Conformational equilibrium in supramolecular chemistry: Dibutyltriuret case

This is an elec onic ep in o he o iginal a icle. This ep in may di e om he o iginal in pagina ion and ypog aphic de ail. Au ho (s): Ti le: Yea : Ve sion: Please ci e he o iginal e sion: All ma e ial supplied ia JYX is p o ec ed by copy igh and o he in ellec ual p ope y igh s, and duplica ion o sale o all o pa o any o he eposi o y collec ions is no pe mi ed, excep ha ma e ial may be duplica ed by you o you esea ch use o educa ional pu poses in elec onic o p in o m. You mus ob ain pe mission o any o he use. Elec onic o p in copies may no be o e ed, whe he o sale o o he wise o anyone who is no an au ho ised use . Con o ma ional equilib ium in sup amolecula chemis y: Dibu yl iu e case M oczyńska, Ka ina; Kaczo owska, Małgo za a; Kolehmainen, E kki; G ubecki, I eneusz; Pie zak, Ma ek; Ośmiałowski, Bo ys M oczyńska, K., Kaczo owska, M., Kolehmainen, E., G ubecki, I., Pie zak, M., & Ośmiałowski, B. (2015). Con o ma ional equilib ium in sup amolecula chemis y: Dibu yl iu e case. Beils ein Jou nal o O ganic Chemis y, 11, 2105-2116. h ps://doi.o g/10.3762/bjoc.11.227 2015 2105 Con o ma ional equilib ium in sup amolecula chemis y: Dibu yl iu e case Ka ina M oczyńska1, Małgo za a Kaczo owska1, E kki Kolehmainen2, I eneusz G ubecki1, Ma ek Pie zak1 and Bo ys Ośmiałowski*1 Full Resea ch Pape Open Access Add ess: 1Facul y o Chemical Technology and Enginee ing, UTP Uni e si y o Science and Technology, Semina yjna 3, PL-85326 Bydgoszcz, Poland and 2Depa men o Chemis y, Uni e si y o Jy äskylä, P.O. Box 35, FI-40014, Jy äskylä, Finland Email: Bo ys Ośmiałowski* - [email p o ec ed] * Co esponding au ho Keywo ds: associa ion; hyd ogen bonding; NMR; o ame ism; sup amolecula chemis y Beils ein J. O g. Chem. 2015, 11, 2105–2116. doi:10.3762/bjoc.11.227 Recei ed: 13 July 2015 Accep ed: 10 Oc obe 2015 Published: 05 No embe 2015 Associa e Edi o : P. R. Sch eine © 2015 M oczyńska e al; licensee Beils ein-Ins i u . License and e ms: see end o documen . Abs ac The associa ion o subs i u ed benzoa es and naph hy idine dianions was used o s udy he complexa ion o dibu yl iu e . The i le molecule is he simples molecule able o o m wo in amolecula hyd ogen bonds. The naph hy idine sal was used o b eak wo in amolecula hyd ogen bonds a a ime while wi h he use o subs i u ed benzoa es he sys ema ic app oach o s udy associa ion was achie ed. Bo h, i a ions and a iable empe a u e measu emen s shed he ligh on he impo ance o con o ma ional equilib- ium and i s in luence on associa ion in solu ion. Mo eo e , he associa es we e obse ed by mass spec ome y. The DFT-based compu a ions o complexes and single bond o a ional ba ie s suppo s expe imen al da a and helps unde s anding he p ope ies o mul iply hyd ogen bonded complexes. 