Dinuclear nickel(II) complexes of reduced asymmetric compartmental ligands

Article abstract of DOI:10.1039/b209437c

The reaction of the reduced asymmetric compartmental proligand 2-{[(2-dimethylaminoethyl)ethylamino]methyl}-4-methyl-6-{[(pyridin-2-ylmethyl)am ino]methyl}phenol, HL⁴ with nickel(II) acetate in the presence of non-coordinating anions gave dinuclear nickel(II) complexes. The crystal structures of [Ni₂L⁶(OAc)(μ-OAc)(OH₂)(CH₃OH)][P F₆], 2, [Ni₂L⁴(μ-OAc)₂(CH₃OH)] [PF₆], 3, [Ni₂L⁴(μ-OAc)₂(OH₂) (CH₃OH)][PF₆], 4, and [Ni₂L⁴(μ-OAc)₂ (CH₃OH)₂][BPh₄], 5, are reported. 2-[(2-Dimethylaminoethylamino)methyl]-6-{[(2-dimethylaminoethyl)ethylamino]methy l}-4-methyl-phenol, HL⁵, was found to react with Ni(ClO₄)₂·4H₂O and NaBPh₄ in the presence of urea. to give [Ni₂L⁵(OH)(OH₂)₂(urea)] [BPh₄]₂, 6, the dinuclear core of which bears some resemblance to that of the active site in urease. The crystal structure reveals the presence of a hydroxo-bridge and an O-bonded molecule of urea.

Full text of DOI:10.1039/b209437c

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Dinuclear nickel(II) complexes of reduced asymmetric
compartmental ligands†
Harry Adams, Scott Clunas, David E. Fenton* and Sharon E. Spey
Department of Chemistry, The University of Sheffield, Dainton Building, Brook Hill, Sheffield,
UK S3 7HF. E-mail: d.fenton@sheffield.ac.uk
Received 25th September 2002, Accepted 2nd December 2002
First published as an Advance Article on the web 15th January 2003
The reaction of the reduced asymmetric compartmental proligand 2-{[(2-dimethylaminoethyl)ethylamino]methyl}-4-
methyl-6-{[(pyridin-2-ylmethyl)amino]methyl}phenol, HL4 with nickel() acetate in the presence of non-coordinat-
ing anions gave dinuclear nickel() complexes. The crystal structures of [Ni2L6(OAc)(µ-OAc)(OH2)(CH3OH)][PF6], 2,
[Ni2L4(µ-OAc)2(CH3OH)][PF6], 3, [Ni2L4(µ-OAc)2(OH2)(CH3OH)][PF6], 4, and [Ni2L4(µ-OAc)2(CH3OH)2][BPh4], 5,
are reported. 2-[(2-Dimethylaminoethylamino)methyl]-6-{[(2-dimethylaminoethyl)ethylamino]methyl}-4-methyl-
phenol, HL5, was found to react with Ni(ClO4)2ؒ4H2O and NaBPh4 in the presence of urea. to give [Ni2L5(OH)-
(OH2)2(urea)][BPh4]2, 6, the dinuclear core of which bears some resemblance to that of the active site in urease.
The crystal structure reveals the presence of a hydroxo-bridge and an O-bonded molecule of urea.
Introduction
X-Ray crystallographic studies on urease isolated from different
sources [Klebsiella aerogenes1,2 and the native and inhibited
metalloenzyme from Bacillus pasterii3–5] have provided a
detailed description of the active site (Fig. 1). The dinickel()
a coordinating anion such as the isothiocyanate anion [NCSϪ]
had provided dinuclear nickel() complexes in which the ligand
remains intact12 it was proposed that the criteria for cleavage of
the iminic pendant arm in metal complexes of unsymmetrical
Schiff base compartmental ligands are the presence of both
Fig. 1 Schematic depiction of the active site in urease.
weakly or non-coordinating anions (ClO4Ϫ, BF4Ϫ) and nickel()
cations. It was also suggested that the reaction proceeded
centre has each nickel() ion ligated by two histidine residues
through the activation of coordinated water at a nickel() ion
from the protein and a carbamylated lysine residue bridges the
or by initial formation of a hydroxo bridge at the dinickel
nickel() ions. Ni(2) is further ligated by an aspartate residue
centre.
and two terminally coordinated water molecules and one bridg-
ing water molecule, or hydroxide ion completes the coordin-
ation environments at the metal ions. Consequently Ni(1) has a
distorted square pyramidal environment and Ni(2) a pseudo-
octahedral environment. The asymmetry of the metallobiosite
and curiosity as to how it functions in the hydrolysis of urea
have determined that urease can serve as a muse for the bio-
inorganic chemist and so inspire the generation of a range of
bio-resemblant coordination chemistry.
