Polymetallic Cu(I) complexes based on bridging phosphine ligands

Chapter 2

Polymetallic Cu(I) complexes based on bridging phosphine ligands Christophe Lescop Univ Rennes, INSA Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes)
UMR 6226, Rennes, France

2.1 Introduction The fluxional coordination behavior of donor ligands in the coordination sphere of monometallic and polymetallic complexes is an important feature in coordination chemistry as it confers many of these derivatives’ specific reactivity and/or stability. Such ligand mobility and the dynamics of the related processes in di-, tri- and polynuclear complexes as well as within bulk solid-state materials, surfaces, or nanomaterials has impacted the development of important areas of study in fundamental and applied research, being crucial in manifold processes such as catalytic reactions, surface modification, and multifunctional optoelectronic properties [1]. Many important families of ligands, such as CO, CNR, and NO, exhibit such fluxional behavior that allows stabilizing a large variety of metal centers and clusters. In particular, interchanges between terminal, semibridging, and symmetrically bridging coordination modes have been reported for these ligands, highlighting the substantial importance of such processes in the stability, properties, and reactivity of the resulting polymetallic assemblies [1c,2]. Therefore, the possibility offered by a specific ligand to present a bridging coordination mode yields polymetallic complexes with many specific and/or valuable characteristics. Importantly, tertiary phosphorus ligands (PR3), while being some of the most widely used donor ligands in coordination chemistry, have been long considered to act exclusively as terminal ligands, precluding their potential involvement in fluxional processes. This phenomenon was assigned to the fact that PR3 ligands are a priori not appropriate to occupy the bridging position between two metal centers and therefore remain as terminal ligands. Copper(I) Chemistry of Phosphines, Functionalized Phosphines and Phosphorus Heterocycles. DOI: https://doi.org/10.1016/B978-0-12-815052-8.00002-6 © 2019 Elsevier Inc. All rights reserved.

21

22

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

However, a series of results have shown how this original postulate is a misperception, revealing more especially the occurrence of bridging phosphanebased ligands to stabilize bimetallic species in which a continuum between bridging and semibridging coordination modes has been observed. In a first step, semibridging phosphane ligands have sporadically appeared in the literature since the early 1990s [3], but this bonding mode was always enforced by an agostic interaction involving a P-H or P-Csp2 moiety and appropriate metal orbitals (complexes A and B, Fig. 2.1) [3]. Finally, after the first speculation on the existence of a bridging phosphane coordination mode [4], breakthroughs were achieved in the early 2000s with the isolation of the first stable complexes bearing bridging phosphane ligands, PR3. Indeed, two different synthetic routes were simultaneously reported for the preparation of dinuclear phosphane-bridged rhodium (I) or palladium(I) complexes. In the first route (Fig. 2.2, route 1), the key step was the formation of a dinuclear rhodium complex with a stibane ligand in a bridging mode that could be subsequently substituted by corresponding phosphane ligands [5]. In the second route (Fig. 2.2, route 2), the key factor was the use of N,P,N phosphole
pyridine ligands having a pincer geometry that eventually results in dinuclear complexes bearing bischelates [6]. In these complexes, the central phosphorus atom was able to coordinate with both the metal centers exhibiting a bridging coordination mode. Starting from these seminary reports, both synthetic routes have subsequently been developed and have given access to a new series of polymetallic complexes stabilized by bridging phosphane ligands. In particular, the second route based on mixed phosphole
pyridine ligands has given access to an unprecedented number of stable Cu(I) polymetallic assemblies bearing bridging phosphanes, details of which are presented in this chapter. 2+ +

Ra Ra

P

L

Pd

Ph

H

Pd P

A

L

Ph2P

Pd

Pd

PPh2

P Rb

Rb

P

Ph

B Ra = tBu, Rb = CH3 , Ph, iPr, L = HPtBu 2

FIGURE 2.1 Examples of complexes bearing semibridging phosphane coordination modes involving a P-H or P-Csp agostic interaction.

Polymetallic Cu(I) complexes Chapter | 2 Route 2

Route 1

Ra

Ra

23

Ra

Sb L

P

Rh L

Rh

X

X

N

Rb

N Pd

Cl Cl a

Ra

Ra

b

Ra P

L

Rh X

Rh L X N

R a = CH3, Ph, iPr X = CPh 2 L = acac, AcO, Cl, Br

Rb Pd

P N Pd N

N P Rb

R b = Ph, Cy

FIGURE 2.2 Preparation of the first described bimetallic complexes of phosphanes bearing symmetrically bridging coordination modes; (a) P(Ra)3, benzene [5]; (b) H2 (20 bar), HC (OCH3)3, MeOH/CH2Cl2, 2NaSbF6 [6].

In the first part of this chapter, a discussion will be devoted to the representation of the three-center two-electron bonds present in transition metal complexes involving phosphanes exhibiting bridging coordination modes. Then, the first examples of polymetallic complexes bearing bridging phosphane ligands will be described. Finally, genuine polymetallic Cu(I) complexes bearing bridging phosphane ligand coordination modes will be exhaustively presented, together with a description of the original compact supramolecular assemblies that were obtained using several of these bimetallic preorganized Cu(I) complexes as very versatile molecular clips for selective coordination-driven supramolecular syntheses.

2.2 The representation of the bridging coordination mode in phosphane complexes The three-center two-electron interaction is frequently observed in molecules bearing multicentered bonding. In order to establish a simple and convenient

24

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

X

µ-Z

µ-X

µ-L

Z

X

X

X

Z

X

Z

Z

or X

X

Z

FIGURE 2.3 Green and Parkin classification of three-center two-electron interactions.

representation of such interaction, Green and Parkin have suggested a classification [7] based on an extension to the classical Covalent Bond Classification that allows description of the different classes of two-center two-electron interactions [8]. In the latter case, ligands are sorted regarding the number of their electrons taking part in their bonding with a metal center (L ligands for two electrons, X ligands for one electron and Z ligands for zero electrons). According to the representation suggested by Green and Parkin, and introducing a “half-arrow” notation, three-center two-electron interactions can be sorted into three classes, dependent on whether the bridging atom contribution to the bonding involves none (μ-Z), one (μ-X) or two electrons (μ-L) (Fig. 2.3). This representation offers a simple and convenient procedure based on electron counting to depict the electronic structure of a given molecule presenting a three-center two-electron interaction. However, in each specific compound, the understanding of the bonding nature deserves an in-depth analysis using sophisticated theoretical methods in order to establish a clear and precise picture of the electronic nature of these multicentered interactions. Regarding this representation, the μ-Z and μ-X bridging atoms are most often encountered in three-center two-electron interactions, for instance in bridging hydride complexes, dihydrogen complexes, or agostic derivatives [2c]. In comparison, the μ-L bridging atoms are much less common. The bridging phosphane ligands range in this class, contributing generally with a little acceptor character to the three-center two-electron interaction.

2.3 The first descriptions of symmetrically bridging phosphine ligands In 2000, a breakthrough in phosphane ligand coordination ligands was achieved with the quasisimultaneous first descriptions of stable bimetallic Rh

Polymetallic Cu(I) complexes Chapter | 2

25

(I) and Pd(I) complexes bearing bridging phosphane ligands. Indeed, tertiary phosphine PR3, while being one of the most widely used donor ligands in coordination chemistry, was previously reported as acting exclusively as terminal L-ligands in low-valent metal complexes [2]. Up to now, an overwhelming number of coordination complexes with an immense structural variety has been reported, with the phosphane ligands behaving as terminal ligands. However, as early as in 1989, Braunstein [4] suggested that terminal phosphane ligands could adopt a transient doubly bridging bonding mode in order to explain the reactivity of several polymetallic complexes bearing terminal phosphane ligands and therefore should not be regarded as thermodynamically unfavorable [9].

