Dependence of the chemical properties of macrocyclic [NiII 2L(μ-O2CR)]+ complexes on the basicity of the carboxylato coligands (L2- = macrocyclic N6S 2 ligand)

Article abstract of DOI:10.1021/ic101574a

The dependence of the properties of mixed ligand [Ni^II ₂L(μ-O₂CR)]⁺ complexes (where L^2- represents a 24-membered macrocyclic hexaamine-dithiophenolato ligand) on the basicity of the carboxylato colig

Full text of DOI:10.1021/ic101574a

11018 Inorg. Chem. 2010, 49, 11018–11029
DOI: 10.1021/ic101574a
Dependence of the Chemical Properties of Macrocyclic [NiII L( μ-O2CR)]þ
2
Complexes on the Basicity of the Carboxylato Coligands
(L2- = macrocyclic N6S2 ligand)
‡
‡
†,||
Ulrike Lehmann,† Julia Klingele,‡ Vasile Lozan,†,§ Gunther Steinfeld, Marco H. Klingele, Steffen Kass,
€
Axel Rodenstein,† and Berthold Kersting*,†
†
€
Institut fu€r Anorganische Chemie, Universitat Leipzig, Johannisallee 29, D-04103 Leipzig, Germany, and
‡
€
Institut fu€r Anorganische und Analytische Chemie, Albert-Ludwigs-Universitat Freiburg, Albertstr. 21,
D-79104 Freiburg, Germany. §Present address: Laboratory of Coordination Chemistry, Institute of Chemistry,
Academy of Sciences of Moldova, Academiei str. 3, MD-2028 Chisinau, Republic of Moldova. ||Present address:
Avancis GmbH & Co. KG, D-04860 Torgau, Germany
Received August 4, 2010
The dependence of the properties of mixed ligand [NiII L( μ-O2CR)]þ complexes (where L2- represents a 24-
membered macrocyclic hexaamine-dithiophenolato ligand) on the basicity of the carboxylato coligands has been
2
examined. For this purpose 19 different [NiII L( μ-O2CR)]þ complexes (2-20) incorporating carboxylates with pKb
2
values in the range 9 to 14 have been prepared by the reaction of [NiII L( μ-Cl)]þ (1) and the respective sodium or
2
triethylammonium carboxylates. The resulting carboxylato complexes, isolated as ClO4- or BPh4- salts, have been
fully characterized by elemental analyses, IR, UV/vis spectroscopy, and X-ray crystallography. The possibility of
accessing the [NiII L(μ-O2CR)]þ complexes by carboxylate exchange reactions has also been examined. The main
findings are as follows: (i) Substitution reactions between 1 and NaO2CR are not affected by the basicity or the steric
2
hindrance of the carboxylate. (ii ) Complexes 2-20 form an isostructural series of bisoctahedral [NiII L( μ-O2CR)]þ
2
compounds with a N3Ni( μ-SR)2( μ-O2CR)NiN3 core. (iii ) They are readily identified by their νas(CO) and
νs(CO) stretching vibration bands in the ranges 1684-1576 cm-1 and 1428-1348 cm-1, respectively. (iv) The
spin-allowed 3A2g f 3T2g (ν1) transition of the NiOS2N3 chromophore is steadily red-shifted by about 7.5 nm per pKb
unit with increasing pKb of the carboxylate ion. (v) The less basic the carboxylate ion, the more stable the complex. The
stability difference across the series, estimated from the difference of the individual ligand field stabilization energies
(LFSE), amounts to about 4.2 kJ/mol [ΔLFSE(2,18)]. (vi) The “second-sphere stabilization” of the nickel complexes is
not reflected in the electronic absorption spectra, as these forces are aligned perpendicularly to the Ni-O bonds. (vii )
Coordination of a basic carboxylate donor to the [NiII L]2þ fragment weakens its Ni-N and Ni-S bonds. This bond
2
weakening is reflected in small but significant bond length changes. (viii) The [NiII L( μ-O2CR)]þ complexes are
2
relatively inert to carboxylate exchange reactions, except for the formato complex [NiII L( μ-O2CH)]þ (8), which reacts
with both more and less basic carboxylato ligands.
2
Introduction
decades,1-3 especially as supporting ligands for the prepara-
tion of model compounds for dinuclear metalloenzymes,4 for
the stabilization of unusual metal oxidation states,5 and to
understand, achieve, and control dinuclear metal reactivity.6,7
Dinucleating Robson-type amino-thiophenolato ligands
have received considerable attention over the past several
*To whom correspondence should be addressed. E-mail: b.kersting@
uni-leipzig.de.
(4) Atkins, A. J.; Black, D.; Blake, A. J.; Marin-Becerra, A.; Parsons, S.;
€
Ruiz-Ramirez, L.; Schroder, M. J. Chem. Soc., Chem. Commun. 1996,
(1) Pilkington, N. H.; Robson, R. Aust. J. Chem. 1970, 23, 2225–2236.
457–464.
€
(2) (a) Atkins, A. J.; Blake, A. J.; Schroder, M. J. Chem. Soc., Chem.
(5) (a) Brooker, S.; Croucher, P. D.; Davidson, T. C.; Dunbar, G. S.;
Commun. 1993, 1662–1665. (b) Branscombe, N. D. J.; Blake, A. J.; Marin-
McQuillan, A. J.; Jameson, G. B. J. Chem. Soc., Chem. Commun. 1998,
€
Becerra, A.; Li, W.-S.; Parsons, S.; Ruiz-Ramirez, L.; Schroder, M. J. Chem.
€
2131–2132. (b) Spencer, D. J. E.; Marr, A. C.; Schroder, M. Coord. Chem. Rev.
Soc., Chem. Commun. 1996, 2573–2574.
2001, 219-221, 1055–1074.
(3) (a) Brooker, S.; Croucher, P. D. J. Chem. Soc., Chem. Commun. 1995,
1493–1494. (b) Brooker, S.; Croucher, P. D. J. Chem. Soc., Chem. Commun.
1995, 2075–2076. (c) Brooker, S.; Croucher, P. D.; Roxburgh, F. M. J. Chem.
Soc., Dalton Trans. 1996, 3031–3037. (d) Brooker, S.; Croucher, P. D. J. Chem.
Soc., Chem. Commun. 1997, 459–460.
(6) Gavrilova, A. L.; Jin Qin, C.; Sommer, R. D.; Rheingold, A. L.;
Bosnich, B. J. Am. Chem. Soc. 2002, 124, 1714–1722.
(7) (a) Brooker, S. Coord. Chem. Rev. 2001, 222, 33–56. (b) Brooker, S.;
Croucher, P. D.; Davidson, T. C.; Dunbar, G. S.; Beck, C. U.; Subramanian, S.
Eur. J. Inorg. Chem. 2000, 169–179.
r
pubs.acs.org/IC
Published on Web 11/10/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 11019
Chart 1
MII = Mn, Fe, Co, Ni, Zn,23 Cd24). The third series com-
prises isostructural zinc(II) carboxylato complexes,
[ZnII2L(μ-O2CR)]þ, differing only in the basicity of the
carboxylato coligand.17 From these studies, we became very
familiar with the coordination chemistry of the amino-
thiophenolato ligand L2- and its various derivatives25 and
have been able to derive magneto-structural9,26 and struc-
ture-reactivity relationships.11,12
As part of this program, we sought to extend our explora-
tion to the syntheses of dinuclear nickel(II) complexes in-
corporating carboxylato coligands of different basicity. Our
motivation was based on the following reasons: (i) Carbox-
ylato ligands are of general importance in coordination
chemistry.27 (ii) Carboxylate ions are biologically relevant
molecules.28-33 (iii) Nickel carboxylate bonding is of im-
portance in bioinorganic34,35 and biomimetic chemistry.36,37
(iv) A systematic study of an isostructural series of carbox-
ylato-bridged nickel(II) complexes has not appeared in the
literature.38 (v) The characterization data of carboxylato-
bridged [NiII2L( μ-O2CR)]þ complexes may serve as invalu-
able references for future studies aimed at regioselective
carboxylate transformations.12,39-41 (vi) Since the immediate
environment of the carboxylate group in the dinuclear com-
plexes is essentially the same, these systems should be well
suited for the study of complex properties as a function of
varying carboxylato coligand basicity.
We are interested in the coordination chemistry of the octaden-
tate amino-thiophenolato liganþd L2- (Chart 1), which supports
mixed-ligand [MII2L( μ-L0)] complexes (L0 = coligand)
with a doubly thiolato-bridged N3M( μ-SR)2( μ-L0)MN3 core
structure.8,9
In this study we describe the synthesis and spectroscopic
properties of 19 bisoctahedral carboxylato complexes of the
type [NiII2L( μ-O2CR)]þ incorporating carboxylates with pKb
values in the range 9-14. It was possible to determine the
crystal structures of most of the complexes by single crys-
tal X-ray diffraction, so that structural and spectroscopic
Several sets of closely related [MII2L(μ-L0)]þ complexes
have now been prepared, and their stability,10 reactivity,11,12
and electronic structure13,14 have been studied as a function
of the type of coligand (or the d electron configuration of thþe
metal ion). One series comprises dinuclear [NiII2L(μ-L0)]
complexes bearing different coligands (e.g., L0 = Cl-, OH-,15
SH-,10 NO2-, NO3-, N3-, N2H4,16 CH3CO2-, HCO3
,
(23) Journaux, Y.; Glaser, T.; Steinfeld, G.; Lozan, V.; Kersting, B. J.
