A Series of Diamagnetic Pyridine Monoimine Rhenium Complexes with Different Degrees of Metal-to-Ligand Charge Transfer: Correlating13C NMR Chemical Shifts with Bond Lengths in Redox-Active Ligands

Article abstract of DOI:10.1002/chem.201600679

A set of pyridine monoimine (PMI) rhenium(I) tricarbonyl chlorido complexes with substituents of different steric and electronic properties was synthesized and fully characterized. Spectroscopic (NMR and IR) and single-crystal X-ray diffraction analyses of these complexes showed that the redox-active PMI ligands are neutral and that the overall electronic structure is little affected by the choices of the substituent at the ligand backbone. One- and two-electron reduction products were prepared from selected starting compounds and could also be characterized by multiple spectroscopic methods and X-ray diffraction. The final product of a one-electron reduction in THF is a diamagnetic metal–metal-bonded dimer after loss of the chlorido ligand. Bond lengths in and NMR chemical shifts of the PMI ligand backbone indicate partial electron transfer to the ligand. Two-electron reduction in THF also leads to the loss of the chlorido ligand and a pentacoordinate complex is obtained. The comparison with reported bond lengths and¹³C NMR chemical shifts of doubly reduced free pyridine monoaldimine ligands indicates that both redox equivalents in the doubly reduced rhenium complex investigated here are located in the PMI ligand. With diamagnetic complexes varying over three formal reduction stages at the PMI ligand we were, for the first time, able to establish correlations of the¹³C NMR chemical shifts with the relevant bond lengths in redox-active ligands over a full redox series.

Full text of DOI:10.1002/chem.201600679

DOI: 10.1002/chem.201600679
Full Paper
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Rhenium Complexes
A Series of Diamagnetic Pyridine Monoimine Rhenium Complexes
with Different Degrees of Metal-to-Ligand Charge Transfer:
13
Correlating C NMR Chemical Shifts with Bond Lengths in Redox-
Active Ligands
Daniel Sieh[a] and Clifford P. Kubiak*[a, b]
Abstract: A set of pyridine monoimine (PMI) rhenium(I) tri-
carbonyl chlorido complexes with substituents of different
steric and electronic properties was synthesized and fully
characterized. Spectroscopic (NMR and IR) and single-crystal
X-ray diffraction analyses of these complexes showed that
the redox-active PMI ligands are neutral and that the overall
electronic structure is little affected by the choices of the
substituent at the ligand backbone. One- and two-electron
reduction products were prepared from selected starting
compounds and could also be characterized by multiple
spectroscopic methods and X-ray diffraction. The final prod-
uct of a one-electron reduction in THF is a diamagnetic
metal–metal-bonded dimer after loss of the chlorido ligand.
Bond lengths in and NMR chemical shifts of the PMI ligand
backbone indicate partial electron transfer to the ligand.
Two-electron reduction in THF also leads to the loss of the
chlorido ligand and a pentacoordinate complex is obtained.
The comparison with reported bond lengths and 13C NMR
chemical shifts of doubly reduced free pyridine monoaldi-
mine ligands indicates that both redox equivalents in the
doubly reduced rhenium complex investigated here are lo-
cated in the PMI ligand. With diamagnetic complexes vary-
ing over three formal reduction stages at the PMI ligand we
were, for the first time, able to establish correlations of the
13C NMR chemical shifts with the relevant bond lengths in
redox-active ligands over a full redox series.
Introduction
and catalysis, and their ability to stabilize uncommon coordina-
tion compounds, and have been reviewed recently.[3,4]
In 1966, Jørgensen used the words “innocent” and “suspect” to
describe the ambiguity in the determination of the oxidation
state of the metal center in nitrosyl complexes.[1] The nitrosyl
ligand can exist as NO+, NO, NOÀ, NO2À, and NO3À in their re-
spective complexes and is therefore “masking” the oxidation
state of the central metal atom.[2,3] Since then, the redox terms
“non-innocent” and “redox-active” have been used to classify
ligands that can participate in the redox chemistry of their re-
spective complexes, even though the former term became
more popular during the 1990s. These types of ligands have
gained much attention in the last decades in both, basic re-
search, as well as for their relevance in bioinorganic chemistry
Bi- and tridentate imine- and/or pyridine-based ligands like
2,2’-bipyridines (BPY), pyridine-2,6-diimines (PDI), and diazabu-
tadienes are among the best studied redox-active ligands and
much work has been put into the experimental and theoretical
exploration of the electronic structures of complexes with
these ligands.[5–8] We are interested in the electrocatalytic con-
version of carbon dioxide (CO2) and have therefore studied the
rhenium BPY system that was initially discovered by Lehn and
others[9–11] to be a catalyst for the transformation of CO2 to CO
in great detail. A common assumption resulting from the work
done by us and others[12–17] is that the catalytic activity stems
from a ligand-based reduction in the catalytically active spe-
cies. More specifically, it is believed that the CO2 carbon atom
binds to a singly occupied dz2 orbital in a Re0 complex having
a mono-reduced BPY ligand.[14,15,18] This mechanistic assump-
tion has recently lead to an accelerated investigation of com-
plexes with bidentate nitrogen-based redox-active ligands for
electrocatalytic CO2 reduction.[19] We reported on molybdenum
pyridine mono(ket)imine (PMI) complexes and could provide
evidence that in this case the redox-active properties of the
ligand were preventing rather than benefiting catalysis be-
cause of ligand–substrate CÀC coupling.[20] Since then, we in-
vestigated the related rhenium complexes in more detail and
will report on their reactivity with CO2 soon. Part of this study
was the isolation of reduced complexes, which will serve as
[a] Dr. D. Sieh, Prof. Dr. C. P. Kubiak
Joint Center for Artificial Photosynthesis
Division of Chemistry and Chemical Engineering
California Institute of Technology
1200 East California Boulevard
Pasadena, CA 91125 (USA)
E-mail: ckubiak@ucsd.edu
[b] Prof. Dr. C. P. Kubiak
Department of Chemistry and Biochemistry
University of California, San Diego
9500 Gilman Drive MC 0358
La Jolla, California 92093 (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201600679.
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reference points for the electro- and spectroelectrochemical
studies and for stoichiometric reactions in a forthcoming publi-
cation.
little affected by the variation of the PMI ligands. The same
conclusion can be made from an inspection of the three car-
bonyl stretching vibrations observed in the FTIR spectra. All
complexes show a high energy stretching vibration at approxi-
mately n˜ =2020 cmÀ1 and two lower energy bands (n˜ ꢀ1920
and 1900 cmÀ1) in acetonitrile (Table S2 in the Supporting In-
formation, also see the Supporting Information for a compari-
son of data measured in MeCN or THF, and as a film). These
values are very close to the related BPY complexes,[16,18] indi-
cating a similar electronic structure. Distinct IR bands at n˜ =
1695, 1410, 1191, and 1140 cmÀ1 can be observed in the IR
spectrum (film) of the complex PrArL-ReTFA, which are absent
in the spectrum of the chlorido congener. These bands are ten-
tatively assigned to the C=O, the CÀO, and the asymmetric
and symmetric CF3 stretches of the TFA ligand, respectively,
based on earlier IR studies on trifluoroacetate compounds.[22]
The facial arrangement of the carbonyl ligands is also con-
firmed in all X-ray structures of the neutral tricarbonyl com-
We herein discuss the isolation and characterization of
singly and doubly reduced Re PMI complexes along with their
ReI PMI0 parent compounds {[Re(PMI)X], X=Cl, trifuoroacetato
(TFA)}. All isolated complexes are diamagnetic and allow for
13C NMR spectroscopic characterization. The NMR and single-
crystal X-ray investigations indicate metal-to-ligand electron
transfer up to two electrons and linear correlations could be
obtained by comparing the 13C NMR chemical shifts of the PMI
ligand with bond length alterations of the same.
Results and Discussion
Synthesis and characterization of the octahedral tricarbonyl
complexes
R
plexes L-Re, which are displayed in Figure 1. The very similar
The octahedral tricarbonyl rhenium chlorido complexes were
synthesized by using the Stiddard method,[21] that is, heating
amount of metal-to-carbonyl-ligand back-bonding that is ob-
served in the IR and NMR spectra is confirmed by the CꢁO
bond lengths in the X-ray structures, which show differences
not larger than 0.013 ꢁ between the seven complexes. The
bond lengths within pyridine mono- (PMI) and diimine (PDI)
ligand frameworks have previously been used to measure the
amount of electron transfer from the metal center to the
redox-active ligand in complexes of this kind.[6–8,23–28] Especially
the imine C=N and the exocyclic CÀC bonds have been found
to be very diagnostic for first-row transition metals. The afore-
R
an excess of the respective ligand L with [Re(CO)5Cl] in tolu-
ene to reflux (Scheme 1).
R
mentioned bond lengths in the neutral complexes L-Re show
only very small deviations from those of the reported crystal
structure of the ligand PrArL[29] (Table S3 in the Supporting Infor-
mation), indicating that the ligands in the octahedral Re com-
R
plexes L-Re are in their neutral stage. Furthermore, only small
differences in these bond lengths can be observed between
the seven octahedral complexes showing that the modifica-
tions made only have a minor influence on the overall elec-
tronic structure. This is also indicated by the 13C NMR resonan-
ces of the imine and the pyridine carbon atoms, which show
differences smaller than 2 ppm.
Scheme 1. Synthesis of the PMI rhenium tricarbonyl complexes with the re-
spective yields given in brackets.
This method could be applied, even in cases where the
ligand was not isolated and yielded the analytically pure,
yellow/orange complexes RL-Re in moderate to good yield
after crystallization. All complexes were characterized by 1H,
13C{1H} NMR, and IR spectroscopy, as well as single-crystal X-ray
Chemical reduction of the ReI complexes
One-electron reduction
R
analysis. Alongside with the chlorido complexes L-Re, the ana-
One-electron reduction was most conveniently achieved by re-
acting the complex MesL-Re with a slight excess of KC8 in THF.
After adding a chilled, yellow/orange solution of MesL-Re to the
reductant it turned pale red quickly. Upon stirring while warm-
ing up to room temperature and finally reducing the volume
logue trifuoroacetato complex PrArL-ReTFA was investigated,
which was obtained by ligand metathesis of the ClÀ ligand by
using Ag(TFA).