2105 In oduc ion The hyd ogen bond (HB) is one o he mos common non-co a- len in e ac ions. Since i s abilizes, o example, he double helix o DNA and in luences pep ide olding i is qui e eason- able o assume ha HBing is c ucial o exis ence o li e. I is also p esen in many small molecules ac ing as an in amolec- ula con igu a ional lock. This is ealized in hyd azones [1], he e ocyclic u ea de i a i es [2], molecules exhibi ing pho oex- ci ed p o on ans e [3] and o he compounds [4-6] epo ed also by us [7-10]. The in amolecula HBing p esen in some he e ocycles esul s in a molecula geome y sui able o associ- a ion by mul iple hyd ogen bonding [11-23] making possible he o ma ion o , in e alia, s able sup amolecula polyme s [24- 30]. Such polyme iza ion needs p ope ly p ea anged mono- me s wi h in e molecula hyd ogen bonding pa e ns i ing be ween molecules. In his sense he con o ma ional eedom is a main limi ing ac o in molecula design. On he o he hand in o ms s abilized by an in amolecula hyd ogen bond i is possible o b eak an in amolecula HB and o ma ion o a com- Beils ein J. O g. Chem. 2015, 11, 2105–2116. 2106 Figu e 1: The compounds used in his s udy and hei a om numbe ing. plex in al e na i e o ame ic s a e. This obse a ion gi es he oppo uni y o con ol such p ocesses [2] making con- o ma ional equilib ium [4,31-35] one o he ac o s, o a ool, ha should be aken in o accoun in molecula design. To he bes o ou knowledge he e a e only ew publica ions ocused on simple molecules capable o o m wo in amolecula HBs [36,37] ha b eak upon associa ion. This phenomenon is s ill unde discussion [37-39]. The said HB b eakage and a con- o ma ional change may only be ealized ia s ong enough in e ac ion be ween hos (H) and gues (G) molecules. This is because, as E e 's ules [40] s a e, in amolecula hyd ogen bonding is s onge han in e molecula one and mo e p obable due o he en opy easons. In o de o cons uc a molecule capable o o m in amolecula hyd ogen bonds one should bea in mind some condi ions: a) such a molecule mus con ain a hyd ogen bond dono and accep o in a close p oximi y and his is especially ue when one assumes ha b) a six-membe ed quasi- ing s abilized by hyd ogen bonds is p e e ed o e he i e-membe ed one and ha c) om he sup amolecula /in e molecula in e ac ions poin o iew he mos e icien associa ion exis s in complexes in which all hyd ogen bond dono s belong o one molecule and all hyd ogen bond accep o s o he o he . The las condi ion was es ed o quad uple hyd ogen bonded associa es [14], while he basis o his phenomenon a e seconda y in e ac ions [41] ha ac diagonally be ween neighbou ing hyd ogen bonding si es. The b eakage o single in amolecula hyd ogen bonding upon associa ion leading o o ame ism was ou mo i a ion o sea ch o mo e complex sys ems han p e iously epo ed [2,9,10]. Since he amide g oup is common in biomolecules we ha e ocused on dibu yl iu e (1) ha con ains NH and CO g oups. This molecule ul ils he needed p ope ies (a–c abo e). The pa en iu e o ms wo in amolecula hyd ogen bonds in he solid s a e [42] and upon cooling [43]. The associa ion o pa en iu e wi h ca ions has al eady been s udied by MS [44] while i s in e ac ion p e e ences wi h anions in solu ion a e no known. On he o he hand he is-u ea de i a i es wi h a space be ween NHCONH g oups we e used in se e al sup a- molecula complexes including hose wi h encapsula ed anions [45,46], sensing ne e agen s [47] o in sel -healing ma e ials [48]. In iu e de i a i es no such space is p esen yielding a DDDD (D – hyd ogen bonding dono ) pa e n in i s linea o m. The syn hesis o dibu yl iu e was p e iously desc ibed [49] bu we used an al e