Ten-Electron Coordination and Reactivity of an Arylphosphinidene Ligand

Article abstract of DOI:10.1021/ja037293r

Photochemical decarbonylation of [Mo₂Cp₂(μ-PR*)(CO)₄] (Cp = η⁵-C₅H₅; R* = 2,4,6-C₆H₂tBu₃) gives [Mo₂Cp₂(μ-κ¹:κ¹,η⁶-PR*)(CO)₂], which shows the first example of a remarkable 10-electron donor arylphosphinidene ligand which bridges two Mo atoms through its phosphorus atom while being π-bonded to one Mo center through the six carbon atoms of the aryl ring. This causes a severe pyramidal distortion of the P-bound C atom. The complex adds CO to give [Mo₂Cp₂(μ-κ¹:κ¹,η⁴-PR*)(CO)₃], which has an 8-electron donor PR* ligand, and then the parent complex [Mo₂Cp₂(μ-PR*)(CO)₄]. Protonation of [Mo₂Cp₂(μ-κ¹:κ¹,η⁶-PR*)(CO)₂] gives the hydride [Mo₂Cp₂(H)(μ-κ¹:κ¹,η⁶-PR*)(CO)₂]⁺, which undergoes P?C bond cleavage and hydride migration, affording the phosphido cation [Mo₂Cp₂(μ-P)(η⁶-R*H)(CO)₂]⁺. Copyright

Full text of DOI:10.1021/ja037293r

Published on Web 10/07/2003
Ten-Electron Coordination and Reactivity of an Arylphosphinidene Ligand
M. Esther Garc´ıa, V´ıctor Riera, Miguel A. Ruiz,* David Sa´ez, Hayrullo Hamidov,†
John C. Jeffery,† and Thomas Riis-Johannessen†
Departamento de Qu´ımica Orga´nica e Inorga´nica/IUQOEM, UniVersidad de OViedo, 33071 OViedo, Spain, and
School of Chemistry, UniVersity of Bristol, Bristol BS8 1TS, UK
Received July 15, 2003; E-mail: mara@sauron.quimica.uniovi.es
The chemistry of metal complexes having phosphinidene (PR,
with R ) alkyl, aryl, etc.) ligands is currently a very active research
field in the frontier between inorganic and organic chemistry.1 The
phosphorus atom in the PR moiety has four valence electrons
available for ligation and can bind from one to four metal atoms
M in many different ways. In mononuclear complexes the M-P
bonds have considerable multiple character, and this confers a high
reactivity to the corresponding molecules. This inherent reactivity,
added to the conceptual analogy established between the chemistry
of multiple M-P and M-C bonds,2 has meant that such complexes
have became very useful synthetic reagents in organophosphorus
and cluster chemistry.1,2
As we pointed out recently, the chemistry of metal-metal bonded
Figure 1. View of the molecular structure of compounds 3 (a) and 6 (b).
The Bu group on C(4) is omitted in both cases.
t
complexes with trigonal phosphinidene bridges remains largely
from planarity for the C atom bonded to phosphorus. Finally, the
unexplored, and this prompted us to study the chemical behavior
PR ligand in this new coordination environment is chemically
of the dimolybdenum complex [Mo2Cp2(µ-PR*)(CO)4] (1) (R* )
active, and unexpected transformations, involving changes in the
2,4,6-C6H2 Bu3).3 In our preliminary studies we found that UV
t
hapticity of the aryl ligand or the cleavage of the P-C bond, can
irradiation of 1 led to the triply bonded derivative [Mo2Cp2(µ-PR*)-
occur under mild conditions.
(µ-CO)2] (2) and a minor unidentified product with a 31P NMR
Compound 3 is best prepared by UV irradiation of a concentrated
resonance at 509.7 ppm. Herein, we report an optimized synthesis
tetrahydrofuran solution of 1, under a nitrogen purge. This leads
of this compound, now characterized as [Mo2Cp2(µ-κ1:κ1,η6-PR*)-
to a mixture of the isomeric complexes 2 (41%) and 3 (19%).4 In
(CO)2] (3) and describe its remarkable structure and reactivity
(Scheme 1). The discovery of this 10-electron donor arylphosphin-
t
the crystal (Figure 1a),5 the complex 3 has a PC6H2 Bu3 ligand
which is η6-bound to Mo(1), while the P atom binds to both Mo
centers. The constraints thus imposed lead to an unprecedented
Scheme 1
situation in which the P atom lies 1.58 Å out of the plane of the
arene ring. This largely exceeds the small related distortions (less
than 0.6 Å) usually found in PR*-bridged dimetal complexes. The
carbon atom C(1) is therefore pyramidal, but rather than being
displaced away from the metal, it is ca. 0.1 Å closer to Mo(1)
[Mo(1)-C(1) ) 2.202(2) Å] than are the remaining aryl ring
carbons [2.315 ( 0.015 Å]. To achieve an electron count of 18 at
each metal atom, the phosphorus atom should formally bind the
metals via P(3)-Mo(1) single and P(3)tMo(2) triple bonds,
respectively. The interatomic distances, however, suggest intermedi-
ate bond orders. Thus, the P(3)-Mo(2) length of 2.2480(6) Å is
intermediate between the reference values found in 1 (formal
PdMo, ca. 2.30 Å)6 and that in [WCl2(PR*)(CO)(PMePh2)2]
(formal PtW, 2.169(1) Å).7 Similarly, the P(3)-Mo(1) distance
of 2.3630(6) Å is longer than a typical ModP bond, but much
shorter than the values observed for comparable M-P single bonds
[i.e. 2.550(3) Å for the single W-P length in [W2Cp2(µ-PR′)-
(CO)4(PH2R′)] (R′ ) 2,4,6-C6H2Me3)].8 These data suggest that
the P-Mo bonds in 3 are best described by contributions from the
following resonance structures: Mo-PR*tMo T Mo+dPR*dMo-.
