AJSTD Vol.32(1)


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ABOUT THE ASEAN JOURNAL ON SCIENCE AND TECHNOLOGY FOR DEVELOPMENT The ASEAN Journal on Science and Technology for Development is a refereed Journal of the ASEAN Committee on Science and Technology (ASEAN COST). It reports on science and technology policies and programmes, and research activities undertaken by COST in support of social and economic development of the ASEAN member countries. The coverage is focused but not limited to, the main areas of activity of ASEAN COST, namely, Biotechnology, Non-Conventional Energy Research, Materials Science and Technology, Marine Sciences, Meteorology and Geophysics, Food Science and Technology, Microelectronics and Information Technology, Space Applications, and Science and Technology Policy, Infrastructure and Resources Development. ABOUT THE ASEAN COMMITTEE ON SCIENCE AND TECHNOLOGY The ASEAN Committee on Science and Technology was established to strengthen and enhance the capability of ASEAN in science and technology so that it can promote economic development and help achieve a high quality of life for its people. Its terms and reference are: ●● To generate and promote development of scientific and technological expertise and manpower in the ASEAN region; ●● To facilite and accelerate the transfer of scientific and technological development among ASEAN countries and from more advanced regions of the world to the ASEAN region; ●● To provide support and assistance in the development and application of research discoveries and technological practices of endogenous origin for the common good, and in the more effective use of natural resources available in the ASEAN region and in general; and ●● To provide scientific and technological support towards the implementation of existing and future ASEAN projects. Information on the activities of ASEAN COST can be obtained at its website http://www.asnet.org DISCLAIMER While every effort is made to see that no inaccurate or misleading data, opinion or statement appears in the Journal, articles and advertisements in the Journal are the sole responsibility of the contributor or advertiser concerned. They do not necessarily represent the views of the Editors, the Editorial Board nor the Editorial Advisory Committee. The Editors, the Editorial Board and the Editorial Advisory Committee and their respective employees, officers and agents accept no responsibility or liability whatsoever for the consequences of any inaccurate or misleading data, opinion or statement. © Copyright 2013: ASEAN Committee on Science and Technology No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form of by any means, without permission in writing from the copyright holder.


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Editorial Board Editor-in-Chief Prof Emeritus Dr Md Ikram Mohd Said School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia Editorial Board Members Malaysia Dr Ahmad Ibrahim Academy of Sciences Malaysia Prof Abdul Halim Shaari Faculty of Science, Universiti Putra Malaysia Prof Thong Kwai Lin Institute of Biological Science, Faculty of Science/ UMBIO Cluster, Institute of Graduate Studies, University of Malaya Brunei Darussalam Rosita Abdullah Senior Special Duties Officer, Head of Science & Technology, Research & International Division, Ministry of Development Assoc. Prof Zohrah Sulaiman Deputy Vice-Chancellor, Universiti Brunei Darussalam Myanmar Dr Zaw Min Aung Director General, Department of Technical and Vocational Education, Ministry of Science and Technology Philippines Dr Carol M. Yorobe Undersecretary for Regional Operations, Department of Science and Technology Zenia G. Velasco Director, Internal Audit Service―DOST Singapore Assoc. Prof Ong Sim Heng Department of Electrical and Computer Engineering, National University of Singapore Assoc. Prof Tan Tin Wee Acting Head, Department of Biochemistry, National University of Singapore Thailand Prof Narongrit Sombatsompop King Mongkut’s University of Technology Prof Prida Wibulswas President, Shinawatra University Cambodia Phal Des Vice-Rector, Royal University of Phnom Penh Indonesia Dr Warsito Purwo Taruno Minister of Research and Technology Nada Marsudi Minister of Research & Technology Lao PDR Malaithong Kommasith Minister of Science and Technology Dr Silap Boupha Director, Ministry of Science and Technology Vietnam Dr Mai Ha Director General, Ministry of Science and Technology