na e me hod (see expe imen al pa ). I is wo h men ioning ha iu e is known as a byp oduc o he u ic acid deg ada ion [44]. The goal o his s udy is o p obe he subjec ed molecule by anionic coun e pa s in o de o ob ain i s in e ac ion scheme and o s udy i s in a- s in e molecula HBing. Fo ha pu pose he anionic coun e pa s chosen a e 4-subs i u ed benzoa es 2–9 and di e abu ylammonium 1,8-naph hy idin-2,7-diola e (10) (Figu e 1). The benzoa e anions o ming wo hyd ogen bonds [10] we e chosen in o de o ob ain a se ies o anions wi h uneable HB accep o p ope ies, while he naph hy idine de i a i e was used o es i he dibu yl iu e is able o exis in linea o m wi hou any in amolecula HBs and s abilized by ou in e molecula ones. In gene al he dibu yl iu e molecule ca ying ou hyd ogen bond dono s (D, ed, Figu e 2) and h ee hyd ogen bond accep- o s (A, blue) can exis in a ious con o ma ions s abilized by one o wo in amolecula hyd ogen bonds and des abilized by elec onic epulsions (black do ) in some o hem [50]. The con o ma ions o 1 co espond o he ollowing hyd ogen- bonding pa e ns: 1a DDDD, 1b DDA, 1c DDDA, 1d DDA, and 1e ADDA. This means ha he subjec ed compound may associa e by quad uple (in 1a, 1c and 1e), iple (in 1b and 1d) and double (all o ms) hyd ogen bonding wi h sui able coun e - pa s. Fo example, DDDD pa e n in 1a should be able o in e ac wi h AAAA o 10. On he o he hand all con o ma ions can associa e wi h 2–9 by in e ac ion wi h a DD pa o lis ed pa e ns. The p incipal in e ac ions in 1 (Figu e 2) o i s Beils ein J. O g. Chem. 2015, 11, 2105–2116. 2107 Figu e 2: Possible con o ma ions o 1. Figu e 3: D i ing o ces in luencing associa ion exempli ied on wo "ex eme" con o ma ions o 1∙∙∙benzoa e. complex (Figu e 3) a e: a) mul iple hyd ogen bonding, b) seconda y a ac i e o epulsi e in e ac ions and c) elec- onic epulsions. As epo ed ecen ly by us he in e molecula elec onic epul- sions [51] can be c ucial because hey in luence on he selec- i i y o associa ion o he ela i e popula ion o o ame s [50]. In he cu en s udy hese o ces coexis wi h o he ones in he same molecule. Resul s and Discussion S udies by NMR echniques P ope ies o 1 The dilu ion s udies o 1 p o ided i s sel -associa ion cons an Ksel = 170 M−1 (CDCl3, , Suppo ing In o ma ion File 1, Figu e S1). The a iable empe a u e (VT) 1H NMR spec a e eal ha NH p o ons a e qui e ine o he empe a u e change. The b oad single obse ed a 8.95 ppm ( ) shi s o 9.12 ppm upon cooling he sample (−40 °C) while he signal obse ed a 7.89 ppm ( ) shi s o 8.17 ppm (−40 °C) spli ing in o a sha p iple a −15 °C e ealing he signal o igina es om H1/H7 wi h 3J(H,H) = 5.42 Hz (–CONH-CH2–). The said sha pening is ela ed o he loss o he molecule’s lexibili y a lowe ed empe a u es caused by in amolecula hyd ogen bonding. A he same ime he in e molecula hyd ogen bonding is bi u ca ed in he (di)mul ime o 1 (Figu e 4). Tha de ini ely also in luences i s chemical shi . I is wo h keeping in mind ha oge he wi h empe a u e lowe ing wo e en s coexis – limi ed dissocia ion o associa e and enhance- men o s abiliza ion o o ms locked by in amolecula hyd ogen bonding making o ms 1e o 1d he majo ones. Since he spec um eco ded a low empe a u e shows he compound is symme ic