Novel dicarbonyl and carbonylnitrosyl tris(μ-triazenide) dirhodium complexes

Article abstract of DOI:10.1039/b109972h

The first neutral, tribridged [Rh₂]³⁺ triazenide complex, [(OC)Rh(μ-RNNNR)₃Rh(CO)] (R = C₆H₄Me-p), reacts with NO gas to give the novel nitrosyl [(OC)Rh(μ-RNNNR)₃Rh(NO)] with an [Rh₂]⁴⁺ core linked to NO^-.

Full text of DOI:10.1039/b109972h

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Novel dicarbonyl and carbonylnitrosyl tris(ꢀ-triazenide) dirhodium
complexes
Neil G. Connelly, Owen D. Hayward, Phimphaka Klangsinsirikul and A. Guy Orpen
School of Chemistry, University of Bristol, Bristol, UK BS8 1TS
Received 31st October 2001, Accepted 4th December 2001
First published as an Advance Article on the web 17th December 2001
The first neutral, tribridged [Rh2]3؉ triazenide complex,
[(OC)Rh(ꢀ-RNNNR)3Rh(CO)] (R ؍ C6H4Me-p), reacts
with NO gas to give the novel nitrosyl [(OC)Rh(ꢀ-
RNNNR)3Rh(NO)] with an [Rh2]4؉ core linked to NO؊.
The cyclic voltammogram (CV) of 1 in CH2Cl2 at a platinum
electrode shows two reversible oxidation waves (0.65 and 1.43 V
vs. SCE) and one reversible reduction wave (at Ϫ0.72 V) corre-
sponding to the formation of the [Rh2]4ϩ monocation, the
[Rh2]5ϩ dication and the [Rh2]2ϩ anion [(OC)Rh(µ-RNNNR)3-
Rh(CO)]Ϫ respectively. A comparison with the CV data for
dibridged [Rh2]z triazenide complexes shows the higher oxid-
ation levels to be stabilised by the lower overall charge of
the tribridged complexes {compare the [Rh2]3ϩ/4ϩ couples for 1
(0.65 V) and [(OC)2Rh(µ-RNNNR)2Rh(CO)2] (1.42 V4).
The room temperature ESR spectrum of paramagnetic 1 in
toluene consists of a broad line (giso ca. 2.11) but the aniso-
tropic spectrum (at 160 K) shows three well separated features
(g1 = 2.215, g2 = 2.117, g3 = 2.016, gave = 2.116) with that at high
field a particularly well defined 1 : 2 : 1 triplet due to hyperfine
coupling to two equivalent Rh nuclei [A1(103Rh) = 6, A2(103Rh) =
6, A3(103Rh) = 17.3 × 10Ϫ4 cmϪ1]. The anisotropic spectrum is
qualitatively similar to those of other [Rh2]3ϩ complexes for
which a correlation between ∆g3/∆〈g〉 (∆g is the difference
between the appropriate g-value and that of the free electron)
and β (half the angle between the two coordination planes of
the [Rh2]3ϩ unit) is observed.3 Applying this correlation to
the ESR spectroscopic data for 1 gives a predicted value of
β (5–10Њ) in good agreement with that determined by X-ray
crystallography (6Њ).
Both tetrabridged [Rh2(µ-L)4] (L = carboxylate, amidinate, etc.),
containing the [Rh2]4ϩ core in a face-to-face structure,1 and
the less rigid dibridged [Rh2]2ϩ triazenide complex [(OC)2Rh-
(µ-RNNNR)2Rh(CO)2] (R = C6H4Me-p throughout this paper)2
are well established species with extensive redox-based chem-
istry linking the [Rh2]zϩ (z = 2–6) cores by a series of sequential
one-electron transfer reactions. We now report the synthesis
and reactions of an unprecedented tribridged [Rh2]3ϩ complex,
namely [(OC)Rh(µ-RNNNR)3Rh(CO)], and its reaction with
NO to give the first triazenide-bridged nitrosyl complex
[(OC)Rh(µ-RNNNR)3Rh(NO)]. The combination of the
redox-active Rh2 core with the NO ligand, a potential ‘electron
sink’, leads to a novel structure best described as containing
NOϪ bound to [Rh2]4ϩ
.
