1,1-Digoldallylium Complexes: Diaurated Allylic Carbocations Indicate

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1,1-Digoldallylium Complexes: Diaurated Allylic Carbocations Indicate New Prospects of the Coordination Chemistry of Carbon Florian F. Mulks, Patrick W. Antoni, Jürgen H. Gross, Jürgen Graf, Frank Rominger, and A. Stephen K. Hashmi J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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Journal of the American Chemical Society

1,1-Digoldallylium Complexes: Diaurated Allylic Carbocations Indicate New Prospects of the Coordination Chemistry of Carbon Florian F. Mulks, Patrick W. Antoni, Jürgen H. Gross, Jürgen Graf, Frank Rominger, and A. Stephen K. Hashmi* Organisch-Chemisches Institut, Heidelberg University, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany Supporting Information Placeholder ABSTRACT:

Reacting (NHC)(cyclopropenyl)gold(I) complexes with cationic gold complexes [(IPr)AuX] afforded extremely reactive allylium-1,1-diido-bridged digold intermediates. We prove the existence and constitution of this structure with FT-ICR-MS/MS, NMR, and UV-Vis-NIR experiments and isolated the nucleophilic addition product [(Me)(Ph)(CCHC){Au(IPr)}2(SOMe2)]NTf2 with DMSO. Our computational investigation unveiled that the bonding situation of this -allylium-1,1-diido digold domain was best described as a three-center four-electron bond with -backbond. The valence orbitals showed extreme delocalization and strong -interactions between the three centers Au1–C1–Au2. The bridging carbon atom C1 was best described as trigonal planar sp-hybridized carbon in this structure. Excitation succeeded in UV-Vis-NIR measurements with energies as low as near IR radiation.

INTRODUCTION Carbon and gold(I) have a highly intertwined history in fundamental chemical research. Schmidbaur has elucidated the existence of unknown coordination properties of carbodiphosphoranes in 1976—well before the rise and great success of stable carbenes in transition metal chemistry (Fig. 1a).1 After alkyl, alkene, and alkyne complexes were long known, carbene and carbyne complexes drew a lot of attention, mainly due to their applications as ligands and in metathesis (Fig. 1c).2–5 Further investigation of bent allenes re-surfaced formally uncharged bivalent carbon centres that were, inter alia, shown to be capable of coordinating two [AuX] units (Fig. 1d). These findings suggested a critical review of our notion of carbons fundamental behavior.6–8 The concept was coined that such carbon centres can be conceived as coordination complexes of carbon in the oxidation state 0.9–16 Cyclopropenes are capable of generating aurated carbocations with cationic gold species.17–21 Direct activation of cyclopropenes with cationic gold species was suggested to follow a vinylcarbene pathway, while the dominance of the shown cationic mesomeric structure II was shown for Fürstner’s example (Fig. 1e).17,19,22–23

RESULTS AND DISCUSSION We have found that our recently published cyclopropenylgold(I) complexes 7 were remarkably preactivated towards thermal vinylcarbene ring-opening to form aurated carbenes.24 We utilized this reactivity to generate

diaurated carbocations in an effort of closing the gap between carbene complexes and gem-diaurated complexes of carbodicarbene, vinyl, or aryl moieties. We added cationic gold(I) complexes to our cyclopropenylgold(I) complexes 7 to induce ring-opening rearrangement reactions towards the -allylium1,1-diido digold complexes III (Fig. 2, the 1,1-diaurated allylic cation). These synthetic experiments were monitored by the observation of the color of the solutions, by 1H NMR spectroscopy and by ESI-MS experiments. Auspicious candidates were then subjected to a further investigation. A variety of ligands, anions, and solvents were investigated to stabilize III under inert conditions.25–26. Successful formation and stabilization of the desired cations was easily visualized by a wide range of deep colors. Characteristic colors of gold carbenes have been discussed—low-energy excitations were suggested to hint at a strong carbene character.27–28. They are constituted mainly of the first singlet excitation S0–S1. We have found mainly L1 = IPr, L2 = IPr or IPr*, X = NTf2, and THF as solvent to be greatly promising, as this setup in a reliable manner delivered colored (green to yellow) solutions at −78 °C (for further details, see SI). As every spectroscopic method demanded different concentrations and lifetimes, the exact methods employed had to be adapted to every different mean of pinpointing the existence of these exciting structures.

