Available online at www.sciencedirect.com

www.elsevier.com/locate/inoche

Inorganic Chemistry Communications 11 (2008) 535–538

Synthesis, characterization and role of a novel dimeric a-diimineiron(II) complex in ATRP of styrene

Rossella Ferro, Stefano Milione *, Loredana Erra, Alfonso Grassi

Dipartimento di Chimica, Universita di Salerno, via Ponte don Melillo, I-84084 Fisciano (SA), Italy

Received 3 December 2007; accepted 7 February 2008Available online 13 February 2008

Abstract

A novel dimeric a-diimine iron(II) complex, [(dab)FeCl-(l-Cl)]2 (dab = 1,4-bis-cyclohexyl-1,4-diazabutadiene) was synthesized andstructurally characterized by single crystal X-ray diffraction and VT 1H NMR spectroscopy. The performances of this complex in atomtransfer radical polymerization (ATRP) of styrene have been studied and related to the molecular structure.� 2008 Elsevier B.V. All rights reserved.

Keywords: Dimeric iron(II) complex; 1,4-Bis-cyclohexyl-1,4-diazabutadiene ligand; ATRP; Styrene polymerization

Atom transfer radical polymerization (ATRP) is a wellestablished methodology for the synthesis of polymericmaterials with defined architecture [1] discovered by Mat-yjaszewski [2] and Sawamoto [3], who first employed copperand ruthenium halides, respectively, to control the polymerchain growth in a radical process. The proposed polymeri-zation mechanism involves the reversible exchange of thehalide atom between the metal complex and the growingradical polymer chain. This lowers the molar concentrationof the radical species in solution and reduces the probabilityof the termination reactions leading to a controlled/livingpolymer chain. In the past decade the contribution of otherresearchers extended this approach to a variety of metal cat-alysts including iron [4], nickel [5], palladium [6], rhodium[7], molybdenum [8] and titanium compounds [9]. Amongthese metals, iron is particularly attractive for biocompati-bility, low cost and easy accessibility to a wide number ofmetal complexes with different ligands [4].

Pioneering studies showed that trialkyl and triaryl phos-phine complexes of iron halides exhibit a good control inATRP of styrene and methyl methacrylate [10]. A series ofmono- and dimeric iron(II) complexes of a-diimine [11] or

1387-7003/$ - see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.inoche.2008.02.002

* Corresponding author. Tel.: +39 0 899 69568; fax: +39 0 899 69603.E-mail address: smilione@unisa.it (S. Milione).

iminopyridine [12] ligands were later on synthesized and suc-cessfully tested under similar conditions. Noteworthy, themonomeric complexes were more effective than the corre-sponding dimeric ones, as expected on the basis of reducedsteric hindrance and easier accessibility to the metal center[11]. However, all factors determining the different reactivityof these iron complexes in ATRP of vinyl monomers are notclear and are topic of current investigation.

Herein, we report on the synthesis and characterizationof the iron(II) dimeric complex [(dab)FeCl-(l-Cl)]2 bearingthe a-diimine 1,4-bis-cyclohexyl-1,4-diazabutadiene (dab)ligand and the role of this compound in ATRP of styrene.

[(dab)FeCl-(l-Cl)]2 was readily prepared according toScheme 1 by stirring a solution of FeCl2(THF)1.5 withone equivalent of the a-diimine ligand in refluxing tetrahy-drofuran followed by extraction with dichloromethane.The expected complex was isolated in high yield asmagenta air stable microcrystalline powder. Single crystalswere grown from concentrated dichloromethane solutionsat �40 �C. The X-ray structure determination revealed adimeric species with crystallographic inversion symmetry(Fig. 1). The geometry at each iron center is best describedas distorted trigonal bipyramid showing a noticeableenlargement of the Cl–Fe–Cl angle [134.87(3)�] and a smallcontraction of the N–Fe–Cl angles [105.01(5)�] in the

Fig. 1. 1H NMR (toluene-d8, 25 �C) spectrum of [(dab)FeCl-(l-Cl)]2. Thesignals labeled with a star are due to the protio impurities of thedeuterated solvent.

