A Trimetallic compound containing Zn-Zr bonds: Cp2Zr(ZnR) 2 (Cp = C5H5; R = C6H 3-2,6-(2,4,6-i-Pr3C6H2)2)

Article abstract of DOI:10.1021/om700319k

m-Terphenylzinc iodides RZnl₂Li(OEt₂)₂ (1) and RZnI (2) (R = C₆H₃-2,6-(2,4,6-i-Pr₃C ₆H₂)₂) were prepared and structurally characterized. The sodium metal re

Full text of DOI:10.1021/om700319k

3054
Organometallics 2007, 26, 3054-3056
A Trimetallic Compound Containing Zn-Zr Bonds: Cp2Zr(ZnR)2
(Cp ) C5H5; R ) C6H3-2,6-(2,4,6-i-Pr3C6H2)2)
Yuzhong Wang, Brandon Quillian, Chaitanya S. Wannere, Pingrong Wei,
Paul v. R. Schleyer,* and Gregory H. Robinson*
Department of Chemistry and the Center for Computational Chemistry, The UniVersity of Georgia,
Athens, Georgia 30602-2556
ReceiVed April 2, 2007
Scheme 1. Synthetic Routes to Compounds 1-3
Summary: m-Terphenylzinc iodides RZnI2Li(OEt2)2 (1) and RZnI
(2) (R ) C6H3-2,6-(2,4,6-i-Pr3C6H2)2) were prepared and
structurally characterized. The sodium metal reduction of Cp2-
ZrCl2 with 2 afforded Cp2Zr(ZnR)2 (3) as the first structurally
characterized compound containing a Zn-Zr bond. The nature
of the unique Zn-Zr bond for 3 was further probed by DFT
computations.
Our pursuit of organometallic chemistry at the main-group-
metal-transition metal interface1-4 led recently to 18-electron
Cp2M(GaR)2 complexes (Cp ) C5H5; M ) Ti, Zr; R ) C6H3-
2,6-(2,4,6-i-Pr3C6H2)2),1,3 where the :GaR units appear to mimic
the neutral 2-electron-donor behavior of :CO ligands. The
isolation of the first compound containing a Zn-Zn bond5
spawned interest in the chemistry of zinc; reports of additional
compounds containing the Zn-Zn bond followed.6-8 Since zinc,
a group 12 metal, and gallium, a main group 13 metal, are
neighbors on the periodic table, we wondered if such Cp2M-
(ZnR)2 (M ) Ti, Zr) complexes could be prepared. Furthermore,
what would be the nature of such Zn-M bonding? Although
compounds containing zinc-transition-metal (mostly groups
6-8) bonds have been known for some time,9-14 compounds
containing Zn-M (M ) group 4: Ti, Zr, Hf) bonds have not
been reported. We now report the synthesis and molecular
structure of Cp2Zr(ZnR)2 (3; R ) C6H3-2,6-(2,4,6-i-Pr3C6H2)2),
the first structurally characterized compound containing a Zn-
Zr bond. The nature of the Zn-Zr bond in 3 was probed by
computations on a model system, Cp2Zr(ZnR′)2 (R′ ) C6H3-
2,6-(C6H5)2) (3a).
Results and Discussion
The lithium adduct RZnI2Li(OEt2)2 (1) was synthesized by
reaction of RLi‚OEt2 with ZnI2 in Et2O. Interestingly, vigorous
stirring of a toluene solution of 1 leads to the formation of
[RZnI]2 (2) as colorless crystals. The moisture- and air-sensitive
title compound, 3, was synthesized by sodium reduction of Cp2-
ZrCl2 with 2 (Scheme 1). Compounds 1-3 were characterized
by 1H NMR, elemental analyses, and single-crystal X-ray
diffraction.
* To whom correspondence should be addressed. E-mail: robinson@
chem.uga.edu (G.H.R.).
(1) Yang, X.-J.; Quillian, B.; Wang, Y.; Wei, P.; Robinson, G. H.
Organometallics 2004, 23, 5119-5120.
(2) Wang, Y.; Quillian, B.; Yang, X.-J.; Wei, P.; Chen, Z.; Wannere, C.
S.; Schleyer, P. v. R.; Robinson, G. H. J. Am. Chem. Soc. 2005, 127, 7672-
7673.
(3) Yang, X.-J.; Wang, Y.; Quillian, B.; Wei, P.; Chen, Z.; Schleyer, P.
v. R.; Robinson, G. H. Organometallics 2006, 25, 925-929.
