Steric enhancement of group 12 metal hydride stability and the reaction of an arylzinc hydride with tetramethylpiperidinyl oxide (TEMPO)

Article abstract of DOI:10.1021/om900005t

The synthesis and characterization of arylzinc hydrides Ar ^*Zn(μ-H)₂ZnAr^* (Ar ^* = C₆H₃-2,6-(C₆H ₂-2,4,6-Prⁱ₃)₂, 5) and {(4-Me ₃

Full text of DOI:10.1021/om900005t

Organometallics 2009, 28, 2091–2095
2091
Steric Enhancement of Group 12 Metal Hydride Stability and the
Reaction of an Arylzinc Hydride with Tetramethylpiperidinyl Oxide
(TEMPO)
Zhongliang Zhu, James C. Fettinger, Marilyn M. Olmstead, and Philip P. Power*
Department of Chemistry, UniVersity of California DaVis, One Shields AVenue, DaVis, California 95616
ReceiVed January 5, 2009
The synthesis and characterization of arylzinc hydrides Ar*Zn(µ-H)2ZnAr* (Ar* ) C6H3-2,6-(C6H2-
2,4,6-Pri3)2, 5) and {(4-Me3Si-Ar*)Zn(µ-H)2Zn(Ar*-4-SiMe3)} (4-Me3Si-Ar* ) C6H2-2,6-(C6H2-2,4,6-
Pri3)2-4-SiMe3, 7) as well as the monomeric arylcadmium hydride Ar*CdH (9) are described. They were
prepared by the transmetalation of the corresponding aryl metal iodides with NaH. The Ar*CdH monomer
displayed significantly greater thermal stability than its recently reported dimeric congener Ar′Cd(µ-
H)2CdAr′ (Ar′ ) C6H3-2,6-(C6H3-2,6-Pri2)2), which decomposed at room temperature to afford Ar′CdCdAr′.
This result supports the proposal that decomposition of the metal hydrides occurs by an associative
mechanism. The reactions of these compounds with TEMPO (2,2,6,6-tetramethylpiperidinyl oxide) were
also examined, but the only crystalline product obtained was 4-Me3Si-Ar*ZnTEMPO, in which the metal
is bound to the TEMPO ligand in a quasi side-on fashion primarily through the oxygen but with a significant
zinc-nitrogen interaction.
Scheme 1. Synthetic Route to Metal-Metal Bonded Group
12 Elements via the Homolytic Cleavage of M-H Bonds
Followed by the Radical Coupling between Two ArM•
Fragments
Introduction
The synthesis of molecular compounds featuring homonuclear
metal-metal bonds between the group 12 elements (Zn, Cd,
and Hg) has proven to be a considerable synthetic challenge
and has attracted great interest in the bonding between these
metal centers.1-8 Recent work has shown that the synthesis of
related organometal hydrides (RMH)2 (M ) Zn or Cd; R )
organic or related substituent) followed by their rearrangement
with the concomitant elimination of H2 can be an effective route
for the formation of M-M bonded compounds (Scheme 1).1b,3,7
However, the pathway in Scheme 1, where a homolytic cleavage
of M-H bonds occurs to produce radical intermediates, is
normally disfavored by a substantial energy barrier and usually
becomes effective only for the heavier metals, where M-H
bonds are weaker.9 Furthermore, as pointed out by Downs and
co-workers, the homolytic cleavage of M-H bonds probably
occurs via an associative mechanism and depends on the ability
of hydrides to form at least a transient bridge between the metal
atoms.10 Recently, our group reported the synthesis and
characterization of the homologous organo group 12 hydrides
(Ar′MH)1 or 2 (M ) Zn, Cd, or Hg; Ar′ ) C6H3-2,6-(C6H3-2,6-
Pri2)2) via a transmetalation of the aryl metal halides (Ar′MI)1
or 2 and NaH or KH.3 Studies on these metal hydrides showed
that the dimeric zinc hydride Ar′Zn(µ-H)2ZnAr′ (1) can be
further reduced by NaH to afford the unusual sodium hydride-
bridged species, Ar′Zn(µ-H)(µ-Na)ZnAr′ (2),3a which features
a new type of Zn-Zn bond, while Ar′Cd(µ-H)2CdAr′ (3) rapidly
rearranges to metal-metal bonded Ar′CdCdAr′ at room
temperature.3b In both reactions a homolytic cleavage of M-H
bonds was proposed, and the coupling of highly reactive radical
intermediates is, apparently, the key step on the pathway to the
final products. However, a greater understanding of the chemical
reactivity of organo group 12 hydrides is limited by their
scarcity.3,7,11,12 We now report the synthesis and characterization
of further examples of the zinc hydrides Ar*Zn(µ-H)2ZnAr*
(5) and {(4-Me3Si-Ar*)Zn(µ-H)2Zn(Ar*-4-SiMe3)} (7) as well
as a monomeric cadmium hydride Ar*CdH (9) (Scheme 2) by
the use of Ar*-type ligands (Ar* ) C6H3-2,6-(C6H2-2,4,6-Pri3)2).
