Venetian Blinds mechanism of solvation/desolvation in palladium(II) wheel-and-axle organic-inorganic diols

Article abstract of DOI:10.1021/ic048896m

A family of organic-inorganic wheel-and-axle diols (Pd(LOH) ₂Cl₂, Pd(LOH)₂(CH₃)Cl, Pd(LOH) ₂(CH₃COO)₂, LOH = α-(4-pyridyl) benzhydrol) and several corresponding solvates are synthesized and characterized by single-crystal X-ray diffraction analysis. Their structures are compared to investigate the factors governing the modes of solid state association, the propensity to clathration, and the structural basis of guest inclusion. In all the complexes, the palladium coordination is a slightly distorted square. The LOH ligands coordinate Pd²⁺ by means of the 4-pyridyl ring. In the chloride complexes solvation occurs with a 1:2 host/guest ratio by hydrogen bonding between the terminal -OH groups of the complex diol and one acceptor atom on the guest, and it is further assisted by guest stacking between host aryl rings. All solvates are organized in layers with practically invariant metrics, while the layers may be assembled in different arrangements. The structures of the nonsolvate compounds are related to the metrics of the solvate forms by rotation of the complex molecules within the layer plane. In all cases the nonsolvates are completely converted into the corresponding crystalline solvate forms by exposure to the vapor of the guest, and conversely they are quantitatively recovered from the solvate upon removal of the guest by mild conditions. On the basis of the structural data, it is proposed that the solvation/desolvation process proceeds by a concerted rotation of the complex molecules in the layer plane. The structural analysis of Pd(LOH) ₂(CH₃COO)₂ and of its tetrahydrofuran monosolvate form suggests that the first step of the solid/gas solvation process may imply the clathration of 1 mol of guest between the aryl rings, which successively triggers the collective reorientation of the host molecules.

Full text of DOI:10.1021/ic048896m

Inorg. Chem. 2005, 44, 431−442
“Venetian Blinds” Mechanism of Solvation/Desolvation in Palladium(II)
Wheel-and-Axle Organic Inorganic Diols
−
Alessia Bacchi,* Elsa Bosetti, Mauro Carcelli, Paolo Pelagatti, Dominga Rogolino, and
Giancarlo Pelizzi
Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica,
UniVersity of Parma, Parco Area delle Scienze, 17A, 43100 Parma, Italy
Received August 11, 2004
A family of organic−inorganic wheel-and-axle diols (Pd(LOH)2Cl2, Pd(LOH)2(CH3)Cl, Pd(LOH)2(CH3COO)2, LOH )
R
-(4-pyridyl)benzhydrol) and several corresponding solvates are synthesized and characterized by single-crystal
X-ray diffraction analysis. Their structures are compared to investigate the factors governing the modes of solid
state association, the propensity to clathration, and the structural basis of guest inclusion. In all the complexes, the
+
palladium coordination is a slightly distorted square. The LOH ligands coordinate Pd2 by means of the 4-pyridyl
ring. In the chloride complexes solvation occurs with a 1:2 host/guest ratio by hydrogen bonding between the
terminal
−OH groups of the complex diol and one acceptor atom on the guest, and it is further assisted by guest
stacking between host aryl rings. All solvates are organized in layers with practically invariant metrics, while the
layers may be assembled in different arrangements. The structures of the nonsolvate compounds are related to
the metrics of the solvate forms by rotation of the complex molecules within the layer plane. In all cases the
nonsolvates are completely converted into the corresponding crystalline solvate forms by exposure to the vapor of
the guest, and conversely they are quantitatively recovered from the solvate upon removal of the guest by mild
conditions. On the basis of the structural data, it is proposed that the solvation/desolvation process proceeds by
a concerted rotation of the complex molecules in the layer plane. The structural analysis of Pd(LOH)2(CH3COO)2
and of its tetrahydrofuran monosolvate form suggests that the first step of the solid/gas solvation process may
imply the clathration of 1 mol of guest between the aryl rings, which successively triggers the collective reorientation
of the host molecules.
Introduction
Their propensity to include small molecules in the crystal
lattice derives from the combination of the hydrogen bond
donor capability of the OH groups with the steric bulkiness
of the terminal diaryl groups.3 The prototypical system is
HO-C(Ar)2-L-(Ar)2C-OH, where L is a suitable rigid
spacer (e.g. fused rings or triple bonds). Our purpose is to
use a metal to build in an innovative way the L spacer, thus
obtaining organic-inorganic wheel-and-axle diols.4 With this
aim, we have synthesized crystalline compounds by coor-
The rational design of new porous solids based on the
combination of soft supramolecular interactions and coor-
dination bond is attracting the interest of many researchers
in the field of crystal engineering, in view of the potential
utilization of these materials for heterogeneous catalysis,
solid-state “green” chemistry, and sensor applications.1 The
“wheel-and-axle” diols have been extensively studied for
their versatility as hosts for a variety of solvent guests.2
Wheel-and-axle diols are organic molecules with a long,
usually linear axis with large, rigid substituents at both ends.
(2) (a) Weber, E.; Skobridis, K.; Wierig, A.; Nassimbeni, L. R.; Johnson,
L. J. Chem. Soc., Perkin Trans. 2 1992, 2123. (b) Johnson, L.;
Nassimbeni, L. R.; Weber, E.; Skobridis, K. J. Chem. Soc., Perkin
Trans. 2 1992, 2131. (c) Caira, M. R.; Jacobs, A.; Nassimbeni, L. R.;
Toda, F. Cryst. Eng. Commun. 2003, 5, 150-153. (d) Caira, M. R.;
Nassimbeni, L. R.; Toda, F.; Vujovic, D. J. Chem. Soc., Perkin Trans.
2 2001, 2119-2124. (e) Csoregh, I.; Brehmer, T., Nitsche, S. I.;
Seichter, W.; Weber, E. J. Inclusion Phenom. Macrocyclic Chem. 2003,
47, 113-121.
* Author to whom correspondence should be addressed. E-mail:
alessia.bacchi@unipr.it.
(1) (a) Yoo, S.-K.; Ryu, J. Y.; Lee, J. Y.; Kim, C.; Kim, S.-J.; Kim, Y.
J. Chem. Soc., Dalton Trans. 2003, 1454. (b) Kaupp, G. Cryst. Eng.
Commun. 2003, 5, 117. (c) Reinbold, J.; Buhlmann, K.; Cammann,
K.; Wierig, A.; Wimmer, C.; Weber, E. Sens. Actuators, B 1994, 18,
77.
(3) Toda, F. Comp. Supramol. Chem. 1996, 6, 465-516.
10.1021/ic048896m CCC: $30.25
Published on Web 12/29/2004
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 2, 2005 431

