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The photoelectron spectra of some iron tricarbonyl complexes of 4π-electron donor ligands
Journal of Electron Spectroscopy and Related Phenomena, 18 (1980) 189-198 0 Elsevler Sclentlflc Pubhshmg Company, Amsterdam - Printed in The Netherlands
THE PHOTOELECTRON SPECTRA OF SOME IRON TRICARBONYL COMPLEXES OF 4r-ELECTRON DONOR LIGANDS
S D WORLEY*
and T R WEBB
Department of Chemrstry, Auburn Unwerslty, Auburn, Alabama 36830
(US A )
D H GIBSON and T -S ONG Department of Chemrstry, Unrverslty of Loursvrlle, Louasvllle, Kentucky
40248
(US A )
(Received 1 July 1979)
ABSTRACT The photoelectron spectra of eighteen compounds which are dlenes or dlenelron tncarbonyl complexes have been investigated A comparison of the photoelectron spectra of the dlenes and correspondmg iron carbonyl complexes has ylelded the values of the perturbation enerses for the two ?r orbltals of the dlene moiety caused by mteractlon with Fe(C0)3 These perturbation energtes are relatively constant (AnI = 0 89 f 0 07 eV, Anz = 0 22 k 0 06 eV) throughout the senes They have been employed to estimate the A lomzatlon energies of the orgamc transient species cyclobutadlene (8 29 and 11 95 eV) and trlmethylenemethane (8 36 and 11 79 eV), two novel molecules which have not been studied successfully by photoelectron spectroscopy to date
INTRODUCTION
Some years ago, Dewar and Worley 12-41 proposed that reasonable estlmates of the perturbation enerees caused by mteractlon of the iron tncarbony1 moiety mth 4n-electron hydrocarbon hgands could be measured by comparing the photoelectron spectra of the complexes to those of the free hgands They compared the photoelectron spectrum of 1,3-butadlene vrnth that of butadlene-iron trlcarbonyl [ 2,4], to obtam estimates of the perturbations mtroduced by the Fe(C0)3 moiety to the dlene a system (An, and Ama), and then applied these estimated perturbation enerses to the measured spectra of cyclobutadlene-uon trlcarbonyl [ 2,4] and tnmethylenemethane-
* Author to whom correspondence should be addressed
190
iron tncarbonyl [3] to predict the ‘ITlomzatlon enerses of the organic transient species cyclobutadlene and trlmethylenemethane Then and now such a procedure seems to be worthwhile, because the electronic spectra of these two transient molecules (which are of considerable interest to theoretical organic chemists [ 53 ) have to date defied direct observation, due to the considerable experimental dlfflcultles encountered m generatmg the transients, even m instruments designed for the study of lablle species [6] However, a maJor and valid crltlclsm could be advanced concernmg the early work of Dewar and Worley, namely, whether an extrapolation of measured perturbation energies for butadlene-Iron trlcarbonyl to the complexes of such transients as cyclobutadlene and tnmethylenemethane IS really valid, aven that the molecular and/or electromc structures of the free hgand may &ffer from those m the complexes Free cyclobutadlene has been shown to be a rectangular singlet m Its ground state [7] In the complex, the rmg 1s known to be square, but does have a singlet ground state [S] Tnmethylenemethane, on the other hand, has a triplet ground state (estabhshed by EPR studies, [ 9]), while its iron tncarbonyl complex 1s a singlet Thus the early estimates by Dewar and Worley for the n electromc structure of trlmethylenemethane probably best refer to the first excited singlet In an attempt to address the possible hmlts of vmatlon m perturbation mthm a series of related molecules, we have studied the photoelectron spectra of eight pairs of 4n-electron dtene-iron tncarbonyl complexes and the correspondmg stable dlene hgands We have also remvestlgated the spectra of cyclobutadleneyron trlcarbonyl and trlmethylenemethane-Iron tncarbonyl, because the early work of Dewar and Worley [ 2-41 was performed mth a low-resolution cylmdrlcal-md-type analyzer It should be noted that several other UPS groups have studied the photoelectron spectra of selected dleneu-on trlcarbonyl complexes [ 10, 11],mcludmg those of cyclobutadlene [12] and trimethylenemethane (XPS only [13] ), but none wth the goals of determmmg perturbation energres and estlmatmg the electronic structures of transients
EXPERIMENTAL
The photoelectron spectra discussed m this work were recorded by a Perkm-Elmer Model PS-18 photoelectron spectrometer In all cases the excltatlon source was the He(I) resonance hne (584 a) The data to be dlscussed represent an average of between three and five spectral runs for each sample, argon and xenon were always employed as mternal cahbrants All samples could be studled readily at room temperature The iron carbonyl complexes used m this work were prepared and purlfled by standard literature methods The dlenes were obtamed from the Aldrich Chemical Company, or from the Chemical Samples Company, and were used mthout additional purlflcatlon
191
RESULTS
AND ASSIGNMENTS
The low-lonlzatlon re@ons of the photoelectron spectra of SIXof the paws of dlenes and their won trrcarbonyl complexes are shown m Figs 1 and 2 The spectra of butadlenelron trlcarbonyl [lo] , cyclobutadlene-uon tmcarbonyl [ 121, and trlmethylenemethane-uon trrcarbonyl [ 11, have been illustrated
, 7
8
9
10
II
I
,
12 1317 8
9
Icmzalmn
IO
Ii
12 I:
7 8
9
IO
II
12
13
potential (eV)
Fig 1 The photoelectron
