Structural and Reactivity Consequences of the Presence of Lone Pairs

Dec 15, 1994 - [Fe3(CO)9(~3-ksFe(CO)4}212-. Robert E. Bachman, Suzanne K. Miller, and Kenton H. Whitmire" ... Received June 24, 1994@. The reaction of...
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Organometallics 1995, 14, 796-803

796

Structural and Reactivity Consequences of the Presence of Lone Pairs in Main-Group-Transition-MetalCluster Compounds: Conversion of [HAs{Fe(C0)4}3l2-into [Fe3(CO)9(~3-ksFe(CO)4}212Robert E. Bachman, Suzanne K. Miller, and Kenton H. Whitmire" Department of Chemistry, Rice University, P.O. Box 1892, Houston, Texas 77251 Received June 24, 1994@ The reaction of NaAsO2 with Fe(C0)dKOWMeOH produces the novel hydrido cluster [HA~(Fe(C0)4}31~-.An X-ray structural study was carried out on the [PPNl+ salt at 173 K in a n attempt to crystallographically confirm the presence of the hydrogen atom. The material crystallizes in the trigonal space group P3 (No. 147) with a = 25.379(4) b = 25.379(4) c = 22.442(4) V = 12518.2(36)A3, and 2 = 6. The reactivity of this unusual cluster anion was probed by pyrolysis, photolysis, and protonation. Addition of acid produced the previously characterized cluster AszFes(CO)22. Pyrolysis and thermolysis both yielded This material was crystallized as its [Et4Nl+ salt, the dianion [F~~(CO)~{,P~-ASF~(CO)~}~]~-. and the structure was determined by single-crystal X-ray diffraction. [ E ~ $ J I ~ [ A S ~ F ~ ~ ( C O ) I ~ I crystallizes in the triclinic space group Pi (No. 2) with a = 12.466(2) b = 13.408(3) c = 15.457(3)A, a = 80.59(3)",p = 72.01(3)",y = 66.66(3)",and V = 2253.7(8) A3. The cluster consists of a distorted-square-pyramidal E2M3 core with the two main-group atoms ligated by Fe(C0)4 fragments. Comparisons of the structure and reactivity of both [PPNl2[HAswith those of other isoelectronic and {Fe(C0)4}31 and [Et4Nl~[Fe~(CO)~{~~-AsFe(C0)~}~l isostructural clusters containing either group 15 or 16 elements has led to a correlation between the observed structure and reactivity patterns and the presence of a lone pair of electrons on the main-group element.

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Introduction Transition-metal clusters which contain main-group fragments have been of interest for many years due to their unusual bonding and reactivity patterns.l The first arsenic-containing transition-metal clusters were prepared in the late 1950s by Hieber and co-workers from the reaction of arsenic oxides with Fe(C0)5 in basic methanol followed by acidification.2 However, the initially formed anionic species from this reaction were not characterized due to limitations in the analytic tools available at the time. Since arsenic lies at the midpoint of the group 15 elements, its study should provide important information about structural and reactivity trends as the main-group element increases in size and in metallic character. This intermediate behavior has been demonstrated by clusters such as Fe3(CO)gASP^)^,^ which has an isostructural counterpart in the analogous, lighter P-containing compound, and As2{W(C0)5}3, which which is similar t o compounds containing heavier antimony and bismuth atoms.4 Abstract published in Advance ACS Abstracts, December 15,1994. (1)For recent reviews see: (a) Scherer, 0. J. Angew. Chem., Int. Ed. Engl. 1985,24,924. (b) Herrmann, W.A. Angew. Chem., Int. Ed, Engl. 1986,25, 56.( c ) Huttner, G. Pure Appl. Chem. 1986,58,585. (d) Whitmire, K. H. J . Coord. Chem. B 1988,17,95.(e) Fenske, D.; Ohmer, J.;Hachgenei, J.;Merzweiler, K. Angew. Chem., Int. Ed. Engl. l988,27,1277.(0 Norman, N. C.Chem. SOC. Rev. 1988,17,269.(g) Scherer, 0.J. Angew. Chem., Int. Ed. Engl. 1990, 29, 1104. (h) Compton, N. A.; Errington, R. J.; Norman, N. C.Adu. Organomet. Chem. 1990,31,91. (i)Whitmire, K.H. J . Cluster Sci. 1991,2,231. (j) Roof, L. C.; Kolis, J. W. Chem. Rev. 1993,93,1037. Gruber, J.;Lux, F. Z. Anorg. Allg. Chem. 1959,300, (2)Hieber, W.; 275. (3)Huttner, G.;Mohr, G.; Frank, A,; Schubert, U. J . Organomet. Chem. 1976.118.C73. (4)Sigwakh, B.;Zsolnai, L.; Berke, H.; Huttner, G. J . Organomet. Chem. 1982,226,C5. @

0276-7333/95/2314-0796$09.00/0

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We recently reported the synthesis and structural characterization of the novel anionic species [HAS{Fe(C0)4)3l2-([I12-).5 In the study of the reactivity of this unusual species and the related clusters [E{Fe(C0)4)3l2- (E = Se, Te),6 it has become apparent that their reactivities are distinctly different even though they are essentially isostructural and isoelectronic. The major feature which distinguishes these sets of clusters is the presence or absence of a lone pair on the maingroup element. This paper explores the effect that the presence or absence of these lone pairs has on both the structure and reactivity of several isoelectronic and isostructural clusters which contain group 15 and 16 elements.

Experimental Procedures General Considerations. All reactions and other manipulations were performed with oven-dried Schlenkware using standard techniques on a Schlenk line or in a Vacuum Atmospheres drybox. All solvents were dried and distilled under nitrogen prior to use: methanol (Mg),THF (NaPhzCO), and hexane (LiAlH4). Bis(triphenylphosphine)nitrogen(l+) chloride, [PPNICl, was prepared according to literature metho d ~ Fe(C0)b .~ (Aldrich), NaAsOz (Baker), [EtSIBr (Janssen), and KOH (EM Science) were used as received without further purification. Solution IR spectra were recorded in 0.1 mm pathlength CaF2 cells on a Perkin-Elmer Model 1640 FT-IR spectrophotometer. IH and 13C NMR spectra were obtained on a Bruker A?$300 spectrometer in THF-da. EI, FAB, and (5)Bachman, R. E.;Miller, S. IC; Whitmire, K. H. Inorg. Chem. 1994,33,2075. (6)Bachman, R.E.; Whitmire, K. H. Inorg. Chem., in press. (7)Ruff, J. K.; Schlientz, W. S. Inorg. Synth. 1975,15, 84.

