Homo- and Heteroleptic Copper(I) Complexes with Diazabutadiene Ligands: Synthesis, Solution- and Solid-State Structural Studies

Article abstract of DOI:10.1002/ejic.201600620

The preparation of novel copper(I) complexes of diazabutadiene (DAB) ligands with aliphatic backbones is reported. [Cu(DAB^R)₂](BF₄), [Cu(DAB^R)(NCMe)₂](BF₄) and [CuCl(DAB^R)] are e

Full text of DOI:10.1002/ejic.201600620

DOI: 10.1002/ejic.201600620
Full Paper
Copper(I) Complexes
Homo- and Heteroleptic Copper(I) Complexes with
Diazabutadiene Ligands: Synthesis, Solution- and Solid-State
Structural Studies
Benjamin Zelenay,[a][‡] Roberto Frutos-Pedreño,[a][‡][‡‡] Jordi Markalain-Barta,[a][‡‡‡]
Esmeralda Vega-Isa,[a][‡‡‡‡] Andrew J. P. White,[a] and Silvia Díez-González*[a]
Abstract: The preparation of novel copper(I) complexes of plexes, which remain scarce in the literature, have been fully
diazabutadiene (DAB) ligands with aliphatic backbones is characterised, and their behaviour both in the solid state as
reported. [Cu(DABR)2](BF4), [Cu(DABR)(NCMe)2](BF4) and well as in solution has been studied by means of X-ray crystal-
[CuCl(DABR)] are easily synthesised and air-stable. These com- lography, NMR and UV/Vis spectroscopy.
Introduction
Schiff bases are ubiquitous ligands for transition metals, and
they have played a major role not only in the development of
their coordination chemistry, but also of their catalytic[1] and
biochemical[2] uses. Copper(I) coordination complexes with aro-
matic diimines gather constant interest due to their important
potential chemical and material applications. Indeed, com-
Figure 1. Structures of bidentate N-ligands.
pounds with either 1,10-phenanthroline (phen) or 2,2′-bipyr-
to the rapid disproportionation of copper(I) to copper(II) and
idine (bipy) derivatives have shown great promise in the prepa-
ration of supramolecular assemblies,[3] as catalysts in organic
reactions,[4] or as luminescent materials.[5] In fact, most of
the emissive copper(I) complexes reported to date contain N-
heterocycles as ligands.
copper(0) species. In these cases, the use of an additional ancil-
lary ligand such as a phosphine is crucial for the preparation
of copper(I) complexes.[8] Hence, despite the intensive efforts
reported in this area it is not so surprising that well-defined
copper complexes with diazadiene ligands remain extremely
rare in the literature.[9] Nevertheless, 1,4-diaza-1,3-butadiene
(DAB) ligands have been shown to have similar σ-donor proper-
ties compared to other common bidentate nitrogen donors
such as bipy or BIAN, while being better π-acceptors.[10]
Herein, we describe the synthesis and characterisation of
novel cationic and neutral copper(I) complexes bearing diaza-
butadiene ligands with different substituents on the imino
groups. Also, the structures of these complexes both in the solid
state and in solution have been carefully examined.
The rigidity of the phen ligands is considered to favour metal
coordination by preventing rotation around the C–C bond on
the backbone. This feature is particularly relevant as many of
these copper(I) complexes exhibit a low configurational stability
in solution, and their speciation is hard to control even in non-
coordinating solvents.[6] Bis(arylimino)acenaphthene (BIAN, Fig-
ure 1) ligands have shown a similar behaviour, while having
markedly different properties as the two exocyclic imine groups
are not part of an heteroaromatic system.[7] The control and/or
understanding of the coordination environment of metal cen-
tres is essential for the design and development of catalysts,
materials or bioinorganic models.
Results and Discussion
On the other hand, complexes with ligands with less re-
stricted backbones, including bipy, might not be accessible due
Preparation and Characterisation of Copper(I) Complexes
[a] Department of Chemistry, Imperial College London,
Exhibition Road, South Kensington, London SW7 2AZ, UK
E-mail: s.diez-gonzalez@imperial.ac.uk
http://www.imperial.ac.uk/people/s.diez-gonzalez
[‡] These authors contributed equally to this work.
[‡‡] Visiting PhD student from Universidad de Murcia (Spain)
[‡‡‡] Erasmus student from Universidad de Barcelona (Spain)
[‡‡‡‡] Visiting PhD student from Universidad de Oviedo (Spain)
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201600620.
[Cu(NCMe)4](BF4) was treated with excess diimine ligands at
room temperature to produce the homoleptic cationic com-
plexes 1–4 (Scheme 1). These were isolated in good yields after
simple recrystallisation. Remarkably, all of these complexes
were indefinitely stable towards oxygen and moisture and
could be stored/handled without the need of any particular
precautions. No mono(diimine)copper(I) complexes could be
accessed when a 1:1 Cu/DABR stoichiometry was used in differ-
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ent solvents (DCM, acetone or MeCN), and only the reported
homoleptic complexes were isolated in all cases.
Scheme 3. Preparation of neutral heteroleptic complexes.
ably shifted to lower field after metal coordination, consistent
with the expected electron donation from the diimine ligands
to the copper centre. This effect is particularly strong in com-
plexes bearing DAB ligands with alkyl substituents (i.e. δ =
8.52 ppm for complex 8, compared to δ = 7.90 ppm in the free
ligand) or with a para-anisyl group (i.e. δ = 8.95 ppm for com-
plex 2, compared to δ = 8.42 ppm in the free ligand). Interest-
ingly, the expected downfield shift for the CH=N signal in the
13C{1H} NMR spectra was only evidenced for complexes 2 and
6 bearing one or two DABAnis ligands. This observation implies
that DABAnis is the best π-acceptor ligand of the series, and in
consequence, there is a larger paramagnetic contribution in
those two complexes from π-back-bonding. Even so, only a very
small deshielding was observed (1.7 and 0.9 ppm for 2 and 6,
respectively). For all the other complexes, a small shift upfield
of 2–3 ppm was observed instead.
Scheme 1. Preparation of cationic homoleptic complexes.
Even though the structure of 1 has been reported in the
literature,[11] no spectroscopic data or yield was provided.
Hence, we treated this complex as all the others in the series.
