Ambidentate character of the 6-aminofulvene-2-aldiminate ligand containing both diimine and cyclopentadienyl donors

Article abstract of DOI:10.1021/om060808d

The 6-aminofulvene-2-aldiminate (AFA) ligand contains both cyclopentadienyl and diimine donors. The ligand preferentially coordinates to transition metals via the nitrogen atoms; however once these are occupied, the cyclopentadienyl ring may be coordinated to a Cp*Ru⁺ unit, providing bi- and trimetallic species. In the tetrahedral [Zn(Ph₂AFA)₂] (2) the metal is located approximately in the planes of the two ligands; however in the square-planar [Pd(Ph₂AFA)₂] (1) there is a severe distortion of the coordination and the metal is located almost 1.3 A out of the planes of the two ligands. This situation arises to avoid the steric interaction of the ligand phenyl substituents. Treatment of 2 with [Cp*Ru(NCMe)₃][BF₄] gives [Cp *Ru(η₅-Ph₂AFA)] (3), providing the first example of the cyclopentadienyl coordination of an AFA ligand in which the nitrogen donors are vacant, and it is thought to form as a result of the fragmentation of an intermediate mixed Zn/Ru species. The X-ray structure of 3 provides some evidence for an interaction between the Ru center and one of the exocyclic carbon atoms, thus suggesting a fulvene form of the ligand in this complex. Similar treatment of 1 with [Cp*Ru(NCMe)₃][BF₄] provides a trimetallic PdRu₂ species (4) in which both of the cyclopentadienyl rings in the complex 2 are coordinated to Ru. The complex [(Ph₂AFA)Pd(Me)PPh₃] (5) is formed on treatment of [(COD)PdMe(Cl)] with NaPh₂AFA in the presence of PPh₃, and preliminary investigations have shown that its activation with [Ni(COD) ₂] as a phosphine scavenger provides species active toward ethylene oligomerization and polymerization.

Full text of DOI:10.1021/om060808d

128
Organometallics 2007, 26, 128-135
Ambidentate Character of the 6-Aminofulvene-2-aldiminate Ligand
Containing Both Diimine and Cyclopentadienyl Donors
Philip J. Bailey,* Michele Melchionna, and Simon Parsons
School of Chemistry, UniVersity of Edinburgh, The King’s Buildings, West Mains Road,
Edinburgh, EH9 3JJ, U.K.
ReceiVed September 5, 2006
The 6-aminofulvene-2-aldiminate (AFA) ligand contains both cyclopentadienyl and diimine donors.
The ligand preferentially coordinates to transition metals via the nitrogen atoms; however once these are
occupied, the cyclopentadienyl ring may be coordinated to a Cp*Ru+ unit, providing bi- and trimetallic
species. In the tetrahedral [Zn(Ph2AFA)2] (2) the metal is located approximately in the planes of the two
ligands; however in the square-planar [Pd(Ph2AFA)2] (1) there is a severe distortion of the coordination
and the metal is located almost 1.3 Å out of the planes of the two ligands. This situation arises to avoid
the steric interaction of the ligand phenyl substituents. Treatment of 2 with [Cp*Ru(NCMe)3][BF4] gives
[Cp*Ru(η5-Ph2AFA)] (3), providing the first example of the cyclopentadienyl coordination of an AFA
ligand in which the nitrogen donors are vacant, and it is thought to form as a result of the fragmentation
of an intermediate mixed Zn/Ru species. The X-ray structure of 3 provides some evidence for an interaction
between the Ru center and one of the exocyclic carbon atoms, thus suggesting a fulvene form of the
ligand in this complex. Similar treatment of 1 with [Cp*Ru(NCMe)3][BF4] provides a trimetallic PdRu2
species (4) in which both of the cyclopentadienyl rings in the complex 2 are coordinated to Ru. The
complex [(Ph2AFA)Pd(Me)PPh3] (5) is formed on treatment of [(COD)PdMe(Cl)] with NaPh2AFA in
the presence of PPh3, and preliminary investigations have shown that its activation with [Ni(COD)2] as
a phosphine scavenger provides species active toward ethylene oligomerization and polymerization.
Introduction
The 6-aminofulvene-2-aldimine (AFAH) system was first
prepared in 1963,1 but its chemistry, in particular its properties
as a ligand, has received very little attention. Our earlier studies
of this system provided two novel magnesium species, one of
which showed the 6-aminofulvene-2-aldiminate (AFA) system
acting as an ambidentate ligand, coordinated to metal centers
through both the nitrogen and the C5-ring donors.2 Two
resonance forms of the ligand may be drawn (Figure 1), and
the ability of the cyclopentadienyl portion of the ligand to
symmetrically coordinate to a metal in a pentahapto mode
indicates that form B makes a significant contribution. This
feature of the ligand provides the potential for the formation of
charge-neutral but zwitterionic analogues of cationic complexes
containing conventional neutral diimine ligands when the
cyclopentadienyl portion of the ligand remains uncoordinated.
This could have particular significance in strongly electrophilic,
positively charged catalysts for which the presence of a
counterion often presents problems by coordinating to the metal
center. One such case is the class of square-planar group 10
transition metal alkene polymerization catalysts formed by Lewis
acid activation of [(1,2-diimine)MMe2] (M ) Ni, Pd) com-
plexes.3 Replacement of the 1,2-diimine ligand in these systems
with an AFA ligand would provide a neutral complex that retains
Figure 1. Resonance forms for the 6-aminofulvene-2-aldiminate
ligand.
significant positive charge at the metal by virtue of the
contribution from resonance form B to the ligand electronic
structure (Figure 2) and would furthermore provide catalysts
that would not require Lewis acid activation. With this in mind
we have initiated a study of the late transition metal coordination
chemistry of the AFA ligand system. We report here the unusual
coordination geometry adopted by the AFA ligand in square-
planar Pd(II) complexes, and the ability of the ligand to
coordinate to Ru(II) via the cyclopentadienyl ring, in complexes
in which the diimine portion of the ligand is either metal
coordinated or vacant. Preliminary results from a study of ethene
oligomerization/polymerization by [(Ph2AFA)Pd(Me)PPh3] (5)
activated by the phosphine scavenger [Ni(COD)2] are also
reported.
