The use of hydrogenated Schiff base ligands in the synthesis of multi-metallic compounds

Article abstract of DOI:10.1016/j.ica.2008.06.026

The synthetic utility of tris-((2-hydroxybenzyl)-aminoethyl)amine (H6TrenSal) and tris-((2-hydroxy-5-bromobenzyl)-aminoethyl)amine (H6Tren5BrSal) are investigated. A range of monomeric complexes with general formula [(H6TrenSal)M][NO₃] with nic

Full text of DOI:10.1016/j.ica.2008.06.026

Inorganica Chimica Acta 362 (2009) 1267–1274
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The use of hydrogenated Schiff base ligands in the synthesis
of multi-metallic compounds
1
*
Abdullahi Mustapha , John Reglinski , Alan R. Kennedy
Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 April 2008
Received in revised form 6 June 2008
Accepted 13 June 2008
Available online 27 June 2008
The synthetic utility of tris-((2-hydroxybenzyl)-aminoethyl)amine (H6TrenSal) and tris-((2-hydroxy-5-
bromobenzyl)-aminoethyl)amine (H6Tren5BrSal) are investigated. A range of monomeric complexes
with general formula [(H6TrenSal)M][NO3] with nickel, copper and zinc are reported and crystallograph-
ically analysed. Nickel adopts three motifs which are different to that observed for copper and zinc. The
use of these species as platforms for the synthesis of more complex systems in conjunction with the lan-
thanoids are explored. Copper and zinc do not follow a similar reaction pathway to nickel. While nickel
forms the expected trimetallic motif [{(TrenSal)Ni}2Ln(HOMe)]+, copper forms a copper trimetallic motif.
In contrast to both nickel and copper, reactions with [(H6TrenSal)Zn]+ produce lanthanoid based products
Keywords:
Multidentate ligands
Nickel
Copper
Zinc
namely [(H6Tren5BrSal)Gd(NO3)3] and [{(H6TrenSal)Ce}2-
l
2-O2].
Ó 2008 Elsevier B.V. All rights reserved.
Controlled organisation
1. Introduction
amine, H6TrenSal, can increase the flexibility around the amino-
ethyl region of the ligand further.
Multidentate ligands such as EDTA, DTPA and TrenSal, with six
or more donor atoms are generally designed to encapsulate metal
cations [1–6]. In this respect the TrenSal motif (Fig. 1) has come to
our attention because its structure contains flexible and inflexible
regions, and it has the ability to change denticity [6,7]. TrenSal is
potentially trianionic and it is thus, unsurprising that it forms dis-
crete hexadentate complexes with trivalent metals [4,5]. However,
with divalent metals [6,7] more complex multi-metallic complexes
are formed in order to balance the charge and satisfy the preferred
coordination geometry of the metal centres (Fig. 1). Thus with
nickel trimetallic complexes where each nickel sits in an octahe-
dral environment are retrieved [6], whereas with copper the ligand
moves to a heptadentate mode and the complex is found to be tet-
rametallic [7]. This change is believed to be driven by the articula-
tion in the apical region of the ligand (centred on N2, Fig. 1).
Intriguingly, hydrogenating the imine thus forming the secondary
A number of complexes employing H6TrenSal have been re-
ported previously [8–11]. These broadly fall into two categories:
simple mono-metallic species [8–10] and more complex trimetallic
complexes [11]. Mono-metallic compounds form with trivalent
metals (e.g. gallium, indium). In these instances the metals sit
within an NyOx (where y = 3, x = 3; y = 4, x = 2, 3) motif preferring
to complex with the ligands apical nitrogen rather than the phen-
oxide donors (Fig. 1) [8]. This is in contrast to the respective Tren-
Sal complexes where the encapsulated metal is held in
a
symmetric N3O3 environment which employs all three phenoxide
donors [6,12]. Thus it would seem that the flexibility of the
H6TrenSal can allow the complexing metals to position themselves
within the pocket formed by the folded ligand. Trimetallic com-
plexes (typically [{(H6TrenSal)Ni}2Ln(OMe)]+: Ln = lanthanoid)
arise from the putative residual charge on the primary [(H6Tren-
Sal)Ni]+ complex and the manner in which the compensating lan-
thanide cation binds to and assembles the two sets of phenoxide
donors [11]. In this way the ligand forms a cage around the nickel
cations with the transition metal sitting within an N4O2 motif. This
motif is less common for Schiff base species but two similar com-
plexes, viz. [{(TEtSal)Ni}2Pb] and [{(TMeSal)Ni}2Gd]+, have recently
been reported [6,12].
Abbreviations: EDTA, ethylenediaminetetraacetic acid; DTPA, diethylenetri-
aminepentaacteic acid; TrenSal, tris-((2-hydroxybenzylidene)-aminoethyl)amine;
Tren5BrSal, tris-((2-hydroxy-5-bromobenzylidene)-aminoethyl)amine; H6TrenSal,
tris-((2-hydroxybenzyl)-aminoethyl)amine; H6Tren5BrSal, tris-((2-hydroxy-5-bro-
mobenzyl)-aminoethyl)amine.
