Synthesis of an anthracene-based bis(pyrazolyl)methane ligand and the structural characterization of its dinuclear tricarbonylrhenium(I) complex

Article abstract of DOI:10.1016/j.jorganchem.2007.03.010

The new anthracene-based, bitopic bis(pyrazolyl)methane ligand 1,8-bis(4-[bis(1-pyrazolyl)methyl]phenyl)anthracene (1,8-[4-CH(pz)₂C₆H₄]₂C₁₄H₈) has been prepared by the cobalt-catalyzed reaction between thionyldipyrazole and 1,8-bis(4-formylphenyl)anthracene. The reaction between 1,8-[4-CH(pz)₂C₆H₄]₂C₁₄H₈ and Re(CO)₅Br yielded the dirhenium complex {μ-1,8-[4-CH(pz)₂C₆H₄]₂C₁₄H₈}[Re(CO)₃Br]₂. The solid state structure of this complex displays extensive noncovalent interactions, particularly CH-π and π-π interactions.

Full text of DOI:10.1016/j.jorganchem.2007.03.010

Journal of Organometallic Chemistry 692 (2007) 3094–3099
www.elsevier.com/locate/jorganchem
Note
Synthesis of an anthracene-based bis(pyrazolyl)methane ligand and
the structural characterization of its dinuclear
tricarbonylrhenium(I) complex
*
Daniel L. Reger , Russell P. Watson, Mark D. Smith
Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208, USA
Received 1 February 2007; received in revised form 7 March 2007; accepted 7 March 2007
Available online 16 March 2007
Abstract
The new anthracene-based, bitopic bis(pyrazolyl)methane ligand 1,8-bis(4-[bis(1-pyrazolyl)methyl]phenyl)anthracene (1,8-[4-CH(pz)2-
C6H4]2C14H8) has been prepared by the cobalt-catalyzed reaction between thionyldipyrazole and 1,8-bis(4-formylphenyl)anthracene. The
reaction between 1,8-[4-CH(pz)2C6H4]2C14H8 and Re(CO)5Br yielded the dirhenium complex {l-1,8-[4-CH(pz)2C6H4]2C14H8}-
[Re(CO)3Br]2. The solid state structure of this complex displays extensive noncovalent interactions, particularly CH–p and p–p interactions.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Bis(pyrazolyl)methane; Tricarbonylrhenium(I); Supramolecular; Anthracene
1. Introduction
We previously reported the structures of multimetallic
rhenium(I) complexes of the arene-linked ligands m-
[CH(pz)2]2C6H4 [6a], p-[CH(pz)2]2C6H4 [6a], and 1,3,5-
[CH(pz)2]3C6H3 [6b]. Crystallographic studies revealed that
CH–p and p–p interactions are dominant in directing the
supramolecular structures of these complexes. To further
explore the importance of such interactions among com-
plexes comprising relatively rigid, arene-based ligands, we
prepared the compound 1,8-bis(4-[bis(1-pyrazolyl)methyl]-
phenyl)anthracene (1,8-[4-CH(pz)2C6H4]2C14H8, L), an
anthracene-based analogue to the phenylene- and mesityl-
ene-linked ligands just described. The additional fused aryl
groups of this ligand as well as its phenylene substituents
increase the potential for both CH–p and p–p interactions.
Herein, we describe the synthesis of L and the synthesis and
structure of a dirhenium complex of this ligand.
The design of solid materials with useful properties relies
on the thorough understanding of the factors that influence
their molecular and supramolecular structures [1]. We have
been exploring the coordination chemistry of poly(pyraz-
olyl)methane compounds that are functionalized at the
carbon ‘‘backbone’’ to give ‘‘third-generation’’ poly(pyraz-
olyl)methane ligands [2,3]. Specifically, in these ligands two
or more poly(pyrazolyl)methyl groups are linked through
organic spacers to yield polytopic ligands. Using flexible
[4], semi-rigid [5], and rigid [6] spacers, we have explored
the roles that ligand size and flexibility play in dictating
the final solid state structures of metal complexes. In addi-
tion, we have found that noncovalent interactions, such as
p–p stacking and weak hydrogen bonds (CH–p and CH–X,
where X = N, O, F) [7], made possible through the func-
tionality built into the ligands, organize these complexes
into supramolecular structures.
