Regioselective and reversible carbon-nitrogen bond formation: Synthesis, structure and reactivity of ureato-bridged complexes [Mo2(NAr) 2(μ-X){μ-ArNC(O)NAr}(S2CNR2) 2] (Ar = Ph, p-tol; X = S, NAr; R = Me, Et, Pr)

Article abstract of DOI:10.1039/b417234p

Reaction of ArNCO with syn-[MoO(μ-O)(S₂CNR₂)] ₂ or syn-[MoO(μ-NAr)(S₂CNR₂)]₂ at 110°C leads to the facile formation of bridging ureato complexes [Mo ₂(NAr)₂(μ-NAr){μ-ArNC(O)NAr}(S₂CNR ₂)₂] (Ar = Ph, p-tol; R = Me, Et, Pr), formed upon substitution of all oxo ligands and addition of a further equivalent of isocyanate across one of the bridging imido ligands. Related sulfido-bridged complexes [Mo₂(NAr)₂(μ-S){μ-ArNC(O)NAr}(S ₂CNR₂)₂] have been prepared from syn-[Mo ₂O₂(μ-O)(μ-S)(S₂CNR₂) ₂]. When reactions with syn-[MoO(μ-NAr)(S₂CNEt ₂)]₂ were followed by NMR, intermediates were observed, being formulated as [Mo₂O(NAr)(μ-NAr){μ-ArNC(O)NAr}-(S ₂CNEt₂)₂], which at higher temperatures convert to the fully substituted products. A crystallographic study of [Mo ₂(N-p-tol)₂(μ-S){μ-p-tolNC(O)N-p-tol}(S ₂CNPr₂)₂] reveals that the bridging ureato ligand is bound asymmetrically to the dimolybdenum centre - molybdenum-nitrogen bonds irons to the terminal imido ligands being significantly elongated with respect to those cis - a result of the trans-influence of the terminal imido ligands. This trans-influence also leads to a trans-effect, whereby the exchange of aryl isocyanates can occur in a regioselective manner. This is followed by NMR studies and confirmed by a crystallographic study of [Mo₂(N-p- tol)₂(μ-N-p-tol)-{μ-p-tolNC(O)NPh}(S₂CNEt ₂)₂] - the PhNCO occupying the site trans to the terminal imido ligands. Ureato complexes also react with PhNCS, initially forming [Mo₂(NAr)₂(μ-S){μ-ArNC(O)NAr}(S₂CNR ₂)₂], resulting from exchange of the bridging imido ligand for sulfur, together with small amounts of [Mo₂(NAr) ₂(μ-S)(μ-S₂)(S₂CNEt₂) ₂], containing bridging sulfide and disulfide ligands. The ureato complexes [Mo₂(NAr)₂(μ-S){μ-ArNC(O)NAr}-(S ₂CNR₂)₂] react further with PhNCS to give [Mo₂(NAr)₂(μ-S)₂(S₂CNR ₂)₂]_n (n = 1, 2), which exist in a dimer-tetramer equilibrium. In order to confirm these results crystallographic studies have been carried out on [Mo₂(N-p-tol)₂(μ-S) (μ-S₂)(S₂CNEt₂)₂] and [Mo ₂(N-p-tol)₂(μ-S)₂(S₂CNPr ₂)₂]₂. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b417234p

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F U L L P A P E R
Regioselective and reversible carbon–nitrogen bond formation:
Synthesis, structure and reactivity of ureato-bridged complexes
[Mo2(NAr)2(l-X){l-ArNC(O)NAr}(S2CNR2)2] (Ar = Ph, p-tol;
X = S, NAr; R = Me, Et, Pr)
Graeme Hogarth* and Idris Richards
Department of Chemistry, University College London, 20 Gordon Street, London,
UK WC1H 0AJ. E-mail: g.hogarth@ucl.ac.uk
Received 10th November 2004, Accepted 16th December 2004
First published as an Advance Article on the web 18th January 2005
Reaction of ArNCO with syn-[MoO(l-O)(S2CNR2)]2 or syn-[MoO(l-NAr)(S2CNR2)]2 at 110 ◦C leads to the facile
formation of bridging ureato complexes [Mo2(NAr)2(l-NAr){l-ArNC(O)NAr}(S2CNR2)2] (Ar = Ph, p-tol; R = Me,
Et, Pr), formed upon substitution of all oxo ligands and addition of a further equivalent of isocyanate across one of
the bridging imido ligands. Related sulfido-bridged complexes [Mo2(NAr)2(l-S){l-ArNC(O)NAr}(S2CNR2)2] have
been prepared from syn-[Mo2O2(l-O)(l-S)(S2CNR2)2]. When reactions with syn-[MoO(l-NAr)(S2CNEt2)]2 were
followed by NMR, intermediates were observed, being formulated as [Mo2O(NAr)(l-NAr){l-ArNC(O)NAr}-
(S2CNEt2)2], which at higher temperatures convert to the fully substituted products. A crystallographic study of
[Mo2(N-p-tol)2(l-S){l-p-tolNC(O)N-p-tol}(S2CNPr2)2] reveals that the bridging ureato ligand is bound
asymmetrically to the dimolybdenum centre—molybdenum–nitrogen bonds trans to the terminal imido ligands being
significantly elongated with respect to those cis—a result of the trans-influence of the terminal imido ligands. This
trans-influence also leads to a trans-effect, whereby the exchange of aryl isocyanates can occur in a regioselective
manner. This is followed by NMR studies and confirmed by a crystallographic study of [Mo2(N-p-tol)2(l-N-p-tol)-
{l-p-tolNC(O)NPh}(S2CNEt2)2]—the PhNCO occupying the site trans to the terminal imido ligands. Ureato
complexes also react with PhNCS, initially forming [Mo2(NAr)2(l-S){l-ArNC(O)NAr}(S2CNR2)2], resulting from
exchange of the bridging imido ligand for sulfur, together with small amounts of [Mo2(NAr)2(l-S)(l-S2)(S2CNEt2)2],
containing bridging sulfide and disulfide ligands. The ureato complexes [Mo2(NAr)2(l-S){l-ArNC(O)NAr}-
(S2CNR2)2] react further with PhNCS to give [Mo2(NAr)2(l-S)2(S2CNR2)2]n (n = 1, 2), which exist in a
dimer–tetramer equilibrium. In order to confirm these results crystallographic studies have been carried out on
[Mo2(N-p-tol)2(l-S)(l-S2)(S2CNEt2)2] and [Mo2(N-p-tol)2(l-S)2(S2CNPr2)2]2.
with [CpCo(CO)2], an X-ray diffraction study confirming
Introduction
the presence of the ureato-bridge.7 Casey and co-workers
Ureato (or ureylene) ligands, [RNC(O)NR]2−, are doubly-
deprotonated ureas and, while moderately common at mononu-
clear metal centres (A), they are surprisingly rare at binuclear
centres,1–12 at which binding modes (B–D) have been identified.
As early as 1964, Manuel isolated a product from the
reaction of Fe3(CO)12 and PhNCO which was incorrectly
formulated as Fe2(CO)6(PhNCO)2,1 but later shown to be the
ureato-bridged iron(I) complex, [Fe2(CO)6{l-PhNC(O)NPh}].2
Soon after, Dekker and Knox produced the methyl analogue,
[Fe2(CO)6{l-MeNC(O)NMe}] from the reaction of methyl azide
with Fe2(CO)9, and showed that the structure closely resembled
the phenyl analogue.3,4 More recently, Hansert and Vahrenkamp
reported the formation of [Fe2(CO)6{l-NPhC(O)NPh}] from
the addition of CO to [Fe2(CO)6(l-PhN2Ph)] under mild
conditions.5 Since this report, Kabir and co-workers have de-
tailed the synthesis of a range of phosphine, [Fe2(CO)5(PPh3){l-
PhNC(O)NPh}], [Fe2(CO)4(l-dppm){l-PhNC(O)NPh}], and
phosphite, [Fe2(CO)6−n{P(OMe)3}n{l-PhNC(O)NPh}] (n = 1,
2) substitution products. These can be formed directly from
[Fe2(CO)6{l-PhNC(O)NPh}] upon addition of the appropri-
ate phosphorus-containing ligand, but are also produced di-
rectly from [Fe2(CO)6(l-PhN2Ph)]; the latter suggesting that
[Fe2(CO)6{l-PhNC(O)NPh}] is initially produced with phos-
phorus substitution occurring on this complex and not directly
onto [Fe2(CO)6(l-PhN2Ph)].6
have also reported the preparation of the related complex,
[Cp*2Co2{l-PhNC(O)NPh}], formed upon addition of PhNCO
◦
to [Cp*3Co3(l-H3)(l-H)] at 55 C; the IR spectrum displaying
a strong band at 1659 cm−1 indicative of C O stretching
=
vibration.8
In all the above examples, the metal is in a relatively low
1
1
oxidation state and the ligand acts in a l–g ,g -manner (C).
In contrast in the binuclear titanium complex, [(Cp2Ti)2{l-
PhNC(O)NPh}], reported by Fachinetti and co-workers, a
structural analysis shows the diphenylureato ligand bridges the
two Cp2Ti units through both the nitrogen and oxygen atoms
(D). Interestingly, the latter is isolated upon thermolysis of
trinuclear, [(Cp2Ti)3{l-PhNC(O)NPh}], which itself contains a
rare example of ureato bonding mode (B).9
A few years ago we reported the synthesis of the dimolyb-
denum(V) ureato-bridged complex, [Mo2(NPh)2(l-NPh){l-
PhNC(O)NPh}(S2CNEt2)2] (1a), produced upon refluxing a
toluene solution of syn-[MoO(l-NPh)(S2CNEt2)]2 with excess
PhNCO.10–11 Formation of 1a involves substitution of both
terminal oxo ligands and addition of a further equivalent of
isocyanate across a bridging imido ligand. At the time we
showed that the latter process was reversible; thermolysis of
1a in the absence of isocyanate affording syn-[Mo(NPh)(l-
NPh)(S2CNEt2)]2, a process which could be readily reversed
upon addition of PhNCO.
Somewhat similar ureato-bridged dicobalt complexes are
known. Thus, the dicobalt(II) complex, [Cp2Co2{l-t-BuNC(O)-
t-Bu}], is a low yield product of the reaction of (t-BuN)2S
At the time, a study of the crystal structure of 1a suggested
that the trans-influence of the terminal imido ligands may lead
to a trans-effect, by way of the regioselective loss of the PhNCO
7 6 0
D a l t o n T r a n s . , 2 0 0 5 , 7 6 0 – 7 7 3
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
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moiety trans to these ligands (in preference to that trans to the
bridging imido group). We have now returned to this area of
chemistry and herein demonstrate that this is indeed the case,
suggesting that this type of ureato-bridged complex are quite
unique in their structure and chemical behaviour.
of cis-[MoO2(S2CNMe2)2] with p-tolylNCO gave [Mo2(N-p-
tol)2(l-N-p-tol){l-p-tolNC(O)N-p-tol}(S2CNR2)2] (1c) in ca.
10% yield. With the latter, the sulfido-bridged ureato com-
plex [Mo2(N-p-tol)2(l-S){l-p-tolNC(O)N-p-tol}(S2CNMe2)2]
(2c) was also formed and isolation of pure 1c was difficult as
the two could not be separated by chromatography. Fortunately,
crystallisation of the mixture from a dichloromethane–methanol
solution gave brown crystals of 1c and orange crystals of 2c that
could be separated mechanically.
Results and discussion
Synthesis of ureato complexes
Synthesis of the ureato complexes, [Mo2(N-p-tol)2(l-N-p-
tol){l-p-tolNC(O)N-p-tol}(S2CNR2)2] (1b–1d), by the above
method is presumed to occur via the initial formation of syn-
[MoO(l-N-p-tol)(S2CNR2)]2. Indeed, refluxing syn-[MoO(l-N-
p-tol)(S2CNEt2)]2 with a further three equivalents of p-tolylNCO
in toluene for 24 h resulted in the formation of 1b in 90% yield.
In a similar manner, [Mo2(N-p-tol)2(l-S){l-p-tolNC(O)N-p-
tol}(S2CNR2)2] (2b and 2d) were prepared in high yields
upon thermolysis of syn-[Mo2O2(l-O)(l-S)(S2CNR2)2] with four
equivalents of p-tolylNCO in toluene for 24 h. Again, this
was the only product recovered after column chromatography.
The reaction of syn-[MoO(l-S)(S2CNEt2)]2 with p-tolylNCO
was also performed, however, in this instance only the starting
material was recovered.
