polymorphism , 
 the phenomenon of a material packing in multiple 
 distinct crystalline forms , is a well - known occurrence in the field 
 of crystallography . in the case of organic compounds
this is particularly 
 well documented : for example , an analysis of 245 polymorph screens 
 carried out at the ssci ( solid state chemical information , west lafayette ) 
 showed that ca .
however , it should be noted that this work was 
 specifically a search for polymorphism and , although showing that 
 it is a widespread phenomenon , the study does not correspond with 
 the overall statistics from the cambridge structural database ( csd ) . 
 in their recent review ,
cruz - cabeza and 
 bernstein present an analysis of conformational polymorphism in organic 
 compounds using the
2011 release ) and found that , of 
 the 351  979 structures in the subset , 2770 corresponded to 
 1297 truly polymorphic organic molecules .
it was also found that increases 
 in the size and conformational flexibility of molecules do not appear 
 to be directly related to whether or not a compound exhibited polymorphism . 

their findings show that polymorphism arises from the presence of 
 multiple competing intermolecular interactions combined with the possible 
 conformations the molecule can adopt : these factors can give rise 
 to a variety of packing systems that may differ only slightly in energy . 

fewer polymorphic systems are reported for coordination complexes : a comparative survey of the csd ( nov .
subset used ) found the ratio of organic 
 to organometallic compounds exhibiting polymorphism to be ca .
one likely reason for this is simply that the phenomenon is not routinely 
 investigated in these complexes : to quote mccrone ,  the number 
 of polymorphic forms for a given compound is proportional to the time 
 and money spent in research on that compound  .
polymorphism is of paramount importance in the pharmaceutical 
 industry where different phases can have vastly different physical 
 properties and so the compounds produced , 
 typically organic in nature , are intensely screened to understand 
 their habits , properties , and preferences .
coordination complexes 
 are often synthesized in a research environment where the aim of the 
 crystallographic study is to characterize the molecular structure : 
 usually there is neither need nor incentive to screen for polymorphs .
a great number of techniques exist to aid the search for different 
 crystal forms , including variation of the solvent system and the use 
 of additives .
nevertheless , it is not 
 unknown for a phase to be obtained and later  disappear  
 following the appearance of a more thermodynamically stable form : 
 seeds of this new form pervade the crystallization environment and 
 prevent the formation of the previously obtained less stable polymorph : 
 this phenomenon of disappearing polymorphs has been described extensively 
 by dunitz and bernstein , who detail the 
 common causes and problems .
examples of such occurrences are taken 
 almost exclusively from the field of organic rather than coordination 
 chemistry , most likely for the reasons mentioned above .
a well - documented 
 case is that of 1,2,3,5-tetra - o - acetyl--d - ribofuranose , for which metastable 
 polymorph a was first prepared in 1947 ; it  disappeared  
 in 1954 following the discovery of the higher - melting polymorph b. 
 polymorph a was then irreproducible across numerous countries until 
 1981 , when scientists in budapest were able to recover it , presumably because their laboratory had remained 
 free from contamination by the more stable b form .
one particularly 
 high - profile case was that of ritonavir which is prescribed as an 
 anti - hiv medication : two years after it was introduced , the formation 
 of a more stable polymorph with lower solubility and bioavailability 
 led to its temporary withdrawal from sale .
one method used for polymorph control is epitaxy , whereby 
 another 
 surface is used to promote the growth of a given phase or orientation 
 of a crystal .
the technique is employed to obtain the correct polarity 
 of gan for electronics and to attach 
 metal  organic frameworks to surfaces , typically using surfaces of a different chemical composition to 
 that of the desired product ; in these examples , sapphire and gold 
 are typically used , respectively .
employed 
 the seeding effect of a powder of the desolvated hydrate phase of 
 [ co(-c5h4cooh)(-c5h4coo ) ] to directly yield otherwise 
 unobtainable single crystals of the nonsolvated phases and , using a single crystal of pimelic acid , 
 ward et al
. showed that the so - called  yn  ( yellow - needle ) 
 metastable phase of 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile 
 can also be obtained by epitaxy , but 
 only on the ( 101 ) faces of the substrate .
rather counterintuitively , 
 it has also been observed that in some cases the stable phase of a 
 compound will bring about growth of the metastable phase during crystal 
 growth , for example , with sulfathiazole and the steroid reported by stoica et al .
polymer heteronuclei may also be used to control polymorphism and the technique has been demonstrated on a 
 variety of organic compounds .
clearly , 
 seeding and epitaxy offer an appealing solution to the challenge of 
 obtaining metastable polymorphs . in most reported examples
it appears 
 that the substrates used differ significantly from the desired phase , 
 but if an isomorphous system of a compound chemically similar to a 
 lost phase were to exist then this would be an obvious candidate . 
 for any such approach to be viable
, the seed phase also needs to be 
 stable to the recrystallization conditions , for example , by being 
 insoluble in the solution used . as an extension of our investigations 
 into the unusual properties 
 of thioether macrocycle complexes
, we have studied complexes 
 of analogous oxathioether macrocycles . while studying [ pdcl2(anes2o ) ] ( anes2o = 1-oxa-4,7-dithiacyclononane ) , 1 , two previously unreported phases , designated -1 and -1 , were discovered : the preparation 
 of the first of these to be revealed ( -1 ) could 
 not be reproduced following discovery of the -1 form and it was therefore eventually designated as a disappearing 
 polymorph . the previously unreported analogous complex [ ptcl2(anes2o ) ] , 2 , was also investigated and 
 a phase suitable for the epitaxial retrieval of the disappearing polymorph 
 -1 was identified .
we report herein these new 
 crystallographic phases and discuss the potential causes of the instability 
 of -1 with reference to structural characteristics 
 and dft calculations .
reagents were used as received from sigma - aldrich , 
 fischer industries , acros organics , alfa aesar , and merck schuchardt . 

