DOI: 10.1002/chem.201405803

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Platinum trans-Bis(borirene) Complexes Displaying Coplanarity and Communication Across a Platinum Metal Center Holger Braunschweig,*[a] Alexander Damme,[a] Rian D. Dewhurst,[a] Hauke Kelch,[a] Bret B. Macha,[a] Krzysztof Radacki,[a] Alfredo Vargas,[b] and Qing Ye[a]

Abstract: Ambient-temperature photolysis of the aminoborylene complex [(OC)5Cr=B=N(SiMe3)2] in the presence of a series of trans-bis(alkynyl)platinum(II) precursors of the type trans-[Pt(CCAr)2(PEt3)2] (Ar = Ph, p-C6H4OMe, and pC6H4CF3) successfully leads to twofold transfer of the borylene moiety [DB=N(SiMe3)2] onto the alkyne functionalities. The alkynyl precursors and resultant bis(borirene)platinum(II) complexes formed are of the type trans-[Pt(B{=N(SiMe3)2}C= CAr)2(PEt3)2] (Ar = Ph, p-C6H4OMe, and p-C6H4CF3). These species have all been successfully characterized by NMR, IR, and UV/Vis spectroscopy as well as by elemental analysis. Singlecrystal X-ray diffraction has verified that these trans-bis(borirene)platinum(II) complexes display coplanarity between the

twin three-membered rings across the platinum core in the solid state and stand as the first examples of coplanar conformations of twin borirene systems. These complexes were modeled using density functional theory (DFT), providing information helpful in determining the ability of the transition metal core to interact with each individual borirene ring system and allowing for the observed coplanarity of these rings in the solid state. This proposed transition metal interaction with the twin borirene systems is manifested in the electronic characterization of these borirene species, which display divergent photophysical UV/Vis spectroscopic profiles compared to a previously published mono(borirene)platinum(II) complex.

Introduction Heteronuclear boron-containing aromatic systems are of active interest to materials chemists as they hold great promise for the development of electronically conjugated materials.[1] Borirene systems (cyclo-BC2R3) are 2p-electron, aromatic, threemembered rings[2] notable for being isoelectronic to the wellknown cyclopropenylium cation (Figure 1).[3] More specifically, borirenes represent the smallest singly-substituted (main group) heteroaromatic compounds allowed by the Hckel rule. The synthetic routes to borirenes are drastically different from those of other boron-containing heteroaromatic species, as traditional silicon– or tin–boron exchanges[4] are unavailable for the synthesis of this class of compound. Instead, borirenes are most often prepared by means of photolytic or thermal transfer of terminal borylene moieties[5, 7a] from a terminal borylene pentacarbonyl complex of the form [(OC)5M=B=N(SiMe3)2] (M = Cr, Mo, or W)[6] onto an alkynyl substituent.[7] Borirene synthesis

[a] Prof. Dr. H. Braunschweig, Dr. A. Damme, Dr. R. D. Dewhurst, H. Kelch, B. B. Macha, Dr. K. Radacki, Dr. Q. Ye Institut fr Anorganische Chemie Julius-Maximilians-Universitt Wrzburg Am Hubland, 97074 Wrzburg (Germany) E-mail: [email protected] [b] Dr. A. Vargas Department of Chemistry, School of Life Sciences University of Sussex, Brighton BN1 9QJ, Sussex (UK) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405803. Chem. Eur. J. 2015, 21, 2377 – 2386

Figure 1. Resonance structures of a borirene (top) and a cyclopropenylium cation (bottom).

is also known to be possible from the photolytic rearrangement of an aryl-substituted alkynylborane;[2e, 7g] however, this route is severely limited in scope by the difficulty in manufacturing the various alkynylborane precursors and the specificity of the subsequent rearrangement. Borylene transfer is a proven method for the “borylenation” of CC triple bonds using mildly reactive, thermally-stable, Group 6 terminal borylene complexes,[5a, 7a] without the superfluous steps of aforementioned E–B exchanges. This methodology is viable for synthesis of borirenes from both organic[5a, 7b] and organometallic[7c,e] alkynyl species with remarkably high yields and under mild conditions. Chujo and co-workers, as well as other prominent research groups,[1g, 8] have been able to successfully synthesize and incorporate a variety of electron-deficient tricoordinate boron moieties into conjugated polymeric and oligomeric systems starting from 1998[1a] up until the present date. However, typi-

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Full Paper cal polymer characterization methods for these species were often inhibited by the sensitive nature of the materials and few of the reported species were able to be fully characterized.[8r,s] These materials typically feature redshifted UV/Vis absorption spectra relative to their monomeric analogues.[1g, 8] This data corresponds to a decrease in the HOMO–LUMO gap of the compounds as higher degrees of electronic delocalization are implied when in a polymeric form.[1g, 8r] Organometallic linkers have been shown to aid in conjugation between independent delocalized units in both polymeric[8o,p, 9a–c] and monomeric[9d–g] systems, and can increase communication between these systems when incorporated in a direct A–B sequential order (see Figure 2 middle). Utilization of this A–B copolymeric arrangement also allows for further tuning of the photophysical profile of the overall polymer as well as to increase solvent tolerance. Recently Scheschkewitz and co-workers have also used p-conjugated phenylene linkers to induce communication between silicon–silicon double bond units (disilenes) and to enforce their solid-state coplanarity.[10] The tunability of the

