4818
Cobalt-Hydrogen Metallocarboranes. Synthesis of closo-[(C H ~ ) ~ C ~ B ~ H ~and ]~COH nido,closo - [(CH3)2C*B3H 5]CoH[(CH 3)2C2B4H4] and Their Conversion to New Cobalt-Cobalt Bonded Metallocarboranes William M. Maxwell, Vernon R. Miller, and Russell N. Grimes* Contributionfrom the Department of Chemistry, University of Virginia, Charlottesuille, Virginia 22901. Received December I , 1975
Abstract: The reaction bf N ~ + [ ( C H ~ ) ~ C ~ Bwith ~ HCoC12 S ] - at 25’ yields the red neutral commo-metallocarborane [ C,C’(CH3)2C2B4H4]2Coi11H.From NMR evidence the hydrogen atom associated with the metal is proposed to be face-bonded to a triangular face on the polyhedral surface. The compound is converted in aqueous acid to a nido,closo species, [C,C’( C H ~ ) ~ C ~ B ~ H ~ J C O H [ C , C ’ - (which C H ~in)turn ~ C ~can B ~beH metal ~ J , deprotonated by NaH to give the.[(CH3)2CzB3H5]C O [ ( C H ~ ) ~ C ~ Bion; ~ Hreaction ~ J - of the latter species with aqueous HCI regenerates the neutral metal-protonated compound. The reaction of the ion with CoCl2 and NaCsHs produces nido,closo-[( C H ~ ) ~ C ~ B ~ H ~ ] C O H [ ( C H ~ ) ~ C ~ B ~ H ~ ] C O ( ~ C5H5) in which a Co-Co bond is postulated. Treatment of [ C , C ’ - ( C H ~ ) ~ C ~ B ~with H ~ J(q5-C5H~)Co(C0)2 ~CO’~~H yields 4,5-(CH3)2-1,2,4,5-(15-C5H5)2C02C2B3H3 and (15-CsHs)Co[(CH3)2C2B3H3JCoH[(CH3)~C2B3H3JCo(~5-C~H~), each C O ” ’ Hthe tetracarbon carborane species having a proposed Co-Co link. Air oxidation of [ C , C ’ - ( C H ~ ) ~ C ~ B ~ H ~ ] ~produces
(CH3)4C4BsHs.
One of the most versatile carborane systems in reactions with metal reagents is 2,3-C2B4H8 (nido-dicarbahexaborane[8]). The variety of main-group and transition metal compounds obtained from the neutral carborane, its bridgedeprotonated ion C2B4H7-, or C-alkyl derivatives of these includes such diverse species as bridged complexes containing B-M-B three-center bonds,] a-bonded terminally substituted derivatives,lb,c,fopen-cage (nido) metallocarboranes,’d,2 and closed polyhedral (closo) mono- and dimetallocarbora n e ~ , l ~ including , ~ ~ , ~ the , ~ triple-decked sandwich complexes 1,7,2,3- and 1,7,2,4-(q5-C5H5)2C02C2B3H5. The present paper describes recent work on the C2B4H8 system which led us into systematic studies of metallocarboranes containing metalhydrogen bonds, produced several new species having adjacent metal atoms in a polyhedral cage, and in addition provided an entry to a previously unknown area of metallocarborane chemistry, the tetracarbon metallocarboranes. Many of our earlier investigations in this area1d,2b,4 utilized reactions of the carborane substrate C2BdH7- with metal ions in the presence of cyclopentadienide ion, C5H5-, giving metallocarboranes containing one or more (q5-C5H5)M moieties in the cage framework, for example, ( $ - C ~ H ~ ) C O C ~ B & . Anticipating a routine extension of this work, we assumed that similar reactions in the absence of C5H5- would generate commo-metallocarborane anions in which the metal is present simultaneously in two polyhedral cages (or, in the “metalcomplex” terminology, the metal atom is face-bound to two carboranyl ligands):
