Guanidinates as chelating anionic ligands for early, middle and late transition metals: Syntheses and crystal structures of [Ti{i]2(NPh)2CNEt2}2Cl2, [Ru{ii2-(NPh)2CNHPh}3] and [Pt{ri2-(NPh)2CNHPh}2]

Article abstract of DOI:10.1039/b001992p

Treatment of the dimeric ruthenium halide complex [Ru(r|-C6H6)Q2]2 with 1,2,3-triphenylguanidine provided the tris-chelate [Ru{r|2-(NPh)2CNHPh}3] with loss of the aromatic ligand and metal oxidation. This product may also be prepared by treatment of the p

Full text of DOI:10.1039/b001992p

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Guanidinates as chelating anionic ligands for early, middle and
late transition metals: syntheses and crystal structures of [Ti{ꢀ2-
(NPh)2CNEt2}2Cl2], [Ru{ꢀ2-(NPh)2CNHPh}3] and [Pt{ꢀ2-(NPh)2-
CNHPh}2]
Philip J. Bailey,* Keith J. Grant, Lindsey A. Mitchell, Stuart Pace, Andrew Parkin and
Simon Parsons
Department of Chemistry, The University of Edinburgh, The King’s Buildings,
West Mains Road, Edinburgh, UK EH9 3JJ. E-mail: Philip.Bailey@ed.ac.uk
Received 13th March 2000, Accepted 26th April 2000
Published on the Web 22nd May 2000
Treatment of the dimeric ruthenium halide complex [Ru(η-C6H6)Cl2]2 with 1,2,3-triphenylguanidine provided the
tris-chelate [Ru{η2-(NPh)2CNHPh}3] with loss of the aromatic ligand and metal oxidation. This product may also
be prepared by treatment of the previously reported [Ru(η-p-PriC6H4Me){η2-(NPh)2CNHPh}Cl] with bases (KOH,
LiNiPr2, NEt3). Complexes containing chelating guanidinate ligands are also available by means of metathesis of
the monolithiated ligands Li[C(NR)2NR2] with metal halide complexes, as exemplified by the synthesis of the
square planar bis-chelate [Pt{η2-(NPh)2CNHPh}2] from [Pt(PhCN)2Cl2] and [Ti{η2-(NPh)2CN(Et)2}2Cl2] from
[TiCl4(THF)2]. The characterisation of these complexes allows a comparison of the co-ordination properties
of chelating guanidinates with early, middle and late transition metals.
Since the first report in 1996 of the ability of the anions
of 1,2,3-trisubstituted guanidines [(NR)2CNHR]Ϫ to act as
chelating guanidinate ligands,1 a large number of such com-
plexes of transition, main group and lanthanide metals have
appeared in the literature.2–14 Guanidinate(1Ϫ) ligands are
related to amidinates by substitution of the central carbon
Fig. 1 Alternative 1,3-diazaallyl (A) and iminium/diamide (B) reson-
with an amino group and, as such, have available to them an
iminium/diamide type resonance structure in which the lone
pair of the unco-ordinated nitrogen atom is delocalised into
the ligand π system (Fig. 1), a possibility clearly unavailable to
the amidinates.1 We have previously reported ruthenium and
rhodium complexes containing chelating guanidinate ligands,1
and the redox pair [Mo2{µ-η2-(NPh)2CNHPh}4]0/ϩ which gave
the first indications of the flexible donor properties of these
ligands,15 and in addition we have characterised a number of
examples of complexes containing guanidines acting as neutral
monodentate ligands16 and main group complexes containing
both guanidinate-(1Ϫ) and -(2Ϫ) ligands.17 Here we report the
synthesis and characterisation of three complexes of early,
middle and late transition metals [TiIV, RuIII and PtII] contain-
ing chelating guanidinate(1Ϫ) ligands and discuss the evidence
available to support the involvement of the unco-ordinated
nitrogen lone pair in the ligand π system.
ance structures for a guanidinate(1Ϫ) anion.
nitrobenzyl alcohol as matrix and CsI as calibrant. Elemental
analyses were conducted by the microanalytical service of this
department.
