Oxorhenium complexes bearing the water-soluble tris(pyrazol-1-yl) methanesulfonate, 1,3,5-triaza-7-phosphaadamantane, or related ligands, as catalysts for Baeyer-Villiger oxidation of ketones

Article abstract of DOI:10.1021/ic400024r

New rhenium(VII or III) complexes [ReO₃(PTA)₂] [ReO₄] (1) (PTA = 1,3,5-triaza-7-phosphaadamantane), [ReO ₃(mPTA)][ReO₄]I (2) (mPTA = N-methyl-1,3,5-triaza-7- phosphaadamantane cation), [ReO₃(HMT)₂][ReO₄] (3) (HMT = hexamethylenetetramine), [ReO₃(η²-Tpm)(PTA) ][ReO₄] (4) [Tpm = hydrotris(pyrazol-1-yl)methane, HC(pz) ₃, pz = pyrazolyl], [ReO₃(Hpz)(HMT)][ReO₄] (5) (Hpz = pyrazole), [ReO(Tpms)(HMT)] (6) [Tpms = tris(pyrazol-1-yl) methanesulfonate, O₃SC(pz)₃^-] and [ReCl ₂{N₂C(O)Ph}(PTA)₃] (7) have been prepared from the Re(VII) oxide Re₂O₇ (1-6) or, in the case of 7, by ligand exchange from the benzoyldiazenido complex [ReCl₂{N ₂C(O)Ph}(Hpz)(PPh₃)₂], and characterized by IR and NMR spectroscopies, elemental analysis and electrochemical properties. Theoretical calculations at the density functional theory (DFT) level of theory indicated that the coordination of PTA to both Re(III) and Re(VII) centers by the P atom is preferable compared to the coordination by the N atom. This is interpreted in terms of the Re-PTA bond energy and hard-soft acid-base theory. The oxo-rhenium complexes 1-6 act as selective catalysts for the Baeyer-Villiger oxidation of cyclic and linear ketones (e.g., 2-methylcyclohexanone, 2-methylcyclopentanone, cyclohexanone, cyclopentanone, cyclobutanone, and 3,3-dimethyl-2-butanone or pinacolone) to the corresponding lactones or esters, in the presence of aqueous H₂O₂. The effects of a variety of factors are studied toward the optimization of the process.

Full text of DOI:10.1021/ic400024r

Article
pubs.acs.org/IC
Oxorhenium Complexes Bearing the Water-Soluble Tris(pyrazol-1-
yl)methanesulfonate, 1,3,5-Triaza-7-phosphaadamantane, or Related
Ligands, as Catalysts for Baeyer−Villiger Oxidation of Ketones
Luísa M. D. R. S. Martins,*,†,‡ Elisabete C. B. A. Alegria,†,‡ Piotr Smolenski,c Maxim L. Kuznetsov,‡
́
and Armando J. L. Pombeiro*,‡
†Chemical Engeneering Departamental Area, Instituto Superior de Engenharia de Lisboa (ISEL), R. Conselheiro Emídio Navarro,
1959-007 Lisboa, Portugal
‡Centro de Química Estrutural, Complexo I, Instituto Superior Tec
́
nico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001
Lisboa, Portugal
cFaculty of Chemistry, University of Wrocław, Ul. F Joliot Curie 14, PL-50383 Wroclaw, Poland
S
* Supporting Information
ABSTRACT: New rhenium(VII or III) complexes [ReO3(PTA)2][ReO4]
(1) (PTA = 1,3,5-triaza-7-phosphaadamantane), [ReO3(mPTA)][ReO4]I
(2) (mPTA = N-methyl-1,3,5-triaza-7-phosphaadamantane cation),
[ReO3(HMT)2][ReO4] (3) (HMT = hexamethylenetetramine),
[ReO3(η2-Tpm)(PTA)][ReO4] (4) [Tpm = hydrotris(pyrazol-1-yl)-
methane, HC(pz)3, pz = pyrazolyl], [ReO3(Hpz)(HMT)][ReO4] (5)
(Hpz = pyrazole), [ReO(Tpms)(HMT)] (6) [Tpms = tris(pyrazol-1-
−
yl)methanesulfonate, O3SC(pz)3 ] and [ReCl2{N2C(O)Ph}(PTA)3] (7)
have been prepared from the Re(VII) oxide Re2O7 (1−6) or, in the case of
7, by ligand exchange from the benzoyldiazenido complex [ReCl2{N2C-
(O)Ph}(Hpz)(PPh3)2], and characterized by IR and NMR spectroscopies,
elemental analysis and electrochemical properties. Theoretical calculations
at the density functional theory (DFT) level of theory indicated that the
coordination of PTA to both Re(III) and Re(VII) centers by the P atom is
preferable compared to the coordination by the N atom. This is interpreted in terms of the Re−PTA bond energy and hard−soft
acid−base theory. The oxo-rhenium complexes 1−6 act as selective catalysts for the Baeyer−Villiger oxidation of cyclic and linear
ketones (e.g., 2-methylcyclohexanone, 2-methylcyclopentanone, cyclohexanone, cyclopentanone, cyclobutanone, and 3,3-
dimethyl-2-butanone or pinacolone) to the corresponding lactones or esters, in the presence of aqueous H2O2. The effects of a
variety of factors are studied toward the optimization of the process.
radiopharmaceuticals, of the obtained complexes.11,12 An
1. INTRODUCTION
interesting feature revealed by certain PTA-Re complexes13 is
Recently, the coordination chemistry of the water-soluble
the ambivalent behavior concerning solubility in water and in
cagelike aminophosphine 1,3,5-triaza-7-phosphaadamantane
organic solvents, which can be adjusted by the ligand protofilic
(PTA) has experienced rapid development, mainly justified
properties. This has important implications for the separation
by the search for new water-soluble transition-metal amino-
and recycling of catalysts under biphasic conditions. Moreover,
phosphine complexes particularly suited to serve as catalyst
precursors in aqueous organometallic catalysis,1,2 water-soluble
trioxorhenium complexes have been regarded as efficient
catalysts for the oxidation of a variety of organic substrates14−21
antitumor agents,1,3−5 or photoluminescent materials,1,6,7a,b and
as the main organic linker in coordination polymers.7c
Among the transition-metal complexes bearing PTA or its
and very useful starting reagents in coordination chem-
istry.21−26
As an extension of the above synthetic and catalytic
derivatives, ruthenium (e.g., [RuCl2(PTA)4] or [RuCl2(CO)-
directions, the present work aims at the easy preparation of
(PTA)3]) and rhodium (e.g., [RhI4(mPTA)2]I, where mPTA =
N-methyl-1,3,5-triaza-7-phosphaadamantane cation) com-
new water-soluble Re complexes bearing PTA-type ligands
plexes8 have attracted considerable interest, because of their
(Figure 1) and probing their catalytic potential in mild
oxidative conversions of ketones. Hence, within the search for
catalytic potential for a variety of processes under aqueous or
a suitable starting material for the synthesis of new water-
biphasic conditions. On the other hand, the coordination
chemistry of PTA to rhenium is also a matter of high current
interest concerning the electrochemical behavior,9 photo-
physical activity,10 therapeutic and diagnostic applications as
Received: January 4, 2013
© XXXX American Chemical Society
A
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Figure 1. 1,3,5-Triaza-7-phosphaadamantane (PTA), its N-methylated derivative (mPTA) salt, hexamethylenetetramine (HMT), hydrotris(pyrazol-
1-yl)methane (Tpm), and lithium tris(pyrazol-1-yl)methanesulfonate Li(Tpms).
soluble Re complexes, we have chosen Re2O7, which is a
relatively inexpensive, accessible, and highly reactive material; in
the presence of the water-soluble cagelike phosphanes PTA or
[Me-PTA]I (N-methyl-1,3,5-triaza-7-phosphaadamantane io-
dide), or of HMT (hexamethylenetetramine), at room
temperature, this material led to [ReO3(PTA)2][ReO4] (1),
[ReO3(mPTA)][ReO4]I (2) and [ReO3(HMT)2][ReO4] (3),
respectively (see reactions a−c in Scheme 1).
