A zinc-zinc-bonded compound and its derivatives bridged by one or two hydrogen atoms: A new type of Zn-Zn bonding

Article abstract of DOI:10.1002/anie.200601926

(Chemical Equation Presented) Unique interactions: Ar′ZnZnAr′ (1; Ar′ = C₆H₃-2,6-(C₆H ₃-2,6-iPr₂)₂), the dimer Ar′Zn(μ-H) ₂ZnAr′ (2), and the unprecedented species Ar′Zn(μ-H) (μ-

Full text of DOI:10.1002/anie.200601926

Angewandte
Chemie
À
and also to show that Zn Zn bonds can in fact be stabilized by
monodentate ligands.Furthermore, the synthesis of the singly
or doubly bridged hydride derivatives of such a species would
À
provide further information on the nature of the Zn Zn
bonding.We now describe the synthesis and characterization
of the zinc–zinc-bonded species Ar’ZnZnAr’ (4) (Ar’ = C6H3-
2,6-(C6H3-2,6-iPr2)2), the hydride-bridged dimer Ar’Zn(m-
H)2ZnAr’ (5), which has a Zn···Zn separation very similar
to that in 4, and the unique compound Ar’Zn(m-H)(m-
Na)ZnAr’ (6), in which the Zn atoms are connected by a
À
new type of Zn Zn interaction.
Dinuclear 4 was synthesized by the reduction of Ar’ZnI
(3) with Na.The hydride, Ar ’Zn(m-H)2ZnAr’ (5), was
prepared by the treatment of 3 with NaH.Rather than
forming zinc–zinc-bonded compound 4 (as indicated in the
indirect route for 1 whereby KH was used to reduce the
[Zn(Cp*)2] species in situ[1b] or whereby [Cp*ZnZnCp*] was
Metal–Metal Bonding
DOI: 10.1002/anie.200601926
obtained by the unforeseen ligand exchange between
[1a]
*
[ZnCp2 ] and ZnEt2
(Scheme 1)), further reduction of 5
A Zinc–Zinc-Bonded Compound and Its
by NaH provided the unprecedented sodium hydride-bridged
species Ar’Zn(m-H)(m-Na)ZnAr’ (6, Scheme 2).
Derivatives Bridged by One or Two Hydrogen
À
Atoms: A New Type ofZn Zn Bonding**
Zhongliang Zhu, Robert J. Wright,
Marilyn M. Olmstead, Eric Rivard, Marcin Brynda, and
Philip P. Power*
Recently, the synthesis and structural characterization of
decamethyldizincocene [Cp*ZnZnCp*] (1; Cp* = h5-C5Me5,
Scheme 1. Synthetic routes to the zinc–zinc-bonded dimer
[Cp*ZnZnCp*] (1).[1]
À
Zn Zn 2.305(3) Š) was reported by Carmona and co-work-
ers.[1] It was the first stable compound that contained a Zn Zn
À
bond.[2] This landmark development was confirmed by
À
calculations which showed that the Zn Zn bond is formed
mainly by the overlap of the Zn 4s orbitals while the Zn
4p orbitals interact with the h5-C5Me5 ligand.[1b,3] A further
À
example of a compound with a Zn Zn bond, [{HC-
(CMeNAr)2}ZnZn{HC(CMeNAr)2}] (Ar= 2,6-iPr2C6H3) (2),
was reported by Robinson and co-workers,[4] and this species
À
has a Zn Zn bond length of 2.3586(7) Š. The zinc atoms are
three-coordinate, and the metal–metal bond has 95% s char-
À
acter.The Zn Zn bond in 2 is about 0.05 Š longer than that in
1 despite its lower metal coordination number.Clearly, the
À
type of ligand exerts a large influence on the Zn Zn bond
length.It is therefore desirable to have zinc–zinc-bonded
species in which the Zn atoms are bound to only one ligand
atom to provide the simplest possible bonding arrangement
Scheme 2. Synthetic routes to 4, 5, and 6.
The crystal structure of 4 (Figure 1)[5] confirms the
À
À
presence of a Zn Zn bond.The observed Zn Zn bond
length in 4 (2.3591(9) Š) is about 0.05 Š larger than that in 1
(2.305(3) Š) and is essentially the same as that in 2
(2.3586(7) Š). The two Ar’ ligands are arranged in a nearly
orthogonal orientation to each other, which provides effective
[*] Z. Zhu, Dr. R. J. Wright, Prof. M. M. Olmstead, Dr. E. Rivard,
Dr. M. Brynda, Prof. P. P. Power
Department of Chemistry
University of California
One Shields Avenue, Davis, CA 95616 (USA)
Fax: (+1)530-752-8995
À
steric protection of the Zn Zn moiety.An almost linear
E-mail: pppower@ucdavis.edu
arrangement is observed for the {C(ipso)-Zn-Zn-C(ipso)}
interior, which is in contrast to the other known metal–metal-
bonded dimers ArMMAr, (M = Cr,[6a] Ga,[6b] Ge,[6c] As,[6d] or
Se[6e]) which for numerous reasons display strongly trans-bent
geometries.
