Heteronuclear chains of four metal atoms including one quadruply bonded dimetal unit. Dichromium and dimolybdenum compounds with appended copper(I) atoms

Article abstract of DOI:10.1021/ic991274u

It is shown that the M₂(DPhIP)₄ molecules (M = Cr, Mo; HDPhIP = 2,6-di(phenylimino)piperidine) can each capture two Cu(I) atoms to form molecules with linear Cu···M-M···Cu chains. In these chains the Cu(I) atoms have only weak interactions with the M atoms (Cr···Cu = 2.628 A; Mo···Cu = 2.615 A) even though their introduction causes marked decreases in the M-M distances (from 2.265 to 1.906 A for Cr and from 2.114 to 2.078 A, for Mo). These seemingly contradictory facts are explained by noting that in the M₂(DPhIP)₄ molecules there are strong destabilizing (to the M-M bonds) donor interactions of the dangling nitrogen atoms with the π* orbitals. When these are eliminated by the introduction of the Cu(I) atoms, the M-M bonds become stronger and shorter.

Full text of DOI:10.1021/ic991274u

Inorg. Chem. 2000, 39, 1743-1747
1743
Heteronuclear Chains of Four Metal Atoms Including One Quadruply Bonded Dimetal
Unit. Dichromium and Dimolybdenum Compounds with Appended Copper(I) Atoms
F. Albert Cotton,*,† Carlos A. Murillo,*,†,‡ Lindsay E. Roy,† and Hong-Cai Zhou†
The Laboratory for Molecular Structure and Bonding, Department of Chemistry, P.O. Box 30012,
Texas A&M University, College Station, Texas 77842-3012, and Department of Chemistry,
Universidad de Costa Rica, Ciudad Universitaria, Costa Rica
ReceiVed October 29, 1999
It is shown that the M2(DPhIP)4 molecules (M ) Cr, Mo; HDPhIP ) 2,6-di(phenylimino)piperidine) can each
capture two CuI atoms to form molecules with linear Cu‚‚‚M&M‚‚‚Cu chains. In these chains the CuI atoms
have only weak interactions with the M atoms (Cr‚‚‚Cu ) 2.628 Å; Mo‚‚‚Cu ) 2.615 Å) even though their
introduction causes marked decreases in the M&M distances (from 2.265 to 1.906 Å for Cr and from 2.114 to
2.078 Å for Mo). These seemingly contradictory facts are explained by noting that in the M2(DPhIP)4 molecules
there are strong destabilizing (to the M&M bonds) donor interactions of the dangling nitrogen atoms with the π*
orbitals. When these are eliminated by the introduction of the CuI atoms, the M&M bonds become stronger and
shorter.
Introduction
Several strategies exist for making molecules that have three
or more metal atoms lined up in direct contact with one another.
One of these is the all-in-one fell swoop (ofs) strategy whereby
one simply combines several (commonly four) ligands with high
enough denticity to hold n metal atoms in place with enough
of a metal reagent to supply n metal atoms, and the derived
L4MnX2 type molecule is obtained directly. One example of
this is the preparation of Cr5(tpda)4Cl21 (eq 1), and even longer
chains have been made in this way.2
reporting here some examples of the latter type of chemistry.
The ligand used in this work is the anion of 2,6-di(phenylimino)-
piperidine (I) for which we used the acronym DPhIP (pro-
nounced deefip). The preparation of this ligand and the
dichromium complex, Cr2(DPhIP)4 (1), has been reported
earlier.6 We now report the molybdenum homologue of 1, Mo2-
(DPhIP)4 (2), and the reaction products of 1 and 2 with CuCl,
namely, [Cr2Cu2(DPhIP)4(CH3CN)](CuCl2)2‚2CH3CN, [3(CH3-
CN)]‚2CH3CN, [Mo2Cu2(DPhIP)4(CH3CN)](CuCl2)2‚2CH3CN,
and [4(CH3CN)]‚2CH3CN.
Another strategy is to employ ligands capable of binding n
metal atoms with a source of <n metal atoms, thereby obtaining
an isolable intermediate to which further metal atoms can later
be added. This step-by-step (sbs) strategy has already been used
(see, for example,3 the conversion of Cr2(dpa)4 to Cr3(dpa)4-
Cl2) to make homoatomic chains (eq 2)), but it has the potential
to make mixed metal chains in a controlled way.4,5 We are
Experimental Section
† Texas A&M University.
