Inorganic and Organometallic Photochemistry - American Chemical

5 Roles of Charge Transfer States in the Photochemistry of Ruthenium(II) Ammine Downloaded by UNIV OF MASSACHUSETTS AMHERST on March 2, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0168.ch005

Complexes P E T E R C. F O R D Department of Chemistry, University of California, Santa Barbara, C A 93106 Ru(II)

complexes

display a variety of electronic

states and the chemistry of these is very rich.

excited

Here, our

recent work with the Ru(II) ammine complexes

will be

summarized with particular emphasis placed on the solution photochemistry and spectroscopy of the pentaammine complexes Ru(NH ) L . 3

The electronic spectrum of Ru(NH )6

2+

5

3

2+

is shown to display both ligand field and charge transfer to solvent absorptions.

Irradiation

of the former

principally

leads to ammonia substitution while that of the latter mostly leads to oxidation of Ru(II) to Ru(III), with stoichiometric formation of H . 2

When L is an unsaturated ligand such as a

substituted pyridine, the visible spectrum is dominated by metal-to-ligand

charge transfer absorptions.

However,

the

presence of spectrally unseen ligand field states appear to dictate the major photoreaction pathway (ligand substitution) in many cases, although the MLCT

state is capable of

undergoing inner sphere electron transfer to another metal center when L contains a second coordination site (e.g., pyrazine). Interest

i n the photochemistry

of R u ( I I )

has been considerable

in

recent years (1,2,3,4) and has been spurred by the discoveries that excited

states of certain aromatic nitrogen, heterocycle R u ( I I )

plexes can undergo either energy transfer (5) w i t h the appropriate substrates.

or electron transfer

com(6)

I n our laboratories, attention has been

focused largely on understanding the solution spectroscopy and photochemistry of the pentaammine

complexes R u ( N H ) L

be either a saturated ligand such as N H

3

3

5

2 +

, where L

can

or H 0 or a ^-unsaturated 2

ligand such as an organonitrile or a substituted pyridine ( 7 - 1 8 ) . 0-8412-0398-9/78/33-168-073$05.00/0 © 1978 American Chemical Society In Inorganic and Organometallic Photochemistry; Wrighton, Mark S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

The

74

INORGANIC AND ORGANOMETALLIC PHOTOCHEMISTRY

spectra of these species are markedly dependent on the nature of the unique ligand (19), and the photochemistries that these complexes display i n fluid solution include pathways that can be assigned logically as characteristic to particular excited state types. Thus, models for the photochemical reactions of various R u ( N H ) L complexes can be described, and it is on the basis of one model that we demonstrated that judicious variation of ligand substituents or of the solvent medium can tune photochemical properties of particular systems (10). Applications of such models and the understanding of the consequences of molecular perturbations on excited state properties are essential i n the design of chemical systems for applications as the conversion of radiant energy to chemical potential energy.

Downloaded by UNIV OF MASSACHUSETTS AMHERST on March 2, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0168.ch005

3

5

2 +

Spectra and Types of Excited States Table I summarizes the aqueous solution absorption spectra of R u ( N H ) \ R u i N H ^ p y ^ , and R u ( N H ) ( C H C N ) , respectively. The spectrum of R u ( N H ) (Figure 1) is typical of that seen for other saturated ammine R u ( I I ) complexes, and the spectra of R u ( N H ) H 0 3

e

2

3

3

e

5

2 +

3

2 +

3

5

2

Table I. Comparison of Spectra of M(NH )5L* Complexes in Aqueous Solution 3

Complex Ru(II) Ru(NH ) * 3

2

e

Ru(NH ) py 3

2

5

Ru(NH ) (CH CN) * 3

5

2

3

Rh(III) Rh(NH ) * 3

Rh

3

e

(NH ) py * 3

3

5

Rh(NH ) (CH CN) * 3

5

3

3

Co (III) Co(NH ) * 3

e

3

Co(NH ) py* 3

5

3

Co(NH ) CH CN * 3

5

3

3

(nm)

(M-'cm- )

390 275 407 244 350 226

35 640 7,770 5,090 163 15,400

305 255 302 259 301 253 472 338 474 340 252 467 333

1

Assignment

Ref.

