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solvent removed by pumping, and the residue distilled under reduced pressure.
The liquid boiling at temperatures between 325 K and 326 K (pressure p = 0.20 kPa)
was collected and allowed to solidify (melting temperature: 294 K to 296 K) in
the condenser. The purity was estimated, after hydrolysis with excess HCl and
subsequent oxidation with H2O2 of the H3PO3 produced, by titrimetric determination
(standard NaOH) of the H3PO4. The difference between the two end-points yields
a value for the mole fraction of P: found, 0.2130; calculated for C5H12N3P, 0.2134.
(Dimethylamino)dicyanophosphine was prepared by refluxing a stoichiometric
mixture of (CH3)2NPCl2(l), prepared as described in reference 1, and AgCN in
acetonitrile for 20 h with stirring. The mixture was filtered under nitrogen, the solvent
removed by pumping, and the product distilled under reduced pressure followed
by two further distillations (boiling temperature: 339 K to 341 K at p = 0.13 kPa).
The purity was assessed by determination of the mole fraction of P by the method
described above: found, 0.2420; calculated for C4H6N3P, 0.2437.
Tricyanophosphine was prepared according to the method described by Jones
and Coskran.(2) A mixture of dry AgCN and freshly-distilled PCl3 was stirred in
acetonitrile (dried over CaH2) for 3 h under nitrogen at room temperature. The
mixture was filtered to remove AgCl, the solvent removed, and the crude product
twice sublimed in vacuo. The mass-fraction purity was assessed as 0.995 by the
method described for the other cyanophosphines.
Enthalpies of hydrolysis of the three compounds were determined using an
isoperibol twin-vessel solution-reaction calorimeter.(3) The calorimeter was operated
at T = 298.15 K and the precision and accuracy of the results were frequently
successfully checked by measurements of the molar enthalpy of neutralization of
tris(hydroxymethyl)aminomethane (THAM) with excess 0.1 mol·dm−3 HCl(aq).
Bis(dimethylamino)cyanophosphine and (dimethylamino)dicyanophosphine both
undergo rapid hydrolysis in water and HCl(aq). To avoid the formation of an
unidentified yellow precipitate which formed when the latter compound reacted
with water alone, 2 mol·dm−3 HCl(aq) was used in the calorimetric experiments on
both compounds.
The hydrolysis of P(CN)3 in 2 mol·dm−3 HCl(aq) has been reported to yield H3PO3
as the only phosphorus-containing product, whereas the reaction with 5 mol·dm−3
NaOH(aq) resulted in approximately 20 per cent of the phosphorus as species other
than HPO23−.(4) The formation of by-products, however, appears to be related to the
violence of the reaction since the aqueous alkaline hydrolysis of a solution of P(CN)3
in diethyl ether produced only HPO23−. In the present study the hydrolysis of P(CN)3
was investigated using (i), de-ionised de-oxygenated water; (ii), water containing
dissolved HCN; and (iii), 0.5 mol·dm−3 NaOH(aq) where, after hydrolysis, Na2HPO3
was assumed to be the only phosphorus-containing product. Titration of the alkaline
hydrolysate with standard HCl(aq) produced the calculated difference between the
first (Na2HPO3) and second (NaH2PO3) end-points, and between the second and
third (NaCN) end-points for the reaction:
P(CN)3 + 5NaOH = Na2HPO3 + 3NaCN + 2H2O.