Cluster compounds containing [Mn.SUB.3.SrO.SUB.4.] and [Mn.SUB.4.SrO.SUB.4.] core structures, preparation method and application thereof

Abstract

[Mn.sub.3SrO.sub.4] cluster compounds are synthesized in a single step from raw materials consisting of simple and inexpensive Mn.sup.2+, Sr.sup.2+ inorganic compounds and carboxylic acids by using permanganate anion as oxidant. This step can be followed by the synthesis of asymmetric biomimetic water splitting catalyst [Mn.sub.4SrO.sub.4] cluster compounds in the presence of water. The [Mn.sub.4SrO.sub.4] cluster compound can catalyze the splitting of water in the presence of an oxidant to release oxygen gas. The neutral [Mn.sub.3SrO.sub.4](R.sub.1CO.sub.2)6(R.sub.1CO.sub.2H).sub.3 cluster compound can serve as precursors for the synthesis of biomimetic water splitting catalysts, and can be utilized in the synthesis of different types of biomimetic water splitting catalysts. [Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4) cluster compounds can serve as artificial water splitting catalysts, can be utilized on the surface of an electrode or in the catalyzed splitting of water driven by an anoxidant.

Claims

1. A compound [Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4) of Formula II ##STR00015## wherein R.sub.1 is H, a C.sub.1-8 linear alkyl, or a C.sub.1-8 branched alkyl; the four ligands L.sub.1, L.sub.2, L.sub.3 and L.sub.4 are the same or different and are each selected from carboxylic acid molecules, pyridine, imidazole, pyrazine, quinoline, isoquinoline, bipyridine, H.sub.2O, alcohol molecules, ketones, nitriles, and esters.

2. A complex comprising two or more compounds [Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4) of ##STR00016## wherein R.sub.1 is H, a C.sub.1-8 linear alkyl, or a C.sub.1-8 branched alkyl; the four ligands L.sub.1, L.sub.2, L.sub.3 and L.sub.4 are the same or different and are each selected from carboxylic acid molecules, pyridine, imidazole, pyrazine, quinoline, isoquinoline, bipyridine, H.sub.2O, alcohol molecules, ketones, nitriles, and esters.

3. A method for preparing the [Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4) compound of claim 1, comprises the following steps: Step 1: dissolving an acid, an oxidant, a Mn.sup.2+ salt, a Sr.sup.2+ salt, and optionally water an acetonitrile solvent to obtain a solution; filtering the solution to obtain a filtrate; and crystallizing the filtrate to obtain crystals; Step 2: dissolving the crystals obtained from Step 1 in an ester solvent; and adding L.sub.1, L.sub.2, L.sub.3 and L.sub.4 to the ester solvent to cause recrystallization to obtain the [Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4) compound, wherein the molar ratio of the acid, the oxidant, the Mn.sup.2+ salt, the Sr.sup.2+ salt, and the optional water is (10-120):(1-10):1:1:(0-20).

4. A method for preparing the complex according to claim 2, comprising the following steps: Step 1: dissolving an acid, an oxidant, a Mn.sup.2+ salt, a Sr.sup.2+ salt, and optional in an acetonitrile solvent to obtain a solution; filtering the solution to obtain a filtrate; and crystallizing the filtrate to obtain crystals; and Step 2: dissolving the crystals obtained from Step 1 in an ester solvent; and adding L.sub.1, L.sub.2, L.sub.3 and L.sub.4 to the ester solvent to cause recrystallization to obtained the complex of claim 3; and Step 3: optionally, dissolving the product obtained from Step 2 in an ester solution, and then subject to rinsing or recrystallization with an alkane, a cycloalkane, or a halogenated hydrocarbon, wherein the molar ratio of the acid, the oxidant, the Mn.sup.2+ salt, the Sr.sup.2+ salt, and the optional water is (10-120):(1-10):1:1:(0-20).

5. A method for biomimetic water splitting, comprising: adding the [Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4) compound of claim 1 and an oxidant; and causing water to split and release oxygen.

6. A method for biomimetic water splitting, comprising: adding the complex according to claim 2 and an oxidant into water; and causing water to split and release oxygen.