nmr spectroscopy was carried out at 400 mhz using bruker av400 and 
 bruker av(iii)400 spectrometers .
ir spectroscopy was carried out using 
 a bruker tensor 27 ftir or thermoscientific nicolet is5 ftir with 
 an id5 atr attachment .
esi mass spectrometry was undertaken using 
 a bruker daltonics microtof electrospray mass spectrometer and maldi 
 mass spectrometry using a bruker ultraflex iii spectrometer equipped 
 with a matrix - assisted laser desorption ionization source and used 
 a dtcb matrix .
dimethylformamide ( 500 ml ) was added to cs2co3 ( 16.9 g , 87.6 mmol ) and heated to 100 c . 
 under vigorous
stirring , 
 solutions of mercaptoethyl ether ( 4.5 ml , 36 mmol ) and dibromoethane 
 ( 3.2 ml , 37 mmol ) in dimethylformamide ( 40 ml , 40 ml ) were added dropwise 
 at a rate of 1.2 ml per hour .
after addition , the solution was allowed 
 to cool and thereafter was stirred for 3 days .
the resulting suspension 
 was filtered and the volume reduced in vacuo to yield a yellow / brown 
 oil that solidified after repressurization .
this oil was dissolved 
 in ch2cl2 ( 100 ml ) and washed with water ( 2 
  50 ml ) , aqueous naoh ( 0.1 m , 4  50 ml ) , and water again 
 ( 4  50 ml ) .
the product was purified by kugelrohr distillation 
 as the first fraction and yielded a colorless oil ( 1.668 g , 10.15 
 mmol , yield 28% ) .
h nmr h / ppm ( 400 mhz ; cd3cn ) : 2.75 ( 4h , t , jhh = 4.0 hz , ch2 ) , 3.00 ( 4h , s , ch2 ) , 3.68 ( 4h , t , jhh = 4.0 hz , ch2 ) .
c nmr c / ppm ( 400 mhz ; cd3cn ) : 34.31 
 ( spc ) , 35.90 ( spc ) , 74.95 ( spc ) .
( chcl3 ) : 3006 ( s ) , 2958 ( s ) , 2919 ( s ) , 2862 ( s ) , 1465 ( w ) , 1410 ( s ) , 1361 
 ( w ) , 1295 ( s ) , 1274 ( s ) , 1255 ( w ) , 1149 ( w ) , 1113 ( s , br ) , 1043 ( w ) , 
 1023 ( w ) , 1007 ( w ) , 911 ( w ) .
1-oxa-4,7-dithiacyclononane 
 ( 150 mg , 0.91 mmol ) in mecn ( 15 ml ) 
 was added to a suspension of pdcl2 ( 142 mg , 80 mmol ) in 
 mecn ( 10 ml ) and stirred for 30 min before filtering .
the solids were 
 washed with mecn ( 10  5 ml ) , the washings combined with the 
 mother liquor and the volume reduced in vacuo ( 10 ml ) .
et2o ( 40 ml ) was added slowly , causing a bright orange powder 
 to precipitate .
this was obtained by filtration and dried in air ( 97.4 
 mg , 0.28 mmol , yield 35% ) .
calcd for c6h12cl2opds2 : c = 21.1 , h = 3.54 , n = 0% . found : c = 20.96 , h 
 = 3.36 , n = 0% .
ir /cm ( atr ) : 2972 ( w ) , 
 2934 ( w ) , 2873 ( w ) , 1489 ( w ) , 1467 ( w ) , 1423 ( w ) , 1414 ( w ) , 1400 ( w ) , 
 1299 ( w ) , 1282 ( w ) , 1227 ( w ) , 1195 ( w ) , 1140 ( w ) , 1122 ( s ) , 1056 ( w ) , 
 1018 ( m ) , 1003 ( m ) , 921 ( m ) , 805 ( w ) , 787 ( w ) .
1-oxa-4,7-dithiacyclononane 
 ( 75 mg , 46 mmol ) in mecn ( 5 ml ) was added 
 to a solution of ptcl2 ( 106 mg , 0.400 mmol ) in mecn ( 20 
 ml ) and the suspension stirred at 60 c for 19 h. upon cooling , 
 a green precipitate formed which was collected by filtration , washed 
 with et2o ( 4  2 ml ) and dried in air ( 124 mg , 0.287 
 mmol , yield 72% ) .
calcd for c6h12cl2opts2 : c = 16.7 , h = 2.81 , n = 0% . found :
ir /cm ( atr ) : 2972 ( w ) , 
 2930 ( w ) , 2873 ( w ) , 1485 ( w ) , 1464 ( w ) , 1422 ( w ) , 1412 ( w ) , 1399 ( w ) , 
 1300 ( w ) , 1281 ( w ) , 1228 ( w ) , 1194 ( w ) , 1141 ( w ) , 1125 ( s ) , 1056 ( w ) , 
 1018 ( m ) , 1001 ( m ) , 916 ( m ) , 803 ( w ) , 787 ( w ) .
saturated solutions of 1 and 2 were prepared 
 by stirring excess compound in 
 meno2 overnight at 35 c before allowing the solution 
 to cool and residual solid to settle .
vapor diffusion recrystallizations 
 were then carried out using the decanted saturated solutions with 
 et2o as the antisolvent .
these were carried out at room 
 temperature ( 1520 c ) to give the  phases and 
 the nonepitaxially obtained -1 ( vide infra ) .
recrystallization 
 at 25 , 30 , or 35 c yielded -1 and -2 from their respective solutions . in all cases ,
these recrystallizations 
 were attempted using either the saturated solution or diluted concentrations 
 ( 75% , 50% , or 25% ) thereof ; crystal growth rates were controlled by 
 varying the number of apertures in the lid of the vessel containing 
 the solution of 1 or 2 , using either standard 
 0.5 dram glass vials or nmr tubes : the temperature appeared to be 
 the only variable that affected which phase was obtained . for 
 the epitaxial growth experiments , a crystal of -2
the crystal was held just above the surface of a saturated 
 solution of 1 in meno2 and a vapor diffusion 
 recrystallization using et2o as the antisolvent was carried 
 out .
crystals obtained were broken off and analyzed using single crystal 
 x - ray diffraction , or dissolved in meno2 for maldi mass 
 spectrometry ( vide infra ) .
