Mixed chloride/amine complexes of dimolybdenum(II,II). 3. Preparation, characterization, and crystal structure of Mo2Cl4(NH2R)4 (R = Et, Prn, But, Cy): First quadruply-bonded dimolybdenum compounds with primary amine ligands

Article abstract of DOI:10.1021/ic981289c

Four new quadruply-bonded dimolybdenum(II) complexes of the formula Mo₂Cl₄(NH₂R)₄ (R = Et (1), Pr_n (2), Bu^t (3), Cy (4)) have been prepared in excellent yield by reduction of the dimolybdenum(III) complex Mo₂Cl₆-(THF)₃ with 2 equiv of sodium amalgam in the presence of the appropriate primary amine. The molecular structures of 2-4 have been investigated by X-ray crystallography. Crystal data are as follows: for 2, orthorhombic space group Ccca with a = 13.328(3) ?, b = 26.639(5) ?, c = 6.774(2) ?, and Z = 4; for 3, monoclinic space group P2₁/c with a = 19.165(1) ?, b = 20.858(1) ?, c = 14.1400(8) ?, β = 99.002(5)°, and Z = 8; for 4, tetragonal space group P4₂2₁2 with a = 15.556(4) ?, c = 6.9368(8) ?, and Z = 2. All of the non-centrosymmetric molecules 2-4 possess the same structure characterized by a Mo₂Cl₄N₄ core with D_2d virtual symmetry and slight deviation from the eclipsed geometry. The Mo-Mo bond lengths for 2, 3, and 4 are 2.118(2), 2.1322(6), and 2.117(1) ?, respectively, which are consistent with the Mo-Mo quadruple bond. In addition to the structural data, IR, UV-vis, and ¹H NMR spectroscopy have been used to characterize the complexes 1-4. Without amalgam, the reactions of the starting material with amines also produce the reduced species but with low yields (not exceeding 25%). The main products of these interactions have been found to be mononuclear molybdenum(III) complexes MoCl₃-(NH₂R)₃. This has been confirmed by a single-crystal X-ray diffraction study for mer-MoCl₃(NH₂Pr_n)₃·1/6THF (2a·1/6THF) with the following crystal data: triclinic space group P1?, a = 12.370(2) ?, b = 17.977(2) ?, c = 25.498(6) ?, α = 95.32(1)°, β= 103.21(2)°, γ = 103.48(1)°, and Z = 12.

Full text of DOI:10.1021/ic981289c

Inorg. Chem. 1999, 38, 2649-2654
2649
Mixed Chloride/Amine Complexes of Dimolybdenum(II,II). 3. Preparation,
Characterization, and Crystal Structure of Mo2Cl4(NH2R)4 (R ) Et, Prn, But, Cy): First
Quadruply-Bonded Dimolybdenum Compounds with Primary Amine Ligands
F. Albert Cotton,* Evgeny V. Dikarev, and Santiago Herrero
Department of Chemistry and Laboratory for Molecular Structure and Bonding,
P.O. Box 30012, Texas A&M University, College Station, Texas 77842-3012
ReceiVed NoVember 9, 1998
Four new quadruply-bonded dimolybdenum(II) complexes of the formula Mo2Cl4(NH2R)4 (R ) Et (1), Prn (2),
But (3), Cy (4)) have been prepared in excellent yield by reduction of the dimolybdenum(III) complex Mo2Cl6-
(THF)3 with 2 equiv of sodium amalgam in the presence of the appropriate primary amine. The molecular structures
of 2-4 have been investigated by X-ray crystallography. Crystal data are as follows: for 2, orthorhombic space
group Ccca with a ) 13.328(3) Å, b ) 26.639(5) Å, c ) 6.774(2) Å, and Z ) 4; for 3, monoclinic space group
P21/c with a ) 19.165(1) Å, b ) 20.858(1) Å, c ) 14.1400(8) Å, â ) 99.002(5)°, and Z ) 8; for 4, tetragonal
space group P42212 with a ) 15.556(4) Å, c ) 6.9368(8) Å, and Z ) 2. All of the non-centrosymmetric molecules
2-4 possess the same structure characterized by a Mo2Cl4N4 core with D2d virtual symmetry and slight deviation
from the eclipsed geometry. The Mo-Mo bond lengths for 2, 3, and 4 are 2.118(2), 2.1322(6), and 2.117(1) Å,
respectively, which are consistent with the Mo-Mo quadruple bond. In addition to the structural data, IR, UV-
vis, and 1H NMR spectroscopy have been used to characterize the complexes 1-4. Without amalgam, the reactions
of the starting material with amines also produce the reduced species but with low yields (not exceeding 25%).
The main products of these interactions have been found to be mononuclear molybdenum(III) complexes MoCl3-
(NH2R)3. This has been confirmed by a single-crystal X-ray diffraction study for mer-MoCl3(NH2Prn)3‚1/6THF
(2a‚1/6THF) with the following crystal data: triclinic space group P1h, a ) 12.370(2) Å, b ) 17.977(2) Å, c )
25.498(6) Å, R ) 95.32(1)°, â ) 103.21(2)°, γ ) 103.48(1)°, and Z ) 12.