‡ Universidad de Costa Rica.
General Procedures. All syntheses and sample manipulations were
carried out under an atmosphere of nitrogen with standard Schlenk and
glovebox techniques. The compounds Mo(CO)6 and anhydrous CuCl
were purchased from Strem Chemicals and stored in a drybox under
argon. H(DPhIP) and Cr2(DPhIP)4 were synthesized following known
procedures.5 Infrared spectra were recorded from KBr pellets on a
Perkin-Elmer 16 PC FT-IR spectrometer. NMR spectra were recorded
on a Varian VXR-300 spectrometer. Elemental analyses were performed
by Canadian Microanalytical Services Ltd.; they were satisfactory.
(1) Cotton, F. A.; Daniels, L. M.; Lu, T.; Murillo, C. A.; Wang, X. J.
Chem. Soc., Dalton Trans. 1999, 517.
(2) Lai, S.-Y.; Lin, T.-W.; Chen, Y.-H.; Wang, C.-C.; Lee, G.-H.; Yang,
M.-H.; Leung, M.-K.; Peng, S.-M. J. Am. Chem. Soc. 1999, 121, 250.
(3) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Pascual, I. Inorg. Chem.
Commun. 1998, 1, 1.
(4) (a) Mashima, K.; Fukumoto, A.; Nakano, H.; Kaneda, Y.; Tani, K.;
Nakamura, A. J. Am. Chem. Soc. 1998, 120, 12151. (b) Mashima, K.;
Nakano, H.; Nakamura, A. J. Am. Chem. Soc. 1996, 118, 9803. (c)
Mashima, K.; Nakano, H.; Nakamura, A. J. Am. Chem. Soc. 1993,
115, 11632.
(5) Mashima, K.; Tanaka, M.; Tani, K. J. Am. Chem. Soc. 1997, 119,
4307.
(6) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Pascual, I.; Zhou, H.-C.
J. Am. Chem. Soc. 1999, 121, 6856.
10.1021/ic991274u CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/17/2000

1744 Inorganic Chemistry, Vol. 39, No. 8, 2000
Cotton et al.
Preparation of Mo2(DPhIP)4, 2. (a) Method A. The compound
H(DPhIP) (0.53 g, 2.0 mmol) was dissolved in THF (20 mL) and
deprotonated at -78 °C by methyllithium (1.0 M in THF, 2.0 mL).
The pale-yellow solution obtained was transferred to a flask containing
Mo2(O2CCH3)4 (0.216 g, 0.50 mmol), and the reaction mixture was
heated at reflux temperature overnight. After filtration, the orange
solution obtained was layered with hexanes. Red crystals of compound
2 and yellow crystals of Mo2(DPhIP)2(O2CCH3)2 7 were obtained after
a week. This preparation was not optimized. IR (KBr, cm-1) for
Mo2(DPhIP)2(O2CCH3)2: 1600 (s), 1498 (vs), 1477 (vs), 1450 (vs),
1437 (vs), 1423 (vs), 1345 (s), 1335 (s) 1316 (m), 1255 (m), 1215 (s),
1183 (s), 1152 (w), 1140 (w), 1108 (w), 1074 (w), 1057 (w), 1028
(w), 1001 (w), 980 (w), 921 (w), 861 (w), 805 (w), 782 (m), 762 (m),
715 (m), 695 (m), 671 (m), 630 (w), 593 (w), 516 (w).
