The preparation and structural characterisation of bis(N,N-diethyl-monothiocarbamato)complexes of cadmium and of the novel tetrameric complex [Et4Zn4(OSCNEt2)2(NEt 2)2]: Two new bonding modes for the monothiocarbamato ligand

Article abstract of DOI:10.1039/a805986a

Bis(diethylmonothiocarbamato) complexes of cadmium (1) and zinc (2) were prepared by the reaction of sodium monothiocarbamate with cadmium acetate or zinc chloride. Compound 1 crystallises as clear needles and a single crystal X-ray analysis showed it to be monoclinic (space group C2/c, no. 15), a = 22.143(2), b = 9.239(2), c = 7.553(3) A, β = 101.41(2)° and Z = 4. The coordination at the cadmium centre is distorted trigonal prismatic with two S,O-bidentate and two O-monodentate diethylmonothiocarbamato ligands. This new, O-binucleating, bonding mode for the monothiocarbamato ligand results in polymeric chains which are co-aligned to give a distorted close-packed hexagonal array. The cage complex [Et₄Zn₄(OSCNEt₂)₂(NEt ₂)₂] 3 is formed as the only isolable product from the reaction of EtZnNEt₂ with carbonyl sulfide. Clear rhombic crystals (space group P2₁/n, no. 14) were formed [a = 10.818(1), b = 14.732(1), c = 11.758(2) A, β = 103.25(1)° and Z = 2]. The cage is comprised of two six-membered Zn₂CNOS metallocycles that are linked by pairs of Zn-S and Zn-O bonds. In 3 a second new bonding mode for the monothiocarbamato ligand is observed in which both the oxygen and sulfur atoms are binucleating. Molecules of 3 pack to form a slightly distorted cubic close-packed array. The variable temperature ₁H NMR of 3 is also reported and shows the cluster to remain intact in solution.

Full text of DOI:10.1039/a805986a

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The preparation and structural characterisation of bis(N,N-diethyl-
monothiocarbamato)complexes of cadmium and of the novel
tetrameric complex [Et4Zn4(OSCNEt2)2(NEt2)2]: two new
bonding modes for the monothiocarbamato ligand
Mohammed Chunggaze, M. Azad Malik, Paul O’Brien, Andrew J. P. White and
David J. Williams
Department of Chemistry, Imperial College of Science, Technology and Medicine,
South Kensington, London, UK SW7 2AZ. E-mail: p.obrien@ic.ac.uk
Received 30th July 1998, Accepted 15th September 1998
Bis(diethylmonothiocarbamato) complexes of cadmium (1) and zinc (2) were prepared by the reaction of sodium
monothiocarbamate with cadmium acetate or zinc chloride. Compound 1 crystallises as clear needles and a single
crystal X-ray analysis showed it to be monoclinic (space group C2/c, no. 15), a = 22.143(2), b = 9.239(2), c = 7.553(3)
Å, β = 101.41(2)Њ and Z = 4. The coordination at the cadmium centre is distorted trigonal prismatic with two
S,O-bidentate and two O-monodentate diethylmonothiocarbamato ligands. This new, O-binucleating, bonding
mode for the monothiocarbamato ligand results in polymeric chains which are co-aligned to give a distorted
close-packed hexagonal array.
The cage complex [Et4Zn4(OSCNEt2)2(NEt2)2] 3 is formed as the only isolable product from the reaction of
EtZnNEt2 with carbonyl sulfide. Clear rhombic crystals (space group P21/n, no. 14) were formed [a = 10.818(1),
b = 14.732(1), c = 11.758(2) Å, β = 103.25(1)Њ and Z = 2]. The cage is comprised of two six-membered Zn2CNOS
metallocycles that are linked by pairs of Zn–S and Zn–O bonds. In 3 a second new bonding mode for the mono-
thiocarbamato ligand is observed in which both the oxygen and sulfur atoms are binucleating. Molecules of 3 pack
to form a slightly distorted cubic close-packed array. The variable temperature 1H NMR of 3 is also reported and
shows the cluster to remain intact in solution.
