J . Phys. Chem. 1987, 91, 4255-4261
4255
Competition between Aquation and H/D Exchange in 1064-nm Laser Excitation of Cobalt(I11)-Amine Complexes: A Search for Vibrational Photochemistry in Solution David M. Goodall,* David B. Hollis,+ and Michael S. White1 Department of Chemistry, University of York, York YO1 5DD. U.K. (Received: May 7 , 1986; In Final Form: February 12, 1987) Spectra of the complex ions [ C O ( N H ~ ) ~ [Coen3I3+, ]~+, and [Co(NH3),Me2SOI3+have been determined in the wavelength range 700-1 100 nm covering the region of spin-forbidden d-d transitions and the N-H stretching second vibrational overtones. and the competition between trans NH3 H/D exchange Kinetic studies are reported of H/D exchange in [Co(NH3)6]3+, and aquation in [Co(NH3),Me2SOI3+.Nd:YAG laser excitation of the complexes at 1064 nm led to photoaquation (normal electronic photochemistry) or H/D exchange (ascribed to vibrational photochemistry following state-selective N-H ( u = 3) preparation of a hot ground state). For [ C O ( N H ~ ) ~ H/D ] ~ + , exchange occurred with 4 = (2 f 2) X lo-,, and a t test showed a probability greater than 80% for a nonzero quantum yield. This laser-stimulatedproton-transfer reaction is probably the first example of vibrational photochemistry for a transition-metal complex in solution. For [Coen3I3+and [Co(NH3),Me2S0I3+,photoaquation was observed with quantum yields (6 i 2) X lo-, (en loss) and (16 f 6) X (Me2S0 loss), respectively.
Introduction The search for bond-specific reactions driven by vibrational excitation has provided a stimulus for laser photochemists for many years.' It is now evident that intramolecular selectivity cannot be controlled in gas-phase decompositions which follow multiphoton2 or single-photon a b ~ o r p t i o n : reaction ~ occurs with vibrational energy statistically distributed over all modes. There are examples of mode-selective behavior for reactions in lowtemperature mat rice^,^ where single-photon excitation drives fundamental vibrational transitions and populates low-lying states in the vibrational manifold. We have concentrated on reactions induced by single-photon vibrational excitation in the liquid phase, arguing that rapid intermolecular vibrational relaxation will augment intramolecular vibrational relaxation in relaxing the initially prepared state, and that it may be possible to see mode-selective reactions in low quantum yield where the decomposition mode is closely coupled in phase space to the pumped mode., The first example of a vibrational photochemical process in the liquid phase was our study of the photoionization of H 2 0induced by a Nd:glass laser.6 Quantum yields for five excited vibrational states of H 2 0have been reported, and the study has been extended to H20-D20 mixtures7 A second example has been documented recently, with mode-specific behavior suggested in the wavelength-dependent decomposition of 1,5-hexadien-3-01.* Is it possible to use vibrational excitation to enhance the reactivity of a solute? Having succeeded in photochemically driving an 0-Ha-0 O-.-+HO proton-transfer reaction in pure liquid N--.+HO and ~ a t e r , we ~ , are ~ now investigating N-Ha-0 C---+HO proton transfers for nitrogen and carbon C-H-0 acids in D 2 0 solution. Nitrogen acids will be considered in this paper; data on C H 3 N 0 2in D 2 0 have recently been p ~ b l i s h e d . ~ The Co(II1) complexes investigated in the present study undergo the following thermal reactions in D 2 0 solution, illustrated for the general case of an acidopentaammine complex (X = C1-, Me2S0, etc.)'OJ1 I. aquation
-
-
-
-
[Co(NH3),XI3++ D 2 0 [Co(NH3),D20I3++ X 11. base-catalyzed H / D exchange [Co(NH3),XI3++ OD-
(1)
[Co(NH3),(NH2)XI2++ HOD (2) [Co(NH3),(NH2)XI2+ DOH(D) [Co(NH3),(NH2D)XI3+ OH-(OD-) (3)
+
6
-
+
Present address: Department of Ceramics, Glasses and Polymers, University of Sheffield, Sheffield, S10 2TZ, U.K. 'Present address: Department of Chemistry, University of Hull, Hull, HU6 7RX, U.K.
