Heteroleptic Ter-Bidentate Cr(III) Complexes as Tunable Optical Sensitizers

Article abstract of DOI:10.1021/acs.inorgchem.8b02530

To exploit Cr(III) coordination complexes as sensitizers in supramolecular energy-converting devices, the latter optical relays should display long-lived excited states, broad emission bands, and tunable spatial and electronic connections to activator units. An ad-hoc versatile strategy has been therefore developed for the preparation of a family of luminescent pseudo-octahedral [CrN₆] chromophores made up of ter-bidentate heteroleptic [Cr(phen)₂(N-N'′)]³⁺ complexes, where phen is 1,10-phenanthroline, and N-N′ stands for α,α′-diimine ligands possessing peripheral substituents compatible with both electronic tuning and structure extensions. As long as the ligand field in these [CrN₆] chromophores remains sufficiently strong to avoid back-intersystem crossing, photophysical studies indicate that the lifetime of the near-infrared emissive Cr(²E) excited state is poorly sensitive to ligand-based electronic effects. On the contrary, a drop in symmetry, the coupling with high frequency oscillators, and the implementation of sterical constraints in heteroleptic [Cr(phen)₂(N-N′)]³⁺ complexes affect both Cr(²E a? ⁴A₂) energies and Cr(²E) lifetimes. Altogether, [Cr(phen)₂(phenAlkyn)]³⁺ (phenAlkyn = 5-ethynyl-1,10-phenanthroline) and [Cr(phen)₂(dpma)]³⁺ (dpma = di(pyrid-2-yl)(methyl)amine) complexes mirror the favorable photophysical properties of homoleptic [Cr(phen)₃]³⁺ and thus emerge as the best heteroleptic candidates for acting as sensitizers at room temperature, and below 100 K, respectively, in more complicated architectures.

Full text of DOI:10.1021/acs.inorgchem.8b02530

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Heteroleptic Ter−Bidentate Cr(III) Complexes as Tunable Optical
Sensitizers
Benjamin Doistau,* Guillaume Collet, Emilio Acun
̃
a Bolomey, Vida Sadat-Noorbakhsh,†
,
†
†,§
†
Cel
́
ine Besnard,‡ and Claude Piguet*
,†
†Department of Inorganic and Analytical Chemistry, University of Geneva, 30 quai Ernest Ansermet, CH-1211 Geneva 4,
Switzerland
‡
Laboratory of Crystallography, University of Geneva, 24 quai Ernest Ansermet, CH-1211 Geneva 4, Switzerland
*
S Supporting Information
ABSTRACT: To exploit Cr(III) coordination complexes as sensitizers in
supramolecular energy-converting devices, the latter optical relays should
display long-lived excited states, broad emission bands, and tunable spatial and
electronic connections to activator units. An ad-hoc versatile strategy has been
therefore developed for the preparation of a family of luminescent pseudo-
octahedral [CrN ] chromophores made up of ter−bidentate heteroleptic
6
3+
[
Cr(phen) (N−N’′)] complexes, where phen is 1,10-phenanthroline, and N−
2
N′ stands for α,α′-diimine ligands possessing peripheral substituents compatible
with both electronic tuning and structure extensions. As long as the ligand field
in these [CrN ] chromophores remains sufficiently strong to avoid back-
6
intersystem crossing, photophysical studies indicate that the lifetime of the near-
2
infrared emissive Cr( E) excited state is poorly sensitive to ligand-based
electronic effects. On the contrary, a drop in symmetry, the coupling with high
frequency oscillators, and the implementation of sterical constraints in
3
+
2
4
heteroleptic [Cr(phen) (N−N′)] complexes affect both Cr( E → A2)
2
2
3+
energies and Cr( E) lifetimes. Altogether, [Cr(phen) (phenAlkyn)] (phenAlkyn = 5-ethynyl-1,10-phenanthroline) and
Cr(phen) (dpma)] (dpma = di(pyrid-2-yl)(methyl)amine) complexes mirror the favorable photophysical properties of
homoleptic [Cr(phen) ] and thus emerge as the best heteroleptic candidates for acting as sensitizers at room temperature, and
2
3+
[
2
3+
3
below 100 K, respectively, in more complicated architectures.
INTRODUCTION
coordination complexes with the preparation of [CrN6]
■
chromophores (strong ligand fields Δ and weak Racah
Cr(III)-containing coordination complexes have recently
attracted a renewal of interest for their magnetic, redox, and
optical properties, which proved to be useful for various
modern applications where this earth-abundant metal could
30−32
parameters B and C)
programmed for acting as sensitizers
in molecular lanthanide-based energy transfer upconver-
33−36
sion.
The associated micro- to millisecond lifetimes (τS)
1
,2
advantageously replace rare ruthenium or platinum ores.
observed for the metal-centered excited-state levels in [CrN ]
6
First, the orbital-free magnetic moments of trivalent chromium
complexes are ideally suited for rationalizing and optimizing (i)
S→A
ET
complexes maximize energy transfer efficiency η
in
3
−6
7−11
37,38
magnetic coupling,
(ii) zero-field splitting,
and (iii)
implemented in poly-
Second, the Cr(III) polypyridyl
complexes are famous for displaying three successive ligand-
sensitizer → activator pair (eq 1)
and are therefore well-
12,13
single-molecular magnet behavior
metallic architectures.
adapted for being harnessed for the sensitization of nearby
1
4−17
rad
S
activators in polymetallic assemblies. (k τ = Φ corresponds
S
S
1
1
18
to the intrinsic quantum yield of the donor emissive level. κ is
the orientation factor between sensitizer and activator dipoles.
centered or one-metal-centered reductions, which allows
19
their incorporation into memory devices. Third, the strongly
2
oxidative Cr( E) spin-flip excited state can be exploited (i) in
n is the refractive index of the medium. NAv is the Avogadro
r
−1
38
photocathodic solar cells (by grafting chromium polypyridyl
number in mol . r is the separation between donor and
acceptor, and J(λ) is the overlap integral between the emission
spectrum of donor (S) and the absorption spectrum of the
20
complexes onto semiconductor surfaces), (ii) for carbon−
21−23
carbon bond formation,
and (iii) for water splitting and
2
4
O2 production. Finally, the remarkable metal-centered
optical properties of Cr(III) complexes associated with
39−45
acceptor (A)).
octahedral [CrO ] units found in solid oxides (weak ligand
6
25−29
fields Δ and large Racah parameters B and C),
which gave
Received: September 5, 2018
rise to the first ruby lasers, can be completely reversed in
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02530
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
−
1
S→A
5
4
6
and gave the starting material [Cr(phen) Cl ]Cl (1), the
i
y
2
2
k
j
128π n N r
z
S→A
ET
j
r
2
Av
z
η
= k
= j1 +
z
crystal structure of which was solved and is shown in Figure 1
(Figure S1 and Tables S1, S2, and S4 in the Supporting
j
j
z
z
ET
S→A
rad
S
+ kS
9(ln 10)κ J(λ)k
τ
ET
k
S {
54
(
1)
Information).
rad
Reasonably assuming comparable radiative rate constant (k )
S
2
for [CrN ] complexes with similar symmetries, the Cr( E)
6
excited-state lifetime τ thus appears as the most relevant
S
tunable parameter with J(λ) and r for optimizing energy
S→A
transfer efficiency (ηET ). Despite these remarkable photo-
physical properties, alternative Ru(II) chromophores, which
3
display short MLCT lifetimes within the microsecond range,
are by far more exploited than Cr(III) as sensitizers in
polymetallic devices; this is mainly because of the complicated
and weakly developed synthetic strategies leading to
31,46,47,42
heteroleptic chromium complexes.
