Exploration of the Suitability of Bicyclic Guanidinates as Ligands in Catalytic Chemistry Mediated by Titanium

Article abstract of DOI:10.1021/om0341092

The synthesis, structure, and reactivity of titanium complexes supported by the bicyclic guanidinate ligand derived from 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) are described. In situ reaction between a THF solution of the lithium salt, (hppLi)_n, and titanium chloride starting materials afforded the series of compounds Ti(hpp)_nCl_4-n(THF)_x [n = 1, x= 1 (1); n = 2, x = 0 (3); n = 3, x = 0 (4)]. The dimeric, base-free complex [Ti(hpp)Cl ₂(μ-Cl)]₂ (2) was synthesized from the reaction of the silylated ligand precursor, hppSiMe₃, with TiCl₄ in CH₂Cl₂. Preliminary olefin polymerization studies of 1-3 using MAO as activator indicated low activities. The reaction between 3 and 2 equiv of AlMe₃ resulted in formation of a novel trimetallic Ti(III) species, Ti{AlMe₂(hpp)₂}Cl(μ-Cl)AlMe₃ (5), highlighting the noninnocent behavior of the ligands. The bis(benzyl) complex Ti(hpp)₂-(CH₂Ph)₂ (6) was synthesized from the alkane elimination reaction between the neutral ligand precursor, hppH, and Ti(CH₂Ph)₄. Attempted generation of the corresponding mono-benzyl cation through reaction with the neutral borane B(C ₆F₅)₃ again afforded reduced species. The synthesis of titanium tert-butylimido compounds [Ti(μ-N ^tBu)(hpp)Cl]₂ (7) and [Ti(N ^t-Bu)(hpp)(μ-hpp)]₂ (8) from the salt metathesis reaction of 1 and 2 equiv of (hppLi)_n, respectively, with Ti(N ^tBu)Cl₂(py)₃ is reported. The molecular structures of compounds 2 and 3 and 5-8 are reported, and the distribution of π-electron density throughout the guanidinate ligands is discussed in detail.

Full text of DOI:10.1021/om0341092

Organometallics 2003, 22, 5201-5211
5201
Exp lor a tion of th e Su ita bility of Bicyclic Gu a n id in a tes
a s Liga n d s in Ca ta lytic Ch em istr y Med ia ted by
Tita n iu m †
Martyn P. Coles* and Peter B. Hitchcock
Department of Chemistry, University of Sussex, Falmer, Brighton BN1 9QJ , U.K.
Received August 13, 2003
The synthesis, structure, and reactivity of titanium complexes supported by the bicyclic
guanidinate ligand derived from 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH)
are described. In situ reaction between a THF solution of the lithium salt, (hppLi)n, and
titanium chloride starting materials afforded the series of compounds Ti(hpp)nCl4-n(THF)x
[n ) 1, x ) 1 (1); n ) 2, x ) 0 (3); n ) 3, x ) 0 (4)]. The dimeric, base-free complex [Ti-
(hpp)Cl2(µ-Cl)]2 (2) was synthesized from the reaction of the silylated ligand precursor,
hppSiMe3, with TiCl4 in CH2Cl2. Preliminary olefin polymerization studies of 1-3 using
MAO as activator indicated low activities. The reaction between 3 and 2 equiv of AlMe3
resulted in formation of a novel trimetallic Ti(III) species, Ti{AlMe2(hpp)2}Cl(µ-Cl)AlMe3
(5), highlighting the noninnocent behavior of the ligands. The bis(benzyl) complex Ti(hpp)2-
(CH2Ph)2 (6) was synthesized from the alkane elimination reaction between the neutral ligand
precursor, hppH, and Ti(CH2Ph)4. Attempted generation of the corresponding mono-benzyl
cation through reaction with the neutral borane B(C6F5)3 again afforded reduced species.
The synthesis of titanium tert-butylimido compounds [Ti(µ-NtBu)(hpp)Cl]2 (7) and [Ti(Nt-
Bu)(hpp)(µ-hpp)]2 (8) from the salt metathesis reaction of 1 and 2 equiv of (hppLi)n,
respectively, with Ti(NtBu)Cl2(py)3 is reported. The molecular structures of compounds 2
and 3 and 5-8 are reported, and the distribution of π-electron density throughout the
guanidinate ligands is discussed in detail.
Sch em e 1. P r in cip a l Reson a n ce Str u ctu r es of th e
In tr od u ction
Gen er ic Gu a n id in a te An ion [R2NC{NR′}2]-
Featured among the many classes of nitrogen-based
ligands that have been investigated as alternatives to
the cyclopentadienyl anion in early transition metal
chemistry are the amidinates [RC{NR′}2]- and guanidi-
nates [R2NC{NR′}2]-.1 Tuning the properties of the
former type of ligand to meet specific requirements has,
to date, largely been restricted to investigations of the
steric demands of the carbon2 and nitrogen substitu-
ents.3 An additional feature associated with the guanid-
inate anions that allows a certain degree of control to
be exercised over the electron donor properties to a
metal center is the presence of a third, “zwitterionic”
resonance structure arising from delocalization of the
Namide lone-pair into the “CN3” core of the ligands (I,
Scheme 1). The extent to which this component is able
to contribute to the overall bonding is dependent upon
the angle (R) between the “R2N-” and “NCN” moieties,
with maximum contribution arising from a parallel
arrangement (R ) 0°), which generates the correct
orbital alignment for π-overlap between the N lone-pair
and the p-orbital of the sp2-carbon atom.
To a varying degree, the steric demands of the
nitrogen substituents have been shown to effect the
angle R and, hence, influence the extent of delocalization
from the amide nitrogen atom. For example, in the
aluminum dichloro complexes Al[R2NC{NiPr}2]Cl2,4
when the R-substituent is a relatively bulky -SiMe3
group, the R-value is relatively large (86.2°), while in
the analogous -NMe2 derivative, R ) 28.4°. It has also
been argued that an increase in the electron-withdraw-
ing nature of the silyl group versus the methyl group
contributes to the observed distribution of π-electron
density within the guanidinate framework. Concurrent
with this reduction in R, there is a pronounced shorten-
† Part of this work, including the crystal structures of compounds 3
and 5, has previously been communicated. See ref 6.
(1) Bailey, P. J .; Pace, S. Coord. Chem. Rev. 2001, 214, 91-141.
Barker, J .; Kilner, M. Coord. Chem. Rev. 1994, 133, 219-300. Edel-
mann, F. T. Coord. Chem. Rev. 1994, 137, 403.
(2) Schmidt, J . A. R.; Arnold, J . J . Chem. Soc., Dalton Trans. 2002,
3454. Schmidt, J . A. R.; Arnold, J . J . Chem. Soc., Dalton Trans. 2002,
2890. Schmidt, J . A. R.; Arnold, J . J . Chem. Soc., Chem. Commun.
1999, 2149. Abeysekera, D.; Robertson, K. N.; Cameron, T. S.;
Clyburne, J . A. C. Organometallics 2001, 20, 5532. Schmidt, J . A. R.;
Arnold, J . Organometallics 2002, 21, 2306.
(3) Coles, M. P.; Swenson, D. C.; J ordan, R. F.; Young, V. G., J r.
Organometallics 1998, 17, 4042. Boere´, R. T.; Klassen, V.; Wolmer-
shauser, G. J . Chem. Soc., Dalton Trans. 1998, 4147.
(4) Aeilts, S. L.; Coles, M. P.; Swenson, D. C.; J ordan, R. F.
Organometallics 1998, 17, 3265.
10.1021/om0341092 CCC: $25.00 © 2003 American Chemical Society
Publication on Web 11/05/2003

5202 Organometallics, Vol. 22, No. 25, 2003
Coles and Hitchcock
Sch em e 2
not reported. We have previously communicated our
initial studies of the [hpp]- anion as a ligand in titanium
chemistry6 and report here further studies into the
synthesis of hpp complexes of titanium, presenting a
detailed examination of the bonding within such com-
plexes.
Exp er im en ta l Section
Gen er a l Exp er im en ta l P r oced u r es. All manipulations
were carried out under dry nitrogen using standard Schlenk
and cannula techniques, or in a conventional nitrogen-filled
glovebox. Solvents were dried over appropriate drying agent
and degassed prior to use. hppH (Fluka), nBuLi (2.5 M solution
in hexanes, Acros), TiCl4 (Aldrich), and AlMe3 (2.0 M solution
in hexanes, Aldrich) were purchased from commercial sources
and used without further purification. TiCl4(THF)2,10 Ti(CH2-
Ph)4,11 and Ti(NtBu)Cl2(py)3 were synthesized using pub-
12
lished procedures. hppSiMe3 was synthesized from the reaction
of (hppLi)n and SiMe3Cl13 and used without further purifica-
tion. Elemental analyses were performed by S. Boyer at
London Metropolitan University. NMR spectra were recorded
using a Bruker Avance DPX 300 MHz spectrometer at 300
(1H) and 75 (13C{1H}) MHz. Proton and carbon chemical shifts
were referenced internally to residual solvent resonances.
Coupling constants, J , are quoted in Hz.
