Olefin Hydrogenation with Iron Carbonyls as Catalysts

O estudo de [Fe(CO)5], [Fe2(CO)9] e [Fe3(CO)12] como precursores catalíticos nas reações de hidrogenação do ciclohexeno e 1-hexeno foi efetuado. Um reator fotoquímico com fluxo de H2 e irradiação UV-vis foi usado no processo. Os espectros infravermelho das amostras mostraram a formação de [Fe(CO)4(olefina)] e [HFe(CO)3(π-alila)] como intermediários. A atividade mostrada do [Fe2(CO)9] na hidrogenação do ciclohexeno foi maior (66,6%) que a atividade dos outros ferrocarbonilas (43,0% [Fe(CO)5], [Fe3(CO)12]). A competição en tre as reações de isomerização e hidrogenação na reação do 1-hexeno foi estudada. A velocidade de isomerização foi muito maior que a velocidade de hidrogenação e as espécies polinucleares promoveram isomerização mais rapidamente que o [Fe(CO)5]. As conversões em alcanos foram: 60,0% ([Fe(CO)5], [Fe3(CO)12]) e 75,4% [Fe2(CO)9], mostrando que, para a hidrogenação do 1-hexeno, uma maior atividade para [Fe2(CO)9] foi também encontrada.


In tro duc tion
The util ity of [Fe(CO) 5 ] and [Fe 3 (CO)  12 ] as photocatalysts in the hy dro ge na tion of alkenes has been known since the 1970s [1][2][3][4] .Many un sat u rated de riv a tives are pro duced in these re ac tions via the loss of CO groups from the iron car bon yls, and the ac tual cat a lyst is prob a bly a re duced car bonyl spe cies, such as [Fe(CO) x ] or [Fe(CO) 3 ] 5-10 .Sur pris ingly, the photocatalytic ac tiv ity of the dinuclear com pound [Fe 2 (CO) 9 ] has never been in ves ti gated.In this pa per, we re port our studies on the photocatalytic ac tiv ity of [Fe 2 (CO) 9 ] in the hy dro gena tion of cyclohexene and 1-hexene.Fur ther more, com par a tive stud ies of the photocatalytic ac tiv i ties of [Fe(CO) 5 ] and [Fe 3 (CO)  12 ] in these pro cesses were also performed.

Ex per i men tal
A pho to chem i cal re ac tor made of Py rex 11 was used, and a mer cury va por lamp (125 W) was adapted to the re actor.
The re ac tions were con ducted un der H 2 flux (P[H 2 ] = 1 atm), at 0 °C, un der con tin u ous ir ra di a tion.All of the prepa ra tory steps were un der taken un der Ar at mo sphere, and the re ac tor was purged with Ar for five min utes be fore use.
The to lu ene so lu tions were 0.01 M in iron and 0.1 M in the ole fin (cyclohexene or 1-hexene).The same iron atom amounts were used for the three iron car bon yls stud ied in or der to al low the data to be com pared.

