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Inorganic Chemistry Communications 7 (2004) 1295–1297

Synthesis, structural characterization and ethylenepolymerization behavior of complex [Ph4P][CrCl3{HB(pz)3}]

[HB(pz)3 = hydrotris(1-pyrazolyl)borate]

Rene Rojas a, Mauricio Valderrama a,*, Guang Wu b

a Departamento de Quımica Inorganica, Facultad de Quımica, Pontificia Universidad Catolica de Chile,

Av. Vicua Mackenna 4860, Casilla 306, Santiago-22, Chileb Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106, USA

Received 2 September 2004; accepted 12 October 2004

Abstract

Reaction of CrCl3(THF)3 with K[HB(pz)3] in THF leads to the formation of the complex K[CrCl3{HB(pz)3}] (1). The salt

metathesis of complex 1 with [Ph4P]Br in CH2Cl2 yields the complex [Ph4P][CrCl3{HB(pz)3}](2). The structure of complex 2 Æ CHCl3has been determined by single crystal X-ray diffraction. In the anion the metal centre shows a distorted octahedral geometry with the

hydrotris(1-pyrazolyl)borate bonded as N,N 0,N00-donor tripod ligand and three chloride atoms completing the co-ordination sphere.

Complex 2 in the presence of MAO leads to the formation of an active catalyst for the polymerization of ethylene.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Chromium; Hydrotris(1-pyrazolyl)borate complexes; Ethylene polymerization; Synthesis

Homogeneous olefin polymerization catalysis based

on half-sandwich cyclopentadienyl chromium(III) com-

plexes has attracted much attention in recent years [1].

These types of complexes commonly stabilized by halide

or alkyl groups and r-donor ligands [2], upon activation

by methylaluminoxane (MAO) provide an excellent

single site catalyst for the polymerization of olefins [3].

Recently we reported the synthesis, structural character-ization and activity in the ethylene polymerization of the

anionic complex [(Me3SiC5H4)CrCl3]�, which shows

high activity, and a large molecular weight distribution

associated to the presence of the three terminal chloride

atoms [4].

In the search of new related polymerization catalysts,

we have prepared chromium(III) complexes containing

1387-7003/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.inoche.2004.10.007

* Corresponding author. Tel.: +56 2 686 4417; fax: +56 2 586 4744.

E-mail address: jmvalder@puc.cl (M. Valderrama).

hydrotris(1-pyrazolyl)borate. This anionic ligand has

been widely used in coordination chemistry as stabiliz-

ing group due to its strong electron donor ability,

capable to form stable complexes with metals in high-

oxidation state [5]. Complexes formed with Group 4

elements, of general formula [TpMCl3] (Tp = hydro-

tris(1-pyrazolyl)borate and its 3,5-dimethyl derivatives;

M = Ti, Zr) exhibit high activities as catalyst in thepolymerization of olefins [6]. In this communication,

we report the synthesis, molecular structure and its

activity in ethylene polymerization reaction of complex

[Ph4P][CrCl3{HB(pz)3}].

The reaction of CrCl3(THF)3 with potassium hydro-

tris(1-pyrazolyl)borate (KHBpz3) in THF solution

leads to the formation of a green solid, characterized

as the complex K[CrCl3{HB(pz)3}] (1), which by treat-ment with tetraphenylphosphonium bromide in CH2Cl2solution yields the complex [Ph4P][CrCl3{HB(pz)3}] (2).

Fig. 1. Molecular structure of complex 2 Æ CHCl3. The hydrogen

atoms are omitted for clarity. Selected bond distances (A) and angles

(�): Cr–Cl(1), 2.3355(11); Cr–Cl(2), 2.3445(11); Cr–Cl(3), 2.3405(11);Cr–N(1), 2.051(3); Cr–N(5), 2.067(3); Cr–N(3), 2.086(3) and Cl(1)–Cr–

Cl(3), 93.75(4); Cl(1)–Cr–Cl(2), 92.34(4); Cl(3)–Cr–Cl(2), 92.79(4) and

N(1)–Cr–Cl(2), 91.67(9); N(5)–Cr–Cl(2), 174.16(10); N(3)–Cr–Cl(2),

89.83(9); N(1)–Cr–N(5), 85.10(12); N(1)–Cr–N(3), 85.37(13); N(5)–

Cr–N(3), 85.06(13); N(1)–Cr–Cl(1), 89.86(10); N(5)–Cr–Cl(1),

92.52(10); N(3)–Cr–Cl(1), 174.82(9); N(1)–Cr–Cl(3), 174.14(9); N(5)-

Cr-Cl(3), 90.13(9).

