Cis-Dioxorhenium(V/VI) Complexes Supported by Neutral Tetradentate N4 Ligands. Synthesis, Characterization, and Spectroscopy

Article abstract of DOI:10.1021/acs.inorgchem.7b02404

A series of cis-dioxorhenium(V) complexes containing chiral tetradentate N₄ ligands, including cis-[Re^V(O)₂(pyxn)]⁺ (1; pyxn = N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)cyclohexane-1,2-diamine), cis-[Re^V(O)₂(6-Me₂pyxn)]⁺ (cis-2), cis-[Re^V(O)₂(R,R-pdp)]⁺ (3; R,R-pdp = 1,1′-bis((R,R)-2-pyridinylmethyl)-2,2′-bipyrrolidine), cis-[Re^V(O)₂(R,R-6-Me₂pdp)]⁺ (4), and cis-[Re^V(O)₂(bqcn)]⁺ (5; bqcn = N,N′-dimethyl-N,N′-di(quinolin-8-yl)cyclohexane-1,2-diamine), were synthesized. Their structures were established by X-ray crystallography, showing Re-O distances in the range of 1.740(3)-1.769(8) ? and O-Re-O angles of 121.4(2)-124.8(4)°. Their cyclic voltammograms in MeCN (0.1 M [NBu₄]PF₆) display a reversible Re^VI/V couple at E_1/2 = 0.39-0.49 V vs SCE. In aqueous media, three proton-coupled electron transfer reactions corresponding to Re^VI/V, Re^V/III, and Re^III/II couples were observed at pH 1. The Pourbaix diagrams of 1·OTf, 3·OTf, and 5·OTf have been examined. The electronic absorption spectra of the cis-dioxorhenium(V) complexes show three absorption bands at around 800 nm (600-1730 dm³ mol^-1 cm^-1), 580 nm (1700-5580 dm³ mol^-1 cm^-1), and 462-523 nm (3170-6000 dm³ mol^-1 cm^-1). Reaction of 1 with Lewis acids (or protic acids) gave cis-[Re^V(O)(OH)(pyxn)]²⁺ (1·H⁺), in which the Re-O distances are lengthened to 1.788(5) ?. Complex cis-2 resulted from isomerization of trans-2 at elevated temperature. cis-[Re^VI(O)₂(pyxn)](PF₆)₂ (1′·(PF₆)₂) was obtained by constant-potential electrolysis of 1·PF₆ in MeCN (0.1 M [NBu₄]PF₆) at 0.56 V vs SCE; it displays shorter Re-O distances (1.722(4), 1.726(4) ?) and a smaller O-Re-O angle (114.88(18)°) relative to 1 and shows a d-d transition absorption band at 591 nm (? = 77 dm³ mol^-1 cm^-1). With a driving force of ca. 75 kcal mol^-1, 1′ oxidizes hydrocarbons with weak C-H bonds (75.5-76.3 kcal mol^-1) via hydrogen atom abstraction. DFT and TDDFT calculations on the electronic structures and spectroscopic properties of the cis-dioxorhenium(V/VI) complexes were performed.

Full text of DOI:10.1021/acs.inorgchem.7b02404

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
cis-Dioxorhenium(V/VI) Complexes Supported by Neutral
Tetradentate N4 Ligands. Synthesis, Characterization, and
Spectroscopy
Vicky Yin-Ming Ng,†,∥ Chun-Wai Tse,†,∥ Xiangguo Guan,† Xiaoyong Chang,† Chen Yang,†
Kam-Hung Low,† Hung Kay Lee,‡ Jie-Sheng Huang,† and Chi-Ming Che*,†,§
†State Key Laboratory of Synthetic Chemistry, Institute of Molecular Functional Materials, and Department of Chemistry, The
University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of Chinaa
‡Department of Chemistry, The Chinese University of Hong Kong, New Territories, Hong Kong, People’s Republic of China
§HKU Shenzhen Institute of Research and Innovation, Shenzhen 518053, People’s Republic of China
S
* Supporting Information
ABSTRACT: A series of cis-dioxorhenium(V) complexes
containing chiral tetradentate N4 ligands, including cis-
[ReV(O)2(pyxn)]+ (1; pyxn = N,N′-dimethyl-N,N′-bis(2-
pyridylmethyl)cyclohexane-1,2-diamine), cis-[ReV(O)2(6-
Me2pyxn)]+ (cis-2), cis-[ReV(O)2(R,R-pdp)]+ (3; R,R-pdp =
1,1′-bis((R,R)-2-pyridinylmethyl)-2,2′-bipyrrolidine), cis-
[ReV(O)2(R,R-6-Me2pdp)]+ (4), and cis-[ReV(O)2(bqcn)]+
(5; bqcn = N,N′-dimethyl-N,N′-di(quinolin-8-yl)cyclohexane-
1,2-diamine), were synthesized. Their structures were estab-
lished by X-ray crystallography, showing Re−O distances in the
range of 1.740(3)−1.769(8) Å and O−Re−O angles of
121.4(2)−124.8(4)°. Their cyclic voltammograms in MeCN
(0.1 M [NBu4]PF6) display a reversible ReVI/V couple at E1/2
=
0.39−0.49 V vs SCE. In aqueous media, three proton-coupled electron transfer reactions corresponding to ReVI/V, ReV/III, and
ReIII/II couples were observed at pH 1. The Pourbaix diagrams of 1·OTf, 3·OTf, and 5·OTf have been examined. The electronic
absorption spectra of the cis-dioxorhenium(V) complexes show three absorption bands at around 800 nm (600−1730 dm3 mol−1
cm−1), 580 nm (1700−5580 dm3 mol−1 cm−1), and 462−523 nm (3170−6000 dm3 mol−1 cm−1). Reaction of 1 with Lewis acids
(or protic acids) gave cis-[ReV(O)(OH)(pyxn)]2+ (1·H+), in which the Re−O distances are lengthened to 1.788(5) Å. Complex
cis-2 resulted from isomerization of trans-2 at elevated temperature. cis-[ReVI(O)2(pyxn)](PF6)2 (1′·(PF6)2) was obtained by
constant-potential electrolysis of 1·PF6 in MeCN (0.1 M [NBu4]PF6) at 0.56 V vs SCE; it displays shorter Re−O distances
(1.722(4), 1.726(4) Å) and a smaller O−Re−O angle (114.88(18)°) relative to 1 and shows a d−d transition absorption band at
591 nm (ε = 77 dm3 mol−1 cm−1). With a driving force of ca. 75 kcal mol−1, 1′ oxidizes hydrocarbons with weak C−H bonds
(75.5−76.3 kcal mol−1) via hydrogen atom abstraction. DFT and TDDFT calculations on the electronic structures and
spectroscopic properties of the cis-dioxorhenium(V/VI) complexes were performed.
characterized structurally and/or spectroscopically include
Os(VI)11,12 and Ru(VI) complexes13,14 as well as limited
examples of Re(V) complexes: cis-[ReV(O)2(bpy)(py)2]+,15 cis-
[ReV(O)2(Me3tacn)(OH2)]+,16 and cis-[ReV(O)2(Hdab)-
(py)2]17 (bpy = 2,2′-bipyridine; Me3tacn = N,N′,N″-trimeth-
yl-1,4,7-triazacyclononane; H2dab = 1,2-diaminobenzene).
We18b and Meyer18a also reported electrochemical generation
of cis-dioxo d2 metal complexes of Ru and/or Os from the cis-
M(OH2)2 (M = Ru, Os) precursors.
INTRODUCTION
■
cis-Dioxo metal complexes constitute an important class of
compounds in metal-catalyzed oxidation reactions,1,2a−h with
an exemplary example being alkene cis-dihydroxylation
involving the [3 + 2] cycloaddition pathway.2a,h For d2 dioxo
systems, the trans configuration is more stable. The cis
configuration is generally favored in d0 and d1 dioxo systems
to maximize π-bonding interactions between dπ(metal) and
pπ(O) orbitals. A large number of trans-dioxo d2 metal
complexes1,2b,i,j,3−10 have been studied; a notable example
with Re(V) ion is trans-[ReV(O)2(py)4]+ (py = pyridine),
which has been demonstrated to show interesting spectroscopic
and photophysical properties by Gray and co-workers.4 cis-
Dioxo d2 metal complexes have been less explored, particularly
for octahedral ones, examples of which that have been
cis-Dioxo d1 metal complexes are even sparse in the literature,
with limited examples belonging to Cr(V)19 and Mo(V)
complexes,20 together with two types of Re(VI) complexes: cis-
Received: September 19, 2017
© XXXX American Chemical Society
A
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[ReVI(O)2(Ar)2] (Ar = xylyl, mesityl)21 and cis-[(Tp)-
ReVI(O)2(X)] (Tp = hydrotris(pyrazol-1-yl)borate; X = Cl−,
Br−, I−)22 bearing mono- and/or tridentate ligands. Notably,
cis-[ReVI(O)2(Ar)2] adopts a tetrahedral geometry; complexes
cis-[(Tp)ReVI(O)2(X)] represent the only examples of
octahedral d1 cis-dioxorhenium(VI) complexes known to date.