Analogous commo species derived from higher carboranes are well k n o ~ n However, .~ our investigation of reactions of the C2B4H7- ion with FeC12 and CoC12 gave no isolable metallocarborane products. The corresponding reactions of the C,C’-(CH3)2C2B4H5- ion, in contrast, gave good yields of metallocarboranes which somewhat surprisingly proved to be neutral molecues rather than salts. In this paper we describe the chemistry of the cobalt species, which differed substantially from that of the iron systems presented elsewhere.6 Journal of the American Chemical Society
98:16
Results and Discussion Reaction of the C,CV-(CH3)2C2B4H5-Ion with CoC12. The (CH3)2C2B4HsV ion reacts with cobalt(I1) chloride at room temperature in tetrahydrofuran (THF) to give the neutral commo-metallocarborane [C,C’-(CH~)~C~B~H~]~CO~~’H (I) in up to 34% yield, obtained as a moderately air-sensitive tomato red crystalline solid. The synthesis of I from CoC12 is accompanied by the formation of a dark ferromagnetic solid assumed to be cobalt metal, but no evolution of gas is observed. The assignment of a formal cobalt +3 oxidation state in I is consistent with its diamagnetism as evidenced by its normal I IB and ] H N M R spectra. The gross structures proposed for I (depicted in Figure 1) and all other new compounds reported herein are based on I’B and IH pulse Fourier transform nuclear magnetic resonance (FTNMR) spectra obtained at 32.1 and 100 MHz, respectively, as well as infrared, electron impact and chemical ionization mass spectra (see Tables I-IV). The location of the “extra” hydrogen in I has not been definitively established, but hydrogens producing unusually high-field7 IH N M R resonances in metallocarborane~~?~ and metalloboraneslnhave been shown via x-ray diffraction s t u d i e ~ or ~ ~assumed8 ’~~ to be bonded to the metal atoms. Furthermore, in the case of I it is observed that the Co-H ‘ H resonance sharpens on IIB decoupling, implying that this unique hydrogen is not bound exclusively to the metal but is associated partially with nearby boron atoms in a face-bonded arrangement. In earlier papers from this laboratory1d$8 a similar proposal was advanced for the related compounds 1,2,3- and 1,2,4-(q5-C5H5)Fe11HC2B4H6.Recently, the elusive face-bonded hydrogen in the octahedral CB5H7 system has been located by an electron-diffraction study,’ following a microwave investigation12 which had given indirect evidence of four-center H-B3 bonding. The electron diffraction results confirm that the extra hydrogen in CB5H7is located above the center of a triangular B3 face; earlier NMR work by OnakI3 had strongly suggested that at room temperature the face-bridging proton rapidly tautomerizes around the molecule, rendering the four equatorial boron atoms equivalent on the NMR time scale. Similar tautomeric behavior is likely in I, and the high symmetry evident from the FTNMR spectra (Tables I and 11) is consistent
1 August 4, 1976
4819 -
excess NaH/THF, -H2 H+ ( CH3),C2B4H,
I (tomato (C5H5)Co(CO),
,
t A
f
II(yellow)
red)
-
a
25" dec
I +
4. (green)
III: (black)
a
-
T U (brown
5'
5
intermediate, not isolated 1
I T (red-brown) OBH OCCH3.CH
OH
Figure 1. Reaction scheme showing proposed structures of new metallocarboranes I-VI Metal-bound hydrogens are shown schematically
Table I.
