Syntheses, structures, spectroscopy, and chromotropism of new complexes arising from the reaction of nickel(II) nitrate with diphenyl(dipyrazolyl) methane

Article abstract of DOI:10.1021/ic061311z

The complexes [(dpdpm)Ni(η²-NO₃)₂] (1), [(dpdpm)Ni(η²-NO₃)(η¹-NO ₃)(CH₃CN)] (2), [(dpdpm)₂Ni(η¹- NO₃)(H₂O)]-NO₃ (3), and [(dpdpm) ₂Ni(H₂O)₂][NO₃]₂ (4) (dpdpm = diphenyl(dipyrazolyl)methane, Ph₂C(C₃N ₂H₃)₂), have been prepared and characterized by IR and UV-vis-NIR spectroscopy and X-ray diffraction studies. X-ray studies have confirmed that complexes 1-4 all adopt variously distorted octahedral structures in the solid state, the largest distortions arising from the small bite-angle of the bidentate nitrate ligand in 1 and 2. Magnetic moment measurements indicate that these solids are paramagnetic with two unpaired electrons. The solution ¹H NMR data show that the paramagnetism is maintained in solution. Absorption spectra of 1-4 show three main bands in the region of 350-1000 nm representing spin allowed (d-d) transitions from the ground state ³A_2g to the excited states ³T _2g, ³T_1g(³F), and ³T _1g(³P). A weak shoulder was also detected at about 700-800 nm in most spectra, representing spin-forbidden transitions ³A _2g → ¹Eg. A comparison of the crystal field parameters 10Dq and B for 1-4 to the corresponding values for related complexes indicated that these parameters are fairly insensitive to structural variations within this family of complexes. The 10Dq/B ratios show greater variations, but no clear correlations are apparent between 10Dq/B and such structural features as the nature of ligator atoms (N:O ratio), the bonding mode of the nitrate ligand, or the overall charge. Complexes 1 (green) and 2 (blue) interconvert as a function of temperature (solutions and solid samples), concentration of CH₃CN (solutions), or CH₃CN vapor pressure (solid samples).

Full text of DOI:10.1021/ic061311z

Inorg. Chem. 2007, 46, 299−308
Syntheses, Structures, Spectroscopy, and Chromotropism of New
Complexes Arising from the Reaction of Nickel(II) Nitrate with
Diphenyl(dipyrazolyl)methane
Natalie Baho and Davit Zargarian*
De´partement de Chimie, UniVersite´ de Montre´al, C. P. 6128, Succursale Centre-Ville, Montre´al,
Que´bec, Canada H3C 3J7
Received July 14, 2006
The complexes [(dpdpm)Ni(
NO3 (3), and [(dpdpm)2Ni(H2O)2][NO3]2 (4) (dpdpm
prepared and characterized by IR and UV vis NIR spectroscopy and X-ray diffraction studies. X-ray studies have
confirmed that complexes 1 4 all adopt variously distorted octahedral structures in the solid state, the largest
η
2-NO3)2] (1), [(dpdpm)Ni(
η
η η
2-NO3)( 1-NO3)(CH3CN)] (2), [(dpdpm)2Ni( 1-NO3)(H2O)]-
)
diphenyl(dipyrazolyl)methane, Ph2C(C3N2H3)2), have been
−
−
−
distortions arising from the small bite-angle of the bidentate nitrate ligand in 1 and 2. Magnetic moment measurements
indicate that these solids are paramagnetic with two unpaired electrons. The solution 1H NMR data show that the
paramagnetism is maintained in solution. Absorption spectra of 1−4 show three main bands in the region of 350−
3
3
3
1000 nm representing spin allowed (d−
d) transitions from the ground state A2g to the excited states T2g, T1g(3F),
and 3T1g(3P). A weak shoulder was also detected at about 700
−
800 nm in most spectra, representing spin-forbidden
3
transitions A2g
f −4 to the corresponding
1Eg. A comparison of the crystal field parameters 10Dq and B for 1
values for related complexes indicated that these parameters are fairly insensitive to structural variations within this
family of complexes. The 10Dq/B ratios show greater variations, but no clear correlations are apparent between
10Dq/B and such structural features as the nature of ligator atoms (N:O ratio), the bonding mode of the nitrate
ligand, or the overall charge. Complexes 1 (green) and 2 (blue) interconvert as a function of temperature (solutions
and solid samples), concentration of CH3CN (solutions), or CH3CN vapor pressure (solid samples).
Introduction
ease with which their steric and electronic properties can be
modified by simple synthetic modifications, thereby allowing
a fine-tuning of ligand properties. The anionic bis- or tris-
(pyrazolyl)borates are the most commonly studied of the
scorpionate ligands. This family of ligands has shown great
versatility for stabilizing a wide range of coordination and
organometallic complexes, in particular those possessing
metals in their less common oxidation states (e.g., Pd(IV)4,5
and Pt(IV)6). By comparison, the neutral bis- or tris-
(pyrazolyl)alkanes have had less impact in organometallic
chemistry.7 The paucity of organometallic compounds based
on poly(pyrazolyl)alkane ligands is particularly evident in
Since Trofimenko’s initial reports on poly(pyrazolyl)-
borates1 and poly(pyrazolyl)alkanes,2 these so-called scor-
pionate ligands have found wide applications in coordination,
organometallic, and bioinorganic chemistry.3 Perhaps the
most important feature of these two classes of ligands is the
* To whom correspondence should be addressed. Phone: 514-343-2247.
Fax: 514-343-2468. E-mail: zargarian.davit@umontreal.ca.
(1) (a) Trofimenko, S. J. Am. Chem. Soc. 1966, 88, 1842. (b) Trofimenko,
S. J. Am. Chem. Soc. 1967, 89, 3170. (c) Trofimenko, S. J. Am. Chem.
Soc. 1967, 89, 6288. (d) Trofimenko, S. J. Am. Chem. Soc. 1969, 91,
588. (e) Trofimenko, S. J. Am. Chem. Soc. 1970, 92, 5118.
(2) Trofimenko, S. J. Am. Chem. Soc. 1970, 92, 1499.
(3) For a few recent reviews on the development of the coordination
chemistry of scorpionate ligands, see: (a) Reger, D. L. Comments
Inorg. Chem. 1999, 21, 1. (b) Sadimenko, A. P. AdV. Heterocycl.
Chem. 2001, 81, 167. (c) Trofimenko, S. Chem. ReV. 1993, 93, 943.
(d) Trofimenko, S. Polyhedron 2004, 23, 197. (e) Otero, A; Ferna`ndez-
Baeza, J.; Antin˜olo, A.; Tejeda, J.; Lara-Sanchez, A. Dalton Trans.
2004, 1499. (f) Pettinari, C.; Pettinari, R. Coord. Chem. ReV. 2005,
249, 525. (g) Pettinari, C.; Pettinari, R. Coord. Chem. ReV. 2005, 249,
663. (h) Bigmore, H. R.; Lawrence, S. C.; Mountford, P.; Tredget, C.
S. Dalton Trans. 2005, 635.
(4) Byers, P. K.; Canty, A. J.; Honeyman, R. T. AdV. Organomet. Chem.
1992, 34, 1. (See in particular pp. 26 and 34.)
(5) Canty, A. J.; Andrew, S.; Skelton, B. W.; Traill, P. R.; White, A. H.
Organometallics 1995, 14, 199.
(6) Reinartz, S.; Brookhart, M.; Templeton, J. L. Organometallics 2002,
21, 247.
(7) Byers, P. K.; Canty, A. J.; Skelton, B. W.; White, A. H. Organome-
tallics 1990, 9, 826.
