Photosensitive MetalâOrganic Systems - American Chemical Society

3 Photochemical Reactions of Organometallic Molecules on Surfaces CO Substitution Chemistry of Surface-Confined Derivatives of (η -C H )Mn(CO)

Downloaded by MONASH UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0238.ch003

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Doris Kang, Eric W. Wollman, and Mark S. Wrighton* Department of Chemistry,MassachusettsInstituteofTechnology, Cambridge, MA 02139

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Derivatives of (η -C H )Mn(CO) attached to SiO , Si or Au surfaces undergo photoreactions that allow the surface to be tailored in a rational manner. Photosubstitution of functionalized phosphines for CO occurs on all substrates, although the scope of the reactionismore limited for the surface-confined species than for the analogous complexes in solution. Flat surfaces modified with the derivatives of (η -C H )Mn(CO) can be patterned pho tochemically, because no thermal CO substitution occur. 5

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THE P H O T O C H E M I S T R Y O F M E T A L C A R B O N Y L S on surfaces is of potentially practical and fundamental importance. Possible applications include microelectronic device fabrication and photoimaging. Recent relevant studies include the formation of Fe thin films on GaAs (I) and Si (2) by U V photolysis of adsorbed Fe(CO) and the assembly of a reversi ble photoimaging system based on poly[(vbpy)Re(CO) ] (vbpy is 4methyl-4'-vinyl-2,2'-bipyridine) (5). These systems exploit photoinduced CO loss from Fe(CO) and metal-metal bond cleavage in the photoexcited Re dimer. 5

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*Corresponding author 0065-2393/93/0238-0045$06.50/0 © 1993 American Chemical Society

In Photosensitive Metal—Organic Systems; Kutal, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

46

PHOTOSENSITIVE M E T A L - O R G A N I C SYSTEMS


Fundamental studies of surface-confined metal carbonyls may lead to new photoreactions or the elucidation of reaction mechanisms. Photoprocesses of metal carbonyls on solid substrates have been investigated in many systems (1—23). Although the chemical and physical characteris tics of the surface may influence the reactivity of the adsorbed species, the primary photochemical events of surface-confined metal carbonyls are often identical to those of the analogous solution complexes. For a variety of mononuclear metal carbonyls in solution, C O loss occurs as the primary photoprocess to generate a coordinatively unsa turated intermediate that can be trapped by another 2e~ donor L (24—26):

M(CO)

rt

M(CO)_

A

x

MCCO^L

(1)

This substitution process has been observed for several surface-supported metal carbonyls and provides a means of functionalizing a surface with L . Importantly, many highly photosensitive metal carbonyls are quite ther mally inert (24). This chapter describes the photochemistry of derivatives of (η · C H ) M n ( C O ) covalently bound to high-surface-area S i 0 , single-crystal Si, and A u . Photosubstitution of functionalized phosphines for C O is observed on all modified surfaces. Photochemical patterning of flat sub strates can be achieved because the (rç -C H )Mn(CO) derivatives are inert toward thermal CO substitution. 5

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Experimental Section Procedures describing general spectroscopic and photochemical methods; han dling of reagents; preparation of manganese compounds; and modification of high-surface-area S i 0 , single-crystal Si, and A u thin films were published in detail elsewhere (27). 2

11-Diphenylphosphinoundecylferrocene. Triethylsilane (1.8 mL, 11 mmol) was added to a solution of 11-bromoundecanoylferrocene (28) (2.2 g, 5.0 mmol) dissolved in 4 mL of trifluoroacetic acid (Aldrich) under Ar. The mixture was stirred for 48 h and then diluted with water. The organic product was extracted with E t 0 , washed with aqueous N a H C 0 , and dried over M g S 0 . The residue obtained upon removal of solvent was chromatographed on silica gel with hexane to give pure 11-bromoundecylferrocene in 62% yield. H N M R (250 MHz, CDC1 ): δ 4.09 (s, 5H), 4.05 (m, 4H), 3.39 (t, 2H), 2.30 (t, 2H), 1.85 (m, 2H), and 1.17-1.52 (m, 16H) ppm. (NMR results are reported as chemical shifts (δ) in parts per million downfield from tetramethylsilane. Abbreviations used are s, singlet; m, multiplet; and t, triplet.) 2

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K A N G ET AL.

