Synthesis, Characterization, and Electrochemical Studies of Iron

Synthesis, Characterization, and Electrochemical Studies of Iron, Cobalt, and Nickel. Complexes of Poly .... reduction of C02 to formic acid6 and C 0 ...
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Inorg. Chem. 1986, 25, 4642-4650

4642

with the result that the C - M o 4 H 2 angles are significantly smaller than 90'. On the other hand, this distortion in the [WO(H20)(CN)4]2-ion together with the large trans effect of the oxo ligand, will promote dissociation of the aquo ligand and thus a dissociative reaction mode. This is in agreement with the positive value of AV(kl). The large trans influence of the 0x0 ligand is clearly in the M ~ - Nbond distances in [MoO(phen)(CN),]- 6 (the corresponding tungsten complex is isomorphous with the molybdenum complex): The Mo-N bond distance trans to the oxo ligand is 2.363 (7) A, whereas the Mo-N bond distance trans to the cyanide ligand is only 2.174 (7) A.

Finally, the results of this investigation clearly demonstrate the close correlation between ground-state structure and transitionstate energetics as facilitated by the trans effect of the oxo ligand,

Admowledgment. The authors gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie, the S.A. Council for Scientific and Industrial Research, and the Research Fund of the University of the Orange Free State* Registry No. trans- [WO,(CN),le, 42720-52-5; trans- [WO(H,O)(CN)4I2-, 105121-19-5; NC, 14343-69-2.

Contribution from the Solar Energy Research Institute, Golden, Colorado 80401

Synthesis, Characterization, and Electrochemical Studies of Iron, Cobalt, and Nickel Complexes of Polyphosphine Ligands Daniel L. DuBois* and Alex Miedaner Received October 11, 1985

The reaction of [M(CH,CN),] (BF4)2 (where M = Fe, Co, and Ni) with P(CH2CH2PPh2),(PP,), PhP(CH2CH2PPh2),(PP,), and Ph2PCH2CH2PPh2(dppe) results in the formation of [Fe(PP,)(CH3CN)2](BF4)2,[Fe(PP,)(CH,CN),](BF,),, [Fe(dppe)2(CH3CN)21(BF4)2. [CO(PP~)(CH~CN)I (BF4)2, [Co(dppeMCH3CN)I (BF4)21 [Ni(PW(CH,CN)I (BF4)2r [Ni(PP2)(CH3CN)I(BF4)2, and [Ni(dppe),] (BF4)2, respectively. Electrochemical studies have been carried out on these complexes to examine the influence of the nature of the polyphosphine ligand on the redox properties of each metal. For [Fe(PP,)(CH,CN),](BF,), the For reversibility of both the Fe(II/III) and Fe(I/O) couples are enhanced relative to those of [Fe(dppe)2(CH3CN)2](BF4)2. ] ( B-1F , ) ~ [CO(PP,)(CH,CN)](BF~)~ the lowest oxidation state accessible in CH,CN is +1, while for [ C O ( ~ ~ ~ ~ ) ~ ( C H , C N ) the oxidation state can be observed. The Ni(I/O) couple is reversible for [Ni(dppe)2](BF4)2and irreversible for [Ni(PP2)(CH3CN)](BF4)2and [Ni(PP,)(CH,CN)](BF4)2. The electrochemical studies of the latter complex have led to the synthesis of a Ni(0) dimer, [Ni(PP,)],.

