Cyanide-Bridged Heterobimetallic Complexes of the Group 6 Metal Carbonyls and Copper(I). X-ray Structures of (CO)5MCNCu(PPh3)3 (M = Cr, W) Derivatives

Article abstract of DOI:10.1021/ic9510627

The cyanide-bridged compounds (CO)₅MCNCu(PPh₃)₃ where M = W in 1, and M = Cr in 2 have been prepared by the reaction of NaM(CO)₅CN with Cu(CH₃CN)₄BF₄ and PPh₃, and their solid-state structures have been determined crystallographically. In 1, the CN bond length was 1.15(2) A?, and the bridge was slightly bent with angles of W-C(6)-N(1) = 175.9(14)° and Cu-N(1)-C(6) = 176(2)°. Crystal data for 1: monoclinic, space group P2₁/n, a = 20.376(4) A?, b = 12.436(3) A?, c = 22.423(5) A?, β= 97.28(3)°, Z = 4, R = 7.90percent. The chromium analog, complex 2, was isomorphous with 1. In addition, the isocyanide-bridged complex (CO)₅WNCCu-(PPh₃)₃ (3) was prepared by the reaction of CuCN with PPh₃ and W(CO)₅(THF) at -78°C. The structure of this compound was determined by infrared and ¹³C NMR measurements. The isocyanide-bridged 3 was found to undergo thermal rearrangement to yield the cyanide-bridged 1. In the ¹³C NMR spectrum of 1 at room temperature, the CN peak is very broad (fwhh = 47 Hz) and can be found at 147.8 ppm. Dynamic ¹³C NMR measurements on 1 revealed that this complex undergoes an equilibrium reaction in THF-d₈ solution. Two possibilities are presented which are consistent with the observations. One is a contact ion pair/solvent-separated ion pair exchange reaction which involves breakage of the Cu-N bond and insertion of a solvent molecule. The second is dissociation/ recoordination of a triphenylphosphine ligand from Cu(I). Differentiation between the two is impossible on the basis of the available data. In the slow-exchange limit, the ¹³CN peak of 1 is found at 149.0 ppm, shows ¹⁸³W satellites (J_wc = 96 Hz), and has a line width of 12 Hz. In the ¹³C NMR spectrum of 3 at room temperature, the CN peak is sharp (fwhh = 0.75 Hz) and can be found at 160.7 ppm. Dynamic ¹³C NMR measurements on 3 revealed no ion pair exchange as high as 40°C. The ¹³C NMR spectrum of Cu¹³CN(PPh₃)₃ was also recorded at room temperature, and the CN peak can be found at 152.3 ppm with a line width of 15 Hz.

Full text of DOI:10.1021/ic9510627

4764
Inorg. Chem. 1996, 35, 4764-4769
Cyanide-Bridged Heterobimetallic Complexes of the Group 6 Metal Carbonyls and
Copper(I). X-ray Structures of (CO)5MCNCu(PPh3)3 (M ) Cr, W) Derivatives
Donald J. Darensbourg,* Jeffrey C. Yoder, Matthew W. Holtcamp,
Kevin K. Klausmeyer, and Joseph H. Reibenspies
Department of Chemistry, Texas A&M University, College Station, Texas 77843
ReceiVed August 11, 1995X
The cyanide-bridged compounds (CO)5MCNCu(PPh3)3 where M ) W in 1, and M ) Cr in 2 have been prepared
by the reaction of NaM(CO)5CN with Cu(CH3CN)4BF4 and PPh3, and their solid-state structures have been
determined crystallographically. In 1, the CN bond length was 1.15(2) Å, and the bridge was slightly bent with
angles of W-C(6)-N(1) ) 175.9(14)° and Cu-N(1)-C(6) ) 176(2)°. Crystal data for 1: monoclinic, space
group P21/n, a ) 20.376(4) Å, b ) 12.436(3) Å, c ) 22.423(5) Å, â ) 97.28(3)°, Z ) 4, R ) 7.90%. The
chromium analog, complex 2, was isomorphous with 1. In addition, the isocyanide-bridged complex (CO)5WNCCu-
(PPh3)3 (3) was prepared by the reaction of CuCN with PPh3 and W(CO)5(THF) at -78 °C. The structure of this
compound was determined by infrared and 13C NMR measurements. The isocyanide-bridged 3 was found to
undergo thermal rearrangement to yield the cyanide-bridged 1. In the 13C NMR spectrum of 1 at room temperature,
the CN peak is very broad (fwhh ) 47 Hz) and can be found at 147.8 ppm. Dynamic 13C NMR measurements
on 1 revealed that this complex undergoes an equilibrium reaction in THF-d8 solution. Two possibilities are
presented which are consistent with the observations. One is a contact ion pair/solvent-separated ion pair exchange
reaction which involves breakage of the Cu-N bond and insertion of a solvent molecule. The second is dissociation/
recoordination of a triphenylphosphine ligand from Cu(I). Differentiation between the two is impossible on the
basis of the available data. In the slow-exchange limit, the 13CN peak of 1 is found at 149.0 ppm, shows 183W
satellites (JWC ) 96 Hz), and has a line width of 12 Hz. In the 13C NMR spectrum of 3 at room temperature, the
CN peak is sharp (fwhh ) 0.75 Hz) and can be found at 160.7 ppm. Dynamic 13C NMR measurements on 3
revealed no ion pair exchange as high as 40 °C. The 13C NMR spectrum of Cu13CN(PPh3)3 was also recorded
at room temperature, and the CN peak can be found at 152.3 ppm with a line width of 15 Hz.