spectra of dlenes B, C, and D (see Table 1) and thew won carbonyl complexes The excitation source was the He(I) resonance hne The posltlons chosen for the vertical lomzatlon potentials listed m Table 1 are indicated
w I’
1
I
8
9
I
IO
I
II
1
I
I
I
I2 I: ‘78
9
Iomzatron
IO
II
I2 I317
8
9
IO
II
12
3
potentbal (eV1
Fig 2 The photoelectron spectra of dlenes F, G, and H (see Table 1) and their won carbonyl complexes The excitation source was the He(I) resonance lme The positions chosen for the vertical ionization potentials listed m Table 1 are indicated
(H) 8 8W1) 8 19(Fe)
Ethyl-trans, trans-2,4-hexadlenoate
Tnmethylenemethannron (TMMFCO)
Cyclobutadlenelron (CBFCO) trlcarbonyl
8 15(Fe)’
7 51(Fe)
GFCO
trlcarbonyl
8 73(n1)’
HFCO
8 49(Fe)’
7 96(Fe)’
FFCO
Blcyclo[ 2 2 l] heptadlene (G)
8 63(Fe)
8 43(Fe)’
9 96ko) 8 73(Fe)
8 34(Fe)
9 59(n2)k
8 48(Fe) 10 84(a2 )’
8 32(n1 )’
(F)
1,3-Cyclohexadiene
10 57(n* )h
7 94(Fe)
8 47(n1 )h
(E)
2-Methyl-trclns-l,3-pentad1ene
8 50(Fe)
10 41(a* )g
EFCO
7 95(Fe)
2,3-Dimethyl-1,3-butadlene
DFCO
8 07(Fe) 8 72(n1 )g
CFCO 8 62(Fe)
8 60(n1)’
trans-1,3Pentadlene (D)
8 62(Fe) 11 ll(n2)f
8 ll(Fe)
BFCO
(C)
8 72(Fe)d 10 98(s, )e
8 87(n1)”
2-Methyl-1,3-butadlene
(B)
11 34(n*)C
9 08(n1 )” 8 22(Fe)d
1,3-Butad1ene (A)
AFCO
12
9 25(al)
9 18(n1)’
9 75(n1)
9 63(n1) 10 53(n(-j)
9 24(nl )’
9 46(n1)
9 47(Rl)
9 Wnl)
9 Wn1)
12 Ol(n2 )
12 17(Q)’
9 93(nco)
9 8fUn2) 11 l3(n2)
llOl(A2)’
10 88(7r2 )
10 67(n2)
1130(n2)
11 48(~,)~
14
10 63(no)
15
AND THEIR IRON TRICARBONYL
9 88(7r1 )d
13
HYDROCARBONS
11
OF SOME 4n-ELECTRON
Compound b
IONIZATION ENERGIES AND ASSIGNMENTS a
TABLE 1
11 47(n2)
I6
COMPLEXES
Previous values are 7 98,8
43 eV (ref 11)
Spectroscopy,
Wiley, New York, 1970
Prewous values are 8 17,8 45,9 21, and 12 81eV (ref 12)
m The first Fe component was not resolved
’
k Previous values are 8 69 and 9 55 eV, E Hedbronner and H D Martin, Helv Chum Acta, 55 (1972) 1490
56,9 33, and 1104 eV (ref 11)
Revlous values are 8 25 and 10 7 eV (ref 14)
’
’
h Pre-nous values are 8 47 and 10 5 eV (ref 14)
g Previous values are 8 62 and 10 2 eV (ref 14)
f Previous values are 8 67 and 11 1 eV (ref 14)
e Previous values are 8 85 and 10 9 eV (ref 14)
52 eV (ref lo), 8 16,8 67,9 82,ll
D W Turner, C Baker, A D Baker and C R Brundle, Molecular Photoelectron
a Previous values are 8 23,8 82,9 93,11
’
b The designation FCO is for the won trlcarbonyl complex of each dlene
a All lomzatlon ener@es are m eV, and represent vertical loruzatlon potent&s
194 m previous pubhcatlons, and smce the current spectra contained no slgmflcant differences from these, they have been omitted here Furthermore, the high-lomzatlon-energy reg;zons of the spectra for all the molecules have been omitted, because at lomzatlon enerses greater than 13 eV the spectra are complex, lmposslble to assign defmltlvely, and not important to the arns of this paper (see Introduction) The lomzatlon enerses measured from the photoelectron spectra studied m this work are gwen m Table 1, as well as suggested band assignments Letter designations for the various molecules which ~11 be used subsequently m the text are also gven m Table 1 As can be seen m Figs 1 and 2, the photoelectron spectra of most of the complexes resemble one another rather closely, havmg a broad band system at low lomzatlon energy (7 5-8 8 eV), urlth two resolved components, and then two addltlonal bands resolved m the 9 Z12 2eV region, the exception bemg the complex HFCO, which has two highenergy oxygen ‘lone-pau-’ orbltals Past mterpretatlons of the photoelectron spectra of dlene-lron trlcarbonyl complexes have concluded that the broad band unth two components must correspond to the overlappmg lomzatlon of orbltals which are pnmmly associated mth the iron moiety [l-4, lo-121 We see no reason to alter that mterpretatlon for the complexes studied m this work This leaves bands three and four m the spectra of the complexes to be assigned to lomzatlon of the perturbed 7r orbltals of the hgands The photoelectron spectra of the esters H and HFCO are somewhat more complex, but nevertheless their mterpretatlon 1s straghtforward The I1 band m the spectrum of H contams vlbratlonal structure wmllar to that of all the other ConJugated dlenes, and hence must be assigned to ionization of an orbital which 1s primarily n m character (n, ) The band structure and energy (8 85 eV) for this process mdlcate that there 1s very little resonance mteractlon between this 7r orbital and those orbltals associated pnmarlly mth the ester group (rco, nco , and no) A small mductlve destablhzatlon of the 7~~ ionic state 1s mdlcated by the slightly higher value of II for H as opposed to that for C. The I4 lomzatlon band m the spectrum of H corresponds to lomzatlon of an orbital which 1s primarily confined to the conJugated dlene moiety (nz ), analogous to the case of C which contams a band at almost the same energy (11 13, 11 11 eV) The remammg two bands, I2 and 1, m the spectrum of H, must be assigned to lomzatlon of the lonepair orbltals ylco and no Extensive earlier work m these laboratorres [1517] and elsewhere [ 181 has shown that the carbonyl-oxygen lone-pau orbital nco always hes higher m the energy scale than does an ester-oxygen lone-paw orbital no, therefore, we assign the I2 band (9 96 eV) for H to lomzatlon of nco, and the I3 band (10 53 eV) to lonrzatxon of 7to The correspondmg bands for the uon trlcarbonyl complex (HFCO) occur at 9 98 eV and 10 63 eV respectively, which mdlcates that the perturbations caused m nco and no by the Fe(C0)3 moiety are very small This would be expected, smce the coordmatlon IS through the dlene 7r orbltals (7~~ and
