Strategies for the Construction of Supramolecular Dimers versus Homoleptic 1D Coordination Polymers Starting from the Diphosphorus [Cp2Mo2(CO)4(η2-P2)] Complex and Silver(I) Salts.

The reactions of the tetrahedral diphosphorus [Cp2Mo2(CO)4(η2-P2)] (1; Cp = C5H5) complex with Ag[Al{OC(CF3)3}4] (AgTEF) (A) and Ag[FAl{OC(C6F5)(C6F10)}3] (AgFAl) (B) were studied. The first reaction led to the formation of the [Ag2(η2-1)2(η1:η1-1)2][TEF]2 (2) dimer and the [Ag2(η1:η1-1)3] n [TEF]2n (3) coordination polymer, whereas the second reaction afforded the [Ag2(η1:η1-1)2(η1-CH2Cl2)2(η2-C7H8)2][FAl]2 (4) or the [Ag2(η2-1)2(η1:η1-1)2][FAl]2 (5) dimer and the [Ag2(η1:η1-1)4] n [FAl]2n (6) coordination polymer. In each case, the products obtained depended on the ratio of the reactants and/or the synthetic procedure.

Introduction

In the past two decades, metal‐directed self‐assembly processes have been widely used to synthesize discrete supermolecules and extended polymeric frameworks.1 Those derivatives are usually obtained from the spontaneous association of organic multitopic ligands bearing N, O, or S donor atoms with a large variety of Lewis acidic metal cations.2 In fact, the directional but reversible coordinative bonds result in a typical equilibrium that exists between the involved molecular components and the various potential products. This feature is affected by the different reaction conditions applied and generally leads to the formation of the thermodynamically most stable product(s).3 In this field, our group developed an alternative approach by utilizing P‐donating organometallic polyphosphorus (P) ligand complexes with flexible coordination modes as connectors between metal ions.4 Using this novel approach, it was possible to synthesize one‐ and two‐dimensional coordination polymers,5 vast fullerene‐like supramolecular spherical aggregates,6 and organometallic nanosized capsules.7 Among the used P ligand complexes, the tetrahedral [Cp2Mo2(CO)4(η2‐P2)] (Cp = C5H5) (1) complex was extensively studied. The reaction of 1 with CuI halides led to one‐dimensional [Cu(µ‐X){Cp2Mo2(CO)4(µ,η2:η1:η1‐P2)}] (X = Cl, Br, I) polymers, and its reaction with AgNO3 resulted in the undulated 1D [Ag2{Cp2Mo2(CO)4(µ,η2:η1:η1‐P2)}3(µ,η1:η1‐NO3)][NO3] polymer.8 The reactions of the AgX salts of weakly coordinating anions {X = BF4, PF6, ClO4, SbF6, Al[OC(CF3)3]4} with 1 led to AgI dimers with the general formula [Ag2{Cp2Mo2(CO)4(µ,η2:η2‐P2)}2][{Cp2Mo2(CO)4(µ,η2:η1:η1‐P2)}2][X]2.9 Some of these dimeric compounds were further treated with multitopic pyridine‐based organic molecules, which in the solid state led to a large variety of unprecedented organometallic–organic hybrid coordination polymers.10 Although the coordination chemistry of 1 towards AgI has been extensively studied, no homoleptic polymeric compounds of 1 and AgI have yet been reported. We present herein a systematic study of the variation of the ratio of the reactants and the reaction procedure for the self‐assembly processes of the P2 ligand complex 1 and the AgI salts of the weakly coordinating anions [Al{OC(CF3)3}4]– (TEF) and [FAl{OC(C6F5)(C6F10)}3]– (FAl). The found conditions allowed fine‐tuning of the formation of either dimers or homoleptic one‐dimensional coordination polymers.

