Triple-Decker Sandwich Complexes of Tungsten with Planar and Puckered Middle Decks.

A triple-decker complex of tungsten, [(Cp*W)2{µ-η6:η6-B4H4Co2(CO)5}(H)2] (1; Cp* = η5-C5Me5), with a planar middle deck has been isolated by thermolysis of an in situ formed intermediate from the reaction of Cp*WCl4 and LiBH4 with Co2(CO)8. In addition, we have also isolated another triple-decker complex, [(Cp*W)2{µ-η6:η6-B5H5Fe(CO)3}(H)2] (4), having a puckered central ring, from a similar reaction with Fe2(CO)9. Clusters 1 and 4 are unprecedented examples of a triple-decker complex having a 24-valence electron with bridging hydrogen atoms.

"Triple-Decker Sandwich" Containing Planar {B2E2Pd} Ring (E = S or Se).

In an effort to generate triple-decker complexes comprising a {PdCl2}moiety in the middle deck, we have explored the reactivity of [(Cp*M)2{µ-B2H2E2}], 1-4 (1: M = Co, E = S; 2: M = Co, E = Se; 3: M = Rh, E = Se; and 4: M = Ir, E = Se; Cp* = η5-C5Me5), with [PdCl2(COD)] (COD = 1,5-cyclooctadiene). The reactions led to the formation of a series of trinuclear heterometallic triple-decker complexes, [(Cp*M)2{µ-B2H2E2Pd(Cl)2}], 5-8 (5: M = Co, E = S; 6: M = Co, E = Se; 7: M = Rh, E = Se; and 8: M = Ir, E = Se). Formation of the complexes 5-8 occurred almost instinctively as a single product with the elimination of the COD ligand. These complexes are examples of novel triple-decker species having a planar bridging palladacycle ligand, in which the Pd metal exists as Pd(II) in an uncommon pseudo-octahedral environment with an elongated M-Pd bonding interaction. The new species, 5-8, have been characterized spectroscopically, and the structures of 5-7 were confirmed by single-crystal X-ray diffraction studies. The structure and bonding of these molecules were further analyzed with the help of density functional theory studies that found a strong electron donation from the B2E2 (E = S or Se) fragment of the middle ring to the axial metals, while a weak bonding interaction between group 9 metals and Pd.

Use of Single-Metal Fragments for Cluster Building: Synthesis, Structure, and Bonding of Heterometallaboranes.

The synergic property of the CO ligand, in general, can stabilize metal complexes at lower oxidation states. Utilizing this feature of the CO ligand, we have recently isolated and structurally characterized a highly fluxional molybdenum complex [{Cp*Mo(CO)2}2{µ-η2:η2-B2H4}] (2; Cp* = η5-C5Me5) comprising the diborane(4) ligand. Compound 2 represents a rare class of bimetallic diborane(4) complex corresponding to a singly bridged C s structure. In an attempt to isolate the tungsten analogue of 2, [{Cp*W(CO)2}2{µ-η2:η2-B2H4}], we have isolated a rare vertex-fused cluster, [(Cp*W)3WB9H18] (5). Having a structural likeness with the dimolybdenum alkyne complex [{CpMo(CO)2}2C2H2], we have further explored the chemistry of 2 with CO gas that yielded a homoleptic trimolybdenum complex, [(Cp*Mo)3(µ-H)2(µ3-H)(µ-CO)2B4H4] (4). In an attempt to replace the 16-electron {Cp*MoH(CO)2} moiety in 4 with isolobal fragment {W(CO)5}, we treated the intermediate, obtained from the reaction of Cp*MoCl4 and LiBH4, with the monometal carbonyl fragment {W(CO)5·THF}. The reaction indeed yielded two bimetallic clusters, [(Cp*Mo)2B4H8W(CO)4] (7) and [(Cp*Mo)2B4H6W(CO)5] (8), that seem to have been generated by the replacement of one {BH} or {BH3} vertex from [(Cp*Mo)2B5H9], respectively. All of the compounds have been characterized by various spectroscopic analyses and single-crystal X-ray diffraction studies. Electron-counting rules and molecular orbital analyses provided further insight into the electronic structure of all of these molecules.

