Redox activation of a B-H bond: A new route to metallaboratrane complexes

Article abstract of DOI:10.1039/b604954k

Oxidative activation of a B-H bond of a coordinated scorpionate ligand provides an unprecedented route to rhodaboratranes. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b604954k

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Redox activation of a B–H bond: a new route to metallaboratrane
complexes
Robin J. Blagg, Jonathan P. H. Charmant, Neil G. Connelly,* Mairi F. Haddow and A. Guy Orpen
Received (in Cambridge, UK) 5th April 2006, Accepted 25th April 2006
First published as an Advance Article on the web 12th May 2006
DOI: 10.1039/b604954k
observed, for example, in [Ba(H2O)2Tm]11 and contrasting
Oxidative activation of a B–H bond of a coordinated
markedly with N2-bonded Tp9 in [Rh(CO)(PPh3)Tp9], has been
scorpionate ligand provides an unprecedented route to
rhodaboratranes.
described as S2H-coordination. More recently, however, the
Rh H–B linkage in [Rh(g -cod){H2B(mt)2}]12 has been described
4
…
The chemistry of hydrotris(pyrazolyl)borate rhodium complexes
such as [Rh(CO)LTp] {L 5 CO or P-donor ligand; Tp 5
HB(pyrazolyl)3, the archetypal scorpionate ligand},1 especially the
photochemical activation of hydrocarbons by the dicarbonyls,2 is
well established. However, attempts to synthesise S-donor scorpio-
nate analogues, using hydrotris(2-thio-1-R-imidazolyl)borate
{HB(mimR)3 or TmR, Scheme 1}, have instead usually led to
cleavage of the B–H bond and formation of well-defined
rhodaboratranes with direct rhodium–boron bonds, e.g.
[RhCl(PPh3){B(mimR)3}] (R 5 Me3,4 or But 5). (Other metalla-
as a three-centre–two-electron (3c–2e) agostic interaction. Cotton
et al.13 noted an analogous B–H Mo interaction in
…
[Mo(CO)2{H2B(3,5-dimethylpyrazolyl)2}(g3-C7H7)] which leads
to an 18-electron count for molybdenum. We note that an axial
…
…
BH Rh or CH Rh interaction in a square planar rhodium(I)
complex might be better described as a 3c–4e interaction given the
presence of the filled Rh dz2 orbital (see below).
As for the Tp9 analogue, cyclic voltammetry at a glassy carbon
electrode in CH2Cl2 shows that [Rh(CO)(PPh3)Tt] 1 is irreversibly
oxidised, with a peak potential of ca. 0.4 V. Treatment of 1
with two equivalents of [Fe(g-C5H5)2][PF6] in the presence of
NEt3 gives, after 30 min, a moderate yield of the red-
orange rhodaboratrane complex [Rh(CO)(PPh3){B(taz)3}][PF6]
(2+[PF6]2).{ The 11B NMR spectrum of cation 2+ shows a well-
defined doublet, {J(11B103Rh) 80 Hz}, contrasting with the very
boratranes include [Ru(CO)(PPh3){B(mimMe)3}],6 [Co(PPh3)
t
{B(mimBu )3}][BPh4]7 and [Ir(CO)(PPh3){B(mimMe)3}].8)
Our interest in rhodium scorpionates has centred on the
preparation of stable Rh(II) species by the one-electron oxidation
of, for example, [Rh(CO)(PPh3)Tp9] {Tp9 5 hydrotris(3,5-
dimethylpyrazolyl)borate, acting as a bidentate N2-donor} to
[Rh(CO)(PPh3)Tp9]+ (where Tp9 is a tridentate N3-donor).9 In
seeking to extend this redox chemistry by using hydrotris(4-
ethyl-3-methyl-5-thioxo-1,2,4-triazolyl)borate {HB(taz)3 or Tt,
Scheme 1} (a potential N-, S- or mixed donor),10 we have
discovered an unprecedented route to rhodaboratranes involving
the oxidative activation of the B–H bond of a coordinated
scorpionate ligand.
