Ind ian Journal of Chemistry Vo l. 42A. September 2003 , pp. 2300-2306

Supramolecular encapsulation of anions in a copper(l) complex

D Saravanabharathi & A G Samuelson*

Department uf Inorganic and Physical Chemistry. Indian Institute of Sc ience, Bangalore 560 012, Ind ia

Received 9 Decelllber 2002

Mi xed li gand copper(I ) complexes of I, lO-phenanthroline (phen) and J ,2-bis(diphenyl)phosphinoethane (dppe) have been prepared with different counter ions [CI04- (1), BF4- (2). SO/- (3), NO]- (4)]. Based on the IH and\J p NM R spectra of these complexes, it is seen that the solution equilibria of the sulfate complex is different from the rest and involves li gand dis oc iation. Competiti on experiments show that the nitra te ion is prefelTed by the cat ion in soluti on. Complex 4 has I H NMR features that are different from those observed in complexes 1 and 2. A Cambridge Structural Database study has h,:cll taken up to show the uniqueness o f phenamhroline in interacting with ani ons, especia lly nitrate ions th rough a prcferential (phen)C2-0(nitrate) illleraction .

Introduction Transition metal complexes of N donor heterocyclic li gands are important due to their interesting photophysical properties'·4, relevance in biological systemsS and more recently in supra molecular chemistr/'. Chelating ligands such as bipyridine and phenanthroline are typical examples of this class of

-donor li gands extensively used in the synthesis of transiti o n metal complexes. In the context of copper(l) chemistry, 2,9 disubstituted phenanthroline offers enhanced ~ tability for CuO) complexes by stabi li z ing the tetrahedral geometry over the square pyramidal or distorted octahedral geometry required by copper(Il ) complexes Ie.d. Since copperO) is a lso stabilised by phosphines, mixed donor complexes with phosphines and heterocyclic N donors are anractive in various ways, including enhanced stabi l i ty and novel photophysical properties. Thus Cu(l ) phosphine complexes (with phenanthroline or bipyridine ligands) have been explored for their use as photocatal ysts6

.8

.

The work reported here deals with [Cu(dppe)(phenanthroline)t complexes that differ by the identity of the counter ions [C104 - (1) ... BF4 - (2), SO~2- (3), NOJ- (4)], in their composition. The

struc ture of the analogous PF6 - complex of the present seri es was crystaJlographical\y characterized by Kitagawa ef al. 9 They have show'1 that both dppe and phenanthroline are chelating the copper ion, whereas the counter anion is non-coordinating (I). As the fCu(dppe) (phenanthroline)t series of complexes possess a coordinativey saturated Cu(I) center, the influence of non-coordinating counter anions in the soluti ons are expected to exert, at best, weak

interactions with the cation. The stability of the nitrate ion complex in solution, is greater than that of other anions in thi s series and suggestive of a supramolecular encapsulation of the ion by the coordinated ligand . To corroborate the results frolll Oltr NMR studi es, we have can'ied out a database analysis of the available structures from the CSD.

Materials and Methods All the reactions were can'ied out under an

atmosphere of purified nitrogen, and lhe solvent dichloromethane was dried with PcOs and d istill ed prior to the reaction. Die thyl ether and petroleum ether were distill ed before use. Phenan th rol ine was obtained from Loba-Chemie, dppe was o btained from Aldrich, USA and used as received.

Spectral measurements 'H NMR spectra were recorded on a Bruker ACF

200 MHz spectrometer with tetramethylsilanc as the internal reference. II p {'H} NMR spectra were reco rded on a Bruker AMX 400 MH z spectr ometer operating at J 62 MHz. H3P04 was used as external reference. Ff-IR spectra were recorded in the solid state di spersed in KBr pellets on a Bruker

+

(I)

SARA V ANAB HARATHI ef al.: SUPRAMOLECULAR ENCAPSULATION IN C:u(I) COMPLEX no I

(EQU INOX 55) spectrometer. The MALDI mass spectra were recorded on a Kratos PCKompact

spectrometer using a a-cyanocinnamic acid matrix prepared from a methanol solution of the complexes.

