Preparation, x-ray structure, and dynamic solution behaviour of N,N′,N′-tris(2,6-diisopropylphenyl)-guanidine, and its reaction with molybdenum carbonyl

Article abstract of DOI:10.1139/v00-142

The reaction of N,N′-bis(2,6-diisopropylphenyl)carbodiimide with lithium 2,6-diisopropylanilide. quenching with water and recrystallization from heptane produces the symmetric guanidine [DipNH]₂C=NDip which crystallizes in the triclinic system, space group P1, a = 10.6513(11), b = 10.8997(11), c = 16.2961(17) A, α = 80.524(12), β = 78.921(13), γ = 70.060(12)°, V = 1735.2(3) A³, Z = 2. The molecule crystallizes with three perpendicular 2,6-diisopropylphenyl groups, which surround and shield the central CN₃ unit, and provide (almost) three-fold symmetry around the central atom. Its dynamic solution behaviour has been studied by VT NMR between -90 and +180°C, and is consistent with three distinct barriers to N - C_Ar rotation. Preliminary estimates of the Gibbs free energy of activation for the lower two barriers are 56 ± 2 and 73 ± 2 kJ mol^-1. Reaction of the title compound with Mo(CO)₆ in refluxing n-heptane produces [DipNH]₂,C=NDip·Mo(CO)₃, a complex in which Mo(CO)₃ is η⁶-coordinated to one of the diisopropylphenyl rings.

Full text of DOI:10.1139/v00-142

1613
Preparation, X-ray structure, and dynamic solution
behaviour of N,N′,N ′′-tris(2,6-diisopropylphenyl)-
guanidine, and its reaction with molybdenum
carbonyl
Robyn E. Boeré, René T. Boeré, Jason Masuda, and Gotthelf Wolmershäuser
Abstract: The reaction of N,N′-bis(2,6-diisopropylphenyl)carbodiimide with lithium 2,6-diisopropylanilide, quenching
with water and recrystallization from heptane produces the symmetric guanidine [DipNH]2C=NDip which crystallizes in
the triclinic system, space group P1, a = 10.6513(11), b = 10.8997(11), c = 16.2961(17) Å, α = 80.524(12), β =
78.921(13), γ = 70.060(12)°, V = 1735.2(3) Å3, Z = 2. The molecule crystallizes with three perpendicular 2,6-
diisopropylphenyl groups, which surround and shield the central CN3 unit, and provide (almost) three-fold symmetry
around the central atom. Its dynamic solution behaviour has been studied by VT NMR between –90 and +180°C, and
is consistent with three distinct barriers to N—CAr rotation. Preliminary estimates of the Gibbs free energy of activa-
tion for the lower two barriers are 56 ± 2 and 73 ± 2 kJ mol–1. Reaction of the title compound with Mo(CO)6 in
refluxing n-heptane produces [DipNH]2C=NDip·Mo(CO)3, a complex in which Mo(CO)3 is η6-coordinated to one of the
diisopropylphenyl rings.
Key words: crystal structure, diisopropylaniline, guanidine, bulky ligands.
Résumé : La réaction du N,N′-bis(2,6-diisopropylphényl)carbodiimide avec le 2,6-diisopropylanilure, suivie d’un
piégeage avec de l’eau et d’une recristallisation à partir de l’heptane permet d’isoler la guanidine symétrique
[DipNH]2C=NDip qui cristallise dans le système triclinique, groupe d’espace P1, avec a = 10,6513(11), b =
10,8997(11) et c = 16,2961(17) Å, α = 80,524(12), β = 78,921(13) et γ = 70,060(12)°, V = 1735,2(3) Å3 et Z = 2. La
molécule cristallise avec trois groupes 2,6-diisopropylphényles perpendiculaires qui entourent et protègent l’unité CN3
centrale et qui assurent une symétrie pratiquement ternaire autour de l’atome central. On a étudié son comportement
dynamique en solution à l’aide de la RMN à température variable entre –90 et 180°C et il est en accord avec trois
barrières distinctes pour la rotation autour de la liaison N—CAr. Sur la base d’évaluations préliminaires, les énergies
libres d’activation de Gibbs des deux barrières les plus faibles sont respectivement de 56 ± 2 et 73 ± 2 kJ mol–1. La
réaction des composés mentionnés dans le titre avec du Mn(CO)6 dans l’heptane au reflux conduit à la formation de
[DipNH]2C=NDip·Mo(CO)3, un complexe dans lequel le Mo(CO)3 est η6-coordiné à l’un des noyaux
diisopropylphényles.
