Pentaammines synthesis

In addition to the high regiochemistry observed, the bulky osmium pentaammine metal center also effectively blocks one face of the pyrrole ring from attack. This directs each transformation carried out on the complex to occur on the same face of the pyrrole ring. For example, in the synthesis of 3-pyrroline complex 74, both protonation and hydride addition occur from the same face of the pyrrole ring, producing 74 exclusively as the c/s-isomer. This feature is also illustrated in the synthesis of pyrrolizinone 109 vide infra), where a stereoselective hydride reduction allows the preparation of 109 as only one diastereomer. 

The synthesis of pentaammine(carbonato)cobalt(III) salts is readily accomplished by an air-oxidation method starting with cobalt(II) nitrate and ammonium carbonate in aqueous ammonia.1 However, a suitable general method for making other types of monodentate amino(carbonato) metal salts is not available in the literature. The air-oxidation technique can be applied only to complexes where the ligands are all ammonia molecules or where the central metal ion is cobalt. 

Until recently ammine complexes of osmium have been little studied compared with their ruthenium analogs. This appears to have been caused by the lack of suitable synthetic routes to them. The discovery of pentaammine(dini-trogen)osmium (II) opened convenient routes to pentaammines of osmium (III), and a convenient synthesis of hexaammineosmium(IIl)2 gave new routes to the previously unknown nitrosyls of osmium(II). Here are given the synthesis of [Os(NH3)s(N2)]l2 and its conversion to [Os(NH3)5l]l2 the synthesis of [Os(NH3)6]l3 and its conversion to [Os(NH3)5(NO)]X3 H20(X = Cl, Br, I) and the preparation of [OsX(NH3)4(NO)]2+(X = OH, Cl, Br, I) from [Os(NH3)5(NO)]3+. 

Complexes of trifluoromethanesulfonate anion with cobalt(III) are labile oward substitution under mild conditions, and they have proved to be useful synthetic precursors to a variety of aminecobalt(III) complexes. The pentaammine-(trifluoromethanesulfonato-O)rhodium(III) ion, which is readily prepared from [Rh(NH3)5Cl]Cl2 in hot CF3SO3H, is also versatile as a synthetic precursor. " Its synthesis and solvolysis to give essentially quantitative yields of the penta-ammineaqua- and hexaamminerhodium(III) ions are described below. The aqua complex has previously been prepared by the base hydrolysis or Ag -induced aquation of [Rh(NH3)5Cl]Cl2 in water, but the present method presents a cleaner and more rapid alternative. The methods for preparation of the [RhCNHj) ] ion have evolved from the procedure of J0rgensen. They involve prolonged reaction of [Rh(NH3)5Cl]Cl2 with ammonia in a pressure vessel at elevated temperature. The solvolysis of [Rh(NH3)5(0S02CF3)](CF3S0j)2 in liquid ammonia is a simple, high-yield, and rapid alternative. 

A reaction time of 3 hr is required for the synthesis of the binuclear complex. Pentaammine(trifluoromethanesulfonato-0)ruthenium(III) trifluoromethanesul-fonate (0.20 g, 0.31 mmol) is dissolved in degassed (30 min, Ar) acetone (AR, 10 mL) contained in a 50-mL bubble flask (Fig. 1) under a continuous stream of At. ( Caution. Acetone is highly flammable and toxic. Due care should... 

The total time required for the synthesis and recrystallization is 1 day. Pentaammine(trifluoromethanesulfonato-0)osmium(III) trifluoromethanesul-fonate, [0s(NH3)s(0S02CF3)KCF3S03)2 (0.10 g, 0.14 mmol), is placed in a 100-mL two-necked round-bottomed flask, and the flask and solid are heated in a vacuum oven at 110°. A second 100-mL two-necked round-bottomed flask is connected to a KOH drying tower and placed in a dish containing Dry Ice. Liquid ammonia (—40 mL) is transferred to this flask, and the flask is stoppered. 

Syntheses reported for the pentaammine(trifluoromethanesulfonato-O) complexes can be readily adapted for other amine or multidentate amine analogs. Syntheses of coIbalt(III) complexes with 1,2-ethanediamine or A -ethyl-l,2-eth-anediamine ligands have been reported earlier in this series. To exemplify the procedures further, trifluoromethanesulfonato-O complexes of cobalt(III), chro-mium(III), and rhodium(III) with unidentate methylamine ligands based on the readily prepared [M(NH3)5Cl]Cl2 precursors are reported here. The following sections report syntheses of 1,2-ethanediamine complexes of Rh(III) and Irflll) and of Ru(II) and Os(II) diimines with trifluoromethanesulfonato ligands. Such syntheses indicate the diversity of the synthesis technique, and the complexes described are excellent precursors for other compounds. 

Ruthenium(II) [Ru(NH3)50H2] can be most efficiently prepared by zinc amalgam reduction of [RuCl(NH3)5]Cl2 in aqueous solution. More recently, an alternative route avoiding Zn + and Cl" ion has been developed based on the aquation of electrochemically reduced [Ru(03SCF3)(NH3)5] " . Alternative routes include the photolysis and acid-catalysed hydrolysis of [Ru(NH3)g] and the reduction of [Ru(NH3)5(OH2)]. The lability of the aqua ligand in these systems makes [Ru(NH3)jOH2] an excellent starting material for the synthesis of substituted pentaammine complexes, and for the study of their kinetics of formation. 

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