N-pyrrolyl phosphines: Enhanced π-acceptor character via carboalkoxy substitution

Article abstract of DOI:10.1021/om970129q

A series of 3,4-dicarboethoxy-N-pyrrolyl phosphine ligands (R₂P-NC₄H₂(CO₂Et)₂) is described. A number of techniques, including reaction calorimetry, infrared spectroscopy, and X-ray crystallography, demonstrate that these ligands are potent π-acceptors and that the electronic substituent parameter χ_i for the 3,4-(EtO₂C)₂C₄H₂N moiety is ca. 16, placing it in a position between fluorine and chlorine.

Full text of DOI:10.1021/om970129q

Organometallics 1997, 16, 3377-3380
3377
N-P yr r olyl P h osp h in es: En h a n ced π-Accep tor Ch a r a cter
via Ca r boa lk oxy Su bstitu tion
Adrian Huang, J ohn E. Marcone, Kate L. Mason, William J . Marshall, and
Kenneth G. Moloy*,†
Central Research and Development,‡ E. I. du Pont de Nemours & Co., Inc.,
Experimental Station, P.O. Box 80328, Wilmington, Delaware 19880-0328
Scafford Serron and Steven P. Nolan*,§
Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148
Received February 21, 1997X
A series of 3,4-dicarboethoxy-N-pyrrolyl phosphine ligands (R2P-NC4H2(CO2Et)2) is
described. A number of techniques, including reaction calorimetry, infrared spectroscopy,
and X-ray crystallography, demonstrate that these ligands are potent π-acceptors and that
the electronic substituent parameter øi for the 3,4-(EtO2C)2C4H2N moiety is ca. 16, placing
it in a position between fluorine and chlorine.
In tr od u ction
for this reason expanding the limits of donor/acceptor
properties of this general class of ligands is an area of
ongoing investigation. In our continued interest in this
general area, we were interested in further enhancing
the π-acceptor character of N-pyrrolyl phosphines by
placing electron-withdrawing groups on the pyrrole ring.
A large body of knowledge exists on the synthesis of
substituted pyrroles, and thus, this represents a viable
approach to a wide variety of ligands in this general
class. In this contribution, we present our recent work
in this area and describe the synthesis and coordination
chemistry of a series of carboethoxy-substituted pyrrolyl
phosphines.
We recently reported the results of several investiga-
tions on the coordination chemistry of N-pyrrolyl phos-
phines.1 A variety of spectroscopic, structural, and
thermochemical measurements showed that these ligands
are exceptional π-acceptor ligands. Our rational for this
π-acceptor character is best demonstrated by resonance
forms A-D. Aromatic delocalization of the nitrogen
lone pair into the five-membered ring places a partial
positive charge adjacent to phosphorus in contributions
B and C and would be expected to render the pyrrolyl
substituent an effective electron-withdrawing group.
Relative to phenyl, resonance form D would also be
expected to contribute in an electron-withdrawing fash-
ion, since a more electronegative nitrogen atom replaces
carbon.
The cumulative evidence, resulting from spectroscopic
(IR) and calorimetric investigations, demonstrates that
the substituent parameter øi for the N-pyrrolyl func-
tionality is approximately 12.1 The π-acceptor character
of these ligands is thus found to exceed that of phos-
phites (-OPh, øi ) 9.7) and fluorinated aromatic phos-
phines (-C6F5, øi ) 11.2) and approaches that found for
fluoroalkylphosphines.2 Fluoroalkylphosphines are re-
ceiving increased attention as a result of their enhanced
π-acceptor and reduced σ-donor properties relative to
more traditional phosphorus ligands.3 Phosphine ligands
find enormous utility in coordination chemistry, orga-
nometallic chemistry, and homogeneous catalysis,4 and
Resu lts a n d Discu ssion
The ligands in this study are readily synthesized by
direct combination of 3,4-bis(carboethoxy)pyrrole with
the appropriate phosphorus chloride, as described previ-
ously for the parent ligand series (eq 1).1a These
reactions proceed cleanly and in high yields; procedures
and spectroscopic details are provided in the Experi-
mental Section. Following this methodology, we pre-
(3) (a) Schnabel, R. C.; Roddick, D. M. Inorg. Chem. 1993, 32, 1513.
