J . A m . Chem. SOC.1982, 104, 3833-3837 211(1) state is 18 kcal/mol below25 the 42-. In addition, the reaction Li2(lzg+) C(3P) C-Li(4Z-) + Li(,S)
-
+
is exothermic by at least 27 kcal/mol, while the corresponding H2 reaction H2(12:g+)+ C(3P)
+
C-H(,II)
+ H(’S)
is endothermic26 by 23 kcal/mol. For Li2C we have found three bound triplet states well below the first singlet. While two of these triplets, the 32; and 3A2have very different geometries, they have essentially identical (at our level of accuracy) energies. We estimate that both are bound relative to C(3P) Li,(’Z,+) by at least 53 kcal/mol. If one reduces the symmetry from C , to C,, these two triplets share the 3A” symmetry and would suffer an avoided crossing. This avoided crossing will have a significant effect on the dynamics of the reaction of ground state C and Liz. Both the ground state of C-Li and the low-lying 3A2state of Li2C are characterized by a donation of electrons from Li or Liz to an “empty” p, orbital on C. Schematically
+
P,
Po 2%
2s
+
completely analogous to the N+ H2case discussed by Dewar.22 This observation prompts the speculation that one might fruitfully consider the recently p r e d i ~ t e d dilithiomethane ~,~ structure as carbenoids resulting from the donation of electrons from the bonding orbital on Li, into a formally empty CJ orbital on CH,. For example, the planar ‘Al state would have the form
Hy&f ”
2 as( a ,)
&(a,)
(25) Kasdan, A.; Herbst, E.; Lineberger, W. C. Chem. Phys. Lett. 1975, 31, 78. (26) Shevlin, P. B. ‘Advances in Reactive Intermediates”; Abramovitch, R. A., Ed.; Plenum Press: New York, 1980; Vol. 1, p 9.
3833
Our calculations on CLi, suggest that in this mode of bonding the Li2 separation would be between 5 and 6 bohr and the Li, end of the molecule positive relative to the C H 2 end. The optimized S C F structure of Ladig and Schaefer6 has the Li2C angle as 101.7O and R(C-Li) = 3.485 bohr (corresponding to a Li, separation of 5.40 bohrs and a C-Li, separation of 2.20 bohrs), with a dipole moment of +4.85 D. As we rotate the Li, group 90’ about the C2 axis to form the “tetrahedral” isomer, we lose the stability due to the delocalization of the carbon pr electrons into the Li p, orbitals (a Li-Li bonding interaction), but we gain the ability to delocalize these electrons into the Li, up* orbital. This should result in an increased Li, separation and LS6 calculated 6.49 bohrs which is an increase of 1.09 bohrs over the planar separation. We can imagine the planar triplet being formed from the planar singlet by exciting an electron from the 2p, on carbon to a primarily ug orbital (a, in C ), on Li,. Qualitatively, this ng orbital would be the out of phase combination of the methylene o and Liz 2ug orbitals. When this orbital is occupied, one anticipates that, relative to the planar singlet, the Li, separation would decrease, the C-Li, separation would increase, and the dipole moment be negative on the Liz side of the molecule. LS6 calculate a Liz separation of 4.684 bohrs (a decrease of 0.721 bohr), a C-Li2 separation of 3.13 1 bohrs (an increase of 0.93 1 bohr), and a dipole moment of -1.22 D (planar lAl was +4.85 D). As with the planar singlet, rotating the Liz group in the 3B, state by 90’ about the C, axis would destroy the delocalization of the carbon 2p, orbital into the Liz K orbitals but will alow the delocalization of this 2p, orbital into the Liz mu* orbital. Because there is only one electron in this K orbital and the Li2 is rather distant (>3 bohrs) from the carbon, this delocalization would increase the Liz separation only slightly. The calculated6 increase is 0.066 bohrs. While the ease with which this very simple model accounts qualitatively for the structural changes in the dilithiomethane is satisfying, the speculations should, of course, be checked with detailed calculations.
Acknowledgment. J.F.H. has benefited from the stimulating environment provided by the members of the Theoretical Chemistry Group at Argonne. The willingness of B. Botch, L. Harding, and T. Dunning, Jr., to advise us on the use of the various codes has been crucial to the completion of this study. Registry No. LiC, 81572-61-4; Li,C, 72023-94-0.