Introduction
Experimental Section
It has recently been found1 that the reaction of the molyb-
denum(III) complex Mo2Cl6(THF)3 with the secondary amine
NHEt2 at room temperature causes reduction of the starting
material to produce the dimolybdenum(II) complex Mo2Cl4-
(NHEt2)4. The yield of the product can be significantly improved
by assisted reduction with sodium amalgam. Complex Mo2Cl4-
(NHEt2)4 was shown2 to be an interesting starting material
offering access to further chemistry of Mo24+. In particular, the
substitution reactions with bidentate phosphines led to the
isolation of a new class of quadruply-bonded dimolybdenum
complexes, Mo2Cl4(η1-diphosphine)4.
General Procedures. All manipulations were carried out under an
atmosphere of dry oxygen-free argon or nitrogen with standard Schlenk
techniques. Solvents were dried and deoxygenated by refluxing over
suitable reagents before use. NH2Et, NH2Prn, NH2But, and NH2Cy were
purchased from Aldrich, Inc. Benzene-d6 and chloroform-d1 were
obtained from Cambridge Isotope Laboratories, Inc., and used as
received. Mo2Cl6(THF)3 was synthesized according to a published
procedure.3 Sodium amalgam was prepared in a drybox by dissolving
a weighed amount of metallic sodium in an approximately measured
quantity of mercury that was pumped under vacuum for at least 1 h in
a Schlenk flask.
(A) Reactions of Mo2Cl6(THF)3 with Primary Amines and Na/
Hg Amalgam. (A1) Synthesis of Mo2Cl4(NH2Et)4 (1). To a suspension
containing 0.15 g (0.24 mmol) of Mo2Cl6(THF)3 in a mixture of 5 mL
of THF and 10 mL of CH2Cl2 at -78 °C was added 2 equiv of sodium
amalgam (0.4%). The reaction system was slowly warmed to -25 °C,
and then 5 mL of a 2 M solution of NH2Et in THF was syringed in. At
a temperature of -10 °C another 70 mL of CH2Cl2 was added, and the
reaction mixture was allowed to reach room temperature. It was stirred
for 15 min, after which the purple solution was filtered and all volatile
components were removed under reduced pressure to leave a homo-
geneous reddish solid. Yield: 0.08 g (65%).
To further study the interaction between MoIII species and
amines we turned to primary amines, NH2R, which have not
been used before for multiply-bonded compounds of Mo. It was
found that the reaction of Mo2Cl6(THF)3 with NH2R (R ) Et,
Prn, But, and Cy) gives low yields of reduced species, both at
room temperature and under reflux conditions, with the major
process being the deposition of mononuclear MoIII complexes
MoCl3(NH2R)3. However, the use of amalgam in this process
results in obtaining dimolybdenum(II) compounds Mo2Cl4-
(NH2R)4 in close to quantitative yields. Four new complexes
Mo2Cl4(NH2R)4 (R ) Et (1), Prn (2), But (3), Cy (4)) have been
prepared and characterized by IR, UV-vis, 1H NMR spectros-
copy, and single-crystal X-ray diffraction studies.
Small red crystals of 1 could be obtained by layering either hexanes
over a dichloromethane solution or ether over a chloroform solution.
Anal. Calcd for Mo2Cl4N4C8H28: C, 18.69; H, 5.49; N, 10.90.
Found: C, 18.44; H, 5.41; N, 10.28. IR data (KBr, cm-1): 3273 (vs),
3225 (s), 2971 (s), 2928 (s), 1628 (m), 1556 (s), 1465 (m), 1381 (w),
(1) Cotton, F. A.; Dikarev, E. V.; Herrero, S. Inorg. Chem. 1998, 37,
1
1261 (m), 1203 (s), 1062 (br, vs), 887 (w), 801 (s), 604 (s). H NMR
5862.
(2) Cotton, F. A.; Dikarev, E. V.; Herrero, S. Inorg. Chem. 1999, 38,
490.
(3) Boyd, I. W.; Wedd, A. G. Aust. J. Chem. 1976, 29, 1829.
10.1021/ic981289c CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/04/1999

2650 Inorganic Chemistry, Vol. 38, No. 11, 1999
Cotton et al.
data (CDCl3, 22° C) δ 3.84 (br, t, NH2), 2.76 (m, CH2), 1.13 (t, CH3);
J(NH2-CH2) ) J(CH2-CH3) ) 7 Hz. UV-vis (THF; λmax, nm): 530
(δ-δ* transition).
was designated as [NH3But][MoCl4(NH2But)2] salt on the basis of
chemical analyses.
(B3) NH2Cy. A mixture of 0.30 g (0.48 mmol) of Mo2Cl6(THF)3, 2
mL (17.4 mmol) of NH2Cy, and 10 mL of THF was stirred at room
temperature for 20 h to form a reddish brown solution. All volatile
components were removed under reduced pressure, and an extraction
was made with a mixture of 10 mL of benzene and 30 mL of hexanes.
The extracted complex was shown to be Mo2Cl4(NH2Cy)4 (4) (0.06 g,
(A2) Synthesis of Mo2Cl4(NH2Prn)4 (2). Two equivalents of sodium
amalgam (0.4%) was added to a suspension of 0.15 g (0.24 mmol) of
Mo2Cl6(THF)3 in 10 mL of THF at -78 °C. The system was slowly
warmed to -25 °C, and 1 mL (12 mmol) of NH2Prn was added. The
reaction mixture was allowed to reach room temperature, and 50 mL
of benzene was then added. After stirring for 20 min at room
temperature, the purple solution was filtered and solvent was removed
in vacuo, leaving a red residue. Yield: 0.11 g (80%).
