Organometallics 1983, 2, 470-472
470
02
Reversible Metal-Metal Bond Cleavage Accompanied by a Geometrical Isomerism. Synthesis and Crystal Structures of Isomers of [Rh(p-t-Bu,P)(CO),],. Catalysis of Alkene Hydroformylation Richard A. Jones" and Thomas C. Wright Department of Chemistry, University of Texas at Austin Austin, Texas 78712
Jerry L. Atwood" and William E. Hunter Department of Chemistry, University of Alabama University, Alabama 35486 Received October 6, 1982
W
03
Summary: The X-ray crystal structures and spectroscopic data for two isomers of [R~(JL-~-BU~P)(CO),]~ show that facile reversible formation and cleavage of the RhRh bond is accompanied by a simple geometrical isomerization. Rh-Rh distances are 2.7609 (9) A vs. 3.717 (1)
A. Conditions that result in the making or breaking of metal-metal bonds are of key interest to the chemistry of dinuclear and cluster complexes.' In some cases it has been possible to demonstrate the reversible cleavage or formation of a metal-metal bond on addition or removal of a small molecule such as CO. Examples include the heterometallic phosphinidene cluster CpMn(CO),Fe2(CO),(p3-PPh) that will reversibly bind CO or PPh3 accompanied by Mn-Fe bond opening and closing.2 Also, reversible Ru-Ru bond fission for Ru3 carbonyl clusters has recently been dem~nstrated.~ -co
Ru~(CO),(CECR)(PP~,) "open"
04
Figure 1. View of isomer 1 viewed from Rh2 along Rh-Rh bond. Key parameters are in ref 10.
can be isolated by using column chromatograph9 followed by separation via fractional crystallization from hexane. Unlike the reaction of LiPPh, with [Rh(CO),Cl], that yields [Rh4(C0)5(p-PPh2)5]-,9the new compounds are neutral and hexane soluble. Although 1 and 2 normally cocrystallize from hexane, by varying the conditions, it is possible to increase the amount of either isomer that is obtained. Thus, if either pure crystalline 1 or 2 is dissolved in hexane and the concentrated (0.015 M) solution cooled as rapidly as possible to -35 "C, small orange crystals of 2 are obtained in good yield. However, if the same solution is allowed to cool slowly over ca. 12 h to -35 "C large, deep red crystals of 1 are the major crystalline product. The solid-state structures of these two isomers have been determined by X-ray crystallography.1° In the deep red isomer 1 (Figure 1)the terminal carbonyls give one rhodium atom (Rh2) a planar coordination as expected for a four-coordinate Rh(1) atom. However, the other rhodium (Rhl) has two carbonyls in the plane perpendicular to that (8)Elution of an alumina column with hexane yields an orange band
from which 1 and 2 may be isolated by fractional crystallization. A deep Here we describe a system that, to our knowledge, is at red band is then obtained from which the trimer (Rh(p-t-Bu2P)COJ3 may present unique. The key feature is the facile reversible be isolated in moderate overall yield (30% from [Rh(CO)2C1]2).5 Both formation and cleavage of a metal-metal bond that occurs 1 and 2 gave satisfactory elemental analyses: yields 1 and 2, 65%; mp (1) 153-155 OC dec, (2) 138-140 OC dec. Spectroscopic data for the with a simple geometrical isomerism. isomeric mixture of 1 and 2: IR (CeH14) 2038 (s), 1987 (s), 1952 ( 8 ) cm-' As part of a program aimed at the study of steric effects (KBr disk), 2025 (s), 1991 (w), 1954 (81, 1923 ( 8 ) cm-'; 'H NMR (C6D6, in transition-metal phosphido c ~ m p l e x e swe , ~ have in90 MHz, 35 "C) 6 1.3 (m); 31P(1H)NMR (in toluene-dy at 32.384 MHz, vestigated the reaction of [Rh(CO),Cl], with ~ - B U ~ P L ~ relative . ~ t o 85% H3P04at ambient temperature) 6 402.80 (t, J ~ h - p= 102 Hz,at -80 "C doublet of doublets IJm-p = 128.2 and 67.1 Hz); lsC(lH) This reaction yields a mixture of complexes including NMR (C6D , 20.2 MHz, 35 "C (relative to Me4Si (6 0.0)) 6 199.8 (doublet [Rh(p.