10.1021/ic061311z CCC: $37.00
Published on Web 12/07/2006
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 1, 2007 299

Baho and Zargarian
the case of nickel, for which no organometallic complex has
been reported.8
complexes is based, primarily, on the coordination of solvent
molecules to the metal center in the initially square planar
species; depending on the donor strength of the solvent or
the steric bulk of the ligands, one or two solvent molecules
can bind to give square pyramidal or octahedral solvato
products, respectively. In some cases, the coordinating ability
of certain anions can modulate the outcome of these
interactions. Similarly, vapochromism arises from metal-
ligand or metal-metal interactions, or more subtle changes
in the compound lattice, when a complex is exposed to
volatile organic compounds; this phenomenon leads to
changes in the absorption or luminescence properties of the
complex. Vapochromism has been studied extensively with
a number of transition metal compounds, including those of
platinum16 and gold,17 but very few studies have been
reported with Ni(II) complexes. For instance, a recent report
outlined the development of sensors based on the vapochro-
mic properties of the complex bis(1,2,6,7-tetracyano-3,5-
dihydro-3,5-diiminopyrrolizinido)nickel(II) dispersed inside
spin coated poly(vinylbutyral) thin films.18 To our knowl-
edge, the vapochromic properties of Ni complexes based on
bis(pyrazolyl)methane type ligands have not been explored.
As a follow-up to our earlier studies, we have prepared a
new series of nickel complexes featuring the ligand dpdpm
(dpdpm ) diphenyl(dipyrazolylmethane)).19 Solid state struc-
tural studies and UV-vis-NIR spectroscopy have been
employed to investigate structural changes in the first
examples of this family of compounds as a function of
temperature, different solvents, and type of anions used.
Herein we report on the synthesis, structural characterization,
and spectroscopy of complexes arising from the reaction of
dpdpm with Ni(NO3)2. The relatively labile coordination of
the nitrate anion has given access to compounds featuring
η2-, η1-, and η0-nitrate moieties. Thus, the nitrate ligand in
the complex [(dpdpm)Ni(η2-NO3)2], 1, undergoes a hapticity
change to give [(dpdpm)Ni(η1-NO3)(η2-NO3)(CH3CN)], 2;
these two compounds interconvert as a function of temper-
ature (solutions and solid samples), acetonitrile concentration
(solutions), or acetonitrile vapor pressure (solid samples).
We have explored the preparation and characterization of
new nickel-poly(pyrazolyl)alkane complexes as precursors
for new organometallic species.9 In previous reports we have
described the synthesis,10 structural characterization,11 and
spectroscopy12 of a series of complexes based on bpm* and
tpm* ligands (bpm* ) bis(3,5-dimethylpyrazolyl)methane,
tpm* ) tris(3,5-dimethylpyrazolyl)methane). All of these
compounds are paramagnetic and some display continuous
thermo-, solvato-, and vapochromic behavior. These proper-
ties arise from the relatively facile changes in the coordina-
tion geometry of these compounds or from the substitution
of one or more ligands by solvent molecules or coordinating
anions. The observation of these phenomena in our com-
plexes implies the existence of closely related structural
derivatives of very comparable energies.
Thermochromic behavior has been observed with other
Ni(II) complexes bearing bis(pyrazolyl)methane or ethyl-
enediamine type ligands. For instance, dichloro(bis(3,5-
dimethylpyrazolyl)methane)NiII undergoes a monomer-
dimer equilibrium (in the solid state) as a function of
temperature: the orange, chloro-bridged dimer is stable
below 130 °C and adopts a square pyramidal geometry, while
the deep purple monomer forms at higher temperatures and
adopts a pseudo-tetrahedral geometry around Ni.1e,13 On the
other hand, the complex [bis(N,N-diethylethylenediamine)-
NiII][NO3]2 and its CuII analogue undergo reversible coor-
dination of the nitrate anions to convert the red square planar
form to a purple octahedral species at high temperatures.13,14
Solvatochromism of Ni(II) species based on ethylenedi-
amine type ligands has been studied intensively.15 These
studies have shown that the solvatochromism of these
(8) For recent reports on organometallic complexes of Pd based on poly-
(pyrazolyl)alkane ligands, see: (a) Sa´nchez, G.; Serrano, L. J.; Pe´rez,
J.; Ramirez de Arellano, M. C.; Lo´pez, G.; Molins, E. Inorg. Chem.
Acta 1999, 295, 136. (b) Jalo´n, A. F.; Arroyo, N.; Go´mez-de La Tore,
F.; Manzano, B. R.; Moreno-Lara. B.; Rodriguez., A. M. J. Organomet.
Chem. 2000, 603, 174. (c) Tsuji, S.; Swenson, D. C.; Jordan, R. F.
Organometallics 1999, 18, 4758.
(9) For representative reports on coordination complexes of Ni featuring
poly(pyrazolyl)alkane ligands, see: (a) Mahon, M. F.; McGinley, J.;
Molloy, K. C. Inorg. Chim. Acta 2003, 355, 368. (b) Wolfgang, K.;
Berghahn, M.; Frank, W.; Reiss, G. J.; Schonherr, T.; Rheinwald, G.;
Lang, H. Eur. J. Inorg. Chem. 2003, 11, 2059. (c) Pettinari, C.;
Marchetti, F.; Cingolani, A.; Leonesi, D.; Colapietro, M.; Margadonna,
S. Polyhedron 1998, 17, 4145. (d) Mann, K. L.; Jeffery, J. C.;
McCleverty, J. A.; Thornton, P.; Ward, M. D. J. Chem. Soc., Dalton
Trans. 1998, 1, 89. (e) Pettinari, C.; Cingolani, A.; Bovio, B.
Polyhedron 1996, 15, 115. (f) Astley, T.; Gulbis, J. M.; Hitchman M.
A.; Tiekink, E. R. T. J. Chem. Soc., Dalton Trans. 1993, 509. (g)
Mesubi, M. A.; Omotowa, B. A. Synth. React. Inorg. Met.-Org. Chem.
1993, 23, 213. (h) Astley, T.; Canty, A. J.; Hitchman, M. A.;
Rowbottom, G. L.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton
Trans. 1991, 1981. (i) Reedijk, J.; Verbiest, J. Transition Met. Chem.
1979, 4, 239. (j) Jansen, J. C.; Van Koningsveld, H.; Van Ooijen, J.
A. C.; Reedijk, J. Inorg. Chem. 1980, 19, 170. (k) Reedijk, J.; Verbiest,
J.; Transition Met. Chem. 1978, 3, 51.
Experimental Section
General Procedure. All starting materials were purchased from
Sigma-Aldrich and used as received. Ni(NO3)2‚H2O was used
without dehydration. The main ligand used in this study, diphenyl-
(dipyrazolyl)methane (dpdpm), was synthesized according to a
published procedure.19b The NMR spectra were recorded on a
(15) (a) Fukuda, Y.; Cho, M.; Sone, K. Bull. Chem. Soc. Jpn. 1989, 62,
51. (b) Bourdin, D.; Lavabre, D.; Beteille, J. P.; Levy, G.; Micheau,
J. Bull. Chem. Soc. Jpn. 1990, 63, 2985. (c) Linert, W.; Gutmann, V.
Coord. Chem. ReV. 1992, 117, 159. (d) Linert, W.; Fukuda, Y.;
Camard, A. Coord. Chem. ReV. 2001, 218, 113. (e) Berger, S. Z. Phys.
Chem. 2002, 216, 1363.
(16) Wadas, T. J.; Wang, Q. M.; Kim, Y. J.; Fraschenreim, C.; Blanton,
T. N.; Eisenberg, T. J. Am. Chem. Soc. 2004, 126, 16841.
(17) Lefebvre, J.; Batchelor, R. J.; Leznoff, D. B. J. Am. Chem. Soc. 2004,
126, 16117.
(10) Michaud, A.; Fontaine, F.-G.; Zargarian, D. Inorg. Chim. Acta 2006,
359, 2592.
(11) (a) Michaud, A.; Fontaine, F.-G.; Zargarian, D. Acta Crystallogr. 2005,
E 61, m784. (b) Michaud, A.; Fontaine, F.-G.; Zargarian, D. Acta
Crystallogr. 2005, E 61, m904.
