Synthesis, properties and reactivity of BCl2 aza-BODIPY complexes and salts of the aza-dipyrrinato scaffold

Article abstract of DOI:10.1039/d0ob00272k

The synthesis and characterisation of the BCl₂-chelated complexes of both archetypal aza-dipyrrin sub-Types are presented. A stepwise halogen exchange, leading to a mixed-halide Cl-B-F intermediate, is implicated in the conversion of F-Aza-BODIPYs to Cl-Aza-BODIPYs upon treatment with BCl₃. The utility of the Cl-Aza-BODIPY scaffold to facilitate substitutions at boron is demonstrated under mild conditions through treatment with aryl Grignard reagents. Additionally, the lability of the B-Cl bond enables facile removal of the BCl₂ group, i.e. deprotection of F-Aza-BODIPYs, under aqueous conditions. Three aza-dipyrrin HX salts were also synthesised and characterised. The pK_a of the protonated aza-dipyrrin was determined to be 4, thereby providing insight regarding the storage and stability of such species.

Full text of DOI:10.1039/d0ob00272k

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Synthesis, properties and reactivity of BCl2
aza-BODIPY complexes and salts of the
Cite this: DOI: 10.1039/d0ob00272k
aza-dipyrrinato scaffold†
Roberto M. Diaz-Rodriguez, a Luke Burke,a Katherine N. Robertson b and
a
Alison Thompson
*
The synthesis and characterisation of the BCl2-chelated complexes of both archetypal aza-dipyrrin sub-
types are presented. A stepwise halogen exchange, leading to a mixed-halide Cl–B–F intermediate, is
implicated in the conversion of F-aza-BODIPYs to Cl-aza-BODIPYs upon treatment with BCl3. The utility
of the Cl-aza-BODIPY scaffold to facilitate substitutions at boron is demonstrated under mild conditions
through treatment with aryl Grignard reagents. Additionally, the lability of the B–Cl bond enables facile
removal of the BCl2 group, i.e. deprotection of F-aza-BODIPYs, under aqueous conditions. Three aza-
dipyrrin HX salts were also synthesised and characterised. The pKa of the protonated aza-dipyrrin was
determined to be 4, thereby providing insight regarding the storage and stability of such species.
Received 7th February 2020,
Accepted 25th February 2020
DOI: 10.1039/d0ob00272k
rsc.li/obc
duced onto the aza-dipyrrin core is limited. Post-modification
of the boron center thus provides an attractive approach to
Introduction
Aza-dipyrrins are a family of fully conjugated, strongly chromo- modifying the photophysical and chemical properties of aza-
phoric heterocyclic compounds consisting of two pyrrolic BODIPYs. Some such modifications on F-aza-BODIPYs have
units bridged by an imino-type nitrogen atom. They are for- been reported, but most involve harsh conditions or bespoke
mally related to the well-known dipyrrin (dipyrromethene) substrates.6,8,10,11
skeleton through replacement of the bridging nitrogen with a
We hypothesised that Cl-aza-BODIPYs would present
carbon-based substituent (Fig. 1). Aza-dipyrrins and their opportunities to introduce substituents at boron via reaction
coordination complexes, particularly the BF2-chelated F-aza- with nucleophiles. Indeed the weaker B–Cl bond of Cl-aza-
BODIPYs (Fig. 1, left), are of interest due to their remarkable BODIPYs cf. the B–F bond of F-aza-BODIPYs should enable
red and near-infrared absorption and emission profiles, very facile substitution under mild conditions.12 To the best of
high extinction coefficients, and excellent photostability. F-aza- our knowledge, only one Cl-aza-BODIPY has been reported,
BODIPYs are consequently attractive for use in an unisolated intermediate en route to an exotic aza-BODIPY
photovoltaics,1,2 biological imaging,3–7 and photodynamic construct featuring alkynylferrocene on boron.9 Herein we
therapy.3 Synthetic routes8 toward aza-dipyrrins produce two synthesise, isolate and characterise the Cl-aza-BODIPYs of
structural supertypes: a tetra-aryl variant, constructed via the both archetypal aza-dipyrrin subtypes, and study the utility of
condensation of an ammonia source and 4-nitro-1,3-diarylbu- the Cl-aza-BODIPY intermediate to facilitate substitutions at
tanones [or the condensation of an appropriate 2-nitrogen sub- boron via reaction with Grignard reagents. In the course of
stituted pyrrole (amino, nitroso) with a pyrrole unsubstituted this work, we also prepare the first series of HX salts of aza-
at the 2-position],8 and a fused-ring variant typically made dipyrrins.
using aryl Grignard reagents and aromatic dinitriles along
with an ammonia source.8,9 Given the constraints of these syn-
thetic approaches, the range of substituents that can be intro-
aDepartment of Chemistry, Dalhousie University, Halifax, Nova Scotia, B3H 4J3,
Canada. E-mail: alison.thompson@dal.ca
bDepartment of Chemistry, Saint Mary’s University, Halifax, NS, B3H 3C3, Canada
†Electronic supplementary information (ESI) available: Characterisation data
(NMR
spectra,
crystallographic
data,
photophysical
data).
CCDC
1955376–1955380. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/d0ob00272k
Fig. 1 Scaffold of F-aza-BODIPY (left) and F-BODIPY (right).
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Results & discussion
Tetraphenyl Cl-aza-BODIPY
Tetraphenyl F-aza-BODIPY 2 5 features the tetra-aryl substi-
tution pattern representative of this class of aza-dipyrrin
ligand (Scheme 1). A solution of 2 in anhydrous dichloro-
methane was treated with 1 equiv. boron trichloride (as a 1.0
M solution in dichloromethane). Upon addition, the colour of
the solution changed immediately from the characteristic teal
of 2 to a deep ultramarine blue. The solution was stirred for
twenty minutes at room temperature, and the solvent then
removed in vacuo to provide Cl-aza-BODIPY 3 as an iridescent
blue residue in quantitative yield. As shown in Fig. 2 (bottom),
the 11B NMR spectrum of the starting F-aza-BODIPY 2 reveals a
triplet arising from 11B coupling to the two fluoro substituents.
