Tetradentate vs pentadentate coordination in copper(II) complexes of pyridylbis(aminophenol) ligands depends on nucleophilicity of phenol donors

Article abstract of DOI:10.1021/ic201536c

The ligand binding preferences of a series of potentially pentadentate pyridylbis(aminophenol) ligands were explored. In addition to the previously reported ligands 2,2′-(2-methyl-2-(pyridin-2-yl)propane-1,3-diyl) bis(azanediyl)bis(methylene)diphenol (H

Full text of DOI:10.1021/ic201536c

ARTICLE
pubs.acs.org/IC
Tetradentate vs Pentadentate Coordination in Copper(II) Complexes
of Pyridylbis(aminophenol) Ligands Depends on Nucleophilicity of
Phenol Donors
Rajendra Shakya,† Zhaodong Wang, Douglas R. Powell, and Robert P. Houser*
Department of Chemistry and Biochemistry, University of Oklahoma, 101 Stephenson Parkway, Norman, Oklahoma 73019-5251,
United States
S Supporting Information
b
ABSTRACT: The ligand binding preferences of a series of potentially pentadentate
pyridylbis(aminophenol) ligands were explored. In addition to the previously reported
ligands 2,20-(2-methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)-
diphenol (H2L1) and 6,60-(2-methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(azanedi-
yl)bis(methylene)bis(2,4-di-tert-butylphenol) (H2L1‑tBu), four new ligands were synthe-
sized: 6,60-(2-methyl-2(pyridine-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)bis-
(2,4-dibromophenol) (H2L1‑Br), 6,60-(2-methyl-2(pyridine-2-yl)propane-1,3diyl)bis(azanediyl)-
bis(methylene)bis(2-methoxyphenol) (H2L1‑MeO), 2,20-(2-methyl-2(pyridine-2-yl)pro-
pane-1,3diyl)bis(azanediyl)bis(methylene)bis(4-nitrophenol) (H2L1‑NO2), and 2,20-(2-
phenylpropane-1,3-diyl)bis(azanediyl)bis(methylene)diphenol (H2L2). These ligands,
when combined with copper(II) salts and base, form either tricopper(II) species or
monocopper(II) species depending on the nucleophilicity of the phenol groups in the
ligands. All copper complexes were characterized by X-ray crystallography, cyclic voltam-
metry, and spectroscopic methods in solution. The ligands in trimeric complexes [{CuL1(CH3CN)}2Cu](ClO4)2 (1),
[{CuL1Cl}2Cu] (1a), and [{CuL2(CH3CN)}2Cu](ClO4)2 (1b) and monomeric complex [CuL1‑tBu(CH3OH)] (2) coordinate in
a tetradentate mode via the amine N atoms and the phenolato O atoms. The pyridyl groups in 1, 1a, and 2 do not coordinate, but
instead are involved in hydrogen bonding. Monomeric complexes [CuL1‑Br] (3a), [CuL1‑NO2] (3b), and [CuL1‑MeONa-
(CH3OH)2]ClO4 (3c) have their ligands coordinated in a pentadentate mode via the amine N atoms, the phenolato O atoms, and
the pyridyl N atom. The differences in tetradentate vs pentadentate coordination preferences of the ligands correlate to the
nucleophilicity of the phenolate donor groups, and coincide with the electrochemical trends for these complexes.
’ INTRODUCTION
of the expected JahnꢀTeller distortion.3 This situation can also
result from steric constraints that prevent the ligand from coordinat-
ing via all donor atoms. For example, Cu(II) complexes from our
laboratory using pyridylbis(imine) or pyridylbis(guanidine) ligands
coordinate such that the pyridyl ring is spatially too far from the
metal center to coordinate.4 Similarly, Fe(II) complexes of a
tetradentate 1,2-dipyridyl-1,2-diaminoethane ligand display k3 den-
ticity because of steric constraints.5 Finally, modifications to the
ligand can lead to changes in denticity. An example of this was
reported where a cyclam ligand goes from k4 to k3 in a Ru(III)
nitrosyl complex when one of the N-atoms of the cyclam was
substituted with a carboxypropyl pendant arm.6 While the authors
did not definitively pinpoint the reason for the change in denticity,
they offer some possible explanations ranging from the reduced σ-
donor ability of the N-atom, to steric and kinetic arguments.6 Here
we describe another example of a ligand system that displays variable
denticity that we believe is electronically based.
Polydentate ligands serve myriad purposes in coordination
chemistry. Important progress in inorganic chemistry, far too
voluminous to enumerate here, has been made at least in part
because of control over structure and reactivity imparted by
polydentate ligands. Polydentate ligands can coordinate to
different metal ions with varying denticity depending on the
coordination number and geometry preferences of the metal.
This principle of “maximum site occupancy”, where coordina-
tively saturated complexes are formed with all ligand binding sites
being used, was recently exploited to form heteronuclear com-
plexes.1 However, it is not uncommon for polydentate li-
gands to coordinate via fewer than all of their ligand binding
sites. Ambiguous coordination possibilities for polydentate li-
gands in naming inorganic complexes are resolved by the IUPAC
nomenclature standards and rules.2 Different coordination pre-
ferences of different metal ions can lead to varying denticity, as
exemplified by a N,N-bis(2-picolyl)amine derivative with a
pendant ethoxyethanol side chain.3 This ligand coordinates with
k5 denticity with Co(II), Ni(II), and Zn(II), but with reduced
denticity (k4 or k3, depending on whether a long axial CuꢀO
interaction is interpreted as a bond or not) with Cu(II) because
Previously we reported a new family of ligands based on the
pyridylbis(acetamide) ligand, H2pp(ac)2,7 that contain phenol
Received: July 19, 2011
Published: October 18, 2011
r
2011 American Chemical Society
11581
dx.doi.org/10.1021/ic201536c Inorg. Chem. 2011, 50, 11581–11591
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Inorganic Chemistry
ARTICLE
Scheme 1
propane-1,3-diamine (ppda),10 2,20-(2-methyl-2-(pyridin-2-yl)propane-
1,3-diyl)bis(azanediyl)bis(methylene)diphenol (H2L1),4a and 6,60-(2-
methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)-
bis(2,4-di-tert-butylphenol) (H2L1‑tBu)9 were synthesized according to
published procedures. 2-Phenylpropane-1,3-diamine (phpda)11 was
synthesized according to a modified procedure, following the method
used for ppda. Solvents used were doubly purified using alumina
columns in a MBraun solvent purification system (MB-SPS). Infrared
spectra were measured from 4000 to 400 cmꢀ1 as KBr pellets on a
NEXUS 470 FTIR spectrometer. 1H NMR spectra were measured using
a Varian 300 MHz instrument. Mass spectra were measured on a Q-TOF
quadrupole time-of-flight mass spectrometer (Micromass, Manchester,
U.K.) equipped with a Z-spray electrospray ionization (ESI) source.
Elemental analyses were performed by Atlantic Microlab, Norcross, GA.
UVꢀvisible spectra were measured using a Shimadzu UV2401PC
spectrophotometer in the range 250 to 1000 nm on solutions ranging
in concentration from 1.0 ꢁ 10ꢀ3 M and 1.0 ꢁ 10ꢀ4 M. Cyclic
voltammetry experiments were performed using a BAS 50W potenti-
ometer and a standard three-electrode cell with a glassy-carbon working
electrode, a Pt-wire auxiliary electrode, and an Ag/AgCl reference
electrode under an inert atmosphere at room temperature. X-band
EPR spectra of the complexes were recorded at 77 K using a Bruker EMX
spectrometer. Magnetic susceptibilities of the complexes in the solid
state were measured at 295 K using a Johnson Matthey magnetic
susceptibility balance (MSBꢀAUTO) with a magnetic field strength
of 4.5 kGauss and a measurement range of (1.999 ꢁ 10ꢀ4 to (5 ꢁ
10ꢀ10 cgs. Solution magnetic susceptibilities were measured at 295 K by
the Evans method.12
groups appended to the nitrogens.4a While not the focus of the
present work, copper coordination chemistry with phenol-con-
taining ligands continues to attract attention because of its
relevance to galactose oxidase.8 The ligands included a
pyridylbis(amidophenol), H4Lamide, a pyridylbis(iminophenol),
H2Limine, and a pyridylbis(aminophenol), H2Lamine (Scheme 1).