Results and Discussion

In a first approach, P2 ligand complex 1 was treated with Ag[TEF] (A), for which the dependence of the composition of the product on the stoichiometry of the reactants was studied by varying the 1/A ratio. If the used reactant stoichiometry 1/A was between 2:1 and 6:1, complex 2 was formed (87 % as the highest yield), which suggests that this product represents a thermodynamic minimum by using such an excess amount of P2 ligand complex 1. However, if the reactant stoichiometry 1/A was between 2:1 and 1:1, compound 3 was obtained with very high selectivity (70 % as the highest yield). Interestingly, with these stoichiometric ratios, compound 2 was formed only if a CH2Cl2 solution of A was slowly added to a CH2Cl2 solution of 1 (if ligand 1 was in reasonable excess at the time of mixing the two reactants), whereas compound 3 was selectively isolated if a CH2Cl2 solution of 1 was added to a CH2Cl2 solution of A (if no excess amount of ligand 1 was present at the time of mixing the two reactants). This effect of the ratio of the reactants together with the reaction procedure on the product composition is clearly reflected in the solid‐state structures of products 2 and 3. Crystals of 2 and 3, grown by layering CH2Cl2 solutions of the crude reaction mixtures with pentane (for 2) and toluene (for 3), were examined by X‐ray structure analysis. Their crystal structures clearly reveal that the formed coordination compounds have different ratios of AgI/1 in the solid state, 1:2 for 2 and 2:3 for 3. Compound 2·1.5CH2Cl2 turned out to be a solvatopolymorph of the silver [Ag2(µ,η1:η1‐1)2(η2‐1)2][Al{OC(CF3)3}4]2 ·CH2Cl2 dimer previously reported by our group,9 whereas 3 is a unique homoleptic one‐dimensional coordination polymer of the general formula [Ag2(µ,η1:η1‐1)3][Al{OC(CF3)3}4]2 (Scheme 1, Figure 1b).

Scheme 1

Reaction of 1 with Ag[Al{OC(CF3)3}4] (A): synthesis of dimer 2 and one‐dimensional coordination polymer 3.

Figure 1

(a) Molecular structure of cationic dimer 2 in the solid state. (b) Section of 1D coordination polymer 3. Cp and CO ligands, hydrogen atoms, counteranions, as well as minor disordered positions are omitted for clarity.

Dimer 2 is surrounded by four P2 ligands 1, two of which possess a bridging µ,η1:η1‐coordination mode and the other two of which show η2‐side‐on coordination (Scheme 1, Figure 1a). Hence, each AgI ion in 2 possesses a distorted tetrahedral coordination sphere consisting of four P atoms. The molecular geometry of this new solvatopolymorph of 2 is comparable to that of the initial structure. In addition, some AgI ions interact with CH2Cl2 solvent molecules [Ag–Cl 3.561(4)–3.647(8) Å], which is not the case in the earlier found structure. The structure of 3 exhibits a one‐dimensional zigzag chain consisting of Ag2(1)2 repeating units interconnected by 1 as an additional ligand. All the P2 ligands 1 in 3 show a bridging µ,η1:η1‐coordination mode. Each AgI ion in 3 possesses a distorted trigonal geometry consisting of three P atoms. The central Ag2P4 six‐membered rings in 3 are nearly planar and show only a slight distortion towards a chair conformation [folding angle 12.5(1)°] as compared to the Ag2P4 six‐membered ring in 2 [folding angle 18.4(1)°]. The P–P bond lengths in 3 [2.082(3)–2.097(2) Å] are comparable to those of noncoordinated ligand 1 [2.079(6) Å]11 and are slightly shortened relative to those of dimer 2 [2.087(2)–2.152(3) Å]. The Ag–P bond lengths inside [2.4464(14)–2.4642(15) Å] and outside [2.4425(16) Å] of the six‐membered rings in 3 are almost identical and are slightly shortened relative to those of dimer 2 [2.4634(16)–2.6885(17) Å]. The Ag···Ag distances exceed 4.8 Å in 2 and 4.34 Å in 3, which suggests no argentophilic interaction.12

Compounds 2 and 3 are well soluble in CH2Cl2, THF, and CH3CN; slightly soluble in toluene; and insoluble in n‐pentane. The room‐temperature 31P NMR spectra of 2 and 3 in CD3CN each show a broad signal centered at δ = –86.1 and –93.0 ppm, respectively, which is upfield shifted relative to that of free P2 ligand complex 1 (δ = –43.2 ppm).11 These observations suggest the presence of a dynamic behavior in solution, which was previously carefully studied for original solvatopolymorph 2.9 Their room‐temperature 1H and 13C NMR spectra show the expected signals attributable to the protons and carbon nuclei of ligand 1 and the TEF anions (for further details, see the Supporting Information).

