cis-Octahedral nickel(II) complexes with symmetric and unsymmetric tripodal tetraamine ligands

Article abstract of DOI:10.1002/1099-0682(200105)2001:5<1279::AID-EJIC1279>3.0.CO;2-F

Nickel(II) complexes with the aliphatic tripodal tetraamine ligands N(CH₂CH₂NH₂)₃ (tren, 1), N[(CH₂CH₂CH₂NH₂)(CH ₂CH₂NH₂)₂] (baep, 2), N[(CH₂CH₂CH₂NH₂)₂ -(CH₂CH₂NH₂)] (abap, 3), and N(CH₂CH₂CH₂NH₂)₃ (trpn, 4) are reported. The tripodal tetradentate N₄ ligands 1-4 react with Ni(NO₃)₂·6H₂O in acetonitrile or methanol to give the blue nickel(II) complexes [Ni(1)(η¹-NO₃)₂] (5a), [Ni(2)(η¹ NO₃)₂] (5b), [Ni(3)(η²-NO₃)]NO₃·CH ₃CN (5c·CH₃CN), and [Ni(4)(η²-NO₃)]NO₃·CH ₃OH (5d·CH₃OH). With NiCl₂·6H₂O, the complexes [Ni(1)Cl(H₂O)]Cl·H₂O (6a), [Ni(2)(μCl)₂Ni(2)]Cl₂·2CH₃OH (6b·2CH₃OH), [Ni(3)Cl(H₂O)]Cl (6c), and [Ni(4)(H₂O)₂]Cl₂·H₂O (6d·H₂O) are obtained. The molecular structures of complexes 5a-d and 6b-d have been determined by X-ray diffraction analysis and they are compared with the molecular structure of the previously characterized complex 6a. Complexes 5a-d and 6b-d exhibit octahedrally coordinated nickel atoms. The tripodal ligands occupy four of the six coordination sites in a pseudo-facial manner. Complexes of the unsymmetrical ligands 2 and 3 possess both five- and six-membered chelate rings. The extension of the ligand arms in 1-4 leads to a systematic variation in the geometric and UV/Vis spectroscopic properties of the complexes depending on the size of the chelate rings formed by the ligands.

Full text of DOI:10.1002/1099-0682(200105)2001:5<1279::AID-EJIC1279>3.0.CO;2-F

FULL PAPER
cis-Octahedral Nickel(II) Complexes with Symmetric and Unsymmetric
Tripodal Tetraamine Ligands
Christian Ochs,[a] F. Ekkehardt Hahn,*[b] and Thomas Lügger[b]
Keywords: Tripodal ligands / Nickel / Structure elucidation / UV/Vis spectroscopy
Nickel(II) complexes with the aliphatic tripodal tetraamine
ligands N(CH2CH2NH2)3 (tren, 1), N[(CH2CH2-
lecular structures of complexes 5a−d and 6b−d have been
determined by X-ray diffraction analysis and they are com-
pared with the molecular structure of the previously charac-
terized complex 6a. Complexes 5a−d and 6b−d exhibit octa-
hedrally coordinated nickel atoms. The tripodal ligands oc-
cupy four of the six coordination sites in a pseudo-facial man-
ner. Complexes of the unsymmetrical ligands 2 and 3 possess
both five- and six-membered chelate rings. The extension of
the ligand arms in 1−4 leads to a systematic variation in the
geometric and UV/Vis spectroscopic properties of the com-
plexes depending on the size of the chelate rings formed by
the ligands.
CH2NH2)(CH2CH2NH2)2] (baep, 2), N[(CH2CH2CH2NH2)2-
(CH2CH2NH2)] (abap, 3), and N(CH2CH2CH2NH2)3 (trpn, 4)
are reported. The tripodal tetradentate N4 ligands 1−4 react
with Ni(NO3)2·6H2O in acetonitrile or methanol to give the
blue nickel(II) complexes [Ni(1)(η1-NO3)2] (5a), [Ni(2)(η1-
NO3)2] (5b), [Ni(3)(η2-NO3)]NO3·CH3CN (5c·CH3CN), and
[Ni(4)(η2-NO3)]NO3·CH3OH (5d·CH3OH). With NiCl2·6H2O,
the complexes [Ni(1)Cl(H2O)]Cl·H2O (6a), [Ni(2)(µ-
Cl)2Ni(2)]Cl2·2CH3OH (6b·2CH3OH), [Ni(3)Cl(H2O)]Cl (6c),
and [Ni(4)(H2O)2]Cl2·H2O (6d·H2O) are obtained. The mo-
Introduction
the unsymmetric ligands 2 and 3 have not hitherto been de-
scribed.
The sterically unhindered tripodal tetraamine ligand tren
(1) is known to form a large number of coordination com-
pounds with different metal ions. Comparatively little is
known about the coordination chemistry of the unsymmet-
ric higher tren homologues, e.g. bis(2-aminoethyl)(3-amino-
propyl)amine (baep, 2), (2-aminoethyl)bis(3-aminopropyl)-
amine (abap, 3), and the symmetric tris(3-aminopropyl)am-
ine (trpn, 4) (Figure 1).