The structures of the copper(II) complexes that formed were
dependent on the nature of the ligands. The copper(II) com-
plexes that were isolated and characterized had varying nuclearity
ranging from mononuclear [Cu(Limine)(CH3OH)], to trinuclear
[{CuLamine(CH3CN)}2Cu], to hexanuclear [Cu6(HLamide)4-
(H2O)2].4a H2Limine forms a monomeric species, [Cu(Limine)-
(CH3OH)], where the ligand has been deprotonated at the
phenol O positions, making the ligand dianionic. The amide
ligand H4Lamide forms a hexacopper cluster, [Cu6(HLamide)4-
(H2O)2], where each ligand has had three protons removed
to form a trianionic ligand. Finally, H2Lamine forms a tri-
copper complex, [{CuLamine(CH3CN)}2Cu] (1). Using the
amine ligand H2Lamine and its tert-butyl substituted derivative
H2LtBu‑amine we similarly explored the iron(III) chemistry of
these ligands.9
Synthesis of H2L1‑Br, H2L1‑NO2, and H2L1‑MeO. The ligands H2L1‑Br
,
H2L1‑NO2, and H2L1‑MeO were prepared by the same general procedure in
which ppda (0.330 g, 2.00 mmol) was condensed with the appropriate
aldehyde (4.00 mmol) in 40 mL of methanol at 50 ꢀC. After 2 h, NaBH4
(0.220 g, 6.00 mmol) was added into the solution in small portions at 0 ꢀC.
The solution was then stirred at room temperature for 2 h, the solvent was
evaporated, and the amine product was extracted with dichloromethane,
dried over anhydrous MgSO4, and isolated after solvent evaporation.
6,60-(2-Methyl-2(pyridine-2-yl)propane-1,3-diyl)bis(azan-
ediyl)bis(methylene)bis(2,4-dibromophenol) (H2L1‑Br). 3,5-
Dibromosalicylaldehyde (1.12 g, 4.00 mmol) was the aldehyde used in
the synthesis of H2L1‑Br (0.826 g, 60% yield). Anal. Calcd for
C23H23Br4N3O2: C, 39.9; H, 3.3; N, 6.1. Found: C, 40.4; H, 3.4; N,
5.8. 1H NMR (300 MHz, CD2Cl2, 300 K) δ 1.46 (s, 3H), 2.91 (d, J = 12
Hz, 2H), 3.06 (d, J = 11.7 Hz, 2H), 3.40 (d, J = 14.4 Hz, 2H), 3.98 (d, J =
14.4 Hz, 2H), 7.10ꢀ7.34 (m, 4H), 7.53ꢀ7.75 (m, 3H), 8.52 (d, J = 9 Hz,
1H) ppm. FTIR (KBr): 3288 (OꢀH), 3071 (s), 2842 (s), 1589, 1569,
1454 (s), 1384, 1282, 1260, 1152, 1089, 993, 860, 787, 748, 684,
651 cmꢀ1. ESI-MS (MeOH): m/z = 693 [H2L1‑Br + H]+.
The amine ligand H2Lamine, henceforth in this paper referred
to as H2L1, coordinates to the copper ions in 1 in a tetradentate
fashion where the pyridyl ring was not coordinated to the copper
center. However, in the iron(III) complexes of H2L1 and
H2LtBu‑amine (henceforth referred to as H2L1‑tBu), the pyridyl
ring does coordinate, rendering the ligands pentadentate chela-
tors. Seeking to understand why H2L1 and H2L1‑tBu act as a
tetradentate ligand in some instances and a pentadentate ligand
in others, we set out to expand the copper(II) coordination
chemistry of the pyridylbis(aminophenol) ligands. Here we
report three new ligands in the H2L1 family of pyridylbis-
(aminophenol) ligands, their copper(II) coordination chemistry,
and provide insight into the tetradentate versus pentadentate
chelating preferences of these ligands on the basis of the
nucleophilicity of the phenol rings.
2,20-(2-Methyl-2(pyridine-2-yl)propane-1,3diyl)bis(azanediyl)-
bis(methylene)bis(4-nitrophenol) (H2L1‑NO2). 2-Hydroxy-5-ni-
trobenzaldehyde (0.668 g, 4.00 mmol) was the aldehyde used in
the synthesis of H2L1‑NO2 (0.654 g, 70% yield). Anal. Calcd for
C23H25N5O6 2CH3OH: C, 56.5; H, 6.3; N, 13.2. Found: C, 56.1; H,
3
5.5; N, 12.6. 1H NMR (300 MHz, CD2Cl2, 300 K) δ 1.45 (s, 3H), 2.99
(d, J = 12 Hz, 2H), 3.14 (d, J = 11.7 Hz, 2H), 3.43 (s, 2H), 3.73 (s, 2H),
4.00 (m, 4H), 6.83 (d, J = 4.2 Hz, 1H); 7.24ꢀ7.37 (m, 4H), 7.78ꢀ8.08
(m, 3H), 8.53 (d, J = 9.0 Hz, 1H) ppm. FTIR (KBr): 3431 (OꢀH), 2930
(s), 1593, 1476 (s), 1335, 1284, 1157, 1089, 995, 865, 787, 752,
650 cmꢀ1. ESI-MS (MeOH): m/z = 468 [H2L1‑NO2 + H]+.
6,60-(2-Methyl-2(pyridine-2-yl)propane-1,3diyl)bis(azan-
ediyl)bis(methylene)bis(2-methoxyphenol) (H2L1‑MeO).
2-Hydroxy-3-methoxybenzaldehyde (0.608 g, 4.00 mmol) was the
aldehyde used in the synthesis of H2L1‑MeO (0.570 g, 65% yield).
’ EXPERIMENTAL SECTION
General Procedures. Unless otherwise stated, all reagents were
used as received from commercial sources. 2-methyl-2-(pyridine-2-yl)
Anal. Calcd for C25H31N3O4 H2O: C, 65.9; H, 7.3; N, 9.2. Found: C,
3
65.6; H, 7.2; N, 8.9. 1H NMR (300 MHz, CDCl3, 300 K) δ 1.49
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Inorganic Chemistry
ARTICLE
(s, 3H), 2.88 (d, J = 11.4 Hz, 2H), 3.14 (d, J = 11.7 Hz, 2H), 3.82 (s, 6H),
3.90 (m, 4H), 6.58ꢀ6.78 (m, 6H), 7.13ꢀ7.64 (m, 3H), 8.50 (d, J = 6 Hz,
1H) ppm. FTIR (KBr): 3418 (OꢀH), 2832 (s), 1589, 1477 (s), 1328,
1232, 1158, 1074, 769, 732, 621 cmꢀ1. ESI-MS (MeOH): m/z = 438
[H2L1‑MeO + H]+.
Syntheses of 2,20-(2-Phenylpropane-1,3-diyl)bis(azanediyl)-
bis(methylene)diphenol (H2L2). The ligand H2L2 was prepared by
the condensation of phpda (0.375 g, 2.50 mmol) with salicylaldehyde
(0.610 g, 5.00 mmol) in a 25 mL MeOH solution. The solution was stirred
at 50 ꢀC for 2 h, yielding a pale yellow solution. NaBH4 (0.285 g, 7.50
mmol) was then added at 0 ꢀC in small portions. The solution was stirred at
room temperature for 2 h, the solvent was evaporated, and the product
amine was extracted with dichloromethane, dried over MgSO4, and isolated
Anal. Calcd for C40H61CuN3O3: C, 69.1, H, 8.8, N 6.0. Found: C, 68.6,
H, 8.8, N, 6.2. UV/vis (CH2Cl2) [λmax, nm (ε, Mꢀ1 cmꢀ1)]: 298
(11,200), 406 (1,600), 597 (855). FTIR (KBr): 2950ꢀ2865 (s), 1617
(s), 1592 (s), 1528, 1471 (s), 1437 (s), 1413, 1384, 1304, 1273, 1202,
1169, 1063, 830, 790, 748, 480 cmꢀ1. ESI-MS (MeOH): m/z = 663
[CuL1‑tBu + H]+. EPR (9.432 GHz, mod. amp. 5.0 G, CH2Cl2, 77 K):
g = 2.225, g^ = 1.998, and A = 184 G. Solution magnetic moment (Evans
method, 19.8 ꢀC, 5.50 ꢁ 10ꢀ3 M, acetonitrile-d3): 1.70 μB/Cu. Solid state
magnetic moment (MSB-Auto, 4.5 kG, 22.0 ꢀC): 1.50 μB/Cu.
