Photoreductive Self-Assembly from [Mo7O24]6- to Carboxylates

Langmuir 2005, 21, 7823-7832

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Photoreductive Self-Assembly from [Mo7O24]6- to Carboxylates-Coordinated {Mo142} Mo-Blue Nanoring in the Presence of Carboxylic Acids Toshihiro Yamase,* Yutaka Yano, and Eri Ishikawa Chemical Resources Laboratory, Tokyo Institute of Technology, R1-21, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, and “Creation of bio-devices and bio-systems with chemical and biological molecules for medical use”, CREST, Japan Science and Technology Agency (JST) Received May 16, 2005 The primary steps of the photoredox reaction between [Mo7O24]6- and carboxylic acid electron (and proton) donors in aqueous solutions are investigated by the chemically induced dynamic electron spin polarization (CIDEP) spectroscopy. The excitation of the O f Mo ligand-to-metal charge-transfer (LMCT) bands of [Mo7O24]6- in the presence of CH3CO2H induces the emissive electron spin polarization (ESP) of •CH2CΟ2Η and •CH3 radicals with an accompanying formation of the one-electron reduced species [Mo7O23(OH)]6-, which is demonstrated by the triplet mechanism involving the O f Mo LMCT triplet states. The prolonged photolysis of the solution containing [Mo7O24]6- and CH3CO2H at pH ) 3.4 leads to the formation of the acetate/propionate-coordinated {Mo142} Mo-blue nanoring, [MoV28MoVI114O429H10(H2O)49(CH3CO2tAc)5(C2H5CO2tPr)]30- (1a) through the formation of the cis-configured dimeric dehydrative condensation to two-electron reduced Mo-blue [(Mo7O23)2]10- (t{Mo14}). 1a is isolated as a [NH4]+/[Me3NH]+-mixed salt which is formulated as [NH4]27[Me3NH]3[MoV28MoVI114O429H10(H2O)49(CH3CO2)5(C2H5CO2)]‚150 ( 10H2O (1) by results of elementary analysis, single-crystal X-ray analysis, 1H NMR, IR, and UV/Vis measurements, and manganometric redox titration. Based on the building-block sequence of {Mo20(Ac)(Pr)1/4}{Mo21(Ac)1/2(Pr)1/4}{Mo20(Ac)}{Mo20(Ac)}{Mo21(Ac)1/2(Pr)1/4}{Mo20(Ac)(Pr)1/4}{Mo20} for 1a, the bottom-up processes from [Mo7O24]6- to the {Mo142} ring in the coexistence of β-[Mo8O26]4- are discussed by (i) the stabilization of the molecular curvature of {Mo14} through both the intramolecular transfer of monomolybdates and the intermolecular transfer of monomolybdates as degradation fragments of β-[Mo8O26]4-, to yield {Mo21} and {Mo20} building blocks, (ii) the outer-ring formation resulting from seven successive two-electron-photoreductive condensations among {Mo21} and {Mo20}, and (iii) inner-ring formation resulting from eight successive dehydrative condensations between monomolybdate linkers attached to the neighboring head Mo sites.

1. Introduction The photolysis of the [Mo7O24]6-/electron (and proton) donor (DH) system in aqueous solutions at pH 5-6 leads to formation of the dimerically condensed Mo-blue, [(Mo7O23)2]10- (t{Mo14}), which is a cis-configured twoelectron reduced species with the molecular curvature of the -O-Mo-O-Mo-O- linkage (for the backbone framework) consisting of 175° as the central Mo-O-Mo bond angle ()ω1) of {Mo14}, 153° as the central Mo-O-Mo bond angle ()ω2) of the half-moiety, and 157° as the O-Mo-O bond angle ()ω3) for linking these two centers.1 Figure 1 shows a scheme for two-electron photoreductive condensation of [Mo7O24]6- to {Mo14} with molecular curvatures. A variety of Mo-blues, the formation of which was wellknown on the reduction of isopolymolybdates in strongly acidic aqueous solutions, has been successfully characterized by Mu¨ller as condensed nanorings of 28- and 32electron species such as [Mo154O448(OH)14(H2O)70]14({Mo154}), [Mo142O432(OH)14(H2O)58]26- ({Mo142}), and [M176O512(OH)16(H2O)80]16- ({Mo176}), where {Mo154} and {Mo176} are intact car-tire-shaped rings with D7d and D8d symmetries, respectively, and {Mo142} is a deficient ring missing six [Mo(H2O)O2(µ-O)Mo(H2O)O2]2+ units (t{Mo2}* To whom correspondence should be addressed. Tel and Fax: +81-45-924-5260. E-mail: [email protected]. (1) Yamase, T. J. Chem. Soc., Dalton Trans. 1991, 3055-3063.

Figure 1. Two-electron photoreductive condensation of [Mo7O24]6- to [(Mo7O23)2]10- ({Mo14}) and its molecular curvature. Mo atoms are indicated by yellow color.

linker units) from {Mo154}.2-5 The formation of the same typed nanorings of {Mo154} and {Mo142} has been also (2) (a) Mu¨ller, A.; Beugholt, C.; Koop, M.; Das, S. K.; Schmidtmann, M.; E.; Bo¨gge, H. Z. Anorg. Allg. Chem. 1999, 625, 1960-1962. (b) Mu¨ller, A.; Krickemeyer, E.; Bo¨gge, H.; Schmidtmann, M.; Beugholt, C.; Das, S. K.; Peters, F. Chem. Eur. J. 1999, 5, 1496-1502. (3) Mu¨ller, A.; Das, S. K.; Krickemeyer, E.; Kuhlmann, C. In Inorganic Synthesis; Shapley, J., Ed.; John Wiley & Sons: New York, 2004; pp 191-200. (4) Mu¨ller, A.; Ko¨gerler, P.; Dress, A. W. M. Coord. Chem. Rev. 2001, 222, 193-218. (5) Ko¨gerler, P.; Mu¨ller, A. In Polyoxometalate chemistry for nanocomposite design; Yamase, T., Pope, M. T., Eds.; KluwerAcademic/ Plenum Pubs.: New York, 2002; pp 1-15.

