Characteristic Physical and Chemical Properties of the 17-Valence

Characteristic Physical and Chemical Properties of the 17-Valence-Electron Alkyl Complexes CpCr(NO)(L)R. Peter Legzdins, Michael J. Shaw, Raymond J...
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Organometallics 1995,14, 4721-4731

4721

Characteristic Physical and Chemical Properties of the 17-Valence-ElectronAlkyl Complexes CpCr(NO)(L)R Peter Legzdins" and Michael J. Shaw Department of Chemistry, The University of British Columbia, Vancouver, British Columbia, Canada V6T 1Zl

Raymond J. Batchelor and Frederick W. B. Einstein* Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6 Received April 5, 1995@

CpCr(NO)(L)R 17-valence-electron complexes (L = Lewis base, R = hydrocarbyl) are preparable from their iodo precursors by metathesis reactions. Their ESR spectra vary greatly in appearance, ranging from featureless singlets to more complicated spectra which reveal varying degrees of delocalization of the unpaired electron. For instance, the X-band ESR spectrum of the prototypal complex CpCr(NO)(PPh3)(CHzSiMes)in hexanes exhibits signals which indicate that the unpaired electron density is delocalized virtually over the entire molecule. Consistently, the solid-state molecular structure of CpCr(NO)(PPhs)(CHzSiMes) contains no unusual bond distances or angles, the intramolecular dimensions being comparable to those exhibited by related 18-electron CpCr(N0)-containing complexes. The redox properties of selected CpCr(NO)(L)R compounds have been established by cyclic voltammetry, and they reveal that these chromium species are difficult t o reduce but relatively easy to oxidize. Neither the oxidation nor the reduction features display reversible behavior on the time scales of the cyclic voltammetry experiments, thereby indicating that these compounds decompose upon conversion to 16-electron cations or 18-electron anions and that the 17-valence-electron configuration is preferred. The chemical properties of the CpCr(NO)(L)R complexes reveal a degree of selective substitutional lability. Thus, CpCr(NO)(PPh3)(CHzSiMe3)is inert to CO, Hz, and CzH4, but it does react with NO to produce the known CpCr(NO)z(CHzSiMes) and with [NOlPFs to form [CPC~(NO)Z(PP~~)IPFS, a previously inaccessible salt. Furthermore, treatment of CpCr(NO)(PPhs)(CHzSiMe3)with 2 equiv of HSnPh3 results in the loss of the alkyl group as Me4Si and subsequent addition of an Sn-H bond to the Cr center to produce CpCr(NO)(PPhdH)(SnPh3). In solutions, the spectroscopic properties of the latter complex are consistent with it being a stannyl hydrido species. However, its solid-state lH NMR spectrum and molecular structure suggest that as a solid it is best viewed as tending toward the q2-stannane complex CpCr(NO)(PPh&,PHSnPh3). Other CpCr(NO)(L)Rcomplexes which are less sterically congested at the metal center than CpCr(NO)(PPb)(CHzSiMe3)react readily with CO to form CpCr(NO)(CO)Zand CpCr(NO)(L)(CO).

Introduction Paramagnetic organometallic complexes of the transition metals have in recent years been the focus of considerable attenti0n.l This interest is due both to the recognition that organometallic radicals play key roles in many stoichiometric and catalytic processes and to the fact that unpaired electrons associated with these Abstract published in Advance ACS Abstracts, September 1,1995. (1)(a)Noh, K. H.; Sendlinger, S. C.; Janiak, C.; Theopold, K. H. J . Am. Chem. SOC. 1989,111,9127. (b)Thomas, B. J.; Noh, S. K.; Schulte, G. K.; Sendlinger, S. C.; Theopold, K. H. J.Am. Chem. SOC.1991,113, 893.(c)Theopold, K. H. Acc. Chem. Res. 1990,23,263 and references cited therein. (d)Aase, T.; Tilset, M.; Parker, V. D. J . Am. Chem. SOC. 1990, 112,4974.(e) Crocker, L. S.; Mattson, B. M.; Heinekey, D. M. Organometallics 1990,9, 1011.(0 Astruc, D.ACC.Chem. Res. 1991, 24,36.Ig) Fortier, S.;Baird, M. C.; Preston, K. F.; Morton, J. R.; Zeigler, T.; Jaeger, T. J.; Watkins, W. C.; MacNeil, J. H.; Watson, K. A.; Hensel, K.; Le Page, Y.; Charlande, J. P.; Williams, A. J. J . Am. Chem. Soc. 1991,113,542.(h) Poli, R.;Owens, B. E.; Linck, R. G. J . Am. Chem. Soc. 1992,114,1302.(i)Luinstra, G. A.; ten Cate, L. C.; Heeres, H. J.; Pattiasina, J. W.; Meetsma, A.; Teuben, J. Organometallics 1991,10, 3227. (i) Fei, M.; Sur, S. P.; Tyler, D. R. Organometallics 1991,10, 419. @

complexes generally facilitate modes of reactivity which are unavailable to related diamagnetic compounds.' In particular, 17-valence-electroncompounds have been studied extensively, and these studies have demonstrated that these species are generally highly reactive.2 For instance, 17-electron complexes can function as potent hydrogen and halogen atom abstraction reagents,2a*b and they are often unstable with respect to dimerization via formation of metal-metal or ligandbased carbon-carbon bonds.2bsc In addition, they are usually quite substitutionally labile, a feature that (2)(a) Brown, T. L. In Organometallic Radical Processes; Trogler, W. C., Ed.; Elsevier: New York, 1990;Chapter 3.(b)Tyler, D.R. Prog. Inorg. Chem. 1988,36,125. (c)Baird, M. C. Chem. Reu. 1988,88,1217. (d) Kowaleski, R. M.; Basolo, F.; Trogler, W. C.; Gedridge, R. W.; Newbound, T. D.; Ernst, R. D. J. Am. Chem. Soc. 1987,109;4860 and references cited therein. (e) Trogler, W. C. In Organometallic Radical Processes; Trogler, W. C., Ed.; Elsevier: New York, 1990;Chapter 9. (0 Coville, N. J. In Organometallic Radical Processes; Trogler, W. C., Ed.; Elsevier: New York, 1990; Chapter 4. (g) Kochi, J. K. In Organometallic Radical Processes; Trogler, W. C., Ed.; Elsevier: New York, 1990;Chapter 7. (h) Kochi, J. K. J. Orgummet. Chem. 1988, 300,139.

0276-733319512314-4721$09.00/00 1995 American Chemical Society

4722 Organometallics, Vol. 14, No. 10, 1995

Legzdins et al.

Table 1. Mass Spectral, Infrared, ESR, and Electrochemical Data ESR complex CpCr(NO)(PPha)(CHzSiMes)

MS (FAB; m l z ) 496 [P+l

IR VNO (THF; cm-l) 1624

g value (THF)

1.998

1.991 1.990

electrochemistry

coupling const (G) acr = 5.3 a c m = 1.0 a c m = 12.0 up = 26.0 acP= 0.6 UN = 5.0 acr = 8 acr = 15.7

EP+3

(V vs SCE)

0.43' (NCMe)

EP$

(V vs SCE) -1.94'

0.29' (THF)

-2.38'

0.19 (THF)

-2.09

8 UN % 8 UCH

CpCr(NO)(pip)(CHzPh) CpCr(NO)(NHzCMe3)(CH2Ph) CpCr(NO)(THF)(CHzSiMe3)

323 [P+l 307 [P+]

1585a 16Ola 1631

1.976 1.975 1.994

%

acr = 17 acr = 15 acr = 15.5 aCm = 1.0 a C w = 10.5 a N = 3.5

CpCr(NO)(THF)(CHzPh) CpCr(NO)(THFJ@-tolyl) CpCr(N0XTHF)Me

1624 1641 1635

1.984 1.977 1.982

acp = 3.5 UTHF = 0.4 ucr = 16 acr = 21 ac,= 22 UCH 5 a N z 5

a

Nujol mull.

Scan rate 0.30 V/s.