0 1995 American Chemical Society

Main-Group-Transition-Metal Cluster Compounds

Organometallics,

Vol.14,No.2, 1995 797

ESI mass spectra were obtained on a VG Analytical Autospec

procedure was repeated with a sample of the cluster salt 3000. FAB mass spectra were performed in a matrix of synthesized in MeOD, the primary off-gases were identified 3-nitrobenzyl alcohol (m-NBA). Carbon monoxide analyses as CH3D. were performed using a Rodder Toepler pump. Protonation of [a2-. An approximately 1 M aqueous solution of HC1 was added slowly dropwise to the methanol CO Analyses Using Toepler Pump. A sample of the reaction mixture prepared above. The addition was continued compound t o be analyzed was weighed into a vacuum flask until all the product had precipitated and the remaining such that the amount of compound used produced a final gas solution was either colorless or a very pale red color, appressure of approximately 200-300 mmHg in a volume of 45proximately 200 mL. The resulting red-black precipitate was 55 mL. A 10-fold excess of pyridinium tribromide, [CsHsNHIcollected by filtration and dried in uacuo overnight. The solids [Brsl, was then weighed into the same flask. The flask was were extracted with 150 mL of hexane for 1-2 h, after which attached to a vacuum line, and approximately 25 mL of the resulting solution was filtered to remove the remaining predried and carefully freeze-thaw-degassed dichloromethane solids. The hexane extract was then allowed to stand for was distilled onto the solids at -196 "C. After the flask was between 1 and 2 weeks, during which time additional solids sealed, the solvent was thawed and the mixture was heated precipitated. These insoluble residues were removed by filtraat between 75 and 80 "C for 2-3 h. The reaction mixture was tion and the solution was concentrated to approximately 50 then cooled to -196 "C and the CO collected and quantified mL. The concentrated solution was passed over a short Florisil utilizing the Toepler pump. column (5 x 2 cm) and cooled t o -20 "C for several days t o Synthesis of Salts of [HAs{Fe(C0)4}sl 2- ([II2-). KOH yield a small amount of very small green crystals. IR (hexane, (1.5 g, 27 mmol) was dissolved in 50 mL of MeOH, and the cm-l): 2112 w, 2098 m, 2066 s, 2058 w, 2044 vs, 1996 m, 1969 resulting mixture was stirred until the base had completely m. MS (EI): m / z 1102 (M+). dissolved and the solution had cooled to room temperature. Synthesis of [ E ~ ~ N ~ Z [ F ~ ~ ( C O ) ~ ~ ~ - A ~ F ~ ( C O ) ~ } Z I ( Fe(C0)5 (1.0 mL, 7.6 mmol) was then added to the solution 1111). Method 1: Photolysis. [Etfilz[I] (0.25 g, 0.21 mmol) rapidly by syringe. This mixture was stirred for 30 min t o was dissolved in 40 mL of THF and placed in a standard 100 yield a light yellow solution. When the solid NaAsOz (0.32 g, mL Pyrex Schlenk flask. The sealed flask was then placed in 2.5 mmol) was added in one portion, the reaction mixture a water-cooled photoreactor apparatus equipped with a 450-W slowly changed from yellow to deep red. The mixture was Hanovia medium-pressure mercury lamp. IR spectroscopy stirred for 4-5 h to assure complete reaction. Excess solid indicated that all the starting material had been consumed [PPNICl (3.5 g) was added t o the reaction solution, and this after approximately 2 h. The major reaction product was mixture was stirred for 30 min, resulting in the formation of [Etfi]z[II]. Unfortunately, the reaction mixture was contamia brick red precipitate. The solids were isolated by filtration nated with a second compound which was difficult to separate and dried in uacuo. The crude product was purified by because of its similar solubilities. A better method was dissolving the solids in minimal THF and filtering the solution therefore sought for the preparation of [Etfi]2[II]. t o remove any insoluble material. The filtered solution was Method 2: Pyrolysis. [Etfi]z[I] (0.25 g, 0.21 mmol) was then layered with approximately 3 volumes of MeOH. Diffidissolved in 50 mL of THF and heated to reflux for 4 or 5 h. sion of the MeOH into the THF results in the formation of The solution was cooled to room temperature and filtered to large red hexagonal plate-like crystals. Yield: 2.85 g (70%). remove a small amount of insoluble material. The THF was IR (THF, cm-l): 2015 w, 1985 s, 1915 s, 1895 s. 'H NMR removed in U ~ C U Oto yield an oily dark red solid. The product (THF-dg, ppm): 7.8-7.5 (m). 13CNMR (THF-dg, ppm): 221.4 was recrystallized by dissolving it in minimal CHzClz and (CO), 134.8, 131.8 (Jc-p = 211 Hz), 128.9, 127.5. MS (FAB, cooling the solution to -20 "C for several days. Yield: 80 mg mNBA): m / z 579 (M+). Calculated for C ~ ~ ~ H ~ I A ~ F ~ ~ N Z(65% O I Zbased P ~ : on As). IR (THF, cm-l): 2010 s, 1993 s, 1971 s, mol of CO/g. Found: 7.22 x mol of CO/g. 7.24 x 1925 m. lH NMR (THF-dg, ppm): 3.21 (2H, q, J = 7.0 Hz), The [ E t a ] + salt was isolated by adding a solution of 1.5 g 1.24 (3H, t, J = 7.0 Hz). 13C NMR (THF-dg, ppm): 220.1, of [ E t a I B r in 100 mL of deaerated water to the above reaction 217.9, 213.0, 52.9, 7.4. MS (ESI, CHzClz): 1295.9 (Msolution in place of the [PPNICl. The fine brick red precipitate, 3[Eta]+). Anal. Calcd for C ~ ~ H ~ & S ~ F ~ ~C, NZ 34.0; O I ,H, : which formed immediately, was isolated by filtration and dried 3.46; N, 2.40. Found: C, 34.6; H, 3.46; N, 2.54. in uacuo overnight. The crude product was purified by Structure of [PPNI2[I]JM€Fat 173 K. A red block of dissolving it in a minimal amount of THF and filtering the approximate dimensions 0.50 x 0.50 x 0.50 mm, which had solution t o remove any insoluble material. The product was been cut from a large hexagonal platelike crystal, was selected then reprecipitated by adding a large excess of hexane. for data collection. The crystal was mounted on a glass fiber with epoxy cement. The unit cell determination and the data Yield: 1.92 g (79% based on As). This material can be further purified by recrystallization from slow diffusion of hexane into collection were performed on an automated Rigaku AFC5S four-circle difiactometer using the TEXSAN data collection a THF solution. IR (THF, cm-l): 2015 w, 1986 s, 1915 sh, 1899 s. package.8 Final unit cell parameters were based on a leastsquares analysis of 25 carefully centered reflections (6.50" 5 Deprotonation of [II2-. Mz[I] (M = Na, K) was synthe28 5 9.50"). The unit cell parameters and space group sized as detailed above and isolated by removing the methanol assignment were consistent-with those found in an earlier under vacuum. The solids were then extracted into 100 mL study conducted at 223 K (P3, No. 147).5No decay correction of ether and filtered to remove the inorganic salts. The ether was applied because only random fluctuations were seen in was removed in uacuo to yield a very air sensitive red powder. the standard intensities. The iron and arsenic positions were Between 400 and 500 mg of this powder was transferred to a determined by direct methods using SHELX-86. The other vacuum flask, and 35 mL of carefully degassed THF was then atoms were found in difference maps after sequential leastvacuum-distilled into the flask. The solution was then frozen squares cycles. The structure refinement was carried out on at -196 "C and 1.3 equiv of methyllithium in ether was F using SHEIXL-93.9 All non-hydrogen atoms in the cluster transferred to the flask under an atmosphere of helium. After and the cations were refined anisotropically. A lattice solvent the transfer the flask was evacuated and sealed before allowing disordered in two major orientations was located in a general it to return to room temperature. M e r sitting at room position and was modeled by constraining the bond metricals temperature for between 30 min and 1h, the flask was again for each orientation t o idealized values and refining occupancy cooled to -196 "C. The head gases were then collected and quantified with a Toepler pump; 1.4 mol of gas was collected (8) Rigaku MSC Automatic Data Collection Control Software, v per mole of cluster. The gases were then identified as being 3.2.1; Molecular Structure Corp., The Woodlands, TX,1987. composed primarily of methane by E1 mass spectrometry. (9) Sheldrick, G. M., SHELXC-93 Univeristat Gijttingen,Gijttingen Weak signals indicative of CO were also observed. When this Germany, 1993.