Unsymmetrical compartmental proligands such as HL1–HL3
provide adjacent, dissimilar binding sites which can each
accommodate a metal and so produce dinuclear complexes
with coordination environments resembling the active site in
In order to encourage hydroxo-bridge formation in dinuclear
urease.6–9
nickel() complexes in the presence of weakly or non-coordin-
Reaction of HL1 and HL2 with copper() perchlorate has
ating anions the asymmetric compartmental proligand HL3 was
been found to give the complexes [Cu2OHLn](ClO4)2 in which
reduced to the di-aminic analogue HL4. In an early study HL4
the integrity of the compartmental is retained and a hydroxo-
was reacted with Ni(ClO4)2ؒ6H2O in ethanol with addition of
bridge spans the Cu() atoms.10 It was anticipated that in the
NaPF6 to facilitate crystallisation; solution of the crystal struc-
corresponding reactions with nickel() perchlorate a bio-
ture of the product revealed that a tetranuclear complex,
resemblant Ni–OH–Ni bridge would be formed however with
[Ni4(L4)2(OH)3(OH2)ClO4](PF6)2ؒ2CH3OHؒ5H2O, had been
HL1–HL3 hydrolytic cleavage of the pendant imine.10,11 As the
formed.13 It was suggested that, by analogy with the formation
reactions of HL1–HL3 with nickel() acetate in the presence of
of [Cu2OHLn]2ϩ noted above, a µ-hydroxo bridged dinickel()
species such as 1 provides the precursor for the tetranuclear
assembly.
† In memoriam Noel McAuliffe (1941–2002).
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 6 2 5 – 6 3 0
625

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residue dissolved in a small volume of water. HCl (2 M) was
added to acidify the solution (pH 3), which was then extracted
with CH2Cl2 (2 × 30 cm3). NaOH (5 M) was then added and the
aqueous layer (pH 10) extracted with CH2Cl2 (3 × 60 cm3). The
solvent was removed under reduced pressure to yield the
product as a colourless oil. Yield: 911 mg (72%).
δH (250 MHz, CDCl3, 298 K): 6.85(1H, d, ArH), 6.63(1H, d,
ArH), 3.71(2H, s, ArCH2), 3.61(2H, s, ArCH2), 2.70–2.32(10H,
m, CH2), 2.11(3H, s, ArCH3), 2.10(12H, s, NCH3). 0.98(3H, t,
CH3). δC (62.5 MHz, CDCl3, 298 K): 154(aromatic, 4 ЊC), 129,
128(aromatic, CH), 127, 126, 122(aromatic, 4 ЊC), 59, 56, 55,
49, 47, 46, 45(CH2), 44, 20, 11(CH3). Selected IR data (ν/cmϪ1):
2970, 2820, 1592, 1474, 1234, 1043, 756. MS (m/z): 336. Anal.
Found: C, 65.02; H, 11.07; N, 15.38. Calcd for C19H36N4OؒH2O:
C, 64.37; H, 10.80; N, 15.80%.
This paper reports an investigation of the reactions of
the reduced proligands HL4 and HL5 with nickel() acetate
and salts of non-coordinating anions thus eliminating the
involvement of the anion in bridge building. The crystal
structures of [Ni2(L4)(OAc)(µ-OAc)(OH2)(CH3OH)][PF6], 2,
[Ni2(L4)(µ-OAc)2(CH3OH)][PF6], 3, [Ni2(L4)(µ-OAc)2(OH2)-
(CH3OH)][PF6], 4, [Ni2(L4)(µ-OAc)2(CH3OH)2][BPh4], 5, and
[Ni2(L5)(OH)(OH2)2(urea)][BPh4]2, 6, are reported.
Complexation reactions
[Ni2L4(OAc)(ꢀ-OAc)(CH3OH)(OH2)](PF6) 2. Ni(OAc)2ؒ
4H2O (139 mg, 0.560 mmol) was added to a solution of HL6
(100 mg, 0.280 mmol) in methanol (10 cm3). After stirring for
30 min, the addition of NaPF6 (94 mg, 0.560 mmol) to the
solution resulted in the formation of green crystals.Yield: 85 mg
(39%).
Experimental
Elemental analyses were carried out by the University of
Sheffield microanalytical service. Infrared spectra were
recorded as KBr discs or as liquid films between NaCl plates,
using a Perkin Elmer 297 (4000–600 cmϪ1) or a Perkin Elmer
1600 (4000–400 cmϪ1) infrared spectrophotometer. 1H nmr
spectra were recorded using a Bruker ACF-250 spectrometer.
Mass spectra (FAB and EI) were recorded using Micromass
PROSPEC spectrometer (the matrix used was 4-nitrobenzyl
alcohol).
