New molybdenum(v) analogues of Amavadin and their redox properties

Article abstract of DOI:10.1039/b102531g

Reactions of [MoO₂(acetylacetonate)₂] with the proligands (N-hydroxyimino)diacetic acid (H₃hidpa), R,R-2,2′-(N-hydroxyimino)dipropionic acid (R,R-H₃hidpa) or R,S-2,2′-(N-hydroxyimino)dibutyric acid (R,S-H₃hidba) yielded the compounds [PPh₄][δ,Λ-Mo(hida)₂]·CH₂Cl ₂ 1, [H₅O₂][Δ-Mo(R,R-hidpa)₂] 2, [PPh₄][Mo(R,S-hidba)₂]. 2H₂O 3a and Na[δ,Λ-Mo(R,S-hidba)₂]·1/4ⁱPr₂ O 3b, respectively. Reactions of H₃hida with a methanolic solution of [PPh₄][MoOCl₄(H₂O)] in the presence of NaOH (ca. pH 8) provided an alternative synthesis for 1. The complex of 1 when transferred into CH₂Cl₂ using [PPh₄]Br yielded brown block-like crystals from a CH₂Cl₂-EtOH solution, however, 2 and 3b were crystallised from H₂O and MeCN solutions with [H₅O₂]⁺ and [Na]⁺ counter cations, respectively. X-Ray crystallography confirmed the same distinctive eight-co-ordinate geometry of the complex anions of 1, 2 and 3b as identified for Amavadin, the form in which vanadium(IV) is bound in Amanita muscaria mushrooms. EPR and UV/vis spectra recorded for 1, 2 and 3a are consistent with the presence of molybdenum(v). Cyclic voltammetric studies using a glassy carbon working electrode in CH₂Cl₂ for 1 exhibited a reversible Mo^VI/Mo^v and a quasi-reversible Mo^V/Mo^IV redox couple at E_1/2 = +0.96 and -0.99 V (vs. a saturated calomel electrode), respectively. Complex 3a also displayed a reversible Mo^VI /Mo^V redox couple at E_1/2 = +0.77 V, whereas the Mo^V /Mo^IV couple was irreversible (E_pc = - 1.28 V). Additional electrochemical studies with 2 recorded a reversible Mo^VI /Mo^V redox couple in Me₂SO (E_1/2 = +0.77 V), however in H²O this one-electron oxidation process is irreversible.

Full text of DOI:10.1039/b102531g

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New molybdenum(V) analogues of Amavadin and their redox
properties
Paul D. Smith,a J. Jon A. Cooney,a Eric J. L. McInnes,b Roy L. Beddoes,a David Collison,*a
Spencer M. Harben,a Madeleine Helliwell,a Frank E. Mabbs,b Alexander Mandel,c
Annie K. Powellc and C. David Garner*d
a Department of Chemistry, The University of Manchester, Oxford Road, Manchester,
UK M13 9PL. E-mail: david.collison@manchester.ac.uk
b EPSRC CW EPR Service Centre, Department of Chemistry, The University of Manchester,
Oxford Road, Manchester, UK M13 9PL
c School of Chemical Sciences, The University of East Anglia, Norwich, UK NR4 7TJ
d School of Chemistry, The University of Nottingham, University Park, Nottingham,
UK NR2 2RD
Received 19th March 2001, Accepted 11th July 2001
First published as an Advance Article on the web 27th September 2001
Reactions of [MoO2(acetylacetonate)2] with the proligands (N-hydroxyimino)diacetic acid (H3hidpa), R,R-2,2Ј-
(N-hydroxyimino)dipropionic acid (R,R-H3hidpa) or R,S-2,2Ј-(N-hydroxyimino)dibutyric acid (R,S-H3hidba)
yielded the compounds [PPh4][∆,Λ-Mo(hida)2]ؒCH2Cl2 1, [H5O2][∆-Mo(R,R-hidpa)2] 2, [PPh4][Mo(R,S-hidba)2]ؒ
2H2O 3a and Na[∆,Λ-Mo(R,S-hidba)2]ؒ¼iPr2O 3b, respectively. Reactions of H3hida with a methanolic solution
of [PPh4][MoOCl4(H2O)] in the presence of NaOH (ca. pH 8) provided an alternative synthesis for 1. The complex
of 1 when transferred into CH2Cl2 using [PPh4]Br yielded brown block-like crystals from a CH2Cl2–EtOH solution,
however, 2 and 3b were crystallised from H2O and MeCN solutions with [H5O2]ϩ and [Na]ϩ counter cations,
respectively. X-Ray crystallography confirmed the same distinctive eight-co-ordinate geometry of the complex
anions of 1, 2 and 3b as identified for Amavadin, the form in which vanadium() is bound in Amanita muscaria
mushrooms. EPR and UV/vis spectra recorded for 1, 2 and 3a are consistent with the presence of molybdenum().
Cyclic voltammetric studies using a glassy carbon working electrode in CH2Cl2 for 1 exhibited a reversible MoVI/MoV
and a quasi-reversible MoV/MoIV redox couple at E1/2 = ϩ0.96 and Ϫ0.99 V (vs. a saturated calomel electrode),
respectively. Complex 3a also displayed a reversible MoVI/MoV redox couple at E1/2 = ϩ0.77 V, whereas the MoV/MoIV
couple was irreversible (Epc = Ϫ1.28 V). Additional electrochemical studies with 2 recorded a reversible MoVI/MoV
redox couple in Me2SO (E1/2 = ϩ0.77 V), however in H2O this one-electron oxidation process is irreversible.
