The X-ray photoelectron spectra of inorganic molecules

401 Joarnul of Molecular

0

Structure,

17 (1973) 401-409

Elsevier Scientific Publishing

Company,

Amsterdam

THE X-RAY PHOTOELECTRON MOLECULES

- Printed in The Netherlands

SPECTRA

OF INORGANIC

VI.* COMPOUNDS CONTAINING RHENIUM-OXYGEN AND RHENIUM-HALOGEN BONDS: RHENIUM 4A CHLORINE 2p AND OXYGEN 1s BINDING ENERGIES AND THEIR CORRELATION WITH STRUCTURE AND OXmATION STATE D. G. TISLEY AND R. A. WALTONt Department

(Received

of Chemistry,

2 November

Pardae

University, Lafayette,

Ind. 47907 (U.S.A.)

1972)

ABSTRACT

Rhenium 4f, chlorine 2p and oxygen 1s binding energies have been recorded for a series of coordination complexes of rhenium(V) and rhenium(V11) containing rhenium-oxygen bonds. Related studies on the rhenium(m) acetates Re2(OAc)4X2(X = Cl, Br) and on several adducts of the rhenium chlorides (Re 4f and Ci 2p binding energies only) are also reported. Chemical shift data are related where possible to the molecular structures of the complexes. Correlations between rhenium 4f binding energies and oxidation state are of diagnostic value only in distinguishing the high oxidation state species (rhenium(V) and (VII)). Differences in rhenium 4f;and oxygen 1s binding energies [6(01s, Re 4f;)] may be useful in probing the nature of the rhenium-oxygen bonds.

INTRODUCTION

In previous papers’ - 3 we have reported our studies on the X-ray photoelectron spectra (ESCA)4 of several Zuluoxidation state derivatives of the rhenium halides. The purpose of these studies was two-fold. (1) To establish whether this technique would provide definitive structural information on metal halide clusters of rhenium(H) and rhenium(ILL)‘* 2, and (2), to determine whether changes in metal core electron binding energies between different oxidation states were of * For Parts I-V see refs. 2, 3, 1 I, 12 and 17. + Address correspondence to this author.

402 sufficient magnitude to justify the use of this technique to assign formal oxidation

for a series of complex rhenium chlorides possessing relatively simple structures and containing only one type of ligand atom, namely chlorine. In the present communication, we report on the X-ray photoelectron spectra of a variety of coordination compounds containing rhenium-oxygen bonds with rhenium in the formal oxidation states 5 + and 7-i-. Since the structures and chemistry of the species which we have studied show many similarities to related molybdenum and tungsten compounds, we believe that our results and conclusions are typical of those to be expected for analogous systems of other heavy elements of the early transition series in high oxidation states. states3,

EXPERIMENTAL

Samples of the complexes were generally available from earlier studies or were prepared by standard literature procedures’ - ’ ‘. The complexes [ReO,py,]I and fRe0,py,]PF6 were kindly supplied by Mr. J. Ebner. ReO

,Cl(bipy

)

and ReO 3Cl(phen)

An aqueous solution of NaReO, (0.1 g in 15 ml) was passed through a Dowex 5OW-X2 cation exchange column. To the resulting solution of per-rhenic acid was added Z,Z’-bipyridyl, such that the HReOb-bipyridyl ratio was 1:1.3, followed by 10-15 drops of concentrated hydrochloric acid. The solution was set aside for 24 h and then evaporated to low volume (3-5 ml). The resulting pale yellow product was filtered off, washed well with diethyl ether to remove excess 2,2’-bipyridyl, and heated at 100 “C in uacuo to sublime off any 2,2’-bipyridinium chloride. Found: C, 28.0; H, 2.1; Re, 43.4; Cl, 8.3. Calculated for ReO,Cl (C,,HsN,): C, 28.2; H, 1.9; Re, 43.7; Cl, 8.3. The related phenanthroline complex ReO,Cl(phen) was prepared by an analogous procedure to that described above. Found: Re, 41.7; Cl, 8.0, calculated for ReO,Cl(C,,H,N,): Re, 41.4; Cl, 7.9. In addition to the usual coordinated ligand absorption bands, the infrared spectra (Nujol mulls) of these two complexes exhibited a characteristic v(Re-0) pattern viz., ReO,Cl(bipy), 944s, 920~s and 91Ovs; ReO,Cl(phen), 944vs, 923vs, 918~s and 910~s. Salts