7. A water splitting catalyst, comprising the [Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4) compound according to claim 2.

8. A water splitting catalyst, comprising the complex according to claim 2.

9. The compound according to claim 1, the carboxylate anion (R.sub.1CO.sub.2) is selected from formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, isovaleric acid, pivalic acid, and hexanoic acid.

10. The compound according to claim 1, the compound of Formula II is [Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4), wherein R.sub.1 is tert-butyl, L.sub.1 is pyridine, and L.sub.2, L.sub.3, and L.sub.4 are pivalic acid.

11. The compound according to claim 10 is of a single crystal of orthogonal system with space group of Pna21, cell parameters of a=19.059(3) Å, b=23.711(4) Å, c=19.416(4) Å, α=90.00°, β=90.00°, and γ=90.00°, Z=4, and volume of 8774(3) Å.sup.3, has a structural formula of Formula II-1: ##STR00017##

12. The complex according to claim 2 is [[Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4)].[[Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4*)], wherein R.sub.1 is tert-butyl, L.sub.1 is pyridine or isoquinoline, L.sub.2, L.sub.3, and L.sub.4 are pivalic acid, and L.sub.4*is ethyl acetate.

13. The compound according to claim 2 is complex 3 or complex 4, wherein: complex 3 is [[Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4).[[Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4*)], wherein R.sub.1 is tert-butyl, L.sub.1 is pyridine, L.sub.2, L.sub.3, and L.sub.4 are pivalic acid, and L.sub.4*is ethyl acetate, and has a single crystal of monoclinic system with space group of P1 2.sub.1/c1, and cell parameters of a=29.9019(5) Å, b=18.9368(3) Å, c=30.1980(4) Å, α=90.00°, β=92.6590(10) °, and γ=90.00°, Z=4, and volume of 17081.1(5) Å.sup.3, and [[Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4)] in complex 3 is of Formula II-1, and [[Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4*)] in complex 3 is of Formula 11-2: ##STR00018## and complex 4 is [[Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4)].[[Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4*)], wherein R.sub.1 is tert-butyl, L.sub.1 is isoquinoline, L.sub.2, L.sub.3, and L.sub.4 are pivalic acid, and L.sub.4*is ethyl acetate, has a single crystal of triclinic system with space group of P-1, and cell parameters of a=14.676(2) Å, b=25.313(3) Å, c=25.601(4) Å, α=76.547(5)°, β=87.559(6)°, and γ=73.153(6)°, Z=2, and volume of 8850(2) Å.sup.3, and [[Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4)] in complex 4 has a structural formula of Formula II-3-1, and the [[Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4*)] in complex 4 has a structural formula of Formula 11-3-2: ##STR00019## ##STR00020##

14. The method according to claim 3, wherein: the Mn.sup.2+ salt is one or more selected from carboxylic acid salts of Mn.sup.2+, Mn(ClO.sub.4).sub.2, MnCl.sub.2, MnSO.sub.4, Mn(NO.sub.3).sub.2, and Mn(CF.sub.3SO.sub.3).sub.2, Mn(ClO.sub.4).sub.2(H.sub.2O).sub.n, MnSO.sub.4(H.sub.2O).sub.n, Mn(NO.sub.3).sub.2(H.sub.2O).sub.n, Mn(CF.sub.3SO.sub.3).sub.2(H.sub.2O).sub.n, and mixtures thereof, wherein n=1-5, and the carboxylate anion (R.sub.1CO.sub.2.sup.−) in the carboxylic acid salts is one or more selected from formate, acetate, propionate, butyrate, isobutyrate, valerate, isovalerate, pivalate, and hexanoate; the Sr.sup.2+ salt is one or more selected from carboxylic acid salts of strontium, Sr(ClO.sub.4).sub.2, Sr(NO.sub.3).sub.2, Sr(CF.sub.3SO.sub.3).sub.2, Sr(ClO.sub.4).sub.2(H.sub.2O).sub.n, Sr(NO.sub.3).sub.2(H.sub.2O).sub.n, Sr(CF.sub.3SO.sub.3).sub.2(H.sub.2O).sub.n, and mixtures thereof, wherein n=1-5 and the carboxylic anion (R.sub.1CO.sub.2.sup.−) in the carboxylic salts of strontium is one or more selected from formate, acetate, propionate, butyrate, isobutyrate, valerate, isovalerate, pivalate, and hexanoate; the oxidant is permanganate anionic oxidant; and the acid is formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, isovaleric acid, pivalic acid, or hexanoic acid.

