Structural variations in dinuclear model hydrolases and hydroxamate inhibitor models: Synthetic, spectroscopic and structural studies

Article abstract of DOI:10.1016/j.ica.2003.08.021

Reactions of the structural model hydrolases [M₂(OAc) ₄(H₂O)(Im)₄]; M=Mn (E^′); M=Co (D^′); M=Ni (B^′) and [M₂(OPiv) ₄(H₂O)(tmen)₂]; M=Mn (E″); M=Co (D″); M=Ni (B″) with a number of hydroxamic acids, RHA (aceto- (R=CH₃), benzo- (R = C₆H₅) and N-phenylacetohydroxamic acid (NPhAHA)) gave a series of hydroxamate dibridged complexes [M₂(OAc)(RA)₂(Im)₄][OTf] and [M ₂(OPiv)(RA)₂(tmen)₂][OTf]; M=Co, Ni, in which the bridging hydroxamates exhibit a novel bonding mode in which the deprotonated hydroxamate hydroxyl bridges the two metal centres only. The formation of this type of structure by NPhAHA is the first example involving a secondary hydroxamic acid. These complexes are good structural models of the acetohydroxamate-inhibited C³¹⁹A variant of Klebsiella aerogenes urease (KAU) and their structures are close to those previously reported for complexes containing tmen capping ligands. Reaction with glutarodihydroxamic acid leads to hydroxylamine elimination and formation of a dimer containing deprotonated N-hydroxyglutarimide as bridging ligand but in this case the structure contains pentacoordinated Co(II) and only one bridging acetate in contrast to the tmen-based series where the analogous complex contains hexacoordinated Co(II) and two bridging acetates. Reaction of [Mn ₂(OAc)₂(μ-OAc)₂(μ-H₂O)(tmen) ₂] with acetohydroxamic acid (AHA) gave the first structurally characterized manganese hydroxamate, [Mn₂(OAc)₃(AA)(tmen) ₂] with the same bridging/chelating mode of hydroxamate bonding as in the analogous cobalt and nickel complexes, although only one bridging hydroxamate occurs in the manganese complex in contrast to the two bridging hydroxamates in the cobalt and nickel complexes. The isolation of the dimanganese hydroxamate bridged complex suggests that hydroxamic acids may also inhibit the dimanganese based metallohydrolase, arginase.

Full text of DOI:10.1016/j.ica.2003.08.021

Inorganica Chimica Acta 357 (2004) 1411–1436
www.elsevier.com/locate/ica
Structural variations in dinuclear model hydrolases
and hydroxamate inhibitor models: synthetic, spectroscopic
and structural studies q
a,
a
a
b
b
D.A. Brown *, W.K. Glass , N.J. Fitzpatrick , T.J. Kemp , W. Errington ,
b
G.J. Clarkson , W. Haase , F. Karsten , A.H. Mahdy
c
c
a,
*
a
Department of Chemistry, Centre for Synthesis and Chemical Biology (CSCB), Conway Institute of Biomolecular
and Biomedical Research, University College Dublin, Belfield, Dublin 4, Ireland
b
Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK
c
Institut fur Physikalische Chemie, Technische Universitat, Darmstadt, Germany
Received 27 August 2003; accepted 31 August 2003
Abstract
Reactions of the structural model hydrolases [M2(OAc)4(H2O)(Im)4]; M ¼ Mn (E0); M ¼ Co (D0); M ¼ Ni (B0) and
[M2(OPiv)4(H2O)(tmen)2]; M ¼ Mn (E00); M ¼ Co (D00); M ¼ Ni (B00) with a number of hydroxamic acids, RHA (aceto- (R ¼ CH3),
benzo- (R ¼ C6H5) and N-phenylacetohydroxamic acid (NPhAHA)) gave a series of hydroxamate dibridged complexes
[M2(OAc)(RA)2(Im)4][OTf] and [M2(OPiv)(RA)2(tmen)2][OTf]; M ¼ Co, Ni, in which the bridging hydroxamates exhibit a novel
bonding mode in which the deprotonated hydroxamate hydroxyl bridges the two metal centres only. The formation of this type of
structure by NPhAHA is the first example involving a secondary hydroxamic acid. These complexes are good structural models of the
acetohydroxamate-inhibited C319A variant of Klebsiella aerogenes urease (KAU) and their structures are close to those previously
reported for complexes containing tmen capping ligands. Reaction with glutarodihydroxamic acid leads to hydroxylamine elimination
and formation of a dimer containing deprotonated N-hydroxyglutarimide as bridging ligand but in this case the structure contains
pentacoordinated Co(II) and only one bridging acetate in contrast to the tmen-based series where the analogous complex contains
hexacoordinated Co(II) and two bridging acetates. Reaction of [Mn2(OAc)2(l-OAc)2(l-H2O)(tmen)2] with acetohydroxamic acid
(AHA) gave the first structurally characterized manganese hydroxamate, [Mn2(OAc)3(AA)(tmen)2] with the same bridging/chelating
mode of hydroxamate bonding as in the analogous cobalt and nickel complexes, although only one bridging hydroxamate occurs in the
manganese complex in contrast to the two bridging hydroxamates in the cobalt and nickel complexes. The isolation of the dimanganese
hydroxamate bridged complex suggests that hydroxamic acids may also inhibit the dimanganese based metallohydrolase, arginase.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Dinuclear nickel; Cobalt; Manganese hydroxamates; Model inhibited complexes; Crystal structures; Magnetic properties
1. Introduction
Abbreviations: OAc, CH3COOꢀ; OPiv, (CH3)3CCOOꢀ; tmen,
N,N,N0,N0-tetramethylethylenediamine; OTf (triflate), CF3SOꢀ3 ; Im,
imidazole; AHA, acetohydroxamic acid; AA, acetohydroxamate
anion; BHA, benzohydroxamic acid; BA, benzohydroxamate anion;
Dinuclear metallohydrolases are an important group
of metalloenzymes, which catalyze the hydrolysis of a
range of peptide and phosphate ester bonds and include
the amidohydrolases, amidinohydrolases and peptide
hydrolases [1]. A common structural feature of these
metallohydrolases is a dinuclear metal active site fea-
turing Zn(II), Ni(II), Co(II) or Mn(II) with carboxylate
bridges which occur respectively in leucine aminopepti-
dase [2], urease [3], methionine aminopeptidase [4] and
NPhAHA, N-phenylacetohydroxamic acid; NPhAA, N-phen-
ylacetohydroxamate anion; GluH2A2, glutarodihydroxamic acid;
GluA2, glutarodihydroxamate dianion.
q Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.ica.2003.08.021.
* Corresponding authors. Tel.: +35317162297; fax: +35317162127.
E-mail addresses: Noel.Fitzpatrick@ucd.ie (D.A. Brown), azad.
mahdy@ucd.ie (A.H. Mahdy).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2003.08.021

1412
D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
arginase [5]. Although crystal structural data are now
available for these metallohydrolases, the mechanism
involved in their catalytic hydrolysis of various sub-
strates is still a matter for discussion. The generally ac-
cepted mechanism is based on that first proposed by
Zerner et al. [6] for the action of urease in which one
Ni(II) centre activates the urea substrate by coordination
of the carbonyl oxygen whilst the other nickel(II) centre
coordinates a water molecule which is deprotonated by a
suitably oriented base of the protein to produce a hy-
droxide which then attacks the activated urea carbonyl
carbon atom to give a tetrahedral intermediate which
decomposes to give eventually ammonia and carbon
dioxide.
More recent discussions stress the importance of
hydrogen bonding to various amino acid residues
by the amino hydrogens of the urea substrate [7], al-
though controversy remains since it has recently been
suggested that one of the amino groups of the urea
approaches the six-coordinated Ni(II) centre and even-
tually bridges the two nickel centres, thereby polarizing
the C–O and C–NH2 bonds and assisting nucleophilic
attack [8]. Some support for this mechanism is pro-
vided by the isolation of complexes such as [L2Ni2(l-
OAc)(urea)](ClO4)2 (L ¼ pyrazolate derivative) where
H-bonding by the NH2 groups to an oxygen atom of a
bridging carboxylate is confirmed by structural data
[9].
observed in the acetohydroxamate-inhibited Cys319Ala
variant of KAU [12].
In view of this structural modelling of hydroxamate
inhibition of dinuclear metallohydrolases, it is important
to determine whether this type of bridging hydroxamate
bonding in dinuclear complexes is general or is it limited
to the examples so far structurally characterized [14–19].
To this end, we report herein studies of the reaction
of a series of hydroxamic acids firstly with imidazole-
based dinuclear models, [M2(OAc)4(H2O)(Im)4]; M ¼
Mn(II), Co(II) and Ni(II) in which the tmen capping
ligand in the previous series has been replaced by two
imidazole molecules, and secondly with the pivalate
bridged series [M2(OPiv)4(H2O)(tmen)2] in which the
bridging acetate groups of the previous series have been
replaced by bridging trimethylacetate (pivalate) groups.
These particular structural changes were made because
the imidazole ligand should be a closer model to the li-
gating histidines in urease [3], and the acetate replaced
by pivalate in order to determine the effect of a richer
electron donor as the bridging ligand; it was also chosen
because of its solubility in a wide range of solvents and
the hope thereby of good crystal growth in this series.
In general it was found, as a result of these structural
variations in the model hydrolases, that their reactions
with hydroxamic acids generally replicated those re-
ported previously [16–19] but with some important
structural differences.
Most recently, it has been suggested on the basis of
model studies that hydrolysis of urea at a dinickel
centre may proceed through initial elimination of
ammonia from the coordinated urea molecule followed
by hydrolysis of the resulting bridged cyanate complex
[10].
2. Results and discussion
2.1. Imidazole model hydrolases [M2(OAc)4(H2O)
(Im)4]; M ¼ Mn (E0); Co (D0); Ni (B0)
The inhibition of metalloenzymes by hydroxamic
acids is important because of therapeutic applications.
Generally the inhibition involves chelation of a mono-
nuclear zinc centre by the hydroxamate function
(RCONR0OH) [11] but in the case of urease the struc-
ture of the acetohydroxamate-inhibited Cys319Ala vari-
ant of KAU shows the deprotonated hydroxyl oxygen of
the hydroxamic acid bridging the two nickel centres with
the carbonyl oxygen bridging one nickel centre only [12].
This unusual mode of bonding by a hydroxamate
bridging two metal centres was modelled first in the
complex [Ni2(H2shi)(Hshi)(pyr)4(OAc)], where Hshi is
deprotonated salicylhydroxamic acid, prepared by self-
assembly [13]. Subsequently, it was shown that the
structural model hydrolases [M2(OAc)4(H2O)(tmen)2]
(M ¼ Mn, Co, Ni) react rapidly with a range of hy-
droxamic acids (e.g., AHA) to give hydroxamate-
bridged dinuclear complexes such as [M2(OAc)
(AA)2(tmen)2][OTf], in which the hydroxamate hy-
droxyl is deprotonated and the oxygen then bridges the
two metal centres whilst the carbonyl oxygen bonds to
one metal centre only, thereby modelling the structure
These complexes were prepared by the reaction of
M(OAc)2 ꢁ 4H2O and imidazole in either ethanol or
methanol at room temperature for 3–15 h. After workup
(see Section 3), slow diffusion of diethylether into filtrates
gave good crystals. These complexes have been described
previously [20–22]. In the case of [Co2 (OAc)4(H2O)
(Im)4] (D0) the above reaction in methanol always gave
the monomeric complex, [Co(OAc)2(Im)2] (1/2D0) whose
structure has been reported previously [23]; however, if
the reaction is carried out in an acetonitrile/water (1:1)
mixture, the major product is the dimer D0. Physical
separation of D0 and 1/2D0 after recrystallization from
deionized water gave crystals of D0 suitable for a crystal
structure determination (Fig. 1). Selected bond distances
and bond angles are given in Table 2.
The structure of D0 contains a dicobalt(II) core bridged
by a water molecule and two bidentate acetates as shown
in Fig. 1; each of the Co(II) ions is further coordinated by
one additional monodentate carboxylate and two N-
donor imidazole ligands, forming a CoN2O4 octahedral
arrangement. The molecules lie on a crystallographically

D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
1413
imposed two-fold axis passing through the bridging aqua
oxygen O10. This structure is very similar to that reported
for the analogous tmen complex [Co2(H2O)(OAc)4
(tmen)2] (D) [24], although the Co–Co distance is slightly
Ni9 Cluster. Although conclusive proof of the structures
of the M9 Clusters (M ¼ Co, Mn) was not obtained, the
results suggest that the above transition metal pivalates
exist as M9 Clusters similar to the fully characterized Ni9
Cluster case.
Because of the cluster formation of the above metal
pivalates, the dinuclear pivalate-bridged complexes were
prepared by reaction of the M9 clusters in methanol and
tmen in a 1:9 ratio for 15 min and subsequent workup
(see Section 3) to give pure products of [Mn2
(H2O)(OPiv)4(tmen)2] (E00); [Co2(H2O)(OPiv)4(tmen)2]
(D00) and [Ni2(H2O)(OPiv)4(tmen)2] (B00) with satisfac-
tory microanalyses.
0
ꢀ
ꢀ
longer in D (3.6877(8) A) compared with 3.597(1) A in
complex D. There is a short intramolecular hydrogen
ꢀ
bond contact 1.79(3) A between O41 and H10 on
O10(water) and its symmetry-related counterpart to the
other hydrogen on O10. There are other short intermo-
lecular contacts between H33 (NH of an imidazole) and
O41 of the monodentate acetate of another molecule and
similarly H22 of the other imidazole and O50 of a bid-
entate bridging acetate. This H-bonding network extends
through the crystal (see Fig. 2). In this process, the car-
boxylate ligand serves as a H-bond acceptor while the
bridging aqua serves as the H-bond donor.
2.3. Crystal structure of [Co2(H2O)(OPiv)4(tmen)2]
(D00)
Previous studies [20] reported in full detail the crystal
structure of the dimanganese dimer [Mn2(OAc)4
(H2O)(Im)4] (E0) and quoted a Co–Co distance for D0
without any further structural details. In a recent paper
[22], the same group reported detailed crystallographic
studies of the series [M2(OAc)4(H2O)(Im)4]; M ¼ Mg,
Ni, Mn but again no structural data were given for the
cobalt dimer. In view of this situation, we have mea-
sured the structure of D0 independently and full details
have been deposited with the Cambridge Crystal Data
Bank. The Co–Co distance quoted in [20] is identical
with that obtained in our work.
The X-ray crystal structure of D00 is shown in Fig. 3
and the crystallographic data is given in Table 1 with
selected bond distances and angles in Table 4. Suitable
crystals of D00 were synthesized by mixing the Co9
Cluster with tmen in a 1:9 ratio in methanol and grown
on standing at room temperature from a mixture of 1:1
ethyl acetate/acetonitrile (see Section 3). The structure of
D00 contains a dicobalt(II) core bridged by a water
molecule and two bidentate pivalates; each of the Co(II)
ions is further coordinated by one additional monod-
entate pivalate and two N-donor tmen ligands, forming
a CoN2O4 octahedral arrangement. The molecules lie on
a crystallographically imposed two-fold axis passing
through the bridging aqua oxygen O11.
2.2. Pivalate-bridged model hydrolases [M2(OPiv)4
(H2O)(tmen)2]: M ¼ Mn (E00); Co (D00); Ni (B00)
Comparison between the reported structure of the
dinuclear cobalt(II) tmen complex D [24] and the piva-
late complex [Co2(H2O)(OPiv)4(tmen)2] (D00) shows that
the structures are similar with two bidentate and two
monodentate pivalates, a water bridging molecule and
four N-donor ligands from two terminal tmen ligands in
Synthesis of these complexes by replacing the metal
acetate by the corresponding metal pivalate should occur
easily; however, reaction of NiCl2.6H2O with potassium
or sodium pivalate is reported to give instead of the
expected simple binary product, Ni(Me3CCOO)2, a no-
nanuclear nickel cluster, [Ni9(Me3CCOOH)4(l4-O)3(l3-
OH)3(Me3CCOO)12] whose structure was confirmed by a
crystal structure determination [25]. In the present stud-
ies, the transition metal pivalates (Mn, Co and Ni) were
prepared by the reaction of a suspension of the metal
carbonate (MCO3) in hot water with a solution of pivalic
acid in methanol in a 1:2 ratio for 12 h. Workup (see
Section 3) gave for the nickel complex, blue crystals of
[Ni9(Me3CCOOH)4(O)3(OH)3(Me3CCOO)12] (Ni9 Clus-
ter). However, in the case of the cobalt complex, pink
crystals of the [Co9(Me3CCOOH)4(O)3(OH)3(Me3C
COO)12] (Co9 Cluster) were obtained, which turned blue
in air. Microanalyses were in error from those calculated
for the Co9 Cluster by about 2%. Suitable crystals for
X-ray crystallography could not be obtained. The anal-
ogousmanganesecluster, [Mn9(Me3CCOOH)4(O)3(OH)3
(Me3CCOO)12] (Mn9 Cluster) gave colourless crystals,
stable in the absence of air with satisfactory microanaly-
ses and IR (KBr) spectrum (Table 3) similar to that of the
00
00
ꢀ
D , although the Co–Co distance in D (3.0518(10) A) is
ꢀ
ꢀ
shorter by ꢂ0.5 A from that reported for D (3.597(1) A)
[24]. The dimeric structures with the bridging waters are
stabilized by intramolecular hydrogen bonding with one
of the oxygens of the monodentate pivalate Me3CCOOꢀ
ꢀ
ligands {O(6). . .O(9)} contacts of 2.565(4), 2.548(4) A in
D. The interatomic distances show that the Co–Co dis-
00
ꢀ
tance in D (3.597(1) A) is longer than that in D
ꢀ
(3.0518(10) A) and this may reflect the greater electron
donating effect of pivalate over acetate.
In D, the Co–OH2–Co distances (2.147(3), 2.117(3)
00
ꢀ
ꢀ
A) are similar to those in D (2.109(6), 2.119(6) A). The
ꢀ
Co–N(tmen) distances in D are equal (ꢂ2.22 A), but
00
ꢀ
ꢀ
shorter by ꢂ0.03 A from those in D (ꢂ2.25(9) A). The
Co–O (bidentate acetate) distances in D and D00 are es-
ꢀ
sentially equal (ꢂ2.065(7) A), whilst the Co–O(monod-
ꢀ
entate acetate) distances in D (ꢂ2.081 A) are shorter
00
ꢀ
than that in D (ꢂ2.125(7) A). The bond angles in D and
D00, such as the Co–O(water)–Co angles in D and D00, are