Experimental Section
All reactions and manipulations of moisture- and air-sensitive
compounds were carried out in an atmosphere of dry nitrogen using
Schlenk techniques or in a conventional nitrogen-filled glovebox
(Saffron Scientific), fitted with oxygen and water scavenging
columns. All the solvents were purified and dried by means of
distillation over an appropriate drying agent prior to use. Toluene,
THF, diethyl ether, hexane, and benzene were all distilled from
Na/benzophenone under a nitrogen atmosphere. NMR solvents were
* Corresponding author. E-mail: Philip.Bailey@ed.ac.uk.
(1) (a) Hafner, K.; Vopel, K. H.; Ploss, G.; Konig, C. Justus Liebigs
Ann. Chem. 1963, 661, 52. (b) Hafner, K.; Vopel, K. H.; Ploss, G.; Konig,
C. Org. Synth. 1967, 47, 52.
(2) Bailey, P. J.; Loron˜o-Gonza´lez, D. J.; Parsons, S. Chem. Commun.
2003, 1426.
(3) (a) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc.
1995, 117, 6414. (b) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV.
2000, 100, 1169.
10.1021/om060808d CCC: $37.00 © 2007 American Chemical Society
Publication on Web 11/30/2006

An Ambidentate Cyclopentadientyl-Diimine Ligand
Organometallics, Vol. 26, No. 1, 2007 129
is consistent with the structure determined by X-ray crystallography
and indicates the absence of carbon-containing impurities.
[(Ph2AFA)2Zn] (2). To a solution of Ph2AFAH (0.40 g, 1.47
mmol) in 30 mL of THF was added NaH, and immediately gas
evolution was observed. The mixture was stirred for 1 h, and a
solution of anhydrous ZnCl2 (0.098 g, 0.735 mmol) in THF (30
mL) was added via cannula. After stirring for 3 h, the solution was
filtered through Celite and the solvent was removed in vacuo. The
solid was extracted with 30 mL of CH2Cl2 and the solvent removed
to yield a yellow crystalline solid. Crystals suitable for X-ray
analysis were obtained by slow evaporation of the solvent from
Figure 2. 1,2-Diimine group-10 metal alkene polymerization
catalysts (C, M ) Ni, Pd; S ) solvent) and the possible neutral
zwitterionic analogue containing an AFA ligand (D).
1
hexane solutions of the product. H NMR (CDCl3, 360 MHz, 20
3
°C): δ 8.05 (s, 4H, H1/H7), 7.01 (d, 4H, H3/H5, J ) 3.65 Hz),
7.00-6.97 (m, 12H, C6H5), 6.84-6.80 (m, 8H, C6H5), 6.39 (t, 2H,
3
H4, J ) 3.65 Hz). 13C{1H} NMR (CDCl3, 90.6 MHz, 20 °C): δ
162.94 (CdN), 150.69 (N-C6H5), 139.82 (C6H5), 128.87 (C6H5),
125.05 (C3/C5), 122.19 (C6H5), 118.67 (C2/C6), 117.99 (C4). Anal.
Calcd for C38H30N4Zn: C, 75.06; H, 4.97; N, 9.21. Found: C, 75.33;
H, 4.58; N, 8.86.
Figure 3. C/H atom numbering scheme used in the assignment of
the NMR spectra of the AFA complexes.
[Cp*Ru(Ph2AFA)] (3). [Cp*Ru(CH3CN)3][BF4] (0.250 g, 0.559
mmol) and (Ph2AFA)2Zn (0.339, 0.559 mmol) were dissolved in
CH2Cl2 (30 mL), and the mixture was stirred for 1 h. After cannula
filtration, 30 mL of diethyl ether was carefully layered onto the
solution. Upon standing for 1 day, crystals suitable for X-ray
analysis had formed, which were isolated and dried under vacuum.
The 1H NMR spectrum of 3 showed it to be protonated (3H+). 1H
degassed using freeze-thaw cycles and stored over 4 Å molecular
sieves. The ligand Ph2AFAH,1 [(COD)PdMe(Cl)],4 and the salt
[Cp*Ru(CH3CN)3][BF4]5 were prepared according to the literature
procedures, while [Ni(COD)2] was purchased from Aldrich and used
as received. Polymer-grade ethylene was purchased from BOC gases
and used as received. All solvents and other reagents were purchased
from Sigma-Aldrich, Fischer, or Acros and used as received unless
otherwise stated. Column chromatography was performed on silica
gel (Merck 60, 7-120 mesh). NMR spectra were recorded on
Bruker AC 250 and 360 MHz spectrometers operating at room
temperature, except for the 13C spectrum of the polyethylene, which
was recorded at 120 °C. 1H and 13C chemical shifts are reported in
ppm relative to Si(CH3)4 (δ ) 0) and were referred internally with
respect to the protio solvent impurity or 13C resonances, respectively.
The C/H atom numbering scheme used in the assignment of the
NMR spectra of the AFA complexes is shown in Figure 3.
Polyethylene GPC analysis was undertaken by RAPRA Technology
(Shrewsbury, UK) using a 30 cm × 10 µm PLgel guard plus 2×
mixed bed-B column at 160 °C in 1,2,4-trichlorobenzene solution.
Refractive index and Viscotek differential pressure detectors were
employed and calibrated with polystyrene.
