Further studies on the substitutional behavior of 1,2-Mo2Br2(CH2SiMe3)4. Alkyl, amide, phosphide, alkoxide, and thiolate for bromide exchange and isomerizations of 1,1- and 1,2-Mo2X2(CH2SiMe3)4 compounds

Article abstract of DOI:10.1021/om0003584

Reactions involving 1,2-Mo₂Br₂(CH₂SiMe₃)₄ and each of LiNMe₂ and HNMe₂ in hexane at -50 °C proceed to give 1,1′-Mo₂(NMe₂)BrR₄, where R = CH₂SiMe₃, as the first product of Br for amide exchange at the Mo₂⁶⁺ center. Further substitution in reactions with LiNMe₂ gives 1,1-Mo₂(NMe₂)₂R₄, but with HNMe₂ the 1,2-Mo₂(NMe₂)₂R₄ isomer is formed. These two do not interconvert thermally (>100 °C) and undergo aminolysis (HNMe₂-d₆) and alcoholysis (^tBuOH) with retention of regiochemistry. 1,1- to 1,2-isomerization of Mo₂(NMe₂)₂R₄ is catalyzed by the presence of Me₂NH₂Br. The mechanisms of these metathetic reactions are discussed in terms of plausible pathways that may promote alkyl group migration across the Mo triple bond Mo bond. Further studies of the bromide substitution by alkyl, aryl, alkoxide, thiolate, and phosphide ligands are described for 1,1′-Mo₂(NMe₂)BrR₄ and 1,2-Mo₂Br₂R₄. The new compounds of formula Mo₂(NMe₂)YR₄ (Y = CH₂SiMe₃, Ph, O^tBu, OⁱPr, S^tBu, PPh₂, and Br) and Mo₂(S^tBu)₂R₄ have been characterized by variable-temperature ¹H and ¹³C{¹H} NMR spectroscopy and mass spectrometry and for Y = PPh₂ by single-crystal X-ray crystallography.

Full text of DOI:10.1021/om0003584

3916
Organometallics 2000, 19, 3916-3924
F u r th er Stu d ies on th e Su bstitu tion a l Beh a vior of
1,2-Mo2Br 2(CH2SiMe3)4. Alk yl, Am id e, P h osp h id e,
Alk oxid e, a n d Th iola te for Br om id e Exch a n ge a n d
Isom er iza tion s of 1,1- a n d 1,2-Mo2X2(CH2SiMe3)4
Com p ou n d s†
Malcolm H. Chisholm,* Damon R. Click, and J ohn C. Huffman
Department of Chemistry and Molecular Structure Center, Indiana University,
Bloomington, Indiana 47405
Received April 26, 2000
Reactions involving 1,2-Mo2Br2(CH2SiMe3)4 and each of LiNMe2 and HNMe2 in hexane at
-50 °C proceed to give 1,1′-Mo2(NMe2)BrR4, where R ) CH2SiMe3, as the first product of Br
for amide exchange at the Mo26+ center. Further substitution in reactions with LiNMe2 gives
1,1-Mo2(NMe2)2R4, but with HNMe2 the 1,2-Mo2(NMe2)2R4 isomer is formed. These two do
not interconvert thermally (>100 °C) and undergo aminolysis (HNMe2-d6) and alcoholysis
(tBuOH) with retention of regiochemistry. 1,1- to 1,2-isomerization of Mo2(NMe2)2R4 is
catalyzed by the presence of Me2NH2Br. The mechanisms of these metathetic reactions are
discussed in terms of plausible pathways that may promote alkyl group migration across
the MotMo bond. Further studies of the bromide substitution by alkyl, aryl, alkoxide,
thiolate, and phosphide ligands are described for 1,1′-Mo2(NMe2)BrR4 and 1,2-Mo2Br2R4.
The new compounds of formula Mo2(NMe2)YR4 (Y ) CH2SiMe3, Ph, OtBu, OiPr, StBu, PPh2,
1
and Br) and Mo2(StBu)2R4 have been characterized by variable-temperature H and13C{1H}
NMR spectroscopy and mass spectrometry and for Y ) PPh2 by single-crystal X-ray
crystallography.
In tr od u ction
metal-ligand bond-forming and exchange reactions
involves so-called “inert” complexes such as simple
mononuclear 18-electron organometallic complexes, e.g.,
Insight into the formation and rupture of carbon-
element bonds has allowed the rapid and remarkable
developments in total synthesis of natural1 and un-
natural products2 in organic chemistry. The reaction
mechanisms of metal-ligand bond formation and rup-
ture (or exchange) are by far less well understood. In
part, this is a natural consequence of the wide variety
of metal ions and ligands and the nature of the medium
in which they may be present.3 For example, despite
their utility4a the nature of organolithium reagents is
still poorly understood with respect to solvation and
complexing reagents.4b,c,d Our best understanding of
M(CO)5, where M ) Fe, Ru, or Os,5 and t2g ML6
6
octahedral complexes, which include the classical Wern-
er complexes of Co(III).6 Also the substitution chemistry
for d8 square-planar ML4 complexes,7 in particular those
of Pt(II) which are relatively slow, have been extensively
studied.
Few mechanistic studies have been undertaken on
dinuclear complexes, and no simple trend has emerged.
For example, for M2(CO)10 compounds (M ) Mn, Re)
carbonyl exchange may occur by two limiting situations
which are independent of the entering ligand, namely,
M-M bond homolysis or M-CO bond dissociation, and
in some cases both of these processes may contribute
to the overall rate.8 There may, in addition, be a
† Dedicated to Professor R. A. Walton on the occasion of his 60th
birthday.
* Corresponding author. Present address: Department of Chemis-
try, The Ohio State University, 100 W. 18th Ave., Columbus, OH
43210. E-mail: Chisholm@chemistry.ohio-state.edu.
(1) Barton, D.; Nakanishi, K.; Meth-Cohn, O.Comprehensive Natural
Products Chemistry; Elsevier Science Ltd: Oxford, U.K., Vols. 1-9,
1999.
(2) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in
Organic Chemistry, 3rd ed.; Harper Collins Publishers: New York,
1987. (b) March, J . Advanced Organic Chemistry, 4th ed.; J ohn Wiley
& Sons Inc.: New York, 1992.
(3) Huheey, J . F.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry,
4th ed.; Harper Collins Publishers: New York, 1993. (b) Rodgers, G.
E. Introduction to Coordination, Solid State and Descriptive Inorganic
Chemistry; McGraw-Hill Inc.: Hightstown, NJ , 1994.
(5) Modi, S. P.; Atwood, J . D. Inorg. Chem. 1983, 22, 26. (b) Unguary
F.; Wojiciki, A. J . Am. Chem. Soc. 1987, 109, 6848. (c) Day, J . D.;
Basolo, F.; Pearson, R. G.; Kangas L. F.; Henry, P. M. J . Am. Chem.
Soc. 1968, 90, 1925.
(6) Kauffman, G. B. Classics in Coordination Chemistry; Dover
Publication: New York, 1968.
(7) Basolo, F.; Pearson, R. G. Mechanisms of Inorganic Reactions,
2nd ed.; J ohn Wiley & Sons: New York, 1967. (b) Wilkins, R. G.
Kinetics and Mechanism of Reactions of Transition Metal Complexes;
VCH: Weinheim, Germany, 1991. (c) Langford, C. H.; Gray, H. B.
Ligand Substitution Processes; W. A. Benjamin, Inc.: New York, 1965.
(d) Tobe, M. L. Inorganic Reaction Mechanisms; Crane, Russak and
Company: Nelson, London, 1972.
(8) Wawersik, H.; Basolo, F. Inorg. Chim. Acta 1969, 3, 113. (b)
Haines L. I. B.; Poe, A. J . J . Am. Chem. Soc. 1980, 102, 3484. (c) Haines
L. I. B.; Hopgood, D.; Poe, A. J . J . Chem. Soc. 1968, A, 421. (d)
Sonnenberger, D.; Atwood, J . D. J . Am. Chem. Soc. 1980, 102, 3484.
