Synthesis and structural characterization of new zinc amidinate complexes

Article abstract of DOI:10.1021/om1008549

The synthesis and reactivity of heteroleptic zinc complexes of the type LZnX (X = Me, I) containing amidinate ligands L of varying steric bulk were investigated. Complexes of the type LZnMe, which were obtained from the homoleptic complexes L₂Z

Full text of DOI:10.1021/om1008549

Organometallics 2010, 29, 6097–6103 6097
DOI: 10.1021/om1008549
Synthesis and Structural Characterization of New Zinc
Amidinate Complexes
†
Sarah Schmidt, Stephan Schulz,* Dieter Blaser, Roland Boese, and Michael Bolte
,†
†
†
‡
€
†Institute of Inorganic Chemistry, University of Duisburg-Essen, 45117 Essen, Germany, and
‡Institute of Inorganic Chemistry, University of Frankfurt, 60438 Frankfurt, Germany
Received September 3, 2010
The synthesis and reactivity of heteroleptic zinc complexes of the type LZnX (X=Me, I) containing
amidinate ligands L of varying steric bulk were investigated. Complexes of the type LZnMe, which were
obtained from the homoleptic complexes L2Zn upon reaction with ZnMe2, react with iodine with subsequent
formation of LZnI. Single-crystal X-ray structures of the amidinate zinc complexes [{MeC(Ni-Pr)2}ZnMe]2,
1, [{MeC(Ni-Pr)2}ZnI]2LiI(OEt)2, 3, [t-BuC(NDipp)2]ZnMe, 5, [t-BuC(NDipp)2]ZnMe(t-BuPy), 6,
[{t-BuC(NDipp)2}Zn( μ-I)]2, 7, and [t-BuC{N(H)Dipp}2][Al{OC(CF3)3}4], 8, are reported.
Introduction
well as in copolymerization reactions of CO2 with epoxides,
respectively.7
Amidines (LH) and amidinates (L-) (L=RC(NR0)2) have
been widely investigated in main group metal, transition metal,
and lanthanide metal chemistry due to their flexible steric and
electronic properties, which can be easily tuned by variation of
the organic substituents (R, R0).1 Moreover, their flexibility to
coordinate either as monodentate two-electron donor (η1) as
well as chelating (η2) or as bridging monodentate ( μ-η1-η1)
four-electron donor2 render these complexes very promising for
technical applications in catalysis,3 often as cyclopentadienyl
replacements, and material sciences.4 Considering these impor-
tant applications, it came to us as a surprise that zinc amidinate
complexes were almost unknown, whereas the corresponding
guanidinate complexes and β-diketiminate complexes have
been studied in more detail. They were shown to be of general
interest for ROP catalysis of lactide5 and ε-caprolactone6 as
Recently, we started to investigate the synthesis of zinc
amidinate complexes in more detail and reported on reac-
tions of ZnMe2 with different carbodiimides C(NR)2 (R=
i-Pr,8 Cy9), which unexpectedly proceeded with formation of
novel cluster-type complexes such as {C[C(NR)2ZnMe]}4
(Figure 1). These are suitable starting reagents for the syn-
thesis of the corresponding halide-substituted complexes
{C[C(Ni-Pr)2ZnX]}4 (X=Cl, Br, I), which were obtained by
Me/halideexchangereaction with AlX3.10 Inaddition, hetero-
and homoleptic zinc complexes of the type [t-BuC(NR)2]ZnX
and [t-BuC(NR)2]2Zn (R = i-Pr, Cy; X = Cl, Br, I) were
synthesized by salt elimination reaction, which represents the
most common synthetic pathway for such complexes. Un-
fortunately, the synthesis of heteroleptic complexes LZnX is
often complicated due to the formation of the corresponding
homoleptic complexes L2Zn.11,12
Herein, we report on a straightforward synthetic route to
heteroleptic zinc amidinate complexes of the type [RC(NR0)2]-
ZnX (X=I, Me), which were expected to be valuable starting
reagents for further reactivity studies. In particular, complexes
containing sterically demanding substituents were expected to
*To whom correspondence should be addressed. Phone: þ49 0201-
1834635. Fax: þ49 0201-1833830. E-mail: stephan.schulz@uni-due.de.
(1) See the following and references cited therein: (a) Edelmann, F. T.
Chem. Soc. Rev. 2009, 38, 2253–2268. (b) Edelmann, F. T. Adv. Organo-
met. Chem. 2008, 57, 133–352. (c) Edelmann, F. T.; Coles, M. P. J. Chem.
Soc., Dalton Trans. 2006, 985–1001. (d) Barker, J.; Kilner, M. Coord.
Chem. Rev. 1994, 133, 219–300.
(2) Further bindings modes see: Cole, M. L. J. Chem. Soc., Chem.
Commun. 2007, 1579–1590.
(7) See the following and references cited therein: (a) Allen, S. D.;
Moore, D. R.; Lobkovsky, E. B.; Coates, G. W. J. Organomet. Chem.
2003, 683, 137–148. (b) Chisholm, M. H.; Gallucci, J. C.; Phomphrai, K.
Inorg. Chem. 2005, 44, 8004–8010. (c) Liu, B. Y.; Tian, C. Y.; Zhanq, L.;
Yan, W. D.; Zhanq, W. J. J. Polym. Sci., Part A: Polym. Chem. 2006, 44,
(3) (a) Coles, M. P.; Swenson, D. C.; Jordan, R. F. Organometallics
1997, 16, 5183–5194. (b) Radzewich, C. E.; Coles, M. P.; Jordan, R. F. J. Am.
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M. P.; Jordan, R. F. J. Am. Chem. Soc. 2000, 122, 274–289. (d) Foley, S. R.;
Zhou, Y.; Yap, G. P. A.; Richeson, D. S. Inorg. Chem. 2000, 39, 924–929.