The 11B NMR spectrum of the Cl-aza-BODIPY 3 features a
singlet shifted downfield cf. the fluorinated starting material
(Fig. 2, top). Interestingly, the 11B NMR spectrum after the
addition of 0.5 equiv. BCl3 to a solution of 2 in chloroform-d,
followed by removal of volatiles and remaining BCl3 and then
redissolution in chloroform-d, revealed unconverted starting
material along with a broad doublet (ca. 48 Hz coupling con-
stant; Fig. 2, middle) that is presumably due to formation of a Fig. 2 11B NMR spectra upon the addition of BCl3 to a solution of 2 in
chloroform-d; 0 equiv. BCl3 (bottom), 0.5 equiv. BCl3 (middle) and 1
equiv. BCl3 (top).
mixed halide intermediate, i.e. Cl–B–F complex (Scheme 1).
Evidence of this mixed halide suggests step-wise halogen
exchange and that, for this aza-dipyrrinato framework, the
initial halogen exchange is fast. Using 19F NMR spectroscopy
50% recovery of the starting F-BODIPY. Presumably, in the
to concurrently monitor the reaction of 2 with 0.5 equiv. BCl3
case of the dipyrrinato framework, the second halogen
reveals a new, low-intensity quartet 7 ppm upfield of the start-
exchange proceeds faster than the first. Clearly the aza-dipyrri-
ing material with the same 48 Hz coupling constant as
observed in the 11B NMR spectrum (Fig. S1 in the ESI†),
nato framework is susceptible towards halogen exchange at
boron, although the mechanistic pathway through which this
occurs differs from that involving BODIPYs.
further supporting the presence of the mixed halide Cl–B–F
intermediate. Similar observations have been reported in the
19F NMR spectra of F–B–CN aza-BODIPYs.13 Addition of the
Salts of tetraphenyl aza-dipyrrin
remaining 0.5 equiv. BCl3 to the reaction mixture resulted in
complete conversion to the Cl-aza-BODIPY 3, as indicated by Cl-aza-BODIPY 3 was found to be unstable. When exposed to
the presence of a singlet as the only 11B resonance. As air, the decomposition of a solution of 3 in chloroform-d was
expected, all 19F signals disappeared upon the addition of 1 monitored using 1H NMR spectroscopy (see Fig. S2 in the
equiv. BCl3. In contrast, our work with meso-carbon ESI†). Cl-aza-BODIPY 3 completely decomposed after three
F-BODIPYs12 showed that treatment with 0.5 equiv. BCl3 pro- days of exposure to air (with periodic agitation), with the onset
duced 50% yield of the corresponding Cl-BODIPY and enabled of decomposition occurring after one hour. Two notable new
Scheme 1 Preparation of Cl-aza-BODIPY 3 via BCl3-mediated halogen exchange of F-aza-BODIPY 2.
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1
signals appeared in the H NMR spectrum over time: a broad
singlet at 14.4 ppm, and a narrow doublet at 8.5 ppm. The
intensity of the characteristic singlet in the 11B spectrum fell
to zero as the decomposition progressed. The singlet at
1
14.4 ppm in the H NMR of the decomposition product inte-
grated to 2H and is reminiscent of the pyrrolic N–H signal,
1
which is typically suppressed in the H NMR spectrum of the
parent aza-dipyrrin 1 due to intramolecular hydrogen bonding.
However, protonation of the Lewis basic nitrogen atom of the
aza-dipyrrin would remove this intramolecular contact and
reveal the two N–H resonances for the corresponding cation.
Given this, and our observed lability of the BCl2 chelate, we
hypothesised the decomposition product to be the hydro-
chloride salt of 1. To date, only one aza-dipyrrin salt has been
reported. However, this salt was produced in only 32% yield.14
In contrast, the HX salts of meso-carbon dipyrrins are well
known: they are easily handled, crystalline derivatives that
impart enhanced stability and are vital to the storage and
manipulation of dipyrrins. Despite this utility, the analogous
aza-dipyrrin salts have not been studied.
Fig. 3 Low-field region of 1H NMR spectra showing the N–H reso-
nances of aza-dipyrrin 3 and three HX salts (A) 1, no observable reso-
nances; (B) 1HCl; (C) 1HBr; and (D) 1HBF4.
With evidence in hand for 1HCl being formed as the
decomposition product of 3 in air, we embarked upon the syn-
thesis of a series of 1HX salts via protonation of free-base aza-
dipyrrins using excess concentrated acid (Scheme 2). In each
case, protonation of 1 afforded an obvious and immediate
colour change from dark blue/purple to an intense dark green.
After treatment of 1 with hydrochloric acid, the proton spec-
trum of the resultant hydrochloride salt (1HCl) featured a
broad, highly deshielded 2H singlet, consistent with protona-
tion at nitrogen (Fig. 3). It is of note that the chemical shift of
this peak is unaffected by concentration, whereas the chemical
shifts of charged N–H resonances often vary substantially with
concentration and solvent. There is also a simplification of the
signals in the aromatic region upon protonation, as shown in
Fig. S3 of the ESI.† This is consistent with an increase in mole-
cular symmetry upon protonation.15 Furthermore, the 1H NMR
spectrum of 1HCl matched that of the decomposition product
of 3, confirming that the Cl-aza-BODIPY decomposes to the
dipyrrin hydrochloride salt upon exposure to air. Two further
salts were synthesised via treatment of 1 with hydrobromic
(1HBr) and hydrofluoroboric (1HBF4) acids. Crystal structures
of the three salts of 1 are included and discussed in the ESI.†
To further investigate the properties of the aza-dipyrrin
salts, protonation was monitored using UV-visible spec-
troscopy. Aza-dipyrrin 1 was titrated with HBr to yield 1HBr,
Fig. 4 Spectrophotometric titration of a 0.1 mM solution of 1 in tetra-
hydrofuran acidified using a 7.0 mM solution of HBr in tetrahydrofuran.
As the concentration of HBr increases, the peak at 630 nm increases in
intensity while the peak at 598 nm decreases in intensity. λmax of 1 =
598 nm and λmax of 1HBr = 630 nm. Note the two isosbestic points at
507 nm and 600 nm.
whereupon protonation was accompanied by a substantial
bathochromic shift of 28 nm (Fig. 4). For comparison, com-
plexation of an aza-dipyrrin with BF2 to give the aza-BODIPY
typically results in a ca. 50 nm red-shift in absorbance.
