New N,N,N-heteroscorpionates based on 2,2′-bis(pyrazolyl)ethanamine and its derivatives. Ligands designed for probing supramolecular interactions

Article abstract of DOI:10.1021/ic060347o

The successful design and synthesis of the new bis(pyrazolyl)-ethanamine ligand and its copper(I) triphenylphosphine complex is reported. The ligand coordinates to the copper(I) center in a fac tridentate fashion, through both the pyrazolyl rings and the nitrogen atom from the NH₂ group. In the solid state, the compound is organized in a 2D noncovalent network by N-H...π and C-H...π interactions and hydrogen bonds. The analogous ligand with a benzyl group substituted on the amine forms a complex with the same copper(I) center that has a similar 2D supramolecular structure and, in addition, is organized by the benzyl synthon into a 3D architecture.

Full text of DOI:10.1021/ic060347o

Inorg. Chem. 2006, 45, 4337−4339
New N,N,N-Heteroscorpionates Based on 2,2′-Bis(pyrazolyl)ethanamine
and Its Derivatives. Ligands Designed for Probing Supramolecular
Interactions
Daniel L. Reger,* Radu F. Semeniuc, James R. Gardinier,† Jennifer O’Neal, Bryn Reinecke, and
Mark D. Smith
Department of Chemistry and Biochemistry, UniVersity of South Carolina,
Columbia, South Carolina 29208
Received March 1, 2006
The successful design and synthesis of the new bis(pyrazolyl)-
ethanamine ligand and its copper(I) triphenylphosphine complex
is reported. The ligand coordinates to the copper(I) center in a
fac tridentate fashion, through both the pyrazolyl rings and the
nitrogen atom from the NH2 group. In the solid state, the compound
alcohol-appended derivative HOCH2C(pz)3, which has allow-
ed the preparation of the multitopic C6H6-n[CH2OCH2C(pz)3]n
(n ) 2, 3, 4, and 6; pz ) pyrazolyl ring) family of semirigid
ligands3 as well as other types4 of systems. Metal complexes
of these ligands have remarkable topologies that maximize
noncovalent interactions of the groups in the ligand back-
bone. We named these ligands “structurally adaptive”
because they are ideal candidates for studying the self-
assembly process and the various factors that might have an
influence over such processes.3b,d,5
It would be highly desirable to prepare amine-appended
poly(pyrazolyl)methanes such as either H2NCH2C(pz)3 or
H2NCH2CH(pz)2 to take advantage of the useful hydrogen-
bonding capabilities, the Schiff-base condensation reactions,
and nucleophilic substitution reactions involving this new
functionality or even for derivatization to potentially water-
soluble ammonium derivatives. Unfortunately, the seemingly
straightforward preparative reactions (conversion of the
alcohol to the halide, followed by nucleophilic displacement
with an amine) do not work as expected, and these appended
scorpionates have thus far remained elusive. We now
communicate our successful strategy at preparing the amine-
appended bis(pyrazolyl)methane compound, NH2CH2CH-
(pz)2, introduce a derivatization route, and take a first look
at the role that the amine (including the relatively acidic NH)
is organized in a 2D noncovalent network by N
−H‚‚‚π and
C−H‚‚‚π interactions and hydrogen bonds. The analogous ligand
with a benzyl group substituted on the amine forms a complex
with the same copper(I) center that has a similar 2D supramolecular
structure and, in addition, is organized by the benzyl synthon into
a 3D architecture.
