Probing the limits of steric constraints in the oligomerization of copper(I) complexes: Structures of two sterically congested oligomers

Article abstract of DOI:10.1016/S0277-5387(98)00371-4

Oligomeric copper(I) clusters are formed by the insertion reaction of copper(I) aryloxides into heterocumulenes. The effect of varying the steric demands of the heterocumulene and the aryloxy group on the nuclearity of the oligomers formed has been probed. Reactions with copper(I)2-methoxyphenoxide and copper(I)2-methylphenoxide with PhNCS result in the formation of hexameric complexes hexakis[N-phenylimino(aryloxy)methanethiolato copper(I)] 3 and 4 respectively. Single crystal X-ray data confirmed the structure of 3. Similar insertion reactions of CS₂ with the copper(I) aryloxides formed by 2,6-di-tert-butyl-4-methylphenol and 2,6-dimethylphenol result in oligomeric copper(I) complexes 7 and 8 having the (aryloxy)thioxanthate ligand. Complex 7 was confirmed to be a tetramer from single crystal X-ray crystallography. Reactions carried out with 2-mercaptopyrimidine, which has ligating properties similar to N-alkylimino(aryloxy)methanethiolate, result in the formation of an insoluble polymeric complex 11. The fluorescence spectra of oligomeric complexes are helpful in determining their nuclearity. It has been shown that a decrease in the steric requirements of either the heterocumulene or aryloxy parts of the ligand can compensate for steric constraints and facilitate oligomerization.

Full text of DOI:10.1016/S0277-5387(98)00371-4

Polyhedron 18 (1999) 949–958
Probing the limits of steric constraints in the oligomerization of copper(I)
complexes: structures of two sterically congested oligomers
*
Christina Wycliff, D. Saravana Bharathi, Ashoka G. Samuelson , Munirathinam Nethaji
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
Received 17 April 1998; accepted 6 October 1998
Abstract
Oligomeric copper(I) clusters are formed by the insertion reaction of copper(I) aryloxides into heterocumulenes. The effect of varying
the steric demands of the heterocumulene and the aryloxy group on the nuclearity of the oligomers formed has been probed. Reactions
with copper(I)2-methoxyphenoxide and copper(I)2-methylphenoxide with PhNCS result in the formation of hexameric complexes
hexakis[N-phenylimino(aryloxy)methanethiolato copper(I)] 3 and 4 respectively. Single crystal X-ray data confirmed the structure of 3.
Similar insertion reactions of CS2 with the copper(I) aryloxides formed by 2,6-di-tert-butyl-4-methylphenol and 2,6-dimethylphenol result
in oligomeric copper(I) complexes 7 and 8 having the (aryloxy)thioxanthate ligand. Complex 7 was confirmed to be a tetramer from
single crystal X-ray crystallography. Reactions carried out with 2-mercaptopyrimidine, which has ligating properties similar to
N-alkylimino(aryloxy)methanethiolate, result in the formation of an insoluble polymeric complex 11. The fluorescence spectra of
oligomeric complexes are helpful in determining their nuclearity. It has been shown that a decrease in the steric requirements of either the
heterocumulene or aryloxy parts of the ligand can compensate for steric constraints and facilitate oligomerization.
Science Ltd. All rights reserved.
1999 Elsevier
Keywords: Copper(I); Aryloxide; Heterocumulene; Oligomeric structures; Fluoresence
1. Introduction
ligands diethyldithiocarbamate and dipropylthiocarbamate
[32,33]. The former, with a S3 coordination yields a
tetrameric complex, while the latter with a S2O coordina-
tion gives rise to a hexameric complex. Furthermore, an
examination of the C–S bond distances in both complexes
show that the tetrameric complex has shorter distances
(1.70–1.72A) compared to the hexamer (1.75–1.81A).
Similarly, in a series of oligomeric complexes formed by
heterocyclic thiones, shorter C–S distances are associated
with the tetrameric complexes while longer C–S bonds are
present in the hexameric complexes [18–21,25–29]. This
suggests that ligands having short C–S bonds would favor
the formation of a tetrameric complex.
Steric factors play a vital role in determining the
nuclearity of clusters with bridging ligands. For example,
the nuclearity of complexes of 6-halo-2-hydroxypyridine
with a methoxide bridge are dependent on the size of the
halogen atom. The chloro substituted ligand yields a
tetramer while the less bulky fluoro substituted ligand
forms a polymeric complex [37]. Similarly, in copper(I)
thiolate complexes, bulky substitution in the ortho position
of thiophenols decreases the nuclearity of the clusters
formed [38,39]. Steric control of the nuclearity has also
The prolific nature of copper(I) to form oligomeric
complexes with nuclearities ranging from two to eight is
well documented [1–30]. A wide variety of coordination
numbers and geometries are encountered in these systems.
The plasticity imparted by the spherical symmetry of the
copper(I) ion permits considerable variations in the steric
and electronic characteristics of the surrounding ligands
[31]. Hence, it is extremely difficult to predict a priori the
nuclearity of a complex that would be formed with a
ligand. Subtle variations in the electronic and steric
characteristics of the substituents on a ligand can result in
changes of nuclearity [18,25,32–42]. A systematic study of
the steric and electronic factors responsible for changes in
the nuclearity of copper(I) complexes would go a long way
in building copper(I) complexes with predefined structure
and properties.
˚
˚
Electronic factors play an important role in determining
the nuclearity of copper(I) clusters [18–21,25–29,32,33].
An example is provided by oligomers formed by the
*
Corresponding author.