Mixing CH2Cl2 solutions of the neutral triazene complexes
[RhCl(CO)2{N(H)RNNR}] and [RhCl(η2-coe)2{N(H)RNNR}]
{formed from the triazene RNNNHR and [{Rh(µ-Cl)(CO)2}2]
or [{Rh(µ-Cl)(η2-coe)2}2] (coe = cyclooctene) respectively} for
25 min, adding one further equivalent of triazene and then
treating the mixture with the base NEt3 gave a red–brown
mixture. Column chromatography on alumina gave a red band
{a mixture of [(OC)2Rh(µ-RNNNR)2Rh(CO)2] and a second
compound, possibly [(OC)(η2-coe)Rh(µ-RNNNR)2Rh(CO)2]}
and a green band from which [(OC)Rh(µ-RNNNR)3Rh(CO)] 1
was isolated as dark green crystals.†
The molecular structure of 1‡ has three triazenide bridges
symmetrically disposed about the (OC)Rh–Rh(CO) unit; the
eclipsed arrangement of the two carbonyls is consistent with
the observation of one strong and one medium weak IR car-
bonyl band, separated by 37 cmϪ1 (Table 1). For each metal
atom, the mutually trans triazenides show an average Rh–N
distance of 2.063 Å; the Rh–N distances for the third triazenide
are longer [Rh(1)–N(4) = Rh(2)–N(6) = 2.087 Å] as a result of
the greater trans influence of CO. Compared with the struc-
tures of dibridged [Rh2]3ϩ complexes {e.g. [(OC)(Ph3P)Rh(µ-
RNNNR)2Rh(CO)(PPh3)]ϩ, Rh–Rh = 2.698 Å3}, the Rh–Rh
distance of 1 (2.542 Å) is much shorter, presumably as a result
of the constraints of the tribridged framework.
Unlike cationic [Rh2]3ϩ complexes such as [(OC)(Ph3P)Rh-
(µ-RNNNR)2Rh(CO)(PPh3)]ϩ, the neutral paramagnetic com-
plex 1 undergoes a ‘radical coupling’ reaction with NO (and
subsequent CO loss). Thus, the dropwise addition of a dilute
CH2Cl2 solution of NO gas to 1 in the same solvent gave a
brown–green solution from which dark brown crystals of
[(CO)Rh(µ-RNNNR)3Rh(NO)] 2 were isolated in good yield.
The X-ray structure of [(OC)Rh(µ-RNNNR)3Rh(NO)] 2
(Fig. 1)‡ is very similar to that of 1, with three triazenide
bridges, one CO ligand on one Rh atom and one NO ligand on
the other. The mutually trans triazenide Rh–N bonds (average
2.053 Å) are slightly shorter than those of 1. The Rh–N bond
distances for the third bridging ligand are different; Rh(1)–
N(4), trans to CO (2.086 Å), is similar to that of 1 whereas
Rh(2)–N(6), trans to NO (2.123 Å), is significantly longer, con-
sistent with the trans labilising effect of the nitrosyl ligand.5
The Rh–Rh distance of 2 (2.518 Å) is slightly shorter than
that of 1. Thus, a formal description of 2 as containing a ‘bent’
Table 1 IR and electrochemical data for tribridged dirhodiumtriazenide complexes
E oЈb/V
Complex
Yield (%)
Colour
ν(CO)a/cmϪ1
[Rh2]2ϩ/3ϩ
[Rh2]3ϩ/4ϩ
0.65
Ϫ0.82
Ϫ0.42(I)
[Rh2]4ϩ/5ϩ
[(OC)Rh(µ-RNNNR)3Rh(CO)] 1
[(OC)Rh(µ-RNNNR)3Rh(NO)] 2
[(OC)Rh(µ-RNNNR)3Rh(CO)I] 4
[(OC)Rh(µ-RNNNR)3Rh(CO)Br] 5
24
64
75
70
Green
Brown
Green
Green
2048, 2011mw
2041, 1705m,brc
2077, 2049m
Ϫ0.72
1.43
—
—
—
0.99(I)
1.25(I)
—
2086, 2052m
—
a In CH2Cl2, strong absorptions unless otherwise stated, m = medium, w = weak, br = broad. b In CH2Cl2, at a platinum electrode, with 0.1 mol dmϪ3
[NBu4][PF6] as supporting electrolyte. For an irreversible (I) process, the peak potential, (Ep)ox or (Ep)red, is given at a scan rate of 200 mV sϪ1
.
All potentials are relative to the saturated calomel electrode, SCE. Under the experimental conditions, E oЈ for the one-electron oxidations of
[Fe(η-C5Me5)2] (calibrant for 1) and [Fe(η-C5H5)2] (calibrant for 2, 4 and 5) is Ϫ0.08 and 0.47 V respectively. c ν(NO).
