Mixed chloride/amine complexes of dimolybdenum(II,II). 6. Stepwise substitution of amines by tertiary phosphines and vice versa: Stereochemical hysteresis

Article abstract of DOI:10.1021/ic9913684

The substitution reactions of primary amines by tertiary phosphines in quadruply bonded, dimolybdenum(II,II) complexes Mo₂Cl₄(NH₂R)₄ have been studied. The exchange reaction has been shown to result at room temperature in disubstituted species Mo₂Cl₄(NH₂R)₂(PR₃)₂ (PR₃ = PMe₃, NH₂R = NH₂Pr(n) (1a), NH²Bu(t) (2a), NH₂Cy (3a); PR₃ = PMe₂Ph, NH₂R = NH₂CY (4a)), while heating is needed to obtain fully substituted complexes Mo₂Cl₄-(PR₃)₄. The crystal structure of disubstituted products has been investigated by X-ray crystallography and revealed that they all belong to the α-isomer, having both phosphine groups at the same Mo atom. Crystal data are as follows: for 1a, tetragonal space group I4₁/a with a = 17.737(2) A, c = 15.6915(6) A, and Z = 8; for 3a, monoclinic space group P2₁ with a = 10.963(3) A, b = 10.117(2) A, c = 13.323(4) A, β = 90.05(2)°, and Z = 2; for 4a, triclinic space group P1 with a = 9.329(3) A, b = 10.206(2) A, c = 18.975(3) A, α = 85.45(2)°, β = 87.10(1)°, γ = 80.88(1)°, and Z = 2. The substitution processes for the direct and reverse reactions have been monitored by ³¹P NMR. They both proceed in a stepwise manner; however, a stereochemical hysteresis is taking place, i.e., the back reaction, the substitution of phosphines by amines, goes through another isomer of Mo₂Cl₄-(NH₂R)₂(PR₃)₂, having phosphine ligands on different Mo atoms. This β-isomer is more thermodynamically stable and can be obtained by thermal conversion of the α-form. All chemical equilibria studied in the paper have been explained as governed by a higher trans effect of PR₃ groups compared to NH₂R groups.

Full text of DOI:10.1021/ic9913684

Inorg. Chem. 2000, 39, 609-616
609
Mixed Chloride/Amine Complexes of Dimolybdenum(II,II). 6. Stepwise Substitution of
Amines by Tertiary Phosphines and Vice Versa: Stereochemical Hysteresis
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 29, 1999
The substitution reactions of primary amines by tertiary phosphines in quadruply bonded dimolybdenum(II,II)
complexes Mo2Cl4(NH2R)4 have been studied. The exchange reaction has been shown to result at room temperature
in disubstituted species Mo2Cl4(NH2R)2(PR3)2 (PR3 ) PMe3, NH2R ) NH2Prn (1a), NH2But (2a), NH2Cy (3a);
PR3 ) PMe2Ph, NH2R ) NH2Cy (4a)), while heating is needed to obtain fully substituted complexes Mo2Cl4-
(PR3)4. The crystal structure of disubstituted products has been investigated by X-ray crystallography and revealed
that they all belong to the R-isomer, having both phosphine groups at the same Mo atom. Crystal data are as
follows: for 1a, tetragonal space group I41/a with a ) 17.737(2) Å, c ) 15.6915(6) Å, and Z ) 8; for 3a,
monoclinic space group P21 with a ) 10.963(3) Å, b ) 10.117(2) Å, c ) 13.323(4) Å, â ) 90.05(2)°, and Z )
2; for 4a, triclinic space group P1h with a ) 9.329(3) Å, b ) 10.206(2) Å, c ) 18.975(3) Å, R ) 85.45(2)°, â )
87.10(1)°, γ ) 80.88(1)°, and Z ) 2. The substitution processes for the direct and reverse reactions have been
monitored by 31P NMR. They both proceed in a stepwise manner; however, a stereochemical hysteresis is taking
place, i.e., the back reaction, the substitution of phosphines by amines, goes through another isomer of Mo2Cl4-
(NH2R)2(PR3)2, having phosphine ligands on different Mo atoms. This â-isomer is more thermodynamically stable
and can be obtained by thermal conversion of the R-form. All chemical equilibria studied in the paper have been
explained as governed by a higher trans effect of PR3 groups compared to NH2R groups.
Introduction
location of substituent atoms on one (IIa) or both ends (IIb) of
the molecule.
The ligand substitution reactions at the quadruply bonded
Mo24+ core were the subject of several studies.1-9 Among them
there are two which examined the reaction of phosphine
interchange in Mo2X4(PR3)4 (X ) Me5 or Cl9) species. In both
cases it has been shown that substitution proceeds in stepwise
manner (Scheme 1); however, different conclusions were drawn
about the reaction mechanism. In the work published by
Andersen et al.5 the dissociative mechanism was proposed on
a base of invariance of the reaction rates on the nature of the
incoming group for phosphine interchange in Mo2Me4(PEt3)4.
In a recently reported study by Chisholm and McInnes,9 a rate
dependence on the entering ligand in phosphine exchange at
Mo2Cl4(PR3)4 was found and an interchange dissociative process
was suggested.
One detail in the above-mentioned studies5,9 was of particular
interest for us: in most of the cases the phosphine exchange
led to the IIb structure, while in two reactions substitution by
trimethylphosphine9 resulted in the IIa isomer. The explanation
given was based on the higher trans effect of the PMe3 group
compared to other phosphines used. Thus we decided to check
how important this feature is by investigating the systems where
the difference in trans effects between outgoing and incoming
groups is pronounced.
