Organometallics 1995, 14, 2806-2812
2806
Aqueous Organometallic Chemistry: Structure and Dynamics in the Formation of (~5-Pentamethylcyclopentadienyl)rhodium Aqua Complexes as a Function of pH Moris S. Eisen,*>'Ariel Haskel,+Hong Chen,+Marilyn M. Olmstead,§ David P. Smith,+Marcos F. Maestre,+and Richard H. Fish*s+ Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720, Department of Chemistry, Technion-Israel Institute of Technology, Haifa, 32000, Israel, and Department of Chemistry, University of California, Davis, California 9561 6 Received February 22, 1995@ The structures of the (~5-pentamethylcyclopentadienyl)rhodium aqua complexes, as a function of pH, were studied by 'H, 13C,170,and 2D NOESY NMR spectroscopic techniques as well as by FAB mass spectrometry and potentiometric titration. The starting complex for our NMR experiments, [Cp*Rh(H2O)&OW2, 1,was structurally characterized by sin lecrystal X-ra crystallography [130 K, Mo Ka radiation, A = 0.710 73 A, a = 23.979(9),! b = 9.726(4) c = 18.257(6) A, 2 = 8, orthorhombic, space group Pna21, 3879 independent reflections, R = 0.0482, R, = 0.10621. Both 'H and 13C NMR titration experiments of the starting complex, 1, were performed by dissolving 1 in H2O (D2O) and obtaining spectra from pH 2-14. From pH 2-5 only one Cp* signal (lH NMR, 1.57 ppm; 13CNMR, 5.78 ppm) was observed, which was attributed to 1. As the pH of the solution with 1was increased from 5 to 7, a dynamic and rapid equilibrium was observed to provide putative [Cp*Rh@OH)(H20)12(0Tf)2,2, and [(Cp*Rh)2@-OH)31(0TE/OH), 3; unfortunately, only one lH or 13C NMR signal for Cp*Rh a t 1.50 (Cp*) or 5.41 ppm (C-CHd, respectively, was found for the latter two species, with broadening of the signals a t pH 5.5-6, indicating that conversion from putative 2 to 3 was very fast on the NMR time scale. As the pH was further increased from 7 to 10, only the lH or 13C NMR signal for 3 was observed at 1.50 or 5.41 ppm, respectively. In addition, starting the equilibrium from 3 (3* 1via putative 2) within the pH range 14-2 provided similar results. The 2D NOESY NMR exchange phasing experiments a t pH 5.8 and 11 showed correlations between the Cp* CH3 groups and the H 2 0 or p-OH groups attached to Rh and between both Cp* CH3 groups of the Cp*Rh aqua complexes, although separate signals for bulk H20 and p-OH or H20 ligands bonded to Rh were not observed due t o a rapid exchange process. A potentiometric titration study gave further evidence that the conversion of 1 3 via putative 2 occurs rapidly with only one pKa of 5.3 being observed, reaffirming the fact that the conversion of 1 3 via putative 2 was extremely fast. The pseudo-first-order rate of conversion of 1 3 a t pH 5.8 was measured by an NMR spin population transfer technique to be k1 = 7.18 s-l (1,0.034 M; 2'1 = 1.6 s), while k-1, 3 1,was found to be 2.93 s-l (2'1 = 1.5 s). The equilibrium constant, Keq,at pH 5.8 for 1 3 was found to be 353. 170NMR studies again showed that H20 molecules bonded to Cp*Rh and those in the bulk solution are in very fast exchange (k > 8150 s-l).
1,
-
-
--
Aqueous organometallic chemistry is a relatively new Moreover, from those results with DNA/RNA biological ligands, we felt it necessary to understand the and exciting area that has primarily focused on catalysis structure and dynamics of the (y5-pentamethylcyclostudied and, to a lesser extent, on the reactions of DNA/ RNA nucleobases, nucleosides, nucleotides, and oligo(2) (a) Smith, D. P.; Baralt, E.; Morales, B.; Olmstead, M. M.; nucleotides,2as well as amino acids (MeOH ~ o l v e n t ) , ~ Maestre, M. F.; Fish, R. H. J . Am. Chem. SOC.1992, 114, 10647. (b) aniline and phenol derivative^,^ and other biological Smith, D. P.; Olmstead, M. M.; Noll, B. C.; Maestre, M. F.; Fish, R. H. Organometallics 1993, 12, 593. (c) Smith, D. P.; Kohen, E.; Maestre, system^.^ Our entry into this new area of organomeM. F.; Fish, R. H. Inorg. Chem. 1993,32,4119. (d) Smith, D. P.; Griffin, tallic chemistry was instigated by our recent studies of M. T.; Olmstead, M. M.; Maestre, M. F.; Fish, R. H. Inorg. Chem. 1993, the reactions of a (y5-pentamethylcyclopentadieny1)- 32, 4677. (e) Kuo, L. Y.; Kanatzdis, M. G.; Sabat, M.; Tipton, A. L.; Marks, T. J. J . Am. Chem. SOC.1991,113,9027 and references therein. rhodium aqua complex(es)with DNA/RNA nucleobases, (0 Presented a t an Advanced NATO Workshop on Aqueous Organometallic Chemistry and Catalysis, Horvath, I., Joo, F., Co-Chairmen, nucleosides, and nucleotides in aqueous solution.2a-d ~
* To whom correspondence should be addressed. ' Technion.
