Organometallics 1983, 2, 460-462
460
A similar four-centered intermediate has been proposed for the oxidative addition of Hz or HzO to two CO(CN)~~-, Scheme I 17-electron radi~a1s.I~~ Also, disproportionation of metal carbonyl radicals in polar solvents is known.lg Mnz(CO)&z -!% ~MXI(CO)~L (1) Evidence supportive of a four-centered concerted process was sought by studying the photochemical reaction of MXI(CO)~L+ MII(CO)~L+ CO (2) aqueous HC1 with Mnz(CO)gPBu3dissolved in 4 % ethanol/isopropyl ether. On the basis of electronic effects, a Mn(C0)3L + HC1- HMII(CO)~LC~ (3) four-centered intermediate should favor formation of HMII(CO)~LC~ + MII(CO)~L HMII(CO)~and Mn(C0)4(PBu3)C1.If steric factors domHM~I(CO)~L + Mn(CO)3LC1 (4) inate, Mn(CO)&21 and HMn(C0)4PBu3 should form. Spectroscopic evidence indicates that only Mnz(CO)lo, MII(CO)~LC~ + CO MII(CO)~LC~ (5) HMn(C0)4PBu3,and Mn(C0)4(PBuJCl form in significant amounts within 20 min. This evidence strongly argues HMII(CO)~LC~HC1- Hz Mn(C0)3LC1z (6) against a four-centered intermediate. Furthermore, irradiation of Mnz(CO)gPBu3in the absence of HC1 (4% Mn(CO)3LC1z+ MII(CO)~L ethanol/isopropyl ether) produces within 15 min a mixture MII(CO)~LC~ +Mn(CO),LCl (7) of Mnz(CO)loand Mnz(C0)8(PBu3)z in equilibrium with MII(CO)~LC~ + CO M ~ I ( C O ) ~ L C ~ (8) Mnz(CO)gPBu3. The appearance of these crossover products coupled with the lack of evidence for any anionic MII(CO)~L+ H C 1 7 MII(CO)~LC~ + HS (9) species, e.g., M~I(CO)~L-, indicate simple metal-metal bond homolysis is occurring without rapid disproportionation. Photochemical bond homolysis (eq 1) is followed by Lastly, it is interesting to briefly compare the thermal facile CO dissociation (eq 2). In step 3 HC1 undergoes a and photochemical reactions of Mnz(CO)8Lz(L = PBu3, two-electron oxidative addition to a coordinatively unP(OEt),, and CO) with HC1. In the absence of light, saturated lbelectron radical. Such a process is common thermal reactions (from 18 to 60 "C) of Mnz(CO)8Lzwith with HC1 and 16-electron species.15 It has also been HC1 show more pronounced ligand effects than the corproposed that oxidative addition of H t and HSn(r~-Bu)~l~ responding photochemical reactions. In all cases where to 15-electron metal carbonyl radicals occurs. The ligand reaction occurs, Mn(CO),(L)Cl always forms, whereas effect, associated with the increase in reaction rate obHMn(C0)4Lnever is detected. The reactivity of L is in served with the substituted compounds, further supports the order PBu3 > P(OEt), >> CO. Therefore, the ligand the oxidative addition pathway. The stronger Lewis bases effect is somewhat similar to that in the photochemical PBu3 and P(OEt)3(compared to the r acid CO) promote process, but the lack of formation of HMII(CO)~Lstrongly facile oxidation of the manganese. Hydrogen atom abindicates a different mechanism is operable. Further straction (eq 4) and CO addition (eq 5) complete the studies including kinetic measurements on both the process. When larger amounts of HC1 are present, a second thermal and photochemical reactions are in progress. HC1 