Table 1
Crystal data and structure refinement for complexes prepared
Parameters
D0
D00
(B,NPhAHA)
(E,AHA)
(D0,Glut)
(B00,NPhAHA)
(D00,NPhAHA)
Formula
Mr
Crys system
C
644.38
20H30N8O9Co2
C32H70N4O9Co2
772.78
C30H51F6N6O6PNi2
854.16
C20H45N5O8Mn2
593.49
C21H25F6N9O11S2Co2
875.48
C33H57F6N6O6PNi2
896.24
C33H57F6N6O6PCo2
896.68
orthorhombic
Aba2
monoclinic
P2(1)/c
orthorhombic
P2(1)2(1)2(1)
15.1949(9)
17.1833(12)
29.3663
monoclinic
P2(1)/c
orthorhombic
Pnma
monoclinic
C2/c
monoclinic
C2/c
Space group
ꢀ
a (A)
8.6450(12)
18.964(2)
16.671(3)
90
11.4038(6)
21.8700(16)
34.7349(16)
90
18.2930(6)
20.5714(5)
17.0645(6)
90
19.0785(10)
19.4628(10)
9.1329(5)
90
17.8088(16)
12.2126(12)
38.495(2)
90
17. 7750(18)
12. 2926(18)
38. 569(2)
90
ꢀ
b (A)
ꢀ
c (A)
a (°)
b (°)
c (°)
90
90
90
90
90.122(3)
90
113.187(10)
90
90
90
94.906(3)
90
94.726(4)
90
90
7667.5(8)
180(2)
3
ꢀ
V (A )
T (K)
Z
2733.1(6)
180(2)
8662.9(9)
180(2)
5902. 9(3)
180(2)
3391.2(3)
180(2)
8341.8(12)
180(2)
8398.6(16)
180(2)
4
8
8
0.71073
8
0.71073
4
8
8
0.71073
ꢀ
c (A)
0.71073
1.566
0.71073
1.185
0.71073
1.715
0.71073
1.427
p(calcld) (M g mꢀ3
)
1.480
0.34 ꢃ 0.32 ꢃ 0.30
1.101
1.336
0.18 ꢃ 0.16 ꢃ 0.04
0.902
1.418
0.20 ꢃ 0.12 ꢃ 0.12
0.902
Cryst size (mm3)
0.44 ꢃ 0.14 ꢃ 0.05
1.276
050 ꢃ 0.16 ꢃ 0.16
0.813
0.20 ꢃ 0.18 ꢃ 0.18
1.201
0.36 ꢃ 0.18 ꢃ 0.08
1.015
l (mmꢀ1
hkl Ranges
)
)11,11; )25,17;
)22,21
8329
)7,14; )28,27;
)42,45
54,456
)16,18; )11,21;
)32,36
44,594
)21,21; )24,24;
)20,20
43,337
)24,24; )24,22; )9,11 )22,20; )15,9;
)49,48
)11,22; )14,15;
)47,49
24,772
Number of reflections
collected
Independent reflections
19,625
24,539
3285 (Rint ¼ 0:0571) 19,778
15,035
(Rint ¼ 0:0402)
0.80, 0.70
10,385
(Rint ¼ 0:2106)
0.97, 0.75
3808 (Rint ¼ 0:0269)
0.84, 0.75
9072 (Rint ¼ 0:0569) 9123 (Rint ¼ 0:0840)
(Rint ¼ 0:1081)
Maximum and
0.93, 0.65
0.88, 0.69
0.94, 0.71
0.90, 0.84
mininmum transmission
RðF Þ½I > 2rðIÞꢄ (%)
RwðF 2Þ (all data) (%)
Goodness of fit on F 2
Largest peak and hole (e A
ꢀ
M–M distance (A)
4.29
7.77
10.68
18.31
4.46
5.45
9.50
22.75
3.44
4.38
6.32
9.21
7.94
12.58
0.972
0.544, )0.476
3.6877(8)
1.162
2.462, )0.789
3.0518(10)
1.092
2.800, )0.477
3.0551(7)
1.106
0.482, )0.530
3.542(18)
1.096
0.461, )0.433
3.6313(6)
1.088
1.248, )1.004
3.0491(8)
1.186
0.608, )0.495
3.0518(10)
ꢀ3
ꢀ
)
[Co2(OAc)4(H2O)(Im)4] (D0); [Co2(OPiv)4(H2O)(tmen)2] (D00); [Ni2(OAc)(NPhAA)2(tmen)2][PF6] (B,NPhAHA); [Co2(OAc){O(N)(O@C)2(CH2)3}(Im)4][OTf]2 (D0,GluH2A2);
[Ni2(OPiv)(NPhAA)2(tmen)2][PF6] (B00,NPhAHA); [Co2(OPiv)(NPhAA)2(tmen)2][PF6] (D00,NPhAHA); [Mn2(OAc)3(AA)(tmen)2] (E,AHA).

D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
1415
Table 2
Selected bond lengths (A) and angles (°) for [Co2(OAc)4(H2O)(Im)4] D
0
ꢀ
Co1–Co1A
Co1–O51
Co1–O40
Co1–N20
Co1–O50
Co1–N30
Co1–O10
O10–Co1#1
N20–C21
N20–C24
C21–N22
N22–C23
C23–C24
3.6877(8)
2.048(2)
2.099(2)
2.124(3)
2.131(3)
2.134(3)
2.154(2)
2.154(2)
1.332(4)
1.360(5)
1.332(5)
1.367(5)
1.358(5)
N30–C31
N30–C34
C31–N32
N32–C33
C33–C34
O40–C41
O41–C41
C41–C42
O50–C51
O51–C51#1
C51–O51#1
C51–C52
1.319(5)
1.355(5)
1.334(5)
1.362(5)
1.374(5)
1.239(4)
1.272(4)
1.519(5)
1.259(4)
1.257(4)
1.257(4)
1.491(5)
N20–C21–N22
C21–N22–C23
C24–C23–N22
C23–C24–N20
C31–N30–C34
C31–N30–Co1
C34–N30–Co1
N30–C31–N32
C31–N32–C33
N32–C33–C34
N30–C34–C33
C41–O40–Co1
O40–C41–O41
O40–C41–C42
O41–C41–C42
C51–O50–Co1
C51#1–O51–Co1
O51#1–C51–O50
O51#1–C51–C52
O50–C51–C52
110.6(3)
108.3(3)
105.3(4)
110.2(3)
105.1(3)
128.6(3)
126.3(2)
112.2(4)
107.3(3)
105.4(4)
110.1(3)
127.8(2)
125.3(3)
118.1(3)
116.6(3)
137.0(2)
137.5(2)
126.0(3)
116.1(3)
118.0(3)
O51–Co1–O40
O51–Co1–N20
O40–Co1–N20
O51–Co1–O50
O40–Co1–O50
N20–Co1–O50
O51–Co1–N30
O40–Co1–N30
N20–Co1–N30
O50–Co1–N30
O51–Co1–O10
O40–Co1–O10
N20–Co1–O10
O50–Co1–O10
N30–Co1–O10
Co1#1–O10–Co1
C21–N20–C24
C21–N20–Co1
C24–N20–Co1
175.17(10)
87.25(11)
88.01(10)
92.22(10)
86.97(10)
91.24(10)
93.35(11)
87.74(11)
92.08(11)
173.65(11)
92.05(9)
92.71(10)
178.91(8)
89.62(10)
87.12(10)
117.71(19)
105.6(3)
125.2(3)
127.6(2)
Hydrogen bonds
D–H
H. . .A
1.79(3)
1.95(4)
1.87(5)
D. . .A
\(DHA)
171(4)
168(4)
178(5)
O10–H10. . .O41
N22–H22. . .O50
N32–H32. . .O41
0.83(3)
0.86(4)
0.87(4)
2.616(3)
2.793(4)
2.739(4)
Table 3
Infrared spectroscopic data (cmꢀ1) for the M9 Clusters
Medium
KBr
(Mn9 Cluster)
(Co9 Cluster)
(Ni9 Cluster)
Assignment
1610, 1573
1425, 1369
1703
1607s, 1572w
1428s, 1361s
3436br
1610s, 1571s
1412s, 1359s
3460br
Asymmetric OCO
Symmetric OCO
Bridging (OH)
3457
1702sh
(Blue Co9 cluster)
1702w
Pivalic acid OCO
CH2Cl2
d.
1574
1600, 1562
1428, 1361
1703
Asymmetric OCO
Symmetric OCO
Pivalic acid OCO
Bridging (OH)
1418, 1362
1716, 1702
3409
3468
(Blue Co9 cluster)
CH3OH
d.
1549
(Pink Co9 cluster)
1548
Monodentate pivalate
[Ni9(Me3CCOOH)4(O)3(OH)3(Me3CCOO)12] (Ni9 Cluster); [Co9(Me3CCOO)4(O)3(OH)3(Me3CCOOH)12] (Co9 Cluster); [Mn9(Me3CCOO)4
(O)3(OH)3(Me3CCOOH)12] (Mn9 Cluster).

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D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
Table 4
00
ꢀ
Selected bond lengths (A) and angles (°) for [Co2(OPiv)4(H2O)(tmen)2] (D )
Co(11)– O(12)
Co(11)–O(14)
Co(11)–O(11)
Co(11)–O(16)
Co(11)–N(12)
Co(11)–N(11)
Co(12)–O(15)
Co(12)–O(13)
Co(12)–O(18)
Co(12)–O(11)
Co(12)–N(14)
Co(12)–N(13)
2.058(6)
2.083(7)
2.109(6)
2.125(7)
2.220(8)
2.264(8)
2.041(7)
2.054(7)
2.093(7)
2.119(6)
2.256(9)
2.262(9)
Co(21)–O(24)
Co(21)–O(22)
Co(21)–O(26)
Co(21)–O(21)
Co(21)–N(22)
Co(21)–N(21)
Co(22)–O(23)
Co(22)–O(25)
Co(22)–O(28)
Co(22)–O(21)
Co(22)–N(24)
Co(22)–N(23)
2.044(7)
2.061(7)
2.118(7)
2.119(6)
2.213(8)
2.256(10)
2.061(6)
2.061(7)
2.092(7)
2.126(6)
2.223(9)
2.259(8)
Co(11)–O(11)–Co(12)
O(12)–Co(11)–O(14)
O(12)–Co(11)–O(11)
O(14)–Co(11)–O(11)
O(12)–Co(11)–O(16)
O(14)–Co(11)–O(16)
O(11)–Co(11)–O(16)
O(12)–Co(11)–N(12)
O(14)–Co(11)–N(12)
O(11)–Co(11)–N(12)
O(16)–Co(11)–N(12)
O(12)–Co(11)–N(11)
O(14)–Co(11)–N(11)
O(11)–Co(11)–N(11)
O(16)–Co(11)–N(11)
N(12)–Co(11)–N(11)
N(14)–Co(12)–N(13)
114.1(3)
94.2(3)
94.0(3)
91.4(3)
89.2(3)
176.5(3)
87.6(3)
87.6(3)
89.3(3)
178.2(3)
91.6(3)
169.8(3)
88.9(3)
95.7(3)
87.9(3)
82.7(3)
83.1(4)
O(15)–Co(12)–O(13)
O(15)–Co(12)–O(18)
O(13)–Co(12)–O(18)
O(15)–Co(12)–O(11)
O(13)–Co(12)–O(11)
O(18)–Co(12)–O(11)
O(15)–Co(12)–N(14)
O(13)–Co(12)–N(14)
O(18)–Co(12)–N(14)
O(11)–Co(12)–N(14)
O(15)–Co(12)–N(13)
O(13)–Co(12)–N(13)
O(18)–Co(12)–N(13)
O(11)–Co(12)–N(13)
O(12)–C(11)–O(13)
O(15)–C(16)–O(14)
94.1(3)
88.5(3)
176.4(3)
92.5(3)
93.7(3)
88.6(3)
89.4(3)
87.7(3)
89.9(3)
177.5(3)
172.0(3)
88.4(3)
88.7(3)
94.9(3)
126.0(9)
124.3(9)
essentially equal (115.1(1)°, 114.1(3)°), whilst the O–C–
theoretical. Attempts to prepare [Co2(urea)(OAc)3
(Im)4][PF6] (C0) also gave an impure air-sensitive dark
pink product. However, the infrared spectra of both A0
and C0 in the carboxylate region are very similar and
close to those of tmen complexes A and C (Table 7).
In the case of the analogous reactions between the
pivalate complexes [M2(OPiv)4(H2O)(tmen)2]; M ¼ Co
(D00), Ni (B00) and urea, no stable pure urea complexes
were obtained. The behaviour of B0, B00, D0 and D00 with
urea may be related to the relatively greater ease of
dissociation of this new series compared to B and D. It
appears from the results described below that, unlike the
hydroxamic acids, urea does not have the same ability to
induce dimerization since it cannot readily take up the
bridging/chelating mode of bonding exhibited by the
hydroxamic acids, although some urea bridging com-
plexes have been described [28,29].
O
(bidentate bridging carboxylate) angles in D
(127.8(5)°, 127.0(4)°) are slightly larger than those in D00
(126.0(9)°, 124.3(9)°).
The O–C–O angles are similar in D, but differ in D00.
The N–Co–N angles in D (82.0(1)°, 81.0(2)°) are slightly
smaller than those in D00 (83.1(4)°, 82.7(3)°). The
O(bidentate bridging acetate)–Co–OH2 angles in D
(92.8(1)°, 89.3(1)°) are smaller than those in D00
(94.0(3)°, 91.4(3)°), perhaps due to the shortening of the
Co–Co distance in D00 compared to that in D by ꢂ0.54
ꢀ
A. The O(monodentate acetate)–Co–OH2 angles in D
(88.9(1)°, 87.8(1)°) are similar to those in D00 (88.6(3)°,
87.9(3)°).
2.4. Urea model hydrolases
Attempts to prepare the urea complexes [M2(urea)
(OAc)3(Im)4][PF6] and [M2(urea)(OPiv)3 (tmen)2][PF6];
(M ¼ Co, Ni) by analogy with the previously reported
urea complexes [M2(urea) (OAc)3(tmen)2][OTf]; M ¼
Ni(A) [27]; M ¼ Co(C) [18] met with only limited suc-
cess.
The reaction of [Ni2(OAc)4(H2O)(Im)4] (B0) with
KPF6 in methanol with an excess of urea (2.5 M) for 12
h gave the dark green complex [Ni2(urea)(OAc)3(Im)4]
[PF6] (A0) with microanalyses approximately 1% from
2.5. Hydroxamate-bridged metal complexes [M2(OAc)
(RA)2(Im)4][X]n; M ¼ Mn, Co, Ni; RA ¼ AA, BA,
NPhAA, GluA2; X ¼ OTf, PF6, OAc; n ¼ 1 or 2
In general, the imidazole-based model hydrolases
[M2(OAc)4(H2O)(Im)4] M ¼ Mn (E0), Co (D0) and Ni (B0)
all react readily to form the above hydroxamate-bridged
complexes analogous to those derived from the tmen
based hydrolases reported previously [16–18] with some