HOMO–LUMO gaps in these molecules was also demonstrated by varying the ring positions occupied by the disilenes at the phenylene bridge (para vs. meta) and examining the solidstate structure as well as UV/Vis spectra to compare the differing degrees of communication.[10b] Even more recently our group has shown that coplanarity between two heterocyclic moieties across borole and diborene linkers is an indicator of the degree of communication with the central p-system.[11] Synthesis of both directly linked and organic spacer-containing multiborirene systems have been successfully shown through previously reported publications.[7b] However, in their solidstate structures, the linked borirene heterocycles were, without exception, non-coplanar. Wang and co-workers have recently presented a system in which a platinum spacer enforces coplanarity between twin boron-containing heterocycles.[9f] In 2009 and 2011, our research group published detailed studies of the first platinum[7c] and iron[7e] monoborirenyl complexes, respectively (see Figure 2 bottom). These systems were synthesized by means of photolytic and thermal transfer of a terminal borylene ligand from a Group 6 metal onto organometallic s-alkynyl precursors. The investigated organometallic borirene systems both displayed redshifted absorption spectra when compared to the alkynyl precursors.[7c,e] In this work we report the synthesis of the first bis(borirene)platinum complexes, and present findings from structural and electronic examination of the role of platinum in allowing increased coplanarity and conjugation of twin borirene systems. This series of platinum-linked bis(borirene) complexes all show coplanarity in the twin ring systems and stand as the first verified structural representations of two coplanar borirene systems across a linking intermediate group. The role of a platinum atom in mediating communication between chromophoric ligands can be generalized by an expected redshift in the absorption spectrum due to an increase in the electronic delocalization between the formerly independent aromatic systems. The transbis(borirene)platinum scaffold serves as a simplified monomeric system that allows us to not only study the effects of transition metals in mitigating electronic conjugation, but also the tunability of the overall photophysical profile of the system by exocyclic augmentation of the three-membered aromatic ring.

Results and Discussion Synthesis of trans-platinum alkynyl precursors

Figure 2. Main-chain boron polymers without (top) and with transition metals (middle) prepared by Chujo et al. Previously reported transfers of terminal borylenes onto transition metal s-alkynyl complexes (bottom). Chem. Eur. J. 2015, 21, 2377 – 2386

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The compound trans-[Pt(CCPh)2(PEt3)2] (2 a) was prepared according to literature methods,[12a] and trans-[Pt(CC-pC6H4OMe)2(PEt3)2] (2 b) and trans-[Pt(CC-p-C6H4CF3)2(PEt3)2] (2 c) were both prepared according to a modified literature procedure[12, 13] in which [cis-PtCl2(PEt3)2] is treated with the alkynes HCC-p-C6H4OMe and HCC-p-C6H4CF3, respectively, in a 1:5 v:v mixture of HNEt2/toluene. The resulting crude species were purified by silica gel column chromatography and the products 2 b and 2 c were isolated as air-stable light yellow crystalline solids. Differential thermal analysis of these solids showed melting points for compounds 2 b and 2 c at 156 and 138 8C, respectively. No thermal decomposition was observed below

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Full Paper 272 Hz (2 c). The a-carbon–platinum coupling values could be subsequently verified by 195Pt{1H} NMR spectroscopy, finding values for 1JPtC interactions of 952 (2 b) and 959 Hz (2 c). It should also be noted that 31P–13C coupling with the alkynyl carbon nuclei could also be detected in the 13C{1H} NMR spectra, showing interaction with both a- and b-carbon nuclei as well as the ipso- and ortho-carbon nuclei of the aryl substituents; however, these signals were too poorly resolved to report.

Figure 3. ORTEP rendered structure of 2 b. Thermal ellipsoids set at 50 % probability. All hydrogen atoms have been omitted for clarity.

Synthesis of trans-bis(borirene)platinum complexes 300 8C for the trans-bis(alkynyl)platinum species investigated in this study. Single crystals suitable for X-ray diffraction were grown from saturated solutions of 2 b and 2 c (Figure 3); however, the structural data for compound 2 c is of insufficient quality for discussion of bond lengths as the disorder resulting from the CF3 groups made anisotropic structural refinement impossible (see Supporting Information for structural connectivity of 2 c). Species 2 b displays an expected CC triple bond length of 1.20(1) . The platinum core is characterized by a PtC bond length of 2.000(9)  and a PtP bond length of 2.305(1)  (Table 1). 31P{1H} NMR spectra of these systems display promi-

The trans-bis(borirene)platinum complexes 3 a–c were synthesized by photolytic transfer of the terminal borylene moiety from the complex [(OC)5Cr=B=N(SiMe3)2] (1) onto the respective alkynyl precursors (Figure 4). Pale yellow benzene or

Table 1. Bond lengths of structures 2 a[12c] and 2 b.

space group Pt1P1 Pt1C1 C1C2 C2C3

2 a[12c]

2b

P21/c 2.289(3) 1.98(1) 1.21(1) 1.43(1)

P1¯ 2.305(1) 2.000(9) 1.20(1) 1.468(6)

nent singlets at d = 11.4 (2 b) and 11.6 ppm (2 c) with 1JPPt coupling constants of 2402 and 2360 Hz, respectively. These coupling constants could be subsequently verified by 195Pt{1H} NMR characterization displaying triplet signals at d = 4759 (2 b) and 4748 ppm (2 c) with 1JPtP coupling constants of 2390 (2 b) and 2350 Hz (2 c). Table 2 lists these coupling values relative to those of 2 a.

Table 2. NMR data acquired for compounds 2 a–c. 31

2a 2b 2c

P (1JPPt)

11.4 (2387) 11.4 (2402) 11.6 (2360)

195

Pt (1JPtP)

13

4755 (2376) 4759 (2390) 4748 (2350)

Ca (1JCPt)

108.5 (961) 105.7 (958) 113.1 (969)

13

Cb (2JCPt)

110.1 (270) 109.3 (270) 109.6 (272)

The 1JCPt and 2JCPt coupling values could also be observed through multinuclear NMR spectroscopy. For the trans-alkynyl systems, the a-carbon–platinum coupling constants (13C{1H}) were 1JCPt 958 (2 b) and 969 Hz (2 c), while the b-carbon-platinum coupling constants were found to be 2JCPt 270 (2 b) and Chem. Eur. J. 2015, 21, 2377 – 2386

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Figure 4. Main reaction scheme for synthesis of trans-bis(borirene)platinum complexes.

hexane solutions of 1 (2.2 equiv) were irradiated in the presence of the trans-bis(alkynyl)platinum precursors 2 a–c for 8 h at room temperature, resulting in the formation of dark brown solutions of the trans-bis(borirene)platinum complexes 3 a–c. Progress and completion of the reaction was ascertained by 31 1 P{ H} NMR spectroscopy of the crude reaction mixtures. Typical reaction monitoring usually witnessed slow consumption of 1 by 11B{1H} NMR spectroscopy (signal at d = 92 ppm), resulting in a mixture of the mono- and bis(borirene)platinum species identified as overlapping resonances appearing at d = 35 ppm. Although the mono- and bis(borirene) species could not be discerned by 11B{1H} NMR spectroscopy, during 31P{1H} NMR spectroscopic monitoring of the reactions for 3 a–c, in all cases a species was observed at about 9 ppm with a 1JPPt coupling constant of approximately 2600 Hz, possibly corresponding to a monoborirene species. Unfortunately we were unable to isolate this species. Signals corresponding to the final products are observed at approximately 5 ppm, with 1JPPt coupling constants of approximately 2750 Hz. Considering that the alkynyl