111. base-catalyzed aquation [Co(NH3),XI3++ OD- -= [CO(NH,),(NH,)X]~'
+ HOD (2)
[Co(NH3),(NH2)XI2+4 [CO(NH,),(NH,)]~++ X (4)
fast
[CO(NH,),(NH,)]~++ 2 D 2 0 [Co(NH3),(NH2D)D20I3+ OD- (5)
+
The aquation reaction I is the most favorable process for acidoammine complexes in acid solution. Base-catalyzed reactions (Schemes I1 and 111) increase in rate with increasing pH and involve the same conjugate base (CB) intermediate. Scheme I1 shows the initial steps in the sequential deuteriation process, and Scheme I11 gives the SNICB aquation mechanism via a pentacoordinated intermediate. Base-catalyzed aquation is never faster than H/D exchange." General base catalysis occurs in exceptional cases with acidic N H functions, e.g., the trans N H in [CotrenenC1]2+,'2 but generally the reactions are subject to specific catalysis by hydroxide ion. In this study we have used laser pumping of N-H stretching modes to reach the conjugate base by the higher energy route with D 2 0 as the acceptor base
+
[ C O ( N H ~ ) ~ X+] ~D+2 0 .+ [CO(NH,),(NH,)X]~+ H D 2 0 + (6) Endothermic proton transfers from those N-H containing com-
(1) Kompa, K. L. Z . Naturforsch. B 1972, 27b, 89. (2) Schulz, P. A,; Sudba, A. S.; Krajnovich, D. J.; Kwok, H. S . ; Shen, Y. R.; Lee, Y. T.Annu. Rev. Phys. Chem. 1979, 30, 379. (3) Jasinski, J. M.; Frisoli, J. K.; Moore, C. B. Faraday Discuss. Chem. SOC.1983, 7 5 , 289. Crim, F. F. Annu. Reu. Phys. Chem. 1984, 35, 657. Chuang, M. C.; Zare, R. N. J . Chem. Phys. 1985, 82, 4791. (4) Frei, H. C.; Pimentel, G. C. Annu. Rel;. Phys. Chem. 1985, 36,491. ( 5 ) Goodall, D.M.; Cureton, C. G. In Advances in Infrared and Raman Spectroscopy, Clark, R. J. H., Hester, R. E., Eds.; Wiley. Chichester, 1983; Vol. 10, p 307. (6) Goodall, D. M.; Greenhow, R. C. Chem. Phys. Lett. 1971, 9, 583. Knight, B.; Goodall, D. M.; Greenhow, R. C. J . Chem. Soc., Faraday Trans. 2 1979, 75, 841. (7) Natzle, W. C.; Moore, C. B.; Goodall, D. M.; Frisch, W.; Holzwarth, J. F. J . Phys. Chem. 1981,85, 2882. (8) Schwebel, A.; Brestel, M.; Yogev, A. Chem. Phys. Lett. 1984, 157, 579. (9) Goodall, D. M.; Chetwood, I. Chem. Phys. Lett. 1986, 129, 291. (10) Basolo, F.; Pearson, R. G. Mechanisms of Inorganic Reactions; Wiley: New York, 1967; 2nd ed. (11) Tobe, M. L. Acc. Chem. Res. 1970, 3, 377. (12) Buckingham, D. A,; Marzilli, P. A,; Sargeson, A. M. Inorg. Chem. 1969, 8, 1595.