2. Experimental section
2.1. General considerations
*
Air sensitive materials were handled under a nitrogen
atmosphere using standard Schlenk techniques. All
Corresponding author. Fax: +1 803 777 9521.
E-mail address: reger@mail.chem.sc.edu (D.L. Reger).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.03.010

D.L. Reger et al. / Journal of Organometallic Chemistry 692 (2007) 3094–3099
3095
solvents were dried and distilled by conventional methods
prior to use. The compounds Re(CO)5Br [8] and 1,8-
bis(4-formylphenyl)anthracene [9] were prepared as previ-
ously described. All other chemicals were purchased from
Aldrich or Fisher Scientific and used as received. Reported
melting points are uncorrected. IR spectra were obtained
on a Nicolet 5DXBO FTIR spectrometer. NMR spectra
were recorded on a Mercury/VX 300 or Mercury/VX 400
spectrometer. All chemical shifts are in ppm and are sec-
ondary-referenced using the solvent signals. Mass spectro-
metric measurements were obtained on a VG 70S
instrument or a Micromass Q-Tof spectrometer.
The yellow solid was isolated by cannula filtration, washed
with 5 mL of Et2O, and dried in vacuo. Yield = 0.26 g
(61%). Mp: 280 ꢁC dec. IR, mC„O (KBr, cmÀ1): 2026, 1908.
1H NMR (400 MHz, DMSO-d6): d 8.74 (s, 1H, 9- or 10-H
anthracene), 8.67 (s, 2H, CH(pz)2), 8.65 (d, J = 1.8 Hz,
4 H, 5-H pz), 8.62 (s, 1H, 9- or 10-H anthracene), 8.25 (d,
J = 1.5 Hz, 4H, 3-H pz), 8.12 (d, J = 6.3 Hz, 2H, 4,5- or
2,7-H anthracene), 7.67 (d, J = 6.3 Hz, 4H, 2,6-C6H4), 7.55
(dd, J = 5.1, 6.3 Hz, 2H, 3,6-anthracene), 7.23 (d, J = 5.1
Hz, 2H, 4,5- or 2,7-H anthracene), 6.97 (t, J = 1.8 Hz, 4H,
4-H pz), 6.24 (d, J = 6.0 Hz, 4H, 3,5-C6H4). Direct Probe
MS m/z (rel. int.%) [assgn]: 1322 (1) [M]+, 1215 (4)
[MÀBrÀCO]+, 972 (10) [MÀRe(CO)3Br]+, 904 (5)
[MÀRe(CO)3BrÀHpz]+, 820 (5) [MÀRe(CO)3BrHÀpzÀ
3CO]+, 622 (30) [L]+, 554 (100) [LÀHpz]+. HRMS ESI (+)
for MÀBrÀ [C46H30BrN8O6Re2]+: calcd. = 1243.0566;
obs. = 1243.0558.