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2010 American Chemical Society
Published on Web 10/28/2010
pubs.acs.org/Organometallics

6098 Organometallics, Vol. 29, No. 22, 2010
Schmidt et al.
Figure 1. Typical structural motifs of zinc amidinate complexes. Amidinate units containing a delocalized π-electron system are
accentuated.
Scheme 1. Synthesis of [{MeC(Ni-Pr)2}ZnMe]2, 1, and [MeC(Ni-Pr)2]ZnI, 2a
a Amidinate units containing a delocalized π-electron system are accentuated.
be promising starting reagents for the synthesis of Zn-Zn
bonded complexes by reductive coupling reaction as well as for
halide and alkyl abstraction reactions.
in [t-BuC(NCy)2ZnBrLiBr(OEt2)2]11 and [Li(OEt2)2( μ-Cl)2-
Zn{N(Dipp)C(Me)]2CH].13 The C-N bond lengths in 1
(C1-N1 1.301(2), C1-N2 1.367(2) A) vary by almost 0.07 A
due to the differently coordinated N atoms, demonstrating the
π-electrons within the NCN unit of the amidinate ligand to be
rather localized than delocalized. The amidinate unit does not
adopt a planar geometry, as is shown by the sum of the bond
angles, which differ significantly. The three-coordinated carbon
and nitrogen atoms in 3 (C1 359.9°; N1 359.0°) are almost
planar, indicating sp2-hybridized centers, whereas the tetra-
coordinated nitrogen atom (N2 334.5°) shows a significantly
smaller sum of bond angles.
The synthesis of heteroleptic zinc amidinate complexes
LZnX via salt elimination reaction is often accompanied by
the formation of the corresponding homoleptic zinc amidinate
complex L2Zn, which is sometimes difficult to separate. There-
fore we became interested in a more straightforward route
to amidinate zinc halide complexes. Hydrolysis of Li[t-BuC-
(NDipp)2] yielded the free amidine 4, which further reacts with
ZnMe2 with elimination of methane and subsequent formation
of [t-BuC(NDipp)2]ZnMe, 5. 5 was isolated in good yield as
a pale yellow crystalline solid. Reaction of 5 with an equimo-
lar amount of I2 yielded the iodine-bridged dimeric heterolep-
tic complex [{t-BuC(NDipp)}2Zn( μ-I)]2, 7, whereas addition
Results and Discussion
Equimolar amounts of [Me(C(Ni-Pr)2]2Zn and ZnMe2
react at ambient temperature under ligand exchange and quan-
titative formation of the heteroleptic complex [{MeC(Ni-Pr)2}-
ZnMe]2, 1. 1 was then converted by reaction with an equimolar
amount of I2 into [MeC(Ni-Pr)2]ZnI, 2. The ate-complex
[{MeC(Ni-Pr)2}ZnI]2LiI(OEt2)2, 3, was obtained as a bypro-
duct in low yield from an impure charge of [MeC(Ni-Pr)2]2Zn
containing LiI, which was formed by the reaction of Li[MeC-
(NDipp)2] with ZnI2.11,12
1H and 13C NMR spectra of 1 showed resonances due to
the amidinate ligand and the methyl group in the expected
1:1 ratio, whereas 2 showed only the resonances due to the
amidinate ligand.
Single crystals of 1 and 3 were obtained from solutions in
toluene (1) and toluene/Et2O (3), respectively, upon storage
at -30 °C. 1 crystallizes in the tetragonal space group P43212
and 3 in the orthorhombic space group P212121. 1 forms a
centrosymmetric dimer in the solid state with bridging
nitrogen atoms and a center of symmetry in the Zn2N2 ring,
as was previously observed for [{t-BuC(Ni-Pr)2}ZnMe]2.12
The zinc atom in 3 is coordinated by one additional iodine
and [Li(OEt2)2]þ as counterion, as was previously observed
(13) Prust, J.; Hohmeister, H.; Stasch, A.; Roesky, H. W.; Magull, J.;
ꢀ
Alexopoulos, E.; Uson, I.; Schmidt, H.-G.; Noltemeyer, M. Eur. J.
Inorg. Chem. 2002, 8, 2157–2162.

Article
Organometallics, Vol. 29, No. 22, 2010 6099
toluene/Et2O (7) upon storage at -30 °C. 5 and 6 crystallize
in the monoclinic space group P21/c; 7 crystallizes in the
triclinic space group P1.
In contrast to previously reported amidinate complexes,14
which typically showed N,N0-chelating amidinate moieties,
the amidinate ligand in 5 coordinates in a rather unexpected
η1-N,η3-arene fashion, clearly showing the expressed flexibility
of amidinate ligands to adopt different coordination modes. As
a consequence, 5 forms a five-membered ring with a localized
C-N single bond (C1-N1 1.3736(16) A) and a CdN double
bond (C1-N2 1.2961(17) A) within the amidinate moiety. In
contrast, the heteroleptic amidinate complexes [MeC(Ni-
Pr)2]ZnMe (1) and [t-BuC(Ni-Pr)2]ZnMe, which contain steri-
cally less demanding amidinate ligands, show the expected N,
N0-chelating modes.12 Obviously, the η1-N,η3-arene binding
mode reduces the ring strain compared to the N,N0-chelating
mode (four-membered ring). A comparable structural motif was
initially observed for the formamidinate complex [K{η6-Mes)-
NC(H)N(Mes)}{η6-Mes)NC(H)NH(Mes)}], in which the po-
tassium-cation coordinates to a single nitrogen atom and an
additional π-coordinated aryl group.15 Recently this binding
mode was also observed for indium and thallium amidinate
complexes.16 While these findings are reasonable for large and
soft (HSAB principle) metal centers, which rather tend to
increase their coordination numbers, it is rather unexpected for
Zn(II) centers. We therefore performed theoretical calculations
on the η1-N,η3-arene (50) and the N,N0 (500) bonding isomers of
5. Both isomers are local minima of the potential energy surface;
however, the N,N0-coordination mode (500) was found to be
energetically slightly preferred by 1.5 kcal. The central structural
parameters calculated for the η1-N,η3-arene isomer (50), which
are summarized in Table 1, agree very well with the experimental
data (full details are given in the Supporting Information).