(b) Method B. The solvent o-dichlorobenzene (25 mL) was degassed
using two freeze-pump-thaw cycles and transferred to a 100 mL flask
charged with Mo(CO)6 (1.34 g, 5.1 mmol) and H(DPhIP) (4.0 g, 15
mmol); hexane (3 mL) was then added to the mixture in order to wash
down Mo(CO)6 that sublimed at the beginning of the reaction. The
reaction mixture was heated overnight to 80 °C and then to reflux
temperature for 24 h. The resulting dark-red suspension was concen-
trated to ca. 5 mL and mixed with diethyl ether (20 mL). The crude
product was collected by filtration, washed with acetone (2 × 20 mL)
and ethanol (2 × 20 mL), and dried under vacuum overnight. Yield:
1.93 g (61%). Crystals suitable for X-ray crystallographic study were
grown from a CH2Cl2 solution of 2 layered with diethyl ether. 1H NMR,
CD2Cl2, δ (intensity, multiplicity, apparent J, assignment): 2.013 (8H,
quintet, 10.2 Hz, piperidine); 2.533 (8H, t, 10.2 Hz, piperidine); 2.821
(8H, t, 10.2 Hz, piperidine); 5.420 (8H, d, 11.1 Hz, phenyl); 6.594-
6.757 (16H, m, 11.2 Hz, phenyl); 6.859 (8H, t, 11.0 Hz, phenyl); 6.980
(8H, d, 11.7 Hz, phenyl). IR (KBr, cm-1): 1623 (s), 1594 (m), 1542
(w), 1499 (vs), 1419 (m), 1330 (m), 1314 (m), 1257 (w), 1216 (s) 1186
(m), 1097 (w), 1069 (w), 1025 (w), 930 (w), 900 (w), 853 (w), 806
(w), 779 (m), 762 (w), 698 (s), 598 (w), 506 (w).
Figure 1. Drawing of the molecular structure of compound 2, Mo2-
(DPhIP)4. Thermal ellipsoids are drawn at the 50% probability level;
hydrogen atoms are omitted for clarity. Axial π* interactions are
indicated by broken lines.
7.096 (8H, t, 7.4 Hz, phenyl); 7.191 (4H, t, 7.4 Hz, phenyl); 7.291
(8H, t, 7.2 Hz, phenyl); 7.399 (8H, d, 7.7 Hz, phenyl). IR (KBr, cm-1):
1591 (w), 1560 (w), 1549 (w), 1497 (vs), 1480 (s, sh), 1448 (m), 1389
(s), 1360 (s), 1342 (m), 1261 (w), 1218 (m), 1191 (s), 1155 (w), 1070
(w), 1026 (w), 1002 (w), 983 (w), 946 (w), 913 (w), 858 (w), 803 (m),
764 (m), 725 (m), 696 (m), 675 (w), 643 (w), 624 (w), 604 (w), 507
(w).
Crystallographic Studies. Data collection for 2 and [3(CH3CN)]‚
2CH3CN was carried out on a Nonius FAST area detector diffractometer
with each crystal mounted on the tip of a glass fiber under a stream of
nitrogen at -60 °C. Cell parameters were obtained by least-squares
refinement of 250 reflections ranging in 2θ from 18.1° to 41.6°. Laue
groups and centering conditions were confirmed by axial images. Data
were collected using 0.2° intervals in φ over the range 0 e æ e 220°
and 0.2° intervals in ω for two different regions in the range 0 e ω e
72°; in this way, nearly a full sphere of data were collected. The highly
redundant data sets were corrected for Lorentz and polarization effects
and for absorption. Data collection for [4(CH3CN)]‚2CH3CN was
performed on a Bruker Smart CCD diffractometer at -120 °C.
The positions of the metal atoms and sometimes the atoms of the
first coordination sphere were located from a direct-methods E-map;
other non-hydrogen atoms were found in alternating difference Fourier
syntheses and least-squares refinement cycles and, during the final
cycles, refined anisotropically. Hydrogen atoms were placed in
calculated positions.
A phenyl ring in 2, an interstitial CH3CN molecule in [3(CH3CN)]‚
2CH3CN, and a CuCl2- anion in [4(CH3CN)]‚2CH3CN are disordered.
Each of them was modeled as having two orientations; the occupancy
of each orientation was refined as an optimal fit determined by
SHELXL.
Crystallographic data for 2, [3(CH3CN)]‚2CH3CN, and [4(CH3CN)]‚
2CH3CN are given in Table 1; selected bond distances and angles for
2, [3(CH3CN)]‚2CH3CN, and [4(CH3CN)]‚2CH3CN are listed in Tables
2, 3 and 4, respectively. Drawings of molecular structures of compound
2 and the cation in 4 are given in Figures 1 and 2, respectively.