Introduction
Mushrooms of the genus Amanita accumulate vanadium to
concentrations of up to 400 mg kgϪ1 (dry weight)1 in the
form of the discrete moiety Amavadin,2 a 1 : 2 complex of
vanadium() with the proligand S,S-N-hydroxyimino-
dipropionic acid (S,S-H3hidpa = HON{CH(CH3)CO2H}2).3–5
The presence of VIV is clearly indicated by EPR spectroscopy6
and the metal centre can be reversibly oxidised to the VV level.7
Thus this species belongs to the group of transition metal
centres in biology which exhibit reversible, one-electron, redox
behaviour. This redox behaviour may account for the biological
role of Amavadin and, in solution, has been shown to mediate
the oxidation of glutathione and other biologically active
thiols.8,9 Recent studies5,10,11 into the chemical nature of
Amavadin have established a novel eight-co-ordinate geometry,
with each S,S-H3hidpa proligand co-ordinated via an η2-NO
group and two unidentate carboxylate groups. Complementary
work 10,12,13 has included the analogues from N-hydroxy-
Scheme 1 Isomerism at the vanadium centre.
through the metal atom. The special co-ordination geometry
and chemical behaviour of Amavadin has generated curiosity
as to whether it is possible to extend this chemistry to
other metals. Therefore, studies of the reactivity of
H3hidpa towards other metal centres, especially those found in
biological systems, were initiated. The synthesis and character-
isation of [PPh4][∆-Mo(R,R-hidpa)(R,S-hidpa)] and its enan-
tiomer,14 that involve molybdenum() bound to two mutually
trans η2-NO groups and four unidentate carboxylates, was
the first example of a chemical analogue of Amavadin. This
complex is capable of both a one-electron reversible oxidation
and a one-electron reversible reduction in CH2Cl2. Other
developments, notably studies of [M(hidca)2]Ϫ/2Ϫ (H3hidca =
N-hydroxyiminodicarboxylic acid, HON{CH(R)CO2H}2, for
R = alkyl) have been reported,15 and the chemistry described
herein has led to several new molybdenum complexes related to
Amavadin.
iminodiacetic acid (H3hida
= HON{CH2CO2H}2) which
provide further examples of this distinctive eight-co-ordinate
structure. This co-ordination environment leads to chirality at
the vanadium (Scheme 1) and the isolated natural product
Amavadin consists of an approximately equimolar mixture of
the ∆- and Λ-helical forms of [V(S,S-hidpa)2]2Ϫ 10,11
These
.
complexes are assigned idealised C2 point symmetry in solu-
tion, where the two-fold axis bisects angles between the
normals to the {VNO} plane from each ligand projected
3108
J. Chem. Soc., Dalton Trans., 2001, 3108–3114
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b102531g

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under vacuum. Yield, 380 mg, 47% (Found: C, 48.74; H, 3.78;
Experimental
N, 3.54; Mo, 11.65; P, 3.9%. Calc. for C33H30N2O10Cl2MoP: C,
48.71; H, 3.72; N, 3.44; Mo, 12.04; P, 3.81%). IR (KBr, cmϪ1):
Reagents and apparatus
1690 [ν(C᎐O)], 1353 [ν(C–O)], 1108 [ν(N–O)]. UV/Vis (MeCN):
᎐
All reagent and solvents were obtained from normal com-
mercial sources and were used without further purification
unless otherwise stated. Tetrahydrofuran (thf ) and Et2O were
distilled from sodium; CH2Cl2 and MeCN were distilled from
CaH2, and all solvents were stored under dinitrogen. The pro-
ligands H3hida and R,R-H3hidpa were synthesised using a
modification of the method16 described by Felcman et al.,17 and
purification was carried out using the procedure outlined by
Koch et al.18 R,S-H3hidba (R,S-N-hydroxyiminodibutyric acid
= HON{CH(C2H5)CO2H}2) was synthesised as described by
Smith et al.15 [MoO2(acetylacetonate)2] was synthesised as
described by Jones,19 by treatment of an aqueous solution of
Na2[MoO4] with HCl (ca. pH 1) and acetylacetone to produce a
yellow precipitate. [PPh4][MoOCl4(H2O)] was prepared as a
green crystalline product from a solution of [MoCl5] in conc.
HCl in the presence of [PPh4]Cl.20 Chemical analyses were per-
formed by The University of Manchester, Microanalytical
Laboratory. IR spectra were recorded on a Perkin-Elmer 1710
FT spectrometer. Electronic absorption spectra were recorded
on a Varian Cary 1E UV/vis spectrophotometer. Mass spectra
were recorded using a KRATOS concept 1S spectrometer
(matrix NBA). X-Band (ca. 9.5 GHz) EPR spectra were
recorded on a Bruker ESP300E spectrometer.
ν/cmϪ1 (ε/molϪ1 dm3 cmϪ1) 26455 (103) and 18181 (19). Mass
spectrum (negative-ion FAB): m/z 390 = [Mo(hida)2]Ϫ.
Method 2. [PPh4][MoOCl4(H2O)] (611 mg, 1.00 mmol) was
dissolved in acetone (15 cm3) and dinitrogen bubbled through
the solution. H3hida (315 mg, 2.10 mmol) was dissolved in
MeOH (6 cm3) and the pH (as measured by Whatman paper
3 pH 1–11 colour indicator paper) adjusted to 8 with NaOH
(5 mol dmϪ3). On addition of the H3hida solution to that of
[PPh4][MoOCl4(H2O)] an instant colour change occurred and a
brown precipitate formed. The reaction mixture was stirred
(for ca. 10 min) and then the solvent removed under vacuum.
The resulting residue was extracted into CH2Cl2 (10 cm3), dried
over anhydrous CaCl2 (for ca. 1 h) and crystals were obtained
from this solution as described in Method 1. Yield, 350 mg,
43% (Found: C, 48.12; H, 3.66; N, 3.38%. Calc. for C33H30N2-
O10Cl2MoP: C, 48.71; H, 3.72; N, 3.44%).