N

N

N

N

N

NFe

Cl

FeCl

Cl

Cl

+ FeCl2(THF)1.5

Scheme 1.

536 R. Ferro et al. / Inorganic Chemistry Communications 11 (2008) 535–538

equatorial plane. The N(2)–Fe–Cl(1*) angle exhibits aslight difference from 180� [176.50(5)�] and the N(1)–Fe–N(2) angle is substantially contracted [77.06(7)�] as a con-sequence of the bite of the five-membered chelate ring.

Fig. 2. ORTEP diagram of [(dab)FeCl-(l-Cl)]2. For clarity, the hydrogen

The Fe–N bond lengths are unexceptional, whereas theaxial Fe–Cl bond is shorter than the bridging equatorialone. Notably the chlorine atom bridges are asymmetricwith bond lengths of (Fe–Cl(1*) 2.5625(8) A and Fe–Cl(1)2.3728(8) A), respectively. The atoms of the five-memberedchelate ring are coplanar with a rmsd of 0.0082. Theplanes Fe(1)–N(1)–N(2)–C(1)–(C2) and Fe(1)–Fe(1*)–Cl(1)–Cl(1*) are almost perpendicular with an angle of88.57(5)�. The molecules pack forms isolated planes per-pendicular to c in which van der Waals interactionsbetween the not-bridging chlorine atoms and the C1 andC2 atoms of the diazabutadienyl ligand of a different com-plex were observed. Each dimeric complex is surroundedby four complexes as a result of these interactions. Themolecular structure of [(dab)FeCl-(l-Cl)]2 is similar tothat of bis((l2-chloro)-chloro-(2-(20-pyridyl)quinoxaline)-iron(II)) [13], described as distorted trigonal bipyramid inwhich a comparable enlargement of the Cl–Fe–Cl angle[133.9(1)�] and asymmetric chlorine bridges [Fe–Cl(2)2.511(1) A, Fe–Cl(2*) 2.391(1) A] are observed.

Despite the paramagnetism of the metal center(leff = 4.85 lB in CD2Cl2 at room temperature), the 1HNMR of [(dab)FeCl-(l-Cl)]2 exhibits well resolved signals(see Fig. 2), which were attributed on the basis of the inte-gral values and downfield and upfield shifts resulting fromthe proximity of the protons to the iron center. The 1HNMR pattern is consistent with a centrosymmetric struc-ture producing couples of not-equivalent cyclohexyl andimino protons. The two imino protons are downfieldshifted to 145.46 ppm and 96.43 ppm as a result of the con-tact with the unpaired electron delocalized in the p orbitalsof the diazabutadienyl bridge: a similar effect was previ-ously observed in a-diimine or iminopyridine iron(II) com-plexes [12]. Moreover, the two cyclohexyl rings give rise tosix upfield shifted signals at �30.18, �14.41, �9.41, �7.79,

atoms are omitted. The ellipsoids are drawn at 50% probability level.

Table 1Styrene polymerization using the [(dab)FeCl-(l-Cl)]2–PECl catalysta

Entry Styrene/1-PECl

Time(h)

Conversion(%)

Mw/Mnb Mn

b Mn(th)c

1 200/1 20 50 1.53 87400 104002 400/1 8 28 1.64 97200 116003 400/1 14 31 1.63 108000 129004 400/1 17 46 1.64 110900 192005 400/1 20 58 1.59 112400 241006 600/1 20 60 1.60 123000 375007 800/1 20 66 1.63 149000 550008 1000/1 20 76 1.66 162000 79200

a 1-PECl (6 ll, 44 lmol), [(dab)FeCl-(l-Cl)]2 (30 mg, 44 lmol), styrene(1–5 ml). T = 120 �C.

b Determined by GPC analysis.c Mn(th) = ([M]0/[PECl]0) �MWSt � conversion.

