Diborane and Triborane Species in the Coordination Sphere of Group-8 Transition Metals.

The synthesis and structural elucidation of a series of ruthenium diborane and triborane compounds are described. Treatment of [Cp*Ru(PPh3)2Cl] (1) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; PPh3 = triphenylphosphine) with [BH3·THF] (THF = tetrahydrofuran) at 60 °C led to the formation of the hydrogen-rich ruthena-octahydrotetraborane arachno-[2-{Cp*Ru(PPh3)B3H8}] (2). The chemistry of 2 is explored with [Fe2(CO)9] at room temperature, which resulted in the formation of a metal-stabilized triborane species, [{Cp*Ru(PPh3)}(µ3-η1:η2:η2-B3H6){Fe2(CO)7}] (3). Compound 3 can be considered as a triborane analogue [B3H6]3- that stabilizes in the coordination sphere of two iron and one ruthenium atoms. Further, the photolysis of nido-[1,2-(Cp*Ru)2(µ-H)2(B3H7)] (4) with [M(CO)5·THF] (M = Mo and W) afforded an arachno-[1,2-(Cp*Ru)(Cp*RuCO)(µ-H)(B3H8)] (5), in which the [M(CO)5·THF] acted as a CO source. In an attempt to convert arachno-5 into a closo or nido species, we have pyrolyzed arachno-5 in toluene at 90 °C for 20 h that afforded nido-[2,3-(Cp*Ru)2(µ-CO)(µ3-H)(B3H6)] (6) having two ruthenium atoms at the basal position. Irradiation of arachno-5 with an intermediate generated from CS2 and [LiBH4·THF] in THF afforded the diborane(5) species [(Cp*RuCO)(Cp*Ru)(µ-H)(µ-η1:η2-B2H4)(CS2H)] (7) in which a dithioformato ligand (SHC═S) is attached to one Ru-B bond. Compound 7 can be considered as a diborane(5) species, which is stabilized by a dithioformato ligand. All the synthesized compounds have been characterized by employing electrospray ionization-mass spectrometry, multinuclear NMR, and IR spectroscopy techniques. The single-crystal X-ray diffraction studies of compounds 2, 3, 6, and 7 helped to establish the structural integrity of these compounds. Further, density functional theory studies were performed to provide insight into the bonding of these metal-stabilized diborane and triborane species.

Hexagonal Planar [B6 H6 ] within a [B6 H12 ] Borate Complex: Structure and Bonding of [(Cp*Ti)2 (µ-ɳ6 : ɳ6 -B6 H6 )(µ-H)6 ].

Isolation of planar [B6 H6 ] is a long-awaited goal in boron chemistry. Several attempts in the past to stabilize [B6 H6 ] 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 -B6 H6 )(µ-H)6 ] (1), which features the first-ever experimentally achieved nearly planar six-membered [B6 H6 ] ring, albeit within a [B6 H12 ] 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 [B6 H6 ] 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.

Metal-Stabilized [B8 H8 ]2- Derivatives with Dodecahedral Structure in the Solid and Solution States: [(Cp2 MBH3 )2 B8 H6 ] (Cp=η5 -C5 H5 ; M=Zr (1-Zr) and Hf (1-Hf)).

Despite the synthesis and structural characterization of closo-hydroborate dianions, [Bn Hn ]2- (n=6-12) more than 50 years ago, some ambiguity remains about the structure of [B8 H8 ]2- . Although the solid-state structure of [B8 H8 ]2- was established by single-crystal X-ray studies in 1969, fast rearrangements in solution at accessible temperatures prevented its detailed characterization. We therefore stabilized a derivative of [B8 H8 ]2- by using Cp2 MBH3 and structurally characterized two new octaborane analogues, [(Cp2 MBH3 )2 B8 H6 ] (Cp=η5 -C5 H5 ; M=Zr (1-Zr) and Hf (1-Hf)), so that the dynamics of the B8 skeleton were arrested. The solid-state structures of both 1-Zr and 1-Hf comprise a dodecahedron core protected by {Cp2 MBH3 } moieties on both sides of the cluster. Spectroscopic characterization (11 B NMR) validates the intactness of the B8 dodecahedron core in solution as well. Theoretical calculations establish that the two exo-{Cp2 MBH3 } fragments provide structural and electronic structural stability to the B8 core and its intact dodecahedral dianionic nature. Furthermore, we propose isodesmic equations for the formation of higher analogues of the Bn core (n>8) guarded by different group 4 transition metals. Our analysis suggests that, as we move to higher polyhedra (n>10), the formation becomes unfavourable irrespective of metal.

Transition Metal Triple-decker Sandwich Complexes Containing Group 13 Elements.

Transition metal triple-decker complexes are an interesting class of sandwich complexes that engrossed great attention due to their structures and properties. Over the decades, synthesis of triple-decker complexes featuring homocyclic, heterocyclic or π-conjugated rings as middle decks have been abundantly reported. In this regard, the chemistry of such complexes bearing boron in the middle deck are well explored due to the ability of boron-containing cycles to readily coordinate bifacially with metal atoms thereby forming triple-decker complexes. On the other hand, electron counting rules and theoretical calculations have strengthened our knowledge of the structure and bonding in these complexes. Further, these complexes can be used as synthons to generate organometallic polymers having interesting electronic, optical and magnetic properties that can be appropriately tuned to cater to a wide range of applications. In our quest for novel metallaboranes and metallaheteroboranes, we have been successful in isolating various triple-decker complexes that feature boron in the middle deck. This review explained elaborately the synthesis, structures, and bonding in such complexes reported by us and others.

