Reactions of diphosphine-stabilized Os3 clusters with triphenylantimony: syntheses and structures of new antimony-containing Os3 clusters via Sb–Ph bond cleavage

The reactivity of the trimetallic clusters [Os3(CO)10(μ-dppm)] [dppm = bis(diphenylphosphino)methane] and [HOs3(CO)8{μ3-Ph2PCH2PPh(C6H4-μ2,σ1)}] with triphenylantimony (SbPh3) has been examined. [Os3(CO)10(μ-dppm)] reacts with SbPh3 in refluxing toluene to yield three new triosmium clusters [Os3(CO)9(SbPh3)(μ-dppm)] (1), [HOs3(CO)7(SbPh3){μ3-Ph2PCH2PPh(C6H4-μ2,σ1)}] (2), and [HOs3(CO)7(SbPh3)(μ-C6H4)(μ-SbPh2)(μ-dppm)] (3). [HOs3(CO)8{μ3-Ph2PCH2PPh(C6H4-μ2,σ1)}] reacts with SbPh3 (excess) at room temperature to afford [Os3(CO)8(SbPh3)(η1-Ph)(μ-SbPh2)(μ-dppm)] (4) as the sole product. A series of control experiments have also been conducted to establish the relationship between the different products. The molecular structure of each product has been determined by single-crystal X-ray diffraction analysis, and the bonding in these new clusters has been investigated by electronic structure calculations.


Introduction
Transition metal clusters containing oxophilic Sn, Sb, and Bi elements can serve as single-source precursors for nanoscale heterogeneous catalysts of commercial interest, especially hydrogenation, dehydrogenation, and oxidation reactions. 1-4 A popular method to incorporate tin and antimony ligands into the coordination sphere of low-valent transition metal clusters is to use organotin hydrides (HSnR 3 ) and triorganostibines (SbR 3 ). The resulting organotin ligands generated by Sn-H oxidative addition of the tin reactant are strong donors and readily survive the calcination process. In comparison, metal cluster-SbR 3 bonds are relatively weak, and the SbR 3 ligand readily dissociates during calcinations to furnish composites with high metal : Sb ratios. 5 As a result, the reactivity of organostibines towards low-valent transition-metal clusters remains relatively unexplored, 6-10 although a large number of antimonycontaining metal clusters have been reported over the years using Sb-Cl, Sb-H, or Sb-Sb bond cleavage reactions, as exemplied by the work of  We have been interested in the reactions of low-valent transition-metal clusters that contain oxophilic main group elements such as tin and antimony for over a decade. 10,14,15 Recently, we have reported our results on several di-and trirhenium complexes containing up to three antimony donor ligands from the reactions of [Re 2 (CO) 9 (NCMe)] and [H 3 Re 3 (-CO) 11 (NCMe)] with SbPh 3 . 10 In continuation of our work on the reactivity of triorganostibines with transition metal clusters, we have examined the reactions of the diphosphine-substituted triosmium clusters [Os 3 (CO) 10 (m-dppm)] and [HOs 3 (CO) 8 {m 3 -Ph 2 PCH 2 PPh(C 6 H 4 -m 2 ,s 1 )}] with SbPh 3 . We have isolated and characterized four new triosmium clusters containing Sb-based ligands from these reactions, which are discussed herein. The solid-state structures for products 1-4 have been determined by X-ray crystallography, and the bonding in these clusters has been explored by electronic structure calculations.

General remarks
All reactions were carried out under an inert atmosphere of nitrogen using standard Schlenk techniques unless otherwise noted. Reagent-grade solvents were dried by standard methods and freshly distilled before use. [Os 3 (CO) 12 ] was purchased from Strem Chemical Inc. and used without further purication. Bis(diphenylphosphino)methane (dppm) and triphenylstibine (SbPh 3 ) were purchased from Sigma-Aldrich and used as received. The starting clusters [Os 3 (CO) 10 (m-dppm)] and [HOs 3 (CO) 8 {m 3 -Ph 2 PCH 2 PPh(C 6 H 4 -m 2 ,s 1 )}] were prepared according to the published procedures. 16 IR spectra were recorded on a Shimadzu FTIR Prestige 21 spectrophotometer, while 1 H and 31 P{ 1 H} NMR spectra were recorded on a Bruker Avance III HD (400 MHz) instrument. All chemical shis are reported in ppm units and are referenced to the residual protons of the deuterated solvents ( 1 H) and external 85% H 3 PO 4 ( 31 P). Elemental analyses were performed by the Microanalytical Laboratories of the Wazed Miah Science Research Center at Jahangirnagar University. All products reported herein were separated in the air by TLC plates coated with 0.25 mm silica gel (HF 254 -type 60, E. Merck, Germany).