We have recently reported a new class of quadruply bonded
dimolybdenum complexes Mo2Cl4N4 with secondary (N )
NHEt2)10 and primary (N ) NH2Et, NH2Prn, NH2But, and NH2-
Cy)11 amines. The compound Mo2Cl4(NHEt2)4 has been shown12
to undergo facile substitution reactions of the amine ligands by
phosphines at room temperature to give species with the same
core structure Mo2Cl4(phosphine)4 (phosphine ) PMe3, PMe2-
Ph, PHEt2, dmpm, dmpe). However, similar substitution of
primary amines does not occur in such an easy manner,
terminating midway at room temperature, and heating is
necessary to accomplish it. Moreover, back reaction (substitution
of phosphines by amines) also takes place.
The substitution reactions proceed without ligand rearrange-
ment; therefore, the mixed-phosphine species I and III (Scheme
1) exist in one form, whereas the disubstituted product II can
adopt two isomeric forms (Scheme 2), which differ by the
(1) Hynes, M. J. J. Inorg. Nucl. Chem. 1972, 34, 366.
(2) Webb, T. R.; Espenson, J. H. J. Am. Chem. Soc. 1974, 96, 6289.
(3) Mureinik, R. J. Inorg. Chim. Acta 1977, 23, 103.
(4) Teramoto, K.; Sasaki, Y.; Migita, K.; Iwaizumi, M.; Saito, K. Bull.
Chem. Soc. Jpn. 1979, 52, 446.
(5) Girolami, G. S.; Mainz, V. V.; Andersen, R. A.; Vollmer, S. H.; Day,
V. W. J. Am. Chem. Soc. 1981, 103, 3953.
We report here the results of our study of stepwise substitution
of amine ligands by tertiary phosphines, and vice versa, and
the structure of intermediate products.
(6) Girolami, G. S.; Mainz, V. V.; Andersen, R. A. J. Am. Chem. Soc.
1982, 104, 2041.
(7) Casas, J. M.; Cayton, R. H.; Chisholm, M. H. Inorg. Chem. 1991, 30,
358.
(10) Cotton, F. A.; Dikarev, E. V.; Herrero, S. Inorg. Chem. 1998, 37,
5862.
(11) Cotton, F. A.; Dikarev, E. V.; Herrero, S. Inorg. Chem. 1999, 38,
(8) Chen, H.; Cotton, F. A. Polyhedron 1995, 14, 2221.
(9) Chisholm, M. H.; McInnes, J. M. J. Chem. Soc., Dalton Trans. 1997,
2735.
2649.
(12) Cotton, F. A.; Dikarev, E. V.; Herrero, S. Inorg. Chem. 1999, 38,
490.
10.1021/ic9913684 CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/20/2000

610 Inorganic Chemistry, Vol. 39, No. 3, 2000
Cotton et al.
Scheme 1
Anal. Calcd for Mo2Cl4N2P2C18H44: C, 31.60; H, 6.48; N, 4.09.
Found: C, 31.96; H, 6.54; N, 4.03. 1H NMR data (benzene-d6, 22 °C)
δ: 4.56 (d, 4H, NH2), 3.25 (m, 2H, CH), 1.8-0.7 (20H, CH2) [J(NH2-
CH) ) 7 Hz]; 1.43 (t, 18H, PMe3) [J(P-CH3) ) 4 Hz]. 31P NMR data
(benzene-d6, 22 °C) δ: -13.70 (s, PMe3).
Scheme 2
(iv) Synthesis of Mo2Cl4(NH2Cy)2(PMe2Ph)2 (4a). The same
procedure as described for Mo2Cl4(NH2Cy)2(PMe3)2 was used for the
preparation of Mo2Cl4(NH2Cy)2(PMe2Ph)2, although 1 day was needed
to complete the reaction with a quantitative (about 95%) yield.
Red-violet blocks of Mo2Cl4(NH2Cy)2(PMe2Ph)2 (4a) were formed
by cooling a saturated solution of the complex in hexanes to -30 °C.
Anal. Calcd for Mo2Cl4N2P2C28H48: C, 41.61; H, 5.99; N, 3.47.
Found: C, 41.56; H, 6.08; N, 3.16. 31P NMR data (benzene-d6, 22 °C)
δ: -3.24 (s, PMe2Ph).
(2) Reactions of Mo2Cl4(NH2R)4 with PR3 under Reflux Condi-
tions. (i) Synthesis of Mo2Cl4(PMe3)4. A toluene solution which
contained 0.3 g (0.41 mmol) of Mo2Cl4(NH2Cy)4 and 10 mmol of PMe3
was heated to its boiling point and kept under reflux for 1 h. The color
of the solution changed from pink-purple to royal blue. The solution
was cooled to room temperature, filtered, and evaporated under vacuum
to dryness. Yield: 0.24 g (92%). The purity of the compound was
Experimental Section
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. Amines were purchased from Aldrich,
Inc., phosphines were provided by Strem Chemicals Inc., and benzene-
d6 was obtained from Cambridge Isotope Laboratories, Inc. Compounds
Mo2Cl4(NH2R)4 (R ) Prn, But, Cy),11 Mo2Cl4(NHEt2)4,10 and Mo2Cl4-
1
checked by H and 31P NMR.12
(ii) Synthesis of Mo2Cl4(PMe2Ph)4. This compound was prepared
similarly to Mo2Cl4(PMe3)4 and isolated as a royal blue powder in 90%
yield. The purity of the compound was checked by 31P NMR.9
(3) Solution 31P NMR Study of the Reaction between Mo2Cl4-
(NH2R)4 and PR3. (i) Reaction of Mo2Cl4(NH2Cy)4 with PMe3. An
excess of PMe3 (7.0 mL of 1 M solution in toluene) was added to a
solution of 0.20 g (0.27 mmol) of Mo2Cl4(NH2Cy)4 in 6 mL of
deuterated benzene. The mixture was stirred at room temperature for
12 h and then was heated at 75 °C for 6 h. The course of the reaction
was monitored by 31P{1H} NMR at 3 h intervals.