Lawrence Berkeley Laboratory. University of California, Davis. @Abstractpublished in Advance ACS Abstracts, May 1, 1995. (1)(a) Barton, M.; Atwood, J. D. J. Coord. Chem. 1991,24, 43. (b) Hermann, W. A.; Kohlpaintner, C. W. Angew. Chem., Int. Ed. Engl. 1993,32, 1524. 4
Debrecen, Hungary, August 29-September 1, 1994. (3) Kramer, R.; Polborn, K.; Robl, C.; Beck, W. Inorg. Chzm. Acta 1992, 198-200, 415 and references therein. (4) (a) Nutton, A,; Maitlis, P. M. J . Chem. SOC.,Dalton Trans. 1981, 2335. (b) Nutton, A,; Maitlis, P. M. J . Chem. SOC., Dalton Trans. 1981, 2339. (c) Espinet, P.; Bailey, P. M.; Maitlis, P. M. J . Chem. SOC.,Dalton Trans. 1979, 1542. (5) (a) Ryabov, A. D. Angew. Chem., Int. Ed. Engl. 1991,30, 931. (b) Jaouen, G.; Vessieres, A,; Butler, I. S. Acc. Chem. Res. 1993, 26, 361.
0276-733319512314-2806$09.00/0 0 1995 American Chemical Society
Aqueous Organometallic Chemistry
Organometallics, Vol. 14, No. 6, 1995 2807 Table 1. Crystallographic Data for 1 compound
v, A3
[Cp*Rh(Hz0)3l(OTDz CizHziFsOgRhSz 590.31 23.979(9) 9.726(4) 18.257(6) 4258(3)
Z
8
cryst syst space group
Pna21
formula fw
A
a, b, A C,
A
orthorhombic
T,K 1,A
130 (2) 0.710 73 1.377 9.4 0.95-0.98 0.0482 0.1062
cl(calcd), g/cm3
p(Mo Ka), cm-l range of transmissn factors R RW
Figure 1. ORTEP drawing of the cationic portion of complex 1 (see Table 3 for selected bond distances and angles).
pentadieny1)rhodium aqua complex(es)as a function of pH.2f In our perusal of the literature, we found that several studies on these Cp*Rh aqua complexes had been reported, the most notable being the pioneering studies of Maitlis and co-workers. They isolated and characterized by X-ray crystallography a product from the hydrolysis of [Cp*RhC1232 at pH 14, [(Cp*Rh)z@OH)3](0H), 3, and also obtained, in another reaction, a complex with an empirical formula of [Cp*Rh(SO& (HzO)& This was thought to be the tris aqua complex [Cp*Rh(H20)3I2+,1; however, attempts t o fully characterize this material were not successful.6 Kolle and coworkers also speculated on this same complex as a lowpH structure, 1, and reported a pK1 value of 3.6.7a Neither of those reported ~ t u d i e s ~focused , ~ " on the structure and dynamics of the Cp*Rh aqua complexes as a function of pH, and, to reiterate, the relationship between the putative low-pH structure, 1, and the highpH structure, 3, defined by X-ray crystallography, has never been unequivocally established; it may be important to state at this time that generally very little is known about the structure and dynamics of organometallic aqua c ~ m p l e x e sin~ ~ comparison ~ ~ ~ , ~ to what has been reported for several inorganic aqua complexes.8 Therefore, in lieu of the previously reported fragmentary results concerning the Cp*Rh aqua complexes, we have performed an in-depth lH, 13C, 170,and 2D NOESY NMR study of the structure and dynamics of the equilibrium of conversion of 1 36 3 via the plausible intermediacy of putative 2 , [Cp*Rh@-OH)(H20)h2+, further studied the structure of 1 by single-crystal X-ray crystallography and fast atom bombardment mass spectrometry (FAB/MS), and also established the relationship between 1 and 3 by utilizing potentiometric titration techniques. (6) Nutton, A,; Baily, P. M.; Maitlis, P. M. J . Chem. SOC.,Dalton Trans. 1981,1997 and references therein. (7) (a) Kolle, U.; Klaui, W. Z. Naturforsch. 1991, 46B, 75 and references therein. (b) Kolle, U.; Flunkert, G.; Gorissen, R.; Schmidt, M; Englert, U. Angew. Chem., Znt. Ed. Engl. 1992, 31, 440 and references therein. (c) Dadci, L.; Elias, H.; Frey, U.;Hornig, A.; Kolle, U.; Merbach, A. E.; Paulus, H.; Schneider, J. S. Inorg. Chem. 1995, 35,306. (8) (a) Cervini, R.; Fallon, G. D.; Spiccia, L. Inorg. Chem. 1991,30, 831.(b) Read, M. C.; Glaser, J.; Sandstrom, M.; Toth, I. Inorg. Chem. 1992,31,4155. ( c ) Wieghardt, K.; Schmidt, W.; Nuber, B; Prikner, B.; Weiss, J. Chem. Ber. 1980,113,36.
