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Chemistry of [PW11NbO40]4−: Structural rearrangement and metal coordination
Accepted Manuscript Chemistry of [PW11NbO40]4−: Structural rearrangement and metal coordination
Ekaterina P. Bushmeleva, Nikolay B. Kompankov, Rishat R. Shiriyazdanov, Albina R. Karimova, Pavel A. Abramov, Maxim N. Sokolov PII: DOI: Reference:
S1387-7003(18)30750-0 https://doi.org/10.1016/j.inoche.2018.10.026 INOCHE 7160
To appear in:
Inorganic Chemistry Communications
Received date: Revised date: Accepted date:
10 August 2018 20 October 2018 23 October 2018
Please cite this article as: Ekaterina P. Bushmeleva, Nikolay B. Kompankov, Rishat R. Shiriyazdanov, Albina R. Karimova, Pavel A. Abramov, Maxim N. Sokolov , Chemistry of [PW11NbO40]4−: Structural rearrangement and metal coordination. Inoche (2018), https://doi.org/10.1016/j.inoche.2018.10.026
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Chemistry of [PW11NbO40]4–: structural rearrangement and metal coordination Ekaterina P. Bushmeleva1,2, Nikolay B. Kompankov2, Rishat R. Shiriyazdanov3, Albina R. Karimova3, Pavel A. Abramov*1,2, Maxim N. Sokolov1,2 1
Nikolaev Institute of Inorganic Chemistry SB RAS, 3 Akad. Lavrentiev Ave., 630090 Novosibirsk. Russia; Novosibirsk State University, 2 Pirogova Str., 630090 Novosibirsk, Russia
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Ufa State Petroleum Technological University, 1 Kosmonavtov str., 450062 Ufa, Russia
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Abstract. Treatment of Cs4[PW11NbO40]∙12H2O with boiling water results in slow degradation of the [PW11NbO40]4– Keggin anion into the trilacunary [A-PW9O34]9– following by aggregation of the latter around three {M=O} (M = Nb, W) fragments, producing [(A-PW9O34)2(MO)3]8– Herve-type anion. The product was isolated as Cs8[(PW9O34)2(WO)(NbO)2]∙17.5H2O (1) and characterized with XRD, XRPD, IR and 31P MAS NMR. Diffusion of diethyl ether into solution of (TBA)4[PW11NbO40] and Pb(NO3)2 in N,N-dimethylformamide (DMF) leads to crystallization of [Pb2(µ2-DMF)3(DMF)8][PW11NbO40]∙DMF (2) with polymeric nature, where the{Pb2} cationic dimers and [PW11NbO40]4– anions form infinite linear chains via Pb - O=M coordination. Complex (2) was characterized with XRD, IR and 31P MAS NMR.
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The chemistry of mixed polyoxotungstates, incorporating Nb or Ta has emerged as an actively pursued research field in last decade due to: i) highly versatile structural behavior [1], ii) rich and complicated chemistry in aqueous solutions, iii) catalytic and photocatalytic activity in oxidation and water splitting. One of the crucial issues of the chemistry of Nb-containing polytungstates is their complicated and unpredictable solution behavior. Many POMs forming this family are stable only in solid state [2], while solution studies demonstrate gradual evolution, very often leading to very complicated mixtures, including new Nb/W species with unknown structures. Accordingly, NMR and ESI-MS data tend to be too complicated for straightforward interpretation. For example, only by coupling 31P NMR with hyphenated HPLC-ICP-AES technique [3] was it possible to interpret the behavior of [PW11NbO40]4– (taken as the simplest representative example of Nb-substituted heteropolytungstate) in aqueous solutions [4]. In this study we found several interesting features: i) absence of Nb-O-Nb dimerization even at very low pH (unlike for other mixed Nb/W POMs [5] and pure polyniobates [6,7]); ii) loss of Nb and not W upon even moderate acidification (pH 4); iii) slow spontaneous rearrangement of [PW11NbO40]4– in weakly acidic solutions into [PW10Nb2O40]4– [4]. In this contribution we report some new aspects of [PW11NbO40]4– chemistry dealing with hydrolytic stability and attempts to differentiate reactivity the donating ability of Nb-oxo and W-oxo sites. Low solubility of Cs+ salts of various Keggin anions, and the ease of separation which this property entails, makes them good heterogeneous catalysts in their own right [8–10]. For example, in the case of Cs+ salts of [PW12O40]3– the amount of cesium significantly influences the crystal structure resulting in the formation of a microporous structure of CsxH3-x[PW12O40] at
