DISUBSTITUTED MANGANESE PENTACARBONYL BROMIDE
Aug. 5, 1963 [CONTRIBUTION FROM
Metal Carbonyls. VI.
THE
2215
CHEMICAL LABORATORIES OF NORTHWESTERN UNIVERSITY, EVANSTON, ILL.]
Isomerism of Disubstituted Manganese Pentacarbonyl Bromide
BY ROBERT J. ANGELICI,FREDBASOLO AND ANTHONY J. POP RECEIVEDFEBRUARY 14, 1963 cis- and trans-Mn( CO)3[P(O C B H ~ ) ~have ] ~ Bbeen ~ prepared and identified by their dipole moments and infraP( OCaH9)3 and P( C4H9)3, have also red spectra. Analogous isomers of Mn( C0)3L2Br, where L = P( C&?S)C~Z, Q always n-butyl). The cis isomer, obtained from the reaction of Mn( CO)sBr with L, been prepared ( C ~ H is isomerizes t o the trans form in solution and the extent of isomerization increases with iilcrease in size of L. The results of a kinetic study suggest an intermolecular mechanism of isomerization involving the dissociation of L.
Disubstituted compounds of the manganese pentacarbonyl halides were first prepared by the reaction between Mn(CO)& (where X- = C1-, Br- or I-) and the desired ligand, L, a t 120' in the absence of solvent.' The products, Mn(CO)yL*X, where L = P(C6&)3, As(C&)3 or C&N, were assigned a structure in which the L's were cis to each other as well as to X. The present study reports the preparation, from organic solvents, of these and other compounds. The compounds obtained were found to exist in two isomeric forms, the relative stabilities of which depend upon L. The kinetics and mechanism of isomerization were also investigated. Experimental Preparation and Purification of Materials.-The starting material, Mn( CO)5Br, was prepared as described.' The preparation and purification of the ligands and solvents were previously reported.)m3 cis-Mn( CO)3 [P(OCBH6)3)2Br,in which the triphenyl phosphite groups are cis t o each other as well as t o Br, was prepared by the reaction of 1.1 g. of P(OC&)a with 0.2 g. of Mn(CO)sBr in approximately 15 ml. of chloroform a t 37" for 12 hr. At the end of this time 30 ml. of n-hexane was added, and the resulting solution was evaporated to approximately 5 ml. under a stream of nitrogen. The yellow crystals obtained were collected on a filter, dissolved in 2 ml. of chloroform and precipitated with 30 ml. of hexane. The precipitate was washed with hexane and allowed t o dry a t room temperature. Anal. Calcd. for M ~ ( C O ) ~ [ P ( O C ~ H ~ )C~, ]55.7; Z B ~H : , 3.58. Found: C, 55.2; H , 3.74. trans-Mn(CO)3[P(OC~H~)3]2Br, in which the triphenyl phosphite groups are trans to each other but cis to Br, was prepared in the same manner except that the reaction was carried out at 55-60' for 16 hr. I t was purified as above. Anal. Found: C, 55.1; H , 3.56. Analogous isomers of Mn(C0)3[P(CeHs)CI2]~Br,Mn(C0)a[P(C4Ho)a]2Br and Mn(.C0)3[P(OCaHs)3]2Br, prepared in, the same manner, were not isolated but identified only by their infrared spectra which were very similar to those of the established M ~ ( C ~ ) ~ [ P ( O C B Hisomers. ~)~]ZB~ Dipole Moment Determinations .-The dipole moments of cis- and trans-Mn( C O ) ~ [ P ( O C B H ~ ) ~ and ] Z BM~~ I ( C O ) ~were B~ determined by the approximate method of Jensen,' using only dielectric constant measurements. The necessary capacitance measurements were made on 0.015 to 0.035 M benzene solutions of the complexes with a General Radio Co. capacitance measuring assembly, type no. 1610-A, set a t 1000 cycles per second. The average deviation of the dipole moments calculated from these data was less than 3 % . Molecular Weight Determination.-The molecular weight of trans-Mn( CO)3[P(OC6H5)3]PBr was determined in benzene by the method of freezing point depression. A solution of 0.5102 g. of the complex in 15 ml. of benzene gave a 0.260' freezing point depression, indicating a molecular weight of 761. Ultraviolet Spectra.-The spectra of the isomers were taken with a Beckman DK2 double beam recording spectrophotometer adjusted for maximum resolution. Corresponding spectra of the isomers were taken on the same paper with identical instrumental settings t o minimize mechanical errors. Determination of Rates of Isomerization.-The rates of cis -+ trans isomerization were determined by following the disappearance of the highest frequency carbonyl absorption of the cis isomer. A Baird Associates Model 4-55 double beam recording spectrophotometer with NaCl optics was used to follow the (1) (2) (3) (4)
E. W. Abel and G. Wilkinson, J . Chem. S O L . 1501 , (1959). R . J. Angelici and F. Basolo, J. A m . Chem. Soc., 84, 2495 (1962). R . J. Angelici a n d F. Basolo, Inorg. Chem., 4, 728 (1963). K . A. Jensen, A c f a Chem. Scand., 8, 479 (1949).
