Mono- versus bis-chelate formation in triazenide and amidinate complexes of magnesium and zinc

Article abstract of DOI:10.1021/ic701061q

Magnesium and zinc complexes of the monoanionic ligands N,N′-bis(2,6-di-isopropylphenyl)triazenide, L¹, N,N′-bis(2,6-di-isopropylphenyl)acetamidinate, L², and N,N′-bis(2,6-di-isopropylphenyl)tert-butylamidinate, L³, have b

Full text of DOI:10.1021/ic701061q

Inorg. Chem. 2007, 46, 9988−9997
Mono- versus Bis-chelate Formation in Triazenide and Amidinate
Complexes of Magnesium and Zinc
Nonsee Nimitsiriwat,† Vernon C. Gibson,*,† Edward L. Marshall,*,† Pittaya Takolpuckdee,†
Atanas K. Tomov,† Andrew J. P. White,† David J. Williams,† Mark R. J. Elsegood,‡ and
Sophie H. Dale‡
Department of Chemistry, Imperial College London, South Kensington Campus,
London SW7 2AZ, U.K., and Chemistry Department, Loughborough UniVersity,
Loughborough LE11 3TU, U.K.
Received May 31, 2007
Magnesium and zinc complexes of the monoanionic ligands N,N
bis(2,6-di-isopropylphenyl)acetamidinate, L2, and N,N -bis(2,6-di-isopropylphenyl)tert-butylamidinate, L3, have been
′ ′-
-bis(2,6-di-isopropylphenyl)triazenide, L1, N,N
′
synthesized, but only L3 possesses sufficient steric bulk to prevent bis-chelation. Hence, the reaction of L1H with
excess ZnEt2 leads to the isolation of (L1)2Zn, 1; L1H also reacts with Bu2Mg in Et2O to afford (L1)2Mg(Et2O), 2.
Similar reactivity is observed for L2H, leading to the formation of (L2)2Zn, 3, and (L2)2Mg, 4. The reaction of L2H
with ZnR2 may also afford the tetranuclear aggregates
the tert-butylamidinate ligand was found to exclusively promote mono-chelation, allowing (L3)ZnCl(THF), 7, [(L3)-
{ }2O, 5 (R ) Me) and 6 (R ) Et). By contrast,
(L2)Zn2R2
Zn(µ
-Cl)]2, 8, (L3)ZnN(SiMe3)2, 9, (L3)MgiPr(Et2O), 10, and (L3)MgiPr(THF), 11, to be isolated. X-ray crystallographic
analyses of 1, 2, 3, 4, 5, 6, 8, and 10 indicate that the capacity of L3 to resist bis-chelation is due to greater
occupation of the metal coordination sphere by the N-aryl substituents.
Introduction
the copolymerization of epoxides with CO2 (zinc,30-47
magnesium48). Accordingly, monoanionic ligand families
A key component of homogeneous polymerization cata-
lysts of the general formula LMX is an ancillary ligand (L),
which possesses sufficient steric bulk to withstand bis-
chelation, as the formation of L2M may represent a deactiva-
tion pathway.1,2 Ideally, however, L should be small enough
so as to not overly obstruct the approach of the monomer to
the active metal site. The identification of appropriate
ligand-metal combinations, which successfully balance
ligand size with metal accessibility, should therefore lead to
new generations of highly active, yet well-behaved, living
polymerization initiators. One example of much contempo-
rary interest is the stabilization of divalent metal centers for
the ring-opening polymerization of cyclic esters (zinc,3-16
magnesium,4,6-11,17-20 calcium,21-23 tin,24-27 and iron28), the
living polymerization of methacrylates (magnesium29) and
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* To whom correspondence should be addressed. E-mail: v.gibson@
imperial.ac.uk (V.C.G.), e.marshall@imperial.ac.uk (E.L.M.).
† Imperial College London.
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‡ Loughborough University.
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9988 Inorganic Chemistry, Vol. 46, No. 23, 2007
10.1021/ic701061q CCC: $37.00
© 2007 American Chemical Society
Published on Web 10/10/2007

Mono- Wersus Bis-chelate Formation
Scheme 1. Selected Monoanionic Ligand Families (from Left to
Right: â-Diketiminates, Salicylaldiminates, Aminotroponiminates,
Amidinates, and Triazenides)
Scheme 2. Ligands Employed in This Study
such as â-diketiminates,49 salicylaldiminates,50 and ami-
notroponiminates51 (Scheme 1) have all been employed in
recent times to good effect.
Amidinate ligands have also acquired renewed appeal,
mainly as a result of the development of simple synthetic
routes to sterically bulky variants.52-64 Of particular relevance
to the study described below, Jordan et al. have shown that
the presence of a sizable substituent on the central carbon
atom forces the two nitrogen substituents to project further
forward, thus improving steric protection of the metal.65 By
contrast, the triazenide family,66 [RNNNR]-, has received
far less recent attention. As noted by Gantzel and Walsh,67
triazenide ligands are expected to donate less electron density
to metal centers than amidinates, an important consideration
in the design of Lewis acidic catalysts for coordinative-
insertion polymerizations.
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Here, we describe the synthesis and characterization of a
series of magnesium and zinc complexes of 2,6-di-isopro-
pylanilido-based triazenide (L1, Scheme 2) and amidinate
(L2 and L3) ligands. Our results demonstrate how finely
balanced the factors influencing mono- versus bis-chelation
can be and should aid the future rational development of
single-site, mono-chelated catalysts.
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Results and Discussion
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Inorganic Chemistry, Vol. 46, No. 23, 2007 9989

Nimitsiriwat et al.
Scheme 3. Postulated Solution State Isomers of
N,N′-Bis(2,6-di-isopropylphenyl)triazene, L1H
Scheme 4
Figure 1. Molecular structure of 1 (50% probability ellipsoids). All of
the isopropyl groups have been omitted for clarity.
Table 1. Selected Bond Lengths (Angstroms) and Angles (Degrees)
for 1
Scheme 5
Zn(1)-N(1)
Zn(1)-N(4)
N(1)-N(2)
N(4)-N(5)
2.033(5)
2.019(5)
1.313(6)
1.317(7)
Zn(1)-N(3)
Zn(1)-N(6)
N(2)-N(3)
N(5)-N(6)
2.068(5)
2.089(5)
1.321(7)
1.302(6)
N(1)-Zn(1)-N(3)
N(1)-Zn(1)-N(6)
N(3)-Zn(1)-N(6)
N(1)-N(2)-N(3)
Zn(1)-N(1)-C(1)
62.30(19)
N(1)-Zn(1)-N(4)
132.61(19)
145.5(2)
62.05(19)
107.9(5)
149.11(19) N(3)-Zn(1)-N(4)
124.34(19) N(4)-Zn(1)-N(6)
107.3(4)
147.6(4)
N(4)-N(5)-N(6)
Zn(1)-N(3)-C(13) 147.4(4)
Zn(1)-N(6)-C(37) 146.6(4)
Zn(1)-N(4)-C(25) 146.3(4)
ature (selected bond data are collated in Table 1). For reasons
of clarity, the isopropyl substituents are not shown in Figure
1 (the complete structure may be viewed in Figure S1 within
the Supporting Information). Although triazenide-1-oxide
zinc complexes have been well studied, triazenide analogues
are much less common,71-73 and to the best of our knowl-
edge, 1 is the first example of a structurally characterized
zinc bis(triazenide) complex.
