Silver(I) coordination chemistry of 2,6-diarylpyrazines. π-stacking, anion coordination, and steric control

Article abstract of DOI:10.1021/ic034277m

The silver(I) coordination chemistry of 2,6-diarylpyrazines is reported. Discrete coordination complexes and two-dimensional coordination networks were characterized. The substitution pattern on the pendant aryl groups controlled the type of coordination chemistry involved. Thus, o-methyl-substituted aryl groups held the aryl groups orthogonal to the central pyrazine ring, opening the hindered nitrogen atoms to complexation, and polymeric networks were characterized. In the absence of the o-methyl groups, discrete coordination complexes were characterized. Thus, a dimeric 2:1 ligand-silver(I) complex was isolated and characterized on reaction of 2,6-bis(3′,5′-dimethylphenyl)-pyrazine with silver(I) trifluoroacetate in acetonitrile solvent, while a 2:2 complex was isolated from dichloromethane solvent. Two trifluoroacetate ligands bridge two silver cations in both complexes. Reaction of the same pyrazine ligand with silver(I) tetrafluoroborate yielded a discrete 2:1 complex. A 2:1 complex was isolated on reaction of 2,6-diphenylpyrazine with silver(I) nitrate. These complexes were interlinked by weakly coordinating nitrate anions to form interwoven one-dimensional ribbons. Two-dimensional networks were obtained on reaction of silver(I) trifluoroacetate with either 2,6-bis(2′,6′-dimethylphenyl)pyrazine or 2-(2′,6′-dimethylphenyl)-6-(3′,5′-dimethylphenyl) -pyrazine. The networks comprised pyrazine-silver(I) strands cross-linked with complex bridged silver(I) trifluoroacetates.

Full text of DOI:10.1021/ic034277m

Inorg. Chem. 2003, 42, 5304−5310
Silver(I) Coordination Chemistry of 2,6-Diarylpyrazines. π-Stacking,
Anion Coordination, and Steric Control
,†
Nate Schultheiss,† Douglas R. Powell,‡ and Eric Bosch*
Chemistry Departments, Southwest Missouri State UniVersity, Springfield, Missouri 65804, and
UniVersity of Kansas, Lawrence, Kansas 66045
Received March 13, 2003
The silver(I) coordination chemistry of 2,6-diarylpyrazines is reported. Discrete coordination complexes and two-
dimensional coordination networks were characterized. The substitution pattern on the pendant aryl groups controlled
the type of coordination chemistry involved. Thus, o-methyl-substituted aryl groups held the aryl groups orthogonal
to the central pyrazine ring, opening the “hindered” nitrogen atoms to complexation, and polymeric networks were
characterized. In the absence of the o-methyl groups, discrete coordination complexes were characterized. Thus,
a dimeric 2:1 ligand−silver(I) complex was isolated and characterized on reaction of 2,6-bis(3′,5′-dimethylphenyl)-
pyrazine with silver(I) trifluoroacetate in acetonitrile solvent, while a 2:2 complex was isolated from dichloromethane
solvent. Two trifluoroacetate ligands bridge two silver cations in both complexes. Reaction of the same pyrazine
ligand with silver(I) tetrafluoroborate yielded a discrete 2:1 complex. A 2:1 complex was isolated on reaction of
2,6-diphenylpyrazine with silver(I) nitrate. These complexes were interlinked by weakly coordinating nitrate anions
to form interwoven one-dimensional ribbons. Two-dimensional networks were obtained on reaction of silver(I)
trifluoroacetate with either 2,6-bis(2′,6′-dimethylphenyl)pyrazine or 2-(2′,6′-dimethylphenyl)-6-(3′,5′-dimethylphenyl)-
pyrazine. The networks comprised pyrazine−silver(I) strands cross-linked with complex bridged silver(I) trifluoro-
acetates.
Chart 1
Introduction
Over the past several years we have been interested in
the coordination chemistry of nitrogen heterocycles in which
the nitrogen atoms are flanked by aryl groups. In this regard
we reported the light-stable silver(I) complex of 2,6-
dimesitylpyridine1 and the one- and two-dimensional coor-
dination networks formed between silver(I) and 2,4,6-
trimesityl-1,3,5-triazine (Chart 1).2 In both ligands the
o-methyl groups on the flanking mesityl groups held the
mesityl group orthogonal to the central heterocycle. With
dimensional coordination polymers with the ultimate goal
this orthogonal orientation the mesityl groups actually
of preparing materials for nonlinear optical applications.3 We
facilitate coordination rather than hinder it.
expected that the steric demands of the pendant aryl groups
In this study we examine the effect of a variety of flanking
would render the coordination selective with the aryl groups
aryl groups on the coordination chemistry of 2,6-disubstituted
oriented in the same direction as shown in Figure 1.
pyrazines with the goal of preparing coordination networks
The coordination chemistry of pyrazines is rich, and one-
dimensional coordination polymers are expected on self-
assembly with silver(I) since silver(I) favors linear coordi-
nation geometry. Indeed, the first structural characterization
according to Figure 1. We planned to prepare ordered one-
* Author to whom correspondence should be addressed. E-mail: erb625f@
smsu.edu.
