Sharpe, Helen R.
(2017)
m-Terphenyl transition metal complexes: catalysis and small molecule activation.
PhD thesis, University of Nottingham.
Abstract
Herein, the synthesis, reactivity and catalytic activity of low-coordinate transition metal complexes bearing the m-terphenyl ligands is reported. Synthesis of the low-coordinate complexes are achieved via metathesis reactions between the aryl lithium dimer [2,6-Ar2C6H3Li]2 and the transition metal dihalide (MX2) in a mixture of toluene and THF. The steric demands of the ligand were altered through variation of the flanking aryl (Ar) by substitution of methyl groups in either the 2,6-positions (2,6-Xyl), 2,4,6-positions (Mes) or 2,4,5-positions (Tmp).
Two-coordinate group 12 complexes (7-9) exhibit conformational isomerism in solution due to rotation of the flanking aryl substituent. The analogous open shell transition metal complexes are three-coordinate where the coordination sphere features two m terphenyl ligands with coordination of a THF molecule to the metal centre (15 and 16).
Coordination of acetonitrile to the two-coordinate complexes 10 and 11 afforded the four-coordinate complexes 17 and 18 highlighting the steric demands of the 2,6 Mes2C6H3 ligand are not great enough to prevent access of small molecules to the metal centre.
Two- and three-coordinate complexes of manganese and iron (10, 11, 15, and 16) were found to be efficient precatalysts for the cyclotrimerisation of primary aliphatic isocyanates. Secondary, tertiary aliphatic and aryl isocyanates were not effective in this reaction where it is proposed that a metal-amidate species forms deactivating the catalytically active species towards cyclotrimerisation. Kinetic measurements between the reaction of 10 with EtNCO revealed the rate to be 1st order with respect to both substrates.
Iron complexes 11 and 16 were efficient precatalysts for the hydrophosphination of isocyanates producing mono- (21) and/or diinsertion (22) products where the latter is a new family of derivatised phosphines. The product obtained is a result of the steric demands of the isocyanate where bulky isocyanates yield the mono-insertion product exclusively and the less bulky aryl isocyanates produce a mixture. The selectivity of the reaction can be controlled with minor modifications to the reaction conditions. Initial coordination chemistry of the diinsertion product 22a was investigated towards [Ru(p cym)Cl2]2.
Manganese complexes 10, 13 and 15 were efficient precatalysts in the dehydrocoupling of Me2NH·BH3 affording the cyclic dimer [Me2N–BH¬2]2 under mild conditions. Small changes in the sterics of the ligand drastically alter the reaction mechanism (homogeneous vs. heterogeneous).
The reactivity of two-coordinate iron complexes 11 and 14 were investigated towards CO under ambient conditions which formed novel squaraines (29Mes and 292,6-Xyl) and diiron carboxylate complexes (302,6-Xyl) as a result of CO bond scission. Carbon-13 labelling studies verified the origin of the carbons from the central C4O2 unit of the squaraine though analysis via EPR, NMR and IR spectroscopies, mass spectrometry and DFT calculations.
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