Dipyrimidylamine and tripyrimidylamine as chelating N-donor ligands

Article abstract of DOI:10.1016/S0020-1693(00)00308-X

Dipyrimidylamine (dipm) and tripyrimidylamine (tripm) are reported. A bidentate tripm ruthenium complex, [RuCl(η⁶-p-cymene)(κ²-tripm)] SbF₆, is synthesized and structurally characterized. The presence of an intramolecular C-H···N hydrogen bond is proposed between the cymene C-H and a ring nitrogen in an uncoordinated pyrimidine ring. IR data on [Mo(CO)₄(dipm)] suggest that the Tolman electronic parameter of the dipm ligand is similar to that of 2 × PPh₃. M-L bonding may be weaker for pyrimidine versus pyridine because κ²-tripm complexation is apparently more favorable relative to the κ³-form than is the case for tripyridylamine.

Full text of DOI:10.1016/S0020-1693(00)00308-X

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 311 (2000) 45–49
Dipyrimidylamine and tripyrimidylamine as chelating N-donor
ligands
Wenbin Yao 1, Konstantinos Kavallieratos 2, Susan de Gala, Robert H. Crabtree *
Sterling Chemistry Laboratory, Department of Chemistry, Yale Uni6ersity, PO Box 208107, New Ha6en, CT 06520-8107, USA
Received 26 July 2000; accepted 29 August 2000
Abstract
Dipyrimidylamine (dipm) and tripyrimidylamine (tripm) are reported. A bidentate tripm ruthenium complex, [RuCl(h6-p-
cymene)(k2-tripm)]SbF6, is synthesized and structurally characterized. The presence of an intramolecular CꢀH···N hydrogen bond
is proposed between the cymene CꢀH and a ring nitrogen in an uncoordinated pyrimidine ring. IR data on [Mo(CO)4(dipm)]
suggest that the Tolman electronic parameter of the dipm ligand is similar to that of 2×PPh3. MꢀL bonding may be weaker for
pyrimidine versus pyridine because k2-tripm complexation is apparently more favorable relative to the k3-form than is the case for
tripyridylamine. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Dipyrimidylamine ligands; Tripyrimidylamine ligands; Ruthenium complexes; X-ray crystal structures
generally thermally stable as hoped, but the complexes
studied were too stable and lacked catalytic activity.
1. Introduction
Catalytic activity for alkane conversion has been found
by Periana et al. [3b] for a 2,2%-dipyrimidine platinum
complex. This soft chelating ligand, proposed to have
greater p acceptor character than 2,2%-dipyridine, was
successful in catalysis at elevated temperature. This
prompted us to examine the possibility of incorporating
additional nitrogen atoms into the pyridine rings of
tripyridylamine. We therefore decided to look at tri-2-
pyrimidylamine (tripm), which we hoped would give
complexes with better catalytic activity than tripyridyl-
amine complexes. Most tripyridylamine complexes also
show low solubility in common solvents, but we ex-
pected tripm complexes would be more soluble because
of the extra nitrogen atoms on the rings allowing
stronger interactions with the solvent.
The design and synthesis of ligands effective in pro-
moting homogeneous catalysis but resistant to thermal
degradation at high temperature is one of the challeng-
ing emerging areas in catalysis. Phosphines are excellent
for promoting catalysis but decompose readily at ele-
vated temperature by PꢀC bond cleavage [1]. Ligand
robustness is particularly desirable in the area of CꢀH
functionalization catalysis [2a], where high tempera-
tures are useful both for improving rates and for in-
creasing the thermodynamic driving force. Jensen and
coworkers [3a] have shown how pincer phosphines can
improve thermal stability for phosphines in alkane de-
hydrogenation and Heck chemistry, which also requires
high temperatures. Mosny and Crabtree [2b] in our
group extensively studied the chemistry of tri-2-pyridyl-
amine (tripyam), and showed how its complexes were
2. Synthesis of dipyrimidylamine (dipm) and tripm
* Corresponding author. Tel.: +1-203-432 3925; fax: +1-203-432
6144.
E-mail address: robert.crabtree@yale.edu (R.H. Crabtree).
1 Present address: Reveo Inc., 8 Skyline Dr., Hawthorne, NY
10522, USA.