the same procedure was also carried out 
 using -1 as the seed , either with saturated solutions 
 of 2 or with solutions which were saturated first with 2 and then with 1 .
all crystallographic experiments were 
 undertaken at 293 k on an agilent supermova system using mirror - monochromated 
 mo k radiation (  = 0.71073  ) detected at an eos 
 ccd area detector .
numerical absorption corrections based on gaussian integration 
 over a multifaceted crystal model were applied .
structure solutions 
 were obtained using shelxs direct methods and refined using shelxl through olex2 .
the relative total energies and 
 cohesive energies ( ces ) of the five phases revealed in this study , 
 as well as those for a hypothetical third phase of 2 ( designated 
 -2 ) which is isomorphous and isostructural with 
 -1 , were calculated using density functional theory 
 ( dft ) with on - the - fly ultrasoft pseudopotentials and the perdew  burke 
ernzerhof 
 ( pbe ) exchange - correlation functional , 
 as implemented in the castep code .
a 
 plane - wave basis set with an energy cutoff of 800 ev was used for 
 the valence electron wave functions .
pack scheme using a ( 3  3  3 ) -centered k - points grid . to account for the dispersion interactions between 
 molecules in the crystal , the semiempirical methods of grimme and tkatchenko - scheffler
the initial atomic coordinates and lattice 
 parameters for the investigated polymorphs were taken from the crystallographic 
 data obtained in this study .
the lattice parameters and atomic positions 
 were subsequently optimized for each polymorph structure . calculated 
 values for selected structural and lattice parameters
diffusion of et2o vapor into a saturated solution of 1 in meno2 at 1520 c yielded clear 
 orange blocky crystals primarily 
 of the phase -1 .
-1 was initially 
 obtained as the minor product of the aforementioned recrystallization , 
 and were separate from the bulk of the crystals which were located 
 lower in the vial .
all further efforts to yield the -1 phase gave either -1 or a second previously 
 unreported phase designated -1 with higher recrystallization 
 temperatures favoring the latter .
crystals of -2 were obtained by diffusion of et2o vapor into a saturated 
 solution of 2 in meno2 at 1520 c , 
 while -2 was obtained by performing these recrystallizations 
 at elevated temperatures of 30 or 35 c .
orange block - shaped 
 crystals of -1 were grown epitaxially using a 
 pale green seed crystal of -2 ( figure 1 ) .
the crystals were readily separated from the 
 seed crystal and were identified ( vide infra ) as being of the same 
 phase as the original crystals ( -1 ) and designated 
 -1-epitaxy to distinguish them from 
 the original crystals of this phase .
crystals appeared to grow from 
 all exposed faces of the seed crystal rather from any one in particular .
( from 
 left to right , top to bottom ) pictures of -1 , 
 -1 , -1 , -2 , and -2 on micromounts , and crystals of -1 grown epitaxially on a seed crystal of -2 stuck to a glass capillary tube viewed with polarized light .
all 
 phases have been observed as growing as rods , plates , and blocks , 
 with the only difference between them being that phases of 1 are deep orange and phases of 2 are pale green in color .
attempts to use the same technique 
 to obtain crystals of the hypothetical 
  phase of 2 using a saturated solution of 2 and crystals of -1 were unsuccessful , 
 resulting in the dissolution of the seed crystal and the recovery 
 of -2 .
the use of a solution which had been sequentially 
 saturated in 2 and 1 yielded the same result .
detailed structural reports of the 
 new complexes and phases discovered during this study , namely , -1 , -1 , -2 , and -2 , along with details of the structure of -1 reported previously are given in supporting information .
relevant structural features 
 of the phases for the discussion are reported below and in all cases z = 1 .
complexes -1 and 
 -2 are isomorphous and isostructural with one 
 another and crystallize in the monoclinic space group p21/n . in these phases
the macrocycle 
 adopts an endodentate conformation with respect to 
 the oxygen donor , yielding an additional apical mo 
 interaction [ pdo = 2.958(4 )  ;
pto 
 = 3.077(4 )  ] to the cis - cl2s2 square planar coordination of the metal centers . the mo 
 distances lie within the sum of the relevant van der waals radii ( 3.15 
  for -1 , 3.27  for -2 ) .
molecules pack into dimers [ for -1 , pdpd = 3.7903(10 )  ; for -2 , ptpt = 3.8237(4 )  ] ( figure 2 ) with the coordination planes parallel to each 
 other and with perpendicular separations of the least - squares planes 
 of 3.625(2 ) and 3.6815(14 )  and shifts of 1.182(2 ) and 1.120(2 ) 
  for -1 and -2 , respectively . 

