Influence of substituents in 1,10-phenanthroline on the structural and photophysical properties of W(CO)4(1,10-phenanthroline-type) complexes

Article abstract of DOI:10.1016/j.ica.2020.120166

In this work, methyl and phenyl substituents were introduced in the 2,9- and 4,7-positions of the phen ligand in order to study its impact on the structural and photophysical properties of [W(CO)₄(phen-type)] complexes [phen-type = 4,7-dimethyl-1,10-phenanthroline (4,7-DMPhen); 4,7-diphenyl-1,10-phenanthroline (BPhen), and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)]. Crystallographic analysis of these complexes allows to investigate the correlation between the geometrical parameters with their photophysical properties measured by steady-state and time-resolved spectroscopies. The (OC)_eq–W–(CO)_eq angle is the most significantly influenced geometrical parameter by the substituents, showing values of 92.5°, 90.2°, and 81.2° for [W(CO)₄(4,7-DMPhen)], [W(CO)₄(BPhen)], and [W(CO)₄(BCP)], respectively. The smallest angle observed for [W(CO)₄(BCP)] is attributed to a steric repulsion exerted by the methyl groups to the equatorial carbonyls. Remarkably, a smaller OC_eq–W–CO_eq angle favors the High-Energy (HE) emission band, as evidenced in the photoluminescence (PL) spectra of [W(CO)₄(BCP)]. Conversely, a greater angle favors the Low-Energy (LE) emission band, as observed in the PL spectra of [W(CO)₄(4,7-DMPhen)] and [W(CO)₄(BPhen)]. The PL lifetime of the LE emission band becomes shorter when decreasing the (OC)_eq–W–(CO)_eq angle. Furthermore, the absorption feature was affected by the 2,9-dimethyl substituents, showing greater contribution from the high-energy shoulder on the visible absorption band. This study shows that the perturbation of the (OC)_eq–W–(CO)_eq angle induced by the substituents in the phen ligand influences on the photophysical properties of [W(CO)₄(phen-type)] complexes.

Full text of DOI:10.1016/j.ica.2020.120166

Inorganica Chimica Acta 516 (2021) 120166
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Inorganica Chimica Acta
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Research paper
Influence of substituents in 1,10-phenanthroline on the structural and
photophysical properties of W(CO)4(1,10-phenanthroline-type) complexes
*
´
´
Danilo H. Jara , Mariano Fernandez, Andres Vega, Nancy Pizarro
´
˜
Universidad Andres Bello, Facultad de Ciencias Exactas, Departamento de Ciencias Químicas, Vina del Mar, Chile
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this work, methyl and phenyl substituents were introduced in the 2,9- and 4,7-positions of the phen ligand in
order to study its impact on the structural and photophysical properties of [W(CO)4(phen-type)] complexes
[phen-type = 4,7-dimethyl-1,10-phenanthroline (4,7-DMPhen); 4,7-diphenyl-1,10-phenanthroline (BPhen), and
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)]. Crystallographic analysis of these complexes allows to
investigate the correlation between the geometrical parameters with their photophysical properties measured by
steady-state and time-resolved spectroscopies. The (OC)eq–W–(CO)eq angle is the most significantly influenced
geometrical parameter by the substituents, showing values of 92.5◦, 90.2◦, and 81.2◦ for [W(CO)4(4,7-
DMPhen)], [W(CO)4(BPhen)], and [W(CO)4(BCP)], respectively. The smallest angle observed for [W
(CO)4(BCP)] is attributed to a steric repulsion exerted by the methyl groups to the equatorial carbonyls.
Remarkably, a smaller OCeq–W–COeq angle favors the High-Energy (HE) emission band, as evidenced in the
photoluminescence (PL) spectra of [W(CO)4(BCP)]. Conversely, a greater angle favors the Low-Energy (LE)
emission band, as observed in the PL spectra of [W(CO)4(4,7-DMPhen)] and [W(CO)4(BPhen)]. The PL life-
time of the LE emission band becomes shorter when decreasing the (OC)eq–W–(CO)eq angle. Furthermore, the
absorption feature was affected by the 2,9-dimethyl substituents, showing greater contribution from the high-
energy shoulder on the visible absorption band. This study shows that the perturbation of the (OC)eq–W–
(CO)eq angle induced by the substituents in the phen ligand influences on the photophysical properties of [W
(CO)4(phen-type)] complexes.
Tungsten complexes
Photoluminescence
Lifetime
Crystal structure
Supramolecular interactions
1. Introduction
3MLCTN-N (2) excited states which are in thermal equilibrium at room
temperature [1,4,5,9,10]. Moreover, this class of compounds display
–
–
[M(CO)4(N N)] complexes (M = Cr, Mo, and W; N N = 1,10-phe-
nanthroline or 2,2′-bipyridine derivatives), have been extensively
studied since the seventies due to their broad absorption in the visible
spectral region, strong negative solvatochromism effect, and unusual
emission properties [1–10]. Also, it has been reported that these com-
plexes exhibit multiple electronic transitions and two photo-
luminescence (PL) bands, namely high-energy (HE) and low-energy (LE)
emission [2,9,10]. The origin of these bands and their emission lifetimes
have been the subject of multiple research works leading, in some cases,
to opposite conclusions. Specifically, discrepancies arise from the origin
of the HE emission band which has been attributed to triplets of Ligand
low PL quantum yield and short excited-state lifetime, limiting their use
in light-induced applications.
For this purpose, Farrel et al. studied the effects of switching the
LUMO character from b1 to a2 symmetries in [W(CO)4(phen)] and [W
(CO)4(tmp)] (tmp: 3,4,7,8-tetramethyl-1,10-phenanthroline) by intro-
ducing methyl substituents in the 3-,4-,7-, and 8-positions of the phen
ligand [7,8]. This structural change led to a significant impact on their
spectroelectrochemical properties; however, little changes were
observed in their electronic and photophysical properties. Notably, a
biexponential decay of the LE emission band, measured in 2-MeTHF
glass at 80 K, was found for both complexes with longer lifetime for
[W(CO)4(tmp)]. This result suggests that a structural modification by
changing substituents in the phen ligand may impact on the PL prop-
erties of [W(CO)4(phen-type)] complexes (phen-type = 1,10-phenan-
throline derivative). Therefore, a better understanding of the excited-
Field (3LF), metal-to-ligand(CO) charge-transfer (3MLCTCO), and
3
–
recently assigned to metal-to-ligand(N N) charge-transfer ( MLCTN-N
)
transitions [8,10–12]. Conversely, the LE emission band has been
unambiguously assigned to two lowest-lying 3MLCTN-N (1), and
* Corresponding author.