Crystal engineering is a topic of intense research interest
and holds promise for revolutionizing materials design and
synthesis.1 An essential step along the path to predicting solid
structures from the substituent groups on the chemical species
of interest is to more clearly elucidate the factors that govern
the noncovalent assembly of molecules or ions into their
ultimate solid-state architectures. Remarkable strides have
been made with both common organic and inorganic
systems.1 Our research focuses on the syntheses and supra-
molecular aspects of the metal complexes of the ubiquitous
scorpionate ligands, first introduced by Trofimenko nearly
40 years ago.2 We have developed synthetic routes to
controllably impart substitution along the scorpionate pe-
riphery, an advancement that is essential for further probing
the role of noncovalent forces in supramolecular organization
involving these ligands.3 Of note is the synthesis of the
(3) (a) Reger, D. L.; Elgin, J. D.; Semeniuc, R. F.; Pellechia, P. J.; Smith,
M. D. Chem. Commun. 2005, 4068. (b) Reger, D. L.; Semeniuc, R.
F.; Rassolov, V.; Smith, M. D. Inorg. Chem. 2004, 43, 537. (c) Reger,
D. L.; Semeniuc, R. F.; Smith, M. D. Inorg. Chem. 2003, 42, 8137.
(d) Reger, D. L.; Semeniuc, R. F.; Smith, M. D. J. Organomet. Chem.
2003, 666, 87. (e) Reger, D. L.; Gardinier, J. R.; Semeniuc, R. F.;
Smith, M. D. Dalton Trans. 2003, 1712.
* To whom correspondence should be addressed. E-mail: reger@
mail.chem.sc.edu.
(4) HOCH2C(pz)3 was used as a starting material for the preparation of
heterotopic ligands or for metallodendritic complexes. For example,
see: (a) Zibaseresht, R.; Hartshorn, R. M. J. Chem. Soc., Dalton Trans.
2005, 3898. (b) Sanchez-Mendez, A.; Silvestri, G. F.; de Jesus, E.; de
la Mata, F. J.; Flores, J. C.; Gomez, R.; Gomez-Sal, P. Eur. J. Inorg.
Chem. 2004, 3287.
(5) (a) Hofmeier, H.; Schubert, U. S. Chem. Commun. 2005, 2423. (b)
Hosseini, M. W. Chem. Commun. 2005, 5825. (c) Keizer, H. M.;
Sijbesma, R. P. Chem. Soc. ReV. 2005, 34, 226. (d) Braga, D.; Maini,
L.; Polito, M.; Tagliavini, E.; Grepioni, F. Coord. Chem. ReV. 2003,
246, 53.
† Present address: Department of Chemistry, Marquette University,
Milwaukee, WI 53201-1881.
(1) (a) Braga, D.; Brammer, L.; Champness, N. R. CrystEngComm 2005,
7, 1. (b) Brammer, L. Chem. Soc. ReV. 2004, 33, 476. (c) James, S.
Chem. Soc. ReV. 2003, 32, 276. (d) Janiak, C. Dalton Trans. 2003,
2781. (e) Sharma, C. V. K. Cryst. Growth Des. 2002, 2, 465.
(2) (a) Trofimenko, S. J. Am. Chem. Soc. 1966, 88, 1842. (b) Trofimenko,
S. Scorpionates: The Coordination Chemistry of Poly-pyrazolylborate
Ligands; Imperial College Press: London, 1999.
10.1021/ic060347o CCC: $33.50
Published on Web 04/28/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 11, 2006 4337

COMMUNICATION
Scheme 1. Synthesis and Functionalization of
2,2′-Bis(pyrazolyl)ethanaminea
a (i) Ethanol, reflux, 12 h. (ii) Hpz, neat, 220 °C. (iii) N2H4‚H2O toluene,
reflux, 12 h. (iv) PhCHO, ethanol, reflux, 12 h. (v) NaBH4, methanol, 12
h.
Figure 1. Structure of the {(PPh3)Cu[(pz)2CHCH2NH2]}+ building block
in 7 and its self-assembly into chains through C-H‚‚‚π and N-H‚‚‚π
interactions (blue dotted lines). Color code: copper, light brown; carbon,
yellow; nitrogen, blue; phosphorus, purple; hydrogen, gray.
has in the supramolecular organization of some model
copper(I) triphenylphosphine complexes.