(18) Flamini, A.; Mattei, G.; Panusa, A. J. Inclusion Phenom. 2004, 33,
377.
(12) Nolet, M.-C.; Michaud, A.; Bain, C.; Zargarian, D.; Reber, C.
Photochem. Photobiol. 2006, 82, 57.
(13) Bloomquist, D. R.; Willett, R. D. Coord. Chem. ReV. 1982, 47, 125.
(14) Fabbrizzi, L.; Micheloni, M.; Paoletti, P. Inorg. Chem. 1974, 13, 3019.
(19) Recent reports have described a number of dpdpm complexes of copper
(a) and silver (b): (a) Shaw, J. L.; Cardon, T. B.; Lorigan, G. A.;
Ziegler, C. J. Eur. J. Inorg. Chem. 2004, 1073. (b) Reger, D. L.;
Gardinier, J. R.; Smith, M.D. Inorg. Chem. 2004, 43, 3825.
300 Inorganic Chemistry, Vol. 46, No. 1, 2007

Preparation and Characterization of Ni(II)-dpdpm Complexes
Bruker Av400 (1H at 400 MHz). The IR spectra were recorded
between 4000 and 400 cm-1 using KBr pellets on a Perkin-Elmer
Spectrum One spectrophotometer using the Spectrum v.3.01.00
software. The UV-vis-NIR spectra were recorded between 1300
and 250 nm with a 1 cm quartz cell on a Varian Cary 500i; baseline
correction was applied prior to recording the spectra. The magnetic
susceptibility measurements were carried out at room temperature
using the Gouy method with a Johnson Matthey Magnetic
Susceptibility Balance calibrated on [HgCo(SCN)4] samples. The
elemental analyses (C, H, and N) were performed in duplicate by
Laboratoire d’Analyse EÄ le´mentaire de l’Universite´ de Montre´al.
Syntheses. [(dpdpm)Ni(η2-NO3)2] (1). Solid Ni(NO3)2‚H2O
(0.45 g, 1.67 mmol) was added to a stirred solution of dpdpm (0.50
g, 1.67 mmol) in CH2Cl2 (40 mL), and the reaction mixture was
heated to reflux for 4 h. The final mixture was cooled to room
temperature, filtered, and evaporated to give a dark turquoise-green
solid. This was washed with hot hexane (2 × 40 mL) and extracted
into CH2Cl2 to remove insoluble impurities. Removal of the solvent
gave a turquoise-green solid (0.67 g, 83% yield). X-ray quality
single crystals (emerald green) were obtained by diffusion of Et2O
royal blue solid (0.60 g, 91% yield). X-ray quality single crystals
(navy blue) were obtained by diffusion of Et2O into a concentrated
CH2Cl2 solution. H NMR (CDCl3): δ 68.4 (br), 62.4 (br), 44.0
1
(br), 37.5 (br), 8.0-7.0 (br), 6.9 (br), plus sharp signals due to free
dpdpm at 7.69 (s), 7.55 (d, J ) 3), 7.37 (psq, J ) 8), 7.07 (d, J )
8), 6.30 (s). IR (KBr): ν (cm-1) 3402 (br), 3135 (w), 3121 (m),
3059 (m), 3035 (m), 2926 (w), 2854 (w), 2427 (w), 1996 (w), 1748
(w), 1631 (m), 1517 (m), 1493 (m), 1448-1438 (s), 1384 (s), 1339
(m), 1326 (m), 1305-1293 (w), 1249 (w), 1219 (m), 1195 (m),
1168 (m), 1112 (s), 1087 (m), 1066 (s), 1038 (w), 1001(w), 991
(w), 939 (w), 921 (w), 891 (w), 873 (w), 863 (w), 826 (w), 757-
748 (vs), 700 (vs), 659 (w), 638 (m), 618 (w), 607 (w), 511 (w).
µeff: 3.36 ΒΜ. mp: 145 °C. Anal. Calcd for C38H34N10O7Ni: C,
56.95; H, 4.28; N, 17.48. Found: C, 56.54; H, 4.26; N, 17.42.
[(dpdpm)2Ni (H2O)2][NO3]2 (4). Solid Ni(NO3)2‚6H2O (0.24 g,
0.83 mmol) was added to a stirred solution of dpdpm (0.50 g, 1.67
mmol) in MeOH (40 mL), and the reaction mixture was refluxed
for 18 h. The final mixture was cooled to room temperature, filtered,
and evaporated to give a dark blue solid. This was washed with
hot hexane (3 × 50 mL) and extracted into CH2Cl2 to remove
insoluble impurities. Removal of the solvent gave a dark blue solid
(0.56 g, 82% yield). X-ray quality single crystals (blue) were
obtained by diffusion of Et2O into a concentrated MeOH solution.
1H NMR (CDCl3): δ 65.7 (br), 52.7 (br), 45.3 (br), 49.0 (br), 7.7-
6.5 (br), plus signals corresponding to free dpdpm. IR (KBr): ν
(cm-1) 3390 (br), 3164 (m), 3130 (m), 33120 (m), 3056 (m), 2418
(w), 1655 (m), 1518 (m), 1492 (m), 1449-1385 (vs), 1332-1306
(vs), 1251(m), 1219 (m), 1189 (s), 1165 (m), 1109 (m), 1086 (m),
1063 (s), 1041 (m), 994 (w), 982 (w), 938 (w), 915 (w), 890 (w),
872 (w), 823 (w), 753 (vs), 698 (vs), 659 (w), 635 (m), 605 (w),
501 (w). µeff: 3.17 ΒΜ. mp: 195-205 °C. Anal. Calcd for
C38H36N10O8Ni: C, 55.70; H, 4.43; N, 17.09. Found: C, 55.25; H,
4.35; N, 17.03.
1
into a concentrated CH2Cl2 solution. H NMR (CDCl3): δ 62.05
(br), 44.43 (br), 7.21 (br), 6.87 (br). IR (KBr): ν (cm-1) 3429.62
(br), 3137.69 (m), 2506 (w), 1768.79 (m), 1522 (s), 1491 (vs), 1452
(s), 1434 (s), 1408 (s), 1384 (s), 1308 (s), 1278 (s), 1260(m), 1217
(w), 1194 (w), 1165(w), 1102 (s), 1087 (w), 1068 (s), 1016 (m),
1001 (w), 958 (w), 940 (w), 921 (w), 891 (w), 861 (w), 807 (w),
785 (m), 750 (s), 699 (s), 659 (s), 636 (w), 604 (w). mp: 240 °C.
µeff.: 3.22 BM. Anal. Calcd for C19H16N6O6Ni: C, 47.24; H, 3.34;
N, 17.40. Found: C, 47.28; H, 3.40; N, 17.00.
[(dpdpm)Ni(η1-NO3)(CH3CN)][NO3] (2). To a solid sample of
complex 1 (0.4 g, 0.828 mmol) was added a sufficiently small
amount of CH3CN (0.55 g, 13 mmol) to avoid complete dissolution
of the solid. The color of the “wet” solid changed immediately
from green to blue. Placing the “wet” blue solid and the residual
blue solution under vacuum for a brief period (1-2 min) gave a
blue solid (0.34 g, 78% yield). X-ray quality crystals (blue) of the
new product were grown by diffusion of Et2O into a saturated CH3-
CN solution. 1H NMR (CD3CN): δ 67.7 (br), 65.8 (br), 54.0 (br),
44.4 (br), 7.32 (br), 7.03 (br), 2.11 (br). (NB: The spectra of aged
solutions also displayed broadened signals for free dppm, and the
broad signal at 2.11 merged with the residual solvent peak.) IR
(KBr): ν (cm-1) 3401 (br), 3151 (m), 3064 (m), (w), 2314 (w),
2288 (w), (w), 1619 (m), 1492 (vs), 1450 (vs), 1435 (s), 1407(s),
1384(s), 1306 (vs), 1277(s), 1220 (m), 1194 (m), 1171(w), 1105
(m), 1085 (w), 1069 (s), (w), 1018 (w), 1000 (m) 939 (m), 921
(m), 890 (m), 871 (m), 808 (m), (s), 752 (vs), 698 (s), 658 (m),
636 (m), 605 (w). µeff:. 3.86 ΒΜ. mp: 238 °C. The elemental
analysis of this compound was problematic because thorough
evaporation of the sample resulted in an analysis identical to that
of complex 1 (i.e., loss of coordinated CH3CN). To minimize the
loss of CH3CN, a batch of crystals was heated for a few seconds
only in a 100 °C oven prior to the analysis, which gave results
consistent with the inclusion of one-half molecule of Et2O:
Found: C, 49.16; H, 3.98; N, 17.53. (Calcd for C21H19N7O6Ni: C,
48.12; H, 3.65; N, 18.71. Calcd for C21H19N7O6Ni + 1/2(C4H10O):
C, 49.23; H, 4.31; N, 17.47.)
