W2Cl4(NR2)2(PR' 3)2 Molecules. 7. Preparation, Characterization, and Structures of W2Cl4(NHR)2(NH2R) 2 and W2Cl4(NHR)2(PMe3) 2 (R = sec-Butyl and Cyclohexyl) and 31P{1H} NMR Studies of Trans-to-Cis

Article abstract of DOI:10.1021/ic961485o

Treatment of W₂Cl₆,(THF)₄ with alkylamine NH₂R (R = Bu^s, Cy) affords a dinuclear species having the stoichiometry W₂Cl₄(NHR)₂NH₂R)₂ (R = Bu^s (1). Cy (2)). This has been confirmed by single-crystal X-ray diffraction studies for 2 with the following crystal data: tetragonal space group P4?2₁c, a = 12.774(2) ?, c = 9.934(2) ?, Z = 2. The molecule possesses an eclipsed structure with strong N-H?Cl intramolecular hydrogen bonding, with disordering of the whole set of ligands containing the amide, amine, and chlorine ligands. Attempts have been made to treat and refine both ligand sets separately for this molecule, and the final refinement converges with reasonable bond distances and angles to R = 0.028 and wR2 = 0.066. In both orientations, the ligand arrangements are the same. Each W atom is surrounded by a trans set of two Cl and two N atoms with a W-W separation of 2.2884(9) ?. Substitution of the amine ligands in 1 and 2 by the monodentate phosphine PMe₃ proceeds smoothly to produce trans-W₂Cl₄(NHR)₂(PMe₃)₂ (R = Bu_s (3), Cy (4)) in high yields. In solution, both 3 and 4 readily undergo isomerization to the corresponding cis-W₂Cl₄(NHR)₂(PMe₃)₂ (R = Bu^s (5), Cy (6)). The characterization of 3-6 has been accomplished by IR, ¹H NMR, and ³¹P{¹H}NMR spectroscopy and mass spectrometry. The crystal data for 5 and 6 are as follows: for 5, monoclinic space group P2₁/a, a = 13.339(3) ?, b = 13.446(3) ?, c = 15.179(3) ?, β= 99.33(2)°, Z = 4; for 6, P2₁/n, a = 8.455(1) ?, b = 25.714(3) ?, c = 13.454(1) ?, β= 104.839(8)°, Z = 4. Each of these phosphine-containing complexes is characterized by a W₂⁶⁺ metal core unit and has an eclipsed W₂Cl₄N₂P₂ conformation. The W-W bond distances for 5 and 6 are 2.321(1) and 2.3229(5) ?, respectively, and these compounds are shown to have PMe₃ ligands cis to the amides. On the other hand, kinetic studies by ³¹P{¹H}NMR spectroscopy show that the trans-to-cis transformation in solution is an irreversible process of the first order, which is different from the reversible process observed in the case of tert-butyl analog. The initial rate constant was 10(1) × 10^-3 min^-1, and the rate constants in the presence of excess PMe₃ were shown to be roughly constant (average 4.5 × 10^-3 min^-1) over a range of [PMe₃]. These observations could be understood if, in addition to a dissociative mechanism, internal flip steps operate as a second mechanism for the process, and the flip barrier is 25-29 kcal/mol.

Full text of DOI:10.1021/ic961485o

3268
Inorg. Chem. 1997, 36, 3268-3276
W2Cl4(NR2)2(PR′3)2 Molecules. 7. Preparation, Characterization, and Structures of
W2Cl4(NHR)2(NH2R)2 and W2Cl4(NHR)2(PMe3)2 (R ) sec-Butyl and Cyclohexyl) and
31P{1H} NMR Studies of Trans-to-Cis Isomerizations of W2Cl4(NHR)2(PMe3)2
F. Albert Cotton,* Evgeny V. Dikarev, and Wai-Yeung Wong
Department of Chemistry and Laboratory for Molecular Structure and Bonding, Texas A&M University,
College Station, Texas 77843-3255
ReceiVed December 12, 1996X
Treatment of W2Cl6(THF)4 with alkylamine NH2R (R ) Bus, Cy) affords a dinuclear species having the
stoichiometry W2Cl4(NHR)2(NH2R)2 (R ) Bus (1), Cy (2)). This has been confirmed by single-crystal X-ray
diffraction studies for 2 with the following crystal data: tetragonal space group P4h21c, a ) 12.774(2) Å, c )
9.934(2) Å, Z ) 2. The molecule possesses an eclipsed structure with strong NsH‚‚‚Cl intramolecular hydrogen
bonding, with disordering of the whole set of ligands containing the amide, amine, and chlorine ligands. Attempts
have been made to treat and refine both ligand sets separately for this molecule, and the final refinement converges
with reasonable bond distances and angles to R ) 0.028 and wR2 ) 0.066. In both orientations, the ligand
arrangements are the same. Each W atom is surrounded by a trans set of two Cl and two N atoms with a W-W
separation of 2.2884(9) Å. Substitution of the amine ligands in 1 and 2 by the monodentate phosphine PMe3
proceeds smoothly to produce trans-W2Cl4(NHR)2(PMe3)2 (R ) Bus (3), Cy (4)) in high yields. In solution,
both 3 and 4 readily undergo isomerization to the corresponding cis-W2Cl4(NHR)2(PMe3)2 (R ) Bus (5), Cy (6)).
1
The characterization of 3-6 has been accomplished by IR, H NMR, and 31P{1H}NMR spectroscopy and mass
spectrometry. The crystal data for 5 and 6 are as follows: for 5, monoclinic space group P21/a, a ) 13.339(3)
Å, b ) 13.446(3) Å, c ) 15.179(3) Å, â ) 99.33(2)°, Z ) 4; for 6, P21/n, a ) 8.455(1) Å, b ) 25.714(3) Å, c
) 13.454(1) Å, â ) 104.839(8)°, Z ) 4. Each of these phosphine-containing complexes is characterized by a
6+
W2 metal core unit and has an eclipsed W2Cl4N2P2 conformation. The W-W bond distances for 5 and 6 are
2.321(1) and 2.3229(5) Å, respectively, and these compounds are shown to have PMe3 ligands cis to the amides.
On the other hand, kinetic studies by 31P{1H}NMR spectroscopy show that the trans-to-cis transformation in
solution is an irreversible process of the first order, which is different from the reversible process observed in the
case of tert-butyl analog. The initial rate constant was 10(1) × 10-3 min-1, and the rate constants in the presence
of excess PMe3 were shown to be roughly constant (average 4.5 × 10-3 min-1) over a range of [PMe3]. These
observations could be understood if, in addition to a dissociative mechanism, internal flip steps operate as a
second mechanism for the process, and the flip barrier is 25-29 kcal/mol.
Introduction
In parts 1 and 2 of this series, we studied the detailed
molecular structures1 and the cis-trans isomerizations2 of W2-
Cl4(NHCMe3)2(PR′3)2-type compounds (R′3 ) Me3, Et3, Prn ,
3
Me2Ph) containing tert-butylamido ligands and various mono-
dentate phosphines (I and II in Figure 1). When the fact that
cis-trans isomerizations of W2Cl4(NHCMe3)2(PR′3)2 molecules
might occur via unimolecular processes was recognized, and
we were led to propose as a mechanism a simple internal
reorientation (or internal flip) of the W2 moiety inside the cavity
formed by the eight ligand atoms,2 our interest in further
investigating this system was stimulated. We have been
endeavoring to obtain additional independent and more direct
evidence to support the internal flip proposal. A challenging
aspect for us at this moment is to find ways to observe the
remaining possible cis isomer of Ci symmetry (III in Figure 1)
if the occurrence of an internal flip is to be corroborated, since
isomer III could be obtained by a mechanism entailing only
one internal flip.2
Figure 1. Three distinct isomeric forms of the W2Cl4N2P2 core.
substituents or PR′3 groups. On the basis of steric consider-
ations, it is believed that ligands of varying steric bulk may
afford new molecules of different stereochemical geometries
and properties. However, previous studies revealed that chang-
ing the PR′3 groups while keeping R ) CMe3 did not lead to
promising results with regard to our goal. We therefore turned
to a study of related systems with different amido groups in
the hope that the unobserved cis isomer might be detected and
even isolated. The work reported here represents the first of
We have now initiated a comprehensive study aimed at further
development of the chemistry of this class of compounds. One
means of accomplishing this is by variation of the amido
two papers describing our recent progress in this area.