* Corresponding author. Tel.: +44 0141 548 2349; fax: +44 0141 552 0876.
E-mail addresses: abmustapha2004@yahoo.com (A. Mustapha), j.reglinski@
strath.ac.uk (J. Reglinski).
We are studying the manner in which ligands support the for-
mation of multi-metallic complexes [13–15]. Our current synthetic
design [6] has been based on the use of multidentate, salicylidene,
Schiff base ligands such as TrenSal. In view of the greater flexibility
1
Permanent address: Department of Chemistry, Kano University of Science and
Technology, Wudil, Kano State, Nigeria.
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.06.026

1268
A. Mustapha et al. / Inorganica Chimica Acta 362 (2009) 1267–1274
Fig. 1. The chemical structure of TrenSal (left) and the X-ray crystal structures of [(Tren5BrSal)2Ni3] (middle) [6] and [(Tren5BrSal)2Cu4O] (right) [7].
of the respective hydrogenated species, we have extended our
interest to H6TrenSal. Although our interests are primarily based
on the use of first row transition metals, the studies of Orvig
et al. [11] suggest that it should be able to encapsulate first row
metals using the lanthanides. This suggested that we should ini-
tially focus on the simple adducts of nickel, copper and zinc with
H6TrenSal and their subsequent combination with lanthanides.
acetate in 100 ml distilled water was added. The product was then
extracted into 3 ꢀ 100 ml chloroform. The extracts were combined,
washed with 100 ml of distilled water and dried over anhydrous
sodium sulphate. The solution was filtered and the solvent re-
moved to give the desired product. Typical yields of 60–80% were
obtained.
2.1.1. H6TrenSal
It was obtained as a viscous oil which was used without further
purification. dH (400 MHz; solvent CDCl3) 7.15 (t, 3H, arom), 7.00
(d, 3H, arom), 6.80 (m, 6H, arom), 4.00 (s, 6H, –Ph–CH2–NH–),
2.70 (t, 6H, –NH2–CH2–CH2–N–), 2.60 (t, 6H, –NH2–CH2–CH2–N–).
dC (100 MHz; solvent CDCl3) 160, 129, 128, 122, 119, 116 (arom),
55 (–Ph–CH2–NH–), 52 (–NH2–CH2–CH2–N–), 46 (–NH2–CH2–
CH2–N–). Mass Spec. (ESI) m/z 465 [MꢁH+].
2. Experimental
All experiments were carried out using standard apparatus and
commercially available chemicals. TrenSal was prepared as
previously reported [6]. NMR analysis was carried out on a Bruker
AMX 400 operating at 400 MHz for 1H and 100 MHz for 13C.
Reflectance spectra (400–900 nm) were recorded on a Photonics
CCD array UV–Vis spectrophotometer. Mass spectra were recorded
in house on a Thermo Finnigan LCQDuo by electrospray ion trap.
Infra red spectra were recorded as potassium bromide discs using
a Nicolet Avatar 360 FTIR spectrometer. Magnetic measurements
were determined at room temperature in the solid state using an
MSB1 magnetic susceptibility balance. All X-ray measurements
were conducted using a Nonius Kappa CCD diffractometer at
2.1.2. H6Tren5BrSal
It was obtained as a solid which was recrystallised from dichlo-
romethane/methanol by vapour diffusion. Crystal suitable for
structural analysis were obtained from this process. Found: C,
45.77; H, 5.40; N, 7.43%. Expected for C27H33N4Br3O3 ꢂ MeOH: C,
45.86; H, 5.09; N, 7.64. dH (400 MHz; solvent CDCl3) 7.22 (dd, 3H,
arom, J1 = 2.3 Hz, J2 = 8.6 Hz), 7.05 (d, 3H, arom, J1 = 2.3 Hz), 6.55
(d, 3H, arom, J2 = 8.6 Hz), 3.9 (s, 6H, –Ph–CH2–NH–), 3.5 (s, 3H, –
NH–), 2.70 (t, 6H, –N–CH2–CH2–NH–), 2.5 (t, 6H, –N–CH2–CH2–
NH–). dC (100 MHz; solvent CDCl3) 157, 131, 130, 124, 118, 111
(arom), 54 (–Ph–CH2–NH–), 52 (–N–CH2–CH2–NH–), 46 (–N–
123 K using graphite monochromated Mo Ka radiation apart from
[(H6Tren5BrSal)Ni], data for which was collected on a Oxford Dif-
fraction Xcalibur CCD diffractometer.
Crystals were coated in mineral oil and mounted on glass fibers.
The heavy atom positions were determined by Patterson methods
and the remaining atoms located in difference electron density
maps. Full matrix least-squares refinement was based on F2, with
all non-hydrogen atoms anisotropic. While the hydrogen atoms
were mostly observed in the difference maps, they were placed
in calculated positions riding on the parent atoms except those in-
volved in hydrogen bonding which were refined isotropically. The
structure solution and refinement used the programs SHELXL and
SHELXS-97 [16] and the graphical interface WinGX [17]. A summary
of the crystallographic parameters is given in Table 1.
