Gold acting as a Lewis base in the formation of hydrogen and halogen bonds

Groenewald, Ferdinand George (2016-03)

Thesis (PhD)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: systematic theoretical investigation was performed to determine the hydrogen bond acceptor ability of the AuI metal centre in anionic and neutral complexes, where it acts as a Lewis base. The study was initiated by revisiting an accepted hydrogen-bond acceptor of gold, i.e. the Au− ion, which is known to form hydrogen bonds in the gas phase and some solutions. Six hydrogen bonded donors were selected, i.e. HF, H2O, NH3, HCN, C2H2 and CH4, with each yielding stable hydrogen bond conformations with the Au− ion. Trends in the interaction energies and geometrical data were identified and used to benchmark the study by comparison to the work of others. The procedure was then repeated for our model complex, the [(Me)2Au]− ion which formed hydrogen bonds to all six hydrogen bond donors, with interaction energies ranging from -2.4kcal/mol to -16.00 kcal/mol. Geometrical data, in conjunction with Atoms in Molecules analysis were consistent with those of the Au− ion and with hydrogen bonding agreeing with the characteristics listed in the IUPAC definition for hydrogen bonding. [(Me)2Cu]− and [(Me)2Ag]− do not yield MI···H interactions, while AuI···H hydrogen-bond formation is dependent on the electrostatic surface potential. The effect of electron-withdrawing and electron-donating ligands, i.e. CF3 − and C(Me)3 −, on gold’s hydrogen bond acceptor abilities, and hence its Lewis basicity, was identified by comparison to the results obtained for the [(Me)2Au]− model complex. Electron donation does not affect the interaction energies due to intermolecular repulsion between the hydrogen-bond donor and the ligand coordinated to Au(I), but influences the AuI···H distances and the properties of the BCP separating Au and H. Conversely, electron-withdrawing ligands affect the interaction energies as well as the calculated geometrical and AIM parameters and is consistent with weakening of the hydrogen bond. The AuI···H interaction energies for both electron-withdrawing and electron-donating ligands were lower than those for the model complex. Nevertheless, they are consistent with the IUPAC definition. A range of neutral N-heterocyclic carbene (NHC) Au(I) complexes, containing different anionic ligands (R, where R = H−, CH3 − , Cl−, OH−) were found to yield stable adducts with H2O, HF and NH3. In addition to the AuI···H-X hydrogen bonds, H-X···H-N hydrogen bonds were also identified, with the second hydrogen bond adding the most stability to the total interaction energy; the most stable hydrogen bonded adduct was found to be Au(NHC)Cl.NH3. Nevertheless, AIM analysis revealed that all AuI···HF and AuI···H2O interactions are hydrogen bonds, while NH3 forms weak dispersion-type AuI···H interactions, with only the Au(NHC)CH3 complex yielding an AuI···H interaction to NH3 that is characteristic for a hydrogen bond. Halogen bonds between the Au− ion and six different halogen bond donors, i.e. ICH3, BrCH3, ClCH3, ICF3, BrCF3 and ClCF3, were identified, where the auride anion behaves similarly to the I− and Br− ions in the formation of the trihalide species, with high interaction energies up to -33.3 kcal/mol and large accumulation of electron density between the Au and X atoms. [(Me)2Au]− was found to yield stable adducts with short AuI···X distances to five of the six halogen-bond donors, along with extremely directional AuI···X-R angles and bond elongation upon X-bond formation. It was found that increasing the X-R bond polarisability influences the bond strength and yielded more stable adducts. All the geometrical and AIM data were consistent with the IUPAC definition of halogen bonding.