[**] The authors thank the National Science Foundation for financial
support. E.R. thanks NSERCof Canada for a postdoctoral fellow-
ship.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 5807 –5810
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5807

Communications
Figure 1. Thermal-ellipsoid plot (ellipsoids set at 30% probability) of 4
without hydrogen atoms. Selected bond lengths [Š] and angles [8]:
Zn(1)-Zn(2) 2.3591(9), Zn(1)-C(1) 1.965(6), Zn(2)-C(31) 1.974(5);
C(31)-Zn(2)-Zn(1) 177.28(18), Zn(2)-Zn(1)-C(1) 177.57(18), C(32)-
C(31)-Zn(2) 120.5(4), C(36)-C(31)-Zn(2) 119.1(4).
Figure 2. Thermal-ellipsoid plot (ellipsoids set at 30% probability) of 5
without hydrogen atoms. Selected bond lengths [Š] and angles [8]:
Zn(1)···Zn(2) 2.4084(3), Zn(1)-H(1) 1.67(2), Zn(1)-H(2) 1.79(3),
Zn(1)-C(1) 1.9301(16); C(1)-Zn(1)-Zn(2) 175.14(5), Zn(1)-Zn(2)-C(31)
175.60(5), C(1)-Zn(1)-H(1) 138.3(9), Zn(2)-Zn(1)-H(1) 45.2(8), C(6)-
C(1)-Zn(1) 121.21(11), C(2)-C(1)-Zn(1) 120.00(12).
À
Well-characterized, stable Zn H species are relatively
rare because of the high tendency of these species to
oligomerize or polymerize.[7] Nonetheless, the synthesis of
5[5] proceeded smoothly and in moderate yield.The bridging
hydrogen atoms in 5 were located in the electron difference
map.The overall appearance of the molecular structure of 5
(Figure 2) and perpendicular orientation of the ligands are
very similar to those of 4 (crystals of 4 and 5 are isomor-
phous), and the Zn···Zn separation in 5 (2.4084(3) Š) is
lengthened by only about 0.05 Š in comparison with that in 4.
In the 1H NMR spectrum a signal corresponding to the
bridging hydride in 5 was located at d = 4.84 ppm, in a
1:1 intensity ratio with the signals of the aryl ligand.A similar
chemical shift was observed at d = 4.59 ppm in the zinc
hydride [RZn(m-H)2ZnR] (R = [{(2,6-iPr2C6H3)N(Me)C}2-
CH]; Zn···Zn 2.4513(9) Š).[7e] It is notable that the Zn···Zn
separation in this hydride species is about 0.1 Š longer than
that seen in the zinc–zinc-bonded b-diketiminate derivative
2.[4]
5
À
ligand-based p levels of the h -C5Me5 ring.In 2, the Zn Zn
bond is the HOMO, and the LUMO is a ligand orbital of
p symmetry.
The sodium hydride bridged compound 6[5] could be
obtained from 3 or 5 by the reaction with the appropriate
À
number of equivalents of NaH (Scheme 2).The Zn Zn bond
length in 6 is 2.352(2) Š (Figure 4), which is essentially the
same as that in 4 (2.3591(9) Š). In contrast to 4 and 5, the two
central aryl rings on the Ar’ ligand are nearly coplanar in 6,
and the bridging sodium and hydride centers also lie near this
plane.The Na + counterion is sequestered by strong
h6 interactions involving the flanking aryl rings on the
Ar’ ligand.The Na +···C(aryl) separations range from 2.843
to 3.108 Š, and the Na centroid distances of the two
flanking aryl rings are 2.658 Š and 2.670 Š, respectively. In
addition, the two sets of Zn···Na+ and Zn···HÀ distances are
3.113(4)/3.130(4) Š and 1.75(6)/1.74(6) Š, respectively. The
1H NMR spectrum of 6 displays a broad signal at d =
2.04 ppm, which was assigned to the bridging hydride.
