Insertion of sodium phosphaethynolate, Na[OCP], into a zirconium-benzyne complex

Article abstract of DOI:10.1039/c7cc01482a

Reaction of the zirconium-benzyne complex [Cp₂Zr(PMe₃)(C₆H₄)] with sodium phosphaethynolate, Na[OCP], affords a zircono-phosphaalkene complex. Notably, unlike reactions of other transition metal complexes with Na[OCP] that yield the products of simple salt metathesis, this transformation represents novel Na[OCP] insertion chemistry and formation of an unusual solid state coordination polymer. The polymer is disrupted upon addition of Me₃SiCl to afford a silyl-capped dimer that retains the zirconophosphaalkene functionality. Protonation of either form of zirconophosphaalkenes results in the formation of benzoylphosphine, PhC(O)PH₂.

Full text of DOI:10.1039/c7cc01482a

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Insertion of sodium phosphaethynolate, Na[OCP],
into a zirconium–benzyne complex†
Cite this:DOI: 10.1039/c7cc01482a
ab
Jerod M. Kieser,a Robert J. Gilliard, Jr.,
Arnold L. Rheingold,c
Hansjorg Grutzmacher*b and John D. Protasiewicz*a
Received 24th February 2017,
Accepted 3rd April 2017
¨
¨
DOI: 10.1039/c7cc01482a
rsc.li/chemcomm
Reaction of the zirconium–benzyne complex [Cp2Zr(PMe3)(C6H4)]
with sodium phosphaethynolate, Na[OCP], affords a zircono-phos-
phaalkene complex. Notably, unlike reactions of other transition metal
complexes with Na[OCP] that yield the products of simple salt metathesis,
this transformation represents novel Na[OCP] insertion chemistry and
formation of an unusual solid state coordination polymer. The polymer
is disrupted upon addition of Me3SiCl to afford a silyl-capped dimer
that retains the zirconophosphaalkene functionality. Protonation
of either form of zirconophosphaalkenes results in the formation
Q
of benzoylphosphine, PhC( O)PH2.
The development of convenient and scalable routes to the phos-
phorus analogue of the cyanate anion, [OCP]ꢀ, has spawned
renewed interest in the chemistry of this fundamental 3-atom
building block.1 An attractive aspect of this synthon is that
through its three dominate resonance structures, [O–CRP]ꢀ,
[OQCQP]ꢀ, and [ORC-P]ꢀ, a wide range of reactivity may be
realized. Indeed, recent reports have shown that this material
Scheme 1 Selected heterocycles produced from Na[OCP].2,9
exhibits novel [4+2] and [2+2] cycloaddition chemistry and may [OCP]–metal complex can be described as salt metathesis reactions
serve as a means to transfer [P]ꢀ or a [OCP]ꢀ unit.2–7 Furthermore, where [OCP]ꢀ acts as a pseudo-halide. Conversely, phosphaalkynes
the stability and scalability of Na[OCP] as a common synthon has (R–CRP) have rich chemistry with transition metal complexes,
led to the development of phosphorus analogs of important small especially for insertion and oligomerization reactions.13–16
molecules, such as urea and cyanuric acid.8,9 Heterocycles a–e
(Scheme 1) serve as an indication of the applicability of Na[OCP]
towards important developments in main-group chemistry.
The reactivity of the phosphaethynolate anion towards transi-
tion metal complexes, however, has remained relatively unexplored,
with only a handful of isolated products known (Fig. 1).10–12
In these cases the scope of reported reactions to produce the
a Department of Chemistry, Case Western Reserve University, 2080 Adelbert Rd.,
Cleveland, Ohio, 44106, USA. E-mail: protasiewicz@case.edu
b
¨
¨
Department of Chemistry and Applied Biosciences, ETH-Honggerberg, 8093 Zurich,
Switzerland. E-mail: gruetzmacher@inorg.chem.ethz.ch
c Department of Chemistry and Biochemistry, University of California, San Diego,
La Jolla, California, 92093, USA
† Electronic supplementary information (ESI) available. CCDC 1531806 and
1531807. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c7cc01482a
Fig. 1 Representative metal complexes derived from Na[OCP].10–12
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Scheme 2 Proposed route to 1,3-benzoxaphospholes.
It is noteworthy that a number of phosphaalkynes are highly
reactive and cannot be stored for long periods of time.15,17
Alternatively, utilization of Na[OCP], a remarkably stable salt,
provides opportunities for the isolation of organometallic products
that were previously inaccessible.
We have been interested in the chemistry of 1,3-benzoxa-
phospholes and related molecules as fluorescent conjugated
materials that feature pp–pp PQC bonds.18–21 During our studies
we have sought new routes to these important species and
recognized that a 9-atom heterocyclic system might be assembled
by reaction of the phosphaethynolate anion with benzyne
(Scheme 2), similar to a previously reported reaction between
benzyne and organic azides.22 Our initial efforts to generate
