Reaction between rhodium(III) bisboryls and diborane(4) compounds: Evidence for a σ-bond metathesis process

Article abstract of DOI:10.1039/a606993b

The rhodium bis(boryl) complex [RhCl(PPh₃)₂{B(cat)}₂] (cat = 1,2-O₂C₆H₄) reacts with the diborane(4) compounds B₂(O₂R)₂ {O₂R = 1,2-O₂C₆H₃Me-4, 1,2-O₂C₆H₂Bu^t₂-3,5 and dimethyl-L-tartrate [OCH(CO₂Me)CH(CO₂Me)O]} affording unsymmetrical metal bis(boryls) [RhCl(PPh₃)₂{B(cat)}-{B(O₂R)}] and diborane(4) compounds (cat)B-B(O₂R), one possible mechanism for which involves σ-bond metathesis.

Full text of DOI:10.1039/a606993b

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Reaction between rhodium(iii) bisboryls and diborane(4) compounds: evidence
for a s-bond metathesis process
Todd B. Marder,*a Nicholas C. Norman,*b Craig R. Riceb and Edward G. Robinsb
a The University of Waterloo, Department of Chemistry, Waterloo, Ontario, Canada, N2L 3G1
b The University of Bristol, School of Chemistry, Bristol, UK BS8 1TS
The rhodium bis(boryl) complex [RhCl(PPh3)2{B(cat)} ] (cat
diborane(4) species B2(1,2-O2C6H3But-4)2 1b to give the
platinum bis(boryl) complex cis-[Pt(PPh3)2{B(1,2-O2C6H3-
But-4)}2] presumably via reductive elimination of 1a followed
by oxidative addition of 1b.1c Reductive elimination of 1a from
the rhodium(iii) bis(boryl) complex [RhCl(PPh3)2{B(cat)}2]
2a1a in the presence of isocyanides has also been observed.1f In
order to gain further insight into the factors which influence the
oxidative addition–reductive elmination reaction of B–B bonds
with transition-metal centres, we undertook a study of the
reaction between complex 2a and a series of diborane(4)
compounds, preliminary results for which are described
herein.
The reaction between 2a {generated in situ from the reaction
between equimolar ratios of either [RhCl(PPh3)3] 3a or
[{RhCl(PPh3)2}2] 3b and 1a in CH2Cl2 solution}1a and 1 equiv.
of the diborane(4) compound B2(1,2-O2C6H3Me-4) 1c was
followed by 31P NMR spectroscopy which revealed the
formation, after 2 h, of three products in a ratio of 1:2:1
(Fig. 1). One of these products was unreacted 2a (d 31.15, JRh–P
112.9 Hz)1a and a second (d 31.45, JRh–P 114.8 Hz) was
identified as the symmetrical bis(boryl) compound
[RhCl(PPh3)2{B(1,2-O2C6H3Me-4)}2] 2b, prepared and iden-
tified independently from the reaction between 3b and 1c. The
third, and major, product (d 31.30, JRh–P 114.8 Hz) we propose
to be the mixed or unsymmetrical bis(boryl) complex
[RhCl(PPh3)2{B(cat)}{B(1,2-O2C6H3Me-4)}] 2c on the basis
of the 31P NMR chemical shift, which is intermediate between
those of the two symmetrical bis(boryls) 2a and 2b, and the
value of JRh–P which is almost identical to those of 2a and 2b.
An identical 31P NMR spectrum to that shown in Fig. 1 was
observed for the reaction between equimolar amounts of 2b and
1a.
2
= 1,2-O2C6H4) reacts with the diborane(4) compounds
B2(O2R)2 {O2R = 1,2-O2C6H3Me-4, 1,2-O2C6H2But -3,5 and
2
dimethyl-L-tartrate [OCH(CO2Me)CH(CO2Me)O]} afford-
ing unsymmetrical metal bis(boryls) [RhCl(PPh3)2{B(cat)}-
{B(O2R)}] and diborane(4) compounds (cat)B–B(O2R), one
possible mechanism for which involves s-bond metathesis.
The oxidative addition of the B–B bond in certain diborane(4)
compounds to low-valent transition-metal centres is now well
established and results in metal bis(boryls) of the general
formula [LnM{BR2}2] some of which are active catalysts for the
diboration of alkenes and alkynes.1 During studies of the
reactivity of metal boryls, we recently demonstrated that
reversible oxidative addition–reductive elimination of B2(cat)2
1a2 (cat = 1,2-O2C6H4) occurs with the rhodium(iii) tris(boryl)
complex [Rh(PMe3)3{B(cat)}3],3 and also that the platinum(ii)
bis(boryl) compound cis-[Pt(PPh3)2{B(cat)}2] reacts with the
MeO2C
MeO2C
CO2Me
CO2Me
O
O
O
O
O
O
O
O
B
B
B B
R4
R4
1a R4 = H
1e
b R4 = 4-But-3,5,6-H3
c R4 = 4-Me-3,5,6-H3
d R4 = 3,5-But2-4,6-H2
O
O
O
B
B
R
O
Whilst the formation of the unsymmetrical bis(boryl) com-
plex 3c was unexpected, the generality of the reaction was
1f O2R = 1,2-O2C6H3Me-4
g O2R = 1,2-O2C6H2But2-3,5
h O2R = tartrate
◆
MeO2C
O
CO2Me
◆
O
O
O
∆
B
B
∆
Cl
Ph3P
Cl
PPh3
O
B
PPh3
O
B
CO2Me
CO2Me
Rh
Rh
*
*
Ph3P
O
O
2a
2f
R
O
O
O
O
B
B
Cl
Ph3P
Cl
PPh3
O
B
PPh3
O
B
33
32
31
30
29
Rh
Rh
δ
R
Ph3P
R
O
O
Fig. 1 A 31P{1H} NMR spectrum of the reaction between 2a and 1c in
CH2Cl2 solution showing the presence of the compounds 2a (*), 2b («) and
2c (/). Chemical shift and coupling constant data are given in the text. All
spectra were recorded on a JEOL GX 400 spectrometer.
2b O2R = 1,2-O2C6H3Me-4
d O2R = 1,2-O2C6H2But2-3,5
2c O2R = 1,2-O2C6H3Me-4
e O2R = 1,2-O2C6H2But2-3,5
g O2R = tartrate
Chem. Commun., 1997
53

