We investigated herein the reactions of (Me3tacn)FeCln (1a: n = 3, 1b: n = 2) with common aluminum hydride reagents and a bulky dihydridoaluminate {Li(ether)2}{Al(OC6H3-2,6-tBu2)}(μ-H)2, which yielded the diamagnetic hydrido complexes 2–4 containing Fe(II) and Al(III). In particular, the use of divalent 1b afforded excellent isolated yields. The structures of 2–4 were determined using spectroscopic and crystallographic analyses. The crystal structures showed distorted octahedral Fe centers and fairly short Fe–Al distances [2.19–2.24 Å]. The structures of cation moiety 2 and neutral complex 4 were further probed using DFT calculations, which indicated a stable low-spin Fe(II) state and strongly electron-donating nature of the (Me3tacn)FeH3 fragment toward the Al(III) center.

Zr–Ir hydrido complexes with ansa-(cyclopentadienyl)(amide) as the supporting ligand in the zirconium fragment, e.g., (L1ZrR)(Cp*Ir)(μ-H)3 [L1 = Me2Si(η5-C5Me4)(NtBu), R = Cl (5), Ph (7), Me (10), alkyl, and aryl] were designed, synthesized, and isolated as tractable early–late heterodinuclear complexes. Despite the presence of the three supporting hydride ligands, Zr–Ir distances in the crystal structures of 5, alkyl, and aryl complexes [2.74–2.76 Å] were slightly longer than the sum of the element radii of Zr and Ir [2.719 Å]. These hydrocarbyl complexes displayed the thermolytic C–H activation of a variety of aromatic compounds and several organometallic compounds. Also, the substrate scope and limitation in the Zr–Ir system were studied. The regiochemical outcomes during the C–H activation of pyridine derivatives and methoxyarenes suggested the in situ generation of a Lewis acidic active intermediate, i.e., (L1Zr)(Cp*IrH2) (III). The existence of III and relevant σ-complex intermediates {L1Zr(η2-R–H)}(Cp*IrH2) (IIR) (R = Me, Ph) in the ligand exchange was demonstrated by the direct isolation of a Et3PO-adduct of III (39b) from 7 and kinetic studies. The structure of the direct Zr–Ir bonds in IIPh, IIMe, III, and 39b were probed using computational studies. The unprecedented strong M–M′ interactions in the early–late heterobimetallic (ELHB) complexes have been proposed herein.

Photochemical reactions of diruthenium tetrahydride complexes containing cyclopentadienyls as auxiliary ligands with carbon dioxide were studied for the effective fixation and reduction of CO2. Whereas the reactions of CpsRu(μ-H)4RuCps (Cps = Cp*, C5Me5; CpEt, C5Me4Et; Cp‡, 1,2,4-C5(tBu)3H2) with CO2 did not proceed under dark and mild conditions, the photochemical reactions under UV (365 nm) irradiation smoothly proceeded to afford two types of products, (i) a μ-formato complex and (ii) a μ-carbonyl-μ-oxo complex, according to the width of the space for the reaction stretching between the two auxiliary cyclopentadienyl ligands.