D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
1417
Fig. 1. Molecular structure of the complex [Co2(OAc)4(H2O)(Im)4]
(D0).
Fig. 3. Molecular structure of the complex [Co2(OPiv)4(H2O)(tmen)2]
(D00).
state (KBr) spectra are absent. This behaviour illustrates
the ability of a range of hydroxamic acids to form stable
dimers based on the bridging/chelating mode of bonding
in accord with their widespread inhibition effects on
metalloenzymes [7,8]. In the case of the complexes
formed by N-phenylacetohydroxamic acid, (B0,NPhA-
HA) and (D0,NPhAHA) these are the first examples of
bridged/chelated hydroxamate complexes formed by a
secondary hydroxamic acid. Since neither of these
complexes formed crystals suitable for crystal structure
determination, the analogous complexes [Ni2(OAc)
(NPhAA)2(tmen)2][PF6] (B,NPhAHA) and [Co2(OAc)
(NPhAA)2(tmen)2][PF6] (D,NPhAHA) were prepared
(see Section 3 for details) and the presence of the
bridging/chelating mode was confirmed for (B,NPhA-
HA) (see Fig. 4).
Fig. 2. Hydrogen bonding network in the structure of
[Co2(OAc)4(H2O)(Im)4] (D0).
structural differences (Fig. 1). Thus, reaction of B0 with 2
moles of acetohydroxamic acid in methanol followed by
1 mole of TMS–OTf was facile and complete within a few
minutes to give the dark green complex [Ni2(OAc)(AA)2
(Im)4][OTf] (B0,AHA) with satisfactory microanalyses.
The analogous cobalt complex [Co2 (OAc)(AA)2(Im)4]
[OAc] ꢁ HOAc ꢁ H2O (D0,AHA) was prepared in the
absence of TMS–OTf. Similar products were obtained
with benzohydroxamic acid (B0,BHA), (D0,BHA) and
N-phenylacetohydroxamic acid (B0,NPhAHA), (D0,
NPhAHA).
It should be noted that the above reactions of B0 (and
D0) with hydroxamic acids occur readily in methanol, a
solvent in which both dissociate as indicated by their
infrared spectra in methanol in which the bidentate
bridging bands at about 1620 cmꢀ1 present in their solid
2.6. Crystal structure of [Ni2(OAc)(NPhAA)2 (tmen)2]
[PF6] (B,NPhAHA)
The structure of (B,NPhAHA) is shown in Fig. 4
together with selected bond distances and bond angles in
Table 5. The structure exhibits the bridging/chelating
mode of bonding very similar to that reported previ-
ously for primary hydroxamic acids [16]. The two
monodentate acetates, water molecule and one bidentate
acetate in [Ni2(OAc)4(H2O)(tmen)2] B have been re-
placed by two N-phenylhydroxamates in the bridging/
chelating bonding mode. The metal atoms are in an
N2O4 distorted octahedral environment with a relatively
ꢀ
short metal–metal distance of 3.0551(7) A. The magnetic

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D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
glutarodihydroxamic acid undergoes a most unusual
rearrangement with loss of hydroxylamine and forma-
tion of a new dinuclear complex containing as bridg-
ing group
a
deprotonated N-hydroxyglutarimide,
[M2(OAc)2{O(N)(O@C)2(CH2)3}(tmen)2][OTf] in which
the deprotonated hydroxy group bridges the two metal
atoms and the two ketonic carbonyl groups each
coordinate to one metal atom [17,18].
The proposed mechanism suggests that the two
metal centres in B [26] and D [24] act as Lewis acids,
polarizing the carbonyl groups in glutarodihydroxa-
mic acid with an attack at the other nucleophile
nitrogen and loss of NH2OH, ring closure and for-
mation of a tetrahedral intermediate, which on de-
protonation forms the cyclic product, a mechanism
similar to that proposed in the cleavage of C–N bonds
in peptide hydrolysis catalyzed by the dizinc centre in
aminopeptidases [30].
Reactions of B0 and D0 with glutarodihydroxamic
acid proceeded similarly to those of B and D with hy-
droxylamine elimination also occurring with this series
of model hydrolases and formation of the cyclized
products [M2(OAc){O(N)(O@C)2(CH2)3}(Im)4][OTf]2,
M ¼ Ni (B0,GluH2A2); M ¼ Co (D0,GluH2A2). The re-
actions proceeded rapidly in methanol (see Section 3)
and gave crystals suitable for structure determination,
which showed that for the imidazole series of model
hydrolases B0 and D0, the cyclized products contain only
one bridging acetate group (Fig. 5) in contrast to B and
D where the cyclized products contain two bridging
acetate groups [18].
Fig. 4. Molecular structure of the complex [Ni2(OAc)(NPhAA)2
(tmen)2][PF6] (B,NPhAHA).
properties of (B,NPhAHA) are discussed below in a
separate section.
2.7. Reactions of B0 and D0 with glutarodihydroxamic acid
(GluH2A2)
In the case of the tmen-based model hydrolases,
[M2(OAc)4(H2O)(tmen)2] M ¼ Co (D) and Ni (B),
Table 5
ꢀ
Bond lengths [A] and angles [°] for [Ni2(OAc)(NPhAA)2(tmen)2][PF6] (B,NPhAHA)
Ni1– Ni2
Ni1–O11
Ni1–O13
Ni1–N11
Ni2–O12
Ni2–O15
Ni2–O13
3.0551(7)
2.009(3)
2.074(3)
2.165(4)
2.023(3)
2.062(3)
2.147(3)
Ni1–O14
Ni1–O15
Ni1–N12
Ni2–O16
Ni2–N14
Ni2–N13
2.130(3)
2.146(3)
2.201(4)
2.029(3)
2.126(4)
2.223(4)
O11–Ni1–O14
O14–Ni1–O13
O14–Ni1–O15
O11–Ni1–N11
O13–Ni1–N11
O11–Ni1–N12
O13–Ni1–N12
N11–Ni1–N12
O12–Ni2–O15
O12–Ni2–N14
O15–Ni2–N14
O16–Ni2–O13
N14–Ni2–O13
O16–Ni2–N13
N14–Ni2–N13
Ni1–O13–Ni2
176.38(13)
80.30(11)
91.93(11)
85.21(14)
173.00(13)
88.77(13)
92.10(13)
82.97(15)
95.06(12)
85.03(15)
178.84(15)
91.32(11)
98.59(14)
90.70(13)
83.99(16)
92.73(11)
O11–Ni1–O13
O11–Ni1–O15
O13–Ni1–O15
O14–Ni1–N11
O15–Ni1–N11
O14–Ni1–N12
O15–Ni1–N12
O12–Ni2–O16
O16–Ni2–O15
O16–Ni2–N14
O12–Ni2–O13
O15–Ni2–O13
O12–Ni2–N13
O15–Ni2–N13
O13–Ni2–N13
Ni1–O15–Ni2
99.71(12)
84.49(12)
82.31(11)
95.11(13)
103.24(13)
94.85(13)
170.37(13)
175.69(12)
80.64(11)
99.27(14)
88.12(12)
82.57(11)
89.64(14)
94.85(13)
176.42(13)
93.10(11)

D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
1419
bridging water, one bidentate acetate and both terminal
acetates in the hydrolase model D0. The deprotonated N-
hydroxy O2 bridges the two divalent cobalt ions Co1
and Co2 and the two carbonyl oxygens O1 and O3, are
each coordinated to their respective cobalt atoms
(Fig. 5). As a consequence of the pentacoordination
N2O3 environment about each cobalt centre and of only
a single acetate bridge, the Co–Co distance in the
ꢀ
0
asymmetric unit is 3.6313(6) A, which is a little shorter
ꢀ
than that of the precursor D (3.6877(8) A) or in the
acetohydroxamate-inhibited Cys319Ala urease (3.7 A)
ꢀ
and longer than that observed in the doubly bridged
ꢀ
salicylhydroxamate complex (3.016 A) [13]. The coor-
dination about the cobalt atoms in (D0,GluH2A2) is
distorted trigonal bypyramidal, which is different from
the precursor D0, which shows a distorted octahedral
environment. The Co–Co separation in the hexacoor-
dinated [Co2(OAc)2{O(N)(O@C)2(CH2)3}(tmen)2][OTf]
Fig. 5. Molecular Structure of [Co2(OAc){O(N)(O@C)2(CH2)3}(Im)4]
[OTf]2 (D0,GluH2A2).
ꢀ
2.8. Crystal structure of [Co2(OAc){O(N)(O@C)2
(CH2)3}(Im)4][OTf]2 (D0,GluH2A2)
(D,GluH2A2) reported [18] is 3.4128(11) A, shorter than
that in the imidazole pentacoordinated (D0,GluH2A2)
ꢀ
(3.6313(6) A), in which the bond angle O15–Co2–O11 is
The structure of (D0,GluH2A2) is shown in Fig. 5 with
crystallographic data (Table 1) and selected bond dis-
tances and angles in Table 6 and contains a deproto-
nated bridging N-hydroxyglutarimide replacing the
91.14(16)° for (D,GluH2A2), i.e., smaller than that in
(D0,GluH2A2) O5–Co2–O2, 98.75(9)°. The change in
geometry from octahedral in the hexacoordinated tmen
complex (D,GluH2A2) to trigonal bipyramidal in the
Table 6
Selected bond lengths [A] and angles [°] for [Co2(OAc){O(N)(O@C)2(CH2)3}(Im)4][OTf]2 (D0,GluH2A2)
ꢀ
Co1–O2
Co1–N2#1
Co1–O1
Co2–O5
Co2–N4
S1–O8
1.975(2)
2.0096(18)
2.205(2)
2.014(2)
2.0192(18)
1.414(2)
1.4392(18)
1.316(4)
1.319(4)
Co1–N2
Co1–O4
Co2–O2
Co2–N4#1
Co2–O3
S1–O7
2.0096(18)
2.016(2)
1.977(2)
2.0192(18)
2.256(2)
1.4389(19)
1.816(3)
1.323(4)
1.221(4)
S1–O6
F1–C14
F3–C14
S1–C14
F2–C14
O1–C1
Co1–O2–Co2
O2–Co1–N2
N2–Co1–N2#1
N2–Co1–O4
O2–Co1–O1
N2#1–Co1–O1
O2–Co2–O5
O5–Co2–N4#1
O5–Co2–N4
O2–Co2–O3
N4#1–Co2–O3
N1–O2–Co2
C11–N4–Co2
C3#1–C2–C1
C2–C3–C4
126.63(10)
124.59(5)
108.21(10)
94.30(6)
75.98(8)
89.75(6)
98.75(9)
95.74(6)
95.74(6)
75.20(8)
87.54(6)
117.13(16)
128.61(16)
115.6(3)
114.3(3)
118.9(3)
117.0(3)
119.3(3)
O2–Co1–N2#1
O2–Co1–O4
N2#1–Co1–O4
N2–Co1–O1
O4–Co1–O1
O2–Co2–N4#1
O2–Co2–N4
N4#1–Co2–N4
O5–Co2–O3
N4–Co2–O3
N1–O2–Co1
C8–N2–Co1
C10–N2–Co1
C3–C2–C1
124.59(5)
97.10(8)
94.30(6)
89.75(6)
173.08(8)
120.98(5)
120.98(5)
113.89(10)
173.95(8)
87.54(6)
116.24(16)
123.59(15)
129.67(16)
115.6(3)
113.1(3)
38.9(4)
C5–C4–C3#1
C3#1–C4–C3
O5–C6–C4
O3–C5–N1
N1–C5–C4
O1–C1–N1
125.6(3)
124.4(3)
O1–C1–C2
Hydrogen bonds
D–H. . .A
d(D–H)
0.88
0.88
d(H. . .A)
2.08
2.00
d(D. . .A)
2.910(3)
2.825(3)
\(DHA)
157.0
156.2
N3–H3C. . .O6#2
N5–H5A. . .O7#3

