3489
J . Am. Chem. SOC.1983, 105, 3489-3494
Raman and Infrared Spectra of Binuclear Platinum( 11) and Platinum(111) Octaphosphite Complexes. A Characterization of the Intermetallic Bonding Paul Stein,*lgMark K. Dickson,lband D. Max Roundhill*" Contribution from the Department of Chemistry, Washington State University, Pullman, Washington 991 64. Received September 7 , 1982
Abstract: Raman and IR spectra of K4[Ptz(pop)4X2](X = C1, Br, I; pop = Pz05H?-) complexes were recorded and the Raman active Pt(II1)-Pt(II1) stretching frequencies observed at 158, 134, and 110 cm-I. Symmetric Pt-X stretching frequencies were assigned at 304, 224, and 194 cm-I, respectively, while the asymmetric Pt-X frequencies occurred at 295, 195, and 118 cm-I. The Raman spectrum of [Pt2(pop)4]din aqueous solution was recorded and the Pt(I1)-Pt(I1) stretching frequency was detected at 116 cm-I. A vibrational analysis, utilizing the X-Pt-Pt-X unit, was performed. This model could reproduce the Raman active vibrations (h2 cm-I) and was sufficient to indicate a strong Pt(II1)-Pt(II1) single bond and a weak Pt(I1)-Pt(I1) bond. The Raman spectrum of [PtZ(pop),CH3Il4-showed u(Pt-Pt) 156 cm-I, v(Pt-I) 172 cm-I, and v(Pt-C) 489 cm-I, the latter frequency shifting to 475 cm-' with a I3CH3isotope. A normal coordinate calculation including the bridging POP atoms gave good reproductions of the Pt-Pt and Pt-X frequencies and predicted an asymmetric ring mode below 200 cm-l that couples with the asymmetric Pt-X stretch. This ring transition was identified in [Pt,(p~p),I,]~-and [Pt2(pop),CH31I4-complexes at 178 and 172 cm-I. The v(Pt-Pt) signal at 116 cm-' in [Pt2(pop)4]4-was found to involve a significant contribution of the symmetric ring bending. The PZO5H?-ligand vibrations were assigned in the IR spectra and the symmetric POP stretch showed a 35-cm-l upshift in the Pt(II1) complexes. By use of Badger's rule, an apparent 4%reduction in platinum-platinum separation is calculated for the excited state in K4[Ptz(p~p)4].
Although few platinum(II1) compounds are known, recent reports of binuclear Pt(II1) complexes with Pt-Pt distances between 2.47 and 2.695 A2,3have attracted attention. We have found that binuclear platinum(II1) complexes of [Pt,(p~p)~Xz]" (X = C1, Br, I) can be readily prepared by oxidative addition reaction (1) with the binuclear Pt(I1) octaphosphite salt [PtzPtz(PoP)4I4- + x2
-
[Ptz(POP)4X2l4-
(1)
( p o ~ ) ~(pop ] ~ - = P205HzZ-).2Simple MO theory4 of binuclear complexes of d7 and d8 metal ions predicts that these reactions may be facilitated by the formation of a Pt-Pt single bond in the product complex. The binuclear d8 ion, [Pt2(pop)4]e, with orbitals (d,)z(d,.)z has a formal bond order of zero, while a single bond is given to the binuclear d7 ions, [Ptz(pop)4Xz]e,with a filled ( d d ) orbital. The Pt-Pt distance in K4[PtZ(pop),Cl2]is 2.695 (1) A which is considerably shorter than that of 2.925 (1) A in K4[Pt2(pop)4].2Hz0.5 Magnetic measurements indicate these complexes are diamagnetic. The 31Pand I g 5 P t N M R spectra show these complexes are stable in solution although second-order splittings of the resonances complicate the evaluation of a quantitative description of the Pt-Pt interactions. Raman spectral measurements in aqueous solution have been reported for the [Pt2(pp)4XZ]4ions, giving v(Pt-Pt) at 158, 134, and 110 cm-', and for [Ptz(pop)4]e at 116 cm-1.6 While v(Pt-Pt) provides a sensitive monitor of the Pt-Pt interaction, the influence of the ligands obscures a direct correlation to Pt-Pt bonding. For
