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Journal of Organometallic Chemistry 767 (2014) 22e26

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Journal of Organometallic Chemistry

journal homepage: www.elsevier .com/locate/ jorganchem

New ionic fluorenylnickel complexes: Synthesis and solid statestructure

Andrzej Kozioł a, Lucjan B. Jerzykiewicz b, Iwona Justyniak c, Tadeusz Lis b,Stanisław Pasynkiewicz a, Antoni Pietrzykowski a, *

a Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warsaw, Polandb University of Wroclaw, Faculty of Chemistry, Joliot-Curie 14, 50-353 Wrocław, Polandc Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland

a r t i c l e i n f o

Article history:Received 3 April 2014Received in revised form20 May 2014Accepted 22 May 2014Available online 2 June 2014

Keywords:NickelFluoreneCrystal structure determination

* Corresponding author. Tel.: þ48 22 234 7116; faxE-mail address: anpietr@ch.pw.edu.pl (A. Pietrzyk

http://dx.doi.org/10.1016/j.jorganchem.2014.05.0260022-328X/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

New fluorenylnickel complexes [M(THF)n]þ[Ni(h5-Flu)(h1-Flu)2]� (M ¼ þ48 Li, 1 and M ¼ Na, 2) havebeen obtained in the reaction of nickel bromide with fluorenyllithium and -sodium. The X-ray molecularstructures of 1 (monoclinic; P21/c) and 2 (orthorhombic; Fdd2) reveal that in both complexes two flu-orenyl ligands are bonded to the nickel atom by h1 bond and the third one is bonded to the nickel atomby h5 bond. Complexes 1 and 2 are the first structurally characterized nickel complexes bearing fluorenylligands.

© 2014 Elsevier B.V. All rights reserved.

Introduction

Fluorenyl group is an attractive ligand for organometallicchemistry for several reasons. It can be bonded to themetal atom invarious modes. Its complexes exhibit catalytic activity in manypolymerization reactions. It is bulky and plane and easily forms freeradicals and carbene species. Many transition (Ti [1], Zr [2,3], Hf [3],Nb [2], Cr [4], Mn [5], Mo [6], Fe [7], Os [8], Re [9], Ru [10], Co [11])and main group (Ba [12], K [13], Li [14], Na [15], Ca [16]) metalcomplexes, bearing one fluorenyl ligand in h5-bonding mode, havebeen described up to now.

Transition (Mo [17], Fe [18], Co [18], Mn [5,18], Zr [19], Zn [20], Y[21], Re [9], Rh [22], Ti [23], Cu [24], Au [25]) and main group (Na[26], Ga [27], Ge [28], Sn [29], Si [14], Al [30], In [27], Mg [31]) metalcomplexes, bonded to the fluorenyl ligand in h1-bonding mode, arealso known. The fluorenyl ligand can also be bonded to metal in h3

and h6 modes [32e34] (Fig. 1).According to our knowledge, transition metal complexes of

groups 5e10 with two h5-bonded fluorenyl ligands have not beenreported up to now. However, Yb(h5-Flu)2(THF)2 [35] and Sm(h5-Flu)(h3-Flu)(THF)2 have been obtained [36]. The other examples ofcompounds with two fluorenyl ligands bonded to metal in h5-

: þ48 22 234 5462.owski).

bonding mode have been known for zirconium [37] and barium[38].

Nickel complexes with fluorenyl ligands have not beendescribed up to now. Ferriera de Silva and co-workers carried out anumber of gas-phase reactions between fluorene and transitionmetals [39]. They reported that fluorenylnickel compounds werenot formed in these reactions.

In this work we present synthesis and structure of novel fluo-renylnickel complexes, in which the nickel atom is bonded to thefluorenyl ligands in h5- and s-bonding modes. As far as we knowthere are not known analogous complexes of other metals. Exam-ples of complexes bearing one s- and one h5-bonded fluorenyl li-gands are known for zirconium [40] and titanium [23]. One s-fluorenyl and two h5-cyclopentadienyl ligands are present in zir-conium complex [19]. The ionic complex [Li(THF)4]þ[Cu(h1-Flu)2PPh3]� was obtained by Wright et al. [24].