While ureato complexes were successfully synthesized with
PhNCO and p-tolylNCO, attempts to produce them with the
more bulky isocyanates, such as 2,6-dimethylphenylisocyanate,
2,6-diisopropylphenylisocyanate and o-tolylNCO proved to be
unsuccessful. When the reaction was undertaken with o-tolyl-
NCO the previously reported tetraimido complex, [Mo(N-o-tol)-
(l-N-o-tol)(S2CNEt2)]2, was the only identifiable product.11
We have previously prepared the ureato-bridged complexes
[Mo2(NPh)2(l-NPh){l-PhNC(O)NPh}(S2CNEt2 )2 ] (1a) and
[Mo2(NPh)2(l-S){l-PhNC(O)NPh} (S2CNEt2)2] (2a) upon
thermolysis a slight excess of PhNCO with [MoO2(S2CNEt2)2]
or syn-[MoO(l-O)(S2CNEt2)]2 and syn-[Mo2O2(l-O)(l-S)(S2-
CNEt2)2] respectively, followed by purification by column
chromatography.10,11 As it was our intention to monitor reactions
of molybdenum(V) ureato complexes by 1H NMR spectroscopy,
it was decided that p-tolyl ureato complexes would be desirable—
the presence of the methyl groups providing a useful spec-
troscopic handle, while also serving to simplify the complex
aromatic region of the spectrum.
The ureato complex [Mo2(N-p-tol)2(l-N-p-tol){l-p-tolNC-
(O)N-p-tol}(S2CNEt2)2] (1b) was prepared upon refluxing syn-
[MoO(l-O)(S2CNEt2)]2 with approximately five equivalents of
p-tolylNCO for 24 h in toluene. The dark red–brown 1b was
obtained in 58% yield after purification by column chromato-
graphy. A similar reaction of p-tolylNCO with syn-[MoO(l-O)-
(S2CNPr2)]2 afforded [Mo2(N-p-tol)2(l-N-p-tol){l-p-tolNC-
(O)N-p-tol}(S2CNPr2)2] (1d) in 12% yield, while thermolysis
D a l t o n T r a n s . , 2 0 0 5 , 7 6 0 – 7 7 3
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With the bulky arylisocyanates, reactions cleanly took place
but timescales were long and stable products could not be
isolated.
Characterisation of ureato-bridged complexes was made on
1
the basis of IR, FAB mass spectroscopic and H NMR data.
The IR spectrum all exhibited a strong band at ca. 1710 cm−1,
=
assigned to the C O stretching mode of the ureato ligand. In
the mass spectrum, all showed molecular ions, together with a
further high mass peak associated with loss of one equivalent
1
of arylisocyanate (see later). The H NMR spectra were quite
characteristic and that of 1b (Fig. 1) was studied in some detail
in order that reactions of it could be monitored by NMR. In the
aromatic region of the spectrum, eight clearly resolved doublets
are apparent, those at higher field being associated with the
terminal imido ligands. In the region between d 2.5–2.0, four
methyl singlets are seen. That at d 2.16 is clearly associated
with the terminal imido ligands (C). A comparison with the
spectra of other ureato complexes described in this paper has
also allowed the unambiguous assignment of the remaining
three methyl resonances; d 2.39 to the bridging imido ligand
(A), d 2.34 to the tolyl (B) group trans to bridging imido ligand
(bottom isocyanate) and d 2.10 to the tolyl group (D) trans to
the terminal imido ligands (top isocyanate). Further, a COSY
spectrum allowed the aromatic resonances associated with each
methyl group to be established (as shown in Fig. 1).
Fig. 2 Molecular structure of [Mo2(N-p-tol)2(l-S){l-p-tolNC(O)N-
˚
p-tol}(S2CNPr2)2] (2d) with selected bond lengths (A) and angles
(◦); Mo(1)–Mo(2) 2.6793(5), Mo(1)–S(5) 2.3509(10), Mo(2)–S(5)
2.3442(10), Mo(1)–N(3) 2.354(3), Mo(2)–N(3) 2.333(3), Mo(1)–N(4)
2.162(3), Mo(2)–N(4) 2.180(2), Mo(1)–N(5) 1.733(3), Mo(2)–N(6)
1.724(3), N(3)–C(15) 1.373(4), N(4)–C(15) 1.436(4), C(15)–O(1)
1.204(4),
Mo(1)–S(5)–Mo(2)
69.59(3),
Mo(1)–N(3)–Mo(2)
69.72(7), Mo(1)–N(4)–Mo(2) 76.19(8), N(3)–C(15)–N(4) 101.0(3),
N(3)–Mo(1)–N(5) 158.44(12), N(3)–Mo(2)–N(6) 151.51(11), N(4)–
Mo(1)–N(5) 103.08(12), N(4)–Mo(2)–N(6) 95.05(12), Mo(1)–N(5)–
C(40) 168.6(3), Mo(2)–N(6)–C(50) 168.2(2).
reversed upon further addition of isocyanate.11 The successful
isolation of [Mo(NPh)(l-NPh)(S2CNEt2)]2 was based on its
relative insolubility in toluene, especially when compared to
that of 1a, shifting the equilibrium to the right hand side.
Heating 1b alone in d8-toluene for 3 h leads to some spectral
changes consistent with the generation of small amounts of
(presumably) [Mo(N-p-tol)(l-N-p-tol)(S2CNEt2)]2, however, its
relative solubility has made it impossible to isolate.
In order to probe further the nature of isocyanate exchange
in these ureato complexes we followed the reactions of [Mo2(N-
p-tol)2(l-N-p-tol){l-p-tolNC(O)N-p-tol}(S2CNEt2)2] (1b) and
Fig. 1 1H NMR assignments for 1b in CDCl3 (and toluene).
[Mo2(N-p-tol)2(l-S){l-p-tolNC(O)N-p-tol}(S2CNEt2)2]
(2b)
with an approximate four-fold excess of PhNCO in d8-toluene
by NMR. The most informative region of the spectra was the
methyl signals associated with the p-tolyl groups. In both cases,
at between 80–90 ◦C new species developed clearly, concurrent
with the generation of one equivalent of p-tolylNCO; as seen by
the growth of a new resonance at d 1.92. This was accompanied
by the decay of resonances at d 2.01 (at room temperature but
moves to slightly higher field upon heating) in 1b and at d 2.06
in 2b. In the case of 2b, at the end of the reaction we were able
We have previously characterised [Mo2(NPh)2(l-NPh){l-
PhNC(O)NPh}(S2CNEt2)2] (1a) crystallographically,10,11 while
Kim and Lee have also crystallographically characterized a
THF adduct.12 The most salient feature of the molecule is the
asymmetric nature of the binding to molybdenum of the two ni-
trogen atoms of the ureato-bridge. Thus, molybdenum–nitrogen
˚
bond lengths trans to the terminal imido ligands (av. 2.387 A)
˚
being around 0.230 A longer than those trans to the bridging
1
˚
imido ligand (av. 2.158 A). In order to confirm that this was a
to isolate a small amount of orange precipitate. The H NMR
general feature of these ureato-bridged complexes, we obtained
a crystal structure of [Mo2(N-p-tol)2(l-S){l-p-tolNC(O)N-p-
tol}(S2CNPr2)2] (2d), the results of which are summarised in
Fig. 2. Importantly, the same feature is seen, molybdenum–
nitrogen bonds trans to the sulfido-bridge [Mo(1)–N(4) 2.162(3),
spectrum of this in CDCl3 showed two methyl resonances in a
1 : 2 ratio at d 2.34 and 2.17, clearly showing that one p-tolyl
group had been lost. When compared to the spectrum of 2b,
which shows three methyl resonances in a 1 : 2 : 1 ratio at d 2.32,
2.15 and 2.07, it is clear that it is the high-field signal that is
lost; i.e. that proposed to be associated with the tolyl group (D)
(Fig. 1) trans to the terminal imido ligands. From the reaction
of 1b, we were also able to isolate the final product as a dark
˚
˚
Mo(2)–N(4) 2.180(2) A] being on average some 0.173 A shorter
than those trans to the terminal imido ligands [Mo(1)–N(3)
˚
2.354(3), Mo(2)–N(3) 2.333(3) A].
1
brown solid which showed three methyl singlets in a H NMR
spectrum run in CDCl3 at d 2.39, 2.34 and 2.16 in a ratio 1 : 1 : 2.
Again comparing this with the spectrum of 1b in CDCl3 (Fig. 1)
which shows four methyl signals in a 1 : 1 : 2 : 1 ratio at d 2.39,
2.34, 2.16 and 2.10, it is again the high-field signal associated
with the tolyl group trans to the terminal imido ligands which
has disappeared.
Regioselective isocyanate exchange
We previously noted that thermolysis of [Mo2(NPh)2(l-NPh){l-
PhNC(O)NPh}(S2CNEt2)2] (1a) led, in part, to the loss of
one equivalent of PhNCO and formation of the tetraimido
complex [Mo(NPh)(l-NPh)(S2CNEt2)]2, a process which can be
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On the basis of these results it seems clear that it is the p-
tolylNCO unit trans to the terminal imido groups that is ex-
changed regioselectively in both instances; i.e. the trans-influence
observed in the crystal structures of 1a and 2d is translated into
a trans-effect in these exchange reactions. This suggests that
the two products observed are [Mo2(N-p-tol)2(l-N-p-tol){l-
PhNC(O)N-p-tol}(S2CNEt2)2] (3a) and [Mo2(N-p-tol)2(l-S){l-
PhNC(O)N-p-tol}(S2CNEt2)2] (3b) respectively. Fortunately, we
have been able to confirm this with the crystallographic char-
acterisation of 3a, the results of with are summarised in Fig. 3.
The molecular structure closely resembles that of 1a and 2b,
and importantly it confirms that it is the p-tolylNCO trans to
the terminal imido ligands which is exchanged. As expected, the
trans-influence of the latter is manifested in the molybdenum–
nitrogen bonds; those trans to the imido-bridge [Mo(1)–N(3)
Scheme 1 Proposed mode of arylisocyanate exchange (dithiocarba-
mates omitted for clarity).
would then occur exclusively to the more exposed “top” face i.e.
that containing the sterically non-demanding dithiocarbamate
ligands. It should be noted that this is a dissociative pathway and
it is easy to rule out an associative pathway since the latter would
lead to an inversion of the stereochemistry at the dimolybdenum
centre, placing the ureato-bridge in the sterically less favourable
site close to the terminal imido ligands.
˚
˚
2.163(6), Mo(2)–N(3) 2.159(6) A] being on average some 0.228 A
shorter than those trans to the terminal imido ligands [Mo(1)–
Heating the NMR sample of 3a and excess PhNCO at 100 ◦C
for 1 h did not result in any further significant changes illustrat-
ing the regioselective nature of the isocyanate exchange process.
However, heating a toluene solution of 1b and excess PhNCO for
24 h leads to the formation of a mixture of inseparable ureato-
containing species as shown by NMR. Further, the integrated
ratio of the eight methylene protons and the methyl groups of the
p-tolyl moieties suggested that there were now only an average of
3.6 p-tolyl groups per molecule, i.e. on average four of the five p-
tolyl groups have been replaced (by phenyl groups). Three major
methyl signals were seen in an approximate 1 : 1 : 1 ratio at d
2.39, 2.34 and 2.15, together with a much smaller resonance at d
2.11 (ca. 15%). This suggests that all three remaining sites have
been substituted under these more forcing conditions.
˚
N(4) 2.394(5), Mo(2)–N(4) 2.383(5) A].
Mode of formation of ureato complexes and characterisation of
an intermediate
As mentioned above, formation of the ureato-bridged complexes
1 from syn-[MoO(l-NAr)(S2CNR2)]2 involves three steps; the
substitution of both terminal oxo groups, and addition of an
equivalent of isocyanate to one of the bridging imido ligands. In
order to further probe these processes, reactions of syn-[MoO(l-
NAr)(S2CNEt2)]2 (Ar = Ph, p-tol) with excess arylisocyanates
were followed by 1H NMR in d8-toluene. The presence of excess
isocyanate and protio-toluene made changes to the aromatic
region of the spectra difficult to follow, but the methyl resonances
of both the dithiocarbamate (d 1.0–0.5) and p-tolyl (d 2.2–1.7)
groups provided particularly useful probes.