these dimers form abab alternating sheets in the bc plane , generating a herringbone - like pattern when viewed 
 along the a axis ( figure 3 ) .
view of the dimer formed in the structure of [ ptcl2(anes2o ) ] in -2 .
view of the herringbone packing of [ ptcl2(anes2o ) ] dimers in -2 viewed along the a axis .
intermolecular ptpt interactions 
 are highlighted with dashed bonds , hydrogen atoms omitted for clarity , 
 and displacement ellipsoids shown at 50% probability .
-2 are isostructural . the -1 and -2 polymorphs 
 are also isomorphous and isostructural , forming in monoclinic space 
 group p21/c , and are 
 conformationally distinct from their respective  phases .
the 
 macrocycle in each  polymorph adopts an exodentate conformation with respect to the oxygen donor : the resulting mo 
 distances [ pdo = 3.404(3 )  ; pto 
 = 3.450(2 )  ] indicate the absence of any significant apical 
 mo interaction to supplement the cis - cl2s2 square planar coordination of the metal 
 center . as in the  phases , molecules in the  phases 

form dimers [ for -1 , pdpd 
 = 3.4946(7 )  ; for -2 , ptpt 
 = 3.5294(2 )  ] ( figure 4 ) with coordination 
 planes parallel to one another , but with decreased perpendicular separation 
 and a shift of the least - squares planes compared to the  phase 
 structures [ for -1 , 3.5237(11 ) and 0.417(2 )  ; 
 for -2 , 3.5471(6 ) and 0.4003(11 )  , respectively ] . 

