Structural, spectroscopic and electrochemical studies of nickel(II) "sandwich" complexes with ligands featuring tethered 1,4,7-triazacyclononane macrocycles

Article abstract of DOI:10.1039/b101766g

A structural and electrochemical study of the nickel(II) complexes, [Ni(tacn)₂](ClO₄)₂ (1), [NiL^eth](ClO₄)₂ (2), [NiL^ox](ClO₄)₂·H₂O (3), [Ni

Full text of DOI:10.1039/b101766g

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Structural, spectroscopic and electrochemical studies of nickel(II)
“sandwich” complexes with ligands featuring tethered 1,4,7-
triazacyclononane macrocycles†
Bim Graham,a Leone Spiccia,*a Alan M. Bond,a Milton T. W. Hearnb and
Christopher M. Keperta
a School of Chemistry, PO Box 23, Monash University, 3800, Victoria, Australia.
E-mail: leone.spiccia@sci.monash.edu.au
b Department of Biochemistry and Molecular Biology, PO Box 130, Monash University, 3800,
Victoria, Australia
Received 23rd February 2001, Accepted 22nd May 2001
First published as an Advance Article on the web 19th July 2001
A structural and electrochemical study of the nickel() complexes, [Ni(tacn)2](ClO4)2 (1), [NiLeth](ClO4)2 (2), [NiLox](ClO4)2ؒ
H2O (3), [Ni2Ldur](ClO4)4ؒ2H2O (4), [Ni3Ldur(H2O)3](ClO4)6ؒ9H2O (5) and [Ni2Lisomes(H2O)3](ClO4)4ؒ7H2O (6)
[tacn = 1,4,7-triazacyclononane, Leth = 1,2-bis(1,4,7-triazacyclonon-1-yl)ethane, Lox = 1,2-bis(1,4,7-triazacyclonon-1-
ylmethyl)benzene, Ldur = 1,2,4,5-tetrakis(1,4,7-triazacyclonon-1-ylmethyl)benzene, Lisomes = 1,2,3-tris(1,4,7-triaza-
cyclonon-1-ylmethyl)benzene] is reported. Single-crystal X-ray crystallography has established that 2 and 6 feature nickel()
centres in a distorted octahedral geometry, which are sandwiched between pairs of tacn rings. In 6, a second nickel()
centre coordinates the third tacn ring of Lisomes and three water molecules complete the distorted octahedral
coordination sphere. For the sandwiched Ni() centres, steric contraints introduced by the ethane and ortho-oriented
tacn rings in Lisomes cause significant distortions to the Ni() coordination spheres. Cyclic, square-wave and near
steady-state voltammetric studies on 1–6 indicate that the sandwiched nickel() centres may be reversibly oxidised to
the nickel() state, with the potential for the nickel()/() couple showing a dependence on the nature of the tether
linking the sandwiching tacn rings. Complex 4 undergoes oxidation in two overlapping one-electron processes [i.e.,
Ni()Ni() → Ni()Ni() → Ni()Ni()], indicating that the two nickel centres within the complex behave in an
essentially independent fashion.
length. The X-ray structures of [CuLeth 2ϩ and [CuLprop 2ϩ
]
]
Introduction
confirmed the formation of intramolecular sandwich-type
structures, whilst the other [CuL]2ϩ complexes were proposed to
be dimeric or oligomeric. For alkyl bridges comprising of ≥4
carbons, intermolecular sandwich structures of the form
[Cu2L2]4ϩ have been postulated.6–8 We have recently reported
further examples of Ni() and Cu() complexes of poly-
(tacn) derivatives.9,10 The Ni() series includes [NiLox](ClO4)2ؒ
H2O (3), [Ni2Ldur](ClO4)4ؒ2H2O (4) and [Ni3Ldur(H2O)3]-
(ClO4)6ؒ9H2O (5) [Lox = 1,2-bis(1,4,7-triazacyclonon-1-yl-
methyl)benzene, Ldur = 1,2,4,5-tetrakis(1,4,7-triazacyclonon-1-
ylmethyl)benzene].9,10 At least one Ni centre in these complexes
is sandwiched between pairs of tacn macrocycles attached to
ortho positions of an aromatic spacer group. Detailed equi-
librium studies on the series of xylyl-bridged ligands, Lox, Lmx
and Lpx, reported by Zompa and co-workers,8 confirmed that
the stability of the sandwich structures decreases in the order
Lox > Lmx > Lpx.
