Nickel(II) complexes with amide ligands: Oxidative dehydrogenation of the amines in a tetradentate diamide-diamine ligand

Article abstract of DOI:10.1039/b107378h

Four complexes were prepared by the reaction of Ni(II) with three pyrrolidine based diamide-diamine ligands. The two amine groups in the ligand N,N′-bis(S-prolyl)-1,2-ethanediamine (S,S-bprolenH₂) were oxidatively dehydrogenated during the prep

Full text of DOI:10.1039/b107378h

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Nickel(II) complexes with amide ligands: oxidative dehydrogenation
of the amines in a tetradentate diamide–diamine ligand†
Colin L. Weeks, Peter Turner, Ronald R. Fenton* and Peter A. Lay
Centre for Heavy Metals Research, School of Chemistry, The University of Sydney, NSW 2006,
Australia
Received 14th August 2001, Accepted 19th December 2001
First published as an Advance Article on the web 21st February 2002
Four complexes were prepared by the reaction of Ni() with three pyrrolidine based diamide–diamine ligands.
The two amine groups in the ligand N,NЈ-bis(S-prolyl)-1,2-ethanediamine (S,S-bprolenH2) were oxidatively
dehydrogenated during the preparation of the Ni() complex in air. The reaction involved O2 to yield a complex
of a tetradentate ligand with 1-pyrroline terminal groups. The Ni() complex with S,S-bprolenH2 was synthesised
under oxygen-free conditions, and the Ni() complexes with the analogous ligands N,NЈ-bis(S-prolyl)-R,R-1,2-
cyclohexanediamine (R,R-(S,S)-bprolchxnH2) and N,NЈ-bis(S-prolyl)-1,2-benzenediamine (S,S-bprolbenH2) were
prepared in air. The complexes were characterised by X-ray crystallography, 1H and 13C NMR spectroscopy and
IR spectroscopy. The Ni(/) reduction potentials of complexes with pyridyl and pyrrolidine based tetradentate
diamide ligands were measured by cyclic voltammetry to assess the stability of the Ni() oxidation state, but
did not show any correlation with the ease of ligand oxidation.
Introduction
Nickel() complexes with several oligopeptide ligands cause
DNA cleavage in the presence of oxidants,1–10 in some instances
with sequence selectivity.2,3,5,7,8,10 It has been postulated that
Ni() and/or Ni() complexes are among the species respons-
ible for this DNA cleavage.1,2,6,10 Studies on the Ni(/) reduc-
tion potentials of nickel–oligopeptide complexes show that an
increase in the number of deprotonated amide donors lowers
the Ni(/) reduction potential.11,12 In the light of this, there is
Fig. 2 Tetradentate diamide–dipyrrolidine ligands
interest in the formation of Ni() complexes with amide ligands
and studies of the ease of oxidation to Ni() analogues, and
the stability of the oxidised products.
Experimental
Vagg and coworkers developed a series of acyclic tetra-
dentate ligands with two central amide groups and two terminal
pyridyl groups (Fig. 1).13,14 These ligands were used to prepare a
Reagents and solvents
Hydrogen (BOC Gases), 1,2-benzenediamine, activated carbon,
HBr in acetic acid (BDH), palladium on activated carbon,
S-proline (Aldrich), benzylchloroformate, 1,2-ethanediamine,
Ni(CH3CO2)2ؒ4H2O, NiCl2ؒ6H2O (Merck) triethylamine,
NaOH (Ajax), iso-butylchloroformate (ICN Biomedicals),
Na2SO4, acetone, methanol, ethanol, ethyl acetate, diethyl ether,
and chloroform were of laboratory grade; argon (BOC Gases),
toluene, acetonitrile (Ajax), Na2CO3 (Prolabo), NaHCO3
(BDH) were AR grade; and were used as received in the prepar-
ation of the ligands and their nickel complexes. Ferrocene
(Aldrich, 98%) was purified by sublimation prior to use in cyclic
voltammetry.
Fig. 1 Tetradentate diamide–dipyridyl ligands
The following compounds were prepared by known methods:
N,NЈ-bis(2-pyridinecarboxamide)-1,2-ethane (bpenH2) I,13 and
N,NЈ-bis(2-pyridinecarboxamide)-1,2-benzene (bpbH2) II,13
carbobenzoxy-S-proline,30 N,NЈ-bis(carbobenzoxy-S-prolyl)-
1,2-ethanediamine,25 and R,R-1,2-cyclohexanediamine.31
range of complexes with M() ions, including several Ni()
complexes.14–24 In this work, we report the synthesis of Ni()
complexes with three tetradentate diamide ligands, III–V.
These have pyrrolidine terminal groups instead of the pyridyl
terminal groups used by Vagg and coworkers (Fig. 2). Ligands
III and IV were reported as intermediates in the synthesis
of tetraamines,25,26 but only III has been used previously as a
ligand for Cu(),27,28 Rh()29 and Ir().29
Physical Measurements
Elemental microanalyses (C, H, and N) were performed at
the Research School of Chemistry at the Australian National
University or the University of Otago, New Zealand. Melting
points were measured on a Gallenkamp melting point appar-
atus and are reported uncorrected. UV-Vis spectra of the
† Electronic supplementary information (ESI): NMR spectra. See
http://www.rsc.org/suppdata/dt/b1/b107378h/
DOI: 10.1039/b107378h
J. Chem. Soc., Dalton Trans., 2002, 931–940
This journal is © The Royal Society of Chemistry 2002
931

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complexes in DMF (Ajax, HPLC grade) or methanol (APS
Finechem, 99.8%) were recorded on a Hewlett Packard 8452A
diode array spectrophotometer. FTIR spectra were recorded
by diffuse reflectance infrared Fourier transform spectroscopy
pump and the residue was washed with chloroform (50 mL).
The combined filtrate and washings were extracted with
water (200 mL), NaHCO3 solution (200 mL, 3%), and water
(200 mL). The organic layer was dried over anhydrous Na2SO4
and filtered. The solvent from the filtrate was removed on a
rotary evaporator to give a white solid, which was recrystallised
from acetone/diethyl ether, (26.37 g, 69%); δH(200 MHz,
CDCl3) 0.7–1.3 (4 H, m), 1.4–2.3 (12 H, m), 3.50 (6 H, m), 4.25
(2 H, t, pyrrolidine H2), 5.18 (4 H, t, C6H5–CH2), 6.3 (1 H, br s,
amide), 6.7 (1 H, br s, amide), 7.35 (10 H, s, C6H5–CH2).
1
(KBr) on a Bio Rad FTS-40 spectrophotometer. H NMR
spectra were recorded at 25 ЊC on a Bruker AMX400 NMR
spectrometer or a Bruker AC200 NMR spectrometer; in some
experiments, a few drops of D2O were added to identify labile
protons. The 1H-decoupled 13C NMR spectra were recorded on
a Bruker AC200 NMR spectrometer. Tetramethylsilane was
used as the internal standard for the 1H and 13C spectra. Cyclic
voltammetry was carried out using a BAS 100B Electro-
chemical Analyzer controlled by BAS 100W software. The
complexes were dissolved in N,N-dimethylformamide (DMF)
(Ajax, HPLC grade) or distilled water with 0.10 M tetra(n-
butyl)ammonium perchlorate (TBAP) (Fluka, electrochemistry
grade) or 0.10 M NaClO4 (Aldrich, 99.99%), respectively, as the
supporting electrolytes. A three-electrode system with a glassy-
carbon working electrode, a Ag/AgCl reference electrode and a
Pt wire auxiliary electrode was used. Full iR compensation was
applied for all scans. The ferrocenium/ferrocene (Fcϩ/0) couple
was used as an internal redox potential standard for the DMF
solutions.