Results and discussion

Trifluorenylnickel complexes, [Li(THF)4]þ[Ni(h5-Flu)(h1-Flu)2]�

1 and [Na(THF)5]þ[Ni(h5-Flu)(h1-Flu)2] 2, were obtained in the re-actions of NiBr2(DME) with fluorenyllithium and -sodium intetrahydrofuran. Two of the fluorenyl ligands are in the s-bondingmode, while the third one is h5-bonded to the nickel atom. Bis-acetylacetonate nickel reacts with fluorenyllithium in the same

Fig. 1. Bonding modes of the fluorenyl ligand to metals.

Fig. 2. Molecular structure of [Li(THF)4]þ[Ni(h5-Flu)(h1-Flu)2]� 1 showing the atomnumbering scheme. Thermal ellipsoids are drawn at the 50% probability level.Hydrogen atoms were omitted for clarity. Selected bonds (Å) and angles (�): Ni(1)eC(9A) 2.013(2); Ni(1)eC(9B) 1.996(2); Ni(1)eC(9C) 2.048(2); Ni(1)eC(10C) 2.228(2);Ni(1)eC(11C) 2.346(2); Ni(1)eC(12C) 2.310(2); Ni(1)eC(13C) 2.180(2); Ni(1)eC(9A)eFluAcent 118.3(1); C(9A)eNi(1)eC(9B) 98.3(1); Ni(1)eC(9B)eFluBcent 117.1(1); C(9B)eNi(1)eFluCcent 133.5(1); Ni(1)eC(9C)-FluCcent 63.4(1); C(9A)eNi(1)eFluCcent 128.1(1).

A. Kozioł et al. / Journal of Organometallic Chemistry 767 (2014) 22e26 23

manner. On the other hand NiCl2(NH3)6 does not react with fluo-renyllithium and -sodium.

The compounds 1 and 2 can be crystallized from tetrahydro-furan at �78 �C forming dark green crystals. They are sensitivetowards oxygen and moisture but in an inert atmosphere (argon)are quite stable up to about 40 �C, where a slow decompositionbegins. 1 and 2 are insoluble in hexane, toluene and acetonitrile.They are soluble in diethyl ether, dichloromethane and dioxane, butsolutions are not stable and decompose after few minutes formingblack nickel powder and 9,90-bisfluorenyl. Solutions of 1 and 2 intetrahydrofuran were stable enough to be characterized by 1H and13C NMR.

1H and 13C NMR spectra of the compounds 1 and 2 are almostidentical. The 1H NMR spectra exhibit only one set of signals fromdifferently bonded fluorenyl ligands: doublet at 7.4 ppm(3JHeH ¼ 8 Hz) from protons bonded to C-4 and C-5, multiplet at~7 ppm from protons at C-2, C-3, C-6 and C-7 and multiplet at~6.3 ppm fromprotons at C-1 and C-8. This suggests a fast exchangein hapticity between three fluorenyl groups. Low temperaturespectra (at �60 �C) did not show any significant change besidessome broadening of signals. A further lowering of the temperatureto �100 �C did not allow us to make a definitive conclusion mainlydue to a very low sample concentration. Signals of protons at C-9were not observed. Signals of complexed tetrahydrofuran mole-cules were observed at 3.54 and 1.69 ppm. 13C NMR spectra exhibitsignals at the range 145e141 ppm for C-80, C-90 and C-40 C-50 carbonatoms, at 123e121 ppm for C-2, C-7 and C-3, C-6, and at119e118 ppm for C-1, C-8 and C-4, C-5. The signal of C-9 appearedat 50.1 ppm (this signal was observed only for the complex 1).Signals of complexed tetrahydrofuran molecules were observed at67.8 and 25.9 ppm.