† Southwest Missouri State University.
‡ University of Kansas.
(1) Bosch, E.; Barnes, C. L. Inorg. Chem. 2001, 40, 3234.
(2) Bosch, E.; Barnes, C. L. Inorg. Chem. 2002, 41, 2543.
(3) For a recent review see: Evans, O. R.; Lin, W. Acc. Chem. Res. 2002,
35, 511.
5304 Inorganic Chemistry, Vol. 42, No. 17, 2003
10.1021/ic034277m CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/30/2003

Ag(I) Coordination Chemistry of 2,6-Diarylpyrazines
Anal. Calcd for C42H40N4O2F3Ag: C, 63.24; H, 5.05; N, 7.02.
Found: C, 63.35; H, 5.05; N, 7.02.
(2,6-Diphenylpyrazine)silver(I) Nitrate, 2. Silver(I) nitrate (15
mg, 0.09 mmol) and 2,6-diphenylpyrazine (22 mg, 0.09 mmol) were
placed in a screw-cap vial along with acetonitrile (2 mL). The vial
was sealed and gently heated until a clear, homogeneous mixture
formed. The vial was allowed to cool in the dark. Colorless cube-
shaped crystals were harvested after 1 day (21 mg, 70%): mp 190-
192 °C. Anal. Calcd for C32H24AgN5O3: C, 60.65; H, 3.82; N,
11.06. Found: C, 60.04; H, 3.78; N, 11.22.
(2,6-Bis(3′,5′-dimethylphenyl)pyrazine)silver(I) Trifluoro-
acetate, 3. Silver(I) trifluoroacetate (12.6 mg, 0.06 mmol) and 2,6-
bis(3′,5′-dimethylphenyl)pyrazine (16.7 mg, 0.06 mmol) were
placed in a screw-cap vial along with dichloromethane (3 mL). The
vial was sealed and gently heated until a clear, homogeneous
solution formed. The vial was allowed to cool in the dark, and
colorless rod-shaped crystals were harvested after one week (17
mg, 58%): mp 230-232 °C. Anal. Calcd for C44H40N4O4F6Ag2:
C, 51.89; H, 3.96; N, 5.50. Found: C, 51.95; H, 3.79; N, 5.43.
(2,6-Diphenylpyrazine)silver(I) Tetrafluoroborate, 4. A ho-
mogeneous solution of silver(I) tetrafluoroborate (27 mg, 0.14
mmol) and 2,6-diphenylpyrazine (20 mg, 0.09 mmol) in a mixture
of nitromethane (6 mL) and toluene (2 mL) was formed after gentle
heating in a sealed screw-cap vial. The vial was stored in the dark.
After one week the cap was loosened and the solvent allowed to
evaporate slowly. Colorless rods were harvested after a further week
(25 mg, 80%): mp >270 °C. Anal. Calcd for C32H24N4BF4Ag‚
CH3NO2: C, 55.07; H, 3.78; N, 9.74. Found: C, 54.94; H, 3.73;
N, 9.52.
Figure 1. Proposed formation of ordered one-dimensional coordination
polymers on self-assembly of 2,6-diarylpyrazines with silver(I) salts.
of a one-dimensional coordination polymer formed between
silver(I) and pyrazine was reported by Vranka and Amma
in 1966 using silver(I) nitrate.4 It should, however, be noted
that although silver(I) has a preference for coordination
number 2 with linear coordination geometry, other coordina-
tion numbers and geometries are possible. Indeed, in 1996
Moore and co-workers reported a compilation of the
coordination geometries about silver(I) using 90 structures
culled from the Cambridge Crystallographic Database.5
Linear coordination geometry was most common with 43
occurrences. Other common geometries included trigonal
planar, tetrahedral, and trigonal pyramidal with occasional
instances of square planar, pyramidal, T-shaped, and bent
geometry. The flexibility in coordination about silver(I) has
also been noted in silver(I)-pyrazine complexation. It is,
for example, noteworthy that Ciani and co-workers reported
the characterization of several different coordination networks
on self-assembly of pyrazine with silver(I) tetrafluoroborate.6
Only one of these networks was a simple one-dimensional
coordination polymer. Both Ciani7 and Moore5 have reported
the formation of one-dimensional coordination networks on
self-assembly of pyrazine with silver(I) hexafluorophosphate.
