Block for the Construction of Hydrogen Bonded Networks

Block for the Construction of Hydrogen Bonded Networks

GiannisS.Papaefstathiou^a, RobbyKeuleers^b, ConstantinosJ.Milios^a, Catherine P.Raptopoulou^c, ArisTerzis^c, HermanO.Desseyn^b, andSpyrosP.Perlepes^a

aDepartmentofChemistry, UniversityofPatras,26504Patras, Greece

bLaboratoriumAnorganischeScheikunde, RijksuniversitairCentrumAntwerpen, Groenenborgerlaan 171,2020Antwerpen, Belgium

cInstitute ofMaterialsScience, NCSR“Demokritos”,15310Aghia ParaskeviAttikis, Greece Reprint requests toProf.H.O.Desseyn, Fax:+32(0)3 2180233

or toProf.S.P.Perlepes,e-mail:perlepes@patreas.upatras.gr Z.Naturforsch.58b,74–84 (2003); receivedSeptember9,2002

The ligandN,N'-dimethylurea(DMU) is usedto propagatethe octahedralcoordination geom- etryof[Co(DMU)6]²⁺into 1D and2D assembliesviaa combination ofcoordinativebondsand interionichydrogen-bonding.Compounds[Co(DMU)6](ClO4)2(1), [Co(DMU)6](BF4)2(2)and [Co(DMU)6](NO3)2(3) havebeen prepared fromthereactionsofDMU andtheappropriate hydratedcobalt(II)saltsin EtOH, MeCNorMe2CO(onlyfor1) inthe presence of2,2-di- methoxypropane.Crystalstructure determinationsdemonstratethe existence of[Co(DMU)6]²⁺

cationsandClO4 , BF4 orNO3 counterions.The great stabilityofthe[Co(DMU)6]²⁺cation inthesolidstate isattributedtoapseudochelate effect whicharisesfromthe existence ofstrong intracationic N-H O(DMU) hydrogenbonds.The[Co(DMU)6]²⁺cationsandcounterions self- assembleto formahydrogen-bonded 1D architecture in1, and different 2Dhydrogen-bonded networksin2and3.The precise nature oftheresultingsupramolecular structure isinfluenced by the nature ofthecounterion.Two main motifsof intermolecular(interionic) hydrogenbonds havebeen observed: N-H O(ClO4 , NO3 ) orN-H F(BF4 )andweakC-H^:::F(BF4 ) or C-H O(NO3 ) hydrogenbonds.Thecomplexes werealsocharacterizedby vibrationalspec- troscopy(IR,far-IR,low-frequencyRaman).Thespectroscopicdata are discussed intermsof the nature ofbondingandthe knownstructures.

Key words:Cobalt(II)/N,N'-Dimethylurea Complexes, Hydrogen-BondedCoordination Complexes, VibrationalSpectroscopy

Introduction

Recent efforts in the field of crystal engineer- ing have focused on the predictable assembly of organic molecular solids via intermolecular inter- actions [1 - 5] and ithasbecome clear thatcertain functional groups,e. g.carboxylic acids, amidesand ureas, are reliable, robust connectors for the for- mation of hydrogen-bonded organic networks [1, 2, 6- 9]. However, little work has been done in constructingassembliesofcoordinationcompounds viadirectional intermolecularinteractions[10- 18].

From a crystal engineering standpoint, the advan- tage ofusing transition metalsis that theshape of the mainbuildingunitcanbecontrolledby usinga metal-ligandsystemthatisknownto exhibita cer- taincoordination geometry[11].Aspecificgeom- etrycanthenbe propagatedthroughout thecrystal

0932–0776/03/0100–0074 $06.00 c 2003Verlag derZeitschriftf¨urNaturforschung, Tubingen¨ http://znaturforsch.com

structure by attaching substituents to the ligands;

thesesubstituentsactasintermolecularconnectors.

Ordered assemblies of transition metalcomplexes (coordination polymers), i. e. supramolecular sys- tems involving coordinative connectivities, attract intense research due to their potential as useful porous, conductiveand magneticmolecularmateri- als[19 -22].

We have recently [23, 24] embarked on a pro- grammewhich hasasashort-term goalthecreation of novel supramolecular structures based on hy- drogenbonding interactionsbetweensimplemetal complexes. Thisproject canberegardedasan ex- tension of our work intheareaofcoordination poly- mers[25 -28].Remarkableanalogiescanbe drawn [29] between hydrogen-bonded networksandcoor- dination polymers.Forhydrogen-bonded networks, the donor (i. e., a protic hydrogen atom) and the

G.S.Papaefstathiouetal.·HydrogenBondedCobalt(II)ComplexesofN,N'-Dimethylurea 75

Fig. 1.Asmall part(whichcontainsonly two molecules) ofthe -networkcommonlyobserved in symmetrically disubstituted ureas (left) and the chemical formula of N,N'-dimethylurea, abbreviatedasDMU.

acceptor (i.e., a region of hight electron density) canbe compared with metal ions and ligands, re- spectively.Furthermore, as notedbyEtter [30],in cases where there are multiple hydrogen-bonding sites,there isaratherhigh degree of predictability concerningwhich donorsandacceptors willbe en- gaged.Therefore,the “nodeandspacer”approach canbe employed equally wellwith hydrogenbonds as withcoordinativebonds[29].Ourlong-term goal is to combine coordination polymers and ligand- based hydrogenbonds tocreate novelsupramolecu- lararchitectures.Suchanapproach has someadvan- tages, asitcombines thestrength ofthecoordination networkandthe flexibilityimpartedby thesofthy- drogen bond interactions. Available strategies for theachievementofthisgoal havebeenrecently re- viewed[13].