15. The method according to claim 3, wherein the volume of the acetonitrile solvent in Step 1 is about 60-100 mL acetonitrile per mmol strontium salt, and the ester solvent used in Step 2 is ethyl acetate, methyl acetate, propyl propionate, n-hexane, isooctane, dichloroethane, or dichloromethane.

16. The method according to claim 4, wherein: the Mn.sup.2+ salt is one or more selected from carboxylic acid salts of Mn.sup.2+, Mn(ClO.sub.4).sub.2, MnCl.sub.2, MnSO.sub.4, Mn(NO.sub.3).sub.2, and Mn(CF.sub.3SO.sub.3).sub.2, Mn(ClO.sub.4).sub.2(H.sub.2O).sub.n, MnSO.sub.4(H.sub.2O).sub.n, Mn(NO.sub.3).sub.2(H.sub.2O).sub.n, and Mn(CF.sub.3SO.sub.3).sub.2(H.sub.2O).sub.n, wherein n=1-5; wherein the carboxylic acid anion (R.sub.1CO.sub.2.sup.−) in the carboxylic acid salts of Mn.sup.2+ is one or more selected from formate, acetate, propionate, butyrate, isobutyrate, valerate, isovalerate, pivalate, and hexanoate; the Sr.sup.2+ salt is one or more selected from carboxylic acid salts of strontium, Sr(ClO.sub.4).sub.2, Sr(NO.sub.3).sub.2, Sr(CF.sub.3SO.sub.3).sub.2, Sr(ClO.sub.4).sub.2(H.sub.2O).sub.n, Sr(NO.sub.3).sub.2(H.sub.2O).sub.n, Sr(CF.sub.3SO.sub.3).sub.2(H.sub.2O).sub.n, wherein n=1-5, and the carboxylic acid anion (R.sub.1CO.sub.2.sup.−) in the carboxylic acid salts of strontium is formate, acetate, propionate, butyrate, isobutyrate, valerate, isovalerate, pivalate, and hexanoate; the oxidant is an permanganate anionic oxidant; and the acid is formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, isovaleric acid, pivalic acid, or hexanoic acid.

17. The method according to claim 4, wherein: the volume of the acetonitrile solvent in Step 1 is about 60-100 mL acetonitrile per mmol strontium salt; and the ester solvent in Step 2 is ethyl acetate, methyl acetate, propyl propionate, n-hexane, isooctane, dichloroethane, or dichloromethane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the crystal structure of compound 1 as prepared in Example 1 of the present invention. For the sake of clarity, the hydrogen atom, the methyl of the tert-butyl and solvent molecules are all omitted.

(2) FIG. 2 shows the crystal structure of compound 2 as prepared in Example 2 of the present invention. For the sake of clarity, the hydrogen atom, the methyl of the tert-butyl and solvent molecules are all omitted.

(3) FIG. 3 shows the crystal structure of complex 3 as prepared in Example 3 of the present invention. For the sake of clarity, the hydrogen atom, the methyl of the tert-butyl and solvent molecules are all omitted. There are two [Mn.sub.4SrO.sub.4] cluster compounds in the figure, and they differ in that one neutral pivalic acid molecule on the strontium ion is replaced by one ethyl acetate molecule.

(4) FIG. 4 shows the crystal structure of complex 4 as prepared in Example 4 of the present invention. For the sake of clarity, the hydrogen atom, the methyl of the tert-butyl and solvent molecules are all omitted. There are two [Mn.sub.4SrO.sub.4] cluster compounds in the figure, and they differ in that one neutral pivalic acid molecule on the strontium ion is replaced by one ethyl acetate molecule.

