Carborane-Based Pincers: Synthesis and Structure of SeBSe and SBS Pd(II) Complexes

Abstract

We report the synthesis of several unique, boron-rich pincer complexes derived from m-carborane. The SeBSe and SBS pincer ligands can be synthesized via two independent synthetic routes, and are metallated with Pd(II). These structures represent unique coordinating motifs, each with a Pd-B(2) bond chelated by two thio-or selenoether ligands. This class of structures serves as the first example of boron-metal pincer complexes, and possesses several interesting electronic properties imposed by the m-carborane cage.Pincer complexes,1 formed from tridentate ligands (the "pincer") and metals or metalloids, are an exciting class of structures with utility in catalysis,2 molecular electronics,3 and medicine. 4 The pincer ligand provides tailorability with respect to 1) anchoring site on the base, the central moiety from which the arms extend,5a 2) metal-binding heteroatoms, which control the electronic nature of the complexed atom, 5b and 3) the auxiliary sites on the arms, 5c which can be modified to control the chirality of the resulting structure and the steric environment around the complexed atom. To date, the base has been made primarily from hydrocarbons, including aromatic groups such as benzene and heteroatom analogs, 6a aliphatic groups,6b-c and, more recently, carbene-type moieties. 6dIcosahedral carboranes represent one class of structures 7 that have not been explored as base units in pincer complexes. This is surprising considering these structures are highly tailorable, 8a exhibit extraordinary stability, 8b and provide a wide range of accessible chemical derivatization pathways. 8c Indeed, Hawthorne et al. have pioneered synthetic methods for chadnano@northwestern.edu. Supporting Information Available: Experimental and characterization for 4-7. X-ray crystallographic files for 5 and 7 in CIF format. Details on DFT studies of 5. These materials are available free of charge via the internet at http://pubs.acs.org. Herein we report the first carborane-based pincer ligand family and the complexes of these ligands metallated with Pd. The structure formed represents a new class of compounds with a previously unobserved Pd-B σ-coordination bond pincer motif. 10 Additionally, through density functional theory (DFT) calculations we show that the electronic structure of this novel complex, derived in part from this new ligand, is substantially different from its hydrocarbon analogues. The carborane-based pincer complex 5 was synthesized in four steps starting with the commercially available m-carborane 1 (Scheme 1a) to form the desired pincer ligand 4. Pincer complex 5 is made in 76% yield by reacting ligand 4 with Pd(CH 3 CN) 4 [BF 4 ] 2 in acetonitrile followed by 2 eq. of ( n Bu) 4 NCl. Importantly, complex 5 is stable indefinitely in air and can be chromatographed on silica. All spectroscopic data are consistent with the proposed structure for 5. ) and an isolated resonance at δ -0.7. Integration of the area under these peaks shows a 9:1 ratio, respectively. The 1 H coupled 11 B NMR spectrum of comple...

1-Selenolato-2-phenyl-o-carborane complexes of palladium(II) and platinum(II): Synthesis, spectroscopy and structures

“…The dihedral angle between "PdPSeCl 2 " planes is 48.02(3) . The two PdeSe distances are essentially similar and are as expected [25,26]. Various angles around each palladium atom are normal and lie in the range reported for [M 2 Cl 2 (m-Cl)(m-ER)(PR 3 ) 2 ] complexes [1].…”

“…Similarly Pd(1)eCl(2) (trans to (2) a CeB distances lie in the region 1.705 (7) and 1.740 (7) A. [25,26]. It is worth noting that while both cis and trans [M(SeCb o Ph) 2 (PR 3 ) 2 ] could be isolated (vide supra), isolation of asymmetric structure 7 reflects severe steric demands of selenocarborane ligand which would be exerted in the expected bis-selenolato bridged complexes.…”

New ligand platforms featuring boron-rich clusters as organomimetic substituents

“…8A) [66]. Surprisingly, it was not until our report in 2009 where icosahedral carboranes were first explored as base units in pincer complexes, where we presented the carborane-based pincer ligand motif and its respective Pd(II) complexes [67,68]. The structures obtained represented a new class of compounds with a previously unobserved Pd–B boryl σ-coordination within a pincer ligand framework.…”

The Organometallic Chemistry of Boron-Containing Pincer Ligands based on Diazaboroles and Carboranes

“…78 The described pincer ligand precursors 74 and 77 contain either phenylthiomethyl-or phenylselenomethyl-tethers at meta positions with respect to each other (Scheme 24). Both pincer ligand precursors were generated from closo-C 2 B 10 H 12 73, which was subjected to a twofold lithiation.…”