The IR and NMR spectroscopic characterization of the neu-
tral tricarbonyl complexes is consistent with a facial arrange-
ment of the three carbonyl ligands. Three resonances for the
carbonyl carbon atoms can be detected in the 13C{1H} NMR
spectra for all seven complexes, two of them close to d=
200 ppm and one slightly upfield at about d=190 ppm
(Table S1 in the Supporting Information). The close proximity
1
of the solution in vacuum it turned dark green. H NMR spec-
troscopic characterization of the raw product revealed two
major products. Whereas the 1H NMR shifts of the first indi-
cates a diamagnetic species with resolved coupling patterns,
1
the H NMR shifts of the second one are broadened, either due
to paramagnetism or a dynamic process (Figure S45 in the
Supporting Information). The green, diamagnetic species, [MesL-
Re]2, could be isolated by recrystallization from a THF/pentane
R
of the CO resonances observed for the different complexes L-
Re shows, that the back-bonding into the CO ligands is very
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Figure 1. ORTEP representations of the molecular structures of the neutral, octahedral complexes RL-Re with their anisotropic displacement parameters
shown at the 50% probability level. Hydrogen atoms (except for MeL-Re) and co-crystallized solvent molecules are omitted for clarity.
mixture at À358C as green crystals. The H and 13C{1H} NMR
spectra of this complex are consistent with an intact ligand
system. Besides that, a remarkable upfield shift of some of the
13C NMR resonances of the pyridine imine ligand can be ob-
served. The imine and pyridine portion of the 13C{1H} NMR
spectrum is displayed in Figure 2 and compared with those of
PrArL-Re and the doubly reduced complex PrArL-ReÀ. The latter
one will be discussed in the next section. A comparison of the
1H NMR resonances is given in Figure S59 in the Supporting In-
formation. The carbonyl, imine, and pyridine 13C NMR resonan-
ces for these complexes and MesL-Re are also listed in Table 1.
By inspection of Table 1 it becomes obvious that the resonan-
ces of the two octahedral chlorido complexes MesL-Re and
PrArL-Re are identical within the errors.
1
While the resonances of the pyridine 6- and 3-carbon atoms
are only slightly shifted, the resonances of the imine and the
other pyridine carbon atoms can be observed at significantly
higher field. The imine carbon atom resonance for example is
shifted 20 ppm upfield from the chlorido starting material. It
should be mentioned that a strong upfield shift of these
13C NMR resonances was also observed for a pyridine monoal-
dimine ligand that was reduced by alkali metals[28] and correla-
Figure 2. Portions of the 13C{1H} NMR spectra of PrArL-Re (bottom), [MesL-Re]2 (middle), and PrArL-ReÀ (top). The resonances of the imine and pyridine carbon
atoms are labeled.
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Table 1. Selected 13C{1H} NMR chemical shifts in [ppm] for selected
carbon atoms of the respective PMI ligand of the complexes PrArL-Re,
MesL-Re, [MesL-Re]2, and PrArL-ReÀ.
PrArL-Re
MesL-Re
[
MesL-Re]2
PrArL-ReÀ
CꢁO
198.3
198.0
189.0
178.7
156.6
129.4
140.2
130.0
154.2
198.7
197.9
189.4
178.3
156.7
129.2
140.3
129.8
154.2
203.2
200.6
190.1
159.1
150.1
127.9
130[a]
120.6
151.8
211.6
CꢁO
CꢁO
C=N
122.6
136.7
122.3
118.4
102.3
154.2
PyC(2)
PyC(3)
PyC(4)
PyC(5)
PyC(6)
Figure 3. ORTEP representation of the molecular structure of the complex
[
[a] The resonance of this carbon atom is nearly overlapping with the res-
onance of one of the aryl carbon atoms and could not be determined
more precisely.
MesL-Re]2 with the anisotropic displacement parameters shown at the 50%
probability level. Hydrogen atoms and the co-crystallized toluene molecule
are omitted for clarity. Selected bond lengths [ꢁ]: Re1ÀRe1I 3.1397(3), Re1À
N1 2.125(3), Re1ÀN2 2.132(3), Re1ÀC20 1.931(4), Re1ÀC21 1.930(4), Re1ÀC22
1.886(4), C20ÀO1 1.145(5), C21ÀO2 1.155(4), C22ÀO3 1.157(5), N1ÀC2
1.319(4), C2ÀC3 1.440(5).
tions of 13C NMR shift based on the electron transfer to the PDI
ligand in Rh and Ir complexes have recently been introduced
by Burger et al.[30] Furthermore, de Bruin et al. have mentioned
a good agreement of the upfield shift of proton and carbon
atom resonances in a pyridine aldimine Ir complex with calcu-
lated Mulliken charge densities[31] and qualitatively an upfield
shift of the resonances based on an electron transfer into
these kinds of ligands has been mentioned in some cases.[32]
Considering this, it is reasonable to assume that the differences
in the chemical shifts indicate significant electron transfer to
the PMI ligand. A more detailed discussion of this is given
below in comparison with the doubly reduced complex PrArL-
ReÀ (see below). The resonances of the carbonyl carbon atoms
are only slightly shifted to higher frequencies (1–4 ppm,
Table 1), indicating that the metal-to-CO-ligand back-bonding
is not strongly increased in [MesL-Re]2.[33,34]
By comparison with the rhenium bipyridine system, the
green color of the diamagnetic complex suggests a metal–
metal-bonded dimeric complex. The Re–Re dimer [BPY-Re]2
has a distinct, strong metal-to-ligand charge-transfer (MLCT)
absorption band with lmax at about 810 nm and less intense
bands at approximately l=605 and 475 nm in the UV/Vis
spectrum.[10,11,16,35,36] Consequently, a strong absorption at l=
804 nm (25 LmmolÀ1 cm) can be observed in the UV/Vis spec-
trum of the isolated complex [MesL-Re]2 measured in THF (Fig-
ure S51 in the Supporting Information), which is accompanied
by bands at l=620 (3), 517 (3), and 420 nm
(15 LmmolÀ1 cm).[37] Furthermore, the IR spectrum of the com-
plex [MesL-Re]2 measured in a THF solution shows five CꢁO
stretching vibrations at n˜ =1991, 1962, 1902, 1885, and
1862 cmÀ1. Although only four IR bands are observed for [BPY-
Re]2, the positions of the IR bands of [MesL-Re]2 fit well with re-
ported values for [BPY-Re]2 in THF[16,35,36] and the observation
of five bands is consistent with the lower symmetry of the PMI
ligand compared to BPY. For a C2-symmetrical dimer a maxi-
mum of six IR bands can be expected based on group theory.
Single crystals suitable for X-ray diffraction were obtained
from pentane diffusion into a toluene solution at À358C. In-
spection of the molecular structure that is displayed in
Figure 3 confirms the interpretation of complex [MesL-Re]2
being a metal–metal-bonded dimer with an idealized C2 sym-
metry. The bond lengths within the carbonyl ligands and the
pyridine imine ligand backbone as well as all ReÀligand bonds
are listed in Table 2 and compared with the doubly reduced
complex PrArL-ReÀ (see below) and the octahedral starting com-
plexes RL-Re. The values for the latter species are averaged
over all ReI starting complexes in Scheme 1 except for MePyL-Re.
By inspection of Table 2 it becomes obvious that the statistical
differences are in the range of crystallographic standard uncer-
tainties (s.u.) of reasonable to good data sets, granting this
analysis. With 3.1397(3) ꢁ the ReÀRe distance is longer by
0.06 ꢁ than in the corresponding BPY system.[38] The higher
steric demand of the PMI ligand MesL compared to BPY is
a tempting and probably valid explanation for this difference
and the very similar HOMO–LUMO gap of approximately
Table 2. Selected bond lengths in [ꢁ] of the complexes L-Re,[a]
and PrArL-ReÀ.
[
MesL-Re]2,
[b]
RL-Re[a]
[
MesL-Re]2
PrArL-ReÀ[b]
C20ÀO1
C21ÀO2
C22ÀO3
Re1ÀN1
Re1ÀN2
Re1ÀC20
Re1ÀC21
Re1ÀC22
C2ÀN1
1.152(3)
1.150(4)
1.155(3)
2.169(16)
2.170(4)
1.922(5)
1.923(6)
1.908(3)
1.294(5)
1.479(3)
1.358(2)
1.389(2)
1.390(4)
1.383(4)
1.387(3)
1.343(2)
1.145(5)
1.155(4)
1.157(5)
2.125(3)
2.132(3)
1.931(4)
1.930(4)
1.886(4)
1.319(4)
1.440(5)
1.357(5)
1.402(5)
1.390(6)
1.380(6)
1.368(6)
1.364(5)
1.160(3)
1.168(2)
1.173(3)
2.0371(15)
2.0946(17)
1.882(2)
1.903(2)
1.931(2)
1.392(2)
1.381(3)
1.409(2)
1.432(3)
1.359(3)
1.424(4)
1.357(3)
1.376(3)
C2ÀC3
C3ÀN2
C3ÀC4
C4ÀC5
C5ÀC6
C6ÀC7
C7ÀN2
[a] Averaged over all ReI starting complexes RL-Re except for MePyL-Re.
Standard uncertainties in brackets are based on the averaging. [b] Crystal-
lographic s.u. in brackets.
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1.53 eV compared to [BPY-Re]2 determined by UV/VIs spectros-
copy indicates a similar electronic structure in solution. Howev-
er, the greater metal–metal separation could also be attributed
to the better electron-accepting abilities of the PMI ligand
system,[39] leading to a stronger delocalization of the electron
in the ReÀRe bond into the backbone of the bidentate ligand
and therefore to a weaker metal–metal bond.
bond lengths are shortened and elongated, respectively by
nearly 0.02 ꢁ, which is within five times the s.u. of these
bonds. In comparison, the singly reduced monoaldimine li-
gands reported by Roesky et al.[28] show bigger deviations from
aromaticity (see Table S4 in the Supporting Information). This
might be taken as another indication that the reduction equiv-
alent in [MesL-Re]2 is not fully located at the PMI ligand.
In accordance with the small differences in the chemical
shifts of the carbonyl carbon atoms comparing [MesL-Re]2 and
It is noteworthy that for a Re,[16] but also Ru,[42] Al,[27] Ge,[43]
and Ga[44] pyridine aldimine complexes, as well as a Zn pyridine
ketimine complex[45] reductive CÀC coupling has been ob-
served, where two PMI ligand units are coupled through the
imine carbon atom. This underscores the ligand-based redox
chemistry of complexes with PMI ligands and consequently,
the singly occupied molecular orbital (SOMO) in the calculated
structure of the reduced chlorido complex [modL-ReCl]À[46] is
almost fully located at the PMI ligand (89%), with only 4% par-
ticipation of the Re metal center according to Mulliken popula-
tion analysis (Figure S71 in the Supporting Information, also
see the Supporting Information for more information concern-
ing the DFT calculations in this manuscript). This CÀC coupling
pathway upon one-electron reduction seems to be kinetically
hindered for the Re pyridine ketimine complex MesL-Re de-
scribed here. After loss of the chlorido ligand the metal partici-
pation in the SOMO is increased (Figure S73 in the Supporting
Information) to 19%. This is especially apparent, when an
open-shell functional like TPSS is used. The structure is more
distorted towards a square pyramid (Figure S73 in the Support-
ing Information) compared to the more trigonal-bipyramidal
structure the BP-86 functional is predicting (Figure S72 in the
Supporting Information), making the molecule suitable for
metal–metal bond formation.
