Structural isomers of the chromium bis(phosphoranimine)methanide complex [(HC(PPh2NSiMe3)2)Cr(μ-Cl)]2

Article abstract of DOI:10.1021/om0108494

The species [(HC(PPh₂NSiMe₃)₂Cr(μ-Cl)₂] was prepared and structurally characterized. It is shown to exist in two structural isomers: one in which the Cr center adopts a pseudo-square-planar coordination geometry and another where the metal coordination sphere is better described as trigonal bipyramidal. Preliminary assessment of the catalytic activity in ethylene polymerization is also reported.

Full text of DOI:10.1021/om0108494

1308
Organometallics 2002, 21, 1308-1310
Str u ctu r a l Isom er s of th e Ch r om iu m
Bis(p h osp h or a n im in e)m eth a n id e Com p lex
[(HC(P P h 2NSiMe3)2)Cr (µ-Cl)]2
Pingrong Wei and Douglas W. Stephan*
School of Physical Sciences, Chemistry and Biochemistry, University of Windsor,
Windsor, Ontario, Canada N9B 3P4
Received September 26, 2001
Summary: The species [(HC(PPh2NSiMe3)2Cr(µ-Cl)2] was
prepared and structurally characterized. It is shown to
exist in two structural isomers: one in which the Cr
center adopts a pseudo-square-planar coordination ge-
ometry and another where the metal coordination sphere
is better described as trigonal bipyramidal. Preliminary
assessment of the catalytic activity in ethylene polymer-
ization is also reported.
species is shown to exist in two isomeric forms, reflect-
ing the conformational flexibility of this ligand.
Exp er im en ta l Section
Gen er a l Da ta . All preparations were done under an
atmosphere of dry, O2-free N2 employing both Schlenk line
techniques and Innovative Technologies, Braun, or Vacuum
Atmospheres inert-atmosphere gloveboxes. Solvents were
purified employing Grubb-type column systems manufactured
by Innovative Technologies. All organic reagents were purified
The longstanding efforts of Dehnicke and co-workers1
have demonstrated the unique nature of phosphoran-
imine ligand complexes for a wide variety of elements.
Recent studies have further increased interest in this
class of ligands. For example, while others have alluded
to the electronic analogy between phosphoranimide and
cyclopentadienyl ligands,1 we have exploited the steric
analogy between these ligands2 to develop active and,
in some cases, remarkably active catalysts for ethylene
polymerization.3 Related systems have also been shown
to prompt multiple C-H bond activations of Ti-CH3
fragments4 and the formation of unprecedented di-
zwitterions.5 In a more fundamental vein, we6 and
Cavell and co-workers7 have reported the synthesis of
the dilithiomethane derivative [Li2C(PPh2NSiMe3)2]2.
Cavell et al. have developed the intriguing chemistry
of this dilithio species, reporting the unique and ex-
tremely interesting carbene complexes of Ti and Zr,8,9
Hf,9 Sm,10 and Cr.11 Related studies of the monolithio
derivative LiCH(PPh2NSiMe3)2 have received much less
attention but are of interest due to the structural
analogy to â-diimine ligands. In this communication, the
ligand bis(phosphoranimine)methanide is used to pre-
pare the complex [HC(PPh2NSiMe3)2Cr(µ-Cl)]2. This
1
by conventional methods. H and 13C{1H} NMR spectra were
recorded on Bruker Avance-300 and -500 spectrometers oper-
ating at 300 and 500 MHz, respectively. Trace amounts of
protonated solvents were used as references, and chemical
shifts are reported relative to SiMe4. 31P NMR spectra were
recorded on a Bruker Avance-300 and are referenced to 85%
H3PO4. Guelph Chemical Laboratories performed combustion
analyses.
