1423
Organometallics 1996,14, 1423-1428
Ligand Substitution at 17-Electron Centers. Electroactivation of Functionalized Cyclopentadienylmanganese Tricarbonyl Complexes to Single- and Double-CO Substitution Y. Huang, G. B. Carpenter, and D. A. Sweigart" Department of Chemistry, Brown University, Providence, Rhode Island 02912
Y. K. Chung and B. Y. Lee Department of Chemistry, Seoul National University, Seoul 151 -742, Korea Received October 3, 1994@
Electrochemical oxidation of (MeCp)Mn(CO)s (1) in t h e presence of P(OEt)3 leads to rapid single- and double-CO substitution. With analogous complexes (3-5) having bulky substituents on t h e cyclopentadienyl ring, the 17-electron species produced by electroactivation also undergo rapid single CO substitution, but at rates significantly reduced from t h a t observed with 1+;substitution of a second CO in 3+-5+ is even more retarded. Digital simulations of a n associative mechanism closely reproduced experimental data and allowed t h e reactivities of t h e 17-electron complexes to be quantified. The ability of bulky substituents to hinder t h e approach of a nucleophile to the metal was examined by determining t h e X-ray structure of (1,3-dimethyl-2-phenylcyclopentadienyl)Mn(CO)~ (3): monoclinic, space group P21/c, a = 11.019(2) b = 7.5631(10) A, c = 17.716(2) p = 103.381(12)O, 2 = 4, 3273 unique reflections with I > 2411, R = 0.0376, wR2 = 0.0912.
A,
A,
Introduction
Ph
The enhanced reactivity of 17-electron organometallic complexes to ligand substitution and atom abstraction is well d ~ c u m e n t e d . l -Published ~ kinetic studies of CO substitution3 indicate that these radicals generally follow an associative mechanism and that the rate enhancement in comparison to 18-electron complexes is many orders of magnitude. In the present study, we show that the normally inert4 (MeCp)Mn(CO)3(l),as well as analogues variously functionalized on the cyclopentadienyl ring (3-5; see Figure 11, undergo extremely rapid substitution of one or two CO ligands Abstract published in Advance ACS Abstracts, January 15, 1995. (1)(a) Tyler, D. R. Prog. Znorg. Chem. 1988,36,125.(b) Baird, M. C. Chem. Rev. 1988,88,1217.(c) Kuksis, I.; Baird, M. C. Organometallics 1994,13, 1551. (d) Huber, T.A.; Macartney, D. H.; Baird, M. C. Organometallics 1993,12,4715. (e) Scott, S. L.; Espenson, J. H.; Zhu, Z. J. Am. Chem. SOC.1993,115,1789. (0 Zhu, 2.; Espenson, J . H. Organometallics 1994,13,1893.(g) Organometallic Radical Processes; Trogler, w. C . , Ed.; Elsevier: Amsterdam, 1990. (2)(a)Hershberger, J. W.; Klingler, R. J.; Kochi, J. K. J . Am. Chem. SOC.1983,105, 61.(b) Zizelman, P. M.; Amatore, C.; Kochi, J . K. J . Am. Chem. SOC.1984,106,3771. (c) Kochi, J. K. J . Organomet. Chem. 1986,300, 139.(d) Poli, R.; Owens, B. E.; Linck, R. G. Znorg. Chem. 1992,31, 662. (e) Legzdins, P.; McNeil, W. S.; Shaw, M. J. Organometallics 1994,13,562. (3)(a) Poe, A. Transition Met. Chem. 1982, 7, 65.(b) Fabian, B. D.; Labinger, J. A. Organometallics 1983,2,659.(c) Shi, Q.Z.; Richmond, T. G.; Trogler, W. C.; Basolo, F. J. Am. Chem. SOC.1984,106, 71.(d) Herrinton, T.R.; Brown, T. L. J . Am. Chem. SOC.1985,107,5700.(e) Turaki, N. N.; Huggins, J. M. Organometallics 1986, 5, 1703. (0 Kowaleski, R. M.; Basolo, F.; Trogler, W. C.; Gedridge, R. W.; Newbound, T. D.; Ernst, R. D. J.Am. Chem. Soc. 1987,109,4860.(g) Watkins, W. C.; Hensel, K.; Fortier, S.; Macartney, D. H.; Baird, M. C.; McLain, S. J. Organometallics 1992,11,2418.(h)Meng, Q.; Huang, Y.; Ryan, W. J.; Sweigart, D. A. Znorg. Chem. 1992,31,4051. (i) Shen, J. K; Freeman, J . W.; Hallinan, N. C.; Rheingold, A. L.; Arif, A. M.; Ernst, R. D.; Basolo, F. J . Am. Chem. SOC.1992, 11, 3215. ( 4 ) h g e l i c i , R. J.; Loewen, W. Znorg. Chem. 1967,6, 682. @
p*n
h W 3
WPh
4
&n(~0)3
5
