Nickel(II)-methyl complexes with water-soluble ligands L [(salicylaldiminato-κ2N,O)NiMe(L)] and their catalytic properties in disperse aqueous systems

Article abstract of DOI:10.1021/om0607191

Neutral (salicylaldiminato)nickel(II) methyl complexes [{6-C(H)=NAr-2,4- I₂C₆H₂O-κ²N,O}NiMe(L)] (Ar = 2,6-{3,5-(F₃C)₂C₆H₃} ₂C₆H₃) with different water-soluble ligands L (2a, L = 1,3,5-triaza-7-phosphaadamantane; 2b, L = hexamethylenetetramine (urotropine); 2c, L -tetraethylammoniumpyridine-3-sulfonate; 2d, L = amino-terminated poly(ethylene glycol) monomethoxy ether) were prepared. 2a - d are potentially water-soluble catalyst precursors for ethylene polymerization, which form a water-insoluble active site [{κ²-N,O}NiR(ethylene) ] (R = growing chain). Only complex 2d was found to be water-soluble (> 2 mmol L^-1); 2c is soluble in water/2-propanol mixtures. In toluene as a reaction medium, only the relatively weakly coordinated tertiary amine complex 2b is polymerization active (1.7 × 10⁴ TO). In aqueous systems 2c,d are also active due to compartmentalization of the active site in the polymer particles and of L in the aqueous phase. Polyethylene particle sizes vary from 18 nm (dispersions formed with 2d) to over 0.5 μm (2c) to suspensions (2b) depending on the initial state of the reaction mixture, correlated with catalyst solubility.

Full text of DOI:10.1021/om0607191

Organometallics 2007, 26, 1311-1316
1311
Articles
Nickel(II)-Methyl Complexes with Water-Soluble Ligands L
[(salicylaldiminato-K2N,O)NiMe(L)] and Their Catalytic Properties
in Disperse Aqueous Systems
Brigitte Korthals, Inigo Go¨ttker-Schnetmann, and Stefan Mecking*
Lehrstuhl fu¨r Chemische Materialwissenschaft, Fachbereich Chemie, UniVersita¨t Konstanz,
UniVersita¨tsstrasse 10, D-78457 Konstanz, Germany
ReceiVed August 8, 2006
Neutral (salicylaldiminato)nickel(II) methyl complexes [{6-C(H)dNAr-2,4-I2C6H2O-κ2N,O}NiMe(L)]
(Ar ) 2,6-{3,5-(F3C)2C6H3}2C6H3) with different water-soluble ligands L (2a, L ) 1,3,5-triaza-7-
phosphaadamantane; 2b, L ) hexamethylenetetramine (urotropine); 2c, L ) tetraethylammonium pyridine-
3-sulfonate; 2d, L ) amino-terminated poly(ethylene glycol) monomethoxy ether) were prepared. 2a-d
are potentially water-soluble catalyst precursors for ethylene polymerization, which form a water-insoluble
active site [{κ2-N,O}NiR(ethylene)] (R ) growing chain). Only complex 2d was found to be water-
soluble (>2 mmol L-1); 2c is soluble in water/2-propanol mixtures. In toluene as a reaction medium,
only the relatively weakly coordinated tertiary amine complex 2b is polymerization active (1.7 × 104
TO). In aqueous systems 2c,d are also active due to compartmentalization of the active site in the polymer
particles and of L in the aqueous phase. Polyethylene particle sizes vary from 18 nm (dispersions formed
with 2d) to over 0.5 µm (2c) to suspensions (2b) depending on the initial state of the reaction mixture,
correlated with catalyst solubility.
mini-3,4 or microemulsions7 of a solution of lipophilic catalyst
precursors in a small amount of hydrocarbon solvent.8 This is
attractive, as most types of catalyst precursors known are
lipophilic compounds. Polymerization with water-soluble com-
plexes is also of interest, however: the absence of organic
solvent simplifies the initial catalyst system as well as the
polymer dispersions obtained in terms of number of phases and
compounds present. This is desirable for, e.g., studies of the
unique particle formation processes in these systems or of
thermal properties of dispersed particles or film formation. In
this context, it is notable that unusually small polymer particles
Introduction
Late transition metal catalysts for polymerization of olefins
have been studied intensely recently.1 Due to their functional
group tolerance, polar monomers can be copolymerized.2 Also,
polymerizations can be carried out in aqueous emulsion to afford
dispersions of submicron polymer particles.3-6 By contrast to
traditional free-radical emulsion polymerization, which is applied
on a large scale for the synthesis of polymer dispersions for,
e.g., environmentally friendly coatings, catalytic polymerization
allows for a control of polymer microstructures and is also
complimentary in terms of monomers polymerizable. Polyeth-
ylene dispersions have been prepared starting from aqueous
(5) Polybutadiene: (a) Ono, H.; Kato, T. J. Polym. Sci., Part. A: Polym.
Chem. 2000, 38, 1083-1089. (b) Monteil, V.; Bastero, A.; Mecking, S.
Macromolecules 2005, 38, 5393-5399. ROMP: (c) Lu, S. -Y.; Quayle,
P.; Booth, C.; Yeates, S. G.; Padget, J. C. Polym. Int. 1993, 32, 1-4. (d)
Ku¨hn, I.; Mohr, B.; Durant, Y.; Schwab, R.; Leyrer, R. (BASF). DE
19859191, 2000. (e) Claverie, J. P.; Viala, S.; Maurel, V.; Novat, C.