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Full Paper precursors 2 a, 2 b, and 2 c all display 1JPPt coupling constants around 2400 Hz, the non-isolated intermediate 1JPPt coupling values lie directly between the trans-bis(alkynyl) precursors and the trans-bis(borirene)platinum products, lending further evidence to the hypothesis that the species at 9 ppm is a trans-bis(phosphine)/monoborirene/monoalkynyl species. Once total consumption of the alkynyl precursor had been confirmed, photolytic irradiation of the samples was halted and the subsequent crude mixtures were purified by removal of the volatiles in vacuo and sublimation of unreacted 1 and the [Cr(CO)6] side product. The crude brown product was subjected to chromatography over pacified silica gel[14] and subsequently crystallized from hexamethyldisiloxane, yielding the air- and moisture-sensitive bis(borirene)platinum complexes in fair yields (3 a: 39 %; 3 b: 45 %; 3 c: 63 %) as yellow crystalline solids. The compounds, when kept in inert atmosphere, can survive indefinitely. Differential thermal analysis (DTA) of bis(borirene) 3 a showed decreased thermal stability when compared to the trans-bis(alkynyl) precursors; the complex appears to melt at 165 8C and readily decomposes with no identifiable thermally-stable intermediate at temperatures in excess of 282 8C. After the successful synthesis of trans-[Pt(B{=N(SiMe3)2}C= CPh)2(PEt3)2] 3 a, analysis of the spectroscopic data revealed dual sets of resonances corresponding to the purified compound. These twin sets of data were detected in 1H, 13C, 29Si, 31 P, and 195Pt NMR spectra and were presumed to correspond to different conformations of the bis(borirene) arms relative to each other across the platinum core. As can be seen in Table 3,

exists between the platinum core and the borirene arms in the periphery of the molecule. This results in two separate conformational isomers in which the borirene moieties are either held on one side of the PtX2L2 plane, or staggered across this plane (syn and anti conformations). Freezing of bond rotation within this system was hypothesized to result from either pseudo-hindered rotation due to steric encumbrance exerted by the phosphine ligands or strong electronic conjugation between the platinum core and the twin borirene aromatic systems. The room-temperature (RT) 1H NMR spectrum displayed two independent resonances for the trimethylsilyl group environments. Variable-temperature (VT) 1H NMR data for 3 a yielded two separate coalescence temperatures corresponding to two different rotational barriers for the molecule, 50 and 72 8C, which can be assumed to apply to the rotational barriers for the BN bond and PtC bond, respectively. As can be seen in Figure 5, the upper rotational barrier is demonstrated by the

Table 3. NMR data showing dual signals obtained for compounds 3 a-c. XTMS = signal corresponding to nuclei of the trimethylsilyl group. 1

3a

3b

3c

HTMS

13

CTMS

11

29

5.46 5.35

5.9 (2765) 5.9 (2762)

4108 (2751) 4110 (2759)

– –

6.0 (2773) 6.0 (2765)

4110 (2755) 4111 (2763)

5.98 5.79

6.2 (2743) 5.8 (2728)

4100 (2719) 4111 (2737)

B

0.603 0.574

4.05 4.02

35.5

0.625 0.603

4.11 4.06

35.4

0.535 0.525

3.94 3.85

35.1

SiTMS

31

P (1JPPt)

195

Pt (1JPtP) Figure 5. VT-NMR diagram for compound 3 a from 80 to 80 8C.

the 31P{1H} NMR data recorded for samples 3 a, 3 b, and 3 c all show two distinct sets of singlets with prominent platinum satellite coupling constants featuring equal values for coupling interactions. The 195Pt{1H} spectra also verify this dual signal data and again show relative equanimity between the two resolved resonances in terms of 31P coupling interactions. The only spectrum that did not display this dual set of signals was the 11 1 B{ H} NMR spectrum; however, data for compounds 3 a– c showed such broad resonances that overlap of multiple signals cannot be disregarded. As can be expected, the a- and bcarbons for compounds 3 a–c could also not be resolved due to severe broadening of the resonances due to 1J and 2J coupling to the 195Pt and 11B nuclei. The duality of the NMR signals observed in these systems indicate that a rotational barrier Chem. Eur. J. 2015, 21, 2377 – 2386

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coalescence of the two independent trimethylsilyl signals into one broad singlet at 72 8C (DG = 75 kJ mol1).[15] The lower thermal rotational barrier at 50 8C (DG = 48 kJ mol1) is in excellent agreement with published data from the study of a platinum monoborirene species by our group in 2009, which featured a VT-NMR coalescence temperature (TC = 55 8C/DG = 45 kJ mol1) that was linked to the rotational barrier around the BN bond.[7c] Through VT 1H NMR studies of 3 a the temperature-dependent rotation of structural elements of the molecule can be explained by observation of four signals corresponding to the four different trimethylsilyl environments at low temperatures (below 50 8C), which coalesce to yield four signals corresponding to the four different trimethylsilyl environments (both BN and PtC bond rotations becoming frozen). Warming of this system to room temperature compromises the BN rotational barrier to yield two signals corresponding to two different trimethylsilyl environments. Finally, high-temperature studies of the system exhibit coalescence of the two signals into one signal corresponding to both rotation-

