IR, proton, and carbon-13 NMR spectral characterization of some chiral and achiral aminophosphines and their selenides

Article abstract of DOI:10.1080/10426500902953946

Though aminophosphines have been known for a century, and a large variety of such compounds has been synthesized for different aspects of their chemistry, until now, no examples are available on phosphines containing three different amino substituents. In this study, the first examples of such chiral tris(amino)phosphines and o-phenylenedioxo(amino)phosphines were successfully synthesized using condensation reactions, and they were converted to their respective selenides using a simple oxidative addition reaction. The compounds are characterized by IR, 1H, and 13C NMR spectral techniques, and the spectral aspects are presented. The spectral studies (i) indicated that they are indeed powerful tools for structural elucidation of compounds; (ii) showed the effect of heavier selenium atom on the P-N bond rotation process; and (iii) further supported the fact that dipolar structure predominates over the -bond structure for the aminophosphine selenides. Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.

Full text of DOI:10.1080/10426500902953946

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IR, Proton, and Carbon-13 NMR Spectral
Characterization of Some Chiral and
Achiral Aminophosphines and Their
Selenides
Janarthanan Gopalakrishnan a & M. N. Sudheendra Rao b
a Department of Applied Sciences, PNG University of Technology,
Morobe Province, Papua New Guinea
b Department of Chemistry, Indian Institute of Technology Madras,
Chennai, Tamil Nadu, India
Published online: 23 Mar 2010.
To cite this article: Janarthanan Gopalakrishnan & M. N. Sudheendra Rao (2010): IR, Proton, and
Carbon-13 NMR Spectral Characterization of Some Chiral and Achiral Aminophosphines and Their
Selenides, Phosphorus, Sulfur, and Silicon and the Related Elements, 185:4, 754-764
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Phosphorus, Sulfur, and Silicon, 185:754–764, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500902953946
IR, PROTON, AND CARBON-13 NMR SPECTRAL
CHARACTERIZATION OF SOME CHIRAL AND ACHIRAL
AMINOPHOSPHINES AND THEIR SELENIDES
Janarthanan Gopalakrishnan1 and M. N. Sudheendra Rao2
1Department of Applied Sciences, PNG University of Technology, Morobe
Province, Papua New Guinea
2Department of Chemistry, Indian Institute of Technology Madras, Chennai, Tamil
Nadu, India
Though aminophosphines have been known for a century, and a large variety of
such compounds has been synthesized for different aspects of their chemistry, until
now, no examples are available on phosphines containing three different amino sub-
stituents. In this study, the first examples of such chiral tris(amino)phosphines and
o-phenylenedioxo(amino)phosphines were successfully synthesized using condensation
reactions, and they were converted to their respective selenides using a simple oxidative
addition reaction. The compounds are characterized by IR, 1H, and 13C NMR spectral
techniques, and the spectral aspects are presented. The spectral studies (i) indicated that
they are indeed powerful tools for structural elucidation of compounds; (ii) showed the effect
of heavier selenium atom on the P–N bond rotation process; and (iii) further supported the
fact that dipolar structure predominates over the π-bond structure for the aminophosphine
selenides.
Supplemental materials are available for this article. Go to the publisher’s online edition of
Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental
file.
Keywords Aminophosphine selenides; aminophosphines; chiral aminophosphine selenides;
chiral aminophosphines; dynamic NMR
INTRODUCTION
Aminophosphines are tricoordinate phosphorus compounds containing one to three
polar and labile P(III)–N, bonds and they are considered as one of the most intriguing in
chemistry. Over the years, aminophosphine research has picked up momentum and has
gained paramount importance due primarily to four reasons: (i) their richness in acting as
versatile ligands in transition metal coordination chemistry;1–8 (ii) their specificity in their
roles as catalysts for various organic conversions;9–14 (iii) their ease in being converted
Received 20 January 2009; accepted 6 April 2009.
J.G. thanks IITM, Chennai, India for financial support. SAIF, IITM, Chennai, India is gratefully acknowl-
edged for NMR facilities.