(4) Quillian, B.; Wang, Y.; Wannere, C. S.; Wei, P.; Schleyer, P. v. R.;
Robinson, G. H. Angew. Chem., Int. Ed. 2007, 46, 1836-1838.
(5) Resa, I.; Carmona, E.; Gutierrez-Puebla, E.; Monge, A. Science 2004,
305, 1136-1138.
X-ray structural analysis (Figure 1) shows that the tricoor-
dinate zinc atom in 1 prefers a trigonal-planar geometry and is
embraced by the sterically demanding m-terphenyl ligand on
both sides of the ZnI2 plane. The essentially identical Zn-I
distances, 2.5910(5) and 2.5950(5) Å, in 1 are only slightly
shorter than those in [{(2,6-i-Pr2C6H3)N(Me)C}2CHZn(µ-I)2Li-
(OEt2)2] (4)15 (2.6142(8) and 2.6648(8) Å). The Zn-C distance
(1.971(3) Å) in 1 is between those of 2 and 3 from 1.952(4) to
1.997(4) Å. While the 2.804(7) and 2.800(7) Å Li-I distances
in 1 are equal, those in 4 (2.784(8) and 2.919(8) Å) differ
significantly.
Like the lithium adduct 1, compound 2 (Figure 2) has two
µ2-bridging iodine atoms. However, in 2 the iodine atoms link
two zinc atoms and help constitute a planar four-membered Zn2I2
ring, which is shielded effectively by the two nearly orthogonal-
oriented m-terphenyl ligands. The C(1)-Zn(1)‚‚‚Zn(2)-C(37)
(6) 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.
(7) Zhu, Z.; Wright, R. J.; Olmstead, M. M.; Rivard, E.; Brynda, M.;
Power, P. P. Angew. Chem., Int. Ed. 2006, 45, 5807-5810.
(8) Grirrane, A.; Resa, I.; Rodriguez, A.; Carmona, E.; Alvarez, E.;
Gutierrez-Puebla, E.; Monge, A.; Galindo, A.; Del Rio, D.; Andersen, R.
A. J. Am. Chem. Soc. 2007, 129, 693-703.
(9) Burlitch, J. M.; Ferrari, A. Inorg. Chem. 1970, 9, 563-569.
(10) Denis, J. S.; Butler, W.; Glick, M. D.; Oliver, J. P. J. Am. Chem.
Soc. 1974, 96, 5427-5436.
(11) Budzelaar, P. H. M.; Boersma, J.; Van der Kerk, G. J. M.; Spek,
A. L.; Duisenberg, A. J. M. Inorg. Chem. 1982, 21, 3777-3780.
(12) Budzelaar, P. H. M.; Boersma, J.; Van der Kerk, G. J. M.; Spek,
A. L.; Duisenberg, A. J. M. Organometallics 1985, 4, 680-683.
(13) Wright, C. A.; Shapley, J. R. Inorg. Chem. 2001, 40, 6338-6340.
(14) Enders, M.; Kohl, G.; Pritzkow, H. Organometallics 2002, 21,
1111-1117.
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Eur. J. Inorg. Chem. 2001, 1613-1616.
10.1021/om700319k CCC: $37.00 © 2007 American Chemical Society
Publication on Web 05/10/2007

Notes
Organometallics, Vol. 26, No. 12, 2007 3055
Figure 3. Molecular structure of 3 (thermal ellipsoids are shown
at the 30% probability level). Selected bond distances (Å) and angles
(deg): Zn(1)-C(1) ) 1.997(4), Zn(1)-Zr(1) ) 2.7721(7); C(1)-
Zn(1)-Zr(1) ) 172.84(13), Zn(1)-Zr(1)-Zn(1a) ) 99.12(3).
