Photochemistry and Redox Catalysis Using ... - ACS Publications

7

Downloaded by PURDUE UNIVERSITY on August 4, 2013 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0238.ch007

Photochemistry and Redox Catalysis Using Rhenium and Molybdenum Complexes Andrew W. Maverick, Qin Yao, Abdul K. Mohammed, and Leslie J. Henderson, Jr.

Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803-1804

Halide complexes of Mo(III), Re(IV), and Re(V) and complexes of Mo(V), Re(V), and Re(VI) containing one oxo ligand luminesce in the near-infrared region in room-temperature solutions. Many of them are photooxidized by electron acceptors; with ReCl 2- and several of the Mo(III) complexes, this is thefirststep in overall photoinitiated two- or three-electron transfers. ReCl 2catalyzes the oxidation and electrooxidation of Cl to Cl and organic oxidations such as the conversion of toluene to benzal dehyde in aqueous HCl solution. The rhenium(V) complex ReCl - appears to be an important intermediate in the catalytic reactions. Thefluorescencesof the oxo-d complexes Mo OX L(X is Cl or Br and L is H O or CH CN) and Re OCl - are surprisingly long-lived (lifetimes to ~100 ns in solution at room temperature), and these species are photoredox-active as well. 6

6

-

2

6

1

v

4

VI

2

3

5

E A R L Y TRANSITION M E T A L C O M P L E X E S are stable in oxidation states with the photophysically attractive d electronic configuration, as well as in a number of adjacent oxidation states. Our goal has been to carry out an initial one-electron photooxidation reaction (eq 1; A is an electron acceptor) of a d starting complex M followed by a second oxida tion (eq 2, 3, or 4). 3

3

0065-2393/93/0238-0131$06.00/0 © 1993 American Chemical Society

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

132

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

M + A M

+

+ A 2M+

M

+

+ A

+ A"

M

+

M

2 +

+ A"

(2)

M

2 +

+ M

(3)

M

2 +

+ A"

(4)

(1)

Previous transition metal photoredox experiments have concentrated on systems such as Ru(II) and Cr(III) polypyridine complexes. These often undergo efficient excited-state redox reactions (I), but they are largely lim ited to overall one-electron processes. The electron-transfer products, for example, R u ( b p y ) (bpy is 2,2'-bipyridine), Cr(phen) (phen is 1,10phenanthroline), reduced acceptors, and oxidized donors, react only very slowly with organic and inorganic substrates. The value of multielectron systems in redox catalysis has been demonstrated elegantly for several R u and Os complexes with labile coor dination sites, which allow for multiple coupled proton and electron transfers. One such species, [(bpy) (H 0)Ru] (^-0) , catalyzes the oxi dation of C P to C l (2, 3) and H 0 to 0 (4). In our work, we wished to combine photochemical and multielectron redox capabilities in the same complex. Early transition metal species appeared to offer the best oppor tunity for this combination. The approaches we pursued for photochemical two-electron oxida tions are outlined in equations 1—4. We have demonstrated that V(phen) is photooxidized to [V (phen)2] (^-0) (5), which is oxi dized spontaneously to V O(phen)^ in basic solution (d). This scheme utilizes reactions 1 and 2, and requires that a single photon be capable of driving an overall two-electron process. A second scheme, represented by the combination of reactions 1 and 3, is illustrated by the disproportionation of M o ( N C S ) ~ following initial photooxidation of M o ( N C S ) ~ " (7). More recently, we also observed (8) disproportionation following photooxidation of the l,4,7-trimethyl-l,4,7-triazacyclononane complexes (Me [9]aneN )MoX3 ([9]ane is 1,4,7-triazacyclononane and X is Br or I). Finally, our interest in the photoredox reactions of R e C l " was stimu lated in part by the possibility that a photogenerated Re(V) complex might itself be photoactive (see eq 4). These latter schemes, in which reaction 1 is followed by reactions 3 and 4, both entail the absorption of two photons to effect a single net two-electron transfer. Three groups of experiments are discussed in this chapter. First, our work using R e C l ~ and its photoinitiated three-electron oxidation is reviewed. Exploration of the mechanism of this photooxidation led directly to the second topic, the discoveiy that R e C l ~ has the surprising ability to catalyze the oxidation of G T to C l . We conclude with a discus sion of the solution photophysics and photochemistry of metal—oxo com plexes with the d and d configurations. 3+

