Syntheses of CuI polymetallic assemblies from reaction of ligands bearing the 2,5-bis(2-pyridyl)phosphole fragment with CuII precursors

Article abstract of DOI:10.1039/c3cc42955e

Upon reaction with ligands A, 1 and 3 bearing the 2,5-bis(2-pyridyl) phosphole fragment, an unexpected conversion of Cu^II metal centers to Cu^I centers is observed affording either bimetallic complexes bearing a bridging phosphane coordination mode or hexametallic metallacycles. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc42955e

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Syntheses of CuI polymetallic assemblies from reaction
of ligands bearing the 2,5-bis(2-pyridyl)phosphole
fragment with CuII precursors†
Cite this: Chem. Commun., 2013,
49, 6158
Received 21st April 2013,
Accepted 21st May 2013
´
´
Mehdi El Sayed Moussa, Fabian Friess, Wenting Shen, Muriel Hissler, Regis Reau
and Christophe Lescop*
DOI: 10.1039/c3cc42955e
www.rsc.org/chemcomm
Upon reaction with ligands A, 1 and 3 bearing the 2,5-bis(2-pyridyl)- coordination mode adopted by ligand A. This study was extended
phosphole fragment, an unexpected conversion of CuII metal centers to also to the reaction of CuII metal centers with the heterotritopic N,X,N
CuI centers is observed affording either bimetallic complexes bearing a ligands 1–3 (Fig. 1: 1, X = S; 2, X = O, 3, X = Se) easily synthesized from
bridging phosphane coordination mode or hexametallic metallacycles. ligand A. Compared to ligand A, these new multitopic ligands present
alternative coordination geometries and modes4 with a central donor
Electron transfer processes in CuI/CuII complexes involve significant chalcogenide atom bearing modulated Lewis basicity that should
internal reorganisation energies due to the geometry constraints impact the reactivity of the CuII precursors.
imposed by the specific coordination spheres of the CuI and the CuII
The room temperature (RT) stoichiometric reaction5 of ligand A
metal centers (respectively, mostly four-coordinate and five- or six- with Cu(OTf )2 (OTf ꢀ: CF3SO3ꢀ) in CH2Cl2 afforded a dark green
coordinate).1 Hence the relative stability of CuI and CuII complexes solution that changed within a few minutes to a light orange clear
can be greatly modulated by the specific features of the ligands solution. After purification upon precipitation, an orange powder of
reacting with the CuI/CuII metal precursors, arousing interest in complex 4 was collected. Despite the introduction of paramagnetic
the investigation of the reaction of these metal centers with new CuII metal centers in the reaction, the RT 31P{1H} NMR spectrum
multitopic ligands. The bis(2-pyridyl)phosphole ligand A2 (Fig. 1) of 4 in CD2Cl2 shows two broad doublets at +17.9 and ꢀ3.5 ppm
1
recently emerged as a powerful 6-electron donor N,P,N-chelate for (JPP = 94.2 Hz). The H NMR spectrum of 4 in CD2Cl2 presents a
the stabilization of a series of CuI bimetallic complexes3a due to the single set of broad signals6 with chemical shifts comparable to those
formation of a very rare bridging phosphane coordination mode B.3 recorded for the free ligand A, bearing therefore no significant
These results motivated us to perform the reaction of divalent CuII chemical shift differences that could witness the formation of
metal centers with ligand A to investigate: (i) the stability of the CuII paramagnetic CuII complexes. These data clearly suggest that ligand
complex compared to the CuI derivative based on ligand A; (ii) the A reacts with the CuII metal centers to afford a new diamagnetic
impact of the introduction of a divalent metal center on the complex 4. However, these multinuclear spectroscopic data do not
allow a structure to be proposed and dark-red single crystals for the
X-ray diffraction study were grown in a 39% yield by diffusion of
pentane in a CH2Cl2 solution of 4.