The molecular structures of the compounds 1 and 2 weredetermined by X-ray (Figs. 2 and 3). The crystal data and structurerefinement parameters for 1 and 2 are collected in Table 1. Thecompound 1 crystallizes in monoclinic crystal system in spacegroup P21/c (Z ¼ 4). The compound 2 crystallizes in orthorhombiccrystal system, space group Fdd2 (Z ¼ 48) with three crystallo-graphically independent anions and cations. Dimensions of the unitcell of 2 are: a ¼ 37.420(9) Å; b ¼ 118.55(3) Å; c ¼ 13.846(3) Å. Thisis a very large unit cell what is unusual for this kind of complexes.The atom numbering scheme for 1 is shown on Fig. 2 and for 2 onFig. 3. Both complexes have ion-separated structures, wherelithium cation is solvated by four THF molecules, and sodium issolvated by five THF molecules.

The compounds 1 and 2 contain the same anion and differencesappearing in the Li or Na salt and between the different moleculesin the Na salt are not chemically relevant. Therefore, the detaileddiscussion is concentrated on the anion of the compound 1.

The NieC9 distances between nickel atom and two fluorenylligands (FluA and FluB) are Ni(1)eC(9A) 2.013(2)Å and Ni(1)eC(9B)

1.996(2) Å. The angles Ni(1)eC(9A)eFluAcent and Ni(1)eC(9B)eFluBcent are 118.3(1)� and 117.1(1)� respectively, indicating s-bonding mode of these two ligands.

The two planes drawn through all carbon atoms of FluA and FluBare almost perpendicular with the angle between them 81.2(1)� in1 and 84.1(1)� in 2. Such arrangement of these ligands allowedminimizing repulsion between them.

The average CeC bond lengths of six-membered rings in FluAand FluB are 1.398 Å and 1.396 Å indicating aromatic character ofthese rings.

Six-membered rings of the FluC ligand are planar. There areunequivocal differences between bond lengths in these rings(Fig. 4). The C(1C)eC(2C), C(3C)eC(4C), C(5C)eC(6C) and C(7C)eC(8C) bonds (av. 1.377 Å) are much shorter than the others (av.1.416 Å) indicating localized double bonds. The plausible reason ofa visible loss of aromatic character of six-membered rings of theFluC ligand is the higher participation of p-electrons of five-membered ring in the bond to the nickel atom. NieFluA and Nie-FluB are s-bonded to nickel, which does not disturb the aromaticnature of the six-membered rings.

Bonds C(9C)eC(10C) and C(9C)eC(13C) (Fig. 4) of FluC five-membered ring are shorter than corresponding bonds in FluA andFluB, bonded to nickel in h1-bondingmode. Those differences are inthe range 0.029e0.040 Å. This indicates an increase of electrondensity in five-membered ring of FluC.

The angle Ni(1)eC(9C)eFluCcent equal 63.4(1)� what indicatesthe position of the nickel atom over the five-membered ring and ischaracteristic for p-bonding mode. The coordination modes of five-membered ring to nickel can be determined by slip-fold distortion[41]. To define a bonding mode of the Ni atom to the fluorenylligand FluC, we have calculated the slip parameter DMeC, i.e. thedifference between the average Ni1eC distances to C11 and C12and those to C10 and C13. The value for perfect h5 coordinationshould be 0, although values till about 0.2 indicate this type ofcoordination. For “true” h3 coordination DM-C values have beenreported of ca. 0.69e0.79 [41]. The value of DM�C in 1 is ca. 0.12 Å,and ca. 0.09 Å in all three independent molecules of the compound2, what indicates h5-bonding mode of Ni to the fluorenyl ligandFluC. It seems that fluorenylnickel complexes prefer 18 valenceelectron (VE) configuration over a possible 16 VE configuration as in

Fig. 3. Molecular structure of [Na(THF)5]þ[Ni(h5-Flu)(h1-Flu)2]� 2 showing the atomnumbering scheme. Thermal ellipsoids are drawn at the 50% probability level.Hydrogen atoms were omitted for clarity. Selected bonds (Å) and angles (�): Ni(1)eC(9A) 2.009(4); Ni(1)eC(9B) 2.021(3); Ni(1)eC(9C) 2.058(3); Ni(1)eC(10C) 2.193(3);Ni(1)eC(11C) 2.294(4); Ni(1)eC(12C) 2.296(4); Ni(1)eC(13C) 2.219(3); Ni(1)eC(9A)eFluAcent 117.1(2); C(9A)eNi(1)eC(9B) 98.9(1); Ni(1)eC(9B)eFluBcent 119.5(3); C(9B)eNi(1)eFluCcent 127.5(1); Ni(1)eC(9C)eFluCcent 62.2(2); C(9A)eNi(1)eFluCcent 133.(1).