Scan rate 0.40 V/s.

permits them to be utilized as intermediates in electrontransfer-induced catalytic cycles.2c-h Two broad classes of 17-valence-electron complexes of chromium that we have been investigating recently are the cationic [Cp'Cr(NO)L2]+ complexes3 and the neutral Cp'Cr(NO)(L)X (Cp' = Cp (175-CsH5),Cp" (a5C5Mes); L = Lewis base; X = halide or alkyl (R))nitrosyl specie^.^ Although some halide-containing members of the latter class have been known for more than 25 years, the characteristic chemical properties of these compounds had remained largely unexplored. Consequently, we decided some time ago to undertake a wideranging investigation of these complexes. We first carried out studies dealing with the synthesis, characterization, and redox properties of representative 17electron halide c~mplexes.~ We next discovered the apparently simple iodide-for-alkyl metathesis reaction summarized in eq 1, in which 2 equiv of the Grignard CpCr(NO)(PPh,)I

+ 2Me3SiCH,MgC1 (in THF)-

CpCr(NO)(PPh,)(CH,SiMe,) (1) reagent is required to completely consume the CpCr(NO)(PPhdI reactant. Of some concern to us initially was the fact that similar reactions could not be utilized to prepare a range of related CpCr(NO)(L)(CH2SiMe3) complexes. A subsequent detailed study of conversion 1 established that it proceeds via initial loss of PPh3 from the chromium's coordination sphere and permitted us to develop a general synthetic route to the desired CpCr(NO)(L)R(R = alkyl) c~mplexes.~ In this paper we present the complete results of our studies concerning the characterization and reactivity of representative examples of these 17-valence-electron alkyl complexes. A portion of this work has been communicated previously.6 (3) Legzdins, P.; McNeil, W. S.; Batchelor, R. J.;Einstein, F. W. B. J. Am. Chem. SOC.1994,116,6021. (4) Legzdins, P.; McNeil, W. S.; Shaw, M. J. Organometallics 1994, 13, 562 and references cited therein. ( 5 ) Legzdins, P.; Shaw, M. J. J.Am. Chem. SOC.1994,116, 7700.

Experimental Section General procedures routinely employed in these laboratories have been described previo~sly.~ All reagents were purchased from commercial suppliers or were prepared according to literature methods. Thus, CpCr(NO)(PPh3)(CH2SiMe3),5 CpCr(NO)(L)R(L = piperidine (pip), NHzCMe3; R = CHzSiMe3, CHzPh)? [CpCr(NO)I12,8[CpCr(NO)(NCMehlPF6,9CpCr(N0)(C0)z,lo HSnPhs," and [Cp$'e]PF612 were prepared by published procedures. The CpCr(NO)(THF)R complexes (R = CHzSiMes, CHzPh, p-tolyl, CH3) were generated in situ by treatment of [CpCr(NO)I]2 in THF with the appropriate Grignard reagent, RMgCL5 The 400 MHz solid-state 'H NMR spectrum was recorded by Ms. Patricia Aroca and Dr. Colin Fyfe of this department. The mass spectral, IR, ESR, and electrochemical data for the CpCr(NO)(L)R complexes investigated during this work are collected in Table 1. Gas Chromatography. Gas chromatography was performed using a Shimadzu GC-14Agas chromatograph equipped with an OV-17 capillary column. The injector temperature and the detector temperature were normally set at 190 and 270 "C, respectively. The column temperature was kept at 40 "C for a period of 1 min after injection of the sample, whereupon it was raised 10 "Clmin until a temperature of 250 "C was reached. The normal sample volume used was 0.1 pL for liquid samples and 10 pL for samples of the atmosphere above reaction mixtures. Compounds were identified by comparison of their retention times with those of authentic samples. Electrochemical Measurements. The detailed methodology employed for cyclic voltammetry (CV) studies in these laboratories has been previously described.13 All potentials listed in Table 1 are reported versus the aqueous saturated calomel electrode (SCE). ESR Measurements. Ambient-temperature X-band ESR spectra of M solutions were recorded using a Varian (6) Herring, F. G.; Legzdins, P.; McNeil, W. S.; Shaw, M. J.; Batchelor, R. J.;Einstein, F. W. B. J . Am. Chem. Soc. 1991,113,7049. (7)Dryden, N. H.; Legzdins, P.; Rettig, S. J.; Veltheer, J. E. Organometallics 1992,11, 2583. (8)Legzdins, P.; Nurse, C. R. Znorg. Chem. 1985,24,327. (9)Chin, T. T.; Legzdins, P.; Trotter, J.;Yee, V. C. Organometallics

1992,11, 913. (10) Chin, T. T.; Hoyano, J. K.; Legzdins, P.; Malito, J. T. Inorg. Synth. 1990,28,196. (11)Eisch, J. J. Orgunomet. Synth. 1981,2,173. (12) Smart, J. C.; Pinsky, B. L. J.Am. Chem. SOC.1980,102,1009.

17-Valence-Electron Alkyl Complexes of Cr E-3 spectrometer or were recorded by Dr. F. G. Herring with the spectrometer and interfaced computer system described by Phillips and Herring.14 Computer simulations were performed to verify the splitting patterns observed in the various ESR spectra. Synthesis of CpCr(NO)(py)(CH2SiMes). Dark green CpCr(NO)(py)I (0.35 g, 1.00 mmol) was dissolved in THF (15 mL), and MeaSiCHzMgCl(2.00 mL, 1.0 M in EtzO, 2.00 mmol) was added. Upon addition of the Grignard reagent, the solution changed from green to red-brown. The solution was stirred overnight, during which time a white solid precipitated and the solution became green-brown. The solvent was removed from the final mixture under reduced pressure, and the residue was extracted with Et20 (3 x 20 mL). The extracts were combined and filtered through a column of alumina (1.5 x 3 cm). The column was washed with Et20 until the washings were colorless and only a brown band remained at the top of the column. The Et20 was removed in vacuo from the bright green filtrate. Pentane (5 mL) was added to the green oily residue, and the resulting solution was cooled to -78 "C. A bright green precipitate formed, was isolated at -78 "C by cannulation, and was washed with cold pentane (2 x 5 mL). This solid was dried in vacuo at -78 "C, but when it was warmed t o room temperature, it melted and formed an olive green oil. Anal. Calcd for C14H21N20SiCr: C, 53.65; H, 6.75; N, 8.94. Found: C, 52.42; H, 6.64; N, 9.20.

Reaction of CpCr(NO)(PPhs)(CHzSiMed with [CpzFeIPF6. To a green solution of CpCr(NO)(PPh3)(CH2SiMe3)(0.25 g, 0.50 mmol) in MeCN (10 mL) was added blue [CpzFeIPFs (0.16 g, 0.50 mmol). The solution immediately turned dark blue, but after being stirred for 18 h, it changed back to green. The final solution was filtered through a plug of Celite (2 x 3 cm) supported on a glass frit and was then cooled to -30 "C overnight. This treatment resulted in the deposition of orange crystals, which were isolated by cannulating the supernatant solution into another vessel. The orange crystals were identified as ferrocene (0.10 g) by comparison of their characteristic IR spectrum with that of a n authentic sample. The green mother liquor was concentrated under reduced pressure to a volume of 5 mL, and then Et20 (5 mL) was added. The solution was cooled to 8 "C for 18 h to induce the formation of a mixture of orange and green crystals. The green crystals were identified as [CpCr(NO)(NCMe)zlPF6by comparisons of its IR and FAB-MS spectra to those exhibited by an authentic ample.^

Reaction of CpCr(NO)(PPhs)(CHzSiMes)with NO. CpCr(NO)(PPh3)(CHzSiMe3)(0.20 g, 0.40 mmol) was dissolved in Et20 (10 mL), and NO was bubbled through the solution for 5 min. The solution was stirred for 1h, whereupon a white precipitate formed. The final mixture was filtered through alumina (2 x 3 cm) supported on a glass frit, and the column was washed with Et20 (3 x 10 mL). The solvent was removed from the combined filtrates, resulting in the isolation of CpCr(N0)2(CHzSiMe3)(0.05 g, 47% yield) as a dark green 0i1.l~This oil was dried under vacuum overnight. IR (Nujol): V N O 1778,1676 cm-'. IR (EtzO): Y N O 1778,1674 cm-l. lH NMR (CDC13): 6 5.39 (s, 5H, C5H5),0.23 (s, 2H, CHz), -0.03 (s, 9H, CH3). 13C NMR (CDC13): 6 99.6 (C5H5), 4.70 (CH2) 1.98 (CH3). EIMS (probe temperature 120 "C): m l z 264 [P+],249 [P+ - Me]. Reaction of CpCr(NO)(PPhs)(CHsSiMes)with [NOIPF6. CpCr(NO)(PPh3)(CHzSiMe3) (0.55 g, 1.1 mmol) was added t o a stirred suspension of [NOlPFe (0.19 g, 1.1mmol) (13)(a) Herring, F. G.; Legzdins, P.; Richter-Addo, G. B. Organometallics 1989,8,1485.(b) Legzdins, P.; Wassink, B. Organometallics 1984,3,1811.(c) Legzdins, P.; Lundmark, P. J.; Phillips, E. C.; Rettig, S. J.; Veltheer, J. E. Organometallics 1992,11, 2991 and references

cited therein. (14)Phillips, P. S.; Herring, F. G. J. Magn. Reson. 1984,57, 43. (15) Legzdins, P.; Richter-Addo, G. B.; Wassink, B.: Einstein. F. W. B.; Jones, R. H. J.Am. Chem. SOC.1989,1 1 1 , 2097.