R
the octahedral starting complexes L-Re (see above), the car-
bonyl CÀO and the ReÀCcarbonyl bonds are not, or only slightly,
affected. On the other hand, both rheniumÀnitrogen bonds
R
are shorter than in L-Re, indicating a stronger interaction of
the PMI ligand with the metal center. The difference is small
(ꢀ0.04 ꢁ), but the shortening is consistent with data reported
for neutral and mono-reduced pyridine aldimine ligands in
a series of first-row transition metal[24] and Al[27] complexes and
also with a further decrease in the doubly reduced complex
PrArL-ReÀ (see below), indicating additional electron density in
R
the PMI ligand compared to the complexes L-Re, which con-
tain a neutral PMI ligand. Note that the bite angle of the PMI
ligand only increases negligibly from approximately 74 (RL-Re)
over 74.5 ([MesL-Re]2) to 758 (PrArL-ReÀ). The bond lengths
within the pyridine imine moiety, particularly the imine C2ÀN1
and the exocyclic C2ÀC3 bonds are of special interest to deter-
mine the degree of participation of the PMI ligand in the
redox chemistry of the complex. Wieghardt et al. suggested
a set of values for neutral, singly and doubly reduced pyridine
aldimine ligands.[24,25] More importantly, besides a few known
41]
crystal structures of neutral pyridine monoaldimine (RLA)[40
,
and monoketimine (RL)[29] ligands, a series of singly and doubly
reduced pyridine aldimine ligands has been reported.[28] Com-
Considering the diamagnetic nature of the dimeric complex
[
MesL-Re]2, two possible scenarios arise for the increased elec-
paring the bond lengths in [MesL-Re]2 with those in the unre-
R
duced complexes L-Re, a shortening of the imine bond (C2À tron density in the PMI ligands in this complex. 1) Two mostly
N1) by 0.025 ꢁ and an elongation of the exocyclic C2ÀC3 bond
by 0.039 ꢁ can be observed. Whereas the difference in the
imine bond is barely larger than five times the standard uncer-
tainty, the CÀC bond is significantly longer in the reduced
complex [MesL-Re]2. Unfortunately, differences in these bond
lengths up to 0.02 ꢁ can be found within the crystal structures
of different neutral pyridine aldimine ligands and the differ-
ence of these to the same bond lengths in the ketimine ligand
PrArL is even larger (see Table S4 in the Supporting Information).
A direct comparison of the bond lengths in reported reduced
ligands with those in the complexes presented here is there-
fore not straightforward. Both, the imine C2ÀN1 and the exo-
ligand-based radicals that strongly antiferromagnetically
couple (one per MesLÀRe unit) or 2) a closed-shell case, where
the increased electron density at the Re center leads to strong
p-back-bonding to the PMI ligand. Whereas the first one is
counter intuitive considering that Re–Re radical coupling has
to be assumed, the later one seems likely considering the
third-row transition metal. However, it is difficult to separate
these as independent effects as they are expected to influence
the bond lengths and NMR shifts in the ligand backbone in
a similar way.[6] DFT calculations on the dimer [modL-Re]2 as
a closed-shell singlet (CSS) state converged smoothly by using
either the BP-86 or the TPSS functional with comparable bond-
ing parameters. Both methods reasonably reproduce the bond
lengths observed in the X-ray structure (Table S12 in the Sup-
porting Information), but slightly underestimate the strength
of the ReÀRe bond. Broken symmetry calculations starting
from a converged triplet wavefunction converged to singlet
structures, indicating that the DFT level of theory does predict
a closed-shell singlet ground state. The DFT-calculated HOMO
of the CSS dimer [modL-Re]2 is completely delocalized (Fig-
ure S78 in the Supporting Information) with the largest portion
(52% according to Mulliken population analysis) being located
at the two PMI ligands. A smaller fraction (16%) is located at
[40]
cyclic C2ÀC3 bond in the pyridine aldimine ligand PrArLA
change by about 0.07 ꢁ upon one-electron reduction,[28]
a value very close to the one suggested by Wieghardt et al. for
these kinds of ligands.[24,25] Considering this and the aforemen-
tioned perturbation of these bonds going from the complexes
RL-Re to the one-electron reduction product [MesL-Re]2, it
seems reasonable to draw two conclusions: 1) there is a signifi-
cant electron transfer to the PMI ligand in [MesL-Re]2, and 2) the
added redox equivalent is not fully located on the PMI ligand.
The bond lengths within the pyridine ring are not strongly
affected by the reduction. Indeed, only the C6ÀC7 and C7ÀN2
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the six CO ligands, leaving 16% participation of each of the shifts observed for these carbon atoms are very similar in both
two Re centers.
Dissolution of the isolated, green dimer in MeCN leads to similar to the previously reported complex PrArL-Mo2À [20]
a color change to yellow/orange. By inspection of the IR spec-
Reduction of the TFA complex PrArL-ReTFA with Na/Hg re-
reduction products PhL-ReÀ and PrArL-ReÀ and also reasonably
.
trum of the complex recorded in MeCN, two bands can be de- sults in the formation of PrArL-ReÀ, the same product that was
tected at n˜ =2009 and 1897 cmÀ1. The low energy band is obtained from the analogous chlorido complex PrArL-Re as con-
broad and probably consists of two overlapped bands. These firmed by 1H NMR spectroscopy (see Figure S61 in the Support-
bands are very close to those reported for a pyridine aldimine ing Information). The isolated dimer could also be further re-
rhenium tricarbonyl butyronitrile radical[16] and some related duced with a slight excess of KC8, resulting in a red species
1
bipyridine complexes[13,47] generated in spectroelectrochemical with similar H NMR chemical shifts for the pyridine protons as
experiments and it seems likely that the dimeric complex [MesL- observed for PhL-ReÀ and PrArL-ReÀ (Figure S62 in the Support-
Re]2 breaks up in more strongly coordinating solvents, like ing Information). However, the reaction proceeded less cleanly
Mes
C
MeCN, to form the solvated radical L-Re(NCMe) .
than the reductions with excess Na/Hg starting from the chlori-
R
do complexes L-Re and we did not attempt to isolate the re-
duced complex MesL-ReÀ this way. The complex PhL-ReÀ was
only prepared in situ in this study and characterized by NMR
and IR spectroscopy, but the complex PrArL-ReÀ was isolated
and further characterized by single-crystal X-ray diffraction and
elemental analysis. Three CꢁO stretching vibrations can be ob-
served in the IR spectrum of the complex PrArL-ReÀ, which are
located at n˜ =1951, 1844, and 1835 cmÀ1 (in MeCN solution)
and are shifted to lower energies by 60–80 wavenumbers rela-
tive to the starting material, indicating higher back-bonding
into the carbonyl ligands. These bands compare well with
those observed for the BPY-ReÀ complexes.[16,17,36,38]
Two-electron reduction
Two-electron reduction was performed by using an excess of
sodium amalgam (Na/Hg) for the complexes PhL-Re, PrArL-Re,
and PrArL-ReTFA in THF (Scheme 2). Reduction with an excess
of KC8 was also successfully applied for PrArL-Re, but was found
to be less reliable for the preparation of clean L-ReÀ.
R
Single crystals suitable for X-ray diffraction of the doubly re-
duced complex PrArL-ReÀ were obtained from a very dilute pen-
tane solution that contained small amounts of THF at À358C.
The asymmetric unit contains one complex molecule and two
half sodium atoms. One sodium atom bridges two complex
molecules and is coordinated by two carbonyl ligands and four
THF molecules. The second Na atom is coordinated by six THF
molecules. Part of the asymmetric unit is displayed in
Figure 4.[48]
Scheme 2. Two-electron chemical reduction of the complexes PhL-Re, PrArL-
Re, and PrArL-ReTFA with Na/Hg.
The Re metal center in the molecular structure of PrArL-ReÀ
displays a pseudo-trigonal-bipyramidal coordination geometry
Reduction of the complexes PhL-Re and PrArL-Re with an with the trigonal plane spanned by the N1, C21, and C22
excess of Na/Hg in THF resulted in a color change to deep red. atoms (ꢀ]=3608) and a distortion towards a Y-shape (C21-
Inspection of the reaction products by NMR spectroscopy indi-
cated the clean formation of a single, diamagnetic product. All
proton resonances of the pyridine hydrogen atoms are ob-
served upfield from their counterparts in the respective start-
R
ing materials L-Re (see Figure S60 in the Supporting Informa-
tion for a comparison for PrArL-Re). Furthermore, inspection of
the 1H and 13C{1H} NMR spectra indicate a higher symmetry
R
than in the octahedral complexes L-Re. For example, only one
set of signals is observable for the isopropyl groups in PrArL-
ReÀ. Consistent with this, only one resonance for the carbonyl
carbon atoms can be observed at d=211.6 ppm in the
13C{1H} NMR spectra of the reduction products, indicating
1) slightly higher back-bonding into the carbonyl ligands and
2) the loss of the chlorido ligand and the fast exchange of the
three carbonyl ligands.[33,34] The 13C NMR resonances of the
imine and all pyridine carbon atoms except that of the C6
Figure 4. ORTEP representation of the molecular structure of the complex
atom are shifted upfield compared to those of the starting
PrArL-ReÀ with the anisotropic displacement parameters shown at the 50%
probability level. Hydrogen atoms and the half Na(THF)6 molecule are omit-
R
R
complexes L-Re and a clear trend is visible going from L-Re
over [MesL-Re]2 to L-ReÀ (Figure 2 and Table 1). The chemical ted for clarity.