The stepwise insertion of a CH2 group into each of the
ligand arms in 1 leads to the ligands 2؊4.[15Ϫ17] Compared
to 1, these ligands possess an unchanged set of donor
groups with expanded linkers between them. This slight to-
pological variation has profound consequences. Ligands
2؊4 are more basic than 1,[17] since the ammonium groups
formed upon protonation are further apart in these ligands
than they would be in tren. The geometric parameters of
the five- and six-membered chelate rings that could be
formed by 2؊4 have been analyzed and it was concluded
that for complexes with both macrocyclic and non-macro-
cyclic ligands, an increase of the chelate ring size from five-
membered to six-membered will increase the stability of
complexes of smaller relative to larger metal ions.[18]
Figure 1. Topology of the tripodal tetraamine ligands studied
A systematic study of these contradicting effects was car-
ried out with a complete series of CuII complexes with the
ligands 1؊4.[16a,17,19] Depending on the ligand topology,
these complexes adopt either a trigonal-bipyramidal or a
distorted square-pyramidal geometry.[16,17]
Complexes of NiII with 1 were first described in 1926.[1]
Since then, various monomeric octahedral,[2Ϫ5] trigonal-
bipyramidal,[6Ϫ9]
and
dinuclear
coligand-bridged
octahedral[10Ϫ12] NiII complexes of 1 or its hexamethylated
analogue Me6tren have been reported. Thermochemical
and spectroscopic studies of aqueous solutions of NiII com-
plexes of 4 have indicated that these complexes are octahed-
ral.[13,14] However, the precise molecular structures of the
(trpn)NiII complexes remain unknown. Nickel complexes of
The stability constants of the copper complexes were
found to decrease systematically over five orders of magni-
tude depending on the number of six-membered chelate
rings present.[17] Studies on the octahedral CoIII complexes
with 1,[20] 2,[16b] 3,[15] and 4[20] revealed differences in the
catalytic activities of these complexes in the hydrolysis of
phosphate esters. It would appear that the stability and re-
activity of complexes with tripodal tetraamine ligands may
be controlled and significantly manipulated by the ligand
topology if the governing principles are fully understood.
This might ultimately prove useful for the modelling of ni-
trogen-rich active sites in metalloproteins. To study the con-
[a]
Institut für Anorganische und Analytische Chemie der Freien
Universität Berlin,
Fabeckstraße 34Ϫ36, 14195 Berlin, Germany
[b]
Anorganisch-Chemisches Institut der Westfälischen Wilhelms-
Universität Münster,
Wilhelm Klemm Straße 8, 48149 Münster, Germany
Eur. J. Inorg. Chem. 2001, 1279Ϫ1285
WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
1434Ϫ1948/01/0505Ϫ1279 $ 17.50ϩ.50/0
1279

C. Ochs, F. E. Hahn, T. Lügger
FULL PAPER
sequences of a variation of the ligand topology of tripodal in trace amounts. We assume that this is due to the low
tetraamines with respect to the properties of octahedral basicity of the central nitrogen atom of the dinitrile pre-
metal complexes, we have prepared the NiII complexes of cursor. It is known that α-cyanomethyl and β-cyanoethyl
1؊4. It was assumed that octahedral complexes with two groups at a nitrogen atom markedly lower its basicity.[22]
monodentate coligands in cis positions would be formed. Under such conditions, decomposition and self-condensa-
Furthermore, we have studied the effects of the counterions tion reactions of the glyconitrile are favored over the con-
by preparing two series of complexes starting from densation reaction with the amine. Consequently, the only
NiCl2·6H2O and Ni(NO3)2·6H2O. The molecular structures possible means of enhancing the condensation reaction is
and spectroscopic properties of the nickel complexes ob- through activation of the glyconitrile. This is best achieved
tained are compared with the data for the previously char- by tosylation and tosylated glyconitrile can easily be pre-
acterized nickel complex with the ligand tren pared from glyconitrile and p-toluenesulfonyl chloride.[23]
[Ni(1)Cl(H2O)]Cl·H2O (6a).[2]
The condensation reaction of TsOCH2CN with
HN(CH2CN)(CH2CH2CN) proceeds at 50 °C in DMF
within 5 d. The resulting ammonium salt was converted
into the free amine with aqueous sodium carbonate. The
crude product could be recrystallized from hot chloroform.
As reported previously,[16] ligand 2 is obtained from the tri-
nitrile by hydrogenation with AlH3.
Results and Discussion
Seven new octahedral NiII complexes with the symmetric
and unsymmetric tripodal tetraamine ligands 1؊4 have
been synthesized and structurally characterized. Depending
on the ligand topology, complexes of type 5 (Scheme 1)
contain either two η1-coordinated nitrate ions or an η2-co-
ordinated nitrate ion and a nitrate counterion. A second
series of complexes 6 has been synthesized from ligands
1؊4 and nickel dichloride in water. In this series, the com-
plex 6a with the ligand 1 has been described previously.[2]
In both series, the ligand topology is systematically varied
by the stepwise insertion of methylene groups into the li-
gand arms. The structural and spectroscopic trends ob-
served in the two series are discussed below.