Table 1Summary of crystallographic data for the complexes 1 and 2.

[Li(THF)4]þ[Ni(h5-Flu)(h1-Flu)2]� 1

[Na(THF)5]þ[Ni(h5-Flu)(h1-Flu)2]� � 0.5 THF 2

Empirical formula C55H59LiNiO4 C61H71NaNiO5.5

Crystal size (mm) 0.14 � 0.09 � 0.03 0.40 � 0.30 � 0.25Crystal system Monoclinic OrthorhombicSpace group P21/c Fdd2a (Å) 9.893(3) 37.420(9)b (Å) 19.378(4) 118.55(3)c (Å) 24.580(5) 13.846(3)В (�) 111.68(3)Volume (Å3) 4379(2) 61423(25)Z 4 48Formula weight 849.68 973.88Dcalc (mg m�3) 1.290 1.264Temperature (K) 100(2) 100(2)Absorption

coefficient (mm�1)0.491 1.024

F(000) 1808.0 24960Radiation MoKa

(l ¼ 0.71073 Å,graphitemonochromator)

CuKa

(l ¼ 1.54180 Å,graphitemonochromator)

q Range for datacollection (�)

1.38e24.71 2.98e77.04

Scan type u u and 4

Absorptioncorrection Tmin, Tmax

None Analytical transmission0.744, 0.824

Index ranges �11 � h � 11�20 � k � 22�28 � l � 28

�28 � h � 46�134 � k � 149�17 � l � 17

Reflections collected 13505 152144Unique data 7470, Rint ¼ 0.0319 31435, Rint ¼ 0.0529Refinement method Full-matrix

least-squareson F2

Full-matrixleast-squareson F2

number of parameters 550 1854Goodness-of-fit on F2a 1.073 1.047Final R indices [I > 2s(I)]b R1 ¼ 0.0427

wR2 ¼ 0.0812R1 ¼ 0.0581wR2 ¼ 0.1391

R indices (all data) R1 ¼ 0.0540wR2 ¼ 0.0848

R1 ¼ 0.0737wR2 ¼ 0.1444

Largest resid. (e �3) 0.54/�0.35 0.49/�0.63Weights a, bc 0, 6.9541 0.0823, 0

a GoodF ¼ S ¼ {P

[u(Fo2 e Fc2)2]/(n � p)}1/2.

b R1 ¼ PjjFoj e jFcj/jPjFoj, wR2 ¼ {

P[u(Fo2 e Fc

2)2]/P

[u(Fo2)2]}1/2.c w ¼ 1/[s2(Fo2) þ (aP)2 þ bP], where P ¼ [2 Fc

2 þ max(Fo2,0)]/3.

A. Kozioł et al. / Journal of Organometallic Chemistry 767 (2014) 22e2624

the structurally related [Ni(h3-allyl)(h1-allyl)2]� anion described byP€orschke et al. [42].

The small fold-angles, defined as an angles between planesderived through C9, C10, C13 and C10, C11, C12, C13, are observed ineach of FluA, FluB and FluC (7.1�, 6.1� and 6.3� respectively), how-ever, due to rigidity of fluorenyl ligands, these values are notcharacteristic and do not allow distinguishing between bondingmodes of the fluorenyl ligands.

The three independent [Ni(h5-Flu)(h1-Flu)2]� anions in 2 havesimilar structures, therefore, the bond lengths and angles listed inFig. 3 caption are for the molecule with Ni1 atom. The bond dis-tances between nickel and carbon atoms in two fluorenyl ligandsFluA, FluB of the crystals 2 are comparable with correspondingdistances of s-bonded ligands in the complex 1.

