ligand Replacement at a Square Planar Metal Center A Kinetic Experiment for the Inorganic Chemistry Laboratory H. Kriiger University of South Africa, P.O. Box 392, Pretoria 0001. Republic of South Africa D. J. A. d e Waal National Chemical Research Laboratoly, Council for Scientific and Industrial Research, P.O. Box 395, Pretoria 0001, Republic of South Africa Ligand replacement in transition metal complexes is an important area of research ( I ) since these reactions are often central to an understanding of the chemical behavior of transition metal complexes in solution and the role of these complexes in homogeneous catalysis. Very few laboratory experiments on ligand replacement have, however, been devised to supplement lectures on inorganic reaction mechanisms for more advanced students. T o fill this need and to provide more practical experience for nur honors students, we have chosen the experiment presented here, which forms part of our research projects (21.I t will, we hope, stimulate the students' interest in the study of reaction kinetics and provide an o.~.~ o r t u n i ttov d e v e l o ~insieht into reaction pathways for transition metal complexes. We have chosen rhe fdlowine svstem in which the lieand triphenyl phosphine is replaced from a rhodium metalcenter by a series of pyridine derivatives: ~~~
[Rh(ccd)L,lf
+ N-N
[Rh(ccd)Lz]++am
-
[Rh(eod)(N-N)LIt + [Rh(cod)(N-N)]' + L [Rh(cod)(arn)Llt L
+
~
(1) (2)
where cod = 1,s-cyclooctadiene, N-N = 2,2'-bipyridine, am = pyridine or 2,6-lutidine, and L = PPh3. This system will introduce students to stopped-flow spectrophotometry as a technique to study the kinetics of venr fast reactions. If a commeriial stopped-flow spectrophotdmeter is not available, afairly inexpensive instrument can be built for student training (3). Since [ R h ( c ~ d ) ( P P h ~ )is~ ] + fairly expensive and requires some time to prepare, i t is advisable to provide the students with samples of this complex prepared by the lecturer. An extremely small amount of complex (less than 30 mg) is needed per student, and the overall experiment will take approximately two days to complete. Experimental Procedure Synthesis of [Rf@cu)(PPhs)2] [PF8] and identification of the Reaction Products The starting complex [Rh(cod)(PPh3)z][PF6] was prepared by reacting 0.5 gof [Rh(cod)Cl]z (as prepared by Chatt ( 4 ) ) with 1.12 g PPh3 for 20 min a t room temperature under Nz atmosphere in 40 mL of a 1:l acetone-methanol solvent mixture containing 1.5 g NH4PF~.Reducing the volume of the solution under reduced pressure (water pump) resulted in the precipitation of [Rh(cod)(PPh3)~][PF6] as an orange powder. The composition of this compound was confirmed bv elemental analvsis. I t is further characterized bv a strnns infrared ahsorption a t 850 cm-I due to the PFs counter i o i and a 31PlHlI NMR sienal (121 MHz. CDCM a t 6 = 27.22 ppm relativeto ~ a ~ ~ ~ ( d o u J& b l e t=, 144?96 Hz) for the two equivalent PPh? ligands. The products of reactions 1 and 2 were confirmed h i comparingthe UV-visible spectra of the reaction products produced in situ in the kinetic 262
Journal of Chemical Educatlon
experiments with the UV-visible parameters of authentic samples of [Rh(cod)(2,2'-hipyridine)][PF6] (A, = 474 nm; r = 4.5 X lo2cm-' mol-I L), [Rh(cod)(PPh3)(pyridine)][C104] (A,. = 514 nm; r = 1.68 X 103 em-' mol-I L) and [Rh(cod)(PPh3)(2,6-lutidine)][CIOa] (A,. = 512 nm; r = 1.5 X lo3 cm-' mol-I L), which were respectively prepared by a literature method (5) and abstracting the halide from [Rh(cod)(PPh3)(C1)](6)with AgC104 followed by the addition of pyridine or 2,6-lutidine. The equilibrium between the four and five coordinate ~ ~ ~ reaction product complexes (5) shown in eq 1is an extremely rapid process (2)and will not influence the kinetics of lieand replacement of [Rh(cod)(PPh3)~]+. Because of this equiEhrium, the absorption peak of [Rh(cod)(N-N)]+ a t X = 474 nm decreases with increasing PPh3. The amount by which the apparent absorption of [Rh(cod)(2,2'-hipyridine)]+ decreases a t a given PPh3 concentration level was exactly reproduced in the final UV-visible spectra of the kinetic experiment& ~
