Zinc-zinc bonded zincocene structures. Synthesis and characterization of Zn2(η5-C5Me5)2 and Zn2(η5-C5Me4Et)2

Article abstract of DOI:10.1021/ja0668217

While, in general, decamethylzincocene, Zn(C₅Me ₅)₂, and other zincocenes, Zn(C₅Me ₄R)₂ (R = H, Bu^t, SiMe₃), react with dialkyl and diaryl derivatives, ZnR′₂, to give the half-sandwich compounds (η⁵-C₅Me₄R) ZnR′, under certain conditions the reactions of Zn(C₅Me ₅)₂ with ZnEt₂ or ZnPh₂ produce unexpectedly the dizincocene Zn₂(η⁵-C ₅Me₅)₂ (1) in low yields, most likely as a result of the coupling of two (η⁵-C₅Me ₅)Zn^. radicals. An improved, large scale (ca. 2 g) synthesis of 1 has been achieved by reduction of equimolar mixtures of Zn(C ₅Me₅)₂ and ZnCl₂ with KH in tetrahydrofuran. The analogous reduction of Zn(C₅Me ₄R)₂ (R = H, SiMe₃, Bu^t) yields only decomposition products, but the isotopically labeled dimetallocene ⁶⁸Zn₂(η⁵-C₅Me₅) ₂ and the related compound Zn₂(η⁵-C ₅Me₄Et)₂ (2) have been obtained by this procedure. Compound 2 has lower thermal stability than 1, but it has been unequivocally characterized by low-temperature X-ray diffraction studies. As for 1 a combination of structural characterization techniques has provided unambiguous evidence for its formulation as the Zn-Zn bonded dimer Zn ₂(η⁵-C₅Me₄Et)₂, with a short Zn-Zn bond of 2.295(3) A indicative of a strong Zn-Zn bonding interaction. The electronic structure and the bonding properties of 1 and those of related dizincocenes Zn₂(η⁵-Cp′)₂ have been studied by DFT methods (B3LYP level), with computed bond distances and angles for dizincocene 1 very similar to the experimental values. The Zn-Zn bond is strong (ca. 62 kcal-mol^-1 for 1) and resides in the HOMO-4, that has a contribution of Zn orbitals close to 60%, consisting mostly of the Zn 4s orbitals (more than 96%).

Full text of DOI:10.1021/ja0668217

Published on Web 12/30/2006
Zinc-Zinc Bonded Zincocene Structures. Synthesis and
Characterization of Zn2(η5-C5Me5)2 and Zn2(η5-C5Me4Et)2
Abdessamad Grirrane,† Irene Resa,† Amor Rodriguez,† Ernesto Carmona,*,†
Eleuterio Alvarez,† Enrique Gutierrez-Puebla,‡ Angeles Monge,‡ Agust´ın Galindo,§
Diego del R´ıo,† and Richard A. Andersen|
Contribution from the Instituto de InVestigaciones Qu´ımicas-Departamento de Qu´ımica
Inorga´nica, UniVersidad de SeVilla-Consejo Superior de InVestigaciones Cient´ıficas, Ame´rico
Vespucio 49, 41092 SeVilla, Spain, Instituto de Ciencia de Materiales de Madrid, Consejo
Superior de InVestigaciones Cient´ıficas, Campus de Cantoblanco, 28049 Madrid, Spain,
Departamento de Qu´ımica Inorga´nica, UniVersidad de SeVilla, Aptdo. 553, 41071 SeVilla, Spain,
and Chemistry Department, UniVersity of California, Berkeley, California 94720
Received September 21, 2006; E-mail: guzman@us.es
Abstract: While, in general, decamethylzincocene, Zn(C5Me5)2, and other zincocenes, Zn(C5Me4R)2 (R )
H, But, SiMe3), react with dialkyl and diaryl derivatives, ZnR′2, to give the half-sandwich compounds (η5-
C5Me4R)ZnR′, under certain conditions the reactions of Zn(C5Me5)2 with ZnEt2 or ZnPh2 produce
unexpectedly the dizincocene Zn2(η5-C5Me5)2 (1) in low yields, most likely as a result of the coupling of two
(η5-C5Me5)Zn• radicals. An improved, large scale (ca. 2 g) synthesis of 1 has been achieved by reduction
of equimolar mixtures of Zn(C5Me5)2 and ZnCl2 with KH in tetrahydrofuran. The analogous reduction of
Zn(C5Me4R)2 (R ) H, SiMe3, But) yields only decomposition products, but the isotopically labeled
dimetallocene 68Zn2(η5-C5Me5)2 and the related compound Zn2(η5-C5Me4Et)2 (2) have been obtained by
this procedure. Compound 2 has lower thermal stability than 1, but it has been unequivocally characterized
by low-temperature X-ray diffraction studies. As for 1 a combination of structural characterization techniques
has provided unambiguous evidence for its formulation as the Zn-Zn bonded dimer Zn2(η5-C5Me4Et)2,
with a short Zn-Zn bond of 2.295(3) Å indicative of a strong Zn-Zn bonding interaction. The electronic
structure and the bonding properties of 1 and those of related dizincocenes Zn2(η5-Cp′)2 have been studied
by DFT methods (B3LYP level), with computed bond distances and angles for dizincocene 1 very similar
to the experimental values. The Zn-Zn bond is strong (ca. 62 kcal‚mol-1 for 1) and resides in the HOMO-
4, that has a contribution of Zn orbitals close to 60%, consisting mostly of the Zn 4s orbitals (more than
96%).
chemistry.8 The use of substituted, bulky cyclopentadienyl
ligands9 has permitted the discovery of new, unexpected
Introduction
The chemistry of metalocenes continues to attract a great deal
of attention1-4 due to their successful applications in many areas
of chemistry such as olefin polymerization catalysis,5 asym-
metric catalysis,6 C-H bond activation,7 or bioorganometallic
structures in mono- and polynuclear metallocenes of both the
transition (d and f) and the main group elements. Figure 1 shows
structural types (a-e) found for divalent metallocenes, MCp′2.
However, despite initial expectations,10 binuclear metallocenes
of structure f (Figure 1), where a pair of directly bonded metal
atoms unsupported by bridging ligands are sandwiched between
two cyclopentadienyl rings whose planes are essentially per-
† Universidad de Sevilla-Consejo Superior de Investigaciones Cient´ıficas.
‡ Campus de Cantoblanco.
§ Departamento de Qu´ımica Inorga´nica, Universidad de Sevilla.
| University of California.
(1) (a) Kealy, T. J; Pauson, P L. Nature 1951, 168, 1039. (b) Miller, S. A.;
Tebboth, J. A.; Tremaine, J. F. J. Chem. Soc. 1952, 633.
(2) (a) Fischer, E. O.; Pfab, W. Z. Naturforsh. 1952, 7b, 377. (b) Wilkinson,
G.; Rosenblum, M; Whiting, M. C.; Woodward, R. B. J. Am. Chem. Soc.
1952, 74, 2125. (c) Wilkinson, G. J. Am. Chem. Soc. 1952, 74, 6146.
(3) Jutzi, P.; Burdford, N. In Metallocenes; Togni, A., Halterman, R. L., Eds.;
Wiley: New York, 1998; Vol. 1, Chapter 1.
(4) For example, see: (a) Yamamoto, A. J. Organomet. Chem. 2000, 600,
159. (b) Pauson, P. L. J. Organomet. Chem. 2001, 637-639, 3. (c) Fischer,
E. O.; Jira, R. J. Organomet. Chem. 2001, 637-639, 7. (d) Rosenblum,
M. J. Organomet. Chem. 2001, 637-639, 13. (e) Whiting, M. C.
J. Organomet. Chem. 2001, 637-639, 16. (f) Cotton, F. A. J. Organomet.
Chem. 2001, 637-639, 18.
(5) Metallocene-Based Polyolefins; Scheirs, J., Kaminsky, W., Eds.; John Wiley
& Sons Ltd., Chichester, 2000; Vols. 1 and 2.
(6) Ferrocenes; Togni, A., Hayashi, T., Eds.; Verlach Chemie: Weinheim,
1995.
(7) For some illustrative examples, see: (a) Arndtsen, B. A.; Bergman, R. G.
Science 1995, 270, 1970. (b) Chen, H.; Schlecht, S.; Semple, T. C.; Hartwig,
J. F. Science 2000, 287, 1995. (c) Pamplin, C. B.; Legzdins, P. Acc. Chem.
Res. 2003, 36, 223. (d) Jones, W. D. Inorg. Chem. 2005, 44, 4475.
(8) Jaouen, G.; Top, S.; Vessie`res, A.; Alberto, R. J. Organomet. Chem. 2000,
600, 23.
(9) (a) Jutzi, P.; Burford, N. Chem. ReV. 1999, 99, 969. (b) Janiak, C.;
Schumann, H. AdV. Organomet. Chem. 1991, 23, 291. (c) Hays, M. L.;
Hanusa, T. P. AdV. Organomet. Chem. 1996, 40, 117.
(10) (a) Schneider, J. J.; Godard, R.; Werner, S.; Kru¨ger, C. Angew. Chem.,
Int. Ed. Engl. 1991, 30, 1124. (b) Abrahamson, H. B.; Niccolai, G. P.;
Heinekey, D. M.; Casey, C. P.; Burstein, B. Angew. Chem., Int. Ed. Engl.