The zinc atom in 1 is distorted severely from an idealized
tetrahedral geometry due to the acute chelate angle of the
triazenide, with angles at the metal ranging from 62.05(19)
[N(4)-Zn(1)-N(6)] to 149.11(19)° [N(1)-Zn(1)-N(6)].
The molecular structure shares many of the gross features
previously described for a zinc bis(benzamidinate), [{PhC-
(NSiMe3)2}2Zn],74 and two zinc bis(guanidinate)s [{(Me3-
Si)2NC(NR)2}2Zn] (R ) iPr, Cy).75 Both N3Zn units approach
planarity [to within 0.064 and 0.119 Å for the N(2) and N(5)-
containing ligands respectively], with the angle between the
two planes ca. 65.3°. The Zn-N bond lengths fall within a
narrow range [2.019(5)-2.089(5) Å], and the anionic charges
isolation of the triazene in 60% yield following recrystalli-
zation from nitromethane. H NMR spectroscopy revealed
the triazene to exist in solution as a mixture of isomers, in
a ratio of ca. 7:1, with the major component believed to be
the E isomer on steric grounds64 and by comparison to the
analogous acetamidine.62
All of our attempts to form mono-chelate zinc and
magnesium alkyls led to the isolation instead of the respective
bis(triazenide) complexes. Thus, the room temperature reac-
tion of L1H with ZnEt2 in toluene resulted in the formation
of (L1)2Zn, 1 (Scheme 4), even if an excess of ZnEt2 was
used (3.0 equiv). Similar attempts to produce a mono-
(triazenide) magnesium alkyl complex from the reaction of
L1H with n/sBu2Mg in Et2O afforded a five-coordinate
etherate, [N(NAr)2]2Mg(Et2O), 2 (Scheme 5), the solvent
remaining intact even after recrystallization from heptane.
We note that Walsh and co-workers70 have previously
synthesized a six-coordinate counterpart, [N(N-p-tolyl)2]2Mg-
(THF)2, the lower bulk of the p-tolyl groups allowing two
solvent molecules to bind to the metal.
1
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9990 Inorganic Chemistry, Vol. 46, No. 23, 2007

Mono- Wersus Bis-chelate Formation
Scheme 6. Selected Literature Examples of Structurally Characterized
Zinc Amidinate Complexes
several studies have highlighted the importance of the
substituent on the central carbon atom of amidinate ligands:
as this group increases in size, so the N-substituents are
increasingly repelled, leading to heightened steric protection
of the metal center. We therefore anticipated that the
acetamidinate ligand L2 would afford greater steric protection
to metals than the triazenide L1, and the chances of obtaining
mono-chelate species would be improved.
Although amidines have been extensively employed as
ancillary ligands in transition metal, lanthanide, and main-
group metal complexation chemistry, only a relatively small
number of amidinate complexes of zinc(II) and magnesium-
(II) have been reported. The known amidinate zinc(II)
complexes are either bis-chelates53,76 or oxygenated tetra-
nuclear zinc aggregates77,78 (Scheme 6). By contrast,
several mono-chelated amidinate magnesium(II) complexes
have been synthesized previously, including [ArC(NiPr)2]-
MgI(THF)2 (Ar ) 2,6-(4-tBu-C6H4)2C6H3),54 [tBuC(NAr)2]-
MgCp(THF)n (Ar ) 2,6-iPr2C6H3, n ) 0 and Ar )
2,4,6-Me3C6H2, n ) 1)63 and [nBuC(NCy)2]Mg{η1-N(Ar)P-
nBuPh}(Et2O) (Ar ) 2,6-iPr2C6H3)79 and the form-
amidinates [{HC(NPh)2Mg(THF)}2(µ-Cl)2(µ-THF)]80 and
[{HC(NAr)2}Mg(µ-Cl)(THF)]2 (Ar
although bis-chelate counterparts are again more
common.53,54,59,63,82-86
Despite the expected increase in steric protection, all of
our attempts to isolate mono-chelated zinc(II) and magne-
sium(II) complexes of L2 were unsuccessful; instead the
major components of all of the product mixtures were the
bis(amidinate) complexes (L2)2Zn, 3, and (L2)2Mg, 4. For
example, lithiation of L2H and the subsequent addition to
ZnCl2 in toluene, or the direct reaction of L2H with
Figure 2. Molecular structure of the C2-symmetric 2.
Table 2. Selected Bond Lengths (Angstroms) and Angles (Degrees)
for 2
Mg-N(1)
2.1174(12) Mg-N(3)
2.0545(18) Mg-N(1A)
2.1568(12) N(1)-N(2)
1.3045(16)
2.1568(12)
2.1174(12)
1.3105(17)
Mg-O(30)
Mg-N(3A)
N(2)-N(3)
N(1)-Mg-N(3)
N(1)-Mg-N(1A)
N(3)-Mg-O(30)
N(3)-Mg-N(3A)
60.22(5)
138.03(8)
98.17(4)
163.66(8)
N(1)-Mg-O(30)
110.98(4)
113.26(5)
113.26(5)
110.98(4)
N(1)-Mg-N(3A)
N(3)-Mg-N(1A)
O(30)-Mg-N(1A)
O(30)-Mg-N(3A) 98.17(4)
N(1A)-Mg-N(3A) 60.22(5)
N(1)-N(2)-N(3)
Mg-N(3)-C(16)
N(2)-N(3)-C(16)
110.20(12) Mg-N(1)-C(4)
149.52(10) N(2)-N(1)-C(4)
115.61(12)
151.74(10)
112.66(12)
)
2,6-iPr2C6H3),81
of both triazenide ligands are delocalized over the N3
backbones with four similar N-N bond lengths observed
[N(1)-N(2) ) 1.313(6), N(2)-N(3) ) 1.321(7), N(4)-N(5)
) 1.317(7), N(5)-N(6) ) 1.302(6) Å].
Diffraction quality crystals of 2 were isolated from a
saturated heptane solution at room temperature; the molecular
structure is shown in Figure 2, and selected bond parameters
are listed in Table 2. The five-coordinate metal adopts a
highly distorted square-pyramidal geometry with the mag-
nesium displaced by ca. 0.53 Å in the direction of the etherate
oxygen from the best-fit plane through N(1), N(3), N(1A),
and N(3A) (the four basal nitrogen atoms being coplanar to
ca. 0.23 Å). Angles between the oxygen atom and the four-
coordinated nitrogens range from 98.17(4) to 110.98(4)°. The
triazenide fragments exhibit unremarkable bond distances and
angles, with the N-N backbone bond lengths [1.3045(16)-
1.3105(17) Å] and the chelate bite angle [N(1)-Mg-N(3)
) 60.22(5)°] being comparable to those observed in the
structure of 1.
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(80) Cotton, F. A.; Haefner, S. C.; Matonic, J. H.; Wang, X.; Murillo, C.
A. Polyhedron 1997, 16, 541-550.
(81) Andrews, P. C.; Brym, M.; Jones, C.; Junk, P. C.; Kloth, M. Inorg.
Chim. Acta 2006, 359, 355-363.
(82) Westerhausen, M.; Hausen, H. D. Z. Anorg. Allg. Chem. 1992, 615,
27-34.
Syntheses of Acetamidinate Complexes. The inability
of the triazenide ligand to stabilize mono-chelates renders it
an unsuitable support for divalent magnesium- and zinc-based
single-site catalysts. To determine how large the ancillary
ligand must be to suppress bis-chelation, we next examined
the acetamidinate ligand L2. Although the direct comparison
of triazenide and amidinate ligands has, to the best of our
knowledge, not been investigated previously in this manner,
(83) 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, 957-963.
