4069
Znorg. Chem. 1993, 32, 4069-4076
Synthetic and Structural Studies on ( R C & ) ~ R Y E ~ ~ /(E ~ += S, Se, Te): Mobile Metal-Metal Bonds within a Mixed-Valence RuIv/Rum Cluster Eric J. Homer, Thomas B. Rauchfuss,' and Scott R. Wilson School of Chemical Sciences, University of Illinois, 505 S. Matthews Avenue, Urbana, Illinois 61 801 Received February 17, 1993
Thermolysisof solutions of (MeCsH4)Ru(PPh3)2EH(E = S, Se) gives the cubane clusters (MeCsH4)4RuqE4. For the case for E = S, the coproducts were shown to be PPh3 and H2. A rational synthesis of PPh4TeH is reported; this salt was employed in the preparation of (MeCsH4)4Ru4Te4. This Ru4Te4 cluster crystallizes in the monoclinic space group C2/c with a = 11.943(6) A, b = 18.623(6) A, c = 12.590(7) A, V = 2792(4) A3, and Z = 8. Structural trends show that the identity of the chalcogen more strongly affects the nonbonding R w R u , R w E , and E-E interactionsthan the bonding interactions. The clusters undergo 2e oxidationsas demonstrated by cyclicvoltammetry studies. Chemical oxidations using (MeCsH&Fe+ gave salts of the dications ( M ~ C S H ~ ) ~ R(E U= ~E ~ +Te). S,~Se, The dicationic S4 and Se4 clusters exhibit dynamic IH NMR properties such that at low temperatures signals for two CH3C5H4 groups were observed while at high temperatures the MeCsH4 groups appear equivalent. On the basis of coalescencetemperatures, the barriers were estimated as -52 kJ/mol. In a test of the possible influence of steric factors on the dynamics, the DNMR properties of the new derivative (MesSiCsH4)4Ru4S4(PF6)2were shown also to be very similar to the MeCsH4 clusters. The Me3SiCsH4 cluster crystallizes in the monoclinic space group P21/c with a = 18.828(2) A, b = 12.421(1) A, c = 21.451(1) A, /3 = 92.442(1)', V = 5012 A3, and 2 = 4. The structure confirms the presence of three Ru-Ru bonding distances. Variable-temperature NMR experiments on (MeCsH4)4Fe4S42+ and (MeC5H4)4Ru4S4gave no evidence of structural dynamics.
Introduction In contrast to the situation for bulk metals, many lowdimensionaland molecular species feature localized metal-metal interactions. The existence of such localized metal-metal interactions has prompted numerous investigations of their structure and stereochemistry. Less studied are the dynamics of metal-metal bonds. Progress on this theme is surveyed in the following paragraphs, beginning with cases of metal-metal bond/ no bond equilibria. The phenomenon of a metal-metal bond/no bond equilibrium is illustrated by Cp2Cr2(C0)6, which spontaneously homolyzes in solution to give the 17especies CpCr(C0)3 (eq 1). The fragility
of Cp*Crz(C0)6 is indicated by the long Cr-Cr bond length of 3.281 A, which arises because of steric crowding.lvz An intramolecular example of metal-metal bond/no bond equilibria was recently proposed by Kolle3 to account for the temperature dependence of the 1H NMR spectrum of (CsRs)2RuzX4, where X = C1, Br. These workers crystallized two forms of the dimer whichdiffer with respect to their Ru-Rudistances (eq 2). Related
~~~~
~
(1) Adams, R. D.; Collins, D. E.; Cotton, F. A. J. Am. Chem. SOC.1974, 96, 749. (2) Leading reference: Hoobler, R. J.; Hutton, M. A.; Dillard, M. M.;
Castellani,M. P.; Rheingold,A. L.; Rieger, A. L.; Rieger,P. H.; Richards, T. C.; Geiger, W. E. Organometallics 1993, 12, 116. (3) KBlle,.U.; Kossakowski, J.; Klaff, N.; Wesemann, L.; Englert, U.; Heberich, G. E. Angew. Chem., Int. Ed. Engl. 1991, 30, 690; Angew. Chem. 1991, 103, 732.
