Rhiann E.
Andrew
,
Caroline M.
Storey
and
Adrian B.
Chaplin
*
Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK. E-mail: a.b.chaplin@warwick.ac.uk
First published on 9th May 2016
With a view to use as carbene transfer agents, well-defined silver(I) and copper(I) complexes of a macrocyclic NHC-based pincer ligand, bearing a central lutidine donor and a dodecamethylene spacer [CNC–(CH2)12, 1], have been prepared. Although the silver adduct is characterised by X-ray diffraction as a dinuclear species anti-[Ag(μ-1)]22+, variable temperature measurements indicate dynamic structural interchange in solution involving fragmentation into mononuclear [Ag(1)]+ on the NMR time scale. In contrast, a mononuclear structure is evident in both solution and the solid-state for the analogous copper adduct partnered with the weakly coordinating [BArF4]− counter anion. A related copper derivative, bearing instead the more coordinating cuprous bromide dianion [Cu2Br4]2−, is notable for the adoption of an interesting tetranuclear assembly in the solid-state, featuring two cuprophilic interactions and two bridging NHC donors, but is not retained on dissolution. Coinage metal precursors [M(1)]n[BArF4]n (M = Ag, n = 2; M = Cu, n = 1) both act as carbene transfer agents to afford palladium, rhodium and nickel complexes of 1 and the effectiveness of these precursors has been evaluated under equivalent reaction conditions.
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Scheme 1 Selected synthesis and reactions of Cu(I) and Ag(I)–NHC complexes.4,5 |
The application of carbene transfer methodology is not limited to monodentate examples and the use of silver-based transmetallation protocols is also prevalent in the coordination chemistry of mer-tridentate “pincer” ligands bearing terminal NHC donors.7,8 While the corresponding silver transfer agents are often generated in situ, well defined and characteristically bimetallic Ag(I)–CEC (E = C, N) complexes such as A–C have been isolated and crystallographically characterised (Scheme 2).9–15 Copper adducts of CEC (E = C, N) ligands have also been prepared (e.g.D and E),10,16–18 however, their application as transfer agents has yet to be realised to our knowledge.
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Scheme 2 Examples of isolated Ag(I) and Cu(I) complexes of CNC pincer ligands.12–17 |
As part of our work involving the organometallic chemistry of macrocyclic CNC pincers,19 we now report the synthesis and characterisation of well-defined Ag(I) and Cu(I) adducts of a lutidine-based pincer ligand bearing a dodecamethylene spacer [CNC–(CH2)12, 1]. The use of these coinage metal species as transfer agents is then detailed for the synthesis of rhodium, palladium, and nickel complexes of 1.
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Scheme 3 Preparation of 2 and related literature precedents.11 |
Single crystals grown from diethyl ether/pentane and analysed by X-ray crystallography enabled structural elucidation of 2 in the solid-state as a dinuclear complex anti-[Ag(μ-1)]2[BArF4]2 (Fig. 1) – as for closely related precedents B, F and G.11,13 Interestingly, while the principle geometric metrics about silver in 2 are directly comparable to the aforementioned precedents (e.g. Ag–NHC = 2.088(4), 2.093(4) Å, B; 2.077(3), 2.080(3) Å, 2; NHC–Ag–NHC = 176.78(9)°, B; 178.99(13)°, 2), the ligand topology is significantly altered. At the heart of the structural difference is the adoption of near orthogonal NHC–Ag–NHC geometries in 2 [N25–C24–C18*–N19* = 100.2(4)°], in contrast to coplanar NHC–Ag–NHC arrangements observed in B, F and G. This change in geometry is presumably necessary to accommodate the long aliphatic chain and as a consequence results in a very large Ag⋯Ag* separation [7.2732(6) cf. 3.7171(5), B; 3.538(2), F; 4.6636(7) Å, G], precluding the adoption of any argentophilic interaction as seen in A and C.12,14,15.