Introduction
Vahrenkamp has provided an excellent example of such
behavior in a chromium-iron system.5 Using Cr(CO)5CN- as
a cyanide-donating “ligand”, the µ-CN complex is formed, but
using Cp(dppe)FeCN as cyanide donor, the so-called µ-NC
complex is obtained. There is no thermal rearrangement of the
cyanide bridge observed at temperatures as high as 150 °C (at
which both complexes decompose).
The pentacarbonyl cyanide fragment is of great synthetic and
spectroscopic utility due to the fact that the carbonyl infrared
stretching frequencies are extremely sensitive probes of the
electronic structure of the complex. With this in mind, the
synthesis of the cyanide-bridged complexes (CO)5M(CN)Cu-
(PPh3)3 (where M ) W in 1 and M ) Cr in 2) was carried out.
In addition, the synthesis of the isocyanide-bridged complex
(CO)5W(NC)Cu(PPh3)3 (3) was carried out at -78 °C with the
reaction being monitored by 13C NMR. Reported here are the
synthetic schemes used, X-ray crystal structures for 1 and 2,
the results of infrared studies on all compounds, and variable-
temperature 13C NMR studies on complexes 1 and 3.
Cyanide has been ubiquitous as a bridging ligand since the
beginnings of coordination chemistry. At the end of the
nineteenth century, Hofmann developed a series of cyanide-
bridged cadmium and nickel compounds1 which could include
solvent molecules in a guest-host type relationship. This area
of research has remained active into the 1990s with the synthesis
of polymeric cadmium cyanide clathrates by Iwamoto.2 Re-
cently, cyanide-bridged, dinuclear transition metal compounds
have seen application in the areas of electron delocalization and
charge transfer.3 Additionally, Holm et al. have prepared several
dinuclear cyanide-bridged assemblies which mimic the active
site of cyanide-inhibited cytochrome c oxidase.4 There are,
however, very few studies in the literature dealing with linkage
isomerization of the cyanide bridge.5-8
X Abstract published in AdVance ACS Abstracts, July 1, 1996.
(1) (a) Hofmann, K. A.; Ku¨spert, F. Z. Anorg. Allg. Chem. 1897, 15, 204.
(b) Hofmann, K. A.; Ho¨chtlen, F. Chem. Ber. 1903, 36, 1149. (c)
Hofmann, K. A.; Arnoldi, H. Chem. Ber. 1906, 39, 339.
(2) (a) Yuge, H.; Iwamoto, T. J. Chem. Soc., Dalton Trans. 1993, 2842
and references therein. (b) Park, K.-M; Iwamoto, T. J. Chem. Soc.,
Dalton Trans. 1993, 1876.
(3) (a) Zhou, M.; Pfennig, B. W.; Steiger, J.; Van Engen, D.; Bocarsly,
A. B. Inorg. Chem. 1990, 29, 2456. (b) Agnus, Y.; Gisselbrecht, R.
L.; Metz, B. J. Am. Chem. Soc. 1989, 111, 1494. (c) Burewicz, A.;
Haim, A. Inorg. Chem. 1988, 27, 1611. (d) Christofides, A.; Connelly,
N. G.; Lawson, H. J.; Loyns, Andrew C. J. Chem. Soc., Chem.
Commun. 1990, 597. (e) Scandola, F.; Argazzi, R.; Bignozzi, C. A.;
Chiorboli, C.; Indelli, M.T.; Rampe, N. A. Coord. Chem. ReV. 1993,
125, 283.
Experimental Section
Methods and Materials. Sodium bis(trimethylsilyl)amide (1 M
solution in THF), cuprous cyanide, and triphenylphosphine were
obtained from Aldrich and used without further purification. Tungsten
and chromium hexacarbonyls were obtained from Strem and used
without further purification. Cuprous [13C]cyanide was obtained from
Cambridge Isotope Labs and used as received. Copper acetonitrile
(4) (a) Lee, S. C.; Scott, M. J.; Kauffmann, K.; Mu¨nck, E.; Holm, R. H.
J. Am. Chem. Soc. 1994, 116, 401. (b) Scott, M. J.; Lee, S. C.; Holm,
R. H. Inorg. Chem. 1994, 33, 4651. (c) Scott, M. J.; Holm, R. H. J.
Am. Chem. Soc. 1994, 116, 11357.
(5) Zhu, N.; Vahrenkamp, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 2090.
(6) Alvarez, S.; Lopez, C. Inorg. Chim. Acta 1982, 64, L99.
(7) Gaswick, D.; Haim, A. J. Inorg. Nucl. Chem. 1978, 40, 437.
(8) Fronczek, F. R.; Schaefer, W. P. Inorg. Chem. 1974, 13, 727.
S0020-1669(95)01062-7 CCC: $12.00 © 1996 American Chemical Society

Cyanide-Bridged Heterobimetallic Complexes
Inorganic Chemistry, Vol. 35, No. 16, 1996 4765
tetrafluoroborate was prepared according to literature procedures.9
Hexane, benzene, and THF were dried in sodium benzophenone stills
prior to use, and rigorous Schlenk techniques were utilized for all
reactions. The sodium salts of tungsten and chromium pentacarbonyl
cyanides were prepared according to published literature procedures.10
Photolysis experiments were performed using a mercury arc 450-W
UV immersion lamp purchased from Ace Glass Co. Infrared spectra
were recorded on a Mattson 6021 FTIR equipped with DTGS and MCT
detectors operating in absorbance mode.