(4) Schlosser, M. Organometallics in Synthesis; J ohn Wiley
&
Sons: New York, 1994. (b) Collum, D. B. Acc. Chem. Res. 1993, 26, 7.
(c) Barr, D.; Clegg, W.; Hodgson, S. M.; Lamming, G. R.; Mulway, R.
E.; Scott, A. J .; Snaith, R.; Wright, D. S. Angew. Chem., Int. Ed. Engl.
1989, 28, 1241. (d) Lucht, B. L.; Collum, D. B. Acc. Chem. Res. 1999,
32, 1035.
10.1021/om0003584 CCC: $19.00 © 2000 American Chemical Society
Publication on Web 09/11/2000

Substitutional Behavior of 1,2-Mo2Br2(CH2SiMe3)4
Organometallics, Vol. 19, No. 19, 2000 3917
1
as 1,2-Mo2(NMe2)2(CH2SiMe3)4 by H NMR spectroscopy: the
dependence on the entering ligand. M2X6 compounds (M
) Mo or W, and X ) a uninegative ligand such as alkyl,
amide, alkoxide) that contain an ethane-like E3MtME3
core (E ) C, N, O, etc.) exhibit numerous substitution
and exchange reactions, but little is known of the
intimate mechanism by which metal-ligand bonds are
formed and broken.9 Some time ago, we noted that Br-
for-NMe2 substitution in 1,2-Mo2Br2R4 could give either
1,1- or 1,2-Mo2(NMe2)2R4 products (R ) CH2SiMe3) and
furthermore that these isomers were persistent or inert
to isomerization once formed.10 Studies of the substitu-
tion chemistry of 1,2-Mo2Br2R4 were impeded at that
time because of the difficulties in obtaining significant
quantities of its precursor complex Mo2(CH2SiMe3)6, a
compound originally prepared by Wilkinson from the
reaction between MoCl5 and LiCH2SiMe3.11 With the
advent of an improved synthesis of Mo2(CH2SiMe3)6,
from MoCl3(dme), where dme is dimethoxyethane,12 we
are now able to re-examine the fundamental substitu-
tion chemistry of 1,2-Mo2Br2(CH2SiMe3)4 and report
here on further findings.
spectrum was identical to that previously reported.10 The 13C-
{1H} NMR spectrum was obtained at -20 °C in touene-d8 and
is reported here: δ 2.84 (s, SiMe3), 3.29 (s, SiMe3), 41.42 (s,
distal Me of NMe2 group), 44.59 (s, CH2), 52.46 (s, CH2), 57.88
(s, proximal Me of NMe2 group).
P r epa r a tion of 1,1′-Mo2(NMe2)(Br )(CH2SiMe3)4. To a
suspension of 1,2-Mo2Br2(CH2SiMe3)4 (0.100 g, 0.143 mmol),
in a 25 mL round-bottomed flask in pentane (15 mL) at 0 °C,
was added HNMe2 (ca. 10 equiv) over 5 min by use of a
calibrated vacuum manifold. This led to the rapid precipitation
of H2NMe2+Br-. The solution was further stirred 15 min with
the last 5 min at room temperature. Filtration through a
medium glass frit with a Celite pad, followed by stripping
the solvent, gave a yellow liquid identified as 1,1′-Mo2(NMe2)-
(Br)(CH2SiMe3)4 (>95% crude yield). NMR data were obtained
in toluene-d8 at ambient temperature. 1H: δ 0.18 (s, SiMe3,
9H), 0.21 (s, SiMe3, 27H), 1.86 (d, CH2, J H-H ) 11.7 Hz, 3H),
2.00 (d, CH2, J H-H ) 12.3 Hz, 1H), 2.08 (d, CH2, J H-H ) 11.7
Hz, 3H), 2.12 (d, CH2, J H-H ) 12.3 Hz, 1H), 2.62 (s, distal Me
on NMe2 group), 3.61 (s, proximal Me on NMe2 group, 3H).
13C{1H}: δ 2.56 (s, SiMe3, 9C), 2.88 (s, SiMe3, 3C), 42.78 (s,
distal Me of NMe2 group), 48.99 (s, CH2), 59.67 (s, proximal
Me of NMe2 group), 65.06 (s, CH2). The observed m/z distribu-
+
Exp er im en ta l Section
tion for the 1,1′-Mo2(NMe2)(Br)(CH2SiMe3)4 ion is in good
agreement with the calculated m/z distribution. In both cases
the molecular ion of maximum ion current was at 667 Da.
P r ep a r a tion of 1,1′-Mo2(NMe2)(P h )(CH2SiMe3)4. To 1,1′-
Mo2(NMe2)(Br)(CH2SiMe3)4 (0.100 g, 0.149 mmol) was added
PhLi (0.0125 g, 0.149 mmol) in a 25 mL round-bottomed flask.
Hexane (15 mL) was added to the mixture at 0 °C, and the
solution was stirred for 2 h while warming to ambient
temperature. Filtration through a medium frit with a Celite
pad gave a clear dark solution. Removing the solvent in vacuo
gave a dark yellow liquid identified as 1,1′-Mo2(NMe2)(Ph)(CH2-
SiMe3)4 in >90% crude yield as determined by 1H NMR
spectroscopy. NMR data were obtained in toluene-d8. 1H at
45 °C: δ 0.01 (s, SiMe3, 27H), 0.13 (s, SiMe3, 9H), 1.03 (d, CH2,
J H-H ) 12.3 Hz, 1H), 1.82 (d, CH2, J H-H ) 11.4 Hz, 3H), 1.98
(d, CH2, J H-H ) 11.7 Hz), 2.78 (d, CH2, J H-H ) 12 Hz), 3.08
(br s, NMe2, 6H), 7.09-7.77 (m, Ph, 5H). 13C{1H} at -45 °C:
δ 2.43 (s, 3 SiMe3), 3.10 (S, 1 SiMe3), 34.81 (s, CH2), 40.63 (s,
distal Me of NMe2 group), 48.31 (s, CH2), 57.65 (s, proximal
Me of NMe2 group) 125-136 m, Ph). The observed m/z
distribution for the 1,1′-Mo2(NMe2)(Ph)(CH2SiMe3)4+ ion is in
good agreement with the calculated m/z distribution. In both
cases the molecular ion of maximum ion current was at 662
Da.
All manipulations were carried out under an inert atmo-
sphere of oxygen-free UHP-grade argon using standard Schlenk
techniques or under a dry and oxygen-free atmosphere of
nitrogen in a Vacuum Atmospheres Co. Dry Lab System.
Hexane was degassed and distilled from sodium benzophenone
ketyl under nitrogen. Pentane was degassed and distilled from
calcium hydride under nitrogen. Toluene-d8 was degassed and
stored over 4 Å sieves for 24 h prior to use. Dimethylamine
was purchased from Aldrich and used as received. Dimethy-
lamine-d6 was purchased from CDN Isotopes and was used
as received. Addition of gases to reaction mixtures was done
by using a calibrated gas vacuum manifold.
1,2-Mo2Br2(CH2SiMe3)4 was prepared according to a modi-
fied literature procedure.10 Lithium salts were prepared by
dropwise addition of nBuLi in hexanes (Aldrich) to a hexane
solution of the appropriate reagent at or below 0 °C.
NMR spectra were obtained on 300 MHz Varian Gemini
2000, 400 MHz Varian UNITYInova, or 400 MHz Varian VXR
spectrometers. All 1H NMR chemical shifts are reported in ppm
1
relative to the H impurity in toluene-d8 at δ 2.09. Elemental
analyses were performed on
a Perkin-Elmer 2400 CHN
analyzer. Elemental analysis samples were prepared and
placed in aluminum capsules in the glovebox prior to combus-
tion. Infrared spectra were recorded on a Nicolet 510P FT-IR
spectrometer. Solid-state spectra were taken as Nujol mulls
between KBr plates. Mass spectra were taken on a Kratos MS-
80 high-resolution mass spectrometer using negative ion CI
with methane (CH4) as reagent gas.