2.3.1 Tertiary phosphanes as bridging ligands in Rh(I) organometallic bimetallic complexes The first example of a symmetrically bridging phosphane in a dirhodium(I) complex 1 (Figs. 2.2 and 2.4) containing bridging triisopropylstibane ligand was reported by Werner et al. [5]. Using this dinuclear rhodium complex, Werner showed that it was possible to introduce by substitution of this stibane ligand a tertiary phosphine ligand PR3 while preserving the bridging coordination mode. According to this approach, Werner reported the preparation of a large series of phosphane- (and arsane-) bridged dinuclear rhodium complexes [A1Rh(μ-CR2)(m-EX3)RhA2] in which A1, A2 5 Cl, κ2-acac; R 5 aryl; E 5 P, As; X 5 Me, Ph, iPr [10]. Notably, depending on the set of A1, A2 terminal ligands, R substituents on the supporting bridging carbenes and the X substituents, almost symmetrically bridging ligands or semibridging binding modes were observed in the solid state, regardless of the gross symmetry of the whole complex. Using quantum chemistry, Werner, Kaupp et al. highlighted a number of factors that favor the occurrence of bridging or semibridging phosphane ligands in such Rh(I) dimers and their putative group 9 Co(I) and Ir(I) analogs [11]. It was thus revealed that a symmetrical bridging mode depends Me Me Me

Me Me

P

O

Rh C Ph

Ph

Me

O Me Ph

Ph

Rh

Cl

C

Me Me

P

O

Rh

O Me

Me

C Ph

Ph

Me Cl

Rh

Cl

C Ph Ph

Me P Rh C

Ph

Ph

Rh

Cl

C Ph Ph

FIGURE 2.4 Examples of dinuclear and tetranuclear Rh(I) complexes featuring bridging phosphane ligands, as reported by Werner et al.

26

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

greatly on the π-accepting ability of the bridging EX3 ligand and the electronegativity supplied by the X substituents, the more electronegative favoring a bridging mode. Werner reported the structure of a tetranuclear Rh(I) complex with bridging phosphane ligands, in which two dimeric Rh(I) units are symmetrically bridged by phosphane ligands and are connected by two μ-Cl ligands. This assembly presumably self-dissociates into dimers in solution and equilibrium is likely to occur between Rh4 and Rh2 species, revealing the high fluxionality of this type of multiple bridged polymetallic species. This result opened a new perspective for the formation of chain-like multimetallic assemblies based on bridging phosphane ligands bearing unusual structural and electronic configurations.

2.3.2 Bridging phosphane ligands in Pd(I) bimetallic coordination complexes based on N,P,N chelates bearing a phosphole ring Almost simultaneously with the initial discovery by Werner, Reau et al. reported the dipalladium(I) complex containing bridging 2,5-bis(2-pyridyl)phosphole ligand 1 [12] (Fig. 2.5). Organophosphorus ligands are generally based on a phosphole ring belonging to the family of the σ3,λ3-phosphane ligands [13]. In the phosphole heterocycle (Fig. 2.5), the phosphorus atom does not readily form sp hybrids and mainly employs “p” electrons for bonding. This leads to a pyramidal geometry of the tricoordinate phosphorus atom and a pronounced “s” 2+, 2 X–

R3

.. N

P

R2 N

R4

..

P R1

P

N R5

M1 N

Ph Ph P

N M2 N

1 2a,b 2a : M1 = M2 = Pd(I) 2b : M1 = Pd(I), M2 = Pt(I) X– = PF6–, SbF6–, OTf – FIGURE 2.5 Molecular structure of the ligand 1 and the complexes 2a,b; general molecular structure of the phosphole ring.

Polymetallic Cu(I) complexes Chapter | 2

27

character of the lone pair. Such geometric and electronic features prevent efficient interaction of the lone pair on the phosphorus atom with the endocyclic diene system. Therefore, phospholes present a weak aromatic character, and a reactive heteroatom. As a result, the coordination chemistry of phosphole rings resembles those of typical two-electron donor tertiary phosphanes (PR3) and it has been extensively studied. In the specific case of the 2,5-bis(2-pyridyl)-phosphole ligand 1 [12], the central phosphole ring is substituted at the 2- and 5-positions by two 2-pyridyl moieties affording a potential six-electron donor N,P,N chelate ligand. The indication that this ligand can exhibit a bridging coordination mode in bimetallic complexes was obtained in the study of the catalytic activity of Pd(II) complexes bearing the 2,5-bis(2-pyridyl)phosphole ligand 1. The Pd(I) dimer 2a, having two identical Pd
P bond lengths, was isolated in this case (Fig. 2.5) [6]. Following this observation, a rational and direct synthesis of 2a was discovered with the reaction of ligand 1 with [Pd2(CH3CN)6][BF4]2 affording a good yield of complex 2a (Fig. 2.5) [6,14]. The X-ray crystal structure of complex 2a revealed that the Pd(I) dication contained two square-planar metal centers capped by two 2,5-bis(2-pyridyl)phospholes 1 performing as a six-electron (μ-1κN:1,2κP:2κN) donor. In addition, theoretical calculations have identified that Pd
Pd and Pd
P bonds in 2a to be highly delocalized with strong interactions in a system of four centers and six electrons. As a consequence, this theoretical description allows assigning of the μ-L interaction and hence the bridging coordination mode as per the classification of Green and Parkin [7]. Phosphole derivative 1, bridging symmetrically the two Pd(I) in 2 ˚ ), is only the second example showing an unprecedented (Δ(Pd
P) 5 0.01 A coordination mode, as reported by Reau et al. in 2001 [6]. Discoveries in the early 2000s that tertiary phosphanes (R3P) can act as bridging ligands revealed that these widely used ligands could no longer be considered as simple terminal ligands. The bridging coordination mode of phosphine/phosphanes can be taken into account to explain the reactivity of many polymetallic systems, such as clusters or nanoparticles. In this context, several studies have subsequently shown that PR3 migration between metal centers via bridging PR3 intermediates or transition states could be detected by dynamic NMR spectroscopies in iron, ruthenium, platinum, or osmium bimetallic complexes or clusters [15]. These studies also revealed that such PR3 migration takes place in the same way that CO migration occurs, but also that higher energy is required for such processes. These observations enabled rationalization of the reason for bridging phosphane ligands having been lately and rarely observed in coordination chemistry and, indeed, both in the case of Werner’s and Reau’s complexes, a templating effect (respectively, additional μ2-carbenes ligands or multiple N,P,N chelates) was found to be vital to induce the bridging coordination mode.

28

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

2.3.3 Toward an extension of the bridging phosphane coordination mode to other polymetallic systems? The characterization of complexes presenting a new bridging coordination mode for a family of phosphine ligands appeared to be a discovery of sufficient importance to justify more in-depth study of this original facet of coordination chemistry of ligand 1. Therefore, in order to confirm phosphanes definitively as versatile binucleating or bridging ligands, efforts have been made to extend the observation of such a coordination mode to a larger variety of stable complexes of various metal centers with different coordination spheres. The synthesis and characterization in the solid state of the first heterobimetallic complex Pd(I)/ Pt(I) 2b (Fig. 2.5) [16] bearing bridging phosphane ligands was first achieved by Reau et al., adapting a stepwise method that was established to study the synthesis and mechanism of 2a [14]. A sequential introduction of the metal centers was thus successfully done. X-ray diffraction studies of this complex revealed geometric parameters almost identical for 2a and 2b. Nevertheless, the geometry of the M2(μ2-P)2 core was markedly different in these two bimetallic assemblies. Indeed, in the heterobimetallic complex 2b each μ-P atom asymmetrically ˚ ). Thus, in spite of the binds the two metal centers (Δ(μP-M): 0.083(3) A multiple chelates formed by the N,P,N-ligand 1, the P atom can adopt either a symmetrical or nonsymmetrical coordination mode in structurally similar derivatives. This suggests that there is no substantial discontinuity between these two coordination modes for μ-PR3 ligands. DFT calculations have shown that very little energy is required for bridging a phosphorus center to move from a symmetrical to a substantially asymmetrical bridging position [16]. Tiny changes, such as the metal’s electronic requirements, the environment about the metal, and/or the crystal packing, can be invoked as the cause for a switch from a symmetrical to an asymmetrical bridging coordination mode in these complexes. These conclusions were substantially confirmed by extensive coordination chemistry developed by the Reau group with Cu(I) metal centers [and to a more limited extent with Ag(I) metal centers] and the 5-bis(2-pyridyl) phosphole ligand 1 or related mixed 2-pyridyl/phosphole-based ligands. Werner’s complexes or the Pd2 or Pd/Pt complexes described by Reau were revealed to have limited coordination chemistry since they generally did not react with a wide variety of ancillary ligands, or afforded new complexes in which the bridging coordination mode was not kept [10,14]. Conversely, ligand 1 was able to stabilize Cu(I) bimetallic complexes in which the coordination spheres of the metal centers were occupied by labile ligands that could be stepwise substituted by a variety of ancillary ligands preserving the polymetallic units bearing the bridging coordination mode [16
18]. This allowed conducting a very detailed experimental survey of the parameters,

29

Polymetallic Cu(I) complexes Chapter | 2

ruling out the stability of bridging phosphane polymetallic assemblies. These complexes also afforded preorganized bimetallic precursors bearing short metal
metal distances that were used as very versatile molecular clips in order to synthesize selectively a large variety of original compact supramolecular assemblies using the synthetic principles of the coordination-driven supramolecular chemistry.