(2,6-Bis(2′,6′-Dimethylphenyl)pyrazine)silver(I) Trifluoro-
acetate, 5. A homogeneous solution of silver(I) trifluoroacetate (32
mg, 0.15 mmol) and 2,6-bis(2′,6′-dimethylphenyl)pyrazine (20 mg,
0.07 mmol) in a mixture of nitromethane (8 mL) and acetonitrile
(0.5 mL) was formed in a screw-capped vial on gentle heating.
The vial was stored in the dark, and colorless rods were harvested
after 2 days (34 mg, 67%): mp 245-247 °C. Anal. Calcd for
C24H20N2O4F6Ag2: C, 39.48; H, 2.77; N, 3.85. Found: C, 39.76;
H, 2.94; N, 4.03.
Experimental Section
All chemicals were purchased from Aldrich and used as received.
The synthesis and characterization of the diarylpyrazines used in
this study are reported elsewhere.8
Silver(I) Complex Formation. (2,6-Bis(3′,5′-dimethylphenyl)-
pyrazine)silver(I) Trifluoroacetate, 1. Silver(I) trifluoroacetate (16
mg, 0.08 mmol) and 2,6-bis(3′,5′-dimethylphenyl)pyrazine (20 mg,
0.07 mmol) were placed in a screw-cap vial along with acetonitrile
(5 mL). The vial was sealed and gently heated until a clear,
homogeneous solution was formed. The vial was allowed to cool
in the dark. After one week colorless rod-shaped crystals were
harvested (19 mg, 69%): mp 235-237 °C. Anal. Calcd for C84H80-
Ag2F6N8O4: C, 63.24; H, 5.05; N, 7.02. Found: C, 63.11; H, 5.23;
N, 7.14. Since elemental analysis indicated that the ratio of ligand
to silver(I) trifluoroacetate was 1:2, the reaction was repeated with
this initial ratio. Thus, a homogeneous solution of silver(I)
trifluoroacetate (8.1 mg, 0.037 mmol) and 2,6-bis(3′,5′-dimethyl-
phenyl)pyrazine (20.8 mg, 0.07 mmol) in acetonitrile (5 mL) was
prepared as described above. After one week thin colorless rod-
shaped crystals were harvested (25 mg, 87%): mp 235-237 °C.
(2-(2′,6′-Dimethylphenyl)-6-(3′′,5′′-dimethylphenyl)pyrazine)-
silver(I) Trifluoroacetate, 6. A homogeneous solution of silver-
(I) trifluoroacetate (29 mg, 0.13 mmol) and 2-(2′,6′-dimethylphenyl)-
6-(3′′,5′′-dimethylphenyl)pyrazine (18 mg, 0.06 mmol) in toluene
(5 mL) was obtained on gentle heating in a screw-cap vial. The
vial was stored in the dark, and colorless needles were harvested
after 3 days (40 mg, 62%): mp 190-192 °C. Anal. Calcd for
C52H40N4O12F18Ag6‚2C7H8: C, 38.08; H, 2.71; N, 2.69. Found: C,
38.35; H, 2.73; N, 2.69.
Crystallography. Crystallographic measurements were per-
formed at 100 K using a Bruker APEX diffractometer with graphite-
monochromated Mo KR (λ ) 0.71073 Å) radiation. Single crystals
of all compounds were coated with Paratone-N oil, attached to glass
fibers, and transferred to the diffractometer. Approximately 11/2
hemispheres of data were collected as ω-scan images. None of the
crystals showed significant decay during data collection. The data
were integrated and corrected for Lorentz and polarization effects
with SAINT9 and were corrected for absorption with SADABS.10
(4) Vranka, R. G.; Amma, E. L. Inorg. Chem. 1966, 5, 1020.
(5) Venkataraman, D.; Lee, S.; Moore, J. S.; Zhang, P.; Hirsch, K. A.;
Gardner, G. B.; Covey, A. C.; Prentice, C. L. Chem. Mater. 1996, 8,
2030.
(6) (a) Carlucci, L. Ciani, G.; Proserpio, D. M.; Sironi, A. J. Am. Chem.
Soc. 1995, 117, 4562. (b) Carlucci, L. Ciani, G.; Proserpio, D. M.;
Sironi, A. Inorg. Chem. 1995, 34, 5698.
(9) (a) Data Collection: SMART Software Reference Manual, Bruker-
AXS, 6300 Enterprise Dr., Madison, WI 53719-1173, 1994. (b) Data
Reduction: SAINT Software Reference Manual, Bruker-AXS, 6300
Enterprise Dr., Madison, WI 53719-1173, 1995.
(7) Carlucci, L. Ciani, G.; Proserpio, D. M.; Sironi, A. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1895.
(8) Schultheiss, N.; Bosch, E. Heterocycles 2003, 60, in press.
(10) Sheldrick, G. M. SADABS. Program for Empirical Absorption
Correction of Area Detector Data, University of Go¨ttingen, Germany,
2000.
Inorganic Chemistry, Vol. 42, No. 17, 2003 5305

Schultheiss et al.
