Nonisomorphous x-ray structures of tritylnitrile and tritylisonitrile

Article abstract of DOI:10.1007/s10870-012-0345-2

The crystal structures of a related series of isosteric compounds, namely tritylnitrile 1, tritylisonitrile 2, and a solid solution of tritylnitrile/tritylisonitrile 3 have been determined by single crystal X-ray diffraction. The bond lengths, angles, and

Full text of DOI:10.1007/s10870-012-0345-2

J Chem Crystallogr
DOI 10.1007/s10870-012-0345-2
ORIGINAL PAPER
Nonisomorphous X-Ray Structures of Tritylnitrile
and Tritylisonitrile
Kelsey M. Skodje
Maria O. Miranda
Daron E. Janzen
•
Lindsay M. Hinkle
Kent R. Mann
•
•
•
Received: 24 February 2012 / Accepted: 13 June 2012
Springer Science+Business Media, LLC 2012
ꢀ
˚
˚
Abstract The crystal structures of a related series of
isosteric compounds, namely tritylnitrile 1, tritylisonitrile
b = 9.5739(8) A, c = 17.8251(11) A, b = 108.5920(10)ꢁ,
3
V = 2917.4(4) A , Z = 8, R = 0.0587, wR = 0.1734.
˚
1
2
2
, and a solid solution of tritylnitrile/tritylisonitrile 3 have
been determined by single crystal X-ray diffraction. The
bond lengths, angles, and intermolecular interactions do
not deviate significantly from previously reported nitriles
and isonitriles. These isosteric compounds pack in non-
isomorphous but very similar structures. Subtle differences
between the packing of 1 and 2 are detailed. The solid
solution of tritylnitrile/tritylisonitrile 3 is isomorphous with
the structure of tritylnitrile 1. This solid solution possesses
substitutional disorder involving the nitrile and isonitrile
Keywords Nitrile ꢀ Isonitrile ꢀ Nonisomorphous ꢀ
Isosteric ꢀ X-Ray crystallography
Introduction
The aromatic isonitrile trityl isonitrile (1,1,1-triphe-
nylmethylisonitrile) has been the focus of several organic
transformations [1–7] as well as a several organometallic
studies involving its role as a ligand [8–10]. However, only
one crystallographically characterized example of a metal
complex incorporating tritylisonitrile as a ligand has been
reported [8] and no structural information is reported for
this isonitrile [11]. The sterically demanding nature of
tritylisonitrile as a ligand was attractive for use in our
0
groups (Z = 2, nitrile/isonitrile occupancy ratios 53.7/46.3
and 54.6/45.4). The structure of tritylisonitrile 2 possesses
0
positional disorder over two sites (Z = 2, occupancy ratio
of 93.1/6.9). A comparison with previously reported nearly
isosteric structures reveals that other chemically dissimilar
compounds crystallize in each of the two molecular pack-
ing arrangements described in this report. Structure 1:
8
investigations involving square planar d metal complexes
˚
monoclinic, P2 /c, a = 18.049(11) A, b = 9.583(6) A,
˚
3
where prevention of metal–metal interactions was desired.
While exploring reactions involving tritylisonitrile, the
room temperature isomerization of tritylisonitrile to trityl-
nitrile in acetonitrile solution was observed. From the fil-
trate of a reaction containing tritylisonitrile, crystals grew
of the tritylnitrile.
1
˚
˚
c = 17.824(11) A, b = 108.680(10)ꢁ, V = 2921(3) A ,
Z = 8, R = 0.0467, wR = 0.1491. Structure 2: triclinic,
1
2
˚
P 1, a = 9.5877(13) A, b = 10.7479(14) A, c = 14.809(2)
˚
˚
A, a = 88.457(2)ꢁ, b = 80.303(2)ꢁ, c = 77.128(2)ꢁ,
3
V = 1466.4(3) A , Z = 4, R = 0.0423, wR = 0.1151.
