4467 could be arranged in the following order in the nephelauxetic series: cyclam > en 2 pn > NH3. Therefore, the 3d electrons of Co(II1) are delocalized to the largest extent in cyclam and least in ammine comSir: plexes. This seems t o indicate a gradual decrease of donor lone-pair donation to Co(II1) along the series. In the course of the structural and mechanistic studies In other words, the metal ion tends to attract the of octahedral Co(II1) complexes containing a macrodonor lone-pair electrons to itself closest in the cyclam cyclic quadridentate amine ligand, 1,4,8,1 l-tetraazaand least in the ammine complexes. This deduction cyclotetradecane, abbreviated as cyclam, a few spectacuis reasonable since all octahedral Co(II1) complexes lar properties have been observed which differ markedly containing saturated alkylamines are structurally, elecfrom those of the corresponding bisethylenediamine, en, tronically, and energetically fairly similar. complexes. l v 2 (I) trans-[Co(cyclam)ACl]+, where A = The earlier mentioned differences in properties beC1, NCS, NO2, and CN, is thermodynamically more tween cyclam and bisethylenediamine complexes could stable than trans-[C~(en)~ACl]+with respect to the now be explained in terms of the greater expansion and transcorresponding trans-[Co(cy~larn)A(H~O)]~+ of the 3d shell and hence a greater demand of donor [CO(~~),A(H,O)]~+, respectively. The cyclam comlone-pair elections by Co(II1) in cyclam complexes. plexes, even supplied as the nitrates, only partially reConsequently, in these complexes, the metal-ligand lease the chloride at equilibrium, whereas under bonds are strengthened. This idea is strongly supsimilar conditions the corresponding bisethylenediamine ported by the observed greater acidity of aquo-cyclam complexes aquate with complete loss of the chloride. 3-5 complexes than of the corresponding aquo-en com(11) trans- and cis-[Co(~yclam)(H~O)~]~+ are more acidic plexes. The relatively greater attraction of water lonethan trans- and ~is-[Co(en),(H~O)~]~+, respe~tively.~~~ pair electrons by Co(II1) in cyclam complexes indirectly This effect is most pronounced for the first pK, of these weakens the 0-H bond of the aquo ligand and, thereaquo complexes. (111) The acid hydrolysis of transfore, increases the acidity of their aquo complexes. This [Co(cyclam)ACl]+ is slower than those of the correeffect of metal-ligand bond strengthening would be 1,2,8 (IV) The base hysponding trans-[C~(en)~ACl]+. more pronounced for negatively charged ligands, such drolyses of trans-[Co(cyclam)C12]+ and trans-[Co(cyas Cland OH-, than for neutral molecules, such clam)OHCl]+ are faster than those of trans-[Co(en)2as OHz, since the electron pairs from the former donors Clz]+and trans-[Co(en),OHCl]+, r e s p e ~ t i v e l y . ~This ~~~~ are relatively more “basic.” This immediately explains communication attempts to correlate these seemingly the observation that chloro-cyclam complexes are more unrelated facts and to explain them in terms of the stable with respect to their aquo complexes than the “thermodynamic” and “kinetic” nephelauxetic effects corresponding bisethylenediamine analogs. That hyof various amine ligands on the central Co(II1) ion. droxo-cyclam complexes would be more stable with The energy separation between the two excited respect to their aquo counterparts would, therefore, singlets, lTIg and lTPg, of octahedral Co(II1)-amine be responsible for the greater acidity of the latter complexes, [ C O ( A ~ ) ~ ] is ~ +given , by Aij = [16 than the corresponding bisethylenediamine analogs. (84B/lODq)]B. For the series of most saturated alkylThis effect would