Introduction This paper is the first of a series investigating the electrochemical properties of transition-metal complexes containing polyphosphine ligands and their use as redox catalysts. Previous electrochemical investigations of metal complexes containing monodentate phosphine ligands have revealed that, in general, the reductions or oxidations of such complexes are irreversible due to cleavage or formation of metal-phosphorus bonds.' The use of chelating diphosphine ligands increases the reversibility of the redox couples of a number of metal complexes when compared to that of their monodentate analogue^.^-^ This tendency of a diphosphine ligand to promote reversible electron-transfer processes can be attributed to their ability to prevent metal-phosphorus bond cleavage. This suggests that other polyphosphine metal complexes may also display enhanced electrochemical reversibility by preventing metal-phosphorus bond cleavage. By systematically varying the nature of the polyphosphine ligand in various metal complexes, we hope to gain a better understanding (1) Bontempelli, G.; Magno, F.; Schiavon, G.; Corain, B. Inorg. Chem. 1981, 20, 2579. Bontempelli, G.; Magno, F.; Corain, B.; Schiavon, G. J. Electroanal. Chem. Interfacial Electrochem. 1979, 103, 243. Olson, D. C.; Keim, W. Inorg. Chem. 1969,8, 2028. Pilloni, G.; Valcher, S.; J. Electroanal. Chem. Interfacial Electrochem. 1972, 40, 63. Pilloni, G.; Zotti, G.; Martelli, M. J. Electroanal. Chem. Interfacial Electrochem. 1975.63, 424. Zotti, G.; Zecchin, S.; Pilloni, G. J. Organomet. Chem. 1983, 246, 61. Corain, B.; Bontempelli, G.; DeNardo, L.; Mazzocchin, G.-A. Inorg. Chim. Acta 1978, 26, 37. Jasinski, R. J. Electrochem. SOC.1983, 130, 834. (2) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 197f4,17,3334. Pilloni, G.; Vecchi, E.; Martelli, M. J. Electroanal. Chem. Interfacial Electrochem. 1973.45, 483. Pilloni, G.; Zotti, G.; Martelli, M. Inorg. Chem. 1982,21, 1284. Kunin, A. J.; Nanni, E. J.; Eisenberg, R. Inorg. Chem. 1985, 24, 1852. (3) Pilloni, G.; Zotti, G.; Martelli, M. J. Electroanal. Chem. Interfacial Electrochem. 1974, 50, 295. (4) Zotti, G.; Zecchini, S.;Pilloni, G. J. Orgammer. Chem. 1979, 181, 375. (5) Martelli, M.; Pilloni, G.; Zotti, G.; Daolio, S . Inorg. Chim. Acta 1974, 11, 155. Bowmaker, G. A.; Boyd, P. D. W.; Campbell, G. K.; Hope, J. M. Inorg. Chem. 1982, 21, 1152. Zotti, G.; Pilloni, G.; Rigo, P.; Martelli, M. J. Electroanal. Chem. Interfacial Electrochem. 1981, 124, 211.

of the factors controlling the stability of different oxidation states of (po1yphosphine)metal complexes. Such understanding could be useful in the rational development of metal phosphine complexes as redox catalysts. Currently transition-metal complexes of phosphine ligands are known to catalyze the electrochemical reduction of C02to formic acid6 and C 0 7 and that of aryl halides to biaryls.8 In this paper we report the synthesis, characterization, and electrochemical studies of Fe, Co, and Ni complexes containing tetradentate, tridentate, and bidentate phosphine ligands as well as weakly coordinating acetonitrile ligands. These complexes permit a comparison of the ability of the various polyphosphine ligands to stabilize different oxidation states for a given metal. Experimental Section Acetonitrile and dichloromethane were dried by distillation from calcium hydride under nitrogen. Toluene and tetrahydrofuran (THF) were distilled from sodium benzophenone ketyl under nitrogen. Except where mentioned all reactions were carried out by using standard Schlenk techniques. All reagents and products were handled with exclusion of air with the exception of the air-stable nickel complexes. P(CH2CH2PPh2)j (PP,), PhP(CHZCH2PPh2)z (PP,), Ph2PCH2CHzPPhZ (dppe), and Ni(COD), (COD is 1,5-cylooctadiene)were purchased from Strem Chemicals. The acetonitrile complexes of Fe, Co, and Ni were prepared as described in ref 9. Infrared spectra were obtained on a Perkin-Elmer 599B spectrophotometer. All of the BF4 salts showed a broad strong infrared absorption between 900-1150 cm-'. A Varian E109 spectrometer was used for obtaining EPR spectra. All EPR spectra were recorded on 1 X lo-' M dichloromethane solutions unless indicated otherwise. A JEOL FX9OQ FT NMR spectrometer equipped with a tunable, variable-temperature probe was used to collect 'H and ,'P NMR spectra. Me4Si was used as an internal reference for all 'H spectra. A capillary filled with phosphoric (6) Slater, S.;Wagenknecht, J. H. J. Am. Chem. SOC.1984, 106, 5367. (7) DuBois, D. L.; Miedaner, A. J. Am. Chem. Soc., in press. (8) Troupel, M.;Rollin, Y.;Sibille, S.;Fauvarque, J. F.;Perichon, J. J. Chem. Res. 1980, 26. Schiavon, G.; Bontempelli, G.; Corain, B. J. Chem. Soc., Dalton Tram. 1981, 1074. (9) Hathaway, B. J.; Holah, D. G.; Underhill, A. E. J. Chem. SOC.1962, 2444.