1992, 31, 471. (c) Kersten, J. L.; Rheingold, A. L.; Theopold, K. H.; Casey,
C. P.; Widenhoefer, R. A.; Hop, C. E. C. A. Angew. Chem.. Int. Ed. Engl.
1992, 31, 1341. (d) Lutz, F.; Ban, R.; Wu, P.; Koetzle, T. F.; Kru¨ger, C.;
Schneider, J. J. Inorg. Chem. 1996, 35, 2698.
9
10.1021/ja0668217 CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 693-703
693

A R T I C L E S
Grirrane et al.
magnetic Zn+ in a microporous crystalline silicoaluminophos-
phate has been reported.18
Organozinc compounds are of historical significance,19 with
Frankland’s synthesis of ZnMe2 and ZnEt2, the first binary metal
alkyls, paving the way for the development of alkyl derivatives
of the main group metals and for their countless applications
in organic synthesis. In recent years, organozinc compounds
have become essential reagents for organic synthesis,20 but
despite extensive research, molecular complexes of the metal-
2+
metal bonded Zn2 unit remain elusive, although the divalent
zincocenes, ZnCp′2, form a well-known family of metallocenes
with the distinct η5/η1(π) structure (d in Figure 1).21 We have
recently communicated the synthesis and structural and elec-
tronic properties of Zn2(η5-C5Me5)2, 1, the first stable molecular
compound of zinc with a zinc-zinc bond.22 This report attracted
the interest of the scientific community23 and was followed
shortly by a number of theoretical studies dedicated to 1 and
also to related species.24,25 Furthermore two new examples of
zinc-zinc bonded compounds have been provided. Robinson
and co-workers have reported the generation26 of Zn2[HC-
(CMeNAr)2]2 (Ar ) 2,6-Pri2C6H3), with three-coordinate zinc
atoms (bidentate â-diketiminate ligands), and very recently
Power and co-workers have characterized27 the zinc-zinc
bonded species Zn2Ar2 for ArdC6H3s2,6-(C6H3s2,6-Pri2)2,
where the Zn22+ unit is supported by monodentate ligands. The
two compounds have essentially identical Zn-Zn bond lengths
of ca. 2.36 Å, approximately 0.05 Å longer than 1, despite their
lower metal coordination number.
Aware of the importance of our initial report, we have focused
our efforts on the synthesis, reactivity, structural properties, and
electronic characteristics of dizincocenes and have succeeded
recently in isolating and characterizing by X-ray crystallography
a second example of a dimetallocene, namely Zn2(η5-C5Me4-
Et)2 2, which features a Zn-Zn bond length of ca. 2.30 Å,
practically identical to that of 1. In this paper we present full
details of the work that has allowed their generation and
structural characterization, as well as our own study of their
electronic structure and bonding properties.
Figure 1. Structural types for divalent metallocenes.
pendicular to the metal-metal bond axis, have remained
unknown until recently.
The capacity of metal atoms to form homonuclear (or
heteronuclear) metal-metal bonds have given rise to the
development of one of the most important areas of modern metal
chemistry, e.g., that of metal clusters.11 This ability changes
considerably along the Periodic Table not only from one group
to another but also for elements of the same group. For instance,
one of the most remarkable and distinct features of the group
12 elements, Zn, Cd, and Hg, is their varying tendency to form
dinuclear compounds containing [M-M]2+ units. Thus, for the
heaviest element, mercury, stable diatomic species are known
in solution and in the solid state,12 whereas for zinc and
cadmium the almost invariable oxidation state is +2, in the form
of mononuclear M2+ ions.12 Only a few exceptions to this rule
are known. The dication Cd22+ has been known for many years,
and it has been structurally characterized by X-ray methods in
Cd2(AlCl4)213 and by 113Cd NMR spectroscopy14 in Cd2TpMe2
2
(TpMe2 ) hydrotris(3,5-dimethylpyrazolyl)borate). There is
2+
evidence for the formation of Zn2 ions in ZnCl2/Zn glasses
at high temperatures15 and in zeolite matrices.16 The dihydride
Zn2H2 has been isolated in argon matrices and characterized
by vibrational spectroscopy, deuterium substitution, and MP2
calculations.17 Recently, the formation of mononuclear, para-
(11) (a) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms,
2nd ed.; Oxford University Press: Oxford, 1993. (b) Schno¨ckel, H. J. Chem.
Soc., Dalton Trans. 2005, 3131.
(12) (a) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; Wiley: New York, 1999; Chapter 15.
(b) Wiberg, N., Ed. Holleman-Wiberg Inorganic Chemistry, 34 ed.;
Academic Press: New York, 2001; Chapter XXIII.
(13) (a) Faggiani, R.; Gillespie, R. J.; Vekris, J. E. J. Chem. Soc., Chem.
Commun. 1986, 517. (b) Staffel, T.; Meyer, G. Z. Anorg. Allg. Chem. 1987,
548, 45.
(14) Reger, D. L.; Mason, S. S. J. Am. Chem. Soc. 1993, 115, 10406.
(15) Kerridge, D. H.; Tariq, S. A. J. Chem. Soc. A 1967, 1122.
(16) (a) Rittner, F.; Seidel, A.; Boddenberg, B. Microporous Mesoporous Mater.
1998, 24, 127. (b) Seff, K.; Zhen, S.; Bae, D. J. Phys. Chem. B 2000, 104,
515.
(17) (a) Greene, T. M.; Brown, W.; Andrews, L.; Downs, A. J.; Chertihin,
G. V.; Runeberg, N.; Pyykko¨, P. J. Phys. Chem. 1995, 99, 7925. (b) Wang,
X. Andrews, L. J. Phys. Chem. A 2004, 108, 11006.
(18) Tian, Y.; Li, G. D.; Chen, J. S. J. Am. Chem. Soc. 2003, 125, 6622.
(19) (a) Frankland, E. Ann. 1849, 71, 213. (b) Frankland, E. J. Chem. Soc. 1850,
2, 297. (c) Seyferth, D. Organometallics 2001, 20, 2940.
(20) Organozinc Reagents; Knochel, P., Jones, P., Eds.; Oxford University Press:
Oxford, 1999.
(21) (a) Fischer, E. O.; Hoffmann, H. P.; Treiber, A. Z. Naturforsch. 1959, 14b,
599. (b) Blom, R.; Boersma, J.; Budzelaar, P. H. M.; Fischer, B.; Haaland,
A.; Volden, H. V.; Weidlein, J. Acta Chem. Scand. 1986, A40, 113. (c)
Fischer, B.; Wijkens, P.; Boersma, J.; van Koten, G.; Smeets, W. J. J.;
Spek, A. L.; Budzelaar, P. H. M. J. Organomet. Chem. 1989, 376, 223. (d)
Burkey, D. J.; Hanusa, T. P. J. Organomet. Chem. 1996, 512, 165. (e)
Budzelaar, P. H. M.; Boersma, J.; van der Kerk, G. J. M.; Spek, A. L.;
Duisenberg, A. J. M. J. Organomet. Chem. 1985, 281, 123. (f) Haaland,
A.; Samdal, S.; Tverdova, N. V.; Girichev, G. V.; Giricheva, N. I.; Shlykov,
S. A.; Garkusha, O. G.; Lokshin, B. V. J. Organomet. Chem. 2003, 684,
351.
Results and Discussion
Zn2(η5-C5Me5)2 (1) and Zn2(η5-C5Me4Et)2 (2). Synthesis
and Reactivity. During the course of our work on different
zincocenes, e.g., Zn(C5Me4SiMe3)2 and related compounds,28
(22) (a) Resa, I.; Carmona, E.; Gutierrez-Puebla, E.; Monge, A. Science 2004,
305, 1136. (b) Del R´ıo, D.; Galindo, A.; Resa, I.; Carmona, E. Angew.
Chem., Int. Ed. 2005, 44, 1244.
(23) (a) Parkin, G. A. Science 2004, 305, 1117. (b) Schnepf, A.; Himmel, H. J.
Angew. Chem., Int. Ed. 2005, 44, 3006.
(24) (a) Xie, Y.; Schaefer, H. F., III; King, R. B. J. Am. Chem. Soc. 2005, 127,
2818. (b) Xie, Y.; Schaefer, H. F., III; Jemmis, E. D. Chem. Phys. Lett.
2005, 402, 414. (c) Timoshkin, A. Y.; Schaefer, H. F. Organometallics
2005, 24, 3343. (d) Liu, Z.-Z.; Tian, W. Q.; Feng, J. K.; Zhang, G.; Li,
W.-Q. THEOCHEM 2006, 758, 127. (e) Merino, G.; Beltra´n, H. I.; Vela,
A. Inorg. Chem. 2006, 45, 1091. (f) Philpott, M. R.; Kawazoe, Y. Chem.
Phys. 2006, 327, 283. (g) Pathak, B.; Pandian, S.; Hosmane, N.; Jemmis,
E. D. J. Am. Chem. Soc. 2006, 128, 10915.
(25) (a) Richardson, S. L.; Barnah, T.; Pederson, M. R. Chem. Phys. Lett. 2005,
415, 141. (b) Xie, Z.-Z.; Fang, W.-H. Chem. Phys. Lett. 2005, 404, 212.
(c) Kang, H. S. J. Phys. Chem. A 2005, 109, 4342. (d) Kress, J. W.
