title: 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.
authors.affiliation: Laboratoire de Chimie du Solide et Inorganique Moléculaire, UMR 6511 CNRS, Université de Rennes 1, Institut de Chimie de Rennes, Avenue du Général Leclerc, 35042 Rennes Cedex, France.
authors.forename: Boris
authors.initials: B
authors.lastname: Le Guennic
authors.forename: Haijun
authors.initials: H
authors.lastname: Jiao
authors.forename: Samia
authors.initials: S
authors.lastname: Kahlal
authors.forename: Jean-Yves
authors.initials: JY
authors.lastname: Saillard
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Halet
authors.forename: Sundargopal
authors.initials: S
authors.lastname: Ghosh
authors.forename: Maoyu
authors.initials: M
authors.lastname: Shang
authors.forename: Alicia M
authors.initials: AM
authors.lastname: Beatty
authors.forename: Arnold L
authors.initials: AL
authors.lastname: Rheingold
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Fehlner