Nickel(II) and zinc(II) dibenzoylmethanates: Molecular and crystal structure, polymorphism, and guest- or temperature-induced oligomerization

Article abstract of DOI:10.1021/ic000981g

Four forms of nickel(II) and two of zinc(II) dibenzoylmethanates have been isolated and characterized with powder and single-crystal X-ray diffraction analyses, differential scanning calorimetry, magnetic susceptibility measurements, and solid-state ¹³C cross-polarization/magic angle spinning NMR. Nickel dibenzoylmethanate, Ni(DBM)₂ (DBM = PhCOCHCOPh^-), forms three polymorphic forms (light-green, brown, and green) and a fourth clathrate form with guest benzene included. The light-green polymorph is metastable. Substituted benzenes induce recrystallization of the polymorph into a stable brown form (C₃₀H₂₂NiO₄; a = 26.502(3) A, b = 5.774(1) A, c = 16.456(2) A, β = 116.03(1)°; monoclinic, C2/c; Z = 4). Unlike the other forms, the brown form is diamagnetic and is comprised of monomers of the low-spin [Ni(DBM)₂] complex. The Ni(II) is chelated by two DBM ligands in a square planar environment by four donor oxygen atoms. When heated, the brown form transforms to a green form which is stable above 202 °C (C₉₀H₆₆Ni₃O₁₂; a = 13.819(2) A, b = 16.252(2) A, c = 17.358(2) A, β = 108.28(1)°; monoclinic, P2₁/n; Z = 2). This polymorph is formed by van der Waals packing of trimers [Ni₃-(DBM)₆] containing linear Ni₃ clusters with an Ni-Ni distance of 2.81 A. The cluster is surrounded by six DBM ligands, providing a distorted octahedral environment about each Ni by six oxygen atoms. Benzene stabilizes the trimeric structure at room temperature, forming a [Ni₃(DBM)₆]·2(benzene) inclusion compound (Ni-Ni distance of 2.83 A) with guest benzene molecules located in channels (C₉₀H₆₆Ni₃O₁₂ + 2(C₆H₆); a = 17.670(2) A, b = 20.945(3) A, c=11.209(2) A, β = 102.57(1)°; monoclinic, P2₁/c; Z = 2). Zinc dibenzoylmethanate has been prepared in two polymorphic forms. The monomeric form contains [Zn(DBM)₂] molecules with the zinc center in a distorted tetrahedral environment of four oxygens from the two chelated DBMs (C₃₀H₂₂O₄Zn; a = 10.288(2) A, b = 10.716(2) A, c = 12.243(2) A, α = 89.19(1)°, β = 75.39(1)°, γ = 64.18(1)°; triclinic, P1 Z = 2). Another, dimeric form contains [Zn₂(DBM)₄] species, with two zinc atoms separated by a distance of 3.14 A and each zinc coordinated by five oxygen atoms (C₆₀H₄₄O₈Zn₂; a = 25.792(3) A, b = 7.274(1) A, c = 24.307(2) A, β = 90.58(1)°; monoclinic, C2/c; Z = 4). The polymorphic variety of the title complexes and the peculiarities of the Ni(II) and Zn(II) coordination environments are discussed in the context of using the complexes as precursors for new metal complex hosts.

Full text of DOI:10.1021/ic000981g

1626
Inorg. Chem. 2001, 40, 1626-1636
Nickel(II) and Zinc(II) Dibenzoylmethanates: Molecular and Crystal Structure,
Polymorphism, and Guest- or Temperature-Induced Oligomerization†
D. V. Soldatov, A. T. Henegouwen,‡ G. D. Enright, C. I. Ratcliffe, and J. A. Ripmeester*
Steacie Institute for Molecular Sciences, National Research Council of Canada,
Ottawa, Ontario K1A 0R6, Canada
ReceiVed August 31, 2000
Four forms of nickel(II) and two of zinc(II) dibenzoylmethanates have been isolated and characterized with powder
and single-crystal X-ray diffraction analyses, differential scanning calorimetry, magnetic susceptibility measure-
ments, and solid-state 13C cross-polarization/magic angle spinning NMR. Nickel dibenzoylmethanate, Ni(DBM)2
(DBM ) PhCOCHCOPh-), forms three polymorphic forms (light-green, brown, and green) and a fourth clathrate
form with guest benzene included. The light-green polymorph is metastable. Substituted benzenes induce
recrystallization of the polymorph into a stable brown form (C30H22NiO4; a ) 26.502(3) Å, b ) 5.774(1) Å, c )
16.456(2) Å, â ) 116.03(1)°; monoclinic, C2/c; Z ) 4). Unlike the other forms, the brown form is diamagnetic
and is comprised of monomers of the low-spin [Ni(DBM)2] complex. The Ni(II) is chelated by two DBM ligands
in a square planar environment by four donor oxygen atoms. When heated, the brown form transforms to a green
form which is stable above 202 °C (C90H66Ni3O12; a ) 13.819(2) Å, b ) 16.252(2) Å, c ) 17.358(2) Å, â )
108.28(1)°; monoclinic, P21/n; Z ) 2). This polymorph is formed by van der Waals packing of trimers [Ni3-
(DBM)6] containing linear Ni3 clusters with an Ni-Ni distance of 2.81 Å. The cluster is surrounded by six DBM
ligands, providing a distorted octahedral environment about each Ni by six oxygen atoms. Benzene stabilizes the
trimeric structure at room temperature, forming a [Ni3(DBM)6]·2(benzene) inclusion compound (Ni-Ni distance
of 2.83 Å) with guest benzene molecules located in channels (C90H66Ni3O12 + 2(C6H6); a ) 17.670(2) Å, b )
20.945(3) Å, c)11.209(2) Å, â ) 102.57(1)°; monoclinic, P21/c; Z ) 2). Zinc dibenzoylmethanate has been
prepared in two polymorphic forms. The monomeric form contains [Zn(DBM)2] molecules with the zinc center
in a distorted tetrahedral environment of four oxygens from the two chelated DBMs (C30H22O4Zn; a ) 10.288(2)
Å, b ) 10.716(2) Å, c ) 12.243(2) Å, R ) 89.19(1)°, â ) 75.39(1)°, γ ) 64.18(1)°; triclinic, P1h; Z ) 2).
Another, dimeric form contains [Zn2(DBM)4] species, with two zinc atoms separated by a distance of 3.14 Å and
each zinc coordinated by five oxygen atoms (C60H44O8Zn2; a ) 25.792(3) Å, b ) 7.274(1) Å, c ) 24.307(2) Å,
â ) 90.58(1)°; monoclinic, C2/c; Z ) 4). The polymorphic variety of the title complexes and the peculiarities of
the Ni(II) and Zn(II) coordination environments are discussed in the context of using the complexes as precursors
for new metal complex hosts.
Introduction
Chart 1
Nickel(II) DBM (DBM ) dibenzoylmethanate, PhCOCHCO-
Ph-) gives rise to a family of metal complex hosts produced
by apical coordination of a variety of pyridine-type ligands (A)
to the basic metal bischelate unit (Chart 1).1,2
The [NiPy2(DBM)2] complex was shown to be trans con-
figured, both in six clathrates belonging to four structural types,
and in its dense, nonclathrate phase.1 In contrast, the analogous
Zn complex (as well as the Cd complex) is cis configured and
therefore probably is unable to include guests.1 The electronic
structure of a metal center thus controls the clathratogenic ability
of this host type.3 Packing and molecular features of the
unmodified metal DBMs could shed some light on what part
of the trans-[MA2(DBM)2] molecule is the most important in
maintaining the host properties.
elucidating fundamental questions related to the entire family
of metal DBM hosts. A structural comparison has been made
to reveal whether the tendency of the [Ni(DBM)2] unit to adopt
a planar bischelate configuration appears on the level of the
precursor. Up to the present, structures for Pd(II), Cu(II), and
Sn(II) DBMs have been reported,4-6 but these metal centers
In this paper we report on the structure and relevant properties
of the nickel(II) DBM as well as of its zinc(II) counterpart,
* To whom correspondence should be addressed.
† Issued as NRCC No. 43874.
(3) In rare cases complex isomerization may be induced by the guest:
(a) Nassimbeni, L. R.; Niven, M. L.; Zemke, K. J. Acta Crystallogr.
1986, B42, 453-461. (b) Soldatov, D. V.; Ripmeester, J. A.; Shergina,
S. I.; Sokolov, I. E.; Zanina, A. S.; Gromilov, S. A.; Dyadin, Yu. A.
J. Am. Chem. Soc. 1999, 121, 4179-4188. (c) Lipkowski, J.; Kislykh,
N. V.; Dyadin, Yu. A.; Sheludyakova, L. A. Zh. Strukt. Khim. 1999,
40, 954-964.
