Synthesis, structure, and coordination chemistry of a tridentate, six-electron-donor amidinate ligand

Article abstract of DOI:10.1021/om990365w

The coordination chemistry of ancillary amidinate ligands with a pendant pyridine functionality is described. Reaction of p-toluoyl- or p-^tBu-benzoyl chloride with 2-amino-ethylpyridine generates the amides 2-Py-(CH₂)₂NHCO(p-RPh) (R = Me 1a; ^tBu 1b); these amides are then converted to the amidines 2-Py-(CH₂)₂NHC(p-RPh)NR′ (R = Me, R′ = Ph (2a) (L^MeH); R = ^tBu, R′ = 3,5-dimethylphenyl (2b) (L^tBuH)) by reaction with PCl₅ followed by R′NH₂. The amidines 2a,b were characterized by ¹H NMR and IR spectroscopy and elemental analyses, and 2b was characterized by X-ray crystallography. Reaction of 2a or 2b with homoleptic metal-alkyls or -amides yields the mono- or bis(amidinate) complexes (L^Me)₂Mg (3a), (L_tBu)AlMe₂ (4), (L_tBu)Zr(CH₂Ph)₃ (5), and (L^Me)₂La[N(SiMe₃)₂] (6). All metal complexes were characterized by ¹H NMR and IR spectroscopy, elemental analyses, and X-ray crystallography. The X-ray crystal structures of compounds 3-6 show them to be monomeric, with the pendant pyridine coordinated intramolecularly in all cases. The tridentate amidinate coordinates meridionally to the metal center except in the case of the lanthanum derivative 6, where an approximate facial geometry is observed.

Full text of DOI:10.1021/om990365w

5360
Organometallics 1999, 18, 5360-5366
Syn th esis, Str u ctu r e, a n d Coor d in a tion Ch em istr y of a
Tr id en ta te, Six-Electr on -Don or Am id in a te Liga n d
Kristi Kincaid, Christopher P. Gerlach, Garth R. Giesbrecht, J ohn R. Hagadorn,
Glenn D. Whitener, Alex Shafir, and J ohn Arnold*
Department of Chemistry, University of California, Berkeley, and Chemical Sciences Division,
Lawrence Berkeley National Laboratory, Berkeley, California 94720-1460
Received May 17, 1999
The coordination chemistry of ancillary amidinate ligands with a pendant pyridine
functionality is described. Reaction of p-toluoyl- or p-tBu-benzoyl chloride with 2-amino-
t
ethylpyridine generates the amides 2-Py-(CH2)2NHCO(p-RPh) (R ) Me 1a ; Bu 1b); these
amides are then converted to the amidines 2-Py-(CH2)2NHC(p-RPh)NR′ (R ) Me, R′ ) Ph
t
(2a ) (LMeH); R ) Bu, R′ ) 3,5-dimethylphenyl (2b) (LtBuH)) by reaction with PCl5 followed
1
by R′NH2. The amidines 2a ,b were characterized by H NMR and IR spectroscopy and
elemental analyses, and 2b was characterized by X-ray crystallography. Reaction of 2a or
2b with homoleptic metal-alkyls or -amides yields the mono- or bis(amidinate) complexes
(LMe)2Mg (3a ), (LtBu)AlMe2 (4), (LtBu)Zr(CH2Ph)3 (5), and (LMe)2La[N(SiMe3)2] (6). All metal
1
complexes were characterized by H NMR and IR spectroscopy, elemental analyses, and
X-ray crystallography. The X-ray crystal structures of compounds 3-6 show them to be
monomeric, with the pendant pyridine coordinated intramolecularly in all cases. The
tridentate amidinate coordinates meridionally to the metal center except in the case of the
lanthanum derivative 6, where an approximate facial geometry is observed.
In tr od u ction
Resu lts a n d Discu ssion
The ability of amidinates [RC(NR′)2]- to act as four-
electron, bidentate ligands for main group elements,
transition metals, and the lanthanides is well estab-
lished.1,2 Amidinates have proven to be versatile ligands
due to the ease with which the steric and electronic
properties of the resultant metal complexes can be tuned
by means of substitution at the nitrogen atoms. Previous
work from our group has expanded the scope of these
ligands by linking two amidines together to produce C2
symmetric ligand systems that are loosely analogous to
ansa-Cp systems.3,4 Here we report the high-yield
synthesis of a series of easily prepared amidines with a
pendant pyridine moiety. In this first account, our aim
is to demonstrate their versatility to act as ligands
toward a variety of metals from across the periodic table.
As shown here, compared to simple amidines, the ability
of these compounds to act as six-electron-donor, three-
coordinate ligands renders them closer analogues to Cp
ligands;5 nonetheless, their tendency to adopt meridi-
onal geometries clearly sets them apart from η5-Cp
ligands where only facial coordination is possible.
Reactivity studies on these and related compounds will
be addressed in forthcoming papers.
Am id in e Syn th esis. Synthesis of the (2-pyridine)-
ethyl-functionalized benzamidinate was accomplished
by conversion of a suitable amide to an imine chloride,
which was then reacted with a substituted aniline to
yield the desired amidine. We employed related meth-
odology previously in the synthesis of linked bis-
(amidines),4 and the route has been used by others to
prepare symmetric, bulky amidinates.6,7 The starting
amide was formed in high yield by reaction of p-R-
benzoyl chloride with 2-aminoethylpyridine in the pres-
ence of triethylamine (eq 1).
(1) Edelmann, F. T. Coord. Chem. Rev. 1994, 137, 403.
(2) Barker, J .; Kilner, M. Coord. Chem. Rev. 1994, 133, 219.
(3) Hagadorn, J . R.; Arnold, J . Angew. Chem., Int. Ed. Engl. 1998,
37, 1729.
(4) Whitener, G. D.; Hagadorn, J . R.; Arnold, J . J . Chem. Soc.,
Dalton. Trans. 1999, 1249.