Fig. 3 Molecular structure of [Mo2(N-p-tol)2(l-N-p-tol){l-p-tolNC-
˚
(O)NPh}(S2CNEt2)2] (3a) with selected bond lengths (A) and an-
gles (◦); Mo(1)–Mo(2) 2.6018(9), Mo(1)–N(3) 2.163(6), Mo(2)–
N(3) 2.159(6), Mo(1)–N(4) 2.394(5), Mo(2)–N(4) 2.383(5), Mo(1)–
N(5) 1.730(6), Mo(2)–N(7) 1.732(6), N(3)–C(11) 1.451(9), N(4)–
C(11) 1.355(9), C(11)–O(1) 1.206(8), Mo(1)–N(3)–Mo(2) 74.03(18),
Mo(1)–N(4)–Mo(2) 66.01(14), Mo(1)–N(6)–Mo(2) 82.8(2), N(3)–
C(11)–N(4) 101.7(6), Mo(1)–N(5)–C(40) 162.9(5), Mo(2)–N(7)–C(50)
173.7(7).
The reaction of yellow syn-[MoO(l-NPh)(S2CNEt2)]2 and
excess PhNCO in d8-toluene occurred over 4 h at room temper-
ature to afford an orange solution identified as [Mo2O(NPh)(l-
NPh){l-PhNC(O)NPh}(S2CNEt2)2] (4a). This assignment was
made initially on the basis of NMR evidence, which clearly
showed the addition of two equivalents of PhNCO. The two ends
of the molecule are now inequivalent being easily established
by the appearance of four triplet resonances between d 0.7–
0.4 associated with the methyl groups of the dithiocarbamate
ligands. We were able to prepare 4a in a bench scale reaction
between syn-[MoO(l-NPh)(S2CNEt2)]2 and two equivalents of
PhNCO at room temperature over 72 h. The IR spectrum
confirmed the presence of the ureato-bridge [m(CO) 1709 cm−1]
Scheme 1 highlights the proposed mode of arylisocyanate
exchange. Exclusive loss of the p-tolylNCO moiety trans to the
terminal imido ligands would generate syn-[Mo2(N-p-tol)2(l-
X)(l-N-p-tol)(S2CNEt2)2]. It is expected that only a minor
structure rearrangement of the Mo2N3X core would take place
during this process, most notably a slight folding of the Mo2(l-
X)(l-N-p-tol) unit would be expected since the fold-angle in
species of this type is typically 145–165◦.13 Alternative loss of
the second p-tolylNCO unit of the ureato-bridge would require a
much more significant perturbation of the Mo2(l-X)(l-N-p-tol)
core in opening up from approximately 90◦ found in the ureato
complexes. Addition of the PhNCO to the Mo2(l-X)(l-N-p-tol)
D a l t o n T r a n s . , 2 0 0 5 , 7 6 0 – 7 7 3
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and the remaining terminal oxo group [m(MoO) 927 cm−1].
Further, the mass spectrum showed a molecular ion, and in line
with all other ureato complexes, a second heavy ion associated
with loss of one equivalent of PhNCO. Unfortunately, we have
been unable to isolate 4a pure, all samples produced in this
way being seemingly contaminated with isocyanate oligomers
(molybdenum–oxo complexes are well-known to facilitate this
reaction). Nevertheless, we believe that we have strong evidence
for the initial formation of 4a and importantly, warming the
mixture of 4a and excess PhNCO to 100 ◦C resulted in the clean
and complete conversion to 1a.
addition of a second equivalent of isocyanate to one of the
bridging imido ligands affording the isolated intermediates 4a–b;
and in the final step the second terminal oxo group is replaced to
give ureato complexes 1. A key question is why the substitution
of terminal oxo ligand results in the facile addition of isocyanate
to a bridging imido ligand? We presume that the Mo2N2 core in
the starting complexes is not electron-rich enough to undergo
addition of the isocyanate- or that at this stage the equilibrium
between the bound and unbound isocyanate states lies strongly
towards the latter. The substitution of oxo for imido ligands by
isocyanates is quite well known and in (almost) all cases there
is a substantial energy barrier—necessitating the use of higher
temperatures.14 However once this has occurred, the stronger
p-donor ability of the imido versus the oxo moiety is expected to
lead to an increase in the electron density at the metal centre—
thus activating it towards isocyanate addition. Clearly, now the
equilibrium lies over towards the bound state; that is, the ureato-
bridge. Finally, the second oxo ligand is substituted at a slightly
higher temperature (possibly due to increased steric constraints)
to afford the final fully substituted product.
Following the experiments described above, we decided to
investigate two crossover reactions with the aim of gaining more
insight into the nature of the isocyanate addition process. Re-
action of syn-[MoO(l-N-p-tol)(S2CNEt2)]2 and excess PhNCO
in d8-toluene occurred at ca. 60 ◦C, the starting materials being
consumed over approximately 20 min at 70 ◦C. Importantly,
spectral changes mirrored those of the two previous experiments
and >90% of new resonances were associated with a single
complex, showing that the transformation is highly regiose-
lective. We propose that this new product, characterised by
the appearance of two new methyl resonances are observed
at d 2.17 and 2.02, is [Mo2O(NPh)(l-N-p-tol){l-PhNC(O)N-p-
tol}(S2CNEt2)2] (5). Unfortunately, we cannot unambiguously
assign each of the tolyl methyl resonances for 1b in d8-toluene
as one is obscured by the toluene signal (at ca. d 2.05). However,
based on the spectrum of 1b in CDCl3 (Fig. 1) and the
observation of four methyl signals for 1b in d8-toluene at d 2.18
(3H), ca. 2.05 (3H), 2.01 (3H) and 1.77 (6H), we favour the
assignment of the signal at d 2.17 to the bridging tolylimido
ligand and that at d 2.02 to one half of the newly created
ureato-bridge. We assign the latter to the tolyl group trans to
the bridging imido ligand. Further warming of 5 and PhNCO at
100 ◦C resulted in substitution of the second oxo group to afford
a single new species (>95%) assigned as [Mo2(NPh)2(l-N-p-
tol){l-PhNC(O)N-p-tol}(S2CNEt2)2] (6), and characterized by
tolyl methyl resonances at d 2.18 and 2.04, and two methyl
triplets associated with the dithiocarbamate ligands.
The reaction of syn-[MoO(l-N-p-tol)(S2CNEt2)]2 and excess
p-tolylNCO in d8-toluene proceeded in a similar manner,
although owing to the relative insolubility of syn-[MoO(l-N-p-
tol)(S2CNEt2)]2 in toluene, no significant reaction was observed
until the temperature reached 60 ◦C. Over about 30 min at
this temperature a new species appeared which was characte-
rised as [Mo2O(N-p-tol)(l-N-p-tol){l-p-tolNC(O)N-p-tol}(S2-
CNEt2)2] (4b). Seven low-field doublets are clearly seen in the
aromatic region of the spectrum (one is obscured) together with
four triplet resonances associated with the methyl groups of the
dithiocarbamates and four singlets between d 2.19–1.80 assigned
to the four inequivalent tolyl groups. As above, further warming
of 4b at 100 ◦C resulted in the clean formation of brown 2b over
about 10 min. Unfortunately, the insolubility of syn-[MoO(l-
N-p-tol)(S2CNEt2)]2 precluded the isolation of a good quality
sample of 4b; after stirring syn-[MoO(l-N-p-tol)(S2CNEt2)]2
and two equivalents of p-tolylNCO at room temperature for
4 days only starting materials and oligiomeric p-tolylNCO were
seen.
These experiments suggest that the mode of formation of
ureato complexes 1 from syn-[MoO(l-NAr)(S2CNR2)]2 is as
shown in Scheme 2. Initial substitution of one of the terminal
oxo groups occurs to afford [Mo2O(NAr)(l-NAr)2(S2CNR2)2]
in a slow, rate-determining, step. This is followed by the rapid
Scheme 2 Proposed mode of formation of ureato-bridges complexes 1 (dithiocarbamates omitted for clarity).
D a l t o n T r a n s . , 2 0 0 5 , 7 6 0 – 7 7 3
7 6 4

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The reaction between syn-[MoO(l-NPh)(S2CNEt2)]2 and ex-
cess p-tolylNCO in d8-toluene did not proceed with the high
level of regioselectivity expected. Upon warming to 70 C two
process may be the isomerisation of 7b–7a at the more elevated
temperatures, however, we favour a second process, namely the
exchange of the labile isocyanate trans to the terminal imido
ligands. In 7a and 8a this will have no net effect, however,
for 7b and 8b it will result in exchange of PhNCO for p-
tolylNCO; for example, 8b will react to form 9. There is
some evidence to suggest that this does occur since in the
final spectrum there are eight methyl resonances associated
with the p-tolyl groups having a total integrated intensity of
nearly 12 protons (against the expected 9). Thus, we favour
the final formation of [Mo2(N-p-tol)2(l-NPh){l-PhNC(O)N-
p-tol}(S2CNEt2)2] (8a) and [Mo2(N-p-tol)2(l-NPh){l-p-tol-
NC(O)N-p-tol}(S2CNEt2)2] (9), although we stress that this is
speculative.
◦
new species developed in an approximate 2 : 1 ratio. That both
were ureato complexes was apparent from the doubling up of all
aromatic resonances giving a spectrum, which was impossible to
unambiguously assign. Nevertheless, on the basis of its general
features and the above experiments we tentatively assign these
changes to the formation of two isomers of [Mo2O(N-p-tol)(l-
NPh){l-PhNC(O)N-p-tol}(S2CNEt2)2] (7a–b).
Formation of two isomers is difficult to reconcile with the
high regioselectivity observed in all previous transformations
of these complexes. Isomer 7a is the expected product, and
formation of 7b would require the folding of the Mo2N2 core
upon addition of the isocyanate. It may be that this is possible
in this instance and we note that the fold angle of 155.1◦ in
syn-[MoO(l-NPh)(S2CNEt2)]2 is significantly less than that of
164.6◦ found for syn-[MoO(l-N-p-tol)(S2CNEt2)]2.13 Further
warming to 100 ◦C resulted in substitution of the remaining
oxo ligand to produce two products in a 1.4 : 1 ratio. The
change in ratio suggests that a secondary process is occurring.
Heating the final mixture for a further 1 h at 100 ◦C resulted in
no significant further changes to the spectrum. This secondary
Reactions of ureato complexes with PhNCS and CS2
Heating [Mo2(N-p-tol)2(l-N-p-tol){l-p-tolNC(O)N-p-tol}(S2-
CNEt2)2] (1b) with an excess of PhNCS for 3 h in toluene led
to a colour change from brown to orange. After removal of
volatiles under reduced pressure addition of a small amount
of dichloromethane gave an orange solution together with a
small amount of a green insoluble solid. Chromatography of the
D a l t o n T r a n s . , 2 0 0 5 , 7 6 0 – 7 7 3
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soluble fraction led to separation of two bands. These afforded
[Mo2(N-p-tol)2(l-S){l-p-tolNC(O)N-p-tol}(S2CNEt2)2] (2b) as
an inseparable mixture with what is believed to be two isomers
of [Mo2(N-p-tol)2(l-S){l-PhNC(O)N-p-tol}(S2CNEt2)2] (10a)
(ca. 17%) (see below), and [Mo2(N-p-tol)2(l-S)(l-S2)(S2CNEt2)2]
(11) (20%). When the same reaction was carried out over a
slightly longer timescale, again small amounts of impure 2b and
11 were obtained together with larger amounts of the insolu-
ble green solid, identified as [Mo2(N-p-tol)2(l-S)2(S2CNEt2)2]2
(12a).
Refluxing 2b with an excess of PhNCS for 2 h also gave
green 12a upon removal of volatiles, showing that 2b is the
primary product of the reaction of 1b with PhNCS. This was
◦
1
further confirmed by a H NMR study. Thus at around 85 C
in d8-toluene, 1b begins to react with PhNCS giving 2b and
one equivalent of p-tolylNCO. Complete consumption of 1b
occurred over ca. 1 h at this temperature, with formation of 2b
being relatively clean, although small amounts of other products
(see below) were noted. Further heating of the solution at 100 ◦C
for 5 h led to consumption of 2b affording a yellow solution with
a green precipitate.
