The chemistry of trityl isoselenocyanate revisited: A preparative and structural investigation

Article abstract of DOI:10.1016/j.jorganchem.2010.02.021

The reaction of trityl chloride with KSeCN gives trityl isoselenocyanate which was structurally characterised by X-ray diffraction. Trityl isoselenocyanate reacts with hydrazine to give trityl selenosemicarbazide and with primary amines to give selenourea derivatives. However, with secondary amines mixtures of selenoureas and substitution products are formed. Trityl selenosemicarbazide undergoes a condensation reaction with salicylaldehyde to give the corresponding trityl selenosemicarbazone. In the case of 2-pyridinecarboxaldehyde, the analogous selenosemicarbazone cannot be isolated, instead a small quantity of the diselenide was isolated from the reaction mixture. The compounds prepared here were fully characterised spectroscopically and several also by X-ray diffraction.

Full text of DOI:10.1016/j.jorganchem.2010.02.021

Journal of Organometallic Chemistry 695 (2010) 1276–1280
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
The chemistry of trityl isoselenocyanate revisited: A preparative
and structural investigation
*
Mounir Ben Dahman Andaloussi, Fabian Mohr
Fachbereich C – Anorganische Chemie, Bergische Universität Wuppertal, 42119 Wuppertal, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of trityl chloride with KSeCN gives trityl isoselenocyanate which was structurally character-
ised by X-ray diffraction. Trityl isoselenocyanate reacts with hydrazine to give trityl selenosemicarbazide
and with primary amines to give selenourea derivatives. However, with secondary amines mixtures of
selenoureas and substitution products are formed. Trityl selenosemicarbazide undergoes a condensation
reaction with salicylaldehyde to give the corresponding trityl selenosemicarbazone. In the case of 2-pyr-
idinecarboxaldehyde, the analogous selenosemicarbazone cannot be isolated, instead a small quantity of
the diselenide was isolated from the reaction mixture. The compounds prepared here were fully charac-
terised spectroscopically and several also by X-ray diffraction.
Received 1 February 2010
Accepted 18 February 2010
Available online 24 February 2010
Keywords:
Organoselenium compounds
Isoselenocyanate
NMR spectroscopy
X-ray crystal structure
Diselenide
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Results and discussion
Isoselenocyanates RN@C@Se are useful synthetic precursors for
a variety of organoselenium compounds containing the –NHC(Se)–
unit including selenoamides and selenocarbamate esters which
can act as monoanionic SeÀ ligands towards a metal upon deproto-
nation. In our laboratories we are studying various aspects of the
coordination chemistry of these types of organoselenium com-
pounds with both transition- and main-group-metals, and have
previously reported some results with gold [1–3]. Isoselenocya-
nates are generally prepared in a one-pot procedure from the cor-
responding formamides, phosgene and Se metal in the presence of
a base [4]. Alternatively, the reaction of an isocyanide with Se me-
tal in CHCl3 also gives reasonable yields of isoselenocyanates [5]. In
either case, use of highly toxic phosgene (or one of its derivatives)
is required. In search for a safer and simple preparation for isose-
lenocyanates we came across a report that trityl isoselenocyanate
could be prepared simply from trityl chloride and KSeCN [6],
although the exact same procedure had previously been reported
to yield trityl selenocyanate [7]. Given this apparent contradiction,
we wished to re-examine the reaction of trityl chloride with KSeCN
and use modern characterisation methods to unambiguously iden-
tify the product. Should the isoselenocyanate indeed be formed, we
wished to examine its reactivity and to see if any organoselenium
compounds suitable as ligands could be prepared from it. The re-
sults of this investigation are reported herein.
Trityl isoselenocyanate (1) was prepared from the reaction of
trityl chloride and KSeCN (Scheme 1) as previously described [6].
The compound was obtained as colourless needles after recrystal-
lisation from hexane. We observed that the compound turned red
upon standing at room temperature in the presence of light over a
period of ca. 12 h. To prevent this decomposition, the material was
stored in amber bottles in the freezer. In solution (acetone or
CH2Cl2) decomposition also occurs: the formation of a red precip-
itate (presumably elemental Se) can be observed after a period of
several hours. Since there has been some debate in the past if the
seleno- or iso-selenocyanate derivative is formed by the reaction
of KSeCN and Ph3CCl, we wished to unambiguously confirm the
identity of the material in the solid state using single crystal
X-ray diffraction. The molecular structure of 1 is shown in Fig. 1,
important bond distances and angles are included in the figure
caption.
