Steric demands in the formation of heteroleptic Cu(I) complexes with α-diimines and triphenylphosphine

Article abstract of DOI:10.1016/j.poly.2018.05.042

The synthesis and the study of the molecular structure of a series of heteroleptic copper(I) compounds with triphenylphosphine and α-diimines presenting a varying degree of bulk at their chelating cite is reported. The most sterically crowded diimine acts as a monodentate ligand towards copper(I) promoting bridging of the halogen atom present in the metal coordination sphere and formation of a dimer. Evidence for this behavior is obtained from the electronic spectra and semiempirical quantum chemical calculations with MOPAC. Steric demands in all the studied compounds preclude coordination of silver(I) under similar reaction conditions.

Full text of DOI:10.1016/j.poly.2018.05.042

Accepted Manuscript
Steric demands in the formation of heteroleptic Cu(I) complexes with α-dii-
mines and triphenylphosphine
Eleni Tseriotou, Dimitris Tzimopoulos, Antonis Hatzidimitriou, Pericles
Akrivos
PII:
S0277-5387(18)30280-8
https://doi.org/10.1016/j.poly.2018.05.042
POLY 13189
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
12 March 2018
18 May 2018
Please cite this article as: E. Tseriotou, D. Tzimopoulos, A. Hatzidimitriou, P. Akrivos, Steric demands in the
formation of heteroleptic Cu(I) complexes with α-diimines and triphenylphosphine, Polyhedron (2018), doi: https://
doi.org/10.1016/j.poly.2018.05.042
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Steric demands in the formation of heteroleptic Cu(I) complexes with α-diimines and
triphenylphosphine
a
a*
a
a
Eleni Tseriotou , Dimitris Tzimopoulos , Antonis Hatzidimitriou , Pericles Akrivos
a Aristotle University of Thessaloniki, Department of Chemistry, Laboratory of Inorganic
Chemistry, P.O.B. 135, GR-54124 Thessaloniki, Greece
Abstract
The synthesis and the study of the molecular structure of a series of heteroleptic copper(I)
compounds with triphenylphosphine and α-diimines presenting a varying degree of bulk at their
chelating cite is reported. The most sterically crowded diimine acts as a monodentate ligand
towards copper(I) promoting bridging of the halogen atom present in the metal coordination sphere
and formation of a dimer. Evidence for this behavior is obtained from the electronic spectra and
semiempirical quantum chemical calculations with MOPAC. Steric demands in all the studied
compounds preclude coordination of silver(I) under similar reaction conditions.
Keywords diazabutenes, copper(I) complexes, triphenylphosphine complexes, crystal structure,
MOPAC.
1. Introduction
The interest in Cu(I) compounds is continuous due to their versatile geometric arrangements, which
give rise to interesting properties, utilized in their applications in photovoltaic cells, supramolecular
constructions and catalytic activity in photoredox systems [1-4]. In most of the above studies, tetra-
+
+
2 2
coordinated copper complexes with the stoicheiometries [Cu(L) ] or [Cu(L)Ρ ] are involved, with
1
0
L representing a chelating diimine ligand and Ρ a phosphine. As a result of the d electron
configuration of the central metal, the actual geometry of the chromophore is the outcome mainly of
the steric repulsions between the diimine and phosphine backbones [5]. It should be noted that some
Cu(I) complexes reveal luminescence, a property tunable due to the versatility of the environment
around the metal, including the polarizability and bridge formation ability of halides when present
in the coordination sphere [6-7]. The chemistry of several diimine coordination compounds has
been extensively studied and reviewed especially as far as 2,2;-bipyridine (bpy), 9-10-
phenanthroline (phen) and their symmetrically substituted derivatives are concerned [5] while
asymmetric analogues like iminopyridines have also been investigated [8].
Both homoleptic and heteroleptic Cu(I) complexes involving diimines and/or phosphines have been
studied extensively in this respect especially in relation to their photophysical, photochemical and
redox behaviour, all due to their low lying MLCT excited states [9] and the possibility of their
control through the manipulation of the steric and electronic properties of the coordinated ligands.
In the current study a group of iminopyridine and diazabutene ligands presented in figure 1 was
synthesized and their coordination to copper(I) was investigated by means of spectroscopic
methods.
Figure 1 near here
The synthesis of the copper complexes is rather straightforward, generally follows the scheme
described in scheme 1 and the isolated products are generally orange to red.
2. Experimental
2.1 Materials and measurements
UV-Vis measurements were performed in 1 cm quartz cells on a Hitachi U-2010 spectrometer using
-
1
UV-Solutions program. IR spectra were recorded in the region 4000 – 200 cm on a Perkin-Elmer
Spectrum One spectrometer using KBr pellets. Computational studies were done with the PM6 [10]
Hamiltonian as implemented in the MOPAC2016 program [11].

