0.004 to −0.035 Å for Re–Npy and from −0.017 to
−0.037 Å for Re–O. Larger deviations, especially for B3LYP
functional, were noticed for Re–Cl bond lengths, but they are
also satisfactory for this level of theory. The Re–Cl elongation
is consistent with the research of Turki et al. The authors
addressed that the drawback of DFT arises from the dynamical
correlation effects, which become very important in complexes
with a polar M–Cl bond.15 This issue can be partly solved
using the dispersion corrected functional in the DFT method,
as indicated by the results for the dispersion corrected B3LYP
Table 3 Atomic charges from the Natural Population Analysis (NPA) for
1 and 3 (PBE1PBE method in combination with LTZ631+)
Atom
1
3
Re(1)
N(1)
N(2)
O(1)
P(1)
Cl(1)
Cl(2)
0.075
0.052
−0.380
−0.205
−0.635
1.327
−0.308
−0.284
−0.376
−0.221
−0.637
1.340
−0.257
−0.223
Table 2 Basis sets employed for valence electrons of the complexes studied in this work
Basis set
Re
N
O
C
H
Cl
P
LTZ631
LANL2TZ(f)
LANL2TZ(f)
LANL2TZ(f)
LANL2DZ
6-31G*
6-31G*
6-31G*
6-31+G*
6-31+G*
6-31G*
3-21+G*
6-31+G*
6-31++G**
6-31+G**
6-31+G*
6-31+G*
6-31+G*
6-31+G*
6-31+G*
6-31+G*
6-31+G*
6-31+G*
LTZ6311
LTZ631+
LDZ631+
6-31+G*
6-31+G*
6-31+G*
6-31+G*
6-31+G*
6-31+G*
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Cl) isomer (3.28 for 1 and 3.46 eV 3) can be found. For this
geometry, the energy level of HOMO level is stabilized whereas
the energy level of LUMO is slightly raised compared with
complex 1.
Table 4 Wiberg bond index matrix in the NAO basis for 1 and 3
(PBE1PBE method in combination with LTZ631+)
Bond
1
3
Re(1)–N(1)
Re(1)–N(2)
Re(1)–O(1)
Re(1)–P(1)
Re(1)–Cl(1)
Re(1)–Cl(2)
0.540
2.087
0.601
0.781
0.852
0.878
0.585
2.060
0.577
0.762
0.890
0.929
Electronic spectra
The complexes 1 and 3 have similar absorption character
and TDDFT transitions well correlate with the experimental
absorptions, as illustrated in Fig. 5.
The low-energy absorption bands observed at 791.2 and
602.4 for 1 and 763.6 and 617.6 nm for 3 are contributed by
transitions HOMO → LUMO and HOMO → LUMO+1 either for
cis-(Cl,Cl) or trans-(Cl,Cl) isomer (Tables S3 and S4†). Analysis
of the corresponding orbitals clearly shows that these tran-
sitions can be described as mixed d→d (Ligand Field; LF),
dπ(Cl)→dRe (Ligand–Metal Charge Transfer; LMCT) and
π(Cl)→π*(p-tol)/π*(py-2-COO) (Ligand–Ligand Charge Transfer;
LLCT) transitions or a delocalized MLLCT (metal–ligand-to-
ligand CT) description also can be used. The blue shift of the
lowest-energy absorption band for 3 correlates well with the
increase in HOMO–LUMO for this complex.
The triple bond character of Re–Nimido in 1 and 3 is also
indicated by the Wiberg bond indices (Table 4). For both com-
plexes 1 and 3, the Wiberg bond indices of Re(1)–N(2) are over
three times larger than those of Re(1)–N(1) in 1 and Re(1)–N(1)
in 3.
All used functionals and basis sets led to the higher stabi-
lity of the cis-isomer, but the energy difference between
trans-(Cl,Cl) and cis-(Cl,Cl) isomers is remarkably small
(1.92–3.74 kcal mol−1), indicating that their formation is kine-
tically rather than thermodynamically controlled.