N,NЈ-Bis(S-prolyl)-R,R-1,2-cyclohexanediamine (R,R-(S,S)-
bprolchxnH2) IV. The carbobenzoxy protecting groups were
removed by the method used for III. The product, a white solid,
was dried over silica gel, (14.27 g, 101%); mp 179–181 ЊC
(Found: C, 62.10; H, 8.88; N, 17.95. C16H28N4O2 requires C,
62.30; H, 9.15; N, 18.17%); νmax/cmϪ1 3315vs (amide NH),
3248w, 2972w (CH), 2935s (CH), 2860s (CH), 1637vs (amide I),
1523vs (amide II), 1510vs, 1456w, 1297w (amide III), 1110w,
880w, 695w (amide V), (DRIFTS in KBr); δH(400 MHz,
CDCl3) 1.25 (2 H, m, cyclohexane H3 and H6), 1.31 (2 H, m,
cyclohexane H4 and H5), 1.66 (4 H, m, pyrrolidine H4), 1.73
(2 H, m, cyclohexane H4Ј and H5Ј), 1.77 (2 H, m, pyrrolidine
H3), 1.92 (2 H, br s, amine), 2.01 (2 H, m, cyclohexane H3Ј and
H6Ј), 2.11 (2 H, m, pyrrolidine H3Ј), 2.92 (4 H, m, pyrrolidine
H5), 3.62 (2 H, m, cyclohexane H1 and H2), 3.66 (2 H, dd,
pyrrolidine H2), 7.59 (2 H, br s, amide); δC(200 MHz, CDCl3)
24.8 (cyclohexane C4 and C5), 26.3 (pyrrolidine C4), 30.8
(pyrrolidine C3), 32.7 (cyclohexane C3 and C6), 47.2 (pyrrolidine
C5), 52.5 (cyclohexane C1 and C2), 60.6 (pyrrolidine C2), 175.3
(CO).
Syntheses
N,NЈ-Bis(S-prolyl)-1,2-ethanediamine (S,S-bprolenH2) III.
The synthesis was carried out by a modification of the method
of Jun and Liu.25 Palladium on activated carbon (∼1 g, 10%)
was added to N,NЈ-bis(carbobenzoxy-S-prolyl)-1,2-ethanedi-
amine (34.6 g) in methanol (450 mL). Hydrogen was gently
bubbled through the mixture until CO2 production ceased
(∼90 h). The mixture was filtered through a Celite pad and the
residue was washed with methanol (∼100 mL). The combined
filtrate and washings were filtered through a filter paper and
the solvent was removed on a rotary evaporator. Ethanol
(∼100 mL) was added and evaporated to remove any residual
water as an azeotrope. The product, a yellow coloured oil, was
stored under nitrogen in the refrigerator for 5 d, during which
time most of it solidified forming white crystals. The product
was recrystallised from methanol/ethyl acetate as the hemi-
hydrate, (7.40 g, 42%); mp 105–108 ЊC (lit.,29 92–93 ЊC) (Found:
C, 54.84; H, 8.39; N, 21.02. C12H23N4O2.5 requires C, 54.73; H,
8.80; N, 21.28); νmax/cmϪ1 3306vs (amide NH), 2965s (CH),
2943s (CH), 2921w (CH), 2860s (CH), 2825w (CH), 1652vs
(amide I), 1541vs (amide II), 1444s, 1311s (amide III), 1255s,
1235s, 1110s, 910s, 692s (amide V), (DRIFTS in KBr) (lit.,29
3278, 1646, 1552 cmϪ1); δH(400 MHz, CDCl3) 1.72 (4 H, m,
pyrrolidine H4), 1.90 (2 H, m, pyrrolidine H3), 2.14 (2 H, m,
pyrrolidine H3Ј), 2.72 (2 H, br s, amine), 2.97 (4 H, m, pyrrol-
idine H5), 3.37 (4 H, t, ethene bridge CH2), 3.76 (2 H, dd,
pyrrolidine H2), 7.95 (2 H, br s, amide) (lit.,29 1.63–1.72 (4 H),
1.82–1.94 (2 H), 2.02–2.16 (2 H), 2.3–2.6 (2 H), 2.88 (2 H), 2.97
(2 H), 3.34–3.36 (4 H), 3.67 (2 H), 7.63–7.81 (2 H)); δC(200
MHz, CDCl3) 26.1 (pyrrolidine C4), 30.7 (pyrrolidine C3), 39.1
(ethene bridge CH2), 47.2 (pyrrolidine C5), 60.5 (pyrrolidine
C2), 175.8 (CO) (lit.,29 25.96, 30.56, 39.05, 47.05, 60.61, 175.81).
N,NЈ-Bis(carbobenzoxy-S-prolyl)-1,2-benzenediamine. Carbo-
benzoxy-S-proline (48.48 g) was dissolved in toluene (400 mL)
and triethylamine (28.5 mL) was added. The solution was
chilled to Ϫ10 ЊC in an ice/acetone bath and iso-butyl-
chloroformate (25.5 mL) was added, followed by toluene
(125 mL). The solution was stirred for 1 h. A cold solution of
1,2-benzenediamine (10.43 g) and triethylamine (28.5 mL) in
chloroform (300 mL) was added. The flask was sealed with a
CaCl2 drying tube and left stirring overnight at room temper-
ature. The reaction mixture was filtered at the pump and the
residue was washed with toluene (∼50 mL). The combined
filtrate and washings were extracted with water (400 mL),
NaHCO3 solution (400 mL, 3%), and water (400 mL). The
organic layer was dried over anhydrous Na2SO4 (∼5 g) and
filtered. The solvent from the filtrate was removed on a rotary
evaporator, giving a very viscous brown coloured oil that
solidified on cooling. The product was dissolved in boiling
ethanol (500 mL) and activated carbon (14.5 g) was added. The
mixture was boiled for 30 min and the hot solution was filtered.
The filtrate was evaporated to dryness to give a pale brown
coloured solid, (46.92 g, 85%); δH(200 MHz, CDCl3) 1.8–2.4
(8 H, m, pyrrolidine), 3.52 (4 H, m, pyrrolidine), 4.49 (2 H,
m, pyrrolidine H2), 5.16 (4 H, m, C6H5–CH2), 7.34 (14 H, m,
benzene rings), 7.64 (1 H, br d, amide), 8.91 (1 H, br d, amide).