In stark contrast, when a diimine with 2,6-diisopropylphenyl
substituents on the nitrogen atoms was used in the preparation
of cationic complexes, only heteroleptic complex 5 was ob-
tained in an excellent yield (Scheme 2). The outcome of the
reaction remained unchanged even when different stoichiome-
tries and/or more forcing reaction conditions were employed.
The IR spectra of these complexes display medium absorp-
tion bands around 1599–1624 cm–1, the typical absorption re-
gion for C=N symmetric stretching vibrations. These values are
shifted to lower wavenumbers with respect to those of the free
diimine ligands, which agrees with the coordination of both
nitrogen atoms of the DAB ligands to the copper centre. Addi-
tionally, cationic complexes 1–5 display a single stretching
band at ca. 1050 cm–1 for the B–F bonds, as expected for com-
plexes without significant interaction with the metal centre.
Mass spectra of all cationic complexes showed peaks for the
expected metal cation, [Cu(DABR)2]+ for 1–4, or [Cu(DABDipp)]+
for complex 5. No molecular peaks were obtained for neutral
complexes 6–8, and values of m/z consistent with
[Cu(DABR)(NCMe)]+ species were systematically observed in-
stead.
Scheme 2. Cationic heteroleptic complex 5.
Compound 5 is a dark red complex that can be handled in
air, but it slowly decomposes over time (i.e. 3 months) if not
stored under an inert atmosphere. A similar reactivity has been
disclosed in the literature for DAB ligands with triphenylmethyl
groups on the nitrogen atoms.[12]
On the other hand, the reaction between an equimolar mix-
ture of a diimine and CuCl afforded the corresponding copper(I)
complexes 6–8 bearing one N-ligand per copper centre
(Scheme 3). These complexes were isolated in fair to excellent
yields after recrystallisation. However, when DAB ligands bear-
ing bulkier aromatic groups (Mes or Dipp) were used, only the
starting materials were recovered, even when the reaction tem-
perature was increased.
Structural Studies
Suitable crystals for single-crystal X-ray diffraction were ob-
tained for complexes 1–7 by slow diffusion of hexane into DCM
solutions, except for complex 4, where an acetone/pentane
combination was used instead. A summary of the crystallo-
graphic data for these compounds is provided in the Support-
ing Information (Table S1). Ball-and-stick representations of the
obtained structures are given in Figures 2, 3, 4, 5, 6, 7 and 8,
and selected bond lengths are provided in the captions of these
These deep red/nearly black complexes did not decompose
over months when stored in the solid state. However, after
3 days, solutions in dichloromethane started showing signifi-
cant broadening of the signals in the 1H NMR spectra, indicative
of oxidation to paramagnetic copper(II) species.
All isolated complexes were fully characterised by spectro- figures. Complexes 3 and 6 contain four independent units (A–
scopic methods as well as elemental analysis and single-crystal D) in their crystals, and their respective bond lengths and an-
1
X-ray diffraction. In the H NMR spectra, the resonances of the gles for the structures not shown (B–D) are provided in the
imino protons on the backbone of the ligands appear consider- Supporting Information (Table S2 and S4).
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Figure 2. Structure of the cation present in the crystal of [Cu(DABMes)2](BF4)
(1). Most hydrogen atoms are omitted for clarity. Selected bond lengths [Å]
and angles [°]: Cu–N(1) 2.0239(16), Cu–N(2) 2.0256(17), Cu–N(21) 2.0268(15),
Cu–N(22) 2.0334(17); N(1)–Cu–N(2) 82.24(7), N(1)–Cu–N(21) 108.44(7), N(1)–
Cu–N(22) 145.46(7), N(2)–Cu–N(21) 145.49(7), N(2)–Cu–N(22) 107.64(7), N(21)–
Cu–N(22) 82.41(6).
Figure 4. Structure of one (3-A) of the four independent cations present in
the crystal of [Cu(DABCy)2](BF4) (3). Most hydrogen atoms are omitted for
clarity. Selected bond lengths [Å] and angles [°]: Cu(1)–N(1) 2.041(3), Cu(1)–
N(2) 2.011(3), Cu(1)–N(21) 2.020(3), Cu(1)–N(22) 2.031(3); N(1)–Cu(1)–N(2)
82.31(12), N(1)–Cu(1)–N(21) 117.44(12), N(1)–Cu(1)–N(22) 121.20(11), N(2)–
Cu(1)–N(21) 135.45(12), N(2)–Cu(1)–N(22) 124.09(12), N(21)–Cu(1)–N(22)
81.47(11).
Figure 3. Structure of the cation present in the crystal of [Cu(DABAnis)2](BF4)
(2). Most hydrogen atoms are omitted for clarity. Selected bond lengths [Å]
and angles [°]: Cu–N(1) 2.0099(17), Cu–N(2) 2.0095(17), Cu–N(21) 2.0292(18),
Cu–N(22) 2.0027(17), N(1)–Cu–N(2) 82.35(7), N(1)–Cu–N(21) 121.46(7), N(1)–
Cu–N(22) 126.91(7); N(2)–Cu–N(21) 120.85(7), N(2)–Cu–N(22) 128.18(7), N(21)–
Cu–N(22) 82.41(6), 145.46(7), N(2)–Cu–N(21) 145.49(7), N(2)–Cu–N(22)
107.64(7), N(21)–Cu–N(22) 82.41(6).
All structures 1–7 (Figures 2–8) show tetracoordinated cop-
per centres. These might be described as tetrahedral, but in
order to quantify the distortion with respect to an ideal tetrahe-
dral geometry, we calculated the τ4 geometry index introduced
by Houser in 2007.[13] This is based on the formula τ4 = {360° –
(α + ꢀ)}/141°, where α and ꢀ are the two largest θ angles in
the four-coordinate species. The values of τ4 range from 1.00
for a tetrahedral to 0.00 for an ideal square-planar geometry.
Figure 5. Structure of the cation present in the crystal of [Cu(DABAd)2](BF4)
(4). Most hydrogen atoms are omitted for clarity. Selected bond lengths [Å]
and angles [°]: Cu(1)–N(1) 2.0374(11), Cu(1)–N(2) 2.0389(11), Cu(1)–N(31)
2.0559(12), Cu(1)–N(32) 2.0267(11); N(1)–Cu(1)–N(2) 81.95(5), N(1)–Cu(1)–
N(31) 120.12(5), N(1)–Cu(1)–N(32) 129.81(5), N(2)–Cu(1)–N(31) 119.50(5), N(2)–
Cu(1)–N(32) 128.66(5), N(31)–Cu(1)–N(32) 81.91(5).