Organometallics, Vol. 14, No. 10, 1995 4723 in CHzClz (10 mL). Within 2 min, the VNO band at 1616 cm-' in the IR spectrum of the supernatant solution was replaced by bands at 1831 and 1738 cm-l and a broad band at 1669 cm-l. The solvent was removed under reduced pressure, and then CHzClz (1mL) was reintroduced into the flask to obtain a green solution. A silica gel column (1 x 10 cm, Fisher, 6080 mesh) was prepared using a CHZClfit20 (97:3) solvent mixture. The green solution was applied t o the top of the column and was eluted with the solvent mixture under a slight pressure of dinitrogen. Two pale brown bands followed by a dark green band were eluted and collected. The IR spectrum of the first eluate exhibited a broad YNO band at 1668 cm-', and that of the second displayed a broad VNO absorption at 1655 cm-'. These solutions were concentrated in vacuo to incipient precipitation and cooled to -30 "C; however, no tractable products were isolable. The IR spectrum of the dark green band displayed two sharp V N O bands at 1831 and 1738 cm-'. The dark green eluate was concentrated under reduced pressure and cooled to -30 "C overnight to induce the deposition of [CpCr(NO)2(PPh3)]PF6(0.38 g, 59% yield) as dark green microcrystals. Anal. Calcd for CZ~HZONZOZF~PZC~: C, 47.27; H, 3.44; N, 4.79. Found: C, 47.11; H, 3.47; N, 4.70. IR (Nujol): V N O 1826, 1730 cm-'. IR (CHzClZ): VNO 1831, 1738 cm-l. 200 MHz 'H NMR (CD2ClZ): 6 7.99-7.05 (m, 15H, P(Ca5)3), 5.70 (8, 5H, C5H5). 50 MHz 13C{'H} NMR (CDzC12): 6 153.97, 153.75, 153.60, 153.54, 153.14, 152.93, 150.93, 150.73, 149.76 (aromatic ring carbons), 122.90 (C5H5). 81 MHz 31P{1H} NMR ~ 710.62 Hz, (CDZC12): 6 68.46 (PPhd, -31.07 (septet, l J p = PFs-1. 188 MHz "F NMR (CDzC12): 6 4.30 (d, l J p = ~ 710.68 Hz, PFS-). FAB-MS m / z 439 [P' - PFs-I.

Reaction of CpCr(NO)(PPhs)(CH2SiMed with HSnPhs. An emerald green solution of CpCr(NO)(PPhd(CHzSiMe3)(0.49 g, 1.0 mmol) in THF (20 mL) was treated with HSnPh3 (0.50 mL, 2.0 mmol). The resulting mixture was refluxed for 2 h, after which time the V N O band in the IR spectrum of the solution had shifted from 1624 to 1639 cm-'. The THF was removed from the green solution in vacuo, and the residue was extracted with Et20 (50 mL). The Et20 extracts were filtercannulated into another vessel and then cooled to 8 "C for 2 days. This treatment resulted in the formation of 0.32 g of a green microcrystalline solid. This solid was placed in the thimble of a Soxhlet extractor and was extracted continuously with pentane for 7 days. This procedure resulted in the formation of large X-ray-quality single crystals of analytically pure CpCr(NO)(PPh3)(H)(SnPhs) (0.10 g, 13%yield). Anal. Calcd for C41H3DOPSnCr: C, 64.76; H, 4.77; N, 1.84. Found: C, 64.90; H, 4.82; N, 1.90. IR (THF): V N O 1639 cm-'. IR (Nujol): VNO 1621 cm-'. 400 MHz 'H NMR (CsD6): 6 8.097.43 (m, 15H, Sn(C&h), 7.40-6.80 (m, 15H, P(Ca5)3), 4.56, (d, 3 J= ~ 3.0 HZ, ~ 5H9 C5H5)1 -2.47 (d, 'JPH = 90.3 HZ, 'JSnH = 23.7 Hz, lH, Cr-H). 50 MHz: 13C{'H} NMR (csD13): 6 147.05, 137.84, 137.61, 133.58,133.46, 129.88, 129.15, 128.52, 128.48 (aromatic rings), 92.07 (C5H5). 81 MHz 31PNMR: 6 87.94 (2Jsnp= 38.6 Hz, PPh3). FAB-MS: m l z 760 [P+ - HI. Reaction of CpCr(NO)(pip)(CHSh) with CO. Bright green CpCr(NO)(pip)(CHzPh)(0.10 g, 0.3 mmol) was dissolved in THF (10 mL). The solution was stirred under an atmosphere of CO for 12 h, whereupon it underwent a gradual color change from green t o orange. The solvent was vacuumtransferred into another vessel, and the resulting clear solution was analyzed by GC. Only one peak other than solvent was detected in the GC trace, and this was identified as toluene by comparison to an authentic sample. A water-cooled sublimation probe was attached to the flask containing the organometallic residue. The flask was evacuated and warmed with a hot water bath. Within 2 h, small bright orange crystals formed on the probe. These were identified as CpCr(NO)(CO)zby IR and EIMS comparisons to authentic samples.1° The remaining residue was recrystallized from Et20 to obtain 0.01 g of a red solid, which was tentatively identified as CpCr(NO)(pip)(CO)on the basis of its spectroscopic properties (Le.

Legzdins et al.

