Inorg. Chem. 1981, 20, 947-949 1"
C20"C I
-65T
I
I
-85'C
10
260
250 240 ppm
230 220
Figure I. CO "C DNMR spectra for (?5-C5H5)2M02(C0)3(C12H~) in a toluene solution.
947
higher temperatures, two of the three C O ligands underwent selective site exchange. These are assigned to C O ligands bonded to the same molybdenum atom.6 Below -65 OC, one of the terminal C O resonances underwent extensive broadening; we propose no specific explanation for the broadening (either another dynamic process or a selective relaxation effect may be extant). The proton N M R spectrum of the isocyanide derivative, ($-C5H5)2M02(CO)[CNC(CH3)3]Z(C12H20), which should be isostructural with 2, showed the presence of two (CH3)& proton resonances of nearly equal intensity. Irrespective of whether the isocyanide ligands are on the same or different molybdenum atoms, there would be two isocyanide ligand environments; hence, the N M R data do not establish which isomer predominates in solution. Because of overlap of some methylene and methyl (metallacycle) resonances, there was no obvious and unique interpretation of all the 'H N M R resonances; note that all ethyl groups should be inequivalent and the protons in each CH2 group should be diastereotopic, irregardless of the isocyanide ligand placement. The structure of the hydride derived from 1 by protonation was not defined by the spectroscopic data. Acknowledgment. This research was supported by a grant from the National Science Foundation. We especially thank Dr. Mamoru Tachikawa for critical comments. Registry No. ( I I ~ - C ~ H ~ ) ~ M O ~ ( C O 761 ) ( C36-86-2; ~ H ~ ~ )($~, C5H5)2Mo2(CO)3(C6Hio)2,76136-87-3; ($-CSHS)~MO~(CO) [CNC(CH3)3]2(C12HN), 76282-00-3; ( T ~ - C ~ H ~ ) ( H ) ~ M O ( $ - C ~ H ~ ) M O (CO)(C12H20)*+(F3CSO3-)2, 76282-02-5; ( $ - C ~ H ~ ) ~ M O ~ ( C O ) ~ (C2HSC2C2H5), 61373-5 1-1. (6) These assignments are based on several considerations. First, the "C
chemical shift of the two CO ligands involved in the low-temperature Metallacycles of structural form 1 have a metal-metal bond exchange have very similar chemical shifts- feature that might be of formal bond order 3; crystallographic data,2J which esexpected for the two CO ligands on the same molybdenum atom (in a tablished short metal-metal bond distances, support this formal sense, the oxidation states of the two molybdenum atoms are different). Second, an intermetal (intramolecular) exchange should tend characterization. We have explored the reactivity of this type to involve all three CO ligands. CO exchange localized at a single metal of multiple bond for the metallacycle derived from 3-hexyne. site is a fairly common low-energy process in polynuclear complexes.' Carbon monoxide rapidly converted the bridged monocarbonyl (7) E. Band and E. L. Muetterties, Chem. Rev., 78, 639 (1978). to the metal-metal singly bonded species ( T ~ - C ~ H ~ ) ~ M O ~ (C0)3(C12H20),with two terminal carbonyl groups on one molybdenum atom and one terminal carbonyl on the other (see below); this reaction was thermally reversible. Analogous was the facile addition of two tert-butyl isocyanide ligands at 25 "C to give ( v ~ - C ~ H ~ ) ~ M O[CNC(CH3)3]2(C12H20). ~(CO) In Contribution from the Department of Chemistry, sharp contrast, neither triethylphosphine nor trimethyl phosh r d u e University, West Lafayette, Indiana 47907 phite reacted with the metallacycle. Molecular models clearly suggest that this lack of reaction is steric in origin. There is One-Electron Oxidations of Quadruply Bonded insufficient room to allow addition of bulky ligands without Molybdenum Dimers' a large increase in the molybdenum-molybdenum separation. Protonation of the metal-metal triply bonded metallacycle did C. Zietlow,2Douglas D. Klendworth, Tayseer Nimry, proceed rapidly to give the dimer ( . I I ~ - C ~ H ~ ) ( H ) , M O ( ~Thomas ~Dennis J. Salmon,' and Richard A. Walton* C5HS)Mo(CO)(C12H20)2'. The tricarbonyl derived from 1 should have structure 2 with
Received May 6, 1980
The molybdenum(I1) dimers M O ~ ( O ~ C(R R )=~ alkyl or aryl) and [ M o ~ ( S O ~may ) ~ ]be ~ oxidized reversibly, both chemically and electrochemically, to [ M o ~ ( O ~ C R ) and ~]+ [Mo2(S04),]3-.4-9 These oxidations correspond to a change 2
essentially a Mo-Mo formal bond order of 1. The infrared spectrum of a solution of 2 showed that all the carbonyls were terminal. All three C O environments should be differentiable by NMR barring dynamic processes that equilibrate C O sites. At - 4 5 OC, the 13Cspectrum of 2 consisted of a three-line pattern consistent with the proposed structure (Figure 1). At 0020-1 66918 111320-0947$01.25/0
(1) Part 28 of the series entitled "Complex Halides of the Transition Metals". (2) Undergraduate research participant from Kalamazoo College, Kalam a m , Mich. 49001. (3) Postdoctoral research associate, 1977-1978. (4) McCarley, R. E.; Templeton, J. L.; Colburn, T.J.; Katovic, V.;Hoxmeier, R. J. Adu. Chem. Ser. 1976, No. 150, 318. ( 5 ) Cotton, F. A.; Pedersen, E. Inorg. Chem. 1975, 14, 399. (6) Cotton, F. A.; Frenz,B. A.; Webb, T. R. J . Am. Chem. Soc. 1973, 95, 4431.