Table 2. Selected Atomic Coordinates ( x lo4) and Isotropic Thermal Parameters (A2 x 103) for 1 X Y z U Rh(1) O(13) O(14) O(15) C(5) C(6) C(7) C(8) C(9) C(10) C(11) C(12) C(13) CU4) Rh(2) O(16) O(17) O(18)
317 (1) 223(3) 1176(3) 270(4) 108(5) -397(5) -328(5) 213(6) 464(5) 235(6) -866(6) -758(5) 425(7) 1023(6) -2857(1) -3717(3) -2751(4) -2876(4)
1441(1) 691(8) 1037(9) -723(8) 3556(11) 2747(10) 2068(12) 2378(11) 3340(12) 4505(14) 2684(15) 1133(13) 1922(15) 3998(13) -1145(1) -801(8) -204(9) 981(9)
O(1) 1138(5) 223(5) -173(4) 81(9) -43(9) -700(6) -1014(7) -526(8) 708(7) 489(10) -1037(8) -1727(8) -639(9) 7276(1) 7021(5) 6213(5) 7597(5)
Table 3. Selected Bond Distances Angles (deg) for 1 Rh(l)-0(13) Rh(l)-0(15) Rh(l)-C(6) Rh(l)-C(8) Rh(2)-0(16) Rh(2)-0(18) 0(14)-Rh(l)-0(13) 0(15)-Rh(l)-0(14)
2.213(8) 2.131(8) 2.134(10) 2.079(12) 2.140(8) 2.150(9) 81.9(3) 84.1(3)
20(1) 28(2) 36(2) 35(2) 31(3) 26(3) 22(2) 26(3) 32(3) 36(3) 44(4) 39(3) 45(4) 46(4) 21(1) 31(2) 30(2) 51(3)
(A) and
Rh(l)-0(14) Rh(l)-C(5) Rh(l)-C(7) Rh(l)-C(9) Rh(2)-0(17) Rh(2)-C(15) 0(15)-Rh(l)-0(13) C(5)-Rh(l)-0(13)
2.137(8) 2.122(11) 2.096(10) 2.112(12) 2.162(8) 2.114(10) 78.9(3) 103.3(5)
Results X-rayCrystal Structure of [Cp*Rh(H20)sl(OTf)~, 1. Clearly, it would be informative to have the unequivocal structure of complex 1, [Cp*Rh(H20)3l(OTfh, to be better able t o define the equilibrium between 1 3. Initial attempts to accomplish this involved reaction of [Cp*Rh(OTf& prepared from [Cp*RhC1232 with AgOTf in CH2C12, with H2O. This provided a complex that was always H2O deficient and gave a formula of [Cp*Rh(H20)2(OTfhk. We recently found, however, that this was a consequence of our recrystallization (CH2C12) and drying procedures, whereby the H20 ligand was partially lost during these processes. Moreover, we found that, by layering the CH2C12 solution with 3 equiv of HzO/Cp*Rh, X-ray-quality crystals were obtained for 1. The X-ray structure of complex 1 is shown in Figure 1,while Table 1shows the crystallographic data, Table 2 provides selected atomic coordinates and isotropic thermal parameters, and Table 3 shows selected bond distances and angles. The structure consists of two
Eisen et al.
