Synthesis and reactivity of copper(i) complexes containing a bis(imidazolin-2-imine) pincer ligand

Article abstract of DOI:10.1039/b703183a

The new pincer ligand 2,6-bis[(1,3-di-tert-butylimidazolin-2-imino)methyl] pyridine (TL^tBu) has been prepared in high yield from 2,6-bis(hydroxymethyl)pyridine (1) and 1,3-di-tert-butylimidazolin-2-imine (3). Reaction of TL^tBu with [

Full text of DOI:10.1039/b703183a

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Synthesis and reactivity of copper(I) complexes containing a
bis(imidazolin-2-imine) pincer ligand†
Dejan Petrovic, Thomas Bannenberg, So¨ren Randoll, Peter G. Jones and Matthias Tamm*
Received 2nd March 2007, Accepted 11th April 2007
First published as an Advance Article on the web 2nd May 2007
DOI: 10.1039/b703183a
The new pincer ligand 2,6-bis[(1,3-di-tert-butylimidazolin-2-imino)methyl]pyridine (TLtBu) has been
prepared in high yield from 2,6-bis(hydroxymethyl)pyridine (1) and 1,3-di-tert-butylimidazolin-2-imine
(3). Reaction of TLtBu with [Cu(MeCN)4]PF6 affords the highly reactive copper(I) complex
[(TLtBu)Cu]PF6, [5]PF6, which forms the stable copper(I) isocyanide complexes [6a]PF6 (mCN
=
2179 cm−1) and [6b]PF6 (mCN = 2140 cm−1) upon addition of tert-butyl or 2,6-dimethylphenyl isocyanide,
respectively. For the cations 6a and 6b, DFT calculations reveal ground-state electronic structures of the
type [(TLtBu-jN1:jN2)Cu(CNR)] with tricoordinate geometries around the copper atoms. Exposure of
[5]PF6 to the air readily leads to trapping of atmospheric CO2 to form the square-planar complex
[(TLtBu)Cu(HCO3-jO)]PF6, [7]PF6, with the bicarbonate ligand adopting a rarely observed
monodentate coordination mode. In chlorinated solvents such as dichloromethane or chloroform,
[5]PF6 rapidly abstracts chloride by reductive dechlorination of the solvent to yield [(TLtBu)CuCl]PF6,
[8]PF6 quantitatively. Reaction of TLtBu with copper(I) bromide or chloride affords complexes 9a and
9b, respectively, for which X-ray diffraction analysis, low-temperature NMR experiments and DFT
calculations reveal the presence of a j2-coordinated ligand of the type [(TLtBu–jN1:jN2)CuX]. In
solution, complex 9b undergoes slow disproportionation forming the mixed-valence
copper(II)/copper(I) system [(TLtBu)CuCl][CuCl2], [8]CuCl2 with a linear dichlorocuprate(I) counterion.
Introduction
Ligands derived from the 1H-imidazole heterocycle currently play
a major role in organotransition metal and coordination chem-
istry. In particular, N-heterocyclic carbenes of the imidazolin-
2-ylidene type1 are nowadays ubiquitous and indispensable to
the development of diverse research areas such as homogeneous
catalysis,2 materials science3 and medicinal chemistry.4 The stabil-
Scheme 1 Mesomeric structures of imidazole-based ligands.
ity of these carbenes can be attributed inter alia to the capability
of the imidazolium ring to stabilize a positive charge, leading to
strongly basic and highly nucleophilic ligands. This behavior can
be transferred to an exocyclic moiety X at the 2-position of the N-
heterocycle, so that for species such as 2-methylen-, 2-imino- and 2-
oxoimidazolines (X = CH2, NH, O) a strong contribution from the
ylidic mesomeric structure B (Scheme 1) must be considered.5,6 The
resulting build-up of negative charge at X affords compounds with
considerably enhanced basicity and nucleophilicity. Accordingly,
the heavier 2-chalcogenoimidazolines (X = S, Se, Te) have been
regarded as neutral analogues of thiolate, selenolate and tellurolate
ligands, respectively.7
2-iminato ligands (X = N−).8 Desilylation of these precursors
allows access to imidazolin-2-imines (X = NH), which are strong
bases that may well exhibit even stronger nucleophilic properties
than related guanidine-type systems.5,9 These imines are valuable
ligands in their own right, and also useful building blocks for the
design and preparation of novel multidentate poly(imidazolin-
2-imine) ligands of type L1 (Scheme 2).10 The first example
of an ethylene-bridged bis(imidazolin-2-imine) system of this
type was introduced by Kuhn et al.,11 however, their synthetic
protocol is confined to the use of the ligand precursor 2-imino-
1,3-dimethylimidazoline, obtained by a multi-step protocol from
2-aminoimidazole.12 Related poly(guanidines) of type L2 have
recently found remarkable applications as “superbasic” proton
sponges and as ligands for transition metal complexation, with
particular emphasis on copper coordination chemistry.13,14
We have exploited this concept in the synthesis of various
novel 2-trimethylsilyliminoimidazolines (X = NSiMe3) and have
shown that these are suitable precursors for the synthesis of
transition metal complexes incorporating ancillary imidazolin-
With this contribution, we wish to present a new pyridine-
bridged bis(imidazolin-2-imine) ligand TLtBu along with an ac-
count of its copper coordination chemistry. Since TLtBu can be
described by the two limiting resonance structures A and B
(Scheme 2), it can be regarded as a potentially more basic analogue
of Schiff-base pyridine-2,6-diimine ligands. The latter are among
Institut fu¨r Anorganische und Analytische Chemie, Technische Uni-
versita¨t Carolo-Wilhelmina zu Braunschweig, Hagenring 30, D-38106
Braunschweig, Germany. E-mail: m.tamm@tu-bs.de; Fax: +49 531 391 5387
† Electronic supplementary information (ESI) available: Details of the elec-
tronic structure calculations and presentations of all calculated molecular
structures together with Cartesian coordinates of their atomic positions.
See DOI: 10.1039/b703183a
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Scheme 2 Mesomeric structures of TLtBu (A and B), novel multidentate
poly(imidazolin-2-imine) ligands of type L1 and related poly(guanidines)
of type L2.
the most popular pincer-type ligands,15 in particular because of
the unexpected discovery that their late transition metal complexes
are exceptionally active olefin polymerization catalysts.16 To date,
only a limited number of reports on copper complexes containing
pyridine-2,6-diimine ligands exist,17 which is surprising in view of
their potential use in dioxygen activation.18 For instance, some un-
expected results were reported upon reaction of copper(I) deriva-
tives with 2,6-bis[1-(2,6-diisopropylphenylimino)ethyl]pyridine.17b
In addition, it should be noted that copper(I) chloride and bromide
complexes with related chelating N-donor ligands have been used
as efficient catalysts in atom transfer radical polymerizations
(ATRP).19,20
Scheme
3
Synthesis of 2,6-bis[(1,3-di-tert-butylimidazolin-2-imino)-
methyl]pyridine, TLtBu
.
