Subscriber access provided by Massachusetts Institute of Technology
J. Phys. Soc. Jpn. 89, 051014 (2020) [9 Pages]
SPECIAL TOPICS: Frontier of Hydrogen Science

Multimetallic Rare Earth and Group 4 Transition Metal Hydrides for Novel Transformations of Small Molecules

+ Affiliations
1Advanced Catalysis Research Group, RIKEN Center for Sustainable Resource Science, Wako, Saitama 351-0198, Japan2Organometallic Chemistry Laboratory, RIKEN Cluster for Pioneering Research, Wako, Saitama 351-0198, Japan

Hydrogenolysis of mono(cyclopentadienyl (Cp))-ligated rare earth metal alkyl complexes with H2 affords a variety of multimetallic rare earth hydride complexes, which can serve as a unique platform for activation of various substrates such as CO, CO2, H2, and unsaturated C–C and C–N bonds. By using the mono-Cp-ligated rare earth species as a building block, novel d–f heteromultimetallic hydride complexes could be prepared, which show some interesting reactivities toward small molecules such as C–O bond cleavage and C–C bond formation of CO, and reversible H2 uptake and release under crystalline form. A titanium hydride complex shows high activity toward inert molecules such as N–N bond cleavage and N–H bond formation of N2, C–C bond cleavage and rearrangement of benzene, and hydrodenitrogenation of pyridines under mild conditions. These reactions are induced by synergistic effects of multiple metal hydrides.

©2020 The Physical Society of Japan
1. Introduction

Transition metal hydride species are important components not only in a wide range of functional materials14) but also in a variety of catalytic and stoichiometric reactions.5) Particularly, their importance in recent synthetic organic and inorganic chemistries cannot be overemphasized. Multimetallic polyhydride compounds are of remarkable interest as they may show synergistic effects through the cooperation of multiple M–H sites. However, studies on molecular multimetallic hydride complexes to date have been mainly based on late transition metals,616) while the chemistry of multinuclear early transition metal polyhydride complexes has remained relatively unexplored.

Rare earth metal hydrides are among the most reactive metal hydride compounds. Previously, a great number of molecular rare earth metal monohydride complexes of the general type [(L)(L′)MH], which are supported by two anionic ancillary ligands (L, L′) per metal, have been synthesized and widely studied.1719) In contrast, well-defined rare earth metal dihydride or polyhydride complexes bearing one anionic ancillary ligand (L) per metal such as [(L)MH2], had remained unknown until 2003, when the isolation and structural characterization of the C5Me4SiMe3-ligated tetranuclear yttrium and lutetium octahydride complexes [{(C5Me4SiMe3)Ln(μ-H)2}4(THF)] (Ln = Y, Lu) were reported,20,21) which show unique reactivities that differ from those of the conventional monohydride complexes.

This article is intended to overview and highlight recent advances in synthesis of rare earth metal hydride complexes and their unique reactions toward small molecules.2224) A new family of d–f heteromultimetallic hydride complexes composed of both rare earth and d-block transition metals are also described. In addition to studies on the rare earth metal hydride complexes, the reaction chemistry of the group 4 transition metal hydride complexes toward various stable molecules, such as N2 and aromatic compounds will also be described. These studies are opening a new avenue of transition metal hydride chemistries.

2. Properties of Rare Earth Metal Hydride Complexes
Synthesis of rare earth metal hydride complexes

Hydrogenolysis of the C5Me4SiMe3-ligated scandium bis(trimethylsilylmethyl) complex [Cp′Sc(CH2SiMe3)2(THF)] (Cp′ = C5Me4SiMe3) (1) with H2 (1 atm) in toluene at room temperature affords the THF-free tetranuclear octahydride complex [{Cp′Sc(μ-H)2}4] (2) (Fig. 1).25) This complex consists of four “Cp′ScH2” units, which are assembled in a tetrahedral form through “intermolecular” Sc–H interactions. There are totally eight hydride ligands in the whole molecule, six of which are edge-bridged in a \(\mu_{2}\)-H–Sc2 style, one is face-capped in a \(\mu_{3}\)-H–Sc3 form, and one is body-centered in a \(\mu_{4}\)-H–Sc4 fashion. The THF ligand coordinated originally to the scandium metal center of the alkyl precursor 1 is extruded from the tetrahedral core structure due to formation of the “intermolecular” Sc–H bonds.


Figure 1. (Color online) Synthesis of multimetallic rare earth metal polyhydride complexes bearing the C5Me4SiMe3 ligand.

Hydrogenolysis of the Cp′-ligated dialkyl complexes of the larger rare earth metals such as Y, Er, Tm, and Lu gives analogously the corresponding tetranuclear octahydride complexes [{Cp′Ln(μ-H)2}4(THF)] (Ln = Y, Er, Tm, Lu) (3), in which one THF ligand is bonded to one of the four metal centers (Fig. 1).25) The overall structure of the mono(THF)-coordinated polyhydride complexes 3 is almost the same as that of the THF-free Sc complex 2.

In the case of the larger metals such as Gd, Dy, and Ho, the analogous tetranuclear hydride complexes 4 were obtained and an agostic interaction26) between the metal center and a methyl group of an adjacent C5Me4SiMe3 ligand is also observed, probably due to greater steric unsaturation of these larger metal centers (Fig. 1).25) These results demonstrate that the precise structures of the tetranuclear octahydride complexes are, to some extent, dependent on the ionic radius of the metal centers.

These tetranuclear octahydride complexes 24 are all soluble in common organic solvents such as hexane, toluene, and THF, and their tetranuclear frameworks are retained in solution. The 1H NMR signals of the hydrides in scandium complex 2 in C6D6 gave a broad peak at \(\delta_{\text{H}}\) 3.91 ppm, while that of the yttrium complex 3 gave a quintet peak at \(\delta_{\text{H}}\) 4.32 ppm (\(J_{\text{Y--H}} = 15.3\) Hz), indicating that all of the hydride ligands are equivalent on the NMR time scale and each hydride ligand couples with four yttrium atoms.