(b) Ernst, M. F.; Roddick, D. M. Organometallics 1990, 9, 1586. (c)
Ernst, M. F.; Roddick, D. M. Inorg. Chem. 1989, 28, 1624. (d) Koola,
J . D.; Roddick, D. M. J . Am. Chem. Soc. 1991, 113, 1450. (e) Brookhart,
M.; Chandler, W. A.; Pfister, A. C.; Santini, C. C.; White, P. S.
Organometallics 1992, 11, 1263. (f) Phillips, I. G.; Ball, R. G.; Cavell,
R. G. Inorg. Chem. 1988, 27, 4038. (g) N-Sulfonylphosphoramides have
recently been investigated as
phosphorus ligands, see: Hersh, W. H.; Xu, P.; Wang, B.; Yom, J . W.;
Simpson, C. K. Inorg. Chem. 1996, 35, 5453.
a new class of strong π-acceptor
(4) See the following and references therein: (a) Wilson, M. R.;
Woska, D. C.; Prock, A.; Giering, W. P. Organometallics 1993, 12, 1742.
(b) Caffery, M. L.; Brown, T. L. Inorg. Chem. 1991, 30, 3907. (c) Dunne,
B. J .; Morris, R. B.; Orpen, A. G. J . Chem. Soc., Dalton Trans. 1991,
653. (d) Corbridge, D. E. C. Phosphorus. An Outline of its Chemistry,
Biochemistry and Technology, 4th ed.; Elsevier: New York, 1990. (e)
Levason, W. In The Chemistry of Organophosphorus Compounds;
Hartley, F. R., Ed.; Wiley: New York, 1990; Vol. 1, Chapter 15. (f)
McCauliffe, C. A. Comprehensive Coordination Chemistry; Wilkinson,
G., Gillard, R. D., McCleverty, J . A., Eds.; Pergamon: Oxford, 1987;
Vol. 2, p 989. (g) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke,
R. G. Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987; p 66. (h) Homoge-
neous Catalysis with Metal Phosphine Complexes; Pignolet, L. H., Ed.;
Plenum: New York, 1983. (i) Alyea, E. C.; Meek, D. W. Adv. Chem.
Ser. 1982, 196.
† E-mail: moloykg@esvax.dnet.dupont.com.
‡ Contribution No. 7514.
§ E-mail: spncm@uno.edu.
X Abstract published in Advance ACS Abstracts, J uly 1, 1997.
(1) (a) Moloy, K. G.; Petersen, J . L. J . Am. Chem. Soc. 1995, 117,
7696. (b) Li, C.; Serron, S.; Nolan, S. P.; Petersen, J . L. Organometallics
1996, 15, 4020. (c) Serron, S.; Nolan, S. P.; Moloy, K. G. Organome-
tallics 1996, 15, 4301. (d) Serron, S.; Nolan, S. P. Inorg. Chim. Acta
1996, 252, 107.
(2) Tolman, C. Chem. Rev. 1977, 77, 313.
S0276-7333(97)00129-5 CCC: $14.00 © 1997 American Chemical Society

3378 Organometallics, Vol. 16, No. 15, 1997
Huang et al.
