Charge-assisted hydrogen bonding and other noncovalent interactions in the self-assembly of the organometallic building block [(η6- hydroquinone)Rh(P(OPh)3)2]+ with a range of counteranions

Article abstract of DOI:10.1021/om0604425

The synthesis and X-ray structures are reported for [(η⁶- hydroquinone)Rh(P(OPh)₃)₂]⁺X~ (X = BF ₄, ClO₄, SbF₆, OTf, OTs, OPf), [(η⁶-resorcinol)Rh(P(OPh)₃)₂] ⁺BF₄^-, and [(η⁶-4,4′- biphenol)Rh(P(OPh)₃)₂]⁺BF₄ ^-. In these complexes, the -OH groups are activated by the electrophilic rhodium moiety to participate in charge-assisted hydrogen bonding to the anionic counterion. The crystal structures feature three kinds of noncovalent interactions - hydrogen bonding, Coulombic attraction, and π-π stacking, which result in an intriguing array of architectures: dimeric, 1-D chain, C₂ helical, and C₃ helical. The nature of the charge-assisted hydrogen bonding and the resulting 3-D structure in these systems are remarkably dependent on the identity of the anion. Robust porous networks are formed rapidly (minutes or less) with [(η⁶- hydroquinone)Rh(P(OPh)₃)₂]⁺X^- (X = BF₄, ClO₄) and [(η⁶-resorcinol)Rh(P(OPh) ₃)₂]⁺BF₄^-. The hydrophobic pores in [(η⁶-hydroquinone)Rh(P(OPh) ₃)₂]⁺ClO₄^- bind toluene reversibly. This work demonstrates that self-assembly of well-designed organometallic building blocks via charge-assisted hydrogen bonding is an effective strategy for the construction of robust porous networks. With counterions containing both oxygen and fluorine, it was found that the former is invariably the hydrogen bond acceptor, a result in agreement with atomic charge calculations. It is anticipated that self-assembly via charge-assisted hydrogen bonding is an approach applicable in many organometallic systems.

Full text of DOI:10.1021/om0604425

5276
Organometallics 2006, 25, 5276-5285
Charge-Assisted Hydrogen Bonding and Other Noncovalent
Interactions in the Self-Assembly of the Organometallic Building
Block [(η6-hydroquinone)Rh(P(OPh)3)2]+ with a Range of
Counteranions
Seung Uk Son,*,† Jeffrey A. Reingold,‡ Gene B. Carpenter,‡ Paul T. Czech,§ and
Dwight A. Sweigart*,‡
Departments of Chemistry, Sungkyunkwan UniVersity, Suwon 440-746, Korea, Brown UniVersity,
ProVidence, Rhode Island 02912, and ProVidence College, ProVidence, Rhode Island 02918
ReceiVed May 19, 2006
The synthesis and X-ray structures are reported for [(η6-hydroquinone)Rh(P(OPh)3)2]+X- (X ) BF4,
ClO4, SbF6, OTf, OTs, OPf), [(η6-resorcinol)Rh(P(OPh)3)2]+BF4-, and [(η6-4,4′-biphenol)Rh(P(OPh)3)2]+BF4-.
In these complexes, the -OH groups are activated by the electrophilic rhodium moiety to participate in
charge-assisted hydrogen bonding to the anionic counterion. The crystal structures feature three kinds of
noncovalent interactionsshydrogen bonding, Coulombic attraction, and π-π stacking, which result in
an intriguing array of architectures: dimeric, 1-D chain, C2 helical, and C3 helical. The nature of the
charge-assisted hydrogen bonding and the resulting 3-D structure in these systems are remarkably dependent
on the identity of the anion. Robust porous networks are formed rapidly (minutes or less) with [(η6-
-
hydroquinone)Rh(P(OPh)3)2]+X- (X ) BF4, ClO4) and [(η6-resorcinol)Rh(P(OPh)3)2]+BF4 . The
-
hydrophobic pores in [(η6-hydroquinone)Rh(P(OPh)3)2]+ClO4 bind toluene reversibly. This work
demonstrates that self-assembly of well-designed organometallic building blocks via charge-assisted
hydrogen bonding is an effective strategy for the construction of robust porous networks. With counterions
containing both oxygen and fluorine, it was found that the former is invariably the hydrogen bond acceptor,
a result in agreement with atomic charge calculations. It is anticipated that self-assembly via charge-
assisted hydrogen bonding is an approach applicable in many organometallic systems.
bonding, which can occur in ionic or zwitterionic systems and
refers to hydrogen bonding accompanied by Coulombic interac-
tions resulting from the inherent electronic charges.5 This can
lead to an exceptionally strong interaction between the op-
positely charged components. For example, depending on the
identity of the anionic component,6 guanidinium cations can
Introduction
The self-assembly of molecules or molecular units into
supramolecular arrays can be driven by covalent bond formation
and/or can be driven by noncovalent interactions such as π-π
stacking, hydrogen bonding, and van der Waals forces.1
Hydrogen bonding has been recognized as a particularly
powerful tool in this regard because of its unique directionality
and specificity.2,3 Supramolecular assemblies predicated on
hydrogen bonding can be reinforced by the cooperative action
of multipoint H bonds or additional cooperative interactions
between the modular components of the assembly.2a,4 An
important example of this is so-called charge-assisted hydrogen
have a cation-anion association constant as high as 106 M-1
.
Similarly, charge-assisted hydrogen bonding can be an effective
strategy for fully utilizing the directional properties of hydrogen-
bonding-mediated assembly in organometallic systems.7,8
(4) For representative examples see: (a) Nadin, A.; Derrer, S.; McGeary,
R. P.; Goodman, J. M.; Raithby, P. R.; Holmes, A. B. J. Am. Chem. Soc.
1995, 117, 9768. (b) Bisson, A. P.; Hunter, C. A. Chem. Commun. 1996,
1723. (c) Adams, H.; Carver, F. J.; Hunter, C. A.; Morales, J. C.; Seward,
E. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 1542. (d) Corbin, P. S.;
Zimmerman, S. C. J. Am. Chem. Soc. 2000, 122, 3779.
