Stereomutation at an octahedral transition-metal center: Energetics of hydride transit between syn and anti faces of the bis(diphenylphosphinopropyl)(methyl)silyl (biPSi) complex RuH(biPSi)(CO)2

Article abstract of DOI:10.1021/om0100939

Reaction of SiH(Me)(CH₂CH₂CH₂PPh₂)₂, bi-PSiH , with Ru(PPh₃)₂(CO)₃ affords quantitatively the phosphinoalkylsilyl complex RuH(biPSi)(CO)₂, as a mixture of the crystallographically characterized syn diastereomer 3-s and its anti correspondent 3-a; subsequent treatment with CCl₄ then LiAl²H₄ provides access to the deuterioisotopomer Ru²H(biPSi)(CO)₂, 3-d₁. Slow stereomutation of pure 3-s to an equilibrium mixture 3-s/3-a (4.5:1, 295 K) occurs with ΔG?₂₉₅, 102(5) kJ mol^-1; ΔH?₂₉₅, 113(7) kJ mol^-1; ΔS?₂₉₅, 37(2) J K^-1 mol^-1; this is not accompanied by incorporation of external ¹³CO, shows only a minor isotope effect vs 3-d₁, and is concluded to be nondissociative.

Full text of DOI:10.1021/om0100939

1898
Organometallics 2001, 20, 1898-1900
Ster eom u ta tion a t a n Octa h ed r a l Tr a n sition -Meta l
Cen ter : En er getics of Hyd r id e Tr a n sit betw een Syn a n d
An ti F a ces of th e
Bis(d ip h en ylp h osp h in op r op yl)(m eth yl)silyl (biP Si)
Com p lex Ru H(biP Si)(CO)2
Xiaobing (“J oe”) Zhou and Stephen R. Stobart*
Department of Chemistry, University of Victoria, British Columbia, Canada V8W 2Y2
Received February 7, 2001
Summary: Reaction of SiH(Me)(CH2CH2CH2PPh2)2, “bi-
PSiH”, with Ru(PPh3)2(CO)3 affords quantitatively the
phosphinoalkylsilyl complex RuH(biPSi)(CO)2, as a
mixture of the crystallographically characterized syn
diastereomer 3-s and its anti correspondent 3-a ; subse-
quent treatment with CCl4 then LiAl2H4 provides access
to the deuterioisotopomer Ru2H(biPSi)(CO)2, 3-d 1. Slow
stereomutation of pure 3-s to an equilibrium mixture
3-s/ 3-a (4.5:1, 295 K) occurs with ∆Gq295, 102(5) kJ
mol-1; ∆Hq295, 113(7) kJ mol-1; ∆Sq295, 37(2) J K-1
mol-1; this is not accompanied by incorporation of
external 13CO, shows only a minor isotope effect vs 3-d 1,
and is concluded to be nondissociative.
comparable behavior. This is not the case, however, as
is described below. In fact on dissolution, pure 3-s slowly
attains equilibrium with its anti correspondent 3-a by
a mechanism that is nondissociative in CO; all attempts
to observe a putative 16-e intermediate (3-i) to connect
3-s with 3-a (i.e., a five-coordinate hydridomonocarbonyl
analogue of 1) have so far failed; the dicarbonyl 3 is
unchanged under all of the conditions that are routinely
used to induce thermal, chemical, or photochemical
decarbonylation; and treatment of the chloromonocar-
bonyl2 1 with hydride transfer reagents leads not to a
hydride analogue, but rather to the detection in solution
of a species concluded on the basis of its NMR properties
to be a novel carbonyl monoanion, cis-[RuH2(biPSi)-
(CO)]- (4, i.e., the H- adduct of 3-i).