P r ep a r a tion of 1,2-Mo2(NMe2)2(CH2SiMe3)4. To a sus-
pension of 1,2-Mo2Br2(CH2SiMe3)4 (0.100 g, 0.143 mmol) in
pentane (15 mL) was added HNMe2 (ca. 10 equiv) by use of a
calibrated vacuum manifold. This led to rapid precipitation of
H2NMe2+Br-. The solution was further stirred 2 h. Filtration
through a medium glass frit with a Celite pad, followed by
stripping the solvent, gave a yellow oil, which was identified
P r ep a r a tion of 1,2-Mo2(StBu )2(CH2SiMe3)4. A mixture of
1,2-Mo2Br2(CH2SiMe3)4 (0.100 g, 0.143 mmol) and LiStBu
(0.028 g, 0.286 mmol) was placed in a 25 mL round-bottomed
flask. Hexane (20 mL) was added, and the solution was stirred
for 5 h. Filtration through a medium glass frit with a Celite
pad gave a clear red solution. Removing the solvent in vacuo
gave a red crystalline solid identified as 1,2-Mo2(StBu)2(CH2-
SiMe3)4. Alternatively, it is possible to obtain the product by
crystallization from hexane (ca. 5 mL) at -20 °C in 70% yield.
NMR data were obtained in toluene-d8 at ambient tempera-
ture. 1H: δ 0.36 (s, SiMe3, 36H), 1.40 (s, StBu, 9H), 1.81 (d,
CH2, J H-H ) 10.8 Hz, 4H), 2.65 (d, CH2, J H-H ) 10.5 Hz). 13C-
{1H}: δ 2.92 (s, SiMe3, 12C), 36.31 (s, StBu), 67.48 (s, CH2,
4C). IR (cm-1): 1459(s), 1377(m), 1364(m), 1363(w), 1256(m),
1244(s), 1154(m), 935(w), 880(m), 846(s), 741(m), 699(w), 681-
(w). The observed m/z distribution for the 1,2-Mo2(StBu)2(CH2-
(9) Chisholm, M. H. J . Organomet. Chem. 1990, 400, 235. (b)
Chisholm, M. H. Acc. Chem. Res. 1990, 23, 419. (c) Chisholm, M. H.
Angew. Chem., Int. Ed. Engl. 1986, 25, 21. (d) Huq, F.; Mowat, W.;
Shortland, A.; Skapski, A. C.; Wilkinson, G. Chem. Commun. 1971,
1079. (e) Mowat, W.; Shortland, A.; Yagupski, G.; Hill, N. J .; Yagupsky,
M.; Wilkinson, G. J . Chem. Soc., Dalton Trans. 1972, 533.
(10) Chisholm, M. H.; Rothwell, I. P.; Folting, K.; Huffman, J . C.
Organometallics 1982, 1, 251.
(11) Huq, F.; Mowat, W.; Shortland, A.; Skapski, A. C.; Wilkinson,
G. Chem. Commun. 1971, 1079.
(12) Gilbert, T. M.; Landes, A. M.; Rogers, R. D. Inorg. Chem. 1992,
31, 3438.
+
SiMe3)4 ion with loss of one tert-butyl group (C(CH3)3) is in
good agreement with the calculated m/z distribution. In both
cases the molecular ion of maximum ion current was at 662
Da. Anal. Calcd for Mo2S2C24H62: C, 40.09; H, 8.69; S, 8.92.
Found: C, 39.34; H, 8.00; S, 8.58.
P r ep a r a tion of 1,2-Mo2(StBu )(Br )(CH2SiMe3)4. To 1,2-
Mo2Br2R4 (0.100 g, 0.143 mmol) was added LiStBu (0.014 g,

3918 Organometallics, Vol. 19, No. 19, 2000
Chisholm et al.
0.143 mmol) in a 25 mL round-bottomed flask. Hexane (20 mL)
was added, and the solution was stirred for 1 h at ambient
temperature. Filtration through a medium frit with a Celite
pad gave a mixture of compounds: 1,2-Mo2Br2R4, 1,2-Mo2(St-
Bu)BrR4, and 1,2-Mo2(StBu)2R4. The characterization data of
1,2-Mo2Br2R4 and 1,2-Mo2(StBu)2R4 have been given previ-
ously. 1H NMR data for 1,2-Mo2(StBu)BrR4 obtained at -20
°C in toluene-d8: δ 0.31 (s, SiMe3, 18H), 0.36 (s, SiMe3, 18H),
0.90 (d, CH2, J H-H ) 10.8 Hz, 2H), 1.39 (s, StBu, 9H), 2.21 (d,
CH2, J H-H ) 10.8 Hz) 2.50 (d, CH2, J H-H ) 10.2 Hz), 4.94 (d,
CH2, J H-H ) 11.1 Hz).
(s, CH2, 6H), 2.35 (d, CH2, J H-H ) 12.3 Hz, 2H), 3.13 (br s,
NMe2, 6H). 13C{1H} at -20 °C: δ 2.24 (s, SiMe3), 2.80 (s,
SiMe3), 47.56 (s, CH2), 57.26 (s, CH2), 61.70 (br s, NMe2). The
+
observed m/z distribution for the Mo2NMe2(CH2SiMe3)5 ion
is in good agreement with the calculated m/z distribution. In
both cases the molecular ion of maximum ion current was at
672 Da.
P r ep a r a tion of 1,1′-Mo2(NMe2)(OiP r )R4. To 1,1′-
Mo2(NMe2)(Br)(CH2SiMe3)4 (0.100 g, 0.149 mmol) was added
LiOiPr (0.010 g, 0.149 mmol) in a 25 mL round-bottom flask.
Hexane (15 mL) was added to the mixture at 0 °C, and the
solution was stirred for 2 h while warming to 20 °C. Filtration
through a medium frit with a Celite pad gave a clear dark
solution. Removing the solvent in vacuo gave a dark liquid,
most of which was identified as 1,1′-Mo2(NMe2)(OiPr)R4 in
>80% crude yield as determined by 1H NMR spectroscopy.
NMR data were obtained in toluene-d8. 1H at 20 °C: δ 0.18 (s,
SiMe3, 9H), 0.20 (s, SiMe3, 27H), 1.33 (d, CH2, J H-H ) 12.9
Hz, 1H), 1.39 (d, CH3, 3 J H-H ) 6 Hz, 3H), 1.47 (d, CH3, 3
J H-H ) 6.3 Hz, 3H), 1.60 (d, CH2, J H-H ) 11.7 Hz, 3H), 1.94
(d, CH2, J H-H ) 11.7 Hz, 3H), 1.99 (d, CH2, J H-H ) 12.9 Hz,
1H), 3.12 (br s, NMe2), 5.42 (septet, CH(CH3)2, 1H). 13C{1H}
at -45 °C: δ 3.27 (s, SiMe3), 3.35 (s, SiMe3), 26.93 (s, OCH-
(CH3)2, 43.17 (s, distal Me on NMe2 group), 58.00 (s, proximal
Me on NMe2 group), 61.67 (s, CH2), 79.06 (s, CH2), 80.74 (s,
OCH(CH3)2. The observed m/z distribution for the Mo2(NMe2)(Oi-
Pr)(CH2SiMe3)4+ ion is in good agreement with the calculated
m/z distribution. In both cases the molecular ion of maximum
ion current was at 644 Da.
P r ep a r a tion of 1,1′-Mo2(NMe2)(StBu )(CH2SiMe3)4. To
1,1′-Mo2(NMe2)(Br)(CH2SiMe3)4 (0.100 g, 0.149 mmol) was
added LiStBu (0.0143 g, 0.149 mmol) in a 25 mL round-
bottomed flask. Hexane (20 mL) was added, and the solution
was stirred for 2 h at ambient temperature. Filtration through
a medium frit with a Celite pad gave a clear dark solution.