Currently limited
to homoleptic complexes, Cr(III)-polypyridyl chromophores
have been only sporadically involved as optical partners in
Figure 1. Molecular structures of the cations in the crystal structures
of [Cr(phen) Cl ]Cl·(CH ) NCHO (1), [Cr(phen) (H O)(OH)]-
48−51
52
polymetallic architectures3
exhibiting light downshifting
2
2
3
2
2
2
or upconverted emission. 3−36 Given that the well-known D
(
CF SO ) (5), and [phenAlkynH](CF SO ) (4) showing H bond
3 3 2 3 3
3
3
+
3+
length. Color code: Cr (yellow), C (gray), N (blue), O (red), S
orange), F (pale green), Cl (green). Ellipsoids are drawn at 50%
probability. The Δ enantiomer is represented for each complex.
symmetrical ter−bidentate [Cr(bipy) ] or [Cr(phen)3]
complexes (i) offer a large array of functionalization
3
(
possibilities due to the rich bipy and phen ligand chemistry,
(
ii) display acceptable metal-centered lifetimes (63 μs ≤ τ ≤
3
56 μs), and (iii) may undergo specific ligand substitutions to
Subsequent reaction of [Cr(phen) Cl ]Cl with trifluorome-
2 2
thanesulfonic acid, followed by precipitation in diethyl ether,
led to the formation of the key intermediate [Cr-
3
0,39,42−44
give heteroleptic derivatives,
the target [Cr-
3+
(
phen) (N−N′)] building blocks emerge as valuable and
2
versatile candidates for working as heteroleptic sensitizers in
extended polymetallic structures. However, the remarkable
(phen) (CF SO ) ]CF SO . Addition of 1,10-phenanthroline
2 3 3 2 3 3
(phen), or of 5-bromo-1,10-phenanthroline (phenBr), ulti-
mately yielded, respectively, 41% and 66% of the target pure
42,43
strategy developed by Kane-Maguire
heteroleptic ter−bidentate chromium complexes does not
tolerate electron-withdrawing ligands,
for the preparation of
homoleptic [Cr(phen)
Tables S1, S3, and S5)
(phen) (phenBr)](PF (3, Figure 2; Figure S5 and Tables
](PF ) (2, Figure 2; Figure S2 and
6 3
55,56
3
42−44
and we are aware of
and heteroleptic [Cr-
a single recent report, in which this limitation was partially
2
)
6 3
19
overcome via overheating under microwave radiation. To
more globally solve this problem, we describe herein a novel
versatile synthetic method that permits the synthesis of a
S7, S8, and S10) complexes after metathesis with hexafluor-
ophosphate anions. The latter synthetic method failed when
using 1,10-phenanthroline possessing weak nitrogen donors as
3
+
complete family of heteroleptic [Cr(phen) (N−N′)] com-
found in 5-nitro-phenantroline (phenNO ) or in a more
2
2
plexes with few, if any, electronic restrictions. This work then
highlights how the variations in ligand-based electronic and
geometrical characteristics (Scheme 1) affect the Cr(III)
delocalized π system as found in 5-ethynyl-1,10-phenanthro-
line (phenAlkyn). In the latter case, reaction of [Cr-
(phen) (CF SO ) ]CF SO with phenAlkyn resulted in the
2 3 3 2 3 3
isolation of single crystals of off-white protonated ligand
phenAlkynH (4, Figure 1; Figure S3 and Tables S6), together
Scheme 1. Chemical Structures of the Bidentate Ligands
Used in This Work
with [Cr(phen) (H O)(OH)](CF SO ) complex (5, Figure
2
2
3
3 2
1
, Figure S4 and Table S6) as red crystals, which suggest the
competitive substitution of triflate anions with water
molecules.
For the prevention of unwanted ligand protonation and
complex hydrolysis, (i) [Cr(phen) Cl ]Cl (1) was carefully
2
2
dried under vacuum; (ii) [Cr(phen) (CF SO ) ]CF SO was
2
3
3 2
3
3
immediately used after synthesis without storage, and last but
not least, (iii) a sacrificial base was added during the last step
for trapping the excess of trifluoromethanesulfonic acid.
Pyridine, but especially sterically constrained 2,6-lutidine, was
found to be well-suited for this purpose, since disastrous base-
catalyzed Cr(III) complex hydrolysis could be avoided with
57
ligand-field and nephelauxetic parameters, together with their
consequences on the degeneracy and lifetime of the emissive
doublet excited state, two parameters which are crucial for
further exploitation as sensitizers.
these weak bases (Scheme 2). It should be highlighted that
tremendous ether washing of [Cr(phen) (CF SO ) ]CF SO
2
3
3 2
3
3
intermediate under argon did not permit avoidance of the use
of sacrificial base.
With this modified procedure in hand, [Cr-
RESULTS AND DISCUSSIONS
■
(phen)
Tables S7, S9, and S11), [Cr(phen) (phenNO )](CF SO )
3 3
2 4 3
(phenAlkyn)](BF ) (6, Figure 2; Figure S6 and
Synthesis and Characterization of Heteroleptic Ter−
Bidentate Cr(III) Complexes. The first step of the Kane-
2
2
3
(7, Figure 2; Figure S7 and Tables S12, S13, and S15),
[Cr(phen) (phenNH )](CF SO ) (8, Figure 2; Figure S8
4
2,53
Maguire strategy
was successfully reproduced in our hands
2
2
3
3 3
B
DOI: 10.1021/acs.inorgchem.8b02530
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
and chelate N−Cr−N bite angles (80.3(1)−81.1(1)°) are
similar, whatever substituents are bound to the phenanthroline
unit. Altogether, the [CrN ] chromophores can best be
6
described as pseudo-octahedrons slightly compressed along
the pseudo-3-fold helical axis (Table S22). As ex-
3
2,18,58,59
pected,
the N−Cr−N bite angle in [Cr-
(
(
(
phen) (dpma)](CF SO ) (10) is larger for dpma
2
3
3 3
85.4(1)°, six-membered chelate rings) than for phen ligand
80.0(1)°, five-membered chelate rings). The consequent
increased overlap between the donor orbitals of dpma and
3
d orbitals of trivalent chromium slightly shortens the Cr−
N(dpma) distances (2.045(4) Å compared with 2.067(4) Å
found for the partner phen ligands, Table S22). Neglecting for
a while the specific peripheral substitution of the phenanthro-
line units, all the [CrN ] chromophores discussed above
6
display local close-to-D symmetry, except [Cr-
3
3
+
(
phen) (dpma)] , for which distortion from 3-fold symmetry
2
becomes significant.
Photophysical Properties of Heteroleptic Ter−Biden-
tate Cr(III) Complexes: Ligand-Field Strength, Neph-
elauxetic Effect, and Symmetry. The key point here
concerns the unusual large energy gap ΔE = Δ − 9B − 3C +
2
4
2
5
0(B /Δ) induced between the Cr( T ) and Cr( E)
2
Figure 2. Molecular structures of the cationic complexes in the crystal
structures of [Cr(phen) ] (PF ) ·6.33CH CN (2), [Cr-
spectroscopic levels in pseudo-octahedral [CrN ] chromo-
phore, which maximizes Cr( E → A ) phosphorescence and
Cr( E) excited-state lifetimes via the reduced probability of
phonon-assisted back-intersystem crossing (Figure 3).
6
2
4
3
2
6
6
3
2
2
(
(
(
(
(
phen) (phenBr)](PF ) ·2CH CN (3), [Cr(phen) (phenAlkyn)]-
2 6 3 3 2
1
8
BF ) ·2CH CN (6), [Cr(phen) (phenNO )](CF SO ) (7), [Cr-
4
3
3
2
2
3
3
3
phen) (phenNH )](CF SO ) ·1.83CH CN (8), [Cr-
2
2
3
3
3
3
phen) (bipy)](CF SO ) (9) and [Cr(phen) (dpma)](CF SO )
2
3
3
3
2
3
3 3
10). Cr (yellow), C (gray), N (blue), O (red), Br (brown).
Ellipsoids are drawn at 50% probability. Hydrogens and counter-
anions are omitted for clarity. The Δ enantiomer is represented for
each complex.
and Tables S12, S14, and S16), [Cr(phen) (bipy)](CF SO )
3 3
2
3
(
[
9, Figure 2; Figure S9 and Tables S17, S18, and S20), and
Cr(phen) (dpma)](CF SO ) (10, Figure 2; Figure S10 and
2
3
3 3
Tables S17, S19, and S21) were obtained in good yields and
without requiring large excess (maximum 2 equiv) of the
entering N−N′ ligands. All complexes were characterized by
elemental analysis, ESI-MS, and X-ray-diffraction studies. In all
cationic complexes, the three bidentate binding units are
wrapped around a pseudo-3-fold axis passing through the
chromium atom. Both Λ and Δ enantiomers are present in
racemic proportion, which has no impact on the photophysical
properties of the mononuclear heteroleptic complexes.
