Expanding molecular transition metal cubane clusters of the form [M 4(μ3-O)4]12+: Syntheses, spectroscopic and structural characterizations of molecules M 4(μ3-O)4(O2P(Bn) 2)4(O4), M = VV and WV

Article abstract of DOI:10.1039/c1dt11375e

Oxidizing the trimer V₃(μ₃-O)(O ₂)(μ₂-O₂P(Bn)₂) ₆(H₂O) in the presence of excess ^tBuOOH results in V₄(μ₃-O)₄(μ₂-O ₂P(Bn)₂)₄(O₄) and heating W(CO) ₆ and bis(benzyl)phosphinic acid in 1:1 EtOH/THF at 120 °C produces W₄(μ₃-O)₄(μ₂-O ₂P(Bn)₂)₄(O₄).

Full text of DOI:10.1039/c1dt11375e

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COMMUNICATION
Expanding molecular transition metal cubane clusters of the form
[M4(l3-O)4]12+: syntheses, spectroscopic and structural characterizations of
molecules M4(l3-O)4(O2P(Bn)2)4(O4), M = VV and WV†
John S. Maass,a Zhichao Chen,a Matthias Zellerb and Rudy L. Luck*a
Received 21st July 2011, Accepted 12th September 2011
DOI: 10.1039/c1dt11375e
Oxidizing the trimer V3(l3-O)(O2)(l2-O2P(Bn)2)6(H2O) in
the presence of excess tBuOOH results in V4(l3-O)4(l2-
O2P(Bn)2)4(O4) and heating W(CO)6 and bis(benzyl)phos-
phinic acid in 1 : 1 EtOH/THF at 120 ◦C produces W4(l3-
O)4(l2-O2P(Bn)2)4(O4).
The synthesis of these compounds turned out to be much more
difficult than one would have predicted, given the various ways
in which the analogous molybdenum tetramers were produced.16
Essentially, the only reliable way we have discovered to produce
the vanadium tetramer, V4(m3-O)4(O2P(Bn)2)4(O4), 1, consists of
the oxidation of the trimer V3(m3-O)(O2)(m2-O2P(Bn)2)6(H2O)19 in
t
the presence of excess BuOOH followed by self assembly of the
desired compound, possibly as outlined in eqn (1).‡
In contrast to the large number of sulfur-containing clusters
known for a wide range of transition metals of the form [M4(m3-
S)4]n+,1 which also extends to heterometallic formulations,2 there
has not been an equivalent number of reports of molecular
oxygen-containing clusters of the type [M4(m3-O)4]n+. Notable
exceptions are tin3 (i.e., [Sn4O4]8+), titanium4 [Ti4O4]8+ and man-
ganese [Mn4O4]6+ clusters5 due to their relevance to the oxygen-
evolving complex in photosystem II.6 There are several reports
of vanadium(IV) in sulfite polyoxometalates,7 and as [P2V4O16]6-
in the mineral phosphovanadylite8 and as vanadium(V) in two
other layered phosphovanadate structures.9 For tungsten, reports
of planar aggregates, e.g. [WO2Cl2(THF)]4,10 and of cubane-like
clusters with tetrameric tungstate(VI) units have been detailed.
The [W4O16]8- cores were stabilized by metal-containing units
4 V3(m3-O)O2)(m2-O2P(Bn)2)6(H2O) + 6 tBuOOH + 2 H2O →
(1)
3 V4(m3-O)4(O2P(Bn)2)4(O4) + 12 HOOP(Bn)2 + 6 tBuOH
The production of HOOP(Bn)2 was verified by IR spectroscopy
and the reaction requires water for it to be successful. The synthesis
of complex 1 via this procedure is based on elemental analysis, IR,
1H and 31P NMR spectra (see ESI†) and the fact that the spectra
are similar to those for tungsten (see below) and the molybdenum16
analogs. Routes involving the reaction of starting vanadium
complexes such as V2O5 or [Bu4N]3[H3V10O28] and HOOP(Bn)2 in
CH3CN with or without [Bu4N][OH] either resulted in intractable
products or the trimer V3(m3-O)(O2)(m2-O2P(Bn)2)6(H2O). Reac-
tions with VO(acac)2 in EtOH under solvothermal conditions or
in THF at room temperature in the presence of HOOP(Bn)2 and
either tBuOOH or H2O2 were not successful, as the EtOH reduces
V(V) to V(IV); these reactions also resulted in the aforementioned
trimer. However, oxidizing a solution of VO(acac)2 in EtOH with
tBuOOH and then adding HOOP(Bn)2 in CH2Cl2 (followed by
stirring for 5 h at room temperature) leads to mixtures (38.5%
yield) of the desired tetramer, the aforementioned trimer and/or
some other complex. The desired orange tetramer may be obtained
in pure form by selective crystallization consisting of the diffusion
of hexane into CH2Cl2 solutions of the crude mixture.