Synthesis of N,NЈ-bis(S-prolyl)-1,2-benzenediamine (S,S-
N,NЈ-Bis(carbobenzoxy-S-prolyl)-R,R-1,2-cyclohexanediamine. bprolbenH2) V. A solution of HBr in acetic acid (130 mL, 45%)
A modification of the method of Jun and Liu26 was used to
synthesise N,NЈ-bis(carbobenzoxy-S-prolyl)-R,R-1,2-cyclohex-
anediamine. Carbobenzoxy-S-proline (33.17 g) was dissolved in
toluene (300 mL) and triethylamine (19.5 mL) was added. The
solution was chilled to Ϫ5 ЊC in an ice/acetone bath and iso-
butylchloroformate (17.3 mL) was added, followed by toluene
(50 mL). The solution was stirred for 1 h. A cold solution
of R,R-1,2-cyclohexanediamine (7.59 g) and triethylamine
(19.5 mL) in chloroform (200 mL) was added. The flask was
sealed with a CaCl2 drying tube, and left stirring overnight at
room temperature. The reaction mixture was filtered at the
was added to N,NЈ-bis(carbobenzoxy-S-prolyl)-1,2-benzene-
diamine (46.53 g). The mixture was heated for 214 h on a steam
bath then left to cool. The reaction mixture was filtered and the
solvent was removed from the filtrate on a rotary evaporator.
Diethyl ether was added to the residue, which was triturated
and left to stand; the supernatant was discarded. The residue
was dissolved by shaking with NaOH solution (200 mL, 1 M;
50 mL, 10 M) and chloroform (200 mL). The aqueous layer was
extracted with chloroform (4 × 100 mL). The combined organic
layers were dried over anhydrous Na2SO4 (∼10 g), filtered and
the solvent was removed on a rotary evaporator. The product
932
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was recrystallised from chloroform/diethyl ether. This gave a
pale brown coloured powder (7.14 g, 29%); νmax/cmϪ1 3312s
(amide NH), 3254s, 2969s (CH), 2946w (CH), 2915w (CH),
1665vs (amide I), 1594s, 1528vs (amide II), 1473s, 1301s (amide
III), 1104s, 906w, 869s, 771s, (DRIFTS in KBr); δH(400 MHz,
CDCl3) 1.72 (6 H, m, pyrrolidine H4 and amine), 2.00 (2 H, m,
pyrrolidine H3), 2.15 (2 H, m, pyrrolidine H3Ј), 2.96 (4 H, m,
pyrrolidine H5), 3.84 (2 H, dd, pyrrolidine H2), 7.09 (2 H, m,
benzene H4 and H5), 7.62 (2 H, m, benzene H3 and H6), 9.64
(2 H, br s, amide); δC(200 MHz, CDCl3) 26.3 (pyrrolidine C4),
30.9 (pyrrolidine C3), 47.4 (pyrrolidine C5), 61.1 (pyrrolidine
C2), 124.0 (benzene C3 and C6), 125.7 (benzene C4 and C5),
129.8 (benzene C1 and C2), 174.2 (CO). The crude product was
used in the synthesis of the nickel complex but a small sample
was purified for microanalysis. A methanol solution of the
crude product was loaded onto a silica column (11 × 2 cm, silica
gel 60, 35–70 mesh) and a yellow band was eluted with meth-
anol, which was discarded. The fraction that was eluted with
methanol after the yellow band and a subsequent fraction
eluted with dichloromethane contained pure S,S-bprolbenH2.
The solvent was removed from these fractions on a rotary
evaporator and the residues were combined. The white powder
contained 0.75 CH3OH of crystallisation (Found: C, 62.34;
H, 7.26; N, 16.69. C16.75H25N4O2.75 requires C, 61.64; H, 7.72;
N, 17.16%), mp 154–159 ЊC.
Method 1. Nickel() acetate tetrahydrate (0.196 g) was dis-
solved in water (10 mL) and a solution of III (0.203 g) in water
(10 mL) was added. A NaOH solution (3 mL, 0.4 M) was added
and the solutions were mixed thoroughly. The water was evap-
orated under reduced pressure, and the residue was dissolved in
methanol (30 mL) and filtered. The filtrate was evaporated
under reduced pressure, and the residue was dissolved in
methanol (20 mL) and filtered. Methanol was added to the
filtrate to increase the volume to ∼40 mL and acetonitrile (15
mL) was added. Slow evaporation of the methanol/acetonitrile
solution over a period of several days under argon did not
produce crystals, so diethyl ether (15 mL) was added and the
mixture was allowed to stand for a further 2 d. The supernatant
was decanted, leaving orange-coloured crystals that were
washed with methanol/acetonitrile in a 1 : 5 ratio (12 mL) and
dried under reduced pressure to give a yellow-coloured powder,
(0.055 g, 21%) (Found: C, 43.25; H, 6.49; N, 16.76. C22H22-
N4O3Ni requires C, 43.67; H, 6.71; N, 16.98%); νmax/cmϪ1 3092s,
2955s (CH), 2881s (CH), 2867s (CH), 1599vs (amide I), 1449w
1425s (amide C–N), 1317w, 1222w, 1071w, 1051w, 1005w, 934s,
765w (DRIFTS in KBr); δH(400 MHz, dmso-d6) 1.54 (2 H,
m, pyrrolidine H4), 1.66 (2 H, m, pyrrolidine H3), 1.80 (2 H,
m, pyrrolidine H3Ј), 1.95 (2 H, m, pyrrolidine H4Ј), 2.67 (2 H, m,
pyrrolidine H5), 2.76 (4 H, s, ethene bridge CH2), 3.14 (2 H, m,
pyrrolidine H5Ј), 3.3 (2 H, m, pyrrolidine H2), 4.37 (2H,
q, amine); δC(400 MHz, dmso-d6) 25.5 (pyrrolidine C4), 29.3
(pyrrolidine C3), 47.0 (ethene bridge CH2), 49.0 (pyrrolidine
C5), 66.2 (pyrrolidine C2), 177.1 (CO).
Method 2. Nickel() chloride hexahydrate (0.279 g) was dis-
solved in water (10 mL) and a solution of III (0.311 g) in water
(10 mL) was added. A NaOH solution (10 mL, 1 M) was
added and the solutions were mixed thoroughly. The water
was evaporated under reduced pressure, the residue was dis-
solved in methanol (25 mL), and the solution was filtered. The
filtrate was evaporated under reduced pressure, the residue was
dissolved in methanol (20 mL) and the solution was filtered.
The filtrate was evaporated under reduced pressure and the
residue was removed from the Ar atmosphere. The remainder
of the synthesis was carried out in air. The residue was dis-
solved in methanol (15 mL) and loaded onto a LH20 lipo-
philic Sephadex column (2.5 × 14 cm) and the complexes
were eluted with methanol. A major fast moving yellow
band and a minor, slower moving light orange band were
eluted. The faster moving yellow band was collected and
slowly evaporated to dryness. The residue was recrystallised
from methanol yielding light orange crystals, (0.188 g, 49%);
λmax/nm (CH3OH) 252 (sh, ε/dm3 molϪ1 cmϪ1 7.9 × 103), 380 (3.9
× 102); δH(200 MHz, dmso-d6) 1.5–1.8 (4 H, m), 1.8–2.1 (4 H,
m), 2.69 (2 H, m), 2.75 (4 H, s), 3.14 (2 H, m), 3.3 (2 H, m), 4.40
(2 H, q).