‡ On leave of absence from the Fontys University of Professional
Education, Eindhoven, The Netherlands.
(1) Soldatov, D. V.; Enright, G. D.; Ripmeester, J. A. Supramol. Chem.
1999, 11, 35-47.
(2) Soldatov, D. V.; Ripmeester, J. A. Supramol. Chem. 2001, 12, 357-
368.
10.1021/ic000981g CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/28/2001

Nickel and Zinc Dibenzoylmethanates
Inorganic Chemistry, Vol. 40, No. 7, 2001 1627
Zinc(II) DBM, Monomeric Form. The method was very similar
to that for the nickel analogue (light-green form). Starting from zinc
acetate dihydrate (2.20 g, 10 mmol), 3.6 g (65%) of pale pearly diaqua
complex was prepared; its dehydration (mass loss of 6.9% compared
to 6.6% calculated from eq 2 for Zn) yielded the white final product.
Anal. Found: C, 70.2; H, 4.30. Calcd for [Zn(DBM)2] (C30H22O4Zn):
C, 70.4; H, 4.33. The compound was recrystallized from chlorobenzene.
Zinc(II) DBM, Dimeric Form. The polymorph of the complex was
prepared as large (up to 5 mm) isometric blocks by slow evaporation
of a solution of the monomeric form in benzene at 70 °C. The crystals
were separated and rinsed with the solvent. Anal. Found: C, 70.8; H,
4.33. Calcd for [Zn2(DBM)4] (C60H44O8Zn2): C, 70.4; H, 4.33.
Powder Diffraction Measurements. Powder patterns were recorded
on a Rigaku Geigerflex diffractometer (Co KR radiation, λ ) 1.7902
Å) in a 5-30° 2θ range, 0.02° step scan, 1 s per step. The observed
diffraction patterns were compared with those calculated from single-
crystal diffraction measurements (using room-temperature unit cell
dimensions).
differ too much from Ni(II) and Zn(II) to allow sound
comparisons. A key problem for the present study was the
preparation of single crystals of nickel DBM suitable for single-
crystal X-ray analysis. The compound has low solubility in
neutral organics, while contact with solvents containing oxygen
or nitrogen donor atoms results in bonding of the solvents to
the coordinatively unsaturated [Ni(DBM)2] unit. The problem
has been overcome by heating the powdered solid under a layer
of aromatic solvent, a procedure initially developed to grow
crystalline inclusion compounds from mutually insoluble mix-
tures.7
Experimental Section
Nickel(II) DBM, Light-Green Form. This form of the complex
salt was prepared in two steps (eqs 1 and 2). A solution of dibenzoyl-
methane, C6H5COCH2COC6H5, (Aldrich, 98%; 4.49 g, 20 mmol) in
90% ethanol (150 mL) was added to a hot solution of nickel acetate
tetrahydrate (2.49 g, 10 mmol) in 90% ethanol (150 mL). The mixture
was allowed to cool while being stirred for at least 1 h. The light-
green crystalline precipitate was separated, rinsed with 90% ethanol,
and air-dried to give ca. 5.1 g (95%) of diaqua complex. This was
dehydrated for 2 h at 130 °C to give the light-green final product; a
mass loss of 7.0-7.3% was observed, compared to 6.7% calculated
from eq 2 for Ni. Anal. Found: C, 70.9; H, 4.33. Calcd for “Ni(DBM)2”
(C30H22NiO4): C, 71.3; H, 4.39.
Differential Scanning Calorimetry (DSC) Measurements. Samples
of 5-10 mg were pressed inside aluminum pans, and the DSC curves
were recorded at a 5 deg/min heating rate using a 2920 modulated
differential scanning calorimeter (TA Instruments). Samples of each
polymorph were used from two independent syntheses.
Magnetic Susceptibility Measurements. Magnetic susceptibilities
(øM) were determined using samples of 70-150 mg, on an Evans
balance.8 Two or three determinations were made, and the experimental
data were corrected for the ligand diamagnetism and the temperature-
independent paramagnetism (NR ) 230 × 10-6 cm3/mol per Ni atom).
Ni(CH3COO)2*4H2O + 2(H-DBM) ethanol/water8
[Ni(H2O)2(DBM)2]V + 2CH3COOH + 2H2O (1)
Magnetic moments were obtained from the relation µeff ) 2.828(øMT)1/2
.
NMR Spectroscopy. 13C cross-polarization/magic angle spinning
(CP/MAS) NMR spectra were obtained at 75.48 MHz at room
temperature on a Bruker AMX300 spectrometer equipped with a Doty
Scientific 5 mm CP/MAS probe. A standard CP pulse program was
used with fixed-amplitude 1H decoupling during signal acquisition. 1H
90° pulse lengths were 2.7 µs, CP times were 1-3 ms, and recycle
times were 2-60 s, depending on the sample. Dipolar dephased spectra9
were obtained by interrupting the 1H decoupling for 40 µs immediately
after the CP sequence and before starting the data acquisition. Chemical
shifts were measured relative to external solid hexamethylbenzene and
then corrected to the TMS scale. Spinning speeds were set in the range
6.1-6.6 kHz to avoid overlap of spinning sidebands with any of the
isotropically shifted lines.
[M(H2O)2(DBM)2] 9
t8 “M(DBM)2” + 2H2Ov (M ) Ni, Zn) (2)
Nickel(II) DBM, Brown Form. A sample of the light-green form
of the complex was placed under a layer of dried warm (70 °C) solvent
(xylenes, toluene, or chlorobenzene) for 30 min or until the light-green
powder completely transformed to brown prismatic crystals. These were
separated from the warm supernatant fluid (drybox), rinsed with solvent,
and dried. Anal. Found: C, 71.3; H, 4.35. Calcd for [Ni(DBM)2]
(C30H22NiO4): C, 71.3; H, 4.39. A single crystal for X-ray study was
obtained from chlorobenzene.
Nickel(II) DBM, Green Form. A sample of the brown form of the
complex was placed in the oven (215 °C) for 30 min or until the brown
prismatic crystals quantitatively transformed into green powder. Anal.
Found: C, 71.7; H, 4.50. Calcd for [Ni3(DBM)6] (C90H66Ni3O12): C,
71.3; H, 4.39. Single crystals for X-ray study were prepared as
follows: A ∼1 g sample of the brown form and ∼10 mL of 1,3,5-
triisopropylbenzene (Aldrich, 97%) were sealed in an ampule which
was first heated for a short time to 230 °C. At this point a small quantity
of the powdered green form under a pale-green solution was observed.
Then, upon slow cooling of the ampule to room temperature at a rate
of 10 deg/h, green blocks grew. The separated crystals showed no signs
of decay in air at room temperature over several weeks.
Single-Crystal Diffraction Analysis. Single crystals were cooled
rapidly to -100 °C, at which temperature full X-ray diffraction
experiments were performed. A Siemens SMART CCD X-ray diffrac-
tometer using graphite-monochromated Mo KR radiation (λ ) 0.7107
Å) was used. Full data sets were collected using the ω scan mode over
the 2θ range of 3-58°. An empirical absorption correction utilized
the SADABS routine associated with the Siemens diffractometer. The
final unit cell parameters were obtained using the entire data set.
The structures were solved and refined using the SHELXTL
package.10 Solution was accomplished by using direct methods followed
by differential Fourier syntheses. Structural refinement was performed
on F2 using all data with positive intensities. Non-hydrogen atoms were
refined anisotropically except for the atoms of the minor orientation
of the benzene guest molecule. Hydrogen atoms were refined isotro-
pically with thermal factors 1.2 or 1.5 times greater than those for the
adjacent carbon atoms. The hydrogen atoms of the minor benzene
orientation in [Ni3(DBM)6]·2(benzene) were fixed in calculated positions
as “riding” on the corresponding carbon atoms. For this structure, two
guest benzene orientations were refined with complementary occupancy.
A summary of the crystal data and experimental parameters is given
in Table 1. For the atom numbering schemes see the figures. Complete
results can be found in the Supporting Information.
Nickel(II) DBM-Benzene Inclusion, 1:2/3. A sample of the brown
form of the complex was placed under dried warm (70 °C) benzene
(sealed ampule) for several days until the brown prisms completely
recrystallized into green plate crystals of the inclusion compound. The
product was separated and kept in a tightly closed container to avoid
escape of the benzene guest. Anal. Found: C, 73.3; H, 4.70. Calcd for
[Ni3(DBM)6]·2(benzene) (C102H78Ni3O12): C, 73.3; H, 4.70.
(4) Shugam, E. A.; Shkol’nikova, L. M.; Knyazeva, A. N. Zh. Strukt.