Results and discussion
Ligand synthesis
symmetric, monomeric structure. Upon coordination of the cop-
per ion, the 1H NMR resonance of the tert-butyl hydrogen atoms
is shifted to higher field, whereas downfield shifts are observed
for all other resonances. Elemental analysis of the yellow solid
clearly suggests that a complex of the composition [(TLtBu)Cu]PF6,
[5]PF6, without incorporation of acetonitrile, must have formed
(Scheme 4). All efforts to obtain single crystals suitable for an X-
ray diffraction analysis study proved unsuccessful, and the solid
state structure of the cation 5 remains unresolved.
The synthesis of the ligand 2,6-bis[(1,3-di-tert-butylimidazolin-
2-imino)methyl]pyridine (TLtBu) is achieved starting from 2,6-
bis(hydroxymethyl)pyridine (1), which can be converted into
the corresponding diester
2 by reaction with p-toluene-
sulfonyl chloride.21 Treatment of 2 with two equivalents of
1,3-di-tert-butylimidazolin-2-imine (3) affords the ditosylate
[(TLtBuH2)(OTs)2] (4) in high yield, from which the free ligand
TLtBu can be almost quantitatively released by deprotonation with
an excess of KOtBu (Scheme 3). TLtBu is obtained as an air-stable
off-white solid, of which the 1H and 13C NMR resonances (in C6D6)
for the imidazoline moiety are very similar to those reported for
the imine 3.9 For instance, the 1H NMR spectrum shows a triplet
and a doublet at 7.69 and 7.55 ppm for the bridging pyridine ring
together with two singlets at 6.35 and 4.85 ppm for the benzyl
CH2 and for the imidazoline CH hydrogen atoms, respectively.
The resonance of the tert-butyl hydrogen atoms is observed at
1.43 ppm.
To gain further structural information about the TLtBu
–
copper(I) system, we have performed DFT (density functional
theory) calculations on the cationic complex 5 and on its acetoni-
trile adduct. All computations were performed using the hybrid
density functional method B3LYP implemented in the Gaussian
03 program.22 For all main-group elements (C, H, N, Cl and Br)
the all-electron triple-f basis set (6-311G**) was used, whereas for
the group 11 transition metal Cu the Ahlrichs TZV basis set was
employed.23,24 The calculated structure of 5 reveals a T-shaped
geometry with both imino moieties and the pyridine function
coordinated to the copper atom (Fig. 1). All Cu–N distances are in
Reactions of TLtBu with [Cu(MeCN)4]X (X = PF6, CF3SO3)
◦
˚
the range 2.018–2.039 A, and a dihedral angle of 9.45 is observed
between the CuN3 plane and the pyridine ring.
The reaction of TLtBu with the copper(I) complex [Cu(MeCN)4]PF6
in acetonitrile affords a yellow, extremely air-sensitive solid after
evaporation of the solvent. The 1H and 13C NMR spectra of this
material in CD3CN or acetone-d6 exhibit just one set of resonances
for the imidazolin-2-imine moieties, indicating the formation of a
Calculations on the acetonitrile adduct 5·NCCH3 afford a
trigonal geometry with the pyridine, the acetonitrile and only
one imino nitrogen atom attached to the metal centre (Fig. 1).
Apparently, the ligand TLtBu does not support a four-coordinate
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copper(I) environment, since a favorable tetrahedral geometry
cannot be adopted because of the sterically demanding tert-butyl
substituents. Accordingly, the reaction enthalpy for the formation
of 5·NCCH3 is found to be slightly exothermic by DH◦
=
−9.3 kcal mol−1.25 It should be noted that similar results have
been obtained by DFT calculations on a related Cu(I) system
with the pincer ligand 2,6-bis(dimethylaminomethyl)pyridine.26
Isocyanide adducts of [5]PF6
The reactions of [5]PF6 with tert-butyl and 2,6-dimethylphenyl
isocyanides in THF afforded the isocyanide adducts [6a]PF6 and
[6b]PF6 in good yields after precipitation or evaporation of the
solvent (Scheme 4). In the 1H and 13C NMR spectra, coordination
of the isocyanides is clearly indicated by the observation of the
expected isocyanide resonances together with significantly shifted
resonances for the TLtBu ligand in comparison to [5]PF6. In
addition, the IR spectra exhibit strong CN stretching vibrations
at 2179 [6a] and 2140 cm−1 [6b] which are significantly shifted
to higher wave numbers with respect to the free isocyanides
[mCN(tBuNC) = 2138 cm−1, mCN(XyNC) = 2115 cm−1] in agreement
with negligible p-back-donation from copper(I) into the isocyanide
ligands.27 Although the 1H and 13C NMR spectra of [6a]PF6 and
[6b]PF6 imply formation of symmetric, four-coordinate copper(I)
complexes, low-temperature NMR studies of [6a]PF6 and [6b]PF6
Scheme 4
◦
in acetone-d6 result in broadening of specific signals at −80 C,
suggesting the existence of asymmetric complexes. However, the
coalescence temperature appeared to be even lower and could not
be reached.
In agreement with the theoretical results obtained for the
acetonitrile complex 5·NCCH3, our DFT calculations on 6a and
6b reveal ground-state electronic structures with tricoordinate
geometries around the copper atoms. As shown for the calculated
structure of the tBuNC derivative 6a in Fig. 1, one ligand
arm is clearly bending away with a copper–nitrogen distance of
˚
˚
3.636 A (3.549 A in 6b, see ESI†) indicating full dissociation.
Accordingly, [6a]PF6 and [6b]PF6 can be regarded as dynamic
[(TLtBu-jN1:jN2)Cu(RNC)]PF6 species. Finally, the enthalpies of
formation from 5 and the respective isocyanide are found to be
DH◦ = −21.5 kcal mol−1 for both 6a and 6b,25 which is significantly
more exothermic than 5·NCCH3 and accounts for the higher
stability of the isocyanide complexes.
Dioxygen and carbon–halogen bond activation
The interesting results reported for dioxygen activation with
related guanidine copper complexes14a,d encouraged us to study the
reactivity of the cationic copper(I) complex 5 towards dioxygen.
Thereby, the bulkiness of the TLtBu ligand might enforce end-on
coordination of dioxygen and prevent the formation of dimeric
species in a similar fashion as recently described with a sterically
congested tetradentate guanidine ◦ligand.14a,d However, oxygena-
tion experiments in acetone at −78 C employing [(TLtBu)Cu]SbF6,
[5]SbF6, prepared by reaction of TLtBu with [Cu(CH3CN)4]SbF6 in
acetonitrile, have not been conclusive. The UV/vis profile of the
reaction suggested that no stable Cu/O2 adduct had formed and
that decomposition occurs even at −78 ◦C.28
Fig. 1 Calculated geometries for the [TLtBuCu]+ cation 5, the acetonitrile
adduct 5·NCCH3 and the tert-butyl isocyanide complex 6a.