These Ar* ligands are bulkier than the previously used Ar′,
as a result of which they provide more steric protection of metal
centers and hence greater stability by blocking associative
decomposition. The derivatized Ar* ligand Ar*-4-SiMe3 (Ar*-
4-SiMe3 ) C6H2-2,6(C6H2-2,4,6-Pri3)2-4-SiMe3) was employed
in the case of zinc in order to obtain crystals suitable for
(3) (a) Zhu, Z.; Wright, R. J.; Olmstead, M. M.; Rivard, E.; Brynda,
M.; Power, P. P. Angew. Chem., Int. Ed. 2006, 45, 5807. (b) Zhu, Z.; Fischer,
R. C.; Fettinger, J. C.; Rivard, E.; Brynda, M.; Power, P. P. J. Am. Chem.
Soc. 2006, 128, 15068. (c) Zhu, Z.; Brynda, M.; Wright, R. J.; Fischer,
R. C.; Merrill, A. W.; Rivard, E.; Wolf, R.; Fettinger, J. C.; Olmstead,
M. M.; Power, P. P. J. Am. Soc. Chem. 2007, 129, 10847.
(4) (a) Yang, X.-J.; Yu, J.; Liu, Y.; Xie, Y.; Schaefer, H. F., III; Liang,
Y.; Wu, B. Chem. Commun. 2007, 2363. (b) Yu, J.; Yang, X.-J.; Liu, Y.;
Pu, Z.; Li, Q.-S.; Xie, Y.; Schaefer, H. F.; Wu, B. Organometallics 2008,
27, 5800.
(5) Fedushkin, I. L.; Skatova, A. A.; Ketkov, S. Y.; Eremenko, O. V.;
Piskunov, A. V.; Fukin, G. K. Angew. Chem., Int. Ed. 2007, 46, 4302.
(6) Tsai, Y.-C.; Lu, D.-Y.; Lin, Y.-M.; Hwang, J.-K.; Yu, J.-S. K. Chem.
Commun. 2007, 4125.
* Corresponding author. E-mail: pppower@ucdavis.edu.
(1) (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. (c) Grirrane, A.; Resa, I.; Rodriguez, A.;
Carmona, E.; Alvarez, E.; Gutierrez-Puebla, E.; Monge, A.; Galindo, A.;
del R´ıo, D.; Andersen, R. A. J. Am. Chem. Soc. 2007, 129, 693.
(2) 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.
(7) Reger, D. L.; Mason, S. S.; Rheingold, A. L. J. Am. Chem. Soc.
1993, 115, 10406.
10.1021/om900005t CCC: $40.75
2009 American Chemical Society
Publication on Web 03/19/2009

2092 Organometallics, Vol. 28, No. 7, 2009
Zhu et al.
Scheme 2. Synthetic Routes to 5, 7, 9, and 10
ZnH), 7.10-7.26 (m, 14H, m-C6H3, p-C6H3, and m-Dipp). 13C{1H}
NMR (C6D6, 75.4 MHz, 25 °C): δ 24.4 (p-CH(CH3)2), 24.9 (o-
CH(CH3)2), 25.1 (o-CH(CH3)2), 30.8 (o-CH(CH3)2), 34.7 (p-
CH(CH3)2), 121.1, 126.4, 141.3, 146.7, 148.1, 149.0, and 155.7
(ArC), one ArC resonance is likely obscured by the C6H6 signal.
IR (Nujol): ν Zn(µ-H)2Zn bands were not identified and may be
very weak or obscured by overlapping ligand absorptions.
crystallographic studies. We also report the reaction between 7
and the radical species TEMPO (2,2,6,6-tetramethylpiperidinyl
oxide), which affords 4-Me3Si-Ar*ZnTEMPO (10) (Scheme 2).
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 Grubbs and degassed prior to use.13
The chemicals used in this study were purchased from Aldrich or
Acros and used as received. The salts LiAr* · OEt2,14 Li(Ar*-4-
SiMe3) · OEt2,15 and {Ar*Zn(µ-I)}2 (4)16 were prepared as previ-
ously described. 1H, 13C, 29Si, and 113Cd NMR spectra were recorded
on 300 or 600 MHz spectrometers, and the 113Cd NMR spectra
were referenced externally to 0.01 M Cd(ClO4)2. 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.
{(4-SiMe3-Ar*)Zn(µ-H)2Zn(Ar*-4-SiMe3)} (7). Li(Ar*-4-
SiMe3) · OEt2 (1.46 g, 2.43 mmol) and ZnI2 (0.78 g, 2.43 mmol)
were combined with diethyl ether (50 mL) and stirred for 2 days.
The solvent was removed under reduced pressure, and the residue
was extracted with toluene (50 mL) and separated from the
precipitate (LiI) by filtration. Removal of the solvent gave {4-SiMe3-
Ar*Zn(µ-I)}2 (6) as a white powder. The procedure for the reaction
of 6 (1.53 g, 1.03 mmol) with NaH (0.074 g, 3.08 mmol) was
conducted in a similar way to that described for 5. The crude
compound was extracted with ca. 50 mL of hexane. Storage of the
concentrated hexane solution (ca. 15 mL) for 2 days in a freezer
(ca. -18 °C) afforded colorless, X-ray quality crystals of 7. Yield:
0.72 g, 48% (based on Li(Ar*-4-SiMe3) · OEt2); mp 212-213 °C.