Chem. Soc., Dalton Trans. 2006, 1738–1748.
-
H2PO4 , ,
- 17 ClO4-, ReO4-, CrO42-, MoO42-, WO42- 18 alkyl
(24) Lozan, V.; Kersting, B. Eur. J. Inorg. Chem. 2005, 504–512.
(25) Gregor, T.; Weise, C. F.; Lozan, V.; Kersting, B. Synthesis 2007,
3706–3712.
carbonates (ROCO2-),19 alkyl carbamates (RNCO2-),20
pyrazolate, tetrazolate,21 and tetrahydridoborate (BH4-)22).
In another set of complexes the divalent metal ions vary while
the coligand remains the same (e.g., [MII2L(μ-O2CCH3)]þ,
€
(26) Chaudhuri, P.; Kataev, V.; Buchner, B.; Klauss, H.-H.; Kersting, B.;
Meyer, F. Coord. Chem. Rev. 2009, 253, 2261–2285.
(27) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M.
Advanced Inorganic Chemistry, 6th ed.; John Wiley & Sons: New York, 1999;
pp 486-488.
€
(28) Lipscomb, W. N.; Strater, N. Chem. Rev. 1996, 96, 2375–2433.
(8) Kersting, B. Z. Anorg. Allg. Chem. 2004, 630, 765–780.
(9) Lozan, V.; Loose, C.; Kortus, J.; Kersting, B. Coord. Chem. Rev. 2009,
253, 2244–2260.
(29) Lee, D.; Hung, P. L.; Spingler, B.; Lippard, S. J. Inorg. Chem. 2002,
41, 521–531.
(30) Lippard, S. J.; Berg, J. M. Principles of Bioinorganic Chemistry;
University Science Books: Mill Valley, CA, 1994.
(31) Tshuva, E. Y.; Lippard, S. J. Chem. Rev. 2004, 104, 987–1012.
(32) Tolman, W. B.; Que, L., Jr. J. Chem. Soc., Dalton Trans. 2002
653–660.
(10) Lozan, V.; Kersting, B. Inorg. Chem. 2008, 47, 5386–5393.
(11) Kersting, B. In Activating Unreactive Molecules - The Role of
secondary Interactions; Bolm, C., Hahn, F. E., Eds.; Wiley-VCH: Weinheim,
2009; Chapter 1, pp 1-17.
(12) Kersting, B.; Lehmann, U. Chemistry of Metalated Container
Molecules. In Advances in Inorganic Chemistry; van Eldik, R., Hubbard,
C. D., Eds.; Elsevier: Amsterdam, The Netherlands, 2009; Vol. 61, pp 407-470.
(33) Que, L.; Dong, Y. Acc. Chem. Res. 1996, 29, 190–196.
(34) Jabri, E.; Carr, M. B.; Hausinger, R. P.; Karplus, P. A. Science 1995,
268, 998–1004.
€
(13) Chaudhuri, P.; Kataev, V.; Buchner, B.; Klauss, H.-H.; Kersting, B.;
(35) Lippard, S. J. Science 1995, 268, 996–997.
Meyer, F. Coord. Chem. Rev. 2009, 253, 2261–2285.
(14) Loose, C.; Ruiz, E.; Kersting, B.; Kortus, J. Chem. Phys. Lett. 2008,
452, 38–43.
(15) (a) Kersting, B.; Steinfeld, G. Chem. Commun. 2001, 1376–1377. (b)
Klingele, M. H.; Steinfeld, G.; Kersting, B. Z. Naturforsch. 2001, 56b, 901–907.
(16) Hausmann, J.; Klingele, M. H.; Lozan, V.; Steinfeld, G.; Siebert, D.;
Journaux, Y.; Girerd, J.-J.; Kersting, B. Chem.;Eur. J. 2004, 10, 1716–1728.
(17) Kersting, B. Angew. Chem. 2001, 113, 4109–4112. Kersting, B. Angew.
Chem., Int. Ed. 2001, 40, 3987–3990.
(18) Lozan, V.; Kersting, B. Eur. J. Inorg. Chem. 2007, 1436–1443.
(19) Gressenbuch, M.; Lozan, V.; Steinfeld, G.; Kersting, B. Eur. J. Inorg.
Chem. 2005, 2223–2234.
(20) Lozan, V.; Holldorf, J.; Kersting, B. Inorg. Chim. Acta 2009, 362,
793–798.
(36) (a) Yamaguchi, K.; Koshino, S.; Akagi, F.; Suzuki, M.; Uehara, A.;
Suzuki, S. J. Am. Chem. Soc. 1997, 119, 5752–5753. (b) Carlsson, H.; Haukka,
M.; Bousseksou; Latour, J.-M.; Nordlander, E. M. Inorg. Chem. 2004, 43, 8252–
8262.
(37) Harrop, T. C.; Mascharak, P. K. In Concepts and Models in
Bioinorganic Chemistry; Kraatz, H.-B., Metzler-Nolte, N., Eds.; Wiley-VCH:
Weinheim, Germany, 2006.
(38) Kumar, N.; Kachroo, P. L.; Kant., R. Bull. Chem. Soc. Jpn. 1980, 53,
1787–1788.
(39) Steinfeld, G.; Lozan, V.; Kersting, B. Angew. Chem. 2003, 115, 2363–
2365. Steinfeld, G.; Lozan, V.; Kersting, B. Angew. Chem., Int. Ed. 2003, 42,
2261–2263.
€
(40) Kass, S.; Gregor, T.; Kersting, B. Angew. Chem. 2006, 118, 107–110.
€
(21) Lozan, V.; Voitekhovich, S. V.; Gaponik, P. N.; Ivashkevich, O. A.;
Kersting, B. Z. Naturforsch. 2008, 63b, 496–502.
Kass, S.; Gregor, T.; Kersting, B. Angew. Chem., Int. Ed. 2006, 45, 101–104.
€
(41) Steinfeld, G.; Lozan, V.; Kruger, H.-J.; Kersting, B. Angew. Chem.
(22) Journaux, Y.; Lozan, V.; Hausmann, J.; Kersting, B. Chem. Com-
mun. 2006, 83–84.
2009, 121, 1988–1991. Steinfeld, G.; Lozan, V.; Kr€uger, H.-J.; Kersting, B.
Angew. Chem., Int. Ed. 2009, 48, 1954–1957.

11020 Inorganic Chemistry, Vol. 49, No. 23, 2010
Lehmann et al.
information is now available for an isostructural series of
[NiII2L(μ-O2CR)]þ compounds with carboxylate pKb values
differing by as much as five. The interplay of the basicities of
the carboxylato coligands and the structural and spectro-
scopic features of the nickel(II) complexes are discussed.
681 m [ν(C-Cl)]; UV/vis (MeCN): λmax/nm (ε/M-1 cm-1)=649
(36), 907 sh, 1088 (77). 3[BPh4]: Yield: 121 mg (95%); Mp
230-232 °C (decomp.). Elemental analysis (%) calcd. for
C40H64Cl4N6Ni2O6S2 (1048.30): C 45.83, H 6.15, N 8.02, S
6.12; found: C 45.67, H 6.23, N 7.93, S 6.06. Elemental analysis
(%) calcd. for C64H84BCl3N6Ni2O2S2 (1268.08): C 60.62, H
6.68, N 6.63, S 5.06; found: C 60.46, H 6.93, N 6.65, S 4.94; IR
(KBr disk): ν/cm-1=1682 s [νas(RCO2-)], 1348 s [νs(RCO2-)],
Experimental Section
h
746 m [ν(C-Cl)], 680 m [ν(C-Cl)]; UV/vis (MeCN): λmax/nm -
(ε/M-1 cm-1)=648 (45), 908 sh, 1087 (85). This compound was
additionally characterized by X-ray crystal structure analysis.
[NiII2L(μ-O2CCHCl2)][ClO4] (4[ClO4]). Yield: 71 mg (70%);
General Methods and Instrumentation. All manipulations
were carried out under an argon atmosphere using standard
Schlenk techniques. Reagent grade solvents were used through-
out. Melting points were determined with a Waters Electro-
thermal 7200 instrument in open glass capillaries and are
uncorrected. IR spectra were recorded on a Bruker TENSOR
27 FT-IR-spectrometer. Electronic absorption spectra were
recorded on a JASCO V670 UV/vis/NIR spectrometer. Ele-
mental analyses were carried out with a VARIO EL elemental
analyzer.
Starting Materials. Complex [NiII2L( μ-Cl)][ClO4] (1[ClO4])
was prepared according to the literature procedure.15 All other
reagents were obtained from standard commercial sources and
used without further purifications. Caution! Perchlorate salts of
transition metal complexes are hazardous and may explode. Only
small quantities should be prepared and great care taken.