1
17%) by H NMR. The residue was separated into two fractions: a
brown solid soluble in benzene (0.04 g, 8%), which has not been
identified, and a yellow product insoluble in benzene (0.29 g, 60%),
which was identified as the mononuclear complex mer-MoCl3(NH2-
Cy)3 (4a) by X-ray diffraction.4
Red crystals of 2 were obtained in a week by layering isomeric
hexanes over a toluene solution at -30 °C.
Anal. Calcd for Mo2Cl4N4C12H36: C, 25.28; H, 6.36; N, 9.83.
Found: C, 25.57; H, 6.35; N, 9.54. IR data (KBr, cm-1): 3276 (vs),
3222 (vs), 2962 (vs), 2879 (s), 1561 (s), 1468 (m), 1385 (w), 1311
(w), 1188 (s), 1070 (sh, m), 1050 (s), 1005 (m), 873 (vw), 751 (w),
606 (m). 1H NMR data (benzene-d6, 22° C): δ 4.03 (br, t, NH2), 2.62
(m, CH2), 1.03 (m, CH′2), 0.59 (t, CH3); J(NH2-CH2) ) J(CH2-CH′2)
) J(CH’2-CH3) ) 7 Hz. UV-vis (THF; λmax, nm): 528 (δ-δ*
transition).
(A3) Synthesis of Mo2Cl4(NH2But)4 (3). A suspension of 0.15 g
(0.24 mmol) of Mo2Cl6(THF)3 in 5 mL of THF was cooled in an
ethanol/dry ice bath. Two equivalents of sodium amalgam (0.4%) and
1 mL (9.5 mmol) of NH2But were added to the suspension, which was
then allowed to reach ambient temperature and was stirred for another
3 h. Benzene (50 mL) was added to the reaction mixture, followed by
filtration. All volatile components were evaporated from the purple
solution under reduced pressure to leave a red solid. Yield: 12 g (79%).
Crystals of 3 were obtained by cooling saturated solutions in hexanes
or diethyl ether to -30 °C and by carefully layering isomeric hexanes
over a toluene solution.
(C) Reaction of Mo2Cl6(THF)3 with NH2But without Na/Hg
Amalgam in Refluxing THF. A mixture of 0.30 g (0.48 mmol) of
Mo2Cl6(THF)3 and 2 mL of NH2But (19 mmol) was refluxed in 20 mL
of THF for 3 h. The reddish brown solution was then evaporated to
dryness, and the complex Mo2Cl4(NH2But)4 (3) was extracted from the
residue with 50 mL of hot hexanes and identified by 1H NMR. Yield:
0.05 g (16%).
Physical Measurements. Infrared spectra were recorded on a Perkin-
1
Elmer 16PC FTIR spectrometer using KBr pellets. H NMR spectra
were obtained on a Varian XL-200E spectrometer; internal standard
resonances were CHCl3 (7.24 ppm) and C6D5H (7.15 ppm). Electronic
spectral data were obtained in THF, using a Cary-17D UV-vis
spectrophotometer. Thermal analysis was performed by a TA 4000
thermogravimetric unit (under nitrogen, heating rate 2.5 °C/min).
Elemental analyses were done by Canadian Microanalytical Services,
Ltd. (Delta, BC V4G 1G7, Canada).
X-ray Crystallographic Procedures. Single crystals of compounds
2, 2a, 3, and 4 were obtained as described above. In each case, a crystal
of suitable quality was affixed to the end of a quartz fiber with grease
in a cold nitrogen stream (-120, -100, or -60° C). X-ray diffraction
experiments were carried out using one of the two fully automated
diffractometers equipped with monochromatized Mo KR radiation,
Enraf-Nonius CAD-4S (2 and 2a) and Nonius FAST (3 and 4). Unit
cell determination and data collection followed routine procedures and
practices of this laboratory.5 Oscillation photographs around principal
axes were taken to confirm the Laue class and axial lengths. All data
were corrected for Lorentz and polarization effects.
The structures were solved and refined using the SHELXTL direct
methods6 and the SHELXL-93 programs7 on a DEC Alpha running
VMS. In each model, hydrogen atoms were included at idealized
positions for the structure factor calculations but were not refined.
Details on data collection and structure refinement are reported in Table
1.
Mo2Cl4(NH2Prn)4 (2). A red plate of dimensions 0.30 × 0.20 ×
0.05 mm was selected for diffraction study. The ω-2θ scan technique
was used to scan data points. There was no significant decay of the
crystal, as indicated by the intensity standards. An empirical absorption
correction based on azimuthal scans of reflections with their ψ angles
near 90° was applied. The crystal was shown to belong to the C-centered
orthorhombic system, and analysis of systematic absences unambigu-
ously identified the space group as Ccca. After initial refinement, it
became apparent that the Mo-Mo unit has three-way disorder, and
atoms Mo(2) and Mo(3) corresponding, respectively, to the second and
the third orientations were located. These atoms were included in the
Anal. Calcd for Mo2Cl4N4C16H44: C, 30.69; H, 7.08. Found: C,
30.93; H, 7.31. IR data (KBr, cm-1): 3278 (w), 3193 (w), 3114 (vw),
2966 (vs), 1566 (m), 1473 (w), 1398 (m), 1373 (s), 1263 (vs), 1208
1
(s), 1127 (vs), 1100 (vs), 1025 (vs), 899 (w), 803 (vs), 636 (w). H
NMR data (benzene-d6, 22° C): δ 4.52 (br, NH2), 1.08 (s, CH3). UV-
vis (THF; λmax, nm): 538 (δ-δ* transition).
