Boron Difluoride Adducts of a Flexidentate Pyridine-Substituted Formazanate Ligand: Property Modulation via Protonation and Coordination Chemistry

Article abstract of DOI:10.1021/acs.inorgchem.7b01984

The synthesis and characterization of a flexidentate pyridine-substituted formazanate ligand and its boron difluoride adducts, formed via two different coordination modes of the title ligand, are described. The first adduct adopted a structure that was typical of other boron difluoride adducts of triarylformazanate ligands and contained a free pyridine subsituent, while the second was formed via the chelation of nitrogen atoms from the formazanate backbone and the pyridine substituent. Stepwise protonation of the pydridine-functionalized adduct, which is essentially nonemissive, resulted in a significant increase in the fluorescence quantum yield up to a maximum of 18%, prompting the study of this adduct as a pH sensor. The coordination chemistry of each adduct was explored through reactions with nickel(II) bromide [NiBr₂(CH₃CN)₂], triflate [Ni(OTf)₂], and 1,1,1,4,4,4-hexafluoroacetylacetonate [Ni(hfac)₂(H₂O)₂] salts. Coordination to nickel(II) ions altered the physical properties of the boron difluoride formazanate adducts, including red-shifted absorption maxima and less negative reduction potentials. Together, these studies have demonstrated that the physical and electronic properties of boron difluoride adducts of formazanate ligands can be readily modulated through protonation and coordination chemistry.

Full text of DOI:10.1021/acs.inorgchem.7b01984

Article
pubs.acs.org/IC
Boron Difluoride Adducts of a Flexidentate Pyridine-Substituted
Formazanate Ligand: Property Modulation via Protonation and
Coordination Chemistry
Stephanie M. Barbon, Jasmine V. Buddingh, Ryan R. Maar, and Joe B. Gilroy*
Department of Chemistry and the Centre for Advanced Materials and Biomaterials Research, The University of Western Ontario,
1151 Richmond Street North, London, Ontario N6A 5B7, Canada
S
* Supporting Information
ABSTRACT: The synthesis and characterization of a flexidentate pyridine-substituted formazanate ligand and its boron
difluoride adducts, formed via two different coordination modes of the title ligand, are described. The first adduct adopted a
structure that was typical of other boron difluoride adducts of triarylformazanate ligands and contained a free pyridine subsituent,
while the second was formed via the chelation of nitrogen atoms from the formazanate backbone and the pyridine substituent.
Stepwise protonation of the pydridine-functionalized adduct, which is essentially nonemissive, resulted in a significant increase in
the fluorescence quantum yield up to a maximum of 18%, prompting the study of this adduct as a pH sensor. The coordination
chemistry of each adduct was explored through reactions with nickel(II) bromide [NiBr2(CH3CN)2], triflate [Ni(OTf)2], and
1,1,1,4,4,4-hexafluoroacetylacetonate [Ni(hfac)2(H2O)2] salts. Coordination to nickel(II) ions altered the physical properties of
the boron difluoride formazanate adducts, including red-shifted absorption maxima and less negative reduction potentials.
Together, these studies have demonstrated that the physical and electronic properties of boron difluoride adducts of formazanate
ligands can be readily modulated through protonation and coordination chemistry.
(e.g., 1) of boron-containing ligands have high phosphorescence
quantum yields.26−29 The complexation of transition metals by
these boron adducts also often results in an increase in the
complexity of the oxidation and/or reduction processes, as is the
case with Ru(bpy)2 complexes of BODIPYs (e.g., 2) and other
metal complexes.30−32 The Hicks group has studied palladium-
(II) complexes of “Nindigo” ligands, which also contains a BF2
adduct (e.g., 3). They demonstrate that the absorption properties
and reduction potential of the BF2 adduct can be tuned via the
coordinated metal.33 The Hong group has also taken advantage
of the ECL of the BODIPY unit to devise BODIPY-zinc(II)
complexes (e.g., 4), which act as sensors for phosphates. In the
absence of phosphates, the BODIPY-zinc(II) complex is ECL-
active. However, when phosphates bind to the complex, ECL is
quenched.34
INTRODUCTION
■
Boron adducts of nitrogen-donor ligands are of significant
interest as a result of their use in a variety of applications.1−3
Common examples, including boron dipyrromethenes (BODI-
PYs),2 aza-BODIPYs,4 and BOPHYs,5 have been used in
photovoltaics6,7 and batteries,8 for cell imaging9,10 and photo-
dynamic therapy,1 and as electrochemiluminescence (ECL)
emitters.11,12 Sensing with boron adducts of nitrogen-donor
ligands is another widely explored area,13−17 although their use in
pH sensing remains relatively understudied.18 Accurate pH
measurements are essential in environmental studies and
biological applications, especially considering that maintaining
a normal intracellular pH is necessary for the regulation of critical
processes such as ion transport and apoptosis.19,20 BODIPYs and
aza-BODIPYs have been studied as pH sensors, some of which
are operative over wide pH ranges.21 Others dually sense the
presence of ions or oxygen and pH,22,23 emit in the near-IR,24
and/or have demonstrated cell permeability and low toxicity.25
The redox activity and emissive nature of many boron adducts
of nitrogen-donor ligands also make them attractive targets for
incorporation into complexes with transition metals. Generally,
the combination of an emissive boron adduct with a transition-
metal results in decreased or quenched emission. However,
Wang and others have demonstrated that platinum complexes
Received: August 7, 2017
© XXXX American Chemical Society
A
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Article
agents,42,43 ECL emitters,44 precursors to unusual BN hetero-
cycles,45,46 aggregation-induced-emission luminogens,47 and
building blocks in polymeric materials has also been explored
in detail.48,49 Herein, we demonstrate the synthesis of a pyridine-
substituted formazanate ligand, its BF2 adducts, and modulation
of the adduct properties via protonation and coordination
chemistry.