General Procedures for the Preparation of the Carboxylato-
Bridged Nickel(II) Complexes 2-20. (a). [NiII2L( μ-O2CR)]-
[ClO4]. To a solution of the chlorido-bridged complex 1[ClO4] (92
mg, 0.10 mmol) in MeOH (30 mL) was added a MeOH solution
of the sodium or triethylammonium salt of the carboxylic acid
(0.20 mmol) in MeOH (5 mL). The reaction mixture was stirred
for 2 to 3 h during which time the color of the solution turned pale
M.p.: 348-350 °C (decomp.); IR (KBr disk): ν/cm-1=1652 s
h
[νas(RCO2-)], 1388 s [νs(RCO2-)], 1217 s, 783 s [ν(C-Cl)], 716 s
[ν(C-Cl)]; UV/vis (MeCN): λmax/nm (ε/M-1 cm-1)=648 (25),
906 sh, 1098 nm (70). Elemental analysis (%) calcd. for
C40H65Cl3N6Ni2O6S2 (1013.86): C 47.39, H 6.46, N 8.29,
S 6.33; found: C 47.28, H 6.42, N 8.05, S 6.41. 4[BPh4]: Yield:
113 mg (92%); Mp 286-288 °C (decomp.); Elemental analysis
(%) calcd. for C64H85BCl2N6Ni2O2S2 (1233.63): C 62.31,
H 6.94, N 6.81, S 5.20; found: C 62.05, H 7.26, N 6.79, S
4.91; IR (KBr disk): ν/cm-1 = 1651 s [νas(RCO2-)], 1389 s
h
[νs(RCO2-)], 784 br [ν(C-Cl)]; UV/vis (MeCN): λmax/nm (ε/
M-1 cm-1) = 649 (34), 906 sh, 1100 (70).
[NiII2L( μ-O2CCtCH)][ClO4] (5[ClO4]). Yield: 87 mg (91%);
Mp 310-311 °C (decomp.); IR (KBr disk): ν/cm-1 =3247 w
h
[ν(H-CtC)], 2097 w [ν(CtC)], 1607 s [νas(RCO2-)], 1375 s
[νs(RCO2-)]; UV/vis (MeCN): λmax/nm (ε/M-1 cm-1) = 653
(33), 907 sh, 1104 (78). Elemental analysis (%) calcd. for
C41H65ClN6Ni2O6S2 (954.96): C 51.57, H 6.86, N 8.80, S 6.72;
found: C 51.43, H 6.95, N 8.72, S 6.58. 5[BPh4]: Yield: 75 mg
(64%); Mp 280-282 °C (decomp.); Elemental analysis (%)
calcd. for C65H85BN6Ni2O2S2 (1174.74): C 66.46, H 7.29,
N 7.15, S 5.46; found: C 66.48, H 7.55, N 6.90, S 5.21;
green. A solution of LiClO4 3H2O (0.80 g, 5.00 mmol) in MeOH
3
(2 mL) was added. The resulting microcrystalline solid was
isolated by filtration and dried in air. The crude product was
dissolved in MeCN (30 mL) and filtered. EtOH (30 mL) was then
added. The solution was concentrated by rotary evaporation
(final volume ca. 10 mL) until incipient precipitation, and was
then allowed to stand for another 6 h. The crude product was
filtered, purified by recrystallization from a mixed MeCN/EtOH
(1:1) solvent system, and dried in air.
IR (KBr disk): ν/cm-1 = 3304, 3281 w [ν(H-CC)], 2101 w
h
[ν(CtC)], 1607 s [νas(RCO2-)], 1376 s [νs(RCO2-)]; UV/vis
(MeCN): λmax (ε)=654 (40), 906 sh, 1104 (76). This compound
was additionally characterized by X-ray crystal structure analysis.
[NiII2L( μ-O2CCH2Cl)][ClO4] (6[ClO4]). Yield: 69 mg (70%);
Mp 352-354 °C (decomp.); IR (KBr disk): ν/cm-1 = 1624 s
h
[νas(RCO2-)], 1406 s [νs (RCO2-)], 784 m [ν(C-Cl)], 691 m
[ν(C-Cl)]; UV/vis (MeCN): λmax/nm (ε/M-1 cm-1)=650 (37),
904 sh, 1107 (76). Elemental analysis (%) calcd. for C40H66-
Cl2N6Ni2O6S2 (979.41): C 49.05, H 6.79, N 8.58, S 6.55; found:
C 49.11, H 6.83, N 8.55, S 6.48. 6[BPh4]: Yield: 102 mg (85%); Mp
298-300 °C (decomp.); Elemental analysis (%) calcd. for
C64H86BClN6Ni2O2S2 (1199.19): C 64.10, H 7.23, N 7.01, S 5.35;
(b). [NiII2L( μ-O2CR)][BPh4]. The tetraphenylborate salts
were obtained as green powders by adding a solution of NaBPh4
(342 mg, 1.00 mmol) in MeOH (3 mL) to a solution of [NiII2L(μ-
O2CR)][ClO4] (0.10 mmol) in MeOH (50-75 mL). The crude
product was filtered, purified by recrystallization from a mixed
MeCN/EtOH (1:1) solvent system, and dried in vacuo.
Characterization Data for the New Complexes. [NiII2L-
( μ-O2CCF3)][ClO4] (2[ClO4]). Yield: 95 mg (95%); M.p.: 353-
found: C 63.99, H 7.50, N 6.92, S 4.86; IR (KBr disk): ν/cm-1
=
/
h
1624 [νas(RCO2-)], 1409 [νsymm(RCO2-)]; UV/vis (MeCN): λmax
354 °C (decomp.); IR (KBr disk): [ν/cm-1=1684 s [νas(RCO2-)],
h
nm (ε/M-1 cm-1) = 650 (30), 905 sh, 1111 (72).
1348 w [νs(RCO2-)], 1208 s, 1144 s [ν(C-F)]; UV/vis (MeCN):
[NiII2L( μ-O2C-C6H4-pNO2)][ClO4] (7[ClO4]). Yield: 61 mg
λ
max/nm (ε/M-1 cm-1) = 646 (36), 910sh, 1094 (73). Elemental
(58%); M.p.: 356-358 °C (decomp.); IR (KBr disk): ν/cm-1
=
analysis (%) calcd. for C40H64ClF3N6Ni2O6S2 (998.94): C 48.09, H
6.46, N 8.41, S 6.42; found: C 47.93, H 6.35, N 8.32, S 6.61. 2[BPh4]:
Yield: 121 mg (99%); M.p.: 299-301 °C (decomp.); Elemental
analysis (%) calcd. for C64H84BF3N6Ni2O2S2 (1218.71): C 63.07, H
6.95, N 6.90, S 5.26; found: C 62.85, H 6.83, N 6.87, S 5.39; IR (KBr
h
1621 m [ν(CdC)], 1590 s [νas(RCO2-)], 1405 s [νs(RCO2-)], 1522
s [νas(RNO2)], 1344 m [νs(RNO2)], UV/vis (MeCN): λmax/nm (ε/
M-1 cm-1)=650 (57), 911 sh, 1110 (78). Elemental analysis (%)
calcd. for C45H68ClN7Ni2O8S2 (1052.03): C 51.37, H 6.51, N
9.32, S 6.10; found: C 51.16, H 6.37, N 9.25, S 5.84. 7[BPh4]:
Yield 102 mg (80%). Mp 328-330 °C (decomp.). Elemental
analysis (%) calcd for C69H88BN7Ni2O4S2 (1271.81): C 65.16, H
6.97, N 7.71, S 5.04; found: C 65.29, H 7.08, N 7.47, S 4.87; IR
disk): [ν/cm-1 = 1684 s [νas(RCO2-)], 1349 w [νs(RCO2-)], 1210 s,
h
h
1147 s [ν(C-F)]. This compound was additionally characterized by
X-ray crystal structure analysis.
[NiII2L( μ-O2CCCl3)][ClO4] (3[ClO4]). Yield: 89 mg (85%);
(KBr disk): ν/cm-1 = 1619 [ν(CdC)], 1588 [νas(RCO2-)], 1404
M.p.: 318-320 °C (decomp.); IR (KBr disk): ν/cm-1 = 1683 s
h
h
[νs(RCO2-)], 1521 [νas(RNO2)], 1344 [νs(RNO2)]; UV/vis
(MeCN): λmax/nm (ε/M-1 cm-1)=651 (48), 911 sh, 1110 (68).
[νas(RCO2-)], 1349 s [νs(RCO2-)], 840 m, 746 m [ν(C-Cl)],
(42) If not otherwise indicated, the values for the dissociation constants
(46) Mansfield, C. H.; Whiting, M. C. J. Chem. Soc. 1956, 4761–4764.
(47) Katon, J. E.; McDevitt, N. T. Spectrochim. Acta 1965, 21, 1717–1724.
(48) Dunn, G. E.; McDonald, R. S. Can. J. Chem. 1969, 47, 4577–4588.
(49) Newman, R. J. Chem. Phys. 1952, 20, 1663–1664.
(50) Matsue, T.; Evans, D. H.; Osa, T.; Kobayashi, N. J. Am. Chem. Soc.
1985, 107, 3411–3417.
€
were taken from: Kortum, G.; Vogel, W.; Andrussow, K. Pure Appl. Chem.
1960, 1, 187–536.
(43) Siebert, H.; Tremmel, G. Z. Anorg. Allg. Chem. 1972, 390, 292–302.
(44) Spinner, E. J. Chem. Soc. 1964, 4217–4226.
(45) Lever, A. B. P; Odgen, D. J. Chem. Soc. 1967, 2041–2048.