Thermolysis of both 1b and 2b with carbon disulfide for
extended periods also resulted in the clean generation of green
12a, although in both cases 11 was notably absent. Heating
green insoluble 12a in toluene affords a clear yellow solution,
which on cooling leads to re-precipitation of the insoluble
green solid. From this observation we surmise that tetrameric
green [Mo2(N-p-tol)2(l-S)2(S2CNEt2)2]2 (12a) is in thermal equi-
librium with dimeric yellow [Mo2(N-p-tol)2(l-S)2(S2CNEt2)2]
(13a). Wentworth and co-workers have previously noted a
similar situation for the analogous complexes [Mo2(N-p-tol)2(l-
S)2(S2CN-i-Bu2)2]n (n = 1, 2).16
In order to confirm the identity of [Mo2(N-p-tol)2(l-S)(l-
S2)(S2CNEt2)2] (11) it was crystallographically characterised
(Fig. 4). The molecule is essentially the same as [Mo2(NAr)2(l-
S)(l-S2)(S2CNEt2)2] (Ar = 2,6-i-Pr2C6H3), previously isolated
as a low yield product of the reaction of [Mo(NAr)2(S2CNEt2)2]
with hydrogen sulfide.15 In 11, the sulfide and disulfide units
are crystallographically disordered, and both sulfur atoms
bind approximately symmetrically to the dimolybdenum cen-
tre. A feature of the crystal structure of [Mo2(NAr)2(l-S)(l-
S2)(S2CNEt2)2] (Ar = 2,6-i-Pr2C6H3) was the presence of weak
intermolecular sulfur–sulfur interactions which are absent in 11.
Reaction of 1d with PhNCS proceeds in a similar manner
to that of 1b, although no disulfide product analogous to 11
was obtained. Chromatography of the soluble fraction afforded
an inseparable mixture of 2d and two isomers of [Mo2(N-
p-tol)2(l-S){l-PhNC(O)N-p-tol}(S2CNPr2)2] (10b) (see below),
while a green solid which precipitated from the reaction mixture
was identified as a mixture of [Mo2(N-p-tol)2(l-S)2(S2CNPr2)2]2
(12b) and [Mo2(N-p-tol)2(l-S)2(S2CNPr2)2] (13b). Crystalliza-
tion from dichloromethane–methanol gave a small number of
red crystals identified by crystallography as (12b) (Fig. 5). The
molecule is almost identical to the di-iso-butyldithiocarbamate
analogue previously characterised by Wentworth.16 The two
dimeric units are held together by triply-bridging sulfido ligands
˚
bound via two short [Mo–S(av) 2.373 A] and one long [Mo–
˚
S(av) 2.715 A] molybdenum–sulfur interactions, values which
˚
compare well with those of 2.38 and 2.69 A found in [Mo2(N-p-
tol)2(l-S)2(S2CN-i-Bu2)2]n (n = 1, 2).16
As detailed above, in both the reaction of 1b and 1d with
PhNCS, one of the products was an inseparable mixture of
the sulfido-bridged ureato complexes [Mo2(N-p-tol)2(l-S){l-
p-tolNC(O)N-p-tol}(S2CNR2)2] (2b and 2d) with two other
species, believed to be isomers of [Mo2(N-p-tol)2(l-S){l-
PhNC(O)N-p-tol}(S2CNR2)2] (10a–b). We have designated these
as cis and trans-relating to the relative positions of the N-tolyl
group of the ureato ligand and those occupying the terminal
sites.
While similar features were seen for the diethyl and dipropyl
complexes, only the latter is discussed fully. The 1H NMR
spectrum clearly showed the presence of 2d together with two
further complexes. For example, the aromatic region was quite
complex, but significantly there were three low-field doublets
all of similar intensities, one being assigned to 2d. In the tolyl-
methyl region of the spectrum, six singlets were observed, in
three pairs being associated with the two ureato and the terminal
Fig. 4 Molecular structure of [Mo2(N-p-tol)2(l-S)(l-S2)(S2CNEt2)2]
(11). The molecule has crystallographically imposed twofold symmetry,
atoms labelled “A” being at the equivalent position (1 − x, y, 0.5 −
◦
˚
z). Selected bond lengths (A) and angles ( ); Mo–Mo(A) 2.7854(15),
Mo–N(2) 1.731(6), Mo–S(3) 2.3750(19), Mo(A)–S(3) 2.387(2), Mo–S(4)
2.528(4), Mo(A)–S(4) 2.618(4), S(3)–S(4) 1.932(4), Mo–S(3)–Mo(A)
71.59(6), Mo–S(4)–Mo(A) 65.51(10), Mo–N(2)–C(10) 169.9(5).
7 6 6
D a l t o n T r a n s . , 2 0 0 5 , 7 6 0 – 7 7 3

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imido sites. Close scrutiny of the relative integrated intensities
of the dithiocarbamate and these resonances showed that rather
than the expected 12 protons for pure 2d, there were only 9.7
protons—suggesting some incorporation of phenyl groups. The
combined integral of the peaks associated with the terminal
imido ligands at d 2.14 and 2.13 (C) was approximately 6
protons, showing that all products contained these groups. In
contrast, those at d 2.32 and 2.31 associated with the tolyl group
trans to the imido bridge (B) integrated to only 1.6 protons,
and at d 2.08 and 2.07 assigned to the tolyl group trans to the
terminal imido ligands (D) to 2.1 protons (Fig. 1). This suggests
that the inseparable mixture contains both 2d and two isomers
of [Mo2(N-p-tol)2(l-S){l-PhNC(O)N-p-tol}(S2CNPr2)2] (10b).
This is substantiated by the FAB mass spectrum which shows
ions at both 1026 and 1012 associated with 2d and 10b,
respectively, together with their respective arylisocyanate loss
peaks at 890 and 876. While we cannot unambiguously assign
all the resonances in the spectrum to cis and trans isomers of
10b we can assign some. Thus, since tolyl-methyl resonances
at d 2.31, 2.14 and 2.07 are associated with 2d, then that at d
2.32 must be associated with cis-10b and that at d 2.08 with
trans-10b. On this basis we can surmise that the ratio of cis :
trans 10b is approximately 1.3 : 1.
Spectral features of the diethyldithiocarbamate complexes,
2b and 10a, show similar features. The ratio of the low-field
aromatic doublets at ca. 1 : 1 : 2 (2b) suggests that this sample
contains a greater amount of 2b. This is also reflected in the
overall integral of the tolyl-methyl resonances at 10.8 (versus the
expected 12). This comprises a 2.3 : 6 : 2.5 ratio (B : C : D)
suggesting that the ratio of cis : trans 10a is nearer 1.1 : 1.
From these results a picture emerges of the reactions of 1
with PhNCS and CS2. The first isolated product is 2, which
is possibly produced as shown in Scheme 3. With PhNCS,
Fig. 5 Molecular structure of [Mo2(N-p-tol)2(l-S)2(S2CNPr2)2]2 (12b).
The molecule has crystallographically imposed twofold symmetry,
atoms labelled “A” being at th◦e equivalent position (x, y, −z). Selected
˚
bond lengths (A) and angles ( ); Mo(1)–Mo(2) 2.8694(5), Mo(1)–N(3)
1.724(3), Mo(2)–N(4) 1.726(4), Mo(1)–S(5) 2.3741(12), Mo(2)–S(5)
2.3732(12), Mo(2A)–S(5) 2.7368(11), Mo(1)–S(6) 2.3732(12),
Mo(2)–S(6) 2.3733(12), Mo(1A)–S(6) 2.6923(11), Mo(1)–S(5)–Mo(2)
74.37(4), Mo(1)–S(6)–Mo(2) 74.33(4), Mo(1)–N(3) C(20) 168.1(3),
Mo(2)–N(4)–C(30) 175.6(4).
Scheme 3 Proposed mode of formation of 2 from 1 and PhNCS or CS2.
D a l t o n T r a n s . , 2 0 0 5 , 7 6 0 – 7 7 3
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Scheme 4 Proposed mode of formation of 12–13 from 2.
substitution of the trans-activated isocyanate could produce
isomeric products, with either the sulfur or nitrogen being metal
bound. The latter mimics the reactivity of the isocyanates, but if
the isocyanate is more strongly bound than the isothiocyanate
then the equilibrium will lie towards the former. In the second
orientation, to which carbon disulfide is limited, sulfur is metal
bound. Molybdenum(V) is well known to be chalcogenophilic
and thus such an orientation is expected to be favourable. Loss of
ArNCX would then afford the sulfido-bridged complex [Mo2(N-
p-tol)2(l-S)(l-N-p-tol)(S2CNEt2)2] to which the released iso-
cyanate can bind to give 2.
containing heterocummulenes are more complex and lead to
the formation of a number of sulfido-bridged species, which
nevertheless can be rationalised in terms of the labile nature of
isocyanate addition–elimination from the ureato ligand.
Experimental
All reactions were carried out under an inert atmosphere unless
otherwise stated. Molybdenum complexes, [MoO2(S2CNR2)2],17
syn-[MoO(l-O)(S2CNR2)]2,18 syn-[Mo2O2(l-O)(l-S)(S2CNR2)2]18
18
and syn-[MoO(l-S)(S2CNR2)]2 were prepared by literature
Formation of the sulfido-bridged complexes [Mo2(N-p-
tol)2(l-S)2(S2CNR2)2]n (n = 1, 2) (12–13) presumably arise from
2 via a similar process as summarised in Scheme 4.
methods. All isocyanates and isothiocyanates were used as
supplied. Chromatography was carried out in air on deactivated
(6% w/w water) alumina columns.
It is noteworthy that in both transformations described in
Schemes 3–4, when PhNCS is used the by-product is the
carbodiimide, PhNCN-p-tol. We have not been able to isolate
this but note that 1b does not react with CyNCNCy even upon
prolonged thermolysis.
Exchange of the aryl-substituents of the ureato-bridged
sulfido complexes leading to the formation of isomers of
10a–b was quite unexpected—yet reproducible with different
dithiocarbamate substituents. Scheme 5 attempts to account for
this. Exchange of p-tolylNCO for PhNCS as shown would be
reversible and have no resultant effect. However, if a competing
pathway involved loss of p-tolylNCS then this would result
in the formation of a new complex, [Mo2(N-p-tol)2(l-NPh)(l-
S)(S2CNR2)2]. This could then undergo further addition of
p-tolylNCO to afford the observed product; isomers perhaps
resulting from a relaxation of the puckered Mo2NS core to a
preferred flatter arrangement.
The precise mode of formation of [Mo2(N-p-tol)2(l-S)(l-
S2)(S2CNEt2)2] (11) remains unclear, and a related dipropy-
ldithiocarbamate product was not seen. It is certainly formed
directly from 1b and PhNCS although it was not identified from
either the reaction of 2b with PhNCS or 1b with carbon disulfide.
This suggests that it is not a sulfur addition product of 13a and
may arise directly from 1b.
Synthesis of [Mo2(N-p-tol)2(l-N-p-tol){l-N-p-tolC(O)N-p-
tol}(S2CNEt2)2] (1b)
syn-[MoO(l-O)(S2CNEt2)]2 (0.56 g, 1.01 mmol) was dissolved
in toluene (50 ml) and p-tolylNCO (0.64 ml, 5.03 mmol)
was added and the mixture was refluxed for 24 h. After
removal of volatiles a very dark product was obtained. This
was absorbed onto alumina and passed down a chromatog-
raphy column. Elution with 40% dichloromethane in petrol
gave a small unidentified yellow band. Elution with 70%
dichloromethane in petrol resulted in a dark red–brown prod-
uct identified as [Mo2(N-p-tol)2(l-N-p-tol){l-N-p-tolC(O)N-p-
tol}(S2CNEt2)2] (1b) (0.62 g, 58%). IR (KBr) m/cm−1 1703s,
1660m, 1596w, 1509vs, 1493s, 1445w, 1437m, 1320m, 1274m,
1255m, 1205w, 1148w, 1075w, 993w, 919w, 816m; 1H (CDCl3) d
7.79 (d, J 8.2, 2H, Ar), 7.39 (d, J 8.2, 2H, Ar), 7.10 (d, J 8.2, 2H,
Ar), 7.05 (d, J 8.1, 2H, Ar), 7.02 (d, J 8.3, 2H, Ar), 6.63 (d, J 8.3,
2H, Ar), 6.59 (d, J 8.1, 4H Ar), 6.34 (d, J 8.2, 4H Ar), 3.88 (m,
4H, CH2), 3.65 (sextet, J 7.1, 2H, CH2), 3.55 (sextet, J 7.1, 2H,
CH2), 2.39 (s, 3H, Me), 2.34 (s, 3H, Me), 2.16 (s, 6H, Me), 2.10
(s, 3H, Me), 1.23 (t, J 7.1, 6H, Me), 1.00 (t, J 7.1, 6H Me); mass
spectrum (FAB), m/z 1044 (M+), 909 (M − p-tolNCO); Anal.
Calc. for Mo2C46H55O1S4N7 C, 53.03, H, 5.28, N 9.41; Found
C, 51.97, H, 5.27, N, 9.20. Complex 1b was also prepared in
90% yield upon refluxing a toluene solution of syn-[MoO(l-N-
p-tol)(S2CNEt2)]2 and 3.5 equivalents of p-tolylNCO for 1 h.
