X-ray photoelectron spectra of palladium and platinum complexes of carbenoid and related ligands

99

Journal of Orgunometaliic Chemistry, @ Elsevier Sequoia S-A., Lausanne -

11-I (1976) 99-106 Printed in The Netherlands i

X-RAY PHOTOELECTRON SI?ECTRA OF PALLADIUM AND PLATINUM COMPLEXES OF CARBENOID AND RELATED LIGANDS

PATRICK BRANT. JOHN H. ENEMARK Dcpartmcnt and ALAN Department

of Chemistry.

*,

L~niversity

of Arizona.

Tucson.

Unicervi?~

of Califorrtia,

_-trizuna 6.i 721

(G.S.4.)

L. BALCH of Chemistry.

Duuis. California

9.56~6

(Li.S._4.)

(Received December 29th. 1975)

Summary S-ray photoelectron spectra have been obtained for a series of palladium and platinum compleses involving carbenoid and related ligands. The Pt 4f7 2 and Pd 3d,-, 2 binding energies indicate that carbenoid ligands are better donors of electron density to the metal than methyl isocyanide. The C 1s binding energies show that the carbenoid carbon atoms are less positively charged than the carbon atom of coordinated methyl ispcyanide. - ..__-- ._..-.. ._

___

Introduction The chemistry of transition metal complexes of carbenoid ligands has developed rapidly in recent years [ 1,2]. In our laboratories we have been particularly interested in the palladium and platinum compleses of ligands obtained by addition of nucleophiles to coordinated isocyanides [3--51. Such complexes have been alternatively described as metal carbenes [1,2] (A) stabilized carbonium ions (B) [ 8,9] or metallated amidinium ions (C) [ 31. ,_, NH R f--C

M --:c

M-_C

‘\ P?HR (A)

+ .I \

NHR

, h-IF7 c-

M-_C

NHR !Ca)

should be addressed.

*--_)

c’(

MHR

03)

* To whom all correspondence

Cl -

Xi’

NHR

%-IR ICW

100

Various physical methods have been employed in an effort to distinguish between the possible bonding schemes and charge distributions suggested by the valence bond structures_ An early infrared study of chromium and tungsten pentacarbonyl aikosycarbenoid complexes [ 61 led to the conclusion that an alkosycarbencid ligand is both a strong u donor and z acceptor_ On the other hand. single crystal S-ray structural results on arninocarbenoid complexes [ 3.71 have been interpreted to indicate that these carbenoid ligands are strong u -don&-s but at best weak ir acceptors. Carbon-13 NMR chemical shifts for carbeno&carbon a :e similar to those found for free carbonium ions leading some investigators [ 8.9) to describe the compleses as a metal stabilized carbonium ions (B). Recently, a “Fe Mossbautr study involving carbenoid compleses was reported [lo] and interpreted to indicate that diamino carbenoid ligands are good o donors but weak x acceptors. The various possible ground state charge distributions in carbenoid compleses make them interesting and appropriate candidates for S-ray photoelectron spectroscopy (XPS) because relative changes in the binding energy at a given atomic center in a closely related series of molecules can often be related directly to changes in the charge distribution of the ground state [ ll--131. The deconvoluted C Is S-ray photoelectron spectrum of Cr(CO),tC(C~i,)(OC~r,, j in the gas phase has been reported [ 141. However, the measurement of the C Is and metal binding energies for a single compound provides lit.tle general insight into the charge distribution in carbenoid complexes and the relative effect of carbenoid ligands upon the electron density on the metal. Therefore. we have obtained high resolution solid-state SPS data for a series of carbcnoid complexes of palladium and platinum_ In addition to the carbenoid compleses investigated. we also report the SPS data for several methyl isocyanide compleses. To our knoxledge, no previous SPS investigations of isocyanide compleses have been reported_ Experimental Xl1 materials investigated were prepared according to previouslv published procedures [5,15-1’71 or purchasedlnd used without further purification_ X11 the compounds investigated are air stable_ S-ray photoelectron spectra were obtained with a McPherson ESCX 36 Photoelectron Spectrometer equipped with a Sargent-Welch turbomolecular pumping system_ Both Xl-h’, (.1486.6 eV) and Mg-K, (1253.6 eV) radiation sources were used in conjunction with a 7.9 cm anode shield. Samples were mounted on doublestick tape (331 Company) or lightly pressed onto aluminium metal planchettes which had been previously etched with a hydrochloric acid solution and rinsed with distilled water. Standardization was achieved using MOO, (MO 34; 2,232_5 eV) and!or a characteristic component of the C 1s spectrum (obtained in some cases by cume deconvohttion) previously standardized with the MO 3dsz1 line in MOO+ The C 1s binding energy in coordinated triphenylphosphine was found to be 285.0 eV. Binding energy values reported are the average of two or more determinations, and the average standard deviation for a given reported binding energy is 20.24 eV or less. Curve deconvolutions and relative area ratios were determined with a DuPont 310 Curve Resolver.