(5) FIG. 5 shows the trace of the change in UV-Vis absorption spectrum of the interaction between compound 2 and water in Example 5 of the present invention.

(6) FIG. 6 shows the electron paramagnetic signal produced after the oxidization of compound 2 in Example 6 of the present invention. The data supports that the four Mn ions of compound 2 in ground state have valence states of +3, +3, +4 and +4, respectively.

(7) FIG. 7 shows the determination of the oxygen released during the spliting of water catalyzed by compound 2 in the presence of oxidant in Example 7 of the present invention.

SPECIFIC MODE FOR CARRYING OUT THE INVENTION

(8) The present invention will be further illustrated with reference to the specific examples below. It should be understood that these examples are only used to illustrate the present invention, but are not intended to limit the scope of the present invention. In addition, it should also be understood that after reading the contents of the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the present invention.

Example 1

Compound 1 [Mn.SUB.3.SrO.SUB.4.](C.SUB.5.H.SUB.9.O.SUB.2.).SUB.6.(C.SUB.5.H.SUB.9.O.SUB.2.H).SUB.3

(9) The preparation method was as follows:

(10) To a 100 ml round bottom flask were added tetrabutylammonium permanganate (Bu.sup.n.sub.4N.MnO.sub.4, 4 mmol), manganese acetate (Mn(CH.sub.3CO.sub.2).sub.2, 1 mmol), strontium acetate (Sr(CH.sub.3CO.sub.2).sub.2, 1 mmol) and pivalic acid ((CH.sub.3).sub.3CCO.sub.2H, 40 mmol). After continuous reaction in acetonitrile at 80° C. for 25 min, the reaction was stopped. The precipitate, which was of a small amount, was removed off by filtration. The resulted brown mother liquor was allowed to stand still at 0° C. for 1-6 days and brown crystals were precipitated out. The resulted crystals were collected and dissolved in n-hexane, and dried under vacuum, with the yield of −60% (according to the mole numbers of Sr ions).

(11) Compound 1 has a structural formula of [Mn.sub.3SrO.sub.4](R.sub.1CO.sub.2).sub.6(R.sub.1CO.sub.2H).sub.3, wherein R.sub.1=tert-butyl.

(12) That is, compound 1 has a structural formula of [Mn.sub.3SrO.sub.4](C.sub.5H.sub.9O.sub.2).sub.6(C.sub.5H.sub.9O.sub.2H).sub.3 and a molecular formula of C.sub.45H.sub.84Mn.sub.3O.sub.22Sr. Theoretical values of elemental analysis are: C, 43.96; H, 6.89; and experimental values are: C, 44.01; H, 6.84. Compound 1 is a single crystal of trigonal system with space group of R-3c, cell parameters of a=10.6575(15) Å, b=10.6575(15) Å, c=87.917(18) Å, α=90.00°, β=90.00°, and γ=120.00°, Z=6, and volume of 8648(3) Å.sup.3.

(13) Compound 1 has a chemical structure as shown in Formula I-1 below, determined specific single crystal parameters as shown in Table 1, and a crystal space structure as shown in FIG. 1.

(14) ##STR00010## wherein R.sub.1=tert-butyl;

Example 2

Compound 2 [Mn.SUB.4.SrO.SUB.4.](C.SUB.5.H.SUB.9.O.SUB.2.).SUB.8.(C.SUB.5.H.SUB.9.O.SUB.2.H).SUB.3.(C.SUB.5.H.SUB.5.N)

(15) The preparation method was as follows:

(16) The first step was the synthesis of the precursor of compound 2. To a 100 ml round bottom flask were added tetrabutylammonium permanganate (Bu.sup.n.sub.4N.MnO.sub.4, 4 mmol), manganese acetate (Mn(CH.sub.3CO.sub.2).sub.2, 1 mmol), strontium acetate (Sr(CH.sub.3CO.sub.2).sub.2, 1 mmol), pivalic acid ((CH.sub.3).sub.3CCO.sub.2H, 40 mmol) and water (H.sub.2O, 1 mmol). After continuous reaction in acetonitrile at 80° C. for 25 min, the reaction was stopped. The precipitate, which was of a small amount, was removed off by filtration. The resulted brown mother liquor was allowed to stand still at 0° C. for 1-2 weeks and brown crystals were precipitated out.