of

the type

[BH]Re04

(I3 = Pyridine,

2,2r-Bipyridy1,

IJO-Phenanthroline

add Ac~~d~n~~

If the above tertiary amines were added to per-rhenic acid solutions, generated as described above, and the solutions evaporated to low volume, or

403 to dryness in the case of the pyridine system, then the per-rhenates [PyH]Re04, [bipyH]ReO,, [phenH]Re04 and [AcrH]Re04 could be isolated. The crystalline materials were washed freely with diethyl ether and, with the exception of byH]Re04, recrystallized from acetone. In all instances their infrared spectra (Nujol mulls) exhibited the appropriate BH+ absorptions and the characteristic v3 band of the per-rhenate anion (usually split) at -900 cm- ‘, viz., [pyH]Re04, 915vs and 875~s; [bipyH]ReO,, 9 17vs and 895vs; [phcnH]Re04, 908vs; [AcrH] Re%, -920sh and 903vs. Physical meastuements The X-ray photoelectron spectra were recorded using a Hewlett-Packard Model 5950A ESCA spectrometer. Monochromatic aluminium K,,., radiation (1486.6 eV) was used as the X-ray excitation source and the powdered samples were dispersed on a gold plated copper surface. Further details of our experimental procedures are described elsewhere”* 12. Infrared spectra were recorded as Nujol mulls on a Beckman IR-12 spectrophotometer.

Rhenium 4f, chlorine 2p, oxygen 1s and carbon 1s binding energies for

twenty-two compounds of rhenium are given in Table 1 and some representative spectra shown in Fig. 1, In all instances, the binding energies are referenced to the C 1s binding energy of graphite at 284.0 eV. In addition to serving as an internal reference check, intimate grinding of the sample with graphite appeared to generally eliminate, or at least reduce to a minimum, sample charging effects”* r2. For the compounds listed in Table 1, the binding energies were usually located with the following precision: Re 4fe -+O.l and 4f;- kO.1 eV; Cl 2pt kO.2 and 2p+ &O. 1 eV; 0 1s f 0.1 eV; C 1s +0.2 eV. Typical rhenium 4f+ and chlorine 2p+ binding energy half-widths (FWHM)* occurred in the ranges 1.2 to 1.6 and 1.2 to 1.5 eV, respectively. The one exception to this was the 2,5-dithiahexane complex Re2C15(DTH)28* ’ 3, for which we observed unusually broad rhenium 4f binding energies, with a FWHM value for the 4f; peak of -2.2 eV. This feature will be considered further in the next section. With the exception of the two complexes containing the Re,O, moiety (Table 1), the other compounds containing Re-0 bonds exhibited oxygen 1s binding energies with FWHM values in the range 1.2-1.6 eV. With Re,O,(dedtc), and Re,03Cl,py4, which contain two types of rhenium-oxygen bonds within the linear O,=Re-O,-Re=O, moiety14*15, * Full width at half maximum.

404 TABLE

1

CORE ELECfRON

BMDING

ENERGIES

(in

ev)

FOR COORDINATION

Re

fReC13(PPh3)12 tReCMDTH)12

ReC13(tu)3 ReCla(tu)3, 1/3 (CH&CO ReCId(PPh3)2 ReJ&(DTH)Z Rel(OAc)aC12 Ret(OAc)JBrz ReOC13(PPh& ReOBrs(PPhs)s ReOClj(PEt,Ph)z

IRe02pyXZ [ReQsyJ[ [ReOsy.JPF~

RezOa(dedtc)b Re20&14pya ReO&l(bipy) ReO&l(phen) IpyHIReO4 [bipyHfRe04 [phenHlReO.+ [AcrH]Re04