2.4 Coordination chemistry of bridging phosphane ligands bearing a phosphole ring with the Cu(I) metal center 2.4.1 Bimetallic Cu(I) complexes bearing a bridging phosphane ligand based on the 2,5-bis(2-pyridyl)phosphole ligand 1 The studies of the coordination chemistry of ligand 1 with the Cu(I) ion appeared as an interesting alternative to the work conducted with Pd(I) and Pt(I) ions. Indeed, while these latter ions are known to have a square-planar coordination sphere in their complexes, the Cu(I) ions present very frequently a tetrahedral coordination sphere. It was therefore interesting to evaluate how the change of the coordination geometry of the metal centers could alter the coordination modes allowed by ligand 1. The extension of the study to the coordination chemistry of ligand 1 with the Cu(I) ion afforded a new family of bridging phosphine ligand complexes [16,17], confirming the ability of ligand 1 to stabilize bimetallic entities via this very unusual bridging phosphine coordination mode. In particular, complex 3 was obtained, where for the first time only one single bridging phosphine ligand was able to stabilize the bimetallic assembly (Fig. 2.6) via the formation of a six-electron μ
1κN:1,2κP:2κN chelate. In derivative 3, two tetrahedral Cu(I) metal centers share a metallophilic interaction 2+

L

Ph

CH 2Cl2 N

2+

Ph N

CH2Cl2

N

Ph

P

L

CH2Cl2

P

N Cu N

Ph

N

5 2+

PF6–

N Cu

N

N P

N

4 dppm

N Ph Cu Cu

N

Ph

Ph2P

N Cu

2+ 2 PF6–

dppm

P

N

CH2Cl2

PPh2 L

8

N Cu

2 PF 6– P

1

2 PF 6–

P

N

P

L = CH3CN

3

Ph

L

2

N

Cu

Ph

L

CH2Cl2

N Cu

P

N

N Cu

L L

2,2′ bipyridine

2 PF6–

1

P

N Cu

2+

2+

2 PF 6–

Cu

Ph

Ph2P

L

6

FIGURE 2.6 Structures of the Cu(I) bimetallic complexes 3
8.

N Cu

PPh2 Ph2P 7

PPh2

30

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

˚ ]. Very importantly, the coordination sphere of the [d(Cu
Cu) 5 2.568(1) A Cu(I) ions in 3 is completed by links with labile acetonitrile ligands that are easily exchangeable with a wide range of ligands (Fig. 2.6) allowing this complex 3 to be used as an unprecedented bimetallic precursor to access a novel family of bridging phosphine ligand complexes [17]. In contrast to the above-described bridging phosphine ligand complexes 2a and 2b (Fig. 2.4), complex 3 could be therefore used as a very versatile precursor to obtain a new family of Cu(I) bridging phosphine ligand complexes. Among these, complexes 4 and 5 were obtained by the stepwise reaction of derivative 3 with one or two equivalents of ligand 1 (Fig. 2.6) [17]. Likewise for complexes 6 and 7, for which one or two equivalents of diphenylphosphinomethane (dppm) were added to 3 (Fig. 2.6) [17]. Finally, the robustness of this bridging phosphane coordination mode for ligand 1 on a Cu(I) dimer was highlighted by the reaction between derivative 3 and 2,20 -bipyridine, which is known to form very stable mononuclear Cu(I) complexes [17]. A new complex 8 (Fig. 2.6) was characterized in which 2,20 -bipyridine substitutes the acetonitrile ligands of 3 without destroying the bimetallic core or modifying the bridging phosphine coordination mode presented by ligand 1. In this series of compounds, the diversity of the coordination chemistry allowed by ligand 1 is illustrated by the fact that it is coordinated according to two different coordination modes to the Cu(I) dimer in complexes 4 and 5 (bischelate N,P,N: ligand μ
1κN:1,2κP:2κN and chelate P,N: ligand μ
1κP:2κN). Moreover, the examination of the different pseudopolymorphic crystallographic structures of these derivatives (in several cases, the existence of different crystalline forms for the same complex was reported, enriching the collection of structural data available, Table 2.1) experimentally confirmed the existence of a continuum between the bridging phosphine and the semibridging phosphine coordination mode (Table 2.1). These observations are also supported by the fact that the solution NMR studies revealed a highly fluxional coordination behavior of the ligands present in these complexes (Fig. 2.7) [17]. In addition, it was proved that it is possible to tune the intermetallic distance depending on the nature of the ancillary ligands accompanying the bridging phosphane ligands in these Cu (I) dimers. It was also possible to react ligand 1 with another source of Cu(I), such as CuCl, and to obtain the neutral Cu4Cl4 derivative 9 in which two equivalents of ligand 1 act as a six-electron donor (μ
1κN:1,2κP:2κN) N,P,N chelate bearing symmetrical bridging phosphane ligand (Fig. 2.8) [17]. The nature of complex 3 (Fig. 2.6) and its ability to preorganize two Cu(I) ions bearing short metal
metal distances was also highlighted by its reaction with one equivalent of the tetrahedral ligand-complex [{CpMo(CO)2}2(μ,η2P2)] acting as a “P2” phosphorus-rich ligand-complex [18]. In this case, the

TABLE 2.1 Selected bond lengths (A˚), angles ( ), and torsion angle of the Cu2(Nμ-PN) moieties of coordination complexes and compact supramolecular assemblies bearing a bridging phosphane ligand.

3

Ref.

Cu-μP

Cu
N

Cu. . .Cu

N-Cu-μP

Cu-μP-Cu

μP-Cu-Cu

[16,17]

2.324(1)

2.040(4)

2.568(1)

85.29(1)

67.25(4)

56.18(4)

2.314(1)

2.041(5)

2.293(1)

2.048(4)

2.386(1)

2.043(4)

2.264(1)

2.148(5)

2.559(1)

2.055(5)

2.289(8)

2.057(2)

2.522(8)

2.061(2)

2.413(8)

2.077(2)

2.402(8)

2.062(2)

2.426(1)

2.106(2)

2.557(1)

2.073(3)

2.500(1)

2.110(3)

2.557(1)

2.097(3)

2.535(1)

2.084(3)

2.494(1)

2.094(3)

[16,17]

4

[16,17]

5

a

[17]

6

7aa

a,b

7b

[17]

[17]

85.55(1) 2.5552(8)

86.14(1)

56.57(4) 66.15(4)

82.85(1) 2.6204(9)

83.15(1)

58.68(3) 65.46(4)

78.22(1) 2.6696(7)

85.80(7)

81.39(6)

2.8329(9)

81.11(7)

67.19(2)

67.69(3)

55.95(2)

69.24(3)

53.20(3)

56.36(2)

79.56(7) 76.35(8)

57.57(2) 69.25(3)

79.27(8) 2.9501(6)

79.45(8) 81.07(9)

52.24(2) 60.56(2)

82.79(6)

2.8743(6)

51.84(4) 62.70(4)

82.78(7) 2.6822(9)

55.17(3)

54.45(2) 56.29(2)

71.83(3)

53.43(2) 54.74(2) (Continued )

TABLE 2.1 (Continued)

a,c

7c

Ref.