Our first goal was to synthesize the potentially chelat-
ing organic ligand, tripm. We have therefore developed
a synthetic pathway to tripm (2) via dipm (1) as inter-
mediate, in 15% overall yield (Eqs. (1) and (2)).
2 Present address: Florida International University, 11200 SW 8th
St., Miami, FL 33199-0001, USA.
0020-1693/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(00)00308-X

46
W. Yao et al. / Inorganica Chimica Acta 311 (2000) 45–49
PCl3 (2072)Bdibenzocyclooctatetraene (dct) (2052)
BP(OPh)3 (2046)Bcod (2038)
BP(OMe)3 (2037)Bdipm (1) (2023)
BPPh3 (2022)Bbpy (2010)
(1)
(2)
The IR data indicate that dipm is indeed substantially
different electronically from 2,2%-bipyridine.
Pyrimidine is indeed a weaker base than pyridine, so
we cannot be certain whether to interpret the difference
in electronic effect as indicating simply weaker s dona-
tion or enhanced p back donation as well. This may mean
that dipm should behave more nearly like two PPh3
groups than like 2,2%-bipyridine, at least as far as the
electronic effects are concerned. Tripm did not give a
clean Mo(CO)4 or Mo(CO)3 derivative, and so we have
not been able to extend this work to tripm itself, but the
electronic effect is not expected to change very much
between dipm and tripm.
In the first step, 2-aminopyrimidine is condensed with
2-chloropyrimidine using K2CO3 at 160°C without a
solvent (Eq. (1)) in a modification of a standard [4] type
of procedure to give dipm (1) in 30% yield. The resulting
dipm is then arylated with 2-bromopyrimidine and
K2CO3 at 160°C (Eq. (2)), also a modification of a known
type of reaction [5], to give tripm in 65% yield. Both dipm
and tripm were isolated by chromatography as white
crystalline solids. Their identities have been confirmed by
1H NMR, GC–MS, and elemental analyses (see Section
7).
4. Synthesis of [RuCl(p-cymene)(k2-tripm)][SbF6]
For comparison with the previously prepared
tripyridylamine analogue 2b, [RuCl(p-cymene)(k2-
tripm)][SbF6], we treated [RuCl2(p-cymene)]2 with
2 equiv. of tripm, and 2 equiv. of AgSbF6 in CH2Cl2. The
product was found to be [RuCl(p-cymene)(k2-
tripm)][SbF6] (3).
1
The H NMR spectroscopic data show six different
tripm resonances in the integral ratio 2:2:2:1:1:1 consis-
tent with k2-binding rather than three resonances in 3:3:3
ratio, which would be consistent with the alternative
[Ru(p-cymene)(k3-tripm)]2+ structure. The uncoordi-
nated ring shows diasterotopic resonances for the 3 and
5 protons.
3. Donor strength of dipyrimidylamine
Tolman [6] has compared the electronic effects of
various monodentate ligands by using the A1 w(CO) IR
vibration of LNi(CO)3 as an indicator. Chelating ligands
cannot readily be studied with LNi(CO)3, so Anton and
Crabtree [7] suggested using the A1 w(CO) band of a more
suitable system: cis-Mo(CO)4L2. These are easily made,
air-stable, and nontoxic complexes, and both chelating
and monodentate ligands can be studied [8]. A very good
correlation between the data for LNi(CO)3 and cis-
Mo(CO)4L2 systems validates the approach and allows
the Tolman electronic parameters to be correlated and
so estimated for bidentate ligands [6,7]. The normal mode
associated with the A1 vibration mainly involves the pair
of trans CO groups, and the bands are always strong
enough to be easily observed and assigned.
We therefore made Mo(CO)4(dipm) by a modification
of a known type of method [8] involving the reaction of
Mo(CO)6 and dipm in refluxing toluene, and measured
its IR spectrum in CHCl3. The A1 frequency was located
at 2023 cm−1, and comparison with other values found
for cis-[L2Mo(CO)4] complexes in the literature [7–9]
gives the following order of donor strength:
The k2-tripm binding mode was found for 3 even
though sufficient Ag+ ion had been provided to remove
both chlorides from the metal. Attempts to remove the
remaining chloride with excess AgSbF6 or TlPF6 under
more vigorous conditions (THF, reflux) were uniformly
unsuccessful, suggesting that the k3-tripm binding mode
may be unfavorable.