we concluded ha he majo o m is 1e. On he o he hand 1a is ano he ex eme o m ha should be aken in o accoun . The sel -associa ed 1a mus ul il wo p e equisi es o exis : a) b eakage o he in amolecula hyd ogen bond(s) and b) associa ion o wo (o mo e) mole- cules o 1a ha a e much less igid s uc u es han o he ones locked by in amolecula HB. The p obabili y o exis ence o o m 1a is low a lowe ed empe a u es. The signal b oadening a oom empe a u e is caused by as in NMR ime-scale equi- lib ium be ween a ious o ms. The NOE expe imen s o 1 (and associa ed 1) a lowe ed empe a u es did no ga e any unequi ocal da a ega ding he shi o equilib ium owa ds any o m. This excludes he exis ence o he dime o mul ime o 1a as he majo o m a low empe a u e. Beils ein J. O g. Chem. 2015, 11, 2105–2116. 2108 Figu e 4: Two possible, ex eme mul iple hyd ogen bonded mul ime ic s uc u es o 1 and VT 1H NMR spec a ( om +25 o −40 °C, low empe a- u es a bo om, CDCl3). Figu e 5: The p oposed s uc u e explaining unusual beha io o he i a ion cu e o 1∙∙∙9 i a ion and aniso opy in luence on me hylene chemical shi . The co ela ion o chemical shi o H1/H7 and H3/H5 wi h empe a u e is high in he ange o −40 °C o ca. +5 °C. The chemical shi s de ia e om linea unc ion abo e +5 °C (Suppo ing In o ma ion File 1, Figu e S2). No e ha H3/H5 beha e i egula ly a highe empe a u es while H1/H7 da a i s well o ano he linea unc ion. This p o es ha a compound 1 exis s as a mix u e o o ms being unde dynamic equilib ium. P o ons H1/H7 beha e linea ly, mos p obably, due o b eaking in amolecula hyd ogen bonds and o ma ion o in e molec- ula ones. I is wo h men ioning ha he nega i e slope o said unc ion a highe empe a u es is mo e han h ee imes highe han ha o lowe empe a u es. Lowe slope o lowe empe a u es is caused by in amolecula hyd ogen bonding making H1/H7 p o on no so sensi i e in 1e o empe a u e change. Associa ion o 1 wi h benzoa es 2–9 Since 1 exis s in a dynamic equilib ium a i was easonable o associa e 1 wi h anionic coun e pa s wi h hei p ope ies Table 1: Associa ion cons an s o 1a [M−1] measu ed wi h he use o H1, H3. Coun e pa (R) KassocH1 KassocH3 2 (NMe2) 110 170 3 (OMe) 100 160 4 (Me) 200 180 5 (H) 280 270 6 (F) 170 120 7 (Cl) 190 160 8 (CF3) 160 220 9 (NO2) 210 250 a[1] = 18.9 mmol dm−3, [2–9] = 10–12 × [1], T = 293 K, sol en CDCl3. uned sys ema ically o he subs i uen e ec . The associa ion cons an s (Kassoc) a e collec ed in Table 1 (see Suppo ing In o ma ion File 1 o igu es). Usually he NH/OH p o ons a e used o ind Kassoc bu in some cases CH chemical shi s a e also use ul [9,52]. He e he p o ons o me hylene a ached Beils ein J. O g. Chem. 2015, 11, 2105–2116. 2109 di ec ly o ni ogen a om we e obse ed du ing he expe imen s (Suppo ing In o ma ion File 1, Figu e S13). Changes in he chemical shi o me hylene a e small bu s ill no iceable. Small changes may be explained by he ac ha only one o hese p o ons alls in he aniso opic cone o he associa ed coun e pa (exempli ied in Figu e 5). S ill i was possible o ind he complexa ion-induced shi (CIS) eliably o H11, which is, o example, equal o 0.14 ppm in he 1∙∙∙6 complex. I is wo h men ioning ha o he 1∙∙∙9 i a ion (Suppo ing In o ma ion File 1, Figu e S10) an unusual beha - io was no