DOI: 10.1039/b109972h
J. Chem. Soc., Dalton Trans., 2002, 305–306
This journal is © The Royal Society of Chemistry 2002
305

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green needles of [(OC)Rh(µ-RNNNR)3Rh(CO)I] 4 were iso-
lated; a similar reaction with Br2 in CH2Cl2 gave dark green
microcrystals of [(OC)Rh(µ-RNNNR)3Rh(CO)Br] 5.
The CV of 4 shows an irreversible oxidation wave at 1.25 V,
and an irreversible reduction wave at ca. Ϫ0.42 V accompanied
by a reversible product wave centred at Ϫ0.74 V. The similarity
in potential of the product wave to that for the formation of
[(OC)Rh(µ-RNNNR)3Rh(CO)]Ϫ 1Ϫ from 1 (E oЈ = Ϫ0.72 V)
suggests that one-electron reduction of
4 to [(OC)Rh-
(µ-RNNNR)3Rh(CO)I]Ϫ 4Ϫ (at Ϫ0.42 V) is followed by iodide
loss to give 1 (which is then reversibly reduced to 1Ϫ).
Complex 5 can also be prepared by adding [Fe(η-C5H5)-
(η-C5H4COMe)][BF4] (E oЈ = 0.74 V) to 1 in the presence of
[PPh4]Br in CH2Cl2 i.e. by a reaction involving one-electron
oxidation. The CV of 1 in the presence of PPh3 also provides
evidence for oxidative activation via 1ϩ. Thus, the first oxidation
wave of 1 becomes irreversible and reversible product waves
appear at 0.38 (oxidation) and Ϫ1.12 (reduction) V, consistent
with the formation of [(OC)Rh(µ-RNNNR)3Rh(PPh3)]ϩ.
Axial coordination to the dibridged [Rh2]zϩ core of triazenide
complexes is enhanced as z is increased.7 The stabilisation of
the higher core oxidation levels [Rh2]zϩ in the novel tribridged
complexes described above should therefore provide access
to further species suitable for exploitation in the systematic
construction of dirhodium-based supramolecular assemblies.8
We thank the Royal Thai Government and the EPSRC for
Studentships (to P. K. and O. D. H. respectively).
Fig.
1
Structure of [(OC)Rh(µ-RNNNR)3Rh(NO)] 2 (hydrogen
atoms omitted for clarity). Important bond lengths and angles: Rh(1)–
Rh(2) = 2.518(1), Rh(1)–N(3) = 2.053(4), Rh(1)–N(4) = 2.086(4),
Rh(1)–N(7) = 2.051(4), Rh(2)–N(1) = 2.045(4), Rh(2)–N(6) = 2.123(4),
Rh(2)–N(9)
1.803(4), N(10)–O(2) = 1.166(5) Å; Rh(1)–C(1)–O(1) = 175.8(5),
Rh(2)–N(10)–O(2) 156.7(4)Њ. The structure of [(OC)Rh(µ-
= 2.064(4), Rh(1)–C(1) = 1.843(6), Rh(2)–N(10) =
Notes and references
=
RNNNR)3Rh(CO)] 1 is similar to that of 2. Important bond lengths
and angles: Rh(1)–Rh(2) = 2.542(1), Rh(1)–N(3) = 2.068(2), Rh(1)–
N(4) = 2.087(2), Rh(1)–N(7) = 2.057(2), Rh(2)–N(1) = 2.057(2),
Rh(2)–N(6) = 2.087(2), Rh(2)–N(9) = 2.068(2), Rh(1)–C(1) = 1.857(2),
Rh(2)–C(2) = 1.857(2) Å; Rh(1)–C(1)–O(1) = 176.8(2), Rh(2)–C(2)–
O(2) = 176.8(2)Њ.
† All new complexes had satisfactory elemental analyses (C, H and N).
‡ Crystal data: [(OC)Rh(µ-RNNNR)3Rh(CO)]
1 (from CH2Cl2–
n-heptane): C44H42N9O2Rh2, M = 934.69, monoclinic, space group
C2/c (no. 15), a = 12.225(2), b = 23.801(4), c = 14.292(3) Å, β = 91.34(2)Њ,
V = 4157.4(13) Å3, Z = 4, T = 173(2) K, µ = 0.842 mmϪ1, R1 = 0.024.
Molecules of 1 lie at sites of C2 symmetry.