4724 Organometallics, Vol. 14, No. 10,1995 IR (Nujol) V N O 1635 cm-', vco 1905 cm-'; FAB-MS m l z 260 [P+l,232 [P+- COI, 202 [P+ - CO - NO]). Reaction of CpCr(NO)(THF')(CHgSiMes)with CO. A solution of [CpCr(NO)Il2 (0.27 g, 0.50 mmol) in THF (15 mL) was treated with Me3SiCHzMgCl (2.0 mL, 1.0 M in EtzO, 2.0 mmol). The resulting dark red solution was stirred overnight and was then filtered through Celite t o remove the white precipitate (presumably Mg salts) which had formed. The filtrate was placed under an atmosphere of CO and was stirred for a further 18 h, whereupon it turned light orange. The solution displayed intense IR bands a t 2016, 1957, and 1701 cm-l. The solvent was removed in vacuo, and the residue was sublimed onto a water-cooled probe to obtain 0.05 g (24% yield) of orange CpCr(NO)(CO)Z, identified by IR and EIMS comparisons t o authentic samples.1° X-ray Crystallographic Analysis of CpCr(NO)(PPhs)(CHzSiMes). Crystals of the complex suitable for an X-ray crystallographic analysis were grown from a saturated solution in pentane maintained at 8 "C for a period of 1week. A dark green block of CpCr(NO)(PPh3)(CHzSiMe3)was mounted in a Lindemann capillary tube under argon. Intensity data (Mo Kafgraphite monochromator) were collected at 200 K with an Enraf-Nonius CAD-4F diffractometer equipped with an extensively in-house-modifiedlow-temperature attachment. The unit cell was determined from 25 well-centered reflections (31" 5 28 5 40"). Two intensity standards were measured every 1 h of acquisition time and showed small periodic fluctuations (f1.5%) during the course of data acquisition. The data were corrected analytically for absorption by the Gaussian integration method. l6 Data reduction included Lorentz and polarization corrections. Peaks corresponding to all the hydrogen atoms but four (on the phenyl rings) were observed in an electron-density difference map after isotropic refinement of all the non-hydrogen atoms ( R = 0.075). The methyl groups were all observed to have staggered orientations about the Si-C bonds. Subsequently, all hydrogen atoms were placed in calculated positions (C-H = 0.95 8)and were assigned isotropic thermal parameters initially 10% larger than those for the corresponding carbon atoms. During refinement, the hydrogen atom coordinate shifts were linked with those of the carbon atoms to which they were bonded. A single parameter was refined for the isotropic thermal motion of the hydrogen atoms of each of the following types: methylene, methyl, phenyl, and cyclopentadienyl. The final full-matrix, least-squares refinement of 293 parameters for 2128 observed data included anisotropic thermal parameters for all non-hydrogen atoms. A weighting scheme based on counting statistics was applied such that was nearly constant as a function of both IFoI and (sin O ) / L Complex scattering factors for neutral atoms17 were used in the calculation of structure factors. The programs used for data reduction, structure solution, and initial refinement were from the NRCVAX Crystal Structure System.lB The program suite CRYSTALS1gwas employed in the final refinement. All computations were carried out on a MicroVAX-I1 computer. Crystallographic data are summarized in Table 2. The final positional and equivalent isotropic thermal parameters for the non-hydrogen atoms are given in Table 3, and selected bond lengths and angles are provided in Table 4. A view of the solidstate molecular structure of CpCr(NO)(PPh3)(CH2SiMes)is shown in Figure 1. Additional experimental details, coordinates, and temperature factors for the hydrogen atoms, as well (16)Busing, W. R.; Levy, H. A. Acta Crystallogr. 1957, 10, 180. (17)International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1975; Vol. IV,p 99. (18) Gabe, E. J.; LePage, Y.; Charland, J.-P.; Lee, F. L.; White, P. S. NRCVAX-An Interactive Program System for Structure Analysis. J . Appl. Crystallogr. 1989, 22, 384. (19)Watkin, D. J.; Carruthers,J. R.; Betteridge, P. W. CRYSTALS; Chemical Crystallography Laboratory, University of Oxford: Oxford, England, 1984.

Table 2. Crystallographic Data for the Complexes CpCr(NO)(PPh3)(CHzSiMes)and CpCr(N0)(PPhd(H)(SnPhd CpCr(NO)(PPha)(CHzSiMe3) CrPONC27H31 496.60 monoclinic P21lc 7.962(3) 15.440(4) 20.883(6)

compd formula fw cryst syst space group a,A b, A

C, A a, deg de€! Y , deg

293 0.048 0.052c

419 0.024 0.033d

90.31 (3)

P7

v, A3

z

ecalcd, dcm3 A(Mo Ka), A p(Mo Ka), cm-l temp, K transmissn factors min-max 28, deg no. of f i n s with Z z 2.5dZ) no. of variables RF' RWFb a

2567.0 4 1.285 0.709 30 (Kal) 5.6 200 0.876-0.930 4-46 2128

tr ic1inic Pi 11.582(2) 11.961(2) 14.174(4) 84.84(2) 68.45(2) 67.89(2) 1689.3 2 1.495 0.709 30 (Kal) 11.3 200 0.769-0.813 4-50 5011

RF = W F O I - IFcl)/TIFolR. w =~[X(w(/F,I - IFc1)2/T(wFo2)]1/2. = [dFo)2 0.00O6Fo2]-'. d w = [dFo)2 0.0002F,2]-1.

+

+

Table 3. Atomic Coordinates ( x lo4)and Equivalent Isotropic Temperature Factors (& x 104) for the Non-HydrogenAtoms of CpCr(NONPPhs)(CH~SiMes) at 200 K atom

xla

Ylb

zlc

6510(1) 5149(2) 6981(2) 4656(6) 3339(6) 7026(7) 4962(8) 8724(8) 7214(9) 3674(7) 2460(7) 1445(7) 1620(8) 2768(8) 3785(7) 3868(7) 2722(7) 1794(9) 1974(10) 3042(9) 4026(8) 6584(7) 7927(7) 9035(8) 8860(8) 7559(8) 6435(7) 9085(7) 9142(8) 7271(8) 8025(8) 7949(7)

3208.7(6) 1945.7(9) 3206U) 3659(3) 4039(3) 2584(4) 3787(4) 4008(4) 2418(4) 2114(4) 1504(4) 1602(4) 2322(4) 2953(4) 2847(4) 1363(4) 1831(5) 1428(6) 552(6) 86(5) 487(4) 1119(3) 873(4) 248(4) -140(4) 112(4) 738(4) 3097(4) 3747(4) 4150(4) 4392(4) 3350(4)

1186.5(4) 1589.8(7) -436.5(7) 994(2) 904(2) 318(2) -571(3) -492(3) -1121(3) 2245(3) 2392(3) 2914(3) 3297(3) 3153(3) 2622(3)

ueq"

271 252 348 325 459 338 503 447 494 277 343 386 395 433 354 274 lOOl(3) 638(3) 415 517 166(3) 503 53(3) 420(4) 548 885(3) 460 1894(3) 259 1508(3) 328 1708(3) 429 2306(3) 427 2695(3) 370 2500(3) 316 380 1643(3) 342 1179(3) 1930(3) 352 1351(3) 385 2109(3) 348 a Ueqis the cube root of the product of the principal axes of the thermal ellipsoid. as the anisotropic temperature factors and selected bond torsion angles, are given as supporting information. X-ray Crystallographic Analysis of CpCr(NO)(PPh& (H)(SnPhs).A green crystal of the complex was mounted on a glass filament with epoxy adhesive. Data were recorded at 200 K in a manner similar t o that described in the preceding

17-Valence-Electron Alkyl Complexes of Cr

Organometallics,

Vol.14,No.10,1995 4725

Table 4. Selected Bond Distances (A) and Angles (deg) for CpCr(NO)(PP&)(CH&iMes)at 200 K Cr-P Cr-N Cr-C(l) Cr-C(51) Cr-C(52) Cr-C(53) Cr-C(54) Cr-C(55) C(ll)-C(12) C(ll)-C(16) C(12)-C(13) C(21)-C(22) C(21)-C(26) C(22)-C(23) C(31)-C(32) C(31)-C(36) C(32)-C(33) C(51)-C(52) C(51)-C(55) C(52)-C(53) Cp-Cr-P C(1)-Cr-P C(1)-Cr-N C(ll)-P-Cr C(21)-P-Cr C(31)-P-Cr C(Bl)-P-C(ll) C(31)-P-C(11) C(31)-P-C(21) C(12)-C(ll)-P

a

2.386(2) 1.678(5) 2.096(5) 2.264(6) 2.255(6) 2.215(6) 2.209(6) 2.247(5) 1.385(8) 1.381(7) 1.368(8) 1.385(8) 1.380(8) 1.377(8) 1.395(7) 1.402(7) 1.372(8) 1.396(8) 1.388(8) 1.385(9) 121.4 91.2(2) 99.2(2) 116.3(2) 114.6(2) 114.2(2) 102.4(2) 104.0(2) 103.6(3) 121.4(4) 120.2(4) 118.3(5) 121.1(6) 119.6(6) 120.7(6) 119.0(6) 121.1(6) 118.0(4) 123.5(4) 118.4(5) 120.6(6) 120.6(6) 119.1(6)

Cr-Cpa P-C(l1) P- C(2 1) P-C(31) Si-C(l) Si-C(2) Si-C(3) Si-C(4) N-0 C(13)-C(14) C(14)-C(15) C(15)-C(16) C(23)-C(24) C(24)-C(25) C(25)-C(26) C(33)-C(34) C(34)-C(35) C(35)-C(36) C(54)-C(53) C(54)-C(55)

1.898 1.827(5) 1.830(5) 1.825(6) 1.845(5) 1.862(6) 1.864(6) 1.888(6) 1.216(6) 1.377(8) 1.371(8) 1.386(8) 1.380(11) 1.351(10) 1.392(8) 1.393(9) 1.376(9) 1.377(8) 1.404(8) 1.398(8)