Theamidinateligandin7 exhibits a N,N0-chelatingbonding
mode, as was observed for [{t-BuC(NDipp)2}Zn( μ-X)]2 (X =
Cl,17 Br18). 7, which is isomorphous to the corresponding
Cl- and Br-substituted complexes, forms an iodine-bridged
dimer in the solid state with a center of inversion within the
central Zn2I2 ring. In contrast, [{t-BuC(Ni-Pr)2}Zn( μ-I)]212 as
well as 3 form N-bridged dimers, clearly reflecting the role of the
N-substituents. Sterically demanding Dipp groups prevent the
formation of a sterically more hindered N-bridged dimer, which
is typically formed in amidinates with sterically less demand-
ing i-Pr substituents. The Zn-I bond lengths only slightly
differ (Zn1-I1 2.6078, Zn1-I2 2.6153 A), and the C-N bond
lengths within the amidinate moiety (C1-N1 1.348(3), C1-N2
1.328(3) A) are in between typical values for a C-N single
bond and a CdN double bond, indicating an almost perfect
delocalization of the π-electrons within the CN2 backbones
of the four-membered rings. Analogous findings were re-
ported for [{t-BuC(NDipp)2}Zn( μ-X)]2 (X=Cl,17 Br18). In
the base-stabilized complex 6, the Zn atom is electronically
saturated by coordination of the Lewis base tert-butylpyridine.
As a consequence, the coordination mode of the amidinate unit
rearranges from a η1-N,η3-arene fashion to a N,N0-chelating
Figure 2. Solid-state structure of 1 (thermal ellipsoids are shown
at 50% probability levels; H atoms are omitted for clarity). Selected
bond length (A) and angles (deg): C1-N1 1.301(2), C1-N2 1.367(2),
Zn1-N1 2.035(14), Zn1-N2 2.043(14), Zn1-C9 1.969(19), N1-
C1-N2 115.04(19), N1-Zn1-N2 60.22(5).
Figure 3. Solid-state structure of 3 (thermal ellipsoids are shown
at 50% probability levels; H atoms are omitted for clarity). Selected
bond length (A) and angles (deg): C1-N1 1.283(4), C1-N2
1.406(4), C9-N3 1.323(4), Zn1-N1 2.073(3), Zn1-N2 2.125(7),
Zn1-I1 2.545(5), Zn2-I2 2.671(4), Zn2-I3 (2.709(4), N1-C1-
N2 112.40(3), N3-C9-N4 119.40(3), N1-Zn1-N2 64.34(11),
I2-Zn2-I3 99.53(14).
of the Lewis base tert-butylpyridine yielded the expected
monomeric, base-stabilized complex [t-BuC(NDipp)2]Zn-
Me(t-BuPy), 6.
(14) Lesikar, L. A.; Richards, A. F. Polyhedron 2010, 29, 1411–1422.
(15) Baldemus, J.; Berghof, C.; Cole, M. L.; Evans, D. J.; Hey-Hawkins,
E.; Junk, P. C. J. Chem. Soc., Dalton Trans. 2002, 2802–2804.
(16) Jones, C.; Junk, P. C.; Platts, J. A.; Rathmann, D.; Stasch, A.
J. Chem. Soc., Dalton Trans. 2005, 2497–2499.
(17) Nimitsiriwat, N.; Gibson, V. C.; Marshall, E. L.; Takolpckdee,
P.; Tomov, A. T.; White, A. J. P.; Williams, D. J.; Elsegood, M. R. J.;
Dale, S. H. Inorg. Chem. 2007, 46, 9988–9997.
1H and 13C NMR spectra of 5 and 6 show the resonances
due to the amidinate ligand and the methyl group (and the
pyridine base 6) in the expected 1:1 intensities, whereas 7
shows only the resonances of the amidinate ligand. Single
crystals were grown from solutions in toluene (5, 6) and
(18) Jones, C.; Furness, L.; Nembenna, S.; Rose, R. P.; Aldridge, S.;
Stasch, A. J. Chem. Soc., Dalton Trans. 2010, 39, 8788-8795.

6100 Organometallics, Vol. 29, No. 22, 2010
Schmidt et al.
Scheme 2. Synthesis of 5, 6, 7, and 8a
a Conditions: (i) ZnMe2, toluene, (ii) I2, toluene, (iii) tert-butylpyridine, toluene, (iv) [H(OEt2)2][Al{OC(CF3)3}4], CH2Cl2. Amidinate units
containing a delocalized π-electron system are accentuated.
Figure 5. Solid-state structure of 6 (thermal ellipsoids are shown
at 50% probability levels; H atoms are omitted for clarity). Selected
bond length (A) and angles (deg): C1-N1 1.358(3), C1-N2
1.328(3), N1-C2 1.424(3), N2-C14 1.426(3), Zn1-N1 2.003(2),
Zn1-N2 2.0022(19), Zn1-I1 2.6153(3), N1-C1-N2 (109.8(2),
C1-N1-C14 39.840(36), C1-N2-C2 133.0(2), N1-Zn1-N2
Figure 4. Solid-state structure of 5 (thermal ellipsoids are shown
at 50% probability levels; H atoms are omitted for clarity). Selected
66.27(8).
