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Attaching a bulky fluoromesityl (FMes) group to boron made particular boroles much more resistant to water and air in tests reported in a 2015 Chemical Science study. The largest improvement was for one pentaarylborole: in wet solvent, it took 10 hours to be fully consumed, compared with less than a minute for its mesityl analogue. That is a result for these compounds and test conditions—not proof that boroles in general are now stable in air.
Why boroles are vulnerable
Boroles are five-membered rings containing boron and four carbon atoms. Their BC4 ring has four π electrons and is antiaromatic, and the boron center and B–C bonds can be susceptible to attack by water and oxygen. In the study, the researchers tested whether a bulky group attached to boron could help shield these vulnerable parts of the molecule.
How much did the FMes group improve water resistance?
The researchers compared a pentaarylborole bearing a 2,4,6-tris(trifluoromethyl)phenyl group—called FMes and designated compound 1—with the mesityl analogue MesBC4Ph4. In wet CD2Cl2, the mesityl compound hydrolysed within one minute, while complete consumption of compound 1 took 10 hours. The authors described compound 1 as more than 600 times less reactive in this comparison. These are reaction times from a wet-solvent test, not a measure of shelf life.
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The proposed explanation is steric shielding: the bulky ortho-CF3 groups of FMes hinder access to the boron center and nearby B–C bonds. The result is associated with this molecular structure; it does not establish that adding FMes will stabilize every borole.
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What happened when the crystals were exposed to air?
After 24 hours in air, the study reported 83% hydrolysis of crystalline MesBC4Ph4 and 13% hydrolysis of crystalline compound 1. The measurements favor compound 1, but the authors cautioned that differences in crystal size distribution or morphology could affect solid-state comparisons. This test is distinct from the wet-solvent experiment, so its percentages should not be treated as interchangeable with the more-than-600-fold reactivity comparison.
Why did a related borole behave differently?
Compound 2 was a related triarylborole, but it differed from compound 1 at the ring’s 3- and 4-positions. It hydrolysed in about 1.5 hours in wet CD2Cl2 and showed 33% hydrolysis after 24 hours in air. It still outperformed the mesityl analogue in the reported tests, but was less protected than compound 1. The authors associated its lower water stability with those structural differences, underscoring that stability depends on more than the group attached to boron.
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Did greater stability eliminate borole reactivity?
No. Compounds 1 and 2 reversibly bound pyridine, and compound 2 isomerized under strongly basic sodium hydroxide (NaOH) conditions. The stability result therefore means improved resistance in the reported water and air tests, not chemical inertness.
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The stability improvement did not come with a loss of the strong electron-accepting behavior reported for these compounds. The paper gave first reduction potentials of −1.52 V versus Fc/Fc+ for compound 1 and −1.69 V versus Fc/Fc+ for compound 2; it also described compound 1 as strongly electron-accepting. Its first reduction was less negative than the mesityl comparator’s reported −1.69 V.
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In CH2Cl2, compound 1 had an absorption maximum at 549 nm, compared with 578 nm for MesBC4Ph4. The authors noted that this blue shift was contrary to the effect they expected from an electron-poor substituent.
For compound 1, the study reported a 5%-weight-loss temperature of 271 °C and a melting point of 199 °C. Compound 2 had a reported 5%-weight-loss temperature of 262 °C. These are characterization results for the studied samples; they do not demonstrate performance in a finished electronic device. The authors discussed optoelectronic potential, but said incorporation into devices remained ongoing work.
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How does FMes substitution compare with other stabilization approaches?
A 2021 review discusses annulation as another way to enhance borole stability. Fused, annulated boroles are electronically different from free boroles, however, so that approach is not a direct, like-for-like comparison with attaching FMes to boron. A useful comparison should identify the molecular structure and distinguish wet-solvent hydrolysis, solid samples exposed to air, and aerated-solution tests; those conditions do not measure the same thing.
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Study source
The measurements and compound comparisons above come from Zhang and coauthors’ 2015 Chemical Science paper, “Taming the beast: fluoromesityl groups induce a dramatic stability enhancement in boroles”, first published 13 July 2015. Broader context on annulated structures comes from the 2021 review “(Hetero)arene-fused boroles: a broad spectrum of applications”.
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