On the chemistry of Zn-Zn bonds, RZn-ZnR (R = [{(2,6-Pri 2C6H3)N(Me)C}2CH]): Synthesis, structure, and computations

Article abstract of DOI:10.1021/ja053819r

Potassium reduction of RZn(μ-I)₂Li(OEt₂)₂ (R = [{(2,6-Prⁱ₂C₆H₃)N(Me)C}₂CH]) affords the second compound with a Zn-Zn bond, RZn-ZnR. The air- and moisture-sensitive title compound was characterized by ¹H NMR, elemental analyses, and single-crystal X-ray diffraction. The Zn-Zn bond was determined to be 2.3586(7) A; this value is only about 0.05 A longer than the Zn-Zn bond reported for Cp*Zn-ZnCp* (Cp* = C₅Me₅), the first reported compound with a Zn-Zn bond. In addition, density functional theory (DFT) computations on related model RZn-ZnR compounds provide insight into the intriguing Zn-Zn bond. Copyright

Full text of DOI:10.1021/ja053819r

Published on Web 08/04/2005
On the Chemistry of Zn-Zn Bonds, RZn-ZnR
(R ) [{(2,6-Pri2C6H3)N(Me)C}2CH]): Synthesis, Structure, and Computations
Yuzhong Wang, Brandon Quillian, Pingrong Wei, Hongyan Wang, Xiao-Juan Yang, Yaoming Xie,
R. Bruce King, Paul v. R. Schleyer, H. Fritz Schaefer, III, and Gregory H. Robinson*
Department of Chemistry and the Center for Computational Chemistry, The UniVersity of Georgia,
Athens, Georgia 30602-2556
Received June 10, 2005; E-mail: robinson@chem.uga.edu
The recent preparation of the first compound containing a Zn-
Zn bond, Cp*Zn-ZnCp* (Cp* ) C5Me5), 1,1,2 is a remarkable
achievement. The characterization of other such compounds is
desirable for further exploration of Zn-Zn bond chemistry.3-6 To
this end, we now report the synthesis and molecular structure7 of
the second compound with a Zn-Zn bond, RZn-ZnR (R ) [{-
(2,6-Pri2C6H3)N(Me)C}2CH]), 2 (Dipp ) 2,6-diisopropylphenyl).
Figure 1. (a) Molecular structure of 2 (thermal ellipsoids are shown at
30% probability levels). Selected bond distances (Å) and angles (deg): Zn-
(1)-Zn(2) 2.3586(7), Zn(1)-N(1) 2.005(3), Zn(1)-N(2) 2.013(3), Zn(2)-
N(3) 2.014(3), Zn(2)-N(4) 2.010(3); N(1)-Zn(1)-N(2) 93.65(13), N(1)-
Zn(1)-Zn(2) 131.12(9), N(2)-Zn(1)-Zn(2) 134.10(9), N(3)-Zn(2)-N(4)
93.43(13), N(3)-Zn(2)-Zn(1) 132.62(9), N(4)-Zn(2)-Zn(1) 132.76(10).
Our route to 2 began with the preparation of the lithium
(b) Space filling model of 2.
derivative, RLi (R ) [{(2,6-Pri2C6H3)N(Me)C}2CH])8 followed by
its reaction with ZnI2 in Et2O to give RZn(µ-I)2Li(OEt2)2.9
close to the 4.56 ppm reported for a tris(pyrazolyl)hydroborato
Potassium reduction of RZn(µ-I)2Li(OEt2)2 affords 2 as colorless,
complex with a terminal Zn-H.26 The fact that no such zinc hydride
air- and moisture-sensitive crystals (eq 1).
1H resonance was observed for 2, coupled with supporting structural
and computational data (Vide infra), further affirms 2.
RZn(µ-I)2Li(OEt2)2 9
K8 2 (1)
The six-membered C3N2Zn rings of 2 are not planar but adopt a
puckered conformation with the zinc atom residing 0.65 Å out of
the N-C-C-C-N plane. A similarly puckered C3N2Zn ring in
RZnN(SiMe3)2 (R ) [{(2,6-Pri2C6H3)N(Me)C}2CH])27 has been
reported. However, the C3N2Zn rings are planar in RZn(µ-H)2ZnR
(R ) [{(2,6-Me2C6H3)N(Me)C}2CH]).25 The zinc atoms in 2 adopt