Mots clés : structure cristalline, diisopropylaniline, guanidine, ligands encombrants.
[Traduit par la Rédaction] Boeré et al. 1619
Introduction
propylphenyl (Dip) substituent on nitrogen. The reactions of
carbodiimides with nucleophiles is a well-established organic
synthetic route for the preparation of amidines, and in partic-
ular for amidinate anions used in coordination chemistry (3),
and we are interested in exploring this route to amidines and
related compounds bearing Dip substituents using the known
N,N′-bis-(2,6-diisopropylphenyl)carbodiimide.
We have recently reported the synthesis of a series of
superamidines (1) and their molybdenum carbonyl adducts
(2) (1, 2), which contain the sterically demanding 2,6-diiso-
Received September 10, 1999. Published on the NRC
Research Press website on November 27, 2000.
There is currently a strong interest in the use of
guanidines (4) and guanidinate anions (5, 6) as ligands in
coordination chemistry. Symmetrically trisubstituted guani-
dines are the nitrogen analogues of carbonic acid. In this
work we report on the preparation of a guanidine (3) with a
Dip substituent on all three nitrogen atoms using the
carbodiimide route. Thermal reaction of the guanidine with
Mo(CO)6 leads to a single product (4) in which Mo(CO)3 is
coordinated in an η6 fashion by one of the Dip groups,
which is the dominant reaction previously observed for Dip-
substituted amidines (1, 2).
R.E. Boeré, R.T. Boeré1, and J. Masuda. Department of
Chemistry and Biochemistry, University of Lethbridge,
Lethbridge, AB T1K 3M4, Canada.
G. Wolmershäuser. Fachbereich Chemie der Universität
Kaiserslautern, Erwin-Schrödinger-Straβe, D-67663
Kaiserslautern, Germany.
1Author to whom correspondence may be addressed.
Telephone: (403) 329-2045. Fax: (403) 329-2057.
e-mail: boere@uleth.ca
Can. J. Chem. 78: 1613–1619 (2000)
© 2000 NRC Canada

1614
Can. J. Chem. Vol. 78, 2000
Fig. 1. ORTEP diagram (50% probability) of the structure of 3
as found by X-ray crystallography, with the atom numbering
scheme. Hydrogen atoms on carbon are omitted for clarity. The
occupancy of H(1) is 30%, and of H(2) 70%.
Fig. 2. ORTEP diagram of the central triazamethane fragment in
3 with directly attached atoms. The disordered H atoms were lo-
cated on a difference Fourier map, and correspond to positional
disorder in the crystal packing of the molecule.
Table 1. Selected interatomic distances
and angles for 3.
Bond lengths
(Å)
N(1)-C(1)
N(1)-C(2)
N(2)-C(1)
N(2)-C(14)
N(3)-C(1)
1.3163(19)
1.4295(18)
1.3480(19)
1.421(2)
Results and discussion
Synthesis and structure of 3
Bis-(2,6-diisopropylphenyl)carbodiimide was prepared
from N,N′-2,6-diisopropylphenylisocyanate according to a
literature method (7). Attempts to prepare the guanidine by
direct addition of the carbodiimide with 2,6-disopropyl ani-
line in refluxing toluene or xylenes failed, although this is
precisely how N,N′,N′′-triphenylguanidine is prepared (5).
However, when the lithium anilide is prepared in situ in THF
(rxn. [1]), it reacts quantitatively with the carbodiimide (rxn.
[2]), to provide, after quenching, the desired guanidine (rxn.
[3]).
