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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.
Transition metal hydride species are important components not only in a wide range of functional materials1–4) 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,6–16) 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.17–19) 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.22–24) 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.
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
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 2–4 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
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
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) (1a–d) 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
The sterically less demanding cyclopentadienyl ligand-coordinated rare earth hydrides have also been reported previously.30) Hydrogenolysis of the C5Me5 (Cp
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
It is widely known that rare earth metal hydrides can be synthesized by the reaction of alkyl complexes with PhSiH331–33) or H2. When the reaction of 1b with PhSiH3 was performed, the hexanuclear polyhydride complex [{Cp
The 1H NMR spectrum of 9 at 110 °C showed a hydride signal at
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
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(
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(
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.
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 LaNi5.44–47) 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
Rare earth dialkyl precursors have been used for the synthesis of d–f heteromultimetallic complexes. The reaction of the C5Me5 (Cp
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)
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.
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
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(
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.56–63) 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(
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-
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(
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-
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.
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).
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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.