Aiming to contribute toward the development of the still
little-explored application of Re compounds as catalyst and/or
catalyst precursor for the oxidation of ketones to the respective
esters or lactones, under relatively mild conditions and with an
environmentally friendly oxidant (H2O2), we now report the
BV oxidation, by aqueous H2O2 in 1,2-dichloroethane, of cyclic
and linear ketones catalyzed by 1−6 (see Scheme 3, presented
later in this paper).
Moreover, the preparation, by some of us,27 of the first water-
soluble transition metal complex bearing simultaneously PTA
and Tpms− (tris(pyrazol-1-yl)methanesulfonate), [Rh(Tpms)-
(CO)(PTA), in high yield, by a single-pot reaction of
[{Rh(CO)2(μ-Cl)}2], PTA and Li(Tpms), prompted us, in
the current work and in pursuit to our interest on the
coordination chemistry of C-scorpionates,28 to react Re2O7
with PTA and Tpm, or HMT and Li(Tpms) (Figure 1), at
room temperature. This has led to the new water-soluble and
stable oxo-rhenium complexes [ReO3(η2-Tpm)(PTA)][ReO4]
(4), [ReO3(Hpz)(HMT)][ReO4] (5) and [ReO(Tpms)-
(HMT)] (6) (see reactions d−f in Scheme 1).
The study we now report also aims to achieve water-soluble
Re compounds prepared, from hydro- insoluble ones, by ligand
exchange reactions, as known29 for the nitridorhenium(V)
[ReNCl2 (PTA)3] and trans-dioxorhenium(V) [ReO2Cl-
(PTA)3] complexes obtained from [ReNCl2(PPh3)2] or
[ReOCl2X(PPh3)2 (X = Cl−, OMe−, OEt−), respectively. The
replacement of the PPh3 and Hpz ligands at our [ReCl2{N2C-
(O)Ph}(Hpz)2(PPh3)2]28b was successful and led to the
formation of the new benzoyldiazenido PTA complex
[ReCl2{N2C(O)Ph}(PTA)3] (7) (see Scheme 2).
Transition-metal-catalyzed Baeyer−Villiger (BV) oxidations,
viz, the transformation of cyclic and acyclic ketones into
lactones and esters, has become an important research topic
recently,30−48 because of the wide applications of the products
in fine chemical industry. For economic and environmental
reasons, a growing amount of attention has been paid to the
replacement of the classical use of organic peroxyacids by more
atom-efficient and environmentally friendly oxidants, such as
molecular oxygen49−51 or hydrogen peroxide.31,38−40,44,52,53
Moreover, the recognized application of the versatile complex
[(Me)ReO3] (MTO) in oxidation catalysis, including the BV
oxidation of ketones,40,53 clearly demonstrates the ability of
rhenium centers to form highly active catalysts for the oxidation
reactions of olefins and other unsaturated substrates. It has
been found that rhenium complexes bearing oxo- or N-ligands
(such as benzoyl-diazenido or benzoyl-hydrazido, C-scorpio-
nates, or pyrazoles) act as selective catalysts (or catalyst
precursors), in homogeneous systems, for the oxidation and
carboxylation of inert alkanes, under mild or moderate
conditions54−56 and are also active in BV oxidation reactions.57
2. EXPERIMENTAL SECTION
Materials and Instrumentation. The synthetic work was carried
out under an oxygen-free dinitrogen atmosphere, using standard
Schlenk techniques. K[ReO4] (Aldrich), Re2O7 (Aldrich), hexame-
thylenetetramine (HMT), pyrazole (Aldrich), 2-methylcyclohexanone
(Aldrich), 2-methylcyclopentanone (Aldrich), cyclopentanone (Al-
drich), cyclohexanone (Aldrich), cyclobutanone (Aldrich), 3,3-
dimethyl-2-butanone (pinacolone) (Aldrich), δ-hexalactone (Aldrich),
ϵ-caprolactone (Aldrich), δ-valerolactone (Aldrich), β-butyrolactone
(Aldrich), tert-butylacetate (Aldrich), 1,2-dichloroethane (Aldrich),
dichloromethane (Fluka), toluene (Fluka), methanol (Fluka), hydro-
gen peroxide (30%) (Fluka), cycloheptanone (Aldrich), diethyl ether
(Riedel-de-Haen), and dinitrogen gas (Air Liquid Portugal) were used
̈
as received from the suppliers. The solvents were dried over
appropriate drying agents and degassed by standard methods.
The C-scorpionates hydrotris(pyrazol-1-yl)methane (Tpm)58 and
lithium tris(pyrazol-1-yl)methane sulfonate (LiTpms),59 the amino-
phosphines 1,3,5-triaza-7-phosphaadamantane (PTA) and N-methyl-
1,3,5-triaza-7-phosphaadamantane iodide ([mPTA]I),60 as well as
[ReCl2(η2-N,O-N2C(O)Ph)(PPh3)2]61 and [ReCl2(N2C(O)Ph)-
(Hpz)(PPh3)2],28b were prepared in accordance with published
procedures. C, H, and N elemental analyses were carried out by the
́
Microanalytical Service of the Instituto Superior Tecnico, Lisbon.
Infrared spectra (4000−400 cm−1) were recorded on a BIO-RAD
Model FTS 3000MX instrument using KBr pellets. Far-infrared
spectra (400−200 cm−1) were recorded on a Bruker Vertex instrument
using polyethylene pellets. 1H, 13C, 13C{1H}, 31P{1H}, and COSY
NMR spectra were measured on Bruker 300 and 400 UltraShield
spectrometers at room and low temperatures. H and 13C chemical
1
shifts (δ) are expressed in ppm, relative to Si(Me)4, whereas δ(31P)
chemical shifts are relative to 85% H3PO4. Coupling constants are
given in units of Hz (abbreviations: s, singlet; d, doublet; t, triplet; m,
complex multiplet; vt, virtual triplet; br, broad). 1H DOSY NMR
spectrum was obtained on a 500 MHz Bruker Avance III spectrometer
from a DMSO-d6 solution of the complex at 23 °C using a 5-mm
probe. In the PFG NMR experiments, a bipolar stimulated echo pulse
sequence was used. The duration of the pulse gradients (1.2 ms) and
the diffusion time (60 ms) were adjusted in order to obtain full
attenuation of signals at 95% of maximum gradient strength. The
gradient strength was incremented from 2% to 95% in a linear ramp
with 16 steps. A delay of 10 s between echoes was used. The
numbering of the pyrazolyl ring is as follows: Re−N(1)N(2)C(3)-
HC(4)HC(5)H. The C, H, N elemental analyses were carried out by
́
the Microanalytical Service of the Instituto Superior Tecnico. The
electrochemical experiments were performed on an EG&G PAR 273A
potentiostat/galvanostat connected to a personal computer (PC)
through a GPIB interface. Cyclic voltammetry (CV) studies were
B
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Scheme 1. Syntheses of Compounds 1−6
undertaken in 0.2 M [nBu4N][BF4]/DMSO, at a platinum-disk
working electrode (d = 0.5 mm) and at room temperature.
Controlled-potential electrolyses (CPE) were carried out in electrolyte
solutions with the above-mentioned composition, in a three-electrode
H-type cell. The compartments were separated by a sintered glass frit
and equipped with platinum gauze working and counter electrodes.