32.1-MHz I1B FTNMR Dataa Compound
6, ppm ( J , Hz)
-6.9(168), -1.7(156) [ ( C H ~ ) ~ C Z B ~ H ~(1) I~COH -5.5(128),' -2.4(128),' +0.9(172), +11.4(168) [(CH~)~C~B~H~ICOHI(CH~)~C~B~HSI (11) (CH3)&"( [(CH3)2C2B&I CO[ ( C H ~ ~ C ~ B1- ~(111) HSI -4.9( 136), - 1.8(140), + 1.8( 144) (CSHS)C~[(CH~)~C~B~H~~COH[(CH~)~C~B~HSI (IV) -30.8(157), -8.4(161), -3.7(146),c -0.8(160)c -45.7( 156), -16.8( 167) 1,~,~,~-(CSHS)~CO~[(CH~)~C~B~H~I (VI (CSH~)CO[(CH~)~C~B~H~]COH[(CH~)~C~B~H~]CO(C~H~) (VI) -61.6,esf-30.0( 150), -9.3e,g
Rel. areas 133 2,c 1,c 3, 1 3,c 2 , c 2 c 2, 1, 2, 1 2, 1 3 1 3 2 ,
a All spectra run in CDC13 solution except where otherwise indicated. Chemical shifts relative to externally referenced BF~.O(C~HS)~. Estimated from overlapped resonances. Spectrum measured in CD3CN solution. e Very broad resonance, "B-H coupling not measurable. f w l p = 360 Hz (undecoupled specrum); w l / 2 = 164 Hz (proton-decoupled spectrum). g w l p = 520 Hz (undecoupled spectrum): w l p = 320 Hz (proton-decoupled spectrum).
with two bridging hydrogens is analogous to the conversion of with this hypothesis. However, a "fixed" (nontautomerizing) ( C S H ~ ) C O C ~ B to & ( C S H ~ ) C O C ~ as B ~reported H~ earlier.2b hydrogen located on a mirror plane cannot be ruled out, nor The proposed structure of I1 is consistent with the FTNMR can one eliminate the possibility that a bridging hydrogen in data (Tables I and 11) and contains a cobalt atom which resides an unsymmetrical fixed position might have so small an effect in both a nido and a closo cage framework. The high-field band on the N M R chemical shifts as to be undetectable. The resoat 7 16.0 in the IH FTNMR spectrum appears as a broad lution of this question in the solid-state structures of I and resinglet and fails to distinguish the B-H-B bridging protons lated molecules may be possible with low-temperature x-ray from the unique hydrogen associated with the cobalt atom. analysis, but is not crucial to the present discussion which However, on IlB decoupling this signal is resolved into a lowconcerns reactions in solution in which rapid proton tautomfield doublet of doublets, attributed to coupling of each of the erism is assumed. Formation of (CH3)4C4BsHs from [ C,C'-(CH~)ZC~B~-B-H-B bridging protons with its neighboring terminal B-H protons, and a separate broad singlet at higher field assigned H&CoH (I). The bright red solid I is thermally stable in vacuo to the Co-H group (Figure 2). This observation indicates that at room temperature but decomposes slowly on exposure to air, forming in 35-4096 yield a colorless crystalline air-stable solid the B-H-B and Co-H protons are not undergoing rapid tauwhich has been characterized as (CH3)&4B&Is, a new cartomeric exchange on the N M R time'scale, although exchange borane system.14 The same product is formed in higher yield at a much slower rate cannot be ruled out. on air oxidation of the ferracarborane [C,C'-(CH3)2The planar cyclocarboranyl ligand in I1 is a C,C'-dimethyl C2B4H4]2FeJ1H2as described in a separate publicatioq6 and derivative of the formal C Z B ~ H system ~ ~ - as found in ($the properties of (CH3)&4 are presented in that conC ~ H S ) C O C ~ and B ~ (C0)3FeC2B3H7,2a H~~~ the latter structext. ture having been established in an x-ray study.lS The Conversion of [ C,C - ( C H ~ ) ~ C ~ B ~ H ~(I)] ~toCNido,Closo OH C Z B ~ H ligand ~ ~ - is a doubly bridge-protonated analogue of Cobaltacarboranes. The treatment of I with acidified aqueous the cyclic planar C2B3Hs4- ring found in the triple-decked T H F gives in high yield the yellow species [(CH3)2sandwich m e t a l l o ~ a r b o r a n e s ~1,7,2,4~ . ~ ~ and 1,7,2,3-($C ~ B ~ H S ] C O H [ ( C H ~ ) ~(11), C ~ as B ~shown H ~ ]in Figure 1. The C5H5)2Co2C2B3H5. Both the C2B3H72- and C2B3Hs4- sysremoval of an apex BH group and its replacement, in effect, tems are isoelectronic with cyclopentadienide ion, C5H5-, Maxwell, Miller, Grimes / Co-H Metallocarboranes
4820 Table 11.
100-MHz 'H FTNMR Dataa
Compound 1
11
I11 (CH3CN)
IV
Figure 2. The 100-MHz proton FTNMR spectra of [C,C'-(CH3)2C ~ B ~ H S ] C ~ H [ C . C ' - ( C H ~ ) ~(11): C~B top~ H spectrum, ~] undecoupled;
middle, partially IB-decoupled; bottom, fully I 'B-decoupled, showing B-H-B multiplet and Co-H singlet (see text). The bottom spectrum is shown in an enlarged horizontal scale.