(17) The second step was the reaction with pyridine. The crystals obtained in the first step were collected and dissolved in ethyl acetate. 2% (volume ratio) of pyridine was added, and after 1-2 weeks, brown crystals were precipitated out. The resulted crystals were collected and dissolved in n-hexane. The undissolved substance, which was of a small amount, was removed off, and the solution was subject to recrystallization. After 1 week, brown crystals were precipitated out and dried under vacuum. The yield was −12% (according to the mole numbers of Sr.sup.2+ ions).

(18) Compound 2 has a structural formula of [Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4), wherein R.sub.1=tert-butyl, L.sub.1=pyridine, and L.sub.2=L.sub.3=L.sub.4=pivalic acid.

(19) That is, compound 2 has a structural formula of [Mn.sub.4SrO.sub.4](C.sub.5H.sub.9O.sub.2).sub.8(C.sub.5H.sub.9O.sub.2H).sub.3(C.sub.5H.sub.5N).(C.sub.6H.sub.14).sub.1.5 (note: the n-hexane is a solvent molecule), and a molecular formula of C.sub.69H.sub.128Mn.sub.4NO.sub.26Sr. Theoretical values of elemental analysis are: C, 48.89; H, 7.61; N, 0.83; and experimental values are: C, 49.00; H, 7.51; N, 0.70. Compound 2 is a single crystal of orthogonal system with space group of Pna21, cell parameters of a=19.059(3) Å, b=23.711(4) Å, c=19.416(4) Å, α=90.00°, β=90.00°, and γ=90.00°, Z=4, and volume of 8774(3) Å.sup.3.

(20) Compound 2 has a chemical structure as shown by Formula II-1 below, determined specific single crystal parameters as shown in Table 2, and a crystal space structure as shown in FIG. 2.

(21) ##STR00011## wherein R.sub.1=tert-butyl;

Example 3

Complex 3 [[Mn.SUB.4.SrO.SUB.4.](C.SUB.5.H.SUB.9.O.SUB.2.).SUB.8.(C.SUB.5.H.SUB.5.N).SUB.1.(C.SUB.5.H.SUB.9.O.SUB.2.H).SUB.3.].[[Mn.SUB.4.SrO.SUB.4.](C.SUB.5.H.SUB.9.O.SUB.2.).SUB.8.(C.SUB.5.H.SUB.5.N).SUB.1.(C.SUB.5.H.SUB.9.O.SUB.2.H).SUB.2.(C.SUB.2.H.SUB.5.O.SUB.2.CCH.SUB.3.).SUB.1.]

(22) The preparation method was as follows:

(23) 0.100 g compound 2 was weighed and dissolved in ethyl acetate. The mixture was allowed to stand still at room temperature for 1-3 weeks and slender brown crystals were precipitated out. The crystals were rinsed with cyclohexane and then vacuum dried. The yield was −70% (according to the mole numbers of Sr.sup.2+ ions).

(24) Complex 3 has a structural formula of [[Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8](L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4)].[[Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8](L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4*)], wherein R.sub.1=tert-butyl, L.sub.1=pyridine, L.sub.2=L.sub.3=L.sub.4=pivalic acid, and L.sub.4*=ethyl acetate.

