Coordination complexes derived from 3,6-di(2-pyridyl)-1,4-dihydro-1,2,4,5-tetrazine (DPDHT). Synthesis and molecular structure of [(DPT)(GaMe2)2]

Article abstract of DOI:10.1139/v98-203

The facile reaction of 3,6-di(2-pyridyl)-1,4-dihydro-1,2,4,5-tetrazine (DPDHT) (1) with trimethylgallium has resulted in the formation of the purple crystalline product, [(DPT)(GaMe₂)₂] (2), via methane elimination. Crystals of [(DPT)(GaMe₂)₂] are orthorhombic, Pbca, a = 15.380(2), b = 21.642(4), c = 11.355(2) A, Z = 8. The structure was solved by heavy-atom Patterson methods and refined by full-matrix least-squares procedures to R = 0.035 for 1570 reflections with I ≥ 3σ(I) (R_w = 0.063 for all 4342 reflections). The [(DPT)(GaMe₂)₂] molecule is roughly planar (apart from the four methyl groups), in contrast to the folded structure of the DPDHT molecule; molecular dimensions are normal.

Full text of DOI:10.1139/v98-203

1800
Coordination complexes derived from
3,6-di(2-pyridyl)-1,4-dihydro-1,2,4,5-tetrazine
(DPDHT). Synthesis and molecular structure of
[(DPT)(GaMe2)2]
Peter N. Preston, Steven J. Rettig, Alan Storr, and James Trotter
Abstract: The facile reaction of 3,6-di(2-pyridyl)-1,4-dihydro-1,2,4,5-tetrazine (DPDHT) (1) with trimethylgallium has
resulted in the formation of the purple crystalline product, [(DPT)(GaMe2)2] (2), via methane elimination. Crystals of
[(DPT)(GaMe2)2] are orthorhombic, Pbca, a = 15.380(2), b = 21.642(4), c = 11.355(2) Å, Z = 8. The structure was
solved by heavy-atom Patterson methods and refined by full-matrix least-squares procedures to R = 0.035 for 1570
reflections with I ≥ 3σ (I) (Rw = 0.063 for all 4342 reflections). The [(DPT)(GaMe2)2] molecule is roughly planar (apart
from the four methyl groups), in contrast to the folded structure of the DPDHT molecule; molecular dimensions are
normal.
Key words: methylgallium, crystal structure, dipyridyldihydrotetrazine.
Résumé : La réaction facile de la 3,6-di(2-pyridyl)-1,4-dihydro-1,2,4,5-tétrazine (DPDHT) (1) avec le triméthylgallium
conduit à la formation du produit cristalline violet, [(DTP)(GaMe2)2] (2) avec élimination de méthane. Les cristaux de
[(DTP)(GaMe2)2] sont orthorhombiques, groupe d’espace Pbca, avec a = 15,380(2), b = 21,642(4) et c = 11,355(2) Å,
Z = 8. On a résolu la structure par la méthode des atomes lourds de Patterson et on l’a affinée par la méthode des
moindres carrés jusqu’à une valeur de R = 0,035 pour 1570 réflexions avec I ≥ 3σ (I) (Rw = 0,063 pour l’ensemble des
4342 réflexions). La molécule de [(DTP)(GaMe2)2] est pratiquement plane (à l’exception des quatre groupes méthyles)
et cette situation est en opposition à celle observée dans la molécule de DPDHT qui comporte une structure repliée; les
dimensions moléculaires sont normales.
Mots clés : méthylgallium, structure cristalline, dipyridyldihydrotétrazine.
[Traduit par la Rédaction] Preston et al. 1804
complex heterocycles and have included for study
3,6-di(2-pyridyl)-1,4-dihydro-1,2,4,5-tetrazine, DPDHT (1),
We have described a number of polymeric transition metal
complexes with pyrazolate (1–4) and, most recently, with
imidazolate (5) ligands. The relative positions of
pyrrole-type and pyridine-type nitrogen atoms in the
ligand are crucial in determining the dimensionality of the
ensuing polymer. Thus a three-dimensional polymetallic
iron-containing imidazolate complex has proven particularly
interesting, exhibiting canted spin antiferromagnetic cou-
pling along chains of tetrahedrally coordinated iron centers
with concomitant molecular magnet properties at low tem-
peratures. We are presently extending our studies using more
deprotonation of which yields a symmetrical tetradentate
ligand, DPT2–, hitherto unused in coordination chemistry. In
this report we illustrate the use of DPDHT in its reaction
with trimethylgallium.