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798 Organometallics, Vol. 14, No. 2, 1995

Bachman et al.

Table 1. Crystal Data and Structure Refinement Details for [PPN12[I].THF and [Et~12[111 fw temp (K) wavelength (A) cryst syst space group unit cell dimens

1728.80 293(2) 0.710 30 A trigonal P3 (No. 147) a = 25.379(4) A, c = 22.442(4) A

V (A) Z density (calcd) (g/cm3) abs coeff (nun-')

12 518.2(36) 6 1.376 1.047 5316 0.4 x 0.2 x 0.4 2.04-22.42 0 5 h 5 24, -21 5 k 5 23,O 5 1 5 23 10 068 9439 (R(int) = 0.0250) fuli-matrix least squares on P 9428/10/997 1.054 R1 = 0.0479, wR2 = 0.1340 R1 = 0.0721, wR2 = 0.1624 1.764 and -0.432

NW cryst size 0 range for data collection deg) index ranges no. of rfln; collected no. of indep rflns refinement method no. of data/restraints/paras goodness of fit on P final R indices ( I > 2a(l)] R indices (all data) largest diff peak and hole (e/&)

Table 2. Selected Atomic Coordinates ( x 104) and Equivalent Isotropic Displacement Parameters (A2x 103) for [PPNI2[11.THP atom

X

Z

0 678(1) 890(3) 1352(2) -346(2) 1556(2) 794(3) 1087(3) 52(3) 1204(3) 3333 2247(1) 2529(2) 2053(2) 229 l(2) 948(2) 243 l(3) 2 137(3) 2293(2) 1463(3) 3333 2267(1) 2695(2) 1897(2) 2289(3) 1025(3) 2548(3) 2062(3) 2298(4) 152l(4)