* To whom correspondence should be addressed. E-mail: sson@skku.edu
(S.U.S.); dwight_sweigart@brown.edu (D.A.S.).
† Sungkyunkwan University.
‡ Brown University.
(5) Ward, M. D. Chem. Commun. 2005, 5838.
§ Providence College.
(6) (a) Schmidtchen, F. P.; Berger, M. Chem. ReV. 1997, 97, 1609. (b)
Schmuck, C. Eur. J. Org. Chem. 1999, 2397, 7. (c) Nelen, M. I.; Eliseev,
A. V. J. Chem. Soc., Perkin Trans. 2 1997, 1359. (d) Linton, B.; Hamilton,
A. D. Tetrahedron 1999, 55, 6027. (e) Russell, V. A.; Etter, M. C.; Ward,
M. D. J. Am. Chem. Soc. 1994, 116, 1941. (f) Dixon, R. P.; Geib, S. J.;
Hamilton, A. D. J. Am. Chem. Soc. 1992, 114, 365. (g) Schrader, T. Chem.
Eur. J. 1997, 3, 1537. (h) Schellhaas, K.; Schmalz, H.-G.; Bats, J. W. Chem.
Eur. J. 1998, 4, 57. (i) Schmuck, C. Chem. Commun. 1999, 843.
(7) (a) Braga, D.; Maini, L.; Grepioni, F. Organometallics 2001, 20, 1875.
(b) Braga, D.; Cojazzi, G.; Emiliani, D.; Maini, L.; Grepioni, F. Organo-
metallics 2002, 21, 1315. (c) Braga, D.; Polito, M.; D’Addario, D.;
Tagliavini, E.; Proserpio, D. M.; Grepioni, F.; Steed, J. W. Organometallics
2003, 22, 4532. (d) Braga, D.; Polito, M.; Bracaccini, M.; D’Addario, D.;
Tagliavini, E.; Sturba, L. Organometallics 2003, 22, 2142. (e) Braga, D.;
Polito, M.; D’Addario, D.; Grepioni, F. Cryst. Growth Des. 2004, 4, 1109.
(f) Braga, D.; Polito, M.; Grepioni, F. Cryst. Growth Des. 2004, 4, 769.
(8) Braga, D.; Grepioni, F. Acc. Chem. Res. 2000, 33, 601.
(1) (a) Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspec-
tiVes; VCH: Weinheim, Germany, 1995. (b) Conn, M. M.; Rebek, J., Jr.
Chem. ReV. 1997, 97, 1647. (c) Fyfe, M. C. T.; Stoddart, J. F. Acc. Chem.
Res. 1997, 30, 393. (d) Bowden, N. B.; Weck, M.; Choi, I. S.; Whitesides,
G. M. Acc. Chem. Res. 2001, 34, 231. (e) Malek, N.; Maris, T.; Perron,
M.-E.; Wuest, J. D. Angew. Chem., Int. Ed. 2005, 44, 4021. (f) Hunter, C.
A.; Lawson, K. R.; Perkins, J.; Urch, C. J. J. Chem. Soc., Perkin Trans. 2
2001, 651.
(2) (a) Prins, L. J.; Reinhoudt, D. N.; Timmerman, P. Angew. Chem.,
Int. Ed. 2001, 40, 2382. (b) Etter, M. C. Acc. Chem. Res. 1990, 23, 120.
(c) Fredericks, J. R.; Hamilton, A. D. In ComprehensiVe Supramolecular
Chemistry; Sauvage, J.-P., Hosseini, M. W., Eds.; Pergamon Press: Oxford,
U.K., 1996; Vol. IX, Chapter 16.
(3) (a) Beatty, A. M. Coord. Chem. ReV. 2003, 246, 131. (b) Adachi,
K.; Sugiyama, Y.; Yoneda, K.; Yamada, K.; Nozaki, K.; Fuyuhiro, A.;
Kawata, Chem. Eur. J. 2005, 11, 6616.
10.1021/om0604425 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 09/26/2006

Self-Assembly of [(η6-hydroquinone)Rh(P(OPh)3)2]+
Organometallics, Vol. 25, No. 22, 2006 5277
Chart 1. Organometallic Building Blocks Utilized in This
Study
organometallic sandwich compounds through charge-assisted
hydrogen bonding.7,8 We reported11 a polymeric network derived
from charge-assisted hydrogen bonding between [(η6-1,4-
2-
hydroquinone)Mn(CO)3]+ and SiF6 and more recently com-
municated preliminary results for the formation of related
networks resulting from interactions between the -OH groups
in [(η6-1,4-hydroquinone)Rh(P(OPh)3)2]+ (1+) and [(η6-1,3-
-
hydroquinone)Rh(P(OPh)3)2]+ (2+) with the BF4 counterani-
on.12 In the latter case, the assembly resulted in the formation
of porous materials containing interesting hydrophobic channels
that consist entirely of phenyl groups from the triphenyl
phosphite ligands.
Herein we report a comprehensive study of noncovalent self-
assembly in the ionic η6-hydroxybenzene complexes illustrated
in Chart 1. These organometallic salts were found to exhibit
differential hydrogen-bonding and π-π stacking interactions,
resulting in an intriguing array of both discrete and polymeric
supramolecular solid-state structures. Solution-phase IR studies
and molecular orbital calculations of atomic charges are
presented to rationalize the structural variations found.
Recently, it has been recognized that the structural and
chemical versatility of organometallic building blocks can be
utilized to prepare supramolecular assemblies with distinct
physical and chemical properties that cannot be replicated in
purely organic systems.9,10 For example, self-assembled coor-
dination networks that feature transition-metal nodes and the
anionic complex [(η4-quinone)Mn(CO)3]- as organometallo-
ligand spacers have been extensively reported.10 In addition to
coordination-mediated self-assembly, there has been a consider-
able interest in supramolecular organometallic assemblies
formed via noncovalent interactions. Braga and co-workers, for
example, have described the self-assembly of a variety of
Results and Discussion
Synthesis of η6-Hydroxybenzene Rhodium Salts. The
complexes [2]BF4 and [3]BF4 were synthesized in good yields
Figure 1. Hydrogen-bonded structural patterns found in the solid state for [1+-3+]X-: (a) dimeric; (b) 1-D chain; (c) C2 helical; (d) C3
helical.