which was recently shown to effectively protonate Cp*2Zn2.20
Even though the formation of methane was observed, the
reaction proceeded with protonation of the amidinate ligand
and subsequent formation of the amidinium salt [t-BuC-
{N(H)Dipp}2][Al{OC(CF3)3}4], 8, rather than with formation
of the expected cationic zinc complex [{t-BuC(NDipp)2}Zn]-
[Al{OC(CF3)3}4]. A comparable finding has been previously
reported for the reaction of the zinc guanidinate with 2,6-di-tert-
butylphenol (HOR),5a even though this reaction occurred
with protonation of only one nitrogen atom and subsequent
formation of the guanidine Me2NC(Ni-Pr)(NHi-Pr), which
bond length (A) and angles (deg): C1-N1 1.3736(16), C1-N2
1.2961(17), N1-C14 1.4387(16), N2-C2 1.4011(17), Zn1-N1
1.9077(11), Zn1-C2 2.4311(12), Zn1-C3 2.7652(14), Zn1-C7
2.7223(13), Zn1-C26 1.9302(14), N1-C1-N2 121.16(12), C1-
N1-C2 120.31(11), C1-N1-C14 125.43(11).
mode (C1-N1 1.3423(16), C1-N2 1.3393(16) A) with almost
perfectly delocalized π-electrons.
Cationic amidinate metal complexes have been investi-
gated in the past due to their capacity to serve as activators
for catalytic reactions.19 We therefore investigated the reac-
tion of 5 with one equivalent of [H(OEt2)2][Al{OC(CF3)3}4],
€
(19) (a) Radzewich, C. E.; Coles, M. P.; Jordan, R. F. J. Am. Chem.
Soc. 1998, 120, 9384–9385. (b) Baugh, L. S.; Sussano, J. A. J. Polym. Sci.
2002, 40, 1663.
(20) Schulz, S.; Schuchmann, D.; Krossing, I.; Himmel, D.; Blaser,
D.; Boese, R. Angew. Chem. 2009, 121, 5859–5862. Angew. Chem., Int.
Ed. 2009, 48, 5748-5741.

Article
Organometallics, Vol. 29, No. 22, 2010 6101
Table 1. Selected Experimental and Calculated Structural Para-
meters of 5 and 50
5
50
Bond length [A]
Zn-C10
Zn-N2
C1-N2
N2-C3
C1-N1
N1-C4
Zn-C4
Zn-C5
Zn1-C6
Zn-C7
Zn-C8
Zn-C9
1.9302(14)
1.9077(11)
1.3736(16)
1.4387(16)
1.2961(17)
1.4011(17)
2.4311(12)
2.7223(13)
3.3046(14)
3.5811(14)
3.3274(16)
2.7652(14)
1.9615
1.9528
1.3749
1.4337
1.3025
1.3889
2.4787
2.779
3.354
3.649
3.424
2.866
Bond Angles (deg)
N1-C1-N2
Zn-N2-C1
C1-N1-C4
C10-Zn-N2
121.16(12)
120.01(9)
120.31(11)
154.48(6)
120.8894
118.7591
123.7574
153.981
Figure 6. Solid-state structure of 7 (thermal ellipsoids are shown
at 50% probability levels; H atoms are omitted for clarity). Selected
bond length (A) and angles (deg): C1-N1 1.3423(16), C1-N2
1.3393(16), N1-C2 1.4223(16), N2-C14 (1.4194(16), Zn1-N1
2.0922(11), Zn1-N2 2.067(11), Zn1-C39 1.9710(15), Zn1-N3
2.1316(12), N1-C1-N2 110.63(11), C1-N1-C2 128.63(11),
C1-N2-C14 130.47(10), N1-Zn1-N2 64.03(4).
Scheme 3
atoms of the amidinate ligand were protonated under these
reaction conditions. Monitoring the reaction by temperature-
dependent NMR spectroscopy starting at -70 °C could not
clarify whether the methyl group or the N atoms are protonated
first. The IR spectrum of 8 showed characteristic absorption
bands of a N-H bond at 3327 and 3290 cm-1
.
Single crystals of 8 were obtained from a solution in
CH2Cl2 at -30 °C. Complex 8 crystallizes in the monoclinic
space group P21. The π-electrons within the NCN backbone
are almost perfectly delocalized (C1-N1 1.326(4), C1-N2
1.331(4) A). The N-C-N bond angle of 116.8(3)° and the
N-Cipso bond distances of 1.452(5) and 1.454(4) A are
similar to those reported for the neutral amidine (N-C-N
119.1(2)°, N-Cipso 1.433(3) A).21
The heteroleptic amidinate zinc halides were expected to
be valuable starting reagents for the synthesis of compounds
containing a central Zn-Zn bond, as was initially reported by
Carmona et al. for dizincocene [Cp*2Zn2].22 Since then, several
complexes containing Zn atoms in the formal oxidation state
þ1 have been prepared, most of them by reductive (Wurtz-
analogous) coupling reaction23 and substitution reaction using
Cp*2Zn2.24 Unfortunately, several attempts to reduce the het-
eroleptic complexes LZnI remained unsuccessful. Reactions of
7 with Na, K, KC8, and Na-naphthalene at different reaction
Figure 7. Solid-state structure of 8 (thermal ellipsoids are shown at
50% probability levels; H atoms are omitted for clarity except for H1
and H2). H1 and H2 were located in the electron difference map.
Selected bond length (A) and angles (deg): C1-N1 1.326(4),
C1-N2 1.331(4), N1-H1 0.873(10), N2-H2 0.885(10), N1-C11
1.452(5), N2-C21 1.454(4), N1-C1-N2 116.8(3), C1-N2-C21
129.2(3), C1-N1-C11 125.8(3), C1-N2-H2 112(3).
coordinates to the zinc alkoxide with formation of [MeZn(OAr)
3
{Me2NC(Ni-Pr)(NHi-Pr)}]. The formation of 8 containing an
amidinium cation clearly shows the improved protonation capa-
bility of [H(OEt2)2][Al{OC(CF3)3}4] compared to the simple
alcohol HOR. The formation of 8 requires three equivalents of
[H(OEt2)2][Al{OC(CF3)3}4], and indeed, when 4 was reacted
with three equivalents of [H(OEt2)2][Al{OC(CF3)3}4], 8 was
formed in almost quantitative yield. The 1H NMR spectrum of
8 showed two new resonances at 7.72 and 7.37 ppm for the N-H
groups. Obviously, both the methyl group and the Lewis-basic N
(21) Xia, A.; El-Kaderi, H. M.; Heeg, M. J.; Winter, C. H. J.