Bacchi et al.
Chart 1. Hybrid Organic-Inorganic Prototypal Diol
was stirred magnetically at room temperature for 3 h; a yellow
powder was filtered off, washed with diethyl ether, and dried in
vacuo. The product was recrystallized from 1,4-dioxane at room
temperature, and yellow crystals were obtained. Yield: 73%. Mp:
300 °C (dec). Anal. Calcd for C36H30Cl2N2O2Pd (Mr ) 699.94):
C, 61.77; H, 4.31; N, 4.00. Found: C, 61.82; H, 4.96; N, 3.52. IR
(KBr, cm-1): ν(O-H) 3403 (m), ν(C-H) 3056, 2919, 2850 (w).
1H NMR (CDCl3, 25 °C, ppm): δ 8.73 (d, JH,H ) 6.67 Hz, 4H,
C(4)-H, C(5)-H), 7.36-7.19 (m, 24H, Ar + C(3)-H + C(6)-
H). 1H NMR ((CD3)2SO, 25 °C, ppm): δ 8.66 (d, 4H, JH,H ) 5.53
Hz, C(4)-H, C(5)-H), 7.46-7.19 (m, 24H, Ar + C(3)-H +
C(6)-H), 6.93 (s, 2H, -OH, D2O exchangeable). 1H NMR
((CD3)2CO, 25 °C, ppm): δ 8.75 (d, JH,H ) 6.82 Hz, 4H, C(4)-H,
C(5)-H), 7.49-7.31 (m, 24H, Ar + C(3)-H + C(6)-H), 3.57 (s,
2H, -OH, D2O exchangeable).
[Pd(LOH)2Cl2]‚2(CH3)2CO (2). A 100 mg amount of 1 was
dissolved in acetone, and the solution was left at room temperature
for some days. Yellow crystals of 2 were filtered off (yield 92%;
mp 300 °C (dec)). Compound 2 was also obtained by exposing
100 mg of 1 to vapors of acetone for 5 days. The quantitative
formation of 2 was assessed by comparing the X-ray powder
diffraction pattern of the crystalline product with the spectrum
calculated on the basis of the single-crystal structure. The experi-
mental pattern contained also some weak peaks belonging to
crystalline LOH, quantified in a few percent impurity. Anal. Calcd
for C42H42Cl2N2O4Pd (Mr ) 816.12): C, 61.83; H, 5.14; N, 3.42.
Found: C, 61.70; H, 5.13; N, 3.50. IR (KBr, cm-1): ν(O-H) 3362
(m), ν(CdO) 1688 (s). 1H NMR (CDCl3, 25 °C, ppm): δ 8.72 (d,
JH,H ) 6.62 Hz, 4H, C(4)-H, C(5)-H), 7.37-7.18 (m, 24H, Ar
+ C(3)-H + C(6)), 2.16 (s, 12H, (CH3)2CO). Thermogravimetric
data: experimental mass loss, 13.67%; calcd for two acetone
molecules, 14.22%.
dinating R-(4-pyridyl)benzhydrol (LOH) to transition metals
with various geometric and chemical properties; the final
structural and clathrating properties will depend on the metal
coordination number, geometry, and configuration, on the
steric and supramolecular characteristics of the counteranions,
and on the hydrogen bond network involving the carbinol
-OH groups (Chart 1). It is also worth of note that the use
of a metal allows one to obtain molecules with a long
principal axle that are interesting to widen the available
network space,5 without the laborious procedures of the
organic synthesis. Metals with propensity to low coordination
numbers are the natural initial choice to design networks with
low steric hindrance in the middle of the molecular axle.
We have recently presented the first results concerning the
rationalization of the supramolecular organization of a series
of silver compounds.6 In this work we report and compare
the crystal organization of a family of palladium complexes,
Pd(LOH)2Cl2, Pd(LOH)2(CH3)Cl, Pd(LOH)2(CH3COO)2, and
several related solvates, to investigate the factors governing
the modes of solid-state association, the propensity to
clathration, and the structural bases of guest inclusion. We
also explore the possibility to convert between the solvate
and the nonsolvate forms via solid/gas processes, and we
propose a mechanism for the observed reversible solvation/
desolvation with retention of crystallinity. The response of
molecular conformation to clathration is also analyzed.
[Pd(LOH)2Cl2]‚2DMSO (3). A sample of [Pd(LOH)2Cl2] was
recrystallized from DMSO, the solution was left at room temper-
ature for some days, and yellow crystals were filtered off. Yield:
42%. Mp: 300 °C (dec). Anal. Calcd for C40H42Cl2N2O2PdS2 (Mr
) 856.12): C, 56.11; H, 4.90; N, 3.27; S, 7.48. Found: C, 56.03;
H, 5.18; N, 3.54; S, 7.85. IR (KBr, cm-1): ν(O-H) 3100 (m),
1
ν(C-H) 3102, ν(SdO) 1049 (s). H NMR (CDCl3, 25 °C, ppm):
δ 8.70 (d, JH,H ) 6.72 Hz, 4H, C(4)-H, C(5)-H), 7.35-7.18 (m,
24H, Ar + C(3)-H + C(6)-H), 2.57 (s, 12H, CH3 (DMSO)).
[Pd(LOH)2Cl2]‚2DMF (4). A 100 mg amount of 1 was dissolved
in DMF, and the solution was left at room temperature for some
days. Yellow crystals of 4 were filtered off. Yield: 73%. Mp: 300
°C (dec). Anal. Calcd for C42H42Cl2N4O4Pd (Mr ) 845.78): C,
59.64; H, 5.20; N, 6.62. Found: C, 59.20; H, 5.24; N, 6.69. IR
(KBr, cm-1): ν(O-H) 3279 (m), ν(C(Ar)-H) 3102, ν(C-H) 2927,
Experimental Section
Synthesis. [Pd(PhCN)2Cl2]7 and [Pd(COD)MeCl]8 were synthe-
sized by following literature methods. The ligand R-(4-pyridyl)-
benzhydrol (Aldrich, 75%) was recrystallized from boiling CH2Cl2.
Proton NMR spectra were recorded at 27 °C on a Bruker 300 FT
spectrophotometer by using SiMe4 as internal standard, while IR
spectra were obtained with a Nicolet 5PCFT-IR spectrophotometer
in the 4000-400 cm-1 range, using KBr disks. Elemental analyses
were performed by using a Carlo Erba model EA 1108 apparatus.
[Pd(LOH)2Cl2] (1). A CH2Cl2 solution (40 mL) of LOH (0.074
g, 2.84 × 10-4 mol) was slowly added to a CH2Cl2 solution (10
mL) of Pd(PhCN)2Cl2 (0.040 g, 1.42 × 10-4 mol). The mixture
1
2874, ν(CdO) 1660 (s). H NMR (CDCl3, 25 °C, ppm): δ 8.72
(d, JH,H ) 6.63 Hz, 4H, C(4)-H, C(5)-H), 7.97 (s, 2H, -COH
(DMF)), 7.36-7.18 (m, 24H, Ar + C(3)-H + C(6)-H), 2.94 (s,
6H, CH3 (DMF)), 2.86 (s, 6H, -CH3 (DMF)). Thermogravimetric
data: experimental mass loss, 17.01%; calcd for two DMF
molecules, 17.28%.
[Pd(LOH)2Cl2]‚2THF (5). A THF solution (40 mL) of LOH
(0.074 g, 2.84 × 10-4 mol) was slowly added to a THF solution
(10 mL) of Pd(PhCN)2Cl2 (0.040 g, 1.42 × 10-4 mol). The mixture
was stirred at room temperature for 3 h, and then the solution was
concentrated and diethyl ether was added (40 mL). After 1 day at
-18 °C, yellow crystals were formed. Yield: 84%. Mp: 300 °C
(dec). Anal. Calcd for C44H46Cl2N2O4Pd (Mr ) 843.90): C, 62.62;
H, 5.45; N, 3.33. Found: C, 62.98; H, 5.18; N, 3.54. IR (KBr,
(4) As far as we know, the only precedent can be found in the following:
Sembirin, S. B.; Colbran, S. B.; Bishop, R.; Craig, D. C.; Rae, A. D.
Inorg. Chim. Acta 1995, 228, 109-117.
(5) Weber, E.; Hens, Th.; Brehmer, Th; Csoregh, I. J. Chem Soc., Perkin
Trans. 2 2000, 235-241.
(6) (a) Bacchi, A.; Bosetti, E.; Carcelli, M.; Pelagatti, P.; Rogolino, D.
Eur. J. Inorg. Chem. 2004, 1985-1991. (b) Bacchi, A.; Bosetti, E.;
Carcelli, M.; Pelagatti, P.; Rogolino D. Cryst. Eng. Commun. 2004,
6, 177-183.
(7) Anderson, G. K.; Lin, M. Inorg. Synth. 1990, 28, 61-62.
(8) Rulke, R. E.; Ernsting, J. M.; Spek, A. L.; Elsevier, C. J.; van Leeuwen,
P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769-5778.
1
cm-1): ν(O-H) 3284 (m), ν(C-H) 2879 (w). H NMR (CDCl3,
25 °C, ppm): δ 8.71 (d, JH,H ) 6.67 Hz, 4H, C(4)-H, C(5)-H),
432 Inorganic Chemistry, Vol. 44, No. 2, 2005