However, it should be noted that further incorporation into
polymetallic architectures might provide complicated mixtures
Figure 3. Energy level diagram for [Cr(III)N ] chromophores in
octahedral symmetry highlighting the lowest spin-allowed absorption
6
3+
(
green arrow), and the lowest spin-forbidden emission (red arrow).
of stereoisomers. Except for [Cr(phen) (dpma)] in 10, in
2
The pertinent energy levels can be computed by using ligand field Δ,
and Racah parameters B and C.
which the dpma ligand introduces an unusual six-membered
chelate ring, all Cr−N bond lengths (2.045(4)−2.063(4) Å)
60,61
3+
Scheme 2. Versatile Synthetic Strategy for the Preparation of Heteroleptic [Cr(phen) (N−N′)] Complexes
2
C
DOI: 10.1021/acs.inorgchem.8b02530
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Inorganic Chemistry
Article
Among a large palette of homoleptic strong-field trivalent
27,62
3+
chromium complexes,
[Cr(ddpd) ] was shown to display
2
the longest room-temperature lifetime (898 μs, Figure 4
1
8
top), which can be further extended by 30% via systematic
58
peripheral deuteration (1.2 ms).
Figure 5. Normalized absorption spectra (full blue traces, solid state
(
ν̅exc
left) and solution (right), 293 K), emission spectrum (λ = 355 nm,
exc
−
1
= 28 170 cm , red trace, frozen solution acetonitrile/
propionitrile (6/4), 10 K) and excitation spectrum (λ = 731 nm
em
−
1
(
ν̅
e
= 13 680 cm ), dashed green line, solid state, 10 K) recorded
m
for [Cr(phen) (phenAlkyn)](BF ) (6). The spectroscopic labels are
2
4 3
those pertinent to idealized octahedral symmetry.
spectroscopic labels assume O symmetry), by intense ligand-
to-metal charge transfer (LMCT) and/or ligand-centered
4
3
4
[
(π* ← π)] transitions accompanied by spin-flip in the A
2
−1
−
1
ground state (22 000−28 000 cm , ε ≈ 500−4000 L mol
cm )
energy (>28 000 cm ; ε ≈ 4000−20 000 L mol cm , Table
S23). It should be noted that an unambiguous discrimination
between the [ (π* ← π)] and Cr( T ← A ) transition is
complicated, but (i) the lack of band structuration and (ii)
their weak intensity (ε < 300 L mol cm ) suggest
assignment to ligand-field transitions.
−
1
66,45,67
and by ligand-centered π* ← π transitions at high
−
1
−1
−1
Figure 4. Chemical structures of homoleptic bis-terdentate (top) and
ter−bidentate (bottom) chromium complexes where the average N−
4
3
4
4
2
2 2
Cr−N chelate angles (blue), and emissive excited Cr( E) lifetimes in
31,18,64,65
solution at room temperature (red), are highlighted.
−
1
−1
66,67
UV excitation into
Those impressive features originate from (i) the weak energy
of the Cr( E) level (12 903 cm ), combined with the strong
2
−1
the ligand-centered absorption bands (λ = 355 nm, ν
=
exc
e
xc
−
1
2
8 170 cm ) provides low-temperature emission spectra with
ligand field induced by minor distortions from local octahedral
4
a single band (accompanied by vibrational progression at lower
symmetry which puts the Cr( T ) level at high energy (22 989
2
2
−
1
2
4
energy) assigned to the famous near-infrared spin-flip Cr( E →
cm ) and (ii) the promotion of E/ A surface crossing at
2
4
A2) luminescence (red trace in Figure 5, Figure S25 and Table
high energy due to weak distortion from octahedra and
6
3,32
S24). At room temperature, thermal equilibrium between the
anharmonic potential misalignment.
The fused six-
2
4
2
2
4
2
T and E states results in dual Cr( T → A ) and Cr( E →
membered chelate rings produced by the ddpd ligand indeed
1
1
2
18
68
fix average N−Cr−N chelate angles of 85.9°, close to 90°
compared with only 78.5° measured for fused five-membered
A
2
) phosphorescence (Figure S26 and Table S25),
a
phenomenon corroborated by excitation spectra recorded
3
+
2
4
chelate rings found in the analogous [Cr(tpy) ] complex
upon analyzing the Cr( E → A ) emissions, which
2
2
3
1
2
(
Figure 4 top). The associated short Cr( E) lifetime
measured for [Cr(tpy)2] (τ < 30 μs) is however much less
demonstrate efficient sensitization originating from the close
3+
2
Cr( T ) level (green trace in Figure 5, Figures S27−S33 and
1
favorable for its use as an optical sensitizer. Moving toward
Table S26). For all [CrN ] complexes displaying D symmetry,
6
3
3
+
3+
2
4
ter−bidentate [Cr(bipy) ] or [Cr(phen)3] complexes (i)
the Cr( T ← A ) transition is split into two components (see
3
1 2
reduces the number of unfavorable constrained 5-membered
chelate rings, thus restoring longer metal-centered lifetimes
the low-energy part of the green trace in Figure 5), which is
diagnostic for a noticeable deviation from pure O symmetry in
64,65
63
(
Figure 4 bottom)
and (ii) offers versatile access to Cr(III)
the [CrN ] chromophores. According to Endicott, the drop
6
complexes due to the rich chemistry of bipy or phen, compared
to the weakly developed ddpd functionalization.
Room-temperature absorption spectra recorded for the
in symmetry induced by the constrained chelate bite angles
(transforming O into D point groups, see previous section)
h
3
results in the splitting of the t orbitals (O symmetry) into a
2g h 1
CrN complexes 2, 3, and 6−10 in the solid state and in
+ e orbitals (D symmetry) separated by a small energy gap.
The interelectronic repulsions accompanying the filling of
6
3
acetonitrile solution (blue traces in Figure 5, Figures S11−
2
4
S24) are dominated by weak spin-forbidden Cr( E ← A ) and
these orbitals with three electrons in Cr(III) complexes
2
2
4
4
Cr( T ← A ) transitions at low energy (13 500−14 500
cm , ε < 15 L mol cm ), by moderately intense spin-
produce four spectroscopic levels: a Cr( E) spin quartet
1
2
−
1
−1
−1
2
ground state; a low-energy doubly degenerate doublet Cr( E)
4
4
allowed Cr( T
←
A ) transitions at medium energy
excited level, reminiscent to that found in O symmetry; and
2
2
−
1
−1
−1
2
2
(
21 000−22 000 cm , ε ≈ 50−500 L mol cm , the
two higher-energy doublet Cr( A ) + Cr( E) levels, reminis-
2
D
DOI: 10.1021/acs.inorgchem.8b02530
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2
cent to the Cr( T ) level found in O symmetry. This explains
1
2
the observation of three spin-flip absorption bands: two Cr( E
4
2
4
2
4
←
E) (arising from Cr( T ← A ) and Cr( E ← A ) in O
1 2 2
2
4
2
point group) and one Cr( A ← E) (arising from Cr( T ←
A ) in O point group) for [CrN ] complexes in D symmetry
Figure 5, low-energy blue trace). Moreover, the energy gap
2
1
4
2
6
3
63
(
between the half-occupied Cr-centered a + e orbitals produces
1
some changes in the bonding parameters between the spin-flip
63
excited states and the ground state. The consequent shift in
the nuclear coordinates is accompanied by minor, but non-
−
1
negligible Stokes shifts (≤38 cm ; 2 nm) together with
2
4
2
4
vibronic progressions for the Cr( E → E) (Cr( E → A ) in O
2
point group) emission bands as observed in [Cr(phen) ]-
3
(
(
(
[
PF6)3 (2), [Cr(phen) (phenBr)](PF ) (3), [Cr-
2 6 3
phen) (phenAlkyn)](BF ) (6), [Cr(phen) (phenNO )]-
2
4
3
2
2
CF SO ) (7), [Cr(phen) (phenNH )](CF SO ) (8), and
3
3 3
2
2
3
3 3
Cr(phen) (bipy)](BF ) (9) (Figure 5 and Figures S11−
2
4 3
S25). A detailed analysis of the vibronic progression, using
anharmonic Morse potentials for modeling the energy gap ΔE
=
E(V + 1) − E(V) between two adjacent vibrational levels of
the ground state (ν is the fundamental frequency and D the
0
69
ground-state dissociation energy), is summarized in eq 2.