Preparation oJ tli e complexes {Cu(C26H2.JP2)(CI2H8N2)}CI04 (1) and { Cur C26HnP2)(C,zH8N2 )}BF4 (2)

About 0 .04g of [Cu(CH3CN)4]CI04 (0.12 mmol) was reacted with 0.05g (0.12 mmol) of dppe in 10 ml of dichloromethane . After the dissolution of the Cu(I) source, 0.02 g of 1, I 0 phenanthroline (monohydrate) (0 .1 0 mmol) was added to the same solution. Addition of phenanthroline immediately resulted in a brilliant yellow orange colouration to the reaction mixture. After stirring further for 1hr, the solvent was removed in vacuum which yielded an oily residue. Addition of diethyl ether to the residue (lnd subsequent washing wi th the same solvent resulted in the formaiion of complex I as a yellow orange free !lowing powder in a ncar quantitative yield. MS (MALOn: lIliz. 641.7 (100%) [Cu(C?6H2.J P2)(CI2HsN2)t, Calcd. 642 .5. IH 1 lMR (COCl ., , 293 K, ppm) 0= 8.74 (d. 2H); 8.64 (d , 2H); g. ll (s, 2H); 7.95 (t, 2 H); 7 .37 (b, 20H); 2.74 (t,

4H). ' 1 P NM R (C OCl.1, 293 K, ppm) 8= -5.2l. Similar procedure excepting the use of

[Cu(CH,CN)4JBF4 resulted in the formation of yeilow orange free flow ing powder of complex 2 in a near quantitative yield. MS (MALDI): 1/1/1. 641.8 (100%) [CU(C26Hc~P2)(CI ]HRN2W, Calcd. 642.5. IH NMR

(C OCI, , 293 K, ppm) 0= 8.74 (d. 2H); 8.64 (d, 2H); 8.11 (~, 2H); 7.95 (t, 2H) ; 7 .37 (b. 20H); 2.73 (t, 4H ).

(CU(C?r,HNP2)(CI2H.~2)hSO.J(3) (fnri

{CII(C26H].JP:)(C'2HsN2)}N0 3 (4) About 0.015g of CuSO" was dissolved in the I: I

mixture of water and acetonitrile ( 10 ml) and stirred for 2 h, with excess o f Cu powder. After the reduction (as indicated by the disappearance of the blue colour), the reaction mixture was filtered under N2 to get a colorless solution of CuO) source. To this, 0.05 g of dppe and 0.024 g of LIO phenanthroline (monohydrate) were subsequently added . The res ulting yellow orange solution was further stirred for 15 min. and concentrated in vacuum. At this stage a minimum amollnt of water (-2ml) was remaining together with the pasty yellow residue, which was extracted in dichloromethane. The dichloromethane was removed in vacuum which yie lded an oily residue. Addition of diethyl ether and subsequent washing to the residue gave complex 3 as yellow

orange free flowing powder in in 85 % yie ld. MS (MALO!): mlz 641.9 (IOOC7n [CU(C26H24 P2)(CI2HsN2Jt, Calcd. 642.5. IH MR (COC I3, 293 K, ppm) 0= 9 . 1- 7.65 (b, 8 H): 7.26 (b. 20 H); 2.69-2.10 (b, 4H) . 31 p MR (COCl .; . 293 K.

ppm) 8= -5.7.(br) About 0.02 g of Cu(N03h was used as the nitrate

source. The cupric nitrate solution dissolved in I () 1111 of acetonitrile was stirred with O. J 5g or copper powder for 2 hr . After the reduction of the cupric nitrate, 0.065g of dppe and 0.032g or 1.lll phenanthroline (monohydrate) were subsequently added to the colorless solution and the resu lting yellow orange solution was further sti rred for IS mill. The solvent was re moved in vac uum wh ich yielded a~ in the case of 3 an o ily residue which was worked lip in a simi lar fashion to give complex 4 as yellow orange free tlowing powder in a near quantitative yield. MS (MALDI): 642.1 111/;' (100) [CU (C16H2" P2)(C I2 HsN2)t, Calcd. 642.5. IH 'MR (COC1}, 293 K. ppm) 0= 8.75 (m. 4H): 8.16 (s . 2H): 7.95 (t, 2H); 7.37 (b, 20H): 2.74 (t. 4H). 'l p NMR (COCh. 293 K. ppm) 8= - 5.26.

' H NMR titration About 0.075 g of complex 1 wa. made lip to 10 III I

in dichlormethane giving a 0.01 M solution (A). Equimolar concentration of tetrabutybl111110nium nitrate was prepared in clichloromethane (0.03 g in I () ml. 0.01 M of titrant) . Solution A (I ml) was t a kl~ n in \0 M different NMR luhes and the titrant was added in increa<;ing amoums starting with 0.\ ml of the titrant. Each tube was sllbjccted to vacuum evaporation of dichloromethane, which resulted in " thick yellow film on the wall of the NMR Tube. CDCI" (0.4 ml) was added before acquisition of the MR spectra.

