Monodisperse Molecular Models for the sp Carbon Allotrope Carbyne; Syntheses, Structures, and Properties of Diplatinum Polyynediyl Complexes with PtC20Pt to PtC52Pt Linkages

Extended conjugated polyynes provide models for the elusive sp carbon polymer carbyne, but progress has been hampered by numerous synthetic challenges. Stabilities appear to be enhanced by bulky, electropositive transition-metal endgroups. Reactions of trans-(C6F5)(p-tol3P)2Pt(C≡C)nSiEt3 (n = 4–6, PtCxSi (x = 2n)) with n-Bu4N+F–/Me3SiCl followed by excess tetrayne H(C≡C)4SiEt3 (HC8Si) and then CuCl/TMEDA and O2 give the heterocoupling products PtCx+8Si, PtCx+16Si, and sometimes higher homologues. The PtCx+16Si species presumably arise via protodesilylation of PtCx+8Si under the reaction conditions. Chromatography allows the separation of PtC16Si, PtC24Si, and PtC32Si (from n = 4), PtC18Si and PtC26Si (n = 5), or PtC20Si and PtC28Si (n = 6). These and previously reported species are applied in similar oxidative homocouplings, affording the family of diplatinum polyynediyl complexes PtCxPt (x = 20, 24, 28, 32, 36, 40 in 96–34% yields and x = 44, 48, 52 in 22–7% yields). These are carefully characterized by 13C NMR, UV–visible, and Raman spectroscopy and other techniques, with particular attention to behavior as the Cx chain approaches the macromolecular limit and endgroup effects diminish. The crystal structures of solvates of PtC20Pt, PtC24Pt, and PtC26Si, which feature the longest sp chains structurally characterized to date, are analyzed in detail. All data support a polyyne electronic structure with a nonzero optical band gap and bond length alternation for carbyne.


■ INTRODUCTION
Carbon is the second most abundant element in the human body on a mass basis and holds fourth place in interstellar space. 1 It exhibits extraordinarily versatile bonding properties, manifested in three limiting hybridization states (sp 3 , sp 2 , and sp).Although phenomena involving carbon and other elements are of immense fundamental importance, there remain major challenges in understanding many systems composed of carbon alone. 2Over the last three decades, the classical and extensively studied polymeric sp 3 and sp 2 allotropes, diamond 3 and graphite, 4 have been augmented by a plethora of new molecular and polymeric forms of carbon.These include, among many others, 2 the fullerenes, 5 nanotubes, 6 and graphene. 7he work reported herein relates to the polymeric sp allotrope of carbon, commonly termed "carbyne" (C ∞ ).There is extensive literature on this material, 8 for which both polyyne (−(C�C−) ∞ ) and cumulated (�(C�C�) ∞ ) electronic structures have received consideration.The former is distinguished by significant bond order or bond length alternation (BLA) 9 and is generally thought to be more stable. 10However, to date there is no undisputed 11 confirmation that carbyne can exist as a pure bulk material.
−32 Many of these efforts have sought to define asymptotic limits for physical properties capable of providing insight into the macromolecule.In addition, there is the purely synthetic challenge of the length of an sp carbon chain that can be accessed.Are limitations more a function of the methodologies developed to date or the intrinsic stabilities of the targets?
Figures 1 and 2 depict the longest members of series of conjugated polyynes that reach at least 20 sp carbon atoms (eicosadecaynes or higher), have been extensively character-ized, and are in nearly all cases isolable in pure form.The first features carbon or silicon endgroups, and the second features transition-metal endgroups.The latter can also be viewed as one of several types of metalated carbynes, a growing class of materials. 33There has been a school of thought that bulkier and more electropositive endgroups enhance stabilities, and the former design element is particularly evident in [D3]-C 20 [D3], Tr*C x Tr* (x = 44, 48), and Py*C 48 Py*.Steric protection is likely a stabilizing factor in the very long sp carbon chains that have been sequestered within carbon nanotubes 34 as well as in supramolecular systems in which C 68 segments are shielded in rotaxanes. 30he systems of Walton in Figure 1 are emblematic of the "bronze age" of polyyne synthesis. 13,14In the triethylsilyl series, SiC 20 Si, SiC 24 Si, and SiC 32 Si were not fully purified and only characterized by UV−visible spectroscopy. 13Samples of t-BuC 24 t-Bu, which can be isolated as red needles that decompose within minutes at room temperature, have been similarly characterized, 14 but pure samples of t-BuC 20 t-Bu have been reported. 15,16The remaining compounds in Figures 1 and  2 were isolated in pure form and characterized by a full array of modern spectroscopies.The longest carbyne models realized to date have been reported by Tykwinski and Anderson and include (1) C 44 and C 48 systems with "supertrityl" endgroups (Tr*) featuring two meta disposed t-butyl groups on each aryl ring 21,30 and (2) a C 48 system with a diarylated C-pyridyl endgroup (Py*) with meta disposed t-butyl groups on each aryl ring. 22n this article, we present a full account of our results with compounds with pentafluorophenylplatinum endgroups, trans,trans-(C 6 F 5 )(p-tol 3 P) 2 Pt(C�C) n Pt(Pp-tol 3 ) 2 (C 6 F 5 ) (PtC x Pt, x = 2n).As shown by Pt′C 28 Pt′ in Figure 2, we have also investigated analogs with p-tolylplatinum endgroups. 28However, this thrust has been paused due to the perception of Figure 1.Polyynes with 10 or more triple bonds bearing stabilizing organic or trialkylsilyl endgroups that have been isolated in pure form except where noted.incrementally higher crystallinities in the pentafluorophenyl complexes, which are capable of attractive C 6 H 4 CH 3 /C 6 F 5 35 πstacking interactions.