Titrations with other strong acids (including HCl, HBF4,
CF3COOH, and H2SO4) revealed that the bathochromic shift
observed upon protonation was independent of anion.
Treatment of 1 with benzoic or tartaric acids failed to proto-
nate the aza-dipyrrin, and treatment of any salt of 1 with water
regenerated the free-base aza-dipyrrin thereby implicating 1HX
salts as weak acids. Indeed, the titration curves presented in
Fig. 4 enabled an apparent pKa of 4.03 to be determined for
1HBr,16 thereby supporting the experimental observations
regarding protonation.
Benzannulated Cl-aza-BODIPY
With the tetraphenyl-substituted Cl-aza-BODIPY 3 synthesised
and characterised, we turned our attention to the ring-fused
subtype of aza-dipyrrins. Treatment of 4 with BF3 gave F-aza-
Scheme 2 Protonation of aza-dipyrrin 1 to yield 1HX salts.
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BODIPY 5. Treatment of a solution of 5 in anhydrous dichloro-
methane with 1 equiv. boron trichloride (Scheme 3), as for 2,
caused an almost imperceptible lightening of the intense
forest green colour of the starting material, but afforded only
75% conversion to the corresponding Cl-aza-BODIPY 6 accord-
ing to analysis using 11B NMR spectroscopy. The course of the
reaction was monitored via integration of the signals in the
corresponding 11B NMR spectrum (Fig. 5): 25% conversion to
6 was achieved upon the addition of 0.5 equiv. BCl3; addition
of 1 equiv. BCl3 resulted in 75% conversion; and complete con-
version was achieved upon treatment of 5 with 1.5 equiv. BCl3.
As for the conversion of 2 to 3, a new doublet was observed
after the addition of 0.5 equiv. BCl3 to 5. This doublet per-
sisted after the addition of 1 equiv. BCl3, accompanied by
similar transient signals in the corresponding 19F NMR
spectra. These results again suggest the formation of a Cl–B–F
complex as an intermediate. Studies of B,N-chelated
BODIPYs17 indicate that more strongly electron-withdrawing
substituents are needed to stabilise the boron center, and our
recent classification of the relative donor strengths of dipyrrins
and aza-dipyrrins18 shows that aza-dipyrrin 4 is a significantly
poorer donor than 1. This suggests that the boron chelate of 5
is more stabilised than that of 2, hence the need for excess
BCl3 in order to drive the halogen exchange to completion.
Compound 6 gave crystals of sufficient quality for X-ray ana-
lysis, the first reported crystal structure of a Cl-aza-BODIPY
(see ESI†).
Fig. 5 11B NMR spectra upon the addition of BCl3 to a solution of 5 in
CD2Cl2 (from bottom to top): 0 equiv., 0.5 equiv., 1 equiv., and 1.5 equiv.
Functionalisation at boron
With a reliable route to Cl-aza-BODIPYS in hand, we investi-
gated their susceptibility to nucleophilic substitution, using
aryl Grignard reagents as model nucleophiles. In a one-pot
procedure, 2 was converted to 3 via BCl3-mediated halogen
exchange and consequent removal of volatile materials. Once
re-dissolved, Cl-aza-BODIPY 3 was treated with nucleophile to
form B-functionalised products 7a–7c (Scheme 4). 7a is a
Scheme 4 Treatment of Cl-aza-BODIPY 3 with aryl Grignard reagents.
known compound, typically accessed via treatment of F-aza- with no purification required. The 11B chemical shift of this
BODIPYs with excess Grignard reagent (3 equiv.) in tetrahydro- species (−2.56 ppm) falls within the range observed for dithie-
furan at reflux temperature.9 However, this method produces a nylboryl BODIPYs,19 which substantially differs from any of
slew of by-products that are difficult to separate via chromato- the other species in the current study.
graphy. In contrast, treatment of Cl-aza-BODIPY 3 with stoi-
It is critical to note that addition of a nucleophile immedi-
chiometric Grignard reagent at room temperature generated ately after in situ formation of the Cl-aza-BODIPY 3 from 2
7a–c with the only byproduct being 1, a remarkable improve- failed to effect B-substitution, instead returning either 2 or the
ment. Some care must be taken when handling 7a, as it is not parent free-base (i.e. deborylated) dipyrrin 1. Removal of the
stable to silica or acidic conditions <pH 2.10 Especially note- solvent and re-dissolution in fresh solvent before adding the
worthy is the high-yielding synthesis of 7c using this method, nucleophile is essential for substitution to proceed. To eluci-
Scheme 3 Preparation of Cl-aza-BODIPY 6 via BCl3-mediated halogen exchange of F-aza-BODIPY 5.
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date this behaviour, 3 was prepared again using a solution of 2
The photophysical properties of the compounds syn-
in chloroform-d, and an aliquot of the reaction mixture was thesised in this work were examined. Table 1 contains spectro-
analysed. The solvent was subsequently removed, then the photometric characterisation data for all aza-dipyrrins and
residue re-dissolved in fresh chloroform-d and again analysed. aza-BODIPYs discussed herein. Complexation of the aza-dipyr-
Fig. 6 shows the presence of four signals in the 11B spectrum rins 1 and 4 to form F-aza-BODIPYs 2 and 5, respectively, leads
before removal of the solvent, one of which is the singlet to a substantial narrowing of the absorbance curves as
expected of 3: after the solvent is removed, only this singlet expected for a molecule with such high symmetry, and a red
remains in the spectrum. This observation supports the postu- shift in absorbance of ca. 60 nm. The addition of BCl3 to 2
late that reactive by-products remain dissolved until the causes a loss of the distinctive greenish hue, which is reflected
solvent is removed and given that the F-aza-BODIPY was regen- in the slight blue shift and curve broadening that accompanies
erated in some cases, one of these by-products is likely BF2Cl. its conversion to 3. Other than a corresponding broadening of
Thus, removal of the solvent and any such by-products is criti- the emission curve and a loss of fluorescence quantum yield
cal to quenching the halogen exchange reaction and achieving attributable to the heavy atom effect of chlorine, the fluo-
full conversion to the desired Cl-aza-BODIPY, a step not rescence profiles of 2 and 3 are very similar, with virtually
required in the synthesis of meso-carbon Cl-BODIPYs.