these pack into sheets in the bc plane with the intradimer 
 mm vector aligned approximately parallel to the a axis , affording alternating hydrophilic and hydrophobic 
 layers ( figure 5 ) . view of the dimers formed 
 between molecules of [ pdcl2(anes2o ) ] in -1 .
. view of the layered packing of dimers of [ pdcl2(anes2o ) ] in -1 , showing two layers running 
 perpendicular to the a axis .
intermolecular pdpd 
 interactions are highlighted with dashed bonds , hydrogen atoms are 
 omitted for clarity and displacement ellipsoids shown at 50% probability . 

the phases -2 and -1 are isostructural . in -1 , which 
 forms in monoclinic space group p21/n ,
the exodentate conformer is again observed , 
 the pdo separation 
 of 3.378(3 )  indicating that there is no significant apical 
 mo interaction .
the molecules do not form dimers 
 in the solid state , but each exodentate o donor is 
 directed toward the base of a neighboring molecule .
the resulting 
 intermolecular pdo interactions [ pdo = 3.212(3 )  ] link molecules into chains that run parallel 
 to the b axis ( figure 6 ) and 
 can be designated as c(5 ) following graph set analysis ; a c 
c3 = 165 ; 
 symmetry code i : 0.5  x , 0.5 + y , 1.5  z ] also appears in this 
 formation and both interactions lie within the sum of the relevant 
 van der waals radii ( 3.15 and 2.95  , respectively )
. view of ( a ) the chain formation and ( b ) the unit cell 
 of [ pdcl2(anes2o ) ] in -1 .
intermolecular 
 pdo interactions are highlighted using dashed bonds , 
 hydrogen atoms are omitted for clarity in ( b ) and displacement ellipsoids 
 shown at 50% probability . the 
 molecules pack as similar dimeric units in the  and  
 phases .
in the  phase , these form a herringbone - like array , 
 whereas in the  phases they align and pack into sheets . despite 
 this difference the two modes of packing are very similar , although 
 a packing overlay routine using mercury gives only a 5/15 molecule match using default parameters .
an overlay 
 of the phases viewed along the co - parallel b axes 
 ( figure 7 ) shows their relation to one another . 
 where the overlap of the dimers at site 1 is maximized , half the dimers can be seen as having rotated through 
 90 about the z axis , as defined , when moving 
 from the  to  phase .
this allows the dimers that remain 
 aligned to 1 to move along the y axis and toward 1 .
little 
 displacement is observed along the x and z axes , resulting in denser packing of molecules .
two - cell 
 deep packing overlay of -1 ( magenta ) 
 and -1 ( green ) , optimized for the overlay of dimers 
 at site 1 .
a destructive phase transition was observed upon 
 cutting a crystal 
 of -1 which transformed its lattice to multiple 
 -1 lattices .
although the supramolecular dimeric 
 motif is retained across the two phases , this change in packing is 
 accompanied by a change in the conformation of the molecule from exodentate to endodentate .
there is little 
 evidence to indicate the order in which these events occur , but both 
 the conformational change and rotation of the dimeric arrangements 
 would significantly alter the intermolecular interactions about the 
 molecule : one could potentially lead to the other , or they could occur 
 concomitantly .
dft calculations 
 with dispersion 
 corrections were carried out using castep to obtain total electronic energies and cohesive energies of the 
 investigated polymorphic structures .
the dispersion interactions were 
 added using semiempirical methods of grimme ( g06 ) and tkatchenko - sheffler ( ts ) . 

the dft - g06 and dft - ts energies returned for 1 were comparable 
 to within 0.005% and the difference between the energies of the phases 
 was only marginally greater with dft - g06 .
table 1 shows the dft - ts relative total energies and cohesive energies ( per molecule ) of different phases 
 of polymorphs 1 and 2 .
the values of the 
 total energy are shown relative to the most stable -phases . 

the cohesive energy is defined as the difference between the total 
 energy of the crystalline material at equilibrium ( per molecule ) and 
 the energy of a free molecule in its ground state .
geometry optimization 
 of individual molecules has been performed in a cubic supercell with 
 dimensions of 20  .
partial atomic charges have been determined 
 from the mulliken population analysis and are provided in the supporting information ( table a3 ) .
a very small number of crystals 
 of the previously unreported phase -1 were originally 
 recrystallized in the upper region of a vial , the lower part of which 
 contained a mass of crystals identified as -1 . 