iced. The chemical shi o H11 dec eases (as in emaining benzoa es) a he beginning o he i a ion and hen inc eases. This may be explained by he al e na i e binding o 9 wi h 1e o m as shown in Figu e 5. The ollowing may be concluded based on unna u al beha io o he i a ion cu e: a) wi hin hyd ogen-bonded o ms p esen in solu ion o a leas one magne ic aniso opy is impo an o chemical shi , b) wo al e na e hyd ogen bonded complexes s abilized by CO−···HN and NO···HN b idges may be p esen in 1∙∙∙9, c) his a angemen is obse ed only o anion ca ying ano he hyd ogen bonding g oup as NO2. The dual cha ac e o he NO2 g oup (elec on-accep ing and hyd ogen bonding) causes unusual H11 beha io e iden (Suppo ing In o ma ion File 1, Figu e S10). This ype o aniso opic in luence on CH2 chemical shi may only exis in 1e o m (shown in Figu e 5). Only in his o m wo in e ac ing molecules a e in such an a angemen as o loca e CH2 p o ons close o he aniso opic cone o he espec i e moie ies in benzoa e – no such e ec is possible in o he o m∙∙∙benzoa e complexes. This sugges s ha said al e na e 1e∙∙∙benzoa e binding may be p esen and ha NO2 g oup has highe aniso opic in luence on CH2 shi han -CO2– one. Associa ion o 1 wi h naph hy idine dianion 10 The Kassoc equal o 300 M−1 o 1∙∙∙10 complex ( ) was ound based on a oma ic double s in 10. Al hough up o ou hyd ogen bonds s abilize he 1∙∙∙10 complex his associa ion is no high due o con o ma ional equilib ia and addi ional s abiliza ion o compe i i e o ms by in amolecula HBing. I was impossible o calcula e he associa ion based on NH chemical shi s because NH p o ons in 1 a e no obse ed a oom empe a u e du ing i a ion when he [G]:[H] (G – gues , H – hos 1) is highe han 0.2. This, again, sugges s as in NMR ime-scale equilib ium. In he 1∙∙∙10 complex he i a ion cu e has also a non-s anda d shape (sigmoidal, see Suppo ing In o ma ion File 1, Figu e S11 (inse )). I is ai o men ion ha he associa ion cons an is loaded wi h an e o highe han in ou p e ious publica ions and should be ea ed as an app oxima e alue. This is due o he ac ha he i a ion cu es we e i ed o ha e he smalles esiduals s a ing om ca. 0.8 [G]:[H] a io ill in ini e gues concen a- ion. Due o he sigmoidal beha io o all i a ion cu es he i ing is no possible o he beginning o he da ase . Also i is no possible o di ide hese da a in o wo sepa a e se s as be o e [9] because no saddle poin ha could be used o ha pu poses is p esen . S ill his p o es ha 1 is in o a ional equilib ium, which is dependen om in e ac ion wi h o he molecules. I is also wo h men ioning ha o he i a ion cu es o [G]:[H] = 0 a he cu e's in lec ion poin , mos p obably, wo (o mo e) sepa a e cu es o e lap. One o hese exhibi s a dec ease o he chemical shi in he beginning o i a ion wi h ela i ely small CIS alue and eaches i s pla eau ela i ely as . The said dec ease o he chemical shi may only be caused by he o ame ism and change o in a- o in e molecula hyd ogen bonding. Subs i uen e ec on associa ion Recen ly we ha e obse ed he co ela ion o he Kassoc and subs i uen cons an in sup amolecula complexes [10]. The lack o such co ela ion o complex o 1 wi h benzoa es may be explained by a) mul iple equilib ium ( o ame ism in 1) and b) opposi e e ec s o