[CuL1‑Br] (3a). H2L1‑Br (0.0693 g, 0.100 mmol) and Cu(OTf)2
(0.036 g, 0.10 mmol) were used (0.0527 g, 65% yield). Anal. Calcd for
C23H21Br4CuN3O2 1/2CH2Cl2: C, 35.4, H, 2.8, N5.3. Found:C, 35.3, H,
3
2.6, N, 5.4. UV/vis (CH2Cl2) [λmax, nm (ε, Mꢀ1 cmꢀ1)]: 306 (10,300),
352 (2,800), 407 (2,380), 648 (180). FTIR (KBr): 3107 (s), 2919 (s),
2361, 2338, 1652, 1575, 1434 (s), 1398, 1300, 1264, 1155, 1090, 1035,
after solvent evaporation (0.68 g, 75% yield). Anal. Calcd for C23H26N2O2
3
1
1/2CH3OH: C, 74.6; H, 7.5; N, 7.4. Found: C, 75.1; H, 7.2; N, 7.1. H
860, 789, 750, 667, 627 cmꢀ1. ESI-MS (MeOH): m/z = 776 [CuL1‑Br
+
NMR (300 MHz, CDCl3, 293 K) δ 2.78ꢀ2.92 (m, 4H), 3.02ꢀ3.08
(m, 1H), 3.78ꢀ3.89 (m, 4H), 6.65ꢀ6.74 (m, 4H), 6.85ꢀ6.88 (m, 2H),
7.05ꢀ7.30 (m, 7H) ppm. FTIR (KBr): 3295 (OꢀH), 3027ꢀ2842 (s),
1615 (s), 1588 (s), 1490 (s), 1458 (s), 1255, 1182, 1151, 1102, 1034, 971,
932, 844, 753, 720, 700, 617, 540, 451 cmꢀ1. ESI-MS (MeOH): m/z = 363
[H2L2 + H]+.
Syntheses of the Complexes. A general synthetic route was used
for the preparation of all complexes in which, under an open air
atmosphere, the appropriate Cu(II) salt was added to a solution of the
respective ligand (0.100 mmol) in acetonitrile for 1b and in methanol for
1a, 2, and 3aꢀc. For complexes 1aꢀb, 2, and 3aꢀb, Et3N (0.040 mL,
0.30 mmol) was used as the base for phenol deprotonation. For 3c, NaH
(0.072 g, 0.30 mmol) was used as the base for phenol deprotonation. In
each case, the resulting dark green solution was stirred for 2 h at room
temperature and filtered to remove any unreacted solids. X-ray quality
crystals of all complexes were obtained after crystallization by appro-
priate methods.
Na]+. EPR (9.442 GHz, mod. amp. 10.0 G, DMSO, 77 K): g = 2.350, g^ =
2.109, and A = 169 G. Solution magnetic moment (Evans method,
19.8 ꢀC, 5.50 ꢁ 10ꢀ3 M, chloroform-d1): 1.73 μB/Cu. Solid state
magnetic moment (MSB-Auto, 4.5 kG, 22.0 ꢀC): 1.60 μB/Cu.
[CuL1‑NO2] (3b). H2L1‑NO2 (0.0467 g, 0.100 mmol) and Cu(ClO4)2
6H2O (0.0370 g, 0.100 mmol) were used (0.0339 g, 60% yield). Anal. Calcd
-
3
for C23H23Cu N5O6 2H2O: C, 48.9, H, 4.8, N 12.4. Found: C, 48.7, H, 4.8,
3
N, 12.4. UV/vis (CH3CN) [λmax, nm (ε, Mꢀ1 cmꢀ1)]: 404 (29,400), 641
(290 sh). FTIR (KBr): 3164 (s), 1652, 1596, 1479 (s), 1437, 1399, 1290,
1182, 1092, 928, 898, 835, 760, 667 cmꢀ1. ESI-MS (MeOH): m/z = 529
[CuL1‑NO2 + H]+. EPR (9.449 GHz, mod. amp. 4.0 G, DMSO, 77 K): g =
2.316, g^ = 2.10, and A = 176 G. Solution magnetic moment (Evans
method, 19.8 ꢀC, 5.50 ꢁ 10ꢀ3 M, dimethylsulfoxide-d6): 1.84 μB/Cu. Solid
state magnetic moment (MSB-Auto, 4.5 kG, 22.0 ꢀC): 1.83 μB/Cu.
[CuL1‑MeONa(CH3OH)2]ClO4 (3c). H2L1‑MeO (0.044 g, 0.10
mmol) and Cu(ClO4)2 6H2O (0.036 g, 0.10 mmol) were used
3
Caution! Perchlorate salts of metal complexes with organic ligands are
potentially explosive. Although no difficulty was encountered during the
syntheses described herein, they should be prepared in small amounts and
handled with caution.
(0.0480 g, 70% yield). Anal. Calcd for C27H37ClCuN3NaO10 H2O:
3
C, 46.0, H, 5.6, N 6.0. Found: C, 45.3, H, 5.0, N, 6.6. UV/vis (CH2Cl2)
[λmax, nm (ε, Mꢀ1 cmꢀ1)]: 283 (17,000), 342 (1,950), 423 (2,210) 594
(450 sh). FTIR (KBr): 2836 (s), 1595, 1569, 1481 (s), 1361, 1239, 1163,
1079, 960, 791, 747, 623 cmꢀ1. ESI-MS (MeOH): m/z = 521
[CuL1‑MeO + Na]+. EPR (9.436 GHz, mod. amp. 5.0 G, CH2Cl2, 77
K): g = 2.295, g^ = 2.086, and A = 176 G. Solution magnetic moment
(Evans method, 19.8 ꢀC, 5.50 ꢁ 10ꢀ3 M, acetonitrile-d3): 1.59 μB. Solid
state magnetic moment (MSB-Auto, 4.5 kG, 22.0 ꢀC): 1.25 μB.
X-ray Crystal Structure Determination. X-ray quality crystals
of 1a and 1b were obtained by ether diffusion into an acetonitrile or
methanol solution of 1a or 1b, respectively. Single crystals of 2, 3b,
and 3c were obtained by slow evaporation of methanol solutions of
the corresponding compounds, whereas X-ray quality crystals of 3a
were isolated by slow evaporation of a dichloromethane solution of
3a. Intensity data for all complexes were collected using a diffract-
ometer with a Bruker APEX ccd area detector.13 Data were collected
for all complexes except 3c using graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å), while for 3c, graphite-monochromated
Cu Kα radiation (λ = 1.54178 Å) was used. The samples were cooled
to 100(2) K. Cell parameters were determined from a nonlinear least-
squares fit of the data. The data were corrected for absorption by
the semiempirical method.14 The structures were solved by direct
methods and refined by full-matrix least-squares methods on F2.15
Hydrogen atom positions of hydrogens bonded to carbons were
initially determined by geometry and refined by a riding model.
Hydrogens bonded to nitrogens or oxygens were located on a
difference map, and their positions were refined independently.
Non-hydrogen atoms were refined with anisotropic displacement
parameters. Hydrogen atom displacement parameters were set to
1.2 (1.5 for methyl) times the displacement parameters of the
bonded atoms.
[{CuL1Cl}2Cu] (1a). H2L1 (0.037 g, 0.10 mmol) and CuCl2 2H2O
3
(0.017 g, 0.10 mmol) were used (0.031 g, 60% yield). Anal. Calcd for
C46H50Cl2Cu3N6O4: C, 54.6; H, 5.0; N 8.3. Found: C, 54.8; H, 5.3; N,
8.2. UV/vis (CH2Cl2) [λmax, nm (ε, Mꢀ1 cmꢀ1)]: 335 (39,000), 396
(3,240), 645 (450 sh). FTIR (KBr): 1597 (s), 1577 (s) 1487, 1456,
1400, 1274, 1261, 1201, 1114, 1042, 995, 875, 787, 751, 732, 605, 467,
413 cmꢀ1. ESI-MS (MeOH): m/z = 439 [CuL1 + H]+, 974
[Cu3(L1)2Cl]+. EPR (9.436 GHz, mod. amp. 5.0 G, CH2Cl2, 77 K):
g = 2.233, g^ = 2.025, and A = 186 G. Solution magnetic moment (Evans
method, 19.8 ꢀC, 6.67 ꢁ 10ꢀ3 M, chloroform-d1): 1.09 μB/Cu. Solid state
magnetic moment (MSB-Auto, 4.5 kG, 22.0 ꢀC): 1.00 μB/Cu.
[{CuL2(CH3CN)}2Cu](ClO4)2 (1b). H2L2 (0.036 g, 0.10 mmol)
and Cu(ClO4)2 6H2O (0.037 g, 0.10 mmol) were used (0.036 g, 65%
3
yield). The sample for elemental analysis was powdered and dried under
vacuum overnight, removing acetonitrile solvent of crystallization from
the complex. However, the complex being hygroscopic, a water molecule
was found to be associated in its elemental analysis. Anal. Calcd for
C46H48Cl2Cu3N4O12 H2O: C, 49.0; H, 4.5; N 5.0. Found: C, 48.7; H,
3
4.5; N, 5.0. UV/vis (CH3CN) [λmax, nm (ε, Mꢀ1 cmꢀ1)]: 285 (27,300),
392 (3,070), 592 (450 sh). FTIR (KBr): 1599 (s), 1578, 1487 (s), 1457
(s) 1400, 1274, 1254, 1203, 1117, 1098, 1087 (ClO4ꢀ), 995, 875, 763,
703, 624, 602, 535, 469, 415 cmꢀ1. ESI-MS (MeOH): m/z = 455
[Cu3(L2)2]2+. EPR (9.444 GHz, mod. amp. 10.0 G, CH3CN, 77 K): g =
2.28, g^ = 2.07, and A = 160 G. Solution magnetic moment (Evans
method, 19.8 ꢀC, 5.50 ꢁ 10ꢀ3 M, acetonitrile-d3): 1.11 μB/Cu. Solid
state magnetic moment (MSB-Auto, 4.5 kG, 22.0 ꢀC): 0.940 μB/Cu.