˚
1
2
˚
Structure 3: monoclinic, P2 /c, a = 18.03464(15) A,
The ability of isonitriles to isomerize to nitriles was
reported as early as 1873 [12]. Others have studied this
type of isomerization, including specifically the trityliso-
nitrile to tritylnitrile transformation [5, 13–15], although in
typical organic solvents this process is normally very slow
at room temperature [13]. Only more recently have studies
shown that this isomerization process can happen more
quickly at room temperature in polar solvents such as
acetonitrile and nitromethane [13]. Kinetic studies of iso-
nitrile isomerization have demonstrated strong effects on
1
K. M. Skodje ꢀ D. E. Janzen (&)
Department of Chemistry and Biochemistry, St. Catherine
University, 2004 Randolph Avenue, St. Paul, MN 55105, USA
e-mail: dejanzen@stkate.edu
L. M. Hinkle ꢀ M. O. Miranda ꢀ K. R. Mann
Department of Chemistry, University of Minnesota, 207 Pleasant
St. SE, Minneapolis, MN 55455, USA
123

J Chem Crystallogr
the rate by factors such as polarity of solvents [13, 15],
addition of salts [13], and addition of catalysts [13]. Of
particular note was the observation that no detectable
isomerization of tritylisonitrile was observed in nonpolar
solvents including cyclohexane [13].
C N
N C
Interest in the crystallographic nature of isomeric nitrile/
isonitrile pairs has led to several studies [16–21] where in
some (but not all) instances these isosteric pairs crystallize
in an isomorphous fashion. Although the exchange of
carbon and nitrogen (nearest neighbors in the Periodic
Table) in the nitrile/isonitrile isomeric pairs is a small
change in terms of molecular shape and volume, the dipole
in the two arrangements is significantly altered. A recent
search of the Cambridge Structural Database (CSD) [22] (v
2
1
Scheme 1 Lewis structures of tritylnitrile (1) and tritylisonitrile (2)
spectrum of crystals of 2 indicated the presence of only
-
1
tritylisonitrile (2,124 cm ). The compound tritylnitrile, 1,
was prepared by isomerization of tritylisonitrile in an
acetonitrile/diethyl ether solution at room temperature over
the course of 1d. The infrared spectrum of crystals of 1
5
.32 including updates through Nov. 2011) for identically
substituted mono-nitrile/mono-isonitrile isomeric pairs
revealed six examples in which at least one polymorph of a
given nitrile or isonitrile has been found to be isomorphous
-
1
indicated the presence of only tritylnitrile (2,234 cm ).
Crystals obtained for structure 3 (solid solution of trityl-
isonitrile and tritylnitrile) indicated the presence of both
1
with its isomeric counterpart [16, 20, 21, 23–29] . Five
additional examples were found for which no isomorphous
-
1
-1
nitrile/isonitrile pairs have been reported [18, 19, 26, 30–
2
5] . All but one of these eleven were aromatic nitrile/
isonitrile (2,125 cm ) and nitrile (2,234 cm ).
3
isonitrile pairs, and all were nearly planar.
X-Ray Analysis
In this paper, we report three X-ray crystal structures
obtained in this study of tritylnitrile (1) and tritylisonitrile
X-ray quality crystals of 1 were grown by slow evaporation
of an acetonitrile/diethyl ether solution of 2. A crystal of 2
suitable for X-ray diffraction was grown by slow diffusion
of diethyl ether into a cyclohexane solution of 2. Crystals
of 3 were obtained by slow evaporation of an NMR sample
of 2 in CD Cl . Table 1 contains crystal data, collection
(
2). (Scheme 1) The common and unique features of these
structures are analyzed and compared with other triphen-
ylmethyl analogues that crystallize in an isomorphous
fashion with either 1 or 2.
2
2
parameters, and refinement criteria for crystal structures 1,
2 and 3. For each of these structures, crystals were placed
on the tip of a 0.1 mm diameter glass capillary and X-ray
intensity data were measured at low temperature (173(2)
K) with graphite monochromated Mo Ka radiation
Experimental
Materials and Measurements
˚
The compound tritylisonitrile, 2, was prepared from the
corresponding tritylamine via an intermediate formamide
derivative using a published method [36]. The infrared
(k = 0.71073 A) on a Bruker SMART Platform CCD
diffractometer [37]. Preliminary sets of cell constants were
calculated from reflections harvested from three sets of 20
frames. These initial sets of frames were oriented such that
orthogonal wedges of reciprocal space were surveyed. This
produced initial orientation matrices determined from
several reflections (1 145 reflections, 2 81 reflections, 3 107
reflections). The frame time for the collections varied (1
20 s, 2 60 s, 3 20 s). For each determination, a randomly
oriented region of reciprocal space was surveyed to the
extent of 1.5 hemispheres (1 and 3) or one sphere (2) to a
1
Isomorphous nitrile/isonitrile pairs with CCDC refcodes: 2,4,6-
trimethylphenyl isocyanide (MOPJUW) [23], 2,4,6-trimethylcyano-
benzene (MESITN) [24], p-decylphenyl isocyanide and p-decylphe-
nylnitrile (EVOSEN and EVOSIR) [16], p-isocyanoaniline (LEGROE)
[20], p-cyanoaniline (BERTOH) [25], 4-iodophenylisonitrile
(IBZICN) [26], 4-iodobenzonitrile (IOBNIT) [27], pentafluoropheny-
lisonitrile (KUMXOF) [28], pentafluorobenzonitrile (HUFVEJ) [29],
trifluorovinylisocyanide and trifluoroacrylonitrile (MELFOY and
MELFIS) [21].