(5) During the preparation of this article, the synthesis of an
The amides 1a ,b were then converted to the corre-
sponding amidines by in situ generation of imine
amidinate ligand with
a pendant dimethylamino group was de-
scribed: (a) Brandsma, M. J . R.; Brussee, E. A. C.; Meetsma, A.;
Hessen, B.; Teuben, J . H. Eur. J . Inorg. Chem. 1998, 1867. An
amidinate ligand with a noncoordinating dimethylamino group has also
been reported: (b) Hao, S.; Gambarotta, S.; Bensimon, C.; Edema, J .
J . H. Inorg. Chim. Acta 1993, 213, 65.
(6) Boere, R. T.; Klassen, V.; Wolmershauser, G. J . Chem. Soc.,
Dalton Trans. 1998, 4147.
(7) Boyd, G. V. In The Chemistry of Amidines and Imidates; Patai,
S., Rappoport, Z., Eds.; J ohn Wiley & Sons: Chichester, 1991; Vol. 2,
pp 339-366.
10.1021/om990365w CCC: $18.00 © 1999 American Chemical Society
Publication on Web 11/12/1999

Tridentate, Six-Electron-Donor Amidinate Ligand
Organometallics, Vol. 18, No. 25, 1999 5361
chlorides via reaction with PCl5; these species readily
reacted with substituted anilines in the presence of
triethylamine to generate amidines 2a ,b in moderate
yields (eq 2).
F igu r e 1. ORTEP view of LtBuH (2b) drawn with 50%
probability ellipsoids.
Ta ble 1. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for LtBu H (2b)
N2-C8
N2-C7
C8-C9
1.362(5)
1.464(5)
1.494(5)
N3-C8
N3-C19
1.301(5)
1.414(5)
N2-C8-N3
N2-C8-C9
C7-N2-C8
120.3(4)
112.9(4)
121.8(3)
N3-C8-C9
C8-N3-C19
126.7(4)
120.9(3)
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for (LMe)2Mg(THF ) (3a )
The amidines 2a (LMeH) and 2b (LtBuH) are soluble
in solvents such as ether, benzene, and toluene and were
isolated as colorless crystals from diethyl ether solu-
tions. Compared to the unsubstituted analogue, incor-
poration of tert-butyl and meta-methyl groups in 2b was
beneficial, as it both increased the solubility of the
N2-C8
1.322(5)
1.321(5)
2.264(4)
2.211(4)
2.110(4)
N3-C8
1.339(5)
1.346(5)
2.104(4)
2.260(4)
2.198(4)
N5-C29
Mg1-N1
Mg1-N3
Mg1-N5
N6-C29
Mg1-N2
Mg1-N4
Mg1-N6
N2-C8-N3
113.4(4)
82.8(1)
62.0(1)
145.3(1)
N5-C29-N6
N1-Mg1-N3
N4-Mg1-N5
N5-Mg1-N6
113.5(4)
144.7(1)
84.0(1)
62.3(1)
N1-Mg1-N2
N2-Mg1-N3
N4-Mg1-N6
1
ligand and provided two convenient H NMR handles.
The proton occupying the 6-position of the pyridine ring
appears as a doublet at ∼8.5 ppm in the 1H NMR
spectrum. The shift of this signal to lower field is
diagnostic of coordination of the pyridine arm to a metal
center (see below). Additionally, the methylene protons
of the ethylene spacer appear as broad singlets at room
temperature in the starting amidines. These signals
sharpen on cooling to 0 °C, where the A2B2 system is
fully resolved; likewise, coordination of the ligand to a
metal center has the same effect. The amidine protons
reside at ∼5.4 ppm for both 2a and 2b, upfield of most
common amines8 but similar to a series of cyclohexyl-
linked bis(amidines) reported earlier.4 IR spectra of the
amidines show characteristic NH stretches in the range
3275-3250 cm-1 along with strong absorbances around
Ta ble 3. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for (LtBu )AlMe2 (4)
N2-C8
1.368(8)
2.218(6)
2.083(5)
2.030(8)
N3-C8
Al1-N2
Al1-C27
1.381(8)
1.898(6)
1.986(8)
Al1-N1
Al1-N3
Al1-C28
N2-C8-N3
117.7(6)
151.4(2)
112.3(2)
119.4(3)
106.9(3)
121.5(3)
N1-Al1-N2
N1-Al1-C27
N2-Al1-N3
N2-Al1-C28
N3-Al1-C28
79.1(2)
88.1(3)
72.3(2)
118.1(3)
81.0(3)
N1-Al1-N3
N1-Al1-C28
N2-Al1-C27
N3-Al1-C27
C27-Al1-C28
Ta ble 4. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for (LtBu )Zr (CH2P h )3 (5)
1600 cm-1 due to νCdN
An ORTEP view of the molecular structure of LtBu
.
N2-C8
1.34(1)
N3-C8
1.328(1)
2.169(6)
2.347(8)
2.331(8)
H
Zr1-N1
Zr1-N3
Zr1-C34
2.393(6)
2.248(6)
2.364(8)
Zr1-N2
Zr1-C27
Zr1-C41
(2b) is shown in Figure 1; bond lengths and angles are
given in Table 1. Complete details of the crystallo-
graphic analyses of compounds 2b-6 are given in Table
6. The two C-N bonds in the amidine (N2-C8, 1.362-
(5) Å) and N3-C8, 1.301(5) Å) are similar in length to
those reported for other bulky amidines,6 indicating the
localized nature of the N-C double bond. Consistent
with this view, electron density in accord with a single
amidine proton (H28) was located on N2. In the solid
state, 2b is monomeric, in contrast to a number of
N2-C8-N3
109.7(6)
137.5(2)
77.3(2)
N1-Zr1-N2
N2-Zr1-N3
N1-Zr1-C34
N2-Zr1-C27
N2-Zr1-C41
N3-Zr1-C34
C27-Zr1-C34
C34-Zr1-C41
Zr1-C34-C35
78.4(2)
59.2(2)
139.2(2)
102.8(2)
101.7(3)
83.1(2)
91.3(3)
87.2(3)
104.0(5)
N1-Zr1-N3
N1-Zr1-C27
N1-Zr1-C41
N2-Zr1-C34
N3-Zr1-C27
N3-Zr1-C41
C27-Zr1-C41
Zr1-C27-C28
Zr1-C41-C42
78.6(2)
142.4(3)
112.5(3)
105.9(3)
141.1(3)
107.4(5)
111.5(5)
(8) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric
Identification of Organic Compounds; J ohn Wiley & Sons: New York,
1991.
amidines that form dimers via intermolecular hydrogen
bonds.6,9

5362 Organometallics, Vol. 18, No. 25, 1999
Kincaid et al.
