Synthesis and spectroscopic studies of group 6 metal carbonyl complexes with a novel cyclotriphosphazane ligand

Article abstract of DOI:10.1081/SIM-200052654

The syntheses and spectroscopic properties of Cr(0) and Mo(0) carbonyl complexes involving the novel cyclotriphosphazane (C₆H ₄N₂)(PPh)₃C₆H₄(NH) ₂ (1) are reported. The cyclotriphosphazane 1 is an interesting cyclic ligand that contains two unique types of phosphorus coordination environments as well as potential N-H reactivity. Reactions of 1 with one, two and excess molar equivalents of M(CO)₅(THF) (M = Cr, Mo and W) were studied, leading to two new cyclophosphazane metal complexes (C ₆H₄N₂)(PPh)₃C₆H ₄(NH)₂·2M(CO)₅ (M = Cr, Mo), where metal coordination is observed only to the phosphorus atoms that have their lone pair of electrons oriented exodentate to the phosphazane ring. The structures for the two new metal complexes are supported by ¹H, ¹³C{ ¹H} and ³¹P{¹H} NMR spectroscopy. The reaction of 1 with W(CO)₅(THF) yields a mixture of ring opened and decomposition phosphazane products. Copyright

Full text of DOI:10.1081/SIM-200052654

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Synthesis and Reactivity in Inorganic, Metal-Organic,
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Synthesis and Spectroscopic Studies of Group 6 Metal
Carbonyl Complexes With a Novel Cyclotriphosphazane
Ligand
Mitchell W. Denker a & Monte L. Helm a
a Department of Chemistry , Fort Lewis College , Durango, Colorado, USA
Published online: 15 Feb 2007.
To cite this article: Mitchell W. Denker & Monte L. Helm (2005) Synthesis and Spectroscopic Studies of Group 6 Metal
Carbonyl Complexes With a Novel Cyclotriphosphazane Ligand, Synthesis and Reactivity in Inorganic, Metal-Organic, and
Nano-Metal Chemistry, 35:3, 227-231, DOI: 10.1081/SIM-200052654
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Synthesis and Reactivity in Inorganic, Metal-Organic and Nano-Metal Chemistry, 35:227–231, 2005
Copyright # 2005 Taylor & Francis, Inc.
ISSN: 0094-5714 print/1532-2440 online
DOI: 10.1081/SIM-200052654
Synthesis and Spectroscopic Studies of Group 6
Metal Carbonyl Complexes With a Novel
Cyclotriphosphazane Ligand
Mitchell W. Denker and Monte L. Helm
Department of Chemistry, Fort Lewis College, Durango, Colorado, USA
Diel et al., 1989; Edwards et al., 2000). Phosphorus-containing
The syntheses and spectroscopic properties of Cr(0) and Mo(0) host molecules should be expected to have different properties
carbonyl complexes involving the novel cyclotriphosphazane
(C6H4N2)(PPh)3C6H4(NH)2 (1) are reported. The cyclotriphos-
phazane 1 is an interesting cyclic ligand that contains two
unique types of phosphorus coordination environments as well
as potential N-H reactivity. Reactions of 1 with one, two and
excess molar equivalents of M(CO)5(THF) (M 5 Cr, Mo and W) molecules are known, many of which contain both phosphorus
than other main group analogs because phosphorus is a soft-
base donor. Also, phosphorus has a higher barrier to pyramidal