Table 1. Crystallographic Data and Data Collection Parameters
1
2
formula
fw
space group
a, Å
b, Å
c, Å
C62H49NO5.5P3WCu
1235.40
P21/n
20.376(4)
12.436(3)
22.423(5)
97.28(3)
5636(2)
4
1.419
2.549
0.710 73
193(2)
C62H49NO5.5P3CrCu
1103.55
P21/n
20.079(11)
12.383(7)
21.975(10)
97.86(4)
5413(5)
4
1.336
0.733
0.710 73
163(2)
â, deg
V, Å3
Synthesis of 1 and 2, (CO)5M(µ-CN)Cu(PPh3)3. In a typical
reaction, Na(CO)5MCN (2 mmol) (M ) W, Cr) was loaded into a 100
mL Schlenk flask with a magnetic stir bar and loaded into an inert-
atmosphere drybox. In the drybox, Cu(CH3CN)4BF4 (2 mmol) was
added to the flask. Tetrahydrofuran (25 mL) was then added via
syringe. The resulting reaction mixture was stirred for approximately
15 min until the formation of a dark brown precipitate was complete.
A solution of triphenylphosphine (6 mmol) in 10 mL of THF was then
transferred via cannula to the dark brown reaction flask mixture. The
resulting suspension was filtered through Celite into a 100 mL Schlenk
flask to give a clear, light yellow solution of (CO)5MCNCu(PPh3)3 (1,
M ) W; 2, M ) Cr). Anal. Calcd for (CO)5WCNCu(PPh3)3‚2C4H8O
(C68H61NO6P3WCu): C, 60.74; H, 4.57. Found: C, 60.84; H, 4.60.
IR (THF): 2121 cm-1 (ν(CN)), 2059 cm-1 (A1), 1929 cm-1 (E), 1904
cm-1 (A1). 13C{1H} NMR (THF-d8, 22 °C): δ 147.8 (br, CN), 197.67
(COeq), 200.26 (COax). Anal. Calcd for (CO)5CrCNCu(PPh3)3‚H2O
(C60H47NO6P3CrCu): C, 66.33; H, 4.36. Found: C, 65.70; H, 4.67.
IR (THF): 2117 cm-1 (ν(CN)), 2056 cm-1 (A1), 1934 cm-1 (E), 1907
cm-1 (A1).
Z
d(calc), g/cm3
abs coeff, mm-1
λ, Å
T, K
transm coeff
0.791-0.984
DIFABS
13.14
28.70
R,a %
7.90
Rw,a %
13.79
a R ) ∑|Fo - Fc|/∑Fo. Rw ) {∑w(Fo - Fc)/∑w(Fo)2}1/2. GOF )
1.016 and 1.049 for complexes 1 and 2, respectively.
no significant trends. Background measurements by stationary-crystal
and stationary-counter techniques were taken at the beginning and end
of each scan for (0.25 of the total scan time for 1 and 0.50 of the total
scan time for 2). Lorentz and polarization corrections were applied to
7030 reflections for 1 and 7353 reflections for 2. Approximately 300
reflections for 1 were lost due to severe crystal icing during the later
stages of data collection. A semiempirical absorption correction was
applied to 1, and a DIFABS correction was applied to 2 (Vide infra).
A total of 9910 unique reflections for 1 and 9578 unique reflections
for 2 with F > 4.0σF for 1 and F > 6.0σF for 2 were used in further
calculations. Structures 1 and 2 were solved by direct methods
[SHELXS, SHELXTL-PLUS program package, Sheldrick (1990)].
Initial refinements of 2 indicated considerable error in the data collection
and absorption correction. Since the crystal quality was questionable
and crystal selection was limited, it was decided to attempt to correct
the absorption problems of the available data set with DIFABS.11 The
DIFABS correction contrived a reasonable data set, and isotropic
refinement produced R ) 20% at convergence. It was then decided to
attempt the anisotropic refinement. Unfortunately the refinement proved
unstable and unusual metric parameters were seen. It was decided to
restrain the bond distances in the phenyl rings to idealize values and
to restrain the thermal parameters to isotropic behavior. The resulting
model was then compared to its isomorphic analog (see the structure
of 1) for any obvious deviations. Full-matrix least-squares anisotropic
refinement for all non-hydrogen atoms yielded [I > 2σI] R(F) ) 0.079,
Rw(F2) ) 0.139, and S(F2) ) 1.02 for 1 and R(F) ) 0.131, Rw(F2) )
0.287, and S(F2) ) 1.05 for 2 at convergence [Sheldrick, 1993].
Hydrogen atoms were placed in idealized positions with isotropic
thermal parameters fixed at -1.2B(adjacent carbon atom). Near the
final stages of refinement for 1 and 2 an included half molecule of
THF was located in an e density map, and its coordinates were added
to the overall atom lists. Neutral atom scattering factors and anomalous
scattering correction terms were taken from the International Tables
for X-ray Crystallography, Vol. C.