Crystallographic Studies. The crystal data for compound 1 were
collected on an Enraf-Nonius CAD4 four-cycle goniometer at 298-
(2) K. The diffraction data were collected with graphite-mono-
chromated Cu KR radiation; the cell parameters were refined using
CAD-4 software on 25 reflections, while NRC-2 and NRC-2A were
used for the data reduction.20 The crystal data for 2, 3, and 4 were
collected on a Bruker AXS SMART 6K diffractometer mounted
with rotating anode Cu KR radiation at 293(2) K (SMART21
software). Cell refinement and data reduction were carried out using
SAINT.22 All structures were solved by direct methods using
SHELXS97,23 and the refinements were done on F2 by full-matrix
(20) (a) Ahmed, F. R.; Hall, S. R.; Pippy, M. E.; Huber, C. P. NRC
Crystallographic Computer Programs for the IBM/360. Accession Nos.
133-147. In J. Appl. Crystallogr. 1973, 6, 309. (b) Bruker (1997).
SHELXTL (1997). Release 5.10. The Complete Software Package for
Single Crystal Structure Determination. Bruker AXS Inc., Madison,
WI. (c) Enraf-Nonius (1989). CAD-4 Software. Version 5. Enraf-
Nonius, Delft, The Netherlands. (d) LePage, Y. J. Appl. Crystallogr.
1987, 20, 264-269. (e) Nonius (1998). Collect Software. Nonius B.V.,
Delft, The Netherlands. (f) Sheldrick, G. M. (1986). SHELXS86.
Program for Crystal Structure solution. University of Gottingen,
Germany. (g) Sheldrick, G. M. (1997a). SHELXS97. Program for
Crystal Structure solution. University of Gottingen, Germany. (h)
Sheldrick, G. M. (1997b). SHELXL97. Program for crystal structure
refinement. University of Gottingen, Germany. (i) Spek, A. L. (2000).
PLATON, 2000 version. Molecular Geometry Program. University
of Utrecht, Utrecht, Holland. (j) Gabe, E. J.; Le Page, Y.; Charlant, J.
P.; Lee, F. L.; White, P. S. J. Appl. Crystallogr. 1989, 22, 384.
(21) SMART (2001). Release 5.059. Bruker Molecular Analysis Research
Tool. Bruker AXS Inc., Madison, WI.
[(dpdpm)2Ni(H2O)(η1-NO3)][NO3] (3). Solid Ni(NO3)2‚H2O
(0.24 g, 0.83 mmol) was added to a stirred solution of dpdpm (0.50
g, 1.67 mmol) in CH2Cl2 (40 mL), and the reaction mixture was
heated to reflux for 4 h. The final mixture was cooled to room
temperature, filtered, and evaporated to give a royal blue solid. This
was washed with hot hexane (2 × 40 mL) and extracted into CH2-
Cl2 to remove insoluble impurities. Removal of the solvent gave a
(22) SAINT (2003). Release 6.06. Integration Software for Single Crystal
Data. Bruker AXS Inc., Madison, WI.
(23) Sheldrick, G. M. SHELXS, Program for the Solution of Crystal
Structures. University of Goettingen, Germany, 1997.
Inorganic Chemistry, Vol. 46, No. 1, 2007 301

Baho and Zargarian
Table 1. Crystallographic Data for Complexes 1-4
1
2‚0.5Et2O
3
4
formula
mol wt
C19H16N6O6Ni
483.09
C21H19N7O6Ni‚0.5C4H10O
561.20
C38H34N10O7Ni
801.46
C38H36N10O8N i
819.48
cryst color
cryst dimens, mm
symmetry
space group
a, Å
green
blue
blue
blue
0.26 × 0.15 × 0.14
monoclinic
P21/n
8.400(2)
14.861(5)
17.029(6)
90
102.17(2)
90
2078.0(11)
4
1.544
CAD-4
293(2)
1.5418
1.798
ω scan
992
0.24 × 0.26 × 0.3
0.33 × 0.26 × 0.13
monoclinic
P21/c
13.5458(8)
16.9361(9)
16.4455(9)
90
104.175(3)
90
3657.9(4)
4
0.16 × 0.14 × 0.12
monoclinic
P21/c
11.6537(2)
16.4081(3)
19.2521(3)
90
92.0130(10)
90
3679.02(11)
4
triclinic
P1h
8.4150(2)
9.4669(2)
16.6064(3)
76.1910(10)
85.0970(10)
86.125(2)
1278.47(5)
2
b, Å
c, Å
R, deg
â, deg
γ, deg
volume, Å3
Z
D(calcd), g cm-1
diffractometer
temp, K
l
1.458
1.455
1.479
Bruker AXS SMART 2K
200(2)
1.5418
1.569
ω scan
582
Bruker AXS SMART 2K
220(2)
1.5418
1.318
ω scan
1664
Bruker AXS SMART 2K
200(2)
1.5418
1.344
ω scan
1704
µ, mm-1
scan type
F(000)
θ
max, (deg)
h, k, l range
69.92
73.01
72.03
68.89
-10 e h e 10
-18 e k e 18
-20 e l e 20
3382
multiscan
SADABS
0.65, 0.79
0.0444, 0.1143
1.042
-10 e h e 10
-11 e k e 11
-20 e l e 20
6433
multiscan
SADABS
0.76, 0.65
0.0525, 0.1296
1.041
-16 e h e 15
-20 e k e 20
-19 e l e 20
6504
multiscan
SADABS
0.94,0.70
0.0506, 0.1238
1.052
-14 e h e 13
-18 e k e 19
-23 e l e 23
5518
multiscan
SADABS
0.80, 0.85
0.0325, 0.0924
1.036
reflns obsvd (I > 2σ (I))
absorption
correction
T (min, max)
R [F2 > 2σ (F2)], wR(F2)
GOF
least-squares.24 All non-hydrogen atoms were refined anisotropi-
cally. The positional parameters for H atoms in water molecules
were refined isotropically, but all other hydrogens were constrained
to the parent atom using a riding model.
were recorded for samples of 1 in CH2Cl2 and CH3CN. The
spectrum of the CH3CN solution of 1 was identical to that of
complex 2 in CH3CN. UV-vis-NIR spectra were also recorded
for CH2Cl2:CH3CN mixtures of various proportions (ca. 6.5 mM).
The narrowest absorptions in the spectra obtained for 100:0 and
0:100 samples (at 381 and 374 nm, respectively) were selected for
monitoring the solvent-induced interconversion of the two com-
plexes. The spectra for the 80:20, 60:40, 50:50, 40:60, and 20:80
samples showed an incremental shift in the peak maximum, as
shown in Figure 7.