6
such as [(1,5-COD)Ir]+,11 [Cp*Rh]2+,12 [Ru(h -C6Me6)]2+,13 and
[Cr(cyclam)]3+.14 So far, dimeric complexes featuring a [W2O4]2+
core have been produced only with tungsten(V)15 exhibiting WV–
15e
WV single bond distances in the range 2.537(1)15b–2.568(1) A.
˚
Our report16 of a readily available route to the previously reported
Mo4(m3-O)4(m2-O2P(C6H5)2)4(O4)17 followed by the syntheses of
this molybdenum tetrameric core stabilized by other phosphinate
ligands, including dibenzylphosphinate,18 raised the obvious ques-
tion as to whether phosphinate ligands (also utilized in many of
the complexes referred to above) would be capable of extending the
number of transition metals that can exhibit this type of bonding
in molecular form. This paper details our success at producing
vanadium and the first reported tungsten(V) tetrameric cubane
clusters of the form M4(m3-O)4(O2P(Bn)2)4(O4).
The tungsten tetramer, W4(m3-O)4(m2-O2P(Bn)2)4(O4), 2, was
synthesized from W(CO)6 and was not obtained using a high
oxidation state compound, e.g., Na2WO4.§ As shown in eqn (2),
this reaction was conducted under solvothermal conditions in a
solvent mixture consisting of equal amounts of EtOH and THF.
aDepartment of Chemistry, Michigan Technological University, 1400
Townsend Drive, Houghton, MI, 49931, USA. E-mail: rluck@mtu.edu
W(CO)6 + 4 HOOP(Bn)2 → W4(m3-O)4(m2-O2P(Bn)2)4(O4)
bDepartment of Chemistry, Youngstown State University, 1 University Plaza,
Youngstown, Ohio, 44555, USA. E-mail: mzeller@ysu.edu
(2)
EtOH:THF, 120 ◦C
W(CO)6 is known to oxidize in solution under O2.20 In pure
EtOH, 2 was obtained on occasion but this is not as reproducible
compared to using the mixture of solvents. Complexes 1 and 2
have very characteristic IR patterns in the range 1012–978 cm-1.
† Electronic supplementary information (ESI) available: Elemental anal-
yses, IR, 1H and 31P NMR spectra, TGA plots for 1–3 and X-ray data
for 1 and 2. CCDC reference numbers 834511 and 834512. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt11375e
11356 | Dalton Trans., 2011, 40, 11356–11358
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This takes the form of two sharp outer peaks with a shoulder
peak in the middle (1012, 1000 and 990 cm-1 for 1; 1012, 1000
and 979 cm-1 for 2) and this pattern was also observed for the
molybdenum analogue (1011, 998 and 980 cm-1).18a The complexes
1
yielded distinctive peaks in their H NMR spectra consisting of
AB patterns for the diasterotopic CH2 protons of the benzylic
ligand at d 2.86 and 2.90 for 1 and d 2.93 and 3.06 for 2. The
1
31P{ H} NMR spectra consist of single resonances at d 60.6 and
68.4 for 1 and 2 respectively.