Ta ble 5. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for (LMe)2La [N(SiMe3)2] (6)
single molecule of unbound THF. The ligands are
coordinated meridionally, with the amidinate nitrogens
and the magnesium center defining two planes at a right
angle to each other (85.61°). The deviation from planar-
ity within the two planes is 0.0276 Å (amidinate
containing N1, N2, and N3) and 0.0806 Å (amidinate
containing N4, N5, and N6). The amidinate nitrogen
bond lengths in 3a (2.104(4)-2.211(4) Å) are similar to
those reported for [iPrNC(Ph)NiPr]2Mg(THF)2 (2.164 Å
(av))10 and [Me3SiNC(Ph)NSiMe3]2Mg(PhCN) (2.125 Å
(av))11 and are slightly shorter than the pyridine-
magnesium bonds (2.260(4) and 2.264(4) Å).
N2-C8
1.319(6)
1.319(6)
2.753(4)
2.609(4)
2.599(4)
2.453(4)
N3-C8
1.338(6)
1.347(6)
2.491(4)
2.747(4)
2.598(4)
N5-C29
La1-N1
La1-N3
La1-N5
La1-N7
N6-C29
La1-N2
La1-N4
La1-N6
N2-C8-N3
N1-La1-N2
N2-La1-N3
N4-La1-N6
N7-La1-N1
N7-La1-N3
N7-La1-N5
114.6(4)
70.4(1)
52.0(1)
91.0(1)
98.3(1)
118.7(1)
108.2(1)
N5-C29-N6
N1-La1-N3
N4-La1-N5
N5-La1-N6
N7-La1-N2
N7-La1-N4
N7-La1-N6
113.8(4)
111.4(1)
69.6(1)
50.9(1)
95.2(1)
87.4(1)
157.7(1)
Yellow crystals of the aluminum derivative were
obtained by layering a saturated CH2Cl2 solution of 4
with THF. An ORTEP view of the molecular structure
is given in Figure 3; selected bond lengths and angles
are presented in Table 3. The structure confirms that
the compound is monomeric with a single amidine
ligand coordinated to the aluminum center in a meridi-
onal fashion. The geometry is distorted trigonal bipyr-
amidal, with N1 and N3 occupying the axial positions
(N1-Al1-N3 151.4(2)°). The N2, C27, and C28 atoms
reside equatorially, forming angles subtended by alu-
minum of ∼120°. The N2-Al1-N3 bite angle (72.3(2)°)
is somewhat larger than those reported for a series of
mono-amidinate dimethylaluminum compounds by J or-
dan et al. (68.2-69.1°).12,13 The C27-Al1-C28 angle of
121.5(3)° is also slightly larger than those cited in the
above series (114.2-118.6°). The N2-C8 and N3-C8
bond lengths are similar (1.368(8) and 1.381(8) Å,
respectively), consistent with delocalization within the
amidine functionality. The amidinate nitrogen-alumi-
num bonds (N2-Al1 ) 1.898(6) Å, N3-Al1 2.083(5) Å)
are shorter than the pyridine nitrogen-aluminum bond
(N1-Al1 2.218(6) Å), but comparable to those of other
similar dialkyl species (∼1.93 Å).12,14 The Al-Me bonds
are longer (C27-Al1 1.986(8) Å, C28-Al1 2.030(8) Å)
than in other mono-amidinate species (∼1.96 Å)12,13 as
a result of the higher coordination number. (For com-
parison, the terminal Al-Me bond distance in Al2Me6
is 1.97 Å.15)
Crystals of (LtBu)Zr(CH2Ph)3 (5) were grown by cooling
a saturated ether solution to -30 °C. An ORTEP
representation is shown in Figure 4; a partial list of
bond lengths and angles is given in Table 4. The
amidinate ligand is coordinated meridionally, with a
mirror plane roughly defined by the Zr1, N1, N2, N3,
and C34 atoms (deviation 0.0204 Å). The six-coordinate
metal is strongly distorted from octahedral, with the
axial C27-Zr1-C41 angle of only 141.1(3)°, and, overall,
the geometry is perhaps better described as approaching
trigonal prismatic. Despite the low electron countsthe
complex is formally 12 electonssthere is no evidence
for agostic interactions, as the Zr-C-C angles are all
close to tetrahedral (104.0(5)-111.5(5)°). For compari-
son, the isoelectronic complex CpZr(CH2Ph)3 is consid-
ered to show Zr-C-C angles substantially less than
tetrahedral due to the presence of agostic CsH- - -Zr
Syn th esis of Meta l Com p lexes. Syntheses of mono-
and bis-ligand metal complexes were easily accom-
plished by alkyl or amine elimination reactions between
amidines 2a ,b and a range of homoleptic metal alkyl
or amide compounds, as shown in eq 3.
xLRH + MR′y ether or THF8 (LR)xMR′y-x + xR′H (3)
3a : M ) Mg, R ) Me, R′ ) Bu, x ) 2, y ) 2.
3b: M ) Mg, R ) tBu, R′ ) Bu, x ) 2, y ) 2.
4: M ) Al, R ) tBu, R′ ) Me, x ) 1, y ) 3.
5: M ) Zr, R ) tBu, R′ ) CH2Ph, x ) 1, y ) 4.
6: M ) La, R ) Me, R′ ) N(SiMe3)2, x ) 2, y ) 3.
The metal derivatives 3-6 were conveniently prepared
in high yield as colorless or yellow crystalline powders.
The relatively low yield of the lanthanum derivative 6
(42% as compared to 75-91% for 3-5) is a reflection of
redistribution chemistry to produce the tri- and mono-
1
substituted products as identified by H NMR spectros-
copy.