Article
[NiII2L( μ-O2CCHdCH2)][ClO4] (12[ClO4]). Yield: 61 mg
Inorganic Chemistry, Vol. 49, No. 23, 2010 11021
Carboxylate Exchange Reactions. All experiments were car-
ried out with a 10-fold molar excess of the sodium or triethyl-
ammonium carboxylate at 50 °C. In a typical experiment,
a solution of complex 8 (93 mg, 0.100 mmol) and sodium
adamantate (202 mg, 1.00 mmol) in MeCN/MeOH (1:1,
100 mL) was stirred at 50 °C. Samples (5 mL) of the reaction
mixture were removed at regular intervals (2 h, 4 h, 8 h).
(64%); Mp 213-216 °C (decomp.); IR (KBr disk): ν/cm-1
=
h
1638 s [ν(CdC)], 1580 s [νas(RCO2-)], 1428 m [νs(RCO2-)]; UV/
vis (MeCN): λmax/nm (ε/M-1 cm-1)=652 (36), 907 sh, 1120 (75).
Elemental analysis (%) calcd. for C41H67ClN6Ni2O6S2 (956.98):
C 51.46, H 7.06, N 8.78, S 6.70; found: C 51.33, H 6.91, N 9.02, S
6.54. 12[BPh4]: Yield: 94 mg (80%); Mp 193-195 °C (decomp.);
Elemental analysis (%) calcd. for C65H87BN6Ni2O2S2 (1176.75):
C 66.34, H 7.45, N 7.14, S 5.45; found: C 66.29, H 7.32, N 6.92, S
LiClO4 3H2O (100 mg, 0.623 mmol) was added, and the
3
sample solution was concentrated under reduced pressure.
The resulting solid was isolated by filtration, washed with
EtOH, dried in vacuo, and then analyzed by IR spectroscopy.
The relative amounts of the starting material and product were
estimated from the relative intensities of the characteristic
νas(CO) signals. This procedure was carried out twice to give
reproducible values for the reaction rates. However, given the
uncertainty of the integration method, the rate data presented
in Table 5 below should be taken as indicative rather than
definite.
5.38; IR (KBr disk): ν/cm-1=1637 [ν(CdC)], 1578 [νas(RCO2-)],
h
1428 [νs(RCO2-)]; UV/vis (MeCN): λmax/nm (ε/M-1 cm-1)=
651 (42), 907 sh, 1122 (77).
[NiII2L( μ-O2CC6H4-pCH3)][ClO4] (13[ClO4]). Yield: 66 mg
(65%); M.p.: 360-362 °C (decomp.); IR (KBr disk): ν/cm-1
=
h
1615 w, 1596 m, 1587 w [ν(CdC)], 1562 s [νas(RCO2-)], 1424 w,
1403 m [νs(RCO2-)]; UV/vis (MeCN): λmax/nm (ε/M-1 cm-1) =
652 (30), 907 sh, 1121 (70)). Elemental analysis (%) calcd. for
C46H71ClN6Ni2O6S2 (1021.06): C 54.11, H 7.01, N 8.23, S 6.28;
found: C 54.26, H 7.27, N 8.34, S 6.13. 13[BPh4]: Yield: 108 mg
(87%); M.p.: 290-292 °C (decomp.); Elemental analysis (%)
calcd. for C70H91BN6Ni2O2S2 (1240.84): C 67.76, H 7.39, N 6.77,
S 5.17; found: C 67.48, H 7.34, N 6.72, S 5.22; IR (KBr disk):
Collection and Reduction of X-ray Data. Crystals of 2[BPh4]
MeCN, 3[BPh4] 2MeCN EtOH, 5[BPh4] 1.5MeCN, and
3
3
3
3
19[BPh4] 3MeCN suitable for X-ray crystallographic analysis
3
were obtained by slow evaporation of mixed MeCN/EtOH (1:1)
solutions. The crystals were removed from the mother liquor
and immediately immersed in a drop of perfluoropolyether
oil. A suitable crystal was selected, attached to a glass fiber,
and placed in a low-temperature nitrogen stream of the
diffractometer. All data were collected at 213(2) K using a
Bruker AXS diffractometer or a STOE IPDS I diffractometer
(for 2) equipped with Mo KR radiation.54 Absorption correc-
tions were applied using the Sadabs program.55 The structures
were solved by Direct Methods56 and refined by full-matrix
least-squares on the basis of all data against F2 using Shelxl-97.57
The program Platon was used to search for higher symmetry.58
All non-hydrogen atoms were refined anisotropically. Split
atom models were used to account for disorder of the CF3 and
tert-butyl groups in complexes 2, 3, 5, and 19 using the SADI
instructions implemented in the Shelxtl program suite. The site
occupancies of the two positions were refined as follows: F(1a)-
F(3a)/F(1b)-F(3b) 0.51(2)/0.49(1) and C(32a)-C(34a)/C(32b)-C-
(34b) 0.60(2)/0.40(2) for 2; C(32a)-C(34a)/C(32b)-C(34b) 0.70(3)/
0.30(3) and C(36a)-C(38a)/C(36b)-C(38b) 0.53(1)/0.47(1) for 5;
C(36a)-C(38a)/C(36b)-C(38b) 0.58(1)/0.42(1) for 19. In the struc-
ture of 19, one MeCN molecule was found to be disordered
over two positions. The site occupancies of the two positions were
refined as follows: N(9)C(54)C(55a)/N(9)C(54)C(55b) 0.79(4)/
0.21(4). In the crystal structure of 3[BPh4] 2MeCN EtOH,
ν/cm-1=1613 w, 1594 sh, 1587 s [ν(CdC)], 1560 s [νas(RCO2-)],
h
1426 m, 1403 m [νs (RCO2-)]; UV/vis (MeCN): λmax/nm (ε/M-1
cm-1)=650 (44), 910 sh, 1122 (78).
[NiII2L(μ-trans-O2CCHdCHMe)][ClO4] (16[ClO4]). Yield:
71 mg (73%); M.p.: 356-358 °C (decomp.); IR (KBr disk):
ν/cm-1 = 1661 m [ν(CdC)], 1575 s, [νas(RCO2-)], 1413 s
h
[νs(RCO2-)]; UV/vis (MeCN): λmax/nm (ε/M-1 cm-1) = 655
(40), 905 sh, 1122 nm (72). Elemental analysis (%) calcd. for
C42H69ClN6Ni2O8S2 (971.00): C 51.95, H 7.16, N 8.65, S 6.60;
found: C 51.76, H 7.07, N 8.56, S 6.48. 16[BPh4]: Yield: 107 mg
(90%); Mp 308-310 °C (decomp.); Elemental analysis (%)
calcd. for C66H89BN6Ni2O2S2 (1190.78): C 66.57, H 7.53, N
7.06, S 5.39; found: C 66.50, H 7.69, N 6.90, S 5.12; IR (KBr
disk): ν/cm-1 = 1659 m [ν(CdC)], 1573 s, [νas(RCO2-)], 1413 s
h
[νs(RCO2-)]; UV/vis (MeCN): λmax/nm (ε/M-1 cm-1) = 657
(45), 902 sh, 1129 (78).
[NiII2L( μ-O2CCH2CH3)][ClO4] (18[ClO4]).Yield: 79 mg (82%);
Mp > 300 °C (decomp.); IR (KBr disk): ν/cm-1 = 1585
h
[νas(RCO2-)], 1424 [νs(RCO2-)]; UV/vis (MeCN): λmax/nm (ε/
M
-1 cm-1) = 650 (22), 903 sh, 1129 (63). Elemental analysis (%)
calcd. for C41H69ClN6Ni2O6S2 (958.99): C 51.35, H 7.25, N 8.76, S
6.69; found: C 51.52, H 7.36, N 8.64, S 6.34; 18[BPh4]: Yield: 91 mg
(77%); Mp 290-292 °C (decomp.); Elemental analysis (%) calcd
for C65H89BN6Ni2O2S2 (1178.77): C 66.23, H 7.61, N 7.13, S 5.44;
found: C 66.44, H 7.52, N 7.01, S 5.23; IR (KBr disk):
3
3
the C and O atoms of the EtOH solvate molecule were refined
isotropically. H atoms were placed at calculated positions and
refined as riding atoms with isotropic displacement para-
meters. Selected details of the data collection and refinement
are given in Table 3. Drawings were produced with Ortep 3 for
Windows.59
ν=1583 [νas(CO2)], 1426 cm-1 [νs(CO2)]; UV/vis (MeCN): λmax
/
h
nm (ε/M-1 cm-1)=651 (24), 907 sh, 1130 (66).
[NiII2L( μ-O2CC10H15)][ClO4] (19[ClO4]). Yield: 83 mg (78%);
Mp 356-358 °C (decomp.); IR (KBr disk): ν/cm-1 = 1567 s
h
[νas(RCO2-)], 1410 s [νs(RCO2-)]; UV/vis (MeCN): λmax/nm (ε/
M
-1 cm-1) = 653 (29), 907 sh, 1122 (65). Elemental analysis (%)
Results and Discussion
calcd. for C49H79ClN7Ni2O6S2 (1065.16): C 55.25, H 7.48, N 7.89,
S 6.02; found: C 54.96, H 7.35, N 7.65, S 5.89. 19[BPh4]: Yield: 112
mg (87%); Mp 354-356 °C (decomp.); Elemental analysis (%)
calcd. for C73H99BN6Ni2O2S2 (1284.93): C 68.24, H 7.77, N 6.54, S
4.99; found: C 68.32, H 7.83, N 6.38, S 4.86; IR (KBr disk):
Preparation of the Metal Complexes. Table 1 lists
the synthesized complexes and their labels. Several of
these have been reported previously, as for instance
the formato (8),22 2-chlorobenzoato (9),60 ferrocene
ν/cm-1 = 1567 s [νas(RCO2-)], 1410 s [νs(RCO2-)]; UV/vis
h
(MeCN): λmax/nm (ε/M-1 cm-1) = 650 (41), 1122 (75). This
compound was additionally characterized by X-ray crystal struc-
ture analysis.
