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

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• 作者：Boris Le Guennic ; Haijun Jiao ; Samia Kahlal ; Jean-Yves Saillard ; Jean-Franç ; ois Halet ; Sundargopal Ghosh ; Maoyu Shang ; Alicia M. Beatty ; Arnold L. Rheingold ; and Thomas P. Fehlner
• 刊名：Journal of the American Chemical Society
• 出版年：2004
• 出版时间：March 17, 2004
• 年：2004
• 卷：126
• 期：10
• 页码：3203 - 3217
• 全文大小：471K
• 年卷期：v.126,no.10(March 17, 2004)
• ISSN：1520-5126

文摘

The reaction of (Cp*ReH₂)₂B₄H₄ with monoborane leads to the sequential formation of(Cp*Re)₂B_nH_n (n = 7-10, 1-4). These species adopt closed deltahedra with the same total connectivitiesas the closo-borane anions [B_nH_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-clusterdistances within the Re-Re single bond range, and formal cluster electron counts three skeletal electronpairs short of that required for a canonical closo-structure of the same nuclearity. An open cluster,(Cp*ReH)₂B₇H₉ (5), is isolated that bears the same structural relationship to arachno-B₉H₁₅ as 1-4 bear tothe closo-borane anions. Chloroborane permits the isolation of (Cp*ReH)₂B₅Cl₅ (6), an isoelectronic chloro-analogue of known open (Cp*WH₂)₂B₅H₅ and (Cp*Re)₂B₆H₄Cl₂ (7), a triple-decker complex containing aplanar, six-membered 1,2-B₆H₄Cl₂ ring. Both are putative five- and six-boron intermediates in the formationof 1. Electronic structure calculations (extended Hückel and density functional theory) yield geometries inagreement with the structure determinations, large HOMO-LUMO gaps in accord with the high stabilities,and ¹¹B chemical shifts accurately reflecting the observed shifts. Analyses of the bonding in 1-4 revealthat the Cp*Re···Cp*Re 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 strongcluster bonding requires a borane fragment shape change to accommodate it, thereby explaining thenoncanonical geometries. Application of the debor principle of borane chemistry to the shapes of 1-4readily rationalizes the observed geometries of 5 and 6. This evidence of the scope of transition metalfragment control of borane geometry suggests the existence of a large class of metallaboranes withstructures not found in known borane or metal clusters.