The triethylsilylpolyynyl building blocks trans-(C 6 F 5 )(ptol 3 P) 2 Pt(C�C) 4 SiEt 3 (PtC 8 Si), 36−38 PtC 10 Si, 38 and PtC 12 Si 38 provide the starting point for this study.As reviewed in Scheme 1, their syntheses entail oxidative heterocouplings of PtC 4 H, PtC 6 H, or PtC 8 H (the last two generated in situ from PtC x Si) with excess HC�CSiEt 3 (HC 2 Si) or HC 4 Si. 28We commonly employ Hay conditions (O 2 , cat.CuCl/ TMEDA), 39 although many different alkyne coupling recipes have been applied in the syntheses underlying Figures 1 and 2. Some of the sequences in Scheme 1 have also been conducted with the "TIPS" analog, H(C�C) 2 Si(i-Pr) 3 . 36While this functions well, no advantages have been found.
Analogous four-carbon sp chain extensions lead to PtC 14 Si, PtC 16 Si, PtC 18 Si, and PtC 20 Si, 38 and a previously unreported conversion of PtC 18 Si to PtC 22 Si is described in the Supporting Information (SI).Importantly, coupling becomes more rapid as the sp chains are extended, as both initial protodesilylation and the oxidation itself are accelerated. 28,29hese trends are likely connected to the well-documented increases in the Brønsted acidities of terminal polyynes R(C� C) n H as the sp carbon chains lengthen. 40ith the aid of new building blocks as described below, it has proved possible to extend our earlier report of PtC 24 Pt 36 in four-carbon increments to the dopentacontahexacosayne PtC 52 Pt.This series of complexes has been characterized by all classical spectroscopies, with PtC 52 Pt representing, at least for the moment, the longest monodisperse polyyne with respect to the prior art in Figures 1 and 2. Importantly, these materials continue to show impressive stabilities when purified, suggesting that homologues with still longer sp chains will be accessible.

Synthesis of H(C�C) 4 SiEt 3 (HC 8 Si).
In previous syntheses of diplatinum polyynediyl complexes PtC x Pt, we employed the two-and four-carbon building blocks HC 2 Si and HC 4 Si for sp chain extensions of monoplatinum precursors PtC x H (Scheme 1).For this new effort, the corresponding eight-carbon synthon HC 8 Si, which Walton reported earlier and characterized by UV−visible spectroscopy, 13,14,41 was generated as depicted in Scheme 2.
First, the Hay homocoupling of HC 4 Si gave SiC 8 Si in 83% yield after workup. 39Then a methanol/pentane solution was Protodesilylation was not very selective, but SiC 8 Si, HC 8 Si, and HC 8 H could largely be separated by column chromatography (silica gel and hexane) if desired.However, the mixture (designated as "crude HC 8 Si") was found to give equally good preparative results and was employed in most cases.Pentane or hexane solutions readily decomposed at room temperature or when concentrated and were stored at −78 to −35 °C and used as promptly as possible.
Oxidative Heterocouplings of HC 8 Si and PtC 8 Si, PtC 10 Si, or PtC 12 Si.As shown in Scheme 3, PtC 10 Si and wet n-Bu 4 N + F − were combined at −78 °C to generate PtC 10 H, as verified by "click" trapping reactions with azides. 38Then Me 3 SiCl was added, a step that has often enhanced yields in the Pt′C x Pt′ series (Figure 2) 29 and is thought to scavenge fluoride ion.Control experiments without Me 3 SiCl are described below.An excess of crude HC 8 Si was added at −35 °C, followed by Hay conditions.Silica gel chromatography gave the known complex PtC 18 Si 38 and a new faster eluting complex PtC 26 Si in 16−31% and 7−13% yields, respectively.
"Double addition" products analogous to PtC 26 Si have occasionally been observed with the shorter building block HC 4 Si. 29The present example likely involves the in situ protodesilylation of PtC 18 Si and coupling with another equivalent of HC 8 Si.Protodesilylation becomes progressively more facile as the sp chain is lengthened.However, some participation of HC 8 H cannot be excluded.Continued elution of the column gave lesser amounts of diplatinum complexes PtC x Pt (x = 20, 28, 36 via PtC 10 H/PtC 18 H homo/ heterocoupling), as assayed by MS and HPLC.
The complexes PtC 24 Si, PtC 26 Si, PtC 28 Si, and PtC 32 Si were dark-brown-orange to red-violet solids that tolerated brief exposures to air but were strongly discolored after several days.DSC measurements with PtC 24 Si and PtC 26 Si showed exotherms with T i values of 79−93 °C (Table s2). 42These and other new complexes below were characterized by microanalyses, NMR ( 1 H, 13 C{ 1 H}, 31 P{ 1 H}), IR, UV−visible spectroscopy, and MS.Key 13 C{ 1 H} NMR and UV−visible properties are summarized in Tables 1−4, and other data are collected in the SI.Molecular ions were always observed by MS.