The metal-complexing properties of the small tridentate macro-
cycle, 1,4,7-triazacyclononane (tacn), have been extensively
studied since its preparation was first detailed in 1972.1 When
reacted with a variety of divalent and trivalent transition metal
ions in a 2 : 1 molar ratio, tacn readily forms “sandwich-type”
complexes, [M(tacn)2]nϩ (Mnϩ = Cr3ϩ, Mn2ϩ, Fe2ϩ, Fe3ϩ, Co2ϩ
,
Co3ϩ, Ni2ϩ, Ni3ϩ), featuring metal centres with octahedral N6
donor sets.2,3 A number of groups have reported that various
bis(tacn) ligands, composed of pairs of covalently linked tacn
macrocycles, can form similar structures, provided the linker
group between the sandwiching macrocycles is sufficiently flex-
ible. The first examples were reported by Takamoto et al.4 and
Wieghardt et al.5 who described a series of mononuclear com-
plexes of the ethane-bridged bis(tacn) ligand [Leth = 1,2-
bis(1,4,7-triazacyclonon-1-yl)ethane; M = Cu2ϩ, Ni2ϩ/3ϩ, Mn2ϩ
Zn2ϩ, Fe3ϩ, Co3ϩ, Cr3ϩ] and the propane-bridged bis(tacn)
ligand,
[Lprop = 1,3-bis(1,4,7-triazacyclonon-1-yl)propane;
,
In this paper, we report the synthesis of a further poly(tacn)
M = Co3ϩ]. Zompa and co-workers6–8 later detailed an in-depth
structural and equilibrium study of the copper() complex-
ation behaviour of a series of bis(tacn) ligands in which alkyl
chains of 2–8 carbons link the tacn rings. It was concluded
that Leth forms only the monomeric [CuL]2ϩ complex in dilute
aqueous solution whilst the other ligands are capable of
forming both [CuL]2ϩ and [Cu2L]2ϩ complexes, with the stabil-
ity of the [CuL]2ϩ species decreasing with increasing chain
ligand,
1,2,3-tris(1,4,7-triazacyclonon-1-ylmethyl)benzene
1,2,3-
(Lisomes), featuring three tacn rings linked by
a
trimethylbenzene unit. It was anticipated that this ligand might
also be capable of forming complexes incorporating a sand-
wiched metal centre by virtue of its ortho oriented tacn rings.
These expectations were confirmed through the isolation of the
binuclear complex, [Ni2Lisomes(H2O)3](ClO4)4ؒ7H2O (6). The
crystal structure of 6 is presented together with that of the
previously reported [NiLeth](ClO4)2 (2). The results of a detailed
electrochemical study on the series of nickel() sandwich com-
plexes 1–6, shown in Fig. 1, are described. These show that
the sandwiched nickel() centres can be reversibly oxidised to
† Electronic supplementary information (ESI) available: representative
cyclic voltammograms for the mononuclear complex 2 and binuclear
complex 4. See http://www.rsc.org/suppdata/dt/b1/b101766g/
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J. Chem. Soc., Dalton Trans., 2001, 2232–2238
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b101766g

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Scheme 1
[Ni2Lisomes(H2O)3](ClO4)4ؒ7H2O (6). A small amount of the tri-
nuclear complex, [Ni3Lisomes(H2O)9]6ϩ, was also formed in the
reaction but attempts to isolate this complex as a crystalline salt
have thus far proved unsuccessful.
The coordination behaviour of Lisomes towards Ni2ϩ ions is
reminiscent to that of the Lox and Ldur ligands,9,10 which also
feature sets of tacn rings attached to the ortho positions of an
aromatic spacer. These either coordinate to different NiII
centres or sandwich a single NiII centre forming the complexes:
[NiLox](ClO4)2ؒH2O (3) and [Ni2Lox(H2O)6](ClO4)4ؒ4H2O from
Lox; and [Ni2Ldur](ClO4)4ؒ2H2O (4), [Ni3Ldur(H2O)3](ClO4)6ؒ
9H2O (5) and [Ni4Ldur(H2O)12](ClO4)8ؒ9H2O from Ldur
.
The preparations of the nickel() sandwich complexes,
[Ni(tacn)2](ClO4)2 (1)12 and [NiLeth](ClO4)2 (2),5 have been
described previously. We found that 2 forms even under condi-
tions of excess metal ion, an observation which can be ascribed
to the fact that a stable five-membered chelate ring, incorporat-
ing the ethane tether, is formed on coordination. Crystals of 2
and 6 suitable for X-ray crystallography were obtained on slow
evaporation of aqueous solutions of the complexes. The crystal
structures of [Ni(tacn)2](NO3)ClؒH2O,12 3,9 410 and 510 have
been described previously.
Fig. 1 Nickel() complexes featuring sandwiched Ni() centres.
the nickel() state. The trends in the redox potentials of the
nickel()/() couples are discussed in terms of the solid-state
structures of the compounds.
Results and discussion
Syntheses
The synthesis of Lisomes was adapted from the method described
recently for the related mesitylene-bridged trimacrocycle, 1,3,5-
tris(1,4,7-triazacyclonon-1-ylmethyl)benzene.11 This involved
reaction of 3 molar equiv. of tacn orthoamide (1,4,7-triaza-
tricyclo[5.2.1.04,10]decane) with 1,2,3-tri(bromomethyl)benzene,
followed by base hydrolytic work-up to yield the crude ligand as
a white solid (Scheme 1). Although further purification of the
ligand by recrystallisation from various solvents and solvent
Crystal structures
The structure of 2 consists of discrete mononuclear [NiLeth 2ϩ
]
cations (Fig. 2, Table 1) and perchlorate anions. The nickel()
centre is coordinated facially by the two tacn rings of Leth, form-
ing an intramolecular sandwich structure. The cation possesses
a C2 axis of symmetry passing through the nickel() centre and
the middle of the C–C bond of the ethane linker. The geometry
about the nickel() centre is distorted from regular octahedral.