Synthesis of CuCN(PPh3)3. In a typical experiment, cuprous
cyanide (0.13 g, 1.4 mmol) and three equiv of triphenylphosphine (1.12
g, 4.3 mmol) were loaded into a 50 mL Schlenk flask in an inert-
atmosphere drybox. Tetrahydrofuran (30 mL) was added via syringe,
and the cloudy solution was stirred until all CuCN dissolved and the
solution was completely clear.
Synthesis of 3, (CO)5W(µ-NC)Cu(PPh3)3. Tungsten hexacarbonyl
(1.4 mmol) in 65 mL of THF was irradiated with UV light in a quartz
photolysis vessel for 45 min yielding an orange solution. This solution
of W(CO)5(THF) was transferred via cannula into an inert, 100 mL
Schlenk flask and cooled to -78 °C. CuCN (1.4 mmol) and 3 equiv
of PPh3 (4.2 mmol) were placed in a 50 mL Schlenk flask in an inert-
atmosphere drybox, and THF (15 mL) was added via syringe. This
slurry was stirred until all CuCN had gone into solution and then was
cooled to -78 °C and transferred via cannula to the flask containing
W(CO)5(THF). The resulting solution was stirred at -78 °C for 2 h
and then allowed to come to room temperature. After overnight stirring,
the solution was a clear yellow, and partial rearrangement of the CN
bridge had occurred forming 1, as evidenced by 13C NMR. However,
in the solution a substantial amount of (CO)5W(NC)Cu(PPh3)3 (3)
remained. IR (THF, -78 °C): 2122 cm-1 (ν(CN)), 2059 cm-1 (A1),
1929 cm-1 (E), 1897 cm-1 (A1). 13C{1H} NMR (THF-d8): δ 161 (s,
sharp, CN).
X-ray Crystallographic Study of 1 and 2. Crystal data and details
of data collection are given in Table 1. A colorless plate for both 1
and 2 was mounted on a glass fiber with epoxy cement at room
temperature and cooled to (193 K for 1 and 163 K for 2) in a cold N2
stream. Preliminary examination and data collection were performed
on a Siemens R3m/V X-ray diffractometer for 1 and a Rigaku AFC5R
for 2 (oriented graphite monochromator; Mo KR λ ) 0.710 73 Å for
both 1 and 2). Cell parameters were calculated from the least-squares
fitting of the setting angles for 25 reflections. ω scans for several
intense reflections indicate acceptable crystal quality. Data for 1 were
collected for 5.0° e 2θ e 50.0° [θ-2θ scans, -24 e h e 0, -14 e
k e 0, -26 e l e 26] at 193 K. Scan width for the data collection for
1 was 2.00° with a variable scan speed from 2.00 to 15.00°/min in ω.
Data for 2 were collected for 4.0° e 2θ e 50.0° at 163 K. Scan width
for the data collection for 2 was [1.60 + 0.3 tan(θ)]° with a variable
scan rate from 4.00 to 16.00°/min in ω. Three control reflections
(collected every 97 reflections for 1 and 150 reflections for 2) showed
13C NMR Spectroscopy. All spectra were recorded on a Varian
XL-200E FT-NMR spectrometer (50 MHz 13C).
A
13C-labeled
analogue of 1 was prepared as above using W(13CO)(CO)5 synthesized
by stirring W(CO)5(THF) under an atmosphere of 13CO for several
hours. This labeled hexacarbonyl was used to prepare NaW(CO)5CN
with 13C label statistically distributed between CO and CN groups. The
labeled complex 1* (100 mg) was dissolved in THF-d8 (1 mL) in a 5
mm quartz NMR tube in an inert-atmosphere drybox. Spectra were
recorded every 20 °C from -80 to +20 °C using liquid N2 as coolant.
The line width at half-height of the 13CN peak was recorded at each
temperature.
13C-labeled Na(CO)5WCN (100 mg) was added to acetone-d6 (1 mL),
and the resulting slurry was filtered into a 5 mm quartz NMR tube,
(9) Heckel, E.; German Patent 1,230,025; Chem. Abstr. 1967, 66, 46487e.
(10) King, R. B. Inorg. Chem. 1967, 6, 25.
(11) Walker, N.; Straut, D. Acta Crystallogr. 1983, A39, 158.

4766 Inorganic Chemistry, Vol. 35, No. 16, 1996
Darensbourg et al.
giving a clear, slightly yellow solution. Cuprous [13C]cyanide tris-
(triphenylphosphine) was prepared from Cu13CN in the same manner
as the nonlabeled compound (Vide infra). Cu13CN(PPh3)3 (100 mg)
was dissolved in CDCl3 (1 mL) in a 5 mm quartz NMR tube in an
inert-atmosphere drybox. Spectra for both of these compounds were
recorded at room temperature, and the line width at half-height of the
13CN peak was recorded.
A 13C-labeled analogue of 3 was prepared as above by substituting
Cu13CN for CuCN. After photolysis of W(CO)6 (0.085 mmol), the
THF was evacuated from the light orange solution of W(CO)5(THF),
resulting in a brown powder. THF-d8 (2 mL) was then added via
syringe, yielding a dark brown solution, which was cooled to -78 °C.
Cuprous [13C]cyanide (0.085 mmol) and PPh3 (0.25 mmol) were placed
in a 5 mm quartz NMR tube in an inert-atmosphere drybox and cooled
to -78 °C. The dark brown solution of W(CO)5(THF) was transferred
via cannula to the NMR tube, and spectra were recorded every 20 °C
from -60 to +40 °C. The line width at half-height of the 13CN peak
was recorded at each temperature. After the spectrum was recorded at
40 °C, the solution was again cooled to -60 °C and a second spectrum
was recorded and found to be identical to the first.