2+


3

3

4+

2

2

2

2+

2

2

2

ni

2+

3

2

w

w

+

2

m

6

3

3

2

6

2

6

2

6

2

1

3

6

2


7.

Redox Catalysis with Rh & Mo Complexes

M A V E R I C K E T AL.

133

Closely related to the last of these three topics, and also to our own interest in the photochemistry of metal—oxo complexes, is the early work of Rillema and Brubaker (9, 10). They showed that the chloro-alkoxo species M C l ( O R ) ~ ( M is M o or W) thermally or photochemically elim inate the appropriate alkyl halides: v

5

v

M Cl (OR)-

Δ θ Γ / ϊ

5

>

v

RCl(g) + M O C l "

(5)

4

This work is an excellent example of a photosensitive metal—organic sys tem. The reaction also produces the oxo complexes M O C l " , some of whose photoredox properties are reported herein.


v

4

Experimental Section Materials and Procedures. Reagents and solvents were of the highest grade commercially available and were used as received. Samples for photochemi cal measurements were prepared by using dry-box, Schlenk, or high-vacuum tech niques. General photochemical, spectroscopic, and electrochemical methods were reported previously (II). The ReCl ~"^~ potential in 1 M HC1 was estimated from cyclic voltammetry (half-wave potential Ey at 20 V/s in order to minimize competing reactions of ReCl ~). Our estimate of the potential for the second oxidation of R e C l ~ (to make ReCl ) in aqueous HC1 was derived from the work of Heath et al. in C H C 1 (12). We used a chemical method to estimate the C e - C e potential in 1 M HC1 because the original value of 1.28 V versus the normal hydrogen electrode (NHE) (13) (based on electrochemical measurements), which is still quoted in reference works (14), was long ago called into question (15). (First, the original measurements were made at a Pt electrode, which is corroded at highly positive potentials in HCl(aq); C e - C e electron-transfer reactions, on the other hand, are often quite slow. Second, solutions of C e in HCl(aq) are unstable with respect to C l evolution (16, 17). The 1.28-V value is likely to be associated with one or both of these competing reactions rather than the desired C e - C e cou ple.) We estimated the value by using the known C1 -C1" potential and the equilibrium constant for the reaction C e + C l ~ τ± Ce + V2Cl (g). The equilibrium constant was measured spectrophotometrically, by mixing solutions of C e and C e separately with 1 M HC1 under 1 atm (101.3 kPa) of C l and show ing that both mixtures yielded the same final [Ce ] to [Ce ] ratio (18). 6

2

2

6

6

6

2

2

I V

I V

m

n i

I V

2

I V

m

2

IV

111

2

IV

m

2

IV

in

Electrocatalysis and Spectroelectrochemistry. Our best results for electrocatalysis with R e C l ~ were obtained at a freshly polished electrode sur face. This approach contrasts with the systems of Meyer and co-workers (2, 3), 2

6


134


which produce C l and 0 most efficiently at highly oxidized glassy carbon elec trodes (19). Our spectroelectrochemical experiments utilized (Bu N) ReCl in C H C N , in a sealed all-quartz cell (1-mm optical path length) fitted with a gold minigrid O T T L E (optically transparent thin-layer electrode) and A g wire pseudoreference electrode; ferrocene was used as the reference redox couple. Electrolysis times with this apparatus were ~2 min. The reaction was studied in C H C N (ReCl " "~: E 0.77 V vs. F c / F c (II); Fc is ferrocene) because the gold minigrid electrode is easily corroded under oxidizing conditions in aqueous H Q . 2