V
VI
Resonance, (re]. area)c
Assignment
~b
7.78(12) 18.7(1)/ w1/2 = 50 Hz 7.82(6) 8.21(6) 16.0(3),d w1/2 = 144Hz 15.59( 2) e f 16.3(l)eJ 8.34(3) 8.03(3) 6.92(6) 16.72(1)' 4.95(5) 7.95( 6) 8.27(6) 15.80(2)i,k 24.3(1)'.k 15.78 (2) m,n 24.3(1)"@ 5.17(5) 5.52(5) 7.85(6) 5.01(5) 5.63(5) 7.34(6) 8.13(6) 25.8(1)P
CH3 CO-H CH3 CH3 CO-H, B-H-B B-H-B
CO-H CH3 CH3 (CH3)4N+ B-H-B C5H5 CH3 CH3 B-H-B
CO-H B-H-B CO-H C5HS C5H5 CH3 C5H5 CsH5 CH3 CH3 CO-H
a All spectra were run in CDC13 solution except where otherwise indicated. Chemical shifts relative to (CH3)4Si = T 10.00. Singlet resonance except where otherwise indicated. Undecoupled spectrum. e I IB-decoupled spectrum. f Doublet of doublets: primary splitting, J' N 8 Hz; secondary splitting, J 3 Hz. g Singlet, w1/2 = 24 Hz. Triplet, J = 0.6 Hz. Doublet of doublets: J' N 7 Hz, J E 4 Hz. J Broad resonance, w 1 p = 126 Hz. Undecoupled spectrum. Broad resonance, w l p = 32 Hz. "B-decoupled spectrum. Doublet of Figure 3. The 100-MHz proton FTNMR spectrum of [(CH3)4doublets: J' N 7.2 Hz, J = 4.0 Hz. w1/2 = 21 Hz. p Singlet, w1/2 N]+[C,C'-(CH~)*C~B~H~]CO[C.C'-(CH~)~C~B~H~]( I 1 I ) . Multiplet = 40 Hz in undecoupled spectrum, 19 Hz in the IIB-decoupled is due to B-H-B protons; note absence of resonance in Co-H region spectrum. (kompare with Figure 2).
Table 111. High Resolution Mass Measurementsa
although unlike the latter species they have not been detected as free ions. Compound Formula Calcd mass Obsd mass Structure I1 can be viewed in another sense, recognizing that the open portion is analogous to nido-2,3-CzBdHg, while the 264.1828 I 59C~12Cg"B710B'H22+ 264.1834 I1 59C012C8 I 1 B7'H23+ 255.1784 255.1786 7-vertex polyhedral cage is electronically equivalent to IV 59C~212C 13' 'B4I0B2364.1329 364.1325 closo-2,3-C2BsH7, with cobalt replacing the apex BH group 'H26+ in both cases. Since 2,3-C2B4H8l6and the analogous molecule 339.0495 V 59C~212C14"B31H20+ 339.0508 1, ~ , ~ - ( $ - C S H ~ ) C O Ceach ~ B lose ~ H ~a ~bridge ~ proton on VI 59C0312C 18' '&'H30+ 489.0902 489.0885 reaction with hydride ion, similar behavior might have been anticipated for 11. Treatment of I1 with excess N a H or K O H a Mass of P + 1 (protonated parent ion) species obtained in methane liberates 1 equiv of H2; however, proton loss occurs not a t a under chemical ionizing conditions. Parent ion (not protonated). B-H-B bridging location but rather a t the Co-H group. The Peaks corresponding to both protonated ( P + 1) and parent (P) ions resulting yellow [(CH~)~C~B~H~]CO[(CH~)~C~B~H~]anion, were observed in all spectra; in each case, the choice of peaks employed for exact-mass determination by peak matching against a known which can be precipitated in aqueous media as a yellow standard was dependent on shape and freedom from overlap with other tetramethylammonium salt (111) exhibits a IH F T N M R peaks. spectrum (Figure 3) which clearly shows that both B-H-B bridges are retained while the proton which was associated with exemplified by [ ( C ~ H ~ ) ~ P ] ~ H R],I7 ~ Cwhich ~ B ~contains HI the metal in the neutral species is no longer present. a formal H- ligand coordinated to the metal atom.Is On the The Co-H proton abstraction is reversible, since treatment of 111 with aqueous or anhydrous HC1 regenerates I1 quantiother hand, the [(C2B9H11)2Fe1lH]- ion, obtained by direct tatively. These results, together with similar findings on the protonation of [ ( C ~ B ~l)2Fe1Il2-, HI may contain a metalbound proton although the 'H N M R spectrum of this species [(CH3)2C2B4H4]2Fe11H2 system6 and on (v5-C5H5)contained no resonance in the high-field region associated with FeI1HC2B4H6,Idshow that the metal-bound hydrogen atoms in these species are protonic rather than hydridic in character. metal-hydrogen groups.19 In this respect these metal-hydrogen metallocarboranes are Cobalt Insertion into the Nido,Closo Monocobalt Anion (III). formally distinct from the hydrido metallocarborane family The nido,closo species I11 undergoes reaction with CoC12 and Journal of the American Chemical Society