Complex 4 is a monocationic CuI-dimer featuring two bis(2-
pyridyl)phosphole ligands A, one acting as a m-1kN:1,2kP:2kN
bridging phosphane donor and one acting as a 1kN:2kP P,N chelate
(Fig. 2). The coordination sphere of the metal ions is completed by
two oxygen atoms of a OTf ꢀ counter-anion acting as a 1kO:2kO
chelate (Cu–O distances, 2.143(5) Å and 2.331(5) Å), while another
non-coordinated OTf ꢀ counter-anion is present in the asymmetric
Fig. 1 Structure of the derivatives A, 1–3 and bridging phosphane coordination
mode B.
unit together with three CH2Cl2 solvent molecules. Metric para-
meters of the (CuI)2(A)2 core in the derivative 4, including the metal–
´
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS - Universite de Rennes
1, 35042 Rennes Cedex, France. E-mail: christophe.lescop@univ-rennes1.fr;
Fax: +33 2-23-23-69-39; Tel: +33 2-23-23-50-02
metal (2.5228(12) Å, revealing a metallophilic interaction between
the metal centers that is among the shortest recorded to date for a
CuI2(A)2 core)3 and metal–mP distances (2.3115(19) Å and 2.341(2) Å)
are similar to those measured for the corresponding dicationic CuI
dimers resulting from the reaction of ligand A with CuI salts.3 Note
that this solid state structure is in agreement with the solution RT
† Electronic supplementary information (ESI) available: Preparation of 3, 4, 5, 50
and 6, spectroscopic and X-ray diffraction data. CCDC 927129–927123. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3cc42955e
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Fig. 2 Views of the crystal structure of the monocationic derivative 4 (H atoms,
counter-anions and included solvant molecules have been omitted).
Fig. 3 (a) View of the X-ray crystal structure of the metallacycle 5 (H atoms,
counter-anions and included molecules have been omitted), (b) view highlight-
ing the m-1kN:1,2kS:2kN coordination mode of the ligand 1 in 5 (the remaining
atoms in the coordination sphere of the CuI atom owing to neighbouring ligands
are shown in white), (c) ‘front’ and ‘side’ views of the Cu6S6 ring observed in 5.
31P{1H} NMR spectrum as the signal at ꢀ3.5 ppm is typical of ligand
A acting as an N,mP,N ligand on a CuI dimer, while the signal at
+17.9 ppm is assigned to ligand A acting as a P,N chelate on the CuI
dimer.3a Therefore, along this reaction, a CuI bimetallic complex is
formed despite the fact that a CuII metal precursor was initially used.
A redox reaction between the CuII metal center and ligand A is not the other molecules included in the unit cells. The building unit of
thermodynamically favored since the redox potential of ligand A derivative 5 is based on two distorted tetrahedral CuI metal centers
(+0.83 V vs. ferrocene/ferrocenium)2 is high compared to the redox tethered by one ligand 1 acting as an 8-electron m-1kN:1,2kS:2kN
potential usually described for the CuII/CuI couple.7 In the first stage donor (Fig. 3b). Each of the metal centers is coordinated to two
of the reaction, the dark green color observed is most probably different ligands 1 affording a metallacycle structure bearing a Cu6S6
because of a preliminary coordination of ligand A to the CuII metal ring (Fig. 3c). The dihedral angles between the coordinated pyridine
center affording a transient CuII complex that may take part in the and the phosphole moieties range from 35.21 to 62.91 and are
formal redox process. Hence, this CuII complex could be the electron markedly larger than those observed in the CuI bimetallic complexes
source that reduces the free CuII center to afford CuI ions leading to bearing ligand A in a bridging phosphane coordination mode (ca.