˚
˚
2
resolution of 0.84 A (1 and 3) or 0.77 A (2). Three major
sections (1 and 3) or four sections (2) of frames were
collected with 0.30ꢁ steps in x at three different u settings
and a detector position of -28ꢁ in 2h. The intensity data
were corrected for absorption and decay (SADABS [38]).
Final cell constants were calculated from the xyz centroids
of strong reflections (1 2,988 reflections, 2 4,054 reflections,
Nonisomorphous nitrile/isonitrile pairs with CCDC refcodes: 2,4,6-
tribromobenzonisonitrile (TBZINT) [19], 2,4,6-tribromobenzonitrile
(
BRBZNT) [30], 4-bromobenzoisonitrile (BBIZCN) [26], 4-bro-
mobenzonitrile (BRBNIT) [31], 2,6-xylylisocyanide (EBONAK)
32], 2,6-dimethylbenzonitrile (JIBDON) [33], p-nitrophenylisocya-
[
nide (EVOSAJ) [34], p-nitrobenzonitrile (PNBZNT) [35], 2,4,6-
trichlorophenylisonitrile and 2,4,6-trichlorobenzonitrile (FUGVAE
and CLBECN03) [18].
1
23

J Chem Crystallogr
Table 1 Crystal data
Compound
1
2
3
Formula
C
20 15
H N
C
20 15
H N
C
20 15
H N
M(g/mol)
269.33
173(2)
269.33
173(2)
269.33
173(2)
Temperature (K)
Crystal system
Space group
Monoclinic
P2 /c
Triclinic
Monoclinic
P2 /c
Pꢀ1
1
1
˚
a (A)
18.049(11)
9.583(6)
17.824(11)
90
9.5877(13)
10.7479(14)
14.809(2)
88.457(2)
80.303(2)
77.128(2)
1466.4(3)
4
18.0364(15)
9.5739(8)
17.8251(11)
90
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
108.680(10)
90
108.5920(10)
90
3
˚
V (A )
2921(3)
8
2917.4(4)
8
Z
3
(g/cm )
D
c
1.225
1.220
1.226
h
min–hmax (ꢁ)
1.19–25.04
0.071
1.40–27.52
0.071
2.32–25.03
0.071
-
1
l (mm
)
Collected reflections
Unique reflections
20,811
5,164
17,423
27,979
5,165
6,615
R
int
0.0271
3,857
0.0278
0.0945
3,774
Observed reflections
Data/restraints/parameters
S
5,622
5164/0/380
1.092
6615/0/540
1.024
5165/8/393
1.074
Final R indices [I [ 2r(I)]
R indices (all data)
CCDC #
R
R
1
1
= 0.0467, wR
= 0.0624, wR
2
2
= 0.1362
= 0.1491
R
R
1
1
= 0.0423, wR
= 0.0501, wR
2
2
= 0.1087
= 0.1151
R
R
1
1
= 0.0587, wR
= 0.0755, wR
2
2
= 0.1569
= 0.1734
867,304
867,305
867,306
3 3,610 reflections) from the actual data collection after
integration (SAINT [37]).
Each structure was solved and refined using SHELXL-
7 [39]. A direct-methods solution was calculated that
9
provided most of the non-hydrogen atoms from the E-map.
Full-matrix least-squares/difference Fourier cycles were
performed that located the remaining non-hydrogen atoms.
All non-hydrogen atoms (except some atoms of the minor
disorder component of 2) were refined with anisotropic
displacement parameters. All hydrogen atoms were placed
in ideal positions and refined as riding atoms with relative
isotropic displacement parameters.