Since the trimethylsilylmethyl group (–CH2SiMe3) is not applicable for the synthesis of the analogous early (much larger) lanthanide complexes due to facile ligand redistribution, bidentate o-dimethylaminobenzyl ligand, CH2C6H4NMe2-o was used instead.27) However, the expected half-sandwich bis(aminobenzyl) complexes [Cp′Ln(CH2C6H4NMe2-o)2] (Ln = La, Ce, Pr, Nd, Sm) could not be isolated in a pure form because the reaction of [Ln(CH2C6H4NMe2-o)3] with C5Me4SiMe3H gave an oil product difficult to purify. However, hydrogenolysis of the in-situ formed bis(aminobenzyl) complexes “[Cp′Ln(CH2C6H4NMe2-o)2]” in THF afforded the corresponding bis(THF)-bonded tetranuclear octahydride complexes [{Cp′Ln(μ-H)2}4(THF)2] (5: Ln = La, Ce, Pr, Nd, Sm) (Fig. 2).25) In the 1H NMR spectra of the lanthanum octahydride complex [{Cp′La(μ-H)2}4(THF)2], the hydride signals are equivalently observed at \(\delta_{\text{H}}\) 8.70 ppm as a broad singlet signal.


Figure 2. (Color online) Synthesis of multimetallic early rare earth metal polyhydride complexes bearing the C5Me4SiMe3 ligand.

On the other hand, the analogous yttrium bis(aminobenzyl) complexes [(C5Me4R)Y(CH2C6H4NMe2-o)2] (R = SiMe3, Me, Et, H) (1ad) were obtained by the treatment of [Y(CH2C6H4NMe2-o)3] with one equivalent of the corresponding C5Me4RH, and isolated in a pure form.28) When the reaction of 1a with H2 was performed under THF-free condition, the pentanuclear yttrium decahydride complex, [{Cp′Y(μ-H)2}5] (6), instead of a tetranuclear species, was obtained (Fig. 3),28) probably because of the absence of a THF ligand in 1a. The pentanuclear yttrium decahydride complex 6 could be generated through a “self-assembly” of five dihydride species “[Cp′YH2]”. Alternatively, it was also generated by a combination of a tetranuclear octahydride complex “[{Cp′YH2}4]”29) with a mononuclear dihydride species “[Cp′YH2]”. In agreement with the above assumption, the hydrogenolysis of 1a in presence of 1 equiv of [{Cp′YH2}4] also afforded 6 selectively.


Figure 3. (Color online) Synthesis of multimetallic yttrium polyhydride complexes. The Cp ligands in the products have been omitted for clarity.

Complex 6 contains a pentanuclear square pyramidal Y5 metal core frame. There are seven \(\mu_{2}\)-H and two \(\mu_{3}\)-H ligands, together with one square-pyramidal \(\mu_{5}\)-H ligand at the center of the four basal yttrium metal atoms. This is the first example of the pentanuclear rare earth metal polyhydride complexes. The 1H NMR spectrum of complex 6 (toluene-\(d_{8}\), 100 °C) showed one sextet signal at \(\delta_{\text{H}}\) 4.60 (\(J_{\text{YH}} = 12.8\) Hz) ppm that was assignable to the hydride ligands, coupling with five yttrium atoms. The chemical shift (\(\delta_{\text{H}}\) 4.60) is similar to that found in 3 (\(\delta_{\text{H}}\) 4.32), whereas the \(J_{\text{YH}}\) coupling constant (\(J_{\text{YH}} = 12.8\) Hz) in 6 is smaller than that in 3 (\(J_{\text{HY}} = 15.3\) Hz).

The sterically less demanding cyclopentadienyl ligand-coordinated rare earth hydrides have also been reported previously.30) Hydrogenolysis of the C5Me5 (Cp\(^{*}\))-ligated yttrium complex 1b with H2 in THF afforded the THF-coordinated pentanuclear decahydride complex [{Cp\(^{*}\)Y(μ-H)2}5(THF)2] (7).28) The analogous pentanuclear complex [{(C5Me4Et)Y(μ-H)2}5(THF)2] was obtained from the hydrogenolysis of 1c with H2 in THF. The basic structural unit of 7 is similar to that of 6 except that 7 contains 2 additional THF ligands.

Hydrogenolysis of the sterically even less demanding C5Me4H-ligated complex 1d with H2 in THF afforded a tetranuclear yttrium polyhydride complex [{(C5Me4H)Y(μ-H)2}4(THF)4] (8) (Fig. 3).28) The structure of 8 contains the four \(\mu_{2}\)-H ligands and the four \(\mu_{3}\)-H ligands. Complex 8 is relatively stable in THF solution, but complex 8 slowly changes when heated in a benzene solution at 50 °C for several hours, suggesting that the four THF ligands in 8 are dissociatively labile and could thereby provide reactive metal sites.

It is widely known that rare earth metal hydrides can be synthesized by the reaction of alkyl complexes with PhSiH33133) or H2. When the reaction of 1b with PhSiH3 was performed, the hexanuclear polyhydride complex [{Cp\(^{*}\)Y(μ-H)2}6] (9) was obtained.28) Complex 9 showed a relatively low solubility in common organic solvents, such as hexane, benzene, and toluene, and no decomposition on heating in toluene at 120 °C for 12 h. The core structure contains 12 hydride ligands: one interstitial \(\mu_{6}\)-H ligand, 8 face-capping \(\mu_{3}\)-H ligands, and 3 edge-bridging \(\mu_{2}\)-H ligands, showing the same coordination mode as that of the previously reported tris(pyrazolyl)borate (Tp)-ligated hexanuclear lutetium polyhydride complex [{(Tp)LuH2}6].34)

The 1H NMR spectrum of 9 at 110 °C showed a hydride signal at \(\delta_{\text{H}}\) 4.00 (\(J_{\text{YH}} = 10.0\) Hz, 12H) as septet form. This coupling pattern suggests that there are six yttrium atoms coupled with each hydride ligand and all of the hydride ligands are equivalent on the NMR timescale. The coupling constant \(J_{\text{YH}}\) (10.0 Hz) for the hydride ligands is the smallest value for any yttrium hydride complexes, indicating that there is a close relationship between the nuclearity of the complex and the value of \(J_{\text{YH}}\). When the nuclearity increases, the value of \(J_{\text{YH}}\) decreases (cf. mononuclear [Cp′2YH(THF)]:35) 75 Hz; dinuclear [{Cp\(^{*}\)2YH}2]:36) 68, 32 Hz, trinuclear [{(Me-PNP\(^{\text{$i$Pr}}\))Y(μ-H)2}3] (Me-PNP\(^{\text{$i$Pr}}\) = {2-(iPr2P)-4-Me-C6H3}2N):37) 17.0 Hz, tetranuclear 3: 15.3 Hz; pentanuclear 6: 12.8 Hz; pentanuclear 7: 12.3 Hz; and hexanuclear 9: 10.0 Hz).