Khim. 1968, 9 (2), 222-227.
(5) (a) Knyazeva, A. N.; Shugam, E. A.; Shkol’nikova, L. M. Zh. Strukt.
Khim. 1969, 10 (1), 83-87. (b) Ma, B. Q.; Gao, S.; Wang, Z. M.;
Liao, C. S.; Yan, C. H.; Xu, G. X. J. Chem. Crystallogr. 1999, 29,
793-796.
(8) Drago, R. S. Physical Methods in Chemistry, Saunders: Philadelphia,
1977.
(9) Opella, S. J.; Frey, M. H. J. Am. Chem. Soc. 1979, 101, 5854-5856.
(10) Sheldrick, G. M. SHELXTL PC, Ver. 4.1. An Integrated System for
SolVing, Refining and Displaying Crystal Structure from Diffraction
Data; Siemens Analytical X-ray Instruments, Inc.: Madison, WI, 1990.
(6) Uchida, T.; Kozawa, K.; Obara, H. Acta Crystallogr. 1977, B33, 3227-
3229.
(7) Udachin, K. A.; Ripmeester, J. A. J. Am. Chem. Soc. 1998, 120, 1080-
1081.

1628 Inorganic Chemistry, Vol. 40, No. 7, 2001
Soldatov et al.
[Zn2(DBM)4]
Table 1. Summary of Low-Temperature Single-Crystal X-ray Experiments
[Ni(DBM)2]
(brown form)
[Ni3(DBM)6]
(green form)
[Ni3(DBM)6]·
2(benzene)
compound
[Zn(DBM)2]
empirical formula
fw
C30H22NiO4
505.2
C90H66Ni3O12
1515.6
C102H78Ni3O12
1671.8
C30H22O4Zn
511.9
C60H44O8Zn2
1023.7
space group
a, Å
b, Å
C2/c (no. 15)
26.502(3)
5.774(1)
P21/n (no. 14)
13.819(2)
16.252(2)
17.358(2)
P21/c (no. 14)
17.670(2)
20.945(3)
11.209(2)
P1h (no. 2)
10.288(2)
10.716(2)
12.243(2)
89.19(1)
75.39(1)
64.18(1)
1168.6(4)
2
1.455
10.87
100
0.028
C2/c (no. 15)
25.792(3)
7.274(1)
c, Å
16.456(2)
24.307(2)
R, deg
â, deg
γ, deg
V, Å3
Z
116.03(1)
108.28(1)
102.57(1)
90.58(1)
2262.7(5)
4
1.483
8.94
100
0.025
0.066
3701.6(8)
2
1.360
8.20
100
0.026
0.067
4049(1)
2
1.371
7.57
100
0.027
0.069
4560(1)
4
1.491
11.14
100
0.024
0.062
F
calcd, g cm-1
µ, cm-1
T, °C
R1(F)a
wR2(Fo )b
0.066
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) ∑[w(Fo - Fc2)2]/{∑[w(Fo )2]}1/2
.
2
2
Table 2. Room-Temperature Unit Cell Parameters from Single-Crystal X-ray Experiments
[Ni(DBM)2]
(brown form)
[Ni3(DBM)6]
(green form)
[Ni3(DBM)6]·
compound
a, Å
b, Å
2(benzene)
[Zn(DBM)2]
[Zn2(DBM)4]
26.741(8)
5.791(1)
16.703(4)
13.928(3)
16.404(3)
17.422(5)
17.930(3)
21.077(4)
11.284(2)
10.307(3)
10.719(4)
12.480(6)
88.32(2)
75.71(1)
63.94(1)
1195.3(8)
2
25.908(4)
7.397(1)
24.367(3)
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
116.64(1)
108.44(1)
102.27(1)
90.47(1)
2312(1)
4
1.451
196
3776(2)
2
1.333
574
4167(1)
2
1.332
373
4670(1)
4
1.456
601
F
calcd, g cm-1
no. of used reflns
1.422
220
Table 3. List of the Forms of Nickel(II) and Zinc(II) DBMs Isolated
compound molecular formula
nickel(II) DBM Ni(DBM)2
stabilitya
color
µ
eff (µB) per metal atom
metastable
light green
brown
green
green
colorless
colorless
3.1(1)
diamagnetic
3.22(5)
3.25(5)
diamagnetic
diamagnetic
nickel(II) DBM
nickel(II) DBM
[Ni(DBM)2]
[Ni3(DBM)6]
stable <202 °C
stable >202 °C
stable in dry benzene
stable >140 °C
stable <140 °C
nickel(II) DBM, clathrate with benzene
zinc(II) DBM
zinc(II) DBM
[Ni3(DBM)6]·2(benzene)
[Zn(DBM)2]
[Zn2(DBM)4]
a Note: This is the thermodynamic stability; all of the compounds exist at room temperature in air for kinetic reasons.
For compatibility with the X-ray powder diffractograms and
comparisons with other structures, unit cell parameters of the single
crystals were measured as well at room temperature, using several dozen
frame ω scans. The parameters are listed in Table 2.
or organic solvents, or induced by heating. Besides pure
polymorphs, nickel DBM forms an inclusion compound with
benzene; its characteristics are also given in Table 3.
Crude nickel and zinc DBMs are prepared by dehydration of
their diaqua complexes, [M(H2O)2(DBM)2],11,12 the latter most
probably possessing molecular structure similar to that of
[M(H2O)2(acac)2] (acac ) acetylacetonate).13 According to the
powder X-ray patterns, the diaqua complexes of nickel and zinc
DBMs are isostructural (Figure 1, patterns 1 and 6). The
dehydrated powder products correspond to stoichiometry
M(DBM)2 and show crystallinity (Figure 1, patterns 2 and 7)
but differ from each other. Both are metastable. The formation
of stable forms apparently requires recrystallization. During the
dehydration process the hydrophobicity and thermal stability
of the compounds inhibit the formation of a liquid interphase
boundary layer that would facilitate recrystallization.
Results and Discussion
Isolation and Stability of Individual Compounds. A list
of isolated polymorphs of nickel and zinc DBMs is given in
Table 3. The gross compositions of the compounds follow from
the mass changes observed during the preparation and/or from
elemental analysis. In each case the composition, along with
the structural properties, is consistent with the single-crystal
X-ray study (five compounds; molecular formulas derived
therefrom are given) and 13C NMR results (for three diamagnetic
compounds). The phase purity of the isolated products is
demonstrated by comparison of their powder patterns with those
calculated from single-crystal X-ray diffraction data (Figure 1).
Qualitatively, the chemical identity of each polymorph can be
seen from properties such as color, magnetic properties, and
relative stability. Although all the compounds have been isolated
and can exist at room temperature in air, some of them are not
thermodynamically stable under these conditions and may
undergo transformation to stable forms catalyzed by humidity
(11) Logvinenko, V. A.; Fedotova, N. E.; Igumenov, I. K.; Gavrilova, G.
V. J. Therm. Anal. 1988, 34, 259-268.
(12) Grigor’ev, A. N.; Kiryukhin, M. V.; Martynenko, L. I. Russ. J. Inorg.
Chem. 1995, 40, 230-233 (pp 242-245 in Russian version).
(13) Co complex: Bullen, G. J. Acta Crystallogr. 1959, 12, 703-708. Ni
complex: Montgomery, H.; Lingafelter, E. C. Acta Crystallogr. 1964,
17, 1481-1482.

Nickel and Zinc Dibenzoylmethanates
Inorganic Chemistry, Vol. 40, No. 7, 2001 1629
Figure 2. DSC thermograms of the brown and green forms of nickel
DBM shown as heat flow (arbitrary units) versus temperature (°C).
The brown-to-green polymorph transformation occurs at 202 °C, and
melting of the green form at 330 °C. Sample mass: 6.50 and 7.56 mg
for the brown and green forms, respectively.
rest of the thermogram is similar to that of the pure green form
(Figure 2b), showing melting at ∼330 °C. The green form
contains trimers, with linear Ni3 clusters surrounded by bridging
and/or chelating DBM ligands. The back conversion of the green
form to the brown form is hindered kinetically and occurs readily
only under a layer of warm solvent.
Unlike substituted benzenes, benzene itself, acting as a
template, is capable of redirecting the whole process into an
essentially new product, [Ni3(DBM)6]·2(benzene). This inclusion
compound contains trimers similar to those found in the green
form. The trimers thus can form a room-temperature stable phase
with the help of an included guest.
Crude zinc DBM contains monomeric [Zn(DBM)2] units; the
crystals of this compound were obtained from chlorobenzene.