195
TABLE
2
PERTURBATION ENERGIES ACTION WITH Fe(CO)a a Compound b
A/AFCO B/BFCO C/CFCO D/DFCO E/EFCO F/FFCO G/GFCO H/HFCO
FOR 4n-ELECTRON
Llgand =I
Complex ni
An, ’
9 08
9 9 9 9 9 9 9 9
0 0 1 0 0 0 0 0
8 8 8 8 8 8 8
87 60 72 47 32 73 85
88 68 62 47 46 24 63 75
80 81 02 75 99 92 90 90
DIENES
AS A RESULT
Llgand
Complex
R2
772
11 34 10 89 11 11 10 41 10 57 10 84 9 59 11 13
1148 1105 1130 10 67 10 88 1101 9 80 1147
An, ’
OF INTER-
Complex Fed
6
0 0 0 0 0 0 0 0
14 16 19 26 31 17 21 34
0 0 0 0 0 0 0 0
50 51 55 55 54 53 83 54
“, All values are in eV See Table 1 for compound desqnatlon i Perturbation energy Sphttmg of the band representing Fe orbital ionization
7r2 are shifted substantially more by complexatlon) The perturbation shifts for the a, and 7r2 orbltals caused by mteractlon with the Fe(C0)3 moiety are presented m Table 2 The splitting (6Fe) between the resolved components of the band m the spectra of the complexes (components which have been assigned to lonlzatlon of orbrtals confined pruntiy to Fe) 1s also @ven m Table 2
DISCUSSION
As stated previously, It has been generally conceded that the first broad band m the photoelectron spectrum of a dlenelron trlcarbonyl complex corresponds to lonlzatlon of the several orbltals which are prunarlly of iron character [lo-121 This broad, band contams a component of lower mtenslty on the low-lonlzatlon-energy edge for all the dlene-u-on carbonyl complexes studed to date It 1s mterestmg to note that the splrttmg between the lowand high-intensity components 1s approxzmately constant (ca 0 5 eV) for all the complexes except GFCO, CBFCO, and TMMFCO The GFCO sphttmg IS sub&a&ally larger (0 8 eV), while those for CBFCO (0 3 eV) and TMMFCO (not resolved) are smaller The GFCO anomaly might be a result of the substantlally smaller degree of mlxmg of the ?r basis orbltals for the noncoqugated G hgand as opposed to the other hgands which are all corqugated However, the two x perturbation energes for GFCO (0 90 and 0 21 eV) are certamly of the same magnitude as those for the other corqugated complexes Since arguments based upon mtensltles of photoelectron spectral bands are
196
d I
G
I
05I I
I
k2 (CO13
GFCO
at best tenuous [ 191, and smce we have no accurate means avdable for perforrmng MO computations for these complexes, we shall refram from further dlscusslon of the iron orbltals The interested reader may wrsh to refer to the work of Connor et al [lo] It should be noted that an interesting trend exists for the ?r ionization energes for the acychc dlene hydrocarbons Namely, the effects of methyl substltutlon at either the l- or Z- posltlons of 1,3-butadlene-uon tncarbonyl (AFCO) to produce DFCO and EFCO are very nearly additive when compared to BFCO and CFCO This 1s true to a lesser extent for the uncomplexed dlenes For both the complexes and the free hgands, the ~1 lontzatlon energy 1s affected most by substltutlon at the l- position, while the 7r2 ionlzatlon energy shifts most upon 2- substltutlon This 1s precisely what one would predict from any calculation of orbital electron den&y for dlenes, 1 e , the ai orbital has its largest contrlbutlons on the l- and 4- carbon atoms of the coqugated dlene, while on the other hand 772has its largest contributlons on the 2- and 3- carbon atoms At present we have msufflclent data to draw any conclusions about bonding m the complexes Perhaps a future XPS study of the complexes would be lllummatmg m this respect Of much greater interest 1s the fact that the perturbation energes (AmI and A7r2 m Table 2) for the eight pairs of &enes and dieneTon tncarbonyl complexes are relatively constant across the series In fact, the average values for An, and A7r2 across the senes are 0 89 + 0 07 eV, and 0 22 + 0 06 eV, respectively We believe that these fmdmgs support the early assumptions of Dewar and Worley [2-41 concerning the estimated lomzatlon eneraes of cyclobutadlene and trlmethylenemethane The prelcted perturbation energies m this early work were smaller (Arl 2~ 0 50 eV, A7r2 Z 0 0 eV), this was a result of the dlfflculty of estnnatmg band posltlons m the low-resolution spectra (only approximate adiabatic lonlzatlon potentrals could be obtained) Furthermore, Dewar and Worley felt that a correction was necessary to account for the fact that 1,3-butahene exists predommantly m the trans- conformation, rather than m the czs- conformation which must be the case for the complex Crude 7r SCF MO calculations mdlcated that czs-l.,Qbutadrene should have an 11 which 1s 0 14 eV higher than that for tians1,3-butadlene However, more sophisticated calculations, and photoelectron spectroscopic data on several geometnc isomers (e g , czs- and trans-2-butene both have I1 values of 9 12 eV [14] ), suggest that czs- and tiuns-butadlene should have approxnnately the same I1 values, therefore, we have not meluded a correction m this work Our revised estimates of the two ?rlomzatlon