CH2–CH2–NH–). FTIR [m
/cmꢁ1 (KBr)]: 3450 (OH br), 1470 (C@C),
1100, 1025, 1265 (C–O), 820 (arom). Mass Spec (ESI) m/z 699,
701, 703, 705 (1:3:3:1) [MꢁH+], m.p. 68 °C.
2.2. Preparation of [Ni(H6TrenSal)(OH2)][NO3], [Ni(H6TrenSal)][NO3]
and [Ni(H6Tren5BrSal)(OH2)][NO3]
A methanolic solution of nickel nitrate (1.1 g, 3.8 mmol) was
added to a solution containing the relevant Schiff base (1.8 g and
2.7 g respectively, 3.9 mmol) dissolved in methanol (50 ml). The
mixture was stirred for 1 h at 50 °C whereupon the resultant pur-
ple solution was allowed to cool and the solvent removed under re-
duced pressure. The sticky purple material was washed with
dichloromethane, water and diethyl-ether to obtain a pale purple
powder. Yields are typically 60–70%.
X-ray quality crystals of [Ni(H6TrenSal)(OH2)][NO3] were
grown by slow evaporation of the product from ethanol on the
open bench.
[Ni(H6TrenSal)(OH2)][NO3]: Anal. Calc. for C27H37N5Ni1O7: C,
53.84; H, 6.19; N, 11.63. Found: C, 54.07; H, 6.43; N, 11.48%. FTIR
2.1. Hydrogenation of TrenSal and Tren5BrSal to give H6TrenSal and
H6Tren5BrSal
Sodium borohydride (3–4 mol equiv.) was added in small por-
tions over a period of 1 h to a solution of the relevant Schiff base
(TrenSal, Tren5BrSal; 1 mol equiv.) in methanol under a blanket
of nitrogen. The yellow solution became colourless. The mixture
was stirred overnight to ensure the reaction was complete. The sol-
vent was removed by rotary evaporation and 6 g of ammonium

Table 1
Crystallographic parameters
H6Tren5BrSal H6TrenSal
CH3OH (NO3)3
[(H6TrenSal)Ni] [(H6TrenSal)Ni(OH2)] [(H6Tren5BrSal)Ni] ꢂ [(H6TrenSal)Cu] [(H6TrenSal)Zn] [(H6Tren5Br)Zn] [{(H6TrenSal}Ni)2- [{(H6TrenSal}Ni)2- [(H6TrenSal)Gd(NO3)3] [(H6TrenSal)Ce]2O2
NO3 ꢂ CH3OH
NO3 ꢂ CH3OH
2CH3OH
NO3 ꢂ 0.7MeOH NO3 ꢂ 1H2O
NO3 ꢂ Et2O
La (HOMe)] NO3
Ce (H2O)]
CH2Cl2 ꢂ CH3OH
2DMF 2Et2O
K0.5(NO3)2.5
Empirical
formula
FW
Crystal
system
Space
group
a (Å)
C
28H37Br3N4O4 C27H39N7O12
C28H39N5NiO7
616.35
C29H43N5NiO8
C29H39Br3N4NiO5
C27.7H37.8N5O6.7 C27H37N5O7Zn1 C31H42Br3N5O7Zn C57.50H80.50La-
N9Ni2 O16
C54H70CeK0.50
N10.50Ni2O15.5
1391.42
-
C29H39Br3Cl2Gd1N7O13 C68H100Ce2N10O12
733.35
monoclinic
653.65
648.943
monoclinic
822.08
monoclinic
611.56
monoclinic
608.99
monoclinic
901.80
orthorhombic
1410.14
triclinic
1161.55
orthorhombic
1529.82
triclinic
orthorhombic orthorhombic
orthorhombic
ꢀ
ꢀ
P21/n
P212121
Pbca
C2/c
P21/n
C2/c
C2/c
Pbca
P1
Fddd
Pbca
P1
9.5986(3)
14.5471(4)
21.6731(7)
9.6620(2)
12.8230(4)
24.4840(8)
15.0819(2)
19.5887(3)
19.6601(3)
25.6110(8)
14.8150(5)
16.5470(4)
13.9434(4)
17.5746(5)
13.9593(4))
26.2890(12)
13.3002(6)
17.1790(6)
26.2391(9)
13.4200(4)
17.1144(5)
12.9220(6)_
17.4571(7)
31.4840(16)
13.0710(3)
14.1409(4)
19.8191(5)
74.535(2)
84.324(2)
67.067(2)
2
17.3039(6)
37.3922(7)
42.9281(14)
15.2796(3)
21.4716(4)
23.8526(5)
10.8974(8)
12.9973(10)
13.7157(11)
88.237(3)
77.694(5)
65.256(5)
2
b (K)
c (K)
a
(°)
b (°)
97.718(2)
93.302(2)
111.146(3)
95.713(2)
95.507(2)
c
(°)
Z
4
4
8
8
4
8
8
8
16
8
V (Å3)
2998.84(16)
4.075
3033.47(15)
0.114
5808.28(15)
0.722
6268.0(3)
0.675
3190.38(16)
4.409
5976.8(4)
0.781
5998.7(3)
0.869
7102.2(6)
4.119
3251.53(14)
1.288
27775.8(14)
1.276
7825.5(3)
4.965
1720.0(2)
1.374
lcalc
(mmꢁ1
)
Number of
reflections
measured
Number of
