Organometallics 1995, 14, 4966-4968
4966
Alkyl(hydrido)platinum(IV) Complexes: The Mechanism of Pt-C Bond Protonolysis Geoffrey S. Hill, Louis M. Rendina, and Richard J. Puddephatt" Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7 Received May 31, 1 9 9 6 Summary: The complexes [PtMedN-NjI {N-N = tbu2bpy (2,Y-bipyribpy (4,4'-di-tert-butyl-2,2'-bipyridine), dine), or phen (1,lO-phenanthroline)} react with HX (X = C1, Br, I, 02CCF3, SO3CF3) to yield [PtXMe(N-N)l and CH4, by an oxidative addition lreductive elimination mechanism. The new alkyl(hydrido)platinum(N) intermediates [PtCUH)Medtbu2bpy)l (l),[PtBr(H)Medtbu2bpy)] (2), [PtH(I)Medtbu2bpy)l (3), and Pt(Cl(H.Me2(bpy)], (a), formed by trans oxidative addition of HX, have been filly characterized at -78 "C by IH NMR spectroscopy. Rapid reductive elimination of CH4 ensues at higher temperatures to yield the corresponding organoplatinum(II) products [PtXMe(N-N)l. The thermal stability of the platinum(N) intermediates follows the sequence X = Cl 'Br > I > 02CCF3, SO3CF3. Introduction The cleavage of metal-carbon a-bonds by electrophiles is of fundamental importance in organometallic chemistry, and the mechanism of this process has been the subject of several thorough investigations.' With non-transition metals, the cleavage is believed to occur via the sE2 mechanism which involves direct electrophilic attack a t the M-C b0nd.l In alkyltransition metal complexes, the highest occupied molecular orbital (HOMO) may be either the M-C bonding orbital or a nonbonding d-orbital. Attack a t the M-C bond gives cleavage by the classical sE2 mechanism, but the same product can be obtained by attack at the metal center (oxidative addition) followed by reductive elimination of the corresponding alkane (Scheme 11.l In the case of protonolysis of alkylplatinum(I1) bonds, arguments have been put forward in favor of both mechanisms.l For example, in the case of [PtRz(PR3)23, studies of selectivity in cleavage of alkyl or aryl groups from platinum(I1) support an oxidative additiodreductive elimination mechanismld whereas kinetic studies indicate an sE2 mechanism.lb The most definitive evidence for the oxidative addition mechanism would be the detection of the proposed alkyl(hydrido)platinum(IV) intermediate, but this has proved difficult owing to the facile reductive elimination of the alkane. There are few examples of hydridoplatinum(W) complexes in the literature,2 and the first alkyl(hydrid0)@Abstractpublished in Advance ACS Abstracts, September 15,1995. (1)(a) Alibrandi, G.; Minniti, D.; Romeo, R.; Uguagliati, P.; Calligaro, L.; Belluco, U. Inorg. Chim. Acta 1986,112,L15. (b) Alibrandi, G.; Minniti, D.; Romeo, R.; Uguagliati, P.; Calligaro, L.; Belluco, U.; Crociani, B. Inorg. Chim. Acta 1985,100,107. (c) Belluco, U.;Michelin, R. A,; Uguagliati, P.; Crociani, B. J.Organomet. Chem. 1983,250,565. (d) Jawad, J . K.; Puddephatt, R. J.;Stalteri, M. A. Inorg. Chem. 1982, 21, 332. (e) Uguagliati, P.; Michelin, R. A.; Belluco, U.; Ros, R. J. Organomet. Chem. 1979,169,115.(D Johnson, M. D. Acc. Chem. Res. 1978,11, 57. (g) Romeo, R.; Minniti, D.; Lanza, S.;Uguagliati, P.; Belluco, U. Inorg. Chem. 1978,17,2813. (h) Belluco, U.;Giustiniani, M.; Graziani, M. J. Am. Chem. SOC.1967,89, 6494. (i) Belluco, U.; Croatto, U.; Uguagliati, P.; Peitropaolo, R. Inorg. Chem. 1967,6, 718. (j) Falk, C. D.; Halpern, J . J. Am. Chem. SOC.1965,87,3523.