identical maxima. This translates to an increased Stokes shift
B-Functionalisation of the benzannulated system was also for 3. However, the addition of BCl3 to 5 produces only a very
achieved via this method (Scheme 5). Treatment of a solution slight colour change that amounts to a lightening of the shade
of 5 in CH2Cl2 with BCl3, followed by removal of solvent, redis- of green of the solution. This is again reflected in the spectra,
solution in CH2Cl2, and treatment with phenylmagnesium with 6 having absorption and emission maxima only slightly
bromide afforded the diphenylboryl aza-BODIPY 8 in moderate red-shifted and broadened cf. those of 5.
yield. As with the preparation of 7a, the only observed by-
Substituting phenyl on boron such as in compound 7a
product was the parent free-base aza-dipyrrin, thus enabling leads to a comparatively large hypsochromic shift of 35 nm
facile purification. Treatment of either 3 or 6 with vinyl compared to the analogous substitution of 2. This substi-
Grignard reagent led to a trace of the desired product along- tution also effectively nullifies the fluorescence of the com-
side bulk decomposition, while treatment with methyl or ethyl pound, as expected.10 This loss of quantum yield can be
Grignard reagents led only to decomposition of the aza-dipyrri- attributed to efficient non-radiative relaxation pathways
nato unit.
offered by the free rotation of the many phenyl groups on a
compound such as 7a, particularly of the β-phenyl moieties.20
Virtually identical results follow for compounds 7b and 7c
bearing other arenes (2-naphthyl and 2-thienyl, respectively)
on boron. Substituting phenyl on boron for the benzannulated
aza-BODIPY 5 yields 8. The incorporation of the phenyl
groups of 8, which is an emerald green colour in solution,
results in a hypsochromic shift of 39 nm compared to 5: the
absorbance curve of 8 is sharp with a noticeable shoulder cen-
tered around 630 nm.
Table 1 Photophysical properties of solutions of aza-dipyrrin deriva-
tives in toluene
λabs
(nm)
ε
Fwhm
(nm)
λem
(nm)
Stokes
shift (nm)
Compound
(log E)
Φa
Fig. 6 11B NMR spectra of the reaction mixture for formation of 3
before (bottom) and after (top) removal of the solvent.
1
2
599
654
649
656
715
719
619
617
622
676
4.39
4.96
4.74
4.43
4.79
4.74
4.62
4.61
4.75
4.89
74
46
55
80
46
52
63
65
61
38
N.f.
677
676
N.f.c
726
729
658
658
659
694
N.f.
23
N.f.
0.32
0.12
3
4
5
6
7a
7b
7c
8
27
N.f.c
11
N.f.c
d
b
b
b
b
b
10
39
41
37
18
a Relative to 3,3′-diethylthiadicarbocyanine iodide in dichloromethane
(Φ = 0.35).21 b So weakly emissive that the quantum yield could not be
accurately measured (Φ < 0.01). c N.f. = not fluorescent at the wave-
lengths available for evaluation using our instrumentation. d Not
determined.
Scheme 5 Synthesis of diphenylboryl benzannulated aza-BODIPY 8 via
Cl-aza-BODIPY 6.
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Cl-aza-BODIPYs with excess water was shown to quickly and
easily deborylate the complex. These studies expand the range
of synthetic modifications that can be performed on the aza-
dipyrrin/aza-BODIPY framework.
Scheme 6 Deborylation of F-aza-BODIPYs via Cl-aza-BODIPY
intermediate.
Experimental section
General methods and materials
All chemicals were used as received unless otherwise noted.
For convenience, reactions involving boron substitution were
performed in a glove box under a positive pressure of nitrogen
gas although Schlenk techniques would be equally applicable.
Prior to use in the glove box, glassware was dried overnight in
an oven at 180 °C and then subjected to dynamic vacuum in
the glove box antechamber overnight. Any items that could not
be oven-dried were simply subjected to dynamic vacuum in the
antechamber overnight. Compounds 2 3 and 5 9,24,25 were syn-
thesised according to literature methods. These were dried
overnight in a vacuum oven, then placed under dynamic
vacuum in the antechamber for a further 18 h before use. All
solids were well treated with static removal devices (electronic
ion generator, anti-static gun) to ensure the discharge of any
static from the material that would interfere with mass deter-
mination.15 Flash and thin-layer chromatography were per-
formed using ultrapure silica (230–400 mesh) or Brockmann
III standard grade neutral alumina (150 mesh). Air-sensitive
samples (in NMR tubes or spectroscopy cuvettes) handled
outside the glove box were sealed with a septum and/or Teflon
tape, and Parafilm if the sample was to spend a prolonged
amount of time outside the inert atmosphere. Mass spec-
trometry was performed using a TOF spectrometer in ESI+ or
APCI mode, as indicated. All NMR spectra were recorded using
a Bruker AVANCE 500 MHz spectrometer unless otherwise
noted. 1H chemical shifts are reported in ppm relative to tetra-
methylsilane using the solvent residual as an internal standard
(δ = 7.26 for chloroform, 5.32 for dichloromethane). 13C NMR
spectra were recorded using the proton-decoupled UDEFT
pulse sequence,26 and chemical shifts are reported in ppm
using the residual solvent as an internal standard (δ = 77.2 for
chloroform and 53.8 for dichloromethane). It has been
reported that carbon atoms directly bonded to boron in aza-
BODIPYs are either weakly or not visible in 13C spectra,27 likely
due to the quadrupolar relaxation of 11B: the spectra of com-
pounds 7a–c are each missing one signal. Dichloromethane-d2
Aza-BODIPY deprotection
Finally, deliberate deborylation of the aza-BODIPY unit was
attempted using a procedure based on our previous work22
involving Cl-BODIPY derivatives of meso-carbon dipyrrins.
