Synthesis and characterization of the homologous M-M bonded series Ar′MMAr′ (M = Zn, Cd, or Hg; Ar′ = C6H 3-2,6-(C6H3-2,6-Pri 2)2) and related arylmetal halides and hydride speci

Article abstract of DOI:10.1021/ja072682x

The synthesis and structural characterization of the first homologous, molecular M-M bonded series for the group 12 metals are reported. The compounds Ar′MMAr′ (M = Zn, Cd, or Hg; Ar′ = C₆H ₃-2,6-(C₆H₃-2,6-Pr

Full text of DOI:10.1021/ja072682x

Published on Web 08/11/2007
Synthesis and Characterization of the Homologous M-M
Bonded Series Ar′MMAr′ (M ) Zn, Cd, or Hg; Ar′ )
C6H3-2,6-(C6H3-2,6-Pri2)2) and Related Arylmetal Halides and
Hydride Species
Zhongliang Zhu, Marcin Brynda, Robert J. Wright, Roland C. Fischer,
W. Alexander Merrill, Eric Rivard, Robert Wolf, James C. Fettinger,
Marilyn M. Olmstead, and Philip P. Power*
Contribution from the Department of Chemistry, UniVersity of CaliforniasDaVis,
One Shields AVenue, DaVis, California 95616
Received April 17, 2007; E-mail: pppower@ucdavis.edu
Abstract: The synthesis and structural characterization of the first homologous, molecular M-M bonded
series for the group 12 metals are reported. The compounds Ar′MMAr′ (M ) Zn, Cd, or Hg; Ar′ ) C6H3-
2,6-(C6H3-2,6-Pri2)2) were synthesized by reduction of the corresponding arylmetal halides by alkali metal/
graphite (Zn or Hg) or sodium hydride (Cd). These compounds possess almost linear C-M-M-C core
structures with two-coordinate metals. The observed M-M bonds distances were 2.3591(9), 2.6257(5),
and 2.5738(3) Å for the zinc, cadmium, and mercury species, respectively. The shorter Hg-Hg bond in
comparison to that of Cd-Cd is consistent with DFT calculations which show that the strength of the Hg-
Hg bond is greater. The arylmetal halides precursors (Ar′MI)1 or 2, and the highly reactive hydrides (Ar′MH)1 or 2
,
were also synthesized and fully characterized by X-ray crystallography (Zn and Cd) and multinuclear NMR
spectroscopy. The arylzinc and arylcadmium iodides have iodide-bridged dimeric structures, whereas the
arylmercury iodide, Ar′HgI, is monomeric. The arylzinc and arylcadmium hydrides have symmetric (Zn) or
unsymmetric (Cd) µ-H-bridged structures. The Ar′HgH species was synthesized and characterized by
spectroscopy, but a satisfactory refinement of the structure was precluded by the contamination of monomeric
Ar′HgH by Ar′H. It was also shown that the decomposition of Ar′Cd(µ-H)2CdAr′ at room temperature leads
to the M-M bonded Ar′CdCdAr′, thereby supporting the view that the reduction of the iodide proceeds via
the hydride intermediate.
Introduction
of formula RMMR were thought to exist only as short-lived
transient species.1,6,7 In 1999, Apeloig and co-workers reported
The synthesis of stable molecular compounds featuring
homonuclear metal-metal bonds between organo derivatives
RMMR (R ) organic or related susbstituent) of the group 12
elements (Zn, Cd, and Hg) has proven to be a considerable
synthetic challenge. For many years, salt-like, ionic species
containing [Hg2]2+ units were the only known group 12
compounds that had been shown to have stable metal-metal
bonds.1 In the 1960s, the work of Corbett and co-workers2 had
shown that the [Cd2]2+ ion existed in aluminum chloride melts,
but it was not until 1986 that the structure of a [Cd2]2+ unit in
[Cd2][AlCl4]2 was reported independently by two different
groups.3 Evidence for the formation of [Zn2]2+ ions was also
reported in 1960s within ZnCl2/Zn glasses at high temperatures4
and later in zeolite matrices.5 Seemingly, M-M bonding could
be stablized only in ionic salts, and molecular M(I) compounds
the first stable, molecular, σ-bonded mercury(I) compound in
the form of the silyl derivative Hg2[Si(SiMe2SiMe3)3]2.8 UV/
vis spectroscopy revealed an absorption band at 530 nm that
was assigned to a HOMO-LUMO transition involving the Hg-
Hg σ and σ* orbitals. Earlier, in 1993, the molecular compound
Cd2TpMe 2 (TpMe ) hydrotris(3,5-dimethylpyrazolyl)borate) was
2
2
synthesized by treatment of CdCl2 with TlTpMe /LiBHEt3 and
2
characterized spectroscopically by Reger and his group.9 The
(5) (a) Rittner, F.; Seidel, A.; Boddenberg, B. Microporous Mesoporous Mater.
1998, 24, 27. (b) Zhen, S.; Bae, D.; Seff, K. J. Phys. Chem. B 2000, 104,
515.
(6) (a) Wardell, J. L. In ComprehensiVe Organometallic Chemistry; Wilkinson,
G., Stone, F. G. A., Albel, E. W., Eds.; Pergamon, Oxford, 1982; Vol. 2,
Chapter 17, p 864. (b) Brodersen, K.; Hummel, H.-U. In ComprehensiVe
Coordination Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J. A.,
Eds.; Pergamon: Oxford, 1987; Vol. 5, Chapter 56.2, p 1050.
(7) (a) Gade, L. H. Angew. Chem., Int. Ed. 1993, 32, 24. (b) Schwerdtfeger,
P.; Boyd, P. D. W.; Brienne, S.; McFeaters, J. S.; Dolg, M.; Liao, M.-S.;
Schwarz, W. H. E. Inorg. Chim. Acta 1993, 213, 233. (c) Kaupp, M.; von
Schnering, H.-G. Inorg. Chem. 1994, 33, 4179. (d) Liao, M.-S.; Zhang,
Q.-E.; Schwarz, W. H. E. Inorg. Chem. 1995, 34, 5597. (e) Liao, M.-S.;
Zhang, Q.-E. J. Mol. Struct. (Theochem) 1995, 358, 195.
(1) Holloway, C. E.; Meln´ık, M. J. Organomet. Chem. 1995, 495, 1.
(2) (a) Corbett, J. D.; Burkhard, W. J.; Druding, L. F. J. Am. Chem. Soc. 1961,
83, 76. (b) Corbett, J. D. Inorg. Chem. 1962, 1, 700. (c) Potts, R. A.; Barnes,
R. D.; Corbett, J. D. Inorg. Chem. 1968, 7, 2558.
(3) (a) Faggiani, R.; Gillespie, R. J.; Vekris, J. E. J. Chem. Soc., Chem.
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548, 45.
(8) Bravo-Zhivotovskii, D.; Yuzefovich, M.; Bendikov, M.; Klinkhammer, K.;
Apeloig, Y. Angew. Chem., Int. Ed. 1999, 38, 1100.
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115, 10406.
(4) Kerridge, D. H.; Tariq, S. A. J. Chem. Soc. A 1967, 1122.
9
10.1021/ja072682x CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 10847-10857
10847

A R T I C L E S
Zhu et al.
existence of a Cd-Cd bond in Cd2TpMe was established by
relatively scarce,9,13,18-20 and the use of the bulky terphenyl
ligands for their stabilization would afford an opportunity to
study their properties.
We now describe the synthesis and characterization of a series
of molecular group 12 compounds, M2Ar′2 (M ) Zn, Cd, or
Hg), as well as the corresponding hydrides (Ar′MH)1 or 2, and
discuss their structures and bonding in detail.
22
the observation of Cd111-Cd113 coupling in its 113Cd NMR
spectrum, although an X-ray structure could not be obtained.
Later, an example of a molecular zinc hydride with a Zn-Zn
bond, Zn2H2,10 was trapped in an argon matrix at 12 K and
characterized by vibrational spectroscopy, deuterium labeling
studies, and theoretical calculations. However, no structurally
characterized stable molecule featuring a Zn-Zn bond was
synthesized until 2004, when the landmark compound, Zn2Cp*2
(Cp* ) η5-C5Me5), was reported by Carmona and co-workers.11
Subsequent work led to the preparation and characterization of
Zn2[HC(CMeNAr)2]2 (Ar ) 2,6-Pri2C6H3)12 and Zn2Ar′2 (Ar′
) C6H3-2,6-(C6H3-2,6-Pri2)2).13 These results showed that a Zn-
Zn bond could be stabilized when suitable ligands were present,
and DFT calculations revealed that the nature of the ligands
used could have a significant influence on the metal-metal
bonding.11-13 More recently, the first structurally characterized
stable molecular species containing a Cd-Cd bond was
reported.14 The currently known metal-metal bonded molecular
compounds of group 12 elements display bond distances that
increase as the group is descended (Zn < Cd < Hg), as
exemplified by the range 2.305(3)-2.3591(9) Å for Zn-Zn
bonds,11-13 2.6257(5) Å for the Cd-Cd bond in Cd2Ar′2,14 and
2.6569(1) Å for the Hg-Hg bond in Hg2[Si(SiMe2SiMe3)3]2.8
However, theoretical calculations including relativistic and
correlation effects, as well as the influence of the lanthanide
contraction, consistently predict that the covalent radius of
mercury should be smaller than that of cadmium.7,15 A similar
contraction trend is also present in neighboring group 11
compounds, which is confirmed by both theoretical calculations
and structural data.16 We were therefore eager to synthesize the
first complete, homologous series of group 12 metal-metal
bonded molecular compounds in order to test the theoretical
prediction. In addition, we thought it desirable to synthesize
the closely related metal hydride derivatives for two reasons:
First, the characterization of such hydrides would allow
experimental comparison of the metal-metal separations in the
metal-metal bonded and metal hydride compounds.17 Second,
well-characterized group 12 molecular metal hydrides are
Experimental Section
General Procedures. All manipulations were carried out by using
modified Schlenk techniques under an atmosphere of N2 or in a Vacuum
Atmospheres HE-43 drybox. Solvents were dried according to the
method of Grubbs21 and degassed prior to use. The chemicals used in
this study were purchased from Aldrich or Acros and used as received.
