Structural transformations in dinuclear zinc complexes involving Zn–Zn
bonds{
Yi-Chou Tsai,*
aDuan-Yen Lu,
aYang-Miin Lin,
aJenn-Kang Hwang
band Jen-Shiang K. Yu
bReceived (in Cambridge, UK) 16th May 2007, Accepted 20th July 2007 First published as an Advance Article on the web 6th August 2007 DOI: 10.1039/b707396h
Reduction of Zn2(m-g2-Me2Si(NDipp)2)2 with 4 equiv. of
KC8 resulted in a dramatic structural transformation into
[(g2-Me2Si(NDipp)2)ZnZn(g 2
-Me2Si(NDipp)2)] 22
featuring a Zn–Zn bond instead of [Zn2(m-g2-Me2Si(NDipp)2)2]22; the
mechanism of the observed structural transformations arising from the Zn–Zn bond formation involving the intermediate of [Zn2(m-g
2
-Me2Si(NDipp)2)2] 2
was elucidated by elaborate computations.
In the field of Zn chemistry, a landmark discovery was recently made with the synthesis and characterization of dizincocene Cp*ZnZnCp* where Cp* = g5-C
5(CH3)5 (1a) or g5-C5(CH3)4
-(C2H5) (1b) by Carmona and co-workers1This was followed by
the synthesis of the coordination dizinc compound (Nacnac)ZnZn-(Nacnac) (Nacnac = [{(2,6-i-Pr2C6H3)N(Me)C}2CH]) (2) by
Robinson and co-workers,2 and ArZnZnAr (3) and the unique
ArZn(m-H)(m-Na)ZnAr (Ar = 2,6-(2,6-i-Pr2C6H3)2C6H3) (4) by
Power’s group.3The most striking feature for the above reported
dizinc complexes (1–4) is that they all possess a Zn–Zn bond axis collinear with rather than perpendicular to their principal axes.4–7
We previously described the ability of a sterically encumbered diamido ligands, Me2Si(NDipp)2 (Dipp = 2,6-i-Pr2C6H3) to
stabilize a low-coordinate and quadruply bonded dimolybdenum complex Mo2(m-g
2
-Me2Si(NDipp)2)2in which the Mo–Mo bond
axis is perpendicular to the principal axis which is defined to coincide with the Si…Si linkage.8It would therefore be interesting to compare the structural properties and electronic characteristics of the Mo–Mo quadruple bond and the Zn–Zn s bond supported by the same ligands. Accordingly, we attempted to synthesize a dinuclear zinc complex supported by two diamido ligands Me2Si(NDipp)2in which the Zn–Zn bond axis is perpendicular
to its principal axis. Moreover, the pursuit of unprecedented Zn2 3+
complexes is of particular interest, due to the possibility that they could be important intermediates for the formation of Zn22+upon
reduction of Zn2+. This communication essentially deals with surprising discovery made en route to an attempted synthesis of the perpendicular dizinc complex on the basis of our prediction of a possible Zn23+species.
In order to achieve our objectives, we started with the preparation of a neutral dinuclear precursor Zn2(m-g2
-Me2Si(NDipp)2)2 (5) in which each of two bidentate amido
ligands coordinates to two Zn atoms in a bridging fashion, and from which a perpendicular dizinc complex [Zn2(m-g2
-Me2Si(NDipp)2)2] 22
was anticipated upon reduction. Compound 5, which is sparingly soluble in hydrocarbon and ethereal solvents, was obtained as a white solid in almost quantitative yield (99%) from the reaction of Li2[Me2Si(NDipp)2] and ZnBr2. X-Ray
crystallography{ was used to confirm the dinuclear nature of 5 depicted in Fig. 1. The unit cell contains two independent but chemically similar asymmetric units, and therefore for the sake of clarity, only one structure is shown here. Both the diamido ligands span two Zn atoms, and each Zn atom thus forms a linear geometry with N–Zn–N bond angles falling in the range between 172 and 176u. The average Zn–N bond distance of 1.819(3) A˚ is comparable to those of monomeric linear Zn amides, Zn[N(SiMe3)Ar]2(Ar = 2,6-Me2C6H3, 2,6-i-Pr2C6H3).