Ar*Zn(µ-H)2ZnAr* (5). {Ar*Zn(µ-I)}2 (2.50 g, 1.87 mmol, 4)
and NaH (0.074 g, 5.61 mmol) were combined with THF (50 mL)
at ambient temperature. After stirring for 2 days, the solvent was
removed, and the residue was extracted with ca. 70 mL of toluene.
The solid was allowed to settle, and the mother liquor was separated
from the precipitate (NaI and excess NaH). The solution was then
concentrated under reduced pressure to afford the product 5 as a
white powder. Yield: 1.32 g, 65% (based on 4). Anal. Calcd for
C72H100Zn2: C, 78.90; H, 9.20. Found: C, 78.16; H, 9.38. 1H NMR
(300 MHz, C6D6, 25 °C): δ 1.08 (d, 24H, o-CH(CH3)2, 3JHH ) 6.9
1
7: H NMR (300 MHz, C6D6, 25 °C): δ 0.29 (s, 18H, Si(CH3)3),
3
1.13 (d, 24H, o-CH(CH3)2, JHH ) 6.6 Hz), 1.22 (d, 24H,
3
3
p-CH(CH3)2, JHH ) 7.2 Hz), 1.39 (d, 24H, o-CH(CH3)2, JHH
)
3
7.2 Hz), 2.94 (sept, 4H, p-CH(CH3)2, JHH ) 6.6 Hz), 3.03 (sept,
3
8H, o-CH(CH3)2, JHH ) 7.2 Hz), 5.08 (s, 2H, ZnH), 7.20 (s, 4H,
m-C6H2), 7.49 (s, 8H, m-Dipp). 13C{1H} NMR (C6D6, 75.4 MHz,
25 °C): δ -0.99 ((CH3)3Si), 24.5 (p-CH(CH3)2), 24.8 (o-CH(CH3)2),
25.1 (o-CH(CH3)2), 30.8 (o-CH(CH3)2), 34.7 (p-CH(CH3)2), 121.0,
131.1, 139.4, 141.6, 146.7, 148.0, 148.3 and 156.0 (ArC). 29Si{1H}
3
NMR (C6D6, 119.2 MHz, 25 °C): δ -4.63. IR (Nujol): ν
Hz), 1.17 (d, 24H, p-CH(CH3)2, JHH ) 7.2 Hz), 1.30 (d, 24H,
(Zn-H)
o-CH(CH3)2, 3JHH ) 7.2 Hz), 2.87 (sept, 4H, p-CH(CH3)2, 3JHH
)
bands were not observed and may be very weak or obscured by
1
6.6 Hz), 2.99 (sept, 8H, o-CH(CH3)2, 3JHH ) 7.2 Hz), 4.93 (s, 2H,
overlapping ligand absorptions. 6: H NMR (300 MHz, C6D6, 25
°C): δ 0.27 (s, 18H, Si(CH3)3), 1.18 (d, 24H, o-CH(CH3)2, 3JHH
)
7.2 Hz), 1.29 (d, 24H, p-CH(CH3)2, 3JHH ) 7.8 Hz), 1.38 (d, 24H,
(8) Bravo-Zhivotovskii, D.; Yuzefovich, M.; Bendikov, M.; Klinkham-
mer, K.; Apeloig, Y. Angew. Chem., Int. Ed. 1999, 38, 1100.
o-CH(CH3)2, 3JHH ) 6.6 Hz), 2.92 (sept, 4H, p-CH(CH3)2, 3JHH
)
7.2 Hz), 3.06 (sept, 8H, o-CH(CH3)2, 3JHH ) 7.2 Hz), 7.24 (s, 4H,
m-C6H2), 7.52 (s, 8H, m-Dipp). 13C{1H} NMR (C6D6, 75.4 MHz,
25 °C): δ -1.04 ((CH3)3Si), 24.4 (o-CH(CH3)2), 24.7 (o-CH(CH3)2),
25.4 (p-CH(CH3)2), 30.7 (o-CH(CH3)2), 35.1 (p-CH(CH3)2), 121.5,
128.9, 131.9, 140.9, 146.8, 147.7, 148.6 and 219.8 (ArC). 29Si{1H}
NMR (C6D6, 119.2 MHz, 25 °C): δ -4.60.
(9) Downs, A. J.; Pulham, C. R. Chem. Soc. ReV. 1994, 23, 175.
(10) Aldridge, S.; Downs, A. J. Chem. ReV. 2001, 101, 3305.
(11) Some organozinc hydride examples other than those in ref 3: (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.
(12) 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.
(13) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
Ar*CdH (9). Compound 9 was prepared in a similar way to
that described for 7 from the reaction of LiAr* · OEt2 (1.54 g, 2.73
mmol) with CdI2 (1.00 g, 2.73 mmol) followed by the transmeta-
lation of {Ar*Cd(µ-I)}2 (8) (1.26 g, 0.88 mmol) with NaH (0.063
g, 2.62 mmol). Storage of a benzene solution for 1 day in a
refrigerator (ca. 7 °C) afforded colorless plates of 9 suitable for
crystallographic studies. Anal. Calcd for C36H50Cd: C, 72.65; H,
8.47. Found: C, 72.01; H 8.48. Yield: 0.42 g, 26% (based on
LiAr* · OEt2); compound 6 is thermally stable at room temperature;
however, at ca. 110 °C it decomposes to a black solid. 9: 1H NMR
(300 MHz, C6D6, 25 °C): δ 1.17 (d, 12H, o-CH(CH3)2, 3JHH ) 7.2
(14) Schiemenz, B.; Power, P. P. Organometallics 1996, 15, 958.