3
NMR (CD2Cl2, 360 MHz, 20 °C): δ 16.51 (bt, N-H+-N, J ) 7
Hz), 8.95 (d, 2H, H1/H7, 3J ) 7 Hz), 7.52-7.39 (m, 10H, C6H5),
3
3
5.66 (d, 2H, H3/H5, J ) 2.99 Hz), 4.20 (t, 1H, H4, J ) 2.99
Hz), 1.85 (s, 15H, CpCH3). 13C{1H} NMR (CD2Cl2, 90.6 MHz,
20 °C): δ 161.37 (CdN), 142.66 (N-C6H5), 130.59 (m-C6H5),
128.45 (p-C6H5), 119.91 (o-C6H5), 92.57 (C5Me5), 85.47 (C3/C5),
84.91 (C2/C6), 79.47 (C4), 11.50 (C5Me5). MS (FAB, m/z): 309
(M+). Treatment of a THF solution of 3H+ with NaH resulted in
the removal of the N-H‚‚‚N proton, and the NMR spectra of 3 in
carefully dried CD2Cl2 could be obtained. 1H-NMR (CD2Cl2, 360
MHz, 25 °C): δ 8.59 (s, 2H, H1/H7), 7.37-7.12 (m, 10H, C6H5),
3
3
5.05 (d, 2H, H3/H5, J ) 2.83 Hz), 4.67 (t, 1H, H4, J ) 2.83
Hz), 1.87 (s, 15H, CpCH3). 13C{1H} NMR (CD2Cl2, 90.6 MHz,
20 °C): δ 159.36 (CdN), 153.09 (N-C6H5), 129.07 (m-C6H5),
124.99 (p-C6H5), 120.67 (o-C6H5), 88.09 (C5Me5), 83.83 (C3/C5),
77.20 (C2/C6), 76.02 (C4), 11.83 (C5Me5). Although attempts at
elemental analysis were made, they were not satisfactory. However,
the 13C NMR spectrum is consistent with the structure determined
by X-ray crystallography and indicates the absence of carbon-
containing impurities.
[(Ph2AFA)2Pd] (1). To a solution of Ph2AFAH (0.30 g, 1.10
mmol) in 25 mL of toluene was added 0.7 mL of CH3Li (1.6 M in
THF) and the mixture stirred for 1 h. A solution of (PhCN)2PdCl2
(0.210 g, 0.55 mmol) in toluene (50 mL) was stirred at 50 °C for
30 min and then added via cannula. The mixture was stirred at
room temperature for 2 h and then filtered through a pad of Celite.
The volume was reduced in vacuo to 10 mL and the solution stored
at -5 °C for 1 day. The dark purple crystalline solid formed was
separated by filtration, washed twice with 2 × 10 mL portions of
hexanes to remove small amounts of free ligand, and dried under
vacuum. Yield: 39.5% (0.217 mmol, 0.141 g). Crystals suitable
for X-ray analysis were obtained from THF solutions upon slow
[(Cp*Ru)2Pd(Ph2AFA)2][BF4]2 (4). [Cp*Ru(CH3CN)3][BF4]
(0.250 g, 0.559 mmol) and (Ph2AFA)2Pd (0.181 g, 0.280 mmol)
were stirred in CH2Cl2 (40 mL) for 1 h, and the solution was filtered
via cannula. After storage at -5 °C for 2 days, crystals of the
desired compound had formed as orange blocks. They were isolated
via filtration and dried in vacuo. Yield: 46.0% (0.167 g, 0.129
mmol). Crystals suitable for X-ray analysis were obtained from
CHCl3 solution via slow evaporation of the solvent. 1H NMR (CD3-
OD, 360 MHz, 25 °C): δ 7.91 (s, 4H, H1/H7), 7.70-7.67 (m, 8H,
1
3
evaporation of the solvent. H NMR (360 MHz, CD2Cl2): δ 7.44
m-C6H5), 7.56-7.53 (m, 12H, C6H5), 5.66 (t, 2H, H4, J ) 2.80
3
(s, 2H, H1/H7), 7.45-7.42 (d, 4H, ortho-C6H5), 7.29-7.25 (t, 4H,
meta-C6H5), 7.20-7.15 (t, 2H, para-C6H5), 6.70 (d, 2H, H3/H5, 3J
) 3.63 Hz), 6.38 (t, 1H, H4, 3J ) 3.63 Hz). 13C{1H} NMR (CD2-
Cl2, 90.6 MHz, 20 °C): δ 160.44 (CdN), 149.03 (N-C6H5), 135.16
(C6H5), 128.53 (C6H5), 125.35 (C3/C5), 122.87 (C6H5), 120.56 (C2/
C6), 117.39 (C4). Although attempts at elemental analysis were
made, they were not satisfactory. However, the 13C NMR spectrum
Hz), 5.40 (d, 4H, H3/H5, J ) 2.80 Hz), 1.58 (s, 30H, CpCH3).
13C{1H} NMR (CD3OD, 90.6 MHz, 25 °C): δ 168.47 (CdN),
147.91 (N-C6H5), 131.02 (C6H5), 130.32 (C6H5), 123.81 (C6H5),
96.01 (C4), 88.11 (C2-C6), 86.18 (C5Me5), 77.58 (C3-C5), 11.57
(C5Me5). Anal. Calcd for C58H60N4B2F8Ru2Pd: C, 53.78; H, 4.67;
N, 4.33. Found: C, 53.39; H, 4.43; N, 4.13.
[(Ph2AFA)Pd(CH3)PPh3] (5). To a solution of Ph2AFAH (0.30
g, 1.10 mmol) in toluene (25 mL) was added 0.7 mL of CH3Li
(1.6 M in THF) and the mixture stirred for 1 h. To a solution of
[(COD)Pd(CH3)(Cl)] (0.290 g, 1.10 mmol) in toluene (10 mL) was
added PPh3 (0.288 g, 1.10 mmol), and after stirring for 5 min, the
(4) Ru¨lke, R. E.; Ernsting, J. M.; Spek, A. L.; Elsevier, C. J.; van
Leeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769.
(5) Fagan, P. J.; Ward, M. D.; Calabrese, J. C. J. Am. Chem. Soc. 1989,
111, 1698.

130 Organometallics, Vol. 26, No. 1, 2007
Table 1. Crystal Data for Compounds 1, 2, 3, 4, and 5
Bailey et al.