Crystal structure determinations
Single crystals of 1-4 suitable for X-ray diffraction study were grown by slow diffusion of n-hexane into a CH 2 Cl 2 solution containing each compound. Suitable crystals were mounted on a Bruker D8 Venture diffractometer equipped with a PHOTON II CPAD detector using a Nylon loop and Paratone oil. The diffraction data were collected at 193(1) K for 4 and 230(1) K for clusters 1, 2, and 3 using Mo-Ka radiation (l = 0.71073). Data  reduction and integration were carried out with SAINT + , 17 and absorption corrections were applied using the program SADABS. 18 The structures were solved with the ShelXT 19 structure solution program using intrinsic phasing and rened with the XL 20 renement package using least-squares minimization within the OLEX2 (ref. 21) graphical user interface. All nonhydrogen atoms were rened anisotropically, and the hydrogen atoms were included using a riding model. Pertinent crystallographic parameters are given in Table 1, and selected bond distances and bond angles for clusters 1-4 may be found in Table S1 (ESI). †

Computational modeling details
All calculations were performed with the hybrid meta exchangecorrelation functional M06, 22 as implemented by the Gaussian 09 program package. 23 The osmium and antimony atoms were described by Stuttgart-Dresden effective core potentials (ECP) and an SDD basis set, 24 while a 6-31G(d ′ ) basis set was employed for the remaining atoms. 25 All calculations included Grimme's dispersion correction. 26 The input data for the optimizations were taken from the coordinates of the experimental structures, and the Hessian matrix for each geometry-optimized structure displayed only positive eigenvalues. The natural charges (Q) and Wiberg bond indices (WBIs) were computed using Weinhold's natural bond orbital (NBO) program (NBO version 3.1). 27,28 Table 2 summarizes the NBO data. The geometry-optimized structures presented here have been drawn with the JIMP2 molecular visualization and manipulation program. 29
The molecular structure of 1 is depicted in Fig. 1. The product contains a triosmium core ligated by nine carbonyls, and dppm and SbPh 3 ligands. The three pnictogen donors lie in the equatorial plane dened by the three osmium atoms with the gross structural features of 1 similar to those of the structurally related phosphine-substituted triosmium clusters [Os 3 (CO) 9 (PR 3 )(m-dppm)] [PR 3 = PPh 3 , P(C 4 H 3 S) 3 , PPh 2 H]. 30,31 The bridging diphosphine ligates the Os(1) and Os(2) atoms, while the SbPh 3 ligand is bonded to the Os(3) atom. The Os-P [mean 2.3299 Å] and Os-Sb [2.5921(3) Å] bond distances are similar to those distances reported in the literature for related clusters. 6,11,12 The SbPh 3 substitution has no effect on the Os 3triangle as the average Os-Os distance in 1 (2.887 Å) is identical to the Os-Os bond distances in [Os 3 (CO) 10 (m-dppm)] (2.885 Å). 32 The solution spectroscopic data of 1 are in accord with the solidstate structure. The 31 P{ 1 H} NMR spectrum displays two doublets at −27.1 and −29.1 ppm (J PP 60 Hz) for the two inequivalent dppm phosphorus atoms, while the 1 H NMR spectrum reveals a virtual triplet at 5.01 ppm (J 10.8 Hz) attributed to the methylene protons of the dppm ligand together with a series of aromatic multiplets due to phenyl protons of the SbPh 3 and dppm ligands.
The bonding in 1 was investigated by DFT. The M06optimized structure of species A is depicted alongside the solid-state structure in Fig. 1. Excellent agreement between the two structures is noted. The three osmium atoms exhibit a negative charge that ranges from −1.44 [Os(2)] to −1.59 [Os(3)] and display a mean Q value (measure of charge) of −1.51. The osmium charges are unremarkable compared to those values reported by us for related osmium clusters. 14c,33 The charge on the antimony atom is 1.89 and is similar in magnitude to that reported for the related cluster [Ru 3 (CO) 9 (SbPh 3 )(m-dppm)]. 34 The mean Wiberg bond index (WBI), which serves a measure of bond strength, for the three Os-Os bonds is 0.44, 0.80 for the Os-Sb bond, and 0.80 for the two Os-P bonds in A. These values are comparable in magnitude to other polynuclear osmium clusters investigated by us. 14c, 33 We also optimized the cluster with an axial SbPh 3 ligand (species A_alt; not shown) and conrmed the thermodynamic preference for A, which is more stable by 2.4 kcal mol −1 (DG).