1.3567(18)
Bond angles
(°)
C(1)-N(1)-C(2)
C(1)-N(2)-C(14)
C(1)-N(3)-C(26)
N(1)-C(1)-N(2)
N(1)-C(1)-N(3)
N(2)-C(1)-N(3)
120.16(12)
124.30(12)
122.81(14)
121.99(13)
119.52(13)
118.47(14)
[1]
[2]
DipNH2 + nBuLi → DipNH–Li+ + nBuH
methane unit in 3 with directly attached atoms. Noteworthy
is the similarity of all the C—N distances [from 1.316(2) to
1.357(2) Å]. A representative C=N bond distance has been
established as 1.283(3) Å (9), and a value close to this has
been obtained for the neutral guanidine (E)-N,N′-dicyclo-
hexyl-4-morpholinecarboxamidine for which the C=N dis-
tance was measured as 1.276(3) Å (10). The related C—N
single bonds in the latter guanidine were measured as
1.414(2) and 1.381(2) Å. Häfelinger and Kuske (11) have
defined the parameter ∆CN = d(C—N) – d(C=N) for the cen-
tral N-C-N linkage found in all amidines (including guani-
dines). We have previously found that two almost identical
Dip-substituted amidine structures, (1, R = 4-CH3C6H4 and
R = 4-CH3OC6H4) possessed the very different ∆CN values
of 0.027(6) and 0.057(3) (1). In the former case, i.e., where
the difference in interatomic distance in the supposed
“double” and “single” bonds was very small, we were able
to detect the presence of two N—H hydrogen atom posi-
tions, and refined a disorder model with 72 and 28%
occupancy of the two structures. We propose that a simi-
lar positional disorder is operative in the crystal structure of
3, for which ∆CN = 0.032(6) Å. Indeed, peaks corresponding
DipNH–Li+ + DipN=C=NDip
→ [DipN]2CNHDip–Li+
[DipN]2CNHDip–Li+ + H2O
→ [DipNH]2C=NDip + LiOH
[3]
The guanidine 3 is easily purified by recrystallization
from acetonitrile, which removes coloured impurities. Since
these crystals tend to retain solvent, we performed a second
recrystallization from n-heptane to give solvent-free colour-
less crystals that have been fully characterized by elemental
analysis, mass spectrometry, and solution NMR.
The structure of 3 has been determined in the solid state
by X-ray diffraction (Figs. 1 and 2, Tables 1 and 2). The side
view of the structure shown in Fig. 1 is particularly instruc-
tive, showing the three Dip groups arranged almost perpen-
dicular to the planar central CN3 unit. A similarly rotated
aryl group disposition was observed previously in the struc-
ture of 2-(2,6-dichlorophenyl)-1-methylguanidine, and seems
to be a feature of ortho-disubstituted aryl amidine or
guanidine derivatives (8). Figure 2 shows the central triaza-
© 2000 NRC Canada

Boeré et al.
1615
Table 2. Crystal data for 3.a
Scheme 1.
Formula
Fw
C37H53N3
539.82
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
triclinic
P1 (no. 2)
10.6513(11)
10.8997(11)
16.296(17)
80.542(12)
78.921(13)
70.060(12)
1735.2(4)
2
Z
dcalc (g cm–3)
Crystal dim. (mm)
Radiation, MoKα (Å)
Temperature (K)
µ (cm–1)
1.033
0.52 × 0.32 × 0.21
0.71073
293
0.60
Absorption correction
Total reflections
Unique reflections
Parameters refined
R (all data)
Rw (all data)
g.o.f. (all data)
Largest ∆/σ
Final diff. map [e Å–3]
none
20 256
5206
382
0.0586
0.1268
1.052
0.01
Reaction of 3 with Mo(CO)6
The reaction of 3 with Mo(CO)6 in refluxing n-heptane,
conditions that were identical to our previous reactions of
superamidines with molybdenum carbonyl (1, 2), resulted in
a slow reaction that deposited yellow crystalline solid at the
reflux temperature. Careful cooling of the solution resulted
in the formation of essentially pure 4, which has been char-
acterized by solution IR, NMR and mass spectroscopies, and
combustion analysis. The similarity in the NMR and IR
spectroscopic parameters with those observed previously for
type 2 complexes strongly suggests that the metal is coordi-
nated in an η6 fashion to the aromatic ring of one of the Dip
groups. A parent ion for the complex is observed in the
0.152, –0.104
aWeighting scheme used: w = 1/[σ 2(Fo2) + (0.0135P)2] where P =
(Fo2 + 2Fc2)/3.
to the three N—H hydrogen atom locations were observed
on a difference Fourier map, and their occupancies were al-
lowed to vary. Convergence was achieved with H(3) as 100,
H(2) 70, and H(1) 30%. In a structure so large we do not put
a great deal of weight in the refinement of these occupan-
cies, and our disorder model is based primarily on the metric
parameters. As Fig. 2 clearly shows, the atoms in the
triazamethane fragment are well-defined with almost
spherical thermal ellipsoids.
