The coordination chemistry of unsymmetric N-capped tripodal NO3 ligands with iron(III), oxo-vanadium(V) and dioxo-molybdenum(VI) metal centres

Article abstract of DOI:10.1016/j.ica.2011.10.054

The synthesis and characterisation of iron(III) [Fe(L¹)] ₂ (1), oxo-vanadium(V) [VO(L¹)] (2) and dioxo-molybdenum(VI) K[MoO₂(L¹)] (3) complexes supported by a partially unsymmetric N-capped tripodal NO₃ ligand (L ¹) are reported. Complexes 1 and 2 were prepared by reactions of a proligand [H₃L¹] and their corresponding metal precursor (FeCl₃ and VOCl₃, respectively) in the presence of triethylamine in THF, while 3 was obtained in a similar manner by treating [MoO₂(acac)₂] (acac = acetylacetonate) with the same proligand and potassium hydroxide in methanol. The X-ray crystal structure of 1 illustrates a dimeric structure with a bis-(μ-phenoxo) Fe₂O ₂ diamond core, and each iron centre bears a rare NO₄ coordination sphere. The preparation of a previously unknown, fully unsymmetric N-capped tripodal NO₃ proligand [H₃L²] featuring three different phenolic arms is also described. Reaction of [H ₃L²] with VOCl₃ yielded the oxo-vanadium(V) complex [VO(L²)] (4). Complexes 2-4 exist as monomers in solution and were characterised by NMR (¹H and ¹³C), IR (for 3), mass spectrometry, and elemental analysis.

Full text of DOI:10.1016/j.ica.2011.10.054

Inorganica Chimica Acta 383 (2012) 91–97
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The coordination chemistry of unsymmetric N-capped tripodal NO3 ligands
with iron(III), oxo-vanadium(V) and dioxo-molybdenum(VI) metal centres
a,
c
a
Lok H. Tong a,b, , Yee-Lok Wong , Hung Kay Lee , Jonathan R. Dilworth
⇑
⇑
a Chemistry Research Laboratory, Department of Chemistry, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, UK
b Department of Inorganic, Analytical and Applied Chemistry, University of Geneva, 30 quai Ernest Ansermet, 1211-Geneva 4, Switzerland
c Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong Special Administrative Region
a r t i c l e i n f o
a b s t r a c t
The synthesis and characterisation of iron(III) [Fe(L1)]2 (1), oxo-vanadium(V) [VO(L1)] (2) and dioxo-
molybdenum(VI) K[MoO2(L1)] (3) complexes supported by a partially unsymmetric N-capped tripodal
NO3 ligand (L1) are reported. Complexes 1 and 2 were prepared by reactions of a proligand [H3L1] and
their corresponding metal precursor (FeCl3 and VOCl3, respectively) in the presence of triethylamine in
THF, while 3 was obtained in a similar manner by treating [MoO2(acac)2] (acac = acetylacetonate) with
the same proligand and potassium hydroxide in methanol. The X-ray crystal structure of 1 illustrates a
Article history:
Received 12 May 2011
Received in revised form 29 August 2011
Accepted 18 October 2011
Available online 23 November 2011
Keywords:
Unsymmetric tripodal ligands
Amine triphenolate
dimeric structure with a bis-(l-phenoxo) Fe2O2 diamond core, and each iron centre bears a rare NO4 coor-
dination sphere. The preparation of a previously unknown, fully unsymmetric N-capped tripodal NO3
proligand [H3L2] featuring three different phenolic arms is also described. Reaction of [H3L2] with VOCl3
yielded the oxo-vanadium(V) complex [VO(L2)] (4). Complexes 2–4 exist as monomers in solution and
were characterised by NMR (1H and 13C), IR (for 3), mass spectrometry, and elemental analysis.
Ó 2011 Elsevier B.V. All rights reserved.