2266(1) 2506(1) 3677(2) 1426(2) 2358(2) 2796(2) 3219(3) 1856(3) 2416(3) 2682(3) 2981(1) 3219(1) 4393(2) 2148(2) 3148(2) 3483(3) 3922(3) 2572(3) 3 161(3) 3378(3) 8423(1) 8657(1) 9812(2) 8596(3) 7574(3) 8968(3) 9351(3) 86 12(3) 8014(4) 8837(4)

~~~

0 -407( 1) 204(2) 247(2) -1653(2) -792(2) -35(3) ~ 3 ) -1157(3) -647(3) 6667 6048(1) 6643(2) 6603(2) 4913(2) 5409(2) 6415(3) 6386(3) 5368(3) 5656(3) 6667 6366(1) 6976(2) 5071(2) 7029(3) 6050(3) 6740(3) 5589(3) 6774(4) 6178(3)

U(ea)

Y

~

a U(eq) is defined as one third of the trace of the orthogonalized Uij tensor.

factors and the isotropic displacement parameters for the nonhydrogen atoms iteratively. The occupancy factors of the two orientations were constrained to sum t o unit occupancy. The hydrogen atoms for the cations and solvent were included in calculated positions using a riding model with fured isotropic displacement parameters. Residual peaks along one of the %fold axes indicated a potential location for yet another lattice solvent molecule of THF. Attempts to model this solvent molecule were not particularly satisfying, and the occupancy appeared to be very low (ca. 16%). Because of the combination of crystallographic disorder and low occupancy, this additional lattice solvent was ignored in the final refinement cycles.

1165.76 173(2) 0.71030 teclinic P1 (No. 2) a = 12.466(2)A, b = 13.408(3)A, c = 15.457(3)A, a = 80.59(3)",p = 72.01(3)', y = 66.66(3)". 225 3.7(8) 2 1.718 3.096 1168 0.3 x 0.3 x 0.5 2.06-25.00 -14 5 h 5 13, - 1 4 5 k 5 0, -18 5 1 5 18 7269 6898 (R(int) = 0.0222) full-matrix least squares on P 6894/0/532 1.025 R1 = 0.0295, wR2 = 0.0651 R1 = 0.0482, wR2 = 0.0725 0.816 and -0.653

Table 3. Selected Bond Lengths and Angles for TPPN12[11.T€W' As(1)-Fe(1) As(2)-Fe(2) As(3)-Fe(3)

Lengths (A) 2.4683(10) Fe-C 2.4554(9) c-0 2.4723(10)

1.754(8)-1.793(7) 1.151(8)-1.188(9)

Angles (deg) Fe( 1)-As( 1)-Fe( 1)* C(14)-Fe( 1)-C(12) C(14)-Fe(l)-C(I 1) C(l2)-Fe(l)-C(ll) C(13)-Fe(l)-As(l) C(lI)-Fe(l)-As( 1) C(24)-Fe(2)-C(22) C(22)-Fe(2)-C(21) C(22)-Fe(2)-C(23) C(24)-Fe(2)-As(2) C(21)-Fe(2)-As(2) Fe(3)-As(3)-Fe(3)* C(34)-Fe(3)-C(32) C(34)-Fe(3)-C(31) C(32)-Fe(3)-C(31) C(33)-Fe(3)-As(3) C(31)-Fe(3)-As(3) a

115.35(2) 93.5(3) 91.7(3) 121.6(3) 89.2(2) 86.6(2) 91.9(3) 122.5(3) 120.4(3) 175.5(2) 86.6(2) 115.61(2) 92.0(3) 9 1.8(3) 117.5(3) 85.6(3) 86.4(2)

C(14)-Fe(l)-C(13) C(13)-Fe(l)-C(12) C(13)-Fe(l)-C(11) C(14)-Fe(l)-As(l) C(12)-Fe(l)-As(l) Fe(2)-As(2)-Fe(2)* C(24)-Fe(2)-C(21) C(24)-Fe(2)-C(23) C(21)-Fe(2) -C(23) C(22)-Fe(2)-As(2) C(23)-Fe(2)-As(2) C(34)-Fe(3)-C(33) C(33)-Fe(3)-C(32) C(33)-Fe(3)-C(3 1) C(34)-Fe(3)-As(3) C(32)-Fe(3)-As(3) Fe-C-0

94.6(3) 118.4(3) 119.0(3) 176.1(2) 84.5(2) 115.38(2) 92.6(3) 94.0(3) 116.4(3) 84.8(2) 90.4(2) 94.2(4) 120.8(4) 121.1(3) 177.8(3) 90.0(2) 175.6(5)177(6)

An asterisk denotes a symmetry related atom.

There was no perceptible effect on the details of the structure whether or not this solvent molecule was included in the refinement. The difference maps consistently revealed three peaks located along the %fold axes, approximately 1.35-1.50 d from each arsenic, which are believed t o be the locations of the hydrogen atoms. Not surprisingly, due to the proximity to the heavy atoms, these positions could not be refined. The hydrogen atoms were therefore included in the found locations but not refined, and no particular inferences can be drawn from the As-H distances. The data collection and refinement parameters are listed in Table 1. Selected positional and displacement parameters are given in Table 2, and selected bond metricals are given in Table 3. Structure of [Etmz[II]. A dark red irregular block (0.3 x 0.3 x 0.5 mm) cut from a larger crystal was chosen for data collection. The crystal was mounted on a glass fiber with epoxy cement. Data collection was carried out on a Rigaku AF'CBS diffractometer.* The unit cell was determined by the careful refinement of 25 random reflections (6.5"5 20 5 14.0"). The crystal was shown to be triclinic and the more common

Main-Group-Transition-Metal Cluster Compounds Table 4. Atomic Coordinates ( ~ 1 0 4 and ) Equivalent Isotropic Displacement Parameters (A2x 103) for [Et4Nlz[W' atom

X

Y

Uea)

Z ~

3927(1) 6516(1) 5623(1) 5002(1) 4866(1) 1889(1) 8647(1) 7837(3) 5894(3) 4179(3) 6477(3) 5204(4) 2712(3) 5348(4) 26 18(3) 613 l(3) 1663(4) 2924(3) 1248(3) -570(4) 8541(3) 8090(3) 9231(3) 11078(3) 6982(4) 5790(4) 4719(4) 5928(4) 5153(4) 3599(4) 5146(4) 3472(4) 5665(3) 1764(4) 2539(4) 1522(4) 395(5) 8572(4) 8294(3) 8979(4) 10129(4)

2572(1) 2133(1) 1734(1) 3756(1) 2770( 1) 2721(1) 1552(1) 1248(3) -402(3) 1963(3) 4924(3) 4176(3) 5571(2) 624(3) 4167(3) 3778(3) 4615(3) 614(3) 2886(3) 2882(4) 3140(4) 1401(3) -558(3) 1378(4) 1452(4) 44~4) 1880(3) 4432(3) 3943(3) 4829(3) 1477(4) 3612(4) 3363(3) 3883(4) 1437(4) 2820(4) 2817(4) 2540(4) 1484(3) 266(4) 1433(4)

7792(1) 6875(1) 8396(1) 7555(1) 6226(1) 8629(1) 6172(1) 8958(2) 7960(2) 10305(2) 6349(3) 9272(2) 7593(2) 5682(3) 5742(2) 4630(2) 9489(3) 9557(2) 6921(3) 9645(3) 7319(3) 4496(2) 7169(2) 5070(2) 8726(3) 8142(3) 9555(3) 6827(3) 8608(3) 7584(3) 5896(3) 5956(3) 5258(3) 9 124(3) 9182(3) 7580(4) 9245(4) 6852(4) 5 152(3) 6782(3) 5517(3)

' U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. centrosymmetric setting, Pi (No. 2), was chosen on the basis of intensity statistics. This choice was shown to be correct by successful refinement of the structure. The structure was solved using the SHEIXTL-PC'O package, which located all the non-hydrogen atoms. Structure refinement on F was carried out with SHELXL-93.9 All the non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were included in calculated positions using a riding model. The refinement converged with Rl(F) = 0.0295 and wR2(F) = 0.0651 for 532 parameters and 5542 observed reflections (I =2 d I ) ) . The data collection and refinement parameters are summarized in Table 1. Positional and displacement parameters are given in Table 4,and selected bond metricals are included in Table 5.

Results Synthesis of [II2-. Methanolic solutions of Fe(C0)d KOH react slowly with solid NaAsOz. During the reaction, the solution changes from pale yellow to deep red. The resulting cluster can be easily isolated as either a [PPNl+ or [Et4Nl+ salt by cation metathesis. The [PPN]+ salt is insoluble in methanol and precipitates directly, while the [ E t a ] + salt can be precipitated (10)SHELXTL PC v 4.2, Siemens Crystallographic Research Systems, Madison, WI, 1990.

Organometallics, Vol. 14, No. 2, 1995 799 Table 5. Selected Bond Lengths and Angles for [Et&[II] As(l)-Fe(3) As(l)-Fe(l) As(l)-Fe(2) As(l)-Fe(4) As(2)-Fe(l) As(2)-Fe(3)

2.3633(10) 2.3673(10) 2.3758(9) 2.4120(10) 2.3447(10) 2.3595(9)