Removing the solvent in vacuo gave a dark yellow oil identified
as 1,1′-Mo2(NMe2)(StBu)(CH2SiMe3)4 in >95% crude yield as
determined by 1H NMR spectroscopy. NMR data were obtained
in toluene-d8. 1H at 40 °C: δ 0.19 (s, SiMe3, 9H), 0.23 (s, SiMe3,
27H), 0.50 (d, CH2, J H-H ) 11.7 Hz, 1H), 1.38 (s, StBu, 9H),
1,81 (d, CH2, J H-H ) 11.1 Hz, 3H), 2.19 (d, CH2, J H-H ) 11.4
Hz, 3H), 2.98 (d, CH2, J H-H ) 11.7 Hz, 1H), 3.24 (br s, NMe2,
6H). 13C{1H} at -60 °C: δ 2.32 (s, 3 SiMe3), 3.15 (s, 1 SiMe3),
37.46 (s, StBu), 42.49 (s, distal Me of NMe2 group), 43.47 (s,
proximal Me of NMe2 group), 46.64 (s, CH2), 58.02 (s, CH2).
The observed m/z distribution for the 1,1′-Mo2(NMe2)(StBu)-
(CH2SiMe3)4+ ion is in good agreement with the calculated m/z
distribution. In both cases the molecular ion of maximum ion
current was at 674 Da.
P r ep a r a tion of 1,1′-Mo2(NMe2)(OtBu )R4. To 1,1′-
Mo2(NMe2)(Br)(CH2SiMe3)4 (0.100 g, 0.149 mmol) was added
LiOtBu (0.012 g, 0.149 mmol) in a 25 mL round-bottom flask.
Hexane (15 mL) was added to the mixture at -50 °C, and the
solution was stirred for 5 h while warming to 20 °C. Filtration
through a medium frit with a Celite pad gave a clear dark
solution. Removing the solvent in vacuo gave a dark liquid
identified as 1,1′-Mo2(NMe2)(OtBu)R4 in >90% crude yield as
determined by 1H NMR spectroscopy. NMR data were obtained
in toluene-d8. 1H at 20 °C: δ 0.17 (s, SiMe3, 9H), 0.19 (s, SiMe3,
27H), 1.41(s, C(CH3), 9H), 1.59 (d, CH2, J H-H 11.7 Hz, 3H),
1.68 (s, CH2, 2H), 1.91 (s, CH2, J H-H ) 11.7 Hz, 3H), 2.6 (br s,
proximal Me on NMe2 group), 3.75 (br s, distal Me on NMe2
group). 13C{1H} at -60 °C: δ 2.59 (s, SiMe3), 2.62 (s, SiMe3),
43.29 (s, distal Me of NMe2 group), 57.82 (s, proximal Me of
NMe2 group), 57.97 (s, CH2), 78.25 (s, CH2). The observed m/z
P r ep a r a tion of 1,2-Mo2(NMe2)(P P h 2)(CH2SiMe3)4. To a
slurry of LiPPh2 (0.028 g, 0.142 mmol) in hexane (20 mL), in
a 25 mL round-bottomed flask at -30 °C, was added 1,1′-
Mo2(NMe2)(Br)(CH2SiMe3)4 (0.095 g, 0.142 mmol) in hexane
(20 mL). The reaction mixture was stirred for 3 h and allowed
to warm slowly to 0 °C. Filtration through a medium glass
frit with a Celite pad gave a clear red solution. The solution
was concentrated until ca. 5 mL of hexane remained, and the
solution was cooled to -20 °C. Over a 24 h period fairly large,
red, X-ray quality crystals were deposited and identified as
1,2-Mo2(NMe2)(PPh2)(CH2SiMe3)4. Yield: 40%. NMR data were
1
obtained in toluene-d8. H at -80 °C: δ -0.41 (d, CH2, J H-H
) 14.8 Hz, 1H), 0.05 (d, CH2, J H-H ) 14.0 Hz, 1H), 0.25 (s,
SiMe3, 9H), 0.31 (s, SiMe3, 9H), 0.36 (s, SiMe3, 9H), 0.42 (s,
SiMe3, 9H), 0.77 (d, CH2, J H-H ) 15 Hz, 1H), 1.74 (d, CH2,
J H-H ) 24.4 Hz, 1H), 2.51 (s, distal Me on NMe2 group, 3H),
2.71 (d, CH2, J H-H ) 16 Hz, 1H), 3.41 (s, proximal Me on NMe2
group, 3H), 3.52 (d, CH2, J H-H ) 15.2 Hz), 3.98 (s, CH2, J H-H
) 15 Hz, 1H), 4.26 (d, CH2, J H-H ) 14.0 Hz, 1H). 13C{1H} at
-60 °C: δ 2.39 (s, 2 SiMe3, 6C), 2.63 (s, SiMe3, 3C), 3.47 (s,
SiMe3, 3C), 41.52 (s, distal Me of NMe2 group), 49.06 (s, CH2)
50.43 (s, CH2), 59.74 (s, proximal Me of NMe2 group), 60.44
(d, CH2, trans J c-p 21.3 Hz), 66.56 (s, CH2), 126-136 (m, PPh2).
31P{1H} NMR spectrum obtained at 20 °C and referenced to
an external H3PO4 standard at 20 °C: δ 208.69 (s, PPh2). The
distribution for the Mo2(NMe2)(OtBu)(CH2SiMe3)4 ion is in
+
good agreement with the calculated m/z distribution. In both
cases the molecular ion of maximum ion current was at 658
Da.
NMR Tu be Rea ction s. (1) To an NMR tube containing
1,1′-Mo2(NMe2)BrR4 (100 mg, 0.149 mmol) and 1 mL of
toluene-d8 was added an excess of HNMe2-d6 (>10 equiv). The
reaction was monitored by 1H NMR spectroscopy at -20 °C
over a period of 4 h. Approximately 2 h after adding HNMe2-
d6, the formation of 1,2-Mo2(NMe2)2R4 was observed. Shortly
thereafter, the formation of HNMe2 was observed. No forma-
tion of 1,1-Mo2(NMe2)2R4 occurred. (2) To an NMR tube
containing 1,1-Mo2(NMe2)2R4 (100 mg, 0.156 mmol) and 1 mL
of toluene-d8 was added an excess of HNMe2-d6 (>10 equiv).
+
observed m/z distribution for 1,2-Mo2(NMe2)(PPh2)(CH2SiMe3)4
ion is in good agreement with the calculated m/z distribution.
In both cases the molecular ion of maximum ion current was
at 771 Da.