Address correspondence to Janarthanan Gopalakrishnan, Department of Applied Sciences, PNG University
of Technology, Private Mail Bag Service, Lae 411, Morobe Province, Papua New Guinea. E-mail: janarthanang@
fastmail.fm and M. N. Sudheendra Rao, Department of Chemistry, Indian Institute of Technology Madras, Chennai
600036, Tamil Nadu, India. E-mail: mns rao@yahoo.com
754

CHARACTERIZATION OF CHIRAL AND ACHIRAL AMINOPHOSPHINES
755
to chalcogenides, which themselves possess ligating behavior,15–20 and hence they find
applications in catalysis, besides being used as carriers of Group 16 elements in electronic
industries;21 and (iv) their diversity in playing vital roles as effective synthons in real-
izing newer variety of inorganic heterocycles,22–28 which has numerous application inter-
ests.29–34 Depending on the substituents attached to phosphorus, drastic changes in catalytic
efficiency have been realized,35–37 and this could possibly be related to the Tolman cone
angle, first suggested by Tolman.38 Hence the tailoring of aminophosphines has become all
the more challenging, and many research groups are currently working to design suitable
aminophosphines for the synthesis of better catalysts with stereospecific applications. Ex-
amples of P-chiral-, C-chiral-, and N-chiral phosphines and partially substituted P-chiral
aminophosphines are reported in the literature,39–48 along with their coordination com-
plexes in some cases, whereas fully substituted P-chiral aminophosphines are not known
until we reported49 the first synthesis, 31P NMR and mass spectral data on chiral, but racemic
compounds of the type [(i-C3H7)2N][(n-C4H9)2N][R2N]P (1) and (o-C6H4O2)(R2N)P (2).
Earlier, single crystal X-ray structures for the compounds (o-C6H4O2)[(c-C6H11)2N]P and
(o-C6H4O2)(R2N)PSe, [R = (i-C3H7)2N; (c-C6H11)2N; (C6H5CH2)2N] were determined,50
which exhibited some interesting structural features, particularly with respect to P N and
P Se bonds. Common spectral techniques such as IR and multinuclear NMR have been
advantageous in elucidating the more precise structures of many of the aminophosphines
and their related compounds.51–55 Aminophosphines also offer interesting examples for the
study of their dynamic behavior, especially in solution using NMR as a tool,56–60 as there
exists a possibility of an inversion process occurring at the pyramidal shaped phosphorus
and nitrogen and rotation around the P–N bond. Though inversion at phosphorus is possibly
less likely an event occurring at room temperature due to the demand of relatively high
activation energy barrier, a probing bond rotation process assumes a more complex scenario
in many cases, as the substituents on nitrogen influence substantially the inversion process
occurring at nitrogen. The substituents on phosphorus as well as nitrogen centers affect
the P–N bond rotation process.[61] However, the geometry around nitrogen in almost all
of the aminophosphines R2P(NRꢁ2) reported so far possess a near planar geometry,51,62,63
1
thus reducing the complexity of the scenario. Herein, we report (i) the IR, H, and 13C
NMR spectral characterization of 1 [R2N = (C2H5)2N (a); OC4H8N (b); C5H10N (c)] and
2 [R2N = (i-C3H7)2N (a); (c-C6H11)2N (b); (C6H5CH2)2N (c)] along with the correspond-
ing selenides [(i-C3H7)2N][(n-C4H9)2N][R2N]PSe (3) [R2N = (C2H5)2N (a); OC4H8N
(b); C5H10N (c)] and (o-C6H4O2)(R2N)PSe (4) [R2N = (i-C3H7)2N (a); (c-C6H11)2N (b);
(C6H5CH2)2N (c)]; and (ii) dynamic 1H NMR behavior of compounds 2a and 4a.
RESULTS
1
IR, H, and 13C NMR data for the phosphines 1(a–c) and 2(a–c) and those of the
phosphine selenides 3(a–c) and 4(a–c) are presented in Tables I and II, respectively. The
set of compounds 1(a–c) and 3(a–c) are the first set of examples containing three different
amino substituents on phosphorus.
DISCUSSION
IR Studies
All 12 compounds synthesized exhibited sharp and strong characteristic bands in
the region 900–1060 cm−1 due to the P–N stretching vibration, which lies well within

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J. GOPALAKRISHNAN AND M. N. SUDHEENDRA RAO
Table I IR, 1H, and 13C NMR data for phosphines 1(a–c) and 2(a–c)
Entry No.