~
Kinetics Solutions containing the appropriate m i n e with concentrations in the range 0.00313-0.2 mol L-I were mixed a t constant temperature with a 2 X mol L-1 solution of [Rh(cod)(PPh&]+ in a stopped-flow spectrophotometer. In the 2,2'-hipyridine study, PPh3 additions were made t o the solution containing [Rh(cod)(PPh3)2]+.Complications due to a fairly slow decomposition of [ R h ( c ~ d ) ( P P h ~ )in~ ]solu+ tion were circumvented hy using only freshly prepared solutions of [Rh(cod)(PPh&lf (aging limited to a maximum of 20 min) and using nitrogen-saturated acetone as solvent. The kinetic experiments were conducted by monitoring the decrease in absorbance of the characteristic absorption peak of [Rh(cod)(PPh&]+ a t X = 440 nm; c = 2.07 X 103 cm-' mol-1 L in the stopped-flow machine. By adhering to pseudo-first-order conditions, that is, keeping the com&ex concentration much lower (at least 10 times) than the amine concentrations, excellent linear plots according to eq 3 were observed: In (A- -A,) = ln (A. -A,) - koWt (3) where the subscripts 0, t, and refer to the absorbance readings at the start of the reaction, a t time t during the reaction, and after completion of the reaction, respectively. Equation 3 is just a more practical form (7)of the more classical equation descrihine a first-order reaction. viz. In C. = In CO- kobdt, where C denotes the specific concentration of the complex undergoing a chemical reaction. The value of kobsd can be calculated from a linear plot of In (A, -A,) vs. t , according to eq 3, or more convenientlv from tr/*= 0.6931 kobad where thehalf life of the reaction & is simpiy read off as the time required for in (A, - A,) to reach a value of in (A, - At)/2. One of the prime objectives of a kinetic study is to determine empirically the rate law that describes the rate behav-
-
~
-
Table 1. Pseudo-First-Order Rate Constants for the Reaction of [Rh(cod)(PPh,),]+ with Pyridine, 2,6-Lutidlne, and 22'-Bipyrldlne in Acetone at 25 ' C [amine].mol L-'
am = py
k . s-' ~ am = 2.6 1ul
am = bipy
ior of the reaction beine studied. If the reaction is first-order in complex concentration, as in the present study, eq R can be used to build fairl\.com~lexrate laws bv determinine the functional dependence of Lobadon all othe; possible concentration variables such as the concentration of the entering and leaving ligands. Rate constants are given inTable 1.The outcome of these kinetic experiments is portrayed in Figure 1,from which the following empirical rate laws can be constructed:
+ bipyl)
kobad,2,? bipyrldino = a[bip~ll(c[PPhd
(6)
where at 25 OC, a = 25 f 1s-1; b = 320 molL-Is-', and c = 26 2. The mathematical form of eq 6 should not be too difficult to establish. To explain the decrease in rate with an increase in [PPhx], a [PPha] term should appear in the denominator, because the rate reaches a limiting value a t high [bipy] and low [PPhx], a [bipy] term should appear in the denominator as well as the numerator of the rate equation. The empirical constants a and c can he determined graphically by linearizing eq 6 by plotting llk,bsd against l/[bipy]. A family of these plots will produce a common intercept of l/a and slope values of c[PPh3]/a.
