Amino Acid Complexes

molecules Article Synthesis, Characterization, and Non-Covalent Interactions of Palladium(II)-Amino Acid Complexes David B. Hobart, Jr. 1,2 , Michael A. G. Berg 1 , Hannah M. Rogers 1 and Joseph S. Merola 1,* 1 Department of Chemistry, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061, USA; [email protected] (D.B.H.J.); [email protected] (M.A.G.B.); [email protected] (H.M.R.) 2 Air Liquide Advanced Materials, 197 Meister Ave., Branchburg, NJ 08876, USA * Correspondence: [email protected] Abstract: The reaction of palladium(II) acetate with acyclic amino acids in acetone/water yields square planar bis-chelated palladium amino acid complexes that exhibit interesting non-covalent interactions. In all cases, complexes were examined by multiple spectroscopic techniques, especially HRMS (high resolution mass spectrometry), IR (infrared spectroscopy), and 1H NMR (nuclear magnetic resonance) spectroscopy. In some cases, suitable crystals for single crystal X-ray diffraction were able to be grown and the molecular structure was obtained. The molecular geometries of the products are discussed. Except for the alanine complex, all complexes incorporate water molecules into the extended lattice and exhibit N-H···O and/or O···(HOH)···O hydrogen bonding interactions. The non-covalent interactions are discussed in terms of the extended lattice structures exhibited by the structures. Citation: Hobart, D.B., Jr.; Berg, Keywords: palladium; chelate; amino acid; hydrogen bonding; non-covalent interaction; X-ray M.A.G.; Rogers, H.M.; Merola, J.S. crystallography Synthesis, Characterization, and Non-Covalent Interactions of Palladium(II)-Amino Acid Complexes. Molecules 2021, 26, 4331. 1. Introduction https://doi.org/10.3390/ Prior work from our group has dealt with the simplest α amino acids, the glycines [1], molecules26144331 and the cyclic amino acids, the prolines [2], respectively. In this paper, we will explore the synthesis, characterization, and non-covalent interactions of the palladium(II) complexes of Academic Editors: Attila Bende and the remaining naturally occurring amino acids. Containing aliphatic R-groups, these amino Claudiu Filip acids are perhaps less unique, but no less important than their previously discussed coun- terparts. In this discussion of these complexes, they are grouped according to the chemical Received: 4 June 2021 nature of the R-group of the amino acid ligand into the following five categories: aliphatic Accepted: 13 July 2021 Published: 17 July 2021 hydrophobic, aromatic hydrophobic, polar neutral, charged acidic, and charged basic. Most of the palladium(II) complexes of the remaining naturally occurring alpha Publisher’s Note: MDPI stays neutral amino acids have been described to some extent in the literature dating back to the 1960s with regard to jurisdictional claims in and 1970s. Many of these syntheses were only studied by a single technique and their published maps and institutional affil- characterization data are incomplete. A goal of our research from the outset has been to iations. completely characterize these complexes to a degree not previously seen or reported in the literature. To this end, we have obtained 1H NMR data, high-resolution time-of-flight mass spectrometric data, elemental analyses, and 13C NMR and X-ray single crystal structures where possible. Of particular interest to us is their ability to form fascinating extended lattice structures primarily due to intermolecular hydrogen bonding, sometimes with the Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. inclusion of water in the H-bonding motif. We recently reported on some incredible motifs This article is an open access article in palladium complexes of beta-amino acids [3]. distributed under the terms and Among the aliphatic hydrophobic amino acids, bis-(alanino)palladium(II) was first conditions of the Creative Commons reported by Sharma [4] in 1964 and characterized solely by infrared spectroscopy. Fa- Attribution (CC BY) license (https:// rooq prepared the complex in 1973 [5], characterizing the product only by potentiometric creativecommons.org/licenses/by/ titration. In 1977 Chernova [6] also reported a synthesis of bis-(alanino)palladium(II) and 4.0/). characterized the product by elemental analysis and molar conductance measurements. Molecules 2021, 26, 4331. https://doi.org/10.3390/molecules26144331 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 4331 2 of 29 prepared the complex in 1973 [5], characterizing the product only by potentiometric titration. In 1977 Chernova [6] also reported a synthesis of bis-(alanino)palladium(II) and characterized the product by elemental analysis and molar conductance measurements. Plet- nev et al. reported the single crystal X-ray structure of