Helical oxidovanadium(IV) salen-type complexes: Synthesis, characterisation and catalytic behaviour

Article abstract of DOI:10.1002/ejic.201300635

Chiral [VO(salen)] complexes (R,R)-7 and (R,R)-8 were synthesised by ligand replacement from [VO(acac)₂] with the resolved (1R,2R)-cyclohexyl ligands 3,3′-[(1R,2R)-1,2-cyclohexane-diylbis(nitrilomethylidyne)]bis-4- phenanthrenol [(R,R)-3] and 3,3′-[(1R,2R)-1,2- cyclohexanediylbis(nitrilomethylidyne)]bisbenz[a]anthracen-1-ol [(R,R)-4], respectively. Similarly, complexes (R)-9 and (R)-10 were synthesised using the (R)-binaphthyl ligands 3,3′-[(1R)-(1,1′-binaphthalene)-2,2′- diylbis(nitrilomethylidyne)]bis-4-phenanthrenol [(R)-5] and 3,3′-[(1R)-(1, 1′-binaphthalene)-2,2′-diylbis(nitrilomethylidyne)]bis-benz[a] anthracen-1-ol [(R)-6], respectively. These complexes were characterised by IR spectroscopy, ESI-TOF-MS, electronic absorption spectroscopy and circular dichroism (CD). Single crystal X-ray analysis of (R,R)-7 and (R,R)-8 revealed distorted square pyramidal geometries and a 1:1 ratio of diastereomeric M and P helical conformers. Solution CD studies in THF combined with DFT calculations indicate that the M conformer dominates in solution for (R,R)-7. Preliminary catalytic oxidation of thioanisole with H₂O₂ and 1 % [VO(salen)] showed good selectivity for sulfoxides over sulfones but low enantioselectivities (8 to 33 %) which are solvent dependent but insensitive to the catalyst used. Chiral helical [VO(salen)] complexes have been synthesised, characterised and examined as catalysts for the asymmetric sulfoxidation of thioanisole with 30 % hydrogen peroxide. Copyright

Full text of DOI:10.1002/ejic.201300635

FULL PAPER
DOI:10.1002/ejic.201300635
Helical Oxidovanadium(IV) Salen-Type Complexes:
Synthesis, Characterisation and Catalytic Behaviour
[
a]
[a]
[a]
Sanmitra Barman, Smita Patil, John Desper,
[a]
[a]
Christine M. Aikens, and Christopher J. Levy*
Keywords: Helical structures / Metallofoldamers / Vanadium / Schiff base ligands / Circular dichroism
Chiral [VO(salen)] complexes (R,R)-7 and (R,R)-8 were syn-
thesised by ligand replacement from [VO(acac) ] with the re-
solved (1R,2R)-cyclohexyl ligands 3,3Ј-[(1R,2R)-1,2-cyclohex-
ane-diylbis(nitrilomethylidyne)]bis-4-phenanthrenol [(R,R)-
complexes were characterised by IR spectroscopy, ESI-TOF-
MS, electronic absorption spectroscopy and circular dichro-
ism (CD). Single crystal X-ray analysis of (R,R)-7 and (R,R)-8
revealed distorted square pyramidal geometries and a 1:1 ra-
2
3] and 3,3Ј-[(1R,2R)-1,2-cyclohexanediylbis(nitrilomethylidy- tio of diastereomeric M and P helical conformers. Solution
ne)]bisbenz[a]anthracen-1-ol [(R,R)-4], respectively. Simi-
larly, complexes (R)-9 and (R)-10 were synthesised using the
CD studies in THF combined with DFT calculations indicate
that the M conformer dominates in solution for (R,R)-7. Pre-
(
R)-binaphthyl ligands 3,3Ј-[(1R)-(1,1Ј-binaphthalene)-2,2Ј-
diylbis(nitrilomethylidyne)]bis-4-phenanthrenol [(R)-5] and
,3Ј-[(1R)-(1,1Ј-binaphthalene)-2,2Ј-diylbis(nitrilomethyl-
2 2
liminary catalytic oxidation of thioanisole with H O and 1%
[VO(salen)] showed good selectivity for sulfoxides over sulf-
ones but low enantioselectivities (8 to 33%) which are sol-
vent dependent but insensitive to the catalyst used.
3
idyne)]bis-benz[a]anthracen-1-ol [(R)-6], respectively. These
Introduction
soluble highly twisted salen ligand system, the metal com-
[
7]
plexes of which gave sulfoxide ee values up to 98%. In the
current study helical vanadyl salen complexes are reported
and their characterisation and use as catalysts for the asym-
metric oxidation of thioanisole with aqueous hydrogen per-
oxide are discussed.
Catalytic asymmetric oxidations are important reactions
[
1]
in the fine chemical and pharmaceutical industries. Many
medicinally useful compounds have chiral sulfoxide moie-
ties in their active structures,[ yet high enantioselectivity
in the catalytic oxidation of sulfides to sulfoxides has
proven to be difficult. Improved selectivity and the develop-
ment of methods that have reduced environmental impact,
such as through the use of benign oxidants, are important
2]
Results and Discussion
[
3]
goals in this field.