0 1981 American Chemical Society
948 Inorganic Chemistry, VoZ. 20, No. 3, 1981
Notes
Table I. E,,, Values for Molybdenum(I1) Halide and Isothiocyanate Complexes in Dichlorornethane" complex Mo,Cl,(PEt3), (1) Mo,Cl,(P-n-Pr,), (11) Mo,Cl,(dppe), (111) Mo,Br,(dppe), (IV) Mo, (NCS), (PEt,), (VI Mo,(NCS),(dppe), (VI) Mo,(NCS),(dppm), (VII)
E,,,-
E,,,-
( 0 ~ ) ~(Red)b
E,,,-
(Red)b
+0.35 +0.38
+OS2
+OS4 +0.80 +0.74' +0.84'
-1.17
-0.85 -0.80
-1.58
-1.60d
" With 0.2 M tetra-n-butylammoniurn hexafluorophosphate (TBAH) as supporting electrolyte. Volts vs. SCE with a Pt-bead working electrode. Oxidation irreversible, with iP,../ip,, < 1 ; Ep,, is quoted rather than E,, . EP,...
16
00
-1 G
Volts vs SCE
in electronic configuration from ( u ) ~ ( T ) ~to( (~U )) ~ ( T ) ~ ( ~ ) ' Figure 1. Cyclic voltammograms in 0.2 M TBAH-dichloromethane of Mo2C1,(P-n-PrJ4: (a) before and (b) after exhaustive electrolysis and therefore to a decrease in the metal-metal bond order from at + O S V. The cyclic voltammogram in (a) was recorded at room 4 to 3.5. On the other hand, with the exception of the retemperature; that in (b) at 0 OC. duction of Mo2(02CCF3),to [ M O ~ ( O ~ C C F ~by) ~pulse ] - radiolysis,'Oneither M O ~ ( O ~ Cnor R ) [~ M O ~ ( S O ~exhibits ) ~ ] ~any simple reversible reduction chemistry. In contrast, the redox chemistry of the analogous rhenium dimers which contain rhenium-rhenium quadruple bonds, e.g., Re2(02CR)4X2(X = C1, Br, or I), is that of one-electron reductions to species (Le., [Re2(02CR)4X2]-)which possess the ( I J ) ~ ( T ) ~ ( ~ ) ~ ( ~ * > ' electronic configuration and thus also contain metal-metal bonds of order 3.511312 In summary, with oxygen-containing anionic bridging ligands such as carboxylates and sulfate, the most significant reuersible redox chemistry of complexes containing the Mo;+ core is that of oxidation to Mo?+, while for isoelectronic ReZ6+reduction to Rez5+is much more accessible. This behavior is also in accord with the electrochemistry of the mixed halide-tertiary phosphine dimers Re&(PR3)2 (X = C1 or Br), where reduction to [Re2X6(PR3),]-,is well \I v i documented.I3 In the case of the dimers M o ~ X ~ ( P Rthere ~)~, 32 34 is to date little information as to their redox chemistry, alkG though we had noted in an earlier report that the chemical Figure 2. X-Band ESR spectrum of a CH2C12glass (-180 "C) oxidation of M o ~ C ~ ~ ( P(R R ~=) ~Et or n-Pr) to containing electrochemically generated [Mo2C1,(P-n-Pr,),]+. (R3PC1),Mo2C19is accomplished by use of refluxing carbon tetrach10ride.l~ To ascertain whether the expected oneMo2X4L4 Mo2X4L4++ e-. For complexes I, 11, and IV, electron oxidation to [ M o ~ X ~ ( P R ~is) indeed ~ ] + possible, we i , , / i , , N 1 for sweep rates ( u ) between 50 and 500 mV/s, have carried out an electrochemical study on several molecules and the ratio ip/u1/2 was constant for this same sweep rate of the type M o ~ X ~ ( P Rand ~ ) ~we , now