Introduction
Dithiocarbamato complexes of metal ions are well known1–6
and find applications in the rubber7 and petrochemical indus-
tries,8 chemical analysis9 and in the deposition of ZnS or CdS
thin films by metal organic chemical vapour deposition.1,6,10–12
Ϫ
The dithiocarbamate (dtc) ligand, R2NCS2 (R = alkyl) is a
formal three-electron donor and has the ability to stabilise
metal centres in a variety of oxidation states.1 There are several
methods for the preparation of such carbamates and the most
popular are the reaction of a metal salt with CE2 (E = O, S or
Se) in the presence of a secondary amine13 or the formal inser-
tion of CE2 across an M–NR2 bond in an alkylamide.14,15 Mixed
alkyl species formed by cadmium or zinc, [RM(E2CNRЈ2)]2
(E = S or Se, R = Me or Et and RЈ = alkyl), are generally
dimeric in the solid state and are readily formed by the reaction
of CE2 with RM(NRRЈ). However, the insertion of CO2 into
such alkylamides leads to unusual species such as, with zinc, the
Scheme 1
tetramers [Me2Zn4(O2CNEt2)6] and [Me4Zn4(O2CNEt2)4].16,17
Here we report further studies of the coordination chemistry
thiocarbamato compound [Et4Zn4(OSCNEt2)2(NEt2)2] 3, a
cage complex obtained as the only crystalline product from the
insertion reactions of carbonyl sulfide (OCS) into zinc–amide
bonds. Compounds 1 and 3 both show new coordination modes
of carbamato ligands. Complexes of monothiocarbamates
(Et2NCOSϪ diethylmonothiocarbamate mtc) have been much
less extensively studied than those of thiocarbamates and only a
handful of reports have appeared. It has been suggested, on the
basis of spectroscopic results, that the monothiocarbamate
group forms chelates with both zinc and cadmium.18 Mono-
for the mtc ligand.
thiocarbamates are known to adopt several modes of co-
ordination; six have been reported so far19–26 (Scheme 1).
The ligands have been shown to form polymeric complexes
involving bridging to adjacent metals through either oxygen
or sulfur.
The present work concerns the preparation and the first
single crystal X-ray structural analyses of [{Cd(OSCNEt2)2}n]
1 and of an unusual tetrameric mixed alkyl/alkylamido/mono-
Experimental
Materials
All chemicals were purchased from Aldrich Chemical Co. Ltd.,
and were used without further purification. All solvents were
dried and distilled before use and reactions were carried
out under an inert atmosphere using standard Schlenk line
techniques.
J. Chem. Soc., Dalton Trans., 1998, 3839–3844
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Preparation of [Et2NH2]؉[SCONEt2]؊
crystals. Microanalysis. Found: C, 33.95; H, 5.78; N, 6.52. Calc.
for C26H60N4O2S2Zn4: C, 35.73; H, 5.75; N, 6.95%. H NMR
1
The salt was prepared by bubbling carbonyl sulfide into
diethylamine (50 mL) while keeping the reaction temperature
below 10 ЊC. The salt precipitates upon saturation of the
diethylamine solution (yield 19.23 g, 42%). The product was
filtered and washed thoroughly with diethyl ether giving a fine
white crystalline solid, and dried under vacuum for ca. 10 min.
The salt sublimes at room temperature under vacuum. The salt
is quite hygroscopic and was converted to the corresponding
sodium salt immediately or reacted with cadmium or zinc salts
to form the respective bis-complexes 1 or 2.
(270 MHz, CDCl3, 80 ЊC): δ 0.43 (q, ZnCH2CH3), 1.36 (t,
ZnCH2CH3), 1.18 (t, ZnNCH2CH3), 2.77 (q, ZnNCH2CH3),
0.91–1.04 (q, OSCNCH2CH3), 3.14–3.50 (q, OSCNCH2CH3).
