The addition of isocyanides to ReS4-: [3 + 1] cycloaddition to S=M=S

Article abstract of DOI:10.1039/b000932f

Isocyanides undergo [3 + 1] cycloadditions to ReS₄- to give dithiocarboimidate derivatives, Re(S)(S₄)(S₂CNR)- and Re₂S₅(S₂CNR)₂^2- , which undergo S-atom transfer and,

Full text of DOI:10.1039/b000932f

2
The addition of isocyanides to ReS4 : [3 + 1] cycloaddition to SNMNS
Daniel E. Schwarz and Thomas B. Rauchfuss*
School of Chemical Sciences and the Frederick Seitz Materials Research Laboratory, University of Illinois at
Urbana-Champaign, Urbana, IL 61801 USA. E-mail: rauchfuz@uiuc.edu
Received (in Irvine, CA, USA) 27th January 2000, Accepted 3rd May 2000
Published on the Web 8th June 2000
Isocyanides undergo [3 + 1] cycloadditions to ReS42 to give
dithiocarboimidate derivatives, Re(S)(S4)(S2CNR)2 and
Re2S5(S2CNR)222, which undergo S-atom transfer and, in
the case of the monometallic species, N-alkylation.
and the dithiocarboimidate (MeNCS222) ligands, a terminal
sulfur atom occupying the apical position. The rhenium atom
lies 0.4570 Å out of the plane formed by the basal sulfur ligands.
The CNN distance is 1.24 Å, which is consistent with a double
bond, and resembling previously described dithiocarbodimidate
complexes.12,13
Analogues of 3 were prepared using tert-butyl isocyanide
(ButNC) and cyclohexyl isocyanide (CyNC) to give the
corresponding derivatives 3b and 3c, respectively.§ No RNC
exchange was observed when solutions of 3a were treated with
an excess of ButNC or solutions of 3b were exposed to MeNC.
Warm solutions of 3a react with 1 and 1 equiv. of MeNC to give
2, indicating that intermetallic S-atom transfer is facile.
Methylation of 3a with MeOTf gave the dithiocarbamate
Re(S)(S4)(S2CNMe2) 4, confirming the relationship between
dithiocarboimidate and the more familiar dithiocarbamate
ligands. Dithiocarbamate complexes are usually prepared from
preformed dithiocarbamates or thiuram disulfides,14 not by N-
alkylation routes.
Cycloadditions to oxo- and thio-metallates represent an im-
portant class of reactions related to atom transfer catalysis, e.g.
by OsO4.1 In these transformations, the substrate adds wholly or
in part to the main group atom, the metal playing a secondary
role. Such cycloadditions to metal sulfides are relevant to
catalyst–substrate interactions in hydrodesulfurization (HDS)
catalysis.2 Virtually all metal-based cycloadditions proceed via
2 + 2 or, more commonly, 3 + 2 pathways,3,4 3 + 1 pathways
have not been observed.
Metal sulfides have been shown to catalyze the conversion of
CO into thioesters and other biologically significant functional-
ities, although the mechanisms of such reactions are unclear.5,6
The pathways for such reactions might be elucidated through
studies on the interactions of isoelectronic analogues of CO
with soluble metal sulfides under well defined conditions. Of all
The mechanism by which 2 and 3 arise involves generation of
a reactive derivative by the addition of sulfur atoms to 1
followed by trapping with RNC. We have previously shown that
such solutions bind nitriles (via a 3 + 2 cycloaddition process9).
Further work is underway to identify this reactive intermediate.
the soluble metal sulfides,7 ReS4 1 exhibits the greatest
2
reactivity toward alkenes and alkynes.8–11 Compound 1 was
therefore selected for an investigation of the reactions of metal
sulfides with isocyanides, which are isoelectronic with CO.
With rigorous exclusion of light and adventitious oxidants,
solutions of 1, as its NEt4+ or PPh4+ salts, are unreactive towards
MeNC. The reaction of MeNC and 1, however, proceeds briskly
when in the presence of elemental sulfur. Addition of 1–2
equivalents of elemental sulfur to solutions of (PPh4)1 and
MeNC afforded brown microcrystalline 2 whose (2)ESI-MS
spectrum shows molecular ions at m/z 372 (z = 22) and 743 (z
22
= 12) corresponding to Re2S9(CNMe)2
(isolated yield:
Scheme 1 Reagents and conditions: i, S8, MeNC (25 °C); ii, excess S8; iii,
1, MeNC; iv, MeOTf.
35%).† The IR spectrum of 2 exhibits peaks at 1586 and 526
cm21 for nCNN and nReNS, respectively. The 1H NMR spectrum
+
in CD3CN shows a PPh4 /Me ratio of 1; a pair of equally intense
Me signals (d 3.25 and 3.274) is attributed to the unsymmetrical
environment of the square-pyramidal Re centers such that the
Me can be trans to persulfide or sulfide (Scheme 1). In part due
to its low solubility, (PPh4)2 2 was not obtained as X-ray quality
crystals.
The addition of further equivalents of S8 to MeCN slurries of
2 gave (PPh4)[Re(S)(S4)(S2CNMe)] 3a over the course of
several hours at room temperature. In this reaction, the two
isomers of 2 are consumed at comparable, but not identical
rates. The new species can be more easily prepared by treatment
of 1 with MeNC in the presence of an excess of sulfur, the yields
being ca. 60%. The poorly soluble side products in this reaction
Fig. 1 Structure of 3a with thermal ellipsoids drawn at the 50% probability
level. Selected distances (Å) and angles (°): Re–S(1) 2.088(1), Re–S(2)
2.257(1), Re–S(5) 2.291(1), Re–S(6) 2.355(1), Re–S(7) 2.344(1), C(1)–
S(7) 1.776(7), C(1)–S(6) 1.802(7), C(1)–N 1.248(8), N–C(2) 1.467(10);
S(6)–C(1)–N 131.2(6), S(7)–C(1)–N 125.0(6), C(1)–N–C(2) 118.2(7),
S(2)–Re–S(5) 92.41(7), S(6)–Re–S(7) 73.66(6).
absorb at ca. 1580 cm21 (nCNN); (2)ESI-MS of these solids
m2
revealed ions corresponding to [ReS4]m[MeNC]n
(where m
and n = 1 and 2). The structure of 3a was established by single
crystal X-ray diffraction (Fig. 1).‡ The rhenium atom is square
pyramidal; the square base is defined by the tetrasulfido bridge
DOI: 10.1039/b000932f
Chem. Commun., 2000, 1123–1124
This journal is © The Royal Society of Chemistry 2000
1123