The structurally characterized cis-dioxorhenium(V) com-
plexes reported so far bear mono-, bi-, and/or tridentate
ligands,15−17,23−27 including the octahedral complexes men-
tioned above15−17 and the five-coordinate complexes cis-
[ReV(O)2I(PPh3)2],24 cis-[ReV(O)2Me(PhCCPh)],25 cis-
[ReV(O)2(R2NCH2CH2S)(PPh3)] (R2N = (OCH2CH2O)C-
(CH2CH2)2N),26a Et2N,26b (o-MeOC6H4)N(CH2CH2)2N26b),
and cis-[ReV(O)2Me(PR3)2] (R = Ph, Cy).27 There is one
report on a cis-dioxorhenium(V) complex containing a
tetradentate ligand. Reaction of cis-[ReV(O)2I(PPh3)2] with
an achiral neutral tetradentate ligand, 1,4,7,10-tetrazacyclodo-
decane (cyclen), was reported to give octahedral cis-
[ReV(O)2(cyclen)]+ on the basis of 1H NMR analysis; however,
this complex has not been characterized by X-ray crystal
analysis and could not be isolated in pure form.28 In contrast to
the structures of cis-dioxorhenium(V) complexes, their spec-
troscopy has been much less studied. cis-[ReV(O)2(bpy)(py)2]+
and its derivatives display an electronic absorption band at
about 490 nm (ε = 4200−4600 dm3 mol−1 cm−1) that was
tentatively assigned to MLCT transitions.15b,c cis-
[ReV(O)2(Me3tacn)(OH2)]+ was only mentioned to show a
low-energy d−d absorption band at ca. 680 nm.16 A quest
remains to explore the spectroscopy of cis-dioxorhenium(V)
complexes and to compare it with that of trans-dioxo
complexes.4,6c
We are interested in studying the chemistry of cis-metal
complexes supported by tetradentate N4 ligands. In the
literature, there are a considerable number of iron and
manganese complexes supported by the neutral N4 ligands
pyxn, pdp, and bqcn (and their substituted forms) that have
been reported to catalyze various oxidation reactions29−31
(pyxn = N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)cyclohexane-
1,2-diamine; pdp = 1,1′-bis(2-pyridinylmethyl)-2,2′-bipyrroli-
dine; bqcn = N,N′-dimethyl-N,N′-bis(quinolin-8-yl)-
cyclohexane-1,2-diamine). Reactive chiral cis-[M(O)2(N4)]n+
species, such as cis-[MnV(O)2(S,S-bqcn)]+,31b have been
suggested to play an important role in the asymmetric
oxidation reaction; however, the isolation and characterization
of cis-[M(O)2(S,S-bqcn)]n+ have not been reported. As high-
valent Re complexes15−17 are considerably less oxidizing, cis-
[ReV(O)2(N4)]+ complexes are likely to be stable and isolable;
fundamental studies on these complexes can provide grounds
for further investigation on the structure of the putative reactive
chiral cis-[M(O)2(N4)]n+ species bearing the same chiral N4
ligand in the Fe and Mn oxidation chemistry.
Chart 1. Structures of Dioxorhenium Complexes 1, 3−5, 1′,
and cis- and trans-2 in This Work
ReVI(O)2(X)],22 the cis-ReV(O)2 counterpart has not been
reported). The acid−base chemistry of 1 and its implication on
the electronic structure are discussed. Notably, for [ReV(O)2(6-
Me2pyxn)]+ (2), both its cis and trans isomers have been
obtained, allowing a direct comparison of the structural and
spectroscopic properties between cis- and trans-dioxorhenium-
(V) complexes having the same auxiliary ligands. DFT and
TDDFT calculations were performed to analyze the electronic
structure and spectroscopic data of these structurally
characterized, chiral N4 ligand supported, octahedral cis-
dioxorhenium(V/VI) complexes.
RESULTS
■
Synthesis and Characterization. Reactions of cis-
[ReV(O)2(PPh3)2I] with the corresponding N4 ligand in
dichloromethane afforded cis-[ReV(O)2(pyxn)]+ (1) and cis-
[ReV(O)2(R,R-pdp)]+ (3) in 16% and 30% yields, respectively.
trans-[ReV(O)2(6-Me2pyxn)]+ (trans-2), cis-[ReV(O)2(R,R-6-
Me2pdp)]+ (4), and cis-[ReV(O)2(bqcn)]+ (5) were synthe-
sized in yields of 13%, 4%, and 3%, respectively, using another
procedure, in which a tetrahydrofuran solution of cis-
[ReV(O)2(PPh3)2I] and the corresponding N4 ligand was
refluxed under argon for 2 h. cis-[ReV(O)2(6-Me2pyxn)]PF6
(cis-2·PF6) was obtained by refluxing trans-2·PF6 in acetonitrile
for 4 h, followed by recrystallization, in 48% yield. cis-
[ReVI(O)2(pyxn)](PF6)2 (1′·(PF6)2) was prepared by con-
stant-potential electrolysis of cis-[ReV(O)2(pyxn)]PF6 (1·PF6)
in MeCN (0.1 M[NBu4]PF6) at 0.56 V vs SCE. cis-
[ReV(O)(OH)(pyxn)]2+ (1·H+) was obtained from the
reaction of 1 with Lewis acids (or protic acids). These
complexes were characterized by MS (+ESI), IR and elemental
analyses. The Re(V) complexes 1−5 are diamagnetic and show
1
well-resolved H and 13C NMR spectra (Figures S1−S5 in the
In this present work, a series of cis-dioxorhenium(V)
complexes and also a cis-dioxorhenium(VI) complex bearing
chiral tetradentate N4 ligands, that is, cis-[ReV(O)2(N4)]+ (N4 =
pyxn (1), 6-Me2pyxn (cis-2), R,R-pdp (3), R,R-6-Me2pdp (4),
bqcn (5)) and cis-[ReVI(O)2(pyxn)]2+ (1′), were synthesized
(Chart 1) and structurally characterized by X-ray crystal
analysis; their electrochemistry and electronic absorption
spectra were examined. The complex cis-[ReVI(O)2(pyxn)]2+
(1′) was obtained by electrochemical oxidation of cis-
[ReV(O)2(pyxn)]2+ (1); the two complexes bearing the same
pyxn ligand allow a direct comparison between octahedral cis-
ReVI(O)2 and cis-ReV(O)2 complexes (in the case of [(Tp)-
Supporting Information). Complexes 3 and 4 bearing R,R-pdp
and R,R-6-Me2pdp ligands, respectively, were characterized by
circular dichroism spectroscopy (Figure S6 in the Supporting
Information). Complex 1′ is paramagnetic with a measured
magnetic moment of μeff = 1.72 μB,32 which is consistent with a
d1 electronic configuration.19b,22,33,34 The EPR spectrum of 1′
at 7 K shows multiple lines with a simulated g value of 1.93
(Figure S7 in the Supporting Information).
The infrared (IR) spectra of cis-[ReV(O)2]+ complexes (1,
−
cis-2, and 3−5 as ClO4 salts except for 4·BPh4) reveal two
absorption bands at 910−930 and 880−890 cm−1, which are
attributed to the symmetric and asymmetric ν(O−Re−O)
B
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stretches, respectively. The νsym(ReO2) stretches are compara-
ble to those of cis-[ReV(O)2(bpy)(py)2]+ (904 cm−1)15b and cis-
[ReV(O)2(Hdab)(py)2] (905 cm−1).17 trans-[ReV(O)2(6-
Me2pyxn)]PF6 (trans-2·PF6) displays an IR absorption band
at 785 cm−1 corresponding to an asymmetric trans-[ReV(O)2]+
stretch, similar to that reported for trans-
[ReV(O)2(diphosphine)2]+ (784.8−786.3 cm−1).8b The 1H
NMR spectra of 1, cis-2, and 3−5 (as PF6 salt except for 4·
BPh4) exhibit aromatic signals at 7−8 ppm (pyridyl/quinolyl),
doublet of doublets/singlet signals at 6−6.7 ppm (bridging
CH2), and aliphatic signals at 0.7−4 ppm (Cy/bipyrrolidyl)
(see Figures S1−S4 in the Supporting Information). The N−
CH3 protons of 1, cis-2, and 5 contribute to the sharp singlet
peaks at 3.3−4.4 ppm, whereas the 6-methyl protons on the
pyridyl groups of cis-2 and 4 appear as singlet peaks at ∼2.7
ppm. The 1H NMR spectrum of trans-2·PF6 displays three sets
of aromatic signals at 7.5−8 ppm (pyridyl), two doublets at
5.2−5.7 ppm (bridging CH2), one singlet at 2.87 ppm (NCH3),
another singlet at 2.55 ppm (6-CH3py), and aliphatic signals at
1.2−4.1 ppm (Cy) (Figure S5 in the Supporting Information).