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x= 2.5 (130 m2/g), while at x below 2 it does not possess any free volume [10–12]. Consequently, further characterization of Cs4[PW11NbO40] is important and possibly could provide a new type of support with different types of catalytic sites. However, poor solubility of Cs salts turns their recrystallization into a difficult task. Attempt to digest Cs4[PW11NbO40]∙6H2O precipitate with boiling water gives a milky suspension which coagulates very slow. 31P NMR spectrum of the solution left in contact with the solid after allowing the suspension to settle for three weeks demonstrates a complicated set of signals with three main resonances at -11.7, -12.3 and -12.6 ppm (Fig. S1), together with the signal from free phosphate. At the same time the specific sets of signals from the mixed Nb/W [PW12-xNbxO40]n– Keggin anions typically presented as isomers mixtures [13] are lacking. In one month colorless blockshaped crystals appeared on the surface and in the bulk of white precipitate, which were picked up and studied by XRD and XRPD (Fig. S2).
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According to the XRD analysis the complex crystallizes in cubic crystal system (P213 space group, a = 20.2937(4) Å, CCDC 1860017) and contains [(PW9O34)2(MO(H2O))3]n– anion. The refinement gives 8 Cs+ cations, and EDS detects Nb and reduced amount of W. Nor other ionic constituents able to affect the charge balance (such as Na+ or Cl-) have been detected. Hence, based on the refinement and EDS data, the composition of the anion is [(PW9O34)2(WO)(NbO)2]8– (Fig. 1).
Fig. 1. The structure of [(PW9O34)2(WO)(NbO)2]8– (mixed Nb/W positions are marked in green). In the crystal structure the mixed Nb/W positions in the equatorial belt are disordered (Fig. 1, marked in green) over two sites with 0.9/0.1 occupancies; they are numbered as W7 and W8 correspondingly (d(“W7”….”W8”) is 1.20(3)Å, d(“W8”….”W8”) is 3.10(5) Å). Refinement gives ca. 1.7 Nb per anion, which is closed to analytical data. This specific disorder should prevent the formation of free space inside the equatorial M3 triangle by shifting metal atom positions inside the structure. The “W7”-O bond distances lie between 2.00(4) - 2.06(5) Å, in addition, d(“W7”-O10W) is 2.26(5) Å. The second W8 position has elongated M-O distances
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which vary from 2.44(5) to 2.60(6) Å, while d(“W8”-O11W(inside the W7 triangle edge)) is 2.26(5) Å, and for the adjacent W7 d(“W7”-O11W) is only 1.92(5) Å, which in fact indicates the presence of M=O group, looking inside the M3 triangle. The second position of O10W, bonded to W8, was not located directly due to small occupancy. It is surprising that degradation of Cs4[PW11NbO40] yields individual [(PW9O34)2(WO)(NbO)2]8–, since attempt at the direct rational synthesis from the structural constituents gives a complicated mixture. In the structure of related Si-based anion [Nb2K(H2O)4(A-α-SiW9O34)2]9–, isolated as K6H3[Nb2K(H2O)4(A-αSiW9O34)2]·23H2O [14], the equatorial Nb–O distances lie between 2.01(1)–2.04(1) Å and axial Nb–O distances vary in 1.60(2)–2.48(2) Å indicating presence of oxo and aqua ligands. The Nb atom is disordered with 60% and 40% occupancy factor over two sites. It seems that disorder of the O=M-H2O fragments is typical for such kind of POM complexes with {O=M-H2O} groups in the equatorial belt.