R
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Re1-C22 86.94(9)8) can be observed. Selected bond lengths are
ing with the complexes RL-Re over [MesL-Re]2 to PrArL-ReÀ. A
summary of the conclusions that can be drawn from the crys-
tal-structure data is depicted in Figure 5.
listed in Table 2. A direct comparison of the bond lengths to
R
and within the carbonyl ligands to those in the complexes L-
Re and [MesL-Re]2 is not fully valid because of the different co-
ordination environment and the coordination of the sodium
atom in the solid-state structure. Least affected by these
changes is the carbonyl ligand C20ÀO1 bond trans to the pyri-
dine nitrogen atom. This ligand shows some shortening of the
Re1ÀC20 bond (0.04 ꢁ), but only a very small, insignificant
elongation of the C20ÀO1 bond (<0.01 ꢁ). The rheniumÀnitro-
gen bond lengths Re1ÀN1 and Re1ÀN2 are further decreased
The bond lengths in the octahedral complexes L-Re (struc-
R
ture A in Figure 5) are only slightly different from those ob-
served in the crystal structure of the free ligand PrArL[29] and the
PMI ligands in these complexes can be interpreted as essential-
ly neutral. In the dimeric complex [MesL-Re]2 (structure B in
Figure 5) obtained by one-electron reduction, the C=N and the
exocyclic CÀC bonds are both distorted towards a ligand-
based radical. The DFT calculations presented above, however,
indicate a closed-shell singlet state with a fully delocalized
HOMO. Therefore, the radical dot in Figure 5 should not imply
a radical, but there is formally one electron delocalized per
RePMI unit between the metal center and the ligand. Most of
the bonds within the pyridine ring are not very different from
R
going from L-Re over [MesL-Re]2 to PrArL-ReÀ showing a stron-
ger interaction of the central metal atom with the PMI ligand.
The bonds within the pyridine imine moiety show significant
R
deviations from the octahedral complexes L-Re. Most noticea-
bly, the imine C2ÀN1 and the exocyclic C2ÀC3 bonds are elon-
gated and shortened by 0.1 ꢁ, respectively, indicating a signifi-
cant electron transfer to the PMI ligand. Although Wieghardt
et al. predict an elongation of the CÀN bond by 0.2 ꢁ for
a doubly reduced pyridine aldimine ligand, the observed short-
ening of the CÀC bond in the complex PrArL-ReÀ is very close
to the predicted value.[24,25] The absolute values are different,
R
the unreduced complexes L-Re. Finally, in the doubly reduced
complex PrArL-ReÀ (structure C in Figure 5), the former imine
CÀN bond is elongated to a C(sp2)ÀN single bond and the exo-
cyclic CÀC bond is closer to a C(sp2)ÀC(sp2) double bond than
a single bond and the aromaticity in the pyridine ring is
broken. This interpretation is widely used for pyridine diimine
however. Furthermore, distortions of 0.15 (CÀN) and 0.10 ꢁ (CÀ complexes and has also been used for some pyridine monoi-
C) were observed in the solid-state structure of the free pyri-
dine aldimine ligand PrArLA upon two-electron reduction.[28] The
larger distortion of the CÀN bond might also be attributed to
sodium coordination to this bond observed in the solid-state
structure. The bond lengths within the pyridine ring in the
crystal structure of the complex PrArL-ReÀ are significantly dif-
ferent from the complexes RL-Re (Table 2) and the expected
values for an aromatic system.[49] In fact, they are much closer
to what is expected[49] for alternating sp2–sp2 single and
double bonds (Table 2 and Figure 5). This deviation is less pro-
nounced in the doubly reduced ligand PrArLA,[28] but again,
sodium coordination to these bonds might introduce some ar-
tifacts.
mine complexes.[23–25,27,50] The development of these bond
lengths and the ReÀN distances are reasonably reproduced in
the DFT-optimized structures (Tables S14 and S15 in the Sup-
porting Information).
Burger et al. were the first one to introduce the use of
13C NMR data to measure the amount of metal-to-ligand elec-
tron transfer in redox-active PDI ligands and showed good cor-
relations of carbon NMR shifts with Hammett parameter and
experimental and theoretical bond lengths in square-planar Rh
and Ir PDI complexes.[30] With a series of diamagnetic com-
plexes varying over three formal reduction stages at the PMI
ligand we are able to correlate the 13C NMR shifts with bond al-
terations over a wider range. Examples of these correlations
are given in Figure 6 (see Figures S64 and S65 in the Support-
ing Information for more correlations). Excellent linear correla-
tions were obtained for the 13C NMR shifts of the imine carbon
atom and the one in the pyridine 2-position with the respec-
tive bonds these two carbon atoms are involved in except for
the CÀNpy bond. A gradual and smooth transition from a C=N
to a C(sp2)ÀN and a C(ar)ÀC(sp2) to a C(sp2)=C(sp2) (for C2ÀC3
or C(sp2)ÀC(sp2) in the case of C3ÀC4) bond can be observed,
respectively, in both, the X-ray and the NMR data. Similarly
good correlations were obtained when the DFT-computed
bond lengths were used instead of the experimentally ob-
served ones (Figures S66 and S67 in the Supporting Informa-
tion). Correlations of the 13C NMR shifts of the other carbon
atoms in the pyridine ring with the bond lengths of the bonds
these atoms are involved in mostly did not give linear relation-
ships. However, the shift of the carbon atom in the pyridine 4-
position could be correlated with the C=N and the exocyclic
CÀC bond lengths (Figure S65 in the Supporting Information).
It is important to mention that the free ligand is not necessari-
ly a reference point in this analysis and no linear correlations
are obtained when its chemical shifts and bond lengths were
Figure 5. Comparison of the bond lengths in [ꢁ] within the pyridine imine
moiety of the complexes A) RL-Re, B) [MesL-Re]2, and C) PrArL-ReÀ.
Correlation of the 13C NMR shifts with the X-ray bond pa-
rameters
As shown above, both, the 13C NMR data as well as the bond
matrices derived from the single-crystal X-ray data indicate an
increasing amount of electron transfer to the PMI ligand start-
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2À
with the doubly reduced free pyridine aldimine ligand PrArLA
reported by Roesky et al.,[28] however. The authors reported an
upfield shift of 54 ppm for the imine carbon atom and of
21 ppm for the one in the pyridine 2-position. These values are
(almost) identical with the differences observed for the two-
electron reductions of the complexes PhL-Re and PrArL-Re. The
values for the other pyridine carbon atoms in the rhenium
complexes do not match the ones in the free ligands that ex-
actly, but the trend is nicely reflected (Tables 1 and S5 in the
Supporting Information). Assuming that the alkali metals in the
2À
reduced free ligand PrArLA are separated by the moderately
coordinating solvent (i.e., [D8]THF) and the NMR chemical
shifts observed are therefore really of a free ligand, an upfield
shift of about 50–55 ppm (C=N) and 20 ppm (PyC(2)) would
correspond to two redox equivalents located on the PMI
ligand.
Conclusion
We have investigated a series of rhenium pyridine ketimine
complexes, which are closely related to the famous rhenium
bipyridine CO2 reduction catalyst. The CO2 reduction properties
of these complexes are going to be reported in a subsequent
publication. One- and two-electron reduction products ob-
tained from these complexes indicate an increasing amount of
electron transfer to the PMI ligand. Perturbations in the bond
lengths of the PMI and PDI ligand frameworks have been used
for more than a decade to measure the amount of ligand re-
duction. Correlations of the 13C NMR chemical shifts of relevant
carbon atoms in the PMI ligands of the complexes studied
here with the bond lengths observed in their crystal structures
show excellent linear dependencies. Together with the recent
report of Burger et al. this provides a new tool to measure the
amount of ligand reduction in redox-active ligands.
Experimental Section
General: Even though we did not observe water or air sensitivity
of the octahedral chlorido complexes, manipulations of all com-
plexes were performed by using standard Schlenk techniques or in
1
a dinitrogen-filled glovebox if not otherwise stated. H, 13C{1H}, and
19F NMR spectra were recorded on Varian 400 or 200 MHz or
a 400 MHz Bruker NMR spectrometer equipped with a cryoprobe.
1H and 13C NMR signals were referenced to the residual solvent sig-
nals and are given relative to (CH3)4Si. 19F NMR signals were refer-
Figure 6. Correlation of the 13C NMR shifts of the complexes RL-Re, [MesL-
Re]2, and PrArL-ReÀ with the bond lengths observed in the corresponding X-
ray structures.
1
enced against CCl3F. The assignments of the H and 13C NMR sig-
1
nals were carried out by combined analyses of 1D and H,1H and
1H,13C correlated 2D spectra (1H,1H COSY, HSQC, HMBC, and ADE-
QUATE as needed). Signals of higher order are reported as first-
order signals whenever the coupling constants could reasonably
be determined. Diffusion coefficients were determined by DOSY
spectroscopy on a 500 MHz Varian NMR spectrometer. Infrared
spectra were recorded as thin films on an ATR cell or as MeCN or
THF solutions with a resolution of 4 cmÀ1 on a Thermo Scientific
Nicolet iS5 FTIR spectrometer. IR peaks are given relative to each
other as: s=strong, m=medium, w=weak, vw=very weak, or
sh=shoulder. Microanalyses were performed by the Midwest Mi-
crolab, LLC. The ligands PhL[20] and PrArL[20,51] were synthesized ac-
cording to literature procedures. Air- and water-free non-deuterat-
included in the correlations shown above. For example, al-
though the 13C NMR resonance of the carbon atom in the 2-
position stays unaffected, the one for the imine carbon atom is
shifted downfield by 10 ppm upon coordination. Coordination
obviously can change the shielding of the carbon atoms signif-
icantly, even if the bond lengths are not affected and the
ligand is considered to be in its neutral state.
No quantification of the amount of electron transfer to the
PMI ligand can be made purely based on the correlations pre-
sented above. An estimate can be obtained by comparison
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ed solvents were obtained from a commercial solvent purification
system. CD2Cl2 was used as received as 1 g ampoules, which were
opened in a N2-filled glovebox. THF and [D8]THF used for the re-
ductions were additionally stirred over Na, filtered, and stored over
sieves in a dinitrogen-filled glovebox prior to use. KC8 was pre-
pared by using a literature procedure.[52] All other reagents were
purchased from commercial sources and used as received.
NCCH3); 13C{1H} NMR ([D8]THF, 100 MHz): d=199.7 (CꢁO), 199.0 (Cꢁ
O), 189.2 (CꢁO), 175.0 (NCCH3), 157.7 (PyC(2)), 154.0 (PyC(6)), 140.2
(PyC(4)), 129.0 (PyC(5)), 128.1 (PyC(3)), 47.5 (NCH3), 16.1 ppm
(NCCH3); IR (THF): n˜ =2018 (s, CꢁO), 1916 (s, CꢁO), 1892 cmÀ1 (s,
CꢁO); IR (MeCN): n˜ =2021 (s, CꢁO), 1915 (s, CꢁO), 1898 cmÀ1 (s, Cꢁ
O); IR (film): n˜ =3078 (vw), 2980 (vw), 2933 (vw), 2869 (vw), 2013 (s,
CꢁO), 1898 (sh, CꢁO), 1878 (s, CꢁO), 1680 (vw), 1626 (vw), 1600
(w), 1565 (vw), 1478 (w), 1443 (w), 1425 (sh), 1378 (w), 1325 (w),
1302 (w), 1257 (w), 1182 (vw), 1166 (w), 1141 (vw), 1099 (vw), 1066
(w), 1027 (vw), 969 (vw), 940 (vw), 907 (w), 778 (w), 752 (w), 646
(w), 630 (w), 610 (vw), 591 cmÀ1 (w); elemental analysis calcd (%)
for C11H10ClN2O3Re: C 30.04, H 2.29, N 6.37; found: C 30.28, H 2.35,
N 6.40.