potentials of cyclobutadlene, based upon the new values of AR] and AX*, are 8 29 and 11 95 eV, respectively, and those for tnmethylenemethane are 8 36 and 11 79 eV, respectively Hedaya et al [20] have reported a value for the first lomzatlon potential of a C4 H4 isomer believed to be cyclobutadlene The appearance potential measured m their electron-Impact work for C4 H4 + was 8 2-8 6 eV Our estimated II for cyclobutadlene (8 29 eV) mdlcates that Hedaya et al were probably successful m generatmg cyclobutadlene, rather than some other isomer such as vinyl acetylene The use of the perturbation-energy corrections m deriving estimates of the m lonlzatlon enerses of the orgamc transients cyclobutafiene and tnmethylenemethane 1s certainly not stnctly ngorous m the theoretlcal sense, for the several reasons mentloned m the Tntroductlon Furthermore, the predicted II and 12 for trlmethylenemethane refer to the first excited smglet, rather than to the ground-state tnplet However, we feel that there are several reasons why our predlcted values vvlllprove to be accurate Fustly, the recent XPS study performed by Jolly et al [13] on the core levels of TMMFCO and AFCO has shown that the overall donor and acceptor characters of the llgands tnmethylenemethane and 1,3-butadlene are very similar, this 1s in spite of the fact that the former 1s a ground-state tnplet, while the latter IS a g-round-state smglet Secondly, the perturbation corrections for GFCO were in accord with those of the coqugated dlene complexes, mdlcatmg a lack of sensltlvlty of the Fe(C0)3 interaction mth ‘ITsystems to dfferences m structure Thirdly, recent work m these laboratones has shown that the complexatlon of Fe(C0)4 to tetramethylallene causes a shift of 0 75 eV towards higher lomzatlon energy [21] This shift 1s of the same order of magmtude as for all the dlene-lron trlcarbonyl complexes considered m this work These facts mdlcate that the mteractlon of iron carbonyl moretles vvlth R systems IS relatrvely msensltlve to rather substantial geometrical and structural alteratrons CONCLUSION
This work has presented the photoelectron spectra for eight pus of 47relectron donor hgands and their non tncarbonyl complexes, as well as for the won tncarbonyl complexes of the theoretically mterestmg transients Estimates of the perturbation cyclobutadlene and tnmethylenemethane enermes An 1 and Anz for the hgand moletles mtroduced by uon carbonyl complexatlon have been made from the photoelectron spectra The direct study of cyclobutadlene and tnmethylenemethane by photoelectron spectroscopy will be a difficult task, because of the short lifetimes of these species and because of the complexity of the spectra when precursor material, side products, and other contammants are present The a loruzatlon energes estimated m this work for these two transients should prove valuable to future workers who are attemptmg to mterpret the complex spectra We believe that our estimates will prove to be accurate to
198
mthm the average deviation of our AnI 0 1eV [22]
and An,
values, 1 e to within
ACKNOWLEDGEMENTS
S D Worley gratefully acknowledges support provided by the Research Corporation for this work T R Webb, D H Gibson, and T -S Ong gratefully acknowledge the support of the donors of the Petroleum Research Fund, admmlstered by the American Chemical Society REFERENCES 1 2 3 4 5
6
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
A preliminary account of portions of this work may be found in S D Worley,T R Webb, D H Gibson and T -S Ong,J OrganometChem,168 (1979) Cl6 M J S Dewar and S D Worley, J Chem Phys ,50 (1969) 654 M J S Dewar and S D WorIey, J Chem Phys ,51 (1969) 263 S D Worley, J Chem Sot , Chem Commun , (1970) 980 W T Borden, E R Davldson and P Hart, J Am Chem Sot , 100 (1978) 388, E R Davldson and W T Borden, J Am Chem Sot ,99 (1977) 2053 and numerous references cited therem For example, an effort was made recently to study the photoelectron spectrum of trlmethylenemethane (J L Beauchamp, 175th Nat1 Am Chem Sot Meet , Anaheim, CA, Phys Abstr 95, March 1978), but overlappmg PES bands from contammants, precursor, side products, etc , have greatly comphcated 1t.smterpretatlon S Masamune, F A Souto-Bachlller, T Machlguchl and J E Bertle, J Am Chem sot * 100 (1978) 4889 R Pettlt, J Organomet Chem , 100 (1975) 205, and references cited therem P Dowd, Act Chem Res , 5 (1972) 242, and references cited therein J A Connor, L M R Derrick, M B Hall, I H Hllher, M F Guest, B R Hlggmson and D R Lloyd, Mol Phys ,28 (1974) 1193 J C Green, P Powell and J van nlborg, J Chem Sot , Dalton Trans , (1976) 1974 M B Hall, I H Hllher, J A Connor, M F Guest and D R Lloyd, Mol Phys , 30 (1975) 839 J W Koepke, W L Jolly, G M Bancroft, P A Malmqulst and K Slegbahn, Inorg Chem, 16 (1977) 2659 G Bier], F Burger, E Hellbronner and J P Maler, kelv Chum Acta, 60 (1977) 2213 S D Worley, S H Gerson, N Bodor, J J Kammskl and T W Flechtner, J Chem Phys, 68 (1978) 1313 S H Gerson, S D Worley, N Bodor and J J Kammskl, J Med Chem , 21 (1978) 686 S H Gerson, S D Worley, N Bodor, J J Kamrnskl and T W Flechtner, J Electron Spectrosc Relat Phenom , 13 (1978) 421 For example, see P R Ollvato, H Vlertler, B Wladlslaw, K C Cole and C Sandorfy, Can J Chem ,54 (1976) 3026 S D Worley, J Electron Spectrosc Relat Phenom , 6 (1975) 157 E Hedaya, R D Miller, D W McNeil, M E Kent, P F D’Angelo and P Schlssel, J Am Chem Sac ,91 (1969) 1875 W E HIl, C H Ward, T R Webb and S D Worley, Inorg Chem , 18 (1979) 2029 It has been pointed out (see D R Yarkony and H F Schaefer, Chem Phys L&t , 35 (1975) 291) that the Ionic state correspondmg to removal of an electron from the ~2 orbital of trlmethylenemethane 1s subject to multlplet sphttmg Our crude analy61s cannotaccountfor this effect, but probably our estimated I2 would correspond to a ‘band center’ of the multlplet structure