Figure 1. Thermal ellipsoid drawing of 1 showing the atom numbering
scheme. Thermal ellipsoids are drawn at the 50% probability level.
Results
This in-situ-formed intermediate exists in a strongly bonded,
contact-ion-pair form. After addition of triphenylphosphine, the
CN stretching frequency shifts back to lower energy at 2121
and 2117 cm-1 for 1 and 2, respectively. This is indicative of
a return to a more solvent-separated form of the bridged
assembly. Precedence exists for this type of behavior in crystal
structures of tris(triphenylphosphine)copper (I) complexes with
noninteracting or very weakly interacting counterions (cf.
Synthesis and Infrared Spectroscopy. The cyanide-bridged
compounds were prepared using sodium cyanopentacarbonyl
metalates as cyanide group donors (eq 1). When the group 6
PPh3
Na(CO)5MCN + Cu(CH3CN)4BF4 THF8
(CO)5M-CN-Cu(PPh3)3 (1)
14
Saturnino’s (Ph3P)3Cu‚‚‚Cl-FeCl3 and references therein).
Also, copper(I), having a completely filled 3d shell will have
labile metal-L bonds with most ligands. The isocyanide
coordination motif presents a relatively hard N-donor atom to
the soft copper(I) ion, making this bond even more labile than
most Cu-L bonds.
The linkage isomer of 1, the so-called “isocyanide”-bridged
compound, 3, was prepared from W(CO)5(THF) using CuCN-
(PPh3)3 as cyanide group donor (eq 2). This reaction was carried
sodium cyanopentacarbonyl metalates were added to copper
acetonitrile tetrafluoroborate, a rapid reaction ensued which
could be followed visually by the precipitation of NaBF4. The
reactions proceeded rapidly to completion, and before dimer-
ization around the copper ion could take place, a 2-fold molar
excess of triphenylphosphine was added to the reaction mixture.
Interestingly, the crystals which grew for both compounds
contained three triphenylphosphine moieties (Vide infra). The
addition of triphenylphosphine did not result in any visible
changes in the reaction mixture but was evidenced in the infrared
spectra by the appearance of the triphenylphosphine bands at
1435 and 1481 cm-1 and by the shift to lower energy of the
cyanide stretch from 2132 to 2121 cm-1 for 1 and from 2129
to 2117 cm-1 for 2.
Infrared spectroscopy proved to be the ideal method for
monitoring the progress of these reactions. The CN stretching
frequency of all compounds was a gauge used to determine
whether coordination had taken place. The infrared spectral
patterns of cyano carbonylates have been analyzed thoroughly,
and the effects on ν(CN) due to coordination by cations and
Lewis acids are well-known.12
(CO)5W(THF) + CuCN(PPh3)3 f (CO)5WNCCu(PPh3)3
(2)
out at -78 °C to avoid possible rearrangement processes leading
to 1. Addition of 1 equiv of preformed CuCN(PPh3)3 to the
W(CO)5(THF) compound resulted in a gradual change of the
solution from light orange to light yellow. An infrared spectrum
taken after 2 h at -78 °C revealed only slight changes from
the IR of 1. The cyanide stretching band is essentially unmoved
at 2122 cm-1 and the higher frequency A1 and E ν(CO) bands
are unchanged. The only significant difference from the IR
spectrum of 1 is that the lower frequency A1 band has shifted
The ν(CN) in THF solution of Na(CO)5WCN is found at 2121
cm-1. THF is a fairly polar solvent (dielectric constant D )
7.4), and therefore this compound exists in the solvent-separated
ion-pair form.12 Upon reaction with Cu(CH3CN)4BF4, the
from 1904 to 1897 cm-1
. This band derives most of its
character from the CO trans to the cyanide group. When the
isocyanide coordination motif is presented to tungsten, a
decrease in back-bonding relative to the cyanide motif results.
Thus, excess electron density on the tungsten is donated to the
CO group trans to the cyanide, lowering its bond order and
ν(CO). Similar IR behavior was noted also in Vahrenkamp’s5
linkage isomers of (CO)5Cr[CN]Fe(dppe)Cp.
cyanide stretching band shifts to higher energy at 2132 cm-1
.
This is indicative of coordination by a Lewis acid. Rather than
pulling electron density away from the group 6 metal through
the cyanide bridge, which would decrease ν(CN), a Lewis acid
allows for cyanide nitrogen lone pair coordination. The electron
pairs of both atoms in the CtN moiety are antibonding with
respect to the CtN bond.13 Hence, the C-N bond order and,
accordingly, the ν(CN) frequency increase with increasing
donation of these lone pairs.
X-ray Crystallography. Crystals suitable for an X-ray
structure analysis were obtained via slow diffusion of hexane
into a concentrated tetrahydrofuran solution of the dinuclear
complex at -20 °C for both 1 and 2. Figure 1 shows a thermal
ellipsoid drawing of complex 1, and Figure 2 shows a thermal
(12) Darensbourg, M. Y.; Barros, H. L. C. Inorg. Chem. 1979, 18, 3286.
(13) Purcell, K. F. J. Am. Chem. Soc. 1967, 89, 247, 1639.
(14) Saturnino, D. J.; Arif, A. M. Inorg. Chem., 1993, 32, 4157.