E-mail address: q.danilohermojara@uandresbello.edu (D.H. Jara).
https://doi.org/10.1016/j.ica.2020.120166
Received 2 September 2020; Received in revised form 24 November 2020; Accepted 26 November 2020
Available online 3 December 2020
0020-1693/© 2020 Elsevier B.V. All rights reserved.

D.H. Jara et al.
Inorganica Chimica Acta 516 (2021) 120166
state dynamic of [M(CO)4(phen-type)] complexes through the structural
and electronic modification of the phen-type ligand is crucial to improve
their photophysical properties for further applications.
2.3. Synthesis of the [W(CO)4(phen-type)] complexes
The synthesis for all complexes was adapted from reported proced-
The obtention of the crystal structure is highly desired to predict the
impact of the structural modification on the optical and PL properties of
one molecule. However, few reports on the crystal structure studies for
[W(CO)4(phen-type)] complexes have been performed, which are also
essential for having precise geometrical parameters to be used in theo-
retical calculations [13]. To our knowledge, [W(CO)4(1,10-phenan-
throline)] [14] and a tetracarbonyl-tungsten with a dipyrido[3,2-a:2′,3′-
c]phenazine derivative ligand [15] are the only crystallographic studies
found in the Cambridge Structural Database.
ures [16–19]. Their molecular structure is shown in Scheme 1.
2.3.1. Synthesis of [W(CO)4(4,7-dimethyl-1,10-phenanthroline)], [W
(CO)4(4,7-DMPhen)]
0.28 mmol (100 mg) of W(CO)6 were added into a one-neck round-
bottom flask with 15 mL of dried THF. Then, the solution was deaerated
with argon for 15 min. After degassing, the solution was irradiated for
40 min with a 365 nm led lamp. The solution changed from pale yellow
to yellowish-green. A stoichiometric amount of 4,7-DMPhen with 5%
excess was dissolved in THF into a separate container and degassed with
argon for 15 to 30 min. After that, the solution was carefully transferred
dropwise with a syringe to the round-bottom flask. The resulting solu-
tion was irradiated for 4 h forming a dark red solution. The product was
then purified on a silica chromatographic column using ethyl acetate as
eluent. Yield: 116 mg (50%). 1H NMR (400 MHz, CDCl3) δ 9.43 (d,
J = 5.3 Hz, H2 and H9), 8.15 (s, H5 and H6), 7.57 (d, J = 5.3 Hz, H3 and
H8), 2.91 (s, 4-CH3 and 7-CH3) (See proton assignment in Fig. S1). Anal.
Calcd. for C18H12N2O4W: C, 42.88; H, 2.40; N, 5.56; found: C, 42.69; H,
2.21; N, 5.40.
In this work, we report the synthesis, structural and photophysical
characterization of a series of [W(CO)4(phen-type)] complexes using
phen-type ligand with phenyl and methyl substituents in the 2,9- and
4,7-positions. A significant impact on the absorption and PL properties
of the complexes is observed when incorporating methyl substituents in
the 2 and 9 positions of the phen ligand. The results are discussed based
on the crystal structure analysis of the complexes. [W(CO)4(phen-type)]
complexes bearing phen-type ligands with substituents in the 2- and 9-
positions have not been synthesized and studied before. Hence, those
new findings will be relevant to reinforce the understanding of the
photophysical properties of [W(CO)4(phen-type)] complexes.
2.3.2. Synthesis of [W(CO)4(4,7-diphenyl-1,10-phenanthroline)], [W
(CO)4(BPhen)].
2. Experimental
The synthesis was carried out using a similar procedure than in 2.3.1.
Yield: 70 mg (39%). 1H NMR (400 MHz, CDCl3) δ 9.64 (d, J = 5.3 Hz, H2
and H9), 8.03 (s, H5 and H6), 7.70 (d, J = 5.3 Hz, H3 and H8), 7.66–7.53
(m, 4-C6H5 and 7-C6H5) (See proton assignment in Fig. S2). Anal. Calcd
for C28H16N2O4W: C, 53.53; H, 2.57; N, 4.46; found: C, 52.97; H, 2.40;
N, 4.31.
2.1. Materials
Tungsten hexacarbonyl 97% was purchased from Merck. 4,7-
dimethyl-1,10-phenanthroline (4,7-DMPhen); 4,7-diphenyl-1,10-phe-
nanthroline
(bathophenanthroline,
BPhen);
2,9-dimethyl-4,7-
diphenyl-1,10-phenanthroline (Bathocuproine, BCP) were purchased
from AK Scientific, Inc. Spectroscopic grade toluene and acetonitrile,
and dried THF were purchased from Merck. All reagents and solvents
were used as received.
2.3.3. Synthesis of [W(CO)4(2,9-dimethyl-4,7-diphenyl-1,10-
phenanthroline)], [W(CO)4(BCP)]
The synthesis was carried out using a similar procedure than in 2.3.1.