THE PHOTOELECTRON SPECTRA OF SOME IRON TRICARBONYL COMPLEXES OF 4r-ELECTRON DONOR LIGANDS
S D WORLEY*
and T R WEBB
Department of Chemrstry, Auburn Unwerslty, Auburn, Alabama 36830
(US A )
D H GIBSON and T -S ONG Department of Chemrstry, Unrverslty of Loursvrlle, Louasvllle, Kentucky
40248
(US A )
(Received 1 July 1979)
ABSTRACT The photoelectron spectra of eighteen compounds which are dlenes or dlenelron tncarbonyl complexes have been investigated A comparison of the photoelectron spectra of the dlenes and correspondmg iron carbonyl complexes has ylelded the values of the perturbation enerses for the two ?r orbltals of the dlene moiety caused by mteractlon with Fe(C0)3 These perturbation energtes are relatively constant (AnI = 0 89 f 0 07 eV, Anz = 0 22 k 0 06 eV) throughout the senes They have been employed to estimate the A lomzatlon energies of the orgamc transient species cyclobutadlene (8 29 and 11 95 eV) and trlmethylenemethane (8 36 and 11 79 eV), two novel molecules which have not been studied successfully by photoelectron spectroscopy to date
INTRODUCTION
Some years ago, Dewar and Worley 12-41 proposed that reasonable estlmates of the perturbation enerees caused by mteractlon of the iron tncarbony1 moiety mth 4n-electron hydrocarbon hgands could be measured by comparing the photoelectron spectra of the complexes to those of the free hgands They compared the photoelectron spectrum of 1,3-butadlene vrnth that of butadlene-iron trlcarbonyl [ 2,4], to obtam estimates of the perturbations mtroduced by the Fe(C0)3 moiety to the dlene a system (An, and Ama), and then applied these estimated perturbation enerses to the measured spectra of cyclobutadlene-uon trlcarbonyl [ 2,4] and tnmethylenemethane-
* Author to whom correspondence should be addressed
190
iron tncarbonyl [3] to predict the ‘ITlomzatlon enerses of the organic transient species cyclobutadlene and trlmethylenemethane Then and now such a procedure seems to be worthwhile, because the electronic spectra of these two transient molecules (which are of considerable interest to theoretical organic chemists [ 53 ) have to date defied direct observation, due to the considerable experimental dlfflcultles encountered m generatmg the transients, even m instruments designed for the study of lablle species [6] However, a maJor and valid crltlclsm could be advanced concernmg the early work of Dewar and Worley, namely, whether an extrapolation of measured perturbation energies for butadlene-Iron trlcarbonyl to the complexes of such transients as cyclobutadlene and tnmethylenemethane IS really valid, aven that the molecular and/or electromc structures of the free hgand may &ffer from those m the complexes Free cyclobutadlene has been shown to be a rectangular singlet m Its ground state [7] In the complex, the rmg 1s known to be square, but does have a singlet ground state [S] Tnmethylenemethane, on the other hand, has a triplet ground state (estabhshed by EPR studies, [ 9]), while its iron tncarbonyl complex 1s a singlet Thus the early estimates by Dewar and Worley for the n electromc structure of trlmethylenemethane probably best refer to the first excited singlet In an attempt to address the possible hmlts of vmatlon m perturbation mthm a series of related molecules, we have studied the photoelectron spectra of eight pairs of 4n-electron dtene-iron tncarbonyl complexes and the correspondmg stable dlene hgands We have also remvestlgated the spectra of cyclobutadleneyron trlcarbonyl and trlmethylenemethane-Iron tncarbonyl, because the early work of Dewar and Worley [ 2-41 was performed mth a low-resolution cylmdrlcal-md-type analyzer It should be noted that several other UPS groups have studied the photoelectron spectra of selected dleneu-on trlcarbonyl complexes [ 10, 11],mcludmg those of cyclobutadlene [12] and trimethylenemethane (XPS only [13] ), but none wth the goals of determmmg perturbation energres and estlmatmg the electronic structures of transients
EXPERIMENTAL
The photoelectron spectra discussed m this work were recorded by a Perkm-Elmer Model PS-18 photoelectron spectrometer In all cases the excltatlon source was the He(I) resonance hne (584 a) The data to be dlscussed represent an average of between three and five spectral runs for each sample, argon and xenon were always employed as mternal cahbrants All samples could be studled readily at room temperature The iron carbonyl complexes used m this work were prepared and purlfled by standard literature methods The dlenes were obtamed from the Aldrich Chemical Company, or from the Chemical Samples Company, and were used mthout additional purlflcatlon
191
RESULTS
AND ASSIGNMENTS
The low-lonlzatlon re@ons of the photoelectron spectra of SIXof the paws of dlenes and their won trrcarbonyl complexes are shown m Figs 1 and 2 The spectra of butadlenelron trlcarbonyl [lo] , cyclobutadlene-uon tmcarbonyl [ 121, and trlmethylenemethane-uon trrcarbonyl [ 11, have been illustrated
, 7
8
9
10
II
I
,
12 1317 8
9
Icmzalmn
IO
Ii
12 I:
7 8
9
IO
II
12
13
potential (eV)
Fig 1 The photoelectron
spectra of dlenes B, C, and D (see Table 1) and thew won carbonyl complexes The excitation source was the He(I) resonance hne The posltlons chosen for the vertical lomzatlon potentials listed m Table 1 are indicated