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Comparison between the IR spectra of Cs4[PW11NbO40]∙6H2O, Cs+ salt of [A-PW9O34]9– (isolated by the addition of CsCl to solution of Na9[A-PW9O34] at native pH) and 1 is shown in SI (Fig. S3). It can be seen that IR spectrum of 1 demonstrates a strong band at 972 cm–1, which can be assigned to the belt M=O groups stretches. 31P MAS NMR of 1 at 15 KHz demonstrates a wide major signal at -12.9 ppm (95%), indicating overlapping of signals from different isomers originating form Nb/W statistical disorder in the belt. In addition, the spectrum has a minor signal at -10.8 ppm (5%) which can be due to shifting of M=O to the central part of the belt. It should be noted that all- tungsten analogue [P2W21O71(H2O)3]6– has only one signal, at -13.3 ppm in solution [15,16], and at -13.5 ppm in 31P MAS NMR [17]. The upfield shift of the 31P signal in the case of Nb substituted POM agrees with our data for Nb-sustituted containing Keggin anions [13]. Unfortunately, 1 is so poorly soluble that 31P NMR spectrum in aqueous solution cannot be recorded.
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The difference in the reactivity between W=O and Nb=O groups in [PW11NbO40]4– was studied by E. Radkov et al. in the reaction of [Cp2Zr(OTf)2]∙THF (Cp = η5-C5H5; OTf– is O3SCF3–) with (Bu4N)4[PW11NbO40] [18]. This reaction yields a single product, where two [PW11NbO40]4– moieties are coordinated to the {Cp2Zr}2+ group via Nb-O-Zr bridges. Moreover, metallation of the anion with Me3EOTf (E = Si, Ge, Sn, Pb; OTf = O3SCF3) results in formation of functionalized POMs with selective coordination of E to O=Nb [19]. It appears that enhance Lewis basicity of this oxo (Nb=O) ligand can be used for construction of hybrid materials involving other metal cations. We studied complexation of [PW11NbO40]4– with Co2+ and Ni2+ in different solvents [4], but failed to detect direct coordination in the solid state due to much stronger salvation of these cations. In this work we chose Pb2+, which is known to be very labile, in the system [PW11NbO40]4–/Pb2+ in DMF. Diethyl ether diffusion into the reaction mixture yielded colorless crystals of [Pb2(µ2-DMF)3(DMF)8][PW11NbO40]∙DMF (2) which were isolated and studied by XRD. Complex [Pb2(µ2-DMF)3(DMF)8][PW11NbO40]∙DMF (2) crystallizes in triclinic crystal system. The best solution of the crystal structure was achieved after choosing P-1 space group (CCDC 1859950). In the structure eight-coordinated Pb2+ cations form dimers [(DMF)4Pb(µ2DMF)3Pb(DMF)4]4+ ({Pb2}) using three bridged DMF molecules (d(Pb-O) = 2.62(4) 2.74(2) Å). Each Pb2+ has four terminal DMF molecules in the coordination sphere (d(Pb-O) = 2.37(6) - 2.68(5) Å). The eight coordination site of each Pb2+ is occupied by an oxo ligand from [PW11NbO40]4– α-Keggin type anion (d(Pb1-O12) = 2.71(2) Å; d(Pb2-O1) = 2.61(2) Å)
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producing an infinite pseudo linear chains (Fig. 2) running along [112] crystal direction (Fig. S5).
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Fig. 2. The structure of [(DMF)4Pb(µ2-DMF)3Pb(DMF)4]4+ dimer in the crystal structure of 2. Disordered fragments of DMF molecules are omitted for clarity.