Yield: 163 mg (47%). 1H NMR (400 MHz, CDCl3) δ 7.85 (s, H3 and H8),
7.67 (s, H5, and H6), 7.61–7.48 (m, 4-C6H5 and 7-C6H5), 3.37 (s, 2-CH3
and 9-CH3) (See proton assignment in Fig. S3). Anal. Calcd. for
2.2. Instrumentation
C
30H20N2O4W: C, 54.90; H, 3.07; N, 4.27; found: C, 54.60; H, 2.92; N,
Ultraviolet–Visible (UV–Vis) absorption spectra were recorded on an
Agilent Cary 8454 Diode-Array spectrophotometer using deaerated so-
lutions at room temperature. Emission spectra were recorded in a Horiba
Jobin-Yvon FluoroMax-4 spectrofluorometer at room temperature and
77 K. Photoluminescence lifetime measurements were carried out in a
PicoQuant FluoTime 300 fluorescence lifetime spectrometer with a
time-correlated single-photon counting technique. Picosecond laser of
375, 405, and 530 nm were employed as pulsed light sources. The NMR
experiment was recorded on a Bruker Advance 400 MHz instrument
using CDCl3 as a solvent. The proton assignations were performed using
1D and 2D NME experiments. The solid-state structures of [W
(CO)4(4,7-DMPhen)], [W(CO)4(Bphen)], and [W(CO)4(BCP)] were
determined by X-ray diffraction at room temperature using a SMART-
APEX II CCD diffractometer system. Data were reduced by using
SAINT(Bruker, A. P. SAINT, version 6.22; Bruker AXS Inc.: Madison, WI,
2004, DOI: https://doi.org//10.1021/jp402550g). The effect of X-rays
absorption was corrected by using a multi-scan or a face-indexed
approach, by using SADABS (Sheldrick, G. M. SADABS, version 2.05;
Bruker AXS Inc.: Madison, WI, 2007). The structures were solved
partially by using direct methods and then completed by Difference
Fourier Synthesis. The structure solution and subsequent refinement by
least-squares used SHELXL (Sheldrick, G. M. A short history of SHELX.
Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112–122;
Sheldrick, G. M. S. N. V. SHELXTL, version 6.12; Bruker AXS Inc.:
Madison, WI, 2000). The positions of the hydrogen atoms were calcu-
lated after each cycle of refinement with SHELXL using a riding model,
4.18.
3. Results and discussion
3.1. Structural characterization
Crystals of the complexes suitable for single-crystal X-ray diffraction
were obtained through liquid–liquid diffusion using acetone/hexane for
[W(CO)4(4,7-DMPhen)] and toluene/hexane for [W(CO)4(Bphen)]
and [W(CO)4(BCP)] at room temperature. The molecular structures of
each complex are depicted in Fig. 1, and selected bond lengths and
angles are listed in Table 1.
Overall, the bond lengths for the three complexes exhibit only slight
differences. The most significant ones are the W–N distances, being
longer for [W(CO)4(BCP)] with values of 2.262(9) and 2.273(7) Å in
comparison to 2.219(2) and 2.220(2) for [W(CO)4(4,7-DMPhen)], and
2.225(4) and 2.229(4) for [W(CO)4(BPhen)].
The three complexes exhibit similar (OC)ax–W–(CO)ax and N1–W–N2
angles with values around 170◦ and 73◦, respectively, which are in
agreement with analogous compounds [14,20,21]. The deviation of the
(OC)ax–W–(CO)ax angle from 180◦ has been attributed to the strong
repulsion between the axial CO orbitals and the occupied orbitals of the
phenanthroline [20]. A smaller than 90◦ N1–W–N2 angle is caused by the
small bite angle of the phen ligand, which usually varies from 70◦ to 80◦.
These deviations contribute to the formation of a distorted octahedral
structure observed for the [W(CO)4(phen-type)] complexes. The most
significant difference is the (OC)eq–W–(CO)eq angle showing values of
–
with C H distance of 0.93 or 0.96 Å. Uiso(H) values were set equal to
1.2 or 1.5 Ueq of the parent carbon atom.
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D.H. Jara et al.
Inorganica Chimica Acta 516 (2021) 120166
Scheme 1. Molecular structure of the [W(CO)4(phen-type)] to be used in this study.
Fig. 1. ORTEP representation of the tungsten complexes with ellipsoids at 50% probability.
92.5◦, 90.2◦, and 81.2◦ for [W(CO)4(4,7-DMPhen)], [W(CO)4(B-
phen)], and [W(CO)4(BCP)], respectively (Fig. S4). The methyl sub-
stituents in the 2- and 9-positions in BCP push the equatorial carbonyls
due to steric repulsion decreasing the (OC)eq–W–(CO)eq angle. Conse-
quently, the angles C26–W1–N1 and C25–W1–N2 become more obtuse
than in the other complexes with values of 102.0 and 104.1, respectively
(Table 1).
BCP ligand can be found in literature, suggesting that the steric hin-
drance produced by phenyl groups makes the crystal packing difficult to
form. Previous works have found that complexes subjected to steric
hindrance avoid it by adopting deformed conformation such as twist,
bow, and tilt [29,30]. This steric strain is minimized in tetracoordinate
complexes by forming a tetrahedral geometry [29,31]. Deviation of
planarity could also be responsible for a longer W–N bond for [W
(CO)4(BCP)] in comparison to the other complexes (Table 1).