he subs i uen on complexes s abili y ( u he discussion in Suppo ing In o ma ion File 1). The use o subs i u ed benzoa e sal s ga e a se o poin s shown in Figu e 6. I is easy o see ha he subs i uen e ec on chemical shi is no as high as be o e (compa e di e ence in CIS alues in N-py idin-2-ylu ea de i a i e [10]). I is mainly exp essed as s eepe cou se o i a ion cu e in case o elec on dona ing subs i uen s han ha in case o elec on accep ing ones. The a iable CIS alues a e di ec ly seen in Figu e 6. Fo a di ec compa ison be ween wo ex eme subs i uen s see Suppo ing In o ma ion File 1, Figu e S12. The highe sigmoidal cha ac e is clea ly obse ed o H1/H7, while o H3/H5 he cu e is linea -like in he [G]:[H] ange whe e he H1/H7 unc ion changes om con ex o conca e. This is caused by di e en cha ac e o H1/H7 s H3/H5 p o ons. The s eepes i a ion cu e was ob ained o 2 (R = NMe2, black ma ke s in Figu e 6), while o 9 (NO2 sal ) he cu e's shape esemble s aigh line in he beginning o i a- ion ( ed ma ke s in Figu e 6). The ac ha he cu e a i s beginning is no alling down, as be o e [9], sugges s he associ- a ion akes place be ween o ms/ o ame s in ol ed in in a- and in e molecula hyd ogen bonding o simila s eng h. P obably he Kassoc is an o de o magni ude highe o he mo e s able Beils ein J. O g. Chem. 2015, 11, 2105–2116. 2110 Figu e 6: Collec i e i a ion cu es (H1/H7 and H3/H5 chemical shi s, CDCl3) o complex o 1 wi h subs i u ed benzoa es. complex and his co esponds o he i a ion da a om ca. 0.6–0.7 [2–9]:[1] a io o in ini y. Tha is unde s andable since 1e may o m doubly hyd ogen-bonded associa e a low [G]:[H] whe e a compe i ion be ween 1e2 s abilized by in amolecula hyd ogen bonds and i s he e ocomplex akes place, while a highe [G]:[H] a ios he p obabili y o o ma ion o in e molec- ula hyd ogen bond is highe . The o ame ism and mul iple kind o equilib ium in 1 cause ha he gene al in e ac ion scheme is non-speci ic yielding a non-linea change o he associa ion cons an . On he o he hand he egula changes o anion p op- e ies a e exp essed by some egula endency in he pa ial i a ion-de i ed da a (discussion in Suppo ing In o ma ion File 1, page S9). The shapes o cu es show ha he e exis wo o mo e associa es a a ime especially in he egion close o he 1:1 mola a io. A his poin he cu e passes h ough he in lec ion poin loca ed a a iable [2–9]:[1] a io. Fo mo e de ailed discussion on in lec ion poin analysis e e o Suppo ing In o ma ion File 1. He e i is enough o men ion ha he posi ion o he in lec ion poin is linea ly dependen om he subs i uen (Hamme ) cons an aken om he publica ion by Hansch and Ta [53]. To sum up he subs i uen e ec on associa ion i is wo h o s ess ha he goal o his wo k was o check how he lexi- bili y o he molecule in luences he associa ion. In his case a subs i uen e ec is no obse ed di ec ly (associa ion cons an s) bu s ill can be seen in he shape o he i a ion cu e, i s in lec ion poin posi ion and CIS alues. VT measu emen s o complexes The VT 1H NMR expe imen s we e conduc ed o ha e a deepe insigh in o he na u e o he o ame ic equilib ium wi hin he complexes. The ollowing sal s we e chosen (a a ious [1]:[benzoa e] a ios [1] = 18.9 mmol dm−3): a) unsubs i u ed 5 (R = H), b) ca ying