a trigonal planar geometry, while the Zn-N bond distances of
2.005(3) and 2.014(3) Å are among the longest on record.28
The Zn-Zn bond of 2 was probed by B3LYP/DZP++ and BP86/
DZP++ density functional theory (DFT) computations on the RZn-
ZnR (R ) [(HNCH)2CH]) model compound, 2H (Figure 2a). As
found in 2, the perpendicular D2d conformation of 2H is slightly
favored, with the D2h rotation transition state being only 0.26
(B3LYP) or 0.37 kcal/mol (BP86) higher in energy than the D2d
minimum. The C3N2Zn rings of 2H are planar, rather than having
the zinc atoms puckered out-of-plane, as observed experimentally
in 2. Indeed, 2H may be regarded as an isoelectronic and a
potentially metalloaromatic29 analogue of biphenyl with a central
Zn-Zn bond: two C-C-C fragments of each ring having been
replaced by isoelectronic N-Zn-N units. Aromaticity was probed
by computing nucleus-independent chemical shifts (NICS)30 on the
simple benzene-like cyclic C3H5N2ZnH monomer. The refined
NICS(0)π zz value of -7.6 (based on the tensor component
perpendicular to the ring)31 reveals weak aromatic character
(compare the -36.6 benzene and the -1.0 1,4-cyclohexadiene
NICS(0)π zz values at the same level).
Supporting computations on related RZn-ZnR systems provide
insight into the nature of the Zn-Zn bond.
Compounds with homonuclear metal-metal bonds of the heavier
group 12 metals, cadmium10,11 and mercury,12 are well-known.
Alkali or alkaline earth metal reduction of metal halides, complexed
by sterically demanding ligands, has proven to be a fruitful synthetic
approach to compounds containing main group metal-metal
bonds13-16 and main group metal-transition metal bonds.17-19 We
applied this approach to the preparation of 2 by utilizing the well-
known sterically encumbered â-diketiminate ligand, [{(2,6-
Pri2C6H3)N(Me)C}2CH]-.20-22 This â-diketiminate ligand has been
used to stabilize an In-In bond, R(Cl)In-In(Cl)R,23 and a recently
reported Mn-Mn bond, RMn-MnR.24
X-ray structure analysis confirms the dimeric nature of 2 and
the central Zn-Zn bond (Figure 1). The two ligands are arranged
in a nearly orthogonal orientation with a N(1)-Zn(1)-Zn(2)-N(3)
torsion angle of 86.6°, thus providing effective steric protection of
the Zn-Zn bond (Figure 1b). The Zn-Zn distance in 2, 2.3586(7)
Å, is only about 0.05 Å longer than that of 2.305(3) Å reported for
1. However, the Zn-Zn bond lengths for 1 and 2 are notably shorter
than the Zn‚‚‚Zn separation of 2.4513(9) Å in the related zinc
hydride dimer, RZn(µ-H)2ZnR (R ) [{(2,6-Me2C6H3)N(Me)C}2-
1
CH]).25 Moreover, the H NMR resonance for the bridging zinc
hydride in RZn(µ-H)2ZnR was found at 4.59 ppm. This value is
9
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J. AM. CHEM. SOC. 2005, 127, 11944-11945
10.1021/ja053819r CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
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Acknowledgment. G.H.R., R.B.K., P.v.R.S., and H.F.S. are
grateful to the National Science Foundation (CHE-0209857).
Supporting Information Available: Full details of the computa-
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