Selected IR data (ν/cmϪ1) using KBr disk: 2996, 2933, 1608,
1443, 1381, 1014, 845. MS (m/z): 589(MHϩ Ϫ CH3OH, 100%).
Anal. Found: C, 39.24; H, 5.46; N, 7.07. Calcd for C26H43F6N4-
Ni2O7P: C, 39.69; H, 5.47; N, 7.12.
[Ni2L4(ꢀ-OAc)2(CH3OH)(OH2)][Ni2L6(OAc)2(CH3OH)]-
(PF6)2ؒ2H2O 3/4. This was obtained as green crystals by an
identical procedure to that above using HL6 (100 mg, 0.280
mmol), Ni(OAc)2ؒ4H2O (139 mg, 0.560 mmol) and NaPF6
(94 mg, 0.560 mmol). Yield: 62 mg, 29%.
Ligand synthesis
Selected IR data (ν/cmϪ1) using KBr disk: 2989, 2899, 1604,
1478, 1043, 855, 761. MS (m/z): 589 (MHϩ Ϫ CH3OH, 100%).
Anal. Found for the bulk sample: C, 38.03; H, 5.06; N, 7.10.
Calcd for C52H88F12N8Ni4O15P2ؒ2H2O: C, 38.42; H, 5.66; N,
6.89%.
The ligand precursor 3-{[(2-dimethylaminoethyl)ethylamino]-
methyl}-2-hydroxy-5-methyl benzaldehyde (A) was prepared as
previously described.10,12
2-{[(2-Dimethylaminoethyl)ethylamino]methyl}-4-methyl-6-
{[(pyridin-2-ylmethyl)amino]methyl}phenol, HL4. 2-(amino-
methyl)pyridine (204 mg, 1.89 mmol) was added to solution of
precursor A (500 mg, 1.89 mmol) in ethanol (40 cm3). The reac-
tion mixture was then heated to reflux for 20 min and allowed to
cool. NaBH4 (144 mg, 3.79 mmol) was added portion wise with
stirring. The reaction was then stirred at rt overnight. The
solvent was removed under reduced pressure and the residue
dissolved in a small volume of water. HCl (2 M) was added
to acidify the solution (pH 3), which was then extracted with
CH2Cl2 (2 × 20 cm3). NaOH (5 M) was then added and the
aqueous layer (pH 10) extracted with CH2Cl2 (3 × 40 cm3). The
solvent was removed under reduced pressure to yield the
product as a colourless oil. Yield: 502 mg (74%).
δH (250 MHz, CDCl3, 298 K): 8.49(1H, s, pyH), 7.55(1H, m,
pyH), 7.28(1H, d, pyH), 7.07(1H, m, pyH), 6.81(1H, d, ArH),
6.66(1H, d, ArH), 3.82(2H, s, ArCH2), 3.76(2H, s, ArCH2),
3.66(2H, s, ArCH2), 2.61–2.38(6H, m, CH2), 2.11(3H, s,
ArCH3), 2.10(6H, s, NCH3). 0.98(3H, t, CH3). Selected IR data
(ν/cmϪ1): 2818, 1467, 1235, 1157, 1041, 780. MS (m/z): 356.
Anal. Found: C, 66.52; H, 8.71; N, 14.00. Calcd for C21H32N4Oؒ
1.25H2O: C, 66.55; H, 9.17; N, 14.78%.
[Ni2L4(ꢀ-OAc)2(CH3OH)2](BPh4) 5. Ni(OAc)2ؒ4H2O (139
mg, 0.560 mmol) was added to a solution of HL6 (100 mg,
0.280 mmol) in methanol (10 cm3). After stirring for 30 min, the
addition of NaBPh4 (192 mg, 0.560 mmol) to the solution
resulted in the formation of green crystals. Yield: 68 mg (25%).
Selected IR data (ν/cmϪ1) using KBr disk: 3055, 2919, 1608,
1481, 1313, 1022, 733. MS (m/z): 589(MHϩ Ϫ 2 CH3OH,
100%). Analysis. Found for the bulk sample: C, 59.13; H, 6.73;
N, 5.50. Calcd for C53H70BN4Ni2O9ؒ2H2O: C, 59.42; H, 6.96; N,
5.25%.
[Ni2L5 (OH)(OH2)2(urea)](BPh4) 6. Ni(ClO4)2ؒ6H2O (326 mg,
0.893 mmol) was added to a solution of HL7 (150 mg, 0.446
mmol) in methanol (10 cm3). After stirring for 30 min, the addi-
tion of urea (134 mg, 2.23 mmol) followed by NaBPh4 (305 mg,
0.893 mmol) to the solution resulted in the formation of green
crystals. Yield: 157 mg (28%).