Exposure of an acetone solution of [5]PF6 to the air at room
temperature resulted in the formation of a green solution, from
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◦
˚
which deep-green crystals of the acetone solvate [7]PF6·C3H6O,
suitable for X-ray diffraction analysis, were isolated in high
yield (Scheme 4). The asymmetric unit contains two structurally
independent moieties, and the molecular structure of one cation
7 is shown in Fig. 2, revealing that a copper(II) complex
[(TLtBu)Cu(HCO3-jO)]PF6 with a monodentate bicarbonate lig-
and has formed. Presumably, the reaction proceeds via formation
of an intermediate Cu(II) hydroxo complex [(TLtBu)Cu(OH)]PF6,
which further reacts with CO2 from the atmosphere to give the
bicarbonate complex [7]PF6. The strong activity of the hydroxo
intermediate in CO2 fixation can be ascribed to the strong
electron-donating property of the ligand TLtBu, enhancing the
nucleophilicity of the coordinated hydroxide, as previously noted
for related systems.29 Several other examples of CO2 fixation and
activation by Cu(II) complexes have been reported,30 although the
major body of related work deals with Zn(II) complexes, which
serve as functional models for carbonic anhydrase, an enzyme
Table 1 Selected bond lengths (A) and angles ( ) for [7]PF6, [8]CF3SO3
and [8]CuCl2
[7]PF6·C3H6O
[8]CF3SO3
[8]CuCl2
1.9364(11)
Cu–N1
Cu–N2
Cu–N3
Cu–X
C8–N2
C11–N3
1.926(2)/1.934(3)
1.957(2)/1.952(3)
1.972(2)/1.962(2)
1.906(3)/1.926(3)
1.343(3)/1.342(4)
1.353(4)/1.348(4)
1.951(3)
1.968(3)
1.987(3)
2.2207(12)
1.355(5)
1.343(5)
81.14(13)
80.76(13)
167.57(9)
101.17(10)
98.56(10)
1.9505(12)
1.9652(14)
2.2339(4)
1.3458(17)
1.3479(18)
81.44(5)
N1–Cu–N2 81.50(10)/81.32(10)
N1–Cu–N3
N1–Cu–X
N2–Cu–X
N3–Cu–X
81.60(9)/81.54(10)
170.81(9)/168.67(11)
96.77(10)/98.09(10)
99.42(9)/99.25(11)
80.82(5))
176.30(3)
99.12(3)
98.83(3)
ligand adopts a perpendicular conformation, so that the structure
of both cations can be regarded as being essentially Cs-symmetric
with each plane containing the respective bicarbonate ligand, the
copper atom and the pyridine nitrogen atom.
−
31
that catalyzes hydration of CO2 and dehydration of HCO3
.
Structurally characterized complexes containing monodentate
bicarbonate ligands are rare,32a and we could only identify five
structures of this type containing copper(II),32 which, except for
˚
one Cu–bis(phenanthroline) system with d(Cu–O) = 1.998(11) A,
˚
reveal elongated Cu–O distances in the range 2.211 to 2.591 A
because of Jahn–Teller distortion. In comparison, [7]PF6 exhibits
˚
significantly shorter distances with Cu1–O1 = 1.906(3) A and
˚
Cu2–O4 = 1.926(3) A. The angles in the bicarbonate units are close
◦
˚
to 120 , and the distances are as follows: C30–O1 = 1.255(4) A,
˚
˚
C30–O2 = 1.233(5) A, C30–O3 = 1.335(5) A (cation 1); C60–O4 =
˚
˚
˚
1.259(4) A, C60–O5 = 1.242(5) A, C60–O6 = 1.332(5) A (cation
2). These pronounced differences clearly indicate that the hydrogen
−
atoms of the HCO3 units are bound to O3 and O6, respectively,
and this could be confirmed by free refinement of these hydrogen
positions. Another pertinent feature of this structure is the
molecular packing, which involves inversion-symmetric dimers of
both independent cations in the asymmetric unit via hydrogen
bonding of the bicarbonate groups, as observed for some other
transition metal bicarbonate complexes (Fig. 2).33 Short O3–
Fig. 2 ORTEP drawing of one of the two independent hydrogen-bridged
dimers of the [(TLtBu)Cu(HCO3-jO)]+ cation 7 in [7]PF6·C3H6O with
thermal displacement parameters drawn at 50% probability, methyl groups
have been omitted for clarity. The lower half of the molecule is generated
by inversion.
˚
˚
H · · · O2 [1.63(5) A] and O6–H · · · O5 contacts [1.72(7) A] are
observed, and the two hydroxy groups point almost linearly
towards the oxygen atoms with O–H · · · O angles of 176(5) and
172(6)◦, which indicates the presence of strong hydrogen bonds.34
Another hint of the high reactivity of the cationic copper(I)
complex 5 arises from its reactivity in chlorinated solvents. For
instance, mixing [Cu(MeCN)4]PF6 and TLtBu in dichloromethane
or chloroform results in an immediate color change from colorless
to deep green, and from both solvents the copper(II) complex
[(TLtBu)CuCl]PF6, [8]PF6, can be isolated almost quantitatively
in pure form (Scheme 4). Similar reactivity towards CHCl3 and
CH2Cl2 has been observed for other copper(I) complexes contain-
ing electron-rich N-donor ligands.17b,35 It had been assumed that
the dechlorination process is radical in nature;36 more recently
published studies on carbon–halogen bond activation, however,
described a reaction mechanism involving (a) an oxidative addition
of the copper(I) complex to the C–X bond of a substrate to
afford a copper(III) organometallic intermediate, followed by (b) a
bimolecular reaction of the intermediate to give the C–C coupled
dimer and the copper(II)–halide complex as products.37 Ligand
effects on the redox reactivity of copper(I) complexes are thus
Both structurally independent complex cations [(TLtBu)Cu-
(HCO3-jO)]+ in [7]PF6·C3H6O display slightly distorted square-
planar environments around the copper center, and the sums
of the four cis angles are 360.2 and 359.3◦, respectively. The
N–Cu–N angles of about 81◦ are significantly smaller than the
N–Cu–O angles of about 99◦ (Table 1). Both cations exhibit a
slight butterfly-like deformation with the copper atoms showing
˚
deviations of 0.06 and 0.05 A from the planes defined by N1,
N2 and N3 and by N8, N9 and N10, respectively, whereas
the metal-bound oxygen atoms O1 and O4 of the bicarbonate
ligands are significantly more displaced by 0.187 and as much
˚
as 0.48 A, which is presumably a consequence of minimizing the
steric interaction with the bulky di-tert-butylimidazolin-2-ylidene
moieties. Accordingly, the N1–Cu1–O1 and N8–Cu2–O4 angles
of 170.81(9)◦ and 168.67(11)◦ deviate significantly from linearity.