Preparation of Nickel Complexes
Addition of stoichiometric amounts of the tripodal li-
gands 1؊4 to solutions of Ni(NO3)2·6H2O in methanol or
acetonitrile gives deep-blue solutions, from which the com-
plexes [Ni(N4)(η1-NO3)2] (5a: N4 ϭ 1; 5b: N4 ϭ 2) and
[Ni(N4)(η2-NO3)2]NO3·X (5c: N4 ϭ 3, X ϭ CH3CN; 5d:
N4 ϭ 4, X ϭ CH3OH) can be crystallized by slow diffusion
of gaseous diethyl ether into them. Reactions of 1؊4 with
Scheme 1. Preparation of NiII complexes of ligands 1Ϫ4
Improved Synthesis of Bis(2-aminoethyl)(3-aminopropyl)-
amine (2)
The previously described preparation of 2,[16a] by addi-
tion of 1 equiv. of acrylonitrile to ammonia followed by
Strecker synthesis, reaction of the resulting dinitrile with
glyconitrile (HOCH2CN),[21] and finally reduction of the
trinitrile with AlH3, has not always given satisfactory re-
sults. The critical step has proved to be the condensation
reaction of the dinitrile with HOCH2CN for the introduc-
tion of the third ligand arm at the central nitrogen atom.
Under the conditions described, the reaction often yields
Figure 2. ORTEP plots of complexes 5a and 5b and of the cations
in complexes 5c and 5d; solvent molecules and counterions in 5c
polymeric products with the desired trinitrile present only and 5d have been omitted for clarity
1280
Eur. J. Inorg. Chem. 2001, 1279Ϫ1285

cis-Octahedral Nickel(II) Complexes
FULL PAPER
NiCl2·6H2O in acetonitrile or methanol yield complexes trans to Ntert vs. NiϪO trans to NH2) diminish in the series
[Ni(N4)Cl(H2O)]Cl (6a: N4 ϭ 1;[2] 6c: N4 ϭ 3), [Ni(N4)(µ- of complexes on going from 5a (0.205 A) to 5c (0.141 A
˚
˚
˚
˚
Cl)2(N4)Ni]Cl2·2CH3OH (6b: N4 ϭ 2), and [Ni(N4)(- and 0.069 A) and are finally reversed in 5d (0.05 A and
H2O)2]Cl2 (6d: N4 ϭ 4). Crystallization of the complexes 0.027 A). We explain this observation in terms of an in-
˚
was achieved in the same manner as described for 5aϪd, or crease in the basicity of the central nitrogen atom of the
by slowly concentrating a methanolic solution. With ace- tripodal ligands. The basicity of Ntert increases from pKa Ͻ
tonitrile as the solvent, all the complexes were found to be 2 for 1 to pKa ϭ 5.8 for 4, while the basicity of the primary
air-stable and soluble in water. A schematic representation amine groups remains essentially unchanged.[17] The dimin-
of complexes of types 5 and 6 is presented in Scheme 1.
ishing difference in the basicities of the primary and tertiary
amine functions in 1 to 4 leads to a less pronounced differ-
ence in the NiϪO distances trans to Ntert and trans to the
Molecular Structures of the Nitrate Salts
The molecular structures of complexes 5a؊d have been NH2 groups. While the NiϪO bond trans to the tertiary
determined by X-ray diffraction methods. In all cases, cis- amine becomes longer on going from 5a to 5d, the NiϪO
octahedral complexes were formed. As can be seen from bond length trans to a primary amine group changes less
Figure 2, the main difference between the complexes lies in and ultimately becomes shorter than the NiϪO bond trans
the coordination mode of the nitrate anions. In 5a and 5b, to Ntert in the case of 5d. From this, it can be concluded that
the cis-octahedral coordination is completed by two η1-co- the ligand topology in complexes 5aϪd determines both the
ordinated nitrate anions, while in both 5c and 5d only one steric and, to a lesser extent, the electronic situation at the
η2-coordinated nitrate anion is found.
metal center.
This difference arises from differences in the steric bulk
and demand of the coordinated tripodal tetraamines. The
relationship between steric demand of the coordinated
Molecular Structures of the Chloride Salts
The molecular structures of complexes 6bϪd are depicted
tetraamine and the coordination mode of the nitrate anions in Figure 3. Again, cis-octahedral complexes with chloride
is best reflected in the NtertϪNiϪNH2 angles. Complex 5a or water coligands were formed. It would appear that the
contains only five-membered chelate rings and all the distribution of the coligands among the two free coordina-
N
tertϪNiϪNH2 angles are smaller than 90° (Table 1). The tion sites at the nickel atom is random and seemingly de-
larger six-membered chelate rings in 5d lead to pends on the conditions used for crystallization of the com-
tertϪNiϪNH2 angles that are all larger than 90°. Com- plexes. Complex 6b, which was obtained by slow concentra-
N
plexes 5b and 5c, containing both five- and six-membered tion of a methanolic solution, crystallizes as a dinuclear
chelate rings, are intermediate between these limiting cases species where two bridging chloride atoms complete the oc-
with NtertϪNiϪNH2 angles larger than 90° for the six- tahedral coordination environments at the nickel atoms. On
membered rings and smaller than 90° for the five-membered the other hand, 6c and 6d were crystallized by diffusion of
ones. A similar relationship between the NtertϪCuϪNH2 diethyl ether into acetonitrile solutions and are mononu-
angles and the different chelate ring sizes has been de- clear complexes. In complex 6c, a water molecule occupies
scribed for trigonal-bipyramidal[16a] and tetragonal-pyram- the trans position and a chloride ion the cis position relative
idal[17] copper complexes. As a consequence of this geomet- to Ntert, while in 6d two coordination sites at nickel are
rical situation, the size of the cavity left for coligands de- occupied by water molecules. The series is completed by
creases on going from 5a to 5d. In 5a and 5b, it is still large compound [Ni(1)Cl(H2O)]Cl·H2O (6a).[2] Its molecular
enough to accommodate two monodentate nitrate anions, structure was first investigated by Jørgensen in 1956.[24] On
while in 5c and 5d only one bidentate nitrate anion can fit the basis of its electronic spectra in aqueous solution, it was
into the cavity and the second nitrate anion is left uncoor- suggested that 6a should be formulated as [Ni(1)(H2O)2]Cl2.