Cu-μP

Cu
N

Cu. . .Cu

N-Cu-μP

Cu-μP-Cu

μP-Cu-Cu

[17]

2.427(2)

2.075(6)

2.8785(14)

77.74(2)

71.46(6)

53.07(5)

2.501(2)

2.079(6)

2.252(2)

2.122(7)

2.728(3)

1.927(6)

2.668(2)

1.959(6)

2.277(2)

2.109(7)

2.254(9)

2.012(2)

2.269(8)

2.054(2)

2.285(2)

2.085(4)

2.299(2)

2.078(4)

2.273(2)

2.085(4)

2.326(2)

2.099(4)

2.2613(15)

2.028(5)

2.3770(14)

2.041(4)

2.3040(13)

1.968(3)

2.4672(11)

2.045(4)

8

[17]

9

[17]

[18]

10

d

15a

[22]

2.3098(12)

77.31(2) 2.9245(14)

80.2(2)

55.48(6) 71.20(7)

77.9(2) 2.8445(13)

77.7(2)

62.00(7) 69.72(7)

79.1(2) 2.5231(8)

86.15(6)

2.566(1)

85.33(12)

67.80(2)

55.81(2)

68.05(4)

54.63(4)

56.39(2)

84.92(12) 83.39(12)

56.59(4) 68.78(4)

84.43(13) 2.536(7)

83.19(11)

55.72(4) 56.23(4)

63.57(4)

86.09(14) 2.5892(7)

48.66(5) 61.62(6)

86.67(6)

2.598(1)

46.80(5)

57.06(4) 59.38(4)

83.73(10)

65.63(3)

54.15(3)

84.59(11)

67.01(3)

60.22(3)

2.6275(7)

54.03(3)

2.4453(11)

1.965(3)

84.08(10)

62.36(3)

58.96(3)

2.1978(12)

2.018(3)

86.01(11)

64.04(3)

48.76(3)

2.7266(11)

68.89(3)

2.1932(12)

2.006(4)

79.48(11)

48.63(3)

2.6966(11)

2.017(4)

80.19(9)

67.33(3)

16

[24]

[27a,b]

17

0

0

17 open

18

180 open0

19

[29]

[27a,b]

[29]

[27a,b]

20

[27a,b]

21

[27b]

e

22

f

23

[27b]

[27b]

2.2997(12)

2.044(4)

2.3265(12)

2.064(4)

2.3118(13)

2.096(4)

2.3815(13)

2.037(4)

2.2594(11)

2.043(3)

2.2891(11)

2.046(3)

2.2935(10)

2.052(3)

2.3858(11)

2.049(3)

2.3176(10)

2.048(3)

2.3659(10)

2.067(3)

2.3994(11)

2.077(3)

2.2871(11)

2.061(3)

2.440(3)

2.086(6)

2.315(3)

2.117(7)

2.2987(16)

2.040(4)

2.3932(14)

2.070(4)

2.347(2)

2.036(6)

2.291(2)

2.034(7)

2.3659(19)

2.032(6)

2.307(2)

2.029(6)

2.4045(19)

2.069(6)

2.281(2)

2.057(6)

2.5730(8)

85.65(10)

67.58(3)

86.42(11) 2.5622(10)

81.10(11)

56.71(3) 66.16(3)

84.46(11) 2.5566(8)

85.48(10)

84.24(9)

66.47(3)

83.70(8)

67.82(3)

83.87(9)

67.78(3)

85.90(18)

2.6019(12)

83.99(11)

68.21(3)

67.99(8)

53.79(6)

67.33(5)

54.60(4)

58.22(8)

85.93(15) 85.3(2)

58.07(4) 67.17(6)

85.32(19) 2.5947(12)

85.06(15)

84.38(15) 84.13(17)

55.39(5) 57.44(6)

67.44(6)

85.32(18) 2.6100(12)

53.87(3) 57.92(3)

85.9(2)

2.5659(13)

55.23(3) 56.99(3)

85.94(10) 2.661(2)

57.77(3) 54.41(2)

85.05(9) 2.6294(6)

55.18(3) 58.35(3)

84.44(9) 2.6117(11)

58.23(4) 55.62(3)

85.67(9) 2.6117(9)

55.71(3)

55.21(5) 57.36(5)

67.64(6)

53.93(5) 58.43(5) (Continued )

TABLE 2.1 (Continued)

24

g

25

26

27

Ref.

Cu-μP

Cu
N

Cu. . .Cu

N-Cu-μP

Cu-μP-Cu

μP-Cu-Cu

[27b]

2.3128(13)

2.050(4)

2.5736(9)

84.95(11)

66.45(3)

58.09(4)

2.3831(13)

2.056(4)

2.328(2)

2.075(6)

2.3502(19)

1.985(6)

2.403(2)

2.031(6)

2.2809(19)

2.071(7)

2.3578(17)

2.042(4)

2.3028(13)

2.023(5)

2.2745(17)

2.064(6)

2.347(2)

2.036(5)

2.2555(10)

2.053(3)

2.4395(11)

2.087(3)

2.2711(13)

2.039(4)

2.4025(11)

2.042(3)

2.2885(16)

2.034(5)

2.2476(17)

2.065(5)

2.2982(19)

2.024(6)

2.396(2)

2.047(5)

2.399(2)

2.020(5)

2.285(2)

2.070(6)

[27b]

[27b]

[27b]

28

[30]

26

[30]

30

c

31

[30]

[30]

83.53(11) 2.6131(12)

85.37(19)

55.47(3) 67.91(5)

85.86(17) 2.6121(14)

84.26(18)

55.64(5) 67.74(6)

85.71(18) 2.5820(9)

84.50(15)

85.03(14)

67.27(4)

82.41(9)

2.5924(10)

83.17(9)

67.08(6)

67.32(4)

53.39(3)

67.31(4)

53.93(3)

59.29(3)

84.72(12) 85.03(15)

58.76(3) 67.76(5)

85.16(14) 2.6127(13)

84.08(16)

84.33(17) 83.60(16)

54.25(4) 57.99(4)

67.61(6)

116.53(18) 2.6110(13)

57.81(6) 55.11(4)

84.24(9)

2.6099(9)

55.35(3) 57.38(4)

83.40(19) 2.6178(11)

53.91(5) 58.35(6)

83.83(11) 2.5539(11)

56.45(5)

54.42(5) 57.98(5)

67.70(6)

58.22(6) 54.08(5)

32

[30]

33

[31]

34

[31]

35

36

37

38

f

39

40

[31]

[31]

[32]

[31]

[32]

[31]

2.3156(9)

2.045(2)

2.6171(6)

84.00(8)

67.77(3)

2.3777(9)

2.055(3)

2.2907(12)

2.064(4)

2.3915(14)

2.044(4)

2.309(3)

2.056(9)

2.398(3)

2.039(9)

2.387(2)

2.030(7)

2.276(2)

2.043(7)

2.3786(17)

2.034(5)

2.2919(17)

2.052(5)

84.72(15)

2.2746(13)

2.054(4)

2.5616(10)

2.3925(15)

2.058(3)

2.3473(17)

2.041(5)

2.2891(16)

2.026(5)

84.30(17)

2.3251(16)

2.039(5)

2.5647(11)

85.93(15)

66.89(5)

2.3273(17)

2.048(6)

2.5610(11)

85.57(15)

66.91(5)

2.3239(17)

2.049(5)

2.3229(16)

2.037(5)

2.3069(11)

2.037(4)

2.3511(11)

2.057(4)

84.23(8) 2.5946(11)

84.93(11)

2.6306(19)

84.9(3)

57.25(2) 67.33(5)

58.22(4)

67.92(9)

57.66(8)

82.67(12)

54.51(3)

83.4(3) 2.6071(16)

85.0(2)

54.42(7) 67.94(7)

84.27(19) 2.6095(12)

84.77(15)

54.99(2)

54.02(5) 58.04(6)

67.91(5)

54.47(4) 57.63(5)

84.32(11)

66.52(3)

85.24(11) 2.5538(11)

85.72(16)

66.83(4)

55.49(5) 57.67(5)

84.99(15) 86.22(14) 2.5740(8)

84.23(10) 84.71(10)

67.08(3)

55.64(3) 57.28(3) (Continued )

TABLE 2.1 (Continued)

a

41

42

43

44

45

46

h

47

Cu. . .Cu

N-Cu-μP

Cu-μP-Cu

μP-Cu-Cu

2.5596(9)

85.19(12)

66.44(4)

Ref.