[CuL1‑tBu(CH3OH)] (2). H2L1‑tBu (0.060 g, 0.10 mmol) and Cu-
(ClO4)2 6H2O (0.037 g, 0.10 mmol) were used (0.046 g, 70% yield).
3
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Inorganic Chemistry
ARTICLE
Table 1. Crystallographic Data for 1a, 1b, and 2
sodium borohydride to the diamine. The H2L1 family of ligands
(H2L1, H2L1‑tBu, H2L1‑Br, H2L1‑NO2, and H2L1‑MeO, Scheme 1)
differ from each other only in the substituents they have on their
phenol rings. While the H2L1 ligand contains no substituents on
the phenol ring, H2L1‑tBu and H2L1‑Br contain either tert-butyl or
bromo substituents, respectively, in the 3- and 5-positions.
H2L1‑MeO possesses a methoxy substituent in the 3-position,
and H2L1‑NO2 possesses a nitro substituent in the 5-position of
the phenol ring. All five ligands contain the same potential N3O2
donor atom set, with pyridyl and amine N-donors, and, when
deprotonated, phenolate O-donors. The diamine ligand H2L2 is
different from the other five ligands in that it has a phenyl group
instead of a pyridyl moiety, giving it a potential N2O2 donor atom
set, with amine N-donors and, when deprotonated, phenolate
O-donors (Scheme 1).
All new copper complexes were synthesized under an open air
atmosphere by combining the copper salt, generally cupric
chloride or cupric perchlorate, with the ligand and a base, either
triethylamine or sodium hydride. The copper(II) complexes iso-
lated in this study fall into two categories: either tricopper complexes
similar to the previously reported [{CuL1(CH3CN)}2Cu](ClO4)2
(1),4a namely, [{CuL1Cl}2Cu] (1a) and [{CuL2(CH3CN)}2-
Cu](ClO4)2 (1b), or monocopper complexes [CuL1‑tBu(CH3-
OH)] (2), [CuL1‑Br] (3a), [CuL1‑NO2] (3b), and [CuL1‑MeO
Na(CH3OH)2]ClO4 (3c). All six new copper complexes were
characterized by elemental analysis, UV/visible spectroscopy,
FTIR, EPR, solution and solid state magnetic susceptibility, and
single crystal X-ray structural analysis.
Trinuclear complexes 1a and 1b are isostructural with
the previously reported trinuclear complex 1 (Scheme 2).4a
The difference between 1 and 1a is simply in the counterion.
The noncoordinating perchlorate anion in 1 leads to acetonitrile
solvent molecules coordinating to two of the copper ions, while
in 1a the chloride anions coordinate (see Scheme 2). The dif-
ference between 1b and the other two is in the ligand: 1b was
synthesized from H2L2 while 1 and 1a were synthesized from
H2L1. As mentioned above, H2L2 contains a phenyl group
instead of a pyridyl group. However, this difference is incon-
sequential to the structures of 1 and 1b since the pyridyl ring does
not coordinate (vide infra), and they are isostructural, including
the perchlorate counterions and the coordinated acetonitrile
solvent molecules.
1a 2H2O
1b 1/3CH3CN
2 CH3OH 1/2H2O
3
3
3
3
formula
fw
C
46H54Cl2
C304H330
Cl12Cu18N38O72
7237.18
C41H66CuN3O4.5
Cu3N6O6
1048.47
736.51
crystal system monoclinic
monoclinic
P21/c
orthorhombic
P212121
12.597(2)
13.704(3)
23.683(5)
90
space group
a (Å)
C2/c
19.352(3)
13.614(2)
19.977(3)
117.984
4647.7(12)
4
21.676(3)
43.659(6)
18.355(6)
114.411(8)
15817(4)
2
b (Å)
c (Å)
β (deg)
V (Å3)
Z
4088.4(14)
4
Fcalcd mg/m3 1.498
1.520
1.197
μ (mmꢀ1
)
1.529
1.368
0.579
θ (deg)
R1,awR2b
1.9 to 28.3
2.1 to 21.7
1.7 to 26.0
0.0273, 0.0769
0.0431, 0.1231 0.0771, 0.2134
[I > 2σ(I)]
GOF on F2
1.001 1.044
1.020
2
a R1 = ∑||Fobs| ꢀ |Fcalc||/∑|Fobs|. b wR2 = {∑[w(Fobs ꢀ Fcalc2)2]/
∑[w(Fobs2)2]}1/2
.
Complex 1a was positioned on a crystallographic center of symmetry,
and restraints on the water OꢀH bond lengths were required. For
complex 1b, the intensity data were truncated to 0.96 Å resolution
because data in higher level shells all had ÆI/σæ < 2. Four independent
cationic trimers were found for complex 1b, with two trimers located in
general positions, and two trimers found to be near or on crystal-
lographic centers of symmetry. The groups containing Cu7 and Cu9
were located near crystallographic centers of symmetry; thus, the
occupancies of the atoms in these groups were set to 0.5. The Cu9
group is an example of “whole molecule” disorder. Most of the perch-
lorate groups were disordered with occupancies refined to 0.847(7) and
0.153(7) for N, 0.661(6) and 0.339(6) for O, 0.903(5) and 0.097(5) for
P, 0.511(7) and 0.489(7) for Q, and, 0.592(7) and 0.408(7) for the R.
The occupancies for the disordered Y acetonitrile refined to 0.544(11)
and 0.456(11) for the unprimed and primed atoms. Restraints on the
positional and displacement parameters of the disordered groups were
required.
For complex 3a, there are three metal complex molecules and two
solvent (CH2Cl2) sites per asymmetric unit of the cell. Both solvent sites
were severely disordered and were best modeled using the Squeeze
program.16 The NꢀH distances were restrained to be approximately
equal. For complex 3b, the intensity data were truncated to 0.94 Å
resolution because data in higher resolution shells all had R(int) > 0.25.
The second water site was so disordered that it was best modeled using
the Squeeze program.16 For complex 3c, the perchlorate anion was
disordered, and was modeled with refined occupancies of 0.591(8) and
0.409(8) for the A and B labeled groups. Restraints on the positional and
displacement parameters of the perchlorate were required. Crystal data
for all complexes are summarized in Tables 1 and 2. Selected bond
lengths and angles for all complexes are summarized in Tables 3ꢀ8.
1, 1a, and 1b all contain three Cu2+ ions arranged in a linear
array with two tetradentate ligands capping each end (Scheme 2
and Figure 1). The phenolato oxygen atoms of the ligands bridge
to connect the terminal CuL1 or CuL2 units together via the
third, central copper ion. The central copper(II) ion links the
terminal units together via four μ2-phenolato oxygen atoms of
the two ligands in a square planar or nearly square planar environ-
ment. The τ4 parameter, a measure of four coordinate geome-
try,17 is zero for centrosymmetric 1a and 1,4a indicating perfect
square planarity, while there is a slightly distorted square planar
environment around the central copper in 1b, with a τ4 para-
meter of 0.09. The CuꢀCu distances in all three complexes are
similar, ranging from 2.940 Å to 3.008 Å.