[H5O2][ꢀ-Mo(R,R-hidpa)2] 2. [MoO2(acetylacetonate)2] (326
mg, 1.00 mmol) was added to a solution of R,R-H3hidpa
(431 mg, 2.10 mmol) in H2O (10 cm3). The reaction mixture
was stirred (for ca. 3 h) at 323 K, whereupon a brown solution
formed, which was allowed to cool (to ca. 293 K) and was
filtered. This solution was concentrated by evaporation under
vacuum (2 cm3) and cooled (to ca. 273 K) then added to cold
(ca. 273 K) thf (20 cm3) and a brown precipitate formed that
was collected and dried under vacuum. Brown block-like
crystals suitable for X-ray diffraction were obtained by slow
evaporation of an aqueous solution of the precipitate at 293 K.
Yield, 240 mg, 52% (Found: C, 30.4; H, 4.27; N, 5.78; Mo,
21.67%. Calc. for C12H21N2O12Mo: C, 29.81; H, 4.38; N, 5.8;
Electrochemical measurements were made with a PAR model
175 waveform generator, a model 173 potentiostat, and a PAR
electrochemistry cell with
a three-electrode configuration
consisting of a glassy carbon working electrode, a saturated
calomel reference electrode (SCE) and a platinum wire second-
ary electrode. Data were recorded on an Advance Bryans Series
6000 XY/t recorder. The cyclic voltammograms were recorded
for solutions of compound (ca. 1 mmol dmϪ3) with [NBun ]-
4
Mo, 20.27%). IR (KBr, cmϪ1): 1650 [ν(C᎐O)], 1366 [ν(C–O)],
[BF4] (ca. 0.2 mol dmϪ3) as supporting electrolyte in non-
᎐
1121 [ν(N–O)]. UV/Vis (H2O): ν/cmϪ1 (ε/molϪ1 dm3 cmϪ1) 26041
(61) and 18181 (15). Mass spectrum (negative-ion FAB): m/z
444 = [Mo(hidpa)2]Ϫ.
aqueous media, prepared from Na[BF4] and [NBun ][HSO3]
4
and recrystallised from toluene.21 KCl (ca. 0.2 mol dmϪ3) was
used as supporting electrolyte for cyclic voltammograms
recorded in H2O. Controlled potential electrolysis (CPE)
experiments were carried out in a two-compartment cell separ-
ated by Vycor porous glass. The working electrode consisted of
a cube of reticulated vitreous carbon, purchased from The
Electrosynthesis Co, Inc., Lancaster, NY 14086, and the
secondary electrode was platinum mesh. All solutions were
deoxygenated by bubbling dinitrogen through them for several
minutes prior to use and all voltammograms were recorded
with solutions under a dinitrogen atmosphere at 293 K. An
optically transparent thin layer electrode (OTTLE) was sup-
ported in a cell and poly(tetrafluoroethylene) block from the
design developed by Heath et al.22 All electrochemical poten-
tials were measured relative to SCE and were corrected for
liquid-junction potentials in non-aqueous media via the use of
the ferrocenium–ferrocene couple as an internal redox standard
([(Cp)2Fe]ϩ/[(Cp)2Fe]; ∆E = 65 mV, ipa : ipc = 0.96).23
[PPh4][Mo(R,S-hidba)2]ؒ2H2O 3a. [MoO2(acetylacetonate)2]
(326 mg, 1.00 mmol) was added to a solution of R,S-H3hidba
(431 mg, 2.1 mmol) in MeOH (10 cm3) and the reaction mixture
was stirred (for ca. 18 h) at 293 K, whereupon a brown solution
formed. [PPh4]Br (419 mg, 1.00 mmol) was added and the solu-
tion evaporated to dryness under vacuum. The resulting residue
was extracted into thf (10 cm3) and filtered to remove any excess
[PPh4]Br. This solution was cooled (to 275 K) and added drop-
wise into cold (275 K) Et2O (50 cm3) and produced a brown
precipitate. The solid product was very hygroscopic and there-
fore was collected in a dinitrogen atmosphere, washed with
Et2O (10 cm3), dried under vacuum and stored under dinitrogen
over silica-gel. Yield, 605 mg, 72% (Found: C, 53.75; H, 5.19; N,
3.17; Mo, 10.80; P, 3.28%. Calc. for C40H48N2O12MoP: C, 54.72;
H, 5.51; N, 3.19; Mo, 11.16; P, 3.53%). IR (KBr, cmϪ1): 1675
[ν(C᎐O)], 1459 [ν(C–O)], 1131 [ν(N–O)]. UV/Vis (MeCN):
᎐
ν/cmϪ1 (ε/molϪ1 dm3 cmϪ1) 26595 (94) and 17730 (10). Mass
spectrum (negative-ion FAB): m/z 502 = [Mo(hidba)2]Ϫ.
Syntheses
[PPh4][ꢀ,ꢁ-Mo(hida)2]ؒCH2Cl2 1. Method 1. [MoO2(acetyl-
acetonate)2] (326 mg, 1.00 mmol) was added to a solution of
H3hida (315 mg, 2.10 mmol) in H2O (10 cm3). The reaction
mixture was stirred (ca. 3 h) at 323 K, whereupon a dark
brown–purple solution formed. After cooling (to ca. 293 K), a
solution of [PPh4]Br (419 mg, 1.00 mmol) in CH2Cl2 (10 cm3)
was mixed with the aqueous phase and after rapid stirring (for
ca. 30 min) the dark brown–purple colour transferred into the
CH2Cl2 layer. The CH2Cl2 phase was separated from the aque-
ous layer and dried over anhydrous CaCl2 (for ca. 1 h), filtered
and then EtOH (10 cm3) was added. Brown–purple block-like
crystals were obtained by slow evaporation (ca. 48 h) at 293 K
from the resulting solution, and these were collected and dried
Na[ꢀ,ꢁ-Mo(R,S-hidba)2]ؒ¼iPr2O 3b. [MoO2(acetylaceto-
nate)2] (326 mg, 1.00 mmol) was added to a solution of R,S-
H3hidba (431 mg, 2.10 mmol) in MeOH (10 cm3), previously
treated with NaOH (40 mg, 1.0 mmol). This reaction mixture
was stirred (for ca. 18 h) at 293 K, whereupon a brown solution
formed and was evaporated to dryness under reduced pressure.
