Synthesis and electronic structure of the first cyaphide-alkynyl complexes

Article abstract of DOI:10.1039/c4dt01108b

The novel complexes trans-[Ru(dppe)₂(C≡CR)(C≡P)] (R = CO₂Me, C₆H₄OMe), the first to incorporate cyaphide as part of a conjugated system, are obtained in facile manner. The electronic structure of these compounds is probed by X-ray, DFT and UV/Vis studies. the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt01108b

Volume 43 Number 24 28 June 2014 Pages 8961–9364
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Ian R. Crossley et al.
Synthesis and electronic structure of the first cyaphide-alkynyl complexes

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Synthesis and electronic structure of the first
cyaphide-alkynyl complexes†
Cite this: Dalton Trans., 2014, 43,
9004
Nicola Trathen, Matthew C. Leech, Ian R. Crossley,* Victoria K. Greenacre and
S. Mark Roe
Received 14th April 2014,
Accepted 22nd April 2014
DOI: 10.1039/c4dt01108b
www.rsc.org/dalton
The novel complexes trans-[Ru(dppe)2(CuCR)(CuP)] (R = CO2Me, (μ–η1–η2-CuP)Pt(PEt3)2],7,8 only in 2006 was Grützmacher able
C6H4OMe), the first to incorporate cyaphide as part of a conju- to isolate the first unequivocal example of a terminal cyaphide
gated system, are obtained in facile manner. The electronic struc- complex, viz. [RuH(dppe)2(CuP)] (1),9 obtained from the
ture of these compounds is probed by X-ray, DFT and UV/Vis η1-phosphaalkyne complex [RuH(dppe)2(PuCSiPh3)]+ (2+)
studies.
by base-induced desilylative rearrangement.9,10 Since this
seminal report, no further examples have been described,
though Russell and co-workers recently inferred the in situ
formation of trans-[Mo(dppe)2(PuCSiMe3)(CuP)]−,11 albeit
unisolated.
We are interested in the chemical and electronic properties
of organometallics that comprise low-coordinate phosphacar-
bons,12 particularly those involving metal-centred conjugation.
To this end, Grützmacher’s methodology presented an intri-
guing opportunity. Herein, we report the synthesis and iso-
lation of the first compounds to incorporate the cyaphide
ligand as part of an extended π-system; we also outline prelimi-
nary investigations into the electronic structure of these
molecules.
The ruthenium alkynyl complexes [Ru(dppe)2(CuCR)Cl]
(R = CO2Me 3,‡ 13 p-C6H4OMe 4) were converted in situ to the
respective triflate salts by reaction with AgOTf, subsequent
treatment with Me3SiCuP affording [Ru(dppe)2(η1-PuCSiMe3)-
(CuCR)]+ (R = CO2Me 5+, p-C6H4OMe 6+) in good yields
(Scheme 1).
Low coordinate phosphacarbons (e.g. RCuP, R2CvPR′) have
long been a source of intrigue,1 being isolobal and isoelectro-
nic analogues of more familiar carbo-centric and nitrogenous
species, yet still embodying appreciable dichotomies. For
instance, the chemistry of phosphaalkynes and phosphaalk-
enes is dominated by the high-energy π-systems (HOMO) akin
to classical alkynes and alkenes, yet the lone pairs remain
accessible (cf. nitriles and imines) to engage in reactivity,2
albeit that those of phosphaalkynes are appreciably stabilised.
Such varied facets render low-coordinate phosphacarbons
attractive moieties to incorporate into electro-active and con-
ducting molecules as a means of moderating orbital distri-
butions and energies, and thus the molecular electronic
properties. Indeed, this is illustrated by numerable examples3
of phosphaalkene4 and phosphole-based5 systems, which
exhibit enhanced electrochemical and photo-electronic
responses in comparison to carbo-centric and nitrogen-doped
analogues.
In respect of phosphaalkynes, however, such application
has yet to be realised, a direct corollary of a lack of intrinsic
kinetic stability within the CuP moiety and resulting difficul-
ties in accessing appropriate derivatives. Indeed, even com-
plexes of the notionally simple cyaphide ligand (CuP−), a
direct analogue of the ubiquitous cyanide, have long evaded
isolation.6 Though first observed by Angelici in 1992, in the
complex “Cl(Et3P)2Pt(CuP)”,7 later trapped as [Cl(Et3P)2Pt-
Department of Chemistry, University of Sussex, Brighton, UK.