The lithium aryl, LiAr′, was prepared as previously described.22 1H,
13C, 113Cd, and 199Hg NMR spectra were recorded on Varian 300 and
600 MHz spectrometers. The 113Cd NMR spectra were referenced
externally to 0.01 M Cd(ClO4)2, while the 199Hg NMR spectra were
referenced externally to a neat sample of HgMe2. (Caution: Cadmium
and mercury compounds are well known for their toxicity. HgMe2 is a
highly dangerous neurotoxin that requires great care in its handling.
Ingestion or skin contact must be rigorously aVoided.) Infrared data
were recorded as Nujol mulls using a Perkin-Elmer 1430 instrument.
Melting points were recorded in sealed capillaries using a Meltemp
apparatus and are uncorrected.
Ar′Zn(µ-I)2ZnAr′ (1). Ar′Li (2.00 g, 4.95 mmol) and ZnI2 (1.59 g,
4.95 mmol) were combined with diethyl ether (50 mL) and stirred for
2 d. The solvent was then removed under a dynamic vacuum, and the
residue was extracted with toluene (50 mL). The resultant slurry was
allowed to settle, and the mother liquor was separated from the
precipitate (LiI) by filtration. The toluene was removed from the filtrate,
and the residue was washed with ca. 10 mL of cold hexane. The isolated
colorless powder was redissolved in warm toluene (ca. 20 mL), followed
by overnight cooling to ca. -18 °C, which afforded X-ray-quality
1
crystals of 1. Yield: 1.82 g, 62%; mp >360 °C. H NMR (300 MHz,
3
C6D6, 25 °C): δ 1.08 (d, 12H, o-CH(CH3)2, JHH ) 6.9 Hz), 1.23 (d,
3
3
12H, o-CH(CH3)2, JHH ) 6.9 Hz), 3.00 (sept, 4H, CH(CH3)2, JHH
)
6.9 Hz), 7.15-7.34 (m, 9H, m-C6H3, p-C6H3, m-Dipp, and p-Dipp (Dipp
) C6H3-2,6-Pri2)). 13C{1H} NMR (C6D6, 100.6 MHz, 25 °C): δ 24.2
(CH(CH3)2), 25.4 (CH(CH3)2), 30.5 (CH(CH3)2), 123.6 (m-Dipp), 127.1
(p-C6H3), 127.9 (p-Dipp), 128.7 (m-C6H3), 142.8 (i-Dipp), 146.8 (o-
C6H3), 148.0 (o-Dipp), 153.4 (i-C6H3).
Ar′Cd(µ-I)2CdAr′ (2). Compound 2 was prepared in a way similar
to that described for 1, starting from Ar′Li (1.11 g, 2.73 mmol) and
CdI2 (1.00 g, 2.73 mmol). After the reaction mixture was stirred for 2
d, hexane (50 mL) was used to extract the crude compound. The hexane
was removed, and the pale yellow residue was extracted again with
hexane (50 mL). The solution was then filtered, and the volume was
concentrated to ca. 10 mL. Storage of the hexane solution for 4 d in a
(10) (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.
(11) (a) Resa, I.; Carmona, E.; Gutie´rrez-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. (c) Grirrane, A.; Resa, I.; Rodr´ıguez, A.;
Carmona, E.; Alvarez, E.; Gutie´rrez-Puebla, E.; Monge, A.; Galindo, A.;
del R´ıo, D.; Andersen, R. A. J. Am. Chem. Soc. 2007, 129, 693.
(12) 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.
(13) Zhu, Z.; Wright, R. J.; Olmstead, M. M.; Rivard, E.; Brynda, M.; Power,
P. P. Angew. Chem., Int. Ed. 2006, 45, 5807.
(14) Zhu, Z.; Fischer, R. C.; Fettinger, J. C.; Rivard, E.; Brynda, M.; Power, P.
P. J. Am. Chem. Soc. 2006, 128, 15068.
(15) (a) Desclaux, J. P. At. Data Nucl. Data Tables 1973, i2, 311. (b) Ziegler,
T.; Snijders, J. G.; Baerends, E. J. J. Chem. Phys. 1981, 74, 1271. (c)
Pyykko¨, P. Chem. ReV. 1988, 88, 563.
(16) (a) Schwerdtfeger, P.; Dolg, M.; Schwarz, W. H. E.; Bowmaker, G. A.;
Boyd, P. D. W. J. Chem. Phys. 1989, 91, 1762. (b) Schwerdtfeger, P.;
Boyd, P. D. W.; Burrell, A. K.; Robinson, W. T.; Taylor, M. J. Inorg.
Chem. 1990, 29, 3593. (c) Bayler, A.; Schier, A.; Bowmaker, G. A.;
Schmidbaur, H. J. Am. Chem. Soc. 1996, 118, 7006.
(17) It is often experimentally difficult to disprove the presence of a bridging
hydride ligand by X-ray crystallography. For further aspects of this problem,
see the following: (a) Schneider, J. J.; Goddard, R.; Werner, S.; Kru¨ger,
C. Angew. Chem., Int. Ed. 1991, 30, 1124. (b) Abrahamson, H. B.; Niccolai,
G. P.; Heinekey, D. M.; Casey, C. P.; Bursten, B. Angew. Chem., Int. Ed.
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. 1992,
31, 1341. (d) Lutz, F.; Bau, R.; Wu, P.; Koetzle, T. F.; Kru¨ger, C.;
Schneider, J. J. Inorg. Chem. 1996, 35, 2698.
(18) Some organozinc hydride examples: (a) Bell, N. A.; Moseley, P. T.;
Shearer, H. M. M.; Spencer, C. B. J. Chem. Soc., Chem. Commun. 1980,
359. (b) Looney, A.; Han, R.; Gorrell, I. B.; Cornebise, M.; Yoon, K.;
Parkin, G.; Rheingold, A. L. Organometallics 1995, 14, 274. (c) Kla¨ui,
W.; Schilde, U.; Schmidt, M. Inorg. Chem. 1997, 36, 1598. (d) Krieger,
M.; Neumu¨ller, B.; Dehnicke, K. Z. Anorg. Allg. Chem. 1998, 624, 1563.
(e) Hao, H. J.; Cui, C. M.; Roesky, H. W.; Bai, G. C.; Schmidt, H.-G.;
Noltemeyer, M. Chem. Commun. 2001, 1118.
(19) Some molecular cadmium hydride complexes: (a) Kimblin, C.; Bridgewater,
B. M.; Churchill, D. G.; Hascall, T.; Parkin, G. Inorg. Chem. 2000, 39,
4240. (b) Dowling, C. M.; Parkin, G. Polyhedron 2001, 20, 285. (c) Philson,
L. A.; Alyounes, D. M.; Zakharov, L. N.; Rheingold, A. L.; Rabinovich,
D. Polyhedron 2003, 22, 3461.
(20) Some spectroscopically characterized organomercury hydrides: (a) Devaud,
M. J. Organomet. Chem. 1981, 220, C27. (b) Bellec, N.; Guillemin, J.-C.
Tetrahedron Lett. 1995, 36, 6883. (c) Greene, T. M.; Andrews, L.; Downs,
A. J. J. Am. Chem. Soc. 1995, 117, 8180. (d) Nakamura, E.; Yu, Y.; Mori,
S.; Yamago, S. Angew. Chem., Int. Ed. 1997, 36, 374.
(21) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers,
F. J. Organometallics 1996, 15, 1518.
(22) Schiemenz, B.; Power, P. P. Angew. Chem., Int. Ed. 1996, 35, 2150.