IR (cm−1
)
1H NMR (δ, ppm)
13C NMR (δ, ppm)
1a
1459(vs), 1370(vs), 1222(w),
1184(vs),1149(w), 1114(m),
1011(vs), 947(s), 915(s), 858(w),
790(m), 691(m), 653(s)
0.86 (t, 6H), 1.00 (t, 6H),
1.10–1.22 (m, 20H),
2.96–3.20 (m, 8H),
3.60–3.76 (m, 2H)
13.6, 18.0, 20.4, 24.1, 29.8,
39.2, 45.4, 46.2
1b
1c
1460(s), 1360(s), 1250(s), 1210(vs),
1180(s), 1110(vs), 1080(vs), 1020(m),
970(s), 930(s), 730(s), 610(m)
1475(vs), 1370(vs), 1230(s), 1190(vs),
1160(s), 1130(s), 1050(s), 1030(s),
960(vs), 940(vs), 920(s), 870(m),
840(w), 710(vs), 680(m), 640(w)
0.90 (t, 6H), 1.20 (d, 12H), 13.9, 20.8, 24.4, 31.0, 43.8,
1.30–1.70 (m, 8H), 2.94
(m, 10H), 3.65 (t, 4H)
44.6, 45.4, 67.9
0.93 (t, 6H), 1.18 (d, 12H), 13.9, 20.3, 21.7, 24.8, 26.3,
1.30 (m, 6H), 1.51 (m,
8H), 2.70 (t, 4H), 2.96
(dq, 2H), 3.04 (dt, 4H)
30.9, 43.9, 45.6, 48.5
2a
2b
2c
1478(vs), 1398(m), 1363(m), 1344(m), 1.30 (d, 12H), 3.40 (ds, 2H), 24.5, 45.0, 111.2, 121.5,
1235(s), 1200(s), 1174(s), 1123(s),
1091(w), 1024(s), 976(s), 912(w),
880(w), 858(m), 829(m), 742(s),
720(w), 678(s), 640(w), 614(w)
1468(vs), 1360(s), 1344(m), 1232(s),
1165(m), 1149(m), 1110(m),
1094(w), 1062(s), 1027(w), 1002(w),
976(m), 906(w), 890(w), 848(w),
826(vs), 736(vs), 669(m), 627(w)
1476(vs), 1381(vs), 1374(m), 1306(w), 3.80 (d, 4H) [3JPH = 14.0
6.90–7.05 (AAꢁBBꢁ, 4H)
147.0
0.90–1.80 (m, 20H),
2.65–2.80 (m, 2H),
25.3, 26.6, 35.5, 54.3,
111.0, 121.3, 147.1
6.75–6.90 (AAꢁBBꢁ, 4H)
47.6, 111.5, 121.8, 127.4,
128.4, 128.5, 136.8,
146.4
1235(s), 1130(w), 1094(m), 1059(m),
1030(w), 1011(w), 944(vs), 909(m),
896(w), 874(m), 822(vs), 774(m),
739(vs), 698(vs), 605(m)
Hz], 6.75–6.95 (AAꢁBBꢁ,
4H), 7.10–7.28 (m, 10H)
the reported range of 780–1100 cm−1 64 Value as high as 1083 cm−1 has been assigned
.
for υPN in the literature.65 The substituents on phosphorus do influence the stretching
frequency and in some cases substantially, as is evident from Table S1 (available online
in the Supplemental Materials). The compounds possessing the dicyclohexylamino group,
(C6H11)2N exhibits a sharp and strong band at 1062 cm−1 for the phosphine 2b and 1053
cm−1 for the phosphine selenide 4b, as reported earlier,66 and this could be attributed to
N–Cring stretching vibration.67 Each family of compounds, viz., 1(a–c), 2(a–c), 3(a–c) and
4(a–c) gave very similar infrared spectra, thus indicating a similar environment around the
phosphorus center. For the (o-phenylenedioxo)phosphine compounds 2 and 4, additional
bands with regard to ortho disubstituted benzene CH wagging vibrations (649–736 cm−1
)
and ortho disubstituted benzene ring bending vibrations (669–698 cm−1); as well as a
strong band around 1229–1254 cm−1 attributable to P–Oaryl stretching frequency could be
identified.67
Comments on P Se stretching vibrations. The aminophosphine selenides 3
and 4 exhibited medium to very strong and sharp bands for υPSe in the range 550–576 cm−1
.