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D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
pentacoordinated (D0,GluH2A2), together with the
presence of one bridging acetate in (D0,GluH2A2) com-
pared with two bridging acetates in (D,GluH2A2), ac-
counts for the shorter Co–Co distance in (D,GluH2A2)
[18] compared with that in (D0,GluH2A2).
The ability of Co(II) to form an analogous series
of complexes to those of Ni(II), including formation of
hydroxamate-bridged complexes and elimination of
hydroxylamine from glutarodihydroxamic acid, is in
accord with studies of the exchange of metal(II) centres
within metalloenzymes, as noted previously, although in
the preliminary study of urease partial replacement of
nickel(II) by cobalt(II) apparently led to a reduction in
urease activity.
The closely related enzyme methionine aminopepti-
dase (MAP) contains a dicobalt(II) active site with ap-
proximately octahedral coordination about each
cobalt(II) centre by both carboxylates and the histidine
nitrogens [4]. Inhibition by epoxide-containing com-
pounds such as fumagilin and ovalicin occurs by mod-
ification of the histidine centre. The epoxide also inhibits
angiogenesis, which may be a therapeutically important
reagent [31]. However, to date inhibition of MAP by
hydroxamic acids has not been proven or structurally
characterized.
anol was reacted with a solution of the hydroxamic acid
also in methanol for about 1–2 h. After work-up (see
experimental section) relatively pure hydroxamate
complexes of aceto-, benzo- and N-phenylacetohydrox-
amic acids of general formula [M2(urea) (OAc)2(RA)
(Im)4][PF6]; M ¼ Co, Ni; RA ¼ AA, BA, NPhAA were
isolated. As in the above case of the NPhAHA com-
plexes of B0 and D0, the analogues of the tmen series
(A,NPhAHA) and (C,NPhAHA) were also synthesized
as pure complexes (see Section 3). Unfortunately, none
of the complexes yielded suitable crystals for structure
determination.
2.10. Hydroxamate-bridged metal complexes containing
bridging pivalates
2.10.1. Reactions of B00 and D00 with hydroxamic acids
To compare the effect of replacing the bridging acetate
groups in the model hydrolases, [M2(OAc)4(H2O)
(tmen)2]; M ¼ Ni (B) and Co (D) by pivalate groups
[M2(OPiv)4(H2O)(tmen)2]; M ¼ Ni(B00) and Co(D00), the
latter were reacted with aceto-, benzo- and N-phen-
ylacetohydroxamic acids and glutarodihydroxamic acid
(Fig. 2). The reactions, which were carried out in dried
methanol with a 2:1 molar ratio of hydroxamic acid to
metal pivalate with 1 mole of TMS–OTf injected into the
reaction mixture, were facile and complete in several
minutes. Satisfactory microanalyses were obtained in
all cases (see Section 3) for the following complexes:
(B00,AHA), (D00,AHA), (B00,BHA), (D00,BHA), (B00,NPhAHA),
(D00,NPhAHA) andcrystals were obtainedin all cases with
crystal structures obtained for the last two complexes (see
Figs. 6 and 7).
2.9. Reactions of urea model hydrolases with hydroxamic
acids
As discussed above, attempts to prepare urea com-
plexes [M2(urea)(OAc)3(Im)4][PF6] M ¼ Ni (A0); Co (C0)
analogous to the previously reported [M2(urea)
(OAc)3(tmen)2][OTf] M ¼ Ni (A) [27] and Co (C) [18]
met with only limited success and gave rather impure
products (by microanalyses) although the infrared
spectra are very similar in the two series (Table 7).
Consequently their reactions with hydroxamic acids
were studied. A solution of impure A0 and C0 in meth-
2.10.2. Crystal structures of (B00,NPhAHA) and (D00,
NPhAHA)
Both complexes showed typical bridging/chelating
hydroxamate structures (Figs. 6 and 7) with selected
Table 7
Infrared spectroscopic data (cmꢀ1) of dinuclear based model hydrolases containing (urea, tmen) and (urea, Im)
Medium
KBr
A
A0
C
C0
Assignment
1669
1621
1553
1669
1621
1559
1669
1618
1559
1669
1622
1543
Urea
Bidentate bridging acetate
Monodentate acetate
CH2CI2
Acetone
MeOH
1664
1617
1549
Ins.
1664
1614
1553
Ins.
Urea
Bidentate bridging acetate
Monodentate acetate
1668
1619
1552
Ins.
1669
1617
1550
Ins.
Urea
Bidentate bridging acetate
Monodentate acetate
1665
1630
1552
1605
1666
1628
1554
1611
1666
1627
1560
1604
1667
1629
1554
1611
Urea
Bidentate bridging acetate
Monodentate acetate
H-bonded acetate
[Ni2(urea)(OAc)3(tmen)2][OTf] (A); [Ni2(urea)(OAc)3(Im)4][PF6] (A0); [Co2(urea)(OAc)3(tmen)2][OTf] (C); [Co2(urea)(OAc)3(Im)4][PF6] (C0).

D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
1421
2.10.3. Reactions of B00 and D00 with glutarodihydroxamic
acid
Both of the pivalate-bridged model hydrolases, B00
and D00 reacted rapidly with glutarodihydroxamic acid
in methanol with KPF6 to give pure products (see Sec-
tion 3) involving elimination of hydroxylamine and
formation of the cyclized N-hydroxyglutarimide with
the deprotonated N–OH bridging the two metal centres
and the two carbonyl groups each coordinating one
metal centre as characterized earlier for the analogous
acetate-bridged series. Unlike the imidazole-based
model complexes (B0 and D0), which gave similar cyc-
lized products, but containing only one bridging acetate
group, the complexes formed by the pivalate group
containing two bridging pivalates in agreement with the
acetate bridging series. Crystal structure determination
of the two complexes gave poor resolution and large R
factors (ꢂ24%) but connectivity information allowed
general structures to be confirmed (see Fig. 8).
2.11. Manganese complexes
No structural studies of manganese hydroxamates
have yet been reported, to the best knowledge of the
authors, perhaps reflecting the instability of these com-
plexes and the ease of oxidation, which is also a feature
of the structural model manganese hydrolases:
[Mn2(OAc)4(H2O)(tmen)2] (E) [4] and [Mn2(OAc)4
(H2O)(Im)4] (E0) [20] investigated in the present work.
For example, no electronic spectra could be recorded for
the pivalate-bridged complex [Mn2(OPiv)4(H2O)
(tmen)2] (E00) due to rapid decomposition in various
Fig. 6. Molecular structure of the complex [Ni2(OPiv)
(NPhAA)2(tmen)2][PF6] (B00,NPhAHA).
Fig. 7. Molecular structure of the complex [Co2(OPiv)(NPhAA)2
(tmen)2][PF6] (D00,NPhAHA).
bond distances and bond angles given in Tables 10 and
11 respectively. The structures are very similar to that of
the related acetate-bridged complex (B,NPhAHA) in
ꢀ
Fig. 4 with metal–metal bond distances of 3.0551(7) A;
(B00,NPhAHA) and 3.0518(10)
ꢀ
ꢀ
3.0491(8)
A
A
Fig. 8. Molecular structure of the complex [Ni2(OPiv)2{O(N)
(O@C)2(CH2)3}(tmen)2][PF6] (B00,GluH2A2).
(D00,NPhAHA).

1422
D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
due to the fact that the crystals were large and had to be
cut-down to size.
This is the first example of a structurally character-
ized manganese hydroxamate and the hydroxamate
group bridges the two manganese centres in exactly the
same bonding mode as occurs in the cobalt and nickel
dimeric series reported. However, in contrast to those
series, reaction of E with AHA results in formation of a
complex with only one bridging hydroxamate and three
bridging acetates in contrast to the cobalt and nickel
complexes formed by the reaction of a hydroxamic acid
with B, D, B0, D0, B00 and D00 all of which contain two
bridging hydroxamates [13,16,18]. This result suggests
that hydroxamic acids may also inhibit the manganese-
based hydrolase, arginase which contains a dimanganese
centre [32].
Fig. 9. Molecular structure of the complex [Mn2(OAc)3(AA)(tmen)2]
(E,AHA).
2.12. Spectroscopic studies
2.12.1. UV/visible spectra
solvents. Similarly, the reaction of E with urea gave only
impure products. However in one case, the reaction of E
with AHA, the bridged hydroxamate complex
[Mn2(OAc)3(AA)(tmen)2] (E,AHA) was isolated and
crystals suitable for X-ray analysis obtained. The
structure is shown in Fig. 9 and selected bond distances
and bond angles are given in Table 8. The R factor for
this structure is somewhat larger than usual, probably
The electronic spectra of both B0 and D0 in methanol
are typical of distorted octahedral Ni(II) and Co(II)
complexes and very close to those of the analogous
tmen-based complexes B and D. Thus the nickel com-
plexes B0 and B show three bands in the region 981–
1075, 637–389 and 373–389 nm assigned to the three
3
3
3
spin-allowed transitions A2gðFÞ ! T2gðFÞ; A2gðFÞ !
3
3
3T1gðFÞ; and A2gðFÞ ! T1gðPÞ for an octahedral d8
Table 8
ꢀ
Selected bond lengths [A] and angles [(°)] for [Mn2(OAc)3(AA)(tmen)2] (E,AHA)
Mn(11)–Mn(12)
Mn(11)–O(13)
Mn(11)–O(17)
Mn(11)–O(11)
Mn(11)–O(15)
Mn(11)–N(12)
Mn(11)–N(11)
3.542(18)
2.133(6)
2.138(6)
2.154(6)
2.162(6)
2.366(7)
2.420(7)
Mn(12)–O(14)
Mn(12)–O(16)
Mn(12)–O(11)
Mn(12)–O(12)
Mn(12)–N(14)
Mn(12)–N(13)
2.131(6)
2.142(6)
2.171(6)
2.196(6)
2.336(7)
2.369(7)
O13–Mn11–O17
O13–Mn11–O11
O17–Mn11–O11
O13–Mn11–O15
O17–Mn11–O15
O11–Mn11–O15
O13–Mn11–N12
O17–Mn11–N12
O11–Mn11–N12
O15–Mn11–N12
O13–Mn11–N11
O17–Mn11–N11
O11–Mn11–N11
O15–Mn11–N11
N12–Mn11–N11
93.0(2)
94.9(2)
99.3(2)
95.4(2)
168.3(2)
88.0(2)
87.6(3)
86.8(2)
173.2(2)
85.5(2)
165.9(3)
87.3(2)
98.9(2)
82.6(2)
78.3(3)
O14–Mn12–O16
O14–Mn12–O11
O16–Mn12–O11
O14–Mn12–O12
O16–Mn12–O12
O11–Mn12–O12
O14–Mn12–N14
O16–Mn12–N14
O11–Mn12–N14
O12–Mn12–N14
O14–Mn12–N13
O16–Mn12–N13
O11–Mn12–N13
O12–Mn12–N13
N14–Mn12–N13
94.0(3)
99.0(2)
97.5(2)
91.3(3)
172.4(3)
76.3(2)
86.6(2)
91.9(2)
168.7(2)
93.8(2)
164.1(2)
85.0(2)
96.9(2)
91.4(2)
77.6(2)
Hydrogen bonds
D–H. . .A
d(D–H)
0.88
0.88
d(H. . .A)
1.90
1.95
d(D. . .A)
2.746(9)
2.770(11)
\(DHA)
160.5
155.2
N(15)–H(15D). . .O(18)
N(25)–H(25D). . .O(28)

D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
1423
complex, whilst the cobalt(II) complexes D0 and D
2.13. Vibrational spectra
2.13.1. Solid state (KBr)
showed two broad bands in the regions 933–1089 and
528–575 nm, assigned to two spin-allowed transitions.
The first band is assigned to the T1g ! T2g transition
The infrared spectra of B0 and D0 in the carboxylate
region are very similar to those of the tmen B and D
complexes. Thus, B, B0, D and D0 showed carbonyl
peaks at 1635, 1615, 1630, 1611 cmꢀ1 respectively, as-
signed to bidentate bridging acetates and at 1542, 1538,
1533, 1537 cmꢀ1 assigned to the monodentate bridging
acetates. Each complex showed two identical weak
broad peaks at 2023, 2111 (B); 1942, 2055 (B0); 2024,
2088 (D) and 2018, 2120 (D0) cmꢀ1, assigned to the
bridging water (Table 9). These two water peaks are
diagnostic in assigning B0 and D0 as dimers. The unusual
low vibrational frequency of the O-H band compared to
the normal water ligand is consistent with their in-
volvement in strong intramolecular hydrogen bonds as
revealed in the crystal structure (Fig. 2).
4
4
and the absorptions in the visible region to the
4
4
4
4T1g ! A2g and T1g ! T1gðPÞ transitions [33].
The electronic spectrum of (B0,GluH2A2) is typical of
distorted trigonal-bipyramidal d8 Ni(II), which showed
two bands for pentacoordination at 641 and 381 nm
3
assigned to d–d transitions 3B1ðFÞ ! EðFÞ and 3B1
3
ðFÞ ! A2, 3EðPÞ, respectively [34,35] and a broad band
at 1061 nm. Similarly, (D0,GluH2A2) showed three broad
bands in the visible region at about 13,000, 18,000,
21,000 cmꢀ1 (1057, 933 and 545 nm), typical for a high
spin d7 Co(II) ion in a trigonal-bipyramidal field [36]
0
0
4
4
4
4
and assigned to d–d transitions of A2ð FÞ ! A2ð PÞ,
0
0
00
0
4A2ð FÞ ! E ð PÞ, A2ð FÞ ! 4E ð FÞ [37].
4
4
4
4
4
4
The electronic spectra of the nickel complexes
(A0,AHA), (A0,BHA), (A0,NPhAHA) and (A,NPhAHA)
are all typical of distorted octahedral d8, Ni(II) whilst
the analogous cobalt complexes are typical of distorted
d7, Co(II).
All of the dibridged hydroxamate complexes
(B,NPhAHA), (D,NPhAHA), (B0,NPhAHA) and
(D0,NPhAHA) showed two bands in the carboxylate
region, ꢂ1620–1630 cmꢀ1 and 1570–1590 cmꢀ1 assigned
to the bridging bidentate acetates and bridging/chelating
hydroxamates respectively (Tables 12 and 13) in accord
with the crystal structure (Fig. 5).
The infrared spectra (KBr) for the urea complexes
showed peaks at 1669, 1621, 1559 cmꢀ1 (A0) and 1669,
1622, 1543 cmꢀ1 (C0), assigned to the coordinated urea,
bidentate bridging acetates and monodentate acetates,
respectively compared to those at 1669, 1621, 1553 cmꢀ1
(A) [27] and at 1669, 1618, 1559 cmꢀ1 (C) [18] (Table 7).
Both (B0,GluH2A2) and (D0,GluH2A2) showed infra-
red peaks at 1618, 1575 and 1643, 1576 cmꢀ1 respec-
tively, assigned to bridging acetates and coordinated
hydroxamates respectively, with additional peaks at
1684 and 1698 cmꢀ1 respectively assigned to the car-
bonyl of the bidentate glutarimide (Table 13).
The electronic spectra of the pivalate complexes B00
and D00 in dichloromethane are typical of distorted oc-
tahedral Ni(II) and Co(II) complexes and very close to
those of the analogous tmen-based complexes B and D.
Thus the nickel pivalate complex B00 showed three bands
in the region 393, 653 and 1085 nm, whilst the analogous
cobalt(II) complex D00 showed two broad bands in the
regions 513 and 1097 nm assigned as above. Unfortu-
nately, no electronic spectra for the analogous manga-
nese [Mn2(OPiv)4(H2O)(tmen)2] (E00) were obtained due
to decomposition of the dimer in all solvents. The elec-
tronic spectra of (B00,GluH2A2) and (D00,GluH2A2) are
similar to those for the previous tmen series. The elec-
tronic spectra are tabulated in the supplementary
materials.
Table 9
Infrared spectroscopic data (cmꢀ1) for dinuclear model hydrolases containing tmen and Im ligands
Medium
KBr
B
B0
B00
D
D0
D00
E
E0
E00
Assignment
1635
1542
2023,
2111
1615
1538
1942,
2055
1622
1516
2016,
2111
1630
1533
2024,
2088
1611
1538
2018,
2120
1619
1523
2016,
2094
1620
1535
2011,
1960
1614
1535
2003,
1946
1620
1519
2010,
2090
Bidentate acetate
Monodentate acetate
Bridging water
CH2CI2
1624
1549
Ins.
1621
1530
1624
1557
Ins.
1616
1540
d.
d.
d.
Bidentate acetate
Monodentate acetate
MeOH
1557
1562
1544
1562
1571
1543
d.
d.
d.
d.
d.
d.
Monodentate acetate
CH3CN
1629
1553
1610
1558
1622
1533
1626
1559
1632
1528
1618
1541
Bidentate acetate
Monodentate acetate
All values in cmꢀ1. d, decomposed. Ins, Insoluble. [Ni2(OAc)4(H2O)(tmen)2] (B); [Ni2(OAc)4(H2O)(Im)4] (B0); [Ni2(OPiv)4(H2O)(tmen)2] (B00);
[Co2(OAc)4(H2O)(tmen)2] (D); [Co2(OAc)4(H2O)(Im)4] (D0); [Co2(OPiv)4(H2O)(tmen)2] (D00); [Mn2(OAc)4(H2O)(tmen)2] (E); [Mn2(OAc)4(H2O)
(Im)4] (E0); [Mn2(OPiv)4(H2O)(tmen)2] (E00).