0277-5387/99/$ – see front matter
PII: S0277-5387(98)00371-4
1999 Elsevier Science Ltd. All rights reserved.

950
C. Wycliff et al. / Polyhedron 18 (1999) 949–958
genated before use. CuCl and [Cu(CH3CN)4]ClO4 (Cau-
tion! Perchlorate salts of metal complexes with organic
ligands are potentially explosive. Only small amounts
should be prepared and this should be handled with great
caution) were freshly prepared before use. Phenols,
MeNCS, PhNCS, CS2 and P(OMe)3 were purified by
distillation. A 60% suspension of NaH in mineral oil was
washed with petroleum ether (b.p. 60–808C) and used. 1H
NMR spectra were recorded with a Bruker ACF 200 MHZ
spectrometer. IR spectra were recorded on a Bio-Rad
FTS-7 spectrophotometer. Samples were examined as KBr
pellets, neat (thin film) or as nujol mull. The absorption
and emission spectra of the oligomeric copper(I) complex-
es in the solid state were recorded by grinding the solid
sample in mineral oil (nujol) to give a fine suspension and
the spectrum of a thin film of this suspension was
recorded. Electronic spectra were recorded on a Hitachi
Model U-3400 spectrophotometer and fluorescence studies
were done on a Hitachi F-2000 fluorescence spec-
trophotometer. Elemental analyses were done with a Carlo
Erba Elemental Analyzer model 1106.
Fig. 1. Possible Interactions Between the –OAr and R Groups in the
Oligomeric Complexes.
been observed in ligands that form three atom bridges. The
copper(I) cluster formed from diethyldithiocarbamate is
tetrameric while the smaller dimethyldithiocarbamate
forms a polymeric copper(I) complex [32].
2.1. Preparation of Copper(I) aryloxides
Copper(I) aryloxides were prepared by the reported
procedure [40–44]. To a stirred suspension of NaH (0.12
g, 3 mmol) in about 40 ml of CH2Cl2 or CH3CN, the
phenol (3 mmol) was added under an atmosphere of dry,
oxygen-free nitrogen. When the evolution of H2 ceased,
CuCl (0.297 g, 3 mmol) was added. A yellow precipitate
of copper(I) aryloxide was formed which was used imme-
diately for further reactions. [Cu(CH3CN)4]ClO4 could be
used instead of CuCl.
Surprisingly, in a recent study on the oligomeric cop-
per(I) complexes of N-alkylimino(aryloxy)methanethiolate
(RN=C(OAr)S2), the nuclearity of the copper(I) clusters
was shown to be dependent on only the bulk of the aryloxy
group present and apparently independent of the substitu-
tion on the nitrogen atom. This is in spite of the fact that
the OAr is fairly distant from the polynuclear copper core
compared to R! [40–42]. When the aryloxy group was
p-cresol, a hexanuclear complex [Cu–S–C(OAr)=NR]6
(R=Ph or Me) was formed. When the bulk of the ligand
was increased by introducing two methyl groups in the
ortho positions of the phenol, the nuclearity of the
resulting cluster was reduced to a tetramer [Cu–S–C(OAr-
)=NR]4(R=Ph or Me). This prompted us to take a closer
look at these systems. In the three atom bridged complex-
es, are the nuclearity of complexes really independant of
the steric influences of R? Can the steric influences of the
group in position 2 be compensated by reduction of steric
demands of the group in position 1? (Fig. 1). What would
be the effect of releasing steric congestion by an extreme
reduction in the bulk of the ligand? In the present paper we
discuss the results of our studies to probe these factors and
the utility of solid state fluorescence data in assigning the
nuclearity of the clusters examined.
2.1.1. Preparation of [Cuhm-SC(=NPh)(OC6H4 –OMe-
2)j]6 (3)
To copper(I) 2-methoxyphenoxide prepared as above in
CH2Cl2, P(OMe)3 (0.35 ml, 3mmol) was added and the
solution stirred for half an hour. Then, PhNCS (0.36 ml, 3
mmol) was added and the stirring was continued for 8h.
The mixture was filtered and the resulting deep orange
colored solution was concentrated to about 10 ml. Layer-
ing with petroleum ether (bp 60–808C) yielded yellow
crystals of 3. Yield 16%. Alternately, [Cu(CH3CN)4]ClO4
could be used instead of CuCl. A bright yellow precipitate
of the aryloxide was formed. PhNCS was added to this and
the solution stirred for 8h. Similar work up of the reaction
mixture yielded yellow crystals of 3. Anal. Calcd for 3: C,
52.25; H, 3.73; N, 4.35. Found: C, 52.57; H, 3.96; N, 4.00.
1H NMR (CDCl3, 298 K): d 7.41–6.61 (m, 9H, –O–
C6H4 –, C6H5NCS), d 3.63 (S, 2.68H, –OMe), d 3.53 (S,
0.31H, –OMe). IR(neat): in cm21 1589(w)(nC=N), 1565(m),
1553(s), 1537(m), 1498(s), 1486(m), 1452(m), 1435(w),
1414(w), 1257(m), 1208(m), 1174(w), 1127(s), 1111(s),
1041(w), 1021(m), 751(s), 692(m).
2. Experimental section
General: Dichloromethane, pyridine and petroleum ether
(bp 60–808C) were purified and dried by conventional
methods. They were distilled under nitrogen and deoxy-

C. Wycliff et al. / Polyhedron 18 (1999) 949–958
951
2.1.2. Preparation of [Cuhm-SC(=NPh)(–OC6H4 –Me-
2)j]6 (4)
2.1.5. Preparation of [Cu(C4H4N2S)]n (11)
2-Mercaptopyrimidine (0.14 g, 1.1 mmol) was added to
a suspension of NaH (0.037 g, 1.1 mmol) in CH3CN. Then
CuCl (0.107 g, 1.1 mmol) was added to this. The mixture
was stirred for 8h and then filtered. A yellow precipitate
(11) was obtained; this was found to be insoluble in all the
usual organic solvents. The reaction was carried out in
ethanol. Sodium (0.019 g, 0.83 mmol) was added to
absolute ethanol and 2-mercaptopyrimidine (0.093 g, 0.83
mmol) was added to this. [Cu(CH3CN)4]ClO4 (0.272 g,
0.83 mmol) or CuCl (0.82 g, 0.83 mmol) was added to the
resulting mixture. The yellow suspension was filtered
when a yellow insoluble precipitate and a colorless solu-
tion were obtained. The IR spectra of this yellow precipi-
tate was identical to the one obtained in the previous
reaction (11). Yield 72%. Anal Calcd. for 11: C, 27.49; H,
1.72; N, 16.04. Found: C, 26.19; H, 1.65; N, 15.02. IR
(nujol) in cm21: 2952 (s), 1565 (s), 1542 (s), 1245 (w),
1171 (s), 767 (m), 744 (m), 647 (m).