Pd(II) Wheel-and-Axle Organic-Inorganic Diols
Table 1. Crystal Data for Compounds 1-10
1
2
3
4
5
formula
fw
cryst system
space group
Z
C36H30Cl2N2O2Pd
699.92
monoclinic
P21/n
C42H42Cl2N2O4Pd
816.08
monoclinic
P21/n
C40H42Cl2N2O4PdS2
856.18
monoclinic
P21/n
C42H42Cl2N4O4Pd
844.10
monoclinic
P21/c
C44H46Cl2N2O4Pd
844.13
triclinic
P1h
4
2
2
2
1
a/Å
b/Å
c/Å
10.736(2)
17.410(2)
16.933(2)
7.2997(9)
18.591(2)
14.631(2)
7.6906(3)
18.5017(8)
14.5046(5)
7.191(8)
15.06(2)
18.84(2)
7.9754(7)
11.220(1)
11.543(1)
100.534(2)
96.889(2)
98.944(2)
991.4(1)
1.414
R/deg
â/deg
95.584(2)
103.959(2)
106.166(2)
88.17(2)
γ/deg
V/Å3
3150.0(8)
1.476
4523
2977
508
1926.9(4)
1.407
4787
4023
237
1982.2(1)
1.434
5123
4115
294
2039(4)
1.375
4774
3997
245
F/Mg m-3
unique reflcns
obsd reflcns [I > 2σ(I)]
params
5280
5009
301
R1 [I > 2σ(I)]
wR2 [I > 2σ(I)]
R1 (all data)
wR2 (all data)
0.0365
0.0706
0.0701
0.0784
0.0619
0.1559
0.0717
0.1588
0.0307
0.0881
0.0411
0.0933
0.1202
0.3252
0.1344
0.3313
0.0250
0.0684
0.0275
0.071
6
7
8
9
10
formula
fw
cryst system
space group
Z
C37H33ClN2O2Pd
679.50
monoclinic
P21/n
C45H49ClN2O4Pd
823.71
triclinic
P1
C43H45ClN2O4Pd
795.66
triclinic
P1h
1
C44H44N2O7Pd
819.21
triclinic
P1
C40H36N2O6Pd
747.11
triclinic
P1
4
1
1
1
a/Å
b/Å
c/Å
10.820(4)
17.386(9)
16.85(2)
8.027(3)
11.255(3)
11.550(2)
100.41(3)
96.37(3)
98.95(4)
1003.4(5)
1.363
7.4639(8)
11.4710(8)
11.7479(8)
101.948(1)
95.957(1)
96.190(1)
970.0(1)
1.362
7.9303(5)
11.1789(7)
11.8089(7)
84.133(1)
88.154(1)
77.3930(9)
1016.2(1)
1.339
7.569(2)
9.995(2)
12.394(3)
89.584(4)
88.114(4)
80.596(3)
924.5(4)
1.342
R/deg
â/deg
94.12(5)
γ/deg
V/Å3
3162(4)
1.428
6907
3255
404
F/Mg m-3
unique reflcns
obsd reflcns [I > 2σ(I)]
params
4847
3065
481
2778
2681
207
8250
7448
498
2123
1934
133
R1 [I > 2σ(I)]
wR2 [I > 2σ(I)]
R1 (all data)
wR2 (all data)
0.0623
0.1325
0.1651
0.1788
0.0464
0.0819
0.0936
0.0955
0.0377
0.1051
0.0389
0.1064
0.0315
0.0705
0.0361
0.0729
0.0834
0.2107
0.0980
0.2343
7.37-7.18 (m, 24H, Ar + C(3)-H + C(6)), 3.72 (m, 8H, THF),
1.82 (m, 8H, THF).
H) 3368 (m), ν(C(Ar)-H) 3054, ν(C-H) 2960, 2883, ν(CdO) 1694
(s). H NMR (CDCl3, 25 °C, ppm): δ 8.74-8.69 (dd, JH,H ) 6
1
[Pd(LOH)2(CH3)Cl]‚2THF (7). A THF dry solution (50 mL)
of LOH (0.153 g, 5.8 × 10-4 mol) was slowly added under N2 to
a THF dry solution (10 mL) of Pd(COD)MeCl (0.079 g, 2.9 ×
10-4 mol). The mixture was stirred magnetically at room temper-
ature for 1 day, and then the solution was evaporate and diethyl
ether was added (20 mL). After 1 day at -18 °C yellow crystals
of 7 were formed. Yield: 76%. Mp: 54 °C (dec). Anal. Calcd for
C45H49ClN2O2Pd (Mr ) 827.21): C, 65.34; H, 5.97; N, 3.39.
Found: C, 65.64; H, 5.95; N, 3.40. IR (KBr, cm-1): ν(O-H) 3289
(m), ν(C-H) 2967, 2876. 1H NMR (CDCl3, 25 °C, ppm): δ 8.75-
8.69 (dd, JH,H ) 6 Hz, 4H, C(4)-H, C(5)-H), 7.35-7.18 (br, 24H,
Ar + C(3)-H + C(6)-H), 3.72-3.68 (m, 4H, THF), 3.00 (s, 2H,
-OH, D2O exchangeable), 1.85-1.80 (m, 4H, THF), 0.77 (s, 3H,
CH3).
[Pd(LOH)2(CH3)Cl] (6). Recrystallization of 7 in CHCl3 gives
partial decomposition of the complex and crystals of 6, which were
characterized by single-crystal X-ray diffraction.
[Pd(LOH)2(CH3)Cl]‚2(CH3)2CO (8). A 100 mg amount of
[Pd(LOH)2(CH3)Cl]‚2THF was recrystallized from acetone, the
solution was left at room temperature for some days, and yellow
crystals were filtered off. Yield: 81%. Mp: 72 °C (dec). Anal.
Calcd for C43H45ClN2O4Pd (Mr ) 795.70): C, 64.91; H, 5.70; N,
3.52. Found: C, 64.52; H, 5.33; N, 3.21. IR (KBr, cm-1): ν(O-
Hz, 4H, C(4)-H, C(5)-H), 7.35-7.18 (br, 24H, Ar + C(3)-H +
C(6)-H + C(21)-H + C(24) + H), 3.13 (s, 2H, OH, D2O
exchangeable), 2.14 (s, 12H, (CH3)2CO), 0.77 (s, 3H, CH3).
[Pd(LOH)2(OAc)2]‚THF (9). A THF solution (50 mL) of LOH
(0.10 g, 3.82 × 10-4 mol) was slowly added to a THF solution (10
mL) of Pd(OAc)2 (0.042 g, 1.87 × 10-4 mol). The mixture was
stirred magnetically at room temperature. After a few minutes a
yellow powder was formed. The mixture was filtered, and the
precipitate was washed with diethyl ether and dried in vacuo.
Yield: 72%. Mp: 179 °C (dec). Anal. Calcd for C44H44N2O7Pd
(Mr ) 819.025): C, 64.52; H, 5.37; N, 3.43. Found: C, 64.26; H,
5.26; N, 3.11. IR (KBr, cm-1): ν(O-H) 3232 (m), ν(C(Ar)-H)
1
3049 (w), ν(C-H) 2973 (w). H NMR (DMSO, 25 °C, ppm): δ
8.51 (d, JH,H ) 6.9 Hz, 4 H, C(4)-H, C(5)-H), 7.42-7.18 (br,
24H, Ar + C(3)-H + C(6)-H), 6.91 (s, 2H, -OH, D2O
exchangeable), 3.61-3.58 (m, 4H, THF), 1.85-1.80 (m, 4H, THF),
1.68 (s, 6H, OAc). Thermogravimetric data: experimental mass
loss (9.18%) corresponds to one THF molecule (calcd: 8.82%).
X-ray-quality crystals of 9 were obtained by recrystallization from
a mixture of CHCl3/THF (1:1).
[Pd(LOH)2(OAc)2] (10). Single crystals of 10 were obtained by
recrystallization of 9 in CHCl3 and were characterized by single-
Inorganic Chemistry, Vol. 44, No. 2, 2005 433