Proton NMR data are useful indicators of complex
formation. For example, the amidine proton is absent
in spectra of 3-6, and a downfield NMR shift of the
ortho pyridine proton is a clear marker of coordination
of the pendant donor arm to the metal, residing at
progressively lower field with increasing metal size (see
Experimental Section). The broad methylene resonances
of the ethylene spacer found in amidines 2a ,b are also
noticeably sharper in metal complexes 3-6, an effect
that appears to be diagnostic of coordination of the
pyridine. IR spectra of the metal derivatives exhibit Cd
N stretches only slightly shifted from the starting
amidines at ∼1600 cm-1
.
All of the metal derivatives were characterized by
X-ray crystallography, which shows them to be mono-
meric species in the solid state. The magnesium complex
(LMe)2Mg crystallized on cooling a saturated THF solu-
tion of 3a to -30 °C overnight. The structure is shown
in Figure 2, with a list of relevant bond lengths and
angles available in Table 2. Despite the strong tendency
of magnesium amidines to coordinate donor solvents
such as THF10 or PhCN,11 the unit cell contains only a
(9) Hafelinger, G.; Kuske, K. H. In The Chemistry of Amidines and
Imidates; Patai, S., Rappoprt, Z., Eds.; J ohn Wiley & Sons: Chichester,
1991; Vol. 2, pp 1-100.
(10) Srinivas, B.; Chang, C. C.; Chen, C. H.; Chiang, M. Y.; Chen, I.
T.; Wang, Y.; Lee, G. H. J . Chem. Soc., Dalton Trans. 1997, 957.
(11) Westerhausen, M.; Hausen, H. D. Z. Anorg. Allg. Chem. 1992,
615, 27.
(12) Coles, M. P.; Swenson, D. C.; J ordan, R. F.; Young, V. G.
Organometallics 1997, 16, 5183.
(13) Coles, M. P.; Swenson, D. C.; J ordan, R. F.; Young, V. G.
Organometallics 1998, 17, 4042.
(14) Lechler, R.; Hausen, H. D.; Weidlin, J . J . Organomet. Chem.
1989, 359, 1.
(15) Vranka, R. G.; Amma, E. L. J . Am. Chem. Soc. 1967, 89, 3121.

Tridentate, Six-Electron-Donor Amidinate Ligand
Organometallics, Vol. 18, No. 25, 1999 5363
Ta ble 6. Cr ysta llogr a p h ic Da ta
2b
26H31N3
385.55
yellow, needle
0.30 × 0.16 × 0.15
orthorhombic
Pnna
22.122(1)
17.516(1)
11.581(1)
90
90
90
4487(1)
8
1.14
3a (THF)
4
5
6
formula
fw
color, habit
crystal size, mm
crystal system
space group
a, Å
b, Å
C
C46H48N6OMg
725.23
yellow, block
0.24 × 0.20 × 0.05
triclinic
P1h
11.7242(7)
12.3840(7)
15.3909(9)
101.034(1)
107.310(1)
96.377(1)
2060(3)
2
C28H36N3Al
441.59
yellow, plate
0.20 × 0.18 × 0.05
monoclinic
P21/c
16.6814(9)
8.3560(5)
19.0415(11)
90
C47H51N3Zr
749.16
yellow, needle
0.22 × 0.15 × 0.08
triclinic
P1h
9.9506(10)
11.0008(2)
19.7062(4)
89.975(1)
80.585(1)
65.909(1)
1937(2)
2
C48H58N7Si2La
928.11
yellow, needle
0.25 × 0.10 × 0.08
monoclinic
C2/c
37.2885(24)
10.8058(7)
23.5483(13)
90
c, Å
R, deg
â, deg
106.224(2)
90
95.958(1)
90
γ, deg
V, Å3
2548(2)
4
1.15
9437(4)
8
1.31
Z
Dc, g/cm3
1.17
1.28
2θmax
46.5
-105
18960
3639
52.0
-128
9838
59.1
-105
12396
52.2
-104
9272
52.3
-102
22946
T, °C
total no. of reflns
no. of indep reflns
6874
4875
6479
8833
Tmax
0.969
1.000
1.000
0.977
0.942
Tmin
0.827
0.67
0.08
0.752
0.85
0.03
0.830
0.99
0.09
0.743
3.20
0.04
0.757
9.92
0.06
µ(Mo KR), cm-1
Rint
R
0.040
0.063
0.061
0.062
0.031
Rw
0.044
0.068
0.063
0.079
0.032
GOF
1.24
1.73
1.66
2.39
0.95
no. of data
1526
3769
1692
5042
4345
F igu r e 3. ORTEP view of (LtBu)AlMe2 (4) drawn with 50%
probability ellipsoids.
F igu r e 2. ORTEP view of (LMe)2Mg(THF) (3a ) drawn with
50% probability ellipsoids. Only the ipso carbons of the
p-tolyl and phenyl groups are shown for clarity. One
cocrystallized molecule of THF is also omitted.
Yellow crystals of (LMe)2La[N(SiMe3)2] (6) were ob-
tained by layering a saturated ether solution with
hexanes. The molecular structure of 6 is presented is
Figure 5 along with a partial list of bond lengths and
angles (Table 5). The large lanthanum center is able to
support two three-coordinate substituted-benzamidi-
interactions.16 Conversely, the bis(amidinate) species
[Me3SiNC(Ph)NSiMe3]2Zr(CH2Ph)2 features larger Zr-
C-C angles of 117.6(2)°and 116.6(2)°.17 The N2-C8 and
N3-C8 bond lengths are identical, again implying a
delocalized double bond. Complex 5 exhibits two short
(2.169(6) and 2.248(6) Å) and one longer (2.393(6) Å)
Zr-N bond, a common feature of all the metal com-
plexes presented here. These compare with bond lengths
of 2.278 (av) and 2.259 Å (av) in related zirconium
amidinates [Me3SiNC(Ph)NSiMe3]2ZrMe217,18 and [Me3-
SiNC(Ph)NSiMe3]2Zr(CH2Ph)2,17 respectively. The Zr-C
bond lengths are similar to those reported for CpZr(CH2-
Ph)316 and [Me3SiNC(Ph)NSiMe3]2Zr(CH2Ph)217 and are
otherwise unremarkable.
