The enthalpies of formation of (dimethylamino)dichlorophosphine, bis(dimethylamino)chlorophosphine, and tris(dimethylamino)phosphine

Article abstract of DOI:10.1006/jcht.1995.0026

The standard molar enthalpies of formation at the temperature 298.15 K of (CH3)2NPCl2 (l), <(CH3)2N>2PCl(l), and <(CH3)2N>3P(l) have been determined by reaction calorimetry as -(286.3 +/- 2.4) kJ*mol^-1, -(220.1 +/- 2.3) kJ*mol^-1, and -(112.8 +/- 2.3) kJ*mol^-1, respectively.Enthalpies of vaporization have been obtained from vapour-pressure measurements and the corresponding values of Δ_fH⁰_m(g) derived as -(245.5 +/- 2.5) kJ*mol^-1, -(174.2 +/- 2.6) kJ*mol^-1, and -(71.3 +/- 2.4) kJ*mol^-1, respectively.The values of the bond enthalpies D(P-Cl) and D(P-N) in substituted phosphines are discussed.

Full text of DOI:10.1006/jcht.1995.0026

M-3085
J. Chem. Thermodynamics 1995, 27, 273–279
The enthalpies of formation of
dimethylamino)dichlorophosphine,
(
bis(dimethylamino)chlorophosphine, and
tris(dimethylamino)phosphine
a
H. M. A. Al-Maydama, Arthur Finch, P. J. Gardner, and
A. J. Head
Department of Chemistry, Royal Holloway and Bedford New College
(
University of London), Egham, Surrey TW20 0EX, U.K.
(Received 22 August 1994)
The standard molar enthalpies of formation at the temperature 298.15 K of (CH
3
)
2
NPCl
2
(l),
{
(CH
3
)
2
N}
2
PCl(l), and {(CH
3
)
2
N}
3
P(l) have been determined by reaction calorimetry as
−1
−1
−1
−
(286.3 2 2.4) kJ·mol , −(220.1 2 2.3) kJ·mol , and −(112.8 2 2.3) kJ·mol , respectively.
Enthalpies of vaporization have been obtained from vapour-pressure measurements
−
1
and the corresponding values of
−
D
f
H°(g) derived as −(245.5 2 2.5) kJ·mol ,
m
−1
−1
(174.2 2 2.6) kJ·mol , and −(71.3 2 2.4) kJ·mol , respectively. The values of the bond
enthalpies D(P–Cl) and D(P–N) in substituted phosphines are discussed.
1. Introduction
The compounds {(CH
3
)
2
N} PCl3 − n , n = 0, 1, 2, 3, all undergo rapid stoichiometric
n
hydrolysis in acidic media to yield well defined products with known enthalpies
of formation. Their enthalpies of formation are, therefore, readily determined by
solution-reaction calorimetry and the results for the series of four compounds
provide information on the enthalpies of the (P–Cl) and (P–N) bonds in substituted
phosphines. This paper reports thermochemical measurements on three members
of the series (n = 1, 2, 3) for which results have not previously been published.
Enthalpies of vaporization at the temperature T = 298.15 K of the three compounds
were derived from vapour-pressure measurements.
2. Experimental
(
Dimethylamino)dichlorophosphine was prepared according to the procedure
(1)
described by Morse et al. Anhydrous dimethylamine vapour was condensed on to
a cold finger and the liquid allowed to drip into a chilled stirred ethereal solution
a
Present address: Faculty of Science, Sana’a University, Sana’a, Republic of Yemen.
0
021–9614/95/030273 + 07 $08.00/0
7 1995 Academic Press Limited

2
74
H. M. A. Al-Maydama et al.
of PCl
3
. The reaction mixture was fractionally distilled and the fraction boiling
between T = 420 K and 423 K collected. The product was re-distilled and the purity
estimated by determination of the chloride ion (Volhard’s method) after hydrolysis
with dilute nitric acid and oxidation of the H
the AgNO by H PO to silver). Mass fraction of Cl: found, 0.485; calculated for
N}PCl , 0.489.