Bacchi et al.
Chart 2. Synthesis of the Complexes 1-10
crystal X-ray diffraction. The homogeneity of the bulk was assessed
by X-ray powder diffraction.
was found. Hydrogen bonds were analyzed with SHELXL9711 and
PARST97,13 and extensive use was made of the Cambridge
Crystallographic Data Centre packages14 for the analysis of crystal
packing. Table 1 summarizes crystal data and structure determi-
nation results.
Thermal Analyses. Thermogravimetry (TGA) measurements
were performed on a Perkin-Elmer thermogravimetrical analyzer
TGA7, and differential scanning calorimetry (DSC) measurements,
on a Perkin-Elmer differential scanning calorimeter DSC7 and
Mettler Toledo Star system. For TGA analyses the samples were
placed in open platinum pans, and for DSC the samples were placed
in closed and vented aluminum pans. The TGA and DSC traces
were obtained on heating the samples at 5 °C/min from 25 to 350
°C for compounds 2 and 4 and from 25 to 250 °C for compound
9.
Results and Discussion
The ligand R-(4-pyridyl)benzhydrol LOH reacts with
Pd(PhCN)2Cl2, Pd(COD)(CH3)Cl, and Pd(OAc)2 in a 2:1
stoichiometry giving the complexes Pd(LOH)2Cl2, Pd(LOH)2-
(CH3)Cl, and Pd(LOH)2(OAc)2 as nonsolvate (1, 6, 10)
or solvate (2-5, 7-9), depending on the reaction con-
ditions (Chart 2). The complexes [Pd(LOH)2Cl2] (1) and
[Pd(LOH)2Cl2]‚2THF (5) are obtained by direct synthesis
in CH2Cl2 and in THF, respectively, while [Pd(LOH)2Cl2]‚
2G (G ) acetone (2), DMSO (3), and DMF (4)) are obtained
through recrystallization of 1 from the corresponding solvent.
Recrystallization from solvents with poorer hydrogen bond
accepting properties (toluene, mesitylene, p-xylene, 1,4-
dioxane, methanol, ethanol, 2-propanol, chloroform, di-
chloromethane, carbon tetrachloride) gives the nonsolvate
Powder Diffraction. X-ray powder diffraction (XRPD) data were
collected on a Philips Analytical X-ray PW3710 BASED diffrac-
tometer with a Cu KR radiation. All XRPD measurements were
carried out at room temperature.
Crystallographic Analysis. Mo KR radiation (λ ) 0.710 73 Å)
was used; T ) 293 K for all compounds (SMART AXS 1000 CCD
diffractometer for 1-5 and 8-10 and Philips PW1100 diffrac-
tometer for 6 and 7). Lorentz, polarization, and absorption correc-
tions were applied.9 Structures solved by direct methods using
SIR9710 and refined by full-matrix least squares on all F2 using
SHELXL9711 implemented in the WingX package.12 Hydrogen
atoms partly located on Fourier difference maps and refined
isotropically and partly introduced in calculated positions. Aniso-
tropic displacement parameters refined for all non-hydrogen atoms,
except for compound 10, where isotropic parameters were used.
In compounds 6 and 8 a positional disorder between Cl and CH3
1
1. The complexes were characterized through IR and H
NMR spectroscopy and single-crystal and powder X-ray
diffractometry. In all the compounds the palladium has a
square planar coordination with two LOH molecules, trans
each other, and two anions as ligands; LOH coordinates by
means of the 4-pyridyl nitrogen. The protons H4 and H5
(Chart 3) of the coordinated pyridyl rings are deshielded with
respect to the free ligand (∆δ ≈ 0.2 ppm); the peak of
alcoholic proton is observed only in hydrogen bond accepting
solvents (DMSO, (CH3)2CO). [Pd(LOH)2(CH3)Cl]‚2THF (7)
(9) (a) SAINT: SAX, Area Detector Integration; Siemens Analytical
Instruments, Inc.: Madison, WI. (b) Sheldrick, G. SADABS: Siemens
Area Detector Absorption Correction Software; University of Goet-
tingen: Goettingen, Germany, 1996.
(10) Altomare, A.; Burla, M. C.; Cavalli, M.; Cascarano, G.; Giacovazzo,
C.; Gagliardi, A.; Moliterni, A. G.; Polidori, G.; Spagna, R. Sir97: A
new Program For SolVing And Refining Crystal Structures; Istituto
di Ricerca per lo Sviluppo di Metodologie Cristallografiche CNR:
Bari, Italy, 1997.
(13) Nardelli, M. J. Appl. Crystallogr. 1995, 28, 659.
(14) (a) Allen, F. H.; Kennard, O.; Taylor, R. Acc. Chem. Res. 1983, 16,
146-153. (b) Bruno, I. J.; Cole, J. C.; Edgington, P. R.; Kessler, M.;
Macrae, C. F.; McCabe, P.; Pearson, J.; Taylor, R. Acta Crystallogr.
2002, B58, 389-397.
(11) Sheldrick, G. Shelxl97. Program for Structure Refinement; University
of Goettingen: Goettingen, Germany, 1997.
(12) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837-838.
434 Inorganic Chemistry, Vol. 44, No. 2, 2005