Synthesis
Salen and related Schiff base ligands with a central unit
backbone) derived from a chiral diamine have been studied
(
The resolved diamines (1R,2R)-diaminocyclohexane
[
4]
as catalysts for asymmetric sulfoxidations. Nakajima et al.
used organic peroxides and V O(salen) complexes derived
from (1R,2R)-diaminocyclohexane to oxidise prochiral sulf-
(DAC) and (R)-BINAM were condensed with the phen-
IV
anthryl and benz[a]anthryl aldehydes 1 and 2 (Scheme 1)
to give four tetradentate salen ligands, 3–6, as previously
[
5]
ides to sulfoxides and achieved up to 42% ee. Corre-
[8]
reported by our group. Metallation of the ligands with
V
+
sponding V O(salen) complexes gave much lower levels of
asymmetric induction. Adão et al. used several VO(salen)
vanadyl acetylacetonate in the presence of excess base
(NaOMe) produced the vanadyl complexes 7–10 [Equa-
and VO(salan) complexes to oxidise thioanisole with hydro-
gen peroxide and achieved up to 50% ee.[
4f,4g]
Bolm et al.
improved the selectivity (up to 85% ee) by incorporating
bulky tert-butyl groups in the ortho positions and electron-
withdrawing nitro groups at the para positions of the aro-
[
6]
matic rings. Recently, Katsuki et al. introduced a water-
[
a] Dept. of Chemistry, Kansas State University,
2
13 CBC Bldg., Manhattan, KS 66506, USA
E-mail: clevy@ksu.edu
http://www.k-state.edu/chem/people/faculty/levy.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300635.
Scheme 1. Ligand precursors used in this work.
5708
Eur. J. Inorg. Chem. 2013, 5708–5717
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FULL PAPER
tions (1), (2), (3) and (4)]. A 2:1 mixture of CH Cl and Characterisation of the Complexes
2
2
EtOH provided sufficient solubility for the deprotonated li-
The compositions of the complexes were determined by
combustion analysis with mass spectrometry confirming the
molecular species (vide infra). Single crystal X-ray struc-
tural analysis was also obtained for (R,R)-7 and (R,R)-8.
IR spectroscopy identified the V=O stretches for the com-
gand and VO(acac) to react in a reasonable amount of
2
time. The excess base resulted in better yields and higher
initial purities. Vanadyl complexes were normally purified
by Soxhlet extraction with CH Cl which was effective at
2
2
dissolving the products away from the NaOMe, Na(acac)
–1
plexes in the 1008–1036 cm range, consistent with pre-
and any unreacted VO(acac) . Complexes (R,R)-7 and
2
[9] 1
vious studies.
H NMR spectra of all the complexes had
(
(
R,R)-8 were isolated as green powders whereas (R)-9 and
R)-10 were brown powders.
IV
broadened signals due to the paramagnetic V centre and
did not provide significant structural information. EPR
data confirmed the paramagnetic nature of the complexes.
Details of the IR, UV/Vis and electronic CD spectroscopic
studies are presented below.
The high-resolution ESI-TOF mass data (Table 1) of
+
+
these complexes showed the M and/or M + H molecular
+
ion peaks and expected isotopic patterns. The dimer M2
was observed for (R,R)-7 and (R,R)-8, and this is likely the
result of μ-oxido bridging of vanadium centres in the gas
phase. The mass data for (R)-9 and (R)-10 provide strong
evidence for the proposed molecular compositions of these
complexes in the absence of crystal structures.
Table 1. ESI-TOF-MS data for vanadyl complexes.
Ion
Formula
Mass
(
Error
(ppm)
obs)[
a]
[b]
[
[
[
[
[
[
[
[
[
(R,R)-7]+
C
36
C
36
C
72
C
44
C
44
C
88
C
50
C
50
C
58
H
28
H
29
H
54
H
32
H
33
H
64
H
30
H
31
H
34
N
2
N
2
N
4
N
2
N
2
N
4
N
2
N
2
N
2
O
3
O
3
O
6
O
3
O
3
O
6
O
3
O
3
O
3
V
V
V
V
V
V
V
V
V
587.1567
588.1624
1174.3140
687.1856
688.1919
1374.3749
757.1611
758.2069
857.2039
4.6
1.0
5.2
0.4
1.7
3.2
11.2
38.9
3.5
+
+
(R,R)-7 + H]
+
(R,R)-7]
(R,R)-8]
2
2
2
+
(R,R)-8 + H]
+
(R,R)-8]
2
(R)-9]+
+
(R)-9 + H]
(R)-10]+
6
[
a] Mass in Daltons. [b] Error = 10 ·(measured mass – theoretical
mass)/theoretical mass.
X-ray Crystal Structures of Complexes (R,R)-7 and (R,R)-8
Single crystals of (R,R)-7 and (R,R)-8 were grown by
slow solvent diffusion of hexane into dichloromethane solu-
tions of the complexes. The crystal data are presented in
Table 2. In each case, the salen ligand is bound in a tetra-
dentate manner to the vanadium centre and two dia-
stereomeric helical conformers (P and M) are present in a
1:1 ratio. Selected metric parameters for the two structures
are presented in Table 3.
The thermal ellipsoid plot of the M conformer of (R,R)-
7
is presented in Figure 1. The geometry is approximately
square pyramidal and vanadium sits above the ONNO
plane by 0.58 Å. Reedijk’s structural index parameter, τ =
0
.29, indicates a small distortion towards trigonal bipyrami-
[
10]
dal geometry. Here N52–V1–O12 (136.2°) is substantially
smaller than N51–V1–O32 (153.8°), at least partially due to
the steric repulsion between the phenanthryl groups.
