Inorg. Chem. 1984, 23, 2019-2025
2019
Contribution from the Departments of Chemistry, Brown University, Providence, Rhode Island 029 12, and University of Otago, Dunedin, New Zealand
Paramagnetic Organometallic Molecules. 16. ESR Studies of Cobalt Acetylene 7r Complexes: A New Class of Fluxional Organocobalt Radicals' LOUIS V. CASAGRANDE," TAICHENG CHEN,Zd-3 PHILIP H. RIEGER,'" JIM SIMPSON,2band STEVEN J. VISCOZd
BRIAN H. ROBINSON,2b
Received November 8. 1983
ESR studies are reported on 19 radicals of the general formula (R2C2)Co(C0)3-,L,where L is a phosphine, phosphite, or arsine and x = 0-3. The radicals are obtained by reduction of THF solutions of acetylene-bridged dicobalt clusters, R2C2C02(CO)bxLx, in the presence of excess ligand L and, for x = 2 or 3, are long-lived at room temperature. ESR line s at 290 K) with width studies show the radicals (Ph2C2)Co(CO)[P(OMe)3]2 to be very rapidly fluxional (7 = 2.2 X instantaneously nonequivalent phosphite ligands. Analysis of the frozen-solutionESR spectrum of (~-BU~C~)CO(CO)~P-~-BU~ permits detailed characterization of the semioccupied molecular orbital. A pseudo-square-planar molecular structure can be inferred from the ESR results. Recent work4 has shown that polynuclear metal carbonyl compounds with a metal-metal backbone can generally be reduced to anions in which the formal 18-electron electronic configuration is exceeded. This redox property arises because there is a relatively accessible LUMO in the neutral species that is centered on the metal cluster b a ~ k b o n e . A ~ detailed study of the radical anions can provide important information on the electronic structure of the polynuclear carbonyl as well as the feasibility of using these species in synthetic and catalytic reactions. We have reported ESR6,7and electrochemical studies8 of the redox properties of the dinuclear acetylene-bridged compounds (RC2R')Co2(CO),. These studies led to the discove#9 of electron-transfer chain-catalyzed (ETC) nucleophilic substitution in organo transition metal clusters. One of the factors that determines the efficiency of the ETC reactions with the acetylene compounds is the importance of the alternative reaction mode to nucleophilic substitution of the radical anions-namely, fragmentation of the radical anions to produce the mononuclear organocobalt radicals (RC2R')Co(CO)3,L,. This new class of radical is produced as summarized in Scheme Iq8 When L = CO, only the fragmentation process leads to net reaction. However, since E3 > E , in this case, monocobalt radicals formed near the electrode are immediately reduced and an overall 2-electron process is observed. However, when L is a phosphine, phosphite, or arsine, both E2 and E3 are more negative than El and good yields of the monocobalt radicals and the substituted dicobalt clusters are obtained. In this paper, we report a variety of ESR experiments designed to characterize these monocobalt radicals and to deduce their electronic and molecular structures. Experimental Section X-Band ESR spectra were recorded on Varian E-4 or Bruker ER-220D spectrometers equipped with variable-temperature units and microwave-frequency counters. The Bruker spectrometer was S.;Peake, B. M.; Robinson, B. H.; Simpson, J. Ausr. J . Chem., in press. (a) Brown University. (b) University of Otago. ACS-PRF Summer Fellow, 1982. Peake, B. M.; Rieger, P. H.; Robinson, B. H.; Simpson, J. Inorg. Chem. 1981, 20, 2540-2543 and references therein. Lauher, J. W. J. Am. Chem. Soc. 1978,100, 5305-5315. Dickson, R. S.;Peake, B. M.;Rieger, P. H.; Robinson, B. H.; Simpson, J. J. Organomet. Chem. 1979, 172, C 6 3 C 6 5 . Peake, B. M.; Rieger, P. H.; Robinson, B. H.; Simpson, J. J. Am. Chem.
(1) Part 15: Huffadine, A.