(15) Wolf, R.; Ni, C.; Nguyen, T.; Brynda, M.; Long, G. J.; Sutton,
A. D.; Fischer, R. C.; Fettinger, J. C.; Hellman, M.; Pu, L. H.; Power, P. P.
Inorg. Chem. 2007, 46, 11277.

Group 12 Metal Hydride Stability
Organometallics, Vol. 28, No. 7, 2009 2093
3
Table 1. Selected Crystallographic Data for 7 · 3C6H14, 9, and10
Hz), 1.24 (d, 12H, p-CH(CH3)2, JHH ) 7.2 Hz), 1.24 (d, 12H,
o-CH(CH3)2, 3JHH ) 6.6 Hz), 2.84 (sept, 2H, p-CH(CH3)2, 3JHH
)
7 · 3C6H14
9
10
7.2 Hz), 3.12 (sept, 4H, o-CH(CH3)2, 3JHH ) 6.6 Hz), 6.79 (s, 1H,
CdH), 7.19 (s, 4H, m-Dipp), 7.24-7.28 (6H, m-C6H3 and p-C6H3).
13C {1H} NMR (C6D6, 75.4 MHz, 25 °C): δ 24.3 (o-CH(CH3)2),
24.4 (o-CH(CH3)2), 24.9 (p-CH(CH3)2), 30.6 (o-CH(CH3)2), 34.8
(p-CH(CH3)2), 121.3, 126.4, 127.9, 142.1, 147.0, 148.5, 148.7, and
167.3 (ArC). 113Cd{1H} NMR (C6D6, 133.1 MHz, 25 °C): δ 410.3.
formula
fw
habit
C96H157.98I0.02Si2Zn2 C36H50Cd
C48H75NOSiZn
775.57
block
1501.66
block
595.16
plate
cryst syst
space group
a, Å
b, Å
c, Å
monoclinic
C2/c
20.7441(10)
17.1921(8)
28.8855(17)
90
110.3960(10)
90
9655.7(9)
4
orthorhombic
Pnma
7.9195(3)
25.5730(11)
16.1398(7)
90
90
90
3268.7(2)
4
monoclinic
P21/n
10.5369(9)
29.196(3)
15.2128(13)
90
93.026(2)
90
4673.5(7)
4
IR (Nujol): ν(Cd-H) 1735 cm-1 (m). 8: H NMR (300 MHz, C6D6,
1
R, deg
25 °C): δ 1.16 (d, 24H, o-CH(CH3)2, 3JHH ) 7.2 Hz), 1.27 (d, 24H,
ꢀ, deg
γ, deg
3
3
p-CH(CH3)2, JHH ) 7.2 Hz), 1.37 (d, 24H, o-CH(CH3)2, JHH
)
V, Å3
3
6.6 Hz), 2.90 (sept, 4H, p-CH(CH3)2, JHH ) 7.2 Hz), 3.06 (sept,
8H, o-CH(CH3)2, 3JHH ) 7.2 Hz), 7.15 (s, 8H, m-Dipp), 7.24-7.26
(6H, m-C6H3 and p-C6H3). 13C{1H} NMR (C6D6, 75.4 MHz, 25
°C): δ 24.6 (o-CH(CH3)2), 24.7 (o-CH(CH3)2), 25.3 (p-CH(CH3)2),
30.6 (o-CH(CH3)2), 35.1 (p-CH(CH3)2), 121.6, 126.6, 127.9, 142.1,
146.6, 148.4, 148.6, and 162.9 (ArC).
Z
cryst dimens,
0.51 × 0.45 × 0.40 0.30 × 0.28 × 0.16 0.28 × 0.24 × 0.14
mm
dcalc, g · cm-3
µ, mm-1
no. of reflns
1.033
0.567
11 088
1.209
0.688
3836
1.102
0.584
10 702
8793
no. of obsd reflns 7918
3402
R1 obsd reflns
wR2, all
0.0536
0.1663
0.0289
0.0710
0.0451
0.1203
{4-Me3Si-Ar*Zn(TEMPO)} (10). A solution of 7 (0.46 g, 0.37
mmol) in toluene (50 mL) was added dropwise to a Schlenk tube
containing the red solid TEMPO (0.18 g, 1.11 mmol) over 10 min
at ambient temperature. The solution was stirred at 70 °C for 1
day, by which time the red color had faded to pale brown. The
toluene solvent and excess TEMPO were then removed under
reduced pressure, and the residue was redissolved in ca. 15 mL of
hexane. Storage for 2 days in a freezer (ca. -18 °C) afforded
colorless, X-ray quality crystals of 10. Yield: 0.26 g, 45%; mp 167
°C. Anal. Calcd for C48H75NOSiZn: C, 74.33; H, 9.75; N, 1.81.
were used for the refinement. A summary of the data collection
parameters for 7, 9, and 10 is provided in Table 1.