The molecular structure of compound 2 is depicted in Fig. 2. The overall structure of 2 is similar to that of [HOs 3 (CO) 8 {m 3 -Ph 2 PCH 2 PPh(C 6 H 4 -m 2 ,s 1 )}], 16 which is prepared from [Os 3 (CO) 10 (m-dppm)] via decarbonylation at 110°C. The cluster core consists of an approximate isosceles triangle of osmium atoms where the Os-Os bond distances range from 2.7720 (3) (2)]. The metalated phenyl group, which may be viewed as a benzylidene moiety, asymmetrically bridges the shortest Os-Os bond [Os(1)-Os(3) 2.7720(3) Å] using the C(10) atom [Os(1)-C(10) 2.232(4) Å and Os(3)-C(10) 2.399(4) Å]. The phosphorus atom associated with the metalated aryl ring occupies an axial coordination site to facilitate this orthometalation. The hydride ligand, which was located and rened crystallographically, spans the same Os-Os vector as the benzylidene-bridged Os(1)-Os(3) edge except that it lies below the metallic polyhedron opposite the bridging benzylidene ligand. The SbPh 3 ligand is bound to Os(3) atom and occupies the equatorial site trans to the Os(2) atom. The mean Os-P bond distance in 2 (2.3302 Å) is almost identical to that found in [HOs 3 (CO) 8 {m 3 -Ph 2 PCH 2 PPh(C 6 H 4 -m 2 ,s 1 )}] 16 (2.3265 Å), while the Os-Sb bond distance of 2.6343(4) Å is similar in magnitude to that observed in 1 and other related clusters. 6,11,12 The optimized structure of species B is shown to the right of cluster 2 in Fig. 2 (3) vector by the benzylidene and hydride ligands in 2 is reproduced in B. This trend is attributed to the ancillary SbPh 3 group, which lengthens (weakens) the proximal Os 3 -C(benzylidene) and Os 3 -H bonds relative to their distal Os 1 -C(benzylidene) and Os 1 -H counterparts. The larger (stronger) WBI for the Os-C(benzylidene) (0.56 versus 0.32) and the Os-H (0.45 versus 0.31) bonds are associated with the Os(1) atom. 36 The solution spectroscopic data of 2 indicate that the solidstate structure persists in solution. The IR spectrum exhibits four absorption bands between 2029-1919 cm −1 , while the 31 P { 1 H} NMR spectrum shows a doublet at −17.7 ppm (J 74 Hz) and a multiplet at −20.9 ppm for the inequivalent phosphorus atoms of the diphosphine ligand. In addition to a series of multiplets in the aromatic region for the phenyl protons, the 1 H NMR spectrum also displays two multiplets at 4.97 and 3.87 ppm attributed to the methylene protons of the diphosphine ligand, and the upeld doublet of doublets at −12.51 ppm (J PH 34.0, 11.6 Hz) for the bridging hydride consistent with the solid-state structure.

