Borole derivatives. 22. Iodine degradation of the triple-decker complexes (μ-C4H4BR)[Rh(C4H4BR)]2 (R = Ph, Me): Synthesis and structural characterization of heterocubanes [Rh(μ 3-I)(C4H4BR)]4 and of bis(borole)rhodium complexes RhI(C4H4BR)21

Article abstract of DOI:10.1021/om970422q

The triple-decker complexes (μ-C₄H₄BR)[Rh(C₄H₄BR)] ₂ (R = Ph, Me) (1) react with elemental iodine in toluene or dichloromethane to produce heterocubanes [Rh(μ₃-I)(C₄H₄-BR)]₄ (2) and bis(borole)iodorhodium complexes RhI(C₄H₄BR)₂ (3). The heterocubanes 2 exchange fragments RhI(C₄H₄BR) at rates which are fast with respect to the NMR time scale. The bis(borole) compounds 3 exist in an equilibrium of rotational isomers C_s-3 = C_2v-3 with ΔRH = (-1.7 ± 0.2) kJ mol^-1 and Δ_RS = (-16 ± 1) J mol^-1 K^-1 for 3b. Activation parameters for the forward and reverse isomerization reactions are reported. The isomerization takes place by the mechanism of ring-ring rotation; this is demonstrated for 3b by TOCSY NMR spectra (¹H NMR, -80 °C, mixing time 150 ms). Crystal structure determinations of 2a, 2b, and 3b are reported. The higher-energy isomer C_s-3b is found in the crystal of 3b. One borole ligand of C_s-3b is planar and η⁵-bonded with a normal Rh-B1 bond distance [236.7(6) pm]. The second ligand shows an unusual folding of 15.8(9)° along the line C2-C5, a very weak bonding interaction Rh-B2 [252.8(6) pm] approaching η⁴-coordination, and a close contact I-B2 [295.3(6) pm]. Both borole ligands show diene-like C-C bond length patterns with weak Rh-to-borole back-bonding.

Full text of DOI:10.1021/om970422q

4292
Organometallics 1997, 16, 4292-4298
Bor ole Der iva tives. 22. Iod in e Degr a d a tion of th e
Tr ip le-Deck er Com p lexes (µ-C4H4BR)[Rh (C4H4BR)]2 (R )
P h , Me): Syn th esis a n d Str u ctu r a l Ch a r a cter iza tion of
Heter ocu ba n es [Rh (µ3-I)(C4H4BR)]4 a n d of
1
Bis(bor ole)r h od iu m Com p lexes Rh I(C4H4BR)2
Gerhard E. Herberich,* Hartmut J . Eckenrath,2 and Ulli Englert
Institut fu¨r Anorganische Chemie, Technische Hochschule Aachen, D-52056 Aachen, Germany
Received May 21, 1997X
The triple-decker complexes (µ-C4H4BR)[Rh(C4H4BR)]2 (R ) Ph, Me) (1) react with
elemental iodine in toluene or dichloromethane to produce heterocubanes [Rh(µ3-I)(C4H4-
BR)]4 (2) and bis(borole)iodorhodium complexes RhI(C4H4BR)2 (3). The heterocubanes 2
exchange fragments RhI(C4H4BR) at rates which are fast with respect to the NMR time
scale. The bis(borole) compounds 3 exist in an equilibrium of rotational isomers Cs-3 )
C2v-3 with ∆RH ) (-1.7 ( 0.2) kJ mol-1 and ∆RS ) (-16 ( 1) J mol-1 K-1 for 3b. Activation
parameters for the forward and reverse isomerization reactions are reported. The isomer-
ization takes place by the mechanism of ring-ring rotation; this is demonstrated for 3b by
TOCSY NMR spectra (1H NMR, -80 °C, mixing time 150 ms). Crystal structure determina-
tions of 2a , 2b, and 3b are reported. The higher-energy isomer Cs-3b is found in the crystal
of 3b. One borole ligand of Cs-3b is planar and η5-bonded with a normal Rh-B1 bond
distance [236.7(6) pm]. The second ligand shows an unusual folding of 15.8(9)° along the
line C2-C5, a very weak bonding interaction Rh-B2 [252.8(6) pm] approaching η4-
coordination, and a close contact I-B2 [295.3(6) pm]. Both borole ligands show diene-like
C-C bond length patterns with weak Rh-to-borole back-bonding.
In tr od u ction
In 1983 we found that dehydrogenative complex
formation of 2,5-dihydro-1H-boroles provides an efficient
and versatile route to C-unsubstituted (borole)metal
complexes. Among the very first examples was the
triple-decker complex (µ-C4H4BPh)[Rh(C4H4BPh)]2 (1a),3
and later the methyl analog 1b was also described.4 In
this paper we report the oxidative degradation by iodine
of the triple-decker complexes 1a ,b. This reaction will
give us two new types of products, the dark red
heterocubanes [Rh(µ3-I)(C4H4BR)]4 (2) and the orange
bis(borole)iodorhodium compounds RhI(C4H4BR)2 (3). Of
these, the compounds 3 should attract particular atten-
tion because of a hitherto unknown bonding situation
of a folded, approximately tetrahapto-coordinated borole
ligand. In later papers we shall show that the com-
plexes 2 are useful as versatile sources of the (borole)-
rhodium fragments Rh(C4H4BR).
9, R ) Ph; b, R ) Me
this oxidation has been established in NMR tube
experiments using d8-toluene as solvent (Scheme 1).
Sch em e 1
(µ-C4H4Br)[Rh(C4H4BR)]2
+ I2 f
1a ,b
1/4[Rh(µ3-I)(C4H4BR)]4 RhI(C4H4BR)2
+
2a ,b
3a ,b
Resu lts a n d Discu ssion
The two types of products 2 and 3 could not be
separated by standard methods. When CO is bubbled
through the reaction mixture, the heterocubanes 2
undergo a fast degradation (0.5 min, 20 °C) to form
labile and more soluble carbonyl derivatives, the dicar-
bonyl RhI(CO)2(C4H4BR) and dinuclear species [Rh(µ-
I)(CO)(C4H4BR)]2.5 The bis(ligand) complexes 3 react
with CO at ambient temperature with noticeable rates.