These complexes represent the most extensive series of unsymmetrically substituted polyynes in the literature, crowned by dotriacontahexadecayne PtC 32 Si.While they are Scheme 2. Synthesis of HC 8 Si Scheme 3. Oxidative Heterocouplings of PtC 8 Si, PtC 10 Si, and PtC 12 Si with HC 8 Si

ACS Central Science
less suitable for some of the goals in the Introduction, there are many trends of interest as noted below.To spare researchers from combing through earlier papers to retrieve data, all tables have been "backfilled" with lower homologues.Of course, some properties, such as the 1 H NMR chemical shifts of the −C 6 H 4 CH 3 signals (Table s3), are essentially "flat" throughout the series.
Oxidative Homocouplings.Although syntheses of PtC 20 Pt and PtC 24 Pt have been reported, these utilized educts with Pt(C�C) n Si(i-Pr) 3 linkages. 36The preparative data in Scheme 4 are restricted to the triethylsilyl precursors PtC x Si, and details for PtC 20 Pt and PtC 24 Pt are given in the SI.In all cases, PtC x Si was first combined with O 2 and the Hay catalyst so that upon subsequent addition of wet n-Bu 4 N + F − the resulting PtC x H would have a better chance of undergoing oxidation as opposed to competing processes. 43−18,19b,21,22,29,38 This occurs sporadically and so far has no mechanistic explanation.This problem was encoun-Table 1. 13   Poor signal-to-noise ratio.d Signal was not observed.e For solubility purposes, this spectrum was recorded in 1:1 v/v C 6 D 5 CD 3 /CDCl 3 ; only 23 signals were observed (theory: 26); that at 62.0 ppm was more intense than the others (Figure 4).tered in our first attempts to prepare PtC 44 Pt and PtC 48 Pt, as assayed by MS data that showed M + , M + −24, and M + −48 ions and HPLC.In second-generation efforts, we have been careful to check the precursors PtC x Si for such ions. 44When absent, the homocoupling products have not so far contained any lower homologues.Since MS is normally not a quantitative technique and lower homologues derived from C 2 loss should be more volatile, we have not interpreted the relative ion intensities of samples.
Regardless, the yields drop to 22−7% for PtC 44 Pt and higher homologues (Scheme 4).When the solvents were removed from crude samples and the residues were analyzed by MS, no ions suggestive of dimeric or other nonoligomeric byproducts were observed.No other materials were eluted from silica gel.Complexes PtC 40 Pt to PtC 52 Pt were dark-red solids with decomposition points of >70 °C (Table s2), and photographs of CH 2 Cl 2 solutions are provided in Figure s1.In the earlier stages of this work, some variability in sample stability was noted, but this is now believed to be a function of purity.For PtC 32 Pt and higher homologues, it is advisible to shield samples from light, but in no case does rapid degradation occur.They are best stored at −35 °C and used within 1 week, although many samples have survived much longer.
NMR Spectra.The 13 C{ 1 H} NMR data in Tables 1 and 2 s2 illustrate the asymptotic approach to the macromolecular limit of PtC ∞ Si and PtC ∞ Pt, but these are obvious from the raw numbers alone (∼111.6 ppm in both cases).The PtC�C signals of PtC x Si vary only from 95.4 to 94.7 ppm (Δδ < 1 ppm), but those of PtC x Pt are better differentiated (Δδ 9.2 ppm).Both series converge to the same 94.7 ppm value.The C�CSi signals of PtC x Si shift downfield with increasing chain length (80.3 to 88.9 or Δδ 8.6 ppm), whereas the C�CSi signals shift slightly upfield (91.2 to 88.8 or Δδ 2.4 ppm) to essentially the same limit.
As illustrated in Figures 3 and 4, the intensities of the PtC� C signals are diminished relative to those of other sp carbon atoms by couplings to 31 P and 195 Pt (33.8% natural abundance,  I = 1/2).These usually remain unresolved. 45With PtC 52 Pt, default solvent CDCl 3 had to be augmented with toluene-d 8 to enhance the solubility and visualize the sp signals.In Figure 3, the C�CSi signals remain quite intense, despite couplings to 29 Si (4.7% natural abundance). 46All of these splittings are evident in an optimized 13 C{ 1 H} NMR spectrum of a close analog of PtC 8 Si. 36Furthermore, DFT computations on TIPS analogs of PtC x Si (x = 8, 12, 16) nicely reproduce the chemical shift trends. 36s shown in Tables 1 and 2 and Figure 4 and in general agreement with the other polyynes in Figures 1 and 2, 31b the remaining C�C signals cluster between 55.9 and 68.2 ppm.Further assignments are challenging. 45It might be simplistically postulated that those furthest upfield correspond to the innermost carbon atoms, but labeling and DFT studies have disproven this conjecture. 18,36To aid additional analyses below, the average chemical shifts of all sp carbon atoms except those of the PtC�C and C�CSi linkages (e.g., PtC�C(C� C) n−2 C�CSi/Pt) are compiled in Figure s3.
