Donor–acceptor-stabilised germanium analogues of acid chloride, ester, and acyl pyrrole compounds: synthesis and reactivity

Abstract

Germaacid chloride, germaester, and N-germaacyl pyrrole compounds were not known previously. Therefore, donor–acceptor-stabilised germaacid chloride (i-Bu)₂ATIGe(O)(Cl) → B(C₆F₅)₃ (1), germaester (i-Bu)₂ATIGe(O)(OSiPh₃) → B(C₆F₅)₃ (2), and N-germaacyl pyrrole (i-Bu)₂ATIGe(O)(NC₄H₄) → B(C₆F₅)₃ (3) compounds, with Cl–Ge=O, Ph₃SiO–Ge=O, and C₄H₄N–Ge=O moieties, respectively, are reported here. Germaacid chloride 1 reacts with PhCCLi, KOt-Bu, and RLi (R = Ph, Me) to afford donor–acceptor-stabilised germaynone (i-Bu)₂ATIGe(O)(CCPh) → B(C₆F₅)₃ (4), germaester (i-Bu)₂ATIGe(O)(Ot-Bu) → B(C₆F₅)₃ (5), and germanone (i-Bu)₂ATIGe(O)(R) → B(C₆F₅)₃ (R = Ph 6, Me 7) compounds, respectively. Interconversion between a germaester and a germaacid chloride is achieved; reaction of germaesters 2 and 5 with TMSCl gave germaacid chloride 1, and 1 reacted with Ph₃SiOLi and KOt-Bu to produce germaesters 2 and 5. Reaction of N-germaacyl pyrrole 3 with thiophenol produced a donor–acceptor-stabilised germaacyl thioester (i-Bu)₂ATIGe(O)(SPh) → B(C₆F₅)₃ (10). Furthermore, the attempted syntheses of germaamides and germacarboxylic acids are also discussed.

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Introduction

The carbonyl group (C=O) in organic compounds such as ketones [RC(O)R], aldehydes [RC(O)H], acid halides [RC(O)X], esters [RC(O)OR], amides [RC(O)NR₂], carboxylic acids [RC(O)OH], and acid anhydrides [RC(O)OC(O)R] is of great importance in organic chemistry (R = alkyl/aryl group; X = halogen). The significance of these carbon compounds provides inspiration for the synthesis of their heavier analogues,^1–3 but the synthetic efforts are typically hampered by the lability of the M=O bond (M = Si, Ge, Sn, Pb). The instability of this bond stems from the σ-bond polarisation and poor π-type overlap between M and O atoms, which usually leads to oligomerisation/polymerisation of compounds containing such M=O bonds.^4–6 Strategies that utilise tailor-made ligands and/or provide donor–acceptor stabilisation to M/O atoms have been applied to address the aforementioned problems and have yielded various stable compounds containing M=O bonds.^7–11 Thus, silanones (silaketones) and germanones (germaketones) with formal Si=O and Ge=O bonds, respectively, were successfully isolated, and the variety of silanones exceeds that of the germanones.^7–10 In addition to silanones, silicon analogues of aldehyde, ester, amide, formyl chloride, carboxylic acid, and acid anhydride compounds were also synthesised via various methods mainly by the groups of Driess, Roesky, Baceiredo, and Kato.¹² Very recently, Aldridge and co-workers reported the generation of a silicon analogue of an acid chloride [(N-nacnac)^ArSi(Cl)=O (I)] through the reaction of the silylene (N-nacnac)^ArSiCl with N₂O (Chart 1) [(N-nacnac)^Ar = HC{(Me₂N)C(Ar)N}₂; Ar = 2,6-i-Pr₂C₆H₃]. The metathesis reactions of I with K[Et₃BH] and KOt-Bu afforded a silaaldehyde [(N-nacnac)^ArSi(H)=O → BEt₃ (II)] and a silaester [(N-nacnac)^ArSi(Ot-Bu)=O (III)], respectively (Chart 1).^12a Surprisingly, such analogues of germanium [LGe(O)Y] [L = a monoanionic ligand; Y = H (germaaldehyde), Cl (germaacid chloride), OR (germaester), NR₂ (germaamide), OH (germacarboxylic acid), and (OGe(O)L) germaacid anhydride] are not yet known, perhaps due to the difficulty in adding an electron-withdrawing Y atom/group to the germanium atom in light of the already heavily polarised Ge=O bond. Owing to our continued interest in the chemistry of germanium, we were able to isolate the Lewis acid (LA) complexes (i-Bu)₂ATIGe(i-Pr)=O → LA (LA = B(C₆F₅)₃ (IV), ZnCl₂ (V), SnCl₂ (VI), and GeCl₂ (VII)) of a germanone¹⁰ starting from a germanium-μ-oxo dimer [ATI = aminotroponiminate, a monoanionic bidentate ligand]. We now understand that this synthetic protocol is exploitable for the synthesis of hitherto unknown germaacid chlorides and germaesters. Consequently, we report in this article the isolation and reactivity of the first examples of a donor–acceptor-stabilised germaacid chloride (i-Bu)₂ATIGe(O)(Cl) → B(C₆F₅)₃ (1), germaester (i-Bu)₂ATIGe(O)(OSiPh₃) → B(C₆F₅)₃ (2), and N-germaacyl pyrrole (i-Bu)₂ATIGe(O)(NC₄H₄) → B(C₆F₅)₃ (3). Compound 3 was obtained during our search for stable germaamides.