At elevated temperature they decompose quantitatively
(80 °C, 5-7 h); one of the two borole ligands is destroyed,
and the formally remaining complex fragments are
stabilized in the form of the same carbonyl species
Syn th eses. The triple-decker complexes 1a ,b readily
react with elemental iodine. Two types of products 2
and 3 are obtained if toluene or CH2Cl2 is used as
solvent at ambient temperature. The stoichiometry of
X Abstract published in Advance ACS Abstracts, September 1, 1997.
(1) Part 21: Herberich, G. E.; Wagner, T.; Marx, H.-W. J . Orga-
nomet. Chem. 1995, 502, 67; 1996, 513, 287.
(2) Eckenrath, H. J . Doctoral Dissertation, Technische Hochschule
Aachen, Aachen, Germany, 1997.
(3) Herberich, G. E.; Hessner, B.; Boveleth, W.; Lu¨the, H.; Saive,
R.; Zelenka, L. Angew. Chem., Int. Ed. Engl. 1983, 22, 996; Angew.
Chem. Suppl. 1983, 1503.
(4) Herberich, G. E.; Bu¨schges, U.; Hessner, B.; Lu¨the, H. J .
Organomet. Chem. 1986, 312, 13.
(5) Herberich, G. E.; Eckenrath, H. J .; Englert, U. To be published.
S0276-7333(97)00422-6 CCC: $14.00 © 1997 American Chemical Society

Borole Derivatives
Organometallics, Vol. 16, No. 20, 1997 4293
mentioned above. In the absence of CO the bis(ligand)
complexes 3 decompose less selectively; inter alia the
triple-decker compounds are also formed. Finally, when
solutions of the carbonyl species are heated in a stream
of dinitrogen (110 °C, 4-8 h) a clean decomposition
takes place to produce the heterocubanes 2.
With these observations in mind we were able to
develop two practical variants. Carbonylation of the
reaction mixture at ambient temperature solubilizes the
less soluble complexes 2; the bis(ligand) complexes 3 can
then be isolated by crystallization at lower temperature.
On the other hand, carbonylation at elevated temper-
ature and subsequent thermal decarbonylation cleanly
gives the heterocubanes 2 as the sole complex products
(Scheme 2).
F igu r e 1. Thermal ellipsoid plot (ORTEP)9 of the molecule
2a . Ellipsoids are scaled to 30% probability.
Sch em e 2
i-iii
(µ-C4H4BR)[Rh(C4H4BR)]2
+ I2 ssf
1a ,b
1/2[Rh(µ3-I)(C4H4BR)]4
2a ,b
i, toluene 20 °C, 2 h; ii, CO (1 bar), 80 °C, 5-7 h;
iii, N2 stream, 110 °C, 4-8 h
An exploratory investigation of 1a by cyclic voltam-
metry revealed a reversible reduction at -1.44 V vs SCE
in DME and a fully irreversible oxidation at 1.19 V in
CH2Cl2. Considering the high anodic potential for the
oxidation, the ready degradation by iodine may seem
surprising. The same apparently paradoxical situation
is found in the chemistry of Fe(CO)5.6 On the basis of
the redox potential of 1a a one-electron oxidation by
iodine is impossible. However, the basicity of the d8
metal center favors an electrophilic attack at one Rh
center. Thus we may speculate that a charge transfer
adduct 1a ‚I2 and an ionic species [(1a )I]I (4) are likely
intermediates. From earlier work we know (i) that
triple-decker complexes with central borole ligands are
prone to nucleophilic degradation,4,7 and (ii) that cat-
ionic species are particularly reactive in this respect.8
Thus we expect that the cation of 4 should readily
undergo nucleophilic degradation by iodide; this degra-
dation would produce 3 and, formally, the monomer of
2.
F igu r e 2. Thermal ellipsoid plot (ORTEP)9 of the molecule
2b. Ellipsoids are scaled to 30% probability.
Ta ble 1. Selected In ter a tom ic Dista n ces a n d Bon d
An gles for 2a
(a) Bond Distances (pm)
I1-Rh1
I1-Rh1′
I1-Rh2
275.52(6)
273.56(6)
277.41(5)
I2-Rh1
I2-Rh2
I2-Rh2′
276.07(5)
272.80(5)
276.20(5)
Rh1-C1
Rh1-C2
Rh1-C3
Rh1-C4
Rh1-B1
217.1(6)
212.9(6)
209.5(6)
215.4(6)
229.8(7)
Rh2-C5
Rh2-C6
Rh2-C7
Rh2-C8
Rh2-B2
214.2(6)
211.4(6)
212.3(6)
216.5(5)
233.3(6)
(b) Bond Angles (deg)
Rh1-I1-Rh1′
Rh1-I1-Rh2
Rh1′-I1-Rh2
92.47(2)
93.61(2)
91.27(2)
Rh1-I2-Rh2
Rh1-I2-Rh2′
Rh2-I2-Rh2′
94.52(2)
91.00(2)
90.44(1)
I1-Rh1-I1′
I1-Rh1-I2
I1′-Rh1-I2
87.43(2)
85.79(2)
89.10(2)
I1-Rh2-I2
I1-Rh2-I2′
I2-Rh2-I2′
86.05(2)
88.29(2)
89.41(1)
Cr ysta l Str u ctu r es of th e Heter ocu ba n es 2a a n d
2b. Both complexes 2a ,b display a heterocubane struc-
ture with a Rh4I4 core (Figures 1 and 2, Tables 1 and
2). Complex 2a crystallizes in space group C2/c and
compound 2b in Pbca. The molecules of 2a possess
crystallographic C2 symmetry.
(6) Shriver, D. F.; Whitmire, K. H. In Comprehensive Organometallic
Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon
Press: Oxford, U.K., 1982; Vol. 4, p 250.
(7) (a) Herberich, G. E.; Bu¨schges, U. Chem. Ber. 1989, 122, 615.
(b) Herberich, G. E.; Hessner, B.; Saive, R. J . Organomet. Chem. 1987,
319, 9.
The Rh-I bond lengths average 275.8 pm [range:
270.49(9)-281.1(1) pm]. This is slightly longer than in
the many known structures with µ2-iodo ligands;10
structures with µ3-iodo ligands are apparently unknown
for rhodium. The angles centered at rhodium average
87.7° [range: 85.11(3)-91.70(3)°], and those centered
(8) Herberich, G. E.; Dunne, B. J .; Hessner, B. Angew. Chem., Int.
Ed. Engl. 1989, 28, 737.