EXPERIMENTAL SECTION
■
General Considerations. Reactions and manipulations were
carried out under a nitrogen atmosphere using standard Schlenk
techniques unless otherwise stated. Solvents were obtained from
Caledon Laboratories, dried using an Innovative Technologies Inc.
solvent purification system, collected under vacuum, and stored under a
nitrogen atmosphere over 4 Å molecular sieves. Reagents were
purchased from Sigma-Aldrich or Alfa Aesar and used as received.
NMR spectra were recorded on 400 MHz (1H, 399.8 MHz; 11B, 128.3
MHz; 19F, 376.1 MHz) or 600 MHz (1H, 599.5 MHz; 13C, 150.8 MHz)
Varian INOVA instruments. 1H NMR spectra were referenced to
residual CHCl3 (7.26 ppm), and 13C NMR spectra were referenced to
CDCl3 (77.2 ppm). 11B NMR spectra were referenced to BF3·OEt2 at 0
ppm, and 19F NMR spectra were referenced to CFCl3 at 0 ppm. Mass
spectrometry (MS) data were recorded in positive-ion mode using a
high-resolution Finnigan MAT 8400 spectrometer and electron-impact
ionization or a Micromass LCT electrospray time-of-flight mass
spectrometer. UV−vis absorption spectra were recorded using a Cary
5000 instrument. Four separate concentrations were run for each
sample, and molar extinction coefficients were determined from the
slope of a plot of absorbance against concentration. Fourier transform
infrared (FT-IR) spectra were recorded using an attenuated-total-
reflectance (ATR) attachment using a Bruker Vector 33 FT-IR
spectrometer. Emission spectra were obtained using a Photon
Technology International QM-4 SE spectrofluorometer. Excitation
wavelengths were chosen based on λmax from the respective UV−vis
absorption spectrum in the same solvent. Fluorescence quantum yields
were estimated relative to [Ru(bpy)3][PF6]2 and corrected for
wavelength-dependent detector sensitivity (Figure S1).50,51
One way to expand the range of properties associated with
boron adducts of nitrogen-donor ligands is by the incorporation
of additional heteroatoms into ligand backbones toward the
realization of multiple coordination modes for a single ligand
(i.e., flexidentate ligand behavior). For example, the Svobodova
group has synthesized flexidentate cyclic enaminone ligands that
form diphenyl boron adducts 5a and 5b. They observed
irreversible conversion of 5a into 5b at elevated temperature.
Similarly, the Aprahamian group has developed a flexidentate
hydrazone ligand,35 which formed BF2 adducts 6a and 6b under
typical conditions for the installation of “BF2” groups.36 The
authors used 6b, which exhibits visible-light E→Z isomerization,
for a variety of applications including measurement of the
solution viscosity15 and detection of acids and bases.13
́
Electrochemical Methods. Cyclic voltammetry (CV) experiments
were performed with a Bioanalytical Systems Inc. (BASi) Epsilon
potentiostat and analyzed using BASi Epsilon software. Electrochemical
cells consisted of a three-electrode setup including a glassy carbon
working electrode, a platinum wire counter electrode, and a silver wire
pseudo reference electrode. Experiments were run at scan rates of 100
mV s−1 in degassed dichloromethane (CH2Cl2) solutions of the analyte
(∼1 mM) and supporting electrolyte (0.1 M [nBu4N][PF6]). Cyclic
voltammograms were referenced against an internal standard (∼1 mM
ferrocene) and corrected for internal cell resistance using BASi Epsilon
software.
X-ray Crystallography Details. The samples were mounted on a
MiTeGen polyimide micromount with a small amount of Paratone N
oil. X-ray measurements were made on a Bruker Kappa Axis Apex2
diffractometer (8a and 8a·H+) or a Nonius Kappa CCD Apex2
diffractometer (8b, 9a, 10a, and 10b) at a temperature of 110 K. Initial
indexing indicated that the sample crystal for 10a was nonmerohedrally
twinned. The twin law of the first fraction was determined to be
−1.000 0.000 0.000
0.000 −1.000 0.001
−0.496 0.090 1.000
which represents a 180.0° rotation about [001]. The twin law of the
second fraction was determined to be
0.940 0.164 0.000
−0.171 1.033 −0.002
−0.009 0.043 1.000
Our group is interested in the chemistry of formazanate
ligands and their boron adducts. We, and others, have previously
shown that these compounds are fluorescent and have interesting
electronic properties, which are tunable through substituent
variation.37−41 Their utility as fluorescence cell-imaging
which represents a 9.3° rotation about [001]. The twin law was included
in the refinement as an adjustable parameter (vide infra).