Å [Os(1)-Os(3)] to 2.8249(3) Å [Os(1)-Os
The ORTEP diagram in Fig. 3 shows the molecular structure of 3. The molecule contains 50 valence electrons and exhibits an expanded metallic polyhedron with two Os-Os single bonds [Os(1)-Os(2) 3.1729(4) Å; Os(2)-Os(3) 2.9725(4) Å]. The >4.30 Å internuclear Os(1)/Os(3) distance precludes any signicant bonding interaction between these osmium atoms. The m 2 -stibene moiety [Sb(1) atom], which serves as a 3e donor, tethers the two non-bonding Os(1) and Os(3) centers. The Sb(2) atom associated with the SbPh 3 ligand is bound by the Os(3) atom and functions as a 2e ligand. The bridging dppm, C 6 H 4 (benzyne), and hydride ligands all span the Os(1)-Os(2) vector and collectively serve to donate 5e to the total electron count. The m-C 6 H 4 ligand is bound to the Os(1) and Os(2) atoms via the C(8) and C(9) carbon atoms, respectively, and the plane containing the benzyne ligand is almost perpendicular to the Os 3 plane based on the dihedral angle of ca.  The M06-optimized structure of species C appears to the right of the X-ray diffraction structure of cluster 3. The Q values for C parallel those data reported for species A and B. The charges on the osmium atoms range from −1. 18 [Os(2)] to −1.94 [Os(3)], with an average Q value of −1.58. The charge on the antimony atoms is sensitive to the nature of the ligand, with the stibine [Sb(2) = 1.88] donor ca. 11% larger than the stibene [Sb(1) = 1.67] bridging moiety. The computed Sb charges for species C are similar in magnitude to those values recently reported by us for a series of ruthenium clusters containing stibine and stibene ligands. 34 The bridging hydride is essentially neutral based on a Q value of 0.07, while the mean Q value for the metalated benzyne carbons is −0.16. The mean charge for P(1) and P(2) atoms is 1.44. The mean WBI for the two Os-Os bonds is 0.30, which is ca. 98% larger (stronger) than the WBI for the non-bonding Os(1)/Os(3) atoms. The WBIs for the three Os-Sb and Os-P vectors are similar in magnitude and exhibit a mean index of 0.76 and 0.79, respectively. Finally, the mean index for the Os-C(benzyne) and the Os-H bonds is 0.71 and 0.39.
The spectroscopic data indicate that cluster 3 retains its solid-state structure in solution. The IR spectrum exhibits six n(CO) bands within the range 2029-1919 cm −1 , consistent with a product containing terminal CO ligands. The 1 H NMR spectrum displays an upeld virtual triplet at −18.70 ppm (t, J 9.6 Hz) for the bridging hydride ligand, and the two multiplets at 2.28 and 2.05 ppm, each integrating to 1H, are attributed to the methylene protons of the dppm ligand. The remaining fortynine hydrogens associated with the ve aryl groups and benzyne moiety are found from 7.43 to 5.85 ppm. Finally, the pair of inequivalent phosphorus resonances appear as two doublets centered at 11.9 and 2.2 ppm (J PP 55 Hz) in the 31 P{ 1 H} NMR spectrum.
The molecular structure of 4 is depicted in Fig. 4. Cluster 4 contains 50 valence electrons, assuming the stibine and stibene ligands collectively contribute 5e to the total electron count. The molecule contains an open triosmium core with two almost equal osmium-osmium edges [Os(1)-Os(2) 3.0211(3) Å and Os(2)-Os(3) 3.0024(3) Å]. The dppm ligand bridges the Os(1)-Os(2) edge, while the non-bonding Os(1)/Os(3) edge is symmetrically bridged by the stibene ligand, exhibiting a mean Os-Sb(1) bond distance of 2.6587 Å. The distance between the non-bonding Os(1)/Os(3) atoms is 4.3501(5) Å which precludes any signicant bonding interaction between these osmium atoms. The SbPh 3 ligand is coordinated to Os(3) [Os(3)-Sb(2) 2.6468(4) Å] and is situated trans to the bridging SbPh 2 ligand. The h 1 -phenyl ligand is coordinated to the Os(1) atom and resides in the equatorial plane dened by three osmium atoms. The DFT-optimized structure of species D is depicted alongside the solid-state structure in Fig. 4. The computed charges and WBIs parallel the data reported for species A-C. These data are included in Table 2.
The NMR spectra of 4 indicate the presence of a pair of stereoisomers in solution. The 31 P{ 1 H} NMR spectrum displays two sets of resonances in a 10 : 1 ratio. The doublets at 13.8 and 0.2 ppm (J PP 66 Hz) are attributed to the major isomer, while the doublets at 15.1 and 1.6 ppm (J PP 66 Hz) are assigned to the minor isomer. Likewise, the aliphatic region of the 1 H NMR spectrum shows two sets of resonances for the methylene protons of the dppm ligand. The virtual triplets at 4.01 ppm (J 10.4 Hz) and 3.94 ppm (J 10.4 Hz) are assigned to the major and minor isomers, respectively. Several possibilities may be proposed for the minor stereoisomer in 4. For example, the minor isomer may arise from a two-site exchange of an axial CO ligand and the s 1 -Ph group at the Os(CO) 2 (SbPh 3 ) moiety to yield a stereoisomer having an axial s 1 -Ph ligand. Alternatively, a tripodal rotation involving two CO ligands and the SbPh 3 donor at the Os(CO) 3 (SbPh 3 ) moiety (Scheme 1) would afford a stereoisomer where the SbPh 3 ligand is situated cis the bridging stibene ligand. Of the three stereoisomers in Scheme 1, the crystallographic structure is the most stable (species D) and is assigned as the major species in solution. The minor stereoisomer is attributed to the cluster containing an axial s 1 -Ph ligand (D_alt1), lying 1.0 kcal mol −1 above species D. The other isomer (D_alt2) exhibits an equatorial SbPh 3 ligand oriented cis to the bridging stibene ligand. D_alt2 lies 3.9 kcal mol −1 above D_alt1.