Ti(h p p )Cl3(THF ) (1). A solution of hppH (0.50 g, 3.6 mmol)
in THF was cooled to 0 °C, and 1 equiv of nBuLi (2.5 M solution
in hexanes) was added. The solution was allowed to warm to
room temperature and stirred for 1 h. The resulting mixture
was added to a solution of TiCl4(THF)2 (1.20 g, 3.6 mmol) in
THF at -78 °C, during which time a color change from yellow
to red was observed. After stirring at ambient temperature
for 15 h, the volatile components were removed and the
product was extracted with CH2Cl2. Removal of the solvent
afforded 1.11 g of 1 (85%) as a red solid. Analytically pure
samples were obtained from slow cooling of a warm (60 °C)
saturated toluene solution to room temperature.
Anal. Calcd for C11H20Cl3N3OTi: C, 36.2; H, 5.5; N, 11.5.
Found: C, 36.0; H, 5.7; N, 11.6. 1H NMR (C6D6, 298 K): δ 3.75
(m, 8H, CH2 + THF), 2.10 (br m, 4H, CH2), 1.36 (br m, 4H,
CH2), 1.10 (m, 4H, THF). 13C NMR (C6D6, 298 K): δ 160.0
(CN3), 70.3 (THF), 49.7 (CH2), 45.4 (CH2), 25.8 (THF), 21.7
(CH2). MS (EI+, m/z, 35Cl): 291 [M - THF]+, 255 [M - THF -
Cl]+.
[Ti(h p p )Cl2(µ-Cl)]2 (2). A solution of hppSiMe3 (1.43 g, 6.8
mmol) in CH2Cl2 was added dropwise to a solution of TiCl4
(1.28 g, 6.8 mmol) in CH2Cl2 at room temperature. The reaction
was stirred for 15 h, during which time formation of a red
precipitate was observed. Filtration and drying under vacuum
afforded 1.22 g of 2 as a microcrystalline solid (62%). Analyti-
cally pure samples were obtained from slow cooling of a warm
(60 °C) saturated toluene solution to room temperature.
ing of the R2N-C distance from 1.400(4) Å in the silyl
derivative to 1.343(5) Å in the methyl derivative,
commensurate with increased delocalization into the
carbon-nitrogen bond.
An alternative method to the incorporation of silyl
substituents by which the π-overlap may be promoted
is to constrain the two components of the ligand into a
cyclic structure such that a conformation approaching
coplanarity is enforced. To investigate the effect of this
imposed geometry, we have initiated a program into the
study of the commercially available bicyclic guanidine
1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a ]pyrimidine
(hppH, A, Scheme 2), as a source of neutral5 and anionic
ligands6 in transition metal chemistry. Previous work
employing [hpp]- as a ligand has principally focused on
its application as a bridging group between multiply
bonded metal complexes,7 although recent accounts of
monomeric and dimeric main-group (Sn, Al) metal
complexes containing bridging and chelating coordina-
tion modes have been reported.4,8 A simple explanation
for the observed tendency of the [hpp]- anion to bridge
two metal centers has been offered, namely, that the
incorporation of the Namidine substituents into the bicyclic
framework of the ligand promotes a parallel projection
of the donor orbitals. In contrast, steric interactions
between substituents in acyclic analogues force the
orbitals to point toward the “mouth” of the ligand,
favoring a chelating coordination mode.
Anal. Calcd for C7H12Cl3N3Ti: C, 28.8; H, 4.1; N, 14.4.
1
A number of group 4 metal complexes incorporating
the [hpp]- anion have previously been presented in the
patent literature and investigated as precatalysts for
the polymerization of ethylene.9 However, structural
analyses of the complexes were not performed, and
detailed discussion concerning the nature of the interac-
tion between the [hpp]- anion and the metal center was
Found: C, 28.7; H, 4.2; N, 14.5. H NMR (CD2Cl2, 298 K): δ
3
3
4.04 (t, J HH ) 5.8, 4H, CH2), 3.29 (t, J HH ) 5.9, 4H, CH2),
1.97 (m, 4H, CH2). 13C NMR (CD2Cl2, 298 K): δ 160.3 (CN3),
50.1 (CH2), 46.2 (CH2), 22.2 (CH2). MS (EI+, m/z, 35Cl): 292
[M]+, 255 [M - Cl]+ (where M ) “Ti(hpp)Cl3”).
Ti(h p p )2Cl2 (3). This compound was prepared by the
procedure outlined for 1, using 1.00 g of hppH (7.2 mmol) and
1.20 g of TiCl4(THF)2 (3.6 mmol). Extraction with CH2Cl2
afforded 1.23 g of 3 (86%) as an orange solid. Crystals were
(5) Oakley, S. H.; Coles, M. P.; Hitchcock, P. B. Inorg. Chem. 2003,
42, 3154. Coles, M. P.; Hitchcock, P. B. Polyhedron 2001, 20, 3027.
(6) Coles, M. P.; Hitchcock, P. B. J . Chem. Soc., Dalton Trans. 2001,
1169.
(7) Cotton, F. A.; Dalal, N. S.; Huang, P. L.; Murillo, C. A.; Stowe,
A. C.; Wang, X. A. Inorg. Chem. 2003, 42, 670, and references therein.
(8) Foley, S. R.; Yap, G. P. A.; Richeson, D. S. Polyhedron 2002, 21,
619.
(10) Manzer, L. E. Inorg. Synth. 1982, 21, 135.
(11) Zucchini, U.; Albizatti, E.; Giannini, U. J . Organomet. Chem.
1971, 26, 357.
(12) Blake, A. J .; Collier, P. E.; Dunn, S. C.; Li, W.-S.; Mountford,
P.; Shishkin, O. V. J . Chem. Soc., Dalton Trans. 1997, 1549.
(13) Kummer, D.; Halim, S. H. A.; Kuhs, W.; Mattern, G. J .
Organomet. Chem. 1993, 446, 51.
(9) Andell, O.; Maaranen, J . World Patent No. WO99/10353, 1999.

Titanium Guanidinate Complexes
Organometallics, Vol. 22, No. 25, 2003 5203
obtained by the slow cooling of a warm (60 °C) toluene solution
to 0 °C.
stirred for 18 h. The mixture was allowed to cool to room
temperature, and the volatiles were removed under reduced
pressure to afford an orange-yellow solid, which was extracted
from LiCl using toluene. Recrystallization from Et2O afforded
an intimate mixture of orange and yellow crystals, from which
a single crystal of the yellow bis(guanidinate) complex 8 was
separated mechanically and the X-ray diffraction study per-
formed.
Anal. Calcd for C14H24Cl2N6Ti: C, 42.6; H, 6.1; N, 21.3.
Found: C, 42.4; H, 6.2; N, 21.1. 1H NMR (C6D6, 298 K): δ 3.86
3
3
(t, J HH ) 5.7, 8H, CH2), 2.26 (t, J HH ) 5.9, 8H, CH2), 1.34
(m, 8H, CH2). 13C NMR (C6D6, 298 K): δ 160.2 (CN3), 47.1
(CH2), 45.7 (CH2), 22.6 (CH2). MS (EI+, m/z, 35Cl): 394 [M]+,
358 [M - Cl]+, 256 [M - hpp]+.
Ti(h p p )3Cl (4). This compound was prepared by the pro-
cedure outlined for 1, using 1.00 g of hppH (7.2 mmol) and
0.80 g of TiCl4(THF)2 (2.4 mmol). Extraction with CH2Cl2 and
recrystallization from hot toluene afforded 0.35 g of 4 (29%)
as an analytically pure orange solid.
P olym er iza tion P r oced u r e. A solution of 0.02 mmol of
the precatalyst was dissolved in toluene (200 mL) and added
to a glass-walled reactor containing 13.3 mL of a toluene
solution of MAO (1.5 M), to generate a Al:Ti ratio of 1000:1.
The mixture was stirred, by means of an overhead mechanical
stirrer, under an atomsphere of 9 bar of ethlyene for 1 h. The
reaction was quenched, and the polymer was collected, washed
with MeOH/H+, and then water, and dried in a vacuum oven
at 60 °C overnight. The yield was recorded and the activity
calculated (g PE mmol-1 h-1 bar-1) for the 1 h period.
Cr ysta llogr a p h y. Details of the crystal data, intensity
collection, and refinement for complexes 2, 3, 5, and 6 are listed
in Table 1, and for complexes 7 and 8 in Table 6. Crystals were
covered in oil, and suitable single crystals were selected under
a microscope and mounted on a Kappa CCD diffractometer.
The structures were refined with SHELXL-97.14 Additional
features are described below.
[Ti(h p p )Cl3]2 (2). C(3) is disordered [0.69:0.31] over two
positions, corresponding to different ring conformations. The
highest occupancy structure is shown in Figure 1.
Ti{AlMe2(h p p )2}Cl(µ-Cl)AlMe3 (5). C(3) is disordered
[0.47:0.53] over two positions, corresponding to different ring
conformations. The highest occupancy structure is illustrated
in Figure 3.
Anal. Calcd for C21H36ClN9Ti: C, 50.66; H, 7.29; N, 25.32.