B
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The data collection strategy included a number of ω and φ scans,
which collected data over a range of angles, 2θ. The frame integration
was performed using SAINT.52 The resulting raw data were scaled, and
absorption was corrected using a multiscan averaging of symmetry-
equivalent data using SADABS (8a, 8a·H+, 8b, 9a, and 10b)53 or
TWINABS (10a).54 The structure was solved by using a dual-space
methodology with the SHELXT program.55 All non-hydrogen atoms
were obtained from the initial solution. Hydrogen atoms were
introduced at idealized positions and allowed to refine isotropically
(8a and 8b) or allowed to ride on the parent atom (10a and 10b), or a
combination of the two methods were used (8a·H+ and 9a). The twin
fractions of 10a refined to values of 0.3337 and 0.1632. The structural
model was fit to the data using full matrix least squares based on F2. The
difference map of 9a and 10a showed regions of electron density that
could not be accurately modeled. Thus, the PLATON SQUEEZE-
calculated structure factors including corrections for the anomalous
dispersion program56 were used, and analysis was continued on these
data. All structures were refined using the SHELXL-2014 program from
the SHELXT suite of crystallographic software.55 See Table S1 and
CCDC 1555851−1555856 for additional crystallographic data.
Formazan 7. In air, p-tolylhydrazine hydrochloride (1.31 g, 8.26
mmol) was dissolved in ethanol (EtOH; 10 mL) with triethylamine
(NEt3; 1.52 g, 2.10 mL, 15.0 mmol) before 2-pyridinecarboxaldehyde
(0.90 g, 0.80 mL, 9.4 mmol) was added, and the solution was stirred for
10 min. After this time, a light-yellow precipitate had formed, and
dichloromethane (CH2Cl2; 75 mL) and deionized H2O (75 mL) were
added to form a biphasic reaction mixture. Sodium carbonate (Na2CO3;
2.98 g, 28.1 mmol) and tetra-n-butylammonium bromide ([nBu4N]-
[Br]; 0.27 g, 0.84 mmol) were added, and the mixture was cooled with
stirring for 30 min in an ice bath to 0 °C. In a separate flask, p-toluidine
(0.90 g, 8.4 mmol) and concentrated hydrochloric acid (HCl; 2.1 mL, 25
mmol) were mixed in deionized H2O (15 mL) and cooled in an ice bath.
A cooled solution of sodium nitrite (0.66 g, 9.6 mmol) in deionized H2O
(5 mL) was added slowly to the amine solution over a 5 min period. This
mixture was then stirred at 0 °C for 30 min, after which time it was added
dropwise to the biphasic reaction mixture described above over a 10 min
period. The resulting solution was stirred for 18 h, gradually turning dark
red over this time. The dark-red organic fraction was then washed with
deionized H2O (3 × 50 mL), dried over magnesium sulfate (MgSO4),
gravity-filtered, and concentrated in vacuo. The resulting residue was
purified by flash chromatography (7:2:1 hexanes/CH2Cl2/EtOAc, silica
gel) to afford a dark-red microcrystalline solid. Yield: 0.88 g, 32%. Mp:
132−135 °C. 1H NMR (400.1 MHz, CDCl3): δ 16.07 (s, 1H, NH), 9.08
(d, 3JHH = 6 Hz, 1H, aryl CH), 8.36 (d, 3JHH = 8 Hz, 1H, aryl CH), 8.27−
8.25 (m, 1H, aryl CH), 7.77 (d, 3JHH = 8 Hz, 4H, aryl CH), 7.56−7.54
(m, 1H, aryl CH), 7.21 (d, 3JHH = 8 Hz, 4H, aryl CH), 2.33 (s, 6H, CH3).
13C{1H} NMR (150.7 MHz, CDCl3): δ 148.7, 144.7, 144.2, 142.4,
140.5, 133.0, 130.5, 122.8, 122.4, 120.4, 21.6. FT-IR (ATR): 3032 (m),
2918 (m), 2857 (w), 1599 (m), 1582 (m), 1564 (m), 1511 (m), 1465
(s), 1428 (m), 1349 (m), 1263 (s), 1235 (s), 1105 (s), 808 (s), 788 (m)
cm−1. UV−vis (CH2Cl2): λmax = 474 nm (ε = 19100 M−1 cm−1). MS (EI,
positive-ion mode). Exact mass calcd for [C20H19N5]+: 329.1640. Exact
mass found: 329.1639. Difference: −0.3 ppm.
CDCl3): δ 152.0, 150.0, 141.7, 140.5, 136.8, 129.8, 129.7, 123.6, 121.0,
110.1, 21.4. 11B NMR (128.3 MHz, CDCl3): δ −0.6 (t, 1JBF = 28 Hz). 19
F
NMR (376.1 MHz, CDCl3): δ −144.8 (q, 1JFB = 28 Hz). FT-IR (ATR):
3014 (m), 2916 (m), 1604 (m), 1584 (m), 1379 (m), 1352 (m), 1316
(s), 1299 (s), 1178 (m), 1109 (s), 1021 (s), 965 (s), 818 (s), 783 (m)
cm−1. UV−vis [CH2Cl2; λmax, nm (ε, M−1 cm−1)]: 507 (26200). MS (EI,
positive-ion mode). Exact mass calcd for [C20H18BF2N5]+: 377.1623.
Exact mass found: 377.1633. Difference: +2.6 ppm. Anal. Calcd for
C20H18BF2N5: C, 63.68; H, 4.81; N, 18.57. Found: C, 63.57; H, 4.84; N,
18.20.