2808 Organometallics, Vol. 14, No. 6, 1995 molecules of 1, which are slightly different crystallographically (supplementary material). The cationic portions of both molecules have the typical piano stool geometry, with the sums of the 0-Rh(1)-0 and O-Rh(2)-0 angles equal to 244.9' and 245.9', respectively. These values are larger than those found in the structure of 3 (227.6' and 227.1'),6 indicating less strain in the oxygen coordinations of 1. Moreover, the mean Rh-0 and Rh-C distances of 2.156 and 2.118 for 1 are comparable to those found in 3 (2.114 and 2.128 A, respectively). When complex 1 was dissolved in H20, the pH was found to be 3.5. Therefore, the in situ preparation of 1 by reaction of [Cp*RhClalz with AgOTf in H2O (measured pH = 3.5) provides a facile method for its formation in aqueous solution. Further attempts on understanding the monomeric nature of complex 1 in solution was provided by running the lH NMR spectrum of the pH 3.5 Cp*Rh aqua complex, 1, in DMSO-&. Thus, the Cp* signal at 1.53 ppm and the broad H2O signal a t 3.38 ppm were found in the ratio Cp*/H20 = 2.5, while a ratio of 2.5 is expected for the aqua complex 1. Thus, we believe that the structure of 1 in water at a pH 3.5 is as shown in the solid state in Figure 1. 'Hand lSCNMFt Spectroscopic Titration Studies of Complexes 1 and 3. An 'H and 13C NMR pH titration study was initiated with [Cp*Rh(H20)31(0Tf)z, 1.2a Complex 1 was dissolved in D20, and the pH was adjusted with either 0.01 M CF3S03D (DOT0 or NaOD. Figure 2 shows the lH NMR titration results from pH 2-10 (in DzO, pD = pH 0.4). From pH 2-5 only one sharp Cp*Rh resonance for 1 is evident at 1.57 ppm, while the signal at 4.73 is that of Hz0. As the pH is increased from 5 to 7, a second signal is observed at 1.50 ppm. Both signals, 1.57 and 1.50 ppm, are clearly broadened in this pH range (5-7) and reflect the possibility of several Cp*Rh aqua complexes being in rapid equilibrium. Further increases in the pH to 10 (Figure 2) and then to 14 (not shown) provide only a sharp signal a t 1.50 ppm. A similar 13C NMR pH titration experiment at pH 2.0 (supplementary material) provided the Cp* ring carbons at 88.73 (doublet, &h-C = 6.1 Hz), while the Cp* CH3 groups appeared at 5.78 ppm. When the pH is raised to 5.5, two sets of signals are evident for both Cp* ring and CH3 carbon atoms. Thus, the Cp* ring carbons appear as doublets at 92.02 (JRh-C = 6.1 Hz) and 88.73 (&-C = 6.1 Hz) ppm, while the cp* CH3 resonances appear at 5.78 and 5.41 ppm (supplementary material). As the pH is raised to 10, only the signal for the Cp* ring carbons is evident a t 92.02 ppm, while the Cp* CH3 carbons show only the 5.41 ppm signal. We also studied the Cp*Rh aqua equilibrium starting from complex 3 at pH 14 and then proceeding to pH 5.2. Figure 3 shows a similar lH NMR pH titration experiment as in Figure 2. Although there is a very slight shift of complex 3 at pH 14, [(Cp*Rh)z@-OH)31+, the pH range from 12.5 to 5.2 provides the same two signals at 1.50 and 1.57 ppm and is further evidence for the equilibrium between the low-, intermediate-, and high-pH Cp*Rh aqua species. Additional Structural Determinations of the Cp*Rh Aqua Complexes 1 and 3 by 2D NOESY Exchange Phasing and FABMS Techniques. The present study defined the structure of the low-pH
JcIIPH =5.5
+
5.0
40
30
20
IO
PPM
Figure 2. IH NMR spectral titration experiments of complex 1,pH 2-10. Cp*Rh aqua complex as that of [Cp*Rh(H20)3I2+,1. In order to further clarify structure 1 as the pH 2-5 Cp*Rh aqua complex (Figure 11, we used a 2D NOESY exchange phasing procedure, the basic premise being that the three H2O molecules complexed to the Rh metal center are in rapid exchange with the bulk H2O molecules (no separate signal for Cp*Rh-complexed H2O was observed even at a lower temperature). Therefore, if structure 1, [Cp*Rh(H20)3I2+,is correct in aqueous acidic solution, then there should be a NOESY crosspolarization effect between the three HzO molecules complexed to the Rh metal center and the CH3 groups of Cp*. The 2D NOESY experiment at pH 5.8 (Figure 4A) clearly verifies a strong correlation between complexed H2O and the Cp* CH3 groups as well as between both Cp* and CH3 in the equilibrium. As well, a similar
Aqueous Organometallic Chemistry
Organometallics, Vol. 14, No. 6, 1995
A
L--- -.-LI
~
*B
~
4.5
50
,
h l
50
,
4.0
l
l
3,O PPM
,
!
2.0
#
4.0
3.5
4.0
l
-
.
3.0
3.0
-.