Ta ble 1. Meta l-Ca r bon yl Str etch in g F r equ en cies
for Rh (I) a n d Mo(0) Com p lexes
νCO
-∆H
entry
ligand
(cm-1
)
(kcal/mol)b
ref
a
trans-RhCl(CO)L2
2044
1
2
3
4
5
6
7
8
9
1a
1b
1c
6.9
23.5
38.8
34.4
35.3
43.8
47.8
50.1
51.7
56.1
58.7
42.6
this work
2022
2001
2024
2007
1990
1984
1982
1978
P(pyrrolyl)3
PPh(pyrrolyl)2
PPh2(pyrrolyl)
P(p-ClC6H4)3
P(p-FC6H4)3
PPh3
P(p-CH3C6H4)3
P(p-OCH3C6H4)3
P(OPh3)3
1c
pared the series of monophosphorus ligands 1a -c and
the chelate 1d , all from readily available starting
materials.
10
11
12
1975
1973
2016
These ligands form coordination complexes by simple
displacement of CO from the appropriate metal carbonyl
precursors. Following on our previous studies, we
prepared the series of rhodium complexes 2a -c via CO
substitution and bridge cleavage from dimeric [RhCl-
(CO)2]2. Complexes 2b and 2c are isolable as micro-
Mo(CO)4(PkP)c
13
14
15
16
17
1d
2052d
this work
1a
3c
(pyrrolyl)2P(CH2)2(pyrrolyl)2 2043e
(C6F5)2P(CH2)2(C6F5)2
(C2F5)2P(CH2)2(C2F5)2
Ph2P(CH2)2PPh2
2041e
2064e
2020e
a
CH2Cl2. bEnthalpy values are reported with 95% confidence
limits. c Only the high-frequency, A1 symmetry band for these C2v
complexes is listed; spectra for the Mo(0) complexes recorded as
d
Nujol mulls. THF. e Nujol mull.
crystalline solids. Complex 2a , however, could not be
isolated and was characterized in situ. The available
evidence shows that 2a loses a CO ligand upon at-
tempted isolation and in this way is reminiscent of the
chemistry of the complexes [R2P(CH2)2PR2]RhCl(CO).
The latter lose CO to yield complexes of the type
{[R2P(CH2)2PR2]RhCl}2 when R is strongly electron
withdrawing (R ) fluoroalkyl,3a N-pyrrolyl1a), indicating
that formation of chlorine bridges becomes more favor-
able as the number of π-acceptor ligands about rhodium
increases. The molybdenum complex Mo(CO)4(1d ), 2d ,
is readily prepared by direct reaction of the chelate with
Mo(CO)6 in refluxing THF.
The infrared spectra of complexes 2a -d demonstrate
that carboalkoxy substitution on the pyrrole ring indeed
results in a potent electron-withdrawing combination
(see Table 1). The carbonyl stretching frequency for
complex 2a is found at 2044 cm-1, 20 cm-1 higher than
that found for the parent complex trans-RhCl(CO)-
[P(NC4H4)3]2 and 66 cm-1 higher than the PPh3 ana-
logue. Examination of Table 1 shows that substitution
of Ph with NC4H2(CO2Et)2 in the series 2a -c results
in a steady 22 cm-1 increase in the carbonyl stretching
frequency. From these data, we estimate the substitu-
ent parameter for the NC4H2(CO2Et)2 group to be ca.
16, placing it between chlorine (øi ) 14.8) and fluorine
(øi ) 18.2) but still less than a fluoroalkyl such as CF3
(øi ) 19.6).2 That ligands 1a -d fall short of the acceptor
character of fluoroalkylphosphines is shown by a com-
parison of the molybdenum complexes Mo(CO)4L; the
A1 band for complex 2d is 10 cm-1 lower in energy than
that found for the (C2F5)2P(CH2)2(C2F5)2 analogue (Table
1, entries 13 and 16).
F igu r e 1. Carbonyl stretching frequency vs enthalpy of
reaction for the trans-RhCl(CO)(PR3)2 system.
data are included in Table 1 for comparison.1c The
thermodynamic data encompass a >50 kcal/mol range
and correlate nicely with the infrared data, as shown
in Figure 1. The good correlation between the infrared
and thermochemical data suggests that, sterically, the
3,4-disubstituted pyrrolyl group does not appear to be
significantly different than the parent unsubstituted
N-pyrrolyl. N-pyrrolyl, in turn, was shown in our
previous studies to be isosteric with phenyl and para-
substituted aryl groups. This is likely a result of both
the remoteness of the carboethoxy groups to the coor-
dination center and the manner in which the pyrrolyl
ring directs these substituents away from the metal.
