Dizincocene as a Building Block for Novel Zn-Zn-Bonded Compounds?

Article abstract of DOI:10.1021/om801155j

The reaction of decamethyldizincocene (Cp*₂Zn₂, 1) with MesnacnacH proceeds with protonation of the Cp* substituent and subsequent formation of the zinc-zinc-bonded complex (Mesnacnac) ₂Zn₂ (3).

Full text of DOI:10.1021/om801155j

1590
Organometallics 2009, 28, 1590–1592
Dizincocene as a Building Block for Novel Zn-Zn-Bonded
Compounds?
Stephan Schulz,*,† Daniella Schuchmann,† Ulrich Westphal,† 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 December 4, 2008
Remarkably, even though the nature of the Zn-Zn bond, in
particular that of the Cp-substituted derivatives, has been
theoretically investigated in detail,11 only very limited informa-
tion on the chemical reactivity of such compounds is available.
Reactions of 1 with H2O, t-BuOH, and NCXyl as reported by
Carmona et al.1 only proceeded with disproportionation and
subsequent formation of elemental zinc and the corresponding
Zn(II) complexes, whereas reactions with R2Zn (R ) Me, Mes)
yielded the corresponding half-sandwich complexes Cp*ZnR.
Moreover, the reaction with iodine occurred with oxidation and
subsequent formation of Cp*2Zn and ZnI2,3 whereas no reaction
was observed with H2, CO, and CO2. Reactions with Lewis
bases such as NMe3, pyridine, PMe3, and others were unsuc-
cessful until we only very recently reported on the reaction of
1 with the strong Lewis base 4-(dimethylamino)pyridine (dmap),
yielding Cp*Zn-Zn(dmap)2Cp* (2), the first Lewis acid-base
adduct of dizincocene.12 Unexpectedly, the two dmap molecules
were found to bind in a geminal binding mode to only one Zn
atom. At the same time, Jones et al. reported on reactions of
several Lewis bases with a low-valent organomagnesium(I)
complex, yielding the corresponding vicinal bis adducts.13
Herein, we report on the reaction of 1 with [((2,4,6-
Me3C6H2)N(Me)C)2CH]H (MesnacnacH) containing an acidic
N-H group.
Summary: The reaction of decamethyldizincocene (Cp*2Zn2, 1)
with MesnacnacH proceeds with protonation of the Cp*
substituent and subsequent formation of the zinc-zinc-bonded
complex (Mesnacnac)2Zn2 (3).
The epoch-making synthesis of decamethyldizincocene
(Cp*2Zn2, 1),1 the first stable molecular compound containing
a central Zn-Zn bond with the Zn atoms formally in the +1
oxidation state, has been the starting point of intensely growing
research activity on low-valent organozinc complexes.2 Since
the report by Carmona et al. in 2004, six new Zn-Zn-bonded
complexes of the type R2Zn2 containing sterically encumbered
organic substituents (R ) EtMe4Cp,3 [(2,6-i-Pr2C6H3)N(Me)C]2-
CH (Dippnacnac),4 2,6-(2,6-i-Pr2C6H3)C6H3,5 [(2,6-i-Pr2C6H3)-
N(Me)C]2,6 Me2Si[N-(2,6-i-Pr2C6H3)]2,7 1,2-bis[(2,6-diisopro-
pylphenyl)imino]acenaphthene8) have been structurally char-
acterized. Moreover, the first molecular compounds containing
Cd-Cd5b,9 and Mg-Mg10 bonds have been described. Except
for 1, which was initially prepared by reaction of Cp*2Zn and
Et2Zn, the new metal-metal-bonded complexes R2M2 were
synthesized by a procedure analogous to the Wurtz coupling
reaction of the corresponding halide-substituted compounds
RMX (X ) Cl, Br, I).
* To whom correspondence should be addressed. Tel: +49 0201-
1834635. Fax: + 49 0201-1834635; e-mail: stephan.schulz@uni-due.de.
† University of Duisburg-Essen.
Results and Discussion
‡ University of Frankfurt.
Solutions of Cp*2Zn2 (1) and MesnacnacH in n-pentane were
combined at 0 °C, and the resulting solution was stirred for
12 h. Mesnacnac2Zn2 (3) precipitated as a colorless crystalline
solid, which was isolated by filtration. Careful evaporation of
the solvent of the remaining mother liquor under vacuum yielded
a waxy solid, which was dispersed in 4 mL of cold pentane
(0 °C) and then filtered. Resonances due to the formation of
Cp*H were clearly observable in the filtrate, whereas the
remaining white solid showed resonances of additional com-
plex 3.
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2004, 305, 1136–1138. (b) del R´ıo, D.; Galindo, A.; Resa, I.; Carmona, E.
Angew. Chem., Int. Ed. 2005, 44, 1244–1247. (c) Carmona, E.; Galindo,
A. Angew. Chem., Int. Ed. 2008, 47, 6526–6536.
(2) It should be noted that Zn2H2, the first molecular zinc complex with
a Zn-Zn bond, was trapped in an Ar matrix at 12 K. Zn2H2 was
characterized by vibrational spectroscopy and theoretical calculations: (a)
Wang, X.; Andrews, L. J. Phys. Chem. A 2004, 108, 11006–11013. (b)
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(3) Grirrane, A.; Resa, I.; Rodriguez, A.; Carmona, E.; Alvarez, E.;
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J. Am. Chem. Soc. 2007, 129, 693–703.
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King, R. B.; Schleyer, P. v. R.; Schaefer, H. F., III.; Robinson, G. H. J. Am.
Chem. Soc. 2005, 127, 11944–11945.
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M.; Power, P. P. Angew. Chem., Int. Ed. 2006, 45, 5807–5810. (b) Zhu,
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Power, P. P. J. Am. Chem. Soc. 2006, 128, 15068–15069.
(10) Green, S. P.; Jones, C.; Stasch, A. Science 2007, 318, 1754–1757.
3 is soluble in organic solvents such as hexane, toluene, and
Et2O. The 1H NMR spectrum of 3 shows resonances due to the
organic groups of the Mesnacnac substituent. No indication for
(11) (a) Kan, Y. J. Mol. Struct. (Theochem) 2007, 805, 127–132. (b)
Velazquez, A.; Ferna´ndez, I.; Frenking, G.; Merino, G. Organometallics
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10.1021/om801155j CCC: $40.75
2009 American Chemical Society
Publication on Web 01/29/2009