reaction as well as to determine the absorptions of the cis and trans isomers. A polystyrene standard was used for calibration. For rate studies of the isomerization of cis-Mn(CO)![P(OC&iS)3]2Br, the solid complex was dissolved in the desired solvent and its rate of isomerization was followed. However, the reaction solutions of cis-Mn( CO)3L2Br, where L = P( C&)C12, P(C4H9)3or P ( O C I H ~ )were ~ , prepared by allowing an excess of L t o react with Mn(CO)5Br in 3 ml. of, sym-tetrachloroethane ( T C E ) a t 38" for 12 t o 20 hr. These solutions were then diluted t o 20 ml., placed in aluminum foil-wrapped flasks, and thermostated in a constant temperature bath. Approximately 5 min. was allowed for the system to attain equilibrium temperature before a sample was withdrawn with a syringe and placed in an infrared cell. Typical changes in spectra with time are shown in Fig. 1. The spectrum a t t = 0 min. is that of cis-Mn(C0)~[P(OCeHS)8]2Br,while that a t t = m is trans-Mn(CO)z[P(OCsH5)3] Br. First-order rate constants were obtained from the slopes of plots of In ( A - A,) vs. t, time, where A is the absorbance a t time t . In all cases A , was zero. All isomerizations continued t o completion and gave linear plots to a t least 807c completion, except for cis-Mn(CO)3[P(CeH5)C1,]2Br for which significant deviations began t o appear after 50y0 completion. The explanation for such a behavior is unknown.
Results The cis and trans isomers of Mn(CO)a[P(OC6H5)3]pBr were prepared and identified by their dipole moments (Table I ) , near-ultraviolet spectra and infrared spectra (Fig. 1 and Table 11). The cis-Mn(CO)BLIBr compounds isomerize to the trans isomers a t rates which depend upon L in the complex and upon the solvent (Table 111). The P(OC&,)r groups in cis-Mn(CO)s[P(OC6H6)3]2Br are displaced by P(OC4H9)3 and P(C6H6)3a t a rate which is independent of the concentration and nature of the entering ligand and which is much faster than the rate of isomerization under the same conditions (Table IV) . TABLE I OF Mn( C 0 ) 3 L ~ BISOMERS r DIPOLEMOMENTS M>
Compound
Debyes
4.78 cis-Mn( C0)3[ P( O C C H ~PBr )~] trans-Mn( CO), [ P( OC~HS),] ZBr 3.53 Mn( CO)sBr 3,39a This value compares with 3.19 f 0.03 D. obtained by a more accurate method by Beck, et ~ 1 . ~ TABLE I1 INFRARED SPECTRA OF cis- A N D trans-Mn( C0)3L2Br I N T C E --trans----
--sic-
L
(s)
(s)
(s)
2058 2004 1953 P(C&)C12 1949 2053 2000 P(OCSH6)3 1969 1927 P(OCafI9)3 2037 1938 1894 2008 P(CrH9)a Measured in P(C4H0)3 solvent.