inversion, which should result in more rigid structures with well
defined cavities. Examples of phosphorus-containing host
were studied, leading to two new cyclophosphazane metal com-
.
plexes (C6H4N2)(PPh)3C6H4(NH)2 2M(CO)5 (M 5 Cr, Mo),
and heteroatom donor sites; however, there are few examples
where the phosphorus atoms are held in fixed positions with
their coordinating electrons in a protected area of reactivity.
Phosphazane compounds, compounds that contain phos-
where metal coordination is observed only to the phosphorus
atoms that have their lone pair of electrons oriented exodentate
to the phosphazane ring. The structures for the two new metal
1
1
complexes are supported by 1H, 13Cf Hg and 31Pf Hg NMR spec- phorus-nitrogen single bonds, have received recent attention
troscopy. The reaction of 1 with W(CO)5(THF) yields a mixture of
ring opened and decomposition phosphazane products.
for their metal coordinating properties (Balakrishna et al.,
1994; Lief et al., 2004; Moser et al., 2003). For example, Krish-
namurthy and co-workers have done extensive studies on the
Keywords Chromium complexes; cyclophosphazane; molybdenum
complexes; pentacarbonyl complexes; phosphazane
metal coordination of diphosphazane ligands with a variety
of transition metals (Balakrishna et al., 1994; Mandal et al.,
2003, 2003a; Raghuraman et al., 2003; Venkatakrishnan
et al., 2003). Molecules that contain cavities or clefts can
offer opportunities for a highly selective coordination of
guest atoms or molecules, and the chemistry within these
INTRODUCTION
Crown type compounds, host macrocycles containing at regions can be unique. Previously reported cyclophosphazanes,
least two donor atoms, have found a wide use in a variety of (C6H4N2)(PPh)3C6H4(NH)2 (1) and [C6H4N2(PR)2]2 (2), are
areas (Bradshaw et al., 1996; Gokel et al., 2004; Lehn 1995;
Nesterov 2000; Pederson et al., 2001). For example, they are
often used to dissolve inorganic salts in organic solvents by
forming coordinate covalent bonds with the metal ion in a mole-
cular cavity. Although there are many sulfur and nitrogen
analogs of the crown ethers, there are few that contain
phosphorus donors (Blower et al., 1999; Baker et al., 2002;
Caminade and Majoral et al., 1994; Coles et al., 1995;
examples of such molecules that possess structure rigidity and
Received 13 October 2004; accepted 10 November 2004.
We thank the Camille and Henry Dreyfus foundation, Research
Corporation (DS0332) and Fort Lewis College for funding that sup-
ported this research.
Address correspondence to Monte L. Helm, Department of
Chemistry, Fort Lewis College, Durango, CO 81301, USA. E-mail:
helm_m@fortlewis.edu.
unique position of the phosphorus donor sites (Barendt et al.,
1991a, 1991b). The cyclotetraphosphazane (2) incorporates
an 8-member P-N ring where two phosphorus atoms, P(1)
and P(4), have their lone pair electrons pointing endodentate
to the phosphazane ring in a well defined cavity. This is in
contrast to the P(2) and P(3) atoms that have their lone pair
227