Fig. 1 Asymmetric unit of tritylnitrile 1 showing anisotropic dis-
placement ellipsoids at the 50 % probability level
Structure of Tritylnitrile (1)
A plot of 1 showing atom numbering and anisotropic dis-
placement ellipsoids is shown in Fig. 1. The structure of
compound 1 was originally refined as tritylisonitrile as the
crystals were harvested from a filtrate of a reaction using
tritylisonitrile as a reactant. The thermal parameters of the
isonitrile, as indicated by the Hirshfeld test [40] were
unusual and the residual electron density peaks were too
-
3
˚
large for an organic structure (0.65e /A ). The refinement
stalled at R = 6.5 %, although the quality of the data
1
indicated a better model should be possible. Upon refining
the structure as tritylnitrile, the thermal parameters of the
nitrile became reasonable, residual electron density peaks
diminished, and R fell to 4.7 %. The infrared spectrum of
1
123

J Chem Crystallogr
the crystals confirms that only nitrile is present in the
structure.
The asymmetric unit consists of two tritylnitrile mole-
cules. Pseudosymmetry appears to be present. An example is
the apparent presence of an inversion center; however the
location of the pseudoinversion is untenable (fractional
coordinates 0.752, 0.132, 0.253). No space group changes
were recommended by the ADDSYM [41, 42] utility of
PLATON [43, 44]. No restraints were used in the refinement.
Structure of Tritylisonitrile (2)
A plot of 2 showing atom numbering and anisotropic dis-
placement ellipsoids is shown in Fig. 2. During investiga-
tion of the isonitrile structure 2, the simplest models of the
Fig. 2 Asymmetric unit of tritylisonitrile 2 showing anisotropic
displacement ellipsoids at the 50 % probability level
0
Z = 2 asymmetric unit led to refinements that stalled for
models involving either two nitriles (R = 7.60 %), two
1
isonitriles (R = 7.05 %), or disorder models in which the
1
two molecules of the asymmetric unit were allowed to
freely vary as nitrile/isonitrile (as a pair) (R = 6.94 %,
1
2
5 % nitrile/75 % isonitrile), all with similar Hirshfeld test
concerns. Analysis of residual electron density not
accounted for in these models led us to search for further
issues. A second minor component composed of a similar
0
Z = 2 asymmetric unit (modeled as isonitrile) partially
overlapping the major component was located. Several
EADP constraints were needed to model this low occu-
pancy component, as many atoms in the minor component
are located in close proximity to the major component
atom positions. In a final step, the minor components were
kept as isonitriles and several models were attempted to
verify the assignment of the major component functional
group as isonitrile. Models with two nitriles in the major
component (R = 4.87 %), C1A-N1A switched from iso-
1
nitrile to nitrile with C1B-N1B remaining as isonitrile
(
R1 = 4.55 %), and C1B-N1B switched from isonitrile to
nitrile with C1A-N1A remaining as isonitrile
R1 = 4.71 %) were all investigated. Hirshfeld concerns
(
with each of these models were present. The final model
containing only isonitriles (major component 93.1 %/
minor component 6.9 %) yielded the best refinement as
Fig. 3 Diagram showing the relative positions of the major and
minor disorder components in the structure of tritylisonitrile 2.
Isonitrile carbons C1A and C1B (lighter shade) indicate the
asymmetric unit molecules of the major component. Isonitrile carbons
C1C and C1D (darker shade) indicate the asymmetric unit molecules
of the minor component
gauged by the R = 4.67 %, small residual electron density
1
-
3
˚
0.279e /A ), and best Hirshfeld test results of all modeled
(
refinements. The infrared spectrum of the crystals confirms
only isonitrile is present in the structure.
The relative positions of the asymmetric units of the
major and minor disorder components of 2 are shown in
Fig. 3. These two positions of the asymmetric units are
related by a translation of ‘ along the a-axis. Additionally,
within the asymmetric unit of structure 2, pseudosymmetry
is present. The two molecules of the asymmetric unit are
Structure of Solid Solution of Tritylnitrile/
Tritylisonitrile (3)
Upon obtaining the unit cell of this crystal, it was observed
that this cell was nearly identical to that of the tritylnitrile
related by a pseudo 2 screw axis roughly parallel with the
1
a-axis at 0.75b, 0.5c.
