Linearly Two-Coordinated Silicon: Transition Metal Complexes with

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Linearly Two-Coordinated Silicon: Transition Metal Complexes with the Functional Groups M#Si-M and M=Si=M Priyabrata Ghana, Marius I. Arz, Uttam Chakraborty, Gregor Schnakenburg, and Alexander C. Filippou J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02902 • Publication Date (Web): 06 May 2018 Downloaded from http://pubs.acs.org on May 6, 2018

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Journal of the American Chemical Society

Linearly Two-Coordinated Silicon: Transition Metal Complexes with the Functional Groups M≡Si-M and M=Si=M Priyabrata Ghana,1 Marius I. Arz,2 Uttam Chakraborty,3 Gregor Schnakenburg1 and Alexander C. Filippou*1 1

Institute of Inorganic Chemistry, University of Bonn, Gerhard-Domagk-Straße 1, D-53121 Bonn, Germany. 2

University of Bristol, School of Chemistry, Cantock's Close, Bristol, BS8 1TS.

3

Institute of Organic Chemistry, University of Regensburg, Universitätsstraße 31, 93040 Regensburg, Germany. *e-mail: [email protected]

ABSTRACT A detailed experimental and theoretical analysis is presented of unprecedented molybdenum complexes featuring a linearly-coordinated, multiply-bonded silicon atom. Reaction of SiBr2(SIdipp) (Sdipp = C[N(C6H3-2,6-iPr2)CH2]2) with Na[Tp’Mo(CO)2(PMe3)] (Na-1) in the ratio

1:2

afforded

the

reddish-brown

metallasilylidyne

complex

[Tp’(CO)2Mo≡Si–

3

Mo(CO)2(PMe3)Tp’] (Tp’ = κ -N,N’,N’’-hydridotris(3,5-dimethylpyrazolyl)-borate) (2), in which an almost linearly coordinated silicon atom (∠(Mo1-Si-Mo2) = 162.93(7)°) is bridging the 15VE metal fragment Tp’Mo(CO)2 with the 17VE metal fragment Tp’Mo(CO)2(PMe3) via a short Mo1-Si bond (2.287(2) Å) and a considerably longer Mo2-Si bond (2.438(2) Å), respectively. The reddish-orange silylidyne complex [Tp’(CO)2Mo≡Si–Tbb] (3) was also prepared from Na-1 and the 1,2-dibromodisilene (E)-Tbb(Br)Si=Si(Br)Tbb (Tbb = C6H2-2,6[CH(SiMe3)2]2-4-tBu) and contains as 2 a short Mo-Si bond (2.2614(9) Å) to an almost linearly coordinated Si atom (∠(Mo-Si-CTbb) = 160.8(1)°). Cyclic voltammetric studies of 2 in diglyme revealed an irreversible reduction of 2 at –1.907 V vs. the [Fe(η5-C5Me5)2]+/0 redox couple. Two-electron reduction of 2 with potassium graphite yielded selectively the 1,3dimetalla-2-silaallene dianion [Tp’(CO)2Mo=Si=Mo(CO)2Tp’]2− (42−−), which was isolated as the bright yellow dipotassium salt [K(diglyme)]2-4. Single crystal X-ray crystallography revealed a centrosymmetric structure of 42−−. The Mo-Si bond length of 42−− (2.3494(2) Å) compares well with those of Mo-Si double bonds and lies in-between the Mo1-Si triple bond and Mo2-Si single bond length of 2. Compounds 2, 3 and [K(diglyme)2]-4 were characterized by elemental analyses, IR and multinuclear NMR spectroscopy. Comparative ELF (electron localization function), NBO (natural bond orbital) and NRT (natural resonance theory) analyses of 2, 3 and 42−− shed light into the electronic structures of these compounds.

1. INTRODUCTION The atomic properties of silicon differ significantly from those of carbon,1 leading to distinct changes in the chemistry of these elements, as exemplified by multiple bonded systems.2 An 1 ACS Paragon Plus Environment

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illustrative example concerns the chemistry of µ2-tetrelido complexes of the general formula LnMEM’Ln’ (E = C – Pb; M,M’ = transition metal, Ln,Ln’ = ligand sphere) in which a single tetrel atom is bridging two metal centers. µ2-carbido complexes (E = C) have been extensively studied as models for surface-bound carbides, which are presumed to be formed upon the dissociative chemisorption of carbon monoxide in the Fischer-Tropsch process.3 Three distinct classes of µ2-carbido complexes have been delineated so far featuring a linear M-CM spine (Figure 1, A – C).4,5,6,7,8 Class A compounds (1,3-dimetallaallenes) includes mainly homo-bimetallic complexes containing mostly identical metal fragments MLn and M’Ln’,4 as exemplified

by

Mansuy’s 4b-e

[(TPP)Fe=C=Fe(TPP)], 6

archetypal

iron

tetraphenylporphyrinato

complexes

5

but also a few hetero-bimetallic examples. Compounds of class

7

B and C, which can be also named metallaalkylidyne (metallacarbyne) complexes, contain electronically disparate metal fragments with an odd (class B) or even number (class C) of valence electrons, to which the bridging carbon atom is bonded via a triple and a single bond, respectively (Figure 1). Many methods have been reported for the synthesis of class B

µ2-carbido complexes, the most prevalent one involving metathetical substitutions of the lithiocarbyne

complexes

[Tp’(CO)2M≡C-Li(THF)n]

(Tp’

=

κ3-N,N’,N’’-hydridotris(3,5-

dimethylpyrazolyl)borate, M = Mo, W),6e,6g with transition metal halides,6g-j whereas class C

µ2-carbido complexes have been prepared from the neutral complex [Ru(≡C:)Cl2(PCy3)2] featuring a terminal, σ-donor carbido ligand7a-e and metal electrophiles.7f-i LnM

C

M'Ln '

L nM

A

C

M'Ln '

L nM

B

(η 5-C 5R 5)(CO)2Mn

E

C

M'L n'

C

Mn(CO)2 (η 5-C 5R 5)

E = Ge, Sn, Pb; R = H, Me

Figure 1. Well known classes of µ2-carbido complexes featuring a linear M-C-M spine (top); isovalent germanium, tin and lead analogues presently known (bottom).