Macromolecules 2001, 34, 382-388. (f) Quenemer, D.; Chemtob, A.;
Heroguez, V.; Gnanou, Y. Polymer 2005, 46, 1067-1075. Also cf.: (g)
Lynn, D. M.; Kanaoka, S.; Grubbs, R. H. J. Am. Chem. Soc. 1996, 118,
784-790. Polyketone: (h) Held, A.; Kolb, L.; Zuideveld, M. A.; Thomann,
R.; Mecking, S.; Schmid, M.; Pietruschka, R.; Lindner, E.; Khanfar, M.;
Sunjuk, M. Macromolecules 2002, 35, 3342-3347. Polyalkyne: (i) Huber,
J.; Mecking, S. Angew. Chem. 2006, 45, 6314-6317. Polycycloolefin: (k)
Chemtob, A.; Gilbert, R. G. Macromolecules 2005, 38, 6796-6805. (l)
Skupov, K. M.; Marella, P. R.; Hobbs, J. L.; McIntosh, L. H.; Goodall, B.
L.; Claverie, J. P. Macromolecules 2006, 39, 4279-4281. Poly(1-olefin):
(m) Wehrmann, P.; Mecking, S. Macromolecules 2006, 39, 5963-
5964.
* To whom correspondence should be addressed. E-mail:
stefan.mecking@uni-konstanz.de.
(1) (a) Ittel, S. D.; Johnson, L. K., Brookhart, M. Chem. ReV. 2000, 100,
1169-1203. (b) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103,
283-316. (c) Bauers, F. M.; Mecking, S. Angew. Chem., Int. Ed. 2001,
40, 534-540. (d) Mecking, S. Coord. Chem. ReV. 2000, 203, 325-351.
(2) For selected examples, see: (a) Mecking, S.; Johnson, L. K.; Wang,
L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 888-899. (b) Younkin, T.
R.; Connor, E. F.; Henderson. J. I.; Friedrich, S. K.; Grubbs, R. H.;
Bansleben, D. A. Science 2000, 287, 460-462. (c) Li, W.; Zhang, X.;
Meetsma, A.; Hessen, B. J. Am. Chem. Soc. 2004, 126, 12246-12247. (d)
Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I. Chem.
Commun. 2002, 744-745. (e) Luo, S.; Jordan, R. F. J. Am. Chem. Soc.
2006, 128, 12072-12073.
(3) (a) Held, A.; Bauers, F. M.; Mecking, S. Chem. Commun. 2000, 301-
302. (b) Bauers, F. M.; Mecking, S. Macromolecules 2001, 34, 1165-
1171. (c) Bauers, F. M.; Mecking, S. Angew. Chem., Int. Ed. 2001, 40,
3020-3022.
(4) (a) Tomov, A.; Broyer, J.-P.; Spitz, R. Macromol. Symp. 2000, 150,
53-58. (b) Soula, R.; Novat, C.; Tomov, A.; Spitz, R.; Claverie, J.; Drujon,
X.; Malinge, J.; Saudemont, T. Macromolecules 2001, 34, 2022-2026. (c)
Soula, R.; Sailard, B.; Spitz, R.; Claverie, J. Macromolecules 2002, 35,
1513-1523.
(6) Review: (a) Mecking, S.; Held, A.; Bauers, F. M. Angew. Chem.,
Int. Ed. 2002, 41, 544-561. (b) Mecking, S.; Claverie, J. P. In Late
transition metal polymerization catalysis; Rieger, B., Baugh, L. S., Kacker,
S., Striegler, S., Eds.; Wiley-VCH: Weinheim, Germany, 2003; pp 231-
278. (c) Mecking, S. Colloid Polym. Sci. 2007, in press (published online
Nov 8, 2006).
(7) Monteil, V.; Wehrmann, P.; Mecking, S. J. Am. Chem. Soc. 2005,
127, 14568-14569.
10.1021/om0607191 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 02/06/2007

1312 Organometallics, Vol. 26, No. 6, 2007
Korthals et al.
Chart 1. Possible Generic Structures of Water-Soluble
Neutral Ni(II) Complexes Suitable as Catalyst Precursors
(WSG: Water-Soluble Group)
mines, can be employed and developed organometallic routes
can be adopted for complex synthesis. Darensbourg et al. have
reported a (salicylaldiminato-κ2N,O)nickel(II)-phenyl complex
with the water-soluble phosphine 1,3,5-triaza-7-phosphaada-
mantane (PTA) coordinating the fourth site, [(N∧O)NiPh(PTA)].
The complex was found to be inactive toward ethylene in a
biphasic toluene/water system.15 We have recently communi-
cated studies of ethylene polymerization by aqueous solutions
of water-soluble (salicylaldiminato)nickel(II) methyl complexes
[(N∧O)NiMe(L)] of sulfonated phosphines (L ) triphenylphos-
phine-3,3′,3′′-trisulfonic acid trisodium salt (TPPTS) or tri-
phenylphosphine-3,3′-disulfonic acid disodium salt (TPPDS))
or polyethylene glycol-substituted primary amine (L ) H2N(CH2-
CH2O)nMe).10 Aqueous dispersions of particles as small as 4
nm, as determined by single-angle dynamic light scattering
(DLS), were obtained. Dissociation of the water-soluble ligand
L was found to be enhanced in water due to a compartmental-
ization of L into the aqueous phase and of the active sites into
the particles formed. We now give a full account of the
synthesis, solubilities, and catalytic properties of a series of
complexes with a range of water-soluble ligands L.
of 5-30 nm size can be prepared by catalytic polymerization.7-10
Such particle sizes are challenging to prepare for any polymer
type and preparation method.