J. Phys. Chem. A 2005, 109, 7757. (e) Zhou, J.; Wang, W.-N.; Fan, K.-N.
Chem. Phys. Lett. 2006, 424, 247. (f) Liu Z.-Z.; Tian, W. Q.; Feng, J.-K.;
Zhang, G.; Li, W.-K.; Cui, Y.-H.; Sun, Ch.-Ch. Eur. J. Inorg. Chem. 2006,
2808.
(26) Wang, Y.; Quillian, B.; Wei, P.; Wang, H.; Yang, X.-J.; Xie, Y.; King,
R. B.; Schleyer, P. v. R.; Schaefer, H. F., III; Robinson, G. H. J. Am. Chem.
Soc. 2005, 127, 11944.
(27) Zhu, Z.; Wright, R. J.; Olmstead, M. M.; Rivard, E.; Brynda, M.; Power,
P. P. Angew. Chem., Int. Ed. 2006, 45, 5807.
9
694 J. AM. CHEM. SOC. VOL. 129, NO. 3, 2007

Zinc−Zinc Bonded Zincocene Structures
A R T I C L E S
Scheme 1. Optimization of the Synthesis of Compound 1
other occasions, for reasons that escape our understanding, only
4a is formed. As discussed below, a convenient and reproducible
procedure for the gram-scale synthesis of 1 has been developed.
Interestingly, while under the conditions specified above for
the synthesis of 1 from 3 and ZnEt2, the organozinc reagents
ZnPri2 and ZnMes2 (Mes ) C6H2-2,4,6-Me3) furnish only the
half-sandwich products (η5-C5Me5)ZnR (R ) Pri, Mes), Zn-
(C6H5)2 behaves like ZnEt2 and gives rise to almost 1:1 mixtures
of Zn2(η5-C5Me5)2 and (η5-C5Me5)Zn(C6H5) 4b (eq 2). In
practice, this method is even more limited than that of Scheme
1 due to the lower solubility in ether of the phenyl zinc
organometallics involved in the reaction. The zincocenes ZnCp′2,
when Cp′ ) C5Me4H, C5Me4SiMe3, and C5Me4But, react with
ZnEt2 in Et2O to afford only the corresponding half-sandwich
ethyl products, (η5-Cp′)ZnEt.
we also attempted the preparation of half-sandwich alkyl or aryl
derivatives (η5-Cp′)ZnR of differently substituted Cp′ ligands
(from now Cp′ is used as a general representation for a
cyclopentadienyl ligand) including C5Me5. Information on these
compounds is sparse, and even if some derivatives of this kind
derived from the parent zincocene were known,29 analogous (η5-
C5Me5)ZnR species had been largely overlooked, despite the
fact that Zn(C5Me5)2 (3) is a well-known compound, structurally
characterized by X-ray crystallography and by gas electron
diffraction studies.
In accord with the reported formation of (η5-C5H5)ZnEt as
the exclusive product of the reaction of Zn(C5H5)2 and ZnEt2,29b
the room temperature interaction of 3 and ZnMe2 gives (η5-C5-
Me5)ZnMe in almost quantitative yield.30a However, the use of
ZnEt2 causes the reaction to take an unexpected course, as a
mixture of two compounds, the expected half-sandwich30a (η5-
C5Me5)ZnEt (4a) and the unexpected dizincocene 1, is generated
(eq 1).
+ Zn(C H ) Et O8
+
2
5
Zn(C5Me5)2
Zn2(η -C5Me5)2
6
5 2
3
1
(η5-C5Me5)Zn(C6H5)
(2)
4b
The formation of 1 and (η5-C5Me5)ZnR (R ) Et, 4a; C6H5,
4b) in the reaction of 3 with the corresponding ZnR2 reagent
takes place through independent, competitive reaction routes.
Thus, a mixture of 1 and 4a or 4b remains unchanged while
stirring at room temperature for 1-2 h and compounds 4 do
not transform into 1 when stirred overnight at 20 °C. It appears
likely that the mixed compounds (η5-C5Me5)ZnR form through
binuclear hydrocarbyl-bridged intermediates similar to those
involved in Schlenk-type equilibria, but a rationalization for the
generation of 1 is more difficult. It has been suggested23b that
active zinc resulting from partial decomposition of ZnEt2 may
reduce 3 to 1, but this appears unlikely as we have found that
Rieke-zinc is not a sufficiently strong reductant to effect this
transformation.
Instead, we have obtained evidence in favor of a radical
process. In a sealed NMR tube experiment equimolar amounts
of 3 and ZnEt2 were mixed at -80 °C in (C2D5)2O and the
mixture was subsequently warmed to -10 °C. Formation of 1
plus smaller amounts of 4a was observed. Minor quantities of
C5Me5H and C2H6 were also detected. However, no ethylene
could be detected. In view of the very high reactivity of the
system toward adventitious water we have not pursued this
experiment any further. Reacting 3 and ZnEt2 in the presence
of 1,4-cyclohexadiene (eq 3) gives only the half-sandwich
complex 4a, and similarly, carrying out the reaction in the
presence of the free radical TEMPO (TEMPO ) 2,2,6,6-
tetramethylpiperidin-1-yloxy) also inhibits the formation of the
dizincocene. Thus in the presence of 0.3 equiv of TEMPO, 4a
Low-temperature 1H NMR monitoring and the use of different
solvents permit optimization of this reaction to obtain one of
the two possible compounds as the major reaction product. As
represented in Scheme 1, effecting the reaction in pentane at
-60 °C yields exclusively the ethyl derivative 4a, while, in Et2O
at -10 °C, dimetallocene 1 becomes the main reaction product,
although it is always accompanied by variable amounts of 4a.
The maximum yield of 1 is seldom higher than 30% and is
often significantly smaller. Indeed this reaction seems to be
sensitive to subtle effects, including the purity of the reagents,
and although if it has been reproduced innumerable times, on
(28) (a) Ferna´ndez, R.; Resa, I.; del R´ıo, D.; Carmona, E.; Gutierrez-Puebla,
E.; Monge, A. Organometallics 2003, 22, 381. (b) Resa, I.; Rodriguez, A.;
Grirrane, A.; Fernandez, R.; Alvarez, E.; Carmona, E.; Gutierrez-Puebla,
E.; Monge, A.; del Rio, D.; Galindo, A., to be submitted.
(29) (a) Aoyagi, T.; Shearer, H. M. M.; Wade, K.; Whitehead, G. J. Organomet.
Chem. 1978, 146, C29. (b) Haaland, A.; Samdal, S.; Seip, R. J. Organomet.
Chem. 1978, 153, 187. (c) Jastrzebski, J. T. B. H.; Boersma, J.; van Koten,
G.; Smeets, W. J. J.; Spek, A. L. Recl. TraV. Chim. Pays-Bas 1988, 107,
263. (d) Strohmeier, W.; Ladsfeld, H. Z. Naturforsch. 1960, 15b, 332.
(30) (a) Resa, I.; Rodriguez, A.; Alvarez, E.; Carmona, E., to be submitted. (b)
Alvarez, E.; Grirrane, A.; Resa, I.; del R´ıo, D.; Rodriguez, A.; Carmona,
E. Angew. Chem., Int. Ed. 2006, in press.
becomes the only organometallic product, and small amounts
9
J. AM. CHEM. SOC. VOL. 129, NO. 3, 2007 695

A R T I C L E S
Grirrane et al.
metal generated in the reaction was suggested to arise from the
decomposition of (C5Me5)Zn• radicals.33 Considering that solu-
tions of 1 are exceedingly sensitive toward atmospheric reagents
with formation of finely divided zinc metal, it appears probable
that had Zn2(C5Me5)2 formed during the reaction of Zn(C5Me5)2
and Ni(cod)2 it would have decomposed under the reaction
conditions (60 °C, 24 h).33
Zn(C5Me5)2 reduction under appropriate conditions has
proved to be a convenient procedure for the preparation of 1 in
a ca. 2 g scale. Low-temperature reduction of 3 itself (eq 5)
with a variety of reagents yields 1. Strong reductants like Na,
Figure 2. ORTEP representation of 5 (30% probability displacement
ellipsoids).
K, and their hydrides, MH, as well as K/naphthalene or CaH2
may be used, but in our hands KH is the most convenient
reagent. To optimize the reduction further, mixtures of 3 and
ZnCl2 are reduced by KH (1:1:2 ratio) employing THF as the
solvent (eq 6).
•
of C10Me10 resulting from the coupling of two C5Me5 radicals31
are also detected. The use of 0.6 or 1 equiv of TEMPO increases
the proportion of C10Me10 in the reaction mixture and produces
also small, albeit isolable amounts of the binuclear compound
5 (eq 4) that contains a TEMPO- derived bridging anionic
ligand, as demonstrated by X-ray studies (Figure 2; see
Supporting Information). It is important to note that the radical
TEMPO does not react separately with either 3 or ZnEt2, so
that the above changes occur only when it is added to the
mixture of the two organozinc compounds, 3 and ZnEt2.
+ ZnCl + 2KH THF, 20 °C8
5
Zn(C5Me5)2
Zn2(η -C5Me5)2
2
3
1
(6)
Extension of this methodology to the synthesis of dizin-
cocenes Zn2Cp′2 of other cyclopentadienyl ligands has met so
far with only limited success. Thus, zincocenes derived from
C5Me4H, C5Me4SiMe3, and C5Me4But yield only zinc metal and
other decomposition products under the conditions of eq 6.