Chart 1 Silicon analogues of an acid chloride I, aldehyde II, and ester III.

To synthesise a germaacid chloride, oxidation of the germylene monochloride¹³ (i-Bu)₂ATIGeCl (G1) with N₂O was carried out in tetrahydrofuran at room temperature. However, germylene G1 did not react with N₂O at room temperature, and therefore, this reaction was performed at higher temperatures. Germylene G1 reacted with N₂O at 60 °C in tetrahydrofuran and afforded the germanium μ-oxo dimer {(i-Bu)₂ATIGe(Cl)(μ-O)}₂ (D1) after 2 h as a yellow solid in 60% yield (Scheme 1).^5,14 It appears that 60 °C is the optimum temperature for this reaction; higher temperatures afforded the ATI ligand salt [ATIH]⁺(Cl)⁻, and lower temperatures resulted in lower yields of μ-oxo dimer D1. Based on the successful conversion of a germanium μ-oxo dimer {(i-Bu)₂ATIGe(i-Pr)(μ-O)}₂ (D) containing Ge–C bonds into donor–acceptor-stabilised germanones IV–VII through the reaction of D with Lewis acids, we planned to react germanium μ-oxo dimer D1 containing Ge–Cl bonds with B(C₆F₅)₃. To our surprise, treatment of μ-oxo dimer D1 with two equivalents of B(C₆F₅)₃ in toluene for 2 h at room temperature yielded the first example of a donor–acceptor-stabilised germaacid chloride (i-Bu)₂ATIGe(O)(Cl) → B(C₆F₅)₃ (1) in quantitative yield (Scheme 1). This accomplishment inspired us to determine whether hitherto unknown germaesters and germaamides could also be isolated using this synthetic strategy of reacting suitable germanium μ-oxo dimers with Lewis acids. Thus, to synthesise a germaester, a germylene siloxide¹⁵ (i-Bu)₂ATIGeOSiPh₃ (G2) was reacted with N₂O in tetrahydrofuran at 60 °C for 2 h to obtain the germanium μ-oxo dimer {(i-Bu)₂ATIGe(OSiPh₃)(μ-O)}₂ (D2). The reaction of μ-oxo dimer D2 containing Ge–OSiPh₃ bonds with two equivalents of B(C₆F₅)₃ in toluene at room temperature afforded the first example of a donor–acceptor-stabilised germaester, namely, (i-Bu)₂ATIGe(O)(OSiPh₃) → B(C₆F₅)₃ (2) (Scheme 2), and demonstrated the suitability of the germanium μ-oxo dimer route for the preparation of germaesters. To extend this route for the synthesis of germaamides, a germanium μ-oxo dimer with Ge–NR₂ moieties is required. Two such germanium μ-oxo dimers, {(i-Bu)₂ATIGeN(H)Ph(μ-O)}₂ (D3) and {(i-Bu)₂ATIGeN(Me)Ph(μ-O)}₂ (D4), were obtained through the reaction of the amidogermylenes (i-Bu)₂ATIGeN(H)Ph (G3) and (i-Bu)₂ATIGeN(Me)Ph (G4) with N₂O at 60 °C for 2 h in tetrahydrofuran (Scheme 3). However, the reaction of μ-oxo dimers D3 and D4 with two equivalents of B(C₆F₅)₃ resulted in the amine → borane adducts PhNH₂ → B(C₆F₅)₃ and Ph(Me)NH → B(C₆F₅)₃, respectively, along with an unidentified oily material instead of the expected germaamides (Scheme 3). These reactions suggest that the synthetic route discussed above is not suitable for the isolation of donor–acceptor-stabilised germaamides. On the basis of the products obtained, it was thought that the lone pairs of electrons on the nitrogen atoms of the NR₂ moieties in D3 and D4 interfered with the expected reaction of these compounds (D3 and D4) with B(C₆F₅)₃. To confirm this hypothesis, a germanium μ-oxo dimer containing amino functional groups with nitrogen atoms that cannot donate lone pairs of electrons to Lewis acids was synthesised and used. As a pyrrole substituent (Py; NC₄H₄) can satisfy the required criterion, the germanium μ-oxo dimer {(i-Bu)₂ATIGe(NC₄H₄)(μ-O)}₂ (D5) with two Ge–NC₄H₄ moieties was synthesised in quantitative yield by the reaction of the N-germylene pyrrole (i-Bu)₂ATIGe(NC₄H₄) (G5) with N₂O in tetrahydrofuran at 60 °C for 2 h (Scheme 4).¹⁶ Treatment of μ-oxo dimer D5 with two equivalents of B(C₆F₅)₃ in toluene at room temperature resulted in the first donor–acceptor-stabilised N-germaacyl pyrrole, (i-Bu)₂ATIGe(O)(NC₄H₄) → B(C₆F₅)₃ (3) in quantitative yield (Scheme 4). The feasibility of isolating N-germaacyl pyrrole 3 as a stable species proves that the aforementioned hypothesis of the interference of lone pairs of electrons on the nitrogen atoms of the NR₂ moieties in μ-oxo dimers D3 and D4 is factually valid.