To copper(I) 2-methylphenoxide prepared using
[Cu(CH3CN)4]ClO4 in CH2Cl2, PhNCS (0.36 ml, 3
mmol) was added and the mixture was stirred for 8h. Then
it was filtered and the resulting yellow colored solution
was concentrated to about 10 ml. Layering with petroleum
ether (bp 60–808C) yielded yellow crystals of 4. Yield
1
19%. H NMR (CDCl3, 298 K): d 7.66–6.21 (m, 9H,
–O–C6H4 –, C6H5NCS), d 1.99 (S, 2.46H, –Me), d
1.89(S, 0.54H, –Me). IR(neat): in cm21 3057(w), 3025(w),
1557(s)(nC5N), 1486(s), 1452(m), 1222(s), 1127(s), 756(s),
690(m), 597(m).
2.1.3. Preparation of [Cuhm-S–C(=S)(–OC6H2 –But –2,6-
Me-4)j]4 (7)
The
cuprous
aryloxide
(2,6-di-tert-butyl-4-
methylphenoxide) was made by adding CuCl to the
suspension of sodium aryloxide in CH2Cl2. P(OMe)3 (3
equiv.) was added followed by CS2 (2 ml). The mixture
was allowed to stir for 8h and then filtered. The solvent in
the yellow–orange filtrate was evaporated and the residue
was extracted with ether. A yellow solution was obtained.
TLC of this solution showed the presence of a major
product (yellow) in addition to unreacted 2,6-di-tert-butyl-
4-methylphenol and some minor products. A column
chromatographic separation was carried out and the yellow
product was isolated. Slow evaporation of the solvent
yielded yellow crystals of 7. Yield 12%. Anal Calcd. for 7:
2.2. Experimental details of crystallography
The reaction mixture from the insertion reaction of
copper(I) 2-methoxyphenoxide and PhNCS was concen-
trated to about 10 ml and layered with petroleum ether
(b.p. 60–808C). Slow evaporation of this mixture yielded
yellow crystals of 3 suitable for the diffraction study. The
crystals were found to be extremely unstable outside the
mother liquor and lost their crystallinity very rapidly.
Hence a crystal of 3 was sealed in a Lindemann capillary
along with the mother liquor and used for data collection.
Intensity data were collected on an Enraf–Nonius CAD4
diffractometer using graphite monochromated Mo Ka
radiation. The intensity control reflections, measured once
in every 3600 s of exposure time to the X-rays showed that
the crystal was degrading with time. Data was collected
with difficulty up to 60% decay. The structure was solved
by the heavy atom method and subsequent Fourier synth-
eses using the SHELX system [43,44] of programs. The
hydrogen atoms were geometrically fixed. Non-hydrogen
atoms (except C12) were refined anisotropically while the
hydrogen atoms isotropically. The refinement was carried
out till a final R factor of 0.0964 was reached.
Crystallization of the yellow complex from a mixture of
petroleum ether (bp 60–808C) and CH2Cl2 after the
chromatographic separation of 7 afforded yellow crystals
suitable for an X-ray crystallographic study. A yellow
crystal was mounted on a glass fiber and data collected. No
significant decay was indicated by the intensity control
reflections. The compound crystallized in the space group
2I41 /a of the tetragonal crystal system. The structure was
solved by the direct method and subsequent Fourier
syntheses showed this to be a tetrameric complex. The
heavy atoms were refined anisotropically. While an attempt
at anisotropic refinement of the carbon atoms showed
disorder, they were refined isotropically. The hydrogen
1
C, 53.54; H, 6.42. Found: C, 52.49; H, 6.63. H NMR
(CDCl3, 298 K): d 7.07(s, 2H, –OC6H2), d 2.33(s, 3H,
Me), d 1.32(S, 18H, CMe3). 13C NMR (CH2Cl2, 298K) d
232.74(S–C=S), d 152.33(ipso –OC6H2), d 141.20(ortho
–OC6 H2), d 135.69(meta –OC6 H2), d 127.81( para
–OC6 H2), d 35.80( para CH3), d 31.74(But), d
21.24(CMe3). IR(nujol): in cm21 1145(s, nC=S),1090(m,
nC–O), 1025(s, nCS2[asym])
2.1.4. Preparation of [Cuhm-S–C(=S)(–OC6H4 –Me2-
2,6)j]n (8)
A yellow suspension of copper(I) 2,6-dimethyl phenox-
ide was made by adding [Cu(CH3CN)4]ClO4 to a suspen-
sion of the sodium aryloxide in CH2Cl2. CS2 (2 ml) was
added to this and the mixture was stirred for about 8h.
Filtration of this mixture yielded a clear orange solution
which was concentrated and an orange solid was obtained
from this on slow evaporation of the solvent. Repeated
attempts to crystallize this orange solid using various
solvent mixtures were unsuccessful. Yield 56%. 1H NMR
(CDCl3, 298K) d 7.1(3H, –OC6H3), d 2.17(6H, CH3).
13C NMR (CH2Cl2, 298K) d 229.40(S–C12,137C=S), d
153.88(ipso –OC6H3), d 129.80(ortho –OC6H3), d
128.87(meta –OC6H3), d 126.84( para –OC6H3), d
15.96(ortho CH3).