A schematic representation of the energy levels with the
partial frontier molecular orbital compositions of 1 and 3 is
presented in Fig. 3. The contours of the selected frontier orbi-
tals are depicted in Fig. 4. As can be seen from Fig. 3 and 4,
only small variations on the frontier molecular orbital energies
and orbital compositions were observed with regard to the
relative position of halide ions in [Re(p-NC6H4CH3)Cl2(py-2-
COO)(PPh3)]. The HOMO of these complexes presents signifi-
cant metallic character and it is mainly constituted by
rhenium dxy orbital (62% for 1 and 65% for 3) in antibonding
relation to chloride occupied p orbitals (30% for 1 and 25%
for 3). For the cis-(Cl,Cl) isomer, this molecular orbital is also
insignificantly contributed by the picolinate ligand (10%).
The HOMO−1 orbital is 1.16 eV for 1 and 0.96 eV for 3
lower in energy in relation to the highest occupied molecular
orbital and presents a high contribution of the p-methylphenyl-
imido ligand (70% for 1 and 72% for 3). The contribution of
this ligand is due to the interaction of a bonding p orbital with
a metallic dπ orbital (14% for 1 and 13% for 3). To a large
extent, the HOMO−1 of 1 and 3 can be considered as πReuNR
orbital. For the cis-(Cl,Cl) isomer, a moderate contribution
from the antibonding interaction of rhenium dπ orbital with
Cl orbitals (12%) is also observed in this molecular orbital. In
turn, the HOMO−1 of 1 is contributed by triphenylphosphine
orbitals (10%). The other high-lying occupied orbitals are
mainly centered on the triphenylphosphine orbitals.
The experimental absorptions at 425.6 nm for trans-(Cl,Cl)
isomer and 418.0 for cis-(Cl,Cl), assigned to HOMO →
LUMO+2 transition can be seen as mixed d→d, π(Cl)→dRe
,
d
Re→π*(py-2-COO)/π*(p-tol) and π(Cl)→π*(py-2-COO)/π*(p-tol)
transitions. The intense bands in the high energy region are
largely attributed to Ligand–Ligand Charge Transfer and interli-
gand (IL) transitions.
Catalytic activity
The catalytic activity of the complexes reported herein was
tested in the reaction of amines with ethyl diazoacetate
leading to N-substituted glycine ethyl esters.4 Aniline (7a) was
selected as the model substrate in order to better assess the
selectivity of the catalysts due to the presence of two N–H
bonds that could lead to products 9a and 10a (Scheme 2 and
Table 5).
Initial studies with the complex trans-Cl,Cl (1) at 70 °C
under air, either in toluene or without a solvent gave poor con-
versions (Table 5, entries 1 and 2). A promising 42% conver-
sion into product 9a was, however, noted when the reaction
was carried out under an argon atmosphere (Table 5, entry 3).
This result could be somewhat improved when using trans-Br,
Br (2) as a catalyst, albeit a substantial amount of product 10a
was obtained (Table 5, entry 4). Interestingly, the formation of
the latter could be prevented when double the amount of the
catalyst was utilized (Table 5, entry 5), whereas microwave
irradiation did not have any influence on the reaction rate
(Table 5, entry 6). Higher conversion was achieved for 9a with
cis-Cl,Cl (3) under solvent-free conditions, which was formed
together with minor amounts of 10a (Table 5, entry 7). Reac-
tions with cis-Cl,Cl (3) conducted either at room temperature
or in manifold organic solvents proved to be inefficient
(Table 5, entries 8–16). Slightly higher conversion was recorded
using double amount of cis-Cl,Cl (3), with concomitant for-
mation of 10a (Table 5, compare entries 7 and 17). Under the
The LUMO of these complexes presents a high contribution
of the p-methylphenylimido ligand, mainly from the inter-
action between the π*-antibonding orbital and the dπ rhenium
orbital. A significant contribution of π*ReuNR orbitals is also
observed in LUMO+1 and LUMO+2 of 1 and 3, unlike LUMO,
however, these MOs are predominantly localized on the
π*-antibonding orbitals of the picolinate ligand.