Summary. It is clear from this work that ureato complexes
[Mo2(NAr)2(l-X){l-ArNC(O)NAr}(S2CNR2)2] (X = S, NAr)
(1–2) display chemical behaviour quite unlike any other bridging
ureato complexes previously studied. This seems to result from
the trans-influence of the terminal imido ligands. The latter
leads directly to the asymmetric binding of the ureato ligand,
which in turn accounts for the regioselective exchange of
one arylisocyanate for another. Transformations with sulfur-
Synthesis of [Mo2(N-p-tol)2(l-S){l-N-p-
tolC(O)N-p-tol}(S2CNEt2)2] (2b)
syn-[Mo2O2(l-S)(l-O)(S2CNEt2)2] (0.20 g, 0.35 mmol) was
dissolved in toluene (ca.50 ml) and p-tolylNCO (0.18 ml,
7 6 8
D a l t o n T r a n s . , 2 0 0 5 , 7 6 0 – 7 7 3

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Scheme 5 Possible route to the formation of mixed-substituent sulfido-bridged ureato complexes 10.
1.41 mmol) was added and the mixture was refluxed for
24 h. After removal of volatiles an orange–red product was
obtained. This was absorbed onto alumina and passed down a
chromatography column. Elution with 90% dichloromethane in
petrol gave a red–orange product, identified as [Mo2(N-p-tol)2(l-
S){l-N-p-tolC(O)N-p-tol}(S2CNEt2)2] (2b) (0.37 g, 84%). IR
(KBr) m/cm−1 1703s, 1654m, 1636w, 1563vw, 1511vs, 1460w,
1437m, 1315m, 1275m, 1251m, 1205w, 1148w, 1079w, 918w,
815m; 1H(CDCl3) d 7.74 (d, J 8.2, 2H, Ar), 7.04 (d, J 8.2, 2H,
Ar), 6.91 (d, 2H, J 8.3, Ar), 6.67 (d, J 8.3, 2H, Ar), 6.57 (d, 8.2,
4H, Ar), 6.46 (d, J 8.2, 4H, Ar), 3.98 (m, 4H, CH2), 3.60 (m,
4H, CH2), 2.32 (s, 3H, Me), 2.15 (s, 6H, Me), 2.07 (s, 3H, Me),
1.29 (t, J 7.1, 6H, Me), 1.05 (t, J 7.1, 6H, Me); mass spectrum
(FAB), m/z 970 (M+), 835 (M − p-tolNCO); Anal. Calc. for
Mo2C39H48O1S5N6 C, 48.34, H, 4.96, N 8.68, S 16.52; Found C,
48.93, H, 4.78, N, 8.68, S, 17.19.
from dichloromethane and methanol afforded crystals of two
types, namely brown crystals of 1c and orange crystals of 2c that
could be separated mechanically. For [Mo2(N-p-tol)2(l-N-p-
tol){l-N-p-tolC(O)N-p-tol}(S2CNMe2)2] (1c): IR (KBr) m/cm−1
1700s, 1686m, 1654m, 1623w, 1542m, 1507m, 1492s, 1395m,
1
1321m, 1260m, 1249m, 1103m, 1018w, 987w, 919w, 814m; H
(CDCl3) d 7.78 (d, J 8.2, 2H, Ar), 7.36 (d, J 8.2, 2H, Ar), 7.15
(d, J 8.2, 2H, Ar), 7.06 (d, 2H, J 8.2, Ar), 6.75 (d, J 8.3, 2H, Ar),
6.57 (d, 8.1, 4H, Ar), 6.31 (d, J 8.1, 4H, Ar), 3.34 (s, 6H, Me),
3.22 (s, 6H, Me), 2.39 (s, 3H, Me), 2.33 (s, 3H, Me), 2.16 (s, 6H,
Me), 2.14 (s, 3H, Me); mass spectrum (FAB), m/z 986 (M+), 852
(M − p-tolNCO); Anal. Calc. for Mo2C42H47O1S4N7.0.5CH2Cl2
C, 49.63, H, 4.67, N 9.53, S 12.46; Found C, 49.37, H, 4.53,
N, 9.42, S, 12.92. For [Mo2(N-p-tol)2(l-S){l-N-p-tolC(O)N-p-
tol}(S2CNMe2)2] (2c): IR (KBr) m/cm−1 1705s, 1532s, 1507vs,
1491s, 1394s, 1315s, 1267w, 1248s, 1140m, 1013w, 982m, 922m,
1
816s; H (CDCl3) d 7.75 (d, J 8.2, 2H, Ar), 7.05 (d, 2H, J 8.2,
Ar), 6.94 (d, J 8.3, 2H, Ar), 6.73 (d, 8.3, 2H, Ar), 6.57 (d, J 8.1,
4H, Ar), 6.46 (d, J 8.1, 4H, Ar), 3.38 (s, 6H, Me), 3.20 (s, 6H,
Me), 2.34 (s, 3H, Me), 2.16 (s, 6H, Me), 2.13 (s, 3H, Me); mass
spectrum (FAB), m/z 913 (M+), 780 (M − p-tolNCO); Anal.
Calc. for Mo2C42H35O1S5N6 C, 46.05, H, 4.39, N, 9.21, S 17.54;
Found C, 45.48, H, 4.84, N, 8.10, S, 17.83.
Synthesis of [Mo2(N-p-tol)2(l-N-p-tol){l-p-tolNC(O)N-p-
tol}(S2CNMe2)2] (1c) and [Mo2(N-p-tol)2(l-S){l-p-
tolNC(O)N-p-tol}(S2CNMe2)2] (2c)
[MoO2(S2CNMe2)] (1.31 g, 3.56 mmol) was dissolved in toluene
(50 ml) and p-tolylNCO (1.60 ml, 12.86 mmol) was added and
the mixture was refluxed for 24 h. After removal of volatiles
a very dark product was obtained. This was absorbed onto
alumina and passed down a chromatography column. Elution
with 30% dichloromethane in light petroleum afforded a purple
band which yielded [MoS2(N-p-tol)(S2CNMe2)2] (0.40 g, 22%).19
Elution with 50% dichloromethane in light petroleum gave a
red–orange product, identified as a mixture of [Mo2(N-p-tol)2(l-
N-p-tol){l-p-tolNC(O)N-p-tol}(S2CNMe2)2] (1c) and [Mo2(N-
p-tol)2(l-S){l-p-tolNC(O)N-p-tol}(S2CNMe2)2] (2c) (combined
yield 0.25 g, ca. 15%). While these complexes could not be
separated by chromatography, crystallisation of the mixture
Synthesis of [Mo2(N-p-tol)2(l-N-p-tol){l-p-tol-NC(O)N-p-
tol}(S2CNPr2)2] (1d)
syn-[MoO(l-O)(S2CNPr2)]2 (1.56 g, 2.57 mmol) was dissolved
in toluene (50 ml), p-tolylNCO (1.60 ml, 12.86 mmol) was added
and the mixture was refluxed for 24 h. After removal of volatiles
a very dark product was obtained. This was absorbed onto
alumina and passed down a chromatography column. Elution
with 35% dichloromethane in petrol gave an unidentified brown
product. Elution with 70% dichloromethane in petrol resulted
D a l t o n T r a n s . , 2 0 0 5 , 7 6 0 – 7 7 3
7 6 9

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1H, Ph), 6.58 (d, J 8.1, 4H, Ar), 6.32 (d, J 8.2, 4H, Ar), 3.85
(m, 4H, CH2), 3.65 (sextet, J 6.8, 2H, CH2), 3.54 (sextet, J 6.8,
2H, CH2), 2.39 (s, 3H, Me), 2.34 (s, 3H, Me), 2.16 (s, 6H, Me),
1.22 (t, J 7.1, 6H, Me), 1.00 (t, J 7.1, 6H, Me); Anal. Calc. for
Mo2C45H53O1S4N7 C, 52.58, H, 5.16, N 9.54, S 12.46; Found C,
51.94, H, 5.08, N, 9.37, S, 11.89.
in a brown product identified as [Mo2(N-p-tol)2(l-N-p-tol){l-
p-tol-NC(O)N-p-tol}(S2CNPr2)2] (1d) (0.34 g, 12%). IR (KBr)
m/cm−1 1703s, 1635br, 1501vs, 1441s, 1373w, 1319m, 1248s,
1
1145m, 1001w, 1030w, 987w, 912w, 812m; H (CDCl3) d 7.67
(d, J 8.2, 2H, Ar), 7.31(d, J 8.2, 2H, Ar), 7.04 (d, J 8.2, 2H, Ar),
6.96 (d, 2H, J 8.2, Ar), 6.88 (d, J 8.3, 2H, Ar), 6.58 (d, J 8.1, 2H,
Ar), 6.48 (d, J 8.1, 4H, Ar), 6.21 (d, J 8.1, 4H, Ar), 3.77 − 3.66
(m, 4H, NCH2), 3.44 − 3.32 (m, 4H, NCH2), 2.35 (s, 3H, Me),
2.30 (s, 3H, Me), 2.13 (s, 6H, Me), 2.07 (s, 3H, Me), 1.61 (m, 4H,
CH2), 1.41 − 1.24 (m, 4H, CH2), 0.81 (t, J 7.4, 6H, Me), 0.70
(t, J 7.4, 6H, Me); mass spectrum (FAB), m/z 1098 (M+), 966
(M+ − p-tolNCO); Anal. Calc. for Mo2C50H63O1S4N7 C, 54.69,
H, 5.74, N 8.93, S 11.67; Found C, 53.94, H, 5.75, N, 8.56, S,
10.87.
Synthesis of [Mo2(N-p-tol)2(l-S){l-PhNC(O)N-p-
tol}(S2CNEt2)2] (3b)
In an NMR tube, [Mo2(N-p-tol)2(l-S){l-p-tolNC(O)N-p-
tol}(S2CNEt2)2] (2b) (10 mg) was partially dissolved in ca. 2 ml of
d8-toluene and the 1H NMR spectrum was recorded. 1H NMR
(d8-toluene) d 8.20 (d, J 8.2, 2H, Ar), 7.53 (d, J 8.4, 2H, Ar), 7.11–
6.88 (obscured), 6.86 (d, J 8.1, 2H, Ar), 6.67 (d, J 8.2, 4H, Ar),
6.28 (d, J 8.0, 4H, Ar), 3.28 (sextet, J 6.8, 2H, CH2), 3.23 (sextet,
J 6.8, 2H, CH2), 2.88 (sextet, J 6.8, 2H, CH2), 2.80 (sextet, J 6.8,
2H, CH2), 2.06 (s, 3H, Me), 1.97 (s, 3H, Me), 1.74 (s, 6H, Me),
0.67 (t, J 7.2, 6H, Me), 0.53 (t, J 7.2, 6H, Me). About four-fold
excess of PhNCO was added which resulted in no significant
change to the spectrum. The solution was slowly warmed to
80 ◦C and significant spectral changes took place. After heating
Synthesis of [Mo2(N-p-tol)2(l-S){l-N-p-tolC(O)N-p-
tol}(S2CNPr2)2] (2d)
syn-[Mo2O2(l-S)(l-O)(S2CNPr2)2] (0.093 g, 0.15 mmol) was
dissolved in toluene (ca.50 ml), p-tolylNCO (0.10 ml, 0.75 mmol)
was added and the mixture was refluxed for 24 h. After removal
of volatiles an orange–red product was obtained. This was
absorbed onto alumina and passed down a chromatography
column. Elution with 40% dichloromethane in petrol gave
a red–orange product, identified as [Mo2(N-p-tol)2(l-S){l-N-
p-tolC(O)N-p-tol}(S2CNPr2)2] (2d) (0.12 g, 79%). IR (KBr)
m/cm−1 1703s, 1506vs, 1433m, 1367w, 1315m, 1246s, 1194w,
◦
for 20 min at 90 C the clean formation of a new product was
apparent together with the formation of one equivalent of p-
tolylNCO (d 1.89, s). This has been identified as [Mo2(N-p-
1
tol)2(l-S){l-PhNC(O)N-p-tol}(S2CNEt2)2] (3b). H NMR (d8-
toluene) d 8.12 (d, J 8.1, 2H, Ar), 7.47 (d, J 8.0, 2H, Ar), 7.26
(d, J 7.8, 4H, Ar), 7.05–6.52 (obscured), 6.23 (d, J 8.1, 4H, Ar),
3.29 (sextet, J 6.8, 2H, CH2), 3.21 (sextet, J 6.8, 2H, CH2), 2.75
(m, 4H, CH2), 1.98 (s, 3H, Me), 1.71 (s, 6H, Me), 0.65 (t, J 7.1,
6H, Me), 0.53 (t, J 7.1, 6H, Me). After standing for a few days,
a small amount of yellow solid was deposited in the NMR tube.