(A4) Synthesis of Mo2Cl4(NH2Cy)4 (4). Procedures similar to those
just described for 2 were followed to prepare Mo2Cl4(NH2Cy)4 (4) using
cyclohexylamine (1 mL, 8.7 mmol). The red solid was isolated with a
yield of 94%.
The orange-red crystals of 4 used for the X-ray experiment were
obtained by keeping a saturated solution of the compound in hexanes
at -30 °C for a week.
Anal. Calcd for Mo2Cl4N4C24H52: C, 39.47; H, 7.18; N, 7.67.
Found: C, 39.47; H, 6.99; N, 7.41. IR data (KBr, cm-1): 3287 (m),
3238 (m), 2930 (vs), 2855 (s), 1568 (s), 1448 (m), 1388 (vw), 1344
(vw), 1261 (s), 1218 (m), 1179 (w), 1124 (s), 1100 (s), 1073 (s), 1046
1
(vs), 962 (w), 891 (w), 803 (s), 634 (w). H NMR data (benzene-d6,
22 °C): δ 4.41 (d, NH2), 3.18 (m, CH), 1.6-0.6 (CH2); J(NH2-CH)
) 7 Hz. UV-vis (THF; λmax, nm): 530 (δ-δ* transition).
(B) Reactions of Mo2Cl6(THF)3 with Primary Amines without
Na/Hg Amalgam at Room Temperature. (B1) NH2Prn. To a
suspension of 0.20 g (0.32 mmol) of Mo2Cl6(THF)3 in 20 mL of THF
was added 0.5 mL (6.0 mmol) of NH2Prn. The mixture was stirred
overnight at room temperature, and the yellow precipitate which
appeared was filtered off. Another 5 mL of tetrahydrofuran was added
to the red solution, and 25 mL of hexanes was layered over it. After 3
weeks, yellow crystals (0.12 g, 49%) were formed and identified by
X-ray as mer-MoCl3(NH2Prn)3 (2a). The dinuclear complex 2, which
(4) Crystal data for mer-MoCl3(NH2Cy)3: monoclinic, P21/c (No. 14), a
) 14.051(6) Å, b ) 35.23(2) Å, c ) 17.333(9) Å, â ) 96.79(4)°, V
) 8716(8) Å3, Z ) 12, T ) 20 °C, Rigaku AFC5R diffractometer
equipped with a rotating Cu radiation source (λ(Cu KR) ) 1.541 84
Å). We do not report the details of the crystal structure of 4a because
of poor refinement.
(5) (a) Bino, A.; Cotton, F. A.; Fanwick, P. E. Inorg. Chem. 1979, 18,
3558. (b) Cotton, F. A.; Frenz, B. A.; Deganello, G.; Shaver, A. J.
Organomet. Chem. 1973, 227. (c) Cotton, F. A.; Dikarev, E. V.; Feng,
X. Inorg. Chim. Acta 1995, 237, 19.
(6) SHELXTL V.5; Siemens Industrial Automation Inc.: Madison, WI,
1994.
(7) Sheldrick, G. M. In Crystallographic Computing 6; Flack, H. D.,
Parkanyi, L., Simon, K., Eds.; Oxford University Press: Oxford, U.K.,
1993; p 111.
1
remained in solution, was detected by H NMR.
(B2) NH2But. To a suspension containing 0.30 g (0.48 mmol) of
Mo2Cl6(THF)3 in 10 mL of THF was added 2 mL (19 mmol) of NH2-
But. The mixture was stirred at room temperature for 3 days. The yellow
precipitate was filtered off (0.22 g, 54%) and identified as mononuclear
complex MoCl3(NH2But)3 (3a) on the basis of chemical analyses. All
volatile components were evaporated from the red-brown solution, and
an extraction with 50 mL of hot hexanes was made. The extract
contained 0.07 g (23%) of Mo2Cl4(NH2But)4 (3), identified by means
1
of H NMR. The brown residue insoluble in hexanes (0.07 g, 16%)

Mixed Chloride/Amine Complexes of Dimolybdenum(II,II)
Inorganic Chemistry, Vol. 38, No. 11, 1999 2651
Table 1. Crystallographic Data for Mo2Cl4(NH2Prn)4 (2), MoCl3(NH2Prn)3‚1/6THF (2a‚1/6THF), Mo2Cl4(NH2But)4 (3), and Mo2Cl4(NH2Cy)4 (4)
2
2a.1/6THF
3
4
formula
fw
space group
a, Å
Mo2Cl4N4C12H36
570.13
Ccca (No. 68)
13.328(3)
26.639(5)
6.774(2)
MoCl3N3O0.17C9.67H28.33
391.64
P1h (No. 2)
12.370(2)
17.977(2)
25.498(6)
95.32(1)
Mo2Cl4N4C16H44
626.23
P21/c (No. 14)
19.165(1)
20.858(1)
14.1400(8)
Mo2Cl4N4C24H52
730.38
P42212 (No. 94)
15.556(4)
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
6.9368(8)
103.21(2)
103.48(1)
5304(2)
99.002(5)
2405(1)
5582.8(5)
1678.6(6)
4
12
8
2
F
calcd, g‚cm-3
1.575
1.471
1.490
1.445
µ, mm-1
1.489
1.183
1.290
1.084
radiation (λ, Å)
temp, °C
Mo KR (0.710 73)
Mo KR (0.710 73)
Mo KR (0.710 73)
Mo KR (0.710 73)
-120
-100
-60
-60
transmission factors
R1,a wR2b [I >2 σ(I)]
R1,a wR2b (all data)
0.8896-0.9983
0.0316, 0.0732
0.0705, 0.0839
0.9381-0.9999
0.0551, 0.1386
0.0869, 0.1579
0.0451, 0.1024
0.0582, 0.1183
0.0374, 0.0918
0.0389, 0.0933
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo - Fc2)2]/∑[w(Fo )2]]1/2
.