The monodentate nitrate ligand of complex 2 showed some
disorder in the positions of the O5 and O55 atoms; the occupancies
of these atoms were initially refined and then fixed at 0.50. This
structure also contained disordered Et2O molecules, which were
refined isotropically using a constrained model; disordered solvent
molecules were then introduced over two positions and refined using
ISOR restrained technique. The crystal structures of 3 and 4 are
stabilized by intermolecular hydrogen bonds. The details on O-H‚
‚‚O distances are available from the detailed structure reports
(Supporting Information). For the sake of clarity, the disordered
Et2O molecules in 2 and the counter ions in 3 and 4 have been
removed from the ORTEP diagrams. All the details concerning the
refinement of the crystal structures are listed in Table 1.
Testing the Reversible Chromotropic Properties of Com-
plexes 1 and 2. A. Solvatochromic Behavior of Complexes 1
and 2. Dissolving ca. 15 mg of green complex 1 in ca. 1 mL of
CH2Cl2 gave a green solution. Evaporating this solution to dryness
gave back the green solid, which was dissolved in ca. 1 mL of
CH3CN to give a blue solution. Evaporation of CH3CN to dryness
gave back the original green solid. Redissolving the green solid in
CH3CN followed by evaporation resulted in the same color changes.
In order to confirm that the observed color changes correspond
to the interconversion of complexes 1 and 2, UV-vis-NIR spectra
B. Reversible Vapochromic Behavior of Complexes 1 and 2.
A small, uncapped vial containing 4 mg of complex 1 was placed
inside a 25 mL round-bottom flask containing ca. 3 mL of CH3-
CN; care was taken to make sure that the solid and the solvent did
not come in direct contact. The flask was then stoppered to allow
a build-up of solvent vapor at room temperature. The turquoise-
green color of complex 1 changed to light blue in 8 min. At this
point, the sample vial was taken out of the flask and placed in a
100 °C oven for 1 min, which caused the original turquoise-green
color of the sample to reappear; the sample did not show any visible
sign of degradation. Exposing the green solid to CH3CN vapors
resulted once again in a green-to-blue color change. Repeating this
procedure five times on the same sample showed the same
observations: samples of 1 obtained by heating 2 for only 1 min
at 100 °C required between 5 and 10 min (the average over five
trials being 7 min) of exposure to CH3CN vapors to undergo the
green-to-blue color change in a convincing fashion.
C. Reversible Thermochromic Behavior of Complexes 1 and
2. Dissolving a solid sample of turquoise-green 1 in CH3CN (ca.
0.02 M) gave a blue solution at ambient temperature. Heating this
solution to 65 °C resulted in a rapid color change to the green color
of CH2Cl2 solutions of 1. This color could be maintained over 45
min of heating without decomposition. Cooling the green solution
to 18 °C regenerated the initial blue color of the sample. Repeating
(24) (a) Bruker (1997). SHELXTL (1997). Release 5.10. The Complete
Software Package for Single Crystal Structure Determination. Bruker
AXS Inc., Madison, WI. (b) Bruker (1999a). SAINT (2003). Release
7.06. Integration Software for Single Crystal Data. Bruker AXS Inc.,
Madison, WI. (c) Bruker (1999b). SMART (2001). Release 5.625.
Bruker Molecular Analysis Research Tool. Bruker AXS Inc., Madison,
WI. (d) Sheldrick, G. M. (1996). SADABS, Bruker Area Detector
Absorption Corrections. (e) Bruker AXS Inc., Madison, WI.
302 Inorganic Chemistry, Vol. 46, No. 1, 2007

Preparation and Characterization of Ni(II)-dpdpm Complexes
Scheme 1
this heating-cooling cycle 10 times confirmed the reversible nature
of this thermochromic behavior. Evaporation of the solution
furnished the green complex 1 unchanged.
In a similar experiment, we recorded the variable temperature
(264-360 K) UV-vis-NIR spectra of a solution of 1 in CH3CN,
and the thermochromic behavior was monitored by following the
high-energy absorption peaks (ca. 370-380 nm). This experiment
showed that higher temperatures result in an incremental shift in
the band maximum of the monitored absorption, from ca. 370 nm
(for 2) to ca. 380 nm (for 1).
Contrary to our expectations, this product was neither
[(dpdpm)2Ni(η2-NO3)][NO3] nor [(dpdpm)2Ni(η1-NO3)2].
Indeed, the final outcome of the reaction depended
on the solvent of recrystallization, producing [(dpdpm)2-
Ni(η1-NO3)(H2O)], 3, from acetone, or [(dpdpm)2Ni(H2O)2],
4, from CH2Cl2, CH3CN, or MeOH. Complexes 3 and 4 were
studied by spectroscopy and X-ray diffraction studies (vide
infra).
The formation of 3 and 4 indicates that the nitrate ligand
in these bis(dpdpm) species is very labile and can be
displaced readily by residual water. In an effort to prevent
the displacement of the nitrate ligands by water, the reaction
of Ni(NO3)2 with 2 equiv of dpdpm was carried out in dry
CH2Cl2 under an atmosphere of dry nitrogen. This reaction
gave a lilac solid that proved to be very sensitive to water:
exposure of this new product to air for a few minutes
converted it to complex 3. We were unable to identify the
lilac solid, but its facile hydration to 3 suggests it might be
one of the anticipated precursors to 3, i.e., [(dpdpm)2Ni(η2-
NO3)][NO3] or [(dpdpm)2Ni(η1-NO3)2].26
Results and Discussion
Syntheses. The reaction of Ni(NO3)2‚H2O with 1 equiv
of dpdpm in CH2Cl2 under reflux over 4 h afforded a green
solution, which was evaporated to give complex 1 as a
turquoise-green solid (Scheme 1). The same result can be
obtained from the room temperature reaction during 18 h.
Washing the turquoise-green solid with hot hexane to remove
any excess of unreacted ligand, followed by extraction into
CH2Cl2 to eliminate unreacted Ni(NO3)2·6H2O, yielded an
analytically pure product. Recrystallization from CH2Cl2/Et2O
gave emerald green crystals that were subjected to spectral
analysis and X-ray diffraction studies (vide infra). Bis(η2-
NO3) complexes of Ni similar to 1 have been reported.25
The analogous reaction of Ni(NO3)2·6H2O with 2 equiv
of dpdpm gave, after workup and purification, a blue solid.
(25) (a) Głwiak, T.; Kurdziel, K. J. Mol. Struct. 2000, 516, 1. (b) Carr, P.;
Piggott, B.; Tinton, H. J. Acta Crystallogr., Sect. C 1985, 41, 372. (c)
Turpeinen, E. Suom. Kemistil. B 1973, 46, 208. (d) Diamantopopoulou,
E.; Zafiropoulos, T. F.; Perplepes, S. P.; Raptopoulou, C. P. Terzis.
A. Polyhedron 1994, 13, 1593. (e) Claramunt, R. M; Domiano, P.;
Elguero, J.; Lavandera, J. L. Bull. Soc. Chim. Fr. 1989, 472.