1
8b. Yield: 0.84 g, 38%. Mp: 221−223 °C. H NMR (400.1 MHz,
CDCl3): δ 8.75−8.69 (m, 2H, aryl CH), 8.19−8.16 (m, 1H, aryl CH),
7.87−7.86 (m, 2H, aryl CH), 7.68−7.67 (m, 2H, aryl CH), 7.61−7.59
(m, 1H, aryl CH), 7.31 (d, 3JHH = 8 Hz, 2H, aryl CH), 7.20 (d, 3JHH = 8
Hz, 2H, aryl CH), 2.44 (s, 3H, CH3), 2.37 (s, 3H, CH3). 13C{1H} NMR
(150.7 MHz, CDCl3): δ 151.4, 143.3, 141.7, 141.5, 141.3, 140.4, 139.4,
135.3, 129.8, 129.5, 123.3, 123.0, 121.8, 120.9, 21.6, 21.0. 11B NMR
(128.3 MHz, CDCl3): δ 0.4 (t, 1JBF = 32 Hz). 19F NMR (376.1 MHz,
CDCl3): δ −137.9 (q, 1JFB = 32 Hz). FT-IR (ATR): 3008 (w), 2914 (w),
1619 (m), 1567 (m), 1476 (m), 1412 (m), 1317 (m), 1277 (m), 1125
(m), 1080 (s), 1028 (s), 972 (m), 820 (m), 774 (s) cm−1. UV−vis
[CH2Cl2; λmax, nm (ε, M−1 cm−1)]: 331 (13700), 457 (26600). MS (EI,
positive-ion mode). Exact mass calcd for [C20H18BF2N5]+: 377.1623.
Exact mass found: 377.1628. Difference: +1.3 ppm. Anal. Calcd for
C20H18BF2N5: C, 63.68; H, 4.81; N, 18.57. Found: C, 63.36; H, 4.80; N,
18.20.
8a·H+. 8a (0.050 g, 0.13 mmol) was dissolved in EtOH (10 mL), and
p-toluenesulfonic acid (p-TsOH; 1.14 g, 5.99 mmol) in EtOH (10 mL)
was added. The solution was stirred for 30 min and then concentrated in
vacuo. The resulting residue was dissolved in CH2Cl2 (25 mL), washed
with deionized H2O (25 mL), dried over MgSO4, gravity-filtered, and
concentrated in vacuo to yield a dark-red microcrystalline solid. Yield:
1
0.060 g, 84%. Mp: 121−123 °C. H NMR (400.1 MHz, CDCl3): δ
9.47−9.46 (m, 1H, aryl CH), 8.45−8.41 (m, 2H, aryl CH), 7.99 (d, 3JHH
= 8 Hz, 4H, aryl CH), 7.88−7.85 (m, 1H, aryl CH), 7.74 (d, 3JHH = 7 Hz,
2H, aryl CH), 7.18 (d, 3JHH = 8 Hz, 4H, aryl CH), 7.10 (d, 3JHH = 7 Hz,
2H, aryl CH), 2.38 (s, 6H, CH3), 2.33 (s, 3H, CH3). 13C{1H} NMR
(150.7 MHz, CDCl3): δ 146.3, 145.8, 143.5, 143.1, 142.6, 141.6, 141.3,
140.4, 130.3, 128.9, 126.3, 125.6, 123.7, 122.9, 21.6, 21.4. 11B NMR
(128.3 MHz, CDCl3): δ −0.6 (t, 1JBF = 30 Hz). 19F NMR (376.1 MHz,
CDCl3): δ −136.1 (q, 1JFB = 30 Hz). FT-IR (ATR): 3357 (br, m), 3102
(m), 2919 (m), 2841 (m), 1599 (s), 1495 (m), 1375 (m), 1334 (s), 1223
(m), 1169 (s), 1116 (s), 1000 (m), 967 (s), 811 (s) cm−1. UV−vis
[CH2Cl2; λmax, nm (ε, M−1 cm−1)]: 533 (23000). MS (ESI, positive-ion
mode). Exact mass calcd for [C20H19BF2N5]+: 378.1701. Exact mass
found: 378.1708. Difference: +1.8 ppm.
9a. In a N2-filled glovebox, adduct 8a (0.050 g, 0.13 mmol) was
dissolved in dry and degassed toluene (10 mL). [Ni(OTf)2] (0.024 g,
0.066 mmol) was suspended in dry and degassed toluene (5 mL) and
added to the solution of 8a. This mixture was heated at 60 °C with
stirring for 18 h before it was removed from the glovebox and exposed to
an ambient atmosphere. The mixture was then concentrated in vacuo to
yield a dark-red solid. This product was recrystallized by the slow
diffusion of pentane into a concentrated CH2Cl2 solution to yield
complex 9a as red crystals. Yield: 0.068 g, 89%. Mp: 249−251 °C. FT-IR
(ATR): 3294 (br, s), 3035 (m), 2921 (m), 2857 (m), 1605 (m), 1588
(m), 1355 (m), 1316 (s), 1296 (s), 1222 (m), 1178 (s), 1128 (m), 1023
(m), 970 (m), 819 (m) cm−1. UV−vis [CH2Cl2; λmax, nm (ε, M−1
cm−1)]: 509 (32300). Anal. Calcd for C42H40B2F10N10O8S2Ni: C, 43.97;
H, 3.51; N, 12.21. Found: C, 43.43; H, 3.77; N, 11.85.
Adducts 8a and 8b. Formazan 7 (1.92 g, 5.83 mmol) was dissolved
in dry toluene (200 mL). Triethylamine (NEt3; 1.77 g, 2.44 mL, 17.5
mmol) was then added slowly, the solution was stirred for 10 min before
BF3·OEt2 (4.11 g, 3.57 mL, 29.0 mmol) was added, and the solution was
heated with stirring at 80 °C for 18 h. The solution gradually turned
from dark red to dark purple during this time. After cooling to 22 °C,
deionized H2O (10 mL) was added to quench any excess reactive boron-
containing compounds. The purple toluene solution was then washed
with deionized H2O (3 × 50 mL), dried over MgSO4, gravity-filtered,
and concentrated in vacuo. The resulting residue was purified by flash
chromatography (CH2Cl2, silica gel) to afford 8a (Rf = 0.35) as a dark-
red microcrystalline solid and 8b (Rf = 0.88) as an orange
microcrystalline solid after removal of the solvent in vacuo.