Tautomeric disorder was postulated (without location of
the H atoms) in the 2-(2,6-dichlorophenyl)-1-methylguani-
dine mentioned above (8). In this system, the three C···N dis-
tances are 1.364(5), 1.339(4), and 1.308(4) Å, only a slightly
larger range than that found in the structure of 3. These au-
thors conclude only that the tautomer that identifies the
1.308(4) Å distance as the C=N bond “is predominant.” In 3,
unlike our previously determined amidine structure (1, R =
4-CH3C6H4) (1), the tautomeric forms are degenerate, and
the apparent bond length averaging can be achieved by sim-
ple positional disorder in the crystal packing, as shown in
the following graphic:
1
high-resolution mass spectrum. In the H NMR spectrum,
the aromatic C—H signals of one of the three Dip rings (a
pseudo doublet and a pseudo triplet) are shifted ~1.5 ppm
upfield as a consequence of π-coordination to the molybde-
num atom. One of the two N—H signals is shifted ~2 ppm
downfield (as observed for 2), while the other retains a
chemical shift similar to that in free 3. The ν(CϵO) bands in
the solution IR spectra are diagnostic of a “piano stool” co-
ordination geometry for the Mo(CO)3 group. Almost cer-
tainly it is the imino Dip group that attaches to the metal, as
seen in all the superamidine complexes. Moreover, among
the three X-ray structures previously determined for type 2
complexes, the ligand environment is extremely similar, and
we note that this same arrangement can be obtained in 4
with the ligand in the conformation obtained for 3 in the
solid state. This proposed structure is indicated in Scheme 1.
If we compare the solution IR data of 4 with those of 2
(Table 3), a distinct pattern emerges (1, 2). The electron
withdrawing CF3 group in 2 (R = CF3) despite being three
bonds removed from the diisopropylphenyl ring, neverthe-
less reduces the electron density at the ring. This leads to
weaker back-bonding to Mo, and hence to about 5 cm–1
higher ν(CϵO) frequencies than in the methyl- and aryl-
substituted amidine complexes 2 (R ≠ CF3). In 4, where an
electron-donating DipNH group is attached to the amidine
backbone, the ν(CϵO) frequencies are about 3 cm–1 lower in
energy than in the methyl- and aryl-substituted amidine
complexes 2 (R ≠ CF3) consistent with stronger back-bonding
by a more electron-rich aromatic ring.
What these results emphasize is that the large Dip groups
dominate the crystal packing and small variations in position
of the remaining atoms can be tolerated with little cost to the
lattice energy.
© 2000 NRC Canada

1616
Can. J. Chem. Vol. 78, 2000
Table 3. Solution IR data for 3, 4, and related compounds.a
Compound
ν(N-H) (cm–1)
ν(N-C-N) (cm–1)
ν(CϵO) (cm–1)
Reference
3
3415, 3392
3391, 3378, 3366, 3350
3453, 3356
3431, 3366
3431, 3366
3403, 3272 brb
3193 brb
1650
1643
1657
1620
1620
1640
1640
1665
1615
1615
—
—
—
—
this work
(1)
(2)
(1)
(1)
this work
(1)
(2)
(1)
(1)
1 (R = CH3)
1 (R = CF3)
1 (R = 4-CH3C6H4)
1 (R = 4-CH3OC6H4)
4
2 (R = CH3)
2 (R = CF3)
2 (R = 4-CH3C6H4)
2 (R = 4-CH3OC6H4)
—
1963, 1893, 1870
1966, 1896, 1876
1971, 1906, 1883
1966, 1896, 1879
1966, 1896, 1879
—
3216 brb
3216 brb
an-Heptane solution in 0.2 mm NaCl solution cell.
bCHCl3 solution in 0.2 mm NaCl solution cell; these weak bands are not observed in n-heptane because of limited solubility of the complexes.
The large difference in NH environments observed for 4
in the NMR spectrum (δ 5.06 and 7.28) is also seen in solu-
tion IR spectra. Thus there is a broad band at 3272 cm–1
quite similar to the NH bands found in 2, but there remains a
sharp NH stretch at 3403 cm–1 for the additional DipNH
group. In summary, the spectroscopic evidence for similarity
with the previously characterized superamidine adducts strongly
supports the proposed structure of 4 shown in Scheme 1.
Fig. 3. NMR spectra of 3 at various temperatures (–40°C to +120°C
in CD3C6D5, 180°C in 1,2-Cl2C6H4). Spectra were monitored at
10 degree intervals, and in the intervening spectra to those dis-
played here, smooth transitions between the peaks were observed.