(84) Sadique, A. R.; Heeg, M. J.; Winter, C. H. Inorg. Chem. 2001, 40,
6349-6355.
(85) Kincaid, K.; Gerlach, C. P.; Giesbrecht, G. R.; Hagadorn, J. R.;
Whitener, G. D.; Shafir, A.; Arnold, J. Organometallics 1999, 18,
5360-5366.
(86) Walther, D.; Gebhardt, P.; Fischer, R.; Kreher, U.; Go¨rls, H. Inorg.
Chim. Acta 1998, 281, 181-189.
Inorganic Chemistry, Vol. 46, No. 23, 2007 9991

Nimitsiriwat et al.
Figure 3. Molecular structure of 3 (50% probability ellipsoids). All of
the isopropyl groups have been omitted for clarity.
Figure 4. Molecular structure of one (I) of the two independent complexes
present in the crystals of 4.
Table 3. Selected Bond Lengths (Angstroms) and Angles (Degrees)
for 3
distortion from tetrahedral coordination attributed to the
minimization of steric clashes between the N-aryl substitu-
ents. The magnesium atom lies in the planes of both NCN
ligand cores to within 0.04 Å for molecule 4-I [0.03 Å for
4-II], and both C(2) and C(32) exhibit sp2 trigonal planar
geometries; the two NCN planes are inclined by ca. 54° for
4-I, and ca. 55° for 4-II. Delocalization of the anionic charge
over the NCN amidinate backbones results in similar bond
lengths for N(1)-C(2) and C(2)-N(3) [1.328(6), 1.321(6)
for 4-I; 1.322(6) and 1.327(6) Å for 4-II], and for N(31)-
C(32) and C(32)-N(33) [1.317(5), 1.342(6) for 4-I; 1.334-
(6), 1.323(6) Å for 4-II]. The four Mg-N bond lengths fall
within a narrow range of 2.039(4)-2.046(4) for 4-I [2.039-
(4)-2.059(4) Å for 4-II], and the chelate rings have acute
N-Mg-N angles [N(1)-Mg-N(3) ) 65.35(15) and N(31)-
Mg-N(33) ) 65.78(15) for 4-I; 65.97(16) and 66.07(16)°
respectively for molecule 4-II], consistent with values
previously reported for other bis(amidinate) magnesium
complexes.53,54,63,82-86
We have also found that the reaction of L2H with excess
quantities of freshly purchased solutions of ZnMe2 or ZnEt2
leads to the clean formation of 3. However, when older
batches of the dialkylzinc reagents were used, alternative
products were obtained, containing both amidinate and alkyl
ligands in a 1:2 ratio (1H NMR). The eventual formulation
of these products as {(L2)Zn2R2}2O (R ) Me, 5; R ) Et, 6)
was facilitated by X-ray crystallographic analyses. As these
two compounds are approximately isostructural, only the
structure of 5 is described here (bond parameters and a full
discussion of the molecular structure of 6 may be found in
the Supporting Information). Both complexes are composed
of a four-coordinate central oxygen ion surrounded solely
by a tetrahedral arrangement of zinc centers (Figure 5). Each
metal atom bears a single alkyl ligand, and both amidinate
ligands bind two zinc ions in a µ-η1:η1 bridging mode. We
have not attempted to identify either the precise conditions
required for the syntheses of 5 or 6 or the source of the
oxygen atom. We do, however, note that the sensitivity of
amidinate zinc complexes to moisture (and the oxophilic
Zn(1)-N(1)
N(1)-C(1)
C(1)-C(2)
2.031(5)
1.328(7)
1.499(7)
Zn(1)-N(2)
N(2)-C(1)
2.038(4)
1.325(7)
N(1)-Zn(1)-N(2)
N(1)-Zn(1)-N(2A)
N(2)-Zn(1)-N(2A)
Zn(1)-N(1)-C(1)
Zn(1)-N(1)-C(3)
N(1)-C(1)-C(2)
C(1)-N(1)-C(3)
C(1)‚‚‚Zn(1)‚‚‚C(1A)
65.86(18)
124.2(2)
147.5(3)
90.7(3)
144.8(4)
123.8(5)
123.1(5)
179.5(3)
N(1)-Zn(1)-N(1A)
N(1)-C(1)-N(2)
Zn(1)-N(2)-C(1)
Zn(1)-N(2)-C(15)
N(2)-C(1)-C(2)
C(1)-N(2)-C(15)
Zn(1)‚‚‚C(1)‚‚‚C(2)
148.2(2)
113.0(4)
90.4(3)
141.8(7)
123.1(5)
123.4(8)
179.4(5)
Zn[N(SiMe3)2]2, resulted in a mixture of 3 and the appropriate
mono-chelated zinc(II) halide or amide complex, and at-
tempts to recrystallize these species repeatedly led to the
fractional precipitation of 3. Similarly, the reaction of L2H
and iPrMgCl in toluene afforded 4, with a minor component
of the crude product mixture tentatively assigned as the
desired mono-chelate (L2)MgCl on the basis of H NMR
spectroscopy.
1
Crystals of 3 were grown by slow cooling of a heptane
solution from 60 °C to room temperature. As shown in Figure
3, complex 3 is structurally similar to 1 with the two L1
ligands coordinated in a chelating η2-fashion, affording a
distorted tetrahedral geometry about the zinc atom (angles
at the metal ranging from 65.86(18) to 148.2(2)°, Table 3)
(the positions of the isopropyl substituents may be viewed
in Figure S3). The four atoms in each ZnN2C unit are
perfectly planar, and the mean Zn-N bond distance (2.035
Å) is similar to that observed in the structure of 1 (2.052
Å). As in the bis(triazenide) counterpart, the monoanionic
charge is delocalized over the amidinate NCN backbones,
as shown by the near-identical C(1)-N(1) and C(1)-N(2)
bond distances [1.325(7) and 1.328(7) Å].