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established with further examples. Thus the reaction between 2a
and B2(1,2-O2C6H2But2-3,5)2 1d afforded both the symmetrical
species [RhCl(PPh3)2{B(1,2-O2C6H2But23,5)}2] 2d (d 31.85,
JRh–P 114.9 Hz)1a and the unsymmetrical compound
[RhCl(PPh3)2{B(cat)}{B(1,2-O2C6H2But2-3,5)}] 2e (d 31.70,
JRh–P 114.9 Hz) as well as unreacted 2a, although in this case 2a
was present as the major species, the ratio of 2a:2e:2d being
ca. 7:1:1, and the reaction was observed to be considerably
slower, taking about 24 h to reach completion.† Similarly, the
reaction between 2a and B2(tart)2‡ [tart = dimethyl l -tartrate,
OCH(CO2Me)CH(CO2Me)O] 1e afforded, in addition to un-
reacted 2a, the symmetrical compound [RhCl(PPh3)2{B(tart)}2]
2f (d 32.05, JRh–P 116.8 Hz) (prepared and identified independ-
ently from the reaction between 3b and 1e) and the un-
symmetrical species [RhCl(PPh3)2{B(cat)}{B(tart)}] 2g (d
31.55, JRh–P 114.9 Hz), although the formation of 2f was very
slow and was only observed after about 9 days at which time the
ratio of 2a:2g:2f was 21:16:1.§ Interestingly, the reaction
between 2f and 1a showed initial formation of 2a and the
unsymmetrical bis(boryl) 2g was only observed after 6 days at
which time the ratio of 2a:2g:2f was 5:1:5.
give the observed rhodium chloride bis(boryls). At this stage,
the mechanism we prefer as the simplest explanation for these
observations is one involving s-bond metathesis, as shown in
Scheme 1. Thus, whilst we have no direct evidence for this
mechanism, we note that such mechanisms have been proposed
by Smith1d to account for the reaction between 1a and
the
platinum
metallacyclopentane
complex
cis-
[Pt(PPh3)2{(CH2)4}] which affords cis-[Pt(PPh3)2{B(cat)}2]
and (cat)B(CH2)4B(cat), and by Hartwig et al. in discussing the
reaction between [RuMe(PMe3)2(h-C5H5)] and HB(cat).5 Fur-
thermore, we note that complexes such as 2a may be compared
to the ruthenium(iv) alkylidene species of the form
[RuCl2(NCR2)(PR3)2] characterised by Grubbs and co-workers6
which are known to be active alkene metathesis catalysts.
Further studies relating to these observations are in progress,
but the preliminary results described herein indicate the
potential mechanistic complexity of systems involving metal
boryls and diborane(4) compounds which will be an important
consideration in any reactions in which metal boryls are
employed as catalysts in diboration reactions.
T. B. M. thanks NSERC of Canada and N. C. N. thanks the
EPSRC, Laporte plc and The Royal Society for research
support, E. G. R. thanks EPSRC for a studentship and T. B. M.
and N. C. N. thank NSERC and The Royal Society for
supporting this collaboration via a series of Bilateral Exchange
Awards. Johnson Matthey plc and E. I. Du Pont De Nemours
and Co., Inc. are also thanked for generous supplies of rhodium
salts.
All of these results imply that a scrambling of the diborane(4)
compounds is occurring in solution in the presence of rhodium,
further evidence for which was the observation and identifica-
tion, by high-resolution mass spectrometry,¶ of the un-
symmetrical diborane(4) compounds (cat)B–B(1,2-O2C6H3Me-
4) 1f, (cat)B–B(1,2-O2C6H2But2-3,5) 1g and (cat)B–B(tart) 1h
in their respective reaction solutions; no such compounds were
observed in the absence of rhodium.
In terms of possible reaction mechanisms, we suspect that an