5278 Organometallics, Vol. 25, No. 22, 2006
Son et al.
Figure 2. Hydrogen-bonded structure of crystalline 1+SbF6- as a diethyl ether solvate (left) and slippage to a π-π-stacked structure upon
drying (right).
Table 1. Crystallographic Data
-
-
-
1+SbF6
1+SbF6-(hex)
1+OTf-
1+OPf-
3+BF4
1+OTs-
1+ClO4
formula
C50H56F6O10-
P2RhSb
1217.55
100(2)
C42H36F6O8-
P2RhSb
1069.31
173(2)
C44H38Cl2F3-
O11P2RhSb
1067.55
100(2)
C42H36F2-
O10P3Rh
934.53
C48H40BF4-
O8P2Rh
996.46
C50H45Cl2O11-
P2RhS
1089.67
293(2)
C42H36Cl-
O12P2Rh
933.01
fw
T, K
100(2)
100(2)
100(2)
cryst syst
space group
a, Å
b, Å
c, Å
triclinic
P1h
triclinic
P1h
triclinic
P1h
triclinic
P1h
monoclinic
P2/c
17.960(5)
11.306(3)
23.267(7)
90
105.502(5)
90
4553(2)
4
orthorhombic
P212121
11.875(3)
17.555(5)
24.061(7)
90
90
90
5016(3)
4
1.443
rhombohedral
R3h
38.625(2)
38.625(2)
15.096(1)
90
90
120
19505(1)
18
1.430
12.834(1)
13.259(1)
17.158(1)
94.600(1)
99.863(1)
116.234(1)
2539.9(3)
2
10.669(2)
14.186(3)
16.885(3)
65.795(4)
85.774(4)
70.863(4)
2196.5(7)
2
10.597(1)
13.952(1)
16.587(1)
74.722(1)
84.604(2)
70.769(1)
2233.7(3)
2
10.843(1)
11.245(1)
17.908(1)
105.394(1)
90.398(1)
107.279(1)
2001.5(3)
2
R, deg
â, deg
γ, deg
V, Å3
Z
D
calcd, g/cm3
1.592
1.617
1.587
1.551
1.454
F(000)
1232
1064
1084
952
2032
2232
8568
cryst size, mm
0.17 × 0.16 ×
0.15 × 0.11 ×
0.10 × 0.09 ×
0.07 × 0.06 ×
0.14 × 0.14 ×
0.14 × 0.11 ×
0.12 × 0.11 ×
0.15
0.05
0.05
0.05
0.10
0.04
0.10
θ range, deg
1.74-28.42
30 290
12 161/6/635
1.65-25.11
21 333
7778/0/505
1.60-26.45
24 103
9123/46/572
1.98-26.55
21 442
8253/0/513
1.82-23.25
35 882
6529/730/618
1.44-25.07
47 263
8890/0/606
1.48-25.10
63 011
7711/0/523
no. of rflns collected
no. of data/restraints/
params
goodness of fit on F2
final R indices (I > 2σ(I))
R1
1.026
1.022
1.128
1.076
1.067
0.930
1.030
0.0377
0.0808
0.0943
0.2578
0.0748
0.1251
0.0725
0.1558
0.1212
0.2917
0.078
0.1451
0.0789
0.2199
wR2
by treatment of the precursor [Rh(P(OPh)3)2Cl]213 with AgBF4
in methylene chloride to generate [Rh(P(OPh)3)2]+ in situ, which
was then reacted with resorcinol and 4,4′-biphenol, respectively.
Consequently, organometallic crystal engineering using these
building blocks may be expected to feature at least three kinds
of noncovalent interactions: hydrogen bonding, Coulombic
attraction, and π-π stacking.
The 1,4-hydroquinone salts [1]X (X- ) BF4-, SbF6-, PF6
,
-
ClO4-,OTs-, OTf-) were synthesized in a similar manner, with
the anion X- deriving from the silver salt (AgX) utilized.
Reactions utilizing silver nitrate and silver sulfate were unsuc-
cessful, as were attempts to coordinate 1,3,5-trihydroxybenzene
and 1,2,4-trihydroxybenzene.
As indicated in Chart 1, complexes 1+-3+ present a number
of noncovalent interaction sites: hydroxyl groups, F and O
atoms in the counteranions, phenyl rings, and electronic charges.
Solid-State Structures and Self-Assembly. Diagrams of the
different types of solid-state structural patterns found in this
study are shown in Figure 1. The cationic hydroxybenzene
complexes (1+-3+) and the anionic companion (X-) can
assemble to generate dimeric, 1-D chain, C2-helical, and C3-
helical motifs, most of which feature charge-assisted hydrogen
bonding. Relevant X-ray crystallographic data are summarized
in Table 1.
Crystals of [(η6-1,4-hydroquinone)Rh(P(OPh)3)2]+SbF6
-
(1+SbF6-) suitable for single-crystal X-ray analysis were
prepared by layering a methylene chloride solution at -20 °C
with diethyl ether or hexane. Cube-shaped orange crystals and
plate-shaped yellow crystals were obtained with diethyl ether
and hexane cosolvents, respectively. The X-ray structure of the
orange crystals revealed that the hydroquinone -OH groups
are hydrogen-bonded to diethyl ether present in the crystal lattice
(O‚‚‚O ) 2.6 Å), as shown in Figure 2 (left). The hydroquinone
rings are arranged in pairs due to an edge-to-edge π-π stacking
interaction involving two carbon atoms of each ring. The average
C‚‚‚C contact between the edges of adjacent rings is 3.3 Å.
The simulated powder XRD pattern of 1+SbF6-‚2Et2O is
shown in Figure 3a. The solid was found to slowly lose the
incorporated solvent molecules. After the solid was dried under
(9) (a) Braga, D.; Grepioni, F.; Desiraju, G. R. Chem. ReV. 1998, 98,
1375. (b) Burrows, A. D.; Chan, C.-W.; Chowdhry, M. M.; McGrady, J.
