Reactions of Atomic and Diatomic Iron with Methylacetylene in Solid

10720

J. Phys. Chem. 1994,98, 10720-10727

Reactions of Atomic and Diatomic Iron with Methylacetylene in Solid Argon David W. Ball Department of Chemistry, Cleveland State University, Cleveland, Ohio 441 I5

Richard G. S. Pong and Zakya H. Kafafi' Naval Research Laboratory, Washington, DC 20375 Received: May 30, 1994; In Final Form: August 17, 1994@

The chemistry of atomic and diatomic iron with methylacetylene (MA) in argon matrices has been studied using Fourier transform infrared spectroscopy. Cocondensation of iron vapors with MA results in the formation of atomic iron and diatomic iron complexes with MA. Vibrational spectra indicate that the atomic Fe(MA) complex is CJ bonding with one or more hydrogen atoms, whereas Fe2(MA) exhibits a spectrum that is characteristic of a n complex. Photolysis of the diiron-methylacetylene complex with low-energy visible light (3, > 670 nm) causes its isomerization into Fe2(H2C=CeCH2). Upon exposure of the matrix to visible light (A > 500 nm), photoisomerization of methylacetylene into allene with the subsequent C-H bond activation of allene is observed. The photoproduct has been characterized as the metal atom insertion compound, propadienyliron hydride (HFeHC=C=CH2). Excitation of the Fe(MA) (T complex with shorter wavelength visible light (A > 400 nm) leads to the activation of the acetylenic C-H bond via metal atom insertion and formation of 1-propynyliron hydride. Photolysis with ultraviolet light (280 nm < 3, < 360 nm) causes activation of one of the methyl C-H bonds as well as the carbon-carbon single bond of MA. The photoproducts have been identified as 3-propynyliron hydride and ethynylmethyliron, respectively. Experiments using various deuterated forms of methylacetylene (CH~CECD, CD&ECH, CD3CECD) support the characterization of these products and their vibrational mode assignments.

Introduction

The chemistry between matrix-isolated metal atoms and unsaturated hydrocarbons is frequently govemed by the presence of n electrons from carbon-carbon multiple bonds.'-l3 The classical Dewar-Chatt-Duncanson m 0 d e 1 , ~ ~initially J~ developed to describe n bonding in Zeise's salt, has been used to represent the interaction between these zerovalent metal atoms and C=C/C=C bonds which are modulated by n electrons. In this model, the n electrons donate into the empty d orbitals of the metal atom, while the electrons from the metal center "backbond" through donation into the n antibonding orbitals of the multiple bond. This model was utilized to describe the bonding in the first matrix-isolated complex of a neutral metal atom and an alkene, namely Al(C2%4), which was reported by Kasai and McLeod in 1975.' The first studies on the reactions between zerovalent metal atoms and carbon-carbon triple bonds yielded different types of chemistry. Kasai et al. performed ESR studies on the A1 C2H2 matrix reaction and reported3x4 not a n complex but a vinylaluminum radical, HzC=CH-Al. The gold-acetylene5 and silver-acetylene6 complexes have since been isolated and characterized by ESR spectroscopy. Recent ESR work13 on the reaction of alkali metal atoms with acetylene showed no interactions between CZHZand ground-state metal atoms. Sideon n complexes between C2H2 and excited-state (2P) metal atoms and vinylidene complexes M:C=CH2 were observed after exposure to light. Fourier transform infrared spectroscopy has been used to characterize the Ni(C2H2) n complex, which photorearranges to form a vinylidene complex Ni=C=CHz.* In a study of the reaction between atomic Fe and acetylene, a hydrogen-bonded or CJ complex was detected and was demon-

+

* To whom correspondence should be addressed. @

Abstract published in Advance ACS Abstracts, September 15, 1994.

strated to be the necessary precursor to C-H bond activation of the parent alkynesg Similar chemistry between atomic iron and ethylene has been reported earlier.1° Carbon-carbon bond activation by matrix-isolated metal atoms has been also achieved in cyclic alkanes with strained ring systems such as cyclopropane. A metallacycle was detected in the photochemistry between Fe16 and Nil7 atoms and cyclopropane. C-C and C - 0 bonds have been also activated in the reactions between photoexcited Fe'* and Nil9 atoms and ethylene oxide. These parent molecules are highly strained rings, however; the ring strain is relieved by metal atom insertion and the formation of the metallacycle. A particularly complex set of interactions between a metal atom and an organic molecule with carbon-carbon multiple bonds has been recently delineatedS2O Monatomic iron was found to spontaneously insert into one of the C-H bonds of allene, CHz=C=CHz, and form propadienyliron hydride. The photochemistry of the metal atom with allene was quite intriguing since it led to the chemistry of Fe with the more stable isomer, methylacetylene. Unlike monatomic iron, diiron chemistry with allene did not lead to any inert bond activation. Only n complexation with allene was observed. This previous study on the Fe/allene system,z0 which indicated rearrangements to make C3& isomers, suggests the necessity that similar experiments be performed using different C3& isomers as reactants. We therefore extend our study on Fe/C3H4 interactions by presenting here the results of the interactions, chemistry, and photochemistry between atomic and diatomic iron and methylacetylene in cryogenic inert gas matrices. Experimental Section The description of the matrix isolation apparatus used in this study was previously reported.20 A gold-plated copper block is mounted on a cold finger of an Air Products Displex Model CSW-202 closed-cycle helium refrigerator. The block has six

This article not subject to U S . Copyright. Published 1994 by the American Chemical Society