These inferences regarding the relative steric sizes
are consistent with the structure of complex 2d , as
determined by X-ray diffraction. An ORTEP drawing
of the molecule is provided in Figure 2, and important
bond lengths and angles are summarized in Table 2.
The structural parameters are unremarkable when
compared to the structure of Mo(CO)4[Ph2P(CH2)2-
PPh2].5 The chelate bite angle is ca. 78°, as expected.
Other angles about molybdenum are not chemically
Also included in Table 1 are the enthalpies (-∆Hrxn
)
for reaction of [RhCl(CO)2]2 with the new ligands to give
complexes 2a -c, as determined by anaerobic solution
calorimetry. We previously reported a complete study
of the thermodynamics of ligand substitution in this
system with a number of other phosphine ligands,
including the parent N-pyrrolyl series, and much of this
(5) Bernal, I.; Reisner, G. M.; Dobson, G. R.; Dobson, C. B. Inorg.
Chim. Acta 1986, 121, 199.

N-Pyrrolyl Phosphines
Organometallics, Vol. 16, No. 15, 1997 3379
only by fluoroalkyl in terms of electron-withdrawing
ability. The simple, high-yield syntheses of these
ligands and the ready availability of starting materials
suggests that they promise general utility as ancillary
ligands in coordination chemistry, particularly when
π-acceptor, “CO-like” ligands are desired.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All manipulations were carried
out under inert atmosphere using a glovebox or Schlenk line.
For synthetic reactions, solvents were purchased from Aldrich
as anhydrous grade and used as received; the solvent used
for calorimetry was dried and distilled before use.6-8 Deuter-
ated solvents were dried over the appropriate agents and
vacuum transferred.6 3,4-(EtO2C)2C4H2NH, PCl3, PhPCl2, Ph2-
PCl, and Cl2PCH2CH2PCl2 were obtained from Aldrich or
Strem and used as received. Et3N was dried over molecular
sieves before use.
Proton and phosphorus NMR spectra were recorded on 300
1
and 500 MHz instruments. The H spectra are referenced to
TMS via solvent peaks, and the 31P spectra are referenced
externally to 85% phosphoric acid. Infrared spectra were
recorded on a Nicolet 205 FTIR spectrometer as solutions or
Nujol mulls with CaF2 cells or plates. Elemental analyses
were performed by Micro-Analysis Inc., Wilmington, DE.
P [NC4H2(CO2CH2CH3)2]3 (1a). A Schlenk flask was charged
with 1.04 g (4.93 mmol) of HNC4H2(CO2CH2CH3)2 and 25 mL
of THF. The solution was cooled to -30 °C, and then 0.13
mL (1.49 mmol) of PCl3 was added, followed by 1.5 mL of Et3N.
After 15 min, the mixture was warmed to room temperature,
whereupon a large amount of colorless solid formed. The
mixture was heated with an oil bath at 60 °C for 6 h. After
the reaction mixture was cooled to room temperature and
filtered, the solids were washed with THF. The combined
filtrates were taken to dryness under vacuum to yield a
colorless oil. Addition of pentane caused the oil to crystallize.
The product was filtered, washed with additional pentane, and
dried under vacuum. Yield: 0.88 g, 90%. 1H NMR (CD2Cl2):
δ 1.31 (t, 18H), 4.27 (q, 12H), 7.33 (d, 6H). 31P{1H} NMR (CD2-
Cl2): δ 82.5.
F igu r e 2. ORTEP drawing of complex 2d (molecule 1);
ellipsoids are drawn at the 40% probability level, and
hydrogen atoms eliminated for clarity. Important bond
distances and angles are provided in Table 2.