0020-1669/86/1325-4642$01.50/0 0 1986 American Chemical Society

Fe, Co, and Ni Complexes of Polyphosphines

Inorganic Chemistry, Vol. 25, No. 26, 1986 4643

NMR (acetonitrile-d,): 50.6 ppm (s). IR: C N stretch, 2250 cm-I (w). acid was used as an external reference for 31P NMR spectra. All "P Anal. Calcd for Cs6Hs4NZB2F8FeP4: C, 60.67; H, 4.92; P, 11.18; N, NMR spectra were proton-decoupled. Electrochemical measurements 2.53. Found: C, 60.44; H, 4.72; P, 10.94; N, 2.39. were carried out with a Princeton Applied Research Model 173 potentiostat equipped with a Model 179 digital coulometer and a Model 175 [COP(CH~CH~PP~~)~(CH,CN)I(BF~)~.CH~C~~, [CO(PP~(CH~CN)Iuniversal programmer. A Houston Instruments Model 2000 X-Y re(BF4)2. A red solution of [CO(CH,CN),](BF,)~(0.96 g, 2.0 mmol) in corder was used for plotting cyclic voltammograms. A silver wire was acetonitrile (30 mL) was added to a solution of PP, (1.34 g, 2.0 mmol) in dichloromethane (30 mL). The resultant dark green solution was dipped in concentrated nitric acid, washed with distilled water, dipped stirred at room temperature for 30 min, and the solvent was removed in in concentrated hydrochloric acid, and rinsed with distilled water. After drying, this wire was used as a pseudoreference electrode. This reference vacuo to produce a dark green solid. This solid was dissolved in dielectrode was separated from the working and counter electrode comchloromethane (50 mL), and hexane (10 mL) was added. Cooling the flask to -20 "C overnight resulted in the precipitation of a dark green partments by a Vycor frit. Ferrocene was used as an internal standard. crystalline solid, which was collected by filtration and dried in vacuo at The potential of ferrocene vs. aqueous SCE in 0.2 N LiC104 solution of acetonitrile is reported to be +0.307 V.I0 All of our measurements were 50 "C for 8 h. The yield was 0.77 g (39%). An EPR spectrum recorded carried out in 0.3 N NEt4BF4solutions of acetonitrile. In this solution at room temperature in dichloromethane consisted of a broad doublet we found the potential of ferrocene to be +0.40 V vs. aqueous SCE. A with g = 2.1 1 and A = 90 G. IR: C N stretches, 2275 (m) and 2315 (w) glassy-carbon disk electrode (IBM) was used as the working electrode, cm-I. Anal. Calcd for C45H47NB2Cl2CoF8P4:C, 52.51; H, 4.60; N, and a platinum wire was used as a counter electrode. All compounds 1.36; Co, 5.73; P, 12.04. Found: C, 52.78; H, 4.71; N, 1.37; Co, 5.91; were studied by cyclic voltammetry over a range of scan rates from 50 P, 12.50. to 500 mV/s. Plots of i, vs. VI/^ were used to establish if the electron[COP(CH~CH~PP~~)~(CH~CN)I(BF~), [CO(PP,)(CH,CN)I(BF~). transfer processes were under diffusion control. Elemental analyses were Acetonitrile (30 mL) was added to a mixture of [Co(CH3CN),](BF4)2 performed by Spang Microanalytical Laboratories. (0.90 g, 1.88 mmol) and PP3 (1.26 g, 1.88 mmol). After the green reaction mixture was stirred for 0.5 h, zinc dust (0.4 g, 6.1 mmol) was [FeP(CH2CH2PPh2),(CH,CN)21(BF4)2, [Fe(PPp)(~,CN),I(BF,),. A colorless solution of IFe(CH3CN)6](BF4)2(0.95 g, 2.0 mmol) in added. The solution turned red within 5 min, and the mixture was stirred acetonitrile (30 mL) was added to a solution of tris(2-(diphenylovernight. The reaction mixture was filtered to remove the zinc dust, and phosphino)ethyl)phosphine (1.34 g, 2.0 mmol) in dichloromethane (20 the solvent was removed from the filtrate in vacuo to yield a red-purple mL). The resultant red solution was stirred at room temperature for 30 residue. This solid was dissolved in dichloromethane (30 mL) and filmin. The solvent was removed on a vaccum line to produce a red solid, tered. Ethanol (30 mL) was added to the filtrate and the volume reduced which was recrystallized from acetone. The yield was 1.67 g (85%). 