(w)
(vs)
(SI
2075 2070 2049 2020
2004 2000 1965 1938
1953 1949 1927 1894'
(1
Discussion Identification of Isomers.-A compound of the composition Mn(C0)a [P(OC&)3]& was prepared by the reaction of Mn(CO)bBr with P(OC&,)3 a t 37'. A solution of this compound when heated to 55' yields a product having a different infrared spectrum but the same composition. During this reaction there is no evolution of carbon monoxide, which suggests (5) W. Beck, W. Hieber and H. Tengler, Chem. R e r . , 94, 862 (1961)
2216
R. J. ANGELICI,F. BASOLO AND A. J. Po0 2000
cn
2000
RATES O F REACTION
k Mt=l22min
-
Isomer
5.0
,
4
,mnr
4.4
lO3[com-
PW,
Added L,
M
M
T. OC!
TABLE IV Mn(CO)3[P(OCsH5)3]~Br WITH L 60.1 IN T C E [Ll, M
Reagent L
AT
104k, sec. -1
5.20 P(C&6)3 0.0406 1 ,4a ,0790 1.4" 4,92 P(CBHS)) CiS 4,86 P( C&)3 0385 5.gb 6.6b P( CeHs)3 ,0776 cas 5.68 P(CeH5)a 0207 19 cas 4.67 cis 5.26 P( CsH5)3 0464 22 CiS 21 P(C&)3 0796 5 06 CZS 5.56 P(OCaHg)3 ,0506 -20~ trans 4.07 P(C&j)3 ,0419 3.4c trans ,0504 2.7c 4.18 P(OC4Hg)a Measured a t 40.0'. Measured a t 50.0". Rate difficult to measure because of overlapping bands in the infrared spectrum. CiS
'
I
5.0
5.6
Fig. 1.-Infrared spectra during the isomerization of cisMn(C0)3[P(OC~Hj)3]2Br in the presence of excess P(OC&)3 in T C E a t 60.1". TABLE I11 RATESOF ISOMERIZATION OF cis-Mn(CO)3L2Rr PRESENCE OF EXCESSL
1 0 3 ~
OF
cis
micrmr ,
0 4.4
Vol. 85
the Br as well as to each other; this isomer is designated cis-llln(C0)3[P(OC&,)3]2Br. The product of the isomerization (1) a t 55' is t m n ~ - M n ( C O ) ~ [ P ( 0 C&)3]&-, in which the P(OC&)3 groups are trans
9' IN
TCE
IN THE
105k, sec. -1
7.11 0.209 42 0 3.08 6.91 ,539 42 0 3.05 5.77 171 502 9.76 5.83 171 50 2 10.2 6.87 436 50 2 10.3 7.69 ,206 60 1 29.6 7.69 ,206 60 1 30.2 7.64 ,539 60 1 29.9 4.80 .o 4.19 50 2 4.14 .o 4.15 50 2 4.68 .o 60 1 16.6 4.84 .o 60 1 17.7" 5.84 .o 60 1 21. O b 5.80 .o 21.9" 60 1 5.95 .o 70 0 56.7 52.0 .o 70 0 46.7 4.68 ,0153 50 2 2.29 4.70 ,0764 50 2 2.28 5.98 ,00116 10.9 60 1 5.80 ,00274 10.8 60 1 5.99 00802 9.89 60 1 6.18 ,0190 60 1 9.65 60 1 5.76 ,0956 9.74 70 0 5.70 ,0190 35.1 6.00 .09,56 70 0 34.5 0.785 5.77 0628 50 2 5.86 ,156 0.776 50 2 6.07 ,0688 60 1 3.45 150 6 15 60 1 3.53 70 0 7.34 0783 13.5 70 0 7.59 ,185 13.6 ... 58.9 2 . 06d 50 2 70 0 47.4 ... 24. 7d a Dielectric constant (20') of T C E is 8.2. * Toluene solvent; dielectric constant (20') is 2.4. Nitrobenzene solvent; dielectric constant (20') is 35.2. Measured in P(C4Hg)a solvent.