According to the crystal structures, the phen ligands bearing phenyl
substituents (BPhen and BCP) are bent from the equatorial plane
(N1–W1–N2) and also exhibit a bow conformation (Fig. S1-3). These
deviations are more significant when methyl groups are also included in
2- and 9-positions (BCP). Conversely, the ligand having only methyl
substituents in the 4- and 7-positions (4,7-DMPhen) exhibits a planar
conformation (Fig. S3). The BPhen and BCP bend in [W(CO)4(BPhen)]
and [W(CO)4(BCP)] was quantitatively measured by the angle formed
between the normal plane and a drawn line from W1 to C5/C6 atoms
(Fig. S5) [22]. The obtained angles were 6◦ and 11◦ for [W(CO)4(B-
Phen)] and [W(CO)4(BCP)], respectively. Additionally, the bow
conformation adopted by BPhen and BCP ligands in their complexes
was measured by the bow angle (ΘB) with values of 12◦ and 19◦,
respectively (Fig. S6). These bent and bow conformations are attributed
to steric effects exerted by the 4,7-diphenyl and 2,9-dimethyl sub-
stituents bound to the phen ligand. However, when comparing with the
reported crystal structure of [Mo(CO)4(neocuproine)], which has only
the 2,9-dimethyl substituents, the ligand lies coplanar with its equatorial
plane [13]. Therefore, we may state that 4,7-diphenyl groups are
responsible for the deviation from coplanarity. Only when those sub-
stituents are present, the 2,9-dimethyls exert an additional distortion on
the ligand, as detected in the crystal structure of [W(CO)4(BCP)]. This
conclusion can be applied only to octahedral complexes, supported by
observing a similar conformation for the BCP ligand in Re and Os
complexes’ crystal structure [23,24]. Conversely, tetrahedral complexes
bearing BCP ligand exhibit a planar conformation [25–28]. It is noted
that few examples of crystal structures for octahedral complexes bearing
For all complexes, the packing is stabilized by intermolecular in-
teractions such as π-π stacking, Mꢀ CO (lone pair)⋯π(aryl) and hydrogen
bond. Ring stacks interactions are shown in Fig. 2, and their geometrical
parameters are summarized in Table 2 and Table 3. The complex [W
(CO)4(4,7-DMPhen)] exhibits an offset π-staked geometry involving
two pyridine fragments of 4,7-DMPhen with centroid–centroid distance
(Cg5⋯Cg5iii, iii: 2–x, y, 3/2–z) of 3.639 Å (Fig. 2A). Conversely, [W
(CO)4(BPhen)] shows
π-π stacking interactions involving a phenyl
groups which stacks almost face-to-face with the central aromatic ring of
BPhen at Cg3⋯Cg1i (i: 1 – x, y – ½, ½ – z) of 3.565 Å (Fig. 2B). An offset
π-π staking is also observed between a phenyl fragment and one pyridine
moiety of BPhen with Cg2⋯Cg3i of 3.824 Å (Fig. 2B). Similarly, the
complex [W(CO)4(BCP)] exhibits a face-to-face π-π stacking between
the same aromatic rings as in [W(CO)4(Bphen)], having a Cg4⋯Cg1ii (ii:
1 – x, y – ½, 1 – z) of 3.625 Å (Fig. 2C). Moreover, the complex [W
(CO)4(4,7-DMPhen)] shows a twofold Mꢀ COax (lone pair)⋯π (pyri-
dine fragment) interaction having the same distance C7–O7⋯Cg2 of
3.106 Å. This interaction is absent in the crystal packing of the [W
(CO)4(Bphen)] and [W(CO)4(BCP)] complexes. However, they exhibit
–
two primary C H⋯O hydrogen bonds with distances values listed in
Table 2. The Mꢀ CO (lone pair)⋯π(aryl) interaction has been found to
contribute significantly to the crystal structure stability of metal
carbonyl complexes, leading to zero- or one-dimensional aggregation
patterns [32,33].
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D.H. Jara et al.
Inorganica Chimica Acta 516 (2021) 120166
Table 1
Selected geometric parameters (Å, ◦) for the tungsten complexes.
[W(CO)4(4,7-DMPhen)]
W1—C8i
1.955(3)
W1—C7
2.019(3)
2.219 (2)
2.220 (2)
92.23(8)
95.72(8)
97.19(10)
170.20(10)
95.72(8)
92.23(8)
73.15(9)
117.2(2)
W1—C8
1.955(3)
W1—N1
W1—C7i
2.019(3)
W1—N1i
C8i—W1—C8
C8i—W1—C7i
C8—W1—C7i
C8i—W1—C7
C8—W1—C7
C7i—W1—C7
C8i—W1—N1
C8—W1—N1
92.51(17)
87.05(10)
86.11(10)
86.12(10)
87.05(10)
170.10(14)
170.20(10)
97.19(10)
C7i—W1—N1
C7—W1—N1
C8i—W1—N1i
C8—W1—N1i
C7i—W1—N1i
C7—W1—N1i
N1—W1—N1i
C1—N1—C6
[W(CO)4(BPhen)]
W1—C25
1.947(6)
1.955(6)
2.017(6)
90.2(2)
W1—C27
2.022(7)
W1—C26
W1—N1
2.225(4)
W1—C28
W1—N2
2.229(4)
C25—W1—C26
C25—W1—C28
C26—W1—C28
C25—W1—C27
C26—W1—C27
C28—W1—C27
C25—W1—N1
C26—W1—N1
C28—W1—N1
C27—W1—N1
C25—W1—N2
C26—W1—N2
C28—W1—N2
C27—W1—N2
N1—W1—N2
90.25(18)
97.25(19)
99.38(19)
170.22(19)
96.60(18)
91.75(18)
72.84(14)
85.2(2)
86.1(2)
88.3(2)
86.5(2)
170.1(2)
170.48(19)
97.8(2)
[W(CO)4(BCP)]
W1—C26
1.94(2)
W1—C27
2.022(19)
2.262(9)
2.273(7)
89.6(5)
W1—C25
1.948(10)
2.018(15)
81.1(9)
W1—N2
W1—C28
W1—N1
C26—W1—C25
C26—W1—C28
C25—W1—C28
C26—W1—C27
C25—W1—C27
C28—W1—C27
C26—W1—N2
C25—W1—N2
C28—W1—N2
C27—W1—N2
C26—W1—N1
C25—W1—N1
C28—W1—N1
C27—W1—N1
N2—W1—N1
87.3(8)
96.1(6)
83.2(9)
102.0(7)
176.2(3)
94.7(7)
87.8(8)
87.2(10)
169.8(7)
173.6(7)
104.1(7)
95.1(8)
72.7(4)
Symmetry code: i: ꢀ x + 1, y, ꢀ z + 3/2.