As presented in Table s1, the 31 P{ 1 H} NMR chemical shifts of PtC x Si and PtC x Pt fall into the same narrow range (17.90− 18.05 and 17.20−18.04ppm for x ≥ 6), with 1 J PPt decreasing approximately monotonically from 2636 to ∼2600 Hz and 2713 to ∼2600 Hz.With the para-substituted arylplatinum bis(phosphine) complexes trans-(p-ZC 6 H 4 )Pt(PEt 3 ) 2 X (X = Br, H), the 1 J PPt values decrease as the Z groups become more electron-withdrawing. 47This provides further support for the enhanced electronegativities and �CH acidities of longer C x chains.
Vibrational Spectroscopy.The IR ν C�C bands of PtC x Si and PtC x Pt (x ≥ 6) are summarized in Table s1.Through x = 26−28, the number of absorptions generally increases with sp chain length, in accord with computational studies of HC x H. 48 Furthermore, at the same sp chain length, unsymmetrically substituted PtC x Si complexes generally exhibit more bands  than PtC x Pt.This is consistent with their nonzero dipole moment along the platinum−silicon vector and inherently noncentrosymmetric nature, which removes the IR/Raman mutual exclusion rule formally applicable to PtC x Pt. 19a Raman spectra of polyynes have received attention as probes of BLA, as elaborated in the Discussion section. 49Although multiple Raman ν C�C absorptions are often detected 27 and/or predicted computationally, 11 spectra are normally dominated by the so-called Я band, corresponding to a collective C�C stretching and �C−C� contraction mode.Thus, Raman spectra were acquired in CH 2 Cl 2 as described in the Experimental Section.Data are summarized in Figure 5, Table 5, and Figure s6 3 and 4 and depicted in Figures 6 and 7.
As recognized since early studies, 13 the absorption envelopes of many polyynes exhibit C�C vibrational fine structure, and Figures 6 and 7 are rich in local maxima.Three trends are apparent, as is also seen with many of the other polyyne series in Figures 1 and 2. First, the longest-wavelength absorptions (λ Emin ) red shift upon increasing the sp carbon chain length.Second, the bands become broader and fine structure weakens as the chains lengthen.Third, the λ max (most intense absorption) transitions from λ Emin for PtC 16 Pt, PtC 18 Pt, PtC 20 Pt, PtC 24 Pt, and PtC 28 Pt to the second longest     wavelength band for PtC 32 Pt and higher homologues.The last trend, also nicely illustrated by Tr*C x Tr*, 21 has been reproduced computationally for trans polyenes 50 and is believed to have a Franck−Condon origin.Aspects of the absorption spectra that probe BLA are analyzed in the Discussion section.
Crystallography.It was sought to crystallographically characterize as many of the preceding complexes as possible.As detailed in Table s7 and the Experimental Section, the structures of three solvates, PtC 26 Si•(CH 2 Cl 2 ), PtC 20 Pt• (CH 2 Cl 2 ), and PtC 24 Pt•(C 6 H 14 ) 2 (CH 2 Cl 2 ) 1.7 , could be solved.These are depicted in Figures 8 and 9. PtC 26 Si represents the longest structurally characterized conjugated polyyne with unlike endgroups, and PtC 24 Pt, the longest with like endgroups.We reported a crystal structure of the fractional solvate PtC 20 Pt•(CH 2 Cl 2 ) 0.8 earlier, 36 but data for the new monosolvate are of much higher quality.The only other crystallographically characterized polyyne with at least 10 triple bonds is t-Bu(C�C) 10 t-Bu. 16ey metric parameters are summarized in Tables s8 and s9, and selected data are analyzed in the Discussion section.The pentafluorophenyl ligand engages in π stacking interactions, but they are not as pronounced as for other complexes in this series, as judged from C 6 H 4 CH 3 /C 6 F 5 /C 6 H 4 CH 3 centroid distances and angles.With PtC 24 Pt•(C 6 H 14 ) 2 •(CH 2 Cl 2 ) 1.7 , the midpoint of the C 24 chain corresponds to the crystallographic inversion center.Chain curvature is always evident, but well within the bounds of other polyynes. 51e crystal packing of conjugated polyynes has been analyzed in detail, 51 and excerpts from each lattice are depicted in Figure 10.In PtC 24 Pt•(C 6 H 14 ) 2 •(CH 2 Cl 2 ) 1.7 , the platinum− platinum vectors are parallel, with a separation of 3.94 Å.Thus, the sp chains remain out of van der Waals contact (radius of sp carbon, 1.78 Å). 52 With PtC 26 Si•(CH 2 Cl 2 ), the platinum− silicon vectors are parallel.Now the separation narrows to 3.57 Å such that contacts between the noticeably curved neighboring chains are apparent.The neighbors pack in head-to-head arrangements with offsets of about 11 sp carbon atoms.With PtC 20 Pt•(CH 2 Cl 2 ), there are two nonparallel sets of parallel platinum−platinum vectors, with none of the nearest parallel chains in van der Waals contact.All of these motifs have precedent in polyyne lattices. 51DISCUSSION Synthetic Methodology and Stabilities.The title complexes PtC x Pt and their supporting cast PtC x Si provide the largest collections of monodisperse symmetrically and unsymmetrically substituted polyynes assembled to date.Eighteen of the 29 complexes bookended by PtC 6 Si/PtC 32 Si and PtC 4 Pt/PtC 52 Pt have been crystallographically characterized (Figure s8 and Table s10), and more are expected to follow soon, including some with much longer sp chains than PtC 24 Pt or PtC 26 Si.