The resulting residue was extracted into MeCN (10 cm3) and
red block-like crystals suitable for X-ray diffraction were
obtained from this solution via the slow vapour diffusion tech-
i
nique using Pr2O at 273 K. Yield, 285 mg, 51% (Found: C,
34.81; H, 5.17; N, 5.07; Mo, 17.77; Na, 4.17%. Calc. for C16H24-
N2O10MoNaؒ2H2O: C, 34.35; H, 5.01; N, 5.00; Mo, 17.17; Na,
J. Chem. Soc., Dalton Trans., 2001, 3108–3114
3109

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Table 1 Crystal data for compounds 1, 2 and 3b
Empirical formula
Formula weight
C33H30Cl2MoN2O10P (1)
812.43
C12H21MoN2O12 (2)
481.25
C39H55N4O20.5Mo2Na2 (3b)
547.32
Crystal colour, habit
Crystal dimensions/mm
Crystal system
a/Å
b/Å
c/Å
Brown, block
0.20 × 0.25 × 0.35
Monoclinic
7.96(1)
28.043(6)
15.47(1)
Brown, block
0.8 × 0.3 × 0.3
Orthorhombic
10.3610(10)
13.9210(10)
6.8470(10)
—
Red, block
0.17 × 0.17 × 0.25
Orthorhombic
28.23(1)
30.29(1)
11.64(1)
β/Њ
91.54(8)
3451(5)
—
9956(17)
V/Å3
987.6(2)
Space group
P21/c (no. 14)
4
1.564
Cu-Kα, 55.76
Rigaku AFC5R
1.54178
P21212 (no. 18)
2
1.618
Mo-Kα, 7.25
Rigaku AFC7R
0.71073
Fddd (no. 70)
16
1.460
Cu-Kα, 49.33
Rigaku AFC5R
1.54178
Z
Dcalc/g cmϪ3
µ/cmϪ1
Diffractometer
λ/Å
T /K
Scan type
2θmax/Њ
296
ω–2θ
120.1
1
293
ω
60
2
293
ω–2θ
158.9
1
No. of reflections measured
Total: 5679
Unique: 5257
(Rint = 0.046)
Lorentz-polarisation
Absorption (DIFABS)24
3765 [I > 3.00σ(I )]
442
Total: 1674
Total:2857
Unique: 2857
Unique: 1674
(Rint = 0.0)
Lorentz-polarisation
Corrections
Lorentz-polarisation
Absorption (ψ-scans)
1583 [I > 2.00σ(I )]
155
No. of observations
No. of variables
Residuals
1674 [I > 2.00σ(I )]
125
R = 0.0279; Rw = 0.0749 [I > 2σ(I )]
R = 0.0297; Rw = 0.0760 (all data)
1.088
R = 0.098; Rw = 0.130
R = 0.064; Rw = 0.053
Goodness of fit indicator
3.58
1.94
4.11%). UV/Vis (H2O): ν/cmϪ1 (ε/molϪ1 dm3 cmϪ1) 24943 (60)
and 17841 (13). Mass spectrum (negative-ion FAB): m/z 502 =
[Mo(hidba)2]Ϫ.
X-Ray crystallography
A summary of the crystallographic information obtained for 1,
2 and 3b is provided in Table 1. The structures were solved by
direct methods using SIR24,25a for 1 and SHELXS25a for 2 and
3b. Non-hydrogen atoms were refined anisotropically. Hydro-
gen atoms were included in the structure factor calculations26 in
idealised positions (C–H = 0.95 Å) and were assigned isotropic
thermal parameters that were 20% greater than the equivalent B
value of the atom to which they were bonded. For 1 and 3b all
calculations were performed using the TEXSAN27a crystallo-
graphic software package. For 2 the data were refined against
F 2 using SHELXL93.27b
Fig. 1 ORTEP39 representation of the structure of the anion present
in 1.
CCDC reference numbers 168519–168521.
See http://www.rsc.org/suppdata/dt/b1/b102531g/ for crystal-
lographic data in CIF or other electronic format.
cell. The Mo–O and Mo–N bond lengths are similar to those
in [∆-Mo(R,R-hidpa)(R,S-hidpa)]Ϫ14 (Table 2), and are each
slightly longer (range 0.13–0.02 Å; av. 0.07 Å) than their
counterparts in [V(hida)2]Ϫ.10 The planes of the two {MoNO}
groups are perpendicular (90.04Њ, as measured from N to O,
see Scheme 1) and each is effectively perpendicular (88.92;
90.49Њ) to the least-squares plane of the molybdenum and
the four-co-ordinated carboxylate oxygen atoms (the “equa-
torial” plane). These oxygen atoms are significantly displaced
from this least-squares plane; O(1) and O(5) sit below (Ϫ0.42
and Ϫ0.38 Å, respectively) and O(6) and O(10) sit above (0.36
and 0.39 Å, respectively) the plane, which contains the
molybdenum.
Initial reactions of [PPh4][MoOCl4(H2O)] with H3hidpa
resulated in fragmentation of the proligand and yielded a
product which contained a novel 2-(oxyimino)propionic acid
moiety, [PPh4][MoO2Cl2(NOC(Me)CO2H)].28 X-Ray crystal-
lography showed that the molybdenum() was bound to two
oxo groups, two chlorides, and the [NOC(Me)CO2H] fragment
co-ordinated via the nitrogen and an oxygen atom in the
{CO2H} group. However, when this reaction was repeated
under basic conditions (see Experimental section), facile
chloride abstraction was promoted and the eight-co-ordinate
Results and discussion
The values for the Mo analysis are reasonably close to the
expected values, however the C, H, N, Na and P are closer and
the spectral and crystallographic data are consistent with the
formulae given.