1
Found: C, 50.60; H, 7.29; N, 25.37. H NMR (C6D6, 298 K): δ
3
3
3.82 (t, J HH ) 5.7, 4H, CH2), 2.64 (t, J HH ) 5.9, 4H, CH2),
1.67 (m, 2H, CH2). 13C NMR (C6D6, 298 K): δ 160.0 (CN3), 46.7
(CH2), 45.7 (CH2), 23.8 (CH2). MS (EI+, m/z, 35Cl): 497 [M]+,
462 [M - Cl]+, 358 [M - hpp]+.
Ti{AlMe2(h p p )2}Cl(µ-Cl)AlMe3 (5). A slurry of 3 (0.40 g,
1.0 mmol) in toluene was cooled to 0 °C, and 2 equiv of AlMe3
(2.0 M solution in toluene) was added. The mixture was
allowed to warm to room temperature and stirred for 15 h.
Removal of the volatiles and extraction with toluene afforded
a green solution, from which 0.15 g of 5 (29%) precipitated.
Anal. Calcd for C19H39Al2Cl2N6Ti: C, 43.5; H, 7.5; N, 16.0.
Found: C, 43.5; H, 7.7; N, 15.8. ESR (toluene, 298 K): giso
)
1.9617 (width 8.3 G). MS (EI+, m/z, 35Cl): 303 [Al(hpp)2]+, 180
[Al(hpp)Me]+.
Ti(h p p )2(CH2P h )2 (6). A solution of hppH (0.50 g, 3.6
mmol) in toluene was added dropwise to a solution of Ti(CH2-
Ph)4 (0.74 g, 1.8 mmol) that had been precooled -78 °C. The
reaction mixture was allowed to warm to room temperature
and stirred for 18 h. Removal of the volatile component
afforded 6 as a red solid, which was crystallized by cooling a
saturated Et2O solution to -35 °C. Yield: 0.57 g, 63%.
1H NMR (C6D6, 298 K): δ 7.26 (m, 8H, CH2Ph), 6.91 (tt,
Ti(h p p )2(CH2P h )2 (6). The absolute structure was defined
by refinement of the Flack parameter in the least squares.
[Ti(µ-NtBu )(h p p )Cl]2 (7b). The dimer lies on a crystal-
lographic inversion center. C(3) is disordered [0.89:0.11] over
two positions, corresponding to different ring conformations.
The highest occupancy structure is illustrated in Figure 5.
Crystal structure data for compounds 3 and 5 have been
previously deposited at the CCDC, reference numbers 160792
and 160793.
4
3
3J HH ) 6.6, J HH ) 2.1, 2H, Ph-Hpara), 3.35 (t, J HH ) 5.7, 8H,
3
CH2), 2.85 (s, 4H, CH2Ph), 2.47 (t, J HH ) 7.0, 8H, CH2),
3
1.31 (quin, J HH ) 5.8, 8H, CH2). 13C NMR (C6D6, 298 K): δ
163.4 (CN3), 153.3 (Ph-Cipso), 127.7 (Ph-Cortho/ meta), 126.3 (Ph-
C
ortho/ meta), 119.5 (Ph-Cpara), 84.0 (CH2Ph), 45.3 (CH2), 43.9
(CH2), 22.7 (CH2).
Resu lts a n d Discu ssion
[Ti(µ-NtBu )(h p p )Cl]2 (7). A solution of hppH (1.00 g, 7.2
mmol) in THF was cooled to 0 °C, and 1 equiv of nBuLi (2.5 M
solution in hexanes) was added. The solution was allowed to
warm to room temperature and stirred for 1 h. The resulting
mixture was added dropwise to a solution of Ti(NtBu)Cl2(py)3
(3.05 g, 7.2 mmol) in THF that was precooled -78 °C. The
resultant orange-red solution was allowed to warm slowly to
room temperature and stirred for 18 h. The volatiles were
removed under reduced pressure, affording a deep red solid,
which was extracted from LiCl using toluene. The volatiles
were then removed to afford pure 7. Yield: 1.37 g, 65%.
Anal. Calcd for C11H21N4ClTi: C, 45.2; H, 7.2; N, 19.2.
Found: C, 45.0; H, 7.4; N, 19.2. 1H NMR (C6D6, 298 K): δ 3.93
Deprotonation of the neutral bicyclic guanidine, hppH,
to the corresponding guanidinate anion, [hpp]-, was
readily achieved in THF at 0 °C, using nBuLi. Although
the Li salt can be isolated as the base-free species
(hppLi)n,15 for the sake of convenience we opted to use
the salt in situ as the THF solution. Accordingly, the
reaction with 1 equiv of TiCl4(THF)2 proceeded smoothly
to afford the mono-ligand compound Ti(hpp)Cl3(THF)
(1, Scheme 2). Although the molecular structure of 1
was not solved, NMR and elemental analysis revealed
that one molecule of THF was retained in the coordina-
tion sphere of the metal, as previously observed in the
amidinate complexes Ti(p-MeC6H4-C{NMe}2)Cl3(THF)16
and Ti(PhC{NSiMe3}{NR*})Cl3(THF) (R* ) myrta-
nyl).17 In contrast to the former complex where the NMe
substituents were inequivalent in solution, assigned as
cis and trans positions with respect to the position of
3
(br t, 4H, CH2), 2.36 (t, J HH ) 5.8, 4H, CH2), 1.45 (m, 4H,
CH2), 1.44 (s, 9H, NCMe3); (CDCl3, 298 K): δ 3.82 (br t, 4H,
CH2), 3.17 (t, 3J HH ) 5.8, 4H, CH2), 1.92 (m, 4H, CH2), 1.11 (s,
9H, NCMe3). 13C NMR (C6D6, 298 K): δ 161.7 (CN3), 72.5
(CMe3), 46.1 (CH2), 44.8 (CH2), 31.8 (CMe3), 23.5 (CH2); (CDCl3,
298 K): δ 161.3 (CN3), 72.6 (CMe3), 46.4 (CH2), 44.4 (CH2),
31.3 (CMe3), 23.4 (CH2). MS (EI+, m/z, 35Cl): 584 [M]+, 569
[M - Me]+.
1
the THF molecule, the H and 13C NMR data for 1 show
[Ti(NtBu )(h p p )(µ-h p p )]2 (8). A solution of hppH (1.00 g,
7.2 mmol) in THF was cooled to 0 °C, and 1 equiv of nBuLi
(2.5 M solution in hexanes) was added. The solution was
allowed to warm to room temperature and stirred for 1 h. The
resulting mixture was added dropwise to a solution of Ti(Nt-
Bu)Cl2(py)3 (1.52 g, 3.6 mmol) in THF at room temperature.
The resultant orange-red solution was heated to 60 °C and
(14) Sheldrick, G. M. SHELXL-97, Program for the Refinement of
Crystal Structures; Go¨ttingen, 1997.
(15) Coles, M. P.; Hitchcock, P. B. Manuscript in preparation.
(16) Flores, J . C.; Chien, J . C. W.; Rausch, M. D. Organometallics
1995, 14, 2106.
(17) Averbuj, C.; Tish, E.; Eisen, M. S. J . Am. Chem. Soc. 1998, 120,
8640.