[NiII(bprolenH؊4)]ؒH2O 1. Method 1. Nickel() acetate tetra-
hydrate (0.196 g) was dissolved in water (15 mL) and III
(0.200 g) was added. The green-coloured solution was heated
on a steam bath then allowed to evaporate slowly. The non-
crystalline residue thus formed was dissolved in water (∼5 mL),
further heated, and NaOH solution (1 M) was added dropwise
until the colour of the solution changed to orange. The solution
was filtered and the filtrate was left to evaporate slowly. Small
yellow needle-like crystals formed over several days and were
collected at the pump and dried under reduced pressure over
silica gel. The product was recrystallised from methanol
(7.3 mg, 3%) (Found: C, 44.39; H, 5.53; N, 17.01. C12H18-
N4O3Ni requires C, 44.34; H, 5.58; N, 17.24); νmax/cmϪ1 3479s,
2940s (CH), 2847s (CH), 1627vs (amide I), 1612vs (imine C᎐N),
᎐
1411s (amide C–N), 1328s, 1032s, 508s, (DRIFTS in KBr);
δH(400 MHz, CD3OD) 2.09 (4 H, quintet, 7.6Hz, 8.0 Hz,
pyrroline H4), 2.71 (4 H, tt, 2.4 Hz, 8.0 Hz, pyrroline H3), 3.24
(4 H, s, ethene bridge CH2), 3.76 (4 H, tt, 2.4 Hz, 7.6 Hz,
pyrroline H5).
Method 2. Nickel() acetate tetrahydrate (0.196 g) was dis-
solved in water (10 mL) and a solution of III (0.208 g) in water
(10 mL) was added. The pH value was raised to ∼8 by the
addition of NaHCO3 solution (10 mL, 1 M) and a fine pre-
cipitate started to form. Sodium carbonate (20 mL, 1 M) was
added to raise the pH value to ∼10, and this resulted in the
precipitation of a large amount of fine solid. The mixture was
heated on a steam bath for an hour, during which most of the
solid dissolved and the colour changed from green–yellow to
bright yellow. The mixture was filtered and the pH value of the
filtrate was ∼10. Slow evaporation of the filtrate gave tiny
needle-like yellow-coloured crystals, which were collected at the
pump, washed with ice-cold water (2 × 2 mL), and dried over
silica gel. A second crop was obtained from the filtrate and
purified by recrystallisation from methanol, (0.055 g, 22%);
λmax/nm (CH3OH) 242 (sh, ε/dm3 molϪ1 cmϪ1, 6.8 × 103), 382
(8.3 × 103), 424 (sh, 3.3 × 103); δH(200 MHz, CD3OD) 2.09 (4 H,
quintet); 2.71 (4 H, tt); 3.24 (4 H, s); 3.76 (4 H, tt); δC(200 MHz,
dmso-d6) 20.3 (pyrroline C4), 32.7 (pyrroline C3), 48.0 (pyrroline
C5), 57.9 (ethene bridge CH2), 164.6 (pyrroline C2), 177.4 (CO).
[NiII(R,R-(S,S)-bprolchxn)]ؒ25H2O 3. Nickel() acetate tetra-
hydrate (0.203 g) was dissolved in water (10 mL) by heating on a
steam bath and a solution of IV (0.252 g) in water (10 mL) was
added. Sodium hydroxide solution (3 mL, 1 M) was added,
which changed the colour of the reaction solution from green
to yellow. The solution was filtered, and the filtrate was left to
evaporate slowly. Orange crystals formed in the filtrate and were
collected at the pump, washed with water (2 × ∼5 mL), and
dried under reduced pressure. A second crop of crystals was
obtained from the filtrate and washings. The product was
recrystallised from methanol (0.165 g, 50%) (Found: C, 47.14;
H, 7.60; N, 13.42. C16H31N4O4.5Ni requires C, 46.86; H, 7.62;
N, 13.66%); λmax/nm (CH3OH) 238 (ε/dm3 molϪ1 cmϪ1 1.4 ×
104), 418 (2.3 × 102); νmax/cmϪ1 3439w, 3364w, 3106s, 2981w
(CH), 2971w (CH), 2934s (CH), 2903w (CH), 2870s (CH),
2855w (CH), 2839w (CH), 1611s, 1575vs (amide I), 1444s,
1417s (amide C–N), 1343s, 1297w, 1238w, 932s (DRIFTS
in KBr); δH(400 MHz, dmso-d6) 0.88 (2 H, m, cyclohexane H3
and H6), 1.03 (2 H, m, cyclohexane H4 and H5), 1.41 (2H, m,
[NiII(S,S-bprolen)]ؒH2O 2. The syntheses were carried out
using Schlenk techniques under an argon atmosphere. All solv-
ents and prepared solutions were degassed thoroughly before
use by repeated evacuation followed by purging with argon.
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Table 1 Summary of crystal data and details of structure refinements for 1, 2, 3 and 4
Compound
1ؒH2O
2ؒH2O
3ؒ3H2O
4ؒD2OؒCD3OD
Model formula
Mw
C12H18N4NiO3
325.00
C12H20N4NiO3
327.03
C16H32N4NiO5
419.17
C17H26N4NiO4
409.13
Crystal system
Space group (no.)
a/Å
b/Å
c/Å
α/Њ
Triclinic
P1 (2)
Trigonal
P3221 (154)
8.5856(14)
8.5856
17.3694(18)
—
—
120.00
1108.8(2)
3
Orthorhombic
P212121 (19)
9.0175(3)
10.1232(3)
21.1448(7)
—
Orthorhombic
P212121 (19)
11.760(2)
18.409(3)
8.571(2)
—
—
—
1855.6(6)
4
¯
9.748(1)
9.923(1)
8.383(1)
105.53(1)
104.40(1)
112.37(1)
664.2(2)
2
β/Њ
—
—
γ/Њ
V/Å3
Z
1930.22(11)
4
T /ЊC
µ/mmϪ1
N
Nobs
R
21
21
20
21
2.245 (Cu-Kα)
2250
1911 (I > 3σ(I ))
0.051
2.017 (Cu-Kα)
1514
1266 (I > 2σ(I ))
0.0389
1.039 (Mo-Kα)
20641
4221 (I > 2σ(I ))
0.0303
1.770 (Cu-Kα)
1931
1808 (I > 2σ(I ))
0.0339
Rw
0.060
0.1093
0.0795
0.0967
cyclohexane H4Ј and H5Ј), 1.51 (2 H, m, pyrrolidine H4), 1.69
(2 H, m, pyrrolidine H3), 1.76 (2 H, m, pyrrolidine H3Ј), 1.88
(2 H, m, pyrrolidine H4Ј), 2.62 (2 H, m, pyrrolidine H5), 2.66
(2 H, m, cyclohexane H1 and H2), 2.82 (2 H, m, cyclohexane H3Ј
and H6Ј), 3.06 (2 H, m, pyrrolidine H5Ј), 3.38 (2 H, q, pyrrol-
idine H2), 4.17 (2 H, q, amine); δC(200 MHz, dmso-d6) 25.2
(cyclohexane C4 and C5 or pyrrolidine C4), 25.4 (pyrrolidine C4
or cyclohexane C4 and C5), 28.8 (pyrrolidine C3), 31.6 (cyclo-
hexane C3 and C6), 49.0 (pyrrolidine C5), 65.8 (pyrrolidine C2),
68.6 (cyclohexane C1 and C2), 177.9 (CO).
X-Ray crystallography
Summaries of the data collection and solutions and refinements
for 1, 2, 3 and 4 are given in Table 1.