The thermal ellipsoid plot of the P conformer of (R,R)-
7
is presented in Figure 2. The vanadium sits above the
ONNO plane by 0.57 Å, similar to that seen for the M con-
Eur. J. Inorg. Chem. 2013, 5708–5717
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Table 2. Crystal data for (R,R)-7 and (R,R)-8.
Table 3. Selected bond lengths, bond angles, torsion angles and cal-
culated parameters for (R,R)-7.[
a]
Compound
(R,R)-7
2 2
(R,R)-8·CH Cl
Bond Length [Å]
(R,R)-7
M isomer P isomer
(R,R)-7
(R,R)-8
(R,R)-8
P isomers
Empirical formula
C
587.54
36
H
28
N
2
O
3
V
1
45 2 2 3 1
C H34Cl N O V
772.58
M isomers[
b]
[b]
M
r
Crystal system
a [Å]
b [Å]
c [Å]
α [°]
β [°]
γ [°]
Unit cell vol. [Å ]
Space group
Z
monoclinic
9.668(3)
10.827(3)
25.450(7)
90
99.717(14)
90
2625.8(13)
P2(1)
4
triclinic
V–Oax[c]
V–O12
V–O32
V–N51
1.581(4)
1.930(3)
1.944(4)
2.030(4)
2.038(4)
1.287(7)
1.271(7)
1.468(5)
1.494(6)
1.574(4)
1.942(3)
1.935(4)
2.052(4)
2.032(4)
1.261(7)
1.279(7)
1.478(5)
1.469(6)
1.598[27]
1.924[3]
1.923[10]
2.051[14]
2.066[5]
1.293[3]
1.290[9]
1.495[6]
1.485[10]
1.590[27]
1.932[24]
1.930[10]
2.028[20]
2.053[2]
1.317[16]
1.298[18]
1.503[33]
1.470[1]
12.0364(9)
16.8357(12)
17.9974(13)
89.1610(10)
77.6150(10)
85.9340(10)
3553.2(4)
P1
V–N52
C25/29ϪN51[
C45/49ϪN52[
C51ϪN51
C52ϪN52
d]
d]
3
4
Angles [°]
T [K] –
173(2)
0.422
100(2)
0.476
1
μ [mm ]
O
O
O
O
a
–V–O12
112.84(18) 107.05(18) 109.3[93]
106.58(18) 109.48(17) 108.4[24]
108.0[41]
108.3[11]
106.4[18]
106.2[37]
87.7[38]
87.1[3]
N
24584
10225
37413
23786
axϪV–O32
ax–V–N51
ax–V–N52
N
ind
98.7(2)
104.5(2)
106.8[6]
R
R
wR
int
0.0695
0.0579
0.1553
0.0830
0.1752
1.012
0.0384
0.0690
0.1670
0.1107
0.1905
1.068
0.02(3)
109.89(18) 104.62(18) 105.9[42]
[
a]
1
[I Ͼ 2σ(I)]
O12–V–O32
O12–V–N51
O32–V–N52
N51–V–N52
O32–V–N51
O12–V–N52
V–O12ϪC12
V–O32ϪC32
V–N51ϪC25/29[
V–N52ϪC45/49[
N52–C52–
90.16(16)
86.34 (15) 85.40(14)
86.59(16)
78.25(17)
91.07(16)
86.85[23]
87.15[7]
86.8[9]
[
a]
2
[I Ͼ 2σ(I)]
(all data)
(all data)
R
1
86.85(16)
78.07(16)
86.4[4]
wR
GoF
Flack parameter
2
78.6[8]
78.8[17]
145.0[27]
145.5[0]
131.6[19]
131.6[24]
126.7[15]
125.8[3]
153.82(17) 145.37(18) 144.4[84]
136.21(16) 147.04(16) 145[13]
0.03(3)
133.4(3)
131.1(3)
126.5(3)
126.1(3)
42.4(5)
130.3(3)
132.7(3)
122.4(3)
127.1(3)
44.1(5)
131.8[16]
130.0[59]
126.4[4]
2
[
F
1 o c o o o 2 o
a] R = Σ||F | – |F ||/Σ|F | for F Ͼ 2σ(F ) and wR = {Σ[w(F –
) ]/Σ[w(F )]} .
c c
2
2
2
1/2
d]
d]
125.5[11]
C51ϪN51
44.6[1]
41.2[44]
21.6[47]
14.2[5]
[e]
arm-arm
32.7
3.96
5.54
29.4
7.52
4.19
22.0[55]
17.4[45]
22.0[43]
[
f]
curv. A
curv. B[
f]
19.9[25]
[a] (R,R)-7: vanadium is V1 for M and V2 for P, (R,R)-8: vanadium
is V1 and V2 for M and V3 and V4 for P. [b] For (R,R)-8 the
average values (VC1 and VC2 for M, VC3 and VC4 for P) are given
with the difference between the two values in brackets. [c] Oax is
the axial oxygen (the numbering changes depending on the mole-
cule). [d] C25 and C45 for (R,R)-7, C29 and C49 for (R,R)-8. [e]
The absolute value of the angle between the mean planes for the
phenanthryl or benz[a]anthryl groups: C11-C24 and C31–C44 for
(
R,R)-7, C11–C28 and C31–C48 for (R,R)-8. [f] Curvature of the
phenanthryl group defined as the angle between the first and last
ring planes in the system. Curv. A is for the ring curved toward
C
6
the axial oxido group and curv. B is for the ring curved away from
the axial oxido group.
