Synthesis, structural characterization, and spectroscopy of the cadmium-cadmium bonded molecular species Ar′CdCdAr′ (Ar′ = C6H3-2,6-(C6H3-2,6-Pr i2)2)

Article abstract of DOI:10.1021/ja066108h

The synthesis and first structural characterization of a cadmium-cadmium bonded molecular compound Ar-CdCdAr- (Ar- = C₆H₃-2,6-(C₆H₃-2,6-Prⁱ₂)₂) are reported. The existence of the Cd-Cd bond was established by ¹¹³Cd NMR spectroscopy and X-ray diffraction (Cd-Cd = 2.6257(5) A). Like its group 12 analogue Ar-ZnZnAr-, DFT calculations showed that Ar-CdCdAr- had significant p-character in the Cd-Cd σ-bonding HOMO. Copyright

Full text of DOI:10.1021/ja066108h

Published on Web 11/04/2006
Synthesis, Structural Characterization, and Spectroscopy of the
Cadmium-Cadmium Bonded Molecular Species Ar′CdCdAr′ (Ar′ )
6 3 6 3 2 2
C H -2,6-(C H ) )
-2,6-Pri
Zhongliang Zhu, Roland C. Fischer, James C. Fettinger, Eric Rivard, Marcin Brynda, and
Philip P. Power*
Department of Chemistry, UniVersity of California, DaVis, One Shields AVenue, DaVis, California 95616
Received August 22, 2006; E-mail: pppower@ucdavis.edu
Molecular compounds featuring homonuclear metal-metal bonds
between group 12 elements Zn and Cd are rare.1 The work of
Carmona and his group, which has resulted in the isolation and
-8
4
characterization of the first stable Zn-Zn bonded compound,
5
Cp*ZnZnCp* (Cp ) η -C
5
Me
5
), has proven that the Zn-Zn bond
can be stabilized by electronic and steric effects. At present, only
two examples of compounds with stable Cd-Cd bonds are
known,6 and detailed structural information exists only for the
,7
6
Me
ionic species [Cd
2
][AlCl
4 2 2
] (1). However, the molecular Cd
Tp 22
Me
2
(2) (Tp
) hydrotris(3,5-dimethylpyrazolyl) borate) has been
synthesized by treatment of CdCl
Me
7
with TlTp
2
3
/LiBHEt . The
2
existence of its Cd-Cd bond was established by the observation
of Cd1 -Cd coupling in its Cd NMR spectrum. Calculations
11
113
113
5
on simple model compounds M
2
(η -C
5
H
5
)
2
(M ) Zn or Cd) have
suggested that molecular compounds with simple organic groups
should be stable and capable of isolation, although the Cd-Cd
bonding was calculated to be weaker than that in the Zn-Zn
species. We now describe the synthesis and characterization of
the molecular species Ar′CdCdAr′ (3) and show that it possesses
considerable thermal stability.
Figure 1. Thermal ellipsoid (30%) drawing of 3; H atoms are not shown.
Selected bond lengths (Å) and angles (deg): Cd(1)-Cd(1A) 2.6257(5), Cd-
(1)-C(1) 2.138(3); Cd(1A)-Cd(1)-C(1) 177.5(3), C(2)-C(1)-Cd(1)
9
116.5(6), C(6)-C(1)-Cd(1) 121.8(6).
Compound 3 was prepared by the reduction of the aryl cadmium
halide Ar′CdI (4).1 The use of several reducing reagents, for
example, Na, Na/naphthalene, and NaH was investigated. Among
them, Na and Na/naphthalene led to over-reduction and the isolation
of Ar′H and Cd metal. However, the addition of 2 equiv of NaH to
Ar′CdI in THF resulted in the synthesis of the title compound
Ar′CdCdAr′ in a low yield in accordance with the eq 1.
(1)-C(1) ) 177.5(3)°], which is very similar to the geometry
observed in the zinc analogue 5.
