Cobalt-catalyzed carbonylation of N-alkylbenzaldimines to 'N-alkylphthalimidines' (=2,3-dihydro-1H-isoindol-1-ones) via tandem C-H activation and cyclocarbonylation

Article abstract of DOI:10.1002/hlca.200690167

The reaction of N-alkylbenzaldimines with carbon monoxide (CO) in the presence of cobalt (Co) catalysts resulted in the formation of N-alkylphthalimidines (Table 1). Their formation is proposed to occur by C-H activation of the aryl ring, migratory insertion of the hydride species into the benzaldimine functionality, CO coordination, and insertion into the Co-C bond, followed by reductive elimination of the N-alkylphthalimidine and regeneration of the starting Co species (Scheme 4). Deuterium (²H)-labeling NMR studies are consistent with this mechanism (Scheme 5).

Full text of DOI:10.1002/hlca.200690167

Helvetica Chimica Acta – Vol. 89 (2006)
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Cobalt-Catalyzed Carbonylation of N-Alkylbenzaldimines to
‘N-Alkylphthalimidines’ (=2,3-Dihydro-1H-isoindol-1-ones) via Tandem
CÀHActivation and Cyclocarbonylation
by Jeffrey K. Funk, Hemant Yennawar, and Ayusman Sen*
Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA
(e-mail: asen@psu.edu)
Dedicated to Professor Giambattista Consiglio on the occasion of his 65th birthday
The reaction of N-alkylbenzaldimines with carbon monoxide (CO) in the presence of cobalt (Co)
catalysts resulted in the formation of N-alkylphthalimidines (Table 1). Their formation is proposed to
occur by CÀH activation of the aryl ring, migratory insertion of the hydride species into the benzaldimine
functionality, CO coordination, and insertion into the CoÀC bond, followed by reductive elimination of
the N-alkylphthalimidine and regeneration of the starting Co species (Scheme 4). Deuterium (2H)-label-
ing NMR studies are consistent with this mechanism (Scheme 5).
Introduction. – The synthesis of 1,4-polyketones by the alternating copolymeriza-
tion of alkenes and carbon monoxide (CO) by late-transition-metal complexes has
been extensively studied and is well-established within the field of carbonylation
chemistry [1–4]. However, the extension of this chemistry from C=C bonds of alkenes
to isoelectronic C=N bonds of aldimines for the formation of polypeptides, as shown in
Scheme 1, has proven to be much less successful.
Previous work in our laboratory [5], as well as by others [6–10], has shown that cat-
ionic PdII complexes used in the successful copolymerization of alkenes and CO
undergo sequential insertion of CO, followed by benzaldimine, into a Pd–alkyl bond.
However, the CO group of the resultant amide functionality coordinates much more
strongly to the Pd center than the corresponding ketone from alkene/CO copolymeri-
zation, prohibiting the subsequent insertion of CO and, thereby, terminating the reac-
tion (Scheme 2) [5].
Related to the copolymerization of aldimines and CO, Jia and co-workers [11–14],
and others [15] have shown that cobalt (Co) complexes can successfully copolymerize
Scheme 1. Alternating Copolymerization of Isoelectronic Olefins andImines with CO to Form Polyke-
tones andPolypeptides, Respectively
© 2006 Verlag Helvetica Chimica Acta AG, Zürich

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Helvetica Chimica Acta – Vol. 89 (2006)
II
Scheme 2. Sequential Insertion of CO andBenzaldimine to Form a Stable Pd
Chelate Structure Pro-
hibiting Further Reaction
N-alkylaziridines and CO for the synthesis of poly-b-peptides (Scheme 3). The mecha-
nism of this transformation has recently been investigated in detail (Scheme 3) [16].
Encouraged by these results, and based on previous results from our own laboratory
for the Co-catalyzed synthesis of e-caprolactam and polycaprolactam [17], we thought
that the above system could also be used for the alternating copolymerization of ben-
zaldimines and CO by an analogous mechanism. Herein, we report our initial results in
this area.
Scheme 3. Copolymerization of N-Alkylaziridines with CO to Form Poly-b-peptides, and Proposed
Mechanism