2.2. Syntheses
2.2.1. 1,8-Bis(4-[bis(1-pyrazolyl)methyl]phenyl)anthra-
cene,1,8-[4-CH(pz)2C6H4]2C14H8 (L)
Sodium hydride (1.20 g, 46.7 mmol) was suspended in
150 mL of THF and cooled in an ice-water bath for
30 min. Pyrazole (3.17 g, 46.6 mmol) was added, and the
resulting solution was stirred at 0 ꢁC for 30 min. Thionyl
chloride (1.7 mL, 23.3 mmol) was added dropwise at
0 ꢁC, and the resulting pale yellow suspension was allowed
to reach room temperature while stirring for 40 min. Solid
1,8-bis(4-formylphenyl)anthracene (1.50 g, 3.88 mmol) and
anhydrous CoCl2 (0.10 g, 0.77 mmol) were added to this
suspension at once, and the system was heated at reflux
for 24 h. After cooling to room temperature, water
(50 mL) was added, and the system was stirred for
40 min. The resulting solution was extracted with CH2Cl2
(1 · 50 mL, 2 · 20 mL) and the combined extracts washed
with dilute aqueous NaCl and dried over MgSO4. This
mixture was filtered, the solvent removed by evaporation,
and the solid taken up in CH2Cl2 for chromatography
(SiO2, CH2Cl2/Et2O 3/1). Yield = 0.74 g (31%). Mp:
252–254 ꢁC. IR (KBr, cmÀ1): 3044, 2958, 1516, 1430,
2.3. Crystal structure determination
X-ray intensity data from a colorless needle of 1 were
measured at 150(1) K on a Bruker SMART APEX CCD-
based diffractometer (Mo Ka radiation, k = 0.71073 A)
˚
[10]. Raw data frame integration and Lp corrections were
performed with SAINT+ [10]. Final unit cell parameters were
determined by least-squares refinement of 7082 reflections
with I > 5r(I) from the data set. Analysis of the data
showed negligible crystal decay during collection. The data
were corrected for absorption effects with SADABS [10].
Direct methods structure solution, difference Fourier calcu-
lations and full-matrix least-squares refinement against F2
were performed with SHELXTL [11].
The compound crystallizes in the space group P21/n as
determined by the pattern of systematic absences in the
intensity data. The asymmetric unit contains one Re com-
plex and a region of disordered solvent (centered at the ori-
gin and equivalent points). Several disorder models were
attempted for these species but none were successful.
Therefore, the disordered solvent was treated with the
Squeeze program in Platon [12]. Electron density from a
1
1385, 739. H NMR (400 MHz, CDCl3): d 8.53 (s, 1H, 9-
or 10-H anthracene), 8.39 (s, 1H, 9- or 10-H anthracene),
8.10 (s, 2H, CH(pz)2), 8.04 (d, J = 8.0 Hz, 2H, 4,5- or
2,7-H anthracene), 7.77 (d, J = 2.0 Hz, 4H, 5-H pz), 7.69
(d, J = 2.0 Hz, 4H, 3-H pz), 7.52 (dd, J = 8.6, 6.8 Hz,
2H, 3,6-anthracene), 7.45 (d, J = 8.0 Hz, 4H, 2,6- or 3,5-
C6H4), 7.39 (d, J = 6.8 Hz, 2H, 4,5- or 2,7-H anthracene),
7.06 (d, J = 8.0 Hz, 4H, 2,6- or 3,5-C6H4), 6.42
(t, J = 2.0 Hz, 4 H, 4-H pz). 13C NMR (75.5 MHz,
CDCl3): d 141.5, 141.2, 139.8, 135.7, 132.0, 130.7, 130.4,
130.2, 129.7, 128.2, 127.6, 127.0, 126.3, 125.6, 123.8,
107.0. HRMS: calcd. for C40H30N8 622.2593, found
622.2597. Direct probe MS m/z (rel. int.%) [assgn]: 622
(40) [M]+, 554 (40) [MÀHpz]+, 486 (100) [MÀ2Hpz]+.
3
˚
volume of 1750.8 A (30.8% total unit cell volume) corre-
sponding to 411 eÀ/cell was removed from subsequent
structure factor calculations by the program. The final tab-
ulated F(000), calculated density, and formula weight
reflect known unit cell contents only. All non-hydrogen
atoms were refined with anisotropic displacement parame-
ters. Hydrogen atoms were placed in geometrically ideal-
ized positions and included as riding atoms (Table 1).