X-ray crystallography: Crystals of the complexes suitable for X-ray
structural determinations were mounted in polybutene oil on
a glass fibre and transferred on the goniometer head to the pre-
cooled instrument. Crystallographic measurements were carried
out with MoKa radiation on a Bruker APEXII or a D8 VENTURE
single-crystal diffractometer. Data were integrated by using
SAINT[53] and a numerical and multi-scan absorption correction was
applied with the program SADABS.[54] The structures were solved
by direct methods by using SHELXS[55] or SIR92[56] and refined
against F2 by full-matrix least-squares by using SHELX97[55] and/or
ShelXle[57] by using all unique data. All non-hydrogen atoms were
anisotropically refined unless otherwise reported; the hydrogen
atoms were placed on geometrically calculated positions and re-
fined by using a riding model. More details of the data collection
and refinement are given in the Supporting Information and the
supplemental cif files.
Synthesis of PhL-Re: The ligand PhL (85 mg; 0.43 mmol) was dis-
solved in toluene (1 mL) and combined with [Re(CO)5Cl] (104 mg,
0.29 mmol) suspended in toluene (6 mL) in a 100 mL Schlenk flask
in a N2-filled glovebox. The flask was taken out of the box and
equipped with a reflux condenser under a dinitrogen atmosphere.
The suspension was heated to reflux under N2 for 13.5 h. An
orange solid formed during this time. The reaction mixture was al-
lowed to cool to RT and was then filtered in a N2-filled glovebox.
The orange solid was washed five times with toluene (1 mL) and
then dried in vacuum. The raw product was then recrystallized by
1
Analytical data of PrArL: H NMR ([D8]THF, 400 MHz): d=8.62 (ddd,
pentane diffusion into a CHCl3 solution leading to 105 mg
3
4
5
3
4
1H; J=4.8, J=1.8, J=1.0 Hz, PyH(6)), 8.36 (ddd, 1H; J=8.0, J=
(0.21 mmol, 72%) of the yellow/orange, crystalline product PhL-Re
as well as single crystals suitable for X-ray diffraction, which were
washed three times with a mixture of pentane/CHCl3 (3:1, 5 mL)
5
3
3
4
1.1, J=1.1 Hz, PyH(3)), 7.83 (ddd, 1H; J=7.9„ J=7.5, J=1.8 Hz,
3
3
4
PyH(4)), 7.40 (ddd, 1H; J=7.5, J=4.8, J=1.2 Hz, PyH(5)), 7.13 (m,
2H; ArH(3,5)), 7.13 (m, 1H; ArH(4)), 2.75 (sept, 2H; 3J=6.9 Hz,
CH(CH3)2), 2.17 (s, 3H; NCCH3), 1.14 (d, 6H; 3J=6.9 Hz, CH(CH3)2),
1.11 ppm (d, 6H; 3J=6.9 Hz, CH(CH3)2); 13C{1H} NMR ([D8]THF,
100 MHz): d=168.1 (NCCH3), 157.4 (PyC(2)), 149.7 (PyC(6)), 147.8
(ArC(1)), 136.4 (ArC(2,6)), 137.2 (PyC(4)), 125.8 (PyC(5)), 121.8
(PyC(3)), 124.5 (ArC(4)), 123.8 (ArC(3,5)), 29.3 (CH(CH3)2), 23.8, 23.2
(2ꢂCH(CH3)2), 17.4 ppm (NCCH3).
1
and dried in vacuum. H NMR (CD2Cl2, 400 MHz): d=9.06 (ddd, 1H;
4
5
3
3
3J=5.4, J=1.6, J=0.8 Hz, PyH(6)), 8.16 (ddd, 1H; J=7.9, J=7.8,
4J=1.6 Hz, PyH(4)), 8.06 (ddd, 1H; 3J=8.1, 4J=1.4, 5J=0.8 Hz,
3
3
4
PyH(3)), 7.68 (ddd, 1H; J=7.7, J=5.4, J=1.4 Hz, PyH(5)), 7.54 (m,
2H; ArH(3), ArH(5)), 7.41–7.30 (m, 2H; ArH(4), ArH(2or6)), 7.09 (m,
1H; ArH(2or6)), 2.43 ppm (s, 3H; NCCH3); 13C{1H} NMR (CD2Cl2,
100 MHz): d=199.0 (CꢁO), 196.6 (CꢁO), 188.1 (CꢁO), 175.1
(NCCH3), 156.4 (PyC(2)), 153.8 (PyC(6)), 150.0 (ArC(1)), 139.8 (PyC(4)),
130.3 (br, ArC(3or5)), 130.1 (br, ArC(3or5)), 129.4 (PyC(5)), 128.2
(PyC(3)), 128.0 (ArC(4)), 122.0 (br, ArC(2or6)), 120.9 (br, ArC(2or6)),
Preparation of MeL: An oven-dried Schlenk flask was charged with
dry CH2Cl2 (20 mL). Methylammonium chloride (300 mg,
4.44 mmol), triethylamine (0.93 mL, 0.67 g, 6.6 mmol), 2-acetylpyri-
dine (0.5 mL, 0.5 g, 4 mmol), and several spatulas of Na2SO4 were
added against a dinitrogen stream. Dry methanol (10 mL) was
added and the reaction mixture was stirred at RT. Over the course
of two days more methylammonium chloride (556 mg, 8.23 mmol)
and triethylamine (1.61 mL, 1.17 g, 11.7 mmol) were added in four
portions while the reaction was stirred at RT. The reaction mixture
was then filtered and the solvent was removed in vacuum. The res-
idue was extracted with benzene (30 mL), filtered, and the benzene
phase was washed two times with deionized water (5 mL). After
being dried over Na2SO4, the benzene was removed in vacuum,
yielding 69 mg of the raw ligand MeL, which was used without fur-
ther purification for the complexation reaction.
Synthesis of MeL-Re: The raw ligand MeL (69 mg, max. 0.51 mmol)
was dissolved in toluene (1 mL) and combined with [Re(CO)5Cl]
(130 mg, 0.36 mmol) suspended in toluene (5 mL) in a 100 mL
Schlenk flask. The reaction mixture was heated to reflux for 22.5 h.
After being cooled to RT, the solvent was removed in vacuum and
the residue was co-evaporated with pentane (2 mL). The yellow
solid was the washed four times with pentane (1.5 mL), followed
by two times with Et2O (1 mL). The raw product was then recrystal-
lized from pentane diffusion into a CH2CL2 solution, which yielded
55 mg (0.13 mmol, 36%) of the complex MeL-Re as well as single
crystals suitable for X-ray diffraction. 1H NMR ([D8]THF, 400 MHz):
d=9.01 (ddd, 1H; 3J=5.4, 4J=1.4, 5J=0.9 Hz, PyH(6)), 8.23–8.16
1
18.8 ppm (NCCH3); H NMR ([D8]THF, 400 MHz): d=9.07 (ddd, 1H;
4
5
3
4
3J=5.4, J=1.5, J=0.8 Hz, PyH(6)), 8.30 (ddd, 1H; J=8.0, J=1.5,
5J=0.8 Hz, PyH(3)), 8.23 (ddd, 1H; 3J=7.9, 3J=7.6, 4J=1.5 Hz,
3
3
4
PyH(4)), 7.74 (ddd, 1H; J=7.5, J=5.4, J=1.4 Hz, PyH(5)), 7.51 (m,
2H; ArH(3), ArH(5)), 7.44 (m, 1H; ArH(2or6)), 7.33 (m, 1H; ArH(4)),
7.08 (m, 1H; ArH(2or6)), 2.44 ppm (s, 3H; NCCH3); 13C{1H} NMR
([D8]THF, 100 MHz): d=199.9 (CꢁO), 197.4 (CꢁO), 188.9 (CꢁO),
176.3 (NCCH3), 157.2 (PyC(2)), 154.3 (PyC(6)), 151.1 (ArC(1)), 140.1
(PyC(4)), 130.5 (br, ArC(3or5)), 130.3 (br, ArC(3or5)), 129.8 (PyC(5)),
129.2 (PyC(3)), 128.1 (ArC(4)), 123.0 (br, ArC(2or6)), 121.4 (br,
ArC(2or6)) and 18.3 ppm (NCCH3); diffusion coefficient ([D3]MeCN,
DOSY): D0 =16.5ꢂ10À10 m2sÀ1; IR (THF): n˜ =2021 (s, CꢁO), 1923 (s,
CꢁO), 1893 cmÀ1 (s, CꢁO); IR (MeCN): n˜ =2022 (s, CꢁO), 1919 (s, Cꢁ
O), 1899 cmÀ1 (s, CꢁO); IR (film): n˜ =2982 (w), 2869 (w), 2017 (s,
CꢁO), 1908 (s, CꢁO), 1889 (s, CꢁO), 1593 (w), 1565 (vw), 1486 (w),
1476 (vw), 1451 (w), 1437 (sh), 1378 (w), 1329 (w), 1305 (vw), 1257
(w), 1219 (vw), 1169 (vw), 1062 (w), 1026 (w), 1002 (vw), 995 (vw),
916 (w), 860 (w), 799 (w), 779 (w), 750 cmÀ1 (w); elemental analysis
calcd (%) for C16H12ClN2O3Re: C 38.2, H 2.41, N 5.58; found: C 38.37,
H 2.50, N 5.37.