Cyanide-Bridged Heterobimetallic Complexes
Inorganic Chemistry, Vol. 35, No. 16, 1996 4767
Figure 2. Thermal ellipsoid drawing of 2 showing the atom-numbering
scheme. Thermal ellipsoids are drawn at the 50% probability level.
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for 1a
N(1)-C(6)
Cu(1)-P(1)
Cu(1)-P(3)
W(1)-C(1)
W(1)-C(3)
W(1)-C(5)
1.15(2)
2.349(6)
2.341(5)
2.01(2)
2.03(2)
2.05(2)
Cu(1)-N(1)
Cu(1)-P(2)
W(1)-C(6)
W(1)-C(2)
W(1)-C(4)
1.998(12)
2.326(5)
2.18(2)
2.00(3)
1.96(3)
C(6)-N(1)-Cu(1)
P(1)-Cu(1)-N(1)
P(3)-Cu(1)-N(1)
P(1)-Cu(1)-P(3)
C(1)-W(1)-C(6)
C(3)-W(1)-C(6)
C(5)-W(1)-C(6)
176(2)
96.2(4)
108.7(4)
112.1(2)
179.2(8)
90.1(6)
N(1)-C(6)-W(1)
P(2)-Cu(1)-N(1)
P(1)-Cu(1)-P(2)
P(2)-Cu(1)-P(3)
C(2)-W(1)-C(6)
C(4)-W(1)-C(6)
175.9(14)
109.6(4)
119.2(2)
109.6(4)
90.3(7)
87.5(8)
89.6(6)
a Estimated standard deviations are given in parentheses.
Figure 3. 50 MHz dynamic 13C NMR spectra of 1 obtained from a
solution of the complex in THF-d8. Numbers indicate temperatures (°C)
at which spectra were obtained.
Table 3. Selected Bond Lengths (Å) and Bond Angles (deg) for 2a
N(1)-C(6)
Cu(1)-P(1)
Cu(1)-P(3)
Cr(1)-C(1)
Cr(1)-C(3)
Cr(1)-C(5)
1.19(2)
2.319(6)
2.328(6)
1.78(3)
1.83(2)
1.76(2)
Cu(1)-N(1)
Cu(1)-P(2)
Cr(1)-C(6)
Cr(1)-C(2)
Cr(1)-C(4)
1.99(2)
2.329(6)
1.97(2)
1.81(2)
1.80(2)
equivalent Cu-P bond lengths (average 2.339[5] Å for 1 and
2.325[6] Å for 2) are as expected for complexes containing three
triphenylphosphine ligands. In both 1 and 2, the group 6 metal
ion is complexed in an almost ideal octahedron. The large
standard deviations in the M-C bond lengths and M-C-O
angles preclude any detailed discussion beyond this.
C(6)-N(1)-Cu(1)
P(1)-Cu(1)-N(1)
P(3)-Cu(1)-N(1)
P(1)-Cu(1)-P(3)
C(1)-Cr(1)-C(6)
C(3)-Cr(1)-C(6)
C(5)-Cr(1)-C(6)
173(2)
N(1)-C(6)-Cr(1)
P(2)-Cu(1)-N(1)
P(1)-Cu(1)-P(2)
P(2)-Cu(1)-P(3)
C(2)-Cr(1)-C(6)
C(4)-Cr(1)-C(6)
178(2)
110.5(5)
95.8(5)
119.2(2)
90.8(10)
87.7(8)
89.1(8)
108.2(5)
108.9(2)
113.1(2)
92.3(9)
The W-C(6) distance in 1 is 2.18(2) Å, whereas, the N(1)-
C(6) bond length is 1.15(2) Å. The latter value is identical to
that of free, gaseous HCN (CtN ) 1.156 Å) and is in the range
of most other gaseous cyanides.16 The infrared CN stretching
frequency of free HCN is 2089 cm-1, while the ν(CN) of 1
appears at 2121 cm-1. This difference is indicative of the fact
that, in 1, both the carbon and nitrogen lone pairs of the cyanide
moiety are complexed. Both of these electron pairs are
antibonding with respect to the CtN bond and thus, when
complexed, will increase the frequency of the CN stretching
band. The cyanide bridge in 1 is very slightly bent with angles
of W-C(6)-N(1) ) 175.9(14)° and Cu-N(1)-C(6) ) 176-
(2)°, whereas, the corresponding values in the 2 are 178(2) and
173(2)°, respectively. These slight deviations from linearity are
not statistically compelling because of the significant errors
associated with them.
13C NMR Experiments. 13C{1H} NMR spectra of 1 were
recorded every 20 °C from -80 to +20 °C. Figure 3 is a stack
plot of the NMR spectra obtained at each temperature. Table
4 contains a listing of the 13CN peak line widths at each
temperature as well as the W-C coupling constants and peak
positions. The line width decreased between -80 and -40 °C
and reached a minimum of 11 Hz at -40 °C. Tungsten satellites
179.0(8)
a Estimated standard deviations are given in parentheses.
ellipsoid drawing of 2. Selected bond distances and angles are
provided for 1 in Table 2 and for 2 in Table 3. Because of the
poorer quality of the data obtained on complex 2, discussion of
metric data will focus on complex 1. Nevertheless, complex 2
is isomorphous with 1 and displays no exceptional bond
distances or angles.