3.

Reactions of Organometallic Molecules on Surfaces

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L i P P h was generated by addition of one equivalent of n-BuLi to a solution of P P h H (Aldrich, 0.4 g, 2.4 mmol) in 17 mL of dry tetrahydrofuran (THF) under A r at - 7 8 °C. 11-Bromoundecylferrocene (1.0 g, 2.4 mmol) dissolved in 4 mL of dry T H F was added to the solution, which was then allowed to warm to room temperature. After 1.5 h of additional stirring, 10 mL of aqueous saturated NH C1 was added to the reaction mixture. The organic layer was collected and dried over M g S 0 . The crude material obtained upon solvent evaporation was chromatographed on silica gel with 9:1 hexane-CH Cl to give pure 11-diphenylphosphinoundecylferrocene as a red-orange oil in 60% yield. *H N M R (300 MHz, CDC1 ): δ 7.29-7.48 (m, 10H), 4.09 (s, 5H), 4.05 (m, 4H), 2.32 (t, 2H), 2.05 (t, 2H), and 1.17-1.52 (m, 18H) ppm. 2

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Preparation of Modified Au Electrodes. A u electrodes were made from 2000 Â of A u (99.999%) evaporated onto 100-mm Si wafers with a 100-Â adhesion layer of Cr. The Au-coated wafers were cut into pieces approximately 0.5 χ 1.0 cm. The pieces were rinsed with hexane and then fiinctionalized by immersing in a 1 m M solution of HS(CH ) (r? -C H )Mn(CO) in hexane over night. The modified A u was rinsed with hexane upon removal from solution. 5

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Electrochemical Methods. Electrochemical measurements were carried out with a Pine Instruments model RDE-4 bipotentiostat. Voltammetric traces were recorded with a Kipp and Zonen model B D 91 XY recorder. Linear sweep cyclic voltammetry was performed in C H C N - 0 . 1 M [n-Bu N]PF at 298 Κ under Ar. Pt gauze was the counterelectrode, and oxidized A g wire was the quasireference. 3

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Studies of Derivatives of (η -C H )Mn(CO) in Solution 5

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( C H ) S i ( f - C H ) M n ( C O ) and H S ( C H ) ( f - C H ) M n ( C O ) exhibit electronic absorption spectra similar to those reported for (η C H ) M n ( C O ) and ( r - C H C H ) M n ( C O ) (Table I). The 330-nm bands are assigned to M n —> (rç -C H R) charge-transfer (CT) transitions, which obscure ligand-field (LF) transitions also present in the same energy region (24). The 330-nm absorption is reported to have some M n - * COTT* CT character as well (24). The complexes (r? -C H R)Mn(CO) (where R = ( C H ) S i - or - ( C H ) S H ) undergo efficient photoinduced CO substitution at 298 Κ upon near-UV irradiation in the presence of excess free phosphine L in alkane solution under Ar. Figures 1 A , 2, and 3A show IR difference spec tra recorded during irradiations of these molecules in solutions containing L = PPh (w-octyl), P P h ( C H ) F c , or PPh Fc (Fc is ferrocenyl). Upon irradiation of the tricarbonyl complexes, initially only 3

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American Chemical Sflciâty Library 1155 16thSystems; SLUW. In Photosensitive Metal—Organic Kutal, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1993. Washington» 0 £ 20Q36

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i?