Different generic structures are conceivable for water-soluble
complexes suited as catalyst precursors for olefin polymerization
(Chart 1). Water-solubility can be introduced by water-soluble
groups covalently attached to the chelating ligand (A in Chart
1), which controls the catalytic properties of the active site and
ideally stays coordinated to the metal site throughout the
catalytic reaction. This would render the active site [(X∧Y)-
NiR(ethylene)] (R ) growing polymer chain) permanently
hydrophilic, and water-soluble when R is a short alkyl chain.
Attachment of a water-soluble moiety to the alkyl group R (B
in Chart 1) would render the catalyst precursor water-soluble.
Upon chain growth by repeated insertion of monomer into the
metal-alkyl bond, eventually a particle would form by collapse
of the growing chain upon itself. The active site is lipophilic in
this case. This is also the case if water solubility of the catalyst
precursor is brought about by a water-soluble ligand L on the
fourth coordination site (C in Chart 1). Note that the function
of L in general is stabilization of the catalyst precursor but
dissociation under polymerization conditions to provide a
coordination site for monomer binding. By contrast to approach
B, reversible coordination of water-soluble L vs monomer
coordination could also intermittently convert the lipophilic
active species into a hydrophilic dormant species, given that
the growing chain is short and does not induce a strong
lipophilicity of the overall complex [(N∧O)NiR(L)]. Such
dormant species as formed, e.g., after chain transfer, could leave
an existing particle and initiate a new particle.
In early work, Flood reported that an aqueous solution of the
water-soluble cationic rhodium complex [(N∧N∧N)RhMe(OTf)2]+
polymerizes ethylene to low molecular weight material (Mw )
5 × 103 g mol-1) with very low activities (average activity ca.
1 turnover/day), the polymer separating macroscopically from
the aqueous phase (N∧N∧N ) triazacyclononane; in terms of
the above classification, this example is analogous to case A).12
Polymerizations in aqueous emulsions, which afforded poly-
ethylene dispersions, have been studied briefly with four-
coordinate, square-planar complexes of types A and B.13,14 This
contribution studies concept C. A practical advantage of route
C is that existing lipophilic chelating ligands, e.g., salicylaldi-
Results and Discussion
Synthesis and Characterization of Complexes. Four (sali-
cylaldiminato-κ2N,O)nickel(II)-methyl complexes [(N∧O)-
NiMe(L)], 2a-d, with various P- and N-coordinating water-
soluble ligands
L were prepared, L ) 1,3,5-triaza-7-
phosphaadamantane (PTA), hexamethylenetetramine (urotropine),
tetraethylammonium pyridine-3-sulfonate, and amino-terminated
poly(ethylene glycol) monomethoxy ether (Scheme 1). All
complexes are based on salicylaldimine 1. Nickel-methyl
complexes of this salicylaldimine (with L ) pyridine or tmeda)
polymerize ethylene to linear, high-molecular weight polyeth-
ylene at high rates, and polymerization starting from aqueous
mini- or microemulsions of these lipophilic complexes affords
polymer dispersions.16
The synthesis of 2a-d requires the reaction of a water-soluble
ligand (L) with the lipophilic salicylaldimine and [(tmeda)-
NiMe2]. The latter is also temperature sensitive, which prohibits
elevated reaction temperatures to increase solubilities and
reaction rates. For this reason, water-soluble ligands L which
are also soluble in toluene were employed. To solid [(tmeda)-
NiMe2] and L, a cold toluene solution (-50 °C) of the
salicylaldimine was added. Gradual warming of the reaction
mixture to 0 °C afforded clear solutions of 2a-c. Notably, also
the ionic complex 2c is soluble in toluene at the typical
concentrations of complex synthesis, 2.5 × 10-2 mol L-1
.
Complex 2d was synthesized from [(tmeda)NiMe2], salicyl-
aldimine, and PEG amine in benzene, as reported.10 Compounds
2a-d were isolated in 63-83% yield as red solids. Of the
complexes active for catalysis (vide infra), 2b is stable to oxygen
and moisture; even after 1 year in air no decomposition was
1
(8) Mecking, S.; Monteil, V.; Huber, J.;Kolb, L.; Wehrmann, P.
Makromol. Symp. 2006, 236, 117-123.
observed by H NMR spectroscopy. 2c,d are hygroscopic and
both decompose within days in air at room temperature.
(9) Kolb, L.; Monteil, V.; Thomann, R.; Mecking, S. Angew. Chem.,
Int. Ed. 2005, 44, 429-432.
1
In H NMR and 13C NMR spectra of all four complexes a
singlet can be observed between -1 and -1.5 ppm and between
(10) Go¨ttker-Schnetmann, I.; Korthals, B.; Mecking, S. J. Am. Chem.
Soc. 2006, 128, 7708-7709.
(11) (a) Candeau, F. In Polymerization in Organized Media; Paleos, C.
M., Ed.; Gordon and Breach Sci. Publ.: Philadelphia, PA, 1992; pp 215-
283. (b) Antonietti, M.; Basten, R.; Lohmann, S. Macromol. Chem. Phys.
1995, 196, 441-466. (c) Pavel. F. M. J. Dispersion Sci. Technol. 2004,
25, 1-16.
