S19002

遷移金属 2 価イオンの水酸化物形成過程における中間化学種の安定化と構造解明
EXAFS study on stability of planar cobalt(II) and nickel(II) hydroxo clusters in water
スギアルト, 林 宜仁
Sugiarto and Yoshihito Hayashi
金沢大学理工研究域物質化学系
Department of Chemistry, Graduate School of Natural Science and Technology, Kanazawa University,Kakuma, Kanazawa 920-1192, Japane-mail: hayashi@se.kanazawa-u.ac.jp
水溶液中でのニッケルおよびコバルト二価イオンの２量体から平面型水酸化物クラスターへの成長を Co および Ni 吸収端を用いた EXAFS 測定により観測した。
We synthesized a group of planar cobalt hydroxide or nickel hydroxide cation clusters Co2M2 and Co6M7(M = Co2+ or Ni2+). Their stability in water is confirmed by EXAFS spectroscopy.
Keywords: EXAFS, cobalt hydroxide, nickel hydroxide, aqueous chemistry
Background and research objectives
Molecular cobalt-oxide and nickel-oxide clusters have attracted attention since Nocera group discoveredthat cobalt-phosphate [1] and nickel-borate [2] oxygen-evolution catalysts are in fact composed of domainsof cobalt or nickel oxo/hydroxo in molecular dimensions [3, 4]. Their group further proposed, on the basisof EXAFS analysis, a planar heptanuclear cobalt-oxo/hydroxo cluster to model the surfaces of cobaltphosphate catalysts [4].
The research aim is to synthesize a planar molecular cobalt(II) or a nickel(II) hydroxo cluster which
dissolves intact in water thus its solution properties can be investigated at a molecular level. To isolate thoselabile hydroxo clusters from aqueous solution, we use a cation complex fac-{Co(tacn)}3+ (tacn = 1,4,7triazacyclononane) as a protecting group [5, 6] to terminate a peripheral site on a metal hydroxo species froma hydrolysis-condensation reaction of cobalt(II) or nickel(II) cations so that the precipitation of the bulk
metal hydroxides in alkaline condition is prevented. As a result, we obtained a new family of planarmolecular cobalt or nickel hydroxide cations Co2M2 and Co6M7 (M = CoII or NiII). Herein, we report anEXAFS study of Co2M2 and Co6M7 in water as well as in solid states.
ExperimentCo2M2 and Co6M7 were synthesized by the reaction of fac-[Co(tacn)(OH2)3]3+ with cobalt(II) ornickel(II) cations in water pH 6 and 12, respectively. Solid-state (BN pellets) and solution-state (in water)
EXAFS spectra of the clusters were collected in Ritsumeikan University SR center BL-3. For each sample,a minimum three sets of data were collected and were merged and processed by using Athena program. The
experimental spectra were analyzed by running FEFF program in Artemis.
Results and discussion
Clusters Co2M2 or Co6M7 (M = CoII or NiII) comprise a {MII2}– or a {MII7}–hydroxo core which iscapped by two or six fac-{Co(tacn)}3+ units, as illustrated in Fig. 1.

= tacn{M7}{M2}

ABFig. 1. Schematic representations of (A) Co2M2 and (B) Co6M7.
Fig. 2 shows solid- and solution-state EXAFS r-space spectra of Co2M2 and Co6M7 (Co K-edge spectra
for Co2Co2 and Co6Co7 and Ni K-edge for Co2Ni2 and Co6Ni7). In general, the spectra display two shells
corresponding to the primary and secondary coordination spheres of the absorber metals. Because the second
shell peaks are observed clearly, it assures the structure frameworks of the metal(II)–hydroxo core aremaintained upon dissolution in water. We tried to extract M···M distances from the experimental spectra byfitting with FEFF program, and we found that the metal–metal distances are close to those of observed in
the data from single-crystal structures. For example, the average Co···Co distance in Co6Co7 in the crystal
structure is 3.03 Å and solution-state EXAFS gives a value of 3.07 Å. Also, the Ni···Ni distance in Co6Ni7
found by crystallography and solution-state EXAFS is 3.06 and 3.03 Å, respectively. This indicates theheptanuclear metal(II)–hydroxo core dissolves intact in water.

Fig. 2. Solid-state (top traces) and solution-state (bottom traces) r-space EXAFS spectra of (A) Co2Co2, (B)
Co2Co7, (C) Co2Ni2, and (D) Co6Ni7. Black dashed and red solid traces are experimental and simulated
spectra, respectively.
In summary, the solution state EXAFS data shows a good agreement with the solid state data which uses
an authentic sample structurally certified by single-crystal X-ray analysis, and the distance parameters
derived from EXAFS were matched well with crystallographic data. The distinct observation of the second
coordination sphere peaks of M····M interactions ensures the retention of each cluster framework when it
dissolves in water. These EXAFS observations are convincing evidence to support our idea that theinteractions of planar metal(II)–hydroxo clusters can be trapped by using fac-{Co(tacn)}3+ as the protecting
group, and these molecular metal hydroxides are stable in water.
References[1] M.W. Kanan, D.G. Nocera, Science, 321 (2008) 1072-1075.
[2] M. Dinca, Y. Surendranath, D.G. Nocera, Proc. Natl. Acad. Sci. U.S.A., 107 (2010) 10337-10341.
[3] D.K. Bediako, B. Lassalle-Kaiser, Y. Surendranath, J. Yano, V.K. Yachandra, D.G. Nocera, J. Am. Chem.
Soc., 134 (2012) 6801-6809.
[4] M.W. Kanan, J. Yano, Y. Surendranath, M. Dinca, V.K. Yachandra, D.G. Nocera, J. Am. Chem. Soc.,
132 (2010) 13692-13701.
[5] Sugiarto, T. Tagami, K. Kawamoto, Y. Hayashi, Dalton Trans., 47 (2018) 9657-9664.
[6] Sugiarto, K. Kawamoto, Y. Hayashi, Front. Chem., 6 (2018) 375.

Output
These results had been presented in the Japan Society of Coordination Chemistry Conference 2019(Nagoya).