2
ΔE = E(V + 1) − E(V) = hν − (V + 1)(
hν0)
0
(2)
2D
Plots of ΔE = E(V + 1) − E(V) as a function of V + 1 indeed
gave straight lines (Figure S34), from which 855 ≤ D ≤ 1920
−
1
−1
cm and 163 ≤ ν /c ≤ 182 cm can be calculated for the
0
69
ground state (Table S27). The vibronic progression induced
by the release in symmetry involves Cr−N stretching modes
variations found for these [CrN ] coordination units in their
crystal structures. As expected, the strong inductive (Hammett
6
−
1
2
4
(
(
163−182 cm ), which altogether spread the Cr( E → E)
2
4
Cr( E → A ) in O point group) emission band over 600
coefficient σ (NO ) = 0.67) and resonance (Hammett
2
I
2
−
1
cm . This represents a considerable advantage for maximizing
coefficient σ (NO ) = 0.16) electron-withdrawing nitro
R 2
the spectral overlap integrals J(λ) between the Cr(III)-based
group, when connected to one phenanthroline unit in
sensitizer and nearby activators (eq 1). [Cr(phen) (dpma)]-
[Cr(phen) (phenNO )](CF SO ) (7), weakens the ligand
2
2
2
3
3 3
(
CF SO ) (10) undergoes a further drop in symmetry
field (Δ decreases) and reinforces the nephelauxetic effect (B
decreases). As rather naive chemists, we originally expected
that the connection of the opposite electron-donating amino
group in [Cr(phen) (phenNH )](CF SO ) (8) should result
3
3 3
because the [CrN ] coordination sphere is now close to C
6
2
symmetry. Within this point group, only monodimensional
irreducible representations are available, and the orbital
2
2
3
3 3
2
4
2
4
degeneracies of the spin-flip Cr( T ↔ A ) and Cr( E↔ A )
in the opposite trend. However, Hammett coefficients teach
chemists that amino substituents, because of the presence of
electronegative nitrogen atoms, are indeed (poor) σ-acceptors
(σ (NH ) = 0.17), while donation concerns only π effects
1
2
2
transitions in O symmetry are completely lifted. We therefore
expect and observe two groups of absorption in the NIR
3+
domain for [Cr(phen) (dpma)] with three (originating from
2
I
2
70
the T term) and two (originating from the E term)
components (Figure 6), respectively. The splitting of the
(σ (NH ) = −0.48). With this in mind, some efficient π-
R
2
overlap between filled phenNH orbitals and Cr(t ) orbitals is
2
2g
2
Cr( E) level is corroborated by the detection of dual NIR
expected to reduce Δ, while electron delocalization is
simultaneously restricted to yield large Racah parameter B, a
prediction in agreement with experimental data summarized in
Figure 7. In addition to the detrimental N−H high-frequency
emission at 293 K (Figure S26). As a matter of clarity,
throughout the rest of the study the O point group will be
assumed for labeling energy levels.
7
1,28,72,73
4
2
As a consequence of the symmetry release, the global near-
infrared emission of [Cr(phen) (dpma)](CF SO ) (10) is
multiphonon relaxation,
the reduced Cr( T )−Cr( E)
2
3+
energy gap in [Cr(phen) (phenNH )] favors back-intersys-
2
3
3 3
2
2
spread over 1300 cm− at 10 K, which makes this complex the
most versatile candidate for optimizing spectral overlap
between Cr(III) sensitizer and potential activators. The
combination of the approximate equations gathered in Figure
1
tem crossing (BISC), two characteristics which limit metal-
centered phosphorescence as previously documented for
analogous nonemissive bis-terpyridine Cr(III) complexes
45
bearing an amino group.
6
0,61
4
3
with the experimentally accessible energies found for the
Emissive Properties of Heteroleptic Ter−Bidentate
2
2
4
Cr( T ), Cr( T ), and Cr( E) levels (relative to the Cr( A )
Cr(III) Complexes: Excited-State Lifetimes. Time-resolved
2
1
2
2
ground state taken as a reference) provide (i) estimations for
the ligand field Δ, and Racah parameters B and C (Table 1)
together with (ii) predictions for the energies of the missing
emissions demonstrate that the Cr( E) emission lifetimes
3
+
measured in heteroleptic [Cr(phen) (N−N′)] complexes
2
strongly depend on temperature (Figure 8 and Tables S28 and
S29). Beyond spontaneous radiative relaxation responsible for
photon emission and modeled with the temperature-
2
4
metal-centered energy levels Cr( T ) and Cr( T ) (Figure 7,
2
1
these approximate calculations assume O point group).
E
DOI: 10.1021/acs.inorgchem.8b02530
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3
+
Table 1. Ligand Fields Δ and Racah Parameters B and C for Pseudo-Octahedral [Cr(phen) (N−N′)] Complexes Deduced by
2
2
2
4
a
Using the Equations Gathered in Figure 3 and the Experimental Cr-Centered E, T , and T Energies
1
2
d
−1
E /cm
N−N
b
N−N
c
Δ/cm−1 B/cm−1 C/cm−1 Δ/B C/B
2E
2T1
4T2
2T2e
4T1e
complex
σ
′
σ
′
I
R
[
[
[
[
[
[
[
Cr(phen) ]3+ (2)
0
0
22 075
22 124
22 321
19 231
16 670
21 930
21 739
779
780
805
747
803
798
816
2700
2697
2642
2815
2809
2663
2579
28
28
28
26
21
27
27
3.5
3.5
3.3
3.8
3.5
3.3
3.2
13 736
13 736
13 717
13 717
13 717
13 717
13 550
14 451
14 451
14 472
14 472
14 722
14 472
14 347
22 075
22 124
22 321
19 231
16 670
21 930
20 347
20 347
20 174
20 174
19 281
20 174
19 743
30 090
30 150
30 565
26 754
24 275
30 085
30 027
3
3
+
Cr(phen) (phenBr)] (3)
0.47
0.29
0.67
0.17
−0.16
0.08
0.16
2
3
+
Cr(phen) (phenAlkyn)] (6)
2
3
+
Cr(phen) (phenNO )] (7)
2
2
3
+
f
f
f
Cr(phen) (phenNH )] (8)
−0.48
2
2
Cr(phen) (bipy)]3+ (9)
2
3
+
Cr(phen) (dpma)] (10)
21 739
2
a
2
2
The spectroscopic labels refer to pure O symmetry. For [Cr(phen) (dpma)](CF SO ) (10), the energies of parent Cr( T ) and Cr( E) levels
2
3
3
3
1
b
were taken as the energy averages of their various components (see text). Hammett parameter estimating the inductive electronic effects of the
70 c
substituent on the N−N′ phenanthroline σ orbitals. Hammett parameter estimating the resonance electronic effects of the substituent on the
70
d
4
e
2
4
N−N′ phenanthroline π orbitals. Energies are given with respect to Cr( A ) ground state. The energies of Cr( T ) and Cr( T ) are computed.
2
2
1
f
2
4
Cr( T ← A ) was masked by LMCT bands. The ligand field Δ and Racah parameter B were determined assuming C = 3.5B.
1
2
2
Figure 8. Temperature-dependent Cr( E) emission lifetimes (τ ),
1
−
3
measured in a mixture of acetonitrile/propionitrile (6/4) (5 × 10
mol L ), of [Cr(phen) ](PF ) (2) (dark blue), [Cr-
−
1
3
6 3
(
(
phen) (phenBr)](PF ) (3) (light blue), [Cr(phen) (phenAlkyn)]-
2 6 3 2
BF ) (6) (green), [Cr(phen) (phenNO )](CF SO ) (7) (yellow,
4
3
2
2
3
3 3
measured on powder samples since degradation occurred in
propionitrile), [Cr(phen) (phenNH )](CF SO ) (8) (orange), [Cr-
2
2
3
3 3
3
+
Figure 7. Energy level diagram for [Cr(phen) (N−N′)] complexes,
(phen)
(bipy)](BF ) (9) (red), and [Cr(phen) (dpma)](CF SO )
2 4 3 2 3 3 3
2
showing low-energy metal-centered levels (experimentally deter-
mined, thick trace; calculated, thin trace).
(10) (purple).
complexes. From 10 to 150 K, the decay curves could be fitted
with double exponential laws because of the formation of
aggregates upon freezing, in which intermolecular energy
transfers resulted in self-quenching and biexponential
7
4−76
independent Einstein’s law,
competitive nonradiative
2
Cr( E) depopulation can be induced by temperature-depend-
ent phonon-assisted pathways such as (i) weak coupling with
vibration modes that requires weak activation energy and
80,38
decays.
Upon melting around 150 K, monoexponential
2
7
persists at low temperature, (ii) crossing with a photo-
decays were recovered in agreement with the existence of
isolated solvated Cr(III) complexes. The double exponential fit
27,28
chemical intermediate in its ground electronic state (GSI),
iii) release of symmetry that induces anharmonic potential
misalignment and the appearance of surface crossing between
(iv) surface crossing due to trigonal
distortion in excited E state induced by ligand steric strain
(
observed in frozen solution can be thus partitioned between a
2
short lifetime (τ ), assigned to partially quenched Cr( E) levels
2
2
4
28,63
2
2
E and A states,
2
found in aggregates, and a long lifetime (τ ) assigned to Cr( E)
1
levels in isolated complexes (Table S28). The [Cr-
2
8,77
2
3
+
relief,
and (v) back-intersystem crossing (BISC) from E
(phen) (phenNH )] complex systematically exhibits the
2 2
4
27,78
2
to T state.
