trans-[Os^(VI)(tmc)(O)_2]^(2+)(tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane) and trans-[Os^(VI)(CN)_4(O)_2]^(2-) are emissive in the solid state and in fluid solutions at room temperature (τ~ 1.0–1.5 µs); the (^3)E_g state of trans-[Os^(VI)(tmc)(O)_2]^(2+) is a powerful one-electron oxidant {[Os^(VI)(O)_2]^(2+) + e^– → [Os^V (O)_2]+, E^0_f > 2.0 V vs. normal hydrogen electrode} in aqueous solution, reacting with PPh3 and ((PhCH_2)_2)S to give O=PPh_3 and ((PhCH_2)_2)S=O, respectively

Che, Chi-Ming

Cho, Kar-Cheong

Gray, Harry B.

Yam, Vivian Wing-Wah

English

Caltech Authors - Main

948 J. CHEM. SOC., CHEM. COMMUN., 1987 
Metal-0x0 Photo-oxidants. Photochemistry and Photophysics of trans- 
[Osvl(tmc)(O)#+ (tmc = 1,4,8,11 -tetramethyl-I ,4,8,11 -tetra-azacyclotetradecane) 
a n d trans- [ OsVl( C N l4 ( 0 I#- 
Chi-Ming Che,*a Vivian Wing-Wah Yam,a Kar-Cheong Cho,b and Harry 6. Gray. 
a Department of Chemistry, University o f  Hong Kong, Pokfulam Road, Hong Kong 
b Department of Physics, The Chinese University o f  Hong Kong, Shatin, New Territories, Hong Kong 
c Arthur Amos Noyes Laboratory, California Institute o f  Technology, Pasadena, California 91 125, U.S.A. 
trans-[OsVl(tm~)(O)~J2+ (tmc = 1,4,8,1 I-tetramethyl-I ,4,8,11 -tetra-azacyclotetradecane) and trans-[O~vl(CN)~(0)~]2- 
are emissive in the solid state and in fluid solutions at room temperature (t - 1.0-1.5 ps); the 3E, state of 
trans-[O~v~(tmc)(O)~J2+ is a powerful one-electron oxidant {[Osv1(0)212+  e- -+ [OsV(0)21+, 150  > 2.0 V vs. 
normal hydrogen electrode} in aqueous solution, reacting with PPh3 and (PhCH2)2S to give O=PPh3 and 
( PhCH2)2S=0, respectively. 
Relatively few long-lived electronic excited states of metal 
complexes are powerful oxidants. 1 Although metalloporphy- 
rin and metal-bipyridine complexes are commonly employed 
as photocatalysts,lf?h these systems suffer from the disadvan- 
tage that long-lived excited states with potentials greater than 
1.6 V (vs. normal hydrogen electrode, n.h.e.) are difficult to 
generate, because of the rapid intramolecular reductive 
quenching that occurs when n-unsaturated ligands are 
present. We describe here a new class of excited-state reagents 
that are powerful oxidants (-2 V vs. n.h.e.) in solution at 
room temperature. lc.2 
The optical spectrum of trans-[O~VI(tmc)(O)~][ClO~]~3 (tmc 
= 1,4,8 , 1 l-tetramethyl- 1,4,8,1l-tetra-azacyclotetradecane) 
(and [Ph4As]2[OsvI(CN)4(0)2]4 in parentheses) in MeCN 
displays vibronically structured absorption bands at 312 (320) 
nm [(dx,,)2 -+ (dxy)l(dn*)l, lAl, -+ 1E,]-f and 355 (370) nm 
[(dx,,)2 -+ (d, )l(dn*)l, lAlg -+ 3E 1.t Excitation of trans- 
[Osv~(tmc)(O~2][C104]2 or [Ph4As]2fOsvr(CN)4(0)2] at 350- 
400 nm results in red emission; photophysical data are 
summarized in Table 1. The large difference in excitation and 
emission energies suggests that the emissive state is not E, 
(3E,); it possibly could be either the B1, or B2, sublevel of 
3Eg.ld,2a,3 Because neither the lifetime nor the intensity of the 
emission of trans-[O~vI(tmc)(O)~][ClO~]~ is quenched by H +  
t d,. = (dxz,dYz); the A,, and E, symbols are for trans-[ML,(O),] 
complexes with Ddh symmetry: see refs. 2(a) and 3. The emission EM 
was estimated from the emission spectrum. The 0s-0 stretching 
frequency in the 3E state was estimated from the vibrational spacing 
in the (dxy)2 + (dxJ1(dx.)l transition. See also ref. 2(a). 
(Table l), the OsO22+ species can be employed as a 
photo-oxidant in both protic and aprotic media. The reduction 
potential (Osvl + e- -+ Osv) of [Ph4As]2[Osv1(CN)4(0)2]$ is 