A
detailed discussion of the structural and spectroscopic properties
of such compounds having sec-butylamido or cyclohexylamido
moieties will be presented in this report. Of prime importance
here is the proper treatment of the disorder of the ligand sets in
the structure refinement of trans-W2Cl4(NHR)2(NH2R)2 (R )
X Abstract published in AdVance ACS Abstracts, July 1, 1997.
(1) Cotton, F. A.; Yao, Z. J. Cluster Sci. 1994, 5, 11.
(2) Chen, H.; Cotton, F. A.; Yao, Z. Inorg. Chem. 1994, 33, 4255.
S0020-1669(96)01485-1 CCC: $14.00 © 1997 American Chemical Society

Study of W2Cl4(NHR)2(NH2R)2 and W2Cl4(NHR)2(PMe3)2
Inorganic Chemistry, Vol. 36, No. 15, 1997 3269
CMe3, Cy) molecules.3 In addition, the results of 31P{1H} NMR
studies of the trans-to-cis isomerizations for the W2Cl4(NHR)2-
(PMe3)2 (R ) Bus, Cy) compounds and the kinetic data for this
process in the case of R ) Cy will be presented.
recrystallized from hexanes to give a homogeneous red crystalline
material. Yield: 0.38 g (76%).
IR data (cm-1): 3249(sh), 3238(w), 1420(w), 1367(m), 1298(m),
1280(ms), 1261(s), 1146(ms), 1093(vs), 1023(s), 955(s), 863(w), 844-
(w), 800(s), 734(w), 710(m), 675(vw).
1H NMR data (benzene-d6, 24 °C, δ): 0.88 (t, J ) 7.5 Hz, CH2CH3),
0.90 (t, J ) 7.5 Hz, CH2CH3), 1.19 (d, J ) 6.6 Hz, CHCH3), 1.23 (d,
J ) 6.4 Hz, CHCH3), 1.40 (d, J ) 8.5 Hz, PMe3), 1.46 (m, CHCH2-
CH3), 1.66 (m, CHCH2CH3), 4.02 (m, CH), 12.05 (m, br, NH).
Experimental Section
General Procedures. All reactions and manipulations of the new
compounds were performed under a nitrogen or argon atmosphere on
a standard double-manifold vacuum line employing Schlenk techniques
and oven-dried glassware. Hexanes, toluene, and tetrahydrofuran (THF)
were purified by distillation under N2 from potassium/sodium ben-
zophenone ketyl. Dichloromethane was distilled under N2 from
phosphorus pentoxide. Chemicals were obtained from the following
sources: PMe3, Strem Chemicals; cyclohexylamine and (()-sec-
butylamine, Aldrich, Inc. They were used as received. Sodium
amalgam was prepared by dissolving a weighed amount of metallic
sodium in an approximately measured quantity of Hg that was pumped
under vacuum for at least 1 h in a Schlenk flask inside a drybox. WCl4
was prepared by refluxing WCl6 and W(CO)6 in chlorobenzene.4
1
31P{1H}NMR data (benzene-d6, 19 °C, δ): -5.30 (s, JW-P ) 116
1
Hz), -5.21 (s, JW-P ) 116 Hz).
FAB/DIP MS (NBA, m/z): 806 ([M]+), 769 ([M - Cl]+), 729 ([M
- PMe3]+), 693 ([M - Cl - PMe3]+), 654 ([M - 2PMe3]+).
(iv) cis-W2Cl4(NHBus)2(PMe3)2 (5). When a solution of trans-W2-
Cl4(NHBus)2(PMe3)2 (3) in isomeric hexanes was kept at room
temperature, the solution became steadily darker, and after 1 week,
red crystals together with some red-brown crystals of 5 were obtained
at the bottom of the Schlenk tube (overall yield, 72%). A structure
determination of the red-brown crystals showed this compound6 to be
polymorphic with that of the red crystals, each containing a different
enantiomeric ratio of R and S configurations of NHBus groups in a
single dinuclear complex. Thus, the molecules pack in slightly different
ways in these two cases, but because of poor refinement, the detailed
dimensions for the red-brown polymorph will not be reported.
IR data (cm-1): 3195(m), 1418(m), 1367(ms), 1342(vw), 1300(m),
1286(ms), 1280(m), 1267(m), 1142(m), 1133(vw), 1119(w), 1106(m),
1090(ms), 1034(w), 1010(w), 960(vs), 857(w), 797(m), 744(ms), 732-
(ms), 673(w), 530(vw).
5
3
Compounds W2Cl6(THF)4 and trans-W2Cl4(NHCMe3)2(NH2CMe3)2
were made according to the literature methods.
Syntheses. (i) trans-W2Cl4(NHBus)2(NH2Bus)2 (1). (()-sec-Bu-
tylamine (0.47 mL, 4.62 mmol) was added by syringe to a stirred
solution of W2Cl6(THF)4, prepared by reduction of WCl4 (0.50 g, 1.54
mmol) with 1 equiv of Na-Hg (0.4%) in THF (15 mL). The solution
turned gradually from greenish-yellow to orange-brown, and the reaction
mixture was allowed to stir for 12 h to ensure complete reaction. The
solution was then filtered through a Celite pad to remove Hg and NaCl,
after which the solvent was removed in Vacuo. The residue was
thoroughly dried and then extracted with warm hexanes (20 mL). An
abundant crop of orange powder of 1 was obtained upon cooling a
concentrated hexanes solution to -15 °C for 1 day. Yield: 0.42 g
(68% based on WCl4). However, several attempts to get suitable single
crystals for X-ray diffraction studies failed.
IR data (cm-1): 3284(w), 3226(m), 1606(w), 1563(m), 1338(vw),
1301(m), 1277(m), 1262(m), 1230(w), 1192(ms), 1158(w), 1134(w),
1086(s), 1061(w), 1035(m), 1011(m), 971(vw), 926(vw), 904(w), 872-
(vw), 817(m), 800(ms), 727(m).
1H NMR data (benzene-d6, 24 °C, δ): 0.53-1.23 (m, CH3), 1.46
(m, CH2), 1.66 (m, CH2), 3.20 (m, CH), 3.65 (m, br, NH2), 4.03 (m,
CH), 4.46 (m, br, NH2), 12.53 (d, J ) 5.6 Hz, NH).
(ii) trans-W2Cl4(NHCy)2(NH2Cy)2 (2). Procedures similar to those
just described were followed to prepare trans-W2Cl4(NHCy)2(NH2Cy)2,
using cyclohexylamine (0.53 mL, 4.62 mmol) instead of sec-butylamine.
Nicely formed orange needles of 2 (0.49 g, 70% based on 0.50 g of
WCl4) of good X-ray quality were obtained by keeping the hexanes
extract at room temperature for 15 h.
IR data (cm-1): 3325(m), 3268(w), 3234(m), 1619(vw), 1551(vs),
1507(vw), 1500(vw), 1340(m), 1314(vw), 1280(w), 1262(m), 1229-
(w), 1207(vw), 1160(m), 1137(m), 1122(m), 1101(s), 1076(vs), 1047-
(vs), 962(m), 921(vw), 890(ms), 845(ms), 805(w), 790(m), 752(ms),
712(ms), 591(m), 532(vw).