The solid-state structure of the magnesium counterpart 4
revealed the presence of two crystallographically independent
molecules (4-I and 4-II) with essentially identical geometries
(Figure 4 and Table 4 in this paper and Figures S4-S7 in
the Supporting Information). The bond angles at magnesium
range from 65.35(15) to 151.56(17)° for molecule 4-I [65.97-
(16) to 151.35(18)° for molecule 4-II], with considerable
9992 Inorganic Chemistry, Vol. 46, No. 23, 2007

Mono- Wersus Bis-chelate Formation
Table 4. Selected Bond Lengths (Angstroms) and Angles (Degrees) for Both Independent Complexes (4-I and 4-II) Present in the Crystals of 4
4-I
4-II
4-I
4-II
Mg-N(1)
2.046(4)
2.039(4)
1.328(6)
1.490(6)
1.342(6)
2.044(4)
2.059(4)
1.322(6)
1.508(7)
1.323(6)
Mg-N(3)
2.046(4)
2.043(4)
1.321(6)
1.317(5)
1.482(6)
2.054(4)
2.039(4)
1.327(6)
1.334(6)
1.509(6)
Mg-N(31)
N(1)-C(2)
C(2)-C(4)
C(32)-N(33)
Mg-N(33)
C(2)-N(3)
N(31)-C(32)
C(32)-C(34)
N(1)-Mg-N(3)
N(1)-Mg-N(33)
N(3)-Mg-N(33)
N(1)-C(2)-N(3)
Mg-N(1)-C(2)
Mg-N(31)-C(32)
N(3)-C(2)-C(4)
N(31)-C(32)-C(34)
Mg-N(1)-C(5)
65.35(15)
151.56(17)
120.96(16)
113.1(4)
90.7(2)
65.97(16)
151.35(18)
121.61(18)
114.7(4)
89.9(3)
N(1)-Mg-N(31)
N(3)-Mg-N(31)
N(31)-Mg-N(33)
N(1)-C(2)-C(4)
Mg-N(3)-C(2)
Mg-N(33)-C(32)
N(31)-C(32)-N(33)
N(33)-C(32)-C(34)
Mg-N(3)-C(17)
Mg-N(33)-C(47)
C(2)-N(3)-C(17)
C(32)-N(33)-C(47)
123.71(16)
151.35(17)
65.78(15)
123.4(4)
90.9(3)
122.73(18)
150.5(2)
66.07(16)
123.0(4)
89.4(3)
91.1(3)
89.1(3)
90.2(2)
90.3(3)
123.5(4)
123.7(4)
147.3(3)
147.6(3)
121.8(4)
121.2(4)
122.3(4)
121.7(4)
147.4(3)
148.1(4)
122.3(4)
122.2(4)
112.9(4)
123.3(4)
147.0(3)
146.7(3)
121.7(4)
122.4(4)
114.5(4)
123.8(4)
148.3(4)
146.5(3)
121.6(4)
122.5(4)
Mg-N(31)-C(35)
C(2)-N(1)-C(5)
C(32)-N(31)-C(35)
Table 5. Selected Bond Lengths (Angstroms) and Angles (Degrees)
for 5
nature of zinc clusters78) has been previously documented
in reports concerning the formation of tetranuclear zinc
clusters of the type shown in Scheme 6.
Zn(1)-N(1)
Zn(3)-N(3)
Zn(1)-O(1)
Zn(3)-O(1)
N(1)-C(1)
N(3)-C(27)
Zn(1)-C(53)
Zn(3)-C(55)
1.9625(17) Zn(2)-N(2)
1.9672(17) Zn(4)-N(4)
1.9500(14) Zn(2)-O(1)
1.9489(13) Zn(4)-O(1)
1.9743(18)
1.9731(16)
1.9510(14)
1.9531(14)
1.332(3)
1.334(2)
1.959(3)
1.956(2)
In 5 (Figures 5 and S8 in the Supporting Information),
the bridging oxygen adopts a distorted tetrahedral geometry
with the angles made by its bonds to the four zinc atoms
ranging from 102.43(6) to 112.68(7)° (Table 5). Each zinc
center exhibits a distorted trigonal-planar conformation, with
the N-Zn-C angles significantly wider than the N-Zn-O
angles (ca. 130 and 108°, respectively). The six atoms of
both of the Zn2N2CO rings are planar to within 0.061 [Zn-
(1), Zn(2)] and 0.085 Å [Zn(3), Zn(4)]. The bond lengths of
C(1)-N(1) and C(1)-N(2) are the same (ca. 1.33 Å),
indicating that the anionic charge is again delocalized over
the ligand backbone. The four Zn-O bond lengths are also
essentially the same (ca. 1.95 Å) and are similar to those of
[HC(NPh)2]6Zn4O,77 [HC(N-p-CH3C6H4)2]6Zn4O,78 and the
triazenide complex [N(NPh)2]6Zn4O.72
1.333(2)
1.332(2)
1.956(3)
1.958(3)
N(2)-C(1)
N(4)-C(27)
Zn(2)-C(54)
Zn(4)-C(56)
Zn(1)-O(1)-Zn(2)
Zn(1)-O(1)-Zn(4)
Zn(2)-O(1)-Zn(4)
N(1)-Zn(1)-O(1)
N(3)-Zn(3)-O(1)
112.68(7)
110.87(7)
108.58(7)
110.43(6)
110.58(6)
Zn(1)-O(1)-Zn(3)
Zn(2)-O(1)-Zn(3)
Zn(3)-O(1)-Zn(4)
N(2)-Zn(2)-O(1)
N(4)-Zn(4)-O(1)
102.43(6)
110.02(7)
112.21(7)
109.32(6)
109.49(6)
N(1)-Zn(1)-C(53) 128.53(10) N(2)-Zn(2)-C(54) 129.04(11)
N(3)-Zn(3)-C(55) 129.93(10) N(4)-Zn(4)-C(56) 130.80(10)
N(1)-C(1)-N(2)
120.87(18) N(3)-C(27)-N(4)
120.84(18)
C(53)-Zn(1)-O(1) 120.97(10) C(54)-Zn(2)-O(1) 121.61(11)
C(55)-Zn(3)-O(1) 119.42(10) C(56)-Zn(4)-O(1) 119.68(10)
N(1)-C(1)-C(2)
119.71(17) N(2)-C(1)-C(2)
119.41(17)
N(3)-C(27)-C(28) 119.95(17) N(4)-C(27)-C(28) 119.21(17)
C(1)-N(1)-C(3)
118.60(17) C(1)-N(2)-C(15)
118.32(17)
C(27)-N(3)-C(29) 119.31(16) C(27)-N(4)-C(41) 117.77(16)
Synthesis of tert-Butylamidinate Complexes. As ob-
served for L1, the acetamidinate ligand L2 evidently possesses
insufficient bulk to act as a suitable ancillary ligand for
single-site zinc or magnesium-based catalysts. In the final
phase of this study, we therefore increased the steric
protection further by switching our attention to the
t-butylamidinate analogue L3.63 In contrast with the behavior
described above, lithiation of L3H followed by reaction with
ZnCl2 in THF yielded (L3)ZnCl(THF), 7. Recrystallization
from toluene resulted in the loss of the coordinated solvent
to form the chloro-bridged dimeric species, [(L3)Zn(µ-Cl)]2,
8. In addition, (L3)ZnN(SiMe3)2, 9, can be prepared from
the reaction of 8 with KN(SiMe3)2. Side-formation of the
bis(chelate) (L3)2Zn was not observed in any of these
reactions, and 7-9 are the first reported examples of mono-
(η2-amidinate) zinc(II) complexes (Scheme 7).
Crystals of 8 suitable for X-ray diffraction were grown
by allowing a saturated toluene solution to stand at room
temperature. Crystallographic analysis revealed that this
complex exists as a chloride-bridged dimer in the solid state
(Figure 6), with the two halves of the molecule being related
by a center of inversion. The central Zn2Cl2 core is coplanar
and is slightly asymmetric, with each zinc having one Zn-
Cl interaction slightly longer than the other [Zn(1)-Cl(1A)
) 2.2928(4), Zn(1)-Cl(1) ) 2.3120(5) Å, Table 6]. Both
of the zinc atoms are coplanar to within 0.026 Å of the
amidinate NCN planes, which are orientated in an ap-
proximately orthogonal fashion to the central Zn2Cl2 core.
Figure 5. Molecular structure of 5 (50% probability ellipsoids). All of
the isopropyl groups have been omitted for clarity (see also Figure S8).
Inorganic Chemistry, Vol. 46, No. 23, 2007 9993

Nimitsiriwat et al.
Figure 6. Centrosymmetric molecular structure of 8 (50% probability
ellipsoids).
Figure 7. Molecular structure of 10.