13C NMR (127 MHz, CDCl3, 20 ЊC): δ 2.59 (ZnCH2CH3),
12.73 (ZnCH2CH3), 13.11 (OSCNCH2CH3), 15.81 (ZnNCH2-
CH3), 42.95 (OSCNCH2CH3), 46.35 (OSCNCH2CH3), 47.28
(ZnNCH2CH3), 180.74 (OSCN), 181.13 (OSCN).
Attempted preparations of RZn(OSCNEt2) (R ؍ Me 4, Et 5)
To a solution of R2Zn (R = Me 4 or Et 5) in toluene was added
dropwise an equimolar equivalent of 2 in toluene at Ϫ78 ЊC.
The mixture was allowed to attain room temperature and left
to stir overnight. The solution was then reduced in vacuo to
leave a very viscous oil. The oil was left to stand for several
Preparation of Na؉[SCONEt2]؊
The diethylammonium salt of diethylmonothiocarbamate
was prepared as above. The diethylammonium salt (45.00 g,
218 mmol) was then dissolved in dry benzene (100 mL) to
which was added slowly sodium hydride (5.00 g, 208 mmol).
The suspension was allowed to stir overnight by which time
a white solid had precipitated out (27.31 g, 84% theoretical).
The solid was isolated by filtration and washed with two
50 ml aliquots of benzene. Microanalysis. Found: C, 38.61;
H, 6.45; N, 8.96. Calc. for C5H10NNaOS: C, 38.70; H, 6.49,
N, 9.03%.
months in pentane, no crystallisation was observed. 1H and 13
C
NMR analysis of the oil in each case showed the presence of
the species 4 or 5.
Physical measurements
IR spectra were recorded on a Mattson-Polaris FT spec-
trometer as Nujol mulls between KBr plates or as KBr pellets.
1H and 13C NMR spectra were recorded using Bruker AM500
or DX400 pulsed FT/NMR instruments, in either CDCl3 or
C6D6. Variable temperature NMR were recorded on a JEOL
EX270. The internal reference used was Me4Si. Mass spectral
data were run on Micromass Autospec and Micromass
Platform2 spectrometers. Microanalysis were carried out at
Imperial College. X-Ray diffraction patterns were obtained
using secondary graphite monochromated Cu-Kα radiation on
a Philips PW1700 series automated diffractometer.
Preparation of Cd[SCONEt2]2 1
To the diethylammonium salt in water was added a stoichio-
metric amount of cadmium acetate, which gave a coagulated
white precipitate. The precipitate was isolated and recrystallised
from chloroform–pentane (1:1) giving colourless needles of
bis(diethylmonothiocarbamato)cadmium(), mp 167 ЊC. The
yield was quantitative.
Microanalysis. Found: C, 32.05; H, 5.04; N, 7.28. Calc. for
C10H20CdN2O2S2: C, 31.88; H, 5.35; N, 7.43%. 1H NMR
(270 MHz, CDCl3, 20 ЊC): δ 1.24 (t, 3H), 1.36 (t, 3H), 3.53
(q, 2H), 3.79 (q, 2H). 13C NMR (127 MHz, CDCl3, 20 ЊC):
δ 13.00 (CH2CH3), 13.00 (CH2CH3), 42.58 (CH2CH3), 46.39
(CH2CH3), 181.37 [OC(S)N]. X-Ray powder results [d(Å)
(% intensity)]: 10.53 (94), 8.3 (100), 5.62 (38), 4.84 (11), 4.2 (13),
3.89 (21), 3.79 (20), 3.48 (24), 2.92 (10), 2.82 (13), 2.69 (13),
2.31 (13), 2.15 (16).
X-Ray crystallographic analysis
Table 3 provides a summary of the crystal data, data collection
and refinement parameters for 1 and 3. The structures were
solved by the heavy atom method and by direct methods for
1 and 3 respectively. In 3 disorder was found in the position
of the terminal methyl group of one of the ethyl groups.