Syntheses
[Ru{ꢀ
2-(NPh) CNHPh} ] 1. Method a. To a suspension of
2
3
[Ru(η-C6H6)Cl2]2 (500 mg, 1.0 mmol) in toluene (30 cm3) was
added 1,2,3-triphenylguanidine (3.44 g, 12.0 mmol). Over a
period of 1 week the stirred solution became dark green and the
brown [Ru(η-C6H6)Cl2]2 dissolved to be replaced by colourless
1,2,3-triphenylguanidinium chloride. The salt was removed by
filtration and the filtrate reduced to dryness under vacuum. The
residue was redissolved in pentane (50 cm3) and filtered through
Celite. Reduction of the solution volume to 10 cm3 and storage
at Ϫ20 ЊC for 2 weeks provided black crystals of complex 1
(0.45 g, 0.47 mmol, 47%).
Method b. To a solution of [Ru{η2-(NPh)2CNHPh}(η-p-
PriC6H4Me)Cl] (250 mg, 0.449 mmol) in THF (20 cm3) cooled
to Ϫ78 ЊC was added 4.40 cm3 of a 0.105 M solution (0.462
mmol) of lithium diisopropylamide in THF. The mixture was
allowed to warm slowly to room temperature with stirring and
after 1 h the green solution was reduced to dryness. The residue
was extracted with pentane (3 × 10 cm3) and the extracts
reduced in volume to 10 cm3. Cooling to Ϫ20 ЊC for 3 weeks
provided black crystals of complex 1 in 16% yield. The isolation
of pure 1 was hampered by its very high solubility in all organic
solvents and a consistent set of microanalytical data could not
be obtained. We attribute this problem to the presence of
residual free 1,2,3-triphenylguanidine which was evident in the
NMR spectrum of crystalline samples of 1 and which could not
Experimental
General
All reactions were carried out under an atmosphere of dry,
oxygen free nitrogen using standard Schlenk techniques and
solvents which were dried and distilled under nitrogen immed-
iately prior to use. The complexes [Pt(PhCN)2Cl2],18 [Ru(η-
C6H6)Cl2]2 and [Ru{η2-(NPh)2CNHPh}(η-p-PriC6H4Me)Cl]19
were prepared according to literature procedures, 1,2,3-
triphenylguanidine and 1,1-diethyl-2,3-diphenylguanidine by
condensation of aniline or diethylamine with diphenylcarbodi-
imide in THF under reflux. NMR spectra were recorded on a
Bruker AC 250 spectrometer, and mass spectra on a Kratos
MS50 TC instrument in positive ion FAB mode using 3-
DOI: 10.1039/b001992p
J. Chem. Soc., Dalton Trans., 2000, 1887–1891
This journal is © The Royal Society of Chemistry 2000
1887

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be separated by chromatography, however the mass spectrum
(ϩFAB) showed a molecular ion at m/z 960 (C57H48N9Ru
requires m/z 959). EPR (CH2Cl2–Toluene): ν = 9.44 GHz,
T = 293 K, axial spectrum, giso = 2.29; ν = 9.44 GHz, T = 150 K,
g|| = 1.809, g⊥ = 2.472, gav = 2.251.
of Pnma), the orientation of the molecules is not consistent
with a mirror plane perpendicular to the z axis of the unit cell
setting given here. Complex 3 contains a molecule of toluene
disordered over two crystallographically unique orientations
both further disordered about an inversion centre; one part-
weight molecule was refined with explicit restraints on the bond
lengths and angles, the other refined as a rigid body.
CCDC reference number 186/1949.
[Pt{ꢀ2-(NPh)2CNHPh}2] 2. To
a
solution of 1,2,3-
triphenylguanidine (600 mg, 2.08 mmol) in diethyl ether (40
cm3) at Ϫ78 ЊC was added 0.85 cm3 of a 2.5 M solution (2.12
See http://www.rsc.org/suppdata/dt/b0/b001992p/ for crystal-
lographic files in .cif format.