very stable bimetallic complexes3 bearing bridging phosphane coor- 301). This demonstrates the high ability of the 2,5-bis(2-pyridyl)phosp-
dination mode B with ligand A.8 This assumption has motivated us hole fragment to adapt its conformation in order to fit the geometric
to slightly modify the core of ligand A by oxidation of its P atom with constraints fixed by the metal ions and by the overall molecular
oxygen, sulfur and selenium, affording the heterotritopic N,X,N geometry. The PQS, the Cu–N and the Cu–S bond lengths (ca. 1.99,
ligands (Fig. 1: 1, X = S; 2, X = O, 3, X = Se) whose coordination 2.01 and 2.35 Å, respectively), as well as the (S–Cu–S) and (N–Cu–N)
chemistry has been less comparatively studied4 and for which the angles (ca. 1191 and 1011 respectively), are standard.2b,3a,4 Conversely,
formation of bimetallic CuI complexes bearing bridging phosphane the S atoms present an original distorted T-shape geometry, with
coordination mode B (Fig. 1) is excluded.
acute (P–S–Cu) angles (ranging from 93.34(8)1 to 100.02(8)1) and
Derivatives 1–3 were obtained in high yields by the oxidation of the obtuse (Cu–S–Cu) angles (from 140.72(7)1 to 162.51(10)1). The S
P-centre of ligand A with elemental sulfur,2b NaIO42b and elemental atoms are located out of the plane defined by the P and Cu atoms
selenium respectively. In the first step, ligand 1 reacted in a 1 : 1 ratio (from 0.21 to 0.63 Å, Fig. 3b). The Cu6S6 ring is non-planar (Fig. 3c,
in CH2Cl2 at room temperature with Cu(OTf )2 affording a dark red maximal deviation from the mean plane: 5a, 0.54 Å; 5b, 0.55 Å) and
clear solution of complex 5.5 In the case of this reaction also, despite presents a distorted ‘6-pointed star’ shape (Fig. 3c) with opposite edge
the introduction of paramagnetic CuII metal centers in the reaction, to edge Cu–Cu and S–S distances ranging from 8.72 Å to 9.31 Å and
derivative 5 in CD2Cl2 exhibits a broad singlet at +50.1 ppm in its from 6.68 Å to 7.68 Å respectively. Using Cu(BF4)2ꢁxH2O instead of
31P{1H} NMR spectrum (shifted to low frequencies relatively to the Cu(OTf )2, a soluble dark red material 50 having multinuclear NMR
signal of free ligand 1, a sharp singlet at d = +52.5 ppm in CD2Cl2). data that compare with those of 5 was obtained. Therefore the
This suggests that coordination of the ligand 1 on metal centers synthesis of the macrocyclic structure 5 is not dependent on the
proceeded during the reaction without a significant modification of templating effect of the counter anion associated with the CuII metal
the P-center hybridization. The 1H NMR spectrum presents one single center. A few examples of high nuclearity (CuI)n metallacycles (n = 6, 8)
set of broad signals comparable to those of the free derivative 1.
