since the development 
 of the first binol - derived phosphoric acids 
 in 2004 by akiyama and terada , brnsted acid catalysts 
 have emerged as a viable catalyst class for a variety of enantioselective 
 reactions . due to their excellent yields and ee values , these 
 catalysts are able to provide a complementary technique to transition 
 metal catalyzed reactions . for the brnsted acid catalyzed enantioselective 
 reduction of imines , rueping et al . postulated a catalytic cycle that 
 involves the formation of a binary complex as a precatalytic state 
 consisting of catalyst 1b and the imine ( see figure 1a ) . subsequent addition of a reduction agent , e.g. , hantzsch 
 ester , leads to a ternary complex . however , the reasons for the high 
 enantioselectivity induced by these catalysts have remained unclear , 
 as structural investigations on the key reactive intermediates are 
 very challenging . ( a ) catalytic cycle for the transfer hydrogenation , as 
 proposed 
 by rueping et al . ( b ) brnsted acid 
 catalysts 1a  c successfully applied 
 for enantioselective reduction of imines or reductive amination . a dft study by himo on the transfer 
 hydrogenation of imines proposed 
 that the enantioselectivity is dictated by the kinetics of the hydride 
 transfer step in the ternary complex . using 
 a truncated phosphoric acid as a catalyst model , the transition state 
 involving the z - imine was predicted to react faster 
 than the e - imine . could further confirm 
 the z - imine transition state as the reactive pathway 
 by using catalysts 1a and 1b ( see figure 1b ) in their calculations . so far , the only detailed experimental 
 data about the binary complex 
 was reported by macmillan and co - workers , who were able to identify 
 the structure of the e - imine/1a complex 
 by a single crystal x - ray analysis . in 
 2013 , w. tang et al . elucidated the structure of a ternary 1c / imine / iridium complex with the help of two noe signals between methyl 
 groups . additional experimental investigations 
 of solvent and temperature effects on the interaction between catalyst 
 and imine were performed by rueping et al . with a combination of dielectric spectroscopy , quantum chemistry , 
 and h nmr , they showed the important role of close - contact 
 ion - pairs in a transfer hydrogenation reaction . with the help of vibrational 
 circular dichroism , merten et al . investigated the influence of different 
 solvents on the transmission of stereochemical information from the 
 chiral brnsted catalyst anion to a mn our first nmr study about the hydrogen bond properties 
 of achiral imine / diphenylphosphate complexes proposed the coexistence 
 of ion paired and neutral hydrogen bond complexes . however , our recent theoretical and nmr investigations 
 on the hydrogen bond situation in binary complexes containing chiral 
 phosphoric acids revealed the ion paired complexes as the dominating 
 species . a landscape of asymmetric single - well 
 potential surfaces was revealed , composed of ion - paired complexes . however , to the best of our knowledge , no detailed experimental structure 
 analysis of any brnsted acid catalyst / substrate complex in solution 
 has been presented so far . in this paper , detailed structural 
 information on chiral brnsted 
 acid / imine complexes is described on the basis of nmr data and theoretical 
 calculations . a high mobility of the imine inside the substrate / catalyst 
 complex is observed for both e- and z - imine . for each isomer , interchanging the orientations 
 is possible by either rotating the imine by 180 or tilting the 
 imine and thereby switching the oxygen that constitutes the hydrogen 
 bond . for e - imine / catalyst complexes , only tilting 
 is observed . for the z - imine / catalyst complex , which 
 is postulated to be the reactive species , the reduced steric hindrance 
 of the imine enables both exchange pathways , canceling the asymmetry 
 of the binary complex . catalyst 1c was used , because it provided the best combination of solubility , h chemical shift dispersion , and performance , compared to 1a , b ( see figure 1b ) . typical aromatic imines used in synthesis 2a  c and 3a  c were 