Fe(3)-As(l)-Fe( 1) Fe(3)-As( 1)-Fe(2) Fe(1)-As( 1)-Fe(2) Fe(3)-As( 1)-Fe(4) Fe( 1)-As( 1)-Fe(4) Fe(2)-As( 1)-Fe(4) Fe( l)-As(2)-Fe(3) Fe( l)-As(2)-Fe(2) Fe(3)-As(2)-Fe(2) Fe( 1)-As(2)-Fe(5) Fe(3)-As(2)-Fe(5) Fe(2)-As(2)-Fe(5) C( 12)-Fe( 1)-C(13) C(12)-Fe(l)-C(ll) C(13)-Fe(l)-C(ll) C( 12)-Fe( 1)-As(2) C( 13)-Fe(l)-As(2) C(ll)-Fe(l)-As(2) C( 12)-Fe( 1)-As( 1) C( 13)-Fe(l)-As( 1) C(11)-Fe(1)-As(1) As(2)-Fe( 1)-As(1) C(12)-Fe( 1)-Fe(2) C( 13)-Fe( 1)-Fe(2) C(ll)-Fe(l)-Fe(2) As(2)-Fe( 1)-Fe(2) C(33)-Fe(3)-As(2) C(32)-Fe(3)-As(2) C(31)-Fe(3)-As( 1) C(33)-Fe(3)-As( 1) C(32)-Fe(3)-As( 1) As(2)-Fe(3)-As( 1) C(31)-Fe( 3)-Fe(2) C(33) -Fe(3) -Fe(2) C(32)-Fe(3)-Fe(2) As(2)-Fe(3)-Fe(2) As(l)-Fe(3)-Fe(2) C(44)-Fe(4)-C(41) C(44)-Fe(4)-C(42) C(41)-Fe(4)-C(42) C(44)-Fe(4)-C(43) C(41)-Fe(4)-C(43)

Lengths (A) As(2)-Fe(2) As(2)-Fe(5) Fe(l)-Fe(2) Fe(2)-Fe(3) Fe-C C-0

2.3819(13) 2.3876(10) 2.7292(12) 2.6984(10) 1.757(5)- 1.798(5) 1.14l(5)- 1.156(6)

Angles (deg) 101.74(4) 55.02(3) As( 1)-Fe(1)-Fe(2) 69.41(3) C(22)-Fe(2)-Fe( 1) 74.92(14) 70.25(3) C(23)-Fe(2)-Fe( 1) 133.00(13) 132.96(4) C(21)-Fe(2)-Fe( 1) 130.14(14) 123.89(3) As( l)-Fe(2)-Fe( 1) 54.73(3) 132.97(3) As(2)-Fe(2)-Fe( 1) 54.10(3) 102.53(3) Fe(3)-Fe(2)-Fe( 1) 85.08(3) 70.53(4) C(22)-Fe(2)-C(23) 98.4(2) 69.38(3) C(22)-Fe(2)-C(21) 99.2(2) 125.07(4) C(23)-Fe(2)-C(21) 96.8(2) 130.55(3) C(22)-Fe(2)-As( 1) 109.0(2) 134.72(3) C(23)-Fe(2)-As( 1) 86.25( 13) 103.9(2) C(21)-Fe(2)-As( 1) 150.9(2) 102.5(2) 112.59(14) C(22)-Fe(2)-As(2) 91.9(2) 148.03(14) C(23)-Fe(2)-As(2) 92.04(13) C(21)-Fe(2)-As(2) 86.12(14) 162.13(14) As( 1)-Fe(2)-As(2) 76.71(3) 92.56(13) C(22)-Fe(2)-Fe(3) 159.80(14) 91.19(14) C(23)-Fe(2)-Fe(3) 93.15(14) 93.79( 14) C(21)-Fe(2)-Fe(3) 95.8(2) 163.52(14) As(l)-Fe(2)-Fe(3) 55.07(3) 77.59(3) 54.92(3) As(2)-Fe(2)-Fe(3) 134.98(13) C(31)-Fe(3)-C(33) 104.9(2) 106.86(14) C(31)-Fe(3)-C(32) 103.8(2) 108.51(14) C(33)-Fe(3)-C(32) 89.4(2) 55.37(4) 93.8(2) C(3 l)-Fe(3)-As(2) 91.67( 13) 121.7(2) C(42)-Fe(4)-C(43) 161.53(14) C(44)-Fe(4)-As( 1) 179.4(2) 98.80(14) C(41)-Fe(4)-As( 1) 87.4(2) 154.54(14) C(42)-Fe(4)-As( 1) 87.57( 14) 94.04( 13) C(43)-Fe(4)-As( 1) 89.80(14) 77.38(3) C(54)-Fe(5)-C(5 1) 91.7(2) 141.6(2) C(54)-Fe(5)-C(53) 99.7(2) 99.31(14) 109.1(2) C(51)-Fe(5)-C(53) 105.95(14) C(54)-Fe(5)-C(52) 89.0(2) 55.70(3) C(5 1)-Fe(5)-C(52) 139.5(2) 55.51(3) 110.7(2) C(53)-Fe(5)-C(52) 93.0(2) 166.3(2) C(54)-Fe(5)-As(2) 92.0(2) C(5 1)-Fe(5)-As(2) 83.6(2) 116.1(2) C(53)-Fe(5)-As(2) 93.98(13) 90.2(2) C(52)-Fe(5)-As(2) 86.31(12) 121.9(2) Fe-C-0 173.1(4)179.8(3)

with the addition of excess water. Both the [PPNl+and [Et4Nl+ salts are insoluble in nonpolar organic solvents such as hexane, toluene, or ether but are soluble in polar solvents such as THF, CHzC12 and CH3CN. The [ E t a ] + salt is also soluble in methanol, while the [PPNl+ salt is not. The simple IR spectrum observed in the CO region for the product is consistent with the presence of a single species containing equivalent Fe(C014 fragments with idealized CsV symmetry. The 13C NMR spectrum contains only a single resonance in the carbonyl region a t 221 ppm. No signals other than those associated with the cation were observed in the 300 MHz lH NMR over a temperature range of -100 to +50 "C and a spectral window of +35 to -70 ppm. FAl3 mass spectral analysis revealed a signal at m / z 579 for the parent ion with the appropriate isotopic distribution pattern. A degassed solution of the initially formed sodium or potassium salt of the cluster reacts with excess methyllithium to produce methane (1.4mol of gas/mol of cluster). If the cluster is prepared in CH3-

Bachman et al.

800 Organometallics, Vol. 14, No. 2, 1995 01231

Ulll'

Figure 1. Diagram of the anion of [PPN]2[1] showing the displacement ellipsoids (50% probability level) and the atomic labeling scheme.

&

01531

Figure 2. Diagram of the anion of [ E t ~ ] 2 [ I Ishowing ] the displacement ellipsoids (50% probability level) and the atomic labeling scheme.

OD, CH3D is the primary off-gas. The excess gas produced in these reactions appears to be CO. When [II2-is pyrolyzed in refluxing THF or photoStructure of [PPNI2[I]-THF. Single crystals of lyzed in the same solvent, the primary product is [III2-. [PPN]z[I]are easily grown from a THF solution layered Photolytic conditions also produce a second unidentified with methanol. It was not possible to confirm the species which is difficult t o separate from the major presence of the hydride ligand by the diffraction study.5 product. Thermal methods are therefore preferred for The asymmetric unit is composed of two independent the preparation of [III2-. The ESI mass spectrum cations in general positions and three independent revealed a parent mass at m I Z 1296, which corresponds cluster anions with crystallographically imposed C3 to an aggregate of one cluster anion and three [Et4Nl+ symmetry. The arsenic atom of each anion lies on a cations. The 13C NMR revealed three carbonyl resocrystallographic 3-fold axis so that only one Fe(C014 nances at 220.2, 217.9, and 213.0 ppm along with group of each is unique (Figure 1). Diagrams of the signals attributable to the cation. [Et4Nlz[II] can be other two independent anions are included with the crystallized from a concentrated CH2C12 solution held supplementary material. Thus, the ratio of cations t o at -20 "C. anions in the unit cell is 2:l. The arsenic displays a Structure of [Et&l2[11]. The asymmetric unit pyramidal geometry with an average Fe-As-Fe angle of one anion and two cations with no significant consists of 115.45".11 The iron atoms adopt the conventional intermolecular contacts. The cations were found to be trigonal-bipyramidal arrangement with the arsenic completely ordered. The anion is isostructural with the atoms occupying axial positions. The As-Fe bonds known [Et4Nlz[SbzFe5(C0)1,1,13 consisting of a slightly average 2.465 A for the three independent anions. distorted square pyramidal core assembled from two Residual electron density could be found along the arsenic atoms and three Fe(C013 fragments. Each %fold axes a t approximately the proper distance from arsenic atom also binds an external Fe(C014 group the arsenic atoms expected for the hydrogen atoms. The (Figure 2). The base of the pyramid is distorted by a peaks could not be refined satisfactorily; therefore, displacement of the arsenic atoms out of the basal plane their contributions were included as fixed positions toward the apical iron atom (Fe(2)) and a deviation of where they were located. The inclusion of the hydrogen the internal basal angles from the ideal value of 90". atoms had little effect on the structure refinement. The basal As-Fe-As and Fe-As-Fe angles average Reactivity of [II2-. When [II2-is protonated with 77.48(15) and 102.1(5)",respectively. Also, the Fe(C014 aqueous HC1, a complex mixture of products results. To groups are bent away from the apex iron slightly. The date the only product which has been unambiguously As-Fe bonds within the cluster core average 2.365(13) identified is the previously characterized A~zFes(CO)zz.