1
2
3
4
5
cryst description
empirical formula
Mw
red block
C42H38N4OPd
721.16
yellow block
C38H30N4Zn
608.03
red block
C31H34Cl2N2Ru
606.57
orange block
C60H62B2Cl6F8N4PdRu2
767.02
dark red block
C38H33N2PPd
655.03
T (K)
150(2)
150(2)
220(2)
150 (2)
150 (2)
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
I2/a
16.7396(7)
19.2569(8)
10.3606(5)
90
90.412(2)
90
3339.7(3)
4
monoclinic
P21/c
10.0428(3)
18.9776(7)
15.4767(6)
90
93.126(2)
90
2945.29(18)
4
monoclinic
P21
11.1538(4)
11.6438(4)
12.7666(4)
90
114.164(2)
90
1512.75(9)
2
monoclinic
P21/c
11.0330(2)
15.3253(3)
19.1861(4)
90
104.6070(10)
90
3139.21(11)
2
monoclinic
P21/n
9.0131(3)
19.7765(8)
17.4028(8)
90
99.393(3)
90
3060.4(2)
4
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
µ(Mo KR) (mm-1
)
0.596
4725 [R(int) )
0.0332]
0.868
6457 [R(int) )
0.0520]
0.716
5391 [R(int) )
0.0311]
1.079
8342 [R(int) )
0.039]
0.688
9135 [R(int) )
0.0333]
no. of indep reflns
no. of data with |F| > 4σ(|F|)
3997
4600
5090
7050
3534
absorp corr
multiscan
(Tmin ) 0.769,
Tmax ) 0.910)
multiscan
(Tmin ) 0.627,
Tmax ) 0.840)
multiscan
(Tmin ) 0.698,
Tmax ) 0.880)
semiempirical
from equivalents
(Tmin ) 0.66,
Tmax ) 0.87)
0.0489
multiscan
(Tmin ) 0.682,
Tmax ) 0.810)
R
Rw
0.0401
0.1002
0.0437
0.1031
0.0337
0.0900
0.0313
0.0826
0.1188
resulting suspension was added by means of a cannula into the
solution of the deprotonated ligand. The mixture was stirred for 3
h and then filtered through Celite. Upon slow evaporation of the
solvent [(Ph2AFA)Pd(PPh3)(CH3)] was obtained as red crystals
provided in Table 1. In 1 the complex sits with the Pd atom on an
inversion center. The structure contains a molecule of THF
disordered about a 2-fold axis. The atom labeled C1S is common
to both disorder components and is full-occupancy. The other atoms
(C2S, C3S, and C4S) are half-weight. Crystals of 3 appeared to
shatter at 150 K, and data were therefore collected at 220 K.
Disordered solvent regions were treated with the Squeeze proce-
dure.9 Compound 4 crystallizes with two molecules of chloroform
in the unit cell.
1
suitable for X-ray analysis (0.346 g, 48%). H NMR (CDCl3, 360
MHz, 20 °C): δ 8.53 (d, 1H, H1, 3J ) 8.20 Hz), 8.00 (s, 1H, H7),
7.52 (d, 2H, C6H5), 7.31-7.39 (m, 9H, P-C6H5), 7.25-7.20 (m,
6H, P-C6H5), 7.12 (t, 1H, C6H5), 7.01 (t, 1H, C6H5), 6.93 (dd, 1H,
3
3
H3, J ) 3.57 Hz), 6.77 (dd, 1H, H5, J ) 3.57 Hz), 6.45 (t, 1H,
3
H4, J ) 3.57 Hz), -0.021 (d, 3H, CH3, J ) 3.57 Hz). 13C{1H}
NMR (CDCl3, 90.6 MHz, 25 °C): δ 160.71 (C1), 158.43 (C7),
150.80 (C8), 150.10 (C12), 134.16 (C17, 2JC-P ) 11.7 Hz), 131.79
(C15), 131.39 (C11), 130.89 (C16, 1JC-P ) 48.9 Hz), 130.09 (C19),
128.77 (C14), 128.30 (C10), 127.94 (C18, 3JC-P ) 10.3 Hz), 124.39
(C3), 123.84 (C5), 122.96 (C13), 121.16 (C9), 119.28 (C2), 119.13
(C6), 118.06 (C4), 3.55 (CH3, JC-P ) 8.5 Hz). Anal. Calcd for
C38H33N2PPd: C, 69.67; H, 5.08; N, 4.28. Found: C, 69.36; H,
4.97; N, 4.15.
General Procedure for Reactivity Studies with Ethylene. A
Bu¨chi glass autoclave (100 mL) was heated at 80 °C under vacuum
for 3 h and then cooled to room temperature under an ethylene
atmosphere. The autoclave was charged with 20 mL of toluene and
then sealed. The pressure of ethylene was set to the desired value,
and the desired temperature was established by means of an oil
bath. After releasing the pressure, a solution of 5 (75 µmol in 5
mL of toluene) and [Ni(COD)2] (150 µmol in 5 mL of toluene)
were added in sequence via cannula. The reactor was sealed and
pressurized with ethylene to the desired reaction pressure. The
reaction mixture was stirred under constant pressure for 6 h, after
which time the pressure was released and the catalyst quenched
with acidified methanol. An aliquot of the mother liquor was
analyzed by EI-MS to detect formation of oligomers, whereas the
solid polymer produced was collected by filtration and dried
overnight under vacuum.