1424
D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
Table 10
00
ꢀ
Bond lengths [A] and angles [°] for [Ni2(OPiv)(NPhAA)2(tmen)2][PF6] (B ,NPAHA)
Ni(1)–O(4)
Ni(1)–O(5)
Ni(1)–O(3)
Ni(1)–N(4)
Ni(1)–O(1)
Ni(1)–N(3)
Ni(2)–O(2)
O(5)–C(29)
2.035(3)
2.035(3)
2.061(3)
2.116(4)
2.151(3)
2.228(4)
2.026(3)
1.259(5)
Ni(2)–O(6)
Ni(2)–O(1)
Ni(2)–N(5)
Ni(2)–O(3)
Ni(2)–N(6)
O(1)–N(1)
O(3)–N(2)
O(6)–C(29)
2.038(3)
2.059(3)
2.134(4)
2.163(3)
2.222(4)
1.401(4)
1.395(4)
1.259(5)
O(4)–Ni(1)–O(5)
O(4)–Ni(1)–O(3)
O(5)–Ni(1)–O(3)
O(4)–Ni(1)–N(4)
O(5)–Ni(1)–N(4)
O(3)–Ni(1)–N(4)
O(4)–Ni(1)–O(1)
O(5)–Ni(1)–O(1)
O(3)–Ni(1)–O(1)
N(4)–Ni(1)–O(1)
O(4)–Ni(1)–N(3)
O(5)–Ni(1)–N(3)
O(3)–Ni(1)–N(3)
N(4)–Ni(1)–N(3)
O(1)–Ni(1)–N(3)
C(23)–N(5)–Ni(2)
C(24)–N(5)–Ni(2)
C(26)–N(6)–Ni(2)
C(27)–N(6)–Ni(2)
C(28)–N(6)–Ni(2)
C(18)–N(3)–Ni(1)
C(17)–N(3)–Ni(1)
C(19)–N(3)–Ni(1)
C(21)–N(4)–Ni(1)
C(22)–N(4)–Ni(1)
C(20)–N(4)–Ni(1)
O(5)–C(29)–O(6)
175.60(12)
80.94(11)
94.67(11)
98.02(13)
86.37(13)
176.27(13)
90.57(12)
88.71(11)
83.49(11)
100.12(13)
90.20(13)
90.27(13)
93.37(12)
83.04(14)
176.61(12)
112.9(3)
O(2)–Ni(2)–O(6)
O(2)–Ni(2)–O(1)
O(6)–Ni(2)–O(1)
O(2)–Ni(2)–N(5)
O(6)–Ni(2)–N(5)
O(1)–Ni(2)–N(5)
O(2)–Ni(2)–O(3)
O(6)–Ni(2)–O(3)
O(1)–Ni(2)–O(3)
N(5)–Ni(2)–O(3)
O(2)–Ni(2)–N(6)
O(6)–Ni(2)–N(6)
O(1)–Ni(2)–N(6)
N(5)–Ni(2)–N(6)
O(3)–Ni(2)–N(6)
N(1)–O(1)–Ni(2)
N(1)–O(1)–Ni(1)
Ni(2)–O(1)–Ni(1)
C(7)–O(2)–Ni(2)
C(15)–O(4)–Ni(1)
N(2)–O(3)–Ni(1)
N(2)–O(3)–Ni(2)
Ni(1)–O(3)–Ni(2)
C(29)–O(6)–Ni(2)
C(29)–O(5)–Ni(1)
C(25)–N(5)–Ni(2)
176.02(12)
80.84(11)
95.34(11)
97.71(13)
86.18(13)
175.83(14)
88.86(11)
89.58(11)
83.26(11)
100.65(13)
90.58(14)
90.74(14)
93.05(13)
83.03(15)
176.31(13)
106.9(2)
113.2(3)
105.7(3)
116.6(2)
92.79(11)
111.8(3)
111.5(3)
108.5(3)
116.0(3)
109.9(3)
115.5(3)
106.6(2)
117.5(2)
105.2(3)
113.7(3)
92.36(11)
128.0(3)
129.6(3)
112.9(3)
103.7(3)
124.7(4)
103.5(3)
Table 11
Bond lengths [A] and angles [°] for [Co2(OPiv)(NPhAA)2(tmen)2][PF6] (D ,NPhAHA)
00
ꢀ
Co(1)–(5)
2.066(3)
2.079(3)
2.079(3)
2.152(4)
2.157(3)
2.278(4)
1.249(6)
Co(2)–O(2)
Co(2)–O(6)
Co(2)–O(1)
Co(2)–N(6)
Co(2)–O(3)
Co(2)–N(5)
C(29)– O(6)
2.055(3)
2.059(3)
2.088(3)
2.170(4)
2.182(3)
2.272(4)
1.254(6)
Co(1)–O(3)
Co(1)–O(4)
Co(1)–N(3)
Co(1)–O(1)
Co(1)–N(4)
C(29)– O(5)
O(5)–Co(1)–O(3)
O(5)–Co(1)–O(4)
O(3)–Co(1)–O(4)
O(5)–Co(1)–N(3)
O(3)–Co(1)–N(3)
O(4)–Co(1)–N(3)
O(5)–Co(1)–O(1)
O(3)–Co(1)–O(1)
O(4)–Co(1)–O(1)
N(3)–Co(1)–O(1)
O(5)–Co(1)–N(4)
O(3)–Co(1)–N(4)
O(4)–Co(1)–N(4)
N(3)–Co(1)–N(4)
O(1)–Co(1)–N(4)
Co(1)–O(3)–Co(2)
O(5)–C(29)–O(6)
94.77(13)
174.14(14)
79.38(13)
86.47(15)
174.45(15)
99.33(15)
88.98(13)
84.48(12)
90.74(13)
100.97(14)
90.27(14)
93.14(14)
89.75(14)
81.44(15)
177.43(14)
91.47(12)
125.4(4)
O(2)–Co(2)–O(6)
O(2)–Co(2)–O(1)
O(6)–Co(2)–O(1)
O(2)–Co(2)–N(6)
O(6)–Co(2)–N(6)
O(1)–Co(2)–N(6)
O(2)–Co(2)–O(3)
O(6)–Co(2)–O(3)
O(1)–Co(2)–O(3)
N(6)–Co(2)–O(3)
O(2)–Co(2)–N(5)
O(6)–Co(2)–N(5)
O(1)–Co(2)–N(5)
N(6)–Co(2)–N(5)
O(3)–Co(2)–N(5)
Co(1)–O(1)–Co(2)
175.12(14)
79.35(13)
95.80(13)
98.10(15)
86.78(15)
173.52(15)
88.83(13)
90.12(13)
83.64(12)
102.33(15)
89.91(15)
90.83(15)
92.55(14)
81.45(16)
176.15(15)
91.92(12)

D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
1425
Table 12
Infrared spectroscopic data (cmꢀ1) for hydroxamate-inhibited dinuclear model hydrolases containing the tmen and NPhAHA ligands
Medium
KBr
(B,NPhAA)
(D,NPhAH)
(A,NPhAHA)
(C,NPhAHA)
Assignment
1669
1609
1594
1669
1608
1594
Urea
Bidentate bridging acetate
Coordinated hydroxamate
1609
1597
1606
1590
MeOH
CH2Cl2
1667
1610
1558
1596
1668
1608
1557
1594
Urea
1610
1577
1598
1607
1574
1587
Bidentate bridging acetate
Monodentate acetate
Coordinated hydroxamate
1663
1609
1577
1596
1664
1609
1574
1594
Urea
1609
1576
1596
1604
1573
1590
Bidentate bridging acetate
Monodentate acetate
Coordinated hydroxamate
[Ni2(urea)(OAc)2(NPhAA)(tmen)2][OTf] (A,NPhAHA); [Ni2(OAc)(NPhAA)2(tmen)2][PF6] (B,NPhAHA); [Co2(urea)(OAc)2(NPhAA)
(tmen)2][OTf] (C,NPhAHA); [Co2(OAc)(NPhAA)2(tmen)2][PF6] (D,NPhAHA).
The infrared spectra of [Ni2(OPiv)4(H2O)(tmen)2]
(B00); [Co2(OPiv)4(H2O)(tmen)2] (D00) and [Mn2(OPiv)4
(H2O)(tmen)2] (E00); in the carboxylate region are very
similar to those of the corresponding acetate complexes
which there is weak interaction between one of the
carboxylate oxygen atoms and the nickel centre (the
carboxylate shift) [38]. Unfortunately, neither B0 nor D0
were sufficiently soluble in CH2Cl2 to give good IR
spectra. In methanol the four dimeric complexes B, B0,
D and D0 all gave a single carboxylate peak in the region
1550–1570 cmꢀ1 assigned to a monodentate acetate
group (Table 9), indicating dissociation of the dimeric
system.
In CH2Cl2, both (B,NPhAHA) and (D,NPhAHA)
showed peaks in the carbonyl region at 1609, 1596, 1576
and 1604, 1590, 1573 cmꢀ1 respectively, assigned to the
bidentate bridging acetate, coordinated hydroxamate
and monodentate acetate. In methanol the above com-
plexes showed peaks at 1610, 1598, 1577 and 1607, 1587,
1574 cmꢀ1 assigned as above (Table 12). In the case of
(B0,NPhAHA) and (D0,NPhAHA), the solution spectra
in methanol showed similar peaks at 1607, 1593, 1558
and 1634, 1584, 1556 cmꢀ1 respectively, assigned as
above. The monodentate acetate arises from the car-
boxylate shift mechanism [38] stabilized by H-bonding
to the methanol solvent. The complex (B0,NPhAHA)
was slightly soluble in acetonitrile and the solution
spectra showed peaks at 1607, 1594 and 1542 cmꢀ1
(Table 13).
In methanol, the solution spectra of (B0,GluH2A2) and
(D0,GluH2A2) showed infrared peaks at 1622, 1581, 1548,
1680 cmꢀ1 and 1628, 1576, 1550, 1693 cmꢀ1 respectively,
assigned to the bidentate bridging acetates, coordinated
hydroxamates and monodentate acetates respectively.
The monodentate acetate peaks are absent in the KBr
spectra. Additional peaks at 1680 and 1693 cmꢀ1 re-
spectively, were assigned to the two carbonyl groups of
the coordinated deprotonated N-hydroxyglutarimide
with two broad peaks in the regions 1754, 1711 and 1753,
1713 cmꢀ1 assigned to free acetic acid. Similar spectra
were obtained in acetonitrile (Table 13).
[Ni2(OAc)4(H2O)(tmen)2]
(B);
[Co2(OAc)4(H2O)
(tmen)2] (D) and [Mn2(OAc)4(H2O)(tmen)2] (E). Thus,
B00, D00 and E00 show carboxylate peaks at 1622, 1619,
1620 cmꢀ1, respectively assigned to bidentate bridging
pivalate and 1516, 1523, 1519 cmꢀ1 assigned to the
monodentate pivalate compared with 1620, 1630, 1635
and 1535, 1533, 1535 cmꢀ1 for B, D and E, respectively.
Each complex showed two identical weak broad peaks
in the region 2000–2200 cmꢀ1 assigned to the bridging
water (Table 9). These broad water peaks are diagnostic
in assigning E, (E00); D, (D00) and B, (B00) as dimers.
Both (B00,AHA) and (D00,AHA) showed peaks in the
carboxylate region at (1627, 1582) and (1628, 1578)
cmꢀ1 respectively, assigned to bridging bidentate piva-
late and bridging/chelating hydroxamates whilst the
corresponding benzo- and N-phenylacetohydroxamate
complexes gave similar spectra (Table 14).
The infrared spectra of (B00,GluH2A2) and
(D00,GluH2A2) complexes showed peaks at (1636, 1589)
and (1637, 1586) cmꢀ1 assigned respectively to bridging
pivalate and coordinated hydroxamate. Additional peaks
at 1688 and 1690 cmꢀ1 were assigned to the asymmetric
stretch of the two carboxylate groups of the coordinated
N-hydroxyglutarimide (Table 14), respectively.
2.13.2. Solution spectra in dichloromethane, methanol and
acetonitrile
The tmen complexes B and D showed peaks in
CH2Cl2 at 1624, 1549 and 1624, 1557 cmꢀ1 (Table 9)
assigned to the carbonyl region of the bidentate bridging
acetate and monodentate acetate, respectively and close
to those in KBr. It has been suggested that in certain
solvents, a bridging bidentate acetate is in equilibrium
with a monodentate bridging intermediate acetate in
In methanol, the infrared spectra of the urea com-
plexes, A0 and C0 are very close to those of the analogous

Table 13
Infrared spectroscopic data (cmꢀ1) for hydroxamate-inhibited dinuclear model hydrolases containing the imidazole ligand
Medium
KBr
(B0,AHA)
(D0,AHA)
(B0,BHA)
(D0,BHA)
(B0,GluH2A2)
(D0,GluH2A2)
(B0,NPhAHA)
(D0,NPhAHA)
Assignment
1628
1586
1624
1576
1612
1575
1607
1570
1618
1575
1684
1643
1576
1698
1607
1591
1612
1593
Bidentate bridging acetate
Coordinated hydroxamate
Bidentate glutarimide
MeOH
1628
1558
1581
1627
1556
1580
1614
1545
1581
1620
1536
1579
1622
1542
1628
1550
1607
1558
1593
1634
1556
1584
Bidentate bridging acetate
Monodentate acetate
Coordinated hydroxamate
Bidentate glutarimide
Free acetic acid
1578
1683
1576
1693
1734, 1715
Ins.
1755,1711
Ins.
1754, 1711
1754, 1710
1753, 1711
1753, 1713
1745, 1712
1756, 1709
Ins.
CH3CN
1632
1546
1581
1632
1523
1570
1607
1542
1630
1543
1578
1691
1607
1542
1594
Bidentate bridging acetate
Monodentate acetate
Coordinated hydroxamate
Bidentate glutarimide
Free acetic acid
1594
1680
1754, 1725
1753, 1725
1754, 1725
[Ni2(OAc)(AA)2(Im)4][OTf] (B0,AHA); [Co2(OAc)(AA)2(Im)4][OAc] ꢁ HOAc ꢁ H2O (D0,AHA); [Ni2(OAc)(BA)2(Im)4][OAc] ꢁ HOAc ꢁ H2O (B0,BHA); [Co2(OAc)(BA)2(Im)4][OAc] ꢁ HOAc ꢁ H2O
(D0,BHA); [Ni2(OAc)(NPhAA)2(Im)4][OAc] ꢁ 2H2O (B0,NPhAHA); [Co2(OAc)(NPhAA)2(Im)4][OAc] ꢁ 2H2O (D0,NPhAHA); [Ni2(OAc){O(N)(O@C)2(CH2)3}(Im)4][OTf]2 (B0,GluH2A2);
[Co2(OAc){O(N)(O@C)2(CH2)3}(Im)4][OTf]2 (D0,GluH2A2).
Table 14
Infrared spectroscopic data (cmꢀ1) for hydroxamate-inhibited dinuclear model hydrolases containing the pivalate ligand
Medium
KBr
(B00,AHA)
(D00,AHA)
(B00,BHA)
(D00,BHA)
(B00,NPhAHA)
(D00,NPhAHA)
(B00,GluH2A2)
(D00,GluH2A2)
Assignment
1627
1582
1628
1578
1610
1578
1614
1579
1606
1590
1606
1591
1636
1589
1688
1637
1586
1690
Bidentate pivalate
Coordinated hydroxamate
Glutarimide hydroxamate
CH2Cl2
1625
1550
1580
1628
1530
1576
1621
1526
1622
1539
1604
1535
1604
1540
1640
1568
1588
1692
1640
1531
1584
1693
Bidentate pivalate
Monodentate pivalate
Coordinated hydroxamate
Glutarimide hydroxamate
Monodentate hydroxamate
1581s,1570w
1592sh,1574s
1592s, 1572sh
1590s, 1571sh
1614
1615
CH3OH
1628
1549
1579
1628
1548
1574
1620
1549
1622
1547
1604
1550
1593
1605
1553
1587
1640
1555
1578
1621s
1641
1550
1587
Bidentate pivalate
Monodentate pivalate
Coordinated hydroxamate
Extra peak
1582s,1571sh
1614s
1594m, 1578s
[Ni2(OPiv)(AA)2(tmen)2][OTf] (B00,AHA); [Co2(OPiv)(AA)2(tmen)2][OTf] (D00,AHA); [Ni2(OPiv)(BA)2(tmen)2][OTf] (B00,BHA); [Co2(OPiv)(BA)2(tmen)2][OTf] (D00,BHA); [Ni2(OPiv)(NPhAA)2
(tmen)2][PF6] (B00,NPhAHA); [Co2(OPiv)(NPhAA)2(tmen)2][PF6] (D00,NPhAHA); [Ni2(OPiv)2{O(N)(O@C)2(CH2)3}(tmen)2][PF6] (B00,GluH2A2); [Co2(OPiv)2{O(N)(O@C)2(CH2)3)}(tmen)2][PF6]
(D00,GluH2A2).