By comparing the FMO energy levels of 1 and 3, an insignif-
icant increase in the energy gap HOMO–LUMO for the cis-(Cl,
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Fig. 3 Schematic representation of the energy levels with the partial frontier molecular orbital compositions of trans-(Cl,Cl)- and cis-(Cl,Cl)-[Re(p-
NC6H4CH3)Cl2(py-2-COO)(PPh3)].
latter conditions, the presence of air was shown to have a dele-
Better results were attained when the secondary amines
terious effect on the performance of the catalyst (Table 5, entry N-methylaniline (7b) and piperidine (7c) were subjected to the
18), as also observed for trans-Cl,Cl (1) (Table 5, entries 1 and conditions in entry 17 (Table 5). A single product was detected
2). Finally, cis-Br,Br (4) behaved similar to trans-Br,Br (2) in both cases, with the piperidine derivative being obtained in
(Table 5, compare entries 4 and 19), with the stereochemistry almost quantitative yield (Scheme 3). It is noteworthy that,
of the complexes having no apparent effect on the yield.
under the aforementioned conditions, complex cis-Cl,Cl-[Re]
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Fig. 4 The contours of selected HOMO and LUMO orbitals of trans-(Cl,Cl)- and cis-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)]. Positive values
of the orbital contour are represented in red (0.04 au) and negative values in green (−0.04 au).
(3) was shown to be superior in the latter reaction (9c, 95% process in which the role of the metal is not yet well under-
yield) when compared with the other complexes studied, stood.17 In the reaction of ethyl diazoacetate with primary
namely, trans-Cl,Cl-[Re] (1) (9c, 31% yield), trans-Br,Br-[Re] (2) amines catalyzed by methylrhenium trioxide, Espenson et al.
(9c, 53% yield), and cis-Br,Br-[Re] (4) (9c, 19% yield).
proposed the formation of a three-membered metallaoxacycle
The transition-metal catalyzed N–H insertion reaction of methylrhenium dioxide (derived from ethyl diazoacetate),
diazocarbonyl compounds is generally considered a stepwise which undergoes nucleophilic attack by the amine at the
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Fig. 5 The experimental (black) and calculated (red) electronic absorption spectra of 1 and 3.
Scheme 2 The reaction of aniline with ethyl diazoacetate catalyzed by the rhenium complexes.
Table 5 Reaction of aniline and ethyl diazoacetate catalyzed by
rhenium complexesa
Solvent/
atmosphere
Conversionb
(9a, 10a) (%)
Entry
Catalyst
1
2
3
4
5
6
7
8
trans-Cl,Cl (1)
trans-Cl,Cl (1)
trans-Cl,Cl (1)
trans-Br,Br (2)
trans-Br,Br (2)c
trans-Br,Br (2)
cis-Cl,Cl (3)
cis-Cl,Cl (3)
cis-Cl,Cl (3)
cis-Cl,Cl (3)
cis-Cl,Cl (3)
cis-Cl,Cl (3)
cis-Cl,Cl (3)
cis-Cl,Cl (3)
cis-Cl,Cl (3)
cis-Cl,Cl (3)
cis-Cl,Cl (3)c
cis-Cl,Cl (3)c
cis-Br,Br (4)
Toluene (air)
Neat (air)
Neat (Ar)
—
17 (0)
42 (4)
53 (17)
54
17
62 (14)
7
—
17
19
10
5
23
7
5
67 (12)
19
54 (8)
Neat (Ar)
Neat (Ar)
Neat (Ar)d
Neat (Ar)
Neat (Ar)e
Acetone (Ar)
CH2Cl2 (Ar)
CHCl3 (Ar)
MeOH (Ar)
MeCN (Ar)
MeNO2 (Ar)
DMF (Ar)
f
Scheme 3 Solvent-free reaction of secondary amines and ethyl diazo-
acetate catalyzed by the rhenium complex cis-Cl,Cl (3).
9
10
11
12
13
14
15
16
17
18
19
carbon atom of the metallacycle.4 In our case, the scenario is
rather different as no RevO bonds are present in the com-
plexes. Therefore, we propose the nucleophilic attack of the
amine occurring on the electrophilic Re carbene 10 to form an
ammonium ylide (11), which undergoes a 1,2-rearrangement
to transfer a proton from nitrogen to carbon with regeneration
of the catalyst (Scheme 4).
The fact that methylrhenium trioxide attained higher yields
than our complexes, in the reaction of primary amines with
ethyl diazoacetate, could be due to the higher electrophilic
character of the carbon-reacting center in the corresponding
Toluene (Ar)
Neat (Ar)
Neat (air)
Neat (Ar)
a Reaction conditions: 7a (0.5 mmol), 8 (0.5 mmol), Re complex
(2.5 mg), solvent (2.0 ml), 70 °C, 24 h, unless otherwise stated.
b Conversion into 9a determined by GLC; the conversion into 10a is
given in parentheses. c 5.0 mg of the rhenium complex were used.
d Reaction performed under microwave irradiation (30 W, 70 °C,
30 min). e Reaction carried out at rt. f The imine derived from acetone
and aniline was formed quantitatively.