unique
reflections
Number
observed
Tmin
Tmax
Rint
Number of
parameters
44168
6901
25317
5360
85884
6319
36666
7181
14745
6006
11484
5888
42845
6606
29830
6223
4061
74773
12789
7758
33014
6143
5023
65140
7691
5224
30802
6657
4333
4958
3975
4967
4961
4328
3393
4066
0.544
1.000
0.0665
381
na
na
0.063
414
0.911
1.000
0.0791
389
na
na
0.057
415
0.375
1.000
0.0402
383
na
na
0.0981
390
na
na
0.071
387
0.730
1.000
0.1393
428
na
na
0.1437
822
na
na
0.0408
385
0.6655
1.0000
0.0815
522
0.552
0.745
0.1646
423
R(I > 2
Rw (all
reflections)
Goodness-
of-fit
r
(I))
0.0451
0.1087
0.048
0.0967
0.0630
0.1157
0.0483
0.1217
0.0499
0.1223
0.0655
0.1594
0.0534
0.1651
0.0922
0.2115
0.0614
0.1483
0.0501
0.1554
0.0416
0.0900
0.0675
0.1686
1.0541
1.044
1.160
1.023
0.996
1.037
1.049
1.083
1.053
1.083
1.010
1.032

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A. Mustapha et al. / Inorganica Chimica Acta 362 (2009) 1267–1274
[(
m
/cmꢁ1 (KBr)]: 3440 (br OH); 2930 (weak NH); 1480 (C@C); 1300
2.5. Preparation of Di-(tris-(2-hydroxybenzyl)aminoethylamine
nickel-methanolo-lanthanum nitrate
(C–O); 755 (arom). Mass spec. (ESI) m/z 521 (70%, LNi+), 465 (100%
LH+). kmax (solid reflectance) 571 nm.
X-ray quality crystals grown from methanol/diethyl-ether by
vapour diffusion were revealed to be the unhydrated product.
[Ni(H6TrenSal)][NO3] ꢂ MeOH: Anal. Calc. for C28H39N5Ni1O7: C,
54.57; H, 6.38; N, 11.36. Found: C, 54.47; H, 6.18; N, 11.47%. FTIR
[Ni(H6TrenSal)(OH2)][NO3] (0.1 g, 0.17 mmol) was dissolved in
15 ml methanol to which five drops of triethylamine were added.
The solution was stirred at 60 °C for 5 min whereupon a methano-
lic solution of lanthanum nitrate hexahydrate (0.04 g, 0.09 mmol)
was added dropwise. The resulting solution was stirred for
15 min, allowed to cool and then filtered. The product forms as
light purple powder crystals that were suitable for analysis by X-
[(m
/cmꢁ1 (KBr)]: 3440 (br OH); 2930 (weak NH); 1480 (C@C); 1300
(C–O); 755 (arom). Mass spec. (ESI) m/z 521 (70%, LNi+), 465 (100%
LH+). kmax (solid reflectance) 581 nm.
X-ray quality crystals grown from methanol/diethyl-ether by
vapour diffusion were revealed to be a neutral product.
[Ni(H6Tren5BrSal)] ꢂ MeOH: Anal. Calc. for C27H31Br3N4Ni1O3: C,
ray methods. Yield 59%. Anal.
Calc. for C54H78La1N8Ni2O12: C,
47.81; H, 5.93; N, 9.12. Found: C, 47.70; H, 5.93; N, 8.92%. FTIR [(m/
cmꢁ1 (KBr)]: 3430 (br OH); 2850 (weak NH); 1595 (NO3), 1485
(C@C); 1295 (C–O); 760 (arom). Mass spec. (ESI) m/z 1173
(10%, (LNi)2La+), 526 (100%, LNi+). kmax (solid reflectance) 550 nm,
920 nm.
42.62; H, 4.47; N, 7.10. Found: C, 42.20; H, 3.56; N, 7.65%. FTIR [(m/
cmꢁ1 (KBr)]: 3425 (br OH); 2930 (weak NH); 1480 (C@C); 1300 (C–
O); 755 (arom). Mass spec. (ESI) m/z 759 (100% LNiH+). kmax (solid
reflectance) 590 nm.
2.6. Preparation of di-(tris-(2-hydroxybenzyl)aminoethylamine nickel)
cerium nitrate
2.3. Preparation of [Cu(H6TrenSal)][NO3] and
[Cu(H6Tren5BrSal)][NO3]
[Ni(H6TrenSal)(OH2)][NO3] (0.2 g, 0.33 mmol) was dissolved in
methanol (20 ml) to which five drops of triethylamine was added.
The solution was stirred for 10 min at 60 °C, whereupon a metha-
nolic solution of cerium(III) nitrate hexahydrate (0.072 g,
0.11 mmol) was added dropwise. The solution turned deep orange.
