Reports: ND450673-ND4: Gas Phase Studies of Stable Carbenes and Triazoles

Jeehiun Katherine Lee, PhD, Rutgers, the State University of New Jersey

In this funding period, we have focused on examining triazoles.  1,2,3-Triazoles have come to the forefront in the chemical field due to 1) the increasing ease of synthesis (through copper "click" chemistry) and 2) their potential use as effective ligands for organometallic catalysts.1-6

Despite the increasing interest in triazoles, their fundamental properties have not been systematically studied.  In this funding period, we have focused on benzotriazoles.  We first examined the parent benzotriazole, which has two possible tautomers (proton on N1, which we call the "N1H tautomer" and the proton on N2, the "N2H tautomer"). Our interest is to characterize the acidic and basic properties of this species, which would allow us both to better understand the efficacy as a ligand, and also to help determine which tautomer prevails in the gas phase (by comparing experimental and theoretical data).

We use mass spectrometry to measure the intrinsic properties of the triazoles, using both a bracketing and a Cooks kinetic method.7-11  Our results for the measurement of the acidity of the parent benzotriazole by bracketing are shown below.

Table 1. Summary of bracketing results for the acidity of benzotriazole

Ref. Compound

DHacid

  Proton Transfer

(kcal mol-1)

Ref. acid

Conj. base

2,4-pentadione

343.8 ± 2.1

+

methyl cyano acetate

340.80 ± 0.60

+

α, α, α-trifluoromethyl-m-cresol

339.3 ± 2.1

+

2-chloropropionic acid

337.0 ± 2.1

+

+

malononitrile

335.8 ± 2.1

+

pyruvic acid

333.5 ± 2.9

+

per-fluoro-tert-butanol

331.6 ± 2.2

+

difluoroacetic acid

331.0 ± 2.2

+

1, 1, 1-trifluoro-2, 4-pentadione

328.3 ± 2.9

+

A “+” indicates the occurrence and a “–” indicates the absence of proton transfer.

The measurement of 337 ± 3 kcal mol-1 is consistent with measurements we also made using the Cooks kinetic method, and also consistent with an earlier experimental result.  This acidity could correspond to either tautomer, however, so we also measured proton affinity:

Table 2. Summary of bracketing results for the more basic site of benzotriazole

Ref. Compound

Proton affinity

Proton Transfer

(kcal mol-1)

Ref. base

Conj. acid

pyridine

222.0 ± 2.0

+

propylamine

219.4 ± 2.0

+

dimethylacetamide

217.0 ± 2.0

+

3-chloropyridine

215.9 ± 2.0

+

+

methylamine

214.9 ± 2.0

+

m-toluidine

214.1 ± 2.0

+

aniline

210.9 ± 2.0

+

2,4-pentadione

208.8 ± 2.0

+

A “+” indicates the occurrence and a “–” indicates the absence of proton transfer.

These results indicate a proton affinity of 216 kcal mol-1; our measurements using the Cooks kinetic method method are consistent. This proton affinity, when compared to calculations, indicate that the tautomer with the proton on the N1 predominates (N1H tautomer). To confirm this conclusion, we also examined 1-methyl benzotriazole and 2-methyl benzotriazole, which allowed us to benchmark the calculations and lend further support for the predominance of the N1H tautomer.

We have also examined N1-substituted benzotriazoles with varying electron-donating and electron-withdrawing substituents to characterize the relationship between proton affinity and binding of these ligands to gold catalysts.

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