Article pubs.acs.org/IC
Weakening of Carbide−Platinum Bonds as a Probe for Ligand Donor Strengths Anders Reinholdt and Jesper Bendix* Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark S Supporting Information *
ABSTRACT: We report the observation of the weakening of the RuC−Pt single bond in (Cy3P)2Cl2RuC−PtCl2−L (RuC−Pt−L) complexes, leading to the incipient formation of the terminal ruthenium carbide complex, (Cy3P)2Cl2RuC (RuC). In the solid state, elongation of RuC−Pt bonds illustrates the degree of weakening, and in solution, decreasing platinum−carbide coupling constants and increasing carbide chemical shifts reveal weaker interaction through the carbide bridge, as the electron donating ability of L becomes progressively stronger. For the bridging carbide ligands, the chemical shifts and coupling constants to platinum are linearly dependent, and NMR data for parent RuC conform to this relationship, providing a spectroscopic means of determining the strength of the RuC−Pt linkages relative to dissociated RuC. The pliancy of the RuC−Pt−L fragment with regard to the identity of L establishes the carbide-bridged complexes as remarkably wide-ranging and sensitive probes for ligand donor abilities.
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INTRODUCTION The MC units in terminal carbide complexes coordinate to metal fragments with σ-donating and π-accepting interactions,1 similar to the interactions between carbonyl ligands and transition metal centers.2 Accordingly, carbide largely acts as a ligand in late transition metal complexes via formation of MC−M′ complexes.3 The bonding in such multiply bound carbide-bridged complexes can be envisaged to be modified through coordination sphere selection around the MC fragments; alternatively, the ligands around M′ may serve in a similar capacity. In terms of resonance structures, these bonding modifications would represent the transition from carbyne-like (MC−M′) to allene-like (MCM′) carbide bridges when bonding persists, and from carbyne-like to terminal carbide (MC + M′) when bonding is disrupted. Though these limiting cases are well documented, transitions between them are difficult to track due to the prevalence of the more stable configuration under experimental conditions. Nevertheless, experimental details about bond formation and breakage in carbide-tethered clusters is relevant to natural nitrogen fixation at nitrogenase cofactors4 and to C−C bond forming reactions at terminal carbide ligands on Fischer− Tropsch catalyst surfaces.5 In parallel to the latter reactivity mode, molecular terminal carbide ligands6 are prone to form bonds to metals6n,7 and main group elements.6e−k,8 In particular, the terminal ruthenium carbide complex (Cy3P)2Cl2RuC (RuC, Cy = cyclohexyl) reacts with platinum group metals to form carbide-bridged structures.6n,7a,b We have demonstrated the formation of the pyridine-containing complex, (Cy3P)2Cl2RuC−PtCl2(py), to be insensitive to © XXXX American Chemical Society
the order in which pyridine (py) and RuC are coordinated to platinum.7a This suggests the central {RuC−Pt} fragment to be rather stable, enabling experimental evaluation of its internal bonding interactions. With this outset, we have incorporated ligands (L, cf. Schemes 1 and 2) with various trans-influence propensities opposite of the {RuC} unit in (Cy3P)2Cl2Ru C−PtCl2−L (RuC−Pt−L) complexes to investigate bonding in the systems.