'H to 20 mL by applying a vacuum. The resulting fine microcrystalline NMR (acetone-d,): Ph, 6.90-7.8 ppm (m); CH,CN resonances, 3.41 (9) precipitate was collected by filtration and dried in vacuo for 3 h. The and 1.02 ppm (m); CH2, 3.7-2.5 ppm (m). 3'P NMR (acetone-d6) (see yield was 1.31 g (80%). 'H NMR (acetonitrile-d3): Ph, 7.16 ppm (s, structure of 1 and text for discussion of assignments): P,, 154.5 ppm (q, br); CH2, 3.5-1.5 ppm (m); CH3CN, 1.95 ppm (s). "P NMR (acetoJ , = Jh = 27 HZ); Pb, 65.8 ppm (td, J a b = 37 HZ); Pa, 49.91 ppm (dd). nittiled,): equatorial phosphorus atoms, 58.5 ppm (d, J = 33 Hz); apical IR: no bands observed between 2100 and 2400 cm-I for coordinated phosphorus atom, 157.2 ppm (q). I R CN stretch, 2245 cm-' (w). Anal. acetonitrile. Anal. Calcd for C46H48N2B2F8FeP4: C, 56.24; H, 4.94; N, Calcd for C.+,H4sNBCoF4P4: C, 61.63; H, 5.29; N, 1.63. Found: C, 2.85; P, 12.61. Found: C, 56.09; H, 4.97; N, 2.86; P, 12.57. 61.72; H, 5.42; N, 1.56. [FeP(CH2CH2PPh2)3(CH3CN)l, [Fe(PP3)(CH3CN)]. A solution of [CO(P~~PCH~CH~PP~~)~(CH,CN)](BF~),.CH~OH, [Co(dppe),tetrahydrofuran (30 mL) and acetonitrile (30 mL) was added to a (1.08 g, 2.25 (CH,CN)](BF,),. A solution of [CO(CH,CN)~](BF,)~ Schlenk flask containing [Fe(CH3CN)]6](BF4)2(0.86 g, 1.81 mmol) and mmol) in acetonitrile (50 mL) was added to a solution of dppe (1.79 g, PP3 (1.21 g, 1.81 mmol). The reaction mixture was stirred for 1 h, and 4.5 mmol) in dichloromethane (70 mL). The reaction mixture was then sodium amalgam containing 0.5 g of sodium was added. The restirred for 1 h and the solvent removed in vacuo to produce an orange action mixture was stirred for 2 h. During this time the solution turned powder. Recrystallization from a mixture of dichloromethane and a deep red-purple. The solution was filtered with a cannula, and the methanol yielded 1.26 g (50%) of an orange microcrystallineproduct. An volume of the filtrate was reduced to approximately 30 mL in vacuo. The EPR spectrum recorded at room temperature in dichloromethane conresulting black solid was collected by filtration and dried in a vaccum for sisted of a broad resonance with g = 2.20 and no resolved hyperfine 3 h. The yield was 0.67 g (48%). Due to the low solubility and instability splitting. IR: CN stretches, 2270 and 2310 cm-I. Anal. Calcd for of this complex in solution, it has not been possible to obtain reliable C, 59.92; H, 5.32; N, 1.27; CO, 5.34; P, 11.24. CSSHSSNB~COFBOP~: or 'H NMR data for this complex. IR: C N stretch, 2202 cm-I. Anal. Found: C, 59.98; H, 5.38; N, 1.48; Co, 5.51; P, 12.13. Calcd for C44H45NFeP4:C, 68.85; H, 5.91; N, 1.82. Found C, 67.21; [NiP(cH2cH2PW2),(~,cN)l(BF,),, [Ni(PP3)(cH,cN)I(BF4),. An H, 5.88; N, 1.76. acetonitrile solution (20 mL) of [Ni(CH3CN),](BF4),.'