Single crystals of the compounds were mounted on a Bruker Kappa APEX2 diffractometer
equipped with a Triumph monochromator using Μο Κα radiation at ambient temperature. Unit cell
dimensions were measured using at least 200 high-angle reflections. The data collected were treated
with Bruker SAINT [12] package and were corrected for absorption using SADABS based on the
crystal dimensions [12]. The structures were solved using SUPERFLIP [13], as implemented in
2
Crystals package [14]. Optimization of the data was performed by full matrix least squares on F
using Crystals version 14.43 [14]. All non hydrogen atoms were found and refined anisotropically
except those of the interstitial solvent molecules. All hydrogen atoms have been found at their
expected positions. Crystal data details are summarized in table S1. Structure representations were
drawn by the Cameron program [15] adopting 50% probability ellipsoids. Further details on the
crystallographic studies as well as atomic displacement parameters are given as Supporting
Information in the form of cif files.
2.2 Synthesis of the ligands
The only difference between the synthesis of the iminopyridine L
1
relative to the diazabutenes L
2
-L
4
is the molar ratio of the reactants, which for the former is 1:1 while for the latter 1:2. The reactions
were carried out in methanol without exclusion of oxygen or water. In a typical reaction, a solution
of the desired aldehyde (10 mmol in 50 mL solvent) was dropwise added to the analogous amount
of amine dissolved in the minimum amount of methanol. Although the formation of a colored
product was evident from the beginning the reaction was carried out under reflux for about 2 h.
After cooling to room temperature, filtration and subsequent washing with small amounts of
methanol and finally with diethylether resulted in the isolation of the ligands in pure form and in
yields around 90%. The compounds have orange color and present substantial solubility in
methanol, dichloromethane and acetonitrile. Their spectroscopic data were confirmed relative to
already published results [16-19].
2.3 Synthesis of the copper compounds
To a solution of the desired copper(I) halide (1 mmol in 20 mL acetonitrile) an equimolar amount of
triphenylphosphine is added under stirring. A white precipitate known to be of the formula
[
3 4 2 2
CuX(PPh )] is immediately formed. The desired diimine is then added (1 mmol in 10 mL CH Cl )
resulting in an immediate change of the color to deep red. The mixture is stirred at room
temperature for about 1 h and then reduced to half its original volume at the rotary evaporator.
Upon cooling dark colored crystals are deposited, which are filtered, washed with small amounts of
diethylether and dried in the atmosphere. In the case of L , after the initial change of the color a
3
turpidity of the solution is evident and a light colored solid is finally deposited, which is every case
was identified as the diimine that was added.
2
.4 Attempted synthesis of silver compounds
Reaction conditions analogous to the ones applied for the copper compounds were also applied
using AgNO instead. Regardless of the application or not of refluxing conditions there was never
3
indication of the formation of an analogous diimine complex. Upon concentration of the reaction
solutions orange residues were isolated which corresponded to the initial ligand added.
Scheme 1 near here
3. Results and Discussion
3.1 Vibrational spectroscopy
The IR spectra of the diimines synthesized (compounds L
sharp imine C=N bond stretching in the region 1600-1650 cm while the characteristic carbonyl
stretching of the initial aldehyde is absent. The imine bands appear at 1626, 1616 and 1608 cm for
1 4
-L ) are dominated by the intense and
-
1
-
1
-1
L
2
, L
3
και L
4
respectively. For L the corresponding band is observed at 1633 cm while the
1

