Synthesis and Characterization of Hypoelectronic Rhenaboranes. Analysis of the Geometric and Electronic Structures of Species Following Neither Borane nor Metal Cluster Electron-Counting Paradigms

Abstract

The reaction of (CpReH(2))(2)B(4)H(4) with monoborane leads to the sequential formation of (CpRe)(2)B(n)()H(n)() (n = 7-10, 1-4). These species adopt closed deltahedra with the same total connectivities as the closo-borane anions [B(n)()H(n)()](2)(-), n = 9-12, but with flattened geometries rather than spherical shapes. These rhenaborane clusters are characterized by high metal coordination numbers, Re-Re cross-cluster distances within the Re-Re single bond range, and formal cluster electron counts three skeletal electron pairs short of that required for a canonical closo-structure of the same nuclearity. An open cluster, (CpReH)(2)B(7)H(9) (5), is isolated that bears the same structural relationship to arachno-B(9)H(15) as 1-4 bear to the closo-borane anions. Chloroborane permits the isolation of (CpReH)(2)B(5)Cl(5) (6), an isoelectronic chloro-analogue of known open (CpWH(2))(2)B(5)H(5) and (CpRe)(2)B(6)H(4)Cl(2) (7), a triple-decker complex containing a planar, six-membered 1,2-B(6)H(4)Cl(2) ring. Both are putative five- and six-boron intermediates in the formation of 1. Electronic structure calculations (extended Hückel and density functional theory) yield geometries in agreement with the structure determinations, large HOMO-LUMO gaps in accord with the high stabilities, and (11)B chemical shifts accurately reflecting the observed shifts. Analyses of the bonding in 1-4 reveal that the CpRe.CpRe interaction generates fragment orbitals that are able to contribute the "missing" three skeletal electron pairs required for skeletal bonding. The necessity of a Re.Re interaction for strong cluster bonding requires a borane fragment shape change to accommodate it, thereby explaining the noncanonical geometries. Application of the debor principle of borane chemistry to the shapes of 1-4 readily rationalizes the observed geometries of 5 and 6. This evidence of the scope of transition metal fragment control of borane geometry suggests the existence of a large class of metallaboranes with structures not found in known borane or metal clusters.

Sub‐Nano Clusters: The Last Frontier of Inorganic Chemistry

“…More composite materials of this kind are certainly underway. (Fehlner et al 2005;Guennic et al 2004). …”

“…The flat B 6 hexagon was incorporated into a solid, Ti 7 Rh 4 Ir 2 B 6 ( Figure 12A) (Fokwa and Hermus 2012). Flat highly-charged boranes made it into salts of transition metals, such as Re and Ru whose examples are shown in Figure 12B (Fehlner et al 2005;Guennic et al 2004). More composite materials of this kind are certainly underway.…”

“…For example, clusters can be catalytic, (Arenz et al 2006;Chrétien and Metiu 2006;Haruta and Date 2001;Heiz et al 1999;Kaden et al 2009;Lee et al 2004;Lopez and Norskov 2002;Molina and Hammer 2005;Remediakis et al 2005;Sanchez et al 1999;Vajda et al 2009;Valden et al 1998;Yoon et al 2005;Zhai et al 2010) some clusters can be incorporated into materials as building blocks or ligands, (Alexandrova et al 2005;Alexandrova et al 2006;Claridge et al 2009;Fehlner et al 2005;Fokwa and Hermus 2012; Guennic et al 2004;Jadzinsky et al 2007;Reber et al 2007) and certain clusters can be high-temperature superconductors (de Heer 1993;Kresin 2012;Kresin andOvchinnikov 2008, 2012). Albeit, these applications often result from exhaustive searches for suitable cluster-containing materials, without much rationale.…”

Computational Methods: Transition Metal Clusters

“…They were initially considered as rule breakers, i.e., main-group-transition metal clusters that possess geometric arrangements that differ from expectations based on the PSEP electron-counting rules, until new rules were proposed. 39 Indeed, with the existence of ''t 2g '' orbitals on the metal fragments, a geometric adjustment is available to metal-containing systems, which is not available to pure maingroup clusters. The manner in which the metal matches the main-group fragments requires more than simplistic electron counting.…”

“…show that some of the metal ''t 2g '' orbitals always participate in the skeletal bonding. 39 The Re 2 fragment, in turn, generates fragment orbitals that are able to contribute the ''missing'' three skeletal electron pairs required for skeletal bonding, leading to sep counts of n + 1. Do these rules apply also to open structures containing two early-transition metal atoms?…”