elec on dono 2 (R = NMe2) and c) ca ying elec on accep o 8 (R = CF3) and 10. The sal 9 was no aken in o accoun because, mos p obably, i o ms wo ypes o complexes as discussed ea lie . Fo sal s 2, 5 and 8 he ollowing [1]:[benzoa e] a ios we e used: 1:0.5, 1:1 and 1:2, while o [1]:[10] 1:0.1, 1:0.5 and 1:1. The empe a u e ange was −40 o +25 °C o benzoa es and −70 o +20 °C o he naph hy idine de i a i e. The benzoa e case The 1:1 [1]:[5] VT 1H NMR da a shows he linea δ [ppm] = (T) cha ac e in case o pa ial da a ( he da a de ia e om linea i y a highe empe a u es as be o e o 1 – Suppo ing In o ma ion File 1, Figu e S2). In Table 2 he co e- la ion coe icien s, linea i y ange (l. . in °), slope (a) and in e - cep (b) o he i ed linea unc ions a e collec ed. Figu e 7 shows he signal labeling. The signals we e assigned as ollows: a) 1d by in eg a ion ( he mos popula ed o m – see compu a ional pa in Suppo ing In o ma ion File 1), b) 1c by signal shape ( he mos b oad due o as “anion shi ” be ween wo o ms o 1c∙∙∙5 wi hin DDD/AA pa e n), c) 1e emaining signal. Some signals a e obse ed a empe a u es highe han −40 °C and wi h a ious a ios (Figu es S17–S25). The com- pa ison o he VT 1H NMR spec a wi h anions ca ying a ious subs i uen s is p esen ed in Suppo ing In o ma ion File 1, Figu e S26a–c. The spec um in Figu e 7 shows ha a leas h ee o ms o 1∙∙∙5 can be obse ed in solu ion a low empe a u es. The signals sugges hese o ms a e symme ic bu i is impo an o keep in mind ha some o a ional equilib ium may s ill be p esen . In 1d associa ed o 5 signals o H1/H7 p o ons a e much sha pe showing a iple a 8.45 ppm. The signal o H1/H7 in asso- cia ed 1e o m a 8.21 ppm lies in he simila egion as in 1 (dime o polyme ) a he same empe a u e and has simila Beils ein J. O g. Chem. 2015, 11, 2105–2116. 2111 Table 2: The linea i ing o he δ [ppm] = (T) unc ion o benzoa es. Fo m(s) l. .a [°C] a (slope) b (in e cep ) Rbδ ch.c 1c low ield −40 – +15 −0.0146 11.97 0.999 down 1e −40 – −5d−0.0130 11.61 0.996 —e 1d −40 – −5 −0.0016 11.06 0.996 up 1c high ield −40 – +5 −0.0118 9.52 0.999 down 1e −40 – −20 −0.0018 8.14 0.995 — 1d −40 – +10 −0.0027 8.34 0.995 up aLinea i y ange, bco ela ion coe icien , cchanges o he chemical shi a e eaching l. . limi , dsignal no seen abo e −5 °C, esignal disappea a highe empe a u es, a empe a u e −5 °C he chemical shi suddenly inc eases (Figu e 8) ha was in e p e ed as 1d+5→o he o m∙∙∙5 isome iza- ion and a e aging o peaks. The simila is ealized o H1/H7 (high ield signal o 1d). Figu e 7: The signal labelling o [1]:[5] in 1:1 a io exempli ied on he spec a eco ded a −40 °C (CDCl3) and s uc u e o complexes. shape (b oadened single ending o iple shape), while H3/H5 signals shi om 9.13 (1) o 12.04 ppm due o in e ac ion wi h an anion. The b oades NH signals come om 1c∙∙∙5 complex ha , mos p obably, exis in a as equilib ium as shown in Figu e 7. In he alipha ic pa o he spec um, excep he me h- ylene o +N(n-Bu)4 ca ion (a 3.09 ppm, ou o he g een box, Figu e 7), wo cha ac e is ic signals a e isible. I is wo h poin ing ou ha hei assignmen based on in eg a ion is in pe ec ag eemen wi h he da a ob ained by in eg a ion o NH p o ons in he same spec um. The a io o 3.597/3.195 = 1.126 while he espec i