CID ESI-FT-ICR-MS/MS We generated III under argon at −40 °C in 10 µM THF solutions and rapidly diluted them to 10−7 M to slow down decomposition pathways and to reach the necessary dilution. The solutions then were injected into an ESI ion source for CID ESIFT-ICR-MS/MS experiments. If not stated otherwise, IIIa–d(*) referred to the cationic intermediate III deriving from the cyclopropenylgold complex 7a–b(*) and [(IPr)Au(NTf2)]. Steric overload stopped the reaction—the (IPr*) cyclopropenyl complexes showed extremely slow conversion with [(IPr*)Au(NTf2)], while the respective derivative of III was not sufficiently stabilised to show increasing concentrations over time. All seven triflimide salts IIIa–d(*) (L1 = IPr, IPr*; L2 = IPr, R1 = Me, Ph, =O; R2 = Me, Ph) were successfully detected, were mass-selected as precursor ions, and dissociated by collisioninduced dissociation (CID) in a RF-hexapole ion trap to gain additional structural insights (Supporting Fig. 1–Supporting Fig. 7) and the discussion thereof).

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a

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selected CAu coordination modes R2

Au(L)

R R

Au(L)

R2

R2

R

R

Au(L) R2

Au(L)

R R1 formally derived from: alkyl alkenyl/ aryl

Au(L)

Au(L)

R

R

R1

alkynyl

AuX AuX

alkenyl/ aryl

this work R Au(L)

R

R

carbene

Au(L)

carbon(0)

allylium-1,1-diyl

gem-diaurated cationic sp2-hybridised species

b

Schmidbaur 2003

Grandberg 1973 Au(PPh3) Au(PPh3)

Fe

Hashmi 2012 R

Au(PPh3)

X

BF4

Fürstner 2010

BF4

(IPr)Au

R = c-propyl, OEt 1c

1b

c

NTf2

(Ph3P)Au

X = S, O 1a

NTf2

(Ph3P)Au

Au(PPh3)

Au(IPr) 1d

gold carbenes/aurated carbocations Fürstner 2014

Widenhoefer 2014

Hashmi 2013

Au(L) (IPr)Au

C C C

Au(L)

R1

OMe

Si

R2 1

L = P(t-Bu)2(o-biphenyl)

2a

R1 R1 1

2

R = R = Me L = IPr** 2d

Si B(C6F5)4

C

R2

2

R = H; R = OMe L = PCy3 2c

2b

d

C

NTf2

R R = OMe, pyrrolidinyl

R1

BF4

OTf

Widenhoefer 2015

Straub 2014

Au(L) L = P(t-Bu)2(o-biphenyl) 2e

complexes of bent allenes Fürstner 2009

Schmidbaur 1976 AuMe AuMe

(Ph)3P

e

AuMe AuMe

(Ph)3P

(Ph)3P

(EtO)2C

3a

3b

gold carbenes/aurated carbocations generated from cyclopropenes

8

Fürstner 2009

Lee 2008

A: [(PPh3)Au(NTf2)] B: [(PPh3)AuCl]/Ag(OTf)

8

DCM, 78 °C

RO

4a

8

or

OR

O

O

[(PPh3)Au(NTf2)]

4b

via: 8

Au(L)

8

Au(L)

O

NTf2 Au(PPh3)

6

5 8

O

d2-DCM, 78 °C

Au(L)

I

Figure 1. Gold complexes have revealed a variety of amazing coordination complexes of carbon.