With respect to the copper coordination sphere, each bicarbonate
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of great importance, and ligands with stronger electron-donor
abilities are more likely to support a high oxidation state proposed
for the copper(III) organometallic intermediate, which results in an
increased reactivity of the respective copper(I) complexes towards
halide substrates. Accordingly, we conclude that this feature
also explains the pronounced reactivity of the TLtBu copper(I)
complexes presented here.
ideally perpendicular imidazole moieties with respect to the
square copper coordination plane (dihedral angles = 89.6 and
87.8◦). This orientation rules out the possibility of p-interaction
between the exocyclic nitrogen atoms and the imidazole rings,
˚
leading to elongated C–N distances [N2–C8 = 1.355(5) A and
˚
N3–C11 = 1.343(5) A], much longer than expected for a N–
2
˚
C(sp ) double bond (1.28 A) and almost in the range of a
2
38
˚
Single crystals of [8]PF6 could be obtained by diffusion of diethyl
ether into a CH2Cl2 solution, the structure, however, could not
N–C(sp ) single bond (1.38 A). In comparison, structurally
characterized 2-iminoimidazolines such as the building block
1,3-di-tert-butylimidazolin-2-imine (3) exhibit much shorter C–N
−
be satisfactorily refined because of a severely disordered PF6
9
˚
counterion. Therefore, the corresponding triflate [8]CF3SO3 was
prepared and characterized by X-ray diffraction analysis, and the
resulting molecular structure of the cation 8 is shown in Fig. 3.
The asymmetric unit consists of a [(TLtBu)CuCl]+ cation and a
bonds [d(C–N) = 1.295(2) A in 3]. Consequently, the imidazole
heterocycles in [8]CF3SO3 also show consistent differences from
the structure of 3, for which significantly longer internal C–N
˚
bonds [1.390(2) and 1.395(2) A] together with a smaller internal
CF3SO3 anion. The copper atom is four-coordinate with three
N–C–N angle [105.3(1)◦] were observed. This indicates an increase
of imidazolium character upon coordination of the imine nitrogen
atom, since the structural comparison of imidazolium salts with
their corresponding free imidazolin-2-ylidenes reveals the same
trend with shortening of the C–N bonds and widening of the
N–C–N angles.39 It should be noted that these considerations are
also valid for all other structurally characterized copper complexes
discussed in this paper.
−
nitrogen donor atoms provided by the chelating ligand. The fourth
ligand is a chloride ion covalently bound to the Cu(II) center, and
this must have been supplied by reductive dechlorination of the
CH2Cl2 solvent (vide supra). The geometry around the copper
center can be regarded as slightly distorted square-planar, and the
sum of the four cis angles is 361.63◦, with the N–Cu–N angles
of 81.14(13) and 80.76(13)◦ being significantly smaller than the
N–Cu–Cl angles of 101.17 (10) and 98.56(10)◦. Compared to
the structural parameters determined for the two independent
cations in [7]PF6, the cation in [8]CF3SO3 exhibits a considerably
more pronounced butterfly-like deformation with the copper and
Reactions of TLtBu with copper(I) halides
Over the past few years, copper(I) halide complexes with nitrogen-
based ligands have attracted much attention because of their
catalytic use in atom transfer radical polymerization (ATRP).19,20
Therefore, we decided to study in detail the reactivity of TLtBu
towards copper(I) bromide and chloride. The reaction of TLtBu
with copper(I) bromide or with the more reactive dimethylsulfide
copper(I) bromide, [(Me2S)CuBr],40 in THF afforded a yellow
solution, from which a yellow powdery material could be isolated
˚
chlorine atoms showing deviations of 0.19 and 0.88 A from the
plane defined by the nitrogen donor atoms N1, N2 and N3.
Accordingly, a comparatively small N1–Cu–Cl angle of 167.57(9)◦
is observed (Table 1).
1
by precipitation with n-hexane. Elemental analysis and H and
13C NMR spectroscopy pointed to the formation of a symmetric
copper(I) complex of the type [(TLtBu)CuBr] (9a). For instance, the
resonances for the pyridine ring protons (7.31–7.45 ppm) and for
the bridging CH2 protons (4.94 ppm) in the product are shifted to
higher field compared to the signals of the free ligand. However,
cooling of an acetone-d6 solution of 9a results in broadening of sig-
nals, suggesting the formation of a compound with an asymmetric
coordination sphere. Since the coalescence temperature could not
1
be reached in acetone-d6, the low-temperature H NMR spectra
Fig. 3 ORTEP drawing of the [(TLtBu)CuCl]+ cation 8 in [8]PF6 with
thermal displacement parameters drawn at 50% probability.
were additionally recorded in THF-d8 exhibiting the coalescence
◦
temperature at −97 C and giving the fully resolved spectrum at
−112 ◦C (Fig. 4). It can be seen that the singlet belonging to the
bridging methylene protons (d) separates into two peaks (dꢀ). The
same behaviour is observed for the singlet from the imidazoline CH
hydrogen atoms (c) and the doublet from the pyridine ring protons
(b), clearly indicating that one ligand arm remains uncoordinated
and that the rapid intramolecular rearrangement can be stopped
at low temperature on the NMR timescale. Line-shape analysis
was used to estimate the barrier of rearrangement, and it is found
to be approximately 8 kcal mol−1 (or 33 kJ mol−1).41
The copper–nitrogen distances in [8]CF3SO3 are Cu–N1 =
˚
˚
˚
1.951(3) A, Cu–N2 = 1.968(3) A and Cu–N3 = 1.987(3) A.
Whereas the Cu–N1 distance between copper and the pyridine-
nitrogen atom falls in the expected range, the Cu–N(imine)
˚
bond lengths are appreciably shorter (approximately 0.1 A) than
those observed in analogous copper(II) complexes containing
related pyridine-2,6-diimine ligands.17 This shortening is concomi-
tant with a lengthening of the copper–chlorine distance [Cu–
˚
Cl = 2.2207(12) A] and reflects the strong electron-releasing
Further support stems from DFT calculations, which for 9a
confirm the presence of a [(TLtBu-jN1:jN2)CuBr] structure in the
gas phase (see ESI† for further details). In addition, the molecular
structure of the acetone solvate 9a·C3H6O could be established
by means of X-ray diffraction analysis, revealing the presence of
capability of the TLtBu ligand, which has been proposed based
on a strong contribution of the ylidic resonance structure B
shown in Scheme 2. Charge separation in coordinated TLtBu
can also be clearly deduced from the observation of almost
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Fig. 4 Variable-temperature 1H NMR study and fluxional behaviour of
[(TLtBu-jN1:jN2)CuBr] (9a) (400 MHz, THF-d8).
a CuBr unit coordinated in a chelating fashion by the pyridine
and the imine nitrogen atom N2 (Fig. 5). As a result, copper
attains coordination number 3 and a trigonal-planar geometry
with the sum of the angles around the copper atom of 359.91◦.