Helvetica Chimica Acta – Vol. 89 (2006)
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Results and Discussion. – 1. Reaction of N-Alkylaldimines with CO. The catalyst
[Co(13CH3C(O))(CO)3(P(o-tol)3)]1) in deuterated 1,4-dioxane was found to produce
AHCTREUNG
not the expected polypeptide, but rather ‘N-alkylphthalimides’ (=2,3-dihydro-1H-iso-
indol-1-ones) as the main products (Table 1). No reaction was observed when either the
Co compound or CO was absent (Entries 5 and 6, resp.). For the reaction involving
1
PhCH=NEt, H-NMR analysis showed the disappearance of the benzaldimine peak
at d(H) 8.29, and the appearance of a new signal at d(H) 4.36 (s). Furthermore, 13C-
NMR spectra taken before and after the reaction also showed the disappearance of
the benzaldimine signals at d(C) 160.4 (N=CH) and 56.3 (NCH2), and the appearance
of new resonances at d(C) 167.7 and 49.5, respectively. The use of 13CO resulted in a
large enhancement of the amide resonance at d(C) 167.7.
Table 1. Reaction of Benzaldimines with CO to Phthalimidines in the presence of the Catalysts
[Co(13CH3C(O))(CO)3(P(o-tol)3] (A) or [Co2(CO)8] (B). All reactions were run in 2 ml of (D8)-1,4-
dioxane, unless noted otherwise.
Entry
Benzaldimine
Conditions
Yield [%]a)
71
1
PhCH=NMe
A (0.02 mmol), aldimine (19 equiv.),
1000 psi CO, 2008, 2 d
2
PhCH=NEt
A (0.02 mmol), aldimine (20 equiv.),
1000 psi CO, 2008, 2 d
71
3
PhCH=NEt
A (0.02 mmol), aldimine (19 equiv.),
65
300 psi 13CO, 2008, 2 d
4b)
5
PhCH=NEt
B (0.06 mmol), aldimine (20 equiv.),
1000 psi CO, 2008, 6 h
No catalyst, aldimine (1 mmol), 2 ml
(1000 psi CO, 2008, 1 d
A (0.02 mmol), aldimine (19 equiv.),
1000 psi N2, 2008, 1 d
A (0.02 mmol), aldimine (21 equiv.),
1000 psi CO, 2008, 1 d
92
PhCH=NEt
n.r.c)
n.r.
62
6
PhCH=NMe
7
PhCH=NPr
8
2,6-Me2C6H3CH=NEt
2,6-F2C6H3CH=NMe
A (0.02 mmol), aldimine (23 equiv.),
1000 psi CO, 2008, 3 d
A (0.02 mmol), aldimine (33 equiv.),
1000 psi CO, 2008, 3 d
n.r.
n.r.
9
1
a) Yields were determined by H-NMR spectroscopy rel. to an external standard of 1,3,5-trioxane in
b
CDCl3. ) In 2 ml of C6
A
At first glance, these new signals are consistent with the formation of an alternating
benzaldimine/CO copolymer. However, further analysis revealed that they were not
due to the copolymer. The HMQC spectrum of the product revealed that the signal
at d(C) 49.5 was correlated with d(H) 4.36. Furthermore, DEPT-135 revealed that
the resonance at d(C) 49.5 was actually derived from a methylene (ÀCH2À) species,
consistent only with the formation of an N-alkylphthalimidine, as corroborated by
1
)
Abbreviation: ‘o-tol’ refers to ortho-tolyl (=2-methylphenyl).