E-mail: i.crossley@sussex.ac.uk; Fax: +44 (0)1273 876687; Tel: +44 (0)1273 877302
†Electronic supplementary information (ESI) available: Full synthetic details
and characterising data for all compounds, computational details, UV/Vis data,
crystallographic data in CIF format and ellipsoid plot of 3. CCDC 962350,
962351 and 990881. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4dt01108b
Scheme 1 Reagents and conditions: (i) AgOTf, CH2Cl2; (ii) PuCSiMe3,
CH2Cl2/C7H7; (iii) KOtBu, thf.
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The identities of 5+ and 6+ follow convincingly from multi-
Treatment of 5+ or 6+ with a single equivalent of KOtBu in
nuclear NMR spectroscopic data. Thus, the 31P{1H} NMR thf solution effects desilylative rearrangement to afford the
spectra exhibit quintet and doublet resonances (5+: δP 108.6; cyaphide complexes 7 and 8 respectively, isolated in excess of
6+: δP 113.1) in 1 : 4 ratio, with mutual couplings of ca. 30 Hz. 60% yield. Notably, this reaction proceeds to completion
A singlet resonance corresponding to the SiMe3 group is within 1 h under ambient conditions; this contrasts the case of
1
apparent in the H NMR spectra, in each case integrating con- 1, for which extended reaction times (14 h) were required. More-
sistently with the dppe backbone, and exhibiting correlation over, while Grützmacher observed a kinetically-favoured “inter-
(HMBC) with a characteristic doublet in the 13C{1H} NMR mediate” (believed to be [Ru(dppe)2{C(SiPh3)vP(OPh)}] (A),
spectra (δC ∼ 190) attributed to the phosphaalkynic centre, and formed by reversible attack of −OPh at phosphorus) no com-
thus confirming the PuCSiMe3 moiety.
parable species are apparent in the formation of 7 or 8.
Retention of the alkynyl functionality is similarly con- Indeed, even in situ NMR studies at −78 °C failed to reveal any
firmed, as is the presence of triflate (δF −78.9), while bulk intermediates, or significantly slow the reaction. Since A was
purity was established by microanalysis. The spectroscopic not considered to lie on the pathway leading to cyaphide,10 we
data resemble those reported for 2+, differences in chemical reason that the faster reactions can be attributed to its
shift being attributable to a more electron withdrawing nature absence, which is presumably the result of diminished electro-
for the metal fragments of 5+ and 6+, and thus differing polari- philicity at phosphorus in 5+/6+, combined with enhanced
zation of the alkynic P and C centres.
facility of direct nucleophilic attack at the smaller SiMe3
The molecular connectivity was further supported by iso- (cf. SiPh3).
lation of X-ray quality crystals of 5·OTf, obtained by slow
Formation of the cyaphide complexes is convincingly esta-
cooling of a saturated CDCl3 solution of the salt (Fig. 1).14 The blished from spectroscopic data, supported in the case of 8 by
cation exhibits the anticipated geometry, with the trans-dis- an X-ray diffraction study (Fig. 2).18 Spectroscopically, a signifi-
posed alkynyl and phosphaalkyne adopting near perfect linearity cant shift to higher-frequency is noted for both alkynic and
(∠C–Ru–P 177.0(3), ∠Ru–CuC 178.0(10); ∠Ru–PuC 175.7(4)°); dppe phosphorus centres (7: δP 161.5, 52.7; 8: δP 159.5, 50.8)
this contrasts the situation observed in 2+ and Jones’ when compared to 5+ and 6+, with concomitant reduction in
[RuH(dppe)2(η1-PuCMe)],15 both of which exhibit appreciably the mutual spin–spin coupling constant (to ∼4 Hz), consistent
3
bent geometries for the phosphaalkyne unit (∠Ru–PuC 165.5(2)8 with increased separation of the interacting nuclei (i.e. JPP vs.