*
Dlscusslon With the empirical rate law for the 2,2'-bipyridine system known, the student should try to find a matching theoretical rate law. The latter is derived from a proposed (theoretical) reaction scheme. All reaction schemes producing a theoretical rate law that matches the empirical rate law, viz. eq 6, are possible reaction mechanisms. Some of these (ideallv all of them except one) can be eliminated as possiblimechHnisms bv carrvina out subsidiaw experiments that will differentiate between these possibilities as will be discussed here. Three of the most likely pathways for the 2.2'-bipvridine system are shown in ~ i i u r 2.i ~ s s u m i n gsteady state (7,8) kineticst for the intermediates (B), (C), and (D), i.e., d[(B)]/ d t = 0, d[(C)]ldt = 0 and d[(D)]ldt = 0, the following theoretical rate laws can be derived2 for pathways (i), (ii), and (iii), respectively:
'
This is a good approximation if the intermediates are very reactive with their rate of formation equal to their rate of decay. The concentration levels of these intermediates will be vely small during the course of the reaction. if the intermediates (B), (C),or (Dl are less reactive than the starting complex (A),biphasic kinetics (consecutive reactions)will be observed and eQ 3 will not produce linear plots. t Rate law 7, for instance, is derived from the equations: d [ ~ ] l d= k3[B][WN]; kl [A][S] = k2[B] [PPh3] ks[B][N-N] assuming d[B]/dt = 0; [Rhl-, = constant = [A] [B] [C] [PI (mass balance equation) and differentiating the latter with respect to time gives -d[A]/df = d[P]/dtsinced[B]ldf = 0 andd[C]ldt= 0.
I t is clear from the above that pathways (i) or (iii) conform to the empirical observation (rate law eq 6) and that the expected direct attack of 2,2'-bipyridine on the metal center of [ R h ( ~ o d ) ( P P h ~ as ) ~depicted ]+ in pathway (ii) is not operative a t all. The empirical constant a can thus be interpreted either as a = kl[S], where the solvent acts as a nucleophile toward the rhodium atom (the solvent is present in excess, and its concentration is invariant), or a = k7, where a triphenyl phosphine ligand dissociates from the starting complex to ~ r o d u c ea threecoordinate 14-electron intermediate. There are some precedents in the literature to support the existence of highly reactive three-coordinate 14electron complexes (9) derived from four-coordinate complexes. The sign and magnitude of the activation entropy AS* can he used to diagnose an activation process as associative or dissociative and hence provide a means of distinguishing between pathways (i) and (iii) of Figure 2. Bimolecular elementary steps usually display large negative AS* values ranging from -10 to -50 cal K-' mol-' whereas the opposite is true for Figure 1. Reaction rate profiles for the replacement of triphenyl phosphine tram [Rh(codXPPh&]+by the amines unimolecular dissociative pyridine, 2,6lutidine,and 2.2'-bipyridine in acetone. The rate data for the monodemate pyridlnes were obtained wlth s t e p s , t h a t is, positive no free triphenyl phosphine added to the reaction solutions. M represents mol L-'.
+ + +
+
Volume 64 Number 3 March 1987
263
AS* values of +I0 to +50 cal K-1 mol-' (I). The s i m of AS* usually depends on whether a net loss or gain in th;! translational degrees of freedom results on going from the reactants to the activated complex. Although not applicable to the present study, caution should he exercised in interpreting ASX values where solvent freezing or defreezing, due to the creation or neutralization of formal electric charge in the activated complex, occurs (7b).The activation parameters obtained from the kinetic data of Table 2 clearly favor pathway (i) over pathway (iii). The activation energy E, was determined from Arrhenius plots, and AS* was calculated a t T = 298.2 "K with the aid of AH* = E, = RT;AG* = RT In I(hlKT)k)and AG* = AH- TAS*,whereR =gas constant in cal K-' mol-I, h = Planck's constant, K = Boltzmann's constant, and k = kl or k7. Rate laws 4 and 5 can also be explain(?din terms of Figure 2 if N-N = am where am represents pyridine or 2,6-lutidine and [Rh(cod)(PPh3)(am)lt the final reaction product. The amine independent terms of rate laws 4 and 5,graphically represented as thr common intrrcept in Figure I , aresimilar
Table 2. Temperature Dependence of the Empirical Constant a Determined independently for the Reaction of 0.1 mol L-' 2,2'Bipy and 0.1 mol L-' 2,6-Lutidlne with 1.0 X lo-' mol L-' [Rh(cod)(PPh&]+ In Acetone with No PPh3 Added to the Reaction Solutions k~
Temperature "C
= a, s-'
2,Z'-bipy
2.B-lutidine
b PYI dne
assuming k, = d[s]
5 = 6.14 kcai moi-' AS' = -31.88 cai K-' mol-' IS1 =
13.6 mol L-'
(concentration d pure acetone).