n
mmol) of BuLi in hexanes. The mixture was allowed to warm
to room temperature and stirred for 30 min. The solution was
recooled to Ϫ78 ЊC and then [Pt(PhCN)2Cl2] (440 mg, 1.02
mmol) solid was added. After 12 h stirring at room temperature
the mixture was filtered providing a yellow solid and a brown
filtrate. The solid was washed on the filter with CH2Cl2 and the
washings were added to the filtrate. The evaporation of the
resulting solution to dryness and recrystallisation of the residue
from CH2Cl2–hexane provided [Pt{η2-(NPh)2CNHPh}2] 2 in
Results and discussion
Our previous observations of the reactivity of 1,2,3-triphenyl-
guanidine with the metal halide complexes [RhCp*Cl2]2 and
[Ru(η-p-PriC6H4Me)Cl2]2 had shown that, in addition to
cleaving the chloro-bridges, the guanidine also acts as a base
to provide a chelating guanidinate ligand with concomitant
formation of the guanidinium chloride (Scheme 1).1 Indeed,
1
53% yield as well formed very pale yellow crystals. H NMR
(250 MHz, CDCl3): δ 7.2–6.8 (m, 30 H) and 5.9 (s, 2 H). 13C
NMR (62.9 MHz, CDCl3): δ 154.5 (CN3), 153.5, 143.6, 137.1,
129.3, 128.6, 125.3, 124.4 and 121.8. MS (ϩFAB): m/z 768
(Mϩ), 480 (Mϩ Ϫ L) and 288 (L). Found C, 58.9; H, 4.28; N,
10.8%. C38H32N6Pt requires C, 59.5; H, 4.17; N, 11.0%.
[Ti{ꢀ2-(NPh)2CNEt2}2Cl2] 3. 1,1-Diethyl-2,3-diphenylguan-
idine (705 mg, 2.64 mmol) was dissolved in diethyl ether (20
cm3) and cooled to Ϫ78 ЊC. n-Butyllithium (2.64 mmol, 1.05
cm3 of a 2.5 M solution) was added and the mixture stirred for
10 minutes then allowed to warm to room temperature. This
solution was recooled to Ϫ78 ЊC, then added to a suspension of
[TiCl4(THF)2] (350 mg, 1.33 mmol) in diethyl ether (20 cm3) at
Ϫ78 ЊC and an instant change to bright red was observed. The
mixture was warmed to room temperature and stirred for 18
hours. The resulting red solution was reduced to dryness under
vacuum leaving a red solid which was dissolved in toluene (10
cm3). Filtration of this solution through Celite yielded a clear
red solution which, upon storage at Ϫ30 ЊC, produced red
needles. These were found to be of insufficient quality for X-ray
diffraction. The solution was re-filtered through Celite and the
resulting solution stored at 5 ЊC. After 3 weeks the solution
Scheme 1
treatment of these dimers with four molar equivalents of the
guanidine in toluene solution rapidly leads to precipitation of
the salt and formation of the guanidinate complexes in good
yield at room temperature. Subsequent to these observations
we have found that reaction of the insoluble dimer [Ru(η-
C6H6)Cl2]2 with 1,2,3-triphenylguanidine in toluene provides
the dark green [Ru{η2-(NPh)2CNHPh}3] 1 in a slow reaction
(1 week), the first example of such a tris-chelate guanidinate
complex, and not the anticipated [Ru(η-C6H6){η2-(NPh)2CN-
HPh}Cl]. The difference in reactivity for the η-p-cymene and
η-benzene complexes can only be explained by a stronger
Ru–arene bond for the p-cymene ligand than for benzene and
the consequent easier displacement of the latter. However,
displacement of the p-cymene ligand from [Ru(η-p-PriC6H4-
Me){η2-(NPh)2CNHPh}Cl] and formation of 1 can also be
achieved by treatment of this species with base (NEt3, KOH,
LiNiPr2), a reaction which was explored in an attempt to
eliminate HCl from the complex and establish a η3 co-
ordination mode for the ligand by use of the third nitrogen
atom. The unexpected formation of an oxidised species in these
reactions is consistent with observations by others who have
found on several occasions that treatment of metal complex
precursors with lithium amidinates or triazinates provides
an oxidised product in the form of the tris-chelate. Examples
of this behaviour are the predominant formation of [Cr-
(η2-PhNNNPh)3] when [Li(THF)4]2[Cr2Me8] is treated with
[LiPhNNNPh],21 and the formation of [Fe{η2-(NPh)2CH}3] on
reaction of Li(NPh)2CH with an iron() precursor.22 However,
such behaviour has never been reported for a reaction in which
the metal precursor is treated with the neutral ligand as in the
current example. One factor which favours the formation of
such oxidised species is the donor characteristics of the lig-
ands which, in the absence of additional π-acid ligands, are
more compatible with M3ϩ than M2ϩ ions. On the basis of
comparisons between the electrochemical behaviour of the
1
afforded a crop of red needles (170 mg, 0.26 mmol, 20%). H
NMR (250 MHz, CDCl3): δ 0.84 (t, J = 7.2, 12 H, CH2CH3),
2.64 (q, J = 7.2 Hz, 8 H, CH2CH3) and 6.93–7.27 (cm, 20 H,
aryl H). 13C NMR (62.9 MHz, CDCl3): δ 12.8 (CH2CH3), 42.2
(CH2CH3), 121.7, 122.6, 124.8, 125.2 (aryl C) and 165.6 (CN3).