-t-Bu2P)(CO),lzfor which two crystalline isomers 1 of triplets (LCO enriched), CO, = 75.1 Hz, 2 J p =~ 10.7 Hz), 45.93 and 2 can be isolated. The relative amounts of each isomer (s, PCMe3), 34.31 (s, PCMeJ. At -80 O C the doublet of triplets split into two distinct signals at 6 202.5 (d, 'JmI< = 83.8 Hz)and 6 194.73 (doublet isolated from solution depend critically on the conditions x Hz, = 47.5 Hz,2 J p ~ c ~ ~ < of doublets of doublets, ' J R ~ =~ 71.6 employed for crystallization. We describe here the syn= 13.4 Hz). thesis and crystal structures of these isomers. (9) Kreter, P. E., Jr.; Meek, D. W.; Christoph, G. G. J. Organomet. Chem. 1980,188, C27. Also poorly characterized [(C0)2RhPh2PIn( n = Interaction of [Rh(CO)zC1]26with 2 molar equiv of 3 or 4) has been reported: Hieber, W.; Kummer, R. Chem. Ber. 1967,100, LiPt-Bu: in tetrahydrofuran at low temperature (-78 OC) 148. gives a deep red solution containing a mixture of com(10) For 1 the space group is monoclinic E1 with a = 9.747 (4) A, b = 12.834 (5) A,c = 11.212 (5) A,,9 = 122.38 (4)", and Z = 2. Leashquares pounds. A mixture of the two dinuclear isomers 1 and 2 (1)See: Balch, A. L. In "Reactivity of Metal-Metal Bonds"; Chisholm,
M. H. Ed.; American Chemical Society: Washington, DC, 1981; ACS Symp. Ser. No. 155, p 166. (2) Huttner, G.; Schneider, J.; Miiller, H. D.; Mohr, G.; vonSeyerl, J.; Wohlfahrt, L. Angew. Chem., Int. Ed. Engl. 1979, 18, 76. (3) Carty, A. J.; MacLaughlin, S. A.; Taylor, N. J. J. Organomet. Chem. 1981,204, C27. (4) Jones, R. A.; Stuart, A. L.; Atwood, J. L.; Hunter, W. E.; Rogers, R. D. Organometallics 1982, I, 1721. (5) Atwood, J. L.; Hunter, W. E.; Jones, R. A,; Wright, T. C. Inorg. Chem., in press. (6) McCleverty, J. A.; Wilkinson, G. Inorg. Synth. 1966, 8, 211. (7) Hoffman, H.; Schellenbeck, P. Chem. Ber. 1966, 99, 1134.
refinement based on 2303 measured (1634 observed) reflections gave a final R = x(IFol - IFcI)/xIFol = 0.026 and R, = 0.030. All the non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms on the methyl groups were located, but not refined. For 2 the space group is monoclinic C2/m with a = 11.999 (4) A, b = 11.139 (4) A,c = 10.927 (4) A,,9 = 120.18 (4)O, and Z = 2. Refinement with 1133 observed reflections gave RI = 0.043 and R, = 0.057. The non-hydrogen atoms were refined with anisotropic thermal parameters. Key bond lengths (A)and angles (deg) are as follows. 1: Rh-Rh 2.7609 (9), Rh(1)-P(2) = 2.244 (2), Rh(2)-P(2) = 2.458 (2) A; Rh(l)-P(l)-Rh(a) = 72.02 (7), C(18)-Rh(l)-C(17) = 111.6 (4), C(29)-Rh(2)-C(19) = 89.2 (4)'. 2: Rh-Rh = 3.717 (l), Rh(1)-P(l) = 2.418 (l), Rh(l)-C(l) = 1.880 (7) A; Rh(1)-P(1)-Rh(1') = 100.49 (6), C(l)-Rh(l)-C(l') = 90.3 (3), P(1kRh(l)-C(l) = 174.5 (2)'.
0276-7333/S3/2302-0470~0~.50/0 0 1983 American Chemical Society
Organometallics, Vol. 2, No. 3, 1983 471
Communications
Figure 2. View of isomer 2 viewed along the Rh-Rh bond. Key parameters are in ref 10.
