Alkyl tetrazoles as diimine (“diim”) ligands for fac-[Re(diim)(CO)3(L)]-type complexes. Synthesis, characterization and preliminary studies of the interaction with bovine serum albumin

Article abstract of DOI:10.1016/j.ica.2020.120244

Herein, we report a new family of luminescent Re(I) complexes with general formula fac-[Re(diim)(CO)₃L]^0/+ in which the role of the diimine-type chelating ligand (diim) is played by alkylated tetrazoles. In particular, the design of

Full text of DOI:10.1016/j.ica.2020.120244

Inorganica Chimica Acta 518 (2021) 120244
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Research paper
Alkyl tetrazoles as diimine (“diim”) ligands for fac-[Re(diim)(CO) (L)]-type
3
complexes. Synthesis, characterization and preliminary studies of the
interaction with bovine serum albumin
a
a
a
a
a
Nicola Monti , Veronica Longo , Stefano Zacchini , Giulia Vigarani , Loris Giorgini ,
a,
b
b,
*
a
,
*
a,*
Eugenia Bonora , Massimiliano Massi , Valentina Fiorini , Stefano Stagni
a
Department of Industrial Chemistry “Toso Montanari”, University of Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy
Curtin Institute for Functional Molecules and Interfaces, School of Molecular and Life Science, Curtin University, Kent Street, Bentley 6102 WA, Australia
b
A R T I C L E I N F O
A B S T R A C T
L]0
/þ
Herein, we report a new family of luminescent Re(I) complexes with general formula fac-[Re(diim)(CO)
3
Dedicated to Professor Maurizio Peruzzini on
the occasion of his 65th birthday.
in which the role of the diimine-type chelating ligand (diim) is played by alkylated tetrazoles. In particular, the
design of the new complexes involved the choice of molecular scaffolds based on 2-pyridyl tetrazole (PTZ) and 2-
quinolyl tetrazole (QTZ) which were decorated with various alkyl residues at N-2 position of the pentatomic ring,
thereby endowing the resulting alkyl tetrazoles PTZ-R and QTZ-R with the proper “bpy-like” coordination
Keywords:
Triscarbonyl Re(I) diimine complexes
Diimine type ligands
Alkyl tetrazoles
ꢀ
attitude. As the “third” ligand (L), pyridine (pyr) or the 5-phenyl tetrazolato anion (Tph) were selected, leading
þ
þ
to cationic species such as fac-[Re(CO) (PTZ-R)(pyr)] , fac-[Re(CO) (QTZ-R)(pyr)] and to the neutrally
3
3
Luminescence
charged “fully tetrazole” complex fac-[Re(CO)
3
(QTZ-Me)(Tph)]. All the new complexes were identified by ESI-
MS spectrometry and fully characterized by IR, 1H and C NMR spectroscopy. The findings that were suggested
13
Protein binding
from the interpretation of the spectroscopic data were further confirmed by X-ray crystallography, with the
3 6
analysis of the molecular structures of the cationic complexes fac-[Re(CO) (PTZ-Me)(pyr)][PF ] and fac-[Re
(
CO) (QTZ-Me)(pyr)][PF ]. Following the in-depth investigation of their photophysical properties, the new
3
6
luminescent Re(I) tetrazole -based complexes were studied for any possible interaction with Bovine Serum Al-
bumin (BSA). The results obtained from this preliminary screening highlighted that, along the series of the Re(I)
þ
tetrazole complexes, the QTZ-R based cationic derivatives fac-[Re(CO)
3
(QTZ-Me)(pyr)] , fac-[Re
t
þ
(
(
CO)
3
(QTZ- Bu)(pyr)]
,
as well as the neutrally charged and “fully tetrazole” complex fac-[Re(CO)
3
(QTZ-Me)
Tph)], displayed the highest affinity to BSA.
1
. Introduction
biology-oriented applicative scenario are explained by their use as
model compounds for the technetium-99 congeners and to their dis-
playing a peculiar environmental sensitivity of the emission stemming
from metal-to-ligand charge transfer (MLCT) excited states [5]. In
particular, the structure-to-properties approach that is commonly
An “archetypal” structure like fac-[Re(CO)
diim) represents a bidentate aromatic diimine and (L) is a monodentate
(diim)(L)]0 , where
/þ
3
(
ancillary ligand, is commonly used to depict Re(I)triscarbonyl diimines,
which are known as one of the most important classes of phosphorescent
(diim)L]0 type species for
/þ
adopted for the design of fac-[Re(CO)
3
6
d -metal complexes that have been – and continue to be - at the centre of
bioimaging purposes relies upon the modulation of their luminescent
outputs by performing chemical modifications onto the chelate diimine
ligand (diim), while the biological targeting is pursued with the choice
of an appropriate “third” monodentate ligand (L) [6]. Actually, the
factors that govern the cellular uptake and the intracellular localization
of luminescent metal complexes are yet far from being fully understood.
Nevertheless, the adoption of a similar design strategy - which also
intense investigation in many areas of science and technology [1]. In
particular, Re(I) triscarbonyl diimines have been extensively studied in
the context of life science, as witnessed by the numerous reports dealing
with their use as luminescent imaging for live cells and tissues and intra
or extra cellular sensing agents for wide range of analytes [2,3,4]. The
reasons that have driven this family of Re(I) complexes to such a
*
Corresponding authors.
E-mail addresses: M.Massi@curtin.edu.au (M. Massi), valentina.fiorini5@unibo.it (V. Fiorini), stefano.stagni@unibo.it (S. Stagni).
https://doi.org/10.1016/j.ica.2020.120244
Received 21 November 2020; Received in revised form 26 December 2020; Accepted 27 December 2020
Available online 7 January 2021
0
020-1693/Crown Copyright © 2021 Published by Elsevier B.V. All rights reserved.

N. Monti et al.
Inorganica Chimica Acta 518 (2021) 120244
(diim)L]0 type complexes
/þ
the -CN
H moiety with an alkyl residue, leading to the alkylated tetra-
included the decoration of fac-[Re(CO)
3
4
with pendant biologically active molecules – has led to an impressively
high number of reports dealing with the use of such kind of Re(I)
complexes as luminescent imaging reagents, among which the mito-
chondria specific Re(I) cationic complex reported by Coogan and co-
workers in 2012 represents one of the most successful examples [7].
In this specific context, we have dedicated intense research efforts to the
design of luminescent markers for cellular imaging based on neutrally
zoles abbreviated as PTZ-R and QTZ-R, respectively, in which the -R
group was introduced at the N-2 position of the tetrazole moiety. In
doing this, the peculiar reactivity that tetrazoles display toward the
addition of electrophiles had to be taken into account. Indeed, the
alkylation reaction of tetrazoles usually leads to the recovery of the
alkylated products as a mixture of regioisomers depending whether the
electrophilic addition takes place at the N-1 or the N-2 position of the
pentatomic ring [10].
3
charged Re(I) complexes with general formula fac-[Re(CO) (diim)
0
/þ
(
L)] , in which the third ligand (L) was represented by variably
As the steric hindrance of the -R group is the most important factor
for determining any eventual regiospecificity of the alkylation reactions,
ꢀ
substituted 5-aryl tetrazolato anion [R-CN
4
]. The results obtained so far
t
have highlighted how the nature of the residue/substituent R repre-
sented a key factor for determining any eventual localization of the
corresponding Re(I) complexes, which could be directed toward lipid
droplets (as in the case of 4-benzonitrile substituted tetrazolato anion) or
to the endoplasmic reticulum, as was observed when the residue R was
represented by the 3-pyridyl ring [8]. Aiming at getting further insights
about the importance of tetrazole ligands for this class of Re(I)-based
luminescent markers, we now extend our studies to the design and the
the addition of the bulky tert-butyl ( Bu) residue to 2-pyridyl tetrazole
(PTZ-H) and 2-quinolyl tetrazole (QTZ-H) led to the exclusive formation
t
t
of the desired N-2 tert-butylated derivatives (PTZ- Bu) and (QTZ- Bu).