Selected IR data (ν/cmϪ1) using KBr disk: 3427, 3054, 1660,
1580, 1427, 736, 707. MS (m/z): 471(MHϩ Ϫ (urea ϩ 2H2O),
100%). Bulk sample analyses were inconsistent due to con-
tamination with urea, even after washing with water. A repre-
sentative analysis gave, Found: C, 61.11; H, 6.96; N, 8.11. Calcd
for C68H88B2N6Ni2O7ؒH2Oؒurea: C, 61.18; H, 7.29; N, 8.27%.
2-[(2-Dimethylaminoethylamino)methyl]-6-{[(2-dimethyl-
aminoethyl)ethylamino]methyl}-4-methylphenol, HL5. N,N-di-
methylethylenediamine (333 mg, 3.79 mmol) was added to
solution of precursor A (1.00 g, 3.79 mmol) in ethanol (70 cm3).
The reaction mixture was then heated to reflux for 20 min and
allowed to cool. NaBH4 (288 mg, 7.57 mmol) was added por-
tion wise with stirring. The reaction was then stirred at rt over-
night. The solvent was removed under reduced pressure and the
X-Ray crystallography
The X-ray crystallographic data are summarised in Table 1.
Data were collected at 150(2) K, using a Siemens SMART CCD
area diffractometer (graphite monochromated Mo-Kα
X-radiation, λ = 0.71073 Å) with an Oxford Cryosystems low
temperature system. The data were corrected for Lorentz and
D a l t o n T r a n s . , 2 0 0 3 , 6 2 5 – 6 3 0
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Table
1
Summary of crystallographic data for [Ni2(L4)(OAc)(µ-OAc)(OH2)(CH3OH)][PF6], 2; [Ni2(L4)(µ-OAc)2(CH3OH)][PF6]ؒ[Ni2(L4)-
(µ-OAc)2(OH2)(CH3OH)][PF6]ؒ2H2O, 3/4; [Ni2(L4)(µ-OAc)2(CH3OH)2][BPh4]ؒ2MeOH, 5; and [Ni2(L4)(OH)(OH2)2(urea)][BPh4]2ؒ2H2O, 6
2
3/4
5
6
Empirical formula
Formula weight
Space group
C26H43F6N4Ni2O7P
786.03
P21/c
C52H88F12N8Ni4O15P2
1590.08
P21/n
C53H70BN4Ni2O9
1035.36
P1
C68H88B2N6Ni2O7
1240.48
P1
¯
¯
a/Å
b/Å
c/Å
αÅ
11.771(2)
16.086(3)
17.687(3)
90
103.915(4)
90
20.653(6)
10.707(3)
31.6276(9)
90
92.049(5)
90
11.2426(17)
13.318(2)
18.752(3)
71.951(3)
82.185(3)
88.779(3)
2644.0(7)
1.300
11.728(4)
14.200(5)
21.057(7)
79.907(7)
75.858(7)
78.937(6)
3306.8(19)
1.246
β/Å
γ/Å
V/Å3
3250.9(10)
1.606
6989(3)
1.511
ρcalc/Mg mϪ3
Z
4
4
2
2
µ/mmϪ1
1.291
1.203
0.769
0.625
Data/restraints/parameters
R
7829 /36 /415
0.0613
16858 /16 /857
0.0629
11960 /26 /673
0.0796
14525 /460 /766
0.0955
wR2
0.1589
0.1702
0.2507
0.3084
polarisation effects and for absorption by semi-empirical
methods based on symmetry-equivalent and repeated reflec-
tions. The structures were solved by direct methods and refined
by full matrix least squares methods on F 2. Hydrogen atoms
were placed geometrically and refined with a riding model and
with Uiso constrained to be 1.2 (1.5 for methyl groups) times Ueq
of the carrier atom. The programs used in the determination
and refinement of the structures were Siemens SMART and
SAINT for control and integration software14 and SHELXTL
as implemented on the Viglen Pentium computer.15
CCDC reference numbers 194355–194358.
See http://www.rsc.org/suppdata/dt/b2/b209437c/ for crystal-
lographic data in CIF or other electronic format.
Results and discussion
In order to remove the risk of hydrolytic cleavage occurring in
reactions of asymmetric compartmental ligands with nickel
salts in the presence of weakly or non-coordinating anions the
Schiff base proligands HL1 and HL3 were reduced. The reduced
proligands were then reacted with Ni(OAc)2 in the presence of
NaPF6 or NaBPh4; NaClO4was avoided as it had induced anion
bridging in complex 1.
The reaction of HL4 with Ni(OAc)2ؒ4H2O and NaPF6 gave
the dinuclear complex [Ni2(L4)(OAc)(µ-OAc)(OH2)(CH3OH)]-
[PF6], 2. The crystal structure was solved and revealed a
basic dinuclear core derived from a bridging cresolate-O atom
and a syn–syn bidentate bridging acetate anion. An ORTEP
view of 2 is shown in Fig. 2 together with the numbering
schemes. Selected bond lengths and angles relevant to the
coordination geometries are given in the caption to the figure.
