Chromium-nitrogen system

Chromium(II) complexes of bipyridyls, terpyridyl and the phenanthrolines have been discussed in Section 35.2.2.1. Complexes of the ligands 2-aminomethylpyridine (pic, 2-picolyl-amine) and 8-aminoquinoline (amq), which have one heterocyclic and one amino nitrogen donor atom, have been prepared by methods similar to those in Scheme 10. The bis(amine) complexes are typical high-spin, distorted octahedral complexes, and the mono(amine) complexes, from their antiferromagnetic behaviour and reflectance spectra, are six-coordinate, halide-bridged polymers (Table 15).103 No tris(amine) complexes could be prepared so the attempt to find spin isomeric systems in octahedral chromium(II) systems was unsuccessful ([Cr(en)3]X2 are high-spin and [Cr(bipy)3]X3 and [CrX2(bipy)2] low-spin). 

Because of the sensitivity of chromium(II) to air oxidation, synthetic work on these systems requires the use either of vacuum-line techniques12 or of a nitrogen-filled box,6 or both. This synthesis describes the preparation of chromium(II) complexes using a combination of a closed ground-glass filter stick 18 and a nitrogen-filled box.6 The filter stick enables preparations, filtrations, and recrystallizations to be carried out... 

After the chromium (II) chloride solution has been transferred to flask B, the flow of ammonia through the reaction vessel should be started. The ammonia delivery tube should approach but not dip below the liquid level in flask B. If tank ammonia is used, the tank should be opened carefully to avoid spattering of liquids by a sudden burst of gas. If ammonia is to be generated, the ammonium sulfate solution should be added carefully to the potassium hydroxide in flask C. It may be necessary to cool flask C with ice at first, then to warm the generator later in order to maintain a reasonably constant flow of ammonia. The use of tank ammonia avoids these problems. If zinc was used in the reduction, a precipitate of zinc hydroxide forms first and redissolves. The violet-blue solution stirred at 0° is saturated with ammonia, then a 2- to 3-g. sample of the platinum catalyst is added rapidly to flask B. A strong countercurrent of nitrogen is used to prevent entrance of air into the system when the catalyst is added. The reaction mixture is allowed to stir for one hour while the flask is cooled with ice. 

This photodissociation of hexacarbonyls was studied by Massey and Orgel (70) in a polymethylmethacrylate matrix. They observed the reversible formation of a deep yellow color on ultraviolet irradiation when as little as 0.1% chromium hexacarbonyl was present in the polymer. Recombination of the Cr(CO)a and CO in the polymer occurs very quickly at 100 °C., in about four hours at room temperature, and not at all at liquid nitrogen temperature. Reversibility is, of course, highly dependent on prevention of diffusion of the dissociated CO out of the polymer system but no problem was noted even at reduced pressures. 

Complex oxides of the perovskite structure containing rare earths like lanthanum have proved effective for oxidation of CO and hydrocarbons and for the decomposition of nitrogen oxides. These catalysts are cheaper alternatives than noble metals like platinum and rhodium which are used in automotive catalytic converters. The most effective catalysts are systems of the type Lai vSrvM03, where M = cobalt, manganese, iron, chromium, copper. Further, perovskites used as active phases in catalytic converters have to be stabilized on the rare earth containing washcoat layers. This then leads to an increase in rare earth content of a catalytic converter unit by factors up to ten compared to the three way catalyst. 

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