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Editorial Advisory Panel Brunei Darussalam Eddie Sunny Deputy Permanent Secretary, Ministry of Development Myanmar Dr Ko Ko Oo National COST Chairman, Deputy Minister, Ministry of Science and Technology Cambodia Dr Om Romny Director, Institute of Technology of Cambodia Philippines Dr Graciano P. Yumul Undersecretary for R&D, Department of Science and Technology Indonesia Prof Syamsa Ardisasmita, DEA Deputy Minister for Science and Technology Network, National COST Chairman Lao PDR Dr Maydom Chanthanasinh Deputy Minister, Ministry of Science and Technology, National COST Chairman Singapore Prof Low Teck Seng National COST Chairman, Managing Director, Agency for Science, Technology and Research Thailand Assoc. Prof Weerapong Pairsuwan Deputy Permanent Secretary, Ministry of Science and Technology Malaysia Dr Noorul Ainur Mohd Nur National COST Chairman, Secretary General, Ministry of Science, Technology and Innovation Vietnam Dr Le Dinh Tien Deputy Minister for Science and Technology, National COST Chairman Editor/Technical Editor Kanesan Solomalai Ex-Academy of Sciences Malaysia Amirul Ikhzan Amin Zaki Academy of Sciences Malaysia Production Manager Kamariah Mohd Saidin Universiti Putra Malaysia Publisher Universiti Putra Malaysia Press


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Contents ASEAN J. Sc. Technol. Dev. Volume 32(1), 2015 Structures, Energies, and Bonding Analysis of Monoaurated Complexes with N-Heterocyclic Carbene and Analogues T. A. N. Nguyen, T. P. L. Huynh, T. X. P. Vo, T. H. Tran, D. S. Tran, T. H. Dang and T. Q. Duong Methanolysis of Crude Jatropha Oil using Heterogeneous Catalyst from the Seashells and Eggshells as Green Biodiesel A. N. R. Reddy, A. S. Ahmed, M. D. Islam and S. Hamdan Decision-making Processes for a Do-not-resuscitate Poisoned Pediatric Patient Admitted to the Department of Emergency and Medical Services — A Case Study R. (III) P. Dioso Short Communication: Sulphur Levels and Fuel Quality in Peninsular Malaysia M. Ramalingam and A. Ahmad Fuad Periphytic Diatoms in the Polluted Linggi (sensu stricto) and Kundor Rivers, Negeri Sembilan, Malaysia I. S. A. Nather Khan 1 16 31 52 60


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ASEAN J. Sci. Technol. Dev.,  32(1): 1 – 15 Structures, Energies, and Bonding Analysis of Monoaurated Complexes with N-Heterocyclic Carbene and Analogues T.A.N. NGUYEN1*, T.P.L. HUYNH1, T.X.P. VO1, T.H. TRAN1, D.S. TRAN2, T.H. DANG3 and T.Q. DUONG4 In this work, we computationally investigated from quantum chemical calculations (DFT) at the BP86 level with the various basis sets def2-SVP, def2-TZVPP, and TZ2P+, chemical bonding issues of the recently described carbene-analogues gold(I) complexes AuCl-NHEMe (Au1-NHE) with E = C – Pb. The optimized structures and the metal-ligand bond dissociation energy (BDE) were calculated, and the nature of the E→Au bond was studied with charge and energy decomposition methods. The equilibrium structures of the system showed that there were major differences in the bonded orientation from the ligands NHC-NHPb to gold(I) complex between the lighter and the heavier homologues. The BDEs results showed that the metal-carbene analogues bonds were very strong bonds and the strongest bond was calculated for Au1-NHC which had the bond strength De = 79.2 kcal/mol. Bonding analysis of Au1-NHE showed that NHE ligands exhibited donoracceptor bonds with the σ lone pair electrons of NHE donated into the vacant orbital of the acceptor fragment (AuCl). The EDA-NOCV results indicated that the ligand NHE in Au1-NHE complexes were strong σ-donors and very weak π donor and the bond order in complexes was Au1-NHC > Au1-NHSi > Au1-NHGe > Au1-NHSn > Au1-NHPb. We also realised that the gold-ligand bond was characterized by a π back-donation component