5204 Organometallics, Vol. 22, No. 25, 2003
Ta ble 1. Cr ysta l Str u ctu r e a n d Refin em en t Da ta for 2, 3, 5, a n d 6
Coles and Hitchcock
2
3
5
6
formula
fw
C
14H24Cl6N6Ti2
C
14H24Cl2N6Ti‚(C7H8)
C19H39Al2Cl2N6Ti
524.32
C28H38N6Ti
506.54
584.89
487.33
temperature (K)
wavelength (Å)
cryst size (mm)
cryst syst
space group
a (Å)
173(2)
0.71073
173(2)
0.71073
173(2)
0.71073
173(3)
0.71073
0.40 × 0.20 × 0.05
monoclinic
P21/n (No. 14)
9.2379(6)
11.3740(8)
10.6759(8)
90.799(4)
1121.6(1)
2
0.20 × 0.20 × 0.05
monoclinic
P21/n (No. 14)
12.4604(6)
8.4351(6)
22.9507(15)
104.261(4)
2337.9(3)
4
0.20 × 0.20 × 0.05
monoclinic
C2/c (No. 15)
32.1612(12)
8.7247(6)
22.3495(11)
120.135(3)
5423.6(5)
8
0.20 × 0.10 × 0.10
tetragonal
P41 (No. 76)
9.3107(3)
9.3107(3)
30.4556(11)
90
2640.2(2)
4
1.27
b (Å)
c (Å)
â (deg)
V (Å3)
Z
D
calc (Mg/m3)
1.73
1.44
1.39
0.62
1.28
0.60
abs coeff (mm-1
)
0.35
θ range for data collection (deg)
no. of reflns collected
3.82 to 23.55
7000
3.73 to 22.97
10 686
3.74 to 25.04
12 618
4.00 to 22.98
6295
no. of indep reflns
1655 [Rint ) 0.086]
3230 [Rint ) 0.070]
4783 [Rint ) 0.060]
3234 [Rint ) 0.040]
no. of reflns with I > 2σ(I)
no. of data/restraints/params
goodness-of-fit on F2
1408
2507
3406
2970
1655/0/137
1.035
3230/0/399
1.006
4783/0/281
1.021
3234/1/316
0.669
final R indices [I > 2σ(I)]
R1 ) 0.036,
wR2 ) 0.077
R1 ) 0.047,
wR2 ) 0.082
0.44 and -0.38
R1 ) 0.041,
wR2 ) 0.083
R1 ) 0.062,
wR2 ) 0.091
0.22 and -0.28
R1 ) 0.046,
wR2 ) 0.093
R1 ) 0.0792,
wR2 ) 0.1058
0.32 and -0.33
R1 ) 0.039,
wR2 ) 0.107
R1 ) 0.044,
wR2 ) 0.112
0.20 and -0.18
R indices (all data)
largest diff peak and
hole (e Å-3
)
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for Ti(h p p )2Cl2 (3)
(d eg) for [Ti(h p p )Cl2(µ-Cl)]2 (2)a
Ti-N(1)
Ti-Cl(1)
Ti-Cl(3)
C(1)-N(1)
C(1)-N(3)
∆C′ N
2.003(3)
2.602(1)
2.266(1)
1.353(4)
1.318(4)
0.037
Ti-N(2)
1.963(3)
2.294(1)
2.423(1)
1.356(5)
0.003
Ti-N(1)
2.014(3)
2.028(3)
2.3455(9)
1.347(4)
1.340(4)
1.338(4)
0.007
Ti-N(2)
2.066(3)
2.043(2)
2.3414(10)
1.340(4)
1.344(4)
1.339(4)
0.006
Ti-Cl(2)
Ti-Cl(1′)
C(1)-N(2)
Ti-N(4)
Ti-N(5)
Ti-Cl(1)
C(1)-N(1)
C(1)-N(3)
C(8)-N(5)
Ti-Cl(2)
C(1)-N(2)
C(8)-N(4)
C(8)-N(6)
∆
CN
∆
CN(1)
∆C′ N(1)
∆
CN(2)
∆C′ N(2)
N(1)-Ti-N(2)
N(1)-Ti-Cl(3)
N(1)-Ti-Cl(1′)
Cl(3)-Ti-Cl(1′)
N(2)-Ti-Cl(1)
Cl(2)-Ti-Cl(1)
N(1)-C(1)-N(2)
N(2)-C(1)-N(3)
C(1)-N(3)-C(5)
66.99(12)
97.32(8)
94.60(8)
90.01(3)
86.89(8)
85.78(4)
107.8(3)
125.8(3)
119.7(3)
N(2)-Ti-Cl(3)
N(2)-Ti-Cl(2)
Cl(3)-Ti-Cl(2)
Cl(2)-Ti-Cl(1′)
N(1)-Ti-Cl(1)
Cl(1′)-Ti-Cl(1)
N(1)-C(1)-N(3)
C(1)-N(3)-C(4)
C(4)-N(3)-C(5)
104.61(9)
90.00(9)
94.78(4)
106.00(4)
86.42(8)
78.81(3)
126.3(3)
119.5(3)
120.8(3)
0.004
0.002
N(1)-Ti-N(2)
N(1)-Ti-N(5)
N(1)-Ti-Cl(1)
N(2)-Ti-Cl(1)
N(1)-Ti-Cl(2)
Cl(1)-Ti-Cl(2)
N(1)-C(1)-N(3)
C(1)-N(3)-C(4)
C(4)-N(3)-C(5)
N(4)-C(8)-N(6)
C(8)-N(6)-C(11)
65.04(11) N(4)-Ti-N(5)
94.68(11) N(2)-Ti-N(4)
65.06(10)
96.74(11)
89.36(7)
110.05(8)
95.93(8)
109.5(3)
125.7(3)
120.5(3)
109.4(3)
125.3(3)
109.53(8)
89.44(8)
89.13(8)
92.65(4)
124.6(3)
120.5(3)
118.7(3)
125.3(3)
120.3(3)
N(4)-Ti-Cl(1)
N(4)-Ti-Cl(2)
N(5)-Ti-Cl(2)
N(1)-C(1)-N(2)
N(2)-C(1)-N(3)
C(1)-N(3)-C(5)
N(4)-C(8)-N(5)
N(5)-C(8)-N(6)
a
Symmetry transformations used to generate equivalent at-
oms: ′ -x, -y+2, -z.
C(8)-N(6)-C(12) 120.5(3)
only three resonances for the methylene groups of
[hpp]-, consistent with an octahedral geometry with the
THF trans to chloride (i.e., a fac-arrangement of chloride
ligands), or the existence of a fluxional process whereby
the two halves of the guanidinate are rendered equiva-
lent.
C(11)-N(6)-C(12) 119.1(3)
performed (Figure 1); crystal data are summarized in
Table 1 and selected bond lengths and angles are
collected in Table 2.
Compound 2 crystallizes from toluene as the chlorine-
bridged dimer [Ti(hpp)Cl2(µ-Cl)]2, with a chelating
guanidinate anion and two terminal chlorides, analo-
gous to the benzamidinate complex.18 The geometry at
titanium is distorted octahedral, with bond angles in
the range 66.99(12)-106.00(4)°, where the smallest
value corresponds to the bite angle of the ligand. The
[hpp]- ligand is positioned such that N(1) is cis to the
chloro-bridges and N(2) is trans, and correspondingly
different Ti-N bond lengths of 2.003(3) and 1.963(3) Å,
respectively, are observed.
To generate the base-free metal complex, the silylated
ligand precursor hppSiMe3,13 prepared from the reaction
of (hppLi)n and SiMe3Cl, was added to a solution of TiCl4
in CH2Cl2 (Scheme 3). The product (2) was isolated as
a red solid, which was purified by crystallization from
1
a warm toluene solution. As for 1, the H and 13C NMR
spectra revealed only three resonances for the [hpp]-
methylene units, consistent with a monomeric species
in solution with an equatorially bound guanidinate, in
agreement with the highest molecular fragment de-
tected in the mass spectrum of 292 atomic mass units.
It has previously been shown however that the analo-
gous benzamidinate complex, [Ti(PhC{NSiMe3}2)Cl2(µ-
Cl)]2, consists of a chloride-bridged dimer in the solid
state.18 To determine which structure was adopted by
2 in the solid state, an X-ray structural analysis was
The distribution of electron density throughout the
“CN3” core of the guanidinate ligand is an important
parameter when discussing the contribution of the
(18) Fenske, D.; Hartmann, E.; Dehnicke, K. Z. Naturforsch. 1988,
43b, 1611.

Titanium Guanidinate Complexes
Organometallics, Vol. 22, No. 25, 2003 5205
Ta ble 4. Selected Bon d Len gth s (Å) a n d An gles
the N(1)-C(1)-N(2) moiety [∆CN ) 0 Å within the
experimental limits of the data], with a significantly
(d eg) for Ti{AlMe2(h p p )2}Cl(µ-Cl)AlMe3 (5)
shorter C-Namide distance [∆′ ) 0.037 Å], which in
Ti-N(1)
2.069(2)
2.055(3)
2.350(1)
2.382(1)
1.968(4)
1.957(3)
1.955(3)
1.326(4)
1.339(4)
1.389(4)
0.053
Ti-N(2)
2.227(2)
2.250(3)
2.460(1)
1.966(4)
1.978(3)
1.959(2)
1.958(4)
1.379(4)
1.328(4)
1.320(4)
0.061
CN
Ti-N(5)
Ti-N(4)
combination with a small R-value between the “NR2”
(amide) and “CN2” (amidine) ligand components [4.7°]
signifies a large contribution from resonance I to the
overall bonding. Therefore we predict an increased
NfM electron donation due to localization of charge
density at both Namidine atoms, realized in the observa-
tion that the titanium-nitrogen bond lengths are
significantly shorter in 2 [1.963(3) and 2.003(3) Å] than
in the corresponding benzamidinate complex [1.986(2)
and 2.072(2) Å]. Caution must be exercised when
comparing such values however, as there is a consider-
able difference in the steric demands of the two ligands,
with the nitrogen methylene substituents in [hpp]-
being effectively “tied-back”, due to incorporation in the
bicyclic framework.