Scheme 5
˚
The Cu–Br distance of 2.2746(5) A and the Cu–N distances of
˚
˚
Cu–N1 = 2.0855(19) A and Cu–N2 = 1.9630(19) A fall in the
expected ranges for these types of bonds. The acetone molecule
forms a C–H · · · O1 hydrogen bond to one of the imidazolium
moieties with an angle of 162.6◦ and an O · · · H distance of
precipitation from the THF filtrate with n-hexane. Formation of 9b
was confirmed by means of 1H and 13C NMR spectroscopy and by
elemental analysis, which is in good agreement with the formula
[(TLtBu-jN1:jN2)CuCl]. As described for the CuBr complex 9a,
low-temperature NMR studies and DFT calculations (see ESI†)
also support the presence of a fluxional tricoordinate structure for
9b in solution with the TLtBu ligand coordinated in a j2-fashion.
Numerous attempts to grow crystals of 9b suitable for the X-ray
diffraction analysis were unsuccessful and led to decomposition
and formation of the deep-green precipitate mentioned above.
By NMR, this material proved to be paramagnetic, and its
elemental analysis is in perfect agreement with the formula
[(TLtBu)Cu2Cl3], suggesting the presence of a mixed-valence copper
system. The result of an X-ray diffraction analysis of crystals
grown from dichloromethane solution confirmed this composition
and revealed the formation of [(TLtBu)CuCl][CuCl2], [8]CuCl2,
containing the copper(II) cation [(TLtBu)CuCl]+ (8) and the
dichlorocuprate(I) anion (Fig. 6). The cationic part of [8]CuCl2
exhibits the same four-coordinate square-planar geometry as
described for the cation in [8]CF3SO3. Comparison of the two
structures reveals that the cation in [8]CuCl2 displays a less
pronounced butterfly-like deformation with the copper and chlo-
˚
2.37 A, which is significantly shorter than the sum of the van
42
˚
˚
˚
der Waals radii of 2.72 A [rvdW(H) = 1.20 A, rvdW(O) = 1.52 A].
Although tricoordinate copper complexes are well established,43
only a limited number of such low-coordinate copper halides
with N-donor ligands have been structurally characterized, and
in particular, copper(I) bromide structures showing an N2CuBr
environment as observed in 9a are exceptionally rare.44,45 Again,
the structure of 9a is a clear manifestation of the inability of
the TLtBu ligand to support a tetrahedral copper(I) environment
on steric grounds, as also discussed for the copper(I) isocyanide
complexes [6]PF6 (vide supra).
˚
rine atoms showing deviations of 0.08 and 0.31 A from the
Fig. 5 ORTEP drawing of [(TLtBu-jN1:jN2)CuBr]·C3H6O (9a·C3H6O)
showing an intermolecular non-covalent contact with thermal displace-
˚
ment parameters drawn at 50% probability. Selected bond distances [A] and
angles [◦]: Cu–N1 2.0855(19), Cu–N2 1.9630(19), Cu–Br 2.2746(5), C8–N2
1.336(3), C11–N3 1.289(3), N1–Cu–N2 83.71(8), N1–Cu–Br 134.21(5),
N2–Cu–Br 141.99(6).
The reaction of TLtBu with copper(I) chloride in THF at ambient
temperature resulted in the initial formation of a yellow solution
followed by progressive formation of a deep-green precipitate
together with the deposition of copper(0) on the wall of the
Schlenk flask (Scheme 5). The deep-green precipitate was isolated
by filtration, and the copper(I) complex 9b was isolated by
Fig. 6 ORTEP drawing of [(TLtBu)CuCl][CuCl2], [8]CuCl2, with thermal
displacement parameters drawn at 50% probability. Only one of the two
independent, inversion-symmetric anions is shown.
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plane defined by the nitrogen donor atoms N1, N2 and N3.
Consequently, a larger N1–Cu–Cl angle of 176.30(3)◦ is observed,
and the sum of the four cis angles around the copper atom is almost
ideal (360.21◦). All three copper–nitrogen distances in [8]CuCl2
blocks for the synthesis of multidentate poly(imidazolin-2-imine)
ligands with exceptional basic and nucleophilic properties, which
should exceed those of related guanidine-type systems.9 As a
result, copper(I) complexes of the pincer ligand 2,6-bis[(1,3-di-
tert-butylimidazolin-2-imino)methyl]pyridine, TLtBu, are highly
reactive and exhibit a pronounced tendency to form stable,
square-planar copper(II) complexes of the type [(TLtBu-j3)CuX]
allowing effective aerobic CO2 fixation (X = bicarbonate, HCO3-
jO), C–Cl bond activation and Cu(I) disproportionation (X =
Cl). These results point toward the possibility of employing this
TLtBu/copper system in oxidation catalysis and in atom transfer
radical polymerization (ATRP), and such studies are underway
in our laboratories. Thereby, the steric and electronic properties
of the poly(imidazolin-2-imine) ligand could be easily tuned by
introducing various available 2-iminoimidazolines9 or by using
different bridging moieties.
˚
˚
[Cu–N1 = 1.9364(11) A, Cu–N2 = 1.9505(12) A, Cu–N3 =
˚
1.9652(14) A] are shorter than those in [8]CF3SO3, associated with
˚
a slightly elongated copper–chloride bond [Cu–Cl = 2.2339(4) A]
(Table 1).
As described for [8]CF3SO3, the imidazole rings in [8]CuCl2 also
adopt an almost perfectly perpendicular arrangement relative to
the square-planar copper coordination sphere (dihedral angles =
89.98 and 88.03◦) together with significantly elongated N2–C8 and
˚
N3–C11 distances of 1.3458(17) and 1.3479(18) A. The asymmet-
ric unit contains two halves of two independent dichlorocuprate
[CuCl2]− moieties with the copper atoms located on inversion
centres. As a consequence, ideally linear orientations with Cl–Cu–
Cl angles of 180◦ are observed, and the Cu–Cl distances in two
˚
independent anions are 2.0954(6) and 2.1027(5) A, respectively,
consistent with previously observed values.46
Experimental
The mixed-valence copper(I/II) complex [8]CuCl2 is obviously
a product of the partial disproportionation of the initially formed
copper(I) complex [(TLtBu-jN1:jN2)CuCl] (9b). Thus, three equiv-
alents of 9b lead to the formation of [8]CuCl2 and elemental copper
in a 1 : 1 ratio, implying that two thirds of the copper(I) atoms
disproportionate to give Cu(II) and Cu(0) with the remaining
All operations were performed in an atmosphere of dry argon
using Schlenk and vacuum techniques. All solvents were purified
by standard methods and distilled prior to use. 1H and 13C NMR
spectra were recorded on JEOL-GX 400 (400 MHz), JEOL-
GX 270 (270 MHz), Bruker AC 200, Bruker DPX 200, Bruker
AV 300 and Bruker DPX 400 devices. The chemical shifts are
given in ppm relative to TMS. The spin coupling patterns are
indicated as s (singlet), d (doublet), t (triplet), q (quadruplet),
m (multiplet), sept. (septet) and br. (broad, for unresolved
signals). Elemental analysis (C, H, N) by combustion and gas
chromatography was carried out with a Vario EL III CHNS, Carlo
Erba Mod. 1106 and a Vario Micro Cube. Mass spectrometry
was performed with a Finnigan MAT 90 and high-resolution
electrospray ionization (ESI) Finnigan MAT 95 XL Trap devices.