There are advantages and disadvantages associated with building block HC 8 Si used in Scheme 3. The multiple additions provide rapid access to mixtures of monoplatinum complexes PtC x Si with long sp carbon chains.Since the  components differ by eight sp carbon atoms, they are easy to chromatographically separate.But unlike SiC 4 Si, which can be cleanly protodesilylated to HC 4 Si, the more remote termini of the precursor SiC 8 Si resist selective functionalization (Scheme 2).With the unsymmetrical bis(trialkylsilyl) tetrayne Me 3 Si-(C�C) 4 Si(i-Pr) 3 , the trimethylsilyl group can be preferentially removed to give H(C�C) 4 Si(i-Pr) 3 , and this has been exploited by Tykwinski for sp chain extensions en route to Tr*C 44 Tr* (Figure 1). 21 key challenge with both heterocouplings and homocouplings is the increasing lability of terminal polyynes PtC x H as the sp chains lengthen.These can be isolated in pure form only through x = 8, 29,37 and some diminution in the efficacy of click trapping is apparent at x = 18. 38 The heterocouplings in Scheme 3 do not seem to have approached any limit arising from the stabilities of PtC x Si.However, the situation with homocouplings in Scheme 4 is less clear.There is an apparent upper yield limit of ∼10% for PtC 48 Pt and PtC 52 Pt, which we attribute to side reactions or decomposition of the precursors PtC x H. Nonetheless, as generations of co-workers have repeated the syntheses of PtC 44 Pt, PtC 48 Pt, and PtC 52 Pt, we have become more sanguine about their stabilities.Pure samples stored as prescribed could be kept for weeks or longer.
As shown in Table s2, the thermal decomposition points of PtC x Pt trend downward for x ≥ 28 but remain above 70 °C.Decomposition may involve sp chain/sp chain cross-linking, reminiscent of solid-state topochemical polymerizations of 1,3diynes 53 and foreshadowed by close contacts in the lattice of PtC 26 Si•(CH 2 Cl 2 ) (Figure 9, top).In any case, we are optimistic that higher homologues of PtC 52 Pt represent viable synthetic targets, although this might be facilitated by replacing the (p-tol) 3 P ligands by bulkier triaryl phosphines. 54Also, some of the above issues can be avoided by the use of "masked alkyne equivalents".As developed by Anderson, π adducts of polyynes and Co 2 (CO) 4 L 2 fragments are used to assemble long C x moieties and the dicobalt moieties removed in the final step. 30roperties of C ∞ Systems.Some extrapolations of the preceding data are more relevant to polymers PtC ∞ Pt and PtC ∞ Si, and others map the dissipation of end-group effects, pointing to values expected of carbyne (C ∞ ).Such limits are best estimated using the Meier equation. 55In the most general form (eq 1), the convergence of a property y of a series of unsaturated oligomers from y n to a constant value at y ∞ can be modeled by the exponential relationship One overarching issue involves the electronic structure of carbyne, for which cumulated (�(C�C�) ∞ ; BLA = 0) and polyyne (nonzero BLA) limits have received extensive consideration.Per the following subsections, our results add to the growing body of evidence from organic models in Figure 1 that support the polyyne formulation.31b This is often couched as a Peierls distortion of the cumulated limit that installs a nonzero band gap. 10 13 C{ 1 H} NMR Data.The limits reached by the chemical shifts of the PtC�C and C�CSi signals with increasing x (Tables 1 and 2; Figure s2) clearly represent attributes of polymers PtC ∞ Pt and PtC ∞ Si.As seen in Figures 3 and 4 for PtC x Pt and PtC x Si, the chemical shifts of the terminal C�C linkages of the other polyynes in Figures 1 and 2 also appear downfield of the remaining sp carbon signals.31b This deshielding is more pronounced with electropositive transition-metal endgroups.
Other chemical shift data map the transition to carbyne.In Figure s3 Raman Data.The Raman Я band of polyynes has been shown to shift to lower energy as the BLA decreases. 49For PtC x Pt, they drop from 1928 cm −1 for x = 20 to 1880 cm −1 for x = 52.As depicted in Figure 11, the Meier equation indicates convergence to 1881 cm −1 (ν ∞ ), requiring a polyyne electronic structure for C ∞ .This protocol also allows the estimation of an effective conjugation length (ECL) of 30 C�C units, derived by limiting y ∞ − y ECL to 1 cm −1 .55b An ECL value represents the point at which the endgroups no longer affect the property of interest.With PtC x Si, some complexes exhibited multiple Raman bands of comparable intensities in this region (Figure s6).This has been seen for other unsymmetrically substituted polyynes, which as noted above are not subject to the Raman/ IR mutual exclusion rule. 20Hence, these were not further analyzed.