1296 R. Rojas et al. / Inorganic Chemistry Communications 7 (2004) 1295–1297

This preparation 1 is somewhat different to those

reported in the literature for the similar compound

[Ph4As][CrCl3{HB(pz)3}]H2O. In the latter case the

potassium intermediate is not isolated and the metath-

esis reaction was carried out in a mixture of methanol–

2N HCl [7].Complexes 1 and 2 are stable in air at room tempera-

ture and are soluble in acetone, dichloromethane and

chloroform.The solid state IR spectra inKBrpellets show

aweak absorption band at the range 2480–2520 cm�1 and

two strong band at the range 340–390 cm�1, assigned to m(BH) and m (CrCl), respectively [6,7]. The spin state of the

complexes was confirmed by magnetic susceptibility

measurement (leff � 3.8 lB), which indicates the presenceof three unpaired electrons in the chromium centre.

As expected from a paramagnetic material, the 1H

NMRspectrumofcomplex2 inCDCl3 showsonlyabroad

multiple signal at d 7.9 ppm, assigned to the phenyl pro-

tonsof the cation.The 31PNMRspectrumexhibits a single

resonance at d 25.6 ppm, indicating that the phosphorus

atom is not in the proximity of the paramagnetic chro-

mium centre [(Ph4P)Br,31P NMR (CDCl3): d 23.2 (s)].

The molecular geometry of complex 2 Æ CHCl3 is con-

firmed by a single-crystal X-ray diffraction analysis. 2

Suitable crystals were obtained from a slow diffusion of

diethyl ether into a chloroform solution. The structure

1 (1) CrCl3(THF)3 (87 mg; 0.5 mmol) and potassium hydrotris (1-

pyrazolyl) borate (126 mg; 0.5 mmol) in THF was stirred at room

temperature for 2 h. The green solution formed was evaporated to a

small volume and the addition of diethyl ether gave a light green solid.

Yield 182 mg (98%). Anal. Found: C, 26.3; H, 2.4; N, 19.9%.

C9H10BCl3CrN6K requires: C, 26.3; H, 2.5; N, 20.5%. leff = 3.8 lB.(2) A solution of tetraphenylphosphonium bromide (210 mg; 0.5

mmol) in dichloromethane was added dropwise to a suspension of 1

(205 mg; 0.5 mmol) in CH2Cl2. After stirring 8 h at room temperature,

the reaction mixture was filtered and the filtrate concentrated under

vacuum. The addition of diethyl ether caused the precipitation of a

green solid. Yield 278 mg (78 %). Anal. Found: C, 55.0; H, 4.3; N,

11.2%. C33H30BCl3CrN6P requires: C, 55.8; H, 4.3; N, 11.8%. leff = 3.7

lB. KM = 129 ohm�1 cm2 mol�1.2 C34H31BCl6CrN6P,M = 830.13, green crystal of 0.15 · 0.15 · 0.10

mm in size, triclinic, space group P�1, a = 11.7402(7), b = 12.8653(7),

c = 13.9771(8) A, a = 79.523(2), b = 71.221(2), c = 81.078(2) �,V = 1959.19(19) A3, Z = 2, Dc = 1.407 mg m�3, l(Mo Ka) = 0.775

mm�1, F(000) = 846. Intensity data were collected on a Bruker Smart

CCD diffractometer at 292(1) K, using graphite-monochromated Mo

Ka radiation (k = 0.71073 A). 2.02 < 2h < 26.38 �, �10 6 h 6 14,

�14 6 k 6 16, �17 6 l 6 16, 8463 reflections of which 6475 were

independent (Rint = 0.0163) are used in all calculations (SMART and

SAINT Software, Version 5.1, Madison, WI, 1999). The structure was

solved by direct methods and refined by full-matrix least-squares on F2

(SHELXTL, Version 6.12, Madison, WI, 2001) with anisotropic

displacement parameters on non-all hydrogen atoms. H atoms were

placed at geometrically idealized positions and refined using a riding

model. Final R(F) = 0.0545 for 5301 reflections [I > 2r(I)]; wR

(F2) = 0.1349 for all data; 449 parameters; goodness-of-fit = 1.198.