Data for 1ؒH2O, 2ؒH2O and 4ؒD2OؒCD3OD were collected
on a Rigaku AFC7R diffractometer employing graphite-mono-
chromated Cu-Kα radiation from a rotating anode generator.
Data for 3ؒH2O were collected on a Bruker SMART 1000
diffractometer with graphite-monochromated Mo-Kα gener-
ated from a sealed tube. In general, crystal data processing and
calculations were undertaken with the TEXSAN32–37 interface.
The data integration and reduction for 3ؒH2O were undertaken
with SAINT and XPREP.38 All data were corrected for Lorentz
and polarisation effects. An empirical absorption correction
based on azimuthal scans of three reflections was applied to the
data for 1ؒH2O and 2ؒH2O and a correction determined from
equivalent reflections with SADABS39 was applied to the data
for 3ؒH2O. An analytical correction was applied to that data
for 4ؒD2OؒCD3OD. Non-hydrogen atoms were modelled with
anisotropic thermal parameters and in general the hydrogen
atoms were included in the refinement at calculated positions
with group thermal parameters.
[NiII(S,S-bprolben)]ؒ2H2O 4. Nickel() acetate tetrahydrate
(0.167 g) was dissolved in water (10 mL) by heating on a steam
bath. A solution of V (0.204 g) in methanol (10 mL) was added
to the hot Ni() acetate solution. The colour of the solution
changed immediately from green to orange. The product crys-
tallised as the solution cooled, and was collected at the pump
after the mixture had stood for 2 d. The dark yellow, crystalline
product was washed with ice-cold water (5 mL), air dried, then
dried under reduced pressure over silica gel, (0.182 g, 69%);
(Found: C, 49.11; H, 5.23; N, 14.45. C16H24N4O4Ni requires C,
48.63; H, 6.12; N, 14.18%); λmax/nm (CH3OH) 216 (ε/dm3 molϪ1
cmϪ1 3.1 × 104), 244 (sh, 1.3 × 104), 274 (1.5 × 104), 294 (1.7 ×
104), 416 (2.1 × 102); νmax/cmϪ1 3456w, 3248w, 3117w, 2972w
(CH), 2872w (CH), 1605vs (amide I), 1569vs (aromatic ring
skeletal vibration), 1481s, 1454s, 1403s, 1033w, 755s (CH
deformation) (DRIFTS in KBr); δH(400 MHz, CD3OD) 1.75
(2 H, m, pyrrolidine H4), 1.96 (2 H, m, pyrrolidine H4Ј), 2.15
(4 H, m, pyrrolidine H3), 2.89 (2 H, m, pyrrolidine H5), 3.45
(2 H, m, pyrrolidine H5Ј), 3.72 (2 H, q, pyrrolidine H2), 4.51
(2 H, q, amine), 6.71 (2 H, dd, benzene H4 and H5), 8.08 (2 H,
dd, benzene H3 and H6); δC(200 MHz, dmso-d6) 25.6 (pyrrol-
idine C4), 29.8 (pyrrolidine C3), 49.4 (pyrrolidine C5), 66.8
(pyrrolidine C2), 118.6 (benzene C3 and C6), 120.3 (benzene C4
and C5), 142.9 (benzene C1 and C2), 177.0 (CO).
1ؒH2O. Slow evaporation of a methanol solution produced
red prismatic crystals of suitable quality for X-ray diffraction
¯
studies. The structure was solved in the space group P1 (no. 2)
by heavy-atom Patterson methods40 and expanded using
Fourier techniques.41 The asymmetic unit includes a water
molecule and the water hydrogens were located and refined with
isotropic thermal parameters.
2ؒH2O. A yellow prismatic fragment cut from a twinned
crystal grown from methanol was used for the structure
determination. The structure was solved in the space group
P3221 (no. 154) by direct methods with SHELXS-97,42 and
extended and refined with SHELXL-97.43
The complex straddles a two-fold axis passing through the
metal centre. The asymmetric unit contains a water molecule
(also located on a two-fold axis), and no hydrogens were
included in the model for the water molecule. The Flack
parameter44–46 refined to 0.00(5).
[NiII(bpen)] 5. The complex was prepared by the published
method17 and recrystallised from methanol (82%); λmax/nm
(CH3OH) 256 (ε/dm3 molϪ1 cmϪ1 1.5 × 104), 382 (7.4 × 103);
νmax/cmϪ1 1633vs (amide I), 1604vs, 1418m (amide III), 755m,
685m (DRIFTS in KBr) (lit.,17 1630 and 1420 cmϪ1).
3ؒ3H2O. Slow evaporation of a methanol/acetonitrile sol-
ution of the product produced orange–yellow prismatic crystals
suitable for X-ray crystallography. The structure was solved in
the space group P212121 (no. 19) by direct methods with
SIR97,47 and extended and refined with SHELXL-97.43 The
hydrogen sites of the water solvate molecules were located and
refined; the O(5) hydrogen bond lengths were restrained. The
[NiII(bpb)] 6. The complex was prepared by the published
method15 and recrystallised from DMF (97%); λmax/nm (DMF)
324 (ε/dm3 molϪ1 cmϪ1 1.9 × 104), 372 (sh, 8.1 × 103), 446 (sh,
3.1 × 103); νmax/cmϪ1 1639vs (amide I), 1604vs, 1576m, 1485m,
1398m, 745vs, 680m (DRIFTS in KBr) (lit.,15 1640, 1605, 1570,
1485, 1390, 750, 680 cmϪ1).
934
J. Chem. Soc., Dalton Trans., 2002, 931–940

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absolute structure was established with the Flack parameter44,45
refining to 0.00(1).
recrystallisation was not required to make the complex. The mp
of III was 13 ЊC higher than the reported value29 indicating that
it was of higher purity. The amide NH stretch also occurred at
a higher frequency (possibly due to the inclusion of 0.5H2O
4ؒD2OؒCD3OD. A CD3OD solution of the complex prepared
for NMR spectroscopy with three drops of D2O added was left
standing in a sealed NMR tube for 2 weeks, and yellow
columnar-like crystals suitable for X-ray crystallography
formed during this time. The structure was solved in the space
group P212121(no. 19) by direct methods with SIR97,47 and
extended and refined with SHELXL-9743 using the XSHELL
interface.48 The two water hydrogens were located and modelled
with isotropic thermal parameters. The absolute structure was
established with the Flack parameter44,45 refining to 0.01(5).
CCDC reference numbers 169589–169592.
1
altering the amide hydrogen-bonding), and the H and 13C
NMR data for III were in good agreement with the literature
1
values.27,29 The H NMR signals of IV and V were assigned
from the 1H COSY NMR spectra (Figs. S2 and S3, Supporting
Information†). The 1H NMR resonances from the cyclohexane
and pyrrolidine rings of IV overlapped in the 1D spectrum but
were easily distinguished in the COSY spectrum as there were
no cross-peaks between the two ring systems. The IR bands
were assigned according to Bellamy49 and by comparison to the
IR spectra of I17 and II.15 The most prominent features of the
IR spectra were the amide NH stretch, the amide I and amide II
bands. This is the first report of the isolation of the chiral
ligands IV and V in a pure form, along with their detailed char-
acterisation. The synthetic scheme can be readily adapted to
produce new ligands of this class by changing the diamine used
as the central bridging group, and has many potential applic-
ations in the production of chiral tetradentate ligands. Com-
plexes with these ligands have the potential to be used in chiral
resolutions,50 catalysis29,51 and selective DNA damage.2,3,5,7,8,10
See http://www.rsc.org/suppdata/dt/b1/b107378h/ for crystal-
lographic data in CIF or other electronic format.
