Mixed Metal Oxide Synthesis by Thermolyses of Simple Heteropolymetallic Precursors in Oxygen
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MIXED METAL OXIDE SYNTHESIS BY THERMOLYSES OF SIMPLE HETEROPOLYMETALLIC PRECURSORS IN OXYGEN GEOFFREY DAVIES, BILL C. GIESSEN AND HUI-LI SHAO Department of Chemistry and The Barnett Institute, Northeastern University, Boston, MA 02115. ABSTRACT Stepwise transmetalation of the neutral molecular target (-O)N 4 Cu 4 C1 (N is a monodentate pyridine) with M(NS) transmealators4 (NS is a monanionic S-methyl hydrazinecarbodithioate ligand, M is Co (n = 2 or 3), Ni or Zn (n = 2)) in aprotic solvents gives a large family of heteropolymetallic products (0 4 O)N4 (Co,Ni,Cu,Zn) 4 C1 6 that contain from one to four different metals. Thermolysis of these solid products in flowing 02 at 220 - 250°C gives uniform, highly dispersed mixtures of the respective metal oxides. For example, thermolysis of bulk precursor (p 4 -O)N4 CoNiCuZnCl 6 gives a uniform mixture of 1 - 2y particles of Co3 0 4 , NiO, CuO and ZnO. TRANSMETALATION Transmetalation is the stoichiometric replacement of the metals in a polymetallic target with other metals from reagents called transmetalators(1]. Typical neutral target (A4 -O)N 4 Cu4 Cl 6 (N is monodentate N,N-diethylnicotinamide, Figure 1), contains four identical copper(II) centers coordinated to the central oxo group and bridged in pairs by chloride. Established neutral transmetalators M(NS) (NS is a monoanionic S-methyl hydrazinecarbodithioate ligandf are shown in Figure 2.
2M
N2 VY
FIGURE 1.
A typical transmetalation target 'CM S.,.c C
FIGURE 2.
S-methyl hydrazinecarbodithioate
transmetalators
Mat. Res. Soc. Symp. Proc. Vol. 249. ©1992 Materials Research Society
88
In a transmetalation reaction, one of the target metal centers is stoichiometrically replaced with a different metal from the transmetalator. The process can be repeated in steps either with the same transmetalator or with one containing a different metal. Alternatively, the target can be reacted with a mixture of transmetalators in the desired proportions. Single heteropolymetallic products such as (p -O)N 4 Cu CoNiCl are obtained because the target reacts with different Gransmefalators with different rates and rate laws. The complete transmetalation product family (p -O)N (Co,NiCuZn) 4 Cl 6 (N is a monodentate pyridine ligand in eq (11) theoretically contains thirty-six members, two (A 4 -o)N 4 Cu 4 Cl
6
+ M(NS)
2
+ M'(NS)
2
-
(A 4 -O)N 4 cu 2 MM'C1
6
+ 2Cu(NS)
2
(1)
of which are the tetranuclear enantiomers (p 4 -O)N 4 CoNiCuZnCl 6 containing four different metals[2]. Transmetalation is the only known route to these products[i,2]. Practical transmetalation reactions have the following characteristics: 1) The target and transmetalators are easy to make and purify and are highly soluble in common solvents; 2) The reactions proceed at practical rates under mild conditions; 3) The products are formed in stoichiometric steps so that discrete heteropolymetallic products are obtained; 4) The products are easy to isolate; and 5) They are members of families of molecules that contain variable numbers of different metals in t
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