4294 Organometallics, Vol. 16, No. 20, 1997
Herberich et al.
Ta ble 2. Selected In ter a tom ic Dista n ces a n d Bon d
An gles for 2b
(a) Bond Distances (pm)
I1-Rh1
I1-Rh2
I1-Rh4
I3-Rh2
I3-Rh3
I3-Rh4
276.5(1)
276.2(1)
274.8(1)
277.7(1)
273.8(1)
276.29(9)
I2-Rh1
I2-Rh2
I2-Rh3
I4-Rh1
I4-Rh3
I4-Rh4
270.49(9)
275.8(1)
276.8(1)
281.1(1)
279.85(9)
276.54(9)
(b) Bond Angles (deg)
Rh1-I1-Rh2
Rh1-I1-Rh4
Rh2-I1-Rh4
Rh2-I3-Rh3
Rh2-I3-Rh4
Rh3-I3-Rh4
87.74(3)
94.57(3)
93.60(3)
91.71(3)
92.93(3)
92.26(3)
Rh1-I2-Rh2
89.03(3)
95.93(3)
91.47(3)
92.88(3)
93.15(3)
90.91(3)
Rh1-I2-Rh3
Rh2-I2-Rh3
Rh1-I4-Rh3
Rh1-I4-Rh4
Rh3-I4-Rh4
I1-Rh1-I2
I1-Rh1-I4
I2-Rh1-I4
I2-Rh3-I3
I2-Rh3-I4
I3-Rh3-I4
91.70(3)
85.53(3)
86.05(3)
88.67(3)
85.11(3)
88.06(3)
I1-Rh2-I2
I1-Rh2-I3
I2-Rh2-I3
I1-Rh4-I3
I1-Rh4-I4
I3-Rh4-I4
90.63(3)
86.45(3)
88.06(3)
87.01(3)
86.75(3)
88.23(3)
1
F igu r e 3. Low-temperature H NMR signals (500 MHz,
CD2Cl2, -80 °C) of a 2a /2b mixture (1.20/1) in the borole
region.
at iodine average 92.2° [range: 87.74(3)-95.93(3)°]. The
cubane faces are almost perfectly planar (the angle sums
deviating from 360° by at most 0.9°).
The structural pattern of the borole ligands is similar
to that established in earlier structural work, as, e.g.,
in RhCl(C4H4BPh)(PPh3)2,11 Fe(CO)3(C4H4BPh),12 and
(µ-C4H4BMe)[Co(C4H4BMe)]2,7b and will not be dis-
cussed in detail. The distance of the Rh atoms to the
best borole planes ranges from 177.4 to 178.3 pm while
the Rh-B bond lengths average 232.5 pm, in agreement
with η5-coordination of the borole ligand.
The observation of a heterocubane structure for 2a ,b
may seem surprising at first glance. The related
complexes of the types [RhX(alkene)2]2 and [RhX-
(diene)]2 (X ) halogen) are dinuclear square planar
complexes of a predominantly d8 metal center.13 The
tetrafluoroethene complex [RhF(C2H4)(C2F4)]x is an
exception; it shows a “dimer-of-dimer” structure in the
crystal but is a dimer in solution.14 As a consequence
of the very strong back-bonding ability of the C2F4
ligand, the metal assumes more d6 character and the
18e configuration becomes more favorable. In the borole
complexes 2a ,b the strong back-bonding ability of the
borole ligand15 results in pronounced metal-ligand
orbital mixing,16 and a description as d8 complexes is
no longer appropriate. Thus, the compounds 2a ,b seem
more akin to complexes such as [Cp*RuCl]4,17 to which
they are isoelectronic in a broad sense.
Con stitu tion of 2a a n d 2b in Solu tion . The
heterocubanes 2a ,b readily undergo degradation reac-
tions with a wide variety of Lewis bases such as, e.g.,
CO, acetonitrile, pyridine, and phosphines.2,5 The car-
bonylation is reversible, and solutions of 2a ,b in aceto-
nitrile regenerate the heterocubanes upon removal of
the solvent in a vacuum.
Osmometric determinations of the molar mass of 2a
in CH2Cl2 give values somewhat below the value
calculated for the heterocubane structure. These mea-
surements confirm that 2a essentially retains its tet-
ranuclear nature in CH2Cl2 solution. On the other
hand, they may be taken as a hint that 2a does undergo
a dissociation in solution, presumably into the dinuclear
species. 1H NMR spectra in CDCl3 taken at ambient
temperature on an 80 MHz spectrometer display one
set of sharp signals consisting of the typical AA′BB′
pattern of a coordinated borole ligand and the signals
of the group R. The same spectra taken on a 500 MHz
spectrometer show broad pseudosinglets in the region
of the AA′BB′ signals. The signals sharpen at higher
temperature (d8-toluene, 80 °C) and also upon cooling
down to -80 °C.
To get more insight we looked at mixtures of 2a and
2b and measured their 1H NMR spectra in CD2Cl2.
Spectra taken at room temperature and at 80 MHz
display only two ligand patterns corresponding to a
1-methyl- and a 1-phenylborole ligand; again, the
spectra are blurred when measured at 500 MHz. At
-80 °C, low-temperature limiting spectra with several
different borole ligand patterns are seen, two of which
are those of 2a and 2b (Figure 3). The spectra could be
simulated with the assumption that five species Rh4(µ3-
I)4(C4H4BMe)4-x(C4H4BPh)x (with x ) 0, 1, ..., 4) are
present, with relative concentrations which correspond
to statistical weights of the five species. The essential
result is the proof that heterocubanes 2 exchange
fragments RhI(C4H4BR) at rates which are fast with
respect to the NMR time scale under the common
experimental conditions.
(9) J ohnson, C. K. Oak Ridge National Laboratory ORNL-5138
(Third Revision, 1976).
(10) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson,
D. G.; Taylor, R. J . Chem. Soc., Dalton Trans. 1989, S1.
(11) Herberich, G. E.; Boveleth, W.; Hessner, B.; Hostalek, M.;
Ko¨ffer, D. P. J .; Negele, M. J . Organomet. Chem. 1987, 319, 311.
(12) Herberich, G. E.; Boveleth, W.; Hessner, B.; Ko¨ffer, D. P. J .;
Negele, M.; Saive, R. J . Organomet. Chem. 1986, 308, 153.