D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
1427
tmen complexes A and C with bands at 1666, 1628, 1554
and 1667, 1629, 1554 cmꢀ1 respectively, assigned to the
coordinated urea, bidentate bridging acetate and
monodentate acetate respectively. The band at
1611 cmꢀ1 is shifted by more than 50 cmꢀ1 compared to
the monodentate bridging possibly due to a monodentate
acetate stabilized by H-bonding to methanol (Table 7).
The solution spectra of the urea hydroxamate com-
plexes (A0,AHA), (A0,BHA), (A0,NPhAHA) and
(A,NPhAHA) in methanol are similar to those in the
solid state. The additional peak at 1520 cmꢀ1 region is
assigned to a monodentate acetate formed by the car-
boxylate shift mechanism [17,38] and stabilized by H-
bonding to the methanol solvent (Tables 12 and 15).
The above cobalt and nickel series of complexes,
[M2(urea)(OAc)2(RA)(Im)4][PF6] showed peaks in the
regions 1670, 1624 and 1577 cmꢀ1 assigned to the co-
ordinated urea, bidentate bridging acetate and coordi-
nated hydroxamates respectively, in agreement with
previous studies of the related tmen-based complexes
[16,18].
In CH2Cl2, the solution spectra of the pivalate com-
plexes B00 and D00 are close to those in KBr. Neither B00
nor D00 showed evidence for a carboxylate shift. No
solution spectra could be obtained for E00 due to de-
composition in all solvents. In methanol, both B00 and
D00 give a single carboxylate peak in the region 1544,
1543 cmꢀ1 respectively, assigned to the monodentate
pivalate group indicating dissociation of the dimeric
system. The solution spectra of the pivalate hydroxa-
mate series also showed peaks in the same regions as in
the KBr spectra and are assigned to the bridging bid-
entate pivalate group and bridging/chelating hydroxa-
mates respectively, but in addition all showed bands at
ꢂ1530–1550 cmꢀ1 assigned to a monodentate carbox-
ylate arising from a carboxylate shift (Table 9).
In CH2Cl2, the solution spectra of (B00,AHA) and
(D00,AHA) showed peaks at 1625, 1580 and 1628, 1576
cmꢀ1, respectively assigned to the bidentate bridging
pivalate and the coordinated hydroxamate, respectively
and peaks at 1550 and 1530 cmꢀ1 assigned to the
monodentate pivalate (carboxylate shift). In methanol
(B00,AHA) and (D00,AHA) showed peaks at (1628, 1579,
1549 cmꢀ1) and (1628, 1574, 1548 cmꢀ1) assigned to the
bidentate bridging pivalate, coordinated hydroxamate
and monodentate pivalate, respectively (Table 14).
The concentrated solution spectra of (B00,BHA) and
(D00,BHA) in CH2Cl2, showed peaks at 1621, 1622 and
1581, 1574 cmꢀ1 respectively, assigned to the bidentate
bridging pivalate and the coordinated hydroxamate, and
peaks at 1526 and 1539 cmꢀ1, assigned to the monod-
entate pivalate. Surprisingly, both complexes showed a
sharp peak at 1614 assigned to a monodentate hy-
droxamate (hydroxamate shift) [38] (Table 14).
The solution spectra of (B00,NPhAHA) and
(D00,NPhAHA) in CH2Cl2 showed peaks at 1604, 1592,

1428
D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
1535 and 1604, 1590, 1540 cmꢀ1 respectively, assigned
again to the bidentate bridging pivalate, coordinated
hydroxamate and monodentate pivalate respectively. In
methanol, the infrared solution spectra of (B00,NPhA-
HA) and (D00,NPhAHA) showed peaks at 1604, 1593,
1550 and 1605, 1587, 1553 cmꢀ1, respectively assigned as
above (Table 14).
The solution spectra of (B00,GluH2A2) and
(D00,GluH2A2) in CH2Cl2 in the carbonyl region showed
peaks at 1640, 1588, 1692 and 1640, 1584, 1693 cmꢀ1
respectively, each set of peaks assigned to the bidentate
bridging pivalate, coordinated hydroxamate and biden-
tate glutarimide, respectively, and peaks at 1568, 1531
cmꢀ1 assigned to the monodentate pivalate. Similar
spectra were obtained in methanol for both complexes
with an extra peak at 1621 cmꢀ1 for (B00,GluH2A2)
(Table 14).
Fig. 11. Susceptibility vm and magnetic moment per dimer vs temper-
ature of complex [Ni2(OPiv)(NPhAA)2(tmen)2][PF6] (B00,NPhAHA).
vðT Þ ¼ ð1 ꢀ xpÞvdimðT Þ þ 2xpvpðTÞ þ 2Na:
With
ð1Þ
2.14. Magnetic susceptibility measurements
NAg2lB2 2eð2J=kTÞ þ 10eð6J=kTÞ
2.14.1. Nickel complexes
vdimðTÞ ¼
The temperature dependencies of the molar suscep-
tibility and the effective magnetic moments of
[Ni2(OAc)(NPhAA)2(tmen)2][PF6] (B,NPhAHA) and
[Ni2(OPiv)(NPhAA)2(tmen)2][PF6] (B00,NPhAHA) are
shown in Figs. 10 and 11, respectively. The effective
magnetic moments increase with decreasing tempera-
ture. Such behaviour can be influenced by the presence
of ferromagnetic exchange interaction between the
nickel(II) centres.
For a quantitative description of the magnetic prop-
erties of the examined dinickel compounds, the data
were fitted by using expression 1 for molar susceptibility
versus temperature based on the isotropic (Heisenberg–
Dirac–van Vleck) spin-exchange Hamiltonian:
kðT ꢀ hÞ 1 þ 3eð2J=KTÞ þ 5eð6J=KTÞ
NAg2l2B
3kT
and
vpðTÞ ¼
:
The Weiss constant h was included to describe phe-
nomenologically the decrease of the magnetic moment
at low temperatures, which is equivalent to including a
zero field splitting parameter D for the dinuclear unit,
which would be more complicated. Due to the ferro-
magnetic coupling, it is not possible to determine accu-
rately the paramagnetic impurity xp, so this parameter
was taken as zero. Na refers to the temperature-inde-
pendent paramagnetism, which is taken as 200 ꢃ 10ꢀ6
cm3/mol per nickel(II) ion [16]. The parameters obtained
for the acetate complex (B,NPhAHA) from the least-
squares fits of the experimental data to Eq. (1) are:
J ¼ 5:5 cmꢀ1 (ferromagnetic), D ¼ 6 cmꢀ1 and g ¼ 2:2,
and for the pivalate complex (B00,NPhAHA) are: J ¼ 5:5
cmꢀ1 (ferromagnetic exchange) and g ¼ 2:2 with no zero
field splitting D ¼ 0.
H ¼ ꢀ2JS1S2
ðS1 ¼ S2 ¼ 1Þ
2.15. Cobalt complexes
The theoretical treatment of dinuclear Co(II) com-
plexes belongs to one of the more difficult chapters of
magnetochemistry [16]. The Co(II) d7 ion is strongly
anisotropic and the first-order orbital momentum is no
longer negligible so the isotropic exchange interaction is
insufficient to discuss these complexes and must be
supplemented by consideration of orbitally dependent
exchange interactions as well as spin–orbit coupling.
This leads to invoking a large number of parameters in
order to attempt a quantitative discussion of the tem-
perature-dependent magnetic susceptibility data. Such
data should be supplemented by complementary tech-
Fig. 10. Susceptibility vm and effective magnetic moment per dimer vs
temperature of complex [Ni2(OAc)(NPhAA)2(tmen)2][PF6] (B,NPhA-
HA).

D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
1429
niques, such as variable temperature and variable mag-
netic field circular dichroism. However, access to such
techniques was not available so we have interpreted the
susceptibility data in terms of the minimum number of
parameters which are physically meaningful.
The complex [Co2(OAc){O(N)(O@C)2(CH2)3}(Im)4]
[OTf]2, (D0,GluH2A2) was magnetically characterized by
measuring the temperature dependence of its molar
susceptibility in the temperature range zero to 300 K
and this exhibits more or less a paramagnetic hyperbola.
The magnetic moment per dimer at room temperature is
higher than the spin-only value. The effective magnetic
moment of the complex decreases with decreasing tem-
perature from ꢂ5.3lB at 300 K to ꢂ2.2lB at 4.4 K.
The magnetic temperature dependence of the mag-
netic susceptibility and effective magnetic moment of
(D0, GluH2A2) is shown in Fig. 13
Fig. 12. Susceptibility vm and magnetic moment per dimer vs tem-
perature of complex [Co2(OPiv)(NPhAA)2(tmen)2][PF6] (D00,NPhA-
HA).
H ¼ ꢀ2JS1S2 þ S1D1S1 þ S2D2S2 þ bHg1S1 þ bHg2S2:
The magnetic data were interpreted in terms of the
above Hamiltonian. The first term is the isotropic ex-
change interaction between the Co(II) ions, the next two
terms are due to the local field splitting and the last two
terms are the Zeeman perturbation.
The orbital contribution of the exchange interaction
together with biquadratic exchange and anisotropic ex-
change interactions have all been neglected. Calcula-
tions were performed on a coupled basis and principal
values of the magnetic susceptibility calculated from the
usual expression:
Fig. 13. Susceptibility vm and effective magnetic moment per dimer vs
temperature of complex [Co2(OAc){O(N)(O@C)2(CH2)3}(Im)4][PF6]2
(D0,GluH2A2).
vl ¼ NKTo2=oHu2 ½ln Ri exp½ꢀEiðHuÞ=kTꢄꢄ
u ¼ x1y1z;
where Ei are the energy levels of the system in the ex-
ternal magnetic field.
The powder average susceptibility was calculated as:
1
3
The calculated values of the magnetic susceptibility
temperature dependencies of the molar susceptibility
and the effective magnetic moments are shown in
Fig. 12, but no satisfactory fitting of the data has yet
been accomplished.
vav
¼
ðvx þ vy þ vzÞ.
and the effective magnetic moment are presented in
Fig. 13 and the best-fit calculated parameters (with or
without spin–orbit coupling) of the local zero field
splitting of D0 ¼ 46 cmꢀ1 is typical for high spin Co(II).
The small value of the isotropic exchange interaction
(J ¼ ꢀ1 cmꢀ1) is in good agreement with the structure
of this complex, whereas the local g (parallel) (2.7), g
(perpendicular) (2.5) and the bridging bidentate acetate
ions do not provide a good pathway for super exchange
interaction, therefore only weak interaction could be
expected [39]. Due to this small value of the isotropic
exchange parameter, the excited states with M ¼ ꢅ1 lie
very close in energy to the antiferromagnetic ground
state and their thermal population is significant even at
very low temperatures. So at T ¼ꢂ 4:5 K, the effective
magnetic moment is about 2.2 Bohr magnetons.
3. Experimental
3.1. General
Reagents and solvents were used as obtained from
commercial sources, without further purification unless
otherwise stated. Methanol, dichloromethane, ethyl ac-
etate, ethanol, diethylether and petroleum ether were
dried by standard procedures.
3.2. Preparation of Hydroxamic Acids
Hydroxamic acids were prepared as described previ-
ously [40,41]. This reaction is carried out in alcoholic
solution in the presence of equimolar quantities of a
In
the
case
of
the
pivalate
complex
[Co2(OPiv)(NPhAA)2(tmen)2][PF6] (D00,NPhAHA), the

1430
D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
base such as sodium alkoxide or sodium carbonate. It is
important to exclude water from the reaction mixture,
as the amide bond of the hydroxamic acid is susceptible
to hydrolysis, which results in several side products. KCl
or NaCl are the major side products produced in these
preparations, which influence the purity of the hy-
droxamic acids prepared.
tered and a solution of 1.63 g (24 mmol) of imidazole in
15 ml acetonitrile was added dropwise over 15 min and
stirred for 12 h. The volume of solvent was reduced and
diethylether slowly diffused into the solution to precip-
itate a dark green solid. Yield: 2 g (62%). Found: C,
37.06; H, 4.59; N, 17.11%. Calculated for C20H30N8
O9Ni2: C, 37.30; H, 4.69; N, 17.40%. FT-IR data (KBr,
cmꢀ1) major peaks 2055–1942br, 1615vs, 1538s.
3.3. [Mn2(OAc)4(H2O)(Im)4] (E0)
3.7. [Mn9(Me3CCOOH)4(O)3(OH)3(Me3CCOO)12]
(Mn9 Cluster)
0.245 g (1 mmol) of Mn(OAc)2 ꢁ 4H2O were added to
a solution of imidazole 0.136 g (1 mmol) in dried de-
gassed ethanol (10 ml) under nitrogen and stirred for 3 h
at room temperature. After filtration, prismatic crystals
suitable for X-ray analysis were obtained by slow dif-
fusion of diethylether into the filtrate. Yield: 43%.
Found: C, 37.4; H, 4.4; N, 17.3%. Calculated for
C20H30N8O9Mn2: C, 37.7; H, 4.7; N, 17.6%. FT-IR data
(KBr,cmꢀ1) 2350–2270br, 1605vs, 1535s, 1416vs, 1069s,
754s, 657s, 649vs and 619m.
20.4 g (200 mmol) of pivalic acid in 50 ml of dried
degassed methanol were added over 30 min under ni-
trogen to a suspension of MnCO3 11.4 g (100 mmol) in
250 ml of dried degassed methanol. The solution was
stirred for 12 h, filtered under suction and nitrogen
passed slowly through the filtrate when colourless air-
sensitive crystals formed as needles which were kept in
closed sample bottles and used freshly. Yield: 8–10 g
(30–38%). Found: C, 48.20; H, 7.40; Mn, 22.31%. Cal-
culated for C80H151O38Mn9: C, 43.37; H, 6.87; Mn,
24.64%. FT-IR data (KBr, cmꢀ1) 3457br, 1703vs,
1610vs, 1573s, 1425vs, 1369s.
3.4. [Co(OAc)2(Im)2] (1/2 D0)
2.48 g (10 mmol) of Co(OAc)2 ꢁ 4H2O in 150 ml of
methanol was filtered and a solution of 1.63 g (24 mmol)
of imidazole in 20 ml methanol added dropwise over 15
min with stirring for 12 h. After filtration the solvent
was reduced under vacuum and slow diffusion of di-
ethylether into the solution gave dark/purple crystals of
[Co(OAc)2(Im)2]. Yield: 2.7 g (73%). Found: C, 38.18;
H, 4.43; N, 17.90%). Calculated for C10H14N4O4Co: C,
3.8. [Co9(Me3CCOOH)4(O)3(OH)3(Me3CCOO)12]
(Co9 Cluster)
20.4 g (200 mmol) of pivalic acid in 50 ml of
methanol were added over 30 min to a suspension of
CoCO3 11.8 g (100 mmol) in 250 ml of hot water and
stirred for 12 h, filtered and on standing, dark-pink
crystals formed. The Co9 Cluster changed colour from
dark pink to dark-blue on drying or on dissolving in
non-alcoholic solvents. Yield: 19.5 g (85%). Found: C,
44.51; H, 6.73; Co, 23.26%. Calculated for the blue
C80H151O38Co9: C, 42.67; H, 6.42; Co, 23.51%. FT-IR
data (KBr, cmꢀ1) 3436br, 1702vs, 1607vs, 1572s,
1428vs, 1361s.
38.35; H, 4.50; N, 17.89%. FT-IR data (KBr, cmꢀ1
)
major peaks 1593vs, 1573vs, 1419vs, 1404vs.
3.5. [Co2(OAc)4(H2O)(Im)4] (D0)
2.48 g (10 mmol) of Co(OAc)2 ꢁ 4H2O dissolved in
150 ml of a 1:1 water/acetonitrile mixture were refluxed
to give a clear solution, filtered and a solution of 1.63 g
(24 mmol) of imidazole in 20 ml of acetonitrile
(CH3CN) added dropwise over 15 min with stirring for
12 h. Filtration and on standing to dryness, gave two
products, a dark purple product, [Co(OAc)2(Im)2] (1/
2D0) and light pink crystals of [Co2(H2O)(OAc)4(Im)4]
(D0). Recrytallization from deionized water gave crystals
of both products, which were physically separated, wa-
shed in diethylether and dried. Yield of the dimer (D0)
2.1 g (62%). Found: C, 38.46; H, 4.93; N, 19.46% Cal-
culated for C20H30N8O9Co2: C, 38.27; H, 4.69; N,
17.39%. FT-IR data (KBr, cmꢀ1) major peaks 2120–
2018(br), 1611vs, 1538s.
3.9. [Ni9(Me3CCOOH)4(O)3(OH)3(Me3CCOO)12]
(Ni9 Cluster)
20.4 g (200 mmol) of pivalic acid in 50 ml of
methanol were added over 30 min to a suspension of
NiCO3 11.8 g (100 mmol) in 250 ml of hot water,
stirred for 12 h and filtered to give a clear solution.
The solution was evaporated under reduced pressure, a
green solid was obtained and dissolved in a 1:2 mixture
of acetone/diethylether and on standing, green needle
crystals formed. Yield: 14.1–15.5 g (50–55%). Found:
C, 42.47; H, 7.10; Ni, 21.42%. Calculated for
C80H151O38Ni9: C, 42.72; H, 6.76; Ni, 23.48%. FT-IR
data (KBr, cmꢀ1) 3460br, 1702vs, 1610vs, 1571s,
1412vs, 1359s.
3.6. [Ni2(OAc)4(H2O)(Im)4] (B0)
2.48 g (10 mmol) of Ni(OAc)2 ꢁ 4H2O were dissolved
in 50 ml of methanol (or in 1:1 water/acetonitrile), fil-