1
The reaction was monitored by H NMR spectroscopy at -20
P r ep a r a tion of Mo2(NMe2)(CH2SiMe3)5. To 1,1′-
Mo2(NMe2)(Br)(CH2SiMe3)4 (0.100 g, 0.149 mmol) was added
LiCH2SiMe3 (0.014 g, 0.149 mmol) in a 25 mL round-bottom
flask. Hexane (15 mL) was added to the mixture, and the
reaction was stirred 12 h at ambient temperature. Filtration
through a medium glass frit with a Celite pad gave a clear
dark solution. Removing the solvent in vacuo gave a dark
liquid identified as Mo2(NMe2)(CH2SiMe3)5 in >95% crude yield
as determined by 1H NMR spectroscopy. NMR data were
obtained in toluene-d8. 1H at 50 °C: δ 0.16 (s, SiMe3, 18H),
0.19 (s, SiMe3, 27H), 0.73 (d, CH2, J H-H ) 11.7 Hz, 2H), 1.81
°C. The formation of protio-HNMe2 was observed after 1 h,
but no isomerization occurred. The reaction was further
monitored at 20 °C for 5 days, and no formation of 1,2-Mo2-
(NMe2)2R4 was observed, but the concentration of HNMe2
steadily increased as the intensity of the protio NMe2 signals
of 1,1-Mo2(NMe2)2R4 decreased. (3) To an NMR tube containing
1,1-Mo2(NMe2)2R4 (100 mg, 0.156 mmol) and 1 mL of toluene-
d8 was added excess nBu4NI. The NMR tube was rotated over
a period of 24 h, and the reaction was monitored intermittently
by 1H NMR spectroscopy. No isomerization occurred. (4) To
an NMR tube containing 1,1-Mo2(NMe2)2R4 (100 mg, 0.156

Substitutional Behavior of 1,2-Mo2Br2(CH2SiMe3)4
Organometallics, Vol. 19, No. 19, 2000 3919
n
Ta ble 1. Su m m a r y of Cr ysta llogr a p h ic Da ta for
1,2-Mo2(NMe2)(P P h 2)(CH2SiMe3)4
mmol) and 1 mL of toluene-d8 was added excess Bu4NBr. The
NMR tube was rotated (gently shaken) over a period of 24 h,
and the reaction was monitored intermittently by 1H NMR
spectroscopy. No isomerization occurred. (5) To an NMR tube
containing 1,1-Mo2(NMe2)2R4 (100 mg, 0.156 mmol) and 1 mL
of toluene-d8 was added TMSBr (0.156 mmol, 20 µL), and the
reaction was monitored intermittently by 1H NMR spectros-
copy. After 1 h the formation of 1,1′-Mo2(NMe2)BrR4 was
detected by 1H NMR spectroscopy. After 24 h complete
conversion had taken place. (6) To an NMR tube containing
1,1-Mo2(NMe2)2R4 (100 mg, 0.156 mmol), 2,6-di-tert-butyl
pyridine (0.156 mmol, 38 µL), and 1 mL of toluene-d8 was
added TMSBr (1 equiv, 20 µL), and the reaction was monitored
empirical formula
fw
C30H60Mo2NPSi4
770.01
space group
a (Å)
b (Å)
P1h
11.179(1)
17.565(2)
21.060(2)
81.23(1)
75.85(1)
84.58(1)
108.15
c (Å)
R (deg)
â (deg)
δ (deg)
temp (K)
Z
4
V (Å3)
3976.27
1.286
1
d
calc (g cm-1
)
intermittently by H NMR spectroscopy. After 1 h the forma-
tion of 1,1′-Mo2(NMe2)BrR4 was detected by 1H NMR spec-
troscopy. After 24 h complete conversion had taken place. (7)
To an NMR tube containing 1,1-Mo2(NMe2)2R4 (100 mg, 0.156
mmol) in tolune-d8, which was submerged into a dry ice
acetone bath and connected to a calibrated vacuum gas
manifold, was added HBr (0.312 mmol). The NMR tube was
slowly allowed to warm to room temperature, and the NMR
tube was rotated for several hours. 1H NMR spectroscopy
revealed that only 1,1′-Mo2(NMe2)BrR4 was present in the
NMR tube. (8) To 1,2-Mo2(NMe2)2R4 (100 mg, 0.156 mmol) in
tolune-d8, which was submerged into a dry ice/acetone bath
and connected to a calibrated vacuum gas manifold, was added
HBr (0.312 mmol). The NMR tube was slowly allowed to warm
to room temperature, and the NMR tube was rotated for
λ (Å)
0.71073
8.089
0.0769
0.0659
1.801
µ (cm-1
R(F)
)
Rw(F)
GoF
with LiNMe2 requires an alkyl group transfer across the
M-M triple bond. The most simple question to be
addressed is the following. At what stage in the NMe2-
for-Br substitution does this occur? Similarly, in the
reaction between 1,2-Mo2Br2R4 and HNMe2 in hexane
one may ask whether the regiochemistry about the M2
template is retained in each NMe2-for-Br substitution.
To address these questions, we have studied the step-
wise substitution chemistry of 1,2-Mo2Br2R4 by low-
temperature reactions in hexane and by NMR studies
in toluene-d8.
The reaction between 1,2-Mo2Br2R4 and LiNMe2, 1
equiv, is complicated by the fact that the second NMe2-
for-Br exchange occurs at a rate not much slower than
the first even at -50 °C. This is evident from the fact
that one obtains a mixture of unreacted 1,2-Mo2Br2R4
and 1,1-Mo2(NMe2)2R4 along with the desired product
Mo2(NMe2)BrR4, which is present as ca. 70% of the
mixture. Note, this is a kinetic distribution of products,
as 1,1-Mo2(NMe2)2R4 can be prepared as described below
and is not labile to ligand scrambling. However, the
NMR data for the latter compound clearly identify it
as the 1,1′-Mo2(NMe2)BrR4 isomer. Thus, the first
NMe2-for-Br substitution proceeds with alkyl group
transfer across the MotMo bond. The second proceeds
with retention of regiochemistry.
Studies of the reaction between 1,2-Mo2Br2R4 and
HNMe2 (ca. 10 equiv) in hexane reveal that there is a
significant difference in the rate of the first NMe2-for-
Br substitution, and at short reaction times, namely 15
min, essentially complete conversion to Mo2(NMe2)BrR4
is attained along with Me2NH2Br, which precipitates
from hexane. This reaction also produces the 1,1′-
isomer. So we now know that in the reactions between
1,2-Mo2Br2R4 and both LiNMe2 and HNMe2 there is a
common intermediate, Mo2(NMe2)BrR4, with the same
regiochemistry. The reaction between Mo2Br2R4 and
HNMe2 which yields Mo2(NMe2)BrR4 has proved useful
in allowing us to interrogate the subsequent substitu-
tion chemistry. A number of substitution reactions
involving lithium reagents are shown in Scheme 2. Here
it is seen that in all cases, except that of LiPPh2, the
regiochemistry is retained. In the latter reaction 1,2-
Mo2(NMe2)(PPh2)R4 is formed.
1
several hours. H NMR spectroscopy revealed the presence of
1,1′-Mo2(NMe2)BrR4.
Attem p ted P r ep a r a tion of Mo2Y2(CH2SiMe3)4 Com -
p ou n d s. To a hexane solution of 1,2-Mo2Br2(CH2SiMe3)4 was
added 2 equiv of LiY at -50 °C. In each case (Y ) Ph, Ct
CSiMe3, CHdCH2, and PPh2) a rapid reaction occurred, but
no single isolable organometallic compound was obtained.
Rather, in each reaction, the solution darkened rapidly and
only insoluble salts were obtained.
Cr ysta l a n d Molecu la r Str u ctu r e of 1,2-Mo2(NMe2)-
(P P h 2)(CH2SiMe3)4. The Bruker-AXS SMART6000 system
was used for data collection. The data were collected using 5
s frames with an omega scan of 0.30°. Data were corrected for
Lorentz and polarization effects and equivalent reflections
averaged using the Bruker SAINT software as well as utility
programs from the XTEL library.
The structure was readily solved using SHELXTL and
Fourier techniques. It was discovered that two independent
molecules were present in the asymmetric unit. Although the
conformation of the two molecules is nearly identical, there
was no evidence of a space group ambiguity (see below
however). The BMFIT program was used to examine the
differences between the two independent molecules, which are
very slight. Near the end of the refinement, it was noticed that
the largest peaks remaining in the difference Fourier cor-
responded to “ghosts” of the metal positions. When these were
included as low-occupancy metal atoms, the residual dropped
dramatically and several atoms that were previously behaving
erratically converged properly. It appears that a slight disorder
(or twinning) may be responsible. The occupancies of the eight
partial molybdenum atoms varied from 2.1 to 3.4%.
A final difference Fourier still had several peaks of intensity
up to 2 e/Å3, mostly in the vicinity of the various metal atoms.
A summary of crystal data is shown in Table 1.