0020-166919311332-4069$04.00/0
singlet-triplet equilibria have been observed for 50e- (CsR5)3C03S2 and 46e- ( C S R ~ ) ~ C O ~ ( CEvidence O ) ~ , ~ for metal-metal bond cleavage involving multinuclear fragments has been presented by Heaton, who employed 195PtNMR spectroscopy to demonstrate site exchangebetween Pt12(C0)a2- and P t ~ ( c 0 ) l ~ ~ (eq 3).5 Other examples of reversible intramolecular metalPt*,,(CO),,"
+ Pt,(C0)l*2- + Pt*,(co),,*-
+ Pt,,(C0),,2-
(3)
metal bond cleavage are depicted in Scheme 1 . H The tetranuclear cluster W4(0iPr)12 exhibitsdynamic behavior as indicated by NMR line broadeningand magnetization transfer experiments. To account for these NMR measurements, Chisholm and co-workers invoke a metallacyclobutadiene-like intermediate which interchanges W-W bonding contacts of 2.50 and 2.73 A (eq 4, R = iPr).9 Highly relevant to our work are the dynamicsof cubane clusters Cp4Fe4SsZ ( z = 0, 2+) and the MeC5H4 analogues.IOJ1 These clusters are derivatives of Cp4Fe4S4 by replacement of one ~ 3 - S (4) Sorai, M.; Kosaki, A.; Suga, H.; Seki, S.; Yoshida, T.; Otsuka, S.Bull. Chem. SOC.Jap. 1971,442364. Pulliam, C. R.; Thoden, J. B.; Stacy, A. M.; Spenctr, B.; Englert, M. H.; Dahl, L. F. J. Am. Chem.Soc. 1991, 113, 7398. Barnes, C. E.; Dial, M. R.; Orvis, J. A.; Staely, D. L.; Rheingold, A. L. Organomerallics 1990,9,1021 and references therein. ( 5 ) Brown, C.; Heaton, B. T.; Chini, P.; Fumagal1i.A.; Longoni, G.J. Chem. Soc., Chem. Commun. 1977, 309. (6) Carmona, D.; Ferrer, J.; Mendoza, A.; Lahoz, F. J.; Reyes, J.; Oro, L. A. Angew. Chem., Int. Ed. Engl. 1991,30, 1171; Angew. Chem. 1991, 103, 1192. (7) Jones,R. A.; Wright,T. C.; Atwwd, J. L.;Hunter, W. E. Organometallics 1983, 2, 470. (8) Bailey, D. A.; Balch, A. L.; Fossett, A.; Olmstead, M. M.; Reedy, P. E., Jr. Inorg. Chem. 1987, 26, 2413. (9) Chisholm, M. H.; Clark, D. L.; Hampden-Smith, M.J. J . Am. Chem. SOC.1989, I l l , 574. (10) Kubas, G. J.; Vergamini, P. J. Inorg. Chem. 1981, 20, 2667. (11) Blonk, H. L.; Mesman, J.; van der Linden, J. G. M.; Steggerda, J. J.; Smits, J. M. M.; Beurskens, G.; Beurskens, P. T.; Tonon, C.; Jordanov, J. Inorg. Chem. 1992,31,962. Blonk, H. L.; van der Linden, J. G. M.; Steggerda, J. J.; Jordanov, J. Inorg. Chfm.Acta 1989,158,239. Other
examplesoflocalizedbondinginFe+Slclustersaredescribedin:Inomata, S.;Tobita, H.; Ogino, H. Inorg. Chem. 1992, 31, 722.
0 1993 American Chemical Society
4070 Inorganic Chemistry, Vol. 32, No. 19, 1993
Scheme 1
Ru-lr distance = 3.66 A
Ru-lr distance = 2.69 A
Houser et al. (MeC5H4)Ru(PPh3)2EH.14The corresponding Cp clusters are virtually insoluble in common organic solvents. In the synthesis of (MeCSH4)4R~4S4 we showed that the coproducts are PPh3 and H2.15 The yield of hydrogen was quantified by conducting the thermolysisreaction in a closed system (eq 6). We also prepared (Me3SiC5H4)4Ru4S4via the same method, starting with (Me3SiCsH4)Ru(PPh3)zSH. The Me3SiCsH4 cluster proved to be extremely soluble in organic solvents.