Notes
Organometallics, Vol. 28, No. 5, 2009 1591
Scheme 1. Lewis Acid-Base Adducts of Low-Valent Org-
anozinc and Organomagnesium Complexes
Scheme 2. Synthesis of Complex 3
Figure 1. Solid-state structure of 3. Thermal ellipsoids are shown
at 50% probability levels; H atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Zn1-Zn1a ) 2.3813(8),
Zn1-N1 ) 2.005(2), Zn1-N2 ) 2.000(3), N1-C1 ) 1.336(4),
N1-C11 ) 1.446(4), N2-C3 ) 1.331(4), N2-C21 ) 1.458(4),
C1-C2 ) 1.396(4), C2-C3 ) 1.416(4); N1-Zn1-N2 ) 93.8(2),
N1-Zn1-Zn1a ) 133.0(1), N2-Zn1-Zn1a ) 133.3(1).
perfectly with the experimental value observed for 3 (2.3813(8)
Å) as well as the calculated value for 6 (2.392 Å).
1
the formation of a zinc hydride species was found in the H
The use of Cp*2Zn2 (1) as the starting reagent for the synthesis
of novel low-valent organozinc complexes may open a general
access to this interesting class of compounds, including com-
plexes which cannot be obtained from procedures analogous to
the Wurtz coupling reactions. For instance, several attempts to
synthesize 3 by reduction reactions of MesnacnacZnX (X )
Cl, I)17 with different reducing agents (Na, K, K-graphite,
Na-napthalenide) only resulted in the formation of Zn(II)
complex (Mesnacnac)2Zn (5) in high yield.15 Reactions of 1
with different substituents containing acidic H atoms are
currently under investigation.
NMR of 3, and the IR spectra also showed no absorption peak
due to a Zn-H group in the typical range from 1600 to 1900
cm-1.14 Moreover, no disproportionation reaction with formation
of elemental zinc and (Mesnacnac)2Zn was observed.15
Single crystals of 3 suitable for an X-ray structure determi-
nation were obtained from a solution in n-pentane/toluene at
-30 °C (Figure 1). The central structural motif of 3 is the
Zn-Zn bond (2.3813(8) Å), which is, surprisingly, slightly
elongated compared to that observed for the sterically more
hindered complex Dippnacnac2Zn2 (6; 2.3586(7) Å)4 containing
the larger Dipp substituents. The torsion angle (N1-Zn1-Zn1′-
N1′ ) 42.8°) observed for 3 is significantly smaller compared
to that of 6 (86.6°). The Zn-Zn bond length previously observed
for R2Zn2 complexes ranges from 2.29 to 2.35 Å, whereas
comparable Zn-Zn bond lengths were observed for the doubly
reduced diimine derivative (R ) [(2,6-i-Pr2C6H3)N(Me)C]2;
2.3994(6) Å)6 and the very recently reported Lewis acid-base
adduct Cp*Zn-Zn(dmap)2Cp* (2; 2.4184(4) Å).12 The C3N2Zn
rings in 3 are almost planar with the Zn atoms slightly out of
the plane, as was previously observed in Dippnacnac-
Experimental Section
General Comments. All manipulations were performed under
an Ar atmosphere. Solvents were dried over Na/K and degassed
1
prior to use. H and 13C{1H} NMR spectra were recorded on a
Bruker Avance 500 spectrometer and are referenced to internal
C6D5H (1H, δ 7.154; 13C, δ 128.0). The IR spectrum of 3 was
recorded on a ALPHA-T FT-IR spectrometer equipped with a
single-reflection ATR sampling module. The melting point of 3
was measured in a sealed capillary and was not corrected. Elemental
analyses were performed at the Elementaranalyse Labor of the
University of Essen.
16
ZnN(SiMe3)2 and 6.4
DFT calculations were performed to improve the understand-
ing of the formation and the bonding situation of 3. The reaction
of 1 with MesnacnacH is exothermic (-48 kcal/mol), and the
calculated structure of 3 is very similar to the calculated structure
of Dippnacnac2Zn2 (6).4 The Zn atoms in 3 carry a partial charge
of 0.86 (0.85 for 6), and the Zn-Zn bond has mainly s character
(s 94.5%, p 4.25%, d 1.29%), as was observed for 6. The
computed Zn-Zn bond distance of 2.378 Å agrees almost
Preparation of (Mesnacnac)2Zn2 (3). A 0.32 g portion of
MesnacnacH (1.0 mmol) dissolved in 5 mL of n-pentane was added
at 0 °C to a solution of 0.20 g of Cp*2Zn2 (0.5 mmol) in 10 mL of
n-pentane and the mixture stirred for 2 h at -4 °C. 3 precipitated
as a colorless crystalline solid, which was isolated by filtration.
Yield: 0.12 g (0.38 mmol, 40%). Melting point: 240 °C. Anal.
Found (calcd) for C46H58N4Zn2 (797.73 g/mol): H, 7.21 (7.33); C,
1
69.07 (69.26); N, 6.94 (7.02). H NMR (500 MHz, C6D6, 25 °C):
(14) Hao, H.; Cui, C.; Roesky, H. W.; Bai, G.; Schmidt, H.-G.;
Noltemeyer, M. Chem. Commun. 2001, 1118–1119.
(15) The formation of (Mesnacnac)2Zn can be excluded, since the 1H
and 13C resonances of this Zn(II) complex are significantly shifted compared
to those observed for 3: Schulz, S.; Eisenmann, T.; Bla¨ser, D.; Boese, R.
Submitted for publication.
(16) Cheng, M.; Moore, D. R.; Reczek, J. J.; Chamberlain, B. M.;
Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 8738–
8749.
δ 1.55 (s, 6 H, CMe), 1.84 (s, 12 H, R o-H), 2.29 (s, 6H, R p-H),
4.86 (s, 1 H, CH), 6.89 (s, 4H, m-H). 13C{1H} NMR (125 MHz,
C6D6, 25 °C): δ 19.2 (R o-C), 21.1 (R p-C), 22.6 (CCH3), 95.5
(CH), 129.0 (m-C), 131.3 (o-C), 132.8 (p-C), 146.7 (CN), 164.7
(CCH3). IR: ν 2905, 1532, 1450, 1386, 1259, 1200, 1146, 1013,
852, 800, 741, 567, 497, 388 cm-1
.
(17) Schulz, S.; Eisenmann, T.; Westphal, U.; Schmidt, S.; Flo¨rke, U.
Z. Anorg. Allg. Chem., in press.
(18) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.