2

4

2

6

3

3

/2

+


6

t/

Photochemistry ofReCl^ We were originally interested in this area because an oxorhenium(V) com plex, if it could be generated photochemically from R e C l " , might itself be photoactive. For example, dioxorhenium(V) complexes such as R e 0 ( p y ) (py is pyridine) are diamagnetic, with the ((xy)^ ground state (see Scheme I): 2

6

+

2

4

2

χΖ-y

xz yz (e) xy(b^ 9

Scheme I. Energy diagram for metal—oxo and trans-ώαχο complexes. 3

1

The lowest-lying E ((xy)\xz yz) ) excited states of these complexes are long-lived (20-22). In contrast to the octahedral R e C l ~ ion, R e O C l " is diamagnetic (23). Thus, it is likely to be electronically similar to R e 0 ( p y ) . The present rhenium system therefore presents an unusual example of two adjacent oxidation states that are both potentially pho toactive. A scheme taking advantage of this reactivity is illustrated in equations 6-8. 9

2

6

5

+

2

4

2

ReCl ~ + A A 6

ReCl - + A " 2

R e C l " + H 0 —> R e O C l ~ + 2 H 6

2

v

2

Re OCl ~ + A A 5

(6)

6

v

5

+

+ CV

R e ^ O Q T + A""


(1) (8)

7.



135

2

Thus, ReCl "~ is a possible starting material for a two-electron sequence in which both steps are photochemical, that is, in which reaction 1 is fol lowed by reaction 4. This sequence is similar to the one we observed for Mo(NCS) ~ (7) and (Me [9]aneN )MoX3 (8) (photoredox followed by disproportionation, or reactions 1 and 3) in that two photons must be absorbed for each net two-electron reaction. Both of these are potentially more versatile than the sequence we discovered for V ( p h e n ) (reactions 1 and 2), which requires that the entire two-electron transfer be driven by a single photon. Because two steps in the "two-photon" schemes can be endothermic, the doubly oxidized products can be more powerful oxidants than those formed in the "one-photon" mechanism. Acceptors such as 2,3,5,6-tetrachloro- (chloranil) and 2,3-dichloro5,6-dicyano-l,4-benzoquinone (DDQ) photooxidize R e C l ~ reversibly. Tetranitromethane, on the other hand, functions as an irreversible oxidant. Irradiation of ReCl ~~ in the presence of C(N0 ) yields R e 0 ~ as the final product, along with reduction products from ( ^ ( Ν Ο ^ (II). This reaction represents an unusual example of a photoinitiated three-electron oxidation. We have not yet determined the mechanism of the photooxidation of R e C l ~ to R e 0 " using C(NO£ . In order to study possible rheniumcontaining intermediates in the absence of the strongly absorbing C(NO^) ~, we treated R e C l ~ with the powerfully oxidizing cerium(IV) in aqueous HC1 solution. However, this experiment is made more compli cated by the fact that C e ^ oxidizes chloride ion: 6

3

6

3

3

2+


3

2

6

2

6

2

4

4

2

6

4

4

2

3

6

Ce (aq) + Cr(aq) .—> Ce (aq) + }ci (g) IV

nl

(9)

2

This reaction occurs slowly under ordinary conditions (16, 17), but we found that it is in fact catalyzed by ReCl ~. (For example, a mixture that initially contains a 100-fold excess of C e , after evolution of chlorine is complete, still shows essentially quantitative recovery of ReCl ~.) In separate experiments without cerium(IV) (24), R e C l ~ also catalyzes the electrochemical oxidation of C P to C L at a glassy-carbon electrode. 2