We initially observed crystals of 1 growing in a tube consisting
of a mixture of the trimers V3(m3-O)(O2)(m2-O2P(Bn)2)6(H2O) and
V3(m3-O)(O2)(m2-O2P(Bn)2)6(py)19 in wet CH2Cl2 and hexane.† The
observation of orange crystals prompted a single crystal X-ray
study, which revealed that 1 was produced and crystallized as
a tetrahydrate, with one crystallographically independent water
molecule of solvation per asymmetric unit, disordered over three
positions; this was refined with the sum of the occupancies
constrained to one. Crystals of 2·3(THF) were obtained by
layering a solution of 2 in THF with hexane. For both 1 and
2, single crystal X-ray diffraction studies¶ reveal cubane core
structures consisting of four diagonally arranged metal atoms and
four triply bridging oxygen atoms at the vertices, with terminal
oxygen atoms on each metal atom (arranged so that the four metal
to oxygen atom bonds are all parallel), and dibenzylphosphinate
ligands bridging the four faces of the cube; see Fig. 1 and 2. We have
previously described these structures as consisting of two [M2O4]2+
halves held together by the bridging phosphinate ligands.18a With
these electron deficient materials, we have distortions from perfect
cubane core geometries resulting in elongations along the axes
formed by the bridging phosphinate groups, in contrast to the al-
most perfect geometries exhibited by more electron rich clusters.22
Fig. 2 Mercury21 drawing of 2 with 50% probability ellipsoids.
Table 1 Selected distances and bond angles for 1, 3 and 2
1 (V)
3 (Mo)
2 (W)
˚
Distances/A
M1 ◊ ◊ ◊ M2
O2 ◊ ◊ ◊ O5
M1 ◊ ◊ ◊ M3
O2 ◊ ◊ ◊ O6
M1–O1
M1–O2
M1–O5
M1–O7
M1–O3
M1–O4
O3¢ ◊ ◊ ◊ O4
Bond angles/◦
O1–M1–O7
M1–O2–M2
O2–M1–O5
O1–M1–O2
O1–M1–O3
O1–M1–O4
O1–M1–O5
2.7610(8)
2.419(2)
3.3300(6)
2.761(2)
1.5786(16)
1.8161(13)
1.8919(13)
2.4392(15)
1.9697(13)
1.9427(13)
2.561(2)
2.6261(5)
2.7560(2)
3.4091(2)
2.7482(2)
1.668(2)
1.9555(19)
1.9612(19)
2.399(2)
2.6354(5)
2.7970(4)
3.4019(4)
2.7262(3)
1.6964(19)
1.9809(16)
1.9825(16)
2.3626(19)
2.0521(17)
2.0742(17)
2.5871(3)
2.0793(19)
2.0586(19)
2.5823(2)
177.40(6)
96.23(6)
81.41(6)
103.16(7)
99.44(7)
99.74(7)
102.31(7)
170.36(8)
84.21(7)
89.44(8)
109.09(9)
97.61(9)
98.92(9)
109.10(9)
168.99(7)
83.35(6)
89.77(7)
110.23(8)
97.66(8)
96.55(8)
110.29(8)
Fig. 1 Mercury21 drawing of 1 with 50% probability ellipsoids.
W(V) species (e.g., anti-[L2W2O4]2+) where shorter distances of
2.565(1) A were noted. This distance is much shorter than that
for the tungsten(VI) cubane clusters11–14 mentioned above, where
15e
˚
Table 1 lists selected bond distances and angles for 1 and 2
and for the complex Mo4(m3-O)4(m2-O2P(Bn)2)4(O4), 3.18a Complex
3 did crystallize in the form of two different solvates, namely
3·2.75C7H8 and 3·2CH2Cl2 and the latter solvate crystallized in
a monoclinic I41/a setting, equivalent to that of 1·4(H2O) and
2·3(THF) and thus the data listed in Table 1 are for isomorphous
compounds, as an examination of the arrangement in the benzyl
groups depicted in Fig. 1 and 2 (and that of 3·2CH2Cl218a) would
attest. The data in Table 1 show that while the metal to metal
˚
distances averaging 3.29(4) A (indicative of no interaction between
the metal atoms) were observed. Evidence of this interaction is also
present in the UV-vis absorption. In this regard complex 3 displays
two absorptions in CH2Cl2 at 314 and 450 nm where the second
transition can be assigned to a Mo–Mo s→s* transition. Notably
complex 1 only displays one intense absorption at 340 nm, which
is presumably a ligand to metal charge transfer band. That for 2 is
at 374 nm with the s→s* transition appearing as a nondescript
buried peak on this large absorption.