Preparation of ClArL: An oven-dried 100 mL Schlenk flask was
charged with 4-chloroaniline (1.16 g, 9.09 mmol) and a spatula tip
of p-toluene sulfonic acid under a dinitrogen atmosphere. Dry ben-
zene (80 mL) and 2-actetylpyridine (1.0 mL, 1.1 g, 9.1 mmol), were
added. The reaction solution was then heated to reflux under dini-
trogen for 14.5 h and was then allowed to cool to RT. The solvent
3
3
4
(m, 2H; PyH(3), PyH(4)), 7.68 (ddd, 1H; J=7.3, J=5.4, J=2.0 Hz,
5
5
PyH(5)), 3.96 (q, 3H; J=1.0 Hz, NCH3), 2.59 ppm (q, 3H; J=1.0 Hz,
www.chemeurj.org
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was removed under reduced pressure. After the yellow, oily residue
had been dried in oil pump vacuum, it was dissolved in Et2O
(10 mL), which was then extracted with three portions of Mili-Q
water (5 mL). The organic phase was dried over Na2SO4, filtered,
and the solvent was removed in vacuum. 1H NMR spectroscopic
characterization revealed contamination of the desired product
with both of the starting materials. The ligand could be used for
the complexation reaction without further purification, however.
three times with a THF/pentane mixture (1:4, 4 mL) and dried in
vacuum. 1H NMR ([D8]THF, 400 MHz): d=9.07 (ddd, 1H; 3J=5.4,
4J=1.6, 5J=0.8 Hz, PyH(6)), 8.29 (ddd, 1H; 3J=8.0, 4J=1.3, 5J=
3
3
4
0.8 Hz, PyH(3)), 8.23 (ddd, 1H; J=8.0, J=7.7, J=1.5 Hz, PyH(4)),
3
3
4
7.75 (ddd, 1H; J=7.6 Hz, J=5.4 Hz, J=1.4 Hz, PyH(5)), 6.99 (m,
2H; ArH(3), ArH(5)), 2.39 (s, 3H; ArC(2or6)CH3), 2.34 (s, 3H; NCCH3),
2.32 (s, 3H; ArC(4)CH3), 2.12 ppm (s, 3H; ArC(2or6)CH3);
13C{1H} NMR ([D8]THF, 100 MHz): d=198.7 (CꢁO), 197.9 (CꢁO), 189.4
(CꢁO), 178.3 (NCCH3), 156.7 (PyC(2)), 154.2 (PyC(6)), 146.5 (ArC(1)),
140.3 (PyC(4)), 137.3 (ArC(4)), 131.2 (ArC(2or6)), 130.3 (ArC(3or5)),
130.7 (ArC(3or5)), 129.8 (PyC(5)), 129.2 (PyC(3)), 128.3 (ArC(2or6)),
21.0 (ArC(4)CH3), 20.6 (ArC(2or6)CH3), 18.5 (ArC(2or6)CH3), 18.3 ppm
(NCCH3); IR (THF): n˜ =2021 (s, CꢁO), 1925 (s, CꢁO), 1891 cmÀ1 (s,
CꢁO); IR (MeCN): n˜ =2022 (s, CꢁO), 1922 (s, CꢁO), 1895 cmÀ1 (s, Cꢁ
O); IR (film): n˜ =2981 (w), 2924 (w), 2863 (w), 2017 (s, CꢁO), 1915
(s, CꢁO), 1887 (s, CꢁO), 1606 (w), 1590 (w), 1562 (vw), 1475 (w),
1444 (w), 1379 (w), 1329 (w), 1299 (vw), 1256 (w), 1209 (w), 1165
(vw), 1157 (vw), 1119 (vw), 1064 (w), 1028 (vw), 991 (vw), 908 (w),
860 (w), 777 (w), 751 cmÀ1 (w); elemental analysis calcd (%) for
C19H18ClN2O3Re: C 41.95, H 3.34, N 5.15; found: C 42.23, H 3.34, N
5.20.
Synthesis of ClArL-Re: The raw ligand ClArL (270 mg, max.
1.17 mmol) was dissolved in toluene (4 mL) and combined with
[Re(CO)5Cl] (302 mg, 0.834 mmol) suspended in toluene (10 mL) in
a 100 mL Schlenk flask. The reaction mixture was heated to reflux
for 16 h under a dinitrogen atmosphere upon which an orange/
yellow solid formed. The reaction mixture was allowed to cool to
RT to maximize the precipitation. The solvent was decanted and
the remaining solid was washed with six portions of Et2O (2 mL)
and dried in vacuum. The product was recrystallized from a THF
solution layered with pentane yielding 297 mg (0.554 mmol, 66%)
of the analytically pure complex ClArL-Re. Single crystals suitable for
X-ray diffraction were obtained from pentane diffusion into a THF
solution. 1H NMR ([D8]THF, 400 MHz): d=9.07 (ddd, 1H; 3J=5.4,
4J=1.5, 5J=0.7 Hz, PyH(6)), 8.31 (ddd, 1H; 3J=8.0, 4J=1.3, 5J=
Synthesis of PrArL-Re: The ligand PrArL (282 mg, 1.01 mmol) was dis-
solved in toluene (4 mL) and combined with [Re(CO)5Cl] (270 mg,
0.746 mmol) suspended in toluene (16 mL) in a 100 mL Schlenk
flask. The suspension was then heated to reflux for 15 h. After the
resulting orange solution was cooled to 408C the solvent was re-
moved in vacuum at this temperature. The yellow solid residue
was washed in portions with pentane (15 mL) followed by Et2O
(2 mL). The product was then recrystallized from a CH2Cl2 solution
layered with pentane. This yielded 353 mg (0.602 mmol; 81%) of
complex PrArL-Re, which was washed three times with a mixture of
pentane/CH2Cl2 (4:1, 5.5 mL) and thoroughly dried in vacuum.
Single crystals suitable for X-ray diffraction were obtained from
slow diffusion of a concentrated toluene solution at RT. 1H NMR
(CD2Cl2; 400 MHz): d=9.07 (ddd, 1H; 3J=5.5, 4J=1.6, 5J=0.8 Hz,
PyH(6)), 8.16 (ddd, 1H; 3J=7.9, 3J=7.8, 4J=1.6 Hz, PyH(4)), 8.09
3
3
4
0.8 Hz, PyH(3)), 8.23 (ddd, 1H; J=8.0, J=7.7, J=1.5 Hz, PyH(4)),
7.76 (ddd, 1H; 3J=7.7, 3J=5.4, 4J=1.4 Hz, PyH(5)), 7.54 (m, 2H;
ArH(3), ArH(5)), 7.44 (m, 1H; ArH(2or6)), 7.10 (m, 1H; ArH(2or6)).
2.46 ppm (s, 3H; NCCH3); 13C{1H} NMR ([D8]THF, 100 MHz): d=199.6
(CꢁO), 197.5 (CꢁO), 188.7 (CꢁO), 177.2 (NCCH3), 157.0 (PyC(2)),
154.4 (PyC(6)), 149.6 (ArC(1)), 140.3 (PyC(4)), 133.6 (ArC(4)), 130.7,
130.6 (br, ArC(3), ArC(5), overlapping), 130.0 (PyC(5)), 129.4 (PyC(3)),
124.8 (br, ArC(2or6)), 123.3 (br, ArC(2or6)), 18.4 ppm (NCCH3); IR
(THF): n˜ =2021 (s, CꢁO), 1923 (s, CꢁO), 1895 cmÀ1 (s, CꢁO); IR
(MeCN): n˜ =2022 (s, CꢁO), 1920 (s, CꢁO), 1900 cmÀ1 (s, CꢁO); IR
(film): n˜ =2981 (w), 2870 (w), 2018 (s, CꢁO), 1910 (s, CꢁO), 1890 (s,
CꢁO), 1598 (w), 1565 (vw), 1485 (w), 1443 (vw), 1403 (vw), 1378
(w), 1327 (w), 1303 (vw), 1257 (w), 1221 (vw), 1168 (vw), 1091 (w),
1062 (w), 1015 (w), 997 (vw), 913 (vw), 873 (w), 831 (w), 778 (w),
751 (w), 725 cmÀ1 (vw); elemental analysis calcd (%) for
C16H11Cl2N2O3Re: C 35.83, H 2.07, N 5.22; found: C 36.00, H 2.12, N
5.30.
3
4
5
3
(ddd, 1H; J=8.0, J=1.4, J=0.9 Hz, PyH(3)), 7.71 (ddd, 1H; J=
3
4
7.8, J=5.4, J=1.5 Hz, PyH(5)), 7.39–7.31 (m, 3H; ArH(3,4,5)), 3.52
(sept, 1H; 3J=6.7 Hz, CH(CH3)2), 2.88 (sept, 2H; 3J=6.7 Hz,
CH(CH3)2), 2.37 (s, 3H; NCCH3), 1.34 (d, 3H; 3J=6.7 Hz, CH(CH3)2),
Preparation of MesL: An oven-dried 100 mL Schlenk flask was
charged with dry toluene (80 mL), 2-actetylpyridine (1.0 mL, 1.1 g,
9.1 mmol), and 2,4,6-trimethylaniline (1.4 mL, 1.3 g, 9.6 mol) under
a dinitrogen atmosphere. A spatula tip of p-toluene sulfonic acid
was added and the flask was then equipped with a dropping
funnel, filled up with dry toluene for the azeotropic removal of
water, as well as a reflux condenser. The solution was then heated
to reflux under N2 for 4.5 h. The solution was allowed to cool to RT
and the solvent was then removed under reduced pressure.
1H NMR spectroscopic investigation of the remaining yellow oil re-
vealed contamination of the desired product with both of the
starting materials. The ligand could be used for the complexation
reaction without further purification, however.
Synthesis of MesL-Re: The raw ligand MesL (560 mg, max.