228
M. W. DENKER AND M. L. HELM
electrons oriented in an exodentate fashion to the ring. 0.101 g (0.19 mmol) of (C6H4N2)(PPh)3C6H4(NH)2 (1) in
Compound 2 has been shown to selectively bind small THF (5.0 mL) was added to [Cr(CO)5THF] solution and
metals, such as Agþ, within the cavity while showing no stirred for 3 h, during which the color changed from orange
affinity for larger metal ions such as Mo(0) (Young et al., to yellow. After removal of the solvent, the dark yellow solid
1995). To date, no such studies have been conducted with was dissolved in THF (10 mL) and filtered through a column
the cyclotriphosphazane (1). Previously reported structural of silica (230–400 mesh, 2 cm ꢀ 30 cm) using THF as the
studies of 1 have suggested the endodentate cavity is larger eluting solvent. The light yellow fractions were collected and
than the observed cavity in the cyclotetraphosphazane (2) solvents removed yielding pure 3 (0.136 g, 78% yield).
1
2
(Helm et al., 1998). The cyclotriphosphazane (1) is also of par- 31Pf Hg NMR (C6D6): d 137.4 [d, area 2, JPP ¼ 24.1 Hz
1
ticular interest because it not only contains phosphorus donor P(2,3)], 106.1 [t, area 1, P(1)]. H NMR (C6D6): d 7.84–7.59
atom sites, but potential N-H reactivity to further derivatize [complex mult., area 4, C6H4], 7.45–7.39 [complex mult.,
the molecule or for use in other metal coordination studies area 4, C6H4], 6.87–6.83 [complex mult., area 15, 3C6H5],
(Spencer et al., 2004; Liang et al., 2004). In an attempt to elu- 4.92 [broad s, area 2, 2NH]. IR (thf, nCO): 2068 w, 2012 vw,
cidate the steric and electronic properties of the coordination 1956 br. m, 1851 m cm21
.
chemistry of 1, we conducted metal complexation studies
.
using group 6 metal carbonyls. We now wish to report these
metal coordination studies of the novel cyclotriphosphazane
ligand (1) with M(CO)5(THF), M ¼ Cr, Mo and W.
Preparation of (C6H4N2)(PPh)3C6H4(NH)2 2Mo(CO)5 (4)
A two-fold excess of [Mo(CO)5THF] was prepared in a THF
solution (54 mL) from Mo(CO)6 (0.219 g, 0.83 mmol). Exactly
0.109 g (0.20 mmol) of (C6H4N2)(PPh)3C6H4(NH)2 (1) in THF
(5.0 mL) was added and the [Mo(CO)5THF] solution and
EXPERIMENTAL
Standard procedures for the manipulation of air-sensitive stirred for 3 h, during which the color changed from light
materials were employed. Unless otherwise stated, all manipu- yellow to dark yellow. After removal of the solvent, the dark
lations were carried out at ambient temperature under an yellow solid was dissolved in THF (10 mL) and filtered
atmosphere of dry nitrogen gas using standard Schlenk, through a column of silica (230–400 mesh, 2 cm ꢀ 30 cm)
syringe and high vacuum-line techniques. Solvents were using THF as the eluting solvent. The light yellow fractions
dried, freshly distilled under nitrogen, and degassed prior to were collected and solvents removed yielding pure 4
1
use. All NMR spectra were recorded on a JEOL GSX-270 (0.155 g, 56% yield). 31Pf Hg NMR (C6D6): d 112.2 [d, area
2
spectrometer (1H NMR, 270.166 MHz; 13C, 67.933 MHz; 31P 2, JPP ¼ 26.8 Hz P(2,3)], 97.1 [t, area 1, P(1)]. 1H NMR
109.365 MHz). The 31P chemical shifts downfield from 85% (C6D6): d 7.96–7.79 [complex mult., area 4, C6H4], 7.51–
H3PO4 (external) are reported as positive (þd). Infrared 7.46 [complex mult., area 4, C6H4], 6.91–6.84 [complex
spectra were recorded with a Thermo Nicolet Avatar 360 FT- mult., area 15, 3C6H5], 5.55 [broad s, area 2, 2NH]. IR (thf,
IR infrared spectrophotometer. The cyclotriphosphazane (1)
was made according to a previously published procedure.
(Helm et al., 1998)
n
CO): 2073 w, 2032 vw, 1985 m, 1950 br. s, 1861 m cm21
.
RESULTS AND DISCUSSION
Reaction of the cyclotriphosphazane (1) with excess
M(CO)5(THF) (M ¼ Cr and Mo) yield the metal complexes
.
Preparation of (C6H4N2)(PPh)3C6H4(NH)2 2Cr(CO)5 (3)
A two-fold excess of [Cr(CO)5THF] was prepared in a THF (C6H4N2)(PPh)3C6H4(NH)2 2Cr(CO)5 (3) and (C6H4N2)
.
.
solution (46 mL) from Cr(CO)6 (0.167 g, 0.76 mmol). Exactly (PPh)3C6H4(NH)2 2Mo(CO)5 (4) (Scheme 1). The presence
SCH. 1.