L
25
~
20
1.0
4
pH=14.0 l
1.0
Figure 3. lH NMR spectral titration experiments of complex 3, pH 14-5.2. NOESY correlation (Figure 4B)for p-OH groups bonded to Cp*Rh and the Cp* CH3 groups was also observed at pH 11,which corroborates that complex 3 is in equilib) complex 1 via putative rium with the low-pH ( ~ 5aqua 2.
In addition, attempts to verify the structure of 1 by FABNS experiments (p-nitrobenzyl alcohol matrix) on acidic solutions (pH < 5) provided data that showed, under the FABNS conditions, that complex 1 forms a presumed p-H2O-bridged dimeric species, [(Cp*Rh(HzO)Z)~C~-H~O)(CF~SO~)ZI+, mle = 1013 (15%), and a [(Cp*Rh)z(CF3SO&I+ ion, a t mle = 923 (100%). This is consistent with our finding that under vacuum complex 1 may lose a water molecule, and this is reflected in the FAB/MS with formation of p-HzObridged dimeric species. Interestingly, complex 3 a t pH 14 shows a FABNS (p-nitrobenzyl alcohol matrix) peak
Figure 4. 4. 2D NOESY for complex 1 at pH 5.8 (A) and pH 11 (B). that provides the replacement of all three p-OH groups by three p-nitrobenzyl alcohol (PNA) groups to give the [(Cp*Rh)z(PNA)31+ion, a t mle = 932. Potentiometric Titration Experiments. A detailed potentiometric titration study of the Cp*Rh aqua complexes would shed light on the equilibrium by determining the PKa values. Complex 1 was dissolved in 0.02 M HOTfand titrated with 0.1 N NaOH. Figure 5A shows the results of this potentiometric titration, and it is clearly the classical titration of a mixture of a strong acid (HOT0 and a weak acid (1). The amount of NaOH from the first equivalence point for HOTf (pKa = 2.5) to the second equivalence point of the Cp*Rh aqua complexes is 1.5 times the amount of starting aqua complex 1;i.e., the ratio of Cp*Rh aqua complex/NaOH is 1:1.5. Surprisingly, there is only one pK, of 5.3 that was observed in this titration for the Cp*Rh aqua species equilibrium. Alternatively, we studied the potentiometric titration of complex 3 from pH 14 to 2 and again
2810 Organometallics, Vol. 14,No. 6, 1995 A
Eisen et al.
'121
I
10-
a-
E
61 4-
0 0
0.5
I
1.5
2.5
2
B
3
I
121
10
3.5
[ml]
NaOH 0.0929M
h 1 . . . , . . . L, . . . , . . . , . . . , I . .
2 ( 0
2
4
HOTf
8
6
10
12
[ml]
0.0182M
Figure 5. Potentiometric titration of complex 1 at pH 2-14 (A) and complex 3 at pH 14-2 (B). found only one pKa of 5.3 (Figure 5B). From the 1:1.5 ratio of complex 1 to NaOH and the fact that we see only one and not two pKa values as we might have expected, we can now formulate the following equilibrium scheme, including the above-mentioned data generated by NMR, single-crystal X-ray, and FAB/MS experiments (eq 1).
I
Rh
2
',dh
H*
ovH+, H
(cis and trans ) 2
Hz0
3
Kinetic and Equilibrium Constants for 1 --L 3. The NMR pH titration experiments (Figures 2 and 3)
established the equilibrium between Cp*Rh aqua complexes 1 and 3 via the plausible intermediate 2 (eq 1). Therefore, we decided t o measure the equilibrium pseudo-first-order rate constants (k1 and k-1) at pH 5.8 by an N M R spin population transfer technique, followed by the use of the Bloch equation k = ~/T~A[Mo+/Mu.. - 11, where T ~ A is the longitudinal relaxation time of nucleus A, MOAis the equilibrium magnetization of the nuclei at site A before perturbation by rf energy, and MU.. is the magnetization of nuclei A after equilibrium magnetization has been reached. This analysis provided the following pseudo-firstorder rate constant for conversion of 1 3 3 at pH 5.8, k, = 7.18 s-l, with a TI for 1 (0.034 M) of 1.6 s, while the rate constant, k-1, for the reverse reaction, 3 1, is 2.93 s-l with a TI for 3 of 1.55 s. From these calculations, we can determine the equilibrium ratio for 1 --L 3 to be 2.45 at pH 5.8, while from the integration ratio the equilibrium constant, Keq, was found to be 353. We also performed 1 7 0 NMR experiments over the pH range from 2 to 14 to elucidate exchange rates of bulk H2O with H20 or p-OH ligands bonded to the Cp*Rh metal center. The spectra showed that the bulk H2O with H2O and y-OH ligands bonded to the Cp*Rh metal center were in the very fast exchange regime, providing only one 170signal; therefore, a rate of exchange can be estimated as &change > 8150 s-l, by simply multiplying the chemical shift of the p-OH groups (-150 ppm, referenced to bulk H20 at 0 ppm) by the Hz/ppm (54.2, frequency of 170)at 400 MHz.'~ This kexchange rate of . '8150 I s-l can be compared to the number recently reported for in situ-generated 1,via variable pressure 170NMR studies, of 1.6 x lo5 s-l, therefore, our reported value appears to be at the lower limits of this fast exchange process.7c It is also noteworthy to mention that in the latter variable pressure 1 7 0 NMR studies7c that only one signal was found for in situgenerated 1, which implies that in solution at pH 2-5 this aqua complex is monomeric.