12
(PHEt2)4 were synthesized according to published procedures.
(1) Reactions of Mo2Cl4(NH2R)4 with PR3 at Room Temperature.
(i) Synthesis of Mo2Cl4(NH2Prn)2(PMe3)2 (1a). Trimethylphosphine
(0.6 mL, 5.90 mmol) was added to a red solution of 0.08 g (0.14 mmol)
of Mo2Cl4(NH2Prn)4 in 5 mL of toluene. After stirring overnight at room
temperature, the color of the solution had changed to blue-violet. The
solution was then filtered, and all volatile components were removed
under vacuum. Yield: 0.08 g (94%).
Brown, plate-shaped crystals of Mo2Cl4(NH2Prn)2(PMe3)2 (1a) suit-
able for X-ray study were obtained by cooling a saturated solution of
the compound in diethyl ether to -30 °C.
Anal. Calcd for Mo2Cl4N2P2C12H36: C, 23.87; H, 6.01; N, 4.64.
Found: C, 24.57; H, 6.10; N, 4.16. 1H NMR data (benzene-d6, 22 °C)
δ: 4.17 (br, t,4H, NH2), 2.80 (m,4H, CH2), 1.13 (m, 4H, CH′2), 0.66
(t, 6H, CH3) [J(NH2-CH2) ) J(CH2-CH′2) ) J(CH′2-CH3) ) 7 Hz];
1.39 (t, 18H, PCH3) [J(P-CH3) ) 4 Hz]. 31P NMR data (benzene-d6,
22 °C) δ: -14.07 (s, PMe3).
(ii) Synthesis of Mo2Cl4(NH2But)2(PMe3)2 (2a). Complex Mo2-
Cl4(NH2But)4 (0.08 g, 0.13 mmol) was dissolved in 50 mL of hexanes,
and 0.6 mL of PMe3 (5.90 mmol) was added to the purple solution.
The color of the solution started to change immediately after the
addition. After 30 min of stirring at room temperature the blue-violet
solution was filtered and evaporated to dryness. Yield: 0.075 g (91%).
Violet block-shaped crystals of Mo2Cl4(NH2But)2(PMe3)2 (2a) were
obtained in different ways: by slow evaporation of a concentrated
benzene solution, or by cooling saturated solutions of the complex in
hexanes or diethyl ether to -30 °C.
Anal. Calcd for Mo2Cl4N2P2C14H40: C, 26.60; H, 6.38; N, 4.43.
Found: C, 26.94; H, 6.32; N, 4.20. 1H NMR data (benzene-d6, 22 °C)
δ: 4.76 (s, br, 4H, NH2), 1.33 (t, 18H, PMe3) [J(P-CH3) ) 4 Hz],
1.19 (s, 18H, CH3). 31P NMR data (benzene-d6, 22 °C) δ: -14.27 (s,
PMe3).
(iii) Synthesis of Mo2Cl4(NH2Cy)2(PMe3)2 (3a). A procedure similar
to that described for the synthesis of Mo2Cl4(NH2Prn)2(PMe3)2 was
followed to prepare Mo2Cl4(NH2Cy)2(PMe3)2 using Mo2Cl4(NH2Cy)4
as starting material. The blue-violet solid was isolated in a yield of
96%.
The violet plates of Mo2Cl4(NH2Cy)2(PMe3)2 (3a) used for the X-ray
experiment were obtained by keeping a saturated solution of the
compound in diethyl ether at -30 °C for a week.
(ii) Reaction of Mo2Cl4(NH2Cy)4 with PMe2Ph. A similar proce-
dure was performed using 40 equiv of PMe2Ph per 1 equiv of Mo2-
Cl4(NH2Cy)4.
(4) Solution 31P NMR Study of the Reaction between Mo2Cl4-
(PR3)4 and NH2R. (i) Reaction of Mo2Cl4(PMe3)4 with NH2Cy. A
solution of 0.10 g (0.16 mmol) of Mo2Cl4(PMe3)4 and 0.72 mL (6.29
mmol) of NH2Cy in 5 mL of toluene was refluxed for 15 h. A
combination of 1H and 31P{1H} NMR spectra was used to estimate the
ratio of the species in solution. In addition, the reaction mixture
immediately after mixing of the reactants was monitored in situ by
31P{1H} NMR spectroscopy at 75 °C and 3 h intervals.
(ii) Reaction of Mo2Cl4(PMe2Ph)4 and Mo2Cl4(PEt2H)4 with
NH2Cy. Similar monitoring procedures were used to follow the
reactions of Mo2Cl4(PR3)4 (PR3 ) PMe2Ph and PEt2H) with ca. 40 equiv
of NH2Cy.