CO substitution in [Os 3 (CO) 10 (m-dppm)] by SbPh 3 gives cluster 1 using either thermal activation or oxidative decarbonylation using Me 3 NO. Control experiments using cluster 1 were conducted to conrm its relationship to clusters 2 and 3. Thermolysis of 1 in reuxing toluene afforded 2 as the major product (62%), together with a minor amount (22%) of the known benzylidene-bridged cluster [HOs 3 (CO) 8 {m 3 -Ph 2 PCH 2 -PPh(C 6 H 4 -m 2 ,s 1 )}]. 16 These results conrm 1 as a precursor to 2 through loss of CO (2 equiv.), coupled with concomitant orthometalation of one of the phenyl rings of the dppm ligand. In the absence of added SbPh 3 , [HOs 3 (CO) 8 {m 3 -Ph 2 PCH 2 -PPh(C 6 H 4 -m 2 ,s 1 )}] forms as a minor product from the competitive loss of SbPh 3 versus CO in 1, followed by orthometalation of one of the aryl rings on the dppm ligand. [HOs 3 (CO) 8 {m 3 -Ph 2 -PCH 2 PPh(C 6 H 4 -m 2 ,s 1 )}] was not observed when [Os 3 (CO) 10 (mdppm)] was treated with SbPh 3 in reuxing toluene. The competitive pathway involving the loss of SbPh 3 in the thermolysis of 1 is efficiently suppressed in the presence of SbPh 3 (2 equiv.). In a separate experiment that was monitored by TLC, we conrmed that the reaction between 1 and SbPh 3 at 110°C also furnished clusters 2 (14%) and 3 (20%) without a trace of [HOs 3 (CO) 8 {m 3 -Ph 2 PCH 2 PPh(C 6 H 4 -m 2 ,s 1 )}]. Control experiments conrmed that cluster 4 is converted into 2 (63%) and 3 (30%) in moderate yield in reuxing toluene. The h 1 -C 6 H 5 ligand in 4 undergoes reductive elimination with the bridging stibene ligand, followed by metalation of an aryl ring on the dppm ligand and release of one SbPh 3 ligand to ultimately give 2. Competitive with this route is the loss of one CO from 4, followed by orthometalation of the h 1 -C 6 H 5 ligand, to give 3. The formation of 4 from [Os 3 (CO) 10 (m-dppm)] and SbPh 3 (excess) at 110°C is short-lived, and it transforms into clusters 2 and 3. Independent experiments on puried samples of 2 and 3 in toluene (110°C) also afforded 4 with signicant material loss noted. These results are understood within a kinetic framework where the rates of decomposition of clusters 2 and 3 are faster than cluster 4.
Facile Sb-Ph bond cleavage was observed in the control reaction involving 1 and SbPh 3 at room temperature. Here cluster 4 is produced in nearly quantitative yield. We have evaluated the thermodynamics for the conversion of 4 / 2 (plus CO and SbPh 3 ) and 3 (plus CO), and electronic structure calculations conrmed 4 as thermodynamically more stable. The formation of 2 along with the liberated CO and SbPh 3 ligands from 4 is endergonic by 28.3 kcal mol −1 , while the formation of 3 and CO lies 33.1 kcal mol −1 above species D (cluster 4). The formation of these products is driven, in part, by entropic contributions involving the release of CO.
The reactivity of the labile cluster [HOs 3 (CO) 8 {m 3 -Ph 2 PCH 2 -PPh(C 6 H 4 -m 2 ,s 1 )}] with SbPh 3 was also investigated. The reaction proceeds rapidly at room temperature to give cluster 4 as the major product. Scheme 3 illustrates this reaction. Reductive coupling of the hydride and benzylidene moieties regenerates the dppm ligand, followed by stibine coordination and oxidative Sb-Ph bond cleavage.

Conflicts of interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to inuence the work reported in this paper.