/
98:16
/
August 4, I976
4821 Table IV.
Io
Infrared Absorptions, cm-l
2960(w), 2925(m), 2885(w), 2578(vs), 1438(m), 1380(m), 11 lO(w), 1045(m), 900(w), 865(m), 812(m) I1 2960(m), 2930(m), 2860(w), 2580(vs), 2545(sh,s), 1890(sh,w), 1865(m), 1655(w), 1585(m), 1455(m), 1375(m), 11 18(w), 1075(w), 1032(w), 1000(m), 985(sh,w), 945(m), 908(w), 885(m), 855(w), 820(w), 798(m), 740(m), 702(m), 620(w) I I I b 3030(m), 2950(m), 2912(m), 2880(m), 2518(vs), 1845(m), 1510(w), 1482(m), 1448(w), 995(m), 945(m), 870(m), 775(m), 738(w) IV" 2950(sh), 2925(s), 2858(w), 2538(vs), 1862(m), 1735(m), 1570(w), 1435(m,br), 1370(w), 1245(w), 1150(m), 1100(m), 1003(m), 945(w), 900(m), 825(s) V c 2958(sh,w), 2920(m), 2850(w), 2485(s), 1732(m), 1430(w), 1lOO(w), lOOO(w), 895(w), 820(m), 725(m) V I u 2922(m), 2870(m), 2512(s), 1738(w)
direct insertion of a second metal atom into a monometallocarborane frequently occurs at a position adjacent to the first metal, particularly in reactions conducted under mild conditions. Further discussion of this point appears later in the paper. Insertion of Cobalt into [ ~ , ~ - ( C H & C ~ B ~ H ~(I). ] ~ The COH reaction of the commo-cobaltacarborane I with (v5-C5H5) Co(CO)2 in T H F under ultraviolet irradiation followed by air oxidation produces a complex mixture of materials, most in low yield. Two of the products have been isolated by thick-layer chromatography and characterized as a green C,C'-dimethyl derivative ( V ) of the previously reported2] 1,2,4,5-(q5C~H~)~CO~CZ and B ~aH black ~, tricobalt species (q5-
CSH~)C~[(CH~)~C~B~H~IC~HW [ (v C 5 -H ~ ) ~ C ~ B ~ H ~
C5H5) (VI). The proposed structures of V and VI, shown in Figure 1, were inferred from IlB and IH FTNMR spectra utilizing arguments similar to those invoked for compound IV above. The spectra of V are very close to those of the parent CH2C12 solution vs. CH2C12. KBr pellet. Thin film, solid 1,2,4,5 isomer21 (e.g., the IlB resonances of the latter comstate. pound occur at 6 -44.7 and -1 1.4, and its C5H5 peaks in the proton spectrum are found at T 5.12 and 5.44). The structure NaCsHs followed by aqueous HC1 to give neutral, red-brown proposed for VI is strongly supported by the I I B FTNMR (v5-CsH5)Co[(CH3)2C2B3H31C~H[(CH3)2C~B3H5l (IV) spectra which combine the features associated with the two in low yield; the same compound is also obtained from I (vide types of 7-vertex cages present. Thus, the area 1 resonance at infra), The proposed structure depicted in Figure 1 is based 6 -61.6 is assigned to the unique boron in the planar C2B3 ring on the following considerations. Assuming that the cage carbon (as in the triple-decked sandwichZb 1,7,2,3-(q5-c5H5)2atoms in each carboranyl unit remain adjacent (elevated C O ~ C ~ B ~and H ~the ) ,area 2 band at 6 -30.0 is attributed to temperatures normally being required to effect carbon-carbon the equivalent pair of boron atoms in the lower CoC2B2 ring, separation in metallocarboranes20$21) and taking into account as in compounds IV, V, and 