This was resolved into two partial occupancy positions, the
major occupancy non-hydrogen atom of which was refined
anisotropically. The remaining non-hydrogen atoms in both
structures were refined anisotropically. In each structure the
C–H hydrogen atoms were placed in calculated positions,
assigned isotropic thermal parameters, U(H) = 1.2Ueq(C)
[U(H) = 1.5Ueq(C-Me)], and allowed to ride on their parent
atoms. Computations were carried out using the SHELXTL
PC program system.27
Preparation of Zn[SCONEt2]2 2
Compound 2 was prepared by reacting a suspension of sodium
N,N-diethylmonothiocarbamate (4.59 g, 29.6 mmol) in benzene,
with anhydrous zinc chloride (2 g, 14.8 mmol) dissolved in dry
diethyl ether. The precipitate (4.19 g, 86% theoretical) was
washed with ethanol and recrystallised from benzene–hexane
(1:1), mp 110 ЊC.
CCDC reference number 186/1163.
Microanalysis. Found: C, 36.81; H, 5.97; N, 8.46. Calc. for
C10H20N2O2S2Zn: C, 36.58; H, 6.14; N, 8.54%. H NMR (270
1
MHz, CDCl3, 20 ЊC): δ 1.09 (t, 3H), 1.20 (t, 3H), 3.37 (q, 2H),
3.67 (q, 2H). 13C NMR (127 MHz, CDCl3, 20 ЊC): δ 13.09
(CH2CH3), 13.09 (CH2CH3), 42.56 (CH2CH3), 46.64
(CH2CH3), 181.53 [OC(S)N]. X-Ray powder results [d(Å) (%
intensity)]: 10.35 (96), 8.19 (100), 5.56 (40), 4.8 (12), 4.17 (13),
3.86 (22), 3.75 (20), 3.46 (25), 2.91 (10), 2.81 (13), 2.68 (12), 2.31
(11), 2.15 (12).
Results and discussion
Synthesis of complexes
Stoichiometric amounts of cadmium acetate and the ammo-
nium salt [Et2NH2]ϩ[Et2NCOS]Ϫ were reacted in water to give
a white precipitate of the bis-mtc complex of cadmium (1)
which was isolated and recrystallised from a 1:1 chloroform
pentane mixture to give colourless needles. The preparation of
Zn(OSCNEt2)2 2 when attempted by the above route resulted in
the formation of a mixture of 2 and a fine insoluble precipitate
characterised as ZnS by X-ray powder diffraction. The bis-mtc
complex of zinc was prepared by a non-aqueous route involving
the metathesis reaction of a suspension of sodium diethyl-mtc
in benzene with zinc chloride solution in diethyl ether. The
solution was filtered and the volume reduced in vacuo to yield
a white powder, which was recrystallised from benzene to give
Preparation of [Et4Zn4(OSCNEt2)2(NEt2)2] 3
One equivalent of diethylamine was added slowly (dropwise) to
diethyl zinc at Ϫ78 ЊC. The resulting clear solution was allowed
to warm to room temperature and refluxed at 40 ЊC for 3 h.
The solution was then cooled to Ϫ78 ЊC and OCS was bubbled
into it. After several minutes the solution turned pale yellow
in colour and a white solid precipitated. Recrystallisation
from toluene afforded clear colourless hexagonal and cubic
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J. Chem. Soc., Dalton Trans., 1998, 3839–3844

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very fine crystals of 2. Crystals of 2 suitable for single crystal
X-ray analysis could not be prepared. The preparation of
RZn(SOCNEt2) (R = Me 4 or Et 5) was attempted by the com-
proportionation reaction28 of 2 with either dimethyl or diethyl
zinc respectively. However, these reactions produced viscous
liquids which when characterised showed the presence of the
above species 4 and 5 (cf. Experimental section). The insertion
of carbonyl sulfide across a metal–amide bond is a well estab-
lished route to many transition metal monothiocarbamates,29
hence bubbling carbonyl sulfide into a solution of ethylzinc
diethylamide30 was expected to afford the mixed alkyl species 5.
However after bubbling carbonyl sulfide through a solution for
several minutes at (Ϫ78 ЊC, toluene) clear crystals of 3 began to
precipitate. Recrystallisation from toluene gave isomorphous
hexagonal and rectangular crystals of [Et4Zn4(OSCNEt2)2-
(NEt2)2] 3. The insertion of OCS gas was also carried out at
higher temperatures and with various ethylzinc diethylamide
concentrations; in each case crystals of 3 began to form. When
left in contact with air, 3 decomposes slowly to give the air
stable bis-complex 2.