9
Complex 5 exhibits short Zn(1)…Zn(2) and Zn(3)…Zn(4) distances of 2.7589(7) and 2.8221(7) A˚ , which are close to that (2.6505(5) A˚ ) of compound [Zn2{N(Dipp)(CH2)3N(Dipp)}2].10
Subsequent reduction of 5 in toluene with 4 equiv. of KC8leads
to the formation of colorless blocks 6 after recrystallization at 235uC. X-ray crystallography{ was used to decipher the dimeric nature and the orientation of the central Zn–Zn bond depicted in Fig. 2. Surprisingly, the newly synthesized dizinc complex turns out to display a coaxial structure K2[(g2-Me2Si(NDipp)2
)-ZnZn(g2-Me2Si(NDipp)2)] and the striking feature in 6 is that
both diamido ligands coordinate to each Zn in a chelating fashion with the Zn–Zn bond axis being collinear rather than perpendicular to the C2 axis, Si(1)–Zn(1)–Zn(2)–Si(2), and two
a
Department of Chemistry, National Tsing Hua University, Hsinchu, 30013, Taiwan, R.O.C. E-mail: [email protected];
Fax: 886-3-5711082; Tel: 886-3-5718232
bDepartment of Biological Science and Technology, National Chiao
Tung University, Hsinchu, 30050, Taiwan, R.O.C
{Electronic supplementary information (ESI) available: Experimental details for the synthesis, X-ray crystallographic data of 5, 6 and (K– C222)2[7] including tables, and details of the computational study. See
DOI: 10.1039/b707396h
Fig. 1 Molecular structure of 5 with thermal ellipsoids at the 30% probability level. One of two crystallographically independent molecules and one half of solvent molecule (THF) found in the asymmetric unit, H-atoms and isopropyl groups have been omitted for clarity.
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planar four-membered rings Zn–N–Si–N in the structure of 6 are attached to each other via the Zn–Zn bond. Owing to the presence of two sandwiched potassium atoms, the two four-membered rings for 6 are almost coplanar with a dihedral angle N(1)–Zn(1)–Zn(2)– N(4) of 10.8u. Both zinc atoms in 6 adopt a trigonal geometry, and the Zn–N bond distances of 1.992(9) and 2.025(8) A˚ are increased by about 0.2 A˚ in comparison with those in 5 and are comparable to those in 2.2The elongation of the Zn–N bond lengths and the short Zn…K separations of 3.795(3) and 3.870(4) A˚ for 6 are presumably due to the increased electron density on the zinc atoms. As is usually the case for metal–metal dimers, the most intriguing metric is the metal–metal bond distances. The value of the distance between two Zn atoms being 2.3695(17) A˚ for 6 is indicative of a substantial Zn–Zn bond, despite being about 0.07 A˚ longer than that of 1a (2.305(3) A˚ ) and 1b (2.295(3) A˚ ), and 0.01 A˚ longer than that of 2, 3 and 4 (2.3586(7), 2.3591(9) and 2.352(2) A˚ respectively). It is, however, shorter than the Zn…Zn separation of 2.4084(3) and 2.4513(9) A˚ in the related zinc hydride dimers.3,11 Apparently, the presence of two embedded potassium ions do not exert significant effect on the length of the Zn–Zn bond though the average distance of K…phenyl ring is about 2.86 A˚ .
In an independent experiment in which the reduction of 5 by 4 equiv. of KC8 was carried out in THF with the presence of
Cryptand [2.2.2] (C222), the complex (K–C222)2[(g2-Me2
Si-(NDipp)2)ZnZn(g2-Me2Si(NDipp)2)], (K–C222)2[7], was isolated
in 79% yield. The Zn–Zn distance of (K–C222)2[7] was determined
to be 2.3634(11) A˚ via X-ray crystallography (Fig. 2),{ only slightly longer than that for 6. The lack of the sandwiched potassium ions dramatically increased the dihedral angle of N(1)–Zn(1)–Zn(2)– N(4) in 722to 50.6(4)u, in contrast to 10.8u in 6. The steady Zn–Zn bond distances in 6 and 722independent of the rotation of metal– metal axes signify that the bonding between the two Zn atoms in 6 and 722is essentially a s bond.