Iron(III) complex
Oxo-vanadium(V) complex
Dioxo-molybdenum(VI) complex
Crystal structure
1. Introduction
(type II). These partially unsymmetric tetradentates featuring
two different arm lengths can form {5,6,6} chelate ring-sizes upon
The coordination chemistry and reactivity of both transition-
metal [1–10] and main-group complexes [11–14] containing tetra-
dentate N-capped tripodal NO3 ligands have attracted considerable
research interests. This versatile class of ligands reacts with simple,
commercially available metal precursors to give metal complexes
which provide opportunities for investigations of catalysis as well
as a number of organic transformations [15]. Symmetric triphenol-
amine ligands (type I, Fig. 1) have been extensively studied due to
well established and easily accessible synthetic routes available for
these ligands. Structural modifications of these tetradentate li-
gands are achievable by introducing various peripheral substitu-
ents onto the aromatic rings such that the electronic and steric
environment around the metal centre can be readily modified. A
triphenolamine ligand with a stereogenic centre incorporated into
one of the methylene carbons has been reported recently and its
potential applications in chiral recognition and asymmetric cataly-
sis have been studied [16,17].
complexation with metals. Recently, we have reported the synthe-
sis and catalytic properties of a series of iron(III) complexes sup-
ported by this type of partially unsymmetric NO3 ligands [18].
The same class of ligands have also been deployed to prepare tita-
nium(IV) and zirconium(IV) complexes for studying catalytic poly-
merization of cyclic esters [19]. More recently, we have also
reported a closely related tripodal ligand with a NO2S donor set.
The latter was prepared by a synthetic route similar to that we
used for the partially unsymmetric tripodal NO3 ligands described
above [20].
As part of a programme to explore the coordination chemistry of
partially unsymmetric tripodal NO3 ligands, we herein report the
synthesis and characterisation of oxo-vanadium(V) and dioxo-
molybdenum(VI) complexes supported by a partially unsymmetric
NO3 ligand (L1). An iron(III) bis-( -phenoxo) dimer derived from L1
l
was also isolated and structurally characterised by X-ray crystallog-
raphy. The diiron complex possessed a Fe2O2 diamond core with a
rare five-coordinate NO4 ligand environment around each iron(III)
centre. In addition, a fully unsymmetric N-capped tripodal NO3 li-
gand (type III) has also been prepared and its coordination chemis-
try with oxo-vanadium(V) was studied. This new ligand bears three
different phenolate arms, and the difference in the substitution pat-
tern on the two benzylic pendant arms introduces an entirely asym-
metric environment around the metal centre.
We have a particular interest in unsymmetric N-capped tripodal
NO3 tetradentate ligands with two different phenol pendant arms
⇑
Corresponding authors at: Department of Inorganic, Analytical and Applied
Chemistry, University of Geneva, 30 quai Ernest Ansermet, 1211-Geneva 4,
Switzerland. Tel.: +41 22 379 60 36 (L.H. Tong).
E-mail addresses: lok.tong@unige.ch (L.H. Tong), ylwong@alumni.cuhk.edu.hk
(Y.-L. Wong).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.10.054

92
L.H. Tong et al. / Inorganica Chimica Acta 383 (2012) 91–97
Fig. 1. Three types of N-capped tripodal NO3 ligands. The coordination chemistry of the partially unsymmetric (type II) and fully unsymmetric (type III) ligands are described
in this work.