Stereomutation at an octahedral metal center is
almost invariably identified with a dissociative pathway
(ID mechanism) and is frequently mediated through an
isolable five-coordinate intermediate.1 We have recently
reported2 that the 16-electron Ru(II) monocarbonyl syn-
Ru(biPSi)(CO)Cl [biPSiH ) SiH(Me)(CH2CH2CH2PPh2)2]
may be isolated as a pure diastereomer, 1-s, that adopts
a severely distorted trigonal bipyramidal (dist TBP2,3
)
geometry in the crystal and is stereochemically rigid in
solution. On decarbonylation of the corresponding oc-
tahedral dicarbonyl trans-Ru(biPSi)(CO)2Cl (2) in re-
fluxing toluene, however, the same complex is formed
as a mixture with its anti counterpart, 1-a . This
establishes a sequence (1-s-2-1-a ) that is intriguing
in the sense that interconversion between two five-
coordinate structures is effectively mediated2 associa-
tively by a coordinatively saturated intermediate. The
hydride counterpart RuH(biPSi)(CO)2 of 2 was crystal-
lographically characterized some time ago as4 the syn
diastereomer 3-s, and we initially expected it to exhibit
The hydridodicarbonyl RuH(biPSi)(CO)2 was origi-
nally isolated as yellow, X-ray quality crystals but in
execrable yield (ca 7%)4 from Ru3(CO)12 and shown to
be a syn diastereomer (3-s), i.e., with the tridentate
biPSi framework meridional and the Ru-H and Si-Me
bonds on the same face of the molecule. We have since
discovered that this complex is formed quantitatively
during reflux over 17 h in toluene solution of the ligand
precursor biPSiH with the mononuclear Ru(0) complex
Ru(PPh3)2(CO)3. Under these conditions, a mixture of
two isomers is generated, of which the more abundant
(NMR4) is 3-s. NMR spectroscopy establishes that the
minor constituent5 also possesses a structure in which
chelate P atoms are equivalent and H is trans to CO,
identifying it as the anti correspondent 3-a of 3-s. These
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10.1021/om0100939 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 04/19/2001

Communications
Organometallics, Vol. 20, No. 10, 2001 1899
36.0) observable in the 31P NMR. Attempts to isolate
any product led only to rapid decomposition, but on
quenching with CO gas, 3 was formed cleanly and im-
mediately in a composition significantly enriched (NMR)
in the less stable anti diastereomer 3-a vs the equilib-
rium ratio (ca. 2.4:1, cf. 4.5:1). Spectral simulations
based on an AA′XX′ spin system (but not AX2 or AXX′)
adequately reproduce the appearance of the δ -9.94
resonance (Figure 1, a), using J AA′ 4.7, J XX′ 11.7, J AX
68.1, J AX′ -24.5, J A′X -27.0, J A′X′ 66.8 Hz (see Figure 1,
b), i.e., are consistent with a cis arrangement of pairs
of hydrogen (A, A′) and phosphorus (X, X′) atoms that
are coplanar (A, X and A′, X′ pairs trans to one another).
Irradiation of the δ -9.94 signal in an NOE-difference
experiment led to enhancement at the SiCH3 and o-C6H5
hydrogen frequencies (i.e., at δ 0.11, 7.8, respectively),
establishing proximity between Ru-H and Si-CH3, i.e.,
a syn relationship of the latter. These data militate
explicitly for an octahedral, anionic structure 4, and
accordingly repetition of the reaction sequence using
1
F igu r e 1. High-field (i.e., low-frequency) region of the H
NMR spectrum for anion 4, generated under conditions
described in the text: (a) observed; (b) simulated (using J
values given in the text); and the H and 31P NMR spectra
of the dideuterio analogue 4-d 2 (c, d) observed; (e, f)
2
simulated.
2
LiAl2H4 led to observation of signals in H and 31P NMR
stereochemical assignments are further supported by
NOE difference measurements,6 which confirm proxim-
ity between H and Me in 3-s (i.e., consistent with its
solid-state geometry) but not in 3-a . The syn isomer
3-s may be obtained free of its anti partner 3-a by
crystallization, or almost so by repeated washing of
the initial product with ethanol then hexanes, but
thereafter in solution slowly reverts to an equilibrium
mixture 3-s/3-a (4.5:1, 295 K; 3.6:1, 318 K).
Reflux (7 h) of the 3-s/3-a mixture with a stoichio-
metric excess of CCl4 in benzene solution yields the
corresponding chloro complex Ru(biPSi)(CO)2Cl (2), as
a mixture7 of two isomers (ca. 1.5:1), of which the major
is that characterized2 previously (i.e., trans). Further
heating in toluene solution (24 h, 95 °C) affords the
diastereomeric monocarbonyl analogue Ru(biPSi)(CO)-
Cl (1), which rapidly takes up mCO (m ) 12 or 13) to
regenerate 2 or its isotopomer 2* (i.e., with m ) 13), as
has been described.2 As was anticipated, subsequent
reactions with LiAlnH4 (n ) 1 or 2; 20 °C, THF solution)
resulted in replacement of chloride by hydride and could
be adapted to regenerate 3 or to obtain each of three
isotopomers (3* and the deuterated analogues 3-d 1,
3*-d 1). By contrast, treatment of 1 with LiAlH4 yielded
an extremely air-sensitive solution showing (reproduc-
ibly, NMR, THF-d8) a single, odd-looking AB-like pat-
tern centered at δ -9.94 in the 1H NMR (high-field
range: see Figure 1, a), which collapsed to a singlet on
irradiation at the frequency of the single resonance (δ
spectra (see Figure 1, c and d) that could both be sim-
ilarly simulated (see Figure 1, e and f) with J AA′ -2.5,
J XX 11.4, J AX 8.4, J AX′ -1.9, J A′X -0.6, J A′X′ 8.6 Hz, i.e.,
′
consistent with the isotopomer all-cis-[Ru2H2(biPSi)-
(CO)]-, 4-d 2.