0022-3654/87/2091-4255$01.50/00 1987 American Chemical Society
4256
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The Journal of Physical Chemistry, Vol. 91, No. 16, 1987
-
revealed efficient radiationless relaxation via the 5T2, state following 3Tl, 'A,, excitation. It was suggested that significant Co-N bond distortions in 3T1, and sT2, retained in crossing to the 'A,, ground state could explain labilization of the metal-ligand bond. Since electronic excitation leads to perturbation at the metal-ligand bonds and causes photoaquation, whereas N-H vibrational excitation should assist proton transfer from the N-H bond and hence H / D exchange, an investigation of competition in 1064-nm photochemistry of the representative complexes [Co(NH3)6]3+,[Coen3I3+,and [ C O ( N H , ) ~ M ~ ~ S should O ] ~ + be diagnostic for vibrational photochemistry.
I
equilibrium position
N -H distance
Figure 1. Schematic diagram of potential energy change along the reaction coordinate for proton transfer from a Co(III)-amine complex, BH3+, to OD- (reaction 2) and D20 (reaction 6 ) . Arrow indicates laser excitation.
plexes were felt to be particularly good candidates for single-photon vibrational photochemistry because the Nd:YAG pump frequency is well matched to the requirements of the decomposition mode. The energetics of the reactions 2 and 6 are conveniently shown in schematic form in Figure 1 by using molecular potential energy curves13to describe potential energy (PE) changes on displacement of the proton from its equilibrium position in the complex, abbreviated BH3+. The N-H (Au = 3) stretching overtone transitions in ammine and amine complexes are centered close to the Nd:YAG laser wavelength, 1064 nm. This is equivalent to photon energy E = 112 kJ mol-', which is comparable to the estimated free-energy change for the proton transfer to solvent D 2 0 in reaction 6. Co-ammine complexes14and D2015have pKa 15, giving 4G2 0 and AG6 = 1 X IOz kJ mol-'. The basic requirement for pumping a single-photon driven reaction, E > E,,5 should therefore be met by excitation of the N-H (Au = 3) transition. Furthermore, the photon energy is deposited directly in the bond to be cleaved, so there should be the close coupling in phase space between the pumped mode and the decomposition mode which is known to be essential for mode-selective We have found that vibrational and electronic photochemistry are competitive for Co(II1) complexes excited in the near-IR. Previous studies of photochemistry following d-d have shown that photoaquation quantum yields decrease with increasing wavelength. Triplet excitation gives qualitatively thermal photochemistry," with acido ligand loss predominating over ammonia loss in halopentaammines. This would be consistent with reaction from a hot ground state following intersystem crossing.'7d Spectroscopic evidenceIga from [C0(NH3),l3+has
-
-
(13) Bell, R. P. The Proton in Chemistry; Methuen: London, 1973; 2nd ed, Chapter IO. (14) (a) Goodall, D. M.; Hardy, M. J. J . Chem. Soc., Chem. Commun. 1975, 919. (b) Hardy, M. J. D.Phil. Thesis, York University, 1975. (15) Covington, A. K.: Robinson, R. A,; Bates, R. G. J . Phys. Chem. 1966, 70, 3820. ( 1 6 ) Oref, I . ; Rabinovitch, B. S. Acc. Chem. Res. 1979, 12, 166. ( 1 7 ) (a) Balzani, V.; Carassiti, V. Photochemistry of Coordinafion Compounds; Academic: New York, 1975; p 193. (b) Zinato, E. In Concepts of Inorganic Photochemistry, Adamson, A. W., Fleischauer, P. D., Eds.; Wiley-Interscience: New York, 1975; Chapter 4. (c) Forster, L. S . Ibid. Chapter 1. (d) Endicott, J . F.; Ferraudi, G. J. J . Phys. Chem. 1976, 80, 949. (18) (a) Langford, C. H.; Vuik, C. P. J. J . Am. Chem. Sor. 1976,98,5409. (b) Langford, C. H.; Sarkihe Malkhasiah, A. Y . J . Chem. Soc., Chem. Commun. 1982, 12 IO.
