molecules Article Structural Evidence for Pnictogen-Centered Lewis Acidity in Cationic Platinum-Stibine Complexes Featuring Pendent Amino or Ammonium Groups † Roberta R. Rodrigues and François P. Gabbaï * Department of Chemistry, Texas A&M University, College Station, TX 77843-3255, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-979-862-2070 † In memory of Alan H. Cowley, a most inspiring Ph.D. advisor, and a great friend. Abstract: As part of our continuing interest in the chemistry of cationic antimony Lewis acids as ligands for late transition metals, we have now investigated the synthesis of platinum complexes featuring a triarylstibine ligand substituted by an o-[(dimethylamino)methyl]phenyl group referred N N to as Ar . More specifically, we describe the synthesis of the amino stibine ligand Ph2SbAr (L) and its platinum dichloride complex [LPtCl]Cl which exists as a chloride salt and which shows weak coordination of the amino group to the antimony center. We also report the conversion of [LPtCl]Cl 3+ into a tricationic complex [LHPt(SMe2)] which has been isolated as a tris-triflate salt after reaction of [LPtCl]Cl with SMe2, HOTf and AgOTf. Finally, we show that [LHPt(SMe2)][OTf]3 acts as a catalyst for the cyclization of 2-allyl-2-(2-propynyl)malonate. Citation: Rodrigues, R.R.; Gabbaï, Keywords: pnictogen bonding; platinum; catalysis F.P. Structural Evidence for Pnictogen-Centered Lewis Acidity in Cationic Platinum-Stibine Complexes Featuring Pendent Amino or 1. Introduction Ammonium Groups . Molecules 2021, The chemistry of stibines is attracted a renewed interest because of applications in 26, 1985. https://doi.org/10.3390/ material science and catalysis [1–4]. Although a parallel is often drawn with the ubiquitous molecules26071985 phosphine ligands, stibines display several atypical traits and can, for example, undergo redox reactions without dissociation of the coordinated metal [5–10]. Moreover, while Academic Editor: Charles L. phosphines mostly act as two-electron donor spectator ligands, stibines have been shown to B. Macdonald display Lewis acidic properties, even when coordinated to a transition metal [11–22]. These Lewis acidic properties manifest in the formation of long contacts between the antimony Received: 3 March 2021 center of cationic complexes and counteranions as in complex [A]+ which was isolated by Accepted: 26 March 2021 Reid (Figure1)[ 11]. Short contacts, akin to polar covalent bonds, can also be formed in the Published: 1 April 2021 case of small halide anions, including the fluoride anion [23]. Such behavior comes to the fore in the case of complexes featuring bis(phosphino)stibine ligands such as [B]+ which is Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in readily converted into C in the presence of a fluoride anion (Figure1)[23]. published maps and institutional affil- A previous article described the platinum complex D, which features a chloride anion iations. coordinated to the antimony center trans from the platinum atom (Figure1)[ 13]. The re- sulting Sb-Cl bond formed by this complex is ~14% longer than the sum of the covalent radii indicating partial ionicity. This lengthening suggested that the simple addition of a donor functionality would suffice in promoting the dissociation of this anion, providing access to a cationic complex. Therefore, based on this simple hypothesis, we have decided Copyright: © 2021 by the authors. to replace the antimony-bound phenyl group of D by an o-[(dimethylamino)methyl]phenyl Licensee MDPI, Basel, Switzerland. This article is an open access article group. This substituent is known to form intramolecular N-Sb bonds when incorporated distributed under the terms and in antimony derivatives [24,25] as demonstrated over two decades ago by Cowley, Jones, conditions of the Creative Commons and Norman [26] as well as by Akiba [27]. In this paper, we report the results of our efforts Attribution (CC BY) license (https:// along with the conversion of the target complex in a triflate derivative that behaves as a creativecommons.org/licenses/by/ carbophilic catalyst. 4.0/). Molecules 2021, 26, 1985. https://doi.org/10.3390/molecules26071985 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x FOR PEER REVIEW 2 of 11 efforts along with the conversion of the target complex in a triflate derivative that behaves as a carbophilic catalyst. Molecules 2021, 26, x FOR PEER REVIEW 2 of 11 Molecules 2021, 26, 1985 2 of 11 efforts along with the conversion of the target complex in a triflate derivative that behaves as a carbophilic catalyst. Figure 1. Structure of representative stibine complexes in which the antimony atom displays Lewis acidic properties. 2. Results and Discussion FigureFigure 1. 