Reactions of Iron with Methylacetylene in Ar

J. Phys. Chem., Vol. 98, No. 42, 1994 10721

TABLE 1: Infrared Frequencies (cm-l) of the Fundamental Vibrational Modes of the Isotopomers of Methylacetylene in Solid Argon vib mode sym species CH3CICH CH3CICD CH3C'CH CDsCECD 2607.5 3323.2.3322.0 2607.7 V I , CH ~ t r 3322.8 2115.8; 2069.6 2116.0,2070.1 2935.1,2867.7 15," CH3 s str 2934.9, 2867.0 v3, c=c str

v4, CH3 s def v5. C-C str Vg, CH3 d str

v7, CH3 d def vg, CH3 rock v9, CH bend

2136.7 1382.9 928.3 2977.9 1444.3 1034.5 628.9, 630.3

2136.3 1141.2 834.0 2231.1 1043.1 838.5 629.3, 630.9

2054.4 1381.4 926.2 2977.1 1444.0, 1445.8 1031.9 493.5,494.4

2002.9 1129.0 789.5 223 1.O 1043.1 828.2 494.2

v2 is in Fenni resonance with 2v7.

polished surfaces available for deposition and a piezoelectric quartz crystal microbalance used to determine molar ratios of reactants and matrix gas. The matrix isolation apparatus is interfaced to a Nicolet 740 Fourier transform infrared spectrometer. Ultrapure iron (Johnson Matthey, 99.9985%), methylacetylene (Pfaltz-Bauer, 97.9%), methylacetylene-dl (MSD, 98%), methylacetylene-d3 (MSD, 99.3%), methylacetylene-& (MSD, 99.4%), and argon (Matheson, 99.9995%) were used without further purification. Iron was placed in an alumina crucible, heated slowly to lo00 "C in vacuo for several hours, and then vaporized between 1300 and 1450 "C. The rates of deposition of iron, methylacetylene, and argon were measured using the quartz crystal microbalance. Deposition of the matrix typically lasted 20-30 min. After deposition, the entire cold finger assembly was rotated 90°, and a single-beam FTIR spectrum was measured. This spectrum was ratioed to a single-beam spectrum of a clean gold-plated copper surface in order to generate an absorbance spectrum. All spectra were measured at a resolution of 0.5 cm-l. Matrices were normally photolyzed after deposition using an Oriel 200-W high-pressure Xe/Hg arc lamp in conjunction with various Coming long-pass and band filters. Results

The infrared spectra of four isotopomers of methylacetylene were measured in cryogenic argon matrices: CH~GCH(abbreviated MA), CH3CECD (DMA), CD3CECH (D3MA), and CD3CECD (D4MA). Methylacetylene is the most stable C3H4 isomer, 1.2 kT more stable than allene and 95 kJ more stable than cyclopropene.22 The infrared spectra of the isotopomers of methylacetylene were therefore not contaminated with isomeric impurities, and assignment of their absorptions was straightforward. Since methylacetylene has C3" symmetry, all 15 of its normal vibrations are infrared-active and are composed of five A1 and five E vibrations. With the exception of v10 (CCC bending mode, symmetry species E) whose IR frequency is beyond the range of the spectrometer, all vibrational modes were identifiable for the four isotopomers. The only ambiguity was the frequency assignment to v5 (C-C stretch) in CD3C+D. This normal vibrational mode is relatively weak in intensity and for CD3C'CD has a frequency very close to that for vg (CD3 rock).21 Several overtones and combination bands were also observed, including 2v7 which is in Fermi resonance with v2 for all four isotopomers. Table 1 lists the infrared frequencies of the fundamental vibrational modes of all isotopomers. The matrix-isolated measured frequencies are fairly close to the values measured in the gas phase.21 In addition, the vibrational mode assignments are consistent with those previously made in the gas phase. Cocondensation Products. When Fe vapors and MA were cocondensed at low concentrations relative to Ar,new absorptions appeared in the infrared spectrum. Figure 1 illustrates

C=C str.

C.H bend

18

16

14

12

10

I

c51

2 08

I

"a

O€

c

04

0;

O( I

3310

800

750

Figure 1. Metal concentration study. FTIR spectra of irodmethylacetylene in solid argon. Molar ratios of Fe:CH3C=CH:Ar are (A) 0.37, (B) 1.2, (C) 2.0, (D) 4.4, and (E) 8.4:5.0:1000.

the changes in the infrared spectra as the iron concentration varies. The new absorption at 3315.5 cm-l in Figure 1 appears in the spectra at low concentrations of iron; two other absorptions, appearing at 1379.2 and 2031.4 cm-', are not shown. These absorptions appeared at low iron concentrations and grew with increasing iron content in the samples. The species giving rise to these absorptions must be associated with an atomic iron and one methylacetylene. At higher iron concentrations, a second set of new absorptions appeared, examples of which are also shown in Figure 1. Figure 1C-E shows the growth of absorptions at 776.7 and 1726.5 cm-' with increasing iron concentration. The strongest of this set of absorptions appeared at 776.7 cm-'. This set of new absorptions maintained the same

Ball et al.

10722 J. Phys. Chem., Vol. 98, No. 42, 1994 .36 7

I

I

.28

.20

.04

-.04

-.12

1

I

925

875

I

1

825

775

I 725

I 650

I

I

1

600

550

500

1 450

Y (cm-1)

Figure 2. Diiron-activatedphotoisomerization of methylacetylene into allene. FTIR difference spectra of Fez(CHz=C=CHz), Fez(CDZ=C=CDz), Fez(CH3C=CH), and Fez(CD3CWD). Molar ratio of Fe:CH2=C=CHz:Ar = 4.8:5.0:1000: (A) no photolysis; (B) photolysis with 1 > 500 nm. Molar ratio of Fe:CD2=C=CDz:Ar = 5.2:5.0:1000: (A') no photolysis; (B') photolysis with 1 > 500 nm. Molar ratio of Fe:CH3C=CH:Ar = 4.9:5.0:1000: (C) no photolysis; (D) photolysis with ,I> 500 nm. Molar ratio of Fe:CD&=CD:Ar = 5.3:4.8:1000: (C') no photolysis; (D') photolysis with 1 > 500 nm.