9
10848 J. AM. CHEM. SOC. VOL. 129, NO. 35, 2007

Homologous M−M Bonded Series for Group 12 Metals
A R T I C L E S
freezer (ca. -50 °C) afforded colorless, X-ray-quality crystals of 2.
extracted with benzene (50 mL) and filtered. Upon removal of the
benzene, a white fine powder was obtained. Dissolving a sample of 6
in C6D6, followed by cooling for 2 d (ca. 8 °C), afforded colorless,
1
Yield: 1.17 g, 67%; mp 287 °C. H NMR (300 MHz, C6D6, 25 °C):
3
δ 1.07 (d, 12H, CH(CH3)2, JHH ) 6.6 Hz), 1.24 (d, 12H, CH(CH3)2,
1
3JHH ) 6.9 Hz), 2.99 (sept, 4H, CH(CH3)2, 3JHH ) 7.2 Hz), 7.13-7.31
(m, 9H, m-C6H3, p-C6H3, m-Dipp, and p-Dipp). 13C{1H} NMR (C6D6,
100.6 MHz, 25 °C): δ 24.5 (CH(CH3)2), 25.2 (CH(CH3)2), 30.6 (CH-
(CH3)2), 123.7 (m-Dipp), 126.7 (p-C6H3), 127.8 (p-Dipp), 128.9 (m-
C6H3), 144.2 (i-Dipp), 146.6 (o-C6H3), 148.2 (o-Dipp), 162.3 (i-C6H3).
113Cd{1H} (C6D6, 133.1 MHz, 25 °C): δ 210.9.
X-ray-quality crystals. Yield: 0.60 g, 18%; mp >360 °C. H NMR
(300 MHz, C6D6, 25 °C): δ 1.05 (d, 12H, CH(CH3)2, 3JHH ) 6.6 Hz),
3
1.12 (d, 12H, CH(CH3)2, JHH ) 6.6 Hz), 2.89 (sept, 4H, CH(CH3)2,
3JHH ) 7.2 Hz), 7.10-7.31 (m, 9H, m-C6H3, p-C6H3, m-Dipp, and
p-Dipp). 13C{1H} NMR (C6D6, 100.6 MHz, 25 °C): δ 24.0 (CH(CH3)2),
24.9 (CH(CH3)2), 30.4 (CH(CH3)2), 122.8 (m-Dipp), 127.1 (p-C6H3),
127.9 (p-Dipp), 128.3 (m-C6H3), 141.8 (i-Dipp), 146.2 (o-C6H3), 147.1
(o-Dipp), 200.0 (i-C6H3). 199Hg{1H} NMR (C6D6, 107.4 MHz, 25 °C):
δ 150.5.
Ar′HgI (3). Method A: Compound 3 was prepared in a way similar
to that described for 1 from Ar′Li (4.00 g, 9.89 mmol) and HgI2 (4.49
g, 9.89 mmol). After the reaction mixture was stirred for 2 d, toluene
(150 mL) was used to extract the crude compound. Upon removal of
the toluene, a white fine powder was obtained. Method B: Compound
3 can also be obtained by the treatment of Ar′MgBr‚2THF (11, 4.00 g,
6.19 mmol) with HgI2 (2.81 g, 6.19 mmol), followed by the same
workup as that of Method A. Storage of the resulting product in hexane
(ca. 0.2 g in 10 mL) for 1 d at ca. -18 °C afforded colorless, X-ray-
quality crystals of 3. Yield: 4.32 g, 60% (Method A) and 4.14 g, 91.9%
Ar′Zn(µ-H)2ZnAr′ (7). The iodide derivative 1 (1.00 g, 1.70 mmol)
and NaH (0.060 g, 2.50 mmol) were combined with THF (50 mL) at
ambient temperature. After the reaction mixture was stirred for 2 d,
the solvent was removed, and the residue was extracted with ca. 50
mL of hexane. The slurry was allowed to settle, and the mother liquor
was separated from the precipitate (NaI and excess NaH). The volume
was concentrated to ca.10 mL, and storage for 2 d in a freezer (ca.
-40 °C) afforded colorless, X-ray-quality crystals of 7. Yield: 0.81 g,
89% (based on 1); mp 234 °C (loss of crystallinity began at 210 °C,
and decomposition of 7 to a black solid was seen at 290 °C). 1H NMR
1
(Method B); mp 223 °C. H NMR (300 MHz, C6D6, 25 °C): δ 1.07
3
3
(d, 12H, CH(CH3)2, JHH ) 6.6 Hz), 1.24 (d, 12H, CH(CH3)2, JHH
)
3
6.9 Hz), 2.99 (sept, 4H, CH(CH3)2, JHH ) 7.2 Hz), 7.13-7.31 (m,
9H, m-C6H3, p-C6H3, m-Dipp, and p-Dipp). 13C{1H} NMR (C6D6, 100.6
MHz, 25 °C): δ 24.5 (CH(CH3)2), 25.2 (CH(CH3)2), 30.6 (CH(CH3)2),
123.7 (m-Dipp), 126.7 (p-C6H3), 127.8 (p-Dipp), 128.9 (m-C6H3), 144.2
(i-Dipp), 146.6 (o-C6H3), 148.2 (o-Dipp), 162.3 (i-C6H3). 199Hg{1H}
NMR (C6D6, 107.4 MHz, 25 °C): δ -1293.6.
3
(300 MHz, C6D6, 25 °C): δ 1.01 (d, 12H, o-CH(CH3)2, JHH ) 6.9
3
Hz), 1.11 (d, 12H, o-CH(CH3)2, JHH ) 6.9 Hz), 2.91 (sept, 4H,
3
CH(CH3)2, JHH ) 6.9 Hz), 4.84 (s, 1H, ZnH), 7.04-7.25 (m, 9H,
m-C6H3, p-C6H3, m-Dipp, and p-Dipp). 13C{1H} NMR (C6D6, 100.6
MHz, 25 °C): δ 24.3 (CH(CH3)2), 25.1 (CH(CH3)2), 30.6 (CH(CH3)2),
123.2 (m-Dipp), 126.0 (p-C6H3), 127.8 (p-Dipp), 128.5 (m-C6H3), 143.4
(i-Dipp), 146.7 (o-C6H3), 148.7 (o-Dipp), 155.7 (i-C6H3). IR (Nujol):
Zn2Ar′2 (4). A solution of 1 (0.93 g, 1.58 mmol) in diethyl ether
(50 mL) was added to a Schlenk tube containing finely cut sodium
(0.036 g, 1.58 mmol) at ambient temperature. After the reaction mixture
was stirred for 2 d, some Zn metal had precipitated. The solution was
then filtered, and the solvent was removed under a dynamic vacuum.
The residue was redissolved in hexane (5 mL), and storage for 2 d in
a freezer (ca. -40 °C) afforded large, colorless, X-ray-quality crystals
of 4. Yield: 0.174 g, 24%; mp >360 °C (when the temperature was
kept near 360 °C for 5 min, 4 decomposed and a black solid was
deposited). 1H NMR (300 MHz, C6D6, 25 °C): δ 1.00 (d, 12H, o-CH-
ν(Zn-H) bands were not observed and may be very weak or obscured by
overlapping ligand absorptions.
Ar′Zn(µ-H)(µ-Na)ZnAr′ (8). Compound 1 (0.50 g, 1.08 mmol) and
NaH (0.039 g, 1.62 mmol) were combined in THF (50 mL) in a Schlenk
tube. The mixture was stirred for 2 d, the solvent was then removed
under a dynamic vacuum, and the residue was extracted with toluene
(50 mL). The slurry was allowed to settle, and the mother liquor was
separated from the precipitate (NaH) using a filter cannula. The solvent
volume was concentrated to ca. 10 mL, and storage of product for 2 d
in a freezer (ca. -18 °C) afforded colorless, X-ray-quality crystals of
8. Yield: 1.03 g, 87% (based on 7); decomposed before melting (gas
evolution at 235 °C, and at ca. 268 °C it became a black solid). Yield:
3
3
(CH3)2, JHH ) 6.9 Hz), 1.12 (d, 12H, o-CH(CH3)2, JHH ) 6.9 Hz),
2.86 (sept, 4H, CH(CH3)2, 3JHH ) 6.9 Hz), 7.06-7.24 (m, 9H, m-C6H3,
p-C6H3, m-Dipp, and p-Dipp). 13C{1H} NMR (C6D6, 100.6 MHz, 25
°C): δ 24.7 (CH(CH3)2), 24.8 (CH(CH3)2), 30.4 (CH(CH3)2), 123.0
(m-Dipp), 125.7 (p-C6H3), 127.6 (p-Dipp), 128.3 (m-C6H3), 142.3 (i-
Dipp), 146.9 (o-C6H3), 147.4 (o-Dipp), 162.4 (i-C6H3).
1
0.40 g, 87% (based on 1). H NMR (300 MHz, C6D6, 25 °C): δ 1.09
(d, 12H, o-CH(CH3)2, 3JHH ) 6.9 Hz), 1.15 (d, 12H, o-CH(CH3)2, 3JHH
) 6.9 Hz), 2.04 (broad peak, 1H, (Ar′Zn)2‚NaH), 2.99 (sept, 4H,
Cd2Ar′2 (5). The iodide derivative 2 (1.00 g, 1.57 mmol) and NaH
(0.075 g, 3.14 mmol) were combined with THF (50 mL) and stirred
for 3 d. The solvent was then removed under a dynamic vacuum, and
the residue was extracted with benzene (50 mL). The slurry was allowed
to settle, and the mother liquor was decanted from the precipitate (NaI
and excess NaH). The volume was concentrated to ca. 10 mL, and
storage for 2 d in a refrigerator (ca. 8 °C) afforded colorless, X-ray-
quality crystals of 5. Yield: 0.22 g, 28% (based on 2); decomposition
3
CH(CH3)2, JHH ) 6.6 Hz), 7.01-7.27 (m, 9H, m-C6H3, p-C6H3,
m-Dipp, and p-Dipp). 13C{1H} NMR (C6D6, 100.6 MHz, 25 °C): δ
23.6 (CH(CH3)2), 25.2 (CH(CH3)2), 30.7 (CH(CH3)2), 122.4 (m-Dipp),
127.3 (p-C6H3), 146.8 (i-Dipp), 148.1 (o-C6H3), 148.8 (o-Dipp), 158.7
(i-C6H3); m-C6H3 and p-Dipp resonances are likely obscured by the
C6H6 signal. IR (Nujol): ν(Zn-H) bands obscured by overlapping ligand
absorptions.
