Unusually weak metal-hydrogen bonds in HV(CO)4(P-P) and their effectiveness as H? donors

Article abstract of DOI:10.1021/ja710455c

The V-H bonds in HV(CO)₄(P-P) are unusually weak (55 to 58 kcal/mol). They transfer H^? to styrene more rapidly than CpCr(CO)₃H does and are effective stoichiometric reagents for the cyclization of appropriate 1,6-dienes. C

Full text of DOI:10.1021/ja710455c

Published on Web 03/12/2008
Unusually Weak Metal-Hydrogen Bonds in HV(CO)4(P-P) and Their
Effectiveness as H• Donors
Jongwook Choi, Mary E. Pulling, Deborah M. Smith, and Jack R. Norton*
Department of Chemistry, Columbia UniVersity, 3000 Broadway, New York, New York 10027
Received November 19, 2007; E-mail: jrn11@columbia.edu
The transfer of H• from a transition metal to an olefin is a key
step in some hydrogenations and hydroformylations,1 in the
reinitiation step of chain transfer catalysis (eq 1),2 and in our Cr-
catalyzed radical cyclization (eq 2).3 Increasing the rate of such an
H• transfer can presumably be accomplished by decreasing the
strength of the M-H bond. While most such bonds lie between 60
and 80 kcal/mol,4 there is evidence suggesting that V-H bonds
are weaker: HV(CO)4(dppm) and related vanadium hydrides
convert dienes into allyl ligands at e0 °C.5 Calculations by Landis
Figure 1. Cyclic voltammogram of 1 mM [Et4N][V(CO)4(dppe)] in
and co-workers on the hypothetical VH5 say that it has the weakest
M-H bond dissociation energy (42.8 kcal/mol) of any “valency-
saturated” neutral hydride complex MHn of the 27 first-, second-,
and third-row metals considered.6 (Landis and co-workers define
a “valency-saturated” compound as one that forms the maximum
number of bonds involving s and d orbitals from the available
valence electrons.6)
acetonitrile/0.1 M n-Bu4NPF6 with a scan rate of 50 mV/s.
for CH3CN pKa values were 18.7(1) for 1a, 17.4(1) for 1b,
17.1(1) for 1c, and 16.7(1) for 1d.
We have therefore investigated the V-H bonds in the seven-
coordinate complexes HV(CO)4(P-P) (P-P ) Ph2P(CH2)nPPh2,
with n ) 1 (dppm) in 1a, n ) 2 (dppe) in 1b, n ) 3 (dppp) in 1c,
and n ) 4 (dppb) in 1d).7 (We expected the high coordination
number to decrease the bond strength further.) We have found that
these V-H bonds are indeed weak and that they are excellent
donors of H• to double bonds.
The Bond Dissociation Enthalpies (BDE values) for the V-H
bonds in HV(CO)4(P-P) (1a-1d) have been determined from their
pKa values in CH3CN and the E°(M-/M•) values of their conjugate
bases in the same solvent (eq 3).8
In order to check the validity of these estimates, we determined
the CH3CN pKa values of HV(CO)4(dppm) (1a) and HV(CO)4(dppe)
(1b) with Et3N and PhCH2NH2, respectively. If we take the pKa of
Et3NH+ in CH3CN as 18.82 and that of PhCH2NH3+ as 16.91 (their
values on the unified scale recently published for CH3CN11), we
obtain pKa values of 18.8(1) for 1a and 17.6(2) for 1b. These results
agree satisfactorily with the indirect pKa values from Keq measure-
ments in CD2Cl2.