(54) ShelXTL, version 5.10; Bruker AXS: Madison, WI, 1998.
(55) SADABS, an empirical absorption correction program part ot the
SAINTPlus NT version 5.0 package; Bruker AXS: Madison, WI, 1998.
(56) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467–473.
(57) Sheldrick, G. M. Shelxl-97, Computer program for crystal structure
€
€
refinement; University of Gottingen: Gottingen, Germany, 1997.
(58) Spek, A. L. Platon - A Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands, 2000.
(51) Wu, H.; Gao, Y.; Yu, K. Transition Met. Chem. 2004, 29, 175–179.
ꢀ
ꢀ
€
ꢀ
(52) Zelenak, V.; Vargova, Z.; Gyoryova, K. Spectrochim. Acta A 2007,
66, 262–272.
(59) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
€
(53) Pethybridge, A. H.; Ison, R. W.; Harrigan, W. F. J. Food Technol.
1983, 18, 789–796.
(60) Hausmann, J.; Kass, S.; Klod, S.; Kleinpeter, E.; Kersting, B. Eur. J.
Inorg. Chem. 2004, 4402–4411.

11022 Inorganic Chemistry, Vol. 49, No. 23, 2010
Lehmann et al.
Table 1. Synthesized Complexes, Their Labels, and Selected IR and UV/vis Spectroscopic Data
νas, νs; Δ/cm-1a
ν2, ν1/nm; Δo/cm-1b
42
compound
pKb
1c
2d
[NiII2L( μ-Cl)]þ
[NiII2L( μ-O2CCF3)]þ
13.77
13.34
12.74
12.1646
11.18
10.55
10.25
10.18
9.8050
9.80
1684, 1348; 336
[1682, 1442; 240]43
1683, 1349; 334
[1677, 1353; 324]44
1652, 1388; 264
[1629, 1370; 259]45
1607, 1375; 232
[1600, 1382; 218]47
1624, 1406; 218
[1595, 1405; 190]45
1590, 1405; 185
[1575, 1392; 183]48
1603, 1369; 234
[1620, 1377; 243]49
1565, 1392; 173
[1554, 1384; 170]48
1571, 1425; 146
[1548, 1393; 155]g
1569, 1407; 162
[1548, 1391; 157]48
1580, 1428; 152
[1560, 1428; 132]51
1562, 1403; 159
[1542, 1389; 153]48
1578, 1406; 172
[1551, 1408; 143]52
1567, 1395; 172
[1563, 1424; 141]g
1575, 1413; 162
[1561, 1433; 128]g
1588, 1428; 160
[1578, 1414; 164]45
1585, 1424; 161
[1558, 1416; 142]45
1567, 1410; 157
[1555, 1408; 147]g
1573, 1400; 173
[n.d.]h
646, 1094; 9141
649, 1088; 9191
648, 1098; 9107
653, 1104; 9058
650, 1107; 9033
650, 1110; 9009
651, 1114; 8977
651, 1116; 8961
649, 1112; 8992
651, 1120; 8929
652, 1120; 8929
652, 1121; 8921
654, 1125; 8889
650, 1120; 8929
655, 1122; 8913
651, 1128; 8865
648, 1129; 8857
653, 1122; 8913
650, 1112; 8993
3d
[NiII2L( μ-O2CCCl3)]þ
4
[NiII2L( μ-O2CCHCl2)]þ
[NiII2L( μ-O2CCtCH)]þ
[NiII2L( μ-O2CCH2Cl)]þ
[NiII2L( μ-O2CC6H4-p-NO2)]þ
[NiII2L( μ-O2CH)]þ
5d
6
7
8c
9c
[NiII2L( μ-O2CC6H4-m-Cl)]þ
[NiII2L( μ-O2C(C5H4)FeCp]þ
[NiII2L( μ-O2CPh)]þ
10c
11c
12
13
14c
15c
16
17c
18
19d
20c
[NiII2L( μ-O2CCHdCH2)]þ
[NiII2L( μ-O2CC6H4-pCH3)]þ
[NiII2L( μ-O2CCHdCHPh)]þ
[NiII2L( μ-O2C(CHdCH)2CH3)]þ
[NiII2L( μ-O2CCHdCHCH3)]þ
[NiII2L( μ-O2CCH3)]þ
9.75
9.63
9.56
9.3353
9.29
9.24
[NiII2L( μ-O2CCH2CH3)]þ
[NiII2L( μ-O2CC10H15)]þe
[NiII2L( μ-O2CR0)]þe
9.11
9.10 f
9.10 f
a The data correspond to the ClO4 salts (see Experimental Section for the BPh4 salts); Δ=νas - νs; values in square brackets refer to the
corresponding sodium carboxylates. b Values obtained from about 10-3 M solutions of the ClO4- salts in MeCN (see Experimental Section for the values
of the BPh4- salts). c The crystal structure of this compound was reported previously. d The crystal structure of this compound is reported in this study.
-
-
e C10H15CO2 = adamantate, R0CO2- = 3,4-dimethyl-6-phenylcyclohex-3-ene carboxylate. f Estimated values. g Determined in this work. h Not
-
determined.
monocarboxylato (10),61 benzoato (11),16 cinnamato (14),40
sorbato (15),40 acetato (17),17 and 3,4-dimethyl-6-phe-
nylcyclohex-3-ene carboxylato bridged compounds
(20).40 The other carboxylato complexes in Table 1
were prepared in a similar fashion (eq 1) by treatment
of the chloro complex [NiII2L( μ-Cl)][ClO4] 1,15 with a
slight excess of the sodium or triethylammonium salt of
the corresponding carboxylate anion, prepared in situ
from the free acid and a base, in methanol at ambient
temperature. The complexes could be readily isolated as
perchlorate salts and were subsequently subjected to anion
metathesis with NaBPh4 to generate the corresponding
tetraphenylborate salts (eq 2). The reaction in eq 1 does
not appear to be affected by the varying basicity or the steric
hindrance of the respective carboxylate ion, as all complexes
are obtained in equally good yields of about 70-90%
(unoptimized). It should be noted that [NiII2L( μ-O2CR)]þ
complexes are less readily prepared by carboxylate exchange
reactions (see further below), which contrasts the behavior
of the related zinc(II) complexes.
½NiII2Lðμ -ClÞꢀ½ClO4ꢀ þ NaO2CR ðor ½NEt3HꢀO2CRÞ
f ½NiII2Lðμ -O2CRÞꢀ½ClO4ꢀ þ NaCl ðor ½NEt3HꢀClÞ
ð1Þ
½NiII2Lðμ -O2CRÞꢀ½ClO4ꢀ þ NaBPh4
f ½NiII2Lðμ -O2CRÞꢀ½BPh4ꢀ þ NaClO4
ð2Þ
All compounds are stable in air, both in solution and
in the solid state. The ClO4- salts exhibit good solubility
in polar aprotic solvents such as MeCN, acetone, and
dichloromethane, but are not very soluble in alcohols
-
and virtually insoluble in water. The BPh4 salts could
be obtained as pale-green crystals by slow evapora-
tion from a mixed MeCN/EtOH (1:1) solvent system.
The products thus purified gave satisfactory elemental
analyses and were characterized by IR and UV/vis spec-
(61) Lozan, V.; Buchholz, A.; Plass, W.; Kersting, B. Chem.;Eur. J.
2007, 13, 7305–7316.
troscopy and by X-ray structure analysis (2[BPh4] MeCN,
3

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 11023
Figure 2. Plot of νas (open circles) and νs (open triangles) for complexes
3, 4, 6, and 17 versus the pKb value of the corresponding carboxylato
ligands.
Figure 1. Overlay of the IR spectra of 3[ClO4] (dashed line) and 5[ClO4]
(solid line).
Table 1 shows that the νas frequencies of the complexes
(except 8) are blue-shifted by up to 25 cm-1 relative to the
sodium carboxylates, an observation that is consistent
with the coordination of the RCO2- group to the [NiII2L]2þ
fragment. Note that the νas and νs values of 2-20 differ
by as much as 100 cm-1 (νas = 1684-1576 cm-1, νs =
1428-1348 cm-1). This scattering reflects the varying
basicity of the respective carboxylate, since the immediate
environment of the carboxylato group in all [NiII2L
(μ-O2CR)]þ complexes is essentially the same. Indeed,
there is a clear correlation between the frequencies and the
carboxylate ion basicity. The frequency νas decreases
while νs increases with increasing carboxylate basicity.
A particularly good νas versus pKb correlation exists for
the trichloro-, dichloro-, chloro-, and acetato bridged
complexes (i.e., 3, 4, 6, and 17) as they are structurally
and electronically related. Here, the correlation is almost
linear (Figure 2).