Interestingly, the parent zincocene, Zn(C5H5)2, does not undergo
decomposition but provides instead crystalline solids. Neverthe-
less the crystalline materials do not correspond to the desired
dizincocene, Zn2(η5-C5H5)2, that remains still an elusive,
unknown molecule but to cyclopentadienyl zincates of composi-
+ ZnEt2 + TEMPO Et O8
+
2
5
Zn(C Me )
(η -C5Me5)ZnEt
5
5
3
4a
[ZnEt(TEMPO)]2
+ C10Me10 (4)
5
As zincocenes ZnCp′2 are good Cp′ transfer reagents, it is
conceivable that in ether solvents C5Me5- transfer from Zn(C5-
Me5)2toZnEt2maygiveriseto[Zn(C5Me5)Sn]+and[Zn(C5Me5)Et2]-
species. In fact, in the presence of the macrocyclic 14N4 ligands
(14N4 ) 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetrade-
cane), ZnEt2 and ZnPh2 are reported to exist in equilibrium32
with [ZnEt(14N4)]+ and [ZnPh2Et]-. The C5Me5-containing zinc
cation and anion could eventually give rise to the Zn2(η5-C5-
Me5)2 product, but in the absence of additional, relevant
experimental data further speculation does not seem to be
justified. Nonetheless, it seems plausible that 1 may result from
the coupling of (C5Me5)Zn• radicals, and in this regard it should
be mentioned that the formation of such radicals had already
been proposed.33 In 1985 Boersma and co-workers found that
the reaction of Zn(C5Me5)2 and Ni(cod)2 yields [(η5-C5Me5)-
Ni]2(C16H24) together with metallic zinc. The C16H24 organic
ligand that bridges the two (η5-C5Me5)Ni units results from the
coupling of two cyclooctadiene-derived radicals, and the zinc
tion Zn(C5H5)3 and Zn2(C5H5)5 to be reported elsewhere.30b
Notwithstanding, Zn(C5Me4Et)2, readily obtained from ZnCl2
and KC5Me4Et,28b undergoes reduction under similar conditions
to give Zn2(η5-C5Me4Et)2 (2), the second example of a dime-
tallocene (eq 7).
-
-
Zn(C5Me4Et)2 + ZnCl2 + 2 KH THF8
5
Zn2(η -C5Me4Et)2
2
(7)
Compounds 1 and 2 can be obtained as crystalline solids.
The two compounds are exceedingly reactive toward oxygen
and water, but 1 has superior thermal stability and it is easier
to handle than 2. Compound 1 is a colorless crystalline solid
that burns spontaneously upon exposure to air. It is indefinitely
stable at room temperature under argon or in a sealed tube in
vacuo and crystallizes readily from pentane or from Et2O
solutions. It decomposes at ca. 110 °C (capillary tube under
argon), but it can be sublimed at temperatures near 70 °C under
a dynamic vacuum of 10-3 mbar. The sublimed solid features
1H and 13C{1H} NMR spectra (vida infra) identical to those of
samples obtained by crystallization from pentane at -20 °C.
The less symmetric compound 2 is more difficult to handle than
(31) Cummins, C. C.; Schrock, R. R.; Davies, W. M. Organometallics 1991,
10, 3781.
(32) Fabicon, R. M.; Pajerski, A. D.; Richey, H. G., Jr. J. Am. Chem. Soc. 1991,
113, 6680.
(33) (a) Fisher, B.; Boersma, J.; Kojˆıc´-Prodic´, B.; Spek, A. L. J. Chem. Soc.,
Chem. Commun. 1985, 1237. (b) Fisher, B.; Boersma, J.; van Koten, G.;
Spek, A. L. New J. Chem. 1988, 12, 613
9
696 J. AM. CHEM. SOC. VOL. 129, NO. 3, 2007

Zinc−Zinc Bonded Zincocene Structures
A R T I C L E S
Scheme 2. Some Reactions of Metallocenes 1 and 3
Scheme 3. Synthesis of 68Zn2(η5-C5Me5)2
1
evolution is clearly observed (confirmed by H NMR experi-
ments) as a result of reduction of H2O by the reactive zinc
produced. After heating the solid at 50 °C under a vacuum,
ZnO is obtained as the only metal-containing product. The
alcoholysis of the zincocenes by ButOH yields related results.
34
Thus 3 affords C5Me5H and [Zn(OBut)2]x (Scheme 2b),
whereas the zinc-zinc bonded metallocene yields a mixture of
C5Me5H, Zn, and [Zn(OBut)2]x. Metallic zinc and the alkoxide
have been identified by X-ray powder diffraction, and the latter
1
has been identified also by comparison of its H and 13C{1H}
NMR spectra with those of an authentic sample.34
Finally, CNXyl reacts with compound 3 to give the iminoacyl
product 6 (Scheme 2c), also identified by comparison of its
spectroscopic properties (IR, 1H and 13C{1H} NMR) with those
of an authentic sample.35 The analogous reaction of 1 yields
the iminoacyl 6 plus metallic zinc, as the only reaction products.
Thus, it is clear that, under the conditions specified in Scheme
2, H2O, ButOH, and CNXyl induce the disproportionation of 1
into Zn and the corresponding Zn(II) compound.
Molecular Structures of Zn2(η5-C5Me5)2 and Zn2(η5-
C5Me4Et)2. Compound 1 has been structurally characterized
by several independent methods, including NMR, mass spec-
trometry, vibrational spectroscopy (IR and Raman studies, to
be reported elsewhere36), and X-ray diffraction studies. A
neutron diffraction investigation on compound 1 has also been
carried out independently and will be reported separately.37 Since
a cobalt complex initially formulated as Co2(η5-C5Me5)210a was
subsequently demonstrated to contain three bridging hydrogen
atoms, [(η5-C5Me5)Co]2(µ-H)3,10b-d we have applied the above
techniques to discard a putative bridging hydride structure for
our compounds, i.e., [(η5-Cp′)Zn]2(µ-H)2. Further verification
of the composition of 1 was provided by mass spectral analysis
of 68Zn2(η5-C5Me5)2, which was prepared from commercially
available 68ZnO via Scheme 3.
1, as it is more difficult to crystallize. In solution it exhibits
moderate thermal stability at room temperature, but as a pure
solid it decomposes slowly at temperatures between 0 and 20
°C and becomes a brown oily material. Heating the pure solid
at 40 °C gives a deposit of metallic zinc. However, crystals of
2 suitable for X-ray studies can be obtained by slow evaporation
of its pentane solutions at -20 °C or by rapidly cooling very
concentrated solutions from room temperature to -78 °C, and
this has allowed its structural characterization by X-ray crystal-
lography. Because of the superior thermal stability of 1 and its
easier isolation and manipulation, it has been used for the
reactivity studies discussed below.
Despite the high reactivity of Zn2(η5-C5Me5)2 toward O2 and
moisture, no reaction occurs with H2 (1-4 atm, 20-70 °C),
carbon monoxide or carbon dioxide (20 °C, 1 atm). Furthermore,
compound 1 is recovered unaltered upon stirring at room
temperature in the presence of NEt3, Me2NCH2CH2NMe2,
pyridine or bipyridine. Similarly, no reaction takes place at 20
°C when 1 is treated with PMe3 or PPh3. In all these cases some
decomposition of 1 to metallic zinc is observed, but this is
probably due to the action of adventitious water. Elemental
iodine oxidizes 1 to a mixture of 3 and ZnI2 (the latter identified
by powder X-ray diffraction) whereas ZnMe2 and ZnMes2
convert 1 into mixtures of the corresponding half-sandwich
compounds (η5-C5Me5)ZnR, and zinc metal.
None of the above techniques have given any indication for
the existence of zinc-bound hydrogen atoms, Zn(µ-H)2Zn.
Instead their application support the dimetallocene formulation,
with a direct Zn-Zn bond. Because of the poor thermal stability
1
of compound 2 its characterization relies on solution H and
13C{1H} NMR data and on a low-temperature X-ray diffraction
study. As discussed below, comparison of the Zn-Zn distance
in 1 and 2 (ca. 2.31 Å) with those found in known Zn2(µ-H)2
compounds27,38 (ca. 2.45 Å), corroborate the proposed formula-
tion. Theoretical calculations presented in the next section and
those already in the literature22b,24,25 give additional strength to
the proposal that 1 and 2 have unique dimetallocene structures.
Comparison of the reactivity of the mononuclear and bi-
nuclear zincocenes, 3 and 1, respectively, toward H2O, ButOH,
and CNXyl (Xyl ) C6H3-2,6-Me2) is useful (Scheme 2). The
mononuclear zincocene reacts with water at -10 °C (a 10 molar
excess, dilute Et2O solution) forming C5Me5H and a white
precipitate of Zn(OH)2 that gives into ZnO when heated at 50
°C under a vacuum. The analogous reaction of 1 generates C5-
Me5H, Zn(OH)2, and highly crystalline zinc metal, as shown
by X-ray powder diffraction, but upon workup at 20 °C, H2
(34) Coates, G. E.; Roberts, P. D. J. Chem. Soc. A 1967, 1233.
(35) Lopez del Amo, J. M.; Buntkowsky, G.; Limbach, H. -H.; Resa, I.;
Fernandez, R.; Carmona, E., submitted.