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Full Paper al barriers becoming compromised and both rotations becoming indiscernible on the NMR timescale. Two-dimensional correlation NMR (1H-29Si) spectra acquired at the lower thermal rotational barrier confirmed this as the barrier for rotation around the BN bond by the presence of four independent trimethylsilyl signals when below the 50 8C coalescence temperature. Once the rotation barrier had been established for the BN bond, attention was focused on examination of the PtC rotational barrier. To probe this influence further, NOESY and ROESY experiments were conducted on the complex in an effort to detect any steric interaction between the ancillary protons of the triethylphosphine and trimethylsilyl arms. Both NOESY and ROESY experiments performed in several solvents and with a variety of mixing times showed no discernable interactions between these protons in 3 a (or indeed any interactions between protons). This evidence suggests that the high rotational energy barrier observed could be primarily due to an electronic interaction between the ligand and the metal rather than any steric hindrance. Single-crystal X-ray crystallographic analysis Single crystals suitable for X-ray diffraction were grown for bis(borirenes) 3 a–c from saturated solutions of the compound in hexane stored at 30 8C. Compounds 3 a–c crystallize as transparent light yellow plates. Although VT-NMR data indicates rotation around the platinum–carbon bond at temperatures in excess of 72 8C in these complexes, all obtained crystalline samples from this class of bis(borirenes) were observed to crystallize with anti-configured borirene units (Figure 6). The crystallographically-derived geometry of 3 c will not be discussed because the structure is heavily disordered due to the presence of ancillary CF3 groups. Of key interest in species 3 a and 3 b are the PtC and borirene CC bond lengths, which indicate the degree of delocalization across both the individual borirene rings as well as the bis(borirene) conjugation across the platinum center. For compounds 3 a and 3 b, the respective borirene CC distances are effectively identical (1.373(1) and 1.34(5) ). These distances indicate that the compounds all possess similar degrees of delocalization across the borirene systems. The respective PtC distances for 3 a and 3 b were found to be 2.046(1) and 2.076(7)  (Table 4).

Table 4. Space group and selected bond lengths and angles for compounds 3 a and 3 b relative to 4.[7c]

space group Pt1C1 [] C1C2 [] C1B1 [] C2B1 [] B1N1 [] C2C3 [] Pt1P1 [] Sa(N) [8] Sa(B) [8]

3a

3b

4[7c]

P1¯ 2.046(4) 1.373(6) 1.505(6) 1.454(6) 1.451(4) 1.480(6) 2.287(1) 359.9 359.6

P1¯ 2.076(3) 1.384(4) 1.522(4) 1.481(4) 1.459(4) 1.476(4) 2.314(1) 357.0 359.3

P21/c 1.974(5) 1.374(7) 1.511(8) 1.482(8) 1.428(7) 1.466(7) 2.279(2) 360.0 359.9

X-ray data for compounds 3 a, 3 b, and 3 c confirm that all species display a coplanar framework between the twin heteroaromatic ring systems across the platinum core. However, disorder within the refined data combined with near identical structural bond lengths and angles make extrapolation of trends within the variants of the bis(borirene) frameworks difficult to discern. What can be confirmed, however, is that all three bis(borirene) systems (3 a–c) under investigation display bond lengths and angles closely akin to a previously reported monoborireneplatinum species (4).

UV/Vis spectroscopy The UV/Vis data for compounds 2 a–c were recorded to establish background data pertaining to characterization studies of the compounds and can be seen in Figure 7 (top) and Table 5.

Table 5. UV/Vis data for compounds 2 a–c.

llocal [nm] llocal [nm] lmax [nm]

2a Ar = Ph

2b Ar = p-C6H4OMe

2c Ar = p-C6H4CF3

263 287 327

263 286 339

269 291 327

Figure 6. ORTEP rendering diagrams for structures of compounds 3 a–c (left to right). Thermal ellipsoids set at 50 % probability. All hydrogen atoms have been omitted for clarity. Chem. Eur. J. 2015, 21, 2377 – 2386

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Full Paper Table 6. UV/Vis data for compounds 3 a–c.

lmax [nm]

3a Ar = Ph

3b Ar = p-C6H4OMe

3c Ar = p-C6H4CF3

4[7c]

257

270

275

247

Table 7. Energy decomposition analysis of compounds 3 a and 4 using the fragment approach. Note: the energies stated for 3 a are the total energies for combination of all three fragments and should be halved to obtain the energies of each borirene fragment interacting with the Pt center.

DEorb [kcal mol1] DEelstat [kcal mol1] DEpauli [kcal mol1] DEint [kcal mol1]

Figure 7. Top: UV/Vis spectra for compounds 2 a (black), 2 b (blue), and 2 c (red). Bottom: UV/Vis spectra and data for compounds 3 a (black), 3 b (blue), and 3 c (red) relative to reported data for 4[7c] (dashed grey).

However, due to spectral overtones resulting from the CC triple bond stretches inherent in this class of compounds, species 2 a–c all display three pseudo-maxima within their absorption spectra. The presence of these triple-bond vibrational artifacts makes discussion of the absorption maxima for all bis(alkynyl)platinum precursors impossible and also renders the data inaccurate for projection to the final bis(borirene) trend profiles. The UV/Vis spectrum of compound 3 a recorded in hexane shows an absorption maximum at 257 nm, which is slightly redshifted relative to that observed for the monoborirene complex 4 (247 nm, spectrum also recorded in hexane). The UV/Vis spectra for compounds 3 b and 3 c recorded in hexane both showed redshifted absorption maxima relative to both 3 a and 4 (Figure 7 (bottom) and Table 6). Compound 3 a was tested for solvatochromism by recording the absorption spectrum in acetonitrile; however, the spectra appears nearly identical to that recorded in hexane, suggesting very little discernable solvatochromism. One feature of note in the spectra of compounds 3 a–c is the presence of a significant shoulder, found to higher wavelength from the main absorption signal. Chem. Eur. J. 2015, 21, 2377 – 2386

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3a

4

310

138

324

171

465

237

170

72

Computational studies Investigation of the bonding in the model bis(borirene) complex trans-[Pt(PEt3)2{cyclo-CC(Ph)BN(SiMe3)2}2] (3 a) was performed using Kohn–Sham density functional theory (DFT) calculations (see Supporting Information for computational details). Table 7 lists all bonding energy contributions observed by fragment approach analysis of the modeling of the bis(borirene) system 3 a relative to the monoborirene 4. The bonding energy decomposition clearly shows that the borirenes are not “independently” bound to the platinum center. The orbital interactions are synergically enhanced, and as expected, the electrostatic contributions and the Pauli contributions become more independent. Furthermore, the orbital projections show that the HOMO of 3 a consists of the borirene s networks, with s-antibonding character between the Pt and C atoms. However, we were unable to observe any MO containing a p interaction between d(Pt) and p(C) orbitals (Figure 8). Time-dependent density functional theory (TD-DFT) methods were used to model the major electronic transitions observed in the absorption spectra of the mono- and bis(borirene) complexes. These calculations indicated that in the model com-