(2) (3) (4) (5) (6)
(7)
SOC.1980, 102, 156-163. (8) Arewgoda, M.; Rieger, P. H.; Robinson, B. H.; Simpson, J.; Visco, S. J. J. Am. Chem. Soc. 1982, 104, 5633-5640. (9) Bezems, G. J.; Rieger, P.H.; Visa, S.J. J. Chem. Soc., Chem. Commun. 1981,265-266. Arewgoda, M.; Robinson, B. H.; Simpson, J. Ibid. 1982,284-285. Bruce, M. I.; Kehoe, D. C.; Matisons, J. G.; Nicholson, B. K.; Rieger, P. H.;Williams, M. L. Ibid. 1982, 442-444.
0020-1669/84/1323-2019$01.50/0
interfaced to an ASPECT-2000 computer. Radicals were prepared by reduction of tetrahydrofuran (THF) solutions of (RC2R')Co2(CO), (R, R' = Ph, r-Bu, CF3; R = CF,, R' = Si(CH3),, 0.01 M) and tetra-n-butylammonium perchlorate, 0.10 M, by in situ electroly~is.~ The sources and/or preparations of the chemicals used in this work are described elsewhere.*
Results Isotropic ESR Spectra. ESR spectra of the anion radicals (RC2R')Co2(C0)6-. were observed when T H F solutions of the parent molecules were reduced electrolytically in a cell maintained at -60 OC in the ESR Radical anions had lifetimes of 1 min or more at this temperature, and well-resolved spectra were obtained. These spectra were the subject of an earlier report.7 When electrolysisof solutions of (R2C2)Co2(CO),(R = Ph, t-Bu) was carried out at higher temperatures, -30 to -50 "C, an 8-line ESR spectrum was obtained, apparently arising from hyperfine coupling to a single s9C0 nucleus ( I = 7/2). These spectra, which were always weak and required prolonged electrolysis to be observable, are assigned to the radicals (R2C2)Co(C0)3. These radicals have lifetimes of a few minutes at -50 OC; no ESR signal was observed on electrolysis at temperatures higher than -30 OC. Reduction of (Ph2C2)Co4(CO)lounder similar conditions gave a weak 8-line spectrum identical with that obtained from (Ph2C2)Co2(CO),. The anion radicals of [CF3C2Si(CH3)3]C02(C0)6 and [(CF3),C2]Co2(CO)6 were relatively stable; no 8-line spectrum was ever observed on electrolysis of these compounds. Reduction of the R = t-Bu compound in the presence of excess P-n-Bu3 or P(OMe)3 at temperatures from -50 to +10 OC gave 16-line spectra (hyperfine coupling to one s9C0and one 31Pnuclei, I = 1/2) that are assigned to the radicals ( r B U ~ C ~ ) C O ( C O ) ~The L . spectra appeared immediately on 0 1984 American Chemical Society
2020 Inorganic Chemistry, Vol. 23, No. 14, 1984
Casagrande et al. n
1
3000 G
!
,
,
I
1
1
3500 G
%We 1* Of (ph2c2)c0(c0) [P(0Me)312 at 270 K: (a) experimental; (b) simulated; (c) simulated constant-width absorption spectrum (stick spectrum) showing second-order splittings of the m,, = 0 lines.