In this complex each nickel() atom is six-coordinate. This
is achieved at Ni(1) by further interaction with the articular
N(Et) atom and terminal N(Me2) of one arm of the ligand and
with a chelating acetate anion. At Ni(2) there is further
coordination from the second ligand arm via the articular N(H)
and pyridinyl N atoms, a water molecule and a methanol mole-
cule. This gives the complex a donor asymmetry,16 both donor
sets are {N2O4} but the precise natures of the donor atoms are
quite diverse.
Chelation from the pendant nitrogen atoms provides
five-membered chelate rings at each nickel() atom. The octa-
hedral coordination at the nickel atoms is distorted with the
greater distortion at the nickel atom, Ni(1), bearing the chelated
acetate [O(4)–Ni(1)–N(1), 177.32(18); N(2)–Ni(1)–O(2),
160.28(18); O(1)–Ni(1)–O(3), 159.52(16)Њ and O(1)–Ni(2)–
O(6), 178.27(16); O(7)–Ni(2)–N(3), 169.74(18); N(4)–Ni(2)–
O(5), 175.16(18)Њ]. The cresolato-bridge is non-symmetric and
the Ni ؒ ؒ ؒ Ni separation is 3.5419(12) Å. The chelating angle,
O(3)–C(22)–O(2), in the monobridging acetate is normal,
Fig. 2 An ORTEP drawing of the molecular structure of 2 showing
the atom labelling; thermal ellipsoids for the non-hydrogen atoms are
drawn at the 50% probability level. Selected bond lengths and angles
at the metal atoms: Ni(1)–O(1), 2.019(4); Ni(1)–O(4), 2.065(4);
Ni(1)–N(2), 2.082(5); Ni(1)–O(2), 2.125(4); Ni(1)–N(1), 2.131(5);
Ni(1)–O(3), 2.134(4); Ni(2)–O(1), 2.042(4); Ni(2)–N(4), 2.042(5);
Ni(2)–O(5), 2.060(4); Ni(2)–O(7), 2.067(4); Ni(2)–N(3), 2.078(5);
Ni(2)–O(6), 2.143(4); Ni(1)–Ni(2), 3.5419(12) Å. Ni(1)–O(1)–Ni(2),
121.40(19); O(4)–C(22)–O(5), 117.3(6); O(4)–Ni(1)–N(1), 177.32(18);
N(2)–Ni(1)–O(2), 160.28(18); O(1)–Ni(1)–O(3), 159.52(16); O(1)–
Ni(2)–O(6), 178.27(16); O(7)–Ni(2)–N(3), 169.74(18); N(4)–Ni(2)–
O(5), 175.16(18)Њ.
120.6(6)Њ whereas the bridging angle, O(4)–C(24)–O(5), of the
syn–syn bridging acetate is more open, 124.9(6)Њ.
The reaction of HL4 with Ni(OAc)2ؒ4H2O and NaPF6 also
gave the dinuclear complex [Ni2(L4)(µ-OAc)2(CH3OH)][PF6], 3,
which co-crystallised with [Ni2(L4)(µ-OAc)2(OH2)(CH3OH)]-
[PF6], 4, and two molecules of water. The crystal structure was
solved and again showed a basic dinuclear core derived from a
bridging cresolate-O atom and a syn–syn bidentate bridging
acetate anion. This was augmented by a second syn–syn bi-
dentate bridging acetate anion. An ORTEP view of 3 is shown
in Fig. 3 together with the numbering schemes. Selected bond
lengths and angles relevant to the coordination geometries
are given in the caption to the figure. In this complex one
nickel() atom, Ni(2) bound by the pyridinyl containing arm, is
D a l t o n T r a n s . , 2 0 0 3 , 6 2 5 – 6 3 0
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Fig. 4 An ORTEP drawing of the molecular structure of 4 showing
Fig. 3 An ORTEP drawing of the molecular structure of 3 showing
the atom labelling; thermal ellipsoids for the non-hydrogen atoms are
drawn at the 50% probability level. Selected bond lengths and angles at
the metal atoms: Ni(1)–O(1), 1.948(4); Ni(1)–O(4), 1.983(4); Ni(1)–
O(2), 2.058(5); Ni(1)–N(1), 2.089(6); Ni(1)–N(2), 2.103(5); Ni(2)–O(1),
2.001(4); Ni(2)–O(5), 2.039(5); Ni(2)–N(4), 2.067(6); Ni(2)–O(3),
2.079(4); Ni(2)–N(3), 2.083(5); Ni(2)–O(7), 2.128(4); Ni(1)–Ni(2),
3.3346(14) Å. Ni(1)–O(1)–Ni(2), 115.2(2); O(4)–C(24)–O(5), 126.9(6);
O(2)–C(22)–O(3), 115.9(6); O(1)–Ni(2)–O(7), 175.20(17); N(4)–Ni(2)–
O(3), 175.1(2); N(3)–Ni(2)–O(5), 170.81(19); N(1)–Ni(1)–O(2),
164.99(19); N(2)–Ni(1)–O(4), 160.5(2); O(1)–Ni(2)–N(1), 96.40(19);
O(1)–Ni(1)–O(4), 99.74(18); O(1)–Ni(1)–O(2), 98.42(18); O(1)–Ni(1)–
N(2), 99.57(19)Њ.