our attempts to cut crystals of -1 induced an 
 incomplete    phase transformation .
the resulting 
 diffraction pattern comprised multiple components , with 90% of reflections 
 indexing to six components all identified as -1 , 4% to a single -1 lattice , and the remaining 
 6% unindexed . in view of this interesting phenomenon ,
it had already been established that the temperature of the laboratory 
 in which the crystals were originally obtained could fluctuate significantly 
 during the day .
the distinctive location of -1 crystals in the upper part of the vial suggested that they grew 
 in the later stages of the experiment following the influx of antisolvent , 
 and on this basis a number of tentative inferences were made .
it is 
 possible that the temperature of the sample varied with the temperature 
 of the surroundings , resulting in the dissolution and subsequent recrystallization 
 of a small amount of material at higher temperatures : in these circumstances , 
 the relative concentrations of complex , solvent , and antisolvent present 
 in the system could be significantly different from those for the 
 recrystallization of -1 . the hypothesis of the 
 temperature and solvent environment playing a key role in the crystallization 
 of these two conformational polymorphs was supported by work carried 
 out by lucas et al . , who showed using temperature - dependent nmr that , 
 when dissolved in dmso , an increase in temperature brings about an 
 increase in fluxionality in the macrocycle of 1 that 
 is not seen when the experiment is carried out in nitrobenzene .
an additional consideration in phase growth 
 is that there could have been an unknown seed crystal present in the 
 recrystallization vial that may have acted as a nucleation site .
consequently , 
 three factors were considered as potentially affecting the phase obtained : 
 the temperature of the recrystallization medium , the relative concentrations 
 of the components , and the presence of seed crystals . to test 
 for location - dependent seeding affects , a possibility discussed 
 by dunitz and bernstein , ambient temperature 
 recrystallizations by vapor diffusion
were undertaken in a number 
 of different laboratories in the same building , including the one 
 in which the original result was obtained : all yielded only the -1 phase . to investigate the effect of temperature on the system , 

similar recrystallizations were carried out at 30 and 35 c , 
 yielding crystals of the new phase -1 , occasionally 
 accompanied by crystals of -1 .
variation of the 
 concentration of the starting solution by dilution of the saturated 
 solution was attempted for all temperatures but there were no systematic 
 effects on the outcome of the recrystallization . noting the similarity 
 between -1 and -1 , we speculated 
 that the presence of a seed crystal of -1 might 
 help to seed the growth of -1 .
recrystallizations 
 were therefore set up which replicated the temperature fluctuations 
 that could have occurred during the original recrystallization after 
 the formation of some -1 , but these attempts yielded 
 only crystals of -1 , -1 , or 
 a mixture of the two phases .
recrystallization of 2 by 
 vapor diffusion at ambient temperature yielded crystals of -2 , while at 30 or 35 c these experiments yielded either 
 -2 or , more commonly , a combination of both -2 and -2 phases .
the solubility of complex 2 in meno2 was 
 found to be significantly lower than that of 1 and , combined 
 with the close phase match of -1 and -2 ( table 2 ) , led us to the conclusion 
 that it should be possible to grow a crystal of -1 directly on a crystal of -2 using epitaxial 
 techniques . by the end of this experiment ,
the pale green seed crystal 
 was surrounded by protrusions of deep orange single crystals ( see 
 figure 1 ) which were readily separated from 
 the seed crystal ( and designated -1-epitaxy to distinguish them from the original -1 crystals ) . 

the experiment also yielded crystals identified as -1 which had grown conventionally on the walls of the vessel . also shown
crystals of -1-epitaxy were characterized 
 by maldi mass spectrometry and single crystal x - ray diffraction to 
 ascertain whether there was any residual contamination by -[ptcl2(anes2o ) ] , either by solid - state diffusion or 
 via partial dissolution of the seed material in the recrystallization 
 solution .
mass spectrometry revealed peaks with isotope patterns corresponding 
 to the pd(ii ) complex , but none from the pt(ii ) complex .
data from 
 one ambient - temperature data collection were refinalized using varying 
 proportions of pd and pt for the absorption correction , and structure 
 solution and refinement of the metal site was in each case carried 
 out .
no reflections were omitted , no application of further extinction 
 correction was carried out and refinement was undertaken using only 
 distance restraints on 1,2-bonded distances and rigid - bond restraints 
 on anisotropic displacement parameters where necessary .
the structures 
 were refined to convergence and the results clearly indicate an absence 
 of pt in the system ( table 3 ) . comparison 
 of the unit cell parameters ( table 2 ) of the 
 epitaxially grown crystals with those of -2 and 
 the original -1 showed a considerably 
 closer correspondence with the latter .
similarly , comparison of intradimer 
 parameters for -1-epitaxy show close 
 matches with those of -1 ( table 4 ) .
although the m  s 
 and m  cl bond lengths do not differ 
 greatly between -1 and -2 , 
 and so do not provide effective discrimination , the intermolecular 
 mm and intramolecular mo separations 
 vary more significantly and therefore provide a clear indicator of 
 the metal present in the crystal .
mm and mo 
 distances in -1-epitaxy are identical 
 to those in -1 to within 1 s.u . but differ greatly 
 from those in -2 ( table 5 ) .
we therefore conclude that -1-epitaxy represents a successful recovery of the previously  lost  