Materials were handled in a conventional high-vacuum
line or in a glove box under an atmosphere of oxygen-free
dry dinitrogen. All solvents were dried and distilled before
use. Gas volumes are at STP. Trimethylgallium was pre-
pared, as previously described, from gallium and dimethyl-
mercury (6). The DPDHT ligand–precursor was synthesized
by a literature method (7).
Received August 18, 1998.
P.N. Preston.1 Department of Chemistry, Heriot–Watt
University, Riccarton, Edinburgh EH14 4AS, U.K.
S.J. Rettig, A. Storr,1 and J. Trotter. Department of
Chemistry, University of British Columbia, 2036 Main Mall,
Vancouver, BC V6T 1Z1, Canada.
Preparation of DPT·2GaMe2
The orange ligand–precursor DPDHT, (1), (0.307 g;
1.3 mmol) was transferred to a 250 mL round-bottomed
flask fitted with a break-seal side-arm. Xylene solvent
(-60 mL) was added to give a pale orange solution. The re-
action flask was then cooled to –196° and evacuated. Me3Ga
(0.296 g; 2.6 mmol) was vacuum-transferred into the reac-
tion vessel which was then flame-sealed. On warming to
1Authors to whom correspondence may be addressed:
PNP: Telephone: 44-131-449-511. Fax: 44-131-451-3180.
E-mail: p.n.preston@hw.ac.uk; AS: Telephone: (604)
822-3962. Fax: (604) 822-2847. E-mail: storr@chem.ubc.ca
Can. J. Chem. 76: 1800–1804 (1998)
© 1998 NRC Canada

Preston et al.
1801
1
Table 1. H NMR assignmentsa for DPDHT ligand (1) and complex [(DPT)(GaMe2)2] (2).
Chemical shiftsb
Coupling constantsc
H3′
H4′
H5′
H6′
J3′, 4′
J3′, 5′
J3′, 6′
J4′, 5′
J4′, 6′
J5′, 6′
Ligand 1d
8.04
7.88
0.16
7.78
6.66
1.12
7.38
6.20
1.18
8.58
7.20
1.38
8.0
8
1.1
1.0
1.1
1.1
7.6
7.7
1.7
1.4
4.9
1.5
Complex 2e
Chemical shift diff.
aAssignments for the complex (2) are made on the basis of decoupling experiments and the magnitude of coupling constants relative to those in the
ligand (1).
bQuoted in ppm with respect to tetramethylsilane.
cQuoted Hz.
dMeasured at 400 MHz in CD2Cl2 solvent at 25°C; the NH resonance appeared at δ 8.53.
eMeasured at 400 MHz in C6D6 solvent at 25°C.
room temperature the orange solution gradually darkened
through amber, golden-brown, dark brown, to deep purple.
On standing overnight at room temperature the solution
deposited dark purple crystals of the product [DPT·2GaMe2]
(2) in quantitative yield. The reaction flask was again cooled
to –196° and the volume of methane evolved during the re-
action measured using a Toepler pump (calcd. 57.8 mL;
found 57.6 mL). The purple crystals were isolated in the
glove box. The purple crystals were stable under a dini-
trogen atmosphere but quickly turned yellow–orange on ex-
posure to air. The infrared spectrum showed characteristic
(8) Ga-C stretching frequencies νasym 590 cm–1 and νsym
535 cm–1 and was devoid of the N-H stretching frequencies
observed at 3299 and 3344 cm–1 in the spectrum of the
ligand–precursor DPDHT. The 1H NMR spectrum of the
complex in C6D6 solution showed one signal for the GaMe2
groups at δ 0.20. The signals due to the protons of the
pyridyl fragments of the ligand precursor, DPDHT, and the
product, [DPT·2GaMe2], are summarized in Table 1 and dis-
cussed below. The mass spectrum (EI) of the purple crystals
displayed the expected three-line pattern for the molecular
ion (M+) and a stronger signal cluster for the M-2Me+ ion.