the atom labelling; thermal ellipsoids for the non-hydrogen atoms are
drawn at the 50% probability level. Selected bond lengths and angles at
the metal atoms: Ni(1A)–O(1A), 1.992(4); Ni(1A)–O(4A), 2.051(5);
Ni(1A)–O(2A), 2.065(5); Ni(1A)–N(1A), 2.161(6); Ni(1A)–O(6A),
2.171(5); Ni(1A)–N(2A), 2.175(6); Ni(2A)–O(5A), 2.014(5); Ni(2A)–
O(1A), 2.023(5); Ni(2A)–N(4A), 2.027(6); Ni(2A)–N(3A), 2.058(5);
Ni(2A)–O(3A), 2.074(5); Ni(2A)–O(7A), 2.145(4); Ni(1A)–Ni(2A),
3.4171(14) Å. Ni(1A)–O(1A)–Ni(2A), 116.7(2); O(4A)–C(24A)–O(5A),
117.9(7); O(2A)–C(22A)–O(3A), 127.1(7); O(2A)–Ni(1A)–N(1A),
173.5(2); O(1A)–Ni(1A)–O(6A), 171.63(19); O(4A)–Ni(1A)–N(2A),
167.4(2); O(1A)–Ni(2A)–O(7A), 175.39(18); O(3A)–Ni(2A)–N(4A),
174.0(2); N(3A)–Ni(2A)–O(5A), 169.7(2)Њ.
six-coordinate and the second nickel() atom, Ni(1) is five
coordinate. At Ni(2) there is further coordination from the
ligand arm via the articular N(H) and pyridinyl N atoms and
a methanol molecule. At Ni(1) there is further interaction with
the articular N(Et) atom and terminal N(Me2) of the second
arm of the ligand. No significant electron density was found
at the vacant site and so the geometry is square pyramidal
[τ = 0.075;17 O(4)–Ni(1)–N(2), 160.5(2); N(1)–Ni(1)–O(2),
164.99(19)Њ]. This gives the complex a coordination number
[5,6] and geometric asymmetry16 with donor sets {N2O3} and
{N2O4}.
Chelation from the pendant nitrogen atoms provides five-
membered chelate rings at each nickel() atom and the octa-
hedral coordination at Ni(2) is more regular than at the nickel
atoms in 2, [O(1)–Ni(2)–O(7), 175.20(17); N(4)–Ni(2)–O(3),
175.1(2); N(3)–Ni(2)–O(5), 170.81(19)Њ]. The cresolato-bridge
is non-symmetric and the Ni ؒ ؒ ؒ Ni separation is 3.3346(14) Å,
shorter than that in 2.
An ORTEP view of 4 is shown in Fig. 4 together with the
numbering schemes. Selected bond lengths and angles relevant
to the coordination geometries are given in the caption to the
figure. The vacant site noted in 3 is occupied by a water mole-
cule in the molecular structure of 4. There is a basic dinuclear
core derived from a bridging cresolate-O atom and a syn–syn
bidentate bridging acetate anion again augmented by a second
syn–syn bidentate bridging acetate anion.
Each nickel() atom is six-coordinate and this is achieved at
Ni(1A) by further interaction with the articular N(Et) atom and
terminal N(Me2) of one arm of the ligand and with a water
molecule. At Ni(2A) there is further coordination from the
second ligand arm via the articular N(H) and pyridinyl N
atoms and a methanol molecule. The complex is thus donor
asymmetrical, both donor sets are {N2O4} but the precise
natures of the donor atoms are quite diverse. Chelation from
the pendant nitrogen atoms provides six-membered chelate
rings at each nickel() atom and the octahedral coordination at
the nickel atoms is distorted [O(2A)–Ni(1A)–N(1A), 173.5(2);
O(1A)–Ni(1A)–O(6A), 171.63(19); O(4A)–Ni(1A)–N(2A),
167.4(2)Њ and O(1A)–Ni(2A)–O(7A), 175.39(18); O(3A)–
Ni(2A)–N(4A), 174.0(2); N(3A)–Ni(2A)–O(5A), 169.7(2)Њ].