˚
distance (M1 ◊ ◊ ◊ M2) is short for 1, that for 2 at 2.6354(5) A
represents a long W–W bond in contrast to those in dimeric
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˚
As expected, the M1–O1 distance in 1 at 1.5786(16) A and the
3062 (vw), 3030 (vw), 1602 (vw), 1496 (m), 1454 (m), 1398 (vw), 1248 (w),
1194 (vw), 1143 (vw), 1087 (m), 1071 (m), 1043 (w), 1012 (s), 1000 (s), 979
(s), 916 (m), 854 (m,br), 779 (m), 694 (s).
˚
corresponding trans M1–O7 at 2.4392(15) A are the shortest and
the longest, respectively, out of the three complexes in Table 1.
The distortion from perfect cubane cores is evident in that there
are much larger distances observed for M1 ◊ ◊ ◊ M3 compared to
the M1 ◊ ◊ ◊ M2 interactions; see Table 1. The distance labeled as
O2 ◊ ◊ ◊ O5 also varies among the three cubes with that for 1 being
¶ Crystal data for 1: C56H56O16P4V4·4(H2O), M = 1376.65, tetragonal, a =
3
˚
˚
˚
˚
22.306(4) A, b = 22.306(4) A, c = 12.824(2) A, V = 6380.7(19) A , T =
100(2)K, space group I41/a, Z = 4, m = 0.738 mm-1, 22 239 reflections
measured, 4828 independent reflections (Rint = 0.0504). R1 = 0.0397 (I >
2s(I)). wR(F2) = 0.0899 (I > 2s(I)). R1 = 0.0691 (all data). wR(F2) = 0.102
(all data). Crystal data for 2: C56H56O16P4W4·3(C4H8O), M = 2060.56,
˚
˚
˚
˚
the shortest at 2.419(2) A, compared to those in 3 and 2 at 2.7560(2)
and 2.7970(4) A, respectively. Somewhat related is the fact that the
tetragonal, a = 22.771(4) A, b = 22.771(4) A, c = 13.107(2) A, V = 6796(2)
3
-1
˚
A , T = 100(2)K, space group I41/a, Z = 4, m = 6.915 mm , 17 299
˚
reflections measured, 5083 independent reflections (Rint = 0.0225). R1
=
O1–M1–O7 angle for 1 at 177.40(6)◦ is very nearly linear whereas
those for 2 and 3 at 168.99(7) and 170.36(8)◦, respectively, deviate
more. These effects may be attributed to the repulsive effects of
the metal to metal atom interaction or more simply the fact that
as the metal atoms form bonds, the oxygen atoms move apart as
a consequence. This also results in the M1–O2–M2 and O2–M1–O5
angles being the largest and smallest, respectively, for 1 compared
to 2 and 3; see Table 1. The last four rows of data in Table 1
illustrate the fact that the geometry at each metal center can be
considered as distorted square pyramidal, with the metal centers
being raised out of the plane defined by the four pseudo-equatorial
oxygen atoms. The listings for the corresponding distances reveal
that those for the oxygen to metal atoms are significantly shorter
in 1 compared to 2 and 3 with the exception of the bond trans to
the multiply bonded oxo ligand. The O3¢ ◊ ◊ ◊ O4 listing in Table 1
illustrates the accommodating nature of the phosphinate bridge
0.0189 (I > 2s(I)). wR(F2) = 0.0425 (I > 2s(I)). R1 = 0.0241 (all data).
wR(F2) = 0.0457 (all data).
1 R. Hernandez-Molina and A. G. Sykes, J. Chem. Soc., Dalton Trans.,
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˚
˚
ranging from 2.561(2) A in 1 to 2.5871(3) A in 3.
In summary, we detail the synthetic routes leading to vanadium
and tungsten tetrameric complexes of the stoichiometry [M4(m3-
O)4]12+. While complexes containing this core were previously
reported for vanadium, albeit in the form of extended clusters, this
is the first report for a tungsten(V) cluster with this core geometry.