Discussion The most surprising aspect of our overall results on elucidating the structure and dynamics of the equilibrium between Cp*Rh aqua complexes from pH 2-14 (eq 1)was that only two species were observed when the equilibrium was started from either 1 or 3, the caveat being that only one broadened NMR signal was observed in the pH range 5-7 for both the putative intermediate 2 and for 3,the tris-p-OH complex. In addition, only one pKa was found for both of these complexes, confirming the rapid equilibria between them. In our scheme for this equilibrium (eq l),we designate the Cp*Rh aqua complex 2 as the most logical intermediate between the conversion of 1 to 3. This can be rationalized by the data generated in the potentiometric titration, by other similar [Cp*Rh(~-0H)(L)12~+ dimer structures reported in the literature, as well as by precedent from inorganic models of the Cp*Rh aqua equilibrium. We believe that a mononuclear [Cp*Rh(H20)2(0H)I+ cationic complex, 4, (eq 2) is formed first from reaction of 2 mol of 1 with 2 mol of NaOH. Complex 4, a monocation, is the logical precursor to the putative dimer complex, 2, a p-OH structure with terminal H20 ligands (eq 11, via a rapid dimerization reaction.
Aqueous Organometallic Chemistry
&12+
Organometallics, Vol. 14, No. 6, 1995 2811
'+
Instrumentation and Materials. Three Bruker NMR spectrometers, two WM 200 MHz instruments, and a WM 400 MHz instrument were used for all lH, 13C, and 2D NMR experiments. The cross-polarization experiments utilized all three spectrometers with different probes and signal-to-noise ratios; similar results were obtained with each instrument. The IH NMR chemical shifts were referenced to an external D2O solution of 3-(trimethylsily1)propionicacid-2,2,3,3-& so-
dium salt. The 13C NMR chemical shifts were referenced to the triflate anion at 120.532 ppm. A Bruker AM 400 spectrometer was used for the 1 7 0 NMR measurements and the 170NMR spectra were recorded at 54.2 MHz for samples in 5 mm sealed J. Young NMR tubes. The 90" pulse width for 1 7 0 NMR spectra was found to be 15.8 ,us. Pulses were used with a short pulse repetition time of 0.1 s. Chemical shifts were referenced to external H2O (distilled)at 298 K (6 = 0 ppm, TI = 5.2 s). The NOESY experiments were performed using the commercial Bruker NOESYPH subroutine with a mixing time of 1.2 s. The FAB/MS data were obtained with an MS-50 instrument. The potentiometric titration experiments were performed on a Corning model 340 pH meter with an Aldrich ultrathin longstem pH electrode; solutions prepared in an oxygen-free environment and those prepared in the presence of air gave similar pH measurements. The potentiometer data was acquired and analyzed on an IBM computer. The acids and bases utilized were standardized with HC1 and NaOH samples of known concentration. Preparation of [Cp*Rh(HzO)&OTf)z,1. A solution of [Cp*RhC12]2 (0.050 g, 0.08 mmol) and AgOTf (0.083 g, 0.32 mmol) in anhydrous CHzClz (4 mL) was stirred at ambient temperature for 3 h, and then it was filtered. The filtrate was carefully layered with H2O (8.7 pL, 0.48 mmol). The layered filtrate was allowed t o stand a t ambient temperature for 12 h to give an amber crystalline product in quantitative yield. The NMR sample was prepared in a Vacuum Atmospheres glovebox by dissolving an appropriate amount of [Cp*Rh(H20)3](OTfl2 in a 5 mm NMR tube in 0.6 mL of DMSO-&, which was dried over activated molecular sieves. The IH NMR spectra were obtained on a Bruker AM 400 spectrometer with the reference peak set at 2.49 ppm for the solvent DMSO-&. The proton integration of H2O was calculated by subtracting the spectrum of a sample of the 0.6 mL of blank DMSO-& solvent containing trace amounts of H2O. The 'H chemical shift of these trace amounts of H2O in the blank DMSO-& solvent is at 3.32 ppm. It is noteworthy t o mention that the IH resonance of HzO in the sample of 1 was downfield-shifted by 0.06 ppm, indicating that the H2O molecules were in an equilibrium process of occupying the coordination sites of Cp*Rh even in the presence of DMSO. The 'H NMR (400 MHz, DMSO-&, 25 "C) 6 3.38 (s, 6H, 3Hz0), 1.53 (s, 