molecule interacts evolving Hz gas (eq 6). Such a Acknowledgment. We are grateful for the William and process has also been proposed for reactions of HI with Flora Hewlett Foundation Grant of Research Corp. that Ir(1) species.l' Chlorine atom abstraction (eq 7) is then provided financial support for this research. followed by CO addition (eq 8). In step 9 one-electron oxidative addition (chloride abRegistry No. Mn2(C0)8(PBu3)2,15609-33-3; Mnz(CO)&P(OEt)3)2,1548&149; HCl, 7647-01-0; HMxI(CO)~PBU~, 56960-19-1; straction) is indicated. However, given the H-C1 bond HMn(C0)4P(OEt)3,84369-08-4;MxI(CO)~(PBU~)CI, 84369-09-5; strength (420 kJ/mol), the solvent would likely play a role. Mn(CO)4(P(OEt)3)Cl,84369-10-8; H2, 1333-74-0;CO, 630-08-0; Alternatively, an initial two-electron oxidative addition of Mn2(CO)lo, 10170-69-1; Mn(CO)5C1, 14100-30-2; HMn(CO)S, HC1 to MII(CO)~Loccurs (producing a seven-coordinate, 16972-33-1; M X I ~ ( C O ) ~ P 24476-71-9; BU~, hexane, 110-54-3; iso19-electronradical) followed by loss of H. There is growing propyl ether, 108-20-3; ethanol, 64-17-5. evidencela that 19-electron radicals are formed via associative processes, especially in disubstituted radicals.lsb (19)Allen, D.M.;Cox, A.; Kemp, T. J.; Sultana, Q.;Pitts, R. B. J. Step 9 predominates only when step 2 is supressed by an Chem. SOC.,Dalton Trans. 1976,1189-1193. atmosphere of CO. An alternative pathway involves the interaction of an HC1 molecule with a pair of solvent-caged radicals forming a four-centered intermediate. Such an interaction may be viewed either as a dinuclear, two-electron oxidative addition of HC1 or as an HC1-promoted disproportionation. Redox-Catalyzed Carbonylatlon of an Iron Methyl the mechanism shown in Scheme I.
+
+ -
(13)When a small amount of oxygen (air) is added to the reaction of Mn2(CO)8L2and HCl, the gfowth of HMn(C0)L is inhibited. The observed effect of added CO is somewhat similar and may reflect trace amounts of O2in the CO. However, the CO (Linde, CP grade) is passed through an activated manganese(I1) oxide, oxygen-scavanging column" prior to use. Further testa are being conducted. (14)Brown, T. L.; Dickerhoof, D. W.; Bafw, D. A.; Morgan, G. L. Reu. Sci. Zmtrum. 1962,33,491-92. (15)(a) Louw, W.J.; deWaal, D. J. A.; Gerber, T. I. A.; Demanet, C. M.; Copperthwaite, R. G. Znorg. Chem. 1982,21,1667-68.(b) Halpern, J. Acc. Chem. Res. 1970,3,386-392 and references therein. (16)Wegman, R.W.;Brown, T. L. Organometallics 1982,1, 47-52. (17)Forster, D. J. Chem. SOC.,Dalton Trans. 1979,1639-1645. (18)(a) Fox, A.; Malito, J.; Po& A. J. Chem. SOC.,Chem. Commun. 1981,1052-1063.(b)McCullen, S.B.; Walker, H. W.; Brown, T. L. J.Am. Chem. SOC.1982,104,4007-4008.
0276-733318312302-0460$01.50/0
Complex Roy H. Magnuson,' Randy Melrowltr, Samu J. Zulu, and Warren P. Glerlng' Department of Chemistry, Boston University Boston, Massachusetts 022 15 Received October 25, 1982
Summaty: ($-C,H,)(PPh,)(CO)Fe(CH,) undergoes a rapid redox-catalyzed migratory insertion that follows the rate law: rate = k [OX],[CO]. 0 1983 American Chemical Society
Communications
0
V.