Results and discussion
Ligand syntheses
The ligands III, IV and V were synthesised from S-proline
(Scheme 1). The published procedure26 for the synthesis of
Nickel complexes
Nickel() complexes with III and IV were prepared by the add-
ition of base to deprotonate the ligand amides. The complex
with V was prepared directly by mixing, since the acetate coun-
ter ion was sufficiently basic to deprotonate the amide groups.
The absence of the amide NH stretching and amide II bands
from the IR spectra of the Ni complexes showed that the amide
groups were deprotonated. The amide I bands occurred at
lower frequencies in the complexes than in the free ligands,
which was consistent with coordination of the ligand via amide
nitrogen atoms.
Two different complexes, 1 and 2, were obtained from the
reaction of III with Ni(), depending on the conditions under
which the reaction took place (Scheme 2). The reaction of Ni()
Scheme 2 Synthesis of nickel() complexes with III
and III in air in the presence of base (OHϪ or CO32Ϫ), produced
a Ni() complex where the two amine groups of the pyrrolidine
rings were oxidised to imines, forming 1-pyrroline rings. When
CO32Ϫ was used (method 2) a large amount of precipitate, that
was presumed to be NiCO3, formed. The precipitate gradually
dissolved as the ligand coordinated, forming a soluble yellow
complex. The Ni(CH3CO2)2/III reaction under Ar, produced 2
in which the ligand was not oxidised. Once isolated, the product
could be handled in air as a solid without significant decom-
position or ligand oxidation. The methanol solution of 2 is also
air-stable, e.g., a CD3OD solution that was exposed to the air
for a month revealed no 1 or other decomposition products by
1H NMR spectroscopy. This allowed the purification of crude
Scheme 1 Synthesis of tetradentate diamide-dipyrrolidine ligands
N,N Ј-bis(carbobenzoxy-S-prolyl)-R,R-1,2-cyclohexanediamine
only used a 1.16 : 1.00 mole ratio of CBZ-S-proline: R,R-1,2-
cyclohexanediamine, but use of the correct 2.00 : 1.00 ratio
increased the yield from 47 to 69%. The removal of the carbo-
benzoxy protecting groups with HBr in acetic acid was con-
siderably faster than the use of H2 over a Pd/C catalyst, but the
latter method produced greater yields with a higher degree of
purity. The yield of crude IV was ∼100% and elemental analyses
and NMR spectra showed that it was of sufficient purity that
J. Chem. Soc., Dalton Trans., 2002, 931–940
935

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Table 2 Selected bond distances (Å) and angles (Њ) of 1 involving the
non-hydrogen atoms
Table 3 Selected bond distances (Å) and angles (Њ) of 2 involving the
non-hydrogen atoms
Ni1–N1
Ni1–N2
Ni1–N3
Ni1–N4
O1–C5
O2–C8
N1–C1
1.893(3)
1.835(3)
1.827(3)
1.898(3)
1.243(5)
1.230(5)
1.478(4)
N1–C4
N2–C5
N2–C6
N3–C7
N3–C8
N4–C9
N4–C12
1.281(5)
1.325(5)
1.461(4)
1.470(5)
1.316(5)
1.285(5)
1.466(5)
Ni1–N1
O1–C5
N1–C4
N2–C6
1.925(2)
1.249(4)
1.503(4)
1.462(4)
Ni1–N2
N1–C1
N2–C5
1.822(2)
1.498(4)
1.314(4)
N1–Ni1–N1Ј
N1–Ni1–N2
N1–Ni1–N2Ј
N2–Ni1–N2Ј
C1–N1–C4
101.5(2)
C4–N1–Ni1
C5–N2–C6
C5–N2–Ni1
C6–N2–Ni1
O1–C5–N2
110.0(2)
86.4(1)
171.4(1)
86.0(2)
107.3(3)
113.7(2)
123.4(2)
119.3(2)
115.8(2)
126.3(3)
N1–Ni1–N2
N1–Ni1–N3
N1–Ni1–N4
N2–Ni1–N3
N2–Ni1–N4
N3–Ni1–N4
Ni1–N1–C1
Ni1–N1–C4
C1–N1–C4
Ni1–N2–C5
84.5(1)
169.1(1)
106.5(1)
84.7(1)
168.5(1)
84.3(1)
136.3(3)
113.2(3)
110.4(3)
116.7(2)
Ni1–N2–C6
C5–N2–C6
Ni1–N3–C7
Ni1–N3–C8
C7–N3–C8
Ni1–N4–C9
Ni1–N4–C12
C9–N4–C12
O1–C5–N2
O2–C8–N3
117.7(3)
123.6(3)
116.7(3)
118.8(3)
124.2(3)
112.6(2)
136.5(3)
110.7(3)
129.0(4)
129.3(4)
C1–N1–Ni1
Symmetry operator: y, x, 1 Ϫ z
product in method 2 in air using column chromatography,
which greatly improved the yield. The crystal used to determine
the structure by X-ray diffraction was obtained by slow evapor-
ation under air of a methanol solution.
X-Ray crystallography
The most significant features of the crystal structure of 1ؒH2O
(Fig. 3) are the N1–C4 and N4–C9 double bond lengths
Fig. 4 ORTEP52 representation of 2 with 25% atom displacement
ellipsoids
The deprotonated Ni–N(amide) bond length of 1.822(2) Å is
significantly shorter than the Ni–N(amine) bond length of
1.925(2) Å. The Ni–N(amide) bond length is slightly, but not
significantly, shorter than the average value in 1, but the
Ni–N(amine) bond length is significantly longer than the
Ni–N(imine) bonds in 1. The coordination about the Ni atom is
square-planar, and it lies within the plane defined by the four
N atoms. The deviation of the nitrogens from the coordination
least-squares planes ranges from 0.037(3) to 0.078(4) Å.
Fig. 3 ORTEP52 depiction of 1 with 25% atom displacement ellipsoids
(Table 2) of 1.281(5) and 1.285(5) Å. The sp2 hybridisation of
N1 and N4 was also shown by their trigonal planar geometry.
The 1-pyrroline rings are almost coplanar with the least-
squares plane defined by the four nitrogen atoms, and the
dihedral angles are 7.0(2) and 4.2(2)Њ. The angle between the
two 1-pyrroline rings is 11.1(2)Њ.
The ORTEP diagram of the complex in 3ؒ3H2O is given in
Fig. 5. The non-hydrogen bond distances and angles are in Table
The coordination geometry about the Ni in 1 is square-
planar, with two Ni–N(amide) and two Ni–N(imine) bonds.
