Dianionic Mononuclear Cyclo-P4 Complexes of Zero-Valent Molybdenum: Coordination of the Cyclo-P4 Dianion in the Absence of Intramolecular Charge Transfer

Article abstract of DOI:10.1002/anie.201908885

Relative to other cyclic poly-phosphorus species (that is, cyclo-P_n), the planar cyclo-P₄ group is unique in its requirement of two additional electrons to achieve aromaticity. These electrons are supplied from one or more metal cent

Full text of DOI:10.1002/anie.201908885

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Accepted Article
Title: Dianionic Mononuclear Cyclo-P4 Complexes of Zero-Valent
Molybdenum: Coordination of the Cyclo-P4 Dianion in the
Absence of Intramolecular Charge Transfer
Authors: Kyle A. Mandla, Michael L. Neville, Curtis E. Moore, Arnold L.
Rheingold, and Joshua S. Figueroa
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10.1002/anie.201908885
Angewandte Chemie International Edition
COMMUNICATION
Dianionic Mononuclear Cyclo-P4 Complexes of Zero-Valent
Molybdenum: Coordination of the Cyclo-P4 Dianion in the
Absence of Intramolecular Charge Transfer
Kyle A. Mandla, Michael L. Neville, Curtis E. Moore, Arnold L. Rheingold and Joshua S. Figueroa*
Dedication ((optional))
Abstract: Relative to other cyclic poly-phosphorus species (i.e. cyclo-
Pn), the planar cyclo-P4 group is unique in its requirement of two
additional electrons to achieve aromaticity. In the coordination
chemistry of cyclo-P4 ligands, these additional electrons are supplied
from one or more metal centers. However the degree of charge
transfer to the cyclo-P4 ligand is highly dependent on the nature of the
metal fragment to which it is bound. Reported here are unique
Scheme 1. Known coordination modes for the [cyclo-P4]2– ligand.
examples of dianionic mononuclear h4-P4 complexes that can be
viewed as the simple coordination of the [cyclo-P4]2– dianion to a
the binuclear inverted sandwich motif, wherein the cyclo-P4 ring
neutral metal fragment. Treatment of the neutral, molybdenum cyclo-
serves as a bridging ligand between two metals, have been
P4
complexes
Mo(h4-P4)I2(CO)(CNArDipp2
)
and
Mo(h4-
2
reported in recent years (Scheme 1).[19-23] In such species, it has
been proposed that the aromaticity of the cyclo-P4 unit is achieved
by multi-center charge transfer to the central phosphacycle from
the two transition metal centers.[20,21,23] Contrastingly,
mononuclear, or so-called “end-deck”, cyclo-P4 complexes
(Scheme 1), in which the bonding interactions between the metal
center and the cyclo-P4 unit are potentially more simplified,
remain uncommon. There are currently only eight structurally
characterized “end-deck” cyclo-P4 complexes, all of which are
either neutral or monoanionic in charge, and are limited to the
metals V, Nb, Ta, Mo, Fe and Co.[14, 24-29] However, the charge of
these mononuclear complexes can create an inherent ambiguity
- rather than simplification - of the bonding interactions between
the metal and the cyclo-P4 unit. This ambiguity centers on the
nominal degree of metal-ligand covalency versus formal metal-to-
ligand charge transfer.In turn, these bonding properties govern
both the extent of aromaticity of the cyclo-P4 unit and the valence
state of the metal center. Indeed, this uncertainty is analogous to
the debate on the iron cyclobutadiene complex, Fe(h4-C-
4H4)(CO)3,[30-32] where metal d8 and d6 electronic configurations
have been proposed as limiting forms.[33-34] Missing from the set
of mononuclear cyclo-P4 complexes are examples of dianionic
species, which can be viewed simply as the coordination of the
P4)(CO)2(CNArDipp2
)
2
with KC8 produces the dianionic, three-legged
piano stool complexes, [Mo(h4-P4)(CO)(CNArDipp2)2]2– and [Mo(h4-
P4)(CO)2(CNArDipp2)]2–, respectively. Structural, spectroscopic and
computational studies on these dianions reveal a distinct similarity to
the classic h6-benzene complex (h6-C6H6)Mo(CO)3 with respect to
both the valence state of the metal center and electronic population of
the p-system of the planar-cyclic ligand.
