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The synthesis, structures, bonding and unusual reactivity of sulfido osmium carbonyl cluster compounds
Poiyh&on Vol. 4, No. 12, pp. 2OOS2025, 1985 Printed in Great Britain
0
0277-5387/85 1985 Peqmm
S3.00+ .OLl F’rcsa Ltd
POLYHEDRON REPORT NUMBER 13 THE SYNTHESIS, REACiWITY
STRUCTURES, BONDING AND UNUSUAL OF SULFIDO OSMIUM CARBONYL CLUSTER COMPOUNDS
RICHARD D. ADAMS Department of Chemistry, University of South Carolina, Columbia, SC 29208, U.S.A.
I. INTRODUCTION.
CONTENTS . . . . . . . . . . . . . . , . . . . . . . . . . 2003
................... II. SYNTHESES AND STRUCTURES 1. Reagents and reactions ....................... 2. Cluster condensation reactions .................... (a) Homonuclear clusters ...................... (b) Mixed-metal clusters ...................... III. BONDING AND UNUSUAL REACTIVITY. .......................... 1. M&clusters 2. M4S clusters ..........................
...............
2004 2004 2010 2010 2016 2020 2021 2022
I. INTRODUCTION The class of low-valent
polynuclear metal complexes that contain metal-metal bonds are known as cluster compounds. These compounds have attracted a great deal of attention with the idea they might serve as models for metal surfaces and heterogeneous catalysts.‘-3 More recently, interest has been focused on the chemical properties that may make them distinct from both heterogeneous and mononuclear metal homogeneous catalysts.4 A major problem in the use of cluster compounds as catalysts is their tendency to undergo degradative fragmentation when subjected to forcing reaction conditions. In an effort to prevent this a number of workers have chosen to employ bridging ligands. GSThe bridging ligands are usually derived from elements of the main groups and may be suitably modified with alkenyl and aryl substituents (e.g. NR, PR, PR,, As, AsR, AsR,, S, SR etc.). In this report attention will be focused on compounds that contain the bridging sulfido ligand and the transition element osmium. In cluster complexes containing three metal atoms, the sulfido ligand invariably serves as a triple bridge, 1.6It contains a lone pair of electrons and, when viewed as an uncharged ligand, it serves as a four-electron donor. Thus, within the framework of the valence-bond description one of the three M-S bonds would be viewed as a two-electron S to M donor bond. It is not a requirement that the M3 group contain three M-M bonds and there are many documented examples where triply bridging sulfido ligands span M3 units which have two or less metal-metal bonds.6 In the presence of four metal atoms the sulf?do ligand frequently serves as a quadruply bridging ligand, 2 or 3. Ifall the metal atoms lie on the same side of a plane that passes through the sulfido ligand, it is regarded as a four-electron donor and it contains a lone pair of electrons (2).* Interestingly, the ligand then has only three orbitals for use in bonding to the four metal atoms. Although the bonding is difficult to describe in terms of valence-bond theory, it fits naturally into the framework of the delocalized bonding scheme of the polyhedral skeletal electron pair (PSEP) theory. In most, if not all cases, the structural type 2 contains an M, cluster with four metal-metal bonds.
*There may be one exception to this definition. This occurs in the complex [HP~OS,(CO)&~-S)(JJ~S)(PPh,C,H,)],
which is described later in this section. 2003
R. D. ADAMS
2004
The structural type 3 contains four metal atoms arranged in a tetrahedral or distorted tetrahedral array about a centralized sulfido ligand. This sulfido ligand, viewed as an uncharged atom, serves as a six-electron donor and two of the four M-S bonds can be viewed as donor-acceptor bonds. There are no examples of type 3 sulfido ligands in the presence of six metal-metal bonds. The sulfido ligand appears to be very flexible and able to adapt to conditions imposed upon it by the arrangements of the metal atoms. For example, in the presence of M-M bonding, M-S-M bond angles as low as 65-75” are common. In the absence of M-M bonding these angles may range from 125 to 140”.’ A compound which imposes an unprecedented degree of distortion upon a quadruply bridging sulfido ligand is ’ The heavy-atom framework of the structure of this CHPtOs,(CO)*(C14-S)(C13’S)(PPh~C~H~)l~. molecule is shown in Fig. 1. The quadruply bridging sulfido ligands, S1 and S,, are believed to serve as six-electron donors of type 3. However, due to the highly strained polycyclic network of M,S rings, all the metal atoms are drawn to one side of a plane which passes through the sulfido ligand. Similar highly strained geometries have been observed for carbon atoms in certain polycyclic hydrocarbon compounds, and these geometries have been described as “inverted” tetrahedral.‘O II. SYNTHESES
AND
STRUCTURES
1. Reagents and reactions Although a number of sulfur-containing compounds have been used in the synthesis of sulfidotransition-metal cluster compounds,6 surprisingly few of these have been used for the synthesis of sulfido osmium carbonyl clusters. Hydrogen sulfide(H,S) would seem to be an ideal source of sulfide, but it has not yet been used with wide success. H,S does react with Os,(CO),, at 125°C to form the cluster 0s3($-I)~3-S)(CO)g (4) in nearly quantitative yields. I1 Both hydrogen atoms are transferred from the sulfur atom to the metal atoms where they bridge adjacent metal-metal bonds. The sulfido ligand serves as a triple bridge.
s,
lo/
’ 4
\ P)q, I0 qos\>Js, s
00s.
/
/
\ \>os: s’ 1
5
4
..s
6
I
+‘: osI’
The reaction of Os,(CO),, with elemental sulfur at 125°C yields compound 4 plus the compounds Os3(&$(CO)g (5) and OS~(,U-H)~~~-S)~(CO)~~@).I2 Compound5 and the selenium homologue of6 have been characterized crystallographically. Compound 5 contains triply bridging sulfido ligands on each side of an open triangular cluster of three metal atoms. I3 The Os,S, framework of6 has the shape of a trigonal prism with only three osmium-osmium bonds, two of which contain bridging hydride ligands.12 At temperatures greater than 2OO”C, the reaction yields the higher-nuclearity sulfido osmium carbonyl clusters, OS&J.+-S)(CO)~~Q, OS~(,U~-S)~(CO)~~(8) and O~i&b-S)2(CO)23 (9).14 These have been characterized structurally by X-ray diffraction methods. ORTEP diagrams of 7 and 8 are shown in Figs 2 and 3, respectively. Compound 7 consists of a square pyramid of five osmium atoms with a quadruply bridging sulfide ligand spanning the square base.” Compound 8 consists of an Os,S, cluster core in the shape of a pentagonal bipyramid and has two Os(CO), units bridging adjacent apical-equatorial edges of the cluster. l6 To date, compound 9 is the highest-nuclearity sulfido osmium carbonyl cluster that has been characterized. Its structure has been described as two square pyramids fused to two trigonal bipyramids that have one Os-Osedge in common.14 Quadruply bridging sulfido
Synthesis, structures, bonding and unusual reactivity of sulfido osmium carbonyl cluster compounds
2005
Fig. 1. An ORTEP drawing of the heavy-atom structure for the compound [HPtOs,(CO)&-S)&,S)(PPh&H& showing the inverted tetrahedral geometry of the sulfido ligands S(1) and S(2). Pt(l)S(l)-OS(~) = 68.3(2)“, Pt(l)-S(lFs(3) = 102.86(2)“, OS(~)-S(l)-OS(~) = 97.8(2)“, Pt(l)-S(l)-Pt(2) = 85.0(2),Pt(2fl(l)-Os(l) = 116.2(2)“,Pt(2~(1)-os(3) = 145.3(3)“,Pt(2)-S(2)-0$4) = 67.3(2)“,Pt(2p(2b OS(~) = 101.2(2)0,Os(4)-S(2m(6) = 97.6(2)“,Pt(2)-S(2bPt(l) = 83.8(2)“,Pt(l)-S(2)4Is(4) = 135.0(33”, Pt(l)-S(2)-Os(6) = 122.2(2)“.
ligands span the square bases of the two square pyramidal units.
Carbon disulfide (CS,) reacts with Os,(CO),, to form the compound Os,&3),(CO),(CS) analogous to 5 but contains a terminally coordinated thiocarbonyl ligand in place of one of the carbonyl ligands. ” CS2 reacts with the lightly-stabilized cluster 0s,gl-H)2~3-S)(CO),(NCMe) to form the compound OS&-H)~~~-S),(CO),(CS) (ll).‘* Compound 11 is an open cluster with two triply bridging sulfido ligands, a bridging hydride ligand across (10) which is structurally
Fig. 2. An ORTEP diagram of Os,~~-S)(CO)Is
(7) showing 50% probability thermal ellipsoids.
2006
R. D. ADAMS
Fig. 3. An ORTEP diagram of OS~~~-S)~(CO)~~ (8) showing 50% probability thermal ellipsoids.
each of the two metal-metal bonds and one linear terminal thiocarbonyl ligand. The reaction of OS,(CO),~ with CS2 at 165°C under CO pressure yields compounds 5,lO and the new compound OS,~L~-S)~&S)(CO)~~ (12)in 8% yield.lg The structure of 12 consists of four osmium atoms arranged in the form of a puckered square. Three of the osmium atoms are bridged by a triply bridging sulfido ligand. The thiocarbonyl ligand serves as a quadruple bridge with the carbon atom bonded to all four osmium atoms and the sulfur bonded to only one. The C-S bond length, 1.757( 18) A, is slightly shorter than a C-S single bond length (i.e. 1.81 A).
St&Fink et ~1.” have reported that the sulfurdimide, Me,SiN=S=NSiMe,, reacts with OSCAR at 125°C to give the compound Os,@,-S)&-NSiMe,)(CO), (13) in 13% yield. The crystal structure analysis of 13 reveals an open triangular structure similar to that of 5 but it contains a triply bridging amido ligand, Me,SiN, and one triply bridging sulfido ligand. The temperature-dependent r3C NMR spectrum of 13 in solution reveals the reversible formation of a second species at high temperature in which all the carbonyl ligands are either equivalent or are rapidly averaged. This was interpreted in terms of an isomeric species, 14, which contains three metal-metal bonds. Although the structure 14 could explain the spectral observations, it should be pointed out that this structure does not adhere to currently accepted cluster bonding theories. Thus, until further evidence is presented it should be regarded as tentative.
Synthesis, structures, bonding and unusual reactivity of sulfide osmium carbonyl cluster compounds
2007
SiMes
SiMe,
I
I
13
14
Of all the routes to sulfidoosmium carbonyl clusters, perhaps the most productive, are those which involve the desullkixation of organosulfur ligands. For example, the arylthiolatotriosmiumdecarbonylhydride clusters readily eliminate arene when heated to 150°C to yield a variety of sulfido osmium carbonyl clusters [i.e. eqn (l)] :‘I Os&-I-W-SPh)(CO),, 15 + Os&-SMCO), 17
150°C _C Os,@3-SMCO), + Os‘&-S)(CO),* 6 6 5 16 z + Os&-S)(CO),
5 + Os&,-S)(WS)(CO),,
7
18
+ Oskk%(CO)Z@ -
(1)
8
The yields of the various products are very sensitive to the reaction conditions. For example, when pyrolyzed in the absence of solvent at temperatures greater than 2OO”C,only compounds 5,16,7 and 8 are obtained.“j When pyrolyzed at 160°C under 100 bar CO pressure, only compound 5 plus three equivalents of Os(CO), is obtained. I5 Compounds 16-18have been characterized by X-ray diffraction methods. Compound 16 consists of a tetrahedral cluster of four osmium atoms with a triply bridging sulfido ligand on one of the OS, faces. 22 Compound 17 consists of a “butterfly” tetrahedral cluster of
four osmium atoms with triply bridging sulfido ligands across the two open triangular faces.21 The structures and bonding of 16 and 17 will be described further in Section III. An ORTEP diagram of
compound 18 is shown in Fig. 4.21The molecule consists of a square pyramid of five osmium atoms with a quadruply bridging sulfido ligand spanning the square base and an Os(CO), group and a triply
Fig. 4.An ORTEP diagram of 0s,~4-S)~3-S)(CO),,
(18) showing SWAprobability thermal ellipsoids.