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n

HS(CH ) (»7 -C H )Mn(CO)L '

2

HS(CH ) (»? -C H )Mn(CO)L

n

HS(CH2) (»/ -C H )Mn(CO) L

2

HS(CH ) ( -C H )Mn(CO)

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(CH ) Si(r/ -C H )Mn(CO)L2

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(CH ) Si(»7 -C H )Mn(CO) L' '/

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(CH ) Si(»; -C H )Mn(CO) L'

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(CH ) Si(r7 -C H )Mn(CO) L

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(CH ) Si(»; -C H )Mn(CO)3

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(rç -C H CH )Mn(CO)

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(r7 -C H )Mn(CO)

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Compound

UV-Visible

b

1830

1827

1931 (1.3), 1871 (1.0)

2022 (1.0), 1942 (1.4)

1837

1932, 1873 330 (1040), 228 (-)

355 (1010), 292(1960)

1938 (7200), 1877 (7200) 1934, 1873

330 (940)

2029 (7100), 1947 (10,500)

330 (-)

330 (1100), 216 (12,000)

Table I. Spectroscopic Data for Relevant Compounds



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1026 (1.0), 1946 (1.5)

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N O T E : All data were recorded at 298 K . A l l data for solution species were recorded in alkane solution. For Si0 -supported species of high surface area, IR spectra were recorded as Nujol mulls. Characteristic frequencies for Si wafer and Au-supported species were obtained from transmission IR spectra. Molar absorptivity (e) or relative optical density (OD). Band positions are given in reciprocal centimeters and extinction coefficients, in parentheses, are in reciprocal (centimeters Molar). Molar absorptivity. Band positions are given in nanometers and extinction coefficients, in parentheses, are in reciprocal (centimeters Molar). Reference 24. L is PPh (n-octyl). L ' is PPh Fc. fL" is P P h ^ C H ^ j F c .

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[Au]-S(CH ) (r? -C H )Mn(CO) L"

n

1920 (1.0), 1850 (1.0)

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2015 (1.0), 1925 (1.3) 1922 (1.0), 1852 (1.0)

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1939 (1.0), 1867 (2.7)

2025 (1.0), 1939 (1.2)

1922, 1870

1931 (1.1), 1861 (1.0)

[Au]-S(CH2) (»? -C H )Mn(CO) L

n

[Au]-S(CH2) (»7 -C H )Mn(CO)

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[Si]-OSi(»7 -C H )Mn(CO) L

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[Si]-OSi(»? -C H )Mn(CO)

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[Si0 ]-OSi(CH ) -(> -C H )Mn(CO) L'

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[SiOJ-OSiiCH^^^-CjH^MniCO)^

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2023 (1.0), 1938 (1.0)

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[SiOJ-OSi(CH ) -(»7 -C H )Mn(CO)

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2036 (1.0), 1960 (1.2)

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Cl Si(»? -C H )Mn(CO)3

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CH OSi(CH ) -(r7 -C H )Mn(CO)3


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PHOTOSENSITIVE M E T A L - O R G A N I C SYSTEMS -ι

A

j

Mn(C0) 2029,1947 cm-'

L.-CO

3

OJO

-

1

j

1

,

r

R Mn(C0) L 1938, 1877 cm"

L.-CO

2

Mn(C0ÏL 1837cm"'

1

2

L = PP^U-octyl); R — S K C H ^

0.05

0.00


•0.05

- 0.10

φ υ

g - 0.15 < Β 0.05 Mn(C0) L '

Mn(C0)L

2

2

L • PPh U-octyl Î 2

0.00

-0.05

-0.10

-0.15

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2100

2000 Wovenumber

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1900 (cm"')

Figure 1. Part A: IR difference spectra accompanying photoreactions of (CH )^Si(rp-C H )Mn(CO) at 2029 and 1947 cm' , with L = PPh (n-octyl) in n-hexane at 25 °C to give (CH )^Si(n -C H^)Mn(CO)^ (absorptions at 1938 and 1877 cm- ) and (CH ) Si(rf-C H )Mn(CO)L (absorption at 1837 cm' ). Part B: IR difference spectra accompanying photoreaction of (CH ) Sifa -C H )Mn(CO) at 1938 and 1877 cm' , with L = PPh (n-octyl) in n-hexane at 25 °C to give (CH )£i(n -C H )Mn(CO)L (absorption at 1837 cm- ). 3

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3. K A N G ET A L

Reactions of Organometallic Molecules on Surfaces

Mn(CO) 2022, 1942 cm"

L r C O

L,

0.03

2-

C

Mn(C0) L " ° 1932, 1873 c m -

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Mn

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