1,2,4,5-(s5-CsH5)2C02C2B3H~. the synthetic route as well as the symmetry indicated from IlB The broad resonance at 6 -9.3 is assumed to arise from overand IH NMR spectra (e.g., only two distinct methyl group lapping signals of the remaining borons, and has a 6 value environments are evident), only two structures seem plausible consistent with their locations. It is interesting to note that for IV: that indicated in Figure 1, and a triple-decked sandwich migration of the equatorial (C5H5)Co group in VI to the adgeometry produced by interchanging the (C5H5)Co and apex jacent apex BH position would produce a quadruple-decked BH groups in the structure shown. A clear preference for the sandwich, of which no known examples currently exist. From adjacent-cobalt geometry given in the figure is afforded by the related work on thermal isomerizations of cobaltacarboranes21 IlB FTNMR spectrum which closely resembles that of the it appears that such a migration might well occur at elevated structurally analogous species 1,2,4,5-(v5-C5H5)2C02- temperatures. The minute quantity of VI obtained in the C2B3H5,21a molecule differing from IV (except for the methyl present work precluded our examining this possibility. groups) only by replacement of the [ ( C H ~ ) ~ C ~ B ~ H ~ ] C OCompounds H V and VI were also obtained when the identical moiety with an electronically equivalent (v5-C5H5)Co group. reagents were refluxed in T H F without ultraviolet irradiation. In contrast, the IlB spectra2b,22of the structurally defined3b The same experiment produced a brown product (VII) which triple-decked sandwich 1,7,2,3-(v5-C5H5)2C02C2B3H5 and exhibited a mass spectrum corresponding to (v5-C5Hs)its C-substituted derivatives are very different from that of IV; C O [ ( C H ~ ) ~ C ~CoH B~H [ (~C]H ~ ) ~ C Z B ~Attempts H ~ ] . to purify in particular, the 1,7,2,3 species each exhibit an area 1 resothis material by chromatography on silica gel led to its denance at very low field (6 -50 to -60 ppm) arising from the composition to IV, and NMR data for VI1 were not obtained. unique equatorial boron in the ring. The spectrum of IV conHowever, its mode of preparation and its decompositign tains no such feature, its lowest-field resonance being an area products suggest that the structure of VI1 is probably that of 2 signal at 6 -30.8 which can be assigned to the pair of VI with a BH group replacing the apex (CsH5)Co unit. equivalent boron atoms in the CoB2C2 ring. The IlB spectra Conclusions of 1,2,4,5-(~~-C5Hs)CoC2B3H5~~ and its C,C'-dimethyl deA principal finding of this work, taken together with our rivative (compound V in this paper) exhibit low-field doublets studies on iron metallocarboranes,1d,6is that the formal of area 2 at 6 -44.7 and -45.7, respectively; the apex boron [Co1I1HI4+and [Fe1[H2l4+units23 can function as isoelecresonances in all three cases appear at much higher field, tronic surrogates for tetrapositive metals such as Ni4+,and are separated by approximately 30 ppm from the low-field douversatile structural members in polyhedral carborane chemblet. istry. A consequence of this fact is the existence of a family of The IIB and IH resonances arising from the open-ring neutral cobalt and iron commo-metallocarboranes incorpoportion of IV are comparable to those of other complexes of rating pairs of dinegatively charged carborane ligands such the formal C2B3H72- ligand.1d,2a,b The broad resonances of as C2B4H62- and C Z B ~ H and ~ ~ the - planar difunctional cyarea 2 at T 15.8 and of area 1 at T 24.3 