There are two variants of the deborylation scheme. The first
uses boron trifluoride diethyl etherate as a Lewis acid to acti-
vate the F-BODIPY BF2 group, which becomes labile in the
presence of ≤3 equiv. of water. The second involves forming
the Cl-BODIPY then dissolving it in a solution of 10% water in
acetone to remove the BX2 moiety. The former method is
entirely ineffective on these F-aza-BODIPYs; no reaction
occurred and the starting materials 2 and 5 were returned
unchanged. Conversely, the latter method works well for these
compounds. Indeed, subjecting 2 to 1 equiv. BCl3, followed by
excess water, gave the free aza-dipyrrin 1 in quantitative yield
after ten minutes of stirring, with no purification required
(Scheme 6). This contrasts with the results for meso-carbon
BODIPYs, which return their hydrochloride salts under the
same conditions. This result is rationalised given the low pKa
of 1HCl and the comparatively large amount of water present.
The same procedure was attempted using F-aza-BODIPY 5.
However, the Cl-aza-BODIPY 6 is insufficiently soluble in the
acetone/water mixture: an alternative mixture using tetrahydro-
furan as the organic component improved the solubility. The
progression of the reaction was noted to be very dissimilar to
the tetraphenyl analogue. Indeed, after ten minutes of stirring,
the desired product was apparent in trace amounts. After
fifteen minutes, the solution began to rapidly change colour,
culminating after one hour with the light blue characteristic of
4. Although conversion was not quantitative, chromatographic
separation to isolate the aza-dipyrrin 4 was facile.
Conclusions
Cl-aza-BODIPYs of the two major classes of aza-dipyrrinato was chosen as a deuterated solvent for derivatives of 1 due to
ligand were prepared via treatment of F-aza-dipyrrins with the chloroform solvent residual obscuring some 1H signals.
BCl3. The F to Cl halogen exchange process was found to occur 11B chemical shifts are reported in ppm, externally referenced
via a step-wise pathway, distinct from that involving meso- to boron trifluoride diethyl etherate (δ = 0.00). 19F chemical
carbon BODIPYs. Use of the BCl2-chelated Cl-aza-BODIPY12,23 shifts are reported in ppm, externally referenced to CFCl3 (δ =
as an intermediate for B-substitutions of aza-BODIPYs was 0.00). UV/Vis spectra were recorded using a 10 mm screw-cap
explored, and was found to be an effective strategy for expedi- quartz cell at room temperature. Spectra of air-sensitive com-
ent replacement of fluorine for aryl moieties. Furthermore, the pounds 3 and 6 were acquired under nitrogen. Fluorescence
first series of aza-dipyrrin HX salts was synthesised and spectra are the composites of ten scans in all cases. Quantum
characterised, and the pKa of the protonated tetraphenyl-sub- yields were calculated relative to 3,3′-diethylthiadicarbocyanine
stituted aza-dipyrrin 1 determined. Finally, treatment of the iodide in dichloromethane (Φ = 0.35)21 using the relationship
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Iunk Astd n2
Aunk Istd nstd
2H, J = 7.3 Hz), 7.32 (s, 2H), 7.27 (t, 4H, J = 7.8 Hz, partially
overlapping solv. residual). 13C {1H} NMR (CD2Cl2, 126 MHz)
δ: 156.4, 148.4*, 136.2*, 132.8, 132.5, 130.6, 129.6, 129.5, 128.6,
116.0, one signal missing [*confirmed via 2D HMBC].
HRMS-ESI+-TOF (m/z): [M − Cl + H]+ calc’d for C32H24N3:
450.1970; found 450.1949.
N-(3,5-Diphenyl-2H-pyrrol-2-ylidene)-3,5-diphenyl-1H-pyrrol-
2-amine hydrobromide (1HBr). Following GP1, the title com-
pound was isolated as a blue-green solid (14 mg, 80% yield).
M.P. 263.9–267.5 °C. 1H NMR (CDCl3, 500 MHz) δ: 13.64 (s,
2H, NH), 8.48 (m, 4H), 7.66–7.54 (m, 10H), 7.43 (t, 2H, J = 7.2
Hz), 7.32 (s, 2H), 7.27 (t, 4H, J = 7.6 Hz, partially overlapping
solv. residual). 13C {1H} NMR (CD2Cl2, 126 MHz) δ: 156.7,
Φ ¼ Φstd
2 , where I represents the integral of the
emission spectrum of the standard (std) or compound under
test (unk), A represents the absorbance at the excitation wave-
length of the standard or test compound, and n represents the
refractive index of the solvent used for the standard or test
compound.28 X-ray crystallography data were collected using a
CCD-equipped diffractometer (30 mA, 50 kV) using monochro-
mated Mo Kα radiation (λ
Experimental parameters for each structure reported herein
are included in the ESI.†
=
0.71073 Å) at 125 K.29
4-Nitro-1,3-diphenyl-1-butanone
Following a literature procedure,3 yet limiting the amount of 148.9, 135.9, 133.1, 132.4, 130.7, 130.0, 129.7, 129.5, 128.6,
nitromethane used, chalcone (6.09 g, 29 mmol), nitromethane 128.1, 116.2. HRMS-ESI+-TOF (m/z): [M − Br + H]+ calc’d for
(7.8 mL, 140 mmol) and diethylamine (15 mL, 140 mmol) C32H24N3: 450.1970; found 450.1957.