that no polymerization has occurred. In addition, the product has an experimentally determined molecqlar weight of 761 which 'compares with 839 calculated for the monomeric compound. The differences between these compounds must, therefjore, lie in the relative positions of the groups in the coordination sphere. The compound prepared a t 37' has been assigned6 a structure in which the P(OC&)3 groups are cis to
I
co
CO C &
e
S
to each other but cis to Br. The following observations support these assignments. A dipole moment study yielded the results shown in Table I. As expected, the trans isomer has a dipole moment close to that of the unsubstituted Mn(C0)6Br. However, the higher moment of the second isomer does not distinguish between the cis isomer and the other possible isomer, in which the P(OC6Hb)3groups are cis to each other but one is trans to Br. The infrared spectrum of the cis isomer has absorption bands in the carbonyl stretching region a t 2053(s), 2000(s) and 1949(s) crn.-', compared with 2070(w), 2000(vs) and 1949(s) for the trans isomer. These spectra are shown in Fig. 1. The trans compound has Czvsymmetry which gives rise to three carbonyl stretching absorptions (2A1 B1) as observed. The normal modes of vibration having these symmetries are shown in Fig. 2 . The AIb mode does not involve a change in dipole moment but is observed because of coupling with the Ala mode or because of distortion of the molecule from CzV symmetry.' Hence the symmetrical stretching mode AIb) should result in a weak absorption as observed in the spectrum. Thus the infrared spectrum confirms the trans isomer assignment previously made on the basis of dipole moment determinations. The Ala mode is assigned to the band a t lowest frequency, since the CO trans to Br is expected to have the greatest degree of Mn=CO *-bonding. I t is interesting that both the position and intensity of this absorption are the same in the cis isomer. The total intensity of the bands (AIb B,) of the trans-CO's is roughly twice that of the single CO trans to the bromide, as expected from very elementary arguments.' The C, symmetry of the cis isomer leads to three expected absorption bands (2A' A"). All modes (Fig. 2) involve dipole changes and hence none should give weak absorptions. The other possible isomer, where one P(OC6H6)3group is trans to Rr, would be
+
+
+
(6) R. J. Angelici. F. Basolo a n d A. J. Poe, Nature, 196, 993 (1962)
(7) J,. E Orgel. I?iova. C k e n . , 1, 25 (1962).
Aug. 5, 1963
DISUBSTITUTED MANGANESE PENTACARBONYL BROMIDE
2217
expected to yield a spectrum with a weak absorption. The observed spectrum containing three strong bands strongly supports the cis assignment to this isomer. As mentioned above, the lowest frequency band is that of the Ab' mode; the remaining two bands then arise from the symmetric and asymmetric stretching vibrations of the two equivalent cis CO's. The total intensity of the absorptions of the two CO's cis to Br is approximately twice that of the CO trans to Br. I t is also known that the CO trans to the bromide is more inert than those cis to the bromide in Mn(C0)6Br,8 and monosubstitution occurs in the cis position. 2 f 3 I t therefore seems reasonable from chemical evidence that substitution cis to Br will occur in the monosubstituted compounds as well. Furthermore, current n-bonding theories suggest that a CO trans to another CO is more readily replaced than CO trans to some Ka A" lesser a-bonding ligand. Thus the chemical evidence Fig. 2.-Normal modes of C-0 stretching vibrations for isomers is generally very much in favor of these cis- and transof Mn( CO)&Br. Mn(C0) [ P(OCBH6)3]zBr assignments. The near-ultraviolet spectra of the ~ i s - M n ( C 0 ) ~ It is noteworthy that i t is not possible to con[P(OC~HF,)~]~B~ in T C E has its absorption maximum struct the sterically crowded ~ i s - M n ( C 0[P(C&,)3]2Br )~ a t 385 mp with e 810 l./mole cm., while the trans compound with Fisher-Taylor-Hirschfelder models, compound absorbs a t 398 mp with E 720. The abwhereas the trans isomer is quite easily assembled. sorption of the cis isomer is more symmetric about the I t can also be seen from models that P(C6Hb)3 is conmaximum than the trans isomer absorption. This is siderably more bulky than those ligands which form reasonable since the cis isomer has a tetragonal ligand cis isomers. field, and therefore the more unsymmetrical rhombic When a chloroform solution of cis-Mn(C0)3[Pfield of the trans compound is expected to result in a (OCH2)3CCH3]~Br2,3 is