From benzene a dimeric form containing [Zn2(DBM)4] mol-
ecules was isolated. The crystalline monomeric form shows no
change until 222 °C, at which temperature a strong endotherm
of 36.3(2) kJ/mol (DSC) indicates melting followed by an
irreversible (exothermal) change above 240 °C. DSC of the
dimeric form shows a small endotherm of irregular shape in
the 140-175 °C range, with the rest of the thermogram identical
to that of the monomeric form. From these observations it is
seen that the dimeric form is thermodynamically stable below
140 °C while the monomeric form is stable at higher temper-
atures.
Figure 1. Powder X-ray diffractograms of (1) [Ni(H2O)2(DBM)2], (2)
Ni(DBM)2 (light-green form), (3) [Ni(DBM)2] (brown form), (4) [Ni3-
(DBM)6] (green form), (5) [Ni3(DBM)6]·2(benzene), (6) [Zn(H2O)2-
(DBM)2], (7) [Zn(DBM)2], and (8) [Zn2(DBM)4]. The vertical bars show
the positions of the main reflections (>5% of the strongest reflection
intensity) calculated from the single-crystal data (where available).
Radiation: Co KR, λ ) 1.7902 Å.
Scheme 1
Magnetic Properties. All the green Ni compounds are
paramagnetic, with the value of µeff indicating two unpaired
electrons per Ni atom (Table 3). This is consistent with a high-
spin-state Ni center coordinated octahedrally. This presumes
sharing of DBM ligands between two or more Ni centers, thus
implying polymerization of coordination units. Indeed, in two
compounds (Table 3) the existence of trimeric units has been
demonstrated. At the same time, the brown form of nickel DBM
is diamagnetic, which indicates a low-spin-state Ni center
characteristic of square planar complexes of Ni(II).
Structure of Brown [Ni(DBM)2]. The structure is mono-
clinic, C2/c. Electrically neutral molecules of [Ni(DBM)2] are
assembled with only van der Waals interactions, with four
molecules per unit cell. Features of the crystal packing are very
similar to those in the isostructural14 Pd(II) and Cu(II) complexes
(reported in refs 4 and 5, respectively) and are not described
here. The nickel atom resides on an inversion center and has a
Phase interconversions for nickel DBM are depicted in
Scheme 1. Although the dehydration product is thermodynami-
cally metastable and shows some dispersity (Figure 1, pattern
2), it is stable in air and nonhygroscopic. In organic solvents
(chlorobenzene, toluene, xylenes) the light-green powder quickly
transforms into brown prisms of a stable polymorph containing
monomeric [Ni(DBM)2] complexes. If left under solvent in air,
the crystals eventually convert back to the diaqua complex.
Organic solvents thus catalyze both polymorphous and hydration
processes, apparently through recrystallization.
Figure 2 shows the DSC thermogram for the brown form: it
is stable up to 202 °C, at which temperature it transforms into
a green form with an enthalpy of the polymorphous transforma-
tion at this temperature of 9.6(3) kJ/mol (of monomer). The
(14) The [Pd(DBM)2] and [Cu(DBM)2] were solved in the I2/c space group.
The primitive unit cell for [Ni(DBM)2] (at room temperature, a )
5.767 Å, b ) 13.567 Å, c ) 16.232 Å; R ) 108.95°, â ) 100.22°, γ
) 102.21°) may be transformed both to monoclinic C (a ) 26.521 Å,

1630 Inorganic Chemistry, Vol. 40, No. 7, 2001
Soldatov et al.
Figure 3. ORTEP drawing of the [Ni(DBM)2] molecule in the brown
form of nickel DBM.
square planar environment defined by four oxygen atoms from
the chelating DBM ligands (Figure 3). The absence of apical
coordination and shortened Ni-O bonds (1.845 Å to O1 and
1.836 Å to O3)15 are consistent with the observed low-spin state
of the nickel center arising from tetragonal distortion charac-
teristic of d8 and d9 cations.
The bischelate fragment is planar; the bond lengths for C-O
(1.28 Å) and C-C (1.40 Å) imply strong conjugation in the
chelate rings. The C-C distance for bonds joining chelate and
phenyl rings (<1.50 Å) is close to a value for the bond between
aryl systems in biphenyls.16 The whole molecule is nearly planar,
with average and maximal deviations of atoms from the least-
squares plane of 0.09 and 0.3 Å, respectively. Due to steric
problems the molecule cannot adopt ideal planarity (there would
be an overlap of the chelate ring hydrogen with the ortho
hydrogens of the phenyl rings). This is avoided partially by a
slight turning of the phenyls; the C11-C16 ring bends by 3.6°
and the C31-C36 ring rotates by 9.0° with respect to the
bischelate fragment. This results in H2-H16 and H2-H32
contacts of 2.09 and 2.13 Å, respectively, which are slightly
shorter than the sum of hydrogen van der Waals radii (2.32
Å).17 The tendency of the molecule to planarity may be
explained by applying a geometry favoring delocalization of
electron density over the local aromatic systems.
Figure 4. ORTEP drawing of the [Ni3(DBM)6] molecules found in
the green form of nickel DBM (a) and in its benzene clathrate (b). H
atoms are omitted.
rings again reveal the tendency to go into the chelate plane,
turning by 4° and 17° (in the stable polymorph) or 8° and 24°
(in the metastable polymorph) to overcome HPh-Hchelate-HPh
repulsion.
In the structure of [Ni(DBM)2] it is the nickel cation that
clamps the ring and closes the delocalized π-system of the
fragment through the nickel orbitals and allows merging of the
two DBM systems to give a common state embracing the entire
molecule. In addition to the isostructural Pd(II)4 and Cu(II)5
DBMs other complexes with substituted DBMs of Ni(II),
Pd(II),20 and Cu(II)21 exhibit similar tendencies in that the
bischelate fragments are planar and the phenyl rings deviate by
no more than 20° from the plane. Sn(II) DBM is a non-
transition-metal analogue that is essentially nonplanar.6
Structure of Green [Ni3(DBM)6]. The structure is mono-
clinic and has been solved in the P21/n space group. This form
of nickel DBM differs dramatically from the brown polymorph
not only in its crystal structure but also in its molecular
organization. There is molecular packing of trimeric nickel DBM
molecules, [Ni3(DBM)6], with two trimers per unit cell.
The observed features are observed also in the crystal
structure of dibenzoylmethane (H-DBM) itself, the molecule
appearing in the enol form in two polymorphic modifications
of the protonated ligand.18,19 Although neither C-O nor C-C
bonds of the chelate ring are equivalent, the conjugation seems
apparent, both from the bond lengths and from the planarity of
the whole fragment clamped by the enol hydrogen. The phenyl
b ) 5.768 Å, c ) 16.474 Å, â ) 116.09°) and to monoclinic I (a )
24.298 Å, b ) 5.768 Å, c ) 16.474 Å, â ) 101.42°). With the latter
choice the isostructural relationship to the Pd and Cu compounds
becomes apparent.
(15) Here, and in the remainder of the text, the derived numerical values
are given with estimated errors less than the last meaningful figure.
(16) CRC Handbook of Chemistry and Physics; Lide, D. R., Ed.; Frederikse,
H. P. R., Assoc. Ed.; CRC Press: New York, 1997; Section 9-1.
(17) The following system of van der Waals radii (Å) has been used: C,
1.71; H, 1.16; O, 1.29 (a); Ni, 1.63; Zn, 1.39 (b). (a) Zefirov, Yu. V.;
Zorkii, P. M. Russ. Chem. ReV. 1995, 64, 415-428 and refs 3-6
therein. (b) Bondi, A. J. Phys. Chem. 1964, 68, 441-451.
(18) Stable form of H-DBM: (a) Williams, D. E. Acta Crystallogr. 1966,
21, 340-349. (b) Templeton, D. H.; Zalkin, A. Acta Crystallogr. 1973,
B29, 1552-1553. (c) Jones, R. D. G. Acta Crystallogr. 1976, B32,
1807-1811. (d) Ozturk, S.; Akkurt, M.; Ide, S. Z. Kristallogr. 1997,
212, 808-810.
The trimer is centrosymmetric (Figure 4a) with a linear Ni3
cluster (Ni-Ni distance of 2.811 Å) surrounded by chelato-
bridging DBM ligands. Each nickel is in a distorted octahedral
environment of six oxygens from four DBM ligands. The Ni-O
distances vary from 1.95 to 2.25 Å; the coordination angles vary
from 76° to 98°. Despite these distortions, the total energy gain
(20) Usha, K.; Sadashiva, B. K. Mol. Cryst. Liq. Cryst. 1994, 241, 91-
102.
(19) Metastable form of H-DBM: Etter, M. C.; Jahn, D. A.; Urbanczyk-
Lipkowska, Z. Acta Crystallogr. 1987, C43, 260-263.