in the proton spectrum clocarboranyl ligand CzB3H54-. One of the more intriguing are readily assigned to the pair of B-H-B groups and the Co-H avenues for exploration that this suggests is the construction proton, respectively. In the I1B-decoup1edspectrum, the T 15.8 of neutral multidecked sandwich complexes from C2B3Hs4peak is resolved into a doublet of doublets (as in the case of I1 and CoH4+ and/or FeH24+ groups, a possibility we are curabove), with J = 4.0 Hz and J' = 7.2 Hz. In this case, unlike rently examining. Certainly the availability of ( C Z B ~ H ~ ) 11, the B-H-B and Co-H resonances are widely separated, Co111H2+,( C ~ B ~ H ~ ) C O ~and ~'H ( C~ Z +B , ~ H ~ ) F ~ "groups H~~+ possibly reflecting association of the metal-bound proton with (or their methyl derivatives) as substitutes for (C5H5)Co2+ both cobalt atoms in IV. and (CO)3Fe2+ units in carborane frameworks enhances the The formation of IV from I11 extends a pattern we had noted already rich chemistry of this area; complexes which are in earlier work and observed several times in the present study: Maxwell. Miller, Grimes / Co-H Metallocarboranes
4822 end-capped with carboranyl ligands are amenable to further reactions such as metal insertion or boron removal, as seen in the behavior of species I and 11. In contrast, C5Hs- is a relatively inert capping group when $-coordinated to a metal ion, as shown by the fact that only one example is known of CsH5pentahapto bonding to two metal ions simultaneously, as occurs in the (a5-C5H5)3Ni2+ cation.24 The apparent tendency toward metal-metal bond formation, in the metal insertion reactions described here, has been noted earlier. In addition to the three cases observed in this work, previous examples of direct insertion which produced species having metals in adjacent polyhedral vertices include the reactions of ( C S H S ) C O C ~ B ~with H S (CsH5)Co(C0)2 at 230°,25 (C0)3FeC2B3H5 with Fe(CO)s at 230°,25 [(CH3)2C2B4H4] 2FeH2 with ($-CSHS)CO(CO)~ under ultraviolet irradiation,6,26C2BsH7 with (C5Hs)Co(C0)2 at 260°,25 and C2B4H7- with NiBr2 and C5H5- at 25°.2bThe detailed nature of metal-metal interactions in polyhedral borane systems is largely unexplored, but a recent investigation of the thermal rearrangements2' of small cobaltacarboranes indicated that complex and subtle factors are operative. Thus, the thermal isomerization of the 9-vertex (s5-CsHs)zCo2C2B5H7system at 240-340' involves a reversible migration of the cobalt atoms between nonadjacent and adjacent vertices resulting in a measureable equilibrium of two isomers, both of which have now been crystallographically ~ h a r a c t e r i z e dThese . ~ ~ earlier results suggested that there are factors favoring metal-metal bonding in metallocarborane systems under moderate conditions, although other influences (such as steric repulsions between bulky ligands) may induce separation of the metal atoms at high temperatures. It may also be pointed out, as other workers have done that the few crystallographically established metal-metal bond lengths in metallocarboranes are at least comparable to those in metal cluster systems; thus, Co-Co distances in (v5-C5H5)2CoX2B6H8, ($-C5H,-)zC O ~ C ~ B ~and H ~ (q5-C5H5)2C02C2B5H7 O, are 2.489 ( 1),28 2.387 (2),29 and 2.444 (2) A27which compare with Co-Co lengths of 2.43-2.55 A in cobalt clusters such as (CO)&o3OH and its derivative^.^^ Much further work remains in order to fully elucidate these early findings on metal-metal interactions in polyhedral metallocarboranes.