Preparation of [Cr2Cu2(DPhIP)4](CuCl2)2, 3. The yellow crystal-
line Cr2(DPhIP)4‚2THF(0.32 g, 0.25 mmol) was dissolved in THF (20
mL), CuCl (0.10 g, 1.0 mmol) was added through a solids addition
tube, and the mixture was heated at reflux temperature for 3 h. The
resulting brown powder was collected, washed with THF (2 × 10 mL),
and dried. Yield: 0.15 g (37%). The orange-yellow square-plate crystals
of [3(CH3CN)]‚2CH3CN suitable for crystallographic study were grown
1
by layering an acetonitrile solution of 3 with diethyl ether. H NMR,
CD3CN, δ (intensity, multiplicity, assignment): 2.611 (8H, t, piperi-
dine); 2.715 (8H, t, piperidine); 6.265 (8H, d, phenyl); 7.065-7.392
(32H, m, phenyl). IR (KBr, cm-1): 1580 (m), 1560 (m), 1537 (vs),
1517 (vs), 1491 (w), 1449 (w), 1437 (s), 1420 (w), 1405 (s), 1371 (s),
1345 (w), 1329 (w), 1263 (w), 1223 (s), 1195 (s), 1156 (w), 1071 (w),
1028 (w), 1004 (w), 972 (w), 948 (w), 916 (w), 867 (w), 803 (m), 764
(m), 722 (m), 697 (m), 607 (w), 510 (w).
Preparation of [Mo2Cu2(DPhIP)4](CuCl2)2, 4. Compound 2 (0.31
g, 0.25 mmol) was dissolved in CH2Cl2 (10 mL), and the dark-red
solution was transferred to another flask containing a CuCl (0.10 g,
1.0 mmol) suspension in CH2Cl2 (10 mL). The mixture was heated at
reflux temperature for 1 h to give an orange crystalline solid, 4, which
was collected by filtration, washed with CH2Cl2 (2 × 5 mL), and dried
under vacuum. Yield: 0.36 g (84%). Orange-red crystals of [4(CH3-
CN)]‚2CH3CN were obtained from a solution of 4 in acetonitrile that
was layered with diethyl ether. 1H NMR, CD2Cl2, δ (intensity,
multiplicity, apparent J, assignment): 2.095 ppm (8H, quintet, 6.6 Hz,
piperidine); 2.789 (8H, t, 6.6 Hz, piperidine); 3.093 (8H, t, 6.3 Hz,
piperidine); 6.240 (8H, d, 7.7 Hz, phenyl); 7.012 (4H, t, 7.4 Hz, phenyl);
Results and Discussion
Syntheses. A common approach to the preparation of
dimolybdenum complexes is reaction of Mo2(O2CCH3)4 and the
lithium salt of the corresponding ligand.8 When this synthetic
route was used for the DPhIP ligand, we found that a mixture
of Mo2(DPhIP)2(O2CCH3)2 and Mo2(DPhIP)4 formed. This was
clearly visible by NMR. Even though we were able to separate
a few crystals of each product, the yields were poor mainly
because the solubilities of both of these compounds are quite
similar in common organic solvents, making efficient separation
difficult. Fortunately, compound 2 can be prepared in high purity
and in good yield by an alternative route involving Mo(CO)6
(7) Crystal data for Mo2(DPhIP)2(O2CCH3)2, M ) 834.62. Triclinic, space
group P1h, a ) 8.791(1) Å, b ) 9.939(2) Å, c ) 10.690(2) Å, R )
69.43(1)°, â ) 85.65(1)°, γ ) 88.90(2)°, V ) 871.9(3) Å3, Dc ) 1.590
g/cm3, Z ) 1, µ ) 7.69 cm-1. Data were collected at 213(2) K on a
FAST area detector system. A total of 2737 unique reflections (2θ e
50°) were measured. Full-matrix least-squares refinement on F2
converges to R1 ) 0.039, wR2 ) 0.099 (all data). The structure
corresponds to a typical paddlewheel with four bridging ligands; two
DPhIP ions are trans to each other with a hanging arm at each end of
the MosMo bond. The MosMo distance is 2.0888(7) Å.
(8) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms,
2nd ed.; Oxford University Press: New York, 1993.