2.2.2. N-(3,5-di-tert-butyl-2-hydroxybenzyl)-N-(20-hydroxybenzyl)-
2. Experimental
N-(200-hydroxy-500-methylphenyl)amine (H3L2)
To a solution of N-(2-hydroxybenzyl)-N-(20-hydroxy-50-methyl-
2.1. General information
phenyl)amine (0.69 g, 3.0 mmol) in THF (30 cm3) at room tempera-
ture was added a solution of 3,5-di-tert-butyl-2-hydroxybenzylbr
omide (0.90 g, 3.0 mmol) in THF (30 cm3) followed by triethylamine
(1.25 cm3, 9.0 mmol). After heating under reflux overnight, the mix-
ture was cooled, and the resulting white solid was filtered and dis-
All reactions were carried out using standard Schlenk-line tech-
niques under an atmosphere of dinitrogen; workups were
performed in air. Tetrahydrofuran (THF) was distilled from so-
dium-benzophenone. Methanol (MeOH) and triethylamine (Et3N)
were distilled from magnesium methoxide and sodium, respec-
tively. Silica gel (70–230 mesh) for flash column chromatography
was purchased from Fluka. All other reagents were purchased from
Aldrich and used as received. All 1H and 13C{1H} NMR spectra were
recorded on a Varian Mercury VX300 spectrometer (1H, 300 MHz
and 13C, 75.4 MHz) using CDCl3 as solvent. Chemical shifts were rel-
ative to internal SiMe4 (d = 0). Atmospheric pressure chemical ion-
isation (APCI) mass spectra were recorded on a Hewlett–Packard
1050 Series Mass Spectrometer. Elemental analyses were per-
formed by the microanalysis laboratory of the Inorganic Chemistry
Laboratory, University of Oxford, UK. N,N-bis(2-hydroxy-3,5-di-
tert-butylbenzyl)-N-(20-hydroxy-50-methyl-phenyl)amine (H3L1)
[18] and 3,5-di-tert-butyl-2-hydroxybenzylbromide [18] were pre-
pared according to literature procedures.
carded. The brown filtrate was concentrated with
a rotary
evaporator and the residue was chromatographed on a silica gel col-
umn using CHCl3 as eluent. The second band was collected and con-
centrated to give a pale yellow solid. Yield: 0.97 g (72%). Mp: 80–
82 °C. 1H NMR: d 7.06–7.12 (m, 3H, ArH), 6.94–6.98 (m, 2H, ArH),
6.72–6.83 (m, 4H, ArH), 4.16 (s, 2H, ArCH2), 4.13 (s, 2H, ArCH2),
2.20 (s, 3H, ArCH3), 1.33 (s, 9H, tBu), 1.27 (s, 9H, tBu). 13C{1H} NMR
(CDCl3): d 154.9, 152.2, 148.5, 141.0, 135.6, 134.9, 130.9, 129.4,
129.2, 126.2, 125.5, 123.3, 122.6, 122.3, 121.3, 120.2, 116.1, 115.8,
56.7, 56.3, 34.8, 34.1, 31.7, 29.7, 20.9. MS (APCI): m/z 448 (100%)
[M+H]+. Anal. Calc. for C29H37NO3: C, 77.8; H, 8.3; N, 3.1. Found: C,
77.6; H, 8.2; N, 3.2%.
2.3. Synthesis of the complexes
2.3.1. [Fe(L1)]2 (1)
To a mixture of [H3L1] (0.56 g, 1.0 mmol) and FeCl3 (0.16 g,
1.0 mmol) in THF (20 cm3) was added triethylamine (0.42 cm3,
3.0 mmol). After heating under reflux overnight, the mixture was
cooled and filtered. The dark filtrate was concentrated with a ro-
tary evaporator. The residue was chromatographed on a silica gel
column using CHCl3/hexane (1:1) as eluent. The second band was
collected and concentrated to give a black crystalline solid. Yield:
0.36 g (30%). Anal. Calc. for C74H100Fe2N2O6: C, 72.5; H, 8.2; N,
2.3. Found: C, 72.6; H, 8.4; N, 2.6%.
2.2. Synthesis of ligands
2.2.1. N-(2-hydroxybenzyl)-N-(20-hydroxy-50-methylphenyl)amine
To a solution of 2-hydroxy-5-methylaniline (1.22 g, 10.0 mmol)
in MeOH (100 cm3) was added salicylaldehyde (1.07 cm3,
10.0 mmol). A bright orange Schiff-base product immediately
formed and the mixture was kept stirring for 1 h at room temper-
ature. NaBH4 (0.76 g, 20.0 mmol) was slowly added and stirring
was continued for a further period of 1 h. All the volatiles were re-
moved under reduced pressure and the residue was quenched with
water (50 cm3). The mixture was then neutralised with glacial ace-
tic acid followed by extraction with CH2Cl2 (3 ꢀ 30 cm3). The com-
bined extracts were dried over anhydrous MgSO4, concentrated
and pumped to dryness to give the product as a pale yellow viscous
liquid. The compound was used without further purification. Yield:
2.23 g (97%). 1H NMR (CDCl3): d 7.12–7.23 (m, 2H, ArH), 6.52–6.89
(m, 5H, ArH), 4.37 (s, 2H, ArCH2), 2.22 (s, 3H, ArCH3). 13C{1H} NMR
(CDCl3): d 156.4, 142.6, 135.4, 130.7, 128.9, 128.5, 123.1, 120.9,
120.0, 116.5, 116.1, 114.6, 48.8, 21.1. MS (APCI): m/z 230 (15%)
[M+H]+.