Syn th esis of [HC(P P h 2NSiMe3)2Cr (µ-Cl)]2 (1). To a stirred
solution of (THF)2CrCl2 (0.040 g, 0.325 mmol) in THF (5 mL)
was added dropwise a solution of LiCH(PPh2NSiMe3)2 (0.325
mmol) in THF (5 mL). The reaction turned dark green and
was stirred overnight. The THF was removed in vacuo, the
residue was extracted into benzene, and the extract was
filtered through Celite. Removal of the solvent gave a green
solid. Yield: 0.11 g, 55%. Green crystals of 1a /1b were obtained
from a slow evaporation of the benzene solution. Anal. Calcd
(found) for C31H39P2N2Si2CrCl‚LiCl: C, 54.15 (53.91); H, 5.71
(6.38); N, 4.07 (3.80). 31P{1H}NMR (C6H6): δ -4.4. 1H NMR
(C6D6): δ 7.68 (8H, br), 7.03 (12H, br), 3.28 (1H, t), 0.26 (18H,
s). 13C{1H} NMR (C6D6; partial): δ 143.8, 142.6, 131.0, 4.7.
µeff ) 4.25 µB. Crystal data for 1a /1b cocrystal: triclinic, P1h, a
) 11.895(8) Å, b ) 16.738(11) Å, c ) 18.430(12) Å, R ) 106.427-
(11)°, â ) 96.891(12)°, γ ) 97.885(13)°, V ) 3437(4) Å3, Z ) 2,
µ ) 0.596 mm-1, R ) 0.0361, Rw ) 0.0815, for 9800 reflections
2
with Fo g 3σ(Fo2).
In a similar fashion performance of the reaction in THF
affords 1b alone. µeff ) 4.29 µB. Crystal data for 1b: triclinic,
P1h, a ) 9.859(6) Å, b ) 11.918(7) Å, c ) 15.560(10) Å, R )
102.603(12)°, â ) 93.100(10)°, γ ) 104.091(12)°, V ) 1719.4-
(18) Å3, Z ) 2, µ ) 0.595 mm-1, R ) 0.0377, Rw ) 0.0861, for
(1) Dehnicke, K.; Krieger, M.; Massa, W. Coord. Chem. Rev. 1999,
182, 19.
(2) Stephan, D. W.; Stewart, J . C.; Guerin, F.; Spence, R. E. v. H.;
Xu, W.; Harrison, D. G. Organometallics 1999, 18, 1116.
(3) Stephan, D. W.; Guerin, F.; Spence, R. E. v. H.; Koch, L.; Gao,
X.; Brown, S. J .; Swabey, J . W.; Wang, Q.; Xu, W.; Zoricak, P.; Harrison,
D. G. Organometallics 1999, 18, 2046.
(4) Kickham, J . E.; Guerin, F.; Stewart, J . C.; Stephan, D. W. Angew.
Chem., Int. Ed. 2000, 39, 3263.
(5) Guerin, F.; Stephan, D. W. Angew. Chem., Int. Ed. Engl. 2000,
39, 1298.
(6) Ong, C. M.; Stephan, D. W. J . Am. Chem. Soc. 1999, 121, 2939.
(7) Kasani, A.; Babu, R. P. K.; McDonald, R.; Cavell, R. G. Angew.
Chem., Int. Ed. Engl. 1999, 38, 1483.
(8) Cavell, R. G.; Babu, R. P. K.; Kasani, A.; McDonald, R. J . Am.
Chem. Soc. 1999, 121, 5805.
(9) Babu, R. P. K.; McDonald, R.; Cavell, R. G. Chem. Commun.
2000, 481.
(10) Aparna, K.; Ferguson, M.; Cavell, R. G. J . Am. Chem. Soc. 2000,
122, 726.
2
4910 reflections with Fo g 3σ(Fo2).
Eth ylen e P olym er iza tion . A 1 L autoclave was dried
under vacuum (10-2 mmHg) for several hours. Toluene (500
mL) was transferred into the vessel under a positive pressure
of N2 and was heated to 30 °C. The temperature was controlled
(to ca. +2 °C) with an external heating/cooling bath and was
monitored by a thermocouple that extended into the polym-
erization vessel. A solution of MAO (500 equiv) in toluene was
injected, and the mixture was stirred for 3 min at 150 rpm.
