| Title: |
Selenium-Containing N-Heterocyclic Carbenesand Their First Palladium(II) Complexes: Synthesis, Structure, andPendent Alkyl Chain Length Dependent Catalytic Activity for Suzuki–MiyauraCoupling. |
| Authors: |
KamalNayan Sharma1, Hemant Joshi1, Alpesh K. Sharma1, Om Prakash1, Ajai K. Singh1 |
| Source: |
Organometallics. Vol. 32 Issue 8, p2443-2451. 9p. |
| Subject Terms: |
*ORGANOSELENIUM compounds, *HETEROCYCLIC compounds, *COMPLEX compounds synthesis, *CATALYTIC activity, *SUZUKI reaction, *CRYSTAL structure |
| Abstract: |
Three new N-alkyl-N′-(2-ethyl-1-selenophenyl)imidazoliumsalts, L1–L3 (differing in the chainlength of alkyl substituent of N), which are precursorsto (Se, CNHC) ligands and their N-heterocyclic carbene(NHC) complexes [Pd(L-HX)Cl2] (1–3) [L= L1–L3, X = I/Br], have been synthesized and characterized bymultinuclear NMR and HR-MS. For the synthesis of Pd complexes 1–3from appropriate imidazolium salts(L1–L3), the route of the silvercarbene transfer reaction has been adopted. Single-crystal structuresof 1and 2have been established with X-raydiffraction. Thermally stable, moisture- and air-insensitive complexes 1–3have been found to be efficient catalystsfor Suzuki–Miyauracoupling reactions (yield up to 96% in 2 h at 80 °C). Nanoparticles(NPs) (size of 80–85% particles ∼2–5 nm) formedin the beginning of these reactions (i.e., when the temperature ofthe reaction mixture reaches 80 °C) appear to be important forcatalytic coupling, probably as dispensers of Pd(0), and contain Pdand Se. The Pd:Se is nearly 3:2, 4:5, and 1:1 in NPs formed from 1, 2, and 3, respectively. The catalyticactivity of 3, having the longest alkyl chain containing(Se CNHC) ligand among the three complexes, has been foundto be higher relative to those of 1and 2. The length of the alkyl chain present in the complex molecule probablycontrols the catalytic activity by influencing the dispersion of NPs(containing Pd and Se) generated in situduring thecatalytic process. The two-phase test has indicated a homogeneousnature of catalytic process, which probably occurs via leaching ofPd(0) from these NPs. This is first example where Pd–Se NPscatalyze Suzuki coupling largely via a homogeneous process, whichis probably combined with oxidative addition to Pd(0) at the surface.The effect of alkyl chain length on catalytic efficiency may be usefulin designing ligands/Pd complexes that efficiently catalyze Suzuki–Miyauracoupling. [ABSTRACT FROM AUTHOR] |
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