952
C. Wycliff et al. / Polyhedron 18 (1999) 949–958
Table 1
Crystallographic details for [Cuhm-SC(=NPh)(O–C6H4 –2-OMe)j]6 3 and [Cuhm-SC(=S)(O–C6H2 –2,6-But2 –4-Me)j]4
7
3
7
Chem formula
FW
Color
C84H72N6O12S6Cu6. 4CH2Cl2
C64H92Cu4O4S8
1436.02
Yellow
Tetragonal
I41 /a
28.56(3)
2270.93
yellow
Monoclinic
P21 /C
13.108(5)
28.402(3)
14.363(3)
116.38(2)
4790(2)
2
1.571
17.23
20
0.7107
4474
Crystal system
Space group
˚
a, (A)
˚
b, (A)
˚
c, (A)
34.31(3)
b, deg
3
˚
V, A
27983(40)
16
1.363
Z
rcalcd, g cm23
m, cm21
T, 8C
14.81
2105
0.7107
9748
0.1173
0.1987
˚
l, A (Mo Ka)
Independent reflections
Final Ra
0.0964
0.1950
Final Rbw
a R(Fo)5(S Fo 2 Fc )/(S Fo ).
bRw(Fo)5[Sw( Fo 2 Fc )2 /(Sw Fo 2)]1 / 2; w5[s2(Fo)1gFo2]21
.
atoms were geometrically fixed. The high value of R
(0.1173) could not be reduced even when the structure was
solved using a fresh data set collected at 2778C. The
inherent disorder in the system can be primarily attributed
to the steric constraints (vide infra). The crystallographic
details and important bond distances and angles are listed
in Tables 1 and 2 for the hexamer 3 and in Tables 1 and 3
for the tetramer 7. Particulars of the crystallographic
analyses including positional coordinates for all the atoms,
thermal parameters, bond distances and angles are given in
Tables S1–S5 and Tables S6–S10 (Supplementary materi-
al) for 3 and 7 respectively.
Table 2
˚
List of Selected Bond lengths [A] and angles [8] for 3 [Cuhm-SC(=NPh)(O–C6H4 –2-OMe)j]6
Bond
Distance
Bond
Distance
2.809(4)
Cu(1)–N(3)
Cu(1)–S(40)
Cu(1)–S(2)
Cu(1)–Cu(20A)
Cu(1)–Cu(39A)
S(2)–Cu(20)
Cu(20)–N(22)
Cu(20)–S(21A)
2.00(2)
Cu(20)–Cu(1A)
Cu(20)–Cu(39A)
S(21)–Cu(20A)
S(21)–Cu(39A)
Cu(39)–S(40)
Cu(39)–S(21A)
Cu(39)–Cu(1A)
Cu(39)–Cu(20A)
2.220(7)
2.268(7)
2.810(4)
2.878(5)
2.266(7)
1.95(2)
3.011(4)
2.222(7)
2.257(6)
2.238(7)
2.257(6)
2.878(5)
3.011(4)
2.222(7)
Atom
Angle
Atoms
Angle
N(3)–Cu(1)–S(40)
N(3)–Cu(1)–S(2)
S(40)–Cu(1)–S(2)
128.8(5)
120.0(6)
107.1(3)
87.8(6)
76.3(2)
129.5(2)
84.0(6)
S(2)–Cu(20)–Cu(1A)
N(22)–Cu(20)–Cu(39A)
S(21A)–Cu(20)–Cu(39A)
S(2)–Cu(20)–Cu(39A)
Cu(1A)–Cu(20)–Cu(39A)
N(41)–Cu(39)–S(40)
N(41)–Cu(39)–S(21A)
S(40)–Cu(39)–S(21A)
N(41)–Cu(39)–Cu(1A)
S(40)–Cu(39)–Cu(1A)
S(1A)–Cu(39)–Cu(1A)
N(41)–Cu(39)–Cu(20A)
S(40)–Cu(39)–Cu(20A)
S(21A)–Cu(39)–Cu(20A)
130.4(2)
84.2(6)
133.4(2)
71.7(2)
68.85(11)
120.2(6)
118.7(6)
115.8(2)
84.2(6)
133.2(2)
74.9(2)
85.7(6)
71.8(2)
135.5(2)
N(3)–Cu(1)–Cu(20A)
S(40)–Cu(1)–Cu(20A)
S(2)–Cu(1)–Cu(20A)
N(3)–Cu(1)–Cu(39A)
S(40)–Cu(1)–Cu(39A)
S(2)–Cu(1)–Cu(39A)
Cu(20A)–Cu(1)–Cu(39A)
N(22)–Cu(20)–S(21A)
N(22)–Cu(20)–S(2)
S(21A)–Cu(20)–S(2)
N(22)–Cu(20)–Cu(1A)
S(21A)–Cu(20)–Cu(1A)
129.7(2)
74.4(2)
67.29(12)
125.9(6)
116.7(6)
112.4(3)
88.0(7)
76.9(2)
(Symmetry transformations used to generate equivalent atoms: 2x,2y,2z).

C. Wycliff et al. / Polyhedron 18 (1999) 949–958
953
Table 3
˚
List of Selected Bond lengths [A] and angles [8] for [Cuhm-SC(=S)(O–
C6H2 –2,6-But2 –4- Me)j]4
7
Bond
Distance
2.636(5)
Cu(1)–Cu(2)
Cu(1)–Cu(3)
Cu(1)–Cu(4)
Cu(2)–Cu(4)
Cu(2)–Cu(3)
Cu(3)–Cu(4)
Cu(2)–S(6)
Cu(2)–S(3)
Cu(2)–S(7)
Cu(3)–S(4)
Cu(3)–S(1)
Cu(3)–S(5)
Cu(4)–S(8)
Cu(4)–S(1)
Cu(4)–S(3)
2.644(5)
2.804(6)
2.663(5)
2.810(6)
2.656(6)
2.259(8)
2.272(9)
2.292(9)
2.246(9)
2.267(9)
2.272(9)
2.245(9)
2.249(9)
2.292(9)
Atoms
Angle
Fig. 2. ORTEP Diagram of the Hexameric Complex 3.
S(2)–Cu(1)–S(7)
S(2)–Cu(1)–S(5)
S(7)–Cu(1)–S(5)
O(1)–C(1)–S(2)
O(1)–C(1)–S(1)
S(2)–C(1)–S(1)
C(1)–O(1)–C(11)
Cu(2)–Cu(1)–Cu(3)
124.2(3)
111.6(3)
123.3(3)
120(2)
109(2)
131(2)
steric congestion is reflected in the distortions present in
the molecular structure.