[(OC)Rh(µ-RNNNR)3Rh(NO)]ؒCH2Cl2 2ؒCH2Cl2 (from CH2Cl2–
propan-2-ol): C44H44N10O2Cl2Rh2, M = 1021.61, monoclinic, space
group C2/c (no. 15), a = 19.293(3), b = 18.266(4), c = 26.178(3) Å,
NOϪ group attached to a [Rh2]4ϩ core seems more appropriate
than linear ‘NOϩ’ attached to [Rh2]2ϩ (where a longer Rh–Rh
distance might be expected). However, the Rh–N–O angle of 2
(156.7Њ) is intermediate between the values usually associated
with bent (120–140) and linear (160–180Њ) nitrosyls.6
β = 102.56(1)Њ, V = 9004(3) Å3, Z = 8, T = 173(2) K, µ = 0.900 mm Ϫ1
,
R1 = 0.0398. CCDC reference numbers 173464 and 173465. See
http://www.rsc.org/suppdata/dt/b1/b109972h/ for crystallographic data
in CIF or other electronic format.
The observation of ν(NO) at 1705 cmϪ1 is also ambiguous
given that ν(NOϩ) usually occurs in the range 1950–1600 cmϪ1
1 T. R. Felthouse, Prog. Inorg. Chem., 1982, 29, 73; L. A. Oro, M. A.
Ciriano, J. J. Perez-Torrente and B. E. Villarroya, Coord. Chem. Rev.,
1999, 193–195, 941.
2 N. G. Connelly, G. Garcia, M. Gilbert and J. S. Stirling, J. Chem. Soc.,
Dalton Trans., 1987, 1403; N. G. Connelly and G. Garcia, J. Chem.
Soc., Dalton Trans., 1987, 2737; N. G. Connelly, P. M. Hopkins, A. G.
Orpen, G. M. Rosair and F. Viguri, J. Chem. Soc., Dalton Trans.,
1992, 2907; N. G. Connelly, T. Einig, G. Garcia Herbosa, P. M.
Hopkins, C. Mealli, A. G. Orpen, G. M. Rosair and F. Viguri,
J. Chem. Soc., Dalton Trans., 1994, 2025.
3 D. C. Boyd, N. G. Connelly, G. Garcia Herbosa, M. G. Hill, K. R.
Mann, C. Mealli, A. G. Orpen, K. E. Richardson and P. H. Rieger,
Inorg. Chem., 1994, 33, 960.
4 N. G. Connelly, P. R. G. Davis, E. E. Harry, P. Klangsinsirikul and
M. Venter, J. Chem. Soc., Dalton Trans., 2000, 2273.
5 B. J. Coe and S. J. Glenwright, Coord. Chem. Rev., 2000, 203, 5.
6 G. B. Richter-Addo and P. Legzdins, Metal Nitrosyls, Oxford
University Press, Oxford, 1992; B. F. G. Johnson, B. L. Haymore and
J. R. Dilworth, in Comprehensive Coordination Chemistry, ed. G.
Wilkinson, vol. 2, ch. 13.3, pp. 101–107, Pergamon Press, Oxford,
1987.
7 T. Brauns, C. Carriedo, J. S. Cockayne, N. G. Connelly, G. Garcia
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and ν(NOϪ) in the range 1720–1520 cmϪ1 6
. However, other
spectrochemical and electrochemical evidence supports the
NOϪ/[Rh2]4ϩ description. First, the FAB mass spectrum of 2
shows the loss of NO before CO from the parent ion; the
nitrosyl ligand, when acting as a strong π-acceptor (i.e. formally
as NOϩ), is usually lost after CO. Second, compound 2 is
formed via an intermediate, 3, with an IR spectrum [ν(CO) =
2078 and 2041 cmϪ1, ν(NO) = 1711 cmϪ1] very similar to that
of [(OC)Rh(µ-RNNNR)3Rh(CO)I] 4 (see below). Assuming,
by analogy with IϪ in 4, an NOϪ ligand in [(OC)Rh-
(µ-RNNNR)3Rh(CO)(NO)] 3, the small change in ν(NO) when
3 loses CO leads to the same assignment for 2. Finally, the CV
of 2 shows an irreversible oxidation wave at 0.99 V and a revers-
ible reduction wave at Ϫ0.82 V. If the nitrosyl group is regarded
as NOϩ, complex 2 would contain an [Rh2]2ϩ core and the waves
at 0.99 and Ϫ0.82 V would be due to the [Rh2]2ϩ/3ϩ and [Rh2]ϩ/2ϩ
couples respectively. As there are no examples known of the
[Rh2]ϩ core, voltammetry also suggests that 2 is a [Rh2]4ϩ com-
plex of NOϪ.
Complexes with the [Rh2]4ϩ core result when 1 couples with
ؒ
halogens (X2, as a source of X ). Thus, addition of solid I2
to 1 in CH2Cl2 gave a dark green solution from which fine
306
J. Chem. Soc., Dalton Trans., 2002, 305–306