In sharp contrast to the plethora of µ2-carbido complexes presented above, isovalent silicon analogues remain elusive and related germanium, tin and lead compounds are confined to the manganese complexes [(η5-C5R5)(CO)2Mn=E=Mn(CO)2(η5-C5R5)] (E = Ge,9 Sn,10 Pb;11 C5R5 = Cp (R = H), Cp* (R = Me), C5H4Me) (Figure 1, bottom). This is not surprising given the anomalous low electronegativity of silicon compared to carbon, which in combination with the high electrophilicity expected for the ‘‘bare’’ silicon atom in the isovalent silicon analogues of the µ2-carbido complexes A – C renders the isolation of these unsaturated silicon compounds a very challenging goal. In fact, all attempts to prepare silicon analogues of A - C failed so far12 and reactions of silanes with metal carbonyls13 or of carbonyl metallates with 2 ACS Paragon Plus Environment

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silicon tetrahalides14 led always to metal carbonyl complexes or clusters containing four- or even higher-coordinate, singly-bonded silicon atoms reflecting the profound tendency of silicon to form σ bonds at the expense of π-bonds to metals for thermodynamic reasons. Obviously, fine stereoelectronic tuning of the metal fragments and steric protection of the reactive silicon atom are indispensable to circumvent follow-up reactions of the targeted M≡Si-M and M=Si=M (M = d-block metal) functional groups. We presumed that d6-metal bonded scorpionato ligands15 would fulfill these requirements as illustrated by the following work describing a systematic approach to the first complexes featuring a linear M≡Si-M (metallasilylidyne complexes) and M=Si=M spine (1,3-dimetalla-2-silaallenes). 2. RESULTS AND DISCUSSION Recent studies in our group have shown that the carbonyl metallate Na[Tp’Mo(CO)2(PMe3)]16 (Na-1) (Tp’ = κ3-N,N’,N’’-hydridotris(3,5-dimethylpyrazolyl)borate) is a powerful metalcentered nucleophile enabling the direct, selective conversion of organotetrel(II) halides to a wide range of tetrylidyne complexes of the general formula [Tp’(CO)2Mo≡E-R] (E = Ge, Sn, Pb; R = singly-bonded aryl, alkyl or amino group) after halide substitution and PMe3 elimination.17 Reaction of (Na-1) with [Si(η5-Cp*)][B(C6F5)4] was also found to give after PMe3 elimination directly the unusual silylidyne complex [Tp’(CO)2MoSi(η3-Cp*)] featuring a Mo-Si bond with partial triple bond character.18 These findings let us presume that similar reactions of Na-1 might occur with NHC-stabilized Si(II) dihalides, SiX2(NHC) (X = Cl, Br, I; NHC = Nheterocyclic carbene),19 leading to the targeted metal-silicon multiple bond systems. Addition of a yellow solution of SiBr2(SIdipp) (Sdipp = C[N(C6H3-2,6-iPr2)CH2]2)17c in fluorobenzene to a suspension of 2 equiv. of yellow Na-1 in fluorobenzene at ambient temperature led to a rapid color change to reddish-brown. Inspection of the reaction by IR and 1H/31P NMR spectroscopy revealed a quite selective metathetical reaction leading to the metallasilylidyne complex [Tp’(CO)2Mo≡Si–Mo(CO)2(PMe3)Tp’] (2) after elimination of SIdipp and PMe3 (Scheme 1).16 A small amount of the 17VE metalloradical [Tp’Mo(CO)2(PMe3)] was also formed during the reaction, which arises probably from a competing one-electron oxidation of Na-1 by SiBr2(SIdipp).20 However, possible reduction products of SiBr2(SIdipp), such as Si2Br2(SIdipp)2 or Si2(SIdipp)2, were not detected by 1H NMR spectroscopy in the reaction mixture except SIdipp (Figure S22, Supporting Information (SI)). After work-up of the reaction mixture, the metallasilylidyne complex 2 was separated from [Tp’Mo(CO)2(PMe3)] and SIdipp by consecutive crystallization from toluene and diethyl ether, and isolated as a reddish-brown, air-sensitive solid in 43% yield. Complex 2 is well soluble in fluorobenzene, DME and THF, and thermally very stable decomposing to a black mass upon heating at 280 – 283 °C. In THF-d8 solution a very slow decomposition of 2 was observed at ambient 3 ACS Paragon Plus Environment


temperature (t1/2 > 7 days) by

1

H and

31

Page 4 of 21

P{1H} NMR spectroscopy leading to

[Tp’Mo(CO)2(PMe3)H] among other products.21

H

N

B + 0.5 SiBr2(SIdipp)

Na

a)

+

N

N N

+ 0.5 Tbb(Br)Si=Si(Br)Tbb

N N

Mo C O

b)

PMe3 CO

Na-1 H

PMe3

N N

N

B

N

N N 0.5

H

N

B

Mo C O

Si

Mo C O C O

C O

N

N

N

N

N

N

N

N N B H

N N

Mo C O

2

Si

Tbb

C O

3

Scheme 1. Synthesis of the metallosilylidyne complex 2 and the silylidyne complex 3; a) −SIdipp, −PMe3, −NaBr, fluorobenzene, r.t., 2.5 h; b) −PMe3, −NaBr, toluene, 110 °C, 30 min.; formal charges are omitted for clarity.

For comparison reasons with 2, the silylidyne complex [Tp’(CO)2Mo≡Si-Tbb] (3) was prepared selectively upon heating a 2:1 molar mixture of Na-1 and the 1,2-dibromodisilene (E)-Tbb(Br)Si=Si(Br)Tbb (Tbb = C6H2-2,6-[CH(SiMe3)2]2-4-tBu)22 in toluene at 110 °C and isolated as a highly air-sensitive, reddish-orange solid in 67% yield (Scheme 1). The silylidyne complex 3 is as 2 a thermally very robust solid, which decomposes upon melting to a dark brown liquid at 283 – 285 °C. The silylidyne complexes 2 and 3 were characterized by elemental analyses, IR and 1H, 13

C{1H},

31

P{1H} and

29

Si{1H} NMR spectroscopy (see SI). In addition, their molecular

structures were determined by X-ray diffraction on red and orange single crystals of the solvates 2·0.5(n-pentane) and 3⋅1.5(benzene-d6), respectively (Figures 2 and 3). The metallasilylidyne complex 2 features an almost linear Mo-Si-Mo spine (∠(Mo1-Si-Mo2) = 162.93(7)°). The bridging, two-coordinated Si atom is connected via a short bond (Mo1–Si = 2.287(2) Å) to the distorted octahedral coordinated molybdenum center Mo1 and a considerably longer bond (Mo2–Si = 2.438(2) Å) to the heptacoordinated molybdenum center Mo2, which features a face-capped octahedral coordination geometry with the Si atom capping the face built up by the PMe3 and the two CO ligands (Figure 2). Both Mo atoms are bonded to a tridentate (κ3-bonded) tris(pyrazolyl)borato ligand, which spans three facial 4 ACS Paragon Plus Environment

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coordination sites with the N−Mo−N bite angles varying in a small range (80.6(2)−87.3(2)°). A mutual syn conformation is adopted by the 15VE metal fragment [Tp’Mo(CO)2] and the 17VE metallyl substituent [Tp’Mo(CO)2(PMe3)] in 2 as evidenced by the torsion angle Cmid(1)-Mo1Mo2-Cmid(2) of -16.5(1)°, where Cmid(1) and Cmid(2) are the midpoints of the carbonyl carbon atoms C16/C17 and C36/C37, respectively. This leads to a roughly Cs symmetric structure of 2 in the solid-state with the symmetry plane passing through the atoms N5, Mo1, Si, Mo2 and N11 (Figure 2 (b)).