(5) Solution 31P NMR Study of the Isomerization of Mo2Cl4-
(NH2R)2(PR3)2. (i) Transformation of Mo2Cl4(NH2Cy)2(PMe3)2 (3a).
A solution of 0.20 g of Mo2Cl4(NH2Cy)2(PMe3)2 (3a) (prepared as
described in (1)) in 5 mL of toluene was refluxed for 3 days. The course
of the transformation was monitored by 31P{1H} NMR at 6 h intervals.
(ii) Transformation of Mo2Cl4(NH2But)2(PMe3)2 (2a). A similar
procedure was employed for a toluene solution of Mo2Cl4(NH2But)2-
(PMe3)2 (2a).
(6) Solution 31P NMR Study of the Reaction between Mo2Cl4-
(NH2R)2(PR3)2 and PR3′. (i) Reaction of Mo2Cl4(NH2Cy)2(PMe3)2
(3a) with PMe2Ph. A solution which contained 0.20 g (0.29 mmol) of
Mo2Cl4(NH2Cy)2(PMe3)2 (3a) (prepared as described in (1)) and 1.60
mL (11.20 mmol) of PMe2Ph in 10 mL of toluene was stirred at room
temperature for 2 days. The equilibrium was monitored by 31P{1H}
NMR spectroscopy at 6 h intervals.

Mixed Chloride/Amine Complexes of Dimolybdenum(II,II). 6
Inorganic Chemistry, Vol. 39, No. 3, 2000 611
Table 1. Crystallographic Data for R-Mo2Cl4(NH2Prn)2(PMe3)2 (1a), R-Mo2Cl4(NH2Cy)2(PMe3)2 (3a), and R-Mo2Cl4(NH2Cy)2(PMe2Ph)2 (4a)
1a
3a
4a
formula
fw
space group
a, Å
Mo2Cl4P2N2C12H36
604.05
I41/a (No. 88)
17.737(2)
Mo2Cl4P2N2C18H44
684.17
P21 (No. 4)
10.963(3)
Mo2Cl4P2N2C28H48
808.30
P1h (No. 2)
9.329(3)
b, Å
c, Å
10.117(2)
13.323(4)
10.206(2)
18.975(3)
85.45(2)
87.10(1)
80.88(1)
1776.9(7)
2
1.511
15.6915(6)
R, deg
â, deg
γ, deg
V, Å3
Z
90.05(2)
4936.6(8)
8
1.625
1477.7(7)
2
1.538
F
calc., g cm-3
µ, mm-1
1.577
1.327
1.117
radiation (λ, Å)
temp, °C
Mo KR (0.710 73)
Mo KR (0.710 73)
Mo KR (0.710 73)
-60
-60
-60
R1,a wR2b [I > 2σ(I)]
R1,a wR2b (all data)
0.0276, 0.0625
0.0326, 0.0670
0.0358, 0.0909
0.0381, 0.0940
0.0547, 0.1222
0.0739, 0.1391
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo - Fc2)2]/∑[w(Fo )2]]1/2
.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
R-Mo2Cl4(NH2Prn)2(PMe3)2 (1a), R-Mo2Cl4(NH2Cy)2(PMe3)2 (3a),
and R-Mo2Cl4(NH2Cy)2(PMe2Ph)2 (4a)
corrected for Lorentz and polarization effects by the MADNES
program.14 Reflection profiles were fitted and values of F2 and σ(F2)
for each reflection were obtained by the program PROCOR.15
All calculations were done on a DEC Alpha running VMS using
programs SHELXTL16 (structure solution) and SHELXL-9317 (least-
squares refinement). Neither metal nor ligand disorder has been
detected. Anisotropic displacement parameters were assigned to all non-
hydrogen atoms. Hydrogen atoms bonded to nitrogen were refined
independently. The rest of the H atoms were included in the structure
factor calculations at idealized positions. Relevant crystallographic data
for complexes 1a, 3a, and 4a are summarized in Table 1, and selected
bond distances and angles are given in Table 2.
1a
3a
4a
Mo(1)-Mo(2)
Mo(1)-N(1)
2.1250(6)
2.240(3)
2.1289(9)
2.240(7)
2.239(6)
2.413(2)
2.414(2)
2.529(2)
2.535(2)
2.432(2)
2.422(2)
160.9(2)
140.82(7)
155.43(7)
146.00(7)
99.4(2)
2.128(1)
2.229(6)
2.250(7)
2.415(2)
2.417(2)
2.527(2)
2.539(2)
2.424(2)
2.425(2)
162.5(3)
139.15(8)
153.17(8)
148.04(8)
98.8(2)
Mo(1)-N(2)
Mo(1)-Cl(1)
Mo(1)-Cl(2)
Mo(2)-P(1)
2.4292(9)
2.539(1)
2.4135(9)
Mo(2)-P(2)
Mo(2)-Cl(3)
Mo(2)-Cl(4)
N-Mo(1)-N
Cl-Mo(1)-Cl
P-Mo(2)-P
Results
161.0(2)
138.36(5)
155.30(5)
144.42(5)
99.51(9)
110.82(2)
102.35(2)
107.79(3)
Mo2Cl4(NH2R)4 + PR3. The reaction between primary amine
complexes Mo2Cl4(NH2R)4 and an excess of tertiary phosphines
proceeds at room temperature as evidenced by the change of
color from red to blue violet. Time of completion, which is
dependent on both entering and leaving groups, increases in
the order PMe3 < PMe2Ph and NH2But < NH2Cy < NH2Prn.