The Я bands similarly converge to 1908 cm −1 for Tr*C x Tr*, 1865 cm −1 for Py*C x Py* (ECL 37), 1790 cm −1 for AdC x Ad, and 1835 cm −1 for transition-metal-substituted ReC x Re (ECL 37).31b There are potential rationales for these modest differences (possible mixing of additional vibrational modes, experimental factors, etc.), but computational studies are likely required for further insight.In any case, the following conclusions emerge: (1) by the Raman criterion, PtC x Pt and all other polyyne series so analyzed exhibit marked BLA, far removed from zero, (2) the range of convergence values, 1790 to 1908 cm −1 , can be viewed as limits on the corresponding absorption of carbyne, and (3) this range nicely agrees with experimental data for polydisperse samples of carbyne encapsulated in different types of carbon nanotubes (1850− 1880 cm −1 ).34b UV−Visible Data.As conjugated oligomers are extended, the λ max and/or λ Emin commonly red shift and increase in intensity.The shift is, of course, expected from basic MO theory.However, the situation with PtC x Pt is more complicated, as revealed by DFT studies of the model PH 3 complexes trans,trans-(C 6 H 5 )(H 3 P) 2 Pt(C�C) n Pt(PH 3 ) 2 -(C 6 H 5 ) (Pt′′C x Pt′′, x = 4−26). 58Two π → π* transitions associated with the sp chain are predicted.That at longer wavelength (lower energy, termed band II) has predominant HOMO−LUMO character.However, its extinction coefficient decreases dramatically with increasing chain length.Experimentally, it is only observed with PtC x Pt with shorter chains (x = 4−12 but not 20), always with characteristic vibrational fine structure. 28,29Analogous series of bands have been found for lower homologues of many of the polyynes in Figure 1, as summarized in a recent experimental and computational study. 23he higher-energy band (band I) becomes markedly stronger with increasing chain length and is multiconfigura-tional with increasing HOMO−LUMO character.These afford the maxima in Figures 6 and 7, and thus the optical band gaps overestimate the true band gaps.The DFT studies furthermore show that the platinum character in the molecular orbitals of most interest (HOMO, HOMO − 1, and LUMO) decreases with increasing chain length, roughly proportional to the Pt/ C x /Pt composition. 57The platinum contribution is significant for shorter sp chains (22% in Pt′′C 6 Pt′′, for which 25% of the atoms are platinum) but drops sharply for later members in the series (6% in Pt′′C 20 Pt′′, 9% of the atoms are platinum).Thus, the orbitals and corresponding transitions in still-higher homologues are almost exclusively carbon-chain-based.
In Figure 12, the longest-wavelength absorptions (λ Emin ) are plotted against the number of triple bonds n using the Meier equation.The excellent fit gives 506 nm for λ Emin∞ , or an optical band gap of 2.50 eV for C ∞ .The ECL (derived by limiting λ Emin∞ − λ Emin(ECL) to 1 nm) is ∼42.These data are quite similar to those obtained for Tr*C x Tr* (λ Emin∞ = 486 nm, ECL = 48), 21 and similar values have been reported for SiC x Si, AdC x Ad, and t-BuC x t-Bu (λ Emin∞ = 501−503 nm).31b There is some play in all of these numbers.For example, when Figure 12 is restricted to data for PtC 16 Pt to PtC 52 Pt (n ≥ 8), the λ Emin∞ drops to 493 nm and the ECL drops to 30.Depending upon how our earlier data with Pt′C x Pt′ are treated, λ Emin∞ ranging from 558 to 636 nm can be obtained.Nonetheless, when all of these limits are taken together, they provide excellent evidence that the λ Emin of C ∞ should be ca.500 nm.Furthermore, as elegantly presented in a recent study, the dramatic attenuation of band II with chain length parallels the evolution to the D ∞h limit of carbyne.23b,31b Crystallographic Data.The wealth of structural data for PtC x Pt and PtC x Si might initially seem to hold promise for defining the BLA trends.However, the esd values that accompany crystallographic bond lengths and angles often constrain comparisons.With our compounds and most other polyynes, they render it virtually impossible to conclude that one C�C or �C−C� linkage is shorter than another.As detailed in reviews, 51 averages drawn from replicate or series of crystal structures can sometimes improve confidence.However, in many cases, computational studies (which are most often gas phase) can be regarded as more accurate.In efforts to skirt these problems, two parameters, BLA(avg) and BLA, have been utilized.The first is the average of the � C−C� bond lengths minus that of the C�C bond lengths.The second is the absolute value of the length of the central carbon−carbon bond minus the length of the two adjacent bonds (the central bond will be C�C when n is odd and � C−C� when even).In Figure s8, these are plotted versus n for all of the crystal structures available for PtC x Si and PtC x Pt.While it is clear that the data are not converging toward a BLA of zero, there is no monotonic trend or Meier correlation.However, a Meier plot of our gas-phase computational data for Pt′′C x Pt′′ gives a BLA(avg) ∞ of 0.092 Å (R 2 = 0.99; Figure s7).
Thus, we suggest that the bulky and polarizable platinum endgroups induce packing forces that alter the bond length patterns from those in the gas phase.However, data from crystal structures so far obtained in the series Py*C x Py* and t-BuC x t-Bu are nicely modeled by the Meier equation, giving BLA(avg) ∞ values of 0.140 and 0.145 Å, respectively.31b In contrast, there is no convergence in the case of TIPSC x TIPS.Given this result and our data in Figure s8, we feel that crystallographic correlations are likely to be uncommon.