Figure 1. Thermal ellipsoid plots (50%) for the M helical isomer
of (R,R)-7. This is designated as molecule 1 in the CIF file and the
labels carry the _1 suffix. Hydrogens are omitted for clarity.
lengths consistent with C=N bonds. The angles between the
salen donor atoms (D) and the oxido group (Oxido-V–D)
former. On the other hand τ = 0.03 indicates essentially no are all significantly larger than 90°, consistent with a metal
trigonal-bipyramidal character. Here the basal angles are centre that is sitting well above the basal plane. The N51–
nearly the same and the geometry is square pyramidal, with V–N52 angle of the five-membered chelate ring in each iso-
the vanadium well above the plane. The shapes of the two mer is substantially smaller than 90°, while the N–V–O
isomers are shown more clearly by the space-filling repre- angles of the six-membered chelates are (86Ϯ1)°. The
sentations in Figure 3. The isomers are only pseudo-mirror angles are similar to what has been seen for other five-coor-
[
16]
images since both contain the (1R,2R)-cyclohexyl group.
dinate complexes of this ligand.
The angle between the averaged planes of the phenan-
Selected bond lengths, bond angles, torsion angles and
calculated parameters for (R,R)-7 are presented in Table 3. thryl rings is 32.7° for the M isomer and 29.4° for the P
The vanadium–oxido bond lengths are consistent with isomer. These values are intermediate between what was
those seen for other VO(salen) complexes.[ The vana- seen for [M (L)(py)] (47.9–54.0° M = Zn, Fe) and [Fe (L)-
dium–phenoxido bonds are significantly shorter that the va- Cl] (11.2–12.0°).[16] The angles correlate with the strength
nadium–imine bonds, consistent with the anionic nature of of ligand bonding as measured by the M–donor distances.
the phenoxido ligands. The imine groups (C25–N51) have For a given complex, the phenanthryl groups have a smaller
11]
II
III
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Figure 2. Thermal ellipsoid plots (50%) for the P helical isomer of
(
R,R)-7. This is designated as molecule 2 in the CIF file and the
labels carry the _2 suffix. Hydrogens are omitted for clarity.
Figure 4. View down the a cell axis for (R,R)-7. The P isomers have
a grey carbon framework and the M isomers have a black carbon
framework.
Figure 3. Space-filling plots (50%) of the diastereomeric helical iso-
mers of (R,R)-7.
interplanar angle as their termini are drawn together by
stronger ligand binding and they are forced into parallel
II
stacking. Weaker binding, such as seen with Zn , results
is reduced steric repulsion and greater freedom for larger
interplanar angles between the phenanthryl groups.
Figure 5. View of one molecular stack viewed along the [111] direc-
Curvature of fused aromatic systems is common if sig- tion of (R,R)-7. The P isomers (bottom row) have a grey carbon
framework and the M isomers (top row) have a black carbon
nificant inter- or intramolecular interactions are present.
The phenanthryl groups in (R,R)-7 show modest curvature
framework.
(4.19–7.52°) as measured by the angle between the calcu-
lated planes for the first and third rings in the carbon dal with the vanadium sitting above the plane (M, 0.577
framework. In the crystal lattice the molecules are arranged and 0.623 Å; P, 0.574 and 0.599 Å). The M isomer shows
in stacks along the a axis. Figure 4 shows nine pairings, greater distortion (τ = 0.18 for molecules 1 and 2) than does
each consisting of columns of M and P isomers that are the P isomer (τ = 0.01, 0.03) as was seen for (R,R)-7. The
interleaved. The relationship between the isomers in one of benz[a]anthryl groups are overlapping in each of the iso-
these stacks is shown in Figure 5 which looks along the mers (Figure 8) but there is no strong π-π interaction since
[
111] direction. For a given closest M/P pair the oxido the centroidϪC distances (3.86–4.30 Å) are outside the nor-
groups are anti and the cyclohexyl group on one isomer sits mal 3.4–3.6 Å range. The shortest distances are 3.89 Å
over the open area between the phenanthryl groups of the (VC1, M) and 3.86 Å (VC3, P). The benz[a]anthryl group
other.
The molecular structure of (R,R)-8 (Figure 6) with the the Zn and Fe complexes of this ligand.
benz[a]anthryl sidearm also showed a 1:1 mixture of helical The structures of (R,R)-7 and (R,R)-8 are similar but
relationships are intermediate between what was seen for
II
II
[8]
isomers. In this case there are four independent vanadyl with a few notable differences. On average, the bonds to
complexes in the unit cell (VC1–VC4, Figure 7) as well as vanadium in (R,R)-8 are longer for V–Oax (+0.017 Å) and
two dichloromethanes of solvation. VC1 and VC2 have the V–N (+0.012 Å) while the V–O (phenoxido) lengths are
M conformation while VC3 and VC4 have the P. Complexes shorter (–0.011 Å). This latter difference is somewhat sur-
of the same conformation have similar structures. The ge- prising, given the more sterically-demanding nature of the
ometry at the metal centres is approximately square pyrami- extended arms in (R,R)-8 and this points to stronger phen-
Eur. J. Inorg. Chem. 2013, 5708–5717
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ture of the aromatic system is larger by 13° on average for
the benz[a]anthryl arms: the range is 14.0 to 24.2° as mea-
sured by the angle between the calculated planes for the
first and fourth rings in the carbon framework. This is con-
sistent with a longer and more flexible aromatic system that
has significant steric repulsions on one face.