5. Crystal structure of [RuCl(p-cymene)(k2-tripm)]-
[SbF6]
To verify the structure of 3 and binding mode of tripm,
an arene ruthenium complex. [RuCl(p-cymene)-
(k2-tripm)][SbF6] (3) was synthesized in 80% yield by
reaction of [RuCl2(p-cymene)]2 with 2 equiv. of tripm,
and 2 equiv. of AgSbF6 in CH2Cl2. Crystals of 3 were
obtained and the X-ray structure solved. The distances
and bond angles established in this molecule are very
similar to those identified in the tripyridylamine ana-

W. Yao et al. / Inorganica Chimica Acta 311 (2000) 45–49
47
logue, [RuCl(p-cymene)(k2-tripyam)][SbF6] 2b (Tables 1
and 2; Fig. 1).
proposal 2b, that k2-binding of tripyam and tripm is
inhibited by resonance stabilization of the coplanar
arrangement for the third ring, is over-simple. In the
new structure (3) we now see that the central N lone
pair can be resonance-stabilized by the two coordinated
rings with the result that in 3 the coordinated ꢀNAr2
system is much closer to coplanar than in the tripyam
analogue. Nevertheless, the resonance stabilization of
the N lone pair, whatever its source, still keeps the three
NꢀAr bonds near coplanar and in that way disfavors
k3-binding because this requires a pyramidal sp3 central
N.
A surprise was the fact that the third, uncoordinated
pyrimidine is orthogonal to the central ꢀNAr2 system,
unlike the near-coplanar arrangement in the k2-
tripyridylamine analogue. A coplanar arrangement fa-
vors resonance stabilization of the central N lone pair
but militates against k3-binding to the metal; an orthog-
onal arrangement prevents resonance but permits k3-
binding because the pyrimidine N of the third ring now
points towards the metal. This suggests that the earlier
Table 1
,
The RuꢀN distances in 3, 2.111(4) and 2.109(4) A,
Crystal data of [RuCl(p-cymene)(k2-tripm)][SbF6] (3)
are somewhat longer than in the tripyridylamine ana-
,
logue (2.106(8) and 2.097(8) A 2b). Although this dif-
Empirical formula
Formula weight
Crystal color/habit
Crystal dimensions (mm)
Crystal system
No. reflections used for unit cell
determination
Lattice parameters
C22H23N7ClRuSbF6
757.73
orange prisms
0.097×0.17×0.31
monoclinic
ference is not very meaningful, it is at least consistent
with pyrimidines binding less well than pyridines, in
line with the lower pKa of pyrimidine controlling the
outcome and not in agreement with the idea that
pyrimidines accept p back-bonding more effectively.
The abnormal orthogonal conformation for the un-
coordinated pyrimidine ring seen in the crystal structure
led us to look for factors that might be responsible. The
endo-nitrogen of the ring, N(5), seemed to be abnor-
25 (10.6–18.0°)
,
a (A)
9.814(4)
19.469(7)
13.936(6)
105.07(3)
2571(3)
P21/a (c14)
4
1.957
1480
18.07
,
b (A)
,
c (A)
i (°)
3
,
V (A )
,
mally close (3.56 A) to C(17) of the arene ring. Assum-
ing a planar arene allows us to place H(17) 1.09 A away
from C(17), this leads to parameters for the
C(17)ꢀH(17)···N(5) system (N···H, 2.70 A; CꢀH···N,
135.4°) that are in the range (N···H, 2.52-2.72 A;
Space group
Z value
,
Dcalc (g cm−3
F000
)
,
v(Mo Ka) (cm−1
)
,
CꢀH···N, \125°) noted by Taylor and Kennard [10] in
their classic study of CꢀH···X hydrogen bonds.
We conclude that this unusual sort of intermolecular
CꢀH···N hydrogen bond may be favored because the
CꢀH proton is expected to be abnormally acidic, both
by binding to Ru(II) and by the net positive charge on
the complex. The tripyridylamine complex 2b does not
show this CꢀH···N structure, so the energy of the
hydrogen bond favoring an orthogonal ring may be
comparable to the resonance energy favoring a copla-
nar ring and the outcome may be determined by pack-
ing effects.