The reaction of RuCl3·3H2O with 1,3,5-C5(tBu)3H3 afforded a dimeric Ru(III) dichloride complex, [Cp‡RuCl(μ-Cl)]2 (1b; Cp‡ = η5-1,2,4-C5(tBu)3H2). The treatment of 1b with gaseous oxygen afforded the tetravalent ruthenium μ-oxo complex [Cp‡RuCl2]2(μ-O) (2b). The treatment of 2b with a primary or secondary alcohol regenerated 1b with concomitant dehydrogenative oxidation of the alcohol to afford the corresponding aldehyde or ketone, respectively. The treatment of 2b with refluxing 2-propanol or an excess of zinc powder in tetrahydrofuran efficiently afforded a novel dimeric Ru(II) monochloride with a butterfly structure, [Cp‡Ru(μ-Cl)]2 (4), with unsaturated ruthenium centers and high reactivity toward electron-donating reagents. The reaction of 1b with 4 afforded the mixed-valence Ru(III)/Ru(II) complex Cp‡Ru(μ-Cl)3RuCp‡ (7). The treatment of the appropriate precursor 1b or 2b with an excess of a hydride reagent such as LiEt3BH or LiAlH4 afforded the novel dinuclear ruthenium tetrahydride-bridged complex Cp‡Ru(μ-H)4RuCp‡ (9b). The treatment of a mixture of [Cp*Ru(μ3-Cl)]4 and 4 with LiEt3BH afforded the analogous dinuclear ruthenium tetrahydride-bridged complex bearing Cp* (Cp* = η5-C5Me5) and Cp‡ ligands, Cp*Ru(μ-H)4RuCp‡ (9c). The molecular structures of the newly synthesized complexes 1b, 2b, 4, 7, 8b, and 9b,c were determined by X-ray diffraction (XRD) studies. The reaction of 9b with ethylene exclusively afforded a μ-ethylidyne μ-ethylidene complex, Cp‡Ru(μ-CCH3)(μ-CHCH3)(μ-H)RuCp‡ (11), whereas the reaction of 9a with excess ethylene selectively afforded a dinuclear ruthenium diviny ethylene complex, Cp*Ru(μ-CHCH2)2(CH2CH2)RuCp* (10). The difference in the reactivities can be attributed to the difference in the spatial size of the reaction sites rather than their electronic properties such as electron density at the ruthenium centers. Irradiation of 11 with UV light (365 nm) effectively afforded a bis(μ-ethylidyne) complex, Cp‡Ru(μ-CCH3)2RuCp‡ (12). The structures of 11 and 12 were also determined by XRD studies.

To construct electronically and sterically anisotropic reaction sites of dinuclear cluster, mixed-ligand diruthenium pentahydrido complexes [Cn*Ru(μ-H)3Ru(H)2(PR3)2]+ (Cn* = 1,4,7-trimethyl-1,4,7-triazacyclononane; R = Cy (3a), iPr (3b), cyclopentyl (Cyp, 3c)) were synthesized by the reaction of [Cn*RuH(H2)2]+ (1) with Ru(H)2(H2)2(PR3)2 (R = Cy (2a) iPr (2b), Cyp (2c)). The treatment of 3a–c with KH afforded the corresponding neutral tetrahydrido complexes Cn*Ru(μ-H)3RuH(PR3)2 (R = Cy (4a), iPr (4b), Cyp (4c)). The structures of 3 and 4 were confirmed by X-ray diffraction studies. Introduction of the Cn* ligand into the cluster increased the electron density at the Cn*-ligated metal center and significantly stimulated reactivitiy toward molecular nitrogen and carbon dioxide. Complexes 3a–c reacted with molecular nitrogen to produce the terminal dinitrogen complexes [Cn*Ru(N2)(μ-H)2RuH(PR3)2]+ (R = Cy (5a), iPr (5b), Cyp (5c)), which readily underwent dimerization to form the bridging dinitrogen complexes [(μ-N2){RuCn*(μ-H)2RuH(PR3)2}2]2+ (R = iPr (6b), Cyp (6c)), through the liberation of N2. Cationic complexes 3a–c reacted with CO2 to produce the bridging formato complexes [Cn*Ru(μ-η1:η1-O2CH)(μ-H)2Ru(H2)(PR3)2]+ (R = Cy (7a), iPr (7b), Cyp (7c)), whereas neutral 4a–c reacted with CO2 to form the carbonyl complexes [Cn*Ru(CO)(μ-H)2RuH(PR3)2]+ (R = Cy (9a), iPr (9b), Cyp (9c)), via cleavage of the C═O bond.

Photoirradiation of a triruthenium alkyne complex containing a μ3-borylene ligand, {Cp*Ru(μ-H)}3(μ3-η2(∥)-PhCCH)(μ3-BH) (Cp* = η5-C5Me5), was examined; whereas a perpendicularly coordinated alkyne complex was obtained by the irradiation with UV light (313 nm), a trimetallic complex containing a three-membered μ3-η3-BC2 ring was preferentially formed as a result of intramolecular borylene transfer by the irradiation with visible light (436 nm).