10.1021/la051301v CCC: $30.25 © 2005 American Chemical Society Published on Web 07/20/2005

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Figure 2. Scheme (represented by Mo frameworks) proposed for the photoinduced self-assembly of {Mo36} to {Mo154} through the dehydrative condensations of {Mo22} with similarity to the photodimeric condensation of [Mo7O24]6- to {Mo14}. Yellow-colored atoms indicate Mo atoms of (Mo7O24) moiety in {Mo36}, and linker, head, 7-fold, and equator-center Mo atoms are defined in a ball-and stick picture of {Mo22}. {Mo2}-linker is derived from the dehydrative condensation between neighboring linker MoO6 octahedra within the same inner-ring.

shown by the photolysis of aqueous solutions containing [Mo36O112(H2O)16]8-(t{Mo36}) and alkylammonium cations as electron donors at pH ) 1 and 2, respectively, as was characterized as [MoV28MoVI126O462(H2O)70]28- and [MoV28MoVI114O404(OH)28(H2O)58]12-.6,7 Since {Mo36} incorporates two Mo7O24 moieties in its centrosymmetric structure which allow 7-fold pentagonal-bipyramidal MoO7 sites,8 it has been proposed that the degradative photoreductive condensation of the half molecules ({Mo18}) proceeds with a mode similar to the {Mo14} formation and results in formation of a two-electron reduced building block, [Mo22O70(H2O)10]10- (t{Mo22}), which undergoes successively two-electron-photoreductive condensations as a cyclic heptamerization to {Mo154}.6,7 The proposed scheme of photoinduced self-assembly to {Mo154} through the dehydrative condensations of {Mo22} is shown in Figure 2 where linker, head, 7-fold, and {Mo2}-linker are indicated for the identification of different types of Mo atoms. Each of the two inner-rings (above and below the equator) of the {Mo154} ring consists of seven head MoO6 octahedra (each of which is bonded to 7-fold MoO7 pentagonal bipyramid) and seven {Mo2}-linker units. {Mo36} is a predominant species in aqueous solutions of isopolymolybdates at pH e 2, although a decrease (to pH ) 2) of solution acidity results in the defect structure of {Mo142}. Therefore, {Mo36} as a starting isopolymolybdate, which incorporates two pentagonal-bipyramidal MoO7 sites, may be essential for the photochemical formation of the Moblue nanorings. As described in this paper, the photolysis of [Mo7O24]6- in the presence of carboxylic acid at the pH level of 3.4 where {Mo36} hardly exists but β-[Mo8O26]4(6) Yamase, T.; Prokop, P.; Arai, Y. J. Mol. Stuct. 2003, 656, 107117. (7) Yamase, T.; Prokop, P. Angew. Chem., Int. Ed. 2002, 37, 466469. (8) Krebs, B.; Stiller, S.; Tytko, K. H.; Mehmke, J. Eur. J. Solid State Inorg. Chem. 1991, t.28, 883-903.

coexists9 leads to the formation of a carboxylatescoordinated {Mo142} ring, [NH4]27[Me3NH]3[MoV28MoVI114O429H10(H2O)49(CH3CO2)5(C2H5CO2)]‚150 ( 10H2O (1), as another family of Mu¨ller’s acetates-coordinated {Mo138}-ring which misses four equator-center ModO atoms.10 This is interesting from the standpoint of the still-unknown self-assembly mechanism of the Mo-blue ring formation, since this finding indicates that [Mo7O24]6is photochemically converted to {Mo142} through the construction of the 7-fold pentagonal bipyramidal MoO7 sites at pH 3.4. The chemically induced dynamic electron spin polarization (CIDEP) technique has also been employed in order to investigate primary steps of the photolysis of the [Mo7O24]6-/carboxylic acids systems. The first work on the magnetic and spin effects in the photoreduction of metaloxide related compounds was done for uranyl compounds and gave details about the intermediates involved in aqueous solutions.11 An observation of emissive electronspin-polarization (ESP) signals of one-electron oxidized donor radicals in the polyoxometalate (POM)/DH ()alcohols and alkylammonium cations) systems showed an involvement of the oxygen-to-metal charge-transfer (O f M LMCT) triplet (3(O f M LMCT)) states (where superscript 3 signifies a spin multiplicity) in the photoredox reaction.6,12,13 The behavior has been interpreted (9) (a) Griffith, W. P.; Lesniak, P. J. B. J. Chem. Soc. A 1969, 10661071. (b) Aveston, J.; Anacker, E. W. Inorg. Chem. 1965, 3, 735-746. (10) Mu¨ller, A.; Maiti, R.; Schmidtmann, M.; Bo¨gge, H.; Das, S. K.; Zhang, W. Chem. Commun. 2001, 2126-2127. (11) Buchachenko, A. L.; Khudyakov, I. V. Acc. Chem. Res. 1991, 24, 177-188. (12) Yamase, T.; Ohtaka, K. J. Chem. Soc., Dalton Trans. 1994, 25992608. (13) Yamase, T. In Polyoxometalate Molecular Science; Borra´sAlamenar, J. J., Coronado, E., Mu¨ller, A., Pope, M. T., Eds.; NATO Science Series; Kluwer Academic Publishers: Dordrecht, Netherlands, 2003; pp 211-229.

Carboxylates-Coordinated {Mo142} Mo-Blue Nanoring

by the triplet mechanism (TM),14,15 i.e., the emissive ESP generated at a sublevel (3T+) of the 3(O f M LMCT) states prior to the Boltzmann distribution is transferred to the triplet state of the radical pair, 3(POM-H‚‚‚•D)+ (where subscript + indicates m ) 1 as one of the magnetic sublevels m ) 1, 0, and -1 for the triplet state, and POM-H denotes a protonated one-electron reduced POM species) which is produced by the fast reaction with DH, followed by formation of two spin-polarized radicals 2(POM-H)+ and 2(•D)+ as doublet states according to the conservation of spin angular momentum in reaction steps. Equation 1 denotes spin description of both entities of POM and DH for the photochemistry of POMs hv

DH

POM981(O f Mo LMCT) f 3(O f Mo LMCT)+ 98 3

{(POM-H)‚‚‚•D}+ f 2(POM-H)++2(•D)+ f (POM-H) + 2(•D)+ (1)