6

IV

2

6

2

6

Mechanism ofReCl -Catalyzed Cl Evolution 6

2

These catalytic experiments are important because they indicate that an oxidized rhenium complex is a rapid oxidant. We first proposed the fol lowing mechanism for C l generation: 2


136

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

2

v

R e C l " —> R e C l - + e~ 6

(10)

6

v

v

2

R e C l " + H 0 —* R e O C l - + 2H+ + C P 6

2

v

(11)

5

2

R e O C l - —» R e ^ O C l s " + e"

(12)

5

2

R e ^ O C l s " + C I " —» [Cl Re(OCl) -]

(13a)

5

H


HOC1 + H

+

C l

I V

* >

2

R e C l " + HOC1

(13b)

6

+ C P —• C l + H 0 2

(14)

2

This mechanism incorporates previously reported oxorhenium(V) and oxorhenium(VI) complexes. It is similar to the one proposed by Meyer and co-workers (3) for [(bpy) (H 0)Ru] (/z-0) -catalyzed C l evolution. The latter mechanism also involved two one-electron oxidations followed by direct attack of C P to form an intermediate containing coordinated hypochlorite (OCP) (see eqs 13a and 13b). The final step, the reaction of hypochlorous acid with HCl(aq) to form C l , is rapid. To test this mechanism, we prepared K^RGYoCI^ (23) and K R e O C l (25) separately. We found that solutions of K^ReOC^ in concentrated HC1 are stable indefinitely (in agreement with the results of Casey and Murmann (26)); even in 1 M HC1, the ion persists for a short period of time. The d complex K R e O C l , on the other hand, is highly moisture-sensitive, reacting rapidly with aqueous HC1 of any concentration to produce R e O C l " " and chlorine gas (27). None of the solutions prepared from K R e O C l or K R e O C l showed measurable catalytic activity for C l formation. Thus, neither of these species is likely to be a part of the catalytic mechanism. We next studied the direct oxidation of R e C l " by spectroelectrochemistry in C H C N . The yellow-orange R e product in this case showed an absorption spectrum (Figure 1) very different from that of R e O C l ~ (26); we assign the new bands to the non-oxo complex R e C l ~ . (Although previous reports on the synthesis of R e C l ~ (28, 29) did not discuss its spectral properties, the bands are similar to those of the isoelectronic OsF (30).) The addition of small amounts of water to these solutions has little effect on the electrolysis. Even when the electrolyzed solutions are added to HCl(aq), no hydrolysis to R e O C l ~ occurs; instead, the spectral features of R e C l ~ persist for several seconds, to be replaced by those of R e C l " . In contrast to the results obtained with oxorhenium complexes, the electrooxidized complex does show catalytic activity in 1 M HC1. Therefore, the singly oxidized species R e C l ~ is likely to be an active member of the catalytic cycle. 4+

2

2

2

2

2

V I

5

1

5

v

2

5

5

5

5

2

2

6

v

3

v

2

5

v

6

v

6

6

v

2

5

v

6

2

6

v

6


7.



137

2

ReCl 7ReCl 6

0-010

I

Mi ι ι [ ι ι ι ι

400

500

ι

6

ιιιι[ιιι ι

ι

ι[

Μ

[ιιιι

ι

ι

M I [ I I M


0.005

Xi

R e C l " + C l 6

(20)