10a. Adduct 8a (0.050 g, 0.13 mmol) was dissolved in CH2Cl2 (25
mL), and [Ni(hfac)2(H2O)2] (0.124 g, 0.130 mmol) was added. The
mixture was stirred overnight before it was filtered and the filtrate
concentrated in vacuo. The resulting red/purple solid was recrystallized
by the slow diffusion of pentane into a concentrated CH2Cl2 solution of
the reaction product to yield complex 10a as red/purple crystals. Yield:
0.081 g, 72%. Mp: 230−232 °C. FT-IR (ATR): 3346 (br, s), 2984 (m),
2871 (m), 1641 (m), 1478 (m), 1255 (m), 1200 (m), 1127 (s), 1055 (s),
894 (m), 818 (w), 790 (s), 750 (s) cm−1. UV−vis [CH2Cl2; λmax, nm (ε,
1
8a. Yield: 0.69 g, 31%. Mp: 162−164 °C. H NMR (400.1 MHz,
CDCl3): δ 8.77−8.76 (m, 1H, aryl CH), 8.15−8.14 (m, 1H, aryl CH),
7.83−7.80 (m, 5H, aryl CH), 7.35−7.33 (m, 1H, aryl CH), 7.26 (d, 3JHH
= 8 Hz, 4H, aryl CH), 2.41 (s, 6H, CH3). 13C{1H} NMR (150.7 MHz,
M
−1 cm−1)]: 535 (23700). MS (EI, positive-ion mode). Exact mass calcd
C
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for [C30H20BF14N5O4Ni]+: 849.0738. Exact mass found: 849.0778.
difference: +4.7 ppm. Anal. Calcd for C30H20BF14N5O4Ni: C, 42.39; H,
2.37; N, 8.24. Found: C, 42.50; H, 2.53; N, 7.94.
10b. Adduct 8b (0.050 g, 0.13 mmol) was dissolved in CH2Cl2 (25
mL), and [Ni(hfac)2(H2O)2] (0.124 g, 0.130 mmol) was added. The
mixture was stirred overnight before it was filtered and the filtrate
concentrated in vacuo. The resulting red solid was recrystallized by the
slow diffusion of pentane into a concentrated CH2Cl2 solution of the
reaction product to yield complex 10b as red/orange crystals. Yield:
0.074 g, 66%. Mp: 241−243 °C. FT-IR (ATR): 3360 (br, s), 3141 (w),
2925 (m), 2870 (m), 1640 (m), 1479 (s), 1415 (w), 1341 (m), 1258 (s),
1196 (s), 1133 (s), 1032 (s), 790 (m), 779 (m), 672 (s) cm−1. UV−vis
[CH2Cl2; λmax, nm (ε, M−1 cm−1)]: 383 (21400), 526 (24100). MS (EI,
positive-ion mode). Exact mass calcd for [C30H20BF14N5O4Ni]+:
849.0738. Exact mass found: 849.0718. Difference: −2.4 ppm. Anal.
Calcd for C30H20BF14N5O4Ni: C, 42.39; H, 2.37; N, 8.24. Found: C,
42.36; H, 2.59; N, 7.85.
Figure 1. Solid-state structures of 8a and 8b. Anisotropic displacement
ellipsoids are shown at 50% probability, and hydrogen atoms have been
removed for clarity.
RESULTS AND DISCUSSION
backbone and a slight displacement (by 0.565 Å) of the boron
atom from the N4 plane. In 8b, the electron density in the π
system of the chelate ring is also delocalized, with the N−N, C−
C, and C−N bond lengths between the expected single and
double bond lengths for the respective atoms involved.59
Electronic delocalization does not extend into the azotoluene
portion of the molecule [C1−N4 = 1.408(2) Å; N4−N3 =
1.239(2) Å]. The boron atom is displaced in the solid-state
structure of 8b from the N1−N2−N5−C16 plane by 0.394 Å.
Adducts 8a and 8b are strongly absorbing over much of the
visible region in CH2Cl2 (Figure 2 and Table 1). Adduct 8a has a
■
Formazan 7 was synthesized by adopting a published procedure
(Scheme S1 and Figures S2 and S3).57 The subsequent reaction
with BF3·OEt2 and NEt3 (Scheme 1) afforded two products,
Scheme 1. Synthesis of the BF2 Adducts 8a and 8b
which could be separated by column chromatography (CH2Cl2,
silica gel). Adduct 8a (Rf = 0.35; 31% yield) resembled other BF2
formazanate adducts, with the aryl-substituted nitrogen atoms
coordinated to the “BF2” fragment.37 Adduct 8b (Rf = 0.88; 38%
yield) contained a “BF2” fragment chelated by one of the aryl-
substituted nitrogen atoms of the formazanate backbone as well
as the nitrogen atom of the pyridine substituent. Several
coordination modes between boron and formazanate ligands
have been reported,45,46 although the connectivity present in 8b
has not been observed previously. When stirred in the presence
of Lewis bases such as 4-(dimethylamino)pyridine, interconver-
sion from 8a to 8b and vice versa was not observed. Adduct 8b
contains a potentially photoactive azotoluene moiety. However,
irradiation with 365−370 or 525−530 nm light and temperature
variation did not result in observable E → Z isomerization. This
behavior was in stark contrast to that observed for
arylazoindazoles, derived from related formazans, which readily
isomerize under similar conditions.58
Figure 2. UV−vis absorption spectra of 8a and 8b in CH2Cl2.