0276-7333f95I2314-4966$09.OQ/O
Scheme 1 r
platinum(IV) complexes { [PtX(H)Rn(dmphen)l,X = C1, Br; R = Me, I-MeOC&; dmphen = 2,9-dimethyl-l,10phenanthroline} were reported very recently.2a>fIn this paper, we describe the reactions of [F'tMe2(N-N)I {N-N = 'buzbpy (4,4'-di-tert-butyl-2,2'-bipyridine),bpy (2,2'bipyridine), or phen (1,lO-phenanthroline)} with HX (X = C1, Br, I, 02CCF3, S03CF3) to ultimately yield [PtMeX(N-N)] and CHI. In several cases, the alkyl(hydrido)platinum(N)intermediate proposed in Scheme 1was detected and fully characterized by low-temperature lH NMR spectroscopy. This work therefore provides further support for the oxidative addition/ reductive elimination mechanism for the electrophilic cleavage of metal-carbon a-bonds.
Experimental Section All lH NMR spectra were recorded at -78 "C using a Varian Gemini 300 MHz spectrometer and are referenced to the residual protons of the deuterated solvents. Chemical shifis are reported in ppm relative to TMS. The complexes [PtMedfbuzbpy)13and [PtMez(bpy)14were prepared by the literature methods. The complexes [PtClMe(bpy)]? [PtClMe(%uzbpy)],6[PtBrMe(tbuzbpy)l,6and [PtIMePbuzbpy)16 were characterized by comparing their 'H NMR spectra to those of authentic samples. Reaction of [F%Medtbuabpy)lwith HCI. To a 5 mm NMR tube charged with a solution of [PtMez('buzbpy)l(O.O3Og, ~
~
~
(2) For example, see: (a) De Felice, V.; De Renzi, A.; Panunzi, A.; Tesauro, D. J. Organomet. Chem. 1996, 488, C13. (b) Wehman-
Ooyevaar, I. C. M.; Grove, D. M.; de Vaal, P.; Dedieu, A.; van Koten, G. h o g . Chem. 1992,31,5484. (c) Ebsworth, E. A. V.; Marganian, V. M.; Reed, F. J. S. J. Chem. Soc., Dalton Trans. 1978, 1167. (d) Ebsworth, E.A. V.; Rankin, D. W. H. J. Chem. Soc., Dalton Trans. 1973,854.(e) Bentham, J.E.; Cradock, S.; Ebsworth, E. A. V. J.Chem. Soc. A 1971,587.(0 Stahl, S.S.; Labinger, J. A.; Bercaw, J. E. J.Am. Chem. Soc., in press. (3)Achar, S.;Scott, J . D.; Vittal, J. J.; Puddephatt, R. J. Organometallics 1993,12,4592. (4)Monaghan, P. K.;Puddephatt, R. J. Organometallics 1984,3,
I..
Add
(5) Rendina, L. M.; Vittal, J. J.; Puddephatt, R. J . Organometallics 1995,14,1030. (6)Note that under rigorously anhydrous conditions oxidative addition of the Si-X bond occurs, but this is much slower. Levy, C. J.; Puddephatt, R. J.;Vittal, J. J. Organometallics 1994,13,1559, and unpublished results.