This is due in part to the constraints imposed by the three edge-
sharing, five-membered chelate rings of the tacn moieties,
which result in acute intra-ring N–Ni–N “bite” angles involving
cis-disposed N atoms [81.48(7)–83.29(7)Њ]. Similar values were
1
mixtures was unsuccessful, both the H and 13C NMR spectra
confirmed the generation of the ligand. The crude Lisomes
ligand was treated with an excess of Ni2ϩ ions to yield a
binuclear complex which, following cation-exchange chromato-
graphy (CEC), could be crystallised as the perchlorate salt,
J. Chem. Soc., Dalton Trans., 2001, 2232–2238
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Table 1 Selected bond distances (Å) and angles (Њ) for 2
Table 2 Selected bond distances (Å) and angles (Њ) for 6
Ni–N(1)
Ni–N(3)
2.123(2)
2.138(2)
Ni–N(2)
2.105(2)
Ni(1)–N(1)
Ni(1)–N(3)
Ni(1)–N(5)
Ni(2)–O(1)
Ni(2)–O(3)
Ni(2)–N(8)
2.118(6)
2.134(6)
2.128(6)
2.054(7)
2.119(6)
2.050(7)
Ni(1)–N(2)
Ni(1)–N(4)
Ni(1)–N(6)
Ni(2)–O(2)
Ni(2)–N(7)
Ni(2)–N(9)
2.099(6)
2.174(7)
2.103(6)
2.083(5)
2.138(6)
2.099(7)
N(1)–Ni–N(1)*
N(1)–Ni–N(2)*
N(1)–Ni–N(3)*
N(2)–Ni–N(3)
N(3)–Ni–N(3)*
84.37(9)
161.33(7)
109.72(7)
82.20(7)
165.3(1)
N(1)–Ni–N(2)
N(1)–Ni–N(3)
N(2)–Ni–N(2)*
N(2)–Ni–N(3)*
83.29(7)
81.48(7)
111.8(1)
89.57(7)
N(1)–Ni(1)–N(2)
81.2(2)
N(1)–Ni(1)–N(3)
82.2(2)
N(1)–Ni(1)–N(4) 102.4(2)
N(1)–Ni(1)–N(6) 173.7(2)
N(2)–Ni(1)–N(4) 171.2(3)
N(1)–Ni(1)–N(5) 103.3(2)
N(2)–Ni(1)–N(3)
N(2)–Ni(1)–N(5)
81.5(3)
89.7(3)
N(2)–Ni(1)–N(6)
96.6(3)
N(3)–Ni(1)–N(4) 106.9(3)
N(3)–Ni(1)–N(5) 168.8(2)
N(3)–Ni(1)–N(6)
N(4)–Ni(1)–N(6)
O(1)–Ni(2)–O(2)
O(1)–Ni(2)–N(7)
O(1)–Ni(2)–N(9)
O(2)–Ni(2)–N(7)
91.6(2)
80.7(2)
90.1(3)
94.3(3)
96.1(3)
96.4(2)
N(4)–Ni(1)–N(5)
N(5)–Ni(1)–N(6)
O(1)–Ni(2)–O(3)
81.7(3)
82.6(2)
86.4(3)
O(1)–Ni(2)–N(8) 178.0(3)
O(2)–Ni(2)–O(3)
O(2)–Ni(2)–N(8)
89.0(2)
91.5(2)
O(2)–Ni(2)–N(9) 173.7(3)
O(3)–Ni(2)–N(7) 174.5(2)
O(3)–Ni(2)–N(8)
N(7)–Ni(2)–N(8)
N(8)–Ni(2)–N(9)
94.8(3)
84.3(2)
82.3(3)
O(3)–Ni(2)–N(9)
N(7)–Ni(2)–N(9)
90.1(3)
84.4(3)
Fig. 2 ORTEP plot of the molecular cation in 2 with thermal
ellipsoids drawn at the 30% probability level.
observed for [Ni(tacn)2](NO3)ClؒH2O [82.2(6)–83.1(6)Њ],13 as
well as other nickel() complexes of tacn-derived ligands (Table
3). In addition to this trigonal distortion, however, the ethane
tether results in further distortion. This is most obvious from
the N(1)–Ni–N(1)* angle subtending the ethane linker
[84.37(9)Њ], which is considerably smaller than the inter-ring cis
N–Cu–N angles observed in [Ni(tacn)2](NO3)ClؒH2O, and the
trans N–Ni–N angles [161.33(7)–165.3(1)Њ], which are also
smaller than those found in the latter (see Table 3). Moreover,
there is considerable variation in the inter-ring N–Cu–N bite
angles [84.37(9)–111.8(1)Њ] and the Ni–N bond lengths
[2.105(2)–2.138(2) Å] in 2. In comparison, the Ni–N bond
lengths in [Ni(tacn)2](NO3)ClؒH2O are close to the optimum
value of 2.08 Å for octahedral NiN6 systems (Table 3), and
reflect the more symmetric coordination environment provided
by the unsubstituted tacn macrocycle.14 In addition to the steric
constraints imposed by the ethane linker, the introduction of
less strongly coordinating tertiary amine nitrogens may be
responsible for the greater variation and lengthening of the
Ni–N bonds in 2 relative to those in [Ni(tacn)2](NO3)ClؒH2O.
This is supported by the presence of a trans influence in 2, viz.
the Ni–N (sec amine) distances trans to the bridgehead NЈs are
shorter [Ni–N(2) 2.105(2) Å] than those for the pair of trans
secondary amines [Ni–N(3) 2.138(2) Å].
Fig. 3 ORTEP plot of the molecular cation in 6 with thermal
ellipsoids drawn at the 30% probability level.
other Ni() is bound to a single tacn unit; its coordination
sphere is completed by three water ligands. This mono-tacn
coordinated Ni() centre lies on the opposite side of the plane
of the benzene ring relative to Ni(1), most likely due to a com-
bination of electrostatic and steric effects, resulting in a large
Ni ؒ ؒ ؒ Ni separation of 7.87 Å.