R. D. ADAMS
2008
bridging sulfido ligand bridging an edge of the square base. The independent syntheses of compounds 7, 8 and M-18, as well as related compounds suggest mechanisms for their formation. These are described in Section 11.2(a). When irradiated under a rapid purge of CO, compound 15 loses benzene and yields the compound O~&-S)(C(~-CO)(CO)~ (19) Ceqn P)l:23 hv/CO Os,(~-H)(~-SPh)(CO),,
_C
6 6
~~,cU3-~~~~3-~~~~~~~~.
(2)
19 Compound 19 consists of a triangular cluster of three osmium atoms with a triply bridging sulfido ligand one side of the 0s3 plane and a triply bridging carbonyl ligand on the other side.
19
Thermal degradation of the benzylthiolato cluster, OS,@-H)&-SBz)(CO),, (20) occurs apparently by two competing mechanisms. 24One which is analogous to that found for 15 results in the elimination of toluene and the formation of compounds 516 and 8. The other mechanism results in formation of dibenzyl and the two isomeric hexaosmium dihydro clusters OS,&H)~~L~“S)&-S)(CO)~~ (21 and 22). ORTEP diagrams of 21 and 22 are shown in Figs 5 and 6. The red isomer 21 contains a quadruply bridging stido ligand of type 3 while the green isomer 22 contains a quadruply bridging sulfido ligand of type 2 and an extended planar array of six osmium atoms. The formation of dibenzyl and dihydrido hexaosmium species suggests a radical mechanism involving an initial homolysis of the sulfur-carbon bond in 20.
Fig. 5. An ORTEP diagram of the red isomer of OS,~C-H)~&&JJ~-S)(CO)~, (21) showing the quadruply bridging stido
ligand S( 1) of type 3.
Synthesis, structures, bonding and unusual reactivity of sulfido osmium carbonyl cluster compounds
Fig. 6. An ORTEP diagram of the green isomer of OS,@H)~(@)@~-S)(CO)~, quadruply bridging sulfido ligand S(1) of type 2.
Fig. I. An ORTEP
diagram
2009
(22) showing the
of OS,~-H),(JL&(JJ~-S)(CO)~~ (23) which is a precursor Os&-H)~o1~-S)tis-S)(CO)~~ (21).
to
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R. D. ADAMS
The principal product formed when compound 20 is irradiated at 25°C is the compound Os,(pH),~~-S)&-S)(CO),, (23). An ORTEP diagram of 23 is shown in Fig. 7. It is similar to 21, and was shown to be a precursor to 21 by its thermal decarbonylation. Thioformamido and dithioformato ligands can serve as precursors for sulfido ligands [eqns (3)25 and (4)26] : R
I
.“Q \, \.szd\
“,N//
I,
\I\9s/ob
rTC
&/o;;I
-co-
‘I\
‘\
24
13)
I\ 26
26
26
A
27
The desulfurization of the thioformamido complex in 24 was found to proceed via a decarbonylated intermediate 26 which was isolated and shown to contain a triply bridging thioformamido ligand. Structural analyses of 24 and 26 showed that the C-S bond length was significantly longer than the C-S bond length of thioformamido ligands in complexes where the sulfur atom was not in a bridging position. Thus, it was inferred that the bridging environment induced a weakening of the C-S bond and this facilitated the bond cleavage. In ligand-splitting reactions such as these both fragments of the ligand usually remain coordinated to the cluster. The desulfurization of the dithioformato ligand in 27 probably proceeds via an intermediate species such as A containing a thioformyl ligand, but this was not observed. The species 28 which contains a bridging thioformaldehyde ligand was isolated. This could be formed by a reductive elimination of C-H from the proposed intermediate. 2. Cluster condensation reactions (a) Homonuclear clusters. The triply bridging sulfido ligand with its lone pair of electrons would seem to be an ideal species for use in cluster size-expansion reactions. By using the lone pair ofelectrons to form coordinative interactions to electron-deficient metal groupings, the sulfido ligand could facilitate the entry of metal-containing units into the cluster [eqn (5)] : M,@,-S)+
M’L, -L_
/1”\
M \A’
B
S -
M’L,_,-
M,M’&-S)L,_,
(5)
C
The paucity of isolated intermediates such as B suggests that these species are less stable than their rearrangement products C.
Synthesis,structures,bonding and unusual reactivityof sulfido osmiumcarbonyl clustercompounds 2011 The low-nuclearity sulfido clusters 5 and 19 have served as starting points for these aggregation or condensation reactions. Osmium pentacarbonyl which is readily decarbonylated to the reactive intermediate Os(CO), when irradiated has been used as a source of monoosmium fragments. The lightly stabilized complex Os,(CO),,(NCMe), has been found to serve as a reagent for the addition of triosmium groupings. By using this methodology most of the cluster complexes synthesized in low yields in the pyrolysis reactions described in Section II. 1 have now been made in much higher yields. In addition, a number of new complexes that were not observed in the pyrolysis reactions have been isolated. Some of these can be converted into the pyrolysis products and thus suggest mechanisms for their formation. The irradiation of solutions of 19 and Os(CO), has resulted in the formation of the highernuclearity clusters, 29 and 7 in 40 and 3%yields, respectively (see Scheme 1).The structure of 29 consists of a nearly planar butterfly cluster of four osmium atoms with a triply bridging sulfido ligand on one triangular face.15 When heated to 125”C, 29 loses 1 mol of CO and condenses to 16 quantitatively. When solutions of 5 and Os(CO), are irradiated the compound OS&,-S),(CO),, (30) is formed in 40% yield16 (see Scheme 1).The structure 3Oconsists of a planar group of four osmium atoms connected by three OS-OS bonds.” Triply bridgi n g sulfido ligands are symmetrically disposed about the 0~ plane. When heated to 68°C 30 loses 1 mol of CO quantitatively and the cluster closes to form 17. The reaction of 5 with 0s3(CO)I,,(NCMe)2 yields the new hexaosmium cluster 0s6(p4-s)2(c0)17 (31) in 29% yield.” The structure of 31 is shown in Fig 8. The molecule is very similar to 8 and contains an Os,S, cluster in the form of a pentagonal bipyramid but has only one Os(CO), group bridging an apical-equatorial edge. ” This compound was also prepared in good yield by the UV irradiation of 19 in an inert solvent. The condensation of 2 mol of 19 via the unobserved intermediate D is shown in Scheme 1. An intermediate such as D is probably also traversed in the formation of 5 as observed in the pyrolysis of 15 under CO pressure.r5 When heated to 125°C compound 31 quantitatively loses 1 mol of CO to yield 18. The CO ligand that is lost probably comes from th CO-rich bridging Os(CO), grouping. A rearrangement that would readily produce condensation of 31 to 18 is shown in Scheme 2.2s
16
+
32
16
/
+Os,KO),,(NCMe),
hu
.
OdCO 1s
8
Scheme 1.
2012
R. D. ADAMS
Fig. 8. An ORTEP diagram of OS&.@)~(CO)~, (31) which is a precursor to OS,(JJ~-S)@~-S)(CO),~ (18)’
The reaction of 16 with 0s,(CO)10(NCMe)2 in refluxing octane yields the highly condensed heptanuclear cluster OS,(,Q-S)(CO)~~ (32) in 23% yield. 22 No intermediates have been isolated. The structure of 32 is shown in Fig. 9. The structure is best described as a fused combination of a square pyramid of five osmium atoms and a trigonal bipyramid of five osmium atoms with one triangular face in common. The base of the square pyramid is spanned by a quadruply bridging cwo-ww~~411 sulfido ligand of type 2. The addition of Os,(CO),,(NCMe), to 17 leads to the formation of 8 in 16% yield.r6 Curiously, it was found that compound 8 could not be prepared by the irradiation of solutions of 31and Os(CO),. It has been found that 0s3(CO)10(NCMe)2 will add to 19 to form the sequence of products OS&,-S)(CO),, (33) and OS&-S)(CO),, (34). Compound 33 is converted into 34 in 23% yield at
-co-w_
Bond breaking
.-.--
Bond making
Scheme 2.
Synthesis, structures, bonding and unusual reactivity of suffido osmium carbonyl cluster compounds
2013
Fig. 9. An ORTEP diagram ofthe fused cluster OS,@,-S)(CO),, (32)showing 50% probability thermal ellipsoids.
1 10”C.2g The molecular structure of 33 is shown in Fig. 10. The molecule consists of a butterfly tetrahedral cluster of four osmium atoms with a triply bridging sulfido ligand across one of the open triangular faces. Os(CO), groupings bridge the two edges of that same open triangular face. The structure of 34 is shown in Fig. 11. This molecule consists of a square pyramid of five osmium atoms with a quadruply bridging sulfido ligand over the square base and a capping Os(CO), group on one of the triangular faces. The transformation of 33 into 34 involves the loss of 2 mol of CO. A proposed rearrangement which involves the loss of one CO from each of the bridging Os(CO), units is shown in Scheme 3. When irradiated under a rapid purge with nitrogen, compound 5 undergoes a self-condensation and forms the green dimer [OS&-S),(CO),], (35). C om p ound 35 has a most unusual structure which is shown in Fig. 12. The central portion of the molecule consists of a butterfly tetrahedron of four osmium atoms [OS(~)-OS(~)-OS(~)-OS(~)] with a dihedral angle of 136”. Exterior Os(CO), groups are attached to the hinge atoms OS(~) and OS(~). Pairs of triply bridging sulfido ligands bridge opposite edges of the butterfly tetrahedron and extended to the exterior Os(CO), groups. The formation of 35
Fig. 10. An ORTEP diagram of OS,~,-S)(CO),~
(33) showing WA probability thermal ellipsoids.
2014
R. D. ADAMS
Fig. 11. An ORTEP diagram of the condensed cluster OS&-S)(CO)17
33
+
Bond breaking
-.-a..
Bond making
(34).
34
Scheme 3.
Fig. 12. An ORTEP diagram of the dimer [OS,~~-S),(CO),]~ (35). OS(~)-OS(~) = 2.834(l) A, OS(~)OS(~) = 2.973(l) A, Os(l)-Os(4) = 2.941(l) A, Os(2)-Os(3) = 2.957(l) A, Os(2)-Os(4) = 2.971(l) A, Os( @-OS(~) = 2.789( 1) A, Os(2bOs(6)
= 2.792( 1) A.
Synthesis, structures, bonding and unusual reactivity of sulfido osmium carbonyl cluster compounds 2015
Scheme 4.
canbeviewedasaback-to-backcouplingoftwoO~,(~~-S),(CO),unitsderivedfrorn5by thelossofone CO ligand from the central osmium atom (see Scheme 4). The self-condensation of compound 25 provides an interesting sequence of products that reveals in great detail the importance of the bridging sulfido ligand to the cluster growth process. When heated to 125°C compound 25 is decarbonylated and yields a mixture of several hexaosmium compounds.30 Of particular importance are the compounds of formula OS&-H)~(~~-S)(,U~-S)(~-HC=NR)~(CO)~ 7 (36) and OS&-H),(&-S)&-S)@-HC=NR),(CO),, (37) (R=CsH5 and p-C,H,F). The former exists in two separable isomeric forms. The latter exists in three separable isomeric forms. Compounds 36 can be converted into 37 by further heating at 125°C. An ORTEP diagram of one of the isomers of 36 is shown in Fig. 13. The molecule consists of two open triosmium clusters linked by a quadruply bridging sulfido ligand of type 3. The Os(l)-Os(2)-Os(3) grouping contains 10 CO ligands which is one more than it contained in the form 25. The OS(~)-Os(S)--Os(6) grouping contains only seven CO ligands. It is not known if the CO ligand acquired by the Os( l)-Os(2)-Os(3) group was transferred intramolecularly or intermolecularly from the OS(~)-Os(5)-Os(6) group. The most important feature of the molecule is the quadruply bridging sulfido ligand S(1) which has brought the two triosmium groups into an
S(2)
n
Fig. 13 . An ORTEP diagram of one of the isomers of 0s&-H)z014-S)&-S)@-HC=NC~H_+F)Z(CO)I (36).The metal atoms are blackened completely.
2016
Fig. 14. An ORTEP diagram of one of the isomers of OS&-H),@-HC=NC,H,),(CO),, metal atoms are blackened completely.