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RESULTS AND DISCUSSION Synthesis. Treatment of Zeise’s salt, K[PtCl3(C2H4)], with ditopic N-heterocyclic ligands affords bridged bimetallic complexes via chloride loss.9 The trans effect of ethylene enforces trans geometries around platinum, which persist when the complexes associate with RuC. Thus, ethylene serves as a directing group and subsequently as a leaving group in the formation of the L[RuCPt]2 complexes in Scheme 1. Though the reaction sequence cleanly installs N-heterocyclic ligands trans to RuC, nucleophilic ligands like phosphines incite the transformation of Zeise’s salt to PtCl2(PR3)2 complexes.10 Consequently, we explored transformations of L[RuCPt]2 complexes that would circumvent such hurdles. In attempted transmetalation reactions, [RhCl(CO)2]2 quantitatively abstracts chloride from (Cy3P)2Cl2RuC−PtCl3− (RuCPt−), providing [RuCPt]2. The reverse reaction proceeds smoothly by addition of (AsPh4)Cl to [RuCPt]2 (Scheme 2, Ph = phenyl). In line with this, treatment of 2 equiv of RuCPt− with Received: August 3, 2017
A
DOI: 10.1021/acs.inorgchem.7b01956 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Scheme 1. Synthetic Routes to Carbide-Bridged L[RuCPt]2 Complexes Starting from Zeise’s Salt and RuCa
a
rily, the dimeric [RuCPt]2 reacts like a masked 3-coordinate PtII complex, and upon treatment with either pym, pyz, or bipy, the corresponding L-bridged L[RuCPt]2 complexes form cleanly. With the establishment of [RuCPt]2 as a versatile ligand scavenger, we examined its stoichiometric conversions with isocyanides, phosphites, phosphines, and arsines,11 which react in the same fashion as N-donor ligands to generate the respective LRuCPt complexes (Scheme 2). Hence, [RuCPt]2 serves as a general entry to RuC−Pt−L fragments with a wide range of carbide−platinum interaction strengths induced by the trans ligands (vide inf ra). Crystallography. The 10 new crystallographically characterized12 carbide-bridged RuC−Pt−L complexes inherit structural features (Figure 1 and Table 1) from their constituent units, including square pyramidal ruthenium, square planar platinum, and linear carbide bridges, characteristic of {RuC−M} complexes derived from RuC.6n,7a The complexes with bridging N-heterocyclic ligands have angles and separations between {RuC−Pt} units governed by the disposition of donor atoms in the ligands. Despite having connectively similar structures, the complexes of monodentate ligands display diversity in bonding interactions. While the RuC triple bonds are similar in length, the Pt−C distances vary significantly (1.87−2.00 Å) with the ligands coordinated trans to the {RuC} units, spanning 29.6% cumulative probability of the Pt−C distance range in the Cambridge Structural Database (v. 1.19, Figure 2). Particularly, the phosphite ligand in (PhO)3PRuCPt induces long Pt−C bonds compared to those induced by the bridging chlorides in [RuCPt]2 (Pt−C elongated by 0.13 Å, 6.9%). On the basis of ligand electron donating abilities (see Figure 3 and discussion below), Cy3PRuCPt should exhibit a longer Pt−C bond than (PhO)3P RuCPt. This mismatch may reflect the rotational flexibility of the P−O−Ph unit, which leads to steric repulsion between phenyl rings and the cyclohexyl groups of the RuC unit in the solid state structure of (PhO)3PRuCPt, accounting for its relatively long Pt−C bond. Overall, the sensitivity of the Pt− carbide bond distances offers a structural quantification of the trans influence of the ligands coordinated opposite of the {RuC} units (Table 1). NMR. The nature of the ligands opposite of the {RuC} units influences the bonding in the RuC−Pt−L fragments. Particularly, the presence of spin 1/2 nuclei allows NMRspectroscopic evaluation of the bonding interactions between platinum and the carbide ligand via one-bond coupling constants and chemical shifts. While 1H and 31P NMR spectra are unexceptional and yield little information about bonding, the variation of 13C NMR resonances displayed by the carbide ligands directly reflects the diversity in bonding in the RuC− Pt−L complexes (Table 2). Compared with representative onebond platinum−carbon coupling constants for organometallic PtII, the interactions within the RuC−Pt−L fragments are relatively strong [e.g., JPt−C in cis-PtCl2(CO)2: 1569 Hz, cisPtPh2(SMe2)2: 879 Hz, cis-PtMe2(PHtBu2)2: 600 Hz, tBu = tertbutyl; Me = methyl).13 As the trans influence of the ligands opposite of the {RuC} unit increases, the coupling constant between platinum and the carbide ligand assumes steadily lower values (600 Hz decrease), implying weaker interaction across the RuC−Pt linkage. Simultaneously, the bridging carbide ligand’s chemical shift increases by 70 ppm, suggesting decreased shielding of the 13C nucleus as backbonding from PtII to carbon-centered π* orbitals on RuC becomes inefficient. This is in overall accordance with the Pt−C bond elongations
®-H+ = Dowex sulfonic acid polymer resin.