(1)
Figure 3.
.2
-
.3
-
ABS.
4 .5
-
d-
V 4
450
400
350
300
WAVELENGTH (nm)
Figure 4. Visible absorption spectra of [Rh(p-t-Bu2P)(CO),]2 in 7.5 X 10" M toluene solution: a, 53 "C; b, 48 "C; c, 38 "C; d, 25 "C.
formed by the Rh2P2core and thus has an unusual tetrahedral environment. The Rh-Rh distance is 2.7609 (9) A, well within the limits expected for a single rhodium-rhodium b0nd.l' The di-tert-butylphosphido bridges are notably asymmetric with the Rh-P distances to the "tetrahedral" (Rhl) end of the molecule being significantly shorter than those to the "planar end" by an average of ca. 0.21 A. The bonding parameters for the central RhzPz core are therefore similar to those found for (COD)Rh(pPh2P)2Rh(PEt3)2(Rh-Rh)12 (see below). In the orange-yellow isomer 2 (Figure 2) all the terminal carbonyls as well as the rhodium and phosphorus atoms lie in the same plane giving both rhodium atoms planar (11)The Rh-Rh bond falls near the center of the normal range for Rh-Rh single bonds; Cowie, M.; Dwight, S. K. Imrg. Chem. 1980,19,209. (12)Meek, D.W.; Kreter, P. E.; Christoph, G. G. J.Orgummet. Chem. 1982,231,C53.
environments. The molecule resides on a 2 / m symmetry site resulting in equal Rh-P bond distances. A major structural difference between 1 and 2 is that the Rh-Rh separation in 2 is now 3.717 (1)A, no longer a reasonable distance for a bonding interaction. The planar dimer 2 thus resembles the planar ptZ(PPh2)2(diphos)~+ cation that has a similar nonbonding Pt-Pt distance of 3.699 (1)A.13 The interesting feature of this system is that a metalmetal bond appears to be reversibly formed and cleaved accompanied merely by a geometrical isomerism: distortion in the bridging phosphido groups and a twist through 90° by two CO groups (Figure 3). The two isomers must be very close in energy for them to interconvert so easily. It is unusual that either one can be so easily and selectively isolated by altering the rate of cooling of solutions. The apparent planarltetrahedral equilibrium for one end of the dinuclear rhodium system is similar to that found for a number of mononuclear nickel(1) systems.14 Although in the latter case metal-metal bond cleavage and formation are not involved. In hydrocarbon solutions 1 and 2 give identical 'H, 31P(1H],and 13C(lH]NMR and IR spectra. The IR (hexane) shows terminal CO vibrations.8 The 31P(1H1NMR in toluene-d8 shows a triplet at room temperature (6 402.8 relative to H3P0,; 'JRh-p = 102 Hz) that is split into a doublet of doublets at -80 OC (lJRh-p = 128.2, 67.1 Hz). Surprisingly both signals have identical chemical shifts. The carbonyl region of the l3CI1HI NMR (13C0 enriched, 50.3 MHz) consists of two signals at -80 OC; a doublet at 6 202.5 ( l J R h 4 = 83.8 Hz) and a doublet of doublets of doublets at 6 194.73 (lJRh2-C = 71.6 ' J p x = 47.5, 13.4 HZ). On warming, at -20 "C the resonances collapse into two broad humps that merge into a single broad resonance at 0 "C. Further warming to +50 O C yields a sharp doublet of triplets 6 199.8 (l&h< = 75.1 Hz, 'JPx= 10.7 Hz). Hydrocarbon solutions of the isomeric mixture are thermochromic. At -80 O C they are deep red and become increasingly lighter on warming until at +50 O C they are pale orange-yellow. The UV-visible spectra (300-500 nm in toluene) from +25 to +53 OC clearly show three isosbestic points (Figure 3), indicating that the species responsible for the absorptions in this region are in equilibrium with each other.15 The spectroscopic data are consistent with a temperature-dependent equilibrium between an unsymmetrical species that is the major component in solution at -80 "C and a symmetrical isomer that predominates at ambient temperatures and above. Although 13C(lH]data are consistent with these species being similar to 1 and 2, the 31P(1H]data appear to contradict this straightforward explanation since in the solid state 1 and 2 have notably different M2P2framework geometries. Thus the species in solution may not necessarily be identical with those found in the solid state. Previous studies on some Ph2Pbridged systems have shown correlations between low-field 31P chemical shifts and the presence of metal-metal bonding and acute M-P-M angles.16 Larger M-P-M angles and no metal bonds were found with 31Pshifts to much higher fields. However, Garrou has noted that not
(13)Carty, A. J.; Hartstock, F.; Taylor, N. J. Inorg. Chem. 1982,21, 1349. (14)See: Cotton, F. A.; Wilkinson, G. "Advanced Inorganic Chemistry"; Wiley: New York, 1980,p 793. (15)Recorded on a Cary 17 instrument: at 25 "C, A- 421 nm (e 6800), 363 (e 3360);a t 53 "C, ,A, 421 nm (e 4560),363 ( e 4200). Unfortunately a t present our equipment cannot be used below ca. 15 "C. (16)Carty, A. J. Adu. Chem. Ser. 1982,No. 196,163.