Results and Discussiotr-Syllthesis and spectroscopic c/taracleri;.atioll

Complexes contmnll1g perchlorate or tetrafluroborate counter anions were synthesized from the direct reaction of the relevant ligands with [Cu(CH]CN)4]CI04 or [Cu(CHlCN).J1 BF~

respectively. However, complexes that contain nitrate or sulphate counter anions were synthesi zed from ill situ reduction of the respective Cu(I1) sa lTS in acetoni tril e (Scheme 1). The products were obtained in pure form by washing repeatedly with diethylethe r. As the complexes 1-4 differ only in the counter anion f rom the known compiex, it is reasonable to expect a

2302 INDIAN J CHEM, SEC A. SEPTEMBER 2003

'Cu(I)' + dppe + 1, 10 Phenanthroline-e--_> (Cu(phenanthroline)(dppe)] X

r Cu powder/CH3CN

CuS04 or CU(N03)2

Scheme 1

similar molecular structure in the present study as well . Indeed , the IR spectra of the complexes suggest no coordinat ion of the anion to copper in the solid state [Solid IR data - Anion , v(cm- I): CI04- , 1093; BF.j- , 1064; SO}-, 1109; N03- , 1370]

The ratio of ligands was estab li shed using IH NMR spectra. The MALDI-mass spectra l analys is supports the mo lecular composition derived fro m the I H NMR studi es .

IH MR of these complexes in CDCb is very informative about their soluti on structure. The integration of the phenanthroline and dppe signals is always I: I, thus confirming the retention of the stoichi o metry em ployed for synthes izing these co mplexes. However the phenanthroline s ignals exhibi t ani on dependent chemical shifts.

Complexes 1 and 2 with CI04- and BF4- counter ani ons respectively, show identical spectral features (Fi g. I a). Noticeably, the signals due to the protons present at the 2, 9 and 4, 7 positions are close to each o ther and resonate as two doublets centered at 8.74 ppm and 8.64 ppm respectively . Whil e the 2,9 signals

show an upfie ld shi ft (~8 frcc-complex 0.5 ppm) with respect to the free ligand, the 4, 7 signals move

downfie ld with a comparable value of ~8. Whereas

the 5, 6 protons, resonate at 8.11 ppm (~8 = 0.3 ppm),

the 3,8 protons show a triplet at 7.95 ppm (~8 = 0.3 ppm). The aromatic region of the dppe ligand displ ays mu ltiplets at 7.36 ppm, whereas the methylenic proto ns (-C H2C Hr of dppe) are magnetically equi val ent and appear as a tripl et due to coupling with P atoms at 2.74 ppm. All of these facts indicate the fo rmat ion of a I: 1 complex si milar to what is known about the PF6 complex.

Interesting ly, the nitrate co mplex, 4, shows a very differen t pro ton MR spectrum. Firstly , virtua l equ iva lence of the 2, 9 and 4 , 7 protons of the phenathrol ine moiety is observed. The sig nals tha t are observed as two doublets, in complexes 1 and 2,

, . , 9.0 8.0 7 0 6.0

1 g'O

, 8.0

, 7.0

, 50

, 4.0

, ,

a

:3 0 20

b

4 0 30 2 0

c

4 .0

Fig. I _ IH NMR spectra of (a) complex 1 (b) complex ~ (c) complex 3. Differences in the 2,9 & 4.7 signG ls are marked.

become a multiplet at 8.75 ppm. However. o the r sig nals (due to protons at 5 , 6 and 3, 8 pos iti o ns) possess comparable chemical shift values to those or complexes 1 and 2. Similarl y the chemi cal \hirt values of the dppe signals are iden ti cal to those or complexes 1 and 2 (Fi g. lb). Cl ea rl y, the nitrate ion

SARA V ANABHARATHI el al. : SUPRAMOLECULAR ENCAPSULATION IN Cu(l) COMPLEX 2303

has interactions with the phenanthroline moiety and has caused significant shifts in the proton resonances.

