Organometallics 1983, 2, 1698-1699
1698
Acknowledgment. We are grateful to the National Science Foundation, the Natural Sciences and Engineering Research Council of Canada, and the University of Alberta for support of this work and to Dr. R. G. Ball for X-ray data collection. Registry No. 4 (R = CHJ, 85534-04-9; 4 (R = C,H,), 85534-05-0;8,87183-21-9;CH3(C2H6)S2CFe2(CO)6, 87183-22-0; CH3(CH2=CHCH,)S2CFe,(C0)6, 87183-23-1; Fe, 7439-89-6. Supplementary Material Available: Tables of positional and thermal parameters, of bond lengths and angles, and of the observed and calculated structure amplitudes (9 pages). Ordering information is given on any current masthead page. O(1 1
Radical Trapping by Alkenes as a Route to Figure 1. Perspective drawing of (~-~2-SCS(CH3)2)Fe2(CO)~, 8. Organoplatlnum(I V ) Complexes and as a Test of Thermal ellipsoids are drawn at the 20% probability level except for hydrogens which are drawn artificially small. Relevant bond Mechanism of Oxidative Addition
parameters are as follows: Fe-Fe = 2.5432 (7) A, Fe(l)-S(2) = 2.2460 (9) A, Fe(2)-S(2) = 2.2351 (8) A, Fe(l)-C(7) = 1.923 (3) A, Fe(2)-C(7)= 1.921 (3) A, S(2)-C(7) = 1.734 (3) A, C(7)-S(1) = 1.730 (3) A, S(l)-C(Me) = 1.790 8, (mean), Fe-CO = 1.782 k, (mean), C-0 = 1.140 A (mean), S(l)-C(7)-S(2) = 126.9 (2)'. relating the two tricarbonyliron ends of the complex. All metric parameters are consistent with the observed geometry, and some of the more important of these are given in the figure caption. Several multinuclear complexes containing the SCX group (X = S, SR, NR, NR2, OR, etc.) have been structurally characterized6*'and are commonly found to exhibit one of two SCX bonding modes; either this group is bound in a side-on manner via the C-S moiety to one metal center6 or it can bridge the two metals in a cis-dimetalated geometry with the C-S function essentially parallel to the metals and coordinated through sulfur to one metal and through carbon to the othere7In these bonding modes the ligands generally function as either two- or three-electron donors depending on the nature of X. The bonding of the SCSMe2group in the present compound is therefore highly unusual in two important ways; first, in order for the metals to achieve 18-electron configurations, it functions as a six-electron donor and, second, the C-S moiety is bound essentially perpendicular to the F e F e bond. This bonding suggests a dipolar ligand formulation as shown in 8B. After this structural study was completed, a structurally analogous thioketene complex, [(Cl&lsCS)Fe2(CO)s], was reported by Behrens and co-workers? In this complex the thioketene group is coordinated perpendicular to the F e F e bond through the C-S functionality, much as in our complex. These authors also regard this group as a six-electron donor and formulate an analogous dipolar structure for the ligand. The present complex [Fe2(CO)6(p-v2-SCSMe2)],is now only the second known example in which the C-S moiety is bound perpendicular to a metal-metal bond and which functions as a six-electron donor.
Patrlck K. Monaghan and Richard J. Puddephatt' Depatfment of Chemistry, University of Western Ontario London, Ontario, Canada N6A 567 Received August 15, 1983
Summary: Dimethyl(1,1 O-phenanthroline)platinum(I I) reacts with a mixture of R I (R = i-Pr or t-Bu) and CH2= CHX (X = CN, CHO, C(=O)Me) to give the platinum(1V) complexes [RIMe,(CHXCH,R)(phen)] in good yield. I t is shown that the products are formed by a free radical mechanism and that product analysis in such systems can give useful mechanistic data.