2.3.2. [vo(L1)] (2)
To a solution of [H3L1] (1.12 g, 2.0 mmol) in THF (40 cm3) was
added VOCl3 (0.19 cm3, 2.0 mmol) followed by triethylamine
(0.83 cm3, 6.0 mmol). The reaction mixture turned immediately
to dark blue. After stirring at room temperature overnight, the
reaction mixture was filtered and the dark blue filtrate was con-
centrated with a rotary evaporator. The residue was chromato-
graphed on a silica gel column using CH2Cl2 followed by ethyl
acetate as eluents. The second band was collected and concen-
trated to give a dark blue crystalline solid. Yield: 0.91 g (73%). 1H

L.H. Tong et al. / Inorganica Chimica Acta 383 (2012) 91–97
93
Table 1
NMR (CDCl3): d 7.22 (d, J = 2.7 Hz, 2H, ArH), 7.14 (d, J = 1.5 Hz, 1H,
Crystallographic data for complex [Fe(L1)]2 (1).
ArH), 6.97 (d, J = 2.4 Hz, 2H, ArH), 6.88 (dd, J = 1.5, 8.3 Hz, 1H, ArH),
6.52 (d, J = 8.1 Hz, 1H, ArH), 4.08 (d, J = 13.5 Hz, 2H, ArCH2), 3.92 (d,
J = 13.5 Hz, 2H, ArCH2), 2.29 (s, 3H, ArCH3), 1.54 (s, 18 H, tBu), 1.28
(s, 18H, tBu). 13C{1H} NMR: d 164.8, 160.8, 145.8, 136.9, 134.9,
132.6, 128.6, 124.7, 123.8, 122.9, 122.5, 112.9, 58.7, 35.2, 34.6,
31.6, 30.0, 21.0. MS (APCI): m/z 624 (100%) [M+H]+. Anal. Calc. for
Compound
Molecular formula
Formula weight
Temperature (K)
Colour
[Fe(L1)]2
C74H100Fe2N2O6
1225.26
150(2)
Black needle
0.32 ꢀ 0.08 ꢀ 0.06
triclinic
Crystal size (mm3)
Crystal system
Space group
a (Å)
C37H50NO4V: C, 71.2; H, 8.1; N, 2.2. Found: C, 71.5; H, 8.3; N, 2.5%.
ꢀ
P1
2.3.3. K[MoO2(L1)] (3)
10.2979(2)
11.7669(3)
14.2121(4)
88.2100(10)
83.0900(10)
82.1920(10)
1693.59(7)
2
b (Å)
A colourless mixture of [H3L1] (0.56 g, 1.0 mmol) and KOH
(0.056 g, 1.0 mmol) in MeOH (80 cm3) was added to [MoO2(acac)2]
(0.33 g, 1.0 mmol). The mixture turned immediately to orange-red.
After stirring at room temperature for 1 h, the solution was filtered
and all the volatiles were removed on a rotary evaporator. The or-
ange-red residue was redissolved in acetone (50 cm3) and the solu-
tion was stirred for 15 min. An insoluble white solid was filtered
and discarded. The filtrate was concentrated to ca. 5 cm3 and tritu-
rated with hexane, yielding an orange-red solid which was isolated
by filtration. The solid was redissolved in ethyl acetate and re-pre-
cipitated with hexane to give the product as a reddish brown solid.