Figure 1. Molecular structure of 1 (thermal ellipsoids are shown
at the 30% probability level). Selected bond distances (Å) and angles
(deg): Zn(1)-C(1) ) 1.971(3), Zn(1)-I(1) ) 2.5910(5), Zn(1)-
I(2) ) 2.5950(5), I(1)-Li(1) ) 2.804(7), I(2)-Li(1) ) 2.800(7);
I(1)-Zn(1)-I(2) ) 103.053(16), Zn(1)-I(1)-Li(1) ) 81.98(13),
Zn(1)-I(2)-Li(1) ) 81.98(14).
symmetric Zn-M-Zn type of Zn-M (M ) transition metal)
bonding. Another reported example of this structural type is
the Zn-Co-Zn linkage in (CpZn)2Co(Cp)PPh3,11 while most
examples contain either symmetric M-Zn-M or unsymmetrical
M-Zn metallic cores.9,10,12-14 The 99.12(3)° Zn-Zr-Zn bond
angle of 3 is similar to the 100.39(4)° Ga-Zr-Ga angle of Cp2-
Zr(GaR)2 (5),3 the 95.06(4)° Sn-Zr-Sn angle of Cp2Zr(SnR2)2
(R ) CH(SiMe3)2) (6),16 and the In-Zr-In angle (95.37(2)°)
for Cp2Zr(InR)2 (7)3 but is significantly greater than the 87.70-
(3)° Ga-Zr-Ga angle in Cp2Zr{Ga[N(Ar)C(H)]2}2][Li(THF)4]17
(Ar ) C6H3Pri2-2,6) and especially the 74.90(2)° Zn-Co-Zn
angle of (CpZn)2Co(Cp)PPh3.11
The novel Zn-Zr bond is the most remarkable structural
feature of 3. The Zn-Zr bond distance, 2.7721(7) Å, is close
to the 2.70 Å sum of the zinc and zirconium covalent radii.
While the central zirconium is four-coordinate with a pseudo-
tetrahedral geometry, the two terminal zinc atoms are two-
coordinate with an almost linear C-Zn-Zr angle of 172.84-
(13)°. This value is nearly identical with the 172.44(16)°
C-Ga-Zr angle of 5 but is between the 171.33(12)° C-In-
Zr bond angle of 7 and the 179.2(1)° C-Ga-Fe bond angle of
RGaFe(CO)4.18
Figure 2. Molecular structure of 2 (thermal ellipsoids are shown
at the 30% probability level). Selected bond distances (Å) and angles
(deg): Zn(1)-C(1) ) 1.960(4), Zn(2)-C(37) ) 1.952(4), Zn(1)-
I(1) ) 2.6180(6), Zn(1)-I(2) ) 2.6300(11), Zn(2)-I(1) ) 2.6218-
(6), Zn(2)-I(2) ) 2.6355(10); I(1)-Zn(1)-I(2) ) 95.21(3), I(1)-
Zn(2)-I(2) ) 94.99(3), Zn(1)-I(1)-Zn(2) ) 85.157(18), Zn(1)-
I(2)-Zn(2) ) 84.65(3).
The nature of the Zn-Zr bond in 3 was explored by
PW91PW91/LANL2DZ density functional theory (DFT) com-
putations on the Cp2Zr(ZnR′)2 (R′ ) C6H3-2,6-(C6H5)2) model,
3a.19 The Zn-Zr bond distance of 2.860 Å for 3a is somewhat
longer than the experimental value of 2.7721(7) Å for 2, but
the C-Zn-Zr angle of 176.0° for 3a compares well with the
172.84(13)° angle of 3. Notably, the computed 62.9° Zn-Zr-
Zn bond angle for 3a is much more acute than the 99.12(3)°
experimental value for 3. This difference may be ascribed to
the substantially less steric crowding of the 2,6-diphenylphenyl
array is almost linear, with C(1)-Zn(1)‚‚‚Zn(2) (175.97°) and
Zn(1)‚‚‚Zn(2)-C(37) (176.26°) angles. The array in RZn-ZnR
(R ) C6H3-2,6-(2,6-i-Pr2C6H3)2) is similar.7 The tricoordinate
zinc atoms in 2 have trigonal-planar geometries. The Zn-I bond
distances in 2, ranging from 2.6180(6) to 2.6355(10) Å, are
comparable to those in 1 and 4.15
Although 3 resides about a 2-fold axis, the most notable
structural feature is the central trimetallic Zn-Zr-Zn core
(Figure 3). This V-shaped Zn-Zr-Zn core is well protected
sterically by two bulky m-terphenyl ligands as well as by two
Cp ligands. Indeed, the Zn-Zr-Zn core represents a rare
(16) Piers, W. E.; Whittal, R. M.; Ferguson, G.; Gallagher, J. F.; Froese,
R. D. J.; Stronks, H. J.; Krygsman, P. H. Organometallics 1992, 11, 4015-
4022.
(17) Baker, R. J.; Jones, C.; Murphy, D. M. Chem. Commun. 2005,
1339-1341.