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Helvetica Chimica Acta – Vol. 89 (2006)
IR analysis. IR Spectra taken before and after the reaction showed the disappearance
of the benzaldimine C=N absorption at 1646 cmÀ1, and the appearance of a C=O
absorption at 1694 cmÀ1, corresponding to the formation of a five-membered lactam
ring. An acyclic amide would be expected to resonate at ca. 1645 cmÀ1. The observed
isotopic shift of the product incorporating a 13C=O group (1652 cmÀ1) was in close
agreement with the calculated value (1657 cmÀ1).
Finally, X-ray crystallography of N-methylphthalimidine (Figure), the product
obtained from the reaction of N-(1-phenylmethylidene)methanamine (PhC(H)=
NMe) with CO, complemented the above analyses. Details of the structure determina-
tion and refinement are given in Table 2 and in the Exper. Part.
Figure. X-Ray crystal structure (ORTEP representation) of N-methylphthalimidine formed in the reac-
tion of N-(1-phenylmethylidene)methanamine with CO in the presence of a cobalt catalyst
2. Mechanistic Considerations. A proposed mechanism for the formation of N-
alkylphthalimidines in the presence of the Co complex is shown in Scheme 4. Analo-
gous to that proposed by Jia and co-workers [16], the initial step of the reaction is likely
to be nucleophilic attack of the benzaldimine on the Co–acyl functionality to generate a
cobaltate species. In the case of Jia’s copolymerization of N-alkylaziridines with CO,
ring opening of N-alkylaziridine relieves steric strain and leads to a new Co–alkyl spe-
cies and eventual copolymer formation (see Scheme 3 above). In our system, the nucle-
ophilic attack of the cobaltate anion on the benzaldiminium species is apparently not as
favorable. Instead, the cobaltate anion coordinates to another benzaldimine and under-
goes oxidative addition of the aryl CÀH bond of the benzaldimine, followed by migra-
tory insertion of the C=N bond. This cobalt–amine species then inserts a CO molecule
to form a Co–acyl species that reductively eliminates the N-alkylphthalimidines to
regenerate the starting cobaltate anion.
In support of this mechanism, it has previously been shown that the cobaltate anion
generated in situ in benzene from [Co2(CO)8] is a very effective catalyst for the synthe-
sis of N-alkylphthalimidines [18–20]. Indeed, using [Co2(CO)8] as catalyst precursor in
benzene also resulted in the formation of N-alkylphthalimidines (Table 1, Entry 4). To
probe this mechanism, the deuterated benzaldimines C
A
G
N

Helvetica Chimica Acta – Vol. 89 (2006)
1691
Table 2. Crystallographic Data for N-Methylphthalimidine (see Figure)
Empirical formula
Formula weight [g/mol]
Temperature [K]
Wavelength [Å]
Crystal size [mm]
Crystal habit
C9
147.17
108(2)
0.71073
0.50×0.25×0.20
yellow brick
A
G
Crystal system
monoclinic
Space group
P 21/c
Unit-cell dimensions:
a=9.1655(12) Å a=908
b=9.0746(12) Å b=114.445(2)8
c=9.6316(12) Å g=908
729.28(16)
V [Å3]
Z
4
Dcalc [g/cm3]
1.340
0.088
312
Absorption coefficient [mmÀ1
F(000)
]
Diffractometer
Radiation source
Generator power
Detector distance
Data collection method
q Range for data collection
Index ranges
CCD area detector
fine-focus sealed tube, MoKa
1600 W (50 kV, 32 mA)
5.8 cm
f and w scans
2.44–28.308
À12ꢀhꢀ11, À12ꢀkꢀ11, À12ꢀlꢀ7
ACHTREUNG
A
N
ACHTREUNG
AHCTREUNG
ACHTREUNG
a CD2 unit, whereas 30% showed a CHD unit. In both cases, there was no evidence
for H/D scrambling in the aromatic ring or the Me group. Together, these studies pro-
vide evidence in support of the proposed mechanism, where a CoÀH species generated
by CÀH activation of the aromatic ring inserts into the C=N bond of the benzaldimine.
3. Modified Aldimines for the Attempted Copolymerization with CO. Since the
copolymerization of benzaldimines with CO did not occur because of the alternative
reaction pathway involving the activation of a ring CÀH bond ortho to the aldimine
functionality, two modified benzaldimines with two blocked ortho-positions were
tested. The use of N-[(2,6-dimethylphenyl)methylidene]ethanamine (2,6-Me2C6H3-
A
was due to the steric bulk of the two Me groups preventing the approach of the cobal-
tate anion to the imine functionality, we also tested the corresponding smaller difluoro
congener. However, there was no reaction in this case either (Entry 9).
Conclusions. – The reaction of N-alkylbenzaldimines with CO in the presence of Co
catalysts results not in the expected formation of the desired alternating copolymer, but
in N-alkylphthalimidines arising from CÀH activation of the aromatic ring of the benz-

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Helvetica Chimica Acta – Vol. 89 (2006)
Scheme 4. ProposedMechanism for the Formation of N-Alkylphthalimidines from Benzaldimines and
CO in the Presence of [Co(13CH3C(O))(CO)3(P(o-tol)3)]1)
Scheme 5. Deuterium Distribution in the Reaction of Isotopically-SubstitutedBenzaldimines with CO
in the presence of [Co2(CO)8]