Dichromium and Dimolybdenum Compounds
Inorganic Chemistry, Vol. 39, No. 8, 2000 1745
Table 1. Crystal Data and Structure Refinement
complex
2
[3(CH3CN)]‚2CH3CN
[4(CH3CN)]‚2CH3CN
chemical formula
C68H64N12Mo2
1341.19
P21/c
14.405(4)
12.45(3)
32.786(8)
90
93.04(5)
90
5872(12)
4
C74H73N15Cl4Cu4Cr2
1672.43
P21/n
16.956(8)
17.409(3)
24.674(5)
90
100.52(2)
90
7161(4)
4
C74H73N15Cl4Cu4Mo2
1760.31
P21/n
17.0279(8)
17.4881(8)
24.672(1)
90
100.780(1)
90
7217.3(6)
4
fw
space group
a, Å
b, Å
c, Å
a, deg
â, deg
γ, deg
V, Å3
Z
data collection instrument
temp, K
radiation λ, Å
F(calcd), g cm-3
µ(Mo KR), cm-1
R1a,b/R1d
wR2b,c/wR2d
Nonius FAST
213(2)
0.710 73
1.404
4.81
0.044/0.052
0.103/0.113
Nonius FAST
213(2)
0.710 73
1.551
16.64
0.070/0.088
0.172/0.193
Smart CCD
153(2)
0.710 73
1.620
16.97
0.033/0.057
0.084/0.094
a R1 ) ∑(|Fo| - |Fc|)/∑|Fo|. b Denotes values of the residuals considering only the reflections with I > 2σ(I). c wR2 ) [∑[w(Fo - Fc2)2]/
2
∑[w(Fo )2]]1/2; w ) 1/[σ2(Fo ) + (aP)2 + bP], P ) [max(Fo or 0) + 2(Fc2)]/3. d Denotes value of the residual considering all the reflections.
2
2
2
Table 2. Selected Interatomic Distances (Å) and Torsional Angles (deg) for Mo2(DPhIP)4, 2
Mo(1)sMo(2)
Mo(1)sN(32)
Mo(1)sN(12)
Mo(1)sN(43)
Mo(1)sN(23)
2.114(1)
2.144(4)
2.157(4)
2.199(5)
2.124(3)
Mo(2)sN(22)
Mo(2)sN(42)
Mo(2)sN(33)
Mo(2)sN(13)
2.157(5)
2.168(5)
2.179(3)
2.196(4)
Mo(1)‚‚‚N(11)
Mo(1)‚‚‚N(31)
2.989(5)
2.998(5)
Mo(2)‚‚‚N(21)
Mo(2)‚‚‚N(41)
2.932(5)
3.085(5)
Mo(1)sN(12)sC(107)sN(11)
Mo(1)sN(32)sC(307)sN(31)
7.3(4)
3.0(4)
Mo(2)sN(22)sC(207)sN(21)
Mo(2)sN(42)sC(407)sN(41)
3.4(4)
4.1(4)
Table 3. Selected Interatomic Distances (Å) and Bond Angles
(deg) for [3(CH3CN)]‚2CH3CN
Cr(1)sCr(2)
Cr(1)sN(2)
Cr(1)sN(8)
Cr(1)sN(12)
Cr(1)sN(6)
Cr(1)‚‚‚Cu(1)
Cu(1)sN(1)
Cu(1)sN(7)
1.906(2)
2.052(5)
2.053(5)
2.072(5)
2.085(5)
2.628(2)
1.873(5)
1.868(5)
Cr(2)sN(11)
Cr(2)sN(5)
Cr(2)sN(9)
Cr(2)sN(3)
Cr(2)‚‚‚Cu(2)
Cu(2)sN(4)
Cu(2)sN(10)
Cu(2)sN(1S)
2.054(5)
2.058(5)
2.077(5)
2.090(5)
2.689(2)
1.906(5)
1.885(6)
2.26(1)
N(7)sCu(1)sN(1)
177.6(2) N(10)sCu(2)sN(4) 169.0(2)
94.1(4) N(4)sCu(2)sN(1S) 96.1(4)
N(10)sCu(2)sN(1S)
N(1S)sCu(2)sCr(2) 159.2(5)
Figure 2. Molecular structure of the cation of compound 4 in [Mo2-
Cu2(DPhIP)4(CH3CN)](CuCl2)2‚2CH3CN. Thermal ellipsoids are shown
at the 50% probability level; hydrogen atoms are omitted for clarity.