Table 2
2
,
Selected bond lengths (A) and angles (°) for [RuCl(p-cymene)(k -
tripm)][SbF6] (3)
Bond angles
ClꢀRuꢀN2
ClꢀRuꢀN6
N2ꢀRuꢀN6
RuꢀN2ꢀC1
RuꢀN2ꢀC2
C1ꢀN2ꢀC2
RuꢀN6ꢀC9
RuꢀN6ꢀC10
C6ꢀN9ꢀC10
87.3(1)
87.4(1)
79.6(2)
118.9(3)
123.3(4)
116.1(5)
120.6(3)
122.3(3)
116.4(4)
6. Preferential k2 conformation of tripm
Bond lengths
RuꢀCl
2.388(2)
2.111(4)
2.109(4)
2.225(5)
2.201(5)
2.198(5)
2.213(5)
2.184(5)
2.161(5)
1.370(6)
1.449(6)
1.403(6)
RuꢀN2
RuꢀN6
RuꢀC13
RuꢀC14
RuꢀC15
RuꢀC16
RuꢀC17
RuꢀC18
N1ꢀC1
It has been reported that tripyridylamine can func-
tion either as a bidentate or a tridentate ligand, when
coordinated to a transition metal [11]. Mosny et al.
[2b,12] have also reported the conversion from a k2-
tripyridylamine to a k3-tripyridylamine form in one
case [12]. Several tripm complexes have now been syn-
thesized, in all of which tripm apparently functions as a
bidentate ligand, even though some of the correspond-
ing tripyridylamine complexes have a tridentate struc-
ture. For example, we have reported [2] that
N1ꢀC5
N1ꢀC9

48
W. Yao et al. / Inorganica Chimica Acta 311 (2000) 45–49
Fig. 1. ORTEP drawing of [Ru(h6-cymene)(k2-trpm)Cl]SbF6 (3).
3
RuCl2(PPh3)3 reacts with tripyridylamine in refluxing
methanol for 24 h to give [RuCl2(PPh3)(k3-tripyam)] [2],
but exactly the same synthetic procedure applied
totripm [12] gives [RuCl2(PPh3)2(k2-tripm)]. The k2-
tripm structure is evident from the close analogy be-
NMR (CD2Cl2): l 10.21 (s br, 1H), 8.68 (d, JHH=
4.8 Hz, 4H), 6.94 (t, 3JHH=4.8 Hz, 2H). Anal. Calc. for
C8H7N5: C, 55.48; H, 4.07; N, 40.44. Found: C, 55.41;
H, 4.12; N, 40.28%. GC–MS: 173 m/z.
1
tween the H NMR spectrum of this complex with that
7.3. Synthesis of tripm
of the crystallographically characterized species, 3. This
shows the higher tendency of tripm over tripyridyl-
amine to adopt a k2-binding mode.
Tripm was prepared by a modification of the method
reported by Faller and Gundersen for making tripyam
[5]. A mixture of dipm (400 mg, 2.30 mmol), 2-bro-
mopyrimidine (2.8 g, 18 mmol), copper (0.2 g), KI
(0.1 g), and K2CO3 (1.0 g) was refluxed at 160°C for
24 h in mesitylene (20 ml). The mixtures were then
filtered through Celite, and to the filtrate were added
hexanes (50 ml) to precipitate a brown solid. This was
then dissolved in CH2Cl2 (40 ml) and eluted through a
basic alumina column to obtain a colorless filtrate,
which was then reduced in volume to afford a white
solid product. Yield: 370 mg (65%). 1H NMR (CD2Cl2):
7. Experimental section
7.1. General
All manipulations were carried out in a dry Ar
atmosphere using standard Schlenk techniques. Chemi-
1
cals were used as received (Aldrich). H NMR spectra
were recorded on a GE Omega 300 spectrometer. Mi-
croanalyses were performed by Atlantic Microlab.
[RuCl2(p-cymene)]2 was synthesized according to the
literature procedure [13].
