molecules Article Coinage-Metal Bond between [1.1.1]Propellane and M2/MCl/MCH3 (M = Cu, Ag, and Au): Cooperativity and Substituents Ruijing Wang, Shubin Yang and Qingzhong Li * The Laboratory of Theoretical and Computational Chemistry, School of Chemistry and Chemical Engineering, Yantai University, Yantai 264005, China * Correspondence: [email protected]; Tel./Fax: +086-535-6902063 Academic Editor: Jennifer R Hiscock Received: 20 May 2019; Accepted: 16 July 2019; Published: 17 July 2019 Abstract: A coinage-metal bond has been predicted and characterized in the complexes of [1.1.1]propellane (P) and M2/MCl/MCH3 (M = Cu, Ag, and Au). The interaction energy varies between 16 and 47 kcal/mol, indicating that the bridgehead carbon atom of P has a good affinity for − − the coinage atom. The coinage-metal bond becomes stronger in the Ag < Cu < Au sequence. Relative to M2, both MCl and MCH3 engage in a stronger coinage-metal bond, both -Cl and -CH3 groups showing an electron-withdrawing property. The formation of coinage-metal bonding is mainly attributed to the donation orbital interactions from the occupied C-C orbital into the empty metal orbitals and a back-donation from the occupied d orbital of metal into the empty C-C anti-bonding orbital. In most complexes, the coinage-metal bond is dominated by electrostatic interaction, with moderate contribution of polarization. When P binds simultaneously with two coinage donors, negative cooperativity is found. Moreover, this cooperativity is prominent for the stronger coinage-metal bond. Keywords: coinage-metal bonding; cooperativity; substituents; NBO; AIM 1. Introduction Small-ring organic molecules have fascinated organic chemists since they exhibit not only unique bonding properties but feature interesting reaction modes and stereochemistry as well [1]. [1.1.1]Propellane is the smallest member of this family but its electronic structure is still under debate [2,3] since its synthesis was reported in 1982 [4]. Jackson and Allen [2] performed an ab initio calculation on the nature of C1-C3 bond in [1.1.1]propellane (Scheme1) and found this bond retains some σ-bridged π pattern. Based on the valence bond theory, Shaik and coworkers ascribed the bridge bond of [1.1.1]propellane to a charge shift (CS) bond [3]. Introducing such small-ring scaffolds can improve passive permeability, aqueous solubility, and metabolic stability, thus [1.1.1]propellane derivatives are extensively synthesized [5]. Very weak C=O H–C hydrogen bonding involving ··· propellane can be detected by atomic force microscopy [6]. Molecular electrostatic potential (MEP) map showed that the inverted-tetrahedral bridgehead atoms of the bridge bond of [1.1.1]propellane possess excess electron density, thus this molecule can be taken as a Lewis base to form a halogen bond with some halogen donor molecules [7]. Molecules 2019, 24, 2601; doi:10.3390/molecules24142601 www.mdpi.com/journal/molecules Molecules 2019, 24, 2601 2 of 11 Molecules 2019, 24, x FOR PEER REVIEW 2 of 11 SchemeScheme 1.1. [1.1.1]propellane. Regium bond is a strongstrong interactioninteraction that occurs between a σ-hole-hole onon aa coinage-metalcoinage-metal (regium) atom andand an an electron electron donor donor [8 ].[8]. Before Before this this name na wasme was proposed, proposed, the Lewis the Lewis acidic characteracidic character of a regium of a atomregium had atom long had been long recognized been recognized [9–11], where [9–11], the corresponding where the corresponding interaction was interaction called gold-bonding. was called Regiumgold-bonding. bonds betweenRegium bonds Mn clusters between (M =MCu,n clusters Ag, Au; (M n = Cu,2–6) Ag, and Au; nucleophiles n = 2–6) and NH nucleophiles3 and HCN are NH in3 aand large HCN part are dominated in a large by part Coulombic dominated attraction, by Coulombic with a attraction, smaller orbital with interactiona smaller orbital contribution interaction [12]. Othercontribution than molecules [12]. Other with than lone-pair molecules electrons, with lone-pairπ molecules electrons, are also π molecules utilized to are bind also with utilized regium to nanoparticlesbind with regium [13,14 nanoparticles] and gold compounds [13,14] and [ 15gold]. If compounds MX (X = halogen) [15]. If bindsMX (X with = halogen) benzene, binds a cation- withπ interactionbenzene, a iscation- characterizedπ interaction since theis characterized oxidation state since of M isthe+ 1oxidation in MX [14 state]. Recently, of M Legonis +1 in and MX Walker [14]. namedRecently, this Legon bond and as a Walker coinage-metal named bondthis bond by analogy as a coinage-metal with the term bond ‘halogen by analogy bond’ [16 with]. Wang the term et al. compared‘halogen bond’ the hydrogen, [16]. Wang halogen, et andal. compared