The elemental analyses of the products were consistent with
the formula Mo2Cl4(NH2R)2(PR3)2. The 31P NMR spectra in
benzene solution showed only a singlet which was slightly
downfield (Table 3) from that for the corresponding Mo2Cl4-
Cl-Mo(2)-Cl
Mo(2)-Mo(1)-N
Mo(2)-Mo(1)-Cl
Mo(1)-Mo(2)-P
Mo(1)-Mo(2)-Cl
109.59(6)
102.28(6)
107.00(6)
110.43(7)
103.41(6)
105.98(6)
(ii) Reaction of Mo2Cl4(NH2But)2(PMe3)2 (2a) with PHEt2. A
similar procedure was carried out employing Mo2Cl4(NH2But)2(PMe3)2
and PEt2H as starting materials in diethyl ether.
(7) Solution 31P NMR Study of the Reaction between Mo2Cl4-
(NHEt2)4 and PMe3. A mixture of Mo2Cl4(NHEt2)4 and PMe3 (1:40)
in toluene was monitored by 31P{1H} NMR over the temperature range
-70 to 20 °C until the spectrum showed Mo2Cl4(PMe3)4 as the only
complex in solution.
1
(PR3)4 species. The H NMR spectra displayed signals for the
NH2R group (close to those in Mo2Cl4(NH2R)4 starting materi-
als) and the phosphine. The integrals showed that the amine/
phosphine ratios were 1:1. Thus we found that the title reactions
stop at the doubly substituted stage at room temperature in
contrast to reactions with the analogous dimolybdenum com-
plexes with secondary amines,10 which exhibit instantly full
substitution under similar conditions.
The X-ray diffraction study has been performed in order to
determine which isomer, IIc or IId, is the reaction product. The
crystal structure investigation revealed that complexes of the
type Mo2Cl4(NH2R)2(PR3)2 (PR3 ) PMe3; NH2R ) NH2Prn
(1a), NH2Cy (3a) and PR3 ) PMe2Ph; NH2R ) NH2Cy (4a))
Physical Measurements. 1H NMR spectra were obtained on a
UNITY-plus 300 multinuclear spectrometer. Resonances were refer-
enced internally to the residual proton impurity in the deuterated solvent.
31P{1H} NMR data were recorded at 22 °C on a UNITY-plus 300
multinuclear spectrometer at 121.4 MHz. Resonances in the 31P{1H}
NMR data were referenced to an external standard 85% H3PO4 (0.00
ppm). Elemental analyses were done by Canadian Microanalytical
Services, Ltd.
X-ray Crystallographic Procedures. Single crystals of compounds
1a, 3a, and 4a were obtained as described above. X-ray diffraction
experiments were carried out on a Nonius FAST diffractometer with
an area detector using Mo KR radiation. Details concerning data
collection have been fully described elsewhere.13 Each crystal was
mounted on the tip of a quartz fiber with silicone grease, and the setup
was quickly placed in the cold N2 stream (-60 °C) of a low-temperature
controller. Fifty reflections were used in cell indexing and about 250
reflections (16° < 2θ < 42°) in cell refinement. Axial images were
used to confirm the Laue group and all dimensions. The data were
(14) Pflugrath, J.; Messerschmitt, A. MADNES, Munich Area Detector
(New EEC) System, version EEC 11/9/89, with enhancements by
Enraf-Nonius Corp., Delft, The Netherlands. A description of MADNES
appears in the following: Messerschmitt, A.; Pflugrath, J. J. Appl.
Crystallogr. 1987, 20, 306.
(15) (a) Kabsch, W. J. Appl. Crystallogr. 1988, 21, 67; (b) 1988, 21, 916.
(16) SHELXTL V.5; Siemens Industrial Automation Inc.: Madison, WI,
1994.
(17) Sheldrick, G. M. In Crystallographic Computing 6; Flack, H. D.,
Parkanyi, L., Simon, K., Eds.; Oxford University Press: Oxford, U.K.,
1993; p 111.
(13) Cotton, F. A.; Dikarev, E. V.; Feng, X. Inorg. Chim. Acta 1995, 237,
19.

612 Inorganic Chemistry, Vol. 39, No. 3, 2000
Cotton et al.