2.35 mmol) was dissolved in toluene (5 mL) and combined with
[Re(CO)5Cl] (465 mg, 1.29 mmol) suspended in toluene (15 mL) in
a 100 mL Schlenk flask. The suspension was then heated to reflux
for 14 h resulting in an orange/red solution from which an orange
solid precipitated upon cooling the solution to 408C. The solvent
was removed at 408C in vacuum and the solid residue was washed
in portions with pentane (15 mL) followed by Et2O (5 mL). The raw
product was then recrystallized from pentane diffusion into a THF
solution, which led to 562 mg (1.03 mmol; 80%) of MesL-Re as well
as single crystals suitable for X-ray diffraction, which were washed
3
1.25 (d, 3H; J=6.6 Hz, CH(CH3)2), 1.08 ppm (m, 6H; 2ꢂCH(CH3)2);
13C{1H} NMR (CD2Cl2; 100 MHz): d=197.6 (CꢁO), 197.3 (CꢁO), 188.2
(CꢁO), 177.4 (NCCH3), 156.0 (PyC(2)), 153.6 (PyC(6)), 144.8 (ArC(1)),
141.1 (ArC(2 or 6)), 139.8 (PyC(4)), 139.4 (ArC(2 or 6)), 129.4
(PyC(5)), 128.6, 128.4, 125.6, 125.2 (ArC(3,4,5), PyC(3)), 28.6, 28.3
(2ꢂCH(CH3)2), 25.4, 25.13, 25.07, 24.95 (4ꢂCH(CH3)2), 20.3 ppm
(NCCH3); 1H NMR ([D8]THF, 400 MHz): d=9.10 (ddd, 1H; 3J=5.4,
4J=1.5, 5J=0.8 Hz, PyH(6)), 8.30 (ddd, 1H; 3J=8.0, 4J=1.6, 5J=
3
3
4
0.8 Hz, PyH(3)), 8.25 (ddd, 1H; J=8.0, J=7.5, J=1.5 Hz, PyH(4)),
7.78 (ddd, 1H; 3J=7.5, 3J=5.4, 4J=1.6 Hz, PyH(5)), 7.35–7.28 (m,
3H; ArH(3,4,5)), 3.71 (sept, 1H; 3J=6.7 Hz, CH(CH3)2), 2.95 (sept,
2H; 3J=6.7 Hz, CH(CH3)2), 2.42 (s, 3H; NCCH3), 1.33 (d, 3H; 3J=
3
6.7 Hz, CH(CH3)2), 1.25 (d, 3H; J=6.5 Hz, CH(CH3)2), 1.07 ppm (m,
6H; 2ꢂCH(CH3)2); 13C{1H} NMR ([D8]THF, 100 MHz): d=198.3 (CꢁO),
198.0 (CꢁO), 189.0 (CꢁO), 178.7 (NCCH3), 156.6 (PyC(2)), 154.2
(PyC(6)), 145.7 (ArC(1)), 141.8 (ArC(2 or 6)), 140.2 (PyC(4)), 140.0
(ArC(2 or 6)), 130.0 (PyC(5)), 129.4 (PyC(3)), 128.8 (ArC(4)), 126.5,
125.4 (ArC(3and5)), 29.0, 28.7 (2ꢂCH(CH3)2), 25.5, 25.3, 25.2 (3ꢂ
CH(CH3)2, overlying with [D8]THF, one hidden), 20.2 ppm (NCCH3);
diffusion coefficient ([D3]MeCN, DOSY): D0 =12.7ꢂ10À10 m2sÀ1; IR
(THF): n˜ =2021 (s, CꢁO), 1925 (s, CꢁO), 1893 cmÀ1 (s, CꢁO); IR
(MeCN): n˜ =2022 (s, CꢁO), 1922 (s, CꢁO), 1897 cmÀ1 (s, CꢁO); IR
(film): n˜ =2971 (w), 2929 (w), 2869 (w), 2017 (s, CꢁO), 1914 (s, Cꢁ
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3
3
O), 1888 (s, CꢁO), 1606 (sh), 1594 (w), 1587 (w), 1562 (vw), 1473
(sh), 1464 (w), 1444 (w), 1385 (w), 1375 (w), 1364 (w), 1321 (w),
1257 (w), 1180 (w), 1165 (vw), 1100 (vw), 1065 (w), 1028 (vw), 991
(vw), 936 (vw), 909 (vw), 856 (vw), 809 (vw), 790 (w), 775 (w), 748
(w), 711 cmÀ1 (w); elemental analysis calcd (%) for C22H24ClN2O3Re:
C 45.08, H 4.13, N 4.78; found: C 45.15, H 4.18, N 4.69.
PyH(6)), 8.35–8.28 (m, 2H; PyH(3,4)), 7.81 (ddd, 1H; J=7.3, J=5.3,
4J=2.1 Hz, PyH(5)), 7.39–7.30 (m, 3H; ArH(3,4,5)), 3.32 (sept, 1H;
3J=6.7 Hz, CH(CH3)2), 2.94 (sept, 2H; J=6.7 Hz, CH(CH3)2), 2.43 (s,
3
3H; NCCH3), 1.33 (d, 3H; 3J=6.7 Hz, CH(CH3)2), 1.20 (d, 3H; 3J=
6.6 Hz, CH(CH3)2), 1.12 (d, 3H; 3J=6.8 Hz, CH(CH3)2), 1.08 ppm (d,
3H; 3J=6.9 Hz, CH(CH3)2); 13C{1H} NMR ([D8]THF, 100 MHz): d=
2
197.6 (CꢁO), 196.8 (CꢁO), 192.5 (CꢁO), 180.3 (NCCH3), 161.2 (q, J=
Preparation of MePyL: In an oven-dried Schlenk flask, 2-acetyl-4-
methylpyridine (0.85 mg, 6.3 mmol) was dissolved in dry toluene
(60 mL). A spatula tip of p-toluene sulfonic acid and aniline
(0.80 mL, 0.82 g, 8.8 mmol) were added and the flask was then
equipped with a dropping funnel filled with dry toluene. The reac-
tion was then heated to reflux under a dinitrogen atmosphere for
22 h. The reaction mixture was allowed to cool to 408C and the
solvent was removed in vacuum at this temperature. The obtained
yellow residue was further dried in vacuum at 408C overnight.
Parts of the raw product were dissolved in dry pentane and placed
in the freezer at À358C for two weeks, which resulted in precipita-
tion of a yellow solid. The solution was decanted and the solid resi-
due was washed three times with small amounts of cold (À358C)
pentane and dried in vacuum. The product obtained this way was
not further isolated and was used for the complexation.
35.9 Hz, CF3COO), 156.3 (PyC(2)), 155.6 (PyC(6)), 145.5 (ArC(1)),
141.4 (ArC(2 or 6)), 141.2 (PyC(4)), 139.9 (ArC(2 or 6)), 130.2
(PyC(3)), 129.3 (PyC(5)), 129.0 (ArC(4)), 126.0, 125.7 (ArC(3and5)),
1
116.7 (q, J=291.3 Hz, CF3COO), 29.0, 28.1 (2ꢂCH(CH3)2), 25.4, 25.0,
24.6 (3ꢂCH(CH3)2, overlying with [D8]THF, one hidden), 19.9 ppm
(NCCH3); 19F NMR ([D8]THF, 376 MHz): d=À74.0 ppm (CF3COO); dif-
fusion coefficient ([D3]MeCN, DOSY): D0 =13.0ꢂ10À10 m2sÀ1
; IR
(THF): n˜ =2026 (s, CꢁO), 1936 (s, CꢁO), 1903 (s, CꢁO), 1716 (w, C=O
(TFA)), 1701 cmÀ1 (w, C=O (TFA)); IR (MeCN): n˜ =2026 (s, CꢁO),
1924 (s, CꢁO), 1905 (s, CꢁO), 1717 (w, C=O (TFA)), 1700 cmÀ1 (w,
C=O (TFA)); IR (film): n˜ =2970 (w), 2932 (w), 2871 (w), 2021 (s, Cꢁ
O), 1916 (s, CꢁO), 1894 (s, CꢁO), 1714 (sh, C=O), 1695 (m, C=O),
1607 (sh), 1594 (w), 1588(w), 1565 (vw), 1466 (w), 1446 (w), 1410
(w, CÀO), 1387 (w), 1376 (w), 1365 (w), 1322 (w), 1258 (w), 1191 (s,
asym. CF3), 1140 (m, sym. CF3), 1104 (vw), 1064 (w), 1029 (vw), 991
(vw), 936 (vw), 908 (vw), 844 (w), 809 (vw), 790 (w), 776 (w), 749
(w), 726 (m), 712 cmÀ1 (vw); elemental analysis calcd (%) for
C24H24F3N2O5Re: C 43.44, H 3.65, N 4.22; found: C 43.47, H 3.61, N
4.22.
Synthesis of MePyL-Re:
A suspension of [Re(CO)5Cl] (104 mg,
0.288 mmol) was combined with the raw ligand MePyL (76 mg, max.
0.361 mmol) in a 100 mL Schlenk flask. The reaction mixture was
heated to reflux under a dinitrogen atmosphere for 17 h and was
then allowed to cool to 408C. The solvent was removed in vacuum
at this temperature and the oily residue was then stirred with pen-
tane (15 mL) overnight. The pentane was decanted and the solid
residue was washed four times with pentane (2 mL). This proce-
dure was repeated one time, yielding 111 mg (0.215 mmol; 75%)
of the yellow complex MePyL-Re. Single crystals suitable for X-ray
diffraction were obtained from a THF/water mixture. 1H NMR
Synthesis of [MesL-Re]2: A solution of MesL-Re (32 mg, 59 mmol) in
THF (4 mL) and KC8 (9 mg, 67 mmol) were separately cooled to
À358C. The cold complex solution was then added to the cold KC8
and the resulting suspension was stirred for 15 min while warming
up before the reaction mixture was filtered. The resulting solution
was reduced to 1 mL and layered with pentane. Green and orange
crystals grew from this mixture at À358C, which were washed
three times with a mixture of THF/pentane (1:4, 1 mL) and dried in
vacuum. The unreduced, orange starting material was removed by
extracting with two time 0.2 mL of MeCN, resulting in 7 mg
(6.9 mmol, 23%) of the green complex [MesL-Re]2. Single crystals
suitable for X-ray diffraction were obtained from pentane diffusion
3
([D8]THF, 400 MHz): d=8.87 (d, 1H; J=5.5 Hz, PyH(6)), 8.14 (s, 1H;
PyH(3)), 7.57 (d, 1H; 3J=5.3 Hz, PyH(5)), 7.49 (m, 2H; ArH(3),
ArH(5)), 7.43 (m, 1H; ArH(2or6)), 7.31 (m, 1H; ArH(4)), 7.07 (m, 1H;
4
ArH(2or6)), 2.58 (s, 3H; PyC(4)CH3), 2.41 ppm (s, 3H; NCCH3); J and
5J couplings in the pyridine ring could not be reasonably resolved,
possibly due to the small additional coupling to the methyl group;
13C{1H} NMR ([D8]THF, 100 MHz): d=200.0 (CꢁO), 197.6 (CꢁO), 189.2
(CꢁO), 176.5 (NCCH3), 156.9 (PyC(2)), 153.5 (PyC(6)), 152.9 (PyC(4)),
151.1 (ArC(1)), 130.5 (br, ArC(3and5)), 130.3, 130.2 (PyC(3and5)),
128.1 (ArC(4)), 123.0 (br, ArC(2or6)), 121.4 (br, ArC(2or6)), 21.4
(PyC(4)CH3), 18.4 ppm (NCCH3); IR (THF): n˜ =2020 (s, CꢁO), 1921 (s,
CꢁO), 1892 cmÀ1 (s, CꢁO); IR (MeCN):n˜ =2021 (s, CꢁO), 1918(s, Cꢁ
O), 1897 cmÀ1 (s, CꢁO); IR (film): n˜ =2981 (w), 2967 (sh), 2869 (w),
2016 (s, CꢁO), 1907 (s, CꢁO), 1887 (s, CꢁO), 1621 (w), 1604 (w),
1593 (w), 1560 (vw), 1486 (w), 1452 (w), 1442 (sh), 1378 (w), 1330
(w), 1304 (vw), 1260 (w), 1224 (w), 1156 (vw), 1088 (sh), 1070 (w),
1030 (w), 1003 (vw), 913 (w), 855 (sh), 834 (w), 775 (w), 741 (w),
699 cmÀ1 (w); elemental analysis calcd (%) for C17H14ClN2O3Re: C
39.57, H 2.74, N 5.43; found: C 39.94, H 2.86, N 5.64.