Anal. calcd. for [DPT·2GaMe2]: C 44.1, H 4.6, N 19.3;
found: C 44.4, H 4.7, N 19.2%.
were processed, corrected for Lorentz and polarization ef-
fects, and absorption (based on azimuthal scans).
The structure was solved by heavy-atom, Patterson methods.
All non-hydrogen atoms were refined with anisotropic ther-
mal parameters. Hydrogen atoms were fixed in calculated
positions with C—H = 0.98 Å and BH = 1.2Bbonded atom. No
secondary extinction correction was required. Neutral atom
scattering factors for all atoms and anomalous dispersion
corrections for the non-hydrogen atoms were taken from the
International Tables for X-Ray Crystallography (9). Final
atomic coordinates and equivalent isotropic thermal parame-
ters, bond lengths and angles appear in Tables 3–5, respec-
tively. Hydrogen atom parameters, anisotropic thermal
parameters, torsion angles, intermolecular contacts, and
least-squares planes are included as supplementary material.2
The reaction of group 13 trialkyls with ligands containing
acidic hydrogen yields adducts which readily eliminate al-
kanes to produce coordinatively unsaturated intermediates
(10). Coordinative saturation is usually achieved by
cyclization, the degree of association being determined by a
balance between steric effects, valency angle strain and en-
tropy factors (8, 10, 11). The methane elimination reactions
of trimethylgallium with pyrazoles to yield dimeric products
containing central six-membered Ga2N4 rings are well docu-
mented (12–14). The reaction of Me3Ga with
2-methylimidazole, on the other hand, results in the forma-
tion of tetrameric species, a consequence of the steric de-
mands of the azolate moiety (15).
X-ray crystallographic analysis
Crystallographic data appear in Table 2. The final unit-cell
parameters were obtained by least-squares on the setting an-
gles for 25 reflections with 2θ = 20.3–25.2°. The intensities
of three standard reflections, measured every 200 reflections
throughout the data collection, decayed by 7.4%. The data
2 Supplementary material may be purchased from: The Depository of Unpublished Data, Document Delivery, CISTI, National Research
Council Canada, Ottawa, Canada, K1A 0S2. Tables of bond lengths and angles have also been deposited with the Cambridge Crystallo-
graphic Data Centre and can be obtained on request from: The Director, Cambridge Crystallographic Data Centre, University Chemical
Laboratory, 12 Union Road, Cambridge, CB2 1EZ, U.K.
© 1998 NRC Canada

1802
Can. J. Chem. Vol. 76, 1998
Table 3. Atomic coordinates and Beq (103 Å2).a
Table 2. Crystallographic data.a
Formula
C16H20Ga2N6
Atom
x
y
z
Beq
Formula weight
Crystal color, habit
Crystal dimensions, mm
Crystal system
Space group
a, Å
b, Å
c, Å
V, Å3
435.81
Dark, prism
0.28 × 0.40 × 0.48
Orthorhombic
Pbca
15.380(4)
21.642(4)
11.355(2)