Ureas have been among the central players in organic crystal engineering [1,6, 31 -33]. In par- ticular, symmetrically disubstituted ureas form - networks with eachureamolecule donatingtwo hy- drogen bonds and “chelating” the carbonyl oxy- gen atoms of the next molecule in the network (Fig. 1). This paper describes the full spectro- scopic andstructural characterization ofthe prod- ucts from the reactions between cobalt(II) per- chlorate,tetrafluoroborateand nitratewithN,N'-di- methylurea(DMU, Fig. 1).

ResultsandDiscussion Preliminaryconsiderations

Incontrast tothe greatnumberofstudiesconcern- ing freeureas[1,6,31 -34],little isknownabout the supramolecular architectures based on hydrogen-

bonding interactions between simple metal-urea complexes.Wecurrently study thesupramolecular structuresofsimple metal-DMU complexes where, in principle, the oxygen atom ofDMU can coor- dinatetothe metal ionand/or provideahydrogen bonding acceptor site. We hope that by reacting metal ions withaligandthat containsbothan effi- cient coordination site andtwo hydrogen bonding functionalities, assemblycan be dictatedby inter- molecular hydrogen bonding interactions. At the outsetof ourefforts wewould liketoreducethe pos- sibilityofcoordination polymerformationthrough anionicligands;for this reason, we employ metal sourcescontainingweaklycoordinatinganions such as ClO4 , BF4 , NO3 etc.Thus,infinite assem- blies based on ligand-counterion hydrogen bonds are expected, though these are not, strictly speak- ing,ligand-based hydrogen-bondedassemblies.

In a recent paper, we described the prepa- ration and crystal structures of complexes [M(DMU)₆](ClO₄)₂ (M = Mn, Ni, Zn) and [Cu(OClO3)2(DMU)4] [24]. The [M(DMU)6]²⁺

cations and ClO₄ anions self-assemble to form a hydrogen bonded 1D architecture in the man- ganese(II) complex and different 2D hydrogen- bonded networks in nickel(II) and zinc(II) com- plexes; the hydrogenbonding functionalitiesonthe molecules of the copper(II) complex create a 2D structure.In order toalso investigatethe influence of the steric requirements and hydrogen bonding abilityofthecounterion onthesupramolecular as- sembly, in the present study the metal ion (Co^II) is kept constant while the counter ion is varied.

Powders withcompositionsCo(ClO₄)₂6DMU and Co(NO3)2 6 DMU had been reported inthe old literature[35 -37], butneither thecrystalstructures nordetailedspectroscopicdataonthesecompounds wereavailable prior tothis work.

The present workcanbealsoregardedasa con- tinuation of oureffortsinthespectroscopyof free ureas[38,39] and inthestudyoftheircoordination chemistry[40- 43].

Synthetic comments

The preparation ofthethreecomplexes reported inthis work is summarized in eq. (1):

CoX26H2O+6DMU

EtOHorMeCN

T DMP ^![Co(DMU)6]X2+6H2O (1)

X=ClO4 (1), BF4 (2), NO3 (3)

Table 1.Selectedbond lengths(A)˚ andangles( ) for the [Co(DMU)6]²⁺ cationspresentincomplexes1,2and3. TheatomsC(2), C(12)andC(22),whichare notlabelled in Fig. 2, are the carbonyl carbon atoms of the three crystallographicallyindependentDMUmolecules.

1 2 3

Co-O(1) 2.098(3) 2.091(2) 2.092(2) Co-O(11) 2.104(3) 2.091(2) 2.089(2) Co-O(21) 2.111(3) 2.098(2) 2.106(2) C(2)-O(1) 1.272(4) 1.257(2) 1.265(3) C(2)-N(1) 1.330(5) 1.329(3) 1.329(3) C(2)-N(2) 1.324(5) 1.325(3) 1.327(3) C(12)-O(11) 1.256(4) 1.262(3) 1.261(3) C(12)-N(11) 1.329(5) 1.322(3) 1.332(3) C(12)-N(12) 1.340(5) 1.334(3) 1.339(3) C(22)-O(21) 1.264(5) 1.254(3) 1.254(3) C(22)-N(21) 1.338(5) 1.329(3) 1.336(4) C(22)-N(22) 1.334(5) 1.338(3) 1.334(3) O(1)-Co-O(11) 92.6(1) 92.6(1) 92.4(1) O(1)-Co-O(21) 91.5(1) 91.5(1) 88.8(1) O(11)-Co-O(21) 91.2(1) 91.3(1) 88.7(1) C(2)-O(1)-Co 130.9(2) 131.1(1) 131.0(1) C(12)-O(11)-Co 128.7(2) 128.4(1) 126.1(2) C(22)-O(21)-Co 130.3(2) 130.1(1) 131.7(2) The employment of 2,2-dimethoxypropane (DMP, aknown dehydratingagent)underheatingwasnec- essary to eliminatethe possibilityof[Co(H2O)6]²⁺

formation insolution.