2I-M

COMPLEXES

Cl

0 Is

4ft

4fz

-Qt

ZP$

44.9 44.8 43.8 44.1 45.5 44.0

42.6 42.5 41.5 41.8 43.0

199.6 199.4 198.9 199.0 199.8 199.4 199.4

198.1 197.9 197.7 197.6 198.3 197.9 197.7

199.0

197.6

46.1 46.2 45.5 45.5 45.3

41.7 43.7 43.8 43.2 43.2 43.0

45.4 45.5 45.5 45.0 45.4 47.8 47.7

43.0 43.1 43.1 42.7 43.0 45.5 45.4

47.6 47.9 47.4 47.5

45.2 45.6 45.0 45.0

198.9 197.7 197.3 195.9

198.9

OF RHENIUM(III),

197.5

199.1 197.5 199-I 197.6

533.9 534.0 532.3 531.9 532.0 530.2 530.8 530.2 530.6 530.8 531.2 531.4 531.3 531.6 530.8 530.8

(IV),

(v)

AND

(VII)

c is

Otherbitzdingenergies

284.3 284.5 284.5

N 1s 400.2 N Is 400.1

284.7 288.8,285.0 288.7,285.0 Br 3~+,~ 188.4,181.7 284.1 284.0 284.2 Br 3pa_~ 188.3,181.4 284.6 I 3ds,$ 630.2, 618.9 284.8 N is 400.6 285.0 284.5 284.3 284.4

284.3

N Is 400.9

a Ligand abbreviations are as foflows: DTH = 2,5_dithiahexane; tu = thiourea; dedtc = diethyldithiocarba-

mate; bipy = 2,2’-bipyridyi; phen = l,lO-phenanthroline;

Acr = acridine.

Fig. 1. Rhenium 4Jbinding energies of coordination complexes of rhenium(V) and rhenium(VI1) containing Re-0 bonds. (a) [ReOzpyd]PFs; (b) ReO&l(bipy); (c) [bipyH]ReOJ.

405 the oxygen 1s binding energies were usually broad (FWHM - 2.0 eV). We attribute this feature to the overlap of the binding energies of the two types of oxygen atoms (0, and 0,). We did not observe a clear resolution of these two components, although the peaks exhibit an asymmetry

on their high binding energy side con-

sistent with an overlap of the binding energies associated with Ob and 0, in the ratio of l/2. By analogy with systems containing fairly strong MCI-M bridges1*‘*t6, we would expect that Ot,, bound to two metal centres, would exhibit a higher binding energy than 0,. For those systems whose carbon 1s binding energies were recorded (Table 1), we observed peaks in the expected range of 284-285 eV4. Also, for the acetate complexes Rez(OAc)4X3, we located an additional carbon 1s binding energy at 288.8 or 288.7 eV, which is assigned to the carbon 1s binding energy of the carboxylate carbon atoms 4*17 . These carbon binding energies will not be considered further.

DISCUSSION

Compounds

of rhenium (III) and ritenium(IV)

not containing

Re-0

bonds

In previous studies’ - 3 we have been principally concerned with rhenium halide species which were of relatively simple stoichiometry and in which we avoided as far as possible the presence of other ligand molecules, such as phosphines and thioethers, which might profoundly affect the charge distribution at the metal centre and thereby complicate a straightforward correlation of oxidation state with binding energy. In order to ascertain whether in fact any correlations exist within a series of adducts of the rhenium halides, we recorded the rhenium 4fand chlorine 2p binding energies of (i) dinuclear [ReC13(PPh3)]3 and [ReCI, (DTH)],, both of which retain the gross structural features of the parent metalmetal bonded Re3CIs2’ anionlo, 18; (ii) ReCl,(tu), and its acetone solvate, which is an authentic mononuclear rhenium(LI1) derivativegS lo, and (iii) trans-ReC14 (PPh3)?, a derivative of rhenium(N) chlorideg. From the data in Table 1, the 4f binding energies are seen to decrease in the sequence trans-ReCI,(PPh,), > [ReCI,(L)], > ReCl,(tu), . However, as discussed previously3* l1 we believe that in comparing metal core binding energies it is preferable to first reference them to some convenient internal standard, thereby minimizing differences in “solidstate” effects between the compounds. As before3* 11, we chose as our reference some convenient value for the chlorine 2p+ level, namely 198.0 eV. Referencing the above rhenium 4f binding energies (Table 1) to this chlorine 2p+ value shows that while ReCl,(tu), possesses the lowest rhenium 4f binding energies, those for are not significantly different. These trends tran+ReC14(PPh3), and [ReCl,(L)], are to be expected from differences in ligand polarizability and changes in stereo-