LF CTTS MLCT IL LF MLCT

18

134 101 170 2,930 158 126

LF LF LF IL LF LF

32

56 46 64 54 3,160 57 54

LF LF LF LF IL LF LF

32

In Inorganic and Organometallic Photochemistry; Wrighton, Mark S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

9 13

33 33

34 U

2 +


5.

75

Roles of Charge Transfer States

FORD

Ol_ 200

250

450

400

300 350 wavelength/nm

Figure 1. Electronic spectrum of Ru(NH )e in aqueous solution ( -) and in 80% aqueous ethanol ( ); 25°C 3

2+

and R u ( e n ) are qualitatively very similar. T h e lower energy, low intensity shoulder at 390 n m can be attributed to the A -> Γι^ d-d transition predicted by ligand field ( L F ) theory. However, the maximum at 275 nm, although situated approximately where the predicted A i - » Τζ L F band might be expected, has an extinction coefficient much larger than that seen for the analogous band of the Co (III) and R h ( I I I ) homologues (see Table I ) . Given this observation, some preliminary photochemical results, and the relative ease of oxidizing R u ( N H ) (E = 0.05V for the R u ( N H ) couple ( 1 8 ) ) , w e have suggested previously a charge transfer to solvent ( C T T S ) assignment (12,15) (vide infra). 3

2 +

x

lg

1

x

χ

g

9

3

h

3

6

6

2 +

3 + / 2 +

Replacement of one N H by a ττ-unsaturated ligand leads to dramatic spectral changes as shown i n the spectrum of R u ( N H ) p y (Figure 2 ) . Thus the bands seen for R u ( N H ) are obscured by the far more intense absorptions centered at 407 and 244 nm. These have been assigned respectively as a metal-to-ligand ( p y ) charge transfer ( M L C T ) band and an internal ligand ( I L ) i r V transition (19). T h e M L C T band position is markedly dependent on the nature of pyridine ring substituents, especially those i n the para position, and on the nature of the solution medium (19). It is this sensitivity that forms the basis of the photochemical tuning mentioned above. Similar M L C T bands are seen for other unsaturated ligands such as the organonitriles. T h e L F band, although unseen i n Figure 2, no doubt contributes i n a minor way to absorption around 400 n m , given the similarity i n the L F strengths of ammonia and pyridine. A n illustration of this is seen i n the spectrum 3

3

3

6

2 +


5

2 +

76


of aqueous R u ( N H ) 5 ( C H C N ) (see Table I ) that displays the expected lower energy L F band at 350 n m as a shoulder on the higher energy M L C T absorption centered at 226 nm. Thus, there are four types of electronic transitions ( C T T S , M L C T , I L , and L F ) w h i c h should draw one's attention when evaluating the photochemistry of pentaammineruthenium(II) complexes. (Ligand-tometal charge transfers unlikely are given the difficulty of reducing aqueous R u ( N H ) (20).) O n a rather qualitative level, one can examine the formal electronic character of these excited states and speculate regarding the reactivity to be expected for each. F o r example, a L F excited state involves an angular redistribution of charge primarily 3


3

e

2 +

3

2 +

8000

WAVELENGTH (NM)

Figure 2.

Electronic spectrum of Ru(NH ) py in aqueous solution s

5

2+

located on the metal [i.e., ( N H ) ( R u * ) L ] , and for the l o w spin d complexes, such a state involves promotion of a t electron to an e^ orbital, σ* w i t h regard to ligand-metal bonds. Thus, it is not surprising that ligand labilization is the most general reaction mode attributed to such states. I n contrast, the I L excited state involves electronic redis tribution localized on the ligand [ ( N H ) R u ( L * ) ] , and reaction pat terns might involve ligand structural changes, reactions w i t h other substrates, etc., similar to those of the free ligands. This is a relatively unexplored area of transition metal photochemistry, but examples such as certain photoreactions of coordinated stilbazole have been attributed to ligand-localized states (21,22). A C T T S state can be represented 3

6

2 +

5

2g

3

5

2 +


5.