IR Spectra
IV
The coordination environment of the V O complexes is
normally reflected ny the V=O stretching frequency,
[
12]
νV=O
.
For octahedral complexes with similar tetradentate
ONNO ligands, the stretching frequency is lower due to the
expected weakening of the V=O bond by the trans axial
ligand. The V=O stretch for octahedral complexes generally
comes in the 970 to 1000 cm–1 range.
[13]
It has been seen
IV
that more electron-withdrawing ligands lead to V O com-
plexes with significantly lower νV=O values due to
·
··V=O···V=O··· interactions that weaken the V=O bond. A
Figure 6. Thermal ellipsoid plot (50% probability) of the M isomer
of (R,R)-8 (VC1).
V
series of V =O(salen)X complexes showed ν
in a nar-
V=O
row 960–990 cm–1 range.
[5]
For complexes (R,R)-7 and (R,R)-8, the V=O stretch is
–1
observed at 1036.8 and 1035.7 cm , respectively (Table 4).
These are characteristic of green-coloured monomeric
IV
square-pyramidal V O(salen) complexes studied pre-
[
14]
viously.
The νV=O values of the binaphthyl complexes
(R)-9 and (R)-10 are lower than seen for the cyclohexyl-
containing complexes (Table 4). These lower frequencies are
consistent with the more electron-withdrawing nature of the
binaphthyl unit and suggest that some ···V=O···V=O··· as-
[
13]
sociation may be present in these complexes.
Table 4. IR stretching frequencies and bond lengths of V=O and
C=N groups in complexes.[
a]
ν
V=O / cm–1
rV=O / Å
ν
C=N / cm–1
rC=N / Å
(
(
R,R)-7
R,R)-8
1036.8
1035.7
1016.0
1008.5
1.581(4)
1.584(5)
1608.0
1609.5
1613.0
1606.0
1.271 (7)
1.291(9)
Figure 7. Unit cell of (R,R)-8 (P1) showing the four independent
vanadyl complexes. The dichloromethanes of solvation have been
removed for clarity.
(R)-9
(R)-10
[
(
a] Crystallographic bond lengths are not available for (R)-9 and
R)-10.
EPR Spectra
The X-band EPR spectrum of (R,R)-7 was recorded at
room temperature in dichloromethane. A broad spectrum
1
with g = 2.0, typical of a paramagnetic d configuration, is
observed (Figure 9). The broadness is likely due to the rapid
interconversion between the M and the P isomers. For com-
1V
plex (R)-9, the X-band EPR spectrum of the V O complex
was collected in DMSO at 77 K (Figure 10) and the spec-
trum simulated using software from Rockenbauer and Ko-
Figure 8. Space-filling plots (50%) of the diastereomeric helical iso-
mers of (R,R)-8: M = VC1, P = VC3.
[
15]
recz. The system was simulated by considering the mole-
oxido donors for the benz[a]anthryl arms. The angle be- cule as having octahedral geometry where the fifth coordi-
tween the aromatic arms is smaller for (R,R)-8 than for nation site is occupied by the axial oxygen and the sixth
1V
(
R,R)-7 by an average of 9.3°. This reflects the fact that the coordination site by the DMSO. For the V O system, we
longer arms are forced in to a more parallel arrangement used the additivity rule to estimate the hyperfine coupling
est
in order to avoid strong edge to face repulsions. The curva- constant Aʈ , based on the contributions of A , of each of
ʈ i
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est
the four equatorial donor groups [Aʈ = ΣA , (i = 1 to 4)]. DMSO occupying the sixth coordination site. Among the
ʈ i
est
–4
–1
The estimated accuracy of the Aʈ isϮ3ϫ10 cm . The three possible geometric isomers, complex (R)-9 shows the
data can be used to establish the most probable binding best match with the β-cis geometry.
mode of the V complex. The influence of the axial ligand
1
V
was not taken into account, since it is not relevant in the
present system. Table 5 shows the comparison between the Solution Property Studies: Electronic and CD Spectra for
simulated and the experimental data. The complex was en- Complexes (R,R)-7 and (R,R)-8
visioned as being in an octahedral environment with
The UV/Vis and CD spectra of the free ligand, (R,R)-3,
[
16]
in THF
contain a signal for π to π* transitions from
the two major chromophores: the imine π bonds and the
phenanthryl sidearms. The negative Cotton peaks around
340 to 400 nm result from the low energy imine πǞπ* ab-
sorption as well as the sidearm L band. The corresponding
b
spectra for the complex (R,R)-7 in THF (Figure 7) contain
all the ligand based peaks, indicating that the ligand stays
intact. However, the ligand based negative Cotton peaks at
3
3
40 and 380 nm are shifted towards lower wavelengths of
10 and 360 nm, respectively, after complexation. The
broad negative peak at 440 nm arise from to the LMCT
transition in the complex. The negative sign of the lowest
energy peak at 440 nm is consistent with the (R,R) configu-
ration of the cyclohexyl diamine backbone seen in our pre-
[
8,16]
vious studies.
The cyclohexyl unit, such as in (R,R)-7 and (R,R)-8, is
not especially effective at directing diastereospecific helix
[
16,17]
formation of metallofoldamers.