2
2
refinement, and their site occupancy factors (sof’s) were allowed to
vary against that of Mo(1) but constrained so that the sum of
molybdenum atoms equaled 1. The sof’s converged to final values of
0.698(2), 0.185(2), and 0.116(2). After anisotropic refinement of all
non-hydrogen atoms, the residuals were R ) 0.032 (for 560 reflections
with I > 2σ(I) and R ) 0.071 (for all 840 independent reflections).
The final difference map was essentially featureless; the largest peak
intact for each Mo-Mo orientation. That disorder was actually found,
with each tertiary carbon atom having its own set of three methyl
groups. Final least-squares refinement of 109 parameters converged
with R ) 0.073. We will use in discussion the structural parameters
for the non-disordered form of 3, presuming them to be more reliable.
Mo2Cl4(NH2Cy)4 (4). A needle of dimensions 0.25 × 0.08 × 0.05
mm was mounted. Indexing based on 50 reflections in the range 18°
< 2θ < 42° resulted in a primitive tetragonal cell, and cell parameters
were further refined against 240 reflections. No absorption correction
was made. Systematic absences in the data uniquely determined the
space group to be P42212. Only Mo, Cl, and N atoms were refined
with anisotropic thermal parameters. A second orientation of the carbon
atoms in the cyclohexyl ring was located, and its site occupancy factor
was converged to 0.26(1). Final refinement of 67 parameters and 15
restraints resulted in R ) 0.037 (for 1062 reflections with I > 2σ(I))
and R ) 0.039 (for all data). The highest peak in the final difference
map was 0.66 e Å-3. The absolute configuration of 4 was established
by the method described by Flack (Flack x parameter ) 0.0(2)).
was 0.71 eÅ-3
.
mer-MoCl3(NH2Prn)3‚1/6THF (2a‚1/6THF). A red block of size 0.40
× 0.30 × 0.20 mm was used for diffraction study. The data were
collected in the range 4° e 2θ e 46° with ω scan motion. The
centrosymmetric space group P1h was selected and confirmed by
successful refinement. Some of the n-propyl groups appeared to be
disordered on two sites. After anisotropic refinement of all non-
disordered atoms, an interstitial molecule of tetrahydrofuran was located
and included in calculations. The final residuals were R ) 0.055 (for
10 628 reflections with I > 2σ(I)) and R ) 0.087 (for all 14 699 data
and 898/25 parameters/restraints). A final difference Fourier map
contained several peaks (largest 1.69 e Å-3), all lying less than 1.0 Å
from Mo atoms.
Results and Discussion
Mo2Cl4(NH2But)4 (3). A red plate with dimensions 0.50 × 0.40 ×
0.10 mm grown from ether solution was selected and affixed to the tip
of a quartz fiber with silicone grease. The diffraction data were collected
on a Nonius FAST diffractometer at -60 °C. With 250 reflections (18°
< 2θ < 42°), the unit cell was shown to be monoclinic primitive.
Calculations in the space group P21/c revealed two crystallographically
independent dimolybdenum molecules each having a second orientation
of the Mo-Mo unit. The occupancies of the minor orientations were
refined as 0.033(1) and 0.077(1), respectively. Also, two of four tert-
butyl groups in the first molecule and three of four in the second
molecule were found to be disordered. Final least-squares refinement
of 492 parameters and 32 restraints led to R ) 0.045 (for 8262
reflections with I > 2σ(I) and R ) 0.058 (for all 9824 data). The highest
peak in the final difference map was 1.10 e Å-3 located 1.05 Å from
Mo(2).
From the toluene/hexane and neat hexane solutions complex 3
crystallized as red needles. Their unit cell was shown to be primitive
cubic, a ) 17.984(2) Å, V ) 5817(1) Å3, Z ) 8. A total of 1283 unique
data were collected in the range 4°< 2θ < 50° on a Nonius FAST
diffractometer at -60 °C. Examination of the systematic absences
narrowed the choice of space groups to P4h3n (No. 218) and Pm3hn
(No. 223). Refinement in the centrosymmetric space group (Pm3hn) was
unsuccessful since it did not provide a separation between ligand (Cl
and N atoms) positions. Calculations in the non-centrosymmetric space
group (P4h3n) revealed a highly disordered structure with two crystal-
lographically independent dimolybdenum molecules each having
threefold disorder. For this reason, the positions of Cl and N atoms are
averaged for three orientations of the dimetal unit and have somewhat
high thermal parameters. This in turn requires three-way disorder of
tertiary carbon atoms of tert-butyl groups to keep Mo-N-C angles
Synthetic Aspects. Reactions of Mo2Cl6(THF)3 with primary
amines, NH2R, at room temperature produce the reduced
dimolybdenum species Mo2Cl4(NH2R)4 (R ) Et (1), Prn (2),
But (3), Cy (4)). The yields of this process were very low (less
than 25%) and did not increase even when the reaction mixture
was refluxed. The main products were mononuclear molybde-
num(III) complexes, MoCl3(NH2R)3, as a result of simple
substitution and metal-metal bond cleavage:
Mo2Cl6(THF)3 + 6NH2R f 2MoCl3(NH2R)3 + 3THF
We have recently reported1 that reaction between Mo2Cl6-
(THF)3 and the secondary amine, NHEt2, also gives the reduced
dimolybdenum(II) complex Mo2Cl4(NHEt2)4. However, the
yield in this case was about 50%, and other products, different
MoIII species, all having a Cl:Mo ratio >3, emerged when
chloride ions, released upon substitution by amines, reacted with
more starting material. Such parallel reactions also take place
for primary amines, despite the fact that the concentration of
Cl- is relatively low.