Results and Discussion
Treatment of N,N′-diphenyl-6-aminofulvene-2-aldimine (Ph2-
AFAH) with butyl lithium in toluene followed by reaction with
[(PhCN)2PdCl2] provides [Pd(κ2-Ph2AFA)2] (1). This is the only
product that may be isolated for either 1:1 or 2:1 (ligand:Pd)
reaction stoichiometries. The crystal structure (Figure 4) shows
that the complex is centrosymmetric and that the two ligands
are therefore equivalent. The coordination of the ligand is highly
unusual, with the mean plane of the ligand atoms being tilted
toward an axial position relative to the square-planar environ-
ment of the metal. Similar, although less marked, deviations from
coplanarity of the metal and ligand atoms have been observed
in complexes of the related AFA ligand [{HNdC(Ph)}2C5H3]-.10
The acute angle between the PdN4 plane and the mean plane
defined by the two NdC imine bonds (θ) in 1 is 53.6°, and the
displacement of the metal from this plane (ω) is 1.27 Å (Figure
5). This unusual coordination geometry is not accompanied by
any significant distortion of the N-Pd-N chelate angle [92.89-
(8)°]. The C-C (C3-C2 and C7-C8) bond lengths within the
chelate ring are crystallographically indistinguishable, as are the
C-N (C2-N1 and C8-N9) bond lengths, thus supporting the
view of the ligand as a cyclopentadienyl-diimine (B, Figure
1). The C5 ring of the ligand is planar; however, the imine CH
carbon atoms lie 0.105 Å (C2) and 0.264 Å (C8) above this
plane, while the nitrogen atoms are 0.165 (N1) and 0.078 (N9)
2
Crystallography. Data were collected at 150 K (220 K for 3)
on a Bruker Smart APEX diffractometer equipped with an Oxford
Cryosystems low-temperature device. The structures were solved
with Patterson methods (DIRDIF)6 and refined using SHELXL or
Crystals.7,8 The crystal data for compounds 1, 2, 3, 4, and 5 are
(7) Sheldrick, G. M. Shelxl-97; University of Gottingen: Germany, 1997.
(8) Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.; Watkin,
D. J. J. Appl. Crystallogr. 2003, 36, 1487.
(9) Sluis, P. v.d.; Spek, A. L. Acta Crystallogr. 1990, A46, 194-201.
(10) (a) Etkin, N.; Ong, C. M.; Stephan, D. W. Organometallics 1998,
17, 3656. (b) Ong, C. M.; Stephan, D. W. Inorg. Chem. 1999, 38, 5189.
(6) Beurskens, P. T.; Beurskens, G.; Bosman, W. P.; de Gelder, R.;
Garcia-Granda, S.; Gould, R. O.; Israel, R.; Smits, J. M. M. The DIRDIF96
Program System, Technical Report of the Crystallography Laboratory;
University of Nijmegen: The Netherlands, 1996.

An Ambidentate Cyclopentadientyl-Diimine Ligand
Organometallics, Vol. 26, No. 1, 2007 131
Figure 4. Molecular structure of [Pd(κ2-Ph2AFA)2] (1). Selected
bond lengths (Å) and angles (deg): Pd-N1 2.028(2), Pd-N9
2.0369(19), N1-C2 1.308(3), C2-C3 1.412(3), C3-C7 1.446(3),
C7sC8 1.413(3), C8-N9 1.309(3), N1-Pd-N9 92.89(8), Pd-
N1-C2 121.55(17), N1-C2-C3 127.6(2), C2-C3-C7 131.4(2),
C3-C7-C8 130.7(2), C7-C8-N9 127.2(5), C8-N9-Pd 118.86-
(16).
Figure 6. Molecular structure of [Zn(κ2-Ph2AFA)2] (2). Selected
bond lengths (Å) and angles (deg): Zn-N11 1.9962(16), N11-
C21 1.313(2), C21-C31 1.411(3), C31-C71 1.459(3), C71-C81
1.410(3), C81-N91 1.132(2), Zn-N91 1.9981(16), N11-Zn-N91
111.62(6), Zn-N91-C81 127.75(13), N91-C81-C71 130.14(18),
C81-C71-C31 135.58(18), C71-C31-C21 136.16(18), C31-
C21-N11 129.99(18), C21-N11-Zn 127.68(13).
distorts to avoid this clash. These observations are in stark
contrast to the structural features of the tetrahedral complex [Zn-
(κ2-Ph2AFA)2] (2), which is formed on treatment of ZnCl2 with
2 equiv of NaPh2AFA. The X-ray crystal structure of this
complex (Figure 6) shows it to contain almost planar AFA
ligands, with the maximum displacement of non-phenyl atoms
from the C5 mean plane of 0.074 Å. The maximum zinc
displacement from the plane defined by the two ligand CdN
bonds (ω, Figure 5) for the two inequivalent ligands is just 0.161
Å (cf. 1.27 Å in 1). The intraligand N-N distances (3.304 and
3.305 Å) are substantially greater in 2 than in the free ligand
and in 1. Given the planarity of the ligands, it is surprising that
the angles within the chelate ring are, in most cases, substantially
greater than the corresponding angles in the free ligand, which
is also planar. For example, the C-C-C angles at the two C5
ring atoms contained within the chelate ring are 136.17°/135.58°
and 136.05°/135.38° in the two independent ligands, which
compares with 131.66°/131.87° in the free ligand and 131.33°/
130.65° in the Pd complex (1). While these angles would not
be expected to be 120° because of the constraints of the five-
membered ring, they are considerably greater than 126°, which
is predicted by simple geometry. The differences in the ligand
coordination geometry between 1 and 2 may be accounted for
by the fact that the tetrahedral coordination of the Zn(II) ion
allows the phenyl substituents of one ligand to interdigitate with
those of the other, while this is not possible in a square-planar
complex.
Figure 5. Parametrization of the Ph2AFA ligand coordination
geometry. θ° ) the acute angle between the plane defined by the
metal and the two ligand nitrogen atoms and the mean plane defined
by the two ligand CdN bonds; ω ) the displacement of the metal
from this plane.
Å below and above it, respectively. The Pd-N-C angles are
close to 120°; the remaining angles in the chelate ring are
however considerably greater than 120°, but do not differ by
more than 1° from the corresponding angles in the free ligand
(Ph2AFAH) in which all the non-phenyl atoms are coplanar and
there is a strong N-H-N intramolecular hydrogen bond.11
Furthermore, the intraligand N1-N9 distance of 2.946 Å in 1
is only slightly greater than the value of 2.791 Å found in the
free ligand. The distortion of the ligand found in 1, and the
unusual coordination geometry, therefore appears not to be due
to a mismatch between the required 90° metal coordination angle
and the preferred ligand bite angle, but is rather due to the steric
effects of the phenyl substituents. The coordination of the ligands
with the metal in the ligand planes would place the phenyl rings
in the same region of space, and the coordination mode therefore
In an attempt to explore the ability of the Ph2AFA ligand to
act as an ambidentate ligand bonding to metals through both
the diimine and C5-ring donors, the zinc complex 2 was treated
with [Cp*Ru(MeCN)3][BF4]. However, there is no zinc in the
product isolated, the mass spectrum of which indicates it to be
[Cp*Ru(Ph2AFA)] (3). The X-ray crystal structure of 3 (Figure
7) shows that Ru coordination occurs via the C5 ring of the
AFA ligand. The fact that the [Cp*Ru]+ fragment coordinates
in this way rather than to the nitrogen donors is consistent with
the known affinity of this species for π-aromatic ligands;5
(11) Ammon, H. L.; Muller-Westerhoff, U. Tetrahedron 1974, 30, 1437.