Crystallography
After the initial investigation with vanadium5,10–13 and recent
results with molybdenum,14 herein we report the structures of
further examples of molybdenum() analogues of Amavadin. 1
forms brown block-like crystals in the space group P21/c (no.
14) (Table 1). As expected, the complex anion exhibits the eight-
co-ordinate geometry found in Amavadin containing a 1 : 2
complex of molybdenum() with hida3Ϫ, bound by two mutu-
ally trans η2-NO groups and four unidentate carboxylates, see
Fig. 1. The asymmetric unit contains half the complex anion
and a [PPh4]ϩ counter cation with a molecule of CH2Cl2. The
anion was refined as the ∆-helical form at the molybdenum()
centre. The space group has a centre of symmetry; therefore the
equivalent Λ-isomer at the metal centre is generated in the unit
3110
J. Chem. Soc., Dalton Trans., 2001, 3108–3114

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Table 2 Comparison of selected bond lengths (Å) and angles (Њ) for 1, 2, 3b and [PPh4][Mo(R,R-hidpa)(R,S-hidpa)] 14
1
2a,b
3ba
[PPh4][∆-Mo(R,R-hidpa)(R,S-hidpa)]
Mo(1)–O(1)
Mo(1)–O(5)
Mo(1)–O(6)
Mo(1)–O(10)
Mo(1)–O(3)
Mo(1)–O(8)
Mo(1)–N(1)
Mo(1)–N(2)
2.01(1)
2.04(1)
2.038(9)
2.074(9)
2.01(1)
2.00(1)
2.09(1)
2.09(1)
2.064(2)
2.053(6)
2.050(7)
2.066(7)
2.017(9)
1.99(1)
2.001(8)
2.003(7)
2.055(8)
2.054(9)
2.062(2)
2.064(2)
2.062(2)
2.002(2)
2.002(2)
2.070(2)
2.070(2)
2.039(6)
2.053(6)
2.039(6)
2.003(6)
2.003(6)
2.101(7)
2.101(7)
N(1)–O(3)
N(2)–O(8)
1.41(1)
1.46(1)
1.394(3)
1.394(3)
1.406(7)
1.406(7)
1.36(1)
1.39(1)
N(1)–Mo(1)–O(1)
N(2)–Mo(1)–O(8)
40.0(4)
41.9(4)
39.99(9)
39.99(9)
40.0(2)
40.0(2)
39.1(3)
40.1(3)
a C2 symmetry at the molybdenum centre, hence N(2), O(6), O(8), and O(10) are generated by symmetry. O(1,5,6,10) are carboxylate O atoms and
O(3,8) are the η2-NO O atoms. b O(3) and O(5) renumbered for comparison.
Fig. 3 ORTEP representation of the structure of the anion present in
Fig. 2 ORTEP representation of the structure of the anion present in
3b.
2.
with significant displacements for O(1) and O(3) (Ϫ0.43 and
Amavadin-style complex was isolated, providing a rapid and
alternative synthesis of 1.
Ϫ0.36 Å, respectively).
3b crystallised in the space group Fddd (no. 70), with the
eight-co-ordinate geometry of Amavadin (Fig. 3).5 The asym-
metric unit contains half of the anion with the Mo atom
located on a two-fold axis. This anion was refined as the ∆-
helical form at the molybdenum() centre and the Λ-form is
generated by the symmetry of the unit cell. The ligand was
refined in exclusively the R,S-chiral form. The asymmetric unit
also contains half a sodium counter cation located on a two-
fold axis. Two “solvent atoms” were found disordered over two
2 crystallised as brown blocks in the space group P21212
(no. 18) (Table 1). The complex anion exhibits the eight-
co-ordinate geometry found in Amavadin, see Fig. 2. The
asymmetric unit consists of half the complex anion with the
∆-form at the molybdenum() centre plus an oxygen atom (O6)
around which three peaks were observed in the final difference
map. These can be assigned to two full occupancy H atoms and
one half occupancy H atom, lying close to the two-fold axis.
The latter H-bonds to the symmetry equivalent water oxygen
(O6A) generated by the two-fold axis generating an [H2O5]ϩ
unit with a slightly disordered central H atom. Thus, through-
out the crystal structure there are complex anion units of
[∆-Mo(R,R-hipda)2]Ϫ linked by [H2O5]ϩ units. The R,R-ligand
can be used as a “lock” on the structure solutions since the
chirality of the ligand is carried directly through to the com-
plex.29 The Mo atom sits on a crystallographic C2 axis in the
lattice, so the corresponding Mo–O and Mo–N bond lengths
are identical within the two ligands and of a similar length to
those in [∆-Mo(R,R-hidpa)(R,S-hidpa)]Ϫ 14 (Table 2), with each
slightly longer (range 0.12–0.03 Å; av. 0.07 Å) than their
conuterparts in [∆-V(S,S-hidpa)2]Ϫ.10 The planes of the two
{MoNO} groups are close to perpendicular (90.2Њ) and effect-
ively perpendicular (91.2 and 91.2Њ) to the “equatorial” plane,
i
sites each, probably arising from Pr2O; it is not clear which
atom is carbon and which is oxygen. As in 2, the Mo atom sits
on a crystallographic C2 axis in the lattice and corresponding
Mo–O and Mo–N bond lengths are identical in the two ligands.
The planes of the two {MoNO} groups are not mutually per-
pendicular (96.5Њ),30 but are effectively perpendicular (88.9,
91.1Њ)30 to the equatorial plane. These oxygen atoms are signifi-
cantly displaced from this plane, as found in 1 and 2; O(1) and
O(5) sit above (0.41 and 0.34 Å, respectively),30 with the two
symmetry equivalent O atoms correspondingly below, as also in
2. The two ethyl groups of the anion of 3b are arranged in an
eclipsed fashion, this is in contrast to the staggered arrange-
ment of these groups in the TaV analogue31 and both eclipsed
and staggered arrangements in the VIV analogue,15 although
there is some disorder in the lattice water molecules.