3. Results and discussion
2.2.2. {l-1,8-[4-CH(pz)2C6H4]2C14H8}[Re(CO)3Br]2 (1)
Re(CO)5Br (0.33 g, 0.81 mmol) and 1,8-[4-CH(pz)2-
C6H4]2C14H8 (L: 0.20 g, 0.32 mmol) were dissolved in
200 mL of acetone, and the solution was heated at reflux.
After 2 d, a yellow precipitate had formed. After a total of
3 d at reflux, the system was cooled to room temperature.
3.1. Syntheses
The new anthracene-based ligand 1,8-[4-CH(pz)2C6H4]2-
C14H8 (L) was synthesized by the cobalt-catalyzed conden-
sation reaction [13] between thionyl dipyrazole, SO(pz)2,
prepared in situ, and the respective, previously reported

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D.L. Reger et al. / Journal of Organometallic Chemistry 692 (2007) 3094–3099
Table 1
CH(pz)2C6H4]2C14H8}[Re(CO)3Br]2 (1) within 2 d (Scheme
2). The complex is virtually insoluble in most common
organic solvents except dimethyl sulfoxide (DMSO), in
which it is freely soluble. Mass spectra of 1 show the parent
ion and ions related to the loss of the [Re(CO)3Br] frag-
ment as well as decomposition of the ligand, L. Infrared
absorption bands indicate facial coordination of the car-
bonyl groups consistent with related compounds previously
studied [6a,6b].
Crystal data and refinement details for {l-1,8-[4-CH(pz)2C6H4]2C14H8}-
[Re(CO)3Br]2 (1)
Empirical formula
Formula weight
Temperature (K)
Crystal system
C46H30Br2N8O6Re2
1323.00
150(1)
Monoclinic
P21/n
Space group
Unit cell dimensions
˚
a (A)
13.6810(7)
14.8887(8)
28.1120(15)
90
97.1940(10)
90
˚
b (A)
˚
c (A)
3.2. Solid state structure of 1
a (ꢁ)
b (ꢁ)
c (ꢁ)
Complex 1 was crystallized by the vapor diffusion of
diethyl ether into DMSO solutions of the solid. ORTEP
drawings showing two views of the complex are shown in
Fig. 1, and selected bond distances and angles are given
in Table 2. The two phenylene rings substituted onto the
anthracene linker are rotated out of the plane of the linker,
and the [Re(CO)3Br] moieties are also rotated away from
each other, presumably reducing steric crowding in the
molecule.
3
˚
V (A )
5681.1(5)
4
1.547
5.710
Semi-empirical from equivalents
2520
0.26 · 0.16 · 0.06
Z
Dcalc (Mg mÀ3
)
Absorption coefficient (mmÀ1
Absorption correction
F(000)
)
Crystal size (mm3)
Theta range for data collection 1.46–25.06
(ꢁ)
Index ranges
À16 6 h 6 16, À17 6 k 6 17,
As observed in our previous studies of arene-linked,
bis(pyrazolyl)methane complexes containing the [Re(CO)3-
Br] moiety [6a,6b], for 1 only one of the possible isomers
about the rhenium is formed. NMR and IR characteriza-
tion confirm the isomeric purity of the bulk products.