2

where the intersystem crossing (ISC) from an excited singlet state 1(O f Mo LMCT) to a triplet state 3(O f Mo LMCT) generates ESP in 3(O f Mo LMCT)+. Thus, the CIDEP spectroscopy of the [Mo7O24]6-/carboxylic acid systems again gives us a good opportunity to discuss the intermediates proposed for the solution photochemistry of the [Mo7O24]6-/CH3CO2H system,16 since the ESP signals reflect unambiguous direct observation, identification, and spin characteristics of the radical species involved in the photoredox reaction between [Mo7O24]6- and carboxylicacid electron (and proton) donor. 2. Experimental Section CIDEP Spectroscopy. CIDEP measurements were done at room temperature by a JEOL X-band ESR spectrometer (JEOL RE-1X) without field modulation.12 A CIDEP sample in a microwave cavity, which was flowed through a quartz flat cell (with 0.3-mm interior space) as a flow rate of ca. 25 cm3 min-1, was irradiated with a Questek 2320 excimer XeCl (308 nm, 50 mJ per pulse, with a repetition rate of 30 Hz) laser. The sample solutions for CIDEP measurements were prepared by adjusting pH levels of the aqueous solution (40-50 mL) containing carboxylic acids (5-20 mL, 50-400 mmol) or arenesulfinates (6-13 g, 30-80 mmol) and [NH4]6[Mo7O24]‚4H2O (3-5 g, 2-4 mmol) or Na2MoO4‚2H2O (4 g, 18 mmol) with HClO4. The ESP signals were amplified by a wide-band preamplifier of a microwave unit and fed to a NF BX-531 boxcar integrator. Spectra were measured at 0.2-1.6 µs after the laser pulse usually under the conditions of gate width of 50 ns and time window of 500 ns. The time resolution of our CIDEP set up was ca. 0.1 µs. The spectra were obtained at a microwave power of 10-40 mW. 1H NMR, IR, and UV/Vis Measurements. 1H NMR spectra were recorded on a JEOL AL-300 300.53 MHz spectrometer at 296 ( 1 K for 100% D2O solutions in 5 mm NMR tubes by using 90° pulse, a scan repetition time of 26 s, and a line-broadening factor of 0.1 Hz before FT treatment. The 1H chemical shifts were referenced via H2O as an external standard. IR (as KBr pellets) and UV/vis spectra were recorded with Jasco FT-IR 5000 and Jasco V-570 UV-vis-NIR spectrometers, respectively. Synthesis of [NH4]27[Me3NH]3[MoV28MoVI114O429H10(H2O)49(CH3CO2)5(C2H5CO2)]‚150 ( 10H2O (1). [NH4]6[Mo7O24]‚4H2O (0.4 g, 0.31 mmol) was dissolved in H2O (20 mL), and the pH level of the solution was adjusted to 3.4 with CH3CO2H. The acidified solution was exposed to UV light from a 500-W superhigh-pressure mercury/xenon lamp. [Me3NH]ClO4 (0.04 g, 0.25 mmol) was added into the deeply blue-colored solution after 2-days irradiation, and rhombohedral dark-blue crystals of 1 (14) McLauchlan, K. A.; Stevens, D. G. Acc. Chem. Res. 1988, 21, 54-59. (15) Turro, N. J.; Kleinmann, M. H.; Karatekin, E. Angew. Chem., Int. Ed. 2000, 39, 4437-4461. (16) Yamase, T.; Kurozumi, T. J. Chem. Soc., Dalton Trans. 1983, 2205-2209.

Langmuir, Vol. 21, No. 17, 2005 7825 together with colorless crystalline of [NH4]2[Me3NH]2[Mo8O26]‚ 2H2O (2) were precipitated at room temperature within 3-7 days. The dark-blue crystalline was collected and washed with cold water and thereafter dried in the air. Yield: 50 mg, 12% based on Mo. Calcd. N 1.67, C 1.05, H 2.25%; found N 1.87, C 1.14, H 2.17%. IR (KBr pellet): ν (cm-1) ) 1622 {m,δ(H2O)}, 1400 {m,δ(NH4+)}, 955(m), 908(w), 744(s), 634(s), 563(s); λ(nm) (M) ) 748 (1.6 × 105 lmol-1 cm-1), 1088 (1.1 × 105 lmol-1 cm-1). Crystal Data for 1. H566Mo142N30O640C22, M ) 25118.0, space group C2/m, a ) 52.500(3), b ) 39.524(2), c ) 16.3239(9) Å, β ) 96.750(2), V ) 33637.6 (33) Å3, Z)2, F ) 2.48 gcm-3, µ ) 26.6 cm-1, F(000) ) 23984. Crystal size ) 0.15 × 0.15 × 0.1 mm3. A crystal was coated with paraffin oil and mounted in a loop. Intensity data were measured on a Rigaku/MSC Mercury CCD diffractometer with the graphite monochromatized Mo KR radiation (λ ) 0.71071 Å) at 113 K. Data collection proceeded using ω-scan at 0.5° scan and χ ) 50° in eight runs (with 360 and 131 frames for first-seventh and eighth runs, respectively) of -75.0° < ω < 105.0°, φ ) 0°; -75.0° < ω < 105.0°, φ ) 90; -75.0° 2σ(I), R ) 0.195 and Rw ) 0.284 for all data: max/min residual electron density 6.54 and -4.38 eÅ-3. No hydrogen was included in the refinement. All of the Mo and O atoms for the anion except for two disordered atoms O66a and O66b were refined anisotropically, and the disordered O66 atoms were refined isotropically with half occupancies. All of the C and crystal-water O atoms were also refined isotropically. Site occupancies of C atoms in the anion were set to be 1/2 and 1/4 for C1-C6 and C7-C9 atoms, respectively, through the refinement of their thermal parameters. Since NH4+- and Me3NH+ammonium N and C atoms and crystal-water O atoms could not clearly be distinguished using X-ray crystal structure analysis, they were refined as crystal-water O atoms. The site occupancies for disordered atoms of these crystal-water atoms were determined based on refinement of their thermal parameters. Atomic position parameters, isotropic and anisotropic temperature parameters, and selected interatomic distances and bond-angles in Tables S1-S3 respectively, as Supporting Information. The calculation of the bond valence sums (Σs) for all the oxygen atoms of Mo-O bonds indicated the coordination of 49 aqua ligands (for Σs e 0.5) and 20 OH- ions (for 0.7 e Σs e 1.1) to the anion, although exact determination of the degree of protonation was a problem.17 The result is given also in Table S4 as Supporting Information. Crystal Data for 2. H32Mo8N4O28C6, M ) 1375.85, space group P-1 (No. 2), a ) 8.2471(9), b ) 10.385(1), c ) 11.019(1) Å, R ) 64.771(3), β ) 71.507(3), γ ) 72.755(4)°, V ) 795.1(1) Å3, Z ) 1, F ) 2.873 gcm-3, µ ) 31.48 cm-1, F(000) ) 656.0. Crystal size ) 0.50×0.50×0.30 mm3. Intensity data for the single-crystal X-ray crystallography of 2 formulated as [NH4]2[Me3NH]2[Mo8O26]‚ 2H2O was measured on a Rigaku RAXIS-RAPID imaging plate diffractometer with a graphite monochromatized Mo KR (λ ) 0.71069 Å) radiation at room temperature of 300 K. A total of 6738 reflections (2.2° < θ < 27.5°) were collected of which 3488 unique reflections (Rint ) 0.047) were used. Lorentz polarization factor were applied and an empirical absorption correction (multiscan) using equivalent reflections was performed with the program (by using the program Numabs and Shape, T. Higashi, Program for Absorption Correction, Rigaku Corporation, Tokyo, 1999). Transmission factors of 0.385-0.389 were found. The structure was solved by direct methods and refined (208 parameters) by using the CrystalStructure software package (SHELXS97) to R1 ) 0.044 for 2446 unique reflections with I >

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Figure 4. pH Dependence of ESP intensities of the central hyperfine components at mI ) 0 and 1/2 for •CH2CO2H and •CH3 radicals, respectively, observed at 0.50 µs after the laser-pulse excitation.

CO2H system are denoted by eq 2 3

(O f Mo LMCT)+ + CH3CO2H

f 3{([Mo7O24H]6-)‚‚‚•CH2CO2H}+ f 2([Mo7O24H]6-)+ + 2(•CH2CO2H)+ f ([Mo7O24H]6-) + 2(•CH2CO2H)+

2

f2([Mo7O24H]6-) + 2(•CH2CO2H) Figure 3. Emissive CIDEP spectra at different delay times after laser irradiation of [Mo7O24]6-/CH3CO2H systems at pH levels 5.7 (a) and 3.2 (b), respectively. Spectral lines (triplet) were assigned to the •CH2CO2H radical and other lines indicated by arrows (quartet), to the •CH3 radical. 2σ(I) and R ) 0.064 and Rw ) 0.143 for all data: max/min residual electron density 1.41 and -1.52 eÅ-3. H atoms were not indicated in the calculation. C, N, O, and Mo atoms were refined anisotropically. The structure of 2 in the unit cell with the atomlabeling scheme, atomic position parameters, anisotropic temperature parameters, and selected interatomic distances and bond-angles are shown in Figure S1, Tables S5-S7 respectively, as Supporting Information.