Ti-Cl(1)
Ti-Cl(2)
Cl(2)-Al(2)
Al(2)-C(18)
N(2)-Al(1)
Al(1)-C(15)
C(1)-N(1)
C(1)-N(3)
C(8)-N(4)
Al(2)-C(17)
Al(2)-C(19)
N(4)-Al(1)
Al(1)-C(16)
C(1)-N(2)
C(8)-N(5)
C(8)-N(6)
∆
CN(1)
∆C′ N(1)
∆CN(2)
∆′CN(2)
0.014
0.039
N(1)-Ti-N(2)
N(2)-Ti-N(4)
Cl(1)-Ti-Cl(2)
N(1)-Ti-Cl(1)
N(2)-Ti-N(5)
N(4)-Ti-Cl(2)
N(5)-Ti-Cl(2)
62.34(9)
81.20(9)
95.23(4)
105.47(8)
90.51(10) N(2)-Ti-Cl(1)
94.47(6)
N(4)-Ti-N(5)
N(2)-Al(1)-N(4)
N(1)-Ti-N(4)
N(1)-Ti-Cl(2)
62.22(9)
96.14(11)
93.39(10)
92.13(7)
98.10(7)
96.26(7)
120.95(4)
N(5)-Ti-Cl(1)
Ti-Cl(2)-Al(2)
110.64(7)
C(15)-Al(1)-C(16) 118.53(18)
Changing the stoichiometry of (hppLi)n:TiCl4(THF)2
allowed access to the base-free, bis-guanidinate complex
Ti(hpp)2Cl2 (3, Scheme 2). 1H and 13C NMR spectro-
scopic analysis of 3 indicated a symmetric environment
for the [hpp]- anions, suggesting a structure with trans-
chlorides. However, the molecular structure (vide infra)
revealed cis-chlorides, indicating that a rapid fluxional
process must be occurring at room temperature, as
studied in detail for the acyclic guanidinate complex Ti-
(Me2NC{NiPr}2)2Cl2.22 The molecular structure of 3 is
illustrated in Figure 2, crystal data are summarized in
Table 1, and selected bond lengths and angles are
collected in Table 3.
Compound 3 exists as a monomer in the solid state,
with two chelating guanidinate ligands and two termi-
nal chlorides. The titanium is distorted octahedral
[angles in the range 65.04(11)-110.05(8)°], with the
main distortion again arising from the bite angle of the
[hpp]- groups. The titanium-nitrogen distances follow
trends similar to those observed in 2, where the Ti-N
bonds trans to a chloride ligand are slightly longer than
those that are cis. Scheme 4 compares the titanium-
nitrogen distances in 3 with those for the benzamidi-
nate23 and acyclic guanidinate analogues,22,24 illustrat-
ing a decreased value in 3 commensurate with increased
electron donation arising from the small values of R [3.8°
and 2.9°]. However, the same steric arguments pre-
sented above for 2 must also be considered in this case.
The ∆CN values in 3 again indicate symmetry across the
amidine moiety, although for this compound, a reduced
contribution from resonance I is implied as indicated
Ta ble 5. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for Ti(h p p )2(CH2P h )2 (6)
Ti-N(1)
Ti-N(4)
2.105(3)
2.064(3)
2.185(4)
1.344(5)
1.355(5)
1.329(5)
0.014
Ti-N(2)
Ti-N(5)
2.080(3)
2.095(3)
2.181(4)
1.330(5)
1.341(5)
1.349(5)
0.012
Ti-C(15)
C(1)-N(1)
C(1)-N(3)
C(8)-N(5)
Ti-C(22)
C(1)-N(2)
C(8)-N(4)
C(8)-N(6)
∆
CN(1)
∆C′ N(1)
∆
CN(2)
∆C′ N(2)
-0.018
-0.014
N(1)-Ti-N(2)
N(1)-Ti-N(5)
63.35(13) N(4)-Ti-N(5)
114.28(13) N(2)-Ti-N(4)
93.80(15) N(5)-Ti-C(15)
107.63(15) N(4)-Ti-C(22)
80.40(14) N(5)-Ti-C(22)
92.06(17) Ti-C(15)-C(16)
115.2(3)
124.2(3)
120.2(3)
119.6(3)
124.8(4)
119.1(3)
63.50(13)
107.67(13)
81.16(15)
95.32(15)
102.37(15)
114.0(3)
110.5(3)
125.3(4)
118.6(3)
110.1(3)
N(1)-Ti-C(15)
N(2)-Ti-C(15)
N(2)-Ti-C(22)
C(15)-Ti-C(22)
Ti-C(22)-C(23)
N(1)-C(1)-N(3)
C(1)-N(3)-C(4)
C(4)-N(3)-C(5)
N(4)-C(8)-N(6)
C(8)-N(6)-C(11)
N(1)-C(1)-N(2)
N(2)-C(1)-N(3)
C(1)-N(3)-C(5)
N(4)-C(8)-N(5)
N(5)-C(8)-N(6)
125.1(4)
C(8)-N(6)-C(12) 119.9(4)
C(11)-N(6)-C(12) 120.2(4)
different resonance forms to the overall bonding. Previ-
ously the ∆CN value,19 defined as the difference in bond
length between the C-N single and CdN double bonds,
has been used as a qualitative indication of the delo-
calization within the amidine unit in metal amidinate
complexes. This value is typically within the range 0 Å
(for fully delocalized systems) to approximately 0.14 Å
for isolated C-Nsingle and CdNdouble bonds to an sp2-
carbon center,20 although we have demonstrated that
a certain degree of flexibility exists depending on
crystallographic packing forces, as illustrated for a
series of linked-bis(N,N′-dialkylamidine) compounds.21
For comparative purposes, we also find it useful to
define a ∆C′ N value as the difference between the average
value of the C-Namidine bond lengths and the C-Namide
distance, where the greater the contribution from
by a significant decrease in the ∆′ values (0.004 and
CN
0.002 Å).
We can find no reports of conventional tris-guanidi-
nate Ti(IV) complexes in the literature, although the
closely related aminopyridinato species Ti{RN-2-C6H4N}3-
Cl (R ) Me, Ph) have been synthesized,25 demonstrating
the possibility of a seven-coordinate titanium center
using chelating ligands of this type. As [hpp]- may be
considered a relatively sterically undemanding guanidi-
resonance I (Scheme 1), the larger the value for ∆′CN
Examination of the C-N distances in the guanidinate
ligand of 2 indicates a symmetrical delocalization across
.
(22) Mullins, S. M.; Duncan, A. P.; Bergman, R. G.; Arnold, J . Inorg.
Chem. 2001, 40, 6952.
(23) Roesky, H. W.; Meller, B.; Noltemeyer, M.; Schmidt, H.-D.;
Scholz, U.; Sheldrick, G. M. Chem. Ber. 1988, 120, 8640.
(24) Bailey, P. J .; Grant, K. J .; Mitchell, L. A.; Pace, S.; Parkin, A.;
Parsons, S. J . Chem. Soc., Dalton Trans. 2000, 1887.
(25) Kempe, R.; Arndt, P. Inorg. Chem. 1996, 35, 2644.
(19) Ha¨felinger, G.; Kuske, K. H. The Chemistry of the Amindines
and Imidates; Wiley: Chichester, 1991; Vol. 2, Chapter 1.
(20) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J . Chem. Soc., Perkin Trans. 2 1987, S1.
(21) Grundy, J .; Coles, M. P.; Hitchcock, P. B. J . Organomet. Chem.
2002, 662, 178.

5206 Organometallics, Vol. 22, No. 25, 2003
Ta ble 6. Cr ysta l Str u ctu r e a n d Refin em en t Da ta for 7a , 7b, a n d 8
Coles and Hitchcock
7a
7b
8
formula
fw
C
22H42Cl2N8Ti2‚C7H8
C
22H42Cl2N8Ti2
C
36H66N14Ti2
677.47
585.34
790.83
temperature (K)
wavelength (Å)
cryst size (mm)
cryst syst
space group
a (Å)
173(2)
0.71073
173(2)
0.71073
173(2)
0.71073
0.30 × 0.15 × 0.05
monoclinic
P21/c (No. 14)
10.8220(5)
10.4833(4)
15.3615(8)
102.882(2)
1698.9(1)
2
0.30 × 0.30 × 0.10
monoclinic
C2/c (No. 15)
17.8443(11)
9.1562(4)
19.2712(10)
115.112(2)
2851.0(3)
4
0.30 × 0.30 × 0.15
orthorhombic
Pbca (No. 61)
10.7160(3)
15.3055(4)
24.9766(5)
90
4096.5(2)
4
1.28
b (Å)
c (Å)
â (deg)
V (Å3)
Z
D
calc (Mg/m3)
1.32
0.66
1.36
0.77
abs coeff (mm-1
)
0.43
θ range for data collection (deg)
no. of reflns collected
3.86 to 22.97
12 813
3.81 to 25.03
9685
3.78 to 27.87
21 448
no. of indep reflns
2353 [Rint ) 0.121]
2513 [Rint ) 0.062]
4866 [Rint ) 0.066]
no. of reflns with I > 2σ(I)
no. of data/restraints/params
goodness-of-fit on F2
1770
2097
3509
2353/0/190
1.062
2513/0/164
1.019
4866/0/235
1.008
final R indices [I > 2σ(I)]
R1 ) 0.058,
wR2 ) 0.106
R1 ) 0.089,
wR2 ) 0.116
0.30 and -0.40
R1 ) 0.033,
wR2 ) 0.080
R1 ) 0.044,
wR2 ) 0.085
0.30 and -0.30
R1 ) 0.043,
wR2 ) 0.099
R1 ) 0.070,
wR2 ) 0.111
0.28 and -0.30
R indices (all data)
largest diff peak and hole (e Å-3
)
F igu r e 2. Molecular structure of Ti(hpp)2Cl2 (3) with
thermal ellipsoids drawn at the 30% probability level.
Hydrogen atoms omitted for clarity.