The compound crystallises in the space group P21/c with two
independent molecules in the asymmetric unit. From the molecu-
lar structure it is immediately apparent that the compound is in-
deed the isoselenocyanate and not the selenocyanate as was
suggested by Rheinboldt and de Campos [7]. Based on the C–N dis-
tance of 1.458(3) Å, which is typical for a nitrogen–carbon-single
bond, an ionic structure [Ph3C][NCSe] can also be ruled out. The
Se@C@N unit is, as expected, virtually linear [178.8(2)°] and the
quite short Se@C and C@N bond distances [1.740(3) and
1.149(3) Å, respectively], are also virtually identical to those ob-
served in the ferrocene derivative [Fe{
other isoselenocyanate that has been structurally characterised
g
5-C5H4NCSe}2], the only
* Corresponding author.
E-mail address: fmohr@uni-wuppertal.de (F. Mohr).
0022-328X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.02.021

M. Ben Dahman Andaloussi, F. Mohr / Journal of Organometallic Chemistry 695 (2010) 1276–1280
1277
Se
H2NNH2
Ph3C
NH2
Ph3C
Ph3CN=C=Se
Cl
KSeCN
+
N
H
N
H
(1)
(2)
N
H
NH2
Se
Se
Ph3C
N
H
N
H
(3)
Ph3C
Ph3C
N
+
N
H
N
Scheme 1.
[8]. These rather short bond distances are indicative of the strong
-interactions between the three atoms of the NCSe unit. The bond
in 2 is longer than that in the isoselenocyanate 1, reflecting the
perturbation of the cumulated -system and the high degree of
p
p
distances and angles of the two independent molecules of 1 are
identical within experimental error, except the Ph3C–N@C angles,
which differ by ca. 6° probably due to packing effects in the crystal.
The reaction of trityl isoselenocyanate with hydrazine hydrate
in cyclohexane affords trityl selenosemicarbazide Ph3CNHC(Se)
NHNH2 (2) (Scheme 1) in good yield as colourless, crystalline solid
[6]. In contrast to the isoselenocyanate, compound 2 can be kept at
room temperature in the absence of light for several weeks with-
out any sign of decomposition. Given this stability both in the solid
state and in solution, we were able to spectroscopically character-
ise this compound for the first time: The 77Se NMR spectrum shows
a singlet at 319.4 ppm, whilst the 13C NMR spectrum of 2 displays a
singlet resonance due to the C@Se carbon atom at 179.20 ppm (in
addition to the signals of the Ph3C unit). We were also able to ob-
tain X-ray quality crystals of 2 and determined the solid state
structure (Fig. 2).
delocalisation. The increase of the C–N bond distances is also con-
sistent with the change of bond order; the Ph3C–N bond distances
in 1 and 2 are essentially identical.
The reactivity of trityl isoselenocyanate with n-butylamine and
diethylamine was also investigated (Scheme 1). In the case of
n-butylamine, the selenourea derivative Ph3CNHC(Se)NHnBu (3)
was obtained as colourless solid in good yield. The compound
was spectroscopically characterised, the data being fully consistent
with the proposed structure (see Experimental). The same reaction
with the Et2NH also gave a colourless solid; however, this material
was shown by 1H NMR spectroscopy to consist of a ca. 1:1 mixture
of the selenourea Ph3CNHC(Se)NEt2 and the substitution product
Ph3CNEt2. This reactivity mimics that reported for the sulphur ana-
logue: Trityl isothiocyanate reacts with n-butylamine to give
exclusively the thiourea derivative whereas Et2NH gives the thio-
urea only in poor yield (the other component is the substitution
product, which is simply removed in the purification process). Fur-
thermore, other secondary amines such as piperidine and morpho-
line react with trityl isothiocyanate to give exclusively the
substitution products [9]. It appears that in the case of the
This compound also crystallises in the space group P21/c with
two independent molecules in the asymmetric unit. The
NHC(Se)NHNH2 unit is held in an E configuration through an intra-
molecular H-bond N(6)HÁÁÁN(5) of ca. 2.09 Å forming a five-mem-
bered quasi cyclic structure. The Se–C bond distance [1.862(6) Å]
Fig. 2. Molecular structure of one of the independent molecules of compound 2.
Ellipsoids show 50% probability levels. Only the NH hydrogen atoms are shown as
spheres with arbitrary radii. Selected bond distances [Å]: C(3)–Se(2) 1.862(6), C(3)–
N(6) 1.310(9), C(3)–N(4) 1.361(7), C(4)–N(6) 1.439(8), N(4)–N(5) 1.393(8). Selected
angles [°]: C(4)–N(6)–C(3) 129.2(7), N(6)–C(3)–N(4) 116.2(6), N(6)–C(3)–Se(2)
126.6(5).