Helvetica Chimica Acta – Vol. 89 (2006)
1693
aldimine. Attempts to copolymerize N-alkylbenzaldimines with CO by blocking the
ortho-positions of the benzaldimine resulted in no reaction. Interestingly, although
CÀH activation was not observed in the attempted copolymerization of benzaldimines
with CO using Pd compounds [5–7], there are numerous examples for CÀH activation
of benzaldimines in the literature [21–29].
We thank the U.S. Department of Energy (Office of Basic Energy Sciences) for financial support. The
X-ray crystallographic facility at Penn State University acknowledges NSF grant CHE-0131112.
Experimental Part
General. (D8)-1,4-Dioxane (Aldrich) and C6
C
AHCTREUNG
phosphine (P(o-tol)3; Strem), and 13CH3I (Aldrich) according to literature procedures [16]. The aldimines
were synthesized according to published procedures [31] by condensation of the appropriate aldehydes
and amines, dried over CaH2 (Acros) and purified by solvent removal (when necessary), followed by dis-
tillation. The aldimines were then degassed (three freeze/pump/thaw cycles) prior to use. [Co2(CO)8]
(Strem) was used as received. CH2Cl2 (EMS) and EtOH (200 proof; Pharmco) were used as received.
GC Analyses were performed on an Agilent 5890 Series II apparatus with an RTX-5 split capillary column
(Restek) connected to an FID detector. IR Spectra were recorded on a Bruker IR spectrometer in 0.05-
mm CaF2 liquid cells (resolution: 0.5 cmÀ1). 1H- (300 MHz) and 13C-NMR (75 MHz) Spectra were
recorded at r.t. on a Bruker DPX-300 spectrometer; d in ppm, J in Hz. All work involving air- and/or
moisture-sensitive compounds was carried out in a dry box under N2 atmosphere.
General Procedure (GP 1): N-(1-Phenylmethylidene)methanamine (C6H5C(H)=NMe). In a 100-ml
round-bottom flask, benzaldehyde (5.0 g, 47 mmol; Aldrich) and excess 2.0M MeNH2 in THF were
reacted until the aldehyde was consumed (GC analysis). The soln. was then dried by careful addition
of CaH2, the solvent (THF) was removed, and the product was purified by distillation. 1H-NMR
(CDCl3): 3.51 (s, Me); 7.40 (3 arom. H); 7.71 (2 arom. H); 8.25 (s, N=CH). 13C-NMR (CDCl3): 48.6
(Me); 128.3, 128.6, 130.9, 136.7 (Ph); 162.8 (N=CH).
General Procedure (GP 2): N-(1-Phenylmethylidene)ethanamine (C6H5C(H)=NEt). In a 100-ml
round-bottom flask, benzaldehyde (5.0 g, 47 mmol) and excess EtNH2 (Aldrich) were reacted until the
aldehyde was consumed (GC analysis). The soln. was then dried by careful addition of CaH2, and the
product was purified by distillation. 1H-NMR (CDCl3): 1.31 (t, J=7.3, Me); 3.66 (qd, J=7.3, 1.3,
NCH2); 7.41 (3 arom. H); 7.75 (2 arom. H); 8.29 (s, N=CH). 13C-NMR (CDCl3): 16.8 (Me); 56.3
(CH2); 128.4, 129.0, 130.9, 136.8 (Ph); 160.8 (N=CH).
N-(1-Phenylmethylidene)propanamine (C6H5C(H)=NPr). Prepared from benzaldehyde and PrNH2
1
(Aldrich) according to GP 2. H-NMR (CDCl3): 0.99 (t, J=7.3, Me); 1.77 (m, J=7.2, NCH2
A
(td, J=6.9, 0.9, NCH2); 7.39 (3 arom. H); 7.75 (2 arom. H); 8.26 (s, N=CH). 13C-NMR (CDCl3): 12.3
(Me); 24.5 (NCH2CH2); 63.9 (NCH2); 128.5, 128.9, 130.8, 136.8 (Ph); 161.1 (N=CH).
N-[(2,6-Dimethylphenyl)methylidene]ethanamine (2,6-Me2C6H3C(H)=NEt). Prepared from 2,6-
dimethylbenzaldehyde (Aldrich) and EtNH2 according to GP 2. 1H-NMR (CDCl3): 1.38 (t, J=7.3,
MeN); 2.43 (s, 2 Me); 3.73 (qd, J=7.3, 1.2, NCH2); 7.07, 7.17 (3 arom. H); 8.61 (s, N=CH). 13C-NMR
(CDCl3): 17.1 (CH2Me); 20.8 (2 Me); 57.3 (NCH2); 128.7, 129.0, 134.9, 137.4 (C6H3); 160.9 (N=CH).
N-[(2,6-Difluorophenyl)methylidene]methanamine (2,6-F2C6H3C(H)=NMe). Prepared from 2,6-
1
difluorobenzaldehyde (Aldrich) and MeNH2 according to GP 1. H-NMR (CDCl3): 3.58 (s, Me); 6.91
(t, J(H,F)=8.5, 2 arom. H); 7.30 (1, arom. H); 8.48 (d, J(H,F)=1.5, N=CH). 13C-NMR (CDCl3): 50.4
(Me); 112.2 (dd, J(C,F)=23.3, 2.4, arom. C); 112.2 (dd, J(C,F)=18.5, 7.2, arom. C); 114.1 (t, J(C,
F)=12.9, arom. C); 131.8 (t, J(C,F)=10.9, arom. C); 153.6 (s, N=CH); 162.1 (dd, J(C,F)=256.3, 6.4,
arom. C).
N-{[(2,3,4,5,6-2H5)Phenyl]methylidene}methanamine (C6
N
G