F igu r e 1. Molecular structure of [Ti(hpp)Cl2(µ-Cl)]2 (2)
with thermal ellipsoids drawn at the 30% probability level.
Hydrogen atoms omitted for clarity.
activity ) 7.4 (1) and 7.1 (2) gPE mmol-1 h-1 bar-1).
The low activity may be due in part to the titanium
procatalyst retaining THF (1) or adopting a dimeric
structure (2), thus hindering activation by MAO and/or
approach of an incoming monomer to the active catalyst
species. It is generally accepted that the active species
in group 4 metal olefin polymerization catalysis consists
of a cationic metal alkyl complex, with a “vacant”
coordination site cis to the M-C bond, supported by
ancillary ligands. As such, we considered that upon
activation by MAO, complex 3 would give rise to the
best catalytic system, based on the crystal structure of
the cis-dichloride precursor. Surprisingly however, even
lower activity (2.2 g mmol-1 h-1 bar-1) was observed
for 3 compared with 1 and 2 under identical reaction
conditions, suggesting that a facile deactivation path-
way is present in these systems.
nate ligand due to the Namidine methylene substituents
being incorporated into the six-membered ring, we
reasoned that it may be possible to coordinate three
such ligands at titanium. Accordingly, the reaction
between 3 equiv of (hppLi)n and TiCl4(THF)2 proceeded
to afford the tris-ligand species Ti(hpp)3Cl (4) as the
base-free complex. Unfortunately, crystals suitable for
an X-ray analysis were not forthcoming, precluding a
study of the structure of 4, although the formulation as
the tris-ligand complex was confirmed by elemental
analysis and the observation of the molecular ion (m/z
497) in the mass spectrum.
Preliminary ethylene polymerization studies using
compounds 1-3 as precatalyst and MAO as activator
were performed to gauge the catalytic potential of
titanium complexes supported by [hpp]- anions. Solu-
tions of 1 and 2 (0.02 mmol) in toluene (200 mL) in the
presence of excess MAO (Al/Ti ) 1000) show low26
polymerization activities at room temperature (9 bar,
To investigate possible explanations for the observed
low activity and to gain insight into the stability of the
(26) Britovsek, G. J . P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. 1999, 38, 428.

Titanium Guanidinate Complexes
Organometallics, Vol. 22, No. 25, 2003 5207
Sch em e 3
Sch em e 4. Com p a r ison of Ti-N Bon d Dista n ces
(Å) in Ti(a m id in a te)2Cl2 a n d Ti(gu a n id in a te)2Cl2
Com p lexes
F igu r e 3. Molecular structure of Ti{AlMe2(hpp)2}Cl(µ-Cl)-
AlMe3 (5) with thermal ellipsoids drawn at the 30%
probability level. Hydrogen atoms omitted for clarity.
F igu r e 4. Molecular structure of Ti(hpp)2(CH2Ph)2 (6)
with thermal ellipsoids drawn at the 30% probability level.
Hydrogen atoms except on the methylene unit of the benzyl
group omitted for clarity.
Sch em e 5
Table 1, and selected bond lengths and angles are
collected in Table 4.
Compound 5 consists of a novel trimetallic complex
containing one titanium and two aluminum centers,
linked through hpp and chloride bridges. The titanium-
(III) center exists in a distorted octahedral geometry,
bound by two chelating hpp groups and two chloride
atoms, one of which bridges to an AlMe3 unit. In
addition to coordinating to the titanium, the guanidinate
anions are linked through one of the nitrogen atoms by
an AlMe2 moiety, resulting in a unit that may be
considered as the η4-monoanionic moiety, [Me2Al(hpp)2]-.
Inspection of the bond lengths and angles throughout
this unit indicates that a redistribution of the π-electron
density has occurred. The N(1) and N(5) atoms retain
F igu r e 5. Molecular structure of [Ti(µ-NtBu)(hpp)Cl]2 (7b)
with thermal ellipsoids drawn at the 30% probability level.
Hydrogen atoms omitted for clarity.
[hpp]- ligands with respect to the alkyl aluminum
component of MAO, the preparative scale reaction
between 3 and AlMe3 was explored (Scheme 5). Addition
of a solution of AlMe3 (2 equiv) to 3 gave an immediate
color change from red-orange to green-red. Upon work-
up, green crystals (5) were isolated. The 1H NMR
spectrum indicated the presence of paramagnetic spe-
cies, confirmed by the presence of an ESR signal at giso
) 1.9617. The elemental analysis was consistent with
the formula 3‚Al2Me5. The molecular structure of com-
plex 5 was determined by a further X-ray diffraction
analysis (Figure 3); crystal data are summarized in
an approximately planar sp2-hybridization [∑angle
)
353.8° and 356.8°, respectively], with distances to
titanium slightly longer than in 1 and 3 [2.069(3) and
2.055(3) Å, respectively]. The N(2) and N(4) atoms have

5208 Organometallics, Vol. 22, No. 25, 2003
Coles and Hitchcock
however undergone rehybridization, with corresponding
increased Ti-N bond distances of 2.227(3) and 2.250-
(3) Å, respectively. In addition, the π-system is no longer
equally distributed throughout the CN3 core of the
ligand, with C(1)-N(2) and C(8)-N(4) bond lengths
[1.379(4) and 1.389(4) Å, respectively] longer than the
remaining C-N distances [1.320-1.339 Å], reflected in
the relatively large ∆CN values [0.053 and 0.061 Å] and
alkyl groups present as η1-benzyl groups. The titanium-
nitrogen distances [range 2.064(3)-2.105(3) Å] are
generally longer than in the chloride complexes 2 and
3, reflecting the increase in electron density at titanium
arising from the presence of the alkyl groups compared
with chlorides. A slight localization of charge exists
within the amidine moiety [∆CN ) 0.014 and 0.012 Å],
and negative values for ∆′CN [-0.018 and -0.014 Å]
indicate a reduced contribution from I to the more
electron-rich titanium center, with R values of 5.1° and
3.4°.
To ascertain the potential for 6 as a catalyst precursor
in olefin polymerization, the NMR scale reaction with
B(C6F5)3 was performed in an attempt to characterize
the cationic species formed during such a reaction.
However, an instant color change from red to green took
place on mixing a C6D6 solution of 6 with a stoichio-
metric amount of the borane activator, and an oily
deposit formed on the walls of the NMR tube over a 1
min period. The 1H NMR spectrum indicated that
paramagnetic species had formed, again suggesting that
the ligands are noninnocent with respect to cation
formation. The precise nature of the product was not
determined, and research into this area was not studied
further.
inconsistent ∆′ values [0.014 and 0.039 Å]. The Ti-
CN
(µ-Cl) distance [2.460(1) Å] is slightly longer than the
terminal titanium chloride distance [2.350(1) Å] typical
for a bridge between titanium and aluminum.27
With reference to the lower olefin polymerization
activity observed for 3, complex 5 serves to illustrate
that the guanidinate ligands are not merely acting in a
supporting role as an ancillary ligand set, but are able
to interact with trimethylaluminum, a known compo-
nent of MAO. While we are not able to definitively say
that this exact process is occurring during the polym-
erization reaction, isolation of 5 does serve to illustrate
the noninnocent nature of the [hpp]- groups. The
bridging nature of the guanidinates between the Ti and
Al centers may also be representative of an intermediate
in a ligand abstraction process, analogous to that
proposed to occur during the polymerization of propy-
lene by a zirconium benzamidinate/MAO system.17 This
hypothesis is further substantiated by mass spectral
analysis of 5, which shows peaks at m/z 303 and 180,
corresponding to [Al(hpp)2]+ and [Al(hpp)Me]+, respec-
tively, where complete transfer of [hpp]- from titanium
to aluminum has occurred.
Tita n iu m Im id o Com p lexes. In view of the appar-
ent facile reduction of the metal center during the
formation of cationic species for attempted olefin po-
lymerization catalysis with both MAO and borane
activators, and the resultant low activities of the
complexes in the former system, we decided to investi-
gate the synthesis of other potentially catalytically
active titanium-based guanidinate species. Titanium-
imido compounds are postulated as the intermediates
in a number of organic transformations including hy-
droamination,29 and a number of supporting ligands
have recently been investigated, including the bis-
(cyclopentadienyl) framework30 and pyrrolyl-supported
systems.31 Of particular relevance to our work, a recent
report by Richeson and co-workers described an inves-
tigation of guanidinate-supported titanium-imido com-
plexes as catalysts for the hydroamination of alkynes,28
guanylation of amines, and transamination of guani-
dines.32 We therefore initiated a study of the coordinat-
ing potential of the [hpp]- anion at titanium-imido
centers, using Mountford’s readily available tert-butyl-
Due to our limited success with the titanium chloride/
MAO system, we decided to focus on the generation of
well-defined alkyl cations, which may be accessed from
the alkyl abstraction reaction of complexes of general
type Ti(hpp)2R2 (R ) methyl, benzyl) with B(C6F5)3. To
synthesize a suitable dialkyl precursor, 2 equiv of hppH
was reacted with Ti(CH2Ph)4 at low temperature in
toluene to afford the bis-benzyl complex Ti(hpp)2(CH2-
Ph)2 (6, Scheme 2). Despite purification by crystalliza-
tion from Et2O, repeated attempts at obtaining accurate
elemental analysis were unsuccessful, with values
consistently low for the carbon content, suggesting
possible formation of nonvolatile titanium carbide dur-
1
ing combustion. The H and 13C NMR spectra of 6 show
one environment for each of the hpp-methylene units,
indicating a fluxional system in solution and rapid
racemization of the C2-symmetric structure (vide supra),
with the benzyl methylene group appearing as a singlet
at 2.85 ppm. This observation contrasts the recently
reported acyclic analogue Ti(Me2NC{NiPr}2)2(CH2Ph)2,
where a stereochemically rigid structure is maintained
in solution, resulting in an AB splitting pattern for the
methylene groups of the benzyl ligand.28 The molecular
structure of 6 is illustrated in Figure 4, crystal data are
summarized in Table 1, and selected bond lengths and
angles are collected in Table 5.