Reactivity of rare earth metal hydride complexes toward small molecules

Some representative reactions of the C5Me4SiMe3-ligated tetranuclear yttrium octahydride complex are summarized in Fig. 4. The reaction of the tetranuclear yttrium octahydride complex [{Cp′Y(μ-H)2}4(THF)] (3) with unsaturated hydrocarbons such as styrene gives the allyl benzylic heptahydride complex 10, in which the phenyl part is bonded to one Y atom in an \(\eta^{2}\)-fashion and the allyl part is bonded to another Y atom in an \(\eta^{3}\)-form (Fig. 4).29) Hydrogenolysis of 10 affords the THF-free yttrium octahydride complex [{Cp′Y(μ-H)2}4] and ethylbenzene. Under H2 (1 atm), 3 or 10 could catalytically hydrogenate styrene into ethylbenzene.


Figure 4. (Color online) Some unique reactions of a tetranuclear yttrium octahydride complex 3.

Carbon dioxide (CO2) is a naturally abundant, less expensive, non-toxic, and inherently renewable carbon resource. Hence, the use of CO2 as a chemical feedstock for organic reactions is of much interest. The reaction of [{Cp′Y(μ-H)2}4(THF)] (3) with CO2 takes place rapidly, but no characterizable product could be isolated from this reaction. In contrast, the tetranuclear yttrium tetrahydride complex 11, which was obtained from reaction of 3 with 1,4-bis(trimethylsilyl)-1,3-butadiyne, reacts with CO2 to afford the characterizable bis(methylene diolate) complex 12 (Fig. 4).21) In this reaction, both of the two C=O double bonds in CO2 are reduced into the C–O single bonds by four hydride ligands, in contrast with the common reactions of transition metal hydrides with CO2, which usually give formate –OC(=O)H species.

Reduction and functionalization of the carbon monoxide (CO) is of critical importance, since it may offer potential applications in the development of homogeneous systems for the selective formation of hydrocarbons and oxygenates. The reaction of [{Cp′Ln(μ-H)2}4(THF)] (Ln = Y, Lu) (3) with CO affords the dioxo/tetrahydride complex [(Cp′Y)4(\(\mu_{3}\)-O)2(μ-H)4(THF)] (13) and ethylene under mild conditions (Fig. 4).38) Stepwise formation of some key reaction intermediates, such as oxymethylene (μ-OCH2), and enolate (μ-OCH=CH2) species, has been confirmed, some of which have been isolated and structurally characterized. Further reaction of 13 with CO gives the corresponding tetraoxo complex [Cp′Ln(\(\mu_{3}\)-O)]4 (14) together with ethylene. Reactions of 3 with various transition metal carbonyl complexes such as [Cp\(^{*}\)Rh(CO)2], [Cp\(^{*}\)Ir(CO)2], or [Cp\(^{*}\)W(CO)2(NO)], were also examined.39) For example, the reaction of 3 with the rhodium carbonyl complex [Cp\(^{*}\)Rh(CO)2] afforded a dioxo/dimethyl complex 15, in which the two C≡O triple bonds of the carbonyl ligands were completely reduced and cleaved by addition of six Y–H bonds from 3 (Fig. 4). The reaction patterns are dependent on the nature of the transition metal carbonyls. These reactions may provide the mechanistic aspects of the Fischer–Tropsch synthesis, which produce hydrocarbons and oxygenates by hydrogenolysis of CO on heterogeneous catalyst.

Metal imide species have received much current interest because of the ability of the M=N group to undergo a wide range of reactions. The reactions of [{Cp′Ln(μ-H)2}4(THF)] (Ln = Y, Lu) (3) with 4 equiv of benzonitrile afford the corresponding tetranuclear cubane-like imido complexes [{Cp′Ln(\(\mu_{3}\)-NCH2Ph)}4] (Ln = Y, Lu) (16) (Fig. 4).29) In these reactions, the C≡N triple bond of benzonitrile is completely reduced to a C–N single bond by double Ln–H addition. This is in sharp contrast to the previously reported reactions of metallocene hydride complexes40,41) with nitriles, which yielded only single-insertion products. The reactions of the benzylimido complexes 16 with an excess amount of benzonitrile give the benzonitrile tetramerization products 18, via benzamidinate-dianion complex 17 (Fig. 4). Further reaction of 18 with excess amount of benzonitrile catalytically affords the benzonitrile-cyclotrimerization product, namely triphenyl triazine (PhCN)3 and complex 18 is recovered from these reactions, suggesting that the benzonitrile-tetramerized complex is the true catalyst.42)

Generally, cationic complexes differ in their structure and reactivity from their neutral analogues. The reaction of the yttrium octahydride complex [{Cp′Y(μ-H)2}4(THF)] (3) with one equivalent of [Ph3C][B(C6F5)4] affords the corresponding cationic heptahydride complex [(Cp′Y)4(μ-H)7(THF)][B(C6F5)4] (19) in high yield (Fig. 4).43) In contrast to the neutral hydride complex 3, the cationic hydride complex 19 is an active catalyst for the syndiospecific polymerization of styrene.

3. Synthesis and Properties of Heteromultimetallic Hydride Complexes Composed of Rare Earth and d-Transition Metals

Structurally well-defined molecular d–f heteromultimetallic hydride complexes consisting of d-block transition metals and lanthanide (f-block) are of much interest because synergistic effects may result from the two substantially different metal centers, and are also of great interest as molecular models for hydrogen storage alloys such as LaNi5.4447) However, such heteromultimetallic complexes have not yet been extensively studied due to difficulty in their synthesis and structural characterization. The reaction of [{Cp′Y(μ-H)2}4(THF)] (3) with the molybdenum pentahydride complex [Cp\(^{*}\)Mo(PMe3)H5] easily afforded the corresponding heteromultimetallic hendecahydride complex [(Cp′Y)4(μ-H)11MoCp\(^{*}\)] (20) (Fig. 4).48) Complex 20 released one H2 molecule under vacuum condition to give a nonahydride complex [(Cp′Y)4(μ-H)9MoCp\(^{*}\)] (21). Unprecedented structural features including a trigonal bipyramidal \(\mu_{5}\)-H atom in 20 and unique reactivities such as hydrogen addition/release reactions between 20 and 21 have been clarified. Monitoring of H2 addition to the complex 21 in a single-crystal-to-single-crystal process by X-ray diffraction has been achieved.48) Density functional theory (DFT) studies have demonstrated that the hydrogen addition process is cooperatively promoted by the Y/Mo interplay, thus offering unprecedented insight into the hydrogen addition and release process of metal hydride species. Some analogous Y/Mo heteromultimetallic polyhydride complexes [{(C5Me4H)Y}4(μ-H)4{(μ-H)5MoCp\(^{*}\)}2] and [{(Cp\(^{*}\)Y)5(μ-H)8}(μ-H)5MoCp\(^{*}\)] bearing the sterically less demanding Cp-ligated yttrium units such as “(C5Me4H)Y” and “(C5Me5)Y” groups were also synthesized and showed the similar H2 release and uptake properties.49)