1H NMR data (benzene-d6, 24 °C, δ): 0.71-1.54 (m, 36H, Cy),
2.05 (m, 4H, Cy), 3.08 (m, br, 2H, NH2), 3.69 (m, 2H, NH2), 4.01 (m,
4H, Cy), 12.51 (d, J ) 8.2 Hz, 2H, NH).
(iii) trans-W2Cl4(NHBus)2(PMe3)2 (3). W2Cl4(NHBus)2(NH2Bus)2
(0.50 g, 0.62 mmol) was dissolved in hexanes (15 mL), and excess
PMe3 (0.19 mL, 1.86 mmol) was added via syringe with stirring of the
solution. The initial orange color turned to red within 5 min. The
reaction was terminated after 0.5 h of stirring, after which the solvent,
liberated sec-butylamine, and excess PMe3 were removed under reduced
pressure. The red product, trans-W2Cl4(NHBus)2(PMe3)2 (3), was
1H NMR data (benzene-d6, 24 °C, δ): 0.76 (t, CH2CH3), 0.81 (t,
CH2CH3), 0.91 (d, J ) 6.34 Hz, CHCH3), 1.04 (d, J ) 6.18 Hz,
CHCH3), 1.31-1.61 (m, br, CHCH2CH3), 1.44 (d, J ) 9.6 Hz, PMe3),
1.45 (d, J ) 9.0 Hz, PMe3), 3.42 (m, CH), 3.54 (m, CH), 11.88 (m, br,
NH).
1
31P{1H}NMR data (benzene-d6, 19 °C, δ): -2.18 (s, JW-P ) 308
3
1
3
Hz, JP-P ) 5.2 Hz), -2.15 (s, JW-P ) 308 Hz, JP-P ) 5.2 Hz).
FAB/DIP MS (NBA, m/z): 806 ([M]+), 769 ([M - Cl]+), 729 ([M
- PMe3]+), 693 ([M - Cl - PMe3]+), 654 ([M - 2PMe3]+), 618 ([M
- Cl - 2PMe3]+).
(v) trans-W2Cl4(NHCy)2(PMe3)2 (4). The synthesis of 4 was
conducted in essentially the same way as that of 3. An orange hexanes
solution of 2 (0.50 g in 20 mL) was prepared, and to this solution was
added an excess of PMe3 (0.17 mL, 1.66 mmol). This mixture was
stirred at room temperature for 30 min, giving a red solution. The
distillation of all volatile components under vacuum from the red
solution resulted in a red residue, which was then redissolved in a
minimum quantity of hexanes. A crop of bright red crystals of 4 was
readily obtained in 2 days at 0 °C. Crystals grown in this way have
been used for the X-ray data collection. The supernatant liquid was
allowed to crystallize further at -15 °C, and a second crop of red
crystalline solid was obtained, for an overall yield of 0.38 g (80%).
IR data (cm-1): 3216(w), 1450(s), 1418(w), 1344(w), 1296(w),
1280(m), 1260(w), 1124(w), 1100(sh), 1079(s), 1019(w), 951(s), 889-
(w), 875(vw), 848(w), 794(m), 738(m), 706(w), 672(vw).
1H NMR data (benzene-d6, 24 °C, δ): 1.03 (m, Cy), 1.19-1.67 (m,
br, Cy), 1.39 (d, J ) 8.2 Hz, 18H, PMe3), 3.99 (m, Cy), 12.08 (d, J )
9.0 Hz, 2H, NH).
1
31P{1H}NMR data (benzene-d6, 19 °C, δ): -5.37 (s, JW-P ) 113
Hz).
FAB/DIP MS (NBA, m/z): 858 ([M]+), 782 ([M - PMe3]+), 704
([M - 2PMe3]+), 684 ([M - NHCy - PMe3]+).
(vi) cis-W2Cl4(NHCy)2(PMe3)2 (6). A solution of trans-W2Cl4-
(NHCy)2(PMe3)2 (4) in hexanes was allowed to stand at room
temperature, and the solution gradually darkened. After a few days,
black crystals of 6 (69%) were found on the wall and at the base of the
Schlenk tube. Single crystals suitable for X-ray diffraction studies were
readily formed by a hexane-toluene layering procedure under ambient
conditions.
(3) Bradley, D. C.; Errington, M. B.; Hursthouse, M. B.; Short, R. L. J.
Chem. Soc., Dalton Trans. 1986, 1305.
(4) Schrock, R. R.; Sturgeoff, L. G.; Sharp, P. R. Inorg. Chem. 1983, 22,
2801.
(5) (a) Chisholm, M. H.; Eichhorn, B. W.; Folting, K.; Huffman, J. C.;
Ontiveros, C. D.; Streib, W. E.; Van Der Sluys, W. G. Inorg. Chem.
1987, 26, 3182. (b) Sharp, P. R.; Schrock, R. R. J. Am. Chem. Soc.
1980, 102, 1430.
(6) Crystal data for the red-brown polymorph of cis-W2Cl4-
(NHBus)2(PMe3)2: W2Cl4P2C14H38N2, M ) 805.90, monoclinic, space
group P21/a, a ) 13.627(2) Å, b ) 12.018(1) Å, c ) 16.440(3) Å, â
) 96.90(1)°, V ) 2673.0(6) Å3, Z ) 4.

3270 Inorganic Chemistry, Vol. 36, No. 15, 1997
Cotton et al.
IR data (cm-1): 3208(w), 3189(sh), 3154(vw), 1608(vw), 1583(vw),
1511(m), 1415(w), 1349(w), 1299(w), 1286(m), 1280(m), 1261(m),
1235(vw), 1221(w), 1184(vw), 1170(vw), 1139(vw), 1124(ms), 1099-
(m), 1065(m), 1045(br, w), 981(w), 954(s), 890(vw), 861(vw), 850-
(w), 804(w), 754(w), 748(w), 738(w), 725(w), 675(vw), 585(w).
1H NMR data (benzene-d6, 24 °C, δ): 0.94 (m, Cy), 1.32 (m, Cy),
1.48 (d, J ) 10.0 Hz, 18H, PMe3), 1.97 (m, Cy), 3.48 (m, Cy), 11.84
(br, NH).
Table 1. Crystallographic Data for trans-W2Cl4(NHCy)2(NH2Cy)2
(2), cis-W2Cl4(NHBus)2(PMe3)2 (5), and cis-W2Cl4(NHCy)2(PMe3)2
(6)
2
5
6
formula
fw
W2Cl4C24H50N4 W2Cl4P2C14H38N2 W2Cl4P2C18H42N2
904.18
805.90
857.98
space group
a, Å
b, Å
P4h21c (No. 114) P21/a (No. 14)
P21/n (No. 14)
8.455(1)
25.714(3)
13.454(1)
104.839(8)
2827.5(5)
4
12.782(2)
12.782(2)
9.943(2)
90
1624.5(5)
2
13.339(3)
13.446(3)
15.179(3)
99.33(2)
2686(1)
4
1
31P{1H}NMR data (benzene-d6, 19 °C, δ): -1.89 (s, JW-P ) 308
3
c, Å
Hz, JP-P ) 5.0 Hz).
FAB/DIP MS (NBA, m/z): 858 ([M]+), 821 ([M - Cl]+), 782 ([M
- PMe3]+), 747 ([M - Cl - PMe3]+), 704 ([M - 2PMe3]+), 684 ([M
- NHCy - PMe3]+).