-
4(RC5H4)Ru(PPh,),EH (RC5H4),Ru4E4 2H,
+
+ 8PPh3 (6)
R = H, Me; E = S,Se, Te A
Rh-Rh distanc&= 2.76 A
The preparation of (MeCsH4)4Ru4Te4was modeled after the corresponding sulfide and selenide clusters, although we did not establish the intermediacy of ( M ~ C S H , ) R U ( P P ~ ~ ) ~The T~H. Ru4Te4and Ru4Se4clusters were characterized by 12sTeand 77Se NMR, which showed a single line. As a tellurium source, we used (PPh4)TeH, whose structure and serendipitousisolation has been described by Haushalter.16 This salt can be prepared in good yield by the addition of PPh4C1 to a partially acidified aqueous solution of NazTe. Orange crystals of (PPh4)TeH are slightly light-sensitiveand highly air-sensitive. The salt is readily Rh-Pd distances = 2.70, 3.1 7 A soluble in polar organic solvents such as acetonitrile, methanol, and DMF. Its solubility characteristics parallel those of PPh41, by p3&. lH NMR studies on Cp4Fe4S5 show a 1:2:1 pattern of with which it is isostructural. The 1H NMR chemical shift for Cp resonances which collapses at higher temperatures to a 1:3 the TeH is -13.4 ppm. pattern indicating that Fe-Fe bonds migrate in step with rotation of the SZgroup (eq 5 ) . Comparabledynamicsoccurin C P , F ~ ~ S ~ ~ + The neutral cubane clusters were doubly oxidized using 2 equiv of (MeC5H4)2FePF6(eq 7, cyclopentadienyl groups omitted). Rh-Rh distance = 3.71
but at higher temperatures. As illustrated in the examples above, when ligand dynamics modify the electron count of a metal center within a cluster, one expects to observe correlated changesin the metal-metal bonding. Conceptually distinct are those examples where dynamics result from redoxeventswithin the~luster.~ This effect is more prevalent for odd-electron ensembles where the structural consequences are often sufficiently subtle that the dynamics can even be observed in the solid state.12 In this paper we provide details13of cluster dynamics resulting from internal redox with the following characteristics: (i) The species are diamagnetic, (ii) the redox events result in substantial structural changes at the affected metals, and (iii) the clusters have been examined under both fastand slow-exchange limits. Results Synthesis of R@4 Cubane Clusters. The R u ~ E (E~= S,Se) clusters are prepared simply by heating toluene solutions of (1 2) Leading references to this active area: Jang, Ho G.; Geib, S. J.; Kaneko, Y.; Nakano, M.; Sorai, M.; Rheingold, A. L.; Montez, B.;Hendrickson, D. N . J . Am. Chem. SOC.1989,1 11, 173. (13) Preliminary account: Houser, E. J.; Amarasekera, J.; Rauchfuss, T. B.; Wilson, S.R. J . Am. Chem. SOC.1991, 113, 7441.
The choice of this oxidant was guided by electrochemical data for the clusters (see below) as well as the high solubility of ( M e c ~ H d ) ~ Fwhich e, facilitates its separation from the ionic products. The dicationic clusters proved to be moderately air stable and were characterized by elemental analyses, mass spectrometry, and *HNMR spectroscopy. The structure of the tetracyanoquinodimethanide, (TCNQ)22-, salt of (MeCsH4)4Ru&*+ has been reported by us previou~ly;~~ this paper presents structural details on the diamagnetic (M ~ ~ S ~ C S H ~ ) ~ R U & ( P F ~ ) ~ . Structure of (MeC&)&%Te4. The structure of (MeCsH4)4Ru4Te4 can be viewed as a nested dimer of (MeC~H4)zRuzTez butterfly subunits each of which features a single Ru-Ru bond and two bridging tellurium atoms (Figure 1). The two bonding Ru-Ru distances average 2.89 A; the four nonbonding Ru...Ru distancesaverage 4.02 A. The metal-metal bonded distance may be compared to 2.76 A for the Ru4S4 ana10gue.l~The nonbonded Rw-Ru contacts are 0.42 A longer than in the Ru4S4 cluster, reflecting the greater size of Te vs S. The average intrabutterfly Te-aTe distance is 3.92 A, while the averageinterbutterfly Te-Te distance is 3.27 A. Intercluster Tea-Te contacts of -4.49 A are (14) Amarasekera, J.; Rauchfuss, T. B. Inorg. Chem. 1989, 28, 3875. Amarasekera, J.; Houser, E. J.; Rauchfuss, T. B.;Wilson, S.R. Inorg. Chem. 1992,31, 1614. Amarasekera, J. Ph.D. Thesis, University of Illinois, Urbana, IL, 1988. (15) Other examples of complexes that remove H2 from H B : Besenyei, G.; Lee, C.-L.;Gulinski, J.; Rettig, S. J.; James, B. R.;Nelson, D. A.; Lilga, M. A. Inorg. Chem. 1987,26,3622.Rabinovich, D.;Parkin, G. J. Am. Chem. Soc. 1991, 113, 5904. (16) Huffman,J.C.;Haushalter,R.C.Pofyhedronl987,8,531. Bjbrgvinsson, M.; Schrobilgen, G. J. Inorg. Chem. 1991, 30, 2540. (1 7 ) (MeC5H+Ru& and (MeC5I&),Ru&: Amarasekera, J.; Rauchfuss, T. B.; Wilson, S.R. J . Chem. Soc., Chem. Commun. 1989, 14.