Electron Transfer to Complex Ligands. Radical Anions and Organomagnesium Radical Complexes of 2,2’-Bipyridines and 1,lO-Phenanthrolines Wolfgang Kaim Contribution from the Chemistry Department, J . W . Goethe Universitaet, Niederurseler Hang, 0 - 6 0 0 0 FrankfurtlMain, West Germany. Received April 24, 1981
Abstract: The bidentate complex ligands 2,2’-bipyridine (l),4,4’-dimethyLZ,Z’-bipyridine (2), 1,lO-phenanthroline (3), and 4,7-dimethyl-1,lO-phenanthroline(4) have been reduced to radical anions by potassium metal and to organomagnesium radical complexes in a single electron transfer (SET) reaction of diphenylmagnesium. Well-resolved ESR spectra were obtained that could be analyzed on the basis of HMO calculations. In the case of the l,l0-phenanthrolines there are two low unoccupied molecular orbitals available to accommodate the additional electron; a comparison of the spin distributions demonstrates that, in contrast to earlier assumptions,it is the 5bl orbital that is singly occupied. This result helps to explain the similar properties of corresponding 2,2’-bipyridine and 1,lO-phenanthroline metal complexes.
2,2’-Bipyridine (bpy) and 1,lO-phenanthroline (phen) have been widely used as “classical” complex ligands for metal ions and 0002-7863/82/1504-3833$01.25/0
inorganic or organometallic fragments.’-3 Apart from their good n-donor complexing properties due to the bidentate coordination, 0 1982 American Chemical Society
Kaim
3834 J . Am. Chem. Soc.. Vol. 104, No. 14. 1982 these systems are distinguished because of their relatively low lying ff* orbitals. As a result of this particular combination of electronic properties these ligands often form highly colored coordination cumpounds because of a low energy charge transfer in the excited state;M they can stabilize labile inorganic or organometallic species by accepting excess negative charge;’.s they also form easily reducible complexes with the additional electron(s) residing often in the ligand T * orbital,l-’ and finally, they can function as negative molecular ions, thus stabilizing metals in formally low oxidation state^.^.^^.^.^^ Among the mmt simple coordination complexes of bpy and phen are the alkali metal “ion pairs”” that form in the reaction of the corresponding heterocycle and an alkali metal in solvating ethers. However, while 2,Y-bipyridine ion pairs were reported as early as 195812and have been frequently studied since then,”.14 the 1.10-phenanthrolinenegative ion has not been well documented so far. Although the electrochemical reduction of phen has been an unequivocal ESR characterization of the reduced species is still missing: Assignments of ESR coupling constants for the phen radical anion were described as either ‘p~obable”‘~ or “not unambiguous”?0 Moreover, the fact that twd molecular orbitals compete for the status of lowest unoccupied M O (LUMO) has not been critically considered before?’ Recent investigations have demonstrated that not only alkali metals but also organ~metallics’~~~’ such as Grignard reage n t ~ ’ can ~ transfer ~ ~ . an ~ electron ~ ~ ~ to ~ suitable N-heterocycles.
( I ) Cotton, F. A,; Wilkiinson, G. ‘Advanad Inorganic Chemistry”. 4th ed.; Wiley-Interscience: New York, 1980; pp 119-121. (2) MeWhinnie, W. R.; Miller. J. D. Ado. Imrg. Rndioehem. 1969. 12, 135. (3) Schilt. A. A. ‘Applications of I,lO-Phenanthroline and Related Compounds”; Pergamon: London, 1969. (4) Cf.: Mason. S . F. Inorg. Chim. Aelo Re”. 1968. 2, 89. Balzani. V.; Carmetti. V. T h e Photochemistry of Coordination Complexes”; Academic Press: New York. 1969. (5) Ito, T.;Tanaka. N.; Hanazaki, 1.; Nagakura, S. Bull. Chem. Sm. Jpn. 1969. 42, 702. (6) (a) Coats, G. E.; Green, S. 1. E.3. Chem. Sm. 1962.3340. (b) Brown, 1. M.; Wcissman. S. 1. 