Unimolecular Precursors to Binary Metal Sulfides: Mechanistic and Structural Correlations
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Pb(OAc) 2 .3 H2 0 + 2 RSH
H2 0, &2OH (1:4 v/v) 60 'C,1 h R = t-Bu, i-Bus-Bu
Pb(SR)2 + 2 HOAc
(1)
refluxing a suspension in decalin (bp 190°C) to produce cubic PbS (Galena). The details of this 3 low temperature route to pure (ESCA), crystalline (Fig. la) PbS have been reported previously. a The formation of PbS from the pyrolysis of compounds 2 and 3 was more challenging. The data obtained from their decompositions allowed a mechanism for the formation of PbS from metal bis(thiolate) precursors to be proposed. Thermogravimetric data for compounds 1-3 can be found in Table I. 85 Mat. Res. Soc. Symp. Proc. Vol. 410 01996 Materials Research Society
While no impurities were found in the XRPD pattern of PbS obtained from the solid-state decomposition of Pb(S-t-Bu) 2 at 250"C, the thermolyses of Pb(S-i-Bu) 2 and Pb(S-s-Bu) 2 , under the same conditions, yielded slightly impure PbS, as determined by XRPD (Fig. lb). These crystalline impurities were identified as Pb. However, if the thermolyses are carried out at a moderately elevated temperature (400"C) for 4h, PbS is the only crystalline phase observed by
Pb(SR) 2
250 "C 0.- PbS/Pb (1) atm
vac, 4 h
-
(2) 500 'C, 1 day
NR k4)
400ooc vac, 4 h
PbS
XRPD (eq 2). If, however, the sample of 2 or 3 initially thermolyzed to 250"C was exposed to the atmosphere before heating to 400*C, the Pb impurity remained, and traces of PbO were found. This oxygen-containing impurity likely results from the oxidation of small RS- and/or R2 S2 complexed elemental lead clusters which are thermodynamically unstable with respect to atmospheric oxidation during post-synthesis atmospheric handling. The presence of elemental lead is more difficult to rationalize, although a comparable process involving the expulsion of RSSR from surface-capping RS- groups can be envisioned.
T Q. >-
En z
TWO- THETA
TWO-
THETA
Figure 1. XRPD patterns of PbS obtained by the decomposition of lead bis(butylthiolate) compounds: (a, left) compound 1 (refluxing Decalin suspension, 3 days); (b, right) compound 3 (250'C, solid state, 4 h). Steigerwald has reported a related process 4 documenting the formation of elemental metal by the reductive elimination of a dialkyldichalcogenide from a metal bis(alkylchalcogernide) species (eq 3). If these complexed lead clusters are present in the samples heated to 400"C, then an oxidative addition of the metal likely occurs. Although well documented by Tuck 5 , oxidative additions of dialkyl or diaryl disulphides to main-group metals traditionally are effected in a hydrocarbon medium (e. g., toluene) (eq 4), not heterogeneously in a vapor phase/ solid phase environment. 86
Hg(TeR) 2
Hg(TeR)2 -
A preferred route
HgTe + TeR 2
Cetibu equilibrium
Hg0 + RTeTeR
RSSR + M
M(SR)n
(3)
(4)
R = n-Bu, p-N0 2 C6 H4 , Ph
M=In, n=3 M= Sn, n=4 M=Tl, n= 1 The present data can be understood by the presence of RSSR surface adsorbed onto crystalline Pb at 250°C for the case of 2 or 3. Exposure of this species to the atmosphere results in an oxidation of the dibutyl disulfide
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