The trans-effect is responsible for the observed facial
arrangement of the CO ligands about rhenium, but of the
two isomers of this type that may be formed upon coordi-
nation by the bis(pyrazolyl)methane donor, only one is
observed for any of these complexes. This isomer, in which
the bromine atom attached to rhenium is oriented cis to the
methine proton in the boat or twist boat conformation of
the six-membered metallocycle, allows a CO ligand on each
rhenium to participate in an intramolecular p–p interac-
tions with the proximate phenylene ring; the other isomer
would place the bromine in this position. In compound 1
reported here, the distances from the phenylene ring cent-
roids to the centers of the proximate C–O bond are 3.13
À33 6 l 6 33
˚
k, Mo Ka (A)
Reflections collected
Independent reflections [Rint
Completeness to h = 25.06ꢁ
Final R indices [I > 2r(I)]
Goodness-of-fit on F2
0.71073
53,835
10,051 [0.0512]
99.8%
R1 = 0.0376; wR2 = 0.0890
1.057
]
dialdehyde [9] (Scheme 1). The ligand is a pale yellow solid
freely soluble in halogenated solvents and in tetrahydrofu-
ran. The reaction of L with Re(CO)5Br in refluxing acetone
resulted in the precipitation of the yellow solid {l-1,8-[4-
2 (pz)2SO
cat. CoCl2
˚
and 3.28 A, and the perpendicular distances from the phe-
N
H
H
N
THF, reflux
-SO2
N
N
nylene planes to the centers of the C–O bond are 3.11 and
N
N
N
N
˚
3.20 A. Since the perpendicular distances of strong p–p
O
H
O
H
L
˚
stacking interactions average 3.3–3.6 A [14], the complexes
presented here exhibit rather strong p–p interactions
Scheme 1. Preparation of 1,8-bis(4-[bis(1-pyrazolyl)methyl]phenyl)-
anthracene (L).
between the CO ligands and the linking phenylene ring.
Re(CO)5Br (excess)
N
H
H
N
N
H
H
N
acetone
reflux
N
N
N
N
N
N
CO
Re
N
N
OC
OC
N
N
N
Br
N
Re
OC
CO
Br CO
L
1
Scheme 2. Preparation of {l-1,8-[4- CH(pz)2C6H4]2C14H8} [Re(CO)3Br]2 (1).

D.L. Reger et al. / Journal of Organometallic Chemistry 692 (2007) 3094–3099
3097
Table 2
Selected bond distances (A) and angles (ꢁ) for {l-1,8-[4-CH(pz)2C6H4]2-
14H8}[Re(CO)3Br]2 (1)
˚
C
Re(1)–N(11)
Re(1)–N(21)
Re(1)–C(51)
Re(1)–C(52)
Re(1)–C(53)
Re(1)–Br(1)
Re(2)–N(31)
Re(2)–N(41)
Re(2)–C(61)
Re(2)–C(62)
Re(2)–C(63)
Re(2)–Br(2)
N(11)–Re(1)–N(21)
N(11)–Re(1)–C(51)
N(11)–Re(1)–C(52)
C(51)–Re(1)–C(52)
C(51)–Re(1)–C(53)
C(51)–Re(1)–Br(1)
N(31)–Re(2)–N(41)
N(31)–Re(2)–C(61)
N(31)–Re(2)–C(62)
C(61)–Re(2)–C(62)
C(61)–Re(2)–C(63)
C(61)–Re(2)–Br(2)
2.183(5)
2.185(5)
1.916(7)
1.909(7)
1.924(7)
2.6502(6)
2.186(5)
2.171(5)
1.904(7)
1.919(6)
1.903(6)
2.6231(6)
84.48(18)
174.7(2)
93.9(2)
88.6(2)
86.9(3)
93.90(18)
84.85(18)
173.7(2)
93.8(2)
88.6(3)
91.4(2)
90.43(18)
supramolecular structure for 1. Helical chains that run
along the b-axis are formed by a series of p–p stacking
interactions, as shown in Fig. 3 (for clarity, the dimeric
units are not shown). This p–p interaction exists between
the N(21)- and N(31)-containing pyrazolyl rings on adja-
cent molecules along the helices (shortest perpendicular
Fig. 1. ORTEP representations of {l-1,8-[4-CH(pz)2C6H4]2 C14H8}-
[Re(CO)3Br]2 (1). Ellipsoids are drawn at the 50% probability level.
Hydrogen atoms are omitted from the bottom view.
We have proposed that weak intramolecular CH–Br
interactions in the transition state of the reacting
[Re(CO)3Br] unit and the ligands direct the formation of
a single geometrical isomer analogous to those found in
this work [15]. It is reasonable to assume that the same
directing force is responsible for the formation of 1 and
that additional stabilizing p–p interactions are also present
between the CO ligands and the phenylene-linkers.