The fact that the copper ions in 1 and 2 are four-coordinate
was surprising since only 2 equiv of phosphine was present in
the solutions from which these molecules crystallized. Indeed,
tris(triphenylphosphine) complexes of copper(I) are rare. How-
ever, the extra stability afforded by tetrahedral coordination can
evidently outweigh steric interactions among the phosphine
phenyl rings.15 The three triphenylphosphine ligands and the
cyanide moiety coordinate to the copper(I) ion in a slightly
distorted tetrahedral geometry in both 1 and 2. The nearly
(15) For an example of a greatly hindered tris(triphenylphosphine) complex
see: Darensbourg, D. J.; Holtcamp, M. W.; Khandelwal, B.; Klaus-
meyer, K. K.; Reibenspies, J. H. Inorg. Chem. 1995, 34, 2389.
(16) Britton, D. Perspect. Struct. Chem. 1967, 1, 109.

4768 Inorganic Chemistry, Vol. 35, No. 16, 1996
Darensbourg et al.
Table 4. Variable-Temperature 13C NMR Data for the 13CN Peak
of 1
13CN peak
line width at
temp, °C
position, ppm
JW-C, Hz
half-height, Hz
-80
-60
-40
-20
0
149.0
149.0
149.1
149.1
148.9
147.8
96
94
96
94
a
13
12
11
15
43
47
20
a
a 183W satellites were not resolved at this temperature.
The line width of the 13CN peak for 1 at room temperature
is 47 Hz. The line width of the 13CN peak for Na(CO)5WCN,
however, is only 2 Hz. Therefore, coupling to the 14N (I ) 1)
quadrupole does not provide an efficient pathway for rapid
relaxation of the perturbed 13C nucleus. In Cu13CN(PPh3)3, the
line width of the 13CN peak is 15 Hz. Copper quadrupolar
broadening is thus somewhat efficient for this complex at room
temperature. However, in 1, the cyanide carbon is separated
from the copper nucleus by an additional bond, and this will
greatly decrease the amount of coupling between the 13C and
Cu nuclei. So, in 1, the mechanism of quadrupolar broadening
should be negligible and definitely less efficient than that in
CuCN(PPh3)3.
One of the two equilibria shown in eq 3 is therefore assumed
to be the dominant mechanism which gives rise to line
broadening of the CN peak in the 13C NMR. Infrared
spectroscopy supports the ion-pair exchange interaction. As
triphenylphosphine is added to the intermediate [(CO)5W-CN-
Cu], the v(CN) band shifts to lower energy. Because this shift
is not due to an increase in the amount of π-back-bonding, it
must indicate that one of the CN lone pairs of electrons is being
held more tightly by the CN moiety. Statistically, then, the
Cu-N σ-bond is weaker after addition of PPh3 to the reaction
mixture.
are evident in the spectra from -80 to -20 °C with JW-CN
≈
96 Hz at every temperature. Above -20 °C, the 13CN peak
broadened rapidly and no 183W satellites were resolved. This
temperature-dependent behavior is reproducible over time with
the same sample. There is also a small peak evident at ca. 143.5
ppm which is possibly due to trace quantities of (CO)5WCNCu-
(PPh3)2.
The isocyanide-bridged complex, 3, was prepared from 13C-
labeled CuCN in an NMR tube at -78 °C, and spectra were
recorded every 20 °C from -60 to +40 °C. Two peaks are
evident in the CN region of the spectrum. One, at 152 ppm, is
possibly due to the presence of slight amounts of unreacted Cu-
(13CN)(PPh3)x. Quantum yield restrictions ensure that not all
W(CO)6 reacts by photolysis, so it is not unusual for small
amounts of Cu13CN(PPh3)x to be present in the reaction mixture.
Alternatively, this peak could be due to the formation of a
dimeric structure such as 4. Fehlhammer et al. reported the
The 13C NMR experiments, however, were ambiguous. The
chemical shift of the solvent-separated complex was taken as
the chemical shift for Na(CO)5WCN. In THF, this complex
exists in the solvent-separated ion-pair form.12 The 13C NMR
spectrum was obtained in THF-d8, and the chemical shift of
the CN peak was 136.4 ppm. The chemical shift of the contact-
ion-pair form of the complex was taken as the chemical shift
of the cyanide carbon of 1 at -60 °C, which is 149.0 ppm.
The 13C NMR experiments carried out on 1 reveal behavior
which is typical for systems that undergo an intermolecular
exchange reaction such as the ones shown in eq 3. As the
temperature was raised from -60 °C, the line began to broaden
slightly. Between -20 and 0 °C, sufficient kinetic energy was
available to the system to shift the equilibria of eq 3 far enough
to the right for the product to be observable on the NMR time
scale. As the temperature was raised past 0 °C, these equilibria
continued to shift toward product. Because there are two species
present in the solution, as the equilibrium shifts to the right, a
concomitant movement of the observed peak should result. At
20 °C, this begins to be apparent as the CN peak shifts upfield
to 147.8 ppm.
formation of a complex of similar structure17 (shown as 5) upon
the addition of 4 equiv of Na(CO)5CrCN to CuSO4 in pH ) 1
water. However, the formation of 4 is unlikely here since 3
equiv of PPh3 was present in the solution. The cyanide peak
of complex 3 is found in the NMR spectrum at 161 ppm. This
peak is downfield from that of complex 1, is much less broad,
and, as expected, shows no 183W satellites. The lack of spin-
spin coupling in this peak was used to assign it positively to
the cyanide carbon of complex 3.