132 Organometallics, Vol. 26, No. 1, 2007
Bailey et al.
replaced by a singlet at 8.59 ppm on treatment of 3 with NaH.
It therefore appears that the two nitrogen atoms in 3 are strongly
basic and the location of a proton chelated by these two atoms
accounts for the spectrum. The two signals for the C-H protons
may therefore be assigned as a doublet (7 Hz) due to coupling
to this added proton; consistent with this a broad triplet (7 Hz)
is observed at 16.51 ppm (1H) for the chelated N-H-N proton.
These NMR characteristics are similar to that found for the free
ligand (Ph2AFAH), in which these C-H protons occur as a
doublet (7 Hz) at 8.29 ppm and the N-H-N triplet (7 Hz) is
observed at 15.59 ppm in CD2Cl2. The source of the proton
must be attributed to traces of water in the NMR solvent;
however there is no evidence for the location of a proton in the
region between the two nitrogen atoms in the X-ray structure
of 3, nor for a counteranion that this would necessitate. The
anhydrous conditions under which 3 was crystallized therefore
provides the neutral complex 3. The high basicity of 3 may be
accounted for by the ability of the two nitrogen atoms to chelate
the added proton in a similar manner to that in well-known
“proton-sponges” such as 1,8-bis(dimethylamino)naphthylene.14
Indeed the hydrogen bonding and tautomerism in R2AFAH
systems have been subject to extensive investigations.15 The
equivalence of the two HCdN-Ph arms of the Ph2AFA ligand
in both 3 and its conjugate acid (3H+) indicates that the Ru-C
interaction with one of the imine carbon atoms observed in the
solid-state structure is either absent in solution or subject to
rapid fluxionality in which the Ru center interacts alternately
with the two imine carbon atoms.
Figure 7. Molecular structure of [Cp*Ru(η5-Ph2AFA)] (3).
Selected bond lengths (Å) and angles (deg): Ru-C31 2.168(7),
Ru-C41 2.195(5), Ru-C51 2.222(4), Ru-C61 2.212(5), Ru-C71
2.160(6), N11-C21 1.312(6), C21-C31 1.402(8), C31-C71 1.442-
(6), C71-C81 1.482(9), C81-N91 1.293(6), N11-C21-C31
124.5(5), C21-C31-C71 131.9(7), C31-C71-C81 128.6(6),
C71-C81-N91 123.7(4).
In contrast to the formation of 3, treatment of the Pd complex
1 with 2 equiv of [Cp*Ru(MeCN)3][BF4] leads to a trimetallic
complex, [Pd(Ph2AFA)2(RuCp*)2][BF4]2 (4), in which the Pd
atom has been retained. The X-ray crystal structure of 4 shows
the complex to have an inversion center at the Pd atom, and
both AFA ligands and Ru centers are therefore equivalent. The
structure shows that the C5 rings of both AFA ligands in the
Pd complex 1 are now coordinated to [Cp*Ru]+ units (Figure
8). The metrical parameters within the [Pd(Ph2AFA)2] part of
the structure are similar to those observed in 1, as discussed
above. The coordination of the C5 ring to Ru is again slightly
asymmetric, with the bonds to the substituted ring atoms being
slightly shorter. The angle between the PdN4 plane and the mean
plane defined by the two NdC imine bonds (θ, Figure 5) in 4
is 52.4°, similar to that in 1, and the metal is displaced 1.107 Å
from this plane (ω, Figure 5). The three C-H bonds of the C5
ring of the AFA ligands are almost coplanar with the C5 ring
plane; however the two C-C bonds to the imine groups lie
0.283 Å (C81) and 0.352 Å (C21) below the plane (on the same
side as the Ru atom). The differences between the chelate ring
C-C bonds are not crystallographically significant and neither
are those between the C-N bonds, and although the Ru
distances to C21 (2.937 Å) and C71 (2.984 Å) are similar to
that considered to be consistent with a weak interaction and a
contribution from the fulvene form of the ligand in 3, the C-C
and C-N bond lengths in the chelate ring are not consistent
however, the possibility that the ligand is coordinated in this
fashion due to the occupation of the nitrogen donors in 2 by
zinc (which is subsequently lost on Ru coordination) cannot be
discounted. An analogous 1,2-diiminoferrocene species has been
prepared by condensation of 1,2-formylferrocene with aniline;
however no structural data have been published.12 There is a
small difference between the C-C bond lengths from the ring
to the imine carbon atoms, with C31-C21 being 0.081 Å shorter
than C71-C81. There is also a smaller difference of 0.019 Å
between the two C-N bonds, with C81-N91 being the shorter.
These observations contrast with the indistinguishability of these
pairs of bonds in the Pd complex 1. There is therefore some
evidence for a contribution from the fulvene form of the ligand
(A, Figure 1) in 3. The distances between the Ru center and
the imine C atoms are 2.972 (C21) and 3.150 Å (C81), which
is considerably longer than 2.282 Å found for the Ru-C bond
to the exocyclic carbon atom in [CpRu(η6-pentafulvene)][CF3-
SO3];13 however C21 is the C atom indicated by C-C bond
length data to have greater fulvene character, so there is perhaps
some weak bonding between this C atom and the Ru center.