were combined and stirred for 6 hours at 60 °C with the use of
N-(3,5-Diphenyl-2H-pyrrol-2-ylidene)-3,5-diphenyl-1H-pyrrol-
an oil bath. The resulting mixture was diluted with methanol 2-amine hydrofluoroborate (1HBF4). Following GP1, the title
(90 mL) and then acidified to pH ≈ 2 using 1 M HCl. The compound was isolated as a yellow-green solid (37.5 mg, 88%
ensuing white precipitate was collected via suction filtration and yield). M.P. 238.0–240.0 °C. 1H NMR (CD2Cl2, 300 MHz) δ:
washed with ice-cold hexanes followed by ice-cold methanol to 11.25 (s, 2H, NH), 8.19–8.07 (m, 4H), 7.69 (m, 10H), 7.51 (t,
yield the title compound as a white fluffy solid (7.4 g, 93% yield). 2H, J = 7.4 Hz), 7.42 (s, 2H), 7.36 (s, 4H). 11B NMR (CD2Cl2,
Rf = 0.46 (EtOAc : Hex 1 : 4). M.P. 94–96 °C. 1H NMR (CDCl3, 96 MHz) δ: −0.82 (s). 19F NMR (CD2Cl2, 282 MHz) δ: −147.28
500 MHz) δ: 7.90 (m, 2H), 7.58 (t, 1H, J = 7.5 Hz), 7.46 (t, 2H, J = (s). HRMS-ESI+-TOF (m/z): [M − BF4 + H]+ calc’d for C32H24N3:
7.7 Hz), 7.32 (m, 2H), 7.28 (m, 3H), 4.86–4.66 (m, 2H), 4.23 (qu, 450.1970; found 450.1962.
1H, J = 7.2 Hz), 3.46 (m, 2H) in accordance with reported data.3
N,N-Dichloroboryl-[N-(3-phenyl-2H-isoindol-1-yl)-N-(3-phenyl-
1H-isoindol-1-ylidene)amine] (6). In the glove box, compound
5 9 (10 mg, 0.022 mmol) was dissolved in anhydrous dichloro-
Synthesis
4,4-Dichloro-1,3,5,7-tetraphenyl-4-bora-3a,4a,8-triaza-s-inda- methane (2 mL). Boron trichloride as a 1.0 M solution in di-
cene (3). In the glove box, F-aza-BODIPY 2 3 (10 mg, chloromethane (33 μL, 0.033 mmol) was added, resulting in a
0.020 mmol) was dissolved in anhydrous dichloromethane lightening of the green hue of the solution. The mixture was
(3 mL). Boron trichloride as a 1.0 M solution in dichloro- stirred for twenty minutes at room temperature, after which
methane (22 μL, 0.022 mmol) was added, causing the teal solu- the solvent was removed in vacuo to give the title compound as
tion to turn deep ultramarine blue. The mixture was stirred for a dark green residue in quantitative yield. 1H NMR (CD2Cl2,
twenty minutes at room temperature, after which the solvent 500 MHz) δ: 8.11 (d, 2H, J = 8.0 Hz), 7.84 (m, 4H), 7.57 (m, 2H),
was removed in vacuo to give the title compound as a shiny 7.53–7.49 (m, 6H), 7.43 (m, 2H), 7.33 (m, 2H). 13C {1H} NMR
golden-blue residue in quantitative yield. 1H NMR (CDCl3, (CD2Cl2, 126 MHz) δ: 154.7*, 137.1*, 132.8*, 131.7, 131.3,
500 MHz) δ: 8.09–8.00 (m, 8H), 7.51–7.42 (m, 12H), 6.93 (s, 131.0, 130.8*, 130.4, 128.1, 127.8, 124.7, 121.2 [*signals eluci-
2H). 13C {1H} NMR (CDCl3, 126 MHz) δ: 161.2, 144.6, 143.4, dated via 2D HMBC]. 11B NMR (CD2Cl2, 160 MHz) δ: 3.17 (s).
132.9, 132.1, 130.50, 130.46, 130.1, 129.7, 128.9, 128.0, 121.4. Mass spectra of this compound show only the free-base aza-
11B NMR (CDCl3, 160 MHz) δ: 2.51 (s). Mass spectra of this dipyrrin 4, indicating that the compound is deborylated
compound show only the free-base aza-dipyrrin 1, indicating during ionisation.
deborylation during ionisation.
4,4-Diphenyl-1,3,5,7-tetraphenyl-4-bora-3a,4a,8-triaza-s-inda-
General procedure 1 (GP1) for the synthesis of HX salts of 1. To cene (7a). In the glove box, compound 2 3 (50 mg, 0.10 mmol)
a stirring solution of 1 (20 mg) in tetrahydrofuran (5 mL), was dissolved in anhydrous dichloromethane (10 mL). Boron
excess concentrated HX (X = Cl, Br, or BF4) (0.1 mL) was trichloride as a 1.0 M solution in dichloromethane (110 μL,
added. Upon protonation, solutions of the free-base aza-dipyr- 0.11 mmol) was added, causing the teal solution to turn deep
rin changed from dark blue/purple to an intense dark green blue and indicating formation of 3. The mixture was stirred for
colour. Solvent and water were removed in vacuo and the twenty minutes at room temperature, after which the solvent
resulting solid was dried in a vacuum oven overnight to yield was removed in vacuo and in an anhydrous fashion. The
salts without need for further purification.