1H and 13C NMR data recorded in CD2Cl2 confirm the expected 1:
1 ligand to anion ratio and reveal time averaged C2v symmetry for 2 at 298 K (500 MHz). Such high symmetry is inconsistent with retention of the solid-state structure and instead implies highly fluxional behaviour in solution, involving fragmentation into [Ag(1)]+ (cf. structure of 4vide infra). Such an assertion is supported by ESI-MS, where only a singly charged species (i.e. integer mass spacing) was evident in the mass spectrum ([Ag(1)]+, 512.1927; calc. 512.1938 m/z). Not surprisingly the carbene signals of 2 were not observed by 13C{1H} NMR spectroscopy at 298 K, although a chemical shift of ca. δ 181 ppm can be inferred from 2D heteronuclear correlation experiments in line with expected values for Ag(I)–NHC complexes.2,3b,21 Progressive cooling to 250 K lead to decoalescence of the (broadened) 1H signals of 2 observed in CD2Cl2 at 298 K and establishment of an equilibrium mixture comprised of three major compounds, one of C2v symmetry and two of apparent Cs symmetry (1
:
0.5
:
1). These species are tentatively assigned as [Ag(1)]+, syn-[Ag(μ-1)]22+ and anti-[Ag(μ-1)]22+, respectively, on the basis of related behaviour observed for F and G involving rapid equilibration between syn- and anti-isomers in solution (Scheme 3); dynamics that necessitate Ag–NHC bond cleavage and invoke coordination of the central lutidine donor through comparison to the less fluxional m-xylylene bridged analogue H.11 In the case of 2, the NMR data suggests that incorporation of the long dodecamethylene spacer destabilises the dinuclear structures relative to entropically favored fragmentation into [Ag(1)]+ at ambient temperature.22 Further cooling to 200 K resulted in a shift in the equilibrium toward the species assigned to anti-[Ag(μ-1)]22+ (ca. 60%) consistent with the pseudo Ci symmetric structure observed in the solid-state being enthalpically favoured in solution. In the context of 2 being used as a carbene transfer agent, these NMR data ultimately demonstrate facile Ag–NHC bond cleavage under conditions relevant to synthesis of other transition metal adducts of 1via transmetallation.
The synthesis of copper adducts of 1 was targeted by low temperature deprotonation of 1·2HBr in THF in the presence of excess copper bromide. In this manner [Cu(1)]2[Cu2Br4] 3 was formed and subsequently isolated in 76% yield (Scheme 4). Further treatment of 3 with Na[BArF4] in toluene resulted in incorporation of the weakly coordinating [BArF4]− anion in place of [Cu2Br4]2− to afford 4 in 59% isolated yield.20,23 In CD2Cl2 solution, the 1H and 13C NMR characteristics of both 3 and 4 point towards simple mononuclear complexes of 1, with sharp resonances and apparent C2v symmetry in the respective spectra at 298 K (400 MHz). Moreover, only minor differences in chemical shift are found for the equivalent 1H (<0.15 ppm) and 13C (<2 ppm) signals of 3/4, and presumably attributed to greater ion pairing in 3. Of most relevance to the coordination of 1, the carbenic centres were readily identified from 13C{1H} NMR spectra by their characteristically high frequency chemical shifts (δ 178.7, 3; δ 180.3, 4).2 Strong parent cation signals are observed by ESI-MS with correct isotope patterns and integer mass spacing (468.2185, 3; 468.2186, 4; calc. 468.2183 m/z), further supporting the presence of discrete [Cu(1)]+ in solution, irrespective of the counter anion.