(14) Reference 9. The in situ catalyst prepared may contain a water-
soluble sulfonated phenyl group either attached to the nickel center (approach
B in Chart 1) or incorporated in the chelating phosphinoenolato ligand
(approach A in Chart 1).
(15) Darensbourg, D. J.; Ortiz, C. G.; Yarbrough, J. C. Inorg. Chem.
2003, 42, 6915-6922.
(16) (a) Zuideveld, M.; Wehrmann, P.; Ro¨hr, C.; Mecking, S. Angew.
Chem., Int. Ed. 2004, 43, 869-873. (b) Bastero, A.; Kolb, L.; Wehrmann,
P.; Bauers, F.; Go¨ttker-Schnetmann, I.; Monteil, V.; Thomann, R.;
Chowdhry, M.; Mecking, S. Polym. Mater. Sci. Eng. 2004, 90, 740-741.
(12) Wang, L.; Lu, R. S.; Bau, R.; Flood, T. C. J. Am. Chem. Soc. 1993,
115, 6999-7000.
(13) References 3a,b. The sulfonated phosphinoenolato complex (cf.:
Klabunde, U.; Ittel, S. D. J. Mol. Catal. 1987, 41, 123-134) possesses a
very low activity in neat aqueous solutions, hampering detailed studies.

Nickel(II)-Methyl Complexes
Organometallics, Vol. 26, No. 6, 2007 1313
Scheme 1. General Synthetic Pathway and Complexes Prepared
Table 1. Polymerization Results
Mn
j
catal
(amt (µmol))
reacn
medium
physical state
before polymerizatn
time
(h)
yield
(g)
TON
(×105 g mol-1
)
colloidal state
after polymerizatn
particle sizek
(nm)
entry
(×103)
(Mw/Mn)
1
2
3
4
5
6
7
8
9
2b (10)
2b (10)
2b (10)
2c (10)
2c (5)
2c (10)
2d (10)
2d (10)
2d (20)
toluenea
H2Ob
solution
2
1
1
1
1
3.5
1
1
3.5
4.71
0.75
2.6
-
0.17
9.0
16.8
2.7
9.4
n/a
0.6
32.3
n/a
9.6
2.0 (2.6)
1.9 (2.1)
3.2 (2.3)
n/a
1.3 (2.5)
2.5 (2.2)
n/a
suspension
n/a
n/a
suspension
suspension
solution
suspension
solution in iPrOH
solution
suspension
H2O/iPrOHc
toluenea
H2Ob
latex + suspensiond
n/a
n/a
500
700
n/a
18
latex + suspensione
H2O/iPrOHc
toluenea
H2Ob
latexf
n/a
solution
solution
2.7
9.4
0.88 (1.7)
0.94 (2.5)
latexg
latexh
H2Ob
16.8
230
a Polymerization conditions: 20 °C; 40 bar of C2H4; 100 mL of toluene. b Polymerization conditions: 20 °C; 40 bar of C2H4; 100 mL of H2O; 0.75 g of
SDS. c Polymerization conditions: 20 °C; 40 bar of C2H4; 98 mL of H2O; 2 mL of 2-propanol; 0.75 g of SDS. d Polymer in form of latex/coagulate (3:1).
j
e Latex/coagulate (1:1). f 6% coagulate. g No coagulate. h 95% as latex. Determined by GPC vs linear PE standards at 160 °C in trichlorobenzene. k Volume
average particle sizes determined by DLS.
-6 and -18 ppm, respectively. This shift is characteristic for
nickel(II)-bound methyl groups. For complex 2a no coupling
between phosphorus and the Ni-bound methyl group is detected.
The 31P NMR resonance is shifted from -100.3 ppm for free
PTA to -51.7 ppm for coordinated PTA. Observation of only
one Me signal for all four complexes indicates that only one
isomer with respect to arrangement of L and the methyl ligand
at the square planar nickel center is present. Solid-state structures
of related complexes revealed coordination of the methyl group
trans to oxygen,15,16a,17 and the similar spectroscopic properties
of the compounds studied in this work suggest that this also
the case for 2a-d. Both bis(trifluoromethyl)phenyl rings are
equivalent in solution, most likely due to rotation around the
aryl-aryl bond. The high fluorine content disturbs elemental
analysis, and satisfactory analyses were obtained only in some
cases.
of absorption, a first-order decay curve can be fitted with a half-
life of 31/2 h. With progressing decomposition, an increasing
scattering of light and formation of a yellow precipitate is
observed. These phenomenological observations show that an
irreversible decomposition reaction occurs, possibly hydrolysis
of the Ni-Me bond.19 The observed half-life of several hours
shows that handling of aqueous solutions of 2d in preparation
of polymerization experiments, which usually takes a few
minutes, is uncritical.
Polymerization Studies. All complexes are soluble in
toluene, enabling a systematic study of their catalytic behavior
in this solvent in addition to studies in aqueous systems (Table
1). In accordance with Darensbourg’s studies, the phosphine
(PTA) complex 2a was found to be inactive under all reaction
conditions studied. This differs from the behavior of related
triphenylphosphine complexes.2b,10 Apparently, the alkylphos-
phine PTA, which also has a rather low steric bulk, does not
dissociate from the metal center. By contrast, the nitrogen
analogue, urotropine, dissociates rather readily. 2b is a precursor
to a very active polymerization catalyst at a mild temperature
of 20 °C (entry 1). This behavior of 2b in toluene solution is in
line with previous observations for complexes with L ) tmeda,
that is, complexes also coordinated by a tertiary amine ligand
L.16 Complex 2c, which is soluble in toluene as well, affords
only traces of polymer under identical conditions (20 °C, 40
atm of ethylene) (entry 4). This again is in line with previous
studies of pyridine complexes, which were found to require
slightly elevated temperatures of 50 °C for activation.16a Notably,
the primary amine-coordinated complex 2d was found to be
entirely inactive toward ethylene at 20 °C in toluene solution
(entry 7).