3.2. Photophysical properties
3.2.1. Electronic absorption properties
The absorption spectra of the three complexes measured in different
solvents are shown in Fig. 3. A broad MLCT band is observed in the
visible spectral region which exhibits a negative solvatochromism up to
100 nm when the polarity of the solvent is increased. The [W(CO)4(4,7-
DMPhen)] and [W(CO)4(BPhen)] complexes exhibit a similar MLCT
band feature, having a shoulder around 400 nm (Fig. 3A and B). The
MLCT band of [W(CO)4(BPhen)] is shifted to lower energy due to
larger conjugation given by phenyl substituents. Remarkably, [W
(CO)4(BCP)] shows a distinct absorption pattern when compared to the
typical absorption spectra of [W(CO)4(phen-type)] complexes [1,4,5]. It
is observed that the electronic transitions on the high-energy side (the
shoulder) of the visible absorption band increase in contribution, lead-
ing to a broader MCLT band with a flat maximum. This characteristic is
clearly attributed to the influence of 2,9-dimethyl substituents, which
decrease the angle OCeq–W–COeq affecting the energy and contribution
of the orbitals involved in the electronic transition. According to Reso-
nance Raman excitation profiles and TD-DFT calculations carried out for
[W(CO)4(phen)] and [W(CO)4(tmp)], multiple allowed electronic tran-
sitions give rise to the visible absorption band of [W(CO)4(phen-type)]
complexes. The low-energy side of the absorption band is a contribution
of pure W ⟶ phen-type MLCT transitions. Conversely, the high-energy
side of the absorption band involves the contribution of W ⟶ Phen-
type and W ⟶ CO MLCT transitions [1–3,7,8]. In general, for [W
(CO)4(phen-type)] complexes, theoretical calculations have shown that
near 30% of the total contribution to the HOMOs corresponds to (CO)eq
and (CO)ax characters. Furthermore, the W ⟶ CO MLCT transition in-
volves a contribution of about 38% of (CO)eq to the HOMO and 88% of
Fig. 2. ORTEP representation at 50% probability showing the π-π stacking in-
teractions for (A) [W(CO)4(4,7-DMPhen)], (B) [W(CO)4(Bphen)], and (C)
[W(CO)4(BCP)], (Symmetry labels: i: 1 ꢀ x, y ꢀ ½, ½ ꢀ z; ii: 1 ꢀ x, y ꢀ ½, 1 ꢀ z.
iii: 2 ꢀ x, y, 3/2–z).
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Inorganica Chimica Acta 516 (2021) 120166
Fig. 3. Absorption spectra of the [W(CO)4(phen-type)] complexes measured in solvents with different polarities. Blue line: ACN, red line: toluene and black line:
hexane/toluene mixture. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
wavelengths (λex) are shown in Fig. 4. Due to the low PL intensity of the
Table 2
emission bands, overlapping, and near IR tail, the PL quantum yield
π-π Interaction geometry for the tungsten complexes. Centroids details: Cg1: C4,
calculation was not possible. As expected for [W(CO)4(phen-type)]
complexes, the low-energy (LE) emission band is observed from 650 nm
to near IR region for the three compounds having a maximum wave-
length around 750 to 800 nm. An emission band tail is also observed at
higher energy, which corresponds to intraligand (IL) emission. In the PL
spectra of [W(CO)4(BPhen)], using λex from 350 to 450 nm, a low-
intensity shoulder band is observed on the tail of the IL emission
(Fig. 4B) which may correspond to the high-energy (HE) emission band.
This band is also observed in the PL spectra of [W(CO)4(BCP)] when
excited from 350 to 475 nm, having a more intense and well-defined
peak (Fig. 4C). For [W(CO)4(4,7-DMPhen)], the HE emission band is
absent or exhibits very low PL intensity (Fig. 4A) which indicates that
this band is not present in all [W(CO)4(phen-type)] complexes as pre-
viously reported [2,4,5,15]. The dual emission band for some of these
types of complexes is a peculiar characteristic, and the origin of the HE
band has been the subject of various research works showing different
conclusions [2,10,15].
C5, C6, C7, C11 and C12; Cg2: C9, C10, C8, C7, C11 and N2; Cg3: C19, C20, C21,
C22, C23 and C24; Cg4: C13 C14 C15 C16, C17 and C18; Cg5: N1, C1, C2, C3, C4
and C12.
[W(CO)4(4,7-DMPhen)]
Interaction
ccd (Å)
ipd (Å)
3.439
Cg5⋯Cg5iii
3.639
[W(CO)4(BPhen)]
3.566
Cg3⋯Cg1i
Cg2⋯Cg3i
3.510
3.529
3.824
[W(CO)4(BCP)]
3.625
Cg4⋯Cg1ii
3.602
Symmetry labels: i: 1 ꢀ x, y ꢀ ½, ½ ꢀ z; ii: 1 ꢀ x, y ꢀ ½, 1–z. iii: 2 ꢀ x, y, 3/2–z.
Table 3
Hydrogen-bond geometry (Å) for [W(CO)4(BPhen)] and [W(CO)4
(BCP)].
[W(CO)4(BPhen)]
For all complexes, excitation spectra recorded at different observed
wavelengths (λobs) show that the LE band is originated from excitation
on the MLCT absorption manifold (Fig. S7). Conversely, the HE emission
band arises from excitation on the high-energy side of the visible ab-
sorption band, as observed in the broad excitation band (300 to 450 nm)
of [W(CO)4(BPhen)] and [W(CO)4(BCP)] (Fig. S7 B and C). Since the
HE emission band appears on the tail of the IL fluorescence, its excitation
band also exhibits contribution from the IL transition.
D—H⋯A
D⋯A
C9—H9⋯O27iv
C2—H2⋯O26v
3.384(7)
3.211(7)
[W(CO)4(BCP)]
3.463(18)
C29—H29B⋯O27vi
C9—H9⋯O25vii
3.493(17)
Symmetry codes: iv: -x, y ꢀ ½, -z + ½; v: ꢀ x, ꢀ y + 1, ꢀ z; vi: ꢀ x
+ 1, y ꢀ ½, ꢀ z + 1; vii: ꢀ x + 1, y + ½, 2 ꢀ z.
3.2.1.1.2. Polar solvent (acetonitrile). The steady-state PL properties
for the three complexes in acetonitrile (ACN) are shown in Fig. 5.
Apparently, the LE emission band is weaker than recorded in toluene for
[W(CO)4(4,7-DMPhen)] and [W(CO)4(BPhen)] (Fig. 5A and B),
while for [W(CO)4(BCP)] this band is almost completely quenched
(Fig. 5C). Conversely, the HE emission band is weakly observed for [W
(CO)4(4,7-DMPhen)] and becomes more predominant for [W
(CO)4(BPhen)] and [W(CO)4(BCP)]. In fact, for the later complex, the
HE emission is the only band observed after excitation from 400 to
500 nm. This result shows that the HE emission band is not quenched by
a polar solvent, as observed for the LE emission band. The excitation
spectra of the HE emission band for the three complexes (Fig. S8) show
an excitation band extending from 300 to 450 nm with a maximum of
around 350 nm. This band confirms that the HE emission originates from
the excitation on transitions contained on the high-energy side of the
absorption spectra of [W(CO)4(phen-type)] complexes with some
contribution of the IL transition.