3. Results 3.1. Primary Steps for the Photoredox Reactions with Carboxylic Acids. Figure 3, panels a and b, shows the CIDEP spectra at different delay times after 308-nm laser-pulse photolysis of the [Mo7O24]6-/CH3CO2H system at pH levels 5.7 and 3.2, respectively. All of the ESP signals of two radicals •CH2CO2H (g ) 2.005, aR-H ) 2.11 mT for 1:2:1 triplet) and •CH3 (g ) 2.004, aR-H ) 2.26 mT for 1:3:3:1 quartet) with hyperfine-structural equal splitting are clearly emissive in the presence of [Mo7O24]6-. Traces of ESP were not evident in the absence of [Mo7O24]6-. The emissive ESP of the •CH2CO2H radical is mainly interpreted by TM which shows the direct abstraction of an R-hydrogen atom of CH3CO2H (with pKa ) 4.76) by 3(O f Mo LMCT)+ of [Mo7O24]6- prior to the short spin-lattice relaxation time (within submicroseconds) for 3(O f Mo LMCT)+. The decay rate of ESP (2(•CH2CO2H)+) of •CH2CO2H was approximately 0.6 µs. The broadening of 2(•CH2CO2H)+ at the early stage (e0.30 µs) after laser-pulse irradiation arises from the spin-spin exchange interaction with 2([Mo7O24H]6-) in the dissociation of the polarized 3 {([Mo7O24H]6-)‚‚‚•CH2CO2H}+ radical pair. The primary reaction steps for generation and disappearance of the TM emission of 2(•CH2CO2H)+ in line with conservation of overall spin angular momentum in the [Mo7O24]6-/CH3-

(2)

The pH dependence of the intensities of 2(•CH2CO2H)+ and 2(•CH3)+ shows an optimum around at pH ) 3.4 without significant change in both g and aR-H values, which bears a striking resemblance to that (an optimum around at pH ) 3.5) of yields of MoV, CO2, CH4, and succinic acid as photoproducts detected under the photostationary condition.16 Figure 4 shows the pH dependence of ESP intensities for the hyperfine components (at mI ) 0 and 1 /2) at 0.50 µs after the laser-pulse excitation for •CH2CO2H and •CH3, respectively. The generation of ESP (2(•CH3)+) of •CH3 results from the ESP transfer of 2(CH3CO2•)+ with “release” of carbon dioxide, which will be produced by the electron/proton transfer from CH3CO2H to 3(O f Mo LMCT)+ of [Mo7O24]6- through the formation of the charge-transfer complex (i).16

Thus, the primary reaction steps for 2(•CH3)+ also in line with conservation of overall spin angular momentum can be denoted by eq 3 3

(O f Mo LMCT)+ + CH3CO2H

f 3{(i)}+ f 3{([Mo7O24H]6-)‚‚‚•O2CCH3}+ f 2([Mo7O24H]6-)+ + 2(CH3CO2•)+ f ([Mo7O24H]6-) + 2(•CH3)+ + CO2

2

f 2([Mo7O24H]6-) + 2(•CH3)

(3)

The ESP signal of 2(•CH3)+ at pH ) 3.2 is much weaker than that of 2(•CH2CO2H)+ and virtually diminished at pH ) 5.7 (Figure 3). Furthermore, 2(•CH3)+ was observed at delays of shorter than 0.6 µs after the laser pulse


Figure 5. Emissive CIDEP spectra measured at 0.50 µs after laser irradiation of [Mo7O24]6- in the presence of electron donors such as CH2(CO2Na)2 (a), CH3CH2CO2Na (b), C6H5SO2Na (c), and p-MeC6H4SO2Na (d) at pH ) 5.7. (e) indicates the absorptive ESP measured at 0.50 µs after the direct excitation of p-MeC6H4SO2Na at pH ) 5.7 in the absence of [Mo7O24].6-

irradiation, in contrast with 2(•CH2CO2H)+ which was observed at the pH range 1.8-7.4 at delays of 0.2-1.3 µs. From the fact that CH4 was a dominant oxidation product at the pH range 1.8-7.4 under the photostationary condition,16 therefore, the weak (or little at pH > 5) signal of 2(•CH3)+ is attributed to shorter relaxation time, as was also suggested by the known spin-lattice relaxation (0.2 µs) of 2(•CH3)+ in the alkaline solution of 1.0 M NaOH.18 Other electron donors CH2(CO2Na)2, CH3CH2CO2Na, C6H5SO2Na, and p-MeC6H4SO2Na also gave the TM emission of one-electron oxidized donor radicals •CH(CO2-)2 (g ) 2.007, aR-H ) 2.01 mT for a 1:1 doublet), •CH(Me)CO2(g ) 2.011, aR-H ) 2.05 mT, aβ-H ) 2.51 mT for a 1: 1 doublet with further splitting into a 1:3:3:1 quartet for each line), •O2S(C6H5) (g ) 2.008, for a singlet), and •O2S(p-MeC6H4) (g ) 2.009 for a singlet) for the laser-flash photolysis of [Mo7O24]6- at pH ) 5.7. Figure 5 shows CIDEP spectra at 0.50-µs delay time after a laser pulse irradiation for the [Mo7O24]6-/other carboxylates or arenesulfinates systems. Hyperfine splittings (about 0.1 mT) due to ortho and para protons for arenesulfonyl radicals19 are too small to be resolved under the experimental conditions. The direct photolysis of arenesulfinates in the absence of [Mo7O24]6- led to the TM absorption signals with an involvement of the 3T- state of arenesulfinates, as was exemplified for 2{•O2S(p-MeC6H4)}- in Figure 5e. The TMemissive ESP signals of carboxylate radicals in both [Mo7O24]6-/CH2(CO2Na)2 and [Mo7O24]6-/CH3CH2CO2Na systems at pH ) 5.7 are due to the R-hydrogen abstraction by 3([Mo7O24]6-)+, and those of the arenesulfonyl radicals in both systems of [Mo7O24]6-/C6H5SO2- (with pKa ) 1.58) and [Mo7O24]6-/p-MeC6H4SO2- (with pKa ) 1.29) are due to the electron transfer from arenesulfinates to 3 ([Mo7O24]6-)+ through the charge-transfer complex formation similar to (i). (17) Brown, I. D.; Wu, K. K. Acta Crystallogr. Sect. B 1976, 32, 19571959. (18) Bartels, D. M.; Lawler, R. G.; Trifunac, A. D. J. Chem. Phys. 1985, 83, 2686-2707. (19) Mcmillan, M.; Waters, W. A. J. Chem. Soc. B 1966, 422-423.