D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
1431
3.10. [Mn2(OPiv)4(H2O)(tmen)2] (E00)
nalyses approximately 1% from theory. Yield: 0.40 g
(49%). Found: C, 28.58; H, 3.93; N, 18.23%. Calculated
for C19F6H29N10O7PCo2: C, 29.57; H, 3.76; N, 18.15%.
FT-IR data (KBr, cmꢀ1) major peaks 1669s, 1622s,
1543s.
2.2 g (1 mmol) of (Mn9 Cluster) were dissolved in 20 ml
of dried degassed methanol and a solution of 1.4 ml
(9 mmol) of tmen in 10 ml of dried methanol added
dropwise over 15 min. After stirring for 30 min, the sol-
vent was evaporated under reduced pressure and the re-
sulting colourless solid dissolved in 50 ml of a 1/1 mixture
of ethyl acetate/diethylether, which on standing afforded
colourless crystals. Yield: 0.51 g (67%). Found: C, 50.30;
H, 9.19; N, 7.33%. Calculated for C32H70N4O9Mn2: C,
50.20; H, 9.22; N, 7.34%.. FT-IR data (KBr, cmꢀ1) major
peaks 2090, 2010br, 1620vs, 1519s.
3.15. [Ni2(OAc)(AA)2(Im)4][OTf] (B0,AHA)
0.643 g (1 mmol) of compound B0 was dissolved in 50
ml of dried methanol and TMS–OTf 0.18 ml (1 mmol)
injected into the solution and stirred for 1 h. A solution
of AHA 0.15 g (2 mmol) in 5 ml of methanol was then
added, and the mixture stirred for 30 min. After filtra-
tion, solvent was removed under vacuum to give an oil,
which was dissolved in 25 ml of hot acetonitrile, filtered
and 50 ml of diethylether added to give a blue green
solid [Ni2(OAc)(AA)2(Im)4][CF3SO3]. Yield: 0.55 g
(74%) Found: C, 31.53; H, 3.73; N, 18.21%. Calculated
for C19F3H27N10O9SNi2: C, 30.59; H, 3.64; N, 18.77%.
FT-IR data (KBr, cmꢀ1) major peaks 1628s, 1586s.
3.11. [Ni2(OPiv)4(H2O)(tmen)2] (B00)
2.25 g (1 mmol) of Ni9 Cluster were dissolved in 20 ml
methanol, and a solution of 1.4 ml (9 mmol) of tmen in
10 ml of methanol added dropwise over 15 min. After
stirring for 30 min, the solvent was evaporated under
reduced pressure and the residue dissolved in a 50/50
mixture of ethyl acetate/acetonitrile. On slow evapora-
tion in air for a period of 3 days; dark green crystals
formed suitable for X-ray crystallography. Yield: 1.8 g
(71%). Found: C, 49.63; H, 9.20; N, 7.26%. Calculated for
C32H70N4O9Ni2: C, 49.74; H, 9.14; N, 7.25%. FT-IR data
(KBr, cmꢀ1) major peaks 2111, 2016br, 1622vs, 1516s.
3.16. [Co2(OAc)(AA)2(Im)4][OAc] ꢁ HOAc ꢁ H2O (D0,
AHA)
0.644 g (1 mmol) of D0 was dissolved in 50 ml of
methanol and a solution of acetohydroxamic acid 0.15 g
(2 mmol) in 5 ml methanol added and stirred for 15 min.
After filtration, solvent was removed under vacuum to
give an oil which was extracted with 3 ꢃ 50 ml of a 1:1
diethyl ether/n-hexane mixture to give the dark pink
3.12. [Co2(OPiv)4(H2O)(tmen)2] (D00)
This was prepared by the same procedure as B00 using
the Co9 Cluster when pink crystals of D00 were obtained.
Yield: 1.63 g (68%). Found: C, 49.72; H, 9.16; N, 7.25%.
Calculated for C32H70N4O9Co2: C, 49.71; H, 9.13; N,
7.25%. FT-IR data (KBr, cmꢀ1) major peaks 2094–
2016br, 1619vs, 1523s.
complex
[Co2(OAc)(AA)2(Im)4][OAc] ꢁ HOAc ꢁ H2O.
Yield: 0.5 g (68%). Found: C, 36.10; H, 4.70; N, 18.56%.
Calculated for C22H36N10O11Co2: C, 36.0; H, 4.94; N,
19.08%. FT-IR data (KBr, cmꢀ1) major peaks 1624s,
1576s.
3.17. [Ni2(OAc)(BA)2(Im)4][OAc] ꢁ HOAc ꢁ H2O
(B0, BHA)
3.13. [Ni2(urea)(OAc)3(Im)4][PF6] (A0)
0.643 g (1 mmol) of B0 was dissolved in 100 ml of dry
methanol under nitrogen and KPF6 0.184 g (1 mmol)
added. The solution was stirred for 1 h, 0.15 g (2.5 mmol)
of urea were added and the solution stirred for a further 12
h and filtered. Solvent was removed under reduced pres-
sure to give an oil, which was extracted with a 1:1 mixture
of diethylether/n-pentane (ꢂ3 ꢃ 50 ml) to give a dark
green solid for which the microanalysis was approxi-
mately 2% from theory. Yield: 0.42g (53.8%). Found: C,
27.33; H, 3.98; N, 17.99%. Calculated for
C19F6H29N10O7PNi2: C, 29.57; H, 3.76; N, 18.12%. FT-
IR data (KBr, cmꢀ1) major peaks 1669s, 1621s, 1559s.
0.643 g (1 mmol) of B0 was dissolved in 50 ml of
methanol, filtered and a solution of benzohydroxamic
acid 0.274 g (2 mmol) in 5 ml methanol added and
stirred for 15 min. Filtration and removal of solvent
under vacuum gave an oil, which was extracted with
3 ꢃ 50 ml of a 1:1 mixture of diethylether/n-hexane to
give the dark green complex [Ni2(OAc)(BA)2(Im)4]
[OAc] ꢁ HOAc ꢁ H2O. Yield: 0.5 g (68%). Found: C,
45.04; H, 4.39; N, 15.39%. Calculated for
C32H40N10O11Ni2: C, 44.79; H, 4.69; N, 16.32%. FT-IR
data (KBr, cmꢀ1) major peaks 1612s, 1575s.
3.18. [Co2(OAc)(BA)2(Im)4][OAc] ꢁ HOAc ꢁ H2O (D0,
BHA)
3.14. [Co2(urea)(OAc)3(Im)4][PF6] (C0)
This was prepared by the same procedure as the
above using D0 in place of B0 and again gave microa-
This was prepared by the same procedure as
(B0,BHA) using D0 in place of B0. Yield: 0.5 g (68%).

1432
D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
Found: C, 44.26; H, 4.42; N, 15.85%. Calculated for
C32H40N10O11Co2: C, 44.76; H, 4.69; N, 16.31%. FT-IR
data (KBr, cmꢀ1) major peaks 1612s, 1575s.
O11S2Co2: C, 28.81; H, 2.87; N, 14.39%. FT-IR data
(KBr, cmꢀ1) major peaks 1698s, 1643s, 1576s.
3.23. [Ni2(OPiv)(AA)2(tmen)2][OTf] (B00,AHA)
3.19. [Ni2(OAc)(NPhAA)2(Im)4][OAc] ꢁ 2H2O (B0,
NPhAHA)
0.772 g (1 mmol) of [Ni2(OPiv)4(H2O)(tmen)2] (B00)
was dissolved under nitrogen in 10 ml of dry methanol
and 0.18 ml, 0.223 g (1 mmol) of TMS–OTf injected and
the solution stirred for 30 min. A solution of acetohy-
droxamic acid 0.15 g (2 mmol) in 5 ml of dried methanol
was then added and the mixture stirred for 30 min. After
filtration, the solvent was removed under vacuum to
give an oil, which was re-crystallized from a warm 1:1
mixture of ethyl acetate/acetonitrile) to give blue green
crystals suitable for X-ray crystallography. Yield: 0.39 g
(33.9%). Found: C, 34.51; H, 6.57; N, 10.98%. Calcu-
lated for C22F3H49N6O9SNi2: C, 35.32; H, 6.60; N,
11.23%. FT-IR data (KBr, cmꢀ1) major peaks 1627s,
1582s.
0.643 g (1 mmol) of compound B0 was dissolved in
50 ml of methanol, filtered and a solution of N-phen-
ylacetohydroxamic acid 0.302 g (2 mmol) in 5 ml
methanol was added and stirred for 15 min. After fil-
tration the solvent was removed under vacuum to give
an oil, which was extracted with 3 ꢃ 50 ml of a 1:1 di-
ethylether/n-hexane mixture to give the dark green
complex [Ni2(OAc)(NPhAA)2(Im)4][OAc] ꢁ 2H2O. Yield:
0.5 g (68%). Found: C, 44.98; H, 4.58; N, 15.81%. Cal-
culated for C32H42N10O10Ni2: C, 45.50; H, 5.03; N,
16.58%. FT-IR data (KBr, cmꢀ1) major peaks 1607s,
1591s.
3.20. [Co2(OAc)(NPhAA)2(Im)4][OAc] ꢁ 2H2O (D0,
NPhAHA)
3.24. [Co2(OPiv)(AA)2(tmen)2][OTf] (D00,AHA)
The analogous cobalt complex was prepared as dark
pink crystals by the same procedure as (B00,AHA) using
D00 in place of B00. Yield: 0.29 g (25%). Found: C, 34.46;
H, 6.60; N, 10.80%. Calculated for C22F3H49N6O9SCo2:
This was prepared by the same procedure as
(B0,NPhAHA) using D0 in place of B0. Found: C, 44.86;
H, 4.32; N, 17.55%. Calculated for C32H42N10O10Co2:
C, 45.50; H, 5.01; N, 16.58%. FT-IR data (KBr, cmꢀ1
)
C, 35.29; H, 6.59; N, 11.22%. FT-IR data (KBr, cmꢀ1
)
major peaks 1612s, 1593s.
major peaks 1628s, 1578s.
3.21. [Ni2(OAc){O(N)(O@C)2(CH2)3}(Im)4][OTf]2
(B0,GluH2A2)
3.25. [Ni2(OPiv)(BA)2(tmen)2][OTf] (B00,BHA)
This was prepared by the same procedure as
(B00,AHA) to give blue green crystals suitable for X-ray
crystallography formed from the filtrate on standing at
room temperature. Yield: 0.43 g (33.75%). Found: C,
43.07; H, 5.94; N, 9.38%. Calculated for C32F3H53
N6O9SNi2: C, 44.06; H, 6.12; N, 9.63%. FT-IR data
(KBr, cmꢀ1) major peaks 1610s, 1578s.
Into a solution of B0 0.643 g (1 mmol) in 50 ml of
dried methanol under nitrogen, TMS–OTf 0.36 ml
(2 mmol) was injected and stirred for 1 h. A solution of
previously refluxed hot glutarodihydroxamic acid 0.223
g (1.25 mmol) in 10 ml of dried methanol was added and
stirred for 30 min. Filtration and removal of solvent
under vacuum gave an oil, which was dissolved in 10 ml
of hot acetonitrile, layered with a 1:1 diethylether/cy-
clohexene mixture to deposit blue green crystals of
3.26. [Co2(OPiv)(BA)2(tmen)2][OTf] (D00,BHA)
[Ni2(OAc){O(N)(O@C)2(CH2)3}(Im)4][OTf]2.
Yield:
The analogous cobalt complex was prepared as dark
pink crystals by the same procedure as (D00,AHA) using
D00 in place of B00. Yield: 0.41 g (32.2%). Found: C,
43.84; H, 6.04; N, 9.52%. Calculated for C32F3H53N6
O9SCo2: C, 44.06; H, 6.12; N, 9.63%. FT-IR data (KBr,
cmꢀ1) major peaks 1614s, 1579s.
0.26 g (60%). Found: C, 29.89; H, 3.38; N, 15.41%.
Calculated for C21F6H25N9O11S2Ni2 C, 28.83; H, 2.86;
N, 14.41%. FT-IR data (KBr, cmꢀ1) major peaks 1684s,
1618s, 1575s.
3.22. [Co2(OAc){O(N)(O@C)2(CH2)3}(Im)4][OTf]2
(D0,GluH2A2)
3.27. [Ni2(OPiv)(NPhAA)2(tmen)2][PF6] (B00,NPhA-
HA)
The analogous cobalt complex was prepared similarly
using D0 in place of B0 as above to give dark pink
crystals of the complex [Co2(OAc){O(N)(O@C)2
(CH2)3}(Im)4][OTf]2, which were suitable for X-ray
crystallography. Yield: 0.25 g (59%). Found: C, 28.08;
H, 2.83; N, 15.72%. Calculated for C21F6H25N9
Under nitrogen, 0.772 g (1 mmol) of B00 was dissolved
in 25 ml of dry methanol, 0.184 g (1 mmol) of KPF6
added and the solution stirred for 30 min. A solution of
NPhAHA 0.302 g (2 mmol) in 5 ml of dried methanol
was then added and the mixture stirred for 30 min. After