On the other hand, the methylation reaction performed onto the same
tetrazole substrates provided the methylated compounds (PTZ-Me) and
(QTZ-Me) as mixtures of both regioisomers, from which the N-2
substituted isomers were isolated following a column chromatography
work up (Fig. 2 and experimental section for further details). In partic-
ular, the clear distinction of the two substitution isomers could be made
from the analysis of the corresponding 13C NMR spectra. In fact, whereas
0
/þ
preparation of series of new fac-[Re(CO)
3
(diim)(L)]
type com-
plexes in which alkylated tetrazoles play the role of the chelating “dii-
mine like” (diim) ligands. For this specific purpose, the 2-pyridyl (PTZ)
and 2-quinolyl tetrazole (QTZ) molecular scaffolds were modified with
the introduction one methyl (PTZ-Me and QTZ-Me) or one tert-butyl
t
the resonance of the tetrazole carbon (C ) of N-1 isomers is typically
found in the chemical shifts range comprised between 151 and 155 ppm,
t
t
(
PTZ- Bu and QTZ- Bu) substituent group in the pentatomic ring, to
afford the precursor species fac-[Re(CO) (PTZ-R)(Br)] and fac-[Re
CO) (QTZ-R)(Br)], respectively. In the successive stage, their molec-
3
(
3
ular architectures was modified either by introducing pyridine as the (L)
ligand, leading to the series of cationic complexes with general formula
þ
þ
fac-[Re(CO)
Fig. 1) or with the replacement of the bromide ion with one phenyl
tetrazolato anion, thereby causing the formation of “fully tetrazole”
charge neutral complex fac-[Re(CO) (QTZ-Me)(Tph)] (Fig. 1). Herein,
3
(PTZ-R)(pyr)]
and fac-[Re(CO) (QTZ-R)(pyr)] ,
3
(
3
the synthesis and the spectroscopic and structural characterization of
the new complexes are described, along with an in-depth analysis of
their photophysical properties. In addition, continuing our recent
studies dealing with the use of Re(I) tetrazole complexes as luminescent
staining agents for proteins [9], the preliminary results about the
investigation of any possible interaction of the new Re(I) complexes with
a model protein such as bovine serum albumin (BSA) are reported.
2
. Results and discussions
Since tetrazoles are commonly considered the nitrogen analogues of
the corresponding carboxylic acids, the use of the pyridyl-(PTZ) and
quinolyl-tetrazole (QTZ) molecular scaffold as neutral diimine type
chelate ligands required the replacement of the fairly acidic hydrogen of
Fig. 2. Functionalization of tetrazole ligands PTZ-H and QTZ-H.
Fig. 1. Re(I) triscarbonyl complexes presented in this work and atom numbering of the tetrazole ring.
2

N. Monti et al.
Inorganica Chimica Acta 518 (2021) 120244
′ ′′
6
], displayed the superimposition of the A (2) and A
the isolation of PTZ-Me and QTZ-Me as the N-2 alkylated regioisomers
(pyr)][PF
stretching modes into a single broad band. In addition, from the com-
parison of the IR profiles of fac-[Re(CO) (PTZ-R)(pyr)][PF ] and fac-
[Re(CO) (QTZ-R)(pyr)][PF with respect to the cationic in-
termediates fac-[Re(CO) (PTZ-R)(NCCH )][PF and fac-[Re
(CO) (QTZ-R)(NCCH )][PF ], it was observed that the substitution of
was confirmed by the significant downfield shifting of the corresponding
C
t
signals to chemical shift values ranging from 164 ꢀ 168 ppm [10].
The reaction of Re(CO) Br with a slight molar excess (1:1.2) of the N-
substituted tetrazole PTZ-R or QTZ-R in toluene at the reflux tem-
3
6
5
3
6
]
2
3
3
6
]
perature led, in all cases, to the isolation of one single and neutrally
charged product (Scheme 1), as suggested by electrospray ionization
mass spectrometry (ESI-MS) experiments. For all of the new Re(I)
complexes, the facial (fac) configuration of the three CO ligands was
suggested by their displaying solid state infrared (IR) spectra (see
Experimental and ESI†, Table S1) consisting of one sharp band at ca.
3
3
6
coordinated acetonitrile molecule in favor of pyridine (pyr) caused the
shift towards lower wavenumbers of the pattern relative to the CO
ꢀ 1
stretchings (ca. 2030, 1930 and 1905 cm ), the occurrence of which
effect is most likely to ascribe due to the stronger
with respect to that of acetonitrile (Fig. 4).
π
-acidity of pyridine
ꢀ
1
2
031 cm , that is assigned to the totally symmetric in- phase stretching
As reported in Fig. 5 and ESI† Figs. S1-S14, the successful introduc-
tion of the pyridine ligand within the first coordination sphere of the Re
(I) center was also confirmed by the appearance of the characteristic
′
ꢀ 1
A (1), followed by two broader bands at ca. 1906 and 1930 cm , which
′
results from the of the totally symmetric out-of-phase stretching A (2)
′′
pattern of signals in the aromatic region in both the 1H and C NMR
spectra of the corresponding fac-[Re(CO) (PTZ-R)(pyr)][PF ] and
fac-[Re(CO) (QTZ-R)(pyr)][PF ] type species.
In a different instance, the intermediate species fac-[Re(CO)
Me)(CH CN)][PF ] was treated with a slight excess of the 5-(phenyl)
13
and the asymmetric stretching A [11].
The presence of PTZ-R or QTZ-R ligands in the structure of the Re(I)
complexes and, in particular, their adopting the desired chelate coor-
3
6
3
6
1
dination to the Re(I) center, was suggested by the comparison of the H
3
(QTZ-
NMR spectra of fac-[Re(CO)
Br)] with those of the free ligands PTZ-R and QTZ-R, respectively. As
for instance, in the exemplar case described in Fig. 3, it was possible to
3
(PTZ-R)(Br)] and fac-[Re(CO)
3
(QTZ-R)
3
6
(
tetrazolato anion, abbreviated as Tph (Scheme 2). The ESI-MS, IR and
NMR spectroscopic characterization of the resulting product provided
results congruent with its occurrence as the neutrally charged and “fully-
t
3
observe that in complex fac-[Re(CO) (QTZ- Bu)(Br)], the bis chelate
t
coordination of QTZ- Bu - while forcing the ligand to a strictly coplanar
tetrazole” complex fac-[Re(CO)
3
(QTZ-Me)(Tph)], as witnessed by the
arrangement - led to the appearance of a different and much better
shift to lower wavenumbers of the fac-type CO stretchings pattern and
the concomitant appearance of two distinct tetrazole carbon (Ct) reso-
nances in the chemical shift region comprised between 164 and 168 ppm
(see ESI† Table S1, and Fig. S14).
t
resolved pattern of aromatic signals than the one recorded from QTZ- Bu
as a “free” ligand, in which the mutual rotation of the tetrazole and the
quinolyl ring is not prevented.
In the successive stage, the obtained complexes fac-[Re(CO)
3
(PTZ-
R)(Br)] and fac-[Re(CO) (QTZ-R)(Br)] were used as starting materials
3
3. X-Ray crystallography
for reactions aimed at the replacement of the coordinated bromide ion
either with a neutrally charged ligand such as pyridine (pyr), or with the
Along the series of the new Re(I)-tetrazole complexes, two cationic
ꢀ
phenyl tetrazolato anion (Tph) . As the preliminary step, both pro-
species – namely, those suggested as fac-[Re(CO)
3
(QTZ-Me)(pyr)]
cedures involved the Ag(I)-assisted substitution of the bromide ion of the
[PF ] and fac-[Re(CO) (PTZ-Me)(pyr)][PF
6
3
6
] – afforded crystals
fac-[Re(CO)
complexes with an acetonitrile solvent molecule. The resulting cationic
intermediates fac-[Re(CO) (PTZ-R)(NCCH )][PF and fac-[Re
CO) (QTZ-R)(NCCH )][PF ] were successively treated with an excess
of pyridine (pyr) in CHCl , leading to the formation the target cationic
complexes fac-[Re(CO) (PTZ-R)(pyr)][PF ] and fac-[Re(CO) (QTZ-
R)(pyr)][PF ], respectively. Along with ESI-MS, these reactions were
3
(PTZ-R)(Br)] and fac-[Re(CO)
3
(QTZ-R)(Br)] precursor
suitable for X-ray diffraction. For both complexes, the analysis of their
molecular structure (Fig. 6 and Table 1, ESI† Table S2) provided results
congruent with the occurrence of the expected octahedral complexes in
which the coordination environment of Re(I) ion consisted of three CO
ligands arranged in a facial (fac) geometry, the pyridyl (PTZ-Me) or
quinolyl tetrazole (QTZ-Me) ligands exerting a bis-chelate coordination,
and was completed by pyridine as the “third” monodentate ligand. The
two complexes show very similar geometries and bonding parameters.