Figure 1. Numbering code for the complexes in this study. upon oxidation in the presence of P(OEt13. The chemistry involved is illustrated in Scheme 1(the nucleophile L is P(0Et)s). The extent of single versus double substitution is markedly influenced by steric congestion in the vicinity of the metal due t o substituents (R) on the cyclopentadienyl ring. Sufficiently bulky groups hinder access of the nucleophile to the metal center and inhibit the rate; the X-ray structure of 3 was determined in order t o examine this effect. As discussed below, oxidatively promoted CO substitution in (RCp)Mn(C0)3is not catalytic; the transformation (RCp)Mn(C0)3 (RCp)Mn(CO)ZL requires an initial stoichiometric oxidation, followed by CO substi-
-
0276-733319512314-1423$09.00/0 0 1995 American Chemical Society
1424 Organometallics, Vol. 14, No. 3, 1995
Huang et al. Table 1. Crystal Structure Data for Complex 3
Scheme 1
formula fw space group cryst dimens, mm scan type a, A b, A c, A P. deg
1'""
L slow
Ci6H13MnO3 308.2 P21/c, monoclinic 0.35 x 0.38 x 0.65 w
11.019(2) 7.5631(10) 17.716(2) 103.381(12) 1436.3(4) 4 1.425 632 Mo K a , 0.710 73 A 9.22 1.90-27.50 4338 3273 181 0.0376 0.0912 0.0589 0.0986 0.905'
v, A3
@-Mn(CO)L2
Z
Ll
.
+ e-
Mn(C0)L:
tution in the 17-electron radical and then stoichiometric reduction. In order for an oxidatively promoted ligand substitution to be catalytic (i.e., electron transfer catalyzed), it is necessary that the 18-electron product be more difficult to oxidize than the reactant, which in turn requires that the reduction potentials in Scheme 1 be in the order Ez" > El". In most substitutions, departing ligands are replaced by ones that increase the electron density at the metal so that El" > Ez". Therefore, catalytic oxidative activation of organometallic complexes to ligand substitution is expected to be uncommon. (The same reasoning implies that catalytic reductive activation should be common.) Rare examples of catalytic oxidatively promoted ligand substitutions have been reported by Kochi et a1.,2a-cwho studied a series of reactions typified by eq 1. Detailed electro-
+
-e-
(MeCp)Mn(CO),(NCMe) PR, (MeCp)Mn(CO),PR,
+ MeCN
(1)
chemical investigations by these workers showed that the 17-electron complex (MeCp)Mn(CO)z(NCMe)+is activated to rapid associative substitution of the MeCN ligand. Experimental Section Syntheses. Complex 1 was purchased from Strem Chemical Co. Photochemical substitution of CO in 1 by P(OEt)3to give 2 was accomplished by a procedure analogous to that reported by Connelly and Kitchen5for the synthesis of (MeCp)Mn(C0)2[P(OMe)3].Thus, W irradiation of 1(2.2 mmol) and P(OEt)3(3.0 mmol) in 60 mL of THF produced a 2:l mixture of 2 and (MeCp)Mn(CO)[P(OEt)3]2, along with other unidentified species. Separation of 2 from the product mixture was effected by TLC on silica gel with diethyl etherhexanes (1: 25) as eluant. The purity of 2 (obtained in 15% yield) was established with certainty by electrochemical, 'H NMR, and IR measurements (IR (THJ?) 1942,1875cm-'). Complexes 3-5 were prepared by published Crystal Structure of Complex 3. A crystal of 3 was grown by coolingof a hexanddiethy1 ether solution. X-ray data collection was carried out using a Siemens P4 single-crystal diffractometer controlled by XSCANS software. w scans were used for data collection, at variable speeds from 10 to 60" (5)Connelly, N. G.; Kitchen, M. D. J . Chem. SOC.,Dalton Trans. 1977. -- 931 (6)Lee, B. Y.; Moon, H.; Chung, Y. K.; Jeong, N. J . Am. Chem. SOC. 1994,116, 2163. (7) Lee, B. Y.; Moon, H.; Chung, Y. K.; Jeong, N.; Carpenter, G. B. Organometallics 1993,12, 3879. - 7
g F(0W radiation p , cm-I 28 limits, deg no. of observns no. of unique data, I > 2u(I) no. of variables RY ( I > 2u(I)) W R ( I ~ 2u(I)) ~ R (all data) wR2 (all data) GOF ecalcdr
fast
R = xllFoi- \Fcll/xlFol. wR2 = [Zw(Fo2- F2)2/ZwF04]"2. Based
on P.