The fact that a 1:1
Figure 9. X-band EPR spectrum of (R,R)-7 in dichloromethane at
room temperature.
mixture of M and P helical conformers is seen in the solid-
state for (R,R)-7 and (R,R)-8 is a clear indication of this. In
previous studies we have been able to use CD spectroscopy
to assess the solution constitution of complexes and found
that one helical form predominates.
the experimental spectrum is compared with the calculated
DFT) spectra of the M and P forms. For (R,R)-7, the ex-
[
8,12]
In this approach
(
perimental spectrum is a good match with that calculated
for the M form and a poor fit with the calculated P spec-
[
18]
trum (Figure 11). This points to M-(R,R)-7 as the domi-
nant species in solution, resulting from a shift in equilib-
rium away from the 1:1 mixture in the solid state
(Scheme 2). Bu et al. observed similar behaviour for a
VO(salen) complex with the (1R,2R)-cyclohexyl backbone
and bulky tert-pentyl substituents on the sidearms.[
11c]
Figure 10. X-band first derivative EPR spectrum of (R)-9 in
DMSO at 77 K.
Table 5. Spin-Hamiltonian parameters calculated for the EPR spec-
trum of (R)-9 in DMSO.
4
–1
4
–1
g
Ќ
A
Ќ
ϫ 10 cm
g
ʈ
A
ʈ
ϫ 10 cm
Binding mode
est
4
–1
(A
ʈ
ϫ 10 cm )
1.97 160
1.97 174
α-cis (165)
β-cis (174.3)
trans (160)
Figure 11. Experimental CD spectrum (THF) of (R,R)-7 and the
DFT calculated spectra of the M and P helical conformers.
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Scheme 3. Sulfoxidation of thioanisole with different catalysts.
2 2
Table 6. Sulfoxidation of thioanisole with 30% H O
.[a]
Catalyst Solvent[b]
Sulfoxide /
%
%
Sulfone /
%
Sulfide /
%
ee[
c]
(
R,R)-7 MeOH/
CHCl
R,R)-7 CH Cl
R,R)-8 MeOH/
CHCl
Cl
MeOH/
CHCl
CH Cl
MeOH/
CHCl
CH Cl
67
30
3
30
3
(
(
2
2
70
68
11
33
7
3
23
29
3
(
(
R,R)-8 CH
2
2
69
89
14
32
8
4
23
7
R)-9
3
(
(
R)-9
R)-10
2
2
91
89
13
33
2
3
7
8
3
(
R)-10
2
2
86
13
10
4
Scheme 2. Redistribution of metallofoldamers upon dissolution.
[
a] 16 h at 0 °C 1 mol.-% catalyst. [b] Two solvent systems: 5%
MeOH in CHCl and pure CH Cl . [c] ee of the product was mea-
3
2
2
Preliminary Results for Sulfoxidations by All the
Complexes
sured by HPLC analysis. HPLC analysis was carried out with a
Daicel OD-H column (2-propanol:hexane 5:95). Retention times
17.82 min for the R and 20.91 min for the S isomer. The major
The vanadyl complexes were examined as selective cata- enantiomer is R in all cases.
lysts for the asymmetric oxidation of a model sulfide,
thioanisole, with 30% aqueous hydrogen peroxide at 0 °C
(Scheme 3). Initial solvent screening indicated that the lysts (R)-9 and (R)-10 reached ca. 90% (Table 6). However,
selectivity was poor in acetonitrile (ee Ͻ 5% in all cases) the catalysts show little variation in enantioselectivity for
but improved in CH Cl and in 5% v/v methanol/chloro- a given set of conditions. Enantioselectivities in methanol/
2
2
form. In all cases the products were R enriched methyl chloroform were considerably higher (30–33% ee) than in
phenyl sulfoxide and sulfone (over-oxidation product). Con- CH Cl (11–14% ee). Still, the overall level of selectivity is
2
2
version essentially stopped after 15–16 h and the reactions fairly low in all cases, likely due to poor facial differentia-
were quenched with sodium sulfite. Conversion with the cy- tion in the sulfide association/oxygen transfer (Fig-
[
19,20]
clohexyl-containing catalysts (R,R)-7 and (R,R)-8 ap- ure 12).
This has been proposed to occur by means of
proached 70% while that of the binaphthyl-containing cata- sulfide coordination (not observed) or direct electrophilic
Figure 12. Proposed mechanism for stereoinduction in the catalytic oxidation of thioanisole with (R,R)-7.
Eur. J. Inorg. Chem. 2013, 5708–5717
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attack of the hydroperoxido group on the sulfur and this Beta Dex 120 30 mϫ0.25 mm capillary column. HPLC was carried
proposal is supported by computational results.[
A key aspect of the proposed mechanism is the proton-
ation and decoordination of a phenoxo donor of the VO-
21]
out using a Gilson 322 Pump and a Hewlett–Packard 1100 UV
detector with a standard flowcell. A Daicel Chiracel OD-H column
–
1
was used with 1.0 mLmin of 5% 2-propanol in hexanes as eluent.
EPR spectra of the vanadium complexes were collected by using a
Varian E-4 EPR spectrometer. FTIR spectra were collected on
powders with an Agilent Cary 630 spectrometer with a diamond
ATR attachment. Some spectra were also collected using KBr pel-
lets and a Nicolet 6700 spectrometer.