(37). The
inseparable relationship. No metal-metal bonds were made and none were broken in the conversion of 25 into 36. An ORTEP diagram of 37 is shown in Fig. 14. This molecule consists of groups of four and two osmium atoms. A quadruply bridging sulfido ligand S(2) joins these two groups. The formation of 37 from 36 has occurred by the transfer of one osmium atom from one group of three metal atoms to the other group of three. In this transformation there is a loss of one CO ligand and a net increase by one in the number of metal-metal bonds. The transformation of36 into 37 must have involved several steps. A mechanism that shows the essential atomic and ligand shifts is shown in Scheme 5. When heated to 15O”C,compound 37 loses another CO ligand plus an equivalent of H, and forms the compound OS&,-S),(p-HC=NR),(CO), 5(38). An ORTEPdiagram of the molecular structure of 38 is shown in Fig. 15. This molecule consists of a butterfly tetrahedron of four osmium atoms with external Os(CO), groups attached to each of the“hinge” atoms ofthe butterfly tetrahedron. Quadruply bridging sulfido ligands bridge the open triangular faces of the butterfly tetrahedron and extend to the external Os(CO), groups. These sulfido ligands are highly strained and conform to the definition of “inverted” tetrahedra (see Section I). There is a bridging carbonyl ligand across the Os(2)-Os(3) hinge bond. A transformation that shows the rearrangements implicit in the conversion of 37 into 38 is given in Scheme 6. The most important feature is that the condensation process has progressed further and at this stage a new cluster grouping (the butterfly tetrahedron) that is quite distinct from the two original triosmium units has assembled and has become a focal point of the molecule. (b) Mixed-metal clusters. Some efforts have been made to investigate the potential for the synthesis ofmixed-metal cluster compounds by adding groupings with transition elements other than osmium to
“-co
36
Scheme 5.
Synthesis, structures, bonding and unusual reactivity of sulfido osmium carbonyl cluster compounds
2017
n
Fig. 15. An ORTEP diagram of OS,&-S)&-HC=NC,H,),(CO),, blackened completely.
(38). The metal atoms are
the sulfido osmium clusters described above. These preliminary studies have been quite successful. The groupings that have received the most attention are platinum phosphine species and tungsten carbonyl complexes. Platinum phosphine complexes are labile and usually add to the sulfido osmium clusters smoothly at room temperature. The tungsten carbonyl complexes are less reactive and usually require application of UV irradiation to initiate the reactions. The reactions ofmononuclear metal species with the clusters results usually in the addition of only one metal unit. The reactions of 19 with Pt(PMe,Ph), yields four tetranuclear products. These are PtOs&-S)(CO)JPMe,Ph)s (39), PtOs&-S)(CO)s(PMezPh), (4% PtOs&+S)(CO)s(PMe,Ph), (41) and PtOs&-S)(CO),(PMe,Ph), (42).31 The last two can be prepared from the first two, in order, by thermal decarbonylation. An ORTEP diagram of 39 is shown in Fig. 16. The molecule consists of a planar butterfly tetrahedron (dihedral angle = 177”) of three osmium and one platinum atoms. The platinum atom contains only two phosphine ligands and the sulfido ligand bridges a PtOs, triangular group instead of an OS, group as it did in 19. Compound 40 is structurally analogous but contains an Os(CO),(PMe,Ph) group instead of the Os(CO), group in 39.31 Of the two decarbonylation products, only compound 42 has been characterized structurally.31 An ORTEP diagram of 42 is shown in Fig. 17. This molecule consists of a closed tetrahedral cluster of three osmium and one platinum atoms, but in this case the sulfido ligand bridges an 0s3 triangle. Compound 41 is believed to be structurally analogous to 42. A mechanism that accounts for the formation of this sequence of products is shown in Scheme 7. It is believed that an interaction of the sulfido ligand
Scheme 6.
2018
R. D. ADAMS
Fig. 16.An ORTEP diagram of PtOs&,-S)(CO),,(PMe,Ph), ellipsoids.
(39)showing 50% probability thermal
with the platinum moiety is an important first step.in the reaction. It should be noted that the facile synthesis of the mixed-metal clusters 39-42 contrasts sharply with similar reactions in which no sulfido ligands are present. For example, the reaction of Os,(CO),, with Pt(PR,), yields only phosphine-substituted derivatives of OS~(CO)~~.~~ The irradiation of solutions of 19 and W(CO),(PMe,Ph) in hexane solvent leads to formation of the
p&QJJQP(3,
-
Fig. 17. An ORTEP diagram of PtOs,&-S)(CO),JPMe,Ph), (42) showing 50% probability thermal ellipsoids. OS(l)-Pt = 2.853(l) A, Os(2)-Pt = 2.740(l) A, Os(3)-Pt = 2.766(l) A, Os(l)-Os(2) = 2.816(l) A, OS(~)-OS(~)= 2.770(l) A, OS(~)-OS(~)= 3.064(l) A.
Synthesis, structures, bonding and unusual reactivity of sulfido osmium carbonyl cluster compounds P
‘Pt’ -t ..
2019
P
Scheme 7.
cluster Os,W(p,-S)(CO),,(PMe,Ph), (43) in 27% yield. The molecule consists of a closed tetrahedral cluster with the sulfido ligand bridging an Os,W triangular face.33 The mechanism of formation of 43 has not been established yet. mixed tungsten-osmium
Compound 5 reacts with Pt(PPh,)&H, to give two tetranuclear products PtOs,~,S),(CO),(PPh,)L (44) (L = CO) and 45 (L = PPh,) and a PPh, substitution product of 5. Compound 44 has acquired one CO ligand, and its yield is significantly increased when the reaction is performed under a CO atmosphere. The structure of 44 is shown in Fig. 18. The molecule contains four metalmetal bonds. Three of these exist in a triangular PtOs, group. The fourth links an Os(CO), group to
n
Fig. 18. An ORTEP diagram of PtOs&,-S),(CO),,(PPh,) (44) showing 50”/, probability thermal ellipsoids. Os(l)-Os(2) = 2.826(l) A, Os(2)-Os(3) = 2.990(l) A, Pt-Os(2) = 2.858(l) A, Pt-Os(3) = 2.905(l)A, os(l)...os(3) = 3.585(l) A, os(l)...pt = 3.401(1) A.
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R. D. ADAMS
the triangular group at one of the osmium atoms. The distance of this external osmium atom to the platinum atom is 3401(l) A, which seems to be too long for a significant bonding interaction. The PtOs, group contains triply bridging sulfido ligands that bridge a Pt-0s and the OS-OS edges and extend to the third osmium atom. The compound appears to have been formed by the insertion of a Ptcontaining group into two adjacent OS-S bonds of 5. Compound 45 has a similar structure. Unlike compound 17 (see below) compounds 44 and 45 are electron-precise [i.e. all the metal atoms obey the effective atomic number (EAN) rule]. When irradiated in the presence of W(CO),PMe,Ph, compound 5 yields four new products: 0s3W(&)2(C0)12(PMe,Ph) (46), Os,W(/@),(CO),,(PMe,Ph), (47), Os,W(,u,S)2(CO),,(PMezPh)z (48) and 0s3W2&-S)&-S)(CO)r,(PMe,Ph), (49).34 The structures and relationships between these molecules are shown schematically in Scheme 8. Compound 46 has a butterfly tetrahedral cluster of four metal atoms with triply bridging sulfido ligands on the two open triangular faces. It is structurally analogous to the homonuclear compound 17. Compound 47 consists of a planar cluster offour metal atoms with triply bridging sulfido ligands on opposite sides of the Os,W plane. Compound 47 can be made from 46 by adding one equivalent of PMe,Ph. Compound 48 is isoelectronic with 46 but it adopts a structure similar to 44 and 45, consisting of triangular group of three metal atoms with an external Os(CO), group linked to the triangle by one metal-metal bond and the bridging sulfido ligands. Compound 48 can be made independently by the irradiation-induced decarbonylation of47. The pentanuclear Os,W, compound 49 contains an Os,W cluster analogous to 48 but has a W(CO),(PMe,Ph) group bridging a W-S edge of the cluster. Compound 49 can be made by the irradiation-induced addition of a W(C0)4(PMe,Ph) moiety to 46. In all of the tungsten-osmium complexes there are bonds between the tungsten atoms and the sulfido ligands. As discussed earlier [cf. eqn (5)], it is believed that preliminary metal-sulfur interactions may be important to the condensation process. The photo-induced reaction of W(CO)6 with 5 yields the product OS,~~-S)@~-S)(CO),CW(CO)J (50), which shows such an interaction in its most fundamental form.35 The structure of 50 is shown in Fig. 19. Most simply, the molecule contains a W(CO)s group attached to one of the triply bridging sulfido ligands of a molecule of 5. The connection involves a simple donor-acceptor bond formed by donation of the lone pair ofelectrons on the sulfido ligand into an empty orbital on the tungsten atom. + PMe, Ph
W(CO),(PMe,Phl
hu
49
48
Scheme 8. III. BONDING
AND UNUSUAL
REACTIVITY
The nature of the metal-metal bonding in polynuclear metal complexes is still not well understood. Localized 2c-2e bonding and application of the EAN rule works well for small clusters (i.e. four metal
Synthesis, structures, bonding and unusual reactivity of sulfido osmium carbonyl cluster compounds 2021
Fig. 19.An ORTEP diagram of 0s,(cr4-S)g13-S)(CO),[W(CO)5](5O)showing50”/,probability thermal ellipsoids.
atoms or less) while the PSEP theory, 36 which employs a delocalized bonding, works better for the larger clusters (i.e. five atoms or more). Recently, Teo has described the topological electron counting theory which incorporates the essential features of both the EAN and the PSEP theories.3’ It should be pointed out, however, that in many cases the EAN rule and the PSEP theory predict the same cormectivities for the metal atoms in both the large and small clusters. 1. M,S, clusters Recent studies have revealed a small number of tetranuclear clusters that conform to the PSEP theory but show unusually high reactivities. 27*3sIn reactions they convert to products that obey the EAN rule. One of these clusters is the sulfido osmium cluster OS~(JJ~-S)~(CO),~(17J2’ An ORTEP diagram of 17 is shown in Fig. 20. The molecule consists of a butterfly tetrahedron of four metal atoms and has sulfido ligands bridging the two open triangular faces. There are five osmium+xmium distances that are short enough (less than 3.10 A) to imply significant bonding interactions. Each metal
Fig. 20. An ORTEP diagram of OS&,-S)~(CO)~~ (17) showing 500/,probability thermal ellipsoids. Os(l)-Os(2) = 2.914(l) A, Os(l)-Os(4) = 3.091(l) A, Os(Z)-Os(4) = 2.935(l) A, Os(Z)-Os(3)= 3.002(l) A, Os(3)-Os(4) = 2.940(l) A, OS(~).. , OS(~)= 3551(l) A.
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R. D. ADAMS
atom contains three carbonyl ligands and, if the sulfido ligands serve as four-electron donors, then the cluster contains 64 valence electrons. This is two in excess of the requirements of the EAN rule. For this reason we have chosen to describe the molecules as electron-rich. If the molecule simply eliminated one metal-metal bond, then all the metal atoms could be made to adhere to the EAN rule. Curiously enough, the isoelectronic compounds 44,45 and 47 do have only four metal-metal bonds, but 46 has five. The reason for this is not clear, but steric effects could be important. Intuitively, it seems that compound 47 is more sterically crowded than 46 and this could mitigate against the formation of the fifth bond. The structures of 17 and 46 are, however, in accord with the PSEP theory. According to the PSEP theory Os(CO), groups contribute two electrons to cluster bonding and triply bridging sulfido ligands contribute four. There are thus 16 or 8 pairs of cluster valence electrons present in 17. The PSEP theory predicts a cluster shape based on a polyhedron with seven vertices. This is the pentagonal bipyramid. The structure of 17can be regarded as a pentagonal bipyramid with one vertex missing (i.e. a nido-pentagonal bipyramid). Thus, the observed structure with five metal-metal bonds is the expected one. However, upon close examination of the metal-metal bonding, it is observed that two ofthe metalmetal bonds, indicated by the dashed connections in Fig. 20, are significantly longer (greater than 3.OOA) than the other metal-metal bonds in 17 and also the metal-metal distances in Os,(CO),, [2.877(3) A]. The latter might best be regarded as OS-OS single bonds. It has not been established if these bond lengthenings are a result of incomplete delocalization or are caused by distortions in the ligand structure. Studies were undertaken to try to establish if compounds 17 and 46, which show similar distortions, might exhibit any unusual reactivity. These studies showed the compounds to be remarkably reactive and most surprisingly, toward the addition of Lewis donors, L. For example, compound 17 adds 1 mol of CO at room temperature under 1 bar pressure to form compound 30.” The addition of phosphines and isocyanides proceeds similarly to yield the derivatives OS&-S)~(CO)~~L (51) (L = PMe,Ph) and (52) (L = CNBU’).~’ The structures of 51 and 52 (see Scheme 9) show that a shift of a carbonyl ligand between the metal atoms must have occurred since each molecule contains an Os(CO), group. Also since phosphine ligands do not engage in intramolecular shifts, it implies that the site of addition was one of the “wing-tip” metal atoms of the cluster of 17. Compounds 30,51 and 52 each contain only three metal-metal bonds. They are thus “electron-precise” and all the metal atoms obey the EAN rule. The formation of these compounds can be achieved most easily by cleaving the two elongated bonds in 17, transferring one CO ligand between a pair of metal atoms, and shifting the attachment of one of the bonds of one of the sulfido ligands to a different metal atom. The reason for the nucleophilic addition at the wing-tip metal atom in 17 could be steric. The wing-tip metal atoms are less crowded being only 6coordinate while the hinge atoms are 7-coordinate. Compounds 51 and 52 lose CO when heated to form substitution derivatives of 17. Compound 46 reacts similarly to 17, but the electron-precise analogs, especially 44 and 45, are considerably less reactive and undergo cluster degradation when subjected to forcing conditions. Surprisingly, none of these compounds will add alkenes or alkynes. Another interesting and important reaction is the addition of hydrogen to 17.40 The reaction requires a temperature of 100°C and cluster degradation due to further reaction occurs but the initial product is compound 6. The mechanism of the reaction is not known, but it involves the addition of 1 mol of H, and the cleavage of the two elongated metal-metal bonds in 17. The end product of the reaction is H20s3(fi3-S)(C0)g.