Table 1. Crystallographic Dataa for 1, 2, 3, 4‚1/2CH3NO2, 5, and 6‚2C7H8
1
2
3
4‚1/2CH3NO2
5
6‚2C7H8
empirical formula
fw
space group
Z
C84H80Ag2F6N8O4
1595.30
P?1
1
C32H24AgN5O3
634.43
Pbcn
C22H20AgF3N2O2
509.27
P?1
2
C33H27AgBF4N5O2
720.28
P?1
1
C24H20Ag2F6N2O4
730.16
P21/n
C66H56Ag6F18N4O12
2086.37
P21/c
4
4
2
a, Å
b, Å
c, Å
9.4603(6)
13.3544(8)
14.7099(9)
82.780(2)
84.554(2)
75.649(2)
1782.20(19)
1.486
7.1361(6)
30.910(2)
11.9478(9)
90
90
90
2635.4(3)
1.599
2.79 (9.26)
4.5443(4)
14.3428(13)
15.3996(14)
89.317(2)
84.737(2)
87.086(2)
998.16(16)
1.694
3.8099(3)
11.8095(9)
16.4554(12)
84.265(2)
83.833(2)
85.556(2)
730.74(10)
1.637
10.5410(11)
21.859(2)
10.9825(11)
90
100.434(2)
90
2488.7(4)
1.653
3.15 (11.51)
11.7717(6)
24.9539(12)
12.8210(6)
90
110.828(2)
90
3520.1(3)
1.968
3.37 (8.12)
R, deg
â, deg
γ, deg
V, Å3
d
calcd, g/cm3
R1 (wR2),b %
2.73 (7.57)
3.45 (8.69)
3.40 (8.76)
2 2
2 2
a All data sets were collected at T ) 100 K using Mo KR (λ ) 0.71073 Å) radiation. b R1 ) ∑||Fo| - |Fc||/∑|Fo|, wR2 ) {∑[w(Fo2 - Fc ) /∑[w(Fo ) ]}1/2
.
Table 2. Selected Bond Lengths, Å, for Compounds 1-6
Compound 1
Ag(1)-N(2)
Ag(1)-O(1)
O(2)-Ag(1)#1
2.2045(14)
2.5584(13)
2.5742(13)
Ag(1)-N(3)
Ag(1)-O(2)#2
Ag(1)-Ag(1)#1
2.2164(15)
2.5742(13)
3.1014(3)
Compound 2
Ag(1)-N(2)
2.2052(14)
Ag(1)-O(1)
2.7051(14)
Compound 3
Ag(1)-N(1)
Ag(1)-O(2)#1
Ag(1)-Ag(1)#2
2.355(2)
Ag(1)-O(1)
2.2312(17)
2.1871(16)
2.9710(4)
2.1871(16)
3.0388(4)
O(2)-Ag(1)#1
Ag(1)-Ag(1)#1
Compound 4
Ag(1)-N(1)
2.1423(16)
Ag(1)-Ag(1)#1
3.810(4)
Compound 5
Figure 2. Perspective view of complex 1 formed on self-assembly of
equimolar amounts of 2,6-bis(3′,5′-dimethylphenyl)pyrazine and silver(I)
trifluoroacetate from acetonitrile solution. The asymmetric unit is shown
with the displacement ellipsoids drawn at the 50% level, and those atoms
are labeled. The hydrogen atoms are omitted for clarity.
Ag(1)-N(1)
Ag(1)O(1)
Ag(2)-O(3)
Ag(2)-Ag(2)#2
2.202(2)
2.457(2)
2.274(2)
2.9137(5)
Ag(1)-N(2)#1
Ag(2)-O(4)#2
Ag(2)-O(1)
2.235(2)
2.229(2)
2.567(3)
Compound 6
Ag(1)-N(1)
Ag(1)-O(2)
Ag(2)-O(3)
Ag(2)-O(5)
Ag(3)-O(5)
Ag(3)-Ag(3)#2
2.2310(2)
2.327(2)
Ag(1)-N(2)#1
2.365(2)
2.3283(19)
2.5273(2)
2.258(2)
2.454(2)
2,6-diarylpyrazines with 1 equiv of a silver(I) salt according
to Figure 1. We reasoned that even the aryl groups that were
not specifically held orthogonal to the pyrazine ring could
easily adopt such a conformation to form an ordered polymer.
We first treated 2,6-bis(3′,5′-dimethylphenyl)pyrazine with
1 equiv of silver(I) trifluoroacetate in acetonitrile and allowed
the components to self-assemble. We isolated colorless rod-
shaped crystals, and elemental analysis of the bulk solid
indicated that a 2:1 ligand-silver complex, 1, had been
formed. A single crystal suitable for X-ray analysis was
selected for analysis and the structure determined. The silver
complex crystallized as a dimeric 2:1 complex. Only half of
the dimer, labeled in Figure 2, was crystallographically
unique.