-
nates as well as a bulky N(SiMe3)2 group. In contrast
to the metal complexes previously described, in 6 the
coordination of the ancillary ligands to the metal center
is intermediate between meridional and facial. This is
due to the increased steric congestion around the metal
because of the bulky amido ligand, which does not allow
for the presence of two meridional amidinate ligands
as in the magnesium derivative, 3a . The ligand contain-
ing N1, N2, and N3 coordinates with a geometry close
to meridional; the sum of the three N-La-N angles is
233.8°. For the other amidine, this sum drops to 211.5°
as the ligand approaches a facial geometry. As expected,
the amidinate nitrogen-lanthanum bonds (2.491(4)-
2.609(4) Å) are substantially shorter than the pyridine
nitrogen-lanthanum bonds (2.747(4) and 2.753(4) Å).
Although numerous lanthanide amidinate species are
(16) Scholz, J .; Rehbaum, F.; Thiele, K. H.; Goddard, R.; Betz, P.;
Kruger, C. J . Organomet. Chem. 1993, 443, 93.
(17) Walther, D.; Fischer, R.; Gorls, H.; Koch, J .; Schweder, B. J .
Organomet. Chem. 1996, 508, 13.

5364 Organometallics, Vol. 18, No. 25, 1999
Kincaid et al.
the metal complexes or to provide additional NMR
handles within the ligand framework.
Overall, the pyridine-substituted amidine described
here appears to be a versatile ligand, forming a wide
range of metal mono- and bis-ligand derivatives via
alkyl or amine elimination reactions. The choice of
amidine (2a or b) allows for a range of solubility and/or
crystallinity of the final product: for example, whereas
both ligand systems yielded highly crystalline magne-
sium salts (3a or b), the p-tolyl-substituted amidine had
to be employed in order to isolate a crystalline lantha-
num derivative. The amidine ligand appears to prefer
meridional coordination as evidenced by the structures
of the Mg, Zr, and Al derivatives; however, as seen in
the crystal structure of the lanthanum species, (LMe)2-
La[N(SiMe3)2] (6), when the metal center is sufficiently
crowded, the ligand can adopt a more facial geometry.
The reaction chemistry of the metal complexes described
here is the subject of ongoing work.
F igu r e 4. ORTEP view of (LtBu)Zr(CH2Ph)3 (5) drawn with
50% probability ellipsoids.
Exp er im en ta l Section
Gen er al Con sider ation s. Standard Schlenk-line and glove-
box techniques were used unless stated otherwise.21 Diethyl
ether, hexanes, dichloromethane, and tetrahydrofuran were
purified by passage through a column of activated alumina
and degassed with argon.22 C6D6 and CDCl3 were vacuum
transferred from sodium/benzophenone and CaH2, respec-
tively. p-Toluoyl chloride, p-tBu-benzoyl chloride, PCl5, Bu2-
Mg, and AlMe3 were purchased from Aldrich and used as
received. Triethylamine was purchased from Aldrich and
distilled from Na. Aniline and 3,5-dimethylaniline were pur-
chased from Aldrich and distilled from CaH2. 2-Aminoethylpy-
26
ridine,23 Zr(CH2Ph)4,24,25 and La[N(SiMe3)2]3 were prepared
according to literature procedures. Melting points were deter-
mined in sealed capillary tubes under nitrogen and are
uncorrected. 1H NMR spectra were recorded at ambient
temperature unless otherwise stated; chemical shifts are given
relative to C6D5H (7.15 ppm) or CHCl3 (7.24 ppm). IR samples
were prepared as Nujol mulls and taken between KBr plates.
Elemental analyses and mass spectral data were determined
at the College of Chemistry, University of California, Berkeley.
In some cases, we routinely experienced carbon analyses that
gave lower than expected values (ca. 1-2%). In these cases,
samples from different preparations were analyzed and more
forcing conditions were employed in the analyzer (longer
combustion times, addition of catalyst). Without exception, the
data were reproducible, and thus we suspect that formation
of refractory carbides is responsible. Single-crystal X-ray
structure determinations were performed at CHEXRAY, Uni-
versity of California, Berkeley.
F igu r e 5. ORTEP view of (LMe)2La[N(SiMe3)2] (6) drawn
with 50% probability ellipsoids. Only the ipso carbons of
the p-tolyl and phenyl groups are shown for clarity. Silyl
methyl groups of the amide have also been removed.
known, there are no lanthanum derivatives for com-
parison, nor has the corresponding pentamethyl-
cyclopentadienyl complex, Cp*2La[N(SiMe3)2],19 been
characterized crystallographically. Nonetheless, the
lanthanum-bis(trimethylsilyl)amide bond length of
2.453(4) Å may be compared to a La-N distance of
2.323(10) Å in Cp*2LaNHCH3(NH2CH3)19 and those in
{La[N(SiMe3)2]3}(PPh3O), which range from 2.38(2) to
2.41(2) Å.20
2-P y-(CH2)2NHCO(p-RP h ) (R ) Me (1a ); tBu (1b)). This
reaction was performed without the exclusion of air. For 1b:
To a solution of p-tBu-benzoyl chloride (24.7 mL, 126 mmol)
and triethylamine (19.3 mL, 139 mmol) in dichloromethane
at 0 °C was slowly added 2-aminoethylpyridine (15.0 mL, 126
mmol) via syringe, generating a yellow precipitate. The
mixture was allowed to warm to room temperature and then
brought to reflux overnight. The dark yellow solution was
cooled to room temperature and the solvent removed under
vacuum. After trituration with water, the yellow solid was
washed with hexanes (2 × 50 mL) and vacuum-dried overnight
to yield a white solid (30.8 g, 86%). 1a was prepared in a
similar fashion. (96%). 1a : Mp: 110-112 °C. 1H NMR (CDCl3,
Su m m a r y a n d Con clu sion s
The synthesis of an amidine ligand with a pendant
pyridine functionality was easily accomplished by utiliz-
ing known synthetic procedures for the generation of
amidines. The synthetic route employed here is attrac-
tive in that the intermediate amide and the resulting
amidine can both be produced in relatively high yield
in multigram quantities. Additionally, the amidines are
easily functionalized to either increase the solubility of
3
300 MHz): δ 8.55 (d, 1H, 6-pyH, J H-H ) 6.6 Hz), 7.68 (m,
(18) Hagadorn, J . R.; Arnold, J . J . Chem. Soc., Dalton Trans. 1997,
3087.
