Hypoelectronic Dimetallaheteroboranes of Group 6 Transition Metals Containing Heavier Chalcogen Elements

Icosahedral m-Carboranes Containing Exopolyhedral B-Se and B-Te Bonds

Chalcogen-containing carboranes have been known for several decades and possess stable exopolyhedral B(9)-Se and B(9)-Te σ bonds despite the electron-donating ability of the B(9) vertex. While these molecules are known, little has been done to thoroughly evaluate their electrophilic and nucleophilic behavior. Herein, we report an assessment of the electrophilic reactivity of m-carboranylselenyl(II), -tellurenyl(II), and -tellurenyl(IV) chlorides and establish their reactivity pattern with Grignard reagents, alkenes, alkynes, enolates, and electron-rich arenes. These electrophilic reactions afford unique electron-rich B-Y-C (Y = Se, Te) bonding motifs not commonly found before. Furthermore, we show that m-carboranylselenolate, and even m-carboranyltellurolate, can be competent nucleophiles and participate in nucleophilic aromatic substitution reactions. Arene substitution chemistry is shown to be further extended to electron-rich species via palladium-mediated cross-coupling chemistry.

Directed Syntheses of CS2- and CS3-Bridged Decaborane-14 Analogues

To establish a procedure for single-cage cluster expansion of open cage dimetallaoctaboranes(12), we have investigated the chemistry of nido-[(Cp*M)₂B₆H₁₀] (η⁵-C₅Me₅ = Cp*, 1: M = Co; 2: M = Rh), with diverse chalcogen-based borate ligands. As a result, treatment of nido-1 and nido-2 with Li[BH₂E₃] (E = S, Se, or Te) yielded 10-vertex nido-[(Cp*Co)₂B₇EH₉] (3: E = S; 4: E = Se; 5: E = Te) along with known 10-vertex nido-[(Cp*M)₂B₆H₆E₂] (6: E = S, M = Co; 7: E = Se, M = Co; 8: E = Te, M = Co; 9: E = Se, M = Rh). The geometries of dimetallachalcogenaboranes, 3-9, are isostructural with decaborane(14). Thermolysis of nido-1 and nido-2 with an intermediate, generated from CS₂ and [LiBH₄]·THF reaction in THF, produced nido-[(Cp*M)₂B₆S₂H₄(CH₂S₂)] (10: M = Co; 11: M = Rh) and nido-[(Cp*M)₂B₆S₂H₄(CS₃)] (12: M = Co; 13: M= Rh). Clusters 10-13 are rare species in which one of the B-B bonds is coordinated with a {CS₂}^2- or {CS₃}^2- ligand, generating di(thioborolane) {B₂S₂CH₂} or di(thioboralane)-thione {B₂CS₃} moieties. To examine further the coordination chemistry of CS₂-bridged decaborane(14) analogue nido-10, photolysis was carried out with {M(CO)₅·THF} (M = Mo or W) that led to the isolation of [(Cp*Co)₂B₆S₂H₄(CH₂S₂){M(CO)₅}] (14: M = Mo; 15: M = W), where the {CH₂S₂} moiety is coordinated with one {M(CO)₅} moiety in η¹-fashion. All the synthesized clusters have been characterized by ESI-mass, multinuclear NMR spectroscopy, and IR spectroscopy and structurally solved by single-crystal XRD. Furthermore, DFT calculations probe the bonding of these CS₂- and CS₃-bridged decaborane analogues.

Polyhedral [M2B5] Metallaborane Clusters and Derivatives: An Overview of Their Structural Features and Chemical Bonding

A large number of metallaborane clusters and their derivatives with various structural arrangements are known. Among them, M2B5 clusters and derivatives constitute a significant class. Transition metals present in these species span from group 4 to group 7. Their structure can vary from oblatonido, oblatoarachno, to arachno type open structures. Many of these clusters appear to be hypoelectronic and are often considered as 'rule breakers' with respect to the classical Wade-Mingos electron counting rules. This is due to their unique highly oblate (flattened) deltahedral structures featuring a cross-cluster M-M interaction. Many theoretical calculations were performed to elucidate their electronic structure and chemical bonding properties. In this review, the synthesis, structure, and electronic aspects of the transition metal M2B5 clusters known in the literature are discussed. The chosen examples illustrate how, in synergy with experiments, computational results can provide additional valuable information to better understand the electronic properties and electronic requirements which govern their architecture and thermodynamic stability.

A combined experimental and theoretical study of bimetallic bis- and tris-homocubane analogues

Various bimetallic bis- and tris-homocubane analogues of group 9 transition metals have been isolated and structurally characterized employing Li[BH₂E₃] (E = S or Se).

Synthesis, Structures and Chemistry of the Metallaboranes of Group 4–9 with M2B5 Core Having a Cross Cluster M–M Bond

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.

Hypo-electronic triple-decker sandwich complexes: synthesis and structural characterization of [(Cp*Mo)2{μ–η6:η6-B4H4E-Ru(CO)3}] (E = S, Se, Te or Ru(CO)3 and Cp* = η5-C5Me5)

Electron-poor triple-decker complexes, [(Cp*Mo)₂{μ–η⁶:η⁶-B₄H₄ERu(CO)₃}] (E = S, Se, Te, Ru(CO)₃) with hexagonal bridging ring composed of boron, ruthenium and chalcogen atoms.

An electron-poor di-molybdenum triple-decker with a puckered [B4Ru2] bridging ring is an oblato-closo cluster

An unprecedented, 22-valence-electron triple-decker sandwich complex [(Cp*Mo)2{μ-η(6):η(6)-B4H4Ru2(CO)6}], 2, has been prepared. In an effort to generate analogous triple-deckers with group 6 metal carbonyl fragments in the middle deck, we have isolated [(Cp*MoCO)2(μ-H)2B4H4], 3, that provides the first direct evidence for the missing link between [(Cp*MoCl)2B3H7] and [(Cp*Mo)2B5H9] clusters.

Synthesis, characterization and crystal structure analysis of cobaltaborane and cobaltaheteroborane clusters

A novel class of cobaltaborane and cobaltaheteroborane clusters, isocloso-[(Cp*Co)₃B₆H₇Co(CO)₂], closo-[(Cp*Co)₂B₂H₅Mo₂(CO)₆I], nido-[(Cp*Co)₂B₂H₂S₂], and closo-[(Cp*Co)₂B₄H₂Br₄] were investigated (see picture).