All liquids were decanted from the tube and the solid was dried
under vacuum in the tube. Redissolution of the solid in CDCl3
allowed a cleaner spectrum to be obtained. 1H NMR (CDCl3) d
7.81 (d, J 8.2, 2H, Ar), 7.34 (d, J 7.7, 2H, Ph), 7.32 (d, J 8.2, 2H,
Ar), 6.91 (t, J 8.3, 2H, Ph), 6.73 (t, J 7.4, 1H, Ph), 6.59 (d, J 8.2,
4H, Ar), 6.50 (d, J 8.2, 4H, Ar), 3.97 (sextet, J 7.1, 2H, CH2),
3.91 (sextet, J 7.1, 2H, CH2), 3.53 (septet, J 6.8, 4H, CH2), 2.34
(s, 3H, Me), 2.17 (s, 6H, Me), 1.26 (t, J 7.2, 6H, Me), 1.05 (t, J
7.2, 6H, Me).
1
1146m, 1089w, 993w, 920w, 814m; H(CDCl3) d 7.73 (d, J 8.3,
2H, Ar), 7.02 (d, J 8.0, 2H, Ar), 6.88 (d, 2H, J 8.4, Ar), 6.64
(d, J 8.2, 2H, Ar), 6.56 (d, 8.2, 4H, Ar), 6.44 (d, J 8.2, 4H, Ar),
3.89 (m, 4H, NCH2), 3.46 (m, 4H, NCH2), 2.31 (s, 3H, Me),
2.14 (s, 6H, Me), 2.07 (s, 3H, Me), 1.74 (m, 4H, CH2), 1.46 (m,
4H, CH2), 0.92 (t, J 7.4, 6H, Me), 0.82 (t, J 7.4, 6H, Me); mass
spectrum (FAB), m/z 1026 (M+), 893 (M − p-tolNCO); Anal.
Calc. for Mo2C43H56O1S5N6 C, 50.39, H, 5.47, N 8.20; Found C,
50.15, H, 5.56, N, 8.03.
Synthesis of [Mo2(N-p-tol)2(l-N-p-tol){l-PhNC(O)N-p-
tol}(S2CNEt2)2] (3a)
In an NMR tube, [Mo2(N-p-tol)2(l-N-p-tol){l-p-tolNC(O)N-
p-tol}(S2CNEt2)2] (1b) (10 mg) was dissolved in ca. 2 ml of d8-
1
1
toluene and the H NMR spectrum was recorded. H NMR
(d8-toluene) d 8.18 (d, J 8.2, 2H, Ar), 7.85 (d, J 8.2, 2H, Ar),
7.51 (d, J 8.2, 2H, Ar), 7.09 (d, J 8.2, 2H, Ar), 6.84 (d, J 8.2,
2H, Ar), 6.70 (d, J 8.2, 2H, Ar), 6.60 (d, J 8.2, 4H, Ar), 6.33 (d,
J 8.2, 4H, Ar), 3.21 (m, 4H, CH2), 2.94 (sextet, J 6.8, 2H, CH2),
2.82 (sextet, J 6.8, 2H, CH2), 2.18 (s, 3H, Me), ca. 2.05 (s, 3H,
Me, obscured), 2.01 (s, 3H, Me), 1.77 (s, 6H, Me), 0.67 (t, J 7.1,
6H, Me), 0.50 (t, J 7.1, 6H, Me). The three singlet resonances
observed for the methyl groups of the tolyl substituents at room
temperature (a fourth being obscured by the toluene) at d 2.18
(3H), 2.01 (3H) and 1.77 (6H) were monitored throughout the
reaction. About four-fold excess of PhNCO was added which
resulted in no significant change to the spectrum. The solution
was slowly warmed and at 65 ◦C a new resonance at d 1.92
began to develop with concomitant reduction of the resonance
at 2.01. After approximately 50 min the ratio of these two
Room temperature reaction of syn-[MoO(l-NPh)(S2CNEt2)]2
with PhNCO-identification of [Mo2O(NPh)(l-NPh){l-
PhNC(O)NPh}(S2CNEt2)2] (4a)
Toluene-d8 was added to syn-[MoO(l-NPh)(S2CNEt2)]2 (ca.
10 mg) in an NMR tube. This gave a pale yellow solution
1
with a significant amount of insolubles. The H NMR showed
the dissolved material to be a ca. 17 : 1 mixture of syn and
anti isomers. An excess of PhNCO was added and after 1 h
a significant number of smaller new resonances had appeared.
After 4 h the solution was now bright orange and resonances
associated with syn-[MoO(l-NPh)(S2CNEt2)]2 had disappeared
(although those due to the anti isomer were still apparent). Some
precipitate remained although this took on an orange appear-
ance. The orange material is characterised as [Mo2O(NPh)(l-
NPh){l-PhNC(O)NPh}(S2CNEt2)2] (4a). 1H NMR (d8-toluene)
d 8.37 (d, J 7.6, 2H, Ph), 7.92 (d, J 7.5, 2H, Ph), 7.39 (d, J 7.7, 2H,
Ph), 7.30 (t, J 8.0, 2H, Ph), 7.05–6.50 (obscured), 3.20 (sextet,
2H, J 6.8, CH2), 3.04 (m, 4H, CH2), 2.83 (sextet, J 6.8, 2H,
CH2), 0.73 (t, J 7.2 3H, Me), 0.60 (t, J 7.1, 3H, Me), 0.51 (t, J
7.1, 3H, Me), 0.44 (t, J 7.2, 3H, Me). Upon heating to 100 ◦C,
clean conversion into 1a occurred with dissolution of all solid
materials.
◦
peaks was ca. 5 : 1 and warming to 90 C resulted in the vir-
tual disappearance of the resonance at d 2.01. In a separate
experiment the resonance at d 1.92 was shown to be associated
with p-tolylNCO. Upon cooling back to room temperature four
resonances were clearly observed at d 2.16 (3H), 2.01 (3H), 1.90
(3H) and 1.76 (6H). Further heating of the solution at 100 ◦C in
an oil bath for 18 h did not lead to any significant change. Hexane
was layered onto the dark brown toluene solution and over ca.
2 weeks small dark red needles developed. These were removed
and dried to afford [Mo2(N-p-tol)2(l-N-p-tol){l-PhNC(O)N-p-
tol}(S2CNEt2)2] (3a). 1H (CDCl3) d 7.78 (d, J 8.2, 2H, Ar), 7.39
(d, J 8.2, 2H, Ar), 7.14 (d, J 8.2, 2H, Ph), 7.11 (d, J 8.2, 2H,
Ar), 7.05 (d, J 8.2, 2H, Ar), 6.88 (t, J 8.3, 2H, Ph), 6.67 (t, J 7.3,
In order to get more data on 4a a bench scale reaction was
carried out. syn-[MoO(l-NPh)(S2CNEt2)]2 (0.12 g, 0.17 mmol)
was placed in a Schlenk tube and degassed toluene (ca. 50 ml)
and PhNCO (0.04 ml, 0.38 mmol) were added. The mixture
was stirred for 72 h by which time a red solution was obtained.
This was filtered and volatiles removed under reduced pressure
7 7 0
D a l t o n T r a n s . , 2 0 0 5 , 7 6 0 – 7 7 3

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resulting in isolation of 4a as an orange–red powder (0.12 g,
80%) slightly contaminated with oligomers of PhNCO. IR (KBr)
m/cm−1 1709s, 1658s, 1591s, 1515vs, 1479s, 1441s, 1385w, 1315m,
Reaction of syn-[MoO(l-NPh)(S2CNEt2)]2 with p-tolylNCO
followed by NMR
In a similar fashion to that described above, the reaction of p-
tolylNCO and syn-[MoO(l-NPh)(S2CNEt2)]2 (ca. 10 mg) was
carried out in d8-toluene. Upon warming to 70 ◦C signifi-
cant spectral changes were observed, being similar to those
described above, but with a clear doubling of all the new
resonances. The spectrum was too complex to fully assign but
in line with the above experiments we ascribe these changes
to the formation of a mixture of [Mo2O(N-p-tol)(l-NPh){l-
PhNC(O)N-p-tol}(S2CNEt2)2] (7a–b) in an approximate 2 :
1 ratio e.g. d 8.40 (d, J 7.5, Ar, minor), 8.34 (d, J 8.1,
Ar, major). Further warming to 100 ◦C resulted in the loss
of these peaks with the concomitant development of new
resonances attributed to the formation of [Mo2(N-p-tol)2(l-
NPh){l-PhNC(O)N-p-tol}(S2CNEt2)2] (8a) and [Mo2(N-p-
tol)2(l-NPh){l-p-tolNC(O)N-p-tol}(S2CNEt2)2] (9) in a 1.4 : 1
1
1265s, 1200w, 1145w, 1078m, 1026m, 927m, 756m, 686m; H
NMR (CDCl3) d 7.95 (d, J 7.6, 2H, Ph), 7.48 (d, J 7.6, 2H,
Ph), 7.43 (t, J 8.2, 2H, Ph), 7.25 (t, J 7.6, 2H, Ph), 7.20 (t, J
7.9, 1H, Ph), 7.14 (m, 2H, Ph), 7.04 (t, J 7.3, 1H, Ph), 6.97 −
6.91 (obscured), 6.74 (m, 1H, Ph), 6.68 (d, J 7.5, 2H, Ph), 3.89–
3.78 (m, 4H, CH2), 3.73–3.51 (m, 4H, CH2), 1.26 (t, J 7.1, 3H,
Me), 1.24 (t, J 7.1, 3H, Me, 1.07 (t, J 7.1, 3H, Me), 1.02 (t, J
7.1, 3H, Me); Mass spectrum (FAB), m/z 897 (M+), 779 (M+ −
PhNCO). All attempts to isolate a pure sample of 4a failed,
with crystals of anti-[MoO(l-NPh)(S2CNEt2)]2 being deposited
from chloroform solutions after extended periods,13 NMR also
showing the formation of syn-[MoO(l-NPh)(S2CNEt2)]2 and
aniline.
1
Reaction of syn-[MoO(l-N-p-tol)(S2CNEt2)]2 with p-tolylNCO
followed by NMR-identification of [Mo2O(N-p-tol)(l-N-p-
tol){l-p-tolNC(O)N-p-tol}(S2CNEt2)2] (4b)
ratio (major isomer remains unclear). H NMR (d8-toluene) d
8.22 (d, J 7.6, 2H, Ar, minor), 8.11 (d, J 8.2, 2H, Ar, major), 7.88
(d, J 7.6, 2H, Ar, minor), 7.76 (d, J 8.2, 2H, Ar, major), 7.40 (d,
J 8.2, 2H, Ar, major), 7.27 (d, J 7.6, 2H, Ar, minor), 7.18 (d,
J 8.3, 2H, Ar, major), 7.07 (d, J 7.9, 2H, Ar, minor), 6.87 (t, J
8.2, 2H, Ph, minor), 6.85 (t, J 8.2, 2H, Ph, major), 6.75 − 6.50
(obscured), 6.41 (d, J 7.8, 2H, Ar, major), 6.39 (d, J 7.7, 2H, Ar,
minor), 3.37 (m, 4H, CH2, major + minor), 3.23 (m, 2H, CH2,
major + minor), 3.12 (m, 2H, CH2, major + minor), 2.19 (s, 3H,
Me, minor), 2.09 (s, 3H, Me, major), 2.06 (s, 3H, Me, minor),
2.01 (s, 3H, Me, minor), 1.86 (s, 6H, Me, major), 1.85 (s, 6H,
Me, minor), 0.81 (t, J 7.2, 6H, Me, major), 0.80 (t, J 7.2, 6H,
Me, minor), 0.67 (t, J 7.2, 6H, Me, major), 0.66 (t, J 7.2, 6H,
Me, minor).