Results

and discussion

Platintcnr binding energies Table 1 lists the XI’S data for the platinum compleses investigated_ All of the c‘arbenoid compleses (3,4.5.9.10,11,13,15,16) are produced by nucleophilic addition to coordinated methyl isocyanide [ 51. The SE dat.a for two isocyanide compleses of platinum, 2 and 14. are also given in Table l_ The Pt 4f7,, binding energies of 2 and 1-t imply a marked decrease ->f electzon density on t.he metal relative to the carbenoid complexesThe addi+_.ion of four equivalents of mcthylamine to 14 leads to compound 7: the only tetrakis-carbenoid platinum complex investigated. Differences in binding energy due to lattice effects should be minimized for 7 and 14 because both are dipositivt cations isolated as their PF,. salts. Thus, the large decrease in the Pt 4/7_2 binding energy (2.0 e\‘) in going from 14 to 7 presumably reflects the much stronger total electron donating ability of the rarbcnoid ligand versus the isocyanide ligand. Additional evidence for the cliffering electron donat.ing and accepting ability of carbenoid ancl isocyanide ligands is provided by compound 2!. This formally platinum(I) compies has a Pt 4i, I binding energy similar to 7, and significantly higher than 3 and 4. Other comparisons of the effect of a carbenoid ligand on the metal charge relatiw to other lipantis can be made from the data in Table 1. Compounds 4, 6, 8 and 12 are all formally square planar platinum(II) compounds containing the cis-PtC1, fr.agtnent. Llariations in relasation effects should be minimizecl in this series, and the differences in metal binding energies should be primarily due to differences in the electron donating ability of the coordinated ligands. The data for 4, 6. S and 12 indicate that. the chelating carbenoid ligand is as good or better donor of electron density than bipyrid_vi, PR\ or Cl-. This result is reinforced by the Pt 4ft L binding energy for compound 3. In light of the - 1 eV decrease in binding encrs when two Cl- ligands are replaced by carbenoid iigands in going from 12 to 4 it is somewhat surprising that the binding energy of 7 is similar to 12. The possibility that there are greater rciasation or reorganization effects on the platinum bincling energies for species

3, 4, 5, and 6 cannot be ruled out. although differences in relasation energies are usually minimized for a closely related series of molecules [lS,19]. It is also important to note that in ‘i each carbenoid ligand must be tram to another carbenoid

”

c

accz

3-?lc? m-c_

103

ligand. Structural studies show that carbenoid ligands exert a strong trans influence [4]_ Table 1 shows that the binding energy for the trans-dicarbenoid comples (13) is about 1 eV larger than the energies for the related cis complexes (9. 10,ll). If the Pt binding energy is indeed sensitive to the stereochemistry of the carbenoid ligands then the expected decrease of -1 eV per pair of Clligands of PtCL’- (12) replaced by carbenoid ligands will be nearly counterbalanced by an -1 eV increase for trans geometry, and the binding energies of 7 and 12 will be little different. Replacing the cis halogens in compounds 3 and 4 Ly CHI\NC produces compleses which are acidic [3,5] and lose a proton to giv+ t.he cationic species 9, 10, and ll- These compounds contain the same cation in different environmentsThe electronic spectra of the solids 9 and 10 contain additional electronic transitions (at 21000 and 17000 cm-’ respectively) that are not present in 9, 10 and 11 in various solvents or in 11 in the solid state. In addition 9 and 10 show color changes from red to green upon dehydration. It is likely that 9 and 10 contain cations which are associated through metal-metal interactions [ 51. However, these interactions do not manifest themselves in the SPS spectra. Not only are the Pt 4fi;z binding energies nearly identical in 9, 10 and 11 but the Pt 4f7,.z and I 3djiz binding energies of 10 in hydrated and dehydrated form are not detectably different. The results in Table 1 also demonstrate the inherent difficulties and ambiguities in using XPS data to probe chemical bonding. For esample, the binding energies of dicarbenoid compleses 3 and 4 are -0.6 eV less than those of 9,10, and 11 which are in turn 0.9 eV less than that of 13. The observed binding energy increases for these three kinds of dicarbenoid compleses could be due to the