(CO)ax to the LUMO [7]. Therefore, due to a significant COeq character
involved in the HOMO of [W(CO)4(phen-type)] complexes, any pertur-
bation on the OCeq–W–COeq angle may strongly influence on its energy
and contribution to the electronic transitions. A change of the (CO)ax
orbitals is excluded since the OCax–W–COax angle exhibits similar values
for the three complexes. Previous works by Farrel et. Al. found that the
absorption features of the [W(CO)4(phen-type)] complexes were unaf-
fected by switching of LUMO character between b1 for a2 symmetries
introducing methyl groups at the 3,4,7 and 8-positions in the phen
ligand [7,8]. This result is likely attributed to the lack of perturbation on
the carbonyls groups. Another explanation for the change in the ab-
sorption feature for [W(CO)4(BCP)] could be the electron-donating
properties of methyl substituents in the 2- and 9- positions. When
methyl substituents are present in these positions, changes in the pho-
tophysical properties have been observed for uncoordinated and coor-
dinated phen-type molecules [34,35].
3.2.1.2. Steady-state PL properties at 77 K. Due to the low PL intensity of
[W(CO)4(phen-type)] at room temperature, steady-state experiments at
77 K in a mixture of methanol/ethanol 4:1 were conducted, and the PL
spectra for the three complexes are shown in Fig. 6. Fluorescence and
phosphorescence bands of the IL transitions are observed in the PL
3.2.1.1. Steady-state photoluminescence (PL) properties at 293 K
3.2.1.1.1. Non-polar solvent (toluene). The steady-state PL proper-
ties for the three complexes were investigated using a non-polar and
polar solvents at room temperature. Toluene was used as a non-polar
solvent, and their PL spectra measured at various excitation
5

D.H. Jara et al.
Inorganica Chimica Acta 516 (2021) 120166
Fig. 4. PL spectra of the [W(CO)4(phen-type)] complexes, measured at various excitation wavelengths (λex) in toluene at 293 K. PL spectra are corrected for
wavelength variations in detector response.
Fig. 5. PL spectra of the [W(CO)4(phen-type)] complexes measured at various excitation wavelengths (λex) in ACN at 293 K. PL spectra are corrected for wavelength
variations in detector response.
Fig. 6. PL spectra of the [W(CO)4(phen-type)] complexes measured at various excitation wavelengths (λex) in methanol/ethanol 4:1 at 77 K. PL spectra are corrected
for wavelength variations in detector response.
spectra of the three complexes after excitation from 300 to 370 nm. For
[W(CO)4(4,7-DMPhen)] and [W(CO)4(BPhen)], the LE emission
band predominates under light irradiation from 400 nm to longer
wavelengths, while the HE emission band is very weak or absent (Fig. 6A
and B). Conversely, the PL spectrum of [W(CO)4(BCP)] shows both HE
and LE emission bands at the same range of excitation wavelengths,
exhibiting maxima of 495 nm and 690 nm, respectively (Fig. 6C). These
results confirm that the 2,9-methyl substituents in BCP favor the HE
emission band also at low temperatures. It is noted that the PL spectrum
of [W(CO)4(BPhen)] at 77 K does not show the HE emission band, as
previously reported by Servaas et al. [3] Probably, this band was
confused with the phosphorescence band from the 3IL excited state since
they have similar energy levels, as observed in the PL spectra of [W
(CO)4(BCP)] (Fig. 6C). Interestingly, the excitation band for [W
(CO)4(BCP)] monitored on the LE emission band shows two distin-
guished bands between 350 and 500 nm (Fig. S9C). This observation
corroborates possible changes in the energy and contribution of the
molecular orbitals involved in the electronic transition of [W
(CO)4(BCP)], as observed in its absorption spectrum.
3.2.1.3. Time-resolved PL properties at room temperature (293 K) and
77 K. The PL lifetime for the three complexes at 293 K and 77 K are
summarized in Table 4. The PL lifetime for the HE emission band was
only detectable at 77 K for [W(CO)4(BCP)], because at room temper-
ature, this band exhibits low PL intensity and is highly overlapped with
the tail of the IL fluorescence band. A multiexponential decay resolved in
three PL lifetimes (Table 4) was determined for this complex upon
excitation at 405 nm and observed at 550 nm. We ruled out the possi-
bility of a biexponential decay because of the elevated value of
χ
2, which
is not due to random error (Fig. S13-16) [36]. The shortest PL lifetimes
may be attributed to the decay of the IL emission. Therefore, the HE
band of [W(CO)4(BCP)] decays with biexponential kinetics, which is in
6

D.H. Jara et al.
Inorganica Chimica Acta 516 (2021) 120166
uncoordinated phen-type ligand. Furthermore, simultaneous emission
from both ligand and MLCT excited states have been previously reported
for Re and Cu complexes, supporting our assignment [37,38].
Table 4
PL lifetimes of the [W(CO)4(phen-type)] measured at different conditions.
τ
(HE)/ns
τ
(LE)/ns [λob]b, (A)a
[λob]b, (A)a
λex = 405 nm
77 K
As discussed, the incorporation of 2,9-dimethyl substituents in the
phen ligand has a strong influence on the photophysical properties of the
[W(CO)4(phen-type)] complexes. To support this finding, we also syn-
thesized and carried out a photophysical analysis of [W(CO)4(2,9-
DMPhen)], where 2,9-DMPhen = 2,9-dimethyl-1,10-phenanthroline.
For this complex, similar absorption features and PL properties to [W
(CO)4(BCP)] were observed (Fig. S5-7 and Table S1). This fact cor-
roborates that the 2,9-dimethyl substituents are the main responsible for
the differences in the photophysical properties of the complexes when
they are compared to analogous complexes without these substituents.