In a similar fashion to that described above, the reaction of p-
tolylNCO and syn-[MoO(l-N-p-tol)(S2CNEt2)]2 (ca. 10 mg) was
carried out in d8-toluene. The insolubility of syn-[MoO(l-N-p-
tol)(S2CNEt2)]2, however, meant that no significant reaction was
observed until the sample was heated to 60 ◦C. At this tempera-
ture, after about 30 min the solution became orange and a new
species was seen tentatively characterised as [Mo2O(N-p-tol)(l-
N-p-tol){l-p-tolNC(O)N-p-tol}(S2CNEt2)2] (4b). 1H NMR (d8-
toluene) d 8.32 (d, J 8.2, 2H, Ar), 7.86 (d, J 8.2, 2H, Ar), 7.34
(d, J 8.2, 2H, Ar), 7.14 (d, J 8.1, 2H, Ar), 6.88 (d, J 8.2, 2H,
Ar), 6.76 (d, J 8.2, 2H, Ar), 6.42 (d, J 8.2, 2H, Ar), one doublet
obscured, 3.22 (m, 2H, CH2), 3.05 (m, 4H, CH2), 2.84 (m, 2H,
CH2), 2.19 (s, 3H, Me), 2.05 (s, 3H, Me), 2.00 (s, 3H, Me), 1.80
(s, 3H, Me), 0.74 (t, J 7.2, 3H, Me), 0.61 (t, J 7.1, 3H, Me),
0.52 (t, J 7.1, 3H, Me), 0.46 (t, J 7.2, 3H, Me). Upon heating
to 100 ◦C, over 10 min clean conversion to 1b occurred with
complete dissolution of all insolubles.
Reactions of [Mo2(N-p-tol)2(l-X){l-p-tolNC(O)N-p-
tol}(S2CNEt2)2] (X = N-p-tol, S) (1b–2b) with PhNCS and CS2
A
mixture of [Mo2(N-p-tol)2(l-N-p-tol){l-p-tolNC(O)N-p-
tol}(S2CNEt2)2] (1b) (0.06 g, 0.06 mmol) and PhNCS (0.08 ml,
0.74 mmol) in toluene (ca. 50 ml) was refluxed for 3 h. This
led to the formation of a green precipitate in an orange solution.
After separation of the precipitate, volatiles were removed under
reduced pressure to give an orange–brown oily product. This was
absorbed onto alumina and passed down a chromatography
column. Elution with 50% dichloromethane in petrol resulted
in isolation of an orange band shown identified as [Mo2(N-p-
tol)2(l-S)(l-S2)(S2CNEt2)2] (11) (0.01 g, 20%). IR (KBr) m/cm−1
1511vs, 1437s, 1381m, 1354m, 1318m, 1274s, 1204m, 1148m,
1070w, 987w, 912vw, 846vw, 815m, 778w: 1H (CDCl3) d 6.62 (d,
J 8.4, 2H, Ar), 6.58 (d, J 8.4, 2H, Ar), 3.99 (m, 4H, CH2), 3.84
(sextet, J 6.9, 2H, CH2), 3.80 (sextet, J 6.9, 2H, CH2), 2.14 (s,
6H, Me), 1.35 (d, J 7.1, 6H, Me), 1.34 (t, J 7.1, 6H, Me); mass
spectrum (FAB) m/z 796 (M+).
Reaction of syn-[MoO(l-N-p-tol)(S2CNEt2)]2 with PhNCO
followed by NMR
In a similar fashion to that described above, the reaction of
PhNCO and sparingly soluble syn-[MoO(l-N-p-tol)(S2CNEt2)]2
(ca. 10 mg) was carried out in d8-toluene. Upon warming
to 60 ◦C significant spectral changes were observed, while at
◦
70 C the starting material was consumed over approximately
20 min (the tube was shaken a few times in order to aid
dissolution) with the formation of a bright orange solution.
By NMR this contained ca. 90% of a single new species
tentatively assigned as [Mo2O(NPh)(l-N-p-tol){l-PhNC(O)N-
p-tol}(S2CNEt2)2] (5). 1H NMR (d8-toluene) d 8.29 (d, J 8.3, 2H,
Ar), 7.83 (d, J 8.2, 2H, Ar), 7.41 (d, J 7.8, 2H, Ar), 7.35 (d, J 7.8,
2H, Ar), 7.11–6.54 (obscured), 3.19 (sextet, J 7.0, 2H, CH2), 3.01
(m, 4H, CH2), 2.80 (sextet, J 7.0, 2H, CH2), 2.17 (s, 3H, Me),
2.02 (s, 3H, Me), 0.72 (t, J 7.2, 3H, Me), 0.58 (t, J 7.2, 3H, Me),
0.51 (t, J 7.2, 3H, Me), 0.44 (t, J 7.2, 3H, Me). Some minor peaks
were seen which may be associated with a second isomer of ca.
10% composition most notably at d 2.16 (s), 2.13 (s), 0.69 (t, J
7.2), 0.62 (t, J 7.2), 0.52 (t, J 7.2) all others being obscured. Upon
warming to 100 ◦C, further spectral changes were apparent
and after 1 h the solution turned dark brown. By NMR this
contained ca. 95% of a single new species tentatively assigned
as [Mo2(NPh)2(l-N-p-tol){l-PhNC(O)N-p-tol}(S2CNEt2)2] (6).
1H NMR (d8-toluene) d 8.19 (d, J 8.2, 2H, Ar), 7.84 (d, J 8.2,
2H, Ar), 7.60 (d, J 7.7, 2H, Ar), 7.37 (d, J 7.9, 2H, Ar), 7.09 (d,
J 7.7, 2H, Ar), 7.07–6.50 (obscured), 3.25 (m, 4H, CH2), 2.95
(sextet, J 6.8, 2H, CH2), 2.84 (sextet, J 6.8, 2H, CH2), 2.18 (s,
3H, Me), 2.04 (s, 3H, Me), 0.71 (t, J 7.2, 6H, Me), 0.55 (t, J 7.2,
6H, Me).
Elution with 60% dichloromethane in petrol afforded an
inseparable mixture of (2b) and two isomers of [Mo2(N-p-tol)2(l-
S){l-PhNC(O)N-p-tol}(S2CNEt2)2] (10a) (0.01 g, ca. 17%). IR
(KBr) m/cm−1 1702s, 1653m, 1634m, 1590m, 1511vs, 1504sh,
1493s, 1439m, 1383w, 1313m, 1261s, 1204w, 1146m, 1097s,
1
1021s, 806s; H (CDCl3) d 7.88 (d, J 8.4, Ar), 7.75 (d, J 8.6,
Ar), 7.74 (d, J 8.2, Ar, 2b), 7.24 (d, J 8.0, Ar), 7.16 (t, J 7.8, Ph),
7.05 (d, J 8.4, Ar), 7.04 (d, J 8.2, Ar, 2b), 6.92 (d, J 8.4, Ar), 6.91
(d, J 8.3, Ar, 2b), 6.87 (t, J 8.3, Ph), 6.68 (d, J 8.4, Ar), 6.67 (d,
J 8.3, Ar, 2b), 6.57 (d, J 8.2, Ar, 2b), 6.56 (d, J 8.2, Ar), 6.47 (d,
J 8.3, Ar), 6.46 (d, J 8.2, Ar, 2b), 6.43 (d, J 8.2, Ar), 3.95 (m,
NCH2), 3.55 (m, NCH2), 2.33 (s, Me), 2.32 (s, Me, 2b), 2.15 (s,
Me, 2b), 2.14 (s, Me), 2.08 (s, Me), 2.07 (s, Me, 2b), 1.293 (t, J
7.1, Me, 2b), 1.290 (t, J 7.4, Me), 1.06 (t, J 7.4, Me), 1.05 (t, J 7.1,
Me, 2b), 0.801 (t, J 7.4, Me); Anal. Calc. for Mo2C38H46O1S5N6
C, 48.07, H, 4.89, N 8.74; Found C, 47.75, H, 4.85, N, 8.40.
The green precipitate was washed with hexane and dried
to afford [Mo2(N-p-tol)2(l-S)2(S2CNEt2)2]n (12a–13a) (0.01 g,
22%). This was better prepared as detailed below. Thermolysis
D a l t o n T r a n s . , 2 0 0 5 , 7 6 0 – 7 7 3
7 7 1

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of a toluene solution of 2b (0.09 g, 0.09 mmol) and PhNCS
(0.11 ml, 0.82 mmol) resulted in the deposition of a large amount
of green precipitate. This was isolated, washed with hexane and
dried to yield (12a–13a) (0.04 g, 56%). The complexes show poor
solubility in all common organic solvents. Addition of warm
chloroform resulted in the dissolution of a small amount of
material giving a yellow solution. Heating in toluene resulted in
the complete dissolution to give a golden yellow solution from
which a green solid was precipitated upon cooling. When the
J 8.2, Ar), 3.87 (m, NCH2), 3.45 (m, NCH2), 2.32 (s, Me), 2.31
(s, Me, 2d), 2.14 (s, Me, 2d), 2.13 (s, Me), 2.08 (s, Me), 2.07 (s,
Me, 2d), 1.72 (m, CH2), 1.23 (m, CH2), 0.93 (t, J 7.4, Me, 2d),
0.92 (t, J 7.4, Me), 0.819 (t, J 7.4, Me), 0.816 (t, J 7.4, Me, 2d),
0.801 (t, J 7.4, Me); mass spectrum (FAB), m/z 1024 (MA+),
+
+
1012 (MB+), 890 (MA − p-tolNCO + MB − PhNCO); Anal.
Calc. for Mo2C42H55O1S5N6 C, 49.85, H, 5.44, N 8.31; Found C,
49.37, H, 5.41, N, 8.03.
Further elution with 40% dichloromethane in light petroleum
gave a yellow–green band which afforded a mixture of
[Mo2(N-p-tol)2(l-S)2(S2CNPr2)2]2 (12b) and [Mo2(N-p-tol)2(l-
S)2(S2CNPr2)2] (13b) (0.44 g, 43%). Red crystals grown from
a dichloromethane–methanol mixture were analysed crystallo-
graphically. Upon addition of chloroform a yellow solution was
generated, shown by NMR to be 13b. IR (KBr) m/cm−1 1655m,
1635m, 1487s, 1460m, 1424m, 1382w, 1308w, 1242m, 1196vw,
1144m, 1099m, 102ovw, 986w, 811m; 1H NMR (CDCl3) d 6.51
(s, 8H, Ar), 3.92–3.81 (m, 8H, NCH2), 2.10 (s, 6H, Me), 1.89–
1.77 (m, 8H, CH2), 1.01 (t, J 7.4, 12H, Me); mass spectrum
(FAB), m/z 819 (M+); Anal. Calc. for Mo2C28H42S6N4 C, 41.08,
H, 5.13, N 6.85; Found C, 41.55, H, 5.42, N, 6.77.
1
solution was tepid it took on a red appearance. H (CDCl3) d
6.46 (s, 8H, Ar), 3.91 (dectet, J 7.1, 8H, CH2), 2.05 (s, 6H, Me),
1
1.88 (t, J 7.1, 12H, Me); H (DMSO) d 6.84 (d, J 8.3, 4H, Ar,
tetramer), 6.77 (d, J 8.3, 4H, Ar, tetramer), 6.58 (d, J 8.2, 4H,
Ar, dimer), 6.84 (d, J 8.3, 4H, Ar, tetramer), 6.46 (d, J 8.3, 4H,
Ar, tetramer), 6.42 (d, J 8.3, 4H, Ar, tetramer), 6.25 (d, J 8.3,
4H, Ar, dimer), 4.03 − 3.86 (m, 8H, CH2, dimer + tetramer),
2.10 (s, 6H, Me, tetramer), 2.07 (s, 6H, Me, tetramer), 2.03 (s,
6H, Me, dimer), 1.87 (s, 6H, Me, tetramer), 1.82 (s, 6H, Me,
tetramer), 1.31 (t, J 7.2, 12H, tetramer), 1.25 (t, J 7.2, 12H,
dimer), 1.15 (t, J 7.2, 12H, tetramer) ratio dimer : tetramer at
room temperature ca. 12 : 1; mass spectrum (FAB)m/z 763 (M+);
IR (KBr) m/cm−1 1700w, 1559w, 1491vs, 1450w, 1428s, 1376w,
1356w, 1331m, 1274s, 1209m, 1141m, 1079w, 993w, 813m; Anal.
Calc. for Mo4C48H68S12N8.CH2Cl2 C, 36.54, H, 4.35, N 6.96, S
23.87; Found C, 36.69, H, 4.31, N, 6.89, S, 23.96.
X-Ray data collection and solution
Complex 1b (10 mg) was dissolved in ca. 2 ml of d8-toluene
and the 1H NMR spectrum was recorded (see above). An excess
of PhNCS was added and the sample warmed to 85 ◦C at which
point new resonances began to appear. After heating for ca.