The observations and conclusions made for the present
complexes are in line with those made for the free acids
and other series of carboxylato complexes.48,72,73 In fact,
for the present complexes, the IR spectra of the nickel-
bound carboxylato ligand are almost superimposable
with those of the free sodium carboxylates, with the
exception of the about 20-25 cm-1 blue-shift of the νas
frequency.74 These observations further suggest that the
Δ criterion63 should be used with care for assignment of
carboxylato binding modes. The pKb dependence of the
separation Δ=νas-νs has to be taken into account.
UV/vis Spectroscopy. The complexes were further stu-
died by electronic absorption spectroscopy to see whether
the electronic transitions of the [NiII2L( μ-O2CR)]þ com-
plexes correlate with the pKb values of the carboxylato
ligands. Electronic absorption spectra for all complexes
were recorded in MeCN solution over the range
300-1600 nm. Figure 3 shows the spectra of MeCN solu-
tions of two representative compounds (trichloroacetato
complex 3 and acetato complex 17) in the 500-1400 nm
region. Table 1 lists selected values and their assignments.
The electronic absorption spectra of 2-20 are similar
but not identical. Each complex exhibits two weak
3[BPh4] 2MeCN EtOH, 5[BPh4] 1.5MeCN, and 19[BPh4]
3MeCN).62
3
3
3
3
Spectroscopic Characterization of the Complexes. In-
frared Spectroscopy. The carboxylate ion displays versatile
coordination behavior,27 and infrared (IR) spectroscopy is
a powerful tool to distinguish between its different coordi-
nation modes.63 In fact, the frequency of the asymmetric
and symmetric CdO stretching modes, νas and νs, respec-
tively, and their separation Δ=νas-νs are often used as
spectroscopic criteria to identify the carboxylate binding
mode.43,64,65 A number of correlations between the car-
boxylate stretching frequencies and the carboxylate bind-
ing mode have been reported in the literature.52,64,66-69
Nevertheless, there are only a few systematic IR spectro-
scopic studies of isostructural carboxylato complexes.
Most of them involve inert cobalt(III) species.43,70 To our
knowledge, a systematic IR study of an isostructural series
of carboxylato-bridged nickel(II) complexes has not been
reported.45,71
FT-IR spectra of the dinuclear nickel(II) complexes 2-20
were recorded over the range 4000-400 cm-1. Figure 1
shows the spectra of the two representative compounds 3
and 5 in the 1800-1200 cm-1 region. Assignments of the
bands are based on comparisons between the individual
complexes and on comparisons with the IR data for the
respective sodium carboxylates (Table 1). In some cases, an
unambiguous assignment could not be made, either because
of poor resolution, low signal intensity, or superpositions of
the signals.
(62) The thiolate complexes were also examined by cyclic voltammetry.
However, their irreversible redox behavior prevented an analysis of the
variation in redox potential as a function of pKb.
(63) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Co-
ordination Compounds, 5th ed.; Wiley: New York, 1997.
(64) Deacon, G. B.; Philipp, R. J. Coord. Chem. Rev. 1980, 33, 227–250.
(65) Martini, D.; Pellei, M.; Pettinari, C.; Skelton, B. W.; White, A. H.
Inorg. Chim. Acta 2002, 333, 72–82.
(66) Edsall, J. T. J. Chem. Phys. 1937, 5, 508–517.
(67) Lewandowski, W.; Kalinowska, M.; Lewandowska, H. J. Inorg.
Biochem. 2005, 99, 1407–1423.
(68) Czakis-Sulikowska, D.; Czylkowska, A.; Malinowska, A. J. Therm.
Anal. Cal 2002, 67, 667–678.
(69) (a) Ishioka, T.; Shibata, Y.; Takahashi, M.; Kanesaka, I.; Kitagawa,
Y.; Nakamura, K. T. Spectrochim. Acta 1998, 54A, 1827–1836.
(70) Wieghardt, K. J. Chem. Soc., Dalton Trans. 1973, 2548–2552.
(71) Kumar, N.; Kachroo, P. L.; Kant., R. Bull. Chem. Soc. Jpn. 1980, 53,
1787–1788.
(72) Lindberg, B. J. Acta Chem. Scand. 1968, 22, 571–580.
(73) Lloyd, H. A.; Warren, K. S.; Fales, H. M. J. Am. Chem. Soc. 1966, 88,
5544–5549.
(74) Shindo, H.; Brown, T. L. J. Am. Chem. Soc. 1965, 87, 1904–1909.

11024 Inorganic Chemistry, Vol. 49, No. 23, 2010
Lehmann et al.
Table 2. Selected UV/vis Data for [NiII2L(μ-L0)]þ Complexesa
complex
coligand
ν2/nm
ν1/nm
Δo/cm-1
ref.
21c,d
22b
23c,d
24c,d
25b
26c,d
27
NO3 -
659 (46) 1049 (77)
578 (129) 1066 (86)
9532
9381
9158
9132
9074
9001
9001
8977
8977
8929
8865
8764
8511
8475
16
18
16
16
10
16
16
16
this work
this work
this work
10
-
ClO4
N3
-
672 (37)
615 (33)
647 (56)
621 (40)
629 (43)
624 (33)
651 (27)
651 (35)
651 (26)
667 (52)
663 (32)
634 (23)
1092 (84)
1095 (62)
1102 (72)
1111 (57)
1111 (57)
1114 (67)
1114 (86)
1120 (66)
1128 (65)
1141 (68)
1175 (49)
1180 (67)
pydz e
S6
2-
-
NO2
phtz e
N2H4
O2CH
O2CPh-
O2CCH3
SPh-
SH-
28c,d
8
11
17
-
27b
28b
29c,d
10
16
pz e
Figure 3. Electronic absorption spectra of 3[ClO4] and 17[ClO4] in
MeCN at ambient temperature at about 10-3 M.
a In MeCN solution. b The data refer to the BPh4 salts. c The
data refer to the ClO4- salts. d In MeOH solution. e pydz = pyridazine,
phtz = phthalazine, pz = pyrazolate.
-
Figure 4. Plotofν1 (opensquares) and Δo (full circles) versuspKb for the
complexes 2-20. The pKb values of the carboxylates in complexes 19 and
20 are estimated to be 4.90.
absorption bands in the 500-1600 nm region which can
be assigned to the spin-allowed 3A2g f 3T1g (ν2) and 3A2g
f 3T2g (ν1) transitions of a nickel(II) (S = 1) ion with Oh
symmetry.75 There is also a weak shoulder around 910 nm
attributable to a spin-forbidden 3A2g f 1Eg(D) transition
which gains intensity because of the lowering of the
symmetry. The 3A1g f 3T1g(P) transition (expected below
400 nm) is obscured in each case by the strong thiolato-
nickel(II) LMCT transitions which occur in that same
spectral region.
Figure 4 shows a plot of the ν1 values for 2-20 against
the pKb value of the carboxylato ligands. As can be seen,
the ν1 values are steadily red-shifted with increasing
carboxylate ion basicity, from 1094 nm in 2 bearing the
weakly basic trifluoroacetate ion (pKb = 14 - pKa =
13.76) to 1129 nm in 18 with the much more basic
propionate ion (pKb = 9.11). No clear trend is observed
for the ν2 values, which scatter randomly between 546 and
555 nm.
Rough estimates of the octahedral splitting parameters
Δo, which are inversely proportional to ν1 (Δo [cm-1] =
107/ν1 [nm]),75 can be obtained from the ν1 transitions.
Figure 4 shows that Δo decreases with increasing carbox-
ylate basicity, from 9141 cm-1 in the trifluoroacetato
complex 2 to 8857 cm-1 in the propionato complex 18.
This is somewhat surprising as the Ni-O bond weakening
is observed upon going from 2 to 18 (vide infra). Given
Figure 5. Secondary CH π bonding interactions in the benzoato
3 3 3
complex 11. This figure was generated using data downloaded from
The Cambridge Crystallographic Data Centre (CCDC) and corresponds
to the structure originally reported in ref 16.
that Δo is determined by the complete N3S2O donor set
and not just the O donors, it can be concluded that
binding of a basic carboxylate donor weakens the macro-
cycle-nickel(II) interactions (i.e., the Ni-N and Ni-S
bonds) and vice versa. This coupling of the Ni-O bonding
interactions to the Ni-N and Ni-S -bonds is indeed
reflected in small but consistent bond length changes across
the series (vide infra). The UV/vis data for other previously
characterized [NiII2L( μ-L0)]þ complexes are also in line
with this assumption. Table 2 lists some representative
examples ordered by decreasing Δo values. As discussed
above, the [NiII2L( μ-L0)]þ complexes bearing weakly
basic coligands (such as the NO3- and ClO4- ions) feature
the highest Δo values. Thus, as discussed above, the N6S2
macrocycle adapts to the bonding preferences of the coli-
gand, which results in varying contributions of coligand and
macrocycle to Δo.