Complexes 1 and 2 seem to be the only prod- ucts from theCoX26H2O/DMUreaction systems (X= ClO₄, BF₄). The solvent (EtOH, MeCN and evenMe2CO inthe case of1) andtheDMU:Co^II reaction ratio (8:1, 6:1, 4:1, 3:1) have no in- fluence on the identity of the complexes. The Co(NO3)26H2O/DMUreaction mixturesinEtOH, Me₂CO and MeCN at low DMU:Co^II ratios (2:1, 1:1)behave inadifferentmannerfromthoseathigh ligand:metalratios(8:1,6:1); the former repeatedly gaveamixture ofared oilandapowderofuncertain nature.

Description ofstructures

Selected bond distances and angles for the [Co(DMU)6]²⁺ cations of complexes 1, 2 and 3 are listed in Table 1. An ORTEP plot of the [Co(DMU)₆]²⁺ cations of complexes 1 and 2 is shown in Fig. 2; since the cations of these com- plexesare almostidentical only one plot isgiven.

The structure of [Co(DMU)6]²⁺ present inthe ni- tratecomplex3isdepicted inFig.3.Detailsofthe

Fig. 2. An ORTEP representation of the cation [Co(DMU)6]²⁺presentincomplexes1and2.Openbonds indicate intramolecular(intracationic) hydrogenbonds.

An identical labellingscheme is used foratomsgener- atedby symmetry.Forclarity,mostcarbonatomsare not labelled.

hydrogenbondsof1,2and3are provided inTables 2,3and 4respectively.

Complexes1,2and3crystallize inthe triclinic space group P¯1. Their structures consist of al- mostperfectoctahedral[Co(DMU)6]²⁺cationsand ClO₄ , BF₄ orNO₃ counterions.TheCo^IIatom sitsonan inversioncentreand is surroundedby six O-bonded DMU ligands. Thebond angles around the metal ionare fairlycloseto 90 ,indicating only a slight distortion of the octahedral coordination sphere.Therearesixintramolecular(intracationic) hydrogenbonds withatomsN(1), N(11)andN(21) [and their symmetry equivalents] as donors, and atoms O(21), O(1) and O(11) [and their symme- tryequivalents] asacceptors.Theseareregardedas

“moderate” hydrogenbonds usingJeffrey’s[44] and Steiner’s[45] classifications.Additionally,incom- plexes1and2therearetwo (symmetryequivalent) intracationic C(23)-H O(21) interactions thatcan beregardedas weak hydrogen bonds. Interactions oftheC-H (O, N)typearecorrectly termed hydro- genbondsbecausetheyare,likethe (N, O)-H (N, O) hydrogenbonds,largelyelectrostaticincharac- ter [1].Weak hydrogenbonds withC-Hgroups as donors arecurrently under intensestudy.Formely

G.S.Papaefstathiouetal.·HydrogenBondedCobalt(II)ComplexesofN,N'-Dimethylurea 77

Fig. 3. An ORTEP representation of the cation [Co(DMU)6]²⁺ present in the nitrate complex 3. Open bonds indicate intramolecular (intracationic) hydrogen bonds.An identical labellingscheme is used foratoms generatedby symmetry.Forclarity, allcarbonatomsare notlabelled.

considered “unusual” or “nonconventional”, they are nowdiscussed ratherfrequentlyin mostfields of structural chemistry [1,6, 45 - 47] andbiology [48,49]. Typical C-H O(N) separations occur in the2.2-3.0 ˚Arangeandthe hydrogenbondangle isinthe 100- 180 range [1] ,while dissociation energiesare0.4 - 4Kcal mol¹,withthe majority

<2Kcal mol¹[45].At the lowenergyend ofthe range,theC-H^:::Ohydrogenbond graduallyfades intoavan derWaalsinteraction[45].The high end of the interaction has not yet beenwell explored;

C-H A bonds stronger than 4Kcal mol¹are pre- dictedto occur whenveryacidic C-Hor verybasic acceptorgroupsare involved[45].

TheCo-ODMUdistancesin1-3areslightlylonger comparedwiththose in othercobalt(II)complexes withurea-type ligands[43], becausethecoordinated oxygen atoms of DMU are involved in hydrogen bonds. All DMU molecules in 1 - 3 are coordi- nated ina bentfashion,withCo-O-C angles rang- ing from 126.1(2) to 131.7(2) . This is the usual wayofcoordination ofurea and itsderivatives[50].

Linearly or approximately linearly coordinated ureas have been observed only in few cases [40, 50,51].

Fig. 4. A view of the 1D network formed by hydro- genbondingbetween[Co(DMU)6]²⁺cationsandClO4

counterionsincomplex1; twocationsareshown.Only the intermolecular(interionic) hydrogenbondsareshown.

