Sains Malaysiana 47(7)(2018): 1491–1499

http://dx.doi.org/10.17576/jsm-2018-4707-17

Synthesis, Structure and Density Functional Theory (DFT) Study of a Rhenium(I) Pyridylpyrazole Complex as a Potential Photocatalyst for CO₂ Reduction

(Sintesis, Struktur dan Kajian Teori Fungsi Ketumpatan (DFT) ke atas Kompleks Renium(I) Piridilpirazol sebagai Fotomangkin untuk Penurunan CO₂)

WUN FUI MARK-LEE¹., YAN YI CHONG¹., KUNG PUI LAW^1,3., ISHAK B. AHMAD¹

& MOHAMMAD B. KASSIM^1,2*

¹School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor Darul Ehsan, Malaysia

²Institut Sel Fuel, Universiti Kebangsaan Malaysia, 43600 UKM, Bangi, Selangor Darul Ehsan, Malaysia

³School of Biosciences, No. 1 Jalan Taylor’s, 47500 Subang Jaya, Selangor Darul Ehsan

Malaysia

Received: 17 September 2017/Accepted: 28 February 2018

ABSTRACT

The Re(I) complex, [Re(PyPzH)(CO)₃Cl] where PyPzH = 2-(1H-pyrazol-3-yl)pyridine, was successfully synthesised and characterised with an infrared (IR), ultraviolet-visible (UV-Vis), 1H and 13C nuclear magnetic resonance (NMR) spectroscopies and X-ray crystallography. The IR spectrum featured three n(C≡O), n(N-H), n(C=N) and n(C=C) signals at (1860-2020), 3137, 1614 and 1513 cm^-1, respectively. The UV-Vis spectrum of the complex exhibited ligand-centred (π®>*) electronic excitations [max = 227 nm, ε = 1.942 x 104 M^-1cm^-1; lmax = 292 nm, ε = 0.853 x 104 M^-1cm^-1] and a metal-to-ligand charge transfer (MLCT) band [lmax = 331 nm, ε = 0.467 x 104 M^-1cm^-1]. The 13C and 1H-NMR spectra exhibited the characteristic signals of the three C≡O (189.0 – 199.0 ppm) and NH (14.84 ppm), respectively. The X-ray structure of [Re(PyPzH)(CO)₃Cl] showed the crystal adopted a monoclinic system with a C2/c space group [unit cell dimensions: a = 27.7422(14) Å, b = 11.1456(5) Å, c = 9.2461(4) Å with α = γ = 90º and β = 92.552(2)º]. Density functional theory (DFT) and time-dependent (TD) DFT calculations were performed to investigate the optimised structural geometry and electronic properties of the title complex. The results showed that the highest-occupied molecular orbital (HOMO) was predominantly found on the dπ-orbitals of Re(I), Cl and CO. While the lowest-unoccupied molecular orbital (LUMO) was located on the PyPzH moiety. The structural and photophysical properties of the [Re(PyPzH)(CO)₃Cl] were established and the reaction enthalpies for the dissociation of Cl atom in the formation of [Re(PyPzH)(CO)₃]• were discussed in view of its potential application for photocatalytic CO₂ reduction.

Keywords: Crystal structure; DFT; photocatalytic CO₂ reduction; pyridylpyrazole; rhenium(I) polypyridine

ABSTRAK

Kompleks Re(I) [Re(PyPzH)(CO)₃Cl] dengan PyPzH = 2-(1H-pirazol-3-il)piridina telah berjaya disintesis dan dicirikan dengan spektroskopi inframerah (IR), ultralembahyung-nampak (UV-Vis) dan resonans magnet nukleus (RMN) 13C dan 1H dan kristalografi sinar-X. Spektrum inframerah menunjukkan kehadiran tiga jalur n(C≡O), n(N-H), n(C=N) dan n(C=C) masing-masing pada (1860-2020), 3137, 1614 dan 1513 cm^-1. Spektrum UV-Vis kompleks menunjukkan peralihan elektronik berpusatkan ligan (π®>*) [lmaks = 227 nm, ε = 1.942 x 104 M^-1cm^-1; lmaks = 292 nm, ε = 0.853 x 104 M^-1cm^-1] dan satu jalur peralihan caj logam kepada ligan (MLCT) [lmaks = 331 nm, ε = 0.467 x 104 M^-1cm^-1]. Spektrum RMN 13C dan 1H masing-masing menunjukkan isyarat cirian untuk tiga isyarat kumpulan C≡O (189.0 - 199.0 ppm) dan NH (14.84 ppm). Struktur X-ray bagi hablur tunggal [Re(PyPzH)(CO)₃Cl] memberikan sistem monoklinik dengan kumpulan ruang C₂/c dengan dimensi sel unit sel a = 27.7422(14) Å, b = 11.1456(5) Å, c = 9.2461(4) Å dengan α = γ = 90º dan β = 92.552(2)º. Pengiraan berdasarkan teori fungsi ketumpatan (DFT) dan DFT bersandar masa (TD) telah dijalankan untuk membangunkan struktur geometri optimum dan ciri elektronik kompleks [Re(PyPzH)(CO)₃Cl]. Keputusan kajian menunjukkan orbital molekul terisi dengan tenaga tertinggi (HOMO) disetempatkan pada orbital-dπ Re(I), Cl dan CO manakala orbital molekul tidak terisi dengan tenaga terendah (LUMO) terletak pada moiety PyPzH. Struktur dan sifat fotofizikal kompleks [Re(PyPzH)(CO)₃Cl] telah dikenal pasti dan entalpi tindak balas untuk penguraian atom Cl untuk pembentukan [Re(PyPzH)(CO)₃]• juga dibincangkan untuk aplikasi sebagai fotomangkin penurunan CO₂ yang berpotensi.

Kata kunci: DFT; fotomangkin penurunan CO₂; piridilpirazol; renium(I) polipiridina; struktur Kristal

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*Corresponding author; email: mb_kassim@ukm.edu.my