A hexameric cluster 5 was formed from an insertion
reactions with MeNCS and the copper(I) aryloxide of
p-cresol just as in the case of PhNCS. This ligand has
steric variation at another position in the ligand; the
terminal nitrogen atom of the N–C–S bridge has a less
bulky methyl group (compared to PhNCS). All the three
hexameric copper(I) complexes, with ligands having vary-
ing steric requirements, have very similar paddle-wheel
structures. Although there is no change in the nuclearity, a
closer examination of their structural parameters reveals
subtle differences that reflect the changes caused by the
steric requirements of the substituents. The hexamer 5
(from MeNCS) crystallizes in the highly symmetric space
118(2)
64.3(2)
3. Results and discussion
3.1. Reactions with PhNCS
Introduction of bulky groups in the ortho positions of
the aryloxy group has been found to hinder the oligo-
merization process and result in the formation of a tetramer
or a dimer. In order to investigate whether mono ortho
substitution will have the same effect, insertion reactions
of PhNCS were carried out using copper(I) aryloxides of
¯
group 2R3-while the other two (1 and 3 from PhNCS)
¯
crystallize in space groups of much lower symmetry (P1
and P21 /c respectively). The hexameric structure of 5 is
symmetry-generated from one-sixth of the molecule by a
three-fold axis and an inversion center whereas the only
symmetry element present in the other two hexamers is an
inversion center in the molecule. 5 is less distorted than 1
and 3 and the six copper atoms are located at the vertices
of an almost perfect octahedron. In 5, the steric repulsion
between the aryloxide and the substituent on the nitrogen
atom is minimal because of the smaller size of the methyl
group and there are no ortho substituents on the –OAr
group. On the other hand, the larger phenyl substituent on
the nitrogen in 1 results in the formation of a more
distorted structure. Steric congestion is maximum in the
case of 3 which has the phenyl substituent on the nitrogen
atom as well as a bulky ortho substituent on the –OAr.
The extent of distortion found in the three hexamers can be
ranked as 3.1.5. This is evident from Table 4 where the
bond angles at the core atoms in the three hexamers are
compared. Reduced steric constraints result in a more
mono-ortho
substituted
phenols-o-methoxyphenol
(guaiacol) and o-cresol. An X-ray crystallographic analysis
of the product formed from guaiacol revealed it to be a
hexameric cluster 3. The molecular structure of the com-
plex has six copper atoms in roughly octahedral geometry
and
the
six
N-phenylimino(2-methoxyphenoxy)-
methanethiolate ligands bridge these copper atoms re-
sulting in a paddle-wheel structure very similar to the
hexamer formed in the reaction with p-cresol. The molecu-
lar structure of this hexameric complex 3 is given in Fig. 2.
A yellow crystalline product 4 was formed in the case of
o-cresol; the yellow color suggests that 4 is also a
hexameric complex (vide infra). The formation of hexa-
meric clusters clearly show that the partial relief in steric
congestion achieved by using a mono-ortho sublstituted
phenol instead of a di-ortho substituted phenol can result
in a cluster with a higher nuclearity. Even though mono
substitution has resulted in the formation of a hexamer, the

954
C. Wycliff et al. / Polyhedron 18 (1999) 949–958
Table 4
List of bond angles(8) for the atoms in the core of the three hexamers
Hexamer 5
Hexamer 1
Hexamer 3
(MeNCS–p-cresol)
(PhNCS–p-cresol)
(PhNCS–guaiacol)
Cu–S–Cu
S–Cu–S
N–Cu–S
Cu–S–C
S–C–N
89.6
115.5
119.8; 117.9
103.5; 103.2
123.6
86.2–92.6
113.5–116.8
114.8–124.5
100.5–107.4
121.1–123.1
117.7–120.9
117.9–119.3
89.1–98.4
106.9–115.6
116.9–127.7
98.6–106.4
121.8–126.0
113.6–121.4
115.4–120.4
N–C–O
S–C–O
117.9
115.5
regular structure and distortions are minimized due to
steric relief. A reduction in the steric strain is reflected by
greater symmetry in the molecular and crystal structure.
constraints an insertion reaction with copper(I) 2,6-di-tert-
butyl-4-methyl phenoxide was carried out on CS2. A
yellow crystalline product 7 was formed and was identified
as a tetramer from an X-ray crystallographic analysis. The
tetramer has a tetrahedral arrangement of the four copper
atoms which are bridged by S–C–S and sulfide bridges.
The structure is similar to that of the tetramer 2 isolated in
the reaction between PhNCS and copper(I) 2,6-dimethyl
phenoxide. The ORTEP diagram of 7 is shown in Fig. 3.
The reaction involving copper(I) 2,6-di-tert-butyl-4-
methylphenoxide and CS2 results in a tetrameric cluster 7
(the corresponding reaction with PhNCS gives only a
dimeric complex). The reduction in the size of the
heterocumulene ligand has compensated for the steric
congestion due to the phenol and consequently oligo-
merization is made feasible.
Subsequently, reactions were carried out with CS2 and
copper(I) 2,6-dimethyl phenoxide which has less bulky
methyl groups. An orange product 8 was obtained which
could not be crystallized and hence its nuclearity could not
be determined from an X-ray study. The orange color of
the complex suggests that it is a hexamer (vide infra). The
role played by steric effects is distinctly brought out by the
insertion reaction of CS2 with copper(I) 4-methylphenox-
ide where there is no ortho substitution in the phenol. In
this case, there is a further reduction in the steric require-
1
The H NMR spectra of these complexes suggest that
the steric effects impact the solution properties of the
complex too. Studies carried out earlier [45] had shown
that the dissociation of the hexamer into a tetramer and a
dimer (Scheme 1) would offer relief from the steric
constraints. Such dissociation into two different species is
1
indicated by the room temperature H NMR spectrum of 3,
4 and 1. Hexamer 3 shows two resonances for the –OMe
group (3.63 ppm; 2.68 H and 3.53 ppm; 0.32H) and the
two peaks have non integral ratios. Hexamer 4 with the
mono ortho methyl substituent also has two signals for the
methyl group with non integral ratios(1.99 ppm, 2.46H and
1.89 ppm, 0.54H). Similarly, there are two peaks corre-
sponding to the p-methyl substituent of the aryloxide in the
1H NMR spectrum of 1 suggesting the presence of two
species in solution. The ratio of the intensities of the two
closely spaced singlets was found to be temperature
dependent. At higher temperatures, the intensities of the
two signals were equal. At lower temperatures, only one
peak was observed indicating the presence of only one
species. These changes could be attributed to a dissociation
of the hexamer to give a tetramer and a dimer. The
presence of very little steric congestion in the hexamer 5
(from MeNCS) is confirmed by its behavior in solution,
1
the H NMR spectrum of 5 does not indicate any dissocia-
tion in contrast to 1, 3 and 4.