E.; Mingos, D. M. P. Chem. Soc. ReV. 1995, 24, 329. (c) Sun, S.-S.; Lees,
A. J. Inorg. Chem. 2001, 40, 3154. (d) Kuehl, C. J.; Yamamoto, T.; Seidel,
S. R.; Stang, P. J. Org. Lett. 2002, 4, 913. (e) Shin, D. M.; Chung, Y. K.;
Lee, I. S. Cryst. Growth Des. 2002, 2, 493. (f) Kim, Y.; Verkade, J. G.
Inorg. Chem. 2003, 42, 4262. (g) Hartnell, R. D.; Arnold, D. P. Organo-
metallics 2004, 23, 391. (h) Dong, Y.-B.; Geng, Y.; Ma, J.-P.; Huang, R.-
Q. Inorg. Chem. 2005, 44, 1693.
(10) (a) Oh, M.; Carpenter, G. B.; Sweigart, D. A. Acc. Chem. Res. 2004,
37, 1. (b) Reingold, J. A.; Son, S. U.; Kim, S. B.; Dullaghan, C. A.; Oh,
M.; Frake, P. C.; Carpenter, G. B.; Sweigart, D. A. Dalton Trans. 2006,
2385.
(11) Sun, S.; Carpenter, G. B.; Sweigart, D. A. J. Organomet. Chem.
1996, 512, 257.
(12) Son, S. U.; Reingold, J. A.; Carpenter, G. B.; Sweigart, D. A. Chem.
Commun. 2006, 708.
(13) Haines, L. M. Inorg. Chem. 1970, 19, 1517.

Self-Assembly of [(η6-hydroquinone)Rh(P(OPh)3)2]+
Organometallics, Vol. 25, No. 22, 2006 5279
remarkable concerted hydroquinone ring slippage of ca. 3 Å
with concomitant loss of hydrogen bonding to the ether and
gain of π-π stacking interactions, all without the loss of
crystallinity.
Crystals of the triflate salt 1+OTf- were grown by layering
hexane on a methylene chloride solution at -20 °C. The solid-
state structure consists of the dimeric unit illustrated in Figure
4 and follows the general pattern depicted in Figure 1a. The
two hydroquinone rings are π-π stacked (3.6 Å), and the -OH
groups are hydrogen-bonded to the sulfonate oxygens of the
triflate anion (average O‚‚‚O ) 2.68 Å). Since the sulfonate
end of the triflate anion contains most of the net negative charge
(vide infra), the hydrogen bonding would be expected to involve
the oxygens rather than the fluorines and may be classified as
charge-assisted. Analogous charge-assisted hydrogen bonding
to the oxygen atoms in triflate has been reported to occur in
[Cp2Ta(OH)2][OTf-].14
The synthesis of 1+PF6-, with AgPF6 as the anion source,
proceeded smoothly and gave a product with a satisfactory
elemental analysis. After slow recrystallization from methylene
chloride, however, it became evident from subsequent single-
crystal X-ray analysis and altered bulk elemental analysis that
hydrolysis of the anion to PF2O2- (OPf-) had occurred during
the recrystallization process. The hydrolysis reaction probably
stems from trace water and may have been accelerated by the
acidic nature of the coordinated hydroquinone. Hydrolysis of
PF6- in this manner has been observed previously.15 The X-ray
structure of 1+OPf- (Figure 5) is very similar to that found for
1+OTf-. Charge-assisted hydrogen-bonding and π-π-stacking
(3.5 Å) interactions dominate the observed dimeric units. Careful
analysis of the X-ray data confirmed that the hydrogen bonding
from the hydroquinone -OH groups is to oxygen and not
fluorine acceptors on the OPf- anion (average O‚‚‚O ) 2.65
Å).
Figure 3. PX-RD patterns: (a) simulated pattern of 1+SbF6-‚2Et2O
from single-crystal data; (b) pattern of 1+SbF6- crystals after drying
under vacuum; (c) simulated pattern from single-crystal data of
-
1+SbF6 grown in CH2Cl2 and hexane.
vacuum for 1 day, the PXRD pattern changed significantly to
that shown in Figure 3b, from which it is inferred that the solid
remains crystalline but undergoes a substantial structural change
upon solvent loss. It proved possible to ascertain the nature of
this change, because the simulated PXRD obtained from single-
crystal data for 1+SbF6- grown with hexane cosolvent (yellow
plates) matched that obtained after drying 1+SbF6-‚2Et2O
(compare parts c and b of Figure 3), suggesting that they have
the same structure. The structure of the former, shown in Figure
2 (right), reveals a π-π-stacked dimeric aggregate with nearly
eclipsed hydroquinone rings that are separated by an average
of 3.5 Å and not involved in any hydrogen bonding. It is
concluded that, upon drying, 1+SbF6-‚2Et2O undergoes a
Figure 4. Dimeric structure of 1+OTf-, which features charge-assisted hydrogen-bonding and π-π-stacking interactions.
Figure 5. Dimeric structure of 1+OPf- (compare to 1+OTf- in Figure 4).

5280 Organometallics, Vol. 25, No. 22, 2006
Son et al.
Single crystals of 1+BF4 and 1+ClO4 were grown by
layering a methylene chloride solution with diethyl ether.12
These two salts have virtually identical structures, which feature
the intriguing C3-helical hydrogen-bonded network shown in
-
-
-
Figure 1d. Structural details for 1+ClO4 are shown in Figure
9. The hydrogen-bonding distances in 1+BF4- are F‚‚‚O ) 2.47,
2.60 Å, and those in 1+ClO4 are O‚‚‚O ) 2.38, 2.88 Å. In
-
each compound, six C3 helices assemble to generate the
hexagonal channels or pores illustrated in Figure 10. The
structure belongs to the centrosymmetric space group R3h, and
the direction of rotation of the helices alternates around the
channels. The channels themselves located at the core of the
six helices consist of hydrophobic phosphite phenyl groups
(Figure 10). Two of the three phenyl groups from each P(OPh)3
ligand contribute to the channels, which have a diameter of ca.