AFRIKAANSE OPSOMMING: ‘n Sistematiese teoretiese ondersoek om te bepaal of die AuI metaal as ‘n waterstofbinding akseptor kan optree is onderneem. Ons begin die ondersoek deur na ʼn voorbeeld van goud te kyk wat alreeds ‘n aanvaarde waterstofbindingakseptor is: die Au− ioon. Die auried anioon is bekend as waterstofbinding akseptor in die gas fase asook in sekere oplosmiddels. Ons ondersoek is gedoen in die gas fase waar ons gevind het dat die auried anioon aan ses verskillende tipes waterstofbindingdonor molekules bind. Die resultate het tendense in die geometriese waardes asook die interaksie energieë gelewer en het ons toegelaat om die akkuraatheid van ons metodes te bepaal en as ‘n maatstaf te gebruik. Hierdie tendense kon toe gebruik word om met ons model kompleks, die [(Me)2Au]− ioon te vergelyk. Dié model kompleks het, net soos die Au− ioon, waterstofbindings met al ses waterstofbinding donors gevorm met interaksie energiëe wat tussen -2.4 kcal/mol en -16.00 kcal/mol wissel. Sowel die geometriese data asook “Atome in Molekules” (AIM) analises dui aan dat die AuI···H interaksie ʼn waterstofbinding is wat met die IUPAC definisie korrieleer. Die [(Me)2Cu]− en [(Me)2Ag]− ione vorm geen MI···H waterstofbondings met die elektrostatiese potensiële oppervlaktes nie wat aandui dat die vorming van die AuI···H interaksie as gevolg van dié oppervlakte is. Effekte van die elektron-donerende- en elektron-ontrekkendegroepe wat aan die goud koördineer was ook ondersoek om te bepaal of dit die Lewis basisiteit van Au(I) beïnvloed. Die resultate stel vas dat die elektron-donerendegroepe geen effek op die interaksie energie het nie; inteendeel dit word verswak indien dit met ons model kompleks vergelyk word, as gevolg van afstotende interaksies tussen die goud kompleks en die waterstofbindingdonor. Die invloed word wel in die Au···H afstande en AIM resultate gesien. Die effekte van die elektron-ontrekkendegroepe is wel meer opvallend met ʼn noemenswaardige destabilisering in die AuI···H interaksie energie. Alhoewel daar duidelike veranderinge in die interaksie energiëe, geometriese data asook die AIM analise is, stem die eienskappe van die interaksie ooreen met waterstofbindings soos deur IUPAC gedefinieer. Vervolgens is die ondersoek vergroot na die N-heterosikliesekarbene (NHC) Au(I) komplekse met ‘n anioniese ligand (R, waar R = H−, CH3 − , Cl−, OH−) wat aan die teenoorgestelde kant koördineer uitgebrei. Stabiele waterstofgebinde produkte het met die H2O, HF en NH3 donor molekules gevorm, alhoewel twee waterstofbindings gevorm het: die AuI···H-X en ʼn H-X···H-N. Die AIM analise het geblyk dat die AuI···H interaksies van H2O en HF waterstofbindings is, terwyl die AuI···H interaksie van NH3 is ʼn swak van der Waalstiepe interaksie is. Die mees stabiele waterstofbindingsproduk is Au(NHC)Cl.NH3, omdat daar gevind is dat die H-X···H-N interaksie die grootste bydrae tot die algehele stabiliteit van die sisteem maak. Om verder te bewys dat Au(I) as ʼn Lewis basis kan optree, is die studie verryk deur na die vermoë van Au(I) om as ‘n halogeenbinding akseptor op te tree te kyk. Om sistematies te bly het ons weereens met die auried anioon begin en gevind dat dit vorm ‘n halogeen binding met elk van die ses donor molekules verder tree Au− soos die I− and Br− haliede in die trihaliede spesies op. Die stabilisering betrokke by die produkte is -33.3 kcal/mol, wat gepaard gaan met ʼn hoë elektrondigtheid tussen die twee atome wat bind. [(Me)2Au]− het getoon dat dit die vermoë het om met vyf van die ses halogeenbindingsdonor molekules te bind met kort AuI···X afstande, duidelike rigtinggewend interaksies en X-R bindingsrekking op lewering van die produk. ʼn Verhoging in die X-R binding se polariseerbaarheid het tot ‘n meer stabiele interaksie energie gelei. Al die geometriese en AIM data het saamgestem met die IUPAC definisie van halogeenbindings.

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