Table 2. Atomic Coordinates ( x 104) and Isotropic Thermal Parameters ( A 2 x 103) for Complex 3 Mn O(1) O(2) o(3) C(1) C(2) C(3) C(4) C(5) C(6) (37) C(8) C(9) C(10) C(11) C(W C(13) C(14) C(15) (316)
X
Y
Z
2978(1) 2386(3) 347(2) 3509(3) 4077(2) 3 136(2) 3352(2) 4393(2) 4840(2) 265 l(2) 4282(3) 2210(2) 1305(2) 513(3) 602(2) 1472(2) 2277(2) 2607(3) 3282(3) 1370(3)
23OO( 1) - 1133) 3238(4) 5174(3) 253(3) 830(3) 2684(3) 3200(3) 1723(3) 3843(3) -1618(3) -318(3) - 1268(3) -2400(3) -2594(3) -1648(3) -517(3) 844(4) 4060(4) 2895(4)
662(1) -666( 1) 514(1) -330(1) 1340(1) 1720(1) 1899(1) 1619(1) 1276(1) 2342(1) 1114(2) 1967( 1) 1451(1) 1717(2) 2501(2) 3017(2) 2760(1) -147(1) 5W) 554(1)
ues
W1) lOl(1) 103(1) 103(1) W1) 38(1) 42~) 48U) 55U) 60( 1) 40(1) 54(1) 63U) 63(1) 58U) 47(1) 61(1) 66U) 66(1)
min-l. Three standard reflections were measured every 97 reflections with no systematic decrease in intensity observed. Data reduction included profile fitting and an empirical absorption correction based on separate azimuthal scans for eight reflections (maximum and minimum transmission 0.398 and 0.350). The structure was determined by Patterson methods and refined initially by use of programs in the SHELXTL 5.1 package. All of the hydrogen atoms appeared in a difference map. The hydrogen atoms were inserted in ideal positions, riding on the atoms to which they are bonded; they were refined with isotropic temperature factors 20% greater than that of the attached atom. All other atoms were refined with anisotropic thermal parameters. Final refinement on F was carried out using SHELXL 93. Relevant structural data are given in Tables 1-3. Electrochemical Studies. Voltammetric experiments were done under a blanket of nitrogen that was saturated with solvent. The electrolyte was 0.10 M B a P F 6 , which was synthesized by metathesis of B a B r and HPF6, recrystallized from CHzClz/hexanes,and dried under vacuum. The solvent in all experiments was CHZC12, which was purchased in HPLC grade from Fisher Scientific (catalog numbers D143-1 and D150-1). Additional "purification" by distillation was not
Ligand Substitution at 17-Electron Centers
Organometallics, Vol. 14, No. 3, 1995 1425
Table 3. Selected Bond Lengths (A)and Bond Angles (deg) for 3 Bond Lengths 2.153(2) Mn-C(2) 2.154(2) Mn-C(4) 2.133(2) Mn-C( 14) 1.790(3) Mn-C( 16) 1.15l(3) C( 15)-0(3) 1.143(3) C(l)-C(5) 1.430(3) c(i)-c(7) 1.445(3) C(Z)-C(S) 1.407(3) C(3)-C(6) 1.412(3) C(WC(9)
Mn-C(l) Mn-C(3) Mn-C(5) Mn-C( 15) C(14)-0(1) C(16)-0(2) C( 1)-C(2) C(2)-C(3) C(3)-C(4) C(4)-C(5) Mn-C( 14)-O( 1) Mn-C( 16)-O(2) Mn-C(2)-C(8) C( l)-C(2)-c(3) C(2)-C( 1)-c(7) C(3)-C(2)-C(8) C(4)-C(3)-C(6) C(2)-C(S)-C(9)
Bond Angles 178.8(3) Mn-C(15)-O(3) 177.1(2) Mn-C(1)-C(7) 129.6(1) Mn-C(3)-C(6) 107.4(2) C(5)-C(l)-C(7) 125.7(2) C(l)-C(2)-C(S) 126.4(2) C(2)-C(3)-C(6) 125.6(2) C(2)-C(8)-C(13) 123.3(2) C(l)-C(5)-C(4)