Compounds with N3PSe framework showed relatively higher value compared to O2NPSe
framework (563–576 vs. 550–560 cm−1), thus indicating that the contribution of dipolar
structure (Figure 1) is more for the latter compared to the former. In fact, variations in P Se
bond length values elucidated from X-ray crystallography in such selenide compounds are
generally attributed to the various extent of contribution of dipolar and π-bond structures.68

CHARACTERIZATION OF CHIRAL AND ACHIRAL AMINOPHOSPHINES
757
Table II IR, 1H, and 13C NMR data for phosphine selenides 3(a–c) and 4(a–c)
Entry no.
IR (cm−1
)
1H NMR (δ, ppm)
13C NMR (δ, ppm)
3a
1459(vs), 1376(vs), 1248(w), 1174(vs), 0.94 (t, 6H), 1.15 (t, 6H),
13.5, 13.9, 20.5, 23.4, 30.3,
40.3, 46.9, 47.7
1117(s), 1094(m), 1018(vs), 973(vs),
922(vs), 867(m), 771(w), 723(m),
672(m), 640(m), 570(s)
1.28 (m, 4H), 1.34 (d,
12H), 1.57 (m, 4H), 3.02
(m, 4H), 3.18 (m, 4H),
3.80 (m, 2H)
3b
3c
4a
4b
1456(vs), 1376(vs), 1251(m), 1174(s),
1155(m), 1114(vs), 1075(m),
1030(m), 973(m), 954(vs), 922(s),
845(m), 752(m), 723(s), 688(w),
640(m), 602(m), 563(s)
1.03 (t, 6H), 1.29 (d, 12H), 13.9, 21.1, 24.5, 31.2, 45.2,
1.39–1.81 (m, 8H), 3.04
(m, 10H), 3.69 (t, 4H)
46.2, 47.1, 68.5
1453(vs), 1379(vs), 1350(vs) 1254(m), 0.88–0.98 (t, 6H), 1.22–1.64 14.1, 20.6, 23.2, 24.8, 26.0,
1203(m), 1171(s), 1158(s), 1114(s),
1056(vs), 1027(s), 973(m), 938(s),
717(vs), 688(m), 640(m), 576(vs),
547(m)
1472(vs), 1414(s), 1379(s), 1334(m),
1267(s), 1254(s), 1152(s), 1098(vs),
1040(vs), 982(vs), 896(vs), 851(m),
797(m), 771(m), 736(vs), 694(vs),
624(m), 557(vs), 531(s), 512(s)
(m, 26H), 2.83–3.07 (m,
8H), 3.65–3.84 (m, 2H)
30.6, 47.0, 47.6, 47.7
1.30–1.34 (d, 12H),
3.91–4.08 (ds, 2H),
6.95–7.10 (AAꢁBBꢁ, 4H)
22.4, 49.7, 112.4, 123.2,
144.7
1472(vs), 1402(m), 1373(w), 1328(vs), 1.16–1.30 (m, 4H),
25.2, 26.6, 32.9, 59.3,
112.4, 123.1, 144.8
1270(m), 1254(s), 1226(vs), 1165(s),
1146(s), 1078(vs), 1053(vs),
1005(vs), 976(vs), 890(s), 864(vs),
838(vs), 819(vs), 771(s), 749(vs),
646(vs), 608(m), 550(vs), 518(m),
496(m), 416(m)
1472(vs), 1450(s), 1354(s), 1328(s),
1229(vs), 1200(m), 1155(w),
1101(vs), 1059(vs), 1027(m),
1008(m), 941(m), 906(s), 848(vs),
787(m), 765(s), 742(s), 694(vs),
643(s), 598(s), 560(m), 496(s),
470(m), 422(s)
1.54–1.82 (m, 16H),
3.44–3.60 (m, 2H),
6.97–7.06 (AAꢁBBꢁ, 4H)
4c
4.25 (d, 4H) [3JPH = 14.4
Hz], 7.07–7.13 (AAꢁBBꢁ,
4H), 7.27–7.39 (m, 10H)
49.5, 112.3, 123.4, 127.9,
128.2, 128.7, 135.6,
145.3
Comments on P–N stretching vibrations. In this study, for the preparation
of tris(amino)phosphines 1 and tris(amino)phosphine selenides 3, the amines used are
indicated along with their pKa values69 in parentheses: (i) (i-C3H7)2NH (11.05); (ii) (n-
C4H9)2NH (11.25); (iii) (C2H5)2NH (10.84); (iv) OC4H8NH (8.50); and (v) C5H10NH
(11.12). The steric bulk as well as the pKa values is different for all these amines. This
causes different extent of P–N interactions, and as a result, subtle differences in the three
P–N bonds arise for 1 and 3. Moreover, based on single crystal X-ray structural studies,
Figure 1 Canonical forms of aminophosphine chalcogenides.