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The important conclusion is that isocyanides add across the
SNMNS functionality7 via an unprecedented 3 + 1 cycloaddition
process. Isocyanides have been shown to add to m-S ligands in
binuclear molybdenum compounds.15
This research was sponsored by the U.S. National Science
Foundation.
Notes and references
† Satisfactory CHN analyses were obtained for 2, 3a–c, and 4.
‡ Crystallographic data for 3a: C26H23NPReS7, M 791.12, T 193(2) K,
monoclinic space group P21/c a = 11.0148(7), b = 19.6065(13), c =
13.6281(9) Å, b = 98.89°, V = 2907.8(3) Å3, Z = 4, m = 4.755 mm21
,
18817 reflections (Rint
= 0.1019), 6944 independent reflections, for
observed data R1 = 0.0427, wR2 = 0.0898, for all data R1 = 0.0947, wR2
= 0.1043.
CCDC 182/1636. See http://www.rsc.org/suppdata/cc/b0/b000932f/ for
crystallographic files in .cif format.
§ 3b: 1H NMR (CD3CN, 500 MHz): d 1.43 (s). (2)ESI-MS: m/z 494. 3c: 1H
NMR (CD3CN, 500 MHz): d 1.3–1.9(m). (2)ESI-MS: m/z 520.
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Chem. Commun., 2000, 1123–1124