Scheme 9. 2. M,S clusters The archetypical molecule of this series is compound 16 which is a 60-electron cluster that consists of a closed tetrahedral cluster of metal atoms with a triply bridging sulfido ligand on one triangular face.
Synthesis, structures, bonding and unusual reactivity of sulfido osmium carhonyl cluster compounds
2023
The molecule is electron-precise and all the metal atoms obey the EAN rule, but careful consideration reveals a feature about the bonding that could be important to its reactivity. Specifically, one of the metal-metal bonds to the unique osmium atom can be viewed as a heteropolar (donor-acceptor) bond.41 In addition, since the sulfido ligand is a four-electron donor, one of the OS-S bonds can be
16
16A
regarded similarly. Of course, in its resonance stabilized state all the metal-metal bonds to the unique osmium atom and all the metal-sulfur bonds will be chemically equivalent. However, the notion of the heteropolar bond suggests something about its potential reactivity that may distinguish it from the homopolar bond. Namely, one of these bonds can be opened 16A without producing a charge separation and a ligand can be added to the vacant site on the unique metal atom. In compound 16 reactions of this type are remarkably facile. 42 For example, 16 adds 1 mol of CO at room temperature under 1 bar pressure to form compound 29. The reaction is quantitative and complete in approximately 3 h. A similar reaction occurs with phosphines and even with amines, and with amines the reaction is complete in a matter of seconds. These additions are also reversible. Compound 16 activates hydrogen, and this reaction proceeds albeit slowly even at room temperature/l bar pressure. The product is OS&H)~(&)(CO)~~ (53) and the ORTEP diagram of structure of 53 is shown in Fig. 21. The molecule consists of a butterfly tetrahedral cluster with the sulfido ligand bridging one of the open triangular faces. The hydride ligands were not located in the structural analysis, but the two metal-metal bonds opposite the sulfido ligand are elongated and this suggests that each may contain one bridging hydride ligand. The formation of 53 has involved the cleavage of one of the homopolar sulfur-bridged metal-metal bonds in 16 and not the “heteropolar” bond. This may be related to the fact that the addition of hydrogen results in a homolytic cleavage of the Hz molecule. The reaction of 16 with alkynes is especially interesting, but to date only reactions with terminal alkynes have been investigated.43 The reaction of 16 with PhECH yields two noninterconvertible products. These have been identified as 0s&3-S)[~4-C=C(H)Ph](CO)lz (54) and OS&~SC(Ph)CH](CO),, (55). The structure of 54 is shown in Fig. 22. The molecule consists of a puckered rhombus of four osmium atoms connected by four metal-metal bonds. A sulfido ligand bridges three of the metal atoms while a phenylvinylidene ligand bridges all four metal atoms on the opposite side of the
Fig. 21.An ORTEP diagram of 0s4~-H)2&-S)(CO)12 (53).The molecule contains a crystallographic plane of symmetry passing through the atoms S, OS(~)and OS(~).Os(l)-Os(2) = 2.8149(6)A, OS(~)OS(~) = 2.9699(9)A, Os(2)-Os(3) = 2.8578(10) I$ OS(~). . . Os(1’) = 3.9295(9)A.
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Fig. 22.An ORTEP diagram of 0s,@3-S)[j14-C=C(H)Ph](CO)Iz (54)showing 50%thermal ellipsoids. OS(~)-OS(~)= 2.850(l) A, Os(l)-Os(4) = 2.859(l) A, Os(2)-Os(3) = 2.787(l) A, OS(~)-OS(~)= 2.866(l) A, C(13)-C(14) = 1.446(16)A. rhombus. The formation of the vinylidene has obviously occurred by a 1,Zshift of the acetylenic hydrogen atom. These rearrangements have been observed previously in a number of cases in reactions of metal complexes with terminal alkynes. 44 The structure of 55 is shown in Fig. 23. The molecule consists of a butterfly tetrahedral cluster of four metal atoms with a quadruply bridging S-CPh=CH thiolato ligand. The sulfur atom of this unusual ligand bridges two osmium atoms while the alkenyl group is a,a-bonded to the other two. The ligand was formed by the formation of a carbon-sulfur bond between the phenyl-substituted carbon of the alkyne and the sulfido ligand and a cleavage of one osmium-sulfur and one osmium-osmium bond in the cluster. Curiously, the hydrogen shift that occurred in the formation of 54 did not occur in the formation of 55. Although the mechanism of this hydrogen shift is not yet known, it is tempting to speculate that the absence of a hydrogen shift in the formation of 55 may somehow be related to the formation of the bonding interaction between the
Fig. 23. An ORTEP diagram of 0sq[~4-SC(Ph)=CH](CO)1z
(55) showing 50% probability thermal ellipsoids. Os(l)-Os(2) = 2.7840(7) A, Os(l)-Os(3) = 2.8490(7) A, Os(l)-Os(4) = 2.8442(7) A, OS(~)OS(~) = 2.8132(7) A, Os(3)-Os(4) = 2.764(l) A, S-C(l4) = 1.817(11) A, C(13)-C(14) = 1.400(15) A.
Synthesis, structures, bonding and unusual reactivity of sulfido osmium carbonyl cluster compounds
2025
alkyne and the sulfido ligand. Such effects might also occur in the sulfur-inhibited reactions of certain heterogeneous transition metal catalysts. In fact a variety of sulfur-ligand interactions similar to those found in 55 could be responsible for and explain a range of the sulfur-poisoning effects that exist for heterogeneous catalysts.45V46 Acknowledgements-Our
research on this subject has been supported by the Office of Basic Energy Sciences ofthe U.S. Department of Energy, and the National Science Foundation.
REFERENCES 1. E. L. Muetterties, Science 1977,1%, 839. 2. E. L. Muetteries, Bull. Sot. Chim. Belg. 1975,&l, 959. 3. R. Ugo, Catal. Reo. 1975,11,225. 4. (a) E. L. Muetterties, Catal. Rev.-Sci. Eng. 1981, 23, 69; (b) R. D. Adams and I. T. Horvath, Prog. Znorg. Chem. 1985,33,127. 5. H. Vahrenkamp, Adv. Organomet. Chem. 1983,22,169. 6. H. Vahrenkamp, Angew. Chem., Znt. Ed. Engl. 1975,14,322. 7. R. D. Adams and T. S. A. Hor, Organometallics 1984,3,1915. 8. K. Wade, In Transition Metal Clusters (Edited by B. F. G, Johnson), John Wiley, New York (1980). 9. R. D. Adams and D. F. Foust, Organometallics 1983,2, 323. 10. K. B. Wiberg, Act. Chem. Res. 1984,17,379. 11. A. J. Deeming and M. Underhill, J. Organomet. Chem. 1972,42, C60. 12. B. F. G. Johnson, J. Lewis, P. G. Lodge and P. R. Raithby, J. Chem. Sot., Chem. Commun. 1979,719. 13. R. D. Adams, I. T. Horvath, B. E. Segmuller and L. W. Yang, Organometallics 1983,2,144. 14. J. P. Attard, B. F. G. Johnson, J. Lewis, J. M. Mace, M. McPartlin and A. Sironi, J. Chem. Sot., Chem. Commun. 1984,595. 15. R. D. Adams, I. T. Horvath, B. E. Segmuller and L. W. Yang, Organometallics 1983,2,1301. 16. R. D. Adams, I. T. Horvath, P. Mathur, B. E. Segmuller and L. W. Yang, Organometallics 1983,2,1078. 17. P. V. Broadhurst, B. F. G. Johnson, J. Lewis and P. R. Raithby, J. Organomet. Chem. 1980,194, C35. 18. P. V. Broadhurst, B. F. G. Johnson, J. Lewis, A. G. Orpen, P. R. Raithby and J. R. Thomback, J. Organomet. Chem. 1980,187,141. 19. P. V. Broadhurst, B. F. G. Johnson, J. Lewis and P. R. Raithby, J. Chem. Sot., Dalton Trans. 1982,164l. 20. G. S&s-Fink, U. Thewalt and H. P. Klein, J. Organomet. Chem. 1982,224,59. 21. R. D. Adams and L. W. Yang, J. Am. Chem. Sot. 1982,104,4115. 22. R. D. Adams, D. F. Foust and P. Mathur, Organometallics 1983,2,990. 23. R. D. Adams, I. T. Horvath and H. S. Kim, Organometallics 1984,3,548. 24. R. D. Adams, I. T. Horvath, P. Mathur and B. E. Segmuller, Organometallics 1983,2,996. 25. R. D. Adams, Z. Dawoodi, D. F. Foust and B. E. Segmuller, Organometallics 1983,2,315. 26. R. D. Adams, N. M. Golembeski and J. P. Selegue, J. Am. Chem. Sot. 1981,103,546. 27. R. D. Adams and L. W. Yang, J. Am. Chem. Sot. 1983,105,235. 28. R. D. Adams, I. T. Horvath and L. W. Yang, J. Am. Chem. Sot. 1983,105,1533. 29. R. D. Adams, I. T. Horvath and P. Mathur, Organometallics 1984,3,623. 30. R. D. Adams, Z. Dawoodi, D. F. Foust and B. E. Segmuller, J. Am. Chem. Sot. 1983,105,831. 31. R. D. Adams and T. S. A. Hor, Znorg. Chem. 1984,X$4723. 32. M. I. Bruce, G. Shaw and F. G. A. Stone, Chem. Commun. 1971,1288. 33. R. D. Adams, T. S. A. Hor and P. Mathur, Organometallics 1984,3,634. 34. R. D. Adams, I. T. Horvath and P. Mathur, J. Am. Chem. Sot. 1984,106,6296. 35. R. D. Adams, I. T. Horvath and S. Wang, Znorg. Chem. 1985,22,1728. 36. K. Wade, In Transition Metal Clusters (Edited by B. F. G. Johnson), Chap. 3. John Wiley, Chichester (1980). 37. B. K. Teo, Znorg. Chem. 1984,23,1251,1257. 38. H. Vahrenkamp and D. Wolters, Organometallics 1982,1,874. 39. R. D. Adams, I. T. Horvath and K. Natarajan, Organometallics 1984,3,1540. 40. R. D. Adams, Z. Dawoodi, D. F. Foust and L. W. Yang, Ann. N. I! Acad. Sci. 1984,415,47. 41. G. R. John, B. F. G. Johnson and J. Lewis, J. Organomet. Chem. 1979,181,143. 42. R. D. Adams and S. Wang, report in preparation. 43. R. D. Adams and S. Wang, Organometallics (in press). 44. M. I. Bruce, Adv. Organomet. Chem. 1983,22,60. 45. C. H. Bartholomew, P. K. Agrawal and J. R. Katzer, Adv. Catal. 1982,31,135. 46. J. Oudar, Catal. Rev.-Sci. Eng. 1980,22, 171.
