Notes and references
y 1–OTf: 1–F (50 mg, 0.191 mmol) was dissolved in CDCl3 (0.5 mL)
and treated with TMSOTf (0.069 mL, 0.382 mmol). Formation of
1
1–OTf was quantitative by NMR. H NMR (399.57 MHz, CDCl3) d
2.42 (s, 6H, CH3-dipyrrin), 2.49 (s, 6H, CH3-dipyrrin), 2.60 (s, 3H,
CH3-dipyrrin), 6.11 (s, 2H, CH-dipyrrin). 19F NMR (375.97 MHz,
CDCl3) d ꢀ78.05 (s, OTf), ꢀ148.96 (bs, B–F). 11B NMR (128.20 MHz,
CDCl3) d 0.92 (bs). [1–DMAP]OTf: A mixture of 1–F (100 mg,
0.382 mmol), TMSOTf (0.138 mL, 0.763 mmol) and DMAP
(93.1 mg, 0.763 mmol) in toluene (5 mL) was stirred at 80 1C for
12 h. The solvent was then removed in vacuo and the residue dissolved
in CH2Cl2 and washed with distilled water. After drying over MgSO4,
reduction of the volume followed by addition of hexanes resulted in
the precipitation of an orange solid. This solid was washed with
hexanes and dried in vacuo to afford [1–DMAP]OTf (161 mg, 82%
yield). 1H NMR (499.91 MHz, CDCl3) d 2.12 (s, 6H, CH3-dipyrrin),
2.47 (s, 6H, CH3-dipyrrin), 2.73 (s, 3H, CH3-dipyrrin), 3.19 (s, 6H,
N(CH3)2), 6.07 (s, 2H, CH-dipyrrin), 6.86 (d, 3J = 7.5 Hz, 2H,
CH-DMAP), 7.89 (d, 3J = 7.5 Hz, 2H, CH-DMAP). 13C NMR
(125.7 MHz, CDCl3) d 14.63, 16.86, 17.54, 107.55, 122.98, 132.51,
Fig. 2 Left: spectral changes in the emission spectrum of a CHCl3
solution containing [1–DMAP]OTf (5.1 ꢂ 10ꢀ5 M) and TBAI (10 eq.)
upon addition of 1 eq. of TBAF. Right: pictures of the solution under
a hand held UV lamp before and after fluoride addition.
The observed coupling constant is comparable to that in 1–F.
This salt crystallizes in the monoclinic P21/n space group
(Fig. 1).z The structure of this derivative confirms the presence
of a DMAP ligand coordinated to the boron centre. The
B(1)–N(3) bond of 1.593(5) A, which can be compared to that
observed in DMAP–BF3 (1.589 A),12 is short thus indicating
the tight coordination of the DMAP ligand. The B(1)–F(1)
bond length of 1.385(5) A is somewhat longer than in 1–OTf
indicating a less electron deficient boron centre.
141.64, 143.98, 153.70, 156.60. 19F NMR (375.99 MHz, CDCl3)
1
d
ꢀ78.09 (OTf), ꢀ167.71 (q, JF–B
=
37 Hz). 11B NMR
1
(128.20 MHz, CDCl3) d 0.91 (d, JB–F = 37 Hz). [1–DMAP]OTf
is hygroscopic and water could not be completely removed for
elemental analysis. (Found: C, 49.87; H, 5.13. C22H29BF4N4SO4
([1–DMAP]OTfꢃH2O) requires C, 49.63; H, 5.49%).
z Crystal data for 1–OTf: C15H17BF4N2SO3, Mr = 392.18, mono-
clinic, P21/c, a = 10.715(4), b = 11.143(4), c = 14.078(5) A,
b = 91.037(7)1, V = 1680.7(10) A3, Z = 4, T = 110(2) K, m =
0.253 mmꢀ1, 10 057 reflections collected, 4022 unique (Rint = 0.0842),
R1 = 0.0517 [I 4 2s(I)], wR2 = 0.1710 (all data). CCDC 689181.
Crystal data for [1–DMAP]OTf: C22H27BF4N4SO3, Mr = 514.35,
monoclinic, P21/n, a = 11.166(2), b = 17.342(3), c = 12.365(2) A,
b = 101.252(4)1, V = 2348.4(8) A3, Z = 4, T = 110(2) K,
As indicated by 1H NMR spectroscopy, the salt [1–DMAP]-
OTf reacts with tetrabutylammonium fluoride (TBAF) in
CDCl3 to afford 1–F. No reaction is observed in the presence
of Clꢀ, Brꢀ or Iꢀ salts. Since the reaction with Fꢀ is fast and
quantitative, we questioned whether it could be used for the
development of a fluoride assay. With this in mind, we first
studied the fluorescence of [1–DMAP]+ in CHCl3 in the
presence of Iꢀ ions. Addition of 10 eq. of tetrabutyl-
ammonium iodide (TBAI) to a solution of [1–DMAP]OTf
(5.1 ꢂ 10ꢀ5 M) in CHCl3 results in a drastic quenching of the
fluorescence of [1–DMAP]+ (see ESIz). This quenching most
probably results from an external heavy atom effect13 whose
effectiveness is increased by the formation of [1–DMAP]+/Iꢀ
ion pairs (Scheme 2). Remarkably, when 1 eq. of TBAF is
added, the fluorescence intensity increases by a factor f =
500%, giving a response than can be easily detected with the
naked eye (Fig. 2). We propose that this increase results from
the formation of 1–F, which as a neutral compound, is not as
sensitive as [1–DMAP]+ to the external spin orbit coupling
effect imparted by Iꢀ. A much weaker response is observed
upon addition of 1 eq. of Clꢀ (f = 48%) or Brꢀ (f = 6%) ions
which compete with the iodide ions in pairing with the
[1–DMAP]+ cation.
m = 0.203 mmꢀ1, 10 736 reflections collected, 3684 unique (Rint
=
0.0317), R1 = 0.0636 [I 4 2s(I)], wR2 = 0.1319 (all data). CCDC
689182. For crystallographic data in CIF or other electronic format
see DOI: 10.1039/b808740g
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In conclusion, we describe a novel approach for the fluores-
cent turn-on sensing of Fꢀ ions. This approach is based on: (i)
the use of a BODIPY boronium cation ([1–DMAP]+) which is
converted into a neutral BODIPY dye (1–F) in the presence of
Fꢀ ions; (ii) the greater sensitivity of cationic [1–DMAP]+
(when compared to neutral (1–F) to the external heavy atom
effects imparted by Iꢀ.
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This work was supported by NSF (CHE-0646916), the
Welch Foundation (A-1423), the Petroleum Research Funds
(Grant 44832-AC4) and the US Army Medical Research
Institute of Chemical Defense. We thank Prof. Warren Piers
for useful discussions and access to unpublished work from his
group.
ꢁc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 4596–4597 | 4597