in magnitude to the limiting rate of the 2,2'-bipyridine system and therefore describe pathway (i) where step kl[S] is rate limiting. Since very little free PPhe is aenerated. effective compet&.ion with am for i n t e r m e d f a t e i ~ )is not possible, that is, ks[am] >> kz[PPhsl, and, since (B) is more reactive than (A), ks[am] >> kl[S]. The second term of rate law 4 represents pathway (ii) where c = ka = 320 s-I M-' with no contribution by step ks due to the low concentration level of
--
free -- PPh" - - -. 0.
The overall picture that emerees is that the rhodium atom of [ R ~ ( C O ~ ) ( P Pis~excepti&ally ~)~~+ sensitive to steric and electronic properties of the entering nucleophile. Pyridine is reactive enough to compete with the solvent as a nucleophile to produce the classical two-term rate law normally found for square-planar complexes. Although 2,6-lutidine should be a more powerful nucleophile than ~vridine,based on oK. values, excessive steric crowding by the two methyl seems to prohibit the formation of a five-coordinate activated complex with [Rh(cod)(PPh<, thus rendering direct substitution (step k,) inoperative. The solvent pathway is preferred instead where it is easier for the relat&ely small acetone nticleuphile to replace one of the bulky PPhx ligands from IRh(cod)iPPh2~.,17. The nucleunhile 2,2 - b i m ~ i d i n e " also piefers to react exclusively via th; solvent pathway in spite of the fact that it is stericallv much less demandine than 2,6-Intidine. It therefore appkars that the electroni; donor properties of 2,Z'-hipyridine fail to induce sufficient
-.
264
.
Figure 2. Possible Dathwavs tor the reaction of .lRhlcodUPPhd,l+ . .. ... wim 2.2'in acetone. A I of me ind vma lelementatyl reactlon steps involve associative actl~aton via a fivesoordinateact valed comp ex. A fa~r-cwtdt"ate activated comp ex s implgl tor steps k , , ke, ana kg, nowever. Step k , r the only elementary step with a dissociative mode of activation. The reaction product (PI is actually an equilibrium mixture, as indicated in eq 1.
Assuming k, = a
Jou'rnal of Chemical Education
..
reactivity to offset the statistical concentration advantage of acetone, which traditionally ( l a ) reacts as a weak nucleophile towards square-planar d8 complexes. The study presented here also demonstrates that by including the concentration of the leavina lieand as a rate variable, a more complex rate law is obta&edfor the solvent path from which more kinetic information can be gleaned. Competition between theleaving ligand PPh3 and the entering ligand 2,2'-bipyridine for reaction with the solvent intermediate (B) can be detectedand the competition ratio established as kzlks = 26. The role of the leaving ligand in the solvent pathway of square planar transition metal complexes is not well documented in the literature. A few systems (lo), worth mentioning are [Rh(cod)P(cy~lohexyl)~Cl] [PtC13(SMez)]- (11) and [Pd(diene)Br]+(12). The experiments presented in this paper orovide an illustration of closely refated reactions that have entirely different rate laws. It is also a good illustration that stoichiometrv and similarity in reactants are never reliahle guides to reacStion rate laws. Literature Cited
York. L970Vnl. 13. n 2 R 1
7. is1 Frost, A. A ; Pearson. R. G.K i n ~ l i r sand Mochonism, 2nd 4.;WilW New York, 1961;p49. (bl Moore, W.J.;Pearson,R. G.Kineficaond Mechonism,3ded.;Wiley: N*Wyark, 1981. (c) Wilkinson, F. Chemical Kineties and Ra=efion Mechanisms; Van Nmtrand Reinhold: New York, 1980; Chapter 4. 8. Pyun,C. W. J. ChamEduc. 1971,48,194. 9. (a1 Halpern, J.; Wong, C. S. J. Chem. So