2.151(2) 2.131(2) 1.779(3) 1.794(3) 1.149(3) 1.414(3) 1.502(3) 1.48l(3) 1.504(3) 1.387(3) 178.2(3) 129.0(2) 127.5(2) 126.2(2) 125.8(2) 127.0(2) 118.7(2) 108.4(2)
necessary (or desirable), as judged by the solvent potential window and the chemical reversibility of the oxidation of (benzene)Cr(CO)s,a complex knowns t o be very sensitive to trace nucleophilic impurities. Once opened, the solvent was kept under argon or nitrogen and contact with the atmosphere was kept to a minimum. Cyclic voltammetry was done with EG&G 173/175/179 potentiostatic instrumentation. The working electrode was a 1 mm diameter platinum or glassy-carbon disk, and the counter electrode was a platinum wire. The reference was a Metrohm Ag/AgCl electrode filled with CHzCld0.10 M BQNClOd and saturated with LiC1; this was separated from the test solution by a salt bridge containing 0.10 M B m F s in CHzClz. IR spectroeledrochemistry was done with an optically transparent thin-layer electrode (OTTLE), the construction . ~ electrolyses were and use of which have been r e p ~ r t e d Bulk done with a platinum-basket working electrode and a platinummesh counter electrode, which was separated from the test solution by a salt bridge. Digital simulation of proposed mechanisms utilized the program DigiSim.lo This program utilizes an efficient fast implicit finite difference algorithm and is able to simulate moderately complex mechanisms within a few minutes on a PC with a 486 chip. Data input includes an E" value for each heterogeneous electron-transferstep and an estimate of the rate constants for any homogeneous reactions. In agreement with experimental data, the relevant heterogeneous electron transfers were taken as Nernstian. The simulationresults were found t o be quite sensitive to the input values of the homogeneous rate constants, which were altered in a systematic manner until the voltammetric shapes and current ratios closely matched the experimental ones. A reasonable estimate of the precision with which the rate constants were determined is &20%. Results and Discussion The results of the voltammetric oxidation of 1 in CH2Cl2 are illustrated in Figure 2. In the absence of a nucleophile and at ordinary scan rates the cyclic voltammogram (CV) corresponds to a one-electron chemically (8)Stone, N. J.; Sweigart, D. A,; Bond, A. M. Organometallics 1966, 5 , 2553. (9)(a) Bullock, J. P.; Boyd, D. C.; Mann, K. R. Znorg. Chem. 1987, 26, 3086. (b) Pike, R. D.; Alavosus, T. J.; Camaioni-Neto, C. A.; Williams, J. C.; Sweigart, D. A. Organometallics 1989, 8, 2631. (10)(a) DigiSim 1.0 program; Bioanaltyical Systems, Inc., West Lafayette, IN. (b) Rudolph, M.; Reddy, D. P.; Feldberg, S. W. Anal. Chem., submitted for publication.