Fig. 1. Molecular structure of one of the independent molecules of compound 1.
Ellipsoids show 50% probability levels. Hydrogen atoms have been omitted for
clarity. Selected bond distances [Å]: Se(1)–C(1) 1.740(3), C(1)–N(1) 1.149(3), N(1)–
C(2) 1.458(3). Selected bond angles [°]: Se(1)–C(1)–N(1) 178.8(2), C(1)–N(1)–C(2)
169.7(3).

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M. Ben Dahman Andaloussi, F. Mohr / Journal of Organometallic Chemistry 695 (2010) 1276–1280
O
OH
Se
Se
Ph3C
NH2
Ph3C
N
N
H
N
H
N
H
N
H
OH
(4)
Scheme 2.
selenium derivative too, substitution accompanied by release of
selenocyanate is also a competing reaction when an amine reacts
with trityl isoselenocyanate. Depending on the nature of the
amine, either exclusively addition products (selenoureas) or mix-
tures of addition and substitution products are formed.
Trityl selenosemicarbazide undergoes a condensation reaction
with salicylaldehyde to give the imine Ph3CNHC(Se)NHN@
CH(C6H4-2-OH) 4 in reasonable yield (Scheme 2).
Scheme 3.
to 90° is typical for a diselenide. This result suggests that the de-
sired condensation product from 2-pyridinecarboxaldehyde and
trityl selenosemicarbazide had formed at some point during the
reaction but this is then transformed into the diselenide which
crystallises out from the reaction mixture accompanied by other
decomposition products. Heimgartner also reported the isolation
of a diselenide obtained by recrystallisation of a 1,2,4-triazole-3-
selenone [10]. We propose that the diselenide 5 was formed by
oxidative coupling of the selenol tautomer of the selenosemicar-
bazone (Scheme 3). Unfortunately, we were not able to recover
any crystals of the compound for solution spectroscopic studies.
In summary, we have confirmed by X-ray crystallography and
by preparing derivatives with amines and hydrazine that the reac-
tion of trityl chloride with KSeCN gives trityl isoselenocyanate and
The compound’s identity could be unambiguously established
by using 1H, 13C and 77Se NMR spectroscopy (see Experimental sec-
tion for details), since no crystals suitable for X-ray diffraction
could be obtained. In contrast, under the same conditions 2-pyri-
dinecarboxaldehyde gave after work-up a dark gum which con-
tained a few large yellow crystals after standing for some time.
Evidently, significant decomposition occurs during the reaction.
The crystals however, could be studied by X-ray diffraction after
manually removing them from the gum, which revealed them to
be the diselenide 5 (Fig. 3)
ꢀ
The compound crystallises in the space group P1. Its overall
geometry i.e. the Se–Se distance [2.3300(3) Å] as well as the C–
Se–Se angles [99.56(6)°] and the C–Se–Se–C torsion angle close
Fig. 3. Molecular structure of compound 5. Ellipsoids show 50% probability levels. Hydrogen atoms have been omitted for clarity.

M. Ben Dahman Andaloussi, F. Mohr / Journal of Organometallic Chemistry 695 (2010) 1276–1280
1279
Table 1
not trityl selenocyanate. We have also examined the preparation of
Crystal data and refinement details of compounds 1, 2 and 5.
functionalised organoselenium compounds derived from trityl
selenosemicarbazide which are potential ligands. We are currently
developing this chemistry further and are examining the use of
these compounds as ligands in various metal complexes.
1
2
5
Empirical formula
Colour
C40H30N2Se2
colourless
696.58
C40H37Se2N6
colourless
759.68
C52H42Se2N8
yellow
936.86
Mr, (g molÀ1
)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/c
20.3163(6)
8.48624(18)
20.0327(6)
90
109.984(3)
90
3245.85(14)
4
monoclinic
P21/c
14.7937(8)
9.6345(5)
24.9993(13)
90
101.075(5)
90
3496.8(3)
4
triclinic
P1
ꢀ
3. Experimental
11.1128(5)
13.8305(3)
16.8335(7)
101.174(3)
99.377(4)
112.017(3)
2272.76(15)
2
3.1. General
a
(°)
1H, 13C and 77Se NMR spectra were recorded on a 400 MHz Bru-
ker Avance spectrometer. Chemical shifts are quoted relative to
external SiMe4 (1H, 13C) and SeMe2 (77Se). Elemental analyses were
performed by staff of the microanalytical laboratory of the Univer-
sity of Wuppertal. All reactions were carried out under aerobic
conditions unless stated otherwise. KSeCN was prepared as de-
scribed in the literature [11] and all other chemicals and solvents
(HPLC grade) were sourced commercially and used as received.