2

2

Scheme IL Possible mechanisms for ReCl ~-catalyzed oxidation of CT to Cl . 6

2

I V / m

-

Electrode potentials for the C e and R e C l ^ couples in 1 M H Q are ~1.46 and 1.70 V vs. N H E , respectively (see Experimental Sec tion); estimates are available for the C l ~ to C P ratio (~2.2 V) (32, 33) and R e C l ~ (>2.5 V) (12). Generation of C l " (reaction 16) is consider ably more favorable than oxidation of ReCl ~ by C e (reaction 19); reac tion 15 might be expected to be faster as well. Also supporting the mechanism of reactions 15-18 are our cyclic voltammograms for R e C l ~ in aqueous H O . These show the most chemically reversible behavior (the ratio of cathodic to anodic peak current is closest to 1) at high scan rates and low [CP]. The less reversible nature of the process at low scan rates and high [CP] is consistent with a model involving scavenging of electrogenerated ReCl ~ by C P (eq 16). Surprisingly, we also observed weak luminescence ( A = 1500 nm; lifetime about 100 ns) in the C H C N solutions containing electrogenerated R e C l " . This discovery suggests that even simple non-oxo d com plexes may be useful for photochemical reactions. In particular, because R e C l ~ is a powerful oxidant even in its ground state, we expect it to be a still more reactive species in its luminescent excited state. 2

0/

6

2

IV

6

2

6

6

m a x

3

2

6

6

Catalytic Oxidation of Other Substrates Oxidation of C P to C l in the chlor-alkali process is perhaps the best example of a catalyzed oxidation carried out in aqueous chloridecontaining solutions. This oxidation was traditionally carried out at gra phite electrodes, which are slowly consumed under the operating condi tions of high potential and current. However, the so-called dimensionally stable anode (DSA), which contains R u 0 as the active catalyst, has become increasingly popular (34). 2

2


7.



139

Both because of the success of the D S A in chlorine production and because of forecasts that the demand for C l is expected to decrease over the next few decades (55), we were interested in using our system to oxi dize substrates other than chloride ion. We therefore prepared a standard catalytic mixture containing Ce(IV) in 0.1 M HC1, with and without K^ReCl^ (The smaller acid concentration was chosen so as to decrease the uncatalyzed reaction rate and make catalytic effects easier to discern.) We then added a variety of organic compounds to these mixtures. Nearly all compounds tested, including C H C 1 and C H C N (which are ordinarily considered to be resistant to oxidation), decolorized the C e ^ test solution at least slowly, even in the absence of R e C l " . How ever, with several substrates, oxidation was markedly accelerated when R e C l ~ was added. Toluene, for example, was oxidized —60 times faster in the presence of ReCl ~" (8 χ 10~ M); under similar conditions, isopropyl alcohol was oxidized about 5 times faster when ReCl ~* was added. The oxidation products in these reactions (primarily benzaldehyde from toluene, with smaller amounts of benzoic acid also detected; exclusively acetone from isopropyl alcohol) were the same whether or not ReCl ~" was added. More detailed mechanistic studies of these systems, for example, via stopped-flow methods, may help to determine whether specific complexes among the species in our proposed Cl -evolution mechanism function as rapid oxidants in the oxidation of organic substrates as well. However, clearly, the simple complex R e C l ~ participates in several unusual types of redox reactions. (Experiments with benzyl alcohol as the substrate give little rate enhancement with Re; however, this molecule is oxidized pri marily to benzoic acid, and very little benzaldehyde is formed. This chemoselectivity, and that for oxidation of toluene to benzaldehyde men tioned previously, may also provide useful mechanistic information.) 2

2

2

3


2

6

2

6

2

4

6

2

6

2

6

2

2

6

Photophysics and Photochemistry of Oxo Complexes The general features of the electronic structure of metal-oxo complexes were discussed extensively in the 1960s, based on absorption spectra of the 'Vanadyl" (V 0 ) and "molybdenyl" ( M o 0 ) ions (36-38). Interest in these species has been rekindled in recent years by the discovery that closely related frans-dioxometal complexes with the d configuration (such as R e 0 ( p y ) and R e 0 ( C N ) " ) phosphoresce in solution and undergo efficient excited-state redox reactions (20—22). A n electronic absorption and emission study of "molybdenyl" (d*) complexes in the solid state (39) has also contributed to this new area of inorganic photochemistry. The d and d species are both found with one oxo ligand, as M O L w