For both CV and CPE experiments, a Luggin capillary connected to a
silver wire pseudo-reference electrode was used to control the working
electrode potential. A Pt wire was employed as the counter-electrode
for the CV cell. The CPE experiments were monitored regularly by
cyclic voltammetry, thus assuring no significant potential drift occurred
along the electrolyses. The solutions were saturated with N2 by
C
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Scheme 2. Synthesis of Complex [ReCl2{N2C(O)Ph}(PTA)3] (7)
[C−N, (mPTA)] cm−1, 910 (ReO). 1H NMR (400 MHz, D2O): δ
5.00 and 4.85 (J(HAHB) = 12 Hz, 4H, NCHAHBN+), 4.80 and 4.27
(J(HAHB) = 13 Hz, 2H, NCHAHBN), 4.52 (s, 2H, PCH2N+), 4.17 and
3.95 (J(HAHB) = 15.0 Hz, 4H, PCHAHBN), 3.18 (s, 3H, MeOH), 2.67
(s, 3H, N+CH3). 13C{1H}NMR (100 MHz, D2O): 82.5 (s, NCH2N+),
70.3 (s, NCH2N), 57.3 (s, PCH2N+), 51.3 (s, CH3), 48.7 (s, PCH2N).
31P{1H}NMR (162 MHz, D2O): −69.1 (s).
Scheme 3. Baeyer−Villiger (BV) Oxidation of Cyclic (1) and
Acyclic (2) Ketones
[ReO3(HMT)2][ReO4] (3). A 2:1 molar amount of HMT (57.8 mg,
0.412 mmol) in MeOH was added to a stirred solution of Re2O7
(100.0 mg, 0.206 mmol) in MeOH (10 mL), yielding a brown solid,
which was isolated by filtration and washed with MeOH (10 mL).
Yield = 46%, based on Re2O7. 3 is soluble in H2O (S25°C ≈ 25 mg
mL−1). C13H28N8O8Re2 (796.8): Calcd. C 19.6, N 14.1, H 3.5; Found
C 19.2, N 14.6, H 3.5. IR (KBr): 1265 [C−N, (HMT)] cm−1, 910
1
(ReO). H NMR (400 MHz, D2O): δ 4.61 (s, br, 6H, NCH2N
(HMT)), 4.40 (s, br, 6H, NcoordCH2N (HMT)), 3.14 (s, 3H, MeOH).
13C{1H}NMR (100 MHz, D2O): 71.2 (s, NCH2N, HMT). H DOSY
1
bubbling this gas before each run, and the redox potentials of the
complexes were measured by CV in the presence of ferrocene as the
internal standard, and their values are quoted relative to the SCE,
NMR (500 MHz, DMSO-d6): log D = −9.52 m2/s.
[ReO3(η2-Tpm)(PTA)][ReO4] (4). An equimolar amount of Tpm
(60.2 mg, 0.206 mmol), and then a 2:1 molar amount of PTA (64.7
mg, 0.412 mmol) in MeOH, were added to a stirred solution of Re2O7
(100.0 mg, 0.206 mmol) in MeOH (10 mL). After ∼10 min, a yellow
solid precipitated from the blue solution. The solid was isolated by
filtration, washed with 5 mL of methanol, and dried in vacuo. 4 is
soluble in H2O (S25°C ≈ 1.88 mg mL−1). Yield = 38%, based on Re2O7.
4, C16H22N9O7PRe2 (855.8): Calcd. C 22.5, N 14.7, H 2.6; Found C
22.2, N 14.1, H 2.6. IR (KBr): 2954 [C−H, (Tpm)] cm−1, 2917 [asym
C−H, (CH2) (PTA)], 2876 [sym C−H, (CH2) (PTA)], 1641 [CC,
(Tpm)], 1516 [CN, (Tpm)], 1414 [asym P−CH2, (PTA)], 1309
[sym P−CH2, (PTA)], 1292 [N−CH2, (PTA)], 1243 [N−C−N,
(PTA)], 1026 [N−C−N, (PTA)], 911 (ReO). 1H NMR (400
MHz, D2O): δ 9.80 (s, br, 1H, HC-(pz)3), 8.72 (s, br, 1H, pz-H3,5)
8.33 (s, br, 2H, pz-H3,5), 8.12 (s, br, 1H, pz-H3,5), 7.76 (s, br, 2H, pz-
H3,5), 6.88 (s, br, 1H, pz-H4), 6.52 (s, br, 2H, pz-H4), 4.59 and 4.56
(J(HAHB) = 13.5 Hz, 6H, NCHAHBN, PTA), 4.40 (s, 6H, PCH2N,
PTA). 13C{1H}NMR (100 MHz, D2O): 140.9 (s, pz-C3,5), 130.2 (s,
pz-C3,5), 106.9 (s, pz-C4), 82.1 (s, HC-(pz)3), 71.2 (s, NCH2N),
46.90 (d, 1J(CP) = 54.0 Hz, PCH2N). 31P{1H}NMR (162 MHz,
D2O): −68.2 (s).
using the [Fe(η5-C5H5)2]0/+ redox couple (E = 0.44 V vs SCE).62
ox
1/2
Gas chromatography (GC) measurements were carried out using a
FISONS Instruments GC 8000 series gas chromatograph with a FID
detector and a capillary column (DB-WAX, column length = 30 m;
internal diameter = 0.32 mm). The temperature of injection was 240
°C. The initial temperature was maintained at 80 °C for 1 min, then
increased at a rate of 10 °C/min up to 140 °C (in the case of
pinacolone oxidation) or 180 °C, and held at this temperature for 1
min. Helium was used as the carrier gas.
Syntheses and Characterization of the Complexes.
[ReO3(PTA)2][ReO4] (1). To a stirred solution of Re2O7 (100.0 mg,
0.206 mmol) in MeOH (10 mL) a 3:1 molar amount of PTA (97.3
mg, 0.619 mmol) in MeOH was added, yielding a yellow solid, which
was isolated by filtration and washed with MeOH (10 mL). 1 is
soluble in H2O (S25°C ≈ 16.7 mg mL−1). Yield = 58%, based on Re2O7.
1, C13H28N6O8P2Re2 (830.8): Calcd. C 18.8, N 10.1, H 3.4; Found C
18.2, N 10.1, H 3.6. IR (KBr): 2919 cm−1 [asym C−H, (CH2)
(PTA)], 2874 [sym C−H, (CH2) (PTA)], 1416 [asym P−CH2,
(PTA)], 1310 [sym P−CH2, (PTA)], 1288 [N−CH2, (PTA)], 1242
[N−C−N, (PTA)], 1014 [N−C−N, (PTA)], 911 [ReO]. 1H NMR
(400 MHz, D2O): δ 4.69 and 4.53 (J(HAHB) = 12 Hz, 12H,
NCHAHBN (PTA)) 4.20 (s, 12H, PCH2N (PTA)), 3.15 (s, 3H,
MeOH). 13C{1H}NMR (100 MHz, D2O): 71.2 (s, NCH2N), 46.90 (d,
1J(CP) = 20.5 Hz, PCH2N). 31P{1H}NMR (162 MHz, D2O): −70.1
[ReO3(Hpz)(HMT)][ReO4] (5). A methanol solution (10 mL) of
HMT (58.3 mg, 0.416 mmol) was added dropwise to a methanol
solution (10 mL) of Re2O7 (100.0 mg, 0.206 mmol). An equimolar
amount (relative to Re2O7) of LiTpms salt (60.4 mg, 0.206 mmol) was
added. The reaction mixture was stirred overnight, the volume of the
solution was reduced under vacuum, and then Et2O was added. Slow
evaporation of the solvent resulted in the formation of a brown
compound (5). Yield = 40%, based on Re2O7. C9H16N6O7Re2 (692.7):
Calcd. C 15.6, N 12.1, H 2.3; Found C 15.3, N 11.7, H 2.8. IR (KBr):
3072 cm−1 [N−H, (Hpz)], 1669 and 1587 [CC and NC, (Hpz)],
(s).