The nickel atom deviates by just 0.033(1) Å from the least-
squares plane defined by the four nitrogen atoms. The deviation
of the nitrogens from the coordination least-squares planes
ranges from 0.018(5) to 0.022(5) Å. The Ni–N(amide) bond
lengths of 1.835(3) and 1.827(3) Å, are within the range
(1.820–2.02 Å) reported for other deprotonated Ni–N(amide)
bonds.18,20,23,24,53–56 The Ni–N(imine) bond lengths of 1.893(3)
and 1.898(3) Å are also typical (1.840–2.065 Å).57–63
The molecular structure of 2ؒH2O (Fig. 4) has a two-fold
symmetry axis through the Ni atom. The non-hydrogen bond
distances and angles are given in Table 3. The crystal structure
showed that the amines in the pyrrolidine rings of the ligand
were preserved during the formation of the complex. The N1–
C4 bond length is 1.503(4) Å, and the geometries about N1 and
C4 are tetrahedral. The amine nitrogen atoms are chiral and
both have the S configuration. The Ni atom is coordinated to
two deprotonated amide nitrogens and two amines in a square-
planar geometry. The pyrrolidine rings of the ligand are non-
planar and are at a 60.3(2)Њ angle to the coordination plane.
Fig. 5 ORTEP52 representation of 3 with 25% atom displacement
ellipsoids
936
J. Chem. Soc., Dalton Trans., 2002, 931–940

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Table 4 Selected bond distances (Å) and angles (Њ) of 3 involving the
non-hydrogen atoms
Table 5 Selected bond distances (Å) and angles (Њ) of 4 involving the
non-hydrogen atoms
Ni1–N3
Ni1–N1
O1–C5
N1–C1
N2–C5
N3–C12
N4–C16
1.839(2)
1.921(2)
1.259(3)
1.486(3)
1.314(3)
1.320(3)
1.507(3)
Ni1–N2
Ni1–N4
O2–C12
N1–C4
N2–C6
N3–C11
N4–C13
1.845(2)
1.932(2)
1.262(3)
1.515(3)
1.482(3)
1.475(3)
1.510(3)
Ni1–N2
Ni1–N1
O1–C5
O3–C17
N1–C4
N2–C6
N3–C11
N4–C13
1.833(3)
1.919(3)
1.243(4)
1.375(7)
1.512(4)
1.416(4)
1.415(5)
1.499(5)
Ni1–N3
Ni1–N4
O2–C12
N1–C1
N2–C5
N3–C12
N4–C16
1.842(3)
1.922(3)
1.250(5)
1.509(5)
1.327(5)
1.323(4)
1.487(5)
N3–Ni1–N2
N3–Ni1–N1
N2–Ni1–N1
N3–Ni1–N4
N2–Ni1–N4
N1–Ni1–N4
C1–N1–C4
C1–N1–Ni1
C4–N1–Ni1
C5–N2–C6
86.70(8)
170.36(8)
86.80(8)
86.68(7)
172.34(8)
100.24(8)
106.4(2)
120.6(2)
109.7(1)
125.6(2)
C5–N2–Ni1
C6–N2–Ni1
C12–N3–C11
C12–N3–Ni1
C11–N3–Ni1
C16–N4–C13
C16–N4–Ni1
C13–N4–Ni1
O1–C5–N2
118.3(2)
113.5(1)
123.4(2)
116.6(2)
113.5(1)
106.1(2)
119.4(2)
109.0(1)
128.2(2)
128.1(2)
N2–Ni1–N3
N2–Ni1–N1
85.6(1)
86.8(1)
172.3(1)
172.0(1)
86.6(1)
101.1(1)
106.3(3)
118.6(2)
108.9(2)
125.9(3)
C5–N2–Ni1
C6–N2–Ni1
C12–N3–C11
C12–N3–Ni1
C11–N3–Ni1
C16–N4–C13
C16–N4–Ni1
C13–N4–Ni1
O1–C5–N2
118.2(2)
114.8(2)
126.8(3)
118.1(3)
114.3(2)
106.0(3)
117.3(2)
108.6(2)
128.7(3)
126.8(4)
N3–Ni1–N1
N2–Ni1–N4
N3–Ni1–N4
N1–Ni1–N4
C1–N1–C4
C1–N1–Ni1
C4–N1–Ni1
C5–N2–C6
O2–C12–N3
O2–C12–N3
4. The Ni atom is bound to two amines and two deprotonated
amide nitrogens and has square-planar coordination geometry.
The deviation of the nitrogens from the coordination least-
squares planes ranges from 0.077(2) to 0.101(2) Å. The devi-
ations of the N atoms from the coordination plane are slightly
larger than those observed in the crystal structures of 1, 2
and 4; perhaps due to the central trans-cyclohexane bridge. The
amine nitrogens are chiral and both have a S configuration.
The Ni–N(amide) bond lengths are 1.839(2) Å and 1.845(2) Å,
and are slightly longer than the 1.822(2) Å Ni–N(amide)
bond length of 2. The Ni–N(amine) bond lengths of 1.921(2)
and 1.932(2) Å are not significantly different from the
Ni––N(amine) bond length of 1.925(2) Å in 2. The longer
Ni–N(amide) bond lengths may be due to distortion caused by
the central trans-cyclohexane bridge.
ation plane is 1.0(1)Њ. The puckered pyrrolidine rings were
not oxidised during the synthesis of the complex; the amine
nitrogens are chiral and both have the S configuration.
In all four crystal structures, the bond angles formed by the
chelate rings about the Ni are less than the ideal value of 90Њ for
a square-planar complex, with the angle between the terminal
nitrogen donors >100Њ to compensate. The uniform S con-
figuration of the amine nitrogens in 2, 3 and 4 shows that
their chirality was determined by the fixed S configuration at
position 2 of the pyrrolidine rings.
NMR spectra of nickel complexes
The 1H NMR spectrum of 1 (Fig. S4, Supporting Information†)
was much simpler than that of III (Figure S1, Support-
ing Information†). There were only four signals, and the indi-
vidual proton–proton couplings were determined along with
the coupling constants by selective decoupling experiments
(Figs. S5–S7, Supporting Information†). The resonances at
2.71 and 3.76 ppm are triplets of triplets; the stronger couplings
are to the two protons on position 4 of the 1-pyrroline ring,
while the weaker triplet splitting is due to coupling to each
other. The resonance at 2.09 ppm is a quintet as the coupling to
the two protons on position 3 of the 1-pyrroline ring and the
coupling to the two protons on position 5 of the 1-pyrroline
ring are approximately equal, so there are four “equivalent”
neighbours.
The ORTEP diagram of the Ni complex in 4ؒD2OؒCD3OD
(Fig. 6) shows that the Ni atom is square-planar and coordin-
The 1H COSY NMR spectrum (Fig. 7) of 2 has a larger num-
ber of signals and a much more complicated spin–spin coupling
pattern than the spectrum of 1. When D2O was added to a
solution of 2 in CD3OD, the signal at 4.37 ppm began to
decrease and the multiplet at 3.6 ppm (corresponding to the
signal at 3.3 ppm in dmso-d6) became a triplet, due to the slow
exchange of the amine protons. An hour after the D2O was
added, the quartet at 4.37 ppm was still present; a day later it
had completely disappeared. The pattern and intensity of the
cross-peaks in the COSY spectrum were used to assign the
remaining signals. The signal at 3.3 ppm in dmso-d6 is obscured
by the water peak, but the cross-peaks to the amine protons
and the axial and equatorial protons on position 3 of the
pyrrolidine rings are observed.