2+

v

3 +

2

+

2

3

4

1

2

4

2

4


140

PHOTOSENSITIVE M E T A L - O R G A N I C SYSTEMS 2

or M O L ; the d species are also found as trans-MO^. A l l of these complexes can be treated with the qualitative energy diagram of Scheme I. The lowest-energy absorption band in both cases is assigned to the b —> e (xy -* xz, yz) transition. (The orbital designations used here are those for C symmetry.) In the d complexes there is only one such transition, and luminescence from the lowest-energy excited state is always fluorescence ( E -+ B£. The d configuration, on the other hand, leads to singlet and triplet excited states. In this case, the two lowest-energy bands are A —• E and, at higher energy, A -» E; phosphorescence is observed from E (20-22). 5

2

1

4 v

2

2

2

1

1

3

1

X

3


1

Oxo-d Systems 2

+

Among the most promising d systems reported so far is R e 0 ( p y ) , whose phosphorescence occurs with a maximum at 640 nm and a lifetime of 10 /*s in C H C N at room temperature. This complex can be photooxidized to the powerful oxidant R e 0 ( p y ) , which is in turn capable of attacking organic substrates, apparently by Η-atom abstraction (22). More recent experiments have dealt with nitridorhenium(V) complexes, whose electronic spectral features are similar to those of the R e 0 species (40). We have now also observed luminescence and photoredox reactions for the monooxo complex Re OCl (CH CN)"~ in acetonitrile solution. (The absorption spectrum of this ion, shown in Figure 2, is similar to that reported for R e O C l ( H 0 ) ~ in concentrated HCl(aq) (26).) We assign the two lowest-energy absorption bands to transitions from A to E and E. (The fact that the intensities of the two bands are similar is probably the result of substantial mixing via spin-orbit coupling.) The maxima in the luminescence (~1300 nm) and absorption spectra (1175 nm) suggest an excited-state energy of approximately 8000 cm"" ; the lifetime of the emitting state is about 100 ns. Cyclic voltammetiy performed on R e O C l ( C H C N ) ~ reveals a quasi-reversible oxidation wave, with half-wave potential Ey ~1.0 V vs. Ag—AgCl. Accordingly, we have also carried out photochemical electrontransfer experiments with the complex. Irradiation (λ >650 nm) in the presence of D D Q (2,3-dichloro-5,6-dicyano-l,4-benzoquinone) leads to formation of the radical anion DDQ~; this ion then disappears with second-order kinetics. These observations are consistent with reversible one-electron transfer: 2

4

3

2+

2

4

+

2

v

4

3

v

4

2

1

3

1

X

1

v

4

3

2

ReOCl (CH CN)" + DDQ A

ReOCl (CH CN) + D D Q "

(21)

R e O C l ( C H C N ) H- D D C T

ReOCl (CH CN)~ + DDQ

(22)

4

4

3

3

4

4

3

3


7.


M A V E R I C K ET AL.

141

ReOCl (CH CN)- in CBUCN 4

3


60 I I III I j 1111111111 M 1111 I I I J I I I I J I I I I J I 1 I I j 111111111111111II I I [ I I I l_

Γι I I 1 1 I I I I 1 I I I I 1 I I 1 I I M I I 1 I I I I 1 I I I I 1 I I I I I I I 300 · 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600

Q

I I

I

I I I I

I

11 1 I

1

I

fn

λ/nm

Figure 2 Electronic absorption (—) and luminescence (M) spectra for (Bu^sf)[Re OCl (CH CN)J in CH CN at room temperature. The emis sion spectrum was recorded by using a 5 χ 10~ M solution, with 436-nm excitation and emission bandpass of ~30 nm. v

4

3

3

2

9

-

1

1

Kinetic analysis indicates a value of 3 χ 10 M s" for k . We are now exploring the reactivity of the photogenerated Re(VI) complex in this sys tem and the suitability of other Re(V) oxo complexes for photoredox reac tions. b

