Dangling Phosphine Complexes of Cr, Mo, and W
Organometallics, Vol. 19, No. 22, 2000 4521
sociation energy of Mo(CO)6 is 167 kJ /mol, considerably
larger than ∆Hq for our system, also suggesting that
bond making is important.14 The activation parameters
may be compared to those obtained previously from
infrared studies for the chelation of (OC)5Mo[η1-PPh2-
CH2CH2PPh2] and (OC)5Mo[η1-PMe2PCH2CH2PMe2].15
Values of 120 kJ /mol and 14 J /(K mol), respectively,
were reported for the former and 118 kJ /mol and -7.0
J /(K mol) for the latter. In those studies it was concluded
that a concerted mechanism of chelation best explains
the results, consistent with our conclusions.
Con clu sion s. Isomerization of (OC)5M[η1-PPh2CH2-
CH2P(p-tolyl)2] and of (OC)5M[η1-P(p-tolyl)2CH2CH2-
PPh2] is faster than chelation for molybdenum and
tungsten complexes but comparable to chelation for
chromium complexes. In all cases bond making in the
transition state appears to be important, but especially
so for the molybdenum and tungsten complexes. Al-
though exchange of coordinated and dangling phos-
phines in pentacarbonyl complexes of group 6 metals
has received little previous attention, it appears to be a
common phenomenon.
F igu r e 4. Plots of concentration versus time: ([) disap-
pearance of (OC)5Mo[ηl-P(p-tolyl)2CH2CH2PPh2] (4Mo); (b)
appearance of (OC)5Mo[η1-PPh2CH2CH2P(p-tolyl)2] (3Mo);
(9) appearance of (OC)4Mo[η2-PPh2CH2CH2P(p-tolyl)2]
(7Mo).
namic comparisons of M-PPh3 with M-P(p-tolyl)3 show
that formation of complexes of PPh3 is less exothermic
than formation of complexes of P(p-tolyl)3. For example,
replacing PPh3 with P(p-tolyl)3 in Fe(CO)3(PR3)2 leads
to a release of 5.4 kJ /mol of energy, or an average of
2.7 kJ /mol per ligand,11 and protonation of P(p-tolyl)3
is more exothermic than protonation of PPh3 by 8.4 kJ /
mol.12 Our isomerization reaction is accompanied by
only a very small increase in entropy (∆S ) 1.5(4) J /K
mol), showing that the difference in disorder for the two
isomers in solution is very small. As shown in Table 1,
the equilibrium constant for isomerization of the mo-
lybdenum complexes is not very temperature-depend-
ent, as expected for a reaction for which ∆H and ∆S
are small.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. Reactions and manipulations of
air-sensitive materials were carried out under a dry nitrogen
atmosphere with standard Schlenk techniques. Preparations
of P(p-tolyl)2H,16 (OC)5Cr[PPh2CHdCH2],17 (OC)5Mo[PPh2CHd
CH2],17 (OC)5W[η1-PPh2CH2CH2P(p-tolyl)2],2 and (OC)5W[η1-
P(p-tolyl)2CH2CH2PPh2]2 were as described previously. NMR
and IR spectra were recorded with GE QE-300 and Nicolet 20
DXB FT-IR spectrometers, respectively. Phosphorus-31 NMR
spectra are referenced to 85% phosphoric acid. Elemental
analyses were performed at the University of Illinois Micro-
analytical Laboratory, Urbana, IL.
The activation enthalpies, ∆Hq, for the isomerization
3Mo a 4Mo were found to be 102.4(6) kJ /mol for the
forward reaction and 103.6(3) kJ /mol for the reverse.
The difference between the two numbers (-1.3 kJ /mol)
is in good agreement with ∆H for the reaction (-1.2 kJ /
mol). Likewise, the values of ∆Sq, -32(2) J /(K mol) for
the forward reaction and -33(1) J /(K mol) for the
reverse reaction, are in agreement with ∆S for the
reaction (1.5 J /(K mol)).
The enthalpies of activation are considerably less than
the average M-P bond dissociation energies reported
for Mo(CO)3(PPh2Me)3 (147 kJ /mol) and Mo(CO)4(PPh2-
Me)2 (158 kJ /mol), supporting a mechanism with a
significant associative contribution.13 The entropies of
activation for both the forward and reverse isomeriza-
tion reactions are significantly negative and also strongly
suggest a mechanism in which there is significant
association in the transition state. Thus, it appears that
both reactions 1 and 2 proceed by interchange mecha-
nisms but, as postulated earlier, the former reaction is
accelerated by the presence of a labilizing short phos-
phine arm.2
(OC)5Cr [P (p-tolyl)2H] (5). A solution consisting of Cr(CO)6
(4.02 g, 18.3 mmol) and THF (250 mL) was irradiated in a
quartz vessel with a 400 W UV lamp for 6 h, after which was
added P(p-tolyl)2H (4.0 mL, 18.6 mmol). The solution was
stirred for 1.5 h, the solvent was removed, and the product
was dissolved in a 1:1 v/v ratio of CH2Cl2/CH3OH. Unreacted
Cr(CO)6 was removed by filtration and, after refrigeration at
-5 °C for 48 h, (OC)5Cr[P(p-tolyl)2H] (1.2 g; 32%) crystallized
(mp 91-92 °C). IR (CH2Cl2): A1 + E, 1943 cm-1; A1 , 2066
1
2
cm-1; B2, 1984 cm-1 31P NMR (CDCl3): δ 30.9 ppm (1J PH
.
)
338 Hz). Anal. Calcd for C19H15O5PCr: C, 56.17; H, 3.72.
Found: C, 56.54; H, 3.60.
(OC)5Mo[P (p-tolyl)2H] (6). A solution of Mo(CO)6 (3.56 g,
36.0 mmol) and dimethoxyethane (60 mL) was heated to reflux
for 1 h. To this solution was added P(p-tolyl)2H (2.9 mL, 36
mmol), and the resulting solution was heated under reflux for
an additional 2.5 h. Recrystallization from a 1:1 v/v ratio of
CH2Cl2 and CH3OH after solvent removal gave (OC)5Mo[P(p-
1
tolyl)2H] (3.60 g, 58.3%; mp 81-82 °C). IR (CH2Cl2): A1 + E,
2
1948 cm-1; A1 , 2075 cm-1
.
31P NMR (CDCl3): δ 4.5 ppm (1J PH
The rates of chelation of 3Mo and 4Mo (Table 1) were
nearly identical for each of the three temperatures
reported. Activation parameters, ∆Hq and ∆Sq, for
chelation of 3Mo were 113.5(5) kJ /mol and -22(15) J /(K
mol), respectively, while for 4Mo they were 115.5(4) kJ /
mol and -16(12) J /(K mol), suggesting a transition state
in which bond making is important. The M-CO dis-
) 332 Hz). Anal. Calcd for C19H15O5PMo: C, 50.69; H, 3.36.
Found: C, 50.59; H, 3.25.
(OC)5Cr [η1-P P h 2CH2CH2P (p-tolyl)2] (3Cr ). A mixture of
(OC)5Cr[PPh2CHdCH2] (1.10 g, 2.72 mmol), P(p-tolyl)2H (0.55
mL, 2.57 mmol), and 2,2′-azobis(isobutyronitrile) (AIBN; 0.15
g) was heated without solvent at 80 °C for 24 h. Recrystalli-
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1984, 106, 3905.
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Inorg. Chem. 1988, 27, 81. (c) Dias, P. B.; Minas de Piedade, M. E.;
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