The IR spectra were recorded on Bruker Vertex 70 and UV-Vis
spectra on Varian Cary 50 devices. Copper(I) chloride, copper(I)
bromide and 2,6-pyridinedimethanol were obtained from Acros,
and Cu(MeCN)4PF6 was obtained from Strem chemicals and used
as received. 2,6-Bis[(tosyloxy)methyl]pyridine21 and 1,3-di-tert-
butylimidazolin-2-imine9 were prepared according to literature
procedures.
−
Cu(I) contained in the CuCl2 anion. This reaction must be
accompanied by release of two equivalents of the free ligand, which
has been confirmed by isolation of TLtBu from the hexane mother
liquor after complete separation of 9b and [8]CuCl2. Presumably,
the instability of 9b is a consequence of both the electronic
and structural properties of TLtBu, which strongly supports the
higher oxidation state of copper because of its strong electron-
releasing capability and its rigid and planar arrangement of the
nitrogen donor atoms. The propensity of copper(I) complexes
to undergo disproportionation is well established,43,47 and is
strongly dependant on the auxiliary ligands35d,48 and solvents.49
Many examples of mixed-valence copper complexes are known
in the literature, but they are mainly obtained either by partial
oxidation of copper(I), partial reduction of copper(II) or by using
redox active ligand.50 Few other examples of cationic Cu(II)
complexes containing the dichlorocuprate(I) counterion have been
described,51 however, we are not aware of any case where such
mixed-valence systems have been accessed by disproportionation
as described here.
[(TLtBuH2)(OTs)2], 4
We propose that the different reactivity of the complexes [(TLtBu
-
A solution of the 2,6-bis[(tosyloxy)methyl]pyridine (4.471 g,
10 mmol) in THF (70 mL) was treated dropwise with 1,3-di-tert-
butylimidazolin-2-imine (3) (3.965 g, 20.3 mmol) in THF (30 mL)
at ambient temperature, and the resulting reaction mixture was
subsequently heated in boiling THF for 24 h. The precipitate was
filtered off, washed with THF (2 × 20 mL) and dried in high
vacuum. The product was isolated as a pale pink solid (4.7 g, 53%).
A subsequent reduction of the volume of the filtrate and further
refluxing for 24 h afforded the second portion of the product
isolated as described above (cumulative yield: 7.3 g, 87%). Found:
C, 61.45; H, 7.23; N, 11.63; S, 7.48. Calc. for C43H63N7O6S2: C,
61.62; H, 7.58; N, 11.70; S, 7.65%; dH (400 MHz, D2O; Me4Si)
7.91 (1 H, t, p-Py), 7.64 (4 H, d), 7.49 (2 H, d, m-Py), 7.40 (4 H, s,
NCH), 7.31 (4 H, d), 4.05 (4 H, s, Py–CH2), 2.35 (6 H, s), 1.63 (36
H, s, CCH3); dC(100.53 MHz, D2O; Me4Si) 156.1, 143.4, 142.5,
jN1:jN2)CuX] (X = Br, 9a; X = Cl, 9b) obtained from the
reactions of TLtBu with copper(I) bromide or chloride, respectively,
could be a consequence of the soft acid behavior of copper(I).44
Accordingly, the chloride anion is considered to be a weaker ligand
for copper(I) than the bromide anion and therefore imposes a
stronger driving force towards disproportionation and formation
of copper(II) species as observed for 9b. In contrast, no such
reactivity is observed for 9a.
Conclusions
With this contribution, we have shown that 2-iminoimidazolines
such as 1,3-di-tert-butylimidazolin-2-imine (3) can be conveniently
derivatized by N-alkylation and are therefore suitable building
2818 | Dalton Trans., 2007, 2812–2822
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139.9, 139.1, 129.6, 125.8, 121.3, 117.4, 61.5, 53.7, 29.1, 20.4; m/z
[(TLtBu-jN1:jN2)Cu(2,6-o-XyNC)]PF6, [6b]PF6
(FAB+) 665 (M+–OTs, 54%), 494.4 (M+–H(OTs)2, 100).
A solution of TLtBu (198 mg, 0.4 mmol) in THF (5 mL) was
added to a solution of Cu(CH3CN)4PF6 (149 mg, 0.4 mmol) in
THF (5 mL) at ambient temperature. Some minutes later 2,6-
dimethylphenyl isocyanide (52.5 mg, 0.40 mmol) was added and
the mixture was stirred for 1 h. Evaporation of the solvent and
washing with n-hexane (2 × 5 mL) afforded the product, which
was isolated as a pale yellow solid (297 mg, 89%). Found: C, 54.85;
H, 6.52; N, 13.10. Calc. for C38H56N8CuPF6: C, 54.76; H, 6.77; N,
13.44; dH (400 MHz, acetone-d6; Me4Si) 8.01 (1 H, t, p-Py), 7.64 (2
H, d, m-Py), 7.41-7.24 (3 H, m, Xy–H), 6.85 (4 H, s, NCH), 5.01
(4 H, s, Py–CH2), 2.36 (6 H, s, Ar–CH3), 1.65 (36 H, s, CCH3); dC
(100.61 MHz, acetone-d6; Me4Si) 164.0 (o-C), 152.9 (NCN), 140.6
(p-C), 136.7, (Xy-o-C) 131.6 (Xy-p-C), 130.1 (Xy-m-C), 122.2 (m-
C), 111.3 (NCH), 61.6 (C-Py), 58.9 (NCMe), 31.2 (CNCtBu), 19.7
(Xyl-Me); mmax(KBr)/cm−1 2140 (C≡N).
2,6-Bis[(1,3-di-tert-butylimidazolin-2-imino)methyl]pyridine, TLtBu
A suspension of 4 (7.25 g, 8.65 mmol) in THF (120 mL) was
cooled to 0 ◦C, and KOtBu (2.329 g, 20.76 mmol) was added.
The reaction mixture was stirred at ambient temperature for 2 h,
filtered and the solvent was removed in vacuo. The product was
extracted with a mixture of toluene (50 mL) and n-hexane (50 mL)
and filtered. Removal of the solvent afforded the product as an
off-white solid (4.1 g, 95%). Found: C, 70.56; H, 9.55; N, 19.83.
Calc. for C29H47N7: C, 70.55; H, 9.59; N, 19.86; dH(400 MHz, C6D6;
Me4Si) 8.02 (2 H, d, m-Py), 7.72 (1 H, t, p-Py), 6.02 (4 H, s, NCH),
5.38 (4 H, s, Py–CH2), 1.43 (36 H, s, CCH3); dC (100.53 MHz, C6D6;
Me4Si) 164.1 (o-C), 146.5 (NCN), 136.2 (p-C), 118.7 (m-C), 108.4
(NCH), 58.1 (C–Py), 54.4 (NCMe), 29.5 (CCH3); dH(400 MHz,
acetone-d6; Me4Si) 7.69 (1 H, t, p-Py), 7.55 (2 H, d, m-Py), 6.35
(4 H, s, NCH), 4.85 (4 H, s, Py–CH2), 1.56 (36 H, s, CCH3); dH
(400 MHz, CD3CN, Me4Si) 7.67 (1 H, t, p-Py), 7.48 (2 H, d, m-Py),
6.33 (4 H, s, NCH), 4.79 (4 H, s, Py–CH2), 1.53 (36 H, s, CCH3);
m/z (CI) 493 (M+, 36%), 437 (M+–tBu, 100), 380 (M+–2tBu, 58).