(21) (a) Usha, K.; Vijayan, K. Mol. Cryst. Liq. Cryst. 1989, 174, 39-48.
(b) Usha, K.; Vijayan, K. Liq. Cryst. 1992, 12, 137-145.

Nickel and Zinc Dibenzoylmethanates
Inorganic Chemistry, Vol. 40, No. 7, 2001 1631
upon formation of these six bonds compared to four bonds in
the brown form must favor the cluster formation.
and Ni734 and cyclic Ni12 clusters35 were recently prepared, with
the Ni-Ni distance varying between 2.3 and 3.1 Å. Linear Ni3
trimers have been found for nickel complexes with acetylac-
etonate,36 its 3-substituted derivatives,37 and some other ligands,38
the Ni-Ni distance varying between 2.4 and 3.1 Å. The
trimerization may be explained as a means of achieving
octahedral coordination, as the trimer is the smallest polymeric
unit in which octahedral coordination of all the nickel atoms
can be accomplished provided there is no intermolecular sharing
of oxygen atoms. Bulky substituents were shown to suppress
trimerization of nickel acetylacetonate derivatives due to steric
hindrance.39,40 Moreover, when passing from the molecular level
to 3D packing, the trimer molecule yields to simpler units to
satisfy packing requirements. This is probably the main reason
only a few trimeric nickel complexes of this type have been
found so far in the solid state.
More than 300 structures with direct chemical Ni-Ni
interactions have been reported. These comprise Ni2 clusters,22
Ni3 bent clusters,23 and triangular,24 square,25 pentagonal,26
hexagonal,27 tetrahedral,28 octahedral,29 cubic,30 and pentagonal
prismatic or antiprismatic clusters,31 with the Ni-Ni distance
varying from 2.3 to 3.3 Å. In organometallic compounds
incorporating Ni(0) clusters the length for the Ni-Ni single bond
is typically between 2.3 and 2.6 Å. In Ni(II) complexes the
nickel clusters are surrounded by polydentate-bridging ligands;
the Ni-Ni distance varies continuously up to the sum of the
atomic van der Waals radii,17 3.3 Å, at which point the chemical
interaction may be considered to be insignificant. Using special
polydentate ligands, complexes comprising linear Ni4,32 Ni5,33,34
There are three types of DBM units in the trimer molecule
(Figure 4a). The first (O1, O3, etc.) and second (O4, O6, etc.)
chelate to the terminal nickels with Ni-O distances of 1.95-
2.02 Å, while the second also bridges one donor oxygen (O6)
to the central nickel (Ni2) at 2.11 Å. As these ligands coordinate
in the cis mode, the chelate rings surrounding the nickel atom
are not coplanar and cannot interact. The third type (O7, O9,
etc.) coordinates to all three nickel atoms, chelating the central
one at 1.97-1.99 Å and bridging the terminal atoms at 2.16
(Ni1A-O7) and 2.25 (Ni1-O9) Å. These ligands chelate to
the central atom in the trans mode, so that their chelate rings
are coplanar and may interact. Qualitatively the trimeric
molecule is quite similar to trimeric nickel acetylacetonate,36
except that in that molecule the Ni-Ni distance is longer, 2.86-
2.88 Å. But, as compared with the [Ni3(acac)6] trimer, the [Ni3-
(DBM)6] molecule has greater conformational freedom; the
phenyl rings rotate from the corresponding chelate rings (which
are planar within 0.1-0.2 Å) by 29.7° (C11···), 37.5° (C31···),
28.2° (C41···), 24.1° (C61···), 30.7° (C71···), and 21.5° (C91···).
This 22-38° range in the angles indicates the molecule’s ability
to respond to crystal packing requirements.
(22) (a) Byers, L. R.; Dahl, L. F. Inorg. Chem. 1980, 19, 680-689. (b)
Jones, R. A.; Stuart, A. L.; Atwood, J. L.; Hunter, W. E.; Rogers, R.
D. Organometallics 1982, 1, 1721-1723. (c) Beck, J.; Strahle, J.
Angew. Chem., Int. Ed. Engl. 1985, 24, 409-410. (d) Ho, N. F.; Mak,
T. C. W.; Luh, T. Y. J. Chem. Soc., Dalton Trans. 1990, 3591-3595.
(e) Walther, D.; Klettke, T.; Imhof, W.; Gorls, H. Z. Anorg. Allg.
Chem. 1996, 622, 1134-1144. (f) Shaposhnikova, A. D.; Teplova,
M. V.; Kamalov, G. L.; Pasynskii, A. A.; Eremenko, I. L.; Struchkov,
Yu. T.; Yanovskii, A. I. Russ. J. Inorg. Chem. 1997, 42, 1824-1827
(pp 1986-1989 in Russian version). (g) Fenske, D.; Maczek, B.;
Maczek, K. Z. Anorg. Allg. Chem. 1997, 623, 1113-1120. (h) Scherer,
O. J.; Vondung, C.; Wolmershauser, G. Angew. Chem., Int. Ed. Engl.
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Robertson, G. B. J. Am. Chem. Soc. 1990, 112, 7047-7048. (b)
Walther, D.; Klettke, T.; Gorls, H. Angew. Chem., Int. Ed. Engl. 1995,
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Imhof, W.; Gunther, W. Chem. Ber. 1996, 129, 1457-1461. (d)
Brechin, E. K.; Harris, S. G.; Parsons, S.; Winpenny, R. E. P. Angew.
Chem., Int. Ed. Engl. 1997, 36, 1967-1969.
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6685. (b) North, T. E.; Thoden, J. B.; Spencer, B.; Bjarnason, A.;
Dahl, L. F. Organometallics 1992, 11, 4326-4337. (c) Berkessel, A.;
Bats, J. W.; Huber, M.; Haase, W.; Neumann, T.; Seidel, L. Chem.
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Florke, U.; Haupt, H.-J. Inorg. Chim. Acta 1999, 289, 199-203.
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1529. (b) Bell, M.; Edwards, A. J.; Hoskins, B. F.; Kachab, E. H.;
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2660.
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Hernandez, C.; Martinez-Ripoll, M. Polyhedron 1989, 8, 1253-1259.
(b) Wark, T. A.; Stephan, D. W. Organometallics 1989, 8, 2836-
2843. (c) Schulbert, K.; Mattes, R. Z. Naturforsch. 1994, 49b, 770-
772.
(28) (a) Simonov, Yu. A.; Dvorkin, A. A.; Matuzenko, G. S.; Yampol’skaya,
M. A.; Gifeisman, T. Sh.; Gerbeleu, N. V.; Malinovskii, T. I. Koord.
Khim. 1984, 10, 1247-1252; Chem. Abstr. 1984, 101, 238414s. (b)
Bochmann, M.; Hawkins, I.; Yellowlees, L. J.; Hursthouse, M. B.;
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Goddard, R.; Kruger, C.; Werner, S.; Metz, B. Chem. Ber. 1991, 124,
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Organometallics 1998, 17, 1305-1314.
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Schowen, R. L. J. Am. Chem. Soc. 1980, 102, 6621-6623. (b) Eguchi,
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Trans. 1997, 479-483.
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Zebrowski, J. P.; Hayashi, R. K.; Bjarnason, A.; Dahl, L. F. J. Am.
Chem. Soc. 1992, 114, 3121-3123.
It should be noted that the green form is thermodynamically
stable above 202 °C and the structure was studied at -100 °C.
Although there should be no significant changes in molecular
structure or packing mode, the metastable phase is more than
300° away from the conditions of its stability. Crystal packing
of the trimers in the structure is shown in Figure 5a. The van
(33) Shieh, S. J.; Chou, C. C.; Lee, G. H.; Wang, C. C.; Peng, S. M. Angew.
Chem., Int. Ed. Engl. 1997, 36, 56-59.
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733-741. (c) Blake, A. J.; Brechin, E. K.; Codron, A.; Gould, R. O.;
Grant, C. M.; Parsons, S.; Rawson, J. M.; Winpenny, R. E. P. Chem.
Commun. 1995, 1983-1985. (d) Elmali, A.; Elerman, Y.; Svoboda,
I.; Fuess, H.; Griesar, K.; Haase, W. Z. Naturforsch. 1996, 51b, 665-
670. (e) Ulku, D.; Ercan, F.; Atakol, O.; Dincer, F. N. Acta Crystallogr.
1997, C53, 1056-1057.
(31) (a) DesEnfants, R. E., II.; Gavney, J. A., Jr.; Hayashi, R. K.; Rae, A.
D.; Dahl, L. F.; Bjarnason, A. J. Organomet. Chem. 1990, 383, 543-
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K.; Dahl, L. F. J. Am. Chem. Soc. 1992, 114, 10369-10379. (c)
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3778.