â, deg
V, Å3
Z
F
calc, g/cm3
1.848
1.993
2.015
19.425
Physical Measurements. The IR spectra were recorded on a Perkin-
Elmer 16PC FT-IR spectrophotometer as Nujol mulls between KBr
plates. 1H NMR spectra were obtained on a Varian XL-200 spectrom-
eter operated at 200 MHz. Resonances were referenced internally to
the residual proton impurity in the deuterated solvent. The 31P{1H}
NMR data were obtained at room temperature on a Varian XL-200
broad-band spectrometer operated at 81 MHz and using an internal
deuterium lock and 85% H3PO4 as an external standard. Positive
chemical shifts were measured downfield from H3PO4/D2O. The
positive FAB/DIP (DIP ) direct insertion probe) mass spectra were
acquired using a VG Analytical 70S high-resolution, double-focusing,
sectored (EB) mass spectrometer. Samples for analysis were prepared
by dissolving the neat solid compound in m-nitrobenzyl alcohol (NBA)
matrix on the DIP tip. The probe was then inserted into the instrument
through a vacuum interlock and the sample bombarded with 8 keV
xenon primary particles from an Ion Tech FAB gun operating at an
emission current of 2 mA. Positive secondary ions were extracted and
accelerated to 6 keV and then mass analyzed.
The kinetic measurements were carried out by 31P{1H}NMR
spectroscopy at 40 °C with about 15 mg of trans-W2Cl4(NHCy)2(PMe3)2
in 2.5 mL of a 1:4 mixture of C6D6 and C7H8. To each solution except
one was added some additional PMe3. Before the temperature was
raised to 40 °C, spectra measured at room temperature were integrated
to obtain the ratio of free to bonded PMe3. The temperature was then
raised as quickly as possible to 40 °C, and the spectra were measured
at regular time intervals. Rate constants were calculated on the basis
of a first-order irreversible rate law, k ) 1/t ln(A0/At).
X-ray Crystallographic Procedures. Single crystals of 2, 5, and
6 were obtained as described herein. The normal crystallographic
procedures we followed have been presented elsewhere.7 For 2, 5,
and 6, suitable crystals were fastened to the end of a glass fiber with
a thin layer of epoxy resin, and intensity data were collected at ambient
temperature on a Rigaku AFC5R diffractometer equipped with a rotating
Cu radiation source (λ Cu KR ) 1.54184 Å). Axial lengths and Laue
classes were confirmed by axial photography. The identification of
the crystal systems, data collection, and structure solutions and
refinements are described below for each individual crystal studied.
All calculations were carried out on a DEC 3000-800 AXP workstation.
Data sets were corrected for decay where necessary and for Lorentz
and polarization effects. An absorption correction based on azimuthal
scans of six reflections with Eulerian angle ø near 90° was applied to
the data using the TEXSAN software package.8 For all structures, the
positions of W atoms were located by direct methods in the SHELXTL
program.9 The remainder of the non-hydrogen atoms were found by
use of a combination of least-squares refinements and difference Fourier
techniques in the SHELXL-93 structure refinement program.10 In each
model, hydrogen atoms were introduced in idealized positions for the
calculations of structure factors, and the entire model was refined to
convergence. Pertinent crystallographic data and refinement results
for each compound are collected in Table 1. A listing of the important
µ, mm-1
16.060
20.391
radiation (λ, Å) Cu KR (1.541 84) Cu KR (1.541 84) Cu KR (1.541 84)
temp, °C
20
20
20
transm factors 1.0000-0.7970
1.0000-0.3815
1.0000-0.8118
R1,a wR2b
0.028, 0.066
0.057, 0.078
0.057, 0.164
0.027, 0.064
[I > 2σ(I)]
R1,a wR2b
0.064, 0.172
0.043, 0.070
(all data)
a R1 ) ∑||Fo| - |Fc|/∑|Fo|. b wR2 ) [∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]]1/2
.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
trans-W2Cl4(NHCy)2(NH2Cy)2 (2)
W(1)-W(1A)
W(1)-N(2C)
W(1)-Cl(2C)
2.2884(9)
1.91(3)
2.38(1)
W(1)-N(1)
W(1)-Cl(1)
2.19(2)
2.37(2)
N(1)-W(1)-Cl(1)
N(1)-W(1)-Cl(2C)
N(1)-W(1)-N(2C)
N(2C)-W(1)-Cl(1)
84(1)
W(1A)-W(1)-N(1)
99.2(5)
79.1(9) W(1A)-W(1)-N(2C) 100.1(8)
160.2(6) W(1A)-W(1)-Cl(1) 102.0(5)
97(1)
W(1A)-W(1)-Cl(2C) 102.7(4)
W(1)-N(1)-C(1)
N(2C)-W(1)-Cl(2C) 92 (1)
113(2)
128(2)
Cl(1)-W(1)-Cl(2C) 151.8(3) W(1)-N(2C)-C(1C)
molecular dimensions for 2 is presented in Table 2. Selected bond
distances and angles and torsion angles about the W-W bond for both
5 and 6 are shown in Table 6. Tables of positional and isotropic
parameters and anisotropic displacement parameters, as well as complete
tables of bond distances and angles and coordinates of hydrogen atoms,
are available as Supporting Information.
trans-W2Cl4(NHCy)2(NH2Cy)2 (2). A well-formed orange needle
with dimensions of 0.05 mm × 0.08 mm × 0.55 mm was mounted for
intensity measurements. Least-squares refinement of 25 carefully
centered reflections in the range 28 < 2θ < 53° resulted in a tetragonal
cell. Laue symmetry 4/mmm was derived from oscillation photographs
on a, b, c, and 110. Systematic extinctions uniquely determined the
space group as the acentric group P4h21c. In order to determine the
absolute configuration, Bijvoet pairs were collected. A total of 2742
data in the range 9 < 2θ < 120° were measured using an ω-2θ scan
technique. The overall change in standard intensities during the period
of data collection was -11%. Only W and Cl atoms were refined
anisotropically. All other ligand atoms and the carbon atoms in
cyclohexyl rings were assigned a site occupancy factor (SOF) of 0.5
each. Final refinement anisotropic for W and Cl atoms of 75 parameters
and 12 restrains converged with R (for 779 reflections with I > 2σ(I)
of 0.028 and R for all 1218 data of 0.057. The highest peak in the
final difference map was 0.37 e/Å3. The absolute configuration of 2
was established by the method described by Flack (Flack x parameters
) -0.07(8)).
cis-W2Cl4(NHBus)2(PMe3)2 (5). Crystals of cis-W2Cl4(NHBus)2-
(PMe3)2 were obtained in two crystalline forms and were separated
under a microscope on the basis of their slightly different colors and
shapes (red blocks and red-brown cubes). A red block-shaped crystal
of 5 having dimensions 0.13 mm × 0.15 mm × 0.25 mm was selected
and mounted. Least-squares refinement of 25 reflections in the range
48 < 2θ < 51° resulted in unit cell parameters having primitive
monoclinic symmetry. The Laue symmetry of 2/m was verified by
axial photographs. The diffraction data were collected at room
temperature in the range 5 < 2θ < 120°. A total of 4189 data were
(7) Cotton, F. A.; Dikarev, E. V.; Wong, W. Y. Inorg. Chem. 1997, 36,
80.
(8) TEXSAN: Crystal Structure Analysis Package, Molecular Structure
Corp., Houston, TX, 1985.
(9) SHELXTL V. 5, Siemens Industrial Automation Inc., Madison, WI,
1994.
(10) Sheldrick, G. M. In Crystallographic Computing 6; Flack, H. D.,
Parkanyi, L., Simon, K., eds.; Oxford University Press: Oxford, UK,
1993; p 111.

