Facile, metal promoted, oxidation of η4-1,3-diphosphacyclobutadiene by water or methanol

Article abstract of DOI:10.1039/a905240b

The new complexes [MoCl(CO)(η⁴-1,3-P₂C₂Bu(t)₂)(η⁵-L)] (L = C₅H₅ 1, C₅Me₅ 2) are reported; facile oxidation of 1 with water or methanol affords a rare example of a mixed valence phosphaphosphonietinyl in [MoCl(CO){η³,λ³,λ⁵-PC₂Bu(t)₂-PH(OR)}(η⁵-C₅H₅)] (R = H, Me), which has also been structurally characterised for R = H.

Full text of DOI:10.1039/a905240b

4
Facile, metal promoted, oxidation of h -1,3-diphosphacyclobutadiene by water
4
5
or methanol: synthesis of [MoCl(CO)(h -1,3-P2C2But2)(h -L)] (L = C5H5,
C5Me5) and [MoCl(CO){h ,l ,l -PC2But2PH(OR)}(h -L)] (L = C5H5,
3
3
5
5
R = H, Me)
Andrew S. Weller,a Christopher D. Andrews,a Andrew D. Burrows,a Michael Green,*a Jason M. Lynam,a
Mary F. Mahona and Cameron Jonesb
a Department of Chemistry, University of Bath, Claverton Down, Bath, UK BA2 7AY. E-mail: a.s.weller@bath.ac.uk
b Department of Chemistry, University of Wales Cardiff, PO Box 912, Park Place, Cardiff, UK CF1 3TB
Received (in Basel, Switzerland) 29th June 1999, Accepted 13th September 1999
4
5
The new complexes [MoCl(CO)(h -1,3-P2C2But2)(h -L)] (L
= C5H5 1, C5Me5 2) are reported; facile oxidation of 1 with
water or methanol affords a rare example of a mixed valence
Complex 2 is remarkably stable to the ambient environment,
C6D6 solutions remaining unchanged (by 1H and 31P{1H} NMR
spectroscopy) on exposure to air or water for 2 weeks. In sharp
contrast, 1 undergoes reaction with water (slowly with atmos-
pheric, ca. 2 days on addition of 10 equiv.) or methanol (7 days)
to afford complexes in which one of the coordinated phosphorus
atoms has undergone formal oxidative addition of HOR (R =
phosphaphosphonietinyl in [MoCl(CO){h ,l ,l -PC2But2-
3
3
5
5
PH(OR)}(h -C5H5)] (R = H, Me), which has also been
structurally characterised for R = H.
3
3
5
We have previously reported on studies directed towards the
H, Me). The resulting complexes [MoCl(CO){h ,l ,l -PC2-
2
5
development of the transition metal chemistry of h (4e)-bonded
But2PH(OR)}L] (L = h -C5H5, R = H 3, or Me 4), are formed
phosphaalkynes.1,2 Mindful of the relationship between phos-
phaalkynes and their isoelectronic alkyne counterparts we have
examined the reactivity of P·CBut towards the molybdenum
in essentially quantitative yield for 3 and in ca. 90% yield for 4
(impurities present are compound 3 and at least two other
unidentified compounds). The solid state structure of 3 is shown
in Fig. 1.‡
5
5
complexes [MoCl(CO)3L] (L = h -C5H5 or h -C5Me5). It has
5
been shown3,4 that [MoCl(CO)3(h -C5H5)] reacts thermally
It is apparent that one of the phosphorus atoms is no longer
coordinated to the metal [Mo(1)…P(1) 2.8745(13) Å]. Moreo-
with PhC2Ph to form the four-electron donor diphenylacetylene
2
5
3
complex [MoCl(h -PhC2Ph)(CO)(h -C5H5)]. In the event,
however, this study has revealed for the first time that a
coordinated 1,3-diphosphacyclobutadiene ligand can undergo a
facile and selective reaction with water or methanol to give a
ver, P(1) is bent away from the plane of the h -phosphaallyl
group [C(12)–P(2)–C(7)] by 25.12° and also bears a hydroxy
group [P(1)–O(1) 1.581(3) Å] and a direct P–H bond. Both the
hydroxy and phosphorus bound hydrogens were located in the
difference map and refined without constraints. The remaining
bond lengths and angles are consistent with the description of 3
3
3
5
mixed valence h ,l ,l -phosphaphosphonietinyl ligand.
Addition of 2.1 equiv. of P·CBut to pentane solutions of
5
5
3
3
5
[MoCl(CO)3L] (L = h -C5H5 or h -C5Me5) and gentle heating
as a h ,l ,l -phosphaphosphonietinyl complex, and as such are
(40 °C) for 78 and 24 h respectively, results in the isolation of
unremarkable.
orange [MoCl(CO)(h -1,3-P2C2But2)L] (L = h -C5H5 1, h -
From a bonding perspective, 3 can be viewed in terms of the
two canonical forms A and B; one in which there is a formal
bond between Mo and P(1) (A) or a zwitterionic alternative in
which there is no bond between these two atoms (B). On the
basis of the long Mo–P(1) distance coupled with the fact that in
the IR spectrum the carbonyl group stretch is observed at > 60
cm21 to lower frequency compared with 1, which is consistent
with a negative charge on Mo, we suggest that B represents the
4
5
5
C5Me5 2) in moderate and good yield, respectively.
A single crystal X-ray diffraction study performed on 2
confirmed the structure, and this will be reported in full at a later
date.5 The NMR spectra of 1 and 2 are also fully consistent with
this description.† Two different environments are observed for
each of the phosphorus and carbon atoms in the 1,3-diph-
osphacyclobutadiene ring and the appended But groups, as a
consequence of the asymmetry introduced by the carbonyl and
chloride ligands. This demonstrates that the 1,3-P2C2But2
ligand does not rotate on the NMR timescale at room
temperature, an observation that has been precluded previously
by the relatively high symmetry of associated metal fragments
in other 1,3-diphosphacyclobutadiene complexes.6,7 However,
the 1,3-diphosphacyclobutadiene ligand in 1 does begin to
rotate on the NMR timescale at higher temperatures, the onset of
Fig. 1 Crystal structure of one molecule within the asymmetric unit of
compound 3 showing the labelling scheme used. Thermal ellipsoids are
represented at the 30% probability level. Selected bonds distances (Å):
Mo(1)–Cl(1) 2.5468(11), Mo(1)–C(1) 2.036(4), Mo(1)–P(1) 2.8745(13),
Mo(1)–P(2) 2.4179(10), P(2)–C(12) 1.848(3), P(2)–C(7) 1.839(3), P(1)–
C(12) 1.742(3), P(1)–C(7) 1.742(3), P(1)–O(2) 1.581(3).
1
coalescence of the But signals being observed in the H NMR
spectrum at +60 °C at 400 MHz.
Chem. Commun., 1999, 2147–2148
This journal is © The Royal Society of Chemistry 1999
2147