Not surprisingly, the coordination geometry of the sand-
wiched Ni(1) centre in 6 is very similar to that of the sand-
wiched nickel() centres in 3,9 410 and 5,11 in which the metal
centres are also bound by ortho oriented tacn moieties (Table 3).
Thus, the Ni–N bond lengths [2.099(7)–2.174(7) Å] show
The crystal structure of 6 consists of discrete binuclear
[Ni2Lisomes(H2O)3]4ϩ cations (Fig. 3, Table 2), perchlorate anions
and waters of crystallisation. The analysis confirms the hybrid
nature of the compound, with Lisomes adopting two distinct
modes of coordination towards nickel(). One Ni() centre is
sandwiched by a pair of ortho-oriented tacn rings while the
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J. Chem. Soc., Dalton Trans., 2001, 2232–2238

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Table 3 Selected structural parameters for sandwiched NiII centres in nickel() complexes
Intra-ring
N–Ni–N bite
angles (Њ)
N–Ni–N
bridgehead
angle (Њ)
Inter-ring
N–Ni–N bite
angles (Њ)
Inter-ring
trans-N–Ni–N
angles (Њ)
Complex
Ni–N distances/Å
[Ni(tacn)2](NO3)ClؒH2Oa
2.093(4)–2.116(4)
2.105(2)–2.138(2)
2.110(4)–2.182(4)
2.086(4)–2.193(6)
1.99(3)–2.21(3)
2.099(7)–2.174(7)
2.089(4)–2.262(4)
82.2(6)–83.1(6)
81.5(1)–83.3(1)
79.3(1)–82.4(1)
80.0(2)–82.6(2)
78(1)–83(1)
80.7(2)–82.6(2)
80.3(2)–81.8 (2)
—
95.1(6)–99.9(6)
84.4(1)–111.8(1)
90.5(1)–105.3(1)
90.3(3)–106.6(3)
88(1)–108(1)
89.7(3)–106.9(3)
90.2(2)–109.0(1)
176.0(6)–177.2(6)
161.3(1)–165.3(1)
169.7(1)–173.9(1)
169.2(2)–173.4(3)
167(1)–174.1(9)
168.8(2)–173.7(2)
170.3(2)–178.8(2)
2
84.4(1)
103.6(1)
103.1(2)
102(1)
102.4(2)
—
3b
4c
5c
6
d
[Ni(Bztacn)2][PF6]2
a Ref. 13. b Ref. 9. c Ref. 10. d Ref. 15.
Table 4 Electronic spectra of nickel() complexes of ligands derived from 1,4,7-triazacyclononane measured in CH3CN
Complex
λmax/nm (εmax/MϪ1 cmϪ1
)
1
327 (5), 506 (5), 560 (sh), 800 (7), 880 (sh)
366 (15), 522 (16), 849 (31), 924 (32)
347 (10), 359 (10), 538 (8), 625 (sh), 840 (sh), 917 (17)
347 (19), 360 (19), 536 (13), 625 (sh), 840 (sh), 917 (29)
335 (sh), 535 (29), 830 (sh), 885 (58)
340 (19), 534 (17), 830 (sh), 884 (35)
390 (18), 532 (6), 840 (sh), 961 (11)
2
3
4
5
6
a
[Ni(Bztacn)2][PF6]2
a Ref. 15.
3
considerably more variation than observed within the structure
of [Ni(tacn)2](NO3)ClؒH2O,12 and the inter-ring cis N–Ni–N
angles involving either one or both of the bridgehead nitrogens
[102.4(2)–106.9(3)Њ] are enlarged relative to those involving
pairs of secondary nitrogens [89.7(3)–96.6(3)Њ]. This indicates
that the aromatic spacer prevents the sandwiching macro-
cycles from approaching the nickel() centre as closely as in
[Ni(tacn)2](NO3)ClؒH2O. Further evidence for the molecular
strain induced in the cation by the aromatic spacer is pro-
vided by the fact that the longest Ni–N bond [2.174(7) Å] is
associated with one of the tertiary bridgehead nitrogens, N(4),
and the considerably enlarged N(4)–C(20)–C(23) angle
[118.5(6)Њ]. The intra-ring cis N–Ni–N angles [80.7(2)–82.6(2)Њ]
and trans N–Ni–N angles [168.8(2)–173.7(2)Њ] are again
reduced significantly from those expected for an undistorted
octahedron.
spin–orbit splitting of the T2g state, or the close approach of
the 1E2g state such that the spin forbidden 3A2g → 1E2g transition
gains intensity through spin–orbit coupling with the T2g
3
state.18,19
If, as is common practice, the wavelength of maximum
absorbance in the 800–1000 nm is taken as a measure of the
ligand field splitting parameter, 10Dq, the data for the sand-
wiched nickel() complexes suggest that the ligand field
strengths follow the order tacn (10Dq = 12500 cmϪ1) > Lox
=
Ldur (10900 cmϪ1 ≈ Leth (10820 cmϪ1) > Bztacn (10400 cmϪ1).
The weaker ligand fields of Leth, Lox, Ldur and Bztacn relative to
tacn can be attributed to steric constraints as well as the conver-
sion of one amine in each tacn ring into a tertiary amine. Both
of these factors combine to reduce the binding ability of the
ligands, as is evident from the Ni–N distances (Table 3). The
estimated 10Dq values for 5 and 6 of 11300 cmϪ1 indicate a
somewhat greater average ligand field strength compared with
2–4 and [Ni(Bztacn)2][PF6]2.