and 153.7(2)° respectively), attributed to steric encum- 2JPP). A significant shift is also noted for the phosphaalkynic
brance. The internal geometry of 5+ is largely unremarkable; carbon centre (ΔδC ∼ 86), similar to that observed by Grützma-
the CuP linkage (1.528(11) Å) is comparable to those of 2+ cher. The 1H and 13C{1H} spectra confirm loss of the SiMe3
(1.530(3) Å)9 and Russell’s trans-[Mo(dppe)2(η1-PuCSiMe3)2] group and retention of the respective alkynyl ligands, which is
(1.540(2) Å),11 which are consistent with prior examples.15,16
A
further supported by infrared data (7: νCO 1660 cm−1 νCC
somewhat short CuC distance is noted (1.153(15) Å; cf. 2040 cm−1; 8: νCC 2032 cm−1); the CuP stretching mode is
1.16–2.25 Å from a CCDC search17), but is mirrored in the also observed in both infrared and Raman spectra (7:
parent alkynyl 3 (1.136(10) Å), and presumably results from 1255 cm−1; 8: 1261 cm−1) and is in good agreement with that
disorder within this unit.
Fig. 1 Molecular structure of 5+ in crystals of 5·OTf, DCM solvate.
Hydrogen atoms are omitted and phenyl rings reduced for clarity; 50%
thermal ellipsoids. Selected bond distances (Å) and angles (°): Ru(1)–
C(91) 2.082(11), C(96)–P(5) 1.528(11), C(96)–Si(1) 1.858(12), C(91)–C(92)
1.153(15), C(92)–C(93) 1.450(18), P(5)–C(96)–Si(1) 178.3(6), C(96)–P(5)–
Ru(1) 175.7(4), P(5)–Ru(1)–C(91) 177.0(3), C(91)–C(92)–C(93) 171.9(12).
Fig. 2 Molecular structure of 8. Hydrogen atoms omitted and phenyl
rings reduced for clarity; 50% thermal ellipsoids. Selected bond
distances (Å) and angles (°): C(1)–P(1) 1.544(4), Ru(1)–C(1) 2.065(4),
Ru(1)–C(2) 2.084(3), C(2)–C(3) 1.205(5), Ru(1)–C(1)–P(1) 172.3(2), Ru(1)–
C(2)–C(3) 174.4(3), C(1)–Ru(1)–C(2) 171.91(14), C(2)–C(3)–C(4) 178.5(4).
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reported for 1 (1239 cm−1) and those calculated for 7 and 8 Ru, 14% πCuC), lying 0.2 eV lower in energy (cf. 0.01 eV for 7).
(∼1240 cm−1; see ESI†).
The LUMO of each molecule is appreciably separated from the
In the solid state, 8 exhibits slight distortion from linearity HOMO (ΔE 3.45 eV 7, 3.7 eV 8) and centred on the dppe
(∠Ru–CuC 174.4(3)°; ∠Ru–CuP 172.3(2)°), as previously ligands (ca. 75%) and Ru dx2–y2 orbital (25%), with appreciable
noted for other trans-bisalkynyls.19 A shorter CuP (1.544(4) Å) Ru–P antibonding character. Higher energy orbitals (up to
and very slightly longer Ru–CP (2.065(4) Å) bond are noted as LUMO+10) are almost exclusively ligand (dppe) based, while
compared with 1 (1.573(2) and 2.057(2) Å respectively), pre- the CuP π* orbitals do not contribute appreciably until
*
sumably reflecting diminished dπ→π(CuP) retrodonation LUMO+18/19; the CuC (and for 8 Ar) π* orbitals feature from
within 8, due to the competing trans-alkynyl. It is, however, LUMO+11.
noteworthy that DFT studies§ indicate greater linearity within
It is noteworthy that the lone-pair of the cyaphide moiety is
the conjugated system of 8, together with a longer CuP appreciably stabilised with respect to the π-system, lying ca.
linkage (1.58 Å), a situation that is mirrored for 7; this would 1.6 eV below the HOMO (HOMO−6 in 7, HOMO−7 in 8). In
perhaps imply incidence of packing effects in the solid state.