Results and Discussion
The compounds 5, 7, and 9 were prepared by the treatment
of the aryl metal halide with NaH in THF. Attempts to grow
crystals of 5 suitable for X-ray crystallography were unsuc-
cessful. The use of the derivatized aryl group Ar*-4-SiMe3
instead of Ar* allowed crystals of 7 to be obtained. We have
established elsewhere15 that derivatization of the para position
of the central aryl ring in terphenyl ligands exerts essentially
no steric and little electronic effect, so that it can be assumed
that structures of ZnAr*-4-SiMe3 derivatives are similar to those
of ZnAr*. The molecular structure of 7 is shown in Figure 1. It
is similar to that previously reported for the Ar′ derivative 1.
The Zn-H distances are 1.62(2) and 1.65(2) Å, which are close
to the 1.67(2) and 1.79(3) Å measured in 1, and the Zn · · · Zn
separation of 2.4102(5) Å is essentially the same as the
previously seen 2.4084(3) Å. The terphenyl ligands in 7 are
oriented perpendicularly to each other, and the central aryl rings
subtend a dihedral angle of 47.1° with respect to the Zn2H2 core.
1
Found: C, 73.89; H, 9.88; N, 1.70. H NMR (600 MHz, C6D6, 25
°C): δ 0.27 (s, 9H, Si(CH3)3), 1.15 (s, 12H, N(C(CH3)2CH2)2CH2),
3
1.19 (d, 12H, o-CH(CH3)2, JHH ) 6.6 Hz), 1.23-1.24 (m, 6H,
N(C(CH3)2CH2)2CH2), 1.32 (d, 12H, p-CH(CH3)2, 3JHH ) 7.2 Hz),
3
1.39 (d, 12H, o-CH(CH3)2, JHH ) 6.6 Hz), 2.87 (sept, 2H,
p-CH(CH3)2, 3JHH ) 6.6 Hz), 3.16 (sept, 4H, o-CH(CH3)2, 3JHH
)
7.2 Hz), 7.19 (s, 4H, m-Dipp), 7.54 (s, 2H, m-C6H2). 13C{1H} NMR
(C6D6, 150.9 MHz, 25 °C): δ -1.04 ((CH3)3Si), 14.4 (N(C(CH3)2-
CH2)2CH2), 17.5 (N(C(CH3)2CH2)2CH2), 24.0 (o-CH(CH3)2), 24.5
(o-CH(CH3)2), 25.4 (p-CH(CH3)2), 30.9 (o-CH(CH3)2), 32.0
(N(C(CH3)2CH2)2CH2), 35.0 (p-CH(CH3)2), 58.6 (N(C(CH3)2CH2)2-
CH2), 121.5, 131.8, 139.1, 142.4, 147.0, 148.6, 148.9, and 151.6
(ArC). 29Si{1H} NMR (C6D6, 119.2 MHz, 25 °C): δ -21.9.
X-ray Crystallography. Crystals of 7, 9, or 10 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 the
N2 cold stream on the diffractometer.17 The data for 7 were recorded
near 180 K, and those of 9 and 10 were recorded near 90 K on a
Bruker APEX instrument (Mo KR radiation (λ ) 0.71073 Å) and
a CCD area detector). The SHELX version 6.1 program package
was used for the structure solutions and refinements. Absorption
corrections were applied using the SADABS program.18 The crystal
structures were solved by direct methods and refined by full-matrix
least-squares procedures. All non-H atoms were refined anisotro-
pically. All carbon-bound H atoms were included in the refinement
at calculated positions using a riding model included in the
SHELXTL program. The two hydrogen atoms bonded to Zn in 7
were located in the electron density map and refined with similar
distance restraints; however, there is a disorder over one hydrogen
position with iodine impurity. This is probably due to incomplete
transmetalation of 6 by NaH. The occupancy of the iodine was
refined to 0.01. The terminal hydrogen atoms bonded to Cd in 9
were also located in the electron density map and refined freely. In
10 the positions of Zn and O atoms are disordered. The occupancy
of the second position refined to 0.11. Distance restraints and equal
atomic displacement parameters of the two atoms of one position
1
A H NMR signal corresponding to the bridging hydride in 7
was located at 5.08 ppm, which is slightly downfield to the 4.84
Figure 1. Thermal ellipsoid (50%) plot of 7. Carbon-bound
hydrogen atoms and isopropyl groups on flanking the rings are not
shown for clarity. Selected bond lengths [Å] and angles [deg]:
Zn(1) · · · Zn(1A) 2.4102(5), Zn(1)-H(1) 1.65(2), Zn(1)-H(2)
1.62(2),Zn(1)-C(1)1.924(2),Si(1)-C(4)1.872(2);C(1)-Zn(1)-Zn(1A)
176.39(7), C(1)-Zn(1)-H(1) 134.1(8), Zn(1A)-Zn(1)-H(1) 43.2(8),
Zn(1A)-Zn(1)-H(2) 42.1(8), C(6)-C(1)-Zn(1) 120.89(15),
C(2)-C(1)-Zn(1) 120.39(15), C(4)-Si(1)-C(37) 109.65(13),
C(37)-Si(1)-C(38) 108.60(16); the dihedral angle between the
Zn2H2 and central aryl planes is 47.1°, and the dihedral angle
between two central aryl planes is 87.1°.
(16) Wang, Y.; Quillian, B.; Wannere, C. S.; Wei, P.; Schleyer, P. v.