J. Chem. Soc., Dalton Trans., 2001, 3108–3114
3111

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Table 3 Comparison of UV/vis absorption bands for 1, 2, 3a, 3b and
[∆-PPh4][Mo(R,R-hidpa)(R,S-hidpa)]14
is presumed to be at much higher energy than the other two d–d
bands, because of ligand-field effects of molybdenum,35 and
hence obscured by the intense charge transfer absorptions.
ε/molϪ1
Compound
Solvent
ν/cmϪ1
dmϪ3 cmϪ1
EPR spectra
1
MeCN
CH2Cl2
H2O
18181
26455
17023
26774
18181
26041
17730
26595
17841
24943
18181
25974
19
103
47
134
15
61
10
94
13
61
18
1, 2 and 3a all exhibit X-band EPR spectra in frozen solutions
(2 in H2O–glycerol; 1 and 3a in CH2Cl2–toluene) at 120 K,
consistent with a Mo(), d1 configuration in each case with the
ligand donor atoms arranged in the eight-co-ordinate geometry
found in Amavadin. The spectra are either axial (2) or near
axial (1, 3a) and are similar to the X-band spectrum reported
for [PPh4][∆-Mo(R,R-hidpa)(R,S-hidpa)].14 These EPR spectra
can be simulated using the spin-Hamiltonian parameters given
2
2
3a
3b
MeCN
H2O
y2
ground state, as
in Table 4, and are consistent with a dx
Ϫ
[PPh4][∆-Mo(R,R-hidpa)-
(R,S-hidpa)]
MeCN
previously shown for the analogous V() compounds.31 Thus,
like their V() counterparts, the EPR spectra of these Mo()
complexes are indicative of the presence of a strong axial ligand
field, with g3 significantly smaller and A3 much larger than
the respective g1, g2, A1 and A2 values. The single-crystal EPR
spectra of [PPh4][Mo(hida)2] and [Ca(H2O)5][V(hida)2]ؒ6H2O
doped in their Nb and Ti analogues, respectively, have been
investigated at X-band, Q-band and 180 GHz.36 In the 180 GHz
spectrum of [PPh4][Nb{Mo}(hida)2], all three g-features are
split into two resonances, indicating the presence of two mag-
netically distinct species in the sample, and is assigned to the
presence of two magnetically distinct, independent molecules in
the asymmetric unit of [PPh4][Nb(hida)2]. These molecules have
similar bond lengths and inter-bond angles. However, the
dihedral angle between the two Nb–η2-NO groups is signifi-
cantly different in the two molecules (98.3 and 95.1Њ). The pres-
ence of two different sets of g- and A-values is consistent with
the ligand-field being dominated by the {M–(η2-NO)2} group
and hence the g- and A-values being sensitive to variations in
the geometric parameters of this group.
89
Amavadin and its analogues crystallise with metal counter
cations such as calcium and uranyl.5,15,32,33 The sodium cation
in 3b binds to two O(2) atoms and two O(4) atoms of the carb-
oxylate groups from four different molybdenum-containing
anions. The four oxygens are arranged around the sodium
in an approximately tetrahedral manner, generating an infinite
lattice; the bond lengths are 2.269(7) and 2.229(7) Å for
Na–O(2) and Na–O(4), respectively, and the O–Na–O inter-
bond angles range from 104.6(4)–110.8(4)Њ.
In each case the η2-NO groups subtend angles at the molyb-
denum, see Table 2, which compare with the range 39.9–41.2Њ
observed for related vanadium systems.9,11 The N–O bond
lengths within these groups show a modest contraction from 1
to 3a (Table 2), this correlates with an increase in the observed
νN–O stretching frequency from 1108 to 1131 cmϪ1. For 1, 2 and
3b the ligating carboxylate oxygen atoms from the same ligand
are mutually trans and occupy sites on the same side of the
least-squares plane defined by the molybdenum and the four
oxygen atoms; this leads to an alternating up-down pattern of
these oxygen atoms with respect to the least-squares plane.
Electrochemical studies
The redox properties of Amavadin and related systems have
been explored in detail7,15 and highlight the interaction between
ligand and solvent in the observed E1/2 values of the VV/VIV
couple. [PPh4][∆-Mo(R,R-hidpa)(R,S-hidpa)]14 is capable of
both a one-electron reversible oxidation and a one-electron
reversible reduction in CH2Cl2 (Table 5). We have investigated
the influence of ligand and solvent on the redox chemistry of
the new eight-co-ordinate molybdenum complexes reported
herein.