It is qualitatively obvious from the time required for
3-s, 3-a to equilibrate in solution (hours) that the
diastereomers are separated by a substantial energy
barrier (although they possess comparable thermody-
namic stability: K ) 0.22, 298 K; ∆G°298 ≈ 5 kJ mol-1).
Starting with material that is predominantly syn (3-s),
the change in diastereomer ratio Asyn/Aanti can be
1
followed to equilibrium by using H NMR (integrated
intensity of Ru-H or Si-CH3 signals at tt, from t0 to
t∞). Data so obtained followed rigorously first-order
kinetics behavior, with k1(syn:anti) 5.25 × 10-6, 3.58 ×
10-5, and 1.72 × 10-4 s-1 at 295, 307, and 318 K,
respectively, yielding an excellent straight line fit of ln-
(k/T) vs T-1 (Eyring plot) and the following activation
parameters: ∆Gq295, 102(5) kJ mol-1; ∆Hq295, 113(7) kJ
mol-1; ∆Sq295, 37(2) J K-1 mol-1. Measured rates were
concentration independent and varied <20% in THF vs
toluene. At 295 K, k1 ) 4.36 × 10-6 s-1 for stereomu-
tation of the monodeuterio isotopomer 3-d 1 (i.e., corre-
sponding to kRuH/kRuD ) 1.20). Significantly, incorpora-
tion of external 13CO (1 atm, 99 atom %, benzene
solution, 295 K) into 3-s/3-a was found to be much
slower than the interconversion between the latter, so
that during appropriate experiments only a trace of
label, i.e., through formation of RuH(biPSi)(CO)(13CO)
(3*), accumulated (NMR) over 24 h.
(5) Minor isomer (3-a ): 1H NMR (C6D6) δ 0.52 (SiCH3), -6.73 (t,
RuH, J PH 22.0), (THF-d8) δ 0.08 (SiCH3), -7.06 (t, RuH); 13C NMR
2
(C6D6) δ 7.5 (SiCH3), 20.5 (SiCH2, SiCH2CH2), 32.4 (t, PCH2), 199.0 (t,
The kinetics data establish that the syn:anti stereo-
mutation 3-s:3-a is intramolecular. By contrast with the
rapid, stereoselective label incorporation into 2,2 the rate
of exchange with external 13CO is negligible compared
with k1, effectively ruling out CO dissociation under
the8-10 strong trans influence of the biPSi silyl group
as the pathway to an unsaturated intermediate. In
RuCO, J PC 11.6), 203.9 (t, RuCO, J PC 7.8); 29Si NMR (C6D6) δ -1.1
(t, 2J SiP 11.2); 31P NMR (C6D6) δ 32.9. The Ph hydrogens were observed
as complex multiplets in the δ 7.0-8.0 range; the methylene hydrogens
for 3-a were obscured by those due to 3-s.
2
2
(6) Irradiation of the 1H NMR signals of 3-s at δ 0.38 or -6.30 each
caused significant intensity enhancement of the other, while no similar
effect was observed on irradiating those of 3-a at δ 0.52 or -6.73. The
irradiation at δ -6.30 or -6.73 also led to intensity enhancement at δ
8.10 or 7.95, respectively, and accordingly the corresponding 1H NMR
signals are assigned to Ph ortho hydrogens in 3-s and 3-a .
(7) Minor isomer: IR (KBr disk, cm-1) 1964s, 2012w (νCO); 1H NMR
(8) Auburn, M. J .; Stobart, S. R. Inorg. Chem. 1985, 24, 318.
Gambaro, J . J .; Hohman, W. H.; Meek, D. W. Inorg. Chem. 1989, 28,
4154. Kapoor, P.; Lo¨vqvist, K.; Oskarsson, Å. Acta Crystallogr. 1995,
C51, 611. Aizenberg, M.; Milstein, D. J . Am. Chem. Soc. 1995, 117,
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(9) Huang, D.; Streib, W. E.; Bollinger, J . C.; Caulton, K. G.; Winter,
R. F.; Scheiring, T. J . Am. Chem. Soc. 1999, 121, 8087.
(C6D6) δ 0.36 (SiCH3); 13C NMR (C6D6) δ 4.0 (SiCH3), 17.0 (SiCH2),
2
21.2 (SiCH2CH2), 27.8 (t, PCH2), 191.6 (t, RuCO, J PC 10.4), 201.7 (t,
RuCO, 2J PC 11.5); 29Si NMR (C6D6) δ 1.7 (t, 2J SiP 16.0); 31P NMR (C6D6)
δ 13.9. The FAB MS for both 1 and 2 (mixtures of isomers) showed as
the fragment at highest m/z (centered on 627 amu) a polyisotopic
pattern attributable to Ru(biPSi)(CO)+.