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POLYHEDRON REPORT NUMBER 13 THE SYNTHESIS, REACiWITY
STRUCTURES, BONDING AND UNUSUAL OF SULFIDO OSMIUM CARBONYL CLUSTER COMPOUNDS
RICHARD D. ADAMS Department of Chemistry, University of South Carolina, Columbia, SC 29208, U.S.A.
I. INTRODUCTION.
CONTENTS . . . . . . . . . . . . . . , . . . . . . . . . . 2003
................... II. SYNTHESES AND STRUCTURES 1. Reagents and reactions ....................... 2. Cluster condensation reactions .................... (a) Homonuclear clusters ...................... (b) Mixed-metal clusters ...................... III. BONDING AND UNUSUAL REACTIVITY. .......................... 1. M&clusters 2. M4S clusters ..........................
...............
2004 2004 2010 2010 2016 2020 2021 2022
I. INTRODUCTION The class of low-valent
polynuclear metal complexes that contain metal-metal bonds are known as cluster compounds. These compounds have attracted a great deal of attention with the idea they might serve as models for metal surfaces and heterogeneous catalysts.‘-3 More recently, interest has been focused on the chemical properties that may make them distinct from both heterogeneous and mononuclear metal homogeneous catalysts.4 A major problem in the use of cluster compounds as catalysts is their tendency to undergo degradative fragmentation when subjected to forcing reaction conditions. In an effort to prevent this a number of workers have chosen to employ bridging ligands. GSThe bridging ligands are usually derived from elements of the main groups and may be suitably modified with alkenyl and aryl substituents (e.g. NR, PR, PR,, As, AsR, AsR,, S, SR etc.). In this report attention will be focused on compounds that contain the bridging sulfido ligand and the transition element osmium. In cluster complexes containing three metal atoms, the sulfido ligand invariably serves as a triple bridge, 1.6It contains a lone pair of electrons and, when viewed as an uncharged ligand, it serves as a four-electron donor. Thus, within the framework of the valence-bond description one of the three M-S bonds would be viewed as a two-electron S to M donor bond. It is not a requirement that the M3 group contain three M-M bonds and there are many documented examples where triply bridging sulfido ligands span M3 units which have two or less metal-metal bonds.6 In the presence of four metal atoms the sulf?do ligand frequently serves as a quadruply bridging ligand, 2 or 3. Ifall the metal atoms lie on the same side of a plane that passes through the sulfido ligand, it is regarded as a four-electron donor and it contains a lone pair of electrons (2).* Interestingly, the ligand then has only three orbitals for use in bonding to the four metal atoms. Although the bonding is difficult to describe in terms of valence-bond theory, it fits naturally into the framework of the delocalized bonding scheme of the polyhedral skeletal electron pair (PSEP) theory. In most, if not all cases, the structural type 2 contains an M, cluster with four metal-metal bonds.
*There may be one exception to this definition. This occurs in the complex [HP~OS,(CO)&~-S)(JJ~S)(PPh,C,H,)],
which is described later in this section. 2003
R. D. ADAMS
2004
The structural type 3 contains four metal atoms arranged in a tetrahedral or distorted tetrahedral array about a centralized sulfido ligand. This sulfido ligand, viewed as an uncharged atom, serves as a six-electron donor and two of the four M-S bonds can be viewed as donor-acceptor bonds. There are no examples of type 3 sulfido ligands in the presence of six metal-metal bonds. The sulfido ligand appears to be very flexible and able to adapt to conditions imposed upon it by the arrangements of the metal atoms. For example, in the presence of M-M bonding, M-S-M bond angles as low as 65-75” are common. In the absence of M-M bonding these angles may range from 125 to 140”.’ A compound which imposes an unprecedented degree of distortion upon a quadruply bridging sulfido ligand is ’ The heavy-atom framework of the structure of this CHPtOs,(CO)*(C14-S)(C13’S)(PPh~C~H~)l~. molecule is shown in Fig. 1. The quadruply bridging sulfido ligands, S1 and S,, are believed to serve as six-electron donors of type 3. However, due to the highly strained polycyclic network of M,S rings, all the metal atoms are drawn to one side of a plane which passes through the sulfido ligand. Similar highly strained geometries have been observed for carbon atoms in certain polycyclic hydrocarbon compounds, and these geometries have been described as “inverted” tetrahedral.‘O II. SYNTHESES
AND
STRUCTURES
1. Reagents and reactions Although a number of sulfur-containing compounds have been used in the synthesis of sulfidotransition-metal cluster compounds,6 surprisingly few of these have been used for the synthesis of sulfido osmium carbonyl clusters. Hydrogen sulfide(H,S) would seem to be an ideal source of sulfide, but it has not yet been used with wide success. H,S does react with Os,(CO),, at 125°C to form the cluster 0s3($-I)~3-S)(CO)g (4) in nearly quantitative yields. I1 Both hydrogen atoms are transferred from the sulfur atom to the metal atoms where they bridge adjacent metal-metal bonds. The sulfido ligand serves as a triple bridge.
s,
lo/
’ 4
\ P)q, I0 qos\>Js, s
00s.
/
/
\ \>os: s’ 1
5
4
..s
6
I
+‘: osI’
The reaction of Os,(CO),, with elemental sulfur at 125°C yields compound 4 plus the compounds Os3(&$(CO)g (5) and OS~(,U-H)~~~-S)~(CO)~~@).I2 Compound5 and the selenium homologue of6 have been characterized crystallographically. Compound 5 contains triply bridging sulfido ligands on each side of an open triangular cluster of three metal atoms. I3 The Os,S, framework of6 has the shape of a trigonal prism with only three osmium-osmium bonds, two of which contain bridging hydride ligands.12 At temperatures greater than 2OO”C, the reaction yields the higher-nuclearity sulfido osmium carbonyl clusters, OS&J.+-S)(CO)~~Q, OS~(,U~-S)~(CO)~~(8) and O~i&b-S)2(CO)23 (9).14 These have been characterized structurally by X-ray diffraction methods. ORTEP diagrams of 7 and 8 are shown in Figs 2 and 3, respectively. Compound 7 consists of a square pyramid of five osmium atoms with a quadruply bridging sulfide ligand spanning the square base.” Compound 8 consists of an Os,S, cluster core in the shape of a pentagonal bipyramid and has two Os(CO), units bridging adjacent apical-equatorial edges of the cluster. l6 To date, compound 9 is the highest-nuclearity sulfido osmium carbonyl cluster that has been characterized. Its structure has been described as two square pyramids fused to two trigonal bipyramids that have one Os-Osedge in common.14 Quadruply bridging sulfido
Synthesis, structures, bonding and unusual reactivity of sulfido osmium carbonyl cluster compounds
2005
Fig. 1. An ORTEP drawing of the heavy-atom structure for the compound [HPtOs,(CO)&-S)&,S)(PPh&H& showing the inverted tetrahedral geometry of the sulfido ligands S(1) and S(2). Pt(l)S(l)-OS(~) = 68.3(2)“, Pt(l)-S(lFs(3) = 102.86(2)“, OS(~)-S(l)-OS(~) = 97.8(2)“, Pt(l)-S(l)-Pt(2) = 85.0(2),Pt(2fl(l)-Os(l) = 116.2(2)“,Pt(2~(1)-os(3) = 145.3(3)“,Pt(2)-S(2)-0$4) = 67.3(2)“,Pt(2p(2b OS(~) = 101.2(2)0,Os(4)-S(2m(6) = 97.6(2)“,Pt(2)-S(2bPt(l) = 83.8(2)“,Pt(l)-S(2)4Is(4) = 135.0(33”, Pt(l)-S(2)-Os(6) = 122.2(2)“.
ligands span the square bases of the two square pyramidal units.
Carbon disulfide (CS,) reacts with Os,(CO),, to form the compound Os,&3),(CO),(CS) analogous to 5 but contains a terminally coordinated thiocarbonyl ligand in place of one of the carbonyl ligands. ” CS2 reacts with the lightly-stabilized cluster 0s,gl-H)2~3-S)(CO),(NCMe) to form the compound OS&-H)~~~-S),(CO),(CS) (ll).‘* Compound 11 is an open cluster with two triply bridging sulfido ligands, a bridging hydride ligand across (10) which is structurally
Fig. 2. An ORTEP diagram of Os,~~-S)(CO)Is
(7) showing 50% probability thermal ellipsoids.
2006
R. D. ADAMS
Fig. 3. An ORTEP diagram of OS~~~-S)~(CO)~~ (8) showing 50% probability thermal ellipsoids.
each of the two metal-metal bonds and one linear terminal thiocarbonyl ligand. The reaction of OS,(CO),~ with CS2 at 165°C under CO pressure yields compounds 5,lO and the new compound OS,~L~-S)~&S)(CO)~~ (12)in 8% yield.lg The structure of 12 consists of four osmium atoms arranged in the form of a puckered square. Three of the osmium atoms are bridged by a triply bridging sulfido ligand. The thiocarbonyl ligand serves as a quadruple bridge with the carbon atom bonded to all four osmium atoms and the sulfur bonded to only one. The C-S bond length, 1.757( 18) A, is slightly shorter than a C-S single bond length (i.e. 1.81 A).
St&Fink et ~1.” have reported that the sulfurdimide, Me,SiN=S=NSiMe,, reacts with OSCAR at 125°C to give the compound Os,@,-S)&-NSiMe,)(CO), (13) in 13% yield. The crystal structure analysis of 13 reveals an open triangular structure similar to that of 5 but it contains a triply bridging amido ligand, Me,SiN, and one triply bridging sulfido ligand. The temperature-dependent r3C NMR spectrum of 13 in solution reveals the reversible formation of a second species at high temperature in which all the carbonyl ligands are either equivalent or are rapidly averaged. This was interpreted in terms of an isomeric species, 14, which contains three metal-metal bonds. Although the structure 14 could explain the spectral observations, it should be pointed out that this structure does not adhere to currently accepted cluster bonding theories. Thus, until further evidence is presented it should be regarded as tentative.
Synthesis, structures, bonding and unusual reactivity of sulfide osmium carbonyl cluster compounds
2007
SiMes
SiMe,
I
I
13
14
Of all the routes to sulfidoosmium carbonyl clusters, perhaps the most productive, are those which involve the desullkixation of organosulfur ligands. For example, the arylthiolatotriosmiumdecarbonylhydride clusters readily eliminate arene when heated to 150°C to yield a variety of sulfido osmium carbonyl clusters [i.e. eqn (l)] :‘I Os&-I-W-SPh)(CO),, 15 + Os&-SMCO), 17
150°C _C Os,@3-SMCO), + Os‘&-S)(CO),* 6 6 5 16 z + Os&-S)(CO),
5 + Os&,-S)(WS)(CO),,
7
18
+ Oskk%(CO)Z@ -
(1)
8
The yields of the various products are very sensitive to the reaction conditions. For example, when pyrolyzed in the absence of solvent at temperatures greater than 2OO”C,only compounds 5,16,7 and 8 are obtained.“j When pyrolyzed at 160°C under 100 bar CO pressure, only compound 5 plus three equivalents of Os(CO), is obtained. I5 Compounds 16-18have been characterized by X-ray diffraction methods. Compound 16 consists of a tetrahedral cluster of four osmium atoms with a triply bridging sulfido ligand on one of the OS, faces. 22 Compound 17 consists of a “butterfly” tetrahedral cluster of
four osmium atoms with triply bridging sulfido ligands across the two open triangular faces.21 The structures and bonding of 16 and 17 will be described further in Section III. An ORTEP diagram of
compound 18 is shown in Fig. 4.21The molecule consists of a square pyramid of five osmium atoms with a quadruply bridging sulfido ligand spanning the square base and an Os(CO), group and a triply
Fig. 4.An ORTEP diagram of 0s,~4-S)~3-S)(CO),,
(18) showing SWAprobability thermal ellipsoids.
R. D. ADAMS
2008
bridging sulfido ligand bridging an edge of the square base. The independent syntheses of compounds 7, 8 and M-18, as well as related compounds suggest mechanisms for their formation. These are described in Section 11.2(a). When irradiated under a rapid purge of CO, compound 15 loses benzene and yields the compound O~&-S)(C(~-CO)(CO)~ (19) Ceqn P)l:23 hv/CO Os,(~-H)(~-SPh)(CO),,
_C
6 6
~~,cU3-~~~~3-~~~~~~~~.