It is clear from Table 1 that the pKa of 1a-d decreases as the
natural bite angle13 of the P-P ligand increases. Presumably the
six-coordinate anion arising from deprotonation prefers an octa-
hedral geometry, with angles close to 90°. In the Cambridge
Structural Database14 the average dppm bite angle in metal
complexes is 71.3°, with a maximum value of 76.2°, implying that
a dppm ligand cannot achieve a P-M-P angle close to the optimal
value of 90°. With increasing bite angle, the strain imposed by
deprotonation decreases and the pKa decreases.15
The E° and pKa values for 1 imply not only BDE values from
eq 3 but also free energies of V-H bond dissociation (∆G°(H•))
from eq 5;8d both are given in Table 1 for 1a-d. Mayer has noted
that ∆S°(M-H) is not always equal to ∆S°(M•), as assumed in the
derivation of eq 3, and has suggested that the free energies be used
instead of BDE values in assessing the thermodynamics of H•
transfer.16
BDE (M-H) ) 1.37 pKa + 23.06 E°(M-/M•) +
59.5 (kcal/mol) (3)
Cyclic voltammetry of [Et4N][V(CO)4(P-P)] (2) has given the E°
values straightforwardly, as the metalloradicals •V(CO)4(P-P) (3)
produced by the one-electron oxidation of these anions are stable9
and the oxidations are therefore reversible. That reversibility is
demonstrated by the peak-to-peak separation, which is close to 60
mV for all four anions 2 and is illustrated for 2b in Figure 1.
Direct measurement of the pKa values of the hydrides 1 in
CH3CN proved impossible in view of the insolubility of HV(CO)4-
(dppp) (1c) and HV(CO)4(dppb) (1d) in that solvent. We estimated
the pKa values of the hydrides 1 in CH3CN from that (17.0)10,11 of
Cp*Cr(CO)3H in the same solvent by measuring Keq for the proton
1
transfer in eq 4 by H NMR in CD2Cl2.12 The resulting estimates
∆G°H• ) 1.37pKa + 23.06E° + 54.9 (kcal/mol) (5)
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J. AM. CHEM. SOC. 2008, 130, 4250-4252
10.1021/ja710455c CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Values of E° for [Et4N][V(CO)4(P-P)] (2) and Values of
pKa, BDE, and ∆G°(H•) for HV(CO)4(P-P) (1); Natural Bite Angles
for P-P
observed, so the kH in Table 2 (obtained from eqs 7 and 8) is a
lower limit.
natural
d[1]
dt
-
) kobs[1]
(7)
(8)
P−M−P
bite anglec
V
−
H BDE
∆
G
°
(H•)
b
P
−P
E
°
a (V)
pKa
(kcal/mol)
(kcal/mol)
(deg)
kobs ) 2kH[Styrene]
dppm
dppe
dppp
dppb
-1.18 (1)
-1.12 (1)
-1.17 (1)
-1.19 (1)
18.7(1)
17.4(1)
17.1(1)
16.7(1)
57.9
57.5
56.0
54.9
53.3
52.9
51.4
50.3
78.1
86.2
98.6
The resulting kH values for 1 (Table 2) are 7 to 20 times faster
than kH to styrene from CpCr(CO)3H. These ratios, however, are
considerably smaller than the ones we would expect if the
differences in kH were entirely the result of the differences in BDE
between V-H and Cr-H. If we compare 1b with CpCr(CO)3H
and assume an R of unity in the Polanyi equation (eq 9),27 the ratio
of kH(1b) to kH(CpCr) will be 4020. Steric effects (the bulk of the
chelating phosphine ligands in the hydrides 1, and crowding within
their seven-coordinate structures) presumably decrease the rate of
H• transfer from vanadium.