5
3
species
[Mo(h -C9H7)(CO)2(h -1,3-PC2But2PFBF3)]
5.
Whereas, in solution, [BF4]2 rapidly dissociates from 5, in the
case of the formation of 3 and 4, trans-nucleophilic attack by
H2O or MeOH is consummated by addition of H+ to the
phosphorus centre. Additional support for this mechanism
5
3
comes from the zwitterionic complex [Ru(h -C5H5)(h -
1,3-PC2But2PFCButP)] 6, generated from a putative inter-
mediate resulting from [PF6]2 attack at a coordinated 1,3-diph-
osphacyclobutadiene ring.10 There is, of course, a more subtle
variation on these two pathways, which depends on hydrogen
bonding (Scheme 1) and involves simultaneous protonation and
nucleophilic attack. In agreement with these suggestions,
involving rate determining breaking of an OH bond, is the
observation of a large positive kinetic isotope effect kD/kH ≈ 3.6
[t1/2(H2O) 5.5 h, t1/2 (D2O) 20 h].
We have reported here the synthesis of two new readily
accessible 1,3-diphosphacyclobutadiene complexes, and for
one, facile oxidative addition of water or methanol, a reaction
without precedent. The robust nature of 2 combined with the
reactivity of 1 presages a wealth of chemistry associated with
this system which is currently under investigation.
mode of bonding in 3. In either canonical form P(1) may be
considered formally as being phosphorus(v).
The NMR data for 3 and 4 are very similar; for both
compounds H(1) is observed as a doublet of doublets in the 1H
NMR spectrum, the very large value of one of the couples
[J(PH) ≈ 500 Hz] confirming the presence of a direct P–H
bond. Although the hydroxy proton in 3 is not observed in the
1H NMR spectrum, presumably owing to fast exchange in
solution, in 4 the methoxy group is clearly seen as an integral 3H
doublet. In the 31P{1H} NMR spectra of 3 and 4 two different
phosphorus environments are observed [3: d(31P) 8.9, 215.8, 4:
d(31P) 10.6, 216.7], shifted significantly to higher field from
those in 1, with the broader, low field resonance, split on
retention of coupling to 1H.
We thank the EPSRC (J. M. L.) and the University of Bath
(C. D. A.) for support.
As previously stated, the formulation of compounds 3 and 4
corresponds to the unprecedented selective oxidative
[P(iii)?P(v)] addition of water or methanol respectively to one
of the phosphorus centres present in the 1,3-diphosphacyclobu-
tadiene complex, 1. Although it has been previously shown that
one of the phosphorus lone pairs of a coordinated 1,3-diph-
osphacyclobutadiene can coordinate onto transition metal
Lewis acid fragments,8 it seems unlikely that the formation of 3
or 4 involves an initial deprotonation of H2O or MeOH (Scheme
1, pathway a,) followed by nucleophilic attack by HO2 or
MeO2 on the resulting [1,3-PP(H)C2]+ ring. More likely is
pathway b of Scheme 1 since it has been previously reported 9
Notes and references