much more negative than that of trans-[OsVI(tm~)(O)~]- 
[ClO,],, -1.52 vs. -0.73 V5 (vs. cp2Fe+/O; cp = cyclo- 
pentadienyl). Values of the standard reduction potential of 
the OsVVOsV couple (0.01 V vs. n.h.e.)3 and the estimated 
emission E%,, (>2 eV)t for trans-[OsVI(tm~)(O)~]2+ suggest 
that its 3E, state [(dxy)l(dn*)l] is an extremely powerful 
one-electron oxidant: Ef"[ OsVI( 0)2]2+ */[ OsV02] + > 2.0 V 
(vs. n.h.e.) in H20 (>1.4 V vs. cp2Fe+/0 in MeCN). The 
excited-state reduction potential of [OSVI(CN)~(O)#- is 
estimated to be -0.5 V vs. cp2Fe+/0 in MeCN, assuming its 
emission EGO is -2 eV.? Thus a series of trans-dioxo- 
osmium(v1) photo-oxidants can be obtained by ligand varia- 
tions. 
tr~ns-[OsvI(en)~( 0)2][PF6]2 (en = 1,2-diamineoethane) 
and trans-[o~~~(NH~),(o)~][PF~]~,~ whose optical spectra are 
virtually the same as that of trans-[OsVI(tmc>(O)212f, ex- 
hibited no emission in the solid state or in fluid solution. It is 
likely that hydrogen bonding between Os=O and N-H groups 
provides an efficient mechanism for nonradiative decay of the 
excited states of these complexes.2" 
Steady-state quenching results for trans-[OsvI(trn~)(O)~]- 
[C1O4I2 with substrates are set out in Table 2. The large 
$. The cyclic voltammogram of [P~,As],[OSV~(CN),(O)~] was 
obtained in 0.1 M-[Bu~N]PF~ acetonitrile solution, working electrode 
pyrolytic graphite. The OsVVOsV couple at -1.52 V vs. cp,Fe+'O is 
quasi-reversible with AEp -70 mV at a scan rate of 100 mV s-1. 
J. CHEM. SOC., CHEM. COMMUN., 1987 949 
Table 1. Photophysical data for trans-[OsV*(trn~)(O),][ClO~]~ and [Ph4As]2[Osv~(CN)4(0)2] .a 
Emissionb 
Complex ~rn,x./nm Lifetime/ps 
trans-[ OsVI( tmc)( O)2][C104]2 Solid, 630 MeCN , 1 .o 
MeCN, 630 H20, 1.4 
[Ph4As12[OsV1(CN)4(O)2I MeCN, 700(br) MeCN, 0.4 
0.1 M - C F ~ C O ~ H ,  1.5 
a The quantum yield of the phosphorescence was S l O - 3  in both cases: standard, quinine sulphate. 
Hitachi 6 5 0 - 4 0  spectrophotometer. 
The emission spectrum was run on a 
quenching rate constants (3-4 x 109 dm3 mol-1 s-1) for 
amines are consistent with the substantial driving forces for 
the one-electron oxidation of NR1R2R3 to +NRlR2R3. Flash 
photolysis of trans-[OsvI(trn~)(O)~]2+ and tmpd (N,N,N’ ,N’- 
tetramethyl-p-phenylenediamine) in degassed acetonitrile 
yields a transient tmpd+ signal (Amax, - 600 nm)7 that decays 
by second-order kinetics. 
The high reduction potential of trans-[OsvI(tmc)( 0),]2+ * 
suggests that the excited-state reagent also can be employed 
for halide oxidations. Exciplex formation was not observed 
between trans-[OsVI(tm~)(O)~]2+ and X- (X = C1 or Br). The 
difference in quenching rates of C1- and Br- (a factor of 100: 
see Table 2) is parallel with the difference in their oxidation 
potentials and indicates that the photosystem might be useful 
for bromide analysis in the presence of chloride. 
The 0s -0  bond in the 3E, state of trans-[OsvI(tmc)(O)~]2+ 
or [ O S V ~ ( C N ) ~ ( O ) ~ ] ~ -  is much weaker than in the ground state 
[v(Os=O): lAl, - 870 cm-1; 3E, -700 cm-l],t implying that 
these osmium complexes might undergo 0x0-transfer photo- 
reactions.3 Oxygen-atom acceptors (stilbene, phosphine, 
phosphite, and dialkyl sulphide) quench the phosphorescence 
of trans-[O~VI(tmc)(O)~]2+ at diffusion-limited rates (Table 
2). The k ,  values for cis- and trans-stilbenes cannot be 
explained by an energy-transfer mechanism, because the 
excited-state energy of the osmium complex (48 2 3 kcal 
mol-1; cal = 4.184 J ) t  is lower than the triplet energy of the 
organic quencher (cis-stilbene, 49; trans-stilbene, 57 kcal 
mol-l).* Although photol.ysis ( h  >300 nm) of trans- 
