Octahedral Co(III) complexes of 2-(phenylimino)pyrrolyl ligands: Synthesis and structural characterisation

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Octahedral Co(III) complexes of 2-(phenylimino)pyrrolyl ligands: Synthesis and structural characterisation
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  This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:http://www.elsevier.com/copyright  Author's personal copy Octahedral Co(III) complexes of 2-(phenylimino)pyrrolyl ligands: Synthesisand structural characterisation Clara S.B. Gomes a , Sónia A. Carabineiro a , Pedro T. Gomes a, ⇑ , M. Teresa Duarte a , M. Amélia N.D.A. Lemos b a Centro de Química Estrutural, Departamento de Engenharia Química e Biológica, Instituto Superior Técnico, Universidade Técnica de Lisboa, Torre Sul, Av. Rovisco Pais,1049-001 Lisboa, Portugal b Instituto de Biotecnologia e Bioengenharia, Centro de Engenharia Biológica e Química, Instituto Superior Técnico, Universidade Técnica de Lisboa, Torre Sul, Av. Rovisco Pais,1049-001 Lisboa, Portugal a r t i c l e i n f o  Article history: Received 20 August 2010Receivedinrevisedform29November2010Accepted 10 December 2010Available online 21 December 2010 Keywords: Iminopyrrolyl ligandsCobalt(III)Octahedral complexesRedox behaviour a b s t r a c t ThereactionsofCoCl 2 withthreeequivalentsof2-(phenylimino)pyrrolylsodiumsalts,performedunderanitrogen atmosphere, lead to the formation of the Co(III) complexes [Co( j 2 N  , N  0 -NC 4 H 3 C(H) @ N–C 6 H 5 ) 3 ]( 2a ), [Co( j 2 N  , N  0 -NC 4 H 3 C(CH 3 ) @ N–C 6 H 5 ) 3 ] ( 2b ) and [Co( j 2 N  , N  0 -NC 16 H 9 C(H) @ N–C 6 H 5 ) 3 ] ( 2c ), accommo-dating three chelating iminopyrrolyl ligands. Complexes  2a – c  were obtained in moderate yields, andtheir characterisation by  1 H,  13 C NMR andX-ray diffraction showthey are diamagnetic and have anocta-hedral geometry about the cobalt centre, respectively. Uncharacterised products were obtained in thesame reaction involving ligand precursors such as 2-(2,6-dimethylphenylimino)pyrrolyl sodium salts,which is attributed to a greater steric hindrance in the coordination of three of these bulkier ligands.The redox behaviour of complexes  2a – c  shows an irreversible reduction wave with a peak potential inthe range   3.2 to   3.7V. Upon reduction, the complexes decompose giving rise, in the case of   2a , to aredox pattern compatible with the formation of [Co( j 2 N  , N  0 -NC 4 H 3 C(H) @ N–C 6 H 5 ) 2 ].   2010 Elsevier B.V. All rights reserved. 1. Introduction Iminopyrrolyl chelating ligands have been widely used in thesynthesisofmetalcomplexes,notonlyforapplicationinareassuchashomogeneouscatalysis[1]butalsoinluminescence[2]ormate- rials science [2c,3]. In the 1960s, Holmet al. reported the first syn-theses of homoleptic Co(II), Ni(II), Cu(II) and Zn(II) complexescontaining these ligands, although only with alkylimino groups[4]. In the case of cobalt, the use of aerobic conditions combinedwith low hindered alkylimino groups led to the formation of oxidised octahedral tris(2-iminopyrrolyl) Co(III) complexes,[Co( j 