A triruthenium complex containing μ3-η2(∥)-ethyne and μ3-methylidyne ligands, (CpRu)3{μ3-η2(∥)-HC≡CH}(μ3-CH)(μ-H)2 (2a), was synthesized via the treatment of {Cp*Ru(μ-H)}3(μ3-H)2 (3) (Cp* = η5-C5Me5) with propene. In the low-temperature region, VT 1H NMR spectra showed that complex 2a is in equilibrium with a nonclassical μ3-vinyl complex, (CpRu)3(μ3-η2:η2-HC═CH2)(μ3-CH)(μ-H) (5a), which features an interaction between one of the β-vinyl protons and a Ru center. The presence of the unusual 3c-2e interaction of the β-C–H bond of the vinyl group is strongly supported by the small JC–H value (98 Hz) found for the β-carbon as well as the small JH–H value for the α-vinyl proton. Treatment of the equilibrated mixture of phenyl-substituted complexes (CpRu)3{μ3-η2(∥)-HC≡CPh}(μ3-CH)(μ-H)2 (2b) and (CpRu)3(μ3-η2:η2-HC═CHPh)(μ3-CH)(μ-H) (5b) with tBuNC afforded a μ-styryl complex, (CpRu)3(μ-η2-HC═CHPh)(μ3-CH)(μ-tBuNC)(μ-H) (6), while analogous treatment of 2a and 5a resulted in the formation of μ3-dimetalloallyl complex (CpRu)3{μ3-η3-CHCHCN(H)tBu}(μ3-CH)(μ-H) (7). These results imply that the coordinatively saturated μ3-η2(∥)-alkyne−μ3-alkylidyne complex can generate a vacant site via the migration of a hydrido ligand onto an alkyne ligand to yield a μ-vinyl intermediate.

Multiple metal nuclei in a transition metal cluster are expected to act as an electron-reservoir, thus they would enable smooth electron transfer between the metal centers and substrate. It has been shown that the perturbation of an electronic state on a metal center sometimes resulted in a considerable change in the coordination mode of a hydrocarbyl ligand. Sometimes, it is shown to involve C–C bond activation. It is also expected that obtained cationic complexes upon oxidation showed high reactivity toward nucleophiles. In this review, we will mention oxidation of triruthenium clusters having (1) face-capping benzene, (2) μ3-diruthenaallyl, (3) nido-ruthenacyclopentadiene, and (4) closo-ruthenacyclopentadiene ligands. These reactions demonstrate unprecedented skeletal rearrangement of the hydrocarbyl groups, and in some cases, reactivity toward nucleophiles, such as water and methanol, is dramatically enhanced leading to the functionalization of a hydrocarbyl ligand.Highlights► Multiple metal nuclei in a cluster are expected to act as an electron-reservoir to operate smooth electron transfer with substrate. ► Redox of a cluster sometimes resulted in a considerable change in the coordination mode of a hydrocarbyl ligand. ► Redox-induced skeletal rearrangements of benzene, dimetalloallyl, and metallacyclopentadiene ligands placed on a triruthenium plane were studied. ► These reactions demonstrate facile C–C bond formation and scission on a trimetallic plane as well as dramatic enhancement of the reactivity toward nucleophiles.

The treatment of a trimetallic complex containing a μ3-borylene ligand, (Cp*Ru)3(μ3-BH)(μ-H)3 (6; Cp* = η5-C5Me5), with 1 atm of acetylene resulted in the incorporation of two molecules of acetylene on the opposite face of the μ3-borylene ligand to form a μ3-η2(∥)-ethyne μ-ethylidene complex, (Cp*Ru)3(μ3-BH)(μ3-η2(∥)-HCCH)(μ-CHCH3)(μ-H) (7a). Although an intermediate was not observed in the reaction of 6 with acetylene, the intermediary monoalkyne complex 8b was obtained by the reaction of 6 with phenylacetylene, which was converted into (Cp*Ru)3(μ3-BH)(μ3-η2(∥)-PhCCH)(μ-CHCH3)(μ-H) (7b) upon subsequent reaction with acetylene. The thermolysis of 7a provided a closo-2-boraruthenacyclopentenyl complex, {Cp*Ru(μ-H)}2{Cp*Ru(−BHCHCMeCH−)} (11), as a consequence of B–C bond formation across the Ru3 plane with the partial breakage of the cluster skeleton. An X-ray diffraction study for 11 clearly showed the formation of a boraruthenacyclopentenyl skeleton, which bisects the triruthenium plane. This transformation shows how important the flexibility of cluster skeleton is for the bond-forming reaction.