Fig. 4 shows the cyclic voltammograms obtained for 1 and 3a
in CH2Cl2, where each complex exhibits a reversible MoVI/MoV
redox couple, analogous to that of [PPh4][∆-Mo(R,R-hidpa)-
(R,S-hidpa)].14 As with Amavadin and its relatives7,15 shifts of
≈100 mV are observed in the E1/2 value of the MoVI/MoV couple
for the hida3Ϫ, hidpa3Ϫ and hidba3Ϫ as ligands within the same
solvent system. The relative increased electron donating ability
of hidba3Ϫ in 3a compared to hida3Ϫ and hidpa3Ϫ gives rise to
the lowest E1/2 value for the MoVI/MoV couple in CH2Cl2. The
trend in E1/2 values reported for these oxidation processes is
consistent with modest variations in the electron donating
ability of the individual ligands and increased solubility in
organic media (Table 5). The nature of the proligand also
affects the MoV/MoIV couple of these complexes. 1 displays a
quasi-reversible37 MoV/MoIV redox couple, however, the
equivalent process for 3a is irreversible (Fig. 4). The MoV/MoIV
couple becomes less reversible on going from 1 to 2 to 3a. These
electron transfer processes are considered to be metal-based as
in the corresponding vanadium systems.15
UV/Vis absorption spectra
Each compound has a brown–purple colour in solution and
the electronic absorption spectra obtained for complexes 1, 2
and 3a showed a broad, low intensity, band centred at around
ν = 18000 cmϪ1 (ε = 10–47 molϪ1 dm3 cmϪ1) and a more domin-
ant feature was a medium intensity band at around ν = 26000
cmϪ1 (ε = 60–134 molϪ1 dm3 cmϪ1) (Table 3). These spectra
closely resemble the UV/vis spectrum obtained for [PPh4]-
[∆-Mo(R,R-hidpa)(R,S-hidpa)].13 By analogy with the elec-
tronic structure proposed for Amavadin and related systems
from discrete variational (DV) Xα calculations,34 these transi-
2
2
tions are assigned as dx
dyz, dxz and dx
dxy,
Ϫ
y2
Ϫ
y2
respectively. This interpretation suggests an “electronic equiv-
alence” between two mutually trans and perpendicular η2-NO
groups and one terminal oxo group.34 The transition dx
2
Ϫ
y2
dxy in [M(hidca)2]nϪ (where M = V, n = 2 and M = Mo, n = 1)
would be expected to be at significantly higher energy for MoV
than VIV, as shown for the comparison of d–d transitions in
[VOCl4]2Ϫ and [MoOCl4]Ϫ.35 This transition is equivalent to 10
Dq for an octahedral complex; 10 Dq increases from 3d to 4d
metals (in the same oxidation state and with the same ligands)
and with increasing oxidation state. However, a similar com-
2
Ϫ
y2
parison does not hold for the dx
dyz, dxz transitions.
Thus, the difference in energy between this band in [VOCl4]2Ϫ
and in [MoOCl4]Ϫ 35 is significantly less than the difference of
ca. 6000 cmϪ1 observed between the equivalent bands of the
[V(hidca)2]2Ϫ and [Mo(hidca)2]Ϫ anions, indicating that the
“electronic equivalence” between M᎐O and {M–(η2-NO) }
A series of CPE experiments were performed for [PPh4]-
[Mo(hidpa)2]14 in CH2Cl2–[NBun ][BF4] (ca. 0.4 mol dmϪ3). The
᎐
2
4
centres is qualitative but not quantitative.34 An intense charge
first coulometric step involved fixing the potential at ϩ1.2 V
(vs. SCE) and carrying out the oxidation process, thus generat-
ing MoVI by exhaustive electrolysis at 293 K. On completion the
transfer band in the region 32000–41000 cmϪ1 was also
observed in [Mo(hidca)2]Ϫ anions. The dx
dz transition
2
2
Ϫ
y2
3112
J. Chem. Soc., Dalton Trans., 2001, 3108–3114

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Table 4 EPR Parameters for 1, 2, 3a and [PPh4][∆-Mo(R,R-hidpa)(R,S-hidpa)]
g3
g1
g2
A3/G
A1/G
A2/G
[PPh4][∆-Mo(R,R-hidpa)(R,S-hidpa)]a
1.894
1.893
1.890
1.897
1.974
1.976
1.973
1.976
1.967
1.969
1.973
1.969
86.0
85.0
86.0
84.0
29.9
31.0
34.0
31.0
35.0
35.0
34.0
35.0
1b
2c
3ab
a Ref. 14. b Dichloromethane–toluene (10 : 1) solution at 100 K. c Water–glycerol (10 : 1) solution at 100 K.
Table 5 E1/2 values recorded in different solvents at a glassy carbon working electrode (vs. SCE) for 1, 2, 3a and [PPh4][∆-Mo(R,R-hidpa)-
(R,S-hidpa)].14 For each reversible couple the current peak intensity (ip) obeyed a linear relationship with the square root of the scan rate (range:
20–700 mV sϪ1
)
MoVI/MoV
E1/2/V
MoV/MoIV
E1/2/V
Compound
Solvent/Electrolyte
∆E/mV
ipa : ipc
∆E/mV
ipa : ipc
1
2
CH2Cl2/[NBun ][BF4]
ϩ0.96a
ϩ0.76b
ϩ0.77a
ϩ0.77a
ϩ0.87a
70
—
70
70
—
0.96
—
0.94
0.95
—
Ϫ0.99c
—
80
—
—
—
—
1.12
—
—
—
—
4
H2O/KCl
Me2SO/[NBun ][BF4]
Ϫ1.13b
Ϫ1.28b
Ϫ1.12a
4
3a
CH2Cl2]/NBun ][BF4]
[PPh4][∆-Mo(R,R-hidpa)(R,S-hidpa)]11
CH2Cl2/[NBun4][BF4]
4
a Reversible process, the current peak intensity, ipa and ipc, correlated with the square root of the scan rate. b Irreversible process. c Quasi-reversible
process.
(vs. SCE). Attempts to carry out this oxidation chemically with
[NO][BF4]38 yielded a pale yellow product that was only spar-
ingly soluble in CH2Cl2 and unstable in solvents such as MeOH
and Me2SO under anhydrous/anaerobic conditions.
A second exhaustive electrolysis experiment was carried out,
by setting the potential to Ϫ1.5 V (vs. SCE), in order to produce
the [Mo(hidpa)2]2Ϫ moiety. This reduction step yielded a dark
red solution, however, a cyclic voltammogram of this solution
revealed a loss of the original sweep profile and the only feature
present was an irreversible oxidation peak at ϩ0.56 V (vs. SCE),
consistent with an electrochemical–chemical process. This
illustrates that the reduced species is not stable during the
coulometric experiment and undergoes chemical reaction or
rearrangement, after an initial metal-based reduction process,
to produce a new product that is detected by the new oxidation
peak.