the purpose of estimating the bond strengths in both the binuclear platinum(I1) and platinum(II1) complexes, we have collected the [Pt2(pop)Jzl4- ( x Raman and IR spectral data of [Pt~(pop)4]~-, = C1, Br, I) and [Ptz(pop)4CH31]4-. Application of a normal coordinate analysis delineates the ligand contributions to the Pt-Pt stretching modes. A characterization of binuclear platinum(I1) and platinum(II1) bond strengths will significantly improve our understanding of the interactions in these molecules and possibly lead to better predictions of their chemistries. The electronic spectral properties of these complexes have been investigated. Intense absorptions for [Pt2(pp)4]4-(367 nm)' and [Pt2(p~p)4X2]4(310-460 nm) have been assigned to 5d2z((r*) 6 ~ , ( ( r ) ~and 3 ~ 5d#(a) 5 d 2 ( ~ *transitions. )~ Electronically similar binuclear rhodium(1) and rhodium(I1) ions [Rhzb4I2- and show comparative [Rh2b4X2]" (b = 1,3-diiso~yanopropane)~~~~~ absorptions. Much interest has been afforded to these d8 binuclear platinum(I1) and rhodium(1) complexes which show efficient phosphorescence with vibrational progressions at 139 and 147 cm-l. These vibrations have been assigned to the metal-metal stretching frequencies in the excited state and they occur with an increase in the metal-metal bond strength over that of the ground state. The Rh-Rh stretching frequencies, 79 cm-' for the ground state and 144 cm-' for the triplet level, have been observed for the binuclear rhodium(1) complex by Raman spectroscopy.12 We now report our vibrational analysis for the binuclear platinum complexes and suggest that it may serve as a point of reference in analyzing similar face-centered binuclear or oligomeric
-
-
structure^.'^ (1) (a) Present address: Department of Chemistry, Duquesne University, Pittsburgh, PA 15282. (b) Shell Development Co. Houston, TX 77001. (c) Department of Chemistry, Tulane University, New Orleans, LA 701 18. (2) 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. (3) Muraveiskaya,G. S.; Orlova, V. S.; Evstafeva, 0. N. Russ. J . Inorg. Chem. 1974,19, 1030-1035. Orlova, V. S.; Muraveiskaya, G. S.; Evstafeva, 0. N. Ibid. 1975, 20, 1340-1346. Muraveiskaya, G. S.; Kukina, G. A,; Orlova, V. S.;Evstafeva, 0. N.; Porai-Koshits, M. A. Dokl. Akad. Nauk SSSR 1976, 226, 596-599. Cotton, F. A,; Falvello, L. R.; Han, S . Inorg. Chem. 1982, 21, 1709-1710. Cotton, F. A,; Falvello, L. R. han, S. Inorg. Chem. 1982,21,2889-2891. Hollis, L. S.; Lippard, S. J. J . Am. Chem. SOC. 1981, 103, 676145763. (4) Mann, K. R.; Lewis, N. S.; Williams, R. M.; Gray, H. B.; Gordon, J. G. Inorg. Chem. 1978, 17, 828-834. ( 5 ) Filomena Dos Remedios Pinto, M. A.; Sadler, P. J.; Neidle, S.; Sanderson, M. R.; Subbiah, A. J . Chem. Soc., Chem. Commun.1980, 13-15. (6) Stein, P. In 'Raman Spectroscopy; Linear and Nonlinear"; Lascombe, J., Huong, P. V., Eds.; Wiley Heyden Publishers, 1982; pp 651-652.
