1882
J . Am. Chem. SOC.1984, 106, 1882-1883
Thermal, Photochemical, and Electrochemical Reactions Involving Binuclear Platinum(I1) and -(III) Pyrophosphite Complexes. Reaction Chemistry of Pt2(P2O5H2):- and the Halide Complexes Pt2 (P205H2)4X24-
Table I. Thermal and Photochemical Halidc Substitution Rcactions of P t z ( P z 0 , H , ) 4 X 2 4 -with Halides Y /I
complcx (25 phi)
halide (1 inM)
conditio& thermal
Samuel A. Bryan,la Mark K. Dickson,Ib and D. Max Roundhill*'"
I1 v
therm a1 17 v
Department of Chemistry, Tulane University New Orleans, Louisiana 701 18 Department of Chemistry Washington State University Pullman, Washington 99164 Received December 7 , 1983 The bridged binuclear complex Pt2(P205H2)44(P205H22-= HO(0)POP(O)OH2-) has recently attracted interest because of its distinctive spectroscopic Halogen addition across this biplatinum(I1) complex yields axially substituted biplatinum(II1) compexes Pt2(P20,H2),X2- (X = C1, Br, 1).lo With only small added quantities of halogen, the mixed-valence Pt"Pt"' complexes Pt2(P205H2)4X4can be obtained in the solid state.!' A simple molecular orbital description of the bonding proposes the orbital occupancies to be Pt'IPt" ( la,,)2( 1a2J2, Pt"Pt"' (1a1,)2(1a2u)', and Pt"'Pt"' (where lal, and la2u are d a and da*), and the electronic transitions between da* pa and d a da* are assigned on the basis of this model.12 No reaction chemistry has yet been published interrelating these Pt"Pt", Pt"Pt"', and Pt"'Pt"' complexes. This communication for the first time describes the solution reactions of these binuclear Pt(II1) complexes, and shows how this chemistry can be related to the spectroscopic models. The compounds Pt2(P205H2)44and Pt2(P2O5H2),Cl,4-are dibasic acids with pKI = 3.10 and 4.95 and pK2 = 6.75 and 7.55, respectively. The monodeprotonated complexes are stable in aqueous solution, but the doubly deprotonated complexes rapidly deposit metallic platinum from solution. Charge differences affect substitution rates since Pt2(P2O5H2),Cl,4-(pH C4; phosphate buffer and constant ionic strength) does not undergo significant substitution of C1- for Br- or I-, but Pt2(P205H2)3(P,05H)X25(X = C1, Br, I) will undergo replacement at pH = 6.5 of X- for Y- (25 OC; X = C1, Y = Br, I; X = Br, Y = I; X = I, Y = Br).I3 This selective axial substitution is followed spectroscopically by changes in the (lalg)1(la2,,)1 (da do*) transitions at 282 (X = Cl), 305 (X = Br), and 338 nm (X = I).1o No mixed halide complex Pt2(P2O5H2),(P2OsH)XY5-is formed. Halide substitution in Pt2(P205H2)4X,4is photochemically accelerated. Comparative thermal and photochemical data are
thermal I1 v
thcrnial I1 v therinal 11 v
thermal /I
25 "C, pH 1.
pH 6 .
u
time
substitiition
75 75 0
200 min 70 s 3h 60 s
60
12 h 1 2 niin 1 2 11 1 2 min
75 4 60
3h 2 min 3d 15 min
I
3 95 0 100
[Cl-1, [ Br-] = 1 0 0 inhl.
-
-
-
-
(1) (a) Department of Chemistry, Tulane University, New Orleans, LA 70118. (b) Shell Development Co., Houston, TX 77001. (2) Sperline, R. P.; Dickson, M. K.; Roundhill, D. M. J . Chem. SOC., Chem. Commun. 1977, 62-63. (3) Filomena Dos Remedios Pinto, M. A,; Sadler, P. J.; Neidle, S.; Sanderson, M. R.; Subbiah, A,; K u r d a , R. J. Chem. SOC.,Chem. Commun. 1980, 13-1 5. (4) Fordyce, W. A.; Brummer, J. G.; Crosby, G. A. J. Am. Chem. SOC. 1981, 103, 7061-7064. ( 5 ) Che, C.-M.; Butler, L. G.; Gray, H. B. J . Am. Chem. SOC.1981,103, 7796-7797. (6) Rice, S. F.; Gray, H . B. J . Am. Chem. SOC.1983, 105, 4571-4575. (7) Stein, P.;Dickson, M. K., Roundhill, D. M. J . Am. Chem. SOC.1983, 105, 3489-3494. (8) Che, C.-M.; Butler, L. G.; Gray, H. B.; Crooks, R. M.; Woodruff, W. H . J . Am. Chem. SOC.1983, 105, 5492-5494. (9) Dickson, M. K.; Pettee, S. K.; Roundhill, D. M. Anal. Chem. 1981, 53, 2159-2160. (10) Che, C.-M.; Schaefer, W. P.; Gray, H. B.; Dickson, M. K.; Stein, P.; Roundhill, D. M. J . Am. Chem. SOC.1982, 104, 4253-4255. (11) Che, C.-M.; Herbstein, F. H.; Schaefer, W. P.; Marsh, R. E.; Gray, H. B. J . Am. Chem. SOC.1983, 105, 4604-4607. (12) Mann, K. R.; Gordon, J. G., 11; Gray, H. B. J . Am. Chem. SOC.1975, 97, 3553-3555. (13) The complexes K4[Pt2(P205H2)4C12] (ref 11) and (n-Bu4N)4[Pt2(P205H2)4X2](X = Br, I) have been structurally characterized (Rheingold, A., personal communication).