Bis(dimethylamino)chlorophosphine was prepared (i), by the reaction between
3
PO
3
with H
2
2
O (to avoid reduction of
3
3
3
{
(CH
3
)
2
2
(
2)
tris(dimethylamino)phosphine (Aldrich) and PCl in ethereal solution; and (ii), by
3
the addition of PCl to dimethylamine in ethereal solution. In both cases the reaction
3
mixture was distilled under dry nitrogen and the fraction boiling between T = 331 K
and 335 K (pressure p = 1.27 kPa) collected. The purity of the re-distilled product
was estimated by determination of the chloride ion produced by acid hydrolysis (see
above). Mass fraction of Cl: found, 0.229; calculated for {(CH
3
)
2
N}
2
PCl, 0.230.
and
NH in ethereal solution, and was also obtained from
Aldrich. It was purified by distillation and the fraction boiling at T = 435 K to 437 K
p = 101.325 kPa) collected. The purity was estimated by hydrolysis with excess HCl,
Tris(dimethylamino)phosphine was prepared by the reaction between PCl
3
(
3)
excess anhydrous (CH
3 2
)
(
followed by potentiometric titration of the hydrolysate with standard NaOH(aq).
The difference between the first and second end-points enabled the phosphorus
content to be determined (as H
(CH N} P, 0.190.
A differential twin-vessel reaction calorimeter, operator in the isoperibol mode
3
PO
3
). Mass fraction of P: found 0.189; calculated for
{
3
)
2
3
(
4)
at T = 298.15 K, was employed. Its performance was frequently checked
by measurements of the enthalpy of neutralization of tris(hydroxymethyl)-
−3
aminomethane (THAM) with excess 0.1 mol·dm HCl(aq). Because of the
sensitivity of the compounds to air and moisture, the glass ampoules were filled in
a dry-box containing nitrogen.
The sample for vapour-pressure measurements was contained in a glass tube
surrounded by a jacket, through which water thermostatted to 20.02 K was
circulated, and connected to a strain-gauge (Schaevitz EM Ltd, model P522-0001,
range of pressure: 0 to 34.7 kPa) and high vacuum system. To prevent condensation
of vapour, all parts of the apparatus outside the thermostatted jacket were electrically
heated to a temperature above that of the thermostat. The apparatus was tested by
measurements of the vapour pressure of conductivity water between T = 278 K and
(
5)
3
18 K; the mean deviation of nine values from those recommended by IUPAC was
(
0.37 2 0.34) per cent.†
3. Results
The hydrolysis of the three substituted phosphines was carried out using
−
3
2
mol·dm HCl(aq). Preliminary experiments with {(CH
3
)
2
N} P, the phosphine
3
most resistant to hydrolysis, indicated that the reaction was complete within the
main period of a calorimetric experiment (about 10 min). This was ascertained by
†
Throughout this paper uncertainty intervals of reported experimental results denote 95 per cent
confidence limits, calculated using Student’s t at the appropriate number of degrees of freedom.

D
f
H°[{(CH
m
3
)
2
N}
n
PCl3 − n , l and g, 298.15 K] for n = 1, 2, 3
275
3
−3
TABLE 1. Hydrolysis of a mass m of (CH
3
)
2
NPCl
2
(l) in a volume 100 cm of 2 mol·dm HCl(aq) X
0
.2(HCl + 27.76H O) at T = 298.15 K
2
a
−1
−1
−1
m/g
−D
r
H
m
/(kJ·mol
)
−DH
m
/(kJ·mol
(dilution)
)
−D
r
H /(kJ·mol )
(corrected)
m
(
observed)
0
0
0
0
0
0
0
0
0
0
.08093
.07992
.07552
.05811
.05807
.04991
.04124
.04047
.02727
.02290
254.66
254.10
254.10
253.27
252.97
254.81
253.63
252.95
253.38
253.73
2.02
2.05
2.04
2.06
2.06
2.05
2.06
2.0
256.68
256.15
256.14
255.33
255.03
256.86
255.69
255.00
255.50
255.83
2.12
2.10
Mean: 255.82
20.45)
(
a
−3
Weighings in air corrected using r/(g·cm ) = 1.35 (estimated from the values for PCl and
3
{
(CH
3
)
2
N} P, where r denotes density.