1
23

J Chem Crystallogr
0
. The structure solution yielded a Z = 2 asymmetric unit
1
with essentially the same orientation of the two trityl
moieties as in the nitrile structure 1 (with either nitriles or
isonitriles). Very similar R statistics and Hirshfeld test
concerns were observed for models where both molecules
1
were assigned as nitriles (R = 6.31 %) or as isonitriles
1
(
R1 = 6.48 %). The best solution was achieved by allow-
ing each molecule of the asymmetric unit to refine with a
unique free variable as a disordered nitrile/isonitrile pair.
The nitrile/isonitrile ratio for the molecule containing
0
0
atoms C1–N1(nitrile) and C1 –N1 (isonitrile) was refined
as 53.6/46.4 (esd 1.7). The nitrile/isonitrile ratio for the
Fig. 4 Asymmetric unit of tritylnitrile/tritylisonitrile solid solution 3
showing anisotropic displacement ellipsoids at the 50 % probability
level
0
molecule containing atoms C21–N2(nitrile) and C21 –
0
N2 (isonitrile) was refined as 54.7/45.3 (esd 1.9). Although
the errors associated with these disorder ratios indicate the
ratios to be the same within the error calculated, this model
with independently refined disorder free variables for each
molecule of the asymmetric unit was retained as there is no
independent evidence that the disorder ratios are identical.
To achieve this stable solution, the thermal parameters of
untenable (fractional coordinates 0.752, 0.132, 0.253). No
space group changes were recommended by the ADDSYM
utility of PLATON.
0
atom pairs C1 and N1 (nitrile carbon and isonitrile nitro-
0
Results and Discussion
gen) and N1 and C1 (nitrile nitrogen and isonitrile carbon)
were fixed to be identical in a pairwise fashion. The same
0
Intramolecular Features
thermal parameter treatment was used for C21 and N2 as
0
well as N2 and C21 pairwise. Also, the nitrile distances
˚
Table 2 contains selected bond distances and angles of
interest. The bond distances and angles observed for both
nitrile and isonitrile moieties are typical values for these
functional groups. The nitrile groups of structure 1 have
were fixed for C1–N1 and C21–N2 (1.15 A), the isonitrile
0
0
0
0
˚
distances were fixed for C1 –N1 and C21 –N2 (1.15 A),
the distances were fixed for the bonds adjacent to the
˚
nitriles C1–C2 and C21–C22 (1.49 A), and the distances
˚
bond distances of 1.145(2) A (N1–C1) and 1.146(2)A (N2–
˚
0
were fixed for the bonds adjacent to the isonitriles C2–N1
C21). The isonitrile groups of structure 2 display bond
0
˚
˚
˚
and C22–N2 (1.47 A). The DFIX command in SHELXL
was used for these bond distance restraints with a standard
uncertainty of 0.02. These fixed bond distance values were
chosen based on the structures of the pure nitrile 1 and pure
isonitrile 2. Attempts to remove the thermal parameter
constraints resulted in Hirshfeld test concerns. Unreason-
able bond distances and an unstable refinement resulted
when the bond length restraints were removed. The final
distances of 1.150(2) A (N1A–C1A) and 1.153(2) A (N1B–
C1B). No comparisons will be made with the minor dis-
order component of structure 2 as the low occupancy yields
larger errors in the bond lengths. Due to the nature of the
refinement of structure 3, which involved restraining the
isonitrile and nitrile bond lengths, no specific bond length
comparisons should be made. For comparison to previously
determined structures, a careful analysis of the CSD was
conducted. Of the 104 structures of unligated organic iso-
model refined with a value of R = 5.87 %. The infrared
1
˚
nitriles, the average carbon nitrogen distance is 1.151 A. A
spectrum of the crystals confirms both isonitrile
-
1
-1
(
2,125 cm ) and nitrile (2,234 cm ) are present in the
structure.
The relative positions of the major and minor disorder
similar analysis of 10,690 structures of unligated organic
˚
nitriles yields an average value of 1.141 A.
The bonds adjacent to the nitrile and isonitrile groups
deviate further from previously measured values in analo-
gous structures. The bond distances of the carbon–carbon
bonds adjacent to the nitrile group in structure 1 are
components of 3 are shown in Fig. 4. The final model of
this solid state solution has the trityl group atom positions
of each part of the asymmetric unit in identical locations
for each of the disorder components (isonitrile and nitrile).