Scheme 1 Synthesis of donor–acceptor-stabilised germaacid chloride 1. Notes: (a) in the alphanumerical numbering pattern, G denotes germylene, and D denotes germanium μ-oxo dimer, and (b) products with a Ge=O → B(C₆F₅)₃/Ge-OTMS → B(C₆F₅)₃ moiety are given a linear/arbitrary numerical numbering pattern (starting from 1).

Scheme 2 Synthesis of donor–acceptor-stabilised germaester 2.

Scheme 3 Attempted synthesis of donor–acceptor-stabilised germaamides that resulted in amine → borane adducts.

Scheme 4 Synthesis of donor–acceptor-stabilised N-germaacyl pyrrole 3.

In all the reactions, germanium μ-oxo dimers D1–D5 were reacted with the Lewis acid B(C₆F₅)₃.¹⁷ To understand the utility of other Lewis acids for the successful conversion of germanium μ-oxo dimers D1, D2, and D5 to the corresponding donor–acceptor-stabilised germaacid chloride, germaester, and N-germaacyl pyrrole, a range of Lewis acids (such as BF₃, GeCl₂, and SnCl₂) were screened. However, all of these reactions were typically unsuccessful until now (see the ESI‡ for details). Surprisingly, the germanium-μ-oxo dimer {(i-Bu)₂ATIGe(i-Pr)(μ-O)}₂ (D) with Ge–i-Pr bonds was insensitive to the nature of the Lewis acid used.¹⁰ Thus, it reacted smoothly with B(C₆F₅)₃, ZnCl₂, SnCl₂, and GeCl₂ to afford the donor–acceptor-stabilised germanones IV, V, VI, and VII, respectively.¹⁰

