The synthesis and structure of a neutral tetranuclear zinc(II) complex [Zn4(L)4] [LH2 = N,N-bis(2-mercaptoethyl)benzylamine]

Article abstract of DOI:10.1039/a901251f

The reaction of benzylamine with ethylene sulfide yields the monoamine-dithiol N,N-bis(2-mercaptoethytyl)benzylamine, LH₂. Reaction of Na₂L with Zn(BF₄)₂ affords the neutral tetranuclear complex [Zn₄L

Full text of DOI:10.1039/a901251f

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The synthesis and structure of a neutral tetranuclear zinc(II)
complex [Zn4(L)4] [LH2 ؍ N,N-bis(2-mercaptoethyl)benzylamine]
Douglas J. E. Spencer,a Alexander J. Blake,a Simon Parsonsb and Martin Schröder*a
a School of Chemistry, The University of Nottingham, Nottingham, UK NG7 2RD
b Department of Chemistry, The University of Edinburgh, West Mains Road, Edinburgh,
UK EH9 3JJ
Received 2nd November 1998, Revised manuscript received 16th February 1999,
Accepted 16th February 1999
The reaction of benzylamine with ethylene sulfide yields
the monoamine–dithiol N,N-bis(2-mercaptoethyl)benzyl-
amine, LH2. Reaction of Na2L with Zn(BF4)2 affords the
neutral tetranuclear complex [Zn4L4], which shows an
unusual Zn4S4 metallacyclic structure.
There are a number of metalloenzymes that incorporate Zn()
thiolates in their prosthetic group. These include alcohol
dehydrogenases,1,2 metallothioneins,3 and zinc fingers.4 The
study of thiolate complexes of Zn() as models for these
enzymes, particularly of LADH (liver alcohol dehydrogenase),2
has therefore become an active area of investigation.5–7 Com-
plexes of thiolates with Zn() show interesting co-ordination
chemistry because of their tendency to form oligomers and
clusters8,9 comprising cages or aggregates of [ZnS4] units
formed via S-bridging between metal centres. Other structures
including binuclear9,10 and tetrameric11 species have been
reported. Monomeric compounds featuring Zn() co-ordinated
to an NS2-donor set have also been reported as models for the
active centre of LADH.6,12,13 These are rare examples of non-
polymeric neutral Zn() complexes of the type Zn(SR)2 con-
taining tetrahedrally co-ordinated Zn() ions bridged by RSϪ.
As part of a study of thiolate-bridged complexes of biological
significance,7,14 we report herein a novel, neutral tetranuclear
Zn() system with bridging thiolates.
The sodium salt of the ligand N,N-bis(2-mercaptoethyl)-
benzylamine (LH2)15,16 (Scheme 1) reacts with Zn(BF4)2 in a
S
C6H6
Fig. 1 Complementary views of the structure of [Zn4(L)4]ؒ0.5Et2O
NH2
N
with numbering scheme adopted. Hydrogen atoms and solvent mol-
ecule are omitted for clarity. (a) View approximately along a axis; (b)
view approximately along c axis.