The reaction of [{RhCl(g4-cod)}2] (cod 5 cycloocta-1,5-diene)
with NaTt gives [Rh(g4-cod)Tt] which reacts sequentially with CO
and PPh3 to give [Rh(CO)(PPh3)Tt] (1).{ The X-ray structure{ of 1
shows a rhodium centre coordinated to two S atoms of the Tt
ligand and the H atom of a B–H group positioned approximately
…
˚
axially above the Rh centre [Rh(1) H(1) 2.41 A], and substantial
angular distortion of the Rh(I) centre from square planar
coordination (Fig. 1). This type of bonding mode, previously
Fig. 1 The molecular structure of [Rh(CO)(PPh3)Tt] 1. (All hydrogen
Scheme 1
atoms except that attached to boron are omitted for clarity). Important
…
…
bond distances and angles: Rh(1) H(1) 2.41, Rh(1) B(1) 3.239, Rh(1)–
S(1) 2.414(1), Rh(1)–S(2) 2.431(1), Rh(1)–P(1) 2.264(1), Rh(1)–C(1)
School of Chemistry, University of Bristol, Bristol, UK BS8 1TS.
E-mail: Neil.Connelly@bristol.ac.uk; Fax: 0117 929 0509;
Tel: 0117 928 8162
…
1.817(3), C(1)–O(1) 1.152(4) A; B(1)–H(1) Rh(1) 139, S(1)–Rh(1)–P(1)
˚
168.86(3), S(2)–Rh(1)–C(1) 162.59(12)u.
2350 | Chem. Commun., 2006, 2350–2352
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Scheme 2 S–N 5 thioxotriazolyl ring.
nitrogen atom of the third, unbound pyrazolyl ring, is converted to
a 2c–3e bond in [Rh(CO)(PPh3)(Tp9-k2)]+.9) At the same time, the
boron-bound hydrogen atom would be polarised from B–Hd2 to
B–Hd+, facilitating proton loss and formation of the B–Rh bond.
The route to 2+ also has implications for the formal depiction of
the Rh–B bond in rhodaboratranes. It has become conventional to
draw this as a dative bond from Lewis basic Rh(I) to Lewis acidic
BR3. However, the need for two equivalents of a one-electron
oxidant to synthesise 2+ from 1, the accompanying large increase in
energy of n(CO) of ca. 90 cm21 (from 1977 cm21 in 1 to 2064 cm21
in 2+, consistent with an increased formal metal oxidation state),
and the octahedral geometry of 2, unexpected for a Rh(I) complex,
suggest an alternative formal description for the Rh–B bond, i.e.
one in which the B atom of the dianion BR322 (isoelectronic with
Fig. 2 Structure of the cation [Rh(CO)(PPh3){B(taz)3}]+ 2+. (Hydrogen
atoms are omitted for clarity). Important bond distances: Rh(1)–B(1)
2.155(5), Rh(1)–S(1) 2.397(1), Rh(1)–S(2) 2.387(1), Rh(1)–S(3) 2.386(1),
˚
Rh(1)–P(1) 2.495(1), Rh(1)–C(1) 1.856(5), C(1)–O(1) 1.147(6) A.
broad singlet previously observed for rhodaboratranes derived
from Tm.3,4
The X-ray structure{ of 2+ (Fig. 2) shows an octahedral
rhodium centre coordinated to the three S atoms and the B atom
˚
of the B(taz)3 ligand, with a Rh–B distance of 2.155(5) A,
comparable to those of Tm-derived rhodaboratranes such as
[RhCl(PPh3){B(mimMe)3}] {2.122(7) and 2.132(6) A, for two
˚
independent molecules in the unit cell},3 [Rh(PMe3)2
2
CR3 and NR3), acts as a donor to a rhodium(III) atom. Such a
bonding description has been noted previously5 as a possibility for
{B(mimMe)3}]+ {2.153(11) and 2.148(10) A} and [RhCl(PPh3)
4
˚
t
5
{B(mimBu )3}] {2.095(3) A}. The phosphine ligand is coordinated
˚
[MCl(PH3){B(mimH)3}] (M 5 Rh or Ir) but was discounted on the
trans to the rhodium–boron bond with the Rh–P bond distance
22
basis that BR3
dianions are unknown (even though DFT
˚
[2.495(1) A] much longer than in 1 [2.264(1) A]. A similarly long
˚
calculations had suggested a d6 configuration for the metal centre).
One of the beauties of organometallic chemistry is the stabilisation
of otherwise unstable ligands.
distance has been observed for (B)Rh–Ptrans in [Rh(PMe3)2
{B(mimMe)3}]+ [2.459(3) and 2.471(3) A for two independent
˚
molecules in the unit cell]. In this complex, the much shorter Rh–
4
PMe3 distance cis to the Rh–B bond [2.293(3) and 2.292(3) A]
We thank the EPSRC for studentships (to R.J.B. and M.F.H.).