II

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R1 R1

R2

R2

[(L2)AuX]

R1 Au(L2)

R1 Au(L2)

Au(L1)

Au(L1)

R2

R2

R1

R2

Au(L2)

Au(L1)

Au(L2)

Au(L1)

X

Au(L1)

III

7 scope of the cyclopropenylgold(I) complexes 7 Me

Me

Me

Au(IPr)

Au(IPr)

7a

Ph

Ph

7b Me

Me

Au(IPr)

7c*

Ph

Ph

O

N

IPr

7d Au(IPr*)

Me

N

O

Ph

Ph

Ph

N

Ph

N

Ph Ph Au(IPr*) 7a*

Au(IPr*) 7b*

Au(IPr*)

Ph

7d*

Ph IPr*

Figure 2. Cyclopropenylgold(I) complexes react with cationic gold(I) complexes to give highly reactive 1,1-digoldallylium complexes. Ring-opening reactions of cyclopropenylgold(I) complexes 7 with cationic gold(I) species afforded the title complexes III. In most cases, the fragments [(L1)Au]+ and [(L2)Au]+ were generated, which confirmed the proposed structure and its “stability”. 3-Methyl substituted complexes eliminated [(L)AuH], [{(L)Au}2H]+, [{(L)Au}2H2]•+and dehydrogenated molecule fragments. The C2–C3 bond seemed to be broken easily. Fragments of the type [{(L)Au}2C=(CH)]•+ were observed in a number of cases.

NMR Spectroscopy We found IIIb to be stable enough and to emerge in sufficient purity to perform 1H and 13C NMR experiments at −38 °C in d8-THF (1H: 600.24 MHz and 400.33 MHz; 13C: 150.93 MHz and 100.66 MHz). Assignment of the signals was supported by COSY, HSQC, HMBC and 1,1-ADEQUATE experiments. We found a 1/1 mixture of (E)- and (Z)-configuration, which isomerised nearly completely to the thermodynamically favoured (Z)-isomer within 2 h (Supporting Fig. 8). Computational analysis supported the identification of the more stable isomer. Due to the stability of this single example it was possible to perform NMR experiments for over 12 h per sample. Therefore, we were able to identify the chemical shifts, the integrals (for H), and the coupling constants of every hydrogen atom and carbon atom in the structure of IIIb. High symmetry and extreme delocalization of the cationic charge were amazing features of this structure, which the NMR data intriguingly underlined. C1 (134.8 ppm) and C3 (133.8 ppm) were indistinguishable by their coupling with neighboring atoms from 1H,13C-HSQC(me) and 1H,13C-HMBC experiments due to their distinct chemical similarity. A single set of signals from both IPr ligands was detected in the 1H NMR spectra. The C2-proton was detected at a chemical shift of 6.50 ppm and the methyl group protons at 2.12 ppm. These unremarkable chemical shifts were comparable to non-ionic organic structures (e.g. -methylstyrene CH3:  (CDCl3, 89.56 MHz, 25 °C) = 2.12 ppm, (1-cyclohexenyl)benzene vinyl-H:  (CDCl3, 399.65 MHz, 25 °C) = 6.10 ppm), thus they suggested a very strong delocalization of the cationic charge.29 The 13C signals underlined this even more, with C1 and C3 of the alkyl substituent showing signals at very similar chemical shifts (C1: 134.8 ppm, C3: 133.8 ppm). We needed to perform 1,1-ADEQUATE experiments to differentiate these carbon atoms

in this strongly delocalized system. Strong shielding propagated past gold to the NHC carbon, which led to a high field shift (CNCN: 164.5 ppm).