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that PPh3 is the ligand exchanged by the carbene derived from
ethyl diazoacetate (we always detect some PPh3 as a by-
product),18 this exchange could be more favoured for a certain
stereochemistry on the basis of steric factors or the strength of
the interaction of PPh3 with the metal. On this basis, PPh3
might interact more strongly with the more positively charged
Re in trans-(Cl,Cl), making the ligand exchange more difficult
and resulting in lower yield than with cis-(Cl,Cl).
Although the methodology of Espenson’s group, involving
MeReO3, produced compound 9a in a higher yield (89%)
under milder reaction conditions, the long reaction times
applied and the use of dry benzene as a solvent curtail its prac-
tical application.4 In contrast, the procedure presented herein
is more environmentally benign, with the reactions proceeding
in moderate-to-excellent yields under solvent-free conditions.
Scheme 4 Proposed reaction mechanism.
Conclusions
intermediate, given that Re(VII) species are involved instead of
the Re(V) of our complexes. Indeed, the calculated charge on
the rhenium atom in 1 and 3 was found to be considerably
lower than the formal charge +5 (see above). However, in the
case of the reaction of ethyl diazoacetate with secondary
amines (Scheme 3), the fact that there is only one reactive N–H
bond together with their higher nucleophilicity could account
for the better yields observed.
A comparison between the reactivity of the complexes 1–4
seems more difficult to establish. On the one hand, the less
positive charge on the rhenium atom in cis-(Cl,Cl) isomer,
when compared with that in trans-(Cl,Cl), points to a favoured
nucleophilic attack in the derived carbene of the latter (more
electrophilic). However, the product conversion was higher for
cis-(Cl,Cl) (67% vs. 42%). At any rate, we must take into account
that the Re atomic charges from the Natural Population Analysis
(NPA) (Table 4) are small and, probably, not different enough to
allow the explanation of the differences found in reactivity. On
the other hand, the NBO studies revealed the existence of one σ
and two π bonds of Re–Nimido in cis-(Cl,Cl) strongly polarized
towards the nitrogen atom, and the lack of σRe–NR natural
orbital in trans-(Cl,Cl), which might indicate a lower stability of
the latter complex affecting its reactivity.
As regards the reactivity of the dibromo complexes 2 and 4
with ethyl diazoacetate and aniline, the conversion was the
same in both cases (54%) and somewhat lower than with cis-
(Cl,Cl) (1). Therefore, the stereochemistry seems not to play an
important role in this particular reaction. The differences in
reactivity were more pronounced in the reaction of ethyl di-
azoacetate and piperidine, a more nucleophilic and bulkier
amine than aniline (Scheme 3). In this case, the higher electro-
philicity conferred to the complex by the two Cl ligands (more
electronegative than Br), together with the lower steric hin-
drance imparted by Cl to the complex when compared with Br,
could explain the higher conversion recorded for the cis-
(Cl,Cl), the stereochemistry being important in this case.
Another aspect that could be considered is the ligand
exchange in the formation of the Re carbene 10. If we suppose
The reactivity of [Re(p-NC6H4CH3)Cl3(PPh3)2] (X = Cl, Br)
towards pyridine-2-carboxylic acid has been examined and
compared with the related exchange reactions performed with
oxocompounds [ReOX3(PPh3)2]. As a result of these studies, six
novel rhenium(V) imidocomplexes have been obtained and
characterized structurally and spectroscopically. In the isomers
trans-(X,X)-[Re(p-NC6H4CH3)X2(py-2-COO)(PPh3)] and cis-(X,X)-
[Re(p-NC6H4CH3)X2(py-2-COO)(PPh3)], the pyridine-2-carboxy-
late ion is coordinated in a chelate way via N- and O-donor
atoms, and the PPh3 molecule is cis-located in relation to the
ReuN moiety, which forces the metal nonbonding d electrons
to lie in the plane perpendicular to the MuN bond axis. The
X-ray studies and NBO analysis confirm a linear coordination
mode of the p-NC6H4CH3 ligand and the triple bond between
the rhenium and the imido ligand. The caboxylate imido
rhenium complexes have been shown to be catalytically active
in the reaction of aniline and ethyl diazoacetate under solvent-
free conditions. The complex cis-(Cl,Cl)-[Re(p-NC6H4CH3)-
Cl2(py-2-COO)(PPh3)] (3) has reached a maximum 67% conver-
sion into N-phenyl ethyl glycine ester, with the product of
double carbene insertion being normally obtained as a side-
product. In contrast, secondary amines have led to single
N-substituted ethyl glycine esters in moderate-to-excellent yields.