Hetero-trimetallic complexes comprising bridging boryl and borylene ligands: an experimental and theoretical study.

In an effort to explore the coordination chemistry of the coordinative sulfur centers in arachno-ruthenaborane [(Cp*Ru)2(B3H8)(CS2H)] (arachno-1), we have thermolyzed arachno-1 with group-6 metal carbonyls [M(CO)5·THF] (M = Cr, Mo and W). The reaction of arachno-1 with [Cr(CO)5·THF] resulted in the formation of hetero-trimetallic triply bridging borylene [(Cp*Ru)2(µ-CO)(µ3-CH2S2-κ2S':κ2S''){Cr(CO)3}(µ3-BH)] (2), bridging boryl-borylene [(Cp*Ru)2(µ-CO){(µ3-BH(CH2S2)-κ2B:κ2S':κ1S'')}{Cr(CO)3}(µ3-BH)] (3), and sulfido bridged hetero-trimetallic complex [(Cp*Ru)2(µ-CO)3{Cr(CO)3}(µ3-S)] (4). In 2, one side of Ru2Cr-triangle features a µ3-BH ligand while the other side is quadruply bridged by a methanedithiolato ligand in an unsymmetrical fashion. Unlike 2, in complex 3, one side of the Ru2Cr-triangle has a µ3-BH ligand while the opposite side is bridged by a boryl ligand BH(CH2S2) in an unsymmetrical way (µ3-κ2:κ2:κ1) to the metal centers. Interestingly, when the similar reactions of arachno-1 were performed with heavier group-6 metal carbonyls [M(CO)5·THF] (M = Mo and W), it led to the formation of methanedithiolato bridged hetero-trimetallic chain complexes, [{Cp*Ru(CO)}2(µ-CO)2(µ3-CH2S2-κ2S':κ2S''){M(CO)2}] (5, M = Mo; 6, M = W) and sulfido-bridged hetero-trimetallic complexes [(Cp*Ru)2(µ-CO)3{M(CO)3}(µ3-S)] (7, M = Mo; 8, M = W), analogous to 4. In complexes 5 and 6, a Ru2M-chain is symmetrically bridged by a methanedithiolato ligand. On the other hand, in complexes 4, 7, and 8, a sulfido ligand coordinates to two ruthenium and one group-6 metal atoms in µ3-fashion. All the complexes have been characterized by 1H NMR, 13C NMR, UV-vis, IR spectroscopy, and mass spectrometry and their structural architectures have been unambiguously established by single crystal X-ray diffraction studies. In addition, theoretical investigations provided valuable insights into their electronic structures and bonding properties.

16-Vertex oblato-hypho-titanaborane [(Cp*Ti)2B14H18].

Although Lipscomb predicted in 1977 that supra-icosahedral boron clusters would be viable, their synthesis has been impeded by the unavailability of appropriate synthetic methodologies. Herein, we report the first examples of the open 16-vertex oblato-hypho-titanaborane clusters [(Cp*Ti)2B14H17R] (1: R = H; 2: R = Me) having a non-Wadean 19-skeletal-electron-pair count. Interestingly, these clusters show a six-membered [Ti2B4] open face, which could lead to closo-19-vertex clusters.

Chalcogen stabilized trimetallic clusters: synthesis, structures, and bonding of [(Cp*M)3(E)6+m(BH)n] (M = Nb or Ta; E = S or Se; m = 0 or 1 or 2; n = 0 or 1).

In an effort to isolate the chalcogen-rich niobium analogue of [(Cp*Ta)3(µ-S)3(µ3-S)3BH], the room temperature reaction of [Cp*NbCl4] (Cp* = η5-C5Me5) with Li[BH2S3] was carried out. Although the objective of isolating the niobium analogue was not achieved, the reaction yielded a homocubane-type cluster [(Cp*Nb)3(µ-S)3(µ3-S)3(µ-S)BH], 1, and a hexa-sulfido cluster [(Cp*Nb)3(µ-S)6], 2. Cluster 1 is a notable example of a homocubane-type cluster in which one of the vertices of the homocubane is missing. Compound 1 may be considered as a hypo-electronic cluster with an electron count of 64 cve (cve = cluster valence electrons), whereas compound 2 shows the presence of two doubly bridging η1-S around each Nb-Nb bond. On the other hand, the room temperature reaction of [Cp*TaCl4] with selenaborate ligand, [LiBH2Se3], led to the formation of [(Cp*Ta)3(µ-Se)4{µ-Se2(Se2)}], 3. Compound 3 is one of the rarest examples having a Ta3Se6 core structure with a unique diselenide bridging fragment. The presence of a short Se-Se bond of this diselenide unit makes this molecule of further interest. All these compounds were characterized by 1H, 11B{1H} and 13C{1H} NMR spectroscopy, infrared spectroscopy, mass spectrometry, and single-crystal X-ray crystallography. Density functional theory (DFT) calculations were carried out to provide insight into the bonding and electronic structures of these chalcogen-rich trimetallic clusters.