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3.2. Structure and Formation of [NH4]27[Me3NH]3[MoV28MoVI114O429H10(H2O)49(CH3CO2)5(C2H5CO2)]‚ 150 ( 10H2O (1). The prolonged photolysis of an aqueous solution containing [NH4]6[Mo7O24]‚4H2O and CH3CO2H at pH ) 3.4 resulted in the formation of {Mo142}-ring Mo blue [MoV28MoVI114O429H10(H2O)49(CH3CO2)5(C2H5CO2)]30(1a) which was isolated as [NH4]+/[Me3NH]+ mixed salts [NH4]27[Me3NH]3[MoV28MoVI114O429H10(H2O)49(CH3CO2)5(C2H5CO2)]‚150 ( 10H2O (1). Figure 6, panels a and b, shows structures of 1a and a quarter molecule as asymmetric unit, respectively. The {Mo142} ring of 1a with approximately 1.1-nm thickness and approximately 3.4nm outer and 2.1-nm inner-ring diameters misses six {Mo2}-linker units (three units in each inner-ring) in the D7d {Mo154}-full ring with a perfect car-tire shape. As discussed below, the formation of {Mo142} is based on the photoreductive condensations among five {Mo20} (missing two monomolybdate-linkers) and two {Mo21} (missing one monomolybdate-linker) building blocks, instead of seven {Mo22} (abundant in four monomolybdate-linkers) building blocks for {Mo154}(Figure 2). In 1a, five acetates (CH3CO2-tAc) and one propionate (C2H5CO2-tPr) are attached to the inner wall of the cavity of {Mo142} with two types of coordinations: the first, each of two Ac and one Pr as bidentate (b-) ligands is coordinated by {Mo2}linker Mo atoms thereby replaces two aqua ligands with the formation of {Mo2(Ac/Pr)}unit, and the second, each of three Ac as tridentate (t-) ligands is coordinated by linker-binding MoO6 octahedron (head), MoO7 pentagonalbipyramid (7-fold), and mono-lacunary Mo6O23 central MoO6 octahedron (equator-center) Mo atoms thereby replaces one aqua and one oxo ligands with the formation of {Mo3(Ac)} unit. Figure 7 shows the coordination geometries at Ac/Pr-coordinating Mo sites. There are X-ray crystallographically eight {Mo2(Ac/Pr)}units and eight{Mo3(Ac)} units, which are disordered; half number of each type of coordinations are with half occupancies and other half with quarter occupancies. Such coordinations let us formally estimate the coordination of five Ac-ligands and one Pr-ligand to the {Mo142} inner-ring. Figure 8 shows the building-block sequence for the {Mo142(Ac)5(Pr)} ring which is formed as a result of the successive photoreductive condensation among {Mo20}, 2{Mo20(Ac)(Pr)1/4}, 2{Mo21(Ac)1/2(Pr)1/4}, and 2{Mo20(Ac)} building blocks. The building blocks {Mo20}, {Mo20(Ac)(Pr)1/4}, {Mo21(Ac)1/2(Pr)1/4}, and {Mo20(Ac)} can be further divided into formally two sub-building blocks for each; 2{Mo10}, {Mo10(b-Ac)1/2(tAc)1/2} and {Mo10(b-Pr)1/4}, {Mo11(t-Ac)1/2} and {Mo10(bPr)1/4}, and {Mo10(b-Ac)1/2(t-Ac)1/2} and {Mo10}, respectively, as shown in Figure 8. The manganometric redox titration [for the determination of the (formal) number of MoV centers] showed the existence of 28 ( 1 MoV centers for 1. None of the [Me3NH]+ cations was found in the crystal structure due to disordering. However, elemental analysis of carbon, hydrogen, and nitrogen indicates that both [Me3NH]+ and NH4+ cations are incorporated. The disorder of both cations and crystal water molecules in the present system prevents a distinction among N, C, and O atoms. Together with results of elemental and thermogravimetric analyses, thus, we formulate 1 as [NH4]27[Me3NH]3[MoV28MoVI114O429H10(H2O)49(CH3CO2)5(C2H5CO2)]‚150 ( 10H2O. The calculated bond-valence sums (20 OH- ions for 0.7 e Σs e 1.1) suggests a high degree of singly protonated oxygen atoms for 1. This implies the strong hydrogen-bonding of [NH4]+/ [Me3NH]+ cations in disorder with 10 ring-oxo atoms rather the incorportion of 20 OH- ligands which gives the number of the anion charge to be 20- in large displacement from the result (the presence of 27 [NH4]+

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Yamase et al.

Figure 6. Structures of [MoV28MoVI114O429H10(H2O)49(CH3CO2)5(C2H5CO2)]30- (1a) with the location of C2 axis and mirror plane (m) (a) and a quarter molecule of [NH4]27[Me3NH]3[MoV28MoVI114O429H10(H2O)49(CH3CO2)5(C2H5CO2)]‚150(10H2O (1) (b) as asymmetric unit. Blue-, violet-, yellow-, red-, and green-colored atoms indicate head, 7-fold, and equator-center Mo atoms, and acetate C and propionate C atoms, respectively.

Figure 7. Coordination environments at acetate (Ac)/propionate (Pr)-binding Mo sites. Blue-, violet-, yellow-, red- and greencolored atoms indicate head, 7-fold, and equator-center Mo atoms, and acetate C and propionate C atoms, respectively. (a) and (b) indicate the coordination of Ac/Pr to {Mo2}-linker Mo atoms, and (c) and (d) the coordination of Ac to head, 7-fold, and equator-center Mo atoms, respectively.

and 3 [Me3NH]+) of the elemental analysis. Each monoclinic unit cell of 1 (space group C2/m) shows the abundance of two {Mo142} rings, which are related by a translation of 1/2, 1/2, and 0. Every {Mo142(Ac)5(Pr)} ring is connected to its neighbors also via both ionic and hydrogen bondings with [NH4]+/[Me3NH]+ cations. The abundance of the coordinated ligands of Ac and Pr is also indicated by NMR spectra. Selected 1H NMR

spectrum of 1 in D2O recorded at room temperature is shown in Figure 9. The 1H NMR spectrum of 1 exhibits three unresolved singlet lines (at δ ) -1.78, -2.73, and -3.59 ppm due to [Me3NH]+, Ac, and Pr, respectively) with an intensity ratio of approximately 10:7:1 close to the number ratio (9:6:1) of [Me3NH]+ Me-hydrogen, AcMe-hydrogen, and Pr-Me-hydrogen atoms for 1, although the intensity of the Pr-CH2-hydrogen atoms


Langmuir, Vol. 21, No. 17, 2005 7829

Figure 8. Structural sequence of building blocks (red-colored) of {Mo20}, 2{Mo20(Ac)(Pr)1/4}, 2{Mo21(Ac)1/2(Pr)1/4}, and 2{Mo20(Ac)} for the {Mo142(Ac)5(Pr)} ring with the location of C2 axis and mirror plane (m). Each of the building blocks is divided into two sub-building blocks where b and t denote the bidentate and tridentate ligands, respectively. Blue-, violet-, yellow-, red-, and green-colored atoms indicate head, 7-fold, and equator-center Mo atoms, and acetate and propionate C atoms, respectively.