After stirring for further 15 min, the solution was filtered and al-
lowed to cool. The product forms as an orange powder. Yield
68%. Crystals suitable for analysis by X-ray crystallography were
A methanolic solution (15 ml) of copper nitrate dihydrate (0.4 g,
0.86 mmol) was added to a solution containing the relevant Schiff
base (0.4 g and 0.6 g respectively, 0.86 mmol) dissolved in metha-
nol (20 ml). Three drops of triethylamine were added and the green
solution stirred for 20 min at 50 °C. The solution was allowed to
cool and then filtered. On prolonged standing (24 h) the product
crystallises from solution. Yield 67%.
[Cu(H6TrenSal)][NO3]: Anal. Calc. for C27H35N5Cu1O6 ꢂ 2H2O: C,
obtained by slow evaporation of orange filtrate. FTIR [(m
/cmꢁ1
51.87; H, 6.29; N, 11.59. Found: C, 51.79; H, 6.77; N, 11.59%. FTIR
(KBr)]: 3440 (br OH); 2870 (weak NH); 1485 (C@C); 1390 (NO3),
1250 (C–O); 760 (arom). Mass spec. (ESI) m/z 1179 (100%,
(LNi)2Ce+). During optimisation of the mass spectral analysis the
dication m/z 589 ((LNi)2Ce2+) can be a prominent feature. kmax (so-
lid reflectance) 540 nm.
[(m
/cmꢁ1 (KBr)]: 3410 (br OH); 2925 (weak NH); 1480 (C@C); 1385
(NO3); 1280 (C–O); 770 (arom). Mass spec. (ESI) m/z 526 (100%,
LCu+). kmax (solid reflectance) 675 nm.
Crystals of [Cu(H6Tren5BrSal)][NO3] were grown from metha-
nol/diethyl-ether by vapour diffusion. Although these were sub-
jected to analysis by X-ray methods the data did not refine to an
acceptable degree. Thus only the identity of the compound and
the connectivity of the atoms can be confirmed. The data was not
deposited with the CCDC but can be found in the Supplementary
material.
2.7. Preparation of di-(tris-(2-hydroxybenzyl)aminoethylamine
copper) copper tetranitrate
[(H6TrenSal)Cu][NO3] (0.4 g, 0.86 mmol) was dissolved in 15 ml
methanol to which five drops of triethylamine were added. The
solution was stirred at 60 °C for 5 min whereupon a methanolic
solution of lanthanum nitrate hexahydrate (0.185 g, 0.43 mmol)
was added dropwise. The resulting solution was stirred for
15 min, allowed to cool and then filtered. The product forms as
brown/purple powder crystals that were subjected to analysis by
X-ray methods. Yield 51%. Anal. Calc. for C54H70Cu3N12O18 ꢂ 4H2O:
C, 44.78; H, 5.23; N, 11.19. Found: C, 44.25; H, 4.92; N, 11.46%. FTIR
[Cu(H6Tren5BrSal)][NO3]: FTIR [(m
/cmꢁ1 (KBr)]: 3410 (br OH);
2935 (weak NH); 1470 (C@C); 1385 (NO3); 1270 (C–O); 820
(arom). Mass spec. (ESI) m/z 764 (100%, LCu+). kmax (solid reflec-
tance) 681 nm, 813 nm.
2.4. Preparation of [Zn(H6TrenSal)][NO3] and
[Zn(H6Tren5BrSal)][NO3]
[(m
/cmꢁ1 (KBr)]: 3440 (br OH); 2930 (weak NH), 1485 (C@C); 1385
A methanolic solution (10 ml) of zinc nitrate hexahydrate
(0.05 g, 0.25 mmol) was added to a solution containing the rele-
vant Schiff base (0.12 g and 0.17 g respectively, 0.25 mmol) dis-
solved in methanol (10 ml). The colourless solution was stirred
for 20 min at 50 °C. The solution was allowed to cool and then fil-
tered. On prolonged standing (24 h) the product crystallises from
solution. Yield 71%.
(NO3); 1260 (C–O); 755 (arom). Mass spec. (ESI) m/z 1116 (100%,
(L2Cu3+) 526 (LCu+). kmax (solid reflectance) 409 nm, 646 nm.
Although the sample was subjected to analysis by X-ray meth-
ods the data did not refine to an acceptable degree. Thus only the
identity of the compound and the connectivity of the atoms can be
confirmed. The data was not deposited with the CCDC but can be
found in the Supplementary material.
[Zn(H6TrenSal)][NO3]: Anal. Calc. for C27H35N5Zn1O6 ꢂ 2H2O: C,
51.72; H, 6.27; N, 11.17. Found: C, 51.79; H, 6.23; N, 11.20%. FTIR
2.8. Preparation of [(H6Tren5BrSal)Gd(NO3)3]
[(m
/cmꢁ1 (KBr)]: 3460 (br OH); 2940 (weak NH); 1455 (C@C); 1385
(NO3); 1280 (C–O); 755 (arom). Mass spec. (ESI) m/z 527 (100%, LZn+).