Cp-Cr-N C(B)-Si-C(l) C(3)-Si-C(1) C(3)-Si-C(2) C(4)-Si-C( 1) C(4)-Si-C(2) C(4)-Si-C(3)

126.4 108.3(3) 108.1(3) 106.5(3) 107.7(3) 119.8(3) 173.6(4) 118.3(5) 123.2(5) 118.4(6) 120.4(7) 120.6(7) 119.4(7) 120.7(7) 120.4(7) 121.0(6) 120.2(6) 108.0(6) 108.3(6) 107.4(6) 108.0(6) 108.3(5)

Cp represents the center of mass of the cyclopentadiene ring.

section with the same instrument. Two standard reflections which were measured every 1 h of exposure time declined systematically in intensity by 10% during the course of the measurements. The data were corrected for absorption by the Gaussian integration method,16and corrections were carefully checked against measured scans. Data reduction included corrections for intensity-scale variation and for Lorentz and polarization effects. Coordinates and anisotropic thermal parameters for all nonhydrogen atoms were refined. Positions of all hydrogen atoms of the phenyl and cyclopentadienyl groups were then clearly revealed in an electron-density difference map. These hydrogen atoms were, nonetheless, placed in calculated positions 0.95 A from their respective carbon atoms and were assigned isotropic temperature factors initially proportionate t o the equivalent isotropic temperature factors of the corresponding carbon atoms. NMR results along with the bond angles about the tin and chromium atoms indicated the presence of a hydrogen atom bridging Sn and Cr. Such a hydrogen atom (H(10))was placed in a calculated position20 with arbitrarily assigned Cr-H (1.65 A) and Sn-H (1.92 A) distances and an isotropic temperature factor of 0.04 A2. This position lay within a broad region of positive residual electron density, as indicated by the difference map. In subsequent cycles of (20) Orpen, A.

G.J . Chem. SOC.,Dalton Trans. 1980,2509.

C(U)

Figure 1. Solid-state molecular structure of CpCr(N0)(PPhB)(CH&Mes) at 200 K. The 50%probability displacement ellipsoids or spheres are shown for the non-hydrogen atoms. refinement, the coordinate shifts for the hydrogen atoms of the phenyl and cyclopentadienylgroups were linked with those for their respectively bound carbon atoms. A mean isotropic temperature factor for all hydrogen atoms was refined, and the shifts were applied to the individual values. A weighting scheme based on counting statistics was applied such that was nearly constant as a function of both IF,I and (sin @)/A. Final full-matrix least-squares refinement of 419 parameters for 5011 data (I, 2 2.5dIO)) converged at RF = 0.024. The coordinates for H(10) were included in this refinement (0.144(3), 0.232(3), 0.204(3) after the last cycle). The absolute value of the final maximum shift/ error was 0.25 for the z coordinate of H( lo), but for all other variables it was 0.01 or less. Computations were carried out on MicroVAX-I1 and 80486 computers utilizing the programs specified in the previous section. Crystallographic details are summarized in Table 2. Final fractional atomic coordinates for the non-hydrogen atoms are listed in Table 5, and selected bond lengths and angles are summarized in Table 6. A view of the solid-state molecular structure of CpCr(NO)(PPhs)(H)(SnPh3) is shown in Figure 2. Tables of additional crystallographic data, the coordinates for the hydrogen atoms, and the anisotropic temperature factors for CpCr(NO)(PPha)(H)(SnPh3) are given as supporting information.

Results and Discussion Synthesis of CpCr(NO)(L)R Complexes. As we have described p r e v i ~ u s l ythe , ~ general synthetic r o u t e to the 17-valence-electron CpCr(NO)(L)R complexes involves (a) effecting the conversion from CpCr(N0)(THF)I to CpCr(NO)(THF)R by treatment with 2 equiv of the appropriate Grignard reagent, RMgC1, in THF, (b) destroying a n y excess organomagnesium species

remaining with MeI, and ( c ) introducing L t o displace THF from the coordination s p h e r e of the CpCr(N0)(THF)R intermediate complex (eq 2).

Legzdins et al.

4726 Organometallics, Vol. 14, No. 10,1995 Table 5. Fractional Atomic Coordinates ( x 104 and Equivalent Isotropic Temperature Factors (1 2 x lo4)for the Non-Hydrogen Atoms of CpCr(NO)(PPhs)(H)(SnPhdat 200 K atom

X

3032.7(2) 1243.0(4) -844.7(6) 979(2) 1088(2) 593(3) 1208(3) 2551(3) 2770(3) 1559(3) -1186(2) -1031(2) -1344(3) - 1802(2) -1954(3) -1661(2) -2268(2) -2065(2) -3137(3) -4407(2) -4618(2) -3551(2) -1150(3) -258(3) -416(4) - 1449(5) -2319(5) -2185(3) 2780(2) 3671(3) 3542(3) 2520(3) 1632(3) 1759(3) 3683(2) 3541(3) 3931(3) 4463(3) 4637(4) 4250(4) 4846(3) 4803(3) 5947(3) 7168(3) 7234(3) 6088(3)

Y

2338.0(1) 1282.1(3) 2797.9(5) 1615(2) 1534(2) 34(2) -557(2) -654(2) -133(2) 298(2) 4343(2) 4527(2) 5683(2) 6669(2) 6493(2) 5340(2) 2535(2) 1489(2) 1317(2) 2183(2) 3224(2) 3398(2) 2951(2) 3271(3) 337x3) 3144(4) 2823(4) 2725(3) 4037(2) 4117(2) 5215(2) 6262(2) 6205(2) 5111(2) 2677(2) 2087(2) 2349(3) 3229(3) 3813(3)

3546(3) 1149(2) 824(2) 122(3) -284(2) 12(3) 719(2)

2

1999.7(1) 2730.9(3) 2867.8(5) 4798(2) 3923(2) 2170(2) 2863(2) 2453(2) 1505(2) 1330(2) 3250(2) 4150(2) 4503(2) 3963(2) 3077(2) 2721(2) 3847(2) 4387(2) 5156(2) 5401(2) 4872(2) 4102(2) 1679(2) 837(2) -91(3) -191(3) 625(3) 1566(2) 2626(2) 3037(2) 3395(2) 3347(2) 2937(2) 2579(2) 392(2) -327(2) - 1342(2) -1657(2) -965(2) 49(2) 2243(2) 3222(2) 3410(2) 2621(2) 1648(2) 1462(2)

U,"" 270 266 217 483 333 357 382 377 329 329 240 297 334 329 330 292 247 287 319 315 335 311 298 428 564 629 628 438 251 295 341 332 355 337 290 313 361 373 487 448 290 365 404 379 419 351

U,,is the cube root of the product of the principal axes of the thermal ellipsoid.

In some cases, the desired CpCr(NO)(L)Rcomplexes may be obtained directly by treatment of CpCr(NO)(L)I with 2 equiv of RMgC1, but these latter conversions probably also proceed via the CpCr(NO)(THF)Rintermediates. The isolable product compounds are bright to dark green solids which, though generally stable in air for short periods of time, are best stored under an atmosphere of dinitrogen. The physical and spectroscopic properties of these complexes are given in Table 1. In THF solution these compounds display ESR signals a t g values less than 2.00 and VNO bands in their IR spectra at approximately 1625 cm-l. Table 1 also contains the IR and ESR spectroscopic data for the CpCr(NO)(THF)R intermediate complexes in which R = Me, CHZPh, CHzSiMe3, and p-tolyl, although none of these THF complexes have yet proven t o be isolable. ESR Spectroscopy of the CpCr(NO)(L)R Complexes. The ESR spectra of these 17-electron alkylcontaining complexes (Table 1)vary greatly in appearance. Some of the complexes display featureless singlets

Table 6. Selected Intramolecular Distances (A) and Angles (deg) for CpCr(NO)(PPhd(H)(SnPhdat 200 K Sn-Cr Sn-C(41) Sn-C(51) Sn-C(61) Sn-H(10) Cr-H(l0) Cr-P P-C(l1) P-C(21) C(41)-Sn-Cr C(51)-Sn-Cr C(51)-Sn-C(41) C(Gl)-Sn-Cr C(61)-Sn-C(41) P-Cr-Sn N-Cr-Sn N-Cr-P C p -Cr- Sn Cp -Cr-P 0-N-Cr C(ll)-P-Cr C(21)-P-Cr C(Bl)-P-C(ll) a