from the Au to the ligand. All investigated complexes in this study were suitable targets for synthesis and gave a challenge in designing Au nano-crystals of narrow size distribution from gold(I) complexes that carried versatile N-heterocyclic carbene-analogues NHE. Key words: N-heterocyclic carbene ligands; bond dissociation energy; EDA-NOCV; gold; DFT calculations The first direct synthesis of metal complexes with N-heterocyclic carbenes (NHCs) as ligand was pendently presented by Hans Werner Wanzlick (Wanzlick 1968) and Karl Ӧfele (1968). After that the break through result of the isolation of stable carbenes was reported by Arduengo et al. (1991). It has been known that NHCs have emerged as an essential class of ligands in inorganic and organometallic chemistry (Bourissou et al. 2000; Hahn & Jahnke 2008; Herrmann 2002). In the report of Jacobsen (Jacobsen et al. 2009) showed that NHCs display particular properties across the wide family of neutral ligands used in catalysis due to strong σ-donating character. Furthermore, ligand NHCs exhibit a special 1 Department of Chemistry, Hue University of Sciences, Hue University, 77 Nguyen Hue, Hue, Vietnam 2 Department of Chemistry, Quang Binh University, 312 Ly Thuong Kiet, Dong Hoi, Quang Binh, Vietnam 3 HCMC University of Food Industry, 140 Le Trong Tan, Tan Phu, Ho Chi Minh, Vietnam 4 Department of Chemistry, Hue University of Education, Hue University, 34 Le Loi, Hue, Vietnam * Corresponding author (e-mail: ainhungnguyen.chem@gmail.com)


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ASEAN Journal on Science and Technology for Development, 32(1), 2015 geometry associated with a flexible structure allowing fine-turning of their steric properties (Poater et al. 2009). In addition to this, NHCs can be very popular used for a large variety of reactions of high synthetic interest and these two-coordinate forms of carbon with two unpaired electrons have been considered as “new” ligands for bioactive coordination compounds (Hermann et al. 2002; Nemcsok et al. 2004). It has been noted that NHCs ligands can be stabilized by two nitrogens and form stable complexes with transition metals (Ag, Au) and with main-group elements (Nemcsok et al. 2004). Although the first stable transition metal carbene complex was investigated in 1964 (Fischer & Maasbӧl 1964), but after a long time, the metal-ligand bonding in complexes of mixed carbene-halogen complexes (NHC-TMX with TM = Cu, Ag, Au and X = F – I) was published for using a charge decomposition analysis which was noticed for the first time by Frenking and Boehme (1998) and group 11 elements (Cu, Ag, Au) called as coinage metals, have aroused intense interest (Zhu et al. 2012). The chemical bonding between NHCs and group 11 metals have been investigated theoretically (Nemcsok et al. 2004; Hu et al. 2004). Moreover, theoretical studies of the electronic structure of transition metal complexes with NHCs ligand have been recently carried out by other group (Schwarz et al. 2000; Weskamp et al. 1999; Lee et al. 2004). The fact was that, the type of NHC ligands that have been developed by Arduengo et al. [1991] have found recent use in the synthesis of molecular gold(I) fluoride and chloride complexes. Particularly in recent years, it has been known that gold could from stable coordination complexes with NHC ligand (Marion & Nolan 2008; Nolan 2011; Zhu et al. 2012). The nature of the Au-NHC binding has been presented from the structures and properties of the complexes (Nemcsok et al. 2004). In this study, we want to choose NHCMe and to extend to the heavier homologues in order to give insight into the structures and bonding situation using NBO and energy decomposition analysis (EDA) with the set of orbitals — the natural orbitals for chemical valence (NOCV) methods. The main purpose of this study was to investigate in details the bond strength of Au-E bond; the nature of the Au-E bond in AuCl-ligands; and the differences in the Au-ligand bonding from the carbene to plumbylene complexes. Scheme 1 shows the overview of the compounds investigated in the presented work and the schematic representation of