Crystals of [Zn(H6Tren5BrSal)][NO3] suitable for X-ray analysis
were grown from methanol/diethyl-ether by vapour diffusion.
[Zn(H6Tren5BrSal)][NO3] ꢂ (C2H5)2O: Anal. Calc. for C31H42Br3N5-
Zn1O7: C, 41.28; H, 4.69; N, 7.77. Found: C, 41.63; H, 4.70; N, 7.98%.
[(H6TrenSal)Zn][NO3] (0.5 g, 0.86 mmol) was dissolved in 10 ml
methanol to which five drops of triethylamine were added. The
solution was stirred at 90 °C for 10 min whereupon a methanolic
solution of gadolinium nitrate hexahydrate (0.024 g, 0.05 mmol)
was added dropwise. The resulting solution was stirred for
15 min, allowed to cool and then filtered. The product forms as
white powder. Crystals suitable for X-ray analysis were obtained
by vapour diffusion of the mother liquors with diethyl-ether. Yield
FTIR [(m
/cmꢁ1 (KBr)]: 3420 (br OH); 2920 (weak NH); 1475 (C@C);
1390 (NO3); 1275 (C–O); 815 (arom). Mass spec. (ESI) m/z 765
(100%, LZn+).

A. Mustapha et al. / Inorganica Chimica Acta 362 (2009) 1267–1274
1271
46%. FTIR [(
(C@C); 1385 (NO3); 1260 (C–O); 755 (arom).
m
/cmꢁ1 (KBr)]: 3430 (br OH); 2940 (weak NH), 1470
tifs one where the coordination sphere is completed with water
(Fig. 3b), another where a second phenol donor (i.e. N4O2) is em-
ployed which is cationic (Fig. 3c) and a third species ([(H6Tren5-
BrSal)Ni] Fig. 3d which also employs two phenolates but which is
now neutral.
2.9. Preparation of [{(H6TrenSal)Ce}2-l
2-O2]
[(H6TrenSal)Zn][NO3] (0.5 g, 0.86 mmol) was dissolved in 10 ml
methanol. The solution was stirred at 90 °C and treated with three
drops of a methanolic solution of KOH followed by cerium nitrate
hexahydrate (0.2 g 0.5 mmol) in methanol (10 ml). The resulting
solution was stirred for 15 min, allowed to cool whereupon orange
crystals form. Crystals that were suitable for analysis by X-ray
methods were grown from DMF/diethyl-ether by vapour diffusion.
Yield 45%. Anal. Calc. for C54H66Ce2N8O8: C, 53.95; H, 5.53; N, 9.32.
As predicted the flexibility of the hydrogenated ligand allows
the metals to sit within the pocket defined by the four nitrogen do-
nors (the N2–M–N angles are all less than 90°, Table 2) and all the
species share the common MN4 motif. Indeed the metrical param-
eters of the MN4 motif for all the compounds isolated are remark-
ably similar (Table 2). Copper and zinc complete their coordination
sphere by binding to a single phenoxide. In contrast nickel was iso-
lated in two hexadentate forms, one which is effectively the cop-
per/zinc structure with an additional coordinated water
molecule, an ‘‘anhydrous” form where a second phenoxide rotates
into the sixth coordination site and a neutral species where the
phenolate has been deprotonated. The two five coordinate com-
plexes (Cu, Zn) are trigonal bipyramidal in nature with a phenoxide
donor filling the fifth position trans to the apical nitrogen (N2,
Table 2). The ambivalent coordination chemistry of TrenSal has
been remarked upon previously by Lui et al. [8] in their reports
on their group 13 complexes. Here three distinct structural types
are reported; an N3O3 coordination mode with aluminium, an
N4O motif with gallium and an N4O3 motif with indium. The struc-
tures reported here for copper and zinc follow that of the adjacent
element gallium (Table 2) in that they also display two pendant
phenoxides. The two structures isolated which contain nickel
(viz. the N4O and N4O2 motifs) are further additions to the H6Tren-
Sal family of complexes. These two structural motifs can be consid-
ered to lie intermediate between the N4O structure of gallium and
N4O3 indium [8] and can thus be considered to be part of a wider
structural progression.