10.5 Å and are separated by ca. 23 Å.
Figure 6. Dimeric structure of 2+BF4-, which does not have π-π
interactions.
The dimeric structures found for 1+OPf- and 1+OTf-
combine in a cooperative manner three types of noncovalent
interactions: charge pairing, hydrogen bonding, and π-π
stacking. Interestingly, a different type of dimeric assembly was
found for [(η6-1,3-hydroquinone)Rh(P(OPh)3)2]+BF4- (2+BF4-).16
In this case, the dimer is held together by charge-assisted
hydrogen bonding but geometric restrictions prevent π-π
stacking between the 1,3-hydroquinone rings (Figure 6). The
The ease of formation of the pore structure shown in Figure
-
10b for 1+ClO4 and 1+BF4 was investigated by comparing
the PXRD pattern of slowly grown macrocrystals with that found
for microcrystals obtained by rapid precipitation. The addition
of diethyl ether to a methylene chloride solution of 1+ClO4
-
led to rapid precipitation of a powder that appeared under a
microscope to consist of good-quality microcrystals. Relevant
-
hydrogen bond distances in 2+BF4 average O‚‚‚F ) 2.8 Å.
-
PXRD patterns for various samples of 1+ClO4 are shown in
[(η6-4,4′-biphenol)Rh(P(OPh)3)2]+BF4- (3+BF4-) forms the
hydrogen-bonding network depicted in Figure 1b. Only one F
atom in the BF4- anion participates in hydrogen bond formation
with the phenolic -OH groups. A 1-D polymeric chain structure
results, shown in Figure 7a, with the hydrogen bond distances
O‚‚‚F ) 2.6 Å and O‚‚‚O ) 2.7 Å. The 3-D crystal structure
features small channels which are lined with phenyl groups from
the triphenyl phosphite ligands that undergo π-π stacking. The
channels were found to be filled with unidentified disordered
solvent molecules, which were located using PLATON software
(Figure 7b).
Figure 11a-c. The powder pattern simulated from single-crystal
X-ray data is given in Figure 11a. The actual pattern obtained
with well-formed macrocrystals of 1+ClO4 is very similar
-
(Figure 11b). Interestingly, the PXRD of microcrystalline
1+ClO4- formed by simple rapid precipitation is similar (Figure
11c), indicating that precipitated 1+ClO4- is (i) indeed crystal-
line and (ii) has the same porous structure possessed by slowly
grown single crystals (Figure 10). We come to the significant
conclusion that the dynamic processes occurring in the assembly
of the organometallic building block 1+ClO4- into an intricate
3-D supramolecular architecture with hexagonal channels oper-
ate on a fast preparative time scale. Thus, the synthesis of
The interesting C2-helical chain motif shown in Figure 1c
was found for the tosylate salt of [(η6-1,4-hydroquinone)Rh-
(P(OPh)3)2]+ (1+OTs-).17 Long rod-shaped single crystals of
1+OTs- were grown by layering a methylene chloride solution
with hexane at -20 °C. The helical hydrogen-bonding network
has C2 projection symmetry (Figure 8). The space group
(P212121) implies the generation of chirality during the crystal-
lization process, which means that the helices pack such that
all possess the same direction of rotation (the Flack parameter
for the structure shown was determined as -0.09(8)). The two
independent hydrogen bonds in 1+OTs- have O‚‚‚O ) 2.47
and 2.63 Å.
-
crystalline porous materials such as 1+ClO4 can be ac-
complished within seconds (precipitation) rather than requiring
days (slow single-crystal growth).
Analogous conclusions obtain for the 1+BF4 analogue
-
(Figure 11d-f), although, due to contamination by an unidenti-
fied species (asterisks in Figure 11e,f), the PXRD patterns are
not as clear-cut as that seen with 1+ClO4. Stronger hydrogen
-
-
bonding in 1+ClO4 compared to that in 1+BF4 (vide infra)
may be responsible for a cleaner crystallization process with
the former.
-
Figure 7. (a) 1-D hydrogen-bonded chain structure in 3+BF4-. (b) Structure of 3+BF4 with disordered solvent (violet) in channels that
are lined with phenyl rings.

Self-Assembly of [(η6-hydroquinone)Rh(P(OPh)3)2]+
Organometallics, Vol. 25, No. 22, 2006 5281
Figure 8. C2-helical hydrogen-bonded structure found in 1+OTs-. The helices all pack with the same twist direction, resulting in a chiral
crystal.
-
-
Figure 9. The C3-helical hydrogen-bonded structure found in 1+ClO4 and 1+BF4
.
-
-
Preliminary experiments were done to probe the possible
interaction of appropriate aromatic molecules with the hydro-
By comparison, Bu4N+ClO4 and Bu4N+BF4 decompose
cleanly, with the tetrafluoroborate salt being more stable, as
expected.
-
phobic channels present in 1+ClO4 (Figure 10). As seen in
Figure 12, the PXRD pattern of solid 1+ClO4 changes
-
IR Studies and Atomic Charge Calculations. The hydrogen-
bonding interactions between the organometallic cations and
the counteranions shown in Chart 1 were studied in methylene
chloride solution via FT-IR. The results are summarized in
Figures14 and 15 and in Table 2. As shown in Figure 14, νOH
in 1+ is red-shifted by hydrogen bonding to the anion. With
1+SbF6-, two IR peaks are seen at 3517 and 3405 cm-1 (labeled
b and c). The peak at 3517 cm-1 is assigned to “free” 1+: i.e.,
the complex not hydrogen-bonded to the counteranion. In
support of this assertion, peak b also appears at the same
significantly after exposure to toluene for 5 days (Figure 12b)
and then reverts to the original pattern after drying under
vacuum. Although the details are unknown, it may be concluded
that toluene interacts reversibly with the host channels in
1+ClO4
.