0 1995 American Chemical Society
Notes
Organometallics, Vol. 14,No. 10,1995 4967 Table 1. Selected 'H NMRa Spectroscopic Data for Complexes 1-4 'H 6
comKI1ex 1 2
3 4 a
In
N-N aromatic 'bu Pt-Me Pt-H 8.75 [d, 2H, 3 J ~ 6 = ~ 65.8, H61,8.17 [br s, 2H, H31, 1.37 [s, 18Hl 1.23 [s, 6H, 2Jpt~ = 67.01 -21.80 [s, 1H, 'JRH = 1589.71 7.63 [br d, 2H, 3 J ~ =~ 5.8, ~ H51 e -20.90 [s, lH, 'JRH = 1630.51 8.76 [d, 2H, 3&6H5 = 5.7, H61, 8.14 [br 8 , 2H, H31, 1.35 [s, 18HI 1.34 [s, 6Hl 7.61 [br d, 2H, 3 J ~ 5 = ~ e5.7, H51 = 67.51 -19.16 [s, 1H, 'JRH = 1655.51 8.77 [d, 2H, 3&6H5 = 5.9, H61, 8.14 [br 8 , 2H, H31, 1.39 [s, 18Hl 1.41 [s, 6H, 2Jpt~ ~ @ 5.9 H51 7.60 [br d, 2H, 3 J ~ 5 = 1.28 [s,6H, 'JRH = 68.31 -21.65 [S, 1H, 'JRH = ca. 15911 8.78 [br m, 2Hl,8.26 [br m, 2H1,8.10 [br m, 2H1, 7.63 [br m, 2Hl CDzClz at -78 "C. Quoted multiplicities do not include 195Pt satellite signals. Coupling constants are given in Hz.
0.061 mmol) in CDzClz at -78 "C was added Me3SiCl (7.70
pL, 0.061 mmol). Distilled HzO (1.1pL, 0.061 mmol) was added t o the cold solution, thus hydrolyzing the MesSiCl to afford 1 equiv of HC1. The progress of the reaction was followed by lH NMR spectroscopy, and, within 5 min, the presence of [PtCl(H)Mez(%uzbpy)I(1) was detected. 'H NMR of 1 in CDzClz: 6 = 8.75 [d, 2H, 'JH~H~ = 5.8 Hz, H61, 8.17 [br s, 2H, H3], 7.63 [br d, 2H, 3 J ~ 5 ~=65.8 Hz, H51, 1.37 [s, 18H, %u], 1.23 [s, 6H, 'Jpt~= 67.0 Hz, Pt-Me], -21.80 [s, lH, 'Jpt~ = 1589.7 Hz, Pt-HI. Compound 1 was indefinitely stable a t -78 "C, but it decomposed rapidly at 0 "C to give CH4 and [PtC1Me(tbuzbpy)].5 Reaction of [PtMe&bu2bpy)l with HBr. In a similar reaction to that described above, [PtMez('buzbpy)l, MeaSiBr, and distilled HzO were reacted in CDzClz at -78 "C to afford (2) within 5 min. 'H NMR of 2 in CDz[PtBr(H)Me~(~buzbpy)] Clz: 6 8.76 [d, 2H, 3 J ~ 6 = ~ 5 5.7 Hz, H61, 8.14 [br 8, 2H, H31, ~ 65.7 Hz, H51, 1.35 [s, 18H, 'bu], 1.34 [s, 7.61 [br d, 2H, 3 J ~ 5 = 6H, lg5Ptsatellite signals are obscured, Pt-Me], -20.90 Is, l H , 'Jpt~= 1630.5 Hz, Pt-HI. Compound 2 decomposed to the yellow [PtBrMe(tbuzbpy)]6within 10 min at -78 "C. Reaction of [PtMe&bu2bpy)l with HI. In a similar reaction to that described above, [PtMez('buzbpy)l,MesSiI, and distilled HzO were reacted in CDzClz at -78 "C to afford [PtH(I)Mez(tbuzbpy)](3). lH NMR of 3 in CDzClz: 6 8.77 [d, 2H, 3 J ~ 6 ~ 55.9 Hz, H61, 8.14 [br s, 2H, H31, 7.60 [br d, 2H, 3 J ~ 5 ~ =6 5.9 Hz, H5], 1.41 [s, 6H, 'JRH = 67.5 Hz, Pt-Me], 1.39 [s, 18H, 'bu], -19.16 [s, l H , 'JRH = 1655.5 Hz, Pt-HI. within Complex 3 decomposed to the yellow [PtIMe('bu~bpy)l~ 5 min a t -78 "C. Reaction of [PtMez(bpy)] with HC1. To a 5 mm NMR tube charged with a solution of [PtMez(bpy)l (0.005 g, 0.013 mmol) in CDzClz at -78 "C was added MesSiCl(1.60 pL, 0.013 mmol). Distilled HzO (0.2 pL,0.013 mmol) was added t o the cold solution, thus hydrolyzing the Me3SiCl to afford 1equiv of HC1. The progress of the reaction was followed by lH NMR spectroscopy, and, within 5 min, the presence of [PtCl(H)MeZ(bpy)] (4) was clearly detected. lH NMR of 4 in CDzClz: 6 8.78 [br m, 2H1, 8.26 [br m, 2H1, 8.10 [br m, 2H1, 7.63 [br m, 2H], 1.28 [s, 6H, 'Jpt~= 68.3 Hz, Pt-Me], -21.65 [s, l H , 'JRH = ca. 1591 Hz, Pt-HI. Compound 4 was indefinitely stable at -78 "C, but it decomposed by reductive elimination of CH4 a t 0 "C to afford [PtC1Me(bpy)l.6