Ta ble 2. Su m m a r y of Im p or ta n t Bon d Dista n ces
a n d An gles for Com p lexes 2d
2d
Mo(CO)4[Ph2P-
molecule 1
molecule 2
(CH2)2PPh2]a
Bond Lengths (Å)
Mo-P
2.457(2)
2.430(2)
2.004(7)
2.012(8)
2.050(8)
2.021(8)
2.471(2)
2.500(2)
2.495(2)
1.999(8)
1.974(8)
2.053(9)
2.030(9)
2.447(2)
2.034(8)
1.992(9)
2.039(8)
2.017(9)
Mo-Ceq
Mo-Cax
P P h [NC4H2(CO2CH2CH3)2]2 (1b). This compound was
prepared similarly, using PPhCl2 (0.5 mL, 3.8 mol),
HNC4H2(CO2CH2CH3)2 (2.0 g, 9.48 mmol), Et3N (1.5 mL), and
30 mL THF. After 5 h at 60 °C, the slurry was filtered and
the solids were washed with THF. The combined filtrates were
taken to dryness, and the resulting product was slurried into
pentane. After filtration and further washing with pentane,
the colorless product was dried under vacuum: mp 129-132
°C. Yield: 1.89 g, 94%. 1H NMR (CD2Cl2): δ 1.30 (t, 12H),
4.25 (q, 8H), 7.23 (m, 2H), 7.43 (d, 4H), 7.55 (m, 3H). 31P{1H}
NMR (CD2Cl2): δ 81.7.
Bond Angles (deg)
P-Mo-P
Ceq-Mo-Ceq
Cax-Mo-Cax
78.76(5)
92.2(3)
177.2(3)
77.77(5)
91.4(3)
171.0(4)
80.2(1)
90.4(3)
176.6(3)
Averages (deg)
trans-P-Mo-Ceq
cis-P-Mo-Ceq
P-Mo-Cax
172.3(3)
94.4(3)
91.1(4)
89.1(6)
171.0(3)
95.5(3)
93.4(5)
87.0(8)
171.4(4)
96.4(4)
88.9(4)
91.0(6)
Ceq-Mo-Cax
a
From ref 5.
P P h 2[NC4H2(CO2CH2CH3)2] (1c). This material was pre-
pared similarly from Ph2PCl (0.71 mL, 3.95 mmol),
HNC4H2(CO2CH2CH3)2 (1.0 g, 4.74 mmol), and Et3N (0.66 mL,
4.74 mmol) in 15 mL of THF. Conversion to the desired
product was complete after stirring overnight at room tem-
perature. The solvent was stripped from the colorless slurry
in vacuo and replaced with 15 mL of toluene. The slurry was
filtered and the colorless solid was washed with toluene. The
combined filtrates were taken to dryness. The resulting
colorless solid was slurried into a mixture of toluene (1 mL)
and pentane (10 mL), filtered, and then washed with pentane
before drying under vacuum. Yield: 1.252 g, 80%. 1H NMR
(CD2Cl2): δ 7.47 (m, 6H), 7.33 (m, 6H), 4.25 (q, 4H), 1.30 (t,
6H). 31P{1H} NMR (CD2Cl2): δ 87.1.
different from those found in the analogous Ph2P(CH2)2-
PPh2 complex, attesting to the minimal steric impact
of the 3,4-dicarboethoxy groups. The only bonding
parameter of note is the Mo-P distance, which is
slightly shorter (ca. 0.05 Å) in 2d than that found in
the Ph2P(CH2)2PPh2 complex. This is consistent with
the greater degree of π-acceptor character of 1d relative
to Ph2P(CH2)2PPh2, as we previously found for com-
plexes of P(pyrrolyl)3.1a
Con clu sion
Placement of strongly electron-withdrawing carbo-
ethoxy groups on N-pyrrolyl phosphines leads to a
significant enhancement in the π-acceptor character of
this class of ligands. The available evidence indicates
that this class of phosphorus substituents is exceeded
(6) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals 3rd ed.; Pergamon Press: New York, 1988.