On comparison of trans-2 and cis-2, the three sets of aromatic
signals with an integral ratio of 1:1:1 in trans-2 convert into
two sets with an integral ratio of 2:1 in cis-2. The bridging CH2
protons of trans-2 (5.2−5.7 ppm) are less deshielded than that
of cis-2 (6−6.2 ppm). On the other hand, the N−C(Cy)−H
proton signal of trans-2 (4.05 ppm) is much more deshielded
than that of cis-2 (2.05 ppm). Insignificant differences were
Figure 2. UV−vis spectral changes over 6 h of an acetonitrile solution
of trans-2 (3 × 10−4 M) incubated at 50 °C. Inset: depletion of trans-2
monitored at 355 nm and formation of cis-2 monitored at 580 nm.
corresponds to the initial concentration of trans-2. A third
1
species was not observed in either the UV−vis or H NMR
spectrum. After reaching equilibrium, the cis/trans ratio
corresponds to an equilibrium constant (Keq) of 2.09. Similarly,
starting with cis-2, the same equilibrium ratio between the two
isomers was reached after incubation at 50 °C for 6 h,
suggesting the reversibility of the isomerization process.
Analogous to other isomerization processes reported in the
literature,35 the trans-cis isomerization of 2 can be treated as a
reversible first-order reaction:
1
observed when the H NMR spectra of freshly dissolved cis-2·
kt‐c
[cis‐2]
[trans‐2]
trans‐2 HooI cis‐2 Keq
=
PF6 or trans-2·PF6 were recorded at 60 °C.
kc‐t
The cis-[ReV(O)2(N4)]+ complexes are stable in the solid
state for months and in acetonitrile solution for days, while
trans-2 undergoes isomerization to cis-2 thermally. Time-
The rates of the forward reaction (kt‑c) and backward
reaction (kc‑t), and Keq at different temperatures (40−70 °C)
are summarized in Table 1. The experimental rate data were
1
dependent H NMR spectroscopy revealed that trans-2·PF6
slowly isomerizes into cis-2·PF6 after standing in acetonitrile
solution at room temperature in the dark (Figure 1), during
Table 1. Rate and Equilibrium Constants for Trans−Cis
Isomerization of 2 in Acetonitrile
temp (°C)
kt‑c (10−4 s−1
)
kc‑t (10−4 s−1
)
Keq
40
50
60
70
0.49
1.67
0.23
0.80
2.98
9.51
2.11
2.09
2.07
2.06
6.17
19.59
used in conjunction with the Eyring equation (see Figure S8
and the Experimental Section in the Supporting Information for
details) to calculate the activation enthalpy (ΔH⧧) and entropy
(ΔS⧧) (Table 2). The Gibbs free energies (ΔG⧧) at 50 °C (323
Table 2. Thermodynamic Activation Parameters for Trans−
Cis Isomerization of 2 in Acetonitrile
ΔH⧧ (kJ mol−1
)
ΔS⧧ (J K−1 mol−1
)
ΔG⧧(323 K) (kJ mol−1
)
t-c
c-t
t-c
c-t
t-c
c-t
Figure 1. Changes in the regions of (a) −0.5 to +8.5 ppm and (b)
1
4.5−8.2 ppm in the H NMR spectra of trans-2·PF6 in MeCN-d3
107.7
108.7
16.0
12.8
102.7
104.7
solution at room temperature.
K) were determined to be 102.7 kJ mol−1 (t-c) and 104.7 kJ
mol−1 (c-t), respectively. The very small Gibbs free energy
difference (ΔΔG⧧) reveals that the stabilities of the cis isomer
and corresponding trans isomer are comparable.
Crystal Structures. The structures of 1, cis-2, trans-2, 3−5,
and 1′ were established by X-ray crystallography (Figures 3 and
5). Diffraction-quality crystals of cis-[ReV(O)2(N4)]+ (1, cis-2,
and 3−5; PF6 anion except for 4·BPh4) were obtained by
which a color change from yellow to brown was observed. The
kinetics and thermodynamics of the isomerization of trans-2 to
cis-2 was followed by UV−vis spectroscopy. Figure 2 depicts
the spectral changes when a solution of trans-2 was incubated
at 50 °C in acetonitrile, showing the depletion of trans-2 and
clean formation of cis-2 with an isosbestic point at 385 nm. The
time-dependent total concentration of trans-2 and cis-2
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Figure 4. α and β angles in cis-dioxorhenium complexes.
while the corresponding angles of cis-[ReV(O)2(bpy)(py)2]+
and cis-[ReV(O)2(Hdab)(py)2] are 70.6(3)°15a and 72.7(3)°,17
respectively. The experimental Re−N1 and Re−N4 distances
are 2.101(3)−2.186(6) Å, which closely match with the
computed values of 2.132−2.197 Å. The experimental N1−
Re−N4 angles are 174.59(13)−179.71(13)°, which were also
well reproduced by the computed values of 176.1−178.7°.
A crystal of cis-[ReVI(O)2(pyxn)](PF6)2 (1′·(PF6)2) suitable
for X-ray diffraction analysis was obtained by slow diffusion of
diethyl ether into an acetonitrile solution of 1′·(PF6)2. The
structure of 1′ is depicted in Figure 3f, which adopts a pseudo-
octahedral geometry with cis-α configuration similar to that of
1. In comparison to 1 (Re−O 1.750(5), 1.753(5) Å), the
experimental Re−O distances of 1′ are slightly shorter
(1.722(4), 1.726(4) Å; DFT-calculated 1.728 Å; Table S14 in
the Supporting Information). These Re−O distances of 1′ are
also slightly shorter than those of the octahedral complex cis-
[(Tp)ReVI(O)2(Cl)] (1.797(11) and 1.741(10) Å).22 The
experimental O−Re−O angle of 114.88(18)° (computed
116.5°, Table S14) of 1′ is smaller than that of 1
(121.4(2)°) but larger than that of cis-[(Tp)ReVI(O)2(Cl)]
(107.1(4)°).22
Figure 3. ORTEP drawings of (a) cis-[ReV(O)2(pyxn)]+ (1), (b) cis-
[ReV(O)2(6-Me2pyxn)]+ (cis-2), (c) cis-[ReV(O)2(R,R-pdp)]+ (3), (d)
cis-[ReV(O)2(R,R-6-Me2pdp)]+ (4), (e) cis-[ReV(O)2(bqcn)]+ (5), and
(f) cis-[ReVI(O)2(pyxn)]2+ (1′). Hydrogen atoms are not shown.
diffusion of diethyl ether into methanol solutions of the
complexes. Selected bond distances and angles are given in
Table 3. All of the cis-dioxorhenium(V) complexes adopt a
pseudo-octahedral geometry with cis-α configuration. The Re−
O distances are 1.740(3)−1.769(8) Å, which are close to the
computed values (1.749−1.754 Å, Tables S9−S13 in the
Supporting Information) in the DFT-optimized ground state
(S0) geometries. These Re−O distances fall within the range
(1.716(6)−1.82(1) Å) of those reported for octahedral
complexes cis-[ReV(O)2(Hdab)(py)2] (1.716(6), 1.723(7)
Å),17 cis-[ReV(O)2(bpy)(py)2]+ (1.733(8), 1.736(7) Å),15a
and cis-[ReV(O)2(OH2)(Me3tacn)]+ (1.78(1), 1.82(1) Å).16
The experimental and computed O1−Re1−O2 bond angles (α
in Figure 4) for cis-[ReV(O)2(N4)]+ (1, cis-2, and 3−5) are
121.4−124.8 and 124.0−127.0°, respectively; both values are
comparable to those of cis-[ReV(O)2(bpy)(py)2]+ (121(4)°)15a
and cis-[ReV(O)2(Hdab)(py)2] (118.2(3)°)17 but larger than
that of cis-[ReV(O)2(OH2)(Me3tacn)]+ (106.7(5)°).16 In the
cases of 1 and 3, introducing methyl groups to the 6-position of
their pyridyl groups to give cis-2 and 4, respectively, slightly
increases the O−Re−O angle (α in Figure 4) (121.4(2) →
123.75(15)° for 1 → cis-2, 122.5(2) → 124.8(4)° for 3 → 4),
probably owing to the repulsion between the oxo ligand and
the 6-methyl group. The N2−Re−N3 angles (β in Figure 4) of
these cis-dioxorhenium(V) complexes are 73.5(3)−77.56(11)°,
For cis-[ReV(O)(OH)(pyxn)](OTf)2 (1·H+·(OTf)2), a
crystal suitable for X-ray diffraction analysis was obtained by
slow diffusion of diethyl ether into an acetonitrile solution of 1
(12 mM) and ScIII(OTf)3 (6 mM) at 4 °C. In the unit cell, a C2
axis passes through the Re center, which suggests disorder of
the proton on the two oxo/hydroxo O atoms. In comparison to
1 (Re−O 1.750(5), 1.753(5) Å), the experimental Re−O
distances of 1·H+ are slightly longer (1.788(5) Å); this is
attributed to the weakening of Re−O π bonding by
protonation. The experimental O−Re−O angle of 112.8(3)°
of 1·H+ is significantly smaller than that of 1. Notably,
hydrogen bonding between the hydroxo group of 1·H+ and the
N atom of cocrystallized acetonitrile is also observed (Figure S9
in the Supporting Information).
The X-ray crystal structure of trans-[ReV(O)2(6-Me2pyxn)]-
ClO4 (trans-2·ClO4) is depicted in Figure 5. The Re−O
distances are 1.768(4) Å, with a O−Re−O angle of 179.5(3)°.