[OsVI(trn~)(O)~]2+ and stilbene in degassed MeCN for 3 h 
produced no epoxide or benzaldehyde, O=PPh3 and (PhCH2)2- 
S O  (yield -31 %) were formed under similar photochemical 
conditions with PPh3 and (PhCH2)2S. Similarly, O=PPh3 was 
formed upon irradiation ( h  >300 nm) of [Ph4Asl2[OsVI- 
(CN),(0)2] and PPh3 in MeCN for 3 h.§ The phosphorescence 
of [OSV*(CN)~(O)~]~-,  which has a lower excited-state reduc- 
tion potential than trans-[OsvI(tm~)(0)~]2+, is not efficiently 
quenched by PPh3 (kq < l o 6  dm3 mol-1 s-1). The relative rates 
of photo-oxidation of PPh3 by the two OS(O)~*+ complexes 
suggest that electron transfer to O S ( O ) ~ ~ + *  is the initial step in 
the reaction pathway. The observation that O=PPh3 is formed 
in these photoreactions confirms that oxygen-atom transfer9 
competes effectively with back electron transfer in the 
photogenerated [ 0 ~ ( 0 ) ~ + ] [ P P h ~ + ]  complex. 
We thank Dr. Janet L. Marshall for experimental assist- 
ance. Support from the Croucher Foundation (C. M. C. and 
§ Steady-state irradiation was performed with a high-pressure mer- 
cury short-arc lamp (350 W) equipped with filters or monochromator. 
Photochemical reaction conditions: trans-[O~VI(tmc)(O),][PF~]~, 6.5 
mg; PPh3, 100 mg; MeCN, 4 ml; trans-[Osvx(tmc)(0)2][PF6]2, 20 mg; 
(PhCH2),S, 30 mg; MeCN, 4 ml. O=PPh3 was identified by its i.r. 
band at 1190 cm-l. (PhCH,),S=O was identified by 1H n.m.r. 
spectroscopy, 6 3.97 in CDC13 with m-toluic acid as the internal 
standard. The yield of (PhCH2)2S=0 is based on the amount of 
osmium complex used. No O=PPh3 or (PhCH2)2S=0 was formed in 
the absence of the osmium complex. 
Table 2. Rate constants for the quenching of 0.1 M trans- 
[Osvx(tmc)(0)2]2+ in degassed 0.1 M-[E~,N]CIO, acetonitrile solution 
at 25 “C. 
Quencher kqaldm3 mol-1 s-1 
tmpd 
Ph2NH 
PhNMe, 
cis-Stilbene 
trans-Stilbene 
PPh3 
(Ph0)3P 
(PhCH2)2S 
B u ~ P  
Et2S 
Thiophene 
NaClb 
NaBrb 
4.4 x 10’ 
3.1 x 109 
3.5 x 109 
4.0 x 109 
4.4 x 109 
1.2 x 109 
7.8 x 108 
2.2 x 108 
2.4 x 108 
1.4 x 107 
1.0 x 106 
1.1 x 108 
2.1 x 109 
a The second-order quenching rate constants were obtained by both 
intensity and lifetime-quenching methods. b In 0.1 M - C F ~ C O ~ H .  
V. W.-W. Y.) and National Science Foundation Grant 
CHE84-19828 (H. B. G.) is gratefully acknowledged. This is 
contribution No. 7573 from the Arthur Amos Noyes Labora- 
tory. V. W.-W. Y. is a Croucher Foundation and Li Po Chun 
Scholar. 
Received, 11th February 1987; Corn. 183 
Refer en ces 
1 Recent work in this area: (a) T. Shimidzu, T. Tyoda, H. Segawa, 
and H. Honda, Nouv. J. Chim., 1986, 10, 213; (b) A. Slama- 
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D. G .  Nocera, J. R. Winkler, A. W. Maverick, C. M. Che, and 
H. B. Gray, Am. Chem. SOC. Symp. Series, 1983, 211, 21; (f) 
K. Kalyanasundaram, Coord. Chem. Rev., 1982,46, 159; (g) R. J. 
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8 S.  L. Murov, ‘Handbook of Photochemistry,’ Marcel Dekker, New 
York, 1973. 
9 See also: B. A. Moyer, B. K. Sipe, and T. J. Meyer, Znorg. Chem., 
1981, 20, 1475. 
1986, 1519. 


Metal-Oxo Photo-oxidants, Photochemistry and Photophysics of trans-[Os^(VI)(tmc)(O)_2]^(2+)(tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane) and trans-[Os^(VI)(CN)_4(O)_2]^(2-)

http://authors.library.caltech.edu/32135/1/CHEjcscc87.pdf

Metal-Oxo Photo-oxidants, Photochemistry and Photophysics of trans-[Os^(VI)(tmc)(O)_2]^(2+)(tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane) and trans-[Os^(VI)(CN)_4(O)_2]^(2-)

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