Pd(II) Wheel-and-Axle Organic-Inorganic Diols
Chart 3. Geometric Descriptors Used in the Analysis of the Molecular
Conformation
so that |Ti| < 90°, according to Dance’s model for tri-
phenylphosphines15). The principal interactions responsible
for the crystal organization are reported in Table 3.
In 1 the Pd(LOH)2Cl2 units are assembled by -OH‚‚‚Cl
hydrogen bonds in chains running along a (Figure 1), and
consequently the terminal C-OH vectors are in a gauche
orientation (ø ) 109°). The terminal triarylcarbinol groups
can be classified as regular rotors.15 The Pd(LOH)2Cl2 units
present a C2 pseudosymmetry, with the pseudo-2-fold axis
passing through the palladium atom, perpendicularly to the
Cl-Pd-Cl and N-Pd-N vectors, and directed along c
(Figure 1). The molecules in the chain are related by
translation; consequently, the chain pitch corresponds to a
) 10.736(2) Å. The molecular long axes form an angle θ )
26° with a, the direction of propagation of the chain. The
chains are associated in layers by translation along b in the
ab plane (Figure 1), with Pd‚‚‚Pd ) b ) 17.410(2) Å and
with a minimum distance C‚‚‚C ) 4.3 Å between adjacent
aromatic rings. The metric of these layers, defined by the
bidimensional lattice a′, b′ and R′, will be the key of the
discussion of the reversible solvation/desolvation processes
shown by 1. The layers have a bump-and-hollow profile,
and the optimal space filling for the three-dimensional
packing is achieved by 2-fold screw rotation around b (Figure
2). Because of the inclination of the molecular long axes
relative to a, the adjacent, symmetry-related layers present
opposite orientation of the molecules in the chains, resulting
in a herringbone motif (Figure 2). The replacement of one
chlorine atom in 1 with the isosteric, but hydrogen-bonding-
inactive, methyl group does not result in any important
modification of the crystal organization. Compound 6 is
isostructural to 1, showing that a single -OH‚‚‚Cl interaction
is sufficient to stabilize the chains, with the assistance of
shape complementarity between complex molecules. The
positions of Cl and CH3 ligands can be exchanged within a
chain, as shown by the disordered population of these two
coordination sites in 6. Pd(LOH)2Cl2 can host in the crystal
lattice solvent molecules (G) in a 1:2 stoichiometry by
generating the solvates 2-5, among which 2 and 3 are
isostructural, while 4 has a cell with edges and angles similar
to 2 but with different unique axis and different orientation
of the symmetry operations in the cell. In all cases the
inclusion is based on the formation of two hydrogen bonds
between the two -OH groups of the complex and the oxygen
acceptor of the guest molecule G, with O‚‚‚O distances
ranging between 2.75 and 2.80 Å (Table 3). The resulting
supramolecular Pd(LOH)2Cl2‚2G unit is invariably centro-
symmetric, the molecular conformation is the same in all
cases, and the positions of the G molecules coincide (Figure
3). In all cases, the complex symmetry imposes strict
coplanarity to pyridines and ideal trans configuration for the
terminal -OH groups. Compared to 1, the triarylcarbinol
groups show a pronounced flattening of one ring (T3 )
6-14°) and a tilting of the other (T2 ) 50-69°). The
Pd(LOH)2Cl2‚2G units are stacked along a in chains where
G is sandwiched between a pair of aryl rings (Ar) tilted to
Table 2. Conformation of the [Pd(LOH)2X2] Molecule in the
Complexes 1-10a
ψ (deg)
ø (deg)
T1 (deg)
T2 (deg)
T3 (deg)
1
48.6(1)
109.6(4)
24.2(5)
30.3(6)
20.4(7)
20.3(2)
15(1)
34.5(2)
29.4(9)
27.2(8)
-41(2)
27(2)
36.1(5)
32.4(6)
60.1(6)
65.6(2)
69(1)
50.5(2)
32.5(9)
37.1(8)
-49(2)
27(2)
39.2(6)
39.1(6)
14.2(7)
14.3(3)
6(1)
7.4(2)
39.7(9)
35.1(8)
-4(2)
9(2)
2
3
4
5
6
0
0
0
0
180
180
180
180
45.2(2)
109.2(6)
7
2.4(9)
178(2)
8
9
0
180
-176(1)
30.1(3)
42(1)
-31(1)
54(1)
58.6(3)
30(1)
-34(1)
22(2)
5.9(3)
32(1)
-31(1)
38(1)
2.3(4)
10
14.9(7)
-141(1)
-26(1)
-33(2)
-51(1)
a ψ ) dihedral angle between pyridines; ø ) reciprocal orientation of
the two C-OH vectors; T1 ) orientation of the C-OH vector relative to
the adjacent pyridine ring; T1-T3 ) conformation of the triaryl group,
following Dance15 (T1 refers to the pyridine ring and T2 to the ring involved
in the Ar‚‚‚G stacking contact).
is obtained by synthesis in THF; recrystallization from
acetone gives [Pd(LOH)2(CH3)Cl]‚2(CH3)2CO (8), while
recrystallization in CHCl3 gives crystals of the nonsolvate
[Pd(LOH)2(CH3)Cl] (6) and partial decomposition of the
starting complex. In the IR spectra, the most significant band
is ν(OH), which follows the trend ν-OH(8) (3368 cm-1) ≈
ν-OH(2) (3362 cm-1) > ν-OH(7) (3289 cm-1) ≈ ν-OH(5)
(3286 cm-1) ≈ ν-OH(4) (3279 cm-1) > ν-OH(3) (3100 cm-1);
the sequence is in accord with the hydrogen bonds distances
obtained by the structural analysis (see X-ray diffraction)
and reflects the strength of the OH‚‚‚G hydrogen bond.
[Pd(LOH)2(CH3COO)2]‚THF (9) was obtained by synthesis
in THF, and the nonsolvate form [Pd(LOH)2(CH3COO)2]
(10) by recrystallization in CHCl3; however, 10 is not very
stable in chlorinated solvents.
X-ray Diffraction. Single crystals of compounds 1-10
were examined by X-ray diffraction. The molecular confor-
mation is described on the basis of the following geometric
descriptors6 (Chart 3, Table 2): the dihedral angle between
pyridines (ψ); the orientation of the C-OH vectors with
respect to the adjacent pyridine (Τ1); the reciprocal orienta-
tion of the two C-OH vectors (ø); the torsion angles Cortho
Cipso-Csp -O (T1, T2, T3) for each aromatic ring (labeled
-
3
(15) Dance, I.; Scudder, M. J. Chem. Soc., Dalton Trans. 2002, 1579.
Inorganic Chemistry, Vol. 44, No. 2, 2005 435