R.; Robinson, G. H. Organometallics 2007, 26, 3054.
(17) Hope, H. Prog. Inorg. Chem. 1995, 41, 1.

2094 Organometallics, Vol. 28, No. 7, 2009
Zhu et al.
Scheme 3. Synthetic Routes to the Zn-Zn Bonded Dimer
Zn2(η6-C5Me5)2 (12)1
It has been reported that the first Zn-Zn bonded compound
(ZnCp*)2 (Cp* ) η5-C5Me5, 12) can be prepared by the reduction
of 1:1 ZnCp*2/ZnCl2 mixtures with KH in THF solution (route 2
in Scheme 3).1b,c The radical rearrangement of the transient species
(Cp*ZnH)2 via homolytic cleavage of the Zn-H bond might be
the key step on the pathway to the final product. A similar radical
pathway might also exist in the stepwise synthesis of Ar′Zn(µ-
H)(µ-Na)ZnAr′ (2), where Ar′Zn(µ-H)2ZnAr′ (1) can eliminate H2
and then form an adduct with NaH.3a However, investigations of
CH3ZnH in argon matrixes showed that this simple zinc hydride
species is photostable and will not yield the CH3Zn• radical when
irradiated with the broadband output of a 500 W medium-pressure
Hg lamp.21Nonetheless we decided to test the reactivity of the
arylmetal hydrides with the radical species TEMPO. The reaction
of compound 7 with TEMPO at 70 °C for 1 day afforded the
derivative 10 (Figure 3) with hydrogen elimination in moderate
yield upon concentration and cooling of the hexane solution to
-18 °C.
Figure 2. Thermal ellipsoid (50%) plot of 9. Carbon-bound hydrogen
atoms are not shown for clarity. Selected bond lengths [Å] and angles
[deg]: Cd(1)-H(1) 1.79(4), Cd(1)-C(1) 2.122(2), C(1)-C(2) 1.404(2);
C(1)-Cd(1)-H(1) 174.2(14), C(2)-C(1)-Cd(1) 120.47(11).
ppm shift in 1, and its intensity was in the correct ratio to the
signals of the aryl ligand. In contrast to the behavior of 1, 7
does not undergo a further reduction with NaH to form a sodium
hydride-bridged species {(4-SiMe3-Ar*)Zn(µ-H)(µ-Na)Zn(Ar*-
4-SiMe3)}. Presumably such a process is prevented by the more
sterically encumbering parent Ar* ligand, which discourages a
closer approach of the metal centers.
The monomeric structure of 9, illustrated in Figure 2, is its
most noteworthy feature. Only two molecular cadmium hydride
structures have been previously reported. These are the weakly
dimerized arylcadmium hydride 3 and the tris(pyrazolyl)borato
complex TpMe2Cd(µ-H)2BH2 (11)7 (TpMe ) tris(3,5-dimeth-
2
ylpyrazolyl)hydroborate). The former species is an unstable,
loosely associated dimer, while the latter complex has two
hydrides bridging the Cd and B atoms. The monomeric structure
of 9 is thus in sharp contrast to these. The coordination geometry
at cadmium is almost linear [C(1)-Cd(1)-H(1) ) 174.2(4)°]
with Cd(1)-C(1) and Cd(1)-H(1) distances of 2.122(2) and
1.79(4) Å, respectively. The Cd-H distance is comparable to
the short Cd-H bond length of 1.78(6) Å in the unsymmetrically
bridged cadmium hydride 3, but is shorter by ca. 0.2 Å than
the bridging 1.973 and 2.126 Å Cd-H distances in 11. Our
previous calculations3c on model compounds PhM(µ-H)2MPh
(M ) Zn, Cd, and Hg) showed that the dimerization of organo
group 12 hydrides becomes less favored as the group is
descended. A similar trend was also found in (Ar′MI)1 or 2 (M
) Zn, Cd, or Hg),3c where arylzinc iodide and arylcadmium
iodide were dimers in the solid state, while arylmercury iodide
existed as a monomer. In 9, the Ar* ligand increases the
preference for a monomeric structure by steric pressure and the
unassociated Ar*CdH is the result. This compound also
exhibited much higher thermal stability than the dimer 3.
Refluxing compound 9 in toluene overnight led only to partial
decomposition of 9 with some precipitation of a black solid.