dinated as a counterion. Not only the number of coordin- Some years later, it was shown that it is
ated nitrate anions, but also the NiϪO distances are at least [Ni(1)Cl(H2O)]Cl·H2O in the solid state,[2] while
partially determined by the topology of the tripodal tetra- [Ni(1)(H2O)2]2ϩ cations exist in water. Interestingly, in the
˚
amine. The shortest NiϪO distance [2.075(3) A] is observed solid state, the coordination of the monodentate ligands in
˚
for 5a and, as expected, the largest values [2.224(4) A and 6a (Cl trans to Ntert) is exactly opposite to that found in 6c
˚
2.195(4) A] are found in 5d. It would appear that this is not (Cl cis to Ntert).
exclusively due to steric effects. In complexes 5a, 5b, and
As for the nitrate salts 5, certain bonding parameters of
5c, the shorter of the two NiϪO distances is found trans to the tripodal tetraamine in complexes 6 (Table 2) are
the tertiary nitrogen atom of the tripod, while the longer strongly dependent on the topology of the ligand. The
˚
NtertϪNiϪLtrans axis is essentially linear for all complexes
one is found trans to a primary amine group [2.075(3) A vs.
˚
˚
˚
2.280(2) A for 5a, 2.087(3) A vs. 2.171(3) A for 5b, 2.139(4) of type 6. As for the nitrate complexes, the NtertϪNiϪNcis
˚
˚
˚
˚
A vs. 2.280(4) A and 2.147(4) A vs. 2.216(4) A for 5c; angles are smaller than 90° for five-membered chelate rings
Table 1]. This trend is reversed in 5d, where the shorter and larger than 90° for six-membered rings.
NiϪO distance in each of the two independent molecules
In contrast to the nitrate complexes of type 5, the mon-
in the asymmetric unit lies trans to a primary amine group odentate ligands in complexes 6 are sterically non-de-
˚
˚
˚
˚
[2.174(4) A vs. 2.224(4) A and 2.168(4) A vs. 2.195(4) A]. manding. The distances between the nickel atoms and these
Moreover, the differences in the NiϪO distances (NiϪO ligands allow an evaluation of the dependence of the trans
Eur. J. Inorg. Chem. 2001, 1279Ϫ1285
1281

C. Ochs, F. E. Hahn, T. Lügger
FULL PAPER
˚
Table 1. Selected bond lengths [A] and angles [°] for complexes of type 5
5a
5b
5c·CH3CN
molecule A
5d·CH3OH
molecule B
molecule A
molecule B
NiϪN1
NiϪN2
2.089(3)
2.055(3)
2.095(3)
2.072(3)
2.075(3)
2.280(2)
83.6(1)
83.5(1)
84.4(1)
96.0(1)
94.0(1)
163.4(1)
172.1(1)
89.3(1)
84.5(1)
2.109(4)
2.061(4)
2.067(4)
2.095(4)
2.087(3)
2.171(3)
94.6(1)
84.4(2)
83.7(2)
94.1(2)
173.0(2)
92.6(2)
166.8(1)
90.4(2)
88.1(1)
2.086(4)
2.035(5)
2.061(5)
2.062(5)
2.139(4)
2.280(4)
96.6(2)
2.099(4)
2.054(5)
2.062(4)
2.068(4)
2.147(4)
2.216(4)
98.3(2)
2.109(5)
2.054(5)
2.098(5)
2.065(5)
2.224(4)
2.174(4)
100.4(2)
94.6(2)
2.111(5)
2.061(5)
2.068(5)
2.079(5)
2.195(4)
2.168(4)
100.9(2)
91.5(2)
NiϪN3
NiϪN4
NiϪO1
NiϪO2 (O4)
N1ϪNiϪN2
N1ϪNiϪN3
N1ϪNiϪN4
N2ϪNiϪN3
N2ϪNiϪN4
N3ϪNiϪN4
O1ϪNiϪN1
O2(O4)ϪNiϪN1
O2(O4)ϪNiϪO1
90.7(2)
91.4(2)
84.5(2)
84.0(2)
90.9(2)
92.8(2)
96.3(2)
94.6(2)
90.3(2)
92.6(2)
173.3(2)
159.9(2)
101.6(2)
58.5(2)
91.8(2)
89.9(2)
97.2(2)
96.5(2)
166.1(2)
164.1(2)
106.4(2)
58.1(1)
168.5(2)
162.3(2)
103.8(2)
58.7(2)
175.0(2)
158.8(2)
99.6(2)
59.3(2)
˚
Table 2. Selected bond lengths [A] and angles [°] for complexes of
type 6 (Nt-N1 ϭ ligand trans to N1, Nc-N1 ϭ ligand cis to N1;
Lt-N1 ϭ Cl (6a,b), H2O (6c,d); Lc-N1 ϭ H2O (6a,d), Cl (6b,c); see
also Figure 2)
6a[a]
6b
6c
6d
NiϪN1
NϪN2
2.105(3)