406 chemistry. The increase in metal core binding energies expected on changing the formal oxidation state from + 3 (in [ReCl,(PPh,)],) to +4(in rrans-ReCI,(PPh,),) is apparently compensated for by an increase in coordination number which results in an increase in the accumulation of electronic charge at the metal centre. For ReCl,(tu), , the coordination of three poIarizabIe sulphur donors is apparently sufficient to reduce the rhenium 4f binding energies below those of the other rhenium(ItL) derivatives [ReCI,(L)‘J2. Th us in addition to the dependence of metal core eIectron binding energies upon Iigand electronegativities (polarizabilities) and x-acceptor ability, changes in coordination number are clearly also important_ The dinuclear complex Re,CI,(DTH), h as rhenium 4fbinding energies which are significantly broader (see ResuIts) than those found for the other rhenium complexes listed in Table 1. We have previously shown13* lg that this complex possesses the staggered dimeric structure depicted in (I), in which the Cl Cl

s,

________~l,J-&,___ -_ 1

/\

CICI

ST b

-

two rhenium environments are quite different and, formally at least, require the assignment of different oxidation states. While two sets of 4fbinding energies are not clearly resolved, the broadening effects are clearly consistent with the structure. Furthermore, this feature in other systems may prove to be of diagnostic value in detecting

rhenium species in different

Compounds containing Re-0

environments.

bonds

The most apparent feature of the data in Table 1 is that the derivatives of rhenium(VI1) namely, the [Re04]anion and the complexes of per-rhenyl chloride ReO,Cl(B) (B = bipy or phen), exhibit much higher rhenium 4fbinding energies than the complexes of rhenium(V). To check that these trends are real for those complexes containing covalent Re-Cl bonds, we can internally reference these binding energies to a value of 198.0 eV for the appropriate chlorine 2p+ binding energy. We then find the following “corrected” values (in eV) for the rhenium 4f; binding energies: ReO,Cl(bipy), 46.0; Re03Cl(phen), 45.8; Re203C14py4, 43.5; ReOCl,(PEt,Ph),, 43.3; ReOCI,(PPh,),. 43.6. Jt is clear from this and previous’ - 3 studies that it is only with the high oxidation states of rhenium (i.e., rhenium(V) and (VII)) that differences in metal core binding energies are sufficient to justify the use of a simple binding energy versus oxidation state correlation, unless comparisons arc made between structurally related species and some suitable internal reference is avaiiable to enable a correction to be made for differences in Madelung potentlrtl. Since in the present study we have available a series of complex species

407 TABLE VALUES

2 OF DIFFERENCES

BETWEEN

OXYGEN

1s

AND

RHENIUM

4f$ BINDING

ENERGIES

(in

ev)

Compound

6(0 Is, Re4fg)

Compound

6(0 Is, Re4fg)

Re2(OAc)&12 Ret(OAc)4Br2 ReOC&(PPh& ReOBr&PPh& ReOC13(PEtpPh)z [ReO2py.JCl- 2HzO

490.2 490.2 489.1 488.7 489.0 487.2 487.7 487.1

Re203(dedtcj4 RezO 3C14py4 Re03Cl(bipy) ReO&l@hen)