a donor-acceptor bonding in Fischer-type carbene complex is shown in Scheme 2. The C atom in NHCMe ligand retains one lone pairs which is formally formed from a carbene ligand in a triplet state to a triplet AuCl fragment. To understand the gold-chloride and NHE ligands interactions in complexes, we have carried out density functional theory calculations. We investigated the bonding situation in complexes and the electronic structure of the molecules was analyzed with charge- and energy-decomposition methods. (a) (b) Scheme 1. Overview of the compounds investigated in the present work: (a) Complexes [AuCl-{NHEMe}] (Au1-NHE) and (b) Ligand NHEMe (NHE) with E = C, Si, Ge, Sn, Pb. COMPUTATIONAL DETAILS AND THEORETICAL ASPECTS The geometries of the gold(I) carbeneanalogues complexes [AuCl-{NHCMe}]− [AuCl-{NHPbMe}] (Au1-NHC − Au1-NHPb) were carried out at the gradient corrected DFT level of theory using Beck’s exchange functional (Becke 1998) in conjunction with Perdew’s correlation functional (BP86) (Perdew 1986). The calculations were carried out using 2


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T.A.N. Nguyen et. al.: Analysis of Monoaurated Complexes with N-Heterocyclic Carbene and Analogues Scheme 2. Schematic representation of donor-acceptor bonding in Fischer-type of AuCl complex that carries NHC ligand. the program package Gaussian03 (Frisch et al. 2004) optimized together with Turbomole 6.01 (Ahlrichs et al. 1989). All geometries were fully optimized without any symmetry constraints. A triple zeta valence basis set (def2-SVP) (Schäfer et al. 1992) was used for all of the main group elements and the relativistic of effective core potentials (ECPs) (Weigend & Ahlrichs et al. 2005) were applied for the heavier group-14 atoms Sn, Pb, and atom Au. The nature of the stationary points was checked by frequency calculations at the same level of theory (BP86/def2-SVP). All the structures of the complexes were verified as minimum by confirming that their respective Hessians were real on the BP86/def2-SVP level. The calculation of the bond dissociation energies (BDEs) and the charge analysis with Wiberg bond indices (WBI) as well as the natural partial charges at the BP86/def2-TZVPP (Snijders et al. 1981) //BP86/def2-SVP level of theory were carried out by using NBO 3.1 partitioning method in Gaussian03. Next, the bonding analysis was considered by using the ZieglerRauk-type energy decomposition analysis (EDA) (Ziegler & Rauk 1977) and natural orbital for chemical valence (NOCV). All complexes on the BP86/def2-SVP optimized structures were re-optimized by using the BP86/ TZ2P+ of core functional/basis set combination as implemented in ADF 2013.01 (Velde et al. 2001). An auxiliary set of s, p, d, f, and g STOs was used to fit the molecular densities and to represent the Coulomb and exchange potentials accurately in each SCF cycle (Krijn & Baerends 1984). Relativistic effects were taken by means of the zeroth-order regular approximation (ZORA) Hamiltonian (Velde et al. 2001; Lenthe et al. 1993; Lenthe et al. 1996) with a small frozen core. The nature of the Au-E bonds in Au1-NHC−Au1-NHPb were investigated at BP86/TZ2P+ with the EDA-NOCV (Mitoraj & Michalak 2007a; Mitoraj & Michalak 2007b; Mitoraj et al. 2009) method which combines the EDA (Nemcsok et al. 2004) with the NOCV (Mitoraj & Michalak 2007b; Mitoraj et al. 2009) under C1 symmetric geometries (without symmetry). Herein, we want to present a detailed theoretical aspect about the EDA-NOCV method. The EDA gave very well-defined energy terms for the chemical bonds in molecules. In the EDA developed independently by different groups (Morokuma 1971; Ziegler & Rauk 1979), and the recently introduced EDA-NOCV, the bond dissociation energy, De, of a molecule was divided into the instantaneous interaction energy ΔEint and the preparation energy ΔEprep: ΔE (= –De) = ΔEint + ΔEprep (1) The preparation energy ΔEprep was the energy which was required to promote the fragments from their equilibrium geometries in the electronic ground state to the geometries and electronic reference state which they had in the molecule. The interaction energy ΔEint could be further divided into three main components: ΔEint = ΔEelstat + ΔEPauli + ΔEorb (2) where, ΔEelstat is the quasiclassical electrostatic interaction energy between the fragments, 3