Figure 2. a) DIAMOND plot of the molecular structure of compound 2 in the crystal lattice of 20.5(n-pentane) at 123(2) K. Thermal ellipsoids are set at 30% probability. Hydrogen atoms, were omitted for clarity except those bonded to the boron atoms. Selected bond lengths [Å] and bond angles [°]: Mo1–Si 2.287(2), Mo2–Si 2.438(2), Mo2–P 2.568(2), Mo1-N1 2.271(5), Mo1-N3 2.256(5), Mo1-N5 2.272(5), Mo2-N7 2.254(5), Mo2-N9 2.252(5), Mo2-N11 2.227(5), Mo1–C16 1.953(5), Mo1–C17 1.946(6), Mo2–C36 1.963(6), Mo2–C37 1.957(6); Mo1-Si-Mo2 162.93(7), N5-Mo1-Si 169.3(1), C16-Mo1-Si 78.5(2), C17-Mo1-Si 84.2(2), C16-Mo1-C17 84.8(2), C36-Mo2-Si 66.7(2), C37-Mo2-Si 65.9(2), P-Mo2-Si 75.93(5); for additional bonding parameters of 2 see Figure S46 (SI). b) Top view along the Mo1-Si-Mo2 direction illustrating the syn conformation of the two metal dicarbonyl fragments (only the Mo1 and Mo2 bonded atoms are depicted for clarity).

The Mo1–Si bond of 2 (2.287(2) Å) is longer than that of the silylidyne complex [Cp(CO)2Mo≡Si-ArTrip]23 (d(Mo≡Si) = 2.2241(7) Å; ArTrip = C6H3-2,6-Trip2, Trip = C6H2-2,4,6iPr3),24 but lies in-between those of the silylidyne complexes [Tp’(CO)2Mo≡Si-Tbb] (3) (2.2614(9) Å) (Figure 3) and [Tp’(CO)2MoSi(η3-Cp*)] (2.3092(4) Å),18 and is markedly shorter by 15 pm (6.6%) than the Mo2–Si bond of 2 (2.438(2) Å), which in turn appears at the short end of Mo–Si single bond lengths reported to date (d(Mo-Si) = 2.413(1) – 2.7140(8) Å).25 All these structural parameters suggest a considerable triple bond character of the Mo1–Si bond and a single bond character of the Mo2–Si bond in 2, as represented by the metallasilylidyne canonical formula A, but also a certain contribution of the 1,3-dimetalla-2-silaallene canonical formula B (Figure 4) in agreement with the results of a theoretical analysis of the electronic structure of 2 (vide infra).

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Si2 Si3 H1

N3 B N1

Si1

Mo

N5

C1 C25

C26 O2

O1 Si4

Si5

Figure 3. DIAMOND plot of the molecular structure of compound 3 at 100(2) K. Thermal ellipsoids are set at 30% probability. Hydrogen atoms were omitted for clarity except that bonded to the boron atom. Selected bond lengths [Å] and bond angles [°] (bond lengths and angles in square brackets are of the second independent molecule found in the crystal lattice of 31.5(benzene-d6)): Mo-Si1 2.2614(9) [2.2543(9)], Si1-C1 1.861(3) [1.853(3)], Mo-N1 2.249(2) [2.246(3)], Mo-N3 2.255(3) [2.257(3)], Mo-N5 2.233(2) [2.221(2)], Mo-C25 1.956(4) [1.971(3)], Mo-C26 1.950(4) [1.950(4)], C25-O1 1.178(4) [1.156(4)], C26-O2 1.168(4) [1.171(4)]; Mo-Si1-C1 160.8(1) [158.5(1)], N1Mo-Si1 105.13(7) [102.85(7)], N3-Mo-Si1 111.01(7) [108.13(7)], N5-Mo-Si1 167.19(7) [168.60(7)], C25-Mo-Si1 80.3(1) [85.94(9)], C26-Mo-Si1 82.24(9) [77.7(1)], C25-Mo-C26 90.3(1) [88.4(1)].

Tp'(CO)2Mo

Si

Mo(CO)2(PMe 3)Tp'

Tp'(CO)2Mo

A

Si

Mo(CO)2(PMe 3)Tp'

B

Figure 4. Canonical formulas of compound 2.

Both silylidyne complexes display a slight bending of the Mo≡Si-R functionality at the Si atom (2: ∠(Mo1-Si-Mo2) = 162.93(7)°, 3: ∠(Mo-Si1-C1) = 160.8(1)°) towards the two CO ligands, which does not require a lot of energy according to quantum chemical calculations. In fact, a relaxed potential energy scan of the hypersurface of 2 revealed a shallow increase of the energy by only 10 kJ/mol for the in-plane bending of the Mo-Si-Mo in the range of 160 – 180° (Figure S55, SI). Notably, the same extent and direction of bending of the R substituent was observed in the carbyne complexes [Tp’(CO)2M≡C-R] (R = C6H4-4-Me (p-Tol): M = Mo, 163.1(3)°; M = W, 163.2(3)°)26 and the heavier Group 14 tetrylidyne complexes [Tp’(CO)2M≡E-R] (R = C6H3-2,6-R12 (R1 = Mes, Trip); Mo, W; E = Ge, Sn, Pb)17 and can be 6 ACS Paragon Plus Environment

Page 7 of 21

attributed to an interplay of steric and electronic factors, such as the steric bulk of the substituent R and the strength of the two non-degenerate M-E π bonds in the overall Cssymmetric complexes (Figure 6). Further structural information for 2 and 3 was obtained from the IR and multinuclear NMR spectroscopic studies (see Sections 2.4 and 2.6, SI). Thus, the IR spectrum of 2 in the solidstate displays three intense absorption bands at 1928, 1861 and 1791 cm−1, which upon comparison with the calculated IR vibrations were assigned to the two combinations of inphase (A´ symmetric) and out-of-phase (A´´ symmetric) CO stretching modes in the two metal dicarbonyl fragments (Figure 5, and Section 5.2, SI). In THF solution, however, more than four ν(CO) absorption bands are observed (Figure 5) suggesting the presence of at least one additional conformer of 2 in solution as predicted by the theoretical calculations

1861

(vide infra).

a) 0,42

c)

1928

Absorbance

ν4

[Mo]

1791

ν1 0,30

PMe3

solid-state IR (ATR)

ν 2+ν 3

0,36

C C

0,24

O

Si

[Mo] C C

O

O

O

ν 1 (A’)

0,18

PMe3

0,12 [Mo]

Si

[Mo]

0,06 C C

C C

0,00 2000

1950

1900 1850 1800 Wavenumbers (cm −1)

1870

b) 0,50

1750

1700

O

O

O

PMe3

Solution IR (THF) [Mo]

Si

[Mo]

C C O

0,20

0,10 0,00

C C O

O

PMe3 [Mo]

Si

[Mo]

C C

2000

1950

1900

1850

O

ν 3 (A’’) 1806 1794

1857

1933

0,30

O

ν 2 (A’)

0,40

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


1800

1750

1700

Wavenumbers (cm −1)

O

C C O

O

O

ν 4 (A’’) 1

Figure 5. FT-IR spectra of 2 in the solid state (a) and in THF solution (b) in the range of 2000 – 1700 cm− ; c) combined CO stretching vibrations of 2 in Cs symmetry ([Mo] = Tp’Mo).



