Organometallics 1996, 14, 5469-5471
5469
Novel Example of Thermally Induced Metal-Metal Bond Homolysis in a Bimetallic Fulvalene Complex, the New Dichromium Compound ($%f-C 10HB)Cr2(C0)4(PMe2Ph)2 Istvan Kovacst and Michael C. Baird* Department of Chemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6 Received September 8, 1 9 9 6 Summary: Treatment of the fulvalene compound FvCrz(CO)4(PMezPh),& with 2 molar equiv of trityl radicals (Ph3c') results i n hydride hydrogen atom abstraction and formation of triphenylmethane and the new chromium -chromium-bonded dimer trans-FvCrz(CO)dPMezPh)z (4). Chemical reactivity patterns and variabletemperature IR and ' H , 13C('H), and 31P{1H}N M R investigations of 4 suggest that this 18-electron compound exists in thermal equilibrium with its 17-electron, biradical isomer PMeZph(CO)zCr(~-Fv)Cr(CO)ZpMe~Ph, at room temperature. The chemistry of 17-electron organotransition-metal radicals continues t o attract interest,l with particular attention being paid in recent years to the cyclopentadienylchromium carbonyl system, in which the 18electron chromium-chromium-bonded dimer exists in facile thermal equilibrium with the corresponding 17electron monomer (eq l ) . l a ~ ~ , ~ [CpCr(CO),],
* 2CpCr(CO),'
(1)
It has also been found that bulky substituents, either on the Cp ligand (Me, Ph) or in place of a CO ligand (phosphines, phosphites) greatly enhance the extent of homolysis of the chromium-chromium bond.1b,2d-h,3 + NATO Science Fellow; Research Group for Petrochemistry of the Hungarian Academy of Sciences, Veszprbm, Hungary. Abstract published in Advance ACS Abstracts, November 1,1995. (1)Some most recent advances: (a)Huber, T. A,; Macartney, D. H.; (b) Richards, T. C.; Geiger, Baird, M. C. Organometallics 1995,14,592. W. E.; Baird, M. C. Organometallics 1994,13,4494. (c) Kuksis, I.; Baird, M. C. Organometallics 1994,13,1551.(d) Koeslag, M. D.; Baird, M. C. Organometallics 1994,13,11. (e) Creutz, C.; Song, J.-S.;Bullock, (0Kiss, G.; Nolan, S. P.; Hoff, C. R. M. Pure Appl. Chem. 1995,67,47. (g) Balla, J.;Bakac, A.; Espenson, D. Inorg. Chim. Acta 1994,227,285. J. H. Organometallics 1994,13, 1073. (h) Zhu, 2.; Espenson, J. H. Organometallics 1994,13, 1893.(i) Atwood, C. G.; Geiger, W. E. J. Am. Chem. SOC.1994,116,10849. (j) Milukov, V.A,; Sinyashin, 0. G.; Ginzburg, A. G.; Kondratenko, M. A,; Loim, N. M.; Gubskaya, V. P.; Musin, R. 2.; Morozov, V. I.; Batyeva, E. S.; Sokolov,V. I. J . Organomet. Chem. 1995,493,221. (k) Huang, Y.; Carpenter, G. B.; Sweigart, D. A.; Chung, Y. K.; Lee, B. Y.Organometallics 1995,14,1423.(1) Avey, A.;Nieckarz, G. F.; Keana, K.; Tyler, D. R. Organometallics 1995,14, 2790. (m) Lang, R. F.; J u , T. D.; Kiss, G.; Hoff, C. D.; Bryan, J. C.; 1994,116,7917.(n) Tilset, M. Inorg. Kubas, G . J. J . Am. Chem. SOC. Chem. 1994,33,3121.For reviews of earlier results, see: (0)Baird, (p) Trogler, W. C., Ed. Organometallic M. C. Chem. Reu. 1988,88,1217. Radical Processes; J. Organomet. Chem. Library 22; Elsevier: Amsterdam, 1990;p 49. (2)(a)Adams, R.D.; Collins, D. E.; Cotton, F. A. J. Am. Chem. SOC. 1974,96,749.(b) Madach, T.; Vahrenkamp, H. Z.Naturforsch. 1979, 1988,110,643. (d) Goh, 34B,573.(c) McLain, S.J. J . Am. Chem. SOC. L. Y.; Lim, Y. Y. J . Organomet. Chem. 1991, 4d2,209.(e) Fortier, S.; Baird, M. C.; Preston, K. F.; Morton, J. R.; Ziegler, T.; Jaeger, T. J.; Watkins, W. C.; MacNeil, J. H.; Watson, K. A,; Hensel, K.; LePage, Y.; Charland, J.-P.; Williams, A. J. J . Am. Chem. SOC.1991,113,542. (0Watkins, W. C.; Jaeger, T.; Kidd, C. E.; Fortier, S.; Baird, M. C.; Kiss, G.; Roper, G. C.; Hoff, C. D. J . Am. Chem. SOC. 1992,114,907. (g) O'Callaghan, K. A. E.; Brown, S. J.; Page, J . A.; Baird, M. C.; (h)Yao, Richards, T. C.; Geiger, W. E. Organometallics 1991,10,3119. Q.; Bakac, A.; Espenson, J. H. Organometallics 1993, 12,2010. (i) McConnachie, C. A.; Nelson, J. M.; Baird, M. C. organometallics 1992, 11, 2521. @