0
With a spin of / and a natural abundance of 12.22%, 113Cd has
1
2
a receptivity relative to 13C of 7.6 and is widely employed as a
13
useful method to characterize cadmium compounds. A solution
1
13
Cd NMR spectrum of aryl cadmium halide 4 displayed only one
signal at 210.91 ppm, which lies between the 328.80 ppm signal
observed for the bis(aryl)cadmium species Cd(C (1.0 M) and
the 55.13 ppm for CdI
(1.0 M).14 In contrast, 3 displayed a signal
at 540.28 ppm with two symmetrically disposed satellite resonances
6
5 2
H )
2
111
in the correct intensity ratio for coupling to a bound Cd nucleus
(
ruled out the possibility of an Ar′CdH species since the aryl
cadmium hydride would yield a 1:1 doublet or 1:2:1 triplet pattern
[for monomeric or doubly hydrogen bridged structures, respectively]
instead of the satellite pattern observed in the spectrum of 3. The
1
113
2
I ) / , 12.81%) (see Figure 2). The Cd NMR spectrum thus
15
Although the mechanism of this reduction is still not clear, the
X-ray crystal structure of 311 (Figure 1) confirmed the presence of
a Cd-Cd bond in the product 3. The observed Cd-Cd distance of
2
.6257(5) Å in 3 is ca. 0.20 Å shorter than the sum of Pauling’s
1
2
111
113
single-bond metallic radii for Cd [2.82 Å]. This shortening is
similar to that in its zinc analogue Ar′ZnZnAr′ (5) [ca. 0.14 Å]5b
and thus is indicative of a considerable Cd-Cd bonding interaction.
The Cd-Cd distance in 3 is also ca. 0.05 Å longer than the value
large Cd- Cd coupling constant of 8650 Hz is well outside
1
113
the reported range of Cd-H couplings: for example, J( Cd-
1
But
But
H) ) 2520 Hz in Tp CdH (Tp ) hydrotris(3-tert-butylpyra-
zolyl)borate),7 2J( Cd- H) of ca. 50 Hz,
113
1
14,16
and J( Cd- H)
3
113
1
6
14,16
of 2.576(1) Å found in the ionic cadmium(I) salt 1 and is ca. 0.02
of ca. 60 Hz in dialkyl cadmium compounds.
The satellite
signals in 3 are thus assigned to 1 Cd- Cd coupling. The intensity
of these two satellite signals was in reasonable agreement with the
theoretical value of ca. 1:7 based on the isotopic ratio. However,
11
113
Å longer than that of 2.605 Å predicted in the as yet unknown
9
compound Cp*CdCdCp* (6). A similar elongation (ca. 0.05 Å) is
5b
found for Ar′ZnZnAr′ (Zn-Zn ) 2.3591(9) Å) versus Cp*ZnZnCp*
Zn-Zn ) 2.305(3) Å).4 The crystal structure of 3 also showed
a
the Cd- Cd coupling constant is less than half the previous
111
113
(
that the two Ar′ ligands are arranged in a nearly orthogonal
orientation to each other which provides effective steric protection
of the [-Cd-Cd-]2 moiety. An almost linear arrangement is
observed for the [C(ipso)-Cd-Cd-C(ipso)] unit [Cd(1A)-Cd-
reported value of 20 650 Hz for 2.7
Spin-restricted DFT calculations were performed on Ar′CdCdAr′
(3) at B3LYP/6-31 g* level using a simplified model where the
isopropyls were replaced by hydrogen atoms and the coordinates
+
15068
9
J. AM. CHEM. SOC. 2006, 128, 15068-15069
10.1021/ja066108h CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
a postdoctoral fellowship. E.R. thanks NSERC of Canada for a
postdoctoral fellowship.
Supporting Information Available: Computation studies and X-ray
data for 2 (CIF) as well as the full citation for ref 17. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
(
1) Parkin, G. Science 2004, 305, 1117-1118.
(2) Corbett, J. D.; Burkhard, W. J.; Druding, L. F. J. Am. Chem. Soc. 1961,
83, 76-80.
(
3) Taylor, M. J. Metal to Metal Bonded States of the Main Group Elements,
Academic: London, 1975; Chapter 2.
(
4) (a) Resa, I.; Carmona, E.; Gutierrez-Puebla, E.; Monge, A. Science 2004,
3
05, 1136-1138. (b) del R ´ı o, D.; Galindo, A.; Resa, I.; Carmona, E.
Angew. Chem., Int. Ed. 2005, 44, 1244-1247.