-1 phase , whereby the isomorphous and isostructural 
 nature of -2 provides a sufficiently complementary 
 interaction set to favor nucleation of the phase on its surface .
the values for -1-epitaxy are generally much 
 closer to those of -1 than of -2 .
the relative stabilities of polymorphs 
 can be inferred from their crystal densities . for molecular crystals ,
it is generally the case that the densest 
 phase will be the most thermodynamically stable , and the densities 
 of the phases can be readily calculated from crystallographic data 
 ( table 6 ) . from these values , it can be inferred 
 that the  phases are significantly more thermodynamically stable 
 than the  phases , and -1 marginally more 
 stable than -1 .
the densities calculated from 
 the dft - ts optimized structures ( table 6 ) are 
 in a good agreement with those derived from the experimentally determined 
 structures and corroborate the same trend .
comparison of the total energies calculated for each 
 phase ( table 1 ) confirms that the endodentate  phases are significantly more stable than the exodentate phases .
it was found that the energy per molecule of -1 is 7.43 kj mol higher than that of -1 , and for -1 it further increased by 
 0.58 kj mol in contrast to the order inferred 
 from the crystal densities and the observation that -1 is the preferred exodentate modification 
 of 1 .
the calculated energy difference between -1 and -1 is , however , too small to make 
 any firm conclusions about the relative stability of these two phases 
 owing to the possible errors associated with the approximations adopted 
 in the computational method . given the structural similarities 
 and the energies of the phases , 
 the appearance of a    phase transformation 
 in a crystal of -1 is perhaps unsurprising .
the 
 existence of such a phase transition in the hindered environment of 
 a macroscopic single crystal suggests that the barrier for this transformation 
 is relatively low .
therefore , it is reasonable to suggest that -1 is not seen because it would be favorable for it to relax 
 to -1 in the early stages of crystal growth . as 
 the energies are so similar for -1 and -1 , the observation of -1 seems less surprising . 