Bacchi et al.
Table 3. Host-Host and Host-Guest Interactions and Layer Metrics (Chain Pitch a′, b′, R′) in the Crystal Organization of the Complexes 1-10 (θ )
Angle That the Molecular Long Axis Forms with a′)
host-host intrachain
host-guest
host-guest stacking
layer metrics
O‚‚‚Cl
(Å)
O-H‚‚‚Cl
(deg)
O‚‚‚O
(Å)
O-H‚‚‚O
(deg)
Ar‚‚‚Ar )
a (Å)
Ar‚‚‚S
(Å)
θ
(deg)
a
(Å)
b
(Å)
R
(deg)
1
3.168(4)
3.159(4)
166(4)
160(5)
26
10.7
17.4
90
2
3
4
5
6
7
2.808(7)
2.707(3)
2.75(1)
158(14)
173(3)
160(17)
171(2)
7.2997(9)
7.6903(3)
7.191(8)
3.76(1)
3.693(3)
3.59(2)
3.890(3)
57
57
59
49
25
49
7.3
7.7
7.2
8.0
10.8
8.0
18.6
18.5
18.8
17.5
17.4
17.5
90
90
88
91
90
91
2.755(2)
7.9754(7)
3.138(5)
178(8)
2.77(2)
2.75(2)
2.865(4)
157
159
171(4)
8.027(3)
3.98(3)
8
7.4639(8)
3.682(7)
52
7.5
18.0
90
O‚‚‚O
(Å)
O-H‚‚‚O
(deg)
Pd‚‚‚O
(Å)
9
2.77(1)
2.79(1)
3.04(2)
2.47(3)
166(4)
145(4)
144(1)
162(1)
3.030(6)
3.018(7)
3.15(2)
3.06(2)
3.71(1)
3.79(1)
34
36
7.9
7.6
15.4
15.9
98
86
10
larger torsion angles (T2) (compound 2, Figure 4); the
stackingdistances Ar‚‚‚G range between 3.59 and 3.89 Å
(Table 3), while the stacking pitches Ar‚‚‚Ar coincide with
the a axes of the cells. The molecular long axes are tilted
relative to the chain direction (θ ) 49-59°). Chains are
translated by similar steps along b for 2 and 3 (b′ ) b), along
the b-c cell diagonal for 5 (b′ ) 17.507 Å) and along c for
4 (b′ ) c), thus originating layers with the same metrics
(average a′ ) 7.6 and b′ ) 18.1 Å; R′ ) 90°). This
arrangement creates channels along a with alternating
aromatic rings allocating solvent molecules between. Solvates
2-5 are differentiated by the mode of association of the
identical layers in the third direction (Figure 5). In 2 and 3
adjacent layers are generated by a screw 21 rotation around
b, while in 5 they are related by simple translation along the
b + c diagonal and in 4 by a c glide plane. The main
consequence of these different arrangements regards the
orientation of the molecular axes in chains of adjacent
layers: a herringbone motif results for 2 and 3, similar to
the one observed in 1, while parallel rows appear in 4 and
5. The effects of the substitution of one Cl atom with a CH3
group have been tested also for 2 and 5. The methylated
THF solvate 7 is isostructural with the dichloro analogue 5,
while, unexpectedly, the methylated acetone solvate 8 is not
arranged in the herringbone pattern found in 2 but it is
isostructural with 7 and 5 (Figure 5), with slight modi-
fications of the cell but retention of the layer metrics (Table
3). Substantially, the Cl/CH3 exchange does not have any
effect on the crystal organization (isostructurality of 1 with
6 and of 5 with 7). The alternation of the solvate layers
can proceed both in the herringbone (2, 3) and in the par-
allel (4, 5, 7, 8) arrangements, and this suggests the
possibility of herringbone/parallel packing polymorphs for
each solvate.
The robustness, relative to a change in size and accepting
character of the anion, of the chain motif observed in 1 and
6 was verified by analyzing the crystal arrangement of
Pd(LOH)2(CH3COO)2 (10); its solvation properties were
examined in the THF monosolvate form 9. As regards
molecular conformation (Table 2), compound 10 is midway
between the centrosymmetric complex found in 2-5 and the
pseudo-C2 conformation typical of 1 and 6. Similarities to
Figure 1. Organization of one molecular layer in the crystal structure of 1 (left) with OH‚‚‚Cl hydrogen bonds dashed and hydrogen atoms omitted,
edge-on view of the layer (right bottom), and molecular conformation (right top) of Pd(LOH)2Cl2 in 1.
436 Inorganic Chemistry, Vol. 44, No. 2, 2005