The terminal copper ions in all three complexes are five
coordinate with slightly distorted square pyramidal geometries
composed of N2O2 donor atoms from the ligand in the basal
plane, and a coordinated chloride (1a) or acetonitrile (1 and 1b)
ligand in the apical position. Similar to the τ4 parameter, the τ5
parameter is a measure of five-coordinate geometry.18 The five-
coordinate copper ions in complexes 1, 1a, and 1b are all nearly
’ RESULTS
Syntheses and Structures. Ligands H2L1 and H2L1‑tBu were
prepared by published procedures.4a,9 All new ligands were
synthesized in a similar manner by the Schiff base condensation
of the precursor diamine, either ppda or phpda, with the
appropriately substituted aldehyde followed by reduction with
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Table 2. Crystallographic Data for 3aꢀ3c
Scheme 2
3a 2/3CH2Cl2
3b 2H2O
3c
3
3
formula
C
71H67Br12
Cl4Cu3N9O6
2433.68
monoclinic
P21/n
C23H27
CuN5O8
565.04
orthorhombic triclinic
C27H37
ClCuN3NaO10
685.58
fw
crystal system
space group
a (Å)
Pbca
P1
13.626(5)
20.992(8)
29.684(12)
90
21.150(5)
10.823(3)
22.675(6)
90
10.6796(12)
11.1968(12)
15.2613(16)
99.326(8)
104.157(8)
114.830(9)
1531.5(3)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
91.599(8)
90
90
90
8487(6)
4
5190(2)
8
Fcalcd mg/m3
1.905
1.446
1.487
μ (mmꢀ1
)
6.574
0.897
2.463
θ (deg)
1.9 to 25.3
2.0 to 25.3
3.1 to 67.1
R1,awR2b [I > 2σ(I)] 0.0522, 0.1328 0.0747, 0.1935 0.0515, 0.1805
GOF on F2
1.005 1.069 1.122
2
a R1 = ∑||Fobs| ꢀ |Fcalc||/∑|Fobs|. b wR2 = {∑[w(Fobs ꢀ Fcalc2)2]/
∑[w(Fobs2)2]}1/2
.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for 1a
susceptibility for 1 was due to its dissociation into paramagnetic
[CuL1] and antiferromagnetically coupled [Cu2L1]2+ species.4a
The only divergence in solution behavior between the new
complexes, 1a and 1b, and 1 is in their electrospray ionization
mass spectroscopy (ESI-MS). Unlike the ESI-MS of 1 where
there was no evidence of the trimeric species in solution,4a
prominent peaks in the ESI-MS of 1a and 1b suggest that they
do not dissociate completely into monomeric species in solution.
Complex 1a displays a peak at m/z = 974 corresponding to
[Cu3(L1)2Cl]+, and a peak at m/z = 439 corresponding to [CuL1
+ H]+. Complex 1b displays a prominent peak at m/z = 455
corresponding to [Cu3(L2)2]2+. However, these differences are
not inconsistent with the hypothesis that all three trimers at least
partially dissociate in solution into paramagnetic [CuL1] and
antiferromagnetically coupled [Cu2L1]2+ species.
Cu1ꢀO1
1.9082(17)
1.9888(17)
1.986(2)
2.9401(4)
100.26(7)
91.74(6)
99.80(7)
91.58(8)
91.65(8)
163.96(8)
Cu1ꢀO28
1.9055(17)
1.9660(18)
2.015(2)
2.5105(8)
79.74(7)
93.97(6)
98.47(7)
76.36(7)
97.64(9)
162.95(8)
Cu2ꢀO1
Cu2ꢀO28
Cu2ꢀN9
Cu1ꢀCu2
Cu2ꢀN20
Cu2ꢀCl1
O1ꢀCu2ꢀO28A
O1ꢀCu2ꢀCl1
N20ꢀCu2ꢀCl1
N9ꢀCu2ꢀO1
O28ꢀCu2ꢀN20
O1ꢀCu2ꢀN20
O28ꢀCu1ꢀO1
O28ꢀCu2ꢀCl1
N9ꢀCu2ꢀCl1
O28AꢀCu2ꢀO1
N9ꢀCu2ꢀN20
O28ꢀCu2ꢀN9
perfectly square pyramidal, with τ5 values of 0.0005,4a 0.017, and
0.053 for 1, 1a, and 1b, respectively. The bond distances and
angles for 1a and 1b, summarized in Tables 3 and 4, respectively,
are very similar to each other and to complex 1.4a Analogous to 1,
the perchlorate counterions in 1b interact weakly with the
terminal copper ions through the axial positions trans to the
acetonitrile ligands. The pyridyl rings in 1 and 1a do not
coordinate to the copper ions, but instead are involved in
intramolecular hydrogen bonding to one of the amine NH
groups of the ligand (see Discussion section).
The solution behavior of 1a and 1b is essentially the same as
that reported for 1.4a All three species have very similar electronic
spectra, with intense absorptions in the UV around 300 nm,
medium intensity (ε ∼ 3000 Mꢀ1 cmꢀ1) absorptions around
400 nm, and weak absorptions (ε ∼ 400 Mꢀ1 cmꢀ1), presumably
d-d transitions, around 600 nm. The EPR spectra of all three
species are also very similar, with axial parameters typical for
tetragonal copper complexes. The solution and solid state
magnetic susceptibilities of all three complexes of around
1.1 μB/Cu are less than the spin-only value of 1.73 μB/Cu, suggest-
ing some antiferromagnetic coupling. Previously we speculated
that the axial EPR signal and lower than expected magnetic
Complexes 2 and 3aꢀc are all mononuclear copper(II)
complexes. In every case, the ligand coordinates to the copper-
(II) ion via the N2O2 donor atoms from the amine and phenolate
groups of the doubly deprotonated ligands. The difference
between complex 2 and the other mononuclear complexes
3aꢀc is that the pyridyl group does not coordinate in 2, while
it does coordinate in 3aꢀc. The X-ray structure of 2 revealed that
it consists of a five coordinate copper(II) ion in an approximately
square pyramidal geometry (τ5 = 0.095), with a solvent methanol
ligand occupying the apical position (Figure 2). The N2O2 donor
set from (L1‑tBu 2ꢀ
occupies the basal plane of the pyramid. The
)
bond distances and angles, summarized in Table 5, are typical of
square pyramidal copper(II) complexes with similar ligands, and
the apical CuꢀOMeOH bond is, not surprisingly, longer than the
bonds in the basal plane. Similar to 1 and 1a, the pyridyl N atom
of the ligand in 2 does not coordinate to the metal center, but
instead is involved with hydrogen bonding between the pyridyl N
atom and the amine NH groups. The six-membered chelate ring
made up of the copper, nitrogen, and propylene carbon atoms of
the ligand adopts the chair conformation, and the amine NH
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Figure 2. Representation of the X-ray structure of 2. All H atoms except
for the amine and CH3OH protons have been removed for clarity.
Figure 1. Representations of the X-ray structures of 1a (top) and 1b
(bottom). All H atoms except for the amine protons have been removed
for clarity. For 1b, one of the four crystallographically independent
copper trimers is shown, only the N atom of the coordinated CH3CN
molecules are shown for clarity, and the perchlorate anions are also
omitted for clarity.
Table 5. Selected Bond Lengths (Å) and Angles (deg) for 2
Cu1ꢀO1
1.9467(12)
2.0328(14)
2.5287(14)
92.25(5)
Cu1ꢀO44
Cu1ꢀN28
1.9182(13)
2.0226(15)
Cu1ꢀN17
Cu1ꢀO1S
O1ꢀCu1ꢀN17
O44ꢀCu1ꢀO1
O1ꢀCu1ꢀN28
N28ꢀCu1ꢀN17
O44ꢀCu1ꢀN28
O44ꢀCu1ꢀN17
87.65(6)
92.61(6)
170.94(6)
Table 4. Selected Bond Lengths (Å) and Angles (deg) for
One of the Crystallographically Independent Copper Trimers
in 1ba
88.02(5)
176.56(6)
Cu7ꢀO1E
1.970(10)
1.956(14)
2.457(12)
1.943(9)
95.7(5)
Cu7ꢀO27E
1.945(12)
2.021(12)
1.896(12)
2.980(14)
92.3(6)
amine NH groups in 3aꢀc are on the opposite side of the apical
coordination site. These structural features are related to the
coordination of the pyridyl ring to the copper, and will be
examined in detail in the Discussion section.
The solid state structure of 3a contains three symmetry-
related subunits of [CuL1‑Br] connected by intermolecular
hydrogen bonding (Figure 3, bottom). Both NH groups in each
complex are hydrogen bonded to a phenoxo O atom of an
adjacent complex, forming a C3 symmetric supramolecular trimer
Cu7ꢀN9E
Cu7ꢀN19E
Cu7ꢀN 1 V
Cu2ꢀO27E
Cu2ꢀO1E
Cu7ꢀCu8
O1EꢀCu7ꢀN9E
O27EꢀCu7ꢀO1E
N9EꢀCu7ꢀN19E
O27EꢀCu7ꢀN 1 V
O1EꢀCu7ꢀN 1 V
O27E0ꢀCu8ꢀO1E
O1E0ꢀCu8ꢀO1E
O27EꢀCu7ꢀN19E
N19EꢀCu7ꢀN 1 V
N9EꢀCu7ꢀN 1 V
O1EꢀCu7ꢀN19E
O1EꢀCu8ꢀO27E
O27EꢀCu7ꢀN9E
77.9(5)
87.0(4)
94.0(6)
91.4(4)
90.8(4)
170.2(6)
79.8(5)
92.7(4)
held together by a total of six NH O hydrogen bonds. Perhaps
100.2(5)
173.4(5)
3 3 3
as a consequence of the steric hindrance introduced by the
179.995(2) O27EꢀCu8ꢀO27E0
180.0(8)
intermolecular H-bonds, the phenolate rings of the (L1‑Br 2ꢀ
)
a Listings of the bond distances and and angles for the other trimer
ligands are folded above the CuN2O2 basal plane of the square
pyramid in 3a. This stands in contrast to isostructural 3b and 3c,
which have the phenolate rings canted above and below the basal
plane in each complex (Figures 4 and 5).
molecules in 1b can be found in the Supporting Information.
bonds are on the apical side of copper’s square pyramidal coordina-
tion sphere. The phenolate rings are canted on either side of the basal
plane of the copper coordination sphere. One tert-butyl substituted
ring lies above the plane, in closer proximity to the coordinated
methanol ligand, while the other ring lies below the plane.