Inorganic Chemistry, Vol. 46, No. 1, 2007 303

Baho and Zargarian
Table 2. Bond Distances and Angles for Complexes 1-4
1
2
3 (X ) O11)
4 (X ) O2)
Ni-N11
2.027(2)
1.994(2)
2.080(2)
2.113(2)
2.081(2)
2.093(2)
89.61(9)
105.89(9)
167.40(9)
97.83(9)
93.28(9)
102.42(9)
91.91(9)
97.75(9)
159.56(9)
61.58(9)
148.73(9)
96.23(9)
94.35(10)
89.65(9)
61.81(8)
-
2.0284(16)
2.0329(16)
2.1293(15)
2.1017(14)
2.0591(16)
-
88.81(7)
103.30(7)
164.25(7)
86.80(7)
-
94.06(6)
90.45(6)
91.00(7)
-
61.06(6)
168.78(6)
-
108.95(6)
-
-
2.0650(18)
92.92(7)
178.21(7)
86.03(7)
88.03(7)
88.60(7)
Ni-N11
2.0630(13)
2.0841(14)
2.0952(14)
2.0567(14)
2.0724(12)
2.1409(13)
86.81(5)
96.65(5)
101.37(5)
175.00(5)
97.31(6)
85.58(5)
170.79(5)
81.83(5)
87.66(5)
87.17(6)
88.44(5)
89.73(6)
86.63(5)
174.59(6)
90.55(5)
-
2.0593(14)
2.0806(14)
2.0971(13)
2.0686(13)
2.1273(12)
2.0925(11)
86.24(5)
94.68(5)
99.46(5)
177.85(5)
95.03(5)
86.74(5)
173.67(5)
88.25(5)
90.95(5)
86.91(5)
87.94(5)
91.18(5)
86.43(5)
172.15(5)
85.93(5)
-
Ni-N21
Ni-N21
Ni-O1
Ni-N31
Ni-O2
Ni-N41
Ni-O4
Ni-O1
Ni-O5
Ni-X
N11-Ni-N21
N11-Ni-O1
N11-Ni-O2
N11-Ni-O4
N11-Ni-O5
N21-Ni-O1
N21-Ni-O2
N21-Ni-O4
N21-Ni-O5
O1-Ni-O2
O1-Ni-O4
O1-Ni-O5
O2-Ni-O4
O2-Ni-O5
O4-Ni-O5
Ni-N3
N11-Ni-N21
N11-Ni-N31
N11-Ni-N41
N21-Ni-N31
N21-Ni-N41
N31-Ni-N41
N11-Ni-O1
N11-Ni-X
N21-Ni-O1
N21-Ni-X
N31-Ni-O1
N31-Ni-X
N41-Ni-O1
N41-Ni-X
O1-Ni-X
-
N11-Ni-N3
N21-Ni-N3
N3-Ni-O1
N3-Ni-O2
N3-Ni-O4
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
A number of observations have indicated that all the
ligands in 3 and 4 are fairly labile and can be displaced by
coordinating solvents. Thus, the UV-vis-NIR spectra of 3
and 4 displayed small but significant variations as a function
of solvent (vide infra), while their NMR spectra in CD3CN
led to a gradual broadening of the original peaks and gave
rise to signals for free dpdpm (vide infra). Unfortunately,
however, we were unable to intercept and isolate any solvato
derivatives of 3 or 4. In contrast, complex 1 reacted readily
with water, MeOH or CH3CN to give blue solvato deriva-
tives, and we succeeded in isolating and characterizing the
acetonitrile derivative [(dpdpm)Ni(η2-NO3)(η1-NO3)(CH3-
CN)], 2 (vide infra).
Figure 1. ORTEP view of complex 1. Thermal ellipsoids are shown at
30% probability.
Finally, we were unable to prepare the homoleptic complex
[Ni(dpdpm)3][NO3]2 from the reaction of excess dpdpm (>5
equiv) with Ni(NO3)2, 3, or 4, whereas reacting 1 with excess
dpdpm gave 3 (Scheme 1). The precise reasons for our failure
to prepare a tris(dpdpm) complex are not known, but we
can rule out steric or enthalpic factors since we have observed
the formation of [Ni(dpdpm)3]2+ from NiI2.27
Crystal Structures. Single crystals of 1, 2, 3, and 4 were
obtained by vapor diffusion of Et2O into solutions of these
complexes in CH2Cl2, acetonitrile, methanol, and acetone,
respectively. To ensure that the crystals retain their integrity
throughout the data collection period, they were coated by
either Paratone-N oil or epoxy glue and mounted rapidly.
All four sets of diffraction data resulted in fairly accurate
structures for the complexes studied, as reflected in the R1
values of ca. 0.0444 (1), 0.0525 (2), 0.0506 (3), and 0.0325
(4). The crystal data and details of data collection are listed
in Table 1, the principal geometric parameters are listed in
Table 2, and ORTEP III28 molecular structures are illustrated
in Figures 1-4.
All four complexes adopt variously distorted octahedral
structures. The small bite angle of the bidentate nitrate
ligands causes the largest distortions from ideal octahedral
geometry in complexes 1 and 2: cis-O-Ni-O angles are
ca. 62°, while trans-O-Ni-O and trans-O-Ni-N angles
are ca. 149-167°. Further distortions are introduced by the
significantly different Ni-O bond distances in 1 (Ni-O1∼
Ni-O4 < Ni-O5 < Ni-O2) and 2 (Ni-O4 < Ni-O2 <
Ni-O1). By comparison, the bis(dpdpm) species 3 and 4
show somewhat less pronounced angular distortions because
(26) Two structurally related mono(η2-NO3) compounds have been reported
recently: [(bpm*)2Ni(η2-NO3)][NO3] (ref 10) and [(pz2thCH)2Ni(η2-
NO3)][NO3] (pz2thCH ) bis(pyrazolyl)(2-thienyl)methane; Astley, T.;
Hitchman, M. A.; Skelton, W. B.; White, A. H. Aust. J. Chem. 1997,
50, 145.)
(27) The preparation and full characterization of this compound and related
species arising from the reaction of nickel halides will be the subject
of an upcoming report.
(28) Burnett, M. N.; Johnson, C. K. (1996). ORTEPIII - Oak Ridge Thermal
Ellipsoid Plot Program for Crystal Structure Illustrations, Technical
Report ORNL-6895. Oak Ridge National Laboratory, TN.
304 Inorganic Chemistry, Vol. 46, No. 1, 2007

Preparation and Characterization of Ni(II)-dpdpm Complexes
other hand, since the difference between Ni-N11 and Ni-
N41 in 3 is statistically insignificant ((3σ), we conclude
that water and η1-NO3 possess very similar trans influences.
Finally, inspection of Ni-Npyrazolyl distances in the structure
of 2 indicates that CH3CN and η2-NO3 possess fairly similar
trans influence values. Combining these observations gives
the following trans influence order: pyrazolyl ∼ H2O ∼ η1-
ONO2 > NCMe ∼ η2-O2NO.
Magnetic and Spectroscopic Studies. Magnetic suscep-
tibility measurements using the Gouy method have estab-
lished that µeff for complexes 1, 3, and 4 are between 3.1
and 3.4 ΒΜ, whereas that of complex 2 is 3.9 ΒΜ. These
magnetic susceptibilities are somewhat higher than the
corresponding values obtained for our previously reported
bis- and tris-(pyrazolyl)methane complexes of nickel (2.8-
3.3 ΒΜ);10 nevertheless, all of these values are in the
expected range for octahedral Ni(II) complexes,29 and they
indicate the presence of two unpaired electrons.
Figure 2. ORTEP view of complex 2. Thermal ellipsoids are shown at
30% probability. Only one of the two positions (0.50:0.50) is shown for
the disordered O5 atom. The disordered Et2O molecule has been omitted
for clarity.
1
The H NMR spectra of these complexes display broad
and, for the most part, featureless signals that serve primarily
as fingerprints for identifying this family of complexes and
provide little structural information. We believe that the
broadness of the signals is caused primarily by the para-
magnetism of the compounds, but this family of bis-
(pyrazolyl)alkane ligands can also undergo a dynamic
exchange process involving the flip of the M-N-N-C-
N-N ring between chair and boat conformations. Moreover,
in the NMR spectra of complexes 4 and 2 the ligand peaks
appear to be somewhat broader when the spectra are run in
CD3CN. Since the peak due to residual solvent signal (CHD2-
CN) is also fairly broadened, we suspect that these observa-
tions are due to a ligand exchange process whereby solvent
molecules displace dpdpm ligand from the coordination
sphere of the complexes. It should be added, however, that
no dpdpm-free complexes have been obtained in the solid
state.
Figure 3. ORTEP view of complex 3. Thermal ellipsoids are shown at
30% probability. The NO3 counterion has been omitted for clarity.