relative peak intensity as the iron concentration varied, indicating that they are to be considered separately from the first set, lowconcentration absorptions. They are assigned to a diatomic iron/ methylacetylene product. Photolysis Products. Subsequent to deposition, selected samples were photolyzed with visible and W light, specifically, with light having I > 670 nm, I > 500 nm, I > 400 nm, and 280 nm < I < 360 nm. Upon photolysis with light having I > 670 nm, the set of absorptions belonging to diatomic iron/ methylacetylene reduced slightly. Upon higher-energy photolysis, the same set of absorptions decreased further while simultaneouslynew absorptions appeared; however, their major distinguishing feature is their initial appearance upon photolysis with 670 nm cutoff light. Difference spectra obtained by comparing spectra before and after photolysis with light having I > 500 nm are given in Figure 2; new peaks show up as positive features, and disappearing absorptions show up as negative features. The left bottom difference spectrum in Figure 2 shows absorptions appearing at 760.5,812.6, and 900.3 cm-'; additional absorptions, not shown, appeared at 557.6, 692.0, 1338.9, and 1642.6 cm-'. The species belonging to these absorptions is assigned as the photoproduct of the diirod methylacetylene product. The right bottom spectrum shows changes in FeD4MA samples upon 670 nm cutoff photolysis; new absorptions appeared at 543.9,598.7, and 627.9 cm-'. For comparison, difference spectra of similar regions from the previously reported Fe/allene matrix isolation study20 are also shown in the top part of Figure 2. There is an exact agreement in frequency and relative intensity between the absorptions of this new photoproduct and the absorptions measured for diironallene x complex in the Fe/allene study.*O Photolysis with light having A > 500 nm caused the absorptions due to Fe(CH3C=CH) to increase. One of these absorptions appearing at 3315.5 cm-' is displayed at the bottom of Figure 3. Concurrent with this increase, several new absorptions appeared in the spectra, one of them is shown in Figure 3B-A. This absorption at 1737.8 cm-l has been

previously identified as belonging to propadienyliron hydride, the atomic iron C-H insertion product in the Fe/allene study.20 It is one of two absorptions belonging to this product; the other is a broad, medium-strength absorption occumng at 1832.0 cm-'. Upon photolysis with light having I > 400 nm, the absorptions due to Fe(CH3CECH) began to bleach, as shown in the middle spectrum in Figure 3. Concurrent with this decrease, previously unidentified absorptions appeared in the spectrum. The absorption at 1721.9 cm-', shown in Figure 3, appears concurrently with a multiplet of absorptions at 21 17.1, 21 14.7, and 2110.5 cm-'. They keep the same relative intensity as other absorptions (not shown in Figure 3) at 911.0, 984.9, 1373.8, and 2881.8 cm-'. Upon photolysis with light having 280 nm < I < 360 nm, two new sets of absorptions were observed. One set has absorptions at 1697.5 and 3314.4 cm-'. Photolysis with visible light (A > 400 nm) caused a slight bleaching in this photoproduct. The other set of absorptions was not sensitive to any visible photolysis and maintained their absolute intensities. Some of the spectral features of this photoproduct are shown in Figure 3; these are absorptions at 1965.4, 1966.7, 1975.4, and 1977.4 cm-'. Figure 4 shows difference IR spectra that reflect the photochemistry of matrix-isolated iron vapors cocondensed with DMA (CH3C=CD) in solid argon. When the cocondensed sample was photolyzed with light having I > 500 nm, the absorptions due to the iron(methylacety1ene) adduct increased; these absorptions appeared at 491.1, 876.0, 982.8, 1378.3, 2031.1, and 2601.8 cm-'. The increase in the peak at 2601.8 cm-I due to the C-D stretching mode is seen in Figure 4, as is a new absorption appearing at 1737.7 cm-'. This band has the same frequency as the band assigned to the Fe-H stretching mode of propadienyliron hydride.20 Hence, it is assigned to the analogous C-H insertion product for the monodeuterated allene. The middle spectrum in Figure 4 shows changes in the infrared spectrum of Fe/DMA matrices upon 1 > 400 nm photolysis. As was noted in the iron/MA photochemistry, absorptions due to the atomic ironlDMA adduct decreased with

Reactions of Iron with Methylacetylene in Ar

J. Phys. Chem., Vol. 98, No. 42, 1994 10723

C-H Str

.14

Fe-H Str

CEC Str

U ,'\

H3C-Fe-CICH

Fe\

.12

D-C .10

.08

a3

2

5 .06

a

9 .04

.02

.oo

II

H

Fe(H3C-CSCH)

- -

7 1

3312

2x

4x

2125

2100

1 1750

1960

1985

1

1

1

1725

1700

1675

v (cm-1)

Figure 3. C-H and C-C bond activation of CH,C=CH. FTIR difference spectra of Fe(CH,C=CH), HFeCH=C==CH2, HFeCWCH3, HFeH2CCICH, and HC=CFeCH3 in solid argon. Molar ratio of Fe:CH,CsCHAr = 0.76:5:1000: (A) no photolysis; photolysis with (B) 1 =- 500 nm, (C) 1 > 400 nm, and (D) 280 nm < 1 < 360 nm.

CZC str.

C-D str

D-Fe str.

H-Fe str.