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Full Paper pounds [cyclo-C(H)C(Ph)BN(SiMe3)2] (A), monoborirene complex trans-[PtCl(PMe3)2{cyclo-CC(Ph)BN(SiMe3)2}] (4), and the phenylsubstituted [cyclo-C(Ph)C(Ph)BN(SiMe3)2] (C), the major electronic transition contributing to the UV/Vis profile is from an

orbital consisting of borirene C=C and B=N p-bonding interactions into orbitals based predominantly on the phenyl substituents. A similar transition from a dz2 orbital on Pt into the same phenyl-based orbital is also a contributor in the bis(borirene) complex 3 a. It can also be seen that in the two metalcontaining complexes 4 and 3 a, the orbital from which the electron originates has antibonding character between the C= C p-bonds and a d orbital based on Pt. The case of 3 a strongly suggests that there is a photophysically active p network spanning the borirenes, albeit one containing four nodes (Figure 8). A key determinant of communication between two electronically communicating centers is the redshifting of transitions associated with delocalized orbitals. The rationalization of this effect stems from the fact that increasing delocalization splits both empty and filled orbitals into further non-degenerate orbitals. This increased splitting narrows the gap between the highest relevant filled orbital and the lowest relevant filled orbital, thus decreasing the transition energy and redshifting the absorption maximum. This is indeed what we see in the aforementioned calculated major transitions for A (262 nm), 4 (276 nm), and C (295 nm); every expansion of the conjugated system results in a further lowering of the transition energy. There is a redshift of 14 nm upon replacing H with the trans{PtCl(PMe3)2} fragment, and a redshift of 33 nm upon replacement of H by extension of the conjugated system over two phenyl rings (Figure 9, left). This predicted redshift for two phenyl substituents is approximately equal to the expected redshift predicted for the bis(borirene) framework (3 a). Figure 9 (right) shows a direct comparison in the simulated UV/Vis absorption spectrum of the mono- versus the bis(borirene) species and predicts an approximate redshift of 18 nm (the experimentally observed redshift was found to be ca. 10 nm).

Conclusion Three trans-bis(borirene) complexes of platinum were prepared by borylene transfer to trans-bis(alkynyl) complexes, the products being the first examples of systems featuring coplanar

Figure 8. Frontier orbitals of 3 a and 4.

Figure 9. Left: Calculated UV/Vis spectra of three augmented borirene species. Right: Calculated UV/Vis spectra of 3 a (green line) and 4 (blue line) with detailed frontier orbital projections for the two dominant contributing excitations computed for compound 3 a. Chem. Eur. J. 2015, 21, 2377 – 2386

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Full Paper borirene rings. The coplanar arrangement of the borirene rings in these complexes, and their redshifted UV/Vis features, implied significant electronic communication between the rings through the Pt center. The photophysically active interborirene p communication in the complexes 3 a–c, while unable to be explicitly observed in the MOs of the complexes, is implicitly suggested by four different findings: 1) the complexes 3 a– c are the first systems in which two borirene rings are mutually coplanar; 2) steric factors have been ruled out by NMR methods as the cause of this coplanarity; 3) the signals observed in both the experimental and calculated UV/Vis spectra show redshifting of the bands between mono- and bis(borirene) complexes; and 4) the transition that predominantly contributes to the calculated UV/Vis profiles of the bis(borirene)platinum model complex 3 a involves a p-network that spans the two borirene units and the platinum. Attempts to exploit this interborirene communication in optoelectronic systems are ongoing in our laboratories.

Experimental Section General information: All manipulations were performed either under an atmosphere of dry argon or in vacuo using standard Schlenk line or glovebox techniques. Deuterated benzene (C6D6) was dried and degassed by refluxing over LiAlH4 under an atmosphere of argon. Deuterated toluene (C7D8) was dried over molecular sieves and degassed by three freeze–pump–thaw cycles before use. Diethylamine (C4H11N) was dried by refluxing over CaH2 under an atmosphere of argon. All other solvents were distilled from appropriate drying agents.[16] Solvents (both deuterated and non-deuterated) were stored under argon over activated molecular sieves. Photolytic experiments were performed in quartz Schlenk flasks or J. Young tubes. The light source was a LOT-Oriel photolysis apparatus with a 500 W Hg/Xe arc lamp equipped with infrared filters, irradiating at 210–600 nm. NMR spectra of isolated compounds were acquired on a Bruker Avance 500 NMR spectrometer. Routine NMR measurements were performed on a Bruker Avance 400 NMR spectrometer. Chemical shifts (d) are given in ppm. 1H and 13C{1H} NMR spectra were referenced to external tetramethylsilane by the residual proton solvent (1H) or the solvent itself (13C). 11B{1H} NMR spectra were referenced to external BF3·OEt2. 19F{1H} NMR spectra were referenced to external CFCl3. 29Si{1H} NMR spectra were referenced to external C4H12Si. 31P{1H} NMR spectra were referenced to external H3PO4. 195Pt{1H} NMR spectra were referenced to external K2PtCl6. Assignment of the signals of the carbon nuclei was aided by 13C-1H NMR correlation spectroscopy. IR data were acquired on a JASCO FT/IR-6200typeA apparatus. UV/Vis spectra were acquired on a JASCO-V660 UV/Vis spectrometer. Differential thermoanalysis were performed with a Mettler Toledo DSC 823. Microanalyses were performed on an Elementar Vario MICRO cube elemental analyser. Phenylacetylene, 4-ethynylanisole, and benzyltriethylammonium chloride were purchased from ABCR (Lot numbers: 1026023, 1184666, and 1167960 respectively). 1-[(trimethylsilyl)ethynyl]-4-(trifluoromethyl)benzene (C12H13F3Si) was purchased from Sigma–Aldrich (Lot number MKBD5828 V). [(OC)5Cr=B=N(SiMe3)2] (1),[6b] trans-[Pt(PEt3)2(CC-Ph)2] (2 a),[12a] cis-[PtCl2(PEt3)2],[17] and 4-ethynyla,a,a-trifluorotoluene (C9H5F3)[18] were synthesized according to literature methods. trans-[Pt(CC-p-C6H4OMe)2(PEt3)2] (2 b): A solution of cis[PtCl2(PEt3)2] (34 mg, 0.068 mmol) in dry toluene/HNEt2 (2.5 mL; 4:1 v:v) was agitated by sonification. After complete solvation of the Chem. Eur. J. 2015, 21, 2377 – 2386