Pd(II) Wheel-and-Axle Organic-Inorganic Diols
Figure 2. Herringbone arrangement of two consecutive layers in 1 (left) and edge-on view of the packing of two complementary layers, obtained by 2-fold
screw rotation around b (right). Different layers are represented by different shades of gray.
Figure 3. Superimposition of supramolecular Pd(LOH)2Cl2.2S units for 2-5. Hydrogen bond donors and acceptors are evidenced as balls.
Figure 4. Organization of one molecular layer in the crystal structure of 2 (left) with OH‚‚‚O(acetone) hydrogen bonds dashed and hydrogen atoms
omitted, edge-on view of the layer (right bottom), and molecular conformation (right top) of Pd(LOH)2Cl2‚2(CH3)2CO in 2. Compound 3 is isostructural to
2. The layer arrangements for 4, 5, 7, and 8 are similar.
the nonsolvate prototype include rotor conformation of
terminal triaryl groups and ø different from 180°; analogies
to solvate forms regard pyridines almost coplanar, opposite
torsion angles for terminal triaryl groups, and -OH groups
pointing toward trans configuration. The overall organization
of the Pd(LOH)2(CH3COO)2 units in 10 is similar to 1, with
association in chains where the -OH‚‚‚O(CO)CH3 hydrogen
bonds replace the -OH‚‚‚Cl interactions (Figure 6). Com-
Inorganic Chemistry, Vol. 44, No. 2, 2005 437

Bacchi et al.
Figure 5. Arrangement of consecutive layers in solvates 2-5 with layers differentiated by shades of gray (solvent molecules omitted): (a) herringbone
pattern for 3 and 2 (left) and edge-on view (right), showing the 21 relation between layers; (b) parallel pattern for 5 (left) and edge-on view (right),
showing that layers are related by translation; (c) parallel pattern for 4 (left) and edge-on view (right), showing that layers are related by a c glide perpendicular
to b.
438 Inorganic Chemistry, Vol. 44, No. 2, 2005

Pd(II) Wheel-and-Axle Organic-Inorganic Diols
Figure 6. Organization of one molecular layer in the crystal structure of 10 (left) with OH‚‚‚O(acetate) hydrogen bonds dashed and hydrogen atoms
omitted and molecular conformation of Pd(LOH)2(CH3COO)2 in 10 (right top) with edge-on views of the layer, in stick and space-filling views, showing
the presence of voids between aromatic rings (right bottom).
Figure 7. Parallel arrangement of two consecutive layers in 10 (left) with edge-on views in stick and space-filling renderings of the packing of two
adjacent layers, related by translation (right). Different layers are represented by different shades of gray.
pared to the chloride, the acetate ligand allows the formation
of shorter hydrogen bonds (Table 3) and the chains are
stabilized by additional O(H)‚‚‚Pd contacts. The chains run
along a and are assembled in layers along the b-c cell
diagonal whose metric (a′ ) 7.57 and b′ ) 15.86 Å; θ )
36°) is similar to the one observed for the solvate forms,
while the molecular orientation within the chains is inter-
mediate between the nonsolvate and the solvate forms.
Moreover, the unit cell of compound 10 is comparable with
those of 3 and 8, even with some angular distortions (Table
1). Interestingly, in 10 the assembly of the layers occurs by
translation (Figure 7), giving the parallel pattern observed
in precedence only for the solvate forms. This arrangement
leaves some space between the chains within the layers
(Figure 7). The uptake of one THF molecule by 10 gives 9,
Figure 8, which could be an intriguing picture of the first
step of the mechanism converting the nonsolvate into the
solvate forms. 9 is quasi-isostructural with 10 (Table 1); there
is no host-guest hydrogen bonding, but THF is clathrated
in the channels running along a. Clathration is assisted by
interchain stacking of THF between two parallel aromatic
rings belonging to adjacent chains, with distances comparable
to the ones typical of the intrachain stacking of disolvates.
As in 10, chains are formed by OH‚‚‚O(acetate) short
hydrogen bonds and weak O‚‚‚Pd contacts (Table 3). The
chain inclination relative to a (θ ) 34°) and the layer metrics
(a′ ) 7.930 and b′ ) 15.409 Å; R′ ) 98°) are similar to 10.
The layers are assembled by translation in the parallel pattern.
On the other hand, the molecular conformation tends to the
one observed for solvates: even if the monosolvation
imposes the P1 space group, the Pd(LOH)2(CH3COO)2
molecule is nearly centrosymmetric (Table 2), and in
particular pyridines are coplanar and -OH groups are trans
each other. The regular rotor conformation of the triaryl
groups is related to the absence of intrachain stacking
interactions involving the guest.
Inorganic Chemistry, Vol. 44, No. 2, 2005 439

Bacchi et al.
Figure 8. Organization of one molecular layer in the crystal structure of 9 (left) with OH‚‚‚O(acetate) hydrogen bonds dashed and hydrogen atoms omitted,
edge-on view of the layer evidencing clathration of guest (right bottom), and molecular conformation of Pd(LOH)2(CH3COO)2‚THF in 9 (right top).
Scheme 1. Desolvation and Solvation Mechanisma
The comparison of of the whole set of compounds 1-10
provides the structural framework which must be taken into
account for the formulation of a mechanicistic model for the
solvation/desolvation process. The observed structures rep-
resent still pictures of the steps of the process; we propose
a way to link those steps by the simplest geometric
rearrangements required to convert from one structure to the
other. All the structures are organized according to the highly
conserved motif represented by the layer of molecules built
by translation on a bidimensional lattice with average values
a′ ) 7.6 Å, b′ ) 18.1 Å, and R′ ) 90° for the solvate (2-5,
7-9) and a′ ) 10.7 Å, b′ ) 17.4 Å, and R′ ) 90° for the
nonsolvate forms (1, 6, 10). In the solvate structures, the
guest molecules are included by host-guest hydrogen-
bonding and stacking interactions in the space between the
chains within the layer; the host molecules form an angle of
θ ) 54° with the a′ axis of the mesh. In the nonsolvate
structures, the molecules interact by OH‚‚‚Cl hydrogen bonds
with θ ) 26°. The release of guest molecules during
desolvation triggers a rotation in θ by 28° of all the molecules
in the plane, with a slight adjustment of the mesh dimensions
(Scheme 1). The breaking of host-guest and the concomitant
formation of host-host hydrogen bonds also implies a
conformational rearrangement, with conversion of ø from
trans to gauche and of the triaryl groups geometry from
flattened to regular rotor conformation. The reversibility of
a Thick bar ) complex; triangle ) terminal OH.
the desolvation/solvation process is possible because of the
minimum displacements required for the single host mol-
ecules and by the modest reorientation necessary for their
molecular long axes. This mechanism accounts for the
conversion 2-3 S 1, where the propagation of chain rotation
in symmetry-related adjacent layers of 2-3 produces the
correct herringbone pattern found in 1. To convert the solvate
forms 4, 5, 7, and 8, characterized by the parallel arrangement
of adjacent layers, into the herringbone nonsolvate 1 and 6
forms, the simultaneous intralayer rotation must be ac-
companied by an overturning of alternate layers, achievable
by tilting of each molecule around the b′ axis. The structural
correlations between solvate and nonsolvate structures in
these complexes recall the concept of “structural filiation”,
proposed to describe dehydration and other solid-solid
transformations of dihydrated copper(II) 8-hydroxyquinoli-
440 Inorganic Chemistry, Vol. 44, No. 2, 2005