2 N,N  0 -2-NC 4 H 3 C(H) @ N–R) 3 ] (R=methyl, ethyl, isopropyl, sec  -butyl)(Chart1,  A ) [5]. However,thepresenceofabulkysubsti-tuent, such as  tert  -butyl, enabled the synthesis and characterisa-tion of the corresponding tetrahedral bis(2-iminopyrrolyl) Co(II)complex, [Co( j 2 N,N  0 -2-NC 4 H 3 C(H) @ N- t  Bu) 2 ] [4].Later on, Sousa et al. reported the electrochemical synthesis of Co(II)andCo(III)complexescontaining2-aryliminopyrrolylligands[6],inareactionwhereanodicmetalliccobaltwasoxidizedtoCo 2+ ,underanitrogenatmosphere,andtheneutral2-arylformiminopyr-role ligand precursor was involved in the generation of the corre-sponding anionic ligand species, with the concomitant reductionof H + to H 2  occurringonthe cathode. The presence of anadditionalbidentate ligand L^L (phenanthroline or bipyridine) led to the for-mation and characterisation of crystalline insoluble compounds of the type [Co( j 2 N,N  0 -2-NC 4 H 3 C(H) @ N–R) 2 (L^L)] (R=C 6 H 5 ; 2-Me–C 6 H 4 ; 4-Me–C 6 H 4 ; 4-Cl–C 6 H 4 ), but in other cases air-evaporationof the solvent resulted in smooth oxidation of Co(II) to Co(III),affording [Co( j 2 N,N  0 -2-NC 4 H 3 C(H) @ N–R) 3 ] (  A ).Recently, we reported the synthesis and characterisation of aseries of Na [7], Co(II) [8], Ni(II) [9], and Zn(II) [2a] complexes containing 2-aryliminopyrrolyl or 2-aryliminophenanthro[9,10- c  ]pyrrolyl ligands. Particularly, in the case of Co(II), the introduc-tion of substituents with increasing bulkiness led to the formationof four-coordinate complexes with tetrahedral and square-planargeometries (Chart 1,  B  and  C , respectively) [8a]. The characterisa-tion data of the cobalt complexes  B  show them to be tetrahedral(R  1  =H, R  2  =H, Me,  i Pr; R  1  =Me, R  2  =H, Me), whichis the preferredgeometry for four-coordinate Co(II) compounds. Nevertheless, asquare-planar geometry was observed in the case of compound  C (R  1  =Me, R  2  = i Pr), DFT calculations suggesting that this type of geometry is slightly more stable than the tetrahedral one, due toa combination of steric and electronic reasons.We also reported the preparation of five-coordinate imino-pyrrolyl 17-electron complexes [Co( j 2 N,N  0 -NC 4 H 3 C(R) @ N-2,6- i Pr 2 C 6 H 3 ) 2 L] (R  1  =H, Me) [8c], which present trigonal bipyramidaland square pyramidal geometries for monodentate L=PMe 3  andTHF (Chart 1,  D  and  E , respectively). Essentially, two alternativesynthetic routes were used, consisting in the reaction of the 0020-1693/$ - see front matter    2010 Elsevier B.V. All rights reserved.doi:10.1016/j.ica.2010.12.032 ⇑ Corresponding author. Tel./fax: +351 218419612. E-mail address:  pedro.t.gomes@ist.utl.pt (P.T. Gomes).Inorganica Chimica Acta 367 (2011) 151–157 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica  Author's personal copy corresponding iminopyrrolyl sodium salts with: (a) [CoCl 2 L  2 ](L=PMe 3 , THF), or (b) anhydrous CoCl 2  and PMe 3 . The relatedbis(ketopyrrolyl) complexes of Co(II) stabilised by trimethylphos-phine ligands were also studied [8b]. The reaction of 2-formyl- and 