change in the overall charge on the complex

from 0 to +l to +2, to the change

in the ligands. and/or to the change in stereochemistry about the metal_ The relative importance of each of these changes to the observed differences in binding energy cannot be specified from the XPS data_ Addition of halogen to complexes 3 and 4 produces the formally platinum(IV) complexes 15 and 16. The increase in platinum binding energy of 0.6 eV on oxidation is much less than the increase of -2.5 eV reported for osidative addition of halogen to other square planar complexes [ 201. Palladium binding energies The SPS data obtained for the palladium complexes are presented in Table 2. A comparison of the Pd 3d5,- binding energies for the compleses 21, 22, and 23 which contain the cis-PdClz unit indicates that the chelating carbenoid is as good a donor of electron density as the other ligands, similar to the results for the platinum series. The relatively high Pd 3d5/: binding energies for the two isocyanide complexes 18 and 25 are consistent with the Pt 4,Fii2 binding energies found for the isostructural platinum complexes, 2 and 14, and support the classification of the &cyanide ligand as a moderate r acceptor of electron density. As found for the platinum analogs, addition of four equivalents of methylamine to 25 produces the tetracarbenoid complex 19, and decreases the Pd 3d,,, binding energy markedly. Other binding energies The C 1s spectra of the methyl &cyanide

complexes

2,14,

18 and 25 dis-

-1:>:

i__--_

A.--_._-._-_ T

-_-__

c

T

..b

1

.o

_,s:c._j”:z?_,_ I

f

. ..

FIN.

of AEB

1. Rot

cbwa-n

LS rhr

rsrimatz=d


error

Pd &fry2 .s. JEB refcrcncr

arbirral~ in thr

i

point.

(t-V)

Thr

Ft 4:;;:.

honrontti

The and

binding

vrniral

energ?; bars

for

of each

SS(bipy)CI~

had

compound

been

indicate

rhr

mrasurvmrnt.

played two distinct peaks at. -25’i and 2S5.0 eV with an area ratio between 1.2 I 0-S and 0.7 : 1.3 (higher Err/lower Ea)_ Relative intensities varied according to the sample support material, binding energy standardization procedure and thickness of the sample_ S-ray photoelectron spectra of CH$C in the solid and gas phase were reported by Barber et al. [ 211. Two peaks were found in the gas phase at 293.8 and 293.1 eV. and two deconvoluted peaks were reported for the solid state at 28’7.5 and 28’7.0 eV. The binding energies found in this investigation for coordinated CH,NC (-287 eV) are approsimately the same as that reported for solid CH3NC. The peaks were not deconvoluted because the FWHM values were not unusually large. AR comp!eses invoking carbenoid ligands gave broad C 1s spectra (FWHM * -2.4 eV) centered at 285.5-285.8 eV. Curve deconvolution for the “simpler” carbenoid compleses such as 5 gave an ave_mge binding energy for the carbenoid carbon of 2865 + O-4 eV_ Similar values were obtained for samples mounted on aluminum plates and for thick samples on double-stick tape. No attempts

BISDISG ESEHGIES ?-OH P.ALLADIUh¶ --_ _..___.._. _.-.-..__ Compound _-_ _

17

Pd

._ ..~_._...

.._~-.

IPd~~(CSCHi)hliPFhli IPd~C(SIICH3~Zi?l[PF~12

20

C~II,oX_rPdBr: C~li~~,X,PdCI~ Pd(bipyG2 KiedC13

-

__ _...

-_.

_.. Pd

trans.Pd[

,.z

~..._.

Ib

IIC tld

__ c

_ ._...-._..__ --..---1s

...~ . . .. . . .

--. --- -----------

full

width

285.0(1.7)

286.41.9).

285.0(!.9)

285.0(2.1) 285.0(1.6) 287_2(2.0).

339.1<1.8> .._...

=

286.9<1.7).

337.9(1.9)

338.2(1.5)

F(C,H&CIz

-.--..

338.1(1.7)

337.8(1.7) 338.0(1.8) 338.1t1.6) 338.2(1.5)

[P~(CSCEI~>.JIJPF&

* FWl1M

365

335.1(1.6)

19

25

_.