However, the calculated τ4 index for complex 1 was consider-
The calculated τ4 values for complexes 2–7 are between 0.88 ably lower (0.49) indicating that this geometry might be de-
(for 4 and 5) and 0.74 (for 2), in accordance to distorted tetrahe- scribed as distorted square-planar. This difference is obvious
dral arrangements.
when examining the obtained crystal structures for homoleptic
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Figure 8. The crystal structure of the Ci-symmetric complex [CuCl(DABCy)] (7).
Most hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and
angles [°]: Cu(1)–Cl(1) 2.3625(5), Cu(1)–N(1) 2.0875(15), Cu(1)–N(2) 2.1025(15),
Cu(1)–Cl(1A) 2.2909(5); Cl(1)–Cu(1)–N(1) 114.93(4), Cl(1)–Cu(1)–N(2) 112.57(4),
Cl(1)–Cu(1)–Cl(1A) 101.846(17), N(1)–Cu(1)–N(2) 79.94(6), N(1)–Cu(1)–Cl(1A)
125.28(5), N(2)–Cu(1)–Cl(1A) 122.07(5), Cu(1)–Cl(1)–Cu(1A) 78.154(17).
Figure 6. Structure of the cation present in the crystal of 5. Most hydrogen
atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Cu–
N(1) 2.0821(12), Cu–N(2) 2.0798(12), Cu–N(30) 1.9424(15), Cu–N(35)
1.9391(14); N(1)–Cu–N(2) 78.82(5), N(1)–Cu–N(30) 112.60(6), N(1)–Cu–N(35)
117.91(5), N(2)–Cu–N(30) 110.21(6), N(2)–Cu–N(35) 117.88(5), N(30)–Cu–N(35)
114.52(6).
interplanar separations of ca. 3.51 and 3.46 Å, rings inclined
by ca. 1°). Overall, τ4 clearly accounts for the intramolecular π-
stacking with the mesityl rings in complex 1. No such interac-
tion was evidenced in [Cu(DABAnis)2](BF4) (2), the other homo-
leptic complex bearing aromatic substituents in this study.
The diisopropylphenyl groups in mono(chelate) complex 5
adopt an almost perpendicular orientation relative to the che-
late plane with torsion angles about the N(1)–C(Ar) and N(2)–
C(Ar) bonds of ca. 78 and 79°, respectively. This arrangement
effectively shields the copper(I) centre above and below the
coordination plane, which make accommodating a second DAB
ligand difficult.
cationic complexes (Figures 2–5). Indeed, for the monomeric
bis(chelate) structures 1–4 the angles between the two CuN2
chelate planes vary, being ca. 54, 89, 84, 81, 87, 88 and 90° for
1, 2, 3-A, 3-B, 3-C, 3-D and 4, respectively. These planes are
almost perpendicular except for [Cu(DABMes)2](BF4) (1). The sig-
nificantly smaller angle seen in this complex is associated with
a pair of intramolecular π–π contacts between the adjacent
mesityl rings on the two coordinated ligands. The N(1)-bound
ring overlays the N(21)-bound ring with centroid···centroid and
mean interplanar separations of ca. 3.63 and 3.53 Å, the two
rings being inclined by ca. 5°, whilst the N(2)- and N(22)-bound
Dimeric structures 6 and 7 both present central Cu2Cl2 rings
rings overlap in a similar fashion (centroid···centroid and mean in which the copper(I) and the chlorine ions each occupy oppo-
Figure 7. Structure of one (6-A) of the four independent complexes present in the crystal of [CuCl(DABAnis)] (6). Most hydrogen atoms are omitted for clarity.
Selected bond lengths [Å] and angles [°]: Cu(1A)–Cl(1A) 2.2773(7), Cu(1A)–N(1A) 2.073(2), Cu(1A)–N(2A) 2.0641(19), Cu(1A)–Cl(2A) 2.3534(7); Cl(1A)–Cu(1A)–
N(1A) 125.29(6), Cl(1A)–Cu(1A)–N(2A) 123.56(6), Cl(1A)–Cu(1A)–Cl(2A) 105.29(2), N(1A)–Cu(1A)–N(2A) 80.53(8), N(1A)–Cu(1A)–Cl(2A) 109.85(6), N(2A)–Cu(1A)–
Cl(2A) 110.54(6), Cu(1A)–Cl(1A)–Cu(2A) 73.58(2).
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site corners. This ring is perfectly flat in 7 as a consequence of was observed (Figure 9). At higher ligand concentrations, a
the centre of symmetry in the middle of the ring, but distinct band at 425 nm appeared and progressively shifted towards
folds of ca. 21, 13, 16 and 26°, respectively, are seen between the wavelength of DABAnis
.
the two CuCl2 planes for the four independent complexes (6-
A–6-D) in structure 6. The observed Cu···Cu distances in each
independent unit (2.74–2.83 Å) are just around the sum of their
van der Waals radii (2.80 Å),[14] which might be indicative of a
weak attractive interaction between the two closed-shell d10
metal ions. No such interaction can be postulated in 7 as the
observed Cu···Cu separation of 2.934 Å is too long.
The sp2 character of the C and N atoms in the chelate ring of
complexes 1–7 is confirmed by imine bond lengths of 1.286(2)–
1.296(3) Å for cationic 2, or 1.280(3) Å for neutral complex 7,
for instance. These values are very similar to standard N(sp2)=
C(sp2) double bonds (1.27 Å).[15]
In an attempt to better understand the steric environment
imposed by these diimine ligands in the solid state, and maybe
also the specific stoichiometries obtained for the prepared com-
plexes, we quantified such effect of DABR using the percent
buried volume (%VB) method, originally developed by Cavallo
and co-workers as a steric probe of N-heterocyclic carbene li-
gands.[16] A simple online tool calculates the percentage of the
volume of a metal-centred sphere of defined radius that is oc-
cupied by a given ligand.[17] For these calculations we used the
crystallographic data obtained for cationic complexes 1–5, and
according to the obtained %VB values (41.4–47.5),[18] the steric
demand of the diimine ligands used in this study follows the
sequence: DABAnis ≈ DABCy < DABAd < DABDipp < DABMes. It was
surprising to find that the sterically most hindered ligand was
DABMes and not DABDipp as the isolation of heteroleptic com-
plex 5 would suggest. These values clearly show both steric
and electronic factors determine the nature of the accessible
complexes for these ligands.