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Two Pb-O=W distances are not equal, probably due to the packing effects, which appear to be very strong in this particular structure. An indication of this is location of the inversion center inside the phosphorus atom, and not in the center of the {Pb}2 unit. This produces initial disorder of {PO4} over two positions (0.5/0.5 occupancies), and subsequent disorder of all [PW11NbO40]4– anions. This induces, in turn, disorder of coordinated DMF in the coordination sphere of each Pb. Due to chaotic orientation of the Keggin anions we have only overlapped picture with elongated thermal ellipsoids even for W. Solution of the crystal structure in P1 shows also two positions of the {PO4} tetrahedra and splitting of each Pb position. Such kind of full disorder of the crystal structure does not allow to discriminate between the Nb and W positions. Recently we reported a coordination polymer based on Keggin-type [SiW12O40]4- and Pb2+ ions, [Pb2(µ2-DMF)2(DMF)8(SiW12O40)], prepared from H4[SiW12O40] and Pb(NO3)2 in DMF. By slight modification of crystallization conditions, a complex with a different coordination modes of the {Pb2} unit and solvate content, [Pb2(µ2DMF)2(DMF)8(SiW12O40)]∙DMF, can be obtained. In this latter case the tungstosilicate, which has the same charge as [PW11NbO40]4– is also coordinated to Pb2+ through a terminal oxygen ligand [20]. In the C2/c phase the {Pb2} unit locates perpendicularly to the chain orientation and oxo ligands of POM units play the role µ2-bridges. In the case of P-1 phase the {Pb2} and [SiW12O40]4– units form zig-zag chains, while both {Pb2} and silicotungstate units reside in common positions. This, change from [SiW12O40]4– to [PW11NbO40]4– follows conversion of the {Pb2} dimer orientation to practically linear mode (Fig. 2). A possible reason of this can be packing effect or, eventually, uneven distribution of the charge density on the anion surface due to the presence of niobium. Comparison of 31P MAS NMR spectra of Cs4[PW11NbO40]∙6H2O, Cs+ salt of [A-PW9O34]9– (isolated by the addition of CsCl to solution of Na9[A-PW9O34] at natural pH) and 2 is shown in
ACCEPTED MANUSCRIPT SI (Fig. S6). Complex 2 has one signal at -14 ppm, which is close to -13.7 ppm for Cs4[PW11NbO40]∙6H2O. A marked difference in reactivity of [PW12O40]3– and [SiW12O40]4– towards DMF-solvated Bi3+ has been recently established, due to differences in the charge density [21]. Only tungstosilicate is able to coordinate directly Bi3+, while tungstophosphate prefers solvent-separated ionic pair, stabilizing previously unknown [Bi(DMF)8]3+ cation. The complex [Bi(DMF)7(HSiW12O40)] represents a rare case of Bi-containing polyoxotungstate complex.
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These complexes of POMs with Pb and Bi can be used as precursors for tuneable bismuth-POM catalysts which are successfully applied in industry. (e.g., BiPMo12O40 for oxidation of n-butane into maleic anhydride) [10].
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This research expands our earlier observations about inherent instability of [PW11NbO40]4– in aqueous solutions, which entail remarkable structural rearrangements. Thus boiling a suspension of Cs4[PW11NbO40]∙12H2O with subsequent maceration in water selectively produces [(PW9O34)2(WO)(NbO)2]8– anion which cannot be accessed in a direct way from [PW9O34]9-. Like [SiW12O40]4–, [PW11NbO40]4– can be used for construction of coordination polymers. In this case, the same overall charge notwithstanding, the presence of Nb can induces subtle changes in the way of linking of the building blocks.
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The NIIC team thanks Federal Agency for Scientific Organizations for funding. This work was done within the framework of implementation of a project part of the state task for 2017-2019 № 10.1448.2017/4.6.
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Graphical abstract
ACCEPTED MANUSCRIPT Highlights Cs4[PW11NbO40] converts into Cs8[(PW9O34)2(WO)(NbO)2]; The product was characterized with XRD, XRPD, IR and 31P MAS NMR; (TBA)4[PW11NbO40] and Pb(NO3)2 combine into a polymer; {Pb2} cationic dimers and [PW11NbO40]4– anions form infinite linear chains;
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Complex was characterized with XRD, IR and 31P MAS NMR.