-1
characteristic carbonyl band of pyridinecarboxaldehyde at 1730 cm is absent. Furthermore, the
-1
isopropyl group CH bands in compounds L
1
and L in region 2970-2860 cm appear split. The
2
observation of coordination shifts is of great value in the study of coordination compounds and in
the present study the coordination shift of the imine C=N bond serves as verification for the
coordination of the diimine to the metal centre. The relative value of the shift may also serve as a
measure of the strength of the metal-ligand bond formed. The shifts observed are all towards lower
-1
1 4 2
wavenumbers by an average of 45, 20 and 10 cm for compounds of ligands L , L and L
respectively, in accordance with their expected hindrance towards the diimine binding site [20-22].
Triphenylphosphine, which is expected to coordinate to the metal centres presents also
characteristic mid-to-low intensity bands, mainly attributable to the P-C bond stretching in the
-1
-1
region 515-540 cm [23-24]. In our case only L
Observation of a split band in the region 518-525 cm is indicative of the coordination of the
phosphine ligand. No such band was observed for compound 2c (CuΙ(L )(PPh )) and in view of the
4
possesses a band in this region at 528 cm .
-1
2
3
similarity of its colour to the rest of the compounds, crystal structure determination seemed to be
desirable for the full characterization of the compound.
3.2 Electronic spectra
Electronic absorption spectra were recorded in dichloromethane and acetonitrile. Besides offering a
substantially larger dielectric constant, the latter also provides a suitable medium for the dissolution
of cuprous halides, forming [Cu(NCMe)
The ligands present a band typical of charge transfer character in the region 300-450 nm with a
broad maximum around 350 nm. For L , the minor steric hindrance of the phenyl ring ortho-
4
]X, something that was utilized in the synthetic procedure.
4
position makes it possible for the ligand to adopt an almost planar geometry, something indicated
by the π-π* character [9, 25] of the low energy absorption bands observed (Figure 2a).
Concerning the spectra of the coordination compounds, there is an obvious extension of the
absorption maxima in the case of 1a-1c an 4a-4c which indicate a charge transfer character for
them, in accordance with literature [25, 26]. This effect has been observed in numerous cases upon
coordination of phosphines to Cu(I), indicating that the charge transfer process involves a
phosphine-centered orbital. On the contrary, compounds 2a-2c reveal spectra quite similar to the
ligand with minor extension towards the low energy edge of the envelope (Figure 2b). This, in line
with the failure to observe characteristic phosphine IR bands, indicates that the stoicheiometry of
the compounds isolated is not identical to that realized for the other ligands.
Figure 2near here
Diminishing of the charge transfer bands and shift of their maxima to higher energies was realized
when their spectra were recorded in the more polar and better electron donating acetonitrile.
3.3 Crystallographic study
Structure of compound 1b
The compound appears solvated with both chloroform and acetonitrile participating in the crystal
formation. The asymmetric iminopyridine provokes the existence of stereoisomers which occur as
1
0
discrete units in the unit cell (Figure 3). The geometry of the metal centre is typical of the d
monovalent copper, i.e. distorted tetrahedral. The angles within the chromophore reveal large
variations, the smaller one realized between the iminopyridine based nitrogen atoms (79.43° and
79.61°) while the largest (122.81° and 120.14°) includes the more sterically hindered heterocyclic
nitrogen and phosphorous in both isomers.
The bond distances in the chromophore are Cu-Br= 2.397 (2.399) Å, Cu-P= 2.196 (2.206) Å and
Cu-N 2.113 (2.072) Å and 2.144 (2.173) Å towards the imino and pyridine nitrogen atom
respectively. Concerning the iminopyridine chelating ligand, the two rings are not coplanar, due