Mo2Cl6(THF)3 + 2[NH3R]Cl + 4NH2R f
2[NH3R][MoCl4(NH2R)2] + 3THF
MoCl3(NH2R)3 + [NH3R]Cl f
[NH3R][MoCl4(NH2R)2] + NH2R

2652 Inorganic Chemistry, Vol. 38, No. 11, 1999
Cotton et al.
Table 2. Spectroscopic Data for the Complexes Mo2Cl4(NH2R)4 (R ) Et (1), Prn (2), But (3), Cy (4)) and Mo2Cl4(NHEt2)4
a
IR (cm-1
)
1H NMR
vis (nm)
compd
ν(N-H)
δ(N-H)
δ-δ* trans
NH2
CH
CH2
CH3
Mo2Cl4(NH2Et)4 (1)
3273 (vs)
3225 (vs)
3276 (vs)
3222 (vs)
3278 (w)
3193 (w)
3287 (m)
3238 (m)
3219 (m)
1465 (m)
1468 (m)
1473 (w)
1448 (m)
1460 (m)
530b
528b
538b
530b
534c
3.84 (t)d
2.76 (m)
1.23 (t)
Mo2Cl4(NH2Prn)4 (2)
Mo2Cl4(NH2But)4 (3)
Mo2Cl4(NH2Cy)4 (4)
4.03 (t)e
4.52 (s)e
4.41 (d)e
4.50 (s)e
2.62 (m)
1.03 (m)
0.59 (t)
1.08 (t)
3.18 (m)
1.6-0.6
f
Mo2Cl4(NHEt2)4
3.26 (s)
0.89 (s)
a KBr pellets. b In THF. c In CH2Cl2. d In CDCl3. e In C6D6. f Reference 1.
We were able to estimate the yields of the different products
for the reactions with tert-butylamine and cyclohexylamine since
it was possible to completely separate compounds based on their
solubility. The MoCl3(NH2R)3:Mo2Cl4(NH2R)4:[NH3R][MoCl4-
(NH2R)2] relative yields (based on the amount of Mo in Mo2-
Cl6(THF)3) are 54:23:16 and 60:17:8 for R ) But and Cy,
respectively.
Despite the difference in reaction products for primary and
secondary amines, the behavior of dimolybdenum(III) species
in reactions of this type contrasts greatly with that of ditungsten-
(III) species. The latter are known to react with a variety of
primary8 and secondary9 amines to produce only nonreduced
compounds of the type W2Cl4(amide)2(amine)2 having both
amido and amino groups. We have no evidence, so far, for the
existence of molybdenum compounds of that type.
The thermal stability of Mo2Cl4(NH2Et)4 (1) was studied
under the same conditions used for Mo2Cl4(NHEt2)4 in order
to compare the data. Although the loss of the amine for Mo2-
Cl4(NHEt2)4 starts at about 65 °C, it is not significant until 90
°C. However, for compound 1 this process starts only at 135
°C. That difference is substantial, especially if the boiling points
of both amines are considered (55 °C for NHEt2 and 16.6 °C
for NH2Et). The thermogravimetric weight loss curve for
complex 1 presents a continuously decreasing line between 135
°C (the beginning of the weight loss) and 325 °C (when the
line becomes flat and the weight loss corresponds to the loss of
the four amines). There are no clear steps in the overall process
as was observed for the diethylamine compound.
The infrared spectra of the complexes 1-4 (Table 2) exhibit
two absorptions due to N-H stretching between 3193 and 3287
cm-1, and one absorption for N-H bending between 1473 and
As in the case of NHEt2, the best results in the synthesis of
reduced dimolybdenum(II) species with primary amine ligands
were achieved by reduction of the starting material with sodium
amalgam, in the presence of the appropriate amine.
1448 cm-1. Corresponding bands appear at 3219 and 1460 cm-1
,
respectively, for Mo2Cl4(NHEt2)4.1 The electronic spectra of the
complexes with primary amines show the maximum of the
δ-δ* transition absorption band between 538 and 528 nm
(Table 2).
Mo2Cl6(THF)3 + 2Na + 4NH2R f
Mo2Cl4(NH2R)4 + 2NaCl + 3THF
The 1H NMR spectra of complexes 1-4 display well-defined
signals (Figure 1), instead of the very broad signals which
characterize the Mo2Cl4(NHEt2)4 spectrum because of the
exchange in solution between the coordinated amines and the
free amine liberated in the decomposition process. No free amine
was detected in the spectra of complexes 1-4. The signals of
the hydrogen nuclei of the NH2 group for compounds 1 and 2
are triplets. Although they are broader than the rest of the signals
in the spectrum, the coupling with the nearest hydrogen nuclei
of the CH2 groups is observed in both cases. For complex 3,
the nearest hydrogen atoms are farther away and no coupling
between hydrogen nuclei of NH2 and CH3 groups is resolved.