2a,b are insoluble in water (<0.0003 mmol L-1). Solubility
of 2c in water is very low (<0.1 mmol L-1), but it is soluble in
2-propanol (>10 mmol L-1). Complex 2d was found to be
water-soluble; solubility in neat water exceeds 2 mmol L-1 (>6
g L-1).
The stability of 2d in aqueous solution was monitored by
UV/vis spectroscopy. In the absence of oxygen,18 a freshly
prepared solution of the complex in neat water shows a clear
absorption spectrum below 600 nm without scattering (see
Figure S1 in the Supporting Information). With time, the broad
absorption band is shifted hypsochromic; correspondingly the
color of the solution changes from bright red to pale yellow.
The appearance of an isosbestic point indicates that a reaction
is occurring. At 342 nm, the wavelength of the largest change
(17) Connor, E. F.; Younkin, T. R.; Henderson, J. I.; Waltman, A. W.;
Grubbs, R. H. Chem. Comm. 2003, 2272-2273.
(18) The measurements were carried out in a cuvette with a septum.
This same setup was successfully used for investigations of triplet lifetimes
of organic compounds, which would be detoriated by any oxygen present.
Thus, the absence of oxygen in the experiments described in this paper can
be safely assumed.
Polymerizations in aqueous systems were carried out in the
presence of sodium dodecylsulfate (SDS) as a surfactant to
(19) Hristov, I. H.; DeKock, R. L.; Anderson, G. D. W.; Go¨ttker-
Schnetmann, I.; Mecking, S.; Ziegler, T. Inorg. Chem. 2005, 44, 7806-
7818.

1314 Organometallics, Vol. 26, No. 6, 2007
Korthals et al.
stabilize polymer particles formed. The state of the initial
reaction mixtures differed with the solubility properties of the
catalyst precursor. Water-insoluble complexes were suspended
in aqueous SDS solutions, supported by ultrasound. For water-
insoluble complexes, which are soluble in a water-miscible
alcohol, suspending in water in a fine fashion was facilitated
by introducing them as a solution in a small volume of alcohol.
From a suspension of 2b in aqueous SDS solution, im-
mediately after stoppage of the ultrasound treatment, a bright
red precipitate visibly separated, with a colorless supernatant
aqueous phase. This catalyst precursor suspension is active for
polymerization of ethylene (entry 2). The activity is about one-
third vs the activity observed in toluene (entry 1). This lowered
activity is not surprising, ethylene solubility in water as well as
in the semicrystalline polymer (not swollen by any solvent)
being rather low, such that mass transfer limitations may
occur.20,21 The activity can be increased by adding 2-propanol
as a cosolvent. Upon sonication of 2b in pure 2-propanol a red
solid precipitates again immediately when sonication is stopped;
however, the liquid phase stays red, indicating a slight solubility
of the complex. On addition of aqueous SDS solution to the
mixture and exposure to ethylene, the polymer is partly formed
as a latex (entry 3).
From a suspension of 2c in aqueous SDS solution, after
stoppage of the ultrasound treatment, 2c also precipitated visibly.
However, the aqueous phase was orange, indicating that to some
degree the complex remained in the liquid phase. Exposure of
this mixture to ethylene resulted in polymerization (entry 5).
This contrasts to the inactivity observed in toluene solution
(entry 4). As observed previously for water-soluble complexes
[(N∧O)NiMe(L)] (L ) water-soluble phosphine or amine), an
incipient compartmentalization of the lipophilic active species
generated by dissociation of L into the polymer particle formed
and of the ligand L into the aqueous phase retards recombination
of the latter with the active species and, thus, enhances activity.10
Figure 1. Particle sizes and size distributions of polymer disper-
sions as determined by dynamic light scattering (173° backscat-
tering). Numbers refer to entries in Table 1.
coordinating amino function of the PEG amine by water could
also contribute to enhanced dissociation and polymerization
activities in water.
In more detail, for the experiment with the lower catalyst
concentration (entry 8), after 1 h the ethylene consumption had
decreased to the detection limit of the mass flow system. For
entry 9 with a higher catalyst concentration, monomer consump-
tion was detected for 3.5 h (cf. Supporting Information, Figure
S2, for mass flow traces). We assume that the decay in
polymerization activity in these experiments is based on the
decomposition of 2d by hydrolysis (vide supra).
At the lower catalyst concentration (entry 8), a dispersion
with extremly small particles with a volume average size of 18
nm was obtained (Figure 1). The calculated number of particles/
nickel center added to the reaction mixture is ca. 0.2 in this
case. Given the inaccuracies in particle size determination, this
is relatively close to unity on an order of magnitude scale. A
single given active site can generate a polymer particle (for
detailed studies and discussion, cf. ref 10).
As outlined, 2c is soluble in 2-propanol. Upon addition of a
solution of 2c in 2-propanol (2 mL) to an aqueous SDS solution
(98 mL), no formation of precipitate was visibly observed.
Attempts to elucidate the nature of these “solutions” by DLS
failed; unreproducable results were obtained for a given sample.