based on bidentate bulky organophosphorus ligands have been
Upon diffusion of pentane vapors into a solution of 5 in CH2Cl2, previously described, but the metal source is always a CuI metal salt.9
single crystals suitable for the X-ray diffraction study having two The unexpected formation of this new CuI macrocycle triggered us to
different morphologies (5a, dark red prisms; 5b, light red plates) were study the impact of the variation of Lewis basicity of the heteroatom
obtained in a comparable amount and in a 27% overall yield. Both grafted on the P-atom of the bis(2-pyridyl)phosphole moiety on the
crystal structure resolutions revealed that 5 is a [Cu6(1)6](OTf )6 CuI reaction’s fate, by reacting ligands 2 and 3 with Cu(OTf )2. The
hexametallic derivative. Unit cells contain in addition two reaction of the oxophosphole ligand 2 with a stoichiometric amount
[Cu(H2O)4(OTf )2] complexes, four CH2Cl2 and six H2O molecules in of Cu(OTf )2 at room temperature in CH2Cl2 did not induce any color
5a, and three CH2Cl2 molecules in 5b. In crystals 5a,b, [Cu6(1)6]6+ units change in the clear solution obtained. The 31P{1H} NMR spectrum
are metallacycles (Fig. 3a) having very similar geometrical parameters shows a sharp singlet at +42.5 ppm at the same chemical shift as
and do not present short contacts with the OTf ꢀ counter anions and recorded for the free ligand 2 in CH2Cl2 demonstrating that this
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by oxidation–reduction processes between the free ligands and
the metal centers. This point is supported by the fact that
ligands 1–3 have been reported to present very similar oxidation
potentials that are markedly higher than those of the related
phosphole derivative.2b Furthermore, as hypothesized for the
formation of complex 4, the pre-coordination of the CuII metal
center in ligands 1 and 3 is probably necessary to afford
transient CuII complexes that can reduce the free CuII metal
centers supplying CuI ions in the reaction media.8 The effect of
the Lewis basicity of the chalcogen atoms carried by the P-atoms
in 1–3 supports the hypothesis that the pre-coordination of the
CuII centers plays a key role in these reactions. In the case of the
thioxophosphole ligand 1 and the selenophosphole ligand 3,
the central chalcogen atoms being soft Lewis donors can fit the
geometrical constraints imposed by the metal ions and the
overall molecular geometry, driving the first step of the reaction
to the formation of a macrocyclic structure bearing distorted
tetrahedral CuI metal centers. This is supported by the observa-
tion that the chalcogen atoms in 5 or 6 tolerate significant bond
lengths and angle differences within the overall similar macro-
cycle architecture. Conversely, the central oxygen atom of the
oxophosphole ligand 2 is a hard Lewis donor that can probably
not tolerate such structural constraints and does not afford
related stable complexes.
Fig. 4 (a) View of the X-ray crystal structure of the metallacycle 6 (H atoms,
counter-anions and included molecules have been omitted), (b) view highlight-
ing the m-1kN:1,2kSe:2kN coordination mode of the ligand 3 in 6 (the remaining
atoms in the coordination sphere of the CuI atom owing to neighbouring ligands
are shown in white), (c) ‘front’ and ‘side’ views of the Cu6Se6 ring observed in 6.
ligand did not react with Cu(OTf )2 (vide infra). Conversely, as observed
in the case of the synthesis of the derivative 5, the stoichiometric
reaction at room temperature of ligand 3 with Cu(OTf )2 in CH2Cl2
solution resulted instantaneously in a dark red clear solution afford-
ing after purification the derivative 6 as a dark red powder.5 The
derivative 6 is characterized at room temperature in CD2Cl2 by a broad
singlet at +34.4 ppm, shifted to high frequencies relatively to the
signal of the free ligand 3 (d = +40.4 ppm in CD2Cl2) and by a single
set of broad signals in the 1H NMR spectrum that compares well with
those of the free derivative 3. Similarly to what was observed in the
case of the derivative 5, single crystals having different morphologies
(6a, dark red blocks 6b, light red plates) were grown in a moderate
yield from the diffusion of pentane vapors into a CH2Cl2 solution of 6.
In both cases, the crystal structure resolution revealed that derivative 6
is a novel [Cu6(3)6](OTf )6 macrocycle having gross geometric para-
meters closely related to those of the thioxophosphole based macro-
cycle 5 (unit cell of 6a is completed by two [Cu(H2O)5(OTf )](OTf )
complexes, six CH2Cl2 and two H2O molecules while the unit cell of
6b is completed by eight CH2Cl2 and two pentane molecules).
In the derivative 6, ligand 3 acts as an 8-electron m-1kN:1,2kSe:2kN
donor on two CuI metal centers. Therefore, the basic units in 5 and 6
are structurally similar. Nevertheless, the PX and Cu–X (P = Se,
ca. 2.14 Å; Cu–Se, ca. 2.50 Å) bonds are, as expected, longer. The
These results highlight how the subtle balance of the stability of
the CuII and CuI oxidation states in the coordination chemistry of
multitopic ligands bearing different heteroatoms can drive the
formation of complex and original molecular architectures. We
are currently investigating the mechanism of the formal redox
processes occurring along the syntheses of the discrete polymetallic
assemblies 4–6 together with the coordination chemistry of ligands
A and 1–3 with other divalent metal centers.