The distorted octahedral geometry of the mono tacn-
coordinated Ni(2) centre in 6 parallels that observed in
[Ni2Lpx(H2O)6](ClO4)4ؒ3H2O
[Lpx = 1,4-bis(1,4,7-triazacyclo-
Hancock and co-workers18 have cautioned that because of
non-1-ylmethyl)benzene]9 and 5.10 Notably, the longest Ni(2)–N
bond is associated with the tertiary bridgehead nitrogen, N(7)
[2.138(6) Å]. The intra-ring N–Ni–N bite angles [82.3(3)–
84.4(4)Њ] are below 90Њ, whilst the trans N–Ni–O angles
[173.7(3)–178.0(3)Њ] are on average closer to the ideal 180Њ than
the trans N–Ni–N angles observed for the sandwiched Ni(1)
centre. This is to be expected since the NiN3O3 coordination
sphere is free of the steric constraints that are introduced by
sandwiching a nickel centre between two linked tacn macro-
cycles.
the mixing of T2g and E2g states, the two bands in the 800–
3
1
1000 nm region cannot be assigned to pure A2g → 3T2g and
3
3A2g → 1E2g transitions. These workers have shown that an
empirical relationship, 10Dq = 10630 ϩ 1370ε1/ε2 exists between
10Dq and the ratio of the coefficients of the uncorrected
3A2g → 1E2g and A2g → 3T2g transitions (ε1/ε2) which can be
3
used to generate more rational values of 10Dq. Thus, whilst the
bands in the 800–1000 nm region for 2 are in similar positions
to those of 3 and 4, their relative intensities are different, indi-
cating a difference in ligand field strength. Application of the
relationship of Hancock and co-workers17 yields corrected
10Dq values of 11940 cmϪ1 for 2 and 11820 cmϪ1 for 3 and 4.
Thus, the ligand field strength of Leth is only marginally greater
Electronic spectra
Electronic spectral data for the nickel() recorded in
acetonitrile are summarised in Table 4 together with that of the
related nickel complex, [Ni(Bztacn)2][PF6]2 (where Bztacn =
1-benzyl-1,4,7-triazacyclononane).15 The data for 1 and 2 are in
excellent agreement with literature data for aqueous solutions
of the complexes.5,16 Each complex shows three maxima typical
of octahedral nickel() complexes which arise predominantly
than that of Lox and Ldur
.
Electrochemistry
Cyclic, square-wave and near steady-state voltammetry were
performed upon the nickel() complexes, 1–6, in the range Ϫ1.2
to ϩ1.2 V vs. the ferrocene–ferrocenium couple (Fc–Fcϩ).
Representative square-wave voltammograms for the mono-
nuclear complex, 2, and the binuclear complex, 4, are illustrated
in Fig. 4, while Table 5 summarises the electrochemical data for
each of the compounds.
3
3
from spin-allowed A2g → 3T2g (1000–800 nm), A2g → 3T1g(F)
(550–500 nm) and 3A2g → 3T1g(P) (300–400 nm) transitions.17,18
In each case, the low energy band is asymmetric or has a
shoulder. This may be attributed to symmetry splitting or
J. Chem. Soc., Dalton Trans., 2001, 2232–2238
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Table 5 Cyclic, square-wave and near steady-state voltammetric data for oxidation of nickel() complexes in acetonitrile (0.1 M Bu4NClO4)a
Cyclicb
Square-wavec
W1/2e/mV
Near steady-stated
106Dg/cm2 sϪ1
nh
red
f
E1/2/V
∆Ep/mV
Ipox/µA
Ipred/µA
Ipox/Ip
Ep/V
Ip/µA
Id /nA
Fc
1
0.00
ϩ0.56
ϩ0.67
ϩ0.87
ϩ0.86
ϩ0.87
ϩ0.87
62
62
58
62
80
54
58
4.15
2.35
2.46
2.74
3.63
1.88
1.98
4.15
2.41
2.49
2.71
3.57
1.85
1.98
1.00
0.98
0.99
1.01
1.02
1.02
1.00
0.00
104
103
108
104
133
104
103
13.0
8.6
8.7
8.6
12.1
7.1
4.77
2.18
2.19
2.19
1.82
0.97
0.91
23
1
1
1
1
2
1
1
ϩ0.56
ϩ0.67
ϩ0.86
ϩ0.87
ϩ0.86
ϩ0.87
10.5
10.6
10.6
4.3
4.8
4.4
2
3
4
5
6
7.2
a Potentials are quoted with respect to the Fc–Fcϩ couple used as an internal reference. The current data are normalised to 1.0 mM concentration (c)
to facilitate comparison of the values. b Cyclic voltammetric data were obtained at a scan rate of 100 mV sϪ1 c Square-wave voltammetric data were
e W1/2 is the estimated width of the
.
measured with a period of 30 ms. d Steady-state voltammetric data were measured at a scan rate of 10 mV sϪ1
.
peak shaped response at half the wave height. f Limiting current under near steady-state conditions. g Calculated from limiting current data and use
of the relation IL = 4nFrcD where n = number of electrons, F = Faraday constant, r = electrode radius and D = diffusion coefficient. h Total number of
electrons associated with process on the basis of self-consistent set of n and D values.