among other facts to the bulky isopropyl substituents. The substituted ring forms a 76.4° (73.5°)
angle relative to the pyridine one.
Figure 3 near here
Structure of compound 1c
The compound appears almost identical to 1b from which it differs in the halogen atom
participating. The compound appears solvated with chloroform and acetonitrile molecules. Due to
the asymmetry of the coordinated iminopyridine two stereoisomers are realized both of which are
present as discrete units in the unit cell (Figure 4).
Figure 4 near here
The structural features resemble the ones presented by compound 1b. The bond distances in the
chromophore are Cu-I= 2.570 (2.560) Å, Cu-P= 2.214 (2.207) Å and Cu-N 2.097 (2.117) Å and
2.126 (2.080) Å towards the imino and pyridine nitrogen atom respectively. The overall
coordination of the diimine appears to be more symmetrical owing in part to the smaller effect from
the halogen atom. The smallest angle within the chromophore is realized between the iminopyridine
based nitrogen atoms (78.58° and 79.00°) while the largest (117.13° and 116.51°) between the
imino nitrogen and phosphorous in both isomers. Concerning the iminopyridine chelating ligand,
the two rings are not coplanar, due among other facts to the bulky isopropyl substituents. The
substituted ring forms a 74.2° (68.4°) angle relative to the pyridine one.
Structure of compound 4c
The compound bears a complete analogy to the previous ones except that the symmetric
diazabutene does not allow for stereoisomer formation. The chromophore is distorted tetrahedral
consisting of one iodine, one phosphorus and two nitrogen atoms (Figure 5). The largest angle is
observed between phosphorus and the nitrogen atoms (126.95° and 122.43°) and consequently the
smallest one (78.71°) is related to the diimine bite angle. The bond distances in the chromophore
are Cu-Ι= 2.587 Å, Cu-P= 2.198 Å, Cu-N 2.099 Å and 2.116 Å respectively. In relation to
compound 1c it appears that the Cu-I bond is slightly weakened with a parallel slight strengthening
of the Cu-P one. The Cu-N bonds are not identical; however their variation is smaller than the ones
observed in the iminopyridine complexes. A notable feature is the almost coplanarity of the two
o
aromatic rings of the diimine with an angle of 4.02 formed between them.
Figure 5 near here
2 2
Structure of compound 2c’, [CuI(L )(NCMe)]
The spectroscopic evidence gathered so far made it mandatory to study a complex of ligand L
2
and
the supposed compound 2c proved to be readily crystallized. The absence of phosphine is apparent,
however its place in the metal coordination sphere is taken up by a solvent molecule (Cu-N distance
2.037 Å), something not unfamiliar in the case of acetonitrile solutions of copper(I) compounds.
Figure 6 near here
An interesting feature is the monodentate character of the diimine ligand (2.098 Å), certainly a
result of the extreme hindrance within the coordination sphere (Figure 6). The ligand adopts a
transoid conformation, much like the one it is expected to do in its free state and the two imine
bonds are only slightly different, being measured as 1.259 Å for the free and 1.266 Å for the
coordinated nitrogen atom. The metal coordination is completed by the formation of a dimer based
on a slightly asymmetrical double iodine bridge (Cu-I distances 2.621 and 2.654 Å respectively)
which forms a Cu
2
I
2
planar central core with a CuI
2
NN’ chromophore which must be coded as 2c’
and its stoicheiometry transformed to [CuI(L
2
)(NCMe)] .
2