I 1.6
I -0.1
E N vs. AglAgCl
Figure 2. Cyclic voltammograms of 1.11mM complex 1 in CH2CldO.1 M Bu4NPF6 at 25 "C with P(OEt)3 present at the following concentrations (mM): (A) none; (B) 0.41; (C) 1.26; (D)3.60. The working electrode was a 1.0 mm diameter glassy-carbon disk, and the scan rate was 0.50 V s-l. All potentials are relative to El,z(ferrocene) = 0.55 V. reversible process (E112= 1.40 V l l ) , in agreement with results from previous ~ t u d i e s . ~We J ~ found, however, that the reversibility is lost in the presence of P(OEt)3 and that in this case the oxidation of 1 leads to the appearence of two new reversible couples at Ey2 = 0.85 and 0.18 Vll (Figure 2B-D). IR spectra taken after the electrochemical experiments verified that there was no reaction in the bulk solution. IR OTTLE experiments suggested that the two new complexes resulted from rapid single- and double-CO substitution in 1+ to give the cations [(MeCp)Mn(CO)3-,[P(OEt)31nl+ ( n = 1 , 2). I n Figure 2B a deficiency of P(OEt)3limits the reaction to the single substitution 1+ 2+, with the reduction wave near 0.85 V being attributed to 2+. As more P(OEt)3 is added, double substitution to yield (MeCpIMn(CO)[P(OEt)&+ becomes competitive (Figure 2C) or dominant (Figure 2D). Verification of these assignments was provided by a comparison of vco bands found in IR OTTLE oxidation and subsequent reduction of [l, P(OEt)3I mixtures in CHzClz with published5 YCO data for [(MeCp)Mn(C0)3-,L,]+/O complexes (L = PR3, P(OR)3) and with IR data for genuine samples of (MeCpIMn(C0)2P(OEt)3 (2) and (MeCp)Mn(CO)[P(OEt)312. Furthermore, voltammetric experiments with 2 gave a reversible couple at exactly the Ell2 value shown in Figure 2B.
-
(11)All potentials reported are relative to ferrocene, E m = +0.55 V.
(12)Denisovich, L. I.;Zakurin, N. V.; Gubin, S. P.; Ginzburg, A. G.
J. Organomet. Chem. 19'75,101, C43.
Huang et al.
1426 Organometallics, Vol. 14, No. 3, 1995
I
I 1.2
-0.2
E I V vs. AgIAgCI
Figure 3. Cyclic voltammograms of 1.00 mM complex 2 in CHZC12/0.1 M BuaPF6 at 25 "C with P(OEt)3 present at the following concentrations (mM): (A) none; (B) 1.20; (C) 5.90. The working electrode was a 1.0 mm diameter platinum disk, and the scan rate was 0.50 V s-l. All potentials are relative to Eldferrocene) = 0.55 V. The replacement of CO in 2+ by P(OEt)3 to give (MeCp)Mn(CO)[P(OEt)3]2+was probed directly by the voltammetric oxidation of 2 in the presence of P(OEt13. Figure 3 gives some results. The electrochemical behavior indicates that 2+ undergoes clean CO substitution. Digital simulation of the observed CV's in 14 experiments covering a scan rate range of 0.50-50 V s-l and a [P(OEt)31range of 1.0 to 20 mM (with 2 at 1.0 mM) established that the rate law for CO substitution in 2+ is first order in both the metal complex and P(OEt)3, with a second-order rate constant of (3.1 & 0.5) x lo3 M-l s-l at 25 "C. We next examined what effect substituents on the cyclopentadienyl ring have on single- and double-CO substitution in the 17-electron cationic radicals. Figure 4 illustrates the behavior of complex 3. It can be seen that one CO in 3+ is readily replaced by P(OEt13. However, by comparison t o the results shown in Figure 2, it is apparent that double-CO substitution is much more difficult in 3+ than in l+. In particular, the C P s in Figure 2D and 4D, which refer to experiments with similar P(OEt)3concentrations, illustrate the difference rather strikingly. Electrochemical experiments performed with complexes 1 and 3-5 showed that all undergo chemically reversible one-electron oxidation at 0.50 V s-l in C H ~ C ~ ~ / B U ~ with N P FE1/z7s G in the range 1.37-1.43 V.ll The propensity for double-CO substitution by P(OEt)3was found to follow the reactivity order 1+ > 3+ a 4+ > 5+ (vide infra). The obvious interpretation of the above order is that it is steric in origin; i.e., it seems likely that bulky substituents inhibit the approach of the nucleophile P(OEt)3 to the metal. This would be particularly true for the second CO substitution, since the presence of the first P(OEt)3would only serve to enhance the steric congestion in the transition state for an associative process. In order to examine the proposed steric effects more closely, the X-ray structure of 3 was obtained.
I 1.55
I -0.1
E/V vs. Ag/AgCI
Figure 4. Cyclic voltammograms of 1.00 mM complex 3 in CHZC12/0.1 M BuaPF6 at 25 "C with P(OEt)3 present at the following concentrations (mM): (A) none, (B) 0.36; (C) 0.79; (D)3.40. The working electrode was a 1.0 mm diameter glassy-carbon disk, and the scan rate was 0.50 V s-l. All potentials are relative to El/a(ferrocene)= 0.55 V.