(25) That is, complex 3 has a structural formula of [[Mn.sub.4SrO.sub.4](C.sub.5H.sub.9O.sub.2).sub.8(C.sub.5H.sub.5N).sub.1(C.sub.5H.sub.9O.sub.2H).sub.3].[[Mn.sub.4SrO.sub.4](C.sub.5H.sub.9O.sub.2).sub.8(C.sub.5H.sub.5N).sub.1(C.sub.5H.sub.9O.sub.2H).sub.2(C.sub.2H.sub.5O.sub.2CCH.sub.3).sub.1], and a molecular formula of C.sub.119H.sub.212Mn.sub.8N.sub.2O.sub.52Sr.sub.2. Theoretical values of elemental analysis are: C, 45.84; H, 6.85; N, 0.90; and experimental values are: C, 45.82; H, 6.81; N, 1.21. Complex 3 is a single crystal of monoclinic system with space group of P1 2.sub.1/c1, cell parameters of a=29.9019(5) Å, b=18.9368(3) Å, c=30.1980(4) Å, α=90.00°, β=92.6590(10)°, and γ=90.00°, Z=4, and volume of 17081.1(5) Å.sup.3. Complex 3 has a chemical structure of [[Mn.sub.4SrO.sub.4](C.sub.5H.sub.9O.sub.2).sub.8(C.sub.5H.sub.5N).sub.1(C.sub.5H.sub.9O.sub.2H).sub.2(C.sub.2H.sub.5O.sub.2CCH.sub.3).sub.1] as shown by Formula II-2 below, determined specific single crystal parameters as shown in Table 3, and a crystal space structure as shown in FIG. 3.

(26) ##STR00012## wherein R.sub.1=tert-butyl;

Example 4

Complex 4 [[Mn.SUB.4.SrO.SUB.4.](C.SUB.5.H.SUB.9.O.SUB.2.).SUB.8.(C.SUB.9.H.SUB.7.N).SUB.1.(C.SUB.5.H.SUB.9.O.SUB.2.H).SUB.3.].[[Mn.SUB.4.SrO.SUB.4.](C.SUB.5.H.SUB.9.O.SUB.2.).SUB.8.(C.SUB.9.H.SUB.7.N).SUB.1.(C.SUB.5.H.SUB.9.O.SUB.2.H).SUB.2.(C.SUB.2.H.SUB.5.O.SUB.2.CCH.SUB.3.).SUB.1.]

(27) The preparation method was as follows:

(28) The first step was the synthesis of the precursor of complex 4. To a 100 ml round bottom flask were added tetrabutylammonium permanganate (Bu.sup.n.sub.4N.MnO.sub.4, 4 mmol), manganese acetate (Mn(CH.sub.3CO.sub.2).sub.2, 1 mmol), strontium acetate (Sr(CH.sub.3CO.sub.2).sub.2, 1 mmol) and pivalic acid ((CH.sub.3).sub.3CCO.sub.2H, 40 mmol). After continuous reaction in acetonitrile at 80° C. for 25 min, the reaction was stopped. The precipitate, which was of a small amount, was removed off by filtration. The resulted brown mother liquor was allowed to stand still at 0° C. for 1-2 weeks and brown crystals were precipitated out.

(29) The second step was the reaction with isoquinoline. The crystals obtained in the first step were collected and dissolved in ethyl acetate. 1% (volume ratio) isoquinoline was added for recrystallization. After 1-2 weeks, black crystals were collected, rinsed with cyclohexane and vacuum dried. The yield was −40% (according to the mole numbers of Sr.sup.2+ ions).

(30) Complex 4 has a structural formula of [[Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4)].[[Mn.sub.4SrO.sub.4](R.sub.1CO.sub.2).sub.8(L.sub.1)(L.sub.2)(L.sub.3)(L.sub.4)], wherein R.sub.1=tert-butyl, L.sub.1=isoquinoline, L.sub.2=L.sub.3=L.sub.4=pivalic acid, and L.sub.4*=ethyl acetate.

(31) That is, complex 4 has a structural formula of [[Mn.sub.4SrO.sub.4](C.sub.5H.sub.9O.sub.2).sub.8(C.sub.9H.sub.7N).sub.1(C.sub.5H.sub.9O.sub.2H).sub.3].[[Mn.sub.4SrO.sub.4](C.sub.5H.sub.9O.sub.2).sub.8(C.sub.9H.sub.7N).sub.1(C.sub.5H.sub.9O.sub.2H).sub.2(C.sub.2H.sub.5O.sub.2CCH.sub.3).sub.1], and a molecular formula of C.sub.132H.sub.226Mn.sub.8N.sub.2O.sub.54Sr.sub.2. Complex 4 is a single crystal of triclinic system with space group of P-1, cell parameters of a=14.676(2) Å, b=25.313(3) Å, c=25.601(4) Å, α=76.547(5)°, β=87.559(6)°, and γ=73.153(6)°, Z=2, and volume of 8850(2) Å.sup.3.