Q276-7333l95I2314-5469$09.QQlQ
Surprisingly, however, while the metal-metal bonds in the very similar fulvalene complexes FvMz(C0)s (M = Cr, Mo, WI4 are all longer than that of [CpCr(C0)31~,~" there has been reported no direct evidence for homolysis of the metal-metal bonds in these fulvalene complexes (eq 21, either thermally or phot~chemically.~~,~
We therefore decided t o investigate the possible effects of CO substitution by bulky phosphines in these compounds and have recently reported the preparation and properties of several (fulva1ene)dimolybdenumcarbonyl dimers, FvMoz(C0)4Lz (L = PPh3, PCy3 (Cy = cyclohexyl), PXy3 (Xy = 3,5-dimeth~lphenyl)).~ In no cases do these compounds undergo thermal molybdenum-molybdenum bond homolysis. We now report preliminary results on the preparation and chemistry of a chromium analogue, FvCrz(CO)4(PMezPh)z,which would be expected t o have a weaker metal-metal bond than the molybdenum analogueslo and which indeed does undergo spontaneous thermal homolysis. The new hydride FvCrz(C0)4(PMezPh)zH2was synthesized essentially as reported previously for the molybdenum analogues FvMoz(C0)4LzHz (L = PMe3, PPh3h5 The carbonylate salt (Et4N)z[FvCrz(CO)sl(lI6,' was protonated with glacial acetic acid in hexanetoluene (1:3) solution, forming the unstable dihydride FvCrz(C0)6Hz (2).697Subsequent in situ reaction of 2 with PMe2Ph a t room temperature resulted in the formation of yellow, crystalline FvCrz(C0)4(PMezPh)zHz (3)(a) Goh, L.-Y.; DAniello, M. J., Jr.; Slater, S.; Muetterties, E. L.; Tavanaiepour, I.; Chang, M. I.; Fredrich, M. F.; Day, V. W. Inorg. (b) Cooley, N.A,; MacConnachie, P. T. F.; Baird, Chem. 1979,18,192. M. C. Polyhedron 1988,7,1965.(c) Watkins, W. C.; Hensel, K.; Fortier, S.; Macartney, D. H.; Baird, M. C.; McLain, S. J. Organometallics 1992, 11,2418.(d) 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. (4)(a)Abrahamson, H. B.; Heeg, M. J. Inorg. Chem. 1984,23,2281. (b) Drage, J. S.; Vollhardt, K. P. C. Organometallics 1986,5,280.(c) Moulton, R.; Weidman, T. W.; Vollhardt, K. P. C.; Bard, A. J. Inorg. Chem. 1986,25,1846.(d) McGovern, P. A,; Vollhardt, K. P. C. Synlett 1990,493.(e) Tilset, M.; Vollhardt, K. P. C.; Boese, R. Organometallics 1994, 13, 3146. (0The compound [CpCr(CO)& catalyzes the 1,4dihydrogenation of conjugated dienes4gvia CpCr(C0)s and the hydride CpCr(C0)3H,l0z4gand thus indirect evidence for the diradical isomer of FvCrz(C0)G is found in observations that this compound and the corresponding dihydride effect similar chemistry: Vollhardt, K. P. C., private communication. ( g ) Miyake, A,; Kondo, H. Angew. Chem., Int. Ed. Engl. 1968,7,631. (5)(a) Kovacs, I.; Baird, M. C. Organometallics 1995,14,4074.(b) Kovacs, I.; Baird, M. C. Organometallics 1995,14,4084. (6)The compounds (Et4N)z[FvCr~(C0)~] and FVCrz(CO)sH2 have also been prepared elsewhere: McGovern, P. A.; Vollhardt, K. P. C., private communication.