For compound 4, the signal of the NH2 group is a doublet,
because the closest carbon atom is bonded to only one hydrogen
atom.
Molecular Structures. Crystal structures of Mo2Cl4(NH2R)4
(R ) Prn (2), But (3), Cy (4)) are very similar. Perspective
drawings of the molecules 2-4 are depicted in Figures 2-4,
respectively. The central portions of each structure consisting
of the non-centrosymmetric Mo2Cl4N4 core are the same, and
each has D2d symmetry. The Mo-Mo bond distances in all of
them (2, 2.118(2) Å; 3, 2.1322(6) Å; 4, 2.117(1) Å) fall within
the range of the bond distances established for the Mo-Mo
quadruple bond with a σ2π4δ2 configuration.10 Each Mo atom
in the molecules is approximately square planar with the two
nitrogen atoms of primary amines in a trans arrangement in each
The Mo2Cl4(NH2R)4 complexes (R ) Et (1), Prn (2), But (3),
Cy (4)) can be isolated from the reaction mixture in nearly
quantitative yield. The ethylamine compound 1 has limited
solubility in all common solvents and therefore requires special
treatment.
Properties of Mo2Cl4(NH2R)4 (R ) Et (1), Prn (2), But
(3), Cy (4)). Complexes 2-4 are all readily soluble in
dichloromethane, THF, toluene, and benzene. Complexes 3 and
4 can even be dissolved in diethyl ether or hexanes. However,
compound 1 is only partially soluble in dichloromethane or
chloroform and slightly soluble in THF. The solutions of these
species are stable under nitrogen or argon atmosphere, and no
sign of decomposition was observed when the solvents were
evaporated. The behavior of an analogous complex with a
secondary amine, Mo2Cl4(NHEt2)4, in solution was found1 to
be completely different: it is stable only in the presence of free
amine and undergoes partial decomposition (polymerization)
when its solutions are under reduced pressure. In the solid state,
compounds 1-4 are also more stable than the diethylamine
derivative. These compounds are not very air sensitive even as
powders and can be handled in the atmosphere for short periods
of time.
(8) (a) Bradley, D. C.; Errington, R. J.; Hursthouse, M. B.; Short, R. L.
J. Chem. Soc., Dalton Trans. 1986, 1305. (b) Cotton, F. A.; Dikarev,
E. V.; Wong, W.-Y. Inorg. Chem. 1997, 36, 3268.
(9) Cotton, F. A.; Dikarev, E. V.; Nawar, N.; Wong, W.-Y. Inorg. Chem.
1997, 36, 559.
(10) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms,
2nd ed.; Oxford University Press: New York, 1993; Table 3.1.1, p
143.

Mixed Chloride/Amine Complexes of Dimolybdenum(II,II)
Inorganic Chemistry, Vol. 38, No. 11, 1999 2653
Figure 3. Perspective drawing of one crystallographically independent
molecule of Mo2Cl4(NH2But)4 (3). Atoms are represented by thermal
ellipsoids at the 40% probability level. Only the major orientation of
the dimolybdenum unit is depicted. Hydrogen atoms of tert-butyl groups
are omitted. Carbon and amine hydrogen atoms are shown as spheres
of arbitrary radii.
Figure 1. 1H NMR spectra of (a) Mo2Cl4(NH2Et)4 (1) in CDCl3, (b)
Mo2Cl4(NH2Prn)4 (2) in C6D6, (c) Mo2Cl4(NH2But)4 (3) in C6D6, and
(d) Mo2Cl4(NH2Cy)4 (4) in C6D6, at room temperature.
Figure 4. Perspective drawing of Mo2Cl4(NH2Cy)4 (4). Atoms are
represented by thermal ellipsoids at the 40% probability level. Only
the major orientation of cyclohexyl rings is displayed. Carbon and
hydrogen atoms are shown as spheres of arbitrary radii.
Table 3. Selected Bond Distances (Å), Angles (deg), and Torsion
Angles (deg) for Mo2Cl4(NH2Prn)4 (2) and Mo2Cl4(NH2Cy)4 (4)
2
4
Mo(1)-Mo(1A)
Mo(1)-N(1)
Mo(1)-Cl(1)
2.118(2)
2.220(4)
2.404(1)
2.117(1)
2.239(5)
2.432(2)
N(1)-Mo(1)-N(1B)
Cl(1)-Mo(1)-Cl(1B)
N(1)-Mo(1)-Cl(1)
N(1)-Mo(1)-Cl(1B)
Mo(1A)-Mo(1)-N(1)
Mo(1A)-Mo(1)-Cl(1)
155.3(3)
148.64(8)
82.8(1)
90.6(1)
102.3(2)
105.68(4)
154.3(3)
156.5(1)
87.3(1)
87.5(1)
102.8(1)
101.77(5)
N-Mo-Mo-Cl
7.7(1)
2.8(2)
Figure 2. Perspective drawing of Mo2Cl4(NH2Prn)4 (2). Atoms are
represented by thermal ellipsoids at the 40% probability level. All three
orientations of the dimolybdenum unit are depicted. Carbon and amine
hydrogen atoms are shown as spheres of arbitrary radii.
which are more than 0.03 Å longer in the case of the compound
with secondary amine ligands.