487.9 487.8 485.7 486.0 486.1 486.0 485.8 485.8

lReO$y~lr [RdbPY.d’Fs

IpyHlReO~ IbipyHlneOG

[phenH]ReOa [AcrH]ReOh

containing several types of rhenium-oxygen unit, we decided to, investigate whether binding energy measurements might yield information on the nature of the rheniumoxygen bonds. Accordingly, in Table 2 we list values of the differences (6) between the oxygen 1s and rhenium 4f3 binding energies. Of considerable interest is the range of 6(Ols, Re4fj;) values found within the oxo-complexes of rhenium(V), the three complexes of the type ReOX3(PR3), exhibiting higher values for this energy difference (by - 1 GV) than those complexes which contain the linear 0 = Re-0-Re=0i4* r5 or linear 0= Re=02’ moieties. We have already mentioned that for those rhenium(V) complexes in Table 1 which contain rhenium-chlorine bonds and for which we can use the chlorine 2~~ binding energy as an internal reference, the “corrected” rhenium 4f+ error, essentially identical, viz., binding energies are, within experimental ReOCI,(PPh,),, 43.6, ReOCls(PEt2Ph)2, 43.3, and Re20,C14py4, 43.5 eV. In other words, the formal charge at the metal centre is similar for these systems. From the structural data already available in the literature for complexes containing these types of rhenium-oxygen linkages14# 15*20-23, it is clear that the observed rhenium-oxygen bond lengths are indicative of multiple bond character, the n-bonding formally being of the type Re(Sd) c O(2p). Since the rhenium 4f binding energies (Table 1) are fairly constant within this series of complexes,the implies that the formal positive high value of S(Ols, Re 4f%) for ReOX,(PR,), charge at the oxygen atom is greater than in those complexes which contain the [Re203] and [ReOz] moieties. This could be interpreted to mean that x-bonding is greater for ReOX3(PR3)2, thereby leading to a decrease in electronic charge at the oxygen. On the other hand for those systems which contain two oxygen atoms per rhenium, the multiple bond character of each Re-0 bond would presumably be less than that within the one Re-0 bond of ReOX3(PR3)2, if we are to preserve the similar charge distribution at the metal centre. This is reflected by the lower values of 6(Ols, Re 4f;) for the complexes containing the [Re,O,] and [ReO,] moieties. The similarity of the rhenium 4f and oxygen 1s binding energies and the

408 cT(Ols, Re 4f;) values for all the rhenium(VI1) derivatives, implies that not only is the charge distribution at the metal centres very similar, but that the Re-0 bond orders are essentially identical. Since Cl’, bipy and phen bond to do rhenium (VH) as c-donors, we suspect that in ReO,Cl(bipy) and ReO,Cl(phen), one or both of the donors are rather weakly bound so that the overall donation of electron density onto the metal is little different froin the situation in [ReO,]-. In other words, the operation of trans-effect in ReO,Cl(B) is a possibility suggested by these binding energy measurements. Structural data for the [ReOBr,(CH,CN)Jand Hz0 and [ReOBr,(H20)]anionsz4? 25, which reveal that the CH,CN ligand molecules trans to the Re=O moiety are only weakly bonded, indicate this to be a real possibility. A single crystal X-ray structure analysis on ReO,CI(B) would be desirable_ Finally, we consider the two rhenium(Hl) acetate derivatives Re,(OAc),X, (X = CI or Br), whose rhenium 4f binding energies are slightly higher than the values found for the complexes of rhenium(V) listed in Table 1. This further illustrates the difficulty of attempting to correlate binding energy with oxidation state for species of quite different structure. 1n these acetates, which possess the structure depicted in (IL), the rhenium-halogen bonds are considerably longer and weaker than normal rhenium-halogen single bonds?, a feature which probr?

ably enhances the formal positive charge at the metal and thereby explains the rather high values of these binding energies.

in part

ACKNOWLEDGEMENTS

This work was supported by the National Science Foundation (Grant No. GP-19422 and MRL Program GH-33574). One of us (RAW) is a recipient of a Camille and Henry Dreyfus Foundation Teacher-Scholar Grant. We also thank the National Science Foundation for providing funds for the purchase of the ESCA spectrometer. We are grateful to Dr. Jon W_ Amy and Mr. William E. Baitinger for their invaluable assistance in solving instrumental problems.

REFERENCES

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