Synthesis, characterization and crystal structure of (cis-P,P′-diphenyl-1,4-diphospha-cyclohexane)molybdenum(0)tetracarbonyl

Article abstract of DOI:10.1016/j.inoche.2010.01.030

The preparation of the novel (cis-P,P′-diphenyl-1,4-diphospha-cyclohexane)molybdenum(0)tetracarbonyl complex is described. The spectral data and X-ray structure of the title complex are reported. The results of the crystallographic work show a distorted octahedral complex around the metal center, the first of its kind reported for the P,P′-diphenyl-1,4-diphospha-cyclohexane ligand.

Full text of DOI:10.1016/j.inoche.2010.01.030

Inorganic Chemistry Communications 13 (2010) 534–536
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, characterization and crystal structure of
(cis-P,P0-diphenyl-1,4-diphospha-cyclohexane)molybdenum(0)tetracarbonyl
Gerard M. Carroll a, Susie M. Miller b, Monte L. Helm a,
*
a Chemistry Department, Fort Lewis College, 1000 Rim Drive, Durango, CO 81301, USA
b Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA
a r t i c l e i n f o
a b s t r a c t
The preparation of the novel (cis-P,P0-diphenyl-1,4-diphospha-cyclohexane)molybdenum(0)tetracar-
bonyl complex is described. The spectral data and X-ray structure of the title complex are reported.
The results of the crystallographic work show a distorted octahedral complex around the metal center,
the first of its kind reported for the P,P0-diphenyl-1,4-diphospha-cyclohexane ligand.
Ó 2010 Elsevier B.V. All rights reserved.
Article history:
Received 8 December 2009
Accepted 30 January 2010
Available online 6 February 2010
Keywords:
P,P0-diphenyl-1,4-diphospha-cyclohexane
Molybdenum tetracarbonyl complexes
Phosphine ligands
Bidentate phosphine ligands
The use of phosphine ligands in organometallic catalysis contin-
ues to be a fertile area of study, especially with respect to struc-
ture/reactivity correlations. For example, there have recently
been a number of articles published regarding the catalytic effect
of various phosphine ligand bite angles on reaction yields and
product distributions [1,2]. Although there exists a large body of
knowledge in the literature on bidentate phosphine ligands with
a single bridging carbon chain (i.e. R2P–C–C–PR2), research on cyc-
lic bidentate phosphines with two connecting carbon chains (i.e.
RP(–C–C–)2PR) has recently received attention [3–7]. Such com-
pounds are of particular interest due to the fact that the phospho-
rus atoms are held in a rigid steric position that could lead to
distorted metal coordination geometries and different reaction
chemistry than seen in their acyclic counterparts.
One cyclic bidentate phosphine ligand that shows potential for
interesting metal coordination behavior is P,P0-diphenyl-1,4-
diphospha-cyclohexane, (dpdpc) [8]. The cis isomer of this com-
pound is especially attractive as the phosphorus atoms are held
in a position where their lone pair electrons are aligned to allow
for cooperative binding to a metal center. A previous study re-
ported by Panunzi et al. on the metal coordination properties of
this ligand demonstrate its versatility towards binding M(II) cen-
ters (M = Co, Ni, Pd and Pt) [9]. Fraldi et al. have shown palladium
complexes of the dpdpc ligand are catalytic active towards C–C
bond formation reactions [3]. Recently, work from our laboratory
on dpdpc coordination reactions with Group 9 metals has identi-
fied a bridging, binuclear intermediate that can be converted to a
monomeric product, illustrating the versatility of coordination
modes of this ligand [10]. Further, the structurally characterized
[Cp Rh(dpdpc)Cl]PF6 compound shows a P–Rh–P bite angle of
71.60°, the most acute bite angle for a two-carbon bridged biden-
tate phosphine ligand at the time of its publication [10].
In this work we have synthesized a new metal complex of the
dpdpc ligand with a Mo(0) metal center and obtained its X-ray
crystal structure. Refluxing a mixture of Mo(CO)6 with one equiv-
alent of the phosphine ligand, dpdpc, in diglyme for 1 h affords
the metal complex (dpdpc)Mo(CO)4 as determined by 31P{1H}
NMR (Scheme 1). Removal of solvent by distillation, followed by
dissolution in CH2Cl2, filtration through celite, and removal of the
solvent results in pure Mo(dpdpc)(CO)4 in 15% yield [11].
*
The 31P{1H} NMR spectrum of (dpdpc)Mo(CO)4 shows the
expected downfield chemical shift from the free ligand (ꢀ27.0
ppm) to 44.3 ppm for the metal complex. The 1H NMR spectra of
the complex shows the expected peaks for the dpdpc protons,
including two sets of multiplets centered at 2.47 and 2.16 ppm
from the protons on the methylene backbone. The 13C{1H} NMR
data shows two distinct multiplets for the carbonyl carbons at
219.5 and 208.5 ppm, attributed to the carbonyl carbons trans-
and cis- to the dpdpc ligand, respectively, in addition to a four-
carbon multiplet centered at 27.6 ppm from the dpdpc methylene
carbon atoms [12,13].
In addition to formation of the monomeric metal complex, the
presence of a bridged, binuclear intermediate can be detected in
the 31P NMR when monitoring the reaction progress (Scheme 1).
Although the binuclear product was not isolated, a single 31P{1H}
NMR resonance at 10.1 ppm is observed before the appearance of
the product peak at 44.3 ppm. These resonances correspond well
* Corresponding author. Tel.: +1 970 247 7635; fax: +1 970 247 7567.
E-mail address: helm_m@fortlewis.edu (M.L. Helm).
1387-7003/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.01.030