Rare earth dialkyl precursors have been used for the synthesis of d–f heteromultimetallic complexes. The reaction of the C5Me5 (Cp\(^{*}\))-ligated lutetium dialkyl complex [Cp\(^{*}\)Lu(CH2SiMe3)2(THF)] (Cp\(^{*}\) = C5Me5) with the ruthenium phosphine trihydride complexes such as [Cp\(^{*}\)Ru(PR3)H3] (R = Me, Ph, Et) affords the Lu/Ru heterobimetallic dihydride complexes 22, 23, and 24, via “deprotonation” of the ruthenium hydride by the lutetium alkyl groups followed by C–H activation of a Me, Ph, or Et group of the phosphine ligands at the rare earth metal center (Fig. 5).50,51)


Figure 5. (Color online) Synthesis of Lu/Ru heterobinuclear dihydride complexes and reactions with CO.

The reaction of the methylene bridged complex 22 with CO provided the tetranuclar Lu2/Ru2 vinylphosphine/oxo/hydride complex 25, while the reaction of the phenylene bridged complex 23 with CO afforded the monomeric ruthenium phospha-benzyl carbonyl complex 26. In both reactions, C–O bond cleavage, hydride transfer, and C–C bond formation with phosphine substituents have been achieved. On the other hand, the reaction of the ethylene bridged complex 24 with CO gave the dinuclear phospha-enolate complex 27 via CO insertion into Lu–C bond and phosphine migration reaction. It is clear from these results that the Ln/Ru heterobimetallic dihydride complexes can indeed demonstrate a unique synergistic reactivity, which is governed by changing the phosphine substituent environment.51)

4. Properties of Group 4 Transition Metal Hydride Complexes

In addition to studies on the rare earth polyhydrides mentioned above, the analogous group 4 transition metal polyhydrides have also been investigated.52) A titanium polyhydride was found to show remarkable reactivity toward various stable molecules, such as cleavage and hydrogenation of dinitrogen, C–C bond cleavage and rearrangement of benzene, and hydrodenitrogenation (HDN) of pyridines under mild conditions. These studies are opening a new chapter of transition metal hydride chemistry, and some representative results are described below in more details.

Synthesis of group 4 transition metal hydride complexes

In the course of our studies on the mono Cp-ligated rare earth polyhydrides, we became interested in the analogous group 4 transition metal hydrides. Previous attempts to prepare a mono-C5Me5-ligated zirconium hydride complex by hydrogenolysis of the corresponding trimethyl precursor such as [Cp\(^{*}\)ZrMe3] (Cp\(^{*}\) = C5Me5) with H2 were reported to give an unidentified product mixture.53) In contrast, hydrogenolysis of the sterically more demanding C5Me4SiMe3-ligated zirconium and hafnium tris(trimethylsilylmethyl) complexes [Cp′M(CH2SiMe3)3] (M = Zr, Hf; Cp′ = C5Me4SiMe3) with H2 gave the structurally characterizable tetranuclear octahydride complexes [{Cp′M(μ-H)2}4] in high yields through tetramerization of the resulting trihydride species “Cp′MH3” with liberation of two molecules of H2 (Fig. 6).54) In this transformation, the oxidation state of zirconium and hafnium was reduced from 4+ to 3+. Two of the eight hydride ligands in the tetranuclear core structure were face-capped in a \(\mu_{3}\)-H fashion and the other 6 hydrides were edge-bridged in a \(\mu_{2}\)-H form. In contrast to the rare earth octahydride analog such as [Cp′4Y4(\(\mu_{4}\)-H)(\(\mu_{3}\)-H)(\(\mu_{2}\)-H)6], no body-centered interstitial \(\mu_{4}\)-H ligand was found in the Zr or Hf tetrahedron cavity. The 1H NMR spectrum of the zirconium and the hafnium octahydride complexes showed a singlet at \(\delta_{\text{H}}\) 0.61 (Zr–H) and \(\delta_{\text{H}}\) 4.86 (Hf–H) ppm, respectively, for the hydride ligands.


Figure 6. (Color online) Synthesis of group 4 transition metal hydride complexes and N2 activation.

In comparison to the selective formation of the tetranuclear zirconium and hafnium octahydride complexes, hydrogenolysis of the analogous titanium trialkyl complex [Cp′Ti(CH2SiMe3)3] with H2 in the presence of N2 afforded the tetranuclear diimide/tetrahydride complex [(Cp′Ti)4(\(\mu_{3}\)-NH)2(μ-H)4] (28) in 90% yield via cleavage of N–N bond and formation of N–H bond (Fig. 6),55) while the formation of an expected tetranuclear titanium octahydride complex [{Cp′Ti(μ-H)2}4] was not observed. In this reaction, one N2 molecule was formally reduced to two [NH]2− imide units by H2. When the hydrogenolysis of [Cp′Ti(CH2SiMe3)3] with H2 was carried out in the absence of N2, a trinuclear heptahydride complex [(Cp′Ti)3(\(\mu_{3}\)-H)(μ-H)6] (29), instead of complex 28, was obtained in 69% yield (Fig. 6). One of the seven hydride ligands in 29 adopted a \(\mu_{3}\)-H bonding mode and six were in a \(\mu_{2}\)-H form. Formally, two of the three Ti atoms was in the 3+ oxidation state and one was in the 4+ oxidation state. In the 1H NMR spectrum, the bridging seven hydrides of 29 appeared equivalently at \(\delta_{\text{H}}\) 2.66 ppm as a singlet signal.