R. Ferro et al. / Inorganic Chemistry Communications 11 (2008) 535–538 537

�5.46 and �4.30 ppm, the half-peak width of the 1H sig-nals increases as the chemical shift is decreased.

The high thermal stability of [(dab)FeCl-(l-Cl)]2 in tol-uene-d8 or C2D2Cl4 solution was assessed by VT 1HNMR experiments, where the invariance of the 1H NMRpattern in the range of temperature 30–120 �C suggests alow tendency to dissociation of the dimeric structure [14].

Polymerization of styrene in bulk catalyzed by [(dab)-FeCl-(l-Cl)]2 in the presence of 1-phenylethyl chloride(1-PECl) was carried out at 120 �C in nitrogen atmosphere.The main results are summarized in Table 1. The number-average molecular weight (Mn) increases with monomer toinitiator ratio and conversion. Moreover the polydispersityindex (PDIs) of the polystyrenes is ca. 1.60. Over all dataset, the Mn are higher than the theoretical molecular weight(Mn(th)) calculated from the equation

MnðthÞ ¼ ½St�=½1-PECl� �MWSt � conversion

The apparent initiator efficiency f (f = Mn(th)/Mn) lowerthan one (0.30 as average value) indicated that the numberof propagating radical species is lower than that expectedon the basis of the molar concentration of the initiator.However, the self initiation process can not be ruled out,because of long polymerization times necessary for achiev-ing high monomer conversion.

Aiming to improve the polymerization results, the sty-rene polymerizations were also investigated using 1-phenyl-ethyl bromide (1-PEBr) as initiator. Typically the use ofbromine as transferable group is recognized to hold advan-tages in the activation reaction, because of lower C–X bondenergy [15]. Although, the PDIs of the polystyrenes wereslightly lower (typically ca. 1.50), apparently suggesting abetter control, the plot of Mn vs. the monomer to initiatorratio in the feed is not linear. Moreover, the Mn are higherthan Mn(th) and hence the average value of f lower thanone.

Previous studies by Gibson et al. showed that the mono-meric Fe(dab)Cl2 behaves as efficient catalyst for wellcontrolled ATRP of styrene and methyl methacrylate.The experimental Mn values are in agreement with the cal-culated ones and the PDIs are typically of ca. 1.30, as

expected with a well controlled polymerization mechanism[11].

In conclusion the results herein reported confirm the com-monly accepted proposal that the dimeric iron complexeslead to a poorer control than the monomeric ones in ATRPprocesses. In some cases, the dissociation of the dimer to themonomer species is postulated as the first step to generate theactive catalyst. The lower efficiency and the kobs, found threetimes lower than that observed for the monomeric isomer[11], permit us to conclude that the involvement of thedimeric form in ATRP of styrene in bulk cannot in principlebe ruled out.

Acknowledgement

We gratefully acknowledge the Ministero dell’Universitae della Ricerca Scientifica (MURST, Roma – Italy) forfinancial support

Appendix A. Supplementary material

CCDC 667971 contains the supplementary crystallo-graphic data for 1. These data can be obtained free ofcharge from The Cambridge Crystallographic Data Centrevia www.ccdc.cam.ac.uk/data_request/cif. Supplementarydata associated with this article can be found, in the onlineversion, at doi:10.1016/j.inoche.2008.02.002.

References

[1] (a) M. Kato, M. Kamigaito, M. Sawamoto, T. Higashimura,Macromolecules 28 (1995) 1721;(b) J.S. Wang, K. Matyjaszewski, J. Am. Chem. Soc. 117 (1995) 5614.

[2] K. Matyjaszewski, J. Xia, Chem. Rev. 101 (2001) 2921.[3] (a) M. Kamigaito, T. Ando, M. Sawamoto, Chem. Rev. 101 (2001) 3689;

(b) F. Simal, A. Demonceau, A.F. Noels, Angew. Chem., Int. Ed. 38(1999) 538.