residue was re-dissolved in fresh anhydrous dichloromethane
N-(3,5-Diphenyl-2H-pyrrol-2-ylidene)-3,5-diphenyl-1H-pyrrol- (10 mL) and phenylmagnesium bromide as a 3.0 M solution in
2-amine hydrochloride (1HCl). Following GP1, the title com- diethyl ether (67 μL, 0.20 mmol) was added. No substantial
pound was isolated as a blue-green solid (20 mg, 92% yield). colour change occurred. The mixture was stirred for 3 hours at
M.P. 284.0–287.6 °C. 1H NMR (CDCl3, 500 MHz) δ: 14.40 (s, room temperature, after which the reaction vessel was capped
2H, NH), 8.50 (d, 4H, J = 6.6 Hz), 7.68–7.53 (m, 10H), 7.43 (t, and transferred out of the glove box. The reaction mixture was
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Organic & Biomolecular Chemistry
extracted into dichloromethane (2 × 70 mL) with water, then added drop-wise. This addition caused a very slight colour
washed with saturated aqueous sodium chloride (70 mL). The change toward blue-green, which persisted throughout. The
organic layer was dried over anhydrous magnesium sulfate mixture was stirred for 3 hours at room temperature. The reac-
and concentrated in vacuo. The crude blue solid was purified tion vessel was capped and transferred out of the glove box,
using flash chromatography on neutral alumina, eluting with and the mixture was extracted into dichloromethane (2 ×
1/99 EtOAc/hexanes to yield the title compound as a copper- 70 mL) with water, then washed with saturated aqueous
1
coloured solid (20 mg, 53% yield). H NMR (CDCl3, 500 MHz) sodium chloride (70 mL). The organic layer was dried over
δ: 8.15–8.10 (m, 4H), 7.47 (t, 4H, J = 7.7 Hz), 7.41 (m, 2H), 7.11 anhydrous magnesium sulfate and concentrated in vacuo to
(t, 2H, J = 7.3 Hz), 7.03–6.93 (m, 10H), 6.90 (t, 4H, J = 7.5 Hz), yield the title compound as bronze-coloured flakes without
6.78 (s, 2H), 6.70–6.64 (m, 4H) in accordance with reported need for further purification (37 mg, 97% yield). 1H NMR
values.10 We report further spectral data as follows: 11B NMR (CDCl3, 500 MHz) δ: 8.14 (d, 4H, J = 7.5 Hz), 7.48 (t, 4H, J =
(CDCl3, 160 MHz) δ: 1.33 (s). HRMS-ESI+ (m/z): [m + H]+ calc’d 7.6 Hz), 7.42 (t, 2H, J = 7.5 Hz), 7.15 (m, 4H), 6.99 (t, 4H, J =
for C44H33BN3: 614.2768; found 614.2762 in agreement with 7.6 Hz), 6.78 (m, 6H), 6.75–6.71 (m, 2H), 6.63–6.60 (m, 2H). 11
the literature.10 NMR (CDCl3, 160 MHz) δ: −2.59 (br s). 13C {1H} NMR (CDCl3,
B
4,4-Di-(2-naphthyl)-1,3,5,7-tetraphenyl-4-bora-3a,4a,8-triaza- 126 MHz) δ: 161.3, 143.9, 142.8, 132.98, 132.96, 131.5, 129.5,
s-indacene (7b). In the glove box, compound 2 3 (30 mg, 129.4, 129.2, 128.81, 128.75, 127.1, 127.0, 126.8, 120.9.
0.060 mmol) was dissolved in anhydrous dichloromethane HRMS-ESI+ (m/z): [m + Na]+ calc’d for C40H28BN3S2: 648.1716;
(10 mL). Boron trichloride as a 1.0 M solution in dichloro- found 648.1710.
methane (67 μL, 0.066 mmol) was added, causing the teal solu-
N,N-Diphenylboryl-[N-(3-phenyl-2H-isoindol-1-yl)-N-(3-phenyl-
tion to turn deep blue and indicating formation of 3. The 1H-isoindol-1-ylidene)amine] (8). In a glove box, compound 5 9
mixture was stirred for twenty minutes at room temperature, (30 mg, 0.067 mmol) was dissolved in anhydrous dichloro-
after which the solvent was removed in vacuo and in an anhy- methane (10 mL). Boron trichloride as a 1.0 M solution in di-
drous fashion. The residue was re-dissolved in fresh anhydrous chloromethane (101 μL, 0.101 mmol) was added, resulting in a
dichloromethane (10 mL) and 2-naphthylmagnesium bromide lightening of the green hue of the mixture and indicating the
as a 0.25 M solution in 2-methyltetrahydrofuran (480 μL, formation of 6. The mixture was stirred for twenty minutes at
0.120 mmol) was added drop-wise. No substantial colour room temperature, after which the solvent was removed in
change occurred. The mixture was stirred for 3 hours at room vacuo and in an anhydrous fashion. The residue was re-dis-
temperature. The reaction vessel was capped and transferred solved in fresh anhydrous dichloromethane (10 mL) and phe-
out of the glove box, and the mixture was extracted into di- nylmagnesium bromide as a 3.0 M solution in diethyl ether
chloromethane (2 × 70 mL) with water, then washed with satu- (45 μL, 0.134 mmol) was added. This addition briefly caused
rated aqueous sodium chloride (70 mL). The organic layer was the solution to turn black, yet the dark green colour returned
dried over anhydrous magnesium sulfate and concentrated over time. The mixture was stirred for three hours at room
in vacuo. The crude blue solid was purified using flash chrom- temperature. The reaction vessel was capped and transferred
atography on neutral alumina, eluting with 1/99 EtOAc/ out of the glove box, where it was extracted into ethyl acetate
hexanes to yield the title compound as a copper-coloured solid (60 mL) with water, then washed with saturated aqueous
1
(16 mg, 37% yield). H NMR (CDCl3, 500 MHz) δ: 8.15 (d, 4H, sodium chloride (60 mL). The organic layer was dried over
J = 7.2 Hz), 7.71 (d, 2H, J = 7.9 Hz), 7.53 (d, 2H, J = 8.3 Hz), anhydrous magnesium sulfate and concentrated in vacuo. The
7.49 (t, 4H, J = 7.7 Hz), 7.42 (t, 4H, J = 7.6 Hz), 7.34 (m, 7H), crude green solid was purified using flash chromatography on
7.28 (s, 1H, partially overlapping solv. residual), 6.95 (t, 2H, J = neutral alumina, eluting with 1/99 EtOAc/hexanes and dried
7.4 Hz), 6.79 (s, 2H), 6.67 (t, 4H, J = 7.7 Hz), 6.58 (d, 4H, J = in vacuo to give the title compound as a copper-coloured solid
1
7.2 Hz). 11B NMR (CDCl3, 160 MHz) δ: 1.60 (s). 13C {1H} NMR (17 mg, 45% yield). H NMR (CD2Cl2, 500 MHz) δ: 8.20 (d, 2H
(CDCl3, 126 MHz) δ: 161.0, 144.2, 142.4, 133.7, 133.2, 133.1, J = 8.1 Hz), 7.52 (m, 2H), 7.21 (m, 6H), 6.99 (m, 6H), 6.88 (m,
133.0, 132.5, 132.0, 129.51, 129.48, 129.1, 128.9, 128.8, 128.4, 8H), 6.53–6.50 (d, 4H). 11B NMR (CD2Cl2, 160 MHz) δ: 1.66 (s).
127.4, 127.0, 126.3, 125.0, 124.8, 120.9. HRMS-APCI (m/z): 13C {1H} NMR (CD2Cl2, 126 MHz) δ: 154.1, 137.8, 134.0, 133.08,
[m + H]+ calc’d for C52H37BN3: 714.3075; found 714.3075.