Comparisons of E1/2 values recorded for Amavadin and its
relatives in H2O and Me2SO revealed shifts of ≈500 mV for
equivalent redox processes,6,17 highlighting the influence of
hydrogen-bonding and other solvation effects in stabilising the
oxidation state of these complexes. Fig. 5 shows the cyclic vol-
tammograms obtained for 2 in H2O and Me2SO; the MoVI/MoV
Fig. 4 Cyclic voltammograms recorded at a glassy carbon electrode
(vs. SCE) for 1 (a) and 3a (b) (ca. 1 mmol dmϪ3) in CH2Cl2/[NBun ][BF4]
4
redox couple is reversible in Me2SO and irreversible in H2O,
(ca. 0.2 mol dmϪ3) at 293 K, scan rate = 200 mV sϪ1
.
(Table 5). This variation in the reversibility of the oxidation
process, between aqueous and organic media, is in contrast to
the redox chemistry of Amavadin, which displays reversible
characteristics in both solvent environments,7 and illustrates the
instability of the MoVI, d0, analogue in the presence of H2O.
Also in contrast to the VV/VIV couples with hidca3Ϫ ligands, the
difference in Epa between H2O and Me2SO as solvent for 2 was
only ca. 35 mV, compared to a value of ca. 500 mV for the
respective V complexes. A sweep to negative potentials with 2 in
H2O revealed only a peak at Ϫ0.42 V (vs. SCE) that is due to the
reduction of protons, which are present as the counter cations
in 2, and this effect was also observed for Amavadin.7 In Me2SO
the reduction process for 2 is irreversible (Table 5), highlighting
again how the solvent influences the reversibility of these redox
couples.
solution became pale yellow and the cyclic voltammetric
experiments were repeated using a glassy carbon working elec-
trode. The initial oxidation–reduction profiles were observed,
confirming the reversibility of the MoVI/MoV couple on the
extended time-scale of the electrolysis experiment. A com-
parison of the frozen solution X-band EPR spectra before
and after the electrolysis revealed a dramatic reduction in the
intensity of the MoV resonance, consistent with the exhaustive
electrolysis achieving 95% completion of oxidation to MoVI.
No new signal was observed in the EPR spectra, providing clear
evidence that the MoVI/MoV couple observed with these com-
pounds is a metal-based one-electron transfer process. Further
experiments using an OTTLE cell22 showed distinctive changes
in the UV/vis absorption spectrum of [PPh4][Mo(hidpa)2] in
CH2Cl2–[NBun ][BF4] (ca. 0.4 mol dmϪ3) upon gradual stepwise
4
oxidation to ϩ1.0 V (vs. SCE), consistent with the loss of the
d–d transitions and the emergence of a charge transfer band
at ν = 28000 cmϪ1 (ε = 790 molϪ1 dmϪ3 cmϪ1). The original
spectrum containing the low intensity d–d absorption bands
was observed after reduction of the solution at ϩ0.5 V
Conclusions
The syntheses of 1, 2, 3a and 3b have further developed
the chemistry of molybdenum analogues of Amavadin and
X-ray crystallography has confirmed the same distinctive
J. Chem. Soc., Dalton Trans., 2001, 3108–3114
3113

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and J. A. McCleverty, Kluwer, The Netherlands, 1993, p. 411.
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245, 131; (b) M. F. C. G. da Silva, J. A. L. da Silva, J. J. R. Fraústo
da Silva, A. J. L. Pombeiro, C. Amatore and J.-N. Verpeaux,
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9 P. M. Reis, J. A. L. Silva, J. J. R. Fraústo da Silva and A. J. L.
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10 E. M. Armstrong, R. L. Beddoes, L. J. Calviou, J. M. Charnock,
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11 E. M. Armstrong, M. S. Austerberry, D. Collison, S. N. Ertok,
C. D. Garner, M. Helliwell and F. E. Mabbs, unpublished work;
S. N. Ertok, Ph.D. Thesis, The University of Manchester, 1993.
12 M. A. A. F. de C. T. Carrondo, M. T. L. S. Duarte, J. C. Pessoa,
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Fig. 5 Cyclic voltammograms recorded at a glassy carbon electrode
(vs. SCE) for 2 (a) = H2O/KCl, (b) = Me2SO/[NBun ][BF4] (ca. 0.2 mol
18 E. Koch, H. Kneifel and E. Bayer, Z. Naturforsch., Teil B, 1986, 41,
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4
dmϪ3) at 293 K, scan rate = 200 mV sϪ1
.
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eight-co-ordinate geometry for the molecular anions. Cyclic
voltammetry has provided a wealth of information on the
redox behaviour of these molybdenum complexes and further
illustrated the significant role of the solvent on the electro-
chemical properties of members of this family of complexes.
For the molybdenum complexes the MoV, d1 state is signifi-
cantly preferred.
Acknowledgements
We thank British Nuclear Fuels plc, (BNFL) for funding
(to P. D. S. and S. M. H.), EPSRC for studentship funding
(to J. J. A. C.) and Dr P. Harston (Company Research
Laboratory, BNFL, Springfields, Preston, Lancashire) for
valuable discussions during the course of this work.
27 (a) Texray Structure Analysis Package, Molecular Structure
Corporation, 1985; (b) G. M. Sheldrick, SHELXL93, Program for
the Refinement of Crystal Structures, University of Göttingen,
1993.
28 S. M. Harben, P. D. Smith, R. L. Beddoes, D. Collison and
C. D. Garner, J. Chem. Soc., Dalton Trans., 1997, 2777.
29 S. M. Harben, Ph.D. Thesis, The University of Manchester, 1996.
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31 S. M. Harben, J. J. A. Cooney, P. D. Smith, M. Helliwell, D. Collison
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32 S. M. Harben, P. D. Smith, R. L. Beddoes, D. Collison and
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