 chosen , to provide different electronic properties ( see figure 2).2a  c derived from aniline were selected , 
 because this allowed for a simple n labeling , which enables 
 the unambiguous assignment of -methyl groups and nh protons . in addition , 3a , c derived from anisidine were investigated 
 to show the effect of an inverse relative electron density of the 
 two aromatic substituents . the trifluoromethyl substituents of 2c and 3a enable f nmr investigations , 
 and imine 3c allows for the direct comparison with the 
 single crystal structure by means of theory . toluene , dichloromethane ( cd2cl2 ) , and freonic 
 mixtures of cdcl2f and cdclf2 were tested as solvents . after extensive optimization of 
 sample preparations , nmr parameters , and solvent screening , we were 
 able to improve the line widths tremendously and altogether enable 
 an investigation by means of nmr ( for spectra , see the supporting information ) . due to the better solubility 
 of the imine/1c complexes in cd2cl2 the nmr measurements at 180 and 130 k were 
 done in cd2cl2 and freonic mixtures , respectively 
 ( see the supporting information ) . brnsted 
 acid catalyst 1c and imines with different 
 functional groups . imines 2a  c and 3a  b were investigated by means of nmr 
 and theory . 2a  c/1c and 3a  c/1c , theoretical calculations were 
 performed to identify the energetic minima . all complexes were optimized 
 in the continuum of cd2cl2 ( smd ) at the dft 
 level of theory using grimme s d3 empirical dispersion parameter 
 ( tpss - d3/def2-svp ) . as the experiment was performed at 180 k , the dielectric constant 
 of the continuum medium was raised ( from 8.93 at rt to 16.20 at 180 
 k ) to adapt to the experimental condition . in all cases , only ion 
 pair minima were obtained , which is in full agreement with the dominating 
 species found in our recent nmr studies . vibrational analysis were performed at the tpss - d3/def2-svp level 
 of theory . in all cases , subsequently , single point calculations ( scs - mp2/cbs//tpss - d3/def2-svp ) and additional solvent corrections ( smd ) on the optimized geometry were performed to 
 determine the energy differences between conformational minima . softwares 
 used were gaussian 09 , version d.01 , for geometry optimization and 
 frequency analysis and orca 3.0.3 for single points . four types of energetic minima 
 with different arrangements of the imines were identified in our theoretical 
 calculations . two structures type i e and type ii e containing e - isomers and two 
 structures type i z and type ii z containing z - isomers of the imine ( figure 3a ) are representatively illustrated 
 by e , z-2a/1c . each of these four structures has at least three low energy conformations 
 of the isopropyl groups without affecting the core structure composed 
 of the imine and the binaphtyl backbone . therefore , in figure 3a and in the further discussion , 
 these isopropyl conformations are boltzmann averaged but not depicted . energies 
 are shown in kj mol relative 
 to type i e. the bis(2,4,6-triisopropylphenyl ) 
 groups are shown transparent for the sake of clarity . ( b ) overlay 
 of global minima of investigated e-2a - c/1c and e-3a - c/1ctype i e complexes . for the sake of clarity , the structural analysis suggests that the methyl substituent 
 in 
 the para - position of the imine is situated above the binaphtyl moiety 
 of the catalyst for conformation type i e , which 
 indicates a ch interaction . in contrast to structure type i e , the imine in conformation type ii e is rotated by 180. the aniline part of the imine is now located 
 proximal to the binaphtyl moiety of the catalyst . the conformational 
 analysis of e - isomer complexes suggests a preference 
 ( ca . 4.59.3 kj / mol ) for structure type i e due to the ch stabilization rather than type 
 ii e ( figure 3 ; e-2a/1c ) . this thermodynamic 
 preference is conserved in the series of the investigated e - imine/1c complexes and seems to be independent 
 of functional group induced electronic modulations , which is demonstrated 
 by superimposed global minima in figure 3b . even the inverted electronic properties 