Study of W2Cl4(NHR)2(NH2R)2 and W2Cl4(NHR)2(PMe3)2
Inorganic Chemistry, Vol. 36, No. 15, 1997 3271
collected using an ω-2θ scan motion. A plot of three representative
reflections measured at regular intervals revealed that 15% of diffraction
intensity was lost during the experiment. Systematic absences in the
data unambiguously determined the space group to be P21/a. After
anisotropic refinement of most atoms, it was apparent that one of the
two amido groups was disordered. We have both R and S enantiomers
(with SOF’s of 0.5 each) in one of the NHBus groups. For another
polymorph,6 there is no disorder, and both amido groups in one
molecule conform to the same enantiomer. It should be noted that the
total ratio of R to S in each unit cell is 0.50:0.50 for both cases. Full
refinement of 207 parameters resulted in residuals R based on 3522
reflections with I > 2σ(I) and R based on all 3989 data of 0.057 and
0.064, respectively. The largest residual peak in the final difference
electron density map was 1.88 e/A3, in the vicinity of W atoms.
trans-W2Cl4(NHCy)2(PMe3)2 (4). A red crystal with dimensions
0.20 mm × 0.30 mm × 0.45 mm was selected and affixed to the tip
of a quartz fiber with silicone grease. The diffraction data were
collected on an Enraf-Nonius CAD-4S diffractometer (λ Mo KR )
0.710 73 Å) at -100 °C. With 25 reflections (20 < 2θ < 30), the unit
cell was shown to be orthorhombic, a ) 9.396(2) Å, b ) 17.980(4) Å,
c ) 8.627(2) Å, V ) 1457.5 Å3, Z ) 2. The Laue symmetry mmm
was displayed by the axial photographs, but disorder streaks were
observed on these photos. When we tried to solve the structure in the
space group Pba2, which was justified by the systematic absences
observed, and several other space groups with lower Laue symmetries
as well, a disorder of the W-W vector (disorder ratio 0.50:0.50) was
always found. As a consequence of this disorder, all the ligands
appeared distorted. At this time, it is pretty clear that we are dealing
with the trans isomer, but we have not yet achieved a satisfactory
solution of this structure.
cis-W2Cl4(NHCy)2(PMe3)2 (6). A red-brown block with ap-
proximate dimensions 0.10 mm × 0.13 mm × 0.25 mm was used for
intensity measurements. A primitive monoclinic cell was derived from
the indexing on the basis of 25 reflections with the 2θ range of 19-
37° and further confirmed by axial photos. The X-ray diffraction data
were gathered at room temperature via an ω-2θ scan method. Periodic
monitoring of three representative reflections revealed no loss in crystal
integrity throughout data collection. The structure was solved and
refined in the monoclinic space group P21/n. After the anisotropic
refinement of heavy atoms had converged, it became obvious that one
cyclohexylamido group was disordered over two sites. This disorder
was modeled by restraining the chemically equivalent bonds to be
approximately equal. Also, the SOF’s were allowed to vary but were
constrained so that the sum was equal to 1. The resulting model gave
reasonable bond distances and angles. All the non-hydrogen atoms
were refined with anisotropic displacement parameters. Final least-
squares refinement of 309 parameters led to residuals of R ) 0.027
for 3491 reflections with I>2σ(I) and R ) 0.043 for all 4212 data. A
final difference Fourier map revealed that the highest remaining peak
of electron density (0.78 e/Å3) was located near W(1).
yields (eq 2). We observed that the initial substitution products
W2Cl4(NHR)2(NH2R)2 + 2PMe3 f
W2Cl4(NHR)2(PMe3)2 + 2NH2R (2)
are pure trans isomers, and thus stereochemistry is preserved
upon substitution of NHR2 by PMe3. However, the trans-W2-
Cl4(NHR)2(PMe3)2 compounds 3 and 4 isomerize readily and
completely in solution to cis-W2Cl4(NHR)2(PMe3)2 (R ) Bus
(5), Cy (6)). The major difference here compared with the tert-
butyl case2 is that the isomerization is found to be complete
and irreversible in the present two cases, whereas an equilibrium
is reached when NHR is NHCMe3.
These new metal-metal-bonded complexes are readily
soluble in aromatic solvents such as benzene and toluene, but
only complexes 1-4 exhibit good solubility in aliphatic
hydrocarbons. It should be noted that they all display rather
low stability in CH2Cl2, even under an inert atmosphere.
Compounds 1 and 2 appear to be extremely air-sensitive in
solution, leading to black solutions upon exposure. In the solid
state, all PMe3-containing compounds 3-6 may be handled in
air for short periods of time. In solution, they are readily
11
oxidized by air to yield WOCl2(PMe3)3 as one of the
decomposition products.
Spectroscopy. The infrared spectra of trans-W2Cl4(NHR)2-
(NH2R)2 (1 and 2) both exhibit characteristic ν(NH) in the range
3225-3325 cm-1, as well as δ(NH2) at 1563 (for 1) and 1551
(for 2) cm-1, corresponding to the coordinated NH2R and NHR
ligands. Compounds 3-6 also show absorption bands in the
IR spectra due to ν(NH) above 3000 cm-1. The presence of
1
NHR ligands in 1-6 is discernible from the H NMR signals
within the short range δ 11.84-12.53. In the cases of 1 and 2,
1
their H NMR spectra give an AB pattern due to the diaste-
reotopic NH2 protons of the coordinated NH2R groups with δA
3.65, δB 4.46 (for 1) and δA 3.08, δB 3.69 (for 2). However,
the signals become broad due to coupling with the protons on
adjacent R groups. Although the intermediate complexes 1 and
2 do not show molecular ion peaks M+ and any other
W2-containing fragment ions in their FAB mass spectra, the
corresponding PMe3 substitution products display mass spectral
peaks assignable to parent ions as well as fragment ions
produced by sequential loss of amido, chlorine, and PMe3
ligands.
The 31P{1H}NMR spectra of trans- and cis-W2Cl4(NHCy)2-
(PMe3)2 in C6D6 are depicted in Figure 2. Each of the spectra
is characterized by a strong central singlet flanked by weak
satellites due to 183W-31P coupling. The data, together with
those of W2Cl4(NHBus)2(PMe3)2, are collected in Table 3.
Because of the stronger trans influence of NHR ligands
compared to Cl ligands, the P-W bonds are notably stronger
(i.e., shorter as shown later) in the cis than in the trans isomer.
Results and Discussion
Synthesis and Properties. The dinuclear W(III)-W(III)
precursors, trans-W2Cl4(NHR)2(NH2R)2 (R ) Bus (1), Cy (2)),
used for the preparation of the W2Cl4(NHR)2(PMe3)2 compounds
are easily synthesized by reaction of the appropriate amines
RNH2 (R ) Bus, Cy) with W2Cl6(THF)4, which is formed in
situ by the reduction of WCl4 with Na-Hg in tetrahydrofuran5
(eq 1). Compounds 1 and 2 can be isolated in high yields (1,
1
The corresponding JW-P values are about 300 vs 110 Hz for
the cis isomer vs the trans isomer. Also, the appearance of the
satellites as doublets in the cis isomers usually serves as a useful
indicator for identifying this kind of isomer in such systems.
For an ABX spin system, PP183W, the splitting observed in the
satellites is proportional to JA-B (i.e., 3JP-P in such cases), and
the 3JP-P values lie in the range 5-6 Hz. For the cis isomers,
the P-P coupling constants are large enough to lead to a
resolved splitting of the satellites, and approximate 3JP-P values
may be readily obtained from the spectra. Obviously, from
Table 3, the chemical shift and coupling constant parameters
show little dependence on the identities of the amido ligands
in these two cases.
W2Cl6(THF)4 + 6NH2R f
W2Cl4(NHR)2(NH2R)2 + 2[RNH3]Cl + 4THF (1)
68%; 2, 70%) as orange solids from orange-brown solutions.