Experimental section
Materials
All chemicals and bidentate ligands were purchased from com-
mercial sources and were used without further purification.
The complexes mer-[Re(p-NC6H4CH3)X3(PPh3)2] (X = Cl, Br)
were prepared according to the literature methods.19
Instrumentation
IR spectra were recorded on a Nicolet Magna 560 spectrophoto-
meter in the spectral range 4000–400 cm−1 with the samples
in the form of KBr pellets. The 1H NMR, 13C NMR and 31P
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NMR spectra were recorded (298 K) on a Bruker Avance 500
NMR spectrometer at a resonance frequency of 500 MHz for
31P NMR: 25.58 ppm.
trans-(Br,Br)-[Re(p-NC6H4CH3)Br2(py-2-COO)(PPh3)] (2). IR
1H NMR spectra, 125 MHz for 13C NMR spectra and 162 MHz (KBr; ν/cm−1): 3051(w), 1677(vs), 1614(m), 1588(s), 1481(m),
for 31P NMR using DMSO-d6 as a solvent and TMS as an 1448(w), 1435(s), 1357(m), 1312(s), 1287(s), 1248(m), 1173(w),
internal standard. The X-ray intensity data of 1–6 were col- 1153 (m), 1092(s), 1055(w), 1017(w), 855(m), 822(m), 747(s),
lected on a Gemini A Ultra diffractometer equipped with an 705(sh), 694(s), 563(w), 528(s), 598(m), 497(m) and 457(w).
Atlas CCD detector and graphite monochromated MoKα radi-
C31H26Br2N2O2PRe (835.53): calcd C 44.56, H 3.14, N 3.35;
ation (λ = 0.71073 Å) at room temperature. Details concerning found C 44.83, H 3.19, N 3.38%.
crystal data and refinement are given in Table 1. Lorentz,
1H NMR: 8.18(d, 1H, 7.8 Hz), 8.09–8.05(m, 1H), 8.02(d, 1H,
polarization and empirical absorption correction using spheri- 5.4 Hz), 7.96–7.92(m, 1H), 7.68–7.59(m, 6H), 7.53–7.45(m,
cal harmonics implemented in SCALE3 ABSPACK scaling 11H), 7.26(d, 2H, 8.2 Hz) and 2.28(s, 3H) ppm.
algorithm were applied.20 The structures were solved by the
13C NMR: 154.3, 148.6, 147.0, 142.8, 142.3, 134.7, 134.6,
Patterson method and subsequently completed by the differ- 131.6, 128.8, 128.2, 121.7, 22.4 ppm.
ence Fourier recycling. All the non-hydrogen atoms were
refined anisotropically using a full-matrix, least-squares tech-
nique. The hydrogen atoms were treated as “riding” on their
parent carbon atoms and assigned isotropic temperature
factors equal to 1.2 (non-methyl) and 1.5 (methyl) times the
value of the equivalent temperature factor of the parent atom.
The methyl groups were allowed to rotate about their local
threefold axis. SHELXS97 and SHELXL97 programs were used
for all the calculations.21 Atomic scattering factors were those
incorporated in the computer programs.
31P NMR: 25.65 ppm.