Scheme 2. Interconversion and Formation of L[RuCPt]2 and L RuCPt Complexes
4,4′-bipyridine affects displacement of the trans-chloride, cleanly generating bipy[RuCPt]2. However, similar conversions of RuCPt− with pyrazine and pyrimidine are incomplete and fail to provide the expected L[RuCPt]2 complexes; rather, onehalf of RuCPt− remains unconverted, whereas the other half is transformed into (Cy3P)2Cl2RuC−PtCl2L (LRuCPt, identified by new carbide resonances; L = pyz: 345.6 ppm, L = pym: 346.4 ppm). These observations reflect the difficulty of chloride displacement as the steric bulk around the N-heterocycles increases, favoring monometalation over dimetalation. ContraB
DOI: 10.1021/acs.inorgchem.7b01956 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 1. Molecular structures of RuC−Pt−L complexes. Thermal ellipsoids correspond to the 50% probability level. Cyclohexyl groups from the RuC unit are shown as wireframe. Hydrogens, solvent molecules, and disordered parts are omitted. Color code: C: gray, N: blue, O: red, P: orange, Cl: green, As: purple, Pt: silver, Ru: turquoise.
Table 1. Average Crystallographic Parameters (Å and deg) for the Carbide Bridges in the RuC−Pt−L Complexes complex
trans ligand
Ru−C
C−Pt
Ru−C−Pt
[RuCPt]2 py RuCPt7a pym [RuCPt]2 pyz [RuCPt]2 bipy [RuCPt]2 pz [RuCPt]2 Ph3As RuCPt t BuN CRuCPt
μ-Cl py pym pyz bipy pz− AsPh3 t BuNC
1.676(8) 1.679(3) 1.678(3) 1.668(6) 1.679(3) 1.678(4) 1.670(2) 1.661(6)
1.871(8) 1.882(3) 1.893(3) 1.895(6) 1.891(4) 1.909(4) 1.949(2) 1.967(6)
179.6(4) 172.9(2) 176.0(2) 176.3(3) 171.4(2) 169.9(2) 171.9(2) 176.5(3)
Cy3P
PCy3 PPh3 P(OPh)3
1.666(2) 1.672(2) 1.659(2)
1.971(2) 1.983(2) 2.001(2)
174.5(1) 173.7(1) 179.3(2)
RuCPt RuCPt (PhO)3P RuCPt Ph3P
Figure 2. Pt−C distances from the Cambridge Structural Database (v. 1.19). The box indicates the range of RuC−Pt distances spanned by the RuC−Pt−L complexes.
observed by X-ray crystallography, suggesting the reduced shielding to reflect poorer orbital overlap between platinum and C
DOI: 10.1021/acs.inorgchem.7b01956 Inorg. Chem. XXXX, XXX, XXX−XXX
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heterocyclic carbene (NHC) ligands in trans-PdBr2(NHC)L complexes15 with the chemical shifts moving downfield as the L ligands become increasingly electron donating. The carbide chemical shifts in the RuC−Pt−L complexes parallel this behavior and respond to variations in trans influence propensities of the L ligands (Figure S13) like Huynh’s carbene ligands. Thus, the RuC−Pt−L complexes represent an alternative to Huynh’s scale for determination of the electron donating abilities of ligands. Compared to the simple reagents used to prepare Huynh’s carbene complexes, the Ru C−Pt−L complexes possess clear availability drawbacks. However, the greater chemical shift range allows for sharper distinction between ligands (16 versus 45 ppm in Figure S13). Yet, the clearest asset of the RuC−Pt−L complexes comes from the information yielded by the Pt−C coupling constants: with these, 13C experiments provide two parameters that relate to the electron donating ability of L, and the interpretation with RuC as the end-member in Figure 3 sets an upper bound to the carbide chemical shift-based scale. Also, assuming validity of the correlations in Figure 3 and Figure S13 allows an approximate experimental estimate of the free carbene chemical shift.16 Between the py and PCy3 complexes, the Pt−C coupling constant diminishes by 31%, whereas the NHC’s carbene chemical shift increases by 16.4 ppm. Extrapolating this to zero coupling constant predicts a 52.2 ppm chemical shift increase from trans-PdBr2(NHC)py to the free carbene, i.e., 212 ppm, which is within 10 ppm of the chemical shifts for similar benzimidazole-derived carbenes.17
Figure 3. Platinum-to-carbide coupling constants versus carbide chemical shifts in RuC−Pt−L complexes (CDCl3). As an extrapolation, RuC is assigned with a zero Pt−C coupling constant. Figure S15 shows a magnification of the graph with L ligands indicated explicitly.