(7) Ojelund, G.; Wadso¨, I. Acta Chem. Scand. 1968, 22, 1691-1699.
(8) Kilday, M. V. J . Res. Natl. Bur. Stand. (U.S.) 1980, 85,467-
481.

3380 Organometallics, Vol. 16, No. 15, 1997
Huang et al.
Ta ble 3. Cr ysta llogr a p h ic Da ta for Com p lex 2d
[ ( C H 3 C H 2 O 2 C ) C 4 H 2 N ] 2 P ( C H 2 ) 2 P [ N C 4 H 2 ( C O 2
-
CH2CH3)2]2 (1d ). A Schlenk flask was charged with 2.05 g
(9.71 mmol) of HNC4H2(CO2CH2CH3)2 and 30 mL of THF.
Cl2P(CH2)2PCl2 (0.51 g, 2.19 mmol) was added; this was
followed by 2.0 mL of Et3N, resulting in formation of a copious,
colorless precipitate. The mixture was heated to 60 °C for ca.
15 h. The 31P NMR spectrum showed a single resonance at δ
77.9 assigned to the title compound. The solution was filtered,
and the solvent was removed under vacuum. the resulting
crude product was extracted three times with 30 mL portions
of hot toluene to give a colorless solid, which was dried under
vacuum: mp 139-149 °C, dec. Yield: 1.27 g, 62%. An
additional 0.48 g of material was recovered from the toluene
extracts (total yield ) 86%). 1H NMR (CD2Cl2): δ 1.29 (t, 24
H), 2.58 (t, 4 H), 4.24 (q, 16H), 7.49 (m, 8H).
Rh Cl(CO)(1a )2 (2a ). A solution of 0.145 g (0.219 mmol) of
1a in 1 mL of CH2Cl2 was added to 0.0204 g (0.105 mmol) of
[RhCl(CO)2]2 in 0.5 mL of CH2Cl2. The solution turned a
darker orange, and gas evolution was observed. After 25 min,
IR analysis showed complete loss of [RhCl(CO)2]2 and forma-
tion of the desired product (νCO ) 2044 cm-1, νester ) 1728
cm-1). 31P{1H} NMR (CD2Cl2, -60 °C): δ 93.6 (d, J Rh-P ) 195
Hz). Attempts to isolate this product resulted in formation of
a material devoid of carbonyl ligands, as determined by IR.
Rh Cl(CO)[1b]2 (2b). [RhCl(CO)2]2 (0.018 g, 0.0473 mmol)
and 1b (0.100 g, 0.189 mmol) were dissolved in 5 mL of CH2-
Cl2, giving immediate gas evolution and a yellow solution.
After 2 h, pentane (10 mL) was added to give an orange
precipitate, which was filtered, washed with pentane, and
dried under vacuum. 1H NMR (CD2Cl2): δ 7.7-7.5 (m, 18H),
4.17 (q, 16H), 1.20 (t, 24H). 31P{1H} NMR (CD2Cl2): δ 97.3
(d, J Rh-P ) 165.1 Hz). Anal. Calcd for C53H58ClN4O17P2Rh:
C, 52.04; H, 4.78; N, 4.58. Found: C, 52.12; H, 4.51; N, 4.56.
Rh Cl(CO)(1c)2 (2c). A solution of 1c (0.180 g, 0.45 mmol)
in 5 mL of CH2Cl2 was added to [RhCl(CO)2]2 (0.041 g, 0.213
mmol), resulting in gas evolution and a color change from
orange to yellow. After 30 min, the solvent was removed under
vacuum to yield a yellow semisolid. Pentane was added to
give a solid product, which was isolated by filtration, washed
with pentane, and dried under vacuum. Yield: 0.125 g, 61%.