Asit was statedabove,we haverecentlycharac- terizedcomplexes[M(DMU)6](ClO4)2,whereM= Mn, Ni, Zn [24]. These complexes, together with [Co(DMU)6](ClO4)2(1) described here, constitute an isostructural series.TheaverageM-O_DMUbond lengths change according to the sequence Mn^II (2.168 A) >˚ Co^II (2.104 A) >˚ Ni^II (2.064 A) <˚ Zn^II(2.095A),˚ i. e.,theyfollow thewell-knownIr- ving-Williams series [52]. As the thermodynamic stability of the complexes increases from man- ganese(II) to nickel(II),thecoordinativebondsbe- comestrongerandthebond lengthsdecrease inthe same order.

Complexes 1 - 3 extend to eleven the num- ber of structurally characterized metal com- plexesof DMU.The eight, previously structurally characterized, complexesare [Mn(NO3)2(DMU)3] [40], [MnBr₂(DMU)₃] [51], [Fe(DMU)₆](ClO₄)₃ [53], [Er(DMU)₆(H₂O)](ClO₄)₃[54], [M(DMU)₆]- (ClO4)2(M=Mn, Ni, Zn)[24], and[Cu(OClO3)2- (DMU)₄] [24].

TheClO4 andBF4 counterions are distorted tetrahedral;forexampletheO-Cl-O bondanglesare intherange 105.3(6) - 114.8(6) .Thesum ofangles intheNO3ion isexactly 360 indicatingaplanar geometry.

We have up to now discussed aspects of the molecular structuresofcomplexes1-3.Figs. 4 -6 provideviews of the hydrogen bonding networks ofthecomplexes.Distancesandangles for the in- termolecular(interionic) hydrogenbondshavebeen included inTables 2- 4.

In 1, two oxygen atoms from each perchlorate, O(2) andO(3), actas hydrogenbondacceptors to NH groups from two DMU ligands belonging to two different[Co(DMU)₆]²⁺cations.These ligand-

H Bond D A H A D-H A OperatorofA N(1)-H(N1) O(21)^a 2.887(5) 2.05(4) 162(5)

N(11)-H(N11) O(1)^a 2.902(5) 2.10(4) 156(4) N(21)-H(N21) O(11)^a 2.935(6) 2.15(5) 161(4) C(23)-H(23A) O(21)^a 2.731(8) 2.43(7) 103(6)

N(2)-H(N2) O(3)^b 3.033(7) 2.27(4) 167(4) 1 –x,–y,–z N(12)-H(N12) O(2)^b 3.156(8) 2.39(7) 150(6) 1 –x,–1 –y,1 –z

Table2.Dimensionsoftheunique hydro- genbonds(distancesinA and˚ anglesin ) forcomplex1.

aIntramolecular(intracationic) hydrogen bonds,seeFig.2;^bthese oxygenatoms belongtothe perchloratecounterionsnot shown inFig.2;A=acceptor;D= donor.

H Bond D A H A D-H A OperatorofA

N(1)-H(N1) O(21)^a 2.876(4) 2.12(2) 157(2) N(11)-H(N11) O(1)^a 2.884(4) 2.09(2) 159(2) N(21)-H(N21) O(11)^a 2.927(4) 2.14(3) 157(2) C(23)-H(23B) O(21)^a 2.730(5) 2.38(5) 101(6)

N(2)-H(N2) F(2)^b 2.928(9) 2.24(3) 162(3) x,1 +y,–1 +z N(12)-H(N12) F(1)^b 3.114(10) 2.41(4) 150(3) 1 +x,–1 +y,z N(2)-H(N2) F(5)^c 2.994(9) 2.31(3) 161(3) 1 –x,1 –y,–1 –z N(12)-H(N12) F(7)^c 3.077(10) 2.42(4) 143(3) 1 –x,–y,–z C(21)^d-H(21B) F(3)^b 3.412(12) 2.54(5) 154(4) –x,1 –y,1 –z

Table3.Dimensionsoftheunique hydro- genbonds(distancesinA and˚ anglesin ) forcomplex2.

aIntramolecular(intracationic) hydrogen bonds,seeFig.2;^bthese fluorineatoms belongtothetetrafluoroboratecounteri- onsnot shown inFig.2;^cdisordered flu- orine atoms; ^d this atom, which is not labelled inFig.2, belongs tothe methyl group ofa DMU ligand;A = acceptor;

D= donor.

H Bond D A H A D-H A OperatorofA

N(1)-H(N1)^:::O(21)^a 2.825(3) 2.06(3) 158(3) –x,–y,–z N(11)-H(N11) O(1)^a 2.861(3) 2.07(3) 158(3)

N(21)-H(N21) O(11)^a 3.036(4) 2.32(4) 154(3) –x,–y,–z N(2)-H(N2) O(33)^b 3.043(4) 2.33(3) 165(3) x,y,–1 +z N(12)-H(N12) O(32)^b 3.115(4) 2.41(4) 156(4) x,–1 +y,z N(22)-H(N22) O(32)^b 3.065(4) 2.41(4) 138(4) 1 –x,–y,–z C(11)^c-H(11B) O(32)^b 3.443(5) 2.59(4) 154(3) x,–1 +y,z

Table 4.Dimensionsoftheunique hydrogen bonds(distancesin A and˚ angles in ) for complex3.

a Intramolecular (intracationic) hydrogen bonds,seeFig.3;^bthese oxygenatomsbe- longtothe nitratecounterionsnot shown in Fig.3;^cthisatom,which isnotlabelled in Fig.3, belongs tothe methyl group ofa DMU ligand;A=acceptor;D= donor.