 of the imine substituents in e-3a/1c , e-3b/1c , 
 and e-3b/1c are not sufficient 
 to alter the relative thermodynamic stability between type 
 i e and type ii e. the similar structures 
 of the aromatic imine / catalyst complexes suggest that the electronic 
 properties of the imine have very little influence on the structure 
 of the binary complex . structurally , 
 the two complexes involving z - imines 
 exhibit either an interaction between the aniline part of the imine 
 and the binaphthyl backbone of the catalyst ( type ii z ) or an interaction between the -methyl group and the catalyst 
 backbone ( type i z ) ( see figure 3a ) . here the conformation type ii 
 z is always thermodynamically favored ( 4.99.5 kj / mol ) 
 over type i z in the series of the investigated z - imine/1c complexes . all investigated binary 
 complexes feature a strong hydrogen 
 bond independent of their structural arrangements , which is marked by the critical point ( s( ) = 0 ) around sgn(2) = 0.06 
 to 0.07 in the plot of density  vs reduced density 
 gradient s( ) ( figure 4 : nci analysis of the global minima of complex e , z-2a/1c ) . this is in full agreement 
 with our current experimental results , which showed a downfield shifted 
 proton in the h spectra around 1519 ppm . the strong 
 hydrogen bond is also depicted by the blue surface which exists in 
 all investigated complexes ( figure 5 ; e , z-2a/1c ) . plot of density ( sgn(2) ) vs reduced density 
 gradient ( s( ) ) showing a large area of van 
 der waals interaction and the strong hydrogen bond in the complex type i e and type ii z of e , z-2a/1c . nci analysis of the 2a/1c complexes 
 ( global 
 minima ) showing weak dispersive interactions ( green surfaces ) and 
 an attractive strong hydrogen bridge between imine and catalyst ( blue 
 surfaces ) . ( a ) type i e shows ch interaction 
 between the p - methyl substituent of the imine and 
 binol of the catalyst . ( b ) the imine is rotated 180 in type ii e and shows the aniline moiety lies on top of the 
 binol aromatic surface and the p - methyl group faces 
 outward from the catalyst . ( c ) ) in type i z , the 
 imine is also rotated and exhibits a ch interaction 
 between the -methyl group and binol . ( d ) similar to type ii e , type ii z shows a  
 interaction between the aniline moiety and binol , as well as a ch 
 interaction between the -methyl group with the 2,4,6-triisopropylphenyl 
 group of the catalyst ; for comparison and clarity , a similar orientation 
 as in figure 3b was 
 adopted . additionally , the plot 
 in figure 4 exhibits 
 also a large region of many weak interactions 
 characteristically indicated by the critical points around sgn(2) = 0.03 to 0.03 , which is expected 
 for complexes with many aromatic surfaces . figure 5a illustrates the nci analysis of the conformation type i e in the complex e-2a/1c . the existence of the weak interactions , to which the 
 ch interaction ( red circle ) also belongs , is indicated 
 by the green surface . these numerous van der waals interactions are 
 contributing significantly to the geometry of the complex and conformational 
 distribution . therefore , the application of the dispersion corrected 
 functional ( d3 ) is indispensable in the 
 simulation . respectively , the chemical shift calculation of the protons 
 of the methyl group predicted a typical shielding by the aromatic 
 ring as discussed below . compared to the conformation type 
 i e , the aniline part of the imine in the conformation type ii e is expected to interact with the binaphtyl moiety 
 of the catalyst . accordingly , a weak  interaction 
 is predicted by the nci analysis ( figure 5b ) . moreover , the para - methyl group of the 
 imine in the conformation type ii e faces outward 
 from the catalyst . similar to the situation in e-2a/1c complexes , several characteristic dispersive 
 interactions 
 can also be identified in z-2a/1c complexes ( figure 5c , d ) . while in the conformation type i z the -methyl 
 substituent has a ch interaction with the binaphthyl 
 moiety of the catalyst , the conformation type ii z has a distinctive  interaction ( t - shape ) between 