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precursor, 4-ethynylanisole (61 mg, 0.46 mmol) was added to the reaction mixture. The reaction was heated at 80 8C for 72 h, after which a color change was observed from colorless to light yellow. After removing the volatiles in vacuo, the residue was extracted with toluene (3  2 mL), concentrated, and subjected to chromatography on a silica gel column (1  10 cm) using toluene as an eluent (Rf = 0.6). The product was discerned as a dark non-ultraviolet-active spot on TLC plates. The product fraction was collected and solvent evaporated in vacuo yielding an air-stable yellow crystalline solid (45 mg, 0.065 mmol, 96 %). Crystals suitable for X-ray diffraction were grown by slow evaporation of a saturated toluene solution at room temperature. 1H NMR (500.13 MHz, C6D6, 297 K): d = 7.50 (m, 4 H, o-CH of C6H4OCH3), 6.78 (m, 4 H, m-CH of C6H4OCH3), 3.29 (s, 6 H, p-C6H4OCH3), 2.04 (m, 12 H, P-CH2), 1.12 ppm (m, 18 H, P-CH2CH3); 13C{1H} NMR (125.77 MHz, C6D6, 297 K): d = 158.0 (s, p-C of C6H4OCH3), 132.3 (t, o-C of C6H4OCH3, 5 JCP = 1.3 Hz), 122.8 (t, ipso-C of C6H4OCH3, 3JCPt = 21 Hz, 4JCP = 1.3 Hz), 114.2 (s, m-C of C6H4OCH3), 109.3 (t, b-C-C-Pt, 2JCPt = 270 Hz, 3 JCP = 2.5 Hz), 105.7 (t, a-C-Pt, 1JCPt = 958 Hz, 2JCP = 15 Hz), 54.8 (s, pC6H4OCH3), 16.9 (virtual quintet, Pt-P-CH2, N1 = 69 Hz), 8.6 ppm (virtual triplet, P-CH2CH3, N2 = 23 Hz) [N1 = j 1JCP + 3JCP j , N2 = j 2JCP + 4 JCP j]; 13C{1H}-DEPT135 NMR (125.77 MHz, C6D6, 297 K): d = 158.0 (s, p-C of C6H4OCH3), 132.3 (t, o-C of C6H4OCH3, 5JCP = 1.3 Hz), 114.2 (s, m-C of C6H4OCH3), 54.8 (s, p-C6H4OCH3), 8.6 ppm (virtual triplet, P-CH2CH3, N = 23 Hz) [N = j 2JCP + 4JCP j]; 31P{1H} NMR (202.46 MHz, C6D6, H3PO4, 297 K): d = 11.4 ppm (s, 1JPPt = 2402 Hz, 1JPC = 18 Hz); 195 Pt{1H} NMR (107.00 MHz, C6D6, K2PtCl6, 297 K): d = 4759 ppm (t, 1 JPtP = 2390 Hz, 1JPtC = 952 Hz); IR (solid): n˜ = 2102 cm1 (CC stretch); UV/Vis (hexane): lmax = 339 nm; elemental analysis calcd (%) for C30H44O2P2Pt: C 51.94, H 6.39; found: C 52.24, H 6.36; DTA: m.p. = 156 8C. trans-[Pt(CC-p-C6H4CF3)2(PEt3)2] (2 c): A solution of cis-[PtCl2(PEt3)2] (32 mg, 0.064 mmol) in toluene/HNEt2 (2.5 mL; 4:1 v:v) was agitated by sonification. After complete solvation of the precursor, 4-ethynyl-a,a,a-trifluorotoluene (120 mg, 0.705 mmol) was added to the reaction mixture. The reaction was heated at 80 8C for 72 h, after which a color change was observed from colorless to light yellow. After removing the volatiles in vacuo, the residue was extracted with toluene (3  2 mL), concentrated, and subjected to chromatography on a silica gel column (1  10 cm) using toluene as an eluent (Rf = 0.75). The product was discerned as a dark non-ultraviolet-active spot on TLC plates. The product fraction was collected and solvent evaporated in vacuo yielding an air-stable yellow crystalline solid (42 mg, 0.055 mmol, 86 %). Crystals suitable for X-ray diffraction were grown by slow evaporation of a saturated toluene solution at room temperature. 1H NMR (500.13 MHz, C6D6, 297 K): d = 7.35 (m, 8 H, m- and o-CH of C6H4CF3), 1.94 (m, 12 H, P-CH2), 1.05 ppm (m, 18 H, P-CH2CH3); 13C{1H} NMR (125.77 MHz, C6D6, 297 K): d = 133.3 (br s, ipso-C of C6H4CF3), 131.2 (br s, o-C of C6H4CF3), 128.4 (s, m-C of C6H4CF3), 127.0 (q, p-C6H4CF3, 1JCF = 31 Hz), 125.5 (q, p-C of C6H4CF3, 2JCF = 3.8 Hz), 113.1 (t, a-C-Pt, 1JCPt = 969 Hz, 2JCP = 15 Hz), 109.6 (br s, b-C-C-Pt, 2JCPt = 272 Hz), 16.9 (virtual quintet, Pt-P-CH2, N1 = 69 Hz), 8.5 ppm (virtual triplet, P-CH2CH3, N2 = 24 Hz) [N1 = j 1JCP + 3JCP j , N2 = j 2JCP + 4JCP j]; 13C{1H}-DEPT135 NMR (125.77 MHz, C6D6, 297 K): d = 131.2 (br s, o-C of C6H4CF3), 128.4 (s, m-C of C6H4CF3), 125.5 (q, p-C of C6H4CF3, 2JCF = 3.8 Hz), 8.5 ppm (virtual triplet, P-CH2CH3, N = 21 Hz) [N = j 2JCP + 4JCP j]; 19 1 F{ H} NMR (376.50 MHz, C6D6, CFCl3, 297 K): d = 61.8 ppm (s, pC6H4CF3); 31P{1H} NMR (202.46 MHz, C6D6, H3PO4, 297 K): d = 11.6 ppm (s, Pt-P-CH2, 1JPPt = 2360 Hz, 1JPC = 14 Hz); 195Pt{1H} NMR (107.00 MHz, C6D6, K2PtCl6, 297 K): d = 4748 ppm (t, 1JPtP = 2350 Hz, 1JPtC = 959 Hz); IR (solid): n˜ = 2100 cm1 (CC stretch); UV/ Vis (hexane): lmax = 327 nm; elemental analysis calcd (%) for

