The mononuclear complex [Ni(meppp)] {H2meppp = meso-1,3-bis[(2-mercaptoethyl)(phenyl)phosphino]propane} was used as an S-bidentate NiP2S2 metalloligand to prepare a series of NiIIMI complexes with various neutral ligands, namely, [Ni(µ-meppp)ML2]PF6 [M = Rh, L = 1/2cod (1a; cod = 1,5-cycloocatadiene), CO (1b), XylNC (1c; Xyl = 2,6-dimethylphenyl), P(OPh)3 (1d); M = Ir, L = 1/2cod (2a), CO (2b), XylNC (2c)]. The reactivities of 1 and 2 toward methyl iodide and tertiary hydrosilanes were examined. Complexes 1c, 2a, and 2b reacted with MeI to afford the oxidative addition products [Ni(µ-meppp)M(L)2(Me)(I)]PF6 [M = Rh, L = XylNC (3c); M = Ir, L = 1/2cod (4a)] and [Ni(µ-meppp)Ir(CO)(Me)(I)2] (5). In the reactions with hydrosilanes, only 2c exhibited an interesting reactivity to afford the NiII(µ-H)IrIII silyl complexes [Ni(µ-meppp)(µ-H)Ir(XylNC)2(Si)]PF6 [Si = SiEt3 (6a), SiMe2Ph (6b), SiMePh2 (6c), SiPh3 (6d)]. The reaction proceeded via the intermediate isomers [Ni(meppp)(µ-H)Ir(XylNC)2(Si)]PF6 (7); intermediate 7d (Si = SiPh3) was characterized and has an NiII(µ-H)IrIII silyl structure with the hydrido ligand nesting in the reverse side of the Ni(µ-S)2Ir pocket to that of 6d. These results demonstrated that the reactivities of the bis(thiolate)-bridged NiIIMI heterodinuclear complexes can be tuned by changing the metal ions and ancillary ligands. In addition, the S-bidentate NiP2S2 metalloligand [Ni(meppp)] plays an important role in the stabilization of the bridging hydrido ligand in both pockets of the NiII(µ-H)IrIII core owing to the flexibility of the hinged Ni(µ-SR)2Ir structure.

The Front Cover shows the transformation of a low-valent dithiolate-bridged heterodinuclear complex into two different types of hydride silyl complexes by treatment with hydrosilanes, where the hydride ligands are stabilized by semibridging between two metal centers of a “dimetal butterfly”. These results demonstrate that flexible dithiolate-bridged heterodinuclear complexes have the potential to stabilize small inorganic and organic ligands in their “butterfly cleft”. More information can be found in the Full Paper by B. Kure, T. Tanase et al. For more on the story behind the cover research, see the Cover Profile.

AbstractInvited for the cover of this issue is the group of Tomoaki Tanase at Nara Women's University, Japan. The cover image shows the transformation of a low-valent dithiolate-bridged heterodinuclear complex into two different types of hydride silyl complexes by treatment with hydrosilanes, where the hydride ligands are stabilized by semibridging between two metal centers of a “dimetal butterfly”.