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Helvetica Chimica Acta – Vol. 89 (2006)
3.52 (s, Me); 8.19 (s, N=CH). 13C-NMR (CDCl3): 48.6 (Me); 127.9 (1:1:1 t, J(C,D)=22.7, arom. C); 128.5
(1:1:1 t, J(C,D)=24.1, arom. C); 130.4 (1:1:1 t, J(C,D)=24.5, arom. C); 136.5 (s, arom. C); 162.8 (N=
CH).
N-{[(2,3,4,5,6–2H5)Phenyl](2H)methylidene}methanamine (C6
A
U
(D6)benzaldehyde (Aldrich) according to GP 1. 1H-NMR (CDCl3): 3.53 (s, Me). 13C-NMR (CDCl3):
48.6 (Me); 127.8 (1:1:1 t, J(C,D)=24.3, arom. C); 128.5 (1:1:1 t, J(C,D)=24.8, arom. C); 130.4
(1:1:1 t, J(C,D)=24.5, arom. C); 136.4 (s, arom. C); 162.6 (1:1:1 t, J(C,D)=24.0, N=CD).
General Procedure (GP 3) for the Reaction of Benzaldimines with CO. Into a thin-necked glass liner
was added [Co(13CH3C(O))(CO)3
(P(o-tol)3)] (20 mg, 0.04 mmol), benzaldimine (20 equiv.), and (D8)-1,
R
4-dioxane (2 ml). The mixture was then placed into a 125-ml Parr high-pressure reactor, which was sealed
and charged with 1000 psi of CO. The reaction was run at 2008 (oil-bath temp.) for the indicated amount
of time (see Table 1). After cooling down and releasing the pressure, the mixture was analyzed by 1H- and
13C-NMR spectroscopy. The formation of the N-alkylphthalimidine main product was confirmed by 1H-,
13C- (DEPT135), and HMQC-NMR spectroscopy, as well as by X-ray crystallography in the case of N-
methylphthalimidine (Figure).
N-Methylphthalimidine (=2,3-Dihydro-2-methyl-1H-isoindol-1-one). [Co(13CH3C(O))(CO)3
(P(o-
N
tol)3)] (20 mg), PhCH=NMe (0.19 g), and (D8)-1,4-dioxane (2 ml) were reacted as described in GP 3
at 2008 for 7 d. The product mixture was concentrated under vacuum, and the Co species were removed
by liq./liq. extraction with H2O/CH2Cl2. The org. phase was evaporated, and crystals were grown from a
conc. EtOH solution at À108. 1H-NMR (CDCl3): 3.20 (s, Me); 4.37 (s, PhCH2); 7.34 (t, J=7.4, arom. H);
7.49 (q, J=7.3, H, 2 arom. H); 7.83 (d, J=7.6, 2 arom. H). 13C-NMR (CDCl3): 29.8 (Me); 52.4 (CH2N);
123.0, 123.9, 128.4, 131.5, 133.3, 141.4 (C6H4); 169.0 (C=O).
Reaction of N-(1-Phenylmethylidene)ethanamine with 13CO. In a thin-necked glass liner was added
[Co(13CH3C(O))(CO)3
(P(o-tol)3)] (20 mg), PhC(H)=NEt (24 equiv.), and (D8)-1,4-dioxane (2 ml). The
A
mixture was placed in a 30-ml Parr high-pressure reactor, sealed, sequentially cooled (acetone/dry ice,
then liq. N2), and then exposed to ca. 300 psi of 13CO. The mixture was warmed to r.t., and then heated
at 2008 for 48 h. Then, the reactor was cooled down, degassed, and the product mixture was subjected to
IR, 1H- and 13C-NMR analyses.
General Procedure (GP 4) for the Reaction of N-(1-Phenylmethylidene)alkanamines with CO Cata-
lyzedby [Co 2(CO)8]. In a thin-necked glass liner was added [Co2(CO)8] (20 mg), the imine (ca. 20 equiv.),
and C6D6 (2 ml). The mixture was placed into a 125-ml Parr high-pressure reactor, which was sealed and
A