The latter mechanism is only significant for
E− T gap weaker than 3400 cm as found in [CrN O ]
complexes, and should be overpassed for most of our [CrN ]
shortest Cr( E) lifetimes of the series at any temperature
2
2
4
−1
2
4
2
(orange trace in Figure 8), because of the combined effect of
2
4
(i) a reversible Cr( E) ↔ Cr( T ) back-intersystem crossing
6
2
2
7,29,78,79
2
complexes.
produced by the weak ligand field in this complex (Figure 7)
2
4
−1
The Cr( E) emission decays were measured in air-
equilibrated acetonitrile/propionitrile (6/4) solution from 10
to 300 K (Figures S35−S45), and in degassed acetonitrile
since the energy gap ΔE( E− T ) = 2950 cm is below the
−1
27,78
2
upper limit of 3400 cm set by Forster,
and (ii) the
presence of unfavorable high-energy N−H oscillators, which
3
+
2
71,28,72,73
solution at 293 K (Figure S46) for the [Cr(phen) (N−N′)]
are famous for quenching the Cr( E) level.
While
2
F
DOI: 10.1021/acs.inorgchem.8b02530
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2
BISC is phonon-assisted and predominant at high temperature,
the presence of N−H vibration modes opens a de-excitation
pathway that requires weak thermal activation and persists at
Table 2. Cr( E) Emission Lifetimes (τ ) for Heteroleptic
1
3+
[Cr(phen) (N−N′)] Complexes
2
3
+
complex
T/K
τ /μs
1
low temperature. All the other [Cr(phen) (N−N′)]
2
a
2
[Cr(phen) ](PF ) (2)
10
2
2100
224
1700
214
2900
259
1400
177
370
17
complexes possess roughly constant Cr( E) lifetimes in the
millisecond range from 10 to 150 K in frozen solutions, which
suggests the predominance of the radiative rate constant for
3
6 3
b
93
a
[
Cr(phen) (phenBr)](PF ) (3)
10
2
6 3
b
b
b
b
b
b
2
293
the de-excitation of the Cr( E) level at these temperatures.
a
[
Cr(phen) (phenAlkyn)](BF ) (6)
10
2
4 3
Upon melting around 150 K, an abrupt decrease of τ results
1
2
93
from the emergence of coupling processes with solvent
vibrational modes and the operation of thermally activated
relaxation processes. However, this abrupt decrease is not
c
[
Cr(phen) (phenNO )](CF SO ) (7)
10
2
2
3
3
3
2
93
a
3+
[Cr(phen)
2
(phenNH
2
)](CF
3
SO
)
3
3
(8)
10
observed for [Cr(phen) (phenNH )] , probably because the
2
2
2
93
deleterious effect of N−H vibration dominates the de-
excitation processes and remains predominant upon solution
melting regarding additional quenching processes produced by
a
[
[
Cr(phen) (bipy)](BF ) (9)
10
3000
208
1400
23
2
4 3
2
08
a
Cr(phen) (dpma)](CF SO ) (10)
10
2
3
3 3
the diffusion of O . At room temperature in oxygen-free
2
2
293
acetonitrile, the decays of the Cr( E) levels in [Cr(phen) (N−
2
a
3
+
Lifetimes measured in air-equilibrated frozen acetonitrile/propioni-
b
N′)] complexes are monoexponential (except N−N′ =
−
3
−1
trile (6/4) solutions (C = 5 × 10 mol L ). Lifetimes measured in
phenNH , Figure 9). They amount to about 200 μs, apart from
2
−4
3+
3+
freeze−pump−thaw degassed acetonitrile solution (C = 10 mol
[
Cr(phen) (phenNH )] and [Cr(phen) (dpma)] which
−1
c
2
2
2
L ). Lifetime measured in solid state.
show lifetime 2 orders of magnitude smaller than those found
at 10 K (Table 2).
inert heteroleptic ter−bidentate Cr(III) complexes [Cr-
3
+
(
phen) (N−N′)] , which may include electron-withdrawing
2
or electron-donating bidentate ligands. Building on this
success, both electronic properties and symmetry effects can
be rationally implemented into [CrN ] chromophores for
6
tuning their optical properties. First, pseudo-D symmetry can
3
be taken as a satisfying model for the analysis of both the
coordination spheres (octahedrons compressed along the
pseudo-3-fold axis) and the luminescence properties (two
2
4
2
bands for the Cr( T → A ) and one band for the Cr( E →
1
2
4
A ) spin-flip transitions derived from O symmetry) in
2
3
+
[
Cr(phen) (N−N′)] . Second, the coexistence of five-
2
membered and six-membered chelate rings in [Cr-
3
+
(
phen) (dpma)] induces further deviation from trigonal
2
symmetry, which significantly broadens the spectral domain
covered by the NIR emission, an advantage for spectral overlap
with low-energy acceptors obtained at the cost of additional
nonradiative relaxation processes. Despite short room-temper-
2
Figure 9. Cr( E) emission decay curves measured in freeze−pump−
thaw degassed acetonitrile (293 K, 10− mol L , λ = 355 nm) for
4
−1
exc
[
Cr(phen) ](PF ) (2, dark blue), [Cr(phen) (phenBr)](PF ) (3,
3 6 3 2 6 3
light blue), [Cr(phen) (phenAlkyn)](BF ) (6, green), [Cr-
2
2
2
4 3
ature Cr( E) lifetime, the combination of long Cr( E) lifetime
(
(
[
phen) (phenNO )](CF SO ) (7, yellow), [Cr(phen) (phenNH )]-
2 2 3 3 3 2 2
(
(
1.4 ms) and large spectral width at 10 K makes [Cr-
CF SO ) (8, orange), [Cr(phen) (bipy)](BF ) (9, red), and
Cr(phen) (dpma)](CF SO ) (10, purple).
3
3
3
2
4
3
3+
phen) (dpma)] compatible with sensitization in these
2
2
3
3 3
specific conditions. With this in mind, [Cr-
(
3
+
phen) (phenAlkyn)] finally emerges as the best and most
2
versatile candidate for being selected as sensitizer in Cr-based
optically active metallo-supramolecular assemblies. At 10 K,
this complex possesses a long Cr ( E) lifetime (2.9 ms)
Close scrutiny at Figure 8 shows that [Cr(phen) (bipy)]-
2
(
BF ) (9) and [Cr(phen) (dpma)](CF SO ) (10) display
4
3
2
3
3 3
III 2
maximum drops in lifetime in the 10−293 K range due to the
additional rotational degrees of freedom found in bipy and
dpma, compared with phen. This provides extra vibration
together with reasonably large spectral NIR emission (spread
−
1
over 600 cm ). At room temperature, its excited-state lifetime
in deoxygenated solution remains long enough (259 μs) for
being exploited as relay, and the synthetic versatility of its
CC triple bond is compatible with its connections to
72
modes for both complexes and larger anharmonic potential
3
+
misalignment and surface crossing for [Cr(phen) (dpma)]
2
justifying its particularly short lifetime (23 μs, Table 2) at 293
31
63,77,28
neighboring activators.
K in O -free solution.
Finally, [Cr(phen) (phenAlkyn)]-
2
2
(
BF ) (6) displays the longest lifetime in frozen solution
4 3
EXPERIMENTAL SECTION
below 150 K (around 3 ms), which remains non-negligible at
room temperature in degassed solution (259 μs).
■
General Procedures. H and 13C NMR spectra were recorded on
a Bruker Avance 400 MHz spectrometer equipped with a variable-
temperature unit. Chemical shifts were given in ppm with respect to
tetramethylsilane Si(CH ) . Pneumatically assisted electrospray (ESI)
1
CONCLUSION
■
3
4
The use of noncoordinating and weakly basic 2,6-lutidine as a
sacrificial base for trapping excess of acid extends the Kane-
Maguire synthetic strategy to the preparation of kinetically
−4
mass spectra were recorded from 10 M solutions on an Applied
Biosystems API 150EX LC/MS system equipped with a Turbo
Ionspray source. Elemental analyses were performed by K. L.