1.Structure Structure of representativeof representative stibine stibine complexes complexes in which in thewhich antimony the antimony atom displays atom Lewisdisplays acidicLewis properties.As acidic a first properties. step, we decided to introduce the o-[(dimethylamino)methyl]phenyl group by reaction of the corresponding lithium salt [27] with the bis(phosphino)chlorostibine 1 2. Results and Discussion [28]2. Results as shown and Discussionin Scheme 1. The reaction was carried out in toluene upon heating to 80 °C. As a first step, we decided to introduce the o-[(dimethylamino)methyl]phenyl group byFollowing reactionAs a first of standard the step, corresponding we workup decided lithium andto introduce column salt [27 the]chromatography, with o-[(dimethylamino)methyl]phenyl the bis(phosphino)chlorostibine the new amino-functionalized group 1ligandby[28 reaction] as 2 shown was of obtainedthe in Schemecorresponding as1. an The air-stable reaction lithium was solidsalt carried[27] in 71%with out yieldthe in toluenebis(phosphino)chlorostibine The presence upon heating of the o-[(dimethyl- 1 ◦ toamino)methyl]phenyl[28] 80 asC. shown Following in Scheme standard group1. The workup reactioncould and be was column easily carried chromatography,confirmed out in toluene by 1H the upon NMR new heating amino-spectroscopy, to 80 °C. which functionalizedFollowing standard ligand 2workupwas obtained and column as an air-stablechromatography, solid in 71% the yieldnew amino-functionalized The presence displays resonances at 1.69 ppm and 3.37 ppm (in C6D6) corresponding to the benzylic of the o-[(dimethylamino)methyl]phenyl group could be easily confirmed by 1H NMR spec- methyleneligand 2 was and obtained dimethylamino as an air-stable groups, solid inrespecti 71% yieldvely. The Integration presence of of the these o-[(dimethyl- two resonances troscopy,amino)methyl]phenyl which displays group resonances could at 1.69be easily ppm andconfirmed 3.37 ppm by (in 1H C 6NMRD6) corresponding spectroscopy, which toproduced the benzylic the methylene expected and intensity dimethylamino ratio when groups, referenced respectively. to Integrationthe resonances of these arising from the displays resonances at 1.69 ppm and 3.37 ppm (in C6D6) corresponding to the benzylic twophenylene resonances protons produced in thethe expected aromatic intensity region ratio of the when spectrum. referenced The to the presence resonances of the phosphino methylene and dimethylamino groups, respectively. Integration of these two resonances arisinggroups from was the easily phenylene confirmed protons by in thethe aromatic detection region of a of single the spectrum. resonance The presence at −8.19 ppm in the 31P ofproduced the phosphino the expected groups wasintensity easily ratio confirmed when by referenced the detection to the of aresonances single resonance arising at from the NMR spectrum. −phenylene8.19 ppm in protons the 31P NMRin the spectrum. aromatic region of the spectrum. The presence of the phosphino groups was easily confirmed by the detection of a single resonance at −8.19 ppm in the 31P NMR spectrum. SchemeScheme 1. Synthesis1. Synthesis of ligand of ligand 2 and its 2 conversion and its conversion into the corresponding into the platinum(II)corresponding complex platinum(II) [3][Cl]. complex [Scheme3][Cl]. 1. Synthesis of ligand 2 and its conversion into the corresponding platinum(II) complex [3][Cl].With this ligand at our disposal, we decided to investigate its reaction with (Me2S)2PtCl2. This reaction, which was carried out in CH2Cl2 at room temperature, proceeded smoothly Molecules 2021, 26, 1985 3 of 11 to afford complex [3][Cl] as an air-stable solid in 89% yield. The 31P NMR spectrum of complex [3][Cl] features a resonance at 49.52 ppm that displays the expected 195Pt satellites. The 1JPt−P coupling constant of 2524 Hz is comparable to that measured for complex D (1JPt−P = 2706 Hz), suggesting the formation of a divalent square planar platinum complex. Other notable spectroscopic features include a downfield of the phenylene pro- ton positioned ortho from the antimony atom from 7.50 ppm in 2 to 8.37 ppm in [3][Cl]. Finally, the benzylic methylene and dimethylamino group give rise to sharp resonances at 1.71 ppm and 3.10 ppm, respectively. Because the chemical shift of these resonances is close to those in the free ligand 2, we resorted to X-ray analysis to elucidate the antimony atom’s coordination environment in [3][Cl] and the eventual presence of a dative N!Sb dative interaction. Single crystals of complex [3][Cl] could be easily obtained upon diffusion of pentane into a CH2Cl2 solution of [3][Cl] at ambient temperature in air. Complex [3][Cl] crystallized in the P21/c space group with one molecule in the asymmetric unit (Table1, Figure2 ). Inspection of the solid-state structure indicated the formation of a cationic complex formed by complete ionization of the Sb–Cl bond, resulting in the presence of a chloride counter anion sitting in the lattice and forming a hydrogen bond with an adventitious water molecule. The cationic complex [3]+ possesses a platinum atom in a square planar geometry with a chloride ligand coordinated trans from the antimony atom as in the case of D [13]. The resulting Sb–Pt bonds of 2.4928(5) Å is notably shorter than that in D (2.5380(8) Å).