Trapping of alkyl radicals by alkenes during oxidative addition has been used as a mechanistic test in several instances, but the method has not been used quantitatively nor has it yielded useful organometallic products.' We now report results which show the potential of this trapping procedure for both synthetic and mechanistic applications. Reaction of [PtMe2(l,10-phenanthroline)] (I) with i-PrI in acetone solution under inert atmosphere gives largely the product of trans oxidative addition [PtIMe2(i-Pr)(phen)] (11)2along with some [Pt12Me2(phen)](111) while the similar reaction of I with t-BuI gave only 111. In both cases a free radical mechanism was indicated. In the presence of excess of reactive alkenes, new roducts were formed in high yield according to eq 1 ( = hen).^^^
A
I
1981, 216, C15. (8) Umland, H.; Edelmann, F.; WormsbHcher, D.; Behrens, U. Angew. Chem., Int. Ed. Engl. 1983, 153.
RI
+
CH2=CHX
-
Me.
I
L
IVa, X = CN, R = i-Pr b, X = CHO, R = i-Pr c, X = C(=O)Me, R = i-F'r d, X = C N , R = t-BU e, X = CHO, R = t-Bu ~~
(6) (a) Ricard, L.; Estienne, J.; Weiss, R. Inorg. Chem. 1973,12, 2183. (b) Dean, W. K.; Vanderveer, D. G. J.Organomet. Chem. 1978,146,143. (c) Cowie, M.; Gibson, J. A. E., manuscript in preparation. (7) (a) Patin, H.; Mignani, G.; Benoit, A.; McGlinchey, M. J. J. Chem. SOC.,Dalton Trans. 1981,1278. (b) Patin, H.; Mignani, G.; Mah6, C.; Le Maroville, J.; Southern, T. G.; Benoit, A.; Grandjean, D. J. Organomet. Chem.1980,197,315. (c) Porter, S. K.; White, H.; Green, C. R.; Angelici, R. J.; Clardy, J. J. Chem. SOC.,Chem. Commun. 1973,493. (d) Cameron, T. S.; Gardner, P. A.; Grundy, K. R. J. Organomet. Chem. 1981,212, C19. (e) Mah6, C.; Patin, H.; Benoit, A.; Le Maroville, J. J. Organomet. Chem.
+
C HXC H pR .N,
~
~~
~
(1) (a) LabingerTJ. A.; Osborn, J. A.;ovillexJ. Inorg. Chem. 1980, 19,3236. (b) Lappert, M. F.; Lednor, P. W. Adu. Organomet. Chem. 1976, 14,345. (c) Kwiatek, J.; Seyler, J. K. J. Organomet. Chem. 1965,3,421. (d) Halpern, J. Pure Appl. Chem. 1979,51, 2171. (2) Ferguson, G.; Parvez, M.; Monaghan, P. K.; Puddephatt, R. J. J. Chem. SOC.,Chem. Commun. 1983, 267. (3) This appears to be the first case in which useful organometallic
products have been formed in oxidative addition of a tertiary alkyl halide. Presumably steric effects hinder attack of the t-Bu. radical at the metal center. Isolated yields were -70%. (4) The reactions were normally carried out in the diffuse daylight. If light was rigorously excluded the reactions occurred much more slowly. The chain reaction (see later) is clearly initiated by a photochemical step, the detailed nature of which is not yet understood.