G
DOI: 10.1021/acs.inorgchem.8b02530
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Buchwalder from the Microchemical Laboratory of the University of
Geneva. Reagent-grade tetrahydrofuran and diethyl ether were
distilled from sodium and benzophenone. Reagent-grade dichloro-
tablets of zinc (1.2 g) were added. The mixture was heated to reflux
during 10 min. The produced green solid was filtered over a PTFE
membrane and dried with Et O. Removal of excess of Zn tabs with
2
methane was distilled from CaH . The 5-nitrophenanthroline
tweezers afforded 93% (1.71 g) of [Cr(phen) Cl ]Cl as a green
2
2
2
81
81
(
phenNO2), 5-aminophenanthroline (phenNH ), and di(pyrid-
-yl)(methyl)amine (dpma) were synthesized according to the
powder. Elemental Anal. Calcd (%) for Cr(phen) Cl ]Cl·1.2H O·
2
2 2 2
82
2
1.5EtOH: C, 53.21; H, 4.53; N, 9.19. Found: C, 53.26; H, 4.33; N,
literature procedures. All other chemicals were purchased from
commercial sources and used without further purification. Silica-gel
plates (Merck, 60 F254) were used for thin-layer chromatography.
Preparative column chromatography was performed using either
neutral alumina gel from Fluka (Typ 507 C) 100−125 mesh or
SiliaFlash silica gel P60 (0.04−0.063 mm).
9.00. Slow diffusion of Et O into a solution of [Cr(phen) Cl ]Cl in
DMF provided green single crystals suitable for XRD.
2
2
2
Preparation of [Cr(phen) (CF SO ) ](CF SO ). Trifluorometha-
2
3
3 2
3
3
nesulfonic acid (7 mL) was added to [Cr(phen) Cl ]Cl (1.5 g, 2.89
2
2
mmol) into a Schlenk tube under argon and heated at 100 °C for 2 h
with argon bubbling. (See refs 30, 42, and 53.) The solution was
cooled on ice and vigorously stirred. Anhydrous distilled diethyl ether
Preparation of 5-Bromo-1,10-phenanthroline. A portion of
1,10-phenanthroline (7 g, 38.9 mmol, 1 equiv) was loaded in a heavy
(40 mL) was added and led to the formation of a pink precipitate.
walled glass reaction tube equipped with a Teflon screw top fitted
with a Viton O-ring. (See ref 83.) The vessel was put into an ice bath,
and oleum (22−30%, 25 mL) and bromine (0.95 mL, 2.95 g, 18.4
mmol, 0.47 equiv) were added. The mixture was heated and stirred at
The solid was filtered under argon, washed twice with anhydrous
diethyl ether, and transferred into the glovebox where the pink
precipitate was stored [Cr(phen) (CF SO ) ](CF SO ) (2.51 g, 2.49
2
3
3
2
3
3
+
mmol, yield 86%). ESI-MS m/z: [Cr(phen) (CF SO ) ] , 710.4.
2
3
3 2
1
35 °C for 16 h. The solution was cooled to room temperature,
Elemental Anal. Calcd (%) for [Cr(phen) (CF SO ) ](CF SO )·
2
3
3
2
3
3
poured into ice, and neutralized with KOH. The excess bromine was
destroyed with sodium thiosulfate. The mixture was extracted three
times with dichloromethane (3 × 200 mL). The combined organic
phase was washed twice with water (2 × 300 mL), dried over MgSO4,
filtered, and evaporated to dryness, providing 8.72 g of crude product.
The solid was dissolved in minimum of dichloromethane, and then,
1
.1HSO CF : C, 32.94; H, 1.68; N, 5.47. Found: C, 33.22; H, 1.85;
3 3
N, 5.25.
Preparation of [Cr(phen) ](PF ) (2). [Cr(phen) (CF SO ) ]-
3
6 3
2
3
3 2
(
CF SO ) (200 mg, 0.232 mmol, 1 equiv), 1,10-phenanthroline (126
3 3
mg, 0.636 mmol, 2.7 equiv), and anhydrous distilled dichloromethane
15 mL) were introduced into a Schlenk tube under argon. The
(
Et O was added until the solution turned turbid. The flask was closed
2
reaction mixture was heated at 45 °C for 90 min. The resulting yellow
and the mixture left to crystallize overnight. The resulting solid was
collected by filtration (5.51 g), and recrystallized using the same
solid was filtered on a PTFE membrane, washed with CH Cl , and
2
2
dried with diethyl ether to give pure [Cr(phen) ](CF SO ) (98 mg,
3
3
3 3
procedure to yield pure 5-bromo-1,10-phenanthroline (4.24 g, 42%).
0.094 mmol, yield 41%). Metathesis resulted from the dissolution of
1
H NMR (400 MHz, CDCl ): δ 9.22 (m, 2H), 8.68 (dd, J = 8.4,
3
[
Cr(phen) ](CF SO ) in a minimum amount of MeOH, followed by
3
3
3 3
1
8
.6 Hz, 1H), 8.20 (dd, J = 8.1, 1.8 Hz, 1H), 8.17 (s, 1H), 7.76 (dd, J =
.4, 4.4 Hz, 1H), 7.66 (dd, J = 8.1, 4.3 Hz, 1H). C NMR (100 MHz,
the addition of a saturated solution of TBAPF6 in MeOH. The
1
3
resulting precipitate of [Cr(phen) ](PF ) was filtered and dried.
3
6 3
CDCl ): δ 150.7, 150.5, 146.5, 145.5, 135.7, 134.9, 129.5, 128.6,
3
+
Slow diffusion of Et O into a solution of [Cr(phen) ](PF ) in
2 3 6 3
1
2
27.7, 123.6, 123.5, 120.6. ESI-MS m/z: [M + H] (C H BrN ):
12 8 2
acetonitrile provided single crystals suitable for XRD. ESI-MS m/z:
59.3, 261.3.
Preparation of 5-Ethynyl-1,10-phenanthroline. Portions of 5-
+
[
[Cr(phen) ](CF SO ) ] , 890.7. Elemental Anal. Calcd (%) for
3 3 3 2
[
Cr(phen) ](CF SO ) ·1.2H O: C, 43.13; H, 2.51; N, 7.92. Found:
3
3
3
3
2
bromo-1,10-phenanthroline (4.0 g, 15.4 mmol, 1 equiv),
PdCl (PPh ) (867 mg, 1.24 mmol, 8 mol %), CuI (472 mg, 2.48
C, 44.05; H, 2.39; N, 7.85. Elemental Anal. Calcd (%) for
[Cr(phen) ](PF ) ): C, 42.08; H, 2.35; N, 8.18. Found: C, 42.25;
2
3 2
(
3
6 3
mmol, 16 mol %), and a mixture of NEt (28 mL)/THF (56 mL)
3
H, 2.38; N, 8.10.
Preparation of [Cr(phen) (phenBr)](PF ) (3). [Cr-
previously degassed by argon bubbling (30 min) were introduced into
a Schlenk tube under argon. Trimethylsilyacetylene (TMSA, 5.35 mL,
2
6 3
(
phen) (CF SO ) ](CF SO ) (51 mg, 0.06 mmol), 5-bromo-1,10-
2 3 3 2 3 3
3
.79 g, 38.5 mmol, 2.5 equiv) was then added. The mixture was
phenanthroline (38 mg, 0.15 mmol), and anhydrous distilled
dichloromethane (7 mL) were introduced into a Schlenk tube. The
reaction mixture was heated at 45 °C for 3 h. The formed yellow solid
was filtered on a PTFE membrane, washed with CH Cl , and dried
stirred at 75 °C for 16 h. After solvent evaporation, the crude product
was dissolved in dichloromethane (150 mL), and 4 mL of tris(2-
aminoethyl)amine was added until the solution turned greenish blue.
The organic layer was diluted to 500 mL, washed by water (4 × 300
2
2
with Et O to give pure [Cr(phen) (phenBr)](CF SO ) (44 mg, 0.04
mL), dried over anhydrous MgSO , and filtered, and the solvent was
2
2
3
3 3
4
mmol, yield 66%). Metathesis resulted from the dissolution of
Cr(phen) (phenBr)](CF SO ) in a minimum amount of MeOH,
evaporated. The resulting crude was purified by plug filtration (Al O :
2
3
[
CH Cl /MeOH (0−3%)) providing 4.5 g of intermediate 5-
2
3
3 3
2
2
followed by the addition of a saturated solution of TBAPF in MeOH.