2.100(2)
2.095(2)
2.114(3)
2.109(2)
2.095(2)
2.093(2)
2.106(2)
2.144(4)
2.093(4)
2.084(4)
2.078(4)
2.162(3)
2.432(1)
83.1(1)
2.147(3)
2.096(3)
NiϪN3
2.114(3)
NiϪN4
NiϪLt-N1
2.084(3)
2.413(3) 2.4449(6)
2.239(2) 2.4840(6)
2.174(3)
NiϪLc-N1
N1ϪNiϪN2
N1ϪNiϪN3
N1ϪNiϪN4
N2ϪNiϪN3
N2ϪNiϪN4
N3ϪNiϪN4
2.165(3)
82.67(8)
84.0(2)
82.2(1)
97.5(2)
93.7(1)
161.0(1)
93.57(9)
90.94(7)
84.76(7)
83.23(7)
91.30(8)
174.02(8)
89.50(8)
96.74(5)
98.43(13)
94.45(14)
94.43(14)
90.74(13)
90.92(14)
170.63(13)
92.59(13)
94.0(2)
93.8(1)
91.7(2)
91.3(2)
171.9(1)
96.22(11)
L
c-N1ϪNiϪN1
t-N1ϪNiϪN1
t-N1ϪNiϪLc-N1 87.16(6)
L
178.61(8) 175.25(5) 174.15(13) 177.09(14)
L
85.59(2)
89.29(9)
84.88(12)
[a]
Data from ref.[2]
˚
trans to Ntert [NiϪO1 2.174(3) A]. However, even the in-
crease in the basicity of the tertiary nitrogen atom on going
from 1 to 4 does not lead to a shortening of the NtertϪNi
distance in either series of complexes due to the concomit-
ant increase in the number of sterically less favored six-
membered chelate rings.
UV/Vis Spectra
Figure 3. ORTEP plots of the cations in complexes 6b؊d; solvent
molecules and chloride counterions have been omitted for clarity
A d8 ion like NiII in an octahedral ligand field gives rise
to four absorption bands.[25] The three strongest ones are
effect of the different amine groups of the tripodal tetra- caused by the spin-allowed transitions from the ground
3
3
3
amine on the ligand topology. For two identical monodent- state A2g to the excited triplet states T2g, T1g(F), and
ate ligands, as found in 6b (ClϪ) and 6d (H2O), the follow- 3T1g(P). A weaker band of low intensity can be attributed
ing trends were found. In 6b, containing the ligand 2, the to the spin-forbidden transition to the singlet state Eg. In
1
bond to the monodentate ligand trans to Ntert [NiϪCl1 practice, it is seldom observed as a discrete band, but more
3
3
˚
2.4449(6) A] is shorter than that to the ligand cis to Ntert often as a shoulder of the A2g Ǟ T2g transition band at
[NiϪCl1* 2.4840(6) A]. This situation is reversed in 6d, about 800 nm.[26,27]
˚
containing ligand 4 with an increased basicity of the central
For complexes of types 5 and 6, three distinct absorption
nitrogen atom.[17] In 6d, the distance to the ligand cis to bands are observed. The transitions from 3A2g to the 3T1g(P)
3
˚
Ntert [NiϪO2 2.165(3) A] is shorter than that to the ligand and T1g(F) excited states appear as sharp bands of high
1282
Eur. J. Inorg. Chem. 2001, 1279Ϫ1285

cis-Octahedral Nickel(II) Complexes
FULL PAPER
in KBr with a PerkinϪElmer IR 983 instrument. Ϫ Elemental ana-
lyses (C, H, N) were performed with a Vario EL Elemental Ana-
lyzer. Ϫ FAB mass spectra were recorded with Finnigan MAT 112
or Finnigan MAT 711 instruments. Ϫ UV/Vis spectra were re-
corded in methanol with a PerkinϪElmer Lambda 9 UV/Vis NIR
spectrophotometer.
intensity in the regions 350Ϫ380 nm and 560Ϫ620 nm, re-
spectively, whereas the absorption band at about
3
3
950Ϫ1100 nm, due to the A2g Ǟ T2g transition, is broad
and of low intensity. A shoulder at about 800 nm is found
for complexes 5a,b and 6a,b, while for the rest of the com-
plexes this feature seems to be superimposed by the long-
wavelength 3A2g Ǟ 3T2g transition. The exact values for the
absorption maxima and the molar extinction coefficients
are listed in Table 3.