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ASEAN Journal on Science and Technology for Development, 32(1), 2015 calculated by means of the frozen electron density distribution of the fragments in the geometry of the molecules. ΔEPauli was referred to the repulsive interactions between the fragments which were caused by the fact that two electrons with the same spin could not occupy the same region in space, and could be calculated by enforcing the Kohn–Sham determinant on the superimposed fragments to obey the Pauli principle by anti-symmetrisation and renormalisation. The stabilising orbital interaction term ΔEorb was calculated in the final step of the energy partitioning analysis when the Kohn–Sham orbitals relaxed to their optimal form. The EDA-NOCV method combined the charge (NOCV) and energy (EDA) partitioning schemes to decompose the deformation density which was associated with the bond formation, Δρ, into different components of the chemical bond. Furthermore, the EDA-NOCV calculations also provided pair wise energy contributions for each pair or interacting orbitals to the total bond energy. NOCV is defined as the eigenvector of the valence operator, n, given by Equation 4: n ψi = υ ψi (4) In the EDA-NOCV scheme the orbital interaction term, ΔEorb, is given by Equation 5: / /N 2 N 2 TEorb = TEkorb = yk 9- F-TSk, - k + FkT,SkC k=1 k=1 (5) in which FTS –k,–k and FTS k,k were diagonal transition-state Kohn–Sham matrix elements corresponding to NOCVs with the eigenvalues –υk and υk, respectively. The ΔEkorb term of a particular type of bond was assigned by visual inspection of the shape of the deformation density, Δρk. The EDA-NOCV scheme thus provided information about the strength of orbital interactions in terms of both, charge (Δρorb) and energy contributions (∆Eorb) in chemical bonds. RESULTS AND DISCUSSION Structures and Energies The theoretically predicted geometries of Au1-NHC−Au1-NHPb with bond length, bond angle, and bending angle are shown in Figure 1 together with Table 1. Complexes Au1-NHC− Au1-NHPb clearly feature η1 coordinated AuCl at the central E atom (E = C − Pb). To the best of our knowledge, there was no experimental geometries for gold(I) complex that carried NHEMe. Note that the theoretical study with geometries and bond dissociation energies of less bulky N-heterocyclic carbene, silylene, and germylene complexes of MCl (M = Cu, Ag, Au) have been investigated by Boehme and Frenking (1998) for the first time in the recent past. Moreover, we have found somewhere else that the theoretical as well as the experimental geometries of related carbene complexes where the substituents at nitrogen was R = hydrogen, benzyl, benzoyl, were in good agreement with our calculated values (Boehme & Frenking 1998; Bovio et al. 1993). The calculated Au-C bond length of Au1-NHC gives the shortest value (1.997 Å) and the theoretically predicted Au-E bond lengths of complexes Au1-NHC− Au1-NHPb in this study increased from 1.997 to 2.708 Å. This could be easily explained by the increasing radii of the group-14 atoms. The calculated equilibrium structures of complexes Au1-NHC−Au1-NHPb in Table 1 show that lighter ligands NHE (E = C – Ge) were bonded in a head-on way to the metal fragment AuCl in which the bending angle was 180°. A comparison of the bending angle of the theoretical structures of Au1-NHCH − Au1NHGeH indicates that our calculated values are quite similar (Boehme & Frenking 1998). In contrast to that the bending angle of Au1-NHE 4