report the salient fearange, in accord with diffusion control. Because of the low tures of this investigation. solubility of I11 in dichloromethane and, as a consequence, the attendant low peak currents, measurements were carried out Results and Discussion at only one sweep rate (200 mV/s).16 As in previous ~ t u d i e s , ~ we ~ J have ~ J ~ carried out the elecThe potential separation, AE,, between the anodic and trochemical measurements on dichloromethane solutions of cathodic peaks was in all cases greater than 60 mV and varied the complexes. Voltammetric half-wave potentials vs. SCE with sweep rate. For example, AE, for I1 varied from 110 to for the molybdenum(I1) dimers M o ~ C ~ ~ ( (R P R=~ Et ) ~or 140 mV for v between 100 and 500 mV/s and between 95 and n-Pr) and M ~ ~ X ~ ( d p(X p e =) ~C1 or Br, dppe = 1,2-bis(di160mV for these same scan rates in the case of IV. Although pheny1phosphino)ethane) are presented in Table I, and the these electrochemical properties can be viewed as being in cyclic voltammogram of M ~ ~ c l , ( P - n - P is r ~shown ) ~ in Figure accord with quasi-reversible electron-transfer processes,20,21 la. In all instances the complexes exhibit a single oxidation,
-
(7) Pernick, A.; Ardon, M. J . Am. Chem. SOC.1975, 97, 1255. (8) Trogler, W. C.; Erwin, D. K.; Geoffroy, G. L.; Gray, H. B. J. Am. Chem. SOC.1978, 100, 1160. (9) Bino, A.; Cotton, F. A. Inorg. Chem. 1979, 18, 1159. (10) Baxendale, J. H.; Gamer, C. D.; Senior, R. G.; Sharpe, P. J. Am. Chem. SOC.1976, 98, 637. (11) Srinivasan, V.; Walton, R. A. Inorg. Chem. 1980, 19, 1635. (12) Cotton, F. A.; Pedersen, E. J. Am. Chem. Soc. 1975, 97, 303. (13) Brant, P.; Salmon, D. J.; Walton, R. A. J. Am. Chem. Soc. 1978,100, 4424.
(14) Giicksman, H. D.; Hamer, A. D.; Smith, T. J.; Walton, R. A. Inorg. Chem. 1976, 15, 2205.
(15) Brant, P.; Glicksman, H. D.; Salmon, D. J.; Walton, R. A. Inorg. Chem. 1978, 17, 3203.
(16) The similarity of the Ell2values for I and I1 is expected because of their close structural ~ i m i 1 a n t y . l ~Both ~ ~ ' complexes possess an ecliped rotational configuration of DU symmetry with r r ~ n r - M o C l ~ ( P Runits. ~)~ While Mo2C14(dppe)2also has an eclipsed configuration, the clectrochemical data we report is for the a isomer, wherein the individual MoC12P2units have a cis geometry." These complexes are in turn different from M q B r , ( d ~ p e ) ~which , has a partially staggered configuration (aclase to 30°), bnd 'ng dppe ligands and a trans arrangement within the MoBr2P2units."~' Accordingly, these complexes all possess a (g)2(~)4(6)2electronic configuration, but structural differences preclude any simple explanation of the trend in E l l 2 value. (17) San Filippo, J., Jr. Inorg. Chem. 1972, 2 1 , 3140. (18) Best, S . A.; Smith, T. J.; Walton, R. A. Inorg. Chem. 1978, 17, 99. (19) Cotton, F. A.; Fanwick, P. E.; Fitch, J. W.; Glicksman, H.D.; Walton, R. A. J . Am. Chem. Soc. 1979, 101, 1752.