(salen) complex upon reaction with hydrogen peroxide. This
breaks the helical conformation of the complex and likely
reduces any impact the helical structure has on the selectiv-
ity of the catalyst. The observed enantioselectivities are not
significantly better than those with simpler vanadyl salen
catalysts or catalysts with related tridentate Schiff-base li-
High-resolution electrospray ionisation spectra were acquired on
a LCT Premier (Waters Corp., Milford MA) time of flight mass
spectrometer. The instrument was operated at 10000 resolution (W
mode) with dynamic range enhancement that attenuates large in-
tensity signals. The cone voltage was 60 eV. Spectra were acquired
at a 16666 Hz pusher frequency covering the mass range 100 to
[
4,5]
gands.
Conclusions
1200 u and accumulating data for 2 seconds per cycle. Mass correc-
Helical vanadyl salen complexes with the (1R,2R)-cyclo-
hexyl chiral unit do not have a strong thermodynamic pref-
tion for exact mass determinations was made automatically with
the lock mass feature in the MassLynx data system. A reference
erence for the M helical conformer over the P conformer, compound in an auxiliary sprayer can be sampled every third cycle
as evidenced by the 1:1 ratio of diastereomers seen in the by toggling a “shutter” between the analysis and reference needles.
solid-state structures. This is consistent with previous re- The reference mass is used for a linear mass correction of the ana-
lytical cycles. Samples were diluted in dichloromethane and deliv-
ered to the instrument in MeOH 0.1% formic acid through direct
sults from our group and others. However, solution CD
studies suggest that the M conformer is favoured in solu-
infusion. Lower resolution mass spectra were collected using a
tion, at least in the case of (R,R)-7. This behaviour was also
Bruker Ultraflex MALDI-TOF/TOF instrument.
II
II
III
seen for Zn , Fe and Fe complexes of these ligands and
has been discussed recently by Akine et al.[
22]
The Amsterdam Density Functional (ADF) program was used for
all DFT calculations.[ The Slater type full core basis sets em-
ployed are of polarised triple-zeta (TZP) quality. The asymptoti-
Complexes
23]
(R)-9 and (R)-10, which are based on the binaphthyl chiral
unit, are almost certainly M-helical, consistent with our
previous findings. Despite this, the stereoselectivity of these
catalysts for thioanisole oxidation with H O are not signif-
[
24,25]
cally correct statistical average of orbital potentials (SAOP)
was utilized in time-dependent density functional theory (TDDFT)
computations to determine energetics and oscillator strengths of
2
2
icantly better than for the catalysts with the cyclohexyl unit.
This implies that it is the poor facial selectivity of the cata-
excited states. Rotatory strengths were also computed using the
[26]
RESPONSE module in ADF.
The lowest 150 singlet excited
lysts, which is similar in all cases, rather than the conforma- states were evaluated. The circular dichroism spectra were fit with
tional flexibility that leads to poor enantioselectivity. Sol- a scaled Gaussian function with a width at half-maximum of
vent polarity and the presence of protic solvents have a 20 nm. When compared with experimental data, it is evident that
the energies are overestimated by an average of ca. 0.1 eV, as has
large impact on the catalytic outcome as well, although this
been noted previously for these functionals.[ In view of the over-
27]
is not surprising given the hydroperoxido species present in
estimation, a 0.1 eV correction was applied to the calculated ener-
the reaction pathway, which can participate in hydrogen-
gies.
bonding.
(
R,R)-7: Ligand (R,R)-3 (0.100 g, 0.192 mmol) and sodium meth-
oxide (0.103 g, 1.92 mmol) were dissolved into a mixture (2:1 ratio)
of dichloromethane (10.0 mL) and ethanol (5.0 mL). Oxidovanadi-
um(IV) acetylacetonate (0.0520 g, 0.192 mmol) was added to the
mixture and the solution was stirred for 16 h at room temperature.
The green precipitate was isolated by filtration, washed with dichlo-
romethane (5 mL) and dried in vacuo to yield (R,R)-7 (0.100 g,
89% yield). Single-crystals suitable for X-ray analysis were grown
by slow liquid-liquid diffusion: hexanes were layered over a solu-
tion of (R,R)-7 in dichloromethane in a sealed tube. IR ν˜ = 1032.45
Experimental Section
General: All reactions were carried out under inert atmospheres
unless otherwise noted. Tetrahydrofuran and dichloromethane were
dried with calcium hydride, ethanol was dried with magnesium,
and chloroform was dried with 4 Å molecular sieves. Solvents were
degassed and vacuum transferred into storage flasks prior to use.
The ligands were synthesised by previously reported methods.[
8,16]
–
1
+
+1
(
4
V=O) cm . ESI-TOF-MS: M (C36
H
28
+1
N
2
O
3
V) 587.1567 (δ =
V) 588.1624 (δ = 1.0 ppm),
1174.3140 (δ = 5.2 ppm). C36
587.57): calcd. C 63.54, H 3.79, N 4.01; found C 63.55, H 3.78, N
.00.
Elemental analyses were carried out by Desert Analytics of Tucson,
Arizona. UV/Vis spectra were obtained on a Varian Cary 500 spec-
trometer, and CD spectra (5 scans, 1.00 mm path length quartz cell)
on JASCO 720 or J-815 spectropolarimeters. Solution samples for
these two techniques were prepared using dried spectroscopic grade
+
29 2 3
.6 ppm), [M + H] , (C36H N O
+
+1
[M
2
]
(C72
H
54
N
4
O
6
V
2
)
28 2 3
H N O V
(
4
–
5
THF at concentrations that ranged between 1.5 and 2.5ϫ10 m.