† Selected NMR data for the new complexes: 1: 1H(CD2Cl2) d 5.66 (s, 5 H,
C5H5), 0.89 (br, 9 H, But), 0.69 (br, 9 H, But). 31P{1H}(CD2Cl2) d 86.9 (s,
1 P), 71.1 (s, 1 P). IR (pentane) 1988 cm21. 2: 1H(CD2Cl2) d 1.90 (s, 15 H,
C5Me5), 0.86 (s, 9 H, But), 0.82 (s, 9 H, But). 31P{1H}(CD2Cl2) d 101.7 [d,
1 P, J(PP) 7 Hz], 74.4 (br, 1 P). IR (CH2Cl2) 1961 cm21. 3 1H [(CD3)2CO]
d 8.11 [dd, 1 H, PH, J(PH) 4, 499 Hz], 5.58 (s, 5 H, C5H5), 1.24 (s, 9 H, But),
1.03 (s, 9 H, But). 31P [(CD3)2CO] d 8.9 [d, 1 P, J(HP) 499 Hz], 215.8 (s,
1 P). IR (KBr) 1921 cm21. 4: 1H(C6D6) d 8.00 [dd, 1 H, PH, J(PH) 5, 496
Hz], 5.41 (s, 5 H, C5H5), 3.90 [d, 3 H, POMe, J(PH) 12 Hz], 1.00 (s, 9 H,
But), 0.80 (s, 9 H, But). 31P[(CD3)2CO] d 10.6 [d, 1 P, J(HP) 496 Hz], 216.7
5
4
that in the solid state structure of [Mo(h -C9H7)(CO)2(h -
1,3-P2C2But2)][BF4] the [BF4]2 anion is bonded to one of the
phosphorus centres resulting in the formation of the zwitterionic
(s, 1 P). IR (KBr) 1889 cm21
.
‡ Crystal data: 3: C16H25ClMoO2P2·0.25CH2Cl2, M = 463.92, l =
0.71069 Å, triclinic, space group P1 (no. 2), a = 10.067(2), b = 13.036(4),
¯
c = 17.169(4) Å, a = 67.90(2), b = 87.66(2), g = 72.60(1)°, U =
1985.5(9) Å3, Z = 4, T = 293(2) K, Dc = 1.552 g cm23, m = 1.028 mm21
,
F(000) = 946; 6974 unique reflections, all data; R1 = 0.0400 wR2
=
0.0771 (all data). CCDC 182/1420. See http://www.rsc.org/suppdata/cc/
1999/2147/ for crystallographic files in .cif format.
1 G. Brauers, M. Green, C. Jones and J. F. Nixon, J. Chem. Soc., Chem.
Commun., 1995, 1125.
2 N. Carr, M. Green, M. F. Mahon, C. Jones and J. F. Nixon, J. Chem.
Soc., Chem. Commun., 1995, 2191.
3 J. L. Davidson and D. W. A. Sharp, J. Chem. Soc., Dalton Trans., 1975,
2531.
4 J. L. Davidson, M. Green, F. G. A. Stone and A. J. Welch, J. Chem. Soc.,
Dalton Trans., 1976, 738.
5 A. S. Weller, C. D. Andrews, A. D. Burrows, M. Green, J. M. Lynam
and M. F. Mahon, unpublished work.
6 (a) P. Binger, R. Milczarek, R. Mynott, M. Regitz and W. Rösch,
Angew. Chem. Int. Ed. Engl., 1986, 25, 644; (b) F. G. N. Cloke, K. R
Flower, P. B. Hitchcock and J. F. Nixon, J. Chem. Soc., Chem.
Commun., 1994, 489; (c) P. B. Hitchcock, M. J. Maah and J. F. Nixon,
J. Chem. Soc., Chem. Commun., 1986, 737.
4
7 The complex [Mo(h -P2C2But2)3] [ref. 6(b)] has static 1,3-diph-
osphacyclobutadiene rings on the NMR timescale, but is also sterically
very crowded.
8 P. B. Hitchcock, M. J. Maah, J. F. Nixon and C. Woodward, J. Chem.
Soc., Chem. Commun., 1987, 844; H. F. Dare, J. A. K. Howard, M. U.
Pilotti, F. G. A. Stone and J. Szameitat, J. Chem. Soc., Chem. Commun.,
1989, 1409.
9 P. B. Hitchcock, M. J. Maah, J. F. Nixon and M. Green, J. Organomet.
Chem., 1994, 466, 153.
10 P. B. Hitchcock, C. Jones, J. F. Nixon, Angew. Chem. Int. Ed. Engl.
1994, 33, 463.
Scheme 1 i + [H]+; ii + [OR]2 (R = H or Me); iii + HOR (R = H or Me),
2[H]+.
Communication 9/05240B
2148
Chem. Commun., 1999, 2147–2148