(18) Su, J.; Li, X.-W.; Crittendon, R. C.; Campana, C. F.; Robinson, G.
H. Organometallics 1997, 16, 4511-4513.
(19) The structure was optimized at the PW91PW91/LANL2DZ DFT
level with the Gaussian 03 program (see the reference in the Supporting
Information).

3056 Organometallics, Vol. 26, No. 12, 2007
Notes
(CH3)2), 1.11 (t, 12H, o-(CH2CH3)2), 1.26 (d, 12H, o-CH(CH3)2),
1.31 (d, 12H, p-CH(CH3)2), 2.86 (m, 2H, p-CH(CH3)2), 3.01 (m,
4H, o-CH(CH3)2), 3.38 (q, 8H, O(CH2CH3)2), 6.94-7.14 (m, 7H,
-C6H3 and -C6H2). Anal. Calcd (found) for C44H69O2I2ZnLi
(956.10): C, 55.27 (55.14); H, 7.27 (7.25).
Compound 2. The white residue of 1 (5.00 g, 5.23 mmol) was
dissolved in 80 mL of toluene, and this solution was stirred
vigorously for 1 day. After filtration, the solution was concentrated
to 10 mL. After 2 days at ambient temperature, colorless crystals
1
of 2 (2.36 g, 67.0% yield) were observed. Mp: 282 °C. H NMR
(C6H6): δ 1.17 (d, 12H, o-CH(CH3)2), 1.27 (d, 12H, o-CH(CH3)2),
1.34 (d, 12H, p-CH(CH3)2), 2.87 (m, 2H, p-CH(CH3)2), 3.06 (m,
4H, o-CH(CH3)2), 7.13-7.24 (m, 7H, -C6H3 and -C6H2). Anal.
Calcd (found) for C72H98Zn2I2 (1348.04): C, 64.14 (64.03); H, 7.33
(7.24).
Compound 3. A 70 mL portion of diethyl ether was added to a
flask containing 2 (3.00 g, 2.22 mmol), Cp2ZrCl2 (0.65 g, 2.23 mmol
(Strem)), and finely cut sodium (0.50 g, 21.7 mmol) at ambient
temperature. After being powerfully stirred over 2 days, the dark
red solution was filtered. The filtrate was concentrated to 10 mL
and then kept standing at ambient temperature. Over 3 days, orange
crystals of 3 (0.57 g, 18.4% yield) were observed. Mp: 273 °C. 1H
NMR (C6H6): δ 1.22 (d, 24H, o-CH(CH3)2), 1.32 (d, 24H, o-CH-
(CH3)2), 1.39 (d, 24H, p-CH(CH3)2), 2.93 (m, 4H, p-CH(CH3)2),
3.22 (m, 8H, o-CH(CH3)2), 4.33 (s, 10H, C5H5), 7.16-7.29 (m,
14H, -C6H3 and -C6H2). Anal. Calcd (found) for C86H118OZn2Zr
(1389.76): C, 74.32 (74.37); H, 8.56 (8.54).
X-ray Crystal Structure Determination of 1-3. Crystals of
1-3 were mounted in glass capillaries under an atmosphere of argon
in the drybox. The X-ray intensity data for 1-3 were collected at
room temperature on a Bruker SMART APEX II X-ray diffracto-
meter system with graphite-monochromated Mo KR radiation (λ
) 0.710 73 Å), using the ω-scan technique. The structures were
solved by direct methods using the SHELXTL 6.1 bundled software
package.23 Absorption corrections were applied with SADABS. All
non-hydrogen atoms were refined anisotropically (except for those
atoms of the disordered diethyl ether solvent molecule in crystals
of 3). Hydrogen atom positions were calculated and allowed to ride
on the attached carbon atoms with the isotropic temperature factors
fixed at 1.1 times those of the corresponding carbon atoms. Crystal
data for 1: C44H69O2I2ZnLi, fw ) 956.10, monoclinic, P21/n (No.
14), a ) 14.1460(12) Å, b ) 14.7776(12) Å, c ) 24.150(2) Å, â
) 102.0940(10), V ) 4936.4(7) Å3, Z ) 4, R1 ) 0.0385 for 7903
data (I > 2σ(I)), wR2 ) 0.1121 (all data). Crystal data for 2:
C72H98Zn2I2, fw ) 1348.04, triclinic, P1h (No. 2), a ) 13.4042(17)
Å, b ) 14.4142(18) Å, c ) 18.520(2) Å, R ) 88.019(2)°, â )
86.503(2)°, γ ) 79.634(2)°, V ) 3512.3(8) Å3, Z ) 2, R1 ) 0.0463
for 8969 data (I > 2σ(I)), wR2 ) 0.1433 (all data). Crystal data
for 3: C86H118OZn2Zr, fw ) 1389.76, orthorhombic, Pbcn (No.