Table 1. Solution Properties of 8a, 8b, 9a, 10a, and 10b in
CH2Cl2
The isolated adducts were characterized by 1H, 11B, 13C{1H},
and 19F NMR, IR and UV−vis spectroscopy, elemental analysis,
and MS (Figures S4−S7). In both 8a and 8b, 1:2:1 triplets and
1:1:1:1 quartets were observed in the respective 11B and 19F
NMR spectra, confirming the presence of the BF2 moiety.
However, the chemical shifts of the signals were quite different
(11B: 8a, −0.6 ppm; 8b, 0.4 ppm. 19F: 8a, −144.8 ppm; 8b,
−137.9 ppm). Both adducts were also characterized by single-
crystal X-ray crystallography (Figure 1). The bond lengths and
angles observed in 8a are structurally similar to those of other
BF2 adducts of triarylformazanate ligands,37 including delocaliza-
tion of the electron density over the entire NNCNN
a
a
a
λmax (nm)
ε (M−1 cm−1
)
Ered1 (V)
Ered2 (V)
Ered3 (V)
b
8a
517
26200
−0.99
−1.53
−0.31
−0.64
−0.92
−2.00
−2.09
b
8b
467, 331
509
26600, 13700
32300
9a
−0.99
−1.66
−1.46
b
10a
10b
539
26700
527, 383
24100, 21400
−1.18
a
CV experiments were conducted in CH2Cl2 containing 1 mM analyte
and 0.1 M [nBu4N][PF6] as the supporting electrolyte at a scan rate of
100 mV s−1. All voltammograms were referenced internally against the
b
ferrocene/ferrocenium redox couple. Irreversible peak; potential at
the maximum cathodic current reported.
D
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wavelength of maximum absorption (λmax) of 507 nm, typical of
BF2 adducts of triarylformazanate ligands.37 Adduct 8b exhibits
two low-energy maxima at 404 and 457 nm, which are of similar
intensity relative to the maxima observed for 8a. The absorption
spectra of 8a and 8b in toluene and tetrahydrofuran (THF) are
unchanged relative to those collected in CH2Cl2, indicating that
the absorption maxima likely arise because of π → π* transitions
rather than charge transfer. Both 8a and 8b are essentially
nonemissive, with fluorescence quantum yields (ΦF) less than
1%.
Adducts 8a and 8b are electrochemically active (Figure 3 and
Table 1). Both adducts exhibit a reversible one-electron
Figure 3. Cyclic voltammograms of 8a and 8b recorded at a scan rate of
100 mV s−1 in 1 mM CH2Cl2 solutions containing 0.1 M [nBu4N][PF6]
as the supporting electrolyte.
reduction wave (8a, Ered1 = −0.99 V; 8b, Ered1 = −1.53 V) and
a second irreversible reduction wave (8a, Epc = −2.00 V; 8b, Epc =
−2.09 V). The separation of these waves was much larger for 8a
(ΔE = 1.01 V) than 8b (ΔE = 0.56 V). The electrochemical
behavior of 8a is consistent with that of other triarylformazanate
BF2 adducts,37 and the electrochemical behavior of 8b is
comparable to that of related azobenzene species.60
Figure 4. (a) Emission spectra of 8a at various pH values in 9:1 H2O/
THF. (b) Change in ΦF of 8a as a function of the pH. The black dashed
line is a line of best fit. The error bars were obtained from three
independent measurements.
isolation of the protonated adduct, the 1:1:1:1 quartet and 1:2:1
triplet in the 19F and 11B NMR spectra were retained, and 1H and
13C NMR shifts were similar (Figures S9 and S10), implying that
the connectivity within the structure of the resulting adduct (8a·
H+) had not changed. Upon protonation of the pyridine moiety
at low pH, inter- or intramolecular hydrogen bonding with
nearby formazanate molecules, counterions, and solvent
molecules is likely. This potentially restricts vibration and
rotation of the pyridine substituent, minimizing nonradiative
relaxation pathways. The presence of hydrogen bonding was
confirmed in the solid state through single-crystal X-ray
crystallography (Figure 5). A hydrogen atom attached to the
pyridyl nitrogen atom was observed. Many hydrogen bonds were
also observed, including from the pyridine NH to a p-
toluenesulfonate counterion [N5−O1 = 2.815(10) Å]. All
bond lengths and angles in the solid-state structures of 8a and
8a·H+ are similar (Table 2). In the case of 8b, no change was
observed in the 1H NMR and UV−vis absorption spectra, even
upon exposure to concentrated HCl and p-TsOH, indicating that
protonation did not occur.
Protonation Chemistry. We have long postulated that the
emission intensity observed for BF2 adducts of triarylformaza-
nate ligands is attenuated by nonradiative relaxation pathways
associated with rotation and/or vibration of the 3-aryl
substituents.39 We therefore chose to explore the protonation
chemistry using p-TsOH as a method for limiting vibration/
rotation in 8a. A change in the color of solutions of 8a and shifts
1
in the corresponding H NMR spectra were observed under
acidic conditions. The fluorescence intensity also increased
significantly upon acidification of solutions of 8a. The pH
dependence of fluorescence was therefore studied, revealing a
linear increase in the fluorescence intensity with decreasing pH
(Figure 4).