The product was collected by filtration and dried in vacuo. Yield:
0.30 g (36%). 1H NMR (CDCl3): d 7.04 (d, J = 2.1 Hz, 2H, ArH), 6.70
(d, J = 2.1 Hz, 2H, ArH), 6.64 (s, 1H, ArH), 6.25 (d, J = 8.1 Hz, 1H,
ArH), 5.97 (d, J = 8.1 Hz, 1H, ArH), 4.76 (d, J = 12.6 Hz, 2H, ArCH2),
3.82 (d, J = 12.6 Hz, 2H, ArCH2), 2.03 (s, 3H, ArCH3), 1.30 (s, 18H,
tBu), 1.21 (s, 18H, tBu). 13C{1H} NMR (CDCl3): d 159.7, 158.1,
141.2, 136.3, 136.1, 127.8, 125.7, 125.0, 123.4, 122.5, 121.4, 115.3,
62.2, 34.8, 34.1, 31.7, 30.1, 20.6. IR (cmꢁ1): 3426m, 2955s, 2903m,
2865m, 1607m, 1498s, 1470m, 1442m, 1414w, 1391w, 1361m,
1294s, 1253s, 1238s, 1203m, 1168m, 1128m, 1040w, 1001w,
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Density (g/cm3)
1.201
0.480
658
l
(mmꢁ1
)
F(000)
Transmission coefficients (minimum, maximum) 0.86, 0.97
Reflections collected
Independent reflections (Rint
Observed data with I P 2
Number of parameters, p
S(GOF)
12127
6640 (0.0358)
5245
408
)
(I)
r
1.144
Final R indices [I P 2
R indices (all data)
r
(I)]
R1 = 0.0474, wR2 = 0.1280
R1 = 0.0676, wR2 = 0.1575
non-hydrogen atoms [21]. Hydrogen atoms were introduced in
their idealised positions and included in structure factors calcula-
tions with assigned isotropic temperature factors [22]. Selected
crystallographic data for 1 are shown in Table 1.
971w, 915s
m(MoO2), 891s m(MoO2), 871s, 851s, 813s, 779w,
755m, 747m, 626w, 601w, 554s, 512w, 472w. MS (ESI+): m/z 726
(25%) [M+K+H]+. Anal. Calc. for C37H50KMoNO5: C, 61.4; H, 7.0; N,
1.9. Found: C, 61.0; H, 7.2; N, 2.1%.
3. Results and discussion
3.1. Synthesis of ligands
The partially unsymmetric tripodal NO3 proligand [H3L1] was
prepared according to published procedure by the reaction of 2-hy-
droxy-5-methylaniline and 3,5-di-tert-butyl-2-hydroxybenzyl bro-
mide in the presence of triethylamine [18]. The new fully
unsymmetric proligand [H3L2] was synthesised by a two-step ap-
proach as outlined in Scheme 1. Treatment of 2-hydroxy-5-methy-
laniline with salicylaldehyde in methanol afforded the
corresponding Schiff-base product as a bright orange precipitate.
The latter was immediately reduced in situ with NaBH4 to give the
corresponding disubstituted amine N-(2-hydroxybenzyl)-N-(20-hy-
droxy-50-methylphenyl)amine in 97% yield. Subsequent reaction of
this disubstituted amine with 3,5-di-tert-butyl-2-hydroxybenzylbr-
omide in the presence of triethylamine gave proligand [H3L2] in 72%
yield.
2.3.4. [vo(L2)] (4)
This compound was synthesised from [H3L2] (0.89 g, 2.0 mmol),
VOCl3 (0.19 cm3, 2.0 mmol), and triethylamine (0.83 cm3, 6.0 mmol)
in THF (40 cm3) using a similar procedure as described above for 2.