Ar′Cd(µ-H)2CdAr′ (9). Compound 9 was prepared in a way similar
to that described for 7 from the iodide derivative 2 (1.00 g, 1.57 mmol)
and NaH (0.045 g, 1.89 mmol). The product was obtained as colorless,
X-ray-quality crystals. Yield: 0.71 g, 88%; compound 9 is temperature
sensitive and at room temperature it partially decomposes into 5 in a
few hours. 1H NMR (300 MHz, C6D6, 25 °C): δ 1.11 (d, 12H, o-CH-
1
of 5 to a gel-like gray solid was seen at 182 °C. H NMR (300 MHz,
3
C6D6, 25 °C): δ 1.05 (d, 12H, CH(CH3)2, JHH ) 7.2 Hz), 1.12 (d,
3
3
12H, CH(CH3)2, JHH ) 6.6 Hz), 2.95 (sept, 4H, CH(CH3)2, JHH
)
6.9 Hz), 7.10-7.27 (m, 9H, m-C6H3, p-C6H3, m-Dipp, and p-Dipp).
13C{1H} NMR (C6D6, 100.6 MHz, 25 °C): δ 24.3 (CH(CH3)2), 24.9
(CH(CH3)2), 30.4 (CH(CH3)2), 122.9 (m-Dipp), 125.8 (p-C6H3), 127.1
(p-Dipp), 128.6 (m-C6H3), 144.0 (i-Dipp), 147.0 (o-C6H3), 147.2 (o-
Dipp), 176.5 (i-C6H3). 113Cd{1H} NMR (C6D6, 133.1 MHz, 25 °C): δ
540.3 (1JCdCd ) 8650 Hz).
Hg2Ar′2 (6). With rapid stirring, a solution of the iodide derivative
3 (2.00 g, 2.76 mmol) in 50 mL of diethyl ether was added dropwise
to freshly prepared KC8 (0.37 g, 2.76 mmol) with cooling in an ice
bath. The resulting dark suspension was stirred for 36 h to ensure
complete reduction. The solvent was removed, and the residue was
3
3
(CH3)2, JHH ) 6.6 Hz), 1.20 (d, 12H, o-CH(CH3)2, JHH ) 6.9 Hz),
3
3.07 (sept, 4H, CH(CH3)2, JHH ) 7.2 Hz), 6.84 (s, 1H, CdH), 7.14-
7.28 (m, 9H, m-C6H3, p-C6H3, m-Dipp, and p-Dipp). 13C{1H} NMR
(C6D6, 100.6 MHz, 25 °C): δ 24.3 (CH(CH3)2), 24.8 (CH(CH3)2), 30.5
(CH(CH3)2), 123.5 (m-Dipp), 126.2 (p-C6H3), 127.9 (p-Dipp), 128.6
(m-C6H3), 144.3 (i-Dipp), 147.0 (o-C6H3), 148.3 (o-Dipp), 166.4 (i-
C6H3). 113Cd{1H} NMR (C7D8, 133.1 MHz, -40 °C): δ 410.7 (broad
peak). IR (Nujol): ν(Cd-H) bands obscured by overlapping ligand
absorptions.
9
J. AM. CHEM. SOC. VOL. 129, NO. 35, 2007 10849

A R T I C L E S
Zhu et al.
Ar′HgH (10). Compound 10 was prepared in a way similar to that
described for 7 starting from the iodide derivative 3 (1.00 g, 1.57 mmol)
and KH (0.075 g, 1.88 mmol). A colorless solid mixture of 10 and
Ar′H was obtained after workup. Yield: 0.25 g, 31% (based on 3, yield
(hereafter described as BP86/TZ2P/ZORA). In the EDA analysis, the
instantaneous interaction energy (∆Eint) between the two fragments of
the molecule is decomposed into three contributions: ∆EP, ∆Eel, and
∆Eo, where ∆EP is the quantum mechanical Pauli repulsive exchange
interaction between electrons possessing the same spin, ∆Eel is the
quasi-classical attractive electrostatic interaction energy between the
frozen charge distributions of the fragments, and ∆Eo is the stabilization
energy corresponding to the relaxation of the Kohn-Sham orbitals to
their final form. The bond dissociation energy (BDE) between the two
fragments is the sum of the above instantaneous interaction energy
(∆Eint) and a preparation energy term (∆Eprep), which correspond to
the energy necessary to promote the two fragments from their
equilibrium geometry and electronic ground state to the geometry and
electronic state corresponding to the bonded compound. The bonding
energy (BE) from the B3LYP/ECP/6-31g* calculations was estimated
as the difference between the energy corresponding to the optimized
geometry of the final compound and twice the energy of the optimized
half of the molecule in the doublet spin state. Further details of the
quantum mechanical calculations are included in the Supporting
Information.
1
1
determined by H NMR). H NMR (600 MHz, C6D6, 25 °C): δ 1.10
3
3
(d, 12H, CH(CH3)2, JHH ) 6.6 Hz), 1.20 (d, 12H, CH(CH3)2, JHH
)
3
6.6 Hz), 3.01 (sept, 4H, CH(CH3)2, JHH ) 7.2 Hz), 7.15-7.32 (m,
1
9H, m-C6H3, p-C6H3, m-Dipp, and p-Dipp), 14.59 (s, 1H, HgH, JHgH
) 2900 Hz). 13C{1H} NMR (C6D6, 100.6 MHz, 25 °C): δ 24.1 (CH-
(CH3)2), 24.8 (CH(CH3)2), 30.6 (CH(CH3)2), 123.2 (m-Dipp), 128.7 (m-
C6H3), 141.8 (i-Dipp), 147.2 (o-C6H3), 147.5 (o-Dipp), 179.7 (i-C6H3);
p-C6H3 and p-Dipp resonances are likely obscured by the C6H6 signal.
1
199Hg{1H} NMR (C6D6, 107.4 MHz, 25 °C): δ -659.0 (d, JHgH
)
2910 Hz).
Ar′MgBr‚2THF (11). Compound 11 was prepared by a modification
of the procedure described for Ar′I.23 A solution of 560 mmol of
DippMgBr (Dipp ) C6H3-2,6-Pri2) in THF (750 mL) was added
dropwise to a stirred solution of 1,3-dichlorobenzene (36.74 g, 250
mmol) in THF (1300 mL) with previously added n-BuLi (100 mL of
2.5 M solution in hexanes) in an dry ice/acetone bath. Stirring was
continued overnight with slow warming to ambient temperature, after
which time the mixture was heated to reflux for 2 h. The solvent was
then removed in Vacuo, and the residue was washed with hexanes (300
mL) and extracted with warm toluene (500 mL). The resultant slurry
was allowed to settle, and the mother liquor was separated from the
precipitate (Mg2 and Li) salts by filtration. The volume was concentrated
to ca. 400 mL, and storage for 2 d in a refrigerator (8 °C) afforded
colorless crystals of 11, which were further washed with a small amount
of cold hexane (50 mL). Second and third crops of 11 were obtained
by continued concentration and cooling of the mother liquor. Yield:
71.3 g, 44% (combined yield); mp 175 °C. 1H NMR (300 MHz, C6D6,
To compare the metal-metal bonding in 4-6, we performed several
theoretical studies on both molecules containing bulky Ar′ ligands and
the model compounds M2Ph2. These include FA, EDA, and the
estimation of the BE using the fragment-oriented approach.27 We also
compared these theoretical results with results of previously reported
calculations on species exhibiting similar M-M bonding.11,12
X-ray Crystallography. Crystals of 1-9 were removed from a
Schlenk tube under a stream of argon and immediately covered with a
thin layer of hydrocarbon oil. A suitable crystal was selected, attached
to a glass fiber, and quickly placed in a N2 cold stream on the
diffractometer.28 The data for 1-6, 8, and 9 were recorded near 90 K
on a Bruker SMART 1000 instrument (Mo KR radiation, λ ) 0.71073
Å, and a CCD area detector), while data for 7 were collected on a
Bruker APEX instrument (Mo KR radiation and a CCD area detector).
For compounds 1-4 and 6-9, the SHELX version 6.1 program package
was used for the structure solutions and refinements. Absorption
corrections were applied using the SADABS program.29 Crystals of 5
were determined to be twinned, and an alternative procedure (see
Supporting Information) was used to “de-twin” the data and afford a
solution. The structure of 5 is also disordered over two positions that
are rotated 82.7° with respect to each other, such that the cadmiums
appear as a “composite” atom. The resultant cadmium libration appears
to bisect these two planes, and the “true” Cd-Cd distance is probably
longer than the refined distance. The crystal structures were solved by
direct methods and refined by full-matrix least-squares procedures. All
non-H atoms were refined anisotropically. All carbon-bound H atoms
were included in the refinement at calculated positions using a riding
model included in the SHELXTL program. A summary of the data
collection parameters for 1-3, 6, and 9 is provided in Table 1. The
corresponding crystallographic details for 4, 5,30 7, and 8 can be found
in refs 13 and 14.