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Scheme 4 Preparation of 3, 4 and a related literature precedent.25 |
Despite similar solution characteristics, in the solid-state the nature of the counter anion impacts significantly on the coordination geometries of 3 and 4 (Fig. 2 and 3). Two independent, but well-separated cation/anion pairs are observed in the solid-state structure of 4. The cationic fragments are structurally similar, displaying near ideal T-shaped coordination geometries with the flexible dodecamethylene spacer notably skewed to one side. Focusing on the metrics associated with the independent cation shown in Fig. 2,24 the complex displays an approximately linear NHC–Cu–NHC angle (176.37(13)°), equivalent Cu–NHC bond lengths within error (1.905(3)/1.906(3) Å), and a Cu–N bond length of 2.233(3) Å. These parameters are in good agreement with those reported for E, although the anionic nature of the component CNC pincer ligand leads to a shorter Cu–N bond length than that in 4 (2.017(2) Å). In the case of 3, an anion bridged dimer/tetranuclear formulation is instead observed, viz [Cu(1)]2{μ-[Cu2Br4]}, featuring two formally bridging NHC donors (Cu1–C24, 1.980(3); Cu2–C24, 2.070(3) Å) and two cuprophilic interactions (Cu–Cu, 2.5209(5) Å). The bonding interaction with the anion results in a significant distortion of the coordination geometry observed in 4, with the NHC–Cu–NHC angle deviating from linearity (167.92(11)°) alongside elongation of the non-bridged Cu–NHC (1.925(3) cf. 1.905(3)/1.906(3) Å) and Cu–N (Cu–N = 2.246(2) cf. 2.233(3)) bonds. The bridging μ2-NHC coordination mode is an unusual feature of 3, but has striking precedence in trinuclear copper clusters, such as I, that contain three ligands in this coordination mode (Cu–NHC ca. 2.026(5)–2.044(5) Å for I).25,26 In these trinuclear clusters, the bridging coordination mode is retained in solution and characterised by δ13C 167–169 for imidazolyline based variants; values to significantly lower frequency than found for 3 (δ13C 178.7).25,26
With well-defined coinage metal complexes 2 and 4 in hand, we turned to evaluation of their capacity to act as transfer agents of macrocyclic 1 in CH2Cl2. As convenient benchmarks we targeted preparation of known and soluble palladium and rhodium adducts of 1, [Pd(1)Cl][BArF4] 5 and [Rh(1)(CO)][BArF4] 6, through reactions with [Pd(NCMe)2Cl2] and [Rh(CO)2Cl]2, respectively (Table 1).19b,c Following these reactions in situ by 1H NMR spectroscopy in CD2Cl2, using the [BArF4]− resonances as a convenient internal standard, revealed rapid and high yielding transmetallation reactions of 2 in both cases (>70% yield). Consistent with these high yields, 5 and 6 have previously been isolated in 58% and 52% yield, respectively, through in situ generation of the silver transfer agent.19b,c In the case of copper, although complete consumption of 4 was apparent within 30 min in both cases, a large difference in selectivity is apparent: rhodium-based 6 was formed with an excellent yield of 98% (and subsequently isolated in 82% yield), while palladium-based 5 was formed with a significantly inferior yield of 23%.
[M′] | M | t /h | T/°C | Product | Yieldc |
---|---|---|---|---|---|
a Reactions carried out in J. Young's NMR tubes, which were periodically placed in a ultrasound bath during the course of the reaction. b Complete conversion unless otherwise noted. c Determined by integration of 1H NMR data. d Incomplete reaction. | |||||
[Pd(NCMe)2Cl2] | Ag | 0.5 | 20 | 5 | 73% |
[Pd(NCMe)2Cl2] | Cu | 0.5 | 20 | 5 | 23% |
[Rh(CO)2Cl]2 | Ag | 0.5 | 20 | 6 | 72% |
[Rh(CO)2Cl]2 | Cu | 0.5 | 20 | 6 | 98% |
[NiCl2(glyme)] | Ag | 20d | 20 | 7 | 22% |
[NiCl2(glyme)] | Ag | 20 | 40 | 7 | 76% |
[NiCl2(glyme)] | Cu | 20d | 20 | 7 | 86% |
[NiCl2(glyme)] | Cu | 5 | 40 | 7 | 90% |
Seeking to expand the scope of this transmetallation methodology, we targeted the preparation of nickel derivative 7 – the lighter and Earth abundant group 10 congener of 5. Using the aforementioned methodology in combination with [NiCl2(glyme)], resulted in slow dissolution of the largely insoluble nickel(II) precursor, and gratifying (albeit gradual) formation of 7 at room temperature over 20 hours. The reaction with copper-based 4 notably proceeded ca. 4 times faster, suggesting the explanation for this behaviour is more complex than low solubility of [NiCl2(glyme)] alone. Repeating under more forcing conditions (40 °C) resulted in complete consumption of 2 (20 h) and 4 (5 h) and formation of 7 in 76% and 90% yield, respectively. The new air and moisture stable complex 7 was subsequently isolated from these reactions in ca. 30% yield following purification on alumina and fully characterised. Alternatively, 7 can also be prepared in similar yield using in situ generation of 2 from 1·2HBr and Ag2O (29% isolated yield).