[ReO3(mPTA)][ReO4]I (2). A 2:1 molar amount of [mPTA]I (123.2
mg, 0.412 mmol) in MeOH was added to a stirred solution of Re2O7
(100.0 mg, 0.206 mmol) in MeOH (30 mL), yielding a brown solid,
which was isolated by filtration and washed with MeOH (10 mL). 2 is
soluble in H2O (S25°C ≈ 4.2 mg mL−1). Yield = 69%, based on Re2O7.
2, C8H19IN3O8PRe2 (815.5): Calcd. C 11.8; H 2.3; N 5.2; Found C
11.7, H 2.3, N 6.0. IR (KBr): 1452 [asym P−CH2, (mPTA)], 1250
1
1265 [C−N, (HMT)], 910 (ReO). H NMR (400 MHz, D2O): δ
8.9 [s, 1H, H(1) Hpz], 7.9 [s, 1H, H(3) or H(5) Hpz], 7.7 [s, 1H,
D
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H(3) or H(5) Hpz], 6.4 [s, 1H, H(4) Hpz], 4.6 [s, br, 6H, NCH2N
(HMT)], 4.4 (s, br, 6H, NcoordCH2N (HMT)). 13C{1H}NMR (100
MHz, D2O): 141.8 [s, C(3) Hpz], 130.6 [s, C(5) Hpz], 107.6 [s, C(4)
Hpz]. 71.4 (s, NCH2N, HMT).
was applied for the molecular cavity. The entropic term in MeOH
solution (Ss) was computed according to the procedure described by
Wertz67 and Cooper and Ziegler68 (see the Supporting Information
for details). The enthalpies and Gibbs free energies in solution (Hs and
Gs, respectively) were estimated using expressions 1 and 2:
[ReO(Tpms)(HMT)] (6). Li(Tpms) (61.8 mg, 0.206 mmol) was
added to a methanolic solution of Re2O7 (100.0 mg, 0.206 mmol, 10
mL of MeOH). Then, a 2:1 molar amount of HMT (57.8 mg, 0.412
mmol) in MeOH was added. After stirring for ∼10 min, a yellow solid
precipitated from the blue solution. The solid was isolated by filtration,
washed with 5 mL of methanol and dried in vacuo. 6 is soluble in H2O
(S25°C ≈ 15 mg mL−1). Yield = 35%, based on Re2O7. 6,
C16H21N10O4ReS (635.7): Calcd. C 30.2; H 3.3; N 22.0; Found C
29.7, H 3.2, N 21.3. IR (KBr): 1515 cm−1 (CC, Tpms), 1272 [C−
Hs = Es + Hg − Eg
(1)
Gs = Hs − TSs
(2)
where Es and Eg are the total energies in solution and in gas phase,
respectively, and Hg is the gas-phase enthalpy calculated at the
corresponding level.
+
The chemical potential (μ) and global softness (S) of the ReO3
1
and mPTA+ species were calculated using eqs 3 and 4:69
N, (HMT), 918 (ReO). H NMR (400 MHz, D2O): δ 7.89 (s, br,
3H, pz-H3,5), 7.67 (s, 3H, pz-H3,5), 6.41 (s, br, 3H, pz-H4), 4.61 (s,
br, 6H, NCH2N (HMT)), 4.44 (s, br, 6H, NcoordCH2N (HMT)).
13C{1H}-NMR (100 MHz, D2O): 141.0 (s, pz-C3,5), 130.0 (s, pz-
C3,5), 107.0 (s, pz-C4), 81.6 (s, O3SC-(pz)3), 73.5 (s, NCH2N).
[ReCl2{N2C(O)Ph}(PTA)3] (7). A 10:1 molar amount of PTA (208.5
mg, 1.33 mmol) was added to a stirred solution of [ReCl2{N2C(O)-
Ph}(Hpz)(PPh3)2] (130.4 mg, 0.13 mmol) in toluene (25 mL). Then,
the reaction mixture was refluxed during 3 h, yielding a reddish brown
solid. The suspension was cooled to room temperature, the solid was
isolated by filtration, and washed with MeOH (15 mL). 7 is soluble in
H2O (S25°C ≈ 8 mg mL−1). Yield = 61%, based on [ReCl2(N2COPh)-
(Hpz)(PPh3)2]. 7, C25H41Cl2N11P3ORe (861.7): Calcd C 34.8, N
17.9, H 4.9; Found C 35.1, N 17.8, H 5.1. IR (KBr): 2919 [m, C−H,
(CH2) (PTA)], 1627 [m, CO, η1-N−N2C(O)Ph], 1579 [s, NN,
η1−N−N2C(O)Ph], 1449 [m, asym P−CH2, (PTA)], 1383 [m, sym
P−CH2, (PTA)], 1284 [m, N−CH2, (PTA)], 1016 and 1242 [m, N−
C−N, (PTA)]. Far IR (polyethylene): 299 and 267 [s, ν(Re−Cl)]. 1H
NMR (400 MHz, CDCl3): δ 7.98 [d, 2H, JHH= 5.4 Hz, o-H (Ph)],
7.58 [vt, 1H, p-H (Ph)], 7.40 [vt, 2H, m-H (Ph)], 4.67−4.37 (m, 24H,
NCH2N + PCH2N (PTA)), 4.17 [s, 12H, PCH2N (PTA)]. 31P{1H}-
NMR (162 MHz, CDCl3): −74.7 (t) and −84.8 (d), 2J(PP) = 9.8 Hz.
Catalytic Studies. Under typical conditions, Re catalysts were
used as stock solutions (for this, 10.0 mg of the compound were
dissolved in 2.00 mL of 1,2-dichloroethane (1,2 DCE)). The required
amount of this stock solution for the desired oxidant/catalyst molar
ratio (1000:1) was transferred to a second flask containing 3.00 mL of
1,2-dichloroethane. Then, 1.7 mmol of H2O2 as a 30% aqueous
solution (102 μL) and 1.7 mmol of substrate were added, and the
reaction solution was stirred for 6 h at the desired temperature
(typically 70 °C) and normal pressure (dinitrogen atmosphere). Then,
120 μL of cycloheptanone (internal standard) and 10.00 mL of diethyl
ether (to extract the substrate and the organic products from the
reaction mixture) were added. The products were analyzed by GC
using the internal standard method.
I + A
μ ≈ −
(3)
(4)
2
1
S ≈
I − A
where I and A are the vertical ionization potential and electron affinity,
respectively. Fukui functions of atom x in a molecule with N electrons,
f+x and f−x (the former representing nucleophilic attack and the latter
representing electrophilic attack), were defined in a finite-difference
approximation using eqs 5 and 6:
fx+ = [qx(N + 1) − qx(N)]
(5)
fx− = [qx(N) − qx(N − 1)]
(6)
where qx is the NBO electronic population of atom x in a molecule.