STUDIES OF GROUP 6 METAL CARBONYL COMPLEXES
229
SCH. 2.
of 3 and 4 in their reaction mixtures can be initially identified metal coordination, whereas the smaller downfield shifts seen
1
by their distinct 31Pf Hg NMR spectral resonances, which in P(1) and P(3) are due to inductive effects. The chemical
2
4
are sharp, well defined resonances shifted downfield from the shift assignments are also supported through JPP and JPP
free ligand. The reactions are carried out at room temperature coupling constant data. This reaction indicates there is
and are complete in 3 h in near quantitative yield, as analyzed relatively no deactivation of the second ‘side’ phosphorus
1
by 31Pf Hg NMR spectroscopy. Separation from excess atom, P(2) or P(3), upon coordination of the first metal
metal carbonyl and purification of the metal complexes is carbonyl. Somewhat surprisingly, even in the presence of
best accomplished by passing the solution through a column excess metal carbonyl, we saw no indication of metal coordi-
of silica. nation to the center phosphorus atom, P(1), indicating this
Reaction of 1 with 1, 2 and excess equivalents of position is most likely too sterically hindered to allow for
W(CO)5(THF) yield only ring opened and decomposed phos- metal coordination of this type.
1
phazane products. Attempts to isolate any of the many
The 31Pf Hg NMR spectra of the two complexes (3 and 4)
reaction products were unsuccessful. Presumably, coordination show a simple doublet/triplet AX2 splitting pattern with peak
of W(CO)5 to 1 must create a steric or electronic environment positions that clearly indicate formation of the metal com-
that facilitates P-N bond cleavage resulting in the decompo- plexes. In the case of 3, the doublet from P(2,3) is shifted
sition of the cyclotriphosphazane ligand.
downfield 44 ppm relative to the free ligand (1), where the
Reactions of 1 with one equivalent of M(CO)5(THF) center phosphorus atom P(1) shows a 33 ppm downfield shift
(M55Cr and Mo) yield a mixture of products including the relative to the free ligand (Table 1). The downfield chemical
bimetallic complexes (3 or 4), mono-metallic complexes shift observed for P(1) is most likely an inductive effect as a
1
(e.g., 5, 6) and un-reacted 1 as analyzed by 31Pf Hg NMR spec- result of metal coordination to the nearby P(2) and P(3)
troscopy (Scheme 2). Attempts to separate these reaction atoms, as also observed in the mono-coordinated complexes
mixtures were unsuccessful. Although not isolated, compounds (5, 6). A similar trend in the downfield shifts are observed
5 and 6 can be identified in the reaction mixtures through their for 4 (Table 1). In addition to the changes in the chemical
1
2
distinctive AMX 31Pf Hg NMR splitting patterns. For example, shifts, the JPP coupling constants increase to 24.1 Hz and
compound 5 shows 38, 15 and 1 ppm downfield chemical 26.8 Hz for 3 and 4, respectively, from 8.3 Hz as seen in the
shifts for P(2), P(1) and P(3) phosphorus atoms, respectively. free ligand (1). The increase in phosphorus-phosphorus
The large downfield chemical shift for P(2) is due to direct coupling constants upon metal coordination has been
TABLE 1
1
1
31Pf Hg NMR, H NMR and infrared data for 1, 3 and 4
31Pf Hg NMR (ppm)
1
1H (ppm)
N-H
IR (cm21
)
n CO
Compound
P(1)
P(2,3)
1
73.2
106.1
93.4
137.4
5.88
4.92
—
2068 w, 2012 vw,
3 (M ¼ Cr)
1956 br. m, 1851 m
2073 w, 2032 vw, 1985 m,
1950 br. s, 1861 m
4 (M ¼ Mo)
97.1
112.2
5.55

230
M. W. DENKER AND M. L. HELM
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1
The 1H and 13Cf Hg NMR spectra also support formation of
3 and 4. The 1H NMR shows the expected aromatic peaks for
the C6H5 and C6H4 rings ranging between 7.84–6.83 ppm and
7.96–6.84 ppm for 3 and 4, respectively. The N-H peak,
appears at 4.92 and 5.55 ppm for 3 and 4, respectively, both
shifted upfield from the free ligand N-H peak, which appears
at 5.88 ppm (Table 1). The observed upfield chemical shift
for the N-H peaks upon metal coordination may be a result
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increasing electron shielding around the hydrogen bonded
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multiple carbonyl stretching frequencies ranging from 2068–
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P(2,3), in 1 are less sterically hindered and more readily
coordinate to group 6 metals than the center phosphorus
atom, P(1). Future studies in our lab aim to use smaller metal
ions to access the center phosphorus atom in 1 for coordination,
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