Aromaticity is a central tenant of molecular orbital theory
and governs the electronic stabilization of unsaturated cyclic
molecules.[1,2] While aromaticity is most well established for cyclic
hydrocarbons, there has been a long standing interest in the
synthesis, development and study of all-inorganic aromatic
compounds, especially those featuring the heavier main group
elements.[3] Of these, cyclic poly-phosphorus aromatic
compounds (i.e. cyclo-Pn) have received considerable attention
due to the proclivity of phosphorus, relative to other heavier main-
group elements, to form homonuclear multiple bonds.[4-6]
Accordingly, there has been a vibrant coordination chemistry of
cyclic poly-phosphorus compounds that mirrors the well-
established coordination chemistry of some Hückel-type 4n+2
aromatic hydrocarbons. Examples of such coordinated poly-
phosphorus rings include the all-phosphorus analogue of the
cyclopentadienyl ion (i.e. [cyclo-P5]–),[7,8] complexes of which have
found wide utility as supramolecular building blocks.[9-11] In
addition, coordination compounds featuring the all-phosphorus
analogue of benzene (i.e. cyclo-P6) have been reported,[12,13] as
have complexes containing the all-phosphorus analogue of the
more enigmatic cyclobutadiene dianion (i.e. cyclo-[P4]2–).[14-16]
For cyclo-P4 complexes in particular, the most common
examples are multinuclear, where the cyclo-P4 ring is bound h4 to
one metal and k1 to up to four other metals via the phosphorus
lone pairs (Scheme 1).[15-18] In addition, examples featuring
[cyclo-P4]2– dianion[35] to
a neutral metal fragment. Such
complexes would obviate the need to invoke a charge-transfer
paradigm within the bonding framework and could serve as a
benchmark for other mononuclear [cyclo-P4]2– complexes.
Accordingly, here we report the synthesis and structures of the
dianionic complexes [(cyclo-P4)Mo(CO)2(CNArDipp2)]2– and
[(cyclo-P4)Mo(CO(CNArDipp2)2]2–
(ArDipp2
=
2,6-(2,6-(i-
Pr2)2C6H3)2C6H3), which are direct electronic analogues of the
classic h6-benzene complex, (h6-C6H6)Mo(CO)3,[36] and allow for
a direct assessment of the effects of metal coordination on the
ostensible aromatic framework of the [cyclo-P4]2– dianion.
We recently reported the diiodo-monocarbonyl complex,
(h4-P4)MoI2(CO)(CNArDipp2
)
(1), which along with its more
2
reduced dicarbonyl counterpart, (h4-P4)Mo(CO)2(CNArDipp2)2 (2),
represented the first examples of monomeric cyclo-P4 complexes
of molybdenum.[28] On account of its high-valent nature, we
reasoned that complex 1 could be used to access a low-valent
and low-coordinate cyclo-P4 complex via chemical reduction.
Treatment of 1 with an excess of potassium graphite (KC8) in THF
solution resulted in a color change to dark red from orange
(Scheme 2). Analysis of the reaction mixture by 1H NMR
[*]
Dr. K. A. Mandla, Dr. Curtis E. Moore, Prof. Dr. Arnold L. Rheingold,
Prof. Dr. J. S. Figueroa
Department of Chemistry and Biochemistry
University of California, San Diego
9500 Gilman Drive, MC 0358, La Jolla, CA 92093 (USA)
E-mail: jsfig@ucsd.edu
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/anie.201908885
Angewandte Chemie International Edition
COMMUNICATION
1650 cm-1) consistent with both a highly reduced metal center and
contact ion pairing between the complex and potassium ions.[37]
Such IR spectroscopic features have been observed previously in
a number of anion transition metal complexes supported by m-
terphenyl isocyanide ligands.[38-40]
Attempts to obtain single crystals of this reduction product
were unsuccessful under a variety of conditions. However,
addition of dibenzo[18-crown-6] to the product enabled the
formation of red single crystals from
Crystallographic structure determination revealed this species to
be the dipotassium salt,
[K2(dibenzo[18-crown-6]][(h4-
a toluene solution.