In the solid-state, 7 shows the expected contraction of metal–ligand bond lengths in comparison to 5 (Ni–Cl, 2.145(2); Ni–N, 1.928(5); Ni–C, 1.898(6), 1.916(6) Å; Pd–Cl, 2.287(4); Pd–N, 2.077(10); Pd–C, 2.036(12), 2.056(13) Å), but is otherwise isostructural with the palladium-based analogue (Fig. 4).19c Most notably, the chloride ancillary ligand is easily accommodated within the macrocyclic ring, which is orientated to maintain pseudo C2 symmetry (cf. twisting observed in 4), with an essentially linear N–Ni–Cl bond angle (179.06(15) cf. 176.2(3)° for the N–Pd–Cl angle in 5). In solution the solid-state structure is fully retained as indicated by the observation of diastereotopic methylene bridge (pyC2) and N-methylene (N-C
2CH2) resonances at δ 5.14/6.30 (2JHH = 15.0 Hz) and δ 3.73/4.73, respectively. A single carbenic carbon signal is observed at δ 162.0 (cf. δ 164.5, 5).
1H NMR (400 MHz, CD2Cl2) δ 7.82 (br, 1H, py), 7.70–7.75 (m, 8 H, ArF), 7.56 (br, 4H, ArF), 7.42 (br, 2H, py), 7.17 (br, 2H, imid), 7.05 (s, 2H, imid), 5.25 (br, 4H, pyC2), 4.06 (app. t, J = 7, 4H, NCH2), 1.79 (app. pent., J = 7, 4H, CH2) 1.10–1.40 (m, 16 H, CH2). 13C{1H} NMR (101 MHz, CD2Cl2) δ 162.3 (q, 1JCB = 50, ArF), 155.1 (s, py), 140.5 (br, py), 135.4 (s, ArF), 129.4 (qq, 2JFC = 32, 3JCB = 3, ArF), 125.3 (q, 1JFC = 273, ArF), 125.0 (br, py), 124.1 (br, imid), 120.2 (br, imid), 118.0 (pent., 3JFC = 4, ArF), 57.1 (s, py
H2), 54 (obscured, NCH2), 31.4 (s, CH2), 27.5 (br, CH2), 25.8 (s, CH2). The carbene resonance was not unambiguously identified in the 13C{1H} NMR spectrum, but can be located at ca. δ 181 from an HMBC experiment. ESI-MS (CH3CN, 180 °C, 3 kV) positive ion: 512.1927 m/z, [M]+ (calc. 512.1938). Anal. Calcd for C57H47AgBF24N5·CHCl3 (1496.05 g mol−1): C, 46.28; H, 3.22; N, 4.64. Found: C, 46.08; H, 3.23; N, 4.58.
1
H NMR (400 MHz, CD2Cl2) δ 7.78 (t, 3JHH = 7.7, 1H, py), 7.39 (d, 3JHH = 7.7, 2H, py), 7.22 (d, 3JHH = 1.7, 2H, imid), 6.98 (d, 3JHH = 1.7, 2H, imid), 5.37 (s, 4H, pyC2), 4.17 (t, 3JHH = 6.8, 4H, NCH2), 1.83 (app. pent., J = 7, 4H, CH2), 1.14–1.34 (m, 16H, CH2). 13C{1H} NMR (101 MHz, CD2Cl2) δ 178.7 (s, NCN), 155.5 (s, py), 139.4 (s, py), 123.2 (s, py), 122.5 (s, imid), 120.8 (s, imid), 56.3 (s, py
H2), 52.1 (s, NCH2), 31.2 (s, CH2), 28.1 (s, CH2), 27.8 (s, CH2), 25.8 (s, CH2). ESI-MS (CH3CN, 180 °C, 3 kV) positive ion: 468.2185 m/z, [M]+ (Calc. 468.2183). Anal. Calcd for C50H70Br4Cu4N10 (1384.96 g mol−1): C, 43.36; H, 5.09; N, 10.11. Found: C, 43.28; H, 4.97; N, 9.99.