Page 8 of 21

In comparison, the IR spectrum of 3 in THF displays only two intense absorption bands at 1911 and 1835 cm−1 arising from the in-phase (A´ symmetric) and out-of-phase (A´´ symmetric) CO stretching vibrations of the two cis-coordinated carbonyl ligands (Figure S35, SI). The ν(CO) bands of 3 appear at lower wavenumbers than those of the isovalent carbyne complex [Tp’(CO)2Mo≡C-(p-Tol)] (ν(CO) in THF: 1982 and 1899 cm−1) pointing to a considerably stronger Mo(dπ)-CO(π*) backbonding in 3. This leads to shorter Mo-CO and consequently longer C-O bonds of 3 (d(Mo-C)mean = 1.957 Å, d(C-O)mean = 1.168 Å) than the respective bonds of [Tp’(CO)2Mo≡C-(p-Tol)] (d(Mo-C)mean = 1.987 Å, d(C-O)mean = 1.143 Å),26 and can be rationalized with the higher σ(donor)/π-acceptor ratio of the silylidyne ligand Si(Tbb) than that of the carbyne ligand C(p-Tol). The 1H and

13

C{1H} NMR spectra of 2 display a double set of signals in the integral ratio of

2:1 for the various 1H and

13

C nuclei of the 3,5-dimethylpyrazolyl arms of the two heterotopic

Tp’ ligands indicating a time-averaged Cs symmetry of complex 2 in solution (Section 2.4, SI). The most distinctive NMR feature of 2 is its markedly deshielded appears in THF-d8 as a doublet due to

31

29

Si NMR signal, which

2

P coupling ( J(Si,P) = 56.6 Hz) at δ = 438.9 ppm

(Figure S30, SI), i.e. at considerably lower field than those of the silylidyne complexes 3 (δ(29Si)= 258.5 ppm (Figure S40, SI)) and [Cp(CO)2Mo≡Si-ArTrip] (δ(29Si)= 320.1 ppm).23 Complex 2 displays also a characteristic flanked by a pair of

29

31

P NMR singlet signal at δ = −4.3 ppm, that is

Si satellites (Figure S29, SI) and is observed at higher field than those

of [Tp’Mo(CO)2(PMe3)Cl] (δ = 33.6 ppm (Figure S7, SI)) and [Tp’Mo(CO)2(PMe3)H] (δ = 8.6 ppm (Figure S34, SI)). The electronic structures of 2 and 3 were studied by quantum chemical calculations (section 5, SI). Geometry optimization of 2 at the RI-JCOSX/B97-D3/def2-TZVP/COSMO(THF)27 level of theory afforded a minimum structure (2calc) displaying well matching bond lengths and angles to 2, but with a Mo-Si-Mo spine closer to linearity (2calc: ∠(Mo1-Si-Mo2) = 174.31 °). Notably, also the observed syn conformation of the metal fragments could be well reproduced by the calculations (2calc: Cmid(1)-Mo1-Mo2-Cmid(2) = −2.4 °) (Section 5.1, SI). A careful search for other rotamers of 2calc was performed by varying the torsion angle Cmid(1)Mo1-Mo2-Cmid(2), which led to two additional minima, the one featuring a anticlinal conformation (2’calc: Cmid(1)-Mo1-Mo2-Cmid(2) = −142.6 °, ∆E = 1.2 kJ/mol) and the other a gauche conformation of the metal dicarbonyl fragments (2’’calc: Cmid(1)-Mo1-Mo2-Cmid(2) = −54.3°, ∆E = 9.1 kJ/mol) (Section 5.1, SI). The small energy difference between the three rotamers suggests a small barrier to rotation of the metallyl substituent Tp’Mo(CO)2(PMe3) about the Mo2-Si bond, rationalizing the time-averaged Cs-symmetric structure suggested in solution by NMR spectroscopy (vide supra). The calculated bond lengths and angles of the geometry-optimized structure of the silylidyne complex 3 (3calc) do also compare well with the 8 ACS Paragon Plus Environment

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experimental values, with the calculated Mo≡Si-R angle being as in 2 larger than the experimental value (∠(Mo-Si-CTbb) = 160.8(1)° (3), 171.27° (3calc) (Section 5.6, SI)). These findings let us assume, that also crystal packing forces may affect the extent of bending of the Mo≡Si-R functionality in 2 and 3. The silylidyne complexes 2calc and 3calc feature a similar set of frontier orbitals, pointing to their isolobal analogy (Figure 6). In both cases the HOMO corresponds to a metal d orbital that is oriented orthogonally to the Mo1-Si axis and is non-bonding with respect to the Mo≡Si bond, the HOMO-1 represents the out-of-plane Mo1-Si π bond, and the HOMO-2 the in-plane Mo1-Si π bond (Figure 6). In case of 2calc both Mo1-Si π-bonding orbitals (HOMO-1 and HOMO-2) display some Mo2-Si π-antibonding character, which compensates the Mo2-Si πbonding component present in HOMO-3 leading in valence bond terms to a major contribution of the canonical formula A (Figure 4).28 Topological analysis of the electron localization function (ELF) of 2calc revealed a valence disynaptic basin V(Mo1,Si) with a considerable higher mean electron population of 5.13e than that of the valence disynaptic basin V(Mo2,Si) (1.91e) (see SI, Table S14), as suggested in valence bond terms by the metallasilylidyne canonical formula A (Figure 4). Natural bond orbital (NBO) analyses of 2calc led to a leading Lewis structure with an occupied σ(Mo1-Si) NBO, that is markedly polarized toward the Si atom (58%(Si)), two π(Mo1-Si) NBOs, that are markedly polarized toward the Mo1 atom (79/76%(Mo)), and one σ(Mo2-Si) NBO, that is less polarized (48%(Si) (Section 5.4, SI)). Similar results were obtained for the silylidyne complex 3calc (Section 5.8, SI). The decreased occupancies of the σ and π NBOs of 2calc and of the π NBOs of 3calc suggest considerable electron delocalization. Comparative natural resonance theoretical (NRT) calculations of [Tp(CO)2Mo≡Si–Mo(CO)2(PMe3)Tp] (Tp = κ3-N,N’,N’’-hydrotris(pyrazolyl)borate) (2modelcalc) and 3calc (Section 5.15, SI) were performed at the same level of theory. NRT analysis of 3calc gave a considerable weight of resonance structures with a Mo≡Si bond (79.6 %) and a small contribution of resonance structures with a Mo=Si bond (14.9%) to the resonance hybrid leading to a total Mo-Si NRT bond order of 2.74. In comparison, a higher weight of resonance structures with a Mo1=Si bond (47.4%) and a smaller weight of resonance structures with a Mo1≡Si bond (27.5%) was found for 2modelcalc leading to a lower Mo1-Si NRT bond order of 1.99 (2modelcalc contains Tp instead of Tp’ ligands). This can be contrasted with the Mo2-Si NRT bond order of 2modelcalc (1.16) resulting from a large input of resonance structures with a Mo2-Si single bond (59.3%) and a smaller weight of resonance structures with a Mo2=Si bond (25.2%). All these findings point in valence bond terms to an appreciable contribution of the 1,3-dimetalla-2-silaallene canonical formula B (Figure 4). A considerable ionic part of the Mo≡Si bonds in 2 and 3 is suggested by the covalent and ionic contribution to the NRT bond order of 2modelcalc (1.11 / 0.89) and 3calc (1.51 / 1.24), and is further reflected in the high positive charge at the silicon 9 ACS Paragon Plus Environment