The local atomic softnesses sx+ and sx− were calculated by the formulas
listed as eq 7:
+
−
sx = fx+S and sx = fx−S
(7)
+
The total energies of the interaction between the ReO3 and mPTA+
fragments (ΔEint) were calculated with the help of the HSAB
formalism,69,70 using the expression depicted in eq 8:
ΔEint = ΔEν + ΔEμ
2
⎡
⎢
⎣
⎤
⎥
(μ
− μ
)
+
+
⎛
⎜
⎞
⎟
⎠
1
2
ReO3
mPTA
1
λ
+
−
⎢
= −
sResx −
sR+e + sx−
4 s+ + sx−
⎥
⎦
⎝
Re
(8)
where ΔEν and ΔEμ are the components of ΔEint at the constant
external potential and chemical potential, respectively, x is an atom of
mPTA+ interacting with Re (P or N), λ is a parameter estimated by eq
9
Computational Details. The full geometry optimization of all
structures has been carried out at the DFT/HF hybrid level of theory
using Becke’s three-parameter hybrid exchange functional in
combination with the gradient-corrected correlation functional of
Lee, Yang, and Parr (B3LYP)63 with the help of the Gaussian 0364
program package. Restricted approximations for the structures with
closed electron shells and unrestricted methods for the structures with
open electron shells have been employed. No symmetry operations
have been applied. The geometry optimization was carried out using a
quasi-relativistic Stuttgart pseudopotential that described 60 core
electrons and the appropriate contracted basis set (8s7p6d)/
[6s5p3d]65 for the Re atom and the 6-31G(d) basis set for other
atoms. The Hessian matrix was calculated analytically for the
optimized structures in order to prove the location of correct minima
(no imaginary frequencies) and to estimate the thermodynamic
parameters, the latter being calculated at 25 °C.
4
4
λ = |
qeq
−
q◦
|
x,ReO3
∑
+
∑
+
x,ReO3
(9)
x=1
x=1
where qexq and q°x are the NBO electronic populations of atom x of the
ReO3 fragment in the 2P or 2N molecule at equilibrium (the first
term) and in the isolated optimized ReO3 molecule (the second
+
+
term).
3. RESULTS AND DISCUSSION
Synthesis and Characterization of the Complexes.
Reactions of Re2O7, in MeOH at room temperature, with the
water-soluble aminophosphine PTA, its derivative [Me-PTA]I,
its N-analogous HMT, or PTA in the presence of the neutral
Tpm, led to new water-soluble and stable trioxo-rhenium(VII)
complexes [ReO3(PTA)2][ReO4] (1), [ReO3(mPTA)][ReO4]I
(2), [ReO3(HMT)2][ReO4] (3), and [ReO3(η2-Tpm)(PTA)]-
[ReO4] (4), respectively, whereas the oxo-complexes
[ReO3(Hpz)(HMT)][ReO4] (5) and [ReO(Tpms)(HMT)]
(6) were obtained in the presence of HMT and the water-
Total energies corrected for solvent effects (Es) were estimated at
the single-point calculations on the basis of gas-phase geometries using
the polarizable continuum model in the CPCM version66 with MeOH
as a solvent. The solvent effects for the calculations of the bond
energies in 2P and 2N were estimated at the CPCM-B3LYP/6-
311+G(d,p)//gas-B3LYP/6-31G(d) level of theory. The UAKS model
E
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soluble Li(Tpms) salt (recall Scheme 1). The formation of the
latter complexes (5 and 6) was found to be very sensitive to the
sequence of reagent additions (see the Experimental Section).
Moreover, the formation of 5 involves the rupture of a C(sp3)−
N(pyrazolyl) bond in Tpms−. Other cases of metal-induced
cleavage of such a type of bond have been reported, namely, the
C−N bond rupture in HC(pz)3 or HC(3,5-Me2pz)3 by the
benzoylhydrazido rhenium(V) chelate [ReCl2{η2-N,O-N2C-
(O)Ph}(PPh3)2],28b in HC(3,5-Me2pz)3 by triethyl vanadate
to form [VO2(3,5-Me2Hpz)3][BF4]28j or in bis(pyrazolyl)-
propane (CH3)2C(pz)2 by Pt(II) complexes,71 e.g.,
[PtCl2(RCN)2] (R = Me or Ph) or K2[PtCl4]. In all of the
reported cases, the way the cleavage reaction occurs has not
been established, but can possibly involve metal-promoted
hydrolysis of Tpms−.28a
The benzoyldiazenido PTA complex [ReCl2{N2C(O)Ph}-
(PTA)3] (7) was prepared by ligand exchange at [ReCl2{N2C-
(O)Ph}(Hpz)(PPh3)2], in a 1:10 metal to PTA molar ratio, in
hot toluene (Scheme 2). For a lower amount of PTA (e.g., 1, 2,
or 5 mol equiv), replacement of the PPh3 or Hpz ligands by
PTA did not occur. The use of more polar solvents, namely,
CH2Cl2 and MeOH, led to the formation of the phosphine
oxide (PTAO).
evidenced by the appearance of three singlets at δ 7.89, 7.66,
and 6.41 (each integrated for one proton, corresponding to the
3, 4, and 5 position, respectively, in the pyrazole ring) and at
8.96, the signal attributed to the NH proton. A similar pattern
was observed for the Re(III) pyrazole complex [ReCl2{N2C-
(O)Ph}(Hpz)(PPh3)2].28b The equivalence of the pyrazolyl
groups in 6 is indicated by both 1H and 13C{1H} NMR spectra
which display a single resonance for each type of proton or
carbon, respectively, i.e., for the 3, 4, or 5 position in the ring.
However, in 4, those groups, as expected, are nonequivalent.
The 1H NMR spectrum of 7 exhibits the expected
resonances of the ortho-, meta-, and para-protons of the phenyl
1
ring of the benzoyldiazenido ligand. In addition, H NMR
spectrum shows a set of resonances in the range δ 4.67−4.17
for N−CH2-N and P−CH2−N methylene of coordinated PTA
ligands with a ratio of 1:2, and the 31P{1H} NMR spectrum
consists of two signals: triplet at δ −74.7 and doublet at −84.8
(2JPP = 9.8 Hz). This is indicative of a meridional octahedral
complex (recall Scheme 2). The resonances are comparable to
those observed for [RuI2(H2O)(mPTA)3]I3·2H2O.8c
The 31P{1H} spectra of Re complexes bearing PTA or
[mPTA]+ exhibit coordination through P atoms to the metal
center, resulting in downfield shifts of 31P{1H} resonances
upon coordination. The 1H NMR spectra confirm the P-
coordination modes. P−CH2−N for 1 and 4 and P-CH2−N+
methylene protons for 2 occur as singlets, although for free
PTA or [mPTA]+ they give doublets (2J(PH) = 10.5 and 6.5
Hz, respectively).8c,d On the other hand, in the case of
coordination through only the N atoms of the adamantane cage
to the metal center, similar spectra to those of the free ligands
would be expected, e.g., as reported previously73 for the
31P{1H} and 1H NMR spectra of [ZnCl2(PTA)2] (its structure
was confirmed by X-ray analysis) with PTA coordinated
through only a N atom.
The obtained complexes were characterized by IR and NMR
spectroscopies, elemental analysis, and electrochemical study.
The IR spectra of 1, 2, 4, and 7 show typical bands of
coordinated PTA (or coordinated [mPTA]+ for 2), in the
2919−2880 and 2876−2874 cm−1 ranges assigned to νas[C−
H(CH2)] and νs[C−H(CH2)]; in the 1452−1416, 1383−1309
and 1292−1284 cm−1 ranges, attributed to νs(P−CH2), νas(P−
CH2) and ν(N−CH2), respectively; and in the 1026−1014
cm−1 and 1250−1242 cm−1 ranges attributed to ν(NCN). For
7, the presence of the chloride ligands is confirmed by the
strong intensity ν(Re−Cl) bands at 299 and 267 cm−1.