Fig. 1 The coordination at cadmium in the structure of 1.
Molecular structure of Cd[SCONEt2]2 1
The X-ray structure of 1 reveals the complex to have crystallo-
graphic C2 symmetry, the cadmium being bound to two
‘bidentate’ diethyl-mtc ligands and ‘monodentately’ to the
oxygen atoms of two further mtc ligands (Fig. 1). The oxygen
atoms of each ligand (all of which are crystallographically
equivalent) are also binucleating and bridge adjacent cadmium
centres to form the extended polymer chain shown in Fig. 2.
This is a new bonding mode for the mtc ligand and is repre-
sented in Scheme 2-I. The geometry at cadmium is distorted
Fig. 2 Part of one of the extended polymer chains present in the
crystals of 1.
Scheme 2
trigonal prismatic (Fig. 3) with Cd–S distances of 2.498(1) Å
and Cd–O distances of 2.587(4) Å (bidentate) and 2.347(4) Å
(monodentate), i.e. the oxygen bridging is asymmetric. The
geometry within the mtc ligand is unexceptional, exhibiting
Fig. 3 The trigonal prismatic coordination polyhedron present in 1.
Table 1 Selected bond lengths (Å) and angles (Њ) for 1
the expected pattern of N–C᎐O bond delocalisation, the C᎐O
᎐
᎐
distance being unaffected by its dual coordinating and bridging
roles (Table 1). The coordination planes of the two ‘bidentate’
mtc ligands subtend an angle of ca. 133Њ at cadmium. Along the
polymer axis (the crystallographic c direction) the Cd atoms
describe a zigzag chain, the Cd ؒ ؒ ؒ Cd vectors subtending an
angle of 151Њ (transannular Cd ؒ ؒ ؒ Cd separation 3.90 Å), and
with adjacent planar Cd2O2 rings being inclined by 52Њ to each
other. Adjacent polymer chains are parallel and are arranged
to give a distorted close-packed hexagonal array (Fig. 4).
The outer surfaces of each chain are dominated by the hydro-
phobic ethyl groups and thus there are no interchain inter-
actions other than normal van der Waals. This absence of
strong intermolecular interactions clearly contributes to the
low melting point of the solid.
Cd–O*
Cd–S
S–C
OЉ–CdЈ
Cd–O Љ
2.347(4)
2.498(1)
1.740(5)
2.347(4)
2.347(4)
Cd–O
C–O
Cd–SЈ
Cd–OЈ
C–N
2.587(4)
1.268(6)
2.498(1)
2.587(4)
1.341(7)
OЉ–Cd–O*
OЉ–Cd–S
OЉ–Cd–O
S–Cd–O
SЈ–Cd–OЈ
C–O–CdЈ
OЉ–Cd–SЈ
O*–Cd–S
O*–Cd–O
83.5(2)
117.6(1)
75.7(1)
61.6(1)
61.6(1)
130.7(3)
94.9(1)
94.9(1)
134.0(1)
OЈ–Cd–OЉ
S–Cd–OЈ
C–O–Cd
O*–Cd–SЈ
SЈ–Cd–S
SЈ–Cd–O
OЈ–Cd–O*
O–Cd–OЈ
Cd–O–CdЈ
134.0(1)
104.8(1)
93.2(3)
117.6(1)
136.9(1)
104.8(1)
75.7(1)
145.2(2)
104.4(1)
In the IR spectrum the ν(C–S) mode is observed at 680 cmϪ1
(medium) and 695 cmϪ1 (medium) for 1 and 2, respectively.
The broad band at 1510–1525 cmϪ1 (very strong) seen for
1 and at 1510–1530 cmϪ1 (very strong) for 2 is assigned to
the ν(C–O) mode that may also include the ν(C–N) mode.