The cresolato-bridge is non-symmetric and the Ni ؒ ؒ ؒ Ni
separation is 3.4171(14) Å, intermediate to the separations in 2
and 3.
[Ni2(L4)(µ-OAc)2(CH3OH)2][BPh4], 5, was isolated when
[BPh4]Ϫ was used as the counter ion in the reaction of HL4 with
Ni(OAc)2ؒ4H2O. The crystal structure was solved and showed
similarity to that of 4. The molecular structure is shown in
Fig. 5 together with the numbering schemes. The significant
difference from 4 is that methanol molecules are coordinated to
Ni(2) and Ni(1) where previously a water molecule and a
methanol molecule were coordinated. The atoms C(9), N(2)
and C(20) of one aminic arm are disordered and were refined to
occupancies of 61.5% : 38.5%.
It is intriguing that complexes 2–4 are crystallised from
solutions prepared under the same reaction conditions in the
presence of [PF6]Ϫ anions. This suggests that there is a similar
speciation in solution with only subtle changes in the solubility
of the complexes or the conditions of crystallisation, for
example variable temperature drop in the laboratory overnight,
leading to the analysed product. The reactions were carried out
in different laboratories at different times of the year; 3/4 was
crystallised in one laboratory during the autumn and 5 in a
refurbished laboratory during the following spring. To explain
our results requires a detailed knowledge of the speciation in
D a l t o n T r a n s . , 2 0 0 3 , 6 2 5 – 6 3 0
628

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Scheme 1 Schematic representations of the dinickel() cores present in 2–5.
Realisation of the formation a hydroxo-bridge occurred in
the absence of acetate via the reaction of HL5 with Ni(ClO4)2ؒ
6H2O, NaBPh4 and urea. Poor quality crystals of [Ni2(L5)-
(OH)(OH2)2(urea)][BPh4]2, 6, were recovered and the crystal
structure solved. An ORTEP view is shown in Fig. 6 together
with the numbering schemes. Selected bond lengths and angles
relevant to the coordination geometries are given in the caption
to the figure. In this complex there is a dinuclear core derived
from a bridging cresolate-O atom and a hydroxide anion. The
nickel() atom, Ni(1), bound by the ligand arm bearing an
articular N(H) atom and a terminal NMe2 group, is six-
coordinate and the second nickel() atom, N(2), bound by the
ligand arm bearing an articular N(Et) atom and a terminal
NMe2 group, is five coordinate. At Ni(1) the six coordination is
completed by a water molecule and an O-bound urea molecule.
Fig. 5 An ORTEP drawing of the molecular structure of 5 showing
the atom labelling; thermal ellipsoids for the non-hydrogen atoms are
drawn at the 50% probability level. Selected bond lengths and angles at
the metal atoms: Ni(1)–O(1), 2.023(3); Ni(1)–N(3), 2.066(4); Ni(1)–
O(3), 2.06(4); Ni(1)–O(5), 2.070(4); Ni(1)–N(4), 2.092(5); Ni(1)–O(6),
2.142(4); Ni(2)–O(2), 2.004(5); Ni(2)–O(1), 2.011(4); Ni(2)–O(4),
2.038(4); Ni(2)–N(1), 2.122(4); Ni(2)–N(2A), 2.149(6); Ni(2)–N(2B),
2.152(7); Ni(2)–O(7), 2.379(6); Ni(1)–Ni(2), 3.4268(11) Å. Ni(1)–O(1)–
Ni(2), 116.31(16); O(2)–C(22)–O(3), 126.1(5); O(4)–C(24)–O(5),
125.6(6); O(1)–Ni(1)–O(6), 175.50(15); N(3)–Ni(1)–O(5), 174.28(16);
O(3)–Ni(1)–N(4), 169.53(17); O(4)–Ni(2)–N(1), 172.13(18); O(2)–
Ni(2)–N(2A), 166.6(3); O(2)–Ni(2)–N(2B), 169.2(52); O(1)–Ni(1)–
O(7), 166.20(16)Њ.
solution and as this cannot be accessed a hypothesis for the
formation of the complexes is advanced. Schematic represen-
tations of the dinickel() cores present in 2–5 are depicted in
Scheme 1.
Complexes 2 and 3 represent extremes of the carboxylate
shift — the movement of a bound carboxlate from being chel-
ated to forming a monodentate bridge and then a bidentate
bridge.18,19 Migration of an O-atom from the chelated acetate to
replace a water molecule on the second nickel atom would
generate a vacant site at the first nickel atom. This could then
be filled by a water molecule to generate 4, or by a MeOH mole-
cule to generate 5. It is probable that in solution the coordinated
solvent molecules are quite labile and so complex 5 could also
from arise from 4 by solvent exchange. In the present study this
complex was isolated and recovered with the [BPh4]Ϫ cation
present — this might be a consequence of solubilities varying
with counterion.