Moreover, 7 exhibits strong bands at 1627 and 1579 cm−1
assigned to ν(CO) and ν(NN), respectively, of the
benzoyldiazenido ligand. Such IR bands for η1-N-NNC(O)Ph
ligand appear at comparable values for the parent benzoyldia-
zenido-Re(III) pyrazole complex [ReCl2{N2C(O)Ph}(Hpz)-
(PPh3)2]28b [ν(CO) 1593 and ν(NN) 1565 cm−1]. The
benzoyldiazenido ligand is expected to behave as a 3-electron
donor (allowing the 18-electron configuration to be achieved
by the complex) and thus to present the linear coordination
geometry RE⇋NNC(O)Ph.28b,72
In addition, although PTA and HMT are able to be
polycoordinated, PTA compounds (1, 2, 4, and 7) are expected
to be monomeric, since we observed (see below) that the
coordination of PTA to both Re(VII) and Re(III) centers by
the P atom is clearly more preferable than the coordination by
the N atom. Pulse-field-gradient spin−echo NMR, which relies
on differences in diffusion coefficients of molecules with
different sizes in solution (see the Experimental Section and
Figure S8 in the Supporting Information),74 has been used in
the analysis of 3, as a HMT representative case, and excluded
the formation of polycoordinated species. The average value of
the diffusion coefficient (D = 3.0 × 10−10 m2 s−1) of 3 is in
accord with a monomeric form of the complex in solution. In
fact, the coordination sphere seems to be saturated around of
metal centers in 1−7; therefore, we postulated the formation of
monomeric form of oxorhenium complexes, especially in
solution.
Complexes 1−6 exhibit, in their IR spectra, a strong and
broad ν(ReO) band at ∼910 cm−1 (less intense for 6 due to
the lower number of ReO bonds), the ν(CH) of the methine
moiety of the hydrotris(pyrazol-1-yl)methane ligand (for 4),
the ν(SO) of the methanesulfonate group (for 6), the ν(C
C) and ν(CN) of the pyrazolyl rings at normal ranges (for 4
and 6) or the typical bands of a tertiary amine, concerning the
HMT ligand (for 3, 5, and 6).
Electrochemical Behavior of 1−7. Complexes 1−5
exhibit, by cyclic voltammetry (CV), at a platinum electrode
at 25 °C in a 0.2 M [nBu4N][BF4]/DMSO solution, a reduction
wave at potential values in the range from −0.31 V to −0.84 V
vs. SCE (see Figures S1−S5 in the Supporting Information)
assigned28a,75 to the Re(VII) → Re(VI) reduction and
comparable to that reported28a for the related [ReO3(Tpms)]
In the 31P{1H} NMR spectrum of 1, the singlet at δ = −70.1
is consistent with the Re metal center bearing two PTA ligands
in the axial positions. 2 displays, in its 1H NMR spectrum, four
types of methylene groups of the [mPTA]+ cationic ligand:
NCH2N, NCH2N+, and PCH2N are all of AB type. For
PCH2N+, a singlet has been observed. The 1H NMR spectra of
[ReO3(HMT)2][ReO4] (4), [ReO3(Hpz)(HMT)][ReO4] (5)
and [ReO(Tpms)(HMT)] (6) show two types of methylene
groups of coordinated HMT, N−CH2−N and Ncoord−CH2−N,
in contrast to the free ligand, which exhibits only one resonance
at δ 4.54. The coordination of one pyrazole ring at 5 is
red
(IE1/2 = −0.83 V vs. SCE). (See Table 1.) For 2 and 3, at a
lower potential, a second reduction is observed at IIEp
=
red
−0.81 and −1.12 V vs SCE, respectively. Neither a metal-
centered oxidation nor a ligand-centered oxidation was
detected for 1−5, the former being in accordance with the
F
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a
Table 1. Cyclic Voltammetric Data for
[ReO3(PTA)2][ReO4] (1), [ReO3(mPTA)][ReO4]I (2),
[ReO3(HMT)2][ReO4] (3), [ReO3(η2-Tpm)(PTA)][ReO4]
(4), [ReO3(Hpz)(HMT)][ReO4] (5), [ReO(Tpms)(HMT)]
(6), and [ReCl2{N2C(O)Ph}(PTA)3] (7)
complex IEp (E1/2
)
IIEp (E1/2
)
IEp (E1/2
ox
)
IIEp (E1/2
)
red
red
red
red
ox
ox
ox
1
2
3
4
5
6
7
(−0.65)
b
(−0.31)
(−0.84)
(−0.62)
(−0.33)
−0.83
(−0.81)
−1.12
0.56
(0.84)
(0.86)
(0.78)
1.44
1.67
−1.56
a
Values given in units of V 0.02 relative to SCE (see Experimental
Section); scan rate of 200 mV s−1. Values for reversible waves are given
in parentheses. The two observed oxidation waves are due to iodide
b
oxidation.
metal d0 Re(VII) electron count. Moreover, the [ReO4]−
counterion is also redox inactive in the studied potential
I
ox
range. For 2, two oxidation waves are observed at Ep = 0.56
and IE1/2ox = 0.84 V vs SCE, which are assigned to the oxidation
of the iodide counterion.
The Re(III) complexes 6 and 7 exhibit a single-electron
I
red
irreversible reduction wave at Ep = −0.83 and −1.56 V vs
SCE, respectively (see Figures S6 and S7 in the Supporting
Information), assigned to the Re(III) → Re(II) reduction.28a
Moreover, a first single-electron partially reversible oxidation
wave at IE1/2ox = 0.86 V (6) and 0.78 V (7) vs SCE is assigned
to the Re(III) → Re(IV) oxidation,28a which is followed, at a
II ox
higher potential, by a second irreversible one at Ep = 1.44
and 1.67 V, respectively, assigned to the Re(IV) → Re(V)
oxidation.28a
Complex 7 and its parent pyrazole triphenylphosphine
complex [ReCl2{N2C(O)Ph}(Hpz)(PPh3)2]28b display com-
parable reduction potentials, what is indicative of an overall
electron-donor character of the three PTA ligands in 7
comparable to that of the pyrazole and two PPh3 ligands in
the latter complex, in accordance with the values of the
electrochemical Lever parameter EL (the stronger the character
of EL, the lower the EL value) for these ligands (0.34 V,76 0.20
V,77 and 0.39 V77 vs NHE, respectively). In fact, ΣEL(3 PTA) =
1.02 ≅ ΣEL(Hpz +2 PPh3) = 0.98 V vs NHE. However, one
should be cautious since an accurate comparison cannot be
established, because the reduction potential is not the
thermodynamic one in view of the irreversibility of the
reduction wave.
Figure 2. Calculated structures for complexes of types 2 and 7 with
relative ΔGs values indicated (given in kcal/mol). P◯ and N◯
denote PTA coordinated via the P and N atom, respectively; L = N
NC(O)Me. The most thermodynamically stable ones are high-
lighted.
Theoretical Considerations. The PTA ligand has two
types of donor atoms (P and N atoms) able to coordinate to a
metal. Complexes with such coordination modes of PTA, i.e.,
via P,28i,29,76,78 via N,79 or both80 are known. With the aim to
investigate the thermodynamic preferences of the PTA
coordination to Re(VII) and Re(III) in the studied complexes,
quantum-chemical calculations at the B3LYP level of theory
have been carried out for complex 2 (see 2P and 2N, Figure 2)
and the 7 model species (7′Pa-f and 7′Na-f; see Figure 2). The
results indicate that all structures with the PTA coordinated via
the P atom are more stable than the corresponding N-PTA
coordinated structures by 10.1−51.0 kcal/mol (Figure 2). This
is taken into account by a higher stability of the Re−PPTA bond,
compared to the Re−NPTA one. Indeed, the calculated Re−PPTA
bond energy in 2P for the MeOH solution (19.5 kcal/mol) is
significantly higher than the Re−NPTA energy in 2N (8.0 kcal/
mol).