The distances between the center of five-membered rings inFluC and Ni atoms in both complexes are comparable (1.858 Å for 1and 1.834 Å for 2). A comparison of those distances with averagedistances for cyclopentadienyl, pentamethylcyclopentadienyl andindenyl ligands (Cp e 1.752 Å, Cp* e 1.757 Å, Ind e 1.815 Å [43])suggests that NieCcent distance increases with an increase of theligand size.

A plausible course of the reaction between fluorenyllithium andnickel bromide is presented on Scheme 1.

An exchange reaction between NiBr2 and MFlu (M ¼ Li or Na)leads to the formation of an unstable 12 valence electron (VE){NiBr(h1-Flu)} complex which undergoes h1 / h5 rearrangementof the fluorenyl ligand formingmore stable 16VE complex {NiBr(h5-Flu)}. This complex reacts with the second molecule of MFluforming 16VE complex {Ni(h5-Flu)(h1-Flu)}. The further reactionproceeds with the next MFlu molecule to form the final, isolatedand fully characterized, “ate complexes” 1 and 2. Such type of anionic complex, Liþ[Ni(h5-Cp)(h1-Cp)(h1-Cp-d5)]�, was postulatedas an intermediate in the reaction of nickelocene with LiC5D5 [44].The structural characterization of 1 and 2 is the first proof that theformation of such “ate complexes” is possible and suggests that themechanism of cyclopentadienyl ligand exchange proposed bySwitzer and Rettig [44] was correct.

Conclusions

Complexes 1 and 2 represent the first structurally characterizedexamples of fluorenylnickel complexes. Fluorenyl ligands arebonded to nickel atom in h5- and s-bonding modes. Contrary tonickelocene and bis-indenylnickel, bis-fluorenylnickel, with paral-lel arrangement of two fluorenyl ligands, cannot be formed. This is

Fig. 4. Bond lengths of FluC in 1.

A. Kozioł et al. / Journal of Organometallic Chemistry 767 (2014) 22e26 25

probably caused by a repulsion of p-electron systems of the fluo-renyl ligands. Arrangements of fluorenyl ligands observed in thecomplexes 1 and 2 allow decreasing these repulsive interactions. Inthese complexes nickel adopts 18 VE configurations. Moreover, theformation of these complexes proves that h5 / s rearrangementsof cyclopentadienyl and similar ligands is possible, and that thistype ionic complexes can be formed as intermediates in ligandexchange reactions of nickelocene and its analogs.

Experimental section

All operations and experiments with metal complexes werecarried out under an atmosphere of purified argon using standardSchlenk techniques. Solvents were distilled from potassiumbenzophenone ketyl. Fluorenyllithium and fluorenylsodium weresynthesized according to the literature procedures [26,45]. Fluo-rene was purchased from Aldrich. 1H NMR (400 MHz) and 13C NMR(100 MHz) spectra were recorded on Mercury-400BB spectrometerin C6D6 at ambient temperature. Electrospray mass spectra wererecorded on Applied Biosystems 4000QTRAP spectrometer.Elemental analyses were made on Vario EL instrument fromElementar.

Reaction of fluorenyllithium with NiBr2(DME)

Fluorenyllithium in THF (21.3 cm3, 0.415 M, 8.86 mmol) wasadded to the solution of NiBr2(DME) (0.91 g, 2.95 mmol) in THF(16 cm3). The reaction mixture was stirred at room temperature for1.5 h, then it was cooled to�78 �C and hexane (60 cm3) was added.After 15 min at room temperature, green crystals were filtered off

Scheme 1.

and washed with cold hexane (20 cm3). The final product was driedunder reduced pressure. [Li(THF)4]þ[Ni(h5-Flu)(h1-Flu)2]� was ob-tained with 94% yield (2.34 g, 2.76 mmol).