Ligand Syntheses: Tris(2-aminoethyl)amine (1) was purchased from
Aldrich and was used without further purification. Ligands 2, 3,
and 4 were synthesized according to the method of Dittler-Klinge-
mann and Hahn.[16a] Alternatively, ligand 3 can be prepared by the
method of Fanshawe and Blackman.[15] We preferred the prepara-
tion outlined in ref.[16] since the free ligand and not the hydro-
chloride is initially obtained in this preparation. For the present
study, the synthesis of 2 was slightly modified compared to the
published procedure.[16a]
Table 3. UV/Vis spectroscopic data (λ [nm], ε [103·molϪ1·cmϪ1] for
complexes of types 5 and 6
3A2g
Ǟ
3T1g(P)
3T1g(F)
3T2g
5a
5b
5c
5d
358 (18.6)
358 (10.7)
360 (33.9)
364 (51.0)
361
349 (22.9)
363 (20.9)
370 (21.0)
380 (33.7)
565 (13.5)
572 (8.9)
570 (22.4)
577 (28.0)
565
570 (9.3)
580 (15.6)
592 (13.0)
613 (17.6)
952 (14.1)
980 (10.1)
980 (11.1)
1070 (8.8)
952
962 (11.1)
984 (20.5)
1025 (8.5)
1050 (4.4)
Modified Preparation of Bis(2-aminoethyl)(3-aminopropyl)amine (2):
Ligand 2 has previously been obtained by Michael addition of ac-
rylonitrile to ammonia followed by a Strecker synthesis to yield
the dinitrile HN(CH2CN)(CH2CH2CN). The addition of a second
ϪCH2CN group to the central nitrogen atom using glyconitrile
(HOCH2CN) proved more difficult and very often failed due to
reasons not yet fully understood. However, reaction of the dinitrile
HN(CH2CN)(CH2CH2CN) with TsOCH2CN[23] in DMF at 50 °C
over a period of 5 d yielded the salt of the desired trinitrile,
[HN(CH2CN)2(CH2CH2CN)]OTs. On addition of an excess of
aqueous sodium carbonate and cooling the resulting mixture to 5
°C, colorless crystals of N(CH2CN)2(CH2CH2CN) separated
within a few days. The crude product could be recrystallized from
hot chloroform. Reduction of the trinitrile with AlH3 as described
previously[16a] gave bis(2-aminoethyl)(3-aminopropyl)amine (2) in
64% yield.
6a[a]
6a[b]
6b
6c
6d
[a]
[b]
Data from ref.[2]
Ϫ
Complex 6a synthesized for this study.
One of the most notable results is the observation of a
bathochromic shift with increasing number of six-mem-
bered chelate rings. This can be ascribed to the weaker li-
gand field generated by the six-membered chelate rings,
which are known to be less stable than five-membered che-
late rings.[17,28] Thus, the distance between the energy suble-
vels is decreased and the transitions are seen to be shifted
to longer wavelengths.
Preparation of Nickel Complexes from Ni(NO3)2·6H2O: The pre-
paration of 5a is described as an example. Ni(NO3)2·6H2O (290 mg,
1 mmol) was dissolved in acetonitrile (5 mL) and then ligand 1
(146 mg, 1 mmol) was added dropwise by means of a syringe. The
blue solution thus obtained was stirred at room temperature for 30
min, filtered, and concentrated to a volume of 3 mL. Slow diffusion
of diethyl ether into the flask led to the deposition of dark-blue
crystals of 5a, which proved to be suitable for X-ray crystallography
without the need for recrystallization. The air-stable crystals were
collected by filtration, washed with diethyl ether, and carefully
dried. This procedure could also be carried out in methanol.
Conclusion
With complexes 5aϪd and 6bϪd we present seven new
complexes forming two series of octahedral NiII complexes
with aliphatic tripodal tertraamine ligands. The series 6bϪd
is completed by compound 6a, which has previously been
characterized by Marzotto et al.[2] The variation of the
lengths of the ligand arms leads to complexes with different
size chelate rings. This causes a distortion of the octahedral
coordination environment, a variation in the space available
for binding and in the strengths of the bonds to monodent-
ate coligands, and a slight shift of the absorption maxima
in the UV/Vis spectra. These effects will allow, at least in
part, control over the binding of monodentate ligands de-
pending on the topology of the tripodal tetraamine without
a change in the nature of the donor groups. Ligands 1Ϫ4
might therefore prove useful for the introduction of minute
changes in the modelling of amine-rich reaction centers in
metalloproteins.
5a: Yield 263 mg [80% based on Ni(NO3)2·6H2O]. Ϫ C6H18N6NiO6
(329.0): calcd. C 21.91, H 5.52, N 25.55; found C 22.18, H 5.40, N
24.93. Ϫ MS (FAB, positive ions, DMSO/glycerol): m/z (%) ϭ 266
(83) [Ni(1)(NO3)]ϩ, 203 (100) [Ni(1) Ϫ H]ϩ, 58 (6), 44 (53).