-
Figure 13 shows TGA curves for 1+ClO4- and 1+BF4-, along
with curves for relevant tetrabutylammonium salts. A DSC
experiment with 1+ClO4 showed the onset of a heat capacity
-
change about 50 °C lower than the TGA decomposition
temperature of ca. 180 °C. TGA experiments generally reflect
gross thermal stability and not more subtle phase changes that
may be occurring. It is suggested that the DSC result signals
the breakup of the C3-helical hydrogen-bonding assembly in
1+ClO4-. The TGA curves show that there is not a great
-
frequency with the counterions BF4 and ClO4-. The much
-
greater intensity of this peak in the case of SbF6 reflects the
relatively poor ability of SbF6- to function as a hydrogen bond
acceptor, a fact also indicated by the X-ray structures (vide
supra).
-
-
difference in the thermal stabilities of 1+ClO4 and 1+BF4
.

5282 Organometallics, Vol. 25, No. 22, 2006
Son et al.
-
Figure 10. (a) 3-D packing of the C3 helices in 1+ClO4 and 1+BF4-. (b) Depiction of the resultant hydrophobic channels (red).
Figure 11. PXRD patterns: (a) simulated pattern of 1+ClO4- from single-crystal X-ray data; (b) experimental pattern of macrocrystals of
-
1+ClO4-; (c) experimental pattern of microcrystals formed by simple precipitation of 1+ClO4-; (d) simulated pattern of 1+BF4 from
single-crystal X-ray data; (e) experimental pattern of macrocrystals of 1+BF4-; (f) experimental pattern of microcrystals formed by simple
-
precipitation of 1+BF4
.
Peaks c-e in Figure 14 are assigned to hydrogen-bonded
-OH groups. The shift of these νOH bands from the “free”
position (peak b) can be used to estimate the strength of the H
bonding between the hydroquinone -OH groups and the
counterion by application of Iogansen’s equation.18 The results,
presented in Table 2, indicate that H bonding between 1+ or
2+ and the counteranion is greater for O-based acceptors than
for F-based acceptors. The hydrogen-bonding strength spans the

Self-Assembly of [(η6-hydroquinone)Rh(P(OPh)3)2]+
Organometallics, Vol. 25, No. 22, 2006 5283
-
-
Figure 12. PXRD patterns: (a) 1+ClO4 freshly precipitated from methylene chloride with diethyl ether; (b) 1+ClO4 after exposure to
-
neat toluene for 5 days; (c) 1+ClO4 after vacuum drying of sample in (b).
Figure 13. TGA curves: (a) 1+ClO4-; (b) 1+BF4-; (c) Bu4N+ClO4-;
-
(d) Bu4N+BF4-. Inset is the DSC curve for 1+ClO4
.
-
range 14-27 kJ/mol and follows the order SbF6-< BF4
<
Figure 14. IR spectra in the νOH region (11 mM, CH2Cl2
solvent): (a) free 1,4-hydroquinone; (b) 1+X- without hydrogen
bonding between 1+ and X-; (c-e) 1+X- with hydrogen bonding
between 1+ and X-.
-
ClO4 e OTf- < OPf-, OTs-.
The νOH bands in the IR spectra of free hydroquinone,
resorcinol, and 4,4′-biphenol were found to be invariant over
the concentration range utilized (3-11 mM), indicating the
absence of intermolecular hydrogen bonding at these concentra-
tions. In contrast, Figure 14 clearly shows that hydrogen bonding
in 1+X- can be extensive at 11 mM. The enhanced hydrogen
bonding in 1+X- can be attributed to (1) the positive charge on
the cation brought about by the electrophilic rhodium fragment
and (2) the obligatory anionic counterion that can act as a
hydrogen bond acceptor. Charge pairing of the species in 1+X-
undoubtedly complements the hydrogen bonding. To probe the
“charge-assisted” nature of the hydrogen bonding, IR spectra
of CH2Cl2 solutions of 1,4-hydroquinone (11 mM) containing
varying amounts of Bu4ClO4 were recorded. As shown in Figure
15, 1 equiv of Bu4NClO4 has little effect on the IR spectrum
and, even with 10 equiv of Bu4NClO4 present, a significant
amount of free hydroquinone remains. We conclude that the
hydrogen bonding observed with 1+X--3+X- has as important
components both ionic charge pairing and electrophilic activa-
tion imparted by coordination to the transition metal.
(14) Quan, R. W.; Bercaw, J. E.; Schaefer, W. P. Acta Crystallogr., Sect.
C: Cryst. Struct. Commun. 1991, C47, 2057.
(15) Kannan, S.; James, A. J.; Sharp, P. R. Inorg. Chim. Acta 2003, 345,
8.
(16) X-ray structures of 1+BF4 and 2+BF4 were reported in ref 12.
(17) Charge-assisted hydrogen bonding of tosylate has been reported:
Doubell, P. C. J.; Olivier, D. W.; Van Rooyen, R. H. Acta Crystallogr.,
Sect. C: Cryst. Struct. Commun. 1991, C47, 353.
-
-
Next, we performed molecular orbital calculations using
Spartan19 to assign atomic charges to the key terminal atoms
for the range of counterions. Atomic charges are notoriously
(18) The Iogansen equation is ∆H° ) -1.28(∆ν)1/2: (a) Kazarian, S.
G.; Hamley, P. A.; Poliakoff, M. J. Am. Chem. Soc. 1993, 115, 9069. (b)
Iogansen, G. A.; Kurkchi, G. A. Furman, V. M.; Glazunov, V. P.; Odinokov,
S. E. Zh. Prikl. Spektrosk. 1980, 33, 460.
(19) Spartan ’04, Version 1.0.3; Wavefunction, Inc., Irvine, CA 2004.

5284 Organometallics, Vol. 25, No. 22, 2006
Son et al.