+
À
DFT calculations[8,9] for 4 (Figure 3) were performed at
the B3LYP/6-31g* level and indicate that the HOMO is
À
localized mainly in the Zn Zn s-bonding region with some
The metal–metal bonding in 6 differs considerably from
that observed in 4.The DFT calculations indicate that there is
a delocalized orbital formed by the bridging hydrogen atom
and two zinc atoms, as illustrated in HOMOÀ17 (Figure 5).In
contrast to 4, 4s orbitals of zinc atoms participate in the
À
additional Zn C(ipso) s-bonding character.The LUMO and
LUMO + 1 consist of two almost degenerate orbitals of
À
p symmetry, which are localized mostly on the central {Zn
Zn}2+ unit.These calculations show that the Zn Zn bond is
À
formed from the overlap of mainly
4pz orbitals in the HOMO, thus
leaving two empty orthogonal px
and py orbitals to form the p-sym-
metric LUMOs.The 4s orbitals of
the zinc centers are mainly
involved in bonding to the ipso car-
bon atoms of the ligands.In con-
trast to 4, the Zn 4s orbitals form
À
À
the Zn Zn bond in 1, and this Zn
Zn bonding orbital is the
HOMOÀ4.The orbitals HOMO
to HOMOÀ3 of
1 are mostly Figure 3. Representation of the key molecular orbitals of 4 from DFT calculations.
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Chemie
Experimental Section
All manipulations were carried out under anaerobic and anhydrous
1
conditions. H and 13C NMR spectra were recorded on a Varian 300
spectrometer and referenced to known standards.
4: A solution of 3 (0.93 g, 1.58 mmol) in diethyl ether (50 mL) was
added to a Schlenk tube containing finely cut Na (0.036 g, 1.58 mmol)
at about 258C.After stirring for 2 days, some Zn metal was
precipitated.The solution was filtered, and the solvent was removed
in a dynamic vacuum.The residue was redissolved in hexane (5 mL),
and storage for 2 days in a freezer (ca. À408C) afforded large
colorless X-ray quality crystals of 4. Yield: 0.174 g, 23.9%; m.p.
> 3608C (when the temperature was kept at ca.360 8C for several
1
minutes, 4 decomposed, and a black solid was deposited); H NMR
3
(300 MHz, C6D6, 258C): d = 1.00 (d, 12H, o-CH(CH3)2, JHH
=
6.9 Hz), 1.12 (d, 12H, o-CH(CH3)2, 3JHH = 6.9 Hz), 2.86 (sept, 4H,
CH(CH3)2, 3JHH = 6.9 Hz), 7.06–7.24 ppm (m, 9H, m-C6H3, p-C6H3,
m-Dipp, and p-Dipp; Dipp = 2,6-iPr2C5H3); 13C{1H} NMR (C6D6,
100.6 MHz, 258C): d = 24.7 (CH(CH3)2), 24.8 (CH(CH3)2), 30.4
(CH(CH3)2), 123.0 (m-Dipp), 125.7 (p-C6H3), 127.6 (p-Dipp), 128.3
(m-C6H3), 142.3 (i-Dipp), 146.9 (o-C6H3), 147.4 (o-Dipp), 162.4 ppm
(i-C6H3).
5: The iodide derivative 3 (1.00 g, 1.70 mmol) and NaH (0.060 g,
2.50 mmol) were combined with THF (50 mL) under an inert
atmosphere of dry argon at ambient temperature.The mixture was
stirred for 2 days, the solvent was then
Figure 4. Thermal-ellipsoid plot (ellipsoids set at 30% probability) of 6
without hydrogen atoms. Selected bond lengths [Š] and angles [8]:
Zn(1)-Zn(2) 2.352(2), Zn(1)-H(1) 1.75(6), Zn(1)-Na(1) 3.113(4),
Zn(1)-C(1) 1.977(5); C(1)-Zn(1)-Zn(2) 170.38(15), Zn(2)-Zn(1)-Na(1)
68.25(5), Zn(2)-Zn(1)-H(1) 47.6(18), Zn(1)-Na(1)-Zn(2) 44.26(7), C(6)-
C(1)-Zn(1) 121.21(11), C(2)-C(1)-Zn(1) 120.00(12); dihedral angles:
between the C1–C6 and C31–C36 planes 1.98, between the C1–C6 and
Zn2HNa planes 8.98, between the C31–C36 and Zn2HNa planes 9.38.
removed in a dynamic vacuum, and the
residue was extracted with hexane
(50 mL).The slurry was allowed to settle,
and the mother liquor was separated from
the precipitate (NaI and excess NaH).The
volume was concentrated to about 10 mL,
and storage for 2 days in
a freezer
(ca. À408C) afforded colorless X-ray
quality crystals of 5. Yield: 0.81 g, 89.1%
(based on 3); decomposed before melting
(loss of crystallinity started at 2108C, and
decomposition of 5 to a black solid was
observed at 2908); 1H NMR (300 MHz,
C6D6, 258C): d = 1.01 (d, 12H, o-CH-
(CH3)2, 3JHH = 6.9 Hz), 1.11 (d, 12H, o-
Figure 5. Representation of the key molecular orbitals of 6 from DFT calculations.