commencement of electrolysis and grew in intensity on continued passage of current. These radicals were unstable at higher temperatures so that a steady-state ESR intensity was rapidly achieved on electrolysis at -10 OC or above. Reduction of the R = Ph compound in the presence of exPPh3, P-n-Bu3, P(C6H1J3, or P(OPh)3 similarly gave 16-line spectra, and with excess AsPh,, a 32-line spectrum was obtained (coupling to one 59C0 and one 15As nuclei, I = 3/2). With a 100-fold excess of P(OMe)3 or P(OEt),, 24-line spectra (Figure la) were obtained (coupling to one 59C0and two 31P nuclei) that are assigned to (Ph2C2)Co(CO)L2. With a stoichiometric amount of P(OMe),, reduction at -30 OC gave only the monosubstituted radical (Figure 2). With a 10-fold excess of P(OMe),, a mixture of mono- and disubstituted radicals was obtained. Reduction of (Ph2C2)Co2(C0)6,[P(OMe),Jx (x = 1, 2) without added P(OMe), also gave the disubstituted radical. The disubstituted radicals are considerably more stable than the unsubstituted or monosubstituted species with lifetimes of several hours at +30 OC. Electrolysis of [CF3C2Si(CH3)3]CO2(Co)6 or [(CF3),C2]C O ~ ( C Oin) ~the presence of P(OMe), at -30 OC gave the usual anion radical speectra, but after prolonged electrolysis at -10 OC, weak spectra were observed that exhibited hyperfme coupling to a single 59C0and to two 31Pnuclei. The dinuclear anion radicals have lifetimes of 1 min or more at this temperature, and their spectra are easily observed in the absence of P(OMe),. Reduction of [ (CF3),C2] C O ~ ( C O[P(OMe),] )~ or (Ph2C2)Co2(CO),[P(OMe),j3 at room temperature in the presence of excess P(OMe), gave 32-line spectra (coupling to one 59C0 and three equivalent 31Pnuclei) that are assigned to (R2C2)Co[P(OMe),l,. Reduction of the R = Ph compound in the presence of an excess of the bidentate ligands Ph2PCH2CH2PPh2,
,
n
3000 G
3500 G
Figure 2. ESR spectra of (Ph2C2)Co(C0)2[P(OMe)3] at 260 K: (a) experimental; (b) simulated; (c) simulated constant-widthabsorption spectrum (stick spectrum). Table I. Isotropic ESR Parameters 2.060 2.057 2.059 2.064 2.07d 2.061 2.06d 2.076
49.2 50.6 43.0 41.5 44.3 45.4 46.2 34.6
2.060 2.057 2.066 2.070 2.064 2.061 2.073 (Ph,C,)Co(CO)[Ph,AsCH,CH,AsPh,] 2.075
47.8 48.0 43.3 43.3 43.0 43.4 40.5 35.6
(Ph,C,)Co(CO), (f-Bu,C,)Co(CO), (Ph, C,)CO(CO),P-TZ-BU, (Ph,C,)Co(CO)zP(c-C, Hi113 (PhzC,)Co(CO), PPh, (Ph, C, )Co(CO),P(OMe), (Ph,C,)Co(CO),P(OPh), (Ph,C, )Co(CO), AsPh, (f-Bu,C,)Co(CO), P-n-Bu, (t-Bu, C,)Co(CO), P(OMe), [CF,C,Si(CH,), ] Co(CO)(P(OMe),], [(CF, ), C, 1 W C O ) [P(OMe), 1 (ph,C,)Co(CO)[P(OEt),l (Ph,C,)Co(CO)[P(OMe), 1 (Ph,C,)Co(CO)[Ph, PCH,CH,PPh,]
,
,
,
(Ph,C,)Co(CO)[Ph,PCH,CH,AsPh,l [ICF,),C, I Co[P(OMe), 1 (Ph,C,)Co[P(OMe), 1
,
,
2.064 43.1 2.074 38.8 2.068 45,O
112.0 (1 P) 110.6 (1 P) 114.1 (1 P) 166.3 (1 P) 170.5 (1 P) 135.4 (1 As) 111.3 (1 P) 165.3 (1 P) 102.9 (2 P) 97.3 (2 P) 105 .o (2 P) 105.1 (2 P) 67.0 (2 P) 135.4 (1 As) 113.2 (1 P) 77.4 (3 P) 78.8 (3 P)
a 50.002, evccpt where noted. Units of cm-' ; unccrtainty. 50.2. Units of loT4cm" ;uncertainty, ~ 0 . 5 . ?0.01.