-
The IR spectra of the complexes show a large number of
absorptions. The very strong signals observed at 1491 and
1278 cm-1 in the IR spectrum of 1 are tentatively attributed
to the η2-NO3 moiety. For comparison, the corresponding
absorptions in the analogous complex [(bpm*)2Ni(η2-NO3)]-
[NO3] were found at 1480 and 1290 cm-1.10,30 The complexes
bearing monodentate nitrate ligands give rise to sharp peaks
at 1448-1438 and 1384 cm-1 (3) and at 1450-1435 and
1384 (2). Finally, the nitrate counter anions in 4 gave rise
to absorptions at 1449-1385 and 1332-1306 cm-1.
The different colors of the complexes under discussion
(green for 1, blue for 2, 3, and 4) hinted at energetic
differences in the electronic transitions for these d8 com-
plexes. Although Laporte rules stipulate that d-d transitions
are forbidden in perfectly octahedral complexes, the geo-
metrical distortions present in our compounds would be
Figure 4. ORTEP view of complex 4. Thermal ellipsoids are shown at
30% probability. The NO3 counter ions have been omitted for clarity.
-
of the absence of a chelating nitrate in these complexes and
the fact that the two dpdpm ligands in these compounds adopt
fairly regular cis and trans N-Ni-N angles (∼86-102° and
175-178°).
Inspection of the Ni-Onitrate distances in the structures of
1 and 2 supports our previously proposed10 trans influence
orders of pyrazolyl > η2-O2NO and η1-ONO2 > η2-O2NO.
For instance, Ni-O1, Ni-O4 < Ni-O2, Ni-O5 in 1, and
Ni-O4 < Ni-O1 in 2. In addition, the pyrazolyl ligand also
appears to have a somewhat greater trans influence than η1-
NO3 and water: Ni-N21, Ni-N31 > Ni-N11, Ni-N41
in 3; Ni-N21, Ni-N31 > Ni-N11, Ni-N41 in 4. On the
(29) Collinson, S. R.; Schro¨der, M. Nickel: Inorganic and Coordination
Chemistry. In Encyclopedia of Inorganic Chemistry, 2nd ed.; King,
R. B., Ed.; Wiley InterScience: 2006; Vol. VI.
(30) For a general discussion of NO3 stretching frequencies, see: Nakamoto,
K. Raman Spectra of Inorganic and Coordination Compounds. Part
B: Applications in Coordination, Organometallic and Bioinorganic
Chemistry, 5th ed.; Wiley: 1997; Chapter 8, pp 87-88.
Inorganic Chemistry, Vol. 46, No. 1, 2007 305

Baho and Zargarian
excited states commonly observed for essentially octahedral
nickel(II) complexes. Weak features due to spin forbidden
3
1
transitions A2g f Eg also occur at energies intermediate
between the A2g f T2g and A2g f T1g(3F) transitions.33
3
3
3
3
The observation of this spin-forbidden transition is thought
3
3
to be facilitated by the close proximity of A2g f T2g and
3A2g f Eg transitions, which leads to a strong mixing
1
through spin-orbit coupling and an increase of the intensity
and bandwidth for the spin-forbidden transition.12
Qualitative analysis of the electronic spectra for these
complexes in different solvents allows us to speculate on
possible solvent-Ni interactions. For instance, the transition
energies are quite similar in the acetone and CH2Cl2 spectra,
but fairly different in the MeOH spectra, especially in the
case of complexes 3 and 4 (Figures 5b and 5c in Supporting
Information); these observations hint at possible MeOH-
Ni interactions in these complexes. As mentioned earlier,
however, no MeOH adducts have been isolated in the solid,
implying that the aquo adducts crystallize more readily.
Moreover, the similarity of the spectra recorded for acetone
and CH2Cl2 samples of 1 (Figure 5) and 2 (Figure 5a in
Supporting Information) suggests that the coordinated CH3-
CN in complex 2 likely dissociates in these solvents, thus
forming 1.
Figure 5. UV-vis-NIR spectra of complex 1 in different solvents.
The well-resolved bands for the three main transitions have
allowed us to determine the 10Dq value and Racha param-
eter, B, for each complex based on the following
equations:34
10Dq ) E(3A2g f 3T2g) ) ν1
(1)
15B ) E(3A2g f 3T1g (3F)) + E(3A2g f 3T1g (3P)) - 3Dq
) ν2 + ν3 - 3ν1 (2)
Figure 6. The UV-vis-NIR spectra of complexes 1, 2, 3, and 4 in
All calculated values for 10Dq and B in various solvents
are listed in Table 3. The 10Dq parameters (∆) for 1-4 are
remarkably similar for all four complexes (e.g., 10805-
10870 cm-1 for spectra of acetone solutions). Interestingly,
the largest 10Dq and B values are found in acetone spectra.
In order to establish a possible correlation between structural
features and crystal field parameters, we have compiled a
list of these parameters for a range of octahedral Ni(II)
complexes in Table 4. Comparison of this data reveals
similarly uniform 10Dq values (within 500 cm-1) for the
analogous complexes of Ni bearing bpm* and tpm* ligands,12
implying that within a family of closely related compounds
the 10Dq value is fairly insensitive to structural variations
such as the nature of ligator atoms (N:O ratio), the bonding
mode of the nitrate ligand, and the overall charge. Indeed,
the only complexes that give rise to significantly different
10Dq values are [Ni(H2O)6]2+ and [Ni(o-phenanthroline)3]2+.
On the other hand, greater variations are observed in the
acetonitrile.
expected to facilitate weak d-d transitions. UV-vis spec-
troscopy was, therefore, used to delineate the differences in
the electronic states of complexes 1-4. Given the possibility
of geometrical changes in solution upon solvent coordination,
the UV-vis-NIR spectra were recorded in acetone, CH2-
Cl2, CH3CN, and MeOH (Figure 5 for complex 1, and
Figures 5a-c in Supporting Information for complexes 2-4).
As expected for octahedral d8 complexes,31 complexes 1-4
displayed three main bands featuring low molar absorptivities
at around 1000, 650, and 380 nm; a weak shoulder was also
detected at about 700-800 nm in most spectra.32 Table 3
lists the band maxima and molar absorptivities for all four
complexes. All bands have been assigned from the Tanabe-
Sugano diagram for octahedral complexes, as follows. The
three main bands represent spin allowed (d-d) transitions
from the ground state 3A2g to the 3T2g, 3T1g(3F), and 3T1g(3P)
(31) Sacconi, L.; Mani, F.; Bencini, A. Nickel. In ComprehensiVe
Coordination Chemistry; Wilkinson, G., Gilard, R., McCleverty, J.,
Eds.; Pergamon Press: Oxford, 1987; Vol. 5, p 58.
(32) Very similar “weak shoulders” have been observed in the absorption
spectra of [Ni(NH3)6]2+. For a discussion, see: Triest, M.; Bussie`re,
G.; Be´lisle, H.; Reber, C. J. Chem. Educ. 2000, 77, 670. Available at
http://jchemed.chem.wisc.edu/jcewww/articles/JCENi/JCENi.html.
(33) For a description of this phenomenon, including detailed analyses of
these transitions based on Tanabe-Sugano diagrams and modern
theoretical models, see: Nolet, M.-C.; Beaulac, R.; Boulanger, A.-
M.; Reber, C. Struct. Bonding 2004, 107, 145.
(34) (a) Reedijk, J.; Van Leeuwen, P. W. N. M.; Groeneveld, W. L. Recl.
TraV. Chim. Pays-Bas 1968, 87, 129. (b) Ko¨nig, E. Struct. Bonding
1971, 9, 175.