Ag(1)-O(4)
2.2603(19)
2.4749(19)
2.287(2)
Ag(2)-O(1)
Ag(3)-O(6)#2
Ag(3)-O(1)
2.91384(4)
Space group assignments were based on systematic absences and
statistical tests and verified by structure refinement. Structures were
solved by direct methods and refined against all data using the
SHELXTL software package.11 All non-hydrogen atoms were
refined with anisotropic displacement parameters. Hydrogen atoms
were assigned positions by geometry and refined with a riding
model using isotropic displacement parameters 1.2 times (1.5 for
methyl) those of the attached atoms. Basic structural parameters
are presented in Table 1. The anionic species of compound 4‚1/2-
CH3NO2 was disordered and modeled in two orientations with
refined occupancies of 0.272(3) and 0.228(3) for the unprimed and
primed atoms. The 0.5 occupancy solvent was also near this same
region of the cell. Restraints on the positional and displacement
parameters of the anion and solvent of 4‚1/2CH3NO2 were required.
Restraints on the positional parameters of the fluorine atoms of
compound 6‚2C7H8 were required. Selected bond lengths from
structures 1-6 are shown in Table 2.
Two of the dimethylphenyl rings are essentially coplanar
with the pyrazine ring with torsional angles of approximately
3.5° and 4.5° for rings C(33)-C(40) and C(3)-C(10),
respectively. The remaining two aryl groups are twisted out
of planarity with the pyrazine ring by 24° and 28° for rings
C(13)-C(20) and C(23)-C(30), respectively. Within each
dimer unit the diarylpyrazine rings are π-stacked with
interplanar distances in the range 3.4-3.6 Å. The two
trifluoroacetate ligands each bridge across two silver atoms
and thereby effectively fill the space between the two pairs
of diarylpyrazine ligands. The silver-nitrogen bond lengths
of 2.2045(14) and 2.2164(15) Å are well within the general
range of 2.18-2.36 Å reported thus far for silver-pyrazine
Results and Discussion
We were specifically interested in the synthesis of ordered
one-dimensional coordination polymers by self-assembly of
(11) (a) Sheldrick, G. M. SHELXTL Version 5 Reference Manual, Bruker-
AXS, 6300 Enterprise Dr., Madison, WI 53719-1173, 1994. (b)
International Tables for Crystallography; Kluwer: Boston, 1995; Vol.
C, Tables 6.1.1.4, 4.2.6.8, and 4.2.4.2.
5306 Inorganic Chemistry, Vol. 42, No. 17, 2003

Ag(I) Coordination Chemistry of 2,6-Diarylpyrazines
Figure 5. Perspective view of the nitrate-bridged coordination network 2
showing the extensive π-stacking of the pyrazyl and phenyl rings.
Figure 3. View of the packing of adjacent complexes 1.
symmetry, the N-Ag-N angle is 180° with N(2)#1-
Ag(1)-O(1) and N(2)-Ag(1)-O(1) angles of 97.14(5)° and
82.86(5)°, respectively. The silver-nitrogen distance of
2.2052(14) Å and the silver-oxygen distance of 2.7051(14)
Å are similar to the distances of 2.214 and 2.722 Å,
respectively, reported by Vranka and Amma for the unsub-
stituted pyrazine-silver(I) nitrate structure.4 The phenyl rings
are slightly twisted with respect to the central pyrazine with
torsional angles of 6° and 26° for phenyls C(11)-C(16) and
C(3)-C(8), respectively. The nitrate counterions effectively
bridge adjacent complexes to form a silver-nitrate ribbonlike
structure as shown in Figure 5. In the structure of 2 the
bridging nitrate anions orient the adjacent ligands so that
they partially overlap to form an intertwined one-dimensional
ribbon. For each pair of adjacent complexes a phenylpyrazine
moiety from one ligand π-stacks with a phenylpyrazine
moiety from the ligand complexing the adjacent silver cation.
The interplanar distance is approximately 3.5 Å. It is
interesting to note that Vranka and Amma reported that
nitrate atoms bridged adjacent one-dimensional pyrazine-
silver(I) strands to form a two-dimensional network in the
unsubstituted pyrazine-silver(I) nitrate structure.
In each of the complexes 1 and 2, at least one of the
pendant aryl rings is almost coplanar with the central
pyrazine ring, thereby rendering the “hindered” N inacces-
sible to complexation. We reasoned that the unreacted
equivalent of silver(I) salt remained in solution in both these
experiments as the acetonitrile solvate. The coordinating
solvent thus actually facilitated formation of the 2:1 com-
plexes. We speculated that we could force the second silver
atom to complex the hindered nitrogen atom by changing to
a less coordinating solvent as shown in Figure 6.