(36) Tang, C.; Downs, A. J.; Koeppe, R.; Resa, I.; Rodriguez, A.; Carmona, E.,
manuscript in preparation.
(37) Ferna´ndez-D´ıaz, M. T.; McIntyre, G.; Gutie´rrez-Puebla, E.; Monge, A.,
manuscript in preparation.
(38) Hao, H.; Cui, C.; Roesky, H. W.; Bay, G.; Schmidt, H. -G.; Moltemeyer,
M. Chem. Commun. 2001, 1118.
9
J. AM. CHEM. SOC. VOL. 129, NO. 3, 2007 697

A R T I C L E S
Grirrane et al.
Figure 3. Molecular ion envelope M+ for 1.
The 1H and 13C{1H} NMR spectra of 1 are extremely simple
and of little structural value. The 1H NMR spectrum shows only
one signal at δ 2.02 due to the Me protons of the C5Me5 ligands.
A corresponding 13C{1H} resonance is found at δ 10.0, and
this is accompanied by the resonance due to the quaternary ring
carbon nuclei with a chemical shift of 108.8 ppm. No 1H signals
attributable to a putative bis(hydride) bridging structure have
been found. For comparative purposes, the structurally charac-
terized bridging hydride complexes [HC(CMeNXyl)2]Zn2(µ-
H)238 and [ArZn]2(µ-H)227 (Ar ) C6H3-2,6-(C6H3-2,6-Pri2)2)
present signals at 4.59 and 4.84 ppm, respectively, due to the
hydride ligands. NMR data for 2 are also simple, and as for 1
they are of little use for structural purposes. These data are listed
in the Experimental Section.
Figure 4. ORTEP representation for 1 (30% probability displacement
ellipsoids). d(Zn-Zn) ) 2.305(3) Å. C5Me5centr-Zn-Zn angles 177.05-
(2)° and 178.88(2)°.
The high-resolution mass spectra of 1 and the 68Zn-labeled
compound provide unequivocal support for the dimetallocene
formulation, Zn2(η5-C5Me5)2, without additional H atoms. Figure
3 shows the molecular ion envelope, M+ around 400 m/e, whose
complexity is due to the existence of several zinc isotopes
(principally 64Zn, 48.6%; 66Zn, 27.9%; 67Zn, 4.1%; and 68Zn,
18.8%39). It is evident that the dimeric molecules of 1 give a
well behaved molecular ion in the gas phase. The exact mass
Figure 5. ORTEP representation for 2 (30% probability displacement
ellipsoids). d(Zn-Zn) ) 2.295(3) Å. C5Me4Etcentr-Zn-Zn angles 173.60-
(1)° and 176.77(2)°.
of the peak at 398 amu is 398.0927, giving an excellent fit for
the calculated mass of 398.0930 for 64Zn2
C
1H30, whereas
12
20
the exact mass of the peak at 400 amu (400.0904) corresponds
to the molecules with isotopic distribution 64Zn66Zn12C20 H30.
1
The artificially prepared 68Zn2(C5Me5)2 gives a simple molecular
two planar, nearly parallel C5Me5 rings that adopt an eclipsed
configuration. The separation between the planes of the rings
is of ca. 6.40 Å, and as the van der Waals radius assigned to a
-CH3 group is of 2.0 Å,40 this large distance prevents steric
interactions between Me groups of the two rings that are
common in mononuclear metallocenes.41 This notwithstanding,
the Me groups bent out of the plane of the rings away from the
metal atoms, the ring plane-C-Me angles varying between
ca. 3° and 6°. Similar bendings have been observed in
mononuclear metallocenes and have been attributed, using
covalency arguments,42 to the match between the radial exten-
sions of the p-orbitals of the metal parallel to the Cp′ ring and
the carbon p-orbitals of the Cp′ ring perpendicular to the Cp′
plane. Alternatively, on an electrostatic description as applied
ion M+ at 406 amu, exact mass 406.0848, due to molecules
with isotopic distribution 68Zn2
C
1H30 (calculated mass
12
20
406.0844). In the two spectra, fragment ion patterns due to Zn-
(C5Me5)2 and 68Zn(C5Me5)2 are respectively observed, suggest-
ing that in the gas phase the molecules of Zn2(C5Me5)2 rearrange
to Zn(C5Me5)2 and Zn.
The crystal and molecular structures of the dimetallocenes 1
and 2 have been determined by X-ray crystallography and are
presented in Figures 4 and 5 (see Experimental Section for X-ray
data). For 1 two independent determinations have been carried
out, the first at -100 °C using graphite monochromated Mo
KR radiation (λ ) 0.710 73 Å), while the second was effected
at -170 °C, with Cu KR radiation (λ ) 1.541 84 Å). In accord
with expectations both determinations provided identical results,
and hence only the original -100 °C study is discussed. As
can be seen in Figure 4, the zinc atoms are sandwiched between
(40) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University
Press: Ithaca, New York, 1960; Chapter 7.
(41) (a) Haaland, A. Acc. Chem. Res. 1979, 12, 415. (b) Haaland, A. Top. Curr.
Chem. 1975, 53, 1.
(39) Emsley, J. The Elements, 2nd ed.; Clarendon Press: Oxford, 1991.
(42) Jemmis, E. D.; Schleyer, P. v. R. J. Am. Chem. Soc. 1982, 104, 4781.
9
698 J. AM. CHEM. SOC. VOL. 129, NO. 3, 2007

Zinc−Zinc Bonded Zincocene Structures
A R T I C L E S
Figure 6. Crystal packing of compound 1 showing one Zn2(η5-C5Me5)2 molecule and the specific orientation of four neighbor molecules (for which only
one C5Me5 ring is shown).
to LiC5H5,43 it has been advanced that a positively charged metal
center will favor greater electron density on its side of the Cp′
ring, achieved by bending the ring substituents away from the
metal.43
Interestingly, the related [Ar′Zn]2(µ-H)(µ-Na) derivative that
exhibits an unusual type of metal-metal interaction has a Zn-
Zn separation of 2.352(2) Å, thus essentially identical to that
of Ar′ZnZnAr′.27
In compound 1 the coordination of the C5Me5 rings is
symmetrical and it is characterized by Zn-Cring distances in
the narrow range of 2.27 to 2.30 Å and by Zn-C5Me5centr
separations of ca. 2.04 Å. Furthermore, the ringcentr-Zn-Zn-
ringcentr distribution is almost linear, with an average angle of
177.4(1)°. The molecular structure of Zn2(η5-C5Me4Et)2 exhibits
similar features (Figure 5) although the ringcentr-Zn-Zn-
A final structural comment that is worthy of note concerns
the molecular packing observed in the crystals of 1 and 2. With
reference for instance to 1 (see Figure 6) each molecule of
Zn2(η5-C5Me5)2 is surrounded by four neighbor molecules with
an orientation such that all of the Zn-Zn vectors are parallel
and consequently the C5Me5 rings are also parallel. If these
molecules are viewed along the Zn-Zn vector of one molecule
of Zn2(η5-C5Me5)2, the cyclopentadienyl ligands are found to
adopt a gearlike arrangement with each C5Me5 group being
located approximately at the midpoint of the Zn-Zn vector and
perpendicular to it.
ringcentr shows some deviation from linearity, with ringcentr
-
Zn-Zn angles of 173.6(1)° and 176.8(1)°.
Doubtless, the most salient feature in the structures of 1 and
2 is the short Zn-Zn separation of 2.305(3) Å in 1 and 2.295-
(2) Å in 2, thus identical within experimental error. For the
two compounds, the Zn-Zn bond length is appreciably shorter
than twice Pauling’s single-bond metallic radius (2.50 Å40).
Comparing this Zn-Zn distance with the M-M bond lengths
Electronic Structure and Bonding Properties of 1 and 2.
Comparison with Other Zn-Zn Bonded Species. The
synthesis and structural characterization of 1 was followed
shortly by a number of theoretical studies aimed at investigating
the electronic structure and the bonding characteristics of Zn2(η5-
C5Me5)2 and of other related species. Here we provide full
details22b of our own computational analysis of Zn2(η5-C5Me5)2
extended to other dizincocenes Zn2(η5-Cp′2)2 and to compounds
of general formulation Zn2R2. The stability of these molecules
has been calculated with the aid of DFT methods by computing
their Zn-Zn bond dissociation energy and the energy of their
disproportionation reaction to metallic zinc and the correspond-
ing Zn(II) organometallic, ZnCp′2 or ZnR2.