One interesting structural feature of molecules 2-4, which
reflects an effect of packing forces, is an orientation of nitrogen-
connected atoms (one carbon and two hydrogens) relative to
the Mo-Mo bond. This orientation of substituents on the amine
ligands can be characterized by the torsion angles Mo-Mo-
N-C and Mo-Mo-N-H. The difference among the title
MoCl2N2 fragment. The molecular dimensions for the three
complexes 2-4 (Tables 3 and 4) are not significantly different
from those of the recently determined1 structure of the analogous
complex Mo2Cl4(NHEt2)4, except for the Mo-N distances,

2654 Inorganic Chemistry, Vol. 38, No. 11, 1999
Cotton et al.
Table 4. Averaged Bond Distances (Å), Angles (deg), and Torsion
Angles (deg) for Two Crystallographically Independent Molecules
Mo2Cl4(NH2But)4 (3)
Mo-Mo
Mo-N
Mo-Cl
2.1309(6)
2.248(4)
2.439(1)
2.1335(6)
2.254(4)
2.431(1)
N-Mo-N
Cl-Mo-Cl
N-Mo-Cl
158.3(2)
153.81(5)
82.8(1), 92.4(1)
160.3(2)
152.31(5)
84.1(1), 91.2(1)
Mo-Mo-N
Mo-Mo-Cl
100.9(1)
103.08(4)
99.9(1)
103.84(4)
N-Mo-Mo-Cl
3.9(1)
2.9(1)
structures is that while in the Mo2Cl4(NH2Prn)4 (2) molecule
the n-propyl group is located “perpendicular” to the metal-
metal axis ( Mo-Mo-N-C is 74°), the tert-butyl (in 3) and
cyclohexyl (in 4) groups are directed “parallel” to the Mo-Mo
vector ( Mo-Mo-N-C vary in the range 144-177°). There-
fore, in the latter two cases one of the two amine hydrogen
atoms is located more or less above the metal-metal axis,
allowing hydrogen bonding with the chlorine atom across the
dinuclear molecule. Indeed, in the structure of Mo2Cl4(NH2-
But)4 (3), where the torsion angles Mo-Mo-N-H are in the
range 20-27°, there are two strong N-H‚‚‚Cl bonds per
molecule, each having an H‚‚‚Cl separation of about 2.4 Å with
corresponding N-H-Cl angles of 135-142°. At the other
extreme, one of the Mo-Mo-N-H angles in propylamine
complex is 56°, providing a long N-H‚‚‚Cl contact of 2.95 Å
and an acute N-H-Cl angle of 107°. Another hydrogen atom
in this structure is directed away from the dimetal unit (Mo-
Mo-N-H angle of 160°), giving an intramolecular hydrogen
bonding with the N-H‚‚‚Cl parameters being 2.67 Å and 158°.
Figure 5. Perspective drawing of one crystallographically independent
molecule of mer-MoCl3(NH2Prn)3 in 2a‚1/6THF. Atoms are represented
by thermal ellipsoids at the 40% probability level. Hydrogen atoms
are shown as spheres of arbitrary radius.
Conclusions
It has been shown already that the newly-synthesized dimo-
lybdenum complex Mo2Cl4(NHEt2)4 can serve effectively as a
starting material for exploring the chemistry of the Mo24+ core.
However, this compound is hard to store in the solid state and
it is rather unstable in solution, losing amine ligands spontane-
ously when kept in the absence of free amine. As a result of
our quest for a more amenable material, the quadruply-bonded
dimolybdenum molecules of the general type Mo2Cl4(NH2R)4,
where NH2R is a primary amine (R ) Et (1), Prn (2), But (3),
Cy (4)), are now accessible by the reduction of an easily
available MoIII species. These complexes have greater stability
than those with secondary amine ligands: they are stable in
solution without excess amine present, and they have higher
decomposition temperatures as solids. These properties make
them more attractive for use in the synthesis of new dimolyb-
denum(II) compounds.
We would like to note that there are just a few known
examples of molybdenum compounds with primary amine
ligands NH2R (R ) Me,11 But,12 Ph13). They include different
oxidation states for Mo atoms from +2 to +6 and display Mo-
NH2R bonds in the range 2.19-2.38 Å.
The compound MoCl3(NH2Prn)3‚1/6THF (2a‚1/6THF) crystal-
lizes in the triclinic space group P1h with the asymmetric unit
composed of six molecules of the mononuclear complex (Figure
5) and one molecule of THF. The propylamine ligands on each
octahedrally-coordinated metal center are in a meridional
conformation, and Mo-N and Mo-Cl distances are in the
ranges 2.198(6)-2.232(6) and 2.400(2)-2.439(2) Å, respec-
tively. The angles at the Mo atom vary in the ranges 83.5(2)-
96.4(2)° and 172.15(7)-179.0(2)°. The octahedral MoIII com-
plex is one of the well-established class of MoCl3L3 compounds
(where L is a neutral donor) known to include both meridional
(L ) py,14a pic,14b THF,14c THT,14d OPMe3,14e PMe2Ph14f) and
facial (L ) OC(NH2)2,14g SC(NH2)2,14h CH3CN,14i 2-SC5H2-
NH-3,6-(SiMe2But)214j) isomers.
On the basis of the relative stability of the complexes Mo2-
Cl4L4 with primary and secondary amines, one can assume that
tertiary amine compounds would be the least stable. Indeed,
we were unable so far to isolate any of those species on the
same reaction route. Further work, including the study of
aromatic amines, is currently underway.
Acknowledgment. We are grateful to the National Science
Foundation for support of this work. S.H. thanks the Spanish
Direccio´n General de Ensen˜anza Superior for financial support.
Supporting Information Available: Four X-ray crystallographic
files, in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
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