1900 Organometallics, Vol. 20, No. 10, 2001
Communications
further support of a profile that channels through a
ruthenium dicarbonyl species, CO:13CO “scrambling” in
3* is observable at a rate that is qualitatively very
similar to k1. In a geometry where the sites trans to the
labilizing Si and H centers are both occupied by uni-
dentate CO, we see no justification for invoking pref-
erential dissociation/reattachment11 of a chelate phos-
phorus (i.e., biPSi) atom at Ru. Rapid incorporation of
external CO would also be expected into any transient
four-coordinate9 Ru(0) species formed by reductive
elimination (i.e., of Si-H); moreover substitution of
Ru-H in 1-s by Ru-D modifies k1 in a way that is
typical for a “normal secondary hydrogen isotope effect”,
thereby suggesting12 a process that may involve a
reduction in coordination number at Ru but ruling
against Ru-H bond scission.13
bond reductive elimination/readdition.17 In this last
context it should be noted, however, that weak KIEs
(kH/kD ) 1-2) have been observed18 for dissociative
addition of dihydrogen to unsaturated transition-metal
centers, as well as for site exchange in the rhodium
cation Rh(H)(nH)[P(CH2CH2PPh2)3]+ (n ) 1 or 2), where
indeed the magnitude of the KIE (kH/kD ) 1.3) has been
taken to imply19 some degree of H-H bonding during
the intramolecular reorganization.
Anionic complexes resembling 4 have remained ex-
ceedingly rare since the investigation by Pez and
Halpern of the catalytically active, o-metalated ana-
logue20 [RuH2(C6H4PPh2)(PPh3)2]- (K+ salt). In fact the
existence of 4 attests to a radical difference in Lewis
acidity between the two 16-e congeners RuX(biPSi)(CO),
X ) Cl (i.e., 1, which2 has been isolated) or H (i.e., 3-i,
which has not), reinforcing the arguments of Caulton
et al.21 concerning the relative stability of members of
the isoelectronic family RuH(X)(CO)(PtBu2Me)2 (5) that
ranges through an extensive series of donors X including
(as its weakest member21) hydride ion. Complexation
of (putative) 3-i with the latter, in preference to adopting
a neutral, unsaturated state like that22 of the silyl
analogue RuH(SiHPh2)(CO)(PtBu2Me)2 (i.e., 5, X )
-SiHPh2), further highlights the scarcity of five-
coordinate Ru(II) centers from which π-donor ligands
are absent.23
On the basis of these observations, we propose that
the diastereoisomerization 3-s:3-a occurs nondissocia-
tively, which requires either (i) concerted exchange of
H and CO or (ii) twisting of the Ru-H bond around the
Ru-Si axis, each of which might be expected to induce
concomitant exchange of the two CO groups, as is
observed (i.e., for 3*). Li and Hall have recently14
concluded on the basis of a density functional theory
analysis (B3LYP) that of possible mechanisms for cis
to trans isomerization of the “pincer” dihydride15 IrH2-
(CO)[C6H3(CH2PiPr2)2], a nondissociative trigonal twist
is feasible at much lower energy than any dissociative
mechanism. The calculated activation barrier of 24.0
kcal mol-1 to complete the required rearrangement, by
two successive rotations of sets of atoms defining
trigonal faces, is remarkably close (i.e., at ∼100 kJ
mol-1) to the energetics reported here for the 3-s:3-a
interconversion, which would be effected by related
consecutive rotations, so accomplishing i above. With
regard to ii, although we searched carefully for direct
Ack n ow led gm en t. This research was supported by
funding from the NSERC, Canada.
OM0100939
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to mediate a crossover between the diastereomeric reaction manifolds
that lead respectively to 3-s and 3-a , rather than following the
microscopic reverse of the elimination step, which would simply
regenerate the original stereochemistry (i.e., 3-s).
1
2
16
evidence (e.g., J Si-H or J P-Si
)
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(12) See: Carroll, F. A. Perspectives on Structure and Mechanism
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(13) In this respect also being inconsistent with H transfer to a cis
CO group, i.e., mediation through a transient five-coordinate RuII
formyl.
(14) Li, S.; Hall, M. B. Organometallics 1999, 18, 5682.
(15) Rybtchinski, B.; Ben-David, Y.; Milstein, D. Organometallics
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