Reaction Mechanism DFT geometry optimizations at the B97D/SDD (+f) (Au, basis set and effective core potential), 6-31G** (other atoms) level of theory with DCM CPCM estimated the (Z)-isomer (L1 = L2 = IMe) (Z)-IIIbIMe to be more stable by 5.50 kcal/mol (the same level of theory applies to all computational data in the main article). Employing the full IPr ligand led to even stronger steric interactions between the ligands and the allylic substituents. In this case, the (Z)-isomer of the product IIIb was calculated to be 11.62 kcal/mol more stable than the (E)-isomer (Fig. 3). We have found the reaction via a -complex to be a valid mechanism for ring-opening (r.-o.) to afford IIIb. The first metastable singlet ground state was the -complex of [(IMe)Au]+ with the cyclopropene double bond of 7bIMe. We found an activation energy of 5.17 kcal/mol (TSr.-o.) for the reaction to the diaurated cation (Z)-IIIbIMe and 8.10 kcal/mol for (E)-IIIbIMe. The barriers for the forward and backward reaction were feasible for the equilibration of the system, which we observed in our NMR experiments. Rotation around C2–C3 appeared to be frozen. The double bond character was too high at this side of the allylic system at equilibrium geometry. Rotation around the C1– C2 bond gave a barrier (TSrot.) of 15.15 kcal/mol for the (Z)-, 9.65 kcal/mol for the (E)-isomer. With 8.76 kcal/mol (relative to the reactants), a total barrier very similar to the ring-opening reaction was found. When we closed the C–Au distance, an auracyclobutene intermediate arose and allowed turning the C3-substituents. This was another example of the rare auracyclobutenes. They are a possible intermediate for allylic gold shift—although other geometries are possible for this. Related auraoxacyclobutanes were proposed as intermediates in gold-catalyzed cyclization reactions.30–37 For these, the isolation of an auraoxacyclobutane intermediate of gold-catalyzed oxidation of norbornene was successful.38 This interesting rotation explains the (E/Z)-isomerization, which we observed in the NMR spectra of IIIb.



-complex

ring-opening reaction

G [kcal/mol]

Au(IMe)

8.10

l (C1C3) = 1.56 Å  = 119.8° trans-IV

Ph

8.76

Au(IMe) ‡

Me

Me

Ph Au(IMe)

Au(IMe) Au(IMe) 1

l (C1C3) = 2.55 Å  = 36.2° (Z)-IIIbIMe

Au(IMe)

3

l (C C ) = 1.82 Å  = 120.9° cis-V cis-TSr.-o.

2.93

0.00

Me

Me

Me

H

‡

Ph

0.89

l (C1C3) = 1.54 Å  = 112.2° cis-IV

1

3

l (C C ) = 1.84 Å  = 120.7° trans-V trans-TSr.-o.

Au(IMe)

l (C1C3) = 2.17 Å  (Ausyn) = 10.1°  (Auanti) = 157.2° VI TSrot.

Au(IMe)

Au(IMe)

‡

Ph Au(IMe)

Au(IMe) (IMe)Au

l (C1C3) = 2.56 Å  = 36.7° (E)-IIIbIMe

Au(IMe)

(IMe)Au

Ph

Me

8.35

Ph

rotation around C1C2 and flip around C2C3

cations III

Ph Me

Page 4 of 11

6.39

distance l (C1C3) [Å]

dihedral angle  (Au2C1C2C3) [°]

Figure 3. The computational reaction energy surface explained the observed reaction and the subsequent isomerization. Relaxed Gibbs energy surface G [kcal/mol] of the ring-opening reaction (r.-o.) and of the (E/Z)-isomerization (rot.) of the complexes 7bIMe with [(IMe)Au]+.

Isolation of a Sulfoxonium-Trapped Digold Complex Dissolving 1 eq. of 7b and [(IPr)Au(NTf2)] in technical grade DMSO in air led to a pure sulfoxonium species. We isolated this complex 8 by precipitation with water from the reaction solution. Filtration in DCM over Celite gave 8 quantitatively as a pure colorless solid (Fig. 4). Due to its stability towards air and water (decomposition at ambient conditions in an open flask took ~ 1 month) this structure was characterized by NMR, IR, UV-Vis-NIR, HR-MS, EA and X-ray crystal structure analysis. We identified and optimized the intermediates arising upon closing the distance of a DMSO molecule to the carbocationic center of IIIbIMe computationally. A metastable Van-der-Waals (VdW) aggregate was found at 3.7 Å C1–S distance (free enthalpy G = −1.8 kcal/mol). A minor barrier at 2.5 Å (activation barrier 2.2 kcal/mol, 0.4 kcal/mol in reference to the reactants) connected the aggregate with the bound structure 8 with a bond length l (C1–S) = 1.8 Å (−12.1 kcal/mol). While this was a significant stabilization, it implied that liberation of DMSO should still be possible. The angles at the diaurated carbon atom were characteristic for sp3-centres ( (C2–C1–S) = 108.6(6)°,  (C2–C1– Au1) = 109.0(5)°,  (C2–C1–Au2) = 111.7(5)°,  (S–C1Au1) = 103.7(4)°,  (S–C1–Au2) = 108.5(4)°,  (Au1–C1– Au2 = 115.0(4)°. The bond lengths of both CNHC–Au bonds were slightly longer than the ones that the carbocation formed (l (Au1– CNHC,1) = 2.011(8) Å, l (Au1–C1) = 2.108(7) Å, l (Au2– CNHC,2) = 2.022(9) Å, l (Au2–C1) = 2.074(8) Å). We detected DMS, DMSO, acetophenone and a mixture of hydrocarbon decomposition products in the gas phase over the solid, both after heating and after letting 8 decompose at r.t. over the course of 2 months with GC-MS measurements. Although we were not able to scavenge defined products yet, this confirms the liberation of the original diaurated carbocation or other cationic