0002-7863/84/1506-1882$01.50/0
collected in Table I. All replacements show photoenhancement, and the replacement of I- by C1- shows 100% photochemical substitution in 15 min as compared to no thermal reaction after 3d (eq 1). These photochemical reactions use Pyrex glassware
and for the case of X = I a sharp cutoff filter (A > 335 nm).14 These photochemical substitutions, which show a minimum enhancement of 200 over the thermal rate, are selectively axial and proceed via the observable XY intermediate under pH 1 conditions. The thermal reactions of Pt2(P205H2)4X$-resemble those of the kinetically inert monomeric platinum(1V) complexes. Two explanations are possible to explain the photoenhancement of substitution. Irradiation in the da da* chromophore will produce a 17e-17e- diradical (lal,)l(la2u)l excited state, and if the triplet state is sufficiently long-lived it may show substitution lability analogous to monomeric 17e- radi~a1s.I~The more likely reason, however, is that the excited state involves X Pt(II1) LMCT character, resulting in a photoinduced Pt'IPt"' intermediate Pt,(P2O5H2),X4-.l6 Lability in such an intermediate would explain our observed photoenhancement. This pathway parallels that proposed by Taube for platinum(1V) ~omp1exes.l~Photoinduced lability of platinum(1V) complexes proceeds via platinum(II1) intermediates, which are formed in a chain process with quantum yields greater than unity.18 Quantum yield measurements in progress on substitutions in Pt2(P20,H2)4X2"should help decide the electronic character of the excited state. Halogens Y2 react with Pt2(P205H2)4X24to exchange X for Y if Y = C1, Br and X = I, or if Y = C1 and X = Br, I. The reverse reactions involving oxidation of Y- by X2 are thermody-
-
-
(14) The apparatus uses an Illumination Industries 200-W mercury lamp in a fan cooled Ealing Corporation housing. Wavelength selection is made with either thick Pyrex glass or with a Schott Corp. sharp cutoff filter. (15) Brown, T. L. Ann. N . Y. Acad. Sci. 1980, 333, 80. (16) We inherently assume hydration may occur at axial positions in these intermediates. (17) Rich, R. L.; Taube, H. J . Aam. Chem. SOC.1954, 76, 2608-2611. (18) Adamson, A. W.; Sporer, A. H. J . Am. Chem. SOC. 1958, 80, 3865-3870. Dreyer, R.; Konig, K.; Schmidt, H. Z . Phys. Chem. (Leipzig) 1964, 227,257-271. Dreyer, R. Z . Phys. Chem. (Frank urt Main) 1961.29, 347-359. Preliminary quantum yields on these Pt"'Pt complexes indicate that Q > I .
f '
0 1984 American Chemical Society
1883
J . Am. Chem. SOC.1984, 106, 1883-1884
namically unfavorable. Scheme I shows the thermal replacement reactions that occur with halogens and halides. If small quantities of Y2 are added to Pt,(P20,H2),X$- the initial formation of Pt,(P205H2),XY4- can be verified (eq 2). With interhalogens
-
Pt2(P205H2),X,4-
y2
y2
Pt2(P205H2)4XY4Pt2(P205H2)4Y24- (2)
(XU) and Pt2(P205H2)44the first product is Pt2(P205H2)4XY4(XU = CH3I,I0ICI, IBr, CNBr, CNI), but if excess XY is added the major product is Pt2(P205H2)4Y24(eq 3).19 No evidence
CNY
Pt2(P2O~H2)4~-
-+
(CN)2 (3)
+ Y,
PH 1
Pt2(P205H2)4XY4+ Y(4)
At this low pH the solutions are stable since [Y-] is low and the substitution rate is slow. Respective ,,A are ClBr 298 nm (e 5.6 X lo4),CII 313 nm (e 4.1 X lo4),BrI 316 nm (e 5.2 X IO4). The 31PN M R spectra correspond with the XY compounds formed by eq 2. Although reversible electrochemistry has not been observed because of electrode adsorption, chemical reduction of Pt2(P205H2)44-to Pt2(P205H2)46occurs with Cr(II).22 Electrochemical oxidation of a solution of Pt2(P20,H2)44and X- (X = CI, Br, I) at pH 1-2 with a Pt gauze electrode at 0.8 V vs. Ag/AgC1 gives Pt(P205H2)4X2,4-.At 0.0 V, or with added H,, H3P02,or Zn/Hg, the reaction is reversed. For conditions where we find Eo (X = C1) = [Pt2(P205H2)44-] = [Pt2(P205H2)4X24-], 0.20, Eo (X = Br) = 0.066, and Eo (X = I) = -0.146 V vs. SCE. This low potential for oxidation of Pt2(P2O5H2)?-correlates with electron loss from a du* HOMO. The one-electron oxidants Ce4+ and IrCI6'- can also be used to effect this oxidation (eq 5 ) , and
+
-e-, xPt2(P205H2)d4- ePt2(P205H2)4X4e-.-x-
Acknowledgment. We thank Pinky Tivari for experimental assistance. We thank the Boeing Co for financial support to purchase the Nicolet 200-MHz N M R spectrometer (WSU).