HS
SH
LH2
Scheme 1
case we observe a rare example9,11,12 of aliphatic amine group
functionality at Zn(). Lippard and co-workers have reported11
a tetranuclear Zn() complex with the ligand N,NЈ-dimethyl-
N,NЈ-bis(2-mercaptoethyl)ethylenediamine (L1H2) (see below)
to give [Zn4Cl4(L1)2], while Darensbourg and co-workers have
1:1 molar ratio in THF to give a white solid after removal of
the solvent.† Colourless crystals suitable for crystallographic
studies were grown by diffusion of Et2O vapour into a solution
of the complex in CHCl3. Elemental analysis, IR spectroscopy
and FAB mass spectrometry confirm the product to have
the stoichiometry [Zn4L4] and this was confirmed by X-ray
diffraction studies on a single crystal of the diethyl ether
hemi-solvate.‡
The complex contains four crystallographically independent
Zn() centres (Fig. 1), each co-ordinated by an equivalent
[NS3] donor set comprising a tertiary amine from one ligand
[Zn–N 2.109(3)–2.161(4) Å], one thiolate donor from the same
ligand co-ordinated terminally [Zn–Sterminal 2.261(4) Å] and a
second thiolate bridging two Zn() ions in two different [ZnL]
units [Zn–Sbridging 2.313(2)–2.3572(14) Å]. Therefore, an overall
Zn4S4 metallacycle is formed within the structure. Most of the
previously reported structures of Zn() with mixed amine–
thiolate ligands involve the N-donors as part of a heteroaro-
matic ring (pyrazoles, imidazoles and pyridines).5,6,12,13,17 In this
prepared9
a
neutral binuclear Zn() complex [Zn2(L2)2]
[L2H2 = N,NЈ-bis(2-mercaptoethyl)-1,5-diazacyclooctane] (see
below). The bond distances in both of these structures9,11 are
similar to those in [Zn4(L)4] with Zn–N bond lengths of
2.079(11), 2.103(11) Å, Zn–Sbridging 2.284(4)–2.356(4) Å,11 and
Zn–N 2.231(2)–2.255(2) Å, Zn–Sterminal 2.327(1) Å, Zn–Sbridging
2.394(1)–2.494(1) Å, respectively.9
N
N
N
N
SH HS
L1H2
SH
HS
L2H2
J. Chem. Soc., Dalton Trans., 1999, 1041–1042
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ϩ 1)ϩ, 579 (64Zn2L2 ϩ 1)ϩ with correct isotopic distribution. m/z (ϩve
ES) 1161 (Mϩ), 874 and 581.
Interestingly, tetrahedral co-ordination at Zn() in [Zn4(L)4]
is highly distorted with rather acute N–Zn–Sterminal and N–Zn–
Sbridging angles, N1B–Zn1–S7B 91.27(10), N1C–Zn2–S7C
‡ Crystal data: C44H60N4S8Zn4ؒ0.5C4H10O, M = 1199.98, triclinic, space
¯
group P1, a = 13.926(3), b = 14.593(3), c = 14.631(5) Å, α = 88.90(2),
92.62(11),
91.70(11),
91.17(11),
N1D–Zn3–S7D
N1B–Zn1–S4B
N1D–Zn3–S4D
91.97(11),
88.86(11),
90.35(10),
N1A–Zn4–S7A
N1C–Zn2–S4C
N1A–Zn4–S4A
β = 88.70(2), γ = 62.674(14)Њ, U = 2640.7(12) Å3, T = 220 K, Z = 2,
Dc = 1.509 g cmϪ3, λ(Cu-Kα) = 1.54184 Å, µ = 5.297 mmϪ1. 9255 unique
data measured and used in all calculations. A molecule of Et2O was
found to be half-occupied and disordered over two sites. Final wR(F2)
was 0.104, R1 = 0.0448. CCDC reference number 186/1354. See http://
www.rsc.org/suppdata/dt/1999/1041 for crystallographic files in .cif
format.
88.69(10)Њ, and expanded Sbridging–Zn–Sterminal angles, S7B–Zn1–
S4B 125.93(6), S4C–Zn2–S7C 125.03(6), S4D–Zn3–S7D
122.29(5), S4A–Zn4–S7A 129.79(5)Њ. This distortion is prob-
ably due to the steric factors inherent in the formation of the
tetranuclear complex and within individual [ZnL] units. The
aromatic rings orientate themselves exo to the central Zn4S8
core and are arranged alternately up and down (Fig. 1b) due to
the inversion of successive ligand units around the metallo-
cyclic [Zn4S4] centre. This also reduces the steric interactions
between the aromatic rings. There is no evidence of π–π stack-
ing either within or between molecules. FAB and electrospray
mass spectrometry confirm the integrity of the complex, at least
in part, in solution with molecular ions observed for the
monomer, dimer, trimer and tetramer.18
1 H. Eklund and C.-I. Bränden, in Zinc Enzymes, ed. T. G. Spiro,
Wiley, New York, 1983, p. 124.