1
Fig. 6 ORTEP52 representation of 4 with 25% atom displacement
ellipsoids
The H COSY NMR spectrum of 3 in dmso-d6 (Fig. S8,
Supporting Information†) was used to assign the structure. The
1
1D H NMR spectrum of 3 in CD3OD was also recorded to
ated to two amines and two deprotonated amide nitrogens. The
Ni–N(amide) bond lengths (Table 5) of 1.833(3) Å and 1.842(3)
Å are significantly shorter than the Ni–N(amine) bond lengths
of 1.919(3) and 1.922(3) Å. The coordinated N atoms are
co-planar, with deviations ranging from 0.018(4) to 0.022(4) Å.
The dihedral angle between the benzene ring and the coordin-
determine the structure of the multiplet hidden under the water
peak in dmso-d6. Partial deuterium exchange of the amine
protons in CD3OD solution; and complete exchange in D2O
resulted in a change in the signal due to the protons on position
2 of the pyrrolidine rings from a quartet to a triplet. The rigid-
ity of the molecule results in proton–proton couplings over
J. Chem. Soc., Dalton Trans., 2002, 931–940
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Table 6 Nickel(/) reduction potentials
decreased from 0.91 to 0.46 and the peak-to-peak separation,
∆Ep, increased from 70 to 87 mV. The decrease in the ratio of
ipc/ipa is counter-intuitive for a reversible process. However, the
Ni() complexes are four-coordinate and have a square-planar
geometry about the Ni, whereas Ni() complexes are usually
six-coordinate,12,64,65 so there is a conformational change when
the oxidation state of the nickel changes. At slower scan rates,
the scan rate was low compared to the rate of conformational
rearrangement, so a single reversible redox couple was
observed. As the scan rate increased, the timescale of the
experiment became comparable to the rate of conformational
rearrangement, i.e., the redox couples for the four-coordinate
species and the six-coordinate species started to separate, and
the reversibility was lost. Although scan rates of up to 2000 mV
sϪ1 were used, distinct redox couples for the four-coordinate
and six-coordinate species could not be resolved within the
potential window.
b
Couple
E1/2/V (vs. NHE)a
E1/2/V (vs. Fcϩ/0
)
1ϩ/0
2ϩ/0
3ϩ/0
4ϩ/0
5ϩ/0
0.950
0.918
0.870
0.316
0.033
0.220
0.311
a Solvent: H2O, supporting electrolyte: NaClO4 (0.1 M) b Solvent:
N,N-dimethylformamide, supporting electrolyte: TBAP (0.1 M)
The Ni(/) reduction potentials determined in this work
were similar to those reported for Ni tripeptide complexes in
water (0.84–0.96 V vs. NHE).12 With tetrapeptides, peptide
amides, and higher order peptides, the values lie in the range
0.79–0.84 V vs. NHE.12 Complexes 1–5 had fairly low Ni(/)
reduction potentials, which indicated that the deprotonated
amides were effective at stabilising the Ni() oxidation state.
The complexes with terminal amine ligands had the lowest
Ni(/) reduction potentials. The presence of a central benzene
bridge made the reduction potential more positive. This was
probably due to delocalisation of the negative charge on the
deprotonated amide groups into the benzene ring. The most
easily oxidised complex was 3, yet it was complex 2 that
underwent oxidative dehydrogenation of the amines during
the synthesis. This indicates that factors besides the Ni()
oxidation potential, such as the strain induced in the ligand
upon coordination, influence whether or not the amine groups
undergo oxidative dehydrogenation.
Fig. 7 1H COSY NMR spectrum of 2 in dmso-d6. Residual solvent
peaks: 2.07 ppm (acetone), 2.48 ppm (dmso-d5), and 3.32 ppm (water).
Two reversible reductions were observed in the CVs of 5
(Fig. 8(b)) and 6 (Fig. S11(b), Supporting Information†). These
quite long paths. There are even weak cross peaks in the COSY
1H NMR spectrum between the resonance due to the protons
on positions 1 and 2 of the cyclohexane ring (2.66 ppm),
and the resonance due to the protons on position 2 of the
pyrrolidine rings (3.38 ppm).
1
The H COSY NMR spectrum of 4 in CD3OD (Fig. S9,
Supporting Information†) did not show exchange of the amine
protons with deuterium. When D2O was added, the quartet due
to the amine protons (4.51 ppm) was still visible after 2 d,
though it partially overlapped the water peak, which had
shifted; therefore, an accurate integration could not be deter-
mined. The lack of deuteration of the amines was established
by the resonance at 3.72 ppm (protons on position 2 of the
pyrrolidine rings), which remained as a quartet. This did not
change to a triplet as the equivalent resonances in the spectra of
2 and 3 did on amine deuteration.
There was no evidence of paramagnetic species in the NMR
spectra, which showed that the Ni complexes retained their
square-planar coordination in solution.
Cyclic voltammetry (CV)
The Ni(/) redox couples for 1–5 were quasi-reversible at a
glassy-carbon working electrode (Table 6); no reversible oxid-
ation was observed for 6. Not all complexes were soluble in a
single solvent, which prevented quantitative comparisons being
made amongst the redox properties in the same solvent. At
lower scan rates, the Ni(/) couple was almost fully reversible,
as the scan rate increased quasi-reversible behaviour was appar-
ent as the anodic and cathodic peaks moved farther apart and
the relative intensity of the cathodic peak decreased (Fig. S10,
Supporting Information†). As the scan rate increased from 10
to 100 mV sϪ1 for 5, the ratio of the peak currents, ipc/ipa,
Fig. 8 CV of (a) I (3.5 mM) and (b) 5 (5 mM) in DMF, supporting
electrolyte: TBAP (0.1 M), glassy-carbon working electrode, at scan
rates of (a) 300 mV sϪ1 and (b) 100 mV sϪ1
938
J. Chem. Soc., Dalton Trans., 2002, 931–940

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reductions were ligand centred; two reductions were also
observed in the cyclic voltammograms of the ligands (Fig. 8(a)
and Fig. S11(a), Supporting Information), though only the
reduction at more negative potential was reversible. The
coordination of the ligands to Ni() stabilised the reduced
forms due to delocalisation of the charge onto the Ni. This
stabilisation caused the reduction potentials to shift to more
positive values and the first reduction became reversible. These
two reductions probably involve the pyridyl rings in the ligand
as no reductions, either reversible or irreversible, were observed
in the CVs of the pyrrolidine-based ligands III–V. A single
reversible reduction was observed for 3 and 4 in DMF at
Ϫ2.808 and Ϫ2.680 V vs. Fcϩ/0, respectively, and an irreversible
reduction of 1 at Ϫ2.178 V vs. Fcϩ/0. The absence of reductions
in the CVs of IV and V indicates that these reductions are likely
to be Ni-based. The CV of 2 was recorded in water (due to its
limited solubility in DMF), so it was not possible to examine its
redox behaviour with a carbon electrode at the more negative
potentials where the other Ni() complexes were reduced.
complex of III indicates that with deprotonated amide nitrogen
coordination, the higher oxidation states of Ni are accessible
using relatively mild oxidants. This may have relevance as a
biomimetic reaction leading to the biological oxidation of
DNA species.
Acknowledgements
Our thanks to Dr Ming Xie and Dr Ian Luck from the NMR
facility in the School of Chemistry, University of Sydney for
recording the 400 MHz NMR spectra. This work has been
supported by funding from Australian Research Council (ARC)
grants, an ARC RIEFP grant for an X–ray diffractometer, and
an APA scholarship to C. L. W.
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