Ekaterina P. Bushmeleva, Nikolay B. Kompankov, Rishat R. Shiriyazdanov, Albina R. Karimova, Pavel A. Abramov, Maxim N. Sokolov PII: DOI: Reference:
S1387-7003(18)30750-0 https://doi.org/10.1016/j.inoche.2018.10.026 INOCHE 7160
To appear in:
Inorganic Chemistry Communications
Received date: Revised date: Accepted date:
10 August 2018 20 October 2018 23 October 2018
Please cite this article as: Ekaterina P. Bushmeleva, Nikolay B. Kompankov, Rishat R. Shiriyazdanov, Albina R. Karimova, Pavel A. Abramov, Maxim N. Sokolov , Chemistry of [PW11NbO40]4−: Structural rearrangement and metal coordination. Inoche (2018), https://doi.org/10.1016/j.inoche.2018.10.026
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Chemistry of [PW11NbO40]4–: structural rearrangement and metal coordination Ekaterina P. Bushmeleva1,2, Nikolay B. Kompankov2, Rishat R. Shiriyazdanov3, Albina R. Karimova3, Pavel A. Abramov*1,2, Maxim N. Sokolov1,2 1
Nikolaev Institute of Inorganic Chemistry SB RAS, 3 Akad. Lavrentiev Ave., 630090 Novosibirsk. Russia; Novosibirsk State University, 2 Pirogova Str., 630090 Novosibirsk, Russia
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Ufa State Petroleum Technological University, 1 Kosmonavtov str., 450062 Ufa, Russia
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Abstract. Treatment of Cs4[PW11NbO40]∙12H2O with boiling water results in slow degradation of the [PW11NbO40]4– Keggin anion into the trilacunary [A-PW9O34]9– following by aggregation of the latter around three {M=O} (M = Nb, W) fragments, producing [(A-PW9O34)2(MO)3]8– Herve-type anion. The product was isolated as Cs8[(PW9O34)2(WO)(NbO)2]∙17.5H2O (1) and characterized with XRD, XRPD, IR and 31P MAS NMR. Diffusion of diethyl ether into solution of (TBA)4[PW11NbO40] and Pb(NO3)2 in N,N-dimethylformamide (DMF) leads to crystallization of [Pb2(µ2-DMF)3(DMF)8][PW11NbO40]∙DMF (2) with polymeric nature, where the{Pb2} cationic dimers and [PW11NbO40]4– anions form infinite linear chains via Pb - O=M coordination. Complex (2) was characterized with XRD, IR and 31P MAS NMR.
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The chemistry of mixed polyoxotungstates, incorporating Nb or Ta has emerged as an actively pursued research field in last decade due to: i) highly versatile structural behavior [1], ii) rich and complicated chemistry in aqueous solutions, iii) catalytic and photocatalytic activity in oxidation and water splitting. One of the crucial issues of the chemistry of Nb-containing polytungstates is their complicated and unpredictable solution behavior. Many POMs forming this family are stable only in solid state [2], while solution studies demonstrate gradual evolution, very often leading to very complicated mixtures, including new Nb/W species with unknown structures. Accordingly, NMR and ESI-MS data tend to be too complicated for straightforward interpretation. For example, only by coupling 31P NMR with hyphenated HPLC-ICP-AES technique [3] was it possible to interpret the behavior of [PW11NbO40]4– (taken as the simplest representative example of Nb-substituted heteropolytungstate) in aqueous solutions [4]. In this study we found several interesting features: i) absence of Nb-O-Nb dimerization even at very low pH (unlike for other mixed Nb/W POMs [5] and pure polyniobates [6,7]); ii) loss of Nb and not W upon even moderate acidification (pH 4); iii) slow spontaneous rearrangement of [PW11NbO40]4– in weakly acidic solutions into [PW10Nb2O40]4– [4]. In this contribution we report some new aspects of [PW11NbO40]4– chemistry dealing with hydrolytic stability and attempts to differentiate reactivity the donating ability of Nb-oxo and W-oxo sites. Low solubility of Cs+ salts of various Keggin anions, and the ease of separation which this property entails, makes them good heterogeneous catalysts in their own right [8–10]. For example, in the case of Cs+ salts of [PW12O40]3– the amount of cesium significantly influences the crystal structure resulting in the formation of a microporous structure of CsxH3-x[PW12O40] at