2 Y. Pocker, in Metal Ions In Biological Systems, ed. H. Sigel and
A. Sigel, Dekker, New York, 1989, vol. 25, p. 335; H. Eklund and
C.-I. Bränden, in Active Sites of Enzymes, ed. F. A. Jurnak and
A. McPherson, Biological Macromolecules and Assemblies, Wiley,
New York, 1987, vol. 3, ch. 2.
3 J. H. R. Kägi, S. R. Himmelhoch, P. D. Whanger, J. L. Bethune and
B. L. Vallee, J. Biol. Chem., 1973, 39, 127.
4 W. Kaim and B. Schwederski, Bioinorganic Chemistry: Inorganic
Elements In the Chemistry of Life, Wiley, New York, 1991, ch. 12.
5 B. Kaptein, L. Wang-Griffin, G. Barf and R. M. Kellogg, J. Chem.
Soc., Chem. Commun., 1987, 1457; B. Kaptein, G. Barf, R. M.
Kellogg and F. Van Bolhuis, J. Org. Chem., 1990, 55, 1890; R. M.
Kellogg and R. P. Hof, J. Chem. Soc., Perkin Trans. 1, 1996, 1651
and refs. therein.
Current work is aimed at further developing thiolate chem-
istry at Zn() and related biologically relevant metal ions.
Acknowledgements
6 C. Kimblin, T. Hascall and G. Parkin, Inorg. Chem., 1997, 36, 5680
We thank the EPSRC for support and the EPSRC Centre for
mass spectrometry at the University of Swansea.
and refs. therein.
7 A. J. Blake, A. Marin-Becerra, N. D. J. Branscombe, W.-S. Li,
S. Parsons, L. Ruiz-Ramirez and M. Schröder, Chem. Commun.,
1996, 2573.
Notes and references
8 K. S. Hagen, D. W. Stephan and R. H. Holm, Inorg. Chem., 1982,
21, 3928; A. Choy, D. Craig, I. Dance and M. Scudder, J. Chem.
Soc., Chem. Commun., 1982, 1246; I. Dance, J. Chem. Soc., Chem.
Commun., 1980, 818; I. Dance, J. Am. Chem. Soc., 1980, 102, 3445;
J. L. Hencher, M. A. Kahn, F. F. Said and D. G. Tuck, Polyhedron,
1985, 4, 1263; R. H. Holm and M. J. O’Connor, Prog. Inorg. Chem.,
1971, 14, 241; F. F. Sai and D. G. Tuck, Inorg. Chim. Acta, 1982, 59,
1.
9 T. Tuntulani, J. H. Reibenspies, P. J. Farmer and M. Y. Darensbourg,
Inorg. Chem., 1992, 31, 3497.
10 D. C. Goodman, T. Tuntulani, P. J. Farmer, M. Y. Darensbourg
and J. H. Reibenspies, Angew. Chem., Int. Ed. Engl., 1993, 32, 116;
C. A. Grapperhaus, T. Tuntulani, J. H. Reibenspies and M. Y.
Darensbourg, Inorg. Chem., 1998, 37, 4052.
11 W. J. Hu, D. Barton and S. J. Lippard, J. Am. Chem. Soc., 1973, 95,
467.
12 S. C. Shoner, K. J. Humphreys, D. Barnhart and J. A. Kovacs, Inorg.
Chem., 1995, 34, 5933.