Bayly et al. have reported the synthesis of an interesting series
of nickel lanthanoid trimetallic species [11]. The synthetic proto-
cols employed in his study were simple and relied on the self-
assembly of the complex from a mixture of the three components
(two metal salts and the ligand). A mechanism for the assembly of
these species can be drawn up which employs the pre-formation of
a moiety such as those described above (Fig. 3). This hypothesis
was tested by treating [(H6TrenSal)Ni]+ with lanthanum and ceriu-
m(III) nitrate. Not only did the spectroscopic data agree with that
previously published by Bayly et al. [11] for [(NiTren-
Sal)2La(OMe)]NO3, consistent with his claims regarding the pro-
Found: C, 53.85; H, 6.60; N, 8.77%. FTIR [(m
/cmꢁ1 (KBr)]: 3405 (br
OH); 2900 (weak NH), 1480 (C@C); 1290 (C–O); 765 (arom). Mass
spec. (ESI) m/z (100%, (LCe+) 601. kmax (solid reflectance) 431 nm. leff
1.3 BM.
3. Results and discussion
The multidentate ligand systems H6TrenSal and H6Tren5BrSal
can be prepared with ease by hydrogenating the Schiff base species
generated from the tris-(2-aminoethyl)amine and salicylaldehyde
(H6TrenSal) or 5-bromosalicylaldehyde (H6Tren5BrSal). X-ray
quality crystals of H6Tren5BrSal can be easily grown from dichlo-
romethane (Fig. 2). In contrast H6TrenSal is received as an oily
material that is used without further purification. However, a small
number of X-ray quality crystals we generated as a bi-product of
the reactions of the nitrate salts discussed below. Taken together
these structures elegantly demonstrate the flexibility of these mul-
tidentate species. The neutral species forms a zwitterion that is
heavily folded due to hydrogen bonding interactions, whereas
the nitrate salt being devoid of a hydrogen bonded network adopts
a more open dendritic form.
Although a number of mono-metallic complexes employing this
ligand type have been reported previously [8], curiously no simple
examples of transition metal complexes have been structurally
characterised. Thus treatment of the two hydrogenated Schiff
bases, H6TrenSal and H6Tren5BrSal, with one equivalent of metal
nitrate (M = Ni, Cu, Zn) leads to the formation of cationic 1:1 ad-
ducts. Although the metal ligand ratios are the same throughout
four distinct structural types are retrieved: a dominant five coordi-
nate N4O motif (Fig. 3a, e.g. copper), and three six coordinate mo-
Fig. 2. The X-ray crystal structure of H6Tren5BrSal (left) and [H6TrenSal][NO3]3 (right nitrates not shown). The entrapped methanol (C28, O4) in the structure of
H6Tren5BrSal can be seen. The molecule folds due to intramolecular hydrogen bonding (dotted lines) between O1 and H31 (1.626 Å) and O1 and H1 (1.820 Å). The hydrogen
from O1 has effectively migrated to N3. The crystals of [H6TrenSal][NO3]3 (right) were obtained as a minor bi-product of the metal nitrate reactions and is included here to
demonstrate the structural extremes available to these ligand systems. Thermal ellipsoids are at 50% probability.

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A. Mustapha et al. / Inorganica Chimica Acta 362 (2009) 1267–1274
Fig. 3. The X-ray crystal structures of [(H6TrenSal)Cu]+ (top), [(H6TrenSal)(H2O)Ni]+ (middle left), [(H6TrenSal)Ni]+ (middle right) and [(H6Tren5BrSal)Ni] (bottom). The
structure of [(H6TrenSal)Zn]+, [(H6Tren5BrSal)Cu]+ and [(H6Tren5BrSal)Zn]+ (not shown) are isostructural with that of [(H6TrenSal)Cu]+. For clarity the nitrate counterions
are not shown. Thermal ellipsoids are shown at 50% probability.
pensity of these species to crystallise we were able to retrieve X-
ray quality crystals directly from the reaction mixture for these
two complexes (Fig. 4). Thus the mechanism for the synthesis of
these species can now be further refined. The lanthanoid initially
chelates to the two coordinated phenoxides (O1, O2, Fig. 4 bottom).
Since the nickel sits within the N4 pocket it is difficult for the third
phenoxide to coordinate to the internalised metal. However, the
chelated lanthanoid adopts a more appropriate position where
Table 2
The metrical parameters of the H6TrenSal and H6Tren5BrSal complexes of nickel, copper, zinc and gallium
M–Napex (Å)
M–N (Å)
\N–M–Napex (°)
\Ophenox–M–Napex (°)
[(H6TrenSal)(H2O)Ni]+
[(H6TrenSal)Ni]+
2.088
2.102
2.127
2.034
2.200
2.216
2.147
2.120, 2.057, 2.189
2.128, 2.138, 2.062
2.119, 2.150, 2.169
2.050, 2.127, 2.120
2.112, 2.062, 2.098
2.054, 2.101, 2.112
2.090, 2.099, 2.115
83.80, 85.36, 81.66
84.16, 82.68, 83.93
81.33, 84.00, 81.04
85.47, 85.61, 84.22
81.39, 84.27, 82.33
82.1, 82.7, 84.2,
177.43
175.11
170.12
175.84
170.97
171.3
[(H6Tren5BrSal)Ni]
[(H6TrenSal)Cu]+
[(H6TrenSal)Zn]+
[(H6Tren5BrSal)Zn]+
[(H6Tren5BrSal)Ga]+
81.7, 82.8, 80.9
172.7

A. Mustapha et al. / Inorganica Chimica Acta 362 (2009) 1267–1274
1273
(cf. Ln, Ni) there is no driving force for the separation of the Cu–Na-
bond and the third phenolic group remains unattached to the
pex
central metal centre (Cu2)
2þ
3½H6TrenSalCuꢄþ þ LaðNO3Þ ! ½ðH6TrenSalÞ Cu3ꢄ
3
2
þ ½H6TrenSalLaðNO3Þ3ꢄ
ð1Þ
The appearance of this copper motif suggests that there can be
competition for the H6TrenSal moiety when lanthanum is intro-
duced into the mixture. In this instance some displacement of cop-
per by lanthanum takes place (equation 1). In contrast to copper
the products obtained from the reaction of [(H6Tren5BrSal)Zn]
with gadolinium and cerium nitrate were all lanthanoid based.