Figure 9. UV/Vis spectra for the titration experiments with [Cu(NCMe)4](BF4)
(15 μ
) and different equivalents of DABAnis in DCM.
M
Plotting of the intensity of each absorption against the
equivalents of ligand used evidenced that the intensity of both
bands increases up to 1.5 equiv. of ligand, and remains constant
at higher concentrations.[18] Furthermore, Jobs plots for these
two bands indicate a mol fraction of 0.64 and 0.62 for DABAnis
,
with both values very close to a 2:3 Cu/L ratio for the species
in solution (Figure 10).
UV/Vis Studies
Figure 10. Jobs plot for the titration experiments with DABAnis in DCM.
All the complexes prepared are red, with shades ranging from
dark pink to almost black. As ligand dissociation had been ob-
This points towards a different stoichiometry of the species
served in related complexes,[6] even in noncoordinating sol- in solution when compared to isolated [Cu(DABAnis)2](BF4) (2).
vents, UV/Vis studies were carried out to determine the behav- It is important to note that the UV/Vis spectrum recorded for 2
iour in solution. We focused our efforts on the homoleptic was found to be virtually identical to that from a titration exper-
cationic complexes as there is some precedent in the literature iment with a 2:3 and not a 1:2 Cu/L ratio as it could have been
for [CuCl(DABR)][9a] complexes, and also the low solubility of expected (Figure 11).
CuCl in organic solvents would impede any titration experi-
ments.
In a first stage, we studied [Cu(DABAnis)2](BF4) (2), whose
absorption spectrum recorded in DCM (15 μ
M) showed an
intense broad band between 420 and 452 nm (ε
=
35280 697 L mol–1 cm–1), as well as a much weaker band at
lower energy (593 nm, 5100 70 L mol–1 cm–1). Both bands
were attributed to dπ–π* metal-to-ligand charge transfer
(MLCT) absorptions as the free ligand, DABAnis, in DCM displays
a band at 375 nm (24810 293 L mol–1 cm–1).
Titration experiments were then carried out with
[Cu(NCMe)4](BF4) solutions in degassed DCM (15 μM) and in-
creasing amounts of free ligand DABAnis (0.2–3 equiv.). Between
0.2 and 1.6 equiv. of ligand, a constant increase of both bands
Figure 11. Comparison of UV/Vis spectra in DCM for the Cu/DABAnis system
([Cu] = 15 μ ).
M
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Figure 12. Comparison of 1H NMR spectra for the Cu/DABAnis system in CD2Cl2.
1
In order to further validate these observations, the H NMR
The titration experiments were performed in degassed DCM
spectrum of a 15 μ
M
solution of 2 in CD2Cl2 was recorded. In- with a constant concentration in [Cu(NCMe)4](BF4) (300 μ
M) and
deed, no signals corresponding to [Cu(DABAnis)2](BF4) were an increasing amount of DABAd (60–900 μ
M
, 0.2–5 equiv., Fig-
found at this low concentration. Instead, the signals corre- ure 14). When 0.2 to 1 equiv. of ligand was used, the band at
sponding to free DABAnis were observed, together with those 530 nm (together with a very small absorption at 326 nm) was
corresponding to a new ligated species with a single chemical observed. Between 1 and 2 equiv. of ligand, the band at 326 nm
environment for all diimine hydrogen atoms present (Figure 12). gradually decreased in intensity, whereas the ones at 430 and
Based on these results, a structure is proposed for the copper 530 nm steadily increased. The spectra remained unchanged
complex formed in the titration experiments, featuring a dicop- when larger excesses of ligand, up to 5 equiv., were employed.
per complex with three bridging diimine ligands (Figure 13).[19] Plots of the intensity of each band against the equivalents of
ligand can be found in the Supporting Information.
Figure 13. Plausible structure for the species formed by the Cu/DABAnis sys-
tem in DCM.
Figure 14. UV/Vis spectra for the titration experiments with [Cu(NCMe)4](BF4)
Pre-formed [Cu(DABAnis)2](BF4) (2) might rearrange into a
similar species at low concentrations in non-coordinating sol-
vents. Measurements at higher concentrations could not be car-
(300 μM
) and different equivalents of DABAd in DCM.
Significantly, the UV/Vis spectrum of [Cu(DABAd)2](BF4) (4)
ried out due to the strong absorbance of this system, which and the one obtained for the titration experiment with a Cu/L
also prevented the use of MeCN as solvent.
ratio of 1:2 are identical.[14] These results led us to tentatively
assign the band at 326 nm to a heteroleptic complex
[Cu(DABAd)(NCMe)2](BF4).
In order to obtain more information about these homoleptic
complexes, [Cu(DABAd)2](BF4) (4) was investigated next.
DABAd has the advantage of not having any absorptions in the
In order to gather additional evidence, we next carried out
visible region, and in DCM only
a band at 228 nm UV/Vis studies in MeCN, a coordinating solvent. In this case, two
(14450 503 L mol–1 cm–1) is observed. In this case all measure- main bands were observed in the spectrum of 4, one at 398 nm
ments were carried out at higher concentrations (typically and a more intense one at 525 nm. When this spectrum was
300 μM) as both the complex and the ligand are significantly recorded at different concentrations (0.15–1.2 mM), it became
less absorbent than in the previous case. However, it is impor- clear that as the concentration was reduced, the band at
tant to note that identical UV/Vis bands were recorded at lower 525 nm decreased in intensity at a faster rate than the band
concentrations, down to 40 μ . The absorption spectrum of 4 at 398 nm (Figure 15). A plot of the bands' intensity vs. the
M
recorded in DCM showed a small absorption at 430 nm and a concentration of 4, showed that both lines eventually cross
much stronger band at 530 nm attributed to dπ–π* metal-to- each other, giving evidence that these two bands do not be-
ligand charge transfer (MLCT) absorptions.
long to the same species in solution. Further proof comes from
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Figure 15. UV/Vis spectra for the titration experiments with [Cu(NCMe)4](BF4) (300 μM
) and different equivalents of DABAd in DCM.
a vastly different shape of normalised spectra recorded at differ- 1.3 × 103. It is important to note that only data obtained at
ent concentrations.[18] These results indicate that MeCN can in copper concentrations between 0 and 0.3 m
could be used
M
fact displace diimine ligands at low concentrations, possibly in these calculations, since at higher concentrations the Beer–
forming [Cu(DABAd)(NCMe)2](BF4) in situ.