a
b
Relative to ferrocene. Estimated in MeCN from measurements in
CD2Cl2. Natural bite angle by molecular modeling.13
c
Table 2. Rate Constants kH for H• Transfer from 1a-d and
CpCr(CO)3H to Styrene at 285 K in C6D6
1
M
−H BDE
k
H (M-1 s-
)
relative
rate
3
(kcal/mol)
×
10-
HV(CO)4(dppm) (1a)
HV(CO)4(dppe) (1b)
HV(CO)4(dppp) (1c)
HV(CO)4(dppb) (1d)
(η5-C5H5)Cr(CO)3H
57.9
57.5
56.0
54.9
62.2a
g17 (1)
20.0
10.6
8.2
6.7
1
log k â + R(∆H)
(9)
g9.0 (5)
7.0 (1)
5.7 (4)
The relative value of the rate constants within the Vanadium series
in Table 2 is opposite that predicted from the V-H bond strengths;
i.e., kH decreases in the order 1a(dppm) > 1b(dppe) > 1c(dppp)
> 1d(dppb). Again, steric effects (the increasing size of the chelate
ligands from dppm through dppb) are presumably responsible. Such
effects are also apparent in the influence of olefin structure on the
value of kH from a given M-H (substituents on the carbon to which
the H• is being transferred decrease kH substantially).26
0.85 (20)b
a
Recalculated from the data in ref 8d on the basis of the unified pKa
scale in ref 11. See ref 28. b Obtained by extrapolation from the data in ref
24b.
Scheme 1
The vanadium hydrides 1 also effect the radical cyclization of
dienes 4(a,b) more rapidly, and under milder conditions, than does
CpCr(CO)3H. With 2 equiv of the Cr hydride the cyclizations of
4(a,b) did not go to completion. Under the same conditions 2 equiv
of 1b (0.034 M) converted 4a (0.017 M) to 5a in 77% yield, along
with 13% of the hydrogenation product 6a and 10% of the
isomerization product 7a (Scheme 2). Similar results were obtained
with 1c and 1d, whereas 1a gave 56% 5a (and 35% of the
hydrogenation product 6a) after a shorter reaction time. (See
Supporting Information for details.)
To our knowledge the results in Table 1 are the first solution
measurements of V-H bond strengths4,17 and imply that the bonds
in 1 are significantly weaker than the M-H bonds in other neutral
hydride complexes. No stable neutral hydride with an M-H BDE
< 57.1 kcal/mol (the BDE of CpFe(CO)2H) is listed in a recent
compilation.4,18
Scheme 2
The BDE and ∆G°(H•) values in Table 1 confirm that the
V-H bonds of the vanadium hydrides 1 are substantially weaker
than the Cr-H bond in CpCr(CO)3H (Table 2).19 The BDE and
∆G°(H•) values for the hydrides 1 are low enough20-22 to call
into question the thermodynamic stability of 1 with regard to
loss of H2 (eq 6).23 The stability of the hydrides 1 at room
temperature is probably the result of a kinetic barrier to hydrogen
evolution.
The higher yield of the hydrogenation product 6a obtained
with 1a reflects the faster rate at which H• is transferred from
that hydride. Presumably khydrog (Scheme 3) for 1a increases along
with kH.
2 HV(CO)4P-P h H2 + 2 •V(CO)4P-P
(6)
Scheme 3
The V-H bonds in 1 do transfer H• more rapidly than does
CpCr(CO)3H.24 The rate constants kH for H• transfer to styrene
(Scheme 1) can be determined by observing the hydrogenation of
styrene by the vanadium hydrides 1.25 The extent of back transfer
(rate constant ktr) can, as we have shown in other studies on such
reactions,24b,26 be established by treating styrene with 1-d1. With
1c-d1 and 1d-d1, 2H NMR shows deuterium incorporation only into
the product (ethylbenzene) and not into recovered styrene; kH is
thus rate determining, and can be computed from eqs 7 and 8. With
1a-d1 and 1b-d1 a small amount (∼10%) of H/D exchange is
Similar reactions between 4b and 2 equiv of 1a, 1b, 1c, or 1d
gave quantitative yields of the cyclization product 5b, reflecting
the faster cyclization (larger rate constant kcycl) resulting from the
Thorpe-Ingold effect when E ) CO2Me.3,29
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C O M M U N I C A T I O N S
(12) The relatiVe pKa of two hydride complexes should be similar in H2O,
CH3CN, or CH2Cl2. (a) Jia, G.; Morris, R. H. Inorg. Chem. 1990, 29,
581-582. (b) Kristja´nsdo´ttir, S. S.; Norton, J. R. Acidity of Hydrido
Transition Metal Complexes in Solution. In Transition Metal Hydrides;
Dedieu, A., Ed.; VCH: New York, 1991; Chapter 9.