The precatalyst in a solution of toluene was then injected while
the reaction mixture was stirred for 3 min further at the same
rate. The rate of stirring was increased to 1000 rpm, and the
vessel was vented of N2 and pressurized with ethylene (33 psi).
(11) Kasani, A.; McDonald, R.; Cavell, R. G. Chem. Commun. 1999,
1993.
10.1021/om0108494 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 02/09/2002

Notes
Organometallics, Vol. 21, No. 6, 2002 1309
Any recorded exotherm was within the allowed temperature
differential of the heating/cooling system. The solution was
stirred for 1 h, after which time the reaction was quenched
with 1 M HCl in MeOH. The precipitated polymer was
subsequently washed with MeOH and dried at 100 °C for at
least 24 h prior to weighing.
X-r a y Da ta Collection a n d Red u ction . The crystals of
1a /1b and 1b were manipulated and mounted in capillaries
in a glovebox, thus maintaining a dry, O2-free environment
for each crystal. Diffraction experiments were performed on a
Siemens SMART System CCD diffractometer. The data were
collected for a hemisphere of data in 1329 frames with 10 s
exposure times. Crystal data are summarized above. The
observed extinctions were consistent with the space groups in
each case. A measure of decay was obtained by recollecting
the first 50 frames of each data set. The intensities of
reflections within these frames showed no statistically sig-
nificant change over the duration of the data collections. An
empirical absorption correction based on redundant data was
applied to each data set. Subsequent solution and refinement
was performed using the SHELXTL solution package.
St r u ct u r e Solu t ion a n d R efin em en t . Non-hydrogen
atomic scattering factors were taken from the literature
tabulations. The heavy-atom positions were determined using
direct methods. The remaining non-hydrogen atoms were
located from successive difference Fourier map calculations.
The refinements were carried out by using full-matrix least-
squares techniques on F, minimizing the function w(|Fo| -
|Fc|)2, where the weight w is defined as 4Fo2/2σ(Fo2) and Fo and
Fc are the observed and calculated structure factor amplitudes.
In the final cycles of each refinement, all non-hydrogen atoms
were assigned anisotropic temperature factors. Carbon-bound
hydrogen atom positions were calculated and allowed to ride
on the carbon to which they are bonded, assuming a C-H bond
length of 0.95 Å. Hydrogen atom temperature factors were
fixed at 1.10 times the isotropic temperature factor of the
carbon atom to which they are bonded. The methine protons
on the central carbon of the bis(phosphanimine)methanide
ligands were located and refined in both structural studies.
The remaining hydrogen atom contributions were calculated,
but not refined. CIF files have been deposited as Supporting
Information.
F igu r e 1. ORTEP drawings of the two conformations of
[(HC(PPh2NSiMe3)2CrCl)2] (1; 30% thermal ellipsoids are
shown). Hydrogen atoms have been omitted for clarity.
Bond distances (Å) and angles (deg) for the two conforma-
tions are as follows: (a) Cr(1)-N(1) ) 2.096(3), Cr(1)-C(1)
) 2.264(3), Cr(1)-Cl(1) ) 2.4054(16), Cr(1)-N(2) ) 2.423-