3
25 °C): δ 1.16 (d, 12H, CH(CH3)2, JHH ) 6.6 Hz), 1.17 (s br, 8H,
3
(CH2CH2)2O), 1.39 (d, 12H, CH(CH3)2, JHH ) 6.6 Hz), 3.24 (s br,
3
8H, (CH2CH2)2O), 3.42 (sept, 4H, CH(CH3)2, JHH ) 6.6 Hz), 7.15-
7.33 (m, 9H, m-C6H3, p-C6H3, m-Dipp, and p-Dipp). 13C{1H} NMR
(C6D6, 100.6 MHz, 25 °C): δ 23.8 (CH(CH3)2), 25.1 (CH(CH3)2), 26.1
((CH2CH2)2O) 30.4 (CH(CH3)2), 69.6 ((CH2CH2)2O), 122.9 (m-Dipp),
124.4 (m-C6H3), 126.9 (p-Dipp), 127.1 (m-C6H3), 147.7 (o-C6H3), 150.7
(o-Dipp), 166.9 (i-C6H3); i-Dipp resonance is likely obscured by the
C6H6 signal.
Computational Methods. Geometry optimizations on the model
species, PhMMPh (M ) Zn, Cd, Hg), were performed using density
functional theory, where the B3LYP functional was combined with a
basis set including the quasi-relativistic pseudopotentials for the metal
centers and a 6-31g* basis set for C and H atoms. The CRENBL-
Large orbital basis set with small core (7s6p6d valence electron space
for Zn and 5s5p4d for Cd and Hg) was used for the metal centers.24
This level of theory is hereafter described as B3LYP/ECP/6-31g*. All
the geometry optimizations as well as the determination of the electronic
structure for Zn2Ar′2 (4), Cd2Ar′2 (5), and Hg2Ar′2 (6) were performed
with the Gaussian 03 package.25
The fragment analysis (FA) and the energy decomposition analysis
(EDA) based on the method of Morokuma and the ETS partitioning
scheme of Ziegler and Rauk, as implemented in ADF 2005.01
software,26 were computed using the pure gradient-corrected BP86
functional combined with the basis set of triple-ú quality with
polarization functions (TZP) using a zero-order relativistic approxima-
tion to the energy Hamiltonian (ZORA) and non-frozen-core option
Results and Discussion
Precursor Halide Derivatives: (Ar′MI)1 or 2 (M ) Zn, 1;
Cd, 2; or Hg, 3). The aryl group 12 halides 1-3 were used as
precursors for the synthesis of the target molecular compounds
Ar′MMAr′ (M ) Zn, Cd, or Hg). These compounds were
obtained by a simple lithium halide elimination route. Attempts
(23) Schiemenz, B.; Power, P. P. Organometallics 1996, 15, 958.
(24) Ross, R. B.; Powers, J. M.; Atashroo, T.; Ermler, W. C.; LaJohn, L. A.;
Christiansen, P. A. J. Chem. Phys. 1990, 93, 6654.
(25) Frisch, M. J.; et al. Gaussian 03, revision B.03; Gaussian, Inc.: Pittsburgh,
PA, 2003.
(27) Rosa, A.; Ehlers, A. W.; Baerends, E. J.; Snijders, J. G.; te Velde, G. J.
Phys. Chem. 1996, 100, 5690.
(28) Hope, H. Prog. Inorg. Chem. 1995, 41, 1.
(26) (a) ADF2005.01; SCM, Theoretical Chemistry, Vrije Universiteit: Am-
sterdam, The Netherlands, 2005 (http://www.scm.com). (b) Morokuma, K.
J. Chem. Phys. 1971, 55, 1236. (c) Kitaura, K.; Morokuma, K. Int. J.
Quantum Chem. 1976, 10, 325. (d) Ziegler, T.; Rauk, A. Theor. Chim.
Acta 1977, 46, 1.
(29) SADABS, An empirical absorption correction program, part of the SAINT-
Plus NT version 5.0 package; Bruker AXS: Madison, WI, 1998.
(30) Initial work on 5 was published in ref 14. However, a slightly improved
refinement was obtained, and hence we decided to resubmit the crystal
data. For more details, see the Supporting Information.
9
10850 J. AM. CHEM. SOC. VOL. 129, NO. 35, 2007

Homologous M−M Bonded Series for Group 12 Metals
A R T I C L E S
Table 1. Selected Crystallographic Data for 1‚C7H8, 2, 3, 6‚2C6D6, and 9
1‚
2C7H8
2
3
6
‚
2C6D6
9
formula
fw
habit
C74H90I2Zn2
1364.04
block
C60H74I2Cd2
1273.79
plate
C30H37IHg
725.09
plate
C72H74D12Hg2
1364.73
plate
C60H76Cd2
1022.01
block
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
monoclinic
P21/c
13.9561(4)
22.5253(6)
24.9183(6)
90
121.3220(10)
90
6691.8(3)
4
orthorhombic
P212121
11.2906(8)
18.9150(12)
26.6677(17)
90
90
90
5695.2(7)
4
orthorhombic
Pnma
16.7028(18)
20.891(2)
7.9572(9)
90
90
90
2776.6(5)
2
orthorhombic
Ccca
17.2416(9)
25.4186(13)
15.1465(8)
90
90
90
6638.1(6)
4
orthorhombic
Pccn
20.5438(8)
15.7065(6)
16.3757(6)
90
90
90
5284.0(3)
4
Z
cryst dimens, mm
0.33 × 0.31 × 0.25
0.16 × 0.19 × 0.12
0.40 × 0.08 × 0.14
0.20 × 0.17 × 0.15
0.27 × 0.20 × 0.17
d
calc, g‚cm-3
1.353
1.486
1.735
1.450
1.285
µ, mm-1
1.679
13168
9819
0.0323
1.865
9723
8272
0.0396
6.670
4176
3644
0.0327
0.0801
4.661
4858
3136
0.0193
0.840
7708
5519
0.0486
0.1438
no. of reflns
no. of obsd reflns
R1, obsd reflns
wR2, all
0.1093
0.0855
0.0504
Table 2. Selected Structural Data for 1‚2C7H8, 2, and 3
arrangement of the central rings of the Ar′ ligands (C1-C6 and
C1A-C6A), while the corresponding aryl rings in Ar′Cd(µ-
I)2CdAr′ (2) are oriented almost perpendicularly to each other,
with a dihedral angle of 82.9° between the C1-C6 and C31-
C36 planes. The Zn(1)-(µ-I)2-Zn(1A) plane in 1 displayed a
torsion angle of 37.6° relative to the central aryl rings, whereas
the Cd(1)-(µ-I)2-Cd(2) plane displayed torsion angles of 30.6°
and 29.6° relative to the C1-C6 and C31-C36 planes,
respectively. Like the known monodentate arylmercury halides,32
the monomeric arylmercury iodide, Ar′HgI, 3 had a nearly linear
C(ipso)-Hg-I unit, with a C(1)-Hg(1)-I(1) angle of 178.14-
(14)°. Geometric computational studies on the simple ionic
model compounds MX2 (M ) Zn, Cd or Hg, X ) halide) show
that relativistic effects reduce the dimerization energy of the
mercury dihalide by about 60-70%.33 A similar effect is
probably responsible for the monomeric structure observed for
3.
parameter
1
‚
2C7H8
2
3
M-I (Å)
2.6240(4)
2.6189(4)
1.961(3)
2.8114(7)
2.8045(7)
2.149(6)
2.118(6)
86.51(2)
86.64(2)
135.82(17)
130.86(18)
93.09(2)
93.66(2)
119.4(4)
120.2(4)
2.5839(4)
C(ipso)-M (Å)
2.082(5)
178.14(14)
118.9(2)
M-I-M (deg)
84.599(14)
133.43(9)
95.401(13)
C(ipso)-M-I (deg)
I-M-I (deg)
C(ortho)-C(ipso)-M (deg)
121.0(2)
120.5(2)
to use commercially available anhydrous ZnCl2 instead of ZnI2
did not result in good yields of a clean Ar′ZnCl product. It was
found to be difficult to separate Ar′ZnCl from the LiCl
byproduct because of the low solubility of Ar′ZnCl in hydro-
carbon solvents. The reaction of the iodides ZnI2, CdI2, or HgI2
with 1 equiv of LiAr′ was then employed to synthesize Ar′Zn-
(µ-I)2ZnAr′ (1), Ar′Cd(µ-I)2CdAr′ (2), and Ar′HgI (3), respec-
tively. Ar′HgI (3) can also be obtained by the treatment of HgI2
with 1 equiv of Ar′MgBr‚2THF (11). All the aryl group 12
halides were isolated as colorless, crystalline solids. Selected
structural data for 1-3 are provided in Table 2. The structures
of 1-3 are presented as thermal ellipsoid plots in Figure 1.