Cu-μP

Cu
N

[32]

2.3291(13)

2.056(5)

2.3433(15)

2.059(4)

2.3060(13)

2.046(3)

2.5863(8)

84.67(11)

67.52(4)

2.3476(14)

2.038(4)

2.5793(8)

84.88(11)

67.08(4)

2.3790(14)

2.050(4)

2.2872(13)

2.063(4)

2.3496(10)

2.042(3)

2.3107(10)

2.050(3)

2.3234(12)

2.052(3)

2.3616(12)

2.057(3)

2.3239(8)

1.978(2)

2.3305(8)

2.052(3)

2.3057(13)

2.041(4)

2.3409(14)

2.056(4)

2.3576(12)

2.040(3)

2.3850(11)

2.040(3)

2.391(4)

2.027(7)

2.411(4)

2.030(6)

2.355(4)

2.062(8)

2.425(4)

2.076(8)

[32]

[32]

[32]

[32]

[24]

[24]

85.43(11)

83.95(10) 83.96(11) 2.5721(9)

84.47(8)

66.99(3)

84.95(8) 2.5939(11)

84.62(9)

67.23(4)

85.02(10) 2.5904(9)

84.75(7)

67.63(3)

85.25(6) 2.5824(11)

85.56(11)

67.52(4)

85.74(12) 2.6415(11)

84.83(9)

67.69(4)

84.91(10) 2.670(3)

2.676(3)

55.66(3) 56.65(3)

81.9(2)

67.7(2)

55.8(2)

81.9(3)

67.9(3)

56.5(2)

82.3(3)

54.8(2)

83.4(3)

57.3(1)

48

[33]

49

[33]

50

[33]

a

51

i

52

53

[34]

[34]

[34]

2.2993(10)

2.097(3)

2.4888(10)

2.117(3)

2.2749(11)

2.061(4)

2.3726(11)

2.062(3)

2.2959(10)

2.039(3)

2.3338(10)

2.052(3)

2.2988(11)

2.055(3)

2.3860(13)

2.071(4)

2.288(4)

2.060(11)

2.376(4)

2.058(15)

2.367(4)

2.099(13)

2.307(4)

2.075(10)

2.354(3)

2.042(8)

2.240(3)

2.047(8)

2.252(3)

2.058(9)

2.279(3)

2.070(8)

2.460(3)

2.092(8)

2.230(3)

2.027(9)

2.3205(10)

2.062(3)

2.3586(12)

2.057(4)

2.6225(6)

82.18(9)

66.28(3)

84.76(9) 2.5508(7)

83.96(10)

2.5653(6)

84.08(10)

60.33(3) 66.54(3)

54.89(3)

67.29(3)

55.65(3)

84.79(12)

58.57(3)

84.73(10) 2.6118(9)

83.41(10)

57.06(3) 67.74(3)

85.73(10) 2.604(3)

85.5(3)

2.5680(17)

83.9(4)

67.85(11)

57.68(11) 54.47(10)

67.54(10)

55.14(10)

86.0(5)

57.32(11)

84.9(2)

57.55(7)

86.2(3) 2.6112(17)

57.72(3) 54.54(3)

83.3(4) 2.598(2)

53.39(2)

67.00(8)

79.9(2) 85.9(3)

55.46(7) 53.88(8)

67.72(9)

58.39(7)

2.5548(17)

82.0(3) 84.5(3)

65.79(9)

61.45(8)

2.5961(7)

83.53(11)

67.39(3)

55.60(3)

85.38(10)

52.76(8)

57.00(3) (Continued )

TABLE 2.1 (Continued)

j

54

55j

a

Ref.

Cu-μP

Cu
N

Cu. . .Cu

N-Cu-μP

Cu-μP-Cu

μP-Cu-Cu

[34]

2.328(2)

2.046(7)

2.5660(14)

85.83(19)

67.29(6)

55.92(7)

2.304(3)

2.062(7)

2.383(3)

2.051(8)

2.306(2)

2.061(8)

2.359(2)

2.053(6)

2.311(2)

2.107(7)

2.337(2)

2.049(6)

2.316(2)

2.056(7)

[34]

84.1(3) 2.5731(16)

85.3(3)

2.5403(13)

83.12(19)

66.55(7)

86.3(3) 65.90(6)

81.4(2) 2.5692(12)

84.4(2) 84.98(18)

Different pseudo-polymorphs were crystallized and their X-ray structures were established. Three independent molecules in the unit cell. c Two independent molecules in the unit cell. d Two first sets of data for the μ
1κN:1,2κP:2κN P donor center; two last sets of data for the μ
1κN:1,2κP P donor center. e Twisted-ribbon geometry. f Two independent supramolecular rectangles in the unit cell. g These supramolecular assemblies are noncentrosymmetric. h Two independent Cu2 dimers based on bisphosphole ligand 16 in the unit cell. i Two independent noncentrosymmetric rectangles in the unit cell. j Two independent CuI2(1)2 fragments in the unit cell. b

55.29(7)

56.16(5) 57.95(6)

67.03(6)

56.10(5) 56.86(6)

Polymetallic Cu(I) complexes Chapter | 2 2+

39

2+

N,P,N chelate P,N chelate N N

Ph

P

N Cu Ph Cu

P

N

L

N

Ph Cu

N

P,N chelate

P

Cu Ph P L

N L

N L

N,P,N chelate

FIGURE 2.7 Fluxional coordination behavior of ligand 1 in a Cu(I) bimetallic fragment. 2+ [X] 2 P N N Ph Cu Cu Cl

Cl Cu

N

Cl

Cl

Cu Ph N P

P N Cu

L P [Mo]

N

Ph

Cu

L

P [Mo]

10 9

[Mo] = [CpMo(CO)2] L = CH 3CN [X] = [BF 4] –

FIGURE 2.8 Structures of complexes 9 and 10.

specific arrangement of the metal centers in precursor 3 allowed the characterization of a new complex 10 bearing an unprecedented coordination mode for this [{CpMo(CO)2}2(μ,η2-P2)] ligand-complex [19]. Indeed, in 10 the P2 fragment is coordinated to the CuUUUCu unit in a μ
1,2κP2:2κP3 “side-on, end-on” coordination mode. Such a coordination mode was unprecedented for a phosphorus ligand-complex and only known for its Sb2 homolog. Concomitantly, the μ
1κN:1,2κP:2κN bridging phosphane coordination mode of ligand 1 on the CuI bimetallic unit is not altered by the coordination of the [Cp2Mo2(CO)4P2] ligand 2 on the metal centers, and is kept symmetri˚ ], despite the large dissymmetry of the coordination cal [Δd(μP-Cu) 5 0.06 A sphere of the Cu(I) metal ions in the solid state [one Cu(I) metal center bearing a distorted tetrahedral coordination sphere with a “P2N2” environment, and the second metal center having a distorted square pyramidal geometry

40

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

with a “P3N2” environment]. This confirms that there is no direct correlation between the symmetry of such a P-donor and that of the bimetallic fragments. Finally, it is worth mentioning that the coordination chemistry of 1 with Ag(I) ions was also studied and similar outcomes were established demonstrating that the bridging phosphane coordination mode can be also observed with trigonal planar metal centers, with a behavior and reactivity for the resulting Ag2 bimetallic complexes similar to Cu(I) analogs [20]. This set of results obtained with ligand 1 clearly establishes that phosphine/phosphane ligands can definitively behave as a bridging ligand family. Moreover, the variety of molecular structures determined in the solid state confirms the existence of a continuum between the symmetrical, semi-, and nonbridging coordination modes. These observations suggest that the energy difference between these different modes of coordination is very small and experimentally confirm the assumption made in 1989 by Braunstein [4] suggesting that the terminal phosphine ligands can adopt a transient bridging coordination mode between the different metal sites in polymetallic complexes, as is known to apply for ligands such as CO, CNR, or NO. Such exchange processes must then be very general for the phosphine ligands in polymetallic complexes and have to be taken into account in order to explain the reactivity of this family of derivatives. The development of the coordination chemistry of the 2,5-bis(2-pyridyl) phosphole ligand 1 has made it possible to considerably increase the number of complexes described integrating a bridging phosphine ligand (Table 2.1). This ligand currently remains unique in coordination chemistry because it is the only ligand belonging to the family of phosphine ligands that allows the synthesis of stable complexes with a bridging coordination mode in a systematic way. Nevertheless, it has to be mentioned that Klausmeyer et al. reported in 2009 similar results reacting phenylbis(2-pyridylmethyl)phosphane ligand 11 (Fig. 2.9) with Cu(I) ions [21]. In this case, the ligand

FIGURE 2.9 Structures of ligand 11 and its Cu(I) bimetallic complexes bearing a bridging phosphane coordination mode 12.