~~ A,while the two bonds to the external Fe(C014 fragThis neutral cluster was isolated in low yield (15-20%) ments are slightly longer at 2.412(1)and 2.388(1)A.The as small green crystals that grew from hexane at -20 Fe-Fe bonds are 2.729(1) and 2.698(1) A,and the Fe"C. It was identified on the basis of its IR and E1 mass (l)-Fe(2)-Fe(3) angle is 85.08(3)". The Fe-C and C - 0 spectra. The initial reaction mixture contained several bonds are all normal. other peaks in the IR, many of which change slowly over several days' time. This continuously changing compoDiscussion sition has made the isolation and characterization of the individual species extremely difficult. Because of these The reaction of main-group oxides with Fe(C0)5 and difficulties, no further information regarding the idenKOH/MeOH has been shown to be a general method for tity of the other products is available at this time. the preparation of main-group-element-containing iron clusters. When NaAsO2 is allowed to react with Fe(C0)b under these conditions, a single carbonylate anion, [II2-, (11)Esd's of average values are calculated with the scatter formula is produced rapidly in good yield. A preliminary report i=N of this compound has appeared, and the rationale for = [ C ( d i- d)Z/(N - 1)11'2 arriving at the formulation [HAs(Fe(CO)4)3l2-with a i=l (7

(12)Arnold, L. J.; Mackay, K. M.; Nicholson, B. K. J . Organomet. Chem. 1990,387,197.

(13)Whitmire, K. H.; Leigh, J. S.; Luo, S.; Shieh, M.; Fabiano, M. D . New J . Chem. 1988,12,397.

Main-Group-Transition-MetalCluster Compounds

hydride ligand attached to As has already been presented and will not be discussed in detail here.5 The solid-state structure of this cluster at 223 K revealed a pyramidal arsenic atom bound by three Fe(CO)4fragments, but the presence of the hydride ligand could not be conclusively determined from the original X-ray data reported in ref 5 due to the naturally weak diffracting power of hydrogen atoms, exacerbated by their attachment to the heavy arsenic atom and location on C3 axes, which cuts their scattering power by a factor of 3. The X-ray diffraction study was repeated at a lower temperature (173 K),but the positioning of the hydrogen atoms was not much improved. As before, persistent residual peaks appeared in the electron difference map approximately 1.5 A away from each of the three unique arsenic atoms and located along the crystallographic 3-fold axis on which the arsenic atoms reside. These positions agree with the expected location of the hydrogen atoms. These three positions could not be refined as hydrogen atoms without constraint and, even with the H-As distances loosely constrained to be equal (common isotropic displacement parameter for the three atoms), the refinement was not completely successful. Ultimately the H atoms were included in their difference map locations but not refined. The formation of [II2- contrasts with the products formed for the heavier group 15 elements antimony and bismuth in which four Fe(C0)4 groups surround the main-group metal and the overall charge on the clusters is 3-. In the case of arsenic, the fourth coordination site is occupied by a hydrogen atom which raises the overall charge by 1. Transition-metal clusters which contain As-H fragments are extremely rare. The only other structurally characterized example is @-HAS){CpMn(C0)212.14 An analysis of the formal oxidation state of the central element of [II2- is ambiguous. As in the related clusters [E{Fe(C0)4}4l3-(E = Sb, Bi),15J6the oxidation state of the main-group element in these two clusters could be assigned values ranging from +5 to -3. If the oxidation state is assigned as +5, then the central main-group atom is ligated by [Fe(CO)4l2- and/or H- fragments. This seems unreasonable from a chemical standpoint, as it requires a very strongly oxidizing center to be attached to four extremely reducing functionalities. At the other extreme the main-group element can be viewed as being in a -3 oxidation state and the Fe(C014 fragments as being zero-valent electron acceptors. Both of these views are consistent with the octet and the 18electron rules. In reality the bonding in [II2- is probably highly covalent and not well-represented by either extreme. Preliminary antimony Mossbauer parameters for [Sb{Fe(C0)4}4I3- are consistent with an oxidation state for Sb close to 0.17 The oxidation state of the maingroup element in the starting reagents does not completely clarify this situation, as exemplified by the antimony case, where either SbCl3 or SbC15 may be used in the preparation of [Sb{Fe(C0)4}4I3-.l5 That the arsenic atom is, or remains, protonated in a strongly basic solution is somewhat surprising but not (14)Henmann, W.A.;Koumbouris, B.; Zahn, T.; Ziegler, M. L. Angew. Chem. Znt. Ed. Engl. 1984,23,812. (15)Luo, S.;Whitmire, K. H. Inorg. Chem. 1989,28,1424. (16)(a) Churchill,M. R.; Fettinger, J. C.;Whitmire, K. H.; Lagrone, C. B. J.Organomet. Chem. 1986,303,99. (b)Whitmire, K. H.; Lagrone, C. B.; Rheingold, A. L. Inorg. Chem. 1986,25,2472. (17)Long, G.; Stevens, J. Private communication.

Organometallics, Vol. 14, No. 2, 1995 801

unreasonable. The strong Lewis basicity of arsenic in transition-metal compounds is readily apparent from the cyclotrimerization of AsCos(C0)g observed in the solid-state structure of [AsC03(C0)813.~~ The conjugate base [As{Fe(C0)4}J3- could be an intermediate in the synthesis of [II2-,and such a highly charged species would be extremely basic. The species generated by deprotonation of [II2- with methyllithium is, unfortunately, unstable and has not yet been conclusively characterized. The structural features of the anion support the presence of the hydrogen ligand on arsenic. The electrondeficient [E{Fe(CO)4}3l2- species (E = Sn, Pb), which have no lone pairs on E, are able to adopt the expected planar arrangement of metal atoms around the central main-group atom.18 That [II2- does not clearly indicates that some fourth group-either a lone or bonding pair of electrons-must be attached to the arsenic atom. The differences between the Fe-E-Fe angles in [II2-(115.5", average) and those of the isoelectronic cluster [Te{Fe(C0)4}3l2- (lO9.S0,averageP can be rationalized using classical VSEPR arguments, which hold that a lone pair has a larger steric requirement than a bonding pair. A larger Fe-As-Fe angle could be favored by having only a single electron on As rather than a lone pair, but having only one electron on As in [II2-would give rise to an odd-electron system and all data indicate that it is diamagnetic. The As-Fe bonds in [PPNMII average 2.465(9)A for the three independent anions located in the asymmetric unit. This distance is between 0.08 and 0.15 A longer than the average distances reported for other clusters