Reactivity of group 4 transition metal hydride complexes toward small molecules

Dinitrogen (N2) is ubiquitous and inexpensive resources, but is largely chemically inert under ordinary conditions. Certain microbial organisms can reduce N2 to NH3 catalyzed by nitrogenase enzymes at ambient temperature and pressure, but this reaction process is not yet well clarified and difficult to imitate artificially. Industrially, N2 cleavage and hydrogenation is achieved in combination with dihydrogen (H2) at high temperatures and pressures on solid catalysts to give NH3 (the Haber–Bosch process). To get more information of the N2 activation and develop catalytic NH3 synthesis under ambient conditions, extensive studies on the N2 activation by well-defined transition metal complexes have been carried out over the past decades.5663) N2 reduction and cleavage could be achieved under milder condition by use of transition metal complexes and strong reducing reagents such as Mg, Na/Hg, KC8, and Cp2Co. Another approach is the N2 reduction by transition metal hydrides, which avoids using special reducing agents and proton sources,52,64) and may give some tips to explore well-defined molecular complexes for the catalytic NH3 synthesis from N2 and H2.

As mentioned above, hydrogenolysis of the titanium alkyl complex in the presence of N2 could afford the imide complex 28 through N–N bond cleavage and hydrogenation, while hydrogenolysis of the titanium alkyl complex in the absence of N2 afforded the trinuclear titanium heptahydride complex [(Cp′Ti)3(\(\mu_{3}\)-H)(μ-H)6] (29), which should be active precursor for N2 activation reaction. Apparently, complex 29 showed high reactivity toward N2, which afforded the nitrido/imido complex [(Cp′Ti)3(\(\mu_{3}\)-N)(μ-NH)(μ-H)2] (30) upon exposure to an N2 atmosphere (1 atm) at room temperature (Fig. 7).55) Hydrogenolysis of complex 30 with H2 in the presence of [Cp′Ti(CH2SiMe3)3] afforded the tetranuclear complex 28, suggesting that N2 activation by H2 and [Cp′Ti(CH2SiMe3)3] should be initiated through the formation of the trinuclear hydride complex 29, followed by its reaction with N2. In situ formed imide/nitride complex 30 immediately “trapped” a mononuclear hydride species “Cp′TiH3” generated from [Cp′Ti(CH2SiMe3)3] and H2 to give the tetranuclear imide/hydride complex 28 (Fig. 7).


Figure 7. (Color online) Dinitrogen cleavage and hydrogenation by a titanium hydride complex 29 and 28.

The low temperature 1H NMR and computational studies have revealed that the dinitrogen reduction from 29 proceeded sequentially through scission of an N2 molecule bonded to three Ti atoms in a μ-\(\eta^{2}\):\(\eta^{2}\):\(\eta^{1}\)-side-on-end-on fashion (31) to give a \(\mu_{2}\)-N/\(\mu_{3}\)-N dinitrido species 32, followed by intramolecular hydrogen transfer from two Ti(IV) sites to the \(\mu_{2}\)-N nitride unit to give 30 (Fig. 7). The six electrons required for N2 cleavage were generated by the release of two molecules of H2 from the hydride (H) complex (which generates four electrons) and the oxidation of one hydride (H) ligand to a proton (H+) (which generates two electrons). This has clearly demonstrated that the hydride ligands in 29 can serve as the source of both electrons and protons.

Heating solid of complex 28 at 180 °C under N2 (1 atm) resulted in the further N2 activation to give a diimido/dinitrido complex [(Cp′Ti)4(\(\mu_{3}\)-NH)2(\(\mu_{3}\)-N)2] (33) in 95% yield (Fig. 7).65,66) Experimental and theoretical studies revealed the mechanistic details. Heating solution of complex 28 at 130 °C under N2 (1 atm) did not activate N2, but released one molecule of H2 to give the dinitrido/tetrahydrido complex [(Cp′Ti)4(\(\mu_{3}\)-N)2(μ-H)4] (34). Hydrogenolysis of the nitride units in 34 with H2 (1 atm) at 80 °C recovered 28 quantitatively, demonstrating that reversible dehydrogenation and hydrogenation reactions between 28 and 34 took place. Heating the dinitrido/tetrahydrido complex 34 at 180 °C in the presence of N2 (1 atm) gave the diimido/dinitrido complex 33 quantitatively, suggesting that N2 activation reaction of 28 to give 33 should be initiated through the dehydrogenation of 28 to give 34. The results demonstrate that the interactions among the hydride, imide, and nitride ligands, including the reversible dehydrogenation/hydrogenation of imide and nitride species, and hydride transfer to the nitride species, played a critically important role for N2 activation.

The trinuclear titanium polyhydride complex 29 was also found as a unique platform for the C–C bond cleavage and rearrangement of benzene (Fig. 8).67) When a benzene solution of 29 was left to stand at room temperature under an argon atmosphere for several days, a methylcyclopentenyl complex [(Cp′Ti)3(C5H4Me)(μ-H)4] (35) was obtained. Complex 35 was formed almost quantitatively, when a benzene solution of 29 was kept over gentle heating at 40 °C for 36 h. Remarkably, a benzene molecule was partly hydrogenated and ring-rearranged from a six-membered ring to a five-membered ring species, [MeC5H4]. This reaction needed to break a stable aromatic C–C bond of benzene and make a new C–C bond. In this transformation, three of the seven hydride ligands in 29 were consumed, two being released as H2 by donating two electrons and one transferred to benzene, thus affording the trianionic [MeC5H4]3− species. The formal oxidation state of the Ti atoms in 29 and 35 remained unchanged; both complexes formally contain two Ti(III) and one Ti(IV) atoms. When a benzene solution of 29 was kept at 10 °C for two days, formation of a new complex assignable to [(Cp′Ti)3(C6H7)(μ-H)4] (36) was observed. When kept at 40 °C for ca. 3 h, 36 was converted quantitatively to the methylcyclopentenyl [MeC5H4]3− complex 35. When 35 was heated at 100 °C for two days, insertion of a Ti atom into a C–C bond of the methylcyclopentenyl ring took place, yielding a titanacyclohexenyl product, [(Cp′Ti)3{μ-\(\eta^{2}\),\(\eta^{5}\),\(\eta^{5}\)-CHC(Me)(CH)3}(μ-H)2] (37). The C–C bond cleavage and ring rearrangement of toluene also occurred in a similar fashion, though some details were different due to the presence of the methyl substituent.


Figure 8. (Color online) Carbon–carbon bond cleavage and rearrangement of benzene by the titanium hydride complex 29.