(
5) Other examples of Zn-Zn bonded compounds can be found in (a) 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, 11944-11945. (b) Zhu, Z.; Wright, R. J.; Olmstead, M.
M.; Rivard, E.; Brynda, M.; Power, P. P. Angew. Chem., Int. Ed. 2006,
45, 5807-5810.
1
13
Figure 2.
Cd NMR spectrum of 3. Signals marked with (/) indicate the
1
1
11
113
Cd- Cd coupling ( JCdCd ) 8650 Hz).
(
6) (a) Faggiani, R.; Gillespie, R. J.; Vekris, J. E. J. Chem. Soc., Chem.
Commun. 1986, 517-518. (b) Staffel, T.; Meyer, G. Z. Anorg. Allg. Chem.
1987, 548, 45-54.
(7) Reger, D. L.; Mason, S. S.; Rheingold, A. L. J. Am. Chem. Soc. 1993,
115, 10406-10407.
(
8) Zn-Zn and Cd-Cd bonded molecular species have been detected
spectroscopically in frozen matrices at low temperature. Greene, T. M.;
Brown, W.; Andrews, L.; Downs, A. J.; Chertihin, G. V.; Runeberg, N.;
Pyykk o¨ , P. J. Phys. Chem. 1995, 99, 7925-7943.
(9) (a) Xie, Z.-Z.; Fang, W.-H. Chem. Phys. Lett. 2005, 404, 212-216. (b)
Kang, H. S. J. Phys. Chem. A 2005, 109, 4342-4351.
(
10) All manipulations were carried out under anaerobic and anhydrous
10a
2
conditions. 4: Ar′Li (1.11 g, 2.73 mmol) and CdI (1.00 g, 2.73 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
by hexane (50 mL). The slurry was allowed to settle, and the mother
liquor was separated from the precipitate (LiI). The hexane was removed,
and the pale yellow residue was extracted again with hexane (50 mL).
Filtration and removal of the hexane yielded 4 as a white powder. Yield:
1
1
1
2
.17 g, 67.2%; mp 287 °C. H NMR (300 MHz, C
6
D
6
, 25 °C): δ 1.07 (d,
3
3
2H, CH(CH
.99 (sept, 4H, CH(CH
, m-Dipp, and p-Dipp). C { H} NMR (C
C): δ 24.5 (CH(CH ), 25.2 (CH(CH ), 30.6 (CH(CH
), 127.8 (p-Dipp), 128.9 (m-C ), 144.2 (i-Dipp),
3
)
2
, JHH ) 6.6Hz), 1.24 (d, 12H, CH(CH
3
)
2
, JHH ) 6.9Hz),
3
3
)
2
, JHH ) 7.2Hz), 7.13-7.31 (m, 9H, m-C
6 3
H ,
1
3
1
p-C
6
H
3
6
D
6
, 100.6 MHz, 25
), 123.7 (m-
°
3
)
2
3
)
2
3 2
)
Figure 3. Representation of the frontier molecular orbitals of 3 from DFT
calculations.
Dipp), 126.7 (p-C
6
H
3
6 3
H
113
1
b
1
46.6 (o-C
6
3 6 3 6 6
H ), 148.2 (o-Dipp), 162.3 (i-C H ). Cd { H} NMR (C D ,
33.1 MHz, 25 °C): δ 210.91. 3: The iodide derivative 4 (1.00 g, 1.57
1
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.
were extracted from the X-ray structure.17 The calculations showed
that the HOMO corresponded to a σ-bond between the cadmium
atoms, which is composed mainly of 5p and 5s orbital character in
a ratio of about 5:3 (see Figure 3). The Cd-C bonds are derived
from in phase and out of phase orbital combinations HOMO-14
and HOMO-3, which also have minor Cd-Cd bonding character
10 mL, and storage over 2 d in a refrigerator (ca. 8 °C) afforded colorless
X-ray quality crystals of 3. Yield: 0.22 g, 27.6% (based on 4);
1
decomposition of 3 to a gel-like grey solid was seen at 182 °C. H NMR
3
(
300 MHz, C D , 25 °C): δ 1.05 (d, 12H, CH(CH ) , JHH ) 7.2Hz),
6
6
3
2
3
3
1.12 (d, 12H, CH(CH
3
)
2
, JHH ) 6.6Hz), 2.95 (sept, 4H, CH(CH
6.9Hz), 7.10-7.27 (m, 9H, m-C , p-C , m-Dipp, and p-Dipp).