Organometallics, Vol. 2, No. 3, 1983 461
SCE
E Figure 1. Cyclic voltammograms in CHzClzcontainin 0.1 M TBAP at a scan of 200 mV s-l and 0 OC. Scan a is ( q?-CsHs)(PPh3)Fe(CO)(CH3)under Ar atmosphere: E , = 480 mV; E = 280; ( E , + E c ) / 2 = 380 mV; E , - E , = 200 mV. Scan b is CsHs)(PPh3)Fe(CO)(COCH3) under CO atmosphere: E , = 570 mV; E , = 370 mV, (E, + E312 = 470 mV; E, - E c 200 mV. Scan c is (lIS-C,H,)(PPh?)Fe(CO)(CH?) under CO atmomhere: E.. = 415 mV; E,--= 570"mV;Ec' = 3fO mV; (Ea2+ Ec)/!2= 470 :V; E,, - E , = do0 m v .
(2
Among metal-mediated ligand reactions, alkyl to acyl migratory insertion is of particular interest, as it is a key step in many important industrial processes.' Despite numerous kinetic and stereochemical studies, thermodynamic information is limited2and the role of the oxidation state of the metal in activation of the reaction has not been systematically addressed. Recently, we reported the thermodynamics of the rapid equilibration between alkyl and solvenbincorporated acyl complexes of formally Fe(III) cation radi~als."~Herein, we wish to present the first example of a redox-catalyzed carbonylation reaction and an assessment of the tremendous rate enhancement associated with a one-electron change in oxidation state of an iron complex. The carbonylation reactions of the formally Fe(I1) complex (Cp)(PPh,)(CO)Fe(CH,) (eq 1) and closely related (Cp)(PPh,)(CO)Fe(CH,) + CO 1 Cp(PPh3)(CO)Fe(COCH,) (1) 2
-
derivatives are known to be thermodynamically favorable! (1) Alkyl to acyl migratory insertion reactions have been reviewed. (a) Collman, J. P.; Hegedus, L. S. "Principles and Applications of Organotransition Metal Chemistry";University Science Books: Mill Valley, CA, 1980; pp 359-288. (b) Kuhlman, E.J.; Alexander, J. J. Coord. Chem. Rev. 1980,33,195-225. (c) Calderazzo, F. Angew. Chem., Znt. Ed. Engl. 1977, 16,299-311. (d) Wojcicki, A. Adu. Organomet. Chem. 1973,11,87-145. (e) Daub, G.W. Prog. Znorg. Chem. 1977,22,409. (2) (a) Fachinetti, G.;Fochi, G.; Floriani, C. J. Chem. SOC.,Dalton Trow.1977,1945-1950. (b) Calderazzo, F.;Cotton, F. A. Proc. Znt. Conf. Coord. Chem., 7th 1962, Paper 6147. (c) Cotton, J. D.; Crisp, G. T.;Latif, L. Znorg. Chim. Acta 1981,47, 171-176. (3) Magnuson, R. H.;Meirowitz, R.; Zulu, S. J.; Giering, W. P. J.Am. Chem. SOC.1982,104, 57W-5791. (4)Magnuson, R. H.; Zulu, 5.;T'sai, W.-M.; Giering, W. P. J . Am. Chem. SOC.1980,102,6887-6888. (5) Flood, T. C.;Downs, H. 'Abstracts of Papers", 176th National Meeting of the American Chemical Society, Miama, FL, Aug 1978, American Chemical Society: Washington, DC, 1978; INOR 27. (6) Brunner, H.;Vogt, H., J. Organomet. Chem. 1981 220,223-36.
but the rate is quite slow. For example, in 5 days no perceptible ( [CO], first-order kinetics were observed, for which the rate constants were independent of the concentration of 1, but linearly dependent on the initial concentration of the oxidizing agent, [0Xli. Thus the experimental rate law is given by rate = k[CO][OXIi
k = 500 f 100 M-’s-’
(2)
in which there is zero-order dependence on [ 11, and k has been evaluated from the slope of the plot in Figure 2. Under conditions where [CO] >> [l], a striking linear decrease in absorbance with time was observed. It continued until the absorbance corresponding to complete conversion to 2 was reached, when a sharp break occurred and the absorbance did not change further. This zero-order behavior is fully consistent with the rate law in eq 2. With assumption of steady-state concentrations of 3 and 4, the reaction sequence in Scheme I leads to the rate expression (3)
Provided he,[ I] >> k,[CO] (a reasonable condition, since the electron self-exchange reaction for related ferrocene derivatives is very r a ~ i d )the , ~ above expression simplifies to eq 2 in which the experimentally determined constant (7) The observed first-order dependence on the concentration of carbon monoxide is consistent with either an associative process or with a two-step mechanism involving a 15-electron intermediate for which the concentration of CO is insufficient to detect rate saturation.