Polymetallic Cu(I) complexes Chapter | 2

41

presents a gross structure that resembles the structure of 1 (despite a priori much more flexible backbone due to the presence of methylene fragments instead of a five-membered rigid phosphole ring), a bimetallic Cu(I) complex 12 (Fig. 2.9) bearing a ligand 11 exhibiting a bridging coordination mode was also characterized.

2.4.2 Polymetallic Cu(I) complexes bearing multiple bridging phosphane ligands In order to enlarge the family of polymetallic complexes bearing bridging phosphine ligands and to explore the access to some potential applications, ligands 13a
c and 14 (Fig. 2.10), associating several phosphole cycles with several pyridine moieties, were prepared by Reau et al. and reacted with Cu (I) salts [22,24]. Thus, ligand 13a, that can be defined as an N,P,N,P,N 10-electron donor extended version of the N,P,N six-electron donor 1, gave access to the first helicate supramolecular structure having multiple bridging phosphane coordination modes as a result of a remarkably diastereoselective self-assembling process [22]. Indeed, ligand 13a was detected by multinuclear NMR in solution bearing an equilibrium between two diastereomers with relatively different cis (RP,SP meso compound) and trans (RP,RP and SP,SP) orientations of the P
Ph groups with respect to the mean plane of the five heterocycles. This effect is the result of the fast inversion in

FIGURE 2.10 Structures of ligand 13a
c and 14.

42

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

FIGURE 2.11 X-ray structure of the tetracationic complex 15a and molecular structure of the supramolecular assembly 15c.

solution of the two stereogenic P-centers of 13a (note that this fast inversion in solution also occurs in the case of 1 but due to the symmetry of 1, it has no impact on its solution NMR spectrum or in its coordination behavior). Nevertheless, upon reaction with the Cu(I) ions, a stereoselective coordination process is highlighted in solution and in the solid state, affording Cu4(13a)2 tetrametallic helicate derivative 15a (Fig. 2.11). [22] Derivative 15a presents pseudo C2-symmetry as a result of the coordination of two ligands 13a (both having P
Ph groups in a cis arrangement) on two Cu(I) dimers. For each ligand 13a, one N,P,N-moiety acts towards one CuI-dimer as a six-electron μ
1κN:1,2κP:2κN, donor with a bridging P-center, while the remaining P,N-moiety acts as a four-electron μ
1κN:2κP donor on the second CuI-dimer. Thus, the two different coordination patterns on a local Cu(I) dimer of 15a by the ligand 13a reproduces those observed in the Cu2(1)2 Cu(I) dimer 4 (Fig. 2.6). In addition, an agostic interaction is observed between one of the Cu(I) centers and the Cipso carbon of the P
Ph moieties of the μ
1κN:2κP donor atoms. Within the tetrametallic complex 15a, the two ligands 13a wind around the two bimetallic units, connecting them in a pseudo-helical array, as evidenced by dihedral angles of B 2 60 degrees between the four-electron donor N,P chelate and the six-electron donor N,P,N chelate moieties of each helicand 13a. In the case of the ligand 13a, no enantioselective synthesis occurs and both the P and the M helicates are present in the racemic centrosymmetric solid-state structure. Overall, tetrametallic assembly 15a affords the first example of a double-stranded helicate derivative

Polymetallic Cu(I) complexes Chapter | 2

43

based on bimetallic units bearing bridging phosphane coordination modes. Considering this outcome, Reau, Crassous et al. combined helicene and helicates chemistry for the first time, allowing molecules to be obtained with unprecedented chiral topology and chiroptical properties. Indeed, the aza [4] helicene and enantiopure P,P- and M,M-[aza(6)helicene] capped N,P,N,P,N 10-electron donor ligands 13b and 13c (Fig. 2.10) were prepared and reacted with Cu(I) precursors according to the synthesis described to obtain 15a. In each reaction, highly stereoselective coordination reactions occurred, affording the new tetrametallic complexes 15b and 15c with good to moderate yields [22]. More importantly, chiroptical data, along with 31P{1H} NMR spectroscopic data, strongly supported the formation of enantiomerically pure helicates (1)-/(2)-15c. A chirality transfer about brought by the enantiopure [6] azahelicene fragments could be thus carried out to obtain helical supramolecular edifice. The study of the chiroptical properties of these derivatives demonstrated finally the “electronically innocent” templating role of the Cu(I) ions for locating in a chiral environment extended π-conjugated systems which are themselves enantiopure. This impacts in an original way on the chiroptical properties of these supramolecular assemblies assigned to ligand-to-ligand intramolecular charge transfer [22]. It was also demonstrated that, upon reaction with Ag(I) ions, ligand 13a also allowed the stereoselective synthesis of a trimetallic Ag3(13a)2 helicate bearing all the P-centers of the ligands 13a involved in bridging phosphane coordination modes [23]. These results confirmed the very general ability of the ligands combining phosphole ring and 2-pyridyl moieties to give access to polymetallic assemblies. In addition, the reaction of bisphosphole 14 (Fig. 2.10) with [Cu (CH3CN)4X] (X2 5 BF42 or PF62) led to the formation a “bis-bimetallic” derivative 16 in which the two phosphorus atoms of ligand 14 also bridge the two metal centers (Fig. 2.12) [24]. Compound 16 possesses two [Cu2(μNPN)(CH3CN)4] cores linked by the central p-phenylene moiety of ligand 14. The two [Cu2(μ-NPN)] moieties present an anti-conformation with respect to the p-phenylene central unit and the metric parameters of the [Cu2(μ-NPN)] fragments of this tetrametallic species 16 (Table 2.1), including metal
metal and metal
μP distances, are similar to those of the dimetallic complex 3 (Fig. 2.6, Table 2.1). Therefore, the two independent [Cu2(μ-NPN)] units in the core of the tetrametallic complex 16 keep the key structural features of complex 3. In the next section, the major developments on Cu(I) polymetallic coordination complexes bearing a bridging phosphane coordination mode that have achieved will be detailed. Several of these Cu(I) derivatives (including 4 and 6; Fig. 2.6 and 16, Fig. 2.12) are excellent preorganized supramolecular precursors and allow the design of an unprecedented family of compact supramolecular assemblies [25].

44

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

FIGURE 2.12 X-ray structure of the tetracationic complex 16.