1632 Inorganic Chemistry, Vol. 40, No. 7, 2001
Soldatov et al.
Figure 6. Cut-away view of the [Ni3(DBM)6]·2(benzene) structure
displaying guest benzene molecules (enlarged atoms) in wavy channels
going along the c axis. H atoms are omitted.
packing of trimeric nickel DBM molecules (host) and benzene
molecules (guest) located in channel-like cavities. There are two
host and four guest molecules per unit cell, resulting in a
molecular formula of “Ni(DBM)2·2/3(benzene)”.
The molecular structure of the host trimer is very similar to
that found in the green form (Figure 4). It is also centrosym-
metric, and the Ni-Ni distance in the linear Ni3 cluster equals
2.831 Å, with the same mode of ligand coordination. The
octahedral environment of the Ni centers is more distorted, with
Ni-O distances from 1.95 to 2.32 Å and coordination angles
from 80° to 100°. Two DBM ligands (O1, O3, etc. and O4,
O6, etc.) chelate to terminal nickels with Ni-O distances of
1.95-2.03 Å, and the second also bridges (by O6) to the central
nickel (Ni2) at 2.14 Å. The third ligand (O7, O9, etc.)
coordinates to all three nickel atoms, chelating the central one
at 1.97-1.99 Å and bridging the terminal atoms at 2.15 (Ni1A-
O7) and 2.32 (Ni1-O9) Å.
The packing problem in the inclusion compound is solved
partially by rotating the phenyls with respect to the correspond-
ing chelate rings by dihedral angles of 29.1° (C11···), 31.3°
(C31···), 20.0° (C41···), 16.9° (C61···), 31.5° (C71···), and 17.3°
(C91···). This 17-32° range is significantly shifted compared
to 22-38° for the green form, indicating improved planarity of
the DBM moieties and thus better predisposition of their
aromatic systems to interact.
Another factor facilitating crystal packing is the guest benzene
filling up the residual cavity space. Figure 5 compares the
packing mode in the green form (a) and in the inclusion
compound (b). A dislocation of the bulky trimeric molecules
leaves cavities which form individual channels stretching along
the c crystallographic axis. The channel walls are composed of
phenyl moieties. The benzene molecules are disordered. Figure
6 shows a wavy section of the channels and the accommodation
of benzene molecules (main orientation) therein. No contacts
implying specific interaction between host and guest molecules
have been found in the structure. The Ni3 cluster is separated
from the benzene molecules by at least 4.4 Å.
Figure 5. Two projections showing the dense packing of [Ni3(DBM)6]
molecules in the green form (a, top) and packing of the same molecules
leaving channels in [Ni3(DBM)6]·2(benzene) (b, bottom). For clarity,
guest atoms are enlarged and H atoms are omitted.
der Waals shape of the trimer may be roughly approximated as
an ellipsoid with prominent phenyl substituents. In the projection
along the a axis the staggered packing order of these bulky
molecules is apparent.
Structure of the [Ni3(DBM)6]·2(Benzene) Inclusion Com-
pound. The structure is monoclinic, P21/c. It shows molecular
Structure of Monomeric [Zn(DBM)2]. The structure is
triclinic, P1h, with two molecules per unit cell. There are only
van der Waals contacts between molecules. The molecule is

Nickel and Zinc Dibenzoylmethanates
Inorganic Chemistry, Vol. 40, No. 7, 2001 1633
Figure 7. ORTEP drawing of the [Zn(DBM)2] molecule in the
monomeric form of zinc DBM. H atoms are omitted.
Figure 8. ORTEP drawing of the [Zn2(DBM)4] molecule in the dimeric
form of zinc DBM. H atoms are omitted.
asymmetric (Figure 7). The zinc atom is chelated with two DBM
ligands, one at 1.947 (Zn-O1) and 1.915 (Zn-O3) Å and the
second at 1.933 (Zn-O4) and 1.971 (Zn-O6) Å. The coordina-
tion polyhedron is a very distorted tetrahedron; the coordination
angles vary from 95° to 124°, and the dihedral angle between
the planes of the chelate rings is 100.6°. The phenyl rings are
rotated from the corresponding chelate rings by +11.0°
(C11···C16), -11.6° (C31···C36), +30.0° (C41···C46), and -39.4°
(C61···C66). The significant inequivalence of the two DBM units
is also apparent in the MAS 13C NMR spectra, as described
below. In general, the observed tetrahedral coordination to
Zn(II) is common and was found, for instance, for the analogous
â-diketonate zinc bis(dipivaloylmethanate).41 The observed
distortions may be attributed to the stress of creating dense
molecular packing.
Structure of Dimeric [Zn2(DBM)4]. The structure is mono-
clinic, C2/c. There are four dimeric molecules per unit cell, and
there are only van der Waals contacts between the dimers. The
dimer is centrosymmetric. Two ligands chelate to zinc by four
oxygen donors while one oxygen also bridges to a neighboring
zinc center (Figure 8). The coordination environment of the zinc
atom is intermediate between trigonal bipyramidal (with central
Zn and apical O1 and O4A) and square pyramidal (with central
Zn and apical O4), with average angular deviations from the
ideal polyhedra of 9.5° and 10.1°, respectively. One ligand (O1,
O3, etc.) chelates to a zinc center at 1.986 (O1) and 1.981 Å
(O3). Another one (O4A, O6A, etc.) chelates to zinc at 2.087
Figure 9. Solid-state 13C CP/MAS NMR spectra of (a) [Ni(DBM)2],
brown form, (b) [Ni(DBM)2], brown form, dipolar dephased, (c) [Zn-
(DBM)2], (d) [Zn2(DBM)4], and (e) H-DBM.
(O4A) and 1.983 (O6A) Å but also bridges to the neighboring
zinc (ZnA-O4A) at 2.071 Å. The Zn-Zn distance is 3.14 Å.
The dihedral angle between two chelate rings of 37.3° excludes
significant interaction between the chelate systems. The phenyl
rings rotate from their adjacent chelate rings by +14.7°
(C11···), +35.0° (C31···), -18.9° (C41···), and +16.5° (C61···).
As described below, the inequivalence of the DBM units in the
dimer is also observed in the MAS 13C NMR spectra.
Analogous zinc dimers have been reported for complexes with
bridge-chelating ligands of other types,42 but for zinc acetyl-
acetonate a trimeric structure has been reported.43
MAS 13C NMR Spectra. 13C CP/MAS NMR spectra of the
diamagnetic metal DBM compounds and the parent H-DBM
are shown in Figure 9, and shifts and assignments are given in
Table 4. As a whole, the data confirm both the gross composition
(42) (a) Barnett, B. L.; Kretschmar, H. C.; Hartman, F. A. Inorg. Chem.
1977, 16, 1834-1838. (b) Yampol’skaya, M. A.; Bourosh, P. N.;
Simonov, Yu. A.; Gerbeleu, N. V. Zh. Neorg. Khim. 1987, 32, 1655-
1660. (c) Wang, X. N.; Zhang, W. X.; Yu, Z. G.; Jiang, D. H.; Dong,
S. L. Acta Chim. Sin. 1996, 54, 562-567. (d) Enders, D.; Zhu, J.;
Raabe, G. Angew. Chem., Int. Ed. Engl. 1996, 35, 1725-1728. (e)
Trosch, A.; Vahrenkamp, H. Eur. J. Inorg. Chem. 1998, 827-832.
(f) Ranford, J. D.; Vittal, J. J.; Wu, D. Angew. Chem., Int. Ed. Engl.
1998, 37, 1114-1116.
(43) Bennett, M. J.; Cotton, F. A.; Eiss, R. Acta Crystallogr. 1968, B24,
904-913.
(41) Cotton, F. A.; Wood, J. S. Inorg. Chem. 1964, 3, 245-251.