The monomeric structure of 9 is consistent with an increased
stability to homolytic M-H cleavage, which is believed to
require an associative mechanism.10 A signal corresponding to
It has been reported that TEMPO can induce fission of
p-block M-H bonds by two different radical pathways (Scheme
4). Metal-metal bonded derivatives of group 14 metals
R3MMR3 (R/M ) Ph/Ge or nBu/Sn) were formed via hydrogen
1
the terminal hydride in 9 was located at 6.69 ppm in the H
NMR spectrum. In the 113Cd NMR spectrum a signal at 410.3
ppm was observed. Both chemical shifts are similar to those of
6.84 and 410.7 ppm observed in the 1H NMR and 113Cd NMR
spectra of 3, suggesting that the compound 3 is dissociated to
monomers in solution. The IR spectrum of 9 displayed a
medium-intensity absorption band of 1735 cm-1 in reasonable
agreement with the 1764 and 1771 cm-1 observed in frozen
matrixes19 or the gas phase.20
Figure 3. Thermal ellipsoid (50%) plot of 10, where Zn and O
atoms are disordered over two positions with occupancies of 0.89
and 0.11 for Zn(1), O(1) and Zn(2), O(2), respectively. Carbon-
bound hydrogen atoms are not shown for clarity. Selected bond
lengths [Å] and angles [deg]: Zn(1)-O(1) 1.8450(15), Zn(2)-O(2)
1.831(8),Zn(1) · · · N(1)2.2039(16),Zn(2) · · · N(1)2.220(3),O(1)-N(1)
1.504(2), O(2)-N(1) 1.552(10), Zn(1)-C(1) 1.9317(18), Zn(2)-C(1)
2.052(3),N(1)-C(40)1.500(3),N(1)-C(44)1.488(2);C(1)-Zn(1)-O(1)
163.29(7), C(1)-Zn(2)-O(2) 174.9(4), Zn(1)-O(1)-N(1) 81.62(9),
Zn(2)-O(2)-N(1)81.6(4),C(1)-Zn(1)-N(1)154.23(7),C(1)-Zn(2)-N(1)
141.32(13), O(1)-N(1)-C(40) 108.04(15), O(2)-N(1)-C(40)
95.2(5),C(44)-N(1)-C(40)118.18(15)C(2)-C(1)-Zn(1)117.67(13),
C(6)-C(1)-Zn(2) 109.51(14).
(18) SADABS, An empirical absorption correction program, part of the
SAINT-Plus NT version 5.0 package; Bruker AXS: Madison, WI, 1998.
(19) Wang, X.; Andrews, L. J. Phys. Chem. A 2004, 108, 11006.

Group 12 Metal Hydride Stability
Organometallics, Vol. 28, No. 7, 2009 2095
Scheme 4. Radical Reaction of TEMPO with p-Block M-H
difference relates to the coordination mode of the TEMPO
ligand, which is fully bidentate in the tranistion metal complexes.
Thus, in the [Mn(CO)3(η2-TEMPO)],26 [Co(CO)2 (η2-TEM-
PO)],27 [(dtbpe)Ni(Cl)(η2-TEMPO)] (dtbpe ) 1,2-bis(di-tert-
butylphosphino)ethane),28 and [CuBr(η2-TEMPO)]29 complexes,
the M-N distances are all relatively short and are less than 2.0
Å. In the manganese complex the Mn-N distance, 1.839(3) Å,
is even shorter than the Mn-O distance, 1.981(3) Å. Further-
more the N-O bond lengths in these complexes are in the range
1.304(8)-1.413(3) Å, suggesting that the TEMPO ligand
remains unreduced. The coordination mode of TEMPO in 10
most resembles that found in the magnesium dimer [Mg2(µ2:
η2-TEMPO)(µ2:η1-TEMPO){N(SiMe3)2}2],whereaweakMg · · · N
interaction (2.395(3) Å) is also observed for one of the two
bridging TEMPO ligands.32 The reaction between TEMPO and
Zn{N(SiMe3)2}2 in the presence of LiN(SiMe3)2 afforded
[Li(TEMPO)4] [Zn{N(SiMe3)2}3], which has no Zn-TEMPO
bonding.32
Bonds in Two Pathways18,19
transfer to TEMPO.22 However, TEMPO complexes of group
13 hydrides [MH2(quin)(TEMPO)] (quin ) quinuclidine, M )
Al (13) or Ga (14)) or [AlH(quin)(TEMPO)2] (15) were obtained
via TEMPO substitution and H2 elimination.23
The formation of 10 apparently occurs by a similar process
to that of the group 13 hydrides. However, instead of forming
a simple monodentate M-O bonded complex as in 13-15,
TEMPO is bound to the Zn center in a quasi “side-on” fashion
with a significant secondary interaction between zinc and the
nitrogen atom to afford a three-membered Zn-N-O metalla-
cycle as shown in Figure 3. The zinc is disordered over two
sites with an occupancy ratio of ca. 9:1. The Zn-N-O
metallacycle has Zn-O, O-N, and Zn · · · N distances of
1.8450(15), 1.504(2), and 2.2039(16) Å (only structural infor-
mation of the major Zn1 and O1 components is listed),
respectively. The Zn · · · N distance is ca. 0.4 Å longer than
Zn-N single bond lengths of 1.81-1.85 Å in monomeric zinc
amides,24 but it is well within the Zn · · · N distance range of
2.04-2.67 Å in various zinc amine adducts.25 The C-Zn-O
angle is 163.29(7)°, and there is an acute Zn-O-N angle of
81.62(9)°. The relatively long N-O distance indicates that the
TEMPO has been reduced so that it behaves as an anionic
[TEMPO]- ligand.