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Notes and references
‡ Synthesis of 1: V3(m3-O)O2)(m2-O2P(Bn)2)6(H2O)19 (0.0500 g, 0.0295
mmol) was dissolved in 10 mL of CH2Cl2 at 20 ◦C. To this light blue-
11 Y. Hayashi, F. Mu¨ller, Y. Lin, S. M. Miller, O. P. Anderson and R. G.
Finke, J. Am. Chem. Soc., 1997, 119, 11401–11407.
t
green solution was added BuOOH (0.016 mL of 5.5 M) in decane upon
which the solution turned dark purple. About 5 drops of de-ionized H2O
were then added. After 3 h of stirring, the solution had turned reddish-
orange in color. The solvent was reduced to about 1 mL, kept at -20 ◦C
overnight and then filtered. To the filtrate was added 30 mL of hexane
and this mixture was kept at -20 ◦C for 2 d resulting in an orange
crystalline precipitate, which was then filtered off, rinsed with hexane and
then dried under a vacuum to give 0.0050 g of 1 (0.0038 mmol, 17.2%
yield with respect to V3(m3-O)O2)(m2-O2P(Bn)2)6(H2O)). Anal. (Galbraith
Laboratories, Knoxville, TN) calcd for C56H56V4O16P4: C 51.24, H 4.30.
Found: C 50.84, H 4.58%. 1H NMR (400 MHz, CDCl3): d (ppm) = 2.86
12 K. Nishikawa, K. Kido, J. i. Yoshida, T. Nishioka, I. Kinoshita, B.
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2
2
2
(dd, 8, JHP = 23.3, JHaHb = 14.7 Hz, CHaP), 2.90 (dd, 8, JHP = 27.7,
1
2JHaHb = 14.7 Hz, CHbP), 7.05–7.36 (m, 40, (C6H5CH2)). 31P{ H} NMR
(162 MHz, CDCl3, relative to H3PO4): d (ppm) = 60.6 (s, 1P). UV-vis
spectrum (CH2Cl2) 340 nm. IR (neat, cm-1): 3062 (vw), 3029 (vw), 1602
(vw), 1496 (m), 1454 (m), 1398 (vw), 1245 (vw), 1190 (vw), 1145 (vw), 1101
(m), 1070 (m), 1012 (s), 1000 (s), 990 (s), 914 (vw), 847 (br), 779 (w), 698
(s).
16 A. Jimtaisong, L. Feng, S. Sreehari, C. A. Bayse and R. L. Luck, J.
Cluster Sci., 2008, 19, 181–195.
§ Synthesis of 2: W(CO)6 (0.100 g, 0.284 mmol) and bis(benzyl)phosphinic
acid (0.070 g, 0.283 mmol) were placed in a sealed tube with 6 mL of
a 1 : 1 mixture of ethanol and THF and heated at 120 ◦C for 36 h. The
resulting dark black-blue solution was then cooled to 20 ◦C. After 1 d the
yellow crystals that had formed were filtered off, rinsed with ethanol and
then dried under vacuum to yield 2 (0.018 g, 0.010 mmol, 13.75% yield
based on W(CO)6). Anal. (Galbraith Laboratories, Knoxville, TN) calcd
for C56H56W4O16P4: C 36.47, H 3.06. Found: C 36.63, H 3.23%. 1H NMR
(400 MHz, CDCl3): d (ppm) = 2.93 (dd, 8, 2JHP = 19.5, 2JHaHb = 14.8 Hz,
17 W. Schirmer, U. Floerke and H. J. Haupt, Z. Anorg. Allg. Chem., 1989,
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submitted.
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2
2
CHaP), 3.06 (dd, 8, JHP = 14.9, JHaHb = 14.8 Hz, CHbP), 7.04–7.22 (m,
1
40, (C6H5CH2)). 31P{ H} NMR (162 MHz, CH2Cl2, relative to H3PO4): d
(ppm) = 68.4 (s, 1P). UV-vis spectrum (CH2Cl2) 374 nm. IR (neat, cm-1):
11358 | Dalton Trans., 2011, 40, 11356–11358
22 J. E. McGrady, J. Chem. Soc., Dalton Trans., 1999, 1393–1400.
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The Royal Society of Chemistry 2011
©