3
12,44
in Cd2(AlCl4)2 and the dimercury dihalides Hg2X2
is
noteworthy. In the Cd2 compound the Cd-Cd distance is 2.58
Å, while twice Pauling’s single bond metallic radius for the Cd
atom is 2.82 Å. The reduction of ca. 8% is comparable to that
corresponding to the Zn2 compounds 1 and 2, while, for
mercury, taking the Hg-Hg separation of 2.53 Å corresponding
to Hg2Cl2 and the metallic radius of 1.44 Å, the reduction is of
about 12%. Comparison with the Zn-Zn separation in the other
two zinc-zinc bonded compounds known and in reported Zn-
(µ-H)2Zn structures is also appropriate. Thus, in Robinson’s Zn2-
One of our initial targets was the investigation of the unusual
Zn-Zn bond. The interaction of a Cp′ ligand with a p block
metal in monomeric Cp′M species (M ) In, Tl, Sn+) was
analyzed by Canadell and Eisenstein.45 Recently, the CpZn unit
has been studied in the parent zincocene,21f for which several
coordination modes have been investigated46 and a number of
quantitative parameters computed.47 Previous to the discussion
of the DFT results and taking into account the classical MO
diagram of a half-sandwich main group metallocene,45,48 the
26
[HC(CMeNAr)2]2 and Power’s27 Zn2Ar′2 (vide supra for the
meaning of Ar and Ar′), the Zn-Zn bond distances are
essentially the same, ca. 2.36 Å, that is, about 0.05 Å longer
than in 1 and 2, despite the lower effective coordination number
of the metals, in comparison with 1 and 2. In the H-bridged
dimers [HC(CMeNAr)2]2Zn2(µ-H)238 and [Ar′Zn]2(µ-H)227 where
there is no zinc-zinc bond, the metal-metal distances are 2.45
and 2.41 Å, respectively, between 0.1 and 0.15 Å longer than
in the Zn2(η5-Cp′)2 compounds 1 and 2. Additionally, it is worth
noting that in the compounds Ar′ZnZnAr′ and [Ar′Zn]2(µ-H)2,
where the aryl ligands remain the same,27 the Zn-Zn distances
of 2.3591(9) Å and 2.4084(3) Å differ by only ca. 0.05 Å.
(45) Canadell, E.; Eisenstein, O. Organometallics 1984, 3, 759.
(46) Lokshin, B. V.; Garkusha, O. G.; Borisov, Y. A.; Borisova, N. E. Russ.
Chem. Bull. Int. Ed. 2003, 52, 831.
(47) Rayo´n, V. M.; Frenking, G. Chem.sEur. J. 2002, 8, 4693.
(48) For example, see: Kwon, C.; McKee, M. L. In Computational Organo-
metallic Chemistry; Cundari, T. R., Ed.; Marcel Dekker: New York, 2001;
p 397.
(43) Waterman, K. C.; Streitwieser, A., Jr. J. Am. Chem. Soc. 1984, 106, 3138.
(44) Dorn, E. J. Chem. Soc., Chem. Commun. 1971, 466.
9
J. AM. CHEM. SOC. VOL. 129, NO. 3, 2007 699

A R T I C L E S
Grirrane et al.
(η5-C5H5)2. The computed electronic energy E(Zn-Zn) (see
Table 2) of about 66 kcal‚mol-1 is comparable to values between
67 and 72 kcal‚mol-1 reported by others for the same
compound25b-d and to those calculated for the dihalides Zn2X2
(X ) F, Cl, Br, I)51,52 and Zn2H2.17a,52 Evidently the 62
kcal‚mol-1 value corresponding to the Zn-Zn BDE in Zn2(η5-
C5Me5)2 is higher by ca. 40 kcal‚mol-1 than that obtained for
the cationic [Zn-Zn]+ species53 that has a lower Zn-Zn bond
order, and it is also significantly higher than the experimental
value of ca. 13 kcal‚mol-1 determined for the cation [Zn-Zn]+
in a gas phase study.55
Figure 7. Optimized structure of Zn2(η5-C5Me5)2 compound and 3D-
isosurface of the HOMO-4, which corresponds to the Zn-Zn interaction.
Since the energy of the disproportionation reaction to Zn(0)
and Zn(II) is a better measure of the stability of the dizinc
compounds than the energy of the Zn-Zn bond,23b,24a the energy
of the disproportionation reaction Zn2Cp′2 f Zn + ZnCp′2 has
been computed, assuming that gaseous zinc is produced. For
the parent Zn2(η5-C5H5)2 the reaction is endothermic by 20.0
kcal‚mol-1, in accord with the value obtained by Schaeffer et
al.24a We have computed an endergonic value of ca. 5 kcal‚mol-1
for ∆G, this result being consistent with the stability of the
compounds Zn2(η5-C5Me5)2 and Zn2(η5-C5Me4Et)2.
In order to gain further knowledge of the dizinc unit in
organometallic compounds, we have extended our theoretical
study to other Zn2R2 molecules (R ) CH3, CF3, SiH3, Ph, CCH).
The related mononuclear ZnR2 compounds had already been
investigated both experimentally and theoretically.56 Optimized
structures for these compounds are presented in Figure 8, while
their structural parameters are included as Supporting Informa-
tion. The energies of the Zn-Zn bond and those of the
disproportionation reactions computed for these compounds are
collected in Table 2. It is clear that the BDEs of between 48
and 66 kcal‚mol-1 found for these simple organometallic
molecules are similar to the corresponding values in the
dimetallocenes Zn2(η5-Cp′)2.
frontier orbitals of a neutral (η5-C5Me5)Zn group are a singly
occupied HOMO, which results from the antibonding combina-
tion of a1 (π, Cp*) and the s and pz metal orbitals and a pair of
degenerate orbitals that are the combination of e1 (π, Cp*) with
the px,y metal orbitals. Thus, from a qualitative point of view,
the Zn-Zn bond in 1 results from the interaction of the singly
occupied HOMOs of the two (η5-C5Me5)Zn fragments. These
qualitative arguments are fully confirmed by our DFT calcula-
tions.22 The MO distribution of Zn2(η5-C5Me5)2 was analyzed
by using the AOMIX program,49,50 to find four quasi-degenerate
occupied orbitals (from the HOMO to the HOMO-3) that are
the combinations (in-phase and out-of-phase) of the degenerate
e1 orbitals of C5Me5 with a very small participation of the p
orbitals of Zn atoms (less than 3%). The HOMO-4 (Figure 7)
corresponds to the Zn-Zn interaction and has a participation
of Zn orbitals close to 60% (s, 96%, and p, 4%, for each Zn
atom). A similar interpretation of the bonding has been provided
by Kress.25d In the â-diketiminate complex Zn2[HC(CMeNAr)2]2
the Zn-Zn bond also has very high s character,26 whereas the
metal-metal interaction of the aryl derivative Ar′ZnZnAr′
results mostly from overlap of the zinc 4pz orbitals.27
Analysis of the bonding and of the MO distribution has also
been performed, using the AOMIX program49,50 (corresponding
figures for selected MOs are included in the Supporting
Information). For Zn2Me2 and Zn2(CF3)2 the main component
of the Zn-Zn bond resides in the HOMO-2 that consists mostly
(86-90%) of the Zn orbitals (about 95% s character and 3-4%
d character). A similar conclusion is obtained for Zn2(SiH3)2
and also for Zn2Ph2, for which related calculations have been
published recently27 with similar results.
The bond distances and angles computed for Zn2(η5-C5Me5)2
(Figure 7) compare favorably with the experimental values
(Table 1). For instance, the calculated Zn-Zn bond distance of
2.33 Å is nearly identical to the experimental value of ca. 2.30
Å found for 1 and 2. Moreover, the calculated Zn-Zn length
agrees well with values computed for a number of Zn(I) species
containing the [Zn-Zn]2+ unit. For instance, the model
Zn2[{HNCH}2CH]2 and the dihalides Zn2X2 (X ) F, Cl, Br, I)
have been computed at various levels of theory with Zn-Zn
bond lengths in the range 2.28-2.39 Å.51,52 In addition, in the
dihydride Zn2H2 the Zn-Zn separation is of the order of 2.37-
2.40 Å.17a,52 Table 1 summarizes the Zn-Zn distances obtained
up to now from both experimental and theoretical studies. As
can be seen, except for the neutral Zn2 and the singly charged
Zn2(, these distances fall in the range 2.28-2.40 Å and are
significantly lower than the 2.60 Å value computed for the [Zn-
Zn]+ cation.53
The BDEs calculated for Zn2R2 molecules suggest that they
have sufficient stability for them to be isolated, but the energies
of the disproportionation reactions indicate otherwise. Thus,
thermodynamic parameters in Table 2 point to exergonic
disproportionation with electron-withdrawing groups, e.g., CF3
vs Me, providing some stabilization. The synthesis of the aryl
derivative Zn2Ar′227 contrasts with a disproportionation energy
of ca. -10 kcal‚mol-1 computed for Zn2Ph2, although it can
reasonably be expected that the bulky o-substituents (Ar′ )
C6H3-2,6-(C6H3-2,6-Pri2) play an important stabilizing role.
In any case, these theoretical predictions should be taken with
caution, as significantly different values have, for example, been
In our preliminary report22b we determined the dissociation
energy of the zinc-zinc bond (BDE) by adopting a fragment-
oriented approach54 and obtained a value of 62.1 kcal‚mol-1
for Zn2(η5-C5Me5)2 and a similar value (62.5 kcal/mol) for Zn2-
(49) Gorelsky, S. I. AOMix program, rev. 5.62. http://www.obbligato.com/
software/aomix
(50) Gorelsky, S. I.; Lever, A. B. P. J. Organomet. Chem. 2001, 635, 187.
(51) Liao, M.-S.; Zhang, Q.-E.; Schwarz, W. H. E. Inorg. Chem. 1995, 34, 5597.
(52) Kaupp, M.; von Schnering, H. G. Inorg. Chem. 1994, 33, 4179.
(53) Gutsev, G. L.; Bauschlicher, C. W., Jr. J. Phys. Chem. A 2003, 107, 4755.