Fig. 5.A viewofthe2Dnetwork formedbyhydrogen bondingbetween[Co(DMU)6]²⁺cationsandBF4 coun- terionsincomplex2.Only the intermolecular(interionic) hydrogen bonds are shown. For clarity, the hydrogen bonds to disorderedtetrafluoroborate fluorineatoms(see Table3) have notbeen drawn.

Fig. 6. A view of the 2D network formed by hydro- genbondingbetween[Co(DMU)6]²⁺cationsand NO3

counterionsincomplex3.Only the intermolecular(inte- rionic) hydrogenbondsareshown.

G.S.Papaefstathiouetal.·HydrogenBondedCobalt(II)ComplexesofN,N'-Dimethylurea 79 Table 5.Mostcharacteristic and diagnostic IRfundamentals(cm ¹) of free^aDMU and itscobalt(II)complexes1-3.

DMU 1 2 3 Assignments

3350 sb 3420-3350 sb 3400-3270 sb 3395 -3275sb (NH)

1628s 1580 s 1576 s 1576 s (CO)

1591s 1634s 1634s 1635s as(CN)amide+as(NH)

1541 m 1451 m 1451 m 1456m s(NH)

1270 sb 1357m 1355 m 1355 m as(NH) + as(CN)amide

1175 m 1183m 1182m 1181 m s(N-CH3)

1040m 1049 m 1043m 1045 m as(N-CH3)

775s 767 s 767 s 767 s (CO)

702 w 651sh 653 sh 648sh (CO)

508w 578 m 571w 570 w (NCN)

aIn itsCccrystal phase;b: broad;m:medium; s:strong; sh:shoulder; w:weak.

counterion hydrogen bonds create infinite one-di- mensional (1D) assemblies (Fig. 4). As a conse- quence of the participation of O(2) and O(3) in hydrogen bonding, the Cl-O(2) and Cl-O(3) bond lengths[1.390(5)and 1.366(5)A,˚ respectively] are slightlylonger thanCl-O(4)andCl-O(5)[1.306(7) and 1.342(6)A,˚ respectively].

The [Co(DMU)₆]²⁺ cations of 2 are linked through BF₄ counterions to generate a 2D net- work. One dimension is created by twocrystallo- graphicallyindependentN-H F(BF₄) interactions, whilethesecond dimension isachievedthrough one uniqueC-H F(BF₄ ) interaction.EachBF₄ uses three fluorineatomsashydrogenbondacceptors to NHgroupsfromtheDMUligandsbelongingto dif- ferent cations and to one methyl group of athird cation.

The[Co(DMU)6]²⁺ andNO3 ionsof 3arear- ranged in infinite2Dnetworks throughthreecrys- tallographicallyindependentintermolecular(interi- onic)N-H O(NO3 )and oneuniqueC-H O(32) hydrogenbonds.Each nitrateacceptsfourhydrogen bonds,onethroughO(33)andthreethroughO(32).

The intermolecular(interionic)N-H O, N-H F hydrogenbondsinthesupramolecular structuresof 1 and 2 can be regarded as “moderate” [44, 45], thoughtheyareweaker thanthe intramolecularN- H^:::O(DMU) hydrogen bonds. The intermolecular C-H F andC-H Ohydrogenbondspresentinthe crystal structuresof2and3,respectively, are defi- nitely weak[44,45].

Vibrationalspectraofthecomplexes

The fullvibrationalanalysisofcrystallineDMU hasbeen published[39].Table 5 givesdiagnostic IR

bandsofthe free ligandand itscobalt(II)complexes 1-3. Low-frequency IR (far-IR) and Raman data for 1-3are presented inTable 6. Assignments in Table 5 have been given in comparison with the dataobtained for the free,i. e.uncoordinated, DMU [39] and itsmanganese(II)[24,40],nickel(II)[24], copper(II) [24] andzinc(II)[24] complexes.Low- frequency assignments(Table 6) were assisted by considering the bands of free DMU inthe far IR and low-frequencyRamanregions, andby studying the variation in band position with changing the counterions (cobalt-ligand bands shouldappear at approximately thesame frequencies)and literature reports[43,53,54].

Thebands with (CN)_amidecharacteraresituated athigherfrequenciesinthespectraof1-3than for free DMU, whereas the (CO) bands showa fre- quency decrease. These shiftsare consistent with oxygen coordination, suggesting the presence of

+N=C-O resonant forms [51 - 55].Uponcoordi- nationviaoxygen,the positivelycharged metal ion stabilizes the negativecharge onthe oxygenatom;

the NCOgroup nowoccurs in itspolar resonance formandthe doublebondcharacteroftheCN bond increases, while the doublebond character of the CO bond decreases,resulting inan increase of the CNstretching frequency withadecrease intheCO stretching frequency[55].