(13) Hughes, R. P. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press;
Oxford, 1982; Vol. 5, p 277, especially p 418, 445, 447, and 456.
(14) Burch, R. R.; Harlow, R. L.; Ittel, S. D. Organometallics 1987,
6, 982.
(15) Herberich, G. E.; Carstensen, T.; Ko¨ffer, D. P. J .; Klaff, N.;
Boese, R.; Hyla-Kryspin, I.; Gleiter, R.; Stephan, M.; Meth, H.; Zenneck,
U. Organometallics 1994, 13, 619 and literature cited therein.
(16) (a) Hyla-Kryspin, I.; Gleiter, R.; Herberich, G. E.; Be´nard, M.
Organometallics 1994, 13, 1795. (b) Gleiter, R.; Hyla-Kryspin, I.;
Herberich, G. E. J . Organomet. Chem. 1994, 478, 95.
(17) Fagan, P. J .; Mahoney, P. S.; Calabrese, J . C.; Williams, I. D.
Organometallics 1990, 9, 1843.
Cr ysta l Str u ctu r e of th e Bis(liga n d ) Com p lex 3b.
Complex 3b crystallizes in the monoclinic space group
(18) Spek, A. L. Acta Crystallogr. 1990, A46, C34.

Borole Derivatives
Organometallics, Vol. 16, No. 20, 1997 4295
F igu r e 4. Thermal ellipsoid plot (PLATON)18 of the
molecule Cs-3b. Ellipsoids are scaled to 30% probability.
F igu r e 5. Space-filling representation (PLATON)18 of the
molecule Cs-3b.18
Ta ble 3. Selected In ter a tom ic Dista n ces a n d Bon d
An gles for 3b
diene plane C4,C4′,C5,C5′ amount to 27.2(7) pm for B2
and to 183.7(1) pm for the Rh atom. Thus the bonding
situation is approaching a tetrahapto-coordination of the
borole ligand. The orientation of this ring brings the
boron atom B2 into contact with the iodine atom. The
distance I‚‚‚B2 [295.3(6) pm] is much larger than typical
B-I bond lengths [BI3‚PMe3: 223.7(9) and 227.2(16)
pm20] but also much shorter than a nonbonding distance
[>360 pm].21 The boron center is no longer trigonal
planar. The methyl group is bent away from the metal,
and the angle between the plane C5,B2,C5′ and the
bond vector B2-C6 amounts to 10.5(2)°. The resulting
distance I‚‚‚C of the B-Me group [335.0(7) pm] corre-
sponds to an at most very weak repulsion. Finally, the
C-C distances in the ring [C4-C4′ 143.4(8), C4-C5
139.9(5) pm] show again a diene-like pattern with
moderate back-bonding.
Overall, two effects seem to be operating here, pro-
nounced ligand-ligand repulsions and a weak bonding
interaction between the boron center B2 and the iodo
ligand. However, the origin of the slight pyramidaliza-
tion at B2 remains unclear since both the I‚‚‚B2 interac-
tion and the distance I‚‚‚C of the B2-Me group are in
the borderline range.
(a) Bond Distances (pm)
I-Rh
273.51(5)
217.2(3)
224.8(4)
236.7(6)
I-B2
295.3(6)
216.0(3)
223.5(4)
252.8(6)
Rh-C1
Rh-C2
Rh-B1
Rh-C4
Rh-C5
Rh-B2
C1-C1′
C2-B1
C4-C4′
C5-B2
146.2(8)
155.3(6)
143.4(8)
156.3(6)
C1-C2
C3-B1
C4-C5
C6-B2
138.7(5)
155.8(9)
139.9(5)
155.6(9)
(b) Bond Angles (deg)
C1′-C1-C2
C1-C2-B1
C2-B1-C2′
C2-B1-C3
109.7(2)
109.7(4)
101.2(5)
129.4(2)
C4′-C4-C5
110.2(2)
108.3(4)
100.3(5)
129.1(2)
C4-C5-B2
C5-B2-C5′
C5-B2-C6
C2/m. The molecule of 3b possesses a bent sandwich
structure with crystallographic Cs symmetry (Figure 4,
Table 3). The tight packing of the ligands around the
Rh center is illustrated by a space-filling representation
(Figure 5). The Rh-I bond length amounts to 273.51(5)
pm and lies in the expected range.10 One of the two
borole ligands shows structural features similar to those
found in RhCl(PPh3)2(C4H4BPh).11 This ring is planar
[plane C1,C1′,C2,C2′,B1; largest vertical displace-
ment: 1.5(6) pm at B1]. The boron atom B1 is slightly
bent away from the metal [vertical distance to plane
C1,C1′,C2,C2′: 4.2(6) pm], yet B1 is a trigonal planar
center [angle sum 360.0(5)°]. The metal-to-ring bonding
shows a vertical distance of the Rh atom to the best
borole plane of 185.8(1) pm, a rather large slip distortion
of 13.7(3) pm,19 and a Rh-B1 distance of 236.7(6) pm
[cf. Rh-B 232.5 pm (av) for 2a and 2b; cf. also RhCl-
(PPh3)2(C4H4BPh):11 slip distortion 11.5 pm, Rh-B
240.0(5) pm]. The C-C bond lengths in the ring [C1-
C1′ 146.2(8), C1-C2 138.7(5) pm] show a diene-like
pattern, indicating rather moderate back-bonding.
Str u ctu r es of 3a ,b in Solu tion . The 1H and 13C
NMR spectra of 3a ,b at ambient temperature display
only one borole pattern, and some atoms show signal
broadening which cannot be caused by the quadrupole
moment of a neighboring boron atom [3a , 3-/4-H, C-3,4;
3b, C-3,4]. We therefore measured NMR spectra at
temperatures down to -90 °C with the aim to unravel
the dynamic behavior of the complexes 3.
1
The low-temperature limiting H NMR spectrum of
3b shows six multiplets in the region of the borole rings,
four of equal intensity and a pair of somewhat lower
intensity (Figure 6). With the help of (1H,1H)-COSY
spectra (taken at -90 °C) these multiplets can be
grouped into three AA′BB′ patterns; the presence of an
ABCD spin system can be excluded unambiguously. The
two AA′BB′ systems of equal intensity are assigned to
the isomer Cs-3b found in the crystal while the third
one is assigned to an isomer C2v-3b. Since the borole
The second ring shows surprising structural features.