Experimental Section [CO(NH3)6]C13,z0a and [Co(NH,),Me,SO](C1O4),.Me2SOz' were prepared according to the literature procedures referenced, and purities confirmed from absorption coefficients of the d-d band^.^^.^^ Note: an EXPLOSION HAZARD exists with [CO(NH~)~M~~SO](C~O~)~.M~~SO. An improved routez3to Me2S0 complexes should be used in any future work. For spectroscopic and photochemical experiments the samples were normally studied as 0.05 M solutions in DzO, with 0.1 M DCI to suppress base-catalyzed H / D exchange.I0 Spectra were determined with a UV-visible-near-IR spectrophotometer (Varian 2300). In photochemical studies samples were irradiated for up to 25 h with 1064-nm radiation from a Nd:YAG laser (JK Lasers, ~~p~~~~~DLPY-3H) operating in the normal mode (non 0 switched, with tvpical Dulse length 0.5 ms) at IO-Hz pulse repetition rate. At t i e sample cell the-beam diameter was typically 6 mm, the mean laser power was 10 W, and the light flux and fluence per pulse were 7 kW cm-z and 4 J cm-*, respectively. Laser power, energy, and pulse shape were monitored by using photodiodes (JK lasers, Laser Associates) calibrated with a Rats' Nest energy meter.24 1-3-cm3 volume samples were irradiated in a 10 mm X 10 mm section infrasil cell, stirred with a magnetic stirrer bean, and thermostatted to 12 OC or below. Thermal control reactions were always run in parallel with photochemical reactions. The temperature in the cell was measured with a thermistor positioned above the laser-irradiated volume. The temperature rise was normally found to be in good agreement with values calculated knowing the power deposition in the cell and assuming conductive flow across the silica cell walls to the cooled metal holder as a heat sink. In initial experiments a cylindrical, unstirred sample cell was used, and the mean temperature rise in the cell calculated by assuming a conductive steady state.2s Reaction progress was monitored by the following techniques: ( I ) 'H NMR (Varian EM.360A). Peak areas were measured before and after reaction, with C&S03-Na+ added as a reference standard in the case of [Co(NH3)J3+,giving a sensitivity of 2% for H / D exchange. (2) j9Co N M R (Jeol FX 90Q). This was applicable to [Coen3]'+ and [Co(NH3),13+,complexes with octahedral symmetry, by using s9C0line shift and broadening on deuteriation:26 the sensitivity was 0.4% for H / D exchange. (3) Near-IR spectra (Varian 2300). Changes in the N-H (hu = 2) band centered at 1550 nm were used to follow H / D exchange2' in [Co(NH3),Me2SOI3+,with a sensitivity 1% of total ~~~
~
~
~~
(19) (a) Wilson, R. B.; Solomon, E. I . J . Am. Chem. SOC.1980, 102,4085. (b) Komi, Y . ;Urushiyama, A. Bull. Chem. SOC.Jpn. 1980, 53, 980. (20) (a) Bjerrum, J.; McReynolds, J. P. Inorg. Syn. 1946, 2, 216. (b) Work, J. B. Ibid. 1946, 2, 221. (21) Piriz Mac-Coll, C. R.: Beyer, L. Inorg. Chem. 1973, 12, 7. (22) Jargensen, C. K. Adu. Chem. Phys. 1963, 5, 94. (23) Dixon, N. E.; Jackson, W. G.; Lancaster, M.J.: Lawrance, G. A.; Sargeson, A. M. Inorg. Chem. 1981, 20, 470. (24) Greenhow, R. C.; Schmidt, A. J. J. Sci. Instrum. 1969, 2, 438. (25) Adapting the analysis of steady-state radial heat flow for CW laser
irradiation (Zitter, R. N.; Koster, D. F.; Cantoni, A,; Pleil, J. Chem. Phys. 1980, 46, 107) using K = a P = 0.6 J s-' m-' K-' for the thermal conductivity of H,O and D,O. (26) Bendall, M. R.; Doddrell, D. M . Ausf. J . Chem. 1978, 31, 141. (27) (a) Palmer, J . W.; Basolo, F. J . Inorg. Nucl. Chem. 1960, 15, 279. (b) Palmer, J. W.; Basolo, F.; Pearson, R. G . J . Am. Chem. SOC.1960, 82, 1073
The Journal of Physical Chemistry, Vol. 91, No. 16, 1987 4257
1064-nm Laser Excitation of Co(1II) Complexes
TABLE I: Wavelengths of Band Maxima and Absorption Coefficients for Electronic and Vibrational Transitions 700-1 100 nm, and Absorption Characteristics at 1064 nm' ~
~~~
~
-
electronic 'T,g
[Coen,]'+ [Co(NH3)613+ [Co(NHJ5Me2S0I3+ "Units: A, nm;
0.16
t,
vibrational N-H (A0 = 3)
'AI,
vibrational N-H ( A u = 3)
A,,
4e)(X,,,)
L a x
(Amax)
(X1064)
electronic 4e)(h4)
7 20 740 865
68 f 1 67 f 1 190 f 10
1064 1052 1052
5.0 f 0.2 8.2 f 0.3 8 f l
5.0 f 0.2 6.1 f 0.4 5 f 1
5.8 f 0.6 12 f 2 140 f 10
io-' m2 mol-'.