2.5 Coordination-driven supramolecular assemblies of compact supramolecular metallacycles based on preassembled Cu(I) precursors bearing a bridging phosphane ligand 2.5.1 Selection of bimetallic Cu(I) complexes bearing a bridging phosphane ligand based on the 2,5-bis(2-pyridyl)phosphole ligand 1 as preassembled precursor for coordination-driven supramolecular synthesis Upon examination of the X-ray structures of complexes 4 and 6 (Fig. 2.6), it was anticipated that these derivatives (if the conformation of these derivatives is stable and preserved in solution) could present appealing geometric characteristics to act as preorganized “U-shaped molecular clips” to construct selectively and straightforward coordination-driven supramolecular assemblies. These two complexes have two coordination sites in the cis position, pointing in the same direction and occupied by labile acetonitrile ligands. Such characteristics correspond to the criteria defined in the synthetic concept of the directional-bonding approach (also called coordination-driven supramolecular chemistry) [26] in order to achieve a rational preparation of supramolecular assemblies. In particular, supramolecular rectangles are expected to be obtained if complexes 4 and 6 could have their acetonitrile ligands substituted by linear homoditopic linkers (Fig. 2.13). Moreover, as a direct consequence of the bridging phosphane coordination mode, the two metal centers are very close to each other in these

Polymetallic Cu(I) complexes Chapter | 2

45

FIGURE 2.13 Coordination-driven supramolecular synthesis strategy toward compact supramolecular assemblies using preassembled molecular clips bearing a bridging phosphane coordination mode.

˚ ; 6: 2.67 A ˚ , implying metallophilic interacprecursors (d(Cu
Cu), 4: 2.57 A tions; Table 2.1) [17], which remains an unprecedented parameter in the bimetallic precursors used to prepare discrete coordination-driven supramolecular assemblies. This characteristic feature gives original structural properties to the derivatives: the linkers introduced in these frameworks could be forced to share lateral intramolecular interactions with each other (Fig. 2.13), via π
π interactions when the connectors used are based on π-conjugated systems, or via interactions of a metallophilic nature if the introduced connectors incorporate a metal ion. As a result, additional stabilization of the self-assembled structures may occur, allowing the formation of compact supramolecular assemblies bearing original geometries. Accordingly, a very general coordination-driven supramolecular approach toward compact supramolecular assemblies was successfully highlighted [25]. This synthetic approach using Cu(I) complexes bearing bridging phosphane ligands was revealed to be extremely efficient and versatile and allowed the characterization of a large number of original compact supramolecular assemblies. This also allowed a very substantial increase in the collection of reported Cu(I)-based molecular scaffolds bearing bridging phosphane ligands in the solid state. The stability and the generality of this unique coordination mode with Cu(I) ions was therefore experimentally

46

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

confirmed, together with its great flexibility toward semibridging and symmetrically bridging configurations (Table 2.1).

2.5.2 Cu(I) compact supramolecular assemblies bearing a bridging phosphane ligand and fully π-conjugated linkers The preassembled Cu(I) bimetallic complexes 4 and 6 were thus reacted by Reau, Lescop et al. with a variety of linear ditopic π-conjugated linkers C17
C24 (Fig. 2.14) bearing nitrile groups as a coordinating function leading in a reproducible and systematic manner to the formation of the compact supramolecular rectangles 17
24 [27]. The formation of these structures was confirmed by X-ray analysis in the solid state and also in solution by NMR studies. The dimensions of these supramolecular objects are nanometric and vary according to the nature of the π-conjugated ˚ with the long ditopic connector used to reach a length of the order of 45 A linkers C21 (Fig. 2.15) [27b]. In these new compact supramolecular assemblies, the π-conjugated systems present in the organic linkers are organized in a roughly “face-to-face” arrangement with respect to each other, and not in a parallel-displaced manner as is generally observed in the solid state in the case of “free” π-conjugated systems. The intramolecular distances imposed within the resulting supramo˚ , which reveals the effective establecular derivatives are of the order of 3.5 A lishment of π
π interactions between the delocalized electronic clouds of these π-conjugated systems. These compact self-assembled scaffolds can be seen as being structural supramolecular analogs of the fully organic 2,2-paracyclophanes, a family of molecules that has been intensively investigated as molecular models for the study of the electronic communication property across the space between the π-conjugated systems they incorporate [28]. Nevertheless, in the case of the fully organic 2,2-paracyclophanes, access to structural variations can be very challenging and/or require costly and timeconsuming multistep syntheses. In the course of these studies, Re´au et al. were able to specify the mechanism of formation of these rectangles [29]. The reactions of the short π-conjugated linkers C17 and C18 with the molecular clip 4, under appropriate reaction conditions coordinated initially to two molecular clips at the ends of a single linker affording “open” tetrametallic assemblies 170 open0 and 180 open0 (Fig. 2.16). Subsequently, a second linker is coordinated allowing the formation of the compact supramolecular assemblies in which the cumulation of lateral π
π interactions greatly favors the second step of the self-assembling process [29]. In the context of this study, Reau et al. have also studied the reactions with the molecular clip 4 of π-conjugated systems that are no longer linear, but “curved” (linkers C25
C27; Fig. 2.14) [27b]. In this case, the characterization of original “curved” metallocyclophanes 25
27 was selectively

FIGURE 2.14 Synthesis of the compact supramolecular assemblies 17
27.

48

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

FIGURE 2.15 View of the X-ray structure of the tetracationic compact supramolecular assemblies 21 and of its extended solid-state columnar π-stack.

FIGURE 2.16 Schematic view of the tetrametallic supramolecular assemblies 170 open0 and 180 open0 ; and their X-ray structures.

achieved. These new assemblies present analogous features compared to those of the compact supramolecular rectangles described above, including intramolecular π
π interactions between the π-coordinated π-conjugated walls of the “curved” linkers. Moreover, in the case of supramolecular assemblies based on “long” π-conjugated linkers (19
24), infinite stacked columns of interacting π-conjugated systems are systematically observed in the crystals obtained (see Fig. 2.15 for the compact supramolecular rectangle 21) [27]. The distance between the π-conjugated systems with a compact rectangle and ˚ ) (in these between two neighboring rectangles is of the same order (3.5 A solid-state crystalline materials, the counter-ions and the solvent molecules included are positioned in the cavities generated between the columns). Such an arrangement can be attributed to the substantial energy gain resulting from the contact of the London dispersion forces associated with the polarizable surfaces of these extended π-conjugated systems. This type of arrangement is not observed in the case of supramolecular rectangles based on “short” or “laterally congested” connectors, presumably because of excessive

Polymetallic Cu(I) complexes Chapter | 2

49

sterical crowding and unsufficient cumulation of stabilizing lateral π
π interactions. Interestingly these π-conjugated systems interacting in infinite columns are not observed when the π-conjugated linkers crystallize alone (when it was possible to crystallize them); instead a variety of organizations in the solid state was observed. Such an organization of π-conjugated systems at the solid scale is favorable for charge transfer in these molecular materials. thus paving the way for further studies for their possible integration into devices such as field effect transistors. Subsequent studies have revealed that stabilization of such π-stacked supramolecular assemblies by lateral interlinkers via π
π interactions is so important that is not even necessary to synthesize a compact supramolecular metallacycle in order to get access to infinite columnar π-stacks. Indeed, this feature can be exploited to gain access to original supramolecular architectures, such as π-stacked “U-shape” supramolecular assemblies upon reaction of 4 with disymmetric π-conjugated systems C28
C32 bearing only one terminal nitrile coordination group (Fig. 2.17) [30]. Extended π-conjugated fragments having large lateral planar and nonplanar terminal units, such as pyrene (C30) and carbo [4] helicene (C31,C32) moieties, were successfully reacted. The resulting discrete π-stacked derivatives self-assembled in the solid state in head-to-tail dimers that finally aggregated within infinite columnar π-stacks in a similar way to that observed in the case of the previously described π-stacked compact metallacycles [27,30].

2.5.3 Cu(I) compact supramolecular assemblies having a bridging phosphane ligand and ditopic linkers bearing flexible linear fragments Regarding the formation of the previously described π-stacked “U-shape” supramolecular assemblies, Lescop, Reau et al. suggested that the driving force leading to the formation of such compact supramolecular assemblies could be sufficient to allow the use of ditopic linkers combining π-conjugated fragments and linear fully aliphatic moieties. The successful introduction of such linkers in coordination-driven supramolecular assemblies would be significantly novel since aliphatic fragments are supposed to present intrinsic low rigidity and directionality that challenge the basic guiding synthetic rules of a coordination-driven supramolecular synthetic approach. Therefore, the reactions of the molecular clip 4 with a series of ditopic linkers C33
C36 bearing central aliphatic flexible fragments and terminal cyano-capped rigid π-conjugated moieties of various lengths were conducted. As a result, compact supramolecular metallacycles having unprecedented “pseudo double-paracyclophane” structures (Fig. 2.18) with ˚ ) were obtained [31]. Moreover, intermolecular large lengths (up to 52.3 A π
π interactions were also generated in the extended solid-state crystal

FIGURE 2.17 Synthesis of the compact supramolecular assemblies 28
32.