Unlike the reactions of carbonyl clusters with pyridine leading to the formation of μ-pyridyl complexes, the reaction of the triruthenium pentahydrido complex {Cp*Ru(μ-H)}3(μ3-H)2 (Cp* = η5-C5Me5) (1) with pyridines provided μ3-η2(//)-pyridyl complexes, (Cp*Ru)3(μ-H)4(μ3-η2(//)-RC5H3N) (2a, R = H; 2b, R = 4-COOMe; 2c, R = 4-COOEt; 2d, R = 4-Me; 2e, R = 5-Me), in which the molecular plane of the pyridyl group was tilted with respect to the Ru3 plane. Electron-rich metal centers of the trimetallic core enabled back-donation to the pyridyl group, which caused the additional π-coordination of the C═N bond. The electron-rich metal centers of 2a–2c also promoted further transformation into face-capping pyridine complexes {Cp*Ru(μ-H)}3(μ3-η2:η2:η2-RC5H4N) (3a, R = H; 3b, R = 4-COOMe; 3c, R = 4-COOEt) upon heating. In contrast, the thermolysis of 2d did not afford a face-capping picoline complex because of the poor electron-accepting ability of the picolyl moiety. Instead, the coordinatively unsaturated μ3-picolyl complex (Cp*Ru)3(μ-H)2(μ3-η2-4-Me-C5H3N) (4d) was obtained. Owing to its unsaturated nature, 4d can react with γ-picoline to yield 4,4′-dimethyl-2,2′-bipyridine. Although the reaction rate was slow, complex 1 catalyzed the dehydrogenative coupling of 4-substituted pyridines containing an electron-donating group. The protonation of 2a also afforded the coordinatively unsaturated pyridyl complex [(Cp*Ru)3(μ-H)2(μ3-H)(μ3-η2:η2(⊥)-C5H4N)]+ (5a), but the coordination mode of the pyridyl group in 5a was completely different from that in 4d. The pyridyl moiety in 5a was coordinated on one of the Ru–Ru bonds in a perpendicular fashion. The methylation of the face-capping pyridine complex 3a, which led to the formation of the N-methyl pyridinium complex [(Cp*Ru)3(μ-H)3 (μ3-η2:η2:η2-C5H5NMe)]+ (7b) was also examined. NMR studies on 7b as well as X-ray diffraction studies suggested enhanced back-donation to the pyridinium moiety because of the localized cationic charge on the nitrogen atom.

A heterobinuclear transition metal complex with a Ta–Ir multiple bond, Cp*(Me3SiCH2)2Ta–IrCp*(H)2 (1), was synthesized using a salt elimination reaction. The isolable complex was characterized by NMR, IR, UV–vis spectroscopy, and X-ray crystallography. Compound 1 features an exceptionally short intermetallic distance (Ta–Ir 2.4457(3) Å). DFT calculation for 1 revealed the presence of a highly covalent bonding nature in spite of the involvement of such disparate d elements. Oxidative additions of C–H, N–H, and O–H bonds to 1 were also investigated.

Early−late heterobimetallic (ELHB) hydrido complexes comprising group 4 metals and iridium with mono(pentamethylcyclopentadienyl) (η5-C5Me5 = Cp*) ligands have been synthesized and structurally characterized. The hydrocarbyl complexes {Cp*M(CH2SiMe3)2}(Cp*Ir)(μ-H)3 (3b, M = Zr; 3c, M = Hf) derived from (Cp*MCl2)(Cp*Ir)(μ-H)3 (1b, M = Zr; 1c, M = Hf) displayed the cooperative C−H activation of methoxyarenes and pyridines.