The IR spectrum of 1 exhibits strong bands at 1080 and 622 cm ¹ due to the ₃(F₂) and

4(F₂) vibrations, respectively, of the uncoordi- nated ClO4 [55]. The broad character and signs of splitting of the band at 1080 cm ¹ both in- dicate the involvement of the ClO4 ion in hy- drogenbonding; thishydrogenbondingwasestab-

Table 6. Cobalt(II)-ligand vibrational frequencies for complexes1-3.

Complex (CoO) (OCoO)

IR Raman IR Raman

1 381 m 390m,366m 236m 221w 2 381 m 390 w,363m 237m 221w 3 377 w 389w,365 m 240m 222 w

lishedcrystallographically(seeabove).The ₃(F₂) [ d(BF)] and 4(F2) [ d(FBF)] vibrations of the tetrahedral (point groupT_d)BF₄ anion appearat 1100 - 1000 (broad band) and at 535 cm ¹, re- spectively,intheIRspectrum of2[55].The pres- ence of ionicnitrates(pointgroupD_3h) in3,estab- lished crystallographically, also follows from the IRspectrum ofthiscompoundthroughtheappear- ance ofthe3(E')[d(NO)],2(A2'')[(NO3)] and

4(E')[ _d(ONO)] bandsat1385,833and695cm ¹, respectively.

The appearance of one IR-active (CoO) [F_1u underO_h] andtwoRaman-active(CoO)[A_1g, E_g under Oh] vibrations in the low-frequency spec- tra of 1-3 reflects their trans octahedral stereo- chemistry [55]. The appearance of only one IR- active (OCO) [F_1u under O_h] and one Raman- active (OCO) [F_2gunder O_h] vibration is also in accordwiththis stereochemistry.

ConcludingCommentsandPerspectives

The CoX2/DMU chemistry (X = ClO4, BF4, NO₃) described inthis work hasfulfilled itspromise asasource of interesting hydrogenbonded networks based onsimple metalcomplexes.Itisimportant to notethat complexes 1and2are the onlyproducts thathavebeen isolated fromthecorrespondingre- action mixtures inEtOH or MeCN, strongly sug- gesting that the reactions and crystallizations are selective.The great stabilityofthe[Co(DMU)6]²⁺

cation inthesolidstatecanbe partlyattributedtoa pseudochelate effect.The “moderate”intracationic hydrogenbondspresentinthestructuresof1-3cre- atesix 6-memberedpseudochelatingCoOCNH O rings per Co^II atom, giving an extra stabilization to the [Co(DMU)₆]²⁺ cation. This confirms the general rule (developed for organic compounds [6])that 6-membered-ring intramolecularhydrogen bondsform in preferenceto intermolecularhydro- genbonds.

This work has shown that the hexakis(N,N'-di- methylurea)cobalt(II)cationcanactasahydrogen- bondingbuildingblockwith multi-foldconnectivity linkingClO₄ anions to generatean 1D architecture (1), andBF4 andNO3 anions toyield different 2D hydrogenbonded networks(2,3).Weare presently pursuing ourprediction that this cation (and other [M(DMU)6]²⁺ cations,seeref.[24]) will form hy- drogen bonding contacts to a varietyof inorganic and organic anions to generate a rich diversityof networks.

Therearetwo main motifsof intermolecular(in- terionic) hydrogen bonds thathavebeen observed in the structures of the Co^II/DMU complexes. A detailed comparison of the crystal structures of 1 and2vs.3indicates that thecounterion does play arole intheassemblyofthesecomplexes.In1and 2, where the anions (ClO4 , BF4 ) are approxi- mately“spherical”andwithrelativelylowcoordi- natingability[11],twocrystallographically unique (i.e.,totallyfour)N-H O(ClO₄ ) orF(BF₄ ) hy- drogen bonds form per [Co(DMU)₆]²⁺ cation,re- sulting inthe formation of 1D(1) or 2D(2) motifs;

the 2D motif in 2 arises from the capability of a fluorineatomto “accept”ahydrogenbond froma methyl group.In other words,eachClO₄ orBF₄ anionuses twoatoms ashydrogenbondacceptors toNHgroups.In3,wherethecounterion (NO₃ ) is moreanisotropicwithstrongerhydrogen-bond ac- ceptors[13],threecrystallographically unique (i. e., totally six) N-H O(NO₃ ) hydrogenbonds form per [Co(DMU)6]²⁺ cation, resulting in the forma- tion ofa2DnetworkwithaweakC-H O(NO₃ ) hydrogen bond also contributing to the stabiliza- tion ofthismotif.EachNO3 anionalsouses(like each ClO₄ or NO₃ ) two oxygen atoms as hy- drogen bond acceptors to NH groups, but one of these atoms, i. e., O(32) “accepts” two hydrogen bonds from NH groups. As a result of the differ- enthydrogenbondingabilityofthecounterions,in 1 and 2 ten (from a total of twelve) NH groups of each [Co(DMU)6]²⁺ building block participate in hydrogenbonds,sixofthem participating in in- tramolecular(intracationic)and fourin intermolec- ular (interionic) hydrogen bonds;atomN(22) and its symmetryequivalentare notengaged in hydro- genbonding interactions,seeTables 2,3andFig.2.