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T.A.N. Nguyen et. al.: Analysis of Monoaurated Complexes with N-Heterocyclic Carbene and Analogues Aul-NHC Aul-NHSi Aul-NHGe Aul-NHSn Aul-NHPb Figure 1. Optimized geometries of complexes Au1-NHC−Au1-NHPb at the BP86/def2SVP level. Bond lengths are given in Å; angles in degrees. The bending angle, α, is the angle X-E-Au where X is the mid-point between the N-N distance: became much more acute when E was heavier (bending angles α of Au1-NHSn = 110.3° and Au1-NHPb = 91.5°). We want to discuss the changes in the geometries of free ligands and AuCl in complexes. Table 1 shows that the bond lengths E-N in the complexes increased from C-N to Pb-N and those were shorter than in the free ligands in which the calculated values for the free ligands NHCMe – NHPbMe have recently been discussed by us (Nguyen et al. 2015). The increase of Au-Cl distances from the lighter to the heavier complexes exhibits the same trend compared with the values in the AuCl-NHEH (E = C – Pb) complexes (Boehme & Frenking 1998). Table 1 also shows the calculated BDEs for the Au-NHEMe bonds. There was a significant decrease from the carbene complex Au1-NHC (De = 79.2 kcal/mol) to the silylene Au1-NHSi complex (De = 67.0 kcal/mol) and continuous decrease for the BDEs of the heavier group-14 ligands (51.9 – 42.7 kcal/mol). The calculations suggest that the NHCMe ligand inAu1-NHC is the strongest bonded while the heavier homologues Au1-NHE where E = Si, Ge, Sn, Pb have weaker bonds which are not much different compared with the BDEs of the complexes in the previous studies (Boehme & Frenking 1998; Nguyen & Frenking 2012). The trend of the theoretically predicted AuCl-carbene and analogues bond energy in this study was significantly higher than the calculated values for the boraneNHEMe complexes (De = 59.8 – 13.8 kcal/mol) (Nguyen et al. 2015), the classical Fischer complex (CO)5W-CH(OH) (De = 75.0 kcal/ mol) (Vyboishchikov & Frenking 1998) as well as the (CO)5W-carbene (De = 54.4 kcal/ mol) and analogues (De = 44.3 – 25.5 kcal/mol) {(CO)5W-NHE with E = C – Pb} (Nguyen & Frenking 2012). This was quite suitable because the metal-NHEMe interactions of NHEMe-AuCl had small NHEMe←AuCl π-back-donation in complexes. From this, it follows that the monoaurated donor-acceptor complexes with carbene, silylene, and germylene ligands could have very strong bonds and the appearance of a small contribution in free ligands←AuCl π-back-donation in complexes would be further explained in bonding analysis. Analysis of the Bonding Situation The bonding situation in the complexes Au1-NHC−Au1-NHPb was analyzed using charge- and energy-decomposition methods. Table 2 shows the results of the NBO partitioning scheme and the Wiberg bond indices as well as the natural partial charges. The calculated partial charges showed that the metal fragment AuCl in the complexes carried always a negative charge which increases from Au1-NHC (–0.31 e) to Au1-NHPb (–0.53 e). 5


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ASEAN Journal on Science and Technology for Development, 32(1), 2015 Table 1: Bond length (Å), bond angle (°), and bending angle (a°) for the optimized neutral molecules of Au1-NHC – Au1-NHPb calculated at the BP86/def2-SVP level, and calculated bond dissociation energy, De (kcal/mol) for the dissociation of one molecule of AuCl from Au1-NHC to Au1-NHPb at the BP86/ def2-TZVPP//BP86/def2-SVP level of theory. Molecule Au1-NHC Au1-NHSi Au1-NHGe Au1-NHSn Au1-NHPb Bonding (Å) C-Au = 1.997 C-N = 1.373 Au-Cl = 2.302 Si-Au = 2.253 Si-N = 1.756 Au-Cl = 2.311 Ge-Au = 2.347 Ge-N = 1.856 Au-Cl = 2.298 Sn-Au = 2.601 Sn-N = 2.182 Au-Cl = 2.327 Pb-Au = 2.708 Pb-N = 2.367 Au-Cl = 2.340 Bonding angle (°) N1CN2 = 104.5 N1CAu = 127.7 N1SiN2 = 90.2 N1SiAu = 134.9 N1GeN2 = 86.6 N1GeAu = 136.7 N1SnN2 = 75.5 N1SnAu = 105.9 N1PbN2 = 71.0 N1PbAu = 91.2 Bending angle (α°) 180.0 De (kcal/mol) 79.2 180.0 67.0 180.0 51.9 110.3 44.2 91.5 42.7 Table 2. NBO results with Wiberg bond indices (WBI) and natural population analysis (NPA) at the BP86/def2-TZVPP// BP86/def2-SVP level for complexes Au1-NHC – Au1-NHPb. The partial charges, q, are given in electrons [e]. Molecule Au1-NHC Bond Au-C C-N1 C-N2 WBI q[AuCl] Atom NPA (q) 0.70 –0.31 Au 0.22 1.25 C 0.13 1.25 N –0.31 Au1-NHSi Au-Si 0.88 –0.43 Au 0.10 Si-N1 0.82 Si 1.16 Si-N2 0.82 N –0.71 Au1-NHGe Au-Ge 0.75 –0.37 Au 0.14 Ge-N1 0.80 Ge 1.07 Ge-N2 0.80 N –0.68 Au1-NHSn Au-Sn 0.62 –0.47 Au 0.04 Sn -N1 0.61 Sn 0.86 Sn -N2 0.61 N –0.56 Au1-NHPb Au-Pb 0.61 –0.53 Au –0.01 Pb-N1 0.49 Pb 0.74 Pb-N2 0.49 N –0.49 6