1000
1050
11
1000
'0.03
1050
1100
0,030
I
A
0.025
0.02
0.12
0.020
A
0.01 0.08
i
0.015
0.04
0 600
800
1000
A/ nm
1200
-
Figure 2. Spectrum of [Coen3]CI3in the region of the 3T 'A absorption band and the Au = 3 vibrational overtones. N-H (Au = 3) band expanded in insert. Concentration = 0.050 M, solvent and reference = DzO, cell path length = 10 cm. Solid line: complex dissolved in 6 X IO-) M DC1/DzO. Dashed line: after neutralization with NaOD (30% in D20),0.04 cm3 in 40 cm'.
600
1000
800
Alnm
1200
-
Figure 3. Spectrum of [CO(NH,)~]C~, in the region of the 'T 'A absorption band and the Au = 3 vibrational overtones. N-H (Au = 3) band expanded in insert. Concentration = 0.029 M, solvent and reference = D20, cell path length = 10 cm. Solid line: complex dissolved in 2 X M DCI/D20. Dashed line: after neutralization with NaOD (30% in D20), 0.3 cm3 in .30 cm'.
laser enhancement of the decomposition rate constant, kL, is given N H protons. Decrease in absorbance, A , was followed through by 8t,,2 of trans N H 3 proton exchange to a final value A f , and the kL = k f - k (8) rate constant kobsddetermined from a plot of In ( A - A f ) vs. t . (The bandwidth of the absorption peak remains constant The pumping rate constant E , was calculated as follows:s throughout reaction.) (4) UV-visible spectra (Varian 2300). Aquation of [Co(9) (NH3)5Me2SO]3+was followed as previously d e s ~ r i b e d . ~ ~ ~ ~ ~ In analyzing photochemical change, initial concentrations of where P is the average laser power sampled by the cell, and A complex (co) and concentrations after laser irradiation for time the cross sectional area of the solution in the cell; i.e. the average t (c,) gave a total reaction rate constant k' power density in the solution is P I A . L is Avogadro's constant, h Planck's constant, c the speed of light, and v the laser waveIn c o / c t = k't (7) ) d ( ~ )where , number. For vibrational (v) photochemistry ~ ( v = Similar analyses of samples reacting at the same temperature but E(V)is the molar natural absorption coefficient of the vibrational without laser irradiation gave the thermal rate constant, k . The band and n the number of N-H bonds per molecule. For electronic (e) photochemistry, €'(e) = €(e), the molar natural absorption Coefficient of the electronic band. This ensures that both kL and (28) Buckingham, D. W.; Marty, W.; Sargeson, A. M. Inorg. Chem. 1974, k, refer to only one ligand or one N-H bond in each molecule. 13, 2165. The quantum yield for photodecomposition, 4, is given by (29) (a) Palmer, D. A.; Kelm, H. Inorg. Chem. 1977, 16, 3139. (b)
Reynolds, W. L.; Birus, M.; Asperger, S. J . Chem. SOC.,Dalton Trans. 1974, 716.