(54) Rosa, A.; Ehlers, A. W.; Baerends, E. J.; Snijders, J. G.; te Velde, G.
J. Phys. Chem. 1996, 100, 5690.
(55) Buckner, S. W.; Gord, J. R.; Freiser, B. S. J. Chem. Phys. 1988, 88, 3678.
(56) Haaland, A.; Green, J. C.; McGrady, G. S.; Downs, A. J.; Gullo, E.; Lyall,
M. J.; Timberlake, J.; Tutukin, A. V.; Volden, H. V.; Østby, K.-A. J. Chem.
Soc., Dalton Trans. 2003, 4356.
9
700 J. AM. CHEM. SOC. VOL. 129, NO. 3, 2007

Zinc−Zinc Bonded Zincocene Structures
A R T I C L E S
Table 1. Review of the Experimental and Calculated Zn-Zn Lengths
Zn
−Zn
compound
length (Å)
comments
reference
Experimental
Zn2(η5-C5Me5)2
2.305(3)
X-ray
X-ray
X-ray
X-ray
this work
this work
26
2
Zn2(η5-C5Me4Et)2
2.295(3)
2.3586(7)
2.3591(9)
Zn2[{(2,6-iPr2C6H3) N(Me)C}2CH]2
Zn2{C6H3-2,6-(C6H3-2,6-Pri2)2}2
Theory
Zn2(η5-C5Me5)2
Zn2(η5-C5H5)2
2.331
2.310
2.287
2.336
2.339
2.328
2.336
2.315
2.307
2.29
no symmetry, B3LYP
D5h, B3LYP
this work
25b
D5h, MP2
25b
D5h, B3LYP
25d
no symmetry, B3LYP
no symmetry, B3LYP
D5h, B3LYP
24d
this work
24a
D5h, B3LYP
24a
D5h, B3LYP
25b
VASP, D5h
25c
2.338
2.338
2.392
D5h, B3LYP
D5h, B3LYP
D2d, B3LYP
several compounds, D∞h
24d
24d
26
51,52
Zn2[{HNCH}2CH]2
Zn2X2
range 2.28-2.36
(X ) F, Cl, Br, I)
Zn2H2
range 2.37-2.40
range 3.18-3.91
D∞h
D∞h, several
17a,52
53
Zn2
exchange-correlation
potentials
+
Zn2-
2.60
range 3.02-3.20
2.487
Zn2
Zn2(η5-P5)2
Zn2Cp(η5-P5)
D5, B3LYP
C5, B3LYP
25f
25f
2.476
Table 2. Calculated Energies (kcal/mol) of Zn2R2 Compounds for the Zn-Zn Bond Dissociation Energy (BDE), with the Computed Energy
Contributions, and for the Disproportionation Reaction
Zn−
Zn bond dissociation energy
disproportionation energy
ZPE
molecule
E(Zn−
Zn)a
2·ER
BSSE
BDEb
∆E
∆E
+
∆H
∆G
Zn2(η5-C5H5)2
Zn2(CH3)2
Zn2(CF3)2
Zn2(SiH3)2
Zn2Ph2
66.7
55.2
59.3
50.2
58.4
66.6
-2.6
-1.4
-4.8
-1.4
-0.7
-0.3
-0.8
-0.5
-0.8
-0.4
-0.6
-0.7
62.5
53.3
53.7
48.4
55.1
65.6
+17.5
-1.3
+3.8
-0.2
-1.1
+2.2
+17.9
-1.3
+3.7
-0.3
-3.2
+2.1
+20.0
-1.2
+3.7
-0.3
-2.8
+2.1
+4.7
-8.2
-2.9
-5.4
-10.1
-4.7
Zn2(CCH)2
a The computed BDE represents only the electronic contribution to the reaction enthalpies and does not include ZPE corrections (details in the Supporting
Information). b Corrected for BSSE.
dizincocenes can be prepared readily in ca. 1-2 g scale. Several
independent structural characterization techniques prove un-
equivocally that they contain two directly bonded (η5-Cp′)Zn
units, with a short Zn-Zn separation of ca. 2.30 Å. Prior to the
2+
synthesis of 1, stable, molecular compounds of the Zn2 unit
had not been reported. But, in addition, the structures of 1 and
2 are unique, as the almost linear Cp′centr-Zn-Zn-Cp′centr
geometry, with a homonuclear metal-metal bond unsupported
by bridging ligands, finds no precedent among metallocenes of
either the transition series or the main group elements. The
results of our DFT analysis of the electronic structure of 1 reveal
that the Zn-Zn bond is strong (ca. 62 kcal‚mol-1), and it is
formed mainly by interaction of the 4s orbitals.
Figure 8. Optimized structures of Zn2R2 compounds (R ) CH3, CF3, SiH3,
Ph, CCH).
obtained25b for the Zn-Zn BDE in 1 using CCSD(T) methods
(ca. 42 kcal‚mol-1) and B3LYP calculations (ca. 67 kcal‚mol-1).
In summary, the original discovery of decamethyldizincocene
(1) has been complemented with the synthesis and X-ray
structural analysis of Zn2(η5-C5Me4Et2)2 (2). Although the latter
compound exhibits lower thermal stability than 1, its formation
suggests that analogous dizincocenes derived from differently
substituted cyclopentadienyl ligands may be isolated. The two
Experimental Section
General Methods. All preparations and manipulations were carried
out under oxygen-free argon using conventional Schlenk and glovebox
techniques. Solvents were rigorously dried and degassed before use.
Microanalyses were obtained at the Microanalytical Service of the
Instituto de Investigaciones Qu´ımicas (Sevilla). Mass spectra were
obtained at the Mass Spectroscopy Facility in the Chemistry Department
9
J. AM. CHEM. SOC. VOL. 129, NO. 3, 2007 701

A R T I C L E S
Grirrane et al.
Scheme 4. Two Different Routes for Calculation of BDE
Table 3. X-ray Data for 1, 2, and 5
1
2
5
formula
fw
crystal system
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
C20H30Zn2
401.18
triclinic
P1h
6.9329(3)
10.8831(5)
13.8384(7)
109.777(1)
101.603(1)
94.201(1)
951.09(8)
2
C22H34Zn2
429.23
triclinic
P1h
7.0428(4)
10.3194(6)
14.8036(9)
86.446(2)
89.191(2)
76.599(2)
1044.58(11)
2
C22H46N2O2Zn2
501.35
triclinic
P1h
8.1668(6)
8.1758(6)
10.7877(1)
67.755(2)
77.755(3)
75.470(2)
639.83(9)
1
at the University of California, Berkeley. Infrared spectra were recorded
on a Bruker Vector 22 spectrometer. NMR spectra were recorded on
Bruker AMX-300, DRX-400, and DRX-500 spectrometers. The 1H and
13C resonances of the solvent were used as the internal standard, and
the chemical shifts are reported relative to TMS.
Z
D
calcd, Mg m-3
1.401
2.517
29.31
173(2)
1.365
2.296
30.51
100(2)
1.301
1.892
30.53
173(2)
µ, mm-1
max, deg
θ
Synthesis of 68ZnCl2. A commercial sample of 68ZnO (0.617 g, 7.31
mmol; g99% enrichment) was dissolved in 22 mL of ethanol, and acetyl
chloride (2.10 mL, 29.34 mmol) was added dropwise with a syringe
while keeping the temperature at 0 °C. The mixture was allowed to
reach room temperature very slowly and was further stirred for 1.5 h.
All the volatiles were removed in a vacuum, and the residue was heated
at 100 °C under reduced pressure for 2 h. 20 mL of chlorotrimethyl-
silane were then added, and the suspension was refluxed for 1.5 h.
Afterward the volatiles were evaporated under reduced pressure, and
the solid was washed with pentane (2 × 10 mL). The desired 68ZnCl2
was obtained as a white solid in 92% yield (0.930 g).
temp, K
no. reflns collected
no. reflns used
no. of param
R1 [I > 2σ(I)]a
wR2 (all data)a
GOF
11 466
4662
209
0.0310
0.0759
0.988
26 394
6267
227
0.0326
0.0833
1.037
10 512
3864
145
0.0383
0.1035
1.079
at temperatures between 0 and 20 °C. It is advisable to store it at
-20 °C (or below) under argon. H NMR (500 MHz, C6D6): δ 0.90
1
(t, JHH ) 7.5 Hz, 3H, CH3-Et), 1.89 (s, 6H, CH3-Cp), 1.90 (s, 6H,
CH3-Cp*), 2.24 (q, JHH ) 7.5 Hz, 2H, CH2-Et). 13C{1H} NMR (125
MHz, C6D6): δ 10.5 (s, CH3-Cp), 10.6 (s, CH3-Cp), 16.1 (s, CH3-
Et), 19.5 (s, CH2-Et), 108.3 (s, Cq-Cp), 112.6 (s, Cq-Cp), 114.2 (s,
Cq-Cp). Yield: 70% (2.0 g).
Compound 5, [ZnEt(TEMPO)]2. To a mixture of Zn(C5Me5)2 (335
mg, 1 mmol) and TEMPO (93 mg, 0.6 mmol) in diethyl ether (20 mL)
at -10 °C, ZnEt2 (1 mL, 1 mmol) was added. The resultant mixture
was stirred at -10 °C for 1 h, and the solvent was removed in vacuo.