St?UCtUre . .- --.--

mscal

18

21 22 23 2-a

.-.

COMPLEXES

at

.-

._~ ..-.. -

half

maximum.

-.

..--.

.-

. ..-

--~.--

-----

----.----.---

285.0(1.9)

105 were made to deconvolute the C 1s spectra of 9, 10 or 11. Since the carbenoid C 1s binding energies reported are determined by deconvolution and involve inherently large standard deviations, internal comparison of these binding energies cannot be made. However, a general comparison of the binding energy difference between isocyanide carbon and carbenoid carbon is valid. As expected, the addition of a nucleophile to the methyl isocyanide ligand decreases the binding energy of the coordinated carbon significantly_ Resonance structures Ca and Cb suggest the possibility of distzibuting positve charge over the hetero atoms of the carbenoid ligand. In this study the atoms bound to the carbenoid carbon were N and S. The N Is binding energy of 399.8 c 0.4 eV for 7 and 19 is slightly higher than those observed for most amines and slightly lower than common values for amides [ 22). The S 2p3!? binding energy of 163.9 eV for 13 is more positive than in thioethers such as (ChH,)S(t-Bu) (162.4 eV) [ 231. It has been demonstrated previously [%I) that a plot- of AE,Pt 4fi,’ versus AEaPd 3dsi2 for corresponding platinum and palladium halide and pseudohalide complexes yields a straight line with a slope of unity. A plot of AE,Pt versus A.E#d (referenced to M(bipy)Clr) for the two series of complexes is shown in Fig. 1. A least-squares fit of the data for the series M(PPh,),Cl,. M(bipy)Cl:, C4H,&MClt, and C,H,,,N,MBrz produces a straight line of slope 0.93 2 0.04. This plot implies that the chelating carbenoid iigand is a slightly better u donor than bipyridyl or triphenylphosphine. The deviations of [M {C(NHCH3)z )*lz+, [MI(CNCH,),] ‘* and [M(CNCH,),]” from the line in Fig. 1 clearly suggest that these dications are fundamentally different from the neutral compounds containing the cis-MX, fragment. The deviations to larger AEB for isocyanide complexes of platinum compared to palladium (Fig. 1) contrast with a previous study [ 241 in which smaller 4ZZa values for platinum were observed for rr-acceptor ligands. Conclusions The dat.a obtained clearly indicate that the carbenoid ligands studied are good donors of electron density to palladium and platinum. It can be inferred that the carbenoids are, at best, weak R acceptors. In comparison, the isocyanide ligands remove charge from the metal with the removal being greater for platinum compleses t.han the palladium analogs. These results are consistent with isocyanides being moderate x acceptors [ 261. The XPS results also show that a carbenoid carbon is less positively charged than the carbon of a coordinated methyl isocyanide molecule. Recent non-parameterized molecular orbital calculations 1271 an (OC),Cr[C(CH,)(OCH,)] have likewise concluded that the carbenoid carbon atom is not positively charged. The primary reasons for postulating a positively charged carbon atom in carbenoid ligands have been the reactivity of such ligands with nucleophiles [1,2,283 and the similarity of the 13C chemical shifts to carbonium ions [S,9]. The reactivity with nucleophiles can be rationalized by the nature of the lowest unoccupied molecular orbital (LUMO) of the complexes 1271, but the ‘% chemical shifts have yet to be adequately explained. The metal binding energies for closely related pairs of compounds increase

106

with increasing formal metal oxidation state *_ Thus, the Pd 3dS12 and Pt 4f,,z binding energies are lower in the metal(I) dimcrs IS and 2 than in the tekakisjmethyl isocyanide)metal(II) species 25 and 14. Similarly 15 and 16 eshibit higher Pt 4f7,: binding energies than do 3 and A_ Finally, there is some evidence from Tables 1 and 2 that cis and trczns complexes of four co0rdinat.e platinum and palladium may exhibit detectably different metal binding energies_ This point needs to be verified by a more estensive of cis and Crons isomers of a series of ligands.

study

Acknowledgements We are indebted to Professor R-D. Feltham and Ms. T-F. Block for several helpful discussior?s. Partial support of ;his work by the Sational Science Foundation is gratefully acknowledged. References t 2