Figure 9. 1H NMR spectrum of [MoV28MoVI114O429H10(H2O)49(CH3CO2)5(C2H5CO2)]30- (1a). The concentration of 1a is 4 × 10-1 mol L-1 in D2O. The integration curves and intensities for lines are also indicated.

expected around at δ ) -2.6 ppm would be too weak to enable them to be detected. Each of the observed singlet lines is shifted toward downfield compared to a corresponding line of [Me3NH]Cl (at δ ) -1.90 ppm), AcNH4 (at δ ) -2.91 ppm), and PrNa (triplet around δ ) -3.76 ppm for CH3-hydrogen and quartet around δ ) -2.65 ppm for CH2-hydrogen). The downfield shift of the 1H NMR spectral lines for 1 may be related to the coordination of Ac and Pr and the hydrogen-bonding of [Me3NH]+ with the {Mo142} ring, but two types of the Ac coordinations could not be distinguished on the 1H NMR spectra. It is interesting that Pr as an oxidation product (by •CH3 + • CH2CO2H f CH3CH2CO2H) of CH3CO2H (Figure 3) is

coordinated into the inner wall of the ring cavity. The capture of the oxidation product of donor molecules in the Mo-blue ring was at first found for the cystine-coordinated full-ring, Na3[Mo154O462(H2O)48H14{HO2C-CH(NH3+)CH2-S-S-CH2CH(NH3+)sCO2}11]‚xH2O(x)250),formed thermally (at 55 °C) by reduction of Na2MoO4 with cysteine hydrochloride at pH ) 1.5.20 The structure of 1a is quite similar to the one of [MoV28MoVI110O416H6(H2O)58(CH3CO2)6]32- which was thermally isolated as NH4+ salt in the [NH4]6[Mo7O24]‚4H2O/N2H4‚H2SO4/CH3CO2H system (at pH ) 3.5) and incorporated two b-Ac ligands coordinated by {Mo2}-linker Mo atoms and four b-Ac ligands coordinated by the head/7-fold Mo atoms.10 However, it is noteworthy that in [MoV28MoVI110O416H6(H2O)58(CH3CO2)6]32- the coordination of the latter b-Ac ligands kicks out four equator-center Mo(dO) atoms together with the terminal O atoms to yield four cubane {Mo5O6} compartments in contrast with the case of 1a where the equator-center Mo(dO) atoms persist on the t-Ac coordinations as the {Mo3(Ac)} units (Figure 7). Thus, 1a can be regarded as a precursor of the hypothetical species missing three equator-center ModO atoms, [MoV28MoVI111O426H10(H2O)49(CH3CO2)5(C2H5CO2)]42-, if the steric strain for the t-Ac coordination would be large enough to expel the equator-center ModO group. The photochemical formation of {Mo142} at pH ) 3.4 where [Mo7O24]6- and β-[Mo8O26]4- are dominant species as isopolymolybdates strongly supports that the ring formation occurs through the successive two-electron reductive condensations of [Mo7O24]6- {corresponding to cyclic tetradecamerization with 28 ()2 × 14)-electron reduction}, “bottom-up approach”, after stabilizing the molecular curvature of the two-electron reduced {Mo14} by both the conformation change (intramolecular transfer of monomolybdates) and the attachment (intermolecular transfer of monomolybdates) of degradation fragments of β-[Mo8O26]4- thereby forming two kinds of building blocks (20) Mu¨ller, A.; Das, S. K.; Kuhlmann, C.; Bo¨gge, H.; Schmidtmann, M.; Diemann, E.; Krickemeyer, E.; Hormes, J.; Modrow, H.; Schindler, M. Chem. Commun. 2001, 655-656.

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{Mo20} (attached by two linkers) and {Mo21} (attached by three linkers), as discussed below. The successive twoelectron photoreductive cyclization among the two-electron reduced building blocks {Mo20} and {Mo21} results in the construction of the outer ring (Figure 2), and the successive condensation between the linkers results in the formation of the inner-rings for {Mo142}.6,7 The value ( ) 1.6 × 105 lmol-1 cm-1) of the molar absorption coefficient at absorption maximum (λmax ) 748 nm) for 1 is much larger than ) 1.1 × 103 lmol-1cm-1 at λmax ) 730 nm for {Mo14}, even if number of the blue electrons is taken into account for the estimation ((2/28) × 1.6 × 105 ) 1.1 × 104 lmol-1 cm-1 per two electrons in 1).1 This suggests that the Mo 4d electrons injected photochemically into the centers of {Mo20} and {Mo21} (Figure 1) are delocalized through MoO-Mo linkages (Mo-O-Mo bond angles of 140-160°) among corner-shared MoO6 octahedra within {Mo142}.21 4. Discussion It is clear that all of the ESP signals observed for the [Mo7O24]6-/carboxylic acid (or arenesulfinic acid) systems are also net TM emissive 2(•D)+ arising from rapid electron/ proton transfer from DH to 3T+ of 3(O f Mo LMCT) through ISC of 1(O f Mo LMCT) of [Mo7O24]6- and that ESP serves as a “tracer” or a very subtle and noninvasive “label” of radical reactions.12-15 The rapid electron/proton transfer (within ∼0.1 µs) from CH3CO2H to 3T+ of 3(O f Mo LMCT) is in agreement with the result of picosecond transient electronic absorption measurements for the [Mo7O24]6-/ CH3CO2H system which showed that the transient species (with lifetime of about 33 nanosecond) reacts with CH3CO2H with a rate constant of 1 × 108 lmol-1 s-1, if the 33-ns species could be assigned as the 3(O f Mo LMCT) state of [Mo7O24].6-22 The observation of 2(•CH3)+ implies the ESP transfer in the decomposition (with a release of CO2) of 2(CH3CO2•)+ through the charge-transfer complex (i) in line with the spin angular momentum conservation rule (eq 3). The termination of 2(CH3CO2•)+ within the time resolution (∼0.1 µs) of the CIDEP setup suggests the rate constant of 107-108 s-1 for the decomposition of 2(CH3CO2•)+ to 2(•CH3)+. 2(•CH3)+ is gone at least after 0.6 µs when 2(•CH2CO2H)+ persists (Figure 3b). The difference of ESP decay between 2(•CH3)+ and 2(•CH2CO2H)+ is associated with the spin-lattice relaxation time of radicals (0.2 µs for •CH3 and 2.0 µs for •CH2CO2H),18 as well as the reactivity to stable products. We recall that the CH3CO2Hderived products detected for the photolysis of the [Mo7O24]6-/CH3CO2H system were CO2, CH4, and succinic acid with an approximate yield-ratio of 10:7:5, and that other expected products such as ethane (C2H6) and propionic acid (C2H5CO2H) were in trace yield.16 In addition, the isotopic labeling experiments by the use of CD3CO2H showed that the CHD3:CD4 ratio of methane products was more than 8:1, indicating that the R-hydrogen abstraction of •CH3 from CH3CO2H was unlikely.16 In fact, the coexistence of both 2(•CH3)+ and 2(•CH2CO2H)+ at the early stage of e0.6 µs after laser excitation (Figure 3b) excludes the possibility of this R-hydrogen abstraction, since the scavenging reaction (ESP transfer) of 2(•CH3)+ by CH3CO2H occurring within the spin-lattice relaxation of 2(•CH3)+ would result in observation of only 2(•CH2CO2H)+. Therefore, the R-hydrogen abstraction (to yield the •CH2CO2H radical as a precursor of succinic acid) is accomplished by 3(O f Mo LMCT) (eq 2), which competes with the •CH3 generation (formally as a result of abstraction of acid hydrogen) through the charge-transfer complex (21) Yamase, T. Chem. Rev. 1998, 98, 307-325. (22) Kraut, B.; Ferraudi, G. Inorg. Chem. 1989, 28, 2692-2694.