60), a ) 15.8883(17) Å, b ) 17.6831(19) Å, c ) 29.505(3) Å, V
) 8289.6(15) Å3, Z ) 4, R1 ) 0.0654 for 5205 data (I > 2σ(I)),
wR2 ) 0.2304 (all data).
Figure 4. Representation of the HOMO of 3a from DFT
computations.
ligands employed in the 3a model compared to the extremely
bulky m-terphenyl ligands (C6H3-2,6-(2,4,6-i-Pr3C6H2)2) in 3.3
DFT computations reveal that the HOMO of 3a (Figure 4)
is a Zn-Zr σ-bonding orbital involving overlap of a 3d Zr orbital
with the 4s orbital of the Zn atom. The Wiberg bond index
(WBI), 0.514, and the 1.57e occupancy of the Zn-Zr interaction
(given by natural bond orbital (NBO) analysis) also support the
presence of a Zn-Zr bond. Notably, the 0.514 Zn-Zr bond
index may be compared with the 0.328 and 0.333 Mo-Mo bond
orders reported for [{Mo(η5-Cp)(CO)3}2] and [{Mo(η5-Cp)-
(CO)2}2(µ-PMe2)],20 respectively, and the Zr-Zr bond order
of 0.453 for [{(t-BuC5H4)(t-BuC5H3)Zr(µ-H)Na}2]4.21 Indeed,
the Zn-Zr bonding mode for 3 (Figure 4) is remarkably
different from those of the rather short E-Zr bonds in 5 (E )
Ga), 6 (E ) Sn), and 7 (E ) In), which may be described as
having EfZr donor-acceptor σ bonds supplemented by ZrfE
π back-bonding.18
In summary, an interesting zirconocene derivative containing
unique Zn-Zr bonds (Figure 3) has been synthesized and its
molecular structure determined. Furthermore, the nature of the
Zn-Zr bonding was probed by DFT computations.
Experimental Section
All reactions were performed under purified argon using Schlenk
techniques and an inert-atmosphere drybox (M-Braun LabMaster
130). Solvents were dried and distilled under argon from Na/
benzophenone prior to use. Elemental analyses were performed by
Complete Analysis Laboratories, Inc. (CALI, Parsippany, NJ). 1H
NMR spectra were recorded on a Varian Mercury Plus 400 MHz
spectrometer.
Syntheses of 1-3. Compound 1. A diethyl ether solution (50
mL) of [(2,4,6-i-Pr3C6H2)2C6H3]Li‚OEt2 (6.04 g, 10.7 mmol),
prepared as previously reported,22 was added to a flask charged
with anhydrous ZnI2 (3.43 g, 10.7 mmol (Aldrich)) and diethyl ether
(20 mL) at -78 °C. The mixture was stirred for 4 h at -78 °C and
then warmed gradually to ambient temperature. After the mixture
was stirred for 2 days at ambient temperature, the solvent was
removed in vacuo. The white residue, recrystallized in Et2O/hexane,
gave colorless crystals of 1. Yield: 8.35 g, 81.4%. Mp: >150 °C
Acknowledgment. We are grateful to the National Science
Foundation and the donors of the Petroleum Research Fund,
administered by the American Chemical Society, for support
of this work.
Supporting Information Available: CIF files giving full details
of the computations and X-ray crystallographic studies. This
material is available free of charge via the Internet at http://
pubs.acs.org.
1
dec (as white solid). H NMR (THF-d8): δ 0.98 (d, 12H, o-CH-
(20) Hong, F.-E.; Chang, Y.-C. Bull. Chem. Soc. Jpn. 2004, 77, 115-
121.
OM700319K
(21) Wang, Y.; Quillian, B.; Wei, P.; Wannere, C. S.; Schleyer, P. v.
R.; Robinson, G. H. Organometallics 2006, 25, 3286-3288.
(22) Schiemenz, B.; Power, P. P. Organometallics 1996, 15, 958-964.
(23) Sheldrick, G. M. SHELXTL 6.1, Crystallographic Computing
System; Bruker Analytical X-Ray Systems, Madison, WI, 2000.