 the aniline part and the binaphthyl moiety of the catalyst . in this 
 case , we expect that some of the protons of the aniline are shielded 
 by the aromatic ring . furthermore , it is also recognizable that the 
 -methyl substituent in type ii z interacts 
 weakly with the 2,4,6-triisopropylphenyl group of the catalyst ( see 
 the supporting information for a rotated 
 picture ) . therefore , it is expected that the protons of the -methyl 
 in both conformations type ii z and type 
 i z experience similar shielding effects . in all conformations 
 ( type i / ii e / z ) , the nci 
 analysis showed also many weak dispersive interactions between the 
 3,3 substituent of the catalyst and the aniline / ketone part 
 of the substrate . this demonstrates that these interactions , which 
 often serve as a target for modulation to achieve high stereoselectivity 
 in synthetic application , are already preformed and pronounced in 
 the binary complexes . however , the nci analysis as a whole suggests 
 that modifications both of the 3,3 substituent and of the 
 binaphthyl backbone are important targets for catalyst design . at 300 k , the 1:1 mixtures of imine/1c complexes show 
 only one averaged set of h signals 
 for the brnsted acid catalyst , whereas two sets of signals are 
 detected for each imine , corresponding to their e- and z - isomers with e / z ratios ranging from 80:20 to 70:30 ( for details , see the supporting information ) . by lowering the temperature 
 to 180 k , two complete sets of signals , one of an e - imine/1c complex and one of a z - imine/1c complex , are observed . even at 180 k , an exchange between e - imine inside the e - imine/1c complex and e - imine outside the complex takes place . this exchange is slow on the nmr time scale and leads to an additional 
 set of signals for the e - imine . an in depth investigation 
 of the hydrogen bonds revealed for all imines a mainly ion - paired 
 character for both e and z complexes 
 at 180 k and a fixed geometry of the hydrogen bond . using various homo- and heteronuclear 2d spectra , h and c chemical shift assignments of the z- and e - imines as well as a complete chemical 
 shift assignment of the h signals of the imine / binaphthyl 
 core structure were achieved ( for details , see the supporting information ) . noe studies were conducted in order 
 to determine the structure of 
 the complexes present in solution . considering the experimentally 
 detected high percentage of e - imine complexes ( 7080% ) 
 and the computationally calculated preference for structure type i e , the noe between the varying functional groups 
 of imines 2a  c and 3a  b and the binaphthyl backbone of 1c should be most pronounced . selective 1d h , h noesy spectra of the p - methyl group of e-2a were used to identify the 
 predicted noes unambiguously ( see figure 6 ) . selective noe buildup curves of the e-2a/1c complex show deviations 
 from linearity even below a 100 ms mixing time , similar to small proteins . selective 1d h , h noesy spectra with 
 mixing times of 150 ms were further used for the detection of intermolecular 
 noes between the catalyst and the imine , in order to gain noe intensity 
 with as little chemical exchange as possible . for complexes 2b/1c and 3a/1c , the same noe pattern was found 
 by exciting the functional groups of the acetophenone parts of the e - imines with selective noesy spectra ( for spectra , see 
 the supporting information ) . for imines 2b and 3a , h , f hoesy 
 spectra with mixing times of 500 ms were successfully used to identify 
 noes between the p - trifluoromethyl groups and the 
 binaphthyl backbone of the catalyst . in conclusion , for every investigated e - imine/1c complex , the same noe pattern can 
 be observed between the different functional groups of the acetophenone 
 moiety of the imine ( p - me , p - meo , 
 and p - cf3 ) and the binaphtyl backbone 
 of 1c ( for 1d h , h noesy spectra 
 and 2d h , f hoesy spectra of the other complexes , 
 see the supporting information ) . thus , 
 independent of the electronic properties of the imines , all complexes 