Experimental Section Materials. 2,3-Dimethyl-2,3-dicarbahexaborane(8) [2,3( C H ~ ) ~ C Z Bwas ~ H prepared ~] from 2-butyne and pentaborane(9) as described e l ~ e w h e r e , and ~ ' purified by GLC on 30% Apiezon L/ Chromosorb W at 60'. All other reagents were reagent grade and used as received. Tetrahydrofuran ( T H F ) was dried over LiAIH4 before use. Spectra. Boron-11 FTNMR spectra at 32.1 M H z and proton F T N M R spectra at 100 M H z were obtained on a JEOL PS-IOOP pulse Fourier transform spectrometer interfaced to a JEOL-Texas Instrument EC- 100 computer system. Unit resolution mass spectra were obtained on a Hitachi-Perkin-Elmer RMU-6E mass spectrometer. High resolution mass spectra were recorded on an AEI MS-902 double-focusing instrument equipped with an SRI chemical ionization source and interfaced to a PDP-81 computer. All high resolution spectra were obtained under chemical ionizing conditions. Infrared spectra were obtained on a Beckman IR-8 instrument. General Procedure. Except where otherwise indicated, all reactions were run in high vacuum systems or in an inert atmosphere. Thin layer and preparative layer chromatography were conducted in air on precoated plates of silica gel F-254 purchased from Brinckmann Instruments, Inc. [ ( C H ~ ) ~ C ~ B ~ H ~(I). ]~C A Otetrahydrofuran H (THF) solution of Na+[(CH&C2B4H5]- was preparedI6 by distillation of (CH3)2C2B4H6 (0.57 g? 5.53 mmol) onto N a H (0.20 g, 8.34 mmol) in 30 ml of T H F in vacuo. The solution was filtered in the vacuum line onto anhydrous CoCl2 (0.394 g, 3.02 mmol) at -196' in a 100-ml roundbottom flask. The reaction vessel was allowed to come to room temperature. After stirring for -10 min, the color of the solution was dark
Journal of the American Chemical Society
green. No evolution of gas was detected. The reaction mixture was stirred magnetically for 2 h. After removal of T H F via vacuum distillation, dry nitrogen was introduced into the reaction vessel, and the vessel was quickly attached to a sublimator. On heating at 60" and Torr the tomato red product began to collect on the dry ice-cooled (-78') cold finger. After 6 h of subliming, nitrogen was introduced into the flask, and the flask removed to a nitrogen-filled glove bag. The red-brown solid I was washed from the cold finger with degassed hexane to produce 216 mg of solid product (34% based on (CH3)2C2B& used). The yield of I has varied considerably. The black material remaining in the reaction flask after sublimation was stirred for 1 h in air, in T H F which had been acidified with ethyl acetate saturated with concentrated aqueous HC1. This solution was filtered and solvent was removed. The brown residue was placed on a column of silica (which had previously been acidified with acetic acid by stirring in a 10% acetic acid-hexane solution and then washed with hexane until all excess acetic acid had been removed) and eluted with hexane. The first band was bright yellow [(CH3)2C2B4H4] C O H [ ( C H ~ ) ~ C ~ (11), B ~ H50~mg, ] and thesecond band obtained was colorless (CH3)4C4BgHa, 60 mg. Conversion of I to ( C H ~ ) ~ C ~ B ~[ H ( CSH . ~ ) ~ C Z B ~ H ~ ]on~ C O H standing in the solid state in air decomposed rapidly (hours) to yield (CH3)4C4BgHg. When stored in vacuo at room temperature, solid I darkened consderably over a period of several months, forming the tetracarbon species in low (