0002-7863/83/ 1505-3489$01.50/0
Experimental Section Complexes K4[Pt2(po~)41*2H~0, K 4 [ P t 2 ( ~ ~ ) 4(X X ~=1C1, Br, I), and K,[Pt2(pop)4CH31]were prepared by published procedure^.^^^^' The compound K4[Ptz(pop)4'3CHpI] was prepared in a similar manner2except ~~
~~
(7) Sperline, R. P.; Dickson, M. K.; Roundhill, D. M . J . Chem. SOC., Chem. Commun. 1977, 62-63. ( 8 ) Fordyce, W. A.; Brummer, J. G.; Crosby, G. A. J . Am. Chem. SOC. 1981, 103, 7061-7064. (9) Che, C. M.; Butler, L. G., Gray, H. B. J . Am. Chem. SOC.1981, 103, 7796-7797. (10) Lewis, N. S.; Mann, K. R.; Gordon, J. G., 11; Gray, H. B. J . Am. Chem. SOC.1976, 98, 7461-7463. (11) Rice, S. F.; Gray, H. B. J . A m . SOC.1981, 103, 1593-1595. (12) Dallinger, R. F.; Miskowski, V. M.; Gray, H. B.; Woodruff, W. H. J . Am. Chem. SOC.1981, 103, 1595-1596. (13) Dickson, M. K.; Fordyce, W. A,; Appel, D. M . Alexander, K.; Stein, P.; Roundhill, D. M. Inorg. Chem. 1982, 21, 3857-3858.
0 1983 American Chemical Society
3490 J . Am. Chem. SOC.,Vol. 105, No. 11, 1983
Stein, Dickson, and Roundhill
Table I. Vibrational Frequencies (cm-') of [Pt,(pop),14' and [Pt,(pop),X,]4-
IR 1329 1085 910 695 5 20 442 360 335 308 278 24 1
1270 1060 940 730 516 455 359 335 3 20 295 275 23 7
1270 1092 930 730 520 456 357 336 318 195 283 237
304 235 158 112
2 24
1270 1090 920 728 515 452 357 335 318 118 (178)d 281 23 7
1270 1083 915 7 25 5 20 453,475 356 3 36 3 20 115 (172)d 287 239
POH bend POterm str POterm st* POP str PO, bend PO, bend ring bending Pt-P str ring bending Pt-X str ring bending ring bending
Raman 232 116
134 95
195 b llOQ b
172 b 156 b
Pt-X strC ring bending Pt-Pt str Pt-Pt-X bending
Not observed. P t C H , in [Pt,(pop),CH3II4- is observed at 489 cm-I a Overtones are shown in Figure 3 at 218 cm-' and 326 c m - ' . as shown in Figure 5. Assigned to ring bending but contain significant Pt-I str contribution (see Table 11).
P
= PO,H
Figure 1. Schematic drawing of the [Pt2(pop)4]4-structure. The [Pt2( p ~ p ) ~ X , structures ]~have, in addition, Pt-X units. that "CH31 (90 atom % 13CH31supplied by MSD Isotopes) was used. Infrared spectra were obtained as Vaseline mull films on a polyethylene plate in an evacuated sample chamber of a Perkin-Elmer F1S3 spectrometer. Deuteration of K4[Pt2(pop),] was achieved by carrying out the synthesis in D 2 0 solvent. Samples for Raman spectroscopy were prepared as saturated aqueous solutions. Raman spectra of the binuclear Pt(II1) complexes were obtained with an argon ion 554 Control laser. A 171 Spectrophysics krypton ion laser was used for the binuclear Pt(I1) sample.