.12-

.02

.oo

-+

-

2806

2596

2125

2100

1750

1725

1700

1

1675

1

1260

B-A

1

1235

1

1210

v (cm-1)

Figure 4. C-D, C-H, and C-C bond activation of CH,C=CD. FITR difference spectra of Fe(CH3CECD), HFeCH=C=CHD, DFeCH=C==CH2, DFeCWCH,, HFeCECCHZD, HFeH2CCWD, and DClCFeCH, in solid argon. Molar ratio of Fe:CH,C=CD:Ar = 1 . 0 5 . 0 1OOO. Photolysis time is 30 min (except for (D), 15 min). (A) No photolysis; photolysis with (B) 1 > 500 nm, (C)1 > 400 nm, and (D) 280 nm < 1 < 360 nm.

the concurrent appearance of new absorptions, most notably a multiplet at 1239.0, 1236.5, and 1234.9 cm-', a doublet at 21 14.8 and 21 11.1 cm-', and a peak at 91 1.9 cm-' (not shown in Figure 4). The multiplet in the 1200 cm-' region is similar

to that observed for the ironlMA photoproduct in the 1700 cm-' region and so represents a shift in a vibrational mode due to the isotopic substitution with deuterium. The top spectrum in Figure 4 shows the photochemistry due to UV photolysis; again,

10724 J. Phys. Chem., Vol. 98, No. 42, 1994

Ball et al.

TABLE 2: Infrared Frequencies (cm-') of the Isotopomers of the Iron-Methylacetylene u Complexes -in Solid Argon

TABLE 3: Infrared Frequencies (cm-') of the Isotopomers of Fez-Methylacetylene II Complexes in Argon Matrices

FeFeFeFe(CHICICH) . , (CHCECD) , , (CDICICH) . , ICDIC=CD) . 491.1 490.3 982.8 978.8

Fez(CH3CeCH) Fez(CHsC=CD) Fe2(CD3CWH) Fez(CD3CZCD)

approx vib mode

C-H bend C-H bend (1st overtone) CH3 s def CH3 d def CEC str C-H str

1379.2

1378.3

203 1.4 3315.5

2031.1 2601.8

1038.0 2059.4 3315.8

1037.0 2060.2 2601.7

two photoproducts were formed and were distinguished on the basis of photoreversibility. The photoreversible product has one absorption that appears at 1697.5 cm-', which is accompanied by a very weak feature at 2600.5 cm-'. The first absorption appears at the same position as that for Fe/MA; the second absorption represents a substantial shift in vibrational frequency upon deuteration. In addition, an absorption appears at 1250.0 cm-' at exactly the same frequency observed for the Fe-D stretch in perdeuteriopropadienyliron hydride.20 Hence, it is assigned to the C-D insertion product in the monodeuterioallene, signaling hydrogeddeuterium scrambling upon C-WD activation. Similarly, a new feature also appears at 1215.1 cm-', in the expected region of an Fe-D stretch. It exhibits a photoreversible behavior, disappearing with visible photolysis (A > 400 nm) and reappearing with UV photolysis; this effect suggests that hydrogen scrambling in this photoreversible photoproduct has also occurred. Discussion Identification of Products. Iron(methylacety1ene) CJ Complex. The infrared absorptions of the atomic irodmethylacetylene product first appeared at low matrix concentrations of Fe and methylacetylene. Given that they appear at even the lowest Fe concentration measured, this product is most likely a complex between a single iron atom and a single methylacetylene molecule: Fe(CH&=CH). A complete list of vibrations identified for the four isotopomers is given in Table 2. Several of the absorptions of the complex are recognizable as perturbations of the vibrations of uncomplexed methylacetylene. For example, a new absorption for all four isotopomers appears in the characteristic energy region for an acetylenic C-H stretch: 3315.5 and 3315.8 cm-' for Fe(CH3C-H) and Fe(CD3CWH), which both have acetylenic hydrogens, and 2601.8 and 2601.7 cm-' for Fe(CH3CECD) and Fe(CD3CWD), which have acetylenic deuteriums. A similar assignment can be made for Fe(CH3C'CD) and Fe(CD3C=CD), for which absorptions at 491.1 and 490.3 cm-' are due to C-D bends, but equivalent absorptions are not found for acetylenic C-H bends and may be hidden under the parent peaks. The absorption at 1379.2 cm-' is a perturbation of the CH3 deformation mode of CH3CGCH; an absorption for Fe(CH3CxCD) is present at 1378.3 cm-', while for Fe(CD3CGCH) and Fe(CD3CECD) a product absorption appears at 1038.0 and 1037.0 cm-', respectively, in the region of a CD3 deformation mode. The perturbation on the CH and CH3 vibrations suggests that the iron atom is interacting with the methylacetylene via hydrogen bonding; that is, Fe(CH3CsCH) is a u complex. During 670 nm cutoff photolysis, all of the absorptions of Fe(CH3CECH) increased markedly, indicating that Fe(CH3CSCH) and its isotopomers were formed upon low-energy photolysis in addition to being formed spontaneously upon cocondensation. Photoaggregation of metal atoms into clusters is a known matrix p h e n o m e n ~ nhere ; ~ ~is~ an ~ ~example of the photolytic formation of an organometallic complex in a matrix.

511.8 681.4 776.7" 851.1 854.0 1033.3 1358.1 1432.7 1726.5 2845.0 2911.5 2915.1 2959.1

438.7 530.3 543.1 546.8 661.7" 899.8 1031.0 1357.8 1631.9 1677.0 1679.4 1682.1 1693.3 2844.1

456.6 507.2 677.3 743.0 746.0 750.1 755.2" 788.5 803.2 841.7 875.4 884.2 886.2 921.8 931.8 933.8 1083.4 1463.7 1677.4 1721.8 1729.3 1742.9 1824.9 2214.5 2218.7

426.9 541.8 642.2" 710.7 797.2 877.2 906.7 915.8 974.2 1078.4 1669.4 1686.0 1701.4 2099.4 2218.3

Indicates most intense absorption.