Yamase et al.

(i) (eq 3). The coordination (Figures 7 and 9) of the Prligand into {Mo142} reveals a coupling between radicals • CH2CO2H and •CH3 with a resultant decrease in the yield ratio of CH4 to CO2. Although the trace yield of C2H6 excludes a dominant possibility of the dimerization of • CH3, thus, the generation of CH4 as one of the major products suggests the hydrogen abstraction of •CH3 from the H2O molecule with an accompanying formation of the • OH radical. This is in agreement with the results of the ESR-spin trapping technique which show an involvement of •OH radicals and imply the abstraction of R-hydrogen from CH3CO2H to yield again •CH2CO2H.16 One might expect the molecular distortion in the {Mo142(Ac)5(Pr)} ring consisting of four kinds of building blocks with different molecular curvatures (Figure 8). By using Mo-O-Mo and O-Mo-O bond angles (ω1, ω2, and ω3) and average Mo-O bond distances (l1 and l2 for ω1 and ω2, respectively) for the incomplete double-cubane-type compartments (in sub-building blocks) of the equator (Figures 1 and 8), we can estimate a molecular diameter of the uniform ring consisting of only one kind of subbuilding block.7 The molecular curvatures for four kinds of sub-building blocks, {Mo10(b-Ac)1/2(t-Ac)1/2}, {Mo10(bPr)1/4}, {Mo10}, and {Mo11(t-Ac)1/2}, were ω1 (156-159°, 158° in average), ω2 (138-141°, 140°), ω3 (156-159°, 158°), l1 (1.86-1.94 Å, 1.89 Å), and l2 (1.95-2.05 Å, 2.01 Å)}, {ω1 (157-159°, 158°), ω2 (140-141°, 141°), ω3 (155-161°, 158°), l1 (1.85-1.92 Å, 1.88 Å), and l2 (1.97-2.07 Å, 2.02 Å)}, {ω1 (156-157°, 157°), ω2 (144°), ω3 (157-159°, 158°), l1 (1.83-1.94 Å, 1.88 Å), and l2 (1.97-2.06 Å, 2.01 Å)}, and {ω1 (156-157°, 157°), ω2 (140-146°, 143°), ω3 (155-157°, 156°), l1 (1.88-1.91 Å, 1.90 Å), and l2 (1.93-2.06 Å, 2.00 Å), respectively (Table S3). Thereby, the formations of 28-electron reduced tetradecameric rings (with the outer diameter of D ≈ 34 Å) and 24-electron reduced dodecameric rings (with D ≈ 29 Å) are expected for the former two and the latter two, respectively. Despite such a mismatch of the molecular curvatures of the sub-building blocks between the former two and the latter two, the formation of the 28-electron reduced ring of {Mo142(Ac)5(Pr)} implies that the condensation between the sub-building blocks with different molecular curvatures proceeds favorably with formation of a larger ring to compensate the distortion due to the structural mismatch. In other word, the Ac and Pr coordinations to the building blocks lead to no significant change in the diameter of the {Mo142} ring. The first step for the {Mo142} formation in the [Mo7O24]6-/ CH3CO2H system at pH ) 3.4 is the formation of the cisconfigured two-electron reduced species {Mo14} (t[(MoVMoVI6O23)2]10-) which is the simplest Mo-blue as a precursor of the building-block for the outer ring (Figure 1).1 Provided that the photoredox reaction of [Mo7O24]6- with CH3CO2H occurs only through the R-hydrogen abstraction by 3(O f Mo LMCT) (to form succinic acid) for simplicity, the {Mo142} formation in the [Mo7O24]6-/CH3CO2H system can be given by eq 4 hv

2[Mo7O24]6- + 2CH3CO2H + 2H+ 98 [(MoVMoVI6O23)2]10- (t{Mo14}) + (CH2CO2H)2 + 2H2O (4) {Mo20}(t[MoV2MoVI18O64(H2O)8]10-) and {Mo21}(t[MoV2MoVI19O67(H2O)9]10-) as building blocks for {Mo142} are formed as a result of the structural stabilization of the molecular curvature (for the central -O-Mo-O-MoO- linkage) of {Mo14} at pH ) 3.4, which would be done by both the intramolecular transfer of two MoO6 octahedra and the intermolecular transfer of the degradation frag-


Langmuir, Vol. 21, No. 17, 2005 7831

Figure 10. Schematic Mo frameworks representation for the bottom-up processes from [Mo7O24]6- to both building blocks {Mo20} (t[MoV2MoVI18O64(H2O)8]10-) and {Mo21} (t[MoV2MoVI19O67(H2O)9]10-) as two-electron reduced species as a result of the structural stabilization of the {Mo14} molecular-curvature in the coexistence of β-[Mo8O26]4- (a) and formation of {Mo41} by the photoreductive condensation between {Mo20} and {Mo21} (b). Black- and red-colored atoms are the monomolybdate Mo atoms involved in the conformation change of {Mo14} to {Mo20} or {Mo21}, which would result from the intramolecular transfer within {Mo14} and the intermolecular transfer of the degradation fragments of β-[Mo8O26],4- respectively.