1634 Inorganic Chemistry, Vol. 40, No. 7, 2001
Soldatov et al.
pyridine
Table 4. Assignments of 13C NMR Resonances in DBM Metal Complexesa
C2 (>C-H)
C1 (C-O)
C11 (ipso)
C12/C13 (o/m)b
C14 (p)b
H-DBM
(SO2 soln at 60 °C)c
H-DBM
(solid)
93.4
185.6
134.3
127.5 (o)
129.2 (m)
128.8
127.5
126.4
2q
133.6
92.6
187.7
182.4
135.6
132.7
131.8
129.5
sd
d
d
d
[Ni(DBM)2]
(brown form)
92.9
179.4
177.4
137.3
135.8
129.6
129.0
125.9
2q
129
127.9
131.2
129
s
d
d
d
[Zn(DBM)2]
97.8
90.6
191.3 (1)e
187.9 (1)
185.9 (2)
q
141.9 (1)
138.8 (2)
137.2 (1)
q
132.8 br d
d
2o
q
[Zn2(DBM)4]
96.5
93.3
191.5 (1)
189.0 (1)
184.2 (2)
q
187.7 (1)
186.3 (2)
182.6 (1)
q
142.9 (1)
138.7 (3)
128.4 br d
134.5
132
d
q
2o
128.3
q
[ZnPy2(DBM)2]f
[CdPy2(DBM)2]f
92.8 br d
142.8 (1)
139.4 (3)
130135
149.6 (o) q
? (m) q
139.0 (p) d
d
94.3
92.0
q
2o
129.6
128.0
q
190.4
188.2
187.1
143.0 (1)
141.6 (1)
140.2 (2)
133.0
131.2
148.8 (o) q
125.0 (m) q
138.7 (p) d
183.7
d
q
q
2o
q
a For the atom numeration scheme see Figure 3. b C12, C13, C14 (o, m, p) ) ortho, meta, and para carbons of the phenyl rings. c Olah, G. A.;
Grant, J. L.; Westerman P. W. J. Org. Chem. 1975, 40, 2102-2108. d Multiplicities expected from X-ray structures: s, d, q, and o refer to singlet,
doublet, quartet, and octet. e Numbers in parentheses refer to the quartet intensity distribution. f The structure of these materials has been discussed
previously, ref 1.
and chemical individuality of the brown Ni form and the two
Zn polymorphs. 13C MAS NMR of powdered samples is
sensitive to crystallographic inequivalence of chemically equiva-
lent sites. Therefore, independent resonances for crystallographi-
cally inequivalent carbons in the asymmetric unit are expected.
In brown [Ni(DBM)2] the two DBM units are identical due to
an inversion center at the Ni atom, but the two halves of each
unit are not equivalent (no mirror plane or C2 axis). Conse-
quently doublets are expected for the C-O bonds (C1, C3) and
the ipso (C11, C31) and para (C14, C34) carbons of the phenyl
rings, and quartets for the ortho and meta carbons (Figure 9a;
for the atom numeration scheme see Figure 3). In the dipolar
dephased spectra, illustrated for [Ni(DBM)2] in Figure 9b, those
carbons which have no attached H atoms show strongly. Carbons
with attached H disappear completely if they are static, but if
there is any motion which causes a reorientation of the C-H
bond, and a consequent reduction of the dipolar coupling, then
usually the resonance appears but at significantly reduced
intensity. For [Ni(DBM)2] the unique C-H bond (singlet), C-O
bonds (doublet), and ipso ring carbon (doublet) are therefore
readily assigned (Table 4). Many resonances of the C-H bonds
of the phenyl rings are not resolved in the 125-135 ppm region
(only peaks and shoulders are given values in Table 4), but a
partial assignment can be made on the basis of the dipolar
dephased spectra. It is quite noticeable in all the dipolar
dephased spectra that there is a small amount of intensity in
this region. Closer inspection reveals that this is only present
for some of the peaks and shoulders. Two-fold reorientation of
phenyl groups in solids is well-known,44 and typically this causes
some intensity for the ortho and meta carbons to appear in the
dephased spectrum, since their C-H bonds change orientation,
whereas the para carbon does not appear since its C-H bond
lies along the 2-fold axis and does not change orientation.
Sufficiently rapid reorientation could also make the two ortho
carbons on the same ring equivalent by exchange, and likewise
the two meta carbons, thus reducing their multiplicity by a factor
of 2 from that expected from the X-ray structure.
[Zn(DBM)2] shows a doubling of all lines (Figure 9c) since
the two independent DBM units are distinctly different, and the
whole complex is asymmetric. Quite a large splitting (7.2 ppm)
of the methanate carbon (C2 and C5; see Figure 7) is observed.
[Zn2(DBM)4] has four DBM units, but only two are crystallo-
graphically inequivalent because of an inversion center; con-
sequently this complex also shows a doubling of lines, Figure
9d.
The H-DBM parent material (the extra H maintains charge
neutrality) is present in the solid phase in the enol form,18,19
and consequently the two halves of the molecule on either side
of the >C-H bond are inequivalent. Its spectrum (Figure 9e)
therefore shows a singlet for the unique >C-H bond, a doublet
for the C-O bonds, and so on as for [Ni(DBM)2].
Also included in Table 4 are assignments of spectra obtained
for the pyridine complexes [ZnPy2(DBM)2] and [CdPy2(DBM)2]
whose structures were discussed previously.1 The present
assignments for the DBM complexes facilitated the assignment
of the DBM and pyridine components in these spectra.
All the NMR powder spectra are fully consistent with the
X-ray structural data, and furthermore the dipolar dephasing
experiments indicate 2-fold reorientational dynamics of the
phenyl rings. A remarkable feature is the decrease in the shift
of the C-O doublet in the [Ni(DBM)2] complex as compared
to all the other materials; the average shift is 6.6-9.4 ppm lower.
Its average ipso carbon shift is also 2.6-4.7 ppm lower than
those of the other complexes (but 2.4 ppm higher than that of
(44) (a) Fyfe, C. A. Solid State NMR for Chemists; CFC Press: Guelph,
Canada, 1983. (b) Buchanan, G. W.; Morat, C.; Charland, J. P.;
Ratcliffe, C. I.; Ripmeester, J. A. Can. J. Chem. 1989, 67, 1212-
1218. (c) Schmidt-Rohr, K.; Spiess, H. W. Multidimensional Solid-
State NMR and Polymers; Academic Press: London, 1994.

Nickel and Zinc Dibenzoylmethanates
Inorganic Chemistry, Vol. 40, No. 7, 2001 1635
H-DBM). The replacement of the enol hydrogen in H-DBM
with a metal cation should lead, due to electrostatics, to an
electron density decrease on all the ligand atoms. This is indeed
observed to a greater or lesser extent for the Zn and Cd
compounds. The opposite effect observed for the C-O carbons
in [Ni(DBM)2] may be a consequence of electron donation from
nickel d orbitals to the pseudoaromatic system of the bischelate
fragment (geometrically favorable due to the planarity of the
fragment and shortened coordination bonds).
[Ni(DBM)2] does not include, complexes of a similar structure
were recently utilized to entrap such bulky molecules as
fullerenes and carboranes.51
Magnetic and structural features of nickel DBM polymorphs
indicate that polymorphism can be accompanied by isomeriza-
tion/polymerization on the molecular level. The question as to
whether molecular structure is responsible for formation of a
given solid phase or vice versa is arguable in this case.
Nevertheless, the guest benzene plays a decisive role in the
formation of trimers in the [Ni3(DBM)6]·2(benzene) inclusion
compound at room temperature as the guest-free green phase
containing analogous trimers is stable only above 202 °C. Other
solvents with chemical and physical properties similar to those
of benzene were not able to stabilize this structure by virtue of
their different geometry, thus resulting in a solid containing
monomeric [Ni(DBM)2] units.
The monomer-trimer equilibrium for nickel acetylacetonate
which occurs in solution is a well-known example in Cotton
and Wilkinson’s textbook.52 The equilibrium depends on
concentration and temperature and reveals a positive enthalpy
of dissociation. In noncoordinating solvents both forms coexist
in commensurable quantities; for more dilute solutions about
25% dissociation occurs at 160 °C.40 Similar observations were
made for other related complexes.39,40 So, on the molecular level
these two situations are similar in energy, and which one appears
in the solid may be a matter of noncovalent interactions present
in a potential solid phase.
Stabilization by noncovalent interactions of the metal complex
molecular isomers3 and unstable molecules53 has been reported
and is an attractive way of isolating unstable or unusual chemical
species. The present study suggests an interesting means of
stabilizing polymeric species by a suitable templating guest in
the solid phase. Perhaps appropriate templates could control the
formation of other species, of tetrameric or larger size. Unlike
square planar Ni(II) complexes those containing octahedrally
coordinated nickel atoms are paramagnetic, and upon formation
of the nickel cluster a ferromagnetic interaction arises,54
suggesting the possibility of controlling the physical properties
Main Conclusions. The structural chemistry of nickel and
zinc DBMs, the metal DBM host-type precursors, is complex.