At pH > 4, ΦF remained constant at ∼1%. However, at lower
pH, ΦF increased to a maximum value of 18%. This represents
the highest reported ΦF to date for a BF2 adduct of a
triarylformazanate ligand and corroborates our previous
hypothesis surrounding the role of the 3-aryl substituent as a
fluorescence attenuator. We also observed a slight red shift in the
wavelength of maximum emission (λem) as the fluorescence
intensity increased, from 626 nm at pH 4 to 633 nm at pH 0. This
is consistent with a change in the wavelength of maximum
absorption upon a decrease of the pH in the absorption spectra
from 507 nm (pH 4) to 540 nm (pH 0) (Figure S8). After
Nickel(II) Coordination Chemistry. Both 8a and 8b have
the potential to act as bidentate nitrogen-donor ligands. We
therefore attempted to study their coordination chemistry as a
method for modulating their physical and electronic properties.
This reactivity was probed using several nickel(II) sources in
E
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[2.103(8) Å], and 10b [2.124(5) Å] were not statistically
different. The smaller angle and longer bonds observed in 10b
may hint as to why coordination is less favorable with this ligand
and unsuccessful when [NiBr2(CH3CN)2] and [Ni(OTf)2] were
employed as nickel(II) sources.
All three nickel(II) complexes absorb strongly between 450
and 600 nm (Figure 7 and Table 1). The molar absorptivity of 9a
is significantly higher than that of 10a or 10b because of the
presence of two BF2 adducts in the complex. The λmax value of 9a
is essentially unchanged from that of the parent ligand 8a (509
and 517 nm, respectively). In contrast, the λmax values are shifted
significantly in the complexes that are coordinated to Ni(hfac)2
(10a, Δλmax = −22 nm; 10b, Δλmax = 50 nm). In complex 10b, a
second intense absorption at a wavelength of 389 nm was also
observed. Despite the rigidification of the pyridine substituent by
nickel(II) coordination, each of the nickel(II) complexes was
nonemissive.
Nickel(II) complexes 9a, 10a, and 10b are electrochemically
active (Figure 8 and Table 1). Complex 9a was reversibly
reduced by two electrons at −0.31 V and reversibly reduced by
one electron at −0.99 and −1.46 V (Scheme 2). The two-
electron reduction corresponds to each of the nickel(II)-bound
BF2 adducts being converted to ligand-centered radical anions.
This reduction takes place at a significantly lower potential than
the free ligand (by ca. 0.68 V). The two additional one-electron
reductions observed correspond to the stepwise reduction of the
BF2 ligands to their ligand-centered dianion forms. The ligand-
centered reduction behavior is consistent with that of other BF2
formazanate adducts,37,41 and the noncoincident reduction
waves imply that the electron-rich nature of the formally
trianionic complex resulting from the reduction at −0.99 V has a
significant influence on the subsequent reduction step. Both
events occur at less negative potentials than the corresponding
events observed for 8a. These findings confirm that the electron-
withdrawing nickel(II) makes the BF2 adduct more electron-
poor and thus easier to reduce.
Figure 5. Solid-state structure of 8a·H+. Anisotropic displacement
ellipsoids are shown at 50% probability, and hydrogen atoms aside from
H5, as well as cocrystallized p-TsOH and H2O molecules, have been
removed for clarity. The dashed line indicates a hydrogen bond.
order to explore the effects of metal coordination on the ligand
properties in the absence of metal-based redox reactivity. Initial
attempts employed [NiBr2(CH3CN)2] as a nickel(II) source.
The resulting UV−vis absorption and NMR spectra collected for
the reaction mixture were unchanged compared to that of
adducts 8a and 8b, and the presence of two distinct solids upon
removal of the solvent in each case indicated that NiBr2 had not
bound to the ligands employed. Next, [Ni(OTf)2] was
employed. In this case, a reaction was observed between
[Ni(OTf)2] and 8a, resulting in the formation of complex 9a.
Single-crystal X-ray diffraction studies revealed a 2:1 ratio
between 8a and nickel(II) (Figure 6), with two H2O molecules
oriented cis to one another, completing the octahedral
coordination sphere of nickel(II). Attempts to alter the ratio of
8a to nickel(II) to 1:1 or 3:1, as well as attempts to isolate a
nickel(II) complex via reaction with [Ni(OTf)2] and 8b, were
unsuccessful. Finally, reactions with [Ni(hfac)2(H2O)2] (hfac =
1,1,1,6,6,6-hexafluoroacetylacetonate) were attempted. BF2
adducts 8a and 8b coordinated to the Ni(hfac)2 fragment and
paramagnetic octahedral complexes 10a and 10b containing one
BF2 adduct and two hfac ligands were isolated in both cases
(Figure 6). The N3−N4 bond lengthened upon coordination to
nickel(II) from 1.3135(17) Å in 8a to 1.326(3) Å in complex 9a,
although there was no statistical difference between the N3−N4
bond lengths in 8a and 10a [1.328(10) Å]. Similarly, the N3−N4
bond lengthened from 1.239(2) Å in 8b to 1.336(6) Å in 10b
(Table 2). The N−Ni−N angle is smaller in 10b [75.5(2)°] than
in 9a [average of 77.90(8)°] and 10a [77.6(3)°], although all are
less than the ideal 90° angle expected for octahedral nickel(II).