We initially chose dichloromethane as our “noncoordi-
nating” solvent and used the sparingly soluble silver(I)
trifluoroacetate salt. In this solvent we were not able to
solubilize the nitrate salt even in the presence of the pyrazine
ligands. Accordingly, we allowed an equimolar mixture of
2,6-bis(3′,5′-dimethylphenyl)pyrazine and 1 equiv of silver-
(I) trifluoroacetate to self-assemble in dichloromethane.
Elemental analysis of the resultant colorless solid indicated
that a 1:1 complex, 3, was indeed formed, and we assumed
that this was the expected coordination polymer. Subsequent
X-ray structural analysis indicated that we did not form a
coordination polymer but rather a trifluoroacetate-bridged
2:2 coordination complex as shown in Figure 7. A key feature
of this structure is that the trifluoracetate anions now bridge
two silver atoms between the two pyrazine ligands. Interest-
Figure 4. View of complex 2 formed on self-assembly of equimolar
amounts of 2,6-diphenylpyrazine and silver(I) nitrate in acetonitrile solution.
The extended coordination of the nitrate anion to adjacent complexes is
shown.
complexes.12 Throughout this paper we have drawn a bond
between silver atoms separated by 3.1 Å or less although
this assignment is controversial. This distance is only slightly
longer than the separation of 2.884 Å in silver metal,13 and
Wheatley recently argued that there is significant metal-
metal bonding at these short distances.14 In contrast it is
interesting to note that Brammer recently reported the
structure of the pyrazine-silver(I) trifluoroacetate complex
and noted a silver-silver separation of 3.99 Å. In that
network a more complex trifluoroacetate bridging motif was
observed.15
The complexes stack in the offset manner shown in Figure
3 with an interplanar distance of approximately 3.3 Å.
In a similar experiment the self-assembly of equimolar
amounts of 2,6-diphenylpyrazine and silver(I) nitrate in
acetonitrile solution yielded colorless crystals. Elemental
analysis of the bulk solid indicated that a 2:1 complex, 2,
had been formed. Single-crystal X-ray analysis indicated that
the 2:1 complex shown in Figure 4 was formed.
As seen in Figure 4 the silver(I) cation has pseudo square
planar geometry. Since the silver sits on a center of
(12) See Vranka, Ciani, and Moore in refs 4-7. Robson reported a bond
length of 2.363 Å in the complex silver(I) tricyanomethanide: Batten,
S. R.; Hoskins, B. F.; Robson, R. New J. Chem. 1998, 22, 173.
(13) (a) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements:
Pergamon: Oxford, 1984; p 1368. (b) Pauling, L. The Nature of the
Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960.
(14) Ahmed, L. S.; Dilworth, J. R.; Miller, J. R.; Wheatley, N. Inorg. Chim.
Acta 1998, 278, 229.
(15) Brammer, L.; Burgard, M. D.; Eddleston, M. D.; Rodger, C. S.; Rath,
N. P.; Adams, H. Cryst. Eng. Commun. 2002, 4, 239.
Inorganic Chemistry, Vol. 42, No. 17, 2003 5307

Schultheiss et al.
Figure 6. Postulated solvent control of complexation and coordination polymer formation.
Figure 9. View of the 2:1 complex 4 formed on self-assembly of 2,6-
diphenylpyrazine with silver(I) tetrafluoroborate from a mixture of toluene
and nitromethane with the thermal ellipsoids drawn at 50% occupancy.
Figure 7. View of the 2:2 complex 3 formed on self-assembly of equimolar
amounts of 2,6-bis(3′,5′-dimethylphenyl)pyrazine and silver(I) trifluoroac-
etate from dichloromethane solution.
crystallize complexes with a variety of silver salts including
the hexafluoroantimonate and tetrafluoroborate in noncoor-
dinating solvents. We were only successful in growing X-ray-
quality crystals using silver(I) tetrafluoroborate. In this
experiment we used a large excess of the silver salt with a
mixed toluene/nitromethane noncoordinating solvent. Color-
less rod-shaped crystals were formed, and elemental analysis
of the bulk solid indicated that a 2:1 ligand-silver complex,
4, was formed. The 2:1 complex crystallized with one
nitromethane solvent molecule included. The structure is
shown in Figure 9.
Figure 8. View of the π-stacking of adjacent 2:2 complexes 3 showing
the short silver-silver contacts between complexes.
The silver-nitrogen bond length of 2.1423(16) Å is
notably shorter than the other silver-nitrogen bond lengths
reported herein. The closest contacts to the silver are 2.839
Å for a nitromethane oxygen, Ag(1)-O(2s), and 2.835 Å
for a tetrafluoroborate fluorine, Ag(1)-F(2). Since the cation
lies on a center of symmetry, the geometry about silver is
perfectly linear. The torsional angles between the two phenyl
rings and the pyrazine are approximately 27° and 32° for
C(5)-C(10) and C(11)-C(16), respectively. It is interesting
to note that a similar short silver-nitrogen bond length of
2.193(3) Å was reported by Ciani for the unsubstituted
pyrazine-silver(I) tetrafluoroborate one-dimensional polymer.6a
These bond lengths are presumably shorter because the
noncoordinating anion does not dissipate the Lewis acid
character of the cation in contrast to coordination by the
trifluoroacetate anion, which reduces the Lewis acidity of
the cation.
