Highly Unsaturated Platinum and Palladium Carbenes PtC3 and PdC3 Isolated and Characterized in the Gas Phase

Abstract Carbenes of platinum and palladium, PtC3 and PdC3, were generated in the gas phase through laser vaporization of a metal target in the presence of a low concentration of a hydrocarbon precursor undergoing supersonic expansion. Rotational spectroscopy and ab initio calculations confirm that both molecules are linear. The geometry of PtC3 was accurately determined by fitting to the experimental moments of inertia of twenty‐six isotopologues. The results are consistent with the proposal of an autogenic isolobal relationship between O, Au+, and Pt atoms.

The importance of industrial catalysis by platinum and palladium has prompted extensive studies of their gas-phase chemistry. [1] Each metal atom is known to initiate cleavage of the C À Ha nd C = Cb onds of hydrocarbon precursors.W e believe that the present study provides the first pure rotational spectra of platinum and palladium carbenes isolated in the gaseous phase.P tC 3 and PdC 3 (each in a 1 S state) were generated through laser vaporization of solid Pt/Pd in the presence of ag as sample undergoing supersonic expansion and containing alow concentration (typically 1%)ofahydrocarbon precursor in ab uffer gas of argon. Analysis of the rotational spectra reveals that each molecule has al inear geometry and an MCCC connectivity (where Mi st he metal atom). Theresults are asuccessful test of amodel proposed by Pyykkç et al. [2] which suggests that platinum can be regarded as the isoelectronic and isolobal counterpart of ac halcogen for the purposes of predicting structure and reactivity trends.
Aw ide range of hydrocarbon precursors,e ach tested individually,w ere found to allow the generation of PtC 3 and PdC 3 .F or PdC 3 ,t he range of effective precursors includes C 3 H 4 (allene), C 2 H 2 ,C 2 H 4 ,CH 4 ,and C 4 H 4 O(furan). ForPtC 3 the range is narrower,including C 3 H 4 (allene), C 2 H 4 ,and CH 4 , all of which were found to be effective.Broadband microwave spectra of the target molecules were recorded between 6.5 and 18.5 GHz (Figure 1) using as pectrometer described previously in detail. [3] Each spectrum was assigned and fitted to the Hamiltonian of al inear molecule using Westerns program PGOPHER. [4] Thelow number of J'!J'' transitions within the bandwidth of the spectrometer required that centrifugal distortion constants be fixed at results calculated ab initio by an approach described previously. [5] Structure optimizations,r eaction energies,a nd orbital energy level diagrams were calculated using the MOLPROp ackage [6] at the CCSD(T) level of theory. [7] Theb asis set combination employed the aug-cc-pwCV5Z basis set for each Catom and the aug-cc-pwCV5Z-PP basis set for each of Pt and Pd. [8] The ECP-28-MDF and ECP-60-MDF effective core potentials were used to account for scalar relativistic effects on Pd and Pt, respectively, [8] with all electrons included in the correlation treatment. Electric dipole moments and centrifugal distortion constants were calculated with the GAUSSIAN 09 package [9] at the MP2 level of theory using ab asis set combination consisting of aug-cc-pVTZ on Catoms and aug-cc-pVTZ-PP on Pd and Pt atoms. [8] Selected results of spectroscopic fits are shown in Table 1w ith complete details for all isotopologues provided in the Supporting Information. Thestandard deviations of all fits are consistent with the measured linewidth (FWHM) of 120 kHz. Neither PtC 2 nor PdC 2 were identified despite acareful search of the spectra. Rotational transitions of both PtC [10] (measured previously) and PdC lie higher in frequency than the upper limit of the spectrometer.W here PdC 3 was generated from af uran precursor,i ntense transitions of PdCO [11] were detected in addition to those assigned to PdC 3 . Spectra were measured for isotopologues of PtC 3 and PdC 3 that contain the 13 Cisotope to ensure assignment of the correct molecular carriers and allow precise determination of the molecular geometries.E xperimental data are available only for the ground vibrational state of each molecule allowing an effective r 0 geometry to be fitted in each case. Theexperimental results are consistent with two possibilities for each molecule:1 )a geometry that is slightly bent at equilibrium but quasilinear in the v = 0s tate,a nd 2) an equilibrium (r e )g eometry that is linear. Thea binitio calculations suggest that both molecules are linear at equilibrium. Thei ntensities of PdC 3 transitions were found to be highly dependent on the choice of precursor,i nt he order C 3 H 4 > C 2 H 4 > CH 4 .T ransition intensities were lower when the population of PdC 3 was divided across many isotopic permutations and isotopically enriched allene is prohibitively expensive.T hese factors prevented measurement of the spectrum of any PdC 3 isotopologue that contains both 12 C and 13 Cisotopes.T he intensities of PtC 3 transitions were insensitive to the choice of precursor and it was possible to generate and record spectra for many isotopic permutations of PtC 3 (from the set of 194 Pt,195 Pt,196 Pt,198 Pt, 12 C, and 13 Catoms) using samples prepared by mixing 12 CH 4 and commercially supplied 13 CH 4 .Itwas also found that PtC 3 can be generated from am ixture of 12 C 2 H 2 and 13 CH 4 precursors with the result that the spectra of 194 Pt 12 C 12 C 13 C, 194 Pt 12 C 13 C 12 C, and 194 Pt 13 C 12 C 12 Cw ere detected with equal intensities.T he observation that the 13 Cisotope does not preferentially occupy an end position of the C 3 subunit strongly implies that the C Cb ond of C 2 H 2 cleaves during the sequence of reactions that generates PtC 3 from this set of precursors.