■ CONCLUSIONS
By use of the labile building block HC 8 Si, the sp carbon chains of PtC x Si could be extended in fewer steps than in previous efforts, thereby allowing access to the new diplatinum polyynediyl complexes PtC 28 Pt, PtC 32 Pt, PtC 36 Pt, PtC 40 Pt, PtC 44 Pt, PtC 48 Pt, and PtC 52 Pt.Excluding supramolecular systems, 30,34 this series terminates with the longest polyyne isolated in pure form to date.NMR, IR, Raman, and UV− visible data as well as 18 crystal structures have been carefully analyzed as a function of chain length.Certain protocols map the attrition and ultimate loss of endgroup effects, affording experiment-based predictions for key properties of carbyne (C ∞ ), which include a polyyne electronic structure with an appreciable bond length alternation and a 2.50 eV optical band gap.For the most part, these are in line with those extrapolated from analogs with organic endgroups, confirming that systems with transition-metal termini also provide valid models for the polymeric sp allotrope.This is of special interest to those who believe, as the authors, that in whatever competition may exist to isolate the longest monodisperse polyyne, transition-metal endgroups will ultimately provide the winners.In that context, PtC 52 Pt certainly does not represent any type of stability boundary, and the methods employed for the compounds in Figures 1 and 2 have only superficially dented the universe of sp/sp coupling methods.Accordingly, syntheses of higher homologues remain under active investigation.
■ EXPERIMENTAL SECTION General Data.Reactions were conducted under dry inert atmospheres using conventional Schlenk techniques, but workups were carried out in air.Sources of chemicals, instrumental methods, and other protocols are summarized in the SI.(Caution!Polyynes normally possess highly positive heats of formation and may be regarded as energy-rich materials that are intrinsically thermodynamically unstable.)Many explosions or rapid exothermic decompositions of polyynes have been reported.These most frequently involve species with (C�C) n X linkages (X = H, halide).Regardless, all polyynes should be treated as potentially explosive, and appropriate safety precautions should be taken.
PtC 18 Si and PtC 26 Si (from PtC 10 Si/HC 8 Si).A three-neck flask was fitted with a gas dispersion tube, charged with PtC 10 Si (0.203 g, 0.168 mmol) and THF (200 mL), and cooled to −78 °C.A Schlenk flask was charged with CuCl (0.240 g, 2.42 mmol), acetone (20 mL), and TMEDA (0.675 mL, 0.523 g, 4.48 mmol) with stirring (0.5 h), after which a green solid separated from the blue supernatant.Then wet n-Bu 4 N + F − (1.0 M in THF, 5 wt % water, 0.08 mL, 0.08 mmol) was added to the three-neck flask with stirring.After 5 min (TLC showed no remaining educt), Me 3 SiCl (0.10 mL, 0.84 mmol) and a −35 °C pentane solution of crude HC 8 Si (ca.30 mL, from 50 equiv of SiC 8 Si) were added.Then oxygen was aspirated through the tube, and the blue supernatant was added with stirring. 44After 50 min, hexanes (150 mL) were added.The suspension was filtered through a pad of silica gel (5 × 7 cm 2 , packed in 1:1 v/v acetone/hexanes), which was rinsed (1:1 v/v acetone/hexanes) until the filtrate became colorless.The solvents were removed from the filtrate by rotary evaporation at <10 °C.The red-brown residue was chromatographed on a silica gel column (4.5 × 30 cm 2 , packed in hexanes, and eluted with hexanes and then a CH 2 Cl 2 gradient until 1:2 v/v CH 2 Cl 2 /hexanes).The solvents were removed from the product-containing fractions by rotary evaporation at <10 °C to give (in order of elution) PtC 26 Si as a brown-orange solid (0.015 g, 0.011 mmol, 7%) and PtC 18 Si as a dark-orange solid (0.035 g, 0.027 mmol, 16%).A later fraction afforded a mixture of Pt 20 Pt, Pt 28 Pt, and Pt 36 Pt, as assayed by MS and HPLC (see the text).
PtC 18 Si. 38The thermal, DSC, microanalytical, NMR, IR, UV−vis, and MS data agreed with those reported previously.PtC 20 Si and PtC 28 Si (from PtC 12 Si/HC 8 Si).A three-neck flask was fitted with a gas dispersion tube, charged with PtC 12 Si (0.195 g, 0.159 mmol) and THF (60 mL), and cooled to −78 °C.A Schlenk flask was charged with CuCl (0.286 g, 3.12 mmol), acetone (8 mL), and TMEDA (0.828 mL, 0.642 g, 5.2 mmol) with stirring (0.5 h), after which a green solid separated from a blue supernatant.Then wet n-Bu 4 N + F − (1.0 M in THF, 5 wt % water, 0.05 mL, 0.05 mmol) was added to the three-neck flask with stirring.After 5 min (TLC showed no remaining educt), Me 3 SiCl (0.05 mL, 0.4 mmol) and a −35 °C pentane solution of crude HC 8 Si (ca.30 mL, from 50 equiv of SiC 8 Si) were added.Then oxygen was aspirated through the tube, and the blue supernatant was added with stirring. 44After 75 min, the suspension was filtered through a pad of silica gel (2.5 × 10 cm 2 , packed in 1:1 v/v acetone/hexanes), which was rinsed (1:1 v/v acetone/hexanes) until the filtrate was colorless.The solvents were removed from the filtrate by rotary evaporation.The red-brown residue was dried by oil pump vacuum and chromatographed on a silica gel column (3.5 × 30 cm 2 , packed in hexanes, and eluted with hexanes and then a CH 2 Cl 2 gradient until 1:1 v/v CH 2 Cl 2 / hexanes).The solvents were removed from the product-containing fractions by rotary evaporation at <10 °C to give (in order of elution) PtC 28 Si as a dark-violet solid (0.030 g, 0.021 mmol, 13%) and PtC 20 Si as a violet-red solid (0.074 g, 0.056 mmol, 35%).