As the germanium analogues of acid halides, esters, and amides were previously unknown, there has been no reactivity study on them. Therefore, the reactivity of the donor–acceptor-stabilised germaacid chloride 1, germaester 2, and N-germaacyl pyrrole 3 was studied with great interest to understand how these compounds behave chemically. It was found that germaacid chloride 1 can react with various lithium salts and afford clean products. Thus, through reaction of 1 with lithium phenylacetylide in toluene for 12 h, a unique example of a germaynone (i-Bu)₂ATIGe(O)(CCPh) → B(C₆F₅)₃ (4) was obtained (Scheme 5). Notably, until now, there has been no example of a silaynone. Furthermore, this reaction reveals that the chloride attached to the germaacyl moiety can be replaced with other functional groups, a reactivity omnipresent among acid chlorides in organic chemistry. Germaacid chloride 1, a heavier analogue of acid halides, exhibits reactivity similar to that of acid halides and silaacid chloride;^12a therefore, this reactivity of 1 was further exploited. The lithium and potassium salts of triphenylsilanol and t-butanol reacted with 1 to result in germaesters 2 and (i-Bu)₂ATIGe(O)(Ot-Bu) → B(C₆F₅)₃ (5), respectively (Scheme 5), which is another route for the isolation of germaesters in addition to that shown in Scheme 2.

Scheme 5 Reactions of germaacid chloride 1 with various lithium/potassium salts.

In a similar fashion, alternate synthetic protocols can be suggested for N-germaacyl pyrrole 3 and germanones. For example, treatment of 1 with lithium pyrrol-1-ide and phenyl/methyl lithium yielded N-germaacyl pyrrole 3 and the germanones (i-Bu)₂ATIGe(O)(Ph) → B(C₆F₅)₃ (6)/(i-Bu)₂ATIGe(O)(Me) → B(C₆F₅)₃ (7) as products, respectively (Scheme 5). Thus, from germaacid chloride 1, germaesters, N-germaacyl pyrrole, and germanones can be derived without the need to isolate the corresponding germanium-μ-oxo dimers. This route was also attempted for the possible isolation of germaamides, and the reactions of germaacid chloride 1 with the lithium salts PhN(H)Li and PhN(Me)Li were carried out. However, these reactions faced the same fate as that of the abovementioned reactions carried out for the isolation of germaamides (shown in Scheme 3) by yielding amine → borane adducts only.

However, another reaction of germaacid chloride 1 with lithium bis(trimethylsilyl)amide, which aimed again at obtaining the elusive germaamide, occurred differently and resulted in the germaimine (i-Bu)₂ATIGe(NTMS)(OTMS) → B(C₆F₅)₃ (9) in quantitative yield (Scheme 6). This result reveals that the desired germaamide [8] was formed as an intermediate, which then underwent 1,3-silyl migration to form the stable compound 9 (Scheme 6).

Scheme 6 Reaction of germaacid chloride 1 with lithium bis(trimethylsilyl)amide.

Reactivity studies with donor–acceptor-stabilised germaesters 2 and 5 demonstrated that an interconversion between these germaesters and germaacid chloride 1 is achievable. Germaesters 2 and 5 reacted with a slight excess of Me₃SiCl in toluene at room temperature and offered germaacid chloride 1 (Scheme 7). As mentioned above (Scheme 5), reactions of germaacid chloride 1 with one equivalent of LiOSiPh₃ and KOt-Bu in toluene at room temperature generated the germaesters 2 and 5, respectively (Scheme 7). This type of interconversion is not known among the analogous silicon compounds.

Scheme 7 Interconversion between germaesters 2/5 and germaacid chloride 1.

The reactivity studies on N-germaacyl pyrrole 3 demonstrated that the thiophenoxide moiety of thiophenol can substitute the pyrrolide of 3. Accordingly, the reaction of N-germaacyl pyrrole 3 with thiophenol at room temperature in toluene for 6 h resulted in the first example of a germaacyl thioester (i-Bu)₂ATIGe(O)(SPh) → B(C₆F₅)₃ (10) in quantitative yield (Scheme 8).

Scheme 8 Reaction of N-germaacyl pyrrole 3 with thiophenol.