Discussion
The cyanide peak in the 13C NMR spectrum of 1 was
extremely broad at room temperature (fwhh ) 47 Hz). Several
exchange mechanisms which could account for the observed
1
line broadening are plausible. Nuclei with I > /2 possess an
electric quadrupole moment which can interact with the electric
field gradients on the nuclei of interest.18 This provides an
additional pathway for relaxation of the NMR nucleus and can
result in line broadening. A second mechanism, the contact
ion pair/solvent-separated ion pair equilibrium, is shown graphi-
cally in eq 3. Also shown in eq 3 is another conceivable
mechanism, dissociation of a phosphine ligand from Cu(I),
affording a three-coordinate copper center.
It is unclear which equilibrium process is broadening the 13CN
peak in the 13C NMR. It is possible that the small peak at 143.5
ppm is (CO)5WCNCu(PPh3)2 and the line broadening is brought
about by phosphine dissociation/recoordination. Alternatively,
the peak at 143.5 ppm could be a small amount of unknown
impurity with the line broadening brought on by Cu(I)-NC
bond breakage/re-formation. The experiments performed to date
are not definitive and do not allow differentiation between these
two scenarios.
(17) Fritz, M.; Rieger, D.; Ba¨r, E.; Beck, G.; Fuchs, J.; Holzmann, G.;
Fehlhammer, W. P. Inorg. Chim. Acta 1992, 198, 513.
(18) Gu¨nther, H. NMR Spectroscopy. An Introduction; John Wiley and
Sons: Chichester, U.K., 1980; pp 9-11, 278-9.
The variable-temperature 13C NMR experiment performed on
complex 3 revealed the CN peak to be at approximately 161

Cyanide-Bridged Heterobimetallic Complexes
Inorganic Chemistry, Vol. 35, No. 16, 1996 4769
ppm at every temperature. Because 3 contains an “isocyanide”
bridge, the electronic environment around the cyanide carbon
will be completely different from that in 1. Copper(I) is a poor
π-donor and thus will not back-bond to an appreciable extent.
Isocyanide is a poorer π-acceptor than cyanide, and so the
tungsten will also not be able to donate much electron density
through π-back-bonding. The M-cyanide σ-bonds in 3 are
apparently weaker than the σ-bonds in 1 due to the fact that
the ν(CN) IR band of 3 is the same as that of 1. The lack of
back-bonding to the cyanide moiety in 3 is completely balanced
by the weaker donation of the N and C lone pairs. Hence, the
same amount of electron density is in cyanide π*-orbitals in 3
and in 1.
The fact that the line width of the cyanide peak is so narrow
in 3 is explained by the fact that there is little or no ion-pair
exchange taking place in this complex. The Cu-C bond will
be relatively strong because the carbon end of the cyanide ligand
is a much softer base and also is much more unstable when
free. Tungsten is still able to π-donate somewhat to the
isocyanide moiety so the W-N bond will also be fairly strong.
The line width of the CN peak of 3 did not change from -60
to +40 °C, indicating a lack of quadrupolar coupling. This is
somewhat unusual in that Cu13CN(PPh3)3 does exhibit quadru-
polar broadening at room temperature. Apparently, the weaker
donation of the lone pairs in 3 is sufficient to effectively
decouple the quadrupolar copper nucleus from the cyanide
carbon.
cyanide group. Also, the Cu-N bond in 1 is labile, which
would permit facile rearrangement of the cyanide bridge. It is
tungsten, therefore, that determines the bonding arrangement
in this particular set of isomers. The increased π-back-bonding
capability of tungsten in isomer 1 leads to a very strong W-C
bond. This strong bond prevents rearrangement of 1 from taking
place once formed. In this way, chance rearrangements of the
cyanide bridge in complex 3 are trapped as isomer 1, and the
thermodynamic product is built up over time.
Conclusion
The syntheses of the linkage isomers of a tungsten cyanide-
bridged assembly have been performed. Also completed was
the synthesis of the chromium analogue of the cyanide-bridged
compound. The solid-state structures of both cyanide-bridged
compounds were confirmed by X-ray structure analysis, and
they were found to be quite similar. The structure of the
tungsten isocyanide-bridged complex was deduced by IR and
13C NMR studies, and this complex was found to be thermally
unstable in THF. The cyanide-bridged isomer of the tungsten
system was thermodynamically preferred due to the increased
ability of C-bound cyanide to accept electron density from
tungsten.
Acknowledgment. Financial support of this research by the
National Science Foundation (Grant CHE91-19737) and the
Robert A. Welch Foundation is greatly appreciated.
In a tetrahydrofuran solution at room temperature, 3 under-
goes rearrangement of the isocyanide bridge to form 1. This
reaction is evidently slow since no rearrangement occurred
during the NMR study of 3 even up to 40 °C. The fact that 1
is the thermodynamic product of both synthetic reactions is
somewhat unusual in that the hard/soft acid-base theory would
predict the soft copper(I) to prefer the softer carbon end of the
Supporting Information Available: Tables of crystal data and
structure refinement details, atomic coordinates and equivalent isotropic
displacement parameters, anisotropic thermal parameters, and complete
bond lengths and angles as well as ball-and-stick drawings for
complexes 1 and 2 (18 pages). Ordering information is given on any
current masthead page.
IC9510627