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x= 2.5 (130 m2/g), while at x below 2 it does not possess any free volume [10–12]. Consequently, further characterization of Cs4[PW11NbO40] is important and possibly could provide a new type of support with different types of catalytic sites. However, poor solubility of Cs salts turns their recrystallization into a difficult task. Attempt to digest Cs4[PW11NbO40]∙6H2O precipitate with boiling water gives a milky suspension which coagulates very slow. 31P NMR spectrum of the solution left in contact with the solid after allowing the suspension to settle for three weeks demonstrates a complicated set of signals with three main resonances at -11.7, -12.3 and -12.6 ppm (Fig. S1), together with the signal from free phosphate. At the same time the specific sets of signals from the mixed Nb/W [PW12-xNbxO40]n– Keggin anions typically presented as isomers mixtures [13] are lacking. In one month colorless blockshaped crystals appeared on the surface and in the bulk of white precipitate, which were picked up and studied by XRD and XRPD (Fig. S2).
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According to the XRD analysis the complex crystallizes in cubic crystal system (P213 space group, a = 20.2937(4) Å, CCDC 1860017) and contains [(PW9O34)2(MO(H2O))3]n– anion. The refinement gives 8 Cs+ cations, and EDS detects Nb and reduced amount of W. Nor other ionic constituents able to affect the charge balance (such as Na+ or Cl-) have been detected. Hence, based on the refinement and EDS data, the composition of the anion is [(PW9O34)2(WO)(NbO)2]8– (Fig. 1).
Fig. 1. The structure of [(PW9O34)2(WO)(NbO)2]8– (mixed Nb/W positions are marked in green). In the crystal structure the mixed Nb/W positions in the equatorial belt are disordered (Fig. 1, marked in green) over two sites with 0.9/0.1 occupancies; they are numbered as W7 and W8 correspondingly (d(“W7”….”W8”) is 1.20(3)Å, d(“W8”….”W8”) is 3.10(5) Å). Refinement gives ca. 1.7 Nb per anion, which is closed to analytical data. This specific disorder should prevent the formation of free space inside the equatorial M3 triangle by shifting metal atom positions inside the structure. The “W7”-O bond distances lie between 2.00(4) - 2.06(5) Å, in addition, d(“W7”-O10W) is 2.26(5) Å. The second W8 position has elongated M-O distances
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which vary from 2.44(5) to 2.60(6) Å, while d(“W8”-O11W(inside the W7 triangle edge)) is 2.26(5) Å, and for the adjacent W7 d(“W7”-O11W) is only 1.92(5) Å, which in fact indicates the presence of M=O group, looking inside the M3 triangle. The second position of O10W, bonded to W8, was not located directly due to small occupancy. It is surprising that degradation of Cs4[PW11NbO40] yields individual [(PW9O34)2(WO)(NbO)2]8–, since attempt at the direct rational synthesis from the structural constituents gives a complicated mixture. In the structure of related Si-based anion [Nb2K(H2O)4(A-α-SiW9O34)2]9–, isolated as K6H3[Nb2K(H2O)4(A-αSiW9O34)2]·23H2O [14], the equatorial Nb–O distances lie between 2.01(1)–2.04(1) Å and axial Nb–O distances vary in 1.60(2)–2.48(2) Å indicating presence of oxo and aqua ligands. The Nb atom is disordered with 60% and 40% occupancy factor over two sites. It seems that disorder of the O=M-H2O fragments is typical for such kind of POM complexes with {O=M-H2O} groups in the equatorial belt.