(32) Complex 4 has a chemical structure as shown by Formulas II-3-1 and II-3-2 below, determined specific single crystal parameters as shown in Table 4, and a crystal space structure as shown in FIG. 4.

(33) ##STR00013## wherein R.sub.1=tert-butyl;

(34) ##STR00014## wherein R.sub.1=tert-butyl;

Example 5

Trace of the UV-Vis Spectrum of the Interaction Between Compound 2 and Water

(35) To a colorimetric cell was added 1 mL solution of 25 μM compound 2 in acetonitrile. Using 1 mL pure acetonitrile as reference, the absorption spectrum was determined in Hitachi U-3900 spectrophotometer type UV-Vis spectrometer (see FIG. 5). This compound had the maximum absorption at 250 nm. With the addition of water (0%, 0.15%, 0.35%, 0.55% and 1.05% water was added respectively), the absorption spectrum changed significantly. Specifically, the absorption at 250 nm decreased significantly, while the absorption at 400 nm increased, indicating that water molecules interacted with compound 2. As compared with [Mn.sub.4CaO.sub.4] cluster compounds, the sensitivity of [Mn.sub.4SrO.sub.4] cluster compounds to water was significantly reduced. The absorption peak of [Mn.sub.4SrO.sub.4] cluster compound at 250 nm was only reduced by 20% when there existed 1% water, whereas the absorption peak of the corresponding [Mn.sub.4CaO.sub.4] cluster compound at 250 nm was reduced by more than 60% when there existed 1% water, indicating that a small amount of water has little effect on the spectrum of [Mn.sub.4SrO.sub.4] but has a greater effect on the spectrum of [Mn.sub.4CaO.sub.4] cluster compounds. [Mn.sub.4CaO.sub.4] cluster compounds belongs to water extremely sensitive substances, and the sensitivity leads to reduced stability and therefore limited the application thereof. To the contrary, the [Mn.sub.4SrO.sub.4] cluster compounds newly synthesized and discovered in the present invention were much more stable than [Mn.sub.4CaO.sub.4] cluster compounds and had a wide application prospect as water splitting catalyst.

Example 6

Electron Paramagnetic Resonance of Compound 2 for Detecting the Valence State of Mn Ions in the Compound

(36) Compound 2 (1 mM) was dissolved in dichloroethane, and then 0.5 mM oxidant [Fe(Phen).sub.3](PF.sub.6).sub.3 (wherein Phen=phenanthroline) was added. The mixture was then rapidly frozen to 77K and the electron paramagnetic signals were detected with Bruker E500 electron paramagnetic resonance instrument at 7K (see FIG. 6). The multiple peak paramagnetic signal at g=2.0 was clearly seen. The occurrence of this signal indicated that after oxidization of the compound, the valence states of the four manganese ions were +3, +4, +4 and +4, respectively. Thus it could be inferred that the valence states of the four Mn ions in the ground state (i.e., the stable state before oxidation) of the compound were +3, +3, +4 and +4, respectively.

Experimental Example 7

Determination of the Oxygen Released During the Water Splitting Catalyzed by Compound 2 in the Presence of an Oxidant

(37) The activity of releasing oxygen during the catalytic water splitting was determined on a Clark-type oxygen electrode (FIG. 7). A rapid release of oxygen to saturation was observed with the addition of 500 μM of compound 2 to an aqueous solution containing an oxidant (hydrogen peroxide, 0.1 M), whereas no visible release of oxygen was observed with the addition of the reference compound (Mn(ClO.sub.4).sub.2). The arrow in the figure showed the loading position of the sample. FIG. 7 indicated that compound 2 had catalytic activity of catalyzing the splitting of water to release oxygen.

(38) The embodiments of the present invention have been described above. However, this invention is not limited to the above embodiments. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.