2-acetylpyrrole sodium salts with cobalt dichloridecompounds afforded stable six-coordinate bis(ketopyrrolyl) Co(II)19-electron complexes [Co( j 2 N,O -2-NC 4 H 3 C(R  1 ) @ O) 2 (PMe 3 ) 2 ](R  1  =H, Me) (Chart 1,  F ). The latter compounds displayed octahe-dral geometries around the metal centre, with the PMe 3  ligandslying in the equatorial plane, in  trans  positions to each other, andtwo bidentate ketopyrrolyl ligands occupying the remaining coor-dination positions, in a transoid conformation, with the oxygenatoms occupying the axial positions.All the previous Co(II) compounds have been obtained by thereaction of cobalt dichloride, or CoCl 2  adducts, with two equiva-lents of the corresponding 2-aryliminopyrrolyl sodium salt. Inthe present work, we report the effect of the addition of a thirdequivalent of these chelating 2-aryliminopyrrolyl or 2-arylimino-phenanthro[9,10- c  ]pyrrolyl Na ligand salts in the latter reaction,which leads to the oxidation of Co(II) to Co(III), under nitrogenatmosphere. The resulting tris(2-aryliminopyrrolyl) Co(III) com-plexes are herein described and characterised by elemental analy-sis,  1 H and  13 C NMR spectroscopy, and X-ray diffraction. 2. Results and discussion  2.1. Synthesis of complexes The imino- or acetiminopyrrole ligand precursors  1a – e  used inthis work (Scheme 1) were prepared and characterised accordingto the methods described in earlier publications [2a,8a,9]. In thecase of 2-formiminopyrrolyl derivatives  1a , c , d , it consists in thecondensation of 2-formylpyrrole or 2-formylphenanthro[9,10- c  ]pyrrole with aniline, employing standard conditions [10]. Ontheother hand, the preparationof acetiminopyrroles  1b , e  involvedharsherconditions,suchastheabsenceofsolventandlongerrefluxperiods [8a].Treatment of the ligand precursors  1a – e  with NaH, in THF, re-sulted in the deprotonation of the pyrrole NH, leading to the in situ  formation of the corresponding sodium salts [Na( N,N  0 -NC 4 H 3 C(R  1 ) @ N-2,6-R  2 -C 6 H 3 )]. The addition of the resulting solu-tion to a suspension of anhydrous CoCl 2  in the same solvent, usinga molar ratio of 3:1 (ligand precursor:CoCl 2 ), under rigorous inertatmosphere conditions, afforded the Co(III) complexes  2a – c (Scheme 1), containing three chelating 2-phenyliminopyrrolyl li-gands. These complexes were extracted in diethyl ether ( 2a , b ) or Chart 1.Scheme 1.  Synthesis of Co(III) complexes containing 2-(phenylformimino)- or 2-(phenylacetimino)pyrrolyl ligands.152  C.S.B. Gomes et al./Inorganica Chimica Acta 367 (2011) 151–157   Author's personal copy in toluene ( 2c ), followed by precipitation at   20  C, in diethylether, yielding brownish-red ( 2a ), orange-reddish ( 2b ) and red( 2c ) crystals, in moderate yields (36–58%).As mentioned in the Introduction, in the works of Chakravortyand Holm et al. [5] and Sousa and co-workers [6], the compounds of the type [Co( j 2 N,N  0 -2-iminopyrrolyl) 3 ] were reported as beingobtained from the aerobic oxidation of Co(II) compounds[Co( j 2 N,N  0 -2-iminopyrrolyl) 2 ]. However, in the