3
3
6
]
(
3
3
6
3
3
6
3
6
monitored by IR spectroscopy, enlightening how the transformation of
the neutrally charged starting compounds into the cationic products was
witnessed by the expected shift to higher wavenumbers of the CO
stretchings, whose number and relative intensities were again congruent
with the facial arrangement of the CO ligands. Following a closer in-
spection of the IR profiles (Fig. 4 and Table S1 ESI†) it was possible to
notice that, as sometimes reported for the family of fac-[Re(N^N)
2
The Re-N distances are in the expected range for Re-N(sp ) interactions
[9,12,14,20]. Re(1)-N(1) distances (2.132(8) and 2.029(6) Å for fac-
3 6 3
[Re(CO) (QTZ-Me)(pyr)][PF ] and fac-[Re(CO) (PTZ-Me)(pyr)]
[PF ], respectively) involving the five-member tetrazolato ring, are
6
slightly shorter than Re(1)-N(5) (2.276(7) and 2.245(7) Å), involving a
condensed six member ring. The Re(1)-N(6) interactions (2.201(7) and
2.210(4) Å) are in the middle, in view of the monodentate nature of the
pyridine ring.
(
(
CO)
CO)
3
3
(L)] -type complexes [12,13], the cationic species fac-[Re
(PTZ-R)(NCCH )][PF ], fac-[Re(CO) (QTZ-R)(NCCH )]
(PTZ-R)(pyr)][PF ] and fac-[Re(CO) (QTZ-R)
3
6
3
3
[
PF
6
], fac-[Re(CO)
3
6
3
+
Scheme 1. Synthetic procedure used for the preparation of fac-[Re(CO)
3
(N^N)(pyr)] -type complexes.
3

N. Monti et al.
Inorganica Chimica Acta 518 (2021) 120244
1
t
t
;. “1′′ and “3” are referred to the
Fig. 3. H NMR (aromatic region from 9 to 7.5 ppm) of (QTZ- Bu) (top) and fac-[Re(CO)
3
(QTZ- Bu)(Br)] (bottom) 400 MHz, CDCl
3
integral value of the resonance above.
t
t
t
Fig. 4. IR spectra of fac-[Re(CO)
3
(PTZ- Bu)(Br)](black trace), fac-[Re(CO)
3
(PTZ- Bu)(CH
3
CN)][PF
6
] (blue trace) and fac-[Re(CO)
3
(PTZ- Bu)(pyr)][PF
6
] (red
trace), FTIR-ATR. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
1
t
t
3 6 3 6 3
Fig. 5. H NMR spectra of fac-[Re(CO) (QTZ- Bu)(Br)][PF ] (bottom) and fac-[Re(CO) (QTZ- Bu)(pyr)][PF ] (top), 400 MHz, CDCl .
4
. Photophysical properties
The relevant photophysical data of the target cationic and neutrally
absorption profiles of the Re(I) complexes – which were obtained from
5
ꢀ
the corresponding dilute (10 M) dichloromethane solutions - displayed
the UV region dominated by intense ligand-centered (LC) transitions
(250–270 nm), followed by metal-to-ligand charge transfer (MLCT)
processes (300–410 nm) tailing off in the visible region (See Table 2,
charged Re(I) complexes described herein are summarized in Table 2.
6
As commonly observed for octahedral d metal complexes [1], the
4

N. Monti et al.
Inorganica Chimica Acta 518 (2021) 120244
3
Scheme 2. Synthetic procedure used for the preparation of fac-[Re(CO) (QTZ-Me)(Tph)].
Fig. 6. (left) Molecular structure of fac-[Re(CO)
3
(QTZ-Me)(pyr)][PF
6
] with key atoms labeled. Displacement ellipsoids are presented at the 30% probability level,
(PTZ-Me)(pyr)][PF ] with key atoms labeled. Displacement ellipsoids are
ꢀ
[
PF
6
]
and H-atoms are omitted for clarity; (right) Molecular structure of fac-[Re(CO)
3
6
ꢀ
presented at the 30% probability level, [PF ] and H-atoms are omitted for clarity.
6
corresponding MLCT features (λ = 370 nm), the cationic fac-[Re
Table 1
þ
þ
(
CO)
3
(PTZ-R)(pyr)] and fac-[Re(CO)
3
(QTZ-R)(pyr)] type species
(QTZ-Me)(Tph)] displayed
◦
Selected bond lengths (Å) and angles ( ) for fac-[Re(CO)
3
(QTZ-Me)(pyr)]
and the neutral complex fac-[Re(CO)
3
[
PF
6
] and complex fac-[Re(CO)
3
(PTZ-Me)(pyr)][PF
6
]
bright luminescence that, in line with the behavior typical of the family
of fac-Re(I) triscarbonyl diimine complexes, was represented by broad
and structureless emission profiles centered between ca. 510 nm and
Selected bond
fac-[Re(CO)
PF
3
(QTZ-Me)(pyr)] fac-[Re(CO)
3
(PTZ-Me)(pyr)]
[
6
]
6
[PF ]
Re(1)-C(1)
1.923(9)
1.907(9)
1.928(10)
2.132(8)
2.276(7)
2.201(7)
1.149(11)
1.164(11)
1.147(11)
1.315(11)
1.309(11)
1.338(11)
1.343(12)
1.325(12)
1.472(12)
1.343(12)
1.457(12)
177.9(7)
179.0(8)
176.6(8)
169.9(3)
175.9(3)
173.4(3)
74.1(3)
1.927(9)
1.939(10)
1.928(11)
2.029(6)
2.245(7)
2.210(4)
1.127(11)
1.132(11)
1.144(12)
1.4200*
1.4200*
1.4200*
1.4200*
1.4200*
1.281(10)
1.3900*
1.366(15)
178.5(9)
177.5(8)
178.3(11)
169.5(4)
179.0(3)
173.4(5)
72.8(4)
6
30 nm (see Fig. 7, right). In particular, if the emissions originating from
Re(1)-C(2)
þ
the PTZ-R- based complexes fac-[Re(CO)
3
(PTZ-R)(pyr)] were found
Re(1)-C(3)
to span from λmax = 514 nm to λmax = 578 nm, those observed from the
series of QTZ-R -based cationic derivatives fac-[Re(CO) (QTZ-R)
Re(1)-N(1)
Re(1)-N(5)
3
þ
Re(1)-N(6)
(pyr)] were quite expectedly found to peak at lower energy, as in the
C(1)-O(1)
þ
cases
CO)
of
for
fac-[Re(CO)
3
(QTZ-Me)(pyr)]
and
fac-[Re
C(2)-O(2)
t
þ
(
3
(QTZ- Bu)(pyr)] , whose emissions being both centred at 604
C(3)-O(3)
nm. The further red shift of the emission maxima (λmax = 632 nm) was
N(1)-N(2)
N(2)-N(3)
observed for the neutrally charged “fully tetrazole” complex fac-[Re
N(3)-N(4)
(
3
CO) (QTZ-Me)(Tph)]. However, in all cases, the emission can be
N(1)-C(11)
confidently described as phosphorescence originating from charge
N(4)-C(11)
3
transfer states of triplet multiplicity, CT, in a manner analogous to what
C(11)-C(12)
C(12)-N(5)
we have reported previously for neutral and ionic Re(I) tetrazolato
N(3)-C(21)
complexes [8,9,12,13]. In fact, the excited state lifetime (
τ) and quan-
Re(1)-C(1)-O(1)
Re(1)-C(2)-O(2)
Re(1)-C(3)-O(3)
C(1)-Re(1)-N(5)
C(2)-Re(1)-N(6)
C(3)-Re(1)-N(1)
N(1)-Re(1)-N(5)
Sum angles at
tum yield (Φ) are sensitive to the presence of dissolved dioxygen
3
(
Table 2). Further in support of the MLCT nature of the emissive excited
states was the significant rigidochromic blue shift of the emission that
was observed on passing from 298 K to 77 K [15], highlighting an effect
that can be ascribed to the ensuing reduction, or almost complete
removal, of vibrational and collisional quenching phenomena (Table 2
and ESI† Figs. S15-S34). It is worth noting that under all the different the
540.0(15)
540.0*
4
N C
experimental conditions in which the measurement were carried out
*
The PTZ-Me ligand of fac-[Re(CO)
3
(PTZ-Me)(pyr)][PF
6
] is disordered and
0/þ
type complexes displayed
(
Table 2), the fac-[Re(CO)
3
(QTZ-R)(L)]
the atoms of the N
4
C ring have been constrained to fit a regular pentagon.
photoluminescence performances superior to those displayed by the
þ
parent fac-[Re(CO)
3
(PTZ-R)(pyr)] derivatives, in terms both of
Fig.
