Znorg. Chem. 1988, 27, 4429-4435
4429
NO3- ion can oxidize the clusters with accessible oxidation pothat considerable multiple-bond character between the rhenium tentials. atoms is maintained in these mixed-metal cluster compounds.This is further supported by the fact that the ESR spectrum of Acknowledgment. This work was supported by the National [AU&(H)6(PPh3)6]+ (I), with respect to g value and hyperfine Science Foundation (Grant CHE-851923) and the donors of the coupling constants, is somewhat similar to that of the oxidized Petroleum Research Fund, administered by the American parent complex prepared by W a l t ~ n . ~ ~ Chemical Society. W e gratefully acknowledge the Johnson Comparison of the present work to the work of Caultonz6and Matthey Co. for a generous loan of gold salts. W a l t ~ nleads ~ ~ us to conclude that the starting gold(1) phosphine complex significantly affects the course of the reaction and deRegistry No. 1(PF6), 107712-43-6; 2(PF6)2, 117226-13-8; 3% termines the final product(s). If an acidic anion is used, then the 3b, 117201-65-7; 4(BF4)2, 117201-66-8; 5(BF4), 117201fully protonated Au2Re2 cluster [ A u ~ R ~ ~ ( H ) ~ ( P (4) P ~ is~ ) ~ ] ~117201-64-6; + 67-9; [(Ph,PAu),O]BF4, 53317-87-6; K(0-t-Bu), 865-47-4; [Fe(Csformed.27 The use of counterions of varying basicity leads to HS)JPF6, 11077-24-0;Re2(H)8(PPh3)4, 66984-37-0; Re, 7440-15-5; Au, clusters A ~ ~ R e ~ ( H ) ~ ( (3) p p or h ~[ )A~U ~ R ~ ~ ( H ) , ( P P (5), ~ ~ ) ~ ] + 7440-57-5; sodium naphthalenide, 3481-12-7. in which or some of the hydride ligands with acidic character have been abstracted. This work shows the noninnocence of the SupplementaryMaterial Available: Figures of cyclic voltametric and NO3- ion in the formation of these gold-rhenium clusters. When spectroelectrochemicalexperiments on [ A U ~ R ~ ~ ( H ) , ( P P(2~ ~pages). )~]+ Ordering information is given on any current masthead page. present in a reaction mixture where H+ is being generated, the
Contribution from the Istituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione, CNR, Via J. Nardi 39, 50132 Florence, Italy, Dipartimento di Chimica dell'universitl di Siena, 53 100 Siena, Italy, and Departamento de Quimica, Universidad de Valencia, Valencia, Spain
Synthesis, Characterization, and Electrochemical Properties of a Family of Dinuclear Rhodium Complexes Containing Two Terminal Hydride Ligands and Two Hydride (or Chloride) Bridges. Stoichiometric and Catalytic Hydrogenation Reactions of Alkynes and Alkenes Claudio Bianchini,*J Andrea Meli,' Franco Laschi,' Josd A. ram ire^,^ Piero Zanello,' and Albert0 Vacca' Received March 31, 1988 Protonation by strong acids and thermal decomposition in solution are two routes by which the trihydride (triphw)RhH, (1) [triphos = MeC(CH2PPh2),] is used to synthesize the tetrahydrido complexes [(triph~s)RhH(p-H)~HRh(triphos)](BPh4)2(2) and [ (triph~s)RhH(pH)~HRh(triphos)] (3), respectively. The bis(pch1oro) dihydride [(triphos)RhH(p-Cl)2HRh(triphos)] (BPh4)2 (6) can be prepared either by protonation of (triphos)RhC1(C2H4)followed by NaBPh4 addition or by H/Cl exchange between 2 and CH2C12. Interestingly, 6 exists in solution as a 1:1 mixture of two geometric isomers. The electrochemical behavior of the tetrahydride derivatives in nonaqueous solvents shows that they can reversibly undergo one-electron-redoxchanges with no change of the primary geometry. By contrast, 6 is unable to reversibly accept or lose electrons. Electrochemical techniques have been used to generate the paramagnetic [(triphos)RhH(p-H),HRh(triphos)]+ derivative, which is not directly obtainable by chemical methods. All of the compounds have been fully characterized by IR, NMR, and ESR techniques. Both the mononuclear trihydride 1 and the dimeric tetrahydride 2 are able to straightforwardly transfer hydrogen atoms to unsaturated substrates such as 3,3-dimethylbut-l-ene,dimethyl maleate (DMMA), or dimethyl acetylenedicarboxylate (DMAD). The effectiveness of 2 and 6 to catalytically hydrogenate DMAD and DMMA is investigated and compared to that shown by the mononuclear species [(triphos)Rh(r-DMAD)]BPh4 and [(triphw)Rh(r-DMMA)]BPh4 as well as a family of homo- and heterobimetallic (pH), complexes of formula [(triphos)Rh(p-H),M(triphos)]"+ (M = Rh, Co; n = 3, 2). All of the compounds prove active catalysts or catalyst precursors for hydrogenation reactions of DMAD and DMMA. The catalyzed alkyne hydrogenation yields largely the olefin. In the catalytic cycles some of the binuclear compounds are resistant to fragmentation and are responsible for the catalysis.