The Re−O distances are slightly longer than that of cis-2
Table 3. Selected Bond Distances (Å) and Angles (deg) of cis-Dioxorhenium Complexes, cis-[ReV(O)(OH)(pyxn)](OTf)2 (1·
H+·(OTf)2), and trans-[ReV(O)2(6-Me2pyxn)]ClO4 (trans-2)
a
a
a
1
cis-2
3
4
5
1′
1·H+
trans-2
Re−O1
Re−O2
Re−N1
Re−N4
Re−N2
Re−N3
O1−Re−O2 (α)
N1−Re−N4
N2−Re−N3 (β)
1.750(5)
1.753(5)
2.121(6)
2.124(6)
2.275(5)
2.276(6)
121.4(2)
175.6(2)
75.8(2)
1.742(2)
1.742(2)
2.176(3)
2.176(3)
2.296(3)
2.296(3)
123.75(15)
174.59(13)
74.41(14)
1.765(4)
1.749(4)
2.122(5)
2.132(5)
2.258(5)
2.252(4)
122.5(2)
176.3(2)
75.47(19)
1.741(8)
1.769(8)
2.186(6)
2.171(7)
2.285(7)
2.258(7)
124.8(4)
174.6(4)
73.5(3)
1.740(3)
1.752(3)
2.101(3)
2.105(3)
2.291(3)
2.284(3)
121.80(13)
179.71(13)
77.56(11)
1.722(4)
1.726(4)
2.109(4)
2.119(4)
2.244(5)
2.244(4)
114.88(18)
172.60(16)
77.00(15)
1.788(5)
1.788(5)
2.097(6)
2.097(6)
2.232(5)
2.232(5)
112.8(3)
174.1(3)
77.5(3)
1.768(4)
1.768(4)
2.197(5)
2.197(5)
2.131(5)
2.131(5)
179.5(3)
119.0(3)
87.3(3)
a
O1i, N2i, and N1i atoms in the ORTEP drawings, which are generated by repeating the symmetric unit to form the whole structure, are presented as
O2, N3, and N4 atoms, respectively.
D
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Table 4. Electrochemical Data of cis-Dioxorhenium(V)
Complexes, 1·H+, and trans-2 in Acetonitrile (0.1 M
a
[NBu4]PF6)
E (V vs SCE)
complex
reduction (Epc)
oxidation (E1/2)
1
−1.74
−1.70
−1.67
−1.69
−1.21
−1.80
−1.60
0.42
0.47
0.39
0.42
0.49
cis-2
Figure 5. ORTEP drawing of trans-[ReV(O)2(6-Me2pyxn)]+ (trans-2).
Hydrogen atoms are not shown.
3
4
5
b
trans-2
1·H+
0.96
(1.742(2) Å) and are comparable to those of trans-
[ReV(O)2(py)4]+ (1.745(12), 1.782(13) Å),36 trans-
[ReV(O)2(en)2]+ (1.761(7), 1.769(7) Å),36 and
[ReV(O)2(diphosphine)2]+ (1.748(4)−1.782(5) Å).8b The
experimental Re−N1(py) and Re−N2(Cy) distances are
2.197(5) and 2.131(5) Å, respectively; the former is slightly
longer than that of cis-2 (2.176(3) Å), whereas the latter is
much shorter than that of cis-2 (2.296(3) Å). Notably, N1−
Re−N4 angle of 119.0(3)° is much larger than 90.0° for an
ideal octahedral geometry. In addition, trans-2 (relative to cis-
2) has a considerably larger N2−Re−N3 (β) angle (87.3(3) vs
74.41(14)°).
Electrochemistry. In Nonaqueous Media. The cyclic
voltammograms of the cis-dioxorhenium(V) complexes in
acetonitrile (0.1 M [NBu4]PF6) display a reversible oxidation
couple at E1/2 = 0.39−0.49 V vs SCE and an irreversible
reduction wave at Epc = −1.21 to −1.74 V vs SCE (Figure 6a
c
0.95
a
In acetonitrile with 0.1 M [NBu4]PF6 as the electrolyte. Glassy
carbon was the working electrode, saturated calomel electrode (SCE)
was the reference electrode, and platinum wire was the counter
b
electrode. E1/2[(Me5Cp)2Fe+/0] = −0.11 V vs SCE. Epa of the
c
irreversible wave is reported. The forward wave is broad with Epa
≈
1.3 V.
1, cis-2, 3, and 4 as a result of the replacement of pyridyl by the
more conjugating quinolyl.
The cyclic voltammogram of trans-[ReV(O)2(6,6′-
Me2pyxn)]PF6 (trans-2·PF6) in MeCN (0.1 M [NBu4]PF6)
exhibits an irreversible oxidation wave at Epa = 0.96 V vs SCE
and an irreversible reduction wave at Epc = −1.80 V vs SCE
(Figure 6a). The oxidation wave is tentatively assigned to
oxidation of ReV to ReVI, the potential of which is less anodic
8d
that that of trans-[ReV(O)2(tmen)2]+ (E1/2 = 1.41 V vs SCE,
Figure S17 in the Supporting Information; tmen = N,N,N′,N′-
8c
tetramethylethylenediamine) and trans-[ReV(O)2(dmpe)2]+
(Epa = 1.53 V vs SCE, Figure S17; dmpe = 1,2-bis-
(dimethylphosphino)ethane). The Epa(Re(V) → Re(VI))
value of trans-2 is more anodic than the E1/2(ReVI/V) value of
cis-[ReV(O)2(N4)]+ complexes by ca. 0.5 V. Presumably, cis-
dioxo d1 complexes are stabilized by additional π interaction
between dπ(Re) and pπ(O) orbitals. The irreversible reduction
wave of trans-2 is ascribed to a ligand-based reduction, as the
reduction potential is similar to that of 1 and cis-2 (Epc = −1.74
and −1.70 V vs SCE, respectively).
The cyclic voltammogram of cis-[ReV(O)(OH)(pyxn)]-
(OTf)2 (1·H+·(OTf)2) in MeCN (0.1 M [NBu4]PF6) shows
distinct features in comparison to those of 1. A broad oxidation
wave was observed at a highly anodic potential (Epa ≈ 1.3 V vs
SCE), ascribable to the oxidation of 1·H+ (Figure 7).
Interestingly, 1·H+ exhibits an additional reversible reduction
couple at −0.31 V vs SCE that is tentatively attributed to one-
electron reduction of 1·H+ to cis-[ReIV(O)(OH)(pyxn)]+. The
ligand-based reduction of 1·H+ occurs at a slightly less cathodic
potential relative to 1 (Table 4).
Figure 6. Cyclic voltammograms of (a) cis-dioxorhenium(V)
complexes in acetonitrile (0.1 M [NBu4]PF6) and (b) cis-
[ReV(O)2(pyxn)]OTf (1·OTf), cis-[ReV(O)2(R,R-pdp)]OTf (3·
OTf), and cis-[ReV(O)2(bqcn)]OTf (5·OTf) in 0.1 M CF3SO3H.
Scan rate: 100 mV s−1.
and Table 4). The oxidation is attributable to a ReVI/V couple,
the potentials of which are less anodic than that of cis-
[ReV(O)2(bpy)(py)2]+ (0.77 V vs SCE).15c The reduction wave
at Epc = −1.21 to −1.74 V vs SCE is likely to be ligand-based
reduction, since the Epc of 5 is much less cathodic than that of
Figure 7. Cyclic voltammograms of cis-[ReV(O)(OH)(pyxn)](OTf)2
(1·H+·(OTf)2) in acetonitrile (0.1 M [NBu4]PF6). Scan rate: 100 mV
s−1.
E
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In Aqueous Media. Variable pH electrochemistry of cis-
dioxorhenium(V) complexes in aqueous media has been
conducted. At pH 1, complexes 1·OTf, 3·OTf, and 5·OTf
show a reversible oxidation couple at E1/2 = 0.44−0.47 V vs
SCE (couple I) and two reduction couples at around E = −0.20
to −0.32 V and −0.70 to −0.83 V vs SCE (couples II and III,
respectively) (Figure 6b and Table 5). The current magnitude
Upon further increasing the pH above 3.3 (1, 3) or 2.6 (5),
the potential of couple I becomes insensitive to pH change,
indicative of a one-electron redox that does not involve proton
transfer. At pH > 3.3 (1, 3) or pH > 2.6 (5), the
electrochemical reactions for the ReVI/V, ReV/III, and ReIII/II
couples are represented by eqs 4, 5, and 6, respectively. In the
Pourbaix diagrams of 1, 3, and 5 (Figure 8), the breakpoint of
Table 5. Electrochemical Data of cis-Dioxorhenium(V)
Complexes in Aqueous Medium at Selected pH Values
a
E1/2 (V vs SCE)
complex
pH
couple III
couple II
couple I
1
1
−0.78
−1.01
−1.20
−0.22
−0.46
−0.64
−0.74
−0.32
−0.46
−0.60
0.44
0.31
0.30
0.30
0.44
0.32
0.31
0.31
0.47
0.37
0.36
0.36
3.3
5.7
7.2
1
b
3
5
−0.83
−1.02
−1.21
3.3
5.7
7.2
1
−0.71
−0.90
−0.20
−0.46
−0.63
−0.75
3.3
5.7
7.2
a
For measurement at pH 1: 0.1 M CF3SO3H as the electrolyte; glassy
carbon was the working electrode, saturated calomel electrode (SCE)
was the reference electrode, and platinum wire was the counter
electrode. For measurement at pH 1.8−12: Britton−Robinson buffer
b
as electrolyte; all other conditions the same as for pH 1.39 Epa of the
irreversible wave is reported. Couple II of 3 is irreversible at pH 1−
2.21 but becomes reversible at pH 2.56−6.37. The reduction couples
of 3 are ill-defined at pH >6.37.