Although crystals for 2 suitable for single crystal X-ray analysis
could not be grown, the powder X-ray data (cf. experimental)
and IR data suggest that the coordinating behaviour of the
mtc ligand to both cadmium and zinc metal centres is similar
and that the two compounds are isomorphous.
Molecular structure of 3
The X-ray analysis of the product of the reaction between
EtZnNEt2 and OCS reveals the formation of the fascinating
J. Chem. Soc., Dalton Trans., 1998, 3839–3844
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Fig. 4 The slightly flattened close-packed hexagonal array of polymer
chains in 1 viewed down the chain axes.
Fig. 6 Part of one of the close-packed hexagonal sheets in the crystals
of 3. Adjacent sheets pack to form an ABC cubic close-packed
arrangement.
being bridged on one side by the nitrogen atom of the original
EtZnNEt2 and on the other bidentately by the mtc ligand
formed in situ. The oxygen and sulfur atoms of this latter ligand
are each binucleating, linking the two metallocycles to form the
cage (within which all four zinc atoms are perfectly coplanar).
The two Zn2OS faces are arranged in a distorted square
prismatic geometry with respect to each other, the two four-
membered rings being non-planar (folded by ca. 28Њ about
the Zn ؒ ؒ ؒ Zn vector) with transannular Zn ؒ ؒ ؒ Zn and S ؒ ؒ ؒ O
separations of 3.28 and 3.27 Å respectively.
The geometry at each zinc centre is distorted tetrahedral with
angles ranging between 87.0(1)–136.2(2)Њ at Zn(1) and 88.5(1)–
126.0(3)Њ at Zn(2), the acute angles being associated with the
four-membered rings whereas the most obtuse ones are for the
N–Zn–Et angles. The Zn–C bond lengths are unexceptional
(Table 2) and the bridging Zn–N distances [1.996(4) and
2.010(4) Å] are comparable to those observed in, for example,
bis(µ-diphenylamido)bis(methylzinc).31 The two independent
Zn–O distances do not differ significantly, but are markedly
longer than those observed in related carbamate species.25
The Zn–S bond lengths on the other hand are noticeably
asymmetric with that within the six-membered metallocycle
[2.466(1) Å] being shorter than that bridging between the two
six-membered rings [2.527(1) Å].
Fig. 5 The cage structure of 3.
Table 2 Selected bond lengths (Å) and angles (Њ) for 3
Zn(1)–C(11)
Zn(1)–S
Zn(2)–O(1Ј)
C(1)–O(1)
Zn(1)–N(6Ј)
Zn(2)–C(13)
1.981(5)
2.527(1)
2.214(3)
1.274(5)
1.996(4)
1.958(6)
Zn(2)–S
2.466(1)
1.329(6)
2.213(3)
2.010(4)
1.762(4)
C(1)–N(1)
Zn(1)–O(1Ј)
Zn(2)–N(6)
S–C(1)
The pattern of bonding within the mtc ligand is little
changed from that observed in 1, there being the expected
delocalisation in the N–C᎐O unit. The C–S distance here is
slightly longer than that seen in 1, reflecting the binucleating
᎐
C(11)–Zn(1)–N(6Ј)
C(13)–Zn(2)–S
C(13)–Zn(2)–N(6)
C(1)–Zn(2)–S
C(1)–S(1)–Zn(2)
O(1)–C(1)–S
Zn(1Ј)–O(1)–Zn(2Ј)
C(11)–Zn(1)–O(1Ј)
N(6Ј)–Zn(1)–S
136.2(2)
108.9(2)
126.0(3)
112.0(2)
102.2(2)
121.0(3)
95.8(1)
105.1(2)
105.0(1)
110.9(2)
N(6)–Zn(2)–S
C(1)–S–Zn(1)
114.1(1)
101.3(1)
129.3(3)
111.6(2)
103.6(1)
87.0(1)
97.1(1)
88.5(1)
82.2(1)
129.3(3)
C(1)–O(1)–Zn(1Ј)
Zn(1Ј)–N(6)–Zn(2)
N(6Ј)–Zn(1)–O(1Ј)
O(1Ј)–Zn(1)–S
N(6)–Zn(2)–O(1Ј)
O(1Ј)–Zn(2)–S
role of the sulfur atom, the C᎐O distance being unaffected.