The S042- contammg complex 3, displays

significantly different IH NMR signals, and shows a drastic broadening compared to the other complexes in this series (Fig. lc). In addition, there are signals at 9.1 ppm, 8.30 ppm, and 7.85 ppm, indicating the presence of free phenanthroline due to dissociation. It is to be noted that the overall integration between the phenanthroline and the dppe signals fits the expected I : 1 ratio on the basis of the stoichiometry used for synthesis. The interaction of the sulfate anion with the cation is presumably by coordination to copper and displacement of the phenanthroline from its coordination sphere.

Thus the IH NMR experiments reveal three different categories of complexes. While the perchlorate and tetrafluroborate containing complexes are si milar to the non-interacting hexafluorophosphate complex, sulphate and nitrate anions are different. The doubly charged sulphate is able to disrupt the coordination sphere, but the nitrate has weak interactions with the phenanthroline (vide infra).

The 31 P NMR of all these complexes displays a si nglet at -5 ppm except in the case of the sulphate anion where broad signals are observed. While the FWHM for complex 3 is 530 Hz, complex 4 exhibits a rel atively sharp signal (FWHM = 360 Hz) . Furthermore, the complex 3 shows minor signals at -10 ppm, indicating the presence of monodentate forms of dppe ligand. The occurrence of a relati vely sharp singlet in the negative region for the other anions is in contrast to the usual observation of a broad peak between 0-10 ppm, a characteristic feature of the chelating dppe fragment. This suggests that the species in soluti on is not the chelated species that is found in the solid state. A bridging dppe or a chelated dppe that is exchanging rapidly on the NMR time scale with a bridging dppe describes the structure better. Since the proton NMR presents a sharp sigri al for the complexes, 1, 2 and 4 we suggest an equilibrium that is rapid on the NMR time scale and one that predomi nantly favours the structure having a bridging dppe.

Relative stabilities of complexes 1-4 NM R titration experiments were performed in

order to compare relative stabilities of these interactions in the systems 1-4 (Fig. 2). It was found that the addi tion of the nitrate anion to the complexes 1, 2 and 3 resu lted in a spectrum indicative of the

"'--____ 9

-'--____ 8

-'-----7

'--____ 6

~----4

~ ______ ~~ __ r~ _____ 3

, , 8.4 8.2 8.0 7.8 7.6

ppm

Fig 2 - Changes at the phenanthroline region on addi ng NO,- to the CI04- complex in CDCl): ( I ) complex 1, (2) 1+ 0.1 equi v. of NO)- , (3) 1+0.2, (4) 1+ 0.3 , (5) 1+ 0.6, (6) 1+ 0.7, (7) 1+0.8. (8) 1 +0.9, (9) 1 + 1 equiv. of NO)-.

formation of 4: Addition of the perchlorate ion to 4 did not result in the formation of 1. In other ca es also, a similar observation was made suggesting the greatest stability for the complex formed by the nitrate anion. The structure where both phenanthroline and dppe are bound to the copper is indeed the most stable one and the nitrate is interacting with the phenanthroline in a non-covalent fashion.

Since the nitrate anion induces thi s change it can be inferred that a solution structure is one where two anions of uni-negative charge are comfortably encapsulated, in a dimeric complex . Nitrate anion bei ng planar in shape and smaller in size (in terms o f thermochemical radii), fits into the cavity more readi ly than the other ani ons in this series. As the solution IR studies suggest the presence of a non­coordinati ng nitrate ion (with a band at 1358 em- I). incl usion should principally result on ly from the non­covalent interactions, (vide infra). In other words, Ihe presence of nitrate could dlive the eq uil ibrium towards a structure in which there is a suitable cavity.

2304 INDIAN J CHEM. SEC A. SEPTEMBER 2003

A plausible explanation for the solution behavior of complexes 1-4

Various models can be invoked to explain the anion dependent proton and phosphorus NMR of solutions of these complexes.

a) Ionic model As the stoichiometry of the ligands suggests, the

li gands present in the complexes offer a tetrahedral coordination for Cu(I) to give a [Cu(N-N)(P-PW coordination unit. In principle one expects no perturbation of the molecular cation, a closed shell species. by the anion. However, from the results presented it is clear that the counter anion has considerable influence on the solution behavior of the [Cu(phenathroline)(dppe)r cation. A purely ionic model is insufficient to explain all the observations .