Figure 11. Accomplishment of 28-electron reduced {Mo142(Ac)5(Pr)} ring through successive two-electron photoreductions among two-electron reduced building blocks and dehydrative condensations between neighboring linkers with an accompanying coordinations of acetate (Ac)/propionate (Pr) ligands into 5{Mo20} and 2{Mo21}. The {Mo142} ring produced through the formation of {Mo40} and {Mo41} according to eqs 5-10 is shown by the schematic represented by Mo-frameworks, and the location of C2 axis and mirror plane (m) is also shown.

ments (of monomolybdates) of coexistent β-octamolybdate, β-[Mo8O26]4- (as was isolated as [NH4]2[Me3NH]2[Mo8O26]‚ 2H2O (2) from the photolyte, Tables S5-S7). Figure 10 shows the formation of both {Mo20} and {Mo21} as twoelectron reduced species with the structural stabilization of the {Mo14} molecular curvature. Each MoO6-octahedron

attached to the {Mo14} skeleton incorporates an aqua ligand (Table S4) as a trail of the moiety transferred for the stabilization, as previously pointed out for {Mo142}.7 {Mo20} misses one monomolybdate linker on each of two head Mo sites in {Mo22}(t[{MoV2MoVI20O70(H2O)10]10-) as a two-electron reduced species, whereas {Mo21} misses it

7832


on either head. The conformation change of {Mo14} by the intramolecular transfer of two Mo(H2O)O5 octahedra leads to a decrease of ω1 from 175° to 157-159°,1 and the intermolecular transfer of β-[Mo8O26]4--derived Mo(H2O)O5-octahedra stabilizes both the O-Mo-O linkage (with ω3 ≈ 157°) and the Mo-O-Mo linkage (with ω2 ≈ 138146°; Table S3). Such a structural stabilization of {Mo14} by the attachment of Mo(H2O)O5-octahedral moieties will allow {Mo20} and {Mo21} to undergo further two-electron photoreductive dimeric condensation similar to the {Mo14} formation: the intermolecular photochemical condensation among the two-electron reduced species {Mo20} and {Mo21} leads to the formation of six-electron reduced species {Mo40}, {Mo41}, and {Mo42} for the outer ring thereby forming the Mo-O-Mo linkage through the intramolecular condensation between the neighboring linkers for the inner-ring. Figure 10 also shows an example of the {Mo41} formation. The intramolecular condensation between linkers seems to be an entropy-driven reaction with a subsequent release of hydration shell into liquid bulk water. Thus, successive condensations of both the intermolecular two-electron photoreduction and the resulting intramolecular dehydration between the linkers for outer and inner-rings, respectively, along the molecular curvature reach the {Mo142} ring. Equations 5-10 exemplify reaction steps for the {Mo142}-ring formation from 5{Mo20} and 2{Mo21} building blocks, coupled with the photooxidation of acetic acid to succinic acid (set for simplification).

{Mo20}(t[MoV2MoVI18O64(H2O)8]10-) + {Mo21}(t[MoV2MoVI19O67(H2O)9]10-) + 2CH3CO2H + hv

6H+ 98 {Mo41}(t[Mo 6Mo 35O127(H2O)17]14-) + (CH2CO2H)2 + 4H2O (5) V

VI

2{Mo20}(t[MoV2MoVI18O64(H2O)8]10-) + 2CH3CO2H + hv

4H+ 98 {Mo40}(t[Mo 6Mo 34O125(H2O)16]16-) + (CH2CO2H)2 + 3H2O (6) V

VI

2{Mo41}(t[MoV6MoVI35O127(H2O)17]14-) + 2CH3CO2H + hv

2H+ 98 V {Mo82}(t[Mo 14MoVI68O252(H2O)34]26-) + (CH2CO2H)2 + 2H2O (7) {Mo40}(t[MoV6MoVI34O125(H2O)16]16-) + {Mo82}(t[MoV14MoVI68O252(H2O)34]26-) + 2CH3CO2H + + hv

4H 98 {Mo122}(t[MoV22MoVI100O374(H2O)50]38-) + (CH2CO2H)2 + 3H2O (8) {Mo20}(t[MoV2MoVI18O64(H2O)8]10-) + {Mo122}(t[MoV22MoVI100O374(H2O)50]38-) + + hv

2CH3CO2H + 4H 98 {Mo142′}(t[MoV26MoVI116O435(H2O)58]44-) + (CH2CO2H)2 + 3H2O (9) {Mo142′}(t[MoV26MoVI116O435(H2O)58]44-) + hv

2CH3CO2H + 4H+ 98 {Mo142}(t[MoV28MoVI114O432(H2O)58]40-) + (CH2CO2H)2 + 3H2O (10)

Yamase et al.

where eq 10 denotes the intramolecular photoreductive condensation and the subsequent condensation between the neighboring linkers for the closing ring. Equation 11 denotes the overall photoredox reaction of two-electron reduced building blocks {Mo20} and {Mo21} with CH3CO2H to yield the 28-electron reduced {Mo142} ring and succinic acid hv

5(Mo20} + 2{Mo21} + 14CH3CO2H + 30H+ 98 {Mo142} + 7 (CH2CO2H)2 + 22H2O (11) Figure 11 shows a schematic presentation of the {Mo142} ring (produced according to eqs 5-10) and coordinations of Ac and Pr ligands through the successive condensations among two-electron reduced building blocks, 5{Mo20} and 2{Mo21}, where the coordinations of Ac- and Pr-ligands proceed with disordering. If {Mo22}, incorporating two monomolybdate-linkers on each head-Mo atom, is formed by the photoreductive degradative condensation between the half moieties of {Mo36} (Figure 1),6,7 then it is easy to understand that both the successive two-electron-photoreductive condensations (for the outer ring) among {Mo22} building-blocks (corresponding to a cyclic heptamerization) and the successive dehydrations (for the inner-ring) between the neighboring linkers would lead to the formation of {Mo154}. Each head-Mo atom in {Mo142} coordinates one or two monomolybdate-linkers for the condensation with the neighboring linker (Figures 8 and 11), indicating that the maximum possible number of {Mo2}-linker defects is three for each inner-ring in {Mo154}, as also pointed out by the structural analysis of [MoV28MoVI110O416H6(H2O)58(CH3CO2)6]32-.10 Furthermore, two different types of coordinations of carboxylates in the inner wall (Figures 7 and 11) allow us predict that the maximum possible numbers of the coordinated carboxylate ligands are 28 and 22 for {Mo154} and {Mo142}, respectively. The coordinations of organic ligands in the inner wall of the ring induce not only the increase in the hydrophobic property of the inner-ring surface, but also ligand-inherent functionalities into the nanoring. Such a possible functional design of the surface structures of the inner-rings is an interesting task for material science, abreast a nanotube landscape exemplified by the {Mo154}-based nanotube which was accomplished by the intermolecular condensation between inner-ring linkers of {Mo154}in the presence of the trivalent lanthanide cation as a scavenger of water molecules liberated on the condensation.6 Acknowledgment. This work was supported by Grants-in-Aid for Scientific Research, No. 99P01201 from RFTF/JSPS and No. 14204067 from the Ministry of Education, Science, Sports, and Culture, and for Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Corporation. Supporting Information Available: Crystallographic data for 1 and 2 in CIF format. Figure S1 of the structure of 2 in the unit cell with the atom-labeling scheme, Tables S1-S4 of atomic positional parameters, site occupancies, isotropic and anisotropic thermal parameters, important interatomic distances and bond angles, and bond valence sums for all of the oxygen atoms of Mo-O bonds for 1. Tables S5-S7 of atomic positional parameters, anisotropic thermal parameters, and important interatomic distances and bond angles for 2. These materials are available free of charge via the Internet at http://pubs.acs.org. LA051301V

Photoreductive Self-Assembly from [Mo7O24]6- to Carboxylates

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