Possibly, absorption of light by the colored solution interfered
with the measurement, or decomposition of the complex may
have occurred due to contact with air, which cannot be excluded
completely with the setup used. Exposure to ethylene resulted
in polymerization with a high rate to afford a polymer dispersion
along with a small amount of coagulate. Latex particle sizes
were rather large (500 and 700 nm, respectively) with a broad
distribution (Figure 1); accordingly, the intransparent dispersions
were with a milky white appearance.
By contrast to 2a-c, complex 2d is water-soluble and
dissolves in neat water upon stirring to form a homogeneous
solution. Exposure of an aqueous SDS solution of 2d to ethylene
pressure resulted in polymerization to form a colloidally stable
dispersion. The observation that 2d is catalytically active in
water but not in toluene is due to incipient compartmentalization
of the lipophilic active sites into the polymer particles and of L
into the aqueous phase. Location of the active sites and PEG
amine in different phases (particle phase vs aqueous phase)
retards recombination and enhances activity. Solvation of the
At the higher catalyst concentration (entry 9) a dispersion
with a polymer solids content of 9 wt % was obtained,
corresponding to a catalyst productivity of 1.7 × 104 TO. In
this case, a small amount of coagulate was formed, likely due
to an insufficient stabilization of the high area of the large
number of small particles formed by the surfactant. Accordingly,
particle sizes of the dispersion were also significantly higher
with a volume average 230 nm (Figure 1). Presumably,
formation of the final particles involves coagulation of already
existing particles.
As expected from previous polymerization studies with
nickel(II) complexes of salicylaldimine 1, the polymers obtained
possess a high molecular weight on the order of 105 g mol-1
.
Molecular weight distributions Mw/Mn around 2 show that
molecular weights are chain transfer controlled and indicate a
single site nature of the active species. Branching of the
polymers, prepared at a reaction temperature of 20 °C, is well
below 10 methyl branches/1000 carbon atoms as observed by
13C NMR spectroscopy. As observed previously for other
polymerizations in aqueous systems and comparative studies
in nonaqueous solutions with Ni(II) catalysts, the reaction
medium has no significant effect on the microstructure of the
polymer formed.
Summary and Conclusions
(20) For ethylene solubility in water, cf.: Plo¨cker, U.; Knapp, H.;
Prausnitz, J. Ind. Eng. Chem Process Des. DeV. 1978, 17, 324-332.
(21) van Krevelen, D. W. Properties of polymers: their correlation with
chemical structure; their numerical estimation and prediction from additiVe
group contributions; Elsevier: Amsterdam, 1994; pp 535-84.
A series of (salicylaldiminato)nickel(II)-methyl complexes
[(N∧O)NiMe(L)] with different water-soluble ligands L has been
prepared. All complexes, 2a-d, are soluble in toluene. 2a-c

Nickel(II)-Methyl Complexes
Organometallics, Vol. 26, No. 6, 2007 1315
1H NMR (400 MHz, C6D6, 25 °C): δ/ppm ) 7.88 (s, 4H), 7.82
are insoluble in water; however, 2c is soluble in 2-propanol. In
contrast, 2d is soluble in water.
4
(d, JHH ) 2.3 Hz, 1H), 7.70 (s, 2H), 7.05 (m, 1H), 6.96 (m, 2H),
4
6.86 (s, 1H), 6.75 (d, JHH ) 2.3 Hz, 1H), 4.1 (m, 6H, PTA), 3.9
In toluene solution, only complex 2b coordinated by the
tertiary amine L ) urotropine is catalytically active for ethylene
polymerization under the reactions conditions studied (20 °C,
40 atm of ethylene), with a productivity of 1.7 × 104 turnovers.
By contrast, in aqueous systems, pyridine and primary amine
complexes 2c,d (L ) tetraethylammonium pyridine sulfonate
or amino-terminated polyethylene glycol) are also catalytically
active. Incipient comparmentalization of the lipophilic active
species into the polymer particle formed and of L into the
aqueous phase appears to retard recombination and thus enhance
activity, as observed previously for similar complexes.10 Com-
plex 2a with the water-soluble alkylphosphine PTA is inactive
in aqueous suspension as well as toluene solution due to the
strong coordination of phosohine, in accordance with a previous
report on a similar compound.15
Polymerization of ethylene by an aqueous SDS solution of
the water-soluble complex 2d affords dispersions with very
small particle sizes of 18 nm. Exposure of suspensions of the
water-insoluble 2b to ethylene pressure (in the presence of
surfactant to potentially stabilize polymer particles formed)
affords polymer suspensions. When 2c is introduced to an
aqueous surfactant solution as a solution in 2-propanol, no
visible precipitate is formed. Exposure to ethylene results in
formation of polymer particles in the form of a latex, with large
particle size. These findings underline that the size of polyeth-
ylene particles obtained by catalytic polymerization in aqueous
systems is correlated to the degree of dispersion of the catalyst
in the initial reaction mixture.
(b, 6H, PTA), -1.50 (s, 3H).