15H, Cp*). FAB/MS (glycerol/HzO), mle (relative intensity): 688.9 (43) {[Cp*Rh(OH)zlz(OTf)- 4H); 540.9 (100) {[Cp*Rh(OH)zlz 3H). Elemental analysis for ClzH2lF609RhSz. Calcd C, 24.4; H, 3.56. Found: C, 24.1; H, 3.52. Alternatively, 1 can be and 4 equiv of AgOTf generated in situ by reacting [Cp*RhC121~ in H20 for 3 h and then filtering through Celite, measured pH = 3.5. X-ray Data Collection, Solution, and Refinement of 1. The X-ray data for 1 were collected by using a Siemens R 3 d v diffractometer equipped with an Enraf-Nonius low-temperature apparatus. Only a random fluctuation of 40.5% in the intensities of two standard reflections was observed during data collection. All calculations were carried out on a MicroVAX 3200 computer using the SHELXTL Plus and SHELXL93 program systems.I' Crystals of 1 were transferred t o a Petri dish and immediately covered with a layer of hydrocarbon oil. A single crystal was selected, mounted on a glass fiber, and immediately placed in a low-temperature N2 stream. Some details of the data collection and refinement are given in Table 1. Further details are provided in the supplementary material. The structure was solved in the space group Pna21 using direct and difference Fourier methods. Solution could not be obtained in the alternative space group, Pnma, because no mirror plane is present in the structure.
(9) Lahoz, F. G.; Carmona, D.; Oro, L. A,; Lamata, M. p.; Puebla, M. P.; Foces-Foces, C.; Cano, F. H. J. Organomet. Chem. 1986,316, 221. (10) Fish, R. H.; Kim, H.-S.; Babin, J. E.; Adams, R. D. Orgunometallies 1988, 7 , 2250.
(11)Tables of neutral atom scattering factors, f and f', and absorption coefficients are from: International Tables for Crystallography; Wilson, A. J. C., Ed.; Kluwer Academic Publishers: Dordrecht The Netherlands, 1992; Vol. C, Tables 6.1.1.3 (pp 500-5021, 4.2.6.8 (pp 219-2221, and 4.2.4.2 (pp 193-1991, respectively.
2
ZOH
2&
___)
I
I
1
4
Rh
(2)
The analogous structure depicted for 2 in eq 1 has been found for several nitrogen ligand complexes with the general formula [Cp*Rh(,~0H)(L)l2~+, determined by single-crystal X-ray analysis, where L, a terminal ligand, was ~ y r i d i n equinoline,1° ,~ or l-methylcytosine.2b In addition, a previous study by Wieghardt and coworkers8bon an inorganic model for the rapid equilibrium between 2 and 3 in eq 1was reported for a (1,4,7triazacyclononane)rhodium(III) cationic system. In this latter study, a trans-diaquabis(D-hydroxo)bis[l,4,7-triazacyclononane)rhodium(III)] cationic complex was isolated and characterized by single-crystal X-ray analysis. In solution, this analogue of complex 2 was converted to the tris(D-hydroxo)bis[1,4,7-triazacyclononane)rhodium(I1I)lcationic complex by a change in pH, a structure analogous to complex 3 that was elucidated by Maitlis and co-workers.6 The surprisingly rapid conversion from putative intermediate 2 to tris-p-OH dimer 3 consumes 1mol of NaOH and thereby explains the overall molar ratio of 1:1.5 of starting Cp*Rh aqua complex, 1,to the amount of NaOH added in the potentiometric titration from pH 2-12. More importantly, we have unequivocally ascertained the monomeric structure of 1 by X-ray crystallographic and lH NMR techniques and have clearly established the dynamic equilibria between Cp*Rh aqua complexes 1 and 3 via the plausible intermediate 2, including pseudo-first-order rate constants, an equilibrium constant, as well as the one pKa value. Finally, these results help further explain the structural studies we have reported and are still pursuing with the Cp*Rh aqua complexes and nucleobases, nucleosides, nucleotides, and oligonucleotides as a function of pH.2a-d As the new field of aqueous organometallic chemistry continues to progress, with emphasis on elucidation of structure and dynamics and catalysis studies, we are hopeful that our present approach to the structure and dynamics of the Cp*Rh aqua complexes will be a model for other studies on organometallic aqua complexes. Experimental Section
