Organometallics 1983, 2, 171-173
171
p7[ ;,*;2*p yy P
P
C'
C1 I /
P
P
P
P
I m Figure 3. Schematic representation of two sequential twists that could convert I to I11 without bond breaking: C = CO; C' = CNR, and P = PR3.
the neutral 18-electron species. From our studies it is clear that the order of rates is kl > k214 The similarity of the electrochemical behavior in various solvents as well as the reversibility established by the coulometric experiments support the notion that the structural rearrangement is nondissociative. Figure 3 illustrates a possible mechanism for the intramolecular conversion of I to 111. Our work provides the first examples of the electrochemically induced isomerization of complexes of the type MX2YzZ,that contain a M(CO), unit, where M represents a group 6 metal. We hope to expand the scope of these investigations to include a wider range of isocyanide and phosphine ligands, in particular focusing upon the constraints imposed by chelating phosphines of the type Ph2P(CH2),PPh2,since the latter will ensure that the isomerization will be different from that which has been studied so far (I 111).
I ',
i
I:
Pi
Acknowledgment. Support from the National Science Foundation (Grant CHE82-06117)is gratefully acknowledged. Registry No. MO(CO)~(CN-~-BU)~(PE~~)~, 80215-95-8; Mo(C0)2(CN-t-Bu)2(P-n-Pr3)2, 80215-96-9; M O ( C O ) ~ ( C N - ~ - B U ) ~ -
I
I
I
I
29
I
32
I
(PMePh2)2,80215-97-0;M O ( C O ) ~ ( C N C ~ H80215-99-2; ~~),, [Mo(CO)2(CN-t-Bu)2(PMePh2)2] +,83897-56-7;C7H7+PF6-,29663-54-5; [ M O ( C O ) ~ ( C N C ~ H ~83897-57-8; ~)~]+, [MO(CO)~(CN-~-BU)~(PMePh2)2]PF6, 83897-58-9; [ M o ( C O ) ~ ( C N - ~ - B U ) ~ ( P E ~ , ) , ~ + , 83897-59-0.
I
3.5
h
kb Figure 2. X-band ESR spectra of frozen CH2C12solutions at -160 "C of (a) [ M O ( C O ) ~ ( C N C ~ Hand ~ ~ )(b) ~ ] +[Mo(CO)~(CN-~-BU)~(PMePh2)2]+.
Scheme I -e M o ~ C O ) ~ ( C N R ) ~ ( P, R~)~
Mo(CO)Z(CNR)Z(PR~I~
-e
E l / Z(111I
(14)Note that for the so-called cross redox reaction
I+ + I11
& I + III+ ka
the order k3 > k3holds; for the appropriate theory see: Feldberg, S. W.; Jeftic, L. J. Phys. Chem. 1972, 76, 2439.
[Mo(CO)~(CNR),( PR 3)21+
[MO(CO)~(CNR)~~PR~I~~'
I11
the g, signal. The hyperfine lines in the gll region are well resolved with A = 50 G. Figure 2b shows an ESR spectrum that is typical of the [Mo(CO),(CNR),(PR&]~cations. For the phosphine complexes, the "perpendicular" signal is broader and also more structured, but only one g value is clearly resolved. The 1:2:1 triplet in gllresults from the interaction of the unpaired electron with two equivalent 31Patoms ( I = l/z). On the basis of the ESR evidence for equivalent phosphines and IR data which suggest that the two carbonyl ligands are trans as are also the two isocyanides, the structure of [MO(CO),(CNR)~(PR~)~]+ is concluded to be all-trans. The electrochemical and spectroscopic data for [MO(CO)~(CNR),(PR~),]~~~+ are in accord with Scheme I. Oxidation of isomer I is followed by rapid isomerization to the all-tram cation (possessing structure 111)that in turn isomerizes back to I upon its electrochemicalreduction to 0276-7333/83/2302-0171$01.50/0
New Stable Dlanions from the Electrochemical Reduction of ConJugated Bis(phenyltrlcarbonylchromlum) Groups Stuart N. Miiiigan and Reuben D. Rieke" Department of Chemistry, University of Nebraska Lincoln, Nebraska 68588 Received August 13, 1982
Summary: Bis(tricarbony1chromium)complexes of arene compounds with two conjugated phenyl rings reduce electrochemically in a two-electron fashion. The process is chemically reversible as shown by cyclic voltammetry and coulometry. (Arene)tricarbonylchromium complexes exhibit redox properties that vary as a function of the arene ligand. When benzene or substituted benzene rings are present, reduction is a two-electron yielding a highly 0 1983 American Chemical Society
Communications
172 Organometallics, Vol. 2, No. 1, 1983
Table I. Electrochemical Results compda
(biphenyl)Cr(CO), (bi~henyl)[Cr(CO),I
(9,lO-dihydrophenanthrene) Cr(CO), (9,lO-dihydrophenanthrene) [Cr(Co),l z (trans-stilbene)Cr(CO), (trans-stilbene)[Cr(CO), 1, (cis-stilbene)Cr(CO), (cis-stilbene)[Cr( CO),],
-1.948 -1.606 -1.982
40 30 33
2.72 2.68 2.52
-1.663
31
2.76
-1.656 -1.385 -1.673 -1.395
37 26 24 17'
1.83 2.82 2.48 2.78
Approximately 1 mM in THF, 0.2 M TBAP, 0 "C, on the DME. Vs. the Ag/AgCl, saturated NaCl ( a s ) reference electrode, i 0.005 V. The slope of the log plot is
"i
23 mV.