758
J. GOPALAKRISHNAN AND M. N. SUDHEENDRA RAO
Table III Comparison of δC values of N C carbon for 1 and 3 and 2 and 4
Phosphine
(1)
Phosphine
selenide (3)
Compound no.
1a & 3a
Groups
(i-C3H7)2N
(n-C4H9)2N
(C2H5)2N
(i-C3H7)2N
(n-C4H9)2N
OC4H8N
45.4
46.2
39.2
44.6
45.4
43.8
45.6
48.5
43.9
45.0
54.3
47.6
46.9
47.7
40.5
46.2
47.1
45.2
47.6
47.7
47.0
49.7
59.3
49.5
1b & 3b
(i-C3H7)2N
(n-C4H9)2N
1c & 3c
C5H10
N
2a & 4a
2b & 4b
2c & 4c
(i-C3H7)2N
(c-C6H11)2N
(C6H5CH2)2N
it was reported70 that in (R2N)3P, (i) two P–N bonds are short, and the geometry of that
nitrogen was found to be planar with reasonable sp2 character, and the third bond is
a longer bond, and the geometry of that nitrogen was found to be pyramidal with sp3
character; (ii) only two nitrogens (with shorter P–N bond lengths) are accommodated such
that their lone pairs are orthogonally placed to that of phosphorus lone pair, and the third
nitrogen (with longer P–N bond) then accordingly fits into the remaining space available;
and (iii) steric effect of nitrogen substituents is one of the important factors responsible for
deciding the geometry of the phosphine. As a result, even for such symmetrically substituted
aminophosphines, (R2N)3P, two types of P–N stretching vibrations were observed and
commented.70 Taking all these arguments along with variations in steric bulk and pKa
values of amines into consideration for the present study, it is very much expected for
compounds 1 and 3 to give three sets of P–N stretching vibrations, but to date, no examples
of aminophosphines and their selenides containing three different amino substituents are
available for comparing and commenting. Indeed, three υPN values could be identified in
the IR spectra of aminophosphines 1 and aminophosphine selenides 3, which are presented
in Table III. The fact that υPN bands generally appear as sharp and strong bands in most of
the cases helped in identifying and presenting them.
1H NMR Studies
1
In almost all the cases, high resolution H NMR spectra of the compounds pre-
pared gave the expected number of proton signals and their intensity, and thus offered
valuable assistance in their characterization. The phenyl protons of compounds 2 and 4
gave the characteristic AA’BB’ pattern expected of 1,2-disubstituted benzene. The chiral
tris(amino)phosphines and their selenides 1 and 3 exhibit all the signals in the narrow
δ-range of 0.9–3.9 ppm, but the splitting patterns were so clear that complete assignment
could be made without ambiguity. For example, Figure 2 provides the assignment made
for various signals observed for the compounds 1c and 3a, respectively. The absence of
phosphorus coupling to NCH2 protons of piperidine ring in 1c suggests that the ring is
puckered. A representative 1H NMR spectrum is given in Figure S1 (available online in the
Supplemental Materials) for the compound 3a.

CHARACTERIZATION OF CHIRAL AND ACHIRAL AMINOPHOSPHINES
759
Figure 2 1H NMR signal assignments for (a) 1c and (b) 3a.
1H NMR evidence for the dipolar structure of aminophosphine selenides.
Compared to phosphines, the phosphine selenides are found to exhibit slightly downfield
proton signals. An interesting observation made is that the lone proton of the cyclohexyl
groups in (o-C6H4O2)[(c-C6H11)2N]P E occurs at ca. 2.75 ppm for the oxide71 and ca. 3.50
ppm for sulfide71 and selenide, suggesting the possibility of the dipolar structure (Figure 1)
of phosphine chalcogenides predominating for the heavier chalcogen atoms sulfur and
selenium, thus supporting the views proposed by Grim and Walton.72 Our earlier spectral
studies (31P, 77Se NMR, and mass) on these compounds have also proven this fact.49
Dynamic 1H NMR studies. Dynamic 1H NMR studies of aminophosphines have
provided us with the scope to understand different dynamic processes associated with P–N
bond(s). In the present study, both an aminophosphine, (o-C6H4O2)[(i-C3H7)2N]P (2a),
and its selenide, (o-C6H4O2)[(i-C3H7)2N]PSe (4a) (Figure 3), have been considered for
variable temperature NMR investigations. The examples chosen offered the advantage of
(i) concentrating on a single trivalent P–N bond as the other two bonds are locked up in a
cyclic skeleton; and (ii) evaluating the effect of pentavalent state of phosphorus achieved
Figure 3 Structures of 4a and 2a.