Considering this reaction, the feasibility of substituting the pyrrolide of 3 with hydroxide from a suitable precursor was investigated, as this might lead to the first example of a donor–acceptor-stabilised germacarboxylic acid. However, the reaction of 3 with water in a 1 : 1 molar ratio for 2 h in toluene resulted in [ATIH]⁺[(OH)(B(C₆F₅)₃)]⁻ and not the expected germacarboxylic acid (Scheme S1; see the ESI‡). The commonality in all of the abovementioned reactions of donor–acceptor-stabilised germaacid chloride 1, germaester 2, and N-germaacyl pyrrole 3 is that these reactants undergo nucleophilic substitution in the presence of suitable substrates without any damage to the Ge=O → B(C₆F₅)₃ moiety.

The germanium-μ-oxo dimers D1 and D3–D5, germaacid chloride 1, germaesters 2 and 5, N-germaacyl pyrrole 3, germaynone 4, germanones 6 and 7, and germaacyl thioester 10 are stable at room temperature in an inert atmosphere of dinitrogen. All these compounds are freely soluble in common organic solvents, such as toluene, chloroform, and dichloromethane. Though the germanium-μ-oxo dimers D1–D5 are also freely soluble in tetrahydrofuran, products 1–7 and 10, containing a Ge=O → B(C₆F₅)₃ moiety, decompose even in tetrahydrofuran dried over a potassium mirror to afford [ATIH]⁺[(OH)(B(C₆F₅)₃)]⁻.

Compounds D1, D3–D5, 1–7, and 10 were characterised through multinuclear NMR spectroscopic (¹H, ¹¹B, ¹³C, ¹⁹F, and ²⁹Si) and single-crystal X-ray diffraction studies in the solution and solid states, respectively (see the ESI‡ for details). In the ¹H NMR spectra of D1 and D5, all the resonances are shifted slightly downfield in comparison to those of the precursor molecules, germylene monochloride G1 and N-germylene pyrrole G5, respectively. This shifting is due to the attachment of germanium atoms to electronegative oxygen atoms and the concomitant increase in the formal oxidation state of germanium atoms from +2 to +4. The resonances of the seven-membered ring protons in 1–7 and 10 are shifted downfield in comparison to the corresponding protons in germanium-μ-oxo dimer D1. Owing to the increased electrophilicity of the germanium atom in the Ge=O → B(C₆F₅)₃ moiety (of 1–7 and 10) in comparison to the germanium atoms in the Ge(μ-O)₂Ge moiety of D1, these shifts are expected. In the ¹³C NMR spectra of D1, D3–D5, 1–7, and 10, the expected numbers of signals were observed. In the ¹¹B NMR spectra of 1–6, and 10, singlet resonances at −2.46, −2.61, −2.72, −2.79, −2.44, −3.12, and −2.73 ppm were observed, respectively (Table 1). In comparison, B(C₆F₅)₃ and the donor–acceptor-stabilised germanone (i-Bu)₂ATIGe(O)(i-Pr) → B(C₆F₅)₃ (IV) showed singlet resonances at −2.30 ppm^18,19 and −4.52 ppm,¹⁰ respectively. These data reveal that the resonances in 1–6 and 10 are in between the resonances of B(C₆F₅)₃ and IV. These results suggest that the electron donation by the germaacyl oxygen atom to the boron atom in 1–6, and 10 is reduced relative to that in IV due to the electron-withdrawing effect of the Cl, OSiPh₃, NC₄H₄, CCPh, Ot-Bu, Ph, and SPh atom/group on the germanium atom, respectively (IV has an electron-donating i-Pr group on the germanium atom). The donor–acceptor-stabilised silaaldehyde L′Si(H)=O → B(C₆F₅)₃ (VIII),^12g silaformyl chloride IPr·SiH(Cl)=O → B(C₆F₅)₃ (IX),^12c silaacid anhydride [{PhC(t-BuN)₂}Si{=O→B(C₆F₅)₃}O–Si(H){=O→B(C₆F₅)₃}(Nt-Bu)(HNt-Bu)CPh] (X),^12d monoalumoxane L*Al=O → B(C₆F₅)₃ (XI),²⁰ and boraacid chloride IPr → B(Cl)=O → B(C₆F₅)₃ (XII)²¹ have B(C₆F₅)₃ as the acceptor in the M=O → B(C₆F₅)₃ moiety (M = Si VIII, IX, X; Al XI; B XII) [L′ = HC[CMeN(Ar)]₂ IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, L* = Et₂NCH₂CH₂NC(Me)CHC(Me)NCH₂CH₂NEt₂]. It may therefore be appropriate to compare the boron and fluorine resonances of these compounds with those of 1–6 and 10 (Table 1). These resonances in compounds VIII, IX, X, XI, and XII are shifted upfield with respect to the corresponding resonances of B(C₆F₅)₃ (Table 1), which indicates the shielding of boron and fluorine atoms due to electron donation by oxygen atoms. This result is similar to that observed for compounds 1–6 and 10, containing a Ge=O → B(C₆F₅)₃ moiety (Table 1), but as revealed by the ¹¹B NMR spectral data (Table 1), the magnitude of the shielding in these compounds is lower than that in compounds VIII, IX, X, and XI. In the ²⁹Si NMR spectra of germaester 2, a signal at −13.62 ppm for the SiPh₃ group is shifted downfield in comparison to that in germylene G2 (−24.72 ppm).¹⁵