Resu lts a n d Discu ssion
It is pertinent to present first the results and then
speculate on reaction mechanisms. Moreover, we must
acknowledge the key pioneering contributions made by
Rothwell,10 which are summarized in Scheme 1. Forma-
tion of 1,1-Mo2(NMe2)2R4 (R ) CH2SiMe3) in the reaction
With the now more accessible availability of 1,2-Mo2-
Br2R4, we have also examined some of its substitution
reactions with regard to 1 equiv and 2 equiv of lithium

3920 Organometallics, Vol. 19, No. 19, 2000
Sch em e 1. Su bstitu tion Rea ction s In volvin g 1,2-Mo2Br 2R4, w h er e R ) CH2SiMe3
Chisholm et al.
a
a
Taken from Chisholm, M. H.; Huffman, J . C.; Rothwell, I. P. Organometallics 1982, 1, 251.
Sch em e 2. Br om id e Su bstitu tion Rea ction s In volvin g 1,1′- Mo2(NMe2)Br (CH2SiMe3)4
reagents. The dinuclear template is far from inert, and
from reactions involving alkynyllithium reagents (LiCt
CBut, LiCtCPh, LiCtCSiMe3), LiPh, and LiPPh2 no
simple organometallic compound was isolable. However,
some interesting and noteworthy results were obtained
from related studies. For example, reactions employing
LiOtBu proceed first to give 1,2-Mo2(OtBu)BrR4 and
then a 30:70 mixture of 1,1- and 1,2-Mo2(OtBu)2R4 (as
noted by Rothwell10). In this instance the second sub-
stitution of Br by OtBu is accompanied by some alkyl
group transfer. In contrast, reactions employing LiSt-
Bu lead in a stepwise manner to 1,2-Mo2(StBu)2R4 with
retention of regiochemistry. These results are of par-
ticular interest in that reactions employing 1,1-Mo2-
(NMe2)2R4 and alcohols or thiols lead to 1,1-M2X2R4
compounds (X ) OR or SR).
In t er con ver sion of 1,1- a n d 1,2-Mo2(NMe2)2R 4
Isom er s. That the formation of these isomers in reac-
tions employing LiNMe2 and HNMe2 should proceed via
a common intermediate, namely, 1,1′-Mo2(NMe2)BrR4
was extremely puzzling to us. We first supposed that
the excess Me2NH employed in the simple amine
substitution was the catalyst in the transformation of
the kinetic isomer to the thermodynamic isomer, eq 1.
Initial studies of reaction 1 employing added HNMe2
gave erratic results, and it soon became apparent that
HNMe2 could not be the catalyst. Only in the case of
adventitious Me2NH2Br or trace quantities of Mo2Br2R4
did we observe the “amine-catalyzed” transformation
shown in eq 1. (In the presence of Mo2Br2R4, the
addition of HNMe2 generates Me2NH2Br.) Labeling
studies involving HNMe2-d6 revealed that 1,1-Mo2-
(NMe2)2R4 merely undergoes aminolysis without any
1,1- to 1,2-isomerization. Finally, we were able to show
that the isomerization of 1,1- to 1,2-Mo2(NMe2)2R4 could
be brought about by the presence of catalytic, i.e., trace
amounts, of Me2NH2Br. At this point, we questioned
whether the role of Me2NH2Br might merely be as a
source of Br- which could associate to the Mo26+ center
and facilitate alkyl group transfer by favoring bridge
formation. There are two precedents in the chemistry
of Mo26+-ethane-like “dimers” that foster such a sus-
picion. (1) It was shown that addition of OR- leads to
Mo2(OR)7-, which has a single OR bridging group in the
t
solid state (R ) CH2 Bu) and is fluxional on the NMR
time scale.13 (2) The addition of PMe3 to 1,2-Mo2(CH2-
Ph)2(OiPr)4 in hydrocarbon solutions has been shown to
reversibly give (PMe3)(PhCH2)2(PriO)MotMo(OPri)3,
wherein one four-coordinate Mo atom and one three-
1,1-Mo2(NMe2)2R4 catalyst8 1,2-Mo2(NMe2)2R4 (1)
(13) Budzichowski, T. A.; Chisholm, M. H.; Martin, J . D.; Huffman,
J . C.; Moodley, K. G.; Streib, W. E. Polyhedron 1993, 12, 343.
hexane

Substitutional Behavior of 1,2-Mo2Br2(CH2SiMe3)4
Organometallics, Vol. 19, No. 19, 2000 3921
F igu r e 1. 1H NMR spectrum of 1,1′-Mo2(NMe2)BrR4
recorded at 20 °C, 300 MHz in toluene-d8.
F igu r e 2. 1H NMR spectrum of 1,2-Mo2(NMe2)(PPh2)R4
recorded at -80 °C, 300 MHz in toluene-d8.
coordinate Mo atom are united by the ModMo bond.14
The reversible formation of the latter compound involves
both benzyl and isopropoxy group transfer reactions,
which we have previously shown to be dependent on
1,1-Mo2(NMe2)2R4 the proximal-distal site exchange is
still not frozen out at -60 °C. For a series of 1,1′-Mo2-
(NMe2)XR4 compounds the rates of NMe proximal h
t
6+
distal exchange follow the order X ) Me2N > BuO >
PMe3 association to the Mo2 center.
tBuS > Ph ≈ CH2SiMe3 > Br, which roughly correlates
To investigate the potential catalysis by halide anion,
we examined the reactivity of 1,1-Mo2(NMe2)2R4 in
with the ability of the X ligand to σ- and π-donate to
6+
n
hydrocarbon solvents toward Bu4N+X-, where X ) Br
the Mo2 center. For X ) Br, which is both a poor
π-donor and electronegative, the sole NMe2 ligand
and I. In neither instance is any isomerization of the
1,1- to the 1,2-Mo2(NMe2)2R4 isomer observed. We,
therefore, conclude that the isomerization reaction
shown in eq 1 is catalyzed by the presence of Me2NH2-
Br, which is present during the reaction between Mo2-
Br2R4, and an excess of HNMe2 (ca. 10 equiv). We shall
return to speculate on the more detailed reaction
pathways that are involved in reactions between Mo2-
Br2R4 and each of LiNMe2 and HNMe2. We first present
the evidence for assignment of regiochemistry and our
formulation of the compounds as presented.
2
2
π-donates to the Mo dx -y /dxy orbital most strongly,
which favors alignment of the NC2 plane and the Mo-
Mo axis and imposes a higher rotational barrier.
The 1H NMR spectrum for 1,2-Mo2(NMe2)(PPh2)R4 at
room temperature shows only two SiMe3 groups in the
ratio 2:2. The CH2 protons are not visible at this
temperature and the NMe2 signal is broadened. How-
ever, this is sufficient to establish the regiochemistry
of substitution as 1,2-Mo2(NMe2)(PPh2)R4, and at -80
1
°C a very informative H NMR spectrum is obtained,
Ch a r a cter iza tion Da ta . Many of the new com-
pounds are yellow or dark yellow viscous oils, which
does not endear them to purification or characterization
by single-crystal X-ray crystallography. However, some
of the new compounds were solids, and at least one,
namely, 1,2-Mo2(NMe2)(PPh2)R4, proved suitable for a
single-crystal X-ray study (vida infra). All compounds
were examined by mass spectrometry and by variable-
temperature NMR spectroscopy, and the latter provided
information pertaining not only to molecular formula
but also to molecular structure. The NMR spectra are
in all cases temperature dependent, as rotations about
the central MotMo bond and the Mo-NC2 bond occur
at rates that are comparable to the NMR time scale.
Full NMR data are given in the Experimental Section,
but it is worth examining the variable-temperature
NMR behavior of two representative compounds.
The room-temperature 1H NMR spectrum of 1,1′-Mo2-
(NMe2)BrR4 is shown in Figure 1. Because of rapid
rotation about the MotMo bond, the SiMe3 groups
appear as two singlets in the integral ratio 3:1, while
the methylene protons which are all diastereotopic
appear as two sets of AB quartets again in the ratio 3:1.