˚
suggests the lengthening in 2+ relative to 1 is a result of the strong
Notes and references
trans influence of the boron donor, as might be expected for the
22
dianion BR3 (cf. the strong trans influence of the isoelectronic
anion [SiR3]2 14) rather than neutral BR3 (see below).
{ All new complexes had satisfactory elemental analyses (C, H and N).
Complex [Rh(CO)(PPh3)Tt] 1; orange crystals; yield 75%; n(CO) (CH2Cl2):
1977 cm21; 1H NMR (CD2Cl2): 7.3–7.8, m, 15H, PPh3; 3.88, q (br), J 5.9,
6H, C2N3S(CH2CH3)Me; 2.31, s, 9H, C2N3SEtMe; 1.21, t, J 7.2, 9H,
C2N3S(CH2CH3)Me; 31P NMR (CD2Cl2): 40.8, d, J(31P103Rh) 158; 11B
NMR (CD2Cl2): 24.95, s. Complex [Rh(CO)(PPh3){B(taz)3}][PF6]
The activation of a B–H bond by a redox reaction is unpre-
cedented but may occur as in the C–H activation reactions of, for
example, [Ru2(m-CH2)(m-CO)(m-Ph2PCH2PPh2)(g-C5H5)2]15 and
[Mo2(m-C8Me8)(g-C5H5)2]16 (which give [Ru2(m-CH)(m-CO)(m-
Ph2PCH2PPh2)(g-C5H5)2]+ and [Mo2(m-C8Me7CH2)(g-C5H5)2]+
respectively), i.e. by a double oxidation–deprotonation mechanism.
A similar mechanism for the conversion of 1 to 2+ (Scheme 2) is
supported by the need for two equivalents of a one-electron
2+[PF6]2; orange-red crystals, yield 26%; n(CO) (CH2Cl2): 2064 cm21; H
1
NMR (CD2Cl2): 7.2–7.5, m, 15H, PPh3; 3.80, q, J 7.3, 4H,
C2N3S(CH2CH3)Me; 3.79, q, J 7.0, 2H, C2N3S(CH2CH3)Me; 2.37, s,
6H, C2N3SEtMe; 2.24, s, 3H, C2N3SEtMe; 1.24, t, J 7.3, 9H,
C2N3S(CH2CH3)Me; 31P NMR (CD2Cl2): 8.0, v.br, PPh3; 2143.9,
2
;
(heptet), J(31P19F) 710, PF6 11B NMR (CD2Cl2): 27.52, d, J(11B103Rh)
80.0.
…
oxidant in the presence of a base. Moreover, if the Rh H–B bond
{ X-ray data were collected on a Bruker SMART diffractometer at 100 K
+
2
˚
involves a 3c–4e interaction (between a filled Rh dz2 orbital and the
B–H bond) stepwise oxidation of 1 would convert this into 3c–3e
and 3c–2e bonds. (Similar behaviour occurs on one-electron
oxidation of [Rh(CO)(PPh3)(Tp9-k2)] where the formally 2c–4e
Rh–N s* interaction, involving the rhodium dz2 orbital and the
(for 1) or 173 K (for 2 [PF6] ) for h , 25u with l 5 0.71073 A. The
structures were solved by direct methods and refined by least-squares
against all F2 values.
Crystal data: [Rh(CO)(PPh3)Tt]?CH2Cl2 1?CH2Cl2 (from CH2Cl2–
n-hexane): C35H42BCl2N9OPRhS3, M 5 916.55, triclinic, space group
˚
P-1 (No. 2), a 5 13.122(3), b 5 13.342(3), c 5 13.744(3) A, a 5 76.38(3),
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3
21
˚
b 5 79.99(3), c 5 62.36(3)u, V 5 2065.9(10) A , Z 5 2, m 5 0.774 mm
,
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R1 5 0.0503. [Rh(CO)(PPh3){B(taz)3}][PF6]?2CH2Cl2 2+[PF6]2?2CH2Cl2
(from CH2Cl2–n-hexane): C36H43BCl4F6N9OP2RhS3, M 5 1145.43, tri-
clinic, space group P-1 (No. 2), a 5 12.899(2), b 5 14.035(2), c 5 14.286(2)
A, a 5 68.38(1), b 5 88.30(1), c 5 87.74(1)u, V 5 2402.1(6) A , Z 5 2,
m 5 0.840 mm21, R1 5 0.0552. CCDC 603867 and 603868. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/b604954k
3
˚
˚
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2352 | Chem. Commun., 2006, 2350–2352
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