b) Covalent model The counter anions might form covalent bonds to

the metal centers. For such situations, it is necessary that the anion replace the Cu-N or Cu-P bonds to enter the coordination sphere. Formation of a covalent bond resulting in a five coordinate Cu(l) is highly unlikely. In this case, where multidentate ligands such as bipyridine are present, we assume the probability to be quite small. But, probably due to its high ionic charge, sulfate forms a bond to the metal center and pushes out the phenanthroline (Scheme 2). The IH NMR of the sulfate complex indicates free

+

Scheme 2

+ +

phenanthroline signals in the NMR spectrum, which is not observed in other cases. Formation of free phenanthroline can be readily explained if intrusion o/" the sulfate ion into the coordination sphere o/" copper(J) causes expUlsion of phenanth roline.

c) Supramolecular model The 31p NMR spectrum of complexes 1-3 suggests

the presence of bridging dppe in solution. Although the solid state structure of the analogous complex in this series has a chelating dppe, possibility of having dppe in the bridging mode in solution is quite hi gh due to dissociation in solution. As shown in Scheme 3, Cu(l) complexes possessi ng a chelated dppe. undergo ring opening to form a bridged species. Such dimeric complexes offer a suitable molecular cavity for ion inclusion. This might be particularly important in the case of solvents having a low-dielectric constant lO

.

Kitagwa et at. have recently8 illustrated the formation of such cavities in the case [(Cu­dppeh {hat-(CN)6} ]2+ (hat refers to hexaazatriphenylene hexacarbonitrile) core. In th is system, there are 3 copper atoms, situated in the molecular plane . of the [hat-(CN)6r ion . The three dppe ligands are coordinating to three Cu(l) center. in the chelating mode. Hence the six phenyl rings of the three dppe ligands create concave cavities on each side of the planar [hat-(CN)6r uni t into which two anions are trapped (Fig. 3). Simi larly Pilloni et ({ /. have shown ll that the [Cu2(dptpfhX2]1l (dptpf = bis(diphenylthiophosphoryl)ferrocene) are capable of traping anions without a covalent bond.

Along similar lines, we propose the formation of a bridged structure that would lead to a cavity in which anions are housed. It has recently come to li ght th at the presence of a partial positi ve charge on carbons can result in "anion-1t interactions".12 Hi ghly electronegative groups on an aromatic ring like F or

2+

20 + 2 ~h Ph ~ (>()~ , (

N Ph Ph '\. \ /

N/CU-PJ + 20~

P Ph Ph

phenanthroline n / \

Ph Ph

Scheme 3

o Anion

SARAVANABHARATHI etal.: SUPRAMOLECULAR ENCAPSULATION IN Cu(I) COMPLEX 2305

® Anion Bis(dlphenyfthiophosphory1)ferrocene

Fig. 3-Supramolecular encapsulation of anions in Refs 8 and J I.

heteroatom substitution can cause sufficient depletion of electron density from the ring to aid anion complexation . In these systems, non-coordinating anions can interact with the ligand and influence the solution behavior of the molecular cation to different extents! In fact, a second possibility is along the lines of our earlier proposal based on work we had carried out with dppe complexes of copper(l).13 The arrangement of phenyl rings of the bridging dppe can create hydrophobic environments above and below the CU2P4 plane that are suitable for anion encapsulation through weak C-H'''11: interactions. To probe the probabilities of these two interactions occurring in established structures, a database analysis was carried out with the Cambridge Structural Database (CSD).

Cambridge structural database analysis In order to probe the generality of anion

interactions with phenanthroline complexes, the database analysis included all structures having nitrate and the 1.10-phenanthroline framework. 14 As there is onl y one heteroatom substitution in the ring, the number of complexes having 11: interactions with phenanthroline, as found with triazine l2b is quite small as expected. Of the 122 structures containing a nitrate anion in conjunction with phenanthroline, only e ight of the complexes had one of the nitrate oxygen atoms at an average distance of 3.5 A from the ring carbons fo rming a 11: complex - as suggested by Frontera et al. (See Fig 4) 12". However, innumerable anion-carbon short contacts i.e. anion carbon distances less than the sum of the Van der Waals di stances of C and 0 (re-a:S 3.22 A) were observed. The preference for the anion to res ide near the carbon atom C2, rather than near other carbons, C3 or C4 is very clear. In most of the structurcs. the anion was found closer to carbon atom