The NMe2 signals appear as two singlets, indicating that
proximal and distal NMe2 group exchange is slow in this
compound. Indeed, only upon heating to +60 °C does
exchange occur with a significant rate to cause the
appearance of a single broad resonance. In contrast, for
as shown in Figure 2. There are now, in addition to the
phenyl proton resonances (δ 6.8 to 7.8), four singlets of
equal intensity for the SiMe3 groups and eight methyl-
ene proton signals, the connectivity of which is easily
established by a COSY spectrum, together with proxi-
mal and distal NMe singlets. At -80 °C in toluene-d8
rotation about the MotMo bond is frozen out and we
observe just the gauche rotamer. An inspection of the
methylene proton signals and, indeed, the distal NMe
signal reveals some small coupling to 31P which is not
fully resolved. The only other point worthy of mention
is that for a compound of formula 1,2-Mo2X2R4 the
appearance of one SiMe3 signal and a single AX or AB
spectrum for the methylene protons cannot distinguish
between the presence of only the anti rotamer or a
situation wherein rotation about the MotMo bond is
rapid. That this is seen for X ) Br and StBu even at
-80 °C leads us to propose that the anti rotamer is
present since for other compounds rotation about the
MtM is slow on the NMR time scale at this tempera-
ture.
Molecu la r Str u ctu r e of 1,2-Mo2(NMe2)(P P h 2)R4.
An ORTEP drawing of the molecule is given in Figure
3, and selected bond distances and bond angles are given
in Table 2. The gauche rotamer is present in the solid-
state structure, and this was seen in the low-tempera-
ture NMR spectra. The metrical parameters for the
Mo2C4NP skeleton are readily reconciled with what has
been seen before for mixed amido alkyl and amido
phosphido compounds.15
(14) Chisholm, M. H.; Huffman, J . C.; Tatz, R. J . J . Am. Chem. Soc.
1984, 106, 5385.

3922 Organometallics, Vol. 19, No. 19, 2000
Chisholm et al.
in any way account for the fact that LiBr is ultimately
formed as a crystalline species. It is, however, the most
useful starting point in that it stresses the importance
of the fact that in the metathetic exchange Li‚‚‚Br bond
formation is probably equally important to Me2N-to-Mo
bond formation. The coordination of Br to Li will weaken
the Mo-Br bond and at the same time weaken the Li-N
bond. As the Me2N-to-Mo bond is formed, the Mo-Br
bond and Li-NMe2 bonds are weakened and ultimately
broken. Since it is known that Mo-halogen bonds are
stronger than Mo-NMe2 bonds, the ultimate thermo-
dynamic driving force is derived from the greater
difference in the thermodynamic stability of LiBr over
LiNMe2 in their respective crystalline and oligomeric
states.
Recognizing the limitations of the simplified σ-bond
metathesis shown in Scheme 3, we propose a hypotheti-
cal reaction sequence shown in Scheme 4. This reaction
pathway recognizes that Li‚‚‚Br bond formation and
Mo‚‚‚NMe2 bond formation may occur at different metal
centers. Furthermore, as LiBr is formed and, to be
more precise, as the bromide ligand is removed from
the metal center, then an alkyl group transfer occurs
to generate a MoR3 center and the 1,1′-Mo2(NMe2)BrR4
isomer.
Figu r e 3. Molecular structure of 1,2-Mo2(NMe2)(PPh2)(CH2-
SiMe3)4.
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for 1,2-Mo2(NMe2)(P P h 2)(CH2SiMe3)4
Mo(1)-Mo(2)
Mo(1)-C(64)
Mo(1)-N(36)
Mo(1)-C(69)
2.1911(9)
2.133(7)
1.917(6)
2.172(6)
Mo(2)-C(54)
Mo(2)-C(59)
Mo(2)-P(3)
2.111(7)
2.127(7)
2.357(2)
There are many structural precedents of NH- - -
halogen bonding across dinuclear M-M bonded cen-
ters,17 and Cotton and co-workers have shown just how
often the type of structural motif shown in A occurs with
bridging ligands such as acetates, formaminidates, and
triazines, where X, Y, Z represent various combinations
of the elements C, O, N, S, and P.18
Mo(2)-Mo(2)-N(36) 105.9(2) Mo(1)-Mo(2)-C(54)
97.2(2)
Mo(1)- N(36)-C(75) 135.1(5) Mo(1)-Mo(2)-C(59) 102.3(2)
Mo(1)-N(36)-C(76) 113.3(4) P(3)-Mo(2)-C(54)
Mo(2)-Mo(1)-C(69) 106.0(2) P(3)-Mo(2)-C(59)
111.8(2)
120.6(2)
101.0(1)
110.3(2)
129.6(2)
104.1(3)
Mo(2)-Mo(1)-C(64)
98.7(2) Mo(1)-Mo(2)-P(3)
N(36)-Mo(1)-C(64) 113.5(2) Mo(2)-P(3)-C(48)
N(36)-Mo(1)-C(69) 111.5(2) Mo(2)-P(3)-C(42)
C(75)-N(36)-C(76)
111.4(6) C(42)-P(3)-C(48)
Sch em e 3. Sim p lified σ-Bon d Meta th esis a t a
Mon on u clea r Meta l Cen ter
Why do reagents such as LiOtBu and LiStBu react
with 1,2-Mo2Br2R4 to give 1,2-Mo2(Y)BrR4, where Y )
OtBu or StBu? This is a difficult point upon which to
speculate. Perhaps the different oligomeric structures
of the alkoxide and thiolate lithium salts cause a σ-bond
metathesis to occur at a single Mo center. However,
we cannot rule out that the initial LiY interaction
is again across the Mo-Mo bond as shown in Scheme
3, but in the act of removing the bromide ligand
with the lithium ion it is Br that migrates instead of
CH2SiMe3.
(a ) Sp ecu la tion a bou t th e In tim a te Mech a n ism
of Su bstitu tion a n d F u r th er Rea ction s by Lith iu m
Rea gen ts. The seemingly simple metathetic reaction
shown in eq 2 below is still well beyond our detailed
understanding.
-50 °C
1,2-Mo2Br2R4 + LiNMe2
8
In the reactions involving organolithium reagents and
1,1′-Mo2(NMe2)BrR4 the products 1,1′-Mo2(Y)BrR4 are
formed when Y ) CH2SiMe3, Ph, NMe2, OtBu, OiPr, and
StBu. Again two possible explanations can be offered.
One is that the initial interaction of the Li-Y reagent
occurs across the Mo-Mo bond, and as LiBr is elimi-
nated, a concurrent or consecutive alkyl and amide
group are transferred such that the NMe2 group that
hexane
1,1′-Mo2(NMe2)BrR4 + LiBr (2)
A simplified picture for a σ-bond metathesis reaction
of the SF2 type (substitution, front-side, bimolecular)
depicted by Scheme 3 fails to address the order in which
N-to-Mo and Li-to-Br bonding occurs, fails to recognize
the unknown oligomeric nature of LiNMe2, and does not
(15) For 1,2-M2(R2)(NMe2)4 see: Chetcuti, M. J .; Chisholm, M. H.;
Folting, K.; Haitko, D. A.; Huffman, J . C.; J anos, J . J . Am. Chem. Soc.
1983, 105, 1163. Chisholm, M. H.; Haitko, D. A.; Folting, K.; Huffman,
J . C. J . Am. Chem. Soc. 1981, 103, 4046. (b) For 1,2-M2(PR2)2(NMe2)4
see: Buhro, W. E.; Chisholm, M. H.; Folting, K.; Huffman, J . C.;
Martin, J . D.; Streib, W. E. J . Am. Chem. Soc. 1992, 114, 557.
(16) Alaimao, P. J .; Bergman, R. G. Organometallics 1999, 18, 2707.