The folding angle between the two planes C5,B2,C5′ and
C5,C4,C4′,C5′ of 15.8(9)° is exceptionally large, resulting
in rather weakened bonding between the Rh atom and
B2 [Rh-B2 252.8(6) pm]; the vertical distances to the
(19) The slip distortion is the distance between the geometric center
of the C4B ring and the projection of the metal atom onto the C4B best
plane.
(20) Black, D. L.; Taylor, R. C. Acta Crystallogr. 1975, B31, 1116.
(21) van der Waals radii: Bondi, A. J . Phys. Chem. 1964, 68, 441.

4296 Organometallics, Vol. 16, No. 20, 1997
Herberich et al.
less symmetric Cs-3b is entropically favored and pre-
dominates in the entire temperature range that is
accessible experimentally.
1
When a 500 MHz H NMR spectrum of the phenyl
compound 3a is measured at -90 °C, the low-temper-
ature limiting spectrum for Cs-3a with resolved fine
structure is observed while the borole signals for C2v-
3a are seen as a pair of weak broad pseudosinglets. In
the 13C{1H} NMR spectrum (126 MHz, -90 °C) only the
signals of Cs-3a could be observed. The equilibrium
constant is about K ≈ 0.1, corresponding to ∆RG183
)
3.5 kJ mol-1
.
The room temperature 11B{1H} NMR spectrum of 3b
displays a single resonance for the equilibrium mixture
at δ 33 ppm while the low-temperature limiting spec-
trum (160 MHz, -80 °C) shows a more intense signal
at δ 37 and a weaker signal at δ 28 ppm. The chemical
shift of the weaker signal corresponds to a normal
1-methylborole ligand in a situation with rather moder-
ate back-bonding,22 and is therefore assigned to the
planar borole ring of Cs-3b. The chemical shift of the
stronger signal is very unusual and indicates consider-
able deshielding at the boron; it belongs to the folded
borole ring of Cs-3b and, because of its higher intensity,
also to the borole rings of C2v-3b. Thus, we have to
conclude that both borole rings in C2v-3b are of the
folded type with weakened Rh-B bonding. In the case
of 3a no splitting of the signal was seen at -80 °C.
1
F igu r e 6. Low-temperature H NMR signals (500 MHz,
CD2Cl2, -90 °C) of 3b in the borole region.
Sch em e 3a
a
Cs-3b h C2v-3b.
Ta ble 4. Equ ilibr iu m Con sta n ts a n d F r ee En er gies
for th e Equ ilibr iu m Cs-3b h C2v-3b
We next turn to the mechanism of the forward and
reverse isomerization reactions. Since the coalescence
temperatures observed are concentration-independent,
the isomerization processes are first-order reactions.
Line shape analysis with the help of the program
DNMR24 was used to estimate isomerization rates and
to calculate activation parameters for both isomers of
3b and for the predominant isomer Cs-3a in the phenyl
case (Table 5). The negative activation entropies ob-
tained demonstrate the nondissociative and conse-
quently intramolecular nature of the reactions observed.
We conclude that the isomerization is brought about by
ring-ring rotation.
T (K)
C2v-3b/Cs-3ba
K
∆RG (kJ mol-1
)
183
193
203
213
223
30/70
29/71
28/72
27/73
26/74
0.43
0.41
0.39
0.38
0.35
1.284
1.430
1.589
1.714
1.946
a
The concentration ratios rely on the assumption that the
intensities of corresponding signals in the 13C{1H} NMR spectrum
are comparable.
Ta ble 5. En er getics of th e Equ ilibr iu m Cs-3 h
C2v-3
(a) Thermodynamic Equilibrium Parameters for 3b
∆RH ) (-1.7 ( 0.2) kJ mol-1
∆RS ) (-16 ( 1) J mol-1 K-1
This mechanism can be verified directly by means of
the TOCSY NMR spectra25 of 3b. The spectra were
recorded at -80 °C with a mixing time of 150 ms. We
describe selective excitations of borole protons in â-posi-
tions to the boron (i.e., 3-/4-H) and the transfer of
magnetization to â-positions in the other borole rings;
R-protons (i.e., 2-/5-H) and transfer of magnetization to
these positions can remain out of consideration. Selec-
tive excitation of the â-protons of one borole in Cs-3b
produces a strong signal for these protons, a less
intensive signal for the â-protons in C2v-3b, and an even
less intensive signal for the â-protons of the second
borole in Cs-3b. Thus the interconversion of the two
borole ligands of Cs-3b takes place through the inter-
mediacy of isomer C2v-3b. On the other hand, if the
â-protons of C2v-3b are excited selectively, then both
â-proton signals of Cs-3b appear with equal intensity.
(b) Activation Parameters for Isomerization of Cs-3b
∆Hq ) (34.0 ( 1.9) kJ mol-1
1
∆Sq ) (-47 ( 9) J mol-1 K-1
1
(c) Activation Parameters for Isomerization of C2v-3b
∆Hq ) (35.6 ( 2.0) kJ mol-1
-1
∆Sq ) (-32 ( 9) J mol-1 K-1
-1
(d) Activation Parameters for Isomerization of Cs-3a
∆Hq ) (33.5 ( 1.5) kJ mol-1
∆Sq ) (-52 ( 7) J mol-1 K-1
patterns coalesce at higher temperature and sharpen
to the single pattern seen at ambient temperature, the
two isomers are in a mobile equilibrium (Scheme 3).
Note that isomer C2v-3b is shown as a syn-eclipsed
conformer with both BMe groups close to the Rh-I bond;
the alternative anti-eclipsed conformer would seem too
congested sterically to be capable of existence.
Signal intensities of corresponding signals in the 13C-
{1H} NMR spectra were used to estimate equilibrium
constants (Table 4) and to calculate thermodynamic
parameters for the isomerization of Scheme 3 (Tables
4 and 5). Thus we obtained ∆RH ) (-1.7 ( 0.2) kJ
mol-1 and ∆RS ) (-16 ( 1) J mol-1 K-1. This result
shows that the lower-energy isomer is C2v-3b while the
23
(22) Cf., e.g., δ 28.5 ppm for Ni(C4H4BMe)2 and also δ 19.6 for
CpRh(C4H4BMe)4 and 22.8 ppm for Ru(CO)3(C4H4BMe).11
(23) Herberich, G. E.; Negele, M. J . Organomet. Chem. 1988, 350,
81.