᎐
Whereas the sulfur has a pronounced contracted tetrahedral
geometry, with angles of 82.2(1), 101.3(1) and 102.2(2)Њ,
the oxygen atom adopts a near trigonal planar arrangement
[angles of 95.8(1), 129.3(3) and 129.3(3)Њ], the oxygen atom
lying 0.23 Å out of the plane of its substituents. For both the
sulfur and oxygen atoms, the smallest angles are those associ-
ated with the Zn2OS four-membered ring. The cage molecules
have an approximately spherical surface and pack to form
an only slightly distorted cubic close-packed array, the shortest
centroid ؒ ؒ ؒ centroid distances within a layer being 10.2, 10.8
and 11.5 Å (Fig. 6).
Zn(2)–S–Zn(1)
C(1)–O(1)–Zn(2Ј)
C(13)–Zn(2)–O(1Ј)
Ci-symmetric cage complex 3 (Fig. 5) which exhibits another
new bonding mode for the mtc ligand (Scheme 2-II). The cage is
comprised of two six-membered Zn2CNOS metallocycles each
with a slightly twisted ‘boat’ conformation, the zinc centres
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and two lowest field multiplets, δ 3.14–3.50, for the methylene
groups. The amidoethyl groups give a well resolved triplet at
δ 1.18 for the equivalent methyl groups (12 protons) and two
broad quartets (δ 2.77–3.25) for non-equivalent methylene
protons [Fig. 7(a), 20 ЊC]. The 2D proton COSY NMR taken at
room temperature confirmed, unequivocally, the correlation of
the high and low field resonances.
The 13C NMR spectrum showed nine resonances. Two were
assigned to the ethyl group attached to the zinc and the mono-
thiocarbamato ligand gave five resonances (the quaternary
carbons of the two mtc ligands are slightly inequivalent, the
methyl groups of the mtc ligand are equivalent), the amido-
ethyl groups gave two environments as expected. The chemical
shift assignments are given in the experimental section.
Variable temperature (VT) 1H NMR analysis of 3
A VT proton NMR study was undertaken to try to under-
stand the broad signals observed at room temperature. Proton
NMR spectra were recorded from 10 to 80 ЊC at 5 ЊC inter-
vals, in [2H6]benzene (C6D6). Four proton NMR spectra taken
at 20, 40, 60 and 80 ЊC are shown [Fig. 7(a)–(d) respectively].
On reducing the temperature below 20 ЊC further line
broadening was observed resulting in the loss of fine structure
which may in part be due to viscosity; at Ϫ10 ЊC in [2H8]toluene
the compound precipitated. Increasing the temperature to
40 ЊC [Fig. 7(b)] results in the sharpening of the signals for
zinc–ethyl groups, but a considerable loss of the fine structure is
observed for all other signals. At 60 ЊC [Fig. 7(c)] the zinc–ethyl
group signals are completely resolved and a similar but smaller
effect occurs for the other resonances. The amido-methylene
signals are resolved into two broad multiplets. Increasing the
temperature to 80 ЊC [Fig. 7(d)] results in the sharpening of
signals for all methyl environments within 3. Two clear signals
are observed for methylene groups of the mtc ligands and one
broad signal for amido-methylene groups.
However, the major effect observed in the VT NMR studies is
on the broad resonance at δ ca. 2.9. This resonance is associated
with the methylene protons of the monothiocarbamato ligand.
This peak splits at 60 ЊC into two distinct resonances and at
room temperature a sharp signal is observed. This observation
is consistent with restrictive rotation along the C–N bond hence
lowering the symmetry of the complex at lower temperatures.
At 80 ЊC the spectra simplifies to that seen in the solid state
from X-ray crystallography. The VT NMR studies hence con-
firm that the tetramer 3 exists in solution, the restriction of
rotation around the C–N bond is typical of dynamic variable
temperature behaviour.