These results may be interpreted in terms of the hard−soft
acid−base (HSAB) theory. The calculations indicate that the
+
Re atom in ReO3 is a rather soft center, which is taken into
account by the significant value of its f+ Fukui function, which is
comparable with the f− value of the P atom in mPTA+, and by
the high global softness of ReO3+ (see Table 2). (An additional
discussion is given in the Supporting Information.) Hence, the
interaction of the relatively soft Re center with the soft P atom
of PTA should be more preferable than that with the much-
harder N atom. The quantum-chemical calculations using the
HSAB formalism (see Computational Details) indicate that
both terms of the Re−PTA interaction energy (ΔEν and ΔEμ)
G
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exhibiting much higher yields (up to 34% for 1, Table 3,
entry 31) than the highest previously observed57 value (12%, by
using the chloro-Re(III) scorpionate complex [ReCl3(Tpm)]).
For the cyclic ketones, the achieved yields are comparable with
those exhibited57 by the Re(V) complex [ReCl2{η2-N,O-
C(O)Ph}(PPh3)2] (e.g., 42%, 27%, 14% or 45% for 2-
methylcyclohexanone, 2-methylcyclopentanone, cyclohexanone
or cyclobutanone, respectively), the most active of a series of
Re(III−V) complexes bearing N- or oxo-ligands. The reported
yields of the [(CH3)ReO3]/H2O2 catalyzed BV oxidation of
cyclic ketones in ionic liquids (up to 98% and 70% for
cyclobutanone and cyclopentanone, respectively)20 are higher
than those obtained in our Re catalytic systems, but for the 6-
membered ring ketones, such as cyclohexanone and 2-
methylcyclohexanone, we have achieved comparable or even
better results. The obtained yields in the present work are also
commonly comparable to those observed for phosphinic Pt(II)
complexes.38,39
No direct relationship of the activity with the reduction
potential (see above) is observed, in contrast to what was
shown38a for series of closely related platinum(II)-diphosphine
complexes of the types [Pt(μ-OH)(LL)]2[BF4]2 (LL =
organodiphosphine or fluoro-substituted derivative) and [Pt-
(OH2)2(LL)](OTf)2, where the activity followed the Lewis
acidity of the metal center. However, such a lack of correlation
for the Re complexes of the present study is not surprising
since, in general, they are not so closely related as those of the
above series of Pt(II) compounds, and also in view of the
irreversible character of the reduction waves, their reduction
potentials thus being not the thermodynamic ones and
depending also on kinetic factors. In addition, steric factors
can also play a role.
Because of the proposed mechanistic pathway for MTO-
catalyzed BV oxidation,20,41,42,82 the reaction can proceed via
the formation of active peroxo-Re species derived from the
starting complexes in the presence of H2O2, which would attack
the electrophilic carbon of the carbonyl group of the ketone
ligand. Complete regioselectivity was observed in our case for
all BV oxidations of unsymmetrical ketones, 2-methylcyclo-
pentanone, 2-methylcyclohexanone, and pinacolone, to afford
δ-hexalactone, 6-methylhexanolide, or tert-butylacetate, respec-
tively, as a result of the formal insertion of the O atom between
the carbonyl and the more-substituted Cα atom. The alkyl
substituent at Cα of the ketone promotes the migration of the
substituted side of the ketone as a carbanion to the peroxo-
atom.48 Other cases of complete regioselectivity have also been
reported in some other metal catalytic systems.38b,39,83
Effect of the Oxidant-to-Catalyst Molar Ratio. The
influence of the peroxide-to-catalyst molar ratio was studied
for the BV oxidation of 2-methylcyclohexanone and 3,3-
dimethyl-2-butanone (pinacolone) in the presence of 1 (see
Figure 3 and Table S1 in the Supporting Information). The
increase of the n(H2O2)/n(catalyst) molar ratio, by decreasing
the catalyst amount, results in higher TONs, e.g., the TON for
the oxidation of the former ketone in the presence of complex 1
is enhanced from 40 to 9.14 × 103 (see Table S1 in the
Supporting Information, entries 1 and 5, respectively) upon
changing that ratio from 100 to 40 × 103. However, the yield
values decline after reaching a maximum, because of
deceleration of the reaction rate upon reducing the catalyst
concentration.
Table 2. Calculated Vertical Ionization Potentials and
Electron Affinities, Chemical Potentials (μ), Global (S) and
Local (s+ and s−) Softnesses, Fukui Functions (f+ and f−), λ
Parameter, ΔEν, ΔEμ, and Total (ΔEint) Interaction Energies
+
ReO3
mPTA+
vertical ionization potential, I
vertical electron affinity, A
chemical potential, μ
0.697
0.377
−0.537
3.130
0.451
0.057
−0.254
2.539
N in mPTA+
global softness, S
P in mPTA+
+
Re in ReO3
f+
0.350
1.095
s+
f−
s−
0.358
0.908
0.126
0.320
Re−P in 2P
0.616 au
Re−N in 2N
λ
0.403 au
ΔEν
ΔEμ
ΔEint
−12.48 kcal/mol
−48.27 kcal/mol
−60.75 kcal/mol
−6.22 kcal/mol
−44.66 kcal/mol
−50.88 kcal/mol
and, hence, the total interaction energy (ΔEint) are significantly
higher for the Re−PPTA bond than for the Re−NPTA one (see
Table 2). The higher negative ΔEν value for the Re−P bond is
taken into account by the much-higher local softness s− of the P
atom in PTA, compared to the N atom (0.908 hartree−1 vs
0.320 hartree−1). In the case of the ΔEμ term, its more-negative
value for the Re−P bond is determined by the PTA → Re
charge transfer [the λ parameter, see eq 9] which is also higher
in 2P than in 2N.
Among the possible geometrical isomers of 7′P and 7′N, the
most thermodynamically stable ones are 7′Pa and 7′Na
although isomer 7′Pb is only 3.0 kcal/mol less stable than 7′Pa.
Application of the New Re Complexes as Catalysts for
Baeyer−Villiger Oxidations of Cyclic and Acyclic
Ketones. We found that the oxo-rhenium complexes 1−6
(recall Scheme 1) can exhibit a remarkable catalytic activity
(e.g., turnover numbers, TONs, up to ∼9 × 103 moles of
product per mole of Re catalyst, for 1), under relatively mild
conditions for the partial oxidation of different simple cyclic (2-
methylcyclohexanone, 2-methylcyclopentanone, cyclohexa-
none, cyclopentanone, cyclobutanone) and acyclic (pinaco-
lone) ketones to the respective lactones and esters, in a single-
pot process (see Scheme 3 and Table 3, as we as Figure S1 in
the Supporting Information and Figure 3). The catalytic
systems are based on the above-described oxo-Re compounds
1−6, using hydrogen peroxide (30% aqueous solution) as the
oxidizing agent and 1,2-dichloroethane (1,2 DCE) as the
solvent, under a typical temperature of 70 °C and a reaction
time of 6 h. 1,2 DCE was chosen as the typical solvent for our
systems, because of its high resistance to oxidizing agents and
also in view of the good solubility of both catalysts and
substrates. It has also been used in other cases as the most
appropriate solvent for BV oxidations.81
In general, the tested Re(VII or III) complexes exhibit higher
activities than the simple rhenium oxides K[ReO4] (Table 3,
57
entries 9, 15, 26, 30 and 34) or Re2O7 (Table 3, entries 10,
16, 21 and 35). The oxo-Re(VII) PTA or m-PTA complexes,
i.e., [ReO3(PTA)2][ReO4] (1) and [ReO3(mPTA)][ReO4]I
(2), provide the most active catalysts under the conditions of
this study for the oxidation of all tested ketones. Moreover, in
the presence of 1 or 2, the BV oxidation of the acyclic
pinacolone to tert-butylacetate proceeds very effectively,
Effect of the Reaction Time, Temperature, and Type of
Solvent. The effect of the reaction time was studied for
H
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a
Table 3. Baeyer−Villiger (BV) Oxidation of Several Ketones Catalyzed by Re Complexes 1−6 (Selected Data)
a
Reaction conditions (unless stated otherwise): rhenium catalyst (1.7 μmol, used as a stock solution in 1,2-dichloroethane), 1.7 mmol of substrate,
H2O2 (1.7 mmol, i.e. 1000:1 molar ratio of oxidant to Re catalyst), 1,2-dichloroethane (3.0 mL), 6 h, 70 °C, under dinitrogen. Yield and TON
b
determined by GC analysis. Molar yield (%) based on the ketone substrate, i.e., number of moles of lactone (or ester) per 100 moles of ketone.