Polymetallic Cu(I) complexes Chapter | 2

51

FIGURE 2.18 Chemical structure of the linkers C33
C36 and a view of the X-ray structure and the crystal packing of derivative 34.

FIGURE 2.19 Synthesis of the assemblies C37
C45 and a view of the X-ray structure of derivatives 42 and 43.

structures of these compact derivatives, which induced their aggregation within infinite π-stacked columns bearing discrete stacks of four aromatic moieties separated by imbricated
(CH2)n
fragments [31]. Finally, Lescop et al. described the first systematic study related to the use of fully aliphatic and flexible aliphatic connectors in coordination-driven supramolecular syntheses (Fig. 2.19) [32]. It has thus been highlighted that controlled and selective self-assembly processes are possible from such ligands which, a priori, supply no provision for self-organization in a coherent manner. This is due to the combination of preorganized precursors 4, having small intermolecular distances and a directional labile coordination position, and sufficiently long flexible ligands for which the cumulation of weak London dispersion interactions can afford a sufficient stabilization of the compact metallacyclic structure. Thus, only due to the reaction of the fully aliphatic longer linkers C41
C45, compact supramolecular assemblies were selectively obtained [32].

2.5.4 Formation of one-dimensional coordination polymers based on Cu(I) supramolecular nodes bearing bridging phosphane ligand Regarding the successful outcomes that were obtained from the use of the preassembled bimetallic molecular clips 4 and 6, the tetrametallic complex 16 (Fig. 2.12), based on the bisphosphole ligand 14 (Fig. 2.10), was also considered as a potential source of “bis-clip” for the formation of extended

52

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

FIGURE 2.20 Syntheses of the one-dimensional coordination polymers 46 and 47.

structures integrating compact supramolecular metallacycles. Upon the reactions of 16 with homoditopic connector ligands carrying π-conjugated systems of variable dimensions, Reau et al. observed control of the geometry of the one-dimensional coordination polymers obtained (Fig. 2.20) [24]. As a result of the creation of infinite columns of π-conjugated systems sharing π
π interactions and of the free rotation allowed by the central p-phenylene moiety of ligand 14 (Fig. 2.12), it was possible to impact in the secondary structure of one-dimensional assemblies having the same primary structure [24]. This behavior reproduces a phenomenon commonly described in the case of biological structures where hydrogen bonds mainly play the role of these weak but cumulative and structuring interactions to control the organization of the secondary structure of biologically relevant scaffolds, such as proteins.

2.5.5 Cu(I) compact supramolecular assemblies bearing a bridging phosphane ligand and ditopic inorganic linkers: “sterical protection effect” and metallophilic interlinker lateral interactions Reau et al. also reported the association of inorganic homoditopic linkers with the preassembled supramolecular precursors 4 and 6. Thus, the reactions of 4 with the CN2 and N32 anions, playing the role of ditopic ligands, afforded a similar supramolecular assembly process. New compact metallacycles 48 and 49 have been successfully characterized (Fig. 2.21) [33]. The selectivity of the self-assembly reactions observed in

Polymetallic Cu(I) complexes Chapter | 2

53

FIGURE 2.21 Views of the X-ray structures of the compact supramolecular assemblies 48, 49, 51, and 52 bearing bridging phosphane ligands.

this case was not assigned to the establishment of weak secondary interactions between the linkers, but was attributed to a steric protection criterion due to the steric hindrance of bridging phosphane ligands in the coordination sphere of Cu(I) ions as well as due to the short length of the linkers. As a result, the formation of compact supramolecular metallacycles rather than linear coordination polymers was favored. This result was also observed when the larger C(CN)32 anion was reacted with 4, affording another example of compact discrete metallacycle 50 in which C(CN)32 anion acts as a ditopic linker leaving one CN fragment uncoordinated [33]. The use of inorganic moieties as ditopic ligands was extended to the reaction of 4 with the anionic linker Au(CN2)2 bearing Au(I) metal ion [34]. In this case, Reau et al. systematically obtained a mixture of different crystals (Fig. 2.21): a compact supramolecular rectangle 51 in which aurophilic inter˚ ] and a speactions between the Au(I) ions are observed [d(Au
Au) 5 3.4 A cies 52 bearing a complex supramolecular structure resulting from the aggregation of two compact supramolecular rectangles via an intermolecular ˚ ], in addition to the intramolecular aurophilic interaction [d(Au
Au) 5 3.2 A aurophilic interactions within the single rectangles. On the basis of these results, the precursor 4 was reacted with the neutral connector Hg(CN)2, the Hg(II) ion being presumed to lead to much weaker energetic interactions than the aurophilic interactions. In this case, metal-rich metallacycles 53 in metal ions could also be obtained, but the reaction is no longer selective

54

Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

toward the formation of compact supramolecular metallacycles, and oligomers 54 and coordination polymers 55 are also characterized together with such metallacycles [34]. This last result reflects the importance of the aurophilic versus mercurophilic lateral interactions in order to allow the selective formation of metal-rich metallacycles. Importantly, all along the structural variety observed in derivatives 53
55, the bridging phosphane coordination mode was preserved in all cases, ranging from symmetrically to semisymmetrically bridging (Table 2.1). This supports the great stability of this bridging coordination mode within preorganized Cu(I) bimetallic units. This series of results reinforces moreover the concept that, as the length of the linear ditopic connector employed increases, cumulation of “weak” interlinker interactions within the resulting self-assembled structures is paramount to allow selective synthesis of compact metallacycles from the molecular clip 4 bearing bridging phosphane coordination mode.

2.6 Conclusion Cu(I) polymetallic complexes stabilized by a bridging phosphane coordination mode based on the phosphole-based N,P,N chelate 1 constitue, to date and from far, the largest family of stable derivatives bearing this original coordination mode. The structural diversity encountered in these derivatives has allowed the occurrence of a continuum between symmetrically bridging and semisymmetrically bridging phosphane coordination modes to be highlighted. This implies that phosphane ligands having a bridging phosphane coordination mode have to be considered as possible intermediate species involved in fluxional processes within multimetallic complexes. Importantly, the specific molecular scaffold of the ligand 1 highly favors the formation of stable bridging phosphane coordination mode and allows the isolation and characterization of polymetallic complexes based on square-planar Pd(I) and Pt(I) metal centers [14,16], trigonal Ag(I) metal centers [20], and tetrahedral Cu(I) metal centers [16
18]. This feature makes the N,P,N-ligand 1 still unique in coordination chemistry, being so far the only reported ligand that exhibits this unusual bridging phosphane coordination mode in many of its coordination complexes. Cu(I) polymetallic complexes based on the ligand 1 opened an avenue to study in detail this new family of derivatives due to their high stability and tolerance toward ligand substitution. Ancillary ligands’ new reactivity could be achieved [18], together with the construction of original chiral polymetallic assemblies [22] and the general and original coordination-driven supramolecular syntheses of compact supramolecular metallacycles [25]. Nevertheless, this specific coordination chemistry also defines a very confined niche which, to date, still needs to be extended to other ligand scaffolds. With this in mind, the results published by Klausmeyer et al. with the coordination chemistry of ligand 11 (Fig. 2.9) [21] clearly suggest that the scope of the coordination chemistry of

Polymetallic Cu(I) complexes Chapter | 2

55

the bridging phosphane ligands with Cu(I) metal centers could be extended to other organophosphorus ligands. Regarding the growing attention currently devoted to the synthesis and study of solid-state highly luminescent polymetallic Cu(I) complexes [35], such ligand systems allowing the construction of stable and original polymetallic Cu(I) derivatives may be attractive for the preparation of innovative new luminescent multifunctional supramolecular materials [36].

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