0 1995 American Chemical Society
Communications
5470 Organometallics, Vol. 14,No. 12, 1995
(3) in good yield. Attempted substitutions of 2 failed with the bulkier PPh2Me and PPh3 but proceeded smoothly with the smaller P M ~ s Room-temperature .~ IR and NMR spectra7 of 3 were fully consistent with its formulation, which was confirmed by elemental and mass spectroscopic ana lyse^.^ Treatment of 3 with trityl radical (Ph3C*,1:2 molar ratio) in toluene at room temperature resulted in a rapid color change from yellow to yellow-brown. The complete consumption of 3 and the concomitant formation of a single heat- and light-sensitive organometallic product were established by spectroscopic methods. The IR spectrum of the reaction mixture exhibited absorbances at 1926 (s) and 1850 (vs) cm-l, at positions very similar t o those of 3 but with different relative inten~ities.~ The 'H NMR spectrum of a reaction mixture in toluene-ds exhibited broad resonances attributable to PMe (6 1-68), fulvalene (6 3.69 and 3.921, and phenyl protons, all significantly different from those of 3,7and a singlet at 6 5.37 attributable to Ph3CH. In addition, the W-vis spectrum exhibited a strong da do* transition at 460 nm, indicative of a metal-metal bond in the product which, on the basis of comparisons with data for the (fulva1ene)molybdenumdimers truns-FvMo2(C0)4L2(L = PPh3, PCy3, P X Y ~ )is, ~tentatively identified as the chromium-chromium-bonded dimer trans-FvCrz(CO)4(PMe2Ph)z (4). Thus, as anti~ipated,~ the reaction between trityl radicals and 3 took place according to eq 3.
-
+
-
FvCr2(CO),(PMe2Ph),H2 2Ph3C' FvCr,(CO),(PMe,Ph),
+ 2Ph3CH (3)
Since 4 was found to be too unstable to be isolated analytically pure, it was also obtained in an alternative way, by the treatment of 3 with 1,3,5-hexatriene. It has been demonstrated that hydrogenation of a number of conjugated dienes by CpM(C0)3H(M = Cr, Mo, W) can (7) Spectroscopic data are as follows. 1: IR (THF) YCO 1889 (vs), 1800 (vs, br), 1717 (vs, br) cm-l; lH NMR (200 MHz, acetone-&) 6 1.36(tt, J, = 7 Hz. X = 2 Hz. 24H. CHq). 3.42 (a. J = 7 Hz. 16H. CHpN). 4.13 ("?,4H, Fv),-4.61 ("t", 4H, Fv);IiC{'H} NMR (100.6 MHZ, acetone-&) 6 7.7 (s, CH3), 52.8 (s,CHzN), 80.0 (s,Fv), 81.7 (S, Fv), 100.9 (8, C-1 Fv), 247.4 (s, CO). 2: IR (hexane) YCO 2014.5 (SI, 1948 (s, sh), 1940 (vs) cm-'; 'H NMR (200 MHz, toluene-&) b -5.46 (s, 2H, CrH), 4.08 ("t", 4H, Fv), 4.34 ("t", 4H, Fv); 13C{lH} NMR (100.6 MHz, toluene-&) 6 83.7 (s, Fv), 85.9 (s, Fv), 99.8 (s, C-1 Fv), 234.7 (br s, CO). 3: IR (toluene) YCO 1924.5 (vs), 1856 (vs), 1846 (sh) cm-'; 'H NMR (200 MHz, toluene-ds) 6 -6.07 (d, JPH= 73 Hz, 2H, CrH), 1.26 (d, JPH= 8.5 Hz, 1 2 ~PCH3), , 4.04 ("q", 4H, Fv), 4.35 Yq", 4H, Fv), -7.06 (m, 6H, m,pPh), 7.30 ("t",4H, o-Ph); l3C{lH) NMR (100.6 MHz, toluene-ds) 6 83.7 (s,Fv),84.8 (s, Fv), 99.9 (s, C-1 Fv), 128.5 (d, J p c = 8.5 Hz, m-Ph), = 36 Hz, 129.2 (s, p-Ph), 129.6 (d, J p c = 8.8 Hz, 0-Ph), 143.0 (d, JPC ipso-Ph) (no PMe and CO resonances were observed); 3'P{'H} NMR (162 MHz, toluene-&) 6 59.9. 