It should be noted that no relationship exists between
the Δo values and the relative binding affinity of the
[NiII2L]2þ fragment for a certain coligand.76,77 In a pre-
vious study, we determined the relative stabilities of some
[NiII L( μ-L0)]þ complexes by exchange experiments.16 The
2
relative stabilities of some of the compounds listed in Table 2
were determined as follows: benzoate > acetate > N3
-
>
(75) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier
Science: Amsterdam, 1984.
pyrazolate (pz) > NO2- > N2H4 > NO3- > phthalazine

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 11025
Table 3. Selected Crystallographic Data for Compounds 2[BPh4] MeCN, 3[BPh4] 2MeCN EtOH, 5[BPh4] 1.5MeCN, and 19[BPh4] 3MeCNa
3
3
3
3
3
2[BPh4] MeCN
3
3[BPh4] 2MeCN EtOH
3
5[BPh4] 1.5MeCN
3
19[BPh4] 3MeCN
3
3
formula
formula weight [g/mol]
space group
C66H87BF3N7Ni2O2S2
1259.78
P1
13.385(2)
15.598(2)
17.406(2)
106.73(1)
92.53(1)
C70H96BCl3N8Ni2O3S2
1396.25
P1
14.256(3)
17.082(3)
17.772(3)
116.10(1)
109.19(1)
94.80(1)
C68H89.5BN7.5Ni2O2S2
1236.32
P21/n
19.208(4)
18.136(4)
20.086(4)
90.00
112.26(3)
90.00
6476(2)
4
1.268
C79H108BN9Ni2O2S2
1408.09
P1
13.173(3)
16.845(3)
18.155(4)
108.79(3)
95.30(3)
94.59(3)
3772(1)
2
1.240
˚
a, A
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
108.49(1)
3263.4(9)
2
3
˚
V, A
3535(1)
2
1.312
Z
Dcalcd., g/cm3
cryst. size, mm3
μ(Mo KR), mm-1
R1b (R1 all data)
wR2c (wR2 all data)
1.282
0.35 ꢁ 0.20 ꢁ 0.15
0.24 ꢁ 0.20 ꢁ 0.20
0.49 ꢁ 0.36 ꢁ 0.32
0.30 ꢁ 0.30 ꢁ 0.18
0.697
0.756
0.696
0.606
0.0327 (0.0651)
0.0597 (0.0692)
0.532/-0.252
0.0502 (0.1143)
0.1055 (0.1296)
0.871/ -0.682
0.0572 (0.2141)
0.1149 (0.1678)
0.523/-0.825
0.0423 (0.1012)
0.0839 (0.1023)
0.602/-0.591
3
˚
Max, min peaks, e/A
P
P
P
P
a Observation criterion: I > 2σ(I). b R1 = ||Fo| - |Fc||/ |Fo|. c wR2 = { w(Fo2 - Fc2)2/ w(Fo2)2}1/2
.
Figure 6. ORTEP views of the structures of the cations [NiII2L( μ-O2CCF3)]þ (2), [NiII2L( μ-O2CCCl3)]þ (3), [NiII2L( μ-O2CCtCH)]þ (5),
and [NiII2L( μ-O2CC10H15)]þ (19). Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms, except the terminal hydrogen
atom of the propiolato coligand in 5, have been omitted for clarity. For rotationally disordered CF3 and/or tBu groups only one orientation
is displayed.
>( phtz) > pyridazine ( pz) > ClO4-. Inspection of the cor-
responding data clearly shows that a correlation between the
complex stabilities and the Δo values does not exist. Other-
wise, the ClO4- complex (which has the highest Δo) would be
the most stable complex, but this is definitely not the case.
Another fact worth considering here is the fact
that complex stability differences due to secondary
bonding interactions such as CH π interactions78
3 3 3
between the organic residues of the supporting ligand
and the coligand, as seen in the set of [ZnII2L( μ-O2-
CR)]þ complexes,17,40 are not reflected in the Δo values of
the present [NiII2L( μ-O2CR)]þ compounds. Thus, the
relative stability constants for the formato-, acetato-,
and benzoato-bridged nickel(II) complexes, estimated
from carboxylate exchange reactions (vide infra), increase
in the order formato (8) < acetato (17) < benzoato
(11), but this ranking is not reflected in the Δo values.
(76) The UV/vis spectral data of 2-18 may be used to estimate a relative
stability difference of the [Ni2IIL( μ-O2CR)]þ complexes. As the complexes
are structurally very similar, one simply needs to determine the difference of
the ligand field stabilization energies (which contributes to the stability of a
transition metal complex) across the series (if it can be assumed that all other
effect cancel out). For the present nickel(II) complexes with a d8 electronic
configuration (t2g6eg2, S = 1), LFSE(Ni2þ) = 1.2 ꢁ Δo. Thus, by taking
complexes 2 and 18 as pivots, the difference ΔLFSE(2,18) = LFSE(2) -
LFSE(18) = 1.2 ꢁ (9141 cm-1 - 8857 cm-1) ꢁ 11.96 J/(mol cm-1) = 4.08
kJ/mol. This is a small value.
(78) Meyer, E. A.; Castellano, R. K.; Diederich, F. Angew. Chem. 2003,
115, 1244–1287. Meyer, E. A.; Castellano, R. K.; Diederich, F. Angew. Chem.,
In. Ed. 2003, 42, 1210–1250.
(77) Holleman, A. F.; Wiberg, N. Lehrbuch der Anorganischen Chemie;
Walter de Gruyter: Berlin, 2007; Vol. 102, pp 1327-1363.

11026 Inorganic Chemistry, Vol. 49, No. 23, 2010
Lehmann et al.

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 11027
Figure 8. IR spectra of the crude product obtained after workup of the
reaction between 8 and sodium adamantate in MeOH/MeCN at 50 °C
after 2 h, 5 h, and 8 h (1700-1400 cm-1 region).
Description of the Crystal Structures. Previously, only
[NiII2L( μ-O2CR)]þ complexes with relatively basic carbox-
ylato coligands (i.e., pKb = 9-10) have been crystallogra-
phically characterized. To correlate Ni-O bond distances
with the pKb values it was necessary to examine the struc-
tures of further [NiII L( μ-O2CR)]þ complexes. In this
2
study, the crystal structures of compounds 2, 3, and 5,
bearing the weakly basic trifluoroacetate, trichloroacetate,
and propiolate (HCtC-CO2-) ions, respectively, have
been determined. The structure of the adamantate complex
19 was also studied to evaluate if the Ni-O bond lengths
depend on steric effects. Table 3 lists the data collection and
processing parameters for the four new structures. ORTEP
views of the structures are presented in Figure 6, and selected
bond lengths and angles are given in Table 4. The metrical
data for the known structures of 8-11, 15, 17, 19, and 20 are
included in Table 4 for comparison. The atom numbering
scheme used for the central N3Ni( μ-SR)2( μ-O2CR)NiN3
core in 2 was applied for all complexes to facilitate structural
comparisons.
All complexes are composed of well-separated [NiII
2
L( μ-O2CR)]þ cations, BPh4- anions, and MeCN or EtOH
solvate molecules. In each case, the macrocycle assumes a
“bowl-shaped”, calixarene-like conformation generating a
( fac-N3)NiII( μ-SR)2( μ1,3-O2CR)NiII( fac-N3) core, as ob-
served in all previously reported [NiII2L( μ-O2CR)]þ
structures.8,14 The complexes are essentially isostructural
and the corresponding bond lengths and angles lie within
very narrow ranges.
Figure 7 represents plots of the average Ni-O, Ni-
Ntrans (Ni-N2 and Ni-N5), Ni-N, and Ni-S bond
lengths against the pKb value of the corresponding free
carboxylate ion. The solid lines represent the best linear
fits. The correlation coefficients of 0.703, -0.403, -0.757,
and 0.570, respectively, are indicative of only poor corre-
lations, but the trends are obvious.79 Thus, as can be seen,
the Ni-O distances decrease steadily with increasing
Figure 7. (a-d) Plot of average Ni-O (a), Ni-Ntrans (b), Ni-N (c), and
Ni-S (d) bondlengthsin the [NiII2L( μ-O2CR)]þ complexes (2, 3, 5, 8-11,
15, 17, 19, 20) versus the pKb value of the carboxylate coligand. The solid
lines represent the best linear fits (correlation coefficients: (a) 0.703, (b)
-0.403, (c) -0.757, (d) 0.570).79
˚
carboxylate basicity, from 2.044(3) A in 2 bearing the
This may be attributed to the fact that the CH
interactions operate perpendicularly to the Ni-O bond-
ing interactions, as indicated in Figure 5. Hence, the
π
3 3 3
˚
(79) Parameters resultant from linear least-squares fits: (a) d(Ni-O)/[A]
CH π interactions do not reinforce the Ni-O bonds
and do not contribute to the crystal field splitting para-
˚
3 3 3
= 1.96(2) þ 0.005(2) ꢁ pKb, R = 0.703; (b) d(Ni-Ntrans)/[A] = 2.18(2) -
˚
0.003(2) ꢁ pKb, R = -0.403; (c) d(Ni-N)/[A] = 2.249(6) - 0.0018(5) ꢁ pKb,
˚
meter Δo.
R = -0.757; (d) d(Ni-S)/[A] = 2.43(2) - 0.004(2) ꢁ pKb, R = 0.57.

11028 Inorganic Chemistry, Vol. 49, No. 23, 2010
Lehmann et al.
