Metal-Free Bond Activation by Carboranyl Diphosphines

Abstract

We report metal-free bond activation by the carboranyl diphosphine 1-PtBu2-2-PiPr2-C2B10H10. This main group element system contains basic binding sites and possesses the ability to cycle through two-electron redox states. The reported reactions with selected main group hydrides and alcohols occur via the formal oxidation of the phosphine groups and concomitant reduction of the boron cage. These transformations, which are driven by the cooperation between the electron-donating exohedral substituents and the electron-accepting cluster, differ from those of “regular” phosphines and are reminiscent of oxidative addition to transition metal centers, thus representing a new approach to metal-free bond activation.

Activation of Alkynes by a Redox-Active Carboranyl Diphosphine and Formation of Boron-Containing Phosphacycles

“…Importantly, the observed reactivity is completely analogous to our recently reported examples of bond-activation of main group hydrides and alcohols by 1, which also led to the formation of zwitterions with two phosphonium centers and a dianionic cluster. 26 The synthesis of 2−5 features regioselective metal-free B−H bond activation with the formation of a direct C−B bond. Notably, the carborane cluster retained all its 12 vertices and was converted to its C 2 -symmetric nido-form through two-electron reduction.…”

“…37 Notably, we have previously demonstrated such reoxidation for the isostructural nido-cluster of the chemically reduced dianionic 1 2− with [FeCp 2 ]PF 6 , which could be conveniently monitored by 31 P NMR spectroscopy, as the dianionic open-cage cluster exhibited two singlets in the spectrum, and the neutral closed-cage featured two doublets due to 31 P-31 P coupling. 26 The reaction of 2 and 2 equiv of [FeCp 2 ]PF 6 in tetrahydrofuran led to a mixture of products, with the predominant component possessing two singlet signals in the 31 P NMR spectra at 30.4 and 71.8 ppm, indicating the absence of coupling between phosphorus centers. A single crystal X-ray diffraction study revealed the structure of zwitterionic P( i Pr 2 )P(CHCPh)( t Bu 2 )(C 2 B 10 H 10 ) 6 (Figure 3).…”

“…Since the nido -product 2 contains a reduced cluster with 14 skeletal electron pairs (SEPs), we decided to probe the possibility of two-electron oxidation back to the 13 SEP closo -form . Notably, we have previously demonstrated such re-oxidation for the isostructural nido -cluster of the chemically reduced dianionic 1 2– with [FeCp 2 ]PF 6 , which could be conveniently monitored by 31 P NMR spectroscopy, as the dianionic open-cage cluster exhibited two singlets in the spectrum, and the neutral closed-cage featured two doublets due to 31 P- 31 P coupling . The reaction of 2 and 2 equiv of [FeCp 2 ]PF 6 in tetrahydrofuran led to a mixture of products, with the predominant component possessing two singlet signals in the 31 P NMR spectra at 30.4 and 71.8 ppm, indicating the absence of coupling between phosphorus centers.…”

“…Recently, we reported an example of metal-free bond activation by the carboranyl diphosphine 1-P t Bu 2 −2-P i Pr 2 -C 2 B 10 H 10 (1). 26 This main group element system utilizes two basic phosphine sites and has the ability to cycle through the two-electron redox states of the carborane cluster to afford the ability to both donate electrons through the nucleophilic phosphine groups and accept electrons through cluster opening resulting in a concomitant increase in the C−P bond order.…”

Cyclic Carbonate Synthesis from Epoxides and CO2 Catalyzed by Aluminum–Salen Complexes Bearing a nido-C2B9 Carborane Ligand

“…The functionalization can be achieved at various vertices, particularly at acidic C­(1,2) positions. 1,2-Dichalcogenolato- o -carborane, 1,2-diphosphino- o -carborane, and 1,2-disila- o -carborane have been developed and extensively studied . We have reported the facile synthesis and functionalization of diamino o -carborane (Chart ).…”