3
3
l 8.63 (d, JHH=4.9 Hz, 6H), 7.11 (t, JHH=4.9 Hz,
3H). Anal. Calc. for C12H9N7: C, 57.37; H, 3.61; N,
39.02. Found: C, 57.35; H, 3.59; N, 39.03%. GC–MS:
251 m/z.
7.2. Synthesis of dipm
7.4. Synthesis of Mo(CO)4(s2-dipm)
A
mixture of 2-aminopyrimidine (1.0 g, 1.1×
10−2 mol), 2-chloropyrimidine (1.2 g, 1.1×10−2 mol),
and K2CO3 (3.0 g, excess) was heated at 160°C for 1 h.
Cold water (200 ml) was added, and the mixture was
filtered to obtain an aqueous layer, which was further
extracted with CH2Cl2 (2×200 ml). The organic layer
was dried over MgSO4 for 0.5 h, and then was reduced
A mixture of Mo(CO)6 (0.15 g, 0.57 mmol) and dipm
(0.11 g, 0.64 mmol) was refluxed in toluene (20 ml) for
5 h. The resulting yellow–orange precipitate was
filtered and washed with hexanes. Recrystallization
from CH2Cl2–Et2O afforded a yellow–orange solid.
Yield: 105 mg (0.28 mmol, 49%). H NMR (CD2Cl2): l
9.10 (s br 1H, NꢀH), 8.65 (d, J=4.5 Hz, 2H), 8.96 (d,
1
1
to afford an off-white product. Yield: 0.55 g (30%). H

W. Yao et al. / Inorganica Chimica Acta 311 (2000) 45–49
49
J=5.4 Hz, 2H), 6.96 (dd, J=4.5, 5.4 Hz, 2H). Anal.
Calc. for C12H7N5MoO4: C, 37.86; H, 4.86; N, 18.38.
Found: C, 37.49; H, 4.69; N, 18.15%.
fects. The structure was solved by the Patterson method
using 4089 observed (I\3|(I)) reflections and 343
variable parameters. The standard deviation of an ob-
servation of unit weight was 1.85. Calculations were
performed using TEXSAN crystallographic software of
Molecular Structure Corp.
7.5. Synthesis of [RuCl(p-cymene)(s2-tripm)][SbF6]
To
a
solution of [RuCl2(p-cymene)]2 (30 mg,
0.050 mmol), tripm (25 mg, 0.10 mmol), and freshly
distilled CH2Cl2 (20 ml) was added AgSbF6 (34 mg,
0.10 mmol). The solution was stirred at room tempera-
ture in the absence of light for 30 min. It was then
filtered through Celite to yield a clear bright yellow
solution. The solvent was reduced in vacuo and hexanes
were added. A precipitate formed and was filtered and
washed with hexanes and dried in vacuo to yield a
yellow solid, which was recrystallized from CH2Cl2–
hexanes. Yield: 61 mg (80%). 1H NMR (CD2Cl2): l
(tripm) 8.97 (d, J=4.8 Hz, 2H), 8.95 (complex, 2H),
8.59 (d, J=4.5 Hz, 1H), 8.57 (d, J=4.8 Hz, 1H), 7.52
(t, J=5.0 Hz, 1H), 7.37 (dd, J=4.8, 4.5 Hz, 2H);
(cymene CꢀH): 5.62 (d, J=6.0 Hz, 2H), 5.51 (d, J=
6.0 Hz, 2H); [CH(CH3)2]: 2.94 (m, 1H); [CꢀCH3]: 2.10
(s, 3H); [CH(CH3)2]: 1.31 (d, J=7.2 Hz, 6H). Anal.
Calc. for C22H23N7ClF6RuSb: C, 34.87; H, 3.04; N,
12.94. Found: C, 34.59; H, 2.99; N, 12.75%.
8. Conclusions
Dipm (1) and tripm (2) can be successfully synthe-
sized in moderate yield. The crystal structure of
[RuCl(h6-p-cymene)(k2-tripm)]SbF6 (3) suggests the
presence of an unusual intramolecular CꢀH···N hydro-
gen bond between the cymene CꢀH and pyrimidine N.