2812 Organometallics, Vol. 14,No. 6, 1995 Disorder in one of the triflate anions was modeled with two sites of relative weight 0.7510.25 for the three oxygen atoms. Hydrogen atoms were added geometrically and refined with a riding model. An absorption correction (XA13S2)12was applied. In the final cycles of refinement, all non-hydrogen atoms except those of the above disorder were refined with anisotropic thermal parameters. The largest feature in the final difference map had a peak value of 0.66 e A-3, 1.3 A from a triflate fluorine atom. Some atom coordinates and isotropic thermal parameters are given in Table 2. Selected bond distances and angles are listed in Table 3. NMR Titration Sample Preparations. Complex 1 was dissolved in an argon-degassed solution of CF3S03D in D2O. The pH adjustments were carried out by adding the necessary amounts of 0.01 M NaOD or 0.01 M CF3S03D. The solutions were placed in J. Young NMR tubes and sealed with the total exclusion of air. The 170enrichment was performed by exchanging the D2O with enriched H2"0 purchased from ISOYEDA Co. Ltd., Israel, (10.2% H2l7O)at pH 2.0, while further pH adjustments were made with a 0.01 M NaOH solution. The 2D NOESY NMR solutions were prepared at different pH values by utilizing a 50% mixture of H2O and D2O to increase cross-polarization. Potentiometric Titration of 1. Complex 1 (10.7 mg, 0.0196 mmol) was dissolved in 0.018 M triflic acid (HOT0 and titrated with a standard solution of 0.093 M NaOH. The first equivalence point, belonging to the neutralization of the strong acid, HOTf, was achieved after addition of 2.2 mL of 0.093 M NaOH, at which point the pH was 4.3. The second equivalence point associated with 1 (i.e., 1 t o [2] and 3)was obtained after an additional 0.3 mL (0.028 mmol) of NaOH was added, the pH was 8.1. The two pK,'s corresponding t o the two equivalence points were calculated to be 2.2 i. 0.1 (triflic acid) and 5.4 i 0.1 (11, respectively. The reverse titration was also performed by dissolving 1 in a 0.093 M NaOH solution and titrating with a 0.0182 M HOTf solution. The first equivalence point is acquired after the addition of 2.59 mL (pH = 8.5) and (12) Program XABS2 calculates 24 coefficients from a least-squares fit of (1/A vs sin2(@)to a cubic equation in sin2(6')by minimization of FQ2and Fc2differences. Parkin, S. Department of Chemistry, University of California, Davis, CA, 1993.
Eisen et al. the second equivalence point was acquired after a total addition of 4.19 mL of HOTf. (A mL = 1.60, 0.029 mmol at pH 4.4). Potentiometric Titration of 3. Complex 3,[(Cp*Rh)&OH)dOTf, 39.2 mg (0.056 mmol) was dissolved in 10 mL of 0.093 M NaOH and titrated with a standard solution of HOTf (0.019 M). The first equivalence point occurred after the addition of 8.68 mL of HOTf (pH = 8.3). The pKa was calculated to be 5.5 0.1. After a n additional 1.80 mL (0.164 mmol) of HOTf had been added, the second equivalence point was obtained at pH = 3.53. The reverse titration was also done by dissolving 3 in a 0.019 M HOTf solution and titrating with a 0.093 M solution of NaOH. The first equivalence point was at pH = 3.83 after the addition of 2.69 mL of NaOH, and the second point was at pH = 7.98 after an additional 1.70 mL of NaOH had been added. The pK was 5.4 i 0.1.
*
Acknowledgment. The studies a t LBL were generously supported by NIH Grant A I 08427 (M.F.M.), Laboratory Directed Research and Development Funds (M.F.M. and R.H.F.), and the Department of Energy under Contract No. DE-AC03-76SF00098, while those a t the Technion were supported by Institute Funds t o M.S.E. We also thank the reviewers for constructive comments concerning this manuscript. Supplementary Material Available: The crystal data, data collection, and solution and refinement of 1; tables of atomic coordinates and equivalent isotropic displacement parameters, bond distances and angles, anisotropic displacement parameters, hydrogen coordinates, and isotropic displacement parameters of 1; figures of two crystallographically different molecules and the unit cell structure of 1; 13CNMR spectra of 1 in acidic solution, pH < 5 (Figure A) (the quartet at 118 ppm is due to the triflate anion, CF3S03- and CH3 signal of Cp* at pH 5.8 (Figure B) (16 pages). See any current masthead page for ordering information. OM950140V