Diiron(methy1acetylene) n Complex. The infrared absorptions of the diirodmethylacetylene product first appeared at low matrix concentrations of methylacetylene but higher concentration of Fe. A most likely candidate product is Fez(CH3CZCH). A complete list of all of the absorptions measured for the four isotopomers of Fe2(CH$=CH) is given in Table 3. Specific assignments for the absorptions of Fez(CH3CSCH) are more difficult than those for the atomic iron adduct. All isotopomers of the diiron-methylacetylene complexes exhibit absorptions between 1670 and 1730 cm-', again relatively independent of deuterium content. These frequencies can be assigned to a strongly perturbed C- stretching modes since these red-shifted frequencies are in the region characteristic of C=C stretching modes. Strong interaction of Fez with the n electrons of the C W bond will cause its weakening and acquiring a doublebond character. This spectral feature is thus characteristic of a strong x complex. All four isotopomers exhibited absorptions in various fingerprint regions of the spectrum, Le., C-H stretching regions, C-H bending regions, etc. However, the most intense absorption for all four Fe dimer complexes follows a discernible pattern. For Fe2(CH3C=CH) and Fe2(CD3C=CH), the most intense absorption appears at 776.7 and 755.2 cm-', respectively; for Fe2(CH3CsCD) and Fe2(CD3C=CD), the most intense absorption is found at 661.7 and 642.2 cm-', respectively. This suggests that this mode is the blue-shifted acetylenic C-H bend, a perturbed 1.9of the parent methylacetylenes. This shift represents a potential 20-34% increase in the energy of this vibration, which would be substantial. This vibrational mode may be best described in terms of an internal coordinate for a triatomic F e e - H or Fe-H-C moiety. This description is consistent with a n bonding between diiron and methylacetylene. Diiron(al1ene) n Complex. When matrices were photolyzed with 670 nm cutoff light, the infrared absorptions of Fez(CH3CECH) decreased slightly; this decrease continued upon photolysis with 500 nm cutoff light with the concurrent increase of new absorptions that are attributed to a rearrangement product of the Fez(CH3CWH) complex, identified as Fe2(CHz=C=CHz).

Reactions of Iron with Methylacetylene in Ar

J. Phys. Chem., Vol. 98, No. 42, 1994 10725

TABLE 4: Infrared Frequencies (cm-') of the Isotopomers of Fez(a1lene) a Complexes in Argon Matrices ~~

FezFez(CHz=C=CHz) (CHz%=CHD) 557.6 692.0 760.5 812.6 900.3 1338.9 1642.6

478.2 559.0 687.1 804.3 888.6 11339.7

Fez-

Fez-

(CDz=C=CHD) (CD2'C=CDz) 543.3 591.3 601.3 657.8 686.2 829.2 843.0 844.2 970.2 1049.3 1378.0 1393.0 1641.7 1679.7

474.6 543.9 598.7 627.9 742.5 824.8 969.8 1626.7 1656.3

Hence, this photoproduct is the iron dimer complexed to an allene molecule.2n Phototautomerism of methylacetylene has occurred. In the previous study, we had reported diiron-allene complexes for only two isotopomers of allene, fully protiated and fully deuterated. Further, two different Fez(al1ene) complexes were detected, only one of which was photostable. The infrared absorptions of Fez(CHZ=C=CHZ) measured in this study were all previously identified as the photostable complex. Thus, it appears that the photostable Fe2(CHz=CECH2) complex is the highly preferred rearrangement product of Fez(CH3CsCH). We have already proposedz0a V-shaped structure for this photostable diiron-allene complex, which is consistent with the structures of several known binuclear allene comp l e ~ e s . ~In~ the - ~ ~present study, we are able to report the frequencies of Fez(al1ene) complexes formed from partially deuterated methylacetylenes: Fe2(CHzEC=CHD) and Fez(CD2=C=CHD). The frequencies for all four isotopomers of diirodallene n complex are listed in Table 4. Propadienyliron Hydride. Upon photolysis of Fe/MA samples with 500 nm cutoff light, new absorptions appeared in the infrared spectra. The absorption at 1737.8 cm-' appears in the characteristic Fe-H stretching region, and for all isotopomers having hydrogen atoms, an absorption appeared at about this frequency. For FeD4MA samples, this absorption was absent, but a new one appeared at 1250.0 cm-l, in the expected region for an Fe-D stretch. For FeDMA samples, an absorption appeared at 1250.0 cm-', but for FeD3MA samples this region was obscured by the presence of a strong parent hydrocarbon absorption. These absorptions identify the presence of an Fe-H bond in this photoproduct. The frequencies of the Fe-H and Fe-D stretching modes coincide exactly with those measured for HFeCH=C=CH2 and DFeCD=C=CD2, respectively.20In addition, the frequencies assigned in the previous study for the C=C=C asymmetric stretch are identical to those obtained in the present study for the MA and D4MA reactions. Hence, these frequencies are assigned to propadienyliron hydride. Table 5 lists all the measured frequencies for the identified vibrational motions of the isotopomers of propadienyliron hydride. Note that the frequencies for the C=C=C asymmetric stretch did not shift much as the level of deuteration increased: from 1832.0 cm-' for the MA reaction, 1809.3 cm-' for the DMA reaction, and 1834.3 cm-' for the D3MA reaction to 1799.0 cm-' for the D4MA reaction. The value of 1834.3 cm-' for the D3MA product was unusual since a C=C=C vibration is not expected to increase in frequency as the substituent groups attached to the C=C=C moiety system increase in mass. We suggest that significant mode mixing is occurring for this partially deuterated molecule, which is shifting this vibration to a slightly higher frequency.