Thepresent study is believed to be the first to characterize MC 3 units by rotational spectroscopy.T ransition-metal dicarbides,such as ScC 2 and YC 2 ,have been studied previously. [12] Thed ipole moments of PdC 3 and PtC 3 are calculated at the MP2 level to be 6.1 and 5.6 D, respectively.T he lengths of bonds within PtC 3 were fitted to experimentally determined rotational constants using Kisiels STRFIT. [13] Spectra were measured for 26 distinct isotopologues of PtC 3 where the set includes every permutation of C 3 that it is possible to generate from 12 Cand 13 Cisotopes.T he bond lengths thus determined are compared with those in isolated PtC,C 3 and OC 3 molecules in Table 2. The r 0 geometry of PtC 3 is in good agreement with the r e geometry calculated at the CCSD(T) level. ThePt ÀCbond in PtC 3 is longer than found in diatomic PtC [10] by 0.053 . There are similarities between r(MC) in PtC 3 and in PtCO, [14] and also in changes when these molecules form from their component Pt and C 3 /CO subunits. The r(MC) parameter in PtC 3 is shorter than the same quantity in PtCO by 0.031 . Thefirst C=Cbond (that which is contiguous with the PtÀCb ond) of PtC 3 is longer than the C=Cbond in isolated C 3 by 0.022 . Theset of isotopologues studied is less extensive for PdC 3 than for PtC 3 and does not permit determination of all bond lengths from the experimental data. If the lengths of C = Cbonds within the molecule are fixed as shown in Table 2, r(PdC) is determined to be  1.79898 (4) . Va lues of vibrational wavenumbers calculated ab initio are provided in the Supporting Information. Thedescribed results confirm that the heavier elements of Group 10 can form linear arrangements similar to that previously identified for Ni 2 C 3 . [15] Thed etected palladium/ platinum carbenes are amongst the smallest to be structurally characterized. [16] There is ac orrespondence between the linear geometries of the MC 3 units identified herein and the linear carbon chains that are interceded by Pt/Pd atoms which are afeature of many synthetic coordination polymers. [17] The results are also interesting in the context of the wider chemistry of metal atoms in hydrocarbon plasmas.E arly transition metals are known to react with hydrocarbon precursors to generate metallocarbohedrynes (met-cars). [18] Late transition metals show no general tendency to form such extended structures.T he present experiment does not unambiguously distinguish the reaction sequences (or networks of competing reactions) that generate PdC 3 and PtC 3 .I ti s possible that af raction of the population of each forms through gas-phase association of individual metal atoms with intact C 3 or other units generated independently of any metal atom. [19] Thee nergy changes accompanying the M + C 3 ! MC 3 association reactions to yield linear MC 3 units are calculated to be À295 kJ mol À1 and À417 kJ mol À1 when M = Pd and M = Pt, respectively (detailed calculations are shown in the Supporting Information). However,i ti sa lso possible that the metals themselves initiate the sequence of chemical reactions that leads to dehydrogenation of the precursor. There is extensive evidence from previous studies that both Pt and Pd atoms undergo bond-insertion and cleavage reactions with hydrocarbons. [1a, 20] MCH 2 and MCCH 2 have both been generated [1a,b,21] previously by al aser vaporization/ supersonic expansion method, characterized by matrix isolation spectroscopy,and are also likely to be generated under the present experimental conditions.T ransition frequencies of MCH 2 are expected to be above the upper frequency limit of the spectrometer and both MCH 2 and MCCH 2 will have comparatively low dipole moments which significantly decrease the intensity of their rotational transitions relative to those of MC 3 .