The yields are comparable to that reported in the literature for
the tert-butyl analog.3 These intermediate complexes, like W2-
Cl4(NHCMe3)2(NH2CMe3)2, were found to adopt a trans ster-
eochemistry. A facile substitution reaction between trans-
W2Cl4(NHR)2(NH2R)2 (1 and 2) and PMe3 occurs to give trans-
W2Cl4(NHR)2(PMe3)2 (R ) Bus (3), Cy (4)) in ca. 76-80%
(11) Chiu, K. W.; Lyons, D.; Wilkinson, G. Polyhedron 1983, 2, 803.

3272 Inorganic Chemistry, Vol. 36, No. 15, 1997
Cotton et al.
a
a
b
b
Figure 3. 31P{1H}NMR spectra of (a) (()-trans- and (b) (()-cis-W2-
Cl4(NHBus)2(PMe3)2 at room temperature in C6D6.
Figure 2. 31P{1H}NMR spectra of (a) trans- and (b) cis-W2Cl4-
(NHCy)2(PMe3)2 at room temperature in C6D6.
Table 3. 31P NMR Data for the Trans and Cis Isomers of the
W2Cl4(NHR)2(PMe3)2 Compounds at Room Temperature
trans
31P shifta
cis
1JW-P
b
R
1JW-P
31P shift
3JP-P
Bus
-5.30
-5.21
-5.37
116
116
113
-2.18
-2.15
-1.89
308
308
308
5.2
5.2
5.0
Cy
a Relative to 85% H3PO4/D2O in C6D6. b In hertz.
Another noteworthy feature is observed in the 31P{1H}NMR
spectra of trans- and cis-W2Cl4(NHBus)2(PMe3)2, as shown in
Figure 3. The trans isomer gives two closely spaced but
separable signals of equal intensities with ∆(δ) 0.03 ppm. Also,
two sets of satellites are observed. Based on the fact that RR/
SS and RS/SR configurations (R and S represent R and S
configurations of the sec-butyl groups) of the sec-butyl groups
are not mirror images but diastereoisomers, one signal probably
arises from W2R2 and W2S2 molecules, while the other is caused
by RS and SR configurations (W2RS and W2SR) in the W2 units.
In addition, the intensities of the NMR signals are expected to
be equal in this case since the proportions of RR/SS and RS/SR
should be 50:50 on a statistical basis. For cis-W2Cl4(NHBus)2-
(PMe3)2, the corresponding spectrum again shows two close
central peaks but with unequal intensities. This is presumably
due to the preferential crystallization of one isomer with a
particular configuration (RR/SS or RS/SR) over the other.
Isomerization and Kinetics. 31P{1H}NMR spectroscopic
studies showed that all the compounds of the trans-W2Cl4-
(NHR)2(PMe3)2 type we have studied here undergo isomeriza-
tion to produce the corresponding cis-W2Cl4(NHR)2(PMe3)2
isomers in solution at room temperature. However, no evidence
of isomerization back from a cis isomer to a trans isomer was
observed for these new compounds, and thus it must be
Figure 4. 31P{1H}NMR spectrum of trans-W2Cl4(NHCy)2(PMe3)2 after
the sample was heated to 40 °C in C6D6/toluene for 1 h.
concluded that the equilibrium ratio, cis/trans, is very high. The
isomerization process was followed as a function of time at 40
( 1 °C by measuring the integrated intensities of the 31P NMR
signals arising from trans and cis isomers of W2Cl4(NHCy)2-
(PMe3)2 (Figure 4). The results of a run for the trans-to-cis
conversion of W2Cl4(NHCy)2(PMe3)2 in toluene/C6D6 are shown
graphically in Figure 5a. It can be seen that, in contrast to the
behavior of the W2Cl4(NHCMe3)2(PR3)2 molecules,2 no equi-
librium is reached in this case and complete conversion to the
cis isomer occurs eventually. Based on first-order kinetics, the
rate constant for the trans f cis process is estimated to be 10-
(1) × 10-3 min-1 (Table 4). During the conversion experiment,
there is usually some unavoidable decomposition to W2Cl4-
(PMe3)4,2 which accounts for a slight decrease in the total
intensity in Figure 5.
On the other hand, for the internal flip mechanism (or any
other unimolecular mechanism) previously proposed2 to be valid,
the rate of isomerization would be expected to be insensitive
to the presence of excess free PMe3 ligand. As before,
quantitative experimental tests have been carried out relating

Study of W2Cl4(NHR)2(NH2R)2 and W2Cl4(NHR)2(PMe3)2
Inorganic Chemistry, Vol. 36, No. 15, 1997 3273
Figure 6. Plot of ln(A0/At) vs time for trans f cis isomerization of
W2Cl4(NHCy)2(PMe3)2 at 40 °C in the presence of 0.01 M PMe3.
Figure 7. Perspective drawing of trans-W2Cl4(NHCy)2(NH2Cy)2 (2).
W and Cl atoms are represented by thermal ellipsoids at the 40%
probability level. Carbon and nitrogen atoms are shown as spheres of
arbitrary radius.
Figure 5. Graphs of the intensity of trans- and cis-W2Cl4(NHCy)2-
(PMe3)2 vs time at 40 °C in C6D6/toluene (a) without free PMe3 ligand
and (b) with 0.01 M PMe3.
summarizes the kinetic results concerning the effect of succes-
sive additions of PMe3 on the rate constants for the trans f cis
isomerization. After an approximately 2-fold decrease in rate
upon addition of 0.01 M PMe3, the rate did not show any further
persistent decrease but remained rather constant over a range
of [PMe3] from 0.01 to 0.25 M.
Table 4. Rate Constants for the trans- to
cis-W2Cl4(NHCy)2(PMe3)2 Isomerizations at 40 °C
[PMe3] (M)
103k (min-1
)
0
10(1)
3.7(3)
5.0(3)
4.7(4)
The interpretation of these observations has previously been
discussed for W2Cl4(NHCMe3)2(PR3)2 molecules.2 Generally
speaking, competing mechanisms, one involving PMe3 dissocia-
tion as the rate-determining step and the other a unimolecular
(probably internal flip) mechanism, are suggested to explain
the observed cis-trans isomerization processes. If we therefore
treat the internal flip process as the second mechanism, with a
rate of ca. 4.5 × 10-3 min-1 at 40 °C (i.e., 7.5 × 10-5 s-1 at
313 K), the activation energy Ea for the rate-determining internal
flip process can be estimated from the Arrhenius equation, k )
0.01
0.20
0.25
to this point. As for the W2Cl4(NHCMe3)2(PR3)2 molecules, it
was found that the initial addition of excess PMe3 caused a
slight decrease (by 2-3-fold) in the trans f cis isomerization
rate, but the effect is a little smaller in this case relative to
that in W2Cl4(NHCMe3)2(PR3)2 compounds (about 4-fold
decrease).2 Figure 5b shows a plot of this run in the presence
of 0.01 M PMe3. The reaction rate was determined using a
first-order rate law. A plot of ln(A0/At) vs time is shown in
Figure 6, where A0 and At are the concentrations of trans-W2-
Cl4(NHCy)2(PMe3)2 initially and at time t, respectively. It is
apparent that the data points can be fitted to a straight line with
correlation coefficient of 0.9981. The slope of the linear
regression line corresponds to the rate constant for the isomer-
ization process, and it was calculated to be 3.7(3) × 10-3 min-1
(Table 4).
A e-E /RT, to be 25-29 kcal/mol, taking the pre-exponential
a
factor as 1013-1016.