The complexes are slip-stacked as shown in Figure 10 with
complex-complex (and long silver-silver) separation of
approximately 3.8 Å.
We were thus unsuccessful in our attempts to form one-
dimensional coordination polymers using “solvent” or “an-
ion” control. At this stage we assumed that the propensity
for π-stacking of the essentially planar 2,6-diarylpyrazines
in the solid state controlled the coordination chemistry of
ingly, we had earlier observed similar trifluoroacetate-bridged
pairs of anions in the one- and two-dimensional networks
obtained on self-assembly of trimesityltriazine with silver-
(I) trifluoroacetate.2 More importantly, Brammer has recently
determined the structure of a host of coordination networks
formed by self-assembly of silver(I) carboxylates with ditopic
N-ligands and concluded that the carboxylate-bridged silver-
silver dimer is a reliable supramolecular synthon.15,16
The silver-silver separation of 2.9710(4) Å is slightly
shorter that that observed in complex 1 (Figure 8). The ligand
is essentially planar with (dimethylphenyl)pyrazine torsional
angles of approximately 7° and 26° for aryl rings C(3)-
C(8) and C(13)-C(18). The 2:1 complexes are slip-stacked
so that Ag(1) overlies Ag(1)#2 with a separation of 3.0388-
(4) Å. The second silver in each complex overlies the
centroid of the pyrazine in the neighboring complex with a
separation of 3.2 Å.
Clearly, the simple idea of “solvent control” outlined in
Figure 6 was unsuccessful due to the coordinating nature of
the trifluoroacetate anion. We therefore decided to study the
self-assembly of the 2,6-diarylpyrazines with silver(I) salts
with noncoordinating anions. Accordingly, we attempted to
(16) Brammer, L.; Burgard, M. D.; Rodger, C. S.; Swearingen, J. K.; Rath,
N. P. Chem. Commun. 2001, 2468.
5308 Inorganic Chemistry, Vol. 42, No. 17, 2003

Ag(I) Coordination Chemistry of 2,6-Diarylpyrazines
Figure 10. Two orthogonal views showing the stacking of the (2,6-diphenylpyrazine)silver(I) tetrafluoroborate complexes with the counterion and solvent
molecule omitted for clarity.
Figure 11. Simple interpretation of the network formed on self-assembly
of silver(I) trifluoroacetate with 2,6-bis(2′,6′-dimethylphenyl)pyrazine.
Figure 12. Backbone of the coordination network 5 formed between silver-
these ligands. We decided to counteract the π-stacking by
(I) trifluoroacetate and 2,6-bis(2′,6′-dimethylphenyl)pyrazine in toluene
solution. The arrows indicate the positions at which the silver-pyrazine
strands are interconnected.
fixing the aryl substituents orthogonal to the pyrazine ring
by introducing methyl substituents in the ortho positions on
the aryl rings as we had previously done with pyridine- and
triazine-based ligands.1,2 Accordingly, we allowed 2,6-bis-
(2′,6′-dimethylphenyl)pyrazine to self-assemble with an
excess of silver(I) trifluoroacetate in toluene along with a
trace of acetonitrile. Colorless rod-shaped crystals were
obtained, and elemental analysis of the bulk solid indicated
that a 1:2 ligand-silver complex, 5, was formed. It is
interesting to note that the same 1:2 ligand-silver complex
was also exclusively formed on self-assembly from solutions
more enriched in silversfor example, with ratios of ligand
to silver as high as 1:4. X-ray structural analysis indicated
that a two-dimensional coordination network was formed.
A schematic representation of the network is shown in Figure
11. In this representation the horizontal linkages are made
of pyrazine-silver strands with alternating silver cations and
pyrazine molecules. The vertical links between these strands
are made by complex bridging disilver(I) tetrakis(trifluoro-
acetate) dianions.
Figure 13. Perspective view of the complex dianion that bridges between
silver atoms from adjacent strands of the silver-pyrazine coordination
polymer. The pendant silver atoms are from adjacent strands of silver-
pyrazine complexes.
to cross-linking coordination by the complex dianion shown
in Figure 13.
The honeycomb-like two-dimensional coordination net-
work is shown in Figure 14 with large hexagonal rings
containing 30 atoms, 10 of which are silver(I) cations.