Table 3. Summary of 31P{1H} NMR Data in Benzene-d6 for the Compounds under Discussion
compound
δ (ppm), J (Hz)
Mo2Cl4(NH2Prn)3(PMe3)
-6.05(s)
-14.07 (s)
R-Mo2Cl4(NH2Prn)2(PMe3)2
Mo2Cl4(NH2But)3(PMe3)
R-Mo2Cl4(NH2But)2(PMe3)2
â-Mo2Cl4(NH2But)2(PMe3)2
Mo2Cl4(NH2But)(PMe3)3
-6.32(s)
-14.27 (s)
-2.33 (s)
0.07 (t, 1PMe3), -10.85 (d, 2PMe3), 3J(PP) ) 22
Mo2Cl4(NH2Cy)3(PMe3)
R-Mo2Cl4(NH2Cy)2(PMe3)2
â-Mo2Cl4(NH2Cy)2(PMe3)2
Mo2Cl4(NH2Cy)(PMe3)3
Mo2Cl4(PMe3)4
-5.88(s)
-13.70 (s)
-2.34 (s)
-0.45 (t, 1PMe3), -10.51 (d, 2PMe3), 3J(PP) ) 22
-8.68 (s)
Mo2Cl4(NH2Cy)3(PMe2Ph)
R-Mo2Cl4(NH2Cy)2(PMe2Ph)2
â-Mo2Cl4(NH2Cy)2(PMe2Ph)2
Mo2Cl4(NH2Cy)(PMe2Ph)3
Mo2Cl4(PMe2Ph)4
3.72(s)
-3.24 (s)
5.92 (s)
6.86 (t, 1PMe2Ph), -0.92 (d, 2PMe2Ph), 3J(PP) ) 21
0.29(s)
Mo2Cl4(NH2Cy)3(PHEt2)
â-Mo2Cl4(NH2Cy)2(PHEt2)2
Mo2Cl4(NH2Cy)(PHEt2)3
Mo2Cl4(PHEt2)4
9.36(s)
12.19 (s)
13.49 (t, 1PHEt2), 2.95 (dd, 2PHEt2), 3J(PP) ) 21
3.97(d)
R-Mo2Cl4(NH2But)2(PHEt2)(PMe3)
R-Mo2Cl4(NH2Cy)2(PMe3)(PMe2Ph)
1.20 (d, 1PHEt2), -13.77 (d, 1PMe3), 2J(PP) ) 165
-5.09 (d, 1PMe2Ph), -11.77 (d, 1PMe3), 2J(PP) ) 163
Mo2Cl4(NHEt2)3(PMe3)
R-Mo2Cl4(NHEt2)2(PMe3)2
â-Mo2Cl4(NHEt2)2(PMe3)2
Mo2Cl4(NHEt2)(PMe3)3
-6.84(s)
-14.61 (s)
-1.94 (s)
1.10 (t, 1PMe3), -10.97 (d, 2PMe3), 3J(PP) ) 22
Scheme 3
are all obtained (Scheme 3) with a structure of type IIc, that is,
with both phosphine groups located on the same molybdenum
atom (Figures 1-3). We designate this isomer as R, by analogy
with chelating diphosphine complexes.18
High temperature is required to complete the substitution of
primary amine ligands by phosphines. While refluxing in
toluene, the color of the solution gradually changes to royal
blue, and complexes Mo2Cl4(PR3)4 can be identified in the
resulting mixture in nearly quantitative yield (Scheme 4).
The progress of the reactions was monitored by 31P{1H} NMR
to follow the sequential substitution of primary amine ligands
by phosphines. At 20 °C within 1 h after the addition of PR3 to
a deuterated benzene solution of Mo2Cl4(NH2R)4, a strong 31P
signal due to the monosubstituted product is observed as well
as a small signal due to the disubstituted compound. As can be
seen in Figure 4, after 12 h at 20 °C reaction to give the Mo2-
Cl4(NH2R)2(PR3)2 product is essentially complete, and there is
no indication of any other product, nor is there any further
change in the spectrum, even though excess PR3 is present. The
Figure 1. Perspective drawing of the R-isomer of Mo2Cl4(NH2Prn)2-
(PMe3)2 (1a). Atoms are represented by thermal ellipsoids at the 40%
probability level. Carbon and hydrogen atoms are shown as spheres of
arbitrary radii.
is much more rapid than the third, and therefore the signals for
Mo2Cl4(NH2R)(PR3)3 never become very strong. All of this can
be seen in the upper three spectra of Figure 4.
Since we have isolated and crystallographically characterized
three of the Mo2Cl4(NH2R)2(PR3)2 molecules prepared by the
reaction of Mo2Cl4(NH2R)4 with PR3 and shown that they all
have the R structure, IIc, it is clear that the overall pathway
from Mo2Cl4(NH2R)4 to Mo2Cl4(PR3)4 traverses this intermedi-
ate, as shown in Scheme 5.
Mo2Cl4(PR3)4 + NH2R. The reverse reaction, i.e., substitu-
tion of phosphine ligands in Mo2Cl4(PR3)4 by an excess of
primary amine, also proceeds in a stepwise manner. Immediately
after heating (to 75 °C) a toluene solution of Mo2Cl4(PMe3)4
1
liberation of NH2R can also be observed by H NMR.
When the reaction mixture is heated to 75 °C, signals due to
Mo2Cl4(NH2R)(PR3)3, a doublet and a triplet as expected,
promptly appear as well as a singlet due to the completely
substituted molecule, Mo2Cl4(PR3)4. The fourth substitution step
(18) Best, S. A.; Smith, T. J.; Walton, R. A. Inorg. Chem. 1978, 17, 99.

Mixed Chloride/Amine Complexes of Dimolybdenum(II,II). 6
Inorganic Chemistry, Vol. 39, No. 3, 2000 613
Figure 2. Perspective drawing of the R-isomer of Mo2Cl4(NH2Cy)2-
(PMe3)2 (3a). Atoms are represented by thermal ellipsoids at the 40%
probability level. Carbon and hydrogen atoms are shown as spheres of
arbitrary radii.
Figure 3. Perspective drawing of the R-isomer of Mo2Cl4(NH2Cy)2-
(PMe2Ph)2 (4a). Atoms are represented by thermal ellipsoids at the
40% probability level. Carbon and hydrogen atoms are shown as spheres
of arbitrary radii.