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Comparison between the IR spectra of Cs4[PW11NbO40]∙6H2O, Cs+ salt of [A-PW9O34]9– (isolated by the addition of CsCl to solution of Na9[A-PW9O34] at native pH) and 1 is shown in SI (Fig. S3). It can be seen that IR spectrum of 1 demonstrates a strong band at 972 cm–1, which can be assigned to the belt M=O groups stretches. 31P MAS NMR of 1 at 15 KHz demonstrates a wide major signal at -12.9 ppm (95%), indicating overlapping of signals from different isomers originating form Nb/W statistical disorder in the belt. In addition, the spectrum has a minor signal at -10.8 ppm (5%) which can be due to shifting of M=O to the central part of the belt. It should be noted that all- tungsten analogue [P2W21O71(H2O)3]6– has only one signal, at -13.3 ppm in solution [15,16], and at -13.5 ppm in 31P MAS NMR [17]. The upfield shift of the 31P signal in the case of Nb substituted POM agrees with our data for Nb-sustituted containing Keggin anions [13]. Unfortunately, 1 is so poorly soluble that 31P NMR spectrum in aqueous solution cannot be recorded.
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The difference in the reactivity between W=O and Nb=O groups in [PW11NbO40]4– was studied by E. Radkov et al. in the reaction of [Cp2Zr(OTf)2]∙THF (Cp = η5-C5H5; OTf– is O3SCF3–) with (Bu4N)4[PW11NbO40] [18]. This reaction yields a single product, where two [PW11NbO40]4– moieties are coordinated to the {Cp2Zr}2+ group via Nb-O-Zr bridges. Moreover, metallation of the anion with Me3EOTf (E = Si, Ge, Sn, Pb; OTf = O3SCF3) results in formation of functionalized POMs with selective coordination of E to O=Nb [19]. It appears that enhance Lewis basicity of this oxo (Nb=O) ligand can be used for construction of hybrid materials involving other metal cations. We studied complexation of [PW11NbO40]4– with Co2+ and Ni2+ in different solvents [4], but failed to detect direct coordination in the solid state due to much stronger salvation of these cations. In this work we chose Pb2+, which is known to be very labile, in the system [PW11NbO40]4–/Pb2+ in DMF. Diethyl ether diffusion into the reaction mixture yielded colorless crystals of [Pb2(µ2-DMF)3(DMF)8][PW11NbO40]∙DMF (2) which were isolated and studied by XRD. Complex [Pb2(µ2-DMF)3(DMF)8][PW11NbO40]∙DMF (2) crystallizes in triclinic crystal system. The best solution of the crystal structure was achieved after choosing P-1 space group (CCDC 1859950). In the structure eight-coordinated Pb2+ cations form dimers [(DMF)4Pb(µ2DMF)3Pb(DMF)4]4+ ({Pb2}) using three bridged DMF molecules (d(Pb-O) = 2.62(4) 2.74(2) Å). Each Pb2+ has four terminal DMF molecules in the coordination sphere (d(Pb-O) = 2.37(6) - 2.68(5) Å). The eight coordination site of each Pb2+ is occupied by an oxo ligand from [PW11NbO40]4– α-Keggin type anion (d(Pb1-O12) = 2.71(2) Å; d(Pb2-O1) = 2.61(2) Å)
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producing an infinite pseudo linear chains (Fig. 2) running along [112] crystal direction (Fig. S5).
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Fig. 2. The structure of [(DMF)4Pb(µ2-DMF)3Pb(DMF)4]4+ dimer in the crystal structure of 2. Disordered fragments of DMF molecules are omitted for clarity.