Conclusion
Two novel group 12 complexes of the monothiocarbamato
ligand have been prepared and structurally characterised.
Compound [{Cd(OSCNEt2)2}n] 1 crystallises as clear needles
and is polymeric. The coordination at the cadmium centre is
distorted trigonal prismatic with two S,O-bidentate and two O-
monodentate diethyl-monothiocarbamato ligands. The unique
Ci-symmetric cage complex [Et4Zn4(OSCNEt2)2(NEt2)2] 3 is
comprised of two six-membered Zn2CNOS metallocycles each
with a slightly twisted ‘boat’ conformation. The zinc centres
are bridged on one side by the nitrogen of the diethylamide,
and on the other bidentately by the mtc ligand. Both com-
pounds 1 and 3 show new bonding modes for the monothio-
carbamato ligand.
Fig. 7 1H Variable temperature NMR of 3 at (a) 20, (b) 40, (c) 60 and
(d) 80 ЊC respectively.
NMR studies
1
The H NMR spectrum of compound 3, at room temperature,
showed evidence for nine or ten resonance signals some of
which were broadened. The cage structure of 3 should lead
to only eight proton environments. The spectrum confirmed
four resonances at high field and another set at lower field
in which the broadening was observed. The highest field
signals correspond to the ethyl groups bound to zinc with the
methylene protons at δ 0.43 and a signal at a lower field, δ 1.36,
for the methyl protons. The monothiocarbamato ligands give
two overlapping triplets at δ 0.91–1.04 for the methyl groups
Acknowledgements
We thank J. Barton, R. N. Sheppard of Imperial College for
EI-MS and VT-NMR respectively and Mr Richard Sweeney
(Imperial College) for X-ray powder diffraction work. Paul
J. Chem. Soc., Dalton Trans., 1998, 3839–3844
3843

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Table 3 Crystal data, data collection and refinement parametersa
Data
1
3
Formula
Formula weight
C10H20N2O2S2Cd
376.8
C26H60N4O2S2Zn4
790.4
Colour, habit
Crystal size/mm
Lattice type
Space group, number
Clear needles
0.43 × 0.13 × 0.13
Monoclinic
C2/c, 15
Clear rhombs
0.33 × 0.27 × 0.22
Monoclinic
P21/n, 14
T/K
293
173
Cell dimensions
a/Å
b/Å
c/Å
22.143(2)
9.239(2)
7.553(3)
101.41(2)
1514.6(6)
4b
10.818(1)
14.732(1)
11.758(2)
103.25(1)
1823.9(3)
2c
β/Њ
V/Å3
Z
Dc/g cmϪ3
1.652
1.439
F(000)
760
Cu-Kα
14.08
832
Radiation used
Cu-Kαd
4.25
µ/mmϪ1
θ range/Њ
4.1–60.0
5.9–63.9
No. of unique reflections
measured
1138
1068
Semi-empirical
0.24, 0.11
79
0.038
0.101
0.057, 4.584
0.61, Ϫ0.74
3005
2510
Semi-empirical
0.29, 0.18
177
0.047
0.118
0.071, 2.442
0.80, Ϫ0.64
observed, |Fo| > 4σ(|Fo|)
Absorption correction
Maximum, minimum transmission
No. of variables
e
R1
f
wR2
Weighting factors a, bg
Largest difference peak, hole/e ÅϪ3
a Details in common: graphite monochromated radiation, ω-scans, Siemens P4 diffractometer, refinement based on F 2. b The molecule has crystallo-
¹
graphic C2 symmetry. c The molecule has crystallographic Ci symmetry. d Rotating anode source. e R1 = Σ Fo| Ϫ |Fc /Σ|Fo|. f wR2 = {Σ[w(Fo2 Ϫ Fc2)2]²/
Σ[w(Fo )2]}. g wϪ1 = σ2(Fo ) ϩ (aP)2 ϩ bP.
2
2
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O’Brien is the Royal Society Amersham International Research
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Materials Chemistry. M. C. would like to thank the EPSRC for
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3844
J. Chem. Soc., Dalton Trans., 1998, 3839–3844