PtC 32 Pt.A three-neck flask was fitted with a gas dispersion tube, charged with PtC 16 Si (0.099 g, 0.077 mmol) and acetone (60 mL), and cooled to −45 °C.A Schlenk flask was charged with CuCl (0.286 g, 3.12 mmol), acetone (8 mL), and TMEDA (0.827 mL, 0.641 g, 5.2 mmol) with stirring (0.5 h), after which a green solid separated from the blue supernatant.Oxygen was aspirated through the tube, and the blue supernatant was added with stirring. 44Wet n-Bu 4 N + F − (1.0 M in THF, 5 wt % water, 0.05 mL, 0.05 mmol) was added with stirring.The dark-green suspension immediately turned reddish-brown.After 5 min, Me 3 SiCl (0.060 mL, 0.45 mmol) was added.After 30 min (TLC showed no remaining educt), hexanes (80 mL) were added.The mixture was filtered through a pad of silica gel (2.5 × 5 cm 2 , packed in 1:3 v/v acetone/hexanes), which was rinsed (1:3 v/v acetone/ hexanes) until the filtrate became colorless.The solvents were removed from the filtrate by rotary evaporation, and the red-brown residue was chromatographed on a silica gel column (1 × 25 cm 2 , packed in hexanes, and eluted with hexanes and then a CH 2 Cl 2 gradient until 1:3 v/v CH 2 Cl 2 /hexanes).The solvents were removed from the product-containing fractions by rotary evaporation and oil pump vacuum at <10 °C to give PtC 32 Pt as a red-brown solid (0.031 g, 0.013 mmol, 34%).

Figure 2 .
Figure 2. Polyynes with 10 or more triple bonds bearing stabilizing metal endgroups that have been isolated in pure form.

Scheme 4 .Figure 3 .
Scheme 4. Oxidative Homocouplings of PtC x H Generated from PtC x Si
and interpreted below.Optical Properties.The colors of the solid monoplatinum complexes PtC x Si progressively deepened from bright yellow (x = 6, 8) to orange-yellow (x = 10) to orange-red (x = 12) to orange-brown (x = 14) to dark red (x = 16) to dark violet-red (x = 18, 20, 24, 26, 28, 32).This trend was paralleled by the diplatinum complexes PtC x Pt, and the colors exhibited by dilute solutions are illustrated in Figure s1.These observations are consistent with the UV−visible spectra summarized in Tables

a
Due to interfering fluorescence, these bands broadened and the values are approximate.
A possible mechanistic solution would involve the direct conversion of −(C�C) n Si or −(C�C) n Sn termini to transition-metal derivatives −(C� C) n L y M(C�C) n − capable of reductively eliminating two alkynyl ligands under mild conditions.
, the averages of all sp signals except the PtC�C and C�CSi linkages (x − 4 values total) are treated by the Meier protocol. 56For PtC x Pt, there is a nearly perfect monotonic trend (ppm) from 61.10 (x = 6) to 63.42 (x = 44) that extrapolates to 63.35 (x = ∞).The data for PtC x Si are quite similar for the corresponding values of x (63.02 for x = ∞).Thus, carbyne (C ∞ ) should exhibit a signal with a mean chemical shift of ∼63.15 ppm, which by analogy to linear polyethylene 57 should be a broad singlet.The sp signals of other polyynes in Figures 1 and 2 have been treated in somewhat different ways, but all point to comparable limits.

Figure 11 .
Figure 11.Extrapolation of the Raman Я band of PtC x Pt to x = ∞ by using the Meier equation.

Figure 12 .
Figure 12.Extrapolation of the λ Emin values (longest-wavelength UV− visible absorption) of PtC x Pt (x ≥ 6) to x = ∞ by using the Meier equation.
In this context, click trapping reactions have not yet been explored beyond PtC 18 H. 38As shown in Scheme 4, reactions of PtC 10 Si and PtC 12 Si at 0 °C gave PtC 20 Pt and PtC 24 Pt in 76−96% yields after silica gel chromatography.Due to the protodesilylation and oxidation rate trends noted above, couplings of higher homologues could be carried out at −45 to −78 °C.
C{ 1 H} NMR Data (δ/ppm, J/Hz, 126 MHz/CDCl 3 ) for PtC x Si Data at 101 MHz are from ref 29.b The chemical shifts of the TIPS analogs are very similar, as reported in ref 36 or 38.c This represents a reversal from the assignment in ref 29, as explained in ref 36 (see closing portion of that discussion section).d Data at 126 MHz are from ref 38. e One fewer than the theoretical number of sp carbon atom signals is observed, presumably due to the overlapping of two signals; that at 62.8 ppm is more intense than the others.f Poor signal-to-noise ratio.g Signal was not observed.h Only 18 distinct signals were observed (theory: 28).
a Data are from ref 29.b Data are from ref 38.c

Table 4 .
UV−Visible Data for PtC x Pt (CH 2 Cl 2 ) Data are from ref 29.b These complexes have been prepared as part of a separate study and will be reported at a later date.c This represents new data from that reported in ref 36; the ε values are viewed as more accurate.d Due to the limited amount of sample, the ε values are considered to be accurate to only two significant digits.