c
d
TON = turnover number (number of moles of product per mole of Re catalyst). Moles of converted (reacted) substrate per mole of substrate.
e
f
g
h
i
Moles of lactone (or ester) per mole of converted substrate. Oxidant/substrate =2:1. Oxidant/substrate =4:1. For comparative purposes.57
4
j
k
days. In the presence of acid, n(HNO3) /n(substrate) = 10. Reaction time = 0.25 h.
complex 2 using 2-methylcyclohexanone and cyclobutanone
(see Figure 4 and Table S2 in the Supporting Information). For
the former substrate, the lactone yield and TON increase
during the first 6 h, achieving maximum values (31% and 307,
respectively) before dropping slightly, which was conceivably
due to product overoxidation or hydrolysis.84 In the case of
cyclobutanone, a lactone yield of ∼27% and a TON of 276 are
achieved after only 0.25 h, while for the larger ring ketones no
significant reaction is observed in the first 2−3 h, which is
consistent with the known higher reactivity of the 4-membered
ring ketone.
I
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Figure 5. Effect of the temperature on lactone yield (mol %, based on
substrate) in the BV oxidation of 2-methylcyclohexanone (red square,
■
), catalyzed by [ReO3(PTA)2][ReO4] (1). Reaction conditions:
Figure 3. Effect of H2O2 relative amount (H2O2/catalyst molar ratio)
catalyst (1.7 μmol), substrate, H2O2 (1.7 mmol) 1,2-dichloroethane
(3.0 mL), 6 h, under dinitrogen.
on TON and yield (mol %, based on substrate) in the BV oxidation of
◆
▲
2-methylcyclohexanone [(black diamond, ) and (gray triangle, )]
and pinacolone [(red square,
■
●
) and ( )], catalyzed by
[ReO3(PTA)2][ReO4] (1). Reaction conditions are the same as
those of the entries in Table S1 of the Supporting Information.
Figure 6. Effect of the type of solvent on lactone yield (mol %, based
on substrate) in the BV oxidation of 2-methylcyclohexanone, catalyzed
by [ReO3(PTA)2][ReO4] (1). Reaction conditions: catalyst (1.7
μmol), substrate, H2O2 (1.7 mmol) solvent (3.0 mL), 6 h, 70 °C,
under dinitrogen. DCM = dichloromethane; 1,2 DCE = 1,2-
dichloroethane.
Figure 4. Effect of the reaction time on TON in the BV oxidation of 2-
■
methylcyclohexanone (red squares, ) and cyclobutanone (black
●
circles, ), catalyzed by [ReO3(mPTA)][ReO4]I (2). Reaction
conditions are the same as those of the entries for Table S2 in the
Supporting Information.
scorpionates Tpm or Tpms, has been easily synthesized using
Re2O7 as a convenient starting material. Moreover, ligand
exchange at an organodiazenido diphosphinic Re(III) complex
also leads to a water-soluble product, a Re(III) complex bearing
three PTA ligands. Theoretical calculations at the density
functional theory (DFT) level of theory indicated that the
coordination of PTA to both Re(VII) and Re(III) centers by
the P atom is clearly more preferable than the coordination by
the N atom. This effect is taken into account by a higher
stability of the Re−PPTA bond, compared to the Re−NPTA one,
and also may be interpreted in terms of the HSAB formalism.
The new oxo-rhenium(VII or III) complexes exhibit a high
activity toward the mild ketone oxidation with H2O2, leading to
good yields of lactones and esters, with remarkably high TONs.
The above ligands are expected to allow coordinative
unsaturation at the metal, in view of their lability, and/or to
promote proton-transfer steps, in view of their basic character;
these are features that are favorable28e−h,j,54−56,85 toward the
occurrence of oxidation catalysis with H2O2. Moreover, the
hydrosolubility of the complexes, together with the mild
operation conditions, are particularly significant, in terms of
developing green catalytic processes for ketone oxidations. The
work also establishes that oxo-rhenium complexes bearing
The temperature is also an important factor and 70 °C
appears to be the most adequate value for the cyclic ketone
oxidation. In fact, e.g., we failed the attempt of performing the
oxidation of 2-methylcyclohexanone in the presence of 1 at
room temperature, whereas the use of 50 °C resulted in an
important lactone yield reduction, relative to that at 70 °C
(from 30% at 70 °C to 9% at 50 °C; see Figure 5). The lactone
yield decreases above 70 °C, conceivably because of
acceleration of the decomposition of the oxidant H2O2 and
of any peroxo intermediates.54,57
Replacement of 1,2-dichloroethane (1,2 DCE) by other
solvents resulted in a decrease of activity. In the case of the 2-
methylcyclohexanone oxidation in the presence of 1, the
product yield was drastically reduced, from 30% (entry 1, Table
3) in 1,2 DCE to 13% in dichloromethane (DCM) or 1% in
methanol (see Figure 6). Moreover, the use of acetonitrile or
water results in a full inhibiting effect.
CONCLUSIONS
■
A series of water-soluble and stable oxo-rhenium(VII or III)
complexes bearing the hydrosoluble phosphines PTA or m-
PTA, the related cagelike tetra-amino HMT, or the C-
J
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hydrosoluble PTA or C-scorpionate-type ligands, such as
tris(pyrazolyl)methane or derivatives, are rather promising
catalyst precursors for such a purpose, and deserve to be
explored further.
́
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ASSOCIATED CONTENT
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Sokolnicki, J.; Smolenski, P.; Pombeiro, A. J. L. Cryst. Eng. Commun.
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2011, 13, 6329−6333. (c) Ruiz, M. S.; Romerosa, A.; Sierra-Martin, B.;
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* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
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Pruchnik, F. P.; Ciunik, Z.; Lis, T. Inorg. Chem. 2003, 42, 3318−3322.
(d) Pruchnik, F. P.; Smolenski, P. Appl. Organomet. Chem. 1999, 13,
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Corresponding Author
*E-mail addresses: lmartins@deq.isel.ipl.pt (L.M.D.R.S.M.),
pombeiro@.ist.utl.pt (A.J.L.P.).
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work has been partially supported by the Fundaca
̧
o para a
̃
Cien
̂
cia e a Tecnologia (FCT), Portugal, and its PTDC/QUI-
QUI/102150/2008, PTDC/EQU-EQU/122025/2010,
PTDC/QUI-QUI/119561/2010 and PEst-OE/QUI/UI0100/
2011 projects. M.L.K. thanks FCT and IST for the research
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authors gratefully acknowledge Dr. Maria Can
̂
cia 2007 scientific programme. The
dida Vaz (IST)
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̈
for the direction of the elemental analysis service, Prof. Jose
Ascenso (IST) for the DOSY NMR experiments and the
Portuguese NMR Network (IST-UTL Center) for providing
access to the NMR facility.
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