benzyne in the presence of phosphaethynolate anion, followed
by trapping reactions with electrophiles, did not yield the
desired product. As ascertained by 31P NMR spectroscopy,
reactions produced numerous products which is the result of
direct reactions of Na[OCP] with typical benzyne precursors,
even combining Na[OCP] with the relatively mild precursor
2-trimethylsilylphenyl triflate resulted in oxidative tetramerization
of [OCP]ꢀ after a few minutes.23 Herein, we describe the synthesis,
structure, and reactivity of a novel zirconophosphaalkene (2).
Notably, formation of compound 2 represents the first example
of insertion of Na[OCP] into a metal–carbon bond.
Addition of Na[OCP]ꢁ(dioxane)2.5 to a solution of the zirconium
benzyne complex [Cp2Zr(C6H4)(PMe3)] (1)24 produces a dark red
solution from which an insoluble red microcrystalline solid (2)
can be isolated in 43% yield (Scheme 3). The solid state 31P MAS
NMR spectrum of 2 reveals a singlet at d = 50.6 ppm. Red crystals
of 2 suitable for a single crystal X-ray study were obtained by
layering a THF solution of Na[OCP] on top of a solution of 1 in
THF in a thin glass tube. Fig. 2 reveals that insertion of the CP
unit of Na[OCP] into one of the zirconium–carbon bonds has
occurred to afford an unusual polymeric structure consisting
of [Cp2Zr{k-C,k2-P-C6H4C(O)P}] dimers bridged by [Na(THF)2]
cations. Interestingly, despite the oxophilicity of zirconium, the
resulting product has formed Zr–P instead of Zr–O bonds and
Fig. 2 Molecular structure of 2 (ellipsoids set at 50% probability; hydrogen
atoms, co-crystallized THF, and second coordination polymer chain omitted for
clarity). Selected bond distances [Å] and angles [1]: P1–Zr1: 2.6151(14); P1–Zr10:
2.7010(14); P1–P10: 2.548(2); P1–C11: 1.743(5); Zr1–C13: 2.406(5); O1–C11:
1.276(6); C11–C12: 1.488(8); P1–Zr1–C13: 71.04(14); Zr1–P1–Zr10: 122.75(5);
Zr1–P1–C11: 109.37(19); Zr10–P1–C11: 127.22(19); P1–C11–O1: 125.6(4).
represents an unusual example of a cyclic bridging metallo-
phosphaalkene.14,26,27 The structure also contains a second
crystallographically inequivalent [Cp2Zr{k-C,k2-P-C6H4C(O)P}]
unit (and corresponding coordination polymer chain) as well
as co-crystallized THF molecules.
Within each dimer the two halves are related by an inversion
center and the [Zr2{C6H4C(O)P}2] array is essentially coplanar. The
average PQC bond length of 1.742 Å is similar to other P-zircono-
phosphaalkenes,25 however the expected opening of the Zr–P–C
angle is not observed due to the cyclic structure of the phospha-
alkene unit.25–27 The insertion of Na[OCP] follows a pathway similar
to that reported for reaction of a zirconium benzyne with aceto-
nitrile,28 however the difference in electronic properties between
R–CRP and [O–CRP]ꢀ results in Zr–P bond formation instead of
the Zr–C bond found in the insertion of t-butylphosphaalkyne.29,30
Addition of Me3SiCl to a suspension of 2 in THF results in the
formation of a new red-orange species 3 as the sole product by 31
P
NMR spectroscopy (d = 194.0 ppm), which could be isolated in 96%
yield (Scheme 3). Analysis of orange single crystals of 3 grown from a
saturated benzene solution revealed silylation of the oxygen atoms
has occurred to replace the bridging sodium atoms of 2 (Fig. 3).
The metallo-phosphaalkene core is retained, although there is some
loss of planarity of the [Zr2{C6H4C(O)P}2] array. A slight shortening of
the PQC bond is observed (1.7046(19) Å) compared to that in 2
(1.743(5) Å), as well as ca. 0.05 Å lengthening of the Zr–P bonds. This
change in P–C and P–Zr bond distances combined with a lengthening
of the C–O bond between 2 (1.276(6) Å) and 3 (1.372(2) Å) suggest that
the oxygen atoms in 2 donate a significant amount of electron density
into the p-system, as shown in resonance form 2d (Scheme 4).
A similar effect was previously observed in the structure of metallo-
phosphaalkenes that contained nitrogen in a trans-orientation to the
metal atom.31 This increased electron density around phosphorus
also explains the significant difference observed between the 31P
resonances of 2 (d = 50.6 ppm) and 3 (d = 193.96 ppm).
Scheme 3 Synthesis of 2 and 3.
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as the potential for using Na[OCP] towards the formation of
interesting main group materials.
We thank the National Science Foundation (CHE-1464855), the
ETH Zu¨rich and the Swiss National Science Foundation (SNF) for
funding. R. J. G. is grateful for a UNCF-Merck Postdoctoral Research
Fellowship and a Ford Foundation Postdoctoral Fellowship.
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Scheme 5 Protonation of zirconophosphaalkene complexes.
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