The X-ray structure analysis showed that the arylzinc iodide
1 and arylcadmium iodide 2 formed iodide-bridged dimers in
the solid state [Ar′M(µ-I)2MAr′], whereas the arylmercury
iodide, Ar′HgI (3), had a monomeric structure with a terminal
iodine atom. A similar reaction between Li[HC(CMeNAr)2] (M
) Zn or Cd, Ar ) 2,6-Pri2C6H3) and MI2 did not result in
LiI elimination; instead, the lithium iodide complexes
[{HC(CMeNAr)2}M(µ-I)2Li‚(OEt2)2] were obtained.31 In con-
trast, the structures of 1 and 2 showed that LiI was completely
eliminated from the isolated product. Besides changes associated
with the larger size of Cd vs Zn, the main difference between
the structures involves the relative orientation of the Ar′ ligands.
The centrosymmetric Ar′Zn(µ-I)2ZnAr′ (1) has a coplanar
The observed M-I distances in the arylmetal iodides varied
from 2.6240(4) and 2.6189(4) Å in 1 to 2.8114(7) and 2.8045-
(7) Å in 2, while a shorter M-I distance of 2.5839(4) Å was
found in 3. The large M-I bond length difference between 1/2
and 3 can be explained on the basis of the bridging and terminal
iodine environments in each compound. Compound 2 displayed
a signal at 210.9 ppm in the 113Cd NMR spectrum, which lies
between the values observed for the bis(aryl)cadmium species
Cd(C6H5)2 (328.80 ppm, 1.0 M solution in p-dioxane) and for
CdI2 (55.13 ppm, 1.0 M solution in D2O).34 The mercury
derivative, 3, displayed one signal at -1293.6 ppm in the 199Hg
NMR spectrum. This value is close to the -1142.6 ppm reported
for Hg(CH3)I (1.0 M solution in DMSO-d6).35
Metal-Metal Bonded Compounds: Ar′MMAr′ (M ) Zn,
4; Cd, 5; or Hg, 6). The syntheses of the dinuclear Zn2Ar′2
(4)13 and Cd2Ar′2 (5)14 have been previously communicated by
(32) (a) Pakhomov, V. I. Kristallografiya 1963, 8, 789. (b) Pakhomov, V. I.
Zh. Strukt. Khim. 1963, 4, 594. (c) Tschinkl, M.; Bachman, R. E.; Gabba¨ı,
F. P. J. Organomet. Chem. 1999, 582, 40.
(33) (a) Kaupp, M.; von Schnering, H.-G. Inorg. Chem. 1994, 33, 2555. (b)
Kaupp, M.; von Schnering, H.-G. Inorg. Chem. 1994, 33, 4718. (c)
Hargittai, M. Chem. ReV. 2000, 100, 2233.
(34) Cardin, A. D.; Ellis, P. D.; Odom, J. D.; Howard, J. W., Jr. J. Am. Chem.
Soc. 1975, 97, 1672.
(31) Prust, J.; Most, K.; Mu¨ller, I.; Stasch, A.; Roesky, H. W.; Uso´n, I. Eur. J.
Inorg. Chem. 2001, 1613.
(35) Sens, M. A.; Wilson, N. K.; Ellis, P. D.; Odon, J. D. J. Magn. Reson.
1975, 19, 323.
9
J. AM. CHEM. SOC. VOL. 129, NO. 35, 2007 10851

A R T I C L E S
Zhu et al.
Scheme 1. Synthetic Routes to the M-M Bonded Dimers M2Ar′2
(M ) Zn, Cd, and Hg)
commercially available Hg2I2 salt with 2 equiv of MgAr′Br‚
2THF (11) for the synthesis of Hg2Ar′2 (6). However, dispro-
portionation occurred to yield the arylmercury(II) halide 3 and
metallic mercury, indicating that the ionic [Hg2]2+ core unit does
not remain intact during its reaction with 11. Fortunately, the
dinuclear Hg2Ar′2 (6) could be prepared by the simple reduction
of 3 with KC8. The synthetic routes to the metal-metal bonded
Ar′MMAr′ (M ) Zn, Cd, or Hg) are summarized in Scheme 1,
and the structures of 4-6 can be found in Figure 2. Selected
structural data for 4-6 are provided in Table 3.
As illustrated in Figure 2, X-ray crystal crystallography
confirmed the presence of a homonuclear Hg-Hg bond in 6.
The observed Hg-Hg distance of 2.5738(3) Å in 6 was ca.
0.30 Å shorter than the sum of Pauling’s single-bond metallic
radii for mercury, 2.88 Å,36 indicating that a considerable Hg-
Hg bonding interaction was present. The Hg-Hg distance in 6
is ca. 0.08 Å shorter than the value of 2.6569(8) Å in silylated
derivative Hg2[Si(SiMe2SiMe3)3]2 (12),8 but it is within the bond
length range of ca. 2.49-2.59 Å in the other related compounds,
37a
such as the ionic species [Hg2(Me6C6)2][AlCl4]2
and the
tetranuclear (np3)Co-Hg-Hg-Co(np3) complex (np3 ) N(CH2-
CH2PPh2)3).37b The significant bond length difference between
compounds 6 and 12 suggests that the ligands exert a large
influence on the nature of Hg-Hg bonding interaction. The
shorter Hg-Hg bond length in 6 relative to 12 and the cadmium
species 5 is consistent with a stronger M-M bonding interaction
and a larger HOMO-LUMO energy gap. It is also notable that
the Hg-C distance (2.098(3) Å) is shorter than that of Cd-C
(2.138(3) Å), consistent with the smaller size of Hg. The pale
yellow color of 12 in comparison to the colorless 6 is consistent
with the presence of a lower HOMO-LUMO gap and a weaker
Hg-Hg bond in 12. The greater electron-withdrawing effect
of the terphenyl ligand in comparison to that of the silyl group
is also seen in the 199Hg NMR spectra. Compound 12 displayed
a signal at -104.5 ppm, while 6 had a more deshielded signal
at 150.5 ppm. The crystal structures of 4-6 showed that the
compounds have very similar geometries, wherein the two Ar′
ligands are arranged in a nearly orthogonal orientation to each
other, providing effective steric protection of the metal-metal
core. An almost linear arrangement is observed for the [C(ipso)-
M-M-C(ipso)] unit, with little or no apparent bending
tendency. The angles of Zn(2)-Zn(1)-C(1), Cd(1A)-Cd(1)-
C(1), and Hg(1A)-Hg(1)-C(1) are 177.5(3) avg., 177.5(3)°,
Figure 1. Thermal ellipsoid (30%) plots of 1-3. Hydrogen atoms are not
shown for clarity.
us. Unlike 4, which was synthesized by the simple reduction
of 1 with sodium, compound 5 was prepared by the addition of
2 equiv of NaH to 2 (see below). A straightforward lithium
halide elimination route was investigated for the reaction of the
(36) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University
Press: Ithaca, NY, 1960; Chapter 7.
(37) (a) Frank, W.; Dincher, B. Z. Naturforsch. B 1987, 42, 828. (b) Ghilardi,
C. A.; Midollini, S.; Moneti, S. J. Chem. Soc., Chem. Commun. 1981, 865.
9
10852 J. AM. CHEM. SOC. VOL. 129, NO. 35, 2007

Homologous M−M Bonded Series for Group 12 Metals
A R T I C L E S
Table 3. Selected Structural Data for 4, 5, and 6‚2C6D6
parameter
4
5
6‚2C6D6
M-I (Å)
C(ipso)-M (Å)
2.3591(9)
1.965(6)
1.974(5)
2.6257(5)
2.138(3)
2.5738(3)
2.098(3)
C(ipso)-MsM (deg)
C(ortho)-C(ipso)-M (deg)
177.28(18)
177.57(18)
120.5(4)
177.5(3)
180
116.5(6)
121.8(6)
120.2(3)
119.1(4)
Fock-Slater one-electron equation, which showed ca. 0.1 Å
contraction in the M-M distance between [Hg2]2+ and [Cd2]2+
(see below for theoretical studies on model species for 4-6).15b
Metal Hydride Derivatives: (Ar′MH)1 or 2 (M ) Zn, Cd
or Hg) and Ar′Zn(µ-H)(µ-Na)ZnAr′. Well-characterized or-
gano group 12 hydrides are relatively rare, and there is a high
tendency of these species to decompose.18-20 Examples of
structurally characterized zinc hydride compounds include
[Me2N(CH2)2N(Me)ZnH]2,18a [{η3-HB(3-RC3N2H2)3}ZnH] (R
) But),18b [(Me3PN)ZnH]4‚4THF,18d [{HC(CMeNAr)2}Zn(µ-
H)]2 (Ar ) 2,6-Pri2C6H3),18e and Ar′Zn(µ-H)2ZnAr′.14 The latter
two species have similar bridged structures, whereas the first
three complexes have terminal hydrogen atoms. At present, only
one example of an organocadmium hydride, Tpt-BuCdH (13)
(Tpt-Bu ) hydrotris(3-tert-butylpyrazolyl)borate),9 has been
synthesized. It was characterized by multinuclear NMR spec-
troscopy, but no crystal structure was obtained. There is no
example of a stable, structurally characterized organomercury
hydride. Studies on highly reactive organomercury hydrides
suggest that these species can rapidly decompose into transient
organomercury radicals.38 The synthesis of metal hydride
derivatives is also of importance to provide supporting evidence
for metal-metal bonds. It is often experimentally difficult to
discount the presence of a bridging hydride ligand on the basis
of X-ray crystallography or spectroscopy. A further reason for
our interest in metal hydrides arises from the fact that the
mechanism of the Ar′Cd(µ-I)2CdAr′ (2) reduction to Cd2Ar′2
(4) by NaH was not clear.14 It was desirable to isolate a putative
Ar′CdH intermediate in the reduction of 2 in order to gain insight
into the reduction mechanism.