Scheme 7. Synthesis of 7-9a
Scheme 8
Table 7. Selected Bond Lengths (Angstroms) and Angles (Degrees)
for 10
Mg-N(1)
2.1089(14)
2.156(2)
1.3425(18)
1.561(2)
Mg-N(3)
Mg-O(35)
C(2)-N(3)
2.0908(14)
2.0740(16)
1.3399(19)
Mg-C(32)
N(1)-C(2)
C(2)-C(16)
a (i) nBuLi, ZnCl2, THF; (ii) recrystallize from toluene; (iii) KN(SiMe3)2.
Table 6. Selected Bond Lengths (Angstroms) and Angles (Degrees)
for 8
N(1)-Mg-N(3)
N(1)-Mg-O(35)
N(3)-Mg-O(35)
N(1)-C(2)-N(3)
N(3)-C(2)-C(16)
Mg-N(3)-C(2)
Mg-N(3)-C(20)
C(2)-N(3)-C(20)
63.58(5)
N(1)-Mg-C(32)
N(3)-Mg-C(32)
C(32)-Mg-O(35)
N(1)-C(2)-C(16)
Mg-N(1)-C(2)
Mg-N(1)-C(4)
C(2)-N(1)-C(4)
128.70(8)
127.13(8)
107.16(9)
124.13(13)
90.93(9)
116.00(7)
107.90(7)
111.13(12)
123.38(12)
91.79(9)
Zn(1)-N(1)
Zn(1)-Cl(1)
C(1)-N(1)
C(1)-C(2)
1.9973(12) Zn(1)-N(2)
2.3120(5)
1.3479(18) C(1)-N(2)
1.540(2)
1.9984(12)
2.2928(4)
1.3386(18)
Zn(1)-Cl(1A)
140.34(10)
128.37(12)
138.64(11)
129.57(13)
N(1)-Zn(1)-N(2)
66.49(5)
N(1)-Zn(1)-Cl(1)
N(2)-Zn(1)-Cl(1)
123.44(4)
122.23(4)
Cl(1)-Zn(1)-Cl(1A) 94.552(16)
N(1)-Zn(1)-Cl(1A) 127.34(4)
N(2)-Zn(1)-Cl(1A) 125.15(4)
Pr as a solvent-free,88 three-coordinate compound and reflects
the smaller steric demands of the amidinate ligand family.
Crystals of 10 suitable for X-ray diffraction were grown
by allowing a saturated heptane solution to stand at room
temperature for several days. As shown in Figure 7, 10 exists
as a four-coordinate magnesium complex with one molecule
of diethyl ether coordinated to the metal, affording a highly
distorted tetrahedral geometry about the magnesium atom
with the range of angles about magnesium being 63.58(5)
to 128.70(8)° (Table 7). The core of the amidinate ligand is
close to planar, with the sum of the angles around N(1), N(3),
and C(2) being 359.64, 360.00, and 358.64°, respectively,
and the magnesium atom lies ca. 0.54 Å out of the amidinate
NCN chelate plane. Both Mg-N bond lengths [2.0908(14),
2.1089(14) Å] and the N(1)-Mg-N(3) angle [63.58(5)°]
are comparable to those found in the solid-state structure of
C(1)-N(1)-Zn(1)
C(1)-N(1)-C(6)
C(2)-C(1)-N(1)
Zn(1)-N(1)-C(6)
92.01(9)
C(1)-N(2)-Zn(1)
92.25(9)
133.40(13)
126.51(13)
134.22(10)
132.74(12) C(1)-N(2)-C(18)
124.24(13) C(2)-C(1)-N(2)
135.24(10) Zn(1)-N(2)-C(18)
Opposing aryl rings thus adopt near-parallel orientations, thus
i
minimizing steric interactions between the Pr substituents:
a similar conformation has been reported previously for the
related â-diiminate complex, [(HC{C(Me)NAr}2)Zn(µ-F)]2.87
Mono-chelate magnesium complexes have also been
isolated using the tert-butylamidinate ligand. Thus, as
i
outlined in Scheme 8, treatment of L3Li with PrMgCl in
Et2O or THF affords (L3)MgiPr(L), 10 (L ) Et2O) and 11
(L ) THF). This observation contrasts with aspects of our
previous work on magnesium â-diketiminate complexes,
specifically the isolation of [HC(CMeN-2,6-iPr2C6H3)2]Mgi-
(87) Hao, H.; Cui, C.; Roesky, H. W.; Bai, G.; Schmidt, H.-G.; Noltemeyer,
(88) Dove, A. P.; Gibson, V. C.; Marshall, E. L.; White, A. J. P.; Williams,
M. Chem. Commun. 2001, 1118-1119.
D. J. Dalton Trans. 2004, 570-578.
9994 Inorganic Chemistry, Vol. 46, No. 23, 2007

Mono- Wersus Bis-chelate Formation
Table 8. M-N-Cipso Bond Angles (Degrees) for 1-4, 8, and 10
Conclusion
complex
1, [N(NAr)2]Zn
2, [N(NAr)2]Mg(Et2O)
3, [MeC(NAr)2]Zn
M-N-aryl Cipso bond angles
By studying three structurally related ligands, we have
confirmed that suitable functionalization of the central carbon
atom allows the steric bulk of an amidinate ligand to be finely
adjusted to favor mono- over bis-chelation. Although the
influence of the amidinate carbon substituent has been
described in previous reports,65 its role in preventing bis-
chelate formation, a key factor in the design of amidinate-
supported catalytic sites for polymerization chemistry, has
not been fully appreciated prior to this work.
146.6(4), 146.3(4), 147.4(4), 147.6(4)
151.74(10), 149.52(10)
141.8(7), 144.8(4)
147.3(3), 147.0(3), 147.6(3), 146.7(3)
147.4(3), 148.3(4), 148.1(4), 146.5(3)
135.24(10), 134.22(10)
4-I, [MeC(NAr)2]Mg
4-II, [MeC(NAr)2]Mg
8, {[tBuC(NAr)2]Zn(µ-Cl)}2
10, [tBuC(NAr)2]Mg(iPr)(Et2O) 140.34(10), 138.64(11)
Scheme 9
These studies further suggest that the 2,6-di-isopropylphe-
nyl substituent is of insufficient size to allow the ready
isolation of mono(triazenide) divalent metal complexes.
Ligand L1 will therefore probably be more suited to the
stabilization of higher oxidation state metals. Indeed, we have
previously disclosed that group 4 derivatives of the type (L1)2-
MX2 (M ) Ti, Zr89) act as olefin polymerization catalysts.90
Experimental Section
(L2)2Mg, 4. However, the average C-N-Cipso angle of
128.97° is notably larger than that in 4 (122.05°), an effect
attributed to the steric demand of the bulkier tBu substituent
on the central carbon of the NCN unit.
All manipulations of air- and/or moisture-sensitive compounds
were performed under an atmosphere of dry nitrogen gas using
standard high vacuum Schlenk and cannula techniques or in an
inert atmosphere glovebox. NMR spectra were recorded on a Bruker
DRX-400 spectrometer (1H and 13C at 400.129 and 100.613 MHz
respectively). Elemental analyses were performed by the microana-
lytical services of the Chemistry Department of London Metro-
politan University.