The electronic structures and bonding for Zn–Zn bonded complexes have been subjected to theoretical studies since the discovery of dizincocene and other related species,1–3,12,13 and
those calculations demonstrate the Zn–Zn bonds are formed through the overlap of either a pair of 4s orbitals in complexes 1a, 1b, 2, 3 or a pair of 4pzorbitals in compound 4. The electronic
structures of 722 were scrutinized by two-layered integrated molecular orbital (ONIOM) computations using the second order Møller–Plesset method (MP2) combined with density functional theories (DFT) on the authentic 722. The HOMO of 722 is localized mainly in the Zn–Zn s-bonding region and detailed natural bond orbital (NBO) analyses indicate that the Zn–Zn s bond has very high s character (94.75%), and slight p (2.59%), and d character (2.66%). In the b-diketiminato complex 2, the Zn–Zn bond also displays very high s character in the HOMO. In contrast to 722, the Zn 4s orbitals form the Zn–Zn bond in the dizincocene 1a. The LUMO of 722consists of an orbital of p symmetry, which is localized on the central Zn22+unit. Both of the contour plots of
HOMO and LUMO are shown in Fig. 3.
A simple qualitative correlation diagram14as shown in Fig. 4 might suggest why the coaxial Zn–Zn bonded structure is more favorable than the perpendicular one supported by chelating diamido ligands. To the left of Fig. 4 are the frontier orbitals for a linear [ZnN2]2unit in 522, and the others on the right side of the
figure are the frontier orbitals for a bent [ZnN2]2 unit in 7 22
. Accordingly, 722is predicted to be energetically lower than 522if both Zn–Zn bonds were roughly assumed to have the same bonding energy. Elaborate computations by virtue of the methods
Fig. 2 Reduction of 5 and molecular structure of 6 and 722with thermal ellipsoids at the 30% probability level. One of two crystallographically independent molecules 6 and all solvent molecules (THF) found in the asymmetric unit, H-atoms, isopropyl groups and two counter-cations (K-2,2,2-cryptand)+of 722have been omitted for clarity.
Fig. 3 The contour plots of HOMO and LUMO of 722.
Fig. 4 The orbital correlation diagram for Zn(I) diamides shows how the energies of the orbitals change as the linear molecule becomes bent.
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stated above were also performed on the hypothetical 522and the prepared 722, and the outcomes shown in Fig. 5 are consistent with our qualitative predictions.
To disclose the mechanism for the intriguing structural transformations from 5 to 6 or 722along with the Zn–Zn bond formations, a possible reduction pathway from the starting 5 to product 722 was computed. As depicted in Fig. 5, notably, the reduction from 5 to 722proceeded through a mixed-valence dizinc Zn23+ species 52 featuring the perpendicular structure. The
computed intermediate (IM2) and transition state (TS22) are shown in the ESI.{
Attempts to characterize the hypothetical mixed-valent dizinc complex 52, which presumably possesses one unpaired electron
and should be EPR active,15have so far failed. Treatment of 5
with one equivalent of KC8 in toluene led to an incomplete
reaction with the observation of 5 and 6 only. No comproportio-nation reaction was observed by mixing equal amount of 5 and 6 or (K–C222)2[7]. On the other hand, addition of one equiv. of
oxidants such as [Cp2Fe]PF6or a weaker 16
oxidant, [C7H7]BF4, to
6 or (K–C222)2[7] gave rise to a mixture of 5 and starting
compounds only. Interestingly, the oxidation of 6 or (K–C222)2[7]
by 2 equiv. of [Cp2Fe]PF6resulted in the formation of 32% of 5,
but 51% of 5 was observed on the basis of1H NMR spectroscopy, when 6 or (K–C222)2[7] was treated with the milder oxidant
[C7H7]BF4(Scheme 1).
In conclusion, three remarkable dinuclear zinc complexes 5, 6 and (K–C222)2[7] were successfully prepared and characterized.
The interactions between the Zn22+ moiety and the chelating
diamido ligand Me2Si(NDipp)2 favor a coaxial structure.
However, the mixed-valence Zn23+ intermediate 52 was
theoretically predicted to display a perpendicular structure. Characterizations of the intermediate complex 52are underway.
We are indebted to the Taiwan, R.O.C. National Science Council for support under Grant NSC 95-2113-M-007-017. Mr Ting-Shen Kuo (National Taiwan Normal University) is acknowl-edged for help with crystallographic details.
Notes and references
{CCDC 647437 (5), 647438 (6) and 647439 ((K–C222)2[7]). For
crystal-lographic data in CIF or other electronic format see DOI: 10.1039/ b707396h
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energetics are plotted according to zero-point corrected MP2 energies at ONIOM (full-MP2/6-31G(d,p):BP86/6-31G(d,p)). (b) The reaction mechanism of structural transformation upon two-electron reduction of 5.
Scheme 1
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