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T.A.N. Nguyen et. al.: Analysis of Monoaurated Complexes with N-Heterocyclic Carbene and Analogues The amount of charge donation to the AuCl fragment is always smaller than the donation to other transition metal moieties such as W(CO)5 and Mo(CO)4 that carry the similar NHEMe ligands in the complexes which have been calculated in the previous papers (Nguyen & Frenking 2012; Nguyen et al. 2014a; Nguyen et al. 2014b) in which the more negative charges in transition metal fragments W(CO)5 and Mo(CO)4 are between –0.47 and –0.77 e from the lighter to the heavier homologues. The Wiberg bond orders for the Au-E bond in Au1-NHC was 0.70 and increased in the Au1-NHSi (0.88) and then decreased from Au1-NHGe to Au1-NHPb which were from 0.75 to 0.61. The bond order for the E-N bond became clearly larger in the complexes Au1-NHC−Au1-NHPb compared with the free ligands NHCMe – NHPbMe. This is in agreement with the change in the E-N bonds which become shorter in the complexes than in the free ligands (Nguyen 2015; Nguyen et al. 2014b). The natural population analysis carried out for complexes found that the electrostatic charges of carbon atom in the NHCMe fragment of Au1NHC complex was nearly neutral whereas Si and Ge carried large positive charges which were 1.16 and 1.07 e and then slightly decreased in the heavier homologues (Sn = 0.86 e and Pb = 0.74 e). As mentioned in the computational methods, all complexes were considered under without any symmetry constraints meant the molecules had C1 symmetry. So there were no genuine σ and π orbitals because there was no mirror plane in the molecular structure. Although the lighter complexes Au1-NHE exhibited the end-on bonded in the NHEMe ligands (E = C – Ge) whereas the heavier ligands NHSnMe and NHPbMe were bonded side-on to the metal fragment AuCl. In order to consider the strength of the π donation NHEMe→AuCl which might be expected from the σ- and π lonepair orbital of the ligand NHEMe into the second vacant coordination side of metal fragment AuCl, we had to visually keep the shapes of Au1-NHSn and Au1-NHPb in one plane to identify σ- and π-type molecular orbitals. Figure 2 shows two occupied molecular orbitals and orbital energies of σ-type and π-type MOs from Au1-NHC–Au1-NHPb at the BP86/ TZVPP level. The energy levels of the π-type donor orbitals of complexes were higher lying than the σ-type donor orbitals. The orbital energy values were particularly large in the Au1-NHC in both σ- and π-type MOs and decreased in the silylene and germylene as well as in the heavier analogues. Especially, the shape of the molecular orbitals which indicated that NHEMe→AuCl not only had significant σ donation but also exhibited a bit π donation in complexes. We can explain that the π donation in complexes due to the strong N→E π donation at the ring of the NHEMe ligands. We showed the frontier orbitals with the plot of the energy levels of the energetically highest lying σ and π orbital of the isolated NHE ligands (Figure 3) in order to know whether the ligands had an occupied π-orbital in the E center atom. The HOMO of NHE ligand had π symmetry, except for NHC, in which the HOMO has σ symmetry whereas the HOMO-1 and HOMO-2 had π symmetry. The σ orbitals of NHE ligand were uniformly lone-pair molecular orbitals (MOs), but the π orbitals were delocalized over the NHE ring atoms. Figure 3 also shows that the energy level of the π orbital increased, whereas that of the σ orbital decreased as atom E became heavier. The trend of the energy levels of the energetically highest-lying σ and π orbitals of NHE ligand rationalize the preference of the heavier ligands NHSn to NHPb for side-on co-ordination to the metal, in which the σ-donation takes place through the π orbital of the ligand (Nguyen & Frenking 2012). The end-on coordination of the lighter homologues NHC to NHGe could be explained by various factors that also influence the bending angle α of the ligands (Table 1). 7