This experimental evidence may be ascribed to a steric constraint of
methyl groups imposed over the equatorial carbonyls in the plane,
resulting in a more acute (OC)eq–W–(CO)eq angle, as evidenced by the
crystal structure analysis. Consequently, a second steric repulsion is
produced between the equatorial carbonyls, leading to a change in the
energy of their molecular orbitals and the contribution to the electronic
transitions where these orbitals participate. A more significant contri-
bution from the high-energy side of the visible absorption band was
observed for [W(CO)4(BCP)] as a consequence of a smaller (OC)eq–W–
(CO)eq angle. This observation suggests the involvement of a W ⟶ CO
MLCT transition on this side of the absorption spectrum for [W
(CO)4(phen-type)] complexes, supporting previous conclusions on this
assignment [7–9,11,12].
λex = 530 nm
293 K
Temperature
Solvent
77 K
MeOH/EtOH
4:1
Toluene
ACN
N.D.
MeOH/EtOH
4:1
[W(CO)4(4,7-
DMPhen)]
N.O.
N.O.
[770 nm]
4.1
[650 nm] (*)
167 ± 4
(70%)
524 ± 10
(27%)
3300 ± 95
(3%)
[W(CO)4(BPhen)]
[800 nm]
2.7
[800 nm]
2.6
[690 nm]
126 ± 5
(79%)
474 ± 38
(19%)
2990 ± 160
(2%)
[W(CO)4(BCP)]
[500 nm]
2.0 ± 0.1 (60%)
11.0 ± 0.2
(23%)
[800 nm]
1.7
N.D.
[690 nm] (*)
12 ± 1 (42%)
70 ± 4 (38%)
268 ± 8
30.0 ± 0.3
(17%)
(20%)
(*) λex = 405 nm.
N.O. = No observed in the PL spectrum.
N.D. = No detected.
The steric repulsion produced by 2,9-dimethyl substituents also af-
fects the PL properties of the tungsten complexes. Our results indicate
that HE emission band is not always present in [W(CO)4(phen-type)]
complexes as observed in the PL spectra of [W(CO)4(4,7-DMPhen)]
and [W(CO)4(BPhen)] in toluene and at low temperature (Figs. 4 and
6); however, in ACN, this band weakly appears. Conversely, for [W
(CO)4(BCP)] and [W(CO)4(2,9-DMPhen)] having 2,9-dimethyl sub-
stituents in the phen ligand, the HE emission band was favored in all
conditions, while the LE emission band is almost completely quenched
in ACN. A similar PL behavior has been observed for Cu(I) complexes
based on phen ligands, where the 3MLCT emission band is quenched
when measured in coordinating solvents such as ACN due to an exciplex
formation [39,40]. A similar process might also explain the quenching of
the LE emission band in the ACN solution for [W(CO)4(phen-type)]
complexes. This hypothesis is supported by the fact that the photo-
substitution mechanism for [W(CO)4(phen-type)] complexes are pre-
dominantly associative under visible light irradiation [6,9]. However, if
the exciplex formation takes place, the relaxation of the excited state
should decay by a different pathway, and the HE emission should also be
quenched.
a
Fractional amplitude.
b
Observed wavelength.
agreement with other [W(CO)4(phen-type)] complexes [5,8].
Conversely, at 293 K a monoexponential kinetics was determined for the
LE emission band decay, with PL lifetime values decreasing in the order
of [W(CO)4(4,7-DMPhen)] > [W(CO)4(BPhen)] > [W(CO)4(BCP)].
This tendency implies that attaching methyl substituents in the 2- and 9-
positions of the phen ligand increases the non-radiative rate constant
from the LE emissive excited state, 3MLCT(phen). At low temperature,
the LE emission decay showed a significant lengthening of the PL life-
times in comparison to room temperature due to rigidochromic effect,
which is in agreement with previous works [5,7,8]. However, the decay
was found to be resolved using a tri-exponential kinetic rather than a bi-
exponential one (higher value of
χ
2). Therefore, our results demonstrate
3
that the LE emission band comprises three MLCT(phen-type) excited
states rather than two, as previously proposed [5,8,10], which are in
thermal equilibrium at room temperature but unequilibrated at 77 K.
Nevertheless, three low-lying 3MLCT(phen-type) excited states are
supported by theoretical calculations and Resonance Raman excitation
profiles where these triplet excited states have been evidenced [3,8].
Another explanation is ascribed to a change in the relative energy
order positions of the 1MLCTs and 3MLCTs excited states as a conse-
quence of a structural change and solvatochromic effect (Scheme 2).
Firstly, we state that the excited state energy of the 1MLCT(phen) is lying
below the 3MLCT(CO) excited state. After excitation on the high-energy
side of the visible absorption band (400–450 nm), the 1MLCT(CO)
excited state is populated and rapidly relaxes to the 1MLCT(phen), and
3MLCT(CO) excited states. Then, the 3MLCT(phen) excited state is pre-
dominantly populated by these two states and decays via radiative
deactivation to the ground state; in contrast, the 3MLCT(CO) relaxes
mainly via non-radiative decay. This mechanism is illustrated in
Scheme 2A and would explain the PL properties observed for [W
(CO)4(4,7-DMPhen)] and [W(CO)4(BPhen)] in toluene, and at low
temperature, where the LE emission is the only band observed (Figs. 4
and 6). A second case (Scheme 2B) occurs when the 1MLCT(phen)
excited state rises in energy due to the solvatochromism effect lying
slightly over the 3MLCT(CO) excited state. Conversely, the 1MLCT(CO)
transition energy is weakly affected by the polarity of the solvent. This
change in energy now allows the 1MLCT(phen) excited state to be
3.3. Overview of the structural influence on the photophysical properties
of the [W(CO)4(phen-type)] complexes
In the PL spectra of [W(CO)4(phen-type)] complexes at room tem-
perature, an emission band tail is observed in the higher energy side of
the HE emission band after excitation below 400 nm. Most of the pre-
vious studies on this type of complexes have shown PL spectra from
500 nm to lower energy, preventing a careful analysis of the UV–Vis
region. Farrell, I.R. et al. speculated that this emission might be attrib-
uted to solvent impurities, scattered laser light, or solvent Raman scat-
tering; however, no experimental evidence was shown [8]. Conversely,
we ascribe this emission band tail to originate from the IL excited state of
the coordinated phen-type ligand. Fig. S17 shows that the free ligands’
emission band tail can extend toward 550 to 600 nm after excitation at
350 nm. The absence of free ligand in the 1H NMR spectra for all com-
plexes rules out the possibility that this emission comes from
3
3
deactivated through the MLCT(phen) and MLCT(CO) excited states.