(3), Cr(1)-Cl(1) ) 2.4365(14), Cr(1)-P(1) ) 2.7310(16),
N(1)-Cr(1)-C(1) ) 74.81(10), N(1)-Cr(1)-Cl(1) ) 98.51-
(7), C(1)-Cr(1)-Cl(1) ) 172.65(8), N(1)-Cr(1)-N(2) )
103.54(9), C(1)-Cr(1)-N(2) ) 70.38(10), Cl(1)-Cr(1)-N(2)
) 108.88(7), N(1)-Cr(1)-Cl(1) ) 149.91(7), C(1)-Cr(1)-
Cl(1) ) 100.03(8), Cl(1)-Cr(1)-Cl(1) ) 87.30(5), N(2)-Cr-
(1)-Cl(1) ) 102.35(7), Cr(1)-Cl(1)-Cr(1) ) 92.70(5), P(1)-
N(1)-Si(1) ) 134.67(15), P(1)-N(1)-Cr(1) ) 94.48(11),
Si(1)-N(1)-Cr(1) ) 130.42(13), P(2)-N(2)-Si(2) ) 139.37-
(16), P(2)-N(2)-Cr(1) ) 89.70(11), Si(2)-N(2)-Cr(1) )
128.98(12), P(1)-C(1)-P(2) ) 126.46(18); (b) Cr(2)-N(4)
) 2.112(3), Cr(2)-N(3) ) 2.121(3), Cr(2)-Cl(2)′ ) 2.4275-
(15), Cr(2)-Cl(2) ) 2.4468(15), N(4)-Cr(2)-N(3) ) 94.29-
(11), N(4)-Cr(2)-Cl(2)′ ) 169.46(7), N(3)-Cr(2)-Cl(2)′ )
93.92(9), N(4)-Cr(2)-Cl(2) ) 92.04(8), N(3)-Cr(2)-Cl(2)
) 167.47(7), Cl(2)-Cr(2)-Cl(2)′ ) 81.19(6), Cr(2)-Cl(2)-
Cr(2)′ ) 98.81(6), P(3)-N(3)-Si(3) ) 127.88(15), P(3)-
N(3)-Cr(2) ) 106.80(12), Si(3)-N(3)-Cr(2) ) 125.31(13),
P(4)-N(4)-Si(4) ) 134.05(16), P(4)-N(4)-Cr(2) ) 106.24-
(13), Si(4)-N(4)-Cr(2) ) 119.69(13), P(4)-C(32)-P(3) )
127.93(19).
Resu lts a n d Discu ssion
Reaction of Li[(HC(PPh2NSiMe3)2] with (THF)2CrCl2
in THF proceeds with a concurrent color change to dark
green. Removal of the solvent and recrystallization of
the residue from benzene affords dark green crystals of
compound 1 in 55% yield. This compound exhibits a
single 31P{1H} NMR resonance at -4.4 ppm. The
corresponding 1H and partial 13C{1H} NMR resonances
suggest the presence of the ligand. The magnetic mo-
ment of 4.25 µB is consistent with that of a Cr(II) species.
The spectroscopic and analytical data suggest the
empirical formula [HC(PPh2NSiMe3)2CrCl], which was
confirmed by a crystallographic study.
Compound 1 crystallizes in the space group P1h with
two units each having the empirical formulation HC-
(PPh2NSiMe3)2CrCl per asymmetric unit. Each of these
units are half of centrosymmetric dimeric molecules in
which two chlorine atoms bridge the pair of related Cr
centers. In each case, the anionic nature of the bis-
(phosphoranimine) ligand was confirmed by both the
formulation and the location and refinement of the lone
protons on the central carbon of the ligands. However,
these two dimers exhibit distinctly different coordina-
tion geometries about Cr. In one of the dimers, 1a
(Figure 1a), the central carbon of the bis(phosphoran-
imine) ligand is coordinated to Cr (Cr-C ) 2.264(3) Å).
This is significantly longer that the Cr-C distance of
2.148(5) Å found in [(C(PPh2NSiMe3)2Cr)2].11 This car-
bon atom is essentially trans to chloride, as the C-Cr-
Cl angle is 172.65(8)°. The Cr-Cl distances are 2.4054-
(16) and 2.4365(14) Å, while the Cr‚‚‚Cr separation is
3.504(1) Å. The Cr-N distances are 2.096(3) and 2.423-
(3) Å, as the N atoms occupy positions that are best
described as part of the equatorial plane of a distorted
trigonal bipyramid. The distorted geometry arises from
the NC chelation, as evidenced by the N-Cr-C angles
of 74.81(10) and 70.38(10)°. It is also noteworthy that
two phenyl rings on the adjacent P atoms are ap-
proximately parallel, suggestive of π-stacking.
