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2.6.7 Complexes with Doubly Bonded Heteroelement Ligands 127<br />
on favors the conversion of imido ligands into oxo ligands, especially when the amine byproduct<br />
is further consumed by protonation in an acidic medium. [152] The synthetically<br />
more useful reverse exchange is favored by trapping water with the chlorotrimethylsilane/triethylamine<br />
combination, leading to hexamethyldisiloxane and triethylammonium<br />
chloride. [25] This reaction could in fact involve the in situ conversion of the primary<br />
amine into a bis(trimethylsilyl)amine which subsequently carries out the exchange process.<br />
The direct use of a trimethylsilyl-substituted amine has also led to satisfactory results.<br />
[157]<br />
The use of cumulenes has practical synthetic use <strong>only</strong> for the conversion of oxo into<br />
imido derivatives by the use of isocyanates. Formation of carbon dioxide provides the necessary<br />
driving force to the reaction. An example is the synthesis of compound 88, where<br />
the incompleteness of the second step limits the yield. [158] A problem of this synthesis is<br />
the potential cycloaddition of the imido product with excess isocyanate, leading to metalated<br />
ureas.<br />
Scheme 39 Syntheses from Other Complexes Containing Doubly Bonded<br />
Heteroelement Ligands [156,158]<br />
H2S, CS2<br />
M = W; R 87<br />
1 = Me 77%<br />
M = W; R1 = CH2TMS 40%<br />
M = Mo; R1 MCp<br />
− H2O<br />
= CH2TMS 77%<br />
∗ R1 ( O) 2 MCp ∗ R1 ( O)( S)<br />
W<br />
R 1 O<br />
O<br />
R2NCO, hexane<br />
140 oC, 1 week<br />
52%<br />
W<br />
R 1 NR 2<br />
O<br />
R2NCO, hexane<br />
125 oC, 20 d<br />
23%<br />
W<br />
R1 NR2 NR2 88<br />
Oxo(ç 5 -pentamethylcyclopentadienyl)sulfido[(trimethylsilyl)methyl]molybdenum(VI)<br />
(87, M = Mo; R 1 =CH 2TMS); Typical Procedure: [156]<br />
A soln of [Mo(CH 2TMS)Cp*(=O) 2] (25 mg, 0.071 mmol) in CS 2 (10 mL) saturated with H 2S<br />
was incubated at rt for 3 d, during which time the color of the soln changed from tinted<br />
yellow to red. The solvent was removed with a stream of N 2. The red residue was spotted<br />
on a 0.25-mm-thick silica gel TLC plate and then developed with Et 2O. The red band was<br />
collected to give [Mo(CH 2TMS)Cp*(=O)(=S)]; yield: 20.2 mg (77%). X-ray crystallographic<br />
quality, dark red crystals were grown from a saturated soln in hexane at –208C.<br />
2.6.7.3 Method 3:<br />
From Complexes Containing Triply Bonded Heteroelement Ligands<br />
This method is restricted to the transformation of terminal nitrides to imide compounds.<br />
A rare example of the application of this method to organometallic substrates is the protonation<br />
of tris(2,2-dimethylpropyl)nitridomolybdenum(VI) to afford the imido complexes<br />
89 (Scheme 40). [103,159]<br />
for references see p 135
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