Inorg. Chem. 1983, 22, 541-546
541
Contribution from the Institute for Physical Chemistry, University of Frankfurt, 6000 Frankfurt/Main, West Germany
Pressure Effects on Photoisomerization/Photosubstitution Reactions of the Rhodium(II1) Complexes cis- and trans-Rh(NH,),XYn+ (X = CI, Br; Y = X, H20) L. H. SKIBSTED,*la W. WEBER, R. VAN ELDIK,* H. KELM, and P. C. FORD*Ib Received May 25, 1982 The effect of pressure up to 200 MPa was studied on the photoisomerization and/or photoaquation reactions of a series - R, h ( N H ~ ) ~ ( H ~(X o )=x ~C1,+ Br), and of tetraamminerhodium(II1) complexes, viz., cis- and t r a n ~ - R h ( N H ~ ) ~~Xi ~s + tran~-Rh(NH~)~(0H)Cl+. The volumes of activation were found to range between -8.8 and +9.3 cm3mol-' for the different reactions, but most systems displayed relatively small absolute values. Partial molar volumes were measured for almost all reactant and product species in their ground state, and from these, reaction volume profiles were calculated. These data are discussed in terms of proposed mechanisms for the excited-state substitution reactions, and it is concluded that the activation volumes observed are consistent with a dissociative pathway for ligand labilization.
+
Introduction
hu
t r u n ~ - R h ( N H ~ ) ~ XH~2+0 t r ~ n s - R h ( N H ~ ) ~ ( H ~ o )XX ~(1) +
+
The cis/trans photoisomerization reactions of transitionmetal complexes in general, and those of rhodium(II1) in hv ~ i s - R h ( N H ~ ) ~ xH~2+0 particular, have received significant attention during recent year^.^-^ Several closely related theoretical models6ss-10have t r a n ~ - R h ( N H ~ ) ~ ( H ~ 0 )XX ~(2) + been proposed to account for the stereochemical rearrangements observed when hexacoordinate d6 complexes are subC ~ S - R ~ ( N H , ) ~ ( H ~ O ) Xt ~r a+n ~ - R h ( N H ~ ) ~ ( H ~ 0 ) X ~ + jected to ligand field (LF) excitation. The key premise of these (3) models is that ligand dissociation from the hexacoordinate excited state occurs prior to stereorearrangement. The product Recent studies on the effect of pressure on some typical stereochemistries are then principally interpreted in terms of photosubstitution reactions of Cr(III),13-15 Rh(III),16 and the comparative energies of the resulting square-pyramidal C O ( I I I ) ' complexes ~ have provided some interesting mecha(SPY), five-coordinate apical and basal intermediates (A* and nistic insight. In this context the present article is concerned B*) as shown in Scheme I. These intermediates were proposed with the effect of pressure on some typical photoisomerizato be triplet excited states capable of isomerization on a time tion/photoaquation reactions of disubstituted tetraamminescale competitive with deactivation followed by trapping of rhodium(II1) complexes in an effort to determine the volumes a solvent molecule. of activation for such processes. In addition, a series of partial An important aspect of these models is that the stronger molar volumes were measured to enable the construction of a-donor ligand should show a strong site preference for the reaction volume profiles1*for model limiting reaction mechbasal position in the SPY intermediate. In this way, various anisms. Although the expansion of the reactant molecules cis to trans as well as trans to cis photoisomerization processes during e x c i t a t i ~ ncomplicates '~~~~ the interpretation of the could be predi~ted.~,"In addition, recent observations12 on volumes of activation in terms of these volume profiles, the the photoisomerization of cis- to t r ~ n s - R h ( N H , ) ~ ( H ~ 0 ) C l ~ +data do provide guidelines as to how volume changes should indicate that water exchange occurs with the same quantum be related to the nature of the intimate reaction mechanism. yield as the isomerization, consistent with the model's reExperimental Section quirement that ligand photolabilization precede isomerization of the coordination sphere. The success of these models Materials. The tetraamminerhodium(II1) complexes cis-[Rhprovides strong circumstantial evidence in support of a limiting B~-'/~H~O, (NH3)4C12]CI.'/2H20,21 c ~ s - [ R ~ ( N H ~ ) ~ B ~ ~ ]transdissociative mechanism for the LF photosubstitution reactions [Rh(NH3)4C12] C1,22trans- [Rh(NH3)4Br2] Br,22cis- and trans-[Rhof d6 complexes. Notably, the dissociative model postulates cis- and trans- [Rh(NH3),(H20)Br]S206,6 (NH3)4(H20)C1]Sz06,6 and ~ i s - [ R h ( N H ~ ) ~ ( H(c104)323 ~ 0 ) ~ 1 were all prepared and recrysa common intermediate, viz., the five-coordinate apical Rhtallized according to published procedures. UV-visible spectra were (NH3)4X2+excited-state species for each of the photoreactions in agreement with earlier published data. All other chemicals were (1)-(3) (X = C1, Br). of analytical reagent grade, and doubly distilled water was used throughout the investigation.