133.07, 131.4, 130.5, 129.8, 129.0, 127.6, 127.3, 126.5, 126.3,
4,4-Di-(2-thienyl)-1,3,5,7-tetraphenyl-4-bora-3a,4a,8-triaza- 123.2, 120.5. HRMS-ESI+ (m/z): [m + H]+ calc’d for C40H29BN3:
s-indacene (7c). In the glove box, compound 2 3 (30 mg, 562.2455; found 562.2449.
0.060 mmol) was dissolved in anhydrous dichloromethane
Deprotection of aza-BODIPYs.22 In the glove box, boron tri-
(10 mL). Boron trichloride as a 1.0 M solution in dichloro- chloride as a 1.0 M solution in dichloromethane (67 μL,
methane (67 μL, 0.066 mmol) was added, causing the teal solu- 0.066 mmol) was added to a solution of 2 3 (30 mg) in anhy-
tion to turn deep blue and indicating formation of 3. The drous dichloromethane (10 mL) and the mixture stirred for
mixture was stirred for twenty minutes at room temperature, twenty minutes. The solvent was removed in vacuo to yield the
after which the solvent was removed in vacuo and in an anhy- corresponding Cl-aza-BODIPY 3 as a residue. The reaction
drous fashion. The residue was re-dissolved in fresh anhydrous vessel was capped and transferred out of the glove box, and
dichloromethane (10 mL) and 2-thienylmagnesium bromide the residue was re-dissolved in a 5.55 M [10% v/v] solution of
as a 1 M solution in tetrahydrofuran (120 μL, 0.120 mmol) was water in acetone (10 mL) and stirred at room temperature for
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Paper
ten minutes. The mixture was extracted into dichloromethane
with water, and the organic layer dried over anhydrous sodium
sulfate. Removal of solvent in vacuo quantitatively yielded the
corresponding aza-dipyrrin 1 as a dark purple-black solid, with
no further purification required.
7 D. Wu, S. Cheung, R. Daly, H. Burke, E. M. Scanlan and
D. F. O’Shea, Eur. J. Org. Chem., 2014, 6841–6845.
8 Y. Ge and D. F. O’Shea, Chem. Soc. Rev., 2016, 45, 3846–
3864.
9 E. Maligaspe, T. J. Pundsack, L. M. Albert, Y. V. Zatsikha,
P. V. Solntsev, D. A. Blank and V. N. Nemykin, Inorg. Chem.,
2015, 54, 4167–4174.
In the glove box, boron trichloride as a 1.0 M solution in di-
chloromethane (67 μL, 0.066 mmol) was added to a solution of
5 9 (18 mg) in anhydrous dichloromethane (10 mL) and the 10 X.-D. Jiang, Y. Fu, T. Zhang and W. Zhao, Tetrahedron Lett.,
mixture stirred for twenty minutes. The solvent was removed in
2012, 53, 5703–5706.
vacuo to yield the corresponding Cl-aza-BODIPY 6 as a residue. 11 A. N. Amin, M. E. El-Khouly, N. K. Subbaiyan,
The reaction vessel was capped and transferred out of the
glove box, and the residue was re-dissolved in a 5.55 M [10%
M. E. Zandler, S. Fukuzumi and F. D’Souza, Chem.
Commun., 2012, 48, 206–208.
v/v] solution of water in tetrahydrofuran (10 mL) and stirred at 12 T. Lundrigan and A. Thompson, J. Org. Chem., 2013, 78,
room temperature for one hour. The mixture was extracted 757–761.
into ethyl acetate with brine, and the organic layer dried over 13 T.-Y. Li, Z. Ma, Z. Qiao, C. Körner, K. Vandewal, O. Zeika
anhydrous magnesium sulfate. The mixture was concentrated and K. Leo, ChemPlusChem, 2017, 82, 190–194.
in vacuo and purified using a short neutral alumina column, 14 A. Loudet, R. Bandichhor, L. Wu and K. Burgess,
eluting with 25/75 dichloromethane/hexanes to yield the aza-
Tetrahedron, 2008, 64, 3642–3654.
dipyrrin 4 as a dark blue powder (8 mg, 58% yield).
15 R. M. Diaz-Rodriguez, K. N. Robertson and A. Thompson,
Chem. Commun., 2018, 54, 13139–13142.
16 L. E. Vidal Salgado and C. Vargas-Hernández, Am. J. Anal.
Chem., 2014, 5, 1290–1301.
17 C. Ray, L. Díaz-Casado, E. Avellanal-Zaballa, J. Bañuelos,
L. Cerdán, I. García-Moreno, F. Moreno, B. L. Maroto,
Í. López-Arbeloa and S. de la Moya, Chem. – Eur. J., 2017,
23, 9383–9390.
Conflicts of interest
The authors declare no competing financial interest.
18 R. M. Diaz-Rodriguez, K. N. Robertson and A. Thompson,
Dalton Trans., 2019, 48, 7546–7550.
Acknowledgements
Financial support for this work was provided by NSERC of 19 S. H. Choi, K. Pang, K. Kim and D. G. Churchill, Inorg.
Canada via a Discovery Grant and the CREATE Training Chem., 2007, 46, 10564–10577.
Program in BioActives (510963), Dalhousie University and the 20 L. Jiao, Y. Wu, Y. Ding, S. Wang, P. Zhang, C. Yu, Y. Wei,
Province of Nova Scotia (Nova Scotia Graduate Scholarship). X. Mu and E. Hao, Chem. – Asian J., 2014, 9, 805–810.
Dr Mike Lumsden and Mr Xiao Feng are thanked for sharing 21 D. N. Dempster, T. Morrow, R. Rankin and
their expertise in NMR spectroscopy and mass spectrometry,
respectively.
G. F. Thompson, J. Chem. Soc., Faraday Trans. 2, 1972, 68,
1479–1496.
22 T. Lundrigan, T. S. Cameron and A. Thompson, Chem.
Commun., 2014, 50, 7028–7031.
23 T. Lundrigan, S. M. Crawford, T. S. Cameron and
A. Thompson, Chem. Commun., 2012, 48, 1003–1005.
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