Ph2AsCH2CH2AsPh2,or Ph2PCHzCH2AsPh2give, respectively, 24-, 32-, and 16-line spectra. Thus, in contrast to the Ph2PCH2CH2PPh2species in which coupling to both donor atoms is observed, coupling to only one donor atom (75Asand ,'P, respectively) is shows by the other two species (Table I). These three radicals have stabilities characteristic of other disubstituted species, and the latter two give spectra with broader lines suggestive of unresolved 75Ascoupling. We have therefore assigned disubstituted structures to all three and conclude that the radicals have two types of substitution sites, one of which results in a large ligand coupling and the other a much smaller coupling. When one or both of the ligand
Paramagnetic Organometallic Molecules
Inorganic Chemistry, Vol. 23, No. 14, 1984 2021
atoms is As, exchange between the two sites is slow, but when two or three phosphorus ligands are present, exchange is fast enough that the 31Pnuclei appear equivalent. Line width effects expected from this exchange process are in fact observed and are discussed below. Line positions were fitted to a second-order solution to the spin Hamiltonian'O to give the isotropic g values and hyperfine coupling constants shown in Table I. In most cases, spectra were recorded at several temperatures. The g values and cobalt coupling constants are independent of temperature within experimental error, as are the phosphorus couplings in the di- and trisubstituted radicals. The phosphorus couplings in the monosubstituted radicals, on the other hand, are somewhat temperature dependent, with a temperature coefficient of about -O.l%/K. The coupling constants in Table I refer in every case to the best resolved spectrum obtained usually a t or above 0 OC. ESR Line Width Studies. Incomplete averaging of the g and hyperfine tensor anisotropies is expected to result in line broadening that is in general expressable as a power series in the nuclear spin quantum numbers'&12 (eq l), where mi is the
nuclear spin quantum number of the ith nucleus. The parameter a includes the line width contributions that are independent of mi, including a term proportional to (gll- g,)27R where T~ is the rotational correlation time. The parameters Pi are proportional to the products of g and hyperfine tensor anisotropies, (gll- g,)(Alli - Ali)~R,whereas yi and E, are related primarily to the hyperfine tensor anisotropies, yi a (Alli - Ali)27Rand cv a (Alli- ALi)(A,(.-A1()7R. In the spectra at hand, we observe a decrease in line width with increasing magnetic field indicating that Pc0 is important and that this parameter has the same sign as (Aco). Although ycois significant, the spectra of the monosubstituted radicals show little or no variation in line width with 31Por 14Asquantum numbers, consistent with the expectation that the 31Pand 75As hyperfine tensors are nearly isotropic. In spectra with two (or three) 31Pcouplings, on the other hand, the line widths are strongly dependent on mp, but with Pp negligible and yp negative. That is, the mp = f l (or *3/2) lines are sharper than the corresponding mp = 0 (or f1/2) lines. This behavior cannot be explained by incomplete averaging of anisotropies; it must be due to exchange of 31Pnuclei between coordination sites with different hyperfine couplings. If we assume that, in the disubstituted radicals, one 31Phas a large coupling and the other small (one large and two small couplings in the trisubstituted radicals), then the contribution to the width (in Gauss) of the mp = 0 (or f 1 / 2 ) lines should be given by" (2), where AA and ABare the coupling constants
at sites A and B (in cm-I), pAand pBare the probabilities that the sites will be occupied by a phosphite ligand, and 7 is the mean lifetime, 7 = TATB/(rA 7B). Note that because of the form of eq 1, the correct mrindependent line width is a yp. Direct measurement of line widths in these spectra is difficult because of extensive overlapping of components, and the usual manual techniques were unsuccessful. Accordingly, a nonlinear least-squares program, written in PASCAL for the ASPECT-2000 computer, was developed that fitted a simu-
+
+
(10) Wilson, R.; Kivelson, D. J . Chem. Phys. 1966, 44, 154-168. (11) Fraenkel, G. K. J. Phys. Chem. 1967, 72, 139-171. (12) Peake, B. M.; Rieger, P. H.; Robinson, B. H.; Simpson, J. Imrg. Chem. 1979, 18, 1000-1005.