2.6690(7) 2.166(2) 2.174(3) 2.179(3) 1.84 1.55 2.364(1) 1.827(2) 1.836(2) 123.45(6) 118.75(7) 99.54(9) 107.22(7) 100.24(9) 105.98(3) 90.94(8) 94.66(8) 114.46 118.03 174.3(2) 118.66(8) 113.30(8) 100.2(1)

Cr-N Cr-C(l) Cr-C(2) Cr-C(3) Cr-C(4) Cr-C(5) Cr-Cpa P-C(31) 0-N C(61)-Sn-C(51) H(lO)-Sn-Cr H(lO)-Sn-C(41) H(lO)-Sn-C(51) H(lO)-Sn-C(61) Cp-Cr-N H(lO)-Cr-Sn H(10)-Cr-P H(lO)-Cr-N Cp-Cr-H(10) Cr-H( 10)-Sn C(311-P-Cr C(31)-P-C(11) C(31)-P-C(21)

1.676(2) 2.220(3) 2.205(3) 2.213(3) 2.243(3) 2.241(3) 1.875 1.825(3) 1.208(3) 105.0(1) 34 112 94 140 127.9 42 69 113 116 104 112.50(9) 104.0(1) 106.9(1)

See footnote a of Table 4.

in their ESR spectra. However, in several cases, the CpCr(NO)(L)R compounds exhibit more complicated spectra which reveal varying degrees of delocalization of the unpaired electron. For instance, the prototypal complex CpCr(NO)(PPh3)(CHzSiMea)was the first of this class of compounds that we discovered t o display evidence of significant delocalization of its unpaired electron. The X-band ESR spectrum of this compound in hexanes a t 25 "C is shown in Figure 3. This remarkable spectrum reveals hyperfine coupling of the signal to the metal and to atoms in all of the ligands in the metal's coordination sphere, thereby indicating that the unpaired electron density is delocalized virtually over the entire molecule. Such extensive delocalization of unpaired electron density has never been previously observed in an organometallic system.21 It has been noted, however, that the unpaired electrons in the bimetallic { [HB(Me~pz)slMo(NO)C1)~(3,3'-bpy) complex (Mezpz = 3,5-dimethylpyrazolyl; 3,3'-bpy = 3,3'-bipyridine) are strongly coupled through the 3,3'-bpy ligand.22 Other isolable CpCr(NO)(L)Rcomplexes also display evidence of delocalization of the unpaired electron density. From the ESR spectra of CpCr(NO)(py)(CHzSiMes) CpCr(NO)(pip)(CHzSiMe3), and CpCr(N0)(pip)(CHzPh) it is clear that the unpaired electron interacts with the ligands even though these spectra possess broader line widths. For example, the ESR spectrum of CpCr(NO)(pip)(CHzSiMes)exhibits coupling of the unpaired electron to two nonequivalent I4N nuclei and to a lH nucleus, the latter probably being on the methylene group. The magnitude of the 53Crcoupling constant reveals that the unpaired electron interacts to a similar extent with the metal center in this complex as it does in CpCr(NO)(PPh3)(CHzSiMes).Other hy(21) The related diamagnetic CpRe(NO)(PPha)Clcomplex displays some evidence of delocalizationof the HOMO over the entire molecule. See: Lichtenberger, D. L.; Rai-Chaudhuri, A,; Seidel, M. J.;Gladysz, J. A.; Agbossou, S. K.; Igau, A.; Winter, C. H. Organometallics 1991,

10, 1355.

(22) McWhinnie, S. L. W.; Jones, C. J.; McCleverty, J. A.; Collison, D.; Mabbs, F. E. J . Chem. Soc., Chem. Comnun. 1990,940.

Organometallics, Vol.14,No. 10,1995 4727

17-Valence-Electron Alkyl Complexes of Cr

C(W

Figure 2. Solid-state molecular structure of CpCr(NO)(PPh3)(H)(SnPh3)at 200 K. The 50% probability displacement ellipsoids are shown for the non-hydrogen atoms.

Figure 3. X-BandESR spectrum of CpCr(NO)(PPhs)(CHzSiMes) in hexanes at 25 "C. perfine couplings, if present, are not evident in this ESR spectrum since they are obscured by the width of the lines. Some of the in situ generated CpCr(NO)(THF)R complexes also appear to possess extensive delocalization of the unpaired electron density throughout the molecule. For example, the ESR signal of CpCr(N0)(THF)(CH2SiMe3)species in THF solution appears as a pattern of 11 main lines, as shown in Figure 5. The overall pattern is the result of hyperfine coupling to the five equivalent protons on the Cp ring, the nitrosyl 14N nucleus, and one of the diastereotopic protons on the methylene group. The coupling constant t o the Cp protons is about the same as that observed for the I4N nucleus, but both are only one-third as large as that found for the methylene proton. The fine structure

evident in each signal appears to result from an overlapping doublet of 1:2:1 triplets. This pattern may be attributed to coupling of the unpaired electron to the second diastereotopic methylene proton and also to two equivalent protons on the THF ligand. Which pair of THF protons interacts with the unpaired electron density would be governed by the orientation of the THF ligand with respect t o the metal center. Nevertheless, the interaction with the THF ligand and the magnitude of the coupling to the Cp protons suggests that the unpaired electron in this system is even more delocalized than it is in the CpCr(NO)(PPhs)(CHzSiMedcomplex (vide supra). Solid-state Molecular Structure of CpCr(N0)(PPhd(CH2SiMes). Among the relatively few 17electron organometallic radical complexes to have been structurally characterized in the solid state, distortions in coordination geometry are c o m m ~ n . In ~ ~contrast, ,~~ in the solid-state molecular structure of CpCr(N0)(PPhs)(CHzSiMes),shown in Figure 1,all bond lengths and bond angles (Table 4) are normal and are comparable to those exhibited by related 18-electron cyclopentadienylchromium complexes which possess undistorted molecular g e ~ m e t r i e s . A ~ ~few bond-torsion angles which describe the conformationalfeatures of the CpCr(NO)(PPh3)(CHsSiMes)molecule are as follows: = -172.6(4)", Si-C(1)-Cr-P = Cr-C(l)-Si-C(4) 144.8(3)",N-Cr-P-C(31) = 179.4(3)", Cr-P-C(l1)(23) Baird, M. C. In Organometallic Radical Processes; Trogler, W . C., Ed.; Elsevier: New York, 1990; Chapter 2. (24) (a) Greenhough, T. J.; Kolthammer, B. W. S.; Legzdins, P.; Trotter, J. Acta Crystallogr., Sect. B 1980,B36, 795. (b) Ball, R. G.; Hames, B. W.; Legzdins, P.; Trotter, J. Inorg. Chem. 1980,19, 3626. (c) Hermes, A. R.; Morris, R. J.; Girolami, G. S. Organometallics 1988, 7 , 2372. (d) Daly, J. J.; Sanz, F.; Sneedon, R. P. A.; Zeiss, H. H. J. Chem. SOC.,Dalton Trans. 1973,1497. ( e )H e m a n n , W. A.; Hubbard, J. L.; Bernal, I.; Korp, J. D.; Haymore, B. L.; Hillhouse, G . L. Inorg. Chem. 1984,23,2978.

Legzdins et al.