0 00121
Figure 5. Structural drawing and atomic numbering scheme for complex 3. Figures 5 and 6 illustrate the molecular geometry, and Tables 1-3 provide pertinent structural data. Bond lengths and most bond angles in 3 are ordinary and require no comment. However, there is an interesting structural feature shown in Figure 6 that is relevant to the present discussion. The cyclopentadienyl ring C(1)C(5) is highly planar (mean deviation 0.003 with attached carbons C(6), C(71, and C(8) being about 0.11 A above this plane and away from the metal. Because of the methyl groups, the phenyl ring is prevented from being coplanar and in conjugation with the Cp ring. The angle between these two planes is 61.6" in the solid. As a result, the phenyl group partially blocks the access of a nucleophile to the metal, and the rate of CO substitution decreases.
A),
Organometallics, Vol. 14, No. 3, 1995 1427
Ligand Substitution at 17-Electron Centers
second-order homogeneous rate constant must be at least lo6 M-l s-l, or a pre-wave will not be seen. Digital simulation of the mechanism represented by eqs 2-4 accurately reproduced experimental results for complexes 3-5. Useful kinetic information was obMn-CO
Mn-CO+
= -eMn-CO+
(2)
kl + P(OEt), Mn-P(OEt)3+ + CO
Mn-P(OEt),+ fe_ Mn-P(OEt), Figure 6. Structural drawing of complex 3 showing the rotation of the phenyl ring from the cyclopentadienyl plane.
1+
In order to quantify the reactivity of the 17-electron radicals 1+ and 3+-S+ with P(OEt13, the pre-wave method of Parker et al.13was employed. In Figures 2B and 4B it can be seen that the principal oxidation wave is split into two. This occurs because the following requirements are fulfilled: (1) the second-order reaction with nucleophile is very rapid and (2) there is a deficiency of nucleophile. Figure 7 illustrates the wave splitting for the oxidation of 3 with the nucleophile to metal complex concentration ratio, [PHMI,equal to 0.55. At the onset of oxidation of 3 to 3+,rapid reaction with P(0Et)B causes a cathodic kinetic shift of the wave. Because of the deficiency of P(OEt)3 at the electrode surface, the supply of nucleophile is quickly exhausted and the remainder of the oxidation wave then occurs at the normal unshifted potential. The result is a splitting of the oxidation into a kinetic pre-wave and the normal Nernstian wave. (If the nucleophile is present in excess, only a single kinetically shifted wave occurs.) The separation of the two waves is a sensitive function of the [PHMI ratio, the rate of the homogeneous second-order reaction, and the scan rate (Figure 7). The pre-wave method is applicable only to very rapid reactions; under ordinary experimental conditions, the (13) Jensen, B. S.; Parker, V. D. Electrochim. Acta 1973, 18, 665. Parker, V. D.; Tilset,M. J.Am. Chem. SOC.1987,109,2521.
(4)
tained (via the pre-wave method) for 0.25 I[PHMI I 0.75 and for scan rates between 0.10 and 10.0 V s-l. The behavior of complex 1 was rather more difficult to simulate because concurrent single- and double-CO substitution had to be considered. However, this was easily accomplished because the second CO substitution was studied independently (via complex 21, as described above. Thus, the only unknown for simulation of the mechanism representing the behavior of 1 was kl, the rate constant for the first CO substitution (eq 3). The simulations produced the following second-order rate constants for CO substitution at 25 "C in CHzCldO.1 M Bu~NPF~:
10-6k,lM-'
Figure 7. Cyclic voltammograms showing splitting of the oxidation wave of 1.00 mM complex 3 in CHzCldO.1 M BQNPF6 at 25 "Cwith P(0Et)Babsent in A but present at 0.55 mM in B-F. The scan rate (V s-l) was (A) 0.050, (B) 0.10, (C) 0.20, (D)0.50, (E) 1.0, and (F) 2.0. The working electrode was a 1.0 mm diameter glassy-carbon disk. The current scale was normalized for clarity.