Found C, 61.9; H, 5.79; N, 13.3%. C34H40Cl2N6Ti requires C,
62.2; H, 6.19; N, 12.9%). The crystal structure shows the
presence of a disordered toluene molecule in the unit cell,
however the elemental analysis indicates that this is absent
from the powder sample analysed, presumably removed by the
vacuum drying to which it was subjected.
X-Ray crystallography
Crystal data for complexes 1, 2 and 3 are presented in Table 1.
All data sets were collected on a Stoe Stadi-4 diffractometer
equipped with an Oxford Cryosystems low-temperature device.
The structures of 1 and 3 were solved by direct methods (SIR
92);20 the structure of 2 was solved by placing a Pt atom at the
unit cell origin. Hydrogen atoms were placed in calculated
positions. Difference maps for 1 and 2 exhibited weak peaks
suggesting that the H atoms attached to non-ligating N atoms
lie in the C–N–C(Ph) plane. The structure of 1 contains a
molecule of pentane exhibiting disorder in two carbon posi-
tions in the ratio 60:40; this ratio was fixed during refinement,
with the part-weight atoms having a common isotropic dis-
placement parameter. Similarity restraints were applied to
chemically equivalent bond lengths and angles. Although 1 has
molecular mirror symmetry, with the Ru atoms conforming to
the centrosymmetric space group Pnam (non-standard setting
1888
J. Chem. Soc., Dalton Trans., 2000, 1887–1891

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Table 1 Crystal data for complexes 1–3
1a
2b
3ؒC6H5Mea
Empirical
C62H60N9Ru
C38H32N6Pt
C41H48Cl2N6Ti
formula
M
1032.26
150(2)
767.80
150(2)
743.65
150(2)
T/K
Crystal system
Space group
Orthorhombic
Pna21
Triclinic
P1
Monoclinic
C2/c
¯
a/Å
b/Å
c/Å
α/Њ
22.248(3)
20.648(4)
11.784(2)
6.455(5)
10.971(8)
11.591(8)
76.11(5)
77.68(4)
86.81(6)
778.49
1
9.813(4)
17.821(7)
23.802(8)
β/Њ
94.95(3)
γ/Њ
V/Å3
5413(2)
4
2.710
4193
4147
4
0.370
3675
Z
µ(Mo-Kα)/mmϪ1
Independent
reflections
Data with
I > 2σ(I)
R1
4.59
3252
3237
2677
2332
0.0554
0.1286
0.0388
0.0394
0.0688
0.2242
Fig. 2 Molecular structure of [Ru{η2-(NPh)2CNHPh}3] 1.
wR2
a Refined on F 2. b Refined on F.
isostructural molybendum dimers [Mo2L4] [L = µ-η2-(NPh)2-
CNHPh or µ-η2-(p-MeC6H4N)2CH] we have established that
the guanidinate ligands are considerably stronger donors and
therefore better at stabilising the higher oxidation states of Mo
than the amidinates.15 We have attributed this to the involve-
ment of a ligand resonance form unavailable to the amidinate in
which the non-ligating nitrogen lone pair is delocalised into the
ligand π system (Fig. 1). On the basis of the above observations,
therefore, it is unsurprising that we observe the ready formation
of the ruthenium() tris-chelate 1 from ruthenium() pre-
cursors, although the mechanism still remains obscure.