 show a strong population of structure type i e. ( a ) calculated 
 structure type i e of e-2a/1c corroborated by 
 inter- ( red arrows ) and intramolecular noes ( blue arrows ) . ( b ) corresponding 
 experimental data from 1d selective noesy spectra at 180 k in cd2cl2 ( see the supporting information for additional data ) . the identification of structure type ii e in complex 2a/1c , which showed an imine orientation 
 similar to the crystal 
 structure , is by far more complicated . according to the calculations , type ii e is less 
 populated and the aniline signals providing the crucial noe information 
 show broad line widths . in addition , the expected noe intensities 
 between the aniline moiety and the binaphtyl backbone of the catalyst 
 are further weakened by rotation . therefore , imines 3a and 3b were used , which enable the measurement of selective 
 1d h , h noesy spectra by exciting the methoxy 
 group of the anisidine moiety . the identified 
 noe pattern reveals a proximity of the anisidine moiety of the imine 
 to the binaphtyl backbone of the catalyst ( see figure 7b ) . ii e are observed simultaneously , a fast 
 exchange between both structures must occur , even at 180 k. in principle , 
 the two structures are related by two potential exchange modes . either 
 the imine rotates by 180 under retention of the hydrogen bond 
 or the imine switches the oxygen that constitutes the hydrogen bond 
 and is thereby tilted inside the complex . due to the extended shape 
 of the e - imine and the steric configuration of the 
 catalyst , the latter is more likely to occur . in comparison to structure type i e , the noe interaction in type ii e takes place on the other naphthyl half of the catalyst backbone 
 ( see figure 7a ) . this 
 experimentally proves that exchange occurs via the tilting mechanism 
 ( an exchange of type i e and type ii e by rotation of the imine would lead to an interaction between both 
 the acetophenone and anisidine moiety with the same naphthyl side 
 of the catalyst ) . for the aniline derived imines 2a  c , even a weak noe between the aniline moiety of the imines 
 and the binaphthyl backbone of 1c was observed with the 
 help of longer mixing times ( 300 ms ) in 2d h , h noesy spectra . this proves the existence of type ii e for all investigated complexes ( see the supporting information for spectra ) . ( a ) tilting 
 of the imine e-3a in relation 
 to the catalyst 1c . the observed 
 noe pattern ( green and purple arrows ) reveals the coexistence of complex 
 structures type i e and type ii e , which are in fast exchange on the nmr time scale at 180 k in cd2cl2 . ( b ) section of a 1d selective noesy of e-3a/1c at 180 k in cd2cl2 at 600 mhz ( excitation on the methoxy group of imine e-3a ) showing intermolecular noes 
 36 corresponding to structure type ii e. 
 ( c ) section of a h , f hoesy spectrum of e-3a/1c at 180 k in cd2cl2 at 600 mhz showing noes 1 and 2 corresponding to structure type i e. in addition to the analysis of the noe pattern , 
 also the proton 
 chemical shift pattern confirms the tilting process ( switching the 
 oxygen that forms the hydrogen bond ) as the only exchange mode for e - imine/1c complexes . the two structures type i e and type ii e exchange fast on 
 the nmr time scale and produce only one set of chemical shifts . within 
 this assignment for any e - imine/1c combination , 
 separated sets of chemical shifts are observed for the two naphthyl 
 parts ( figure 8 , blue 
 and green colored peaks ; for assignment and details , see the supporting information ) . in general , this chemical 
 shift pattern can only be observed if one of the two described exchange 
 processes ( tilting or rotation ) occurs exclusively . for the e complexes , tilting takes place ( see above and figure 7a ) . in cases where 
 both exchange processes ( tilting and rotation ) are active , the complex 
 would be symmetrized and an averaged set of signals for both halves 
 of the binaphtyl backbone would be observed ( see z - imine/1c complexes and figure 10 below ) . h chemical shifts of the protons 
 of the binaphthyl 
 backbone of complexes 2a  c/1c and 3a  b/1c at 180 k in cd2cl2 at 600 mhz . in every investigated 