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(a) Department of Chemistry, The Royal Veterinary and Agricultural University, Copenhagen V, Denmark. (b) Department of Chemistry, University of California, Santa Barbara, CA 93106. Strauss, D.; Ford, P. C. J . Chem. SOC.,Chem. Commun. 1977, 194. Ford, P. C. Coord. Chem. Reu. 1982, 44, 61. Martins, E.; Sheridan, P. S . Inorg. Chem. 1978, 17, 3631. Martins, E.; Kaplan, E. B.; Sheridan, P. S. Inorg. Chem. 1979,18, 2195. Skibsted, L. H.; Strauss, D.; Ford, P. C. Inorg. Chem. 1979, 18, 3171. Skibsted, L. H.; Ford, P. C. Inorg. Chem. 1980, 19, 1828. Vanquickenborne, L. G.; Ceulemans, A. Inorg. Chem. 1978,17, 2730. Purcell, K. F.; Clark, S . F.; Petersen, J. D. Inorg. Chem. 1980, 19, 2183. Petersen, J. D. Inorg. Chem. 1981, 20, 3123. Skibsted, L. H.; Ford, P. C. J . Chem. SOC.,Chem. Commun. 1979,853. Skibsted, L. H.; Mansted, L., submitted for publication in Acta Chem. Scand., Ser. A.
0020-1669/83/1322-0541$01.50/0
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(13) Angermann, K.; van Eldik, R.; Kelm, H.; Wasgestian, F. Inorg. Chem. 1981, 20, 955. (14) Angermann, K.; van Eldik, R.; Kelm, H.; Wasgestian, F. Inorg. Chim. Acta 1981, 49, 247. (15) Angermann, K.; Schmidt, R.; van Eldik, R.; Kelm, H.; Wasgestian, F. Inorg. Chem. 1982, 21, 1175. (16) Weber, W.; van Eldik, R.; Kelm, H.; Dibenedetto, .I.Ducommun, ; Y.; Offen, H.; Ford, P. C. Inorg. Chem., in press. ( 1 7) Kirk, A. D.; Porter, G. B. J . Photochem. 1981, 17, 170. (18) Palmer, D. A.; Kelm, H. Coord. Chem. Rev. 1981, 36, 89. (19) Wilson, R. B.; Solomon, E. I. J . A m . Chem. SOC.1980, 102, 4085. (20) Hakamata, K.; Urushlyama, A.; Kupka, H. J . Phys. Chem. 1981, 8 5 , 1983. (21) Hancock, M. P. Acta Chem. Scand., Ser. A 1975, A29, 468. (22) Poe, A. J.; Twigg, M. V. Can. J . Chem. 1972, 50, 1089. (23) Skibsted, L. H.; Ford, P. C. Acta Chem. Scand., Ser. A 1980, A34, 109.
0 1983 American Chemical Society
542 Inorganic Chemistry, Vol. 22, No. 3, 1983
Skibsted et al.