or carbene species. Experiments aiming at liberating DMSO by heating solutions of 8 for employing the released cationic moiety for other reactions failed—i.e. depronotation reactions and nucleophilic addition reactions (similar reagents as used for direct trapping experiments, see SI). This was a gem-dimetalated organic sp3-moiety. It still showed one organic bond to the allylic moiety as opposed to a four-fold coordination complexes of carbon.1,6-7,11,39–44 The isolation of this compound gave an indirect proof of the proposed structure of the intermediary cationic complexes III, which was supported by our other spectroscopic and computational data.

Me

Ph

R1

Au(L) NTf2 SO(Me)2 Au(L)

DMSO, 5 min r.t.

(IPr)Au 7b

R2

[(IPr)Au(NTf2)]

quant.

8

CNHC,1

CNHC,2

N1 N1‘

Au1 S

C1

N2

Au2 O

N2‘

Figure 4. Nucleophilic addition caught the reactive cation in the act. Trapping the cation IIIb with DMSO gave the digold complex 8 (thermal ellipsoids were shown at 50% probability level, hydrogen atoms were omitted for clarity). Orange: gold, blue: nitrogen, grey: carbon, yellow: sulfur, red: oxygen.


Orbital Analysis

coordination complexes of a carbon atom. Aurophilic interactions played major roles in some of these.1,6-7,11,39–44 2.5

524

2.0 1.5 1.0

798

729

0.5 0.0 400

600

800

1000

Wavelength [nm]

0.20

0.15

729

0.10

798

Further insight into the electronic structure of the generated cations III was gained by ultraviolet/visible/near infrared (UVVis-NIR) spectrophotometry. 500 M solutions of 1:1 mixtures of the complexes 7 with different gold cation precursors were prepared under argon. The full range of our seven precursor complexes 7a–d(*) was investigated with [(IPr)Au(NTf2)] in THF at −40 °C under argon. As our earlier experiments suggested, we found IIIb to be the only cation arising in sufficient purity to observe absorptions other than the overlapping ligand signals (Fig. 5). The desired absorption bands showed low extinction coefficients. We had to raise the concentration of our reactants to 10 µM to afford visible near infrared signals. We found three absorption maxima of the complex IIIb ( [nm] ( [L/mol/cm]) = 524 (164), 729 (10), 798 (5)). Absorptions of the ligand system appeared at < 500 nm with very high extinction. The phenyl substituent showed absorption at 524 nm. We found two absorption maxima in the near IR spectrum low extinction coefficients. The extinction coefficients, however, were not a quantitative measure herein, due to the low purity of the reaction solution. We supported assignment with TD-DFT calculations (B97D/SDD (+f) (Au, basis set and effective core potential), 631G** (other atoms) level of theory with DCM CPCM) on the simplified compound IIIIMe with L1 = L2 = IMe and without the anion. A bathochromic shift was discussed as a hint at carbene character in contrast to ylide character. Our computational data matched very well with the found experimental absorption wavelengths of the transition with the highest wavelength ((E)-IIIb: theor. [nm] = 817; (Z)-IIIb: theor. [nm] = 735). The frontier orbitals, which we computed, were barely comparable to similar carbene monogold compounds reported earlier.28 However, antibonding -interaction was visible in the S0HOMO, while the S0-LUMO showed respective -interaction. The remarkable difference to reported structures was, that these interactions were visible symmetrically to both gold atoms—an in-plane p-orbital of C1 facilitated this. Judging just from the bathochromic shift, these intermediates showed by far the highest carbene character reported so far (which showed the respective absorption in the visible light spectrum at 642 nm.28 This functional group was unknown by now, with two metal fragments showing antibonding -interaction with an empty carbon p-orbital.