On thecontrary, in3all thetwelveNHgroups of each[Co(DMU)6]²⁺ cation participate in hydrogen bonds(sixintramolecular,sixintermolecular).

G.S.Papaefstathiouetal.·HydrogenBondedCobalt(II)ComplexesofN,N'-Dimethylurea 81 The N-H hydrogens of symmetrical disubsti-

tuted ureas prefer to adopt ananti, anti (or trans, trans [39]) relationship to the carbonyl groups [6, 33], see Fig. 1. Coordination of DMU to cobalt(II) has twoconsequences. First, whileboth lone pairsonthecarbonyl oxygenatom of freeDMU [39,56] actashydrogenbondacceptors,in1-3one pair actsas a“moderate” [44, 45] hydrogen bond acceptor and the other participates in the forma- tion of the Co-O coordinative bond. Second, the N-Hhydrogens of eachDMU ligandadopt a syn, anti (or cis, trans) relationship tothecoordinated carbonyl group; the syn (or cis) configuration of oneNHgroup isnecessaryfor the formation ofthe 6-membered-ring intramolecular hydrogen bonds, whereas theconfiguration ofthe otherNHgroupre- mainsanti(ortrans)relativetothecarbonyl group to participate in intermolecular(interionic) hydro- genbonds withthecounterions.Thus,thetopology oftheDMUsheet structure[39,56]is significantly perturbed bycoordination. Based on the above, it appears feasibletocontrolthe extentofthe pertur- bation by variation ofthe nature of the metal ion andthecounterion.

The NHgroups inthe complexes reported here are “moderate”[44,45](and not weak) donorsbe- causethe oxygenatomacceptsbothahydrogenand a coordinativebond; thus,the polar resonantforms of the DMU ligands, i. e., (CH₃)HN⁺=C-O , are stabilized.Sincethe polarization occurs through bonds,thiseffecthasbeencalled -bondcoopera- tivity[44,45].

Wecurrently work on neutral (and notcationic) metal complexes of DMU using halides as co- ligands totakeadvantage ofthe facts that there is more space available for guest molecules and no possibility for unpredictable hydrogen bonding to thecounterion.Thesynthesisof neutralcomplexes might favour the construction of motifs based on prevailingDMU-DMUinteractions.We dobelieve that N,N'-disubstituted ureas would prove central playersinthe field of hydrogenbonded networksof coordinationcomplexes.

ExperimentalSection

All manipulations were performedunderaerobic con- ditions usingstarting materials(Merck)andsolventsas received.Elementalanalyses(C, H, N)wereconducted by theUniversity ofIoannina, Greece, Microanalytical Service.IRspectra(4000- 500cm ¹)wererecorded on

a BrukerIFS113vFT spectrometer withsamplespre- pared as KBr pellets. Far-IR spectra (500 - 50 cm ¹) were recorded on thesameBruker spectrometer using polyethylene pellets.FT Raman datahavebeencollected ona BrukerIFS66vinterferometerequippedwitha FRA 106Ramanaccessory, a CW Nd-YAGlaser sourceanda liquid-nitrogenGe detector.

[Co(DMU)6](ClO4)2(1)

A pink-red solution of Co(ClO4)2 6 H2O (0.73 g, 2.0mmol) inEtOH(20ml)and2,2-dimethoxypropane (DMP, 2.5 ml)was refluxed for 30 min, cooled down to roomtemperature andthen treatedwithsolid DMU (1.29 g,14.6 mmol).No noticeable colourchange oc- curred.Thereaction flaskwas storedat4 Cfor 3dand the obtained pink microcrystalline solid was collected byfiltration,washedwithEt2O(2 5 ml)and driedin vacuooverP4O10.Theyieldwas55%.Crystals suitable for single-crystalX-raycrystallography were obtainedby diluting the originalsolution with moreEtOH(15 ml) and layering it withEt2O/n-hexane (1:1v/v,40ml). – C18H48N12O14Cl2Co (786.51): calcd.C27.48, H6.16, N 21.37;foundC27.80, H6.06, N21.12.Caution!Perchlo- ratesaltsare potentiallyexplosive.Although no detona- tiontendencieshave been observed in ourexperiments, caution isadvisedand handling of only small quantities is recommended.

[Co(DMU)6](BF4)2(2)

Apinksolution ofCo(BF4)26H2O(0.68 g,2.0mmol) in EtOH (30 ml) and DMP (2.5 ml) was refluxed for 20 min, cooled to 35 C and thentreatedwith solid DMU(1.20g,13.6mmol).No noticeablecolourchange occurred.Thereaction mixturewas refluxed forafurther 15 min, cooledto roomtemperature and layered with Et2O(35 ml).Slowmixing gave pinkcrystals suitable for X-raycrystallography,whichwerecollectedbyfiltration, washedwithcoldEtOH(2ml)andcopiousamountsof Et2O, and driedinvacuooverP4O10.Typicalyields were inthe75 - 83%range. –C18H48N12O6B2F8Co (761.23):

calcd.C28.40, H6.37, N22.08;foundC28.56, H6.29, N21.78.