Scheme I N
Table 1. Partial Molar Volume Data for Some Disubstituted Tetraamminerhodium(II1) Complexes in Aqueous Solution at 25 "C
r
partial molar vol, cm3 mol-' N
N
8asal*
Apical
~
ts
N
*
( R trans)
*
NA; "4: :--& 1
I
t +s
1kn
N
N
N
S
N
N
P trans
R trans
P PRODUCT
N NH3
R
S
N P
CIS
REACTANT
complex
cis- [ Rh(NH,),CI,]Cl Pans-[Rh(NH3),C1,]C1
complex
108.9 f 0.3 129.4 0.4 cis-[Rh(NH,),(OH,)CI]S,O, 149.2 f 0.4 trans-[Rh(NH3),(OH,)C1]S,06 151.7 1.0 cis- [ Rh(NH,),Br, ]Br 125.5 5 0.8 trans-[ Rh(NH3) ,Br ] Br 142.2 f 1.0 C ~ ~ - [ R ~ ( N H ~ ) , ~ O H , ) B ~ ] S , O156.8 , k 0.8 t~an~-[Rh(NH3),(OHZ)Br]SzO, 157.3 i 0.9
* +_
cationa 87.1
f
0.3b
107.6 k 0.4b
*
83.0 1.5' 85.5 f. 1.8' 96.1 f 0.8d 112.8 l.Od 90.6 i 1.7' 91.1 t 1.7'
*
Single-ion volumes calculated on the basis that F ( H t ) = -4.5 cm3 mol-' .la v ( C I - ) = 21.8 cm3 mol-' v(S,06 ,-) = 66.2 f 1.5 cm3 mol'', determined in this study. v(Br-) = 29.4 cm3 mol-'
SOLVENT
stereochemical orientation on the partial molar volumes and Instrumentation. The photolysis was carried out in a thermostated enable the calculation of overall reaction volume data. The (25.0 h 0.2 "C) high-pressure cell24situated inside a black metal experimental results are summarized in Table I along with the compartment placed on top of a magnetic stirrer. Three-milliliter partial molar volumes of the ionic species calculated according (determined at ambient pressure) quantities of test solution were to standard procedures.28 It is seen that the values for cisirradiated with use of a "pillbox" cell25and vigorously stirred during and t r a n ~ - R h ( N H ~ ) ~ ( 0 H ~are ) C lalmost ~ + equal and very the photolysis with the aid of a 2 X 6 mm Teflon-coated magnetic close to the value of 82.3 f 0.5 cm3 mol-' reportedz9 for the bar. Light at 366,405, or 436 nm was selected from a high-pressure R h ( N H 3 ) Q 2 + species. A similar agreement exists for the mercury lamp (Osram HBO 100/2) using Oriel interference filters. corresponding bromo complexes, whose values also agree very The light beam was focused with the aid of quartz lenses through a well with the value of 91.5 f 1.6 cm3 mol-' reported29for the pinhole into the high-pressure cell. A PRA TX7 electronic feedback system, coupled to a PRA M302 Rh(NH3)5Br2+species. These tendencies illustrate the conlamp power supply, was employed to control the light flux by monsistency of the data in Table I. itoring a small constant fraction of the light beam split off with a quartz Second, the pressure dependencies of the photochemical plate. The light transmitted through the photolysis cell was monitored quantum yields for a series of ligand substitution processes, with a calibrated PIN 25 photodiodez6connected to a dc amplifier of which some are stereoretentive and others stereomobile, were and a strip chart recorder. determined, and the results are summarized in Table 11. In UV-visible spectra were recorded on a Perkin-Elmer 5 5 5 specaddition, the effect of pressure on the course of the phototrophotometer. pH measurements were performed on a Radiometer chemical reactions under consideration was also studied, and Model PHM 64 instrument using a combination glass electrode. the findings can be summarized as follows. Partial molar volumes were determined from density measurements on a Paar DMA 02/C digital precision density apparatus at 25.00 cis- and f"-Rh(NH3)4(&+ and C~S-IUI(NH~)~(H~O)CI~+. f 0.002 o c . Ligand field photolyses of these three complex ions in dilute Calculation of Quantum Yields. Photolysis experiments were acidic aqueous solution at ambient pressure are known to give performed at five different pressures (including ambient or very close the common principal product tr~ns-Rh(NH,)~(H~0)Cl~+.~~~ to ambient, i.e.