The DFT studies suggest that the reaction between the hydride 29 and benzene is initiated by benzene coordination to one of the three Ti atoms, followed by H2 release, C6H6 hydrometallation, repeated C–C and C–H bond cleavage and formation to give a MeC5H4 unit (such as 35), and the subsequent insertion of a Ti atom into the C–C bond of the 5-membered ring MeC5H4 unit with release of H2 to give a titanacycle product (such as 37).68) Apparently, the facile liberation of H2 from the titanium hydride complex to give electrons and change charge population at the titanium centers, in connection with the flexible titanium–hydride bondings and dynamic redox behavior of the trinuclear titanium framework, has enabled this unusual transformation to occur. These results not only provided mechanistic details of the activation and transformation of benzene over a multimetallic framework but may also give some hints in the design of new well-defined molecular catalysts for the activation and transformation of stable aromatics.

The HDN of N-heteroaromatics such as pyridines is an important process in the industrial petroleum refining to remove nitrogenous impurities from crude oil. However, due to the stable C–N bonds of N-heteroaromatics, it causes difficulties to break the bonds under ordinary conditions. The industrial HDN is carried out at high temperatures and pressures (300–500 °C, ∼200 atm) on solid catalysts. In view of the fact that metal hydrides are likely the true active species in the industrial HDN process, the investigation of the reactivity of molecular transition metal hydrides with N-heteroaromatics is of great interest and importance.

Trinuclear titanium polyhydride 29 can ring-open and denitrogenate pyridines under mild conditions. The nitrogen atom in a pyridine ring was extruded through the dehydrogenative reduction of the C=N unit (leading to complex 38) followed by cleavage of the two C–N bonds at a trimetallic titanium framework (leading to complex 39; Fig. 9).69) Both linear and cyclic nitrogen-free hydrocarbons such as cyclopentadiene were selectively generated from pyridines under appropriate conditions. This work represents the first example of HDN of N-heteroaromatics by a well-defined molecular system, and may help better understand the industrial HDN process and guide designing new catalysts.


Figure 9. (Color online) Hydrodenitrogenation of pyridines by the titanium hydride complex 29.

5. Summary and Future Outlook

It has been known that metal hydride species play an important role in a variety of the catalytic and stoichiometric reactions. Particularly, multimetallic hydrides can serve as a unique platform for activation and transformation of inert molecules due to the synergistic effects of the multiple metal hydride sites. Our study on the rare earth and group 4 transition metal hydrides has led to the opening of a new frontier in the chemistry of multimetallic hydride complexes.

Mono(cyclopentadienyl)-ligated tetranuclear rare earth octahydride complexes showed unique reactivities towards various substrates such as CO, CO2, H2, and unsaturated C–C and C–N bonds. By using these rare earth hydride complexes as a building block, novel heteromultimetallic hydride complexes could also be prepared. Some of the heteromultimetallic hydride complexes were found to undergo unique reversible H2 uptake and release in a single-crystal-to-single-crystal process. In addition, the synthesis of the analogous group 4 transition metal hydride complexes has been explored, and a titanium hydride was found to show remarkable reactivity toward various stable molecules such as cleavage and hydrogenation of dinitrogen, C–C bond cleavage and rearrangement of benzene, and hydrodenitrogenation of pyridines under mild conditions. The DFT studies have revealed that the cooperation of multiple metal hydrides plays an important role in these novel transformations.

These multimetallic rare earth and group 4 metal polyhydride complexes will react with a wide variety of molecules in an unprecedented fashion, and therefore the synthesis and reactivity exploration of various types of multimetallic polyhydride complexes with different transition metals (such as group 5, 6, and later) and ligand systems are of particular importance. Recently, the progress was found in the synthesis and chemistry of the analogous group 4 transition metal hydride complexes bearing the PNP-pincer type ligands, which showed cleavage and hydrogenation of N2 in combination with H2.70) As more multimetallic hydride complexes are prepared and further investigations proceeded, the more chances to discover novel and unique reactions and chemical transformations could be expected.

Acknowledgment

This work was supported in part by JSPS KAKENHI Grant-in-Aid for Scientific Research on Innovative Areas “Hydrogenomics” (JP18H05517) and (C) (JP17K05823).