Only the carbon and nitrogen atoms involved in the nitrile
and isonitrile differ for each molecule in the asymmetric
unit.
˚
˚
1.489(2) A (C21–C22) and 1.490(2) A C1–C2. The bond
distances of the nitrogen–carbon bonds adjacent to the
˚
isonitrile group in structure 2 are 1.473(5) A (N1A–C2A)
˚
and 1.470(2) A (N1B–C2B). Due to the nature of the
Pseudosymmetry appears to be present in the structure
of 3. An example is the apparent presence of an inversion
center; however, the location of the pseudoinversion is
refinement of structure 3, which involved restraining the
bonds adjacent to the isonitrile and nitrile groups, no spe-
cific bond length comparisons should be made. The same
123

J Chem Crystallogr
˚
Table 2 Selected bond lengths (A) and bond angles (ꢁ) for structures 1, 2, and 3
a
2
1
3
Bond distances
Nitriles
C1–N1
C21–N2
–
1.145(2)
1.146(2)
–
C1–N1
1.148(13)
1.158(14)
1.149(17)
1.159(18)
1.487(14)
1.490(14)
–
C21–N2
0
0
Isonitriles
N1A–C1A
N1B–C1B
N1A–C2A
N1B–C2B
1.150(2)
1.153(2)
1.473(5)
1.470(2)
N1 –C1
0
0
–
N2 –C21
C1–C2
Adjacent bonds (C–C or C–N)
C1–C2
C21–C22
1.490(2)
1.489(2)
C21–C22
Bond angles
Nitriles
N1–C1–C2
179.6(2)
179.8(2)
–
N1–C1–C2
171(3)
N2–C21–C22
–
N2–C21–C22
0 0
C1 –N1 –C2
173(4)
174(4)
171(4)
Isonitriles
–
–
C1A–N1A–C2A
C1B–N1B–C2B
179.4(1)
179.7(2)
0
0
C21 –N2 –C22
a
Minor component angles and distances excluded from this table
Not applicable
–
CSD analysis as previously described provides an average
carbon–carbon bond distance adjacent to the nitrile moiety
˚
of 1.439 A, while the analogous average carbon–nitrogen
minimizes the strength of such possible interactions [45].
No face-to-face aromatic p-p interactions are found in the
structure of 1, but several short edge-to-face C–Hꢀꢀꢀp
interactions are present. The shortest among these C–Hꢀꢀꢀp
˚
bond distance adjacent to the isonitrile moiety is 1.422 A.
˚
All of these structures have nearly linear nitrile or iso-
nitrile bond angles. The nitriles of structure 1 have angles
of 179.6(2)ꢁ (N1–C1–C2) and 179.8(2)ꢁ (N2–C21–C22).
The isonitriles of structure 2 have angles of 179.4(1)ꢁ
distances are 2.700 A (C28–H28AꢀꢀꢀC37(2-x,0.5 ? y,
˚
0.5-z)), 2.736 A (C16–H16AꢀꢀꢀC7(1-x, -0.5 ? y, 0.5-z))
˚
and 2.780 A (C31–H31AꢀꢀꢀC25(2-x,-y,1-z)).
In the isonitrile structure 2, the closest parallel and
coplanar isonitrile groups (related by inversion) are found
(
C1A–N1A–C2A) and 179.7(2)ꢁ (C1B–N1B–C2B). The
˚
at a perpendicular distance of 4.558(2) A. These isonitriles
average angles of these groups from the CSD analysis
yielded 178.04ꢁ (nitrile) and 177.65ꢁ (isonitrile). The bond
angles of both the nitriles (N1–C1–C2 171(3)ꢁ, N2–C21–
are oriented such that the angle N1A–C1AꢀꢀꢀC1A(1-x,
1-y,-z) is 92.5(1)ꢁ. This arrangement is very similar to that
of 1. Again, possible dipolar interactions are likely weak
due to the distance and orientation of the isonitrile groups.
Like the nitrile structure, structure 2 has no face-to-face
aromatic p-p interactions, but several short edge-to-face C–
Hꢀꢀꢀp interactions are present. The shortest among these C–
0
0
0
C22 173(4)ꢁ) and isonitriles (C1 –N1 –C2 174(4)ꢁ, C21 –
0
N2 –C22 171(4)ꢁ) of structure 3 show more deviation from
linearity and larger standard errors compared with 1 and 2.