(2)
19 Compound 19 consists of a triangular cluster of three osmium atoms with a triply bridging sulfido ligand one side of the 0s3 plane and a triply bridging carbonyl ligand on the other side.
19
Thermal degradation of the benzylthiolato cluster, OS,@-H)&-SBz)(CO),, (20) occurs apparently by two competing mechanisms. 24One which is analogous to that found for 15 results in the elimination of toluene and the formation of compounds 516 and 8. The other mechanism results in formation of dibenzyl and the two isomeric hexaosmium dihydro clusters OS,&H)~~L~“S)&-S)(CO)~~ (21 and 22). ORTEP diagrams of 21 and 22 are shown in Figs 5 and 6. The red isomer 21 contains a quadruply bridging stido ligand of type 3 while the green isomer 22 contains a quadruply bridging sulfido ligand of type 2 and an extended planar array of six osmium atoms. The formation of dibenzyl and dihydrido hexaosmium species suggests a radical mechanism involving an initial homolysis of the sulfur-carbon bond in 20.
Fig. 5. An ORTEP diagram of the red isomer of OS,~C-H)~&&JJ~-S)(CO)~, (21) showing the quadruply bridging stido
ligand S( 1) of type 3.
Synthesis, structures, bonding and unusual reactivity of sulfido osmium carbonyl cluster compounds
Fig. 6. An ORTEP diagram of the green isomer of OS,@H)~(@)@~-S)(CO)~, quadruply bridging sulfido ligand S(1) of type 2.
Fig. I. An ORTEP
diagram
2009
(22) showing the
of OS,~-H),(JL&(JJ~-S)(CO)~~ (23) which is a precursor Os&-H)~o1~-S)tis-S)(CO)~~ (21).
to
2010
R. D. ADAMS
The principal product formed when compound 20 is irradiated at 25°C is the compound Os,(pH),~~-S)&-S)(CO),, (23). An ORTEP diagram of 23 is shown in Fig. 7. It is similar to 21, and was shown to be a precursor to 21 by its thermal decarbonylation. Thioformamido and dithioformato ligands can serve as precursors for sulfido ligands [eqns (3)25 and (4)26] : R
I
.“Q \, \.szd\
“,N//
I,
\I\9s/ob
rTC
&/o;;I
-co-
‘I\
‘\
24
13)
I\ 26
26
26
A
27
The desulfurization of the thioformamido complex in 24 was found to proceed via a decarbonylated intermediate 26 which was isolated and shown to contain a triply bridging thioformamido ligand. Structural analyses of 24 and 26 showed that the C-S bond length was significantly longer than the C-S bond length of thioformamido ligands in complexes where the sulfur atom was not in a bridging position. Thus, it was inferred that the bridging environment induced a weakening of the C-S bond and this facilitated the bond cleavage. In ligand-splitting reactions such as these both fragments of the ligand usually remain coordinated to the cluster. The desulfurization of the dithioformato ligand in 27 probably proceeds via an intermediate species such as A containing a thioformyl ligand, but this was not observed. The species 28 which contains a bridging thioformaldehyde ligand was isolated. This could be formed by a reductive elimination of C-H from the proposed intermediate. 2. Cluster condensation reactions (a) Homonuclear clusters. The triply bridging sulfido ligand with its lone pair of electrons would seem to be an ideal species for use in cluster size-expansion reactions. By using the lone pair ofelectrons to form coordinative interactions to electron-deficient metal groupings, the sulfido ligand could facilitate the entry of metal-containing units into the cluster [eqn (5)] : M,@,-S)+
M’L, -L_
/1”\
M \A’
B
S -
M’L,_,-
M,M’&-S)L,_,
(5)
C
The paucity of isolated intermediates such as B suggests that these species are less stable than their rearrangement products C.
Synthesis,structures,bonding and unusual reactivityof sulfido osmiumcarbonyl clustercompounds 2011 The low-nuclearity sulfido clusters 5 and 19 have served as starting points for these aggregation or condensation reactions. Osmium pentacarbonyl which is readily decarbonylated to the reactive intermediate Os(CO), when irradiated has been used as a source of monoosmium fragments. The lightly stabilized complex Os,(CO),,(NCMe), has been found to serve as a reagent for the addition of triosmium groupings. By using this methodology most of the cluster complexes synthesized in low yields in the pyrolysis reactions described in Section II. 1 have now been made in much higher yields. In addition, a number of new complexes that were not observed in the pyrolysis reactions have been isolated. Some of these can be converted into the pyrolysis products and thus suggest mechanisms for their formation. The irradiation of solutions of 19 and Os(CO), has resulted in the formation of the highernuclearity clusters, 29 and 7 in 40 and 3%yields, respectively (see Scheme 1).The structure of 29 consists of a nearly planar butterfly cluster of four osmium atoms with a triply bridging sulfido ligand on one triangular face.15 When heated to 125”C, 29 loses 1 mol of CO and condenses to 16 quantitatively. When solutions of 5 and Os(CO), are irradiated the compound OS&,-S),(CO),, (30) is formed in 40% yield16 (see Scheme 1).The structure 3Oconsists of a planar group of four osmium atoms connected by three OS-OS bonds.” Triply bridgi n g sulfido ligands are symmetrically disposed about the 0~ plane. When heated to 68°C 30 loses 1 mol of CO quantitatively and the cluster closes to form 17. The reaction of 5 with 0s3(CO)I,,(NCMe)2 yields the new hexaosmium cluster 0s6(p4-s)2(c0)17 (31) in 29% yield.” The structure of 31 is shown in Fig 8. The molecule is very similar to 8 and contains an Os,S, cluster in the form of a pentagonal bipyramid but has only one Os(CO), group bridging an apical-equatorial edge. ” This compound was also prepared in good yield by the UV irradiation of 19 in an inert solvent. The condensation of 2 mol of 19 via the unobserved intermediate D is shown in Scheme 1. An intermediate such as D is probably also traversed in the formation of 5 as observed in the pyrolysis of 15 under CO pressure.r5 When heated to 125°C compound 31 quantitatively loses 1 mol of CO to yield 18. The CO ligand that is lost probably comes from th CO-rich bridging Os(CO), grouping. A rearrangement that would readily produce condensation of 31 to 18 is shown in Scheme 2.2s
16
+
32
16
/
+Os,KO),,(NCMe),
hu
.
OdCO 1s
8
Scheme 1.
2012
R. D. ADAMS
Fig. 8. An ORTEP diagram of OS&.@)~(CO)~, (31) which is a precursor to OS,(JJ~-S)@~-S)(CO),~ (18)’
The reaction of 16 with 0s,(CO)10(NCMe)2 in refluxing octane yields the highly condensed heptanuclear cluster OS,(,Q-S)(CO)~~ (32) in 23% yield. 22 No intermediates have been isolated. The structure of 32 is shown in Fig. 9. The structure is best described as a fused combination of a square pyramid of five osmium atoms and a trigonal bipyramid of five osmium atoms with one triangular face in common. The base of the square pyramid is spanned by a quadruply bridging cwo-ww~~411 sulfido ligand of type 2. The addition of Os,(CO),,(NCMe), to 17 leads to the formation of 8 in 16% yield.r6 Curiously, it was found that compound 8 could not be prepared by the irradiation of solutions of 31and Os(CO),. It has been found that 0s3(CO)10(NCMe)2 will add to 19 to form the sequence of products OS&,-S)(CO),, (33) and OS&-S)(CO),, (34). Compound 33 is converted into 34 in 23% yield at
-co-w_
Bond breaking
.-.--
Bond making
Scheme 2.
Synthesis, structures, bonding and unusual reactivity of suffido osmium carbonyl cluster compounds
2013
Fig. 9. An ORTEP diagram ofthe fused cluster OS,@,-S)(CO),, (32)showing 50% probability thermal ellipsoids.
1 10”C.2g The molecular structure of 33 is shown in Fig. 10. The molecule consists of a butterfly tetrahedral cluster of four osmium atoms with a triply bridging sulfido ligand across one of the open triangular faces. Os(CO), groupings bridge the two edges of that same open triangular face. The structure of 34 is shown in Fig. 11. This molecule consists of a square pyramid of five osmium atoms with a quadruply bridging sulfido ligand over the square base and a capping Os(CO), group on one of the triangular faces. The transformation of 33 into 34 involves the loss of 2 mol of CO. A proposed rearrangement which involves the loss of one CO from each of the bridging Os(CO), units is shown in Scheme 3. When irradiated under a rapid purge with nitrogen, compound 5 undergoes a self-condensation and forms the green dimer [OS&-S),(CO),], (35). C om p ound 35 has a most unusual structure which is shown in Fig. 12. The central portion of the molecule consists of a butterfly tetrahedron of four osmium atoms [OS(~)-OS(~)-OS(~)-OS(~)] with a dihedral angle of 136”. Exterior Os(CO), groups are attached to the hinge atoms OS(~) and OS(~). Pairs of triply bridging sulfido ligands bridge opposite edges of the butterfly tetrahedron and extended to the exterior Os(CO), groups. The formation of 35
Fig. 10. An ORTEP diagram of OS,~,-S)(CO),~
(33) showing WA probability thermal ellipsoids.
2014
R. D. ADAMS
Fig. 11. An ORTEP diagram of the condensed cluster OS&-S)(CO)17
33
+
Bond breaking
-.-a..
Bond making
(34).
34
Scheme 3.
Fig. 12. An ORTEP diagram of the dimer [OS,~~-S),(CO),]~ (35). OS(~)-OS(~) = 2.834(l) A, OS(~)OS(~) = 2.973(l) A, Os(l)-Os(4) = 2.941(l) A, Os(2)-Os(3) = 2.957(l) A, Os(2)-Os(4) = 2.971(l) A, Os( @-OS(~) = 2.789( 1) A, Os(2bOs(6)
= 2.792( 1) A.
Synthesis, structures, bonding and unusual reactivity of sulfido osmium carbonyl cluster compounds 2015
Scheme 4.
canbeviewedasaback-to-backcouplingoftwoO~,(~~-S),(CO),unitsderivedfrorn5by thelossofone CO ligand from the central osmium atom (see Scheme 4). The self-condensation of compound 25 provides an interesting sequence of products that reveals in great detail the importance of the bridging sulfido ligand to the cluster growth process. When heated to 125°C compound 25 is decarbonylated and yields a mixture of several hexaosmium compounds.30 Of particular importance are the compounds of formula OS&-H)~(~~-S)(,U~-S)(~-HC=NR)~(CO)~ 7 (36) and OS&-H),(&-S)&-S)@-HC=NR),(CO),, (37) (R=CsH5 and p-C,H,F). The former exists in two separable isomeric forms. The latter exists in three separable isomeric forms. Compounds 36 can be converted into 37 by further heating at 125°C. An ORTEP diagram of one of the isomers of 36 is shown in Fig. 13. The molecule consists of two open triosmium clusters linked by a quadruply bridging sulfido ligand of type 3. The Os(l)-Os(2)-Os(3) grouping contains 10 CO ligands which is one more than it contained in the form 25. The OS(~)-Os(S)--Os(6) grouping contains only seven CO ligands. It is not known if the CO ligand acquired by the Os( l)-Os(2)-Os(3) group was transferred intramolecularly or intermolecularly from the OS(~)-Os(5)-Os(6) group. The most important feature of the molecule is the quadruply bridging sulfido ligand S(1) which has brought the two triosmium groups into an
S(2)
n
Fig. 13 . An ORTEP diagram of one of the isomers of 0s&-H)z014-S)&-S)@-HC=NC~H_+F)Z(CO)I (36).The metal atoms are blackened completely.
2016
Fig. 14. An ORTEP diagram of one of the isomers of OS&-H),@-HC=NC,H,),(CO),, metal atoms are blackened completely.