bis-(valinato)palladium(II) showing the cis geometry [7]. Bis-(isoleucinato)palladium(II) was prepared by Patel [8] using DL- leucine in 1996 for studying pulse radiolysis and characterized only by elemental microa- nalysis. Hooper [9] synthesized versions of the bis-isoleucinato palladium complexes using DL-isoleucine and characterized them by infrared spectroscopy. The bis-(leucinato) Molecules 2021, 26, 4331 2 of 31 and bis-(valinato) complexes were prepared by Farooq [5]. Once again, potentiometric titration was the only method of characterization reported. Jarzab [10] reported a room- Pletnevtemperature et al. reported crystal the structure single crystal of X-raythe bis-(valin structure ofato) bis-(valinato)palladium(II) complex in 1973, however, show- no hydrogen ingatom the positionscis geometry were [7]. Bis-(isoleucinato)palladium(II) included in the refinement, was nor prepared was an byy Patelother [8 ]characterization using data DL-leucinepresented in in 1996 the for paper. studying pulse radiolysis and characterized only by elemental mi- croanalysis.The aromatic Hooper [9] synthesizedhydrophobic versions amino of the acid-containing bis-isoleucinato palladium palladium(II) complexes complexes previ- usingously DL-isoleucine reported in and the characterized literature include them by the infrared bis-(phenylalaninato) spectroscopy. The bis-(leucinato) and bis-(tyrosinato). The and bis-(valinato) complexes were prepared by Farooq [5]. Once again, potentiometric titrationcrystal wasstructure the only of method bis-(tyrosinato)palladium(II) of characterization reported. was Jarzab reported [10] reported by Jarzab a room- [11] with no other temperaturecharacterization crystal structuredata. As of before, the bis-(valinato) the structure complex was in obtained 1973, however, at room no hydrogen temperature and with atomno hydrogen positions were atom included positions in the reported. refinement, Cherno nor wasva any [6], other using characterization the same methods data mentioned presentedabove with in the the paper. alanine complex, reported the bis-(phenylalaninato) complex in 1976. You The aromatic hydrophobic amino acid-containing palladium(II) complexes previously reportedet al. also in the reported literature the include structure the bis-(phenylalaninato) of phenylalanine and pa bis-(tyrosinato).lladium(II) compounds The crystal [12]. structureAll of the bis-(tyrosinato)palladium(II) complexes formed from was polar reported neutra by Jarzabl amino [11] acid with noligands other char- were originally re- acterizationported in the data. 1970s, As before, however, the structure the trend was of obtained sparse at characterization room temperature data and withcontinues. The pal- noladium(II) hydrogen atombis-chelates positions of reported. asparagine, Chernova methionine, [6], using the and same serine methods were mentioned reported by Farooq [5], abovewho withagain the used alanine a potentiometric complex, reported titration the bis-(phenylalaninato) of ligand concentration complex in to 1976. characterize You the com- et al. also reported the structure of phenylalanine palladium(II) compounds [12]. plexes.All theVagg complexes [13] followed formed from up polar with neutral a room amino temperature acid ligands crystal were originally structure of the bis- reported(serinato) in thecomplex 1970s, however, in 1979. the Hydrogen trend of sparse atom characterization positions were data continues.not reported, The nor were any palladium(II)other data were bis-chelates given. of Kollmann asparagine, [14] methionine, reported and on serinethe synthesis were reported of gold(III), by Fa- palladium(II), rooqand [platinum(II)5], who again used complexes a potentiometric of threonine; titration only of ligand melting concentration point data to characterize were provided. Krylova the complexes. Vagg [13] followed up with a room temperature crystal structure of the bis- 1 13 (serinato)[15] added complex to the in 1979. data Hydrogen available atom for positions bis-(threoninato)palladium(II) were not reported, nor were any with other H and C NMR datadata were for given.both the Kollmann cis and [14 trans] reported isomers, on the as synthesis well as ofroom gold(III), temperature palladium(II), crystal and structures. Bis- platinum(II)(glutaminato)palladium(II) complexes of threonine; and only bis-(cysteinato)palladium(II) melting point data were provided. were Krylova both [15 reported] in 1979 1 13 addedby Graham to the data [16] available and Pneumatikakis for bis-(threoninato)palladium(II) [17], respectively. with GrahamH and Crelied NMR solely data on elemental for both the cis and trans isomers, as well as room temperature crystal structures. Bis- analysis for the characterization

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