Table 5. Carboxylate Exchange Reactions of Various [NiII2L( μ-O2CR)]þ Complexesa,b
entry
R
R0
solvent
T
time
[Ni2L( μ-O2CR0)]þ/[Ni2L( μ-O2CR)]þc
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
Ph
Ph
Ph
C10H15
C10H15
C10H15
C10H15
CH3
MeOH/MeCN
MeOH/MeCN
MeOH
25 °C
50 °C
50 °C
50 °C
50 °C
50 °C
50 °C
50 °C
50 °C
24 h
8 h
6 h
4 d
8 h
8 h
8 h
8 h
8 h
ca. 80:20
ca. 95:5
ca. 95:5
ca. 50:50
ca. 95:5
ca. 95:5
no reaction
no reaction
no reaction
MeCN
MeOH/MeCN
MeOH/MeCN
MeOH/MeCN
MeOH/MeCN
MeOH/MeCN
CCl3
C10H15
CH3
CCl3
a The data refer to the perchlorato complexes. b The carboxylates were either supplied as sodium (in MeOH or 50:50 MeOH/MeCN) or
triethylammonium salts (in MeCN). c The [NiII2L( μ-O2CR’)][ClO4]/[NiII2L( μ-O2CR)][ClO4] ratio was determined by visual integration of the νas(CO)
bands in the IR spectrum of the crude product after workup.
˚
weakly basic trifluoroacetate ion to 2.003(2) A in 17 with
the much more basic acetate ion [d(Ni-O)/d(pKb) =
presented in Table 5. Given the uncertainty of the visual
integration method used these data should be taken as
indicative rather than definitive.
˚
-0.005(2) A]. The decrease in the Ni-O bonds is coupled
with Ni-N and Ni-S bond length changes. Thus, the
average Ni-N (or Ni-Ntrans) distances increase and the
Ni-S bond length decreases with increasing basicity of
the carboxylate. The average Ni-N and Ni-S bond
length changes (per pKb unit) are smaller than variation
The most important results of these studies are as
follows: (i) The [NiII2L( μ-O2CR)][ClO4] complexes are
relatively inert to substitution reactions. As can be seen
from entries 1-4, the best reaction conditions include
the polar, protic solvent MeOH, an excess of the sodium
carboxylate and elevated temperatures 50 °C. Even
under these optimized conditions, about 8 h are required
to drive the respective substitution reactions to comple-
tion (entries 1-6). (ii) Of the two complexes investigated
exemplarily, only the formato-bridged complex 8 under-
went substitution reactions (entries 1-6), whereas no
reaction was observed with the benzoato complex
(entries 7-9). (iii) The formato coligand could be re-
placed by both more and less basic carboxylate ions
(entries 5 and 6).
˚
of the Ni-O distances (ca. 0.002-0.004 A), but add up to
a significant value of about 0.04 A over the full pKb range.
˚
There is also a clear correlation between the Ni-S-Ni
bond angle and the pKb value (not shown in Figure 7).
With increasing pKb value, the Ni-S-Ni angle steadily
contracts from 91.15(2)° in 2 (trifluoroacetate) to
89.59(4)° in 19 (adamantate). This in turn leads to a
˚
decrease of the Ni Ni distance by about 0.10 A. All
3 3 3
of these trends are also seen for complexes coligated by
sterically demanding carboxylate ions (i.e., 10, 19,
and 20). This observation excludes the possibility of bond
length changes due to steric effects. Overall, the observed
correlation between pKb value and Ni-N and Ni-S
bond distances is significant and in good agreement with
the conclusions drawn from the UV/vis spectroscopic
investigations. The UV/vis spectroscopic differences of
the carboxylate complexes are directly related to the
bonding features of the carboxylate coligands.
½NiII2Lðμ -O2CRÞꢀ½ClO4ꢀ þ R0CO2
-
f ½NiII2Lðμ -O2CR0Þꢀ½ClO4ꢀ þ RCO2
ð3Þ
-
The observations made for the formato complex 8 can
be rationalized by taking into account the UV/vis spectro-
scopic results described above showing only small stabi-
lity differences of the various carboxylato complexes76
and the thermodynamic control of the equilibrium de-
scribed in eq 3. If, to a first approximation, the relative
stabilities Krel = exp(ΔΔG/RT) of a pair of two com-
plexes is related to the difference in LFSE (i.e., ΔΔG =
Carboxylate Exchange Reactions. Using IR spectrosco-
py it has also been possible to study carboxylate exchange
reactions. Note that these reactions are less readily inves-
tigated by other methods. To probe whether the complexes
undergo substitution reactions, two selected complexes,
namely, the formato and the benzoato species 8 and 11,
were allowed to react with three different carboxylate ions
(adamantate, acetate, trichloroacetate) according to eq 3.80
Samples (5 mL) of the reaction mixtures were removed at
regular intervals, an excess of LiClO4 was added, and the
solution concentrated to about 2 mL to precipitate the
reaction product and unreacted starting material. The
solid was then filtered, dried and analyzed by IR spec-
troscopy. The relative amounts of starting materials and
products were estimated from the relative intensities of
the corresponding νas(CO) bands.81 A typical set of
spectra is presented in Figure 8. The reaction data are
Δ
LFSE), one can calculate equilibrium constants of Krel =
3.44, 0.65, and 0.28 for the exchange reactions of 8 with
trichloroacetate, acetate, and adamantate (C10H15CO2-),
respectively.82 It can easily be shown for all three equili-
bria that the reaction conditions employed (i.e., 10-fold
molar excess of the carboxylate ions) are sufficient to drive
the reaction to near completion, as also established by IR
spectroscopy.83
The observations made for the benzoato complex are
unexpected. If one uses the same arguments as above,
(82) ΔΔG(3,8) = ΔLFSE(3,8) = 1.2 ꢁ [Δo(3) - Δo(8)] = 1.2 ꢁ 214 cm-1
ꢁ
(80) The sodium or triethylammonium carboxylates were added in a 10-
fold molar excess. At lower molar ratios, the reactions were too slow and
inconvenient to study.
(81) All complexes have very similar solubility properties. It is therefore
very unlikely, that the equilibrium in eq 3 is influenced by solubility
differences.
11.96 J/(mol cm-1) = 3.06 kJ/mol; ΔΔG(17,8) = 1.2 ꢁ (-112 cm-1) ꢁ 11.96
J/(mol cm-1) = -1.61 kJ/mol; ΔΔG(20,8) = 1.2 ꢁ (-48 cm-1) ꢁ 11.96
J/(mol cm-1) = -0.688 kJ/mol; Krel = exp(ΔΔG/(RT)).
(83) x = [HCO2-], [3], [19], or [20]; Krel = [x][x]/[c0-x][10co-x]. For c0 =
0.001, x ∼ 0.00097, 0.00087, and 0.00077 (corresponding to 97%, 87%, and
77% conversions).

Article
Inorganic Chemistry, Vol. 49, No. 23, 2010 11029
exchange reactions of complex 11 should be thermody-
namically feasible in all cases as well. The observed “inert-
ness” is most likely due to kinetic factors such as steric
hindrance, which slow down the reactions, or to secondary
interactions (and hence unfavorable low equilibrium con-
stants) orboth. Thetruthmustawaitfurtherkineticstudies,
which are the focus of current investigations.
fact attributable to kinetic factors or to the absence (8) or
presence(11) of stabilizing secondary CH π host-guest
3 3 3
interactions. (v) [NiII2L( μ-O2CR)]þ complexes are readily
identified by their νas(CO) and the νs(CO) stretching frequen-
cies which lie in the ranges 1684-1576 cm-1 and 1428-1348
cm-1, respectively. The scattering can be attributed to the
varying basicity of the bridging carboxylato coligand, since
their immediate environment in the dinuclear complexes is
practically the same. (vi) UV/vis spectroscopic examinations
Concluding Remarks
3
3
reveal that the spin-allowed A2g f T2g (ν1) transition is
steadily red-shifted across the series, from 1094 nm in 2
bearing the weakly basic trifluoroacetate ion to 1129 nm in
18 with the muchmore basic propionate ion. (vii) The binding
of a basic carboxylato donor affects the macrocycle-nickel-
(II) interactions. This is reflected in the solid state structures
of the complexes by small but significant bond length changes
(i.e., the Ni-N bonds shorten and the Ni-S bonds lengthen).
The main findings of the present work can be summarized
as follows: (i) The macrocycle L2- allows preparation and
isolation of a series of isostructural [NiII2L( μ-O2CR)]þ com-
plexes with different carboxylate pKb values (pKb = 9-14).
(ii) The [NiII2L( μ-Cl)]þ complex 1 reacts readily with sodium
or triethylammonium carboxylates and represents a versatile
starting material for the [NiII2L( μ-O2CR)]þ species. Substi-
tution reactions are not affected by varying basicity or steric
hindrance of the entering carboxylate groups. (iii) In contrast
to the corresponding [ZnII2L( μ-O2CR)]þ complexes which
typically exchange their carboxylates on the NMR time
scale,8 the [NiII2L( μ-O2CR)]þ complexes are relatively inert
to substitution reactions attributable (in part) to the ligand
field effects associated with the d8 electronic configuration of
the nickel(II) (S = 1) ions. An excess of sodium carboxylate,
elevated temperatures of 50 °C, and long reaction times
(>8 h) are required to drive the carboxylate exchange
reactions to completion. (iv) The formato coligand in 8 can be
replaced by both more and less basic carboxylates. The
benzoato-bridged complex 11 does not react in any case, a
Acknowledgment. We are thankful for Prof. Dr. H.
Vahrenkamp and Prof. Dr. H. Krautscheid for providing
facilities for spectroscopic and X-ray crystallographic
measurements. This work was supported by the Deutsche
Forschungsgemeinschaft (Projects KE 585/4-1,2,3), the
University of Freiburg and the University of Leipzig.
U.L. thanks the graduate school “BuildMona” for a
fellowship support.
Supporting Information Available: X-ray crystallographic
data in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.