w I’
1
I
8
9
I
IO
I
II
1
I
I
I
I2 I: ‘78
9
Iomzatron
IO
II
I2 I317
8
9
IO
II
12
3
potentbal (eV1
Fig 2 The photoelectron spectra of dlenes F, G, and H (see Table 1) and their won carbonyl complexes The excitation source was the He(I) resonance lme The positions chosen for the vertical ionization potentials listed m Table 1 are indicated
(H) 8 8W1) 8 19(Fe)
Ethyl-trans, trans-2,4-hexadlenoate
Tnmethylenemethannron (TMMFCO)
Cyclobutadlenelron (CBFCO) trlcarbonyl
8 15(Fe)’
7 51(Fe)
GFCO
trlcarbonyl
8 73(n1)’
HFCO
8 49(Fe)’
7 96(Fe)’
FFCO
Blcyclo[ 2 2 l] heptadlene (G)
8 63(Fe)
8 43(Fe)’
9 96ko) 8 73(Fe)
8 34(Fe)
9 59(n2)k
8 48(Fe) 10 84(a2 )’
8 32(n1 )’
(F)
1,3-Cyclohexadiene
10 57(n* )h
7 94(Fe)
8 47(n1 )h
(E)
2-Methyl-trclns-l,3-pentad1ene
8 50(Fe)
10 41(a* )g
EFCO
7 95(Fe)
2,3-Dimethyl-1,3-butadlene
DFCO
8 07(Fe) 8 72(n1 )g
CFCO 8 62(Fe)
8 60(n1)’
trans-1,3Pentadlene (D)
8 62(Fe) 11 ll(n2)f
8 ll(Fe)
BFCO
(C)
8 72(Fe)d 10 98(s, )e
8 87(n1)”
2-Methyl-1,3-butadlene
(B)
11 34(n*)C
9 08(n1 )” 8 22(Fe)d
1,3-Butad1ene (A)
AFCO
12
9 25(al)
9 18(n1)’
9 75(n1)
9 63(n1) 10 53(n(-j)
9 24(nl )’
9 46(n1)
9 47(Rl)
9 Wnl)
9 Wn1)
12 Ol(n2 )
12 17(Q)’
9 93(nco)
9 8fUn2) 11 l3(n2)
llOl(A2)’
10 88(7r2 )
10 67(n2)
1130(n2)
11 48(~,)~
14
10 63(no)
15
AND THEIR IRON TRICARBONYL
9 88(7r1 )d
13
HYDROCARBONS
11
OF SOME 4n-ELECTRON
Compound b
IONIZATION ENERGIES AND ASSIGNMENTS a
TABLE 1
11 47(n2)
I6
COMPLEXES
Previous values are 7 98,8
43 eV (ref 11)
Spectroscopy,
Wiley, New York, 1970
Prewous values are 8 17,8 45,9 21, and 12 81eV (ref 12)
m The first Fe component was not resolved
’
k Previous values are 8 69 and 9 55 eV, E Hedbronner and H D Martin, Helv Chum Acta, 55 (1972) 1490
56,9 33, and 1104 eV (ref 11)
Revlous values are 8 25 and 10 7 eV (ref 14)
’
’
h Pre-nous values are 8 47 and 10 5 eV (ref 14)
g Previous values are 8 62 and 10 2 eV (ref 14)
f Previous values are 8 67 and 11 1 eV (ref 14)
e Previous values are 8 85 and 10 9 eV (ref 14)
52 eV (ref lo), 8 16,8 67,9 82,ll
D W Turner, C Baker, A D Baker and C R Brundle, Molecular Photoelectron
a Previous values are 8 23,8 82,9 93,11
’
b The designation FCO is for the won trlcarbonyl complex of each dlene
a All lomzatlon ener@es are m eV, and represent vertical loruzatlon potent&s
194 m previous pubhcatlons, and smce the current spectra contained no slgmflcant differences from these, they have been omitted here Furthermore, the high-lomzatlon-energy reg;zons of the spectra for all the molecules have been omitted, because at lomzatlon enerses greater than 13 eV the spectra are complex, lmposslble to assign defmltlvely, and not important to the arns of this paper (see Introduction) The lomzatlon enerses measured from the photoelectron spectra studied m this work are gwen m Table 1, as well as suggested band assignments Letter designations for the various molecules which ~11 be used subsequently m the text are also gven m Table 1 As can be seen m Figs 1 and 2, the photoelectron spectra of most of the complexes resemble one another rather closely, havmg a broad band system at low lomzatlon energy (7 5-8 8 eV), urlth two resolved components, and then two addltlonal bands resolved m the 9 Z12 2eV region, the exception bemg the complex HFCO, which has two highenergy oxygen ‘lone-pau-’ orbltals Past mterpretatlons of the photoelectron spectra of dlene-lron trlcarbonyl complexes have concluded that the broad band unth two components must correspond to the overlappmg lomzatlon of orbltals which are pnmmly associated mth the iron moiety [l-4, lo-121 We see no reason to alter that mterpretatlon for the complexes studied m this work This leaves bands three and four m the spectra of the complexes to be assigned to lomzatlon of the perturbed 7r orbltals of the hgands The photoelectron spectra of the esters H and HFCO are somewhat more complex, but nevertheless their mterpretatlon 1s straghtforward The I1 band m the spectrum of H contams vlbratlonal structure wmllar to that of all the other ConJugated dlenes, and hence must be assigned to ionization of an orbital which 1s primarily n m character (n, ) The band structure and energy (8 85 eV) for this process mdlcate that there 1s very little resonance mteractlon between this 7r orbital and those orbltals associated pnmarlly mth the ester group (rco, nco , and no) A small mductlve destablhzatlon of the 7~~ ionic state 1s mdlcated by the slightly higher value of II for H as opposed to that for C. The I4 lomzatlon band m the spectrum of H corresponds to lomzatlon of an orbital which 1s primarily confined to the conJugated dlene moiety (nz ), analogous to the case of C which contams a band at almost the same energy (11 13, 11 11 eV) The remammg two bands, I2 and 1, m the spectrum of H, must be assigned to lomzatlon of the lonepair orbltals ylco and no Extensive earlier work m these laboratorres [1517] and elsewhere [ 181 has shown that the carbonyl-oxygen lone-pau orbital nco always hes higher m the energy scale than does an ester-oxygen lone-paw orbital no, therefore, we assign the I2 band (9 96 eV) for H to lomzatlon of nco, and the I3 band (10 53 eV) to lonrzatxon of 7to The correspondmg bands for the uon trlcarbonyl complex (HFCO) occur at 9 98 eV and 10 63 eV respectively, which mdlcates that the perturbations caused m nco and no by the Fe(C0)3 moiety are very small This would be expected, smce the coordmatlon IS through the dlene 7r orbltals (7~~ and