of couple II is about twice that of couples I and III. As the pH
increases from 1 to 3.3 (1, 3) or 2.6 (5), the potentials for
couples I−III shift cathodically with a slope of ca. 60 mV/pH,
as depicted in Figure 7. Rotating disk electrode voltammetry of
1·OTf, 3·OTf, and 5·OTf at pH 2 revealed that the coulombic
stoichiometry in couples I−III has a ratio of 1:2:1 (Figures
S18−S20 in the Supporting Information). At pH 1−3.3 (1, 3)
or 1−2.6 (5), couple I is reasonably attributed to the ReVI/V
couple (eq 1). The ReVI/V potential is less anodic than that of
cis-[ReV(O)2(bpy)(py)2]+ (E1/2 = 0.62 V vs SCE at pH 2).15b
Couple II is assigned as a two-electron reduction of Re(V) to
Re(III) (eq 2), while couple III is attributable to the reduction
of Re(III) to Re(II) (eq 3). The slope of 60 mV/pH is in
accordance with one-electron one-proton reaction (eqs 1 and
3) and two-electron two-proton reaction (eq 2) according to
the Nernst equation.37,38
Figure 8. Pourbaix diagrams of (a) cis-[ReV(O)2(pyxn)]OTf (1·OTf),
(b) cis-[ReV(O)2(R,R-pdp)]OTf (3·OTf), and (c) cis-
[ReV(O)2(bqcn)]OTf (5·OTf).
the plot of couple I is logically the pKa value of cis-
[ReV(O)2(N4)]+/cis-[ReV(OH)(O)(N4)]2+ proton-transfer re-
action, which was found to be 3.3, 3.3, and 2.6 for 1, 3, and 5,
respectively.
At pH 1−3.3 (1) or pH 1−2.6 (5):
couple I: cis‐[ReVI(O)2(L)]2+ + e− + H+
At pH >3.3 (1) or pH >2.6 (5):
→ cis‐[ReV(O)(OH)(L)]2+
couple I:
(1)
cis‐[ReVI(O)2(L)]2+ + e− → cis‐[ReV(O)2(L)]+
(4)
couple II: cis‐[ReV(O)(OH)(L)]2+ + 2e− + 2H+
couple II: cis‐[ReV(O)2(L)]+ + 2e− + 2H+ → cis‐
→ cis‐[ReIII(OH)(OH2)(L)]2+
(2)
[ReIII(OH)2(L)]+
(5)
couple III: cis‐[ReIII(OH)(OH2)(L)]2+ + e− + H+
→ cis‐[ReII(OH2)2(L)]2+
couple III: cis‐[ReIII(OH)2(L)]+ + e− + H+ → cis‐
[ReII(OH)(OH2)(L)]+
(3)
(6)
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Molecular Orbital Diagram. With reference to Vanquick-
enborne’s work on cis-dihalogenotetraamine osmium(II)
complexes,40 the d orbitals of cis-dioxo metal complexes with
2
2
2
C2v symmetry are in the order of dxz < dyz < dxy < dz < dx −y
(Figure 9).41 In this work, the DFT-calculated molecular orbital
Figure 11. Molecular orbital diagram of cis-[ReVI(O)2(pyxn)]2+ (1′) at
doublet ground state.
Figure 9. Molecular orbital diagram of cis-dioxo d2 complex assuming
a C2v symmetry.41
Figure 12. Experimental and simulated UV−vis absorption spectra of
cis-[ReV(O)2(pyxn)]+ (1) in acetonitrile solution.
Table 6. UV−Visible Absorption Data of cis-Dioxorhenium
Complexes, (1·H+·OTf)2, and trans-2 in Acetonitrile
a
Solution
complex
λabs (nm) (ε (dm3 mol−1 cm−1))
1
800 (600), 580 (1740), 465 (6050), 345 (sh, 340), 258 (7380)
702 (450), 580 (700), 459 (4000), 270 (10250)
cis-2
3
4
5
784 (700), 572 (2520), 462 (5570), 348 (200), 257 (7050)
688 (640), 564 (1300), 461 (4860), 268 (13470)
846 (1730), 591 (5580), 523 (sh, 3170), 384 (1080), 206 (10240),
297 (sh, 9420), 268 (6160)
Figure 10. Molecular orbital diagrams of the cis-dioxorhenium(V)
complexes 1, 3, and 5 at ground state (S0).
1′
591 (77), 340 (2270), 296 (4580), 249 (10080)
238 (12200), 309 (5450), 900 (120)
1·H+
trans-2 449 (1010), 355 (1660), 310 (1600), 267 (9860)
diagrams for 1, 3, and 5 (Figure 10) at ground state show that
the HOMO is mainly composed of the dxz orbital while the
LUMO and L+3 are mainly dyz- and dxy-based orbitals,
respectively. The L+1 and L+2 are pyridyl/quinolyl-based
orbitals.
The DFT-calculated molecular orbital diagram of cis-
[ReVI(O)2(pyxn)]2+ (1′) is shown in Figure 11. The singly
occupied molecular orbital (SOMO) is mainly composed of
dxz(Re) orbital, supporting the assignment of a d1 Re(VI)
electronic configuration. The dπ*(Re) orbitals of 1′ (−6.5 to
−3.7 eV) are at an energy lower than that of its Re(V)
counterpart (1: −4.5 to −2 eV), in accordance with the higher
electron affinity of Re(VI) relative to Re(V).
a
Measured in acetonitrile at 298 K.
The spectra of cis-[ReV(O)2(pyxn)]PF6 (1·PF6), cis-
[ReV(O)2(pyxn)]PF6 (cis-2·PF6), cis-[ReV(O)2(R,R-pdp)]PF6
(3·PF6), and cis-[ReV(O)2(R,R-6-Me2pdp)]BPh4 (4·BPh4) in
MeCN are similar but distinctively different from that of cis-
[ReV(O)2(bqcn)]PF6 (5·PF6) (Table 6). Complexes 1, cis-2, 3,
and 4 show three absorption bands at 688−800 nm (band I),
∼570 nm (band II). and ∼460 nm (band III). On the basis of
the results of TDDFT calculations (Figure 12 and Figures
S21−S23 and Table S16 in the Supporting Information), the
weak absorption (band I) at ∼750 nm with an extinction
coefficient of 450−700 dm3 mol−1 cm−1 is derived from the
UV−Visible Absorption Spectra. The UV−vis absorption
spectra of the cis-dioxorhenium(V) complexes are depicted in
Figures 11 and 12, and the spectral data are given in Table 6.
1
HOMO → LUMO transition attributable to a [dxz(Re) →
dyz(Re)] transition, which is at a slightly lower energy in
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1[dxz(Re) → dxy(Re)] (HOMO → L+3), mixed with a
1[dxz(Re) → π*(quinolyl)] transition.
comparison to the assigned d−d transition of cis-
[ReV(O)2(OH2)(Me3tacn)]+ (∼680 nm).16 A similar low-
energy absorption band at 750 nm (ε = 250 dm3 mol−1 cm−1)
has also been observed in the electronic absorption spectrum of
cis-[OsVI(O)2(bpy)2]2+.12a The absorption band at ∼580 nm
The UV−vis absorption spectrum of cis-[ReVI(O)2(pyxn)]-
(PF6)2 (1′·(PF6)2) displays three absorption bands in the UV
spectral region at 340, 296, and 249 nm (ε = 2270, 4580, and
10080 dm3 mol−1 cm−1, respectively) and a weak absorption
band at 591 nm (ε = 77 dm3 mol−1 cm−1) (Figure 14). For this
complex, the lowest-energy transition is derived from the
1
(band II) likely comes from a [dxz(Re) → dxy(Re)] transition
mixed with 1[dxz(Re) → π*(L)] character. The intense
absorption band at ∼465 nm (band III) is mainly derived
from the 1[dxz(Re) → π*(L)] transition, though TDDFT
calculations show mixed character from a 1[dxz(Re) → dxy(Re)]
transition. As a result, these two bands, mainly MLCT in
nature, display relatively larger extinction coefficients of 700−
2600 and 4000−6000 dm3 mol−1 cm−1, respectively. In
addition, band I of cis-2 and 4, having 6-methyl substituents
on the pyridyl group, are slightly blue shifted in comparison to
those of 1 and 3, respectively. This could be attributed to the
larger O−Re−O angle (α) in cis-2 and 4 (Table 3), which leads
to LUMO destabilization, as reported in the work by
Demanchy and Jean.41,42 This is also consistent with
calculations that the LUMOs of cis-2 and 4 are at higher
energy than those of 1 and 3 (1, −2.81 eV; cis-2, −2.58 eV; 3,
−2.77 eV; 4, −2.53 eV).