3780(1)
8
1.532
1760
28.6
0.92–1.00
ω-2θ
1.10 + 0.35 tan θ
16 (up to 9 scans)
+h, +k, +l
55
7.4%
4852
4342
0.04
1570
217
0.035
0.063
Ga(1)
Ga(2)
N(1)
N(2)
N(3)
N(4)
N(5)
N(6)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
C(9)
C(10)
C(11)
C(12)
C(13)
0.05675(3)
0.26458(3)
0.1134(2)
0.0684(2)
0.2065(2)
0.2500(2)
0.1386(2)
0.1836(2)
0.1189(3)
0.2005(3)
0.0802(3)
–0.0079(3)
–0.0362(3)
0.0234(4)
0.1106(3)
0.2398(3)
0.3268(3)
0.3577(3)
0.3001(3)
0.2140(3)
0.0251(3)
0.19977(2)
0.06009(2)
0.1547(2)
0.12807(14)
0.0935(2)
0.11847(13)
0.04063(14)
0.20379(14)
0.0997(2)
0.1464(2)
0.0681(2)
0.0633(2)
0.0290(2)
0.0007(2)
0.0073(2)
0.1748(2)
0.1723(2)
0.2013(2)
0.2311(2)
0.2317(2)
0.2827(2)
0.1500(2)
0.16226(4) 4.418(11)
0.58552(4) 5.098(13)
0.2862(3)
0.3830(3)
0.4496(3)
0.3518(3)
0.6289(3)
0.1029(3)
0.4553(3)
0.2781(3)
0.5575(4)
0.5786(4)
0.6735(5)
0.7442(5)
0.7231(4)
0.1741(3)
0.1483(4)
0.0489(4)
5.50(11)
3.88(9)
6.69(12)
3.64(8)
4.40(9)
3.69(8)
3.60(11)
3.25(11)
3.68(11)
4.76(12)
5.74(14)
6.5(2)
5.81(15)
3.21(10)
4.24(12)
4.91(13)
5.44(14)
4.79(13)
6.99(15)
5.98(14)
6.77(15)
8.1(2)
Z
ρcalc, g/cm3
F(000)
µ(MoKα), cm–1
Transmission factors
Scan type
Scan range, deg in ω
Scan speed, deg/min
Data collected
2θ max, deg
Crystal decay
Total reflections
–0.0241(4)
0.0049(4)
0.2118(4)
0.0520(4)
0.5547(4)
0.6970(4)
Total unique reflections
Rmerge
C(14) –0.0109(3)
C(15)
C(16)
0.3242(3)
0.2972(3)
-0.0174(2)
0.1246(2)
Reflections I > 3σ(I)
No. of variables
R (F, 1570 refelctions)
Rw (F2, 4342 reflections)
gof
Max ∆/σ (final cycle)
Residual density, e/Å3
aTemperature 294 K, Rigaku AFC6S diffractometer, MoKα radiation
(λ = 0.71069 Å), graphite monochromator, takeoff angle 6.0°, aperture 6.0
× 6.0 mm at a distance of 285 mm from the crystal, stationary
aBeq = (8/3)π2ΣΣ(Uijaiaj(aiaj)).
Table 4. Bond lengths (Å), with estimated standard deviations in
parentheses.
1.16
0.0003
–2.47 to +1.27 (near Ga)
Bond
Length
Bond
Length
Ga(1)—N(1)
Ga(1)—C(13)
Ga(2)—N(3)
Ga(2)—C(15)
N(1)—N(2)
N(2)—C(1)
N(3)—C(1)
N(5)—C(3)
N(6)—C(8)
C(1)—C(3)
C(3)—C(4)
C(5)—C(6)
C(8)—C(9)
C(10)—C(11)
1.921(3)
1.943(4)
1.924(3)
1.944(5)
1.421(4)
1.286(4)
1.355(4)
1.348(4)
1.339(4)
1.473(5)
1.380(5)
1.363(6)
1.370(5)
1.374(5)
Ga(1)—N(6)
Ga(1)—C(14)
Ga(2)—N(5)
Ga(2)—C(16)
N(1)—C(2)
N(3)—N(4)
N(4)—C(2)
N(5)—C(7)
N(6)—C(12)
C(2)—C(8)
C(4)—C(5)
C(6)—C(7)
C(9)—C(10)
C(11)—C(12)
2.066(3)
1.952(4)
2.042(3)
1.951(5)
1.355(4)
1.405(4)
1.282(4)
1.360(5)
1.349(4)
1.463(5)
1.378(5)
1.371(6)
1.376(5)
1.365(5)
background counts at each end of the scan (scan/background time ratio
2:1), σ2(F2) = [S2(C + 4B)]/Lp2 (S = scan rate, C = scan count, B =
normalized background count), function minimized Σw(|Fo| – |Fc|2 where w
2
2
= 4Fo /σ2(Fo ), R = Σ||Fo| – |Fc||Σ|Fo|, Rw = (Σw(|Fo| – |Fc|)2/Σw(|Fo|2)1/2, and
gof = [Σw(|Fo| – |Fc|)2/(m – n)]1͞2
.