be more obvious for a triply charged amine ligands, such as Am = NH, and ‘/*(en), since diaquo complex than for a lower positively charged lODq and the Racah interelectronic parameter B are hydroxo-aquo complex, thus explaining the pronounced of the order 23,000 and 600 cm-’, respectively,1° it is difference in the first pK, of the diaquo complexes. quite safe to infer that the larger the An, the larger is These are “thermodynamic nephelauxetic effects.” the value of B. In order to estimate the relative posiA “kinetic nephelauxetic effect” is realized when tion of cyclam in the nephelauxetic series as defined by the aquation rates of trans-[Co(cyclam)ACl]+ are found proved to be Jorgensen,l0 trans-[Co(~yclam)(NH~)~]~+ slower than those of the corresponding bisethylenedithe best model closest to a “perfect” octahedral Co(II1) amine analogs. Ethylenediamine complexes, which complex containing cyclam. As will be shown in have a smaller reduction in the Racah electronic re~]~+ Table I, since An in t r a n s - [ C ~ ( e n ) ~ ( N H ~ ) (8300 pulsion parameter B, would have a greater demand ern-')'' is midway between those in [ C O ( N H ~ ) ~and ]~+ to expel the leaving group in order to gain a greater [Co(en),]3+, it is reasonable to suggest that the value delocalization of the 3d electrons into the vacated of B for cyclam differs even greater from that for so that the ground-state electronic repulsion orbital N H 3 than the average value as observed in transcould be reduced in the transition state. The reaction [Co(cy~lam)(NH~)~]~+. From the subsequent Table I, rate is therefore faster.12 In both series, the labilizing it is clear that some of the common amine ligands power of A follows the same pattern: NH2 > OH > NO2 > C1 CN > NCS. Since C1 and CN are so vastly different in their 7-conjugative ability, being (1) K. S. Mok and C. K. Poon, Znorg. Chem., in press, and references therein. operative in opposite directions, and in their ligand (2) C. K . Poon and M. L. Tobe, J . Chem. SOC.A, 2069 (1967). field strength, one of the obvious factors for group(3) S. Asperger and C. K . Ingold, ibid., 2862 (1956). ing these two ligands together in their labilizing power (4) S. C. Chan and M. L. Tobe, ibid., 514 (1963). (5) R. S. Nyholm and M. L. Tobe, ibid., 1707 (1956). is their similarity in their nephelauxetic effect on Co(6) C. K. Poon and M. L.Tobe, Inorg. Chem., 7,2398 (1968). (111). w (7) J. Bjerrum and S. E. Rasrnussen, Acru Chem. Scand., 6, 1275 (1952). The faster base hydrolysis of trans-[Co(cyclam)Clz]+ (8) C. K . Poon and M. L. Tobe, Coord. Chem. Reu., 1,81 (1966). and trans-[Co(cyclam)OHCl]+ could also be predicted. (9) C. K . Poon, Ph.D. Thesis, London, 1967, p 153. Influence of Nephelauxetic Effect on Thermodynamic and Kinetic Stability of Octahedral Cobalt(II1)-Amine Complexes
-
(10) C. K . Jorgensen, “Absorption Spectra and Chemical Bonding in Complexes,” Pergamon Press, London, 1962, p 136. (11) F. Basolo, J . Amer. Chem. SOC.,72,4393 (1950).
(12) C. H. Langford and H . B. Gray, “Ligand Substitution Processes,” w. A. Benjamin, New York, N . Y., 1965, p 72.
Communications to the Editor
4468 Table I. Some Thermodynamic and Kinetic Data of Octahedral Co(II1)-Amine Complexes ~~~
Amine ligand,a Am “3
‘Idrn-bn) I/?(dl-bn) ’/z(pn) l/z(en) l/z(N-me.en) l/dcycIam)
A;: cm-1
PKd
8500
5.79n
...
...
... ... ...
pKwc .
...
...
4 I45h
I . 94h
8000’
2.82i
7.22i
...
...
1.8 x 4.2 x 1.5 x 6.2 x 3.2 x 1.7 x 1.1 x
I
8200 8100
k o d (25O), M-1 sec-1
10-2 10-3
1 . 6 X 106
10-4
5.0 X 4.5 x 2.5 X 3.0 X 3.0 x
1.8 x 9.8 x 2.1 x 2.3 x
~ H , (25”), o ~ sec-l
... .
kexe(25”), A 4 - I sec-1
...
10-6 10-5 10-5 10-6
...