Results and Discussion The reaction of MesSiX (X = C1, Br, I) and H2O (to generate HX in situ) with [PtMe2(N-N)I (N-N = %u2bpy, bpy) affords the novel alkyl(hydrido)platinum(IV) complexes 1-4 (eq 1h6 Compounds 1-4 are all unstable
I
X n N N = 'buzbpy; X = CI (l),X = Br (9,X = I (3) = bpy; X = CI (4)
a t room temperature owing t o the easy decomposition by reductive elimination of CH, to give the corresponding platinum(I1) complex [PtMeX(N-N)], and so they were characterized by lH N M R spectroscopy at -78 "C. The evolution of CH4 was confirmed by GC-MS. Selected lH NMR spectroscopic data for complexes 1-4 are presented in Table 1. The lH NMR spectrum of 1 is presented in Figure 1. The proposed trans stereochemistry for the alkyl(hydrido)platinum(IV) complexes 1-4 is strongly supported by lH NMR spectroscopic evidence. The spectra of 1-3 show the expected three sets aromatic resonances due to the equivalent nature of the two pyridyl moieties of the %uzbpyligand. Similarly, the spectrum of 4 displays four sets of aromatic signals. In each of the complexes only one PtMe resonance is observed a t high field, again only consistent with the trans stereochemistry. These PtMe resonances possess 1g5Ptsatellite signals with the magnitude of 2 J p ~(67.0-68.3 Hz) indicative of an organoplatinum(lV) enter.^.^ There is only one tertbutyl resonance in each of the lH NMR spectra of complexes 1-3, thus further supporting a trans arrangement of HX about the platinum. In contrast to our results, the only other examples of six-coordinate alkyl(hydrido)platinum(IV) species in the literature ([PtX(H)R2(dmphen)l,X = C1, Br; R = Me, 4-MeOCsH4; possess dmphen = 2,9-dimethyl-l,lO-phenanthroline) cis stereochemistry of the H and X ligands about the metal center.2a This difference is certainly a consequence of the diimine ligand employed. The bulky methyl groups of dmphen introduce large interligand steric constraints in the platinum-(N-N) plane that are absent when tbu2bpy and bpy are employed.1° We believe that these constraints most likely promote the cis geometry observed in complexes of the type [PtX(H)Rz(dmphen)l, since the hydride occupies a more sterically encumbered site. The lH N M R spectra of 1-4 show the expected low-frequency hydride resonances with lg5Ptsatellite signals. The large Vpt~(1589.71655.5 Hz) is similar t o that reported in the literature for related compounds,2a,band, as expected, its magni(7) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987; p 326. (8)(a) Borkovskii, N. B.; Kovrilov, A. B.; Lipnitskii, I. V.; Umreiko, D. S. Koord. Khim. 1982, 8, 523. (b) Mortimer, C. T.; Wilkinson, M. P.; Puddephatt, R. J. J . Organomet. Chem. 1979, 165, 269. (9)For example, see: (a) Anderson, C. M.; Crespo, M.; Jennings, M. C.; Lough, A. J.; Ferguson, G.; Puddephatt, R. J . Organometallics 1991,10,2672. (b)Monaghan, P. K.; Puddephatt, R. J. J. Chem. SOC., Dalton Trans. 1988, 595. ( c ) Crespo, M.; Puddephatt, R. J. Organometallics 1987,6,2548. (d)Jawad, J.; Puddephatt,R. J. J. Chem. Soc., Dalton Trans. 1977,1466. (e) Kuyper, J. Inorg. Chem. 1977,16,2171. (0 Jawad, J.; Puddephatt, R. J. J . Organomet. Chem. 1976, 117, 297. (10) Albano, V. G.; Natile, G.; Panunzi, A. Coord. C h e n . Reu. 1994, 133, 67.