P4)Mo(CO)(CNArDipp2)2] ([K2(db18-c-6)][3]; Figure 1). Accordingly,
we formulate the initial product of reduction as a THF-solvated
salt of K2[(h4-P4)Mo(CO)(CNArDipp2)2] (K2[3]), which represents a
formal 4e– reduction of complex 1 accompanied by the loss of two
iodide ligands. In the solid state, the Mo center of [K2(db18-c-6)][3]
adopts a three-legged piano stool motif that is most recognizably
associated with neutral (h6-arene)ML3 complexes of the Group 6
metals (L = 2e– donor ligand). One potassium ion is partially
encapsulated by the crown ether and makes a secondary
interaction to one phosphorus atom of the cyclo-P4 ring. The other
K+ ion is engaged in contact-ion interactions with the CºN and
arene portions of the CNArDipp2 ligands as suggested from the IR
spectroscopic data.[38-40] Treatment of [K2(db18-c-6)][3] with up to
five additional equivalents of dibenzo[db18-crown-6] did not
succeed in the encapsulation of the second K+ ion, thereby
indicating that contact-ion pairing in this complex is substantial.
Scheme 2. Synthesis of dianionic molybdenum cyclo-P4 complexes.
spectroscopy revealed the formation of a single product featuring
two distinct environments for the CNArDipp2 iso-propyl group
methyl and methine resonances. However, only one chemical
environment was present for the central aryl group, thereby
indicating a product with Cs symmetry. In addition, 1H-coupled, 31
P
NMR spectroscopic analysis of the reaction mixture revealed one
singlet resonance centered at d = +105 ppm. This chemical shift
is within the range found for complexes 1 and 2 (d = +90 and +60
ppm for 1; d = +113 ppm for 2),[28] which suggested that the cyclo-
P4 ligand remained intact upon treatment of 1 with KC8. Notably,
FTIR spectroscopy of this product in C6D6 solution revealed an
asymmetric set of broad, low-energy nCN and nCO bands (1900 –
Figure 1. Molecular Structures of K2(dibenzo[18-crown-6]][(h4-P4)Mo(CO)(CNArDipp2)2] ([K2(db18-c-6)][3]; left), the dianionic portion of K2[(h4-P4)Mo(CO)2(CNArDipp2)]
(K2[4]; middle) and the neutral paramagnetic complex (h4-P4)MoI(CO)(CNArDipp2)] (6; right).[49]
Table 1. Metrical and Spectroscopic Parameters for h4-P4 Molybdenum Complexes. av = average; cent = centroid; E = O or N.
d(Mo-(h4-P4)cent
)
nCE
Compound/Parameter
d(P-P)av
(Å)
d(Mo-C) av
(Å)
Proposed
Mo
Valence
4
(cm-1)
(Å)
(h4-P4)MoI2(CO)(CNArDipp2
)
2.157(±0.005)
2.163(±0.015)
2.160(±0.012)
2.161(±0.021)
2.169(±0.010)
2.168(±0.020)
Ciso – 2.119(±0.001)
CCO – 1.988(3)
n
n
CO – 2007
CN – 2142, 2104
2.077(3)
2.075(1)
2.112(2)
2.102(3)
2.051(2)
2.049(4)
2
(1)
(h4-P4)Mo(CO)2(CNArDipp2
(2)
)
2
Ciso –2.109(±0.009)
CCO – 2.013(±0.013)
vCO – 2001, 1941
vCN – 2130, 2101
2
0
0
2
3
[K2(18-c-6)][(h4-P4)Mo(CO)(CNArDipp2)2]
([K2(18-c-6)][3])
Ciso – 1.962(±0.019)
CCO – 1.973(3)
n
n
CO – 1654 1635
CN – 1877, 1780
K2[(h4-P4)Mo(CO)2(CNArDipp2)]
(K2[4])
n
n
CO –1760, 1679
CN –1981, 1915
Ciso – 1.972(3)
CCO – 1.935(±0.007)
[Cp2Co][(h4-P4)MoI(CO)(CNArDipp2)2]
([Cp2Co][5])
Ciso –2.074(±0.009)
CCO – 1.984(4)
n
n
CO – 1884
CN – 2050, 1997
(h4-P4)MoI(CO)(CNArDipp2
)
Ciso – 2.116(±0.011)
n
CO – 1984
2
(6)
CCO – 2.004(5)
nCN – 2137, 2121
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10.1002/anie.201908885
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Whereas (h4-P4)MoI2(CO)(CNArDipp2
)
(1) possesses
2
iodide ligands that are readily lost, the dicarbonyl complex (h4-
P4)Mo(CO)2(CNArDipp2)2 (2) can also serve as a precursor to
dianionic cyclo-P4 complexes upon reduction. Treatment of 2
with an excess of KC8 in THF solution followed by analysis by
31P NMR spectroscopy revealed the formation of K2[3] along
with an equimolar quantity of another cyclo-P4 product, as
indicated by an additional 31P singlet centered at d = + 121 ppm
(Scheme 2). Dissolution of this mixture in 10:1 n-pentane/THF,
solubilized K2[3], leaving a dark red precipitate that could be
crystallized from a THF solution stored at –40 ˚C. Structure
determination on single crystals produced in this manner
revealed the dicarbonyl salt, K2[(h4-P4)Mo(CO)2(CNArDipp2)]
(K2[4]), resulting from the formal loss of one CNArDipp2 ligand
from complex 2. Similar to the bis-isocyanide salt [K2(db18-c-
6)][3], the molybdenum core of dicarbonyl K2[4] adopts a three-
legged piano stool motif (Figure 1). However, in the solid state,
multiple contacts between the [(h4-P4)Mo(CO)2(CNArDipp2)]2–
unit and the K+ ions organize K2[4] into a linear coordination
polymer (Figure S4.3). Notably, treatment of the K2[3]/K2[4]
reaction mixture with 10.0 equivalents of CO at room
temperature results in the complete formation of K2[4] and the
corresponding amount of free CNArDipp2 after 1h of stirring.