Pd(II) Wheel-and-Axle Organic-Inorganic Diols
nates.16 According to this model, the desolvation mechanism
described in this paper can be tentatively classified as a
cooperative reorganization with transmission of structural
information: in both the herringbone and the parallel cases,
the rotation and rotation/overturn mechanisms (filiation step
1) lead to a nonsolvate material which is the precursor of
the stable crystalline form 1, finally obtained by sliding
adjacent layers to compact the packing (filiation step 2)
(Scheme 1).
Scheme 2. Results of the DSC Experiments for 2, 4, and 9
On the other hand, insights regarding the solvation process
may be found by analyzing the relations among the acetate
complexes 10 and 9 and their 1 and 5 chloride counterparts.
While the apohost 1 needs to rearrange its building blocks
to accommodate the guest and give the solvate forms,17 in
10 the intralayer packing of the chains is looser than in 1,
due to the slightly larger θ and to the layer metrics (a′ )
7.9 Å, b′ ) 15.9 Å, R′ ) 86°) similar to 5. In this sense, 10
is fitter to accommodate a guest, which in fact is included
by clathration in 9, without significantly disrupting the layer
network. 9 depicts the first event of the structural rearrange-
ment accompanying the solvation process, showing the
incipient uptake of one THF molecule between the chains;
the -OH groups may be directed to hook the guest and stack
it between two rings, converting to the situation in Figure 4.
The higher stability of the hydrogen-bonded chains involving
the acetate accounts for the difficulty of proceeding further
in the solvation process for 9, as the inclusion of the second
guest molecule would require the breaking of the O-H‚‚‚O
and Pd‚‚‚O intrachain interactions. Moreover, the parallel
arrangement of layers in 10 points to the possible existence
of supramolecular isomers also for the nonsolvate forms 1
and 6.
Scheme 3. Solvation/Desolvation Experiments
Thermal Analysis and Solvation/Desolvation Experi-
ments. The thermal stability of 2 (herringbone packing), 4
(parallel packing), and 9 (clathrate monosolvate) was tested
by thermogravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC). TGA scans revealed that 2 releases
both the acetone guests in a single narrow step (80-115 °C),
and similarly 4 loses both DMF molecules simultaneously
between 110 and 160 °C. On the other hand, compound 9
releases the single THF molecule on a broad temperature
range (80-160 °C), as a consequence of the much less
specific host-guest interactions found in this compound
compared with 2 and 4. DSC measurements show endotherm
peaks in correspondence with the desolvation events; how-
ever, the DSC trace of 2 evidences also an exothermic
phenomenon which persists upon variation of the scan speed,
under the endothermic process. This could be related to the
molecular reorientation among and within the layers with
formation of new hydrogen bonds that is concomitant to the
leaving of the guest, during the conversion of 2 into the
apohost 1. The related process for 4 could be hidden by the
desolvation endotherm. Scheme 2 summarizes the results of
the DSC experiments for 2, 4, and 9.
has been demonstrated by performing the desolvation/
solvation cycles shown in Scheme 3. At every stage, the
identity of the products was confirmed by infrared spectros-
copy and X-ray powder diffraction. 1 transforms into the
acetone (2) or DMF (4) solvate both by recrystallization from
the guest solvent and by solid-state inclusion; 1 does not
include vapors of THF. The process is reversible: by heating
under vacuum, 2 and 4 lose solvent and give the apohost 1.
The crystalline powders of 1 obtained in this way are
identical with the product of the direct synthesis, and they
include vapors of acetone to give microcrystalline powder
of 2.
Conclusions
Pd(LOH)2Cl2 represents a novel type of organic-inorganic
wheel-and-axle diol with good inclusion properties. Its crystal
form 1 hosts several hydrogen bond accepting solvents, either
via dissolution/ricrystallization or via gas/solid absorption,
giving crystalline solvates with Pd(LOH)2Cl2‚2G stoichiom-
etry. The crystalline nonsolvated apohost 1 is recovered by
The capability of the apohosts 1 and 10 to uptake solvent
vapors to convert into 2, 4, or 9 with reversible processes
(16) Petit, S.; Coquerel, G. Chem. Mater. 1996, 8, 2247-2258.
(17) Nassimbeni, L. R. Acc. Chem. Res. 2003, 36, 631-637.
Inorganic Chemistry, Vol. 44, No. 2, 2005 441

Bacchi et al.
guest removal upon heating under vacuum. On the basis of
structural evidence, a path describing the rearrangements
required to convert between the solvated and unsolvated
compounds implying modest structural alterations has been
formulated. The structural analysis shows that the apohost
is preorganized for inclusion and that solvent uptake/release
involves a “venetian blinds” mechanism based on concerted
oscillations of the complex molecules around the palladium
atoms. These molecules are organized in layers with practi-
cally invariant metrics, and the oscillation occurs with slight
conformational rearrangements, implying 1h S C2 conversion
in the molecular symmetry. The solvate layers assemble in
two isomeric forms: with parallel or herringbone orientation
of the molecules between different layers. A further long-
range rearrangement, along with intralayer reorientation, is
required to convert between parallel and herringbone patterns.
As suggested by the analysis of Pd(LOH)2(CH3COO)2, the
first step of the solvent uptake could be the insertion of one
guest molecule in the space between the chains within a layer.
This would trigger the molecular reorientation necessary to
realize the host-guest hydrogen-bonding and stacking
interactions, thus leaving space for the incoming of the
second solvation molecule. Variable-temperature X-ray
powder diffraction experiments and accurate coupled DSC/
TGA measurements are planned to clarify the nature of the
desolvation process and to inspect for possible polymorphism
of these systems.
Acknowledgment. We thank the “Centro Interfacolta`
Misure Giuseppe Casnati” and the “Laboratorio di Struttur-
istica Diffrattometrica” of the University of Parma.
Supporting Information Available: CIF data, including final
atomic coordinates with esd’s for all non-H atoms, calculated
coordinates for all atoms introduced in idealized positions, aniso-
tropic thermal parameters, and a full list of bond distances and
angles for 1-10, perspective views of compounds 1-10, thermal
analysis (TGA and DSC) traces, and X-ray powder diffraction
characterization of compounds involved in the solvation/desolvation
experiments. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC048896M
442 Inorganic Chemistry, Vol. 44, No. 2, 2005