[
(trimethylsilyl)ethynyl]-1,10-phenanthroline. The latter compound
6
The precipitate of [Cr(phen) (phenBr)](PF ) was filtered and dried.
was introduced into a round-bottom flask, dissolved in CH Cl (50
2
6 3
2
2
Slow diffusion of Et O into a solution of [Cr(phen) (phenBr)](PF )
mL), and completed with a solution of tetrabutylammonium fluoride
2
2
6 3
TBAF, 1 mol L− in THF, 21.2 mL, 21.2 mmol, 1.3 equiv). The
1
in acetonitrile provided single crystals suitable for XRD. ESI-MS m/z:
(
+
[
[Cr(phen) (phenBr)](CF SO ) ] , 970.6. Elemental Anal. Calcd
mixture was stirred at room temperature for 15 min, and then diluted
2 3 3 2
in CH Cl (400 mL) and washed by water (4 × 300 mL) to afford
(%) for [Cr(phen)
N, 7.28. Found: C, 40.48; H, 2.51; N, 7.14.
General Preparation of [Cr(phen) (N−N′)](CF
plexes. [Cr(phen) Cl ]Cl (1 equiv) and trifluoromethanesulfonic
2 3 3 3 2
(phenBr)](CF SO ) ·2H O: C, 40.57; H, 2.36;
2
2
2
.58 g of crude product. Purification by column chromatography
(
Al O : CH Cl /acetone (0−50%)), followed by trituration and
2
3
SO ) Com-
3 3
2
3
2
2
stirring of the final solid during 2 h in Et O (25 mL), yielded pure 5-
2
2
2
acid (1 mL per 120 mg of [Cr(phen) Cl ]Cl) were introduced into a
ethynyl-1,10-phenanthroline (1.428 g, 7.0 mmol, 45%) as an off-white
2
2
1
Schlenk tube under argon and heated at 100 °C for 2 h with argon
bubbling. The solution was cooled on ice and vigorously stirred.
Anhydrous distilled diethyl ether (10 mL per 200−300 mg of
solid. H NMR (400 MHz, CDCl ): δ 9.23 (dd, J = 4.3, 1.7 Hz, 1H),
3
9
.21 (dd, J = 4.4, 1.7 Hz, 1H), 8.77 (dd, J = 8.2, 1.7 Hz, 1H), 8.23
(
dd, J = 8.1, 1.7 Hz, 1H), 8.09 (s, 1H), 7.73 (dd, J = 8.3, 4.3 Hz, 1H),
.66 (dd, J = 8.1, 4.4 Hz, 1H), 3.55 (s, 1H). C NMR (100 MHz,
1
3
7
[Cr(phen)
precipitate. The solid was filtered under argon, washed twice with
anhydrous diethyl ether, rapidly dissolved in distillated CH Cl (10
mL), and transferred under argon into a Schlenk tube containing the
Cl (10
2 2
Cl ]Cl) was added, thus leading to the formation of a pink
CDCl ): δ 151.3, 150.9, 146.5, 146.0, 136.0, 134.7, 131.9, 128.4,
3
+
1
(
27.9, 123.6, 123.5, 119.0, 83.3, 80.2. ESI-MS m/z: [M + H]
C H N ), 204.4; [2 M + H] (C H N ), 409.1.
14 9 2 28 17 4
2
2
+
Preparation of [Cr(phen) Cl ]Cl (1). CrCl anhydrous (0.56 g,
.55 mmol, 1 equiv), 1,10-phenanthroline (2.00 g, 11.13 mmol, 3.1
N−N′ ligand, pyridine or 2,6-lutidine, and distillated CH
2
2
2
2
3
3
mL). The resulting mixture was stirred at 45 °C during 2−16 h until
formation of a large amount of precipitate. The solid was collected by
filtration on a PTFE membrane, washed with CH Cl , and dried with
equiv), and absolute ethanol (20 mL) were introduced into a round-
bottom flask. (See ref 84.) The mixture was heated to reflux, and three
2
2
H
DOI: 10.1021/acs.inorgchem.8b02530
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Et O to give [Cr(phen) (N−N′)](CF SO ) complexes as a
Preparation of [Cr(phen) (dpma)](CF SO ) (10). The com-
plex was synthesized according to the general synthetic method
2
2
3
3
3
2
3
3 3
microcrystalline solid.
Preparation of [Cr(phen) (phenAlkyn)](BF ) (6). This com-
plex was synthesized according to the general synthetic method
described for [Cr(phen) (N−N′)](CF SO ) complexes.
2
4 3
2
3
3 3
[Cr(phen) Cl ]Cl (200 mg, 0.387 mmol, 1 equiv), trifluorome-
2 2
described for [Cr(phen) (N−N′)](CF SO ) complexes.
thanesulfonic acid (2 mL), di(pyrid-2-yl)(methyl)amine (143 mg,
0.772 mmol, 2 equiv), and 2,6-lutidine (90 μL, 0.772 mmol, 2 equiv)
were reacted with a reaction time of 2 h. This method afforded pure
[Cr(phen) (dpma)](CF SO ) (241 mg, yield 60%). Slow diffusion
2
3
3 3
[
Cr(phen) Cl ]Cl (200 mg, 0.387 mmol, 1 equiv), trifluorome-
2 2
thanesulfonic acid (2 mL), 5-ethynyl-1,10-phenanthroline (79 mg,
.387 mmol, 1 equiv), and pyridine (62 μL, 0.774 mmol, 2 equiv)
0
2
3
3 3
were reacted with a reaction time of 2 h.
of Et O into a solution of [Cr(phen) (dpma)](CF SO ) in methanol
2 2 3 3 3
This method afforded pure [Cr(phen) (phenAlkyn)](CF SO )
provided single crystals suitable for XRD. ESI-MS m/z: [[Cr-
2
3
3 3
+
(
[
300 mg, 73%). Metathesis resulted from the dissolution of
Cr(phen) (phenAlkyn)](CF SO ) in a minimum amount of
(phen)
[Cr(phen)
Found: C, 43.31; H, 2.55; N, 9.25.
2
(dpma)](CF
3
SO
3
3
)
2
] , 895.3. Elemental Anal. Calcd (%) for
(dpma)](CF
2
SO ·0.35H O: C, 43.42; H, 2.66; N, 9.33.
)
3
2
3
3 3
3
2
MeOH, followed by the addition of a saturated solution of TBABF4
in MeOH. The precipitate of [Cr(phen) (phenAlkyn)](BF ) was
Photophysical Measurements. Absorption spectra in solution
were recorded using a Lambda 1050 PerkinElmer spectrometer
(quartz cell path length 1 cm or 1 mm, 300−800 nm domain).
Absorption spectra in the solid state were recorded on a Lambda 900
PerkinElmer spectrometer equipped with an integration sphere
(background recorded on MgO). Emission spectra (excitation at
355 nm) and excitation spectra were recorded for frozen solution
2
4 3
filtered and dried. Slow diffusion of Et O into a solution of
2
[
Cr(phen) (phenAlkyn)](BF ) in acetonitrile provided single
2 4 3
crystals suitable for XRD. ESI-MS m/z: [[Cr(phen) (phenAlkyn)]-
2
+
(
(
7
CF SO ) ] , 914.5. Elemental Anal. Calcd (%) for [Cr-
3 3 2
phen) (phenAlkyn)](CF SO ) ·2.5H O: C, 44.41; H, 2.64; N,
2
3
3
3
2
.58. Found: C, 44.37; H, 2.53; N, 7.63. Elemental Anal. Calcd (%)
−
3
−1
for [Cr(phen) (phenAlkyn)](BF ) ·1.2H O: C, 50.91; H, 2.96; N,
samples (acetonitrile/propionitrile 6/4 at C ≈ 5 × 10 mol L ), for
2
4
3
2
9
.35. Found: C, 50.91; H, 3.00; N, 9.23.
powder samples at 10 K or for freeze−pump−thaw degassed
−
4
−1
Preparation of [Cr(phen) (phenNO )](CF SO ) (7). The
acetonitrile solution (C ≈ 10 mol L ) at 293 K, with a Fluorolog
(Horiba Jobin-Yvon) instrument, equipped with iHR320, a xenon
lamp 450 W illuminator (FL-1039A/40A), and a water-cooled
photomultiplier tube (PMT Hamamatsu R2658 or R928). The
spectra were corrected for the spectral response of the system. For
time-resolved experiments, the decay curves were collected on
2
2
3
3 3
complex was synthesized according to the general synthetic method
described for [Cr(phen) (N−N′)](CF SO ) complexes.