atom (2calc: +0.76; 3calc: +1.20) and the considerable negative charge at the Mo atom (2calc: q(Mo1) = −0.59; 3calc: q(Mo) = −0.59, Section 5.4 and 5.8, SI).

Figure 6. a) and b): Selected Kohn-Sham orbitals of 2calc and 3calc, respectively, and their corresponding energy −3

eigenvalues; isosurface value: 0.04 e bohr .

Cyclic voltammetric studies of 2 in diglyme at room temperature revealed a quasi reversible one-electron oxidation of 2 at an (Epa+Epc)/2 value of +0.183 V (Epa, Epc: anodic and cathodic peak potential) as well as an irreversible reduction of 2 at Epc = –1.907 V (all potentials vs. the [Fe(η5-C5Me5)2]+/0 redox couple/0.1M (NBu4)PF6 in diglyme; scan rate = 100 mV/s) (Section 4, SI). In order to get a closer insight into the underlying redox chemistry, the metallasilylidyne complex 2 was treated with 2.1 equivalents of potassium graphite (KC8) in DME at ambient temperature. A rapid conversion of 2 was observed by IR spectroscopy leading after two-electron reduction and PMe3 elimination to the 1,3-dimetalla-2-silaallene dianion [Tp’(CO)2Mo=Si=Mo(CO)2Tp’]2− (42−−), which after extraction and crystallization from diglyme was isolated as the analytically pure, bright yellow dipotassium salt [K(diglyme)]2-4 in 59% yield (Scheme 2).


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H B

H

N

B

N

N N N Mo N

Me 3P Mo

Si

C C O O

N N N N B

H

a)

[K(diglyme)]2

N

N N

C O C O

O OC C

N N N N Mo N

Si

C C O O

Mo N

N N N N

B

N

H

[K(diglyme)]2 −4

2

Scheme 2. Synthesis of [K(diglyme)]2-4 upon two-electron reduction of 2; a) + 2 KC8, −PMe3, diglyme, r.t..

[K(diglyme)]2-4 is an extremely air sensitive, but thermally very robust solid, which decomposes upon heating to a black solid at 328 – 329 °C. It is moderately soluble in diglyme and sparingly soluble in DME, but rapidly decomposes in THF. The molecular structure of the dianion [Tp’(CO)2Mo=Si=Mo(CO)2Tp’]2− (42−−) was determined by X-ray diffraction on orange-yellow single crystals of the potassium salt [K(diglyme)3]2-4. Remarkably, this is the first silicon compound containing two cumulated double bonds to a linearly coordinated Si atom (Figure 7).[29] Each dianion is well separated from the two symmetry-equivalent [K(diglyme)3]+ cations, in which the potassium ions are encapsulated by three coordinated diglyme molecules, and features a crystallographic center of inversion at the Si atom leading to a linear Mo-Si-Mo spine and an antiperiplanar conformation of the 15VE Tp’(CO)2Mo fragments (Figure 7). The Mo-Si bond length of 2.3493(2) Å lies inbetween the Mo-Si triple

H1 O1#

N3

B

O2# C16# C17#

N1 Si

Mo

Mo#

N5#

N5 C17

N1#

C16 O2

N3#

O1

B# H1#

2−

Figure 7. DIAMOND plot of the molecular structure of the 4

dianion in the crystal lattice of [K(diglyme)3]2-4.

Thermal ellipsoids are set at 30% probability. H atoms, except those bonded to B, are omitted for clarity. Selected bond lengths [Å] and bond angles [°]: Mo–Si 2.3493(2), Mo#–Si 2.3494(2), Mo–N1 2.296(2), Mo–N3 2.303(2), Mo–N5 2.326(2), Mo–C16 1.918(2), Mo–C17 1.916(2), C16-O1 1.192(2), C17-O2 1.186(2); Mo-Si-Mo# 180.0,



N1-Mo-Si 103.61(4), N3-Mo-Si 107.73(4), N5-Mo-Si 173.00(4), C16-Mo-Si 78.47(6), C17-Mo-Si 82.42(6), N1-MoN3 81.48(6), N1-Mo-N5 80.18(6), N3-Mo-N5 78.50(6), C16-Mo-C17 88.42(8).

bond length of 3 (2.2614(9) Å) and the Mo2-Si single bond length of 2 (2.438(2) Å), and compares well with those of Mo-Si double bonds (d(Mo=Si)mean = 2.323 Å).[30] Further structural information for 42−− was gained by IR spectroscopy. Thus the IR spectrum of [K(diglyme)]2-4 shows two intense absorption bands at 1765 and 1696 cm−1, which upon comparison with the calculated IR vibrations were assigned respectively to the IR allowed, Bu symmetric combination of the in-phase CO stretching modes, and the Au symmetric combination of the out-of-phase CO stretching modes of the two metal dicarbonyl fragments (Figure 8). The ν(CO) bands of 42−− appear at considerably lower wavenumbers than those of 2 indicating a considerably stronger Mo(dπ)-CO(π*) backbonding in 42−−, which is also evidenced by the shorter Mo-CO and longer C-O bonds of 42−− (d(Mo-C)mean = 1.917 Å, d(CO)mean = 1.189 Å) compared with 2 (d(Mo-C)mean = 1.955 Å, d(C-O)mean = 1.165 Å).

b)

2– O

1696

a)

0,28

2–

O

[Mo]

Si

C C

[Mo]

[Mo]

C C

O

0,22

O

O

C C

1765

0,34

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

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Si

[Mo]

C C O

O

O

ν 1 (Ag )

ν 2 (Bu) 2–

0,16

2–

O

O

C C

C C

0,10

[Mo]

Si

[Mo]

O

O

[Mo]

Si

[Mo]

C C

C C

0,04 1900

1850

1800

1750 1700 1650 Wavenumbers (cm−1 )

1600

O

O

O

O

ν 3 (A u)

ν 4 (Bg )

1

Figure 8. a) ATR-IR spectrum of [K(diglyme)]2-4 in the solid state in the range of 1900 – 1550 cm− ; b) combined 2−

CO stretching vibrations of 4

in C2h symmetry ([Mo] = Tp’Mo).