Preparation of complexes 1b and 3
[Re(p-NC6H4CH3)Cl3(PPh3)2] (0.48 g, 0.54 mmol) was added to
pyridine-2-carboxylic acid (0.07 g, 0.60 mmol) in methanol
(60 ml) and the reaction mixture was refluxed for 4 h. The
resulting solution was reduced in volume to ∼10 ml and
allowed to cool to room temperature. A green crystalline pre-
cipitate was filtered off and dried in air. X-ray quality green
crystals of trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)]
(1b) and cis-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)] (3)
were obtained by slow recrystallization from methanol. The
crystals of 1b and 3 were manually separated using a micro-
scope and they were collected in 25 and 65% yield, respecti-
vely. Both compounds crystallize in a monoclinic system (1b in
Cc and 3 in P21/n space group), but they belong to different
crystal classes, 1b to the monoclinic-domatic class, whereas 3
belongs to the prismatic crystal class. Different shapes and
larger dimensions of the crystals 1b facilitate hand-sorting of
the crystals 1b and 3. The photos of the measured monocrys-
tals of 1b and 3 with their dimensions are shown in Fig. 6.
trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)] (1b). IR
(KBr; ν/cm−1): 3063(w), 1679(vs), 1641(sh), 1610(sh), 1590(m),
1564(sh), 1483(m), 1468(w), 1435(s), 1361(w), 1314(s), 1282(s),
1253(m), 1234(w), 1187(m), 1172(w), 1151(m), 1141(m),
1116(w), 1094(s), 1055(w), 1030(w), 1019(w), 853(s), 824(m),
768(m), 754(sh), 746(s), 709(s), 694(s), 527(s), 510(m), 493(w),
456(w).
CCDC 899354 (for 1a), 936243 (for 1b), 899355 (for 2),
899356 (for 3), 899357 (for 4), 936244 (for 5) and 936245 (for 6)
contain the supplementary crystallographic data for this paper.
Preparation of complexes 1a and 2
mer-[Re(p-NC6H4CH3)Cl3(PPh3)2] (0.43 g, 0.54 mmol) or [Re(p-
NC6H4CH3)Br3(PPh3)2] (0.52 g, 0.54 mmol) was added to pyri-
dine-2-carboxylic acid (0.07 g, 0.60 mmol) in acetonitrile
(60 ml) and the reaction mixture was refluxed for 4 h. The
resulting solution was reduced in volume to ∼10 ml and
allowed to cool to room temperature. A green crystalline
precipitate of trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)-
(PPh3)]·MeCN (1a) or trans-(Br,Br)-[Re(p-NC6H4CH3)Br2(py-2-
COO)(PPh3)] (2) was filtered off and dried in air. X-ray quality
green crystals of 1·MeCN and 2 were obtained by slow recrystal-
lization from acetonitrile, and collected in 80 and 75% yield,
respectively.
trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)]·MeCN
(1a). IR (KBr; ν/cm−1): 3063(w), 1679(vs), 1605(w), 1590(s),
1482(m), 1462(w), 1434(s), 1361(m), 1313(s), 1282(s), 1252(m),
1172(w), 1141(m), 1094(s), 1055(w), 1018(w), 853(m), 824(m),
767(w), 746(s), 705(sh), 693(s), 574(w), 527(s), 510(m), 493(sh),
and 456(w).
C33H29Cl2N3O2PRe (787.66): calcd C 50.32, H 3.71, N 5.33;
found C 50.59, H 3.62, N 5.24%.
1H NMR: 8.18(d, 1H, 7.8 Hz), 8.07(td, 1H, 7.7 Hz, 1.4 Hz),
7.98(d, 1H, 5.5 Hz), 7.91(ddd, 1H, 8.0 Hz, 4.7 Hz, 1.9 Hz),
7.61(ddd, 6H, 11.3 Hz, 7.9 Hz, 1.5 Hz), 7.51–7.40(m, 11H),
7.25(d, 2H, 8.1 Hz), 2.27(s, 3H) and 2.09(s, 3H) ppm.
13C NMR: 154.2, 148.2, 147.0, 143.0, 141.9, 134.6, 134.5,
131.8, 131.7, 131.5, 131.3, 129.8, 128.9, 128.8, 126.4, 121.9,
118.5, 22.4, 1.62 ppm.
Fig. 6 The photos of the measured monocrystals of 1b and 3.
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C31H26Cl2N2O2PRe (746.61): calcd C 49.87, H 3.51, N 3.75;
found C 50.01, H 3.67, N 3.63%.
cis-(Br,Br)-[Re(p-NC6H4CH3)Br2(py-2-COO)(PPh3)]·H2O (4).