760
J. GOPALAKRISHNAN AND M. N. SUDHEENDRA RAO
through selenium insertion on the dynamics of phosphorus–nitrogen bond. In addition
to these, the chosen isopropyl moiety serves as an excellent candidate for studying and
understanding the dynamic features with respect to phosphorus and nitrogen, because the
methine proton [ CH(CH3)2] gives a septet pattern in its 1H NMR spectrum.
1
In the room temperature H NMR spectra, as expected, the lone CH proton of the
isopropyl group appears as a 14-line pattern (doublet of septet, ds) due to both H–H
and P–H coupling interactions. The ds pattern for the compound 2a is shown in Figure
S2 (Supplemental Materials). The coupling constant values for the phosphine have been
determined to be, 3JHH = 7.0 Hz and 3JPH = 10.0 Hz, and those for the phosphine selenide
3
3
to be JHH = 7.0 Hz and JPH = 8.0 Hz. The values lie close to the values reported in
the literature.54,73 The methine proton signal of the isopropyl group corresponding to the
phosphine selenide shows a relatively lower coupling interaction compared to that of the
parent phosphine, especially with respect to 3JPH values (8 and 10 Hz respectively). In an
analogous situation, namely, (binaphtholyl)(diisopropylamino)phosphine,74 the coupling
of the methine proton with phosphorus was either not observed or not reported. A careful
1
analysis of the dynamic H NMR spectra of both the compounds reveals some salient
features.
Cowley et al.56,57 have observed an eight-line pattern at −50◦C for the methyl groups
in the chiral chlorophosphine, (C6H5)[(i-C3H7)2N]PCl, and this has been rationalized on
the basis of restricted rotation about the P–N bond, which renders the four methyl groups
anisochronous and each of those that couple to a methine proton to account for four
doublets. The same compound gave a four-line pattern at room temperature, and this has
been explained on the basis of the diastereotopic nature of the two isopropyl groups of the
molecule. However, in the present study, only one set of isopropyl signals is seen for both
of the compounds at room temperature, implying their rapid rotation about the P–N bond.
As the temperature is lowered, significant changes are observed with the signals.
1
The dynamic H NMR spectra for the isopropyl group of the compounds 2a and 4a are
presented in Figure S3 (Supplemental Materials). In the case of phosphine, both CH and
CH3 of isopropyl group undergo a significant change simultaneously with the lowering of
temperature, whereas only the methine signal (CH) reveals the temperature effect more
prominently in the case of phosphine selenide. The coalescence temperature (Tc) is around
−60 ◦C for the phosphine, and it is around −40 ◦C for the selenide. This indicates that the
rotation about the P–N bond becomes restricted faster in the case of selenide, which may
possibly be due to two factors: (i) the pentavalent phosphorus facilitating a multiple-bond
character at the expense of nitrogen lone pair of electrons; and (ii) the increased steric
crowding around the phosphorus of the phosphine selenide due to presence of heavier
selenium atom, in addition to the other groups. The NMR feature at −40 ◦C and below seen
for the selenide indicates that the CH signal selectively has become distinguishable while
that of CH3 group has not. This may be the result of the interaction of the bulky selenium
atom with one of the CH moieties. Another important observation is, at lower temperature,
another doublet at ca. 1.40 ppm appears to be growing in intensity for phosphine and the
origin of which, though not clearly known, seems to suggest that another conformation for
the molecule may become visible at this temperature.