7
right and ESI† Figs. S15-S34). Upon excitation of the
higher quantum yields (ΦAr) and longer emission lifetimes (
τ). This
5

N. Monti et al.
Inorganica Chimica Acta 518 (2021) 120244
Table 2
Relevant Absorption and Emission data of all the target Re(I) complexes described in this work.
Absorption
abs (nm) (10
Emission 298 K
em (nm)
Emission 77 K
ꢀ
4
ꢀ 1
ꢀ 1
a
a
b
b
c
c
Complex CH
2
Cl
2
as solvent
λ
ε
) (M cm
)
λ
τ
air (ns)
τ
Ar (ns)
φ
air (%)
φ
Ar (%)
λ
em (nm)
τ (µs)
þ
þ
fac-[Re(CO)
3
(PTZ-Me)(pyr)]
271 (4.82), 362 (0.93)
262 (5.41), 337 (1.34)
578
514
604
604
632
154
465
1025
876
407
494
2.1
2.4
3.0
3.0
1.5
7.6
6.4
518
6.00
4.06
t
fac-[Re(CO)
fac-[Re(CO)
fac-[Re(CO)
fac-[Re(CO)
3
3
3
3
(PTZ- Bu)(pyr)]
2059
3226
4040
721
482
þ
þ
(QTZ-Me)(pyr)]
256 (7.96), 328 (1.38), 373 (0.92)
256 (7.49), 328 (1.32), 373 (0.88)
253 (9.68), 324 (1.43), 368 (0.90)
18.0
19.0
9.0
532, 566
538, 568
536, 574
11.90
9.80
t
(QTZ- Bu)(pyr)]
(QTZ-Me)(Tph)]
10.50
a
:
:
:
“Air” means air equilibrated solutions, “Ar” means deoxygenated solutions under argon atmosphere;
[Ru(bpy) ]Cl /H = 0.028) [37];
O was used as reference for quantum yield determinations (Φ
in frozen CH Cl
b
c
3
2
2
r
2
2
.
þ
t
þ
Fig. 7. Normalized absorption profiles (l) and emission profiles (r) of fac-[Re(CO)
3
(PTZ-Me)(pyr)] (black solid line), fac-[Re(CO)
3
(PTZ- Bu)(pyr)] (black
þ
t
þ
dotted line), fac-[Re(CO)
98 K, CH Cl
3
(QTZ-Me)(pyr)] (red solid line), fac-[Re(CO)
3
(QTZ- Bu)(pyr)] (red dotted line) and fac-[Re(CO)
3
(QTZ-Me)(Tph)] (blue solid line),
2
2
2
. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ꢀ
ꢀ 5
trend did become even more evident on passing from air equilibrated to
-free solutions, and was quite unexpected in consideration of the
solvent, 2.1*10 3 M) to 2 mL of BSA as 1*10 M solution in aqueous
PBS buffer-(see ESI† Figs. S35-S49). As reported in the literature, [23]
the absorption profile of BSA consists of an intense transition centered at
220 nm followed by a weaker band peaking at 280 nm, which are
usually assigned to the secondary structure of the protein and the aro-
matic residues of amino acids, respectively.
O
2
energy gap law, since the emission maxima of all the QTZ-R based
complexes did peak at significantly lower energies than what observed
for the PTZ-R analogous species. Whereas the more extended -conju-
π
gation across the QTZ backbone reasonably accounts for the red shifted
emission of the QTZ-R based complexes, their displaying higher quan-
Accordingly, whereas the perturbations of the secondary structure of
BSA usually lead to a decrease of the absorbance centred at 220 nm, the
alterations localized in the surroundings of aromatic residues of amino
acids are associated to changes in the process found at 280 nm. Upon
tum yields and longer emission lifetimes might be explained by
þ
3
assuming a likely higher rigidity of the fac-[Re(CO) (diim)(pyr)]
structure that is possibly brought by the benzofused QTZ scaffold with
respect to what happens in the presence of the PTZ-based diimine
ligand.
ꢀ
3
incremental additions of 2.1*10 M solutions of Re(I)-tetrazole based
ꢀ 5
species to 2 mL of a 10 M BSA solution, the resulting absorption pro-
files presented an ever-growing process at 220 nm, while at higher
5
. Interaction with BSA – Bovine serum albumin
wavelengths (250–300 nm), overlapped processes from both BSA and Re
t
(
I) quenchers were observed, (Fig. 8, left for fac-[Re(CO)
3
(PTZ- Bu)
þ
Within the general framework of our extensive studies concerning
(pyr)] ).
the use of Re(I)-tetrazolato complexes in life science, [8,16–22], we have
recently reported the very first examples of Re(I)-based luminescent
markers for proteins purified by SDS-PAGE (Sodium Dodecyl Sulphate -
PolyAcrylamide Gel Electrophoresis) [9]. In the first stage of those
studies, the new Re(I) complexes - which were designed with the general
The intrinsic fluorescence of BSA is often referred to the presence of
two tryptophan residues, whose accessibility is strictly correlated to the
intensity of such emission, as well as the polarity of the solvent used
[24]. The occurrence of changes in emission intensity of BSA upon the
addition of successive aliquots of a quencher (i.e. Re(I) complexes) might
be indicative for alteration of the environment that surrounds the pro-
tein, suggesting therefore the occurrence of binding interactions be-
tween BSA and the quencher. For all the Re(I) complexes presented
herein, a decrease in the emission intensity of BSA was always observed
(diim)(L)]0 , where diim could be either bath-
/ꢀ
formula fac-[Re(CO)
3
ophenanthroline (BP) or bathophenanthroline disulfonate (BPS) and L
ꢀ
was represented by the 5-(phenyl)tetrazolato anion [Tph ] - were suc-
cessfully screened with respect the luminescent detection of bovine
serum albumin (BSA), which is considered as a model protein due to its
structural homology with HSA (human serum albumin). Relying on
these premises we have endeavored to preliminarily investigate the
occurrence of any possible interaction involving BSA and the new
ꢀ 4
as a result of the increased concentration of quencher (0–10 M).
t
Instead, only in the case of the cationic complex fac-[Re(CO)
3
(PTZ- Bu)
þ
(pyr)] (Fig. 8, right) and the neutrally charged species fac-[Re
3
(CO) (QTZ-Me)(Tph)] (Fig. 9, left) an appreciable increase of the Re(I)-
0
/þ
complexes fac-[Re(CO)
3
(diim)(L)]
presented herein, namely the
based emission intensity was observed.
þ
cationic species fac-[Re(CO)
3
t
(PTZ-R)(pyr)] and fac-[Re(CO)
3
(QTZ-
To get more insights about the quenching mechanism that occurs
between BSA and our Re(I) – based quenchers, Stern-Volmer analyses
þ
R)(pyr)] (R is -Me or - Bu) and the neutral complex fac-[Re
(
CO)
3
(QTZ-Me)(Tph)]. To this end, both absorption and emission
L (DMSO as the
3
were carried out (Fig. 9 right for fac-[Re(CO) (QTZ-Me)(Tph)], ESI
titration experiments were performed by adding 20 × 5
μ
Figs. S35-49 for all the other reported species). In all cases, experimental
6

N. Monti et al.
Inorganica Chimica Acta 518 (2021) 120244
ꢀ
5
t
þ
ꢀ 3
Fig. 8. (l) Absorption Titration of BSA 10 M/PBS buffer vs fac-[Re(CO)
3
(PTZ- Bu)(pyr)] , 20x5
μ
L 2.1*10 M, PBS/DMSO 95:5, 298 K. (r) Emission Titration of
ꢀ 5
t
þ
ꢀ 3
BSA 10 M/PBS buffer vs fac-[Re(CO)
3
(PTZ- Bu)(pyr)] , 20x5
μ
L 2.1*10 M, PBS/DMSO 95:5, 298 K. λemi = 538 nm. λemi BSA = 346 nm. λexc = 280 nm.
ꢀ 5
(QTZ-Me)(Tph)], 20 × 5 μ
L 2.1*10ꢀ 3M, PBS/DMSO 95:5, 298 K. λemi = 614 nm. λemi BSA
Fig. 9. (l) Emission Titration of BSA 10 M/PBS buffer vs fac-[Re(CO)
346 nm. λexc = 280 nm; (r) Stern Volmer plot.