containing an arsenic-iron single bond such as As2Fe3(C0)g (2.348(2)A),19 Fez(C0)6(hMe2)2(2.319(10)A),20 Fe2(CO)&sFes(CO)&1 (2.36(1) A),21 AsFe3(CO)gCH (2.370(10) A),22 (CO)5CrAsFes(CO)gCH(2.354(6) Fe3(CO)g(StBu)(AsMe2)(2.378(6) A),23 and Fez(C0)6[tBuAs(NSN)AstBul(2.3241(10)A).24 In the spirocyclic cluster As2Fe6(C0)2212 the Fe-As bonds between the arsenic and the Fe2(CO)6 are similar to the others reported in the literature (2.340(14)A); however, the Fe-As bonds between the Fes(C0)~ fragments and the arsenic atoms are of length comparable (2.407(12) A) to those seen in [II2-. This lengthening of the As-Fe bonds in [II2- is probably caused by steric crowding of the carbonyls on the iron fragments. This phenomenon also occurs in other main-group-iron carbonyl clusters with open frameworks such as the related group 15 clusters [Et4Nl3[E{Fe(C0)4}41(E = Sb, Bi)15J6and the related isoelectronic cluster [Te{Fe(C0)4}312-.6 The reactivities of El2- and isostructural and isoelectronic clusters such as [Te{Fe(C0)4}312- and [Se{Fe(C0)4}3l2- which possess lone pairs of electrons at the main-group elements differ markedly. In contrast to [II2-, [Te{Fe(C0)4}3I2- slowly converts to [TeFe3(CO)gl2and [Se{Fe(C0)4}3I2- exists only briefly in solution, (18)Cassidy, J. M.; Whitmire, K. H. Inorg. Chem. 1989,28,2494. (19)Delbaere, L. T.J.; Kruczynski, L. J.; McBride, D. W.J. Chem. SOC.,Dalton Tram. 1973,307. (20)Keller, E.;Vahrenkamp, H. Chem. Ber. 1977,110,430. (21)Huttner, G.;Mohr, G.; Pritzlaff, B.; von Seyerl, J.;Zsolnai, L. Chem. Ber. 1982,115,2044. (22)Caballero, C.; Nuber, B.; Ziegler, M. L. J . Organomet. Chem. 1990,386,209. (23)Winter, A.;Zsolnai, L.; Htittner, G. J. Organomet. Chem. 1983, 250,409. (24)Herberhold, M.; Schamel, K.; Henmann, G.; Gieren, A.; RuizPBrez, C.; Hubner, T. 2.Anorg. Allg. Chem. 1988,562,49.

Bachman et al.

802 Organometallics, Vol. 14, No. 2, 1995 rapidly converting to [SeFe3(C0)g12-.6This is true even though the As-Fe bonds are ca. 2.46 A compared with 2.64 A for the Te-Fe bonds in [Te{Fe(C0)4}312-,6 implying that [II2- should be much more sterically encumbered by the Fe(C014 fragments. The comparatively large stability of this arsenic analog is probably the result of a combination of electronic and steric considerations related to the absence of the lone pair on the main-group element. The lone pair on the [Te{Fe(C0)4}3l2-dianion results in a smaller Fe-EFe angle than that in [II2-. As a result of this, the Fe(C0)4 groups are pushed closer together, facilitating the formation of the metal-metal bonds. Conversely, by tying up the lone pair, the hydride ligand in PI2allows for wider Fe-E-Fe angles, helping to relieve the steric interactions of the Fe(C014 groups. While [E{Fe(C0)4}3l2-species (E = Se, Te) also follow a simple CO loss reaction pathway under both photolytic and pyrolytic conditions,6[II2-reacts by a fragmentation of arsenic-iron and arsenic-hydrogen bonds to create a higher nuclearity cluster, [III2-. These reactivity patterns are followed by other clusters of the same general classifications. For example, the open-framework compound Bi{ Co(C0)4}3undergoes the same closing reaction as [E{Fe(CO)4}3l2-(E = Se, TeP7 and open intermediates may be involved en route to PCo3(CO)g and AsCoa(C0)g but have not yet been observed. CO loss from such intermediates would be expected to be rapid, on the basis of the results for the Se and Te clusters. Other complexes in which no lone pair is found at E do not show simple closing but more complicated fragmentatiodrecombinationreactions. These include [E{Fe(C0)4}4I3-(E = Sb, Bi),15J6[XSb{Fe(C0)4)3l2-(X = C1, Br),25and E{Co(C0)4}4 (E = Sn, Pb).26 Isoelectronic and isostructural [Te{Fe(C0)4}412- has been mentioned,lJ but its reactivity is as yet unknown. This is not to say that under differing circumstances open-framework compounds with a central four-coordinate main-group element cannot be induced to undergo a closing reaction. We have seen that [Sb{Fe(CO)4}4l3can be induced under mild protonation conditions to close to give [HFe3(CO)g{SbFe(C0)4}12-,but this reaction is in competition with a fragmentatiodreorganization process which yields [Fe3(CO)g{SbFe(C0)4}2I2-.l5 This latter cluster, of course, is virtually identical with the [III2-reported here. Similar chemistry has been noted for bismuth-containing clusters where the oxidation route yielding Bi2Fes(CO)gdominates the protonatiodclosing pathway where only minor amounts of H3BiFe3(CO)g have been isolated.16b It is intriguing to note that, when [Te{Fe(C0)4}3l2- is protonated, the resultant product, Fes(CO)gTez, contains the same cluster core found for [I112-.2s Protonation of [Te{Fe(C0)4}3l2- could be expected to occur at tellurium to produce “[HTe{Fe(C0)4}31-”(isoelectronicand isostructural with [II2-),and its thermal decomposition would be expected to proceed similarly. Unfortunately we could find no evidence, even fleeting, for the existence of a tellurium hydride as an intermediate in the protonation reaction. (25) Luo, S.; Whitmire, K. H. unpublished results. (26) Leigh, J. S.; Whitmire, K. H. Acta Crystallogr., Sect. C 1990, 46,732. (27) (a) Etzrodt, G.; Boese, R.; Schmid, G.Chem. Ber. 1979,112, 2574. (b) Whitmire, K. H.; Leigh, J. S.; Gross, M. E. J. Chem. SOC., Chem. Commun. 1987,926. (28) Bachman, R. E. Ph.D. Thesis, Rice University, 1994.

Table 6. Comparison of Basal Angles (deg) for Square-Pyramidal Clusters with E2M3 Cores Fe

cluster

M-E-M angle (A)

SezFedCOb 96.6 96.45 TezFedCOh [As~Fes(C0)17]~- 102.14 [Sb2Fes(C0)17l2- 100.86 [Bi2Fe3(CO)9lZ96.85 [BizFe4(C0)13lZ97.38, 101.91

E-M-E angle (B)

E. *E,8,

ref

82.4 83.4 77.48 78.99 83.15 80.36

3.10 3.364 2.952 3.201 3.538 3.398

31 32 this work 13 30 29

Monitoring of the pyrolysis of [II2- by lH NMR indicated the formation of a metal hydride signal at -8.3 ppm which is consistent with the formation of [HFe(C0)41-, This anion could arise via reductive elimination from [II2-. Without a detailed kinetic study it is not possible to draw incontrovertible conclusions about this process, but it suggests-using the principle of microscopic reversibility-that an oxidative addition of [HFe(C0)41- could be involved in the formation of [II2-. The speculative sequence is given in eqs 1-3.

+ [HFe(CO),]- - [OAsFe(CO),l- + OH[OAsFe(CO),l- + [HFe(CO),I- [AS(F~(CO)~},I+ OHAs0,-

(1)

(2)