Synthesis, Structure, Bonding, and Reactivity of Metal Complexes Comprising Diborane(4) and Diborene(2): [{Cp*Mo(CO)2 }2 {µ-η2 :η2 -B2 H4 }] and [{Cp*M(CO)2 }2 B2 H2 M(CO)4 ], M=Mo,W.

The reaction of [(Cp*Mo)2 (µ-Cl)2 B2 H6 ] (1) with CO at room temperature led to the formation of the highly fluxional species [{Cp*Mo(CO)2 }2 {µ-η2 :η2 -B2 H4 }] (2). Compound 2, to the best of our knowledge, is the first example of a bimetallic diborane(4) conforming to a singly bridged Cs structure. Theoretical studies show that 2 mimics the Cotton dimolybdenum-alkyne complex [{CpMo(CO)2 }2 C2 H2 ]. In an attempt to replace two hydrogen atoms of diborane(4) in 2 with a 2e [W(CO)4 ] fragment, [{Cp*Mo(CO)2 }2 B2 H2 W(CO)4 ] (3) was isolated upon treatment with [W(CO)5 ⋅thf]. Compound 3 shows the intriguing presence of [B2 H2 ] with a short B-B length of 1.624(4) Å. We isolated the tungsten analogues of 3, [{Cp*W(CO)2 }2 B2 H2 W(CO)4 ] (4) and [{Cp*W(CO)2 }2 B2 H2 Mo(CO)4 ] (5), which provided direct proof of the existence of the tungsten analogue of 2.

Heterometallic Triply-Bridging Bis-Borylene Complexes.

Triply-bridging bis-{hydrido(borylene)} and bis-borylene species of groups 6, 8 and 9 transition metals are reported. Mild thermolysis of [Fe2 (CO)9 ] with an in situ produced intermediate, generated from the low-temperature reaction of [Cp*WCl4 ] (Cp*=η5 -C5 Me5 ) and [LiBH4 ⋅THF] afforded triply-bridging bis-{hydrido(borylene)}, [(µ3 -BH)2 H2 {Cp*W(CO)2 }2 {Fe(CO)2 }] (1) and bis-borylene, [(µ3 -BH)2 {Cp*W(CO)2 }2 {Fe(CO)3 }] (2). The chemical bonding analyses of 1 show that the B-H interactions in bis-{hydrido (borylene)} species is stronger as compared to the M-H ones. Frontier molecular orbital analysis shows a significantly larger energy gap between the HOMO-LUMO for 2 as compared to 1. In an attempt to synthesize the ruthenium analogue of 1, a similar reaction has been performed with [Ru3 (CO)12 ]. Although we failed to get the bis-{hydrido(borylene)} species, the reaction afforded triply-bridging bis-borylene species [(µ3 -BH)2 {WCp*(CO)2 }2 {Ru(CO)3 }] (2'), an analogue of 2. In search for the isolation of bridging bis-borylene species of Rh, we have treated [Co2 (CO)8 ] with nido-[(RhCp*)2 (B3 H7 )], which afforded triply-bridging bis-borylene species [(µ3 -BH)2 (RhCp*)2 Co2 (CO)4 (µ-CO)] (3). All the compounds have been characterized by means of single-crystal X-ray diffraction study; 1 H, 11 B, 13 C NMR spectroscopy; IR spectroscopy and mass spectrometry.

A covalently linked dimer of [Ag25(DMBT)18].

We report the first example of a covalently bound dimer of monolayer protected atomically precise silver nanocluster [Ag25(DMBT)18]- (DMBT stands for 2,4-dimethylbenzenethiol). Covalently linked dimers could be important to design new cluster assembled materials with composite properties.