/,CH3CN (1 .OO [FePhp(CH2CH2PPh2)2(CH,CN),I(BF4)2,[Fe(PP2)(CH3CN),I(Bg, 2.0 mmol) was added to a dichloromethane solution (30 mL) of PF4)2. A solution of PhP(CH2CH2PPh2)2(1.07 g, 2.0 mmol) in di(CH2CH2PPh2)3(1.34 g, 2.0 mmol). The resulting purple solution was chloromethane (20 mL) was added to a solution of [Fe(CH,CN),](BF,), stirred at room temperature for 1 h, and the solvent was removed on a (0.95 g, 2.0 mmol) in acetonitrile (30 mL). The resultant red solution rotary evaporator. The crude product was dissolved in dichloromethane was stirred for 2 h at room temperature, and the solvent was removed (100 mL) in air, and the solution was filtered. Ethanol (100 mL) was on a vaccum line to give a light red solid. The product was recrystallized added to the fdtrate, and the volume of the solution was reduced to 50 from a dichloromethane/THF mixture by slowly removing the solvent. mL on a rotary evaporator. The deep purple microcrystalline product The product was collected by filtration and dried in vacuo. The yield was that formed was collected by filtration and dried in vacuo. The yield was 1.42 g (80%). The product is a mixture of facial and meridional isomers. 1.70 g (94%). 'H NMR (dichloromethane-d,): Ph, 7.2-7.4 ppm (m); 'H NMR (acetone-d6): Ph, 7.0-8.1 ppm (m); CH2, 2.5-3.5 ppm (m); CH2CH2,2.72 (m) and 2.91 ppm (m); CH3CN, 2.34 ppm (q, 5JpH= 2 CH3CN resonances, 3.04, 2.76, 1.82, 1.53, and 1.61 ppm. 31PNMR Hz). 31PNMR (acetonitrile-d3): central phosphorus atom, 146.3 ppm (acetone-d,): central phosphorus atoms of facial and meridional isomers, (q, 2J = 27 Hz); terminal phosphorus atoms, 44.4 ppm (d). IR: CN 119.5 (t, J = 30 Hz) and 110.2 ppm (t, J = 31 Hz); terminal phosphorus Anal. Calcd for stretches, 2310 (w) and 2285 (m) cm-I. atoms of facial and meridional isomers 66.9 (d) and 62.1 ppm (d). IR. C44H4SNB2F8NiP4:C, 55.98; H, 4.81; N, 1.48; F, 16.09; P, 13.12. C N stretches, 2250 (w), 2280 (w), and 2310 (w) cm-I. Anal. Calcd for Found: C, 55.94; H, 4.70; N, 1.44; F, 15.92; P, 12.72. C40H42N3B2F8FeP3: C, 54.14; H, 4.78; N, 4.74; P, 10.51. Found: C, [NiPhP(CH2~2PPh2)2(CH,CN)I(BF4)2, [Ni(PP2)(CH3CN)I(BF4)2. 53.83; H, 4.90; N, 4.57; P, 9.98. A blue solution of [Ni(CH3CN)6](BF4)2.'/2CH3CN (1.00 g, 2.0 mmol) [Fe(Ph2PCH2CH2PPh2)2(CHICN)21(BF~)2, [ F ~ ( ~ P P ) ~ ( C H ~ C N ) ~ I in - CH3CN (20 mL) was added to a dichloromethane solution (50 mL) (BF4)2. A solution of Ph2PCH2CH2PPh2(1.59 g, 4.0 mmol) in toluene of PhP(CH2CH2PPh2)2(1.07 g, 2.0 mmol). The resultant red solution (30 mL) was added to a solution of [Fe(CHpCN),](BF4), (0.95 g. 2.0 was stirred for 1 h, and the solvent was removed in vacuo. The crude mmol) in acetonitrile (20 mL). The red reaction mixture was stirred product was redissolved in dichloromethane (50 mL),and ethanol (70 overnight, and the solvent was removed in vacuo to give a red powder. mL) was added. A yellow precipitate was obtained by reducing the Two 31PNMR resonances were observed at 50.6 (major product) and volume of the solution to -20 mL in vacuo. The yellow precipitate was 74.2 ppm (minor product). The product was washed with dichlorocollected by filtration and dried on a vacuum line at 50 "C for 5 h. The methane (100 mL), and the filtrate was discarded. The remaining red yield was 1.4 g (86%). 'H NMR (dichloromethane-d2): Ph, 7.4-7.8 product was recrystallized from acetone, and dried in vacuo at 50 "C for ppm; CH2CH2, 2.4-3.5 ppm; CH3CN, 2.05 ppm (br s). ,IP NMR 5 h. The yield was 1.1 g (49%). IH NMR (dichloromethane-d2): Ph, (acetonitrile-d3): terminal phosphorus atoms, 55.7 ppm (d, J = 50 Hz); 6.9-7.5 ppm (m); CH,, 3.01 ppm (m); CH,CN, 1.91 ppm (m). 3'P central phosphorus atom, 108.8 ppm (t). IR: CN stretches, 2310 (w) and 2280 (m) cm-'. Anal. Calcd for C,6H3,NB2F8NiP3:C, 53.52; H, 4.50; N, 1.73; F, 18.81; P, 11.50. Found: C, 53.54; H, 4.51; N, 1.67; (10) Bard, A. J.; Faulkner, L. R. Elecrrochemical Methods; Wiley: New York, 1980; pp 701, 35,433, and 451-453. F, 18.79; P, 11.63.