Maximum peak and hole in final Fourier difference map: +1.119 and

�0.970 e A�3, respectively. Additional crystallographic data are

available (CCDC No. 240825) from the Cambridge Crystallographic

Data Centre.

of the complex, as shown in Fig. 1, consists of discrete

[Ph4P]+ and [CrCl3{HB(pz)3}]

� ions. In the anionic com-

plex themetal centre shows anoctahedral geometrywith a

N,N 0,N00-tridentatepyrazolylborate ligandand three chlo-ride atoms completing the co-ordination sphere. The

Cr–Cl [av. 2.3400(11)) A] and Cr–N [av. 2.068(3) A] bond

distances, and the Cl–Cr–Cl [range 92.34(4)–93.75(4)�]and N–Cr–N [range 85.05(13)–85.36(13)�] bond angles

are comparable to those found in similar complexes ob-

tained with different cations, such as [HPMe3][CrCl3(Tp)]

and [Cr(Tp)2][CrCl3(Tp)] [as example: Cr–Cl: av. 2.324(2)

A; Cr–N, av. 2.068(4) A; Cl–Cr–Cl, range 93.4(1)–94.2(1)�; N–Cr–N, range 83.9(2)–87.0(2)�] [8].

Complex 2 in the presence of methylaluminoxane

(MAO) was employed as an initiator for ethylene

polymerization. 3 The catalytic behavior and proper-

3 The polymerizations were carried out by charging a 100 ml Parr

autoclave reactor with toluene and the desired amount of cocatalyst

(methylaluminoxane, MAO) and catalysts 2, in a glove box under an

inert atmosphere. The reaction vessel would then be sealed, brought

out of the glove box, and placed in an ice-water bath for temperature

control. Ethylene gas was introduced to the reaction, and the

consumption of monomers was monitored by a mass flow controlled

inline with the ethylene feed. At the end of the reaction (30 min) the

ethylene feed was removed, the vessel vented, and the reaction

quenched with HCl–Methanol (20% v/v). The polymers produced

were isolated by filtration and washed several times with acetone. The

polymers were dried overnight under vacuum, and the polymerization

activities were calculated from the mass of product obtained. The

polymers were characterized by GPC (in a High Temperature

Chromatograph, Pl-GPC 200). The polymer melting points were

measured on a Differential Scanning Calorimeter instrument; model

DSC 2920, at a rate of 10 �C/min for three cycles using a temperature

range of 50–200 �C.

Table 1

Ethylene polymerization dataa

Entry Al/Cr Pressure bar Yield (g) Activityb Tm (�C) Mw (·103) Mn Mw/Mn

1 1000 6.89 0.30 14 132.0 89 2200 39

2 1000 20.68 0.94 15 132.0 91 3200 28

3 1000 41.37 2.30 18 134.5 285 20900 14

a Solvent, toluene, 30 ml; precatalyst, 6 lmol; polymerization time, 30 min; stirrer rate, 1000 rpm; reaction temperature, 20 �C.b Kg polymer/(molCr)(h)(bar).

R. Rojas et al. / Inorganic Chemistry Communications 7 (2004) 1295–1297 1297

ties of the polymers are summarized in Table 1.

From the analysis of the data, this new systems per-

formed good activity in comparison to Ti and

Zr(IV)-hydrotris(pyrazolyl)borate complexes [9]. The

polymer yield increases almost linearly with pressure.

The polymers produced by 2/MAO exhibit a broad

(multimodal) molecular weight distribution (Mw/Mn =

39–14),which decreases when the pressure is in-creased. Broad polydispersities can occur if more

than one active species is present, or if the ratio

of the chain propagation to the chain transfer rate

(Rprop/Rtrans) changes during the time of polymeriza-

tion. Thus, in this case, the observation of high

molecular weight and narrower molecular weight dis-

tribution, when the pressure was increased, suggests

that the chain transfer process was depressed dueto the fast insertion of ethylene units because of

the high ethylene concentration in the reactor. There-

fore, there was probably more than one ethylene unit

coordinated to the catalytic site. In relation to the

thermal properties, the melting point of these poly-

mers was over 132 �C, indicating the absence of

branching in the polymer chain.

Acknowledgements

We thank ‘‘Fondo de Desarrollo Cientıfico y Tec-

nologico’’, (FONDECYT, Project No. 1020529), Chile,

for financial support. The authors are grateful to Dr. G.

Bazan from the University of California at Santa Barb-

ara for GPC facilities.

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