13 D. T. Corwin, Jr. and S. A. Koch, Inorg. Chem., 1988, 27, 493.
14 A. J. Atkins, A. J. Blake and M. Schröder, J. Chem. Soc., Chem.
Commun., 1993, 1662; A. J. Atkins, A. J. Blake, D. Black, A. Marin-
Becerra, S. Parsons, L. Ruiz-Ramirez and M. Schröder, Chem.
Commun., 1996, 457.
15 G. J. Colpas, M. Kumar, R. O. Day and M. J. Maroney, Inorg.
Chem., 1990, 29, 4779.
† Synthesis of LH2 and Na2L. Benzylamine (5 g, 0.0467 mol) in
benzene (5 cm3) was placed in a Schlenk tube flushed with N2. Ethylene
sulfide (5.9 g, 0.098 mol) in benzene (5 cm3) was added dropwise and the
1
resulting solution stirred at 65 ЊC. After 48 h analysis by H and 13C
NMR spectroscopy confirmed that a mixture of starting material and
mono-substituted product was present. A further two equivalents of
ethylene sulfide (5.9 g, 0.098 mol) were added and the solution left
stirring under N2 at 65 ЊC. After a further 48 h, analysis by NMR
spectroscopy revealed that the reaction had gone to completion. The
bulk solution was filtered and the excess solvent removed in vacuo to
yield a foul-smelling yellow oil. The oil was redissolved in CH2Cl2, the
solution filtered through a plug of silica to remove polymeric impur-
ities, and the excess solvent removed in vacuo to yield a clear oil (5.32 g,
0.023 mol, 52%) which was stored under N2. IR spectroscopy νmax/cmϪ1
(neat) 3059w, 3025w, 2962m, 2935m, 2803m, 2552w, 1600w, 1493m,
1369w, 1293w, 1260m, 1109m, 1028, 734, 698m (Found: C, 57.35; H,
7.71; N, 5.81. C11H17NS2 requires C, 58.15; H, 7.49; N, 5.81%). δH
(CDCl3) 1.68 (2H, s, CH2SH), 2.64 (4H, m, NCH2CH2SH), 2.70 (4H,
m, NCH2CH2SH), 3.64 (2H, s, PhCH2N) and 7.34 (5H, m, H of Ph). δC
(CDCl3) 22.87 (CH2SH), 57.13 (NCH2CH2SH), 58.63 (PhCH2N),
127.28, 128.42 and 128.92 (CH of Ph) and 138.91 (ipso C). m/z (EI)
225 (Mϩ). CAUTION: The ligand has been found to cause severe
allergic reactions and contact with skin should be avoided.
Na2L was prepared in quantitative yield by reaction of NaH (0.127 g,
5.29 mmol) with LH2 (0.4 g, 1.76 mmol) in THF.
16 S. A. Mirza, M. A. Pressler, M. Kumar, R. O. Day and M. J.
Maroney, Inorg. Chem., 1993, 32, 977.
17 L. F. Lindoy and D. H. Busch, J. Chem. Soc., Chem. Commun., 1972,
683.
Preparation of [Zn4L4]. Reaction of Na2L with Zn(BF4)2 (1:1 molar
ratio) in THF gave a white solid after removal of the solvent. The solid
was dissolved in CHCl3 and the solution filtered to remove sodium salts.
The solution was reduced in volume and the complex crystallised by
addition of Et2O (Found: C, 44.25; H, 5.61; N, 4.44. Calc. for
18 C. K. Meng and J. B. Fenn, Org. Mass Spectrom., 1991, 26, 542.
C46H65N4S8O0.5Zn4: C, 44.04; H, 5.42; N, 4.67%). IR (KBr)/cmϪ1
:
3025w, 2921w, 2849w, 1629s, 1494s, 1452s, 1310m, 1095m, 1003m,
825m, 721w, 668w. m/z (ϩve FAB) 1156 (64Zn4L4)ϩ, 868 (64Zn3L3
Communication 9/01251F
1042
J. Chem. Soc., Dalton Trans., 1999, 1041–1042