[(TLtBu)Cu(HCO3-jO)]PF6, [7]PF6
A solution of [5]PF6 (50 mg, 0.0712 mmol) in acetone (3 mL)
was exposed to the air at room temperature and immediately
changed color from light yellow to deep-green. Slow evaporation
of the solvent over a period of two days afforded deep-green
crystals, which were isolated, washed with diethyl ether (2 × 5 mL)
and dried in vacuo (46 mg, 85%). Found: C, 47.49; H, 6.03; N,
12.87. Calc. for C30H48N7CuPF6O3: C, 47.21; H, 6.34; N, 12.85;
kmax(acetone)/nm 371 (e/dm3 mol−1 cm−1 1764) and 622 (286);
mmax(KBr)/cm−1 3209, 2983, 1618, 1580, 1488, 1448, 1425, 1371,
1340, 1299, 1234, 1195, 1107, 1084, 1052, 965, 835, 786, 703 cm−1.
[(TLtBu)Cu]PF6, [5]PF6
A solution of TLtBu (198 mg, 0.4 mmol) in CH3CN (5 mL) was
added to the solution of Cu(CH3CN)4PF6 (149 mg, 0.4 mmol)
in CH3CN (5 mL) at ambient temperature. The reaction mixture
turned yellow upon addition of the ligand. After stirring for 45 min
at room temperature, the solvent was removed under vacuum to
give a yellow solid material (263 mg, 94%). Found: C, 49.49; H,
7.34; N, 13.58. Calc. for C29H47N7CuPF6: C, 49.60; H, 6.75; N,
13.96; dH(400 MHz, acetone-d6; Me4Si) 7.98 (1 H, t, p-Py), 7.73
(2 H, d, m-Py), 6.56 (4 H, s, NCH), 5.01 (4 H, s, Py–CH2), 1.44
(36 H, s, CCH3); dC (100.53 MHz, acetone-d6; Me4Si) 164.2 (o-C),
151.2 (NCN), 139.5 (p-C), 122.6 (m-C), 112.1 (NCH), 61.2 (C–
Py), 57.6 (NCMe), 31.0 (CCH3); dH( 400 MHz, CD3CN, Me4Si)
7.82 (1 H, t, p-Py), 7.46 (2 H, d, m-Py), 6.69 (4 H, s, NCH), 4.77
(4 H, s, Py–CH2), 1.58 (36 H, s, CCH3).
[(TLtBu)CuCl]PF6, [8]PF6
A solution of TLtBu (278 mg, 0.563 mmol) in CH2Cl2 (10 mL)
was added dropwise to a solution of Cu(CH3CN)4PF6 (200 mg,
0.537 mmol) in CH2Cl2 (10 mL) at ambient temperature, the
reaction mixture turned deep green. After 3 h stirring, solvent
volume was reduced to 5 mL and the product was precipitated
with addition of n-hexane (20 mL). Filtration, washing with n-
hexane (2 × 10 mL) and drying in vacuo afforded the product
as a deep-green solid (370 mg, 94%). Found: C, 47.01; H, 6.36;
N, 13.16. Calc. for C29H47N7CuClPF6: C, 47.22; H, 6.42; N, 13.29;
kmax(acetone)/nm 371 (e/dm3 mol−1 cm−1 1815) and 627 (271); m/z
(FAB+) 592.7 (M+–PF6, 82%), 493.9 (TLtBu+, 6).
[(TLtBu-jN1:jN2)Cu(tBuNC)]PF6, [6a]PF6
A solution of TLtBu (198 mg, 0.4 mmol) in THF (5 mL) was
added to a solution of Cu(CH3CN)4PF6 (149 mg, 0.4 mmol) in
THF (5 mL) at ambient temperature. Some minutes later tBuNC
(47 lL, 0.41 mmol) was added and the mixture was stirred for
1 h. Evaporation of the solvent and washing with n-hexane (2 ×
5 mL) afforded the product, which was isolated as a pale yellow
solid (286 mg, 91%). Found: C, 51.56; H, 7.00; N, 13.94. Calc.
for C34H56N8CuPF6: C, 52.00; H, 7.19; N, 14.27; dH (400 MHz,
acetone-d6; Me4Si) 7.96 (1 H, t, p-Py), 7.59 (2 H, d, m-Py), 6.85
(4 H, s, NCH), 4.91 (4 H, s, Py–CH2), 1.65 (36 H, s, CCH3),
1.48 (9 H, s, tBuNC); dC (100.53 MHz, acetone-d6; Me4Si) 164.1
(o-C), 152.8 (NCN), 140.4 (p-C), 137.0 (CNCtBu), 122.1 (m-C),
113.2 (NCH), 61.5 (C-Py), 59.1 (CNCtBu), 58.8 (NCMe), 31.27
(CNCtBu), 31.23 (CCH3) ppm, mmax(KBr)/cm−1 2179 (C≡N); m/z
(FAB+) 639.8 (M+–PF6, 3%), 556.8 (M+–tBuNC–PF6, 3), 493.9
(TLtBu+, 10).
[(TLtBu-jN1:jN2)CuBr], 9a
A solution of TLtBu (262 mg, 0.53 mmol) in THF (10 mL)
was added dropwise to a suspension of [(Me2S)CuBr] (103 mg,
0.50 mmol) in THF (5 mL) at 0 ◦C. During the addition of
the ligand the reaction mixture turned yellow and the starting
material dissolved. The solution was allowed to warm to room
temperature over 15 min, the volume of THF was reduced to
5 mL and the product was precipitated with n-hexane (40 mL).