306 Inorganic Chemistry, Vol. 46, No. 1, 2007

Preparation and Characterization of Ni(II)-dpdpm Complexes
Table 3. Absorption Energies, Molar Absorptivities, and Crystal Field Parameters for Spin-Allowed Bands of UV-Vis-NIR Spectra for Complexes
1-4 in Different Solventsa
E (cm-1), ꢀ (M-1 cm-1
)
complex
solvent
ν1 ) 10Dq
Eg
ν2
ν3
B (cm-1
)
10Dq/B
1
acetone
CH2Cl2
CH3CN
MeOH
acetone
CH2Cl2
CH3CN
MeOH
acetone
CH2Cl2
CH3CN
MeOH
acetone
CH2Cl2
CH3CN
MeOH
10784, 8
10639, 8
10633, 9
10595, 6
10786, 8
10685, 7
10537, 8
10391, 4
10835, 15
10820, 13
10883, 13
10485, 7
10805, 11
10779, 9
10744, 11
10614, 6
12946, 6
12892, 5
13122, 5
13378, 4
12743, 6
12781, 4
13205, 4
13360, 3
12817, 8
-
13839, 3
12921, 3
13231, 6
-
13989, 4
13280, 2
16043, 17
16001, 18
16588, 18
16638, 9
16315, 12
16075, 11
16483, 17
16531, 7
16605, 20
16803, 14
17249, 14
16805, 9
16437, 18
16643, 13
16998, 15
16770, 9
26395, 28
26268, 30
26894, 32
26669, 16
26650, 18
26593, 20
26706, 24
26355, 10
26986, 32
27122, 32
27625, 21
26776, 13
26948, 33
27097, 28
27610, 27
27210, 17
672
690
772
767
707
707
772
781
739
764
815
808
731
760
824
809
16.0
15.4
13.8
13.8
15.2
15.1
13.6
13.3
14.7
14.1
13.4
12.97
14.8
14.0
13.0
13.1
2
3
4
a The spectra were recorded at room temperature. 10Dq and B values were calculated from eqs 1 and 2 in the text.
Table 4. Crystal Field Parameters for 1-4 and Related Compounds
compound
[Ni(H2O)6]2+
N:O
10Dq (cm-1
)
B (cm-1
)
10Dq/B
ref
0:6
2:4
3:3
3:3
4:2
4:2
4:2
5:1
6:0
6:0
6:0
6:0
8580
10633
10537
10300
10170
10883
10744
10420
11670
10650
10730
12690
929
772
772
837
869
815
824
891
786
843
830
710
9.2
13.8
13.6
12.3
11.7
13.4
13.0
11.7
14.8
12.6
12.9
17.9
32
complex 1
complex 2
this work
this work
(tpma)Ni(η2-NO3)(η1-NO3 )
[(bpma)2Ni(η2-NO3)]+
complex 3
12
12
this work
this work
complex 4
[(bpma)(tpma)Ni(η1-NO3)] +
[Ni(tpma)2]2+
12
12
36
32
33
[Ni(pyrazol)6]2+
[Ni(NH3)6]2+
[Ni(o-phenanthroline)3]2+
a All spectra have been recorded for acetonitrile solutions.
Scheme 2
10Dq/B ratios for all the complexes (Table 4). Thus, the
values of ca. 13-16 for complexes 1-4 are intermediate
between the corresponding values reported for the homoleptic
dicationic complexes [NiL6]2+ bearing strong-field ligands
such as o-phenanthroline (17.9)33 and weak-field ligands such
as water (9.2).35 Nevertheless, there appears to be no simple
correlation between the 10Dq/B ratio and the structural
parameters.
Solvato-, Vapo-, and Thermochromic Properties of
Complexes 1 and 2. As mentioned earlier, dissolving 1
(green) in CH3CN gave a blue species that was identified as
the CH3CN adduct 2 arising from the η2 f η1 hapticity
change of a nitrate ligand. A series of experiments established
that the interconversion of 1 and 2 is reversible (Scheme 2).
Thus, evaporation of blue solutions obtained by dissolving
1 in CH3CN gave back green samples of 1, which produced
blue solutions when redissolved in CH3CN. This process
could be repeated several times. UV-vis spectroscopy
showed that the presence of greater proportions of CH3CN
in CH2Cl2 solutions of 1 resulted in an incremental blue-
shift in the high-energy band maximum, from ca. 381 nm
for 1 (100% CH2Cl2) to ca. 374 nm for 2 (80% CH3CN), as
shown in Figure 7. Similar green-to-blue color changes were
noted in CH2Cl2/MeOH and acetone/water mixtures, but the
new species arising from the interaction of MeOH and water
with the Ni center in 1 have not been identified.
(35) Bussie`re, G.; Reber, C. J. Am. Chem. Soc. 1998, 120, 6306.
(36) Reimann, C. W. J. Phys. Chem. 1970, 74, 561.
Inorganic Chemistry, Vol. 46, No. 1, 2007 307

Baho and Zargarian
reversible interconversion of 1 and 2 took place over several
cycles without degradation.
Conclusion
The preparation and characterization of complexes 1-4
has allowed us to study the structural and spectroscopic
properties of a new series of Ni(II) complexes bearing the
dpdpm ligand. The solid state structures of these complexes
are fairly similar to those of the analogous bpm* complexes
reported earlier,10 implying that the different sterics of these
two ligands have little or no influence over the structures
adopted by their complexes. These closely related complexes
also display fairly comparable UV-vis-NIR spectra, imply-
ing that the electronic structures of this family of complexes
are not very sensitive to subtle structural differences.
The most intriguing aspect of the present study is the
solvato-, thermo-, and vapochromism of complex 1, which
is caused by the reversible coordination of acetonitrile to
the Ni center in 1 and the resulting conversion of one of the
η2-NO3 to η1-NO3. That the formation of complex 1 is
favored at higher temperature is presumably due to entropic
factors that tend to favor the chelation of the nitrate ligand
and the dissociation of a molecule of acetonitrile. Future
investigations will probe the vapo- and thermochromic
properties of thin films produced from polymer matrices
containing complex 1.
Figure 7. Shift in the absorption wavelength of the high-energy band in
the UV-visible spectra of 1 as a function of solvent composition (CH2-
Cl2:CH3CN).
The temperature-dependence of the CH3CN-induced η2 f
η1 hapticity change of one of the nitrate ligands in 1 was
studied briefly, as follows. The blue solution obtained by
dissolving a sample of 1 in CH3CN was heated gradually to
ca. 80 °C and its color was monitored. A gradual, blue-to-
green color change was observed up to ca. 65 °C, at which
point the green color was dominant. Further heating to ca.
80 °C only deepened the green color, whereas allowing the
solution to cool to room temperature re-established the
original blue color. Repeating this heating/cooling cycle 10
times showed that the heat-induced interconversion of 1 and
2 was reversible. On the other hand, variable temperature
UV-vis spectra of a CH3CN solution of 1 confirmed that
complexes 1 and 2 are in equilibrium, the major species being
complex 2 below 0 °C (band maximum for ν3 at 372 nm)
and complex 1 above 70 °C (band maximum for ν3 at 378
nm).
That this CH3CN-induced η2 f η1 slippage of one of the
nitrate ligands in 1 can take place in solutions raised the
question of whether or not this process might take place by
exposure of solid samples to CH3CN vapor. Tests showed
that green samples of 1 turned blue when exposed to CH3-
CN vapor, while the resulting blue solid (or independently
prepared samples of 2) turned green when heated briefly
(1-2 min at ca. 100 °C) or placed under vacuum. This
Acknowledgment. The Natural Sciences and Engineering
Research Council of Canada and Fonds Que´be´cois de la
Recherche sur la Nature et les Technologies are gratefully
acknowledged for their financial support. We are grateful
to Profs. G. Hanan and C. Reber and their research groups
for their valuable help with the UV-vis-NIR spectra.
Supporting Information Available: UV-vis spectra of com-
plexes 2-4 in different solvents (Figures 5a-c). This material is
available free of charge via the Internet at http://pubs.acs.org.
Crystallographic data for the structural analysis have been deposited
with the Cambridge Crystallographic Data Centre, CCDC No.
614549 (1), 614551 (2), 614552 (3), 614550 (4). Copies of
this information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.
(Fax: +44-1223-336-033. E-mail: deposit@ccdc.cam.ac.uk or
www: http://www.ccdc.cam.ac.uk).
IC061311Z
308 Inorganic Chemistry, Vol. 46, No. 1, 2007