301

UV-Vis-NIR Spectrophotometry

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Absorbance

Page 5 of 11

0.05

0.00

700

750

800

Wavelength [nm]

816.64 nm

(E)-HOMO

816.64 nm

(E)-LUMO

(Z)-HOMO

734.59 nm

We were interested in gaining a deeper understanding of the [Au1–C1–Au2]-moiety. One way to paraphrase molecular orbitals in a fashion, which (E)-HOMO is more intuitive to the synthetic chemist, is (E)-LUMO NBO analysis. We optimized the geometry of intermediate III with DFT and submitted them to NBO analyses, allowing for strong delocalization of the desired orbitals (Supporting Fig. 14). NBO analysis (even with allowed three-center two-electron bonds) modelled a sp2-hybridised C1. However, second order perturbation theory analysis of the Fock matrix in the NBO basis showed 195.7 kcal/mol stabilization for the mesomeric interaction of the *-NBO’s of C1–Au1 and C1–Au2. This clearly demonstrated that this was a poor description. Similar structural abnormalities have been observed for olefinic and aromatic carbon atoms in three-center two-electron bonds and in bent-allenes or in other structures with strong donors on carbon, which led to structures that were best described as

(Z)-HOMO

(Z)-LUMO

Figure 5. Extremely low energies led to electronic excitations in UV-Vis-NIR spectra. UV-Vis-NIR spectrum (absolute absorbance) of 10 mM IIIb at −40 °C in THF (a: full spectrum, b: extract). The dent in the spectra at ~ 840 nm originated from baseline-correction errors at the point at which the lamps were swapped. c: TD-DFT transition energies for S0 to S1 and the ground state HOMO’s and LUMO’s for IIIbIMe. Yellow: gold, blue: nitrogen, grey: carbon, white: hydrogen. Isovalues in e/Å3: (E)-HOMO: 0.072, (E)-LUMO: 0.045, (Z)-HOMO: 0.068, (Z)-LUMO: 0.065.

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We analyzed the molecular orbitals as computed by DFT. Molecular orbital coefficients of MO’s, which were rich of C1– Au1 and/or C1–Au2 interactions, were selected (Fig. 6 and Table 1). Orbitals with at least total amounts of the C1 orbital Au C

Page 6 of 11

coefficients of > 0.25 and total amounts of the C1, Au1, and Au2 com-

N H

Legend

[a]MO

58[a]

MO 72

MO 83

MO 88

MO 100

MO 48

MO 54

MO 57

MO 63

MO 64

MO 71

MO 75

MO 82

MO 86

MO 87

MO 98

MO 99

[a]MO

73[a]

MO 85

[a]MO

90[a]

MO 102

[a]MO

105 (HOMO)[a]

[a]MO

106 (virtual, LUMO)[a]


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Figure 6. Molecular orbital surfaces demonstrated carbon’s symmetrical coordination. Molecular orbital surfaces of IIIbIMe (isovalues: 0.045 e/Å3). Orbitals with at least total amounts of the C1 orbital coefficients > 0.25 and total amounts of the C1, Au1, and Au2 combined orbital coefficients > 1 were chosen. Polarisation orbitals (n+1, same sub-shell and magnetic quantum number) were included in these calculations. [a] For better visibility, isovalues of 0.065 e/Å3 were chosen.