3
calculating the cooling constant after different main periods and only at 10 min was
the cooling constant identical to that obtained for an electrical calibration.
−
The stoichiometry was established by determination of Cl (using Volhard’s
method) and H
(CH
n(CH
3
PO
3
(potentiometrically) in the products. The general equation is
{
3
)
2
N}
n
PCl3 − n (1) + {(2n − 3 + m)HCl + (p + 3)H
NH Cl + H PO + mHCl + pH O},
2
O} =
{
3
)
2
2
3
3
2
and the results obtained for the three compounds are listed in tables 1 to 3. Because
HCl is either produced (n = 1) or consumed (n = 2, 3) in the reaction and water is
always consumed, these tables include a dilution or concentration term which is
to be added to the observed reaction enthalpy to give the value which corresponds
3
−3
TABLE 2. Hydrolysis of a mass m of {(CH
3
)
2
N} PCl(l) in a volume 100 cm of 2 mol·dm HCl(aq) X
2
0
.2(HCl + 27.76H O) at T = 298.15 K
2
a
−1
−1
−1
m/g
−D
r
H
m
/(kJ·mol
)
DH
m
/(kJ·mol
)
−D
r
H /(kJ·mol )
(corrected)
m
(
observed)
(concentration)
0
0
0
0
0
0
0
0
.01588
.01653
.01818
.02282
.02911
.03458
.04729
.05226
280.87
281.33
280.22
280.37
280.78
280.56
280.48
280.75
1.52
1.57
1.63
1.63
1.66
1.65
1.65
1.60
279.35
279.76
278.59
278.74
279.12
278.90
278.83
279.15
Mean: 279.06
20.31)
(
a
−3
(2)
Weighings in air corrected using r/(g·cm ) = 1.060.

2
76
H. M. A. Al-Maydama et al.
3
−3
TABLE 3. Hydrolysis of a mass m of [(CH
3
)
2
N]
3
P(l) in a volume 100 cm of 2 mol·dm HCl(aq) X
O) at T = 298.15 K
0.2(HCl + 27.76H
2
a
−1
−1
−1
m/g
−D
r
H
m
/(kJ·mol
)
DH
m
/(kJ·mol
)
−D
r
H /(kJ·mol )
(corrected)
m
(
observed)
(concentration)
0
0
0
0
0
0
0
0
.01793
.02096
.02247
.03930
.04638
.04673
.04978
.05059
348.69
348.76
348.76
348.76
348.74
348.72
348.72
348.69
5.29
5.34
5.18
5.26
5.30
5.34
5.32
5.28
343.40
343.42
343.58
343.50
343.44
343.38
343.40
343.41
Mean: 343.44
20.06)
(
a
−3
(15)
Weighings in air corrected using r/(g·cm ) = 0.898.
−
3
to solution of the products in 2 mol·dm HCl(aq) (HCl + 27.76H
calculate molar enthalpies of formation of the the phosphorus compounds from
these results, molar enthalpies of solution of (CH NH Cl(cr) and of H PO (cr) in
2
O). In order to
3
)
2
2
3
3
−3
2
mol·dm HCl(aq) were also measured (tables 4 and 5). Thus, neglecting any
3
TABLE 4. Molar enthalpy of solution of a mass m of (CH
3
)
2
NH
2
Cl(cr) in a volume 100 cm of
−3
2
mol·dm HCl(aq) X 0.2(HCl + 27.76H
2
O) at T = 298.15 K
−1
m/g
n(H O)/n{(CH NH Cl}
2
3
)
2
2
D
sol
H°/(kJ·mol )
m
0
0
0
0
0
0
.13646
.15928
.16610
.21284
.24243
.28222
3317
2842
2725
2127
1867
1604
3.22
3.25
3.24
3.25
3.23
3.23
Mean: 3.24 2 0.01
3
TABLE 5. Molar enthalpy− of solution of
a
mass
m
of
2
H
3
PO
3
(cr) in
a
volume 100 cm of
3
2
mol·dm HCl(aq) X 0.2(HCl + 27.76H
O) at T = 298.15 K.