 sample , the asymmetry of the catalyst is preserved for the e - imine/1c complexes but broken for the z - imine/1c complexes . both the experimental and theoretical chemical shift analyses 
 of 
 the e - imine/1c complexes reveal strong 
 high field shifts of the p - methyl group and the nearby 
 aromatic protons again corroborating type i e ( see figures 5a and 6a for structures and figure 9 for spectra ) . in the structure of e-2a/1c , the p - methyl group is effectively shielded by the naphthyl backbone of 
 the catalyst , while , with hbf4 as the noncoordinating protonating 
 agent , no shielding can occur . this results in a high field shift 
 of the p - methyl group in e-2a/1c compared to e-2a / hbf4 ( exp , 0.37 ppm ; 
 calc , 0.66 ppm ) ( see figure 9 and the supporting information ) . since the p - methyl group in type ii e is not shielded ( see figure 5b ) , the exchange between type i e and type ii e leads to a reduction of the shielding effect caused 
 by the binaphthyl moiety in type i e. in the calculation 
 of the chemical shielding analysis , only type i e is considered , resulting in a larger chemical shift difference ( calc : 0.66 ppm ) . thus , the offset between calculated and experimentally 
 determined  corroborates the coexistence of type i e and type ii e. shielding effects on 
 methyl groups of 2a/1c due to 
 ch interactions between the methyl groups and the extended 
 aromatic systems of the catalyst . free protonated imine was simulated 
 with hbf4 , and the chemical shifts were compared to the 
 imine/1c complexes ( 180 k in cd2cl2 , 600 mhz ) . the calculated energy differences between 
 the two structures type ii z and type i z ranging from 4.9 
 to 9.5 kj / mol are similar to those between type i e and type ii e. again , a preference for type ii z is expected . similar to the e - imine/1c complex , the two structures type ii 
 z and type i z are in principle related 
 by rotation of the imine by 180 without breaking the hydrogen 
 bond , or by tilting the imine inside the complex and thereby switching 
 the oxygen that forms the hydrogen bond . in comparison to type i e and type ii e , only one averaged 
 set of signals can be observed for both naphthyl halves of the catalyst 
 backbone ( see figure 8) . by exciting the -methyl group of z-2a in the 2a/1c complex in a 
 selective 1d h , h noesy spectrum , structures type ii z and type i z were identified 
 simultaneously ( see figure 10b ) . due to the 
 reduced steric hindrance of the z - imine compared 
 to the e - imine , an exchange between the two complex 
 structures type ii z and type i z is possible by rotation , as well as tilting of the imine ( see figure 10a ) . this explains 
 the broken asymmetry of the catalyst in the z - imine/1c complexes ( see figure 8) . with the help of selective 1d h , h noesy spectra , the exact same situation was identified for 
 all investigated z - imine/1c complexes 
 ( see the supporting information for spectra ) . ( a ) 
 noe pattern ( purple arrows ) found for the investigated z-2a/1c complexes . the intramolecular noes could not be assigned to a single structure type ii z or type i z but instead show 
 that an equilibrium of both structures exists in solution ( see the supporting information for additional parameters ) . ( b ) section of a selective 1d h , h noesy excited 
 on the -methyl group ( yellow circle in part a ) of z-2a at 180 k in cd2cl2 at 600 mhz . moreover , the comparison of experimental and theoretical 
 chemical 
 shifts of z-2a/1c fits to the boltzmann weighted average of type ii z and type i z. both structures show a close proximity 
 of the -methyl group to either the 2,4,6-triisopropylphenyl 
 moiety ( type ii z ) or the binaphthyl backbone of 
 the catalyst ( type i z ) , which leads 
 to a strong high field shift for the respective methyl group due to 
 ch interactions ( exp , 0.75 
 ppm ; calc , 0.74 ppm ; see figure 9 ) . by 
 means of nmr and theoretical calculations , we were able to identify 
 the structure of binary complexes , consisting of the chiral phosphoric 
 acid 1c and e-/z - isomers 
 of aromatic imines . all e - imine / catalyst complexes 