12
imido complex Ti(NtBu)Cl2(py)3 as a convenient start-
ing material.
The salt metathesis reaction between (hppLi)n and
Ti(NtBu)Cl2(py)3 proceeds to afford a dark red solu-
tion, from which base-free [Ti(NtBu)(hpp)Cl] (7) can
1
be isolated (Scheme 6). In contrast to the H and 13C
NMR data for the acetamidinate complexes [Ti(µ-NtBu)-
(MeC{NCy}2)X]2 (X ) Cl33 or Me34), where the two
(29) Bytschkov, I.; Doye, S. Eur. J . Org. Chem. 2003, 935.
(30) Haak, E.; Bytschkov, I.; Doye, S. Angew. Chem., Int. Ed. 1999,
38, 3389. J ohnson, J . S.; Bergman, R. G. J . Am. Chem. Soc. 2001, 123,
2923.
(31) Cao, C.; Ciszewski, J . T.; Odom, A. L. Organometallics 2001,
20, 5011. Shi, Y.; Hall, C.; Ciszewski, J . T.; Cao, C.; Odom, A. L. Chem.
Commun. 2003, 586.
Compound 6 exists as the monomeric bis-ligand,
dialkyl in the solid state, with the overall geometry at
titanium best described as a distorted octahedron
[ligand bite angles ) 63.35(13)° and 63.50(13)°] and both
(32) Ong, T.-G.; Yap, G. P. A.; Richeson, D. S. J . Am. Chem. Soc.
2003, 125, 8100.
(27) Kelly, D. G.; Toner, A. J .; Walker, N. M.; Coles, S. J .; Hurst-
house, M. B. Polyhedron 1996, 15, 4307.
(28) Ong, T.-G.; Yap, G. P. A.; Richeson, D. S. Organometallics 2002,
21, 2839.
(33) Stewart, P. J .; Blake, A. J .; Mountford, P. Inorg. Chem. 1997,
36, 3616.
(34) Stewart, P. J .; Blake, A. J .; Mountford, P. J . Organomet. Chem.
1998, 564, 209.

Titanium Guanidinate Complexes
Organometallics, Vol. 22, No. 25, 2003 5209
Ta ble 7. Selected Bon d Len gth s (Å) a n d An gles
Sch em e 6
(d eg) for [Ti(µ-NtBu )(h p p )Cl]2 (7b)a
Ti(1)-N(4)
Ti(1)-N(1)
Ti(1)-Cl
1.845(2)
2.078(2)
2.3301(7)
1.335(3)
0.008
Ti(1)-N(4′)
Ti(1)-N(2)
C(1)-N(1)
C(1)-N(3)
1.967(2)
2.052(2)
1.343(3)
1.344(3)
-0.005
C(1)-N(2)
∆
∆′
CN
CN
N(4)-Ti(1)-N(4′)
N(2)-Ti(1)-N(4′)
N(1)-Ti(1)-N(2)
N(4)-Ti(1)-Cl
85.25(8)
93.46(7)
64.32(7)
112.44(6)
136.39(6)
110.31(18) N(1)-C(1)-N(3)
125.1(2)
119.09(19) C(4)-N(3)-C(5)
N(2)-Ti(1)-N(4) 109.44(7)
N(1)-Ti(1)-N(4) 107.65(7)
N(1)-Ti(1)-N(4′) 156.80(7)
N(4′)-Ti(1)-Cl
N(1)-Ti(1)-Cl
101.44(5)
91.51(6)
124.6(2)
120.0(2)
120.88(19)
N(2)-Ti(1)-Cl
N(1)-C(1)-N(2)
N(2)-C(1)-N(3)
C(1)-N(3)-C(5)
C(1)-N(3)-C(4)
a
Symmetry transformations used to generate equivalent at-
oms: ′ -x, -y, -z.
In contrast to the benzamidinate species Ti(NtBu)-
(PhC{NSiMe3}2)Cl(py)2, compound 7 exists as the dimer-
ic, base-free species [Ti(µ-NtBu)(hpp)Cl]2, with each
titanium atom bound by terminal [hpp]- and chloride
ligands and bridged by imido groups. The geometry at
the metal is distorted toward tetragonal (τ ) 0.34),38
where N(4) defines the apical position and the remain-
ing atoms form the base of the pyramid, with angles in
the range 64.32(7)-101.44°. Similar arrangements of
ligands are found in the acetamidinate complexes [Ti-
(µ-tBu)(MeC{NCy}2)Cl]233 and the methyl analogue [Ti-
(µ-tBu)(MeC{NCy}2)Me]2,34 although more pronounced
distortion toward C4v-symmetric structures are evident
in these cases (τ ) 0.15 and 0.21, respectively). The Ti-
cyclohexyl substituents are inequivalent in solution, the
methylene units of the [hpp]- ligand appear as three
signals in the NMR spectra, again suggesting a fluxional
system in solution. Investigation of the difference in 13C
N
imido bond lengths exhibit significant asymmetry, [Ti-
t
NMR shifts of the carbon atoms of the Bu group (∆δ
(1)-N(4′) ) 1.967(2) Å; Ti(1)-N(4) ) 1.845(2) Å], which
may be rationalized using purely geometrical argu-
ments, given that N(4) is present in an apical site, while
N(4′) may be considered as basal. An alternative expla-
nation based on simple Hu¨ckel calculations that has
been used to describe a similar difference within [M2-
(µ-imido)] bond lengths for the group 6 metal complexes
[M(µ-NtBu)(NtBu)(NMe2)2]2 (M ) Mo, W) attributes the
cause to second-order J ahn-Teller distortions.39 The
value) has been used as a measure of the electron
t
density at the nitrogen atom in Bu-imido compounds,35
and we have recently applied this technique to titanium-
imido complexes incorporating boron-substituted alkox-
ide ligands.36 If the [hpp]- ion is truly acting as an
electron-rich guanidinate through enhanced electron
density due to contribution from I, we predict lower
values for ∆δ compared with conventional acyclic guanid-
inate complexes. The ∆δ value for 7 (CDCl3) of 41.3 ppm
is slightly lower than the value for the analogous
∆
CN
and ∆′ values of 0.008 Å and -0.005 Å, respec-
CN
tively, indicate approximately equal delocalization
throughout the CN3 core of the guanidinate ligand, with
an R-value of 3.0°. Once again the Ti-Nguanidinate dis-
tances [Ti(1)-N(1) ) 2.078(2) Å; Ti(1)-N(2) ) 2.052(2)
Å] are shorter than in the corresponding amidinate
complex [2.112(3) and 2.076(3) Å], commensurate with
contribution from I to the overall bonding situation.
Previous reports have been presented on the synthesis
and isolation of arylimido titanium complexes supported
by amidinate and guanidinate ligands. For example,
Mountford was able to access the benzamidinate com-
plex Ti(NAr)(PhC{NSiMe3}2)Cl(py)2 (Ar ) 2,6-Me2C6H3;
2,6-iPr2C6H3) either by a metathetical route using the
lithium salt of the benzamidine and the (arylimido)-
titanium dichloride starting material or by an imido
exchange reaction of the tert-butylimido derivative Ti-
(NtBu)(PhC{NSiMe3}2)Cl(py)2 with the free aniline.33
Attempted reactions between (hppLi)n and Ti(NAr)Cl2-
complex [Ti(µ-tBu)(MeC{NCy}2)Cl]2 in the same sol-
33
vent (∆δ ) 42.7), in agreement with the above argu-
ment. However, this argument appears counterintuitive
when explaining the fluxionality observed in complex
7 but not in the acetamidinate species and may be owing
to the preferred parallel orientation of the donor orbitals
of [hpp]- (vide supra) facilitating a low-energy barrier
to rotation via an N-bound η1-intermediate, which
equilibrates the methylene units of the [hpp]- ligand.
To try to gain a better understanding of the bonding
within 7, determination of the molecular structure by
X-ray diffraction was performed (Figure 5). The complex
crystallizes in the monoclinic crystal system as the
toluene solvate (7a , space group P21/c) or as the solvate-
free complex (7b, space group C2/c), with similar bond
lengths and angles. Crystal data for each structure are
summarized in Table 6, but for the purpose of comparing
bond lengths and angles, only the latter structure 7b is
described (Table 7).37
(37) Full crystal data for 7a can be found in the Supporting
Information.