˚
distance = 3.41 A) and is the only p–p stacking present
in the crystal structure. The helical chains are re-enforced
by two CH–Br hydrogen bonds between Br(1) and the
˚
methine hydrogen atom H(2) (2.75 A) and between
Br(1) and the 5-pyrazolyl hydrogen atom H(41)
˚
(2.82 A). Finally, a CH–O hydrogen bond (not pictured)
The molecules of 1 are organized into a three-dimen-
sional supramolecular structure. CH–p interactions pro-
duce the dimeric units shown in Fig. 2. The two
molecules of each dimer exhibit two distinct, reciprocal
CH–p interactions between phenylene rings of one molecule
(hydrogen atom donors: H(75) and H(93)) and the anthra-
cene linker of the other molecule (hydrogen atom acceptors;
H-centroid distances of 2.95 A and 2.75 A). Interestingly,
although the planes containing the two anthracene linkers
within the dimers are nearly parallel, the centroids of the
arene rings within the anthracene linkers have ‘‘slipped’’
out of alignment such that no p–p stacking takes place.
Instead, the phenylene substituents rotate out of the planes
of the anthracene rings to be in position for the observed
CH–p interactions. Given the possibility for extensive p–p
interactions, it is somewhat surprising that such stacking
between anthracene units does not occur in 1.
between another 5-pyrazolyl hydrogen atom, H(21), and
˚
˚
A combination of p–p and CH–X (X = Br, O) interac-
tions, in addition to the CH–p interactions creating the
dimers just described, organize the three-dimensional
Fig. 2. Dimeric unit in 1 formed from reciprocal CH–p interactions
(dashed lines).

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D.L. Reger et al. / Journal of Organometallic Chemistry 692 (2007) 3094–3099
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Acknowledgements
[4] (a) D.L. Reger, R.P. Watson, J.R. Gardinier, M.D. Smith, Inorg.
Chem. 43 (2004) 6609;
(b) D.L. Reger, J.R. Gardinier, R.F. Semeniuc, M.D. Smith, J.
Chem. Soc. Dalton Trans. (2003) 1712.
[5] (a) D.L. Reger, R.F. Semeniuc, V. Rassolov, M.D. Smith, Inorg.
Chem. 43 (2004) 537;
We thank the National Science Foundation (CHE-
0414239) for financial support and Dr. Radu Semeniuc
for helpful discussion. We also thank the Alfred P. Sloan
Foundation for support of R.P.W. The Bruker CCD Single
Crystal Diffractometer was purchased using funds pro-
vided by the NSF Instrumentation for Materials Research
Program through Grant DMR: 9975623.
(b) D.L. Reger, R.F. Semeniuc, M.D. Smith, Inorg. Chem. 42 (2003)
8137;
(c) D.L. Reger, R.F. Semeniuc, I. Silaghi-Dumitrescu, M.D. Smith,
Inorg. Chem. 42 (2003) 3751;
(d) D.L. Reger, R.F. Semeniuc, M.D. Smith, J. Chem. Soc. Dalton
Trans. (2003) 285;
Appendix A. Supplementary material
(e) D.L. Reger, R.F. Semeniuc, M.D. Smith, J. Organomet. Chem.
666 (2003) 87;
(f) D.L. Reger, K.J. Brown, M.D. Smith, J. Organomet. Chem. 658
(2002) 50;
(g) D.L. Reger, R.F. Semeniuc, M.D. Smith, Eur. J. Inorg. Chem.
(2002) 543;
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Trans. (2002) 476;
(i) D.L. Reger, R.F. Semeniuc, M.D. Smith, Inorg. Chem. 40 (2001)
6545;
(j) D.L. Reger, K.J. Brown, J.R. Gardinier, M.D. Smith, Organo-
metallics 22 (2003) 4973.
CCDC 632674 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be
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2007.03.010.
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