47 2
Organometallics 1983, 2, 472-474
enough data have been accumulated to allow unequivocal structural assignments to be made solely on this basis.17 The data that we have accumulated so far for a considerable number of p-t-Bu,P complexes also permit no simple correlations to be made.ls In any case one would expect considerably different 31Pchemical shifts for species with different MzP2framework geometries such as 1 and 2. One explanation for the 31Pdata is that all of the species observed by NMR in solution have very similar M2P2 frameworks. The observed spectral changes would then be only due to changes in carbonyl geometry that should not greatly affect the 31Pchemical shifts. If this is so then the formation or cleavage of the Rh-Rh bond in 1 and 2, respectively, could be taking place only on crystallization from solution. Further speculation on the species in solution is unwarranted at this stage. Although few other dinuclear rhodium bis(phosphid0) complexes have been reported,lg there is some similarity between the structure of isomer 1 and that of the complex (COD)Rh(p-Ph,P),Rh(PES), recently reported by Meek.12 Like 1, this complex contains both "tetrahedral" and "planar" rhodium atoms, a rhodium-rhodium bond, and two notably asymmetric phosphido bridges. The bonding has been described as arising from a [Rh(p-Ph2P),(PEtJ2]anion chelating to a [(COD)Rh]+cation with a donor bond between the rhodium atoms. It is possible that the bonding in 1 could be described in similar terms. However, the structure 1 is most unusual since all four terminal ligands (CO) are identical. We are currently investigating the structures of 1 and 2 and related complexes using theoretical calculations.20 In solution [Rh(p-t-Bu2P)(C0),l2readily undergoes 13CO/12C0exchange21indicating that the CO ligands are quite labile. The presence of labile CO ligands suggests the possibility of catalytic activity. [Rh(p-t-Bu2P)(C0)212 does not catalyze hydrogenation under mild conditions; however, it does form a catalytically active system for hydroformylation of hex-l-ene.22 There appears to be no apparent decomposition. I t has been noted that some catalytic systems involving triphenylphosphine complexes are deactivated by the formation of p-Ph2P compounds.23 However, some Ph,P-bridged rhodium complexes are active hydrogenation cataysts,lg and other examples of catalysis by phosphido-bridged complexes are k n ~ w n . ' ~ , ' ~ The rhodium p-t-Bu2Psystem remains catalytically active at elevated temperatures even for prolonged periods.23 Under these conditions new phosphido-bridged species are formed and they are currently under investigation.
Acknowledgment. We thank the Dow Chemical Co., Midland, MI (R.A.J.) and the National Science Foundation (J.L.A. and R.A.J.) for financial support. We thank Johnson-Matthey Ltd. for a loan of RhC13.xH20. We also thank Professor D. W. Meek, The Ohio State University, for a copy of his paper on diphenylphosphido-bridged rhodium complexes prior to publication. Supplementary Material Available: Tables of final fractional coordinates, anisotropic thermal parameters, and bond distances and angles for compounds 1 and 2 and listings of structure factor amplitudes for 1 and 2 (23 pages). Ordering information is given on any current masthead page.