Conclusions
The organometallic salts [(η6-hydroxybenzene)Rh-
(P(OPh)3)2]+X- undergo noncovalent self-assembly dominated
by charge-assisted hydrogen-bonding and π-π-stacking interac-
+
tions. The electrophilic Rh(P(OPh)3)2 moiety activates the
phenolic -OH groups to participate in strong hydrogen bonding
to the anionic counterion X-. The nature of the hydrogen-
bonding interactions and the resulting 3-D structures are
remarkably dependent on the identity of the anion. The X-ray
structures of [(η6-hydroquinone)Rh(P(OPh)3)2]X salts (X ) BF4,
ClO4, SbF6, OTf, OTs, OPf) display an assortment of dimeric,
C2-helical, and C3-helical motifs. This work demonstrates that
self-assembly of well-designed organometallic building blocks
via charge-assisted hydrogen bonding is an effective strategy
for the construction of robust porous networks that are formed
rapidly (minutes or less). With counterions containing both
oxygen and fluorine, it was found that the former is invariably
the hydrogen bond acceptor, a result in agreement with atomic
charge calculations. It is anticipated that self-assembly via
charge-assisted hydrogen bonding is an approach applicable to
many organometallic systems.
Figure 15. IR νOH peaks for 1,4-hydroquinone (H2Q, 11 mM, CH2-
Cl2) in the presence of Bu4NClO4. Peak a is due to free H2Q, and
-
peak b is due to H2Q hydrogen bonded to ClO4
.
Experimental Section
Table 2. Summary of IR Study of Hydrogen Bondinga
OH (cm-1
General Considerations: All reactions were carried out under
N2 in flame-dried glassware. HPLC-grade methylene chloride and
diethyl ether solvents were used as received without further
purification. [Rh(COD)Cl]2 was provided by Strem Chemicals. The
1H NMR spectra were recorded on Bruker (300 MHz) spectrom-
eters. Elementary analyses were performed by Quantitative Tech-
nologies Inc. (QTI, Whitehouse, NJ). Thermogravimetric analyses
(TGA, Q500 from TA Instruments) and differential scanning
calorimetry (DSC, DuPont DSC 2910) were performed at scan rates
of 5 and 10 °C/min using N2, respectively. X-ray powder diffraction
(PXRD) data were recorded on a Bruker D8 ADVANCE instrument
at 40 kV and 40 mA with Cu KR radiation (λ ) 1.540 50 Å), a
scan speed of 0.3°/s, and a step size of 0.1° in 2θ.
ν
)
compd
free
H bonded
shift
-∆Hb (kJ/mol)
hydroquinone
resorcinol
3585c
3580
4,4′-biphenol
3598
-
1+ClO4
3517d
3517d
3517d
3517d
3517d
3517d
3505d
3573e
3231
3170
3078
3058
3330
3405
3321
3296
286
347
439
459
187
112
184
21.6
23.8
26.8
27.4
17.5
13.5
17.4
1+OTf-
1+OPf-
1+OTs-
-
1+BF4
-
1+SbF6
-
2+BF4-
3+BF4
a Data obtained using 11 mM solutions in methylene chloride. b Calcu-
lated with the Iogansen equation.18 c Peak unchanged in 3 mM solution.
d νOH peak in coordinated hydroquinone, which is not hydrogen bonded.
e The νOH peak in the uncoordinated phenol ring.
[(η6-1,4-hydroquinone)bis(triphenyl
phosphite)Rh]SbF6-
(1+SbF6-). After the glassware was flame-dried, [Rh(P(OPh)3)2Cl]212
(0.36 g, 0.24 mmol) and AgSbF6 (0.19 g, 0.56 mmol) were mixed
for 1 h at room temperature in methylene chloride (5 mL). While
the mixture was stirred, a white precipitate formed on the bottom
of the glassware, after which 1,4-hydroquinone (0.10 g, 0.91 mmol)
was added to the reaction mixture. After the mixture was stirred
for 2 h at room temperature, the solvent was removed by rotary
evaporation. The residue was dissolved in methylene chloride (3
mL) and slowly dropped into an ether solution through a Celite
pad. A yellow solid formed in the ether solution and was collected
by filtration (washed with diethyl ether, 10 mL, three times). The
isolated yield was 83% (0.42 g, 0.39 mmol). Crystals were grown
by dissolving 1+SbF6- (30 mg) in methylene chloride (1.0 mL) in
a 5 mL vial and layering with 3 mL of diethyl ether. The solution
was placed in a refrigerator for 2 weeks, after which yellow crystals
formed on the wall of the vial. 1H NMR (CD2Cl2): δ 7.37 (t, J )
7.8 Hz, OPh, 12H), 7.27 (t, J ) 7.6 Hz, OPh, 6H), 7.03 (d, J ) 7.8
Hz, OPh, 12H), 6.11 (br s, OH, 2H), 5.68 (s, hydroquinone ring,
4H). Anal. Calcd for vacuum-dried C42O8H36P2RhSbF6: C, 47.18;
H, 3.39. Found: C, 47.85; H, 3.48.
Table 3. Calculated Atomic Chargesa
CH3C6H4-
SO3
O
-
-
CF3SO3
F
PF2O2
-
-
-
-
SbF6
F
BF4
F
ClO4
O
O
F
O
elec -0.55 -0.55 -0.71
mull -0.49 -0.53 -0.66
nat -0.67 -0.61 -0.94
-0.81
-0.71
-1.08
-0.30 -0.76 -0.43 -0.90
-0.42 -0.68 -0.47 -0.75
-0.39 -1.06 -0.62 -1.13
a Using Spartan ’04 with a 3-21G(*) Gaussian basis set. Abbreviations:
elec, electrostatic potential; mull, Mulliken population analysis; nat, natural
bond orbital population analysis.
difficult to define,20 which led us to include the results from
three differing approaches. However, regardless of the charge
partitioning scheme used, the oxygen atoms are calculated to
be more electron rich than the fluorine atoms (see Table 3).
These results are in agreement with the observed preference
for charge-assisted hydrogen bonding to oxygen over fluorine
in OTf- and OPf-, as well as the trends observed in the IR
spectra.