Re sults and Dis cus sion
As pre vi ously re ported 10 , the ac tiv ity of the cat a lytic precur sors [Fe(CO) 5], [Fe 2(CO) 9] and [Fe 3(CO) 12 ] in the cyclohexene hy dro ge na tion re ac tion shows that [Fe 2 (CO) 9 ] is the most ac tive of the iron car bon yls while the con ver sion values ob tained for [Fe(CO) 5 ] and [Fe 3 (CO) 12 ] are quite sim i lar (Ta ble 1).
Al though [Fe 2 (CO) 9 ] is highly in sol u ble in to lu ene, no insol u ble ma te rial was seen in the re ac tor soon af ter the ir ra di ation started (~1 min).The ab sence of a het er o ge neous com po nent was con firmed by a mer cury test 14 .The pres ence of an het er o ge neous com po nent would have been de tected by the de crease in ac tiv ity due to amal gam for ma tion.
One pos si ble ex pla na tion for the higher ac tiv ity of [Fe 2(CO) 9] is that upon pho to chem i cal ac ti va tion, the dinuclear spe cies [Fe , this kind of in ter me di ate might be pres ent in the sys tem, lead ing to high ac tiv ity.In Scheme 1 it is suggested that the ole fin dinuclear com pound is part of the cy cle as sug gested by Wrighton and Mitchener 17a .Thus, mono and dinuclear iron spe cies could be pres ent in the three sys tems.In the case of [Fe 2 (CO) 9 ], the con cen tra tion of the dimeric species would be the high est, be cause of the pho to chem i cally induced CO dis so ci a tion frag men ta tion, whereas in the other cases, the dimeric spe cies would be formed upon the con densa tion of two mononuclear frag ments.
The IR spec trum of the so lu tion from the cyclohexene reac tion with [Fe(CO) 5 ] af ter 1h, shows bands which are char acter is tic of [Fe(CO) 4 (ole fin)] (2080, 2025, 2001, and 1960 cm -1 ).The bands at 2060 and 2025 cm -1 cor re spond to [Fe 3(CO) 12 ].These bands are pres ent in all of the re ac tions stud ied, and the for ma tion of [Fe 3 (CO) 12 ] may be due to the de com po si tion of the cat a lyst.There is a shoul der at 1945 cm -1 , sug gest ing the pres ence of [HFe(CO) 3 (π-allyl)] (2064, 1994, and 1945 cm -1 ), but the over lap with the band at 1960 cm -1 ([Fe(CO) 4(ole fin)]) does not al low a con clu sive band assign ment.As ob served in the case of [Fe(CO) 5 ], the re ac tion with [Fe 2 (CO) 9 ] as the cat a lyst also showed bands due to [Fe(CO) 4(ole fin)] and [Fe 3(CO) 12 ].Fur ther more, in this case, a well-defined band at 1945 cm -1 ([HFe(CO) 3(π-allyl)]) was ob served.Dur ing the pro cess, the in ten sity of this band does not change ap pre cia bly.In the case of the re ac tion with [Fe 3 (CO) 12 ], the bands due to [Fe(CO) 4 (ole fin)] and [HFe(CO) 3 (π-allyl)] (approx.1945 cm -1 ) ap pear again.In all of the iron car bonyl re ac tions the pres ence of [Fe 2(CO) 6(olefin) 2] could not be char ac ter ized due to the su per im po si tion of the 2059 and 1982 bands on other bands in the spec tra.All IR bands are in agree ment with the data in the lit er a ture 5,17 .Fig. 1 shows the IR bands for the [Fe 2(CO) 9] re ac tion.isomerization and only 11% was con verted to hex ane.Af terwards, the hy dro ge na tion pro cess pre dom i nated (Ta ble 1).Polynuclear spe cies re quire less en ergy to form compounds such as [Fe(CO) 4 (ole fin)], be cause the cleav age of the iron-iron bond de mands less en ergy than that of the iron-CO bond 15 , which ex plains the lower ac tiv ity of [Fe(CO) 5 ] for isomerization .In the step where the hy dro ge na tion re ac tion pre dom i nates (af ter 1h of re ac tion), sim i lar re sults were found for cyclohexene.In Scheme 1, pos si ble mech a nisms are suggested for these hy dro ge na tion and isomerization pro cesses, based on Refs. 5 and 17.It is sug gested here that the step involv ing the equi lib rium be tween [Fe(CO) 3 (ole fin)] and [Fe(CO) 3 (ole fin)] 2 is more im por tant in the hy dro ge na tion cycle, which ex plains why [Fe 2 (CO) 9 ] is more ac tive in hy dro gena tion than in isomerization re ac tions: when [Fe 2 (CO) 9 ] is used, there is an in crease in the [Fe(CO) 3 (ole fin)] 2 con cen tration, and con se quently the [Fe(CO) 3 (ole fin)] con cen tra tion increases.
The IR spec tra for the 1-hexene re ac tion of the three iron com pounds show a pat tern which is anal o gous to that for the cyclohexene re ac tion.The bands char ac ter is tic of [Fe(CO) 4 (1-hexene)], [HFe(CO) 3 (π-allyl)] and [Fe 3 (CO) 12 ] can be ob served.
All at tempts to iden tify the wine-colored com pound which pre cip i tates un der vac uum at -78 °C failed.As soon as the temper a ture rises , the wine-colored solid com pound be comes green and the IR spec trum shows only [Fe 3 (CO) 12 ] bands.

Con clu sions
The pho to chem i cal ac ti va tion of [Fe(CO) 5], [Fe 2(CO) 9], and [Fe 3(CO) 12 ] at low tem per a ture (0 °C) and in the pres ence of olefins led to the for ma tion of compounds that cat a lyze isomerization and hy dro ge na tion processes.
Isomerization is faster than hy dro ge na tion, but does not have any in flu ence on the cat a lyst ac tiv ity, as far as hy droge na tion is con cerned, which con firms that there are in terme di ates com mon to the two pro cesses.
[Fe 2(CO) 9] shows the high est ac tiv ity in the hy dro ge nation of both cyclohexene and 1-hexene.This leads to the con clu sion that the dinuclear spe cies [Fe 2(CO) 6(ole fin) 2] is im por tant in the hy dro ge na tion pro cess.

Ac knowl edg ments
Thanks to Prof. Ian S. But ler (McGill Uni ver sity -Canada) for the help ful sug ges tions, and to CNPq for fel lowships and grants.
Re gard ing the three iron car bon yls, it was ob served that dur ing the first hour of re ac tion, 1-hexene suf fered mainly Ta ble 1. Prod uct con ver sions in the cyclohexene and 1-hexene hy dro ge na tion re ac tions us ing [Fe(CO )5], [Fe 2(CO) 9], and [Fe 3(CO) 12 ] as cat alytic pre cur sors.

Fig ure 1 .
Fig ure 1.In fra red spec tra in the ν(CO) re gion of the cyclohexene hydro ge na tion us ing [Fe 2(CO) 9] as the cat a lyst.