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Full Paper C30H38F6P2Pt: C 46.82, H 4.98; found: C: 47.68, H: 5.10; DTA: m.p. = 138 8C. trans-[Pt({m-(B=N(SiMe3)2)C=C}Ph)2(PEt3)2] (3 a): In a quartz J. Young NMR tube, [(OC)5Cr=B=N(SiMe3)2] (30 mg, 0.083 mmol) and trans-[Pt(CCPh)2(PEt3)2] (21 mg, 0.033 mmol) were dissolved in benzene (1.5 mL). The NMR tube was then irradiated for 6 h at room temperature after which full conversion to the bis(borirene) was confirmed by 31P{1H} NMR spectroscopy. The volatile components of the reaction mixture were removed in vacuo and the brown residue was extracted with hexane and filtered through a plug of pacified silica gel (0.5  2 cm). The light yellow fraction was collected and dried in vacuo to yield trans-[Pt(m-{B=N(SiMe3)2}C=CPh)2(PEt3)2] (13 mg, 0.013 mmol, 39 %) as a light yellow crystalline solid. Crystals suitable for X-ray diffraction were grown by slow evaporation of a hexamethyldisiloxane solution stored at 30 8C for 1 week. 1 H NMR (500.13 MHz, C6D6, 297 K): d = 8.51 (m, 4 H, m- or o-CH of C6H5), 7.50 (m, 4 H, m- or o-CH of C6H5), 7.26 (m, 2 H, p-CH of C6H5), 1.41 (m, 12 H, P-CH2), 0.72 (m, 18 H, P-CH2CH3), 0.60 (s, 18 H, Si-CH3), 0.57 ppm (s, 18 H, Si-CH3); 13C{1H} NMR (125.77 MHz, C6D6, 297 K): d = 130.0 (s, m-C of C6H5), 129.6 (s, o-C of C6H5), 128.6 (s, p-C of C6H5), 128.6 (br s, ipso-C of C6H5), 16.7 (virtual triplet, Pt-P-CH2, N = 34 Hz), 16.7 (virtual triplet, Pt-P-CH2, N = 34 Hz), 8.2 (br s, P-CH2CH3), 4.1 (s, 18 H, Si-CH3), 4.0 ppm (s, 18 H, Si-CH3) [N = j 1JCP + 3JCP j]; 11 1 B{ H} NMR (160.47 MHz, C6D6, BF3·OEt2, 297 K): d = 35.5 ppm (br s, m-{B=N(SiMe3)2}C=C); 29Si{1H} NMR (79.49 MHz, C6D6, C4H12Si, 297 K): d = 5.5 (s, N-Si-CH3), 5.4 ppm (s, N-Si-CH3); 31P{1H} NMR (202.46 MHz, C6D6, H3PO4, 297 K): d = 5.9 (s, Pt-P-CH2, 1JPtP = 2765 Hz), 5.9 ppm (s, Pt-P-CH2, 1JPtP = 2762 Hz); 195Pt{1H} NMR (107.00 MHz, C6D6, K2PtCl6, 297 K): d = 4108 (t, Pt-P-CH2, 1JPtP = 2751 Hz), 4110 ppm (t, Pt-P-CH2, 1JPtP = 2759 Hz); IR (solid): n˜ = 1610 cm1 (BCC stretch); UV/Vis (hexane): lmax = 257 nm; elemental analysis calcd (%) for C40H76B2N2P2PtSi4 : C 49.22, H 7.85, N 2.87; found: C 49.64, H 8.10, N 2.52; DTA: m.p. = 165 8C. (3 b): In trans-[Pt({m-(B=N(SiMe3)2)C=C}-p-C6H4OMe)2(PEt3)2] a quartz J. Young NMR tube, [(OC)5Cr=B=N(SiMe3)2] (23 mg, (15 mg, 0.063 mmol) and trans-[Pt(CC-p-C6H4OMe)2(PEt3)2] 0.022 mmol) were dissolved in hexane (1.5 mL). The NMR tube was then irradiated for 6 h at room temperature after which full conversion to the bis(borirene) was confirmed by 31P{1H} NMR spectroscopy. The volatile components of the reaction mixture were removed in vacuo and the brown residue was extracted with hexane and filtered through a plug of pacified silica gel (0.5  2 cm). The light yellow fraction was collected and dried in vacuo to yield trans[Pt(m-{B=N(SiMe3)2}C=C-p-C6H4OMe)2(PEt3)2] (10 mg, 0.010 mmol, 45 %) as a light yellow crystalline solid. Crystals suitable for X-ray diffraction were grown by slow evaporation of a hexamethyldisiloxane solution stored at 30 8C for one week. 1H NMR (500.13 MHz, C6D6, 297 K): d = 8.55 (m, 4 H, o-CH of C6H4OCH3), 7.14 (m, 4 H, mCH of C6H4OCH3), 3.39 (s, 3 H, C6H4OCH3), 3.39 (s, 3 H, C6H4OCH3), 1.44 (m, 12 H, P-CH2), 0.75 (m, 18 H, P-CH2CH3), 0.63 (s, 18 H, Si-CH3), 0.60 ppm (s, 18 H, Si-CH3); 13C{1H} NMR (125.77 MHz, C6D6, 297 K): d = 159.8 (s, p-C of C6H4OCH3), 131.7 (s, o-C of C6H4OCH3), 128.6 (s, ipso-C of C6H4OCH3), 114.0 (s, m-C of C6H4OCH3), 54.9 (s, C6H4OCH3), 16.7 (virtual triplet, Pt-P-CH2, N = 33 Hz), 16.7 (virtual triplet, Pt-PCH2, N = 34 Hz), 8.2 (br s, P-CH2CH3), 4.1 (s, 18 H, Si-CH3), 4.1 ppm (s, 18 H, Si-CH3) [N = j 1JCP + 3JCP j]; 13C{1H}-DEPT135 NMR (125.77 MHZ, C6D6, 297 K): d = 131.7 (s, o-C of C6H4OCH3), 114.0 (s, m-C of C6H4OCH3), 54.9 (s, C6H4OCH3), 8.3 (br s, P-CH2CH3), 4.1 (s, 18 H, SiCH3), 4.1 ppm (s, 18 H, Si-CH3); 11B{1H} NMR (160.47 MHz, C6D6, BF3·OEt2, 297 K): d = 35.4 ppm (br s, m-{B=N(SiMe3)2}C=C); 31P{1H} NMR (202.46 MHz, C6D6, H3PO4, 297 K): d = 6.0 (s, Pt-P-CH2, 1JPtP = 2773 Hz), 6.0 ppm (s, Pt-P-CH2, 1JPtP = 2765 Hz); 195Pt{1H} NMR (107.00 MHz, C6D6, K2PtCl6, 297 K): d = 4110 (t, Pt-P-CH2, 1JPtP = Chem. Eur. J. 2015, 21, 2377 – 2386