G.M. Carroll et al. / Inorganic Chemistry Communications 13 (2010) 534–536
535
Ph
P
CO
OC
reflux, 1h
diglyme
Mo
CO
Ph
P
P
Ph
Mo(CO)6
+
+
2 CO
OC
P
Ph
dpdpc
OC
CO
P
OC
Mo
Ph
OC
CO
Ph
P
CO
OC
OC
CO
CO
Scheme 1.
to the previously reported Rh dimer/monomer peaks which oc-
curred at 18.0 and 67.0 ppm, respectively [10]. The 31P{1H} NMR
spectra of aliquots of the reaction mixture taken every 5 min show
an initial accumulation of the bridged intermediate, which is then
converted to the product as the reaction proceeds.
Crystals of the metal complex, suitable for X-ray diffraction
studies, were grown by slow evaporation from a saturated CH2Cl2
solution of the compound [14]. The structure of the complex shows
a distorted octahedral geometry around the metal center from the
four carbonyl ligands and two phosphine donors of the dpdpc li-
gand (Fig. 1). The distortion from octahedral geometry revolves al-
most solely around the Mo-dpdpc interaction. The P–Mo–P bite
angle observed in the structure is 67.130(16)°, the most acute bite
angle reported, to date, for this ligand. The cis C–Mo–C angles aver-
age 90.00(8)° and the trans C–Mo–C angle is 179.41(8)°, indicating
the carbonyls maintain a nearly perfect octahedral positions
around the metal. Clearly the rigid boat confirmation of the dpdpc
ligand results in increased structure distortion on the metal center
geometry compared to other, more flexible, singly bridged carbon
chain (R2P–C–C–PR2) bidentate ligands. To our knowledge, this
complex represents the first reported octahedral complex contain-
ing the dpdpc ligand.
The X-ray structure of the complex also indicates the dpdpc li-
gand does not participate in p-backbonding as strongly as the car-
bonyl ligands they replaced. The X-ray data show shorter Mo–C
bonds, and longer C–O bonds in the carbonyls trans from the phos-
phorus atoms, compared to the carbonyls trans from each other (cis
from the dpdpc ligand). These bond distances indicate an increase
in p-backbonding to the carbonyls trans to the dpdpc ligand, rela-
tive to the carbonyl ligands it replaced.
In summary, a new molybdenum tetracarbonyl complex with
the dpdpc ligand was synthesized, representing the first structur-
ally characterized octahedral complex containing the dpdpc ligand.
As shown in the crystal structure, the dpdpc ligand is a sterically
rigid, bidentate phosphorus ligand that results in an acute P–M–P
bond angle upon metal complexation. Similar to previous reports,
our studies show the dpdpc ligand is capable of forming both a
bridged, dimeric complex with two metal centers or a single mono-
meric metal complex. We are currently in the process of studying
the synthesis and reactivity of similar complexes with a view to-
wards their catalytic activity.
Acknowledgement
We thank Fort Lewis College for funding that supported this
research.
Appendix A. Supplementary material
CCDC 756961 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Center via http://www.ccdc.ca-
m.ac.uk/data_request/cif. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/j.inoche.
2010.01.030.
Fig. 1. View of the molecular structure of (dpdpc)Mo(CO)4. Thermal ellipsoids are
drawn at the 50% probability level. Selected bond distances (Å): Mo(1)–P(1)
2.5020(5); Mo(1)–P(2) 2.5038(5); Mo(1)–C(1) 2.037(3); C(1)–O(1) 1.147(3); Mo(1)–
C(4) 2.039(2); C(4)–O(4) 1.148(3); Mo(1)–C(2) 1.991(2); C(2)–O(2) 1.152(3);
Mo(1)–C(3) 1.984(2); C(3)–O(3) 1.158(3). Selected bond angles (°): P(1)–Mo(1)–
P(2) 67.130(16); C(3)–Mo(1)–C(2) 90.74(8); C(3)–Mo(1)–C(1) 91.94(9); C(2)–
Mo(1)–C(1) 89.53(9); P(1)–Mo(1)–P(2) 67.130(16); C(3)–Mo(1)–C(4) 87.51(9);
C(2)–Mo(1)–C(4) 90.27(9); C(1)–Mo(1)–C(4) 179.41(8).
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data). Rint = 0.0462, R1 = 0.0259, wR2 = 0.0557, GOF = 1.015 (49 059 total
reflections, 6454 unique reflections where I > 2r(I)).