Table 2. 13C NMR Chemical Shifts (δC) and Coupling Constants (JPt−C) for the RuC−Pt−L Complexes in CDCl3 complex
trans ligand
δC (ppm)
JPt−C (Hz)
[RuCPt]2 [RuCPt]2 pyz [RuCPt]2 bipy [RuCPt]2 py RuCPt7a pz [RuCPt]2 Ph3As RuCPt CyNC RuCPt t BuNC RuCPt
μ-Cl pym pyz bipy py pz− AsPh3 CyNC t BuNC
326.23 341.36 342.48 348.27 350.34 355.09 374.68 376.04 376.26
1469.7 1365.2 1346.8 1276.9 1283.4 1168.8 1177.4 1105.3 1103.2
(PhO)3P
P(OPh)3 PPh3 PCy3
387.54 388.81 395.77
952.4 968.9 882.7
pym
RuCPt RuCPt Cy3P RuCPt Ph3P
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CONCLUSION
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ASSOCIATED CONTENT
We have demonstrated elaboration of the terminal ruthenium carbide complex (Cy3P)2Cl2RuC (RuC) with Zeise’s salt and a variety of ligands to provide carbide-bridged complexes containing RuC−Pt−L fragments. Variation of the L ligands affects the interaction between the {RuC} unit and platinum significantly as evident from metric (Pt−C bond distance variation by 0.13 Å) and spectroscopic data (JPt−C and δC variation by 600 Hz and 70 ppm). δC and JPt−C exhibit a linear relationship where δC increases toward the value for parent RuC as JPt−C approaches zero. This allows spectroscopic quantification of the weakening of the RuC−Pt bond as the L ligands become increasingly electron donating. The sensitivity to the nature of L establishes the RuC−Pt−L fragments as molecular probes for ligand donor strengths; compared to {NHC−Pd−L}-based probes, the greater chemical shift range and the added information from coupling constants are appealing properties of the RuC−Pt−L complexes.
carbon. Thus, the chemical shift tends to increase with the Pt− C bond distance (Figure S14); however, steric effects easily differ between solution and the solid state (e.g., the long Pt−C bond in (PhO)3PRuCPt; see above), which introduces scatter in Figure S14. Separately, the variations in JPt−C and δC reflect weakening of the RuC−Pt linkage as the trans influence of L increases, and in fact, there is a linear correlation between δC and JPt−C (Figure 3). An interesting interpretation of this correlation comes from assigning the parent terminal carbide, RuC, with a zero platinum−carbide coupling constant. Thereby, RuC provides an extrapolation of the chemical shifts of the RuC−Pt−L complexes as their coupling constants approach zero, and by this spectroscopic measure, the interaction strength across the RuC−Pt linkage decreases by 40% on going from [RuCPt]2 to Cy3PRuCPt. Hereby, the incipient transition from a singly bonded {RuC−Pt−L} complex to a dissociated terminal {RuC} complex can be observed by the elongation of the RuC−Pt bond and the concomitant increase of the carbide’s chemical shift as the Pt− C coupling constant approaches zero. These changes are brought about by variation of the ligand sphere around platinum, and thus, the gradual weakening of the RuC−Pt bond can be observed via changes in isolable molecules. By tradition, linear relationships between coupling constants and chemical shifts in pentafluorophenyl compounds are ascribed to variations in the electron donating abilities of the substituents.14 Recently, Huynh introduced a measure of ligand electron donating abilities on the 13C chemical shifts of N-
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01956. Experimental details, synthetic procedures, spectral figures, additional graphs, and X-ray data for the Ru C−Pt−L compounds (PDF) Accession Codes
CCDC 1564909−1564918 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The D
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jesper Bendix: 0000-0003-1255-2868 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.inorgchem.7b01956 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
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DOI: 10.1021/acs.inorgchem.7b01956 Inorg. Chem. XXXX, XXX, XXX−XXX