a
−1
m/g
n(H
2
O)/n(H
3
PO
3
)
D
sol
H°/(kJ·mol )
m
0
0
0
0
0
0
.06443
.08205
.08299
.10501
.12148
.27669
7064
5547
5484
4334
3747
1645
7.94
7.76
8.19
7.80
8.05
7.91
Mean: 7.94 2 0.17
a
−3
(16)
Weighings in air corrected using r/(g·cm ) = 1.651.

D
f
H°[{(CH
m
3
)
2
N}
n
PCl3 − n , l and g, 298.15 K] for n = 1, 2, 3
277
3
TABLE 6. Molar enthalpy of solution of a mass m of (CH
at T = 298.15 K.
3
)
2
NH Cl(cr) in a volume 100 cm of water
2
−1
m/g
n(H
2
O)/n{(CH
3
)
2
NH
2
Cl}
D
sol
H°/(kJ·mol )
m
0
0
0
0
0
0
.08009
.10042
.23930
.27493
.28477
.28542
5652
4508
1892
1646
1591
1586
2.03
2.07
2.09
2.02
2.04
2.07
Mean: 2.05 2 0.03
enthalpy of mixing between {H
HCl + 27.76H O)}, the enthalpy of the final state in all the hydrolysis experiments:
PO and (CH NH Cl in (HCl + 27.76H O)} was, therefore, known. There was
3
PO
3
in (HCl + 27.76H
2
O)} and {(CH
3
)
2
NH Cl in
2
(
2
{
H
3
3
3
)
2
2
2
no evidence for significant enthalpies of dilution in any of the determinations and,
although the values reported in tables 4 and 5 refer to more concentrated solutions
than were obtained in the hydrolysis experiments, no corrections have been applied.
−1
Although D
reference 6, the corresponding value for the crystalline compound is not listed.
It was necessary, therefore, to determine the molar enthalpy of solution of
CH NH Cl(cr) in water and the results obtained are presented in table 6.
From the mean value:
f
H
m
°{(CH
3
)
2
NH
2
Cl in 2000H O} = −287.16 kJ·mol is given in
2
(6)
(
3
)
2
2
−
1
D
sol
H
m
{(CH NH Cl in nH
°{(CH NH Cl, cr, 298.15 K] = −(289.2120.03) kJ·mol , neglecting the
unknown uncertainty on the NBS value. The quantity D °(H PO ,cr) =
(957.1 2 2.3) kJ·mol was derived from the molar enthalpy of oxidation of
3
)
2
2
2
O, 1600 Q n Q 5600} = (2.05 2 0.03) kJ·mol ,
−1
D
f
H
m
3
)
2
2
f
H
m
3
3
−1
−
H
(7)
(8)
3
PO
3
(cr) to H
3
PO
4
(aq), D
f
H
m
°(H
3
PO
4
in 4OH
2
O), and the enthalpy of dilution
O) was derived from
H°(Cl ,a) = −(167.0820.10) kJ·mol , and molar enthalpies of dilution
as −(164.4420.10) kJ·mol . From the above ancillary quantities and the results
reported in tables 1 to 5, the molar enthalpies of formation of the three phosphorus
compounds were obtained and are listed in table 7.