In a second step, P2 ligand complex 1 was treated with Ag[FAl] (B) in various stoichiometric ratios. The 1:1 reaction in CH2Cl2 and subsequent layering with toluene gave 4 as an orange crystalline solid in moderate yield (39 %; Scheme 2, Figure 2a). However, if the 2:1 stoichiometry was used, compound 5 or 6 was formed depending on the order in which one reactant was added to the other. Product 5 was formed selectively (76 % yield) only if a CH2Cl2 solution of B was slowly added to a CH2Cl2 solution of 1 (in other words, if there was an excess of ligand 1 present). On the contrary, compound 6 was isolated (89 % yield) if the reaction order was reversed, that is, if ligand 1 was the limiting reactant at the time of mixing the two reactants. Single‐crystal X‐ray structure analysis of 4, 5 and 6 reveals composition ratios of 1:1 (for 4) and 1:2 (for 5 and 6) of AgI/1 in the solid state. Among these, compounds 4 and 5 represent dimers with general formulas of [Ag2(µ,η1:η1‐1)2(η1‐CH2Cl2)2(η2‐C7H8)2][FAl]2 and [Ag2(µ,η1:η1‐1)2(η2‐1)2][FAl]2, respectively, whereas derivative 6 is a homoleptic one‐dimensional coordination polymer with the formula [Ag2(µ,η1:η1‐1)4][FAl]2 (Scheme 2, Figure 2). Dimer 4 is composed of two AgI ions with a short Ag···Ag contact of 3.0532(10) Å and is capped by two bridging µ,η1:η1‐coordinating P2 ligands of 1 to form a central P2Ag2P2 ladder‐shaped structural motif with nearly identical P–Ag bond lengths [2.4523(14)–2.4550(13) Å]. Additionally, each AgI ion is η2‐coordinated by a toluene molecule [Ag–(C=C) 3.010(6) Å] and η1‐coordinated by a CH2Cl2 molecule [Ag–Cl 3.402(3) Å]. Interestingly, these labile ligands can be easily substituted by the addition of another equivalent of 1, which leads to dimer 5. The cationic part of dimer 5 is very similar to that of dimer 2, which was isolated from a similar reaction with the use of AgI salt A. In this case, the central Ag2P4 six‐membered ring shows a slight distortion towards a chair conformation [fold angle 20.53(10)°], and no Ag–Ag interaction is observed in 5 [d(Ag···Ag) > 4.85 Å]. The Ag–P distances inside the six‐membered Ag2P4 ring [2.4871(11) and 2.4834(11) Å] are significantly shorter than the Ag–P distances to the end‐on coordinated ligands 1 [2.7112(11) and 2.5988(11) Å]. The η2‐coordinating P–P bond [2.1395(16) Å] is slightly elongated, and the η1:η1‐coordinating P–P bonds [2.091(2) Å] are almost unchanged relative to those of free ligand 1 [2.079(2) Å]. The structure of 6 exhibits a one‐dimensional coordination polymer of infinite interconnected Ag2P4 six‐membered ring repeating units. Most probably, as a result of the steric hindrance of the Cp and CO groups on the Mo centers, two consecutive Ag2P4 repeating units along the polymeric chain of 6 are oriented in an angle of 85.82(6)° towards each other. The Ag2P4 rings themselves are almost planar and show only a slight distortion towards a chair conformation [fold angles 10.50(6)–11.00(6)°]. The P–P distances in 6 [2.1004(12)–2.1005(12) Å] are elongated relative to those of free ligand 1 [2.079(6) Å], and the Ag–P distances [2.5775(8)–2.6086(8) Å] are also elongated relative to those of dimers 4 and 5.