(35) Nugent, W. A.; McKinney, R. J .; Kasowski, R. V.; van-Catledge,
F. A. Inorg. Chim. Acta 1982, 65, L91.
(36) Cole, S. C.; Coles, M. P.; Hitchcock, P. B. J . Chem. Soc., Dalton
Trans. 2002, 4168.
(38) Addison, A. W.; Rao, T. N.; Reedijk, J .; van Rijn, J .; Verschoor,
G. C. J . Chem. Soc., Dalton Trans. 1984, 1349.
(39) Thorn, D. L.; Nugent, W. A.; Harlow, R. L. J . Am. Chem. Soc.
1981, 103, 357.

5210 Organometallics, Vol. 22, No. 25, 2003
Coles and Hitchcock
Ta ble 8. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for [Ti(NtBu )(h p p )( µ-h p p )]2 (8)a
Ti-N(1)
Ti-N(4)
Ti-N(7′)
C(5)-N(4)
C(12)-N(5)
C(12)-N(6)
∆C′ N(1)
1.7143(16)
2.1573(17)
2.1506(16)
1.331(2)
1.325(2)
1.355(2)
Ti-N(2)
2.1475(16)
2.0715(16)
1.331(3)
1.376(2)
1.361(2)
0
Ti-N(5)
C(5)-N(2)
C(5)-N(3)
C(12)-N(7)
∆
∆
CN(1)
CN(2)
-0.045
-0.012
0.036
∆C′ N(2)
N(1)-Ti-N(2)
N(1)-Ti-N(5)
N(2)-Ti-N(4)
N(2)-Ti-N(5)
N(4)-Ti-N(7′)
N(2)-C(5)-N(4)
N(4)-C(5)-N(3)
C(5)-N(3)-C(9)
103.52(7)
99.75(7)
61.94(6)
151.09(7)
150.08(6)
112.65(16) N(2)-C(5)-N(3)
123.55(18) C(5)-N(3)-C(8)
118.77(17) C(8)-N(3)-C(9)
N(1)-Ti-N(4)
N(1)-Ti-N(7′)
N(2)-Ti-N(7′)
N(4)-Ti-N(5)
N(5)-Ti-N(7′)
100.85(7)
101.50(7)
93.52(6)
97.08(6)
98.46(6)
123.81(18)
119.44(17)
116.67(16)
N(5)-C(12)-N(7) 115.05(16) N(5)-C(12)-N(6) 123.28(17)
N(7)-C(12)-N(6) 121.42(17) C(12)-N(6)-C(15) 121.80(17)
C(12)-N(6)-C(16) 123.18(18) C(15)-N(6)-C(16) 114.66(17)
F igu r e 6. Molecular structure of [Ti(NtBu)(hpp)(µ-hpp)]2
(8) with thermal ellipsoids drawn at the 30% probability
level. Hydrogen atoms omitted for clarity.
a
Symmetry transformations used to generate equivalent at-
oms: ′ -x, -y, -z.
(py)3 under a variety of experimental conditions however
afforded no clean product; similar observations were
made employing the lithiated N,N′-bis(cyclohexyl)ac-
etamidinate ligand.33 In addition, attempted NMR scale
reactions between 7 and 2,6-iPr2C6H3NH2 showed no
exchange after 1 week at room temperature, and gentle
heating of the reaction led to decomposition. A plausible
explanation for the lack of reactivity in this case may
be due to the inability of the aniline to initially
coordinate at the metal center prior to “H” transfer,40
due to retention of the dimeric structure in solution. In
contrast, the Ti(NtBu)(PhC{NSiMe3}2)Cl(py)2 is able to
readily lose a labile pyridine group, allowing coordina-
tion of the aniline to occur and the reaction to proceed.
It has been demonstrated that two amidinate or
guanidinate ligands may be incorporated around the
metal center in titanium(imido) complexes, affording
monomeric species with terminal imido groups which
are more closely related to the proposed active species
in hydroamination catalysis. For example, the bis-
(benzamidinate) complex has been reported as both the
of titanium-imido species, compound 8 exists as the
dimeric complex [Ti(NtBu)(hpp)(µ-hpp)]2, in which one
of the [hpp]- groups chelates to titanium in the pre-
dicted fashion and the other bridges two metal centers,
with the coordination sphere completed by a terminal
imido ligand (Figure 6). Considering the imido group
as the apex, the geometry at titanium approaches
regular tetragonal with a τ value of 0.02, the main
distortion again arising from the bite angle of the
chelating guanidinate [61.94(6)°]. The core of the ligand
consists of an eight-membered metallacycle present in
a pronounced chair-type conformation, with a similar
“out-of-the CN3-plane” bridging coordination of the
[µ-hpp]- group to that previously observed in the
8
dimeric tin and aluminum complexes [Sn(µ-hpp)Cl]2
and [Al(µ-hpp)Me2]2.4
The bonding within the chelating guanidinate ligand
is consistent with equal delocalization across the ami-
dine unit (∆CN ) 0 Å), with only a small contribution
from the zwitterionic resonance structure (∆C′ N ) -0.045
Å) and a value of 8.0° for R. The bonding within the
amidine unit is much more localized in the bridging
guanidinate ligand [C(12)-N(5) ) 1.325(2) Å; C(12)-
N(7) )1.361(2) Å], with the corresponding Ti-N(5) and
Ti-N(7′) distances of 2.0715(16) and 2.1506(16) Å,
indicating a nonsymmetrical bonding situation for the
[µ-hpp]- group. The Ti-Nimido bond length of 1.7143-
(16) Å is typical for a terminal tert-butylimido group,
with the distance lying slightly toward the long end of
the range normally quoted for such species (ca. 1.67-
1.73 Å),43 which may reflect a slight increase in electron
density at the titanium. The angle at the imido nitrogen
atom [169.67(15)°] is unremarkable.
In summary, we have synthesized a range of titanium
complexes subtended by an anionic guanidinate ligand,
derived from the bicyclic guanidine precursor, hppH. In
solution the NMR analysis suggests a more fluxional
system than reported for acyclic guanidinate complexes
of titanium, provisionally attributed to the preferred
parallel orientation of the donor orbitals facilitating a
41
base-free complex Ti(NtBu)(PhC{NSiMe3}2)2 and the
pyridine adduct,42 while recent reports of bis(guanidi-
nate) species include the monomeric arylimido com-
pounds Ti(NAr)(Me2NC{NiPr}2)2 (Ar ) Ph22 or 2,6-
Me2C6H328). We attempted therefore to generate the
bis(hpp) complex by reacting 2 equiv of (hppLi)n with
Ti(NtBu)Cl2(py)3 in THF. Unfortunately, even under
relatively forcing conditions (60 °C) we were unable to
cleanly make the bis(guanidinate) complex, with the
reaction always generating an intimate mixture of
compounds (heating to higher temperature resulted in
decomposition). However, a single crystal of the desired
complex, [Ti(NtBu)(hpp)(µ-hpp)]2 (8), was mechanically
separated from the mixture and the molecular structure
solved using X-ray diffraction. Crystal data are sum-
marized in Table 6, and selected bond lengths and
angles collected in Table 8. In contrast to the previously
reported bis(amidinate) and bis(guanidinate) complexes
(40) Coles, M. P.; Dalby, C. I.; Gibson, V. C.; Clegg, W.; Elsegood,
M. R. J . Polyhedron 1995, 14, 2455.
(41) Hagadorn, J .; Arnold, J . Organometallics 1998, 17, 1355.
(42) Stewart, P. J .; Blake, A. J .; Mountford, P. Organometallics 1998,
17, 3271.
(43) Allen, F. H.; Kennard, O. Chem. Des. Autom. News 1993, 8,
31. Fletcher, D. A.; McMeeking, R. F.; Parkin, D. J . Chem. Inf. Comput.
Sci. 1996, 36, 746.

Titanium Guanidinate Complexes
Organometallics, Vol. 22, No. 25, 2003 5211
low-energy barrier to rotation. Investigation of the
potential of several metal chloride species to serve as
catalyst precursors for the polymerization of ethylene
upon activation by MAO was performed with the
observed low activities most probably arising from the
facile reduction of the metal center and noninnocent
nature of the supporting ligands. A similar reaction
pathway was suggested from the reaction of the bis-
(benzyl) complex with the well-defined borane activator
B(C6F5)3. Extension to titanium-imido complexes re-
sulted in formation of the mono-guanidinate complex
containing a bridging imido unit in the solid state and
the bis-guanidinate complex with a terminal imido unit
and bridging [µ-hpp]- anion.
Ack n ow led gm en t. We wish to thank the University
of Sussex for financial support. We also wish to ac-
knowledge the use of the EPSRC’s Chemical Database
Service at Daresbury, and Charlotte Hendy for the
crystallization of compound 7a .
Su p p or tin g In for m a tion Ava ila ble: Crystal structure
data for 2 and 3 and 5-8, including tables of atomic positional
parameters, anisotropic thermal parameters, all bond dis-
tances and angles, and hydrogen atom coordinates. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM0341092