2
SOMO → LUMOα transition attributable to [dxz(Re) →
dxy(Re)].
The absorption spectrum of cis-[ReV(O)2(bqcn)]PF6 (5·
PF6) in MeCN exhibits three absorption bands at around 846
nm (band I), 591 nm (band II), and 523 nm (band III) (Figure
13b). Band I can be correlated to the HOMO → LUMO
Figure 14. Experimental UV−vis absorption spectrum of cis-
[ReVI(O)2(pyxn)](PF6)2 (1′·(PF6)2) in acetonitrile solution.
For cis-[ReV(O)(OH)(pyxn)]2+ (1·H+), its UV−vis absorp-
tion spectrum in the visible spectral region is featureless (Figure
15). Intense absorption bands are located in the UV spectral
Figure 15. Experimental UV−vis absorption spectrum of cis-
[ReV(O)(OH)(pyxn)](OTf)2 (1·H+·(OTf)2) in acetonitrile solution.
region at 309 and 238 nm (ε = 5450 and 12200 dm3 mol−1
cm−1, respectively). The weak absorption band at ∼900 nm (ε
= 120 dm3 mol−1 cm−1) is assignable to a 1[dxz(Re) → dyz(Re)]
transition. The high-energy absorption bands are attributable to
a mixture of MLCT and LMCT transitions such as 1[dπ(Re) →
Figure 13. Experimental UV−vis absorption spectra of cis-
dioxorhenium(V) complexes (a) cis-2·PF6, 3·PF6, and 4·BPh4 and
(b) 5·PF6 in acetonitrile solution.
1
π*(py)] and [pπ(O) → dπ(Re)].
1
transition attributable to [dxz(Re) → dyz(Re)], though with a
The electronic absorption spectrum of trans-[ReV(O)2(6-
Me2pyxn)]PF6 (trans-2·PF6) displays two broad absorption
bands at 449 and 355 nm with ε = 1010 and 1660 dm3 mol−1
cm−1, respectively (Figure 16). The low-energy band at 449 nm
is derived from the HOMO → LUMO transition attributable to
larger ε value (1730 dm3 mol−1 cm−1) in comparison to those
of 1, cis-2, 3, and 4. Band II at 591 nm (ε = 5580 dm3 mol−1
cm−1) is derived from the HOMO → L+2 transition
1
attributable to a [dxz(Re) → π*(quinolyl)] transition. Band
1
III at 523 nm (ε = 3170 dm3 mol−1 cm−1) can be assigned to
a [dxy(Re) → π*(py)] MLCT transition. The high-energy
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Figure 16. Experimental UV−vis absorption spectrum of trans-
[ReV(O)2(6-Me2pyxn)]PF6 (trans-2·PF6) in acetonitrile solution
(inset: expanded region at 280−340 nm).
Figure 17. UV−vis titration of an acetonitrile solution of 1·PF6 (0.1
mM) with CF3SO3H.
band at 355 nm is mainly composed of a 1[dxy(Re) → π*(py)]
1
transition, mixed with [dxy(Re) → π*(d(Re)-p(O))] tran-
1
sition. The vibronic band at 310 nm is attributed to a [pπ(O)
→ π*(dπ(Re)-p(O))] LMCT transition. The progressional
spacing of ca. 684 cm−1 corresponds to the weakened Re−O
bond in the excited state in comparison to the ground state
v
asym(ReO2) stretch (785 cm−1) in the IR spectrum. The
intense absorption band at 267 nm is mainly composed of
transitions involving electron promotion from the H-4/H-5
orbital to LUMO/L+1 orbital, which are assigned to 1[π(py) →
π*(d(Re)-p(O))] transitions.
Reactivity. The oxygen-atom transfer reactivity of the cis-
[ReV(O)2]+ complexes was evaluated from the reaction of 1·
PF6 with triphenylphosphine, thioanisole, and styrene. In
acetonitrile under aerobic conditions, 1 was not able to oxidize
the tested substrates even at elevated temperatures (up to 60
°C) for 4 h. No oxidation product such as phosphine oxide,
sulfoxide, or epoxide could be detected by GC-MS. UV−vis
absorption measurements of the reaction mixtures revealed that
1 remained intact.
Figure 18. Experimental UV−vis absorption spectrum of cis-
[ReV(O)2(pyxn)]OTf (1·OTf; 1 × 10−4 M) in aqueous solution at
different pHs (Britton−Robinson buffer).39
cis-[ReV(O)2(pyxn)]+ (1; 0.1 mM) reacts with excess Lewis
acids (10 mM) including Sc3+, Zn2+, Li+, and CF3SO3H in
acetonitrile to give a pink solution. UV−vis spectroscopy
showed that a common species that absorbed at 238 and 308
nm was produced, irrespective of the choice of Lewis acids. ESI-
MS measurement of the pink solution revealed the formation of
doubly charged species at m/z 272.11, assignable to the
protonated form of 1, cis-[ReV(O)(OH)(pyxn)]2+ (1·H+).
Complex 1·H+ has been crystallographically characterized (vide
supra, Figure S9 in the Supporting Information).
For example, in acetonitrile, 1′ was observed to be reduced
back to 1 (>80% by using UV−vis spectroscopy) in the
presence of a few equivalents of triethylamine or triethanol-
amine. Another cis-dioxorhenium(VI) complex, [(Tp)-
ReVI(O)2(Cl)], has also been mentioned to be decomposed
by bases.22
Interestingly, 1′ was found to oxidize hydrocarbons with
weak C−H bonds (bond dissociation energy (BDE): 75.5−76.3
kcal mol−1) stoichiometrically (Table 7). For instance, reaction
of 1′ (1.0 equiv) with 1,4-cyclohexadiene (CHD) in acetonitrile
at 80 °C for 1 h afforded benzene (0.485 equiv) as an
aromatization product. The product yield based on Re was
97%, considering a reaction stoichiometry of Re:CHD = 2:1.
The reaction was accompanied by a color change from green to
purple-pink. UV−vis measurement of the resultant solution
showed absorption bands at 238 and 308 nm characteristic of 1·
H+. Upon treatment with triethylamine, 1 was quantitatively
produced (quantified at 465 nm) on the basis of the initial
concentration of 1′. In a control experiment where 1′ was
replaced by 1, no benzene could be detected after 3 h at 80 °C.
Complex 1′ similarly reacted with 9,0-dihydroanthracene
(9,10-DHA) to give anthracene in 98% yield. A competitive
reaction between 9,10-DHA and 9,10-DHA-d4 suggested a
primary kinetic isotope effect (KIE) of kH/kD = 4.9. Reaction of
1′ with xanthene gave xanthone in 90% yield (kH/kD = 5.8).
Under identical conditions, no substrate conversion was
noticed in the reaction of 1′ with fluorene, tetralin, or styrene.
The formation of 1·H+ from the reaction of 1 with
ScIII(OTf)3 was followed by UV−vis titration, featuring an
isosbestic point at 370 nm (Figure S27 in the Supporting
Information). Only 0.5 equiv of ScIII(OTf)3 was sufficient to
quantitatively protonate 1 (the proton likely comes from
reaction of ScIII(OTf)3 with a trace amount of water in the
solvent). When CF3SO3H was used as the titrant, 1.0 equiv was
required for complete reaction (Figure 17). The reaction was
reversible; 1·H+ could be converted back to 1 by addition of 1.0
equiv of triethylamine. When weaker Lewis acids such as
Zn(OTf)2 and LiOTf were used as titrants, the reaction was not
complete at a low titrant concentration (e.g., up to 20 equiv),
suggesting that a binding equilibrium exists.
In aqueous solutions, 1·OTf exhibits pH-dependent
absorption profiles in the range of pH 1.8−4.8 (Figure 18).
cis-[ReVI(O)2(pyxn)]2+ (1′) is thermally stable; insignificant
decomposition was observed by heating its acetonitrile solution
at 80 °C for 18 h. However, 1′ is sensitive to aliphatic amines.
I
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Despite the reports of a number of isolated and structurally
characterized d2 cis-dioxorhenium(V) complexes, the majority
of these are five-coordinate complexes adopting a trigonal-
bipyramidal configuration,23−27 with three examples of
structurally characterized octahedral ones.15−17 The complex
cis-[ReV(O)2(cyclen)]+, bearing an achiral N4 ligand and
Table 7. Stoichiometric Oxidation of Hydrocarbons by cis-
[ReVI(O)2(pyxn)](PF6)2 (1′·(PF6)2) in Acetonitrile at 80 °C
1
formulated by H NMR analysis, was not isolated in pure
form.28 To the best of our knowledge, structurally characterized
(by X-ray crystallography) examples of cis-ReVO2 supported by
a tetradentate ligand have not been reported in the literature,
nor have d2 cis-[M(O)2(N4)]n+ complexes (N4 = pyxn, pdp,
bqcn, and their derivatives) been isolated.