13C NMR Studies
Carbon-13 NMR data acquired on all the compounds both in the proton coupled
and decoupled modes helped in unambiguous assignments of the signals. In case of the

CHARACTERIZATION OF CHIRAL AND ACHIRAL AMINOPHOSPHINES
761
phosphines 2(a–c) and their selenides 4(a–c), aromatic carbons of o-phenylenedioxo moiety
gave the expected number of signals (three numbers) and show practically the same chemical
shift values. In these compounds, the most deshielded signal between 144.7 and 147.1 ppm
appears as a doublet in the proton decoupled spectrum, which clearly indicates that these
carbons (2 numbers in each compound) are the closest to phosphorus, which is coupled
(2JPNC = ca. 9 Hz62). In the proton-coupled mode, this signal remained unchanged, whereas
the other two signals clearly showed a doublet each due to coupling with directly bonded
hydrogen (1JCH = ca. 150 Hz73). Each line in the doublet at 121 ppm for the phosphine
and 123 ppm for the phosphine selenide appears to be further split into a doublet due to
coupling to phosphorus compared to 111 and 112 ppm signals, respectively. This further
assisted in the spectral assignments.
In the alkyl region, 2JCNP is observable in all the cases in the proton decoupled spectra
(2JCNP = ca. 11–25 Hz), and their coupled mode spectra showed the expected multiplet
patterns in all the cases. Similarly, compounds 1(a–c) and 3(a–c) showed the expected
number of signals and multiplicities in their proton-decoupled spectra, but eluded from
calculation of J values due to almost-identical environments and narrow range.
13C NMR evidence for the dipolar structure of aminophosphine sel-
enides. In almost all the cases, compared to the phosphines 1 and 2, the N–C carbon
is deshielded in the phosphine selenides 3 and 4 (Table III). Further, in case of 2b, that
is, (o-C6H4O2)[(c-C6H11)2N]P, compared to its oxide and sulfide (56.4 and 58.4 ppm, re-
spectively),71 the N–C carbon in the selenide is deshielded the most (59.3 ppm), thus again
emphasizing the importance of the dipolar structure over π-bond structure for the heavier
chalcogenide (selenide in this case), where a positive sign resides on the phosphorus center
(Figure 1).
CONCLUSION
The study has brought out the IR, 1H, and 13C NMR spectral assignments for the first
examples of tris(dialkylamino)phosphines and their selenides, containing three different
amino substituents on phosphorus center. For such chiral compounds, three different P–N
1
stretching frequencies were identified. Both H and 13C NMR data indicated the higher
contribution of the dipolar structure for the phosphine selenide compared to the π-bond
structure, among the two canonical forms. The dynamic 1H NMR study revealed the effect
of the heavier selenium atom and increased multiple bond character of P–N bond on the
phosphorus–nitrogen bond rotation. In the free phosphine, Tc occurred at much lower
temperature than compared to that of the phosphine selenide.
EXPERIMENTAL
All manipulations were done under inert atmosphere (dry N2 or Ar gas) condi-
tions. Solvents and amines were purified by standard methods.75 PCl3 (Aldrich, 98%)
and selenium (CDH) were used as received. Catechol (CDH) was recrystallized from
hot toluene before use. Reported procedures49,76 were employed for synthesizing (o-
phenylenedioxo)chlorophosphine, (o-C6H4O2)PCl, and the series of compounds 1, 2,
3, and 4 by employing multistep or one-step condensation reactions between (i) PCl3
and respective amines and (ii) (o-C6H4O2)PCl and respective amines; and oxidative
addition reactions of 1 and 2 with elemental selenium under benzene reflux condi-
tions (Scheme 1). The yields were found to be between 58–78% for the phosphines

762
J. GOPALAKRISHNAN AND M. N. SUDHEENDRA RAO
Scheme 1 Synthesis of compounds 1–4(a–c).
and 61–78% for the phosphine selenides. IR spectra were recorded on a Perkin Elmer
1430 spectrophotometer using KBr windows, either as nujol mull (4000–600 cm−1),
neat sample (4000–600 cm−1), or KBr disc (4000–400 cm−1). However, only the peaks in
the finger print region (1600–600 cm−1 for 1 and 2, and 1600–400 cm−1 for 3 and 4) are
1
presented. H NMR spectra (400 MHz) and 13C NMR spectra (100 MHz) were recorded
on JEOL JNM GSX-400 spectrometer. The room temperature spectra were recorded as
1
CDCl3 solutions and variable temperature H NMR spectra were recorded as acetone-d6
solution using tetramethylsilane as the internal standard. Upfield shifts are negative. All the
chemical shift values (δ) are quoted in ppm.
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