Dynamic processes in solution
The room temperature 1H NMR spectra of both the
guanidine 3 and its metal complex 4 contain exchange-
broadened signals as well as many sharp peaks. The room
temperature 13C spectrum of 3 also contains exchange-
broadened peaks. Thus, there is some sort of dynamic pro-
cess operative in both the guanidine and its complex with
Mo(CO)3. In this respect, the Dip-substituted guanidine dif-
fers from the Dip-substituted amidines 1 and their metal
complexes 2. The predominant isomers of 1 and the sole iso-
mers of 2 have sharp, nonexchanging room temperature
NMR signals. We have undertaken a preliminary VT-NMR
1
study of the H NMR spectra of 3 to try and sort out the
complexities in the spectra. No attempt has been made to an-
alyze the VT 13C spectra of this compound; the RT spectrum
is reported in the experimental section. Metal complex 4 is
far too unstable in CDCl3 solution to allow for VT-NMR
studies.
1
The room temperature H NMR spectrum of 3 in CDCl3
solution as reported in the experimental section, or in
CD3C6D5 as shown in Fig. 3 at 30°C, is quite complex. Two
very distinct NH proton signals, each integrating to one, are
observed at δ 5.00 and 4.76. The isopropyl CH protons occur
as three different septets, two of which have almost coinci-
dent chemical shifts, upfield of the remaining set. The most
unusual feature is the isopropyl CH3 protons, which exist as
four sharp doublets in a 1:1:1:1 ratio, as well as a large ex-
change-broadened singlet between the inner two sharp dou-
blets. These data strongly suggest that 3 exists in solution as
a single isomer in which some sort of internal exchange is
occurring that is slow on the NMR timescale. The NMR pa-
rameters of 3 closely resemble the predominant Z-anti iso-
mer found for 1 (this conformation is that drawn in
Scheme 1). This is the predominant solid-state form when
H-bonding is not involved, as well as the lowest energy iso-
mer by gas-phase computation (AM1 method) (1). More-
over, this is the geometry uniquely adopted by the Mo(CO)3
adducts 2 in the solid-state and in solution (1, 2). Indeed, the
solid-state structure of 3 can be considered as an amidine in
the Z-anti geometry, with an additional DipNH group at-
tached as the “R” group on the C atom of the backbone.
© 2000 NRC Canada

Boeré et al.
1617
Our VT-NMR spectroscopic study was conducted in a
variety of solvents, namely CD2Cl2 and CDCl3 for the low-
temperature range, CD3C6D5 for the intermediate, and 1,2-
Cl2C6H4 for the high-temperature range. There is indeed
some solvent dependence of the chemical shifts of the sig-
nals, but the general process seems to be the same, and is
clearly of an intramolecular nature. The solvent that covers
the widest temperature range is CD3C6D5. We independently
checked the low-temperature range of this solvent by using
the less viscous deuterated halocarbons, and find that essen-
tially the same effects occur in all solvents studied. For sim-
plicity we discuss the dynamic NMR behaviour using the
data collected in CD3C6D5 (except at the highest tempera-
tures beyond the boiling point of this solvent, where 1,2-
Cl2C6H4 is used.)
Consider the representative NMR spectra presented in
Fig. 3. At –40°C, rotational exchange of the Dip groups is
frozen out, and in CD3C6D5 solution there are five sharp
methyl doublets in 2:1:1:1:1 ratio corresponding to two sets
of two symmetry equivalent methyl groups of the imino Dip
groups labeled “C” in the figure (δ 1.15 and 1.41), as well as
two sets of two for each of the amino Dip groups, labeled
“A” (δ 0.89 and 1.29) and “B” (δ 0.95 and 1.41). These data
are consistent with three different Dip environments and a
single plane of symmetry in the molecule, as seen in the
solid-state structure. The shift equivalence of the deshielded
B and C resonances is accidental (in CD2Cl2 their shifts are
different). We note also that over the temperature range –90
to +80°C, the NH resonances remain sharp and distinct, with
remarkably temperature-independent chemical shifts. This
also agrees with the structure of 3 shown in Fig. 1 in which
there are two different kinds of NH environments [i.e., H(2)
and H(3)], and supports the notion that there is only a single
geometrical isomer in solution over the whole temperature
range that we have studied. There are of course several pos-
sible conformations that 3 may adopt in solution. We have
modeled several of these by gas-phase AM1 calculations, and
find that the geometry found in our crystal structure is indeed
the lowest energy isomer we could find, lying 26.5 kJ mol–1
below the next higher form. Since this structure is fully con-
sistent with the dynamic NMR data we will for the sake of
argument consider this to be the isomer in solution, though
we recognize that the actual solution geometry has not been
proven.