With gadolinium we retrieved [(H6Tren5BrSal)Gd(NO3)3] where
the trivalent lanthanoid coordinates to the external O3 motif of
the polydentate ligand and didentate with the three nitrate ligands
(Fig. 6). Species similar to this have been reported previously [10].
The significance of this metal complex to this study is the cavity
that remains within the ligand. Although it is clearly large enough
to continue accommodating the zinc, the ligand is now devoid of
zinc. It is unlikely that the differences between nickel and zinc
arise from the Lewis acidity of these metals. A plausible explana-
tion lies in the preferred geometries of the two metals. Zinc, having
a preference for lower coordination numbers allows the secondary
amines more latitude to protonate. This allows a build-up of posi-
tive charge in the vicinity of the cavity which assists the migration
of the zinc from the pocket and prevents the ingress of the lantha-
noid. A contribution to this process may also stem from the man-
ner in which the lanthanoid arranges the oxygen donors such
that the pocket is held in an open configuration weakening the
zinc-donor interactions. Taken together the products obtained
from these copper and zinc precursors can be used to support
the process described by equation 1. However the analogous tri-
zinc species has not been isolated from our reactions. Instead only
monomeric starting material, {(H6TrenSal)Zn], and the lanthanum
complex shown are obtained.
Fig. 4. The X-ray crystal structure of [(H6TrenSalNi)2La(OMe)]+. The complex is
isostructural with the nickel lanthanoid complexes reported by Bayly et al. [11]. The
thermal ellipsoids are shown at 50% probability.
the pendant phenoxide is able to bind independently to lanthanoid
centre. Thus in contrast to the Schiff base species the binding of the
heavy metal is not symmetric.
Our ability to generate the simple compounds of copper and
zinc similar to nickel (Fig. 2) suggested that the reaction reported
by Bayly et al. [11] might also give rise to an interesting series of
lanthanoid complex of these metals. Unfortunately the reaction
failed to generate the desired isostructural species presumably be-
cause, unlike nickel, the phenoxides do not form a chelating cleft
(Fig. 2). From the reaction of [(TrenSal)Cu]+ with lanthanum nitrate
we obtained a homotrimetallic (M3L2) copper complex (Fig. 5)
where three coppers are found to sit in two unique environments.
The central copper (Cu2) is chelated between two [(TrenSal)Cu]
species and sits in an essentially planar environment. The coppers
encapsulated by TrenSal remain five coordinate employing an N3O2
donor set. The distance to the apex nitrogen is now somewhat lar-
ger (ꢃ2.37 Å, cf. Table 2) indicative that the encapsulated copper
(Cu1) is being pulled forward due to the presence of the central
metal (Cu2). A pendant phenolic group is still present except that
it now includes its attendant secondary amine which is protonated.
As an O3 octahedral motif is not demanded by the central copper
The reaction of [(H6TrenSal)Zn]+ with lanthanoids was investi-
gated further using cerium nitrate to confirm the reaction profile.
However, in this instance we obtained a nine coordinate cerium
complex (Fig. 7). It would seem that on complexing to the exterior
of the ligand the cerium oxidises and divests itself of the attendant
nitrates and becomes internalised within the ligand (Fig. 7). The
internalisation of lanthanoids by this family of ligand has been re-
ported previously for the eight coordinate species [H6TrenSalGd]2
[18]. However, coordination expansion in the manner observed
here is a new and novel facet of cerium chemistry. At this stage
Fig. 5. The X-ray crystal structure of [(H6TrenSal)2Cu3] 4NO3. The data failed to
refine to an acceptable degree. Thus only the identity of the compound and the
connectivity of the atoms can be confirmed. The data was not deposited with the
CCDC but can be found for inspection in the supplementary material.
Fig. 6. The X-ray crystal structure of [(H6Tren5BrSal)Gd(NO3)3]. The thermal
ellipsoids are given at 50% probability.

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A. Mustapha et al. / Inorganica Chimica Acta 362 (2009) 1267–1274
Appendix A. Supplementary data
X-ray crystallographic data in CIF format. This material is avail-
able free of charge via the internet at http://www.ccdc.cam.ac.uk
quoting deposition numbers 684063 – 684074. Data was collected
for [(H6TrenSal)2Cu3] 4NO3 and [(H6TrenSal)Cu] NO3. However,
the data failed to refine to an acceptable degree. This data was
not deposited with the CCDC but can be found for inspection in
the supplementary material. Also shown are the expected and
found mass spectral data for the key ions. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.ica.2008.06.026.
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Acknowledgments
A.M. would like to express his gratitude to Kano University of
Science and Technology, Wudil for financial assistance. Thanks
are due to the EPSRC X-ray Crystallography Service at the Univer-
sity of Southampton for data collection.