3.2. Reactions with CS2
Further decrease in the steric requirements of the ligand
was brought about by employing a ligand with no sub-
stituents on the terminal atom. To probe the limits of steric
Scheme 1.
Fig. 3. ORTEP Diagram of the Tetrameric Complex 7.

C. Wycliff et al. / Polyhedron 18 (1999) 949–958
955
ment of the ligand and as a result, multiple insertions take
place. The products that are finally isolated are either the
orthocarbonate [C(OAr)4] or the thioncarbonate
[S=C(OAr)2] and cuprous sulfide [46–49]. The absence of
steric impediments in both p-cresol and CS2 lead to
multiple insertions since there is no hindrance to the
insertion reaction. Attempts were made to prevent multiple
insertion by synthesizing the ligand (a thioxanthate) and
then reacting it with copper(I) instead of an in situ
insertion reaction of the heterocumulene with copper(I)
aryloxide. However only decomposition products (phenol
and cuprous sulfide) could be isolated.
An alternative mode of reducing steric requirements of
the ligand is to covalently link the substituents as in the
case of heterocyclic thione systems. These have an N–C–S
fragment similar to the systems mentioned earlier and the
ligand would have minimum or negligible steric effects.
Reactions were carried out with copper(I) and pyrimidine-
2-thiolate. The resulting product 11 is highly insoluble in
all common organic solvents which suggests a polymeric
structure. This polymeric structure could be broken down
with pyridine. However, the highly air sensitive and non
crystalline nature of the products formed precluded further
characterization. Therefore it can be concluded that when
there is a great release in the steric congestion, oligo-
merization is made so facile that polymerization results. A
possible sheet-like structure for this polymeric copper(I)
complex 11 is shown in Fig. 4. These results clearly
demonstrate the role played by steric factors in controlling
the oligomerization process in copper(I) clusters.
confirming the nuclearities of these oligomers. Even in
cases where single crystals suitable for diffraction work are
not available, it is possible to assign a nuclearity to the
complex. In the solution phase all complexes had a broad
absorption from 250 nm tapering down to 350 nm indicat-
ing dissociation of the oligomeric structures. However, the
spectra recorded in the solid state were more informative
and are discussed below.
An examination of the emission characteristics of these
clusters (Table 5) shows that 1, 3, 4, 5, 8, 9 and 10 are
very similar. Also, these cluster compounds are all either
orange–yellow, orange or red in color. The structures of 1,
3, and 5 are confirmed to be hexameric in the solid state
from crystallographic studies. From the similarity in the
absorption and emission patterns of these molecules (ab-
sorbances from 270–500 nm and a fluorescence emission
band at the long wavelength region from 600–700 nm) and
their color, it can be concluded that 4, 8, 9 and 10 are also
hexamers. The absorption and emission characteristics of
the hexamers are markedly different from 2, 6 and 7. The
latter are colorless/yellow compounds, have absorbances
between 270–350 nm and do not have any emission band
beyond 600nm. The structures of 2 and 7 are known to be
tetrameric from single crystal structure determinations. On
the basis of these similarities it might be concluded that 6
is also a tetramer.
The color in copper(I) compounds with sulfur donor
ligands has been assigned to a ligand-to-metal charge
transfer transition [50]. The difference in the emission
pattern and the color of the hexameric and tetrameric
complexes can be attributed to the difference in their
structures. The Cu–S–Cu angles in the hexamers are
around 908 while those in the tetrameric structure are acute
indicating closed three centre bonding. As a result the
interaction between the ligand and the metal is better in the
case of the hexamers which is reflected in shorter Cu–S
distances. Table 6 compares the Cu–S–Cu angles and the
Cu–S distances in the structurally characterised hexamers
and tetramers. This structural difference could be respon-
sible for the very different emission properties of the
hexamers and tetramers.
3.3. Fluorescence spectra
The absorption and fluorescence spectra of these oligo-
meric complexes were found to be useful in finding out or
3.4. Modeling studies
Modeling studies [51] were also helpful in understand-
ing the role of steric effects in controlling the nuclearity of
oligomeric copper(I) complexes. These studies reveal the
fact that although monosubstitution increases the nuclearity
of the cluster, the resulting structure is considerably
strained. In the hexamer 3 that is formed in the reaction
with guaiacol, free rotation about the O–C(aryloxide) bond
is not possible and severe intramolecular bumping is
encountered, the hydrogen atoms of the –OMe group and
the phenyl ring of the N-phenylimino(aryloxy)-
methanethiolate belonging to two neighboring ligands
Fig. 4. A Possible Sheet-Like Structure for the Polymeric Complex 11.

956
C. Wycliff et al. / Polyhedron 18 (1999) 949–958
Table 5
Fluorescence Spectral Data
No.
Structural
Data
X=C=Y–ArOH
Color
Fluorescence spectra
lex nm
lem nm
1
Hexamer
PhNCS–p-cresol
Orange
274
383
421
284
365
400
414
374
275
310
497
275
312
348
363
375
322
441
494
311
387
455
486
310
491
418, 471, 494, 629
639, 777
629
384
617
603, 626
621
438, 631
424, 474, 496
470, 491, 533
747
351, 388, 470, 493
471, 534
413, 469, 520, 571
413, 469, 513, 570
416, 468, 508, 567
416, 436, 468, 528, 552
656
2
3
Tetramer
Hexamer
PhNCS–DMPa
PhNCS–guaiacol
White
Orange–
Yellow
4
5
Hexamerc
Hexamer
PhNCS–o-cresol
MeNCS–p-cresol
Yellow
red–orange
6
7
–
MeNCS–DMPa
CS2 –BHTb
White
Yellow
Tetramer
8
9
–
–
CS2 –DMPa
red–orange
red–orange
747
401, 470
642
682
PhNCS–p-
chlorophenol
731
10
–
MeNCS–p-
chlorophenol
red–orange
404, 468, 532, 549
739
a DMP52,6-dimethylphenol.
b BHT52,6-di-tert-butyl-4-methylphenol.
c Unpublished.