The two doublets C remain sharp, which we assign to the
more rigid imino ring. Thus there are again five different
CH3 environments, integrating in a 1:1:2:1:1 ratio. At
120°C, line broadening of the NH signals to produce two ex-
tremely broad lines has (finally) set in, indicating the onset
of slow NH tautomerism. This exchange process
interconverts the amino and imino Dip sites, breaking down
the rigidity of the latter. Now wholesale exchange of all the
CH3 and the CH environments sets in, the latter signals hav-
ing remained sharp at lower temperatures, and all resonances
are either coalesced or almost coalesced. Finally, at 180°C,
NH tautomerism and rotation of the Dip groups are all fast
on the NMR time-scale, and a simple high-symmetry spec-
trum containing a single Dip environment occurs. These
changes are also reflected in the aromatic region, but we
have not interpreted the observed patterns in detail; they are
partly obscured by the residual toluene signals, and have
thus been omitted from Fig. 3 for the sake of clarity.
An important piece of additional evidence for the interpre-
tation of the dynamic phenomena put forth above for 3 is the
RT spectrum in CDCl3 of the metal complex 4. The NMR
signals that are sharp are virtually identical to those obtained
for the two Dip groups of the type 2 amidine complexes, i.e.,
a coordinated imino and an uncoordinated amino group (1,
2). The CH3 resonances of the third DipNH group are ex-
change-broadened at RT. This suggests that one amino
DipNH group is undergoing slow rotation on the NMR time-
scale, while the other is not, and that it must be the DipNH
group on the amidine backbone that is dynamic. Sample in-
stability has prevented us from performing the same kind of
VT studies with 4 that were done with 3. Ligand fluxion-
allity in a Pt{COD} complex of the dianion of N,N′,N′′-
triphenylguanidine has recently been reported, but the mech-
anism of intramolecular motion in this complex is not di-
rectly comparable to that observed in 4 (6).
A preliminary estimate of the activation energies for some
of these dynamic processes can be obtained from the half-
height line widths of the NMR signals at the coalescence
temperature, using the classical expression of Gutowsky and
Holm (12). Eyring absolute rate theory (13) leads to a value
of ∆G‡ = 56 ± 2 kJ mol–1 for the interconversion of the A set
of signals, which we have attributed to the rotation of one of
the Dip groups about an N—CAr bond. Similarly, ∆G‡ =
73 ± 2 kJ mol–1 for the interconversion of the B set of sig-
nals. The final process that interconverts A, B, and C signals
at the highest temperature is too complex to analyze by sim-
ple coalescence-point measurement. Although a full under-
standing of the dynamic processes that operate in 3 will
require complete line-shape analysis and spectral simulation,
we note that similar free energies of activation have been
measured for hindered rotation about P—CAr bonds bearing
the related bulky supermesityl substituent. Thus in
Mes*P=C=PMes*, ∆G‡ = 59 ± 1 kJ mol–1 (14), while in
Mes*As=C=PMes*, ∆G‡ = 57 kJ mol–1 (15).
Rotation of an amino Dip group exchanges the “inner”
i
and “outer” methyl groups of the Pr substituents, which ap-
pear to possess quite distinct shielding environments. On
warming to 30°C, the two doublets labeled B have coalesced
to a broad singlet at δ 1.15, indicating slow rotation of a sin-
gle amino Dip group. This leaves four sharp doublets labeled
A and C now in a 1:1:1:1 ratio for the shielded and
deshielded methyl groups of the remaining amino and imino
Dip groups, respectively. Spectra were obtained at 10 deg.
intervals, and the changes in peak intensities and locations
between the selected spectra presented in Fig. 2 underwent
smooth transitions in the intermediate stages. At 70°C the
coalesced signal B has grown into a sharp doublet at δ 1.14
(effectively the average of the two separate signals at –40°C)
due to the four methyl groups on the rapidly rotating Dip
substituent, while the second amino Dip CH3 signals have
collapsed into two exchange-broadened singlets labeled A.
In conclusion, an N,N′,N′′-trisubstituted guanidine bearing
super-bulky Dip substituents has been prepared by the
carbodiimide route, and shows interesting conformational ef-
fects due to hindered rotation in solution as shown by NMR
spectroscopy. It forms a piano stool complex with Mo(CO)3,
in which the imino aryl ring acts as a six-electron donor to
the organometallic fragment.