4: IR (toluene) YCO 1926 (s), 1850 (VS) cm-1; UV-vis (THF) i,,,(da do*) 460 (vs) nm; 'H NMR (200 MHz, toluene-ds) b 1.68 (br s, 12H, PCHB),3.69 (br m, 4H, Fv), 3.92 (br m, 4H, Fv), -7.05 (br m, 4H, Ph), 7.41 (br m, 6H, Ph); 13C{1H}NMR (100.6 MHz, acetone-&, 25 "C) 6 88.4 (s, Fv), 82.6 ( 6 , Fv), 129.3, 130.0, 130.6 (s, all Ph) (no quaternary or PMe resonances were observed); 13C{'H} NMR (100.6 MHz, acetone-&, -50 "C) 6 18.7 (d, Jpc = 32 Hz, PMe), 82.5 (s, Fv), 87.3 (s,C-1 Fv), 88.1 (s, Fv), 129.0, 129.7, 129.8 (8, all Ph), 143.8 (d, J p c = 33.5 Hz, ipso-Ph), 254.8 (d, J p c = 35 Hz, CO); 31P{1H} NMR (162 MHz, toluene-&, -40 "C) 6 66.0. (8)Kovacs, I.; Baird, M. C., unpublished results. (9) Anal. Calcd for C30H32Cr20dP2: C, 57.88; H, 5.18. Found: C, 57.30, H, 5.33. EI-MS ( m l z (%)I: no [MI' appeared; [M - 2H - COP, 592 (0.2);[M - 2H - ZCO]', 564 (0.8); [M - 2H - 3COl+, 538 (2.2); [M - 2H - 4COl+, 508 (0.5);[M - 2H - PMezPhI+,482 (0.4); [M - 2H - PMezPh - CO]', 454 (0.2); [M - 2H - PMe2Ph - 2COl+, 426 (0.2); [M - 2H - PMezPh - 3CO1+, 398 (0.2); [M - 2H - PMe2Ph - 4COl+, 370 (3.3); [FvCrHl+, 181 (11); [O=PMezPh]+, 154 (69); [PMezPhHI+, 139 (100); [PMe2PhIC, 138 (58); [FvI*, 128 (33).
-
20
,
oc
j--jvy*
,,
,
'O
JL,- 4 0 2 PPM
Figure 1. Stacked variable-temperature 31P{lH} NMR spectra (toluene-ds)of compound 4. be utilized to prepare the corresponding dimers, [CpM(C0)31~,in excellent y i e l d ~ . ~ ~ , ~ g J ~ Interestingly, the 'H,13C{lH}, and 31P{1H} NMR spectra of 4 are all strongly temperature-dependent. A variable-temperature 'H NMR experiment revealed that both the fulvalene and the PMe resonances shifted downfield and broadened significantly as the temperature was raised from -80 to +80 "C. Although decomposition became evident above 50 "C, the line broadening and the chemical shift changes were both clearly reversible. Similar effects, resulting from dissociation to paramagnetic monomer, are observed in the lH NMR spectrum of [CpCr(C0)312.2d,i The room-temperature l3C{lH) NMR spectrum of 4 in acetone-& exhibited only tertiary carbon resonances in both the fulvalene (6 82.3, 87.5) and the phenyl regions. The quaternary carbon (C-1 Fv, ipsoBh, CO) and PMe resonances were not observed at room temperature but were at -50 "C.7 Furthermore, the 31P{lH} NMR spectrum of 4 exhibited no phosphorus resonance at room temperature, but a very broad resonance appeared at 6 66.0 at 10 "C and gradually sharpened upon further cooling to appear as a normal 31P{1H}NMR spectrum at -40 "C (Figure 1). A complementary Variable-temperature IR experiment was also carried out in the temperature range 2580 "C. As the temperature was raised, the absorbances of 4 a t 1926 (8) and 1850 (vs) cm-l weakened and two new vco bands emerged at 1949 (SI and 1800 (s, br) cm-l, very similar to those of CpCr(C0)dPMezPh) (1925 (5) and 1807 (m, br) ~ m - l ) Only . ~ ~ the new absorbances were present at 80 "C. The most plausible rationale for the temperature dependence of the IR and NMR spectra is that the chromium-chromium-bonded form of 4 exists in a facile (10) Miyake, A.; Kondo, H. Angew. Chem., Int. Ed. Engl. 1968, 7, 880.