Heptane and toluene were dried by passing through a column of
commercially available Q-5 catalyst (13% copper(I) oxide on Al2O3)
and activated Al2O3 (pellets, 3 mm) under a stream of dry nitrogen
gas and were then stored over a potassium mirror prior to use. THF
and CH2Cl2 were distilled from potassium and CaH2 respectively
and stored over activated 3 Å molecular sieves. The amidines62,63
were synthesized in accordance with literature procedures and all
of the other reagents were used as supplied. Bu2Mg is a com-
mercially available reagent (Aldrich) containing equimolar quanti-
ties of n-butyl and sec-butyl alkyl groups.91
N,N′-Bis(2,6-di-isopropylphenyl)triazene, L1H. Isoamylnitrite
(13.42 cm3, 1.0 × 10-1 mol) was added over 90 min to a cooled (0
°C) solution of distilled 2,6-di-isopropylaniline (9.41 cm3, 5.0 ×
10-2 mol) in 100 cm3 Et2O. The reaction was then stirred at room
temperature overnight (18 h). Volatiles were removed under reduced
pressure at room temperature (the product is thermally sensitive)
to afford an oil, which was dissolved in the minimum amount of
nitromethane and chilled to -20 °C. The crystalline material thus
obtained was filtered cold and washed successively with small
quantities of cold nitromethane until it turned white. After drying
in vacuo at 35 °C, 5.5 g (1.5 × 10-2 mol, 60%) product was
obtained, which was stored at -20 °C. The compound exists as a
7:1 mixture of isomers. Anal. Calcd for C24H35N3: C 78.85, H 9.65,
Comparison of the Steric Properties of the Three
Ligands. Our results clearly show that, while the tert-
butylamidinate ligand is capable of supporting magnesium
and zinc mono-chelate derivative chemistry, the correspond-
ing acetamidinate and triazenide are not. To quantify the
steric protection afforded by each of these three ligands to a
metal center, Table 8 summarizes the M-N-Cipso bond
angles (Scheme 9) measured for 8 and 10. With the caveat
that the N-aryl substituents in the bis-chelates (i.e., 1-4) will
be subject to intramolecular ligand-ligand repulsions, the
angles reported here are nonetheless consistent with the
N-aryl rings being projected increasingly toward the metal,
as the substituent upon the central atom of the triatomic
ligand backbone becomes larger. For example, comparing
data for the isostructural zinc bis-chelates 1 and 3 indicates
that the central MeC unit in the ligand backbone of 3 reduces
the Zn-N-Cipso angles by ca. 4° more than the correspond-
ing triazenide ligand does (average angle ) 147.0 for 1 and
143.3° for 3; see Figures S12 and S13 in the Supporting
Information for overlay diagrams). Although a direct com-
parison between 2 and 4 is complicated by the five-coordinate
nature of the former, a similar effect is nonetheless observed
(average Mg-N-Cipso angles: 150.6 for 2; 147.2 for 4-I;
147.6° for 4-II). Consistently, the structures of the four-
coordinate L3 complexes 8 and 10 exhibit notably smaller
angles (134.7 and 139.5°, respectively). These values com-
pare well with structural data reported for [RC(NR′)2]AlMe2
complexes, where the corresponding angles fall from ca. 144
to ca. 134°, on increasing the R group from methyl to
t-butyl.65 Although the difference between the M-N-Cipso
angles for methyl versus t-butyl backbone substituents may
seem relatively small, it is clear that it is sufficient to favor
mono-chelation over bis-chelation, a crucial factor for
stabilizing well-behaved molecular catalysts.
1
N 11.49. Found C 79.07, H 9.40, N 11.53. H NMR (CDCl3): δ
9.15 (br s, 1H, NH) 7.26-7.03 (m, 4H, Hmeta), 6.84 (d, 2H, Hpara),
3.20 (m, 3.5 H, CHMe2), 2.96 (m, 0.5H, CHMe2), 1.19 (d, 3H,
CH(CH3)2), 1.11 (d, 21 H, CH(CH3)2). 13C NMR (CDCl3, major
isomer resonances only): δ 132.4 (Cipso), 127.1 (Cortho), 123.3
(Cmeta), 118.5 (Cpara), 28.0 (CHMe2), 23.5 (CH(CH3)2).
(89) Guzei, I. A.; Liable-Sands, L. M.; Rheingold, A. L.; Winter, C. H.
Polyhedron 1997, 16, 4017-4022.
(90) Gibson, V. C.; Reardon, D. F.; Tomov, A. K. WO 2004063233.
(91) Duff, A. W.; Hitchcock, P. B.; Lappert, M. F.; Taylor, R. G.; Segal,
J. A. J. Organomet. Chem. 1985, 293, 271-283.
Inorganic Chemistry, Vol. 46, No. 23, 2007 9995

Nimitsiriwat et al.
(L1)2Zn, 1. A suspension of L1H (1.01 g, 2.76 × 10-3 mol) in
20 cm3 toluene was added dropwise to a solution of ZnEt2 (7.5
cm3, 1.1 M in toluene, 8.29 × 10-3 mol, 3.0 equiv), which had
previously been diluted with a further 20 cm3 toluene, and was
chilled to -78 °C. The reaction mixture was then allowed to warm
to ambient temperature and stirred for a total of 18 h before volatile
components were removed under reduced pressure. The crude
product was recrystallized by allowing a saturated pentane solution
to stand overnight (18 h) at -30 °C to give 1 as pale-yellow crystals
(0.598 g, 7.53 × 10-4 mol, 55% yield based on L1H). Crystals
suitable for X-ray crystallography were grown by allowing a
saturated pentane solution to stand at room temperature for several
days. Anal. Calcd for C48H68N6Zn: C 72.57, H 8.63, N 10.58.
to room temperature to afford 4 as colorless crystals (0.761 g, 0.98
mmol, 71% yield based on L2H). Crystals suitable for X-ray
crystallography were grown by slowly cooling a saturated heptane
solution from 70 °C to room temperature. Anal. Calcd for C52H74N4-
Mg: C 80.13, H 9.57, N 7.19. Found C 80.02, H 9.68, N 7.10. 1H
NMR (C6D6): δ 7.06 (m, 12H, C6H3), 3.29 (br sept, 8H, CH(CH3)2),
1.37 (s, 6H, CH3), 1.18 (d, 24H, 3JHH 6.8 Hz, CH(CH3)2), 1.03 (br,
24H, CH(CH3)2). 13C NMR (C6D6): δ 176.81 (NCN), 143.14 (2
+ 6-C6H3), 142.75 (1-C6H3), 124.72 (4-C6H3), 123.30 (3 + 5-C6H3),
28.84 (CH(CH3)2), 23.58 (CH(CH3)2), 14.97 (CH3).
[(L2)Zn2Me2]2O, 5. A solution of L2H (0.503 g, 1.32 mmol) in
toluene (30 mL) was added dropwise to a solution of ZnMe2 (2 M
in toluene, 3.40 mL, 6.61 mmol) in toluene (30 mL) cooled to
-78 °C. The mixture was then stirred for 3 h at room temperature,
after which the volatile components were removed under reduced
pressure. The crude product was recrystallized by cooling a hot
heptane solution from 60 °C to room temperature to afford 5 as
colorless crystals (0.203 g, 0.19 mmol, 29% yield based on L2H).
Crystals suitable for X-ray crystallography were grown by slowly
cooling a hot heptane solution from 60 °C to room temperature.
Despite repeated attempts, it was not possible to obtain satisfactory
1
Found C 72.61, H 8.75, N 10.42. H NMR (C6D6): δ 7.14-7.07
3
(m, 12H, Hmeta, Hpara), 3.48 (sept, 8H, JHH ) 6.9 Hz, CHMe2),
3
1.15 (d, 48H, JHH ) 6.9 Hz, CH(CH3)2). 13C NMR (C6D6): δ
143.80 (Cortho), 141.49 (Cipso), 127.04 (Cpara), 123.59 (Cmeta), 28.87
(CHMe2), 23.76 (CH(CH3)2).