Compound 3 is isostructural.
Table 4. Selected Interatomic Distances (Å) and Bond Angles
(deg) for [4(CH3CN)]‚2CH3CN
Mo(1)sMo(2)
Mo(1)sN(32)
Mo(1)sN(12)
Mo(1)sN(43)
Mo(1)sN(23)
Mo(1)‚‚‚Cu(1)
Cu(1)sN(11)
Cu(1)sN(31)
Cu(2)sN(51)
2.0778(3)
2.132(2)
2.133(2)
2.178(2)
2.181(2)
2.6149(4)
1.904(2)
1.906(2)
2.139(3)
Mo(2)sN(22)
Mo(2)sN(42)
Mo(2)sN(33)
Mo(2)sN(13)
Mo(2)‚‚‚Cu(2)
Cu(2)sN(41)
Cu(2)sN(21)
2.135(2)
2.141(2)
2.199(2)
2.192(2)
2.7248(4)
1.978(2)
1.980(2)
Reactions of 2 or its chromium analogue (1) with stoichio-
metric amounts of CuCl produce the corresponding [M2Cu2-
(DPhIP)4](CuCl2)2 ionic species, M ) Cr (3) or Mo (4),
according to
∆
M2(DPhIP)4 + 4CuCl solvent8 [M2Cu2(DPhIP)4](CuCl2)2
N(11)sCu(1)sN(31) 175.80(9) N(41)sCu(2)sN(21) 161.73(9)
N(41)sCu(2)sN(51)
98.4(1) N(21)sCu(2)sN(51) 98.81(9)
N(51)sCu(2)sMo(2) 163.11(8)
The choice of the reaction solvent is important. For the
synthesis of 4, CH2Cl2 gives better yields than THF; the
solubility of the starting material 2 is significantly higher in
CH2Cl2 than in THF. Compound 4 can be purified by washing
with THF and is soluble in a variety of solvents such as CH3-
CN, acetone, and CH2Cl2.
The synthesis of 3 cannot be carried out efficiently in an
analogous way. Even though CH2Cl2 is a better solvent for the
starting material, the product appears to react with this solvent
at the reflux temperature necessary for the reaction to occur,
and a “dirty-green” suspension of an uncharacterized material
and the neutral H(DPhIP) compound in refluxing o-dichlo-
robenzene:
2Mo(CO)6 + 2H(DPhIP)4 o-dichlorobenzene8
Mo2(DPhIP)4 + 2H2 + 12CO
To ensure complete reaction, a slight excess of the ligand must
be present in solution. The excess ligand can be easily removed
from the resulting product by washing with acetone.

1746 Inorganic Chemistry, Vol. 39, No. 8, 2000
Cotton et al.
is obtained. Thus, THF is better as the reaction solvent, although
because of solubility factors, the yield of the product is less
than that of the molybdenum analogue, but the purity is
the Cu‚‚‚MosMo‚‚‚Cu chain, the Cu atoms are 2.6149(4) and
2.7248(4) Å from the dimolybdenum unit and the MosMo bond
length is 2.0778(3) Å, which is almost the same MosMo
distance as that found in Mo2(DPhAP)4. This again is an
indication of the removal of axial π* coordination and more
importantly no significant Cu‚‚‚Mo interaction.
1
excellent, as attested by the H NMR spectrum.
Axial π* Coordination vs MetalsMetal Communication.
In the two compounds 1 and 2, there are four DPhIP ligands
surrounding the quadruply bonded dimetal unit; these alternate
their directions around the MsM bond to form an S4 type
arrangement. Each ligand has a pendant arm, and the two
pendant arms of two of the DPhIP’s at each end of the molecule
are opposite to each other, thus providing possible bites for two
more metal atoms and the potential of forming a linear chain
of four metal atoms. This is indeed possible by addition of CuCl.