However, extended reaction times and/or heating up to 80 ˚C
does not liberate the remaining CNArDipp2 ligand from K2[4].
Like the inability to encapsulate the second K+ ion in [K2(db18-
c-6)][3], we attribute this lack of additional reactivity between
K2[4] and CO to the strong p-arene/cation binding[41]
capabilities of the CNArDipp2 ligand.
Figure
2.
Highest-lying
filled
molecular
orbitals
for
(h4-
P4)Mo(CO)2(CNArDipp2)]2– ([4]2–; top) and (h6-C6H6)Mo(CO)3 (bottom).
The metrical parameters and IR spectroscopic features
of [K2(db18-c-6)][3] and K2[4] are consistent with the
coordination of a dianionic [cyclo-P4]2– ligand to a low-valent
molybdenum center. In addition, these data are especially
informative when further compared to the neutral h4-P4
complexes 1 and 2. For example, there is a remarkable
consistency in average P-P bond distances across the series.
For [K2(db18-c-6)][3] and K2[4], these average distances are
identical and are essentially the same as complexes 1 and 2
(Table 1).[28] This similarity strongly suggests that the cyclo-P4
units of these complexes have a common electronic structure,
irrespective of the valency of these molybdenum centers. In
addition, these average P-P bond distances are slightly longer
than those of the uncoordinated cyclo-tetraphosphide salt,
Cs2[cyclo-P4]·2NH3 (2.147(±0.002) Å),[35,42] thereby reflecting
that coordination to Mo decreases P-P p-bonding character to
a measurable extent.[43] However, there is a pronounced
increase in the Mo-(h4-P)centroid distance for [K2(db18-c-6)][3]
and K2[4] relative to complexes 1 and 2 (Table 1) which is
consistent with a decrease in metal valence state and reflects
the presence of additional electron density on the metal center.
Complexes [K2(db18-c-6)][3] and K2[4] also display Mo-C bond
distances to both the isocyanide and CO ligands that are 0.1 Å
and 0.2 Å shorter than those within 2 and 1, respectively (Table
1). These short Mo-C bond lengths coincide with nCN and nCO
IR signatures of [K2(18-c-6)][3] and K2[4] that are considerably
lower in energy than those of 1 and 2, thereby providing
additional support for increased p-backbonding interactions in
the salts and, correspondingly, the presence of low-valent Mo
centers.
Figure 3. Occupied molecular oribitals representing the interaction between
molybdenum and the cyclo-P4 ligand in (h4-P4)Mo(CO)2(CNArDipp2)]2– ([4]2–
left) and benzene in (h6-C6H6)Mo(CO)3 (right).
;
molybdenum center.[44] These MOs correlate remarkably well
with the e(xy, x2-y2) and a1(z2) non-bonding orbital set of C3v-
symmetric (h6-C6H6)Mo(CO)3 (Figure 2), with the exception
that the dx2-y2 orbital of [4]2– is stabilized relative to its dxy orbital.