195
TABLE
2
PERTURBATION ENERGIES ACTION WITH Fe(CO)a a Compound b
A/AFCO B/BFCO C/CFCO D/DFCO E/EFCO F/FFCO G/GFCO H/HFCO
FOR 4n-ELECTRON
Llgand =I
Complex ni
An, ’
9 08
9 9 9 9 9 9 9 9
0 0 1 0 0 0 0 0
8 8 8 8 8 8 8
87 60 72 47 32 73 85
88 68 62 47 46 24 63 75
80 81 02 75 99 92 90 90
DIENES
AS A RESULT
Llgand
Complex
R2
772
11 34 10 89 11 11 10 41 10 57 10 84 9 59 11 13
1148 1105 1130 10 67 10 88 1101 9 80 1147
An, ’
OF INTER-
Complex Fed
6
0 0 0 0 0 0 0 0
14 16 19 26 31 17 21 34
0 0 0 0 0 0 0 0
50 51 55 55 54 53 83 54
“, All values are in eV See Table 1 for compound desqnatlon i Perturbation energy Sphttmg of the band representing Fe orbital ionization
7r2 are shifted substantially more by complexatlon) The perturbation shifts for the a, and 7r2 orbltals caused by mteractlon with the Fe(C0)3 moiety are presented m Table 2 The splitting (6Fe) between the resolved components of the band m the spectra of the complexes (components which have been assigned to lonlzatlon of orbrtals confined pruntiy to Fe) 1s also @ven m Table 2
DISCUSSION
As stated previously, It has been generally conceded that the first broad band m the photoelectron spectrum of a dlenelron trlcarbonyl complex corresponds to lonlzatlon of the several orbltals which are prunarlly of iron character [lo-121 This broad, band contams a component of lower mtenslty on the low-lonlzatlon-energy edge for all the dlene-u-on carbonyl complexes studed to date It 1s mterestmg to note that the splrttmg between the lowand high-intensity components 1s approxzmately constant (ca 0 5 eV) for all the complexes except GFCO, CBFCO, and TMMFCO The GFCO sphttmg IS sub&a&ally larger (0 8 eV), while those for CBFCO (0 3 eV) and TMMFCO (not resolved) are smaller The GFCO anomaly might be a result of the substantlally smaller degree of mlxmg of the ?r basis orbltals for the noncoqugated G hgand as opposed to the other hgands which are all corqugated However, the two x perturbation energes for GFCO (0 90 and 0 21 eV) are certamly of the same magnitude as those for the other corqugated complexes Since arguments based upon mtensltles of photoelectron spectral bands are
196
d I
G
I
05I I
I
k2 (CO13
GFCO
at best tenuous [ 191, and smce we have no accurate means avdable for perforrmng MO computations for these complexes, we shall refram from further dlscusslon of the iron orbltals The interested reader may wrsh to refer to the work of Connor et al [lo] It should be noted that an interesting trend exists for the ?r ionization energes for the acychc dlene hydrocarbons Namely, the effects of methyl substltutlon at either the l- or Z- posltlons of 1,3-butadlene-uon tncarbonyl (AFCO) to produce DFCO and EFCO are very nearly additive when compared to BFCO and CFCO This 1s true to a lesser extent for the uncomplexed dlenes For both the complexes and the free hgands, the ~1 lontzatlon energy 1s affected most by substltutlon at the l- position, while the 7r2 ionlzatlon energy shifts most upon 2- substltutlon This 1s precisely what one would predict from any calculation of orbital electron den&y for dlenes, 1 e , the ai orbital has its largest contrlbutlons on the l- and 4- carbon atoms of the coqugated dlene, while on the other hand 772has its largest contributlons on the 2- and 3- carbon atoms At present we have msufflclent data to draw any conclusions about bonding m the complexes Perhaps a future XPS study of the complexes would be lllummatmg m this respect Of much greater interest 1s the fact that the perturbation energes (AmI and A7r2 m Table 2) for the eight pairs of &enes and dieneTon tncarbonyl complexes are relatively constant across the series In fact, the average values for An, and A7r2 across the senes are 0 89 + 0 07 eV, and 0 22 + 0 06 eV, respectively We believe that these fmdmgs support the early assumptions of Dewar and Worley [2-41 concerning the estimated lomzatlon eneraes of cyclobutadlene and trlmethylenemethane The prelcted perturbation energies m this early work were smaller (Arl 2~ 0 50 eV, A7r2 Z 0 0 eV), this was a result of the dlfflculty of estnnatmg band posltlons m the low-resolution spectra (only approximate adiabatic lonlzatlon potentrals could be obtained) Furthermore, Dewar and Worley felt that a correction was necessary to account for the fact that 1,3-butahene exists predommantly m the trans- conformation, rather than m the czs- conformation which must be the case for the complex Crude 7r SCF MO calculations mdlcated that czs-l.,Qbutadrene should have an 11 which 1s 0 14 eV higher than that for tians1,3-butadlene However, more sophisticated calculations, and photoelectron spectroscopic data on several geometnc isomers (e g , czs- and trans-2-butene both have I1 values of 9 12 eV [14] ), suggest that czs- and tiuns-butadlene should have approxnnately the same I1 values, therefore, we have not meluded a correction m this work Our revised estimates of the two ?rlomzatlon