[4] (a) T. Ando, M. Kamigaito, M. Sawamoto, Macromolecules 30(1997) 4507;(b) K. Matyjaszewski, M. Wei, J. Xia, N.E. Mc Dermott, Macro-molecules 30 (1997) 8161;(c) R. Ferro, S. Milione, V. Bertolasi, C. Capacchione, A. Grassi,Macromolecules 40 (2007) 8544;(d) B. Gobelt, K. Matyjaszewski, Macromol. Chem. Phys. 201 (2000)1619;(e) J. Louis, R.H. Grubbs, Chem. Commun. (2000) 1479;(f) M. Wakioka, K.-Y. Beak, T. Ando, M. Kamigaito, M. Sawamoto,Macromolecules 35 (2002) 330.

[5] (a) C. Granel, P. Dubois, R. Jerome, P. Teyssie, Macromolecules 29(1996) 8576;(b) H. Uegaki, Y. Kotani, M. Kamigaito, M. Sawamoto, Macro-molecules 30 (1997) 2249;(c) H. Uegaki, Y. Kotani, M. Kamigaito, M. Sawamoto, Macromol-ecules 31 (1998) 6756;(d) G. Moineau, M. Minet, P. Dubois, P. Teyssie, T. Senninger,R. Jerome, Macromolecules 32 (1999) 27.

[6] P. Lecomte, J. Drapier, P. Dubois, P. Teyssie, R. Jerome, Macro-molecules 30 (1997) 7631.

[7] G. Moineau, C. Granel, P. Dubois, R. Jerome, P. Teyssie, Macro-molecules 31 (1998) 542.

[8] (a) E. Le Grognec, J. Claverie, R. Poli, J. Am. Chem. Soc. 123 (2001)9513;

538 R. Ferro et al. / Inorganic Chemistry Communications 11 (2008) 535–538

(b) F. Stoffelbach, D.M. Haddleton, R. Poli, Eur. Polym. J. 39 (2003)2099.

[9] (a) A.D. Asandei, I.W. Moran, J. Am. Chem. Soc. 126 (2004)15932;(b) A.D. Asandei, Y. Chen, I.W. Moran, G. Saha, J. Organomet.Chem. 692 (2007) 3174.

[10] (a) X.-P. Chen, K.-Y. Qiu, J. Appl. Polym. Sci. 77 (2000) 1607;(b) X.-P. Chen, K.-Y. Qiu, Polym. Int. 49 (2000) 1529;(c) X.-P. Chen, K.-Y. Qiu, Macromolecules 32 (1999) 8711.

[11] (a) R.K. O’Reilly, M.P. Shaver, V.C. Gibson, A.J.P. White, Macro-molecules 40 (2007) 7441;(b) V.C. Gibson, R.K. O’Reilly, W. Reed, D.F. Wass, A.J.P. White,D.J. Williams, Chem. Commun. (2002) 1850;

(c) V.C. Gibson, R.K. O’Reilly, D.F. Wass, A.J.P. White, D.J.Williams, Macromolecules 36 (2003) 2591;(d) M.P. Shaver, L.E.N. Allan, H.S. Rzepa, V.C. Gibson, Angew.Chem., Int. Ed. 45 (2006) 1241.

[12] V.C. Gibson, R.K. O’Reilly, D.F. Wass, A.J.P. White, D.J. Williams,Dalton Trans. (2003) 2824.

[13] S. Kasselouri, A. Garoufis, S.P. Perlepes, F. Lutz, R. Bau,S. Gourbatsis, N. Hadjiliadis, Polyhedron 17 (1998) 1281.

[14] The plots of representative hyperfine shifted signals vs. 1/T for[(dab)FeCl-(l-Cl)]2 were linear in the range 30–120 �C (see supple-mentary material).

[15] M.B. Gilles, K. Matyjaszewski, P. Norrby, T. Pintauer, R. Poli,P. Richard, Macromolecules 36 (2003) 8551.