(37). The
inseparable relationship. No metal-metal bonds were made and none were broken in the conversion of 25 into 36. An ORTEP diagram of 37 is shown in Fig. 14. This molecule consists of groups of four and two osmium atoms. A quadruply bridging sulfido ligand S(2) joins these two groups. The formation of 37 from 36 has occurred by the transfer of one osmium atom from one group of three metal atoms to the other group of three. In this transformation there is a loss of one CO ligand and a net increase by one in the number of metal-metal bonds. The transformation of36 into 37 must have involved several steps. A mechanism that shows the essential atomic and ligand shifts is shown in Scheme 5. When heated to 15O”C,compound 37 loses another CO ligand plus an equivalent of H, and forms the compound OS&,-S),(p-HC=NR),(CO), 5(38). An ORTEPdiagram of the molecular structure of 38 is shown in Fig. 15. This molecule consists of a butterfly tetrahedron of four osmium atoms with external Os(CO), groups attached to each of the“hinge” atoms ofthe butterfly tetrahedron. Quadruply bridging sulfido ligands bridge the open triangular faces of the butterfly tetrahedron and extend to the external Os(CO), groups. These sulfido ligands are highly strained and conform to the definition of “inverted” tetrahedra (see Section I). There is a bridging carbonyl ligand across the Os(2)-Os(3) hinge bond. A transformation that shows the rearrangements implicit in the conversion of 37 into 38 is given in Scheme 6. The most important feature is that the condensation process has progressed further and at this stage a new cluster grouping (the butterfly tetrahedron) that is quite distinct from the two original triosmium units has assembled and has become a focal point of the molecule. (b) Mixed-metal clusters. Some efforts have been made to investigate the potential for the synthesis ofmixed-metal cluster compounds by adding groupings with transition elements other than osmium to
“-co
36
Scheme 5.
Synthesis, structures, bonding and unusual reactivity of sulfido osmium carbonyl cluster compounds
2017
n
Fig. 15. An ORTEP diagram of OS,&-S)&-HC=NC,H,),(CO),, blackened completely.
(38). The metal atoms are
the sulfido osmium clusters described above. These preliminary studies have been quite successful. The groupings that have received the most attention are platinum phosphine species and tungsten carbonyl complexes. Platinum phosphine complexes are labile and usually add to the sulfido osmium clusters smoothly at room temperature. The tungsten carbonyl complexes are less reactive and usually require application of UV irradiation to initiate the reactions. The reactions ofmononuclear metal species with the clusters results usually in the addition of only one metal unit. The reactions of 19 with Pt(PMe,Ph), yields four tetranuclear products. These are PtOs&-S)(CO)JPMe,Ph)s (39), PtOs&-S)(CO)s(PMezPh), (4% PtOs&+S)(CO)s(PMe,Ph), (41) and PtOs&-S)(CO),(PMe,Ph), (42).31 The last two can be prepared from the first two, in order, by thermal decarbonylation. An ORTEP diagram of 39 is shown in Fig. 16. The molecule consists of a planar butterfly tetrahedron (dihedral angle = 177”) of three osmium and one platinum atoms. The platinum atom contains only two phosphine ligands and the sulfido ligand bridges a PtOs, triangular group instead of an OS, group as it did in 19. Compound 40 is structurally analogous but contains an Os(CO),(PMe,Ph) group instead of the Os(CO), group in 39.31 Of the two decarbonylation products, only compound 42 has been characterized structurally.31 An ORTEP diagram of 42 is shown in Fig. 17. This molecule consists of a closed tetrahedral cluster of three osmium and one platinum atoms, but in this case the sulfido ligand bridges an 0s3 triangle. Compound 41 is believed to be structurally analogous to 42. A mechanism that accounts for the formation of this sequence of products is shown in Scheme 7. It is believed that an interaction of the sulfido ligand
Scheme 6.
2018
R. D. ADAMS
Fig. 16.An ORTEP diagram of PtOs&,-S)(CO),,(PMe,Ph), ellipsoids.
(39)showing 50% probability thermal
with the platinum moiety is an important first step.in the reaction. It should be noted that the facile synthesis of the mixed-metal clusters 39-42 contrasts sharply with similar reactions in which no sulfido ligands are present. For example, the reaction of Os,(CO),, with Pt(PR,), yields only phosphine-substituted derivatives of OS~(CO)~~.~~ The irradiation of solutions of 19 and W(CO),(PMe,Ph) in hexane solvent leads to formation of the
p&QJJQP(3,
-
Fig. 17. An ORTEP diagram of PtOs,&-S)(CO),JPMe,Ph), (42) showing 50% probability thermal ellipsoids. OS(l)-Pt = 2.853(l) A, Os(2)-Pt = 2.740(l) A, Os(3)-Pt = 2.766(l) A, Os(l)-Os(2) = 2.816(l) A, OS(~)-OS(~)= 2.770(l) A, OS(~)-OS(~)= 3.064(l) A.
Synthesis, structures, bonding and unusual reactivity of sulfido osmium carbonyl cluster compounds P
‘Pt’ -t ..
2019
P
Scheme 7.
cluster Os,W(p,-S)(CO),,(PMe,Ph), (43) in 27% yield. The molecule consists of a closed tetrahedral cluster with the sulfido ligand bridging an Os,W triangular face.33 The mechanism of formation of 43 has not been established yet. mixed tungsten-osmium
Compound 5 reacts with Pt(PPh,)&H, to give two tetranuclear products PtOs,~,S),(CO),(PPh,)L (44) (L = CO) and 45 (L = PPh,) and a PPh, substitution product of 5. Compound 44 has acquired one CO ligand, and its yield is significantly increased when the reaction is performed under a CO atmosphere. The structure of 44 is shown in Fig. 18. The molecule contains four metalmetal bonds. Three of these exist in a triangular PtOs, group. The fourth links an Os(CO), group to
n
Fig. 18. An ORTEP diagram of PtOs&,-S),(CO),,(PPh,) (44) showing 50”/, probability thermal ellipsoids. Os(l)-Os(2) = 2.826(l) A, Os(2)-Os(3) = 2.990(l) A, Pt-Os(2) = 2.858(l) A, Pt-Os(3) = 2.905(l)A, os(l)...os(3) = 3.585(l) A, os(l)...pt = 3.401(1) A.
2020
R. D. ADAMS
the triangular group at one of the osmium atoms. The distance of this external osmium atom to the platinum atom is 3401(l) A, which seems to be too long for a significant bonding interaction. The PtOs, group contains triply bridging sulfido ligands that bridge a Pt-0s and the OS-OS edges and extend to the third osmium atom. The compound appears to have been formed by the insertion of a Ptcontaining group into two adjacent OS-S bonds of 5. Compound 45 has a similar structure. Unlike compound 17 (see below) compounds 44 and 45 are electron-precise [i.e. all the metal atoms obey the effective atomic number (EAN) rule]. When irradiated in the presence of W(CO),PMe,Ph, compound 5 yields four new products: 0s3W(&)2(C0)12(PMe,Ph) (46), Os,W(/@),(CO),,(PMe,Ph), (47), Os,W(,u,S)2(CO),,(PMezPh)z (48) and 0s3W2&-S)&-S)(CO)r,(PMe,Ph), (49).34 The structures and relationships between these molecules are shown schematically in Scheme 8. Compound 46 has a butterfly tetrahedral cluster of four metal atoms with triply bridging sulfido ligands on the two open triangular faces. It is structurally analogous to the homonuclear compound 17. Compound 47 consists of a planar cluster offour metal atoms with triply bridging sulfido ligands on opposite sides of the Os,W plane. Compound 47 can be made from 46 by adding one equivalent of PMe,Ph. Compound 48 is isoelectronic with 46 but it adopts a structure similar to 44 and 45, consisting of triangular group of three metal atoms with an external Os(CO), group linked to the triangle by one metal-metal bond and the bridging sulfido ligands. Compound 48 can be made independently by the irradiation-induced decarbonylation of47. The pentanuclear Os,W, compound 49 contains an Os,W cluster analogous to 48 but has a W(CO),(PMe,Ph) group bridging a W-S edge of the cluster. Compound 49 can be made by the irradiation-induced addition of a W(C0)4(PMe,Ph) moiety to 46. In all of the tungsten-osmium complexes there are bonds between the tungsten atoms and the sulfido ligands. As discussed earlier [cf. eqn (5)], it is believed that preliminary metal-sulfur interactions may be important to the condensation process. The photo-induced reaction of W(CO)6 with 5 yields the product OS,~~-S)@~-S)(CO),CW(CO)J (50), which shows such an interaction in its most fundamental form.35 The structure of 50 is shown in Fig. 19. Most simply, the molecule contains a W(CO)s group attached to one of the triply bridging sulfido ligands of a molecule of 5. The connection involves a simple donor-acceptor bond formed by donation of the lone pair ofelectrons on the sulfido ligand into an empty orbital on the tungsten atom. + PMe, Ph
W(CO),(PMe,Phl
hu
49
48
Scheme 8. III. BONDING
AND UNUSUAL
REACTIVITY
The nature of the metal-metal bonding in polynuclear metal complexes is still not well understood. Localized 2c-2e bonding and application of the EAN rule works well for small clusters (i.e. four metal
Synthesis, structures, bonding and unusual reactivity of sulfido osmium carbonyl cluster compounds 2021
Fig. 19.An ORTEP diagram of 0s,(cr4-S)g13-S)(CO),[W(CO)5](5O)showing50”/,probability thermal ellipsoids.
atoms or less) while the PSEP theory, 36 which employs a delocalized bonding, works better for the larger clusters (i.e. five atoms or more). Recently, Teo has described the topological electron counting theory which incorporates the essential features of both the EAN and the PSEP theories.3’ It should be pointed out, however, that in many cases the EAN rule and the PSEP theory predict the same cormectivities for the metal atoms in both the large and small clusters. 1. M,S, clusters Recent studies have revealed a small number of tetranuclear clusters that conform to the PSEP theory but show unusually high reactivities. 27*3sIn reactions they convert to products that obey the EAN rule. One of these clusters is the sulfido osmium cluster OS~(JJ~-S)~(CO),~(17J2’ An ORTEP diagram of 17 is shown in Fig. 20. The molecule consists of a butterfly tetrahedron of four metal atoms and has sulfido ligands bridging the two open triangular faces. There are five osmium+xmium distances that are short enough (less than 3.10 A) to imply significant bonding interactions. Each metal
Fig. 20. An ORTEP diagram of OS&,-S)~(CO)~~ (17) showing 500/,probability thermal ellipsoids. Os(l)-Os(2) = 2.914(l) A, Os(l)-Os(4) = 3.091(l) A, Os(Z)-Os(4) = 2.935(l) A, Os(Z)-Os(3)= 3.002(l) A, Os(3)-Os(4) = 2.940(l) A, OS(~).. , OS(~)= 3551(l) A.
2022
R. D. ADAMS
atom contains three carbonyl ligands and, if the sulfido ligands serve as four-electron donors, then the cluster contains 64 valence electrons. This is two in excess of the requirements of the EAN rule. For this reason we have chosen to describe the molecules as electron-rich. If the molecule simply eliminated one metal-metal bond, then all the metal atoms could be made to adhere to the EAN rule. Curiously enough, the isoelectronic compounds 44,45 and 47 do have only four metal-metal bonds, but 46 has five. The reason for this is not clear, but steric effects could be important. Intuitively, it seems that compound 47 is more sterically crowded than 46 and this could mitigate against the formation of the fifth bond. The structures of 17 and 46 are, however, in accord with the PSEP theory. According to the PSEP theory Os(CO), groups contribute two electrons to cluster bonding and triply bridging sulfido ligands contribute four. There are thus 16 or 8 pairs of cluster valence electrons present in 17. The PSEP theory predicts a cluster shape based on a polyhedron with seven vertices. This is the pentagonal bipyramid. The structure of 17can be regarded as a pentagonal bipyramid with one vertex missing (i.e. a nido-pentagonal bipyramid). Thus, the observed structure with five metal-metal bonds is the expected one. However, upon close examination of the metal-metal bonding, it is observed that two ofthe metalmetal bonds, indicated by the dashed connections in Fig. 20, are significantly longer (greater than 3.OOA) than the other metal-metal bonds in 17 and also the metal-metal distances in Os,(CO),, [2.877(3) A]. The latter might best be regarded as OS-OS single bonds. It has not been established if these bond lengthenings are a result of incomplete delocalization or are caused by distortions in the ligand structure. Studies were undertaken to try to establish if compounds 17 and 46, which show similar distortions, might exhibit any unusual reactivity. These studies showed the compounds to be remarkably reactive and most surprisingly, toward the addition of Lewis donors, L. For example, compound 17 adds 1 mol of CO at room temperature under 1 bar pressure to form compound 30.” The addition of phosphines and isocyanides proceeds similarly to yield the derivatives OS&-S)~(CO)~~L (51) (L = PMe,Ph) and (52) (L = CNBU’).~’ The structures of 51 and 52 (see Scheme 9) show that a shift of a carbonyl ligand between the metal atoms must have occurred since each molecule contains an Os(CO), group. Also since phosphine ligands do not engage in intramolecular shifts, it implies that the site of addition was one of the “wing-tip” metal atoms of the cluster of 17. Compounds 30,51 and 52 each contain only three metal-metal bonds. They are thus “electron-precise” and all the metal atoms obey the EAN rule. The formation of these compounds can be achieved most easily by cleaving the two elongated bonds in 17, transferring one CO ligand between a pair of metal atoms, and shifting the attachment of one of the bonds of one of the sulfido ligands to a different metal atom. The reason for the nucleophilic addition at the wing-tip metal atom in 17 could be steric. The wing-tip metal atoms are less crowded being only 6coordinate while the hinge atoms are 7-coordinate. Compounds 51 and 52 lose CO when heated to form substitution derivatives of 17. Compound 46 reacts similarly to 17, but the electron-precise analogs, especially 44 and 45, are considerably less reactive and undergo cluster degradation when subjected to forcing conditions. Surprisingly, none of these compounds will add alkenes or alkynes. Another interesting and important reaction is the addition of hydrogen to 17.40 The reaction requires a temperature of 100°C and cluster degradation due to further reaction occurs but the initial product is compound 6. The mechanism of the reaction is not known, but it involves the addition of 1 mol of H, and the cleavage of the two elongated metal-metal bonds in 17. The end product of the reaction is H20s3(fi3-S)(C0)g.