Filtration, washing with n-hexane (2 × 10 mL) and drying in vacuo
afforded the product as a yellow solid (286 mg, 90%). Found: C,
54.79; H, 7.61; N, 14.72. Calc. for C29H47N7CuBr: C, 54.66; H,
7.43; N, 15.39; dH (200.13 MHz, C6D6; Me4Si) 7.45-7.31 (3 H, m),
6.25 (4 H, s, NCH), 4.94 (4 H, s, Py–CH2), 1.43 (36 H, s, CCH3);
dC (50.32 MHz, C6D6; Me4Si) 162.8 (o-C), 150.7 (NCN), 136.8
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Table 2 Crystallographic data for [7]PF6, [8]CF3SO3, [8]CuCl2 and 9a
[7]PF6 ·C3H6O
[8]CF3SO3
[8]CuCl2
9a·C3H6O
Empirical formula
FW
C33H54CuN7O4F6P
821.35
C30H47ClCuF3N7O3S
741.82
17.8950(4)
8.7368(2)
24.0711(6)
90
105.9805(9)
90
3617.96(15)
173
C29H47Cl3Cu2N7
727.19
14.9281(12)
10.1533(8)
22.6904(16)
90
98.363(3)
90
3402.6(5)
133
C32H53BrCuN7O
695.26
˚
a/A
9.3457(1)
16.0642(1)
28.0336(2)
89.1268(3)
81.5051(3)
74.4997(5)
4009.85(6)
173
10.3778(14)
10.8771(14)
15.850(2)
78.792(3)
76.017(3)
87.240(3)
1703.0(4)
133
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
T/K
qcalc/g cm−1
1.361
1.362
1.419
1.356
Crystal system
Space group
Z
Triclinic
P(−1)
Monoclinic
Cc
Monoclinic
P21/n
Triclinic
P(−1)
4
4
4
2
l/mm−1
0.656
2.94–50.76
62895
14679
12242
0.040
0.0509
0.1281
1.04
0.791
3.52–50.72
9718
6063
5530
0.045
0.0479
0.0957
1.026
1.516
3.08–61.08
66907
10401
8936
0.022
0.0365
0.0781
1.046
1.850
2.70–61.02
37076
10339
6676
0.053
0.0404
0.0930
1.011
2h range/◦
Total reflections
Unique data
‘Observed’ data {I > 2r(I)}
Rint
R1 (‘observed’ data)
wR2 (all data)
GoF
No. of parameters
Res. electr. dens./e A
987
−0.59/0.69
428
−0.346/0.609
407
−0.327/0.66
393
−0.395/0.548
−3
˚
w = 1/[r2(Fo2) + (aP)2 + bP], where P = (Fo + 2 Fc2)/3, and a and b are constants set by the program.
2
(p-C), 119.8 (m-C), 110.6 (NCH), 59.0 (C-Py), 56.6 (NCMe), 29.9
(CCH3).
Single crystal X-ray structure determination of compounds [7]PF6,
[8]CF3SO3, [8]CuCl2 and 9a
Crystal data and details of the structure determinations are
presented in Table 2. Crystals of the compounds [7]PF6 and
[8]CF3SO3 were transferred to a Lindemann capillary. Mea-
surements were carried out at 143 K (Oxford Cryosystems) on
an area detector system (NONIUS, MACH3, j-CCD) at the
window of a rotating anode (NONIUS, FR591) with graphite
[(TLtBu-jN1:jN2)CuCl], 9b and [(TLtBu)CuCl][CuCl2], [8]CuCl2
A solution of TLtBu (262 mg, 0.53 mmol) in THF (10 mL) was
added dropwise to a suspension of CuCl (50 mg, 0.50 mmol) in
THF (10 mL) at ambient temperature. During the addition of the
ligand the reaction mixture turned yellow and the starting material
dissolved. The mixture was stirred for 20 h at ambient temperature,
during which time a formation of a green precipitate, accompanied
by a mirror of elemental copper on the flask wall, was observed.
Filtration, washing of the precipitate with THF (2 × 5 mL) and
drying in vacuo afforded the compound [8]CuCl2 as a deep green
solid. The volume of the pale yellow filtrate was reduced to 5 mL
and the compound 9b was precipitated with addition of n-hexane
(30 mL). Filtration, washing with n-hexane (2 × 10 mL) and drying
in vacuo afforded the compound 9b as a pale yellow solid.
˚
monochromated Mo-Karadiation (k = 0.71073 A). Raw data were
corrected for latent decay and absorption effects. The structures
were solved by a combination of direct methods and difference
Fourier syntheses. The structures were refined on F2 using the
program SHELXL-97 (G. M. Sheldrick, University of Go¨ttingen,
Germany). All non-hydrogen atoms were refined with anisotropic
displacement parameters. Hydrogen atoms were included using
a riding model or with rigid methyl groups allowed to rotate
but not tip, exceptions were the freely refined hydrogens of
hydrogencarbonate in compound [7]PF6 or disordered methyls
(see below). Exceptions/special features: For [7]PF6, the tert-
butyl methyl groups at the atoms C18 and C48 were disordered
over two positions, which were refined ideally staggered with
49.9/50.1% occupancy and 83.9/16.1% occupancy, respectively,
and appropriate similarity restraints. For [8]CF3SO3, the structure
was refined as a racemic twin with a scale factor of 0.465(12).
Crystals of the compounds [8]CuCl2 and 9a were mounted
on glass fibers and measured on a Bruker SMART 1000 CCD
[(TLtBu-jN1:jN2)CuCl], 9b
(115 mg, 39%). Found: C, 58.41; H, 8.03; N, 16.08. Calc. for
C29H47N7CuCl: C, 58.76; H, 7.99; N, 16.54; dH(200.13 MHz, C6D6;
Me4Si) 7.51 (1 H, t, p-Py), 7.30 (2 H, d, m-Py), 6.09 (4 H, s, NCH),
5.19 (4 H, s, Py–CH2), 1.48 (36 H, s, CCH3); dC (50.32 MHz, C6D6;
Me4Si) 162.7 (o-C), 150.5 (NCN), 136.8 (p-C), 119.6 (m-C), 110.6
(NCH), 58.6 (C-Py), 56.5 (NCMe), 31.2 (CCH3).
˚
diffractometer at 133 K (Mo-Ka radiation, k = 0.71073 A).
[(TLtBu)CuCl][CuCl2], [8]CuCl2
Absorption corrections were performed using multi-scans
(program SADABS). Hydrogen atoms were included using rigid
methyl groups allowed to rotate but not tip, or a riding model.
Exceptions/special features: For [8]CuCl2, the tert-butyl methyl
(65 mg, 36%, based on Cu). Found: C, 47.80; H, 6.51; N, 13.48.
Calc. for C29H47N7Cu2Cl3: C, 47.80; H, 6.52; N, 13.30; m/z (ESI)
593.5 (M+–CuCl2, 91%).
2820 | Dalton Trans., 2007, 2812–2822
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groups at the atoms C22 and C26 were disordered over two
positions, which were refined ideally staggered with 75.8/24.2%
occupancy and 74.4/25.6% occupancy, respectively. One improba-
bly short intramolecular H · · · H contact for the minor component
at C26 may be attributable to a non-ideally staggered geometry;
dimensions of disordered groups should in any case be interpreted
with caution. For the disordered structures, a series of similarity
restraints were used to improve stability of refinement. For the
same reason, [8]CF3SO3 was additionally subjected to restraints
of displacement parameters.
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CCDC reference numbers 638693–638696.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b703183a
Acknowledgements
This work was financially supported by the Deutsche Forschungs-
gemeinschaft (DFG Ta 189-6/1). D. P. thanks the Bavarian Science
Foundation (Bayerische Forschungsstiftung) for a scholarship
(2003–2006). We would like to thank Prof. Dr William B. Tolman
and his group for helpful discussions.
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