An empirical model proposed by Pyykkç et al. [2a] provides ac hemical rationalization for an enhanced stability of MC 3 relative to MC 2 or MC 4 .C alculations of the geometries of CAu 2+ ,C Au 3+ ,P t 2 C, Pt 2 C 3 ,a nd Au 2 C 2 revealed analogies between the behavior of each of Au + and Pt and achalcogen atom such as O. [2a] Within this model, the s hole on platinum arising from the 5d 10 6s 0 configuration is analogous to the 2ps 0 hole on oxygen, and the 5dp orbitals of platinum participate in p-bonding interactions analogous to those involving the 2pp orbital of oxygen. Theexistence of afamily of stable molecules was thus predicted. An orbital energy level diagram for PtC 3 is presented in Figure S1 in the Supporting Information. There are striking similarities between the geometries of MC 3 measured during the present work and that reported earlier for OC 3 by Brown et al. [22] Applying the model of Pyykkç et al.,P tCO,P tC,a nd Pt 2 C 3 are analogues of the well-known, stable oxocarbons carbon dioxide,monoxide,and suboxide,respectively,each of which have been known since the 19th century.Similarly,PtSi [23] can be regarded as an analogue of SiO.T he oxocarbon analogue of Pt 2 C 2 would be ethylene dione, [24] at ransient species characterized, only through spectroscopy,for the first time in 2015. This model can thus explain why PtCO,PtC,and PtC 3 , but not yet PtC 2 ,h ave been detected. Ther esults of the present work thus support the suggestion that platinum can be regarded an isoelectronic,isolobal counterpart of oxygen. The proposal can be further assessed with reference to previous works.
Reports of clusters containing multiple carbon and platinum or palladium atoms are scarce.T he adsorption of, and reactions of,C H 4 and CO on Pt n clusters [1e] and the structures of Pt n O m clusters [1d] have been studied. Harding et al. identified aPt 3 C + cluster ion [25] for which the geometry is analogous to acarbonate ion and hence consistent with the prediction of the model provided by Pyykkç et al. [2] The geometries of other platinum/carbon clusters,which have not yet been observed or characterized, may perhaps be predicted by analogy with other oxocarbons.F or example,m ellitic anhydride (C 12 O 9 )i sk nown to be stable,s uggesting that Pt 9 C 12 might be generated in an equivalent structural form. An experimental study [26] of AuC n + and CuC n + revealed ion intensities in the mass spectra that are significantly stronger where n = 3t han for clusters of other sizes. [26] Some caution must be exercised in drawing conclusions about the thermodynamic stability of AuC 3 + relative to other cluster sizes from these results.A si nt he present work, the experiment performed by Ticknor et al. [26] did not unambiguously distinguish between various factors that contribute to observed spectral intensities.I ti sl ikely that C 3 was generated with as ignificantly higher abundance [19] than C 2 within the expanding gas sample and this may cause the generation of AuC 3 + to be favored over the generation of clusters of other sizes,r egardless of the thermodynamic stability of AuC 3 Indeed, during ap revious study,s ignals for NiC 3 + and NiC 6 + were detected in mass spectra with higher intensity than units containing 1, 2, 4, or 5carbon atoms, [27] although the Ni + ion is not isoelectronic and isolobal with O. However,the reported fragmentation behavior of AuC n + is also notable.C lusters where n is odd lose only the metal atom on photodissociation whereas those with an even value of n display an additional loss channel corresponding to the loss of an odd number of carbon atoms.The overall result is that chains (either isolated or attached to the metal ion) containing an odd number of carbon atoms tend to be formed during photofragmentation, consistent with the proposal of Pyykkç et al. Theperspective thus emerging from the collected results of spectroscopic experiments is that the proposal [2a] of an autogenic isolobal relationship of Pt and Au + centers with the Oatom is powerful and useful with respect to structural trends in gasphase clusters that contain Pt, Pd, Au + ,and Cc enters. the EPSRC UK National Service for Computational Chemistry Software (NSCCS) at Imperial College London. A.C.L. thanks the University of Bristol for aS enior Research Fellowship.D .P.Z. thanks Newcastle University for the award of aF aculty of SAgE Research Fellowship.D .P.T. thanks the Royal Society for aU niversity Research Fellowship.