Table 1. Orbital energies and orbital coefficients. MO 48

MO 54

MO 57

MO 58

MO 63

−295.56−

−272.54−

Orbital Energies  [kcal/mol] −375.61−

−338.25−

−304.77− Orbital Coefficients[a]

C1

2s

+0.184

3s

+0.156

C1

MO 64

2s

+0.149

3s

+0.157

C1

MO 71

2s

−0.126

3s

−0.155

C1

MO 72

2s

+0.103

C1

2px

MO 73

MO 75

−247.70−

−239.17−

−0.112


−256.54−


C1 Au1

2py 6d−2

−0.119

C1

−0.106

Au2

2py 6d+2

+0.123

C1

−0.120

Au1

2py

Au2

+0.139

C1

2px

−0.161

C1

2py

−0.111

6d−2

+0.127

Au1

6d0

+0.160

Au2

6d+2

+0.155

6d+2

−0.110

6d−2

+0.283

7d−2

+0.100

6d0

−0.145

6d+2

+0.191

6d−2

+0.180

Au2

MO 82

MO 83

MO 85

MO 86

MO 87

−201.39−

−198.85−


−219.69−


Au1

6d−2

−0.156

none

C1

2py 3s

+0.102 −0.150

C1

2py 3pz

+0.208 +0.111

Au1

6d+1

+0.111

Au1

6d+1

−0.294

6d+2

−0.184

6d−1

+0.319

6d−2

+0.177

7d−1

+0.102

6d+1

+0.382

6d−1

+0.115

7d+1

+0.122

Au2

Au2

MO 88

MO 90

MO 98

C1

2py 3s

−0.164 +0.108

3py

−0.109

Au1

6d+2

+0.333

7d+2

+0.103

6d−2

−0.274

Au2

MO 99

MO 100

−157.00−

−152.41−


−178.32−


Au1

6d+2

+0.168

C1

2px

−0.103

Au1

6d+1

−0.170

6d+2

+0.367

7d+2 Au2

MO 102

C1

2py

−0.213

3s

+0.158

6d0

−0.176

+0.116

6d+2

−0.331

6d+1

+0.235

6d−2

−0.136

6d+2

−0.295

7d+2

6d−2

+0.379

7d−2

+0.115

Au1

Au2

MO 105

C1

2px

−0.195

C1

2py

−0.112

3px

−0.131

Au1

6d0

+0.155

6d+2

−0.102

6d+2

−0.179

6d−2

+0.235

6d−2

+0.355

6d0

−0.163

7d−2

+0.107

−0.108

6d+2

+0.376

6d0

+0.128

6d0

−0.138

6d−2

+0.148

6d+2

−0.304

6d+2

−0.174

7d+2

+0.111

6d−2

+0.377

7d−2

+0.121

Au1 Au2

Au2

MO 106 (virt.) Orbital Energies  [kcal/mol]

−150.13−

−121.13−


C1

2pz

−0.154

3pz

−0.105

C1

2px

−0.331

3px

−0.243

C1

2pz

+0.358

3pz

+0.277



Au1 Au2

6d+2

−0.105

6d−2

−0.134

6d−2

−0.123

Au1 Au2

6d+2

−0.141

6d−2

−0.112

6d−2

−0.172

Au1 Au2

6d+1

+0.143

6d−1

−0.121

6d+1

−0.184

Page 8 of 11

Orbital energies  and orbital coefficients of MO’s of Orbitals with at least total amounts of the C1 orbital coefficients > 0.25 and 1 1 2 total amounts of the C , Au , and Au combined orbital coefficients > 1 were chosen. [a] Only orbital coefficients of valence shell and polarization orbitals (n+1, same sub-shell and magnetic quantum number) with an absolute value > 0.1 were displayed. IIIbIMe.