b (°)
c
(°)
V (Å3)
Z
Dcalc (g cmÀ3
)
1.425
2.309
1408
1.443
2.153
1548
1.369
1.672
956
l
(mmÀ1
)
F (0 0 0)
Crystal size (mm3)
0.22 Â 0.10 Â 0.11 Â 0.04 Â 0.14 Â 0.08 Â
0.02
h Range for data collection (°) 2.70–29.52
Reflections collected
Independent reflection
Abs. corr.
Max./min. trans.
Parameters
Goodness-of-fit
0.03
0.05
3.07–29.51
18330
2.97–29.51
17860
8225
empirical
1.00/0.88
436
0.842
0.0517
0.1120
19232
7677
empirical
1.00/0.60
397
0.915
0.0394
0.0968
10498
3.2. Trityl isoselenocyanate (1)
empirical
1.00/0.88
559
0.862
0.0318
0.0581
This was prepared by a slightly modified literature procedure:
Trityl chloride (7.2 g, 29.5 mmol) was added to a solution of KSeCN
(3.6 g, 34.3 mmol) in acetone (80 ml). The mixture was stirred at
room temperature for ca. 3 h. The precipitated KCl was removed
by filtration and the filtrate was evaporated to dryness. The residue
was recrystallised from hexane to afford the product as colourless
needles in 64% (6.5 g) yield. The compound should be stored in an
amber bottle in the freezer. X-ray quality crystals were selected
from the bulk sample.
R1 [I > 2r(I)]
wR2 (all data)
Largest diff. peak/hole (e ÅÀ3
)
1.021/À1.277 1.961/À0.434 0.513/À0.474
2.81 (quart., J = 6.9 Hz, 4 H, CH2 Ph3CNEt2), 3.89 (quart., J = 7.1 Hz, 5
H, CH2 Ph3CNHC(Se)NEt2), 7.13–7.48 (m, 33 H, Ph3C, NH).
3.5. Reaction of trityl selenosemicarbazide with salicylaldehyde
3.3. Trityl selenosemicarbazide (2)
To
a
solution of trityl selenosemicarbazide (0.300 g,
0.789 mmol) in hot EtOH (40 mL) was added salicylaldehyde
(0.08 mL, 0.789 mmol) and the mixture was heated to reflux for
ca. 4 h. The resulting solution was filtered and the filtrate taken
to dryness in vacuum. The resulting pale yellow solid was ex-
tracted into CH2Cl2 and passed through Celite. The filtrate was con-
centrated in vacuum and the solid which was isolated by filtration
and recrystallised from EtOH. 0.208 g (54%) of a pale yellow solid
was obtained. 1H NMR (CD3Cl) d = 6.91 (dt, J = 7.5/1.0 Hz, 1 H,
H4), 6.97 (d, J = 8.3 Hz, 1 H, H3), 7.09–7.31 (m, 19 H, Ph3C, NH,
H5, H6), 7.84 (s, 1 H, HC@N), 10.46 (s, 1 H, OH). 13C NMR (CD3Cl)
d = 74.19 (CPh3), 116.61 (C3), 118.09 (C1), 119.43 (C5), 127.14 (p-
Ph), 127.96 (Ph), 128.93 (Ph), 131.36 (C6), 131.57 (C4), 144.01
(ipso-Ph), 146.63 (C@N), 157.88 (COH), 158.53 (C@Se). 77Se NMR
(CD3Cl) d = 402.58.
This was prepared by a slightly modified literature procedure:
To a solution of tityl isoselenocyanate (0.500 g, 1.44 mmol) in
cyclohexane (15 mL) and Et2O (10 mL) was added hydrazine hy-
drate (0.1 mL). The mixture was stirred at room temperature for
ca. 10 min. The colourless solid that formed was isolated by filtra-
tion and dried. Recrystallisation from EtOH gave the product as col-
ourless crystals in 76% yield (0.415 g). 1H NMR (acetone-d6)
d = 7.21–7.37 (m, 19 H, Ph3C, NH), 9.21 (br. s, 1 H, NH2), 9.52 (br.
s, 1 H, NH2). 13C NMR (acetone-d6) d = 73.62, 127.79, 128.49,
130.35, 145.51 (CPh3), 178.62 (C@Se). 77Se NMR (acetone-d6)
d = 319.36.