Scheme 4
Figure 4. 31P {1H} monitoring of the reaction between Mo2Cl4(NH2-
Cy)4 and PMe3 at various time intervals and temperatures in C6D6.
After 3 h of heating, the complex Mo2Cl4(NH2Cy)3(PMe3)
was detected in solution (Figure 5), but the reaction is very slow
at this point. Even after 15 h of reflux it was not completed,
and the 31P and 1H NMR results taken together show the
presence of three compounds in solution: â-Mo2Cl4(NH2Cy)2-
(PMe3)2, Mo2Cl4(NH2Cy)3(PMe3), and Mo2Cl4(NH2Cy)411 in an
estimated ratio of 13:37:50.
All the results in Figures 4 and 5 are for PMe3. When the
phosphine is not as basic as PMe3, its substitution by amine
occurs without heating. After 1 day the reaction of Mo2Cl4-
(PEt2H)4 with NH2Cy at room temperature shows a 1:1 mixture
of starting material and â-Mo2Cl4(NH2Cy)2(PEt2H)2, and re-
sidual amounts of other species.
and NH2Cy, the doublet and triplet signals of the complex Mo2-
Cl4(NH2Cy)(PMe3)3 (Figure 5) as well as that of the free
phosphine start to appear. Almost at the same time a new singlet
begins to grow. Its position (δ ) -2.34) (Table 3) is quite
different from the one for the R-isomer of Mo2Cl4(NH2Cy)2-
(PMe3)2 (δ ) -13.70). It is located between those for the PMe3
groups in the MoCl2(NH2Cy)(PMe3) parts of the Mo2Cl4(NH2-
Cy)3(PMe3) (δ ) -5.88) and Mo2Cl4(NH2Cy)(PMe3)3 (δ )
-0.45) species. We assign this signal to the â-isomer of Mo2-
Cl4(NH2Cy)2(PMe3)2 (type IId) in which the phosphine groups
are on different molybdenum atoms.

614 Inorganic Chemistry, Vol. 39, No. 3, 2000
Cotton et al.
Figure 5. 31P {1H} monitoring of the reaction between Mo2Cl4(PMe3)4 and NH2Cy at 75 °C in toluene at 3 h time intervals.
Scheme 5
The results displayed in Figures 4 and 5 clearly show that
the overall interconversion of Mo2Cl4(NH2R)4 and Mo2Cl4(PR3)4
compounds proceeds through isomeric intermediates of com-
position Mo2Cl4(NH2R)2(PR3)2, depending on the direction, as
shown in Scheme 5. This phenomenon can be designated as a
“stereochemical hysteresis”.
r f â Isomerization of Mo2Cl4(NH2R)2(PR3)2. The R-i-
somers of Mo2Cl4(NH2R)2(PR3)2 are relatively stable in solution
at room temperature, even in the presence of free phosphine or
amine. However, soon after heating a solution of the pure
compound in toluene, signals corresponding to Mo2Cl4(NH2R)3-
(PR3), Mo2Cl4(NH2R)(PR3)3, Mo2Cl4(PR3)4, and Mo2Cl4-
(NH2R)411 (the latter was detected by 1H NMR) appear (Figure
6). The â-isomer of Mo2Cl4(NH2R)2(PR3)2 soon becomes the
major component of the mixture, but even after 3 days of reflux
the concentrations of other species (except the R-isomer) are
still significant (Figure 6). The main source of the â-isomer is,
evidently, the complex Mo2Cl4(NH2R)(PR3)3 which, we know,
can undergo a substitution reaction (Scheme 5) with free amine.
Another possible mechanism for isomerization, an intermolecu-
lar “internal flip” of a dimolybdenum unit within the cubic cage
of ligands (Scheme 6), which comes to mind by analogy with
R f â isomerization of diphosphine complexes Mo2Cl4(P-P)2,19
is not a likely route in this case. Attempts to see if this might
occur in the solid state were unsuccessful because of the low
thermal stability of the R-Mo2Cl4(NH2R)2(PR3)2 species.
r-Mo2Cl4(NH2R)2(PR3)2 + PR′3. After 3 h of stirring a
mixture of R-Mo2Cl4(NH2Cy)2(PMe3)2 and an excess of PMe2-
Ph in toluene at room temperature, two doublets (Table 3) of
equal intensity appeared. Shortly after that the signal corre-
(19) (a) Agaskar, P. A.; Cotton, F. A. Inorg. Chem. 1986, 25, 15. (b) Cotton,
F. A.; Kitagawa, S. Polyhedron 1988, 7, 463. (c) Cayton, R. H.;
Chisholm, M. H. Inorg. Chem. 1991, 30, 1422. (d) McVitie, A.;
Peacock, R. D. Polyhedron 1992, 11, 2531.

Mixed Chloride/Amine Complexes of Dimolybdenum(II,II). 6
Inorganic Chemistry, Vol. 39, No. 3, 2000 615
Figure 6. 31P {1H} spectra of R-Mo2Cl4(NH2Cy)2(PMe3)2 at different time intervals after reflux in toluene.