˚
come as close as 0.55 A. These constraints are responsible
for the distortions observed in the molecular structure.
The nuclearity of complex 6 that was formed in the
insertion reaction between copper(I) 2,6-dimethylphenox-
ide and MeNCS could not be ascertained from an X-ray
crystallographic study. Modeling studies were carried out
on this system by introducing two ortho-methyl sub-
stituents in the –OAr groups of the hexamer 5. Contrary to
the results obtained in the PhNCS systems, the intro-
duction of methyl groups appeared to be feasible. While
the possibility of a hexamer being formed is suggested by
the modeling studies, fluorescence studies indicate that
complex 6 is indeed a tetramer. Similarly, the orange
product formed from copper(I) 2,6-dimethyl phenoxide
and CS2 could not be crystallographically characterized.
Modeling studies carried out on this system by construct-
ing a hypothetical hexameric complex showed that the
formation of a hexameric complex would not be restricted
by steric factors. In this case, the results from fluorescence
spectra confirm that a hexamer is indeed formed in this
reaction.
The nuclearity of the cluster formed with a three atom
bridge in a tricoordinate geometry around the metal is
limited by topological factors. The oligomeric cluster
complexes can be viewed as a combination of tub shaped
dimeric (CuNCS)2 units. These tubs are brought together
in an orientation where the trigonal coordination require-
ment of copper(I) is met. The three (CuNCS)2 tubs are in
a parallel orientation in the hexamer while the two dimeric
tubs are oriented orthogonal to each other in the tetramer.
An attempt to form an octanuclear cluster by bringing
together four tubs increases the Cu–S–Cu angle signifi-
cantly. Large deviations of the S–Cu–S angle would be
energetically unfavorable. On the other hand, the formation
of a non planar sheet structure does not require deviations
from trigonal coordination. Hence, in the absence of steric
Table 6
Comparison of the Cu–S–Cu angles and Cu–S distances
˚
Cluster
Cu–S–Cu (8)
Cu–S (A)
Tetramer 2
Tetramer 7
Hexamer 1
Hexamer 3
Hexamer 5
70.1–73.0
70.8–72.1
86.2–92.6
89.1–98.4
89.6
2.25–2.32
2.24–2.30
2.23–2.28
2.21–2.27
2.24–2.24

C. Wycliff et al. / Polyhedron 18 (1999) 949–958
957
[5] S.P. Neo, Z.-Y. Zhou, T.C.W. Mak, T.S.A. Hor, J. Chem. Soc.,
Dalton Trans. (1994) 3451.
constraints, a hexamer or a sheet structure is preferred to
an octanuclear complex.
[6] M.D. Janssen, J.G. Donkervoort, S.B. van Berlekom, A.L. Spek,
D.M. Grove, G. van Koten, Inorg. Chem. 35 (1996) 4752.
[7] Ch.P. Rao, J.R. Dorfman, R.H. Holm, Inorg. Chem. 25 (1986) 428.
[8] D.M. Ho, R. Bau, Inorg. Chem. 22 (1983) 4079.
[9] N. Bresciani, N. Marsich, G. Nardin, L. Randaccio, Inorg. Chim.
Acta 10 (1974) L5.
4. Conclusions
[10] G. Nardin, L. Randaccio, E.J. Zangrando, J. Chem. Soc., Dalton
Trans. (1975) 2566.
[11] J. Diez, M.P. Gamasa, J. Gimeno, L. Elena, A. Aguirre, S.G.
Granda, Organometallics 12 (1993) 2213.
[12] V.W.-W. Yam, W.K. Lee, K.-K. Cheung, B. Crystall, D.J. Phillips, J.
Chem. Soc., Dalton Trans. (1996) 3283.
[13] C.-K. Chan, C.-X. Guo, R.-J. Wang, T.C.W. Mak, C.-M. Che, J.
Chem. Soc., Dalton Trans. (1995) 753.
[14] A.F. Stange, K.W. Klinkhammer, W. Kaim, Inorg. Chem. 35 (1996)
4087.
[15] T. Kaluo, G. Hong, X. Xiaojie, Z. Gongdu, T. Youqi, Scientia.
Sinica 27 (1984) 456.
[16] S.L. Lawton, W.J. Rohrbaugh, G.T. Kokotailo, Inorg. Chem. 11
(1972) 612.
[17] T.K. Sowa, M. Munakata, M. Miyazaki, M. Kaekawa, Polyhedron
14 (1995) 1003.
[18] A.L. Crumbliss, L.J. Gestaut, R.C. Rickard, A.T. McPhail, J. Chem.
Soc., Chem. Commun. (1974) 545.
[19] D.M.L. Goodgame, G.A. Leach, A.C. Skapski, K.A. Woode, Inorg.
Chim. Acta 31 (1978) L375.
[20] S. Kitagawa, Y. Nozaka, M. Munakata, S. Kawata, Inorg. Chim.
Acta 197 (1992) 169.
[21] E.S. Raper, J.R. Creighton, W. Clegg, Inorg. Chim. Acta 183 (1991)
179.
[22] T. Greiser, E. Weiss, Chem. Ber. 109 (1976) 3142.
[23] M. Pasquali, P. Fiaschi, C. Floriani, P.F.J. Zanazzi, J. Chem. Soc.,
Chem. Commun. (1983) 613.
[24] G.A. Ardizzoia, C. Cenini, G. LaMonica, N. Masciocchi, M. Moret,
Inorg. Chem. 33 (1994) 1458.
From this study with ligands of varying steric demands,
it can be concluded that the steric requirements of the
ligand play a major role in determining the structure and
hence the nuclearity of copper(I) complexes. It has been
demonstrated that mono-ortho substitution instead of dis-
ubstitution results in the formation of a hexamer. The
replacement of a phenyl group by a methyl group on the
nitrogen atom of the –NCS– bridge does not bring about a
change in nuclearity although it does affect the structure.