Table 1 Comparison of the ¹¹B and ¹⁹F NMR spectral resonances of boron and fluorine atoms and the O–B bond distances in compounds 1–6 and 10 with B(C₆F₅)₃ and other related compounds of group 13–14 elements with an M=O → B(C₆F₅)₃ moiety(s) (M = Ge, Si, Al, B)

S. no.	Compound	^11B NMR chemical shift (ppm)	^19F NMR chemical shift (ppm)	O–B bond length (Å)	Reference
a In CDCl3. b In CD2Cl2. c In THF-d8. d In C6D6/THF-d8. e In C6D6.
1	Germanone, (i-Bu)2ATIGe(i-Pr)(O) → B(C6F5)3 (IV)	−4.52^a	(−134, −161, and −166)^a	1.473(4)	10
2	Silaaldehyde, L′Si(H)=O → B(C6F5)3 (VIII)	−4.70^b	(−132, −162, and −165)^b	1.503(3)	12g
3	Silaformyl chloride, IPr·SiH(Cl)=O → B(C6F5)3 (IX)	−5.28^c	(−134, −163, and −168)^c	1.492(3)	12c
4	Silaacid anhydride, [{PhC(t-BuN)2}Si{=O→B(C6F5)3}O–Si(H) {=O→B(C6F5)3}(Nt-Bu)(HNt-Bu)CPh] (X)	(−3.99, and −5.46)^c	(−134, −135, −164, −165, −167, and −168)^c	1.493(3), and 1.488(3)	12d
5	Monoalumoxane, L*Al=O → B(C6F5) (XI)	-4.83^d	(-134, -164, and -166)^d	1.444(3)	20
6	Boraacid chloride, IPr → B(Cl)=O → B(C6F5) (XII)	-2.7^e	(-131, -160, and -165)^e	1.518(3)	21
7	B(C6F5)3	−2.30^a	(−127, −143, and −160)^a	—	19
8	Germaacid chloride, (i-Bu)2ATIGe(O)(Cl) → B(C6F5)3 (1)	−2.46^a	(-133, -159, and -165)^a	1.493(5)	This work
9	Germaester, (i-Bu)2ATIGe(O)(OSiPh3) → B(C6F5)3 (2)	−2.61^a	(-132, -160, and -165)^a	1.497(3)	This work
10	N-Germaacyl pyrrole, (i-Bu)2ATIGe(O)(NC4H4) → B(C6F5)3 (3)	−2.72^a	(−133, −159, and −165)^a	1.494(6)	This work
11	Germaynone, (i-Bu)2ATIGe(O)(CCPh) → B(C6F5)3 (4)	−2.79^a	(−133, −161, and −165)^a	1.489(4)	This work
12	Germaester, (i-Bu)2ATIGe(O)(Ot-Bu) → B(C6F5)3 (5)	−2.44^a	(−132, −160, and −165)^a	1.505(3) and 1.502(3)	This work
13	Germanone, (i-Bu)2ATIGe(O)(Ph) → B(C6F5)3 (6)	−3.12^a	(−133, −160, and −165)^a	1.481(3)	This work
14	Germaacyl thioester, (i-Bu)2ATIGe(O)(SPh) → B(C6F5)3 (10)	−2.73^a	(−133, −160, and −165)^a	1.501(5)	This work