The AFA ligand itself is not planar, the maximum deviation of
non-Ph atoms from the C5 plane being 0.34 Å (N11). The N-N
separation is just 2.651 Å, shorter than in the free ligand, in
which an N-H-N hydrogen bond is present.
1
The H NMR spectrum of 3 at 293 K in CD2Cl2 contains
two signals at 8.96 and 8.94 ppm, which may be assigned to
the NdC-H protons. We initially attributed this inequivalence
to the structurally observed Ru-C interaction with one of the
imine carbon atoms and the observed coalescence of the two
signals at 311 K to the fluxionality of this Ru-fulvene carbon
interaction. However, the two NdCH carbon atoms are equiva-
(14) Alder, R. W.; Bowman, P. S.; Sleele, R. W. S.; Winterman, D. R.
J. Chem. Soc., Chem. Commun. 1968, 723.
(15) (a) Perrin, C. L.; Ohta, B. K. J. Mol. Struct. 2003, 644, 1. (b) Perrin,
C. L.; Ohta, B. K. Bioorg. Chem. 2002, 30, 3. (c) Sanz, D.; Perez-Torralba,
M.; Alarcon, Sergio H.; Claramunt, R. M.; Foces-Foces, C.; Elguero, J. J.
Organometal. Chem. 2002, 67, 1462. (d) Pietrzak, M.; Limbach, H.-H.;
Perez-Torralba, M.; Sanz, D.; Claramunt, R. M.; Elguero, J. Magon. Reson.
Chem. 2001, 39, S100. (e) Claramunt, R. M.; Sanz, D.; Alarcon, S. H.;
Perez Torralba, M.; Elguero, J.; Foces-Foces, C.; Pietrzak, M.; Langer, U.;
Limbach, H.-H. Angew. Chem., Int. Ed. 2001, 40, 420. (f) Ackman, L. M.;
Trewella, J. C.; Haddon, R. C. J. Am. Chem. Soc. 1980, 102, 2519. (g)
Mueller-Westerhoff, U. J. Am. Chem. Soc. 1970, 92, 4849.
1
lent in the 13C NMR spectrum, and the two H signals are
(12) Watanabe, M.; Okada, T. Toso Kenkyu, Gijutsu Hokoku 2004, 48,
15.
(13) Barlow, S.; Cowley, A.; Green, J. C.; Brunker, T. J.; Hascall, T.
Organometallics 2001, 20, 5351.

An Ambidentate Cyclopentadientyl-Diimine Ligand
Organometallics, Vol. 26, No. 1, 2007 133
Figure 9. Molecular structure of [(κ2-Ph2AFA)PdMe(PPh3)] (5).
Selected bond lengths (Å) and angles (deg): Pd-C1 2.0436(17),
Pd-P 2.2157(5), Pd-N11 2.0945(15), Pd-N912.1335(14), N11-
C21 1.300(2), C21-C31 1.424(2), C31-C71 1.459(3), C71-C81
1.405(3), C81-N91 (2), C31-C41 1.406(3), C41-C51 1.399(3),
C51-C61 (3), C61-C71 1.416(2), N11-Pd-N91 87.62(6), C1-
Pd-N11 87.24(7), C1-Pd-P 86.49(6), P-Pd-N91 97.95(4).
adopt the “side-on” type of coordination mode, on this occasion
induced by the steric bulk of the coordinated PPh3 ligand. The
steric interaction between the PPh3 ligand and the phenyl
substituent of the Ph2AFA ligand is indicated by the large
P-Pd-N angle of 97.95(4)°; all of the other coordination angles
at Pd are substantially less than 90° to compensate. This
interaction also results in the PPh3 ligand lying substantially
out of the square plane of the complex, the P atom being 0.551
Å out of the mean plane defined by the other three donor atoms,
which are effectively coplanar with the metal. The Pd-N bond
trans to the methyl ligand [2.1335(14) Å] is substantially longer
than that trans to the phosphine [2.0945(15) Å], reflecting the
greater trans-influence of the alkyl. This longer bond is also
that cis to the PPh3 ligand, so steric effects may also make a
contribution to this difference in Pd-N bond distances. Both
of these Pd-N bonds are significantly longer than in the other
Pd complexes (1 and 4), which presumably reflects the extreme
distortion of the ligand coordination geometry forced by the
bulk of the PPh3 ligand. The angle between the mean plane of
the square-planar Pd coordination environment and that defined
by the two NdC imine bonds (θ, Figure 5) in 5 is 72.5°, and
the palladium center is displaced 1.393 Å from this plane (ω,
Figure 5). The plane of the C5 ring is almost perpendicular to
the metal square plane (85.1°). The distortion of the Ph2AFA
ligand coordination geometry is therefore even more extreme
in this complex than in either 1 or 4. The 1H NMR spectrum of
5 shows a rather large coupling of one of the imino protons
with the phosphorus of the PPh3 ligand (J ) 8.34 Hz), also
confirmed by H-P correlated NMR experiments. Coupling with
the phosphorus is also observed for the methyl group, which
appears as a doublet at -0.06 ppm with a coupling constant of
3.65 Hz.
Figure 8. Molecular structure of the dication [Pd(Ph2AFA)2-
(RuCp*)2]2+ (4). Selected bond lengths (Å) and angles (deg): Pd-
N11 2.030(3), Pd-N91 2.034(3), N11-C21 1.296(5), C21-C31
1.434(5), C31-C71 1.472(5), C71-C81 1.430(5), C81-N91 1.290-
(5), Ru-C31 2.152(4), Ru-C41 1.299(4), Ru-C51 2.245(4), Ru-
C61 1.191(4), Ru-C71 2.162(4), N11-C21 1.296(5), C21-C31
1.434(5), C31-C71 1.472(5), C71-C81 1.430(5), C81-N91 1.290-
(5), N11-C21-C31 124.5(5), C21-C31-C71 131.9(7), C31-
C71-C81 128.6(6), C71-C81-N91 123.7(4).
with this picture in 4. Perhaps surprisingly, the coordination of
the [Cp*Ru]+ fragments to the C5 rings of the AFA ligands of
1 to provide 4 does not significantly affect the Pd-N bond
distances (2.028/2.037 Å in 1 and 2.030/2.034 Å in 4). This,
coupled with the rather symmetrical η5-bonding of the AFA C5
rings to the Ru centers, suggests the electronic isolation of the
two coordination sites in the ligand, thus indicating a predomi-
nant cyclopentadienyl-diimine form for the ligand (Figure 1,
B).