Mononuclear complexes with a P2S2 ligand, [M(meppp)] (M = Ni (1a), Pd (1b), Pt (1c); H2meppp = meso-1,3-bis[(mercaptoethyl)phenylphosphino]propane), were treated with [M′Cp*Cl2]2 or [M′Cp*(NO3)2] (Cp* = η5-pentamethylcyclopentadienyl) to afford a series of bisthiolate-bridged MIIM′III heterodinuclear complexes, [M(μ-meppp)M′Cp*X]X′ (M = Ni, Pd, Pt; M′ = Rh, Ir; X = Cl, NO3; X′ = Cl, PF6, NO3). The nitrate complexes [M(μ-meppp)-M′Cp*(NO3)]NO3 (M′ = Rh ([4a–c]NO3), Ir ([5a–c]NO3); M = Ni (a), Pd (b), Pt (c)) further reacted with sodium formate in water or methanol to be transformed into bisthiolate- and hydride-bridged complexes, [M(μ-meppp)(μ-H)M′Cp*]NO3 (M′ = Rh ([6a–c]NO3), Ir ([8a–c]NO3); M = Ni (a), Pd (b), Pt (c)). Complexes [6a]NO3 (M = Ni, M′ = Rh) and [8a]NO3 (M = Ni, M′ = Ir) were characterized by X-ray analyses to reveal that a hydride is stabilized in a semibridging mode on the heterometal centers. In the PdIIRhIII ([6b]NO3) and PtIIRhIII ([6c]NO3) complexes, the hydrides were extremely unstable and were likely to undergo an unusual metal-to-Cp* ring hydrogen transfer, resulting in η4-C5Me5H MIIRhI complexes, [M(μ-meppp)Rh(η4-C5Me5H)]NO3 (M = Pd ([7b]NO3), Pt ([7c]NO3)). The property of the hydride was drastically switched by varying the anchoring metal ions of the M′ site (Rh, Ir); that of [6a]NO3 (M′ = Rh) is not protic and decomposes in water below pH 4, while those of [8a–c]NO3 (M′ = Ir) are protic, subject to H+/D+ exchange reactions, and stable below pH 4. [6a]NO3 reacted with phenylacetylene to give [Ni(μ-meppp)RhCp*(C≡CPh)]NO3 ([10a]NO3), which is in contrast with the inertness of the NiIIIrIII hydride complex [8a]NO3. The reaction is assumed to involve an alkenyl complex, [Ni(μ-meppp)RhCp*(CH═CHPh)]NO3 (9a), formed through an insertion of phenylacetylene into the metal–hydride bond. Analogous MIIRhIII alkynyl complexes, [M(μ-meppp)RhCp*(C≡CPh)]NO3 (M = Pd ([10b]NO3), Pt ([10c]NO3)), were synthesized by treating [4b,c]NO3 with phenylacetylene in basic media, and the structural differences among [10a–c]NO3 were discussed. These results interestingly demonstrated that the structures, properties, and reactivities of the nesting hydride on the {MM′(μ-meppp)} cores were tuned by varying metal ions of the M and M′ sites.

New N3S2 dithiolato ligands, 1,4-bis(2-mercaptoethyl)-7-R-1,4,7-triazacyclononane (RTACN-S2H2; R = Ts, iPr) were synthesized and reacted with NiII ion to give mononuclear complexes [Ni(RTACN-S2)] (R = Ts (1a), iPr (1b)). Complexes 1a and 1b were further transformed by treatment with RhIII species into a series of NiIIRhIII heterodimetallic compounds, [Ni(RTACN-S2)RhCp*X]X′ (R = Ts, X = Cl, X′ = Cl, OTf ([2a]X′); R = Ts, X = X′ = NO3 ([3a]NO3); R = iPr, X = Cl, X′ = Cl, NO3, PF6 ([2b]X′)). Complexes [3a]NO3 and [2b]NO3 were readily reacted with H2 (0.1 MPa) in water at room temperature to afford hydride-bridged NiIIRhIII dinuclear complexes, [(RTACN-S2)Ni(μ-H)RhCp*]NO3 (R = Ts ([4a]NO3), iPr ([4b]NO3)), which were successfully characterized by X-ray crystallography. Upon the heterolytic activation of H2, the Ni–Rh interatomic distances were dramatically decreased from 3.2130(6)–3.262(2) Å ([2a]OTf, [2b]NO3, [3a]NO3) to 2.6921(6)–2.7228(4) Å ([4a]NO3, [4b]NO3), resulting in semibridging Ni(μ-H)Rh cores. In addition, the stability of the hydride of [4a]+ and [4b]+ were interestingly tuned by varying the R group of the TACN ligand through trans influence at the NiII center (Ni–NR = 2.188(3) Å ([4a]+), 2.056(2) Å ([4b]+). In fact, [4a]+ was quite stable and capable of reducing benzaldehyde in water, although [4b]+ quickly decomposed under similar conditions. Catalytic hydrogenation of aldehydes and CO2 in water has been established by using [3a]NO3 and [2a]Cl as precursors of an active species, [4a]+, and found to be interestingly contrasted to the inactive precursor [2b]NO3 with R = iPr. These results indicated that the properties and reactivity of the Ni(μ-H)Rh complexes can be controlled by changing the substituents of the N3S2 supporting ligands.