4258
The Journal of Physical Chemistry, Vol. 91, No. 16, 1987
Goodall et al.
3-
A
400
500
700 k l n m
800
2-
800
900
1000
1100
0.09 A
0.07
Figure 5. Ultraviolet-visible absorption spectra of [Co(NH,),] CI, (-) and [Co(ND3)6]C13 (---). Solvent 1 X lo-) M DCI/DzO,cell path length 1 cm. Concentration 3 X M for 300-800 nm, 3 X M for 200-300 nm for both complexes. Insert shows 300-800-nm difference spectrum AA = A[CO(NH,)~]~+ - A[Co(ND3),13+.
0.05
TABLE 11: H/D Exchange in [Co(NH3),$+ in DCI/D,O Solutions at 90 OC
0.03 1500
1600 A/nm
[Co(NH,),CI,I/ M 0.13 0.12 0.1 I
1700
Figure 4. Spectrum of [Co(NH,),MezSO](CI0,)3~MezS0 over the wavelength ranges 8OC-1100 and 1400-1800 nm. Concentration = 0.050 M, solvent and reference = 0.1 M DCI/D20, cell path length = 1 cm. (a) ,A2 + ,E lAlgabsorption with N-H (Au = 3) band at 1052 nm. (b) N-H (Ac = 2) at 1550 nm and C-H first overtones at 1650-1800 nm.
-
Results Spectroscopy. Near-IR spectra of [Coen313+,[CO(NH3)6I3+, and [ C O ( N H ~ ) ~ M ~ ~are S Ogiven ] ~ + in Figures 2-4, and absorption coefficients at the band maxima and at the laser wavelength 1064 nm are tabulated in Table I. The N-H (Au = 3) stretching vibrational transitions are centered at 1052-1064 nm, and absorption coefficients were obtained after subtracting underlying contributions from the tails of the electronic and lower energy vibrational transitions. Figures 2 and 3 show that the N-H absorption disappears on making [Coen3I3+ and [ C O ( N H ~ ) ~solutions ]~+ basic in D20, due to H / D exchange, and HOD peaks appear at 960 and 1200 nm. C-H (Au = 3, Au = 4) transitions are found in [Coen313+at 1170 and 890 nm. Half-height band widths, AD^,^, are very similar for all N-H (Ac = 3) spectra. Integrated band intensities, I , were estimated per N-H bond by using the equation I =
~'(~nlax)A~1/2
(1 1)
Values of I/m2 mol-' cm-I are 1 . 1 , 1.4, 1.6 for [Coen,l3+, [C0(NH3)6l3+,and [ C O ( N H ~ ) ~ M ~ ~ SAll O ]Z~values + . are similar, as has been noted for other overtone spectra which have predominantly local-mode character,30and roughly double that for C-H (Au = 3) bands (I = 0.5 m2 mol-' cm-I using natural absorption coefficient^).^^ The N-H (Au = 2) and C-H (Au = 2) peaks of [Co(NH,),Me2SO]3+(C104)3.Me2S0 in D 2 0 solution are shown in Figure 4b. For N-H (Au = 2), parameters of interest are A,, = 1550 nm, c(A,,,) = 0.30 m2 mol-', = 2.17 X lo3 cm-', I = 43 mz mol-' cm-I. The ratio of integrated band intensities I(Au = 2)/Z(Au = 3 ) = 27. Decrease in absorbance of the N-H (Au = 2) peak was used to measure H / D exchange for the di(30) Sage, M. L.; Jortner, J. Adu. Chem. Phys. 1981, 47, 293.