Scheme 9. 2. M,S clusters The archetypical molecule of this series is compound 16 which is a 60-electron cluster that consists of a closed tetrahedral cluster of metal atoms with a triply bridging sulfido ligand on one triangular face.
Synthesis, structures, bonding and unusual reactivity of sulfido osmium carhonyl cluster compounds
2023
The molecule is electron-precise and all the metal atoms obey the EAN rule, but careful consideration reveals a feature about the bonding that could be important to its reactivity. Specifically, one of the metal-metal bonds to the unique osmium atom can be viewed as a heteropolar (donor-acceptor) bond.41 In addition, since the sulfido ligand is a four-electron donor, one of the OS-S bonds can be
16
16A
regarded similarly. Of course, in its resonance stabilized state all the metal-metal bonds to the unique osmium atom and all the metal-sulfur bonds will be chemically equivalent. However, the notion of the heteropolar bond suggests something about its potential reactivity that may distinguish it from the homopolar bond. Namely, one of these bonds can be opened 16A without producing a charge separation and a ligand can be added to the vacant site on the unique metal atom. In compound 16 reactions of this type are remarkably facile. 42 For example, 16 adds 1 mol of CO at room temperature under 1 bar pressure to form compound 29. The reaction is quantitative and complete in approximately 3 h. A similar reaction occurs with phosphines and even with amines, and with amines the reaction is complete in a matter of seconds. These additions are also reversible. Compound 16 activates hydrogen, and this reaction proceeds albeit slowly even at room temperature/l bar pressure. The product is OS&H)~(&)(CO)~~ (53) and the ORTEP diagram of structure of 53 is shown in Fig. 21. The molecule consists of a butterfly tetrahedral cluster with the sulfido ligand bridging one of the open triangular faces. The hydride ligands were not located in the structural analysis, but the two metal-metal bonds opposite the sulfido ligand are elongated and this suggests that each may contain one bridging hydride ligand. The formation of 53 has involved the cleavage of one of the homopolar sulfur-bridged metal-metal bonds in 16 and not the “heteropolar” bond. This may be related to the fact that the addition of hydrogen results in a homolytic cleavage of the Hz molecule. The reaction of 16 with alkynes is especially interesting, but to date only reactions with terminal alkynes have been investigated.43 The reaction of 16 with PhECH yields two noninterconvertible products. These have been identified as 0s&3-S)[~4-C=C(H)Ph](CO)lz (54) and OS&~SC(Ph)CH](CO),, (55). The structure of 54 is shown in Fig. 22. The molecule consists of a puckered rhombus of four osmium atoms connected by four metal-metal bonds. A sulfido ligand bridges three of the metal atoms while a phenylvinylidene ligand bridges all four metal atoms on the opposite side of the
Fig. 21.An ORTEP diagram of 0s4~-H)2&-S)(CO)12 (53).The molecule contains a crystallographic plane of symmetry passing through the atoms S, OS(~)and OS(~).Os(l)-Os(2) = 2.8149(6)A, OS(~)OS(~) = 2.9699(9)A, Os(2)-Os(3) = 2.8578(10) I$ OS(~). . . Os(1’) = 3.9295(9)A.
2024
R. D. ADAMS
Fig. 22.An ORTEP diagram of 0s,@3-S)[j14-C=C(H)Ph](CO)Iz (54)showing 50%thermal ellipsoids. OS(~)-OS(~)= 2.850(l) A, Os(l)-Os(4) = 2.859(l) A, Os(2)-Os(3) = 2.787(l) A, OS(~)-OS(~)= 2.866(l) A, C(13)-C(14) = 1.446(16)A. rhombus. The formation of the vinylidene has obviously occurred by a 1,Zshift of the acetylenic hydrogen atom. These rearrangements have been observed previously in a number of cases in reactions of metal complexes with terminal alkynes. 44 The structure of 55 is shown in Fig. 23. The molecule consists of a butterfly tetrahedral cluster of four metal atoms with a quadruply bridging S-CPh=CH thiolato ligand. The sulfur atom of this unusual ligand bridges two osmium atoms while the alkenyl group is a,a-bonded to the other two. The ligand was formed by the formation of a carbon-sulfur bond between the phenyl-substituted carbon of the alkyne and the sulfido ligand and a cleavage of one osmium-sulfur and one osmium-osmium bond in the cluster. Curiously, the hydrogen shift that occurred in the formation of 54 did not occur in the formation of 55. Although the mechanism of this hydrogen shift is not yet known, it is tempting to speculate that the absence of a hydrogen shift in the formation of 55 may somehow be related to the formation of the bonding interaction between the
Fig. 23. An ORTEP diagram of 0sq[~4-SC(Ph)=CH](CO)1z
(55) showing 50% probability thermal ellipsoids. Os(l)-Os(2) = 2.7840(7) A, Os(l)-Os(3) = 2.8490(7) A, Os(l)-Os(4) = 2.8442(7) A, OS(~)OS(~) = 2.8132(7) A, Os(3)-Os(4) = 2.764(l) A, S-C(l4) = 1.817(11) A, C(13)-C(14) = 1.400(15) A.
Synthesis, structures, bonding and unusual reactivity of sulfido osmium carbonyl cluster compounds
2025
alkyne and the sulfido ligand. Such effects might also occur in the sulfur-inhibited reactions of certain heterogeneous transition metal catalysts. In fact a variety of sulfur-ligand interactions similar to those found in 55 could be responsible for and explain a range of the sulfur-poisoning effects that exist for heterogeneous catalysts.45V46 Acknowledgements-Our
research on this subject has been supported by the Office of Basic Energy Sciences ofthe U.S. Department of Energy, and the National Science Foundation.
REFERENCES 1. E. L. Muetterties, Science 1977,1%, 839. 2. E. L. Muetteries, Bull. Sot. Chim. Belg. 1975,&l, 959. 3. R. Ugo, Catal. Reo. 1975,11,225. 4. (a) E. L. Muetterties, Catal. Rev.-Sci. Eng. 1981, 23, 69; (b) R. D. Adams and I. T. Horvath, Prog. Znorg. Chem. 1985,33,127. 5. H. Vahrenkamp, Adv. Organomet. Chem. 1983,22,169. 6. H. Vahrenkamp, Angew. Chem., Znt. Ed. Engl. 1975,14,322. 7. R. D. Adams and T. S. A. Hor, Organometallics 1984,3,1915. 8. K. Wade, In Transition Metal Clusters (Edited by B. F. G, Johnson), John Wiley, New York (1980). 9. R. D. Adams and D. F. Foust, Organometallics 1983,2, 323. 10. K. B. Wiberg, Act. Chem. Res. 1984,17,379. 11. A. J. Deeming and M. Underhill, J. Organomet. Chem. 1972,42, C60. 12. B. F. G. Johnson, J. Lewis, P. G. Lodge and P. R. Raithby, J. Chem. Sot., Chem. Commun. 1979,719. 13. R. D. Adams, I. T. Horvath, B. E. Segmuller and L. W. Yang, Organometallics 1983,2,144. 14. J. P. Attard, B. F. G. Johnson, J. Lewis, J. M. Mace, M. McPartlin and A. Sironi, J. Chem. Sot., Chem. Commun. 1984,595. 15. R. D. Adams, I. T. Horvath, B. E. Segmuller and L. W. Yang, Organometallics 1983,2,1301. 16. R. D. Adams, I. T. Horvath, P. Mathur, B. E. Segmuller and L. W. Yang, Organometallics 1983,2,1078. 17. P. V. Broadhurst, B. F. G. Johnson, J. Lewis and P. R. Raithby, J. Organomet. Chem. 1980,194, C35. 18. P. V. Broadhurst, B. F. G. Johnson, J. Lewis, A. G. Orpen, P. R. Raithby and J. R. Thomback, J. Organomet. Chem. 1980,187,141. 19. P. V. Broadhurst, B. F. G. Johnson, J. Lewis and P. R. Raithby, J. Chem. Sot., Dalton Trans. 1982,164l. 20. G. S&s-Fink, U. Thewalt and H. P. Klein, J. Organomet. Chem. 1982,224,59. 21. R. D. Adams and L. W. Yang, J. Am. Chem. Sot. 1982,104,4115. 22. R. D. Adams, D. F. Foust and P. Mathur, Organometallics 1983,2,990. 23. R. D. Adams, I. T. Horvath and H. S. Kim, Organometallics 1984,3,548. 24. R. D. Adams, I. T. Horvath, P. Mathur and B. E. Segmuller, Organometallics 1983,2,996. 25. R. D. Adams, Z. Dawoodi, D. F. Foust and B. E. Segmuller, Organometallics 1983,2,315. 26. R. D. Adams, N. M. Golembeski and J. P. Selegue, J. Am. Chem. Sot. 1981,103,546. 27. R. D. Adams and L. W. Yang, J. Am. Chem. Sot. 1983,105,235. 28. R. D. Adams, I. T. Horvath and L. W. Yang, J. Am. Chem. Sot. 1983,105,1533. 29. R. D. Adams, I. T. Horvath and P. Mathur, Organometallics 1984,3,623. 30. R. D. Adams, Z. Dawoodi, D. F. Foust and B. E. Segmuller, J. Am. Chem. Sot. 1983,105,831. 31. R. D. Adams and T. S. A. Hor, Znorg. Chem. 1984,X$4723. 32. M. I. Bruce, G. Shaw and F. G. A. Stone, Chem. Commun. 1971,1288. 33. R. D. Adams, T. S. A. Hor and P. Mathur, Organometallics 1984,3,634. 34. R. D. Adams, I. T. Horvath and P. Mathur, J. Am. Chem. Sot. 1984,106,6296. 35. R. D. Adams, I. T. Horvath and S. Wang, Znorg. Chem. 1985,22,1728. 36. K. Wade, In Transition Metal Clusters (Edited by B. F. G. Johnson), Chap. 3. John Wiley, Chichester (1980). 37. B. K. Teo, Znorg. Chem. 1984,23,1251,1257. 38. H. Vahrenkamp and D. Wolters, Organometallics 1982,1,874. 39. R. D. Adams, I. T. Horvath and K. Natarajan, Organometallics 1984,3,1540. 40. R. D. Adams, Z. Dawoodi, D. F. Foust and L. W. Yang, Ann. N. I! Acad. Sci. 1984,415,47. 41. G. R. John, B. F. G. Johnson and J. Lewis, J. Organomet. Chem. 1979,181,143. 42. R. D. Adams and S. Wang, report in preparation. 43. R. D. Adams and S. Wang, Organometallics (in press). 44. M. I. Bruce, Adv. Organomet. Chem. 1983,22,60. 45. C. H. Bartholomew, P. K. Agrawal and J. R. Katzer, Adv. Catal. 1982,31,135. 46. J. Oudar, Catal. Rev.-Sci. Eng. 1980,22, 171.