3.4. 1-Trityl-3-n-butylselenourea (3)
3.6. Reaction of trityl selenosemicarbazide with 2-
pyridinecarboxaldehyde
To a solution of trityl isoselenocyanate (0.348 g, 1.00 mmol) in
cyclohexane (10 mL) was added n-butylamine (0.08 g, 1.10 mmol)
and the mixture was heated at ca. 40° for a few hours. The resulting
precipitate was isolated by filtration and recrystallised from EtOH
to give 0.190 g (45%) of a colourless solid. 1H NMR (acetone-d6)
d = 0.70 (t, J = 7.8 Hz, 3 H, CH3), 0.86 (sext., J = 7.6 Hz, 2 H, CH3CH2),
1.05 (quint., J = 7.6 Hz, 2 H, CH3CH2CH2), 3.38 (quart., J = 6.8 Hz, 2
H, CH2N), 5.71 (br. s, 1 H, NH), 7.28–745 (m, 15 H, Ph3C), 7.69 (s,
1H, NH). 13C NMR (acetone-d6) d = 13.86, 20.15, 31.33, 49.22
(nBu) 74,72, 128.79, 129.37, 129.53, 143.85 (CPh3), 180.61 (C@Se),
77Se NMR (acetone-d6) d = 245.52.
The same reaction carried out with Et2NH gave a ca. 1:1 mixture
(before recrystallisation) of the selenourea and the substitution
product Ph3CNEt2, as determined by 1H NMR spectroscopy and
comparison with an authentic sample of Ph3CNEt2. Data for the
mixture is given here: 1H NMR (acetone-d6) d = 1.15 (t, J = 6.9 Hz,
6 H, CH3 Ph3CNEt2), 1.26 (t, J = 7.3 Hz, 7 H, CH3 Ph3CNHC(Se)NEt2),
To
a
solution of trityl selenosemicarbazide (0.300 g,
0.789 mmol) in EtOH (10 mL) was added 2-pyridinecarboxalde-
hyde (slight excess) and the mixture was heated to reflux for ca.
4 h. The resulting dark-yellow solution was filtered and the filtrate
was concentrated in vacuum. On cooling a dark gum remained
which was found to contain some large yellow crystals after stand-
ing for a few days. The crystals were manually separated from the
gum, cleaned and subsequently used for the X-ray diffraction
experiment.
3.7. X-ray crystallography
Diffraction data were collected at 170 K using an Oxford Diffrac-
tion Gemini E Ultra diffractometer, equipped with an EOS CCD area

1280
M. Ben Dahman Andaloussi, F. Mohr / Journal of Organometallic Chemistry 695 (2010) 1276–1280
detector and a four-circle kappa goniometer. For the data collec-
tion the Mo source emitting graphite-monochromated Mo K radi-
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2010.02.021.
a
ation (k = 0.71073 Å) was used. Data integration, scaling and
empirical absorption correction was carried out using the CrysAlis
Pro program package [12]. The structure was solved using Direct
Methods and refined by Full-Matrix-Least-Squares against F2. The
non-hydrogen atoms were refined anisotropically and hydrogen
atoms were placed at idealised positions and refined using the rid-
ing model. All calculations were carried out using the program
Olex2 [13]. Important crystallographic data and refinement details
are summarised in Table 1.
References
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[4] D.H.R. Barton, S.I. Parekh, M. Tajbakhsh, E.A. Theodarakis, C.L. Tse, Tetrahedron
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Acknowledgements
ˇ
[5] E. Bulka, K.D. Ahlers, E. Tucek, Chem. Ber. 100 (1966) 1367–1372.
[6] C.T. Pedersen, Acta Chem. Scand. 17 (1963) 1459–1461.
[7] H. Rheinboldt, H.V. de Campos, J. Am. Chem. Soc. 72 (1950) 2784.
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(2008) 1434–1438.
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(2005) 698–708.
We thank RETORTE GmbH, Germany for a generous donation
of selenium metal and the DFG for a grant to purchase the
diffractometer.
Appendix A. Supplementary material
[12] CrysAlis Pro version 171.33.42, Oxford Diffraction Ltd. (2009).
[13] O.V. Dolomanov, L.J. Bourhis, R.J. Gildea, J.A.K. Howard, H. Puschmann, J. Appl.
Cryst. 42 (2009) 339–341.
CCDC compounds 1, 2 and 5 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of