Attempts to synthesize copper(II) complexes with H2L1‑MeO
using triethylamine as the base for phenol deprotonation were
unsuccessful, but using NaH as the base resulted in the hetero-
metallic binuclear copper/sodium complex [CuL1‑MeONa-
(CH3OH)2]ClO4 (3c). As mentioned above, the coordination
sphere around the copper ion is isostructural with 3a and 3b. The
six-coordinate sodium ion is chelated by the phenolate and
methoxy O atoms from both phenol rings, plus two coordinated
CH3OH ligands, forming a distorted octahedral geometry. The
phenolate O atoms bridge between the copper and sodium ions
such that the methanol ligands are trans to each other. The ESI-
MS of 3c suggests that the heterometallic complex is at least
partially present in solution, since a peak at m/z = 521 corre-
sponding to [CuL1‑MeO + Na]+ was observed.
Complexes 3aꢀc all contain a five-coordinate copper(II) ion
with slightly distorted square pyramidal geometries, having τ5
values of 0.097, 0.00083, and 0.0053, respectively. In contrast to
1, 1a, or 2, the copper(II) ions in 3aꢀc are coordinated with
N3O2 donor atom sets that include two amine N atoms and two
deprotonated O, and, importantly, also the pyridyl N atom
(Figures 3ꢀ5). The amine N and phenolate O atoms occupy
the basal plane and the pyridyl N atom occupies the apical
position of the square pyramid. Whereas the six-membered
chelate ring made up of the copper, nitrogen, and carbon atoms
in complexes 1, 1a, 1b, and 2 all adopted the chair conformation,
in 3aꢀc the chelate ring is in the boat form. Additionally, the
Cyclic Voltammetry. The electrochemical behavior of all six
complexes was studied by cyclic voltammetry in acetonitrile
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Figure 4. Representation of X-ray structure of 3b with all H atoms
except for amine protons removed for clarity.
Figure 5. Representation of X-ray structure of 3c with all H atoms
except for amine protons removed for clarity.
Table 6. Selected Bond Lengths (Å) and Angles (deg) for
One Independent Molecule of 3a
Cu1BꢀO1B
1.942(5)
2.046(6)
2.325(5)
89.44(19)
174.5(2)
Cu1BꢀO28B
Cu1BꢀN20B
1.951(4)
1.996(6)
Cu1BꢀN9B
Cu1BꢀN18B
Figure 3. Representation of the X-ray structure of 3a (top), and the
intermolecular H-bonding interactions in the trimeric units of 3a
(bottom). All H atoms except for the amine protons have been removed
for clarity.
O1BꢀCu1BꢀO28B
O1BꢀCu1BꢀN20B
N20BꢀCu1BꢀN9B
O1BꢀCu1BꢀN18B
O28BꢀCu1BꢀN18B 93.80(19)
N20BꢀCu1BꢀN18B 89.0(2)
84.3(2)
95.40(19)
O28BꢀCu1BꢀN20B 93.5(2)
O1BꢀCu1BꢀN9B
O28BꢀCu1BꢀN9B
91.9(2)
168.72(19) N9BꢀCu1BꢀN18B
97.2(2)
(1, 1b, 2, 3a, and 3b), dichloromethane (1a, 2, and 3c), or DMF
(3a). In some cases, it was not possible because of poor solubility
to obtain cyclic voltammograms (CVs) in the same solvent. For
example, 1a was only soluble enough in dichloromethane to
obtain electrochemical data, while for 3a the CV in acetonitrile
had a very large ΔE but much a smaller ΔE in DMF. Interest-
ingly, the redox behavior of 2 was very solvent dependent,
displaying a well behaved CuII/CuI redox couple in acetonitrile,
while in dichloromethane having no metal-centered redox activ-
ity (vide infra).
clearly indicated by the increasingly negative E
values. The
1/2
electron-withdrawing bromo- and nitro-groups in 3a and 3b,
respectively, lead to less negative half-wave potentials of ꢀ897
mV and ꢀ680 mV versus Ag/AgCl in acetonitrile. The E1/2 of
ꢀ713 mV versus Ag/AgCl for 3a in DMF is slightly less than in
acetonitrile, and ΔE is significantly smaller for 3a in DMF. The
more electron donating tert-butyl groups on the ligand in 2 lead
to the largest negative E1/2 of ꢀ1055 mV versus Ag/AgCl. The
redox potential for 1b, which contains the (L2)2ꢀ ligand, is
similar to the E1/2 for 1, which is not surprising since they are,
with the exception of the pyridyl ring, isostructural.
The electrochemical behavior of 2 in dichloromethane differs
significantly from 2 in acetonitrile. While the E1/2 for 2 in
acetonitrile is ꢀ1055 mV versus Ag/AgCl, no metal-centered
redox wave was observed for 2in dichloromethane from 0 to ꢀ1800
mV at all scan rates. This phenomena was also observed in the oxo-
bridged diiron complex of the same ligand, [(FeL1‑tBu)2(μ-O].9
The CVs of all six complexes (Figure 6) showed quasi-
reversible CuII/CuI redox waves at potentials ranging from
ꢀ680 mV to ꢀ1055 mV versus Ag/AgCl (Supporting Informa-
tion, Figures S1ꢀS6, and Table 9). As can be observed in Table 9,
the half-wave potentials (E1/2) of complexes 1, 1a, 2, and 3aꢀc
become decreasingly less negative as the ligand becomes more
electron-withdrawing: H2L1‑tBu < H2L1 ≈ H2L1‑MeO < H2L1‑Br
phenol rings of the ligands on the CuII/CuI redox couple is
<
H2L1‑NO2. The influence of the substituents present on the
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Table 7. Selected Bond Lengths (Å) and Angles (deg) for 3b
Cu1ꢀO1
1.935(4)
Cu1ꢀO28
Cu1ꢀN20
1.976(4)
1.995(5)
Cu1ꢀN9
2.062(5)
Cu1ꢀN18
2.226(4)
O1ꢀCu1ꢀO28
O1ꢀCu1ꢀN20
O28ꢀCu1ꢀN20
O1ꢀCu1ꢀN9
O28ꢀCu1ꢀN9
84.65(16)
171.51(18)
92.97(19)
93.31(17)
171.33(18)
N20ꢀCu1ꢀN9
O1ꢀCu1ꢀN18
O28ꢀCu1ꢀN18
N20ꢀCu1ꢀN18
N9ꢀCu1ꢀN18
87.82(19)
98.66(17)
100.63(17)
89.78(18)
88.00(18)
Table 8. Selected Bond Lengths (Å) and Angles (deg) for 3c
Cu1ꢀO1
1.950(3)
Cu1ꢀO28
Cu1ꢀN20
1.923(3)
2.041(4)
Cu1ꢀN9
2.028(4)
Cu1ꢀN18
2.246(4)
O28ꢀCu1ꢀO1
O1ꢀCu1ꢀN9
N9ꢀCu1ꢀN20
O28ꢀCu1ꢀN18
O28ꢀCu1ꢀN9
84.46(14)
92.70(16)
88.50(17)
98.94(14)
170.88(16)
O1ꢀCu1ꢀN18
O28ꢀCu1ꢀN20
N9ꢀCu1ꢀN18
N20ꢀCu1ꢀN18
O1ꢀCu1ꢀN20
101.12(14)
92.90(15)
90.10(16)
88.24(15)
170.56(15)
Figure 6. CVs of the complexes with the H2L1 family of ligands soluble
in acetonitrile: 1, 2, 3a, and 3b in CH3CN containing 0.1 M TBAPF6 and
at a scan rate of 100 mV/s.
copper center (3aꢀc) or not coordinated and involved in
hydrogen bonding (1, 1a, and 2). Similar complexes with pyridyl-
containing ligands have shown this same phenomenon of the
pyridyl ring not coordinating: for example, [CuLimine(CH3-
CN)}],4a and a series of copper(II) complexes with a related
pyridylbis(guanidine) ligand.4b In these species, the sp2 hybridi-
zation of the N atoms in the imine and guanidine ligands result in
a more planar six-membered chelate ring, leading to the inability
of the pyridyl ring to coordinate to the copper atom. However, in
the case of complexes of the diamine ligands, the explanation is
not as simple.