IR data for Mo(CO)4(dipm) suggest that dipm has a
Tolman electronic effect close to that of a pair of PPh3
ligands. The MꢀL bonding may not be as strong as in
tripyridylamine because the k2-form of tripm is appar-
ently the common binding mode, as in [RuCl2(PPh3)2-
(k2-tripm)], unlike the tripyridylamine analogue 2b,
[RuCl2(PPh3)(k3-tripyam)], which is k3-bound. Reso-
nance stabilization of the k2-form, together with the
weaker s-donor character of tripm versus tripyridyl-
amine, is thought to be responsible.
7.6. Synthesis of [RuCl2(PPh3)2(s2-tripm)]
Acknowledgements
To a solution of RuCl2(PPh3)3 (1.6 g, 1.68 mmol) in
dry degassed methanol (30 ml) was added tripm (0.58 g,
2.5 mmol). The stirred solution was refluxed for 15 h.
After cooling, the resulting orange precipitate was
filtered, washed with MeOH (10 ml), and ether (10 ml)
and dried in vacuo. Recrystallization from CH2Cl2–
We thank Dr. R. Periana and Catalytica Corp. for
their interest, Professor J. Faller for discussions and the
National Science Foundation for funding.
References
1
hexanes gave the product (80%, 1.4 g, 1.3 mmol). H
NMR (CD2Cl2): l (tripm) 9.0 (d, J=4.8 Hz, 2H), 8.95
(multiplet, 2H), 8.65 (d, J=4.5 Hz, 1H), 8.55 (d, J=
5 Hz, 1H), 7.0–8.0 (tripm, PPh3 broad, 32H). Anal.
Calc. for C48H39N7Cl2P2Ru: C, 60.85; H, 4.12; N,
10.34. Found: C, 60.69; H, 3.99; N, 10.24%.
[1] P.E. Garrou, Chem. Rev. 85 (1985) 171.
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R.H. Crabtree, Inorg. Chem. Acta 247 (1996) 93.
[3] (a) F.C. Liu, E.B. Pak, B. Singh, C.M. Jensen, A.S. Goldman, J.
Am. Chem. Soc. 121 (1999) 4086. (b) R.A. Periana, D.J. Taube,
S. Gamble, H. Taube, T. Satoh, H. Fujii, Science 280 (1998) 560.
[4] D.J. Brown, Heterocyclic Compounds: The Pyrimidines, vol. 52,
p. 372, New York, Interscience Publishers, 1962 and Wiley, c
1994.
7.7. X-ray study on [RuCl(p-cymene)(s2-tripm)][SbF6]
[5] J.W. Faller, L.L. Gundersen, Tetrahedron Lett. 34 (1993) 2275.
[6] C.A. Tolman, J. Am. Chem. Soc. 92 (1970) 2953.
[7] D.R. Anton, R.H. Crabtree, Organometallics 2 (1983) 621.
[8] (a) J. Muller, D. Goser, M. Elian, Angew. Chem., Int. Ed. Engl.
8 (1969) 374. (b) E.W. Abel, M.A. Bennett, G. Wilkinson, J.
Chem. Soc. (1959) 2323.
[9] (a) J. Chatt, H.R. Watson, J. Chem. Soc. (1961) 4980. (b) F.T.
Delbeke, E.G. Claeys, G.P. van der Kelan, Z. Eeckhart, J.
Organomet. Chem. 25 (1970) 213, 219. (c) S. Ernst, W. Kaim, J.
Am. Chem. Soc. 108 (1986) 3578.
[10] R. Taylor, O. Kennard, J. Am. Chem. Soc. 104 (1982) 5063.
[11] G.C. Kulasingam, W.R. McWhinnie, J. Chem. Soc. A (1967)
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233.
The crystal data are reported in Table 1. A crystal of
3 was mounted on a glass fiber and data collected at
−90° on an Enraf–Nonius CAD-4 diffractometer with
graphite-monochromated Mo Ka radiation and the
ꢀ–2q scan technique. The space group was based on
the systematic absences and the successful refinement.
Of the 5701 reflections collected, 5387 were unique
(Rint=0.181). The intensities of three representative
reflections were measured after every 60 min and re-
mained constant. The linear absorbtion correction was
18.1 cm−1, but azimuthal scans of several reflections
indicated no need for an absorption correction. The
data were corrected for Lorentz and polarization ef-
.