An X-ray structural study of [Et4NMIII revealed a square-pyramidal cluster with the arsenic atoms occupying trans, basal positions. Each arsenic atom also binds an additional Fe(C0)4 fragment through its external lone pair. The cluster is therefore isostructural and isoelectronic with the known [Et4N12[SbzFes(C0)1,1.13 With the arsenic atoms contributing three electrons each to the cluster bonding, the total number of skeletal electrons is 14, as expected for a nido octahedron. Although the cluster is electron-precise, the core geometry is significantly distorted from ideal square-pyramidal geometry. The basal plane is puckered slightly with the arsenic atoms displaced toward the apical iron atom. Additionally, the base of the pyramid is distorted from a square into a parallelogram with obtuse angles at the arsenic atoms and acute angles at iron atoms. Examination of structural data for [1112-,its isostructural antimony analog, [ B ~ ~ F ~ ~ ( C O [BizFes)I~I~-,~~ (CO)912-,30 and Fe3(CO)gE2(E = Se,Te)31,32 show further the effects of the presence of lone pairs on the structural parameters of the cluster core geometries. Data for these molecules are compared in Table 6. There are (29) Whitmire, K. H.; Raghuveer, K. S.; Churchill, M. R.; Fettinger, J. C.; See, R. F. J. Am. Chem. SOC.1986,108,2778. (30) Eveland, J. R.; Whitmire, K. H. Manuscript in preparation. (31)Dahl, L. F.; Sutton, P. W. Inorg. Chem. 1963,2,1067. (32) Schumann, H.; Magerstadt, M.; Pickardt, J. J. Organomet. Chem. 1982,240,407.

Organometallics, Vol. 14,No. 2, 1995 803

Main-Group-Transition-Metal Cluster Compounds

clearly two distinct types of environments seen, depending upon whether the main-group atom has a nonbonding lone pair or whether the lone pair is donated to an external metal fragment. The largest Fe-E-Fe and smallest E-Fe-E angles are found for [III2- and its Sb analog. The Fe-E-Fe angles in [EzFe3(C0)gln- ( n = 0, E = Se, Te; n = 2 , E = Bi) fall in a different range, being noticeably closer t o 90" than those of [III2-. The E-Fe-E angles undergo an opposite and complimentary change as expected. One cluster, [BizFe4(C0)13lZ-, is a hybrid. It has one naked Bi atom and one with a bound lone pair but again the parameters are consistent with the ranges seen for the other molecules.29 It is readily apparent that steric factors are not the major force causing the distortion of these cluster cores. If that were the case, the trends in bond angles should be opposite of those observed. The steric bulk of the external Fe(C014 groups should cause a contraction in the Fe-E-Fe angle and thereby lessen the CO-CO repulsions at the cluster core. An alternative explanation based on electronic considerations has been put forth by Webster and co-workers, as well as others, to explain a similar distortion in phosphorus-containing clusters such as Fe3(C0)9(PPh)~.~~ In these cases, the distortion is attributed t o an attractive, through-space interaction of the main-group elements. This interaction was postulated because the E-E distances were found to be only slightly longer (approximately 0.3 A) than the sum of the covalent radii. If present, the structural result of this interaction would indeed be a decrease in the E-E distance and, consequently, an opening of the Fe-E-Fe angle. However, the consistency of the structural parameters for the two types of bismuth atoms in the hybrid [BizFe4(C0)13lZ-with the ranges for the other ligated and unligated main-group atoms in the other clusters implies that the structural distortions may be independent of a direct Bi. .Bi interaction. An alternative interpretation of the cause of the distortion can be based upon hybridization arguments. In those clusters with lone pairs the Fe-E-Fe angles are closer to go", implying that the orbitals used to make these bonds contain more p character and therefore the lone pair is more s like. This observation is consistent with the "inert pair" concept for heavy post-transition metals.34 The inert pair effect was first put forth to explain the tendency of heavier post-transition elements to forego the use of their s valence electrons for bonding. Relativistic orbital energy calculations have suggested that this tendency is caused by a contraction of the s orbitals and a concurrent lowering of their energy.35 (33) Cook, S. L.; Evans, J.; Gray, L. R.; Webster, M. J . Oganomet. Chem. 1982,236,367. (34) Huheey, J. E. Inorganic Chemistry: Principles of Structure and Reactivity 3rd. ed.; Harper and Row: New York, 1983, pp. 843-845. (35) F'yykko, P. Chem. Rev. 1988,88,563 and references contained therein.

Implied in both of these descriptions is the concept that this inert pair should reside in an orbital which is principally s in character and, hence, be nondirectional. However, more recently it has been suggested that the stereochemical activity or inactivity of the lone pair is not simply determined by the relativistic effects which give rise to the inert pair effect but also by other intraand intermolecular forces present.35 These concepts agree very well with the bonding patterns seen in the clusters under consideration. When the lone pair is unused for bonding, it resides in an orbital having largely s character. As a consequence of this, the orbitals used by the main-group element for bonding to the iron atoms in the cluster core have more p character. This allows the Fe-E-Fe angles to more closely approach the idealized value of 90" expected for a squarebased pyramid. However, when the lone pair is used to bind an ancillary metal fragment, the less favorable energetics of rehybridization to sp3 is offset by the energy released by bonding to the external metal fragment. The validity of these ideas is being probed by theoretical calculations. While this paper was in review, other workers indepently reported the preparation of [IF'- and the corresponding antimony compound via similar though not identical meth~dology.~~

Conclusions In heteronuclear clusters such as those discussed above, electrons on the main-group fragments not directly involved in the cluster core bonding may play a critical role in determining the observed structure and reactivity patterns of the cluster. The presence of a lone pair of electrons on a main-group-atom vertex in an open-framework compound is generally found to promote simple ligand loss and formation of a closo compound, while tying up the fourth pair of electrons by bonding t o an external functionality yields more complicated fragmentation and reorganization patterns. These effects are rationalized on the basis of rehybridization processes occurring at the main-group atom. Acknowledgment. K.H.W. wishes to thank the National Science Foundation and the Robert A. Welch Foundation for financial support of this work. R.E.B. wishes to thank the NSF for a Predoctoral Fellowship. VG Analytical is acknowledged for providing the mass spectral analyses. Supplementary Material Available: Diagrams of the other two independent cluster anions for [PPNlz[I] and complete tables of bond metricals as well as positional and anisotropic displacement parameters for [PPNI2[11 and [Etalz[II] (36pages). Ordering information is given on any current masthead page. OM940498B (36) Henderson, P.; Rossignoli, M.; Burns, R. C.; Scudder, M. L.; Craig, D. C. J. Chem. SOC.,Dalton Trans. 1994, 1641.