No abstract

A series of triborane and tetraborane analogues have been isolated and structurally characterized utilizing chalcogenatoborate ligands Li[BH3(EPh)] (E=S or Se). Thermolysis of [Cp*TaCl4] in the presence of Li[BH3(SPh)] afforded bimetallic tantallaheteroborane [(Cp*Ta)2(μ‐η3:η3‐B2H4S)(μ‐η2:η2‐SBH3)] (1) and hexasulfido trimetallic complex [(Cp*Ta)3(μ‐S)4(μ‐SPh)2] (2). Compound 1 is a fused ditantallaheteroborane, in which both di‐ and triborane analogues are stabilized by two tantalum atoms. In an attempt to synthesize the selenium analogues of 1 and 2, room‐temperature reaction of [Cp*TaCl4] with Li[BH3(SePh)] was carried out, which afforded bimetallic tantallaheteroborane [(Cp*Ta)2{μ‐η3:η3‐B3H6(SePh)}{μ‐SePh}2] (3), monometallic tantallaheteroboranes [Cp*Ta(SePh)2{B4H8‐n(SePh)n}] (4: n=0, 5: n=1), and trimetallic species [(Cp*Ta)3(μ‐Se)4{μ‐Se2(Se)}] (6). Compound 3 is the rarest example of triborane analogue {B3H6(SePh)} in the coordination sphere of two tantalum atoms. Whereas compounds 4 and 5 are examples of unsaturated metallaheteroboranes, in which the tetraborane analogues are stabilized in the coordination sphere of tantalum. One of the unique features of 3 and 5 is the presence of terminal B‐SePh. Compound 6 has similar Ta3Se6 trisbutterfly core as that of 2 with additional bridging selenide unit. All the compounds have been characterized by NMR spectroscopy, mass spectrometry, IR spectroscopy and single‐crystal X‐ray diffraction studies.

An unusual trimetallic octaselenide complex, [(Cp*Ta)3(μ-Se)4{μ-Se2(Se2)}], has been synthesized and structurally characterized.

A series of metal-rich metallaboranes of groups 7 and 9 comprising triply bridging borylene and boride units have been isolated and structurally characterized. Thermolysis of nido-[(RhCp*) 2 B 3 H 7 ] (1; Cp* = η 5 -C 5 Me 5 ) with [Co 2 (CO) 8 ] led to the isolation of tetrametallic [(μ 3 -BH)(RhCp*) 2 (μ-CO)(μ 3 -CO){Co 2 (CO) 4 }] (2), featuring a triply bridging borylene unit, and the trimetallic cluster [(μ 3 -BH)(μ-H)(RhCp*) 2 (μ-CO) 3 {Co(CO)}] (3) that contains a triply bridging hydrido(borylene) unit. The borylene {BH} unit of 2 is coordinated to a deltahedral face of a tetrametallic tetrahedron in a μ 3 fashion. Cluster 3 is a rare example of a tetrahedral metallaborane featuring a hydrido(borylene) unit. In an attempt to synthesize the Mn analogues of 2 and 3, a similar reaction was carried out with [Mn 2 (CO) 10 ] that afforded the trimetallic cluster [(μ 3 -BH)(RhCp*) 2 (μ-CO) 3 {MnH(CO) 3 }] (5) having a triply bridging borylene moiety, the two heterometallic μ 9 -boride clusters [(RhCp*) 3 {Rh(CO)} 3 (μ-CO) 3 {MnH(CO) 3 }B 3 H 2 ] (6) and [(RhCp*) 3 {Mn-(CO) 3 } 2 Rh(CO) 2 B 4 H 3 ] ( 7) and the unusual tetrametallic complex [(RhCp*) 2 (μ-CO) 2 (μ 3 -η 3 -CO 2 ){Mn 2 (CO) 9 }] (8). Clusters 6 and 7 are both unusual heterometallic metal-rich boride clusters, where the boride boron atom is encapsulated inside a tricapped trigonal prism depicting a μ 9 -bonding mode. Compound 8 is a unique example of a metal carbonyl compound in which a CO 2 group is bridging two Rh atoms and one Mn atom in a μ 3 -η 3 fashion. To explore this chemistry with a heavier transition metal, we have carried out the thermolysis of arachno-[IrCp*H 2 (B 3 H 7 )] ( 9) with [Mn 2 (CO) 10 ], which afforded the face-fused iridaborane cluster [(IrCp*) 3 {Ir(CO) 2 } 3 (μ-CO)(μ 3 -CO)B] (10). Compound 10 can also be viewed as a boride cluster, where the naked boron is coordinated to iridium centers in a unique μ 5 coordination mode. All of the compounds have been characterized by 1 H, 11 B, and 13 C NMR spectroscopy and mass spectrometry, and the structures of 2, 3, 5, 6, 8, and 10 have been unambiguously established by crystallographic analyses. Computational studies show that a substantial amount of overlap occurs between the metal frameworks and borylene/boride units.

Isolation of planar [B6H6] is a long‐awaited goal in boron chemistry. Several attempts in the past to stabilize [B6H6] were unsuccessful due to the domination of polyhedral geometries. Herein, we report the synthesis of a triple‐decker sandwich complex of titanium [(Cp*Ti)2(μ‐η6 : η6‐B6H6)(μ‐H)6] (1), which features the first‐ever experimentally achieved nearly planar six‐membered [B6H6] ring, albeit within a [B6H12] borate. The small deviation from planarity is a direct consequence of the predicted structural pattern of the middle ring in 24 Valence Electron Count (VEC) triple‐decker complexes. The large ring size of [B6H6] in 1 brings the metal–metal distance into the bonding range. However, significant electron delocalization from the M−M bonding orbital to the bridging hydrogen and B−B skeleton in the middle decreases its bond strength.