Notes
4968 Organometallics, Vol. 14, No. 10, 1995
R-H
Me,SiC
X 1 I I T
r7r
9
17,-7,,
,
8
I
,
,
,
,
, , , , , , , , , , ,, , , , , , , , , , , , , , , , , , ,, ,, , , , , , , , , , , , , , , , , ,
rT-Trq-,
1
6
5
4
3
~
2
I
0 WPPm)
Figure 1. The lH N M R spectrum of 1 in CD2Cl2 at -78 "C. The x and the 0 denote the residual protons of the solvent and [PtClMe(%u2bpy)I,respectively. tude is strongly dependent on the ligand trans t o the hydride (XI. For complexes 1-4, the magnitude of ~JFW follows a similar trend to that reported in the literature and increases as the nature of X is changed from C1 to Br to Studies with [PtMez(N-N)] (N-N = bpy, X = Br, I; N-N = phen, X = C1, Br, I) complexes were more difficult or impossible owing to the low solubilities of both the reactants and products a t low temperatures. Indeed, the tert-butyl groups in %uzbpy greatly assist in the solubility of both the intermediates and the products in CD2C12. The stability of the organoplatinum(IV) species toward reductive elimination appears t o be largely a function of the nature of the ligand trans to the hydride, i.e., the X group. Qualitatively, the observed rate of reductive elimination for the platinum(IV) species follows the order 1 FZ 4 < 2 < 3. In addition, attempts to detect the hydridoplatinum(IV) intermediate in reactions of [PtMedtbu2bpy)l with HX (X = 02CCF3, so3CF3), in which cases the anion X- is a poor ligand for platinum, have been unsuccessful even in reactions carried out at -90 "C. Hence, the first step of the (11)Appleton, T. G.;Clark, H. C.; Manzer, L. E. Coord. Chem. Rev. 1973,10, 335.
reductive elimination is most likely to be the dissociation of the Pt-X bond, yielding a five-coordinate intermediate [PtHMez(N-N)]+, which then undergoes reductive eliminati~n;~ the rate then follows the order of Pt-X bond strengths, namely Pt-C1 > Pt-Br > Pt-I > Pt-02CCF3, Pt-03SCF3.6t8 In conclusion, the reactions of HX with a series of [PtMez(N-N)I compounds to ultimately afYord CH4 and [FWMe(N-N)I complexes were studied as a function of X and N-N. In several cases, alkyl(hydrid0)platinum(IV)complexes were detected and fully characterized by low-temperature lH NMR spectroscopy. These compounds possess exclusively trans stereochemistry of the H and X ligands about the metal center. Complexes of this type are proposed intermediates in the mechanism of platinum(I1)-carbon a-bond protonolysis, and their detection provides further insight into this fundamentally important area of organometallic chemistry.
Acknowledgment. We thank the NSERC (Canada) for financial support to R.J.P., for a Canada International Fellowship to L.M.R.,and for a Postgraduate Scholarship to G.S.H. OM950404D