[Co(DMU)6](NO3)2(3)

A pink-redsolution of Co(NO3)2 6 H2O (0.58 g, 2.0 mmol) inMe2CO (22 ml)and DMP (2.5 ml) was refluxed for15 min and treated, while hot, with solid DMU(1.29 g,14.6mmol).DMUsoon dissolvedto give ahomogeneous solution ofthesamecolour,whichwas further refluxed for 20min.Aftercoolingtoroomtem- perature, thesolution was layered with Et2O (20 ml).

Table7.Summaryofcrystal data,data collectionandstructurerefinementforX-raydiffractionstudiesofcomplexes 1,2and3.

Compound 1 2 3

Chemical formula C18H48N12O14Cl2Co C18H48N12O6F8B2Co C18H48N14O12Co

Formulaweight 786.51 761.23 711.61

Colour,habit pink prisms pink prisms pink plates

Crystalsystem triclinic triclinic triclinic

Space group P¯1 P¯1 P¯1

a(A)˚ 8.058(8) 7.993(8) 7.858(3)

b(A)˚ 10.71(1) 10.67(1) 10.742(4)

c(A)˚ 11.69(1) 11.62(1) 11.089(4)

( ) 68.53(3) 67.88(3) 67.92(1)

( ) 81.21(4) 80.67(3) 82.45(1)

( ) 81.32(3) 81.86(3) 81.74(1)

V(A˚³) 924(1) 902.3(2) 855.3(6)

Z 1 1 1

Dcalc(gcm ³) 1.413 1.401 1.382

(mm ¹) 0.681 0.565 0.574

F(000) 413 397 377

Radiation (A)˚ Mo-K (=0.71073) Mo-K (=0.71073) Mo-K (=0.71073)

Temperature (K) 298 298 298

Scan mode/speed (min ¹) -2/2.3 -2/3.5 -2/2.0

Range () 1.9 -24.0 1.9 -25.0 2.0-25.0

h,k,lRanges –9^!0,–12^!12, 0^!9,–12^!12, –9^!9,–12^!0,

–13^!13 –13^!13 –13^!12

Measuredreflections 3148 3442 3184

Uniquereflections 2909 (Rint=0.0094) 3192(Rint=0.0065) 3007(Rint=0.0228)

Reflections used[I>2(I)] 2533 2959 2632

w^a a=0.0647;b= 1.0037 a=0.0512;b=0.1958 a=0.0420;b=0.3060

GoF(onF²) 1.051 1.059 1.098

R1^b[I>2(^I)] 0.0522 0.0342 0.0392

wR2^c[I>2(^I)] 0.1357 0.0974 0.0978

()max/()min(eA˚ ³) 0.538/–0.437 0.400/–0.170 0.316/–0.300

aw= 1/[²(Fo2) + (aP)²+bP] andP= (max(Fo2,0) +2Fc2)/3;^bR1 =Σ(^jFo^j–^jFc^j)/Σ(^jFo^j);

cwR2=^fΣ[w(Fo2–Fc2)²]/Σ[w(Fo2)²]^g1=2.

Slowmixing gavewell-formed, X-ray-qualitypinkcrys- tals,whichwerecollectedbyfiltration,washedwithco- piousamountsofEt2O and driedinvacuoover silicagel.

Theyieldwas70%.The productappears slightlyhy- groscopic. –C18H48N14O12Co (711.61): calcd.C30.38, H6.81, N27.56;foundC30.62, H6.99, N27.87.

Theabove preparationsarerepresentative.Complexes 1 - 3 can be isolated using a variety of solvents and DMU:Co^IIratios,see “Synthetic comments”above.

Crystalstructure determinations

Pink prismatic crystalsof1(0.100.350.60mm), 2 (0.10 0.15 0.40 mm) and 3 (0.20 0.45 0.50mm)were mounted inair.Diffraction measurements were made ona CrystalLogic DualGoniometerdiffrac- tometer using graphite monochromatedMo-K radiation.

Crystal data and full detailsofthe data collectionand data

processingare listed inTable7.Unitcell dimensions were determinedand refinedby usingtheangular settingsof 25 automatically centredreflections in the range 11 <

2<23.Threestandardreflections,monitored every97 reflections,showed less than3% intensity variationand no decay.Lorentz,polarizationand scan (onlyfor3) absorptioncorrections wereappliedusingCrystalLogic software.

The structures were solved by direct methods us- ingSHELXLS-86[57] andrefinedbyfull-matrixleast- squares techniques on F² with SHELX-93 [58]. For all three structures, all non-H atoms were refined us- inganisotropicthermal parameters,except tetrafluorobo- rate fluorineatomsF(1), F(2), F(3)andF(4) of2which were found disorderedandrefinedanisotropicallyintwo orientations with occupation factorsfreetovary.AllH atoms of 1- 3,except those onC(13)and C(23) in2 whichwere introduced incalculated positionsas riding