The full synthetic pathway to NH2CH2CH(pz)2 and some
derivatives is depicted in Scheme 1 (for experimental details,
see the Supporting Information). We have previously de-
scribed the synthesis of N-[2,2-bis(pyrazolyl)ethane]-1,8-
naphthalimide (2) and the remarkable ability of this ligand
to form robust π-π-stacked dimers in its Re(CO)3Br
complexes3a that survive both in solution and under elec-
trospray mass spectral conditions. We found that the reaction
of the free ligand 2 with hydrazine hydrate liberates the
desired bis(pyrazolyl)ethanamine (3), along with N-amino-
naphthalimide (4) as the byproduct. The amine is soluble in
water and almost all organic solvents. Condensation of 3
with benzaldehyde followed by reduction with NaBH4
produced the secondary amine 6.6 This indirect route to the
monobenzylated product avoids the difficulties in separating
the dibenzylated derivative, a potential byproduct of the more
direct SN2 reaction between the amine and benzyl bromide.
The equimolar reaction of H2NCH2CH(pz)2 and [Cu-
(PPh3)2]NO3 yielded a pale-yellow powder with the com-
position {(PPh3)Cu[(pz)2CHCH2NH2]}(NO3) (7). The struc-
ture of 7, depicted in Figure 1, shows that the ligand is
tridentate, coordinated to the copper(I) center with both the
pyrazolyl rings and the NH2 group. The fourth coordination
site of the pseudotetrahedral copper(I) atom is occupied by
a triphenylphosphine ligand. The bond lengths and angles
are those expected for a compound of this nature.7 As can
also be seen in Figure 1, the presence of the acidic NH
directly influences the crystal packing, as was originally
intended in the design of this ligand system. Here, two
concerted D-H‚‚‚π interactions (D ) N and C; blue dotted
lines in Figure 1) result in the assembly of the cationic
building blocks into chains.
Figure 2. 2D organization of 7 via D-H‚‚‚π interactions (D ) C, N;
blue dotted lines) and D-H‚‚‚O hydrogen bonds (red dotted lines).
H-centroid and N-centroid distances are 2.59 and 3.50 Å,
respectively, with a N-H-centroid angle of 171°. The
hydrogen atom is positioned almost above the center of the
ring, as demonstrated by the H-perpendicular distance to
the ring of 2.53 Å and the small slip angle of 11.8°. The
phenyl ring that serves as the proton acceptor also acts as a
proton donor in the second D-H‚‚‚π interaction: one
hydrogen atom is oriented toward a second phenyl ring from
the triphenylphosphine moiety, which is coordinated to the
copper(I) center within the first building block. The H-cen-
troid and C-centroid distances are 2.69 and 3.47 Å,
respectively, with a C-H-centroid angle of 140°. The
hydrogen atom is positioned exactly above the center of the
ring, as demonstrated by the H-perpendicular distance to
the ring of 2.68 Å and the very small slip angle of 4.8°.
These chains are further connected into a 2D sheet, by
-
hydrogen bonds involving bridging NO3 counterions and
the acidic NH and CH groups on the scorpionate periphery
(Figure 2). Specifically, the second hydrogen atom from the
NH2 group is involved in a N-H‚‚‚O hydrogen bond; the
H-O and N-O distances are 2.18 and 3.08 Å, respectively,
with an N-H-O angle of 165°. The second hydrogen bond
One hydrogen atom from the NH2 group is oriented toward
one of the phenyl rings from the triphenylphosphine moiety
containing C(51)-C(56), from another building block. The
-
in which the NO3 anion is engaged involves the methine
hydrogen from the -CH(pz)2 moiety. The H-O and C-O
distances are 2.38 and 3.29 Å, respectively, with an N-H-O
angle of 151°. These two N-H‚‚‚O and C-H‚‚‚O hydrogen
bonds build up the 2D, sheetlike structure of 7.
The benzyl-functionalized amine ligand 6 reacted with the
same copper(I) starting material to yield {(PPh3)Cu[(pz)2-
(6) A very recent publication has reported the synthesis and metal
complexes of a related, sterically hindered anilide ligand via a different
synthetic pathway: Adhikari, D.; Zhao, G.; Basuli, F.; Tomaszewski,
J.; Huffman, J. C.; Mindiola, D. J. Inorg. Chem. 2006, 45, 1604.
(7) Reger, D. L.; Semeniuc, R. F.; Smith, M. D. ReV. Roum. Chim. 2002,
47, 1037 (printed in 2003).