1H NMR (400 MHz; THF-d8; 25 �C; ppm): d ¼ 7.40 (d;3JHH ¼ 8 Hz; 6H; 4;5-Ar); 6.87e6.80 (m; 12H; 2;3;6;7-Ar); 6.34(broad; 6H; 1;8-Ar); 3.54 (m; 16H; eOCH2e); 1.7 (m; 16H; eCH2e).13C NMR (100MHz; THF-d8; 25 �C; ppm): d¼ 145.3 (80; 90-Ar); 141.9(40; 50-Ar); 122.7; 121.3 (2;7; 3; 6-Ar); 119.0; 118.3 (1; 8; 4; 5; 8-Ar);67.80 (eOCH2e); 50.1 (9-Ar); 25.9; (eCH2e). Elemental analysis:calcd. for C55H59LiNiO4: C, 77.75; H, 7.00; found: C, 77.03; H, 7.06.Attempts to get ESI mass spectra were unsuccessful.

Reaction of fluorenylsodium with NiBr2(DME)

Fluorenylsodium in THF (38 cm3, 0.41 M, 15.63 mmol) wasadded to the solution of NiBr2(DME) (1.6 g, 5.2 mmol) in THF(10 cm3). The reaction mixture was stirred at room temperature for50 min and then hexane (60 cm3) was added. Green crystals werefiltered off andwashedwith hexane (20 cm3). The final product wasdried under reduced pressure. [Na(THF)5]þ[Ni(h5-Flu)(h1-Flu)2]�

was obtained with 78% yield (3.79 g, 4.05 mmol). 1H NMR(400 MHz; THF-d8; 25 �C; ppm): d ¼ 7.40 (d; 3JHH ¼ 4 Hz; 6H; 4;5-Ar); 6.87e6.81 (m; 12H; 2;3;6;7-Ar); 6.35 (broad; 6H; 1;8-Ar); 3.53(m; 20H; eOCH2e); 1.69 (m; 20H; eCH2e). 13C NMR (100 MHz;THF-d8; 25 �C; ppm): d ¼ 143.7 (80;90-Ar); 142.2 (40;50-Ar); 122.8;121.5 (2;7;3;6-Ar); 119.1; 118.4 (1;8; 4;5-Ar); 67.4 (eCH2e);26.0 (eOCH2e). Elemental analysis: calcd. for C59H67NaNiO5: C,75.56; H, 7.20; found: C, 74.43; H, 7.34. Attempts to get ESI massspectra were unsuccessful.

Reaction of fluorenyllithium with Ni(acac)2

Fluorenyllithium in THF (5.1 cm3, 0.56 M, 2.88 mmol) was addedto the solution of Ni(acac)2 (0.24 g, 0.93 mmol) in THF (25 cm3). Thereaction mixture was stirred at room temperature for 1 h, and thenit was cooled to �78 �C and left for 3 days forming green crystals.The crystals were filtered off at �78 �C and dried under reducedpressure. [Li(THF)4]þ[Ni(h5-Flu)(h1-Flu)2]� was obtained with 57%yield (0.45 g, 0.53 mmol).

Crystal structure determination

Data collection and cell refinement were carried out using KumaKM-4 CCD for 1 or Xcalibur PX CCD diffractometer at 100 K for 2with graphite monochromated Mo Ka or Cu Ka radiations,respectively. The diffraction intensities were corrected for Lorentzand polarization factors. The analytical absorption corrections for 2were made by using CrysAlis-RED. The structures were solved bydirect methods and refined with all data on F2 with SHELXTL [46].All non-hydrogen atoms were refined anisotropically. The positionsof hydrogen atoms were calculated and treated as riding atomswith fixed thermal parameters. In crystals of 2 one Ni atomsexhibited extremely prolate displacement ellipsoids. This apparentdisorder, which was modeled by splitting Ni(2) atom between twosites e occupancy factors for these two positions were set as 0.95and 0.05, respectively.

Acknowledgments

This work was financially supported by National Science Centre(grant no. N N205 012234). AK is grateful for support of this workby the European Union within the framework of European SocialFund through the Warsaw University of Technology DevelopmentProgramme, realized by Centre for Advanced Studies.

A. Kozioł et al. / Journal of Organometallic Chemistry 767 (2014) 22e2626

Appendix A. Supplementary material

CCDC 972103 and 972102 contain the supplementary crystal-lographic data for this paper. These data can be obtained free ofcharge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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