Most organic charge-transfer complexes, when dissolved in a solvent of low ionising-power, appear to exist almost entirely as neutral electrondonor-acceptor pairs. Such complexes involve very little transfer of charge in the ground state. This type of behaviour is even observed for complexes formed by the combination of electron donors of low ionisation potential with electron acceptors of high electron affinity which in the solid phase have a significant degree of ionic character 1. However, when such strongly interacting systems (or mixtures of their components) are dissolved in solvents of high ionising power, ionic species are formed. It is suggested that the driving force for this ionisation is simply the energy of solvation of the ions produced 2 . This may be represented as:where (AD)cT is the charge-transfer complex, and A-and D+ are the two ions formed by single electron transfer from the electron donor (D) to the electron acceptor (A).If the ionic products in solution are the result of solvation, it should be possible reversibly to change the position of equilibrium between the ionic and the neutral charge-transfer molecular species by altering the nature of the solvent.The absorption spectrum of an equimolecular mixture of N , N , N ' , N'tetramethyl-p-phenylenediamine (TMPD) and chloranil in acetonitrile is shown in Fig. l(a). The absorptions at 428,448 nm are assigned to the dianion of the hydroquinone derived from chloranil, and the bands with maxima at 568, 618 nm to TMPD+ 3. The broad, low-intensity band at 843 nm is the intermolecular charge-transfer band of the molecular complex 3 . On R. Foster and T.
Association constants for various charge-transfer complexes formed between 1,3,5-trinitrobenzene, fluoranil (tetrafluoro-p-benzoquinone), or 1,4-dinitrobenzene and various aromatic hydrocarbons have been determined from n.m.r. chemical-shift measurements. By means of a spectrophotometric method, association constants for a series of tetracyanoethylene-aromatic hydrocarbon complexes have also been determined. The effect on the association constant of increasing the methylation of the benzene, of increasing the length of the side-chain in the mono-n-alkylbenzenes. and of the presence of two or more phenyl groups in the donor is considered.WE report the results of a series of determinations of association constants, mainly for complexes of 1,3,5-trinitrobenzene, 1,4-dinitrobenzene, and tetrafluoro-pbenzoquinone (fluoranil), determined from n.m.r. chemical shifts. We also include some optically determined constants for a series of tetracyanoethylene complexes. RESULTS AND DISCUSSIONThe n.m.r. method used to determine association constants ( K = [AD]/[A][D]) for the equilibrium A + D + AD, where AD is the 1 : 1 complex formed from the electron acceptor (A) and the electron donor (D), has been described previously. The chemical shift of the measured nucleus in the complex, relative to the chemical
Strategies commonly used for the synthesis of functionalised bicyclo[1.1.1]pentanes (BCP) rely on the reaction of [1.1.1]propellane with anionic or radical intermediates. In contrast, electrophilic activation has remained ac onsiderable challenge due to the facile decomposition of BCP cations, which has severely limited the applications of this strategy. Herein, we report the electrophilic activation of [1.1.1]propellane in ah alogen bond complex, which enables its reaction with electron-neutral nucleophiles such as anilines and azoles to give nitrogen-substituted BCPs that are prominent motifs in drug discovery.Adetailed computational analysis indicates that the key halogen bonding interaction promotes nucleophilic attackwithout sacrificing cage stabilisation. Overall, our work rehabilitates electrophilic activation of [1.1.1]propellane as av aluable strategy for accessing functionalised BCPs.
The association constants for a number of charge-transfer complexes of 1,3,5-trinitrobenzene with anilines, and with aza-aromatic compounds, particularly pyridines, have been determined by an n.m.r. method. In many cases the magnitude of the association constant is accounted for, a t least qualitatively, by ortho-steric effects, both in the aniline and in the pyridine series. There is evidence that a major contribution, at least, of the interaction of the pyridines is via an n-electron. For some of the aniline complexes, the enthalpy and entropy of complex formation have also been determined.1 ,~,~-TRINITROBENZENE can act as an electron acceptor (A) to form charge-transfer complexes (AD) with electron donors (D) such as the anilines and aza-aromatic compounds. However, most attempts to measure quanti-
duf Veranltlssung-des Herrn Prd. I, o t h a r 81 t y e r hebe ich ekie Reilio von Vnrsuctien ausgcfuhrt, bei welchen Gemische VOR chrumsaurm rind liohlensaurem, und von schwefelsaurem und kohlensaurt?n: Kaliurn in verschiedenen Verhiiltnksen mit telvt Chlorbaryum ausgefaflt wurden. An d i e m Stellr? sei es rnir gestattpt, diesem meinem hochverehrten Lehrer fiiir die stels bereitwillige Unterstiitzung, die er mir hei dieqr Arbeit EU Theil mrden lieh, ineinen innig-$ten Uank auszuspreehen. in seinem Y ersirch, die cheniisckeo Erscheiniingen srif die 'Irundprincipien der Mechanik znruckxnfiibren, behauptete Berh o 11 t> t , dai's clas Product der gegenseitigen Einwirkung der &-per ihrw Masse und Affinitat proportional sei. Diese B e avptung B e r 1 h o 1 ii! t 's hat sich experimental nicht ganz so, ie sit: arifpste!lt wurde, bestiitigt. Aie hedurf'te einer wesente) &n~iiguraldissiwtntion, Tiibiagefi 1879. Die weitere VerMbntlichcng dieser schon vor mohr a h zwei Jabren ale Dbetrttion gedruckhn Arbeit babo ich hisher verzligert in der Hoffiung, die Beautwortuug ainiger duroh dieselbe niaht 8nfgcklMen Fragun nach dou Cfusetzen dar Masseuwirkung intiwinchen bewirken zu k6UIi611, hesondern der I"rage, ub etwa dss G u l dberg-TV a agekche Ckwtz einer Besch&rrkung oder Erweitamng budiirfo. Nachdern Herr O a t w e l d (J. pr. Ohem. pa] aa, 48.61 jetat dim0 Frage von einor ctwrs audoreu Seite in An@@ genommen hot, wiil icb rliosa Arheit nicht linger anntakhalten. Lothar $$eyer. Aanabm dar Obanrle 213. Bd.Iiidcin i d i RUF die m r Prdung und Berichtigung dieser Lehre , anp::;tellten \-'ersa~che von Buns e R Y R I I '~ Hofi-f-f), B e t t c n d o r f +$+), S e t t c g a s t *), Malaguti b*j und Maargueritte*++) verweise, will ich nw bei der Ttleorie yon G ul d b erg und W a a g e +) einen Augcnblick verweilen. Diese Porscher liaben bebaniitlich euie Hy-*) Dbse Annalen 8 8 , 187; Bnnren'a gwmetrirche Metboden, 2. Aufl., 8. 348. *) Diese AIsnalen S 6 , 103; #a, 156; 8 0 , 238. %**) Phil. Trms., Jshrg. 1865, 6. 179. $) Die60 Annalen suypl. a, 326.
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