n2lect - Scission of Dinitrogen by a Molybdenum(III...

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Scission of Dinitrogen by a Molybdenum(III) Xylidene Complex CHM 5.33 Fall 2005
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Introduction The experiment is based on research performed in the laboratory of Professor Cummins during the early 90’s. Refer to: Laplaza, Catalina, and Christopher Cummins. "Dinitrogen Cleavage by a Three-Coordinate Molybdenum (III) Complex." Science 269 (1995): 861-863.
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Aspects of the Lab The experiment involves basic organic synthesis and introduces some rudimentary aspects of the manipulation of air and moisture sensitive materials. In addition, students use GC-MS and NMR to characterize the compounds made during the experiment. The experiment provides a very good context for the discussion of electronic structure and bonding in transition metal complexes.
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Chemistry of Nitrogen N 2p 2s N 2 ± ² 2p ± ³ ² ³ 2p Free gaseous atom is a ground-state quartet (2s 2 2p 3 ) 2 S + 1 ± 2(3/2) + 1 = 4 Nitrogen is trivalent - prefers to form three bonds (NH 3 , HCN, N 2 ) “three electron oxidant” Extremely important naturally and synthetically (amino acids, DNA, polymers, fertilizers) N 2 (dinitrogen) is the most abundant molecule in the earth’s atmosphere comprising ~80% The very stable triple bond ( ² H dis = 225 kcal/mol) renders N 2 practically inert N 2 does combine with metals to form nitrides
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Nitrogen is a common component of many important natural and synthetic compounds We need a way to utilize the natural abundance of N 2 as a synthetic feed stock N 2 + 3H 2 2NH 3 + 3H 2 Thermodynamically, the reaction of N 2 with H 2 is favorable ± E a The problem lies in N 2 overcoming the kinetic barrier ± G ~ -4 kcal/mol 2NH 3
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Biological Nitrogen Fixation In nature, N 2 is converted to ammonia by bacteria which grow in different forms of plant life and algae. These bacteria contain the “nitrogenase” enzyme which catalytically reduces N 2 to metabolically useful NH 3 The enzymes operate anaerobically at ambient temperature and pressure using a two component metalloprotein: the fi
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