The desired product was isolated as a dark blue solid. Yield: 0.77 g
(75%). 1H NMR (CDCl3): d 7.27 (d, J = 2.4 Hz, 1H, ArH), 7.20 (d,
J = 1.5 Hz, 1H, ArH), 7.13 (d, J = 2.1 Hz, 1H, ArH), 6.73–6.78 (m, 2H,
ArH), 6.44–6.49 (m, 3H, ArH), 6.03 (d, J = 8.1 Hz, 1H, ArH), 5.43 (d,
J = 15 Hz, 1H, ArCH2), 4.81 (d, J = 15 Hz, 1H, ArCH2), 4.69 (d,
J = 11.7 Hz, 1H, ArCH2), 3.46 (d, J = 11.7 Hz, 1H, ArCH2), 2.26 (s, 3H,
ArCH3), 1.43 (s, 9 H, Bu), 1.29 (s, 9H, Bu). 13C{1H} NMR: d 162.0,
159.8, 154.5, 143.8, 139.2, 134.6, 132.8, 130.2, 129.6, 128.8, 123.5,
123.0, 121.8, 121.0, 120.5, 116.4, 115.2 (one peak overlapping),
64.0 (one peak overlapping), 35.8, 35.0, 32.5, 30.2, 21.8. MS (APCI):
m/z 512 (100%) [M+H]+. Anal. Calc. for C29H34NO4V: C, 68.1; H, 6.7;
N, 2.7. Found: C, 68.2; H, 6.8; N, 3.0%.
t
t
3.2. Synthesis and coordination chemistry of iron(III), oxo-
vanadium(V) and dioxo-molybdenum(VI) complexes
2.4. X-ray crystallographic analysis
The coordination chemistry of tripodal ligands (L1) and (L2) with
vanadium, molybdenum and iron were studied in this work. Re-
cently, we have reported on the synthesis of a mononuclear iron(III)
complex derived from (L1) [18]. This complex was prepared by the
reaction of FeCl3 and [H3L1] in the presence of triethylamine and 1-
methylimidazole (Im). The solid-state structure of [Fe(L1)(Im)] was
established by X-ray crystallography. The Fe(III) centre was bound
by one (L1) ligand and one imidazole molecule, which formed a
five-coordinate ligand environment around the metal centre. The
imidazole nitrogen occupies the fifth coordination site in the axial
position trans to an apical nitrogen of the tripodal ligand. The coor-
dination chemistry of (L1) with Fe(III) was re-examined in the
Single crystals of [Fe(L1)]2 (1) were obtained by slow evapora-
tion of a THF solution of the complex. Crystals suitable for X-ray
diffraction analysis were covered with perfluoropolyether oil,
mounted on glass capillaries, and cooled rapidly to 150 K in a
stream of cold N2 using an Oxford Cryosystems CRYOSTREAM unit.
Diffraction data were measured using an Enraf–Nonius KappaCCD
diffractometer with graphite-monochromated Mo K
a radiation
(k = 0.71073 Å). Computations were performed using the SHELX-97
PC programme package on a PC computer. The structures were
solved by direct phase determination and refined by full-matrix
least squares with anisotropic thermal parameters for the

94
L.H. Tong et al. / Inorganica Chimica Acta 383 (2012) 91–97
Scheme 1. Synthesis of the new fully unsymmetric N-capped tripodal NO3 proligand [H3L2].
absence of imidazole and, interestingly, a dimeric complex [Fe(L1)]2
(1) was isolated in 30% yield (Scheme 2). Fig. 2 shows the solid-state
structure of 1 as determined by X-ray diffraction analysis. Selected
bond lengths and angles are given in Table 2.
trianionic pentadentate ligands containing a bis-(l-phenoxo)-
bridge has been reported [23]. The Fe(III) centre in the latter
complex exhibits a distorted octahedral geometry with a N2O4
coordination environment. A number of Fe(III) calixarene com-
plexes have recently been published [24] but to our knowledge,
the uncommon NO4 ligand environment around the iron(III) centre
in a phenoxo-bridged iron(III) complex has only been reported pre-
viously in a dimeric calix[4]arene complex [25]. The Fe(III) centre
in the calixarene complex exhibited a distorted trigonal pyramidal
geometry with an FeꢂꢂꢂFe separation (3.258(2) Å) almost identical
to that observed in 1.
ꢀ
Compound 1 crystallises triclinic in the space group P1. The di-
meric complex is located at a crystallographic centre of inversion.