5b: Yield 255 mg [74% based on Ni(NO3)2·6H2O]. Ϫ C7H20N6NiO6
(343.0): calcd. C 24.51, H 5.88, N 24.50; found C 24.31, H 5.80, N
24.38. Ϫ MS (FAB, positive ions, DMSO/glycerol): m/z (%) ϭ 280
(100) [Ni(2)(NO3)]ϩ, 217 (90) [Ni(2) Ϫ H]ϩ, 58 (13), 44 (60).
5c·CH3CN: Yield 278 mg [78% based on Ni(NO3)2·6H2O] of
slightly air-sensitive (loss of solvent) crystals. Ϫ C10H25N7NiO6
(398.1): calcd. C 30.18, H 6.33, N 24.63; found C 30.25, H 6.37, N
24.39. Ϫ MS (FAB, positive ions, DMSO/glycerol): m/z (%) ϭ 294
(84) [Ni(3)(NO3)]ϩ, 232 (100) [Ni(3) Ϫ H]ϩ, 174 (48), 143 (30), 58
(37), 44 (48).
Experimental Section
Materials and Methods: All manipulations were carried out under
air. Solvents were purified by standard methods and freshly dis-
tilled prior to use. Ϫ Infrared spectra were recorded from samples
5d·CH3OH: Yield 269 mg [72% based on Ni(NO3)2·6H2O] of
slightly air-sensitive (loss of solvent) crystals. Ϫ C10H28N6NiO7
(403.1): calcd. C 29.80, H 7.00, N 20.85; found C 29.77, H 6.94, N
Eur. J. Inorg. Chem. 2001, 1279Ϫ1285
1283

C. Ochs, F. E. Hahn, T. Lügger
Table 4. Summary of crystallographic data for 5a, 5b, 5c·CH3CN, 5d·CH3OH, 6b·2CH3OH, 6c, and 6d·H2O
5a 5b 5c·CH3CN 5d·CH3OH 6b·2CH3OH 6c
FULL PAPER
6d·H2O
Empirical formula
C6H18N6NiO6 C7H20N6NiO6 C10H25N7NiO6 C10H28N6NiO7 C16H48Cl4N8Ni2O2 C8H24Cl2N4NiO C9H30Cl2N4NiO3
Molecular mass [amu]
328.97
8.284(2)
11.800(2)
12.825(3)
90
90
90
343.00
12.228(5)
15.742(4)
13.730(5)
90
90
90
398.08
403.09
17.436(2)
12.605(2)
18.056(3)
90
118.602(11)
90
3484.1(9)
8
P21/n
1.537
643.84
8.3694(6)
10.8197(9)
15.9196(9)
90
100.135(5)
90
1419.1(2)
2
P21/c
1.507
1.732
153(2)
5Ϫ50
2485
321.92
10.283(3)
11.125(4)
12.388(5)
90
97.46(3)
90
371.98
8.0390(10)
14.698(2)
14.574(6)
90
94.44(3)
90
1716.9(8)
4
Cc
1.439
1.450
293(2)
5Ϫ52
1789
˚
a [A]
8.946(3)
9.650(3)
20.963(8)
99.71(3)
98.11(3)
100.56(2)
1725.7(10)
4
˚
b [A]
˚
c [A]
α [°]
β [°]
γ [°]
3
˚
V [A ]
1253.7(5)
4
2643(2)
8
1405.2(9)
4
Z
¯
Space group
P212121
1.743
1.583
293(2)
5Ϫ45
1628
Pbca
1.724
1.506
150(2)
5Ϫ50
2320
P1
P21/c
1.522
1.749
293(2)
4Ϫ50
2462
ρcalcd. [g/cm3]
1.532
1.167
293(2)
4Ϫ45
4500
2946
4.11
9.10
434
µ [mmϪ1
]
1.159
Data collection temp. [K]
2Θ range [°]
No. of unique data
173(2)
5Ϫ50
6104
No. of obsd. data, I Ն 2σ(I) 1588
1651
4557
2121
1842
1735
R (%)[a]
3.09
7.67
173
4.16
6.17
16.42
434
2.71
4.24
10.30
144
3.03
wR (%)[a]
9.89
7.00
7.34
No. of variables
243
242
173
[a]
R ϭ Σ||Fo| Ϫ |Fc||/Σ|Fo|, Rw ϭ [Σw||Fo| Ϫ |Fc||2/Σw|Fo|2]1/2, all refinement parameters were calculated for observed data only.
20.70. Ϫ MS (FAB, positive ions, DMSO/glycerol): m/z (%) ϭ 308
in air, most probably due to loss of solvent of crystallization. How-
(38) [Ni(4)(NO3)]ϩ, 245 (56) [Ni(4) Ϫ H]ϩ, 143 (26), 58 (75), 44 ever, those obtained by crystallization from acetonitrile proved to
(38).
be more stable. Complexes 5c, 5d, 6b, and 6d were crystallized as
solvates 5c·CH3CN, 5d·CH3OH, 6b·2CH3OH, and 6d·H2O. Suit-
able crystals were mounted on an EnrafϪNonius CAD-4 diffracto-
meter at low temperature (5b, 5d·CH3OH, 6b·2CH3OH) or at room
temperature. Salient crystal and data collection details are listed in
Table 4. Routine search and autoindexing procedures yielded the
dimensions of the unit cell for all seven compounds. Diffraction
data were collected using ωϪ2θ scans. Raw data were reduced to
structure factors[29] (and their e.s.d. values) by correcting for scan
speed, Lorentz and polarization effects. No decay corrections were
necessary. Empirical absorption corrections (based on Ψ scans)
were applied to the raw data for 5b, 5d·CH3OH, and 6b·2CH3OH.