3. Am. Chem. Soe. 1963,85. 2528, J . Chem. Phys. 1%5,42. 1105. (e) Watsan, S.C.; Eastham, J. F.J. olgonomel. Chem. 1%7, 9. 165. (7) Yamamoto. A,; Morifuji, K.; Ikcda, S.; Saito. T.; Uchida, Y.; Misono, A. 3. Am. Chem. Sm. 1965,87, 4652. Wilke, G.;Hcrrmann. 0. Angew. Chem.. Inl. Ed. Engl. 1966, 5, 581. (8) Chishoim. M. H.;Huffmman, J. C.; RothweII, 1. P.; Bradley, P. G.; KICSS,N.; Woodruff, W. H. J . Am. Chem. Sm. 1981,103. 4945. (9) Cf. the list of compounds given in ref 2. p 190. (IO) Henag, S.‘Neuere Entwicklungcn der anargankhen Chemie”; VEB Deutscher Verlag der Wissensehaften: Berlin, 1974; p 366. (1 I ) Szwarc, M.,Ed. “Ions and Ion Pairs in Organic Reactions”; WilcyInteneience: New York, 1972;Vol. I, especially thc contribution by Sharp, J. H.; Symans, M. C. R.. p 177. (12) Elschner. B.; Hermg, S. Arch. Sei. 1958. I f , spectrum No. 160. (13)Cf. the litmaturecited in: Hanson, P. Ad”. Hetermwl. Chem. 1979, 25, 205. (14) Kaim, W. Chem. Ber. 1981, 114. 3789. (15) Tabncr. B. 1.; Yandlc, J. R. 3. Chem. Sm. A 1968, 381. (16) HOnig. S.;G r w . J.; Lier. E. F.; Quaa, H. Liebigs Ann. Chem. 1973, 339 and literature cited therein. (17) GOrlier, 0.; Dictz. K. P.; Thomas, P. 2. Anorg. Allg. Chem. 1973, 396, 217. (18) Rusina. A,; Vlcek, A. A.; Zalis, S. 2. Chem. 1979, 19. 27. (19) Dmy. R. E.;Charkoudian, J. C.; Rhcingold. A. L.3. Am. Chem. Soe. 1972.94.738. Identical ESR coupling m s t a n u were reported by: El-Shady. M. F. lmrg. Chim. Aelo 1978. 26, 173. (20) Gooijer, C.; Velthorst. N. H.; MacLcan. C. Mol. Phys. 1972. 24. 1361. 121) Kaim.. W. J. Oreonomet. Chem. 1981.222. . . C17 (22) Kaim. W. Angew. Chem., Inl. Ed, Engl. 1982.21, 141; Angew. Chem. Suppl. 1982, 298. (23) . . Kaim. W. 2. Naturforsch., in ~ r a s . (24) Kaim, W. Angew. Chem., Int. Ed. Engl. 1982. 21. 140: Angew. Chem. Suppl. 1982,289.
Figure 1. ESR spcctrum of the radical complex formed in the reaction of 2,2’-bipyridine with diphenylmagnesium in T H F (A) and iu computer simulation (B). The line width is 0.010 mT. Table 1. ISR Coupling Constantsox (mT) of 2.2’-Bipyridine Radical Comdexes
.. 2a la Zb Ib
0.229 0.261 0.262 0.293
0.153
0.09P
0.122 0.120 0.067
0.106 0.16P 0.194
0.463 0.464 0.429 0.387
0.061 0.057 0.049
0.031
thiswork 32 this work 14,24
Methyl proton coupling constant Table 11. ESR Coupling Constantsax (mT) of 1.1 O-Phenanthroline Radical Complexes radical 4a 3a
4b 3b
0.262 0.280 0.255 0.290
0.054 0.041 b 0.021
OH(,)
OH(.)
0.428 0.360 0.305 0.290
0.11P 0.054
Methyl proton coupling constant. to H(2) or H(5) uncertain.
0.280 0.255“ 0.326
OH(+)
0.041 0.049 0.064
thiswork this work this w=rk 21
Not observed. assignment
thereby forming organometallic radical ~ o m p l e x e s . ’ ~ ~ ~ESR ’-’~ studies have exhibited characteristic changes of the spin distribution in these radical complexes upon organometal cwrdinati~n.“.~’”’ Since neither complexation by RMg+2’.24.2S nor the loose association with a potassium cation further complicates the
~
(25) Kaim W. 2. Norurforsrh., B 1981, 368. I I IO. (26) Kaim, W. J . Organomel. Chem. 1980,201.C5; 1981,215,325and 337; 2.Nolurjorseh., B 1981,368.617. (27) Cf. aim: Kaim, W. Inorg. Chim. Acln 1981.53.L151;Chrm. B p i . 1982. 115,910.
J . Am. Chem. SOC..Vol. 104, No. 14, 1982 3835
Electron Transfer to Complex Ligands l
l
Figure 2. ESR spectrum of the radical complex formed in the reaction of 4,4’-dimethyL2,2’-bipyridinewith diphenylmagnesium in THF (A) and its computer simulation (B). The line width is 0.013 mT.
ESR hyperfine structure of the reduced complex ligands, these two reduction methods have been used to prepare paramagnetic species from 2,2’-bipyridine (l),4,4’-dimethyL2,2’-bipyridine (2), 1,lO-phenanthroline (3), and 4,7-dimethyl- 1,lO-phenanthroline (4).
1
I::,
-
-
:< I
I’~,R”lqtl
2
l~a , 121, _ _ :