Arylmetal hydrides 7, 9, and 10 were each prepared by the
treatment of the corresponding arylmetal halide with alkali metal
hydride in THF (Scheme 2). NaH is a suitable hydride source
for the synthesis of 713 and 9; however, the use of a more ionic
hydride source, KH, was required to obtain 10. As mentioned
in a previous communication,13 7 can be further reduced by
NaH to afford the unusual sodium hydride bridged species,
Ar′Zn(µ-H)(µ-Na)ZnAr′ (8), which featured a new type of Zn-
Zn bond in which the core unit,
Figure 2. Thermal ellipsoid (30%) plots of 4-6. Hydrogen atoms are not
can be viewed as a σ-antiaromatic ring.13 It was found that the
arylcadmium hydride Ar′Cd(µ-H)2CdAr′ (9) is quite temperature
sensitive, and it partially decomposes in a few hours to the Cd-
Cd bonded 5 upon standing at room temperature. This observa-
tion showed that the mechanism of Ar′Cd(µ-I)2CdAr′ reduction
shown for clarity.
and 180.0°, respectively. The M-M bonds distances varied from
2.3591(9) Å for Zn-Zn in 4 and 2.6257(5) Å for Cd-Cd in 5,
to 2.5738(3) Å for Hg-Hg in 6. The most striking feature of
the data is the shortening of ca. 0.05 Å in the metal-metal
bond between the Cd-Cd bonded 5 and the Hg-Hg bonded 6.
This observation is consistent with the calculations on model
dications [Cd2]2+ and [Hg2]2+ using the relativistic Hartree-
(38) (a) Quirk, R. P.; Lea, R. E. Tetrahedron Lett. 1974, 15, 1925. (b) Hill, C.
L.; Whitesidcs, G. M. J. Am. Chem. Soc. 1974, 96, 870. (c) Quirk, R. P.;
Lea, R. E. J. Am. Chem. Soc. 1976, 98, 5973. (d) Russell, G. A.; Guo, D.
Tetrahedron Lett. 1984, 25, 5239.
9
J. AM. CHEM. SOC. VOL. 129, NO. 35, 2007 10853

A R T I C L E S
Zhu et al.
Scheme 2. Synthetic Routes to the Aryl Group 12 Hydride
(Ar′MH)1 or 2 (M ) Zn, Cd, or Hg) and Ar′Zn(µ-H)(µ-Na)ZnAr′
by NaH involves the initial formation of 9, followed by the
decomposition of 9 to form the more stable molecular compound
5, with the concomitant elimination of hydrogen. Compounds
7-9 were obtained as colorless crystals, and their structures
can be found in Figure 3. However, during the preparation of
the arylmercury hydride 10, we observed significant amounts
of elemental mercury as well as the formation of Ar′H
byproduct. We were unable to separate 10 from Ar′H and could
not obtain a satisfactory structure for 10 in the solid state.
Nevertheless, the formation of 10 as a monomer in solution
1
was shown by H NMR and 199Hg NMR spectroscopy.
The arylcadmium hydride 9 has an unsymmetrically bridged
structure with different Cd-H distances of 1.78(6) and 2.27(6)
Å, suggesting that 9 is a loosely associated dimer. This is in
contrast to the arylzinc hydride 7,13 where the hydrides
symmetrically bridge the two zinc atoms (Zn-H distances )
1.67(2)-1.79(3) Å; H-Zn-H angles ) 87.9(11)-92.0(11)°).
In 9, the terphenyl ligands are oriented in a nearly coplanar
arrangement, whereas in 7 and the Cd-Cd bonded compound
5, they are approximately orthogonal. The Cd‚‚‚Cd separation
of 2.9196(5) Å in 9 is ca. 0.3 Å longer than that in the Cd-Cd
bonded species 5, in contrast to the relatively close Zn‚‚‚Zn
distances in 4 and 7, and this is also suggestive of a weak Cd-
H-Cd bridging interaction.
Both arylmetal hydrides 9 and 10 were analyzed by multi-
nuclear NMR studies. For 9, a signal corresponding to the
bridging hydride in 9 was located at 6.84 ppm in the 1H NMR
spectrum in the correct intensity ratio to the aryl ligand
resonances. This chemical shift is similar to that of 6.30 ppm
observed in the only previously reported molecular cadmium
hydride, 13.9 Compound 9 also displayed a 113Cd NMR chemical
shift of 410.7 ppm, similar to that of 15 (348.2 ppm). However,
unlike 13, which has an observable 113Cd-1H one-bond coupling
of 2527 Hz, 9 displayed a broad signal in solution which could
not be further resolved at low temperatures (-40 °C). The
1
mercury hydride 10 displayed a signal at 14.59 ppm in the H
NMR spectrum (Figure 4), which lies in the chemical shift range
reported for alkylmercury hydrides (11.9-17.2 ppm).20 Two
symmetrically disposed satellite resonances were observed
1
because of 199Hg-1H coupling (199Hg, I ) /2, 16.68%), and
the 199Hg-1H coupling constant was found to be 2900 Hz,
which is similar to the value of 3218 Hz observed in hydri-
domercury (Z)-2-p-nitrophenylacrylate.20d In addition, the 1JHg-H
coupling constant observed in the 199Hg NMR spectrum (2910
Hz) is essentially the same as that found in the 1H NMR
spectrum. The observation of a doublet in the 1H-coupled 199Hg
Figure 3. Thermal ellipsoid (30%) plots of 7-9. Carbon-bound hydrogen
atoms are not shown for clarity.
9
10854 J. AM. CHEM. SOC. VOL. 129, NO. 35, 2007

Homologous M−M Bonded Series for Group 12 Metals
A R T I C L E S
Figure 4. 1H and 199Hg NMR spectra of 10.
p character of the HOMO gradually decreases as one descends
the group: 18.8% for Zn, 16.5% for Cd, and 15.8% for Hg. In
addition, the heaviest congener, Hg, also has a non-negligible
NMR spectrum is consistent with a monomeric structure for
10 in solution. It is also notable that the hydride signals
progressively move to lower field, from 4.84 ppm in Ar′Zn(µ-
H)2ZnAr′ (7) to 6.84 ppm in Ar′Cd(µ-H)2CdAr′ (8) to 14.59
ppm in Ar′HgH (10), as the group is descended.
2
(5%) contribution to the HOMO from an orbital of dz parentage.
Using the fragment-oriented approach, we have computed the
BEs for both the M2Ar′2 species and the M2Ph2 models at the
B3LYP/ECP/6-31g* level. The calculated energies are in good
agreement with previously reported theoretical data for parent
congeners of the M2Ar′2 species, HMMH,7c,10 and the two stable
Zn-Zn bonded species, 14 (62.1 kcal/mol)11 and 15 (65.2 kcal/
mol) (Table 5).12 In addition to the fragment-oriented approach,
we have also performed EDA at the BP86/TZ2P/ZORA level
for M2Ph2.27 The BDE of M2Ph2 is similar to that of the M-M
bonded compounds M2Ar′2 and is in agreement with the
calculated BDEs of M2Me2, M2Cp2 (Cp ) η5-C5H5), and M2-
Cp*2 at the same level of theory.39 The energy of the Cd-Cd
bond in Cd2Ph2 was found to be 52.7 kcal/mol, ca. 8 kcal/mol
lower than the value (60.9 kcal/mol) found in the parent Zn-
Zn bonded species. The Hg-Hg bond strength in Hg2Ph2 was
determined to be 54.4 kcal/mol, which lies between the energies
of the Zn-Zn and the Cd-Cd bonded species. The same trend
was observed for the energies computed in M2Ar′2 (56.3, 48.8,
and 51.0 kcal/mol for M ) Zn, Cd, and Hg, respectively). One
might expect the Hg-Hg bonding energy to be the lowest of
the triad because of the increase in the total Pauli repulsion,
which is induced by the higher quantum number of the Hg
electrons. However, the EDA of the Hg2Ph2 model clearly shows
that the presence of the relativistic effects stabilizes the
electrostatic and orbital terms, ∆Eel and ∆Eo, which overcomes
the increase in the total Pauli repulsion term, ∆Ep, resulting in
Theoretical Calculations on Group 12 Metal-Metal
Bonded Compounds. As previously reported in our original
communications on Zn2Ar′2 (4) and Cd2Ar′2 (5), the HOMO is
the primary metal-metal bonding orbital.13,14 This is also true
for Hg2Ar′2 (10) where the M-M bonding has significant
contributions from metal s and pz hybrids (Figure 5). The
s-orbital character of the HOMO is, in all cases, less than 50%,
with the remaining contributions from pz and ligand-based
orbitals. The two lowest unoccupied degenerate orbitals (LUMO
and LUMO+1) are of π-symmetry, are localized on the metal-
metal axis, and are derived from a combination of the empty
metal px and py orbitals.