The iron atoms (Fe(1) and Fe(1A)⁄) are linked by two
l
-coordinated
phenoxo oxygen atoms O(1) and O(1A)⁄, forming a planar Fe2O2
diamond core ( bond angles = 360°). The Fe–O bond distances
R
are 2.005(1) (Fe(1)–O(1)) and 2.031(1) (Fe(1)–O(1A)), whereas
the Fe(1)–O(1)–Fe(1A) and O(1)–Fe(1)–O(1A) angles are
107.34(8)° and 72.66(8)°, respectively. The Fe(1)ꢂꢂꢂFe(1A)⁄ distance
is 3.252 Å. It is noteworthy that the two phenolate oxygens (O(1)
and O(1A)⁄) belong to the ‘‘ortho-substitution-free’’ phenolate pen-
dants. An identical structural feature has been observed in the so-
lid-state structure of a dimeric zirconium(IV) complex derived
from the same ligand [19].
Each iron(III) centre in complex 1 has a five-coordinate NO4
coordination environment. The coordination geometry around
the iron centres can be described as distorted trigonal bipyramidal
with the equatorial plane being occupied by phenolate oxygen
atoms (O(1), O(2) and O(3) for Fe(1)), whereas the two axial sites
are occupied by a bridge-head nitrogen (N(1) for Fe(1)) and a
bridging oxygen atom (O(1A) for Fe(1)). The observed N(1)–
Fe(1)–O(1A) angle is 151.27(8)°. The iron atom is sitting slightly
above the equatorial plane (ca. 0.09 Å), leaning toward the bridging
phenoxo oxygen atom O(1A). The two disubstituted benzyl pheno-
late pendants on each (L1) ligand form two six-membered chelate
rings with the metal, whilst the remaining phenolate pendant on L1
forms a five-membered metallacyclic ring. The geometric features
of the five-membered ring display differences with respect to those
in the six-membered systems as observed previously in the closely
related mononuclear iron(III) complex [Fe(L1)(Im)] [18]. All the
equatorial Fe–O and the axial Fe–N bond distances and angles ob-
served in 1 are comparable to those in [Fe(L1)(Im)].
Reaction of [H3L1] with oxo-vanadium(V) trichloride in the
presence of triethylamine in a THF solution afforded the corre-
sponding oxo-vanadium(V) complex [VO(L1)] (2) in 73% yield
(Scheme 2). The 1H NMR spectrum of 2 shows that the methylene
protons on the (L1) ligand occur as discrete diastereotopic doublets
at 4.08 and 3.92 ppm with a coupling constant of 13.5 Hz (Fig. 3). A
number of oxo-vanadium(V) complexes supported by symmetric
amine triphenolate ligands have been reported and found to be
catalytically active towards ethylene polymerization [26].
Treatment of [MoO2(acac)2] with [H3L1] and potassium hydrox-
ide in methanol yielded the dioxo-molybdenum(VI) complex salt
K[MoO(L1)] (3) in 36% yield (Scheme 2). The IR spectrum of the di-
oxo complex showed two strong absorptions at 915 and 891 cmꢁ1
,
which are assignable to symmetric and asymmetric Mo@O
stretches, respectively, in a cis-dioxo moiety [27,28]. The 1H and
13C NMR spectra of 3 are consistent with the proposed octahedral
structure of the complex as shown in Scheme 2. Complex 3 pos-
sesses an internal mirror plane and upon complexation the two
methylene protons on the phenolate pendants are no longer equiv-
alent as expected: the 1H NMR spectrum of 3 showed two doublets
at d 4.76 and d 3.82 ppm with a germinal coupling constant of
12.6 Hz (Fig. 3). In fact, the 1H NMR spectrum of 3 is similar to that
of 2 as both complexes possess two diastereotopic methylene pro-
tons. Attempts to obtain single crystals of 3 suitable for X-ray dif-
fraction analysis have been unsuccessful, but the solid-state
Reports on structurally characterised bis-(
l-phenoxo)-bridged
diiron(III) complexes are rare. A diiron(III) complex supported by
structures of
a number of dioxo-molybdenum(VI) complexes

L.H. Tong et al. / Inorganica Chimica Acta 383 (2012) 91–97
95
Scheme 2. Synthesis of complexes 1–4 derived from unsymmetric N-capped tripodal NO3 proligands H3L1 and H3L2.
bound by symmetric amine triphenolate ligands have been re-
ported [5,6].