HOMOÀ17 bonding interaction to yield an orbital that has
3
CH(CH3)2, JHH = 6.9 Hz), 2.91 (sept, 4H,
CH(CH3)2, 3JHH = 6.9 Hz), 4.84 (s, 1H,
ZnH), 7.04–7.25 ppm (m, 9H, m-C6H3, p-C6H3, m-Dipp, and p-
Dipp); 13C{1H} NMR (C6D6, 100.6 MHz, 258C): d = 24.3 (CH(CH3)2),
25.1 (CH(CH3)2), 30.6 (CH(CH3)2), 123.2 (m-Dipp), 126.0 (p-C6H3),
127.8 (p-Dipp), 128.5 (m-C6H3), 143.4 (i-Dipp), 146.7 (o-C6H3), 148.7
(o-Dipp), 155.7 ppm (i-C6H3); IR (nujol): n˜(Zn-H) bands are likely
obscured by overlapping of ligand vibrations.
À
À
both Zn Zn and Zn H bonding character.The HOMO of 6
has considerable electron density in the region between the
Na+ ion and the zinc atoms, and between the zinc centers.
There is also a smaller region of electron density of opposite
phase near the bridging hydrogen atom such that the orbital
À
resembles a distorted Zn Zn bonding interaction of p sym-
[9]
metry.However, the Wiberg bond order
calculations
6: Compound 5 (0.50 g, 1.08 mmol) and NaH (0.039 g, 1.62 mmol)
were combined in THF (50 mL).The mixture was stirred for 2 days,
the solvent was then removed in a dynamic vacuum, and the residue
was extracted with PhMe (50 mL).The slurry was allowed to settle,
and the mother liquor was separated from the precipitate (NaH) by a
filter cannula.The solvent volume was concentrated to about 10 mL,
and storage for 2 days in a freezer (ca. À188C) afforded colorless X-
ray quality crystals of 6. Yield: 1.03 g, 87.3% (based on 5);
decomposed before melting (loss of crystallinity started at 2358C,
À
indicate that the Zn Zn bond order (0.81) in 6 is slightly
less than that (0.95) in 4 with a considerable bonding
interaction between the zinc centers and sodium atom
(bond order 0.68).
In conclusion, a new zinc–zinc-bonded compound sup-
ported by monodentate ligands has been synthesized by a
straightforward procedure.The related hydrogen-bridged
À
dimer shows that, although it has no Zn Zn bond, the
and decomposition of 6 to a black solid was seen at 2688C); 1H NMR
metal–metal separation is only about 0.05 Š longer. In
3
(300 MHz, C6D6, 258C): d = 1.09 (d, 12H, o-CH(CH3)2, JHH
=
contrast to the zinc–zinc-bonded compounds 1 and 2,[1,4] the
6.9 Hz), 1.15 (d, 12H, o-CH(CH3)2, 3JHH = 6.9 Hz), 2.04 (br, 1H,
Zn2NaH), 2.99 (sept, 4H, CH(CH3)2, 3JHH = 6.6 Hz), 7.01–7.27 ppm
(m, 9H, m-C6H3, p-C6H3, m-Dipp, and p-Dipp); 13C{1H} NMR (C6D6,
100.6 MHz, 258C): d = 23.6 (CH(CH3)2), 25.2 (CH(CH3)2), 30.7
(CH(CH3)2), 122.4 (m-Dipp), 127.3 (p-C6H3), 146.8 (i-Dipp), 148.1
(o-C6H3), 148.8 (o-Dipp), 158.7 ppm (i-C6H3), m-C6H3 and p-Dipp
À
Zn Zn bond in Ar’ZnZnAr’ (4) is formed by using mainly
4pz orbitals.DFT calculations and the structure parameters
indicate a new type of Zn Zn bond in 6.Future work will
focus on investigations of the reactivity of the Zn Zn bond in
À
À
compounds 4, 6, and related species.
Angew. Chem. Int. Ed. 2006, 45, 5807 –5810
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www.angewandte.org 5809

Communications
resonances are likely obscured by the C6H6 signal; IR (nujol): n˜(Zn-H)
bands are likely obscured by overlapping of ligand vibrations.
replaced with protons.The single point (SP) calculations were
performed by using Gaussian03 software (Gaussian03 (Revi-
sionB.03): M. J. Frisch et al. (see the Supporting Information).
The representations of Kohn–Sham orbitals were generated using
the MOLEKEL package (MOLEKEL, P.Flukiger, H.P.Luthi, S.
Portmann, J.Weber, MOLEKEL43. , Swiss Center for Scientific
Computing, Manno, Switzerland, 2000–2002), and the Wiberg
bond orders were computed from SP B3LYP/6-31g* calculations
using the AOMix program (AOMIX, S.I. Gorelsky, AOMix
program, Rev.5.44, http://www.obbligato.com/software/aomix/).
Received: May 16, 2006
Published online: August 14, 2006
Keywords: density functional calculations · hydride ligands ·
.
steric hindrance · structure elucidation · zinc
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5810
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