A DFT calculation9 on 3 gave minimized structural parameters
in good agreement with those obtained from the X-ray diffraction
study. The four frontier HOMOs for 3 are close in energy and are
largely metal d-orbital in character. The latter orbital (Figure 2a)
has a significant antibonding interaction between the pyramidal
carbon C(1) and its adjacent ring carbons, which is consistent with
idene ligand is relevant for different reasons apart from novelty:
first, most of the PR complexes known to date have R ) phenyl or
2,4,6-substituted phenyl, so that the new coordination mode now
disclosed should be taken into account when analyzing the chemical
transformations of any arylphosphinidene ligand at di- and poly-
nuclear centers. Second, the simultaneous coordination of the
bridging P and the aryl ring in a dinuclear complex is totally
unexpected on geometrical grounds, as it implies a strong deviation
† University of Bristol.
9
13044
J. AM. CHEM. SOC. 2003, 125, 13044-13045
10.1021/ja037293r CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
The new coordination modes and transformations of the arylphos-
phinidene ligand here described may be of relevance in the field
of transition-metal phosphides. These materials are currently under
intense research both because of their electric properties17 and
catalytic activity in processes such as hydrodesulfurization (HDS)
or dehydrodenitrogenation (HDN) of fuels.18 It is conceivable that
intermediate species having phosphorus environments similar to
those found in complexes 3 to 6 might be involved in the synthesis
of metal phosphides by chemical vapor deposition (CVD) tech-
niques (for instance, MoP from MoCl5 and PCyH2)19 or as
chemisorbed species in the metal phosphide-catalyzed HDS or HDN
of fuels.
Figure 2. The HOMO-3 (a) and HOMO-4 (b) orbitals of compound 3,
from the DFT calculation, with Bu groups omitted for clarity.
t
the lengthening of ca. 0.04 Å that is found for the C(1)-C(n) bonds
(n ) 2,6) in the crystal. The presence of a π-bonding interaction
between Mo(1) and P, inferred from the X-ray interatomic distances,
is supported by the nature of the HOMO-4 orbital, which shows a
major contribution from a phosphorus p-orbital overlapping with
d orbitals on both metals (Figure 2b).
The decarbonylation of 1 which affords 3 is reversible, and CO
addition to 3 proceeds stepwise through the tricarbonyl complex
[Mo2Cp2(µ-κ1:κ1,η4-PR*)(CO)3] (4) which then adds a further CO
ligand to give 1. Spectroscopic data for 4 reveal that a CO ligand
has been added to the Mo(1) atom with a concomitant change in
the hapticity of the aryl ligand, which is now π-bonded to the metal
through just four of the ring C-atoms (η4-mode).10 This is readily
deduced from the 13C{1H} NMR spectrum, which exhibits just four
strongly shielded ring resonances.
Acknowledgment. We thank the MCYT of Spain for a grant
(to D.S.) and financial support (Project BQU2000-0944).
Supporting Information Available: Experimental procedures,
spectroscopic and microanalytical data for new complexes (PDF), and
crystallographic data for compounds 3 and 6 (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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The coordination mode of the phosphinidene in complex 4 makes
this ligand act as an 8-electron donor, which has no precedent in
the literature. Moreover, its formation from 3 suggests that a
6-electron coordination mode (µ-κ1:κ1,η2-PR*) might occur in the
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(4) Selected spectroscopic data for 3: ν(CO) (CH2Cl2) 1891 (vs), 1816 (s)
cm-1 31P{1H} NMR (121.57 MHz) δ 509.9 (s, µ-PR*). 13C{1H} NMR
.
(100.63 MHz) δ 242.2 (d, JCP ) 7, 2 × CO), 112.1 [s, 2,6C(C6H2)], 99.3
[s, 4C(C6H2)], 92.5, 88.0 (2 × s, Cp), 84.2 [d, JCP ) 74, 1C(C6H2)], 80.2
[s, 3,5C(C6H2)].