Results and Discussion 1. Vibrational Symmetries. In analyzing the present vibrational data, it is useful to begin with the structure in Figure 1 which gives the atomic arrangement for the ion [Pt2(pop),le and shows fourfold symmetry about the Pt-Pt axis. Twenty seven normal modes are classified in the D4hpoint group:
r = 4AI, + lAzs + 3Bl, + 2B2, + 4E, + lAl, + 3Azu + 1B1, + 3B2,
+ 5E,
The AZuand E,, type vibrations involve both P-0 and Pt-P stretches and ring bending modes and are infrared active. Raman active vibrations involving platinum motions transform with At, or E, symmetry. The [Pt,(p~p),X,]~-ions give additional VIbrations due to Pt-X linkages as classified: r = IA,, + lE, + 1A2, + IE, Despite the PtPOPPt units being puckered and the P 0 2 H groups reducing the symmetry from D4h,Figure 1 is useful in analyzing the infrared and Raman active vibrations. A list of number and descriptions of the expected modes are given as follows: Ai,
Eg
A,u
EU
description
1 I 1
1 1 2 0 0 1
1 1 1 0 1
1 1 3 0 0 1
P-0-P stretch Pt-P stretch ring bending Pt-Pt stretch Pt-X stretch Pt-X bend
1 1 0
0
As seen, the Pt-Pt stretching coordinate occurs only in Al,.symmetry and v(R-Pt) is expected to be a mixture of this coordinate,
the symmetric Pt-X, and bridging ligand coordinates. The A,, and AZutype vibrations can couple in [Pt,(p0p),CH,1]~- where the symmetry reduces to C40. 2. Bridging Ligand Vibrations. The IR spectra of the ions [Pt2(pop),I4- and [Pt2(pop),X2I4- are dominated by vibrational modes from the P205H22-units. Their frequencies are listed in Table I. The [Pt2(pop)4X2]4complexes show nearly equivalent sets of frequencies which are interpreted as evidence for little variation of their bridging units. While a comparison between the binuclear Pt(II1) and Pt(I1) complexes show vibrational frequency shifts, the vibrational patterns remain the same. The P-0-P stretching and P02Hgroup vibrations are expected above 400 cm-l while the ring bending modes and the Pt-X stretches occur below 400 cm-l. Symmetric and asymmetric P-0-P stretches have been assigned at 670 and 915 cm-' in pyropho~phites,'~ and at 710-750 and 910-1025 cm-' in diphosphates.l5 The asymmetric P-0-P stretch (A2,) is obscured by the P 0 2 H vibrations. The symmetric P-0-P stretch (E,) is observed at 695 cm-l in [Pt2(pop),le and at 730 cm-I in the [ P t , ( p ~ p ) ~ X complexes. ~]~Although the measured P-0 distances in [PtZ(pop),l4-and [Pt2(pop)4C1z]4-are 1.61 f 0.01 a 35-cm-' shift to higher frequency is observed for the symmetric P-0-P stretch between the Pt(I1) and Pt(II1) ions. This difference can be ascribed to kinematic alterations which arise from a change in the POP bridging angles and does not indicate potential energy differences in the P205H22-bridge. A comparison of the K4[Pt2(pop),].2Hz0 and K4[Pt2(pop),C12] crystal structures indicates an approximate &loo change in the POP angle which apparently accompanies the change in Pt-Pt distance.16 An increase in the symmetric P-0-P stretching and decrease in the asymmetric P-0-P stretching frequencies of -40 cm-I are predicted with this angle change. The P02H vibrations involve P-0 stretches, PO, bends, and POH bending. The latter is identified at 1320 cm-' in [Pt2(pop),le and at 1270 cm-' in the [Pt2(pop),X2I4-complexes, which shift to lower frequency (-300 cm-') in the perdeuterated complexes. The symmetric and asymmetric P-O,,,, stretches occur at 1100 &235
(14) Ebert, M.; Kawan, L.; Pelikanova, M. Collect. Czech. Chem. Commun. 1978, 43, 3317-3323 and references therein. (15) Palmer, W. G. J . Chem. SOC.1961, 1552-1562. Steger, E.; Leukroth, G. Z . Anor. Allg. Chem. 1960,303, 169-176. Muck, A,; Petri, F. Z . Chem. 1971,11,29-30. Etcheverry, S . B.; Baran, E. S. Z . Anorg. Allg. Chem. 1971, 457, 197-202. (1 6) An idealized model where all Pt-Pt-P angles are 90' and all bridging ligand distortion is accommodated by changes in the P-0-P angle was assumed in the calculations of section 3b, giving a 15" change in P-O-P angle
-
between Pt(I1)-Pt(I1) and Pt(II1)-Pt(II1) complexes. Actually the angle change is somewhat smaller (-8") and there is also some increase 1-2') in the Pt-Pt-P angles above 90" for the complex K4[Pt2(pop),CI2](Schaefer, W. P., personal communication).