k is to be equated with the Fe(II1) carbonylation rate constant kc. Thus,the rate of carbonylation of the formally iron(II1) methyl complex 3 is conservatively estimated to be 107-108 faster than the carbonylation rate of the formally Fe(I1) species. This difference between the rates of carbonylation in the Fe(I1) and Fe(II1) states is most likely attributable to a substantial reduction in the d a donor ability of the metal center on oxidation, which allows a bent (or perhaps side-on) CO configuration in the activated complex to be energetically much more a c c e s ~ i b l e . The ~ ~ ~crucial role of a back-bonding in the migratory insertion is also attested to by the ability of an external entering group, which is a strong a acid such as CO, to drive the reaction on Fe(I1) thermodynamically much farther than a weaker a acid such as NCCH, .(Kco/KNcc~s > 1015).’0 It must be pointed out that parts per million concentration of an oxidant is, in principle, sufficient for the redox-catalyzed pathway to carry the entire reaction, so that interpretation of kinetic and stereochemical results must be done with caution. As this type of redox catalysis is most likely not restricted to iron, nor even to carbonylation reactions, the implications for organometallic reactivity may well be far-reachingal’ Acknowledgment. We wish to thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Graduate School of Boston University for support of this research. Registry No. 1, 12100-51-5;2, 12101-02-9. (8) Pladziewicz, J. R.; Espenson, J . H. J. Am. Chem. SOC.1973, 95, 56-63. (9) Hoffman, R.; Berke, H. J. Am. Chem. SOC.1978,100,7224-7236. (10) KAN= W0M-I3 and Kco must be >lo5 since no 1 is detectable after the catalyzed carbonylation of 1. (11) Enhanced reactivity toward ligand substitution has been reported in other 17-valence-electron organometallic species. (a) Hershberger, J. W.; Klingler, R. J.; Kochi, J. K. J.Am. Chem. SOC.1982,104,3034-3043. (b) McCullen, S. B.; Walker, H. W.; Brown, T. C. Zbid. 1982, 104, 4008%4010. (c) Hepp, A.’F.;Wrighton, M. S. Ibid. 1981,103,1258-1261. (d) Absi-Halabi, M.; Brown, T. L. Zbid. 1977, 99, 2982-2988.
Cp2M(s-trans q4-butadiene) Complexes from Zlrconocene and Hafnocene Dlhalldes and “Magnesium Butadiene” Ulrlch Dorf, Klaus Engel, and Gerhard Erker” Abteilung fur Chemie der Ruhr-Universitat 0-4630 Bochum 1, Germany Received September 28, 1982
Summary: Under the kinetic control reaction of “magnesium butadiene” (2) with Cp,MCI, complexes la-c (Cp = q-C5H5, q-C,Me,; M = Zr, Hi) gives (strans -q4-butadiene)metallocenecomplexes 4a-c, the products expected from a stepwise reaction through Cp,MCI-substituted crotylmagnesium halide intermediates 3.
(2-Butene-1,4-diyl)magnesium(2; magnesium butadiene), conveniently obtained as an oligomer upon treatment of butadiene with magnesium metal in THF,l has been shown to be a versatile reagent for the preparation of (1) Pujita, K.; Ohnuma, Y.; Yasuda, H.; Tani, H. J . Organomet. Chem. 1976, 113, 201.
0276-7333/S3/2302-0462$01.50/00 1983 American Chemical Society