The electronic structure of 42−− was analyzed by quantum chemical studies (Section 5, SI). Geometry optimization of 42−− at the RI/B97-D3/def2-TZVP/COSMO(THF)27 afforded the experimentally observed centrosymmetric anti conformer as a minimum structure ((42−)calc) of C2h symmetry displaying well matching bond lengths and angles (Section 5.9, SI). A second minimum structure was also found on the energy hypersurface featuring a gauche conformation of the metal dicarbonyl fragments ((42−)’calc: Cmid(1)-Mo1-Mo2-Cmid(2) = 68.9°, ∆E = -10.2 kJ/mol). The small energy difference between the two rotamers suggested a small barrier to rotation of the Tp’Mo(CO)2 fragments around the Mo-Si-Mo axis. This was verified 12 ACS Paragon Plus Environment

Page 13 of 21

by a relaxed potential energy hypersurface scan varying the torsion angle C16-Mo-Mo#C16# in steps of 10° (Figure 9, top). d(C9-C4#) = 326 pm

A [Tp'Mo(CO)2Mo=Si=Mo(CO)2MoTp']2–

d(O1-C9#) = 329 pm

(42─)

40.0

Energy / kJmol─1

∆E(140°) = 37.7 kJmol

A

35.0

–1

B

30.0 25.0

d(C9-C4#) = 330 pm

20.0 15.0

d(C4-C9#) = 340 pm

B

10.0

d(O1-O2#) = 330 pm

5.0 0.0 0

40

80

120

d(O2-O1#) = 331 pm

160

torsion angle C16-Mo-Mo#-C16#) / °

∆E(0°) = 35.1 kJmol

–1

∆E(10°) = 15.6 kJmol–1

C

[TpMo(CO)2Mo=Si=Mo(CO)2MoTp]2– (42─)model

20.0

C 15.0

Energy / kJmol─1

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


10.0

5.0

0.0 0

40

80

120

160

torsion angle C16-Mo-Mo#-C16#) / °

2−

Figure 9. Relaxed rotational energy profile and least stable rotamers of 4 2− − model

(top) and of (4 ) 2−

rotamer of 4

with selected intramolecular contacts

(bottom) at the RI-B97-D3/def2-TZVP/COSMO(THF) level of theory. The most stable 2

is found at a C16-Mo-Mo#-C16# torsion angle of 60° ((4 −)’calc.)

The scan revealed a low barrier to rotation of the Tp’Mo(CO)2 fragments about the Mo-Si-Mo axis of 42−, which amounts only ca. 38 kJmol─1 and can be attributed mainly to steric effects as indicated by the short intramolecular contacts found between the methyl groups of the Tp’ ligands and the carbonyl groups in the least stable rotamers of 42− at C16-Mo-Mo#-C16# torsion angles of 0° and 140°, respectively (Figure 9, top right). Consequently, the barrier to rotation

is

decreased

[Tp(CO)2Mo=Si=Mo(CO)2Tp]

to 2−

ca. 2− − model

(4 )

16

kJmol─1

in

the

model

compound

bearing the less sterically demanding Tp ligands

(Figure 9, bottom left) with the least stable conformer of (42−−)model being found at a C16-MoMo#-C16# torsion angle of 10° (Figure 9, bottom right). In this context, the electronic structure of the dianion 42−− is reminiscent of that of Cp(CO)2Re=C=Re(CO)2Cp,4l but differs from that of organic allenes (R2C=C=CR2), in which the CR2 termini are orthogonally fixed with a substantial barrier to internal rotation (ca. 310 kJ/mol for the parent allene).31 This difference becomes evident by a closer inspection of the frontier orbitals of the C2h symmetric rotamer 42−calc (Figure 10). 13 ACS Paragon Plus Environment


[Mo]

Si

Page 14 of 21

[Mo]

LUMO+4

LUMO

HOMO

HOMO-1

HOMO-2

HOMO-3

HOMO-4

HOMO-5

HOMO-6 x y

HOMO-27

z –3

Figure 10. Selected Kohn-Sham frontier orbitals with their energy eigenvalues (isosurface value: 0.04 ebohr ) 2

and symmetry race (in curly brackets) of the C2h symmetric rotamer (4 −)calc (top). schematic representation of the contributing metal and silicon atomic orbitals (bottom); the cartesian coordinate system was oriented arbitrarily so that the z-axis corresponds to the Mo-Si-Mo axis and the xz plane intersects both groups of CO ligands.

It can be rationalized with the presence of two orthogonal, occupied metal-centered d(π)orbitals at each Tp’Mo(CO)2− fragment available for π bonding with the two orthogonal p(π) 14 ACS Paragon Plus Environment

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orbitals at silicon leading to an overall 3-center-8-electron Mo-Si-Mo π system, which contrasts the 3-center-4-electron π system of allenes. In fact, mixing of the symmetryadapted Mo dπ orbitals (dxz and dyz) with the Si p(π) (px and py) gives a pair of bonding orbitals of Bu and Au symmetry (HOMO-6 and HOMO-5) leading to two orthogonal 3-center2-electron Mo-Si-Mo π bonds, a pair of Mo-centered Si non-bonding orbitals of Ag and Bg symmetry with a nodal plane at silicon (HOMO-1 and HOMO), and a pair of Mo-Si-Mo π antibonding orbitals (LUMO+4 and LUMO), that are mainly concentrated at the Si atom (Figure 10).28 Due to the presence of the four π levels (HOMO-5, HOMO-6, HOMO-1 and HOMO), rotation of the Tp´Mo(CO)2 fragments about the silicon atom does not lead to a disruption of the two Mo-Si-Mo π bonds at any torsion angle providing a rationale for the low barrier to rotation about the Mo-Si-Mo axis (Figure 9). Natural bond orbital (NBO) analyses of (42−)calc led to a leading Lewis structure with one occupied σ(Mo-Si) and π(Mo-Si) NBO for each Mo-Si bond, the σ(Mo-Si) being slightly polarized toward the Si atom (52%(Si)), and the π(Mo-Si) NBO being markedly polarized as in 2calc and 3calc toward the Mo atom (85%(Mo)) (Section 5.12, SI). NRT analysis of (42−)modelcalc ((42−)modelcalc contains Tp instead of Tp’ ligands) gave a considerable weight of resonance structures with Mo=Si and Mo#=Si bonds (70%) leading to a total Mo-Si NRT bond order of 1.84 (mean value of the two Mo-Si bonds) (Section 5.15, SI). Furthermore, topological analysis of the electron localization function (ELF) of (42−)calc revealed for each Mo-Si bond a valence disynaptic basin with a mean electron population of 3.28e, lying inbetween those of the Mo1-Si triple bond (5.13e) and Mo2-Si single bond (1.91e) of 2. In addition, an analysis of of the electron density difference upon building the Mo-Si bond between the two singlet fragments [Tp’Mo(CO)2]− and [SiMo(CO)2Tp’]− in (42−)calc was carried out by exploiting the properties of the Natural Orbitals for Chemical Valence (NOCV, eigenvectors of the corresponding one electron density difference matrix).32