An alkene polymerization catalyst of the type depicted in
Figure 2 (D) requires a single AFA ligand coordinated to the
metal, an alkyl ligand, and a labile neutral ligand to provide
the site for alkene coordination. The ideal palladium precursor
for this system would be a halide-bridged dimer of the form
[(κ2-Ph2AFA)Pd(µ-X)]2, which could subsequently be methy-
lated in a donor solvent to provide [(κ2-Ph2AFA)PdMe(S)], or
potentially the methyl-bridged dimer [(κ2-Ph2AFA)Pd(µ-Me)]2
in the absence of a donor. However, unfortunately all attempts
to synthesize such a species from PdCl2 or [PdCl2(NCR)2] (R
) Me, Ph) resulted only in the formation of the bis-chelate
complex 1. This is perhaps surprising given the steric hindrance
present in 1, which results in the unusual coordination mode of
the ligands in this complex. The separation of the metal centers
by the bridging halides in the dimeric system would relieve this
problem even though the two ligands would occupy the same
plane. Nevertheless, we have been able to prepare a complex
containing a single Ph2AFA ligand by treating [(COD)PdMe-
(Cl)] with NaPh2AFA in the presence of PPh3, which provides
the complex [(κ2-Ph2AFA)PdMe(PPh3)] (5). The X-ray crystal
structure of 5 (Figure 9) shows the Ph2AFA ligand again to
Reactivity toward Ethylene. We have undertaken prelimi-
nary studies of the reactivity of 5 toward ethylene at low
pressure. The methyl complex 5 showed no reactivity toward
ethylene (5 bar, 25 and 50 °C) in toluene solution; however
this is not unexpected given the coordinative saturation of the
complex provided by the PPh3 ligand. For neutral nickel(II)-
based ethylene polymerization catalysts [Ni(COD)2] has been
used as an effective phosphine scavenger,16 and the addition of

134 Organometallics, Vol. 26, No. 1, 2007
Scheme 1. Synthetic Routes to the Ph2AFA Complexes Reported
Bailey et al.
2 molar equiv of this complex to a solution of 5 in toluene
solution provided a system that showed some reactivity toward
ethylene. Under the same reaction conditions [Ni(COD)2] alone
showed no reactivity toward ethylene. Both linear polyethylene
(Mn ) 8400, Mw ) 140 000, PDI ) 17) and a mixture of
oligomers up to C44 were produced by the system. However,
the activity is extremely low [0.02 kg(mol‚h‚atm)-1 for the
polymer], and the formation of both polymer and oligomers and
the broad molecular weight distribution of the former indicate
the presence of more than one active species. A full description
of the details of this system will need to await further work,
and we hope to report on this in the near future.
result of the 1,4-relationship of the imine donors within the AFA
ligand framework, which contrasts with the more common 1,2-
diimine systems, and the electronic flexibility conferred by the
availability of the fulvene form of the ligand (Figure 1, A). The
almost symmetric coordination of the Cp*Ru+ unit to the C5
ring of the [Ph2AFA]- ligand, when the nitrogen donors are
either occupied or vacant, indicates the presence of significant
charge within the C5 ring of the AFA ligand. The slight bond
alternation within the N-C-C-C-C-N portion of the ligand
in 3 and the apparent interaction between the Ru center and the
fulvene carbon atom thus implied provide evidence for a
contribution from the fulvene form of the ligand (Figure 1, A)
in this complex. However this is absent in 4, where the nitrogen
atoms are Pd coordinated. The AFA ligand system thus appears
to be rather flexible, both sterically and electronically, capable
of providing either a bis-imino or imino-amido donor at the
nitrogen sites and either a cyclopentadienyl or fulvene donor at
the C5 ring site, along with exceptional flexibility of coordination
geometry at the nitrogen donor sites. This suggests that the
ligand should be compatible with a wide range of metals in a
variety of oxidation states and be capable of accommodating a
range of metal steric environments. The identity of the species
active in oligomerization and polymerization of ethene formed
on activation of 5 with [Ni(COD)2] remains to be characterized,
and our work in this area continues.
Conclusions
The synthetic routes to the complexes reported are shown in
Scheme 1. The ability of the Ph2AFA ligand to coordinate to
metals in such a wide range of geometries is unprecedented for
a chelating diimine ligand, which normally favors complexation
of the metal within the plane of the ligand.17 This must be a
(16) (a) Wang, C.; Friedrich, S.; Younkin, T. R.; Li, R. T.; Grubbs, R.
H.; Bansleben, D. A.; Day, M. W. Organometallics 1998, 17, 3149. (b)
Klabunde, U.; Ittel, S. D. J. Mol. Catal. 1987, 41, 123. (c) Klabunde, U.;
Mulhaupt, R.; Herskovitz, T.; Janowicz, A. H.; Calabrese, J.; Ittel, S. D. J.
Polym. Sci., Part A: Polym. Chem. 1987, 25, 1989.
(17) Van Koten, G.; Vrieze, K. AdV. Organomet. Chem. 1982, 21, 151.

An Ambidentate Cyclopentadientyl-Diimine Ligand
Organometallics, Vol. 26, No. 1, 2007 135
obtained on activation of 5 with [Ni(COD)2] and treatment with
ethylene; and the gel permeation chromatogram of the polyethylene
also obtained from this system. This material is available free of
charge via the Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by a research
studentship (to M.M.) funded by the EPSRC (grant GR/S04659/
01), whose support is gratefully acknowledged.
Supporting Information Available: Crystallographic informa-
tion files (CIF) for compounds 1, 2, 3, 4, and 5; 13C NMR spectra
for compounds 1 and 3; mass spectrum of the oligomer mixture
OM060808D