Scheme 6
sponding to the R-Mo2Cl4(NH2Cy)2(PMe2Ph)2 complex started
to grow. We, therefore, identified the first complex as R-Mo2-
Figure 7. 31P {1H} spectrum of reaction mixture R-Mo2Cl4(NH2Cy)2-
Cl4(NH2Cy)2(PMe3)(PMe2Ph) because the large P-P coupling
constant shows that both phosphine groups are on the same
molybdenum atom. After 55 h the starting material was almost
consumed and the ratio of two PMe2Ph containing complexes
was 1:4 (Figure 7). Thus, at room temperature, only the
exchange of phosphine groups has been observed (Scheme 7),
but on heating, further substitution of the amine ligands
proceeds.
(PMe3)2 and PMe2Ph after 55 h at room temperature in toluene.
In an attempt to isolate R-Mo2Cl4(NHEt2)2(PMe3)2 we tried
a reaction with only 2 equiv of phosphine. However, we
observed in the reaction mixture all species (Table 3), including
the â-isomer of Mo2Cl4(NHEt2)2(PMe3)2 which, most likely,
results from the substitution of one PMe3 group in Mo2Cl4-
(NHEt2)(PMe3)3 by amine.
Mo2Cl4(NHEt2)4 + PMe3. Reactions of the secondary amine
complex Mo2Cl4(NHEt2)4 with phosphines have already been
shown12 to proceed at room temperature instantly giving Mo2-
Cl4(PR3)4 complexes. The 31P{1H} NMR monitoring of the
reaction between Mo2Cl4(NHEt2)4 and an excess of PMe3 at
low temperatures showed that the substitution process is similar
to that in the case of primary amine complexes (Scheme 5).
The monosubstituted product and R-Mo2Cl4(NHEt2)2(PMe3)2,
which we identified by analogy with the corresponding NH2R
species (Table 3), appear at about -40 °C. The replacement of
the third group is not important until about 10 °C, and after
that the substitution was quickly completed.
Discussion
All equilibria reported in this paper can be explained by the
pronounced difference in the trans effects of NH2R and PR3
groups. Reaction of a dimolybdenum complex with primary
amine ligands, Mo2Cl4(NH2R)4, with tertiary phosphines at room
temperature stops at the disubstituted stage. After the first amine
has been replaced by a PR3 ligand, the next NH2R group to be
exchanged is that opposite to phosphine, thus giving the
disubstituted product as the R-isomer (two phosphine groups
at the same Mo atom). At low temperature (20 °C) the reaction

616 Inorganic Chemistry, Vol. 39, No. 3, 2000
Cotton et al.
Scheme 7
Scheme 8
does not proceed further although the two PR3 groups continue
to exchange with free phosphine in solution. That has been
proven by the reaction of R-Mo2Cl4(NH2R)2(PR3)2 with PR′3
in which it is phosphines, but not amines, that are replaced.
First and second substitutions at room temperature give R-Mo2-
Cl4(NH2R)2(PR3)(PR′3) and R-Mo2Cl4(NH2R)2(PR′3)2, respec-
tively, even if the less basic phosphines (PR′3 ) PMe2Ph, PEt2H)
are used against PMe3. The third substitution in Mo2Cl4(NH2R)4
must involve an NH2R group trans to another amine ligand and
requires heating to proceed. The compound Mo2Cl4(NH2R)-
(PR3)3 is never present in the reaction mixture in high
concentration. Once it appears, the replacement of the last NH2R
group, which is now activated by being located trans to a PR3
ligand, proceeds rapidly.
The reverse reaction, the substitution of phosphines by
amines, also proceeds in a stepwise manner, but takes a slightly
different route. It requires both heating and a long time to be
accomplished. The second substitution in this process occurs
at the other molybdenum atom, because it is much easier to
replace the ligand which is opposite to the PR3 group. That gives
the other isomer of Mo2Cl4(NH2R)2(PR3)2, which we call the â
isomer. The latter is, apparently, more thermodynamically stable
and can be obtained by thermal conversion of the pure R-form.
We have demonstrated that the main route to the â-form is by
substitution of phosphine by amine in Mo2Cl4(NH2R)(PR3)3
which, in turn, resulted from the replacement of NH2R by PR3
in the R-form:
Further substitution of phosphine ligands by amines in the
â-isomer of Mo2Cl4(NH2R)2(PR3)2 encounters difficulties and
goes very slowly as both remaining PR3 groups are now trans
to the NH2R groups. The complex Mo2Cl4(NH2R)3(PR3) is
relatively stable in these conditions and could be clearly seen
in the reaction mixture.
The substitution of secondary amine ligands in Mo2Cl4-
(NHEt2)4 by phosphines goes much faster than corresponding
reactions with primary amine compounds, and monitoring at
low temperature is required to observe the process. We have
noted earlier12 the partial dissociation of NHEt2 groups in
solution of starting material. That could make a difference in
the reaction mechanism, although the substitution sequence
seems to be the same as in the case of primary amines.
The “stereochemical hysteresis” observed here, as shown in
Scheme 5, has an interesting formal relationship (and has its
origin in the same fundamental concept of the trans effect) as
the classic chemistry of obtaining cis- or trans-PtCl2(NH3)2 that
is presented in most textbooks of inorganic chemistry (Scheme
8). In this classic case the controlling factor is that the trans
effect order is Cl > NH3, whereas in the present case we depend
on the trans effect order PR3 > NH2R.
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: Three X-ray crystallographic
files, in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
2Mo2Cl4(NH2R)2(PR3)2 a
Mo2Cl4(NH2R)3(PR3) + Mo2Cl4(NH2R)(PR3)3
IC9913684