In the present study the reaction of Me3Ga with DPDHT
has been investigated to gain insights into the potential of
this heterocyclic system to act as a ligand precursor in the
preparation of polymeric transition metal compounds. The
chemistry of DPDHT has received little attention (16). A
useful aspect of its reactivity with regard to coordination
chemistry, however, is sequential oxidation and alkyne dipolar
cyclcoaddition which leads to substituted bis(pyridyl)
pyridazines, of value as ligating heteroarenes (17). In the
present study the initial adduct formation between trimethyl-
gallium and DPDHT, probably via the lone pair on the
pyridyl nitrogen atoms, places the trialkyl moieties in prox-
imity of the N-H groups of the dihydrotetrazine ring. Facile
methane elimination, at or even below room temperature,
leads to the formation of the five-membered heterocyclic,
GaN2C2, rings observed in the purple crystalline product.
(N-2), with definitive assignment of H-6′ at 8.58 ppm. (See
Table 1.)
The magnitudes of coupling constants are in accord with
those anticipated for substituted pyridines, a notable feature
of which is the relatively low value of J5′6′ (= 4.9 Hz) in-
volving coupling to the proton on the carbon adjacent to the
pyridine nitrogen (19). Assignments of chemical shifts in the
1H NMR spectrum of the complex (2) were made on the ba-
sis of decoupling experiments and on the relative magni-
tudes of coupling constants in respect to (1). Complexation
of the ligand causes shifts to high field for all the ring pro-
tons, but particularly (>1 ppm) for H-4′, H-5′, and H-6′. The
1
The H NMR spectrum of the ligand has been previously
reported but without definitive assignments (18). In this
work, long range N-H correlation experiments provided as-
signments of 300 (pyrimide-N), 275 (N-1), and 259 ppm
© 1998 NRC Canada

Preston et al.
1803
Table 5. Bond angles (deg), with estimated standard deviations in parentheses.
Bond
Angle
Bond
Angle
N(1)-Ga(1)-N(6)
N(1)-Ga(1)-C(14)
N(6)-Ga(1)-C(14)
N(3)-Ga(2)-N(5)
N(3)-Ga(2)-C(16)
N(5)-Ga(2)-C(16)
Ga(1)-N(1)-N(2)
N(2)-N(1)-C(2)
Ga(2)-N(3)-N(4)
N(4)-N(3)-C(1)
Ga(2)-N(5)-C(3)
C(3)-N(5)-C(7)
Ga(1)-N(6)-C(12)
N(2)-C(1)-N(3)
N(3)-C(1)-C(3)
N(1)-C(2)-C(8)
N(5)-C(3)-C(1)
C(1)-C(3)-C(4)
C(4)-C(5)-C(6)
N(5)-C(7)-C(6)
N(6)-C(8)-C(9)
C(8)-C(9)-C(10)
C(10)-C(11)-C(12)
80.33(13)
115.6(2)
108.51(15)
80.24(13)
111.7(2)
103.6(2)
123.5(3)
118.7(3)
123.9(3)
118.2(3)
113.3(3)
119.7(4)
127.9(3)
128.2(4)
113.1(4)
114.1(3)
114.2(4)
124.7(4)
119.3(4)
120.2(5)
121.4(4)
119.5(4)
118.9(4)
N(1)-Ga(1)-C(13)
N(6)-Ga(1)-C(13)
C(13)-Ga(1)-C(14)
N(3)-Ga(2)-C(15)
N(5)-Ga(2)-C(15)
C(15)-Ga(2)-C(16)
Ga(1)-N(1)-C(2)
N(1)-N(2)-C(1)
Ga(2)-N(3)-C(1)
N(3)-N(4)-C(2)
Ga(2)-N(5)-C(7)
Ga(1)-N(6)-C(8)
C(8)-N(6)-C(12)
N(2)-C(1)-C(3)
N(1)-C(2)-N(4)
N(4)-C(2)-C(8)
N(5)-C(3)-C(4)
C(3)-C(4)-C(5)
C(5)-C(6)-C(7)
N(6)-C(8)-C(2)
C(2)-C(8)-C(9)
C(9)-C(10)-C(11)
N(6)-C(12)-C(11)
111.7(2)
107.0(2)
124.2(2)
113.5(2)
108.2(2)
127.8(2)
117.8(3)
113.1(3)
117.4(3)
114.5(3)
126.8(3)
113.1(3)
118.9(3)
118.7(4)
127.2(4)
118.7(4)
121.1(4)
119.1(4)
120.5(5)
114.7(4)
124.0(4)
119.2(4)
122.1(4)
ligand exhibits statistically significant deviations from exact
planarity, but is roughly planar (maximum displacement
from mean plane = 0.192(5) Å for C(6); maximum angle be-
tween planes of the various rings = 7.0°). This is in contrast
to the parent DPDHT molecule, which is folded with a
boat-shaped tetrazine ring, in both the triclinic (20) and
orthorhombic (21) forms. The torsion angles in the tetrazine
ring of the DPDHT molecule are as high as 44°, while the
corresponding angles in the DPT2– ligand have a maximum
of 3.0(6)°. The Ga atoms both lie 0.24 Å above the mean
plane of the ligand.