The extent of the p character in the metal-metal bonds of
the M2Ph2 model compounds as well as in the M2Ar′2 species
was investigated by fragment analysis of the various orbital
contributions to the total M-M bonding. These contributions
were calculated at the BP86/TZ2P/ZORA level of theory (Table
4), and this analysis clearly shows that, for all three M-M
bonded M2Ph2 species, there is an important involvement of
the metal pz orbitals in the HOMO. This is contrasted with the
bonding situation in Zn2Cp*2 (14), where the Zn-Zn bond is
formed almost exclusively from 4s orbitals (99% s character at
HOMO-4).11 The Zn-Zn bond in Zn2Ph2 also has a more
pronounced p character of ca. 30% (only metal centers were
considered), as compared to that in the Zn2[HC(CMeNAr)2]2
species (15) (95% s, 4% p, 1% d).12 For the M2Ph2 series, the
(39) (a) Pandey, K. K. J. Organomet. Chem. 2007, 692, 1058. (b) Kan, Y. J.
Mol. Struct. (THEOCHEM) 2007, 805, 127.
9
J. AM. CHEM. SOC. VOL. 129, NO. 35, 2007 10855

A R T I C L E S
Zhu et al.
Table 5. Calculated M-M Bonding Energy (BE, in kcal/mol) for
the M2Ar′2 and the M2H2 Species at the B3LYP/ECP/6-31g* Level
of Theory Compared to Previously Reported M-M Bond Energies
E(M
−M)
BSSE
BEb
BDEc
(kcal/mol)
correction
(kcal/mol)
(kcal/mol)
HZnZnH
-60.0 (-59.1)a
-56.6d
PhZnZnPh
Ar′ZnZnAr′
HCdCdH
PhCdCdPh
Ar′CdCdAr′
HHgHgH
-60.9
-52.7
-54.4
-56.3
0.7
0.6
0.9
-55.6
-48.2
-50.1
-58.6 (-55.4)a
-48.7d
-48.8
-64.1 (-61.4)a
-49.9d
PhHgHgPh
Ar′HgHgAr′
-51.0
a Data from ref 7c. b Bonding energy corrected for BSSE. c Bond
dissociation energy (BDE) of the model PhMMPh species obtained at
species at the BP86/TZ2P/ZORA level of theory (details in Supporting
Information). d Bonding energy (BE) obtained from fragment analysis at
the B3LYP/ECP/6-31g* level of theory.
Figure 5. Representation of the frontier molecular orbitals of 6, obtained
from DFT calculations (for more details, see the Supporting Information).
Table 4. Contribution of Various Metal and Ligand Orbitals to the
HOMO of the Model PhMMPh Species at the BP86/TZP/ZORA
Level of Theory
% character
metal s
metal p
metal d
Cipso
s
Cipso
p
othera
Zn-Zn
Cd-Cd
Hg-Hg
45.4
47.2
36.7
18.8
16.5
15.8
3.1
3.6
4.7
14.1
18.2
22.6
18.6
14.5
14.9
5.1
Figure 6. Optimized structures of phenyl group 12 hydrides.
a Contributions from the ligand-derived orbitals in the molecule to the
HOMO orbital.
calculated metal-metal bond lengths are almost identical to
previously computed data for the M2H2 model species,7c,10 yet
they are slightly overestimated (max. 0.1 Å) when compared
to the experimental parameters for the M2Ar′2 species obtained
by the X-ray crystallography. Geometry optimizations were also
performed on phenyl-substituted group 12 iodides, including
three PhMI monomers and three PhM(µ-I)2MPh dimers. All the
optimized structures of the monomeric iodides exhibit linear
C(ipso)-M-I bonds, and a planar phenyl-to-phenyl geometry
was found for all the optimized structures of the dimers. In
addition, unlike the crystal structures of 1 and 2, where the
a higher Hg-Hg BE than that of a Cd-Cd bond. A similar BE
trend within M2X2 salts (X ) halogen) was also reported by
Kaupp et al.7c and Schwerdtfeger et al.7b
The geometries of the M2Ph2 model species were optimized
in both perpendicular (D2d) and planar (D2h) phenyl-to-phenyl
geometries. The D2d perpendicular species correspond to the
lowest energy form (for calculation details and structural
parameters, see Supporting Information); however, the D2h
rotational conformers lie only 0.12 (Zn), 0.21 (Cd), and 0.32
(Hg) kcal/mol higher in energy than the D2d minima. The
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10856 J. AM. CHEM. SOC. VOL. 129, NO. 35, 2007

Homologous M−M Bonded Series for Group 12 Metals
A R T I C L E S
central M-(µ-I)2-M plane is tilted from two central aryl rings,
the iodine atoms in the optimized PhM(µ-I)2MPh dimers are
coplanar with respect to the phenyl rings. Calculations also
revealed that the mercury species has the lowest dimerization
energy of 4.0 kcal/mol, while for zinc and cadmium the
dimerization energies are 5.3 and 9.7 kcal/mol, respectively, at
the B3LYP/ECP/6-31g* level of theory. The theoretically
modeled phenyl group 12 hydrides have optimized planar
phenyl-to-phenyl geometries similar to those of their iodide
analogues, with the hydride moieties arranged in a coplanar
fashion with respect to the phenyl rings (Figure 6). However,
unlike the symmetrically disposed iodine atoms in the phenyl
group 12 iodides models, the optimized geometries of the
corresponding cadmium and mercury hydrides have an asym-
metric arrangement of the hydrogen atoms. An interesting trend
in the “side displacement” of the Ph-M fragments is therefore
observed in the optimized structures on going from Zn to Hg
model species; the same trend was also found in corresponding
arylmetal hydride species Ar′Zn(µ-H)2ZnAr′ (7) and Ar′Cd(µ-
H)2CdAr′ (9).
kcal/mol) lies between those of the Zn-Zn and the Cd-Cd
bonded species at the B3LYP/ECP/6-31g* level of theory. The
related arylmetal halide, Ar′MI, and the very reactive hydride,
Ar′MH, precursors were also synthesized and fully characterized
by X-ray crystallography and multinuclear NMR spectroscopy.
The synthesis of Ar′CdCdAr′ via reduction of (Ar′CdI)2 with
NaH proceeds through the weakly associated (Ar′CdH)2 dimer,
which decomposes to Ar′CdCdAr′. The metal-metal separations
observed in the dimer zinc and cadmium hydrides are signifi-
cantly larger than those in the metal-metal bonded dimers.
Acknowledgment. We thank the National Science Foundation
and the U.S. Department of Energy Hydrogen Storage Center
of Excellence for financial support. R.C.F. thanks the Max Kade
foundation for a postdoctoral fellowship. E.R. thanks NSERC
of Canada for a postdoctoral fellowship. R.W. thanks the
Alexander von Humboldt Foundation for a Feodor Lynen
Research Fellowship.
Note Added in Proof. The synthesis and characterization of
a further Zn-Zn bonded species [{Na(THF)2}2{MeCN-
(C6H3-2,6-Pri2)}2Zn2] (Zn-Zn ) 2.3994(6) Å) has been pub-
lished recently: Yang, X-J.; Yu, J.; Liu, Y.; Xie, H. F.; Schaefer,
H. F.; Liang, Y.; Wu, B. Chem. Commun. 2007, 2363.
Conclusion
The first synthesis and structural characterization of the
homologous M-M bonded series Ar′MMAr′ (M ) Zn, Cd, or
Hg; Ar′ ) C6H3-2,6-(C6H3-2,6-Pri2)2) have been described.
These compounds possess almost linear C-M-M-C core
structures with two-coordinate metals. The M-M bond distances
varied from 2.3591(9) Å for Zn-Zn to 2.6257(5) Å for Cd-
Cd and 2.5738(3) Å for Hg-Hg. DFT calculations showed that
the M-M bonds have significant p character in the M-M
σ-bonding HOMO, and a systematic loss of p character in the
bonds was calculated as one descends the group. The bond
energies for both the M2Ar′2 species and M2Ph2 models were
calculated using a fragment-oriented approach. It estimates a
Cd-Cd bond strength of 48.2 kcal/mol, which is ca. 7 kcal/
mol lower than the value of 55.6 kcal/mol for the parent Zn-
Zn species, while the calculated Hg-Hg bond enthalpy (50.1
Supporting Information Available: X-ray data for 1, 2, 3,
5, 6, and 9 (CIF); complete ref 25; details of the refinement of
5; selected Kohn-Sham orbital surfaces of 6; optimized
structures of the PhMMPh (M ) Zn, Cd, or Hg) model species
at the B3LYP/ECP/6-31g* level; calculation details of the M-M
bonding energy decomposition for the model PhMMPh species
obtained at the BP86/TZP/ZORA level of theory; structural
parameters obtained from geometry optimizations of the PhM-
MPh, (PhMI)1 or 2 and PhM(µ-H)2MPh model species at the
B3LYP/ECP/6-31g* level. This material is available free of
charge via the Internet at http://pubs.acs.org.
JA072682X
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J. AM. CHEM. SOC. VOL. 129, NO. 35, 2007 10857