1592 Organometallics, Vol. 28, No. 5, 2009
Notes
Single-Crystal X-ray Analysis. Data were collected on a Bruker
AXS SMART APEX CCD diffractometer, (Mo KR radiation, λ )
0.710 73 Å; T ) 173(2) K). The structure was solved by direct
methods (SHELXS-97)18 and refined by full-matrix least squares
on F2. A semiempirical absorption correction was applied. All non-
hydrogen atoms were refined anisotropically, and hydrogen atoms
were refined by a riding model (SHELXL-97, Program for Crystal
Structure Refinement).19
Computational Calculations. DFT calculations were carried out
with the Gaussian03 suite of programs (M. J. Frisch, et al., Gaussian
03, Revision D.02; Gaussian Inc., Pittsburgh, PA, 2003; the
complete reference is given in the Supporting Information). The
molecular structure of 3 was 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 3, which
were calculated from NBO population analyses, are given in the
Supporting Information.
Data for 3: C46H58N4Zn2, M ) 797.73, colorless crystal (0.15 ×
0.11 × 0.04 mm), monoclinic, space group P2/n, a ) 13.4527(10)
Å, b ) 8.3904(7) Å, c ) 19.4814(14) Å, ꢀ ) 103.768(6), V )
2135.8(3) Å3, Z ) 2, µ ) 1.158 mm-1, Fcalcd ) 1.240 g cm-3, 16 314
reflections (2θmax ) 50°), 3753 unique reflections (Rint ) 0.0809),
243 parameters; largest maximum/minimum in the final difference
Fourier synthesis 0.269/-0.336 e Å-3, maximum/minimum trans-
mission 0.9552/0.8455; R1 ) 0.0428 (I > 2σ(I)), wR2 (all data)
) 0.0782.
Acknowledgment. S.S. thanks the German Science Foun-
dation (DFG) for financial support. D.S. is grateful to the
Fonds der Chemischen Industrie for a doctoral fellowship.
Supporting Information Available: Tables, text, figures, and a
CIF file giving X-ray crystallographic data, absolute energies and NBO
analyses of 3, and the full citation for Gaussian 03. This material is
available free of charge via the Internet at http://pubs.acs.org.
(19) Sheldrick, G. M. SHELXS-97; Universita¨t Go¨ttingen, Go¨ttingen,
Germany, 1997.
OM801155J