Molecular Structures. Compound 2 forms crystals in the
noncentrosymmetric tetragonal space group P4h21c with two
molecules per unit cell. A perspective view of the molecule is
depicted in Figure 7, and the key molecular parameters are given
in Table 2. This compound provides only the second example
of W2Cl4(NHR)2(NH2R)2 molecules in an eclipsed geometry
and a trans stereochemistry, the other being the tert-butylamido
analog, W2Cl4(NHCMe3)2(NH2CMe3)2.3 A closely related
intermediate complex in a cis configuration, W2Cl4(NEt2)2-
However, complications again arise when free PMe3 con-
centrations continue to increase in the mixture. Table 4

3274 Inorganic Chemistry, Vol. 36, No. 15, 1997
Cotton et al.
and wR2 ) 0.060, and the corresponding selected bonding
parameters are listed in Table 5. The W-N distance is 2.069-
(8) Å, which is midway between the normal distances for NHR
(1.91(3) Å) and NH2R (2.19(2)Å) groups in 2 (Vide infra).
Obviously, this model without separation of ligands yields
molecular dimensions comparable to those of the reported
structure, W2Cl4(NHCMe3)2(NH2CMe3)2 (Table 5).
To surmount this problem, efforts were made to separate the
ligands into two sets, which requires disordering of all the amide,
amine, and Cl groups. This situation is best illustrated in Figure
9, which shows one tungsten fragment with two ligand sets.
The atoms N(1) and N(2) correspond to the amino and amido
nitrogens, respectively, and each of them, together with Cl(1)
and Cl(2), was assigned a site occupancy factor (SOF) of 0.5.
Atoms N(1) and N(1C), N(2) and N(2C), and so on are related
by symmetry elements in space group P4h21c, so that the sum
of SOF’s equals 1 for each atom. In this way, we were able to
distinguish the amine and amide groups from their distinctive
W-N distances (W(1)-N(1), 2.19(2) Å; W(1)-N(2), 1.91(3)
Å, respectively), which are in close agreement with the
corresponding values observed in W2Cl4(NEt2)2(NHEt2)2 (2.198-
(7) and 1.908(7) Å)12 (Table 5). Based on the knowledge
already established in W2Cl4(NEt2)2(NHEt2)2 ( Cl-W-Namide
12
>
Cl-W-Namine
)
and cis-W2Cl4(NHCy)2(PMe3)2 (Vide
infra) (Table 5), we assign Cl(1) and Cl(2C) as the correspond-
ing chlorine atoms in the present model. The two W-Cl
distances are comparable to each other in the resulting model.
In view of the successful refinement and reasonable re-
sulting bond parameters using the separated ligand model, we
decided to revisit the structure of W2Cl4(NHCMe3)2(NH2-
3
CMe3)2 by the same approach. As expected, this new model
affords bond distances and angles comparable to those in 2
(Table 5).
As far as the conformation of 2 is concerned, it maintains an
eclipsed geometry due to the presence of intramolecular
hydrogen bonding with small twist angles of 4.4 and 9.2°. The
N(2C)-H‚‚‚Cl(2B) and N(2B)-H‚‚‚Cl(1) distances are both
2.46 Å.
Compounds 5 and 6 crystallize in the monoclinic space groups
P21/a and P21/n, respectively, with four molecules (constituting
two enantiomeric pairs of molecules) per unit cell for each
complex. Perspective views of the molecular structures of 5
and 6 are shown in Figures 10 and 11. In fact, we note that
there are only two prior examples of this type of molecule
in a cis configuration in the literature, namely cis-W2Cl4-
(NHCMe3)2(PR3)2 (R3 ) Me3, Me2Ph).1,13 Both of the mol-
ecules 5 and 6 are chiral and have an idealized C2 symmetry.
The core structure, consisting of the set of eight ligands, is
essentially the same in the two compounds. Each molecule
possesses an effectively eclipsed geometry (Figure 8b), with
Figure 8. Views of (a) trans-W2Cl4(NHCy)2(NH2Cy)2 (2) and (b) cis-
W2Cl4(NHCy)2(PMe3)2 (6) directly down the W-W axis. For clarity,
carbon atoms are not labeled.
(NHEt2)2, has also been reported recently.12 The molecular
structure of 2 itself consists of two WCl2(NHCy)(NH2Cy)
fragments united by a W-W triple bond with the amide and
amine groups trans to each other in each fragment. The two
WCl2N2 units are eclipsed to each other, with nitrogen atoms
opposite chlorine atoms (Figure 8a). The W-W bond distance
of 2.2884(9) Å is very similar to that observed in W2-
Cl4(NHCMe3)2(NH2CMe3)2 (2.288(3) Å) prepared by Bradley
et al.3 The present structure was initially solved and refined
using a procedure similar to that adopted by Bradley et al., in
which the amide and amine groups were treated as the same
unit without taking disordering into account. In this way, the
unique W, N, and Cl atoms were located. All non-hydrogen
atoms were refined with anisotropic thermal parameters. Again,
as in the published structure for the tert-butyl analog, attempts
to locate and refine the hydrogen atoms on N atoms were
unsuccessful. The final refinement converged with R ) 0.029
its two WCl2NP ligand sets arranged in a fashion akin to that
6+
encountered in the related W2
complexes, cis-W2Cl4-
(NHCMe3)2(PR3)2 (R3 ) Me3, Me2Ph).13 In all cases, each W
atom of the (WtW)6+ center is four-coordinate, and the two
chlorine atoms are in a cis arrangement in each W unit. In no
case was disorder of the W2 unit in the crystal encountered.
The W-W bond distances are 2.320(1) and 2.3229(5) Å
respectively for 5 and 6 (Table 6), which are consistent with a
W-W triple bond with a σ2π4 ground state electronic config-
uration.14 These distances do not vary significantly from those
(13) Bradley, D. C.; Hursthouse, M. B.; Powell, H. R. J. Chem. Soc., Dalton
Trans. 1989, 1537.
(14) Cotton, F. A.; Walton, R. A. Multiple Bonds between Metal Atoms,
2nd ed.; Oxford University Press: New York, 1993; see also references
therein.
(12) Cotton, F. A.; Dikarev, E. V.; Nawar, N.; Wong, W. Y. Inorg. Chem.
1997, 36, 559.

Study of W2Cl4(NHR)2(NH2R)2 and W2Cl4(NHR)2(PMe3)2
Inorganic Chemistry, Vol. 36, No. 15, 1997 3275
Table 5. Distances (Å) and Angles (deg) in trans-W2Cl4(NHR)2(NH2R)2 (R ) But, Cy) and Related Compounds
trans-
W2Cl4(NHCy)2-
(NH2Cy)2
trans-
W2Cl4(NHCy)2-
(NH2Cy)2
trans-
trans-
cis-
cis-
W2Cl4(NHBut)2-
W2Cl4(NHBut)2-
W2Cl4(NEt2)2-
(NHEt2)2
W2Cl4(NHCy)2-
a,c
b,c
b,3
a,c
12
c
(NH2But)2
(NH2But)2
(PMe3)2
W-W
2.2884(9)
2.19(2)
1.91(3)
2.286(1)
2.069(8)
2.069(8)
2.359(3)
2.288(3)
2.11(2)
2.11(2)
2.352(5)
2.284(1)
2.24(2)
2.00(3)
2.345(8), 2.388(8)
2.3084(5)
2.198(7)
1.908(7)
2.384(2), 2.473(2)
2.3229(5)
W-Namine
W-Namide
W-Cl
1.901(5), 1.907(5)
2.384(2), 2.393(2),
2.414(2), 2.421(2)
2.37(2), 2.38(1)
Cl-W-Namine
Cl-W-Namide
W-Namine-C
W-Namide-C
84(1), 79.1(9)
97(1), 92(1)
113(2)
89.4(4), 86.5(4)
89.4(4), 86.5(4)
120.3(6)
89.0(5)
89.0(5)
132(1)
132(1)
83.8(6), 82.8(6)
92(1), 93(1)
131(2)
79.7(2)
92.9(2)
111.4(5), 117.9(5)
112.2(6), 135.6(6)
96.5(2), 97.1(2)
128(2)
120.3(6)
135(3)
127.1(4), 120.5(6)
a Based on a model with separated amine and amide groups. b Based on a model without separation of amine and amide groups. c This work.