it is also worth noting that the exodentate conformation 
 of the macrocycle in -1 gives rise to specific 
 intermolecular opd and c 
interactions that are not observed in -1 and may 
 stabilize the exodentate conformer during growth 
 of -1 via electrostatic attractions given the 
 favorable partial atomic charges ( table a3 of supporting information ) .
this , coupled with there being no 
 evidence of a route for it to convert to the stable  phase , 
 would provide a rationale as to why -1 is observed 
 when -1 is not . in the case of -2 , which was the only exodentate modification 
 of 2 obtained in the 
 study ,
the total energy was found to be 8.59 kj mol higher than -2 . a calculation on a hypothetical 
 phase of 2 , which is isomorphous and isostructural to 
 -1 and is designated -2 , yielded 
 an energy 2.6 kj mol higher than -2 .
the total difference in energy of 11.19 kj mol between -2 and -2 suggests 
 that the latter is not observed as it is less favored thermodynamically .
the cohesive energies ( ces ) were also calculated for these systems 
 ( table 1 ) and these again found the  
 phases to be the most thermodynamically favorable , with ces of 225.29 
 and 226.26 kj mol for -1 and 
 -2 , respectively . for 1 , the exodentate phases were again found to be very similar ( 188.92 
 and 188.34 kj mol for -1 and 
 -1 , respectively ) with the difference in ce between 
 -2 and the hypothetical -2 phases being slightly larger ( 188.92 and 186.41 kj mol , respectively ) .
these data corroborate the conclusions drawn above . in -1 the charge on the pd(ii ) center is calculated 
 as being 0.72 e , whereas in -2 the charge on the 
 pt(ii ) center is predicted to be only 0.36 e ( table a3 of supporting information ) ; the charge on the macrocyclic 
 o donor is found to be the same in both phases ( 0.49 e ) .
a stronger 
 mo interaction would be expected in -1 than in -2 , since a hard o donor would 
 experience greater electrostatic attraction to the more highly charged 
 pd(ii ) center .
given that the metal centers are otherwise shielded 
 from their surroundings by the remaining atoms in the molecule , and 
 the comparatively small differences in the charges of these other 
 atoms between the phases , the chain - forming mo interaction 
 constitutes the principal distinction between the intermolecular interactions 
 in the two phases .
the ces indicate a slightly weaker set of intermolecular 
 interactions in -2 compared with -1 ( 186.41 and 188.34 kj mol , respectively ) . 
 these comparisons lend weight to the inference that the chain - forming 
 interaction between the metal center and macrocyclic o donor of the 
  phase would be weaker for 2 than it is for 1 .
given the likely role of the mo interaction 
 in the formation of the  phase , its relative weakness in 2 provides further rationalization for the absence of the 
 -2 phase .
in 
 two new phases of [ pdcl2(anes2o ) ] ( 1 ) , designated -1 and -1 in order of their discovery , the anes2o macrocycle 
 adopts exodentate conformations , in contrast to the endodentate conformation found in the previously reported 
 -1 polymorph .
we also report two phases of the 
 previously unreported complex [ ptcl2(anes2o ) ] ( 2 ) , designated -2 and -2 , which are isomorphous and isostructural with the correspondingly 
 designated phases of 1 . the inability to reproduce the 
 crystallization of -1 by standard methods led 
 us to the conclusion that -1 is a  disappearing 
 polymorph  , with -1 being the preferred exodentate form of 1 .
in contrast , -2 is the only phase of 2 obtained in this investigation 
 which exhibits an exodentate conformation for the 
 macrocycle .
no polymorph of 2 corresponding to -1 has been isolated thus far despite attempts to seed its 
 growth using a crystal of the latter phase . a crystal of -2 was successfully used to seed 
 the epitaxial growth of the disappearing polymorph -1 , and the resulting crystals were shown by maldi mass spectrometry 
 and single crystal x - ray diffraction to be free from any significant 
 content of 2 .
dft analysis reveals the order of 
 phase stability to be  
 >    for 1 and  > 
  
 >  for 2 ( where the -2 phase 
 is hypothetical and based on the structure of -1 ) . there is a greater difference between the energies for polymorphs 
 of 2 than for polymorphs of 1
however , 
 from these results alone , the polymorphic behavior observed for 1 can not be fully understood .
-1 exhibits 
 a partial    phase transformation brought about 
 by the mechanical pressure of cutting the crystal , whereas no such 
 phase transition on cutting is observed for -2 .
this suggests that the  phase lies in a potential energy 
 well close to that of the stable endodentate  
 phase and that the energy barrier between them is particularly low 
 in the case of 1 .
the -1 form is , 
 therefore , not recoverable by conventional recrystallization as the 
    transformation is facile in the early stages 
 of crystal growth . in -1 , not only do intermolecular 
 pdo and c  hcl interactions 
 give rise to chains of molecules , they may also stabilize the exodentate conformer of the macrocycle during crystal growth ; 
 these interactions appear weaker in -2 and so 
 would not stabilize growth of this phase to the same extent .
 recrystallization of [ pdcl2([9]anes2o ) ] ( [ 9]anes2o = 1-oxa-4,7-dithiacyclononane ) , 1 , and [ ptcl2([9]anes2o ) ] , 2 , by diffusion of et2o vapor into solutions of the complexes 
 in meno2 yielded three phases of 1 and two 
 phases of 2 . the known phase of 1 , herein 
 designated -1 , was obtained under ambient conditions . 
 
 a second phase , 
 designated -1 , was initially also obtained by 
 this method ; however , following the advent of a third phase , -1 , all subsequent efforts over a period of a year to crystallize 
 -1 yielded either -1 , obtained 
 by carrying out the recrystallization at elevated temperature , or 
 -1 , commonly found throughout the study . 
 this 
 persistent absence of a phase which could initially be crystallized 
 with ease led us to the conclusion that -1 was 
 an example of a  disappearing polymorph  . the first 
 phase obtained of 2 , designated -2 , was obtained by recrystallization under ambient conditions and 
 is isomorphous and isostructural with -1 . the 
 second phase -2 was obtained by slight elevation 
 of the recrystallization temperature and was found to be isomorphous 
 and isostructural with -1 . 
 subsequently , -2 was used to seed the growth of the disappearing polymorph 
 -1 . 
 no third phase of 2 ( -2 ) has been isolated thus far . 
Introduction
Experimental
Section
Results and Discussion
Conclusions