P
Notes
Inorganic Chemistry, Vol. 20, No. 3, 1981 949
we believe that they are best considered in a qualitative sense nitrile solutions of V. While neither of the reductions of M~,(NCS),(dpprn)~is reversible, the orange monoanion of as reversible. Measurements of the known reversible couples Mo,(NCS),(dppe), may be generated ( n = 1.04 by coulo(C5H5)2Fe/(C5H5)2Fe+ and R ~ ( b p y ) ~ ~ + / R u ( b p yunder )~~+ metry) upon electrolysis at -1.3 V.25 these same experimental conditions22gave E I I Zvalues of +0.43 and 1.42 V, respectively, and AEp values of 90 and 85 mV The absence of reductions in the electrochemistry of the halide dimers of Mo:+ suggests that those which are exhibited at u = 100 mV/s. Since AE, increased with increase in u, by the isothiocyanate complexes in the potential range -0.80 being ca. 120 mV at 400 mV/s, the behavior of these systems to -1.60 V do not involve processes which utilize the metalparallels closely that which we observe for the molybdenum based u, a,and 6 orbital sets. On the other hand, the existence complexes. of readily accessible one-electron oxidations with the halide Our attempts to generate the monocations of the halide and isothiocyanate dimers I-VI1 is certainly in accord with complexes (I-IV) were restricted to Mo2C1,(P-n-Pr3), and the ease of oxidizing Mo2(02CC3H7),to its monocation and M~,Br,(dppe)~. In the case of IV, the room-temperature controlled-potential electrolysis at +0.8 V led to the decom[ M O ~ ( S O ~to) [~M] ~O ~ ( S O ~ )These ~ ] ~ -oxidations . involve the configuration change ( U ) ~ ( T ) ~ to( ~( )U~) ~ ( T ) ~ and ( ~ ) dem~ position of a major portion of the complex. Decomposition, albeit less rapid, also occurred upon electrolyzing a dionstrate the close electronic relationship and similarity in redox behavior between M O ~ C ~ ~ ( P R MqX,(dppe),, ,)~, Mo2(02CR),, chloromethane solution of Mo2C14(P-n-Pr3),at room temand [MO~(SO,),]~. perature, and an attempt to chemically oxidize Mo2C14(P-nPr3), with use of an acetonitrile solution of NOPF623led to Experimental Section the same result. However, when the electrolysis of Mo2C14(P-n-Pr,), was carried out at 0 “C, the blue solution turned The complexes M o ~ C I ~ ( P (R R ~=) ~Et or n-Pr), M ~ ~ C l ~ ( d p p e ) ~ , dark green and the oxidation ( n = 0.98 by coulometry) was Mo2Brddppe)2, MoANCS)dPEtJ4, Mo2(NCSMdppe)2, and Moz(NCS)4(dppm)2 were available from previous investigation^.'^^'^^^^ found to proceed smoothly to afford stable solutions of the Tetra-n-butylammonium hexafluorophosphate (TBAH) was obtained monocation. The cyclic voltammogram of the resulting soby reacting tetra-n-butylammonium iodide with KPF6 in hot water. lution (Figure lb) indicated that the structure was intact, and The product was recrystallized from aqueous ethanol and dried in coulometry revealed that the single reversible wave now corvacuo. responded to a reduction. When this solution was warmed to Solvents used for electrochemical experiments were of the highest room temperature, the cation decomposed fairly slowly to purity commercially available (Fisher Scientific Co.) and were used afford unidentified products.24 The X-band ESR spectrum without further purification. The CH2C12used in obtaining the ESR of a dichloromethane glass of [MozCl4(P-n-Pr3),]+which had glass spectra was freshly distilled and stored under N2 in the dark prior to use. been generated electrochemically is shown in Figure 2. Two X-Band ESR s p t r a of CH2C12glasses were recorded at -180 OC gvalues were observed at 1.95 and 1.99, the latter exhibiting with a Varian E- 109 spectrometer. Electrochemical measurements some poorly resolved hyperfine structure. The origin of the were made on dichloromethane solutions containing 0.2 M tetra-nbroad reproducible feature at 3200 G is unknown. The butylammonium hexafluorophosphate (TBAH) as supporting elecspectrum of [MoC14(P-n-Pr3),]+ has the appearance of an trolyte. El,1 values (taken as (Epa+ E,,J/2) are referenced to the axially symmetric system (i-e., gll= 1.99 and g, = 1.95). The saturated potassium chloride calomel electrode (SCE) at 22 f 