References

  • 1 D. Meng, M. Sakata, K. Shimizu, Y. Iijima, H. Saitoh, T. Sato, S. Takagi, and S. Orimo, Phys. Rev. B 99, 024508 (2019). 10.1103/PhysRevB.99.024508 CrossrefGoogle Scholar
  • 2 C. Tassel, Y. Goto, Y. Kuno, J. Hester, M. Green, Y. Kobayashi, and H. Kageyama, Angew. Chem., Int. Ed. 53, 10377 (2014). 10.1002/anie.201405453 CrossrefGoogle Scholar
  • 3 G. Kobayashi, Y. Hinuma, S. Matsuoka, A. Watanabe, M. Iqbal, M. Hirayama, M. Yonemura, T. Kamiyama, I. Tanaka, and R. Kanno, Science 351, 1314 (2016). 10.1126/science.aac9185 CrossrefGoogle Scholar
  • 4 S. Orimo, Y. Nakamori, J. R. Eliseo, A. Züttel, and C. M. Jensen, Chem. Rev. 107, 4111 (2007). 10.1021/cr0501846 CrossrefGoogle Scholar
  • 5 J. R. Norton and J. Sowa, Chem. Rev. 116, 8315 (2016), See also the related reviews in this special issue. 10.1021/acs.chemrev.6b00441 CrossrefGoogle Scholar
  • 6 T. Takao and H. Suzuki, Bull. Chem. Soc. Jpn. 87, 443 (2014). 10.1246/bcsj.20130294 CrossrefGoogle Scholar
  • 7 H. Suzuki, T. Kakigano, K. Tada, M. Igarashi, K. Matsubara, A. Inagaki, M. Oshima, and T. Takao, Bull. Chem. Soc. Jpn. 78, 67 (2005). 10.1246/bcsj.78.67 CrossrefGoogle Scholar
  • 8 G. S. McGrady and G. Guilera, Chem. Soc. Rev. 32, 383 (2003). 10.1039/b207999m CrossrefGoogle Scholar
  • 9 H. Suzuki, Eur. J. Inorg. Chem. 2002, 1009 (2002). 10.1002/1099-0682(200205)2002:5%3C1009::AID-EJIC1009%3E3.0.CO;2-0 CrossrefGoogle Scholar
  • 10 F. Maseras, A. Lledόs, E. Clot, and O. Eisenstein, Chem. Rev. 100, 601 (2000). 10.1021/cr980397d CrossrefGoogle Scholar
  • 11 S. Sabo-Etienne and B. Chaudret, Chem. Rev. 98, 2077 (1998). 10.1021/cr9601066 CrossrefGoogle Scholar
  • 12 R. Bau and M. H. Drabnis, Inorg. Chim. Acta 259, 27 (1997). 10.1016/S0020-1693(97)89125-6 CrossrefGoogle Scholar
  • 13 Z. Lin and M. B. Hall, Coord. Chem. Rev. 135–136, 845 (1994). 10.1016/0010-8545(94)80084-7 CrossrefGoogle Scholar
  • 14 R. H. Crabtree, in Comprehensive Coordination Chemistry, ed. G. Wilkinson, J. A. Gillar, and J. A. McCleverty (Pergamon, Oxford, U.K., 1987) Vol. 2, p. 689. Google Scholar
  • 15 G. G. Hlatky and R. H. Crabtree, Coord. Chem. Rev. 65, 1 (1985). 10.1016/0010-8545(85)85020-7 CrossrefGoogle Scholar
  • 16 L. M. Venanzi, Coord. Chem. Rev. 43, 251 (1982). 10.1016/S0010-8545(00)82099-8 CrossrefGoogle Scholar
  • 17 W. J. Evans, J. H. Meadows, A. L. Wayda, W. E. Hunter, and J. L. Atwood, J. Am. Chem. Soc. 104, 2008 (1982). 10.1021/ja00371a035 CrossrefGoogle Scholar
  • 18 G. Jeske, H. Lauke, H. Mauermann, P. N. Swepston, H. Schumann, and T. J. Marks, J. Am. Chem. Soc. 107, 8091 (1985). 10.1021/ja00312a050 CrossrefGoogle Scholar
  • 19 M. E. Thompson, S. M. Baxter, A. R. Bulls, B. J. Burger, M. C. Nolan, B. D. Santarsiero, W. P. Schaefer, and J. E. Bercaw, J. Am. Chem. Soc. 109, 203 (1987). 10.1021/ja00235a031 CrossrefGoogle Scholar
  • 20 O. Tardif, M. Nishiura, and Z. Hou, Organometallics 22, 1171 (2003). 10.1021/om021014b CrossrefGoogle Scholar
  • 21 O. Tardif, D. Hashizume, and Z. Hou, J. Am. Chem. Soc. 126, 8080 (2004). 10.1021/ja047889u CrossrefGoogle Scholar
  • 22 Z. Hou, M. Nishiura, and T. Shima, Eur. J. Inorg. Chem. 2007, 2535 (2007). 10.1002/ejic.200700085 CrossrefGoogle Scholar
  • 23 T. Shima and Z. Hou, in Recent Development in Clusters of Rare Earths and Actinides: Chemistry and Materials, ed. Z. Zheng (Springer, Berlin/Heidelberg, 2016). Google Scholar
  • 24 M. Nishiura and Z. Hou, Nat. Chem. 2, 257 (2010). 10.1038/nchem.595 CrossrefGoogle Scholar
  • 25 M. Nishiura, J. Baldamus, T. Shima, K. Mori, and Z. Hou, Chem.—Eur. J. 17, 5033 (2011). 10.1002/chem.201002998 CrossrefGoogle Scholar
  • 26 M. Brookhart, M. L. H. Green, and G. Parkin, Proc. Natl. Acad. Sci. U.S.A. 104, 6908 (2007). 10.1073/pnas.0610747104 CrossrefGoogle Scholar
  • 27 W. Zhang, M. Nishiura, T. Mashiko, and Z. Hou, Chem.—Eur. J. 14, 2167 (2008). 10.1002/chem.200701300 CrossrefGoogle Scholar
  • 28 T. Shima, M. Nishiura, and Z. Hou, Organometallics 30, 2513 (2011). 10.1021/om1012055 CrossrefGoogle Scholar
  • 29 D. Cui, O. Tardif, and Z. Hou, J. Am. Chem. Soc. 126, 1312 (2004). 10.1021/ja039324o CrossrefGoogle Scholar
  • 30 Z. Hou, Y. Zhang, O. Tardif, and Y. Wakatsuki, J. Am. Chem. Soc. 123, 9216 (2001). 10.1021/ja010555+ CrossrefGoogle Scholar
  • 31 I. Castillo and T. D. Tilley, Organometallics 20, 5598 (2001). 10.1021/om010709u CrossrefGoogle Scholar
  • 32 S. Arndt, P. Voth, T. P. Spaniol, and J. Okuda, Organometallics 19, 4690 (2000). 10.1021/om000506q CrossrefGoogle Scholar
  • 33 A. Z. Voskoboynikov, I. N. Parshina, A. K. Shestakova, K. P. Butin, I. P. Beletskaya, L. G. Kuz’mina, and J. A. K. Howard, Organometallics 16, 4041 (1997). 10.1021/om970363g CrossrefGoogle Scholar
  • 34 J. Cheng, K. Saliu, G. Y. Kiel, M. J. Ferguson, R. McDonald, and J. Takats, Angew. Chem., Int. Ed. 47, 4910 (2008). 10.1002/anie.200705977 CrossrefGoogle Scholar
  • 35 Y. Takenaka and Z. Hou, Organometallics 28, 5196 (2009). 10.1021/om900453j CrossrefGoogle Scholar
  • 36 K. H. Den Haan, Y. Wielstra, and J. H. Teuben, Organometallics 6, 2053 (1987). 10.1021/om00153a004 CrossrefGoogle Scholar