(Fig. 3). To the best of our knowledge, complex 4 is the first metal
complex coordinated by an N-capped tripodal ligand containing
three different phenolic pendant arms. Complexes 2–4 were also
characterised by mass spectrometry. The APCI and ESI spectra of
these complexes showed the respective [M+H]+ and [M+K+H]+
peaks.
In summary, we have reported the coordination chemistry of a
partially unsymmetric N-capped tripodal NO3 ligand with iron(III),
oxo-vanadium(V) and dioxo-molybdenum(VI). The synthesis and
characterisation of an oxo-vanadium(V) complex supported by a
The coordination chemistry of the fully unsymmetric proligand
[H3L2] with VOCl3 was also examined. Complex [VO(L2)] (4) was
obtained in 75% yield, using a similar procedure as described for
2 (Scheme 2). The 1H NMR spectrum of 4 elicited four doublets sig-
nals (falling within the range of d 5.43–3.46 ppm), which are
assignable to two types of chemically inequivalent methylene
groups present on the (L2) ligand. These methylene protons be-
come diastereotopic upon complexation of (L2) to the metal

96
L.H. Tong et al. / Inorganica Chimica Acta 383 (2012) 91–97
Fig. 2. ORTEP representation (50% thermal ellipsoid) showing the molecular structure of [Fe(L1)]2 (1) with atom labeling. Only one orientation of the twofold disordered tert-
butyl groups is shown. Hydrogen atoms are omitted for clarity.
Table 2
Selected bond distances (Å) and angles (°) for
complex [Fe(L1)]2 (1).
[Fe(L1)]2
Fe(1)–O(1)
2.005(1)
2.031(1)
1.857(1)
1.861(1)
2.204(2)
2.031(1)
72.66(8)
130.70(8)
102.68(8)
115.40(8)
104.59(8)
113.21(9)
79.20(7)
151.27(8)
91.22(8)
92.42(8)
107.34(8)
Fe(1)–O(1A)⁄
Fe(1)–O(2)
Fe(1)–O(3)
Fe(1)–N(1)
Fe(1A)⁄–O(1)
O(1)–Fe(1)–O(1A)⁄
O(1)–Fe(1)–O(2)
O(1A) –Fe(1)–O(2)
⁄
O(1)–Fe(1)–O(3)
O(1A)⁄–Fe(1)–O(3)
O(2)–Fe(1)–O(3)
O(1)–Fe(1)–N(1)
O(1A)⁄–Fe(1)–N(1)
O(2)–Fe(1)–N(1)
O(3)–Fe(1)–N(1)
Fe(1)–O(1)–Fe(1A)⁄
Fig. 3. 1H NMR spectra (300 MHz, CDCl3) of (a) oxo-vanadium(V) complex 2, (b)
dioxo-molybdenum(VI) complex 3 and (c) oxo-vanadium(V) complex 4.
new, fully unsymmetric N-capped tripodal NO3 ligand featuring
three different phenolic arms have also been described. We believe
that the various spatial requirements of each pendant arm of the
totally unsymmetric environment of the ligand provides different
steric environment around the metal centre. This can be consid-
ered as a plausible way for controlling routes of substrates to ap-
proach a catalytically active metal centre when a metal complex
of interest is stabilised by a fully unsymmetric ligand. The fact that
complex 4 is prochiral due to the intrinsic asymmetry of the ligand
suggests that the vanadium complex as formed is racemic but it
might be possible to resolve the enantiomers using a suitable chiral
column. It would have to be seen if the asymmetry at the metal
reaction site is sufficient to orientate the substrate appropriately
to give an excess of one isomer. We envisage possible applications
with the new tripodal ligand in obtaining asymmetric catalysts.
Acknowledgements
We thank the Croucher Foundation (Dr. Yee-Lok Wong) for
financial support. We are grateful to Dr. Andrew Cowley and Dr.
Chi-Keung Lam for crystallographic assistance.
Appendix A. Supplementary material
CCDC 808981 contains the supplementary crystallographic data
for for [Fe(L1)]2 (1). These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.ica.2011.10.054.

L.H. Tong et al. / Inorganica Chimica Acta 383 (2012) 91–97
97
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