13C NMR (100 MHz, C6D6, 25 °C): δ/ppm ) 167.0, 163.4,
150.9, 149.2, 142.7, 141.2, 133.6, 131.8 (q, 2JCF ) 33 Hz), 130.8,
1
130.6, 126.9, 123.8 (q, JCF ) 273 Hz), 121.5, 119.3, 96.9, 73.5,
73.2, 50.0, -18.1. 31P NMR (162 MHz, C6D6, 25 °C): δ/ppm )
-51.7. Anal. Calcd for C36H27F12I2N4NiOP: C, 39.20; H, 2.47; N,
5.08. Found: C, 40.37; H, 2.77; N, 4.85.
Synthesis of 2b. In a Schlenk tube 30.8 mg (0.15 mmol) of
[(tmeda)Ni(CH3)2] and 21.0 mg (0.15 mmol) of hexamethylentet-
ramine were combined and a cold toluene solution (5 mL) of 131
mg (0.15 mmol) of salicylaldimine 1 was added at -50 °C. Over
3 h the red solution was allowed to warm to 0 °C. The solvent was
evaporated under vacuum. The solid was washed at -30 °C three
times with 3 mL of pentane, and residual solvent was again
evaporated under vacuum. To remove excess hexamethylentetra-
mine, the complex was washed in air with 10 mL of water. After
drying of the sample in vacuo, 135 mg (83%) of a red solid was
isolated.
1H NMR (400 MHz, C6D6, 25 °C): δ/ppm ) 7.87 (s, 4H), 7.83
4
(s, 2H), 7.79 (d, JHH ) 2.1 Hz, 1H), 6.98 (m, 1H), 6.88 (m, 2H),
6.64 (d, 4JHH ) 2.1 Hz, 1H), 6.33 (s, 1H), 4.3 (b m, 9H), 3.9 (b m,
3H), -1.40 (s, 3H). 13C NMR (100 MHz, C6D6, 25 °C): δ/ppm )
2
167.8, 163.0, 150.6, 150.4, 141.5, 141.4, 132.9, 132.0 (q, JCF
)
33 Hz), 130.7, 130.4, 126.7, 123.6 (q, 1JCF ) 273 Hz), 121.4, 120.0,
95.9, 75.0, 73.2, 73.1, -14.7. Anal. Calcd for C36H27F12I2N5NiO:
C, 39.81; H, 2.51; N, 6.45. Found: C, 40.38; H, 2.95; N, 5.88.
Synthesis of 2c. A 1 g amount of 3-pyridinesulfonic acid was
dissolved in a mixture of 35 mL of water and 5 mL of methanol,
and three drops of a 1% solution of bromothymol blue were added.
An aqueous solution of tetraethylammonium hydroxide was added
dropwise until the color changed to green. After addition of an
equal volume of ethanol, the solvents were removed azeotropically
on a rotary evaporator at 50 °C. The residual solid was first dried
in vacuum and then stored over phosphorus pentoxide. Tetraethyl-
ammonium 3-pyridinesulfonate was obtained quantitatively as a
white, hygroscopic solid.
Experimental Section
Methods and Materials. Unless noted otherwise, all syntheses
of organometallic compounds were carried out under an argon
atmosphere (99.999% pure argon supplied by Messer). Toluene,
benzene, and diethyl ether were distilled from sodium, and pentane
was distilled from calcium hydride under argon. Demineralized
water was deoxygenated by distillation under nitrogen. NMR
analysis were conducted on a Varian Unity INOVA 400. Chemical
shifts were referenced to the residual solvent signal or, in the case
of 31P NMR, to an external standard of 85% H3PO4. Elemental
analysis were performed up to 950 °C on an Elemental Vario EL.
GPC analyses were carried out on a Polymer Laboratories PL220
instrument equipped with Mixed B columns at 160 °C in 1,2,4-
trichlorobenzene. Data are referenced to linear polyethylene
standards. Dynamic light scattering (DLS) was performed using a
Malvern nano ZS (173° backscattering). UV/vis studies were
performed on a Carey 50.
4
1H NMR (250 MHz, CD3OD, 25 °C): δ/ppm ) 8.93 (dd, JHH
5
3
4
) 2.2 Hz, JHH ) 0.8 Hz, 1H), 8.59 (dd, JHH ) 4.9 Hz, JHH
)
1.6 Hz, 1H), 8.19 (ddd, 3JHH ) 8.0 Hz, 4JHH ) 2.2 Hz, 4JHH ) 1.6
Hz, 1H), 7.51 (ddd, 3JHH ) 8.0 Hz, 3JHH ) 4.9 Hz, 5JHH ) 0.8 Hz,
1H), 3.27 (q, 3JHH ) 7.3 Hz, 8H), 1.26 (tt, 3JHH ) 7.3 Hz, 3JHN
)
1.9 Hz, 12H). 13C NMR (100 MHz, CD3OD, 25 °C): δ/ppm )
1
150.5, 146.8, 142.5, 134.8, 124.2, 52.4 (t, JNH ) 3.1 Hz), 6.7.
A 40 mg (0.195 mmol) amount of [(tmeda)Ni(CH3)2] and 56
mg (0.195 mmol) of tetraethylammonium 3-pyridinesulfonate were
placed in a Schlenk tube. A cold solution of 170 mg (0.195 mmol)
of the salicylaldimine 1 in 8 mL of toluene was added at -50 °C.
The red solution was allowed to warm to 0 °C in 3 h, and the
solvent was removed under vacuum. The solid was washed under
argon with 20 mL of degassed water. A 67 mg (70%) amount of
the red complex was isolated.