Lambert law is not fulfilled anymore, most probably because of
additional equilibria present in the system.
Titration experiments in degassed MeCN with [Cu(NCMe)4]-
(BF4) and DABAd led to results very similar to those obtained in
DCM.[18] The Jobs plot derived from this data showed that the
maximum for the 398 nm band lays within 0.5 mol fraction of
the ligand, pointing towards a 1:1 Cu/L ratio (Figure 16). For the
band at 525 nm, the maximum absorbance was between 0.6
and 0.7, which could be attributed to a 1:2 or a 2:3 copper/
ligand ratio. Considering the NMR and X-ray crystallography
data previously collected, a 1:2 stoichiometry for the copper
complex was favoured.
Scheme 4. Proposed equilibrium.
All attempts to isolate the proposed heteroleptic
[Cu(DABAd)(NCMe)2](BF4) failed. No crystals suitable for X-ray
analysis could be grown, and upon concentration, only homo-
leptic complex 4 was observed by NMR spectroscopy. However,
1
further evidenced could be obtained by H NMR analysis. The
spectrum of [Cu(DABAd)2](BF4) (4) was recorded in CD3CN at
different concentrations. As it can be seen in Figure 17, as the
concentration decreased from 12 to 0.3 mM, the resonance of
the imine hydrogen atoms in the copper complex shifted up-
field from δ = 8.3 ppm to δ = 8.0 ppm. This signal also became
very broad with dilution, indicating a quick exchange process
of the diimine ligands on the NMR time scale. When the same
spectrum was recorded at –40 °C, three imine environments
were evidenced, homoleptic complex 4 (δ = 8.3 ppm), free
DABAd ligand (δ = 7.8 ppm), and a new signal (δ = 8.0 ppm).
Next, a mixture 1:0.2 [Cu(NCMe)4](BF4)/DABAd was considered
as only the band at 398 nm was observed under these condi-
tions during the titration experiments (see Figure 14). Signifi-
cantly, a single resonance at δ = 8.0 ppm was obtained at room
temperature for this experiment. Overall, these results show
that even if they might not be isolable, [Cu(DABR)(NCMe)2](BF4)
complexes can be formed as diluted solutions either by dilution
of pre-isolated [Cu(DABR)2](BF4) complexes or by the reaction
of the ligand with a large excess (i.e. 5 equiv.) of the copper
source.
Figure 16. Jobs plot for the titration experiments with DABAd in MeCN.
The extinction coefficient for the absorption band at 398 nm,
attributed to heteroleptic [Cu(DABAd)(NCMe)2](BF4), could be
calculated from titration experiments with a large excess of
[Cu(NCMe)4](BF4) (up to 1000 equiv.) to prevent the formation
of the bis(DAB) complex.[18] Under these conditions, no band at
525 nm was observed and an extinction coefficient of
Importantly, the only heteroleptic cationic complex of this
2191 22 L mol–1 cm–1 was obtained. On the other hand, the kind we could prepare during our synthetic studies, [Cu-
dilution of 4 (0.06–0.6 m
) in solutions of acetonitrile saturated (DABDipp)(NCMe)2](BF4) (5), has a similar UV/Vis spectrum, and
M
in DABAd led to UV/Vis spectra with only the 525 nm band, and the band of the lowest energy for 5 appeared at 353 nm
the extinction coefficient found for this species was (1709 4 L mol–1 cm–1) in MeCN. Quite surprisingly, this partic-
7448 255 L mol–1. With this data in hand, the equilibrium con- ular compound is as stable as the rest of the complexes re-
stant for the reaction depicted in Scheme 4 was found to be ported in this work.
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Figure 17. Comparison of 1H NMR spectra for the Cu/DABAd system in CD3CN.
Finally, in order to study the possible fluorescence emission natively unsaturated species prone to act as a catalyst. In fact,
of the prepared compounds, [Cu(DABAnis)2](BF4), [Cu(DABAd)2]- we recently reported that these complexes are competent cata-
(BF4), as well as [CuCl(DABAnis)] and [CuCl(DABAd)] complexes lysts for the copper(I)-catalysed Click azide-alkyne cycloaddition
were excited in the MLCT or the π–π* bands, but no fluores- reaction,[20] with [Cu(DABCy)2](BF4) (3) as the best performing
cence emissions were observed at room temperature in DCM catalyst. Further studies on the application of these complexes
or MeOH, neither in the presence of oxygen nor in degassed are currently ongoing in our laboratory.
solutions.
Experimental Section
Conclusions
General Considerations: All reagents were used without any fur-
ther purification. DCM was dried by passing it through columns of
molecular sieves in a solvent purification system. All reactions were
carried out in anhydrous solvent under nitrogen using oven-dried
glassware. Melting points (uncorrected) were determined with an
Electrothermal Gallenhamp apparatus. NMR spectra were recorded
with Bruker AVANCE 400 spectrometers at room temperature. 1H
Three related families of copper(I) complexes bearing diaza-
butadiene ligands have been prepared and fully characterised:
homoleptic [Cu(DABR)2](BF4), heteroleptic [Cu(DABR)(NCMe)2]-
(BF4) and neutral [CuCl(DABR)] complexes. Interestingly, the Cu/
L ratio in these complexes is dictated by the chosen copper
source and the diimine ligand, but not the stoichiometry of the
reactions. All of these can be easily handled with no particular NMR shifts in δ (ppm) are referenced to tetramethylsilane (δ =
0.00 ppm). 13C NMR shifts are referenced to the central peak of the
precautions to exclude oxygen or moisture, and – with the ex-
ception of 5 – they are indefinitely stable in the solid state. The
prepared complexes displayed a tetracoordinate copper centre
in the solid state, with different degrees of distortion from the
CDCl3 triplet, the (CD3)2SO septuplet or the CD3CN septuplet, set at
δ = 77.00, 39.50 or 1.32 ppm, respectively. 19F NMR shifts are refer-
enced to the monofluorobenzene singlet, set at δ = –113.15 ppm.