Accordingly, this breaking of the degenerate e-orbital set in
[4]2– is a consequence of the fact that the four-fold symmetric
nature of the cyclo-P4 ligand can accept only one d-
backbonding interaction from the three-fold symmetric MoL3
fragment.[45] However, with respect to the primary bonding
interactions between the cyclo-P4 unit and Mo, the electronic
similarity of [4]2– and (h6-C6H6)Mo(CO)3 are readily apparent
(Figure 3). Both complexes possess a filled a-type molecular
orbital
interaction
between
the
totally-symmetric
representation of the cyclic ligand to the Mo center (Figure 3).
This feature is accompanied by two filled e-symmetry
interactions between the molybdenum dxz and dyz orbitals and
the cyclic ligand, which for the cyclo-P4 unit satisfies the criteria
for Hückel-type, 4n+2 aromaticity (n = 1). Correspondingly,
calculation of the diatropic ring current for [4]2– 1 Å above the
cyclo-P4 centroid resulted in a NICS(1) value of –7.0 ppm
(NICS = nucleus-independent chemical shift).[46-47] Notably,
negative NICS(1) values have been strongly correlated with
the presence of aromaticity in cyclic compounds.47 Accordingly,
this NICS(1) value for [4]2– can be compared with that
calculated for (h6-C6H6)Mo(CO)3 (NICS(1) = –16.3 ppm), with
Density functional theory (DFT) investigations provided a
more detailed picture of the electronic structure of these cyclo-
P4 dianions and revealed a distinct electronic relationship to
the classic h6-benzene complex, (h6-C6H6)Mo(CO)3. As shown
in Figure 2, the three highest-lying molecular orbitals
calculated for [(h4-P4)Mo(CO)2(CNArDipp2)]2– ([4]2–) are a metal-
based non-bonding orbital set and are consistent with the d6
electronic configuration of a zero-valent
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Keywords: Phosphorus • Aromaticity • cyclo-P4 •
Molybdenum • Isocyanide
Conflict of Interest
The authors declare no conflict of interest.
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paramagnetic cyclo-P4 complex, (h4-P4)MoI(CO)(CNArDipp2
)
2
(6), which was prepared according to the route shown in
Scheme 3 through the intermediacy of the salt [Cp2Co][(h4-
P4)MoI(CO)(CNArDipp2)2] ([Cp2Co][5]). Monoiodide 6 exhibits
average P-P bond distances that are similar to the other
mononuclear molybdenum cyclo-P4 complexes, thereby
signifying the presence of a [cyclo-P4]2– unit (Table 1; Figure
1). However, the Mo-(h4-P)centroid distance in 6 is shorter than
those for [K2(db18-c-6)][3] and K2[4], while its n(CN) and n(CO)
stretches are intermediate between those of complexes 1 and
2. These data indicate that complex 6 possesses an Mo center
in a higher-valent state than [K2(db18-c-6)][3], K2[4] and 2, but
not diiodide 1.
Accordingly, we believe this series represents a unique
case where metal-based valence modulation has been
identified for cyclo-P4 complexes featuring the same metal
center and a similar ancillary environment. Given that cyclo-P4
complexes are desirable initial products in the activation of
white phosphorus (P4) by transition-metal centers, the ability to
understand and benchmark metal-based redox changes within
these complexes may aid in the development of new
phosphorus-atom functionalization processes. We are
currently investigating this potential with the aim of developing
transformations for [K2(db18-c-6)][3] and K2[4] that
cooperatively involve redox equivalents from both the Mo
center and the cyclo-P4 ligand.
Acknowledgements
We are grateful to the U.S. National Science Foundation for
support of this work (CHE-1802646) and to the Alexander von
Humboldt Foundation for a Fellowship to J.S.F.
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Entry for the Table of Contents (Please choose one layout)
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More Charge, More Clarity:
K. A. Mandla, M. L. Neville, C.
E. Moore, A. L. Rheingold and J.
S. Figueroa
Like
the
cyclobutadiene
dianion,
the
cyclic,
polyphosphorus
cyclo-P4
Page No. – Page No.
dianion creates an electronic
structure ambiguity when found
in neutral transition metal
complexes. Here, dianionic
Dianionic Mononuclear Cyclo-
P4 Complexes of Zero-Valent
Molybdenum: Coordination of
the Cyclo-P4 Dianion in the
Absence of Intramolecular
Charge Transfer
cyclo-P4
complexes
are
reported which can be viewed
as the simple binding of [cyclo-
P4]2– to a metal.
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