coinage-metal the bondshydrogen, between halogen, small πandmolecules coinage-metal and HX/ YXbonds/MX (Xbetween= F, Cl, small Br, I;π Ymolecules= Cl, Br; and M = HX/YX/MXCu, Ag, Au) (X [=17 F,]. Cl, The Br, distribution I; Y = Cl, Br; of Mσ -holes= Cu, Ag, on theAu) surface [17]. The of nanostructureddistribution of goldσ-holes can on be unveiledthe surface by of MEPs nanostructured [18]. The presence gold can of σ -holesbe unveiled in nanoparticles by MEPs of[18]. gold The is helpfulpresence to of explain σ-holes its in catalytic nanoparticles properties. of gold is helpful to explain its catalytic properties. Cooperativity isis anan important important property property of of noncovalent noncovalent interactions interactions since since it largely it largely determines determines the applicationsthe applications of noncovalent of noncovalent interactions interactions in crystal in engineering,crystal engineering, molecular molecular recognition, recognition, and biological and functionsbiological [functions19–21]. Generally,[19–21]. Generally, both interactions both interactions are strengthened are strengthened if the middle if the moleculemiddle molecule acts as theacts Lewisas the acid Lewis and acid base, and respectively. base, respectively. Both interactions Both interactions are weakened are weakened when the when middle the molecule middle servesmolecule as aserves double as Lewis a double acid /Lewisbase. Coinage-metalacid/base. Coinage-metal bond also exhibitsbond also cooperativity exhibits cooperativity with other types with of interactions [11,22–28]. In FCCF AgCCX NCH⋅⋅⋅ (X =⋅⋅⋅Cl, Br, I), both coinage-metal bond and other types of interactions [11,22–28].··· In FCCF··· AgCCX NCH (X = Cl, Br, I), both coinage-metal halogenbond and bond halogen are simultaneously bond are simultaneously strengthened, strengthened, although AgCCX although is a AgCCX double Lewisis a double acid [23 Lewis]. That acid is, coinage-metal[23]. That is, coinage-metal bond sometimes bond displays sometimes some displays abnormal some cooperativity. abnormal cooperativity. InIn thisthis paper,paper, we we studied studied the the coinage-metal-bonded coinage-metal-bonded complexes complexes between between [1.1.1]propellane [1.1.1]propellane (P) and (P) Mand2/MCl M2/MCl/MCH/MCH3 (M3 =(MCu, = Cu, Ag, Ag, Au). Au). The The coinage-metal coinage-metal bonds bonds formed formed by by M M2 2and andMCl MCl/MCH/MCH33 werewere comparedcompared toto studystudy the the influence influence of of substituents. substituents. The The dependence dependence of of coinage-metal coinage-metal bonding bonding strength strength on theon naturethe nature of a coinage-metalof a coinage-metal atom wasatom explored. was expl Theored. nature The ofnature the coinage-metal of the coinage-metal bond was bond unveiled was byunveiled means ofby atoms means in moleculesof atoms (AIM),in molecules natural bond(AIM orbital), natural (NBO), bond and orbital energy (NBO), decomposition and energy (ED) analyses.decomposition Based (ED) on theanalyses. binary Based Ag systems, on the bi sixnary ternary Ag systems, systems six of Agternary2-P-Ag systems2, AgCH of3 -P-AgCHAg2-P-Ag32, AgCl-P-AgCl,AgCH3-P-AgCH Ag3, 2AgCl-P-AgCl,-P-AgCH3, Ag Ag2-P-AgCl,2-P-AgCH and3, Ag AgCH2-P-AgCl,3-P-AgCl and wereAgCH designed3-P-AgCl towere investigate designed the to cooperativityinvestigate the of cooperativity the coinage-metal of the bond. coinage-metal bond. 2. Theoretical Methods The geometriesgeometries ofof the the binary binary systems systems were were first firs optimizedt optimized at theat the MP2 MP2/aug-cc-pVDZ/aug-cc-pVDZ level. level. For For the coinagethe coinage atom, atom, an aug-cc-pVDZ-PP an aug-cc-pVDZ-PP [29] basis [29] set basis was set adopted was adopted to account to foraccount relativistic for relativistic effects. To effects. ensure thatTo ensure all structures that all corresponded structures corresponded to the true minima to th one thetrue potential minima energy on the surfaces, potential harmonic energy frequencysurfaces, calculationsharmonic frequency were performed calculations at the were same performed level. The at the geometry same level. optimization The geometry of the optimization binary systems of wasthe alsobinary performed systems at the MP2was/aug-cc-pVTZ(PP) also performed and wB97X-Dat the/aug-cc-pVTZ(PP) MP2/aug-cc-pVTZ(PP) levels, where and the ! integrationwB97X-D/aug-cc-pVTZ(PP) grid of B97X-D methodlevels, where was 75,302 the inintegration Gaussian grid 09. The of geometriesωB97X-D method of the ternary was 75,302 systems in wereGaussian only optimized09. The atgeometries the wB97X-D of /aug-cc-pVTZ(PP)the ternary systems level. Becausewere only MP2 neglectsoptimized three-body at the intermolecularwB97X-D/aug-cc-pVTZ(PP)