Compounds 5 and 6 show signals with almost exactly the
same form (∆Ep ≈ 58 mV) and position (E1/2 ≈ 0.87 V) as com-
pound 3, which incorporates a similar aromatic tether between
the sandwiching tacn macrocycles. This suggests that these
signals correspond to the reversible oxidation of the sand-
wiched nickel() centres within these multinuclear complexes.
Near-steady state data obtained under microelectrode condi-
tions enable the diffusion coefficients of all complexes to be
calculated (Table 5). As expected, the higher molecular weight
compounds, 5 and 6 have significantly smaller diffusion
coefficients than compounds 1–3. After allowance for the
variation of diffusion coefficients it is clear that all data in
Table 5 are consistent with reversible one-electron oxidation
processes. Irreversible processes evident at very positive
potentials probably correspond to the oxidation of the other
non-sandwiched nickel centres.
The cyclic and square-wave voltametric response observed
for 4 are somewhat broader (∆Ep = 80 mV, W1/2 = 133 mV) than
those for the other compounds. After analysis of steady-state
data and calculated diffusion coefficients (D, Table 5) it is
concluded that the voltammetry of 4 is composed of two
overlapping oxidation–reduction waves, one for each of the
sandwiched nickel centres [eqns. (2) and (3)].
Ni()Ni() → Ni()Ni() ϩ eϪ
Ni()Ni() → Ni()Ni() ϩ eϪ
(2)
(3)
Fig. 4 Square-wave (period = 30 ms) voltammogram of complex 2 (a)
and complex 4 (b) recorded in acetonitrile (0.1 M Bu4NClO4) using a Pt
macrodisk electrode at 20 ЊC. The initial process is the Fc0/ϩ couple used
as internal reference potential standard. The potential axis used is
relative to the Ag–Agϩ reference. The broader square-wave voltammo-
gram observed for the binuclear complex 4 is concluded to be composed
of two overlapping oxidation–reduction waves, one for each of the
Ni() centres.
Richardson and Taube 20 have derived a working curve which
allows the determination of ∆E1/2 = E1/2(2) Ϫ E1/2(1) for a two-
step electrochemical process from the peak-to-peak separation
(∆Ep) of the cyclic voltammogram. Application of this working
curve yields a ∆E1/2 value of ca. 50 mV for the processes sum-
marised in eqns. (2) and (3). For two non-interacting metal
centres, ∆E1/2 will be equal to 36 mV after allowance is made for
the statistical factor, RT/F ln 4.5,21 Thus, the value obtained for
4 suggests that the two sandwiched nickel() centres within the
compound have some influence on each other. This can be
rationalised on the basis that following oxidation of the first
nickel centre, the second nickel centre becomes more difficult to
oxidise because of the higher charge on the adjacent oxidised
centre, and hence the two redox processes, [eqns. (2) and (3)],
occur at slightly different potentials. This behaviour has also
been observed for the binuclear nickel() complexes of a series
of bis(pentadentate) ligands derived from bis(tacn) macro-
cycles.22 Despite this, similar E1/2 values are observed for the
sandwich complexes, 3–6.
The mononuclear complexes, 1–3, all exhibit redox waves in
their cyclic voltammograms corresponding to the reversible
oxidation of the sandwiched nickel() centres to the nickel()
state. The chemical reversibility of the observed redox processes
is indicated by the ratios of the peak currents accompanying
oxidation and reduction (Ipox/Ipred), which are all close to
unity. This is further supported by the fact that the form of the
voltammograms did not change on repeated cycling of the
potential. The anodic and cathodic peak potential differences
(∆Ep) are close to the theoretical value of 57 mV expected for a
fully reversible 1eϪ process,20 and are in agreement with those
for the completely reversible Fc0/ϩ process measured under the
same conditions. The data are consistent with the presence
of only one sandwiched nickel centre within the structures
[eqn. (1)].
The nickel()/() couples for 2–6 are all shifted to more
positive potentials relative to the [Ni(tacn)2]2ϩ/3ϩ couple, indicat-
ing substantial destabilisation of the nickel() state. This is
Ni() → Ni() ϩ eϪ
(1)
2236
J. Chem. Soc., Dalton Trans., 2001, 2232–2238

View Article Online
postulated to be a consequence of the additional strain induced
in the bridged ligands upon accommodating the requirements
of the smaller nickel() ion, combined with the substitution of
secondary amine nitrogens for tertiary nitrogens, which would
be expected to stabilise the reduced state. The finding that com-
plexes 3–6 require the most positive potentials for oxidation is
not unexpected in view of the X-ray structural data (Table 3). In
particular, the large N–Ni–N angles subtending the aromatic
bridges in these complexes indicate that the nickel() centre is
already too small to fit optimally between the sandwiching
macrocycles because of the spacer size. An even greater degree
of strain would be expected on moving to the nickel() state.