(19) Gagne, M. R.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1992,
114, 275.
(20) Bradley, D. C.; Ghotra, J . S.; Hart, F. A.; Hursthouse, M. B.;
Raithby, P. R. J . Chem. Soc., Dalton Trans. 1977, 1166.
(21) Shriver, E. F.; Bredzon, M. A. The Manipulation of Air-Sensitive
Compounds; J ohn Wiley & Sons: New York, 1986.
(22) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J . Organometallics 1996, 15, 1518.
(23) Magnus, G.; Levine, R. J . Am. Chem. Soc. 1956, 78, 4127.

Tridentate, Six-Electron-Donor Amidinate Ligand
Organometallics, Vol. 18, No. 25, 1999 5365
3
3
3
1H), 7.65 (m, 1H), 7.61 (t, 1H, J H-H ) 4.8 Hz), 7.49 (br s, 1H,
2H, 6-pyH, J H-H ) 4.0 Hz), 7.30 and 7.15 (AB d, 8H, J H-H
)
3
t
NH), 7.21-7.13 (m, 4H), 3.84 (q, 2H, CH2, J H-H ) 5.4 Hz),
8.2 Hz, Bu-Ph), 6.80 (s, 4H, o-3,5-dimethylphenylH), 6.77 (t,
3
3
3.08 (t, 2H, CH2, J H-H ) 5.4 Hz), 2.37 (s, 3H, p-MePh). IR
2H, 5-pyH, J H-H ) 5.7 Hz), 6.44 (s, 2H, p-3,5-dimethylphen-
(cm-1): 1649 (s), 1613 (m), 1593 (m), 1567 (m), 1542 (s), 1504
(s), 1463 (s), 1437 (s), 1424 (m), 1377 (s), 1351 (m), 1318 (s),
1302 (s), 1256 (m), 1200 (m), 1188 (m), 1153 (m), 1102 (w),
1055 (w), 1000 (m), 901 (w), 873 (m), 841 (s), 790 (m), 767 (s),
755 (s), 728 (s), 659(s), 632 (m), 565 (w), 517 (w), 491 (w), 462
(w), 409 (w). 1b: Mp: 105-108 °C. 1H NMR (C6D6, 300
ylH), 6.35 (m, 4H), 3.84 (t, 4H, CH2, J H-H ) 6.3 Hz), 2.97 (t,
3
3
4H, CH2, J H-H ) 6.3 Hz), 2.20 (s, 12H, 3,5-Me2Ph), 1.13 (s,
18H, p-tBu-Ph). IR (cm-1): 1611 (s), 1590 (s), 1526 (s), 1463
(s), 1377 (m), 1317 (m), 1297 (w), 1134 (w), 994 (w), 880 (w),
837 (m), 765 (w), 701 (w). Anal. Calcd for C55H67N6Mg: C,
78.97; H, 8.07; N, 10.05. Found: C, 78.78; H, 8.50; N, 9.79.
[2-P y-(CH2)2NC(p-tBu P h )N(3,5-Me2C6H3]AlMe2 (4). To a
-78 °C solution of 3b (500 mg, 1.30 mmol) in ether (20 mL)
was added AlMe3 (1.55 mL of a 0.86 M hexane solution, 1.30
mmol) slowly via syringe. The reaction mixture was left to
warm to room temperature with stirring. The solvent was
removed under vacuum and the yellow solid dissolved in CH2-
Cl2. Layering with an equal volume of THF afforded yellow
3
MHz): δ 8.35 (d, 1H, J H-H ) 3.2 Hz), 7.93 and 7.21 (AB d,
4H, tBu-Ph, 3J H-H ) 3.9 Hz), 7.59 (br s, 1H, NH), 6.98 (t, 1H,
3J H-H ) 3.6 Hz), 6.64 (d, 1H, 3-pyH, J H-H ) 8.0 Hz), 6.58 (m,
3
1H), 3.87 (q, 2H, CH2, 3J H-H ) 3.9 Hz), 2.85 (t, 2H, CH2, 3J H-H
) 3.9 Hz), 1.14 (s, 9H, p-tBuPh). IR (cm-1): 1629 (s), 1588 (w),
1542 (s), 1501 (m), 1377 (m), 1322 (m), 1271 (m), 1152 (w),
1107 (w), 1050 (w), 991 (w), 852 (w), 772 (w), 750 (w), 722 (w).
1
crystals of 4 (460 mg, 80% yield). Mp: 225 °C (dec). H NMR
2-P y-(CH2)2NHC(p-RP h )NR′ (R ) Me, R′ ) P h (2a ); R
) tBu , R′ ) 3,5-Dim eth ylp h en yl (2b)). For 2b: HCl (86 mL
of a 1.85 M ether solution, 160 mmol) was added to a
dichloromethane solution of 1b (30.8 g, 110 mmol) and left to
stir at room temperature for 45 min. PCl5 (27.3 g, 131 mmol)
was added to the solution followed by 3,5-dimethylaniline (40.8
mL, 327 mmol), which turned the yellow solution red upon
contact. After 30 min, the slurry was heated to reflux and
stirring was continued overnight. The dark red solution was
cooled to room temperature and filtered away from a yellow
precipitate. The solvent was removed and the resulting red
solid triturated with an aqueous solution of 2 M KOH until
the pH had reached ∼12. The solid was extracted into ether
and washed with 2 M KOH (3 × 150 mL), brine (3 × 150 mL),
and 2 M KOH (3 × 150 mL). The ether layer was dried over
magnesium sulfate and the solvent concentrated to form pale
yellow crystals. Further product was obtained by cooling the
solution to -30 °C overnight (26 g, 63%). 2a was prepared
analogously (52%). 2a : Mp: 121-124 °C. 1H NMR (C6D6, 300
3
(C6D6, 300 MHz): δ 8.61 (d, 1H, 6-pyH, J H-H ) 3.3 Hz), 7.29
and 7.14 (AB d, 4H, 3J H-H ) 5.1 Hz, tBu-Ph), 6.83 (s, 2H, o-3,5-
3
dimethylphenylH), 6.77 (t, 1H, 5-pyH, J H-H ) 4.2 Hz), 6.50
(s, 1H, p-3,5-dimethylphenylH), 6.43 (t, 1H, 4-pyH, J H-H
3
)
3
4.2 Hz), 6.29 (d, 1H, 3-pyH, J H-H ) 4.2 Hz), 3.35 (t, 2H, CH2,
3J H-H ) 3.6 Hz), 2.49 (t, 2H, CH2, J H-H ) 3.6 Hz), 2.09 (s,
3
6H, 3,5-Me2Ph), 1.13 (s, 9H, p-tBu-Ph), 0.08 (s, 6H, AlMe2). IR
(cm-1): 1608 (m), 1597 (m), 1570 (m), 1477 (s), 1450 (s), 1424
(sh), 1360 (m), 1340 (m), 1260 (w), 1179 (w), 1167 (w), 1159
(w), 1135 (w), 1120 (w), 1103 (w), 1021 (m), 850 (m), 799 (w),
767 (m), 755(m), 709 (m), 692 (m), 665 (m). Anal. Calcd. for
C28H36N3Al: C, 76.16; H, 8.22; N, 9.52. Found: C, 75.85; H,
8.59; N, 9.51.