(L1)2Mg(Et2O), 2. A 30 cm3 Et2O solution of L1H (1.010 g,
2.76 × 10-3 mol) was added dropwise to a 10 cm3 Et2O solution
of n/sBu2Mg (2.90 mmol) chilled to -78 °C. The reaction mixture
was stirred for 2 h at room temperature after which time the
volatile components were removed under reduced pressure. The
crude product was recrystallized by allowing a saturated heptane
solution to stand at room temperature for 18 h to give 2 as pale-
yellow crystals (0.657 g, 7.90 × 10-4 mol, 57% yield based on
N[N(2,6-iPr2C6H3)]2H). Crystals suitable for X-ray crystallography
were grown by allowing a saturated heptane solution to stand at
room temperature for several days. Anal. Calcd for C48H68N6Mg:
1
elemental analysis for this complex. H NMR (C6D6): δ 7.10-
7.03 (m, 12H, C6H3), 3.46 (sept, 8H, 3JHH 6.9 Hz, CH(CH3)2), 1.35
(s, 6H, CH3), 1.31 (d, 24H, 3JHH 7.0 Hz, CH(CH3)2), 1.16 (d, 24H,
3JHH 6.9 Hz, CH(CH3)2), -0.47 (s, 12H, ZnCH3). 13C NMR
(C6D6): δ 171.33 (NCN), 143.38 (1-C6H3), 143.26 (2 + 6-C6H3),
126.43 (4-C6H3), 124.17 (3 + 5-C6H3), 28.50 (CH(CH3)2), 24.17
(CH(CH3)2), 23.75 (CH(CH3)2), 19.17 (CH3), -12.59 (ZnCH3).
[(L2)Zn2Et2]2O, 6. A solution of L2H (3.350 g, 8.85 mmol) in
toluene (30 mL) was added dropwise to a solution of ZnEt2 (1.1
M in toluene, 24.00 mL, 26.55 mmol) in toluene (30 mL) cooled
to -78 °C. The mixture was then stirred for 3 h at room
temperature, after which the volatile components were removed
under reduced pressure. The crude product was recrystallized by
cooling a saturated heptane solution from 60 °C to room temperature
to afford 6 as colorless crystals (2.251 g, 1.96 mmol, 44% yield
based on L2H). Crystals suitable for X-ray crystallography were
grown by slowly cooling a saturated heptane solution from 60 °C
to room temperature. Anal. Calcd for C60H94N4Zn4O: C 62.72, H
8.25, N 4.88. Found C 62.61, H 8.12, N 4.84. 1H NMR (C6D6): δ
7.12-7.05 (m, 12H, C6H3), 3.49 (sept, 8H, 3JHH 6.9 Hz, CH(CH3)2),
1
C 76.52, H 9.10, N 11.15. Found C 76.37, H 9.20, N 11.16. H
NMR (C6D6): δ 7.18-7.12 (m, 12H, Hmeta, Hpara), 3.52 (br q, 4H,
3
O(CH2CH3)2), 3.30 (sept, 8H, JHH ) 6.7 Hz, CHMe2), 1.12 (d,
3
3
48H, JHH ) 6.7 Hz, CH(CH3)2), 0.69 (t, 6H, JHH ) 6.9 Hz,
O(CH2CH3)2). 13C NMR (C6D6): δ 144.70 (Cipso), 143.51 (Cortho),
125.96 (Cpara), 123.64 (Cmeta), 64.93 (O(CH2CH3)2), 28.60 (CHMe2),
24.60 (CH(CH3)2), 13.57 (O(CH2CH3)2).
(L2)2Zn, 3. A solution of L2H (0.98 g, 2.60 mmol) in toluene
(20 mL) was added dropwise to a solution of Zn[N(SiMe3)2]2 (1.00
g, 2.60 mmol) in toluene (20 mL) cooled to -78 °C. The reaction
was then stirred for 18 h while warming to room temperature, after
which the volatile components were removed under reduced
pressure. The crude product was recrystallized by cooling a saturated
heptane solution from 60 °C to room temperature to afford 3 as
colorless crystals (0.552 g, 0.67 mmol, 52% yield based on L2H).
Crystals suitable for X-ray crystallography were grown by slowly
cooling a saturated heptane solution from 60 °C to room temper-
ature. Anal. Calcd for C52H72N4Zn: C 76.12, H 9.09, N 6.83. Found
C 76.21, H 9.11, N 6.71. 1H NMR (C7D8): δ 7.15-6.90 (m, 12H,
C6H3), 3.53 (br, 4H, CH(CH3)2), 3.07 (br, 4H, CH(CH3)2), 1.33
(br, 12H, CH(CH3)2), 1.27 (s, 6H, CH3), 1.13 (br, 24H, CH(CH3)2),
0.49 (br, 12H, CH(CH3)2). 13C NMR (C7D8): δ 174.56 (NCN),
143.96 (2 + 6-C6H3), 141.87 (1-C6H3), 124.86 (3 + 5-C6H3), 123.31
(4-C6H3), 28.74 (CH(CH3)2), 23.87 (CH(CH3)2), 22.90 (br, CH-
(CH3)2), 14.16 (CH3).
3
1.34 (d, 24H, JHH 6.9 Hz, CH(CH3)2), 1.33 (s, 6H, CH3), 1.27 (t,
3
3
12H, JHH 8.1 Hz, ZnCH2CH3), 1.15 (d, 24H, JHH 6.9 Hz, CH-
(CH3)2), 0.41 (q, 8H, 3JHH 8.1 Hz, ZnCH2CH3). 13C NMR (C6D6):
δ 171.12 (NCN), 143.71 (1-C6H3), 143.40 (2 + 6-C6H3), 126.44
(4-C6H3), 124.27 (3 + 5-C6H3), 28.50 (CH(CH3)2), 24.40 (CH-
(CH3)2), 24.04 (CH(CH3)2), 19.08 (CH3), 12.89 (ZnCH2CH3), 2.17
(ZnCH2CH3).
[(L3)ZnCl(THF)], 7, and [(L3)Zn(µ-Cl)]2, 8. A solution of L3-
Li (3.030 g, 7.10 mmol) in THF (30 mL) was added dropwise to
a solution of ZnCl2 (0.985 g, 7.23 mmol) in THF (30 mL) cooled
to -78 °C. The mixture was then stirred for 18 h while warming
to room temperature. The colorless solution was filtered, and the
solvent was then removed under reduced pressure. The crude
product, (L3)ZnCl(THF), 7, was recrystallized by cooling a saturated
toluene solution from 70 °C to -30 °C to afford {(L3)Zn(µ-Cl)}2,
8, as colorless crystals (2.526 g, 4.85 mmol, 68% yield). Crystals
suitable for X-ray crystallography were grown by allowing a
saturated toluene solution to stand at room temperature for several
days. Anal. Calcd for C58H86N4Zn2Cl2: C 66.92, H 8.33, N 5.38.