The reaction between ZnEt2 and TEMPO has been reported by
Carmona and co-workers.1c However, in this case, a TEMPO-
bridged compound [ZnEt(TEMPO)]2 was obtained, which is the
only previous known Zn-TEMPO complex. The same authors have
reported two synthetic routes (Scheme 3) to the Zn-Zn bonded
compound 12. TEMPO can inhibit the synthetic route 1 by a
competition reaction with ZnEt2. This inhibition suggests the
involvement of radicals in this route. A computational study of
(ZnCp*)2 formation also supported this view.33 The fact that
TEMPO can induce H2 elimination by 7 also suggests a possible
radical formation via Zn-H cleavage. The tendency of zinc
hydrides to eliminate H2 gas and form Zn-Zn bonds thus becomes
feasible via this cleavage type. Zinc hydride 5 and cadmium hydride
9 can also react with TEMPO at 70 °C. However, both reactions
did not result in any crystals suitable for X-ray crystallographic
studies. An attempt to react Zn-Zn bonded compound Ar′ZnZnAr′
with TEMPO did not result any new species. Analysis of the
The “side-on” bonding mode for TEMPO and the Zn center
differs from that found in TEMPO complexes of unsaturated
transition metals (Mn,26 Co,27 Ni,28 Cu,29 Mo,30 and Pd31). The
1
reaction mixtures by H NMR spectroscopy afforded resonances
assignable only to starting materials.
(20) Shayesteh, A.; Yu, S.; Bernath, P. F. Chem.-Eur. J., 2005, 11,
4709.
In summary, dimeric {(4-SiMe3-Ar*)Zn(µ-H)2Zn(Ar*-4-
SiMe3)} (7) and monomeric Ar*CdH (9) were synthesized and
structurally characterized. The reaction between 7 and TEMPO
was examined. Experimental results showed that the zinc
hydride undergoes a Zn-H cleavage to form a complex with
the reduction of the radical species TEMPO. This observation
supports the involvement of a radical pathway in the synthesis
of compounds 2 and 11. The fact that the monomeric compound
9 is more stable than its dimeric analogue 3 supports the view
that homolytic M-H cleavage of group 12 M-H bonds to form
M-M bonds requires association of the hydride species.
(21) Greene, T. M.; Andrews, L.; Downs, A. J. J. Am. Chem. Soc. 1995,
117, 8180.
(22) Lucarini, M.; Marchesi, E.; Pedulli, G. F.; Chatilialoglu, C. J. Org.
Chem. 1998, 63, 1687.
(23) Jones, C.; Rose, R. P. New J. Chem. 2007, 31, 1484.
(24) (a) Power, P. P.; Ruhlandt-Senge, K. S.; Shoner, C. Inorg. Chem.
1991, 30, 5013. (b) Elias, A. J.; Schmidt, H.-G.; Noltemeyer, M.; Roesky,
H. W. Eur. J. Solid State Inorg. Chem. 1992, 29, 23. (c) Rees, W. S., Jr.;
Green, D. M.; Hesse, W. Polyhedron 1992, 11, 1697. (d) Rees, W. S., Jr.;
Just, O.; Schumann, H.; Weimann, R. Polyhedron 1998, 17, 1001. (e)
Schumann, H.; Gottfriedsen, J.; Dechert, S.; Girgsdies, F. Z. Anorg. Allg.
Chem. 2000, 626, 747. (f) Margraf, G.; Lerner, H.-W.; Boltea, M.; Wagner,
M. Z. Anorg. Allg. Chem. 2004, 630, 217. (g) Tang, Y.; Felix, A. M.; Boro,
B. J.; Zakharov, L. N.; Rheingold, A. L.; Kemp, R. A. Polyhedron 2005,
24, 1093.
(25) (a) Westerhausen, M.; Wieneke, M.; Rademacher, B. B.; Schwarz,
W. Chem. Ber. 1997, 130, 1499, and references therein. (b) Coward, K. M.;
Jones, A. C.; Steiner, A.; Bickley, J. F.; Smith, L. M.; Pemble, M. E.
J. Chem. Soc., Dalton Trans. 2000, 3480, and references therein.
(26) Jaitner, P.; Huber, W.; Hunter, G.; Scheidsteger, O. J. Organomet.
Chem. 1983, 259, C1.
Acknowledgment. We thank the National Science Foun-
dation (CHE-0641020) and the Department of Energy Office
of Basic Energy Sciences (DE-FG02-07ER4675) for financial
support. Z.Z. thanks Dr. Eric Rivard for helpful discussions.
(27) Jaitner, P.; Huber, W.; Gieren, A.; Betz, H. J. Organomet. Chem.
1986, 311, 379.
(28) Mindiola, D. J.; Waterman, R.; Jenkins, D. M.; Hillhouse, G. L.
Inorg. Chim. Acta 2003, 345, 299.
Supporting Information Available: X-ray data (CIF) for 7, 9,
and 10. This material is available free of charge via the Internet at
http://pubs.acs.org.
(29) Caneschi, A.; Grand, A.; Laugier, J.; Rey, P.; Subra, R. J. Am.
Chem. Soc. 1988, 110, 2307.
OM900005T
(30) Jaitner, P.; Huber, W.; Gieren, A.; Betz, H. Z. Anorg. Allg. Chem.
1986, 538, 53.
(31) (a) Dickman, M. H.; Doedens, R. J. Inorg. Chem. 1982, 21, 682.
(b) Porter, L. C.; Doedens, R. J. Acta Crystallogr., Sect. C: Cryst. Struct.
Commun. 1985, 41, 838.
(32) Forbes, G. C.; Kennedy, A. R.; Mulvey, R. E.; Rodger, P. J. A.
Chem. Commun. 2001, 1400.
(33) Hepperle, S. S.; Wang, Y. A. J. Phys. Chem. A 2008, 112, 9619.