Fig. 6 An ORTEP drawing of the molecular structure of 6 showing
the atom labelling; thermal ellipsoids for the non-hydrogen atoms are
drawn at the 50% probability level. Selected bond lengths and angles at
the metal atoms: Ni(1)–O(1), 2.016(5); Ni(1)–O(2), 2.032(4); Ni(1)–
O(3), 2.038(6); Ni(1)–N(3), 2.054(6); Ni(1)–N(4), 2.110(6); Ni(1)–O(5),
2.153(7); Ni(2)–O(2), 1.940(5); Ni(2)–O(1), 1.960(5); Ni(2)–O(4),
2.041(7); Ni(2)–N(2), 2.069(7); Ni(2)–N(1), 2.091(7); Ni(1)–Ni(2),
3.0495(18) Å. Ni(1)–O(1)–Ni(2), 100.2(2); Ni(1)–O(2)–Ni(2), 100.3(2);
N(4)–Ni(1)–O(5), 172.8(3); O(1)–Ni(1)–O(3), 171.7(2); N(3)–Ni(1)–
O(2), 168.8(2); N(1)–Ni(2)–O(2), 167.1(2); N(2)–Ni(2)–O(1), 152.8(2);
O(4)–Ni(2)–N(1), 100.3(3); O(4)–Ni(2)–O(2), 91.7 (3); O(4)–Ni(2)–
O(1), 104.1(3); O(4)–Ni(2)–N(2), 103.0(3)Њ.
D a l t o n T r a n s . , 2 0 0 3 , 6 2 5 – 6 3 0
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At Ni(2) five coordination is completed by a water molecule
and the geometry is distorted square pyramidal [τ = 0.24;17
O(2)–Ni(2)–N(1), 167.1(2); N(2)–Ni(2)–O(1), 152.8(2)Њ]. This
gives the complex a coordination number [5,6] and geometric
asymmetry with donor sets {N2O3} and {N2O4}.
Chelation from the pendant nitrogen atoms provides five-
membered chelate rings at each nickel() atom and the octa-
hedral coordination at Ni(1) is distorted, [N(4)–Ni(1)–O(5),
172.8(3); O(1)–Ni(1)–O(3), 171.7(2); N(3)–Ni(1)–O(2),
168.8(2)Њ]. The cresolato-bridge is non-symmetric and the
Ni ؒ ؒ ؒ Ni separation is 3.0495(18) Å, reduced from the values
found for 2–5 and reflecting the reduced connectivity of the
bridges. The angles at the monoatomic bridges are similar,
100.2(2)Њ [Ni(1)–O(1)–Ni(2)] and 100.3(2)Њ [Ni(1)–O(1)–Ni(2)].
Both water molecules lie on the same side of the Ni(2)–O(1)–
Ni(1)–O(2) molecular plane and the urea molecule is cis- to the
water molecule at Ni(1).
We have not yet been able to detect any hydrolysis of urea
using complex 6 despite its having biosite resemblance through
the bridging hydroxide and coordinated water molecules. It has
been suggested that the unique single atom (µ2-κO) bridging
mode of urea detected in 8 might be of relevance to the enzyme
urease in that the urea molecule would be better activated by
coordination to two nickel atoms rather than simply to one —
at this point the bonding mode of urea which permits the
catalytic reaction in urease is not known.21 The lack of reactiv-
ity with complex 6, in which there is only a η1(O)-coordinated
urea, can be regarded as providing circumstantial support for
this hypothesis.
Acknowledgements
We thank the EPSRC for support (to S.C.) and for funds
towards the purchase of the diffractometer.
Complexes incorporating urea have been synthesised and
used as models to help elucidate possible binding modes of the
substrate to the dinickel() centre in urease.20 Several model
complexes have been prepared with urea bound to a nickel at
the dinuclear through its carbonyl oxygen atom (η1(O)-
coordinated). Koga et al., prepared and structurally character-
ised [Ni2L6(µ-CH3COO)(CH3COO)(urea)]BPh4, 7 from the
Schiff base compartmental ligand HL6.7 Like complex 6 it is
coordination number [5,6] and geometry asymmetric but with
donor sets {N4O1} and {N4O2}. The geometry at Niiminic is
square pyramidal and that of Naminic pseudo-octahedral, the
Ni ؒ ؒ ؒ Ni separation is 3.155 Å.
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Together these results established
a precedence for the
hydrolysis of urea via a cyanate intermediate as an alternative
mechanism for the urease-catalysed hydrolysis of urea.
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