With CpCr(CO)3H we were able to make the cyclizations of
4(a,b) catalytic by performing them under hydrogen pressure, which
regenerates the hydride from Cr• as in eq 10.3
(13) (a) Casey, C. P.; Whiteker, G. T. Isr. J. Chem. 1990, 30, 299-304. (b)
Kranenburg, M.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Eur. J.
Inorg. Chem. 1998, 25-27. (c) Dierkes, P.; van Leeuwen, P. W. N. M.
J. Chem. Soc., Dalton Trans. 1999, 1519-1529.
2 CpCr(CO)3H h H2 + 2 CpCr(CO)3•
(10)
(14) CSD version 5.28 (November 2006).
However, when we treated 4a or 4b with a catalytic amount of
1(a, b, c, or d) under 80 psi of H2 at 50 °C, little or no 5 was
formed. Apparently the reaction in eq 6 does not proceed at an
appreciable rate in either direction under ordinary conditions.
(15) Angelici and co-workers proposed a similar explanation for the fact that
cis-Mo(CO)2(dppm)2 is more basic than its dppe and dppp analogues.
Sowa, J. R., Jr.; Bonanno, J. B.; Zanotti, V.; Angelici, R. J. Inorg. Chem.
1992, 31, 1370-1375.
(16) Mader, E. A.; Davidson, E. R.; Mayer, J. M. J. Am. Chem. Soc. 2007,
129, 5153-5166.
(17) The dissociation energy of V-H in the gas phase has been measured by
guided ion beam mass spectrometry; the BDE obtained was 49.0(16) kcal/
mol. (a) Chen, Y.; Clemmer, D. E.; Armentrout, P. B. J. Chem. Phys.
1993, 98, 4929-4936. (b) Armentrout, P. B.; Kickel, B. L. Gas Phase
Thermochemistry of Transition Metal Ligand Systems: Reassessment of
Values and Periodic Trends. In Organometallic Ion Chemistry; Freiser,
B. S., Ed.; Kluwer: Boston, 1996; pp 1-45.
Acknowledgment. This work has been supported by DOE Grant
DE-FG02-97ER14807. The authors are grateful to Prof. J. Ellis
for a generous donation of [Et4N][V(CO)6], to Prof. Ellis and Prof.
C. D. Hoff for helpful discussions, and to the reviewers for useful
comments.
(18) Smaller values of ∆G°(H•) have been reported for two cationic hydride
complexes: (a) 52.6 kcal/mol for Ni(Ph2PCHdCHPPh2)2+, in Berning,
D. E.; Miedaner, A.; Curtis, C. J.; Noll, B. C.; DuBois, D. L. Organo-
metallics 2001, 20, 1832-1839. (b) 49.9 kcal/mol for the paramagnetic
[HCo(dppe)2]+, in Ciancanelli, R.; Noll, B. C.; DuBois, D. L.; DuBois,
M. R. J. Am. Chem. Soc. 2002, 124, 2984-2992.
Supporting Information Available: Synthetic details, kinetic
procedures and data, and details of pKa measurements and electro-
chemistry. This material is available free of charge via the Internet at
http://pubs.acs.org.
(19) The relatiVe BDE values of our M-H bonds have an uncertainty, derived
from the variation in pKa and E° results over repeated measurements of
(0.3 kcal/mol.
References
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values with those from calorimetry22 and considerations of internal
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CH3CN scale,11 the pKa of CpCr(CO)3H becomes 13.55. Using this pKa
in eq 3 gives the Cr-H BDE in CpCr(CO)3H as 62.2 kcal/mol, which we
have shown for consistency in place of the previously published values
of 62 kcal/mol (thermodynamic cycle)8d and 61.5 (calorimetric).22
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