(24) Haegele, G.; Lenzen, T.; Fuhler, R. Comput. Chem. 1995, 19,
277.
(25) Braunschweiler, L.; Ernst, R. R. J . Magn. Reson. 1983, 53, 521.

Borole Derivatives
Organometallics, Vol. 16, No. 20, 1997 4297
then dissolved in hot acetonitrile (400 mL), the solution was
filtered, and the solvent was removed in a vacuum. The
microcrystals so obtained were collected on a frit, washed with
acetone, and dried in a vacuum to give 2a (5.31 g, 85%) as
dark red microcrystals: mp 223 °C dec; in solution somewhat
air-sensitive, soluble in acetonitrile, moderately soluble in CH2-
Cl2, CHCl3, THF, and toluene, insoluble in acetone, ether, and
hexane. Anal. Calcd for C40H36B4I4Rh4: C, 32.48; H, 2.45.
Barriers to internal rotation of borole ligands have
previously been observed.23,26 We note that the present
case is the first nondegenerate example that could be
analyzed.
Con clu d in g R em a r k s. In this paper we have
concentrated on the oxidative degradation reaction of
the triple-decker complexes 1 and the identification and
the structures of the products 2 and 3. For the
preparation of the heterocubanes 2 we have devised an
optimized procedure which circumvents the separation
of the products 2 and 3 and gives the heterocubanes 2
as the sole product in high yields (80-85%). In the next
paper of this series we shall describe reactions of the
heterocubanes 2 with a wide variety of Lewis bases. We
shall then be able to comment in detail on the role of
CO, acetonitrile, and pyridine in the oxidative degrada-
tion reaction.
Found: C, 32.35; H, 2.50. Molar mass: Mcalc 1479.21 g mol-1
;
in acetonitrile Mobs [molarity (g/L)] 391 (5.548), 445 (11.77),
464 g/mol (13.05); in CH2Cl2 Mobs [molarity (g/L)] 1351 (3.475),
1430 g/mol (5.313). SIMS (DMBA): negative ions, m/ z (Irel
)
1110 (1, [Rh3I3(C4H4BPh)3]-), 740 (8, [Rh2I2(C4H4BPh)2]-), 497
(70, [RhI2(C4H4BPh)]-), 357 (26, [RhI2]-), 127 (100, I-).
1
2a : H NMR (500 MHz, CDCl3) δ 7.63 (m, 2 Ho), 7.37 (m,
Hp), 7.35 (m, 2 Hm), borole 5.03 (br, 3-/4-H), 4.18 (br, 2-/5-H),
and borole signals at 80 MHz 5.03 (m, N ) 5.9 Hz, 3-/4-H),
4.18 (m, N ) 5.2 Hz, 2-/5-H). 13C{1H} NMR (126 MHz, CDCl3,
o
20 C) δ 135.91 (Co), 129.14 (Cp), 127.62 (Cm), borole 89.22 (C-
3,4), 74 (br, C-2,5); 11B{1H} NMR (CDCl3) δ 20.
Exp er im en ta l Section
Syn th esis of [Rh (µ3-I)(C4H4BMe)]4 (2b). A solution of
complex 1b4 (310 mg, 0.705 mmol) and elemental iodine (179
mg, 0.705 mmol) in toluene (100 mL) was stirred at 20 °C for
2 h. Carbonylation (5 h) and decarbonylation (4 h) until the
CO band at 2059 cm-1 had disappeared, followed by removal
of the volatiles, gave a raw product. This was dissolved in
CH2Cl2 (80 mL), and the solution was filtered, concentrated
to a small volume (3 mL), and stored at -30 °C for 16 h. The
dark red microcrystals formed were collected, washed wih
acetone (2 × 2 mL), and dried in a vacuum to give 2b (350
mg, 81%): mp 168 °C dec; in solution somewhat air-sensitive,
soluble in acetonitrile, CH2Cl2, CHCl3, THF, and toluene,
insoluble in acetone, ether, and hexane. Anal. Calcd for
Gen er a l P r oced u r es. Reactions were carried out under
an atmosphere of dinitrogen by means of conventional Schlenk
techniques. Hexane was distilled from potassium, CH2Cl2
from CaH2, and toluene from sodium. Acetone was heated
under reflux with boric acid anhydride for 24 h and distilled
under dinitrogen. Acetonitrile was filtered through a column
with activated alumina and distilled under dinitrogen. Carbon
monoxide was dried with concentrated sulfuric acid. NMR
spectra were recorded on a Varian Unity 500 spectrometer (1H,
500 MHz; 13C{1H}, 125.7 MHz; 11B{1H}, 160.4 MHz) and a
Bruker WP-80 PFT (1H, 80 MHz) spectrometer. If not stated
otherwise, chemical shifts were measured at ambient temper-
ature and are relative to internal TMS for 1H and 13C and
relative to BF3‚Et2O as external reference for 11B. The
B-phenyl compounds 2a and 3a are poorly soluble in nonco-
ordinating solvents such as toluene or CH2Cl2; this was
frequently a limiting factor in the recording of NMR spectra.
Secondary ion mass spectra (SIMS) were recorded on a
Finnigan MAT-95 spectrometer. Elemental analyses and
determinations of molecular mass were performed by Mik-
roanalytisches Labor Pascher, D-53424 Remagen-Bandorf,
Germany. Melting points were determined in sealed capil-
C
20H28B4I4Rh4: C, 19.52; H, 2.29. Found: C, 19.52; H, 2.23.
SIMS (K/T): negative ions, m/ z (Irel) 743 (13, [Rh2I3(C4H4-
BMe)2]-), 616 (12, [RhI(C4H4BMe)]2-), 435 (62, [RhI2(C4H4-
BMe)]-), 127 (100, I-).
2b: 1H NMR (500 MHz, CDCl3) δ 5.09 (br, 3-/4-H), 3.66 (br,
2-/5-H), 0.50 (s, Me), and borole signals at 80 MHz 5.10 (m, N
) 5.2 Hz, 3-/4-H), 3.67 (m, N ) 5.2 Hz, 2-/5-H). 13C{1H} NMR
(126 MHz, CDCl3) δ 88.44 [d, 1J (Rh-C) ) 8.8 Hz, C-3,4], 73.51
(br, C-2,5), -1.91 (br, Me). 11B{1H} NMR (CDCl3) δ 24.