This raises the question as to whether there is any communica-
tion between the two axially placed metal atoms and the
quadruply bonded dimetal unit in such heteronuclear tetrametal
chains. A way to find out if this type of MsM communication
exists is to use a dichromium unit in the middle of the tetrametal
chain because CrsCr distances are known to be very sensitive
to axial coordination. In different dichromium complexes, the
CrsCr separations range from 2.689(3) Å 9 in compounds
having strong axial coordination to 1.828(2) Å 10 in those
without it. Even axial coordination from benzene molecules can
lengthen a CrsCr bond significantly.11 In our previous report,6
four imino nitrogen lone pairs in compound 1 were found to
donate electron density to the π* orbitals of a dichromium unit,
giving a CrsCr distance of 2.265(1) Å. For direct comparison,
the compound Cr2(PhIP)4 (PhIP is the anion of 2-(phenylimino)-
piperidine), in which there are no pendant arms, was made and
was found to have a significantly shorter CrsCr distance of
1.858(1) Å.
In compound 3, two Cu atoms are incorporated into the chain
where they occupy the two axial positions in 1, forming a
heteronuclear Cu‚‚‚CrsCr‚‚‚Cu chain. The four imino nitrogen
lone pairs used to provide π* donation in compound 1 are no
longer available because of CusN bond formation, and the Crs
Cr distance here is only 1.906(2) Å. This is much shorter than
the CrsCr distance of 2.265(1) Å in 1, and only 0.048(2) Å
longer than that of Cr2(PhIP)4. These data seem to suggest that
the Cu‚‚‚Cr contacts of 2.628(2) and 2.689(2) Å found in 3 are
too weak to affect significantly the CrsCr separation by axial
coordination.
CH3CN Molecule at an Axial Position. Since compounds
3 and 4 are isotypic, compound 4 will be used as a representative
for the following discussion. In compound 4, one axial CH3CN
molecule is coordinated to one of the two Cu atoms. The CH3-
CN-coordinated Cu has a T-shaped ligand arrangement, while
the other Cu atom is linearly coordinated. The rod-shaped CH3-
CN molecule is not aligned along the metal-metal vector; the
N(51)sCu(2)sMo(2) angle is 163.11(8)°. In contrast we have
found that in compound [Cr3(DPhIP)4(CH3CN)](PF6)2,13 the
CH3CN molecule is aligned with the three Cr atoms. The reason
for this peculiar arrangement of the CH3CN ligand in [4(CH3-
CN)]‚2CH3CN is the repulsion from the two opposite phenyl
rings above and below the axial ligand. The axial coordination
closes one end of the molecule but leaves the other one open.
This has implications in the metal-to-ligand distances; i.e., the
Mo(2)sN bond distances are longer than the Mo(1)sN separa-
tions (Table 4).
We have tentative crystallographic evidence that the axial
CH3CN molecule in compound [4(CH3CN)]‚2CH3CN can be
removed by crystallizing 4 from CH2Cl2 or acetone, but in each
case the molecules of 4 are highly disordered. They can be
modeled as the superposition of two oppositely orientated
molecules, but no satisfactory refinement of either structure14
has yet been achieved.
For compounds 2 and 4, we see a small decrease in the Mos
Mo distance (from 2.114 to 2.078 Å) for which the following
explanation is proposed. In 2, the MosMo quadruple bond is
slightly weakened by donation from the pendant nitrogen atoms
into the π* orbitals. On addition of the CuI atoms, these
interactions are abolished, and since the copper atoms do not
act as donors to the molybdenum atoms, the MosMo bond
length remains as short as one would expect for an unmolested
quadruple bond.
For compounds 1 and 3, we have a similar situation as with
2 and 4 except greatly exaggerated because the π* donation by
the dangling nitrogen atoms in 1 is extensive, lengthening the
CrsCr distance to 2.265 Å. When those interactions are
abolished by the introduction of the CuI atoms, which do not
themselves have any significant donor function, the CrsCr bond
contracts to 1.906 Å, which is appropriate for an unperturbed
quadruple bond.
Compound 2 is the molybdenum analogue of compound 1.
The average distance from the four imino nitrogen atoms to
the corresponding molybdenum atoms is 3.00 Å, and the average
direction angle6 (the MosNsCsN torsional angle, which
should be close to 0° in order to have efficient overlap between
the π* orbital and the nitrogen lone pair orbitals) is only 4.5°.