reactive dianion as shown by the irreversibility of the cyclic voltammograms. Naphthalene complexes, on the other hand, also reduce with two electrons, but the dianion is extremely persistent.' Phenyl ketone complexes undergo only one-electron reduction, but the odd electron is localized primarily on the ketone functionality of the ligand.4-7 Except for the phenyl ketone systems, which do not really involve the tricarbonylchromium group, all (arene)tricarbonylchromium complexes reduce in a characteristic two-electron per chromium atom manner. We report here reductive electrochemical studies demonstrating that bis(tricarbony1chromium) complexes of aromatic compounds containing two conjugated phenyl rings reduce with a total of two electrons. This is formally one electron per chromium atom. In addition, the dianions produced are very persistent. Mono- and bis(tricarbony1chromium) complexes of biphenyl, 9,10-dihydrophenanthrene, and cis- and t r a n s stilbene were chosen because the two phenyl rings are conjugated. As expected, the mono complexes exhibit reductive characteristics that are analogous to the tricarbonylchromium complexes of substituted benzenes. They generally reduce at about the same potential as (benzene)tricarbonylchromium with some allowance for extra conjugation. The reduction products are also quite reactive. The cyclic voltammograms are irreversible until scan rates in excess of 1 V/s at which point a very small anodic peak begins to appear. The redox properties of the bis(tricarbony1chromium) complexes associated with these conjugated ligands stand in marked contrast to their mono counterparts. Each of the bis complexes is reduced approximately 300 mV positive of its corresponding mono complex (see Table I). This significant decrease in reduction potential suggests coupling between the tricarbonylchromium moieties in the same way that extended conjugation in an organic molecule lowers the reduction potential. The two phenyl rings must be conjugated to experience this shift since dimethylbis(phenyltricarbony1chomium)tinreduces at approximately (1) Rieke, R. D.; Arney, J. S.;Rich, W. E.; Willeford, B. R.; Poliner, B. S. J . Am. Chem. SOC.1975,97,5951-5953. (2) Dessey, R. E.; Story, F. E.; King, R. B.; Waldrop, M. J. Am. Chem. SOC.1966,88, 471-476.
(3) Dessey, R. E.; King, R. B.; Waldrop, M. J. Am. Chem. SOC. 1966, 88, 5112-5117.
(4) Khandkarova, V. S.; Gubin, S . P. J. Organomet. Chem. 1970,22, 149-152. (5) Gubin, S.P. Pure Appl. Chem. 1970, 23, 463-487. (6) Ceccon, A.; Corvaja, C.; Giacometti, G.; Venzo, A. J. Chim. Phys. 1975, 72, 23-24. (7) Ceccon, A.; Corvaja, C.; Giacometti, G.; Venzo, A. J . Chem. SOC., Perkin Trans. 2 1978, 283-288.
-1.5
-1.7
-1.9
Figure 1. A cyclic voltammogram of (9,lO-dihydrophenanthrene)bis(tricarbonylchromium)in THF (0.2 M TBAP, 0 "C, 50 mV/s, HMDE).
Table 11. Infrared Carbonyl Bands compd (bi~hen~l)[Cr(CO),l,
(9,lO-dihydr0pnenanthrene)[ 0(cO)312
(trans-stilbene)[ Cr( CO), 1, (cis-stilbene)[Cr(CO),j
Cr ox. CO bands state 1963,1899 0 1881, 1788, -1 1765 1960, 1895 0 1878,1785, -1 1765 1962,1895 0 1888,1797, -1 1777 1964,1898 0 1896,1802 -1 1780
the same potential as (benzene)tricarbonylchromium. The longevity of the resulting dianions of the bis complexes contrasts with the high reactivity of the dianions derived from the mono complexes. An anodic to cathodic current ratio of unity in the cyclic voltammograms is observed for the bis complexes even with scan rates as low as 20 mV/s (see Figure 1). They also appear to have half-lives on the order of several hours when bulk coulometric studies are performed. In this respect the bis complexes are similar to (naphtha1ene)tricarbonylchromium.' Polarographic, voltammetric, and coulometric studies all indicate that the reduction process of bis complexes of arene ligands with conjugated phenyl rings involves a total of two e1ectr0ns.l~This one electron per chromium atom reduction distinguishes this class of tricarbonylchromium complexes from b e n ~ e n e l and - ~ naphthalene' complexes. Studies of the carbonyl stretching bands were also performed (see Table 11). The bands of the unreduced bis complexes are quite similar to those of other (areneb tricarbonylchromium complexes; two sharp bands are usually observed a t approximately 1960 and 1895 cm-' in THF solutions. Exhaustive electrochemical reduction results in the disappearance of these original bands and the appearance of new ones at about 1885,1790,and 1770 cm-'. The small peaks remaining at the original positions are most likely due to trace amounts of oxygen introduced upon transfer of the dianion. When exhaustive reoxidation is performed on the solution either electrochemically or by air, the low wavenumber bands disappear and the or-
Organometallics, Val. 2, No. I , 1983- 173 Scheme 1
Figure 2. Infrared carbonyl stretching bands for (biphenyl)bis(tricarhony1chromium): left; before reduction; center; after electrochemical reduction; right, after electrochemical reoxidation.