kept a t 50' for 24 hours, it greater number of transitions. In the ultraviolet attains equilibrium containing only 20% of the trans region, both isomers have an absorption maximum a t isomer. This ligand, having low steric requirements, 283 mp with e 1100 l./mole cm. for the cis and E 1200 allows greater stability of the cis isomer relative to that for the trans. of the trans form. The higher stability of the cis Analogous cis- and trans-Mn (CO)3L2Brisomers were isomer can be attributed to strong Mn-C a-bonding. prepared in situ for L = P(OC4H9)3, P(C4H9)3and In this isomer the CO's compete for the metal dnP(CGHF,)C~Z.These were identified by their infrared orbitals with the relatively poorly n-bonding ligands, L. spectra (Table 11), which have the same form as those The trans isomerlis then of lower stability, since the of the isomers of Mn(CO)3[P(OC6H6)3]2Br.The prepatrans-CO's must share the same dn-orbitals, resulting could only be ration of tran~-Mn(C0)3[P(C~H~)~]~Br in weaker Mn-C bonds. In complexes bearing bulky accomplished in P(C4H9)3 as the solvent because of ligands, the cis isomer is also probably favored by decomposition in TCE when converting the cis comhigher Mn-C bond energies, but the over-all equilibrium pound to the trans form a t higher temperatures. lies far toward the trans isomer as a result of entropy Attempts to prepare ~ i s - M n ( C 0[P(C&)3]2Br )~ were effects. The higher entropy of the trans form could not successful. When a TCE solution containing a result from an increase in freedom of motion of the mixture of Mn(C0)6Br and P(C6H6)3is kept a t 37" for two days, only Mn(CO)4P(C~H6)3Br results.2 ligands in the less hindered trans positions and/or from greater solvation of the more polar cis isomer. However, if the temperature is raised to 55', then a I t was likewise found in the cis-trans isomerization reaction begins to take place yielding a compound of Mo(C0)4[P(GH6)3]2, that a t 45' the cis form having an infrared spectrum very similar to transisomerizes to an equilibrium mixture consisting of 80% M ~ ( C O ) ~ [ P ( O C B H ~ ) ~The ] Z Breaction ~. of Mn(C0)6trans.g In this system the trans structure is also Br and P(CeH6)s in the absence of solvent a t 120' gives thermodynamically more stable. In the same study, Mn(C0)3 [ P ( C ~ H S ) ~ ] ZasB ~was , reported previously both cis- and trans-Mo(CO)4 [P(c~H6)3]2 were prepared. by Abel and Wilkinson.' Their reported frequencies The cis isomer probably exists because, compared to were reproduced on this compound. However, the Mn, the larger Mo atom reduces crowding of the band intensities again correspond to the trans structure P(CeH6)3 groups. rather than to the cis structure originally proposed. A related study of the cis trans equilibrium of The earlier cis assignment was based on the similarity Pt[P(C2Hb)3]2C12 gives AH = 2.5 kcal./mole and A S = of the infrared absorptions of the Mn(C0)3L2Brcom13.3 e.u.l0 Here too the bond energies of the cis pounds and Mn(C0)3(bipy)Br, for which the structure isomer are estimated to be higher than those of the must be cis. The reaction of Mn(C0)gBr and pyridine trans. That the equilibrium lies far toward transin solution of 37" gives c i ~ - M n ( C O ) ~ ( p y ) ~asB rshown Pt[P(C2H6)3]2C12 is a result of the large positive enby its three almost equally intense infrared absorptions. tropy of isomerization which is attributed to greater Upon heating to 68' no rearrangement to the trans solvation of the more polar cis isomer. In this case, structure occurs, as noted from the spectrum. Hence steric interactions were considered to be negligible Abel and Wilkinson's Mn(CO)3(py)2Br structural assignment is correct. C ~ S - M ~ ( C O ) ~ ( C ~ H ~beN H Zbecause ) ~ B ~ ~of the large platinum(I1). Kinetics and Mechanism of Isomerization.-The haves like the similar pyridine compound. Like transcis to trans isomerization of MII(CO)~[P(oC&,)3]2Br M ~ I ( C O[)P~( C ~ H & ] Z Bthe ~ , analogous A s ( C ~ H ~and )~ as shown by eq. 1 was studied kinetically and found Sb(C6H6)3compounds are probably also trans, since the to proceed a t conveniently measurable rates a t 40-70' disubstituted products do not form a t the lower temperature (37") used for the preparation of the cis isomers. (9) R. Poilblanc and M. Bigorgne, Bull. sac. chim., 1301 (19621
*
( 8 ) A Wojcicki and F Basolo, J A m Chem Sac , 88, 525 (1961)
(10) J. Chatt and R. G. Wilkins, J. Chem. Sac., 4300 (1952)
R. J. ANGELICI,F. BASOLOAND A. J. Po&
2218
do
L 39.0 ked. mole
13- ------
Fig.