CNY
Pt2(P2O5K2),CNY4Pt2(P205H2)4Y,4-
is found for Pt2(P205H2),X2(X = CH,, CN, I (from ICI, IBr)), which correlates with Eo(Cl2/C1-) > Eo(Br2/Br-) > Eo(I2/1-) > Eo((CN)2/CN-). Using techniques of Pt(1V) chemistry,20we can use a novel method to prepare stable aqueous solutions of the mixed complexes Pt2(P205H2),XY4-(X = C1, Y = Br, I; X = Br, Y = I) in high yield. This complementary redox process2' involves treating an aqueous mixture of halide (X-) and Pt2(P205H2)44at low pH with a small quantity of halogen Y2 (X = C1, Y = Br, I; X = Br, Y = I) (eq 4). Pt2(P2O5H2)2-+ X-
Since solutions of Pt2(P205H2)4X4rapidly disproportionate' I the product Pt2(P205H2)4X2 can result either from this reaction or from transfer of a second electron to the oxidant followed by halide ion capture. We are currently doing kinetic measurements and quantum yield experiments to mechanistically probe these reactions.
-e; e
x, -x-
Pt2(P205H2)4X?- (5) a Ce4+ titration verifies that n = 2 for the oxidation.23 If we assume an initial I-electron process, removal of a la2,, (du*) gives Pt2(P205H2)43-, which with electron from Pt2(P205H2)44excess X- will form the mixed-valence complex Pt2(P2O5H,),X4-, (19) For Pt2(P205H2),CNBr4-: A,, = 279, 344 nm; u(CN) = 2152.6 cm-l: 31PNMR 6 26.08 (1J(195Pt31P) = 2230 Hz), 16.58 (1J(195Pt31P) = 1993 Hz); 19'Pt NMR -4593 (1J(195Pt31P) = 2224, 2J(19sPt3'P)= 102 Hz), -4101 = 1988, 2J(195Pt31P) = 67 Hz). For Pt,(P20sH ),CN14': A,, (1J(19SPt31P) = 294, 356 nm; v(CN) = 2151.7 cm-I; 3 ' P N M R 6 19.96 (1J(193Pt31P) = 2219 Hz, 15.40 (1J(19SPt31P) = 2006 Hz); I9'Pt NMR 6 -5087 ('J(19sPt3'P) = 2218, 2J(195Pt31P)= 92 Hz), -4028 (1J(195Pt3'P) = 1934, 2J(195Pt31P) = 57 Hz). For Pt2(P2O5H2),BrCI4-:"P NMR 6 27.42 (1J('95Pt31P)= 2151 Hz), 24.38 ('J('95Pt31P)= 2153 Hz). For Pt2(P2O5H2),,ICI4-:"P N M R 6 26.47 (IJ(195Pt3'P)= 2236 Hz), 19.46 (1J('95Pt"P) = 2175 Hz). (20) Peloso, A. Coord. Chem. Reo. 1973, I O , 123-181. (21) Chanon, M.; Tobe, M. L. Angew. Chem., I n t . Ed. Engl. 1982, 21, 1-23. (22) Alexander, K., A.; Stein, P.; Hedden, D. B.; Roundhill, D. M. Polyhedron 1983, 2, 1389-1 392. (23) Since the Ce4+oxidation of CI- is kinetically slow (see: Skoog, D. A,; West, D. M. 'Fundamentals of Analytical Chemistry", 3rd ed.;Holt, Rinehart and Winson: New York, 1976; p 347), we can discount the pathway where CI- is oxidized rather than Pt,(P205H2),4-.
0002-7863/84/ 1506-1883$01.50/0
Virtual Transition State for the Acylation Step of Acetylcholinesterase-Catalyzed Hydrolysis of o -Nitrochloroacetanilide' Daniel M. Quinn* and Michael L. Swanson Department of Chemistry, University of Iowa Iowa City, Iowa 52242 Received November 18, 1983 Acetylcholinesterase (AChE) catalysis' occurs via an acylenzyme mechanism involving nucleophilic attack by serine on the substrate, with general acid-base assistance by histidine. Rosenberry2x3suggested that small solvent deuterium isotope effects (