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4644 Inorganic Chemistry, Vol. 25, No. 26, 1986 [NiP(CH2CH2PW2),]2,[Ni(PP,)L. A cold (-80 "C) THF solution (50 mL) of Ni(COD)2 (0.83 g, 3.0 mmol) was added via cannula to a cold (-80 "C) THF solution (50 mL) of P(CH2CH2PPh2),(2.0 g, 3.0 mmol). Warming the reaction mixture slowly to room temperature resulted in a clear orange solution. The solvent was removed in vacuo to produce a yellow solid, which was washed with hexanes. The solid was collected by filtration and dried in vacuo at 50 "C for 2 h. The yield was

DuBois and Miedaner

occurs through the metal atom, as has been observed previously." N o bands assignable to CN stretches are observed in the infrared spectrum of [Fe(PP3)(CH3CN)2](BF4)2. However, 'H N M R spectra taken in acetone-d6 show two methyl resonances a t 3.41 and 1.02 ppm, which are assigned to coordinated acetonitrile. If the spectrum is recorded with acetonitrile-d3 as the solvent, the resonance at 1.02 ppm shifts to the position of free 1.95 g (89%). IH NMR (toluene-d8): Ph, 6.9-7.9 ppm (m); CH2CH2, acetonitrile. This indicates that one of the acetonitrile ligands 1.2-2.8 ppm (m). ,lP NMR: (see structure 4 of text for labeling of P is labile and exchanges with solvent within the time of sample atoms) Pa, 33.6 ppm (ddt, J,, = 49 HZ, Ja