Organomet. Chem. 2003, 682, 224–232.
(22) Resa, I.; Carmona, E.; Gutierrez-Puebla, E.; Monge, A. Science
2004, 305, 1136–1138.

6102 Organometallics, Vol. 29, No. 22, 2010
Schmidt et al.
temperatures (25, 60, 110 °C) and times (2, 4, 12 h) in different
solvents (toluene, THF, hexane) yielded only elemental zinc and
so far unidentified products. Comparable findings have very
recently been reported by Jones et al. for reduction reactions of
[{t-BuC(NDipp)2}Zn( μ-Br)]2.18
formed at the “Elementaranalyse Labor” of the University of
Essen.
[{MeC(Ni-Pr)2}ZnMe]2, 1. ZnMe2 (4.2 mL, 1.2 M in toluene,
5 mmol) was added to a solution of [Me(C(Ni-Pr)2]2Zn (1.86 g, 5
mmol) in 20 mL of toluene at ambient temperature and stirred
for an additional 2 h. The solution was concentrated under
vacuum and stored at -30 °C. Colorless crystals of 1 were
formed within 24 h.
Conclusion
Yield: 2.01 g (91%). Melting point: 76 °C. Anal. Found
(calcd) for C18H40N4Zn2 (443.28 g/mol): H, 9.2 (9.1); C, 49.0
(49.0); N, 12.5 (12.5). 1H NMR (300 MHz, C6D6, 25 °C): δ -0.14
(s, 3H, ZnCH3), 1.10 (d, 3JHH = 6.3 Hz, 12H, CH(CH3)2), 1.53
Heteroleptic zinc amidinate complexes LZnMe (1, 5) were
synthesized in good yields either by ligand exchange reaction
between L2Zn (L = amidinate) and ZnMe2 or by methane
elimination reaction of ZnMe2 with LH and structurally char-
acterized. Unexpectedly, the amidinate ligand in 5 coordinates
in a η1-N,η3-arene mode, as was shown by single-crystal X-ray
diffraction. The corresponding iodine-substituted zinc amidi-
nate complexes (2, 3, 7) were accessible from reaction between 1
and 5, respectively, with iodine. 5 was also found to react with
the Lewis base tert-butylpyridine with formation of the Lewis
acid-base adduct 6, in which the amidinate group adopts an N,
N0-chelating binding mode. Reaction of 5 with [H(OEt2)2][Al-
{OC(CF3)3}4] occurred with protonation of both N atoms and
subsequent formation of complex 8.
3
(s, 3H, CH3), 3.23 (sept, JHH = 6.3 Hz, 2H, CH(CH3)2). 13C
NMR (125 MHz, C6D6, 25 °C): δ -8.6 (ZnCH3), 14.3
(CH(CH3)2), 25.5 (CH3), 48.4 (CH(CH3)2), 171.0 (NCN). IR: ν
2961, 2930, 2896, 2867, 1555, 1509, 1452, 1377, 1361, 1342, 1314,
1287, 1259, 1172, 1135, 1122, 1011, 808, 640, 575, 520, 485, 448
cm-1
.
[MeC(Ni-Pr)2]ZnI, 2. A solution of I2 (1.27 g, 5 mmol) in
20 mL of Et2O was added dropwise to 1 (2.21 g, 5 mmol)
dissolved in 20 mL of Et2O at -30 °C. The resulting mixture
was warmed to ambient temperature and stirred for an additional
12 h. The solution was concentrated under vacuum and stored at
-30 °C. 2 was isolated as a colorless, crystalline solid.
Yield: 1.58 g (95%). Melting point: >220 °C. Anal. Found
(calcd) for C8H17N2IZn (333.51 g/mol): H, 5.2 (5.1); C, 28.8
Experimental Details
1
(28.8); N, 8.3 (8.4). H NMR (300 MHz, C6D6, 25 °C): δ 1.14
3
All manipulations were performed in a glovebox (MBraun)
under Ar atmosphere or using standard Schlenk line techniques.
Dry solvents were obtained from a solvent purification system
(MBraun) and degassed prior to use. A 1.2 M solution of ZnMe2
in toluene and I2 were obtained from Acros and used as received,
whereas tert-butylpyridine (Arcos) was distilled and carefully
dried over activated molecular sieves prior to use. Li amidinates
Li[RC(NR0)2] (R = Me, t-Bu; R0 = i-Pr, Dipp) were generally
prepared by reaction of the carbodiimide with the correspond-
ing organolithium compound,3a and [MeC(Ni-Pr)2]2Zn was pre-
pared by reaction of two equivalents of Li[MeC(Ni-Pr)2] with
ZnI2.8a [H(OEt2)2][Al{OC(CF3)3}4] was prepared according to a
procedure described by Krossing et al.25 A Bruker DMX 300 was
used for NMR spectroscopy. 1H and 13C{1H} NMR spectra were
referenced to internal C6D5H (1H: δ = 7.154; 13C: δ = 128.0),
CD2Cl2 (1H: δ = 5.32; 13C: δ=53.5), and THF-d8 (1H: δ=3.580,
1.730; 13C: δ=25.2, 67.4). IR spectra were recorded on a Bruker
ALPHA-T FT-IR spectrometer equipped with a single reflection
ATR sampling module. Melting points were measured in sealed
capillaries and were not corrected. Elemental analyses were per-
(d, JHH = 6.3 Hz, 12H, CH(CH3)2), 1.40 (s, 3H, CH3), 3.23
(sept, 3JHH = 6.4 Hz, 2H, CH(CH3)2).13C NMR (125 MHz, C6D6,
25 °C): δ 15.3 (CH(CH3)2), 25.0 (CCH3), 48.4 (CH(CH3)2), 176.0
(NCN). IR: ν 3058, 3029, 2963, 2123, 2103, 1649, 1622, 1584, 1544,
1484, 1439, 1320, 1296, 1201, 1168, 1155, 1097, 1070, 1025, 898, 826,
750, 687, 620, 598, 529, 513, 490 cm-1
.
[{MeC(Ni-Pr)2}ZnI]2LiI(OEt)2, 3. 3 was obtained as a by-
product in the synthesis of [MeC(Ni-Pr)2]ZnI, 2. Colorless
crystals of 3 were formed within 24 h in less than 5% yield. In
addition, 3 was synthesized by reaction of 2 with an equimolar
amount of LiI in refluxing Et2O for 24 h. The reaction solution
was filtered, and all volatiles were removed under vacuum,
yielding a colorless solid. Unfortunately, this reaction product
also contained 2 and the homoleptic complex [MeC(Ni-Pr)2]2Zn
to some extent (15-20%), as was shown by 1H NMR spectros-
copy. This finding also explains the unsatisfactory elemental
analysis that was obtained for 3.
Melting point: 80 °C (dec). Anal. Found (calcd) for C24H54N4I3-
LiO2Zn2 (949.09 g/mol): H, 5.3 (5.7); C, 29.1 (30.4); N, 5.3 (5.9). 1H
NMR (300 MHz, C6D6, 25 °C): δ 1.05 (d, 3JHH = 6.0 Hz, 12H,
3
CH(CH3)2), 1.11 (t, 6H, OCH2CH3), 1.26 (d, JHH = 6.7, 12H,
(23) (a) Grirrane, A.; Resa, I.; Rodriguez, A.; Carmona, E.; Alvarez, E.;
Gutierrez-Puebla, E.; Monge, A.; Galindo, A.; del Rıo, D.; Andersen, R. A.
´
CH(CH3)2), 1.82 (s, 3H, CH3), 2.02 (s, 3H, CH3), 3.38 (q, 4H,
OCH2CH3), 3.38 (sept, 2H, CH(CH3)2), 3.76 (sept, 3JHH = 6.5 Hz,
2H, CH(CH3)2). 13C NMR data could not be obtained due to the
very low solubility of 3. IR: ν 2962, 2926, 2890, 2869, 1605, 1496,
1455, 1378, 1362, 1319, 1260, 1174, 1121, 1089, 1060, 1010, 877, 793,
J. Am. Chem. Soc. 2007, 129, 693–703. (b) Wang, Y.; Quillian, B.; Wei, P.;
Wang, H.; Yang, X.-J.; Xie, Y.; King, R. B.; Schleyer, P. v. R.; Schaefer, H. F., III;
Robinson, G. H. J. Am. Chem. Soc. 2005, 127, 11944–11945. (c) Zhu, Z.;
Wright, R. J.; Olmstead, M. M.; Rivard, E.; Brynda, M.; Power, P. P. Angew.
Chem., Int. Ed. 2006, 45, 5807–5810. (d) Zhu, Z.; Brynda, M.; Wright, R. J.;
Fischer, R. C.; Merrill, W. A.; Rivard, E.; Wolf, R.; Fettinger, J. C.; Olmstead,
M. M.;Power, P. P.J. Am. Chem. Soc. 2007, 129, 10847–10857. (e) Yang, X.-J.;
Yu, J.; Liu, Y.; Xie, Y.; Schaefer, H. F., III; Liang, Y.; Wu, B. J. Chem. Soc.,
Chem. Commun. 2007, 2363–2365. (f ) Tsai, Y.-C.; Lu, D.-Y.; Lin, Y.-M.;
Hwang, J.-K.; Yu, J.-S. K. J. Chem. Soc., Chem. Commun. 2007, 4125–4127.
(g) Fedushkin, I. L.; Skatova, A. A.; Ketkov, S. Y.; Eremenko, O. V.; Piskunov,
A. V.; Fukin, G. K. Angew. Chem., Int. Ed. 2007, 46, 4302–4305. (h) Yu, J.;
Yang, X.-J.; Liu, Y.; Pu, Z.; Li, Q.-S.; Xie, Y.; Schaefer, H. F.; Wu, B.
Organometallics 2008, 27, 5800–5805. (i) Yang, P.; Yang, X.-J.; Yu, J.; Liu,
Y.; Zhang, C.; Deng, Y.-H.; Wu, B. J. Chem. Soc., Dalton Trans. 2009, 5773–
5779. ( j) Liu, Y.; Li, S.; Yang, X.-J.; Yang, P.; Gao, J.; Xia, Y.; Wu, B.
Organometallics 2009, 28, 5270–5272.
642, 602, 577, 516, 475 cm-1
.
t-BuC(NHDipp)(NDipp), 4. A solution of Li[t-BuC(NDipp)2]
(2.13 g, 5 mmol) in 30 mLof Et2O was added to a mixture of 80 mL
of Et2O and 20 mL of H2O and vigorously stirred for 30 min. The
organic phase was separated, and the aqueous layer was extracted
with CH2Cl2 (3 ꢀ 50 mL). The combined organic phases were dried
with MgSO4 and filtered, and all volatiles were evaporated under
vacuum, yielding 4 as a colorless, crystalline solid.