Bond lengths and angles (Tables 4 and 5) are close to ex-
pected values. In particular, the C(1)—N(2) and C(2)—N(4)
bond distances of the tetrazine ring (1.286(4) and 1.282(4)
Å) correspond to double bonds. The Ga—N(pyridine) bonds
(2.066(3) and 2.042(3) Å) are significantly longer than the
Ga—N(tetrazine) bonds (1.921(3) and 1.924(3) Å); this situ-
ation has analogy in the molecular structure of
Ga{2-[NCH2C6H5]NC5H4}2Me (cf. 2.216(3), 2.236(3) Å
Fig. 1. View of the [(DPT)(GaMe2)2] molecule; 33% probability
thermal ellipsoids are shown for the non-hydrogen atoms.
C(11)
C(12)
N(6)
C(13)
C(10)
C(9)
C(8)
C(2)
Ga(1)
N(1)
N(2)
N(4)
C(16)
C(14)
N(3)
C(1)
Ga(2)
C(3)
for Ga—N(pyridine) bonds, and 1.933(3)
Å
for
N(5)
C(7)
C(4)
C(5)
Ga—N(amine) bonds), although it should be noted that the
gallium in the latter compound is pentacoordinate (22).
The crystal packing is dominated by van der Waals forces,
the shortest intermolecular contact between non-hydrogen
atoms being a C···C distance of 3.28(1) Å.
C(15)
C(6)
disparity in electronic influence of the gallium moieties on
chemical shifts of H-3′ vs. H-4′, 5′ and 6′ is curious, and war-
rants further theoretical studies of complex (2) and related
compounds.
3,6-Di(2-pyridyl)-1,4-dihydro-1,2,4,5-tetrazine (1) reacts
readily with trimethylgallium to provide the novel penta-
cyclic heteroaromatic complex (2) in quantitative yield.
Studies by X-ray crystallography and NMR spectroscopy
suggest considerable bond localization in the tetrazine ring,
The X-ray analysis unambiguously establishes the molec-
ular structure of [(DPT)(GaMe2)2] (Fig. 1). The DPT2–
© 1998 NRC Canada

1804
Can. J. Chem. Vol. 76, 1998
8. A. Storr and B.S. Thomas. J. Chem. Soc. A, 3850 (1971).
9. (a) International tables for X-ray crystallography. Kynoch
Press, Birmingham, U.K. (present distributor Kluwer Aca-
demic Publishers, Boston, Mass.). 1974. Vol. 4. pp. 99–102;
(b) International tables for crystallography. Kluwer Academic
Publishers, Boston, Mass. 1992. Vol. C. pp. 200–206.
10. G.E. Coates and K. Wade. Organometallic compounds: vol. 1,
the main group elements. 3rd ed. Methuen, London. 1967.
Chap. 3.
and marked electronic influence of the gallium moieties on
chemical shift values of the pyridine protons.
A key feature to note from the work is that the known
DPDHT is easily synthesized (7) in multigram quantities
from commercially available materials; applications of the
ligand for use in the synthesis of coordination polymers can
be envisaged.
11. M.F. Lappert, M.K. Majumdar, and B.P. Tilley. J. Chem. Soc.
A, 1590 (1966).
We thank Dr. A.S.F. Boyd for assistance with the NMR
spectroscopy, Mr. P. Borda for elemental analysis, and the
Natural Sciences and Engineering Research Council of Can-
ada for financial support.
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