The frozen glass EPR spectrum of complex 1 at 150 K in a
CH2Cl2–toluene solvent mixture displays an axial spectrum
with g|| = 1.809 and g⊥ = 2.472 (giso = 2.29 at 293 K) and no evi-
dence for coupling of the unpaired electron to nitrogen. The
molecular structure of 1 is shown in Fig. 2 and selected bond
distances and angles given in Table 2. The co-ordination geom-
etry may be described as severely distorted octahedral resulting
from the very small bite angle of the chelating ligands (intra-
ligand N–Ru–N ca. 63Њ). The co-ordination of the ligand to the
metal results in a distortion of the angles around the ligand
central carbon atom such that the N–C–N angle between
the ligating nitrogens is considerably less than 120Њ [range
109.1(10)–109.7(9)Њ] whilst the remaining angles are greater
than 120Њ [range 121.2(9)–128.9(10)Њ]. However, the sum of
the angles around the central carbon for each ligand is 360Њ
indicating no deviation from planarity.
amidinato)platinum() complex is however monomeric in all
phases.25 The structure adopted by bis(guanidinato)platinum()
complexes is therefore open to speculation.
The treatment of 1,2,3-triphenylguanidine with one molar
equivalent of n-butyllithium in diethyl ether followed by
[Pt(PhCN)2Cl2] provides a yellow solution from which [Pt{η2-
(NPh)2CNHPh}2] 2 may be isolated. A crystal structure
determination shows 2 to have a monomeric, square planar
structure with a crystallographically imposed twofold axis
relating the two ligands. The structure is shown in Fig. 3 and
selected bond distances and angles are given in Table 2. The
structure is similar to that of bis(diphenylbenzamidinato)-
platinum(),25 but differs from that adopted by [Pd{η2-
(PhN)2CNHPh}{µ-η2-(PhN)2CNHPh}]2 which features both
chelating and bridging triphenylguanidinate ligands, a structure
intermediate between the square planar bis-chelate and the
tetra-bridged tetragonal prismatic structures.9 Features similar
to those we have previously observed for complexes with che-
lating guanidinate ligands are observed in the structure of 2.
Thus, the C–N bonds to the ligand central carbon show a
pattern indicating delocalised bonding within the co-ordinated
ؒ ؒ ؒ ؒ ؒ ؒ
RN
C NR unit [C–N(1) 1.348(7), C–N(2) 1.323(8) Å]
᎐᎐ ᎐᎐
and a marginally longer bond to the unco-ordinated N(3)
[1.373(7) Å].
Guanidinates have recently attracted attention as ligands for
more electropositive transition, main group and lanthanide
metals. Their potential high basicity conferred by the delocal-
isation of the unco-ordinated nitrogen lone pair into the ligand
π system (Fig. 1) means that they are well suited to stabilisation
of higher oxidation state metal ions with strong π-acceptor
properties and other electron deficient metals. The steric flex-
ibility available through variation of the nitrogen substituents
allows for a high degree of steric shielding of the metal centre to
be built into the ligand and preliminary investigations into the
use of such bulky guanidinates as ancillary ligands in alkene
polymerisation catalysts where such factors are significant have
been reported.26 Three synthetic routes to guanidinate com-
plexes of these metals have been reported and involve either
treatment of the metal dialkylamide complex [M(NR2)n] with
the required guanidine,6 in situ formation of the guanidinate
ligand in the co-ordination sphere of the metal by insertion of
a carbodiimide into a metal–dialkylamido ligand bond7 or
metathesis of a metal halide complex with a lithium guanidin-
ate.2,11 We have investigated the latter route for the synthesis
of titanium complexes. Treatment of [TiCl4(THF)2] with two