(R,R)-8: Ligand (R,R)-4 (0.600 g, 0.965 mmol) and sodium meth-
oxide (0.521 g, 9.65 mmol) were dissolved into a mixture (2:1 ratio)
of dichloromethane (20.0 mL) and ethanol (10.0 mL). The solids
were dissolved by stirring. Oxidovanadium(IV) acetylacetonate
(0.261 g, 0.965 mmol) was added to the mixture and the solution
was stirred for 16 h at room temperature. The green precipitate was
isolated by filtration and purified by Soxhlet extraction with dichlo-
1
13
H and C NMR spectra were obtained on a Varian Unity
00 MHz spectrometer employing residual solvent protons or TMS
4
as an internal standard. Crystallographic data were collected using
either a Bruker SMART 1000 CCD or a Bruker-AXS SMART
APEX CCD diffractometer. Gas chromatographs were collected on
a Varian 3900 instrument with a 39XL FID detector and a Supelco
Eur. J. Inorg. Chem. 2013, 5708–5717
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romethane. The solvent was removed by vacuum transfer and the
resultant solid was dried in vacuo to give (R,R)-8 (0.49 g, 81%
Acknowledgments
+
+1
yield). ESI-TOF-MS:
M
(C44
H
32
+1
O
3
N
2
V)
V) 688.1919 (δ = 1.7 ppm),
1374.3749 (δ = 3.2 ppm). IR ν˜ = 1008.50
687.1856 (δ =
This work is supported by the US National Science Foundation
NSF) under grant number CHE-0349258 and by Kansas State
+
0
33 3 2
.4 ppm), [M + H] (C44H O N
(
+
+1
2 64 4 6 2
[M ] (C88H O N V )
University. J. Tomich is thanked for use of the CD spectroscopy
facilities, L. Maurmann for assistance with NMR and HPLC
analyses and A. Borovik for the EPR spectra. The authors also
wish to thank Dr. Todd D. Williams, Mr. Robert Drake and Mr.
Larry Seib of the University of Kansas mass spectrometry labora-
tory for their efforts in acquiring the ESI spectra. The Waters LCT
Premier mass spectrometer was purchased with support from the
National Institutes of Health (NIH) Shared Instrumentation Grant
Program (SIG S10 RR019398) (T. D. W.).
–1
(V=O), 1606.50 (C=C) cm . UV (in THF) shows a signal for
–5
LMCT at 350 nm. The CD spectrum (in THF at 10 m) shows the
presence of the M helix in the solution. X-ray quality crystals were
grown from the diffusion method from dichloromethane and hex-
ane. C44
32 2 3
H N O V (687.69): calcd. C 76.85, H 4.69, N 4.07; found
C 76.82, H 4.70, N 4.70.
(R)-9: Ligand (R)-5 (0.100 g, 0.145 mmol) and sodium methoxide
(0.086 g, 1.60 mmol) were dissolved into a mixture (2:1 ratio) of
dichloromethane (10.0 mL) and ethanol (5.0 mL). Oxidovanadi-
um(IV) acetylacetonate (0.039 g, 0.145 mmol) was added to the
mixture and the solution was stirred for 16 h at room temperature.
The brown precipitate was isolated by filtration and purified by
Soxhlet extraction with dichloromethane. The solvent was
removed by vacuum transfer and the resultant solid was dried in
[
[
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+
vacuo to give (R)-9 (0.089 g, 91% yield). ESI-TOF-MS: M
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+
+
1
1
+
(
(
C
C
50
H
H
30
N
N
2
O
O
3
V)
757.1611 (δ
=
11.2 ppm), [M
+
H]
50
31
2
3
V) 758.2069 (δ = 38.9 ppm). IR ν˜ = 1016.03 (V=O),
–
1
1573.28 (C=C) cm ). C50
30 2 3
H N O V (757.74): calcd. C 79.09, H
3.99, N 3.80; found C 79.01, H 3.91, N 3.70.
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3
542–3561; g) P. Adão, M. L. Kuznetsov, S. Barroso, A. M.
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+
give (R, R)-10 (0.480 g, 80% yield). ESI-TOF-MS:
M
+
1
(C
58
H
34
N
2
–
O
3
V)
857.2039 (δ = 3.5 ppm). IR ν˜ = 1035.71
H N O V (757.74): calcd. C 81.21, H 3.99, N
30 2 3
[5] K. Nakajima, K. Kojima, M. Kojima, J. Fujita, Bull. Chem.
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1
(V=O) cm . C50
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3.27; found C 80.91, H 4.02, N 3.31.
[
General Procedure for Sulfoxidations: 1 mol.-% of catalyst was dis-
solved in dichloromethane (3.0 mL) in a two neck flask under ar-
gon and stirred for 10 min. Thioanisole (0.591 mL, 5.00 mmol) was
added and the solution was stirred for 15 min. The reaction was
cooled to 0.0 °C in a circulating immersion bath fitted with a mag-
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2
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670–1671; c) M. Pasquali, F. Marchetti, C. Floriani, S. Mer-
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aqueous sodium sulfite (5 mL) was added to quench the reaction.
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at 17.82 and 20.91 min, respectively, under the conditions used.
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www.ccdc.cam.ac.uk/data_request/cif.
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