present work, it isshownthattheadditionofathirdequivalentofsodiumiminopyrr-olyltoCoCl 2  leadstotheoxidationofCo(II)toCo(III),evenwiththereaction being performed under rigorous nitrogen atmosphereconditions, and despite the different substituents on the iminiccarbon or in the pyrrolyl framework. The products obtained arediamagnetic octahedral Co(III) complexes containing three imino-or acetiminopyrrolyl ligands. The  1 H NMR spectra of these com-plexes do not show the pyrrole NH resonance of the free ligand,at  ca.  9.5ppm, which attests to the presence of the ligand coordi-natedtothemetal centreinabidentatechelatingmode. Therefore,this oxidation reaction appears to be preferred over the formationof a six-coordinate 19-electron monoanionic Co(II) species of thetype [Co( j 2 N,N  0 -2-iminopyrrolyl) 3 ]  .Attempts to synthesise complexes corresponding to  2d , e , con-taining the bulkier 2,6-dimethylphenyl substituent at the iminogroup were also performed. However, due to the steric hindranceintroduced by the two methyl substituent groups, these attemptswere unsuccessful, leading to the formation of uncharacterisedmixtures of compounds. The corresponding NMR spectra revealedparamagnetic features, very likely due to the presence of a signifi-cant amount of the corresponding bis(2-iminopyrrolyl) Co(II)derivative, [Co( j 2 N,N  0 -NC 4 H 3 C(R) @ N-2,6-Me 2 C 6 H 3 ) 2 ] (R=H, Me),meaning that the coordination of a third bulky ligand in the Cocoordination sphere is strongly hampered.  2.2. X-ray diffraction Crystals suitable for X-ray diffraction were obtained for all syn-thesised complexes  2a – c . The molecular structures of complexes 2a – c  are presented in Fig. 1 and the corresponding selected bonddistances (Å) and angles (  ) are listed in Table 1. All of them showthe cobalt atom coordinated to three iminopyrrolyl chelating li-gands, in which the metal-pyrrole Co1–N1 bond distances are al-ways shorter than the metal-imine Co1–N2 ones (see Table 1),the corresponding values being in agreement with those foundfor the single Co(III) aryliminopyrrolyl complex structure reportedintheliterature[6]andforothersix-coordinatedCo(III)complexescontaining other anionic  N  -heterocyclic five-membered ring li-gands [11]. All the complexes  2a – c  can be described as  fac   (facial)isomers, since they display always the Co1–N pyrrole  bonds  trans  tothe Co1–N imine  ones, likely reflecting a better stereochemicalarrangement. The dihedral angles  a  between the chelation planesof each ligand, which are defined by the cobalt and two nitrogenatoms, are close to 90   (Table 1), whereas the angles betweentwo  trans  nitrogen atoms and the metal centre are  ca.  180   (Table1), indicating an octahedral geometry around the cobalt atom. ThebiteanglesN1–Co1–N2ofallbidentateligandsareintherange81–83  ,matchingthevaluesoftheliteratureforotherCo(II)andCo(III)aryliminopyrrolyl complexes [6,8].For these compounds, the pyrrole rings of   2a , b  or the phenan-thro[9,10- c  ]pyrroleringsin 2c  showplanarbackboneswithsimilarfeatures. According to Table 1, the shortest