Notes and references
1 (a) B. Rorabacher, Chem. Rev., 2004, 104, 651; (b) F. Durola and
J.-P. Sauvage, Angew. Chem., Int. Ed., 2007, 46, 3537; (c) S. Durot,
F. Reviriego and J.-P. Sauvage, Dalton Trans., 2010, 39, 10557.
´
2 (a) D. Le Vilain, C. Hay, V. Deborde, L. Toupet and R. Reau, Chem.
Commun., 1999, 345; (b) C. Hay, M. Hissler, C. Fischmeister, J. Rault-
´
Berthelot, L. Toupet, L. Nyulaszi and R. Reau, Chem.–Eur. J., 2001, 7, 4222.
´
T-shape geometry of the chalcogen atoms in 6 is less distorted than in 3 (a) B. Nohra, E. Rodriguez-Sanz, C. Lescop and R. Reau, Chem.–Eur. J.,
´
2008, 14, 3391; (b) Y. Yao, W. Shen, B. Nohra, C. Lescop and R. Reau,
Chem.–Eur. J., 2010, 16, 7143; (c) S. Welsch, C. Lescop, G. Balazs,
R. Reau and M. Scheer, Chem.–Eur. J., 2011, 17, 9130.
5: the Se atoms are kept out of the plane made of the Cu and the
P atoms (from 0.01 Å to 0.78 Å), the (P–Se–Cu) angles (from 87.08(7)1
´
to 95.91(5)1) are markedly smaller while the (Cu–Se–Cu) angles are 4 The chemistry of organophosphorus compounds. Phosphine oxides,
sulphides, selenides and tellurides, ed. F. R. Hartley, John Wiley &
Sons, Chichester, 1992, vol. 2.
5 Modification of the ligand/metal salt ratio afforded the same derivative
similar (from 139.52(4)1 to 175.99(3)1) to those observed in the
derivative 5. Within these macrocycles, a non-planar Cu6Se6 ring
(Fig. 4c, maximal deviation from the mean plane: 6a, 0.51 Å; 6b,
0.52 Å) is formed. This ring is markedly distorted compared to
the ‘6-pointed star’ shaped Cu6S6 ring in 5a,b with opposite
edge to edge Cu–Cu and Se–Se distances ranging from 8.82 Å to
10.29 Å and from 6.70 Å to 9.23 Å respectively. Therefore, the
reaction of the selenoxophosphole N,Se,N ligand 3 affords a
hexametallic CuI macrocycle that is similar to the derivative 5
obtained from the thioxophosphole N,S,N ligand 1.
in a lower yield and non-reacted precursors in excess.
6 Hemilabile behaviour for ligand A has been encountered in bimetallic
CuI-complexes. As a result, 1H NMR spectra bearing broad signals are
observed, with a pseudo-symmetry that is not indicative of the real
molecular symmetry.
7 F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, John
Wiley & Sons, New York, 1988, 5th edn.
8 This assumption is supported by the isolation of a few crystals of a
tricationic monometallic complex based on a copper ion, see part S3
in the ESI† for details.
9 (a) E. D. Blue, T. B. Gunnoe and N. R. Brooks, Angew. Chem., Int. Ed.,
2002, 461, 2571; (b) D. Suresh, M. S. Balakrishna and J. T. Mague,
Dalton Trans., 2008, 3272.
The difference in reactivity of the ligands 1–3 towards a CuII
metal center clearly suggests that these reactions are not driven
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6160 Chem. Commun., 2013, 49, 6158--6160
This journal is The Royal Society of Chemistry 2013