The residue was extracted with pentane (20 mL) and crystallized from
Synthesis of 68Zn(C5Me5)2. A mixture of 68ZnCl2 (0.550 g, 3.95
mmol) and KC5Me5 (1.37 g, 7.91 mmol) was suspended in 30 mL of
THF and stirred for 2.5 h at room temperature. The solvent was
evaporated under a vacuum, and the residue was extracted with pentane
(40 mL) yielding 68Zn(C5Me5)2 as a pale yellow solid (0.804 g, 62%),
with 1H and 13C{1H} NMR spectra identical to those of Zn(C5Me5)2.21b
Compound 1, Zn2(η5-C5Me5)2. To a mixture of Zn(C5Me5)2 (3.0 g,
8.9 mmol), ZnCl2 (1.21 g, 8.9 mmol), and KH (0.72 g, 17.9 mmol)
was added THF (70 mL). The suspension was stirred at room
temperature for 50 min, and afterward all the volatiles were removed
in vacuo. The residue was dissolved in pentane (120 mL), and the
solution was filtered off. A white crystalline solid was obtained by
evaporation of the pentane under reduced pressure. The compound can
be crystallized from pentane or from diethyl ether solutions, at
-20 °C. It sublimes at temperatures between 70 and 80 °C, 10-3 mbar.
Despite its thermal stability, due to its extreme reactivity toward oxygen
and water it is best kept at -20 °C under an argon atmosphere. Yield:
1
the same solvent yielding the organometallic compound 4a and 5. H
NMR (500 MHz, C6D6): δ 0.8 (q, JHH ) 8 Hz, 4H), 1.17 (s, 24H),
1.44 (m, 8H), 1.54 (t, JHH ) 8 Hz, 6H).
The half-sandwich compounds (η5-C5Me5)ZnR (R ) Me, Et, Ph,
Mes) and other related derivatives containing different cyclopentadienyl
ligands will be reported separately.30
Computational Details. The geometries of all calculated complexes
were computed within the density functional theory at the B3LYP
level.57For Zn2R2 (R ) CH3, CF3, SiH3, and CCH) the 6-311++G**
basis set was used for all atoms. For Zn2R2 (R ) C5H5, C6H5) the
6-311G* basis set was used. In all cases for the calculation of reaction
energies single-point calculations using the 6-311++G** basis set were
done on the previously optimized geometries. All the optimized
structures were characterized as real minima by diagonalization of the
analytically computed Hessian (Nimag ) 0). All the calculations were
performed using the Gaussian-98 or Gaussian-03 package.58 The MO
analysis has been performed on the optimized structures with the
6-311G** basis set using the AOMIX program.49,50 Figures and pictures
of selected MO have been obtained using Molekel.59
For the calculation of the bond dissociation energy of the Zn-Zn
bond a fragment-oriented approach was adopted54 in order to evaluate
the dissociation energy of the zinc-zinc bond (BDE) in species of
general formula Zn2R2. Two different procedure routes were envisaged
(Scheme 4). Primary, the BDE is calculated directly by subtracting the
energies of the optimized binuclear complex and the two mononuclear
fragments that are allowed to relax to their equilibrium geometries after
1
2.08 g (58%). H NMR (500 MHz, C6D6,): δ 2.02 (s, CH3). 13C{1H}
NMR (125 MHz, C6D6): δ 10.0 (C5Me5), 108.8 (C5Me5).21b Compound
1 can be also obtained following the same synthetic procedure but using
CaH2 as the reducing agent: To a mixture of Zn(C5Me5)2 (1.0 g, 3.0
mmol), ZnCl2, (0.40 g, 3.0 mmol) and CaH2 (0.12 g, 3.0 mmol) was
added THF (40 mL), and the mixture stirred for 2 h and 45 min. After
workup 1 was isolated in 48% yield (0.540 g). HRMS m/z calculated
for C20H30Zn2 (M+): 400.0899, found: 400.0904.
The labeled 68Zn2(C5Me5)2 dimetallocene was obtained following
the same synthetic route described for 1 but using 68Zn(C5Me5)2 and
1
68ZnCl2 as the starting materials. The H and 13C{1H} NMR spectra
68
are identical to those observed for 1. HRMS m/z calculated for C20H30
Zn2 (M+): 406.0783, found: 406.0848.
-
Compound 2, Zn2(η5-C5Me4Et)2: A mixture of KC5Me4Et (3.10 g,
36.6 mmol) and ZnCl2 (2.5 g, 18.6 mmol) was dissolved in THF (50
mL) and stirred for 4 h at room temperature. The solvent was evaporated
in vacuo, and the residue was extracted with pentane (3 × 15 mL).
After removing the pentane, 2 was obtained as a yellow oil in ca. 70%
yield. Crystals of 2 suitable for X-ray studies were obtained by slow
evaporation of its pentane solutions at -20 °C or by low-temperature
(-80 °C) crystallization of concentrated pentane solutions. Compound
2 is also highly reactive toward O2 and H2O and has lower thermal
stability than 1 decomposing slowly, particularly in the form of a solid,
(57) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Wang, Y.;
Parr, R. G. Phys. ReV. B 1988, 37, 785.
(58) Frisch, M. J., et al. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford,
CT, 2004.
(59) Portmann, S.; Lu¨thi, H. MOLEKEL: An Interactive Molecular Graphics
Tool. Chimia 2000, 54, 766.
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702 J. AM. CHEM. SOC. VOL. 129, NO. 3, 2007

Zinc−Zinc Bonded Zincocene Structures
A R T I C L E S
the metal-metal bond rupture, D(Zn-Zn). Alternatively, the BDE is
calculated in two steps. In the first the energy needed for “snapping”
the metal-metal bond, E(Zn-Zn), is computed and then, in the second
step, the energy gained when the isolated fragments relax from the
conformation taken up in the original molecules to their optimal ground-
state structure, ER, is evaluated. Taking into account the importance
of the basis set superposition error (BSSE) associated with metal-
metal bond energy calculations, the second procedure was selected,
since the BSSE can be computed directly using the counterpoise
method.60 The E(Zn-Zn) value was obtained by building the Zn2R2
molecule from two ZnR fragments that have the geometry they adopt
in the complex.
scans with narrow frames. Instrument and crystal stability were
evaluated from the measurement of equivalent reflections at different
measuring times, and no decay was observed. The data were reduced
(SAINT)61 and corrected for Lorentz and polarization effects, and a
semiempirical absorption correction was applied (SADABS).62 The
structure was solved by direct methods (SIR-2002)63 and refined against
all F2 data by full-matrix least-squares techniques (SHELXTL-6.12)64
2
minimizing w[Fo - Fc2]2. All the non-hydrogen atoms were refined
with anisotropic displacement parameters. The hydrogen atoms were
included from calculated positions and refined as riding contributions
with isotropic displacement parameters 1.5 (methyl groups) times those
of their respective attached carbon atoms.
X-ray Crystal Structure Analyses of 1, 2, and 5. A single crystal
of each representative compound (colorless prism 1, colorless prism
2, and colorless block 5) of suitable size was mounted on a glass fiber
using perfluoropolyether oil (FOMBLIN 140/13, Aldrich) in the cold
N2 stream of the a low-temperature device attachment. A summary of
crystallographic data and structure refinement is reported in Table 3.
Intensity data for 1 were collected on a Siemens SMART 1 K
diffractometer CCD area detector, equipped with a graphite mono-
chrotomator λ(Mo KR1) ) 0.710 73 Å and an Oxford Cryosystem low-
temperature device, whereas the data collections for complexes 2 and
5 were performed on a Bruker-AXS X8Kappa diffractometer APEX-
II CCD area detector, equipped with a graphite monochromator Mo
KR1 (λ ) 0.717 03 Å) and a Bruker Cryo-Flex low-temperature device.
The data collection strategy used in all instances was phi and omega
Acknowledgment. We thank Dr. Angel Justo (Instituto de
Ciencias de Materiales, Sevilla) for X-ray powder diffraction
analysis of Zn and ZnO samples. We are also grateful to the
X-ray diffraction service of the Universidad Auto´noma de
Madrid and to Mr. Ce´sar J. Pastor as responsible for this service,
for X-ray data collection at -170 °C employing Cu KR radiation.
I.R. thanks the Ministry of Education for a research grant. D.d.R.
thanks the sixth framework program of the EU for an MC-OIF
fellowship. Financial support from the DGESIC (Project CTQ
2004-409/BQU, FEDER support) and from the Junta de
Andaluc´ıa is gratefully acknowledged.
Supporting Information Available: X-ray crystallographic
data (CIF files). Table with selected bond distances for Zn2R2
optimized complexes and figures with selected MOs. Complete
ref 58. This material is available free of charge via the Internet
at http://pubs.acs.org.
(60) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553.
(61) SAINT 6.02; BRUKER-AXS, Inc.: Madison, WI 53711-5373, USA, 1997-
1999.
(62) Sheldrick, G. SADABS; Bruker AXS, Inc.: Madison, WI, USA, 1999.
(63) Burla, M. C.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; Giacovazzo,
C.; Polidori, G.; Spagna R. SIR2002: the program. J. Appl. Crystallogr.
2003, 36, 103.
(64) SHELXTL 6.14; Bruker AXS, Inc.: Madison, WI, USA, 2000-2003.
JA0668217
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J. AM. CHEM. SOC. VOL. 129, NO. 3, 2007 703