bond occurs betweenN1 and C5, with values ranging from 1.335(7) to 1.347(6)Å. Thelongest bond in the pyrrole ring is that between C3 and C4 in  2c ,whereas for  2a  and  2b  C2–C3 and C4–C5 have similar lengths. Inthese molecules, the angles centred at the pyrrole N1 are in therange104.3(6)–108.6(6)  , whilsttheanglesattheimineN2arebe-tween 114.8(5)   and 120.4(6)  . The imine N2–C6 distance variesbetween 1.295(2) and 1.321(8)Å. The torsion angles N2–C6–C2–N1 of    1.8(2)  , 0.8(2)   and   0.2(2)   ( 2a ), 0.6(9)  , 3.1(8)   and2.1(9)   ( 2b ), and 2.0(7)  , 0.9(7)   and 0.3(7)   ( 2c ) show that the Fig. 1.  ORTEP III diagram of complexes (a)  2a , (b)  2b  and (c)  2c , using 50%probability level ellipsoids. Hydrogen atoms and, in  2c , a co-crystallised Et 2 Omolecule were omitted for clarity. C.S.B. Gomes et al./Inorganica Chimica Acta 367 (2011) 151–157   153  Author's personal copy pyrrole ring and the formimino group are nearly coplanar.Distances C2–C6 in the range 1.396(9)–1.438(9)Å are shorter thanthenormalvaluesfortypicalC–Csinglebonds,indicatinganexten-sionofthepyrrolering p -electronicdelocalisationtowardsthefor-mimino substituent. These values are in agreement with thosefound previously for the ligand precursors [2a,7,8a].For compounds  2a  and  2b , the phenyl substituent of the iminicfragmentisalmostperpendiculartotheiminopyrrolylplaneintwoof the ligands, the third ligand showing a more pronounced rota-tion (dihedral angles of 82.58  , 82.76   and 66.15  , for  2a , and80.77  , 85.84   and 74.02  , for  2b ). Conversely, in compound  2c ,the phenyl groups of two ligands present dihedral angles in be-tween perpendicular and coplanar to the iminopyrrolyl planes,whereas the remaining ligand shows its phenyl ligand almost per-pendicular (dihedral angles of 65.26  , 87.06   and 59.0  ).  2.3. Electrochemical studies The redox behaviour of the Co(III) complexes [Co( j 2 N  , N  0 -NC 4 H 3 C(H) @ N–C 6 H 5 ) 3 ] ( 2a ), [Co( j 2 N  , N  0 -NC 4 H 3 C(CH 3 ) @ N–C 6 H 5 ) 3 ]( 2b ) and [Co( j 2 N  , N  0 -NC 16 H 9 C(H) @ N–C 6 H 5 ) 3 ] ( 2c ), were investi-gated using cyclic voltammetric techniques (Fig. 2). The studywas performed at several scan rates (from 50 to 2000mV/s) at aPt disc electrode in a [NBu 4 ][PF 6 ]/THF solution.ItwasestablishedthattheseCo(III)complexescouldbereducedat verycathodic potentials, in a nonreversible process (evenat thehigher scan rates), attesting the chemical instability of the com-plexes formed after the reduction step.All of the complexes present the same reduction pattern with apeakpotentialintherange  3.2to  3.7V, 2a  beingtheeasiestand 2b  the hardest toreduce(see Table2), whichis certainlyrelatedtothe donating influence of the imino substituent methyl group onthe iminic coordinating nitrogen electron density of   2b .In the case of   2a , it is possible to observe a second reductionwave,whichisnotvisibleinthecyclicvoltammogramsoftheother  Table 1 Selected bond distances (Å) and angles (  ) for complexes  2a – c . 