For the Group 10 metals Ni, Pd and Pt the bis-amidinate and
-triazinate complexes [M(L–X)2] show an interesting balance
between monomeric square planar and dimeric ligand-bridged
tetragonal prismatic Cu2(OAc)4 type structures, the structure
adopted being dependent upon the identity of both the
metal and ligand. Thus, both bis(diphenyltriazinate) complexes
of NiII and PdII (L–X = PhNNNPhϪ) have been found to
adopt the dimeric structure,23 and the bis(formamidinate)
palladium() complexes [L–X = (RN)2CHϪ] are also reported
to be dimeric.24 In contrast, bis(acetamidinato)palladium()
complexes [L–X = (RN)2CMeϪ] are found to adopt the square
planar structure.24 However, replacement of the ligand central
CMe group by a CPh effects a structural transformation
and the bis(benzamidinato)palladium() complexes [L–X =
(RN)2CPhϪ] are found to be dimeric in the vapour (by mass
spectrometry) and solid phases but to exist as an equilibrium
mixture of mono- and di-meric species in solution.24 The benz-
amidinate ligand also favours a tetrabridged dimeric structure
for the bis(benzamidinato)nickel() complex, although this
species appears to be monomeric in solution.25 The bis(benz-
J. Chem. Soc., Dalton Trans., 2000, 1887–1891
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Table 2 Selected bond lengths (Å) and angles (Њ) for complexes 1–3
1
2
3
Ru–N(11)
Ru–N(21)
Ru–N(12)
Ru–N(22)
Ru–N(13)
Ru–N(23)
C(1)–N(11)
C(1)–N(21)
C(1)–N(31)
C(2)–N(12)
C(2)–N(22)
C(2)–N(32)
C(3)–N(13)
C(3)–N(23)
C(3)–N(33)
2.099(8)
2.064(7)
2.082(9)
2.061(9)
2.055(9)
2.095(8)
1.309(13)
1.335(13)
1.387(8)
1.313(12)
1.347(13)
1.396(14)
1.33(3)
Pt–N(1)
Pt–N(2)
2.028(4)
2.061(5)
1.348(7)
1.323(8)
1.373(7)
1.399(7)
1.427(7)
1.430(7)
1.399(7)
1.427(7)
1.430(7)
Ti–N(1)
Ti–N(2)
Ti–Cl(1)
C(1)–N(1)
C(1)–N(2)
C(1)–N(3)
N(1)–C(11)
N(2)–C(21)
N(3)–C(31)
N(3)–C(33)
2.056(4)
2.078(4)
2.2957(15)
1.341(6)
1.343(6)
1.341(6)
1.427(6)
1.423(6)
1.474(6)
1.478(7)
C(1)–N(1)
C(1)–N(2)
C(1)–N(3)
N(1)–C(11)
N(2)–C(12)
N(3)–C(13)
N(1)–C(11)
N(2)–C(12)
N(3)–C(13)
N(1)–Ti–N(2)
63.39(15)
146.9(2)
92.31(16)
94.20(8)
122.2(4)
127.5(4)
120.3(4)
121.3(4)
118.3(4)
N(1)–Pt–N(2)
64.0(2)
108.4(5)
128.0(5)
123.6(5)
128.8(5)
122.0(4)
124.9(5)
N(1)–Ti–N(1A)
N(1)–Ti–N(2A)
Cl(1)–Ti–Cl(1A)
C(1)–N(1)–C(11)
C(1)–N(2)–C(21)
C(1)–N(3)–C(31)
C(1)–N(3)–C(33)
C(31)–N(3)–C(33)
1.325(14)
1.394(14)
N(1)–C(1)–N(2)
N(1)–C(1)–N(3)
N(2)–C(1)–N(3)
C(1)–N(1)–C(11)
C(1)–N(2)–C(12)
C(1)–N(3)–C(13)
N(11)–Ru–N(21)
N(12)–Ru–N(22)
N(13)–Ru–N(23)
62.6(3)
63.3(3)
62.9(3)
The restricted bite angle [63.39(15)Њ] of the chelating guan-
idinate ligands imposes a severe distortion upon the geometry
although the interligand N(1)–Ti–N(2A) and N(1A)–Ti–N(2)
[92.31(16)Њ] and the Cl–Ti–Cl [94.20(8)Њ] angles approach the
ideal. The structure of 3 is very similar to those of the corre-
sponding amidinate complexes [Ti{(NSiMe3)2CPh}2Cl2]27 and
[Zr{(CyN)2CMe}2Cl2]28 and the guanidinate complexes [M-
{η2-(NR)2CN(SiMe3)2}2Cl2] (R = iPr or cyclo-C6H11; M = Zr or
Hf).2 In these latter complexes steric crowding between the
SiMe3 groups and the nitrogen alkyl substituents results in
the dihedral angle formed between the planar NSi2 unit and the
CN3 ligand plane falling in the region 86–88Њ, and it was there-
fore argued that the almost orthogonal arrangement negated
the possibility of any conjugation of this nitrogen into the lig-
and π system. These guanidinate ligands may thus be regarded
as electronically similar to amidinates. In 3 the dihedral angle
between the NC2 [N(Et)2] and CN3 planes is 30.4Њ indicating the
less severe steric crowding in this system and, although not
optimal, some π conjugation might be anticipated (see below).