Syn th esis of Rh I(C4H4BP h )2 (3a ). Complex 1a 3,4 (520 mg,
0.831 mmol) was dissolved in CH2Cl2 (50 mL), and elemental
iodine (211 mg, 0.831 mmol) was added. The reaction mixture
was stirred for 2 h at ambient temperature. Dry CO was then
passed through the flask for 15 min. The orange solution was
concentrated to 5 mL under reduced pressure and then stored
at -30 °C for 16 h to give orange crystals. After removal of
the mother liquor, dissolution in CH2Cl2 (30 mL), and filtra-
tion, the solution was again concentrated to 3 mL under
reduced pressure and stored at -30 °C for 16 h. The crystals
were collected on a frit, washed with acetone (3 × 2 mL), and
dried in a vacuum to give 3a (270 mg, 64%) as orange
needles: mp 168 °C dec; in solution somewhat sensitive,
moderately soluble in CH2Cl2, CHCl3, and toluene, insoluble
in acetone, acetonitrile, ether, and hexane. Anal. Calcd for
laries on
uncorrected.
a Bu¨chi 510 melting point apparatus and are
CV Da ta of 1a . Cyclic voltammetry was carried out using
an EG & G 175 voltage scan generator and an EG & G 173
potentiostat. For the measurements a conventional three-
electrode cell with a platinum-inlay working electrode, a
platinum sheet counter electrode, and a saturated calomel
(SCE) reference electrode was used. Solutions were ca. 10-3
M in electroactive species and 0.1 M in tetrabutylammonium
hexafluorophosphate (TBAH) as supporting electrolyte. FeCp2
a
served as internal standard. CV (CH2Cl2, 100 mV/s) Ep +1.19
V, irreversible oxidation; (DME, 100 mV/s) E1/2 -1.44 V, ia/ic
0.99, reversible reduction.
Syn th esis of [Rh (µ3-I)(C4H4BP h )]4 (2a ). Elemental io-
dine (2.19 g, 8.629 mmol) was added to a solution of 1a 3,4 (5.40
g, 8.63 mmol) in toluene (700 mL). The dark red reaction
mixture was stirred at 20 °C for 2 h. Then a slow stream of
dry CO was passed through the flask while the temperature
was kept at 80 °C for 7 h. Then the orange solution was heated
under gentle reflux while a slow stream of N2 was passed
through the flask until the CO band at 2055 cm-1 had
disappeared. After cooling to 20 °C the volatiles were removed
under reduced pressure. The raw product was suspended in
acetone (30 mL), transferred to a frit, and washed with acetone
until the color of the filtrate was of a pale red. The solid was
C
20H18B2IRh: C, 47.12; H, 3.56. Found: C, 47.08; H, 3.39.
SIMS (NBA): positive ions, m/z (Irel) 626 (<1, [Rh2(C4H4BPh)3]-),
383 (100, [Rh(C4H4BPh)2]+).
3a : 1H NMR (500 MHz, CD2Cl2) δ 7.70 (m, 2 Ho), 7.41 (m,
2 Hm), 7.33 (m, Hp), borole 5.88 (br, 3-/4-H), 4.70 (m, 2-/5-H),
and borole signals at 80 MHz 5.88 (m, N ) 7.3 Hz, 3-/4-H),
4.71 (m, N ) 6.8 Hz, 2-/5-H); 13C{1H} NMR (126 MHz, CD2-
Cl2) δ 136.36 (Co), 130.54 (Cp), 128.28 (Cm), borole 103.74 (br,
C-3,4), 81.47 (br, C-2,5). 11B{1H} NMR (CD2Cl2) δ 28.
VT-NMR sp ectr a of 3a : 1H NMR (500 MHz, CD2Cl2, -90
oC) Cs isomer (90%) δ 7.84 (m, 2 Ho), 7.53 (m, 2 Ho), 7.43-7.49
(m, 2 Hm + Hp), 7.33 (m, 2 Hm), 7.20 (m, Hp), 6.29 (m, 3-/4-H)
and 4.93 (m, 2-/5-H) for first borole, 5.33 (m, 3-/4-H) and 4.45
(m, 2-/5-H) for second borole; C2v isomer (10%) δ 6.02 (s br,
3-/4-H), 4.52 (s br, 2-/5-H), Ph protons hidden. 13C{1H} NMR
(26) (a) Herberich, G. E.; Negele, M.; Ohst, H. Chem. Ber. 1991, 124,
25. (b) Herberich, G. E.; Englert, U.; Hostalek, M.; Laven, R. Chem.
Ber. 1991, 124, 17.

4298 Organometallics, Vol. 16, No. 20, 1997
Ta ble 6. Cr ysta l Da ta , Da ta Collection P a r a m eter s, a n d Con ver gen ce Resu lts
Herberich et al.
2a
2b
3b
formula
system
space group (no.)
a, pm
b, pm
c, pm
C40H36B4I4Rh4
monoclinic
C2/c (15)
1572.6(9)
1387.4(5)
1956.1(5)
105.56(4)
4.112(3)
4
C20H28B4I4Rh4
orthorhombic
Pbca (61)
1692.8(5)
1706.2(2)
2015.6(3)
2015.6(3)
5.822(2)
8
C10H14B2IRh
monoclinic
C2/m (12)
993.6(4)
891.4(3)
1414.0(3)
104.55(3)
1.2123(7)
4
â, deg
V, nm3
Z
d
calc, g cm-3
2.371
2.809
2.112
T, K
203
4521
71.073 (Mo KR)
28
0.19 × 0.14 × 0.12
numerical
0.442
0.622
6143
293
33.65
56.087 (Ag KR)
24
0.25 × 0.20 × 0.08
empirical
rel 0.606
rel 0.999
10 799
203
38.65
71.073 (Mo KR)
28
0.35 × 0.26 × 0.05
numerical
0.395
0.899
5820
µ, cm-1
λ, pm (radiation)
θ
max, deg
cryst dimens, mm3
abs cor
min trans
max trans
no. of reflns
no. of indep obs rflns I > 1.0σ(I)
3712
236
0.035
0.036
5555
290
0.070
0.055
1311
104
0.028
0.032
no. of vars
Ra
Rw
b
resd el dens, 10-6 e pm-3
0.8
1.8 (close to Rh)
1.1 (close to I)
b
2
aR ) ∑||Fo| - |Fc||/∑|Fo|. Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2
.