1
into a toluene solution at À358C. H NMR ([D8]THF, 400 MHz): d=
3
4
5
8.33 (ddd, 1H; J=8.4, J=1.4, J=1.9 Hz, PyH(3)), 7.61 (ddd, 1H;
3
4
3
3
3J=8.5, J=7.1 J=1.5 Hz, PyH(4)), 7.17 (ddd, 1H; J=7.1, J=5.8,
4J=1.4 Hz, PyH(5)), 6.94, 6.90 (m, 2H; ArH(3), ArH(5)), 6.21 (ddd,
3
4
5
1H; J=5.9, J=1.4, J=1.0 Hz, PyH(6)), 2.51 (s, 3H; NCCH3), 2.30
(s, 3H; ArC(4)CH3), 2.08 (s, 3H; ArC(2or6)CH3), 1.97 ppm (s, 3H; Ar-
C(2or6)CH3); 13C{1H} NMR ([D8]THF, 100 MHz): d=203.2 (CꢁO), 200.6
(CꢁO), 190.1 (CꢁO), 159.1 (NCCH3), 151.8 (PyC(6)), 150.1 (PyC(2)),
149.1 (ArC(1)), 136.0 (ArC(4)), 131.0, 130.0, 129.9, 129.72, 129.66
(ArC(2), ArC(3), ArC(5), ArC(6), PyC(4)), 127.9 (PyC(3)), 120.6 (PyC(5)),
21.0 (ArC(4)CH3), 20.3 (ArC(2or6)CH3), 18.7 (ArC(2or6)CH3), 17.5 ppm
(NCCH3); IR (THF): n˜ =1991 (s, CꢁO), 1962 (s, CꢁO), 1902 (m, CꢁO),
1885 (s, CꢁO), 1862 cmÀ1 (m, CꢁO); IR (film): n˜ =2984 (w), 2922 (w),
2859 (w), 1986 (m, CꢁO), 1955 (s, CꢁO), 1898 (m, CꢁO), 1875 (s, Cꢁ
O), 1861 (sh, CꢁO), 1595 (w), 1540 (vw), 1486 (w), 1457 (w), 1437
(w), 1401 (m), 1381 (w), 1343 (m), 1254 (m), 1209 (w), 1157 (w),
1082 (w), 1067 (vw), 1013 (vw), 996 (m), 982 (w), 953 (vw), 859
(vw), 761 (w), 735 cmÀ1 (w); elemental analysis calcd (%) for
C38H36N4O6Re2: C 44.87, H 3.57, N 5.51; found: C 44.64, H 3.60, N
5.53.
Synthesis of PrArL-ReTFA: Silver trifluoroacetate (56 mg; 0.25 mmol)
was dissolved in THF (3 mL) in an amber vial. To this stirred solu-
tion a solution of PrArL-Re (59 mg, 0.10 mmol) in THF (2 mL) was
added. The reaction mixture was stirred for 2 h in the dark, filtered,
and the solvent was removed in vacuum. The solid residue was co-
evaporated two times with pentane (2 mL) and then suspended
into a mixture of Et2O/THF (1:1). After filtration and removal of the
solvent, the raw product was recrystallized by pentane diffusion
into a THF solution, yielding 45 mg (0.068 mmol, 68%) of complex
PrArL-ReTFA. Single crystals suitable for X-ray diffraction were ob-
tained from pentane diffusion into a CH2Cl2 solution. 1H NMR
([D8]THF, 400 MHz): d=9.19 (ddd, 1H; 3J=5.4, 4J=1.5, 5J=0.7 Hz,
Preparation of PhL-ReÀ: The complex PhL-Re (13 mg, 26 mmol) was
dissolved in THF (4 mL) and the solution was added to freshly pre-
pared Na/Hg (18 mg, 0.78 mmol, Na in 2.646 g Hg) upon which the
color of the solution quickly turned green. The mixture was stirred
for one hour and became deep red after this time. The solution
Chem. Eur. J. 2016, 22, 1 – 14
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400 MHz): d=8.59 (ddd, 1H; 3J=6.7, 4J=1.2, 5J=1.2 Hz, PyH(6)),
7.12 (dd, 2H; 3J=8.2, 3J=7.3 Hz, ArH(3,5)), 7.01 (dd, 2H; 3J=8.2,
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3
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4J=1.2 Hz, ArH(2,6)), 6.91 (tt, 1H; J=7.3, J=1.2 Hz, ArH(4)), 6.64
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3
4
5
3
(ddd, 1H; J=9.2, J=1.2, J=1.2 Hz, PyH(3)), 5.92 (ddd, 1H; J=
3
4
3
3
4
9.2, J=5.9, J=1.1 Hz, PyH(4)), 4.96 (ddd, 1H; J=6.7, J=5.9, J=
1.3 Hz, PyH(5)), 1.96 ppm (s, 3H; NCCH3); 13C{1H} NMR ([D8]THF,
100 MHz): d=211.6 (CꢁO), 158.9 (ArC(1)), 153.8 (PyC(6)), 136.5
(PyC(2)), 127.9 (ArC(3,5)), 127.2 (ArC(2,6)), 123.4 (NCCH3), 123.3
(ArC(4)), 122.5 (PyC(3)), 118.4 (PyC(4)), 102.7 (PyC(5)) and 14.7 ppm
(NCCH3); IR (THF): n˜ =1954 (s, CꢁO), 1851 (s, CꢁO), 1810 cmÀ1 (s,
CꢁO).
Preparation of PrArL-ReÀ: The chlorido complex PrArL-Re (45 mg;
0.077 mmol) was dissolved in THF (3 mL) and this solution was
added to sodium (96 mg, 4.2 mmol) dissolved in Hg (14 g). The re-
action mixture was stirred for 14.5 h and filtered. The solvent was
then removed in vacuum and the residue was co-evaporated with
pentane (2 mL) and dried in vacuum. NMR spectroscopic character-
ization indicated the clean formation of the complex PrArL-ReÀ.
Single crystals suitable for X-ray diffraction and the analytically
pure complex were obtained from a diluted pentane solution,
which contained traces of THF (ꢀ10:1) at À358C, yielding 38 mg
(0.041 mmol; 53%) of the complex PrArL-ReÀ·5THF. Although the
asymmetric unit of the crystal structure contains five molecules of
THF per complex molecule, elemental analysis of the crystalline
material after excessive drying in vacuum indicated complete loss
of these solvent molecules. 1H NMR ([D8]THF, 400 MHz): d=8.61
(ddd, 1H; 3J=6.7, 4J=1.2, 5J=1.2 Hz, PyH(6)), 7.04–6.93 (m, 3H;
3
4
5
ArH(3,4,5)), 6.67 (ddd, 1H; J=9.3, J=1.2, J=1.2 Hz, PyH(3)), 5.93
3
3
4
3
(ddd, 1H; J=9.3, J=5.9, J=1.1 Hz, PyH(4)), 4.99 (ddd, 1H; J=
6.9, 3J=5.9, 4J=1.1 Hz, PyH(5)), 2.72 (sept, 2H; 3J=6.9 Hz,
CH(CH3)2), 1.83 (s, 3H; NCCH3), 1.15 (d, 3H; 3J=6.8 Hz, CH(CH3)2),
1.01 ppm (d, 3H; 3J=7.0 Hz, CH(CH3)2); 13C{1H} NMR ([D8]THF,
100 MHz): d=211.6 (CꢁO), 154.2 (PyC(6)), 153.4 (ArC(1)), 136.7
(PyC(2)), 142.7 (ArC(2,6)), 124.7 (ArC(4)), 122.6 (NCCH3), 122.5
(ArC(3,5)), 122.3 (PyC(3)), 118.4 (PyC(4)), 102.3 (PyC(5)), 27.7
(CH(CH3)2), 25.9, 24.3 (2ꢂCH(CH3)2), 14.4 ppm (NCCH3); IR (MeCN):
n˜ =1951 (s, CꢁO), 1844 (s, CꢁO), 1835 cmÀ1 (s, CꢁO); IR (THF): n˜ =
1952 (s, CꢁO), 1851 (s, CꢁO), 1810 cmÀ1 (s, CꢁO); elemental analysis
calcd (%) for C22H24N2NaO3Re: C 46.06, H 4.22, N 4.88; found: C
45.26, H 4.55, N 4.97.
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Acknowledgements
This material is based upon work performed by the Joint
Center for Artificial Photosynthesis, a DOE Energy Innovation
Hub, supported through the Office of Science of the U.S. De-
partment of Energy under Award Number DE-SC0004993. Mi-
chael Takase, Lawrence M. Henling and David G. VanderVelde
are thanked for their invaluable work in the X-ray (M.T. and
L.M.H.) and NMR (D.G.V.V) facilities at Caltech and a lot of help-
ful discussions. The authors are indebted to Natascha Junker,
Bernhard E. C. Bugenhagen, and especially Prof. Peter Burger
for help with the DFT calculations and generous computation
time on the IAAC cluster at the University of Hamburg.
Keywords: alpha diimines · NMR spectroscopy · redox-active
ligands · rhenium · X-ray crystallography
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Received: February 14, 2016
Published online on && &&, 0000
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FULL PAPER
Linear correlation: A series of diamag-
netic pyridine monoimine rhenium com-
plexes with different degrees of metal-
to-ligand charge transfer was investigat-
ed by 13C NMR spectroscopy and single-
crystal X-ray diffraction and linear corre-
lations were observed correlating rele-
vant bond length with the carbon NMR
shifts of the respective carbon atoms.
&
Rhenium Complexes
D. Sieh, C. P. Kubiak*
&& – &&
A Series of Diamagnetic Pyridine
Monoimine Rhenium Complexes with
Different Degrees of Metal-to-Ligand
Charge Transfer: Correlating 13C NMR
Chemical Shifts with Bond Lengths in
Redox-Active Ligands
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Chem. Eur. J. 2016, 22, 1 – 14
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ÝÝ These are not the final page numbers!