e sum o NH (H3/H5) in eg a ion in 1c and in eg a ion o NH in 1e di ided by in eg a ion o NH in 1d (H3/H5) is 1.411/1.255 = 1.124. Figu e 8 shows a apid change o he chemical shi obse ed a empe a u es ca. 0 o +10 °C o 1d∙∙∙5. This is caused by as equilib ium be ween o ms in solu ion. In his case he signal is a e aged and since i was shown in Figu e 7 ha o he han 1d∙∙∙5 o ms a e ep esen ed by highe NH chemical shi s he men ioned signal a e aging causes inc ease o he chemical shi and signal b oadening a highe empe a u e (Suppo ing In o ma ion File 1, Figu e S21, o example). This may be especially ue i one ealize ha NH g oups a e in ol ed in in amolecula hyd ogen bonding a e dissocia ion o he complex a highe empe a u es. Mo eo e , he p oposed o ms ha a e p esen a low empe a u es we e Beils ein J. O g. Chem. 2015, 11, 2105–2116. 2112 Figu e 8: The a iable empe a u e (+20 o −40 °C, CDCl3) dependence o he main signals ( he highes in eg al alues) in 1d∙∙∙5 complex. calcula ed and hei ene ge ic ela ions a e in ag eemen wi h obse ed da a (see da a in Suppo ing In o ma ion File 1 and Figu e S34 o he ene gy diag am). The naph hy idine case The signal obse a ion di icul ies (disappea ance o NH peaks) in 1∙∙∙10 we e o e come by obse a ion o a oma ic double s o 10. The lack o NH signals in he spec um may be caused by: a) as equilib ium o b) he p o on ans e be ween 1 and naph- hy idine dianion 10. While he p o on ans e is mo e p ob- able a highe empe a u es and lowe ing he empe a u e causes he inc ease o he popula ion o o m wi h in amolecula hyd ogen bond we op o a gumen "a". Figu e 9 shows he changes in he spec a upon cooling ( op spec um ep esen s +20 °C, spec a we e eco ded in 5° s eps; he las spec um a bo om ep esen s a empe a u e o −70 °C). The coalescence empe a u e o complex 1∙∙∙10 is −15 °C. A empe a u es below −25 °C he equilib ium is slow enough o obse e an asymme ic complex. I is impo an o keep in mind ha hese spec a a e eco ded o a 1:0.1 mola a io o 1 and 10 o be su e ha mos o 10 is associa ed. F om VT 1H NMR measu emen s he Gibb's ee ene gy equal o 66.6 kJ/mol was ob ained (Ey ing equa ion). Two o ms in 1∙∙∙10 complexes a e, mos p obably 1a∙∙∙10 (high empe a u e) and 1c∙∙∙10 (low empe a u e). Be ween hese wo o a ional ansi ion s a es exis and one o m ep esen ed by a local ene gy minium (Figu e 10). These a e ansi ion s a es ela ed o a) con o ma ional change o 1 and b) shi o he anion 10 along he hyd ogen bonding pa e n o minimize he elec on epulsion. Along he same con- Figu e 9: The VT (+20 o −70 °C, CDCl3) 1H NMR s acked spec a (low emp. a bo om) o 1∙∙∙10 in 1:0.1 mola a io (on he le spec- um a small sa elli e signal om esidual chlo o o m is seen). o ma ional pa h ano he o m o 1c∙∙∙10 ( o m 1) is p esen . This o m is, howe e , less s able han 1c∙∙∙10 ( o m 2) due o elec onic epulsion be ween basic cen e s in he complex. Fo de ailed discussion ha suppo s expe imen al indings e e o he compu a ions sec ion in Suppo ing In o ma ion File 1. Mass spec ome y To ha e a mo e comple e iew o p ope ies o 1 we applied he combina ion o so ioniza ion echnique elec osp ay ioniza- ion (ESI) wi h high esolu ion mass spec ome y (HRMS) o he s udy o he associa ion p ocesses o 1 wi h benzoa e 5 and