No abstract

The introduction of B ← N coordinate bondisoelectronic to C−C single bondinto π-systems represents a promising strategy to impart exotic redox and electrochromic properties into conjugated organic molecules and macromolecules. To achieve both reductive and oxidative activities using this strategy, a cruciform ladder-type molecular constitution was designed to accommodate oxidation-active, reduction-active, and B ← N coordination units into a compact structure. Two such compounds (BN-F and BN-Ph) were synthesized via highly efficient N-directed borylation. These molecules demonstrated well-separated, two reductive and two oxidative electron-transfer processes, corresponding to five distinct yet stable oxidation states, including a rarely observed boron-containing radical cation. Spectroelectrochemical measurements revealed unique optical characteristics for each of these reduced/oxidized species, demonstrating multicolor electrochromism with excellent recyclability. Distinct color changes were observed between each redox state with clear isosbestic points on the absorption spectra. The underlying redox mechanism was elucidated by a combination of computational and experimental investigations. Single-crystal X-ray diffraction analysis on the neutral state, the oxidized radical cation, and the reduced dianion of BN-Ph revealed structural transformations into two distinct quinonoid constitutions during the oxidation and reduction processes, respectively. B ← N coordination played an important role in rendering the robust and reversible multistage redox properties, by extending the charge and spin delocalization, by modulating the π-electron density, and by a newly established hyperconjugation mechanism.

In search of ligand platforms, which can be used to remotely control the catalytic activity of a transition metal, we have investigated the coordination noninnocence of ambiphilic L/Z-type ligands containing a trifluorostiborane unit as a Lewis acid. The known dichlorostiboranyl platinum complex (( o-(PhP)CH)SbCl)PtCl (1) reacts with TlF in the presence of acetonitrile (MeCN) and cyclohexyl isocyanide (CyNC) to afford the trifluorostiborane platinum complexes 2 ((( o-(PhP)CH)SbF)Pt-NCMe) and 3 ((( o-(PhP)CH)SbF)Pt-CNCy), respectively. Formation of these complexes, which results from a redistribution of the halide ligands about the dinuclear core, affects the nature of the Pt-Sb bond. The latter switches from covalent in 1 to polar covalent (or dative) in 2 and 3 where the trifluorostiborane moiety engages the platinum center in a Pt → Sb interaction. The polarity of the Pt-Sb bond can be modulated further by abstraction of an antimony-bound fluoride ligand using B(CF). These reactions afford the cationic complexes [(( o-(PhP)CH)SbF)Pt-NCMe] ([5]) and [(( o-(PhP)CH)SbF)Pt-CNCy] ([6]) which have been isolated as [BF(CF)] salts. These complexes possess a highly Lewis acidic difluorostibonium moiety, which exerts an intense draw on the electron density of the platinum center. As a result, the latter becomes significantly more electrophilic. In the case of [5], which contains a labile acetonitrile ligand, this increased electrophilicity translates into increased carbophilicity as reflected by the ability of this complex to promote enyne cyclization reactions. These results demonstrate that the coordination noninnocence of antimony Z-ligands can be used to adjust the catalytic activity of an adjoining metal center.

With the view of developing self-activating electrophilic catalysts, we are now investigating complexes with a Lewis acidic moiety in the immediate vicinity of the transition metal center. Toward this end, we have synthesized a platinum complex in which the metal is connected to a Lewis acidic bis(triflato)stiboranyl ligand. This complex, ((o-(PhP)CH)SbOTf)PtCl (2), which was obtained by treatment of ((o-(PhP)CH)SbCl)PtCl (1) with 2 equiv of AgOTf, is surprisingly air stable. Yet, it promptly reacts with cyclohexylisocyanide to afford the dicationic chlorostibine complex [((o-(PhP)CH)SbCl)PtCNCy] ([3]) as a bis-triflate salt. Formation of [3] occurs through abstraction of the platinum-bound chloride ligand by the adjacent Lewis acidic antimony center. This halide migration reaction leads to activation of the platinum center. In turn, 2 behaves as a self-activating catalyst in reactions involving alkynes and readily mediates both enyne cyclization and intramolecular hydroarylation reactions, at room temperature, without addition of a chloride abstracting reagent. These results demonstrate that the coordination non-innocence of antimony ligands can be exploited for the purpose of electrophilic catalysis.

Organoborane compounds are versatile reagents for organic synthesis. Various methods have been developed to incorporate boryl groups into organic frameworks. Among others, borometallation of alkynes, such as silaboration and stannaboration, has received substantial attention in recent years. [1] These reactions can introduce multiple functionalities in one step, providing an efficient way for the stereoselective synthesis of highly substituted alkenes. Whereas silaboration and stannaboration of alkynes often require transition-metal complexes as catalysts (Scheme 1 a), [1] the alkynes were reported to be able to directly insert into the M À B bond of boryl transition-metal complexes, resulting in the formation of (2-borylalkenyl)metal complexes (Scheme 1 b). [2][3][4][5][6] One drawback of the abovementioned approaches is the requirement of boryl metal species, some of which are not easily accessible. Thus it will be highly interesting to develop a method through which the borometalation reaction can be directly achieved with hydroborane and metal complexes (Scheme 1 c).Recently, the chemistry of frustrated Lewis pairs (FLP) has provided a new avenue for small-molecule activation. [7, 8] Particularly, Stephan [9] and Berke [10] et al. discovered that FLPs can activate terminal alkynes, which can lead to the formation of alkynylborates. Inspired by their work, we became interested in applying such a concept to the borometallation of alkynes. Herein, we describe the reaction of terminal alkynes with a FLP comprised of HBAr F 2 (1, Ar F = 2,4,6-tris(trifluoromethyl)phenyl)) and 1,4-diazabicyclo-[2.2.2]octane (DABCO) [11] and its application in catalystfree boroauration of terminal alkynes.When 1 was mixed with phenylacetylene in hexane solution at room temperature, the hydroboration reaction took place in a very slow manner, and only 80 % conversion was obtained after 5 h, which is possibly due to the large steric bulk around the boron center in 1. When DBAr F 2 was applied to phenylacetylene, the formation of E-PhC(D) = C(H)BAr F 2 was observed, confirming that the resulting alkenylborane was obtained through 1,2-hydroboration (Scheme 2).The slowness of this hydroboration reaction allowed us to investigate the FLP reactivity of 1 and DABCO against terminal alkynes. When 1 was added to the mixture of the alkyne 2 a or 2 b and DABCO in the hexane solution at 0 8C, a white precipitate was immediately observed. After 3 h, the precipitate was isolated in 75 % (from 2 a) or 72 % (from 2 b) yield, and subsequently identified as ammonium alkynylhydridoborate salts 4 a and 4 b resulting from the deprotonation of the corresponding alkynes (Scheme 2). [9,10] In the 11 B NMR spectra, a doublet (À21.6 ppm with 85 Hz for 4 a and À22.1 ppm with 82 Hz for 4 b) was observed, indicating the existence of the hydridoborate moiety. Both elemental analysis and 1 H NMR spectrum suggest that each ammonium alkynylhydridoborate salt molecule contains one half of a neutral DABCO molecule. The salt 4 b was further characterized by X-ra...