iginal carbonyl stretching bands are seen once again (see, for example, Figure 2). The carbonyl stretching bands for the dianions of the bis complexes were compared to values for other chromium carbonyl compounds obtained in ow research group' and by Hayter! The results indicate that both chromium atoms are formally in the -1 oxidation state. No mixedvalance interactions are indicated, and this further confirms a strong coupling between the tricarbonylchromium moieties. We propose that the following mechanism occurs during the reduction of these complexes. This mechanism is of the ECE type which is w d y associated with two-electron processes. Beginning with the neutral bis complex of hiphenyl, a one-electron reduction forms the radical anion making one tricarhonylchromium group a 19-electron moiety. This unfavorable situation can be relieved with an isomerization step. It is proposed that this occurs by changing from qs to q5 bonding in the 19-electron metal portion. This chromium would now be bound to five carbons, which is a pentadienide system and leaves carbon 1 no longer bonded to the chromium atom. Thus this chromium atom returns to the stable 18-electron configuration. However, this has left an unpaired electron in a benzylic position with regard to the unreduced ring. Resonance structures can be drawn so as to pair up this electron with the electron on the carbon 1' position. This leads to q5 bonding of the unreduced chromium atom and therefore a 17-electron species. Immediate reduction by a second electron occws hy virtue of the applied potential being quite negative of what is required to reduce this 17-electron species. The final result is that each chromium atom is part of an 18-electron system, bound to the ring in a q5 fashion and in the -1 oxidation state. A carboncarbon double bond is postulated to form between the phenyl rings. Precisely the same mechanism can be used to explain reduction of the (sti1bene)tricarbonylchromium complexes except that a butadiene moiety forms between the phenyl rings (see Scheme I). The proposed q5 bonding has several precedents. Cyclopentadienyltricarbonylchromium complexes have been known for some Semmelhack and co(8)Rieke, R. D.;h e y , 3. s.; Henry, w. p., unpublished resulta. (9)Hayter, R. G . J. Am. Chem. Soc. 1966.88.43764362. (10) Nesmsyanov, A. N.; Ustynyuk, N. A,; Novikova, L. N.; Andrianov, V. G.;Struehkov. Y. T.; Ustynyuk, Y. A.; Oprunenko, Y. F.;Luzikov, Y. N. J. Orgonomet. Chem. 1982,226,239-250 and references therein.
workerI2 have demonstrated the existence of a q5-bound species in work on nucleophilic substitution of (benzene)tricarbonylchromium. Recently, the compound with the structure shown below was r e p ~ r t e dthat ' ~ is analogous to the proposed structure of the dianion. r
.
We are presently attempting to isolate the dianions so as to characterize them further. We are also exploring the chemistry associated with these dianions.
Acknowledgment. We gratefully acknowledge support of this work by the National Science Foundation, Grant No. CHE78-06661. We also like to acknowledge a duPont Fellowship administered by the University of Nebraska Chemistry Department to SNM. Dr. Indu Tucker synthesized (9,10-dihydrophenanthrene)bis(tricarbonylchromium). The mono- and bis(tricarbony1chromium) complexes of cis- and trans-stilbene were supplied by Dr. Bennett R. Willeford. Registry No. (hiphenyl)Cr(CO),,12111-60-3;(biphenyl)[Cr(CO)s12. 33010-84-3; (9,10-dihydrophenanthrene)Cr(C0)3, 12094-55-2;(9,10-dihydrophenantrene)[Cr(C0)31~, 57395-32-1; (trans-stilbene)Cr(CO),. 12155-45-2: (trans-stilhene)ICr(CO)d,. 83861-09-0;(cis-stilbene)Cr(CO),,12155-44-1;(cis-s&ene) (CO)slz,83861-10-3.
[e;:
(11) Piper,T. S.;Willdnson,G.J. Inorg. Nuel. Chem. 1956,3,104-124. (12) Semmelhack, M. F.; Hall, H. T.;Yoshifuji, M.J. Am. Chem. Soc. 19'76,98,6387-6389. (13) Weber, L.; Wewers, D.Z . Naturforsch., B: Anorg. Chem., Org. Chem. 1982,37,68-72. (14) Polarographic and voltammetric experimentswere performed by using iR feedback compensation.