charged with 1000 psi of CO. The reaction was run at 2008 for 6 h. The reactor was cooled down,
degassed, and the solvent was evaporated under a stream of air. The products were dissolved in
CH2Cl2, and the Co species were removed by extraction with 1% aq. HCl (2×10 ml). The org. layer
was dried (Na2SO4) and concentrated under a stream of air. The resulting product was recrystallized
from EtOH at À108, and the crystals were dried under vacuum and then subjected to GC/MS, 1H-
NMR, and 13C-NMR analyses.
X-Ray Crystal Structure of N-Methylphthalimidine (=2,3-Dihydro-2-methyl-1H-isoindol-1-one)2). A
yellow brick-shaped crystal (ca. 0.20×0.25×0.50 mm) of N-methylphthalimidine, was used for the X-ray
crystallographic analysis. Intensities were recorded at a temp. of 108(2) K (Rigaku-MSC X-Stream 2000
cooler) on a Bruker SMART APEX CCD area-detector system equipped with a graphite monochroma-
tor and an MoKa fine-focus sealed tube (l=0.71073 Å) operated at 1600 W (50 kV, 32 mA). The detector
was placed at a distance of 5.8 cm from the crystal. A total of 1,850 frames were collected, with a scan
width of 0.38 in w and an exposure time of 5 s/frame; the total data-collection time was ca. 5 h. The frames
were integrated with the Bruker SAINT software package using a narrow-frame integration algorithm.
The integration of the data using a monoclinic unit cell yielded a total of 4,636 reflections to a maximum q
angle of 28.308 (0.90-Å resolution), of which 1,758 were independent (completeness: 96.7%, Rint =0.0216,
2
)
The crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication number CCDC-612554. Copies of the data can be obtained, free of
charge, via the internet (http://www.ccdc.cam.ac.uk/data_request/cif), by e-mail (data_request@
ccdc.cam.ac.uk), or fax (+44-1223-336033).

Helvetica Chimica Acta – Vol. 89 (2006)
1695
Rsig =0.0252), 1,610 reflections being greater than 2s(I). Final cell constants: a=9.1655(12),
b=9.0746(12), c=9.6316(12) Å, a=90, b=114.445(2), g=908; V=729.28(16) Å3. The data are based
on refinement of the XYZ centroids of 2,787 reflections>20s(I), with 2.4418<q<28.2988. Analysis of
the data showed negligible decay during data collection. The data were corrected for absorption effects
using the multiscan technique (SADABS). The ratio of minimum to maximum apparent transmission was
0.875721. The structure was solved and refined with the Bruker SHELXTL (vers. 6.1) software package,
using the space group P21/c, with Z=4 for the formula unit (C9H9NO). The final anisotropic full-matrix
least-squares refinement on F2 with 101 variables converged at R1=4.07% (obs. data) and wR2=10.71%
(all data). The goodness-of-fit was 1.040. The largest peak on the final difference map was 0.301 eÀ/Å3,
and the largest hole was À0.251 eÀ/Å3. Based on the final model, the calculated density of the crystal
was 1.340 g/cm3, and F(000) amounts to 312 electrons.
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ReceivedMarch 16, 2006