  • 37 J. Cheng, T. Shima, and Z. Hou, Angew. Chem., Int. Ed. 50, 1857 (2011). 10.1002/anie.201006812 CrossrefGoogle Scholar
  • 38 T. Shima and Z. Hou, J. Am. Chem. Soc. 128, 8124 (2006). 10.1021/ja062348l CrossrefGoogle Scholar
  • 39 Y. Takenaka, T. Shima, J. Baldamus, and Z. Hou, Angew. Chem., Int. Ed. 48, 7888 (2009). 10.1002/anie.200903660 CrossrefGoogle Scholar
  • 40 W. J. Evans, J. H. Meadows, W. E. Hunter, and J. L. Atwood, J. Am. Chem. Soc. 106, 1291 (1984). 10.1021/ja00317a020 CrossrefGoogle Scholar
  • 41 J. E. Bercaw, D. L. Davies, and P. T. Wolczanski, Organometallics 5, 443 (1986). 10.1021/om00134a009 CrossrefGoogle Scholar
  • 42 D. Cui, M. Nishiura, and Z. Hou, Angew. Chem., Int. Ed. 44, 959 (2005). 10.1002/anie.200461939 CrossrefGoogle Scholar
  • 43 X. Li, J. Baldamus, M. Nishiura, O. Tardif, and Z. Hou, Angew. Chem., Int. Ed. 45, 8184 (2006). 10.1002/anie.200603450 CrossrefGoogle Scholar
  • 44 G. J. Kubas, Chem. Rev. 107, 4152 (2007). 10.1021/cr050197j CrossrefGoogle Scholar
  • 45 L. Schlapbach and A. Züttel, Nature 414, 353 (2001). 10.1038/35104634 CrossrefGoogle Scholar
  • 46 M. Takimoto and Z. Hou, Nature 443, 400 (2006). 10.1038/443400a CrossrefGoogle Scholar
  • 47 F. Schüth, B. Bogdanović, and M. Felderhoff, Chem. Commun., 2249 (2004). 10.1039/B406522K CrossrefGoogle Scholar
  • 48 T. Shima, Y. Luo, T. Stewart, R. Bau, G. J. McIntyre, S. A. Mason, and Z. Hou, Nat. Chem. 3, 814 (2011). 10.1038/nchem.1147 CrossrefGoogle Scholar
  • 49 T. Shima and Z. Hou, Chem.—Eur. J. 19, 3458 (2013). 10.1002/chem.201203495 CrossrefGoogle Scholar
  • 50 T. Shima and Z. Hou, Chem. Lett. 37, 298 (2008). 10.1246/cl.2008.298 CrossrefGoogle Scholar
  • 51 D. Kawai, T. Shima, M. Nishiura, and Z. Hou, J. Organomet. Chem. 847, 74 (2017). 10.1016/j.jorganchem.2017.02.042 CrossrefGoogle Scholar
  • 52 T. Shima and Z. Hou, Dinitrogen Fixation by Transition Metal Hydride Complexes. In Nitrogen Fixation. Topics in Organometallic Chemistry, ed. Y. Nishibayashi (Springer, New York, 2017) Vol. 60, p. 23. 10.1007/3418_2016_3 CrossrefGoogle Scholar
  • 53 P. T. Wolczanski and J. E. Bercaw, Organometallics 1, 793 (1982). 10.1021/om00066a006 CrossrefGoogle Scholar
  • 54 S. Hu, T. Shima, Y. Luo, and Z. Hou, Organometallics 32, 2145 (2013). 10.1021/om400012a CrossrefGoogle Scholar
  • 55 T. Shima, S. Hu, G. Luo, X. Kang, Y. Luo, and Z. Hou, Science 340, 1549 (2013). 10.1126/science.1238663 CrossrefGoogle Scholar
  • 56 Transition Metal-Dinitrogen Complexes, ed. Y. Nishibayashi (Wiley-VCH, Weinheim, 2019). CrossrefGoogle Scholar
  • 57 N. Stucke, B. M. Flöser, T. Weyrich, and F. Tuczek, Eur. J. Inorg. Chem. 2018, 1337 (2018). 10.1002/ejic.201701326 CrossrefGoogle Scholar
  • 58 Nitrogen Fixation. Topics in Organometallic Chemistry 60, ed. Y. Nishibayashi (Springer, New York, 2017). Google Scholar
  • 59 R. J. Burford and M. D. Fryzuk, Nat. Rev. Chem. 1, 0026 (2017). 10.1038/s41570-017-0026 CrossrefGoogle Scholar
  • 60 R. J. Burford, A. Yeo, and M. D. Fryzuk, Coord. Chem. Rev. 334, 84 (2017). 10.1016/j.ccr.2016.06.015 CrossrefGoogle Scholar
  • 61 M. D. Walter, Adv. Organomet. Chem. 65, 261 (2016). 10.1016/bs.adomc.2016.03.001 CrossrefGoogle Scholar
  • 62 S. F. McWilliams and P. L. Holland, Acc. Chem. Res. 48, 2059 (2015). 10.1021/acs.accounts.5b00213 CrossrefGoogle Scholar
  • 63 H. P. Jia and E. A. Quadrelli, Chem. Soc. Rev. 43, 547 (2014). 10.1039/C3CS60206K CrossrefGoogle Scholar
  • 64 J. Ballmann, R. F. Munhá, and M. D. Fryzuk, Chem. Commun. 46, 1013 (2010). 10.1039/b922853e CrossrefGoogle Scholar
  • 65 M. M. Guru, T. Shima, and Z. Hou, Angew. Chem., Int. Ed. 55, 12316 (2016). 10.1002/anie.201607426 CrossrefGoogle Scholar
  • 66 T. Shima, G. Luo, S. Hu, Y. Luo, and Z. Hou, J. Am. Chem. Soc. 141, 2713 (2019). 10.1021/jacs.8b13341 CrossrefGoogle Scholar
  • 67 S. Hu, T. Shima, and Z. Hou, Nature 512, 413 (2014). 10.1038/nature13624 CrossrefGoogle Scholar
  • 68 X. Kang, G. Luo, L. Luo, S. Hu, Y. Luo, and Z. Hou, J. Am. Chem. Soc. 138, 11550 (2016). 10.1021/jacs.6b03545 CrossrefGoogle Scholar
  • 69 S. Hu, G. Luo, T. Shima, Y. Luo, and Z. Hou, Nat. Commun. 8, 1866 (2017). 10.1038/s41467-017-01607-z CrossrefGoogle Scholar
  • 70 B. Wang, G. Luo, M. Nishiura, S. Hu, T. Shima, Y. Luo, and Z. Hou, J. Am. Chem. Soc. 139, 1818 (2017). 10.1021/jacs.6b13323 CrossrefGoogle Scholar

Author Biographies


Takanori Shima was born in Osaka, Japan, in 1972. He obtained his Ph.D. degree in 2001 from Tokyo Institute of Technology on a study of multinuclear polyhydrido complexes, under the supervision of Professor Hiroharu Suzuki. He undertook postdoctoral research in the group of Professor John A. Gladysz (Erlangen, Germany) as an Alexander- von-Humboldt fellow in 2002. In 2004, he joined RIKEN as a special postdoctoral researcher in the Dr. Zhaomin Hou's group. He was promoted senior research scientist in 2011. His current research interests include development of new multimetallic polyhydrido complexes and their use for activation of small molecules.