The reaction between an iron atom and methylacetylene that leads to the formation of propadienyliron hydride involves two specific processes. One step is the rearrangement of methylacetylene (CsC-C) into allene (C=C=C), and the other is the oxidative insertion of atomic iron into one of the C-H bonds. The choice of hydrogen must from a priori concerns be considered random, and the infrared spectra of the reaction products of the partially deuterated methylacetylenes support the randomness of the rearrangement. For instance, in DMA absorptions due to vibrational stretching modes of both Fe-H which may be due to HFeCD=C=CH2 or HFeCH=C=CHD and Fe-D from DFeCH=C=CHz were observed. The fact that two CH2 motions were identified in the partially deuterated methylacetylene reactions confirms this scrambling of H atoms. Random rearrangement would for D3MA produce CHD and CD2 terminal groups from the products, DFeCD=C=CHD, DFeCH=C=CD2 and HFeCD=C=CDZ listed in Table 5. 1 -Propynyliron Hydride. Upon 400 nm cutoff photolysis, the Fe(CH3CWH) complex was bleached and a new set of infrared absorptions appeared. Among these absorptions was a peak at 1721.9 cm-', characteristic of an Fe-H stretch. Absorptions appeared at this wavelength for both isotopomers of methylacetylene having an acetylenic hydrogen: MA and D3MA. For D4MA, no absorption appeared at this position but instead at 1234-1239 cm-', the expected position for a Fe-D stretch. These results indicate that the acetylenic C-H bond has been activated and Fe has inserted into it, forming C-Fe-(WD). In addition, absorption bands appeared in the spectral region expected for the CGC stretching modes and were detected for all isotopomers. Their frequencies ranged from 2109.5 to 21 17.1 cm-', varying little among all the isotopomers of methylacetylene. Other identified vibrational modes include a carbon-carbon stretch at 911.0 cm-' for the MA reaction, which shifted upon deuteration to doublets at 895. U898.0 and 895.1B97.0 cm-I for D3MA and D4MA, respectively. A CH3 deformation was noted at 1373.8 cm-' for MA, shifting to doublets at 1112.8/1111.9 and 1113.2A112.2 cm-' for D3MA and D4MA, respectively. The analogous absorption was not observed for DMA. For MA and D4MA, CH3 symmetric stretching vibrations were also identified at 2881.8 and 2036.9 cm-', respectively. The presence of an intact CH3 group with CEC, C-C, and Fe-H bonds strongly suggests that the insertion product is CH3C=CFeH, or 1-propynyliron hydride (the 1- indicating that the iron atom is bonded to the number one carbon in the alkyne chain). The measured vibrations of this species agree with those previously obtained from the iron/ allene study;2nonce again, additional isotopomers were useful in confirming the characterization of this insertion product. Similar to the formation of propadienyliron hydride, this photorearrangement showed scrambling of the hydrogens in the making of the final product. This is shown in Table 6, which lists the measured frequencies for all the isotopomers. Note that for the reaction with CH3CWD an Fe-H stretching frequency is measured at 1722.1 cm-' and assigned to the scrambled product CDHzCECFeH. Similarly, for the reaction with CD&SCH, activation of an acetylenic C-D was observed. 3-Propynyliron Hydride. Upon photolysis with UV light, new sets of absorptions appeared in the infrared spectra of FeMA matrices. One set showed photoreversible behavior; Le., they were bleached with light having ;1 > 400 nm and were formed with back UV photolysis. The absorptions at 1697.5 and 3314.4 cm-' are associated with this photoreversible product. The absorption at 1697.5 cm-' is present in the UV-photolyzed Fe/ DMA samples but is absent in equivalent FeD4MA samples; instead, an absorption appears at 1204.3 cm-' indicative of an

10726 J. Phys. Chem., Vol. 98, No. 42, 1994

Ball et al.

TABLE 5: Infrared Frequencies (cm-I) of the Isotopomers of Propadienyliron Hydride in Solid Argon

approx vib mode

HFeCH=C=CHZ

Fe-C str C(WD)Zwag Fe-(H/D) str C=C=C asym str

428.6,431.4 820.2 1737.8 1832.0

HFeCD=C=CH*, HFeCH%=CHD, DFeCH%=CH2

HFeCD=C=CD, DFeCH=C=CDZ, DFeCD=C=CHD

763.7,767.6 1737.7, 1250.0 1809.3

807.8.605.1 1736.2 1834.3

DFeCD=C=CDz

585.5 1250.0 1799.0

TABLE 6: Infrared Frequenices (cm-') of the Isotopomers of 1-PropynylironHydride in Solid Argon

approx vib mode

c-c

str

CH3 rock CH3 def Fe-(WD) str c=c str CH3 sym str

CH3CrCFeD, CDHZCWFeH

CH3CWFeH 911.0 984.9 1373.8 1721.9 2110.5,2114.7, 2117.1 288 1.8

CD~CECF~H, CHDZCECFeD

911.9

895.1, 898.0

1234.9, 1236.5, 1239.0; 1722.1 2111.1, 2114.8

CD~CECF~D 895.1, 897.0

1111.9, 1112.8 1222.9, 1234.9; 1716.8, 1718.2, 1721.9 2109.8, 2113.5

1112.2, 1113.2 1234.9, 1236.2, 1239.0 2109.5,2113.5 2036.9

TABLE 7: Infrared Frequencies (cm-') of the Isotopomers of 3-Propynyliron Hydride in Solid Argon

mode

HFeHZCCeCH

HFeHzCC'CD, HFeHDCW H , DFeH2CCICH

Fe-(WD) str C-H str

1697.5 3314.4

1215.1, 1222.6; 1697.5 2600.5

approx vib

DFeDZCCECH, DFeHDCWD, HFeDZCCeCD

DFeDZCC=CD

1204.1, 1215.2; 1690.1

1204.3

TABLE 8: Infrared Frequencies (cm-l) of the Isotopomers of Ethynylmethyliron in Solid Argon

approx vib mode CH3 rock Fe-C str C-H bend CH, sym def c=c str

DCWFeCH3, HC WFeCHzD

HCeCFeCH3 540.5,546.4, 550.6, 556.0 566.1, 564.8 669.8 1161.7, 1163.6 1965.4, 1966.7, 1975.4, 1977.4