potentials of cyclobutadlene, based upon the new values of AR] and AX*, are 8 29 and 11 95 eV, respectively, and those for tnmethylenemethane are 8 36 and 11 79 eV, respectively Hedaya et al [20] have reported a value for the first lomzatlon potential of a C4 H4 isomer believed to be cyclobutadlene The appearance potential measured m their electron-Impact work for C4 H4 + was 8 2-8 6 eV Our estimated II for cyclobutadlene (8 29 eV) mdlcates that Hedaya et al were probably successful m generatmg cyclobutadlene, rather than some other isomer such as vinyl acetylene The use of the perturbation-energy corrections m deriving estimates of the m lonlzatlon enerses of the orgamc transients cyclobutafiene and tnmethylenemethane 1s certainly not stnctly ngorous m the theoretlcal sense, for the several reasons mentloned m the Tntroductlon Furthermore, the predicted II and 12 for trlmethylenemethane refer to the first excited smglet, rather than to the ground-state tnplet However, we feel that there are several reasons why our predlcted values vvlllprove to be accurate Fustly, the recent XPS study performed by Jolly et al [13] on the core levels of TMMFCO and AFCO has shown that the overall donor and acceptor characters of the llgands tnmethylenemethane and 1,3-butadlene are very similar, this 1s in spite of the fact that the former 1s a ground-state tnplet, while the latter IS a g-round-state smglet Secondly, the perturbation corrections for GFCO were in accord with those of the coqugated dlene complexes, mdlcatmg a lack of sensltlvlty of the Fe(C0)3 interaction mth ‘ITsystems to dfferences m structure Thirdly, recent work m these laboratones has shown that the complexatlon of Fe(C0)4 to tetramethylallene causes a shift of 0 75 eV towards higher lomzatlon energy [21] This shift 1s of the same order of magmtude as for all the dlene-lron trlcarbonyl complexes considered m this work These facts mdlcate that the mteractlon of iron carbonyl moretles vvlth R systems IS relatrvely msensltlve to rather substantial geometrical and structural alteratrons CONCLUSION
This work has presented the photoelectron spectra for eight pus of 47relectron donor hgands and their non tncarbonyl complexes, as well as for the won tncarbonyl complexes of the theoretically mterestmg transients Estimates of the perturbation cyclobutadlene and tnmethylenemethane enermes An 1 and Anz for the hgand moletles mtroduced by uon carbonyl complexatlon have been made from the photoelectron spectra The direct study of cyclobutadlene and tnmethylenemethane by photoelectron spectroscopy will be a difficult task, because of the short lifetimes of these species and because of the complexity of the spectra when precursor material, side products, and other contammants are present The a loruzatlon energes estimated m this work for these two transients should prove valuable to future workers who are attemptmg to mterpret the complex spectra We believe that our estimates will prove to be accurate to
198
mthm the average deviation of our AnI 0 1eV [22]
and An,
values, 1 e to within
ACKNOWLEDGEMENTS
S D Worley gratefully acknowledges support provided by the Research Corporation for this work T R Webb, D H Gibson, and T -S Ong gratefully acknowledge the support of the donors of the Petroleum Research Fund, admmlstered by the American Chemical Society REFERENCES 1 2 3 4 5
6
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
A preliminary account of portions of this work may be found in S D Worley,T R Webb, D H Gibson and T -S Ong,J OrganometChem,168 (1979) Cl6 M J S Dewar and S D Worley, J Chem Phys ,50 (1969) 654 M J S Dewar and S D WorIey, J Chem Phys ,51 (1969) 263 S D Worley, J Chem Sot , Chem Commun , (1970) 980 W T Borden, E R Davldson and P Hart, J Am Chem Sot , 100 (1978) 388, E R Davldson and W T Borden, J Am Chem Sot ,99 (1977) 2053 and numerous references cited therem For example, an effort was made recently to study the photoelectron spectrum of trlmethylenemethane (J L Beauchamp, 175th Nat1 Am Chem Sot Meet , Anaheim, CA, Phys Abstr 95, March 1978), but overlappmg PES bands from contammants, precursor, side products, etc , have greatly comphcated 1t.smterpretatlon S Masamune, F A Souto-Bachlller, T Machlguchl and J E Bertle, J Am Chem sot * 100 (1978) 4889 R Pettlt, J Organomet Chem , 100 (1975) 205, and references cited therem P Dowd, Act Chem Res , 5 (1972) 242, and references cited therein J A Connor, L M R Derrick, M B Hall, I H Hllher, M F Guest, B R Hlggmson and D R Lloyd, Mol Phys ,28 (1974) 1193 J C Green, P Powell and J van nlborg, J Chem Sot , Dalton Trans , (1976) 1974 M B Hall, I H Hllher, J A Connor, M F Guest and D R Lloyd, Mol Phys , 30 (1975) 839 J W Koepke, W L Jolly, G M Bancroft, P A Malmqulst and K Slegbahn, Inorg Chem, 16 (1977) 2659 G Bier], F Burger, E Hellbronner and J P Maler, kelv Chum Acta, 60 (1977) 2213 S D Worley, S H Gerson, N Bodor, J J Kammskl and T W Flechtner, J Chem Phys, 68 (1978) 1313 S H Gerson, S D Worley, N Bodor and J J Kammskl, J Med Chem , 21 (1978) 686 S H Gerson, S D Worley, N Bodor, J J Kamrnskl and T W Flechtner, J Electron Spectrosc Relat Phenom , 13 (1978) 421 For example, see P R Ollvato, H Vlertler, B Wladlslaw, K C Cole and C Sandorfy, Can J Chem ,54 (1976) 3026 S D Worley, J Electron Spectrosc Relat Phenom , 6 (1975) 157 E Hedaya, R D Miller, D W McNeil, M E Kent, P F D’Angelo and P Schlssel, J Am Chem Sac ,91 (1969) 1875 W E HIl, C H Ward, T R Webb and S D Worley, Inorg Chem , 18 (1979) 2029 It has been pointed out (see D R Yarkony and H F Schaefer, Chem Phys L&t , 35 (1975) 291) that the Ionic state correspondmg to removal of an electron from the ~2 orbital of trlmethylenemethane 1s subject to multlplet sphttmg Our crude analy61s cannotaccountfor this effect, but probably our estimated I2 would correspond to a ‘band center’ of the multlplet structure