Inorganic Chemistry, Vol. 32, No. 19, 1993 4071 Table II. Distances (A) and Angles (deg) for
(MesSiCsH4)4Ru4S,(PFs)z and (MeCsH4)&@4(TCNQh
Ru-RU Ru.-Ru RuS Ru-RCp(centroid)
s.-s
2.807,2.797, 2.800(1) 3.53 2.31 1.88 3.25 75.1 98.3 102.0 77.7
2.7848,2.7937,2.7836(6) 3.50 2.3 1 1.87 3.26 74.8 97.7 102.5 78.1
RuS-Ruc Ru-S-Rud S-RuS' S-RuSd a Except for the close Ru-Ru contacts, the angles and distances are averages. Esd's are not included as they are small compared to the averaged values. Reference 13. TCNQ is tetracyanoquinodimethanide. Angles which occur on a Ru-Ru bonded cluster face. Angles which occur on a Ru-Ru nonbonded cluster face.
Figure 1. Structure of (MeCsH4)4Ru4Te4with thermal ellipsoids drawn at the 35% probability level. Table I. Average Distances (A) and Angles (deg) for
(MeC~H4)4RuqE4(E = S,Te)' Ru-Rub Ru...Ruc Ru-Cp (centroid) Ru-Eb Ru-E'
2.76 3.60 1.87 2.31 2.37 2.96 3.45 73.3 100.9 96.4 77.8
2.89 4.02 1.88 2.59 2.65 3.27 3.92 67.9 100.5 98.7 77.4
E..Eb Ru-E-Ru~ Ru-E-Ru' E-Ru-Eb E-Ru-E' Esd's are not included;these are average distanceswith small esd's. b Parameters for (MeCsH&RuzE2 "butterfly" subunitswherein the two Ru atoms are mutually bonded. e Parameters for (MeC~H4)2RuzEz rhombs which feature nonbonded RwRu contacts. From ref 9.
E=Se
2ul T
A
E=Te
E (VOLT)
Figure 3. Cyclic voltammograms of ( M ~ C ~ H ~ ) ~(ER=NS, ESe, ~ Te). See. Table 111 for details.
distances in (MepSiC5H4)4R~4S42+ range 2.267(3)-2.375(2) A with an average R u S distance of 2.31 A, the same as in (MeC5H4)4R~&34(TCNQ)2.For (M~~S~CSH&RU&~+, the RuS-Ru and S-RuS angles on Ru-Ru bonded cluster faces average 75.1 and 102.0°,while those on the faces lacking Ru-Ru bonds W are 98.3 and 77.7'. These parameters are within 2 O of those in F w e 2. Structure of the dication in (MesSiC5H,)rR@4(PF6)z with ( M ~ C S H ~ ) ~ R(Table U ~ S 11). ~~+ thermal ellipsoids drawn at the 35% probability level. Electrochemistry. The cyclicvoltammograms of the MeCSbcontaining clusters were recorded under identical conditions presumed to be unimportant. The structural parameters for (Figure 3). We observed that the average of the two oxidation (MeC~H4)4RudTe4are presented in Table I along with those for potentials (E,& varied by only -30 mV upon changing from (M~CSH~~RU~S~. S to Se to Te (Table 111). The chalcogen exerts a more profound Structure of (Me$3iCJ4)&u&(PF~)~. The dication was effect found to be similar to that found in ( M ~ C S H ~ ) ~ R U ~ S ~ ( T C N Q ) ~ , on A&p, the difference between the first and second oxidation waves. For E = S two distinct processes can be observed as described in a preliminary communication.13 The cubane core 121 mV, while for E = Te only a single with AEl12 is distorted from idealized T d symmetry by virtue of three Ruelectrochemical event is observed with hE, = 94 mV. Ru bonds and three Ru-Ru nonbonding distancesaveraging 2.80 Cyclic voltammetry measurements were also conducted on salts and 3.53A, respectively (Figure 2). The corresponding distances of the doubly oxidized clusters. These cyclic voltammograms in ( M ~ C ~ H & R U ~ S ~ ( T Care N Q2.79 ) ~ and 3.50 A. The R u S
Houser et al.
4012 Inorganic Chemistry, Vol. 32, No. 19, 1993 Table 111. Elj2 Values (mV) for (MeaSiCsH4)4Ru& and MeC