IR spectra were recorded with a Perkin–Elmer 100 series FT-IR spec-
expected tetrahedral geometry. Furthermore, the behaviour in
trometer, equipped with a beam-condensing accessory (samples
solution was studied by means of UV/Vis and NMR spectro-
were sandwiched between diamond compressor cells). Single-crys-
scopy. The obtained results show that even if they might only
be stable under diluted conditions, heteroleptic cations
[Cu(DABR)(NCMe)2]+ appear to be intermediates in the forma-
tal X-ray diffraction was performed with Oxford Diffraction Xcalibur
PX Ultra (λ = 1.54184 Å) (1, 2, 3, 6 and 7) and Xcalibur 3 (λ =
0.71073 Å) (4 and 5) diffractometers, and the structures were re-
tion of the isolated homoleptic [Cu(DABR)2](BF4) complexes. fined using the SHELXTL, SHELX-97, and SHELX-2013 program sys-
tems.[21] The absolute structure of 1 was determined by a combina-
Moreover, in diluted solutions one of the diimine ligands in
+
–
tion of R-factor tests (R1 = 0.0315, R1 = 0.0393) and by use of the
Flack parameter [x+ = 0.000(17), x– = 1.030(17)]. CCDC 1409952 (for
1), 1409953 (for 2), 1409954 (for 3), 1409955 (for 4), 1409956 (for
5), 1409957 (for 6), and 1409958 (for 7) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre.
UV/Vis spectra were recorded with a Perkin–Elmer Lambda 20 spec-
trometer in nondegassed DCM or MeCN solutions. Mass spectra
were recorded with a Micromass Autospec Premier, Micromass LCT
the homoleptic complexes can be displaced by a coordinating
solvent, or the initial complex might rearrange into dinuclear
species. This is in line with the known lability of copper–diimine
complexes in solution. While this feature is often regarded as a
drawback for the exploitation of the photochemical properties
of these compounds, it can also be regarded as a very easy
activation mode for catalytic applications. Hence, by simply
adding a sub-stoichiometric amount of such copper complexes
to a solution, the copper complex will evolve to a new coordi- Premier or a VG Platform II spectrometer using EI, CI or ESI tech-
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niques. Elemental analyses were carried out by the Science Techni-
cal Support Unit at London Metropolitan University.
Complex 4: A mixture of [Cu(NCMe)4](BF4) (400 mg, 1.50 mmol)
and
N,N′-bis(adamantyl)-1,4-diaza-1,3-butadiene
(427
mg,
0.79 mmol) was stirred in DCM (15 mL) at room temperature over-
night. The resulting dark red solution was then concentrated to
dryness and the precipitate dissolved in the minimum amount of
DCM. Addition of diethyl ether led to the precipitation of a dark
red solid. This was washed with diethyl ether and hexane, dried
under reduced pressure and recrystallised from DCM and diethyl
ether (yield 580 mg, 80 %). Single crystals for X-ray diffraction were
grown from acetone/pentane. M.p. 325 °C. 1H NMR (400 MHz,
CDCl3): δ = 8.51 (s, 4 H, H–C=N), 2.18 (s, 12 H, CHAd), 184–1.53 (m,
48 H, CH2Ad) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 155.6 (CH,
Complex 1: A mixture of [Cu(NCMe)4](BF4) (108 mg, 0.34 mmol)
and N,N′-bis(mesityl)-1,4-diaza-1,3-butadiene (209 mg, 0.72 mmol)
in CH2Cl2 (20 mL) was stirred at room temperature overnight. This
resulted in a dark green solution that was concentrated to ca. 5 mL,
and pentane (ca. 30 mL) was added, which led to the precipitation
of a nearly black solid. This was collected by filtration, washed with
pentane (3 × 10 mL) and dried under reduced pressure (yield
179 mg, 71 %). Single crystals for X-ray diffraction were grown from
1
DCM/hexane. M.p. 297 °C. H NMR (400 MHz, CDCl3): δ = 8.30 (s, 4
H, H–C=N), 6.61 (s, 8 H, HAr), 2.28 (s, 12 H, 4-Me), 1.97 (s, 24 H, 2,6-
Me) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ = 159.3 (CH, CH=N),
144.3 (C, i-CAr), 136.7 (C, o-CAr), 129.3 (CH, m-CAr), 129.1 (C, p-CAr),
20.8 (CH3, p-Me), 18.8 (CH3, o-Me) ppm. 19F NMR (377 MHz, CDCl3):
CH=N), 59.9 (C, CAd), 44.3 (CH2, CAd), 35.9 (CH2, CAd), 29.5 (CH, CAd
)
ppm. 19F NMR (377 MHz, CDCl3): δ = –151.6 (BF4–) ppm. IR: ν =
˜
2902, 2847, 1621 (C=N), 1558, 1531, 1453, 1378, 1304, 1191, 1040,
1090, 1048, 896, 813, 735, 695 cm–1. UV/Vis: λmax/nm (ε/ –1 cm–1) =
M
δ = –153.4 (BF4–) ppm. IR: ν = 2947, 2910, 1610 (C=N), 1475, 1455,
˜
230 (7100 160), 252 (8960 1540), 532 (9560 60). HRMS: calcd.
for C44H64CuN4 [Cu(DABAd)2]+ 711.4427, found 711.4431.
C45H68BCuF4N4 (815.41): calcd. C 66.28, H 8.41, N 6.87; found C
66.18, H 8.62, N 6.73.
1379, 1299, 1206, 1087, 1048 (BF4–), 886, 850, 723, 650, 587, 519,
418, 336 cm–1. UV/Vis: λmax/nm (ε/ –1 cm–1) = 228 (31170 150),
M
398 (13600 20), 727 (3150 60). HRMS: calcd. for C40H48CuN4
[Cu(DABMes)2]+ 647.3175, found 647.3150. C40H48BCuF4N4 (735.19):
calcd. C 74.10, H 7.46, N 8.64; found C 74.03, H 7.51, N 8.59.