7

D.H. Jara et al.
Inorganica Chimica Acta 516 (2021) 120166
Scheme 2. Excited-state deactivation mechanism explaining the PL behavior for the [W(CO)4(phen-type)] complexes. The diagram shows only the primary excited
state levels involved in the deactivation and the prominent non-radiative decay. Bold arrows indicate the primary process involved in the excited state deactivation.
The latter having enough population to relax through radiative decay.
This mechanism is depicted in Scheme 2B. In this case, the HE emission
band is weakly observed, and the LE emission band decreases in in-
tensity, as shown in the PL spectra of [W(CO)4(4,7-DMPhen)] and [W
(CO)4(BPhen)] in ACN. A similar mechanism would explain the PL
properties of [W(CO)4(BCP)] and [W(CO)4(2,9-DMPhen)] in toluene,
where both HE and LE emission band are observed. However, in abso-
lute terms, the HE emission band is more emissive than the LE band;
hence, the 2,9-dimethyl substituents in the phen ligand may increase the
radiative rate constant from 3MLCT(CO) to the ground state. Finally, a
third mechanism, illustrated in Scheme 2C, would explain the PL prop-
erties of [W(CO)4(BCP)] and [W(CO)4(2,9-DMPhen)] in ACN. Here,
the 1MLCT(phen) excited state rises more in energy due to structural
influence and solvent effect, lying above the 3MLCT(CO) excited state.
Consequently, 1MLCT(phen) excited state deactivates efficiently to
3MLCT(CO) state which relaxes to the ground state via radiative decay.
Therefore, LE emission is quenched, and the HE emission is the only
band observed in the PL spectra of [W(CO)4(BCP)] and [W(CO)4(2,9-
DMPhen)] measured in ACN, as observed in Fig. 3.
for the three complexes displays a single exponential time decreasing in
the order of [W(CO)4(4,7-DMPhen)] > [W(CO)4(BPhen)] > [W
(CO)4(BCP)]. Therefore, to extend the PL lifetime of the LE emission for
[W(CO)4(phen-type)] complexes, no substitution at the 2- and 9-posi-
tions should be attached, while the HE emission band is favored when
introducing substituents in those positions. The PL behavior of these
complexes was explained based on a change in the position of the
excited state energy levels due to structural and solvatochromism effect.
In summary, we have demonstrated that the perturbation on the
(OC)eq–W–(CO)eq angle by substituents in the 2- and 9-positions of the
phen ligand dictates the favoring of the HE and LE emission band in the
PL spectrum of [W(CO)4(phen-type)] complexes.
CRediT authorship contribution statement
Danilo H. Jara: Conceptualization, Methodology, Investigation,
Writing - original draft, Visualization, Funding acquisition, Project
´
´
administration. Mariano Fernandez: Investigation. Andres Vega:
Investigation, Visualization. Nancy Pizarro: Resources.
4. Conclusions
Declaration of Competing Interest
In conclusion, we have demonstrated that the incorporation of
methyl and phenyl substituents in different positions of the phen ligand
has an impact on the structural and photophysical properties of [W
(CO)4(phen-type)] complexes. Crystal structure analysis of [W
(CO)4(4,7-DMPhen)], [W(CO)4(Bphen)], and [W(CO)4(BCP)]
showed a bent of the phen ligand from the equatorial plane and a bow
conformation when phenyl groups are attached to the 4- and 7-positions.
Furthermore, the crystal packing revealed that these complexes are
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
D.H.J. would like to thank to postdoctoral FONDECYT grant
3180327. N.P. acknowledges partial financial support from FONDECYT-
Chile (Grant 1160749) and CONICYT FONDEQUIP (EQM160099). A.V.
acknowledges Financiamiento Basal Proyecto FB0807 (CEDENNA) for
financial support.
stabilized by π-π, Mꢀ CO (lone pair)⋯π(aryl) and hydrogen bond in-
teractions. Remarkably, methyl substituents in the 2- and 9-positions
impact substantially on the (OC)eq–W–(CO)eq angle exhibiting values
of 92.5◦, 90.2◦, and 81.2◦ for [W(CO)4(4,7-DMPhen)], [W(CO)4(B-
phen)], and [W(CO)4(BCP)], respectively. As a consequence, a change
in the absorption feature and a higher contribution of the HE emission
band was observed for [W(CO)4(BCP)]. The 2,9-dimethyl substituents
in the phen ligand exert a steric effect on the equatorial carbonyls
changing their orbital energy and contribution to the electronic transi-
tions of the complex. This fact was confirmed by the preparation and
photophysical analysis of [W(CO)4(2,9-DMPhen)], which exhibits
similar properties to [W(CO)4(BCP)]. For [W(CO)4(4,7-DMPhen)]
and [W(CO)4(BPhen)], the LE emission band is predominant, and no
evidence of the HE emission band was found in toluene at 293 K and in
MeOH/EtOH at 77 K, contrary to previous reports. Furthermore, the LE
emission band was partially quenched for [W(CO)4(4,7-DMPhen)] and
[W(CO)4(Bphen)], and completely quenched for [W(CO)4(BCP)] in
ACN. The PL lifetime of LE emission band measured in toluene at 293 K
Appendix A. Supplementary data
Supplementary data associated with this article can be found in the
online version. CCDC 1987994, 1987995 and 1987996 contains the
crystallographic data and can be obtained free of charge from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.
ac.uk. Supplementary data to this article can be found online at htt
ps://doi.org/10.1016/j.ica.2020.120166.
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