2
3
3 3
[
Cr(phen) Cl ]Cl (200 mg, 0.387 mmol, 1 equiv), trifluorome-
2 2
thanesulfonic acid (2 mL), 5-nitro-1,10-phenanthroline (176 mg,
.772 mmol, 2 equiv), and 2,6-lutidine (45 μL, 0.386 mmol, 1 equiv)
were reacted with a reaction time of 16 h. The resulting yellow
0
−
3
solution samples (acetonitrile/propionitrile 6/4 at C ≈ 5 × 10
−1
powder was suspended in CH Cl , stirred thoroughly for 24 h, filtered
mol L ) within the 10−300 K range, with a photomultiplier
(Hamamatsu R2658 or R928) and a digital oscilloscope (Tektronix
MDO4104C). Pulsed excitation at 355 nm was obtained with the
third harmonic of a pulsed Nd:YAG laser (Quantel Qsmart850). Low
temperatures were achieved with a closed-cycle cryosystem (Janis,
CCS-900/204N) with the sample sitting in exchange gas (helium) for
efficient cooling. Microcrystalline samples or acetonitrile/propionitrile
2
2
on a PTFE membrane, washed with CH Cl , and dried with Et O to
2
2
2
give pure [Cr(phen) (phenNO )](CF SO ) (186 mg, yield 44%).
2
2
3
3 3
Slow diffusion of Et O in a solution of [Cr(phen) (phenNO )]-
2
2
2
(
CF SO ) in methanol provided single crystals suitable for XRD.
3 3 3
+
ESI-MS m/z: [[Cr(phen) (phenNO )](CF SO ) ] , 935.5. Elemen-
2
2
3
3 2
tal Anal. Calcd (%) for [Cr(phen) (phenNO )](CF SO ) ·2.5H O:
2
2
3
3
3
2
−
3
−1
C, 41.46; H, 2.50; N, 8.68. Found: C, 41.41; H, 2.38; N, 8.63.
Preparation of [Cr(phen) (phenNH )](CF SO ) (8). The
(6/4) solutions (C ≈ 5 × 10 mol L ) were mounted in a glass
capillary (1.2 mm interior diameter, glass cutoff at 320 nm) and stuck
on copper plates with Agar Scientific silver paint (G3691). The
copper plates were attached to the sample holder with silver paint as
well. The oxygen free decay curve measurements were done at 293 K
on acetonitrile solutions of the complex (C ≈ 10 mol L ). The
complexes in a quartz tube were dissolved in acetonitrile and then
degassed by freeze−pump−thaw, and filled with argon.
2
2
3
3 3
complex was synthesized according to the general synthetic method
described for [Cr(phen) (N−N′)](CF SO ) complexes.
2
3
3 3
[
Cr(phen) Cl ]Cl (200 mg, 0.387 mmol, 1 equiv), trifluorome-
2 2
−
4
−1
thanesulfonic acid (2 mL), 5-amino-1,10-phenanthroline (90 mg,
.463 mmol, 1.2 equiv), and pyridine (50 μL, 0.521 mmol, 1.35
0
equiv) were reacted with a reaction time of 16 h. The resulting brown
powder was thoroughly washed with THF, filtered on a PTFE
membrane, washed with CH Cl , and dried with Et O to give pure
X-ray Crystallography. All data were collected on a Rigaku
Supernova diffractometer equipped with a CCD ATLAS detector
using Mo or Cu Kα radiation. The crystals were mounted on Mitegen
cryoloops and held at 180 K in the cold stream of a cryostream
2
2
2
[
Cr(phen) (phenNH )](CF SO ) (260 mg, yield 64%). Slow
2 2 3 3 3
diffusion of Et O into a solution of [Cr(phen) (phenNH )]-
2
2
2
(
Oxford Cryosystems). Structures were solved using the dual-space
(
CF SO ) in acetonitrile provided single crystals suitable for XRD.
3 3 3
85
+
algorithm implemented in SHELXT, or using direct methods within
the SIR2004 program. They were refined using the full-matrix least-
squares method on F in the SHELXL program, within the
framework of Olex2 software.
ESI-MS m/z: [[Cr(phen) (phenNH )](CF SO ) ] , 905.5. Elemen-
2
2
3
3 2
86
tal Anal. Calcd (%) for [Cr(phen) (phenNH )](CF SO ) ·1.5H O:
2
2
3
3
3
2
2
87
C, 43.30; H, 2.61; N, 9.06. Found: C, 43.40; H, 2.70; N, 8.96.
Preparation of [Cr(phen) (bipy)](BF ) (9). The complex was
88
2
4 3
synthesized according to the general synthetic method described for
Cr(phen) (N−N′)](CF SO ) complexes.
[
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02530.
2
3
3 3
■
[
Cr(phen) Cl ]Cl (300 mg, 0.578 mmol, 1 equiv), trifluorome-
2 2
*
S
thanesulfonic acid (2.2 mL), bipyridine (181 mg, 1.16 mmol, 2
equiv), and pyridine (93 μL, 1.16 mmol, 2 equiv) were reacted with a
reaction time of 2 h. This method afforded pure [Cr(phen) (bipy)]-
2
(
CF SO ) (444 mg, yield 76%). Metathesis resulted from the
3 3 3
dissolution of [Cr(phen) (bipy)](CF SO ) in a minimum amount of
2
3
3 3
Crystallographic data; absorption, excitation, and
emission spectrum; list of transition bands; list of
lifetimes; and decay curves (PDF)
MeOH, followed by the addition of a saturated solution of TBABF in
4
MeOH. The precipitate of [Cr(phen) (bipy)](BF ) was filtered and
2
4 3
dried. Slow diffusion of Et O into a solution of [Cr(phen) (bipy)]-
2
2
(
CF SO ) in acetonitrile provided single crystals suitable for XRD.
3 3 3
+
Accession Codes
ESI-MS m/z: [[Cr(phen) (bipy)](CF SO ) ] , 866.5. Elemental
2
3
3 2
CCDC 1865017−1865025 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Anal. Calcd (%) for [Cr(phen) (bipy)](CF SO ) ·1.3CH Cl : C,
2
3
3
3
2
2
4
0.85; H, 2.38; N, 7.46. Found: C, 40.70; H, 2.39; N, 7.61. Elemental
Anal. Calcd (%) for [Cr(phen) (bipy)](BF ) ·1.3H O: C, 47.91; H,
2
4
3
2
3.15; N, 9.86. Found: C, 47.87; H, 3.16; N, 9.83.
I
DOI: 10.1021/acs.inorgchem.8b02530
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(10) Padhi, S. K.; Saha, D.; Sahu, R.; Subramanian, J.; Manivannan,
V. Synthesis, Structure, Optical and Magnetic Properties of [CrL-
(
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AUTHOR INFORMATION
Corresponding Authors
E-mail: benjamin.doistau@unige.ch.
E-mail: claude.piguet@unige.ch.
■
(
̈
*
prints for Redox-Active Terpyridine in [Cr(tpy) ](PF ) (n = 3−0),
2
6 n
1
−
*
and the Remarkable Electronic Structure of [Cr(tpy) ] J. Inorg.
2
Chem. 2012, 51, 3718−3732.
ORCID
(12) Mallah, T.; Thiebaut, S.; Verdaguer, M.; Veillet, P. High-TC
́
Benjamin Doistau: 0000-0002-4767-1149
Molecular-Based Magnets: Ferrimagnetic Mixed-Valence Chromium-
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1993, 262, 1554.
13) Verdaguer, M.; Bleuzen, A.; Marvaud, V.; Vaissermann, J.;
Cel
́
ine Besnard: 0000-0001-5699-9675
(
Present Address
§
(
G.C.: Centre de Biophysique Molec
́
ulaire (CBM); UPR
ns cedex 2,
Seuleiman, M.; Desplanches, C.; Scuiller, A.; Train, C.; Garde, R.;
Gelly, G.; Lomenech, C.; Rosenman, I.; Veillet, P.; Cartier, C.; Villain,
F. Molecules to Build Solids: High TC Molecule-based Magnets by
Design and Recent Revival of Cyano Complexes Chemistry. Coord.
Chem. Rev. 1999, 190−192, 1023−1047.
CNRS 4301, Rue Charles Sadron, F-45071 Orlea
France.
́
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
(14) McInnes, E. J. L.; Piligkos, S.; Timco, G. A.; Winpenny, R. E. P.
Studies of Chromium Cages and Wheels. Coord. Chem. Rev. 2005,
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Funding
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Financial support from the Swiss National Science Foundation
is gratefully acknowledged.
chromium(III) complexes: Versatile Tectons in Designing Hetero-
metallic Coordination Compounds. Coord. Chem. Rev. 2011, 255,
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Notes
The authors declare no competing financial interest.
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D.; Howard, J. A. K.; Capelli, S. C.; Hauser, A. A Thermal Spin
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Transition in [Co(bpy) ][LiCr(ox) ] (ox = C O ; bpy = 2,2′-
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