3
=
results exhibited a non-linear behavior of the I
the emission intensities of BSA with and without quencher, respectively;
Q] is the concentration of the Re(I)-quencher considered), while the
decay time of BSA decreased with increasing concentration of quencher
0
/I vs [Q] plot (I
0
and I are
prevalent in the interaction between BSA and the presented Re(I)
complexes. On these basis, the affinity (binding constant, k ) and the
b
[
number of binding sites (n) of BSA towards our Re(I) complexes were
determined according to the Scatchard equation [32–34]. The obtained
values (Table 3, ESI Figs. S35–S49) denote an efficient interaction be-
tween our Re(I) complexes – in particular, the ones containing the QTZ-
(
(
Table 3). Usually upward curvature of Stern–Volmer plot may be due to
i) static as well as dynamic quenching mechanisms that occurs simul-
taneously and/or (ii) high extent of quenching at higher concentration
based diimine ligands - and BSA, as the optimum range for K
b
to be
4
6
ꢀ 1
region of quencher [25–27]. Thus, a modified Stern-Volmer equation
indicative for an efficient process is considered to be 10 –10 L mol
[35].
[
28] which accounts for the positive curvature observed was used (F
0
/F
=
(1 + K
D
[Q])(1 + K
S D S
[Q])). Attempts to obtain K and K values (dy-
namic and static quenching constants, respectively) using the latter
relation were unsuccessful because the resulting quadratic equations
were in all cases unsolvable (Δ < 0) [29]. This may be due to the minor
6. Conclusions
N-alkylated tetrazoles such as those based of the 2-pyridyl (PTZ) and
2-quynolyl molecular scaffolds, can actually play the role of diimine
“diim” ligands for triscarbonyl Re(I) complexes with general formula
contribution of dynamic quenching in the overall process, that was
found to range from 102 to 103
ꢀ 1
M
D
from lifetime data (K , Table 3).
Moreover, as the resulting bimolecular quenching constants (k
q
,
(diim)(L)]0 . In particular, the analysis of the photo-
/þ
fac-[Re(CO)
3
Table 3) are higher than the maximum scatter collision-quenching
constant of diverse kinds of quenchers for biopolymers fluorescence
physical behaviour of the new Re (I) complexes, both in the form of
þ
cationic species complexes fac-[Re(CO)
3
(PTZ-R)(pyr)] , fac-[Re
1
0
ꢀ 1 ꢀ 1
þ
(
2*10
M
s
), [30,31], the static quenching mechanism seems to be
(CO)
3
(QTZ-R)(pyr)] , where pyr is pyridine, and of the neutrally
Table 3
BSA-Re(I) binding experiments data, 298 K.
a
a
a
BSA
b
D
ꢀ 1
b
q
ꢀ 1 ꢀ 1
c
b
ꢀ 1
c
Complex PBS/DMSO 95:5, 298 K
λ
em (nm)
τ
0 BSA (ns)
τ
BSA (ns)
τ
0/
τ
K
(M
)
K
(M
s
)
K
(M
)
n
þ
2
11
5
fac-[Re(CO)
3
(PTZ-Me)(pyr)]
586
538
580
614
604
6.7
6.7
6.7
6.7
6.7
6.1
6.2
5.1
4.5
4.5
1.1
1.1
1.3
1.5
1.5
9.8*10
8.4*10
3.3*10
5.0*10
4.8*10
1.46*10
1.25*10
4.92*10
1.49*10
7.10*10
8.8*10
1.6*10
2.8*10
1.5*10
6.8*10
1.3
1.2
1.0
1.3
1.5
t
þ
þ
2
3
3
3
11
11
11
11
5
6
6
6
fac-[Re(CO)
fac-[Re(CO)
fac-[Re(CO)
fac-[Re(CO)
3
3
3
3
(PTZ- Bu)(pyr)]
(QTZ-Me)(pyr)]
(QTZ-Me)(Tph)]
t
þ
(QTZ- Bu)(pyr)]
a:
b
c
±
8%, ±16%. :
τ
0
/
τ
= 1 + K
D
[Q]. : Derived from Log (I
0
-I/I) = Log K + n Log[Q].
7

N. Monti et al.
Inorganica Chimica Acta 518 (2021) 120244
charged “fully tetrazole” derivative fac-[Re(CO)
3
(QTZ-Me)(Tph)]
sample considered. The Lambert-Beer law was assumed to remain linear
at the concentration of the solutions. The degassed measurements were
obtained after the solutions were bubbled for 10 min under Ar atmo-
displayed results in excellent agreement with those usually described for
the family of fac-Re(I) triscarbonyl diimines. Among the new Re(I)
complexes, in particular, unexpectedly efficient emissive performances
3 2
sphere, using a septa-sealed quartz cell. Air-equilibrated [Ru(bpy) ]Cl /
(QTZ-R)(L)]0
/þ
H
O solution (Φ = 0.028) [38] was used as reference. The quantum yield
were exhibited by the whole series of the fac-[Re(CO)
3
2
type complexes, enlightening an effect that we have preliminary
ascribed to the likely more rigid environment that is brought by the
QTZ-based diimine ligands in the corresponding complexes. Another
suggestion for the influence played by the QTZ-R scaffolds in deter-
mining the extent of the various properties of the Re(I) complexes was
provided by the results obtained from the preliminary studies about the
interaction of the new Re(I) complexes with Bovine Serum Albumin
determinations were performed at identical excitation wavelengths for
the sample and the reference, therefore deleting the I(λr)/I(λs) term in
the equation. Emission lifetimes ( ) were determined with the single
τ
photon counting technique (TCSPC) with the same Edinburgh FLSP920
spectrometer using pulsed picosecond LED (EPLED 360, FWHM < 800
ps) as the excitation source, with repetition rates between 1 kHz and 1
MHz, and the above-mentioned R928P PMT as detector. The goodness of
0
/þ
(
BSA), as the K
b
values for fac-[Re(CO)
3
(QTZ-R)(L)]
type com-
fit was assessed by minimizing the reduced 2 function and by visual
χ
plexes were found to be higher than those relative to fac-[Re
inspection of the weighted residuals. To record the 77 K luminescence
spectra, the samples were put in quartz tubes (2 mm diameter) and
inserted in a special quartz Dewar filled with liquid nitrogen. The sol-
vent used in the preparation of the solutions for the photophysical in-
vestigations was of spectrometric grade. Experimental uncertainties are
estimated to be ± 8% for lifetime determinations, ±20% for quantum
yields, and ± 2 nm and ± 5 nm for absorption and emission peaks,
respectively.
þ
(
CO)
3
(PTZ-R)(pyr)] series. It is also worth noting that in the cases of
t
þ
fac-[Re(CO)
3
(PTZ- Bu)(pyr)] and fac-[Re(CO) (QTZ-Me)(Tph)],
3
beyond to the BSA quenching, an appreciable Re(I)-based emission was
still observed. Taken together, these preliminary results pave the way for
the further development of our Re(I) tetrazole complexes by investi-
gating their interaction toward a wider range of protein targets and by
exploring their possible use as luminescent markers for proteins.
Ligand synthesis Tetrazole derivatives are used as components for
explosive mixtures [10]. In this lab, the reactions described here were
only run on a few grams scale and no problems were encountered.
However, great caution should be exercised when handling or heating
compounds of this type. Following the general method reported by
Koguro and co-workers [39], tetrazole ligands [H-Tph], [H-QTZ] and
7
. Experimental section
General considerations All the reagents and solvents were obtained
commercially (Sigma Aldrich/Merck, Alfa Aesar, Strem Chemicals) and
used as received without any further purification, unless otherwise
specified. When required, the reactions were carried out under an argon
atmosphere following Schlenk protocols and the purification of the Re(I)
complexes was performed via column chromatography with the use of
[H-PTZ] were obtained in almost quantitative yield.
1
[H-Tph] H NMR (DMSOd
6
, 400 MHz) δ (ppm) = 8.06–8.03 (m, 2H,
1
H2, H6), 7.62–7.60 (m, 3H, H3, H4, H5); [H-PTZ] H NMR, (DMSO‑d
6
,
SiO
2
as the stationary phase. ESI-mass spectra were recorded using a
400 MHz) δ (ppm): 8.79–8.77 (m, 1H, H6), 8.22–8.20 (m, 1H, H3),
1
Waters ZQ-4000 instrument (ESI-MS, acetonitrile as the solvent). Nu-
8.08–8.04 (m, 1H, H4), 7.63–7.60 (m, 1H, H5); [H-QTZ] H NMR,
1
13
clear magnetic resonance spectra (consisting of H and C) were always
(DMSO‑d
6
, 400 MHz) δ (ppm) = 8.65 (d, 1H, JH-H = 8.79 Hz, H4), 8.31
1
13
recorded using a Varian Mercury Plus 400 ( H, 399.9; C, 101.0 MHz).
