molecules Article Complexes of Formaldehyde and α-Dicarbonyls with Hydroxylamine: FTIR Matrix Isolation and Theoretical Study Barbara Golec 1,* , Magdalena Sałdyka 2 and Zofia Mielke 2 1 Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland 2 Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland; [email protected] (M.S.); zofi[email protected] (Z.M.) * Correspondence: [email protected]; Tel.: +48-22-343-3410 Abstract: The interactions of formaldehyde (FA), glyoxal (Gly) and methylglyoxal (MGly) with hydroxylamine (HA) isolated in solid argon and nitrogen were studied using FTIR spectroscopy and ab initio methods. The spectra analysis indicates the formation of two types of hydrogen-bonded complexes between carbonyl and hydroxylamine in the studied matrices. The cyclic planar complexes are stabilized by O–H··· O(C), and C–H··· N interactions and the nonplanar complexes are stabilized by O–H··· O(C) bond. Formaldehyde was found to form with hydroxylamine, the cyclic planar complex and methylglyoxal, the nonplanar one in both argon and nitrogen matrices. In turn, glyoxal forms with hydroxylamine the most stable nonplanar complex in solid argon, whereas in solid nitrogen, both types of the complex are formed. Keywords: carbonyls; hydroxylamine; hydrogen bond; matrix isolation; vibrational spectroscopy; computational chemistry Citation: Golec, B.; Sałdyka, M.; Mielke, Z. Complexes of Formaldehyde and α-Dicarbonyls with Hydroxylamine: FTIR Matrix 1. Introduction Isolation and Theoretical Study. Chemical and photochemical reactions often proceed through many intermediate Molecules 2021, 26, 1144. https:// stages between the substrates and products. The reactive intermediates formed at the doi.org/10.3390/molecules26041144 first stages are usually not observed at normal conditions but can be stabilized in an inert environment. The excellent technique for such a purpose is a low-temperature matrix Academic Editors: Rui Fausto, isolation method, which enables the isolation of unstable intermediates in the rare gas Sylvia Turrell and Gulce Ogruc Ildiz matrix [1]. Many reactions begin with the formation of the molecular complexes between the substrates, which also can be isolated in the matrices. Using the FTIR spectroscopy and Received: 26 January 2021 ab initio methods, we are able to distinguish the nature of interactions and the structures Accepted: 18 February 2021 Published: 20 February 2021 of the isolated complexes [2,3]. In this work, we used the FTIR matrix isolation spectroscopy and MP2 calculations to Publisher’s Note: MDPI stays neutral investigate the structures of the complexes formed between simple carbonyl (formaldehyde) α with regard to jurisdictional claims in and -dicarbonyl compounds glyoxal and methylglyoxal, with hydroxylamine in argon published maps and institutional affil- and nitrogen environment. Identification and characterization of these complexes is the first iations. step of our study on isolation of highly unstable intermediate, called hemiaminal, that is formed in oxime formation reaction [4,5] and will be the subject of our next paper. Oximes are widely used compounds in synthetic chemistry in biomedical fields, for example, in the coupling reactions of peptides, proteins, oligosaccharides, and oligonucleotides and in bioconjugation reaction [6–11]. On the other hand, to explain the acid-catalyzed Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. reaction of nucleophiles with the carbonyl group, the formation of weak hydrogen bonds This article is an open access article between the carbonyl group of aldehydes and ketones with proton donors has been distributed under the terms and postulated [12]. Therefore, the deep understanding of the hydrogen bond interaction of conditions of the Creative Commons carbonyl compounds with nucleophilic hydroxylamine is an important issue. Attribution (CC BY) license (https:// Some binary complexes between formaldehyde or α-dicarbonyls and proton donors creativecommons.org/licenses/by/ have been investigated. The most studied are the complexes of formaldehyde with wa- 4.0/). ter [13–31]. Additionally, there are some words about the interaction of formaldehyde with Molecules 2021, 26, 1144. https://doi.org/10.3390/molecules26041144 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 1144 2 of 20 Some binary complexes between formaldehyde or α‐dicarbonyls and proton donors Molecules 2021, 26, 1144 2 of 19 have been investigated. The most studied are the complexes of formaldehyde with water [13–31]. Additionally, there are some words about the interaction of formaldehyde with hydrogen halides and cyanide [32–36], formamide [37,38], nitroxyl [39], ammonia [33], methane [40],hydrogen and