bined orbital coefficients of > 1 were chosen. Polarization orbitals (n+1, same sub-shell and magnetic quantum number) were included in these calculations. The bonding situation in this moiety was much better described with an sp-hybridized C1 than with the sp2-hybridisation, which NBO analysis suggested. MO 63 displayed a symmetric bonding situation between the three atoms Au1–C1–Au2, which was dominated by a dz2--overlap of both gold atoms interacting with the px-orbital of the C1–C2 bond. MO 64 and 71 were strongly dominated by a bonding linear combination of orbitals of all three atoms Au1–C1–Au2 with the C1(py)-orbital and d±2-orbitals of gold. MO 73 was nicely showing binding three-center interaction of the C1(px)-orbital with d0-orbitals, MO 99 with d±2-orbitals. MO 85 and 87 demonstrated sp-interaction with d±2-orbitals. MO 86 showed an interesting four-centre pure -delocalization of both gold’s 6d±1-orbitals with C1(py) and C2(py). This underlined the importance of the substitution pattern for the stabilization of this kind of electronic structure. MO 100 and 105 (HOMO) constituted antibonding interactions within this moiety. These were dominated by the overlap of gold orbitals with an sp- and a p-orbital of C1, respectively, with unmatched phase.

similar to the carbon in M2(-C≡X) (X = O, N, CR, …) or in linear allenes at first glance. However, both bonding and antibonding interactions with the cationic -system contribute towards the structure of these complexes. We have found a new coordination mode of carbon and a new display of carbon’s capability of forming metal-like coordination compounds to form cationic gem-dimetalated complexes. L

L

L

Au

Au

Au

R

+ R

Au L -backdonation

three-center four-electron bond + R

R2

L

1

Au +

H

Au L four-center -contribution

...

+ R2

CONCLUSION

R1 H

Our results question the classification of coordination compounds of carbon based on their formation reaction. Cyclopropenylgold(I) complexes—to a certain extent—are preformed for the observed coordination at an sp-hybridized carbon. The conceivable fragments of the resulting coordination complex, however, range from zero- or monovalent carbon(0) to divalent carbon(II), but none of these notions would forecast the electronic structure of our 1,1-digoldallylium complexes (Fig. 7). It is mandatory that future studies of the coordination chemistry at carbon centers investigate reversible coordination/ dissociation processes to achieve an accurate classification of such coordination complexes. As Fürstner stated in his excellent publication on the subject: “The concept of ‘coordination chemistry at carbon’ is […] more general than previously anticipated.”11 The coordination is similar to divalent carbon(0) compounds, but the roles of carbon’s binding partners are swapped herein.45 Four-electron three-center interaction was observed for the central carbon atom with the two coordinating gold moieties, while an sp- and a porbital remain for the interactions with the neighboring carbon atom. From this atypical point of view, the discussed complexes can be described as carbon complexes of bivalent carbon(I), while the usual separation of organic and metal moiety leads to digold complexes of monovalent carbon(I). This unusual coordination resulted in a formal trivalent trigonal-planar complex of carbon(I) with two gold units and one vinyl moiety. We are looking forward to future discoveries on the properties of these new structural motifs. The reactivity of this kind of complex has yet to be further investigated. Carbon herein seems

+ R

Au L

Au L

Au(L) + R2 Au(L)

-bond

R1 Au(L) H

Au(L)

-backdonation

Figure 7. Coordination chemistry at carbon—the point of view is important for covalent/ coordination-hybrid descriptions of carbon in the role of a central atom. While cyclopropenylgold(I) complexes are pre-hybridized towards future sp-coordination, conceivable coordination fragments poorly describe the title 1,1-digoldallylium complexes. Such compounds must be carefully studied to understand their coordination chemistry. The -backdonation contributions displayed (top right and bottom right) are extracts of the same virtual MO, i. e. the LUMO.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.XXXXXXX. Additional details regarding the syntheses and spectroscopy of 1,1-digoldallylium complexes and experimental and computational details (PDF).

AUTHOR INFORMATION Corresponding Author E-Mail: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT


Page 9 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


This article is dedicated to Moritz T. Mulks. The authors want to thank M. Sc. Florian Schoen, M. Sc. Roxana Lorenz, and M. Sc. Jana Elias (Anorganisch-Chemisches Institut, Ruprechts-Karls Universität Heidelberg) for the excellent support with the presented UV-Vis-NIR measurements. The authors acknowledge the support by the state of Baden-Württemberg through bwHPC.

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1,1-Digoldallylium Complexes: Diaurated Allylic Carbocations Indicate

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