3.-Mechanism
--___
of isomerization of where L = P(OC&)a.
cis-Mn( CO)&Br
Vol. 85
[P(OC&6)3]2Br, whereas eleven return to re-form the cis isomer. The activation parameters for the dissociation of P(OC&)3 have been determined from the reaction of cis-Mn(CO)o[P(OC&)3]2Br with P(C&)3 a t several temperatures (Table IV): E. = 27.4 f 1.4 kcal./ mole (average deviation limits) and AS* = 9.1 e.u. (Fig. 3). The activation energy for the formation of the trans isomer from the five-coordinated intermediate is approximately 1.6 kcal./mole higher than that for the formation of the cis isomer. Since 7-bonding of the metal d,,, d,, and d,, orbitals with CO will tend to keep the OC-Mn-CO angle a t 90°, energy will be required to widen this angle during the formation of the trans isomer. On the other hand, formation of the cis isomer takes advantage of the most stable n-bonding configuration. Added P(OC&)3 (Table 111) depresses the rate of isomerization of C ~ S - M ~ ( C ~ ) ~ [ P ( O C by~ Ha~ ) ~ ] ~ B ~ factor of about two. Even small amounts of P(0C,&)3 decrease the rates almost to the minimum value, and reactions in which the ratio of P(OC&)3 concentration to complex concentration is a t least one have the same slow rates. The effect is small but suggests an association between ~ i s - M n ( C 0[ )P~( O C ~ H S ) ~ ] ~ B T and P(OCsH6)a. The associated species then undergoes isomerization with a rate of about half that of the free complex. The effect of solvent on the rate of isomerization of c i s - M n ( C 0 )[P(OC&)3]&h' ~ is also very small (Table 111), and this too supports the non-ionic mechanism postulated above. However, in the absence of solvent, no measurable isomerization occurs a t these temperatures, so the solvent must mediate the transfer of P(OC&,)3, but is not involved in the activated complexes to a significant extent. The rates of isomerization of ci~-Mn(C0)~L2Br comZ, P(OC&)3 plexes, where L = P ( C ~ H S ) C ~P(C4H9)3, and P(OC4H9)3,have been measured and their activation parameters determined (Tables I11 and V).
in a variety of organic solvents. In the solid state a t 55', the reaction was immeasurably slow. Data in Table I11 show that the reaction is first order in complex, for there is only a very small change in the rate constant accompanying an almost tenfold change in concentration. Added P(OCaH6)3 depresses the rate significantly, but this will be interpreted later as a slight modification of the primary mechanism. The compound czs-Mn(C0)3 [P(OC6H6)3]2Br in T C E a t 60' isomerizes with a rate constant of 1.71 X sec.-l (Table 111). However, when isomerization occurs in the presence of P(CeH6)3, a much faster reaction (2.06 X sec.-l, without evolution of CO) occurs. The rate of this reaction is independent of the P(C&)3 concentration (Table IV). The reaction results in the formation of an unknown product which could not be isolated by solvent precipitation and which decomposed during an attempted separation on a silica gel chromatography column. The infrared spectrum of this compound is different from that of TABLE V either cis- or t r a n ~ - M n ( C O ) ~ L ~and B r has absorption FOR THE ISOMERIZATION OF ACTIVATIONPARAMETERS bands a t 2092(m), 2008(s) and 1972(s) ern.-'. This cis-Mn( C 0 ) 3 L B r I N T C E same product was obtained from the reaction of transE., ASS L kcal,/mole" e.u. Mn(C0)3[P(OC&,)3]2Br with P(CeH6)3, but a t a sec.-l). much slower rate (3.4 X p(CeH5)CIz 26.4 f 2 . 3 * 3.0 If cis-Mn(CO)3[P(OCsH6)3]zBris allowed to react P(C4Ha)a 2 7 . 7 f 0 . !jbmC 3.8 with P(OC4H9)3,a stepwise replacement of the P(0P(OCeH5)3 29.0 f1 . l d 9.1 CaH6)3groups occurs as shown by the broadening of P(0CsHs)a 30.2 f0.7b 12.1 the infrared absorption bands during the formation of P( OCiHy )3 31.8 f 0 . 3 b 14.3 Determined from limiting Limits are average deviations. the mixed complex, cis-Mn(C0)3[P(OC~H6)3] [P(oMeasured in P( CIHI))Isolrates in the presence of excess L. C4H9)3]Br, followed by narrowing again with the final vent. d I n absence of added P(OCeH5)z. formation of cis-Mn(CO)a [P(OC4H9)3;2Br. This isomer The activation energies increase with changing L in then isomerizes more slowly to trans-Mn(CO)s [P(Othe order: P(CeH6)cl~< P(OC&.)3 < P(OC4HJ3. C4Hg)3]2Br with the expected rate, 3.4 X low6set.