Spectra of Pt(II) and Pt(III) Octaphosphite Complexes
J . Am. Chem. SOC.,Vol. 105, No. 11, 1983 3491
232
_ a -433
3 x
io3
I'
_
%\IAVEWUP.I:aER (CIS- '#
'-5
WAVEMUMBER !CM?
Figure 3. Raman spectra are shown for -2-mM solutions of [Pt2-
( p ~ p ) ~ X(X ~ ]=~ C1, - Br, I) in H 2 0 , using 5145-A laser excitation. An arbitrary intensity scale is given with each spectrum. The relative intensities of u(Pt-Pt) are 1:6:143. Intensities of u(Pt-Pt) are measured (not shown) against the 932-cm-' signal in solutions with NaCIO,. An Figure 2. Far-infrared spectra of binuclear platinum(I1) and platinumM [Pt2(pop),IJ4(111) complexes K ~ [ P ~ ~ ( P o P ) K ~X ~ [~PI ,~ ~ ( P o P ) ~ CKI~~[ JPI~, ~ ( P o P ) ~ I *intensity ratio of 0.38 occurs in a solution of 7.8 X vs. 0.8 M NaC104. Relative intensities of u(Pt-Pt) with 4880-A laser 2 H 2 0 . The marked (t) signals correspond to the Pt-X modes. excitation are 1.3:8.1:314.
and 910 cm-I and are characteristic of a hybridized PO, unit without a well-defined P==Obond, which is usually observed above 1200 (3m-l.I' Partial double-bonded P-O,,,, units are also suggested by the X-ray data which give P-O,,,, distances in the range 1.51-1.55 A.16The P 0 2 H vibrational bands are broad and may indicate disorder in the hydrogen positions. The vibrations at -525 and 450 cm-I are compared with POz bending frequencies observed in phosphites and pyrophosphites.'* These modes are not expected to influence the Pt-Pt stretching frequencies significantly. Infrared assignments below 400 cm-' are more tentative, although the Pt-X frequencies can be confidently identified at 295, 195, and 118 cm-' for [Pt2(pop),X2I4- (X = C1, Br, I). We assign the 335-336-cm-I bands to the Pt-P (E,,) stretch as compared to the range 310-360 cm-' for complexes PdX2(PMe3),.I9 The far-infrared spectra of our complexes, given in Figure 2, show intense lines at 178 and 172 cm-l for [Ptz(pop)412]4-and [Ptz(pop),CH3II4- in addition to the Pt-I stretches; these vibrations are assigned to asymmetric (A2,,) ring-bending vibrations. Their intensity may be attributed to mixing with the Pt-I motions. 3. Pt-Pt and Pt-X Vibrations. The Raman spectra of the [Pt2(pop),X2]'- ions shown in Figure 3 and that of [Pt2(pop),le shown in Figure 4 are characterized in aqueous solution by intense signals between 110 and 158 cm-l and are assigned to the Pt-Pt stretching frequencies. A weaker signal in the ions [Pt2(pop),X2]4' at 304, 224, and 194 cm-l is due to the symmetric Pt-X stretch. Depolarization ratios show these signals to be polarized. The [Pt2(pop),Cl2le and [Pt2(pop),Brz]" ions have broad bands which are depolarized and are assigned to E, ring bending and Pt-Pt-X bending modes. The band at 232 cm-I in [Pt2(pop),I4- may be the corresponding E, ring mode or possibly an overtone of v(Pt-Pt) at 116 cm-I. Modes due to the phosphorus ligands are not observed in these spectra. The intense v(Pt-Pt) signal in [Pt2(pop),121e is attributed to a preresonance enhancement with the u-u* excitaion at 438 nm. The relative intensity of v(Pt-Pt) in [Pt2(pop),IZl4-against that ~~
(17) Bellamy, L. J.; Beecher, L. J . Chem. SOC.1952, 475-483. (18) Tsuboi, M. J . Am. Chem. SOC,1957, 79, 1351-1354. (19) Park, P. J. D.; Hendra, P. J. Spectrochim. Acta, Part A 1969, ZSA, 909-916.