0276-7333/83/2302-1698$01.5Q~0 0 1983 American Chemical Society
Communications
Organometallics, Vol. 2, No. 11, 1983 1699
Scheme I. Mechanism of the Reaction F%Me,(phen)
+ i-PrI + C H , = C H C N
CHyCHX
Pt =[PtNe21phen)l
To obtain a maximum yield of complexes IV, it is necessary to optimize the concentration of alkene. With too little alkene (e.g., 0.02 M alkene with 0.004 M complex I and i-PrI in acetone) mixtures of I1 and IV were formed, whereas with very high alkene concentrations some polymer was formed (e.g., when X = COMe, with 2.1 M alkene). With the alkenes styrene or methyl methacrylate, which readily polymerize via free radical^,^ polymer was always formed and no complex IV could be detected. The complexes IV were characterized unambiguously by elemental analysis and infrared and 'H and 13C NMR spectroscopy.6 In the case of IVa the NMR assignments were confirmed by a 2-D heteronuclear lH-13C chemical shift correlated experiment, this being desirable because the asymmetric carbon center adjacent to platinum caused nonequivalence of the MezPt, CH2, and CHMe, groups6 in this and other complexes of IV. The closest analogy to the reactions of eq 1 appears to be in the free radical chain process for reaction of trialkylboranes with alkenes (eq 2 and 3, R' = H or M).7 R* CHz----CHC(O)R' [RCHZCHCOR']. (2) [RCHZCHCOR']. + R3B RCH&H=CR'OBRz + R. (3) However, in this case the radical is trapped by boron as the oxygen-centered resonance form to give an enolate derivative. The spectroscopic data for complexes IV leave no doubt that the softer platinum center traps the carbon-centered radicals to form the a-substituted alkylplatinum(1V) derivatives.6p8 The reactions with i-PrI occur largely by a free radical chain mechanism,'^^ Scheme I, as shown by the inhibition by the radical scavenger 4-methoxyphenol (Figure 1,supplementary material). Given this mechanism, the product analyses shown in Table I give useful quantitative data.
+
-
4
(5) Eastmond, G. C. In "Comprehensive Chemical Kinetics"; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1976; Vol. 14a. (6) For examde. IVa: 'H NMR (CDCM 8 1.78.11.70. s. WPtH) = 71 Hz,' MePt), 2.26'(dd, 3J(HH) = 4, 13 Hz, Ij(PtH)'= 92 Hz, PtCH(CN)I, 0.65 (ddd, 3J(HH) = 4, 11 Hz, V(HH) = 13 Hz, CHaHb-i-Pr],0.08 (ddd, V(HH) = 4, 13 Hz = 'J(HH) 13 Hz, CH"Hb-i-Pr],0.40, 0.70 Id, ,J(HH) = 6 Hz, CHMe2], 1.43 (m, CHMe,]; 13C NMR (CDCI,) d -4.15, -4.45 ('J(PtC) = 638,634 Hz, MePt), 10.9 ('J(PtC) = 679 Hz, PtCH(CN)J,41.1 I2J(PtC) = 37 Hz, CHzJ, 27.0 I3J(PtC) = 71 Hz, CHMe,], 20.8, 23.2 ICHMek IR 2200 cm-' ldCN)I. NMR data for other complexes of IV were similar. (7) Kabalka, G. W.; Brown, H. C.; Suzuki, A.; Honma, S.; Ar-e, A.; Ioth, M. J . Am. Chem. SOC. 1970, 92, 710. (8) For example, IVb: IR 1650 cm-' Iv(C0)); 'H NMR 6 8.76 (d, 3J(HH) = 3 Hz, CH=O): 13C NMR 202.3 (d, CH=O). IVc: IR 1665. cm-' IV(C0)). (9) Note that the formation of IV is not consistent with an SN2 mechanism or with a free radical nonchain mechanism with recombination within the primary radical pair IIPtIMez(phen)].i-Pr.). Oxidative addition of Et1 to I, which is thought to occur by the SN2 mechanism, gave only [PtIMe,Et(phen)] even in the presence of high concentrations of acrylonitrile. For a discussion of cage effects in free radical chemistry see: Koenig, T.; Fischer, H. in 'Free Radicals"; Kochi, J. K., Ed.; Wiley, New York, 1973; Vol. 1, Chapter 4.