A sufficient number of guanidinate complexes have now been
structurally characterised to allow a comparison of the metrical
parameters of this ligand system in a variety of metal environ-
ments, and in particular to address the question of the conju-
gation of the unco-ordinated nitrogen atom into the ligand π
system, i.e. the relative contributions from the two resonance
forms A and B in Fig. 1. The intraligand C–N bond distances
within the CN3 ligand core for a variety of guanidinate(1Ϫ)
complexes are provided in Fig. 5. Unfortunately the nitrogen
substituents in the ligands vary between the complexes and this
cannot be ruled out as the origin of some of the variations
observed, and furthermore some of the errors in the distances
are rather large meaning that some of the apparent variations
are not statistically significant. Nevertheless, taken together the
data provide a picture in which the extent of conjugation of
the unco-ordinated nitrogen lone pair into the ligand π system
is consistent with the anticipated electronic requirements of
the metal centre. As previously discussed, the steric bulk of the
SiMe3 and cyclohexyl groups in 7 dictate that the NSi2 plane is
almost orthogonal to the CN3 plane. This complex can there-
fore be regarded as a system in which such π overlap is absent,
and this fact is reflected in the pattern of C–N bond lengths,
with the bond to the unco-ordinated nitrogen atom being
longer than those to the other two by a significant margin. In
the tantalum() complex 8 the SiMe3 groups have been replaced
by methyl groups although the other substituents remain
unchanged. In this system it could be argued that the reduced
bulk of the methyl groups would allow coplanarity of the NC2
Fig. 3 Molecular structure of [Pt{η2-(NPh)2CNHPh}2] 2.
Fig. 4 Molecular structure of [Ti{η2-(NPh)2CNEt2}2Cl2] 3.
molar equivalents of Li(NPh)2CNEt2 in diethyl ether provides
[Ti{η2-(NPh)2CNEt2}2Cl2] 3 in 20% yield on crystallisation
from toluene solution. The 1H and 13C NMR spectra of 3 indi-
cate the equivalence of the two guanidinate ligands and also the
equivalence of the phenyl and ethyl groups within the ligands,
containing only one set of signals for each. Single crystal X-ray
diffraction shows that in the solid state the complex adopts a
severely distorted octahedral co-ordination geometry as shown
in Fig. 4. The guanidinate ligands are symmetry related by a
twofold rotation axis passing through the Ti atom and bisecting
the Cl–Ti–Cl angle and are thus crystallographically equivalent.
1890
J. Chem. Soc., Dalton Trans., 2000, 1887–1891

View Article Online
distances thus supporting a greater contribution from B. It is
therefore evident that bridging guanidinates are also capable of
considerable electronic flexibility.
Acknowledgements
We thank the Engineering and Physical Science Research
Council (EPSRC) for studentships (L. A. M. and S. P.) and
provision of a 4-circle diffractometer, The Royal Society, The
Leverhulme Trust and The Nuffield Foundation for financial
support. We also thank Dr E. McInnes and the EPSRC EPR
spectroscopy service at the University of Manchester for the
spectrum of complex 1.
Fig. 5 Intraligand C–N bond distances for guanidinate ligands in
a number of structurally characterised complexes. [Ru{η2-(NPh)2-
CNHPh}3] 1; [Pt{η2-(NPh)2CNHPh}2] 2; [Ti{η2-(NPh)2CNEt2}2Cl2] 3;
[Ru(η-p-PriC6H4Me){η2-(NPh)2CNHPh}Cl] 4;1 [Mo2{µ-η2-(NPh)2CN-
HPh}4] 5;15 [Mo2{µ-η2-(NPh)2CNHPh}4]ϩ 6;15 [Zr{η2-(NCy)2CN-
(SiMe3)2}2Cl2] 7;2 [Ta{η2-(NCy)2CNMe2}(NMe2)4] 8.7 * Mean values
for the number of crystallographically independent guanidinate ligands
in the complex.
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these planes is 80.7Њ, furthermore the unco-ordinated nitrogen
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