However, 2 in dichloromethane exhibits two quasi-reversible one-
electron redox couples at 579 mV (ΔE = 179 mV) and 1022 mV
(ΔE = 108 mV), versus Ag/AgCl. These features are attributed to
the oxidation of the tert-butyl substituted phenolate groups leading
to the formation of phenoxyl radicals. These ligand-based redox
features for 2 in acetonitrile appear as irreversible oxidations. None
of the other complexes in this study displayed any of this type of
ligand-centered redox process, which is typically seen for complexes
with tert-butyl substituted salen-type ligands.8a,d,20
The substituent effects on the redox potentials of 1, 1a, 2, and
3aꢀc were analyzed by Hammett analysis. A plot of E1/2 versus
Hammett σp parameter19 for the reduction of Cu(II) center is
shown in Figure 7. Since the CVs of all complexes were not possible
in acetonitrile, the Hammett analysis is most relevant for complexes
1, 2, 3a, and3b (represented by squares in Figure 7). The data for 1a
and 3c in dichloromethane (triangles in Figure 7) are included to
illustrate the similar redox potentials in that solvent; however, a
direct comparison in the Hammett analysis is not warranted because
of solvent dependent differences in redox potentials. Furthermore, it
is important to point out that the Hammett analysis compares the
E1/2 with the Hammett parameter for the para-position, σp. The
ortho-substitution of the methoxy groups in HL1‑MeO does not affect
σp, and indeed the σp value used 3c is the same as for the complexes
with H-substituted para positions in their ligands.
A linear relationship for E1/2 versus σp is obtained for these
complexes, with a calculated slope of 382 mV. The Hammett plot
supports the electronic effects that the substituents on the H2L1
family of ligands have on the redox potentials of their copper
complexes. The more electron-rich copper centers in 1, 1a, and 2
have larger negative potentials and are hence more difficult to
reduce, while the electron deficient copper centers in 3a and 3b
have relatively smaller negative potentials and are thus easier to
reduce. These electronic effects are also key to understanding the
structural differences in these complexes.
The ligands studied in this paper (i.e., H2L1 and its derivatives
plus H2L2) have in common the 1,3-propanediamine backbone
attached to the methylenephenol groups. The 2-position of the
propyl backbone contains either the pyridyl group (H2L1 family
of ligands) or the phenyl group (H2L2). When the prochiral
amine groups coordinate to a metal center, they lock in either the
R or S conformation on each amine. The possible metal complex
isomers include the R,R and S,S enantiomeric pair, and the
meso-(s) and meso-(r) diastereomers (Scheme 3). The meso
isomers impose pseudochirality on the 2-position of the propyl
backbone, giving rise to the two diastereomers. Because of the
rapid pyramidal inversion of the secondary amines in solution
prior to coordination,21 presumably all isomers can be accessed,
but only the most thermodynamically stable conformations will
prevail.
The six-membered chelate ring that is formed when the ligand
coordinates to a metal ion can adopt either a chair or boat
conformation. All of the structures reported in this paper have
the ligand coordinated in either the meso-(s) or the meso-(r)
conformation, and in every case the amine H atoms are in axial
positions on the cyclohexane-like ring, as illustrated in Scheme 3.
Scheme 3 shows the two meso diastereomers in the chair
conformation on the left and the boat conformation on the right.
The key difference between the meso-(s) and meso-(r) diastere-
omers can be seen when comparing them in the chair conforma-
tion: in the case of meso-(s)-chair the methyl group is axial while
the meso-(r)-chair has the pyridyl group in the axial position.
The preferred conformations of the meso-(s) or meso-(r) dia-
stereomers, highlighted in boxes in Scheme 3, can now be seen.
’ DISCUSSION
The complexes in this study synthesized with the H2L1 family
of ligands have the pyridyl group either coordinated to the
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Table 9. Electrochemical Data for 1, 1aꢀb, 2, and 3aꢀc
c
complexa
solvent
E1/2 (mV) b
ΔE (mV)
σp
reference
4a
[{CuL1(CH3CN)}2Cu](ClO4)2 (1)
[{CuL1Cl}2Cu] (1a)
[{CuL2(CH3CN)}2Cu](ClO4)2 (1b)
[CuL1‑tBu(CH3OH)] (2)
CH3CN
CH2Cl2
CH3CN
CH3CN
CH2Cl2
CH2Cl2
CH3CN
DMF
ꢀ914
ꢀ1040
ꢀ875
ꢀ1055
579
139
260
170
199
179
108
430
232
210
367
0
0
0
this work
this work
this work
this work
this work
this work
this work
this work
this work
ꢀ0.2
1022
[CuL1‑Br] (3a)
ꢀ897
ꢀ713
ꢀ680
ꢀ954
0.23
[CuL1‑NO2] (3b)
[CuL1‑MeONa(CH3OH)2]ClO4 (3c)
CH3CN
0.78
0
CH2Cl2
a Sample concentrations were all approximately 1.0 mM. b Scan rate = 100 mV sꢀ1; 0.1 M TBAPF6 supporting electrolyte. c Values obtained from ref 19.
Scheme 3
Figure 7. Plot of E1/2 vs the Hammett σp parameter for compounds 1,
1a, 2, and 3aꢀc in CH3CN (9) or CH2Cl2 (2).
The meso-(s)-boat conformation places the pyridyl group within
coordination distance of the copper ion, allowing for pentaden-
tate coordination by the ligand. The chair form of the same
diastereomer, that is, meso-(s), would be less favorable, since no
intramolecular coordinate or hydrogen bonds involving the
pyridyl ring are possible. Conversely, it is not possible to
coordinate the pyridyl ring to the metal center in the meso-(r)
diastereomer. While the meso-(r)-chair form places the pyridyl
ring in proximity of the NH groups, allowing hydrogen bonding,
it is too far away from the copper center to coordinate. Folding
the meso-(r) diastereomer into the boat form moves the pyridyl
ring even farther away from the copper, and eliminates the
possibility of hydrogen bonding with the amine NH groups.
Therefore, the meso-(s)-boat and the meso-(r)-chair conforma-
tions (Scheme 3, in boxes) are the two most stable structures
possible for these ligands. This conclusion is borne out in the
structures of the complexes reported in this study.
The axial positions of the square pyramids are occupied by
CH3CN solvent ligands (1 and 1b) or chloride ligands (1a). The
pyridyl rings of the ligands in 1 and 1a do not coordinate to the
Cu ions. The pyridyl ring N atoms in 1 and 1a are involved in
hydrogen bonding to one of the amine NH groups of the same
ligand (see Figure 1), adopting the meso-(r)-chair conformation.
Although ligand H2L2 does not contain the pyridyl group, the
structure of 1b is very similar to those of 1 and 1a, with the
exception of the H-bonding to the amine NH groups. Likewise,
in mononuclear, square pyramidal complex 2 with ligand
H2L1‑tBu, the pyridyl ring does not coordinate and instead forms
hydrogen bonds with both amine NH groups in the meso-(r)-
chair conformation.
In contrast, complexes 3aꢀcall contain the ligand coordinated to
the copper ion in the meso-(s)-boat conformation with the pyridyl
ring coordinating. The coordination spheres in the equatorial planes
of the square pyramidal copper centers in 3aꢀc are also made up of
the two N-atoms and two O-atoms from the amine and phenolate
groups of the ligands. One consequence of the meso-(s)-boat
coordination mode is that the amine NH groups are now no longer
involved in intramolecular hydrogen bonding with the pyridyl ring.
In the case of 3a, this allows for intermolecular hydrogen bonding to
The tricopper complexes (1, 1a, and 1b) are all composed of
two five-coordinate copper(II) species linked via bridging phe-
nolate O-atoms to a third, four-coordinate copper ion (Scheme 2
and Figure 1). The ligands coordinate through the two amine
N-atoms and two phenolate O-atoms in the basal plane of
the nearly perfect square pyramids of the terminal copper ions.
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ARTICLE
occur which leads to the H-bonded cyclic trimers seen in its solid
state structure (see Figure 3, bottom).
yet it has the meso-(s)-boat coordination mode. This may be
partially due to the coordination of the methoxy substituents
to the sodium ion in 3c, which may draw electron density away
from the copper center and therefore favor the coordination of
the pyridyl group in the meso-(s)-boat coordination mode.