k
lo6 106 lo6 108 109
3.0
x
1.1 x 6.7 x
103 103 103 103 103 104 104k
m-bn = meso-butylenediamine, dl-bn = dl-butylenediamine, pn = propylenediamine, N-me .en = N-methylethylenediamine. * A; = energy separation between ‘TI, and lTz, of [cO(h1)6]~+in aqueous solution. In the case of cyclam, Aij is for trans-[Co(cyclam)(NH3),13+. Data were taken from M. Linhard and M. Weigel, 2.Anorg. Allg. Chem., 266,49 (1951); 271,101 (1952), except as indicated. c pK,,j and pK[*j are the first and second pK., respectively, for trurzs-[C~(Am)~(H~O)~]~+ in aqueous solution. Complicated by cis-trans isomerization reactions, not many reliable data have been reported. kE,o = first-order aquation rate constant for tran~-[Co(Am)~Cl~]+. Data were taken 78,709 (1956), except as indicated. e k , , = second-order aminefrom R. G. Pearson, R. E. Meeker, and F. Basolo, J. Amer. Chem. SOC., proton exchange rate constant for [CO(Am)6]3+in D O In the case of cyclam, k e x is the estimated value for the complex rrans-[Co(cyclam)82, 1073 (1960), except as indicated. (HzO)z]3++.Data were taken from F. Basolo, J. W. Palmer, and R. G. Pearson, J. Amer. Chem. SOC., J koH = second-order base hydrolysis rate constant for trans-[C~(Am)~Cl~]+ in aqueous solution. Data were taken from Pearson, et a/., footnoted, except as indicated. 0 The gross acid dissociation for the cis-trans equilibrium. The equilibrium cisltruns ratio is 0.17; Pearson, et ul., footnote d and R. G. Yalman and T. Kuwana, J. Phys. Chem., 59,298 (1955). B. Bosnich, C. K. Poon, and M. L. Reference 7. Tobe, Inorg. Chem., 4,1102 (1965). j Reference 6 . Reference 2. Estimated from ref 6. a
Assuming an SNlcb mechanism, l 8 , I 4 the rate constant is directly proportional to the product of k, and kcb, where k, is the acid ionization constant of the conjugate acid and kcb is the rate constant for the dissociation of the conjugate base. The greater tendency by Co(II1) to attract donor electron density in cyclam complexes, on one hand, indirectly weakens the N-H bond and, therefore, increases the value of k, while, on the other hand, promotes k c b by enhancing the a-donating ability of the amido group in the conjugate base. Both effects jointly increase the rate of base hydrolysis of the two cyclam complexes. The secondorder amine proton exchange rate constant for trans[Co(cyclam)(HZO)Z]3+could be estimated to be 3 X lo9 (M-I sec-I) at 250.15 This is very much faster than that of the equally charged [ C ~ ( e n ) ~(Table ] ~ + I). Similarly, both “thermodynamic” and “kinetic” nephelauxetic effects could also explain satisfactorily the properties of other Co(II1)-amine complexes and to correlate them with the cyclam and bisethylenediamine analogs. Some of these properties are collected in Table I. It is obvious from Table I that as Ai; decreases down the amine series p& and kHzOdecrease while k,, and koH increase. This systematic variation is fully consistent with the above discussion. In the absence of available data on A? and pK[v for m-bn, dl-bn, and N-me. en, these ligands are inserted into the series according to a decreasing order of kH,O. As a conclusion, it seems worthwhile noting that the nephelauxetic effect is still only one of the many factors affecting the thermodynamic and kinetic stability of coordination compounds. Its importance may be relatively more pronounced in octahedral low-spin d6 systems, such as Co(II1)-amine complexes. It is in these systems that there are a maximum number of electrons in the nonbonding or antibonding ?r orbitals to display the maximum benefit out of this d electron repulsion effect. Even in this Co(II1)-amine system, however, other factors, under favorable conditions, (13) F. Basolo and R. G. Pearson, “Mechanisms of Inorganic Reactions,” 2nd ed, J. Wiley, New York, N. Y., 1967, p 177. (14) C. K. Poon and M. L. Tobe, Chem. Commun., 156 (1968). (15) Estimated from suitable data in ref 6.
may overshadow the nephelauxetic effect. A higher kHIOand koH for (m-bn) than for (dl-bn) is a good indication of the influence of steric effect in the reactions of these two compounds. The two amine ligands are electronically similar but sterically different. In the meso form, the methyl groups are cis and very nearly maximally opposed. Consequently, the dissociative rate constants of the complex and of the conjugate base are higher than those in the corresponding dl isomers. The steric effect, however, cannot be more important than the nephelauxetic effects in Co(111)-cyclam complexes; otherwise, both kHzOand koH of these complexes wouId be changed in the same direction when compared to those of the bisethylenediamine analogs. Acknowledgment. The author thanks the Committee on Higher and Research Grants of the University of Hong Kong for financial support. C. K. Poon Department of Chemistry Unicersity of Hong Kong, Hong Kong Received April 20, 1970
Synthesis and Characterization of a Nitrogen-Bridged [12]Annulene Sir:
0 t h and Schroder and coworkerslagb have recently reported the synthesis of [12]annulene in which the cis and trans double bonds alternate. Farquhar and LeaverZ have described the preparation of the related cyc1[3.3.3]azine. We now wish to report the synthesis of the [ 12lannulene derivative 8b,8c-diazacyclopent[fglacenaphthylene (4) in which, because of the presence of a central N-N bridge, the double bonds are situated trans-trans-cis-trans-trans-cis.
Scheme I delineates the reactions which have led t o the formation of this compound and its structural interrelationship with the starting compound 3,4,7,8tetrahydro-8b,8c-diazacyclopent[fg]acenaphthylene (2). (1) (a) J. F. M. Oth, H. Rottelle, and G. Schroder, Tefrahedron Lett., 61 (1970); (b) Jean F. M. Oth, J. M. Gilles, and G. Schroder, ibid., 67 (1970). (2) D. Farquhar and D. Leaver, Chem. Commun., 24 (1969).
Journal of the American Chemical Society / 92:14 / July 15, 1970