Communications
Organometallics, Vol. 14,No. 12, 1995 5471
equilibrium with its biradical isomer, as shown in eq 4.
(4)
While the NMR studies established that rapid exchange between the 18-electron dimer and 17-electron biradical occurs above -40 "C, a t least, as in eq 4,the IH NMR and IR studies demonstrate that the equilibrium is shifted considerably to the right above room temperature such that the predominant species in solution at higher temperatures is the biradical isomer. although Thus, 4 behaves much like [CpCr(C0)312,2d,i the changes in the 'H NMR spectrum (downfield shifts of -1 ppm) are much smaller. The radical-like properties of 4 were further supported by studies on its reactions with activated alkyl halides. It has been shown that the solution chemistry of [CpCr(CO)312 is dominated by the reactivity of the small amount of monomer present in solution and that [CpCr(CO)& takes part in halogen atom abstraction reactions with a wide variety of alkyl bromides and iodides. The primary products are CpCr(C0)3X (X = Br, I) and CpCr(C0)3R(R = alkyl) if the starting halide contains no /3-hydrogen atom(s);1a,2iJ1 the halochromium product arises from initial halogen abstraction by one chromium-centered radical and the alkylchromium product from coupling of the resulting alkyl radical with a second chromium-centered radical. We have found that 4 reacts instantaneously at room temperature in the dark with ICH2C02Et, allyl iodide, CHBr3, BrCHzCN, and cc4, but only slowly (several hours) with PhCH2( l l ) ( a ) Cooley, N. A,; Watson, K. A.; Fortier, S.; Baird, M. C. Organometallics 1986,5,2563.(b) Goulin, C. A.; Huber, T. A,; Nelson, J. M.; Macartney, D. H.; Baird, M. C. J. Chem. SOC.,Chem. Commun. 1991, 798. (c) Huber, T. A.; Macartney, D. H.; Baird, M. C. Organometallics 1993, 12, 4715.
Br and BrCH2C02Me. Reactions with unactivated alkyl iodides occur very slowly (1day), in the order Et1 < i-PrI < t-BuI. IR and 'H and 31P(1H}NMR spectroscopic studies showed that the major or sole products in the fast reactions were the dihalides FvCrz(CO)4(PMezPh12X.2 (X = C1, Br, I), identified spectr~scopically.~ There remains the question of why fulvalene compounds, with very long metal-metal bonds, do not undergo spontaneous thermal homolysis as do analogous cyclopentadienyl compounds with much shorter metal-metal bonds. The factors inducing compounds with metal-metal bonds to undergo spontaneous thermal homolysis are not well understood, as there are as yet few examples of homologous series from which to draw conclusions.2f Mention has already been made of the effects of steric factors, metal-metal bonds often being destabilized with respect to homolytic dissociation on substitution of small ligands by more sterically demanding ligands, e.g. CO by tertiary phosphines and v5-C5H5 by v5-C5Me5 and q5-C5Ph5. While it seems implicit in this apparent correlation that the principal factor at play here is weakening of metal bonds through steric strain, in fact such does not appear to be the case with [CpCr(CO)& and [Cp*Cr(C0)312.2fAlthough the latter has a much longer Cr-Cr bond and dissociates much more extensively in solution, the enthalpies of dissociation of the two dimers in toluene are identical within experimental error and the differing degrees of dissociation are related primarily to entropic factors. Thus, dissociation of [Cp*Cr(C0)312apparently gives rise to a much greater release of steric strain than is the case with [CpCr(CO)3la, making possible degrees of ligand rotational motion in monomeric Cp*Cr(CO)snot possible in [Cp*Cr(C0)312.2fThis type of increase in entropy would be much less of a factor in the fulvalene complexes under consideration here. While increased rotation of the ML3 fragments relative t o the cyclopentadienyl rings would occur, as with [CpCr(CO)&, the increase in rotational motion around the fulvalene central C-C bond would not be equivalent to the effect of dissociation of [CpCr(CO)sle, nor would there be entropic factors equivalent to the probable increase in methyl rotational effects in [Cp*Cr(CO)3]2.
Acknowledgment. We thank NATO for a Science Fellowship to I.K. (administered by the Natural Sciences and Engineering Research Council), the NSERC for a Research Grant to M.C.B., and the Hungarian Science Fund for support provided by Grant No. OTKA F7419. OM950719T