(d, 1H, JH-H = 8.40 Hz, H5), 8.17 (d, 1H, JH-H = 8.40 Hz, H8), 8.12 (d,
1H, JH-H = 7.99 Hz, H3), 7.90 (t, 1H, H7), 7.74 (t, 1H, H6).
1
13
H and C chemical shifts were referenced to residual solvent reso-
nances. Infrared Spectra (IR) were acquired with a Perkin Elmer Spec-
trum One FTIR instrument using the ATR (attenuated total reflectance)
module.
Ligand Functionalization The ligands PTZ-Me (N-3), QTZ-Me (N-3)
t
t
and PTZ- Bu, QTZ- Bu were obtained according to previously reported
literature procedures [40].
Photophysics Absorption spectra were recorded at room temperature
using a Perkin Elmer Lambda 35 UV/vis spectrometer. Uncorrected
steady-state emission and excitation spectra were recorded on an
Edinburgh FLSP920 spectrometer equipped with a 450 W xenon arc
lamp, double excitation and single emission monochromators, and a
Peltier-cooled Hamamatsu R928P photomultiplier tube (185–850 nm).
Emission and excitation spectra were acquired with a cut-off filter (395
nm) and corrected for source intensity (lamp and grating) and emission
spectral response (detector and grating) by a calibration curve supplied
with the instrument. The wavelengths for the emission and excitation
spectra were determined using the absorption maxima of the MLCT
transition bands (emission spectra) and at the maxima of the emission
bands (excitation spectra). Quantum yields (Φ) were determined using
the optically dilute method by Crosby and Demas [36] at excitation
wavelength obtained from absorption spectra on a wavelength scale
PTZ-Me (N-3) 1H NMR, (CDCl
3
, 400 MHz) δ (ppm): 8.67–8.65
(m,1H, H6), 8.13–8.10 (m, 1H, H3), 7.77–7.73 (m, 1H, H4), 7.30–7.26
1
3
(m, 1H, H5), 4.34 (s, 3H, CH
164.78 (C ), 150.18 (Cipso), 146.60 (C6), 137.00 (C4), 124.73 (C3),
122.20 (C5), 39.59 (CH ) [40].
QTZ-Me (N-3) 1H NMR, (CDCl
3H, H8, H5, H4), 7.91–7.88 (m, 1H, H7), 7.82–7.76 (m, 1H, H3),
3
). C NMR, (CDCl
3
, 100 MHz) δ (ppm):
t
3
3
, 400 MHz) δ (ppm): 8.38–8.36 (m,
1
3
7.65–7.61 (m, 1H, H6), 4.52 (s, 3H, CH
(ppm): 164.98 (C ), 148.01 (Cipso), 146.49 (C10), 137.25 (C4), 130.01
(C8), 129.93 (C5), 128.32 (C6), 127.54 (C7), 127.42 (C9), 119.54 (C3),
39.71 (CH ) [40].
3
). C NMR, (CDCl
3
, 100 MHz) δ
t
3
t
1
PTZ- Bu H NMR, (CDCl
3
, 400 MHz) δ (ppm): 8.81–8.80 (d, 1H, JH-H
= 4.00 Hz, H6), 8.28–8.26 (d, 1H, JH-H = 7.60 Hz, H3), 7.88–7.84 (m,
t
13
1H, H4), 7.40–7.37 (m, 1H, H5), 1.84 (s, 9H, - Bu). C NMR, (CDCl
100 MHz) δ (ppm): 164.1 (C ), 155.2 (Cipso), 149.4, 137.2, 124.2, 123.7,
73.1 (C-(CH ), 28.3 ((CH
3
,
t
[
[
nm] and compared to the reference emitter by the following equation
37]:
3
)
3
3
)
3
).
t
1
QTZ- Bu H NMR, (CDCl
3
, 400 MHz) δ (ppm): 8.38 – 8.30 (m, 3H,
[
][l
][ ][
]
H4, H5, H8), 7.87 (m, 1H, H3), 7.77 (m, 1H, H7), 7.60 (m, 1H, H6), 1.87
2
n
s
A
A
r
(λ
(λ
r
s
)
)
r
(λ
r
s
)
)
D
D
s
t
13
ϕ = ϕr
(s, 9H, - Bu). C NMR, (CDCl
3
, 100 MHz) δ (ppm): 163.5 (C
ipso), 144.2 (C10), 137.3 (C4), 129.8 (C8), 128.3 (C6), 127.1 (C7),
25.3 (C5), 121.0 (C9), 119.5 (C3), 72.9 (C-(CH ), 28.6 ((CH ).
t
), 157.4
s
2
n
r
s
l
s
(λ
r
(
C
1
3
)
3
3 3
)
where A is the absorbance at the excitation wavelength (λ), I is the in-
tensity of the excitation light at the excitation wavelength (λ), n is the
refractive index of the solvent, D is the integrated intensity of the
luminescence, and Φ is the quantum yield. The subscripts r and s refer to
the reference and the sample, respectively. A stock solution with an
absorbance > 0.1 was prepared, then a 10 times diluted solution was
obtained, resulting in absorbance of about 0.07/0.08 depending on the
7
.1. General procedure for the preparation of fac-[Re(CO)
3
(N^N)-
+
(
pyr)] -type complexes and fac-[Re(CO)
3
(QTZ-Me)-(Tph)]
þ
3
(N^N)-(pyr)] -type complexes
The preparation of fac-[Re(CO)
was accomplished by following a multistep procedure which involved at
first the formation of the neutral fac-[Re(CO) (N^N)(Br)] by refluxing Re
3
8

N. Monti et al.
Inorganica Chimica Acta 518 (2021) 120244
(
CO)
toluene for 24 h. The crude was allowed to cool to room temperature and
the addition of Et O induced the precipitation of bright yellow to orange
5
Br (1 eq) with the appropriate (N^N) ligand (1.1 eq) in 20 mL of
8.03–8.00 (m, 2H, pyr), 7.86–7.80 (m, 2H, pyr), 7.70–7.66 (m, 1H, QTZ-
Me), 7.23–7.20 (m, 1H, QTZ-Me), 4.46 (s, 3H, CH
3
). ESI-MS (m/z):
+
ꢀ
ꢀ 1
2
[M] = 561; [M] = 145 (PF
6
). IR-ATR
ν
(cm ): 2045 (CO), 1940 (CO).
), 152.53 (Cipso),
1
3
solids collected by filtration, air-dried and used for the successive steps
without any further purification. In a two neck round bottomed flask
C NMR, (CD
3
CN, 100 MHz) δ (ppm): 166.78 (C
t
147.44 (QTZ-Me), 143.73 (pyr), 139.94 (pyr), 138.34 (QTZ-Me), 134.84
(QTZ-Me), 131.02 (QTZ-Me), 130.81 (QTZ-Me), 130.44 (QTZ-Me),
130.31 (QTZ-Me), 129.40 (QTZ-Me), 128.94 (QTZ-Me), 128.76 (QTZ-
protected from light, 1 eq of the desired neutral fac-[Re(CO)
and 1.5 eq. of AgPF were combined in 20 mL of CH CN and refluxed for
hrs. The crude was cooled to room temperature, filtered over a celite
3
(N^N)(Br)]
6
3
3
Me), 128.18 (QTZ-Me), 126.93 (pyr), 119.99 (QTZ-Me), 42.39 (CH
3
). Y
705 g/mol; 0.088 mmol). Anal. Calcd. For
(705.53) C 32.35, H 2.00, N 11.91. Found: C 30.39,
pad to remove AgBr and successively combined with pyridine (5 eq.) in
=
34% (MW
=
◦
CHCl
3
(20 mL) .The solutions were heated over reflux (70 C) under an
C
19
H
14
N
6
O
3
P
1
F
6
Re
1
argon atmosphere for 15 hrs, after which the pale to bright yellow
precipitates formed were collected by filtration and washed with Et O.
In the case of fac-[Re(CO) (QTZ-Me)(Tph)], 1.2 eq. of H-Tph were
H 2.04, N 10.63.
fac-[Re(CO)
t
1
2
3
(QTZ- Bu)-(pyr)][PF
6
] H NMR, (CDCl
3
, 400 MHz) δ
t
3
(ppm): 8.87–8.85 (d, 1H, JH-H = 8.80 Hz, QTZ- Bu), 8.80–8.78 (d, 1H,
t
t
added to the reaction mixture instead of pyridine after the halide
extraction step. The crude was then purified by column chromatography
J
H-H = 8.40 Hz, QTZ- Bu), 8.53–8.51 (d, 1H, JH-H = 8.40 Hz, QTZ- Bu),
t
8.20–8.14 (m, 2H, pyr), 8.00–7.98 (d, 2H, JH-H = 5.20 Hz, QTZ- Bu),
t
over SiO
2
eluted with a CH
2
Cl
2
/Acetone 9:1 mixture. Product in F1.