4728 Organometallics, Vol. 14, No. 10, 1995 C(16) = -21.6(3)', Cr-P-C(21)-C(22) = -49.7', and Cr-P-C(31)-C(32) = -49.3'. Even though the ESR spectrum of CpCr(N0)(PPh3)(CHzSiMe3) reveals that the unpaired electron has an unusually large coupling to one of the hydrogen atoms on the methylene group (vide supra), there is no structural evidence for any agostic interactions involving the methylene a-protons which were found during the structure d e t e r m i n a t i ~ n . ~ The ~ large difference between the hyperfine coupling constants to the two diastereotopic protons on the methylene group may thus be attributed to their different torsional relationships to the groups on Cr about the Cr-C(l) bond. Finally, it may also be noted that since the CpCr(NOXPPh3XCHzSiMe3) molecule is asymmetric and contains strong crystal-field ligands, it is not subjected to a Jahn-Teller distortion of the type observed for CpCr(CO)z(L)(L = PPh3, PMe3) systems in which the ligands forming the "legs" of the piano-stool structure are drawn toward each ~ t h e r . ~ g ~ ~ ~ ~ ~ ~ ~ ~ ~ Redox Properties of CpCr(NO)(L)R Complexes. We have investigated the redox properties of selected CpCr(NO)(L)Rcomplexes by cyclic voltammetry, and the results of these studies are collected in Table 1. These properties of these compounds differ in two important aspects from those displayed by their analogous halide complexes. First, the reduction potentials are much more negative for the alkyl complexes than for the halide species (Table 114The shifts in reduction potentials are typical of those observed previously for the replacement of a halide ligand with an alkyl group in an organometallic complex.27 The actual values of r r e d for the CpCr(NO)(L)R complexes investigated range from -1.9 to -2.3 V vs SCE, a feature which makes this series of compounds about as difficult t o reduce as [Cp(+C6Me6)Fe]+ salts.28 An important difference between these two systems, however, is that while the 18-electronFe salts are reduced to 19-electron radicals, the 17-electron Cr complexes are ostensibly reduced to diamagnetic 18-electronspecies. The second difference is that the CpCr(NO)(L)R species display oxidation waves at fairly low positive potentials, in contrast to the CpCr(NO)(L)I complexes which do not display any oxidation features in THF. The end result, therefore, is that the CpCr(NO)(L)R complexes are difficult to reduce and are easy to oxidize. Interestingly, neither the oxidation nor the reduction features display reversible behavior on the time scales of the cyclic voltammetry experiments, thereby revealing that these electronically stable 17-electron compounds decompose upon conversion t o 16-electron cations or 18-electron anions. This increase in reactivity upon formation of a complex with an even number of valence electrons is opposite t o the usual trend observed between 17electron and related 18-electron compound^.^^^^ A cyclic voltammogram of CpCr(NO)(PPhs)(CHzSiMe3) in MeCN is shown in Figure 4. After initial oxidation of the complex, features appear at negative potentials due to reduction of the oxidation products. (25)The related CpZTiCHzR (R = alkyl) systems are known to possess significant agostic interactions.li (26)MacConnachie, C. A,; Nelson, J. M.; Baird, M. C. Organometallics 1992,1 1 , 2522. (27)Connely, N. G.; Geiger, W. E. Adu. Organomet. Chem. 1984, 23,1.

(28) Astruc, D.Chem. Rev. 1988,88, 1189

I---/

2.00

1.00

0.00

-1.w

-1.00

vdo V I scf

Figure 4. Cyclic voltammograms of CpCr(NO)(PPhs)(CHzSiMe3) in MeCN at 25 "C. I

I

Figure 6. X-Band ESR spectrum of CpCr(NO)(THF)(CH2SiMe3) in THF at 25 'C.

These new features occur at potentials similar to those observed for the reduction of [CpCr(NO)(MeCN)zlPFs (Table 1). Upon oxidation of CpCr(NO)(PPhs)(CHzSiMes), and probably the other CpCr(NO)(L)R complexes as well, it thus appears that the Cr-C a-bond to the alkyl ligand is broken and the CHzSiMe3 radical is lost. Chemically, this oxidation process can be effected by treating CpCr(NO)(PPh3)(CHzSiMe3)with [CpzFelPF6 as summarized in eq 3.

The loss of both the PPh3 ligand and the alkyl group and subsequent formation of the known 17-electron radical cation complex [CpCr(NO)(MeCN)z1PFs9is further evidence for the increased substitutional labilify imparted by electron transfer from these CpCr(NO)(L)R complexes. Reactivity of CpCr(NO)(PPb)(CHtSiMes).The CpCr(NO)(PPh3)(CHzSiMe3) complex is a fairly sterically hindered molecule, as has been established by

17-Valence-Electron Alkyl Complexes of Cr

Organometallics, Vol. 14, No. 10, 1995 4729

X-ray crystallography (vide supra). The steric protection of the metal center provided by the phosphine and the alkyl ligands results in this compound being less sensitive to air and moisture than the other complexes in its class. Indeed, CpCr(NO)(PPh3)(CH2SiMe3)appears to be stable in air as a solid for several weeks. Furthermore, its reactivity with small neutral diamagnetic molecules is very limited. For instance, this complex does not react with carbon monoxide even when exposed to 600 psig of CO for 48 h in THF. Dihydrogen and ethylene do not react with the Cr complex under conditions similar t o those used for CO. To delineate the characteristic reactivity of this interesting complex, we have treated CpCr(N0)(PPhs)(CHzSiMes)with several different reagents. We did not, however, investigate its reduction chemistry, given its relatively high EO'red value. The paramagnetic alkyl compound has been reacted with the small paramagnetic molecule NO, the electrophilic reagent [NO]PFs, and a potential source of a hydrogen atom, HSnF'h3. The outcomes of these reactions are considered in the following sections. (a) With NO. Solutions of CpCr(NO)(PPhs)(CHzSiMe3) in Et20 react with NO as summarized in eq 4.

?

NO

Ph3Yy'CH2SiMq N 0

Et,O - PPh3

F

ON'T'CA2SiMes N 0

2 Hsm,

T r

1

r

HSnPh3

1

ing the expected 18-valence-electron CpCr(N0)(PPhs)(H)(CHzSiMes)complex (for which a W analogue is known, from a different route),31this reaction (eq 6) produces the novel CpCr(NO)(PPhs)(H)(SnPhs)compound.

(4)

This transformation results in the formation of the stable 18-valence-electron dinitrosyl species through substitution of the 2-electron phosphine ligand by the 3-electron nitrosyl ligand. The yield for this reaction appears to be quantitative by IR spectroscopy, which clearly reveals the conversion of the mononitrosyl starting material into the well-known dinitrosyl-containing product CpCr(N0)2(CH2SiMe3).l5 (b) With [NOIPFa. Treatment of CpCr(N0)(PPhs)(CHzSiMes)in CH2Ch with the NO+ electrophile results in Cr-C a-bond cleavage rather than insertion, as occurs with CpCr(N0)zR systems.29 The reaction which occurs (eq 5) also results in the loss of the alkyl group from the metal center.

The CHzSiMe3 group is presumably lost as a radical during the reaction and is detected as a peak due to Me&i observed in the GC trace of the atmosphere above the final reaction mixture. The organometallic product of reaction 5 is interesting because it cannot be synthesized by treatment of the known [CpCr(N0)~1+ cationic species with PPh3.30 Nevertheless, [CpCr(NO)a(PPhs)lPFs can be purified by chromatography on silica gel and is air-stable once isolated. ( c ) With H S n P b . The reactivity of CpCr(N0)(PPhs)(CHzSiMes)with HSnPhs has also been investigated with a view to utilizing the tin reagent as a hydrogen atom source.2c However, rather than produc~~

Scheme 1

~~

(29) Legzdins, P.; Wassink, B.; Einstein, F. W. B.; Willis, A. C. J. Am. Chem. SOC.1986,108, 317. (30)Regina, F.J.;Wojcicki,A. Inorg. Chem. 1980, 19, 3803.

The conversion is effected in refluxing THF, and 2 equiv of HSnPh3 is required t o completely consume the organometallic reactant. The organometallic product of conversion 6 was the first chromium nitrosyl hydride ever isolated,6 and it is obtained as a green, air-stable, diamagnetic solid.32 A possible mechanistic pathway for its formation is outlined in Scheme 1. Under the experimental conditions necessary t o effect this transformation, the Cr reactant could abstract a hydrogen atom from the stannane. The resulting alkyl hydride could then undergo reductive elimination, thereby forming a 16-electron, coordinatively unsaturated [CpCr(NO)(PPh3)]fragment. This fragment could next oxidatively add another 1 equiv of stannane to form the final organometallic product. Support for these mechanistic ideas is provided by the detection of Me&i in the atmosphere above the final reaction mixture by gas chromatography. Furthermore, the production of the hexaphenyldistannane byproduct during reaction 6 can also be confirmed by IR spectroscopic and EIMS comparisons of the isolated material with authentic samples of Sn2Phs. The physical properties of the CpCr(NO)(PPhs)(H)(SnPhs) product merit some discussion. The solution lH NMR data for CpCr(NO)(PPh3)(H)(SnPhs) in C6Ds indicate that there is little interaction between the (31)Legzdins,P.; Martin, J. T.; Einstein, F. W. B.; Jones, R. H. Organometallics 1987, 6, 1826. (32)The existence of HCr(C0)dNO)at -40 "C was reported prior to our work (Mantell, D. R.; Gladfelter, W. L. J. Organomet. Chem. 1988, 347, 333),and several trans,tran~-Cr(H)(CO)~(NO)(PR3)~ complexes have been isolated subsequently (see: van der Zeijden, A. H.; Biirgi, T.; Berke, H. Inorg. Chim. Acta 1992, 201, 131.Peters, J. C.; Hillhouse, G . L.; Rheingold, A. L. Polyhedron 1994,13, 1741.

