print pageprinter friendly
Reports: DNI354504-DNI3: Electron-Donating Carboranyl Ligands for Iron-Catalyzed Hydrocarbon Oxidation

Dmitry V. Peryshkov, PhD, University of South Carolina

The goal of this ACS PRF New Doctoral Investigator project is to synthesize and study a new class of electron-rich chelating carborane cluster-based ligands and their transition metal complexes. Icosahedral carboranes (closo-dicarbadodecaboranes, C2B10H12) are remarkably robust neutral boron clusters with two boron vertices replaced by carbon atoms. The electron delocalization of the cluster contributes to its high chemical stability, which, along with extreme steric bulk, makes them attractive ligands for transition metals. While C-metalated carborane clusters are relatively widely studied, B-metalated clusters are underexplored. In this project, we set to prepare C- and B-metalated carborane clusters employing chelating directing groups and to study reactivity of the metalated complexes in organic transformations with the emphasis on the cooperative involvement of the cluster in reactivity. In Year 1 of this project, we attempted to synthesize unsupported carboranyl complexes of Fe(II) by the transmetallation of deprotonated carborane clusters with FeCl2 in THF . The reaction resulted in partial deboronation of the cluster and the formation of anionic Fe(II) dicarbollide complex. The molecular structure of the complex is shown in Figure 1. The K+ cation is solvated by THF molecule and additionally coordinated to six B–H bonds of the clusters with K∙∙∙H distances of 2.65(1)–2.81(1) Angstroms.

Figure 1. The molecular structure of K2(THF)2[Fe(C2B9H11)2].

Thus, we turned to the use of chelating groups attached to the boron cluster that would direct the metalation process and prevent deboronation. As the initial target, we have synthesized a novel carboranyl phosphinite ligand precursor 1,7-OP(i-Pr)2-m-carborane (POBOP-H) bearing two diisiosopropyl phoshpinite arms. For metalation studies, Wilkinson`s catalyst Rh(PPh3)3Cl was used as a low-valent metal source resulting in the clean selective formation of the B–metalated cluster pincer complex (POBOP)Rh(H)(Cl)(PPh3) (Scheme 1). The single-crystal X-ray structures along with spectroscopic characterization confirmed the formation of proposed product. This compound is the first example of structurally characterized B-metalated m-carboranyls of Rh.

Scheme 1. The reactivity of (POBOP)Rh(H)(Cl)(PPh3) complex.

We started to explore reactivity of the supported carboranyl complex with the emphasis on the possible involvement of the carborane cage (Scheme 1). The reductive elimination of HCl from (POBOP)Rh(H)(Cl)(PPh3) in the presence of NEt3 led to the selective formation of the square-planar (POBOP)Rh(PPh3) complex. This electron-rich 16-electron complex was found to participate in the multitude of oxidation addition reactions. The reaction of (POBOP)Rh(PPh3) and PhI led to the selective formation of the five-coordinate (POBOP)Rh(Ph)I. The single-crystal X-ray diffraction revealed extraordinary geometry distortions in this complex (Figure 2). The exohedral B2–B1–Rh1 angle in this compound is 85.2(1) deg, which remarkably deviates from the expected 120 deg exohedral angle for the icosahedral cluster. In addition, the unusually acute B2–B1–Rh1 angle resulted in the short metal-vicinal boron atom contact Rh1–B2 of 2.582(2) Angstroms and the short Rh1–H2A(B2) contact of 2.20(1) Angstroms, which confirmed the possibility of the supporting three-center two-electron B–H∙∙∙Rh interactions proposed in this project. Complex (POBOP)Rh(Ph)I is the first example of an isolated B-carboranyl aryl metal complex, which is a proposed intermediate in metal-promoted B–C coupling reactions.

Figure 2. The molecular structure of (POBOP)Rh(Ph)I complex.

The close contact between the metal center and the vicinal boron atom and the uniquely high strain of cage-metal bond in (POBOP)Rh(Ph)(I) led us to the hypothesis that Rh can easily activate the adjacent B–H bond. Indeed, heating of (POBOP)Rh(Ph)(I) in the presence of CH3CN in C6D6 led to the unprecedented cascade reductive elimination/oxidative addition process resulting in the aryl group transfer to the boron atom of the cage, subsequent B–H activation at the vicinal boron atom, followed by re-formation of the Rh(III) carboranyl hydride. (Scheme 1). The formation of this complex is the first example of reductive elimination of an organic group to the metalated carborane cage from the isolated metal aryl- carboranyl intermediate. Incidentally, complex (POB(BPh)OP)Rh(H)(I)(CH3CN) is the first example of a selective derivatization of the B1(B2) boron atom of the m-carborane cage by an aryl group. In the case of our three-dimensional carboranyl-based pincer system, the presence of the vicinal B–H bond is a unique feature, which allowed for the sequential reductive elimination/oxidative addition process and the re-formation of the Rh(III) metal center. The formation of this complex demonstrated that the carboranyl unit can act not only as a supporting spectator ligand but also can be involved in cooperative ligand-metal reactivity.

PRF grant is critical in supporting the PI`s career development by funding our work on the use of icosahedral borane clusters as electron-donating ligands for transition metals. This project grew into the initially unanticipated area of chelating pincer complexes. These results will serve as the foundation for the PI`s research program that aims to develop novel catalysts featuring ligand-metal cooperativity. As a result of Year 1, the manuscript, co-authored with the graduate students, describing the uncovered reactivity in the POBOP-Rh system is under review. We are expanding the scope of this project by the synthesis of analogous pincer complexes of the first row transition metals, including iron. The graduate students supported by PRF fund obtained extensive training in synthetic organometallic chemistry, work with air-sensitive compounds, and related safety practices.