7.96–7.92 (m, 1H, QTZ- Bu), 7.79–7.76 (t, 1H, JH-H = 15.60 Hz, JH-H =
1
t
fac-[Re(CO)
3
(PTZ-Me)(Br)] H NMR, (CDCl
3
, 400 MHz) δ (ppm):
8.00 Hz, JH-H = 7.60 Hz, pyr), 7.30–7.26 (m, 2H, pyr), 1.96 (s, 9H, - Bu).
+
ꢀ
ꢀ 1
(cm ): 2030
9
7
5
.11–9.09 (m, 1H, H6), 8.28–8.25 (m, 1H, H3), 8.16–8.11 (m, 1H, H4),
ESI-MS (m/z): [M] = 603, [M] = 145 (PF
6
). IR-ATR
(CO), 1926 (CO), 1940 (CO). C NMR (CDCl
, 100 MHz) δ (ppm):
), 151.73 (Cipso), 147.18 (QTZ- Bu), 143.42 (pyr), 140.21
ν
+
13
.65–7.62 (m, 1H, H5), 4.59 (s, 3H, CH
3
). ESI-MS (m/z): [M + Na]
=
3
+
ꢀ 1
t
34, [M + K] = 550. IR-ATR
ν
(cm ): 2033 (CO), 1934 (CO), 1907
167.60 (C
(pyr), 134.33 (QTZ- Bu), 130.83 (QTZ- Bu), 130.50 (QTZ- Bu), 130.15
t
t
t
t
(
CO). Y = 69% (MW = 511 g/mol, 0.35 mmol).
t 1
t
t
t
fac-[Re(CO)
3
(PTZ- Bu)(Br)] H NMR, (CDCl
3
, 400 MHz) δ (ppm):
(QTZ- Bu), 128.55 (QTZ- Bu), 127.26 (QTZ- Bu), 121.08 (pyr), 105.01
t
t
t
9
7
5
.10–9.08 (m, 1H, H6), 8.30–8.26 (m, 1H, H3), 8.14–8.10 (m, 1H, H4),
(QTZ- Bu), 70.01 ( Bu), 29.63 ( Bu). Y = 12% (MW = 748 g/mol; 0.016
mmol). Anal. Calcd. For C22 Re (747.61) C 35.35, H 2.70,
N 11.24. Found: C 36.27, H 2.91, N 11.03.
fac-[Re(CO)
(QTZ-Me)(Tph)] 1H NMR, (CDCl
(ppm): 8.99–8.97 (d, 1H, JH-H = 8.80 Hz, QTZ-Me), 8.59–8.57 (d, 1H, JH-
t
+
.63–7.59 (m, 1H, H5), 1.88 (s, 9H, - Bu). ESI-MS (m/z): [M + Na]
=
H
20
N
6
O
P
3 1
F
6
1
ꢀ 1
75. IR-ATR
ν (cm ): 2032 (CO), 1933 (CO), 1905 (CO). Y = 19% (MW
=
552 g/mol; 0.05 mmol).
fac-[Re(CO)
(QTZ-Me)(Br)] 1H NMR, (Acetone‑d
ppm): 9.01–8.99 (d, 1H, JH-H = 8.80 Hz, H8), 8.87–8.85 (d, 1H, JH-H
3
3
, 400 MHz) δ
3
6
, 400 MHz) δ
(
=
H
= 8.40 Hz, QTZ-Me), 8.33–8.31 (d, 1H, JH-H = 8.40 Hz, QTZ-Me),
8
.80 Hz, H4), 8.53–8.51 (d, 1H, JH-H = 8.40 Hz, H5), 8.34–8.32 (d, 1H,
8.07–8.03 (m, 1H, QTZ-Me), 8.00–7.98 (m, 1H, QTZ-Me), 7.86–7.84
(m, 2H, Tph), 7.82–7.78 (m,1H, QTZ-Me), 7.31–7.24 (m, 3H, Tph), 4.64
J
H-H = 8.00 Hz, H3), 8.24–8.20 (t, 1H, JH-H = 16.00 Hz, JH-H = 8.40 Hz,
ꢀ 1
J
H-H = 7.60 Hz, H7), 8.00–7.96 (t, 1H, JH-H = 15.2 Hz, JH-H = 8.00 Hz,
(s, 3H, CH
(CO), 1932 (CO). C NMR, (Acetone‑d
(CO), 194.13 (CO), 193.09 (CO), 168.91 (C
3
). ESI-MS (m/z): [M + H] = 628 m/z. IR-ATR
ν
(cm ): 2036
, 100 MHz) δ (ppm): 197.34
, QTZ-Me), 162.65 (C , Tph),
+
13
J
H-H = 7.20 Hz, H6), 4.83 (s, 3H, CH
3
). ESI-MS (m/z): [M + Na] = 584
6
ꢀ 1
m/z. IR-ATR
ν
(cm ): 2031 (CO), 1931 (CO), 1910 (CO). Y = 73% (MW
t
t
=
561 g/mol, 0.51 mmol).
148.08 (Cipso), 147.57 (Cipso), 133.54 (QTZ-Me), 130.22 (QTZ-Me),
130.17 (QTZ-Me), 129.77 (QTZ-Me), 129.74 (QTZ-Me), 129.56 (Tph),
128.41 (Tph), 128.30 (QTZ-Me), 128.14 (QTZ-Me), 125.81 (Tph),
t
1
fac-[Re(CO)
3
(QTZ- Bu)(Br)] H NMR, (CDCl
3
, 400 MHz) δ (ppm):
8
.93–8.91 (d, 1H, JH-H = 8.00 Hz, H8), 8.58–8.56 (d, 1H, JH-H = 8.0 Hz,
H4), 8.35–8.33 (d, 1H, JH-H = 8.40 Hz, H5), 8.10–8.01 (m, 2H, H3, H7),
119.46 (QTZ-Me), 41.69 (CH
Anal. Calcd. For C21
Found: C 37.14, H 2.27, N 17.72.
Absorption and Emission Titration Experiments PBS buffer (1 L)
was prepared by dissolving Na HPO (1.44 g), KH PO (0.245 g), NaCl
(8 g) and KCl (0.2 g) in 0.8 L of H O. After complete mixing, the solution
3
). Y = 48% (MW = 627 g/mol; 0.12 mol).
t
+
7
.90–7.81 (m, 1H, H6), 1.93 (s, 9H, - Bu). ESI-MS (m/z): [M + Na]
=
H
14
N
9
O
3
Re (626.6) C 40.25, H 2.25, N 20.12.
1
+
ꢀ 1
(cm ): 2031 (CO), 1930 (CO), 1907
6
26, [M + K] = 642. IR-ATR
ν
(
CO). Y = 73% (MW = 553 g/mol, 0.60 mmol). Y = 41% (MW = 603 g/
mol, 0.14 mmol).
fac-[Re(CO) (PTZ-Me)-(pyr)][PF
2
4
2
4
] 1H NMR, (Acetone‑d
3
6
6
, 400
2
ꢀ 4
MHz) δ (ppm): 9.56–9.54 (m, 1H, PTZ-Me), 8.55–8.52 (m, 3H, pyr),
was topped up to volume. Final pH = 7.4. Then, a 100 mL 4.4*10
M
8
1
.51–8.46 (m, 1H, PTZ-Me), 8.11–8.07 (m, 1H, PTZ-Me), 8.02–7.97 (m,
BSA (Sigma Aldrich, MW = 66463 g/mol) solution was prepared by
H, PTZ-Me), 7.48–7.45 (m, 2H, pyr), 4.75 (s, 3H, CH
3
). IR-ATR
ν
dissolving 2.9 g of BSA in 100 mL of PBS buffer. The final concentration
ꢀ 1
+
ꢀ
ꢀ 5
(
cm ): 2036.2 (CO), 1913.6 (CO). ESI-MS (m/z): [M] = 511; [M]
=
of BSA used in the absorption and emission titration was 1*10
M
1
3
ꢀ 4
1
1
1
45 (PF
6
). C NMR, (Acetone‑d
6
, 100 MHz) δ (ppm): 167.18 (C
t
),
(45.45
μ
L of 4.4*10 M BSA to Vtot = 2 mL). The Re(I)-complexes so-
55.29 (Cipso), 152.65 (PTZ-Me), 144.56 (pyr), 142.47 (PTZ-Me),
40.16 (pyr), 130.44 (PTZ-Me), 127.07 (PTZ-Me), 124.84 (pyr), 42.03
lutions were prepared by dissolving 1.2–1.4 mg of complex in 1 mL of
DMSO, resulting in concentrations of 2.1*10 M. 2 mL of the BSA/PBS
solution were placed in a quartz cuvette and successive aliquots of Re(I)
ꢀ 3
(
CH
3
). Y = 75% (MW = 655 g/mol; 0.303 mmol). Anal. Calcd. For
Re (655.47) C 27.49, H 1.85, N 12.82. Found: C 24.8,
C
15
H
12
6
N O
3
P
1
F
6
1
complexes solutions in DMSO were added with a micropipette (20x5 L).