Schröder and co-workers15 have investigated the electro-
chemistry of the nickel() sandwich complex, [Ni(Bztacn)2]-
[PF6]2, and also noted a positive shift in the potential of the
nickel()/() couple relative to that of [Ni(tacn)2]2ϩ/3ϩ. The
observed shift of 0.21 V, in this case ascribed to the steric bulk
of the pendant benzyl groups, is greater than that for the com-
plex of Leth, but is not as pronounced as those observed for the
complexes of Lox, Ldur and Lisomes. Interestingly, however, this
oxidation–reduction process15 was only found to be chemically
reversible when the temperature was lowered to 235 K. The
unstable nickel() species, [Ni(Bztacn)2]3ϩ, was proposed to
undergo a decomposition process involving loss of the benzyl
groups to generate [Ni(tacn)2]3ϩ. For the complexes of Lox, Ldur
and Lisomes, the linkage of the sandwiching tacn rings appears to
enhance the chemical robustness of the nickel() species, lead-
ing to reversible redox behaviour, even though more positive
potentials are required for their generation.
analyzer. Cyclic, square-wave and steady-state voltammograms
were recorded on a Cypress CS 1090 system. All measurements
were made under an atmosphere of nitrogen using a three-
electrode potentiostatted system in dry, degassed acetonitrile
containing ca. 1 mM sample and 0.1 M Bu4NClO4 as the
supporting electrolyte. A Pt macrodisk working electrode
(r = 0.5 mm), Pt auxiliary electrode and Ag/Agϩ (10 mM
AgNO3) reference electrode were employed to measure cyclic
and square-wave voltammograms. Near steady-state voltam-
mograms were measured in a Faraday cage using a Pt micro-
electrode (r = 5 µm), Pt auxiliary electrode and Ag/Agϩ (10 mM
AgNO3) reference electrode at a scan rate of 10 mV sϪ1. The
redox potentials (E1/2 values) obtained from cyclic voltam-
mograms were taken as the average of the oxidation (Epox) and
reduction (Epred) peak potentials, and are given with respect to
the ferrocene–ferrocenium couple (Fc–Fcϩ), which was used as
an internal standard (1 mM).
CAUTION: Although no problems were encountered in this
work, transition metal perchlorates are potentially explosive
and should be prepared in small quantities and handled with
care.
Syntheses
1,2,3-Tri(bromomethyl)benzene. A mixture of 1,2,3-trimethyl-
benzene (31.6 g, 0.263 mmol) and N-bromosuccinimide (150.0
g, 0.843 mmol) in carbon tetrachloride (700 mL) was refluxed
overnight whilst being illuminated with a halogen lamp. After
cooling to room temperature, the precipitated succinimide,
which formed as a crust on the surface of the reaction mixture,
was removed by filtration and the volume of the filtrate reduced
on a rotary evaporator until a white solid began to form. The
mixture was refrigerated overnight and the precipitate collected
by filtration. Recrystallisation from a chloroform–light petrol-
eum (bp 60–70Њ C) ether mixture afforded a white crystalline
product (30.7 g, 33%). δH(200.13 MHz): 4.62 (4 H, s, CH2), 4.83
(2 H, s, CH2), 7.24–7.38 (3 H, m, aromatic CH); δC (50.32
MHz): 24.69, 29.85 (CH2), 129.72, 131.83 (aromatic CH),
135.70, 138.02 (aromatic quaternary C).
Conclusions
The sandwiched nickel() centres in the series of complexes,
1–6, may be reversibly oxidised to the nickel() state at a poten-
tial that is tuned by the ability of the tether to accommodate
the requirements of the smaller nickel() ion. The [Ni(tacn)2]3ϩ
complex has been found to be a convenient outer-sphere one-
electron oxidant in electrochemical studies because of the
absence of any proton-related equilibria.12,23 The results of this
study indicate that the nickel() sandwich complexes of Leth
,
1,2,3-Tris(1,4,7-triazacyclonon-1-ylmethyl)benzene (Lisomes). A
hot solution of 1,2,3-tri(bromomethyl)benzene (0.55 g, 1.54
mmol) in acetonitrile (10 mL) was added to a stirred solution of
1,4,7-triazatricyclo[5.2.1.04,10]decane (0.65 g, 4.67 mmol) in
acetonitrile (10 mL), resulting in the almost immediate form-
ation of a white precipitate. After stirring for 3 days, the precipi-
tate was filtered off; washed with acetonitrile (2 × 30 mL) and
diethyl ether (2 × 50 mL) and dried in a vacuum desiccator. The
solid was then dissolved in water (50 mL) and refluxed for 3 h.
Sodium hydroxide pellets (1.50 g, 37.5 mmol) were carefully
added in portions to the solution and reflux continued for a
further 4.5 h. After cooling to room temperature, the solution
was extracted with chloroform (4 × 50 mL), the combined
extracts dried over magnesium sulfate, filtered and reduced to
dryness on a rotary evaporator. This yielded the crude ligand
as a fluffy white solid (0.57 g, 74%), which was used directly
in the preparation of the nickel() complex. δH(200.13 MHz):
3.07 (8 H, t, CH2), 3.34 (8 H, t, CH2), 3.48 (4 H, s, CH2),
3.72 (12 H, m, CH2), 3.93 (2 H, s, CH2), 4.28 (4 H, m, CH2),
5.03 (2 H, s, CH2), 5.16 (2 H, s, CH2), 7.42–7.46 (1 H, m,
aromatic CH) 7.53–7.56 (2 H, m, aromatic CH); δC(50.32
MHz): 42.55, 42.72, 43.24, 44.34, 45.96, 48.60, 56.23, 58.20
(CH2), 130.24, 131.06 (aromatic CH) 132.51, 132.66 (aromatic
quaternary C).
Lox, Ldur and Lisomes might prove similarly useful as strong
oxidants. In particular, the binuclear nickel() complex of Ldur
could be of practical use in synthetic chemistry (e.g. oxidation
of organic compounds), by virtue of its ability to undergo a
two-electron reduction at essentially a single potential.