Legzdins et al.

4730 Organometallics, Vol. 14,No.10, 1995

I

I

-2.3

-2.4

I

-2.5

PPU

I

-2.6

Figure 6. Hydride region of the lH NMR spectrum of CpCr(NO)(PPhs)(H)(SnPh3) in C6D6 at ambient temperature. hydride and the tin atoms and that the complex is correctly formulated as a stannyl hydrido species. Thus, the signal for the hydride a t 6 -2.47 ppm (Figure 6) exhibits coupling to both phosphorus WPH= 90.3 Hz) ~ 23.7 Hz). From the work of Schubert and tin ( V s n = and co-workers on a series of similar complexes such as Cp’Mn(CO)z(H)(SnPhs) (Cp’ = $-CsH&te), it is known that the magnitude of the Sn-H coupling constant is a direct measure of the amount of direct Sn-H bonding extant in the ~ o m p l e x . In ~ ~complexes ,~~ where there is an agostic interaction between the transition metal and the Sn-H bond, the value of ~ J S , H ranges from 1500 to 1800 Hz. Since the value of JS,H evident in the ‘H NMR spectrum of CpCr(NO)(PPhs)(H)(SnPh3) is less than 2% of the minimum value expected for an agostic interaction, it indicates that in solution there is no bond between the H and the Sn atoms in this complex. CpCr(NO)(PPhs)(H)(SnPh3)has also been subjected to an X-ray crystallographic analysis, and the solid-state molecular structure established by this analysis is shown in Figure 2. This 18-valence-electron complex effectively possesses a three-legged piano-stool structure, and its intramolecular metrical parameters (Table 6) are comparable to those exhibited by related cyclopentadienylmetal nitrosyl c o m p l e ~ e s . Evidently, ~ ~ * ~ ~ the steric effects of the PPh3 and SnPh3 ligands are sufficiently large and that of the H ligand sufficiently small to force this molecular geometry on the complex rather than the expected four-legged piano-stool-typearrangement. Interestingly, the solid-state molecular structure of this complex indicates the possibility of an interaction between the Sn and H atoms; i.e., it may be viewed as ~

(33) Schubert, U.; Kunz, E.; Harkers, B.; Willnecker, J.; Meyer, J. J . Am. Chem. SOC.1989,111,2572. (34)Schubert, U. Adu. Organomet. Chem. 1990,30,151.

approaching an +stannane complex in the solid state. While the hydridic atom H(10) was poorly defined by this diffraction study, the bond angles a t both the tin and chromium atoms (Table 6) corroborate its approximate location. The approximate hydride position and the arrangement of the other groups on both Sn and Cr atoms are consistent with the existence of some three-center, two-electron SnH-Cr bonding analogous to that invoked for C ~ ’ M ~ ( C O ) Z ( H ) ( SUnfortu~P~~).~~ nately, the solid-state lH NMR spectrum of CpCr(N0)(PPha)(H)(SnPha)does not provide a definitive answer as to the electronic structure of the molecule. Most of the signal intensity in this spectrum appears as a very broad featureless singlet made up of the signals for the aromatic and cyclopentadienyl protons, but there is an unusually sharp signal a t 6 +2.50 ppm which is attributable to the hydride resonance. Reactivity of Other CpCr(NO)(L)R Complexes with CO. As noted earlier in this report, CpCr(N0)(PPhs)(CHzSiMes)is inert to carbon monoxide even when exposed to high pressures of CO. However, less sterically congested CpCr(NO)(L)R complexes react readily with CO. Thus, exposure of a THF solution of CpCr(NO)(pip)(CHzPh)to an atmosphere of CO results in the formation of CpCr(NO)(CO)zand CpCr(NO)(pip)(CO). It is unlikely that this reaction proceeds by reduction of the organometallic reactant by CO, since the amine complexes are more difficult to reduce than CpCr(NO)(PPh3)(CHzSiMes). It is more likely that the carbonyl product complexes are formed via initial CO coordination t o the paramagnetic metal center, thereby forming CpCr(NO)(pip)(CO)(CHzPh) as a 19-electron intermediate. This intermediate can then lose benzyl radicals prior t o forming the very stable 18-electron products is0lated.3~These benzyl radicals then probably abstract H atoms from the solvent, thereby forming the toluene which is detectable by GC in the final reaction mixture. In a similar manner, exposure of in situ generated CpCr(NO)(THF)(CHzSiMes)t o CO results in the formation of CpCr(NO)(CO)z. Interestingly, there is no evidence that insertion-type reactions occur with any of these complexes upon their being exposed to CO. This fact is in contrast to the facile CO-insertion processes undergone by the corresponding 16-electron dialkyl systems Cp’M(N0)Rz (M = Mo, W).36137Nevertheless, the facility of ligand substitutions in these CpCr(NO)(L)R complexes is typical of the reactivity of 17-electron metal-centered radicals in general.2 The unusual feature about the reactions of the CpCr(NO)(L)Rsystems with CO is that the electron count a t the metal center increases by 1 as a result of the substitution of a 1-electron-donorligand for a twoelectron ligand. Summary

This work has established that the 17-valenceelectron CpCr(NO)(L)Rcompounds are unusual in that (35)A similar loss of benzyl radicals has been invoked to occur during substitution reactions of the paramagnetic benzylchromium complexes Cp*Cr(L)(CHzPh)z;see: Bhandari, G.;Kim, Y.; McFarland, J. M.; Rheingold, A.L.; Theopold, K. H. OrganometaZlics 1995,14,738. (36)(a) Dryden, N. H.; Legzdins, P.; Lundmark, P. J.; Riesen, A.; Einstein, F. W.B. Organometallics 1993,12,2085.(b) Debad, J. D.; Legzdins, P.; Batchelor, R. J.;Einstein, F. W. B. Organometallics 1993, 12, 2094.

(37)Richter-Addo, G. B.; Legzdins, P. Metal NitrosyZs; Oxford University Press: New York, 1992;Chapter 4.

17-Valence-Electron Alkyl Complexes of Cr

in some cases the unpaired electrons are extensively delocalized throughout the metal’s coordination sphere. These alkyl-containing complexes are easier to oxidize than are their iodo analogues, but they are harder t o reduce. The reactivity that they display toward small molecules is somewhat dependent upon the steric congestion at the metal center. The ligand substitution reactions of the CpCr(NO)(L)Rcomplexes are dominated by their ability to lose ligands selectively to achieve an 18-valence-electron configuration in the final products. In particular, the loss of alkyl radicals occurs frequently during these latter transformations.

Acknowledgment. We are grateful t o the Natural Sciences and Engineering Research Council of Canada

Organometallics, Vol. 14, No. 10, 1995 4731

for support of this work in the form of grants to P.L. and F.W.B.E. We also thank Mr. K. M. Smith for helpful discussions and technical assistance and Professor F. G. Herring for assistance with the measurement and interpretation of some of the ESR spectra. Supporting Information Available: Tables of supplementary crystallographic data, coordinates for the hydrogen atoms, anisotropic thermal parameters, and selected intramolecular torsion angles for the complexes CpCr(N0)(PPha)(CHzSiMes)and CpCr(NO)(PPhs)(H)(SnPh3) and tables giving additional bond distances and angles and least-squares planes for CpCr(NO)(PPha)(H)(SnPha)(19 pages). Ordering information is given on any current masthead page. OM950245Y