[2-P y-(CH2)2NC(p-tBu P h )N(3,5-Me2C6H3]Zr (CH2P h )3 (5).
A diethyl ether solution (100 mL) of Zr(CH2Ph)4 (5.00 g, 10.5
mmol) was added via cannula to a solution of 2b (4.07 g, 10.5
mmol) in Et2O (750 mL) to generate a red solution and a yellow
precipitate. After stirring at room-temperature overnight, the
mixture was filtered and the resultant yellow powder dried
in vacuo. Concentration of the filtrate and cooling to -30 °C
induced further crystallization (yield 5.75 g, 75%). Mp: 165
°C (dec). 1H NMR (C6D6, 300 MHz): δ 8.69 (d, 1H, 6-pyH, 3J H-H
) 4.2 Hz), 7.22-7.10 (m, 15H), 6.96-6.88 (m, 6H), 6.70 (t, 1H,
3
MHz): δ 8.51 (d, 1H, 6-pyH, J H-H ) 4.4 Hz), 7.09 (m, 2H),
3
6.97 (m, 4H), 6.80 (t, 1H, 5-pyH, J H-H ) 7.2 Hz), 6.74 (m,
3
4H), 6.55 (t, 1H, 4-pyH, J H-H ) 7.2 Hz), 5.41 (br s, 1H, NH),
3.99 (br q, 2H, CH2), 2.98 (br t, 2H, CH2), 1.99 (s, 3H, p-MePh).
IR (cm-1): 3274 (m), 1621 (s), 1590 (m), 1541 (s), 1335 (s), 1330
(m), 1300 (m), 1142 (m), 824 (w), 722 (w). Anal. Calcd for
3
5-pyH, J H-H ) 6.0 Hz), 6.56 (s, 1H, p-3,5-dimethylphenylH),
3
3
C
21H21N3: C, 80.22; H, 6.41; N, 13.36. Found: C, 79.98; H,
6.30 (t, 1H, 4-pyH, J H-H ) 6.0 Hz), 6.15 (d, 1H, 3-pyH, J H-H
6.76; N, 13.16. 2b: Mp: 113-115 °C. 1H NMR (C6D6, 300
) 7.8 Hz), 2.90 (s, 6H, CH2Ph), 2.43 (t, 2H, CH2, J H-H ) 5.7
3
3
MHz): δ 8.31 (d, 1H, 6-pyH, J H-H ) 3.9 Hz), 7.29 (br d, 1H,
3-pyH), 7.00 (m, 3H), 6.72 (m, 2H), 6.68 (s, 2H, o-3,5-
dimethylphenylH), 6.55 (t, 1H, 4-pyH, J H-H ) 3.9 Hz), 6.48
(s, 1H, p-3,5-dimethylphenylH), 5.38 (br s, 1H, NH), 4.03 (br
q, 2H, CH2), 3.04 (br t, 2H, CH2), 2.09 (s, 6H, 3,5-Me2Ph), 1.04
(s, 9H, p-tBu-Ph). IR (cm-1): 3249 (w), 1611 (s), 1590 (s), 1526
(s), 1463 (s), 1377 (m), 1317 (m), 1297 (w), 1134 (w), 994 (w),
880 (w), 837 (m), 765 (w), 701 (w). Anal. Calcd for C26H31N3:
C, 81.00; H, 8.10; N, 10.90. Found: C, 80.75; H, 8.02; N, 10.96.
Hz), 2.12 (s, 6H, 3,5-Me2Ph), 1.52 (t, 2H, CH2, 3J H-H ) 5.7 Hz),
1.07 (s, 9H, p-tBu-Ph). IR (cm-1): 1606 (m), 1590 (s), 1565 (w),
1423 (m), 1376 (w), 1345 (m), 1314 (m), 1205 (s), 1118 (m),
1023 (m), 964 (s), 951 (m), 917 (s), 885 (w), 849 (w), 836 (m),
793 (m), 743 (s), 697 (s), 629 (w), 539 (w), 516 (w). Anal. Calcd
for C47H51N3Zr: C, 75.35; H, 6.86; N, 5.61. Found: C, 72.39;
H, 6.41; N, 5.65.
3
[2-P y-(CH2)2NC(p-MeP h )NP h ]2La[N(SiMe3)2] (6). 2a (1.00
g, 3.18 mmol) in ether (30 mL) was added dropwise via cannula
to an ethereal solution (20 mL) of La[N(SiMe3)2]3 (983 mg, 1.59
mmol) maintained at -78 °C. The solution was left to warm
to room temperature with stirring. The volume was reduced
and the reaction solution layered with hexanes. Upon standing
overnight, pale yellow crystals of 6 formed (603 mg, 42% yield).