(24) See: Ryan, R. C.; Pittman, C. U.; Connor, J. P. J.Am. Chem. Sac. 1977,99, 1986 for hydroformylation using Co,(CO)lo(p-PPh)z. See also the following references for recent examples of stoichiometric reactions related to homogeneous catalysis: Carty, A. J.; Taylor, N. J.; Smith, W. F. J. Chem. SOC., Chem. Commun. 1979,750. Carty, A. J.; MacLaughlin, S.A,; Taylor, N. J. J. Am. Chem. SOC. 1981, 103, 2456. Breen, M. J.; Duttera, M. R.; Geoffroy, G. L.; Novotnak, G. C.; Roberta, D. A.; Shulman, P. M.; Steinmetz, G. R. Organometallics 1982, I, 1008. MacLaughlin, S. A.; Carty, A. J.; Taylor, N. J. Can. J. Chem. 1982, 60, 87. Carty, A. J. Pure Appl. Chem. 1982,54, 113.
A Novel Palladium Complex with Iron-Palladium Dative Bondlng Derived from 1,2,3-Trlthia[3)errocenophane, (Ph,P)PdFe( SC,H,),~0.5CBH,CH, Dletmar Seyferth,' Barry W. Hames,' and Thomas G. Rucker2 Department of Chemistry Massachusetts Institute of Technology Cambridge, Massachusetts 02 139
Martin Cowie' and Raymond S. Dickson Department of Chemistry, The University of Alberta Edmonton, Alberta, Canada T6G 2G2 Received November 3, 1982
(17) Garrou, P. E. Chem. Reu. 1981,81, 229 and references therein. (18) Fof example, in [R~(~-(~-BUCH~),P)(CO)~]~, which has a planar structure simllar to 2 and a nonbonding Rh-Rh distance of 3.741 A,S(31P) equals 260 ppm while in [ I ~ ( ~ - ~ - B U , P ) ( Cwith O ) ~ ]two ~ 'tetrahedral" cobalt atoms and a short Ir-Ir separation of 2.544 A 6(31P)equals 262 ppm. (19) Kreter, P. E.; Meek, D. W. Znorg. Chem. in press. (20) In collaboration with Professor T. A. Albright, University of Houston. (21) On treatment of either 1 or 2 with 13C0 (2 tm, 30 min) a t room temperature in hexane new IR bands are observed for 1 a t 2000 (w), 1939 (s), 1928 (s), and 1895 (m) cm-'. The change is reversed on treatment with 'ZCO. (22) Typical conditions: 1 or 2 (0.034 mmol) and hex-1-ene (5.0 mL, 40.0 mmol) stirred in a glass pressure vessel under CO/Hz (l:l), 2 atm, and ambient temperature. After 10 h, l-heptanal(4870) and 2-methylhexanal (45%) are formed (GLC, mass spectroscopy) in approximately 95% conversion. We have so far not optimized the conditions for yield or product ratio. (23) For example, RhH(CO)(PPh3)3decomposes in solution on heating, giving p-Ph,P complexes; see ref 9 above and also ref 9 in: Kurtev, K.; Ribola, D.; Jones, R. A.; Cole-Hamilton, D. J. Wilkinson, G. J. Chem. SOC.Dalton Trans. 1980, 55.
0276-7333183/2302-0472$0l.50 J O
Summary: The reaction of Pd(PPh,), with 1,2,3-trithia[3]ferrocenophane affords the unusual heterobinuclear species (Ph,P)PdFe(SC,H,),. An X-ray structure determination of this complex as the hemitoluene solvate reveals a dative Fe-Pd bond (2.878 (1) A).
The insertion of low-valent, coordinatively unsaturated species of type L2M (M = Ni, Pd, Pt; L = tertiary phosphine) into the S-S bond of (p-S2)Fez(CO)ewas reported Such insertion into S-S bonds of organic disulfides is a characteristic reaction of such transition(1) Natural Sciences and Engineering Research Council (Canada) Postdoctoral Fellow, 1981-1982. (2) Undergraduate (UROP) research participant. (3) Seyferth, D.; Henderson, R. S.; Gallagher, M. G. J . Organomet. Chem. 1980,193, C75. (4) Day, V. W.; Leach, D. A.; R a u c h , T. B. J.Am. Chem. SOC.1982, 104, 1290.
0 1983 American Chemical Society