Introduction Polynuclear PlYhYdrides4 are of interest because of their effective role in homogeneous catalytic hydrogenations and their
ability to model surface chemistryes A perusal of the large body of experimental information on filynuciear polyhydridesreveals the Daucitv of electrochemical data on these comDounds6as well (5) (a) Parshall, G. W. Homogeneous Cafalysis;Wiley-Interscience: New
(1) CNR. (2) UniversitH di Siena. (3) Universidad de Valencia.
(4) (a) Moore, D. R.; Robinson, S . D. Chem. Soc. Rev. 1983,12,415. (b) Hlatky, G. G.; Crabtree, R. H. Coord. Chem. Reu. 1985, 65, 1. (c) Soloveichik, G. L.; Bulydev, B. M. Russ. Chem. Reu. 1982,51,286. (d) Bau, R.; Carroll, W. E.; Hart, D. W.; TeIler, R. G.; Koetzle, T. F. In Transition Metal Hydrides; Bau, R.,Ed.; Advances in Chemistry 167;, American Chemical Society: Washington, DC, 1978; p 73.
York, 1980. (b) Jamts, B. R. Homogeneous Hydrogenation; WileyInterscience: New York, 1973. (c) Maitlis, P. M. Acc. Chem. Res. 1978, 11, 301. (d) Sivak, A. J.; Muetterties, E. L. J . Am. Chem. SOC. 1979, 101, 4878. (e) Crabtree, R. H; Felkin. H.; Morris, G. E. J. Organomet. Chem. 1977, 141, 205. (f) Burch, R. R.; Muetterties, E. L.; Day, V. W. Organometallics 1982, 1, 186. (g) Chaudret, B.; Devillers, J.; Poilblanc, R. Ibid. 1985, 4, 1727. (h) Wan& H.-H.; Casalnuovo, A. L.; Johnson, B. J.; Mueting, A. M.; Pignolet, L. H. Inorg. Chem. 1988, 27, 325.
0020-1669/88/1327-4429$01 .50/0 0 1988 American Chemical Society
4430 Inorganic Chemistry, Vol. 27, No. 24, 1988
Bianchini et al.
Scheme I
B 'red'UA
2 5
3 as the scarcity of systematic comparisons of the catalytic activity of polymetallic species vs related mononuclear complexes. It is with the synthesis, characterization, and electrochemistry of a family of polyhydrido dirhodium complexes and with their hydrogenation reactions of dimethyl maleate (DMMA) and dimethyl acetylenedicarboxylate (DMAD) that the present paper is largely concerned. We also compare the catalytic activity of bimetallic species with that of related mononuclear complexes. A preliminary account of part of this work has already appeared.' Results Dihydrido-Bridged Complexes. In addition to the donor-acceptor reaction pathway recently reported by usk and other authors? the trihydride (triphos)RhH, (1)9 can function as precursor to polynuclear polyhydride derivatives by two additional routes, namely (i) protonation by strong acids, followed by NaBPh4 addition and (ii) thermal decomposition in T H F (Scheme I). In both cases, the system evolves H2 through reductive elimination reactions. The yellow orange, terminal-bridged tetrahydrido complex [(tripho~)RhH(p-H)~HRh(triphos)] (BPh4)2(2) is obtained by treatment of 1 in CH2C12or THF with HOS03CF3, followed by addition of NaBPh4 in ethanol. Compound 2 is diamagnetic, quite stable in the solid state and in deoxygenated DMF, CH2C12,and EtNO, solutions in which it behaves as a 1:2 electrolyte. The presence of terminal hydride ligands is evidenced by a strong IR absorption at 1980 cm-'. The compound is stereochemically nonrigid in solution; the bridged-terminal interconversion of the four hydride ligands is rapid on the N M R time scale to -60 OC so that each hydrogen atom appears magnetically equivalent with all rhodium and phosphorus atoms. As a matter of fact, the 'H NMR spectrum (CD2C12)in the hydridic hydrogen region exhibits an unresolved multiplet at 6 -10.6 (4 H) also at low temperature, and the 31P{1H1 NMR (DMF, 203 K) spectrum consists of a broad doublet centered at 25.24 ppm (Jm = 90 Hz). A dimeric complex strictly related to 2 is the neutral product obtained by refluxing 1 in THF (Scheme I). The liver red complex, of formula [(tripho~)RhH(p-H)~HRh(triphos)](3), is diamagnetic and very poorly soluble in common organic solvents so as to preclude a meaningful characterization by spectroscopic (a) Allison, J. D.; Walton, A. J . Am. Chem. Sot. 1984, 106, 163. (b) Moehring, G. A.; Walton, R. A. J . Chem. Soc., Dalton Trans. 1987, 715. (c) Bianchini, C.; Meli, A.; Zanello, P. J . Chem. SOC.,Chem. Commun. 1986.628. (d) Klingher, R. J.; Huffman, J. C.; Kochi, J. K. J . Am. Chem. SOC.1980, 102, 208. Bianchini, C.; Mealli, C.; Meli, A.; Sabat, M. J. Chem. Sot., Chem. Commun. 1986, 777. (a) Rhodes, L. F.; Huffman, J. C.; Caulton, K. G. J . Am. Chem. SOC. 1983, 105, 5137. (b) Rhodes, L. F.; Huffman, J. C.; Caulton, K. G. Zbid. 1984, 106, 6874. (c) Rhodes, L. F.; Huffman, J. C.% Caulton, K. G. Zbid. 1985, 107, 1759. (d) Lehner, H.; Matt, D.; Togni, A,; Thouvenot, R.; Venanzi, L. M.; Albinati, A. Znorg. Chem. 1984, 23, 4254. (e) Albinati, A.; Lehner, H.;Venanzi, L. M. Ibid. 1985,24, 1483. (f) Geerts, R. F.; Huffman, J. C.; Caulton, K. G. Zbid. 1986, 25, 590. (9) Albinati, A.; Emge, T. J.; Koetzle, T. F.; Meille, S. V.; Musco, A.; Venanzi, L. M. Zbid. 1986,25, 4821. (h) Lemmen, T. H.; Huffman, J. C.; Caulton, K. G. Angew. Chem., Znf.Ed. Engl. 1986, 25, 262. (i) Abrahams, S. C.; Ginsberg, A. P.; Koetzle, T. T.; Marsch, P.; Sprinkle, C. R. Inorg. Chem. 1986, 25, 2500. (9) Ott, J.; Venanzi, L. M.; Ghilardi, C. A,; Midollini, S.; Orlandini, A. J . Organomet. Chem. 1985, 291, 89.
5 ox
NPC
E/V
V
D
Figure 1. Cyclic voltammogram recorded at a platinum electrode on a MeCN solution containing 2 (5.0 X IO4 mol cm-)) and [NEtJClO, (0.1 mol d d ) . Scan rate: 0.2 V s-I. H
100
1
-
DPPH
I;
Figure 2. X-Band ESR spectrum at 300 K of electrogenerated [(triphos)RhH(p-H),HRh(triphos)]+ in MeCN.
techniques. The IR spectrum (Nujol mulls) exhibits a strong v(Rh-H) vibration a t 1960 cm-'. There is no doubt that 2 and 3 form a two-electron redox couple as demonstrated by electrochemical and chemical investigations. Figure 1 reports the cyclic voltammetric response exhibited by 2 in deaerated MeCN solution. Two successive reduction processes are displayed (peaks A and B), each of which shows a directly associated reoxidation response in the reverse scan (peaks D and C, respectively). Controlled-potential coulometric tests reveal that each step involves a one-electron process. Analysislo of the cyclic voltammetric peak system A/D with scan rates, u, varying from 0.02 to 50 V s-l shows that (i) the ipe/ipcratio is constantly equal to unity, (ii) the i , d 2 term is constant, and (iii) the difference Epa- E , = AE,is equal to 60 mV up to 2 V s-l, then gradually increases up t o 150 m V at 50 V s-l, which is likely due to some uncompensated solution resistances. The same features are displayed by the second reduction process. These data indicate that the redox activity of the complex cation [RhH(p-H)2HRh]Z+is consistent with the two uncomplicated one-electron reduction steps: [RhH(fi-H)'HRh]
'+