[2-P y-(CH2)2NC(p-RP h )NR′]2Mg (R ) Me, R′ ) P h (3a );
R ) tBu , R′ ) 3,5-Dim eth ylp h en yl (3b)). For 3b: To a THF
solution of 2b (4.95 g, 12.8 mmol) was slowly added Bu2Mg
(6.7 mL of a 0.965 M heptane solution, 6.5 mmol) via syringe,
generating an orange solution. After stirring overnight, the
solvent was removed in vacuo and the resulting yellow powder
extracted with hexanes. Yellow crystals of 3b(hexanes)0.5 were
obtained upon cooling to -30 °C overnight (4.61 g, 91% yield).
3a (THF) was synthesized similarly and crystallized directly
from the reaction solvent (74% yield). 3a : Mp: 200-202 °C.
1
Mp: 220 °C (dec). H NMR (C6D6, 300 MHz): δ 9.61 (d, 2H,
3
3
6-pyH, J H-H ) 4.2 Hz), 7.07 and 6.66 (AB d, 8H, J H-H ) 7.8
Hz, Me-Ph), 6.76 (m, xH), 6.50 (t, 2H, 3-pyH, 3J H-H ) 7.2 Hz),
6.30 (d, 2H, 2-pyH, 3J H-H ) 7.2 Hz), 3.27 (br s, 4H, CH2), 2.95
(br s, 4H, CH2), 1.88 (s, 6H, p-MePh), 0.60 (s, 18H, N(SiMe3)2).
IR (cm-1): 1600 (m), 1591 (m), 1566 (m), 1347 (m), 1327 (m),
1314 (m), 1261 (m), 1236 (m), 1179 (w), 1152 (w), 1103 (w),
988 (s), 870 (m), 824 (s), 768 (w), 735 (w), 695 (w), 662 (w),
597 (w). Anal. Calcd for C48H58LaN7Si2: C, 62.12; H, 6.30; N,
10.56. Found: C, 60.08; H, 5.86; N, 10.16.
Gen er a l P r oced u r es for X-r a y Cr ysta llogr a p h y. Per-
tinent details for the individual compounds can be found in
Table 6. A crystal of appropriate size was mounted on a glass
capillary using Paratone-N hydrocarbon oil. The crystal was
1H NMR (C6D6, 300 MHz): δ 8.57 (d, 2H, 6-pyH, J H-H ) 4.0
Hz), 7.27 and 6.87 (AB d, 8H, Me-Ph, J H-H ) 7.8 Hz), 7.14-
3
3
7.08 (m, 6H), 6.79 (m, 6H), 6.37-6.39 (m, 4H), 3.78 (br t, 4H,
CH2), 3.57 (br t, 8H, OCH2CH2), 2.89 (br t, 4H, CH2), 1.99 (s,
6H, p-MePh), 1.41 (br t, 8H, OCH2CH2). IR (cm-1): 1614 (m),
1584 (m), 1534 (m), 1295 (w), 1254 (w), 1144 (w), 1132 (w),
818 (w), 759 (w), 716 (m). Anal. Calcd for C46H48MgN6O: C,
76.18; H, 6.67; N, 11.59. Found: C, 73.29; H, 6.34; N, 12.02.
1
3b: Mp: 185-188 °C. H NMR (C6D6, 300 MHz): δ 8.70 (d,

5366 Organometallics, Vol. 18, No. 25, 1999
Kincaid et al.
transferred to a Siemens SMART diffractometer/CCD area
detector,27 centered in the beam, and cooled by a nitrogen flow
low-temperature apparatus which had been previously cali-
brated by a thermocouple placed at the same position as the
crystal. Preliminary orientation matrix and cell constants were
determined by collection of 60 10-s frames, followed by spot
integration and least-squares refinement. A hemisphere of
data was collected, and the raw data were then integrated (XY
spot spread ) 1.60°; Z spot spread ) 0.60°) using SAINT.28
Cell dimensions reported in Table 6 were calculated from all
reflections with I > 10σ. Data analysis and absorption correc-
tion were performed using Siemens XPREP29 and SADABS.
The data were corrected for Lorentz and polarization effects,
but no correction for crystal decay was applied. The reflections
measured were averaged. The structures were solved and
refined with the teXsan software package30 using direct
methods31 and expanded using Fourier techniques. All non-
hydrogen atoms were refined anisotropically, unless stated
otherwise. Hydrogen atoms were assigned idealized positions
and were included in structure factor calculations, but were
not refined, unless stated otherwise. The final residuals were
refined against the data for which F2 > 3σ(F2). The quantity
minimized by the least-squares program was ∑w(|Fo| - |Fc|)2,
where w is the weight of a given observation. The p factor,
used to reduce the weight of intense reflections, was set to
0.03 throughout the refinement. The analytical forms of the
scattering factor tables for the neutral atoms were used, and
all scattering factors were corrected for both the real and
imaginary components of anomalous dispersion.
Ack n ow led gm en t. We thank the NSF for the
award of a predoctoral fellowship to G.D.W. and the
DOE for support of this work.
(24) Zucchini, U.; Giannini, U.; Albizatti, E.; D’Angelo, R. Chem.
Commun. 1969, 1174.
(25) Zucchini, U.; Giannini, U.; Albizzati, E. J . Organomet. Chem.
1971, 26, 357.
(26) Evans, W. J .; Golden, R. E.; Ziller, J . W. Inorg. Chem. 1991,
30, 4963.
(27) SMART Area-Detector Software Package; Madison, WI, 1993.
(28) SAINT: SAX Area-Detector Integration Program; Madison, WI,
1995.
Su p p or tin g In for m a tion Ava ila ble: Tables of positional
and thermal parameters and bond distances and angles for
all crystallographically characterized compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM990365W
(29) XPREP: Part of the SHELXTL Crystal Structure Determina-
tion Package; Madison, WI, 1995.
(30) TeXsan: Crystal Structure Analysis Package; Molecular Struc-
ture Corporation: The Woodlands, TX, 1992.
(31) SIR92: Altomare, A.; Burla, M. C.; Camilli, M.; Cascarano, M.;
Giacovazzo, C.; Guagliardi, A.; Polidori, G. J . Appl. Crystallogr. 1993,
26, 343.