(126 MHz, CDCl3, -90 °C) Cs isomer δ 135.30 and 135.80 (Co),
127.14 and 127.94 (Cm), 129.86 and 130.02 (Cp), borole 100.88
and 106.99 (C-3,4), 75.35 and 85.55 (C-2,5); C2v isomer not
observed. Simulation of line shapes: temperature range 193-
303 K; signals of borole carbons C-3,4 and of phenyl carbons
Co, Cm, and Cp used.
solution of 2a and 2b (1/1) in (E)-1,2-dichloroethene was cooled
to -30 °C. Crystals of 2b were obtained when a saturated
solution in CH2Cl2 was cooled from 20 °C to -30 °C. In the
case of 3b the compound was dissolved in the minimum
amount of hexane at 20 °C; the solution was kept at -30 °C
to give crystals suitable for the structure determination.
The data collection was performed on ENRAF-Nonius CAD4
diffractometers equipped with a graphite monochromator.
Crystal data, data collection parameters, and convergence
results are given in Table 6.
The major source of systematic error in all three structure
determinations stems from absorption: The numerical method
by Gaussian integration27 was used for the crystals of 2a and
3b which could readily be face-indexed whereas an empirical
absorption correction28 was applied in the case of 2b. The
structures were solved by direct methods29 and refined on F
with the local version of the SDP system.30 In the final least
squares refinements non-hydrogen atoms were assigned aniso-
tropic displacement parameters; in the case of 3b hydrogen
atoms were refined isotropically, whereas the hydrogen atoms
were included in riding geometry with fixed displacement
parameters (C-H ) 98 pm, Uiso(H) ) 1.3Uiso(C)) in the
refinement of the structural model for the other compounds.
Syn th esis of Rh I(C4H4BMe)2 (3b). A solution of complex
1b4 (840 mg, 1.911 mmol) and elemental iodine (485 mg, 1.911
mmol) in CH2Cl2 (50 mL) was stirred at ambient temperature
for 2 h. The volatiles were then removed in a vacuum. The
brown residue was extracted with hexane (5 × 20 mL). The
combined solutions were filtered, concentrated to 50 mL, and
then carbonylated (20 °C, 15 min). The resulting orange
solution was concentrated to 10 mL and stored at -30 °C for
16 h to complete the crystallization. The crystals were
collected, washed with hexane (2 × 1 mL), and dried in a
vacuum to give 3b (510 mg, 69%) as orange needles: mp 93
°C dec; in solution somewhat sensitive, very soluble in CH2-
Cl2 and toluene, moderately soluble in ether and hexane. Anal.
Calcd for C10H14B2IRh: C, 31.14; H, 3.66. Found: C, 30.84;
H, 3.69. SIMS (NBA): positive ions, m/ z (Irel) 440 (28,
[Rh2(C4H4BMe)3]+), 386 (100, [RhI(C4H4BMe)2]+), 259 (100,
[Rh(C4H4BMe)2]+).
3b: 1H NMR (500 MHz, CD2Cl2) δ 5.87 (br, 3-/4-H), 4.13
(m, 2-/5-H), 1.05 (s, Me), and borole signals at 80 MHz 5.86
(m, N ) 7.3 Hz, 3-/4-H), 4.14 (m, N ) 6.7 Hz, 2-/5-H); 13C{1H}
NMR (126 MHz, CD2Cl2) δ 101.17 (C-3,4), 83.04 (br, C-2,5),
2.74 (br, Me); 11B{1H} NMR (CD2Cl2) δ 33.
Ack n ow led gm en t. This work was generously sup-
ported by Fonds der Chemischen Industrie.
Su p p or tin g In for m a tion Ava ila ble: Tables of fractional
coordinates of all atoms, anisotropic displacement parameters,
and bond distances and angles for 2a , 2b, and 3b (14 pages).
Ordering information is given on any current masthead page.31
VT-NMR Sp ectr a of 3b: 1H NMR (500 MHz, CD2Cl2, -90
°C) Cs isomer (70%) δ 6.03 (m, 3-/4-H) and 4.34 (m, 2-/5-H) for
first borole, 5.69 (m, 3-/4-H) and 3.89 (m, 2-/5-H) for second
borole, 1.17 (s, Me), 0.61 (s, Me); C2v isomer (30%) δ 5.83 (m,
3-/4-H), 3.93 (m, 2-/5-H), 1.17 (s, 2 Me); 13C{1H} NMR (126
OM970422Q
1
MHz, CDCl3, -90 °C) Cs isomer (70%) δ 106.43 [d, J (Rh-C)
(27) Coppens, P.; Leiserowitz, L.; Rabinovich, D. Acta Crystallogr.
1965, 18, 1035.
(28) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.
1968, A24, 351.
) 6.0 Hz, C-3,4] and 88.06 (C-2,5) for first borole, 99.25 [d,
1J (Rh-C) ) 6.0 Hz, C-3,4] and 77.17 (C-2,5) for second borole,
1
4.40 and -1.38 (2 Me); Cs isomer (10%) δ 94.81 [d, J (Rh-C)
(29) Sheldrick, G. M. SHELXS-86, Program for Crystal Structure
Solution; University of Go¨ttingen: Go¨ttingen, Germany, 1986.
(30) Frenz, B. A. ENRAF-Nonius SDP Version 5.0, Delft, The
Netherlands, 1989, and local programs.
(31) Further details of the crystal structure analyses are available
on request from the Fachinformationszentrum Karlsruhe, Gesellschaft
fu¨r wissenschaftlich-technische Information mbH, D-76344 Eggenstein-
Leopoldshafen, Germany, on quoting the depository numbers CSD-
470290 for 2a , CSD-470291 for 2b, and CSD-470292 for 3b, the names
of the authors, and this journal citation.
) 6.1 Hz, C-3,4], 79.53 (C-2,5), 4.67 (Me). Simulation of line
shapes: temperature range 193-223 K; signals of all carbons
used, 1J (Rh-C) couplings taken into account, quadrupole
broadening of C-2,5 and B-Me neglected. 11B{1H} NMR (CD2-
Cl2, -90 °C): δ 37 (higher intensity) and 28.
Cr ysta l Str u ctu r e Deter m in a tion of 2a , 2b, a n d 3b.
Crystals of 2a can be obtained from hot toluene solutions.
Crystals of better quality were obtained incidentally when a