The second fragment 1b in the asymmetric units
(Figure 1b) also constitutes half of a centrosymmetric

1310 Organometallics, Vol. 21, No. 6, 2002
Notes
The best description of the difference between the two
isomers is based an examination of the geometry of the
chelate ring. In 1a CrN2P2C adopts a pseudo-boat
conformation resulting from the proximity of the Cr and
the transannular C atom. Presumably, the interaction
of Cr with C results in ligand adjustments and the
observed pseudo-trigonal-bipyramidal coordination sphere
about Cr (Figure 2a). In the second isomer 1b the
chelate ring adopts a more pseudo-chairlike conforma-
tion (Figure 2b). This increases the Cr-C distance and
gives rise to the observed pseudo-square-planar Cr
coordination sphere. This geometry is similar to that
seen in Zn and alkali-metal bis(phosphoranimine)-
methanide complexes.13,14 The observation of both struc-
tural isomers in a single cocrystal suggests that the
energy barrier between these two forms is low and that
packing forces dictate the cocrystallization from solu-
tion.
This notion was further supported by the repetition
of the reaction in THF. In this case, isolation of the
product and the subsequent crystallographic study
revealed the formation of 1b alone. The metric param-
eters of 1b found in this case were indistinguishable
from those found in the initial crystallographic deter-
mination.
Preliminary screening shows 1 in the presence of 500
equiv of MAO catalyzes ethylene polymerization, gen-
erating 135 g of PE/(mmol h). GPC data for the resulting
polymer reveal an Mw value of 344 000. However, the
molecular weight distributions are very broad (PDI )
47), suggesting multisite catalysis. While the accessibil-
ity of different structural isomers may be only part of
the rationale for this observation, it may account for the
reactive nature of bis(phosphoranimine)methanide com-
plexes in general.13,14
F igu r e 2. ORTEP drawings of the chelate ring geometry
for the structural isomers 1a and 1b.
chloro-bridged dimer. The coordination sphere about Cr
is comprised of the two chlorine atoms as well as two
nitrogen atoms in a distorted-square-planar geometry.
This is demonstrated by the pseudo-trans N-Cr-Cl
angles of 169.46(7) and 167.47(7)°. The bite angle of the
chelate ligand, i.e. N-Cr-N, is 94.29(11)°. The Cr-N
distances are similar, averaging 2.116(4) Å, similar to
that seen in the donor-acceptor species (Me3SiNPMe3)2-
CrCl2 (2.117(2) Å).12 These are similar to the shorter of
the two Cr-N distances in 1a . The Cr-Cl distances of
2.4275(15) and 2.4468(15) Å differ slightly and are
similar to those seen in isomer 1a . Another notable
difference from 1a is the significantly longer Cr-C
distance of 2.921(3) Å in 1b. The Cr‚‚‚Cr separation in
1b is 3.704(3) Å, longer than in 1a , and these are
consistent with the differing ligand binding geometries.
In the pseudo-trigonal-bipyramidal case 1a , due to steric
demands, the nitrogen-bound SiMe3 substituents are
pulled away from the Cr center, whereas in the pseudo-
square-planar case 1b, these groups tend to crowd the
Cr coordination sphere.
The metric parameters of the ligand backbones are
also distinct in these two isomers. The P-N distances
range from 1.575(3) to 1.608(3) Å, with the shortest P-N
distance corresponding to the weaker Cr-N bond in 1a .
The P-C distances average 1.763(3) Å in 1a and 1.722-
(3) Å in 1b. The longer P-C distances in 1a are
consistent with both the presence of a formal Cr-C bond
and strained four-membered rings formed by chelation.
The P-C-P angles also differ slightly, being 126.46-
(18)° in 1a and 127.93(19)° in 1b.
Ack n ow led gm en t . Financial support from the
NSERC of Canada is gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: Tables giving crys-
tallographic data. This material is available free of charge via
the Internet at http://pubs.acs.org.
OM0108494
(13) Kasani, A.; McDonald, R.; Cavell, R. G. Organometallics 1999,
18, 3775.
(12) Miekisch, T.; Mai, H. J .; Meyer zu Koecker, R.; Dehnicke, K.;
Magull, J .; Goesmann, H. Z. Anorg. Allg. Chem. 1996, 622, 583-588.
(14) Babu, R. P. K.; Aparna, K.; McDonald, R.; Cavell, R. G.
Organometallics 2001, 20, 1451.