The [MA2(DBM)2] host compounds have coordinatively satu-
rated metal centers, and the structural stability becomes a matter
of appropriate 3D packing. The possibility of the host molecule
to form a trans or cis isomer is the most notable molecular
feature. As the metal(II) DBMs possess a deficiency of donors
for the metal acceptors, they have a potential to transform into
polymeric species by sharing donor sites of DBM ligands. The
diversity of metal(II) â-diketonates, including DBMs, is remark-
able as monomeric,4,5,6,20,21,41 dimeric,45 trimeric,36,43,46 tet-
rameric,47 and even polymeric48 entities have been studied in
the solid state. Further, equilibria between monomeric and
polymeric forms of nickel â-diketonates in solution have been
studied,39,40 and both monomeric and trimeric forms were
recently reported for a nickel complex with 3-phenyl-substituted
acetylacetonate.37 The structural diversity revealed in the current
work is unprecedented as one inclusion plus three guest-free
polymorphs of one compound of nickel DBM have been isolated
and characterized. For zinc DBM two polymorphs, essentially
different at the molecular level, have also been prepared. On
the basis of examination of all these materials, the following
general conclusions may be stated.
As well as [MA2(DBM)2] hosts, the nickel and zinc DBMs
experience packing problems manifested as a tendency to
structural variability (for both molecular species and poly-
morphs) instead of one very preferable stable form. In a broad
sense the inclusion compounds also can be considered as
polymorphic modifications of the host component, which are
stable only in the presence of a templating guest.49 From this
viewpoint, conventional polymorphism and affinity to include
are closely related to each other, and it is not surprising that
they are often found together.50 The trimeric nickel DBM species
form an inclusion compound with benzene, and possibly can
include other solvents as there are no specific interactions of
the included species with the host matrix. Though monomeric
(50) A few examples of versatile organic hosts displaying remarkable
polymorphism. Gossipol (seven guest-free polymorphs): (a) Ibragi-
mov, B. T.; Talipov, S. A. J. Inclusion Phenom. 1994, 17, 325-328.
(b) Gdanec, M.; Ibragimov, B. T.; Talipov, S. A. In ComprehensiVe
Supramolecular Chemistry; MacNicol, D. D., Toda, F., Bishop, R.,
Eds.; Pergamon: Oxford, 1996; Vol. 6, pp 117-145 and refs 48 and
68 therein. Hydroquinone (three guest-free polymorphs): Mak, T. C.
W.; Bracke, B. R. F. In ComprehensiVe Supramolecular Chemistry;
MacNicol, D. D., Toda, F., Bishop, R., Eds.; Pergamon: Oxford, 1996;
Vol. 6, pp 23-60 and refs 6, 13, and 14 therein. 4,5-Bis(4-
methoxyphenyl)-2-(4-nitrophenyl)-1H-imidazole (three guest-free
polymorphs): Sakaino, Y.; Fujii, R.; Fujiwara, T. J. Chem. Soc., Perkin
Trans. 1 1990, 2852-2854. Perhydrotriphenylene (two guest-free
polymorphs): Allegra, G.; Farina, M.; Immirzi, A.; Colombo, A.;
Rossi, U.; Broggi, R.; Natta, G. J. Chem. Soc. B 1967, 1020-1028.
(51) (a) Andrews, P. C.; Atwood, J. L.; Barbour, L. J.; Nichols, P. J.; Raston,
C. L. Chem. Eur. J. 1998, 4, 1384-1387. (b) Croucher, P. D.; Nichols,
P. J.; Raston, C. L. J. Chem. Soc., Dalton Trans. 1999, 279-284. (c)
Andrews, P. C.; Atwood, J. L.; Barbour, L. J.; Croucher, P. D.; Nichols,
P. J.; Smith, N. O.; Skelton, B. W.; White, A. H.; Raston, C. L. J.
Chem. Soc., Dalton Trans. 1999, 2927-2932. (d) Croucher, P. D.;
Marshall, J. M. E.; Nichols, P. J.; Raston, C. L. Chem. Commun. 1999,
193-194. (e) Corden, J. P.; Errington, W.; Moore, P.; Wallbridge,
M. G. H. Acta Crystallogr. 1999, C55, 706-707.
(45) (a) Shibata, S.; Onuma, S.; Iwase, A.; Inoue, H. Inorg. Chim. Acta
1977, 25, 33-39. (b) Cotton, F. A.; Rice, G. W. New J. Chem. 1977,
1, 301-305. (c) Baxter, I.; Drake, S. R.; Hursthouse, M. B.; Malik,
K. M. A.; McAleese, J.; Otway, D. J.; Plakatouras, J. C. Inorg. Chem.
1995, 34, 1384-1394.
(46) (a) Shibata, S.; Onuma, S.; Inoue, H. Inorg. Chem. 1985, 24, 1723-
1725. (b) Petrov, G.; Alexiev, A.; Angelova, O.; Macicek, J. J. Coord.
Chem. 1992, 25, 101-110.
(47) Elder, R. C. Inorg. Chem. 1968, 7, 1117-1123.
(48) Maslen, E. N.; Greaney, T. M.; Raston, C. L.; White, A. H. J. Chem.
Soc., Dalton Trans. 1975, 400-402.
(49) In some cases microporous host structures remain intact upon guest
removal; this property is characteristic of zeolites. Other systems
displaying such a behavior (“zeolite analogues”): (a) Allison, S. A.;
Barrer, R. M. J. Chem. Soc. A 1969, 1717-1723. (b) Ibragimov, B.
T.; Talipov, S. A.; Aripov, T. F. J. Inclusion Phenom. 1994, 17, 317-
324. (c) Ung, A. T.; Gizachew, D.; Bishop, R.; Scudder, M. L.; Dance,
I. G.; Craig, D. C. J. Am. Chem. Soc. 1995, 117, 8745-8756. (d)
Kondo, M.; Yoshitomi, T.; Seki, K.; Matsuzaka, H.; Kitagawa, S.
Angew. Chem., Int. Ed. Engl. 1997, 36, 1725-1727. (e) Russel, V.
A.; Evans, C. C.; Li, W.; Ward, M. D. Science 1997, 276, 575-579.
(f) Kepert, C. J.; Rosseinsky, M. J. Chem. Commun. 1999, 375-376.
(g) Reference 3 (b). (h) Soldatov, D. V.; Ripmeester, J. A. Chem.
Mater. 2000, 12, 1827-1839.
(52) Cotton, F. A.; Wilkinson, G. F. R. S. AdVanced Inorganic Chemistry;
Interscience Publishers: New York, 1966; pp 888-889.
(53) (a) Dyadin, Yu. A.; Kislykh, N. V. MendeleeV Commun. 1991, 134-
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1636 Inorganic Chemistry, Vol. 40, No. 7, 2001
Soldatov et al.
of the material. Two more relevant examples were recently
reported on the remarkable redox properties of the Cr3 cluster
in a complex with bis(2-pyridyl)amine55 and a mixed-valence
1D chain of rhodium atoms, a “solvated molecular wire”.56
Another conclusion is that, as for [MA2(DBM)2] hosts, the
tendency to planarity of the bischelate fragment is observed for
nickel but not for zinc. This is quite evident from a comparison
of the monomeric forms of the metal DBMs, the nickel complex
showing square planar geometry and the zinc complex showing
tetrahedral geometry. The remarkable planarity of [Ni(DBM)2]
demonstrates that the trans configuration adopted by the [MA2-
(DBM)2] complexes when M ) Ni is controlled by the nickel
bischelate unit itself. In the green form and [Ni3(DBM)6]·
2(benzene) this tendency yields to providing each nickel atom
with octahedral coordination and is observed only for the central
nickel atom. The zinc complex does not follow this tendency
at all. Note a similar distinction between trimeric nickel36 and
zinc43 acetylacetonates; while the former incorporates a linear
Ni3 cluster with a planar bischelate fragment about the central
nickel atom, the latter contains a bent Zn3 cluster with no planar
bischelate fragment present.
The present study has shown that the main properties
responsible for the ability of metal DBM hosts to form
supramolecular architectures are already observable in their
precursors. These properties, packing problems, and the ten-
dency to certain conformational and isomeric states become
more striking upon modification of the basic metal DBM unit
to yield this family of versatile host complexes.
Acknowledgment. We kindly thank J. L. Reid for suggesting
and assisting in the magnetic susceptibility measurements at
Ottawa-Carleton Chemistry Institute and Dr. K. Mast (The
Steacie Institute) for fruitful discussions. D.V.S. is grateful for
support in the form of a Visiting Fellowship.
Supporting Information Available: Full data for the structure
determination of five studied compounds (see Table 1) and 13C CP/
MAS NMR spectra of [MPy2(DBM)2] (M ) Zn, Cd). This material is
available free of charge via the Internet at http://pubs.acs.org.
(55) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Pascual, I. J. Am. Chem.
Soc. 1997, 119, 10223-10224.
(56) Finniss, G. M.; Canadell, E.; Campana, C.; Dunbar, K. R. Angew.
Chem., Int. Ed. Engl. 1996, 35, 2772-2774.
IC000981G