The MosMo bond length is 2.114(1) Å, which can be compared
In conclusion it is pertinent to make a comparison of our
results with those reported by Mashima et al.4,5 These workers
used the ligand 6-(diphenylphosphino)-2-pyridonate, pyphos.
Insofar as the Mo2(pyphos)4(PdCl2)2, Mo2(pyphos)4(PdBr2)2, and
Mo2(pyphos)4(PtCl2)2 compounds4b are concerned, the situation
is quite similar to what we found for [Mo2(DPhIP)4Cu2]2+. The
added metal atoms have very little interaction with the molyb-
denum atoms, and the Mo&Mo bond is left essentially
unperturbed. Mashima et al. were able to reduce the PdII and
PtII atoms to PdI and PtI, giving Mo2(pyphos)4(MX)2 molecules
in which MosPd and MosPt bonds were postulated, although
12
to the MosMo bond length of 2.0812(5) Å in Mo2(DPhAP)4
(DPhAP is the anion of 2,6-di(phenylamino)pyridine). The
DPhAP ligand is structurally very similar to DPhIP except that
the lone pair orbital is on an amino N and points away from
the π* orbitals of the dimolybdenum unit. Thus, the bond
lengthening by axial π* coordination in 2 is only 0.033(1) Å,
which is insignificant compared to the 0.406(2) Å bond
elongation in compound 1. This is in accord with the well-known
insensitivity of a MosMo bond to axial coordination.
Compound 4, which is the molybdenum analogue of 3, has
also been made and characterized by X-ray crystallography. In
(13) Cotton, F. A.; Murillo, C. A.; Zhou, H.-C. Unpublished results.
(14) Crystal data for 4 (crystallized from acetone): monoclinic, space group
C2/c, a ) 21.263(3) Å, b ) 19.087(4) Å, c ) 20.132(3) Å, â )
110.54(2)°, Z ) 4, V ) 7651(2) Å3. Crystal data for 4 (crystallized
from CH2Cl2): monoclinic, space group C2/c, a ) 27.301(4) Å, b )
16.600(3) Å, c ) 16.903(2) Å, â ) 98.388(9)°, Z ) 4, V ) 7578(4)
Å3. Data were collected at 213(2) K on a FAST area detector system.
(9) Cotton, F. A.; Murillo, C. A.; Zhou, H.-C. Inorg. Chem., submitted.
(10) Cotton, F. A.; Koch, S. A.; Millar, M. Inorg. Chem. 1978, 17, 2014.
(11) Cotton, F. A.; Feng, X.; Kibala, P. A.; Matusz, M. J. Am. Chem. Soc.
1988, 110, 2807.
(12) Cotton, F. A.; Murillo, C. A.; Lei, P.; Wang, X. Unpublished results.

Dichromium and Dimolybdenum Compounds
Inorganic Chemistry, Vol. 39, No. 8, 2000 1747
the MosMo distances increase very slightly (by ca. 0.02 Å)
while the MosPd and MosPt distances hardly change. With
[Mo2(DPhIP)4Cu2]2+ no comparable experiment has been done.
In the case of Cr2(pyphos)4(PtMe2)2,5 there is a marked
contrast with our work. Here, there are PtII atoms that interact
strongly with the Cr2 unit, increasing the CrsCr distance from
2.015 Å in Cr2(pyphos)4 to 2.389 Å, even though the CrsPt
distances are relatively long, 2.81 Å. This is in contrast to the
situation in [Cr2(DPhIP)4Cu2]2+ where the CrsCr distance is
only 1.906 Å even though the Cr‚‚‚Cu distance is 2.628 Å.
So far we do not have an entirely satisfactory explanation as
to why the interactions of quadruply bonded species with a first-
row closed-shell d10 metal atom are so different from those of
heavier d9 metal atoms having the same oxidation state or why
a PtMe2 unit can be such a good electron donor.
Acknowledgment. We are grateful to the National Science
Foundation for financial support.
Supporting Information Available: Four figures showing thermal
ellipsoids of 2, 3(CH3CN)‚2CH3CN, [Cu2Mo2(DPhIP)4(CH3CN)2+, and
Mo2(DPhIP)2(OAc)2. An X-ray crystallographic file, in CIF format. This
material is available free of charge via the Internet at http://pubs.acs.org.
IC991274U