HCWFeCD3, DC=CFeCDzH

549.5

DCWFeCD3 416.4,421.8

555.6, 559.0

520.5, 523.2

517.8, 519.2 520.4

665.0 1163.4 1850.4, 1852.4, 1856.1, 1857.3; 1965.5, 1966.8, 1970.2, 1971.1

Fe-D stretch. The absorption at 3314.4 cm-I is in the characteristic region of the acetylenic C-H stretch; a frequency for a C-D stretch is seen in the reaction of Fe with DMA. These results provide evidence for C-H bond activation of one of the methyl C-H bonds. The photoproduct is identified as HFeCH2CsCH, or 3-propynyliron hydride (the 3- indicating that the iron atom is bonded to the number three carbon in the alkyne chain; c j 1-propynyliron hydride above). The list of frequencies detected for the isotopomers of this species is given in Table 7. Note that, for the reaction with CD3C'CH, an Fe-H stretching frequency is measured at 1690.1 cm-l and assigned to the scrambled product HFeD2CCsCD. Similarly, for the reaction with CH3CsCD, activation of a methyl C-D was observed. Ethynylmethyliron. The spectrum of the second UV photoproduct, distinguished from that of the previous photoproduct by its photoirreversiblebehavior, did not exhibit any absorption in the Fe-H or a Fe-D stretching region. However, absorptions appearing at 566.1 and 564.8 cm-' for the MA reaction and at 555.6 and 559.0 cm-' for the DMA reaction, shifting to 520.5 and 523.2 cm-I for D3MA and 517.8, 519.2, and 520.4 cm-I for D4MA, have been assigned to an Fe-C stretching mode. A very characteristic set of features appeared at 1977.4, 1975.4, 1966.7, and 1965.4 cm-' for the MA reaction and shifted to 1864.3, 1861.4, 1851.5, and 1850.8 cm-l for D4MA. Similar frequencies have been observed for the C s C stretch in HFeCsCH and DFeC=CD, respectively.g Hence, this photoproduct must have a FeCECH moiety. The other group is CH3, and its presence is confirmed by the identification of a CH3 rock at 540-556 cm-', shifting slightly to 549.5 cm-' for DMA

1850.8, 1853.4, 1861.4, 1864.3; 1965.3, 1975.5, 1977.3

912.5 1850.8, 1851.5, 1861.4, 1864.3

and occurring at 416.4 and 421.8 cm-I for D4MA, and also the characterization of a CH3 symmetric bend at 1163.6/1161.7 cm-' for MA and at 1163.4 cm-l for DMA, shifting to 912.5 cm-' for D4MA. An acetylenic C-H bend is noted for the two methylacetyleneisotopomers having hydrogen atoms in the acetylenic positions: 669.8 cm-' for MA and 665.0 cm-1 for D3MA products. The combination of identifiable atomic groupings indicates that this photoproduct has an ethynyl group and a methyl group attached to an iron atom identified as ethynylmethyliron, of HCEC-Fe-CH3. The frequencies of the vibrations for ethynylmethyliron are listed in Table 8. Though not interfering with the identification of this product, the C S stretching region provides some additional information. In the cases of both partially deuterated methylacetylenes, absorptions for both -CGCH and - C W D groupings appeared, i.e., multiplets in the 1850-60 and 1960-70 cm-' ranges. In the cases of MA and D4MA, only -CGCH and -C=CD groups were seen in the VC-C region. This result suggests that hydrogen scrambling of methylacetylene has occurred during the formation of this photoproduct. This would suggest formation of HCGC-Fe-CHZD and DCEC-Fe-CHD2. Metal Atom Chemistry in Matrices vs Organometallic Chemistry. The present study shows a very rich chemistry between iron atoms or dimers and MA. In particular, the stability of allene over methylacetylenein the formation of Fez(CHz=C=CHz) and HFeCH=C=CH2 is surprising in light of the stability of MA over allene. There is precedent, however. There are several examples of metal complexes that have terminal alkyne ligands, and some of them are known to undergo

Reactions of Iron with Methylacetylene in Ar 1.

Fe

J. Phys. Chem., Vol. 98, No. 42, 1994 10727 C=CH2, CH3CWFeH, HFeCH2CSH, and HCsC-Fe-CH3. Characterization of the products was confirmed by using mono-, tri-, and perdeuterated methylacetylenes. It is further supported by our previous study on the chemistry and photochemistry of iron and allene in cryogenic matrices.20

+ H-CIC-CA' H +H

KIAr

Fe (CHs CZCH)

hv

Acknowledgment. This work was supported by the Office of Naval Research. D.W.B. expresses appreciation to Robert L. R. Towns for arranging travel assistance in the completion of this project. The assistance of Charles Memtt in the spectral plotting and analysis is greatly appreciated.