Complex 5: A mixture of [Cu(NCMe)4](BF4) (250 mg, 0.79 mmol) and
N,N′-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (299 mg,
0.79 mmol) was stirred in DCM (30 mL) at room temperature for
4 h. After filtration of the resulting dark solution, the filtrate was
concentrated to ca. 5 mL, and hexane (20 mL) was added, leading
to the precipitation of a dark brown solid. This was collected by
filtration, washed with pentane (5 × 3 mL) and dried under reduced
pressure product as a dark red powder (yield 463 mg, 96 %). Single
crystal for X-ray diffraction were grown from DCM/hexane. M.p.
123 °C. 1H NMR (400 MHz, CDCl3): δ = 8.33 (s, 2 H, H–C=N), 7.34–
7.26 (m, 6 H, HAr), 2.95 (sept, J = 6.8 Hz, 4 H, CHMe2), 2.20 (s, 6 H,
NCMe), 1.26 (d, J = 6.8 Hz, 24 H, CHMe2) ppm. 13C{1H} NMR
(101 MHz, CDCl3): δ = 160.0 (CH, CH=N), 144.9 (C, i-CAr), 138.4
(C, o-CAr), 127.1 (CH, p-CHAr), 123.8 (CH, m-CHAr), 28.2 (CH, CHMe2),
Complex 2: A mixture of [Cu(NCMe)4](BF4) (206 mg, 0.65 mmol)
and N,N′-bis(4-methoxylphenyl)-1,4-diaza-1,3-butadiene (354 mg,
1.32 mmol) in DCM (30 mL) was stirred at room temperature for
4 h. After filtration of the dark solution, the solution was concen-
trated to ca. 5 mL, and petroleum ether (40 mL) was added, which
led to the precipitation of a black solid. This was collected by filtra-
tion, washed with petroleum ether (8 × 3 mL) and dried under re-
duced pressure (yield 367 mg, 82 %). Single crystals for X-ray diffrac-
tion were grown from DCM/hexane. M.p. 205–207 °C. 1H NMR
(400 MHz, CDCl3): δ = 8.99 (s, 4 H, H–C=N), 7.50 (d, J = 9.0 Hz, 8 H,
HAr), 6.85 (d, J = 9.0 Hz, 8 H, HAr), 3.79 (s, 12 H, OMe) ppm. 13C{1H}
NMR (101 MHz, CDCl3): δ = 161.5 (CH, CH=N), 150.8 (C, p-CAr), 139.1
(C, i-CAr), 124.6 (CH, CAr), 115.2 (CH, CAr), 55.6 (CH3, OMe) ppm. 19F
23.9 (CH3, CHMe2) ppm. 19F NMR (377 MHz, CDCl3): δ = –153.1 (BF4
–
NMR (377 MHz, CDCl3): δ = –152.5 (BF4–) ppm. IR: ν = 2836, 1599
) ppm. IR: ν = 2965, 2871, 2277 (C≡ N), 1634 (C=N), 1591 (C=N), 1466,
˜
˜
1435, 1365, 1334, 1256, 1186, 1050 (BF4–), 797, 753, 699 cm–1. UV/
(C=N), 1562, 1503, 1455, 1438, 1253, 1164, 1096, 1058, 1021, 939,
Vis: λmax/nm (ε/M
–1 cm–1) = 342 (2630 70) 565 (256 5); 353
885, 841, 826, 799, 658, 639, 550, 519, 426, 389 cm–1. UV/Vis: λmax
/
(1709 4), 464 (357 22), 524 (265 19) (MeCN). LRMS (ES+): m/z =
480 [Cu(DABDIPP)(NCMe)]+. C30H42BCuF4N4 (609.04): calcd. C 59.16,
H 6.95, N 9.20; found C 59.07, H 6.67, N 9.38.
nm (ε/M
–1 cm–1) = 251 (22100 500), 451 (36100 500), 593
(5100 70). LRMS (ES+): 599 [Cu(DABAnis)2]+. C32H32BCuF4N4O4
(686.98): calcd. C 55.95, H 4.70, N 8.16; found C 55.85, H 4.60, N
8.05.
Complex 6: A mixture of CuCl (123 mg, 1.24 mmol) and N,N′-bis(4-
methoxyphenyl)-1,4-diaza-1,3-butadiene (364 g, 1.39 mmol) in DCM
(30 mL) was stirred at room temperature overnight. The resulting
black suspension was filtered through anhydrous MgSO4, and the
resultant dark filtrate was concentrated to 15 mL. Addition of petro-
leum ether (40 mL) led to the precipitation of a nearly black powder,
which was collected by filtration, washed with petroleum ether, and
dried under reduced pressure (yield 378 mg, 83 %). Single crystals
for X-ray diffraction were grown from DCM/hexane. M.p. 220–
223 °C. 1H NMR (400 MHz, CDCl3): δ = 8.79 (br. s, 2 H, H–C=N), 7.48–
7.46 (m, 4 H, HAr), 6.98–6.96 (m, 4 H, HAr), 3.87 (s, 6 H, OMe) ppm.
13C{1H} NMR: could not be recorded due to the low solubility of 6.
Complex 3: A mixture of [Cu(NCMe)4](BF4) (174 mg, 0.55 mmol)
and N,N′-bis(cyclohexyl)-1,4-diaza-1,3-butadiene (240 g, 1.09 mmol)
was stirred in DCM (30 mL) at room temperature overnight. After
filtration of the resulting dark red solution, the solvent was evapo-
rated to dryness, the residue dissolved in THF (10 mL) and petro-
leum ether (40 mL) added leading to the precipitation of a red
brownish solid, which was collected by filtration, washed with a
mixture of DCM/petroleum ether (1:4, 10 × 5 mL) and dried under
reduced pressure (yield 208 mg, 64 %). Single crystals for X-ray dif-
fraction were grown from DCM/hexane. M.p. 150–153 °C. 1H NMR
(400 MHz, CDCl3): δ = 8.41 (s, 4 H, H–C=N), 3.60 (br., 4 H, CHCy),
1.88–1.68 (m, 20 H, CH2), 1.41–1.08 (m, 20 H, CH2) ppm. 13C{1H}
NMR (101 MHz, CDCl3): δ = 157.2 (CH, CH=N), 67.1 (CH), 34.9 (CH2),
25.1 (CH2), 24.7 (CH2) ppm. 19F NMR (377 MHz, CDCl3): δ = –153.3
IR: ν = 2835, 1611 (C=N), 1586, 1503, 1436, 1297, 1253, 1165, 1111,
˜
1026, 825, 798, 636, 528, 406 cm–1. UV/Vis: λmax/nm (ε/
M
–1 cm–1) =
232 (12900 360), 393 (14300 300), 589 (1250 90). HRMS: calcd.
for C18H19CuN3O2 [Cu(DABAnis) + MeCN]+ 372.0773, found 372.0788.