(17) 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.
(18) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal
Atoms, 2nd ed.; Clarendon Press: Oxford, U.K., 1993.

Substitutional Behavior of 1,2-Mo2Br2(CH2SiMe3)4
Organometallics, Vol. 19, No. 19, 2000 3923
6+
Sch em e 4. Sim p lified σ-Bon d Meta th esis a t th e Mo2 Cen ter Lea d in g to Alk yl Gr ou p Tr a n sfer
6+
Sch em e 5. Am in olysis a t th e Mo2 Cen ter Lea d in g to Alk yl Gr ou p Tr a n sfer
Sch em e 6. Me2HN2Br -Ca ta lyzed Isom er iza tion of 1,1-Mo2(NMe2)2R4 to 1,2-Mo2(NMe2)2R4
was originally at the Mo(NMe2)BrR center is trans-
ferredto the previous MoR3 center. This seems unlikely
to us, although we cannot rule it out. We prefer the
second alternative, which we think is simpler. The
presence of both the Br and NMe2 groups at one metal
affords a chelate to the entering lithium atom of the LiY
reagent, and exchange/metathesis occurs just at one
metal center. That 1,1-Mo2(NMe2)2R4 reacts with R′OH
to give 1,1-Mo2(OR′)2R4 (R′ ) Bu and Pr) and with
HNMe2-d6 to give 1,1-Mo2(NMe2-d6)2R4 can also most
easily be explained by a SF2 reaction (σ-bond metath-
esis) at a single metal center. Certainly such reactions
are well documented for mononuclear M(NMe2)n com-
plexes. The reaction between LiPPh2 and 1,1′-Mo2-
(NMe2)BrR4, which generates 1,2-Mo2(NMe2)(PPh2)R4,
may do so because this is the thermodynamically
favored product and the PPh2 ligand at the Mo26+ center
may facilitate bridge formation leading to mutual
exchange of PPh2 and R groups. If the LiPPh2 interac-
tion occurred across the Mo-Mo bond, then it is not
clear why R rather than amide group transfer would
occur.
(b) By Am in olysis. A similar pictorial representa-
tion of the aminolysis reaction, can be envisaged and
is shown in Scheme 5. Once again it is recognized
that HNMe2 in a nonpolar solvent will exist as hydrogen-
bonded oligomers and H-to-Br hydrogen bonding may
well occur in an initial “outer-sphere” hydrogen-bonding
process prior to Me2N-to-Mo bond formation.16 Also,
in Scheme 5, it is recognized that HBr does not leave
but rather Me2NH2+Br- is formed, which is likely to
be hydrogen bonded with HNMe2 prior to crystalliza-
tion. One thing we can attest to is the chemical
persistence of 1,1-Mo2(NMe2)2R4 in solution in the
absence of trace quantities of Me2NH2Br, which yield a
slow isomerization. Also the reaction between 1,1′-Mo2-
(NMe2)BrR4 and HNMe2 in toluene-d8 proceeds to give
1,2-Mo2(NMe2)2R4 without any apparent formation of
1,1-Mo2(NMe2)2R4. This aminolysis occurs more slowly
than that which produced 1,1′-Mo2(NMe2)BrR4 from
Mo2Br2R4.
(c) By HBr a n d TMSBr . We speculate that the role
of Me2NH2Br is one of delivering an equivalent of HBr
to 1,1-Mo2(NMe2)2R4 to form 1,1′-Mo2(NMe2)BrR4, which
in the presence of HNMe2 (2 equiv) then reacts as shown
in Scheme 6. This proposed pathway involves the
formation of a Mo2BrHN five-membered ring, and as
in Schemes 4 and 5, the departure of the bromide ligand
promotes an alkyl group transfer. The reaction is a
slower process because the nitrogen lone pair must
coordinate to the Mo(CH2SiMe2)3 center, which is steri-
cally crowded and does not contain any electron-
withdrawing (electronegative) groups. It is, however,
coordinatively unsaturated and certainly capable of
Lewis base association. Further evidence that the role
of Me2NH2Br is one of delivering an equivalent of HBr
comes from the reaction between 1,1-Mo2(NMe2)2R4 and
2 equiv of HBr, which yields 1,1′-Mo2(NMe2)BrR4. The
latter two reactions prompted us to speculate that
TMSBr may also react with 1,1-Mo2(NMe)2R4 via a sim-
ilar mechanism, namely, that HBr would be generated
in situ and react with 1,1-Mo2(NMe)2R4. Our speculation
was based on the amine-catalyzed chloro-for-dimethy-
lamide exchange reactions known to be operative in the
reaction between W2(NMe2)6 and TMSCl.19 The reaction
t
i
(19) Akiyama, M.; Chisholm, M. H.; Cotton, F. A.; Extine M. W.;
Murillo, C. A. Inorg. Chem. 1977, 16, 2407.

3924 Organometallics, Vol. 19, No. 19, 2000
Chisholm et al.
Sch em e 7. Rea ction Sch em e for
1,1-Mo2(NMe2)2(CH2SiMe3)4
Mo2(NMe2)BrR4. Though the outcome of this reaction
does not seem intuitively obvious, one must recognize
that it is simply the reverse of the reaction by which
1,2-Mo2(NMe2)2R4 is formed (see Scheme 2).
Con clu d in g Rem a r k s
The present findings bear testimony to our lack of
understanding of seemingly some of the simplest of
σ-bond metathesis reactions at dinuclear centers. Hope-
fully these results and our speculation will inspire
others to ponder on this subject and to devise experi-
ments that could elucidate further on these matters. We
note for example that a compound of formula 1,1′-Mo2-
(NMe2)BrR4 contains a chiral Mo center. Thus the
introduction of a chiral amide or other ligand Y* could
yield a pair of diastereomers 1,1-Mo2Br(Y*)R4 which
could be readily distinguished by NMR spectroscopy and
separated by their different physical properties. In this
way a pure enantiomer 1,1′-Mo2Br(Y*)R4 could be
employed to interrogate the nature of the second
substitution step. Further studies along these lines
could prove fruitful just as studies of (1,3-C3R2H3-η5)M-
(CO)(L)X piano-stool complexes did in the past.20
sequence we envisage is shown in eq 3.
Me3SiBr + HNMe2 H Me3SiNMe2 + HBr (3)
LnMoNMe2 + HBr H LnMoBr + HNMe2
However, we must concede that the reaction involving
TMSBr and 1,1-Mo2(NMe2)2R4 may proceed by a direct
SF2 σ-bond metathesis reaction since when the reaction
is done in the presence of a proton sponge (2,6-di-tert-
butylpyridine), 1,1′-Mo2(NMe2)BrR4 is still obtained and
without any apparent change in the rate of formation.
These reactions involving 1,1-Mo2(NMe2)2R4 are sum-
marized in Scheme 7.
Ack n ow led gm en t. We thank the National Science
Foundation for support of this work.
Su p p or tin g In for m a tion Ava ila ble: CIF data for 1,2-
Mo2(NMe2)(PPh2)(CH2SiMe3)4 along with representative mass
spectra for the latter and 1,1′-Mo2(NMe2)(Br)(CH2SiMe3)4 are
available. 1H NMR spectra for 1,2-Mo2(StBu)2(CH2SiMe3)4, 1,1′-
Mo2(StBu)(NMe2)(CH2SiMe3)4, Mo2(NMe2)(CH2SiMe3)5, 1,1′-
Mo2(NMe2)(Ph)(CH2SiMe3)4, and 1,1′-Mo2(OtBu)(NMe2)(CH2-
SiMe3)4 are also available. This material is available free of
charge via the Internet at http://pubs.acs.org.
One last reaction to note is that between 1,2-Mo2-
(NMe2)2R4 and 2 equiv of HBr, which yields 1,1′-
(20) Reich-Rohrwig, P.; Wojiciki, A. Inorg. Chem. 1974, 13, 2457.
OM0003584