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2755 Hz), 4111 ppm (t, Pt-P-CH2, 1JPtP = 2763 Hz); UV/Vis (hexane): lmax = 270 nm; elemental analysis calcd (%) for C42H80B2N2O2P2PtSi4 : C 48.69, H 7.78, N 2.70; found: C 48.16, H 7.91, N 2.54. trans-[Pt({m-(B=N(SiMe3)2)C=C}-p-C6H4CF3)2(PEt3)2] (3 c): In a quartz J. Young NMR tube, [(OC)5Cr=B=N(SiMe3)2] (18 mg, 0.050 mmol) and trans-[Pt(CC-p-C6H4CF3)2(PEt3)2] (12 mg, 0.016 mmol) were dissolved in hexane (1.5 mL). The NMR tube was then irradiated for 6 h at room temperature after which full conversion to the bis(borirene) was confirmed by 31P{1H} NMR spectroscopy. The volatile components of the reaction mixture were removed in vacuo and the brown residue was extracted with hexane and filtered through a plug of pacified silica gel (0.5  2 cm). The light yellow fraction was collected and dried in vacuo to yield trans-[Pt(m-{B= N(SiMe3)2}C=C-p-C6H4CF3)2(PEt3)2] (11 mg, 0.010 mmol, 63 %) as a light yellow crystalline solid. Crystals suitable for X-ray diffraction were grown by slow evaporation of a hexamethyldisiloxane solution stored at 30 8C for 1 week. 1H NMR (500.13 MHz, C6D6, 297 K): d = 8.32 (m, 4 H, o-CH of C6H4CF3), 7.73 (m, 4 H, m-CH of C6H4CF3), 1.29 (m, 12 H, P-CH2), 0.64 (m, 18 H, P-CH2CH3), 0.54 (s, 18 H, Si-CH3), 0.53 ppm (s, 18 H, Si-CH3); 13C{1H} NMR (125.77 MHz, C6D6, 297 K): d = 139.9 (s, ipso-C of C6H4CF3), 129.6 (s, o-C of C6H4CF3), 129.1 (s, p-C6H4CF3), 128.8 (s, m-C of C6H4CF3), 125.7 (s, pC of C6H4CF3), 16.6 (m, Pt-P-CH2), 8.1 (m, P-CH2CH3), 3.9 (s, 18 H, SiCH3), 3.9 ppm (s, 18 H, Si-CH3); 13C{1H}-DEPT135 NMR (125.77 MHz, C6D6, 297 K): d = 129.6 (s, o-C of C6H4CF3), 128.8 (s, m-C of C6H4CF3), 125.7 (s, p-C of C6H4CF3), 8.1 (m, P-CH2CH3), 3.9 (s, 18 H, Si-CH3), 3.9 ppm (s, 18 H, Si-CH3); 11B{1H} NMR (160.47 MHz, C6D6, BF3·OEt2, 297 K): d = 35.1 ppm (br s, m-{B=N(SiMe3)2}C=C); 19F{1H} NMR (376.50 MHz, C6D6, CFCl3, 297 K): d = 61.6 (s, p-C6H4CF3), 61.7 ppm (s, p-C6H4CF3); 29Si{1H} NMR (79.49 MHz, C6D6, C4H12Si, 297 K): d = 6.0 (s, N-Si-CH3), 5.8 ppm (s, N-Si-CH3); 31P{1H} NMR (202.46 MHz, C6D6, H3PO4, 297 K): d = 6.2 (s, Pt-P-CH2, 1JPtP = 2743 Hz), 5.8 ppm (s, Pt-P-CH2, 1JPtP = 2728 Hz); 195Pt{1H} NMR (107.00 MHz, C6D6, K2PtCl6, 297 K): d = 4100 (t, Pt-P-CH2, 1JPtP = 2719 Hz), 4111 ppm (t, Pt-P-CH2, 1JPtP = 2737 Hz); IR (solid): n˜ = 1603 cm1 (BCC stretch); UV/Vis (hexane): lmax = 275 nm; elemental analysis calcd (%) for C42H74B2F6N2P2PtSi4 : C 45.36, H 6.71, N 2.52; found: C 45.37, H 6.74, N 2.52.

Acknowledgements We are grateful to the European Research Council for the award of an advanced grant to H.B. Keywords: borirene · boron · density functional theory · platinum · UV/Vis spectroscopy

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Received: October 24, 2014 Published online on November 27, 2014

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Platinum trans-Bis(borirene) complexes displaying coplanarity and communication across a platinum metal center.

Ambient-temperature photolysis of the aminoborylene complex [(OC)5 Cr=B=N(SiMe3 )2 ] in the presence of a series of trans-bis(alkynyl)platinum(II) pre...
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