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).
Synergic property of the CO ligand in general can stabilize metal complexes at lower oxidation states. Utilizing this feature of CO ligand, we have recently isolated and structurally characterized a highly fluxional molybdenum complex [{Cp*Mo(CO) 2 } 2 {µ-η 2 :η 2-B 2 H 4 }] (Cp* = η 5-C 5 Me 5) (2) comprising diborane(4) ligand. Compound 2 represents a rare class of bimetallic diborane(4) complex corresponding to a singly bridged Cs structure. In an attempt to isolate the tungsten analogue of 2, [{Cp*W(CO) 2 } 2 {µ-η 2 :η 2-B 2 H 4 }], we have isolated an rare vertex-fused cluster [(Cp*W) 3 WB 9 H 18 ] (5). Having a structural alikeness with dimolybdenum alkyne complex [{CpMo(CO) 2 } 2 C 2 H 2 ], we have further explored the chemistry of 2 with CO gas that yielded homoleptic tri-molybdenum complex, [(Cp*Mo) 3 (µ-H) 2 (µ 3-H)(µ-CO) 2 B 4 H 4 ] (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*MoCl 4 and LiBH 4 , with mono metal carbonyl fragment {W(CO) 5 .thf}. The reaction indeed yielded two bimetallic clusters [(Cp*Mo) 2 B 4 H 8 W(CO) 4 ], 7 and [(Cp*Mo) 2 B 4 H 6 W(CO) 5 ], 8 that seem to have generated by the replacement of one {BH} or {BH 3 } vertex from [(Cp*Mo) 2 B 5 H 9 ] 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 these molecules.
The reaction of [(Cp*Mo) (μ-Cl) B H ] (1) with CO at room temperature led to the formation of the highly fluxional species [{Cp*Mo(CO) } {μ-η :η -B H }] (2). Compound 2, to the best of our knowledge, is the first example of a bimetallic diborane(4) conforming to a singly bridged C structure. Theoretical studies show that 2 mimics the Cotton dimolybdenum-alkyne complex [{CpMo(CO) } C H ]. In an attempt to replace two hydrogen atoms of diborane(4) in 2 with a 2e [W(CO) ] fragment, [{Cp*Mo(CO) } B H W(CO) ] (3) was isolated upon treatment with [W(CO) ⋅thf]. Compound 3 shows the intriguing presence of [B H ] with a short B-B length of 1.624(4) Å. We isolated the tungsten analogues of 3, [{Cp*W(CO) } B H W(CO) ] (4) and [{Cp*W(CO) } B H Mo(CO) ] (5), which provided direct proof of the existence of the tungsten analogue of 2.
In an attempt to expand the library of M2B5 bicapped trigonal-bipyramidal clusters with different transition metals, we explored the chemistry of [Cp*WCl4] with metal carbonyls that enabled us to isolate a series of mixed-metal tungstaboranes with an M2{B4M’} {M = W; M’ = Cr(CO)4, Mo(CO)4, W(CO)4} core. The reaction of in situ generated intermediate, obtained from the low temperature reaction of [Cp*WCl4] with an excess of [LiBH4·thf], followed by thermolysis with [M(CO)5·thf] (M = Cr, Mo and W) led to the isolation of the tungstaboranes [(Cp*W)2B4H8M(CO)4], 1–3 (1: M = Cr; 2: M = Mo; 3: M = W). In an attempt to replace one of the BH—vertices in M2B5 with other group metal carbonyls, we performed the reaction with [Fe2(CO)9] that led to the isolation of [(Cp*W)2B4H8Fe(CO)3], 4, where Fe(CO)3 replaces a {BH} core unit instead of the {BH} capped vertex. Further, the reaction of [Cp*MoCl4] and [Cr(CO)5·thf] yielded the mixed-metal molybdaborane cluster [(Cp*Mo)2B4H8Cr(CO)4], 5, thereby completing the series with the missing chromium analogue. With 56 cluster valence electrons (cve), all the compounds obey the cluster electron counting rules. Compounds 1–5 are analogues to the parent [(Cp*M)2B5H9] (M= Mo and W) that seem to have generated by the replacement of one {BH} vertex from [(Cp*W)2B5H9] or [(Cp*Mo)2B5H9] (in case of 5). All of the compounds have been characterized by various spectroscopic analyses and single crystal X-ray diffraction studies.
Triple-decker complex of tungsten [(Cp*W)2{µη 6 :η 6 -B4H4Co2(CO)5}(H)2], 1 (Cp* = η 5 -C5Me5) with a planar middle-deck has been isolated by the 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-η 6 :η 6 -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 triple-decker complex having 24-valence electron with bridging hydrogens. Density-functional theory (DFT) calculations account for their stability and structural variation.
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