[(η6-1,4-Hydroquinone)bis(triphenyl
phosphite)Rh]OTf-
(1+OTf-). The same procedure was followed using AgOTf instead
of AgSbF6. The isolated yield was 91%. Crystals of 1+OTf- were
grown by layering a methylene chloride solution with hexane and
cooling in a refrigerator for 2 days. Yellow crystals formed on the
wall of the vial. 1H NMR (CD2Cl2): δ 8.26 (br s, OH, 2H), 7.31(t,
J ) 8.0 Hz, OPh, 12H), 7.21 (t, J ) 7.9 Hz, OPh, 6H), 6.97 (d, J
(20) Hehre, W. J. A Guide to Molecular Mechanics and Quantum
Chemical Calculations; Wavefunction: Irvine, CA 2003; Chapter 16.

Self-Assembly of [(η6-hydroquinone)Rh(P(OPh)3)2]+
Organometallics, Vol. 25, No. 22, 2006 5285
) 8.0 Hz, OPh, 12H), 5.47 (s, hydroquinone ring, 4H). Anal. Calcd
for C43O11H36P2RhSF3: C, 52.56; H, 3.69. Found: C, 53.08; H,
3.63.
[(η6-1,4-Hydroquinone)bis(triphenyl
phosphite)Rh]BF4-
(1+BF4-). The synthesis and characterization of this compound is
reported in ref 12.
[(η6-1,4-Hydroquinone)bis(triphenyl phosphite)Rh]PF2O2
(1+OPf-). The same procedure was followed using AgPF6 instead
[(η6-1,4-Hydroquinone)bis(triphenyl phosphite)Rh]ClO4-
(1+ClO4-). The same procedure was followed using AgClO4 instead
of AgSbF6. The isolated yield was 79%. Crystals of 1+ClO4- were
grown by layering a methylene chloride solution with hexane and
-
of AgSbF6. Before recrystallization, the complex had PF6 as the
counteranion. Anal. Calcd for C42O8H36P3RhF6: C, 51.55; H, 3.71.
Found: C, 52.04; H, 3.69. During recrystallization from methylene
1
cooling in a refrigerator for 4 days. H NMR (CD2Cl2): δ 7.37(t,
-
chloride, however, hydrolysis of the anion to PF2O2 (OPf-)
J ) 7.9 Hz, OPh, 12H), 7.25 (t, J ) 7.8 Hz, OPh, 6H), 7.01 (d, J
) 7.8 Hz, OPh, 12H), 6.96 (br s, OH, 2H), 5.67 (s, hydroquinone
ring, 4H). Anal. Calcd for C42O12H36P2RhCl: C, 54.07; H, 3.89.
Found: C, 54.08; H, 4.01.
occurred to afford 1+OPf- in a 66% isolated yield. 1H NMR (CD2-
Cl2): δ 9.59 (br s, OH, 2H), 7.37(t, J ) 8.0 Hz, OPh, 12H), 7.20
(t, J ) 7.9 Hz, OPh, 6H), 6.98 (d, J ) 8.0 Hz, OPh, 12H), 5.50 (s,
hydroquinone ring, 4H). Anal. Calcd for C42O10H36P3Rh1F2: C,
53.98; H, 3.88. Found: C, 53.50; H, 3.73.
Single-Crystal X-ray Structure. X-ray data collection was
carried out using a Bruker single-crystal diffractometer equipped
with an APEX CCD area detector and controlled by SMART
version 5.0 software. Collection was done at either 100 or 293 K.
Data reduction was performed with SAINT version 6.0 software.
The structures were generally determined by direct methods and
refined on F2 by use of programs in SHELXTL version 5.0. Most
hydrogen atoms appeared in a difference map, or they were
generally inserted in ideal positions, riding on the atoms to which
they are attached.
[(η6-Resorcinol)bis(triphenyl phosphite)Rh]BF4 (2+BF4-). The
synthesis and characterization of this compound are reported in ref
12.
[Rh(η6-4,4-Biphenol)bis(triphenyl phosphite)]BF4 (3+BF4-).
The same procedure was followed using 4,4-biphenol instead of
hydroquinone. The isolated yield was 87%. Crystals of 3+BF4- were
grown by layering a methylene chloride solution with hexane and
cooling in a refrigerator for 3 days. Orange crystals formed on the
wall of the vial. 1H NMR (CD2Cl2): δ 8.39 (br s, OH, 1H), 7.26 (t,
J ) 7.5 Hz, OPh, 12H), 7.22 (t, J ) 7.5 Hz, OPh, 6H), 6.90 (t, J
) 7.6 Hz, OPh, 6H), 6.83 (d, J ) 8.9 Hz, biphenol, 2H), 6.75(d,
J ) 6.75 Hz, biphenol, 2H), 6.00 (br s, OH, 1H), 5.92 (s, biphenol,
4H). Anal. Calcd for C48O8H40P2RhBF4: C, 57.86; H, 4.05.
Found: C, 57.74; H, 3.91.
Acknowledgment. S.U.S. is grateful for support by the
Sungkyunkwan University Faculty Research Fund-2005. D.A.S.
is pleased to acknowledge the donors of the Petroleum Research
Fund, administered by the American Chemical Society, for
support of this research.
[(η6-1,4-Hydroquinone)bis(triphenyl
phosphite)Rh]OTs-
(1+OTs-). The same procedure was followed using silver tosylate
instead of AgSbF6. The isolated yield was 95%. Crystals of 1+OTs-
were grown by layering a methylene chloride solution with hexane
and cooling in a refrigerator for 3 days. 1H NMR (CD2Cl2): δ 7.38
(d, J ) 7.5 Hz, OTs, 2H), 7.27 (t, J ) 7.8 Hz, OPh, 12H), 7.25 (d,
J ) 7.5 Hz, OTs, 2H), 7.15 (t, J ) 7.6 Hz, OPh, 6H), 6.95 (d, J )
7.8 Hz, OPh, 12H), 6.69 (br s, OH, 2H), 5.55 (s, hydroquinone
ring, 4H), 2.39 (s, OTs methyl, 3H). Anal. Calcd for C50O11H43P2-
RhS: C, 54.81; H, 3.94. Found: C, 54.66; H, 3.86.
Supporting Information Available: CIF files for all X-ray
structures. This material is available free of charge via the Internet
at http://pubs.acs.org. These files have also been deposited with
the Cambridge Crystallographic Data Centre as registry numbers
CCDC-299584-299590.
OM0604425