C16H16ClCuN2O2 (367.31): calcd. C 52.32, H 4.39, N 7.63; found C
52.55, H 4.60, N 7.85.
(BF4–) ppm. IR: ν = 2928, 2854, 1627 (C=N), 1541, 1450, 1348, 1257,
˜
1063 (BF4–), 1026, 873, 855, 520, 465, 412 cm–1. UV/Vis: λmax/nm
(ε/
C28H48CuN4
M
–1 cm–1) = 241 (7250 130), 516 (4360 10). HRMS: calcd. for
[Cu(DABCy)2]+
503.3175,
found
503.3180.
Complex 7: A mixture of CuCl (133 mg, 1.34 mmol) and N,N′-bis-
(cyclohexyl)-1,4-diaza-1,3-butadiene (296 mg, 1.34 mmol) was
stirred in DCM (30 mL) at room temperature overnight. The result-
C28H48BCuF4N4 (591.06): calcd. C 56.90, H 8.19, N 9.48; found C
56.86, H 8.17, N 9.39.
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[3] B. Champin, P. Mobian, J.-P. Sauvage, Chem. Soc. Rev. 2007, 36, 358–366.
[4] For selected recent examples, see: a) Z. Gonda, S. Kovacs, C. Weber, T.
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ing mixture was filtered through anhydrous MgSO4, the dark red
filtrate was concentrated to 5 mL, and petroleum ether (50 mL) was
added to precipitate a dark red solid, which was collected by filtra-
tion, washed with a mixture of Et2O/n-hexane (1:3) and vacuum-
dried (yield 220 mg, 86 %). Single crystals for X-ray diffraction were
grown from DCM/hexane. M.p. 166–170 °C. 1H NMR (400 MHz,
CDCl3): δ = 8.36 (br. s, 2 H, H–C=N), 3.63 (br. t, 2 H, CHCy), 2.20–1.51
(m, 10 H CH2), 1.51–1.12 (m, 10 H, CH2) ppm. 13C{1H} NMR: could
not be recorded due to the low solubility of 7. IR: ν = 2926, 2852,
˜
1644 (C=N), 1569, 1456, 1441, 1393, 1379, 1155, 1102, 956, 869, 854,
584, 464, 406 cm–1. UV/Vis: λmax/nm (ε/ –1 cm–1) = 230 (7060 50),
M
382 (856 9), 515 (1830 20). HRMS: calcd. for C16H27CuN3
[Cu(DABCy) + MeCN]+ 324.1501, found 324.1494. C14H24ClCuN2
(319.36): calcd. C 52.99, H 6.99, N 8.83; found C 52.88, H 6.78, N
8.73.
Complex 8: A mixture of CuCl (75.3 mg, 0.761 mmol) and N,N′-
bis(adamantyl)-1,4-diaza-1,3-butadiene (241.0 mg, 0.743 mmol) in
DCM (20 mL) was stirred at room temperature overnight resulting
in a red mixture. This was concentrated to ca. 5 mL, and pentane
(30 mL) was added, which led to the precipitation of a dark red
solid, which was filtered, washed with pentane and dried under
vacuum (yield 176 mg, 56 %). M.p. 237 °C. 1H NMR (400 MHz, CDCl3):
δ = 8.56 (br. s, 2 H, H–C=N), 2.26 (br. s, 6 H, HAd), 1.90–1.72 (m, 24
H, HAd) ppm. 13C{1H} NMR: could not be recorded due to the low
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solubility of 8. IR: ν = 2901, 2848, 1624 (C=N), 1581, 1538, 1451,
˜
–
1367, 1343, 1306, 1090, 981, 890, 813 cm–1. UV/Vis: λmax/nm (ε/
M
1 cm–1) = 232 (6900 200), 384 (900 11), 530 (1290 50). HRMS:
calcd. for C24H35CuN3 [Cu(DABAd) + MeCN]+ 428.2127, found
428.2132. C22H328ClCuN2 (721.85): calcd. C 62.39, H 7.62, N 6.61;
found C 62.23, H 7.65, N 6.59.
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Acknowledgments
This research was financially supported by the Imperial College
London. The Royal Society (RG140579) and the Engineering and
Physical Sciences Research Council (EPSRC) (EP/K030760/1) are
acknowledged for further funding. R. F. P. and E. V. I. thank the
Ministerio de Educación (Spain) for funding, J. M. B. thanks the
Universidad de Barcelona (Spain) for an Erasmus scholarship. Dr.
Marina Kuimova is acknowledged for helpful discussions.
[17] www.molnac.unisa.it/OMtools/sambvca.php.
[18] See the Supporting Information for further details.
[19] For relevant examples of bridging DAB ligands with transition metals,
see: a) H. Van der Poel, G. Van Koten, K. Vrieze, J. Organomet. Chem.
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Keywords: Charge transfer · Copper · N ligands · Schiff
bases · Transition metals
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Received: May 24, 2016
Published Online: ■
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Full Paper
Copper(I) Complexes
The preparation of novel copper(I)
complexes with aliphatic diazabutadi-
ene (DAB) ligands is reported. [Cu-
(DABR)2](BF4), [Cu(DABR)(NCMe)4](BF4)
and [CuCl(DABR)] are easily synthesised
and air-stable. These complexes, which
remain scarce in the literature, were
fully characterised, and their behaviour
both in the solid state as well as in so-
lution has been studied by means of
X-ray crystallography and UV/Vis spec-
troscopy.
B. Zelenay, R. Frutos-Pedreño,
J. Markalain-Barta, E. Vega-Isa,
A. J. P. White,
S. Díez-González* ............................ 1–11
Homo- and Heteroleptic Copper(I)
Complexes with Diazabutadiene Li-
gands: Synthesis, Solution- and
Solid-State Structural Studies
DOI: 10.1002/ejic.201600620
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11
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim