Dinuclear versus mononuclear zinc compounds from reduction of LZnCl 2 (L = α-diimine ligands): Effects of the ligand substituent, reducing agent, and solvent

Article abstract of DOI:10.1021/om800405m

A Zn-Zn-bonded compound, [K(THF)₂]₂[(L ^iPr)Zn-Zn(L^ipr)] (2) and three mononuclear zinc compounds, [Zn(L^Me)₂Na₂(Et₂O)₂] (3), [Zn(L^Et,)₂Na₂(THF)₂] (4), and [Zn(L^Et)₂K₂]., (5), with N-aryl substituted a-diimine ligands L^iPr, L^Me, and L^Et (L = [(2,6-R₂C₆H₃)N(Me)C]₂, R = ⁱPr, Me, Et, respectively) have been synthesized from the reduction of the LZnCl₂ precursors by the alkali metal Na or K. X-ray structural analyses show that the compounds have a [Zn₂L ₂]^2- (2) or [ZnL₂]^2- (3-5) core incorporating Na⁺ or K⁺ ions solvated by THF or Et ₂O molecules, except for 5, which displays a 2D polymeric structure formed by intermolecular K-C bonds due to the lack of K - solvent interactions. In compound 2, the formal Zn²⁺ ion in the precursor is reduced to Zn⁺, while in 3-5 it remains unreduced. The neutral ligands in the precursor, however, are doubly reduced to a dianion, L^2-, in all compounds, as evidenced by the bond lengths of the N-C=C-N moiety of the ligands. Effects of the ligand substituent, reducing agent, and solvent on the products have been studied, which reveal that the steric bulk of the ligand is the most important factor that determines whether the Zn-Zn bond can be formed. DFT computations show that the Zn-Zn bond in 2 is formed mainly by the 4s orbitais of zinc. The results of natural charge and orbital analyses also confirm the reduction of the ligands to their dianionic forms.

Full text of DOI:10.1021/om800405m

5800
Organometallics 2008, 27, 5800–5805
Dinuclear versus Mononuclear Zinc Compounds from Reduction of
LZnCl2 (L ) r-Diimine Ligands): Effects of the Ligand Substituent,
Reducing Agent, and Solvent
Jie Yu,†,¶ Xiao-Juan Yang,*,†,‡ Yanyan Liu,†,¶ Zhifeng Pu,§ Qian-Shu Li,§ Yaoming Xie,
Henry F. Schaefer, and Biao Wu*,†,‡
State Key Laboratory for Oxo Synthesis & SelectiVe Oxidation, Lanzhou Institute of Chemical Physics,
CAS, Lanzhou 730000, People’s Republic of China, State Key Laboratory of Applied Organic Chemistry,
Lanzhou UniVersity, Lanzhou 730000, People’s Republic of China, Institute for Chemical Physics, Beijing
Institute of Technology, Beijing 100081, People’s Republic of China, Center for Computational Chemistry,
Department of Chemistry, UniVersity of Georgia, Athens, Georgia 30602, and Graduate School of Chinese
Academy of Sciences, Beijing 100049, People’s Republic of China
ReceiVed May 6, 2008
A Zn-Zn-bonded compound, [K(THF)2]2[(LiPr)Zn-Zn(LiPr)] (2) and three mononuclear zinc
compounds, [Zn(LMe)2Na2(Et2O)2] (3), [Zn(LEt)2Na2(THF)2] (4), and [Zn(LEt)2K2]n (5), with N-aryl
i
substituted R-diimine ligands LiPr, LMe, and LEt (L ) [(2,6-R2C6H3)N(Me)C]2, R ) Pr, Me, Et,
respectively) have been synthesized from the reduction of the LZnCl2 precursors by the alkali metal Na
or K. X-ray structural analyses show that the compounds have a [Zn2L2]2- (2) or [ZnL2]2- (3-5) core
incorporating Na+ or K+ ions solvated by THF or Et2O molecules, except for 5, which displays a 2D
polymeric structure formed by intermolecular K-C bonds due to the lack of K-solvent interactions. In
compound 2, the formal Zn2+ ion in the precursor is reduced to Zn+, while in 3-5 it remains unreduced.
The neutral ligands in the precursor, however, are doubly reduced to a dianion, L2-, in all compounds,
as evidenced by the bond lengths of the N–CdC–N moiety of the ligands. Effects of the ligand substituent,
reducing agent, and solvent on the products have been studied, which reveal that the steric bulk of the
ligand is the most important factor that determines whether the Zn-Zn bond can be formed. DFT
computations show that the Zn-Zn bond in 2 is formed mainly by the 4s orbitals of zinc. The results of
natural charge and orbital analyses also confirm the reduction of the ligands to their dianionic forms.
derivatives,3,4 the ꢀ-diketiminate HC[C(Me)N(C6H3-2,6-iPr2)]2,5
Introduction
terphenyl ligands,6 and the N-donor ligand η2-Me2Si(NDipp)2.7
Metal-metal bonding is an important and fundamental
dimension of chemistry.1 One of the effective approaches for
generating metal-metal (multiple) bonds is reduction of suitable
metal-ligand-halide precursors with various reducing agents,
especially alkali metals. Appropriate protective ligands are often
essential in the stabilization of metal-metal bonds, and different
ligand substituents can have a great impact on the structures of
the products.2 Zn-Zn-bonded compounds have attracted con-
siderable recent attention since the report of the first Zn-Zn
bond in 2004.3 A variety of supporting ligands have been used
for the synthesis of [RZn-ZnR] compounds, including Cp
Recently we reported
a
Zn-Zn-bonded compound,
[Na(THF)2]2[(LiPr)Zn-Zn(LiPr)]
(1; [(2,6-
LiPr
)
iPr2C6H3)N(Me)C]2), obtained by reduction of L0ZnCl2 (L0 is
the neutral R-diimine ligand) with sodium metal. The neutral
ligand has been doubly reduced to L2-.8 A similar compound
using another R-diimine ligand, [(dpp-Bian)Zn-Zn(dpp-Bian)]
(dpp-Bian ) 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene),
has also been synthesized, in which the ligand is the radical
anion L-.9 To further explore R-diimine-supported metal-metal
bonding, we have examined the reduction of LZnCl2 precursors
bearing R-diimine ligands with 2,6-substituents on the N-phenyl
ring (LiPr, LMe, LEt). Herein we report experimental and
* To whom correspondence should be addressed. E-mail: yangxj@lzb.ac.cn
(X.-J.Y.); wubiao@lzb.ac.cn (B.W.). Fax: (+)86 931 4968286.
† Lanzhou Institute of Chemical Physics.
(4) Del R´ıo, D.; Galindo, A.; Resa, I.; Carmona, E. Angew. Chem., Int.
Ed. 2005, 44, 1244–1247.
(5) Wang, Y.; Quillian, B.; Wei, P.; Wang, H.; Yang, X.-J.; Xie, Y.;
King, R. B.; Schleyer, P. v. R.; Schaefer, H. F.; Robinson, G. H. J. Am.
Chem. Soc. 2005, 127, 11944–11945.
‡ Lanzhou University.
§ Beijing Institute of Technology.
University of Georgia.
¶ Graduate School of Chinese Academy of Sciences.
(1) Cotton, F. A.; Murillo, C. A.; Walton, R. A., Multiple Bonds between
Metal Atoms, 3rd ed.; Springer: New York, 2005.
(6) (a) Zhu, Z.; Wright, R. J.; Olmstead, M. M.; Rivard, E.; Brynda,
M.; Power, P. P. Angew. Chem., Int. Ed. 2006, 45, 5807–5810. (b) 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.
(7) Tsai, Y.-C.; Lu, D.-Y.; Lin, Y.-M.; Hwang, J.-K.; Yu, J.-S. K. Chem.
Commun. 2007, 4125–4127.
(8) Yang, X.-J.; Yu, J.; Liu, Y.; Xie, Y.; Schaefer, H. F.; Liang, Y.;
Wu, B. Chem. Commun. 2007, 2363–2365.
(9) 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.
(2) See for example: (a) Li, X.-W.; Pennington, W. T.; Robinson, G. H.
J. Am. Chem. Soc. 1995, 117, 7578–7579. (b) Su, J.; Li, X.-W.; Crittendon,
R. C.; Robinson, G. H. J. Am. Chem. Soc. 1997, 119, 5471–5472. (c)
Nguyen, T.; Sutton, A. D.; Brynda, M.; Fettinger, J. C.; Long, G. J.; Power,
P. P. Science 2005, 310, 844. (d) Wolf, R.; Brynda, M.; Ni, C.; Long, G. J.;
Power, P. P. J. Am. Chem. Soc. 2007, 129, 6076–6077.
(3) (a) Resa, I.; Carmona, E.; Gutierrez-Puebla, E.; Monge, A. Science
2004, 305, 1136–1138. (b) Grirrane, A.; Resa, I.; Rodriguez, A.; Carmona,
E.; Alvarez, E.; Gutierrez-Puebla, E.; Monge, A.; Galindo, A.; Del R´ıo,
D.; Andersen, R. A. J. Am. Chem. Soc. 2007, 129, 693–703.
10.1021/om800405m CCC: $40.75
2008 American Chemical Society
Publication on Web 10/15/2008

Dinuclear Versus Mononuclear Zinc Compounds
Organometallics, Vol. 27, No. 22, 2008 5801
Scheme 1. Synthesis of the Compounds 2-5
theoretical (DFT) studies of the resultant Zn-Zn-bonded
compound 2 and the mononuclear Zn compounds 3-5, the latter
representing very rare examples of ZnII complexes with two
doubly reduced R-diimine ligands (Scheme 1). The influence
of ligand system, reducing agent, and solvent on the structure
of the product is also discussed.
bond length in 2 (2.393(1) Å) is almost identical with that in the
sodium analogue (2.399(1) Å). As in 1, the formal divalent Zn2+
ion is reduced to Zn+, and the neutral R-diimine ligand is doubly
reduced to the dianion [LiPr]2-, whose negative charges are further
compensated by a solvated K+ ion. Notably, the ionic
[(LiPr)Zn-Zn(LiPr)]2- core in the two compounds (1 and 2) is
significantly different from that of a similar Zn-Zn-bonded
compound bearing the R-diimine ligand dpp-Bian, [(dpp-
Bian)Zn-Zn(dpp-Bian)], in which the dpp-Bian ligand is only
singly reduced to a radical anion, leading to a neutral [LZn-ZnL]
compound.9
Switching the phenyl ligand substituents from 2,6-diisopropyl
to the less crowded 2,6-dimethyl led to the mononuclear compound
[Zn(LMe)2Na2(Et2O)2] (3). The zinc atom is four-coordinated by
the nitrogen donors of two LMe ligands in a distorted-tetrahedral
geometry (Figure 2). The mean Zn-N bond length (2.05 Å) for 3
is comparable to those in 1 (2.02 Å) and 2 (2.01 Å). The two
N-Zn-N bite angles are nearly the same (82.3 and 82.7°), but
the N1-Zn-N4 angle of 144.6(1)° is significantly widened. In
contrast to the parallel/coplanar arrangement for the two ligand
molecules in the Zn-Zn-bonded 1 and 2, the two C2N2 planes are
nearly perpendicular to each other (dihedral angle 84.7°). The zinc
Results and Discussion
Synthesis. The LZnCl2 compounds were obtained by the
reaction of ZnCl2 with the corresponding ligand L. The
precursors were reduced by different reagents such as Li, Na,
K, Mg, and Ca in a variety of solvents (THF, diethyl ether, and
toluene), which led to mono- or dinuclear Zn compounds with
different structures (Scheme 1).
The reduction of (LiPr)ZnCl2 by potassium in THF resulted
in the Zn-Zn-bonded compound [K(THF)2]2[(LiPr)Zn-Zn(LiPr)]
(2), as in the case of the sodium reduction.8 When the 2,6-
dimethyl-substituted precursor (LMe)ZnCl2 was used, the reduc-
tion by Na in Et2O gave
a mononuclear compound,
[Zn(LMe)2Na2(Et2O)2] (3), as yellow crystals. Reaction of the
diethyl precursor (LEt)ZnCl2 with Na or K also yielded mono-
nuclear zinc compounds. The reduction by Na in THF resulted
in the monomeric compound [Zn(LEt)2Na2(THF)2] (4), which
is similar to 3 with LMe. However, using toluene as solvent,
the reduction of (LEt)ZnCl2 by K led to an orange-red, polymeric
compound, [Zn(LEt)2K2]n (5). These compounds are highly air-
and moisture-sensitive. The compounds 3 and 4 are quite
thermally stable up to 80 °C (decomposition), while 2 and 5
slowly decompose upon storage at room temperature for several
days under argon. The reduction of the precursors by Li, Mg,
or Ca led to a red solution in some cases. Unfortunately, we
have not isolated any products from the solutions.
X-ray Crystal Structures. Single crystals suitable for X-ray
diffraction studies were obtained in diethyl ether (3), THF (2
and 4), and toluene (5). The molecular structures of 2-5 are
depicted in Figures 1-4, while the selected bond lengths and
angles are given in Tables 1-4.
The compound [K(THF)2]2[(LiPr)Zn-Zn(LiPr)] (2) is indeed
isomorphous with its sodium analogue 18 with similar structural
features (Figure 1, Table 1), except that the C2N2 planes of the
two ligands are nearly coplanar in 2 while they are parallel in 1
(vertical distances of 0.70 and 1.28 Å, respectively). The Zn-Zn
Figure 1. Molecular structure of 2 (thermal ellipsoids at the 20%
probability level, with atoms of the solvent THF drawn as smaller
spheres and hydrogen atoms omitted for clarity).

5802 Organometallics, Vol. 27, No. 22, 2008
Yu et al.
Compounds 4 and 5, obtained from the 2,6-diethyl-substituted
precursor (LEt)ZnCl2, which has medium steric bulk between
the isopropyl and methyl analogues, also feature the mono-
nuclear [ZnL2]2- core. The compound [Zn(LEt)2Na2(THF)2] (4)
is structurally similar to 3 with LMe. The Zn2+ center is
tetrahedrally coordinated by two ligand molecules, with the
N1-Zn1-N2 bite angle being 82.4(2)° and the N1-Zn1-N1A
angle widened (149.9(3)°) (Table 3). The two N–CdC–N planes
in 4 deviate slightly from the orthogonal conformation (dihedral
angle 79.2° compared to 84.7° in 3). The major differences
between 3 and 4 are that the latter possesses a C2 axis bisecting
the N1-Zn-N1A and N2-Zn-N2A angles (while 3 lacks the
crystallographically imposed C2 symmetry), and the coordina-
tion sphere of the Na ions is completed by a THF molecule
instead of Et2O (Figure 3). As in 3, the additional Na-C(aryl)
bonds in 4 resulted in a Na-Zn-Na angle of 95.2° and
Na · · · Na separation of 4.21 Å.
In contrast to the monomeric manner of 4, the other
compound (5) with the LEt ligand exhibits a polymeric structure,
[Zn(LEt)2K2]n. The coordination sphere of the zinc atom in the
[Zn(LEt)2K2] unit resembles that in 3 and 4, and there is also a
2-fold axis in the [Zn(LEt)2K2] moiety (Figure 4a). The K atoms
are η4-bonded by the C2N2 moiety and are in contact with two
additional C atoms of one phenyl ring in another ligand, with
a K-Zn-K angle of 104.2° and K · · · K separation of 5.14 Å
(Table 4). However, due to the lack of a coordinating solvent
molecule, the “unsaturated” K+ ion forms two intermolecular
K-C bonds with the phenyl 3- and 4-carbons of another unit.
Thus, each [Zn(LEt)2K2] unit is linked to four adjacent mol-
ecules, resulting in a two-dimensional network in the ab plane
(Figure 4b). This aggregation is remarkably different from the
other four compounds (1-4) obtained in either THF or ether.
The polymeric zinc(II) complexes [K(THF)ZnMe(L2-)]n and
[K(Et2O)1/2Zn(CH2Ph)(L2-)]n (L ) t-BuNCHCHN-t-Bu) have
been reported; however, both of them show a linear chain
structure (one-dimensional), where further extension is prevented
by the solvation of the K+ ion.10
Figure 2. Molecular structure of 3 (thermal ellipsoids at the 20%
probability level, with the atoms of Et2O drawn as smaller spheres
and hydrogen atoms omitted).
The mononuclear compounds (3-5) and the dinuclear
compounds (1 and 2) have different charge distributions. The
diimine ligands are doubly reduced in all cases, but the zinc
remains a formal Zn2+ dication in 3-5 with a composition of
[Zn2+(L2-)2(M+)2] (M ) Na, K), while in 1 and 2 the Zn2+ is
reduced to Zn+, [(Zn+)2(L2-)2(M+)2]. The synthesis of the
dinuclear compounds is represented by eq 1, in which 3 equiv
of the alkali metal is consumed to provide one electron to the
Zn2+ center and two electrons to the neutral ligand. The
formation of the mononuclear species, on the other hand, is due
to the disproportionation of the first formed ZnI compounds to
the ZnII products and zinc metal (eq 2), as confirmed by the
observation of Zn metal in the reaction system. To further prove
this mechanism, we carried out the reduction of the (LMe)ZnCl2
precursor with Na in the presence of equimolar free ligand to
convert all Zn to the desired mononuclear product, as shown in
eq 3. As expected, only a trace of zinc was formed in this case,
indicating a greatly lowered degree of the disproportionation
(though this did not completely disappear).
Figure 3. Molecular structure of 4 (thermal ellipsoids at the 20%
probability level, with the atoms of THF drawn as smaller spheres
and hydrogen atoms omitted).
atom deviates slightly from the C2N2 plane (∼0.45 Å). Furthermore,
compound 3 also incorporates two sodium ions, as in 1 and 2, but
each is solvated by only one Et2O molecule. The Na atoms are
located over the C2N2Zn ring and are η4-bonded by the N–CdC–N
moiety with Na-C distances in the range 2.677(4)-2.734(4) Å
and Na-N distances in the range 2.488(4)-2.621(3) Å. There are
two additional Na-C contacts (Na1-C25, Na1-C30, Na2-C13,
and Na2-C18) of each Na atom with the ipso and 2-position
carbon atoms of the phenyl ring of another ligand (Figure 2, Table
2). These contacts, although longer (∼2.96 Å) than the
Na-C(C2N2) bonds, drive the two solvated Na(OEt2)+ units to sit
above two triangular faces of the ZnN4 tetrahedron with a
Na-Zn-Na angle of 100.6° and a Na · · · Na separation of 4.41
Å.
2LZnIICl2 + 6M f [ZnI2L2M2] + 4MCl
[ZnI2L2M2] f [ZnIIL2M2] + Zn0
(1)
(2)
(3)
LZnIICl2 + L + 4M f [ZnIIL2M2] + 2MCl
The existence of the ligand as a dianionic species is evidenced
by a change in the NdC–CdN moiety. Upon uptake of electrons

Dinuclear Versus Mononuclear Zinc Compounds
Organometallics, Vol. 27, No. 22, 2008 5803
Figure 4. (a) Molecular structure of 5 (thermal ellipsoids at the 20% probability level, with hydrogen atoms omitted). (b) The two-
dimensional network in the ab plane formed by intermolecular K-C bonds.
Table 1. Selected Bond Distances (Å) and Bond Angles (deg) for
Compound 2a
Table 4. Selected Bond Distances (Å) and Angles (deg) for 5a
Zn(1)-N(1)
2.090(6) K(1)-C(5A)
2.008(5) K(1)-C(10A)
3.256(2) K(1)-C(17B)
2.956(7) K(1)-C(18B)
2.911(6) C(2)-C(3)
2.941(8) N(1)-C(2)
2.956(7) N(2)-C(3)
3.171(7)
Zn(1)-Zn(1A)
Zn(1)-N(1)
Zn(1)-N(2)
Zn(1) · · · K(1)
K(1)-N(1)
2.3934(8) K(1)-O(2)
2.015(3) K(1)-C(2)
2.005(3) K(1)-C(3)
3.340(1) C(2)-C(3)
2.940(3) N(1)-C(2)
2.950(3) N(2)-C(3)
2.626(4)
2.721(7)
2.897(4)
2.907(4)
1.363(5)
1.409(5)
1.410(5)
Zn(1)-N(2)
Zn(1) · · · K(1)
K(1)-N(1)
3.080(8)
3.30(1)
3.10(1)
1.36(1)
1.412(8)
1.426(9)
K(1)-N(2)
K(1)-C(2)
K(1)-N(2)
K(1)-C(3)
K(1)-O(1)
N(1)-Zn(1)-N(2)
83.0(2)
N(1)-Zn(1)-N(1A) 113.0(3)
N(1)-Zn(1)-Zn(1A) 138.61(9)
N(2)-Zn(1)-Zn(1A) 137.82(9)
N(1)-Zn(1)-N(2A) 122.8(3)
N(2)-Zn(1)-N(2A) 135.3(3)
N(1)-Zn(1)-N(2)
82.89(12)
a Symmetry codes: (A) 1 - x, y, 0.5 - z; (B) 0.5 - x, y - 1.5, 0.5
- z.
a Symmetry code: (A) 1 - x, 2 - y, -z.
Table 2. Selected Bond Distances (Å) and Angles (deg) for 3
the dianion. In the present work, the C-N bond lengths of 1-5
fall in the range 1.409-1.431 Å, which are significantly longer
than those for the free ligands (1.275-1.283 Å). Meanwhile,
the C-C bond is shortened to 1.347-1.358 Å, in comparison
to the length in the free ligands (1.498-1.504 Å).11 These
distances correspond to the dianionic enediamido form (N–Cd
C–N), as is confirmed by the DFT computations described
below. In all compounds 1-5, half of the negative charge of
the dianion, -1, is compensated by a Na+ or K+ ion attaching
to the N–CdC–N moiety. The average Zn · · · Na and Zn · · · K
contacts are 2.86 and 3.30 Å, respectively. Mononuclear Zn
compounds with other diimine ligands in different combinations
of the three oxidation states (neutral, radical anionic, and
dianionic), e.g. [ZnII(L2-)(L-)]-, [ZnII(L-)2], and [ZnII(L0)-
(L-)]+, have been reported.12-14 However, structures of
[Zn(L2-)2]2- compounds that contain two dianionic ligands are
not yet known.13
It would be very instructive to correlate the reaction conditions
(e.g., the ligand environment, reducing agent, and solvent) with
the products from the reduction of LZnCl2. Generally the
electronic factor and steric bulk of the ligands can have a
significant effect on the structure of the resulting metal
compounds. In the present work the ligand steric effect is
dominant and plays a critical role in the formation of the
Zn(1)-N(1)
2.014(3)
2.096(3)
2.099(3)
2.001(3)
2.850(2)
2.879(2)
2.502(4)
2.571(4)
2.684(5)
2.711(5)
2.266(4)
2.949(4)
3.021(5)
82.3(1)
Na(2)-N(3)
2.621(3)
2.488(4)
2.734(4)
2.677(4)
2.282(4)
2.945(4)
2.912(5)
1.350(6)
1.347(5)
1.410(5)
1.431(5)
1.429(5)
1.424(5)
Zn(1)-N(2)
Na(2)-N(4)
Zn(1)-N(3)
Zn(1)-N(4)
Na(2)-C(22)
Na(2)-C(23)
Na(2)-O(2)
Na(2)-C(13)
Na(2)-C(18)
C(2)-C(3)
Zn(1) · · · Na(1)
Zn(1) · · · Na(2)
Na(1)-N(1)
Na(1)-N(2)
Na(1)-C(2)
C(22)-C(23)
N(1)-C(2)
Na(1)-C(3)
Na(1)-O(1)
N(2)-C(3)
Na(1)-C(25)
Na(1)-C(30)
N(1)-Zn(1)-N(2)
N(1)-Zn(1)-N(3)
N(1)-Zn(1)-N(4)
N(3)-C(22)
N(4)-C(23)
N(2)-Zn(1)-N(3)
N(2)-Zn(1)-N(4)
N(3)-Zn(1)-N(4)
109.7(1)
118.8(1)
82.7(1)
118.6(1)
144.6(1)
Table 3. Selected Bond Distances (Å) and Angles (deg) for 4a
Zn(1)-N(1)
1.993(4) Na(1)-O(1)
2.278(6)
Zn(1)-N(2)
2.103(4) Na(1)-C(15A)
2.958(6)
2.911(7)
1.347(8)
1.418(7)
1.432(7)
Zn(1) · · · Na(1)
Na(1)-N(1)
Na(1)-N(2)
Na(1)-C(2)
2.849
Na(1)-C(20A)
2.474(5) C(2)-C(3)
2.585(5) N(1)-C(2)
2.644(6) N(2)-C(3)
2.695(7)
Na(1)-C(3)
N(1)-Zn(1)-N(2)
82.4(2)
N(1)-Zn(1)-N(1A) 149.9(3)
N(2)-Zn(1)-N(2A) 110.3(3)
N(1)-Zn(1)-N(2A) 115.4(2)
a Symmetry code: (A) 1 - x, y, 0.5 - z.
the C-N bonds are elongated and the central C-C bond
shortened from the neutral ligand to monoanion L- and then to
(10) Rijnberg, E.; Boersma, J.; Jastrzebski, J. T. B. H.; Lakin, M. T.;
Spek, A. L.; van Koten, G. Organometallics 1997, 16, 3158–3164.
(11) (a) Cope-Eatough, E. K.; Mair, F. S.; Pritchard, R. G.; Warren, J. E.;
Woods, R. J. Polyhedron 2003, 22, 1447–1454. (b) Kuhn, N.; Steimann, M.;
Walker, I. Z. Kristallogr.: New Cryst. Struct. 2001, 216, 319.
(12) Cardiner, M. G.; Hanson, G. R.; Henderson, M. J.; Lee, F. C.;
Raston, C. L. Inorg. Chem. 1994, 33, 2456–2461.
Figure 5. HOMO-2 of the model compound 2H.

5804 Organometallics, Vol. 27, No. 22, 2008
Yu et al.
Table 5. Crystallographic Data and Refinement Details for Compounds 2-5
2
3
4
5
empirical formula
fw
C72H112K2N4O4Zn2
1306.70
C48H68N4Na2O2Zn
844.41
C56H80N4Na2O2Zn
952.59
C48H64K2N4Zn
840.66
cryst syst
space group
a/Å
b/Å
c/Å
monoclinic
P21/n
monoclinic
P21/n
monoclinic
C2/c
monoclinic
C2/c
13.467(2)
20.206(2)
14.077(2)
91.367(2)
3829.4(8)
2
11.5189(5)
23.1977(9)
18.0425(7)
96.279(2)
4792.3(3)
4
24.712(9)
12.745(4)
19.001(6)
117.492(5)
5309(3)
16.507(3)
13.602(3)
19.924(4)
101.92(3)
4377(2)
4
ꢀ/deg
V/Å3
Z
4
Dcalcd/g cm-3
cryst size/mm3
F(000)
1.128
1.170
1.192
1.276
0.32 × 0.30 × 0.30
0.40 × 0.40 × 0.38
0.48 × 0.40 × 0.30
0.46 × 0.33 × 0.30
1392
1808
2048
1792
µ/mm-1
0.779
1.76-28.28
22 840
8943
0.0556
0.570
1.98-26.84
28 054
10229
0.0686
0.522
1.85-28.34
15 081
6171
0.0777
0.789
1.96-27.93
12 551
4925
0.2481
θ range/deg
no. of rflns collected
no. of indep rflns
R(int)
no. of obsd rflns (I > 2σ(I))
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
GOF (F2)
4395
4988
3048
1475
0.0569, 0.1550
0.1326, 0.1951
0.989
0.0590, 0.1439
0.1383, 0.1826
0.964
0.0909, 0.2516
0.1759, 0.2984
1.007
0.0912, 0.2052
0.2626, 0.2698
0.819
metal-metal bond. The isopropyl-substituted ligand LiPr pro-
vided proper protection for the Zn-Zn bond in 1 and 2, while
the less crowded LEt and LMe resulted in the mononuclear
compounds 3-5, in which Zn2+ replaces the [Zn-Zn]2+ core.
The two alkali metals Na and K as reducing agents led to similar
molecular structures with the same ligand (in the cases 1, 2
and 4, 5), although further conclusions are hindered by the
absence of the products reduced by other agents. The solvent
could affect the aggregation state of the products, as in the case
of 4 and 5 with the same LEt ligand. A THF molecule
coordinates to the Na+ ion to form the monomeric 4, while the
nonsolvated K+ ion in 5 is further bonded by the carbon atoms
of another molecule, leading to a two-dimensional polymeric
species.
Density Functional Theory (DFT) Studies. Theoretical
studies were performed at the DZP BP86 level for the Zn-Zn-
bonded compound 2 and the mononuclear zinc compounds 3-5.
The model of K2[(CHNH)2Zn-Zn(NHCH)2] (2H) is used for
2, and the results are similar to those for the sodium analogue
1/1H.8 The theoretical Zn-Zn bond distance (2.396 Å) com-
pares well with that observed for 2 (2.393(1) Å) and is close to
that calculated for 1H (2.373 Å). The HOMO and HOMO-1
for 2H (see Figure S2 in the Supporting Information) are ligand-
based orbitals, and HOMO-2 represents the Zn-Zn σ bond
(Figure 5). The Wiberg bond index (WBI) gave a Zn-Zn bond
order of 0.64 for 2H (close to the value 0.69 for 1H).8 Similar
to the case for 1H, this σ bond is formed mainly by the 4s
orbitals, with 94.7% s, 4.2% p, and 1.1% d character. The natural
charges on Zn and K are +0.71 and +0.86, respectively, while
each N–CdC–N moiety of the ligand has a total negative charge
of -1.58, with the computed C-N and C-C distances (see
Table S2 in the Supporting Information) corresponding to a
dianionic ligand.14
C-C (1.385 Å) bond distances are close to the crystal structure
data for 3-5 (C-N, 1.410-1.432 Å; C-C, 1.347-1.358 Å)
(Table S5, Supporting Information), which clearly indicate a
formal doubly reduced ligand, as is also confirmed by the natural
charges on L (-1.72). The ligand-based orbitals HOMO and
HOMO-1 (Figure S4, Supporting Information) show the π-bond
character between the two carbon atoms of the N–CdC–N
moiety. As mentioned above, the zinc atom bears a formal +2
valence (natural charge +1.55) and the charge on Na is +0.94.
Conclusions
We have described here the structures of a series of di- and
mononuclear zinc compounds (2-5) of R-diimine ligands
bearing different N-aryl substituents from the reduction of the
LZnCl2 precursors by the alkali metal Na or K. The proper steric
bulk of the substituents on the aryl ring (2,6-diisopropyl groups
in this work) is crucial for the stabilization of the zinc-zinc
bond, and the utilization of different solvents can vary the
aggregation state of the products. In the Zn-Zn-bonded
compounds 1 and 2, the formal Zn2+ ion in the precursor is
reduced to Zn+, while in the mononuclear compounds 3-5 the
+2 valence remains. However, in all compounds, the reducing
agent donates two electrons to the neutral ligand to form a
relatively stable dianion, L2-. DFT computations reveal that
the Zn-Zn bond in 2 contains mainly the 4s orbitals of zinc.
The existence of the dianionic ligands in 1-5 is confirmed by
the natural charge analysis (charges on L: -1.58 for 2H and
-1.72 for 3H) as well as the frontier molecular orbitals, which
show the CdC double-bond character of the N–CdC–N moiety
of the ligands.
Experimental Section
General Considerations. All manipulations with air- and
moisture-sensitive compounds were carried out under argon with
standard Schlenk or drybox techniques. The solvents (toluene, Et2O,
and THF) were dried using appropriate methods and were distilled
under argon prior to use. Benzene-d6 was dried over Na/K alloy.
NMR spectra were recorded on a Mercury Plus-400 spectrometer
in benzene-d6. Elemental analyses were performed with an El-
ementar VarioEL III instrument. The complexes LZnCl2 were
prepared according to published procedures.15
The model compound ZnL2Na2(H2O)2 (3H; L ) PhNCHCH-
NPh) was used to theoretically examine the electronic structure
of the mononuclear zinc compounds, and this model gave a
minimum structure with C2 symmetry (Figure S3, Supporting
Information). This is the case for 4 and 5; but compound 3 lacks
the C2 symmetry. The theoretical C-N (1.401, 1.412 Å) and
(13) Rijnberg, E.; Richter, B.; Thiele, K. H.; Boersma, J.; Veldman,
N.; Spek, A. L.; van Koten, G. Inorg. Chem. 1998, 37, 56–63.
(14) Muresan, N.; Chlopek, K.; Weyhermuller, T.; Neese, F.; Wieghardt,
K. Inorg. Chem. 2007, 46, 5327–5337.
[K(THF)2]2[(LiPr)Zn-Zn(LiPr)] (2). (LiPr)ZnCl2 (1.00 g, 1.85
mmol) and potassium (0.22 g, 5.56 mmol) were combined with 40

Dinuclear Versus Mononuclear Zinc Compounds
Organometallics, Vol. 27, No. 22, 2008 5805
mL of THF at room temperature. The mixture was stirred for 7
days and filtered to yield a dark red solution, which was concen-
trated to about 10 mL and stored at ca. -20 °C. Orange crystals
chromated Mo KR radiation (λ ) 0.710 73 Å). An empirical
absorption correction using SADABS was applied for all data.16
The structures were solved by direct methods using the SHELXS
program.17 All non-hydrogen atoms were refined anisotropically
by full-matrix least squares on F2 by the use of the program
SHELXL.17 Hydrogen atoms bonded to carbon were included in
idealized geometric positions with thermal parameters equivalent
to 1.2 times those of the atom to which they were attached.
Crystallographic data and refinement details for 2-5 are given in
Table 5.
Crystallographic data for the compounds of this work have been
deposited with the Cambridge Crystallographic Data Centre with
reference numbers CCDC 684906-684909. Copies of this informa-
tion may be obtained free of charge from The Director, CCDC, 12
Union Road, Cambridge CB2 1EZ, U.K. (fax, +44 1223 3360333;
e-mail, deposit@ccdc.cam.ac.uk; web, http://www.ccdc.cam.ac.uk).
Computational Methods.18 The DFT computations were per-
formed at the DZP BP86 level of theory with the Gaussian 94
program. The model compound K2[(CHNH)2Zn-Zn(NHCH)2]
(2H), where the N-2,6-diisopropylphenyl group and the methyl
group on the central carbon atoms are replaced by hydrogen atoms,
is used for the dinuclear compound 2, and the simplified model
ZnL2Na2(H2O)2 (3H; L ) PhNCHCHNPh) is used for the mono-
nuclear compounds. The Cartesian coordinates and structures of
the optimized geometry, selected theoretical bond lengths, and
frontier orbitals are given in the Supporting Information.
1
(0.75 g, 31%) were isolated after 2 weeks. H NMR (400 MHz,
C6D6): δ 1.05 (d, 24H, J ) 6.8 Hz, CH(CH3)2), 1.29 (m, 24H, J )
6.8 Hz, CH(CH3)2), 1.38 (t, 16H, J ) 6.4 Hz, THF), 2.11 (s, 12H,
CCH3), 3.55 (t, 16H, J ) 6.4 Hz, THF), 3.90 (m, 8H, CH(CH3)2),
6.54 (t, 4H, J ) 7.6 Hz, p-ArH), 6.89 ppm (d, 8H, J ) 7.6 Hz,
m-ArH). 13C NMR (100.6 MHz, C6D6): δ 16.1 (CH3), 22.7
(CH(CH3)2), 23.8 (CH(CH3)2), 25.7 (THF), 28.1 (CH(CH3)2), 67.7
(THF), 118.0 (m-C6H3), 120.0 (p-C6H3), 123.0 (o-C6H3), 144.7 (i-
C6H3), 154.8 ppm (N-CCH3). Anal. Calcd for C72H112K2N4O4Zn2
(1306.70): C, 66.18; H, 8.64; N, 4.29. Found: C, 66.36; H, 8.58;
N, 4.42.
[Zn(LMe)2Na2(Et2O)2] (3). (LMe)ZnCl2 (1.00 g, 2.30 mmol) and
sodium (0.16 g, 6.90 mmol) were stirred in 50 mL of Et2O at room
temperature for 6 days. The mixture was filtered, and the dark red
filtrate was concentrated to about 10 mL and stored at ca. -20 °C
to yield yellow crystals of the product (0.58 g, 28%) after 1 week.
1H NMR (400 MHz, C6D6): δ 0.98 (d, 12H, J ) 6.8 Hz, Et2O),
1.36 (s, 12H, ArCH3), 1.64 (s, 6H, ArCH3), 1.93 (s, 6H, ArCH3),
2.03 (s, 6H, CCH3), 2.12 (s, 6H, CCH3), 3.60 (m, 8H, Et2O), 6.78
(t, 4H, J ) 6.8 Hz, p-ArH), 7.01 ppm (d, 8H, J ) 6.8 Hz, m-ArH).
13C NMR (100.6 MHz, C6D6): δ 15.0 (CCH3), 15.3 (Et2O),
(CH(CH3)2), 15.9 (CCH3), 20.2 (ArCH3), 20.4 (ArCH3), 21.1
(ArCH3), 21.7 (ArCH3), 25.3 (Et2O), 65.5 (Et2O), 68.3 (Et2O), 115.7
(p-C6H3), 119.4 (p-C6H3),120.2 (m-C6H3), 121.7 (m-C6H3), 127.3
(m-C6H3), 128.6 (i-C6H3), 130.5 (m-C6H3), 131.2 (o-C6H3), 135.3
(o-C6H3), 139.2 (o-C6H3), 140.3 (o-C6H3), 154.1 (N-CCH3), 156.2
ppm (NCCH3). Anal. Calcd for C48H68N4Na2O2Zn (844.41): C,
68.27; H, 8.12; N, 6.63. Found: C, 68.31; H, 8.28; N, 6.67.
[Zn(LEt)2Na2(THF)2] (4). (LEt)ZnCl2 (1.00 g, 2.06 mmol) and
sodium (0.14 g, 6.20 mmol) were stirred in 50 mL of THF at room
temperature for 6 days. The mixture was filtered, and the filtrate
was concentrated to about 10 mL and stored at ca. -20 °C for
several days to give yellow crystals of the product (0.47 g, 24%).
1H NMR (400 MHz, C6D6): δ 1.08 (t, 24H, J ) 8.0 Hz, CH2CH3),
1.40 (t, 8H, J ) 6.0 Hz, THF), 2.08 (s, 12H, CCH3), 2.38 (m, 16H,
CH2CH3), 3.58 (t, 8H, J ) 6.0 Hz, THF), 7.02-7.10 ppm (m, 12H,
ArH). 13C NMR (100.6 MHz, C6D6): δ 14.0 (CH2CH3), 25.3
(CH2CH3), 25.8 (THF), 67.8 (THF), 124.0 (m-C6H3), 126.7 (o-
C6H3), 130.7 (p-C6H3), 148.1 (i-C6H3), 168.1 ppm (N-CCH3). Anal.
Calcd for C56H80N4Na2O2Zn (952.59): C, 70.60; H, 8.46; N, 5.88.
Found: C, 70.43; H, 8.70; N, 5.94.
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (NSFC Grant
No. 20771103), the “Bairen Jihua” project of Chinese
Academy of Sciences, and the 111 Project (No. B07012) in
China and by the U.S. National Science Foundation (No.
CHE-0749868).Wethankthereviewersforhelpfulsuggestions.
Supporting Information Available: CIF files, giving X-ray
structural data for complexes 2-5, Tables S1 and S3, giving the
Cartesian coordinates of the optimized geometry for 2H and 3H,
Tables S2 and S4, giving the theoretical bond distances and angles
for 2H and 3H, Table S5, giving a comparison of the experimental
and theoretical bond parameters, Figures S1 and S3, showing the
optimized structures of 2H and 3H, and Figures S2 and S4, showing
the selected frontier orbitals of 2H and 3H, respectively. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
[Zn(LEt)2K2]n (5). (LEt)ZnCl2 (0.50 g, 1.03 mmol) and potassium
(0.20 g, 5.15 mmol) were combined with 40 mL of toluene at
ambient temperature. The mixture was stirred for 15 days and
filtered to yield a dark red solution, which was concentrated to about
10 mL and stored at ca. -20 °C for several days to yield orange-
red crystals (0.25 g, 19%). The NMR spectra of 5 are not available
due to its poor solubility. Anal. Calcd for C48H64K2N4Zn (840.66):
C, 68.58; H, 7.67; N, 6.66. Found: C, 68.55; H, 7.99; N, 6.45.
X-ray Crystal Structure Determination. Diffraction data for
the complexes 2-5 were collected on a Bruker SMART APEX II
diffractometer at room temperature (293 K) with graphite-mono-
OM800405M
(15) Kervern, G.; Pintacuda, G.; Zhang, Y.; Oldfield, E.; Roukoss, C.;
Kuntz, E.; Herdtweck, E.; Basset, J. M.; Cadars, S.; Lesage, A.; Coperet,
C.; Emsley, L. J. Am. Chem. Soc. 2006, 128, 13545–13552.
(16) Sheldrick, G. M. Program SADABS: Area-Detector Absorption
Correction; University of Go¨ttingen, Go¨ttingen, Germany, 1996.
(17) Sheldrick, G. M. SHELXS-97 and SHELXL-97, Programs for
Crystal Structure Analysis; University of Go¨ttingen, Go¨ttingen, Germany,
1997.
(18) The DFT computations for the model compounds were performed
at the DZP BP86 level of theory with the Gaussian 94 program (full citations
are given in the Supporting Information).