In this work, the new class of cis-dioxorhenium(V) complexes
cis-[ReV(O)2(N4)]+ supported by different neutral N4 ligands
(pyxn, R,R-pdp, bqcn, 6-Me2pyxn, R,R-6-Me2pdp) and trans-
[ReV(O)2(6,6′-Me2pyxn)]PF6 were synthesized. All complexes
(1, cis-2, 3−5, trans-2, 1′) have been structurally characterized
by X-ray crystallography. The coordination geometry is affected
by the ligand structure. For complexes bearing an N4 ligand
with a rigid bipyrrolidyl backbone (R,R-pdp in 3 and R,R-6-
Me2pdp in 4) and those bearing a cyclohexyl backbone without
o-methyl substitution (pyxn in 1 and bqcn in 5), the cis-α
isomer was exclusively obtained, consistent with the cis
coordination mode adopted by such N4 ligands in their
octahedral Fe and Mn complexes.29−31 However, for 2 bearing
an N4 ligand with a cyclohexyl backbone and with o-methyl
aromatic substitution (6-Me2pyxn), both cis-α and trans
isomers were obtained. In the preparation of 2, the trans
isomer was isolated as the major product after purification. This
could be associated with the relatively high flexibility of the
cyclohexyl backbone45 (coupled with the steric effect of the 6-
Me substituents on the pyridyl groups), which allowed the
ligand to adopt multiple coordination modes to the Re(V)
center. trans-2 was found to isomerize into a mixture of trans-2
and cis-2 under thermal conditions. At equilibrium, the cis/
trans ratio was ∼2.1 (at 50 °C). The kinetics and
thermodynamics of the isomerization have been determined,
showing an activation barrier (ΔG⧧) of 102.7−104.7 kJ mol−1
and a small free energy difference (ΔΔG⧧) of 2.0 kJ mol−1
between the two isomers. The fact that the cis isomer is slightly
more stable is quite surprising for a d2-M(O)2 system, because
the electronic repulsion between the two oxo ligands can be
relaxed in the trans configuration. Perhaps, in the trans
geometry of 2, the highly strained bonding angles greatly
destabilize the molecules and outweigh the effect of the
electronic repulsion.
The isolation of trans-2 bearing the 6-Me2pyxn ligand is
rather unusual. Previously, several examples of structurally
characterized octahedral metal complexes with this ligand have
been reported, including [FeII(Hdbc)(6-Me2pyxn)]OTf (Hdbc
= 3,5-di-tert-butyl-1,2-catecholate monoanion),46a [FeIII(dbc)-
(6-Me2pyxn)]OTf,46a [FeII(OTf)2(6-Me2pyxn)],29a [FeII(H(4-
nc))(6-Me2pyxn)]OTf (H2(4-nc) = 4-nitrocatechol),46b and
[MnII(H(4-nc))(6-Me2pyxn)]OTf,46b which all adopt a cis
configuration. In addition, the cis configuration observed for
these Fe and Mn complexes of 6-Me2pyxn exclusively belongs
to cis-β, in contrast to the cis-α configuration found for cis-2 in
this work. Thus, the d2 cis-ReV(O)2 unit appears to have a
significant effect on the coordination mode of the 6-Me2pyxn
ligand.
a
b
Values adopted from ref 52. Product yield calculated on the basis of
1′ acting as a one-electron oxidant and oxidation of CHD to benzene
as a two-electron process, 9,10-DHA to anthracene as a two-electron
process, and xanthene to xanthone as a four-electron process.
DISCUSSION
■
There have been extensive works on trans-dioxo d2 metal
complexes,1,2b,i,j,3−10 whereas their cis-dioxo counterparts have
been comparatively less explored, with the octahedral species
having a few well-characterized examples.11−17 Interesting types
of cis-dioxo d2 metal complexes to be developed are the
octahedral species bearing chiral tetradentate ligands currently
employed in asymmetric oxidation reactions, such as the
neutral C2-symmetric N4 ligands pyxn, pdp, and bqcn and their
derivatives, which have been increasingly used in iron and
manganese catalysis.29−31 We have endeavored to try to explore
a synthetic route to chiral d2 cis-[M(O)2(N4)]n+ (M = Fe, Mn)
species; such attempts, however, have not been successful. In
the literature, there have been a lack of isolated examples for cis-
FeVIO2 and cis-MnVO2 complexes, owing to their high
reactivity/instability. Nam and co-workers reported the
generation of the monooxo d3 manganese(IV) complex
[MnIV(O)(OH2)(bqcn)]2+ from [MnII(OTf)2(bqcn)] by oxi-
dation with cerium(IV) ammonium nitrate in the presence of
water;43a this monooxo complex can also be generated by
oxidation with [RuIII(bpy)3]3+ in aqueous solution or by
photocatalytic generation with water as an oxygen source, as
reported by Nam and Sun.43b In view of the evidence, provided
by ESI-MS analysis, for the formation of cis-[MnV(O)2(S,S-
bqcn)]+ intermediate in the reaction of [MnII(Cl)2(S,S-bqcn)]
with Oxone31b and the periodic relationship between Mn and
Re, together with the fact that high-valent Re-oxo complexes
are relatively nonoxidizing and more stable, Re was the metal of
choice in this work for the generation of relatively stable cis-
dioxo metal complexes having d2 (and also d1) electronic
configurations supported by tetradentate N4 ligands.
Note that, in addition to cis-[MnV(O)2(S,S-bqcn)]+ bearing
the C2-symmetric N4 ligand bqcn, a cis-[M(O)2(N4)]n+
intermediate with another type of neutral N4 ligand, N,N′-
dimethyl-2,11-diaza[3.3](2,6)pyridinophane (L-N4Me4), i.e. cis-
[FeV(O)2(L-N4Me4)]+, was previously proposed, with support
by ESI-MS analysis and DFT calculations, in cis-
[FeIII(Cl)2(N4)]+-catalyzed cis-dihydroxylation of alkenes with
Oxone.44 This cis-FeV(O)2 species was also highly reactive and
has not been isolated.
Complexes trans-2 and cis-2 bearing the same ligand
contribute a unique pair of structurally characterized trans
and cis isomers of d2 ReV(O)2 complexes. In previously
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Article
reported cis-ReV(O)2 complexes,15−17,23−27 the ligands em-
ployed are different from those in the trans-ReV(O)2 complex-
es.3,4,5a,b,6c,7g,8−10 For example, trans-[ReV(py)4(O)2]+ 3 bears
monodentate pyridine ligands, whereas a bidentate bpy ligand
was used for isolation of the corresponding cis analogue (cis-
[ReV(O)2(bpy)(py)2]+).15
values. The sensitivity of the MLCT transition energy and
reversibility of the spectral changes of cis-dioxorhenium(V)
with pH suggest that the latter may be used as a colorometric
pH sensor.
The ReVI/V potentials and basicity of the Re(V) complexes
(in aqueous medium) are valuable information, as they allow
access to the hydrogen-atom affinity of cis-[ReVI(O)2(N4)]2+
complexes. According to the thermochemical method devel-
oped by Mayer and Bordwell (eq 7),47 the bond dissociation
energies (DO−H) of cis-[ReV(O)(O−H)(L)]2+ to form cis-
[ReVI(O)2(L)]2+ (L = pyxn, PDP, bqcn) are calculated to be
74.5 (1 and 3, Scheme 1) and 75.1 kcal mol−1 (5). From these,
Electrochemical studies revealed some interesting properties
of the cis-dioxorhenium(V) complexes. In nonaqueous medium
(acetonitrile), complexes 1, cis-2, and 3−5 all show a ReVI/V
oxidation couple at relatively low potentials (0.39−0.49 V vs
SCE), being ca. 0.3 V less than that for the oxidation of cis-
[ReV(O)2(bpy)(py)2]+ (0.77 V vs SCE).15c In comparison to
the latter complex, the greater stabilization of the ReVI
oxidation state in the cis-[ReVI(O)2(N4)]2+ complexes is
ascribed to the stronger σ-donor strength of tertiary amine
N-donor atoms. Meyer and co-workers previously showed that
cis-[MVI(O)2(bpy)2]2+ (M = Ru, Os) complexes are accessible
through successive proton-coupled electron transfer (PCET)
reactions of cis-[MII(OH2)2(bpy)2]2+.18a For 1, 3, and 5, in
Scheme 1. Thermochemical Cycle of cis-[ReVI(O)2(pyxn)]2+
(1′)
aqueous media, three PCET reactions corresponding to ReVI/V
,
ReV/III, and ReIII/II couples are observed at pH 1 over a potential
range of ca. 1.2 V. The pKa values of cis-[ReV(O)2(N4)]+/cis-
[ReV(OH)(O)(N4)]2+ (3.3 for 1 and 3, 2.6 for 5) are higher
than that of cis-[ReV(O)2(bpy)(py)2]+/cis-[ReV(OH)(O)-
(bpy)(py)2]2+ (pKa < 1, based on the Pourbaix diagram
showing ReVI/V potential independent of pH in the range of 1−
11,15b i.e. protonation of cis-[ReV(O)2(bpy)(py)2]+ to give cis-
[ReV(O)(OH)(bpy)(py)2]2+ was not observed over this pH
range), which is attributable to stronger σ donation of Namine in
comparison to that of Npy. The present finding shows that the
oxo ligands of 1, 3, and 5 are more basic in comparison with
those of cis-[ReV(O)2(bpy)(py)2]+. For an additional compar-
ison, the pKa value of the trans-[ReV(O)2(4-NMe2py)4]+/trans-
[ReV(OH)(O)(4-NMe2py)4]2+ proton-transfer reaction was
determined to be 2.3 by a spectrophotometric titration
method.15c The basicity of the oxo ligands is reflected in the
reaction of 1 with Lewis acids. Upon treatment of a
stoichiometric amount of a strong Lewis acid such as ScIII or
a protic acid such as CF3SO3H, 1 is readily protonated to give
cis-[ReV(O)(OH)(pyxn)]2+ (1·H+), which has been isolated
and structurally characterized.