2 OC lower symmetry of this cation (DU)relative to that of the and are uncorrected for junction potentials. Cyclic voltammetry formally isoelectronic [Mo,(O~CC~H,)~]+ and [ M O , ( S O ~ ) ~ ] ~ experiments were performed with use of a BioAnalytical Systems Inc. ion^,^^^ for which gll= g, = 1.9415and gll= 1.891 and g, = Model CV-1A instrument in conjunction with a Hewlett-Packard 1.909,6 respectively, should be reflected in ESR spectral difModel 7035B X-Y recorder. Potential control for coulometric experiments was maintained with a potentiostat purchased from ferences. Of particular note is the decrease in the orbital BioAnalytical Systems Inc. Values of n, where n is the total number contribution in [ M O ~ C ~ ~ ( P - ~ - compared P ~ , ) ~ ] +to that in the of equivalents of electrons transferred in exhaustive electrolyses at propionate and sulfate-bridged dimers, the g values of the constant potentials, were calculated after measuring the total area phosphine cation being closer to 2.0. under current vs. time curves for the complete reactions. The reactions The oxidations of the isothiocyanate complexes VI and VI1 were judged to be complete when the current had fallen below 1% (dppe = 1,2-bis(diphenylphosphino)ethane and dppm = bisof the initial value. All voltammetric measurements were made at (dipheny1phosphino)methane) were irreversible, the i , , / i , , a platinum-bead electrode in solutions deaerated with a stream of dry ratios having values much less than 1 in both cases. For nitrogen. The platinum electrode was cleaned with the chromic acid Mo2(NCS),(PEt3), this ratio was close to 1 for sweep rates and aqua regia procedures described by ad am^.^' of 100 mV/s or more, but became less than 1 at low sweep Acknowledgment. The work was supported by the National rates. Upon controlled-potential electrolysis at +0.92 V, the Science Foundation through Grant No. CHE79-09233. The blue solution V turned black ( n > 1) and the cyclic voltamVarian E-139 ESR spectrometer was purchased with a grant mogram of the resultant solution confirmed that the complex from the National Science Foundation (Grant No. BMS 75has decomposed. In contrast to the halide dimers, these iso19127). We thank Professor R. R. Schrock for providing us thiocyanate complexes also exhibited one or two reductions with the information described in ref 24 and Professor D. R. (Table I). For V, the reduction at Ellz = -1.17 V was reMcMillin for discussions concerning the interpretation of the versible, the blue solution turning green upon electrolysis at ESR spectral data. -1.32 V ( n = 0.92 by coulometry). This anion reverts slowly to the blue unreduced Mo2(NCS),(PEt3), upon standing. A Registry NO. I, 38832-71-2; 11, 39043-51-1; 111, 64490-77-3; IV, similar reduction ( E 1 / ,= -1.07 V) was observed for aceto64508-29-8; V, 64784-48-1; VI, 64784-51-6; VII, 64784-41-4.
+
(20) Murray, R. W.; Reilley, C. N. “Electroanalytical Principles”; Interscience: New York, 1963. (21) Nicholson, R. S. Anal. Chem. 1965, 37, 1351. (22) We thank a reviewer for suggesting we carr out these measurements. (23) This reagent has been used by us previ~usl$~to oxidize Re2C4(PRJ4 (R = Et or Pr) to [Re2C14(PR3)4]+. (24) We have been informed by Professor R. R. Schrock, Massachusetts Institute of Technology, that he has successfully generated solutions of [ M o ~ C I ~ ( P R ~ by ) ~ ]the + low-temperature chemical oxidation of M o & I ~ ( P R ~in ) ~acetonitrile. Accordingly, we abandoned any further attempts to isolate this cation.
(25) The cyclic voltammograms of dichloromethane solutions of ( B U , N ) ~ M O ~ ( N C S ) , ( P Ewere ~ ~ ) ~also recorded. A quasi-reversible reduction occurs a t E l l 2 = -1.61 V (-1.54 V in acetonitrile), but controlled-potential electrolysis at -1.78 V led to decomposition. An irreversible oxidation at E,, = +0.24 V (+0.23 V in acetonitrile) exhibited no cathodic component on the reverse sweep using a switching potential of +0.5 V. (26) Nimry, T.; Walton, R. A. Inorg. Chem. 1978, 17, 510. (27) Adams, R. N. “Electrochemistry at Solid Electrodes”; Marcel Dekker: New York, 1969; p 206.