IR (KBr; ν/cm−1): 3063(w), 1682(vs), 1589(s), 1482(m), 1467(m),
1H NMR: 8.19(d, 1H, 7.7 Hz), 8.07(td, 1H, 7.6 Hz, 1.4 Hz), 1434(s), 1360(m), 1314(s), 1280(s), 1253(m), 1172(w), 1139(m),
7.98(d, 1H, 5.4 Hz), 7.92(ddd, 1H, 7.1 Hz, 5.6 Hz, 1.3 Hz), 1096(s), 1055(w), 1019(m), 852(m), 823(m), 766(sh), 744(s),
7.59(ddd, 6H, 9.8 Hz, 7.7 Hz, 1.7 Hz), 7.49(dd, 7.4 Hz, 2.0 Hz, 693(s), 574(w), 527(s), 508(m), 494(m) and 456(w).
9H), 7.42(d, 2H, 8.3 Hz), 7.25(d, 8.2 Hz, 2H) and 2.26(s, 3H) ppm.
C31H28Br2N2O3PRe (853.54): calcd C 43.62, H 3.31, N 3.28;
13C NMR: 169.2, 154.2, 148.2, 147.0, 143.0, 141.9, 134.6, found C 43.42, H 3.26, N 3.35%.
134.5, 131.8, 131.7, 131.5, 131.3, 128.9, 128.8, 121.9, 31.2,
1H NMR: 7.94(dd, 1H, 6.6 Hz, 5.1 Hz), 7.75–7.59(m, 4H),
22.4 ppm.
7.51–7.39(m, 10H), 7.36–7.29(m, 5H), 7.17–7.11(m, 3H), 3.33(s,
2H) and 2.23(s, 3H) ppm.
31P NMR: 25.57 ppm.
cis-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(py-2-COO)(PPh3)]
(3). IR
13C NMR: 169.1, 156.5, 154.2, 148.2, 147.0, 143.0, 141.9,
(KBr; ν/cm−1): 3055(w), 2920(w), 1690(vs), 1607(w), 1591(m), 134.6, 134.5, 134.1, 134.0, 131.8, 131.7, 131.5, 131.3, 129.8,
1482(m), 1464(w), 1435(s), 1357(w), 1314(s), 1275(s), 1249(w), 129.2, 129.1, 128.9, 128.8, 127.6, 126.6, 126.4, 125.9, 123.5,
1149(m), 1094(s), 1056(w), 1018(w), 857(m), 819(m), 752(m), 122.9, 121.9, 118.6, 22.4 ppm.
718(sh), 694(s), 559(w), 528(s), 455(w).
31P NMR: 25.63 ppm.
C31H26Cl2N2O2PRe (746.61.53): calcd C 49.87, H 3.51,
N 3.75; found C 49.99, H 3.43, N 3.64%.
Preparation of complexes 5 and 6
1H NMR: 7.91–7.88(m, 1H), 7.72–7.61(m, 3H), 7.46–7.39(m,
9H), 7.33(td, 6H, 7.9 Hz, 2.6 Hz), 7.15(d, 2H, 8.1 Hz), 7.04(d,
2H, 8.4 Hz), 2.25(s, 3H) ppm.
[Re(p-NC6H4CH3)Cl3(PPh3)2] (0.48 g, 0.54 mmol) or [Re(p-
NC6H4CH3)Br3(PPh3)2] (0.52 g, 0.54 mmol) was refluxed in
methanol (60 ml) for 4 h. The resulting solution was reduced
in volume to ∼10 ml and allowed to cool to room temperature.
13C NMR: 154.2, 152.9, 148.1, 146.2, 143.0, 141.9, 134.6,
134.5, 131.8, 131.5, 131.3, 131.0, 129.7, 129.3, 129.2,
129.1, 128.9, 128.8, 126.4, 125.9, 124.3, 121.9, 31.1, 22.4,
22.2 ppm.
A
brown crystalline precipitate of trans-[Re(p-NC6H4CH3)-
Cl2(OMe)(PPh3)2] (5) and trans-[Re(p-NC6H4CH3)Br2(OMe)-
(PPh3)2] (6) was filtered off and dried in air. X-ray quality
brown crystals of 5 and 6 were obtained by slow recrystalliza-
tion from methanol, and collected in 80 and 85% yield,
respectively.
31P NMR: 25.62 ppm.