The average N−Ni bond lengths for 9a [2.115(2) Å], 10a
Similarly, two ligand-centered reductions were observed at
potentials of −0.64 and −1.58 V in the cyclic voltammogram
collected for 10a. Again, both of these reduction processes occur
more easily than those observed for 8a. However, 10a is not as
easy to reduce as 9a (8a, Ered1 = −0.99 V; 9a, Ered1 = −0.31 V;
10a, Ered1 = −0.64 V). Two reduction events were also present in
the CV of 10b, albeit at much less negative potentials. Similar to
the nickel(II) complexes of the BF2 adduct 8a, each process was
observed at less negative potentials (shifted by 0.61 and 0.91 V
respectively) compared to the free ligand. As a result, the second
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 8a, 8a·H+, 8b, 9a, 10a, and 10b
9a
8a
8a·H+
8b
ligand 1
ligand 2
10a
10b
N1−N2
N3−N4
C1−N2
C1−N4
C1−C16
C16−N5
N4−Ni1
N5−Ni1
N1−Ni1
N4−Ni1−N5
N1−Ni1−N4
1.3160(17)
1.3135(17)
1.337(2)
1.340(2)
1.487(2)
1.363(2)
1.309(6)
1.307(6)
1.326(7)
1.346(7)
1.473(8)
1.352(7)
1.337(2)
1.239(2)
1.312(2)
1.408(2)
1.456(3)
1.354(2)
1.306(3)
1.325(3)
1.345(3)
1.340(3)
1.473(3)
1.348(3)
2.136(2)
2.070(2)
1.304(3)
1.327(3)
1.342(3)
1.342(3)
1.473(3)
1.345(3)
2.176(2)
2.079(2)
1.309(10)
1.328(10)
1.330(11)
1.344(11)
1.481(13)
1.331(12)
2.171(8)
1.269(7)
1.336(6)
1.387(7)
1.324(8)
1.432(9)
1.367(7)
2.157(5)
2.035(8)
2.091(5)
75.5(2)
78.06(8)
77.74(8)
77.6(3)
F
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 6. Solid-state structures of 9a, 10a, and 10b. Anisotropic displacement ellipsoids are shown at 50% probability, and hydrogen atoms as well as the
triflate counterions (9a) and cocrystallized solvent (9a and 10a) have been removed for clarity. The second BF2 adduct of 9a and the hfac ligands of 10a
and 10b have been made wireframe for clarity.
10a and 10b, an irreversible reduction is observed, which can be
attributed to a reduction of the hfac ligands (Figure S11).61
CONCLUSIONS
■
In conclusion, we have synthesized a pyridine-substituted
triarylformazanate ligand that can adopt two different coordina-
tion modes upon reaction with sources of “BF2”. The first adduct,
8a, coordinated to BF2 through the aryl-substituted nitrogen
atoms of the formazanate backbone and contained a pyridine
substituent. The second adduct, 8b, coordinated through one of
the formazanate nitrogen atoms, as well as the pyridine nitrogen,
and contained an azotoluene substituent. The free pyridine
moiety in 8a could be protonated, and upon protonation, ΦF
increased substantially to a maximum of 18%. The use of
complex 8a as a fluorescent pH sensor was also demonstrated,
Figure 7. UV−vis absorption spectra of 9a, 10a, and 10b in CH2Cl2.
with linear fluorescence responses observed below pH 4. Finally,
both BF2 adducts were shown to act as redox-active ligands when
coordinated to nickel(II) ions. In the case of Ni(hfac)2
complexes 10a and 10b, the absorption profiles were red-shifted
significantly from adducts 8a and 8b and ligand-centered
reduction events occurred at much less negative potentials.
This work, for the first time, has demonstrated that the properties
of BF2 formazanate adducts can be modulated using protonation
or coordination chemistry, paving the way for the rational design
of future generations of functional materials derived from
adducts of formazanate ligands.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
Figure 8. Cyclic voltammograms of 9a, 10a, and 10b recorded at a scan
chem.7b01984.
rate of 100 mV s−1 in 1 mM CH2Cl2 solutions containing 0.1 M
[nBu4N][PF6] as the supporting electrolyte.
X-ray diffraction data collection and refinement details,
synthesis of formazan 7, wavelength-dependent emission
correction, 1H and 13C{1H} NMR and UV−vis absorption
spectra, and cyclic voltammograms (PDF)
reduction was reversible in metal complex 10b, whereas it was
irreversible in 8b. Upon scanning to further negative potentials in
a
Scheme 2. Electrochemical Reduction of Nickel(II) Complex 9a
a
The charge on ligand 8a has been specified for clarity.
G
DOI: 10.1021/acs.inorgchem.7b01984
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Accession Codes
CCDC 1555851−1555856 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: joe.gilroy@uwo.ca. Tel: +1-519-661-2111 ext. 81561.
■
ORCID
Joe B. Gilroy: 0000-0003-0349-7780
(16) Roubinet, B.; Massif, C.; Moreau, M.; Boschetti, F.; Ulrich, G.;
Ziessel, R.; Renard, P.-Y.; Romieu, A. New 3-(Heteroaryl)-2-
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Boron Diiminates with Unique Optical Properties: Multicolor Tuning
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the University of Western Ontario, the Natural Science
and Engineering Research Council (NSERC) of Canada (Grant
DG 435675 to J.B.G., CGS D scholarships to S.M.B. and R.R.M.,
and a NSERC USRA scholarship to J.V.B.), the Ontario Ministry
of Research and Innovation (Grant ER14-10-147 to J.B.G.), and
the Canada Foundation for Innovation (Grant JELF 33977 to
J.B.G.) for funding this work. Finally, we thank Prof. Elizabeth R.
Gillies for access to instrumentation in her laboratory.
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DOI: 10.1021/acs.inorgchem.7b01984
Inorg. Chem. XXXX, XXX, XXX−XXX