[1 ,10lPhenanthroline

Fig .4--Parameters used in CSD analysi s

2 (71 hits) than to any other carbon (38 hits for C3 and 47 hits for C4). The anion approaches the carbon at an acute angle with respect to the centroid X indicating the presence of the well established C-H .. O interaction 15. Ab Initio molecular orbital calculations on phenanthroline provide an explanation for the preference exhibited by the anion for C2. Calculated charges show the presence of positively charged carbons next to the N and positi ve charges on all the H atoms of phenanthroline. On coordination to a positively charged metal ion , the posi tive charge on carbon atoms adjacent to the N atoms are enhanced but other carbons still bear a net negative charge. The utility of the database analysis was borne out by examination of systems with other anions and phenanthroline. Tetrafluoroborate, hexafl uorophos­phate and perchlorate also indicate a large number of short contacts with the C adjacent to the N, although the percentage of short contacts in these structures to C2 was less than that found for the nitrate ion . Surprisingly , only ten structures with sul fate and phenanthroline have been characterized. Among these structures, only fo ur short contac ts were observed between carbons of phenanthroline and sulfate oxygens.

To find out if nitrate has a greater preference for phenanthroline compared to phenyl rings as in PPh1. a search of nitrate ion containing structures wi th phenyl groups were probed for short contacts. Only 134 hit~ (short contacts between nitrate and C of arene ring) were registered in 644 structures (containin g both nitrate and phenyl groups). The reduced rat io of short contacts to number of avail ab le structures emphasizes the enhanced preference of the ani on for phenanthroline in comparison with a pheny l group . Interestingly, the interaction of a nitrate with bipyridine reveals a simil ar trend. A detailed anal ysis of all an ions with bipyridine and phenanthro[ine is underwayl 6.

2306 INDIAN 1 CHEM, SEC A, SEPTEMBER 2003

Conclusions The results suggest supramolecular interaction of

the counter anion with the ligand in solution, which is an important finding. Most studies involving C-H .. 1t

interactions discuss the solid state structural data. The present results suggest that these effects persist in solution. They also point out the flexibility of the Cu(l) center III optimizing the coordination environments and the choice of ligands. The stoichiometry of the ligands, present III the [Cu(dppe)(phenanthrolineW series 'of complexes offers a tetrahedral coordination for the Cu(I) center with a [Cu(N-N)(P-PW coordination unit. However, the solution behavior and spectroscopic features of the e complexes reveal the flexibility in the structure leading to the formation of a bridged dimer in the presence of counter ions. 1 Hand 31 P NMR experiments reveal that in solution, two anions of uni­negative charge interact very differently from a doubly charged anion like sulfate. Apparently, the two nitrate ions can interact with the coordinated phenanthroline more effectively than the sulfate which will not be able to span the two bridged copper(I) centers present. The presence of sulfate leads to disruption of the structure by formation of covalent bonds and dissociation of the phenanthroline unit. The database study is useful in confirming the dearth of sulfate ions interacting with the phenanthroline. It also confirms the preference of the anions for the carbon adjacent to the nitrogen, which is consistent with the chemical shift Ll8 in the proton attached to the same carbon.

Acknowledgement The research described here was supported by

grants from the Council of Scientific and Industrial Research, Govt. of India. We also thank the Chairman, Bioinformatics Center, Indian Institute of Science, Bangalore, India, for access to the CSD. We are thankful to R Ahuja and C S Sivasankar for help with the database analysis and computation res pecti vel y.

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13 Bera J K, Ph. D. Thesis. Indian InstilUte of Science 1998: (b) Saravanabharathi 0, Ph. D. Thesis, Indian Institute of Scielll'<.:. 2002; (c) Saravanabharathi 0, Monika. Venugopalan P & Samuelson A Goo Polyhedron (2002) In press.

14 Cambridge Structural Database, version 5.23. April 2002. Search criteria were the following: (a) Occurrence of the 1, 10-phenanthroline framework and nitrate in the struclUre registered a hit. (b) C-O COIllacts were considered short if the distance between C and 0 was less than 3.22 A. (~) IT

interactions were those with an oxygen perpendicu lar 10 the plane of the aromatic ring at a di stance of 3.5 A.

15 Desiraju G R, J chelll Soc Chelll COIIIIIIII/I. (1990)-15'+. (b) Desiraju G R, Crystal engineering. The desigll (!f org({llic solids, (Elsevier, Amsterdam) 1989: Desi raj u G R. AngclI ' Chelll In/ Ed Engl. 34 (1995)2311.

16 Ahuja R & Samuelson A G, Unpubli shed results.