3.4 MOPAC calculations
Semi-empirical quantum chemical calculations are being used in order to investigate systems not
readily accessible or intermediates of a reaction. Their utility also includes verification of the
observed spectroscopic characteristics of compounds. In our case it seemed interesting and
intriguing to investigate the hypothetical molecule with the stoicheiometry [CuI(L
2
)(PPh )]. Its
3
geometry was obtained through a full geometry optimization, like it was done for the rest of the
compounds. The final geometry obtained, is presented in Figure 7 and is interesting in the sense that
the diimine is monodentate towards the metal centre and adopts a transoid conformation, giving rise
to a three-coordinate metal centre. Of course the computation only assumed the existence of a single
halide per metal centre, a single phosphine molecule and an initially chelating diimine in a
hypothetical gas phase environment. However, it is expected that if such a structure would occur in
solution, an immediate dimerization through double halide bridge formation would follow. In such
a core and in view of the well documented equilibrium between coordinated and free
triphenylphosphine, an almost anticipated next step would promote the steric stability of the system
by displacing one bulky phosphine with a much smaller acetonitrile ligand, something that was
actually observed.
Figure 7 near here
The distribution and constitution of the frontier molecular orbitals of the iodo compounds was
studied computationally in view of the interesting and non-typical behavior of compound 2c. The
three HOMOs (Figure S4) possess mainly iodine character, in the HOMO some delocalization
towards the metal centre and phosphorous is observed. The LUMOs are located in the diimine
region of the molecule. These observations account for the MLCT and/or LLCT character of the
lowest energy bands in the visible spectrum.
The parent diimines were also studied in the same way (Figure S3). Inspection of the FMOs of
ligand L reveals that the steric hindrance around diimine nitrogen prevents the systems from
1
adopting a coplanar arrangement for the aromatic rings. Furthermore HOMO is centered at the
substituted ring along with LUMO+3, while the other two occupied MOs considered (HOMO-1 and
HOMO-2) are mainly located on the pyridine ring as are the two lowest energy LUMOs. This is an
indication for the presence of several charge transfer excitations at low energies something that is
supported by the general pattern of the recorded visible spectrum. The spectrum of L appeared
4
somewhat different in the form of more intense and sharp absorption bands at longer wavelengths.
The previously applied inspection of its molecular orbitals reveals that LUMO is centered in the
diimine region of the molecule and the next two orbitals on the terminal rings while HOMO is
widely delocalized and the following two occupied orbitals are also centered on the terminal rings
indicating that the majority of the low energy excitations will incorporate considerable π-π*
character.
4. Conclusions
The reaction of anilines with glyoxal produce readily and almost quantitatively the corresponding
diimines. The diimines react under mild conditions with copper(I) halides in the presence of
thriphenylphosphine giving a variety of results. The iminopyridine studied (L
hindered coordination site and affords monomeric complexes with a distorted tetrahedral metal
environment. Analogous is the behavior of the least hindered diazabutene (L ). Contrary to this, the
,6-dimethyl substituted diazabutene (L ) does not react even under severe conditions while the
corresponding 2,6-diisopropyl ligand (L ) coordinates to copper in a mondentate fashion expelling
triphenylphosphine from the coordination sphere which then is completed by the addition of an
1
) presents the less
4
2
3
2
1
acetonitrile molecule giving rise to dimeric compounds of the formula [{Cu(NCMe)(η -L
2
2
)} (µ-
X) ]. None of the diimines reacts with AgNO solution either in the presence or in absence of
2
3
triphenylphosphine. Semiempirical computations are paralleling the observations as of the
coordination preference of L2 and furthermore verify the CT character of the lower energy visible
bands observed for both the organic ligands and their copper coordination compounds.

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Figure captions
Fig. 1. The diimine ligands studied and their notation in the text.
Scheme 1. Syntheric route for the compounds studied.
Fig. 2. Electronic absorption spectra a) of the ligands in methanol, 1(solid), 2 (dotted), 4 (dashed)
and b) of the copper compounds in acetonitrile.
1 3
Fig. 3. Molecular structure of compound [CuBr(L )(PPh )], 2c, produced by Χ ray diffraction. The
CAMERON draw presents ellipsoids at 50% probability level. Hydrogen atoms, solvent molecules
and the second stereoisomer present in the asymmetric unit have been omitted for clarity.
1 3
Fig. 4. Χ ray diffraction molecular structure of compound [CuI(L )(PPh )], 1c. The CAMERON
draw presents ellipsoids at 50% probability level. Hydrogen atoms, solvent molecules are the
second stereoisomer present have been omitted for clarity.
4 3
Fig. 5. Molecular structure of compound [CuI(L )(PPh )], 4c, found by Χ ray diffraction. The
CAMERON draw presents ellipsoids at 50% probability level.
Fig. 6. Molecular structure of compound [CuI(L
The CAMERON draw presents ellipsoids at 50% probability level and no hydrogen atoms for
clarity. The existence of a central planar Cu ring is evident.
2 2
)(NCMe)] , 2c’, produced by X ray diffraction.
2 2
I
2 3
Fig. 7. Optimized geometry of [CuI(L )(PPh )], 2c, as obtained by the application of PM6
Hamiltonian via the MOPAC2016 program.
1 4
Figure S2. Frontier orbitals for L (top) and L (bottom) as obtained from MOPAC 2016
computations. In the left column are the occupied and in the right the vacant energy levels in
qualitative energy order.
Figure S3. Frotnier orbitals for compound 4b presented in the fashion described in the previous
figure.

Appendix A. Supplementary Data
CCDC 1827330-1827333 contains the supplementary crystallographic data for 1b, 1c, 2c and 4c
respectively. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.

Several diimines are reacting with Cu(I) halides in the presence of triphenylphosphine to form a
variety of coordination compounds including monodentate coordination or complete lack of
coordination. Structural investigation results are supported by semiempirical computations.