D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
1433
filtration, the solvent was removed under vacuum and
the solid re-crystallized from 30 ml of warm 1:1 ethyl
acetate/acetonitrile to give blue green crystals suitable
for X-ray crystallography. Yield: 0.52 g (58%). Found:
C, 44.13; H, 6.37; N, 9.30%. Calculated for
C33F6H57N6O6PNi2: C, 44.22; H, 6.41; N, 9.37%. FT-IR
data (KBr, cmꢀ1) major peaks 1606s, 1590s.
[Ni2(urea)(OAc)2(AA)(Im)4)][PF6]. Yield: 0.45 g (57%).
Found: C, 27.99; H, 4.06; N, 19.03%. Calculated for
C19F6H30N11O7PNi2: C, 29.00; H, 3.84; N, 19.58%. FT-
IR data (KBr, cmꢀ1) major peaks 1670s, 1624s, 1577s.
3.32. [Co2(urea)(OAc)2(AA)(Im)4)][PF6] (C0,AHA)
This was prepared by the same procedure using C0 in
place of A0 to give the dark pink complex (C0,AHA).
Yield: 0.32 g (47%). Found: C, 28.51; H, 4.36; N,
19.18%. Calculated for C19F6H30N11O7PCo2: C, 28.90;
H, 3.84; N, 19.57%. FT-IR data (KBr, cmꢀ1) major
peaks 1670s, 1624s, 1576s.
3.28. [Co2(OPiv)(NPhAA)2(tmen)2][OTf] (D00, NPhA-
HA)
The analogous cobalt complex was prepared as dark
pink crystals by the same procedure using D00 in place of
B00. Crystals suitable for X-ray structural analysis were
obtained. Yield: 0.47 g (37.5%). Found: C, 44.13; H,
6.37; N, 9.30%. Calculated for C33F6H57N6O6PCo2: C,
3.33. [Ni2(urea)(OAc)2(BA)(Im)4)][PF6] (A0,BHA)
0.771 g (1 mmol) of A0 in 50 ml of dried methanol were
mixed with a solution of BHA 0.137 g (1 mmol) in 5 ml
methanol and stirred for 1 h, filtered, concentrated and
mixed with 50 ml diethylether when precipitation oc-
curred to give the blue green solid (A0,BHA). Yield: 0.45 g
(57%). Found: C, 34.88; H, 4.24; N, 17.23%. Calculated
for C24F6H32N11O7PNi2: C, 33.95; H, 3.79; N, 18.15 %.
FT-IR data (KBr, cmꢀ1) major peaks 1669s, 1612s, 1575s.
44.22; H, 6.41; N, 9.37%. FT-IR data (KBr, cmꢀ1
)
major peaks 1606s, 1590s.
3.29. [Ni2(OPiv)2{O(N)(O@C)2(CH2)3}(tmen)2][PF6]
(B00,GluH2A2)
To a solution of B00 0.772 g (1 mmol) in 50 ml of
methanol, KPF6 0.184 g (1 mmol) added and the solu-
tion stirred for 1 h. A solution of previously refluxed hot
glutarodihydroxamic acid 0.223 g (1.25 mmol) in 10 ml
of methanol was added, stirred for 30 min, filtered and
solvent removed under vacuum to give a solid which was
dissolved in 10 ml hot 1:1 mixture of acetonitrile/ethyl
acetate, filtered and on standing, blue green crystals
formed, or a concentrated solution in methanol layered
with a 1:1 mixture diethylether/cyclohexene, deposited
blue green crystals of (B0,GluH2A2) suitable for X-ray
crystallography. Yield: 0.57 g (48.5%). Found: C, 38.75;
H, 6.65; N, 8.48%. Calculated for C27F6H56N5O7PNi2:
3.34. [Co2(urea)(OAc)2(BA)(Im)4)][PF6] (C0,BHA)
This was prepared by the same procedure using C0 in
place of A0 to give a dark pink solid (C0,BHA). Yield:
0.32 g (47%). Found: C, 31.75; H, 3.89; N, 16.93%.
Calculated for C24F6H32N11O7PCo2: C, 33.98; H, 3.79;
N, 18.14%. FT-IR data (KBr, cmꢀ1) major peaks 1670s,
1604s, 1572s.
NH
NH
O
O
C, 39.29; H, 6.84; N, 8.48%. FT-IR data (KBr, cmꢀ1
)
HN
H
O
H
O
O
N
N
N
major peaks 1688s, 1636s, 1589s.
M
M
O
N
O
O
O
3.30.[Co2(OPiv)2{O(N)(O@C)2(CH2)3}(tmen)2][PF6]
(D00,GluH2A2)
HN
M= Co(D`), Ni(B`)
RHA
NH
KPF6,
Urea
HN
The analogous cobalt complex was prepared similarly
using D00 in place of B00 as above to give dark pink
crystals of (D00,GluH2A2), which were suitable for X-ray
crystallography. Yield: 0.5 g (42.5%). Found: C, 38.05;
H, 6.78; N, 8.30%. Calculated for C27F6H56N5O7PCo2:
PF6
NH
NH2
H2N
O
N
HN
O O
M
N
N
O
O
M
N
O
O
O
N
N
M
O
N
N
N
O
M
O
O
NH
N
O
NH
HN
TMS-OTf,
RH2A2
C, 39.27; H, 6.83; N, 8.48%. FT-IR data (KBr, cmꢀ1
)
(RHA= AHA, BHA, NPhAHA)
HN
major peaks 1690s, 1637s, 1586s.
RHA
PF6
NH2
[OTf]2
NH
H2N
O
3.31. [Ni2(urea)(OAc)2(AA)(Im)4)][PF6] (A0,AHA)
HN
O
NH
N
N
O
N
O
M
HN
N
O
N
M
O
N
N
O
N
M
N
0.771 g (1 mmol) of impure A0 was dissolved in 50 ml
of dried methanol, mixed with a solution of AHA 0.075
g (1 mmol) in 5 ml methanol and stirred for 1 h, filtered,
concentrated and mixed with 50 ml diethylether when
precipitation occurred to give the blue green complex
O
M
NH
O
O
N
O
O
HN
NH
HN
(RH2A2= GluH2A2)
(RHA= AHA, BHA, NPhAHA)
Scheme 1.

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D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
3.35.[Ni2(urea)(OAc)2(NPhAA)(Im)4)][PF6]ꢁHOAc,
(A0,NPhAHA)
lated for C30F6H51N6O6PCo2: C, 42.18; H, 6.01; N,
9.83%. FT-IR data (KBr, cmꢀ1) major peaks 1606s,
1590s.
0.771 g (1 mmol) of A0 in 50 ml of dried methanol was
mixed with a solution of NPhAHA 0.151 g (1 mmol) in
5 ml methanol and stirred for 1 h then filtered, con-
centrated and mixed with 50 ml diethylether when pre-
cipitation occurred to give the blue green solid
(A0,NPhAHA). Yield: 0.47 g (59%). Found: C, 37.50; H,
4.55; N, 17.84%. Calculated for C26F6H36N11O9PNi2: C,
3.39. [Mn2(OAc)3(AA)(tmen)2] (E,AHA)
0.598 g (1 mmol) of [Mn2(OAc)4(H2O)(tmen)2] (E)
in 5 ml of dried degassed methanol were mixed with a
solution of 0.113 g (1.5 mmol) AHA in 5 ml methanol
under nitrogen and the mixture stirred for 30 min.
The solvent was removed under reduced pressure
when an oily residue was obtained which was then
extracted with 3 ꢃ 50 ml of n-hexane resulting in a
white solid. Several pale white crystals suitable for X-
ray crystal structure analysis were obtained from the
filtrate on standing at room temperature. Yield: 0.37 g
(63%). Found: C, 40.41; H, 7.67; N, 11.68%. Calcu-
lated for C20H45N5O8Mn2: C, 40.47; H, 7.64; N,
11.80%. FT-IR data (KBr, cmꢀ1) major peaks 1630s,
1610s, 1578s.
35.13; H, 4.14; N, 16.99%. FT-IR data (KBr, cmꢀ1
)
major peaks 1669s, 1607s, 1590s.
3.36. [Co2(urea)(OAc)2(NPhAA)(Im)4)][PF6].H2O
(C0,NPhAHA)
This was prepared by the same procedure using C0 in
place of A0 to give the dark pink complex (C0,NPhAHA).
3.37. [Ni2(OAc)(NPhAA)2(tmen)2][PF6](B,NPhAHA)
0.644 g (1 mmol) of [Ni2(OAc)4(H2O)(tmen)2] (B) in 5
ml of methanol were mixed with a solution of 0.184 g (1
mmol) KPF6 in methanol, stirred for 30 min and a so-
lution of 0.302 g (2 mmol) NPhAHA in methanol added
and the mixture stirred for a further 30 min. The solu-
tion was filtered and volume reduced and then layered
with a 1:1 mixture of diethylether/cyclohexene for a
week to deposit blue green crystals suitable for X-ray
crystallography. Yield: 0.35 g (41%). Found: C, 42.13;
H, 5.89; N, 9.53%. Calculated for C30F6H51N6O6PNi2:
3.40. [Ni2(urea)(OAc)2(NPhAA)(tmen)2][OTf] (A,
NPhAHA)
0.736 g (1 mmol) of [Ni2(urea)(OAc)3(tmen)2][OTf] (A)
in 10 ml of dried CH2Cl2 were mixed with a solution of
NPhAHA 0.151 g (1 mmol) in 10 ml of dried CH2Cl2 and
stirred for 12 h, filtered, solvent volume reduced and then
layered with 1:1 mixture of diethylether/cyclohexene for a
week to deposit dark green crystals Yield: 0.51 g (69.3%).
Found: C, 37.12; H, 5.95; N, 10.47%. Calculated for
C26F3H50N7O10SNi2: C, 37.70; H, 6.05; N, 11.86%. FT-
IR data (KBr, cmꢀ1) major peaks 1669s, 1609s, 1594s.
C, 42.18; H, 6.01; N, 9.83%. FT-IR data (KBr, cmꢀ1
)
major peaks 1609s, 1597s.
3.38. [Co2(OAc)(NPhAA)2(tmen)2][PF6](D,NPhAHA)
3.41. [Co2(urea)(OAc)2(NPhAA)(tmen)2][OTf] (C,
NPhAHA)
The analogous cobalt complex was prepared by the
same procedure using [Co2(OAc)4(H2O)(tmen)2] (D) in
place of B. Found: C, 43.35; H, 5.99; N, 9.42%. Calcu-
The analogous cobalt complex was prepared using
[Co2(urea)(OAc)3(tmen)2][OTf] (C) in place of A. Yield:
0.53 g (70%). Found: C, 37.31; H, 6.03; N, 10.64%.
Calculated for C26F3H50N7O10SCo2: C, 37.30; H, 6.05;
N, 11.86%. FT-IR data (KBr, cmꢀ1) major peaks 1669s,
1608s, 1594s.
O
O
O
N
O
H
H
N
M
M
O
N
O
N
O
O
3.42. Crystal structure determination of the complexes
studied
O
M = Co (D``); Ni (B``)
K PF6
GluH2A2
X
RHA
O
Crystals suitable for X-ray analysis were obtained
directly from the above preparation methods. Data were
collected using a Siemens SMART CCD area-detector
diffractometer. A full hemisphere of reciprocal space
was scanned by a combination of three sets of expo-
sures; each set has a different / angle for the crystal and
each exposure of 10 s covered 0.3° in x. The crystal to
detector distance was 5.01 cm. Crystal decay was mon-
itored by repeating the initial frames at the end of the
[PF6]
[X-]
O
O
N
O
N
N
O
O
N
O
N
M
O
O
M
M
M
N
O
N
N
O
N
O
N
O
N
X = KPF6 ; X-= PF6; RHA= NPhAHA
X =TMS-OTf; X- = OTf; RHA = AHA,BHA
Scheme 2.

D.A. Brown et al. / Inorganica Chimica Acta 357 (2004) 1411–1436
1435
data collection and analyzing the duplicate reflection; a
multi-scan absorption correction was applied using
The reactions of the above model hydrolase analo-
gous based series with urea gave only impure products
and no stable products with the pivalate series, sug-
gesting that with the pivalate group, stereo-electronic
factors disfavour attack by the relatively weak urea
nucleophile. Nevertheless, reactions of the impure urea
complexes, [M2(urea)(OAc)3(Im)4][PF6] with the above
hydroxamic acids (RHA) gave the same type of hy-
droxamate-bridged complexes [M2(urea)(OAc)2(RA)
(Im)4][PF6] as observed previously for the tmen series
[M2(urea)(OAc)2(RA)(tmen)2][OTf] [16,18].
In the case of glutarodihydroxamic acid, both series
of model hydrolases B0, B00, D0 and D00 undergo hy-
droxylamine elimination and formation of dinuclear
complexes with a deprotonated N-hydroxyglutarimide
as bridging ligand as first reported for the tmen series
[16,18]. However, whilst the pivalate model hydrolases
B00 and D00 gave a complex with a very similar structure
to those reported for the tmen series [17,18], in the case
of the imidazole-based hydrolases B0 and D0, the re-
sulting complex [Co2(OAc){O(N)(O@C)2(CH2)3} (Im)4]
[OTf]2 (D0,GluH2A2) contains only one bridging acetate
and a five coordinated cobalt(II).
Finally, although the model hydrolases B0 and D0
tend to dissociate in methanol, reactions in this solvent
with hydroxamic acids occur readily with formation of
hydroxamate-bridged dimers, illustrating the strong
force behind this mode of bonding in accord with the
widespread inhibition of metalloenzymes by hydroxamic
acids. The isolation and structural characterization of
the dimanganese complex [Mn2(OAc)3(AA)(tmen)2]
(E,AHA) containing a hydroxamate with the bridging/
chelating mode of bonding gives further confirmation of
the widespread occurrence of this mode and also sug-
gests that hydroxamic acids may inhibit the dimanga-
nese metallohydrolase, arginase.
Tables of electronic spectra. Crystallographic data of
the structures described in this paper have been depos-
ited with the Cambridge Crystallographic Data Centre,
as supplementary publication CCDC (D0), (D00),
(B,NPhAHA), (E,AHA), (D0,GluH2A2), (B00,NPhAHA),
and (D00,NPhAHA). Copies of this data are available
free of charge from: The Director, CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK. (Fax: +44-1223-
336033; e-mail: teched@chemcrys.com.ac.uk).
SADABS [42]. The structures were solved by direct
methods using SHELXTL-PC [43] and refined by full
matrix least-squares on F 2 for all data using SHELXL 97.
Hydrogen atoms were added at calculated positions and
refined using a riding model [44]. Anisotropic tempera-
ture factors were used for all non-H atoms; H-atoms
were given isotropic temperature factors equal to 1.2 (or
1.5 for methyl or NH hydrogens) times the equivalent
isotropic displacement parameter of the atom to which
the H-atom is attached.
3.43. Magnetic measurements
The magnetic susceptibilities of powdered samples of
(B,NPhAHA), (B00,NPhAHA), (D00,NPhAHA) and
(D0,GluH2A2) were recorded on a Faraday-type mag-
netometer consisting of a Cahn RG electrobalance, a
Leybold Heraeus VNK 300 helium flux cryostat, and a
Bruker BE25 magnet connected with a Bruker B-Mn
200/60 power supply in the temperature range 4.4–310
K. The applied magnetic field was about 1.5 T. details of
the apparatus have been described elsewhere [45,46].
The experimental susceptibility data were corrected for
underlying diamagnetism in the usual manner using
PascalÕs constants [47].
4. Conclusions
Reactions of the model hydrolases, [M2(OAc)4(H2O)
(Im)4] (M ¼ Co, Ni) and [M2(OPiv)4 (H2O)(tmen)2]
(M ¼ Co, Ni) with a number of monohydroxamic acids
RHA (AHA, BHA and NPhAHA) gave a series of
dibridged hydroxamate complexes [M2 (OAc)(AA)2
(Im)4][OTf] and [M2(OPiv)(AA)2(tmen)2] [OTf] in which
the hydroxamate groups take up the bridging/chelating
mode of bonding observed previously in the series based
on [M2(OAc)4(H2O)(tmen)2], where M ¼ Ni [26] and
M ¼ Co [24], thereby showing that this mode of bonding
involving deprotonation of the hydroxamate hydroxyl
and bridging by the resulting oxygen and coordination
of the ketonic oxygen to just one of the metal centres in
a dinuclear complex is a general mode of bonding in
dimeric model inhibited hydrolases and models
closely that observed in the acetohydroxamate-inhibited
Cys319Ala variant of KAU [12].
Acknowledgements
The secondary hydroxamic acid, NPhAHA, which
has not been studied previously, also forms hydroxa-
mate bridged/chelated dinuclear complexes with both
the new series of model hydrolases and those previously
studied based on tmen as capping ligand [24]. This result
suggests further synthetic possibilities of designing sec-
ondary hydroxamate inhibitors with a range of substit-
uents on the hydroxamate nitrogen.
We are pleased to acknowledge the support of the
EUCOST program, Project D 21/ 0001/ 00.
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