The decreased congestion is evidenced by more symmetri-
cal structural parameters. When the size is reduced further
by exchanging the =NR group by a =S, the resulting
–SCS– bridge leads to complexes of higher nuclearity.
When the steric demands of the ligand are drastically
reduced, by tying up the two ends of the ligand through
covalent bonding, a polymeric sheet structure results. The
thermodynamic stability of the oligomers seems to follow
the order: hexamerltetramerldimer since release in steric
strain results in a hexamer in all cases. Fluorescence
studies could be used for determining nuclearity of some
of these complexes that could not be studied using X-ray
crystallography. Modeling studies are valuable in gauging
the source of steric congestion in these structures, although
they are not conclusive in predicting the nuclearity accu-
rately. The structures of these copper(I) clusters are
predominantly controlled by steric factors and fine tuning
of steric parameters can lead to complexes of predictable
nuclearity.
´
´
´
[25] R. Castro, M.L. Duran, J.A. Garcıa-Vazquez, J. Romero, A. Sousa,
E.E. Castellano, J. Zukerman-Schpector, J. Chem. Soc., Dalton
Trans. (1992) 2559.
[26] S. Kitagawa, M. Kawata, Y. Nozaka, M. Munakata, J. Chem. Soc.,
Dalton Trans. (1993) 1399.
[27] S. Kitagawa, Y. Nozaka, M. Munakata, M. Kawata, Inorg. Chim.
Acta 197 (1992) 169.
[28] E. Block, M. Gernon, H. Kang Zubieta, J. Angew. Chem., Int. Ed.
Engl. 27 (1988) 1342.
[29] S. Kitagawa, M. Munakata, H. Shimono, S. Matsuyama, H. Masuda,
J. Chem. Soc., Dalton Trans. (1990) 2105.
[30] V.W.-W. Yam, W.K.-M., Fung, K.-K. Cheung, Chem. Commun.
(1997) 963.
[31] B.J. Hathaway, in: G. Wilkinson, R.G. Gillard, J.A. McCleverty
(Eds.), Comprehensive Coordination Chemistry, vol. 5, Pergamon
Press, Oxford, 1987.
Acknowledgements
The financial support of CSIR, India through a grant to
AGS and to CW through a Senior Research Fellowship is
gratefully acknowledged. Low temperature data collection
by Prof. Ward Robinson is gratefully acknowledged.
[32] R. Hesse, Akriv. Kemi. 20 (1962) 481.
[33] R. Hesse, U. Aava, Acta Chem. Scand. 24 (1970) 1355.
[34] M.B. Hursthouse, O.F.Z. Khan, M. Mazid, Motevalli, P.M. O’Brien,
Polyhedron 9 (1990) 541.
References
`
[35] R. Castro, M.L. Duran, J.A. Garcia-Vazquez, J. Romero, A. Sousa,
[1] E. Block, M. Gernon, H. Kang, S. Liu, J.J. Zubieta, Chem. Soc.,
Chem. Commun. (1988) 1031.
E.E. Castellano, J. Zukerman-Schpector, J. Chem. Soc., Dalton
Trans. (1992) 2559.
`
[2] P. Fiaschi, C. Floriani, M. Pasquali, A. Chiesi-Villa, C. Guastini,
Inorg. Chem. 25 (1986) 462.
[3] L.P. Battaglia, A.B. Corradi, M. Nardelli, M.E.V.J. Tani, Chem. Soc.,
Dalton Trans. (1976) 143.
[36] R. Castro, J.A. Garcia-Vazquez, J. Romero, A. Sousa, A. Cas-
tinerias, W. Hiller Strahle, Inorg. Chim. Acta 211 (1993) 47.
[37] A.J. Blake, C.M. Grant, S. Parsons, R.E.P. Winpenny, J. Chem. Soc.,
Dalton Trans. (1995) 1765.
[4] R.J. Haines, R.E. Wittrig, C.P. Kubiak, Inorg. Chem. 33 (1994)
4723.
[38] E. Block, M. Gernon, H. Kang, G. Ofori-Okai Zubieta, J. Inorg.
Chem. 28 (1989) 1263.

958
C. Wycliff et al. / Polyhedron 18 (1999) 949–958
[39] E. Block, M. Gernon, H. Kang, S. Liu, J. Zubieta, J. Chem. Soc.,
Chem. Commun. (1988) 1031.
[46] N. Narasimhamurthy, Ph.D. Thesis, Indian Institute of Science,
Bangalore, 1989.
[40] C. Wycliff, A.G. Samuelson, M. Nethaji, Inorg. Chem. 35 (1996)
5427.
[47] N. Narasimhamurthy, A.G. Samuelson, Tetrahedron Lett. 27 (1986)
991.
[41] S.P. Abraham, N. Narasimhamurthy, M. Nethaji, A.G. Samuelson,
Inorg. Chem. 32 (1993) 1739.
[48] N. Narasimhamurthy, A.G. Samuelson, Proc. Indian Acad. Sci. 55A
(1989) 383.
[42] N. Narasimhamurthy, A.G. Samuelson, H. Manohar, J. Chem. Soc.,
Chem. Commun. (1989) 1803.
[49] N. Narasimhamurthy, A.G. Samuelson, Tetrahedron Lett. 29 (1988)
827.
[43] G.M. Sheldrick, SHELXS-86 Program for Crystal Structure De-
[50] V.W.-W. Yam, W.-K. Lee, T.F. Lai, J. Chem. Soc., Chem. Commun.
(1993) 1571.
¨
termination, University of Gottingen, Germany, 1986.
[44] G.M. Sheldrick, SHELX-93 Program for Crystal Structure Refine-
[51] Modelling results obtained using software programs from Biosym
Technologies of San Diego and graphical displays using InsightII.
¨
ment, University of Gottingen, Germany, 1993.
[45] S.P. Abraham, N. Narasimhamurthy, M. Nethaji, A.G. Samuelson,
Proc. Indian Acad Sci., Chem. Sci. 107 (1995) 255.