2a 2b 2c Ligand 1 Ligand 2 Ligand 3 Ligand 1 Ligand 2 Ligand 3 Ligand 1 Ligand 2 Ligand 3 Distances (Å) N1–C2 1.379(2) 1.375(2) 1.373(2) 1.386(8) 1.352(8) 1.392(8) 1.371(7) 1.393(7) 1.382(7)N1–C5 1.345(2) 1.342(2) 1.340(2) 1.338(8) 1.342(8) 1.339(8) 1.344(7) 1.335(7) 1.347(6)C3–C2 1.395(3) 1.396(2) 1.393(2) 1.397(9) 1.397(9) 1.377(9) 1.401(8) 1.412(7) 1.414(7)C3–C4 1.387(3) 1.384(3) 1.389(3) 1.392(10) 1.393(9) 1.395(10) 1.391(8) 1.407(8) 1.418(8)C5–C4 1.395(3) 1.396(3) 1.397(3) 1.383(9) 1.391(9) 1.379(9) 1.417(8) 1.405(8) 1.398(8)C6–C2 1.414(3) 1.412(2) 1.406(3) 1.438(9) 1.396(9) 1.429(9) 1.416(8) 1.401(8) 1.408(8)N2–C6 1.295(2) 1.301(2) 1.300(2) 1.302(8) 1.321(8) 1.297(8) 1.308(7) 1.309(7) 1.309(7)N2–C7 1.431(2) 1.437(2) 1.431(2) 1.447(8) 1.464(8) 1.426(8) 1.433(7) 1.441(7) 1.444(7)C6–C8 – – – 1.489(9) 1.484(8) 1.506(9) – – –Co1–N1 1.8926(15) 1.9000(14) 1.8953(14) 1.902(6) 1.893(5) 1.860(5) 1.906(4) 1.895(4) 1.915(4)Co1–N2 1.9746(14) 1.9720(14) 1.9794(15) 1.978(5) 1.959(5) 1.987(5) 1.963(5) 1.958(5) 1.964(4)  Angles (  ) C6–N2–C7 119.86(15) 119.72(15) 118.83(15) 118.7(6) 117.9(5) 120.4(6) 115.9(5) 118.2(5) 114.8(5)N1–C2–C6 112.95(16) 113.77(15) 113.45(15) 114.5(7) 115.8(6) 112.4(6) 111.1(5) 111.9(5) 113.1(5)N2–C6–C2 117.03(16) 117.12(16) 116.94(16) 114.4(6) 113.1(6) 115.9(7) 118.6(5) 117.5(5) 117.8(5)C3–C2–C6 137.25(18) 137.05(17) 137.21(17) 136.9(7) 135.8(7) 136.2(8) 138.8(5) 139.1(5) 136.3(5)N1–C2–C3 109.80(17) 109.18(16) 109.26(16) 108.6(7) 108.5(6) 111.4(6) 110.0(5) 108.9(5) 110.2(5)C2–C3–C4 105.71(17) 106.48(17) 106.22(17) 106.8(7) 107.1(7) 105.4(7) 105.7(5) 105.3(5) 104.2(5)C5–C4–C3 107.82(17) 107.09(16) 107.25(16) 106.7(7) 106.0(7) 106.7(7) 107.4(5) 108.0(5) 108.3(5)N1–C5–C4 109.50(17) 109.80(17) 109.45(17) 110.8(7) 109.8(7) 112.2(7) 109.0(5) 109.3(5) 109.4(5)C5–N1–C2 107.16(16) 107.44(15) 107.83(15) 107.1(6) 108.6(6) 104.3(6) 107.9(5) 108.5(5) 107.8(4)N1–Co1–N2 82.22(6) 82.79(6) 82.02(6) 82.2(3) 81.1(2) 82.1(2) 82.06(19) 82.01(19) 82.77(18)N1–Co1–N trans  174.17(6) 172.73(6) 175.56(6) 173.7(2) 173.8(3) 173.3(2) 173.94(19) 173.36(18) 172.02(19)Dihedral  a  89.84(6) a 89.08(5) b 88.71(5) c 89.50(21) a 89.47(18) b 89.96(21) c 89.34(15) a 88.85(17) b 88.85(16) ca Dihedral angle between the chelation planes of ligands 1 and 2, defined by the atoms N pyrrole , Co and N imine . b Dihedral angle between the chelation planes of ligands 2 and 3, defined by the atoms N pyrrole , Co and N imine . c Dihedral angle between the chelation planes of ligands 3 and 1, defined by the atoms N pyrrole , Co and N imine . Fig. 2.  Baseline corrected voltammograms of complexes  2a ,  2b ,  2c  and  B (R  1  =R  2  =H) performed at 200mV/s, at a Pt disk electrode, in [NBu 4 ][PF 6 ]/THFsolution.  Table 2 Peak potentials for the reduction of Co(III) complexes  2a ,  2b  and  2c , and Co(II)complex  B  (R  1  = R  2  = H), measured in [NBu 4 ][PF 6 ]/THF versus ferrocene/ferroceniumcouple redox pair. Complex  I  E red p  /V  II  E red p  /V 2a   3.22   3.60 2b   3.70 – 2c   3.34 – B  (R  1  =R  2  =H)   3.64 –154  C.S.B. Gomes et al./Inorganica Chimica Acta 367 (2011) 151–157 
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