11
[S]/M" [DCI]/M mol kg-I 0.35
0.26 0.26
0.10 0.12
0.46
1 .O 0.9 1.2
lo4 k,/s-' 1.23 f 0.03 1.17 f 0.13 0.31 f 0.09
" S = C,H,SO,Na peak area standard
methyl sulfoxide complex, the C-H peaks being time invariant, as discussed in the next section. Figure 5 shows the effect of deuteriation on the UV-visible spectrum of the hexaammine complex. Spectra of [CO(NH,),]~+ and [ C O ( N D ~ ) ~and ] ~ +a difference spectrum were obtained by adding identical microliter volumes of NaOD and excess DCI to two aliquots of the same solution of [Co(NH3),]C13 in slightly acidic D20, the only difference being the order of addition of acid and base. Exchange occurred only in the case where the base was added first. Deuteriation is seen to lead to a decrease in intensity of the charge transfer and d-d transitions and to give a shift to lower wavelength of the high wavelength side of each absorption band. This is consistent with the decrease in absorbance in the tail of the electronic band in the near-IR on deuteriation seen in Figure 3. Thermal Kinetics. H/D exchange of [Co(NH3),13+was studied in D 2 0 solutions containing 0.1-0.5 M DC1 at 90 OC, a temperature chosen to give reasonable exchange followed by IH NMR. Results are given in Table 11, and the rate constant for exchange, k,, was analyzed by using the equation kx = kDzO + kOD-[OD-] = kDIO + (kOD-Kw/[D+I) (12) The ion product of D,O on the molarity scale, K , = 5.6 X was evaluated at 90 O C by extrapolation of the equation of best fit for pK, of D2O.IS Rate constants at 90 OC calculated from eq 9 are koD- = (2.2 f 0.4) X lo8 M-' s-l and kDIO= (7 17) X s-l. This shows that OD- catalysis is dominant in the thermal reaction under the conditions studied. For [Coen313+,H/D exchange was followed in 0.1 M DCI/D,O at 33 OC by using 59C0peak broadening, giving k, = 1.7 X lo-' SKI. Analysis of ' H spectra before and after reaction showed no loss of ethylenediamine from the complex and proved that in the thermal reaction there was no competition between H/D exchange and aquation. The kinetic investigation of [ C O ( N H ~ ) ~ M ~ ~quantified SO]~+ the competition between aquation and H / D exchange (eq 1-5). The aquation rate constant, k,, was measured at 29 "C to be (4.0 i 0.3) X s-I, in good agreement with the literature value29
*
The Journal of Physical Chemistry, Vol. 91 No. 16, 1987 4259
1064-nm Laser Excitation of Co(II1) Complexes
I
TABLE 111: Fractional H/D Exchange and Rate Constants for Trans H/D Exchange and Aquation of [ C O ( N H ~ ) ~ M ~ ~ in S O0.10 ] ~ +M DCI/D,O 1OSkx/s-' 1OSkobsd/s& 1Osk,/s-' T/OC F 7.0 f 0.5 3.8 f 0.3 29.0 0.65 & 0.05 10.8 & 0.4 6.8 0.8 f 0.1 0.52 f 0.10 0.12 & 0.06Q 0.4 f 0.1 "Using activation parameters from ref 29.