-'. The P(C4H9)3compound has not been included in this The rate constant for replacement of P(OC&)3 iS approximately 2 X sec.-l, as estimated from the, series since i t decomposes in T C E but isomerizes in P(C4Hg)3solvent. The above trend seems to indicate rate of band broadening and narrowing. It thus that the best *-bonding ligands" isomerize most appears that the same rate-determining step is involved in the reactions of ~ ~ S - M ~ ( C O ) ~ [ P ( O C ~ Hreadily ~ ) ~ ! to Z B the ~ trans structure. This is reasonable since the better a-bonding ligands tend to stabilize with either P(C!eH6)3 or P(OC4H9)3 (Table IV). Since to a greater extent an activated complex in which the both reactions are independent of the nature and conentering ligand approaches from the direction trans to centration of the entering ligand, the rate of dissociation the phosphine remaining in the five-coordinated interof P(OC6H6)3from the complex is presumably the slow mediate. However, these activation energies are a step common to both reactions. However, the overall rate of isomerization of c ~ ~ - M ~ ( C ~ ) ~ [ P ( O Ccomposite ~ H ~ ) ~ of ] Zthe B ~activation energy for the dissociation of L from the cis complex and the difference in activadepends not only upon the rate of P(OC&)3 dissociation energies between the reactions of the five-coordition but also upon the rates of forming the cis and nated intermediate with L to form the cis or trans trans isomers from the resulting five-coordinated isomer. As seen from the discussion for ~ i s - M n ( C 0 ) ~ intermediate (Fig. 3). Thus, according to the above [P(OC&)3]2Br, the activation energy of the dissociarates, for every twelve dissociations of P(OC&,)3 only one P(OC&)3 'enters to form trans-Mn(C0)3(11) M . Bigorgne, Bull. SOC. chim., 1986 (1960).
MERCURY (3P1)-SENSITIZED PHOTOLYSIS
Aug. 5 , 1963
tion is the larger term, but whether i t determines the above trend is not known. An attempt to measure the activation energy for the dissociation of P(OC4H9)3 from cis-Mn(C0)~ [P(OC4H9)3]2Br was made using the method employed for the P(OC&)3 complex. Thus P(C6H5)awas added to a solution of czs-Mn(CO)3[P(OC4H9)3]2Br, but in this case no reaction occurred, and the isomerization proceeded as usual. [CONTRIBUTION FROM
THE
DEPARTMENT O F CHEMISTRY, CARNEGIE
OF
LIQUIDALKANES
2219
Acknowledgments.-This research was supported in part by a National Science Foundation Grant, NSFG12872, and a U. S.Atomic Energy Commission Contract, At(l1-1)-1087. The authors are also thankful for a National Science Foundation fellowship (to R. J. A.) and for the granting of leave of absence from the Imperial College of Science and Technology, London (to A. J. P.). INSTITUTE O F
TECHNOLOGY, PITTSBURGH 13, PENNSYLVANIA]
The Merc~ry(~P,)-Sensitized Photolysis of Some Liquid Alkanes at 25” B Y ROBERT R. KUNTZAND GILBERT J. MAINS RECEIVEDJANUARY 14, 1963 The liquid phase mercury-photosensitized decompositions of n-hexane, n-pentane, isopentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane have been investigated. The mechanism of this liquid phase photolysis is similar to t h a t of the low temperature vapor phase investigations. The photolysis products are hydrogen, parent olefin, and parent dimer with no carbon-carbon cracking products. One primary process associated with the quenching of Hg( 3P1)atoms is the formation of a mercury complex. This complex and product olefins alter the “effective” mercury concentration while trace impurities completely remove a large fraction of it, Quantum yields for hydrogen production are all nearly one, indicating complete quenching of excited mercury atoms. Olefin products have a quantum yield of about 0.4 for hydrocarbons containing only secondary and primary H-atoms. The presence of tertiary H-atoms increases the olefin yield considerably. This increase is reflected in the kdlsp/koomb which is less than 1 for secondary and primary radicals and greater than 2.5 for tertiary radicals. The precursors for dimer products are tertiary, secondary, and primary radicals. Tertiary-tertiary dimers are most important, but primary radical combinations become significant as the ratio of primary H atoms t o secondary and tertiary H-atoms increases.
