Clarifying the complex chemistry of cobalt(II) thiocyanate‐based tests for cocaine using single‐crystal X‐ray diffraction and spectroscopic techniques

Abstract

Cobalt(II) thiocyanate‐based tests are routinely used to screen cocaine products, with the formation of a blue species interpreted as a positive response. An array of other organic bases has been identified as false positives – including well‐documented cocaine product adulterant lidocaine and its salt. False positives prompt continued test development, though improvements are hindered by unresolved product structures and reaction pathways. Toward greater clarity, cobalt(II) thiocyanate reactions with cocaine hydrochloride, along with lidocaine and its salt, were investigated using multiple analytical techniques. Reactions involving cocaine hydrochloride yielded glassy, amorphous blue material while reactions of lidocaine hydrochloride monohydrate produced larger, needle‐like crystals whose structure was determined via single‐crystal X‐ray diffraction to be an ion pair (Hlidocaine+)2([Co(SCN)4]2−)·H2O. While the blue precipitate isolated from reactions involving cocaine hydrochloride was unsuitable for crystallographic structure determination, comparative ultraviolet–visible, attenuated total reflectance infrared, and Raman spectroscopic analysis – along with elemental analysis – supports that this solid is comprised of a comparable ion pair (Hcocaine+)2[Co(SCN)4]2−. Pink crystals isolated from lidocaine reaction vessels were identified as coordination compounds cis‐[CoL2(SCN)2] and trans‐[CoL2(SCN)2] where L = lidocaine, while pink crystals from both cocaine hydrochloride and lidocaine hydrochloride monohydrate reaction vessels were the coordination polymer trans‐[Co(H2O)2(SCN)2]·H2O. The results presented herein enable reaction optimization to favor a desired product, whether ion pair or coordination species.

Polymorphism of Bis(benzimidazole)bis(thiocyanato-N)cobalt(II) and Its Relevance to Studies of the Chief Color Test for Cocaine

“…The materials and methods used for this work were first detailed in our recent publication [8] and are also provided here for ease of reference.…”

“…Our recent study probing the product species resulting from reactions between cobalt(II) thiocyanate-based reagents and cocaine hydrochloride, plus the well-documented cocaine product adulterant and color test false-positive lidocaine and its salt, supported that an ion pair is the hallmark blue species formed for these nitrogenous bases [8]. In this work, we expand on our exploration of cobalt(II) thiocyanate-based color tests by studying the blue species produced upon exposure of benzimidazole to a classic cobalt(II) thiocyanatebased reagent (2% w/v cobalt(II) thiocyanate in distilled water; hereafter, 'classic test').…”

“…It is well established that the stretching vibrational frequency and/or peak intensity of various nitriles are sensitive to their local hydrogen-bonding and electrostatic environment in addition to temperature [25,26] However, the low signal-to-noise ratio observed for γ-[Co(Hbzim)2(SCN)2] may be due to the sensitivity of the compound to the laser beam. As detailed in our recent work [8], classic test reactions involving lidocaine monohydrate hydrochloride produced blue needle-like crystals (Hlidocaine) 2 ([Co(SCN) 4 ])•H 2 O, while cocaine hydrochloride yielded a glassy, blue material whose amorphous nature made it unsuitable for crystallographic structure determination. However, elemental analysis along with comparative UV-vis, attenuated total reflectance infrared, and Raman spectroscopic analysis strongly supported that the blue material isolated from cocaine hydrochloride reaction vessels was an ion pair (Hcocaine infrared, and Raman spectroscopic analysis strongly supported that the blue material isolated from cocaine hydrochloride reaction vessels was an ion pair (Hcocaine)2[Co(SCN)4].…”

“…For non-bridging thiocyanate ions, we found 1226 Co-NCS hits, but only 15 hits for Co-SCN. Examples of Co-SCN compounds are potassium (ethylenediamine-N,N',N'-triacetato)-thiocyanato-cobalt(III) sesquihydrate [31] (CSD code DIRLOF) and, in bridging mode, thus bonding to both S and N, the yellow coordination polymer [Co(SCN) 2 (H 2 O) 2 ] [8,32].…”