Yield: 1.83 g (87%). Melting point: 138 °C. Anal. Found
(calcd) for C29H44N2 (420.68 g/mol): H, 10.3 (10.5); C, 83.0
(82.8); N, 6.7 (6.7). 1H NMR (300 MHz, C6D6, 25 °C): δ 1.35 (d,
3JHH = 6.8Hz, 12H, CH(CH3)2), 1.44(s, 9H, C(CH3)3), 3.40 (sept,
2H, CH(CH3)2), 3.58 (sept, 3JHH = 6.4 Hz, 2H, CH(CH3)2), 5.55
(s, 1H, NH), 7.00-7.44 (m, 6H, ArH). 13C NMR (125 MHz, C6D6,
25 °C): δ 21.7 (CH(CH3)2), 24.3 (CH(CH3)2), 25.9 (CH(CH3)2),
29.0 (CH(CH3)3), 30.1 (C(CH3)3), 39.6 (C(CH3)3), 136.4 (Ar-C),
138.5 (Ar-C), 144.8 (Ar-C), 147.9 (Ar-C), 159.1 (NCN). IR: ν 3337
(24) (a) Schulz, S.; Schuchmann, D.; Westphal, U.; Bolte, M.
Organometallics 2009, 28, 1590–1592. (b) Carrasco, M.; Peloso, R.;
ꢀ
Rodríguez, A.; Alvarez, E.; Maya, C.; Carmona, E. Chem.;Eur. J. 2010,
16, 9754–9757. (c) Schulz, S.; Gondzik, S.; Schuchmann, D.; Westphal, U.;
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1989.

Article
Organometallics, Vol. 29, No. 22, 2010 6103
(N-H), 3064, 2959, 2867, 2166, 1656, 1610, 1584, 1457, 1432, 1321,
[t-BuC{N(H)Dipp}2][Al{OC(CF3)3}4], 8. A solution of
[H(OEt2)2][Al{OC(CF3)3}4] (3.28 g, 3 mmol) in 10 mL of CH2Cl2
was added dropwise at -50 °C to 5 (0.50 g, 1 mmol) dissolved in
10 mL of CH2Cl2 and stirred for 1 h at -50 °C. The resulting
solution was warmed to ambient temperature, stirred for an
additional 60 min, concentrated under vacuum, and stored at
-30 °C. Colorless crystals of 8 were formed within 48 h.
1255, 1211, 1097, 799, 755, 724 cm-1
.
[t-BuC(NDipp)2]ZnMe, 5. ZnMe2 (4.2 mL, 1.2 M in toluene,
5 mmol) was added to a solution of 4 (2.10 g, 5 mmol) in 20 mL
of toluene at ambient temperature and stirred for 2 h. The
solution was concentrated under vacuum and stored at -30 °C.
Colorless crystals of 5 were formed within 24 h.
Yield: 2.38 g (95%). Melting point: 143 °C. Anal. Found (calcd)
for C30H46N2Zn (500.06 g/mol): H, 9.3 (9.3); C, 72.5 (72.1); N, 5.6
(5.6). 1H NMR (300 MHz, C6D6, 25 °C): δ -1.06 (s, 3H, ZnCH3),
1.04 (d, 3JHH = 6.8 Hz, 6H, CH(CH3)2), 1.27 (d, 3JHH = 6.9 Hz,
6H, CH(CH3)2), 1.28 (d, 3JHH = 7.1 Hz, 6H, CH(CH3)2), 1.33 (d,
3JHH = 6.8 Hz, 6H, CH(CH3)2), 1.38 (s, 9H, C(CH3)3), 3.24 (sept,
3JHH = 6.9 Hz, 2H, CH(CH3)2), 3.58 (sept, 3JHH = 6.9 Hz, 2H,
CH(CH3)2), 6.97-7.27 (m, 6H, ArH). 13C NMR (125 MHz, C6D6,
25 °C): δ 1.4 (ZnCH3), 21.8 (CH(CH3)2), 22.2 (CH(CH3)2), 24.1
(CH(CH3)2), 26.3 (CH(CH3)2), 28.3 (CH(CH3)2), 28.9 (CH-
(CH3)2), 31.4 (C(CH3)3), 41.6 (C(CH3)3), 123.4 (Ar-C), 125.4
(Ar-C), 126.1 (Ar-C), 128.1 (Ar-C), 168.1 (NCN). IR: ν 3027,
Yield (isolated crystals): 0.83 g (60%). Melting point: >220 °C.
Anal. Found (calcd) for C45H45AlF36N4O4 (1388.81 g/mol): H, 3.1
(3.2); C, 38.1 (38.1); N, 3.9 (4.0). H NMR (300 MHz, CD2Cl2,
1
25 °C): δ 1.03 (d, 3JHH = 6.7 Hz, 6H, CH(CH3)2), 1.27 (d, 3JHH
=
6.7 Hz, 6H, CH(CH3)2), 1.39 (d, 3JHH = 6.7 Hz, 6H, CH(CH3)2),
1.44 (s, 9H, C(CH3)3), 1.47 (d, 3JHH = 6.7 Hz, 6H, CH(CH3)2),
2.84 (sept, 3JHH = 6.8 Hz, 2H, CH(CH3)2), 2.92 (sept, 3JHH = 6.8
Hz, 2H, CH(CH3)2), 7.37 (s, 1H, NH), 7.24-7.62 (m, 6H, Ar-H),
7.64 (s, 1H, NH). 19F{1H} (235 MHz, CD2Cl2, 25 °C): δ -74.6. IR:
ν 3327, 3290, 2970, 1560, 1576, 1351, 1297, 1274, 1239, 1212, 1173,
970, 832, 803, 726, 559, 535, 444 cm-1
.
Single-Crystal X-ray Analyses. Crystallographic data for 1, 3,
5, 6, and 7 were collected on a Bruker AXS SMART APEX
CCD diffractometer (Mo KRradiation, λ= 0.71073 A;T= 173(2)
K). Data for 8 were collected with a STOE IPDS-II diffractometer.