∆ρ1 = –0.81φ-12 + 0.81φ12 –1

(∆E = –307.9 kJmol )

∆ρ2 = –0.61φ-22 + 0.61φ22 –1

(∆E = –90.7 kJmol ) 2

∆ρ3 = –0.55φ-32 + 0.55φ32 –1

(∆E = –94.5 kJmol )

Figure 11. 3D isosurface plots of the NOCV’s of (4 −)calc. Blue color denotes charge depletion upon bond formation, yellow color shows charge concentration.



Plots of the NOCV deformation density pairs along with their eigenvalues and energy contributions are depicted in Figure 11. The bonding between the singlet fragments [Tp’Mo(CO)2]− and [SiMo(CO)2Tp’]− can be described by three interactions involving a) a charge donation of ca. 0.8e− from the lone pair orbital at [SiMo(CO)2Tp’]− into a vacant metal orbital with dz² character of the [Tp’Mo(CO)2]− fragment, and b) two energetically nearly degenerated back donations of ca. 0.6 e− from two metal-centered orbitals with mainly dxz and dyz character of the [Tp’Mo(CO)2]− fragment to the Si-centered acceptor orbitals in the [SiMo(CO)2Tp’]− fragment. All these findings corroborate in valence bond terms the 1,3dimetalla-2-silaallene formula depicted in Scheme 2. The anion 42- features a linear Mo=Si=Mo spine and differs thereby from the 2-silaallenes presently known, a prominent example being Kira´s trisilaallene R2Si=Si=SiR2,29b-c which display a bent skeleton.29 The origin of the linearity of the Mo=Si=Mo skeleton in 42- was addressed using the CGMT (Carter-Goddard-Malrieu-Trinquier) model, which was developed to predict the geometry of “heavy alkenes” R2E=E´R´2 (E = Si, Ge, Sn, Pb; R,R´ = singlybonded substituent). According to this model, a “classical” planar structure results, when the sum of the singlet-triplet energy differences (Σ∆EST) of the fragments ER2 and E´R´2 is smaller than half of the bond cleavage energy (BCE, a measure for Eσ+π) of the E=E´-bond (Σ∆EST < ½ BCE), whereas a trans-bent distortion is observed if Σ∆EST > ½ BCE. A second criterion was also proposed predicting the existence of a direct link E=E´ in R2E=E´R´2, be it planar or trans-bent, according to which a E=E´ bonded structure exists only if Σ∆EST < BCE.33 The CGMT model was applied to (42−−)calc. For this purpose the energy required to cleave only one Mo=Si double bond in (42−−)calc leading to the fragments [Tp’(CO)2MoSi]− and [Mo(CO)2Tp’]− in the triplet state was calculated and compared with the singlet-triplet energy difference ET-ES (∆EST) of each fragment. The following values were obtained: BCE = 455.6 kJmol–1, ∆EST([Tp’(CO)2Mo]−) = 63.3 kJmol–1 and ∆EST([Tp’(CO)2MoSi]−) = 122.6 kJmol–1. A comparison of these values shows that Σ∆EST (185.9 kJmol–1) < ½BCE (227.8 kJmol–1) suggesting the formation of a “classical” linear allene-like structure. Alternative fragmentation modes were also considered (Section 5.14, SI) and in all cases a “classical” linear (allenelike) structure was predicted by the calculations, suggesting that both the considerable Eσ+π Mo-Si bond energy and the comparably small singlet-triplet energy difference ∆EST of the Mo fragments ([Tp’(CO)2Mo]− and [Tp’(CO)2MoSi]−) account for the linearity of the Mo=Si=Mo spine in 42-.


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3. Summary The synthesis of the metallasilylidyne complex [Tp’(CO)2Mo≡Si–Mo(CO)2(PMe3)Tp’] (2) from the tailor-made metallate Na[Tp’Mo(CO)2(PMe3)] (Na-1) and SiBr2(SIdipp), and its selective reduction to the dianion [Tp’(CO)2Mo=Si=Mo(CO)2Tp’]2− (42−−) not only provides a rare example for the use of SiX2(NHC) (X = halogen; NHC = N-heterocyclic carbene) as synthetic equivalents of SiX2, but also substantiates our approach of using stereoelectronically properly designed metal fragments to stabilize novel, unsaturated silicon-centered functional groups bearing a high synthetic potential. In fact, various reactivity studies of 2 reveal a rich and unexpected follow-up chemistry,17 that is presently explored.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Crystallographic

data

for

([Tp’Mo(CO)2(PMe3)Cl]·2(THF)),

[Tp’Mo(CO)2(PMe3)](toluene),

20.5(n-pentane),

Na(DME)2-1·0.5(DME),

[Tp’Mo(CO)2(PMe3)H]·(benzene-d6),

31.5(benzene-d6) and [K(diglyme)3]2-4 (also deposited with the Cambridge Structural Database under the deposition numbers CCDC 1828654−1828660) (CIF) Syntheses, analytical data and illustrations of the IR, 1H NMR, and

29

13

C{1H} NMR,

31

P{1H} NMR

Si{1H} NMR spectra of [Tp’Mo(CO)2(PMe3)Cl], Na[Tp’Mo(CO)2(PMe3)] (Na-1),

[Tp’Mo(CO)2(PMe3)],

[Tp’(CO)2Mo≡Si–Mo(CO)2(PMe3)Tp’]

(2),

[Tp’Mo(CO)2(PMe3)H],

[Tp’(CO)2Mo≡Si–Tbb] (3) and [K(diglyme)]2[Tp’(CO)2Mo=Si=Mo(CO)2Tp’] ([K(diglyme)]2-4), cyclic voltammetric studies of 2, and quantum chemical calculations of 2, 3 and 42−− (PDF) Cartesian Coordinate Data (XYZ) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Alexander C. Filippou: 0000-0002-6225-8977 Marius I. Arz: 0000-0002-1271-6343 Notes The authors declare no competing financial interest.



ACKNOWLEDGMENTS The authors thank the University of Bonn for the financial support of this work and Dr. Ujjal Das for helpful discussions. The authors thank Dr. Senada Nozinovic for the support with the NOE experiments, and Mrs. Kühnel-Lysek, Mrs. Anna Martens, Mrs. Hannelore Spitz, Dipl.Ing. Karin Prochnicki and Mrs. Charlotte Rödde for their contributions to elemental analyses, NMR measurements and single-crystal X-ray diffraction experiments.