77

Roles of Charge Transfer States

FORD

formally as having an oxidized metal and a solvated electron [ ( N H ) R u L . . . e ~], and reduction of solvent or other solution substrate b y the e " is the reactivity most strongly suggested. Reactions characteristic of an oxidized metal center might occur also. Lastly, representation of the M L C T state as an oxidized metal coordinated to a radical i o n [ ( N H ) ( R u ) (L~)] + suggests reactivity such as electron transfer from another substrate to the oxidized metal center (23) or reactivity characteristic of the radical ligand. I n the latter category, there might be electron transfer to another substrate (6,14), structural rearrangement of the ligand (21,22), or bimolecular reactions, such as electrophilic substitution on an aromatic L ( 9 ) . Notably, for M L C T states of various R u ( I I ) , all of the above reactivity types have been reported. Complicating such simple considerations are the possibilities of the interaction and mixing of various excited state types, internal conversion from states initially populated to states of different multiplicities a n d / o r different orbital parentages, and intermolecular energy transfer and quenching. Also, isoenergic crossing into the ground electronic state would give a highly excited vibrational state with considerably more energy than the activation energies of most thermal reactions seen for that complex or molecule. I n case of such a mechanism, however, i t appears that the reaction trajectory taken b y such a hot ground state still reflects the nature of the electronic excited state from which i t was derived (24). 3

3 +

5

e

s


3

2

n i

5

i

Ru(NH ) 3

2+

6

As noted above, preliminary results (7,13) suggested the shorter wavelength band of the R u ( N H ) spectrum to have C T T S character. Among these was the observation that the U V photolysis of several aqueous R u ( N H ) L ( L = N , C H C N , py, N H , or H 0 ) gives R u ( I I I ) products. The production of molecular hydrogen also was noted for some of these cases. I n contrast, U V photolysis of the isoelectric Rh(NH ) gives aquation products only. C T T S absorptions have been noted to be dependent on the nature of the solvent medium (25), and Figure 1 illustrates the difference between the R u ( N H ) spectrum i n aqueous solution and i n 8 0 % ( v / v ) aqueous ethanol. Notably, the lower energy band is unchanged, either i n extinction coefficient or i n position, consistent with its L F assignment. However, there is a shift of the shorter wavelength band to 264 nm, and as a consequence there is much better definition i n 8 0 % ethanol of a relatively weak shoulder at 310 nm, barely suggested i n the aqueous spectrum. T h e shoulder very likely represents the A T L F absorption predicted for this octahedral d complex. ( O n the basis of this assignment, calculations using the 3

3

3

e

2 +

5

6

2

2 +

3

3

2

3 +

3

x

l0

x

e

2 +

2g


6

78


approximate relationship C = 4B for the Racah parameters give crystal field terms Δ = 26.8 k K , Β = 454 cm" , C = 1816 cm' .) C T T S bands have been assigned previously on the basis of their linear energy correlation with the solvent dependence of the C T T S band of iodide (25,26). This correlation is not seen for the charge transfer band of R u ( N H ) (18), an unsurprising observation given that pre vious successes were seen only for anionic complexes. However, linear dependence of the energy of the charge transfer band with the mole fraction of water i n various mixed solvents is seen (Figure 3 ) ; this suggests that solvation of the cationic R u ( I I ) and R u ( I I I ) species is the dominant factor determining the shift of the C T T S band i n different solvent environments. Photolysis of acidic ( p H ~ 3 ) aqueous R u ( N H ) gives several products but analysis of these indicates two primary photoreactions: ammonia aquation (Reaction 1) 1


3

e

1

2 +

3

Ru (NH ) 3

e

2 +

hv

+ H 0 2

• Ru (NH ) H 0 3

5

2

2 +

e

2 +

+ NH

3

(1)

and oxidation of R u ( I I ) to R u ( I I I ) (Reaction 2)

Ru (NH ) 3

hv 6

2 +

> Ru (NH ) 3

e

(2)

3 +

Quantum yields measured over various irradiation wavelengths ( A r ) , 405-214 nm (Figure 4), show that photoaquation dominates (as expected) ir

40 *

39

\

CVJ RU(III) and the related decrease i n 313 nm (31.9 k K ) , both Φ (ΐπ) (0.03 ± 0.01 mol/einstein) and

Inorganic and Organometallic Photochemistry - American Chemical

Recommend Documents