(3.6 f 0.2) X s-I at this temperature. H / D exchange was studied at 29 and 7 "C by near-IR spectro~copy.~'The area of the N-H (Av = 2) peak centered at 1550 nm (Figure 4b) decreased by less than 20% during reaction. This is consistent with the absence of exchange in the four NH, groups cis to the M e 2 S 0 ligand. In closely related complexes the NH, group trans to the acido ligand has been shown to undergo rapid H / D exchange in comparison with cis NH, g r o ~ p s . ' ~ , ' ~The ,~' observation that not all the trans N H 3 protons exchange may be understood by knowing that aquation is in competition with exchange and that the product aquo complex would be expected to undergo only slow H / D exchange. With the assumption of no exchange in the product on the time scale of the experiment, kinetic analysis of the competition scheme shows that the observed exchange rate constant, koMris related to the rate constant for trans NH3 exchange, k,, by the equation kobsd
=
ka
+ kx
(13)
Defining the fractional exchange, F, as the ratio of the change in N-H peak area to the change expected for complete trans exchange, it may be readily shown that F = kx/(k, + k,)
(14)
The values of the exchange rate constants given in Table 111 were determined by using eq 13 and 14 from measurements of koM,k,, and F. It can be seen that in 0.1 M DC1/D20 trans H / D exchange is faster than M e 2 S 0 loss. The similarity of ka and k, makes the complex a suitable candidate for competition studies, as discussed in the section on photochemistry. Photochemistry. [ Coen313+. In initial experiments, extensive loss of 'H N M R N-H proton signal in [Coen313+in a stainless steel cell was found to be associated with metal ion catalyzed release of ethylenediamine followed by H/Dexchange of free en. In a glass cell metal ion catalyzed decomposition was removed, but photochemical production of en was still detectable from changes in the area of bands due to free and bound ligand C H protons. The thermal reaction gave no detectable en release. With ~ ]en,32 ~+ the photochemical reaction [Coen3I3+ [ C 0 e n ~ ( D ~ 0 ) + the en release (1.7 f 0.4% over 25 h) corresponds to 5.1 f 1.2% reaction and kL= (5.8 f 1.4) X lO-'s-'. Ligand loss was ascribed to an electronic process. The pumping rate constant calculated from t'(e) (Table I and eq 9) gave the quantum yield for electronic photochemistry @(e)= (6 f 2) X No differences between 59C0 spectra were found between the thermal and irradiated complexes, indicating the absence of H / D exchange within the 0.4% detectivity limit of 59C0N M R bandwidths. This allowed an upper bound to be established for kL, from which the upper bound for any vibrational photochemical reaction was calculated to be @(v) < 7 X The temperature for these experiments was 2.5 "C. [Co(NH3),I3+. For [CO(NH,),]~' there was evidence for H / D exchange, using 59C0spectra as previously described.26 59C0band broadening and peak shift provide a sensitive measure of H / D exchange which is specific to a symmetric complex such as [Co(NH3),13+and unaffected by the presence of other Co complexes. In H / D exchanged samples in H 2 0 / D 2 0 mixtures with low deuterium content the 59C0shift was found to be -20 Hz per 1% exchange and the broadening 19 Hz per 1% exchange. Typical
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difference between thermal and irradiated samples were 10 Hz, corresponding to 0.5% exchange with the reproducibility for peak position f 1 0 H z and for broadening f 6 Hz. This additional exchange is to be compared to the almost undetectable exchange of a thermal sample after a 10-h period at the same temperature. Four runs were conducted, one in the unstirred cell and three in the stirred spectrophotometer cell, with 5 "C as the equilibrium temperature in the irradiated cell and in the control cell. In the stirred cell the calculated and observed temperature increment during irradiation agreed as 4 f 1 O C . The photochemical quantum yield was calculated as described for [Coen,l3+. The vibrational component of the observed absorption coefficient (Table I) gives ~'(v)= (3.4 f 0.2) X m2 mol-' and was the value used in eq 9 to calculate the quantum yield for the vibrational photochemical process of H / D exchange, d(v) = (2 f 2) X [CO(NH,),M~,SO]~+.In the experiments with Nd:YAG laser pumping, solutions of the complex in 0.10 M DC1/D20 were irradiated in a cooled, stirred sample cell. The temperature in the cell measured during irradiation was 6.8 "C, 1.7 OC higher than the cell temperature when the laser was turned off. Laser enhancement of Me2S0 loss was found, with rate constant (eq 8) kk = (1.0 f 0.4) X s-'. This is an order of magnitude higher than the thermal value k, = 1 X lo-, s-' at 7 OC (Table 111). The enhancement was ascribed to electronic photochemistry and gave a quantum yield @(e)= (17 f 7) X 1OW5. A second series of experiments at -2 "C gave @(e) = (14 f 3) X and the best estimate for the quantum yield averaged at the two temTests for NH, loss3, showed peratures is @(e) = (16 & 6) X no free N H 3 in solution indicating that no detectable fraction of the complex (