Table 11. Line Width Parameters for (Ph,C,)Co(CO)[P(OMe), ]
TlK
a + 7p
290 270 250
13.8 14.1 17.0
Pco
YCO
yp
101'P/s
-2.03 -2.77 -3.81
0.48 0.68
-3.06 -4.67 -9.25
2.2 3.3 6.5
0.90
Line width parameters, in units of Gauss, refer to the width between extrema of first derivative lines. Mean lifetime computed from yp and an estimate of (A* - A B ) ' . See text. a
lated spectrum to a digitized experimental spectrum with a,
Pa, ya, and yp as adjustable parameters. The simulation used values of (g) , (Ac"),and (Ap),which were separately obtained
from a least-squares analysis of the line positions in the same spectrum. Average fitting errors ranged around 0.3%, and the resulting simulated spectra are very good matches to the experimental spectra (Figure 1). Spectra of (Ph2C2)Co(CO)[P(OMe)3]2at 250,270, and 290 K were analyzed in this way; the line width parameters are given in Table 11. The parameters cy + yp, Pcor and ycoare seen to increase with decreasing temperature as expected if the line widths are due primarily to slow tumbling of the radicals in solution, 7 R a N / T . Indeed the temperature variation of the parameters Pa and yc3give Arrhenius activation energies of 9.5 f 0.3 and 9.4 f 0.7 kl mol-', respectively. The temperature variation of a yp is considerably less, suggesting other major contributions that are independent of mi.13 In order to determine 7 from the measured line width contributions, we must estimate the couplings at the two sites. We can approximate these couplings by comparing the 31P couplings in (Ph2C2)Co(CO),-,[P(OMe)3]x for x = 2 and 3 (see Table I). If A, is the coupling at site A and AB is the coupling at site B and we assume that there is a site A phosphite in both cases, one site B ligand for x = 2, and two site B ligands for x = 3, then the average couplings for x = 2 and 3 are
+
A2 A3
= 1/2(AA + AB)
(3a)
= )/3(AA + 2AB)
(3b)
Fitting the experimental values of A2 and A3 to eq 3 gives A, = 184 X lo4 cm-' and AB = 26 X cm-'. Substitution of these coupling constants into eq 2, together with the experimental values of yp, gives the values of 7 given in Table 11. The temperature dependence of 7 corresponds to an activation energy of 17 f 2 kJ mol-'. The estimate of the site A coupling is about 10%larger than the 31Pcoupling in (Ph2C2)Co(CO)2[P(OMe)3],suggesting that the monosubstituted radicals may exist as an equilibrium mixture of isomers with phosphorus ligands at sites A and B. This is probably the reason for the small temperature dependence of the 31Pcoupling in monosubstituted radicals. Frozen-Solution ESR Spectra. ESR spectra of frozen T H F solutions of the monocobalt radicals, generated as described above, varied considerably in resolution and in signal-to-noise ratio and were often complicated by low-intensity spectral features apparently due either to other radical species or to other radical conformations. The best quality spectrum was that of (?-Bu~C~)CO(CO)~P-~-BU~, and this spectrum was analyzed in detail. The spectrum, shown in Figure 3a, shows a fairly obvious series of features, spaced about 100 G apart, which correspond to the largest component of the 59C0hyperfine tensor. The corresponding 31Phyperfine tensor component is also approximately 100 G. A second series of features can be identified with a 59C0coupling of about 60 G and a 31Pcoupling of about 110 G . Several features can be (13) Contributions to a from the spin-rotation interaction and unresolved hyperfine coupling are probably most important. See ref 10-12.
2022 Inorganic Chemistry, Vol. 23, No. 14, I984 a
Casagrande et al. and s9Cohyperfine tensor components are quite insensitive and simulated spectra are virtually superimposable. Thus, we can say only that the 31Ptensor axes are within about 30° of the g tensor axes and that the major axes of both hyperfine tensors (i.e., the largest 31Pand the positive 59C0components) are approximately colinear. Other frozen-solution spectra were qualitatively similar to that described above. In those cases where spectral features can be assigned with certainty, the g tensor and 59C0nuclear hyperfine tensor components differ only slightly from those given in Table 111. Spectra of the di- and trisubstituted radicals are considerably more complex. There is some indication that the fluxional behavior responsible for the line width effects described above is still fast enough at 120-1 40 K to cause the features from some nuclear spin states to be in the fast-exchange limit, some to be in the slow-exchange limit, and some to be in the intermediate-exchange region. Although series of features can still be identified that permit determination of the two largest 59C0hyperfine tensor components (which are similar to those given in Table 111), determination of the 31Phyperfine tensor is not possible. Experiments at liquidhelium temperature are planned.
h
'i
Discussion
i .
8"
lil,,
Ib,O
>,,n r i , r ,
h,,,
*rm
,',&"I *'