(5) X-ray data for 3: Black crystals, monoclinic (P21/n), a ) 9.952(2) Å,
b ) 19.642(4) Å, c ) 14.316(3) Å, â ) 90.39(3)°, V ) 2798.3(10) Å3,
T ) 100 K, Z ) 4, R ) 1.94, GOF ) 1.052.
Protonation of 3 also leads to unexpected results. Reaction with
[H(OEt2)2][BAr′4], [Ar′ ) 3,5-C6H3(CF3)2], occurs rapidly at 233
K to give initially the hydride complex [Mo2Cp2(H)(µ-κ1:κ1,η6-
PR*)(CO)2][BAr′4] (5).11 Compound 5 exists in solution as an
equilibrium mixture of cis and trans isomers, and at room
temperature it rearranges to give the phosphido-bridged complex
[Mo2Cp2(µ-P)(CO)2(η6-HR*)][BAr′4] (6) in high yield.12 An X-ray
study of this product (Figure 1b)13 confirmed the P-C cleavage in
the phosphinidene ligand and formation of a new C-H bond,
resulting in a π-bound arene HR* and a phosphido ligand bridging
the Mo atoms in an almost linear fashion [Mo(1)-P(3)-Mo(2)
169.31(4)°]. Formal Mo-P bond orders should again be one and
three, respectively. In agreement with this, the Mo(2)-P(3) length
is very short, 2.1685(9) Å. However, the Mo(1)-P(3) distance of
2.3573(9) Å is at least 0.1 Å shorter than expected for a single
Mo-P bond (for example, the single and triple W-P lengths in
the phosphido complex [{N3N)WtP}2W(CO)4] [N3Nd(Me3-
SiNCH2CH2)3N] were ca. 2.46 and 2.20 Å, respectively).14
Although a few dimetallic, linearly bridged phosphido complexes
related to 6 are known,15 the H+ induced P-C cleavage responsible
for the formation of 6 is largely unprecedented in phosphinidene
chemistry. We note, however, that the phosphinidene complex [W2-
(µ-PCp*)(CO)10] (Cp* ) C5Me5) is believed to transform into the
transient phosphido complex [W2Cp*(µ-P)CO)8] in refluxing
toluene.16a Moreover, a sterically induced P-C bond cleavage is
thought to be responsible for the formation of [Zr2Cp*4(µ-P)] from
[ZrCp*2Cl(PR*H)].16b DFT calculations correctly predict that the
energy of 6 is some 104 kJ mol-1 below that of 5, and further
studies are now in progress to attempt to identify the path by which
5 rearranges to the final complex 6.
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(9) The structures were optimized using unrestricted B3LYP theory, together
with standard 6-31G* on all atoms except Mo, for which the Jaguar triple-ú
form of the standard Los Alamos ECP basis set (LACV3P) was used.
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(Schro¨dinger, Inc., Portland, OR, 1995-2002).
(10) Selected spectroscopic data for 4: ν(CO) (CH2Cl2) 1961 (s), 1899 (vs),
1823 (s) cm-1 31P{1H} NMR (121.53 MHz) δ 476.1 (s, µ-PR*). 13C{1H}
.
NMR (75.47 MHz) δ 243.9 (d, JCP ) 16, CO), 238.9 (d, JCP ) 17, CO),
234.5 (d, JCP ) 13, CO), 158.5 [s, 6C(C6H2)], 126.3 [s, 5C(C6H2)], 109.8,
108.9 [2 × s, 2C and 4C(C6H2)], 91.7, 91.4 (2 × s, 2 × Cp), 87.2 [s,
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cm-1. Data for trans-5: 1H NMR (400.13 MHz, CD2Cl2, 213 K) δ 5.99
(s, 2H, C6H2), 5.65, 5.52 (2 × s, 2 × 5H, Cp), -3.91 (d, JHP ) 20, 1H,
Mo-H). 31P{1H} NMR (162.09 MHz, 213 K) δ 644.6 (s, µ-PR*).
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cm-1
.
31P{1H} NMR (121.55 MHz) δ 908.5 (s, µ-P). 13C{1H} NMR (75.47
t
MHz) δ 230.2 (s, br, 2 × CO), 118.7 [s, 2,4,6C(C6H3 Bu3)], 92.1, 86.1 (2
× s, 2 × Cp), 84.4 [s, 1,3,5C(C6H3 Bu3)].
t
(13) X-ray data for 6: Black crystals, monoclinic (P21/c), a ) 15.1853(3) Å,
b ) 21.1676(5) Å, c ) 19.4760(4) Å, â ) 98.6950(10)°, V ) 6188.3(2)
Å3, T ) 100 K, Z ) 4, R ) 3.29, GOF ) 0.910.
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JA037293R
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