Preparation of complexes 2 and 4
trans-(Cl,Cl)-[Re(p-NC6H4CH3)Cl2(OMe)(PPh3)2]
(5). IR
A procedure similar to that for 1b and 3 was used with [Re(p-
NC6H4CH3)Br3(PPh3)2] (0.52 g, 0.54 mmol), py-2-COOH (0.07 g,
0.60 mmol) and methanol (60 ml). Crystals of trans-(Br,Br)-
[Re(p-NC6H4CH3)Br2(py-2-COO)(PPh3)] (2) and cis-(Br,Br)-
[Re(p-NC6H4CH3)Br2(py-2-COO)(PPh3)]·H2O (4) were manually
separated using a microscope and collected in 30 and 60%
yield, respectively. Both compounds crystallize in a monoclinic
system (3 in Cc and 4 in C2/c space group) but they belong to
(KBr; ν/cm−1): 3055(w), 2897(w), 2804(w), 2094(w), 1586(w),
1571(w), 1481(m), 1434(s), 1337(w), 1315(sh), 1191(w), 1151(w),
1097(vs), 1029(w), 1014(w), 990(m), 822(m), 745(s), 706(sh),
693(s), 521(s), 512(sh), 499(sh), 443(m).
C44H40Cl2NOP2Re (917.81): calcd C 57.58, H 4.39, N 1.53;
found C 57.39, H 4.27, N 1.47%.
1H NMR: 7.66–7.53(m, 30H), 7.52–7.43(m, 2H), 7.31(d, 1H,
8.4 Hz), 7.26(d, 1H, 8.4 Hz), 3.17(s, 3H), 2.33(s, 3H) ppm.
13C NMR: 133.6, 132.7, 132.5, 131.9, 130.6, 129.3, 129.2,
123.5, 49.2, 21.0 ppm.
different crystal classes,
3 to monoclinic-domatic class,
whereas 4 belongs to the prismatic crystal class. Different
shapes and larger dimensions of crystals 2 additionally facili-
tate hand-sorting of the crystals. The photos of the measured
monocrystals of 2 and 4 with their dimensions are shown in
Fig. 7.
31P NMR: 25.62 ppm.
trans-(Br,Br)-[Re(p-NC6H4CH3)Br2(OMe)(PPh3)2]·1/2H2O (6).
IR (KBr; ν/cm−1): 3054(w), 2905(w), 2807(w), 1593(w), 1571(w),
1481(m), 1433(s), 1332(w), 1313(sh), 1185(w), 1172(sh),
1149(w), 1090(vs), 1028(w), 1015(w), 995(m), 822(m), 745(s),
693(vs), 521(sh), 511(vs), 495(sh), 468(m), 445(m).
C44H42Br2NO2P2Re (1016.74): calcd C 51.97, H 4.16, N 1.38;
found C 51.23, H 4.18, N 1.31%.
1H NMR of 6: 7.66–7.53(m, 30H), 7.32(d, 2H, 8.1 Hz),
7.27–7.24(m, 2H), 3.17(s, 3H), 3.54(s, 1H) and 2.33(s, 3H) ppm.
13C NMR: 138.4, 132.5, 132.0, 131.9, 129.3, 129.2, 123.5,
49.0, 21.0 ppm.
31P NMR: 25.65 ppm.
Catalytic experiments
All reactions were performed using screw-cap tubes in a multi-
Fig. 7 The photos of the measured monocrystals of 2 and 4.
reactor system. In a typical experiment: aniline (7a, 0.5 mmol,
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49 μL) and ethyl diazoacetate (8, 0.5 mmol, 53 μL) were added
to a reactor tube containing the complex cis-[Re(p-NC6H4CH3)-
Cl2(py-2-COO)(PPh3)] (3, 2.5 mg, 0.0034 mmol, 0.68 mol%)
without a solvent and under argon. The reaction mixture
was warmed to 70 °C and monitored by TLC and/or GLC
until steady conversion of the starting materials. EtOAc
(2 ml) was added to the resulting mixture, followed by fil-
tration through celite and washing with additional EtOAc
(4 ml). The solvent was evaporated at reduced pressure and
the resulting reaction crude was purified by column chrom-
atography (silica gel, hexane–EtOAc) to give the pure N-phe-
nylglycine ethyl ester (9a) as a pale orange solid [39.4 mg,
71% yield (referred to the amount converted into 9a)]. Com-
pounds 9a–9c were characterized by comparison of their
physical and spectroscopic data with those reported in the
literature.4
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