30 min, all 1b was consumed and a clear orange solution was
formed. NMR revealed that one equivalent of p-tolylNCO was
generated (d 1.92, s) together with a number of new species one
of which 2b accounted for ca. 80% of the material. 1H NMR (d8-
toluene) d 8.07 (d, J 8.2, 2H, Ar), 7.39 (d, J 8.2, 2H, Ar), 7.23
(d, J 8.2, 2H, Ar), 7.12 (d, J 8.2, 2H, Ar), 6.98–6.50 (obscured),
6.31 (d, J 8.2, 4H, Ar), 6.21 (d, J 8.2, 2H, Ar), 3.51–2.96 (m, 8H,
CH2), 1.98 (s, 3H, Me), 1.78 (s, 6H, Me), 1.75 (s, 3H, Me), 0.75
(t, J 7.2, 6H, Me), 0.64 (t, J 7.2, 6H, Me). Further heating of the
solution at 100 ◦C for 2 h gave a clear yellow–green solution fr◦om
which 11 was noticeably absent. After a further 5 h at 100 C,
copious amounts of green precipitate were deposited making the
acquisition of good quality NMR spectra impossible.
Thermolysis of toluene solutions of 1b and excess CS2 resulted
in the formation of 2b and 12a–13a. Precise amounts of each
varied depending on the amount of CS2 used and the reaction
time—with longer reaction times (>3 h) leading to the formation
of more 12a–13a. Likewise, heating a toluene solution of 2b and
a 10 fold excess of CS2 overnight resulted in the formation of
12a–13a in 73% yield.
For all complexes, a single crystal was mounted on a glass
fibre and all geometric and intensity data were taken from
this sample. For [Mo2(N-p-tol)2(l-S)(l-S2)(S2CNEt2)2] (11) data
collection was made on an automated Nicolet R3mV four-circle
˚
diffractometer equipped with Mo-Ka radiation (k = 0.710 73 A)
at 293 2 K. Lattice parameters were identified by application
of the automatic indexing routine of the diffractometer to the
positions of a number of reflections taken from a rotation
photograph and centred by the diffractometer. The x − 2h
technique was used to measure reflections and three standard
reflections (re-measured every 97 scans) showed no significant
loss in intensity during data collection. The data was corrected
for Lorenz and polarisation effects and unique data with I ≥
2r(I) were used to solve and refine the structure. This was solved
by direct methods and developed by using alternating cycles
of least-squares refinement and difference-Fourier synthesis.
All non-hydrogen atoms were refined anisotropically. Hydrogen
˚
atoms were placed in idealised positions (C–H 0.96 A) and
2
˚
assigned a common isotropic thermal parameter (U = 0.08 A ).
The sulfide and disulfide units were disordered and this was
modelled by fixing the occupancy of S(4) at 50%. Structure
solution used SHELXTL PLUS program package on an IBM
PC.
For [Mo2(N-p-tol)2(l-S){l-p-tolNC(O)N-p-tol}(S2CNPr2)2]
(2d), [Mo2(N-p-tol)2(l-N-p-tol){l-p-tolNC(O)NPh}(S2CNEt2)2]
(3a) and [Mo2(N-p-tol)2(l-S)2(S2CNPr2)2]2 (12b) data collection
was made on a Bruker SMART APEX CCD diffractometer us-
Reaction of [Mo2(N-p-tol)2(l-N-p-tol){l-p-tol-NC(O)N-p-
tol}(S2CNPr2)2] (1d) with PhNCS
˚
ing graphite-monochromated Mo-Ka radiation (k = 0.71073 A)
Heating a toluene solution (50 ml) of 1d (0.129 g, 0.12 mmol)
with PhNCS (0.16 ml, 1.34 mmol) for 3 h resulted in a
colour change from brown to orange. Removal of volatiles
gave an orange–red solid. This was absorbed onto alumina
and passed down a chromatography column. Elution with 40%
dichloromethane in light petroleum gave a yellow band which
afforded an orange solid (0.03 g, ca. 27%) identified as an
inseparable mixture of [Mo2(N-p-tol)2(l-S){l-p-tol-NC(O)N-
p-tol}(S2CNPr2)2] (2d) and isomers of [Mo2(N-p-tol)2(l-S){l-
PhNC(O)N-p-tol}(S2CNPr2)2] (10b). IR (KBr) m/cm−1 1712s,
1593w, 1508vs, 1493s, 1433m, 1371w, 1311s, 1244s, 1192w,
at 293 2 K (2d and 12b) and at 150 2 K (3a). Data reduction
was carried out with SAINT+ and absorption correction
applied using the program SADABS. Structures were solved
by direct methods and developed by using alternating cycles
of least-squares refinement and difference-Fourier synthesis.
All non-hydrogen atoms were refined anisotropically except
for atom C(7) in 12b which had a large isotropic thermal
parameter. In 3a, some of the carbon atoms of one of the phenyl
groups [C(50)–C(55)] were disordered over two positions which
shared two atoms [C(50) and C(53)]. This was successfully mod-
eled using 50% occupancy for sites C(51)/C(81), C(52/C82),
C(54)/C(84) and C(55)–C(85). Hydrogens were generally placed
in calculated positions (riding model). Structure solution used
the SHELXTL PLUS V6.10 program package.
1
1146m, 1104w, 993w, 816m; H (CDCl3) d 7.86 (d, J 8.0, Ar),
7.74 (d, J 8.2, Ar), 7.73 (d, J 8.3, Ar, 2d), 7.23 (d, J 8.1, Ar),
7.16 (t, J 7.5, Ph), 7.03 (d, J 8.0, Ar, 2d), 7.02 (t, J 8.4, Ph), 6.89
(d, J 8.4, Ar), 6.88 (d, J 8.4, Ar, 2d), 6.84 (t, J 8.3, Ph), 6.65 (d,
J 8.4, Ar), 6.64 (d, J 8.2, Ar, 2d), 6.56 (d, J 8.2, Ar, 2d), 6.55
(d, J 8.2, Ar), 6.44 (d, J 8.2, Ar, 2d), 6.43 (d, J 8.3, Ar), 6.42 (d,
CCDC reference numbers 253354–253357.
See http://www.rsc.org/suppdata/dt/b4/b417234p/ for cry-
stallographic data in CIF or other electronic format.
7 7 2
D a l t o n T r a n s . , 2 0 0 5 , 7 6 0 – 7 7 3

View Article Online
observed [I > 2.0r(I)]. At final convergence, R1 = 0.0472, wR2 =
0.1000 [I > 2.0r(I)] and R1 = 0.0625, wR2 = 0.1057 (all data),
for 356 parameters.
Crystallographic data for [Mo2(N-p-tol)2(l-S){l-p-tolNC-
(O)N-p-tol}(S2CNPr2)2] (2d); orange block, dimensions 0.12 ×
¯
0.10 × 0.09 mm, triclinic, space group P1, a = 12.9948(9), b =
˚
13.0247(9), c = 16.4809(11) A, a = 88.154(1), b = 69.828(1),
◦
3
˚
c = 65.095(1) , V = 2353.8(3) A , Z = 2, F(000) = 1054, dcalc
=
References
1 T. A. Manuel, Inorg. Chem., 1964, 3, 1703.
2 J. A. Jarvis, B. E. Job, B. T. Kilbourn, R. H. B. Mais, P. G. Ouston
and P. F. Todd, J. Chem. Soc., Chem. Commun., 1967, 1149.
3 M. Dekker and G. R. Knox, J. Chem. Soc., Chem. Commun., 1967,
1243.
1.445 g cm−3, l = 0.794 mm−1, Tmax/Tmin = 0.932/0.911. 20955
reflections were collected, 10851 unique [R(int) = 0.0219] of
which 7712 were observed [I > 2.0r(I)]. At final convergence,
R1 = 0.0486, wR2 = 0.1102 [I > 2.0r(I)] and R1 = 0.0728, wR2 =
0.1126 (all data), for 521 parameters.
4 R. J. Doedens, Inorg. Chem., 1968, 7, 2323.
Crystallographic data for [Mo2(N-p-tol)2(l-N-p-tol){l-p-
tolNC(O)NPh}(S2CNEt2)2] (3a); red needle, dimensions 0.15 ×
0.08 × 0.02 mm, monoclinic, space group C2/c, a = 35.949(2),
5 B. Hansert and H. Vahrenkamp, J. Organomet. Chem., 1993, 459,
265.
6 S. E. Kabir, M. Ruf and H. Vahrenkamp, J. Organomet. Chem., 1998,
571, 91.
7 Y. Matsuura, N. Ysauoka, T. Veki, N. Kasai and M. Kakudo, Bull.
Chem. Soc. Jpn., 1969, 42, 881.
8 C. P. Casey, R. A. Widenhoefer and R. K. Hayashi, Inorg. Chem.,
1995, 34, 1138.
9 G. Fachinetti, C. Biran, C. Floriani, A. Cheisi-Villa and C. Gustini,
J. Chem. Soc., Dalton Trans., 1979, 792.
◦
˚
b = 16.1750(11), c = 17.8306(12) A, b = 112.957(1) , V =
3
9546.8(11) A , Z = 8, F(000) = 4184, dcalc = 1.424 g cm−3, l =
˚
0.741 mm−1, Tmax/Tmin = 0.985/0.897. 29627 reflections were
collected, 11157 unique [R(int) = 0.0723] of which 5984 were
observed [I > 2.0r(I)]. At final convergence, R1 = 0.0804, wR2 =
0.1942 [I > 2.0r(I)] and R1 = 0.1573, wR2 = 0.2296 (all data),
for 576 parameters.
10 G. D. Foster and G. Hogarth, J. Chem. Soc., Dalton Trans., 1993,
2539.
Crystallographic
data
for
[Mo2(N-p-tol)2(l-S)(l-
S2)(S2CNEt2)2] (11); orange block, dimensions 0.42
×
11 T. A. Coffey, G. D. Foster and G. Hogarth, J. Chem. Soc., Dalton
0.27 × 0.25 mm, monoclinic, space group C2/c, a = 18.516(4),
Trans., 1995, 2337.
◦
˚
12 G. Kim and S. W. Lee, Bull. Korean Chem. Soc., 1998, 19, 1211.
13 G. Hogarth and I. Richards, Inorg. Chim. Acta, 2005, 358, 707.
14 (a) I. S. Kolomnikov, Yu. D. Koreshkov, T. S. Lobeeva and M. E.
Volpin, Chem. Commun., 1970, 1432; (b) E. A. Maatta, Inorg. Chem.,
1984, 23, 2650; (c) A. J. Nielson, Inorg. Synth., 1986, 24, 194;
(d) M. L. H. Green, G. Hogarth, P. Konidaris and P. Mountford,
J. Chem. Soc., Dalton Trans., 1990, 3781.
b = 12.012(2), c = 14.994(3) A, b = 90.13(3) , V = 3334.9(11)
A , Z = 4, F(000) = 1608, dcalc = 1.583 g cm−3, l = 1.211 mm−1,
3
˚
T
max/Tmin = 0.988/0.741. 2980 reflections were collected, 2886
unique [R(int) = 0.0231] of which 2104 were observed [I >
2.0r(I)]. At final convergence, R1 = 0.0539, wR2 = 0.1147 [I >
2.0r(I)] and R1 = 0.0776, wR2 = 0.1319 (all data), for 172
parameters.
15 T. A. Coffey and G. Hogarth, Polyhedron, 1997, 16, 165.
16 K. L. Wall, K. Folting, J. C. Huffman and R. A. D. Wentworth, Inorg.
Chem., 1983, 22, 2366.
17 F. W. Moore and M. L. Larson, Inorg. Chem., 1967, 6, 998.
18 F. A. Schultz, V. R. Ott, D. S. Rolison, D. C. Bravard, J. W. McDonald
and W. E. Newton, Inorg. Chem., 1978, 17, 1758.
19 T. A. Coffey, G. D. Foster and G. Hogarth, J. Chem. Soc., Dalton
Trans., 1996, 183.
Crystallographic data for [Mo2(N-p-tol)2(l-S)2(S2CNPr2)2]2
(12b); red block, dimensions 0.12 × 0.11 × 0.10 mm, tetragonal,
˚
space group P43212, a = b = 12.4558(5), c = 49.648(3) A, V =
3
7702.8(6) A , Z = 4, F(000) = 1672, dcalc = 1.412 g cm−3, l =
˚
0.999 mm−1, Tmax/Tmin = 0.907/0.890. 68980 reflections were
collected, 9229 unique [R(int) = 0.0501] of which 7565 were
D a l t o n T r a n s . , 2 0 0 5 , 7 6 0 – 7 7 3
7 7 3