Figure 10. Perspective drawing of cis-W2Cl4(NHBus)2(PMe3)2 (5).
Some atoms are represented by thermal ellipsoids at the 40% probability
level. Carbon atoms are shown as spheres of arbitrary radius. The
Figure 9. Schematic diagram showing the structural model for 2 with
separated ligand sets in one W fragment.
disordered carbon atom C(10X) in one of the NHBus groups is shown.
observed in cis-W2Cl4(NHCMe3)2(PMe3)2 (2.3267(6) Å).1,13 The
average W-N bond distances of 1.89(1) Å in 5 and 1.904(5)
Å in 6 represent normal W-NH(R) bond lengths of formal
bond order 2. The mean W-P bond lengths for 5 and 6 are
respectively 2.500(4) and 2.512(2) Å. The W-Cl distances
trans to N are longer than those trans to P. This difference in
W-Cl distances indicates that NHR groups exert a greater
structural trans effect than the phosphines. In both 5 and
6, there are two hydrogen bonds across the W-W triple
bond (average NsH‚‚‚Cl distance, 2.41 Å in 5 and 2.46 Å
in 6), and the mean torsion angles involving the hydrogen-
bonded N and Cl atoms are 16.2 and 13.3° for 5 and 6,
respectively.
For 5, another notable structural feature is that one of the
coordinated NHR ligands exhibits disordered sec-butyl carbon
atoms with equal occupancy of two sets of positions. Thus,
three configurations are possible, namely W2R2, W2RS, and
W2S2, in the crystal lattice of 5 (R and S represent R and S
configurations of the sec-butyl groups). In a separate X-ray
diffraction experiment on another red-brown crystal from the
same reaction, the structure analysis revealed that it also contains
four molecules of cis-W2Cl4(NHBus)2(PMe3)2 in a unit cell of
comparable cell volume,6 but the unit cell parameters are slightly
different. In view of the fact that the two chiral carbon atoms
of the sec-butyl moieties in the asymmetric unit adopt the same
configuration (either RR or SS) in this case, we might expect
that these two polymorphic forms give rise to different packing
Figure 11. Perspective drawing of cis-W2Cl4(NHCy)2(PMe3)2 (6).
Atoms are represented by thermal ellipsoids at the 40% probability
level. Carbon atoms are shown as spheres of arbitrary radius. Only
the main orientation for the C(13)-C(18) cyclohexyl ring is displayed.
in the unit cells, which will lead to different lattice parameters.
Since the same numbers of R and S centers exist in both cases,
there is no racemization of Bus groups in the reaction products.

3276 Inorganic Chemistry, Vol. 36, No. 15, 1997
Cotton et al.
ate complexes with PMe3 is reported. It was shown by 31P-
{1H} NMR spectral data that the initial products are pure trans
isomers, but these compounds readily isomerize in an irrevers-
ible manner to the cis-C2 isomers (see Figure 1). In a future
article, results concerning our continuing efforts in the search
of the unknown cis isomer (cis-Ci) by using the least sterically
bulky amide ligands will be presented.
Table 6. Selected Bond Distances (Å) and Angles (deg) and
Torsion Angles (deg) for cis-W2Cl4(NHBus)2(PMe3)2 (5) and
cis-W2Cl4(NHCy)2(PMe3)2 (6)
5
6
W(1)-W(2)
W(1)-P(1)
W(2)-P(2)
W(1)-N(1)
W(2)-N(2)
W(1)-Cl(1)
W(2)-Cl(4)
W(1)-Cl(2)
W(2)-Cl(3)
2.320(1)
2.487(4)
2.513(3)
1.88(1)
2.3229(5)
2.507(2)
2.516(2)
1.901(5)
1.907(5)
2.384(2)
2.393(2)
2.414(2)
2.421(2)
Perhaps one of the most interesting aspects here is the
structural characterization of trans-W2Cl4(NHCy)2(NH2Cy)2, in
which the disordering of the ligands around W atoms due to
the difference in amine and amide groups is carefully studied
and formulated. The earliest example of this type of molecule
was the analogous compound trans-W2Cl4(NHCMe3)2(NH2-
CMe3)2 reported by Bradley et al.3 While there was surely
no reason to doubt that this compound had been correctly
formulated, the crystallographic support for the structure
itself was a bit troubling in that both amide and amine ligands
were refined as a single group, leading to a W-N distance of
2.11(2) Å. Our successful resolution of the ligand sets in 2
opened the possibility of reexamining the structure of W2-
Cl4(NHCMe3)2(NH2CMe3)2. An analogous form of disorder
was shown to exist in W2Cl4(NHCMe3)2(NH2CMe3)2, and,
with separation of the ligands, reasonable molecular dimensions
are obtained. It would appear that such a disorder is likely to
occur in all compounds of the trans-W2Cl4(NHR)2(NH2R)2
type, but, when the structure is refined so that this is included
in the model, entirely satisfactory molecular dimensions are
obtained.
1.90(1)
2.370(3)
2.383(3)
2.424(3)
2.419(3)
P(1)-W(1)-N(1)
87.6(4)
159.1(1)
80.1(1)
96.5(4)
146.9(4)
85.6(2)
96.73(9)
97.4(3)
103.0(1)
114.3(1)
86.6(4)
79.9(1)
160.9(1)
145.0(4)
99.2(4)
85.2(1)
95.73(9)
97.1(4)
116.2(1)
101.53(9)
19.3(1)
17.4(4)
14.9(4)
18.7(1)
88.0(2)
P(1)-W(1)-Cl(1)
P(1)-W(1)-Cl(2)
N(1)-W(1)-Cl(1)
N(1)-W(1)-Cl(2)
Cl(1)-W(1)-Cl(2)
W(2)-W(1)-P(1)
W(2)-W(1)-N(1)
W(2)-W(1)-Cl(1)
W(2)-W(1)-Cl(2)
P(2)-W(2)-N(2)
159.92(6)
79.21(6)
96.5(2)
145.0(2)
86.07(7)
95.85(4)
98.2(2)
102.80(5)
115.35(5)
88.1(2)
79.88(6)
160.62(7)
144.9(2)
97.1(2)
85.31(6)
95.85(4)
97.6(2)
116.22(5)
101.90(5)
15.23(6)
13.5(2)
P(2)-W(2)-Cl(3)
P(2)-W(2)-Cl(4)
N(2)-W(2)-Cl(3)
N(2)-W(2)-Cl(4)
Cl(3)-W(2)-Cl(4)
W(1)-W(2)-P(2)
W(1)-W(2)-N(2)
W(1)-W(2)-Cl(3)
W(1)-W(2)-Cl(4)
P(1)-W(1)-W(2)-Cl(3)
N(1)-W(1)-W(2)-Cl(4)
Cl(1)-W(1)-W(2)-N(2)
Cl(2)-W(1)-W(2)-P(2)
Acknowledgment. We thank the National Science Founda-
tion for support.
13.1(2)
16.00(7)
Supporting Information Available: Three X-ray crystallographic
files, in CIF format, are available. Access information is given on
any current masthead page.
Concluding Remarks
The preparation of W2Cl4(NHR)2(PMe3)2 (R ) Bus, Cy)
molecules by reaction of W2Cl4(NHR)2(NH2R)2-type intermedi-
IC961485O