(3)
s-1
100
3+ 40
4+ 14
5+ 7.0
It seems reasonable t o conclude that the reactivity order 1+ > 3+ > 4+ > 5+ reflects increasing steric congestion near the metal, which causes a decrease in the rate of associative CO substitution in the 17-electron radicals. The steric effects are probably larger than indicated by the k1 values because of concomitant electronic effects, which would be expected to influence the rates in approximately the opposite direction. Whatever the quantitative interplay of steric and electronic effects, it is pertinent to note that the rate with even the most sterically congested complex (5+) is very large (7.0 x lo6 M-l s-l). Thus, 1+-5+ conform to the generali~ationl-~ that 17-electron organometallic complexes are vastly more reactive than their 18-electron analogues. As a striking example of this, consider that 1 does not undergo thermal CO substitution by PPh3 at 140 "Cover 3 days: yet it reacts with P(OEt)3within milliseconds when oxidized to a 17-electron species. Substitution of the second CO in 1+ by P(OEt)3 is slower than the first substitution by a factor of kdk1 = (3.1 x 103)/(1x lo8) = 3 x Such rate lowering is normal behavio+ for associative reactions and is dependent on the nature of the nucleophile. If the steric congestion argument made above for k l is valid, one would expect that the decrease in reactivity for the second CO substitution (k2) would not be constant in the series 1+,3+-5+. Rather, the ratio kdk1 should be smaller (or kl/k2 larger) in the sterically demanding complexes 3+-5+. By matching of digital simulations to experimental data at relatively high concentrations of P(OEt)3, a t which a discernable amount of doubleCO substitution occurred, it was possible to estimate (ik30%)the rate constant (k2) for this step:
Huang et al.
1428 Organometallics, Vol. 14, No. 3, 1995
km-ls-' 104kllk2
1+
3+
3100
40
3
100
4+ 40 40
5+ 6 100
In agreement with expectations, the k1/k2 ratio indeed increases significantly in 3+-5+ as compared t o l+. In conclusion, we have demonstrated the electroactivation of (cyclopentadienyl)Mn(C0)3 complexes to facile CO substitution by P(OEt13. Similar electroactivation of (arene)M(CO)s complexes (M = Cr, Mo, W) has recently been reported;3h in both systems oxidation to 17-electron species initiates the substitution process. It is important to note that the redox-promoted reactions reported herein are stoichiometric (as expected) and not electron transfer catalyzed, as often obtains14J5 in reductively initiated substitutions.
Acknowledgment. This work was supported by grants fron the National Science Foundation (Grant Nos. CHE-8821588 and CHE-9400800). The X-ray equipment was purchased with assistance from an
instrument grant from the National Science Foundation (Grant No. CHE-8206423) and a grant from the National Institutes of Health (Grant No. RR-06462). SupplementaryMaterial Available: Tables of crystallographic data, bond lengths, bond angles, H atom positional parameters, and thermal parameters and a packing diagram for 3 (5 pages). Ordering information is given on any current masthead page. OM940764L (14) (a) Bezems, G. J.;Rieger, P. H.; Visco, S. J.Chem. SOC.,Chem. Commun. 1981,265. (b) Darchen, A,; Mahe, C.; Patin, H. J. Chem. SOC., Chem. Commun. 1982, 243. (c) Miholova, D.; Vlcek, A. A. J. Organomet. Chem. 1985,279,317. (d) Hinkelmann, K.; Mahlendorf, F.; Heinze, J.; Schacht, H.-T.; Field, J. S.; Vahrenkamp, H. Angew. Chem., Int. Ed. Engl. 1987,26,352. (e) Neto, C. C.; Baer, C. D.; Chung, Y. R; Sweigart, D. A. J. Chem. Soc., Chem. Commun. 1993, 816. (0 Neb, C. C.; Kim, S.; Meng, Q.; Sweigart, D. A. J . Am. Chem. SOC. 1993, 115, 2077. (g) Huang, Y.; Neto, C. C.; Pevear, IC A.; Banaszak-Holl, M. M.; Sweigart, D. A.; Chung, Y. K. Znorg.Chim. Acta 1994,226,53. (h) Pevear, K. A.; Banaszak-Holl, M. M.; Carpenter, G. B.; Rieger, A. L.; Rieger, P. H.; Sweigart, D. A. Organometallics 1995, 14,512. (15) See ref 2a-c for rare examples of oxidatively promoted ligand substitutions that are catalytic.