(
6)
of H
3
PO
4
(aq). The enthalpy of formation of (HCl in 27.76H
2
−
−1 (9)
(6)
D
f
m
−1
Enthalpies of vaporization were derived from the linear plots (correlation
coefficients, q99.8 per cent) of lg(p/Pa) against K/T. Because the mean temperatures
of the measurements of the vapour pressure of {(CH
3
)
2
N} PCl3 − n were close to
n
TABLE 7. D
f
m
H°(l), Dvap
m
H°(l), and D
f
m
H°(g) for {(CH
3
)
2
N}
n
PCl3 − n (n = 1, 2, 3) at T = 298.15 K
−
kJ·mol
D
f
H
m
°(l)
D
vap
H
kJ·mol
m
°(l)
−D
f
H°(g)
kJ·mol
m
n
−
1
−1
−1
1
2
3
286.3 2 2.4
220.1 2 2.3
112.8 2 2.3
40.80 2 0.68
45.91 2 1.21
41.48 2 0.58
245.5 2 2.5
174.2 2 2.6
71.3 2 2.4

2
2
78
H. M. A. Al-Maydama et al.
98.15 K (300 K, 310 K, 301 K for n = 1, 2, 3 respectively), no temperature
corrections, which lay within the uncertainty intervals, were applied. The values of
°(298.15 K) and the derived values of D °(g) for the three compounds are
listed in table 7.
D
vap
H
m
f
H
m
4. Discussion
Of the three compounds for which molar enthalpies of formation have been
determined, a value for only {(CH N} P(l) has been previously reported, based on
the reaction between (CH NH(l) and PCl
differs considerably from that obtained
(112.8 2 2.3) kJ·mol . An independent estimate of D
obtained from the relation:
3
)
2
3
(
10)
−1
3
)
2
3
(l). This value: −184.4 kJ·mol ,
in the present study:
°[{(CH N} P, g] can be
−1
−
f
H
m
3
)
2
3
D
f
H
m
°[{(CH
3
)
2
N}
3
P, g] = D
f
H
m
°[{(C
2
H
5
)
2
N}
3
P, g] −
NH, g}],
3[{D
f
H
m
°(C
2
H
5
)
2
NH, g} − {D
f
H
m
°(CH
3 2
)
which assumes the (P–N) bond enthalpies in the two phosphorus compounds are
−
1
identical. Taking D
f
H
m
°[{(C
2
H
5
)
2
N}
3
P,g] =−(225.6 2 10.5) kJ·mol
(Pilcher’s
(11)
(12)
calculation
(72.521.2) kJ·mol ,
°[{CH N}
of Fowell and Mortimer’s result ),
D
f
H°{(C
m
2
H
5
)
2
NH, g}=
−
1 (13)
−1 (13)
−
and
D
f
H
m
°{(CH
3
−1
) NH, g}=−(18.6 2 0.8) kJ·mol :
P, g]=−(63.9 2 11.3) kJ·mol , which does not differ significantly
2
D
f
H
m
3
)
2
3
−1
from the reported value of −(71.3 2 2.4) kJ·mol .
A simple self-consistency test for the standard molar enthalpies of formation of
a series of related compounds is to compare the magnitudes of the molar enthalpies
(11)
of redistribution reactions:
1
3
2
3
PCl
3
(g) + {(CH
3
)
2
N}
3
P(g) = {(CH
3
)
2
N} PCl(g);
2
−1
D
r
H
m
= −(31.3 2 3.2) kJ·mol ,
2
3
1
3
PCl
3
(g) + {(CH
3
)
2
N}
3
P(g) = (CH
3
)
2
NPCl (g);
2
−1
D
r
H
m
= −(30.9 2 3.1) kJ·mol .
If the (P–N) and the (P–Cl) bond enthalpies were constant and transferable,
then these redistribution molar enthalpies would be zero. That they are not reflects
the enhanced stability of the substituted phosphines relative to the symmetrical
analogues. Comparison of these redistribution molar enthalpies with corresponding
but limited results for other P(III) compounds suggests that our results are
(
11)
satisfactorily consistent.
(CH N} P is readily evaluated as (274 2 8) kJ·mol , the main source of
uncertainty arising from D
bond-dissociation enthalpies for P(III) compounds have been tabulated by Pilcher
The mean bond-dissociation enthalpy for (P–N) in
−1
{
3
)
2
3
−1 (14)
f
H°{(CH
m
3
)
2
N, g}
=
(145 2 8) kJ·mol .