The structure solutions were found by standard Patterson methods.
The positional parameters for all non-hydrogen atoms were refined
by using first isotropic and then anisotropic thermal parameters.
Difference Fourier maps calculated at this stage showed the posi-
tional parameters of the hydrogen atoms. However, hydrogen atoms
were added to the structure models in calculated positions and were
refined as riding atoms. No hydrogen positions were calculated for
non-coordinated solvent molecules. Calculations were carried out
with SHELXL-93[30] (refinement on F2). ORTEP[31] was used for
all molecular drawings. The asymmetric unit for 5c·CH3CN and
5d·CH3OH contains two independent molecules, while 6b·2CH3OH
possesses a crystallographic inversion center and the asymmetric
unit contains 1/2 of the complex. Table 1 and Table 2 list selected
bond lengths and angles for complexes 5 and 6, respectively. Crys-
tallographic data (excluding structure factors) for the structures re-
ported in this paper have been deposited with the Cambridge Crys-
tallographic Data Centre as supplementary publication nos.
CCDC-149397 (5a), -149398 (5b), -149399 (5c·CH3CN), -149400
(5d·CH3OH), -140401 (6b·2CH3OH), -149402 (6c), and -149403
(6d·H2O). Copies of the data may be obtained free of charge from
the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. [Fax (in-
ternat.) ϩ 44-1223/360-033; E-mail: deposit@ccdc.cam.ac.uk].
Preparation of Nickel Complexes from NiCl2·6H2O: The prepara-
tion of the previously synthesized[2] 6a is described as an example.
To a solution of NiCl2·6H2O (236 mg, 1 mmol) in methanol (5 mL)
was added 1 (146 mg, 1 mmol). After stirring for 30 min at room
temperature, the mixture was concentrated to a volume of 3 mL.
To initiate crystallization, 1 mL of acetonitrile and a few drops of
diethyl ether were added to the concentrated mixture and the re-
sulting purple-blue solution was cooled to 5 °C. After a few hours,
needle-shaped crystals had formed, which were collected by filtra-
tion and washed with diethyl ether. Recrystallization from meth-
anol by slowly concentrating the solution gave blue, plate-shaped
crystals suitable for X-ray analysis.
6a: Yield 132 mg (45% based on NiCl2·6H2O). Ϫ C6H20Cl2N4NiO
(only directly coordinated water was left after drying, 293.9): calcd.
C 24.53, H 6.86, N 19.07; found C 24.68, H 6.79, N 18.91. Ϫ MS
(FAB, positive ions, DMSO/glycerol): m/z (%)
ϭ 239 (75)
[Ni(1)Cl]ϩ, 203 (24) [Ni(1) Ϫ H]ϩ.
6b·2CH3OH: Yield 309 mg (48% based on NiCl2·6H2O).
Ϫ
C16H48Cl4N8Ni2O2 (643.8): calcd. C 29.85, H 7.51, N 17.40; found
C 29.76, H 7.49, N 17.32. Ϫ MS (FAB, positive ions, DMSO/gly-
cerol): m/z (%) ϭ 253 (70.2) [{Ni(2)Cl}2]2ϩ, 217 (17.53) [Ni(2) Ϫ
H]ϩ.
6c: Yield 132 mg (41% based on NiCl2·6H2O). Ϫ C8H24Cl2N4NiO
(321.9): calcd. C 29.85, H 7.51, N 17.40; found C 29.95, H 7.37, N
17.32. Ϫ MS (FAB, positive ions, DMSO/glycerol): m/z (%) ϭ 267
(89) [Ni(3)Cl]ϩ, 232 (47) [Ni(3)]ϩ, 174 (36).
6d·H2O: Yield 176 mg (50% based on NiCl2·6H2O).
Ϫ
C9H28Cl2N4NiO2 (only directly coordinated water was left after
drying, 353.9): calcd. C 30.54, H 7.97, N 15.83; found C 30.60, H
7.84, N 15.71. Ϫ MS (FAB, positive ions, DMSO/glycerol): m/z
(%) ϭ 281 (53) [Ni(4)(H2O)(OH)]ϩ, 246 (34) [Ni(4)]ϩ, 188 (28).
UV/Vis Spectra: The UV/Vis spectra of all the complexes
were recorded from 250 nm (40000 cmϪ1) to 1400 nm
Crystal Structure Analyses: Crystals of 5a؊d and 6b؊d obtained
by crystallization from methanol were found to slowly decompose (7100 cmϪ1) at room temperature in methanol solution. The
1284
Eur. J. Inorg. Chem. 2001, 1279Ϫ1285

cis-Octahedral Nickel(II) Complexes
FULL PAPER
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Ligands 2؊4: A. M. Dittler-Klingemann, F. E. Hahn, In-
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tinction coefficients of the absorption bands are listed in
Table 3.
[16b]
org. Chem. 1996, 35, 1996. Ϫ
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1285