The bond length restraints used in the refinement of 3 are
likely related to these deviations.
˚
˚
HꢀꢀꢀC distances are 2.694 A (C20–H20AꢀꢀꢀC13A), 2.731 A
˚
Intermolecular Features
(C8A–H8AAꢀꢀꢀC5B(1-x, y, z)) and 2.786 A (C11B–
H11BꢀꢀꢀC17B (1-x,2-y,1-z)).
The intermolecular features of all these structures include
dipolar interactions and C–H…p (edge-to-face) interac-
tions of the nitriles or isonitriles.
As the solid solution of tritylnitrile/tritylisonitrile, 3,
adopts a structure isomorphous with 1, the intermolecular
interactions are the same as those observed in 1.
The orientation of the trityl moieties precludes close
interactions between nitrile groups in the nitrile structure 1.
The closest parallel and coplanar nitrile groups (related by
inversion) are found at a perpendicular distance of 4.586(3)
Packing Features
The structure 3, which includes both nitrile and isonitrile
disorder components, is isomorphous with the nitrile
structure 1. Of note are the nitrile/isonitrile ratios deter-
mined by disorder modeling as 53.6/46.4 and 54.7/45.3.
The major nitrile component of 3 seems to dominate the
structure, and this particular solid solution crystallizes in
the same fashion as the pure nitrile structure 1. This mix-
ture of isonitrile and nitrile cocrystallized from a solution
˚
A. These nitriles are oriented such that the angle C1–
N1ꢀꢀꢀN1(1-x,1-y,-z) is 91.8(1)ꢁ. This pairwise orienta-
tion may arise from attractive dipolar interactions, although
the dipoles are significantly displaced along the C–N bond
vectors from positions of maximum attraction, assuming
equal and opposite charges localized on the C and N
atoms [16]. The distance between these nitriles probably
1
23

J Chem Crystallogr
of tritylisonitrile in CD Cl (NMR sample). The isomeri-
2
2
zation from isonitrile to nitrile occurred at an appreciable
rate as this solvent is sufficiently polar; however, as
CD Cl has a low boiling point, the isomerization was
2
2
incomplete before crystallization occurred.
The structure of pure isonitrile 2, however, is not iso-
morphous with the structure of pure nitrile 1. For the
purposes of this discussion, only the major disorder com-
ponent of 2 will be considered. Although the space group
and unit cells are different for 1 and 2, these structures are
strikingly similar. From an overall perspective, comparison
of the intermolecular features of structures 1 and 2 suggest
the strengths of the intermolecular forces in these structures
are very similar. The molecular volumes of 1 and 2 (1
3
˚
65.1, 2 366.5 A ) and the average equivalent isotropic
3
thermal displacement parameters (non-H atoms) of each
3
2
structure are both remarkably similar (1 41, 2 35 A 9 10
˚
2
3
˚
major disorder component only, 40 A 9 10 both disorder
components). The melting points of these substances are
also comparable (1 127–128 ꢁC, 2 130–133 ꢁC). Only
upon careful inspection of these structures can the differ-
ences in packing be identified. Structures 1 and 2 both
show packing of isonitriles or nitriles in columns. These
columns pack in a hexagonal fashion (see Fig. 5a (1), b
(
2)). In structure 1, the columns extend parallel with the
b-axis, while in structure 2 these columns run along the
a-axis. Unit cell views looking down these columns (down
the b-axis of 1 and the a-axis of 2) appear nearly identical.
The difference in packing in 1 and 2 is observed most
clearly by viewing these columns perpendicular to the
stacking direction (for 1, view nearly along the a-axis,
Fig. 6a; for 2, edge-on view of a plane that includes the bc
face-diagonal, Fig. 6b). In the structure of 1, the columns
are translated  along the b-axis from the neighboring
columns. The pattern alternates ? b and -b between
the columns, forming an ABAB pattern of translation of
columns along the b-axis. In the structure of 2, there is no
translation of neighboring columns along the stacking axis
a. The pattern of translation of columns along the stacking
a-axis can be described as AAAA.