.5C
I30
93
I70
210
2
WAVENUMBER (C'vl-'!
Figure 4. Raman spectrum of -5"
aqueous solution of [Pt2(pop)4]4-,
using 4067-A laser excitation.
in [Pt2(pop)4C1z]eis approximately 140 times, and the appearance of the two v(Pt-Pt) overtones characterize the resonance effect in the [ P t , ( ~ o p ) ~ I , ]ion. ~ - These overtone frequencies (218 and 326 cm-l) indicate a reasonably harmonic mode for u(Pt-Pt). The v(Pt-Pt) and v(Pt-X) stretches are now analyzed by normal coordinate calculations both with and without the bridging units. A comparison of these results allows a realization of the influence of the bridging units. (a) Vibrational Analysis of the X-Pt-Pt-X Unit. A simple analysis is offered to estimate the difference in Pt-Pt bond strength between the binuclear Pt(I1) and Pt(II1) complexes. We find that a linear four-atom unit, X-Pt-Pt-X, is sufficient to reproduce v(Pt-Pt) and symmetric v(Pt-X) observed for the [Pt2(pop),XJ4ions. An analytical solution has been obtained for v(Pt-Pt) and the symmetric v(Pt-X) in terms of three force constants, K(Pt-Pt), K(Pt-X), and I(Pt-Pt; Pt-X). Details are given in the Appendix and frequencies are shown in Table 11. Although a unique fit is not possible, a reasonable solution can be chosen with K(Pt-Pt) = 1.7 mdyn/A, I(Pt-Pt; Pt-X) 5: 0.16 mdyn/A, and K(Pt-X) = 1.65, 1.45, and 1.2 mdyn/A for X = CI, Br, and I, respectively. When the K(Pt-Pt) force constants are assumed to be linearly
3492 J . Am. Chem. Soc.. Vol. 105, No. 11, I983
Stein, Dickson, and Roundhill
Table 11. Observed and Calculated Frequencies and Force Constants of Binuclear Platinum Complexes A . Normal Modes with
Pt-Pt and Pt-X Contributions
frequency, cm-'
symbolb obsd
compd [Pt,(PoP)4~z14-
[Pt,(pop),Br214'
[Pt,(PoP),Iz
1,-
[Pt,(p0p),CH,1]~'
[Pt,@OP), 1 4 -
A,,
calcdd
304 158 295
304 (304) 153 (157) A,,, 285 (304) 154 A,, 224 218 (222) 134 1 3 2 ( 1 3 4 ) A z U 195 201 (206) 143 A,, 194 187 (195) 110 107 (109) A,U 178 171 (161) 118 121 A , 489e 488e 1 7 2 177 156 159 115 116 A,, 116 115 (116) Azu 162
PED:
%
Pt-X
Pt-Pt
bridgeC
78 1 85 1 77 17 78 17 49 45 37 60 93e 48
4 69
1 25
0 57
1 22 51 47 26
1 35 24 12 38
85 .~ 10 18 16 67 18
9 56 22 1 34 57 16 58 89
B. Additional Modes for the A,, and AzUSymmetries of the [Pt,(pop),X,I4- Complexes (X = C1, Br, and I) and [PtZ(pop),l4sym- [Pt (POP),1,; bo1 calcd, cm-
-
*
1,
A,,
-
, POP),^,, 14-
[Pt calcd, cm-
696
758
403
3 85
292 963 372
294 923 372
[Pt,(PoP),CH, I1 4calcd, cm-' 719 411 295 958 376
y I
L
3c3
230
4c3
A
522
ViAVE NUMB E R (CM 1 Figure 5. Raman spectra of [Pt2(pop)4CH31],+and [Pt2(p~p)413CH31]4-
in -2" aqueous solutions are shown. Signals from trace amounts NMR measurements, are of [Pt2(pop),I2I4-.