The ease of protonation of 1 in both nonaqueous and
aqueous media gives rise to interesting spectroscopic
observations of the acid−base pair. Even though there is only
a slight difference in the Re−O distances of 1 (1.750(5) and
1.753(5) Å) and 1·H+ (1.788(5) Å), tremendous differences
exist in both the UV−vis absorption spectra and the
electrochemistry. Upon protonation, the MLCT transition
band of 1 substantially blue-shifts from 465−580 to 309 nm in
acetonitrile (Figure 17) or from 429−584 to 307 nm in
aqueous solutions (Figure 18), suggesting significant stabiliza-
tion of the dπ(Re)-based orbitals as a result of breaking one
π(Re−O) bond (Figure 8); the extent of the blue shift is even
comparable to that resulting from one-electron oxidation of 1
to 1′. In accordance with this, the cyclic voltammogram of 1·H+
in acetonitrile shows a reversible Re(V)/Re(IV) couple at
−0.31 V vs SCE (Figure 7), which is otherwise not observed for
1. In addition, the Re(VI)/Re(V) oxidation potential shifted
anodically by more than 0.5 V. Such features are unprecedented
for cis-dioxorhenium(V) complexes.15−17 As depicted in Figure
18, 1·OTf exhibits reversible pH-dependent absorption profiles,
with varying relative intensities of the absorption bands at 429
nm (for 1) and 307 nm (for 1·H+) at moderately acidic pH
the cis-[ReVI(O)2(N4)]2+ complexes are estimated to be mild
hydrogen-atom abstractors with slightly smaller DO−H values in
48
comparison to permanganate (MnVIIO4 ; 80 kcal mol−1).
−
DO−H = 23.06E + 1.37pKa + C
(7)
In accordance with the calculated hydrogen-atom affinity, cis-
[ReVI(O)2(pyxn)]2+ (1′) was found to stoichiometrically
oxidize hydrocarbons with weak C−H bonds (BDE: 75.5−
76.3 kcal mol−1, three examples). A primary kinetic isotope
effect (KIE) of 4.9−5.8 in intermolecular competitions is
indicative of hydrogen atom abstraction at the rate-determining
step or product-determining step.49 Notably, substrates with
C−H bond dissociation energies of >80 kcal mol−1 (e.g.,
fluorene, 82 kcal mol−1; tetralin, 82.9 kcal mol−1) could not be
oxidized effectively.
Relative to trans-dioxo d2 complexes,1,2b,i,j,3−10 there have
been limited reports on the electronic spectroscopy of cis-dioxo
d2 complexes. Theoretically, there are two singlet transitions
within the Re(5dπ) orbitals for cis-dioxo d2 metal complexes
1
having C2v symmetry, which are [dxz(Re) → dyz(Re)] and
1[dxz(Re) → dxy(Re)], respectively. On the other hand, there is
1
only one [dxy(Re) → π*(d(Re)-p(O))] transition for trans-
dioxo d2 metal complexes with D4h symmetry. According to the
conventional molecular orbital diagrams for C2v and D4h
systems,50 the d−d splitting is expected to be smaller in cis-
dioxo metal complexes than in trans-dioxo metal complexes. In
the UV−vis absorption spectra of cis-[ReV(O)2(N4)]+ com-
1
plexes, the [dxz(Re) → dyz(Re)] transition can be assigned to
the absorption band at 680−850 nm with ε values of 450−1730
1
dm3 mol−1 cm−1, while the [dxz(Re) → dxy(Re)] transition
contributes to the two absorption bands at 400−700 nm with ε
values of 700−6050 dm3 mol−1 cm−1, the latter have substantial
1[dxz(Re) → π*(L)] character. In the case of trans-2, the
1[dxy(Re) → π*(d(Re)-p(O))] transition is mixed with a
1[dxy(Re) → π*(py)] MLCT transition (355 nm, ε = 1660 dm3
mol−1 cm−1). These mixed ligand-field and MLCT transitions
are not suitable for direct comparison. Therefore, only the
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1
wavelengths of the [dxz(Re) → dyz(Re)] transition of cis-
on the experimentally observed electronic absorption proper-
ties of the cis-dioxorhenium(V) complexes and cis-
[ReVI(O)2(pyxn)]2+.
[ReV(O)2(N4)]+ are used to compare with that of the 1[dxy(Re)
→ π*(d(Re)-p(O))] transition for trans-[ReV(O)2(py)4]+,
trans-[ReV(O)2(en)2]+, and trans-[ReV(O)2(CN)4]3− (350−
600 nm; ε = 50−1200 dm3 mol−1cm−1);4b this matches with
the hypothesis that the lowest-energy d−d transition in cis-
dioxo d2 metal complexes is smaller than that in trans-dioxo
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02404.
1
counterparts. In addition, the [pπ(O) → dπ*(Re)] LMCT
absorption band at 250−400 nm of trans-dioxorhenium(V)
complexes shows characteristic vibronic structure with vibra-
tional progressions of 600−800 cm−1 corresponding to excited
state νasym(O−Re−O) stretches.4b On the basis of TDDFT
Experimental procedures, characterization data, computa-
tional details and results, Figures S1−S27, and Tables
S1−S42 (PDF)
1
results, the calculated LMCT [pπ(O) → π*(dπ(Re)-p(O))]
transitions of cis-[ReV(O)2(N4)]+ complexes are at ca. 240−280
nm but are difficult to observe in the experimental electronic
absorption spectra due to the presence of intense ligand-
centered absorption bands of the N4 ligand at similar energies.
Structural and UV−vis absorption spectral data of cis-dioxo
d1 metal complexes19−22 are also sparse in the literature. The
cis-dioxo d1 complex 1′ and its cis-dioxo d2 counterpart 1 bear
the same tetradentate pyxn ligand, and both adopt a pseudo-
octahedral geometry. Isolation of the cis-ReVI(O)2 complex 1′
was realized by electrochemical oxidation of the cis-ReV(O)2
complex 1. Previously, Mayer and co-workers prepared cis-
[(Tp)ReVI(O)2(X)] (X = Cl−, Br−, I−), the only prior examples
of octahedral d1 cis-dioxo Re(VI), by chemical oxidation of the
monooxo complexes [(Tp)ReV(O)(X)(OTf)] with pyridine N-
oxide.22 The crystal structure of 1′, in comparison to that of 1,
reveals shorter Re−O distances and a smaller O−Re−O angle
attributable to removal of one electron from the antibonding
molecular orbital of dxz(Re) and pπ(O). Such an observation is
in line with the slightly increased calculated Re−O bond orders
of 2.2963 and 2.2956 based on NBO analyses of 1′ (Table S18
in the Supporting Information) relative to those of 2.2508 and
2.2505 for 1 (Table S17 in the Supporting Information). In
contrast, the Re−O distances of trans-[Re(O)2(dmap)4]n+
(dmap = 4-(dimethylamino)pyridine) are virtually the same
in both the ReV and ReVI oxidation states (n = 1 and 2,
respectively), as the HOMO dxy orbital is nonbonding in trans-
dioxo d2 complexes with D4h symmetry.51 In the UV−vis
absorption spectrum of 1′ (ReVI), in comparison to that of 1
(ReV), the MLCT [dxz(Re) → π*(py)] transition bands
significantly blue shift to λabs <400 nm. This finding suggests
that the dπ(Re) orbitals of 1′ are at lower energies in
comparison to those of 1, resulting from the stronger electron
affinity of the Re(VI) ion. Similarly, the d−d transition band
also blue-shifts from 800 nm (for 1) to 591 nm (for 1′),
indicating a larger d−d splitting that arises from the oxidation
of Re(V) to Re(VI).
Accession Codes
CCDC 1524881, 1551929−1551934, and 1560139 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail for C.-M.C.: cmche@hku.hk.
■
ORCID
Xiaoyong Chang: 0000-0003-1394-6164
Jie-Sheng Huang: 0000-0003-1724-4362
Chi-Ming Che: 0000-0002-2554-7219
Author Contributions
∥V.Y.-M.N. and C.-W.T. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Hong Kong Research Grants
Council (HKU 700813, 17303815), the National Key Basic
Research Program of China (No. 2013CB834802), and the
B as i c R e s e a r c h P r o g r a m o f S h e n z h e n (N o .
JCYJ20160229123546997). This research was conducted in
part using the research computing facilities and/or advisory
services offered by Information Technology Services, The
University of Hong Kong, and the HKU ITS research
computing facilities that are supported in part by the Hong
Kong UGC Special Equipment Grant (SEG HKU09).
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■
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