References (1)

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The Chemistry of Organic Silicon Compounds, Vol. 3 (Eds.: Z. Rappoport, Y. Apeloig), John Wiley & Sons Ltd, Chichester, England, 2001, pp. 1-163. (2)

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(a) Trofimenko, S. Chem. Rev. 1993, 93, 943. (b) Trofimenko, S. Scorpionates: The Coordination

Chemistry of Polypyrazolylborate Ligands, Imperial College Press, London, 1999: (c) Tellers, D. M.; Skoog, S. J.; Bergman, R. G.; Gunnoe, T. B.; Harman, W. D. Organometallics 2000, 19, 2428. (16)

Na-1 was prepared by two-electron reduction of [Tp’Mo(CO)2(PMe3)Cl] with two equiv. of sodium naphthalenide. The synthesis, spectroscopic and structural characterization by single-crystal X-ray



crystallography

of

Na-1,

its

precursor

[Tp’Mo(CO)2(PMe3)Cl]

Page 20 of 21

and

its

follow-up

products

[Tp’Mo(CO)2(PMe3)], [Tp’Mo(CO)2(PMe3)H], 2, 3 and [K(diglyme)]2-4 is provided in the Supporting Information. (17)

Ghana, P. Synthesis, Characterization and Reactivity of Ylidyne and µ-Ylido Complexes Supported by

Scorpionato Ligands, Dissertation, University of Bonn, 2017. (18)

Ghana, P.; Arz, M. I.; Schnakenburg, G.; Straßmann, M.; Filippou, A. C. Organometallics 2018, 37, 772.

(19)

(a) Ghadwal, R. S.; Roesky, H. W.; Merkel, S.; Henn, J.; Stalke D., Angew. Chem. Int. Ed. 2009, 48, 5683. (b) Filippou, A. C.; Chernov, O.; Schnakenburg, G. Angew. Chem. Int. Ed. 2009, 48, 5687; Angew. Chem. 2009, 121, 5797; (c) Filippou, A. C.; Chernov, O.; Schnakenburg, G. Chem. Eur. J. 2011, 17, 13574; (d) Filippou, A. C.; Lebedev Y. N.; Chernov O.; Straßmann M.; Schnakenburg G., Angew. Chem. Int. Ed. 2013, 52, 6974.

(20)

The yellow air-sensitive metalloradical [Tp’Mo(CO)2(PMe3)] was prepared independently by one-electron reduction of [Tp’Mo(CO)2(PMe3)Cl] with sodium powder in the presence of a catalytic amount of naphthalene and was fully characterized (see Supporting Information). It can be also obtained via oneelectron oxidation of Na-1 with [FeCp2][B{C6H3-2,6-(CF3)2}4].

(21)

The off-white hydrido complex [Tp’Mo(CO)2(PMe3)H] was prepared independently by protonation of Na-1

(22)

Agou, T.; Hayakawa, N.; Sasamori, T.; Matsuo, T.; Hashizume, D.; Tokitoh, N. Chem. Eur. J. 2014, 20,

with HCl in diethyl ether at 0 °C and fully characterized (see Supporting Information).

9246. (23)

Filippou, A. C.; Chernov, O.; Stumpf, K. W.; Schnakenburg, G. Angew. Chem. Int. Ed. 2010, 49, 3296;

Angew. Chem. 2010, 122, 3368. (24)

A series of tetrylidyne complexes of the general formula [Tp’(L)2M≡E-R] and [Cp(L)2M≡E-R] and (M = Mo, W; E = Ge, Sn, Pb; L = CO, PMe3) have been isolated in our group in recent years. In all cases the M≡E bond lengths of the scorpionato complexes were found to be longer by roughly 3-5 pm than those of the cyclopentadienyl complexes. We attribute this to a more directional trans-influence and the larger steric bulk of the Tp’ ligand: a) ref. 17. (b) Lindlahr, C. Novel Tetrylidyne Complexes of Group 6 Metals with

Trialkylphosphane Ligands, Dissertation (ISBN 978-3-8439-2126-8), University of Bonn, Verlag Dr. Hut, 2015. (25)

A CSD survey (01.03.2018) gave 86 compounds with Mo–Si single bonds. The Mo–Si single bond lengths ranged from 2.413(1) to 2.7140(8) Å with a median and mean value of 2.559 Å and 2.564 Å, respectively.

(26)

Wadepohl, H.; Arnold, U.; Pritzkow, H.; Calhorda, M. J.; Veiros, L. F. J. Organomet. Chem. 1999, 587, 233.

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RI-JCOSX approximation: (a) Neese, F. J. Comput. Chem. 2003, 24, 1740; (b) Neese, F.; Wennmohs, F.; Hansen, A.; Becker, U. Chem. Phys. 2009, 356, 98. B97-D3 functional: (c) Grimme, S.; Ehrlich, S.; Goerigk, L. J. Comput. Chem. 2011, 32, 1456; (d) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem.

Phys. 2010, 132, 154104. def2-TZVP basis set: (e) Schäfer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571; (f) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297. COSMO solvation model: (g) Klamt, A. WIREs Comput. Mol. Sci. 2011, 1, 699. 2-

(28)

A more detailed description of the frontier orbitals of 2calc and 4

(29)

All other compounds containing cumulated double bonds to silicon display a bent geometry at the Si atom:

calc

is presented in the sections 5.3 and

5.11 of the Supporting Information, respectively.

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Organometallics 2005, 24, 197; (d) Kira, M.; Iwamoto, T.; Ishida, S.; Masuda, H.; Abe, T.; Kabuto, C. J. Am. Chem. Soc. 2009, 131, 17135; (e) Tanaka, H.; Inoue, S.; Ichinohe, M.; Driess, M.; Sekiguchi, A.


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Organometallics 2011, 30, 3475; (f) Mondal, K. C.; Roesky, H. W.; Schwarzer, M. C.; Frenking, G.; Niepötter, B.; Wolf, H.; Herbst-Irmer, R.; Stalke, D. Angew. Chem. Int. Ed. 2013, 52, 2963; Angew. Chem. 2013, 125, 3036; (g) Xiong, Y.; Yao, S.; Inoue, S.; Epping, J. D.; Driess, M. Angew. Chem. Int. Ed. 2013,

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Angew. Chem. Int. Ed. 2017, 56, 9920; Angew. Chem. 2017, 129, 10052. (30)

A CSD survey (01.03.2018) gave 7 compounds with Mo=Si bonds. The Mo=Si bond lengths ranged from

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2.287(1) to 2.3872(8) Å leading to a median and mean value of 2.317 Å and 2.323 Å, respectively.

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Linearly Two-Coordinated Silicon: Transition Metal Complexes with

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