Mean
(
11)
−1
and range from ꢀDꢁ(P–F) = 506 kJ·mol
to ꢀDꢁ{P–CH
2
(CH
)=274 kJ·mol lies at the lower end of
this range and is similar in magnitude to ꢀDꢁ(P–Br) and ꢀDꢁ(P–CH ).
2
)
2
CH }=
3
−1
−1
1
94 kJ·mol ; our value for ꢀDꢁ(P–NMe
2
3

D
f
H°[{(CH
m
3
)
2
N}
n
PCl3 − n , l and g, 298.15 K] for n = 1, 2, 3
279
Evaluation of bond enthalpies for the substituted phosphines requires the
assumption of bond-enthalpy transferability of either ꢀDꢁ(P–N) from {(CH N}
; these approaches are equivalent. Assuming that
3
)
2
3
P
or of ꢀDꢁ(P–Cl) from PCl
3
1
−
ꢀ
Dꢁ(P–Cl) = 322.2 kJ·mol is transferable, then ꢀDꢁ(P–N) in (CH
3
)
2
NPCl
2
and
−1
−1
in {(CH
3
)
2
N} PCl are 305 kJ·mol
2
and 290 kJ·mol , respectively. Ancillary
quantities for these bond-enthalpy calcualtions are
−1 (11)
D
f
H°(PCl
m
3
, g) = −(286.2 2 2.4) kJ·mol ,
−1 (9)
D
f
H
m
°(P, g) = (316.5 2 1.0) kJ·mol , and
−1 (9)
D
f
H°(Cl, g) = (121.30 2 0.01) kJ·mol .
m
Financial support (to H.A.A.A-M) from the British Council is gratefully
acknowledged.
REFERENCES
1
2
3
4
5
. Morse, J. G.; Cohn, K.; Rudolph, R. W.; Parry, R. W. Inorganic Syntheses 1967, 10, 147.
. Noth, H.; Vetter, H. J. Chem. Ber. 1963, 96, 1109.
¨
. Mark, N. Organic Syntheses 1966, 46, 42.
. Peake, S. J. Ph.D. Thesis, University of London, 1976.
. Recommended reference materials for the realization of physicochemical properties. K. N. Marsh:
editor. Blackwell: Oxford. 1987.
6
. NBS Tables of Chemical Thermodynamic Properties. Wagman, D. D.; Evans, W. H.; Parker, V. B.;
Schumm, R. H.; Halow, I.; Bailey, S. M.; Churney, K. L.; Nuttall, R. L.: editors. J. Phys. Chem.
Ref. Data 1982, 11, Supplement No. 2.
7. Finch, A.; Gardner, P. J.; Hussain, K. S.; Sen Gupta, K. K. Chem. Comm. 1968, 872.
8. Head, A. J.; Lewis, G. B. J. Chem. Thermodynamics 1970, 2, 701.
9. CODATA Key Values for Thermodynamics. Cox, J. D.; Wagman, D. D.; Medvedev, V. A.: editors.
Hemisphere: New York. 1989.
1
0. Hartley, S. B., Holmes, W. S., Jacques, J. K. Mole, M. F., McCoubrey, J. C. Quart. Rev. 1963, 17,
04.
1. Pilcher, G. Thermochemistry of phosphorus(III) compounds. Chap.
2
1
5
in The Chemistry of
Organophosphorus Compounds. Hartley, F. R.: editor. Wiley: London. 1990.
2. Fowell, P. A., Mortimer, C. T. J. Chem. Soc. 1959, 2913.
3. Thermochemical Data on Organic Compounds. Pedley, J. B.; Naylor, R. D.; Kirby, S. P.: editors.
Second edition. Chapman and Hall: London. 1986.
1
1
1
4. Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys.
Chem. Ref. Data 1988, 17, Supplement No. 1.
1
1
5. Aldrich Chemical Company Catalogue. Gillingham, Dorset. 1989.
6. Thomsen, J. in Thermochemistry. Longmans Green: London. 1908, p. 47.