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ASEAN Journal on Science and Technology for Development, 32(1), 2015 Aul-NHC (π) HOMO-6 –7.157 (eV) Aul-NHSi (π) HOMO-2 –6.150 (eV) Aul-NHGe (π) HOMO-2 –6.123 (eV) Aul-NHSn (π) HOMO-2 –5.660 (eV) Aul-NHPb (π) HOMO-2 –5.361 (eV) Aul-NHC (σ) HOMO-9 –8.136 (eV) Aul-NHSi (σ) HOMO-5 –7.460 (eV) Aul-NHGe (σ) HOMO-7 –7.674 (eV) Aul-NHSn (σ) HOMO-6 –7.320 (eV) Aul-NHPb (σ) HOMO-9 –7.810 (eV) Figure 2. Molecular orbitals and orbital energies of σ-type and π-type MOs from Au1-NHC–Au1-NHPb at the BP86/TZVPP level. Orbital energies are given in eV. We also want to point out the orbitals at the Au side carried a little NHEMe←AuCl backdonation and mainly exhibited NHEMe→AuCl σ-donation. We suggested the scheme illustration showing the mixing of the empty s and occupied dz2 orbitals of Au(I) which was graphically shown in Figure 4a. Note that the Au cation had an s0d10 electron configuration but the s orbital was mostly filled in AuCl due to the ionic Au+−Cl− bond and the lowest lying empty orbitals were at the Au might be p orbitals. Although in the gold(I) complex, the relativistic effects were responsible for the very small charge transfer from Au to Cl but when the AuCl-NHEMe was formed, there was no further charge transfer from the Au atom to chlorine. From this it could be asserted that there was the mixing of the valence s orbital with occupied dz2 orbital of the Au. The competition for the empty Au s orbital between donation from chlorine and the carbene-analogues σ-lone pair might lead to the longer Au-Cl bonds in the complexes (Figure 1). Furthermore, the three filled orbitals arose from the E atom to Au and to Cl (E−Au−Cl) σ-interactions which were illustrated in Figure 4b. The highest energy had dz2 symmetry along the Au-Cl axis which could be considered as σ-antibonding character toward chlorine. The middle level of energy showed an anti-state combination of the Cl pz and an sp2 hybrid on the E atom and also revealed that there was no bonding toward Au. The lowest energy of E−Au−Cl σ-bonding orbitals exhibited a constructive overlap of the Au(I) 5dz2 orbital that carried the σ-symmetry on Cl and E atoms. We could point out that the covalent part of the metal bonding in AuCl-NHEMe had the patent of the familiar 6-electron-3 center interactions with the fully important dz2 orbital which significantly contributed to the Au-Cl 8


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T.A.N. Nguyen et. al.: Analysis of Monoaurated Complexes with N-Heterocyclic Carbene and Analogues E(eV) NHEMe –4 –3.810 –4.055 –4.381 –4.626 –5 –4.844 Molecule NHC NHSi NHGe NHSn NHPb π-bond HOMO-1 HOMO HOMO HOMO HOMO σ-bond HOMO HOMO-1 HOMO-1 HOMO-2 HOMO-2 –5.524 –6 –5.823 –6.204 –6.340 –6.613 –7 NHC NHSi NHGe NHSn NHPb Figure 3. Plot of the energy levels of the energetically highest lying σ and π orbital of ligands NHE (E = C − Pb). and AuCl-NHEMe bonding (Figure 4b). Note that the transition metal complexes MCl with M = Au, Ag, Cu that carry the less bulky NHC ligand has been recently described by Nemcsok et al. (2004) using EDA method. In this study, we used the EDA-NOCV calculations in order to give a thorough insight into the nature of the metal-ligand bonding in Au1-NHE. This led to a donor-acceptor description of the Au-E bond in the system. Table 3 showed the results of EDA-NOCV when considering NHCMe as the donor fragments and AuCl as the acceptor fragment. Table 3 shows that EDA-NOCV results at the BP86/TZ2P+ level for compound Au1-NHC–Au1-NHPb using the moieties [AuCl] and [NHEMe] as interacting fragments. The Au-E bond dissociation energies trend in Au1-NHE decreased from the lighter to the heavier homologues (Au1-NHC: De = 78.1 kcal/ mol; Au1-NHPb: De = 42.6 kcal/mol). The trend of the bond dissociations energies (BDEs) De for the Au-E bond in Au1-NHE system was Au1-NHC > Au1-NHSi > Au1NHGe > Au1-NHSn > Au1-NHPb. The decrease of the BDEs from the lighter to heavier adduct was determined by the intrinsic strength of the Au-ligand bonds ΔEint. The carbene adduct had a smaller preparation energy of the interacting fragments (ΔEprep = 1.2 kcal/mol) and stays nearly the same in the heavier homologues (ΔEprep = 2.0 – 2.5 kcal/mol) and the largest 9



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