4338 Inorganic Chemistry, Vol. 45, No. 11, 2006

COMMUNICATION
Figure 3. Structure of the {(PPh3)Cu[(pz)2CHCH2NHCH2Ph]}+ building
block in 8. The color code is the same as that in Figure 1.
Figure 5. 3D architecture of 8, based on a C-H‚‚‚π interaction between
the benzyl group and a pyrazolyl ring. The phenyls marked with a star are
those that act as proton receptors in the chain formation shown in Figure 4.
respectively, with an N-H-O angle of 157°. The second
hydrogen bond in which the counterion is engaged, with the
methine hydrogen from the -CH(pz)2 moiety, has the
following geometrical characteristics: the H-O and C-O
distances are 2.36 and 3.31 Å, respectively, with a C-H-O
angle of 156°.
A more substantial difference caused by the introduction
of the benzyl functional group is an increase in the
dimensionality of the noncovalent network of the copper(I)
compound, from a 2D sheet structure in 7 to a 3D structure
in 8. The added benzyl group acts as a donor and the
pyrazolyl ring as an acceptor in a CH-π interaction, as
pictured in Figure 5. The H-centroid and C-centroid
distances are, in this case, 2.99 and 3.73 Å, respectively,
with a C-H-centroid angle of 137°.
In summary, we have successfully prepared a new amine-
appended heteroscorpionate synthon H2NCH2CH(pz)2 that
can be easily derivatized for use as a supramolecular probe
or that can be potentially used for a broad range of other
interests, from catalysis, through crystal engineering, and
ending with the rational design of compounds with special
spin-crossover properties.8 Our endeavors to use 3 to prepare
ligands of the general formula C6H6-n[CH2NRCH2CH(pz)2]n
and covalent networks thereof will be forthcoming.
Figure 4. 2D supramolecular organization of 8 via C-H‚‚‚π interactions
and N-H‚‚‚O and C-H‚‚‚O hydrogen bonds.
CHCH2NHCH2Ph]}(NO3) (8). The local cation structure of
8 is similar to that of 7, with a tridentate scorpionate and a
triphenylphosphine ligand completing a pseudotetrahedral
copper(I) coordination sphere (see Figure 3). The benzyl
group is oriented away from both the copper center and the
phenyl rings of the triphenylphosphine ligand.
While the replacement of one hydrogen atom from the
amine group with the benzyl group did not change the
immediate environment around copper(I), it did have a major
influence on the supramolecular structure compared to 7. One
major difference between 7 and 8 is a change in the 1D chain
structure where the N-H‚‚‚π interaction is not present in
the case of 8. As can be seen in Figure 4, the cationic
building blocks are still arranged in chains, but these chains
are held together by only C-H‚‚‚π interactions.
Acknowledgment. The financial support from the Na-
tional Science Foundation (Grant CHE-0414239) is greatly
appreciated.
One of the phenyl rings from the triphenylphosphine group
acts as a proton donor, and another one from an adjacent
triphenylphosphine group serves as a proton acceptor. The
H-centroid and C-centroid distances are, in this case, 2.97
and 3.86 Å, respectively, with a C-H-centroid angle of
159°. The hydrogen atom is positioned above the C(53) atom,
therefore imposing a H-perpendicular distance to the ring
of 2.84 Å and a slip angle of 16.6°. As in the case of 7, the
Supporting Information Available: Synthetic details and
crystallographic data for 7 and 8. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC060347O
-
NO3 anions link the chains into a 2D network. The
(8) (a) Reger, D. L.; Gardinier, J. R.; Gemmill, W. R.; Smith, M. D.;
Shahin, A. M.; Long, G. J.; Rebbouh, L.; Grandjean, F. J. Am. Chem.
Soc. 2005, 127, 2303. (b) Reger, D. L.; Gardinier, J. R.; Smith, M.
D.; Shahin, A. M.; Long, G. J.; Rebbouh, L.; Grandjean, F. Inorg.
Chem. 2005, 44, 1852.
association pattern is the same as that with 7; the hydrogen
atom from the NH group is involved in a N-H‚‚‚O hydrogen
bond; the H-O and N-O distances are 2.21 and 2.97 Å,
Inorganic Chemistry, Vol. 45, No. 11, 2006 4339