μ
H 1.69, N 10.85.
fac-[Re(CO)
For absorption titration, absorption spectra were collected from 230 to
800 nm after each addition. For emission titration, emission spectra
were collected from 300 to 800 nm by monitoring the 346 nm maxima
(BSA emission) upon 280 nm excitation.
t
1
3
(PTZ- Bu)-(pyr)][PF
6
] H NMR, (CDCl
3
, 400 MHz) δ
t
(
ppm): 9.17–9.16 (d, 1H, JH-H = 5.60 Hz, PTZ- Bu), 8.44–8.42(d, 1H, JH-
t
H
= 7.20 Hz, pyr), 8.34–8.30 (m, 1H, PTZ- Bu), 8.26–8.24 (d, 2H, JH-H =
t
5
.20 Hz, PTZ- Bu), 7.91–7.81 (m, 2H, pyr), 7.43–7.40 (m, 2H, pyr), 1.92
X-ray crystallography Crystal data and collection details for fac-[Re
t
+
ꢀ
(
(
(
s, 9H, - Bu). ESI-MS (m/z): [M] = 553; [M] = 145 (PF
6
). IR-ATR
ν
(PTZ-Me)(CO)
(CO) (pyr)][PF
recorded on a Bruker APEX II diffractometer equipped with a PHOTON2
(fac-[Re(QTZ-Me)(CO) (pyr)][PF ) or CCD (fac-[Re(PTZ-
]⋅CHCl
Me)(CO) (pyr)][PF Cl ) detector using Mo–K radiation. Data
]⋅CH
3
(pyr)][PF
6
]⋅CH
2
Cl
2
and
fac-[Re(QTZ-Me)
cm ): 2045 (CO), 1940 (CO). 13C NMR, (Acetone‑d
ꢀ 1
]⋅CHCl
are reported in ESI†, Table S2. Data were
6
, 100 MHz) δ
3
6
3
t
ppm): 166.98 (C
t
), 155.22 (Cipso), 152.61 (PTZ- Bu), 144.93 (pyr),
t
t
t
1
1
0
2
42.37 (PTZ- Bu), 140.20 (pyr), 130.32 (PTZ- Bu), 127.06 (PTZ- Bu),
3
6
3
t
t
24.84 (pyr), 69.36 (C Bu), 40.55 (- Bu). Y = 19% (MW = 698 g/mol;
.05 mmol). Anal. Calcd. For C18 Re (697.55) C 30.99, H
.60, N 12.05. Found: C 29.43, H 2.45, N 11.94.
fac-[Re(CO) (QTZ-Me)-(pyr)][PF
1H NMR, (CD
3
6
2
2
α
H
18
N
6
O
3
P
1
F
6
1
were corrected for Lorentz polarization and absorption effects (empir-
ical absorption correction SADABS) [41]. The structures were solved by
direct methods and refined by full-matrix least-squares based on all data
using F2 [42]. Hydrogen atoms were fixed at calculated positions and
refined by a riding model. All non-hydrogen atoms were refined with
3
6
]
3
CN, 400 MHz)
δ (ppm): 8.95–8.88 (m, 1H, QTZ-Me), 8.49–8.47 (d, 1H, JH-H = 8.0 Hz,
QTZ-Me), 8.31–8.27 (m, 2H, QTZ-Me, pyr), 8.17–8.15 (m, 1H, QTZ-Me),
9

N. Monti et al.
Inorganica Chimica Acta 518 (2021) 120244
ꢀ
anisotropic displacement parameters. The PTZ-Me ligand and [PF
6
]
complexes, Inorg. Chem. 50 (2011) 1229–1241, https://doi.org/10.1021/
ic1015516.
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S. Stagni, Proton-induced reversible modulation of the luminescent output of
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anion of fac-[Re(PTZ-Me)(CO)
3
(pyr)][PF
6
]⋅CH
2 2
Cl are disordered.
[
[
They have been split into two positions and refined using one occupancy
factor per disordered group. CCDC 2,044,691 and 2,044,692 contains
the supplementary crystallographic data for fac-[Re(PTZ-Me)
(
2014) 229–243, https://doi.org/10.1021/ic402187e.
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Methylated Re(I) tetrazolato complexes: photophysical properties and light
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(
CO)
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(pyr)][PF
6
]⋅CH
2
Cl
2
and fac-[Re(QTZ-Me)(CO)
3
(pyr)][PF
6
]⋅
CHCl
3
. These data can be obtained free of charge via http://www.ccdc.
cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
[
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15] L. Flamigni, A. Barbieri, C. Sabatini, B. Ventura and F. Barigelletti, Photochemistry
and Photophysics of Coordination Compounds: Iridium. In: V. Balzani, S.
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Declaration of Competing Interest
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.
1
0.1039/c9dt02198a.
[
17] J.L. Wedding, H.H. Harris, C.A. Bader, S.E. Plush, R. Mak, M. Massi, D.A. Brooks,
B. Lai, S. Vogt, M.V. Werrett, P.V. Simpson, B.W. Skelton, S. Stagni, Intracellular
distribution and stability of a luminescent rhenium(I) tricarbonyl tetrazolato
complex using epifluorescence microscopy in conjunction with X-ray fluorescence
imaging, Metallomics 9 (2017) 382–390, https://doi.org/10.1039/c6mt00243a.
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A. Brooks, Imaging nuclear, endoplasmic reticulum and plasma membrane events
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Acknowledgments
[
[
[
[
[
The authors wish to thank the Italian Ministry of Education, Uni-
versity and Research (MIUR) for financial support (PRIN project: To-
wards a Sustainable Chemistry: Design of Innovative Metal-Ligand
Systems for Catalysis and Energy Applications). The Overseas (grant for
E.B.) and Marco Polo (grant for N.M.) programmes joint by University of
Bologna and the Curtin University of Perth are gratefully acknowledged.
3
468.12365.
19] C.A. Bader, E.A. Carter, A. Safitri, P.V. Simpson, P.J. Wright, S. Stagni, M. Massi, P.
A. Lay, D.A. Brooks, S.E. Plush, Unprecedented staining of polar lipids by a
luminescent rhenium complex revealed by FTIR microspectroscopy in adipocytes,
Mol. BioSyst. 12 (2016) 2064–2068, https://doi.org/10.1039/c6mb00242k.
20] C.A. Bader, T. Shandala, E.A. Carter, A. Ivask, T. Guinan, S.M. Hickey, M.
V. Werrett, P.J. Wright, P.V. Simpson, S. Stagni, N.H. Voelcker, P.A. Lay, M. Massi,
S.E. Plush, D.A. Brooks, A molecular probe for the detection of polar lipids in live
cells, PLoS One 11 (2016), https://doi.org/10.1371/journal.pone.0161557.
21] V. Fiorini, A.M. Ranieri, S. Muzzioli, K.D.M. Magee, S. Zacchini, N. Akabar,
A. Stefan, M.I. Ogden, M. Massi, S. Stagni, Targeting divalent metal cations with Re
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2020.120244.
(
1
I) tetrazolato complexes, Dalton Trans. 44 (2015) 20597–20608, https://doi.org/
0.1039/c5dt03690a.
22] M.V. Werrett, P.J. Wright, P.V. Simpson, P. Raiteri, B.W. Skelton, S. Stagni, A.
G. Buckley, P.J. Rigby, M. Massi, Rhenium tetrazolato complexes coordinated to
thioalkyl-functionalised phenanthroline ligands: synthesis, photophysical
characterisation, and incubation in live HeLa cells, Dalton Trans. 44 (2015)
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