methanol halides [41]. and For cyanide α‐dicarbonyls, [32–36], formamide the interactions [37,38 ],of nitroxyl glyoxal, [ 39methyl], ammonia‐ [33], glyoxal andmethane diacetyl [ 40with], and water methanol [42], methanol [41]. For [43,44],α-dicarbonyls, hydrogen the peroxide interactions [45,46], of glyoxal, and methyl- hydroperoxylglyoxal radical and [47] diacetyl have withbeen waterstudied. [42 The], methanol results of [43 these,44], hydrogeninvestigations peroxide show [45 ,46], and that the waterhydroperoxyl complexes radical of formaldehyde [47] have been or α‐ studied.dicarbonyls The trapped results ofin theseargon investigations matrices show are stabilizedthat by the the water OH∙∙∙ complexesO(C) hydrogen of formaldehyde bond formed or betweenα-dicarbonyls water and trapped carbonyl in argon ox‐ matrices ygen. The mostare stabilized stable structure by the of OH the··· methanolO(C) hydrogen complex bond with formed formaldehyde between is wateralso pre and‐ carbonyl dicted to beoxygen. stabilized The by most a hydrogen stable structure bond. However, of the methanol for the methanol complex withcomplexes formaldehyde with is also α‐dicarbonyls,predicted the ab to initio be stabilized calculations by a predict hydrogen the bond. non‐hydrogen However,‐ forbonded the methanol structures complexes as with the most stableα-dicarbonyls, ones. Experimental the ab initio data calculations show that predict in the Ar the matrix, non-hydrogen-bonded glyoxal forms the structures as non‐hydrogenthe‐ mostbonded stable structure ones. with Experimental methanol, data but methylglyoxal show that in the and Ar diacetyl matrix, form glyoxal the forms the planar cyclicnon-hydrogen-bonded structures stabilized by structure O–H∙∙∙O(C) with and methanol, C–H∙∙∙ butO(C) methylglyoxal interactions. and diacetyl form the ··· ··· Hydroxylamineplanar cyclic dimers structures and their stabilized complexes by O–H with O(C)water and and C–H ammoniaO(C) isolated interactions. in argon and nitrogenHydroxylamine matrices have dimers been and studied their by complexes Yao and with Ford water [48–50]. and The ammonia dimer isolated in argon and nitrogen matrices have been studied by Yao and Ford [48–50]. The dimer (NH2OH)2 has a cyclic structure stabilized by two O–H∙∙∙N hydrogen bonds. In hydrox‐ (NH2OH)2 has a cyclic structure stabilized by two O–H··· N hydrogen bonds. In hydroxy- ylamine complexes with H2O and NH3, the OH group of NH2OH acts as a proton donor lamine complexes with H O and NH the OH group of NH OH acts as a proton donor to the oxygen or nitrogen atom of water2 or ammonia,3, respectively. These2 complexes are to the oxygen or nitrogen atom of water or ammonia, respectively. These complexes are relatively strong; the ν(OH) band of hydroxylamine shifts by 100 and 300 cm−1 to lower relatively strong; the ν(OH) band of hydroxylamine shifts by 100 and 300 cm−1 to lower wavenumbers in the spectra of the H2O and NH3 complexes, respectively. A much wavenumbers in the spectra of the H O and NH complexes, respectively. A much weaker weaker complex formed between hydroxylamine2 and carbon3 monoxide stabilized by complex formed between hydroxylamine and carbon monoxide stabilized by O–H··· C=O O–H∙∙∙C=O interaction has been observed in the Ar matrix. In this complex, the ν(OH) interaction has been observed in the Ar matrix. In this complex, the ν(OH) band shifts band shifts 39 cm−1 towards lower energies [51]. For the isocyanic acid‐hydroxylamine 39 cm−1 towards lower energies [51]. For the isocyanic acid-hydroxylamine complex, only complex, only the hydrogen‐bonded structure with the NH group of HNCO attached to the hydrogen-bonded structure with the NH group of HNCO attached to the oxygen atom the oxygen atom of the NH2OH molecule was identified in solid argon [52]. of the NH2OH molecule was identified in solid argon [52]. 2. Results and2. Results Discussion and Discussion 2.1. Formaldehyde–hydroxylamine2.1. Formaldehyde–hydroxylamine Complexes Complexes Ab Initio Calculations.Ab Initio Calculations. The MP2/6‐311++G(2d,2p) The MP2/6-311++G(2d,2p) calculations predict calculations the stability predict of the stability five differentof structures five different of a structures hydrogen of‐bonded a hydrogen-bonded complex between complex formaldehyde between formaldehydeand hy‐ and droxylaminehydroxylamine with stoichiometry with stoichiometry 1:1. Their structures 1:1. Their and structures binding energies and binding (ΔECP energies(ZPE) in ( DECP(ZPE) kJ mol−1) arein presented kJ mol−1) in are Figure presented 1 and in Figure Figure S11 andin Supplementary Figure S1 in Supplementary Material. Material. Figure 1. The optimized structures of the HCHO–NH2OH complexes, IFH–VFH.