10

11 12 13 1-s

15 16 I? 18 19 20 21 ‘Z 23 24 1-s 26 27 26 29 30

31

F.A. Cocmn and C.Xl. Lukrhnrt. Frog_ Inore. Chrnl.. 16 (1972) .tHf. (a) D.J. Cardin. B. Crrinkaya and ?rl.F. Lappert. Chem. Rev._ 72 (1972) 54-1: fh) D.J. Cardin. H. Cerinkaya. M.J. Doyle and M.F. Lapprrf. Chrm. Ser. Rev., 2 (1973) 99. N-X. Isutter. J.tt. Enemark. J. Parks and A.L. Batch. tnorg. Chcnx._ 12 (1973) 451. W.N. Butter and J-H. Encmark. Inorg_ Chcm.. 12 (1973) 540. A-L. Rakh and J-E. Parks. J. Amer. Chrm. Ser.. 96 (1914) 111-I. M-Y. Darensbours and D.J. Darcnsbourg. Ifior~. Chem., 9 (1970) 39. R.F. Ste~anuk and S.C. Payne. Inor~. Chem.. 13 (195-l) 79;. L-F. Farm-Il. E.W. RxxialI rnd E. Rosenbcrx. Chrm. Cnmmun.. (197X, 1036. G-M. Rodner. S-R. Kaht. K- Bark. R-S. St**rhoff. J-E. Wulirr and L.J. Tndd. Ino;~. Chen:.. 1’2 (1973) 1071. G-M. Bancroft and P.L. Sears. Inorg. Chcm.. I-1 (1975) 2716. F. itobborr.W_ Beck and H.D. Bartunik. Chem. Phys. Le’tt.. IA (1973) 217. C-D. Cook. K-Y. Wan. U. G&us. K. HAmrin. G. Johannson. E. Otsson. tf:Swabahn. C. Sordiing and K- Siegbahn. J. Amer. Chem. Ser.. 93 (1971) 1904. 1. Adams. J-31. Thomas. (;.A!. Bancroft. K-D. Butter and Xl. Barber. Chrm. Commun.. (1973) 751. u’-B- Perw_ T-F_ Schsaf. W-L_ Jo&. L.J. Todd and D-L_ Cromn. tnorp. Chem.. 13 t19i.t) 203%. G.B. Kaufman and T.A. Tctrr. tnnr~. Srn;h._ 7 (1963) 245. L. Xata~esta and C. Carirlto. J. Chrm. Sac.. (1958) 2323. F-G. Mann and D. Purdie_ J. Chcm. SM.. (1935) t549. R-L. Martin and D.A. Shirley. J. Amer. Chum. Sot.. 96 (X97-1) 5299. D.A. Shirks-. Chcm. Phys. I.ecz.. 16 (1952) 220. W’. Riu. Anal. Chem.. 44 (1972) 830. 31. Barber. P. Baybutt. .J.A. Connor. I.ti. tlitlirr. W.N.E. ?rtcrrdith and V.R. Sandrrr. m I,...\. Shirk=? (Ed-). Etecrron Spectroscopy_ Amercian Elscvirr. Ner York. 1972. p. 753. R-J. Lindbrrp and J_ tIedman_ Chrm. Scripta. 7 i t915) 155. S. Pignacaro. L- Lunaz7i. C-A. Boicclti. R. Di.htarmn. -4. Ricri. A. Xtrncini. R. Dame11 atd ‘2. Emd~o. Tetrahedron Lat.. 52 (1372) 5341. W.E. Moddeman. J-R. Btackbum. G. Kumar. K.X. Morgan. R.G. Albridpr and M.M. Jonrs. trm~. Chem.. 1 t (19X) 1715. D.J. Doonan. A-L_ Batch_ S.Z. Goldberg. R. Eirenberg and J. Bliilrr. J. Amer. Chem. Sot.. 97 (1975) 1961. A.C. Sarapu and R.F. Fenske. Inox. Chem.. 1-I (1975) 247. T.F. Block. R.F. Fenske and C-P. Cases. J. Amer. Chem. Sot.. 98 (1936) 44 1. A. Da&an and D-L_ Reger. J. Amer. Chrm. Sot.. 94 (1972) 923;. G_J_ Leigh. Inorg. Chim- Acta. 11 (1975) L35_ G. Schiin. J. Ettcwon Spectroscopy. i (1973) 377. G. Johansson. J. Hedman. A. Bernduson. 31. Khsson and R. XiLwm. J. Electron Specwoscopg. 2 (1973) 295.

- Simitar resutrs have been observed for other complexes transition series_ See for example ref. 29.

of metals of rhe second and third