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Can. J. Chem. Vol. 78, 2000
Preparation of N,N′N′′-tris(2,6-
diisopropylphenyl)guanidinotricarbonylmolybdenum(0)
(4)
Experimental section
Starting materials and general procedures
A mixture of 3 (0.482 g, 0.895 mmol) and Mo(CO)6
(0.236 g, 0.895 mmol) was heated in n-heptane (24 mL) to
reflux. After 18 h, a solid precipitate formed. On cooling,
the solid was filtered off, and recrystallized from a minimum
quantity of boiling heptane, hot filtering, and cooling to
–35°C to give small, bright yellow crystals of 4 which were
analytically pure. Yield: 0.17 g, (0.24 mmol, 26%), mp
All experimental procedures were performed under a ni-
trogen atmosphere using modified Schlenk techniques. 2,6-
Diisopropylaniline and n-butyllithium (Aldrich) were com-
mercial products and used as received. Bis-2,6-diisopropyl-
phenylcarbodiimide was prepared using the published
procedure (7). Solvents were reagent grade, or better, and
were used as received (acetonitrile) or dried and distilled un-
der a nitrogen atmosphere from LiAlH4 (n-heptane) and so-
dium–benzophenone (tetrahydrofuran). 1H and 13C NMR
spectra were recorded on a Bruker AC250/Tecmag Macspect
spectrometer operating at 250.13 and 62.90 MHz, respec-
tively. The 1H and 13C chemical shifts δ (ppm) for CDCl3 so-
lutions are referenced to TMS, while CD3C6D5 solutions
were referenced to the residual CHD2 signal at δ 2.09. VT-
NMR spectra were recorded using the Bruker VT accessory,
and temperatures are considered accurate to ±2° below RT
and ±1° above RT. IR spectra were recorded on a Bomem
MB-100 spectrometer as KBr pressed pellets (solids) or in
NaCl solution cells. Mass spectra were undertaken in the
Department of Chemistry, University of Alberta and elemen-
tal analyses by MHW Laboratories, Phoenix, AZ.
1
210°C dec. H NMR (CDCl3) δ: 0.91 (d, J(H-H) = 6.8 Hz,
6H), 1.1 (br s, 12H), 1.20 (d, J(H-H) = 6.8 Hz, 6H), 1.29 (d,
J(H-H) = 6.8 Hz, 6H), 1.39 (d, J(H-H) = 6.8 Hz, 6H), 2.87
(sept, J(H-H) = 6.8 Hz, 2H), 3.11 (sept, J(H-H) = 6.8 Hz,
2H), 3.49 (sept, J(H-H) = 6.8 Hz, 2H), 5.06 (s, 1 H), 5.50 (d,
J(H-H) = 6 Hz, 2 H), 5.77 (t, J(H-H) = 6 Hz), 1 H), 7.06–
7.37 (m, 6H), 7.28 (s, 1 H). 13C NMR spectrum not obtained
due to sample instability. MS (70 eV) reported on 98Mo
isotopomers: m/z 721.31377 (M+ within 1.8 ppm, 2%),
635.31446 (M – 3CO – 2H+ within 0.2 ppm, 17%); 539
(ligand+, 62%), 496 (ligand–iPr+, 100%). IR (KBr) (cm–1):
ν(N-H) 3400, 3252 br; ν(CϵO) 1952, 1882, 1833; ν(C=N)
1635. Anal. calcd. for C40H53MoN3O3 (719.83): C 66.74, H
7.42, N 5.84%; found: C 67.00, H 7.16, N 5.95%.
X-ray analysis
The crystal structure of 3 was determined using a Stoe
IPDS diffractometer. 277 Frames with an angle increment of
1.3° were measured. Therefore the full φ range of 360° was
covered and each reflection was thus measured twice. Exper-
imental details are summarized in Table 1. The structure was
solved by direct methods using SHELXS-97 (16) and refined
by full-matrix least-squares on F2 using SHELXL-97 (17)
using all reflections. The non-hydrogen atoms were refined
anisotropically, and although most of the H atoms on C were
located on the difference map, they were refined using a rid-
ing model (C—H = 0.95 Å). For CH3 groups, a geometri-
cally fixed trigonal pyramidal unit was fitted to the electron
density for H by refining a torsion angle that defined the ro-
tation of the group on the C—C bond (a standard procedure
in SHELXL). The H atoms attached to N were refined
freely. Isotropic U values were bound to 1.2 times the U val-
ues of the corresponding N atoms. Disorder is encountered
with respect to the position of one such H atom. The disor-
der ratio was freely refined (the sum was fixed to 1). The re-
finement converged with occupancy of 0.3 for the hydrogen
attached to N(1) and 0.7 for N(2). Crystallographic data (ex-
cluding structure factors) for the structures in this paper have
been deposited.2
Preparation of N,N′,N′′-tris(2,6-
diisopropylphenyl)guanidine (3)
A solution of 0.495 g (2.75 mmol) 2,6-diisopropylaniline
in 10 mL of anhyd THF was treated with 3.03 mmol of
2.25 M nBuLi in hexanes at 0°C, and stirred for an addi-
tional 20 min at the same temperature. After warming to RT,
1.00 g (2.75 mmol) of N,N′-bis(2,6-diisopropylphenyl)car-
bodiimide in 6 mL of THF was added dropwise to the or-
ange solution, whereupon the mixture was refluxed for 2 h.