Crystallographic data for 1, 3, 5, 6, 7, and 8 are given in the
Supporting Information. The structures were solved by direct
methods (SHELXS-97)26 and refined by full-matrix least-squares
on F2. Semiempirical absorption corrections were applied. All non-
hydrogen atoms were refined anisotropically and hydrogen atoms
by a riding model (SHELXL-97).27
2961, 1534, 1495, 1459, 1435, 1081, 1030, 726, 693, 463 cm-1
.
[t-BuC(NDipp)2]ZnMe(t-BuPy), 6. tert-Butylpyridine (0.63 g,
5 mmol) was added dropwise to 5 (2.50 g, 5 mmol) dissolved in
20 mL of toluene. The resulting solution was stirred for 2 h,
concentrated under vacuum, and stored at -30 °C. Colorless
crystals of 6 were formed within 24 h.
Yield: 2.86 g (90%). Melting point: >220 °C. Anal. Found
(calcd) for C39H59N3Zn (635.26 g/mol): H, 9.2 (9.4); C, 73.2
(73.7); N, 6.5 (6.6). 1H NMR (300 MHz, C6D6, 25 °C): δ -1.06
3
(ZnCH3), 0.98 (s, 9H, t-BuPy), 1.02 (d, JHH = 6.8 Hz, 12H,
CH(CH3)2), 1.12 (s, 9H, C(CH3)3), 1.41 (d, 3JHH = 6.8 Hz, 12H,
Computational Calculations
3
CH(CH3)2), 3.87 (sept, JHH = 6.8 Hz, 4H, CH(CH3)2), 6.82
(dd, 3JHH = 6.2 Hz, 3JHH = 2.8 Hz, 2H, t-BuPy), 7.00 (t, 4H,
Ar-H), 7.25 (d, 3JHH = 6.8, 4H, Ar-H), 8.60 (dd, 3JHH = 6.2 Hz,
2H, 3JHH = 2.8 Hz, 2H, t-BuPy). 13C NMR (125 MHz, C6D6,
25 °C): δ 1.1 (ZnCH3), 21.5 (CH(CH3)2), 22.0 (CH(CH3)2), 23.8
(CH(CH3)2), 26.0 (CH(CH3)2), 28.7 (CH(CH3)2), 30.0 (CH-
(CH3)2), 31.1 (C(CH3)3), 41.6 (C(CH3)3), 120.4 (Ar-C), 123.1
(Ar-C), 139.0 (Ar-C) 149.9 (Ar-C), CN2 not observed. IR: ν
2963, 2866, 1614, 1432, 1404, 1360, 1304, 1275, 1262, 1240, 1211,
1172, 1095, 1075, 1019, 973, 830, 802, 759, 727, 655, 632, 572,
DFT calculations were carried out with the Gaussian03 suite
of programs (M. J. Frisch, et al., Gaussian03, Revision D.02;
Gaussian Inc., Pittsburgh, PA, 2003). The molecular structures
of 50 and 500 were obtained by performing a complete energy
optimization of all geometric parameters at the b3lyp/svp level;
SVP is the split-valence basis set with the additional polarization
functions of Ahlrichs et al. Atomic charges of 50 and 500, which
were calculated from NBO population analyses, are given in the
Supporting Information.
The crystallographic data of the structures (excluding structure
factors) have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication nos. CCDC-785891 (1),
CCDC-785892 (3), CCDC-785893 (5), CCDC-785895 (6), CCDC-
785894 (7), and CCDC-785458 (8). Copies of the data can be
obtained free of charge on application to The Director, CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (fax: int.code_(1223)336-
033; e-mail for inquiry: fileserv@ccdc.cam.ac.uk; e-mail for deposi-
tion: deposit@ccdc.cam.ac.uk).
521, 428 cm-1
.
{[t-BuC(NDipp)2]Zn(μ-I)}2, 7. A solution of I2 (1.27 g, 5 mmol)
in 20 mL of toluene was added dropwise at -30 °C to 5 (2.50 g,
5 mmol) dissolved in 20 mL of toluene. The resulting mixture was
warmed to ambient temperature and stirred for an additional 12 h.
The suspension was filtered, and the resulting solid was recrystal-
lized from a solution in toluene/Et2O at -30 °C. Colorless crystals
of 7 were formed within 24 h.
Yield: 5.45 g (89%). Melting point: >220 °C. Anal. Found
(calcd) for C58H86N4Zn2 (1223.85 g/mol): H, 7.2 (7.1); C, 57.0
(56.9); N, 4.5 (4.6). 1H NMR (300 MHz, THF-d8, 25 °C): δ 0.95
(s, 9H, C(CH3)3), 1.20 (d, 3JHH = 6.8 Hz, 12H, CH(CH3)2), 1.33
(d, 3JHH = 6.7 Hz, 12H, CH(CH3)2), 3.62 (sept, 3JHH = 6.8 Hz,
4H, CH(CH3)2), 7.01-7.19 (m, 6H, Ar-H). 13C NMR (125 MHz,
C6D6, 25 °C): δ 23.0 (CH(CH3)2), 27.6 (CH(CH3)2), 29.1 (CH-
(CH3)2), 30.9 (C(CH3)3), 41.6 (C(CH3)3), 123.9 (Ar-C), 125.0
(Ar-C), 129.0 (Ar-C), 129.7 (Ar-C), CN2 not observed. IR: ν 2957,
2924, 2865, 1617, 1491, 1462, 1438, 1363, 1310, 1254, 1235, 1211,
Acknowledgment. S. Schulz thanks the German Science
Foundation (DFG) for financial support.
Supporting Information Available: This material is available
free of charge via the Internet at http://pubs.acs.org.
(26) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.
(27) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure
€ €
Refinement; Universitat Gottingen, 1997.
1178, 1098, 1053, 1030, 975, 932, 800, 755, 706, 431, 410 cm-1
.