oxidative addition–reductive elimination pathway, analogous to
those described in the introductory paragraph, does occur since,
in the reaction between 2f and 1a, the symmetrical bis(boryl)
compound 2a was observed prior to the appearance of the
unsymmetrical species 2g. The fact that unsymmetrical bis-
(boryls) and diborane(4) compounds are observed in all
reactions, however, implies that an additional reaction mecha-
nism must also be operating since simple oxidative addition–
reductive elimination cannot afford such mixed products.
Possible mechanisms which might account for the formation of
the observed products include a double diborane(4) oxidative
addition to give a rhodium(v) tetraboryl species such as
[RhCl(PPh3)n{B(cat)}2{B(O2R)}2] (O2R = 1,2-O2C6H3Me-4,
1,2-O2C6H2But2-3,5 and dimethyl-l -tartrate), or reductive
elimination of ClB(cat) from a symmetrical bis(boryl) (for
example, 2a) followed by oxidative addition of diborane(4) to
give a rhodium tris(boryl) complex [Rh(PPh3)n{B(cat)}-
{B(O2R)}2], reductive elimination from either of which could
afford the observed products. Whilst we cannot rule out either of
these two possible mechanisms at this stage (particularly in
view of the slowness of reactions involving 1d and 1e where the
presence of trace amounts of undetected intermediates is a
possibility), we note firstly the unlikelihood of the formation of
a rhodium(v) complex in the former mechanism {although
compounds which are formally rhodium(v) such as
[RhH2(SiEt3)2(h-C5Me5)] have been described by Fernandez
and Maitlis},4 and, secondly, the lack of any evidence for
oxidative addition of ClB(cat) to rhodium(i) triphenylphosphine
centres (in this and other systems studied) which would
subsequently be required in the latter mechanism in order to
Footnotes
† We attribute the fact that the reaction between 2a and 1d is considerably
slower than that between 2a and 1c to the fact that 1d is much more
sterically demanding than 1c, consistent with which is the related
observation that whilst 3a,b react in minutes with 1a, the corresponding
reaction between 3a,b and 1d takes nearly 1 h to reach completion; steric
congestion at the rhodium centre is also likely to favour a preponderance of
the bis-B(cat) complex 2a as observed.
‡ Compound 1e was prepared from B2(NMe2)4 and dimethyl-l -tartaric acid,
HOCH(CO2Me)CH(CO2Me)OH, according to the general preparative
routes described in ref. 2 and references therein; full details will be
described in a future publication.
§ The slowness of this reaction is consistent with the observation that the
reaction between 3a,b and 1e takes several days to go to completion.
¶ (cat)B–B(1,2-O2C6H3Me-4) 1f C13H10B2O4, (M+) calc. m/z 252.076.
Found m/z 252.076; (cat)B–B(1,2-O2C6H2But2-3,5) 1g C20H24B2O4, (M+
2
12C1H3) calc. m/z 335.163. Found m/z 335.163.
References
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the press.
3 C. Dai, G. Stringer, T. B. Marder, A. J. Scott, W. Clegg and
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4 M. Fernandez and P. M. Maitlis, J. Chem. Soc., Chem. Commun., 1982,
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116, 1839.
6 P. Schwab, R. H. Grubbs and J. W. Ziller, J. Am. Chem. Soc., 1996, 118,
100 and references therein.
2+
M
BR2
M
BR2
M
BR2
+
+
BR′2
BR′2
R′2B
BR′2
R′2B BR′2
Scheme 1
Received, 14th October 1996; Com. 6/06993B
54
Chem. Commun., 1997