1
H NMR (400 MHz, CD2Cl2) δ 7.83 (t, 3JHH = 7.8, 1H, py), 7.73 (bs, 8H, ArF), 7.56 (br, 4H, ArF), 7.42 (d, 3JHH = 7.8, 2H, py), 7.10 (d, 3JHH = 1.2, 2H, imid), 7.02 (d, 3JHH = 1.2, 2H, imid), 5.19 (s, 4H, pyC2), 4.20 (t, 3JHH = 7.6, 4H, NCH2), 1.87 (app. pent., J = 7, 4H, CH2), 1.25–1.48 (m, 16 H, CH2).13C{1H} NMR (101 MHz, CD2Cl2) δ 180.3 (s, NCN), 162.3 (q, 1JCB = 50, ArF), 153.8 (s, py), 140.2 (s, py), 135.4 (s, ArF), 129.5 (qq, 2JFC = 32, 3JCB = 3, ArF), 125.2 (q, 1JFC = 271, ArF), 124.1 (s, py), 123.2 (s, imid), 119.6 (s, imid), 118.1 (pent., 3JFC = 4, ArF), 54.9 (s, py
H2), 52.9 (s, NCH2), 31.4 (s, CH2), 27.2 (s, CH2), 27.1 (s, CH2), 25.8 (s, CH2), 25.6 (s, CH2). ESI-MS (CH3CN, 180 °C, 3 kV) positive ion: 468.2185 m/z, [M]+ (Calc. 468.2183). Anal. Calcd for C57H47BCuF24N5 (1332.33 g mol−1): C, 51.38; H, 3.56; N, 5.26. Found: C, 51.48; H, 3.47; N, 5.34.
1
H NMR (500 MHz, CD2Cl2): δ 7.82 (t, 3JHH = 7.7, 1H, py), 7.68–7.76 (m, 8H, ArF), 7.55 (br, 4H, ArF), 7.45 (d, 3JHH = 7.7, 2H, py), 7.11 (d, 3JHH = 1.7, 2H, imid), 6.87 (d, 3JHH = 1.7, 2H, imid), 6.30 (d, 2JHH = 15.0, 2H, pyC2), 5.14 (d, 2JHH = 15.0, 2H, pyC
2), 4.73 (app. t, J = 12, 2H, NCH2), 3.68–3.78 (m, 2H, NCH2), 1.94 (br, 2H, CH2), 1.66 (br, 2H, CH2), 1.18–1.50 (m, 14H, CH2), 1.09 (br, 2H, CH2). 13C{1H} NMR (126 MHz, CD2Cl2): δ 162.3 (q, 1JCB = 50, ArF), 162.0 (s, NCN), 156.5 (s, py), 140.9 (s, py), 135.3 (s, ArF), 129.4 (qq, 2JFC = 32, 3JCB = 3, ArF), 125.2 (q, 1JFC = 271, ArF), 125.1 (s, py), 123.0 (s, imid), 121.4 (s, imid), 118.0 (pent., 3JFC = 4, ArF), 55.0 (s, py
H2), 51.3 (s, NCH2), 30.8 (s, CH2), 28.7 (s, CH2), 27.5 (s, CH2), 23.7 (s, CH2). ESI-MS (CH3CN, 180 °C, 4 kV) positive ion: 498.1929 m/z, [M]+ (calc. 498.1929). Anal. Calcd for C57H47BClF24N5Ni (1362.95 g mol−1): C, 50.23; H, 3.48; N, 5.14. Found: C, 50.62; H, 3.74; N, 5.05.
Footnote |
† Electronic supplementary information (ESI) available: 1H and 13C{1H} NMR, and ESI-MS spectra of new complexes. Selected 1H NMR data for transmetallation reactions of 2 and 4. CCDC 1470494–1470497. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6dt01263a |
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