After cooling to RT, 1 mL H2O was added, the mixture dried
with anhyd MgSO4 and evaporated to dryness. Crystalliza-
tion from CH3CN afforded 1.05 g (1.94 mmol, 70.5% yield)
of 3, mp 261–262°C. Additional product could be won from
the mother liquor, but this tended to be slightly discoloured.
Analytical material and data crystals were obtained by
recrystallization from n-heptane. MS (70 eV): m/z
+
539.42351 (M+ within 0.8 ppm, 100%), 524 (M – CH3 ,
i
8%); 496 (M – Pr+, 92%). IR (KBr) cm–1: ν(N-H) 3407,
1
3398; ν(C=N) 1646. H NMR (CDCl3): δ 0.98 (d, J(H-H) =
6.8 Hz, 12H), 1.09 (br s, 12H), 1.31 (d, J(H-H) = 6.8 Hz,
6H), 1.36 (d, J(H-H) = 6.8 Hz, 6H), 3.26 (sept, J(H-H) =
6.8 Hz, 2H), 3.31 (sept, J(H-H) = 6.8 Hz, 2H), 3.46 (sept,
J(H-H) = 6.8 Hz, 2H), 4.77 (s, 1 H), 5.00 (s, 1 H), 6.93–7.30
(m, 9H). 13C NMR δ: 22.55, 23.00, 24.31 (br), 24.88, 26.04,
28.66, 28.80, 28.89, 122.54, 123.07, 124.09, 127.30, 128.76,
132.47, 133.92, 141.15, 144.15, 145.63, 147.09, 147.92.
Anal. calcd. for C37H53N3 (539.9): C 82.32, H 9.90, N
7.78%; found: C 82.12, H 9.89, N 7.93%.
Electronic structure calculations
All calculations were performed using the AM1 method
as implemented in HyperChem 5.1 running on a Pentium III
computer under Windows NT 4.0 (18).
2 A detailed structure report including atom positions and the anisotropic temperature factors has been deposited. Copies of materials on de-
posit may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research Council Canada, Ottawa,
ON, Canada K1A 0S2 (http://www.nrc.ca/cisti/irm/unpub_e.shtml for information on ordering electronically). Crystal data, atomic coordi-
nates, bond lengths and angles, and hydrogen coordinates of the structure(s) reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre. Copies of the data can be obtained, free of charge, on application to the Director, CCDC, 12 Union Road,
Cambridge, CB2 1EZ, U.K. (Fax: 44-1223-336033 or e-mail: deposit@ccdc.cam.ac.uk).
© 2000 NRC Canada

Boeré et al.
1619
8. A. Carpy, J.-M. Leger, C.-G. Wermuth, and G. Leclerc. Acta
Crystallogr. Sect. B: Struct. Crystallogr. Cryst. Chem. B37,
885 (1981).
9. G. Argay, A. Kalman, A. Karpor, and B. Ribar. Acta
Crystallogr. Sect. B: Struct. Crystallogr. Cryst. Chem. B36,
363 (1980).
Acknowledgments
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) for financial support (RTB),
and the University of Lethbridge for purchase of the NMR
spectrometer and the Macspect controller.
10. F. Bellucci, V. Bertolasi, V. Ferretti, and G. Gilla. Acta
Crystallogr. Sect. C: Cryst. Struct. Commun. C41, 544 (1985).
11. G. Häfelinger and K. H. Kuske. In The chemistry of amidines
and imidates. Vol. 2. Edited by S. Patai and Z. Rappoport.
Wiley, Chichester. 1991. Chap. 1. pp. 1–100.
12. H.S. Gutowsky and C.H. Holm. J. Chem. Phys. 25, 1228
(1956).
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© 2000 NRC Canada