Scheme 2

Reaction of 1 with Ag[FAl{OC(C6F5)(C6F10)}3] (B): synthesis of dimers 4 and 5 and one‐dimensional coordination polymer 6.

Figure 2

Molecular structures of cationic dimers (a) 4 and (b) 5 in the solid state. (c) Section of 1D coordination polymer 6. Cp and CO ligands, hydrogen atoms, counterions, as well as minor disordered positions are omitted for clarity.

Compounds 4–6 are well soluble in polar solvents such as CH3CN, are slightly soluble in CH2Cl2, and are insoluble in other common organic solvents such as THF and n‐pentane. Their room‐temperature 31P NMR spectra in CD3CN show signals varying significantly between δ = –74.6 and –92.6 ppm, which are upfield shifted relative to those of free P2 ligand complex 1 (δ = –43.2 ppm). The room‐temperature 1H and 13C NMR spectra of 4–6 reveal the expected signals corresponding to the protons and carbon nuclei of ligand 1 and the FAl anion (for further details, see the Supporting Information).

Experimentally, the effect of the reaction procedure on the formation of compounds 5 and 6 (which contain the same ratio of ligand 1/AgI) is not clear. Structurally, the differences between 5 and 6 are the coordination modes of terminal ligands 1 (η2‐coordination mode in 5 and η1:η1‐coordination mode in 6). To better understand the reaction mechanism with respect to the difference in product formation, DFT calculations were performed at the B3LYP/def2‐TZVP level of theory. The calculations show that by adding the AgI salt to 1, species D is present in solution, as the dissociation of the expected species C (local excess of 1) to D and 1 is thermodynamically favored by –13.9 kJ mol–1 (Scheme 3). Additionally, the reaction in Scheme 3 shows that if P2 ligand 1 is in excess relative to AgI, the η2‐coordination mode of “terminal” P2 ligand 1 is preferred over the η1‐coordination mode.

Scheme 3

Dissociation of hypothetical intermediate C into D and 1.

On the basis of these DFT calculations and the experimental observations, we assume that, in both cases, the Ag2P4 six‐membered ring motif is formed first, for example, compound 4. In the case of a local excess of P2 ligand 1, upon adding Ag[FAl] to 1, dimer 5 is formed as a result of the preference of 1 for the η2‐coordination mode under these conditions. While as long as an excess amount of AgI salt is present, upon adding 1 to Ag[FAl], possibly by preformation of oligomers containing linear P–Ag–P scaffolds with bridging ligand 1, these intermediates can probably be further transformed into the coordination polymer 6 as long as more ligand 1 is added.

Conclusions

The obtained results present a smooth strategy for the construction of either dimers (i.e., 2, 4, and 5) or homoleptic one‐dimensional coordination polymers (i.e., 3 and 6) on the basis of a diphosphorus ligand complex, [Cp2Mo2(CO)4(η2‐P2)] (1), and AgI metal ions. The coordination polymers are the first homoleptic polymeric compounds of 1 and AgI. The variation in product formation was discovered to be due to the flexible coordination modes of ligand complex 1, which could adopt either an η2‐ or η1:η1‐coordination mode according to the ratio of ligand 1/AgI present in solution at the time the reaction was initiated. These results, in addition to the large variety of assemblies based on ligand 1,8, 9 show the advantage of our approach, in which P‐donating ligand complexes are used in metal‐directed self‐assembly, as it can lead to a very rich library of supramolecular compounds with large structural diversity. Current investigations involve reactions based on heavier analogues [Cp2Mo2(CO)4(η2‐E2)] (E = As, Sb, Bi) to study the possibility of utilizing these rarely used ligands as connectors between metal ions in addition to the well‐established study of using P2 ligand 1.

CCDC 1551486 (for 2), 1551487 (for 3), 1551488 (for 4), 1551489 (for 5), and 1551490 (for 6) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.

Supporting Information (see footnote on the first page of this article): NMR spectroscopy data, elemental analysis, mass spectrometry, as well as crystal‐structure refinement data for compounds 2–6.

Supporting Information

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