1H NMR (400 MHz, acetone-d6, 25 °C): δ/ppm ) 8.80 (s, 1H),
8.38 (s, 4H), 8.35 (m, 1H), 8.16 (s, 2H), 8.02 (m, 1H), 7.84 (m,
1H), 7.70 (m, 2H), 7.58 (m, 1H), 7.31 (m, 1H), 7.25 (m, 1H), 3.50
1,3,5-Triaza-7-phosphaadamantane (PTA) was prepared accord-
ing to Daigle.22 Complex 2d was synthesized as reported previ-
ously.10
Synthesis of 2a. In a Schlenk tube, 50 mg (0.244 mmol) of
[(tmeda)Ni(CH3)2] and 41 mg (0.244 mmol) of PTA were com-
bined, and a toluene solution (5 mL) of 213 mg (0.244 mmol) of
salicylaldimine 1 was added at -50 °C. The orange solution was
allowed to warm to 0 °C gradually over the course of 3 h. The
solvent was evaporated under vacuum. The remaining solid was
washed three times with 3 mL of pentane at -30 °C, and residual
solvent was again removed in vacuum. To remove any unreacted
PTA, the complex was washed in air with 10 mL of water. After
drying of the sample in vacuo, 170 mg (63%) of an orange solid
was isolated.
3
3
3
(q, JHH ) 7.2 Hz, 8H), 1.26 (tt, JHH ) 7.2 Hz, JHN ) 1.2 Hz,
12H), -1.00 (s, 3H). 13C NMR (100 MHz, acetone-d6, 25 °C):
δ/ppm ) 170.6, 165.2, 152.3, 151.7, 150.9, 150.8, 143.6, 143.4,
136.9, 135.0, 133.1 (q, 2JCF ) 33 Hz), 133.0, 132.6, 130.4, 129.0,
1
125.5 (q, JCF ) 273 Hz), 124.5, 123.1, 121.8, 98.2, 73.2, 54.0 (t,
1JNH ) 2.9 Hz), 8.7, -6.3. Anal. Calcd for C43H39F12I2N3NiO4S:
C, 41.84; H, 3.18; N, 3.40. Found: C, 39.96; H, 3.25; N, 3.37.
Polymerization experiments were carried out in a 250 mL
stainless steel pressure reactor with mechanical stirrer (500 rpm)
and a heating/cooling jacket controlled by a thermostat connected
(22) Daigle, D. J.; Pepperman, A. B.; Vail, S. L. J. Heterocyl. Chem.
1974, 11, 407-408.

1316 Organometallics, Vol. 26, No. 6, 2007
Korthals et al.
to a thermocouple dipping into the reaction mixture. The reactor is
connected to a gas supply, which allows for application of constant
ethylene pressure and monitoring of the ethylene uptake via mass
flow meters.
µmol of complex 2b. After 10 min of sonication, complex 2b partly
dissolved affording an orange solution with an orange precipitate.
This mixture and a solution of 0.75 g of SDS in 98 mL of water
were transferred to the reactor under an ethylene atmosphere. For
polymerization with the water-soluble complex 2d, a solution of
0.75 g of SDS in 90 mL of water was transferred into the reactor
under an ethylene atmosphere. A 10 µmol (20 µmol, respectively)
amount of the complex was dissolved in 10 mL of water, affording
a homogeneous red solution, and also transferred into the reactor.
Polymerization in Toluene. A 90 mL volume of toluene was
transferred via cannula into the reactor under an ethylene atmo-
sphere. A 10 µmol amount of the respective complex was dissolved
in 10 mL of toluene, affording a homogeneous orange to red
solution. This solution was transferred into the reactor, which was
then closed and rapidly pressurized with 40 bar of ethylene. The
temperature was adjusted to 20 °C. Under a constant pressure of
ethylene, the reaction was allowed to proceed. After 60 or 120 min,
the ethylene feeding was stopped and the reactor was carefully
vented. The content of the reactor was poured into 250 mL of
vigorously stirred methanol. After 1 h, the precipitated polymer
was filtered out, washed three times with 30 mL of methanol, and
dried at 50 °C under vacuum.
The temperature was adjusted to 20 °C, and the reactor was
pressurized with 40 bar of ethylene. After 60 or 210 min,
respectively, the ethylene feeding was stopped, the reactor was
vented, and the latex was filtered through glass wool. An aliqout
of 10 mL was poured in 50 mL of vigorously stirred methanol,
resulting in precitipation of the polymer. The preciptiate was filtered
through a nylon membrane, washed twice with 10 mL of methanol
and twice with 10 mL of water, and dried at 50 °C under vacuum.
Polymerization in Aqueous Media. Due to the different
solubility of the complexes in water, the preparation varied for each
complex. For suspension-type polymerization, 10 µmol (5 µmol)
of the water-insoluble complex 2b,c, respectively, and 0.75 g of
SDS were sonicated in 3 mL of water for 10 min. The resulting
red slurry and an additional 97 mL of water were transferred via
cannula into the reactor under an ethylene atmosphere. For
polymerization with complex 2c with 2-propanol as a cosolvent, 2
mL of 2-propanol was added to 10 µmol of complex 2c, affording
a red, transparent solution. On addition of 10 mL of water, no
precipitate was observed. This mixture and a solution of 0.75 g of
SDS in 88 mL of water were transferred to the reactor under an
ethylene atmosphere. For polymerization with complex 2b with
2-propanol as a cosolvent, 2 mL of 2-propanol was added to 10
Acknowledgment. Financial support by the BMBF (Project
03X5505) is gratefully acknowledged. S.M. is indebted to the
Fonds der Chemischen Industrie and to the Hermann Schnell-
foundation for financial support.
Supporting Information Available: Figure S1, giving UV-
vis spectra of complex 2d in water, and Figure S2, giving plots of
ethylene consumption over time for polymerization with 2d. This
material is available free of charge via the Internet at http://
pubs.acs.org.
OM0607191