1H NMR (CD2Cl2): δ 7.7-7.5 (m, 24H), 4.25 (q, 8H), 1.29 (t,
12H). 31P{1H} NMR (CD2Cl2): δ 79.7 (d, J Rh-P ) 143 Hz).
Anal. Calcd for C45H44ClN2O9P2Rh: C, 56.47; H, 4.63; N, 2.93;
Cl, 3.70. Found: C, 56.42; H, 4.72; N, 3.17; Cl, 3.41.
formula
fw
space group
a, Å
b, Å
c, Å
MoP2O20N4C46H52
1138.82
Cc (No. 9)
24.212(1)
22.029(1)
22.692(1)
109.94(1)
11 377.2
3.48
-112
8
0.043
0.044
â, deg
V, Å3
µ(Mo KR), cm-1
temp, °C
Z
Ra
Rw
a
no. of refined params
no. of data collected
no. of data unique, I > 3σ
Rmerge
1313
30702
8051
0.016
R ) ∑(||Fo| - |Fc||)/∑|Fo|; Rw ) ∑w(|Fo| - |Fc|)2/∑w|Fo| .
a
2
1974 (sh), and 1954 cm-1. Ester carbonyl bands were observed
at 1732 and 1714 cm-1
.
The solvent was removed under
vacuum to yield a pale yellow solid. The product was dissolved
in toluene; a small amount of colorless solid was removed by
filtration, and the solvent was removed under vacuum to give
a glassy product. Pentane was added to give a slurry of light
yellow microcrystals. The product was isolated by filtration,
washed with pentane, and dried under vacuum. Yield: 0.346
g, 77%. 1H NMR (CD2Cl2): δ 7.44 (s, 8H), 4.18 (q, 16H), 3.05
(m, 4H), 1.22 (t, 24 H). 31P{1H} NMR (CD2Cl2): δ 149.6 (s).
Anal. Calcd for C46H52MoN4O20P2: C, 48.52; H, 4.60; N, 4.92.
Found: C, 48.05; H, 4.38; N, 4.89.
Solu tion Ca lor im etr y. Calorimetric measurements were
performed using a Calvet calorimeter (Setaram C-80). This
calorimeter and experimental procedures for its use under
inert atmosphere have been previously described.1c,9 Experi-
mental enthalpy data are reported with 95% confidence limits.
X-r a y Str u ctu r a l An a lysis of 2d . Details on the struc-
tural analysis of 2d are given in Table 3 and provided as
Supporting Information. The complex crystallizes with two
crystallographically independent molecules per unit cell. Unit
cell information: space group, Cc (No. 9); a ) 24.212(1) Å; b
) 22.029(1) Å; c ) 22.692(1) Å; â ) 109.94°; Z ) 8; Rw ) 0.044.
Ack n ow led gm en t. The National Science Founda-
tion (Grant No. CHE-9631611) and Du Pont (Educa-
tional Aid Grant) are gratefully acknowledged for
support of this research.
Mo(CO)4(1d ) (2d ). A mixture of Mo(CO)6 (0.104 g, 0.394
mmol), 15 mL of THF, and 0.394 g of 1d (0.427 mmol) was
brought to reflux under an argon atmosphere, whereupon the
colorless solution gradually turned pale yellow. After ca. 15
h, IR analysis showed that no Mo(CO)6 remained and only
bands attributable to the title complex were observed at 2052,
Su p p or tin g In for m a tion Ava ila ble: Tables of fractional
coordinates, anisotropic thermal parameters, hydrogen fixed
atom coordinates, interatomic distances, interatomic angles,
and intermolecular distances for 2d (15 pages). Ordering
information may be found on any masthead page.
(9) (a) Nolan, S. P.; Hoff, C. D.; Landrum, J . T. J . Organomet. Chem.
1985, 282, 357-362. (b) Nolan, S. P.; Lopez de la Vega, R.; Hoff, C. D.
Inorg. Chem. 1986, 25, 4446-4448.
OM970129Q