H,C-Fe-CIC-H

.H

2

Fer + H-C:C-CLH

References and Notes

I O K / A ~N

H

(1) Kasai, P. H.; McLeod, Jr., D. J. Am. Chem. SOC.1975, 97, 5609. (2) Kasai, P. H.: McLeod, Jr.. D. J . Am. Chem. SOC.1975, 97, 6602. (3) Kasai, P. H.; Watanabe, T.; McLeod, Jr., D. J . Am. Chem. SOC. 1977, 99, 3521. (4) Kasai, P. H. J . Am. Chem. SOC. 1982, 104, 1165. PO nm (5) Kasai, P. H. J . Am. Chem. SOC. 1983, 105, 6704. Fer (CH2=C=CH1) (6) Kasai, P. H. J. Phys. Chem. 1982, 86, 4092. Figure 5. Observed reaction pathways and photochemistry of atomic (7) Ozin, G. A.; McIntosh, D. F.; Power, W. J.; Messmer, R. P. Inorg. and diatomic iron with methylacetylene in solid argon. Chem. 1981, 20, 1782. (8) Kline, E. S.; Kafafi, Z. H.; Hauge, R. H.; Margrave, J. L. J . Am. Chem. SOC. 1987, 109, 2402 a photoassisted rearrangement from a n-bonded alkyne complex (9) Kline, E. S.; Kafafi, Z. H.; Hauge, R. H.; Margrave, J. L. J . Am. to a a-bonded vinylidene complex: Chem. SOC. 1985, 107, 7559. (10) Kafafi, Z . H.; Hauge, R. H.; Margrave, J. L. J. Am. Chem. SOC. 1985, 107, 7550. L,M(R-C'CH) -I- hv L,M=C=C(H)R (11) Jones, P. M.; Kasai, P. H. J . Phys. Chem. 1988, 92, 1060. (12) Manceron, L.; Andrews, L. J. Phys. Chem. 1989, 93, 2964. (13) Kasai, P. J . Am. Chem. SOC. 1992, 114, 3299. An intermediate, thought to be an acetylenohydride L,MH( (14) Dewar, M. J. S. Bull. SOC. Chim. Fr. 1951, 18, C71. C'C-R), was not detected when phenylacetylene was reacted (15) Chatt, J.; Duncanson, L. A. J . Chem. SOC. 1953, 2939. (16) Kafafi, Z. H.; Hauge, R. H.; Fredin, L.; Margrave, J. L. J . Chem. with ruthenium-phosphine complexes; a high yield of the SOC., Chem. Commun. 1983, 1230. vinylidene product was r e p ~ r t e d . ~Alonso ' et ~ 2 1 related .~~ a (17) Kline, E. S.; Hauge, R. H.; Kafafi, Z. H.; Margrave, J. L. similar rearrangement in a series of Ru(PRs)(acetylene) and RuOrganometallics 1988, 7, 1512. (PR3)(alkylacetylene) complexes. Recently, Gibson et ~ 2 2 . ~ ~ (18) Kafafi, Z. H.; Hauge, R. H.; Margrave, J. L. J . Am. Chem. SOC. 1987, 109, 4775. reacted CP*2TaH3 with allene to form Cp*2Ta(q3-C3Hs),which (19) Kline, E. S.; Hauge, R. H.; Kafafi, Z. H.; Margrave, J. L. High rearranged photolytically to Cp*2Ta=C=C(H)CH3. NMR Temp. Sei. 1990, 30, 69. studies of the rearrangement indicated that the intermediate was (20) Ball, D. W.; Pong, R. G. S.; Kafafi, Z. H. J . Am. Chem. SOC. 1993, 115, 2864. in fact the methylacetylene complex, Cp*2TaH(HCEC-CH3), (2 1) Shimanouchi, T. Tables of Molecular Vibrational Frequencies; with the methyl group of the methylacetylene preferentially NSRDS-NBS 39; National Bureau of Standards: Washington, DC, 1972; distant from the hydrido ligand. Such behavior of C& ligands Vol. 1 and references therein. is by no means assured, however. Pu, Peng, Arif, and G l a d y ~ z ~ ~ (22) Yoshimine, M.; Pacansky, J.; Honjue, N. J . Am. Chem. SOC.1989, 111, 4198. recently reported on the chemistry of CpRe(NO)(PPh3)(allene)+; (23) Klotzbucher, W. E.; Ozin, G. A. J . Am. Chem. SOC. 1978, 100, in the presence of strong base and at -80 "C, this complex 2262. rearranges to a methylacetylene complex at a yield of >90%. (24) Ozin, G. A.; Klotzbucher, W. E. Inorg. Chem. 1979, 18, 2101. (25) Cotton, F. A,; Wilkinson, G. Advanced Inorganic Chemistry, 4th ed.; John Wiley and Sons: New York, 1980. Summary (26) Bailey, W. I., Jr.; Chisolm, M. H.; Cotton, F. A,; Murillo, C. A,; Rankel, L. A. J . Am. Chem. SOC. 1978, 100, 802. (27) Lewis, L. N.; Huffman, J. C.; Caulton, K. G. J . Am. Chem. SOC. The chemistry and photochemistry of iron atoms and iron 1980, 102, 403. dimers with methylacetylene has been examined using matrix (28) Casey, C. P.; Austin, E. A. Organometallics 1986, 5, 584. isolation FTIR spectroscopy and is summarized in Figure 5. (29) Hoel, E. L.; Ansell, G. B.; Leta, S . Organometallics 1986,5, 585. Atomic iron and diatomic iron complexes have been identified. (30) Cayton, R. H.; Chisolm, M. H.; Hampden-Smith, M. J. J . Am. Chem. SOC. 1988, 110, 4438. Vibrational spectral indicate that the atomic Fe(MA) complex (31) Wolf, J.; Werner, H.; Serhadli, 0.;Ziegler, M. L. Angew. Chem., is a bonding with one or more hydrogen atoms, whereas FezInt. Ed. Engl. 1983, 22, 414. (MA) exhibits a spectrum that is characteristic of a n complex. (32) Alonso, F. J. G.; HBhn, A.; Wolf, J.; Otto, H.; Werner, H. Angew. Chem., Int. Ed. Engl. 1985, 24, 406. Photolysis of the diiron-methylacetylene complex with low(33) Gibson, V. C.; Parkin, G.; Bercaw, J. E. Organometallics 1991, energy visible light (A > 670 nm) causes its isomerization into 10, 220. Fez(HzC=C=CHz). The Fe(CH3C=CH) complex undergoes (34) Pu, J.; Peng, T. S.; Arif, A. M.; Gladysz, J. A. Organometallics a complex route of photolytic rearrangement, forming HFeCH= 1992, 11, 3232.

t

Fer (CHsCZCH)

-