57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

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

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

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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