Oxo~d Systems As part of our study of the photochemical properties of octahedral Mo(III) complexes, we attempted to prepare H B ( M e p z ) M o B r ~ ( M e ^ z H = 3,5-dimethylpyrazole), for comparison with the previously reported H B ( M e p z ) M o a ~ (41). Reaction of HB(Me pz) Mo(CO) ~ with concentrated HBr(aq) yields M o O B r ( H 0 ) " instead of the desired complex; however, solutions prepared from this Mo(V) product are luminescent! Electronic absorption and emission spectra of M o O B r ( C H C N ) ~ , obtained by dissolving ( B u N ) [ M o O B r ( H 0 ) ] in C H C N , are illustrated in Figure 3. Absorption (36—38) and solid-state luminescence (39) studies of these oxo-d species have established that the lowest-energy absorption and emission bands are associated with the B «-• E transition. However, these species have not previously been reported to fluoresce in solution at III

2

3

3

III

2

3

3

2

4

3

3

2

v

4

4

4

2

3

1

2

2

2


3

11.1 11


142


Figure 3. Electronic absorption (—) and correctedfluorescence(M) spec tra for (BuJSi)[Mo OBr (CH CN)] in CH CN at room temperature. The emission spectrum was recorded by using a 1 χ l€r M solution, with 436nm excitation and emission bandpass of ~15 nm. v

4

3

3

3

v

room temperature. (The fluorescence quantum yields for M o O C l (CH CN)~ and M o O B r ( C H C N ) - are -4.4 χ 1 ( Γ and 1.4 χ Ι Ο " mol/einstein, respectively.) The photophysical properties of several Mo(V) and Re(VI) oxo com plexes are presented in Table I. The most important features of these complexes are as follows. First, the longer-lived excited states are well suited to bimolecular reactions; in separate experiments, we showed that both M o O C l ( C H C N ) ~ and MoOBr (CH CN)~ undergo photooxidation in the presence of acceptors such as tetracyanoethylene (42). Second, the aqua complexes M o O X ( H 0 ) ~ are also fluorescent in CH C1 . These are among the very few known aqua complexes that luminesce in roomtemperature solution. Third, we were surprised to find relatively intense fluorescence with these species, since Winkler (39) found that M o O C l ~ (in which the axial ligand trans to the oxo group is absent) did not fluoresce measurably in solution. We repeated Winkler's work, and we found that the fluorescence of M o O C l ~ in C H C 1 solution, although present, is indeed much weaker than that of the solvated species MoOCl (L)~. Fourth, and again unexpectedly, complexes with chelating ligands such as Me [9]aneN (see Table I) are also poorer emitters than MoOCl (L)~. And finally, other d oxo complexes, such as those of Re(VI) (data for ReOCl ~ are included in Table I and Figure 4) and W(V) (43), also fluoresce. These results make it likely that a large number of other d species will prove to be effective photoredox agents. 4

v

4

3

4

4

3

4

4

4

3

3

2

2

2

4

4

2

2

4

3

3

1

4

5

1



2

3

2

+

3

3

2

2

2

750 750 720 700 630 920

CH CN CH CN CH C1 CH CN crystal HOAc-Ac 0 3

Abs.

Solvent

c

950 900 950 1100 890 ca. 1110

Fluor.

2

1

11,700 12,100 12,000 11,100 13,100 ca. 10,000

Ε ( E) (cm' )"

N O T E : All values are given for solutions at room temperature, unless otherwise noted. "Estimated from average of energies of absorption and emission maxima. *Data are taken from ref 39. 'Uncorrected. ''Not measured.

3

3

4

4

3

4

MoOCl (CH CN)MoOBr (CH CN)' MoOCl (H 0)~ [(Me [9]aneN )MoOBr ] MoOCl, -b *" ReOClc

Complex

Table I. Photophysical Properties of Mo(V) and Re(VI) Oxo Complexes


100 40 60

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