Experimental
Materials and reagents
Reagent or analytical grade chemicals, obtained from
commercial suppliers, were used without further purification.
Literature methods were used to prepare [Ni(tacn)2](ClO4)2
(1),12 [NiLeth](ClO4)2 (2),5 [NiLox](ClO4)2ؒH2O (3),9 [Ni2Ldur]-
(ClO4)4ؒ2H2O (4)10 and [Ni3Ldur(H2O)3](ClO4)6ؒ9H2O (5).10
Physical measurements
1H and 13C NMR spectra were recorded in CDCl3 on a Bruker
AC200 spectrometer and are referenced relative to tetramethyl-
silane. The electrospray ionization (ESI) mass spectrum of 6
was recorded in a 1:1 CH3CN–H2O mixture on a Micromass
Platform quadrupole mass spectrometer. The quoted m/z values
correspond to the most intense peak in each signal envelope.
Infrared spectra were recorded as KBr pellets on a Perkin-
Elmer 1600 FTIR spectrometer and electronic spectra on a
Cary 5G UV-Vis-NIR spectrophotometer. Electron micro-
probe analysis was made with a JEOL JSM-1 scanning electron
microscope through an NEC X-ray detector and pulse-
processing system connected to a Packard multichannel
[Ni2Lisomes(H2O)3](ClO4)4ؒ9H2O (6). To an aqueous solution
(50 mL) of Lisomes (0.25 g, 0.5 mmol) and Ni(NO3)2ؒ6H2O (0.50
g, 1.72 mmol), heated on a steam bath, a solution of 2 M
sodium hydroxide was added dropwise until a small amount of
nickel hydroxide precipitate appeared which would not dissolve
J. Chem. Soc., Dalton Trans., 2001, 2232–2238
2237

View Article Online
Table 6 Crystal data for [NiLeth](ClO4)2 (2) and [Ni2Lisomes(H2O)3](ClO4)4ؒ7H2O (6)
Formula
C14H28Cl2N3NiO8
542.05
C27H71Cl4Ni2N9O26
1197.11
M/g molϪ1
Crystal system
Space group
a/Å
b/Å
c/Å
α/Њ
β/Њ
Orthorhombic
Pbcn (no. 60)
14.1402(2)
9.6023(1)
16.4655(2)
90
Triclinic
P1(no. 2)
¯
9.5317(3)
15.6344(5)
16.8035(4)
92.521(5)
91.116(2)
105.705(1)
2407.0(1)
2
90
90
γ/Њ
U/Å3
2235.66(9)
4
Z
T /K
123
0.71069
1.610
11.60
3679
2518
0.037
0.042
Ϫ0.46, 1.36
123
0.71069
1.652
10.99
12062
6366
0.092
0.104
Ϫ1.35, 1.39
λ/Å
Dc/ g cmϪ3
µ(Mo–Kα)/ cmϪ1
No. of reflections measured
No. of reflections (I ≥ 3σ(I ))
Ra
RЈb
ρmin,ρmax /e ÅϪ3
2
a R = Σ(|Fo| Ϫ |Fc|)/Σ|Fo|. b RЈ = [Σw(|Fo| Ϫ |Fc|)2/ΣwFo ]1/2, where w = [σ2(Fo)]Ϫ1
.
on prolonged heating. Sufficient 2 M hydrochloric acid was
then added to just dissolve this precipitate and the resulting
purple–blue solution heated for a further 15 min before being
diluted to 2 L with water and loaded onto a Sephadex SP-C25
column (Hϩ form, 15 × 4 cm). After washing the column with
water and 0.1 M sodium perchlorate (to remove a green band of
excess Ni2ϩ), a purple–pink band was eluted with 0.5 M sodium
perchlorate. On standing overnight this fraction yielded purple–
pink needles of 6, which were collected by vacuum filtration,
washed with methanol and air-dried (0.27 g, 45%). Micro-
analyses: C27H71Cl4N9Ni2O26 {[Ni2Lisomes(H2O)3](ClO4)4ؒ7H2O}
requires C, 27.1; H, 5.9; N, 10.6. Found: C, 26.2; H, 5.1; N,
10.4%. ESI mass spectrum: m/z 916.2 [Ni2Lisomes(ClO4)3]ϩ, 408.6
[Ni2Lisomes(ClO4)2]2ϩ, 239.2 [Ni2Lisomes(ClO4)]3ϩ. Electron micro-
probe: Ni and Cl present. Selected IR bands (νmax/cmϪ1): 3421s
(br), 3320s, 2937m, 1627m (br), 1492m, 1461m, 1365w, 1145s,
1087s, 628s.
Acknowledgements
This work was supported by ARC grants to A. M. B., M. T. W.
H. and L. S. B. G. was the recipient of an Australian Post-
graduate Award.
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Crystallography
Intensity data for a pink crystal of 2 (dimensions ca.
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and 6, respectively, and processed using the Nonius software.24
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26
molecules. ORTEP perspectives of the complexes are pre-
sented in Fig. 2 and 3, and selected bond lengths and angles in
Tables 1 and 2.
CCDC reference numbers 159103 and 159104.
See http://www.rsc.org/suppdata/dt/b1/b101766g/ for crystal-
lographic data in CIF or other electronic format.
26 C. K. Johnson, ORTEP II, Report ORNL-5138, Oak Ridge
National Laboratory, Oak Ridge, TN, 1976.
2238
J. Chem. Soc., Dalton Trans., 2001, 2232–2238