, 100.6 MHz, 25 °C): δ 24.3 (CH(CH ), 24.9 (CH-
), 30.4 (CH(CH ), 122.9 (m-Dipp), 125.8 (p-C ), 127.1 (p-Dipp),
28.6 (m-C ), 144.0 (i-Dipp), 147.0 (o-C ), 147.2 (o-Dipp), 176.5
) , JHH
3 2
13
)
{
H
6 3
H
6 3
C
(
+
see Figure S1, Supporting Information). The LUMO and LUMO
1 combinations are almost degenerate and are formed mainly
from the cadmium 5p and 5p orbitals. The metal-metal bonding
1
H} NMR (C
6
D
6
3 2
)
(CH
3
)
2
)
3 2
6 3
H
1
(
6
H
3
6 3
H
x
y
113
1
b
1
6 3 6 6
i-C H ). Cd { H} NMR (C D , 133.1 MHz, 25 °C): δ 540.28 ( JCdCd
) 8650Hz). (a) Schiemenz, B.; Power, P. P. Angew. Chem., Int. Ed. 1996,
35, 2150-2152. (b) Chemical shifts are reported in ppm with respect to
in 3 is thus similar to that in Ar′ZnZnAr′. This is in contrast to
Cp*ZnZnCp* where the Zn-Zn bond is of mainly 4s character.4
It seems likely that the influence of the different ligands on the
4 2
external 0.01 M Cd(ClO ) .
(11) Crystallographic data for 3 at 223 K with Mo KR radiation (λ ) 0.71073
Å). 3: monoclinic, space group C2/m, a ) 15.2430(9), b ) 17.3351(11),
c ) 15.0018(9) Å, â ) 120.5340(10)°, Z ) 2, R1 ) 0.0363 for 2872 (I
> 2σ(I)) data, wR2 (all data) ) 0.1018.
metal-metal bonding may likewise account for the large differences
Me
2
in the Cd-Cd coupling constant between 3 and Cd .
Tp 22
(
12) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University
We have described the synthesis and first structural characteriza-
tion of a molecular compound containing a Cd-Cd bond. The
existence of the metal-metal bond was confirmed by 1 Cd NMR
spectroscopic studies. Similar to the Zn-Zn bonded compound
Ar′ZnZnAr′, DFT calculations showed that Ar′CdCdAr′ (3) had
significant p-character in the Cd-Cd bonding orbital. Work to
elucidate the details of the mechanism of the reduction of 4, the
isolation of the putative aryl cadmium hydride intermediate Ar′CdH,
and the synthesis and reactivity of 3 and other Cd-Cd bonded
compounds are in hand.
Press: Ithaca, NY, 1960; Chapter 7.
1
13
(13) A review of Cd NMR spectroscopy can be found in: Summers, M. F.
13
Coord. Chem. ReV. 1988, 86, 43-134.
(
14) Cardin, A. D.; Ellis, P. D.; Odom, J. D.; Howard, J. W., Jr. J. Am. Chem.
Soc. 1975, 97, 1672-1679.
(
15) Full details of the preparation and structure of the aryl cadmium hydride
intermediate species Ar′CdH will form part of a full account of our work
on Cd-Cd bonded compounds.
(
(
16) Turner, C. J.; White, R. F. M. J. Magn. Reson. 1977, 26, 1-5.
17) The single point (SP) calculations were performed using Gaussian 03
software: Frisch, M. J.; et al. Gaussian 03, revision B.03; Gaussian,
Inc.: Pittsburgh, PA, 2003 (see Supporting Information for full reference).
The representations of Kohn-Sham orbitals were generated using
MOLEKEL package: Flukiger, P.; Luthi, H. P.; Portmann, S.; Weber, J.
MOLEKEL, version 4.3; Swiss Center for Scientific Computing: Manno,
Switzerland, 2000-2002.
Acknowledgment. We thank the National Science Foundation
for financial support. R.C.F. thanks the Max Kade foundation for
JA066108H
J. AM. CHEM. SOC.
9
VOL. 128, NO. 47, 2006 15069