n
(L2)2Mg, 4. A solution of BuLi (2.5 M in hexane, 1.10 mL,
2.76 mmol) was added to a solution of L2H (1.042 g, 2.75 mmol)
in toluene (30 mL) cooled to 0 °C. After being stirred for 3 h at
room temperature, the mixture was cooled to -78 °C and a solution
of iPrMgCl (2.0 M in Et2O) (1.40 mL, 2.80 mmol) was added. The
mixture was then stirred for 18 h while warming to room
temperature. The colorless solution was filtered, and the solvent
was removed under reduced pressure. The crude product was
recrystallized by cooling a saturated heptane solution from 70 °C
1
Found C 67.04, H 8.32, N 5.29. H NMR (C6D6): δ 7.08-6.98
3
(m, 6H, C6H3), 4.06 (sept, 4H, JHH 6.8 Hz, CH(CH3)2), 1.27 (d,
3
3
12H, JHH 6.8 Hz, CH(CH3)2), 1.15 (d, 12H, JHH 6.8 Hz, CH-
9996 Inorganic Chemistry, Vol. 46, No. 23, 2007

Mono- Wersus Bis-chelate Formation
(CH3)2), 0.93 (s, 9H, C(CH3)3). 13C NMR (C6D6): δ 178.89 (NCN),
143.49 (2 + 6-C6H3), 142.18 (1-C6H3), 125.17 (4-C6H3), 123.39
(3 + 5-C6H3), 41.97 (C(CH3)3), 29.85 (C(CH3)3, 28.87 (CHCH3)2),
25.88 (CH(CH3)2), 22.34 (CH(CH3)2).
(q, 4H, 3JHH 7.1 Hz, O(CH2CH3)2), 1.78 (d, 6H, 3JHH 7.8 Hz, MgCH-
(CH3)2), 1.42 (d, 12H, 3JHH 6.9 Hz, CH(CH3)2), 1.26 (d, 12H, 3JHH
3
6.7 Hz, CH(CH3)2), 1.03 (s, 9H, C(CH3)3), 0.71 (t, 6H, JHH 7.1
Hz, O(CH2CH3)2), 0.34 (sept, 1H, 3JHH 7.8 Hz, MgCH(CH3)2). 13
C
Synthesis of (L3)ZnN(SiMe3)2, 9. A solution of KN(SiMe3)2
(0.216 g, 1.28 mmol) in toluene (20 mL) was added dropwise to a
solution of 8 (0.530 g, 1.02 mmol) in toluene (20 mL) cooled to
-78 °C. The mixture was then stirred for 18 h while warming to
room temperature. The colorless solution was filtered, and the
solvent was then removed under reduced pressure. The crude
product was recrystallized by allowing a saturated heptane solution
to stand at -30 °C for several days to afford (L3)ZnN(SiMe3)2, 9,
as colorless crystals (0.273 g, 0.423 mmol, 42% yield). Anal. Calcd
for C35H61N3Si2Zn: C 65.13, H 9.53, N 6.51. Found C 64.99, H
9.50, N 6.53. 1H NMR (C6D6): δ 7.08-7.03 (m, 6H, C6H3), 6.58
(sept, 4H, 3JHH 6.9 Hz, CH(CH3)2), 1.32 (overlapping d, 12H, 3JHH
NMR (C6D6): δ 178.97 (NCN), 145.28 (1-C6H3), 141.94 (2 +
6-C6H3), 123.46 (3 + 5-C6H3), 123.40 (4-C6H3), 66.89 (O(CH2-
CH3)2), 44.71 (C(CH3)3), 31.12 (C(CH3)3), 28.57 (CH(CH3)2), 25.77
(CH(CH3)2), 25.08 (MgCH(CH3)2), 23.07 (CH(CH3)2), 13.62
(O(CH2CH3)2), 10.71 (MgCH(CH3)2).
Synthesis of (L3)MgiPr(THF), 11. 11 was prepared in a similar
manner to that outlined for 10, but using L3Li (1.035 g, 2.41 mmol),
iPrMgCl (2.0 M in THF, 1.21 mL, 2.41 mmol), and THF as a
solvent. The crude product was recrystallized by allowing a
saturated heptane solution to stand at -30 °C for 18 h to afford
(L3)MgiPr(THF), 11, as colorless crystals (0.493 g, 0.882 mmol,
37% yield). Anal. Calcd for C32H80N2Mg(C4H8O): C 77.33, H
10.46, N 5.01. Found C 77.53, H 10.72, N 5.37. 1H NMR (C6D6):
δ 7.15-7.05 (m, 6H, C6H3), 3.68 (sept, 4H, 3JHH 6.9 Hz, CH(CH3)2),
3
6.4 Hz, CH(CH3)2), 1.30 (overlapping d, 12H, JHH 6.4 Hz, CH-
(CH3)2), 0.95 (s, 9H, C(CH3)3), 0.11 (s, 18H, Si(CH3)3). 13C NMR
(C6D6): δ 180.75 (NCN), 142.93 (2 + 6-C6H3), 142.64 (1-C6H3),
125.43 (4-C6H3), 123.70 (3 + 5-C6H3), 41.75 (C(CH3)3), 29.97
(C(CH3)3, 28.89 (CHCH3)2), 25.26 (CH(CH3)2), 22.66 (CH(CH3)2),
5.58 (Si(CH3)3).
3
3.49 (br m, 4H, THF), 1.68 (d, 6H, JHH 7.9 Hz, MgCH(CH3)2),
1.40 (d, 12H, 3JHH 6.9 Hz, CH(CH3)2), 1.25 (d, 12H, 3JHH 6.7 Hz,
CH(CH3)2), 1.12 (br m, 4H, THF), 1.01 (s, 9H, C(CH3)3), 0.45 (sept,
3
1H, JHH 7.8 Hz, MgCH(CH3)2). 13C NMR (C6D6): δ 179.21
Synthesis of (L3)MgiPr(Et2O), 10. A solution of iPrMgCl (2 M
in Et2O, 1.45 mL, 2.90 mmol) was added dropwise to a solution of
L3Li (1.211 g, 2.84 mmol) in diethyl ether (30 mL) cooled to
-78 °C. The mixture was then stirred for 18 h while warming to
room temperature, after which the solvent was removed under
reduced pressure and the product was extracted into toluene (30
mL). After the removal of the solvent, the crude product was
recrystallized by allowing a saturated heptane solution to stand at
room temperature for 18 h to afford (L3)MgiPr(Et2O), 10, as
colorless crystals (0.820 g, 1.46 mmol, 51% yield). Crystals suitable
for X-ray crystallography were grown by allowing a saturated
heptane solution to stand at room temperature for several days.
Despite repeated attempts it was not possible to obtain satisfactory
(NCN), 145.31 (1-C6H3), 142.14 (2 + 6-C6H3), 123.41 (4-C6H3),
123.22 (3 + 5-C6H3), 69.53 (THF), 44.41 (C(CH3)3), 30.89
(C(CH3)3), 28.69 (CH(CH3)2), 26.03 (CH(CH3)2), 25.25 (THF),
24.99 (MgCH(CH3)2), 22.63 (CH(CH3)2), 9.55 (MgCH(CH3)2).
Acknowledgment. This work was supported by scholar-
ships from the Institute for the Promotion of Teaching
Science and Technology (IPST), Thailand (to N.N. and P.T.).
Supporting Information Available: Full details of the crystal-
lographic analysis of 1-6, 8, and 10 (including additional figures)
and CIF files. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
elemental analysis for this complex. H NMR (C6D6): δ 7.15-
7.04 (m, 6H, C6H3), 3.69 (sept, 4H, 3JHH 6.8 Hz, CH(CH3)2), 2.97
IC701061Q
Inorganic Chemistry, Vol. 46, No. 23, 2007 9997