The reason why the meso-(r)-chair coordination mode is
chosen in some cases (1, 1aꢀb, and 2) while the meso-(s)-boat
coordination mode is chosen in others (3aꢀc) can be rationa-
lized on the basis of the electronic properties of the ligands. The
nucleophilicity of the phenolato oxygens on the ligands correlate
with the electron donating or withdrawing nature of the substit-
uents in the position para to the OH group on the phenol rings,
and these can be quantified in a qualitative way by their Hammett
σp parameters, tabulated in Table 9.19 The Hammett σp para-
meters and nucleophilicity of the phenolate oxygens of the
’ ASSOCIATED CONTENT
S
Supporting Information. X-ray structural information
b
(CIF) and CVs for 1a, 1b, 2, and 3aꢀ3c. This material is
available free of charge via the Internet at http://pubs.acs.org.
ligands increase in the following order: H2L1‑NO2 < H2L1‑Br
<
,
’ AUTHOR INFORMATION
H2L1 ≈ H2L1‑MeO < H2L1‑tBu. Several of the ligands (H2L1‑tBu
H2L1‑MeO, and H2L1‑Br) have substituents in the ortho positions
relative to the phenol OH group, but this is of minor conse-
quence to the nucleophilicity of the phenolato oxygen. The
nucleophilicity of the ligands correlates nicely with the electro-
chemical potentials of the Cu(I)/Cu(II) couples (Figure 7).
The possibility that steric factors influence the determination
of meso-(r)-chair or meso-(s)-boat coordination mode has been
considered as well. The most sterically hindered ligand, the tert-
butyl substituted H2L1‑tBu, coordinates to copper in the meso-(r)-
chair mode in 2, suggesting that the bulky tert-butyl groups in the
2-positions of the phelolate rings might interfere with the ability
of the pyridyl ring to coordinate to the copper in the meso-(s)-
boat coordination mode. However, inspection of the structure of
2 reveals that the phenolate rings are disposed on opposite sides
of the basal plane of the copper coordination sphere, and
potential interactions between the 2-position tert-butyl group
on the side of the basal plane that would accept the pyridyl group
in the meso-(s)-boat mode would be longer than 3 Å. The
argument that the ligand coordination geometry where the
phenolate rings are disposed on either side of the basal plane
might impose geometric constraints that prevent the meso-(s)-
boat coordination mode is weakened by the fact that this very
example is observed in complex 3b. The phenolate groups in 3b
are canted above and below the copper basal plane, but the
complex adopts the meso-(s)-boat coordination mode with the
pyridyl ring coordinated to the copper. Finally, the unsubstituted
ligand H2L1, where the least amount of steric hindrance would be
expected, coordinates in the meso-(r)-chair mode in 1 and 1a,
suggesting an alternate explanation for the coordination prefer-
ences of the ligand.
Corresponding Author
*E-mail: houser@ou.edu.
Present Addresses
†Department of Chemistry, Wayne State University, 5101 Cass
Ave, Detroit, MI 48202.
’ ACKNOWLEDGMENT
This work was supported by the National Science Foundation
(NSF CHE-0616941) and the University of Oklahoma. Addi-
tionally, we thank the NSF for the purchase of a CCD-equipped
X-ray diffractometer (CHE-0130835).
’ REFERENCES
(1) Fenton, H.; Tidmarsh, I. S.; Ward, M. D. Dalton Trans. 2009,
4199.
(2) Nomenclature of inorganic chemistry IUPAC recommendations
2005; RSC Publishing: Cambridge, U.K., 2005.
(3) Kirin, S. I.; Happel, C. M.; Hrubanova, S.; Weyhermuller, T.;
Klein, C.; Metzler-Nolte, N. Dalton Trans. 2004, 1201.
(4) (a) Shakya, R.; Jozwiuk, A.; Powell, D. R.; Houser, R. P. Inorg.
Chem. 2009, 48, 4083. (b) Pal Chaudhuri, U.; Powell, D. R.; Houser,
R. P. Inorg. Chim. Acta 2009, 362, 2371.
(5) Malassa, A.; Gorls, H.; Buchholz, A.; Plass, W.; Westerhausen,
M. Z. Anorg. Allg. Chem. 2006, 632, 2355.
(6) Doro, F. G.; Castellano, E. E.; Moraes, L. A. B.; Eberlin, M. N.;
Tfouni, E. Inorg. Chem. 2008, 47, 4118.
(7) Yang, L.; Powell, D. R.; Klein, E. L.; Grohmann, A.; Houser, R. P.
Inorg. Chem. 2007, 46, 6831.
(8) (a) Mukherjee, C.; Weyhermuller, T.; Bothe, E.; Chaudhuri, P.
Inorg. Chem. 2008, 47, 11620. (b) Roy, N.; Sproules, S.; Bothe, E.;
Weyhermuller, T.; Wieghardt, K. Eur. J. Inorg. Chem. 2009, 2655.
(c) Orio, M.; Jarjayes, O.; Kanso, H.; Philouze, C.; Neese, F.; Thomas,
F. Angew. Chem., Int. Ed. 2010, 49, 4989. (d) Storr, T.; Verma, P.; Pratt,
R. C.; Wasinger, E. C.; Shimazaki, Y.; Stack, T. D. P. J. Am. Chem. Soc.
2008, 130, 15448. (e) Whittaker, J. W. Chem. Rev. 2003, 103, 2347.
(f) Rolff, M.; Schottenheim, J.; Decker, H.; Tuczek, F. Chem. Soc. Rev.
2011, 40, 4077. (g) Benisvy, L.; Wanke, R.; da Silva, M.; Pombeiro,
A. J. L. Eur. J. Inorg. Chem. 2011, 2791. (h) Arora, H.; Philouze, C.;
Jarjayes, O.; Thomas, F. Dalton Trans. 2010, 39, 10088.
(9) Shakya, R.; Powell, D. R.; Houser, R. P. Eur. J. Inorg. Chem.
2009, 5319.
The complexes with the meso-(r)-chair coordination mode
(1, 1aꢀb, and 2) contain the more nucleophilic phenolato
groups in ligands H2L1‑tBu and H2L1, while the complexes with
the meso-(s)-boat coordination mode (3aꢀc) have, in the
cases of H2L1‑Br and H2L1‑NO2, less nucleophilic phenolate
oxygens because of the electron withdrawing nature of Br and
NO2. We surmise that the less nucleophilic phenolato donors
(H2L1‑Br and H2L1‑NO2) leave the copper ion somewhat
electron deficient, making the coordination of the pyridyl ring
more favorable and leading to the meso-(s)-boat coordination
mode. In the case of the more nucleophilic donors (H2L1‑tBu
and H2L1), the copper center has more electron density
and hence there is less energetic benefit from the coordination
of the pyridyl ring than is gained from hydrogen bonding
between the pyridyl ring and the amine NH groups. The case
of complex 3c, which also has the meso-(s)-boat coordination
mode, is noteworthy in that its H2L1‑MeO ligand’s phenolato
oxygen nucleophilicity is expected to be close to that of H2L1,
(10) Friedrich, S.; Schubart, M.; Gade, L. H.; Scowen, I. J.; Edwards,
A. J.; McPartlin, M. Chem. Ber./Recl. 1997, 130, 1751.
(11) Busto, E.; Gotor-Fernandez, V.; Montejo-Bernardo, J.; Garcia-
Granda, S.; Gotor, V. Org. Lett. 2007, 9, 4203.
(12) Evans, D. F. J. Chem. Soc. 1959, 2003.
(13) (a) SMART Software Reference Manual; Bruker-AXS: Madison,
WI, 1998; (b) SAINT Software Reference Manual; Bruker-AXS: Madison,
WI, 1998.
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ARTICLE
(14) Sheldrick, G. M. SADABS, Program for empirical absorption
correction of area detector data; University of G€ottingen: G€ottingen,
Germany, 2002.
(15) (a) Sheldrick, G. M. SHELXTL Version 6.10 Reference Manual;
Bruker AXS Inc.: Madison, WI, 2000; (b) International Tables for
Crystallography; Kluwer: Boston, MA, 1995; Vol. C.
(16) van der Sluis, P.; Spek, A. L. Acta Crystallogr., Sect. A 1990,
A46, 194.
(17) Yang, L.; Powell, D. R.; Houser, R. P. Dalton Trans. 2007, 955.
(18) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor,
G. C. J. Chem. Soc., Dalton Trans. 1984, 1349.
(19) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
(20) (a) Benisvy, L.; Blake, A. J.; Collison, D.; Davies, E. S.; Garner,
C. D.; McInnes, E. J. L.; McMaster, J.; Whittaker, G.; Wilson, C. Dalton
Trans. 2003, 1975. (b) Maki, T.; Araki, Y.; Ishida, Y.; Onomura, O.;
Matsumura, Y. J. Am. Chem. Soc. 2001, 123, 3371.
(21) Rauk, A.; Allen, L. C.; Mislow, K. Angew. Chem., Int. Ed. 1970,
9, 400.
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