We were intrigued by the possibility that it would be
sufficient to have only one of the aryl groups held orthogonal
while allowing the second ring free rotation. Accordingly,
we repeated the aforementioned experiment with the mixed
arylpyrazine 2-(2′,6′-dimethylphenyl)-6-(3′′,5′′-dimethylphe-
nyl)pyrazine and obtained a crystalline solid that had a
complex elemental analysis that indicated a 3:1 silver-ligand
complex, 6, was formed. Single-crystal X-ray analysis
revealed a complex corrugated two-dimensional coordination
network. The network comprises the familiar silver-pyrazine
strands cross-linked by bridging silver(I) trifluoroacetates.
The zigzag silver-pyrazine strands shown in Figure 15 are
similar to those found in structure 5.
A view of one of the one-dimensional strands of pyrazine-
bound silver cations is shown in Figure 12. As expected the
o-methyl groups effectively hold the aryl rings almost
orthogonal to the central pyrazine ring with torsional angles
of approximately 78° and 82° for C(13)-C(19) and C(3)-
C(9), respectively. This, more rigid, conformation facilitated
complexation at the hindered nitrogen atom with concomitant
formation of a coordination network as opposed to discrete
coordination complexes. The silver-nitrogen bond lengths
are similar at 2.202(2) and 2.235(2) Å for Ag(1)-N(1) and
Ag(1)-N(2)#1, respectively.
As shown in Figure 12 the silver-pyrazine backbone has
a zigzag shape with a N-Ag-N bond angle of 146.46(9)°.
The orientation of the aryl substituents opens the silver atoms
A more complex silver(I) trifluoroacetate bridge was
formed between adjacent strands of the silver-pyrazine
Inorganic Chemistry, Vol. 42, No. 17, 2003 5309

Schultheiss et al.
Figure 17. View of a portion of the silver trifluoroacetate linkage
emphasizing the Ag-π interactions between Ag(1) and the pendant
dimethylphenyl ring and Ag(3) and the included solvent molecule, toluene.
Figure 14. Two orthogonal views of the two-dimensional network 5 with
the trifluoromethyl groups, the pendant dimethylphenyl rings omitted for
clarity.
Figure 18. Two orthogonal views of the two-dimensional network 6 with
the dimethylphenyl rings, incorporated toluene, and CF3 groups omitted
for clarity.
Figure 15. View of the backbone of the coordination network 6 formed
between silver(I) trifluoroacetate and 2-(2′,6′-dimethylphenyl)-6-(3′′,5′′-
dimethylphenyl)pyrazine in toluene solution.
toluene with a slightly longer separation of C(4S)-Ag(3) )
2.707 Å. These interactions are shown in Figure 17. The
weaker silver-arene interactions observed here may be a
result of the multiple silver-oxygen interactions that reduce
the Lewis acidity of the silver(I) cation.
The two-dimensional network is shown in the two
orthogonal views in Figure 18.
In conclusion, we have prepared coordination networks
by self-assembly of silver salts with 2,6-diarylpyrazines in
which at least one of the aryl groups is held orthogonal to
the pyrazine ring due to an ortho substituent. With other aryl
groups discrete complexes were obtained presumably due
to the preferred π-stacking of the essentially planar triaryl
ligands in the solid state. All the complexes with silver(I)
trifluoroacetate included the dimeric silver(I) trifluoroacetate
supramolecular synthon.
Figure 16. View of the complex silver trifluoroacetate bridge between
adjacent strands of the silver-pyrazine coordination network 6.
complex and is shown in Figure 16. In this figure the terminal
silver atoms, Ag(1), are also part of the adjacent silver-
pyrazine strands.
Acknowledgment. We thank the Petroleum Research
Fund, administered by the American Chemical Society (Grant
No. 37506-B3), and the Graduate College at SMSU for
partial funding of this research, the National Science
Foundation (NSF) (Grant CHE-0079282) and the University
of Kansas for funds to purchase the X-ray instrument and
computers, and the NSF for a GK-12 Fellowship for N.S.
(Grant No. DGE-0086335).
A silver atom is η1 coordinated, or σ-bound, to the pendant
2′,6′-dimethylphenyl ring with C(15)-Ag(2) ) 2.499 Å. In
their seminal paper Lindeman, Rathore, and Kochi analyzed
all reported crystal structures containing silver-arene inter-
actions and reported that the separation of silver(I) from the
mean plane of the coordinated benzene lies in a narrow
range.17 This distance was found to be 2.41 ( 0.05 Å. The
distance reported here is thus marginally longer. Another
silver atom is η1 coordinated to the incorporated solvent
Supporting Information Available: Six X-ray crystallographic
information files in CIF format. This material is available free of
charge via the Internet at http://acs.pubs.org.
(17) Lindeman, S. V.; Rathore, R.; Kochi, J. K. Inorg. Chem. 2000, 39,
5707.
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5310 Inorganic Chemistry, Vol. 42, No. 17, 2003