Table I. Products Formed by Reaction of [PtMe,(phen)] w i t h i-PrI (0.4 M ) a n d C H , = C H C N in Acetone at 2 0 "C
l o 3[PtMe,(phen)], M
[CH,=CHCN],
M
p r o d u c t ratio IVa/II
4.2 3.2 2.1 4.2 4.2 4.2 4.2 4.2
0.12 0.12 0.12 0.40 0.20 0.10 0.06 0.02
2.4 3.8 4.7 7.6 4.6 2.5 1.7 1.0
From the scheme it can be seen that the product ratio of IVa/II can be equated to kz[CH2=CHCN]/k1[PtMez(phen)]. Thus decreasing [PtMe2(phen)]or increasing [CH,=CHCN] should increase the product ratio IVa/II, and this is shown to be the case (Table I). From the data in Table I a mean value of kl/kz = 20 f 10 can be calculated. Since k2 = 2 X lo5 L mol-1 s-l,lo it follows that kl = 4 X lo6 L mol-l s-l. As far as we are aware, this is the first estimate of a rate constant for attack of a radical a t a diamagnetic transition metal center"J2 and shows that such addition reactions can be extremely rapid. In a second competition experiment, complex I, with a high acrylonitrile concentration, was treated with mixtures of i-PrI and t-BuI. Now the selectivity for formation of IVd or IVa is given by the relative rate constants for abstraction of iodine by the five-coordinate platinum(II1) species [PtMez(CH(CN)CH2R)(phen)]from t-BuI or i-PrI. The mean value for k(t-BuI)/k(i-PrI) = 1.7 f 0.2. Abstraction from the tertiary alkyl halide is faster, as expected from the weaker C-I bond, but the selectivity is low.l3 In summary, the alkene trapping technique developed here, which is clearly applicable to other organometallic systems involving radical intermediates, has given a synthesis of functionally substituted organoplatinum complexes, a useful qualitative test for a free radical mechanism, and is capable of yielding quantitative mechanistic data. Acknowledgment. We thank NSERC (Canada) and the University for financial support, and Professor A. G. Davies for helpful discussion. Registry No. I, 52594-55-5;11,87318-07-8 111,86407-72-9; IVa, 87338-36-1; IVb, 87338-37-2; IVC, 87350-68-3; IVd, 87338-38-3; IVe, 87338-39-4; i-PrI, 75-30-9; t-BuI, 558-17-8; CH,=CHCN, 107-13-1; CH,=CHCHO, 107-02-8; CH,=CHC(=O)Me, 78-94-4.
Supplementary Material Available: Figure 1, absorbance vs. time during the reaction of [PtMez(phen)] with i-PrI a n d CHz=CHCN in acetone at 21 O C (2 pages). Ordering information is given on any current masthead page. ~~
~~~
(10) Abell, P. I. In "Comprehensive Chemical Kinetics"; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1976; Vol. 18, Chapter 3. This figure was calculated from kinetic data for Et. CH,=CHCN
+
at high temperatures and under different experimental conditions. Radical additions of Et. and i-Pr. to double bonds have very similar rate constants, but there will be a large uncertainty in k2 The value calculated for kl should therefore be considered an order of magnitude quantity only. (11) Similar rate constants have been found for S H reactions ~ of main-group metal alkyls, in which radical addition at the metal center is probably rate determining. Davies, A. G.; Roberta, B. P. 'Free Radicals"; Kochi, J. K., Ed.; Wiley: New York, 1973; Chapter 10. (12) In principle, a similar experiment at high alkene concentration, measuring the relative yields of IV and poly(alkene), could give the relative rate constants for attack of RCHZCHX.at platinum or alkene. We have been unable to obtain reproducible data in such experiments. (13) For example, bromine atom abstraction by metal radicals gives k(t-BuBr)/k(i-PrBr) in the range 2-8, for reactions of Bu,Sn., Cr(II), Ag(O), and [Co(CN),]". Kuivila, H. G. Adu. Organomet. Chern. 1964,1, 47. Tamura, M.; Kochi, J. K. J.Am. Chern. SOC. 1971,93,1483. Halpern, J. Ann. N.Y. Acad. Sci. 1974,239, 2.
