X-ray Snapshot Observation of Palladium-Mediated Aromatic Bromination in a Porous Complex

Abstract

Pd-mediated aromatic bromination is intriguing to synthetic and organometallic chemists due to both its synthetic utility and, more importantly, a proposed mechanism involving an uncommon Pd(IV)/Pd(II) catalytic cycle. Here, we report an X-ray snapshot observation of a Pd reaction center during a Pd-mediated aromatic bromination in a single crystal of a porous coordination network crystalline scaffold. Upon treatment of a single crystal with N-bromosuccinimide, sequential X-ray snapshots revealed that the aryl-Pd(II)-L species embedded in the network pores was converted to the brominated aryl product through a transient aryl-Pd(II)-Br species, which is normally unobservable because of its rapid dimerization into insoluble Pd2(μ-Br)2 species. Though the reaction pathway may be biased by the crystalline state, the new X-ray snapshot method relies on crystalline flasks to provide important mechanistic insight.

An Iodine‐Vapor‐Induced Cyclization in a Crystalline Molecular Flask

“…The former strategy allows a modular approach to be employed by modifying the planar aromatic handle 8. This provides an anchor for postsynthetic modification (PSM) reactions that occur in the solid state and can be monitored crystallographically 9, 10, 11, 12, 13, 14, 15. In the absence of an aromatic handle, matching of the spatial dimension of the pore cavity is also a viable strategy to observe guests using crystallographic methods.…”

“…Herein, we report an iodine‐vapor‐mediated cyclization of 1,8‐bis(2‐phenylethynyl)naphthalene (bpen) to 7‐iodo‐12‐phenylindeno[2,1‐α]phenalene (ipp; Scheme 1). This reaction makes use of a well‐known CMF with the composition [(ZnI 2 ) 3 (tpt) 2 ]⋅ x (G), where G=guest 9, 10, 11, 12, 13, 14, 15. Cyclization of bpen to ipp in the presence of iodine vapor was found to occur quantitatively, under mild conditions, without complicated activation of the CMF, and in the absence of solvent.…”

An Iodine‐Vapor‐Induced Cyclization in a Crystalline Molecular Flask

“…[8] This provides an anchor for postsynthetic modification (PSM) reactions that occur in the solid state and can be monitored crystallographically. [9][10][11][12][13][14][15] In the absence of an aromatic handle,m atching of the spatial dimension of the pore cavity is also av iable strategy to observe guests using crystallographic methods.This approach has been used to probe photoswitching, [16] change conformational isomerization ratios, [17,18] observe unusual molecular interactions, [19] and identify organic species, [20,21] with the notable advantage of requiring only nanogram to microgram quantities of the analyte.[3]Arange of chemical reactions have been observed within CMFs,i ncluding the conversion of amines into imines, [9,10] Huisgen cycloadditions, [11] Diels-Alder reactions, [14] and metal-catalyzed methylation [12] and bromination.[15] All of these processes occurred upon immersing aC MF that contained am odified intercalated triphenylene moiety into solutions with appropriate chemical reactants.O nly one example of achemical reaction combining gas-phase reagents and aCMF has been reported, and involved the conversion of avinyl group into epoxidation/oxidation products with the aid of ar adical initiator. [14] This reaction, however,r equired loading of the CMF with 2,2'-azobis(isobutyronitrile) and the use of forcing conditions,whereby an emulsion of crystals was heated at 80 8 8Cf or 24 hours in air.T ot he best of our knowledge,a ttempting chemical transformations in CMFs using gas-phase reagents in the absence of solvents is unprecedented.…”

“…[9][10][11][12][13][14][15] Cyclization of bpen to ipp in the presence of iodine vapor was found to occur quantitatively,under mild conditions,without complicated activation of the CMF,a nd in the absence of solvent.…”

Visualization of Solution Chemistry by X-ray Crystallography Using Porous Coordination Networks

“…When inclusion complex PCN-2¢12 was treated with N-bromosuccinimide (NBS) in acetonitrile at r.t. for 3 h, X-ray crystallography revealed that a square-planar ArPdBr intermediate 13 was formed as a result of the dissociation of methylxanthate ligand ( Figure 7). 39 Intermediate 13 is a well-known species but has rarely been observed by X-ray analysis because of its tendency to dimerize in solution upon crystallization to form [Pd 2 (¯-Br) 2 ]-type complexes. After washing the crystals containing intermediate 13 with fresh acetonitrile, we monitored the formation of arylbromide product 14 by time-dependent X-ray diffraction analysis.…”