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Two Pb-O=W distances are not equal, probably due to the packing effects, which appear to be very strong in this particular structure. An indication of this is location of the inversion center inside the phosphorus atom, and not in the center of the {Pb}2 unit. This produces initial disorder of {PO4} over two positions (0.5/0.5 occupancies), and subsequent disorder of all [PW11NbO40]4– anions. This induces, in turn, disorder of coordinated DMF in the coordination sphere of each Pb. Due to chaotic orientation of the Keggin anions we have only overlapped picture with elongated thermal ellipsoids even for W. Solution of the crystal structure in P1 shows also two positions of the {PO4} tetrahedra and splitting of each Pb position. Such kind of full disorder of the crystal structure does not allow to discriminate between the Nb and W positions. Recently we reported a coordination polymer based on Keggin-type [SiW12O40]4- and Pb2+ ions, [Pb2(µ2-DMF)2(DMF)8(SiW12O40)], prepared from H4[SiW12O40] and Pb(NO3)2 in DMF. By slight modification of crystallization conditions, a complex with a different coordination modes of the {Pb2} unit and solvate content, [Pb2(µ2DMF)2(DMF)8(SiW12O40)]∙DMF, can be obtained. In this latter case the tungstosilicate, which has the same charge as [PW11NbO40]4– is also coordinated to Pb2+ through a terminal oxygen ligand [20]. In the C2/c phase the {Pb2} unit locates perpendicularly to the chain orientation and oxo ligands of POM units play the role µ2-bridges. In the case of P-1 phase the {Pb2} and [SiW12O40]4– units form zig-zag chains, while both {Pb2} and silicotungstate units reside in common positions. This, change from [SiW12O40]4– to [PW11NbO40]4– follows conversion of the {Pb2} dimer orientation to practically linear mode (Fig. 2). A possible reason of this can be packing effect or, eventually, uneven distribution of the charge density on the anion surface due to the presence of niobium. Comparison of 31P MAS NMR spectra of Cs4[PW11NbO40]∙6H2O, Cs+ salt of [A-PW9O34]9– (isolated by the addition of CsCl to solution of Na9[A-PW9O34] at natural pH) and 2 is shown in
ACCEPTED MANUSCRIPT SI (Fig. S6). Complex 2 has one signal at -14 ppm, which is close to -13.7 ppm for Cs4[PW11NbO40]∙6H2O. A marked difference in reactivity of [PW12O40]3– and [SiW12O40]4– towards DMF-solvated Bi3+ has been recently established, due to differences in the charge density [21]. Only tungstosilicate is able to coordinate directly Bi3+, while tungstophosphate prefers solvent-separated ionic pair, stabilizing previously unknown [Bi(DMF)8]3+ cation. The complex [Bi(DMF)7(HSiW12O40)] represents a rare case of Bi-containing polyoxotungstate complex.
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These complexes of POMs with Pb and Bi can be used as precursors for tuneable bismuth-POM catalysts which are successfully applied in industry. (e.g., BiPMo12O40 for oxidation of n-butane into maleic anhydride) [10].
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This research expands our earlier observations about inherent instability of [PW11NbO40]4– in aqueous solutions, which entail remarkable structural rearrangements. Thus boiling a suspension of Cs4[PW11NbO40]∙12H2O with subsequent maceration in water selectively produces [(PW9O34)2(WO)(NbO)2]8– anion which cannot be accessed in a direct way from [PW9O34]9-. Like [SiW12O40]4–, [PW11NbO40]4– can be used for construction of coordination polymers. In this case, the same overall charge notwithstanding, the presence of Nb can induces subtle changes in the way of linking of the building blocks.
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Acknowledgments
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The NIIC team thanks Federal Agency for Scientific Organizations for funding. This work was done within the framework of implementation of a project part of the state task for 2017-2019 № 10.1448.2017/4.6.
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Graphical abstract
ACCEPTED MANUSCRIPT Highlights Cs4[PW11NbO40] converts into Cs8[(PW9O34)2(WO)(NbO)2]; The product was characterized with XRD, XRPD, IR and 31P MAS NMR; (TBA)4[PW11NbO40] and Pb(NO3)2 combine into a polymer; {Pb2} cationic dimers and [PW11NbO40]4– anions form infinite linear chains;
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Complex was characterized with XRD, IR and 31P MAS NMR.