Comparison of the different packing arrangements
found in this series of structures with other related isosteric
structures yields surprising results. As the tritylhalides tri-
tylchloride, tritylbromide, and trityliodide are nearly isos-
teric with trityl nitrile and tritylisonitrile, a comparison
with these reported structures can be made. None of the
reported trityl halide structures are isomorphous with either
the tritylnitrile structure 1 or tritylisonitrile structure 2. A
monoclinic polymorph of trityl iodide is reported [46], and
two unique triclinic polymorphs are reported for trityl-
chloride [47], yet none of these match the packing of 1 or
Fig. 5 Packing diagram of the structure of tritylnitrile 1 (a) and
tritylisonitrile 2 (b) including the unit cell axes. Structure 1 view is
down the b-axis. Structure 2 view is down the a-axis. Each column is
surrounded by six neighboring columns. Shades of molecules indicate
symmetry equivalence
packing has been reported for the trityliodide. Upon
searching the unit cell dimensions of 1 and 2 using the
CSD, additional molecular structures isomorphous with 1
and 2 were discovered. Triphenylphosphine sulfide and
triphenylphosphine selenide each have a reported triclinic
structure isomorphous with the tritylisonitrile structure 2
2. Interestingly, a hexagonal polymorph is common to
tritylbromide [48] and tritylchloride [49], but no such
123

J Chem Crystallogr
The nonisomorphous behavior observed in the structures
of tritylnitrile 1 and tritylisonitrile 2 is unlikely due to
steric differences, as these are essentially isosteric mole-
cules. The temperatures of crystal growth and the crystal
data collection were identical as well. Without detailed
calculations or modeling of these structures, a few possible
reasons why the structures of 1 and 2 are nonisomorphous
can be proposed. The crystallization solvents were differ-
ent, which could strongly influence the polymorphic out-
come of the structures. Crystals of 2 were obtained from a
cyclohexane solution of 2, in which isomerization to 1 was
prohibited, but crystals of 1 were obtained by evaporation
of an acetonitrile/diethyl ether solution of 2, in which
isomerization to 1 was allowed. The differences in dipolar
interactions possible for nitrile and isonitrile could be a
driving factor in these different packing arrangements;
however, the reports of related isomorphous structures with
significantly different dipolar interactions (triphenylphos-
phine sulfide, triphenylphosphine selenide, triphenylphos-
phine methylene) argue against large dipolar interaction
influences on the structures. Also, the minor disorder
component in the structure of 2 may play a role in stabi-
lizing this unique packing arrangement of 2 compared with
1
. In the absence of other stronger or competitive inter-
molecular forces (such as hydrogen bonding or face-to-face
aromatic p–p stacking) to dominate packing, the different
solvents used for crystallization are the most likely source
of this nonisomorphous behavior.
Supplementary Material
Fig. 6 Packing diagram of the structure of tritylnitrile 1 (a) and
tritylisonitrile 2 (b) including the unit cell axes. Structure 1 view has
the same collection of packed molecules as Fig. 5a, but with a view
Crystallographic data (excluding structure factors) have
been deposited at the Cambridge Crystallographic Data
Centre as supplementary publication numbers CCDC
867304 (1), 867305 (2), and 867306 (3). These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/
retrieving.html.
o
down the a-axis (90 rotation around the horizontal dimension).
Structure 2 view has the same collection of packed molecules as
Fig. 5b, but with an edge-on view of a plane that includes the bc face-
diagonal. Shades of molecules indicate symmetry equivalence
Acknowledgments This work was supported by a 3M Faculty/
Student Collaborative Research Grant through the Center of Excel-
lence for Women, Science and Technology, St. Catherine University.
The authors thank Dr. Victor G. Young Jr., Director of the X-ray
Crystallographic Laboratory of the Department of Chemistry, Uni-
versity of Minnesota, and Professor Doyle Britton of the Department
of Chemistry, University of Minnesota for assistance.
[
50, 51], and a reported monoclinic structure isomorphous
with the tritylnitrile structure 1 [52, 53]. In addition, a
structure reported of a solid solution of triphenylphosphine
sulfide/triphenylphosphine oxide [54] is isomorphous with
that of tritylisonitrile 2. The majority component is the
triphenylphosphine sulfide. Interestingly, none of the
multiple polymorphs reported of pure triphenylphosphine
oxide are isomorphous with 1 or 2. One final structure,
triphenylphosphine methylene [55], was found to be iso-
morphous with the tritylnitrile structure 1. Although atom
types, bond types, dipoles, and other features vary con-
siderably amongst these structures, the common feature of
these structures found to be isomorphous with either 1 or 2
is that they are all nearly isosteric with 1 or 2.
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