each case, NBO calculations reveal the lone-pair to be held in
The frontier orbitals of 7 and 8 (Fig. 3) are similar to those an orbital of ca. 75% s and 25% p character, with polarisation
typically seen in alkynyl and bis(alkynyl) complexes.20 Thus, of the CuP moiety in the sense Pδ+–Cδ−. In this regard, the
the HOMO and HOMO−1 in each case derive from the out-of- cyaphide closely resembles classical phosphaalkynes.
phase mixing of the Ru (dxy, dxz), CuC (π) and CuP (π) orbi-
Both 7 and 8 were further studied by a combination of UV/
tals, with an appreciable contribution from the cyaphide Vis spectroscopy and TD-DFT (calculating the first 100 excited
moiety. This is most pronounced for 7 (50% π(CuP), 35% Ru) states; see ESI† for details). Both exhibit strong absorptions
in which the electron-withdrawing methylpropiolate ligand around 250 nm (40 000 cm−1) arising from ligand → ligand
contributes only ca. 10% to either orbital. In contrast, the charge transfer (LLCT) between the π-CP/CC and dppe π* orbi-
more donating CuCC6H4OMe ligand contributes significantly tals. For 7, a further feature around 275 nm (36 363 cm−1) is
to the HOMO of 8 (24% πCuC, 17% πAr cf. 30% Ru, 24% πCuP), again dominated by LLCT transitions but also involves some
*
leading to reduced involvement of the cyaphide, which in turn intraligand transitions (ILCT) centred on π(CuP)→π(CuP). A
dominates the orthogonally-lying HOMO−1 (43% πCuP, 35% weaker feature around 300 nm (33 333 cm−1) is again domi-
nated by LLCT. In contrast, while a dominance of LLCT is also
apparent for 8, a strong feature around 298 nm (33 550 cm−1
)
involves significant contributions from ILCT, centred on π→π*
transitions of the alkynyl (HOMO→LUMO+11) and CuP
(HOMO→LUMO+18/19) ligands;
a
smaller contribution
from metal→ligand charge transfer (MLCT) is evident between
ruthenium and the dppe π* orbitals (HOMO→LUMO+5,
9, 10).
In conclusion, we have described the first organometallic
complexes to incorporate the terminal cyaphide ligand as part
of an extended π-system; this also represents only the second
unequivocal report of a terminal metal-cyaphide complex.
Structural and theoretical studies reveal a modestly screened
cyaphide moiety with a stabilised, but nonetheless accessible,
lone-pair akin to classical phosphaalkynes. The cyaphide con-
tributes significantly to the HOMO and HOMO−1, with an
influence that is clearly moderated by the trans-alkynyl ligand.
The molecules absorb strongly in the UV region, their elec-
tronic spectra being dominated by LLCT transitions to the
dppe ligands, though ILCT π→π* transitions within the CuP
moiety also contribute, most significantly so in 8 for which
further ILCT occurs within the CuCC6H4OMe ligand. These
molecules are the first of a novel class of conjugated, organo-
metallic hetero-ynyl complexes that we continue to explore and
develop.
This work was supported by the Royal Society, Leverhulme
Trust (F/00 230/AL; studentship to NT) and University of
Sussex (studentship to VKG). IRC gratefully acknowledges the
award of a Royal Society University Research Fellowship. We
thank Dr A. K. Brisdon (Manchester) for collection of Raman
scattering data and Dr I. J. Day (Sussex) for useful discussion.
Fig. 3 Frontier molecular orbitals for 7 (left) and 8 (right).
9006 | Dalton Trans., 2014, 43, 9004–9007
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12 N. Trathen, V. K. Greenacre, I. R. Crossley and S. M. Roe,
Organometallics, 2013, 32, 2501.
13 CCDC 962350: see ESI† for full details and ellipsoid plots.
14 CCDC 962351: see ESI† for full details.
15 C. Jones, C. Schulten and A. Stasch, Eur. J. Inorg. Chem.,
2008, 1555.
Notes and references
‡The novel complex 3 was prepared by a modification of established synthetic
routes to ruthenium alkynyl complexes. See ESI† for full details and characteris-
ing data.13
§Calculations used the B3LYP hybrid-functional, with the lanl2dz basis set for
Ru and 6-31G** for all other atoms; see ESI† for full details.
16 P. B. Hitchcock, M. J. Maah, J. F. Nixon, J. A. Zora,
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