 possess two structures , which are in fast exchange on the nmr time 
 scale even at 180 k. due to the extended steric properties of the e - imine , this exchange occurs by tilting the imine inside 
 the complex and thereby switching the oxygen that constitutes the 
 hydrogen bond . additionally , the structure of the z - imine / catalyst complexes was determined for the first 
 time . for all z - imine / catalyst complexes , a similar 
 exchange process between two structures was observed . however , the 
 compact structure of the z - imine enables an additional 
 exchange pathway by rotation , symmetrizing the catalyst . for all investigated 
 imine / r - trip ( 3,3-bis(2,4,6-triisopropylphenyl)-1,1-binaphthyl-2,2-diyl 
 hydrogen phosphate ) complexes , dispersive interactions between substrate 
 and catalyst were identified not only at the 3,3 substituents 
 of the catalyst but at the binaphthyl backbone as well , revealing 
 the importance of modifications at this moiety for catalyst design . the four presented core structures are highly preserved ; i.e. , these 
 core structures were found to be independent of electron density and 
 substituent modulations of the aromatic imines . this invariance of 
 the structures across all imine substrates reflects the strength of 
 this class of catalysts . extended dispersive interactions between 
 catalyst and substrate , which are postulated to provide high stereoselectivities 
 in the transition states , are preformed in the binary complex , while 
 the strong hydrogen bond allows for a high substrate tolerance . for 
 example , for phosphoramidites , it is known that variations in the 
 electron density of the substrate often have to be compensated by 
 changes in the ligand structure . the 
 structural investigations presented here show that in brnsted 
 acid catalysis the strength of the hydrogen bond overrides the variations 
 in the ch and  interactions caused 
 by different functional groups of the substrates . assuming that these 
 structural features of the binary complexes are preserved in the transition 
 states , brnsted acid catalysis seems to combine both high stereoinduction 
 due to dispersive interactions and structural invariance caused by 
 the hydrogen bond .
despite 
 the huge success of enantioselective brnsted acid 
 catalysis , experimental data about structures and activation modes 
 of substrate / catalyst complexes in solution are very rare . here , for 
 the first time , detailed insights into the structures of imine / brnsted 
 acid catalyst complexes are presented on the basis of nmr data and 
 underpinned by theoretical calculations . 
 the chiral brnsted 
 acid catalyst r - trip ( 3,3-bis(2,4,6-triisopropylphenyl)-1,1-binaphthyl-2,2-diyl 
 hydrogen phosphate ) was investigated together with six aromatic imines . 
 for each investigated system , an e - imine / r - trip complex and a z - imine / r - trip complex were observed . 
 each of these complexes consists of 
 two structures , which are in fast exchange on the nmr time scale ; 
 i.e. , overall four structures were found . 
 both identified e - imine / r - trip structures feature a strong 
 hydrogen bond but differ in the orientation of the imine relative 
 to the catalyst . 
 the exchange occurs by tilting the imine inside the 
 complex and thereby switching the oxygen that constitutes the hydrogen 
 bond . 
 a similar situation is observed for all investigated z - imine / r - trip complexes . here 
 , an additional 
 exchange pathway is opened via rotation of the imine . for 
 all investigated 
 imine / r - trip complexes , the four core structures 
 are highly preserved . 
 thus , these core structures are independent 
 of electron density and substituent modulations of the aromatic imines . 
 overall , this study reveals that the absolute structural space of 
 binary imine / trip complexes is large and the variations of the four 
 core structures are small . 
 the high mobility is supposed to promote 
 reactivity , while the preservation of the core structures in conjunction 
 with extensive  and ch interactions 
 leads to high enantioselectivities and tolerance of different substrates .