may be explained in terms of solvent stabilization efI. Introduction fects, but the absence of olefin products in the n-decane Information regarding the reactions of large alkyl system is surprising. radicals in the liquid phase is meager. Production of This investigation was initiated in an effort to these radicals by radiolysis and by chemical means evaluate the applicability of mercury-photosensitized usually is accompanied by the simultaneous production decompositions to the liquid phase and to clarify the of a variety of products which obscure the radical reacreaction mechanism by careful product analysis on a tions. A study of the liquid phase mercury-photonumber of saturated hydrocarbon systems. Because sensitization of n-decane2 indicated that the sole preadditional information concerning the photosensitizacursors of the final products were parent alkyl radicals tion process is needed, a third important facet of this and H-atoms. This technique thus should be suited investigation was an attempt to elucidate this mechfor unambiguous study of alkyl radicals in the liquid anism. phase. 11. Experimental In order for mercury-photosensitization to be a pracThe light source used in these experiments was a n Sc2537 low tical tool for the study of liquid phase decompositions, pressure mercury vapor lamp manufactured by the Hanovia it is necessary that mercury be sufficiently soluble in the Manufacturing Co. I t is constructed of a Vycor envelope solvent to absorb significant quantities of incident light shaped into a four-coil spiral, a geometry identical with those during a reasonable period of irradiation. Some solulamps described in an earlier research from this Laboratory.’ Its is 95% at 2537 A. with small intensities at longer wave tions of mercury in hydrocarbons have been s h o ~ n ~ .output ~ lengths. to extinguish much of the 2537-A. mercury resonance Three reaction vessels were employed. All rate studies were radiation. Solubilities of mercury in various hydrocarried out in a vessel consisting of a Vycor tube closed a t one carbons4 all lie in the micromolar concentration range end and surrounded by a Vycor water jacket. The inner tube was connected to a Pyrex-Vycor graded seal to facilitate manipuand vary only slightly from one hydrocarbon solvent to lation. A small side arm was attached to the vessel above the another. Absorption spectra of mercury in the hydrograded seal so mercury might be isolated from the system during carbons used for this investigation were found to be filling and photolysis. The reaction volume of this vessel was similar to one another and to exhibit the pressure 6.7 cc. Experiments requiring optical density measurements were performed in a vessel having similar construction, but with a broadenings and Stark effect splittings observed prereaction volume of 60 cc. and a 100-mm. quartz absorption cell viously. 2, These hydrocarbon-mercury solutions all fused to the vessel below the graded seal. A few experiments absorb approximately 4% of the incident 2537-A. radiawhich involved high conversions were made in a 200-cc. Vycor tion for each centimeter of path length. vessel without a water jacket. This cell was attached to a 300cc. ballast vessel to prevent the build-up of product pressure. Phibbs and Darwent2 did not observe any C-C Lamp intensity was measured by the uranyl oxalate actinometer “atomic” cracking products or olefin products in the using the modification of Pitts, et a1.8 A number of measureliquid phase mercury-photosensitized decomposition of ments at several optical densities indicated a linear variation of n-decane, while similar vapor phase decompositions absorbed intensity with optical density in the cylindrical vessels at the low optical densities possible in these experiments. Maxishow that both types of products are important. The mum output of the lamp was reached in 2 min., and corrections absence of liquid phase C-C “atomic” cracking products for this build-up time were negligible compared to photolysis is in agreement with the low temperature vapor phase times. For quantum yield determinations, it was necessary to investigations of Taylor and Bates5 and Back,6 and take a time average of the absorbed intensity by measurement (1) Based upon thesis research submitted b y Robert R. Kuntz in partial fulfillment of the requirements for the degree of Doctor of Philosophy, Carnegie Institute of Technology. (2) M. K. Phibbs and B . deB. Darwent, J . Chem. P h y s . , 18, 679 (1950). (3) H. Reichardt and K. F. Bonhoeffer, Z. Physik, 67, 780 (1931). (4) H. C . Moser and A. Voigt, U. S. Atomic Energy Comm. I.S.C.-892 (1957).
of the variation in optical density of the mercury-hydrocarbon system. (5) H. S . Taylor and J. R . Bates, Proc. Nati. Acad. Sci., 12, 714 (1926) (6) R . A . Back, Trans. Faraday Soc., 54, 512 (1958). (7) D. B . Peterson and G . J. Mains, J , A m . Chem. SOL.,81, 3610 (1959). (8) J. N. Pitts, Jr., J. D . Margerum, R . P . Taylor and W. Brum, i b i d . , 77, 5499 (1955).