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In a preliminary study of optical properties, the UV-vis spectra of compounds 1, 2, and 10 were recorded in toluene at room temperature. Compounds 1, 2, and 10 showed an absorption maximum in the visible region at approximately 420 nm (Fig. 1). Theoretical studies suggested that these absorptions in compounds 1, 2, and 10 are essentially due to , , and transitions, respectively (Table S1; see the ESI‡ for details). Furthermore, there are two high-energy transitions in each of these compounds with λ_max values of approximately 350 and 285 nm (Fig. 1), which are due to multiple transitions (Table S1; see the ESI‡ for details). The optical properties of compounds with formal M=O → LA moieties (M = Ge, Si) have rarely been studied. For germanone VII with a Ge=O → GeCl₂ moiety, optical properties have been reported. In comparison to compounds 1, 2, and 10, the absorption maximum of VII in the visible region (437 nm) is slightly redshifted, and this absorption is due to a transition. Most likely, a different Lewis acid in compound VII altered the composition of the HOMO.

Fig. 1 UV-vis spectra of compounds 1, 2, and 10 (30 μM solution) in toluene.

The structures of compounds D1, D3–D5, 1–7, 9, and 10 in the solid state were determined by single-crystal X-ray diffraction analysis (Fig. 2–4 and S53–S62, Tables S2–S5, and Experimental section; see the ESI‡).²² Compounds 1–4 and 6 crystallised in the triclinic space group P1̄ (Tables S3 and S4; see the ESI‡). Compounds 5, 7, and 10 crystallised in the monoclinic, orthorhombic, and monoclinic space groups P2₁/n, P2₁2₁2₁, and P2₁/c, respectively (Table S4; see the ESI‡).

Fig. 2 Molecular structure of germaacid chloride 1 with thermal ellipsoids at the 50% probability level. All hydrogen atoms and a solvent molecule (dichloromethane) are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ge1–O1 1.698(2), O1–B1 1.493(5), Ge1–Cl1 2.117(1), Ge1–N1 1.831(3), Ge1–N2 1.846(3); O1–Ge1–N1 111.60(1), O1–Ge1–N2 116.79(1), O1–Ge1–Cl1 112.25(9), B1–O1–Ge1 134.6(2), N2–Ge1–N1 87.46(1), N1–Ge1–Cl1 116.19(1), N2–Ge1–Cl1 110.52(1). Data collection temperature: 100 K.

Fig. 3 Molecular structure of germaynone 4 with thermal ellipsoids at the 50% probability level. All hydrogen atoms and a solvent molecule (dichloromethane) are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ge1–O1 1.708(2), O1–B1 1.489(4), Ge1–C16 1.856(3), Ge1–N1 1.860(2), Ge1–N2 1.845(2); O1–Ge1–N1 114.10(1), O1–Ge1–N2 110.17(1), O1–Ge1–C16 113.63(12), B1–O1–Ge1 131.46(2), N2–Ge1–N1 86.91(1), N1–Ge1–C16 112.42(2), N2–Ge1–C16 116.98(1). Data collection temperature: 100 K.

Fig. 4 Molecular structure of germaacyl thioester 10 with thermal ellipsoids at the 50% probability level. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ge1–O1 1.698(3), O1–B1 1.501(5), Ge1–S1 2.199(2), Ge1–N1 1.864(4), Ge1–N2 1.866(4); O1–Ge1–S1 116.19(1), B1–O1–Ge1 144.0(3), N2–Ge1–N1 85.72(2). Data collection temperature: 100 K.

The molecular structures of compounds 1–7 and 10 [Fig. 2 (1), 3 (4), 4 (10), S57 (2), S58 (3), S59 (5), S60 (6), and S61 (7)‡] confirmed the presence of a (Y)Ge=O → B(C₆F₅)₃ moiety [Y = Cl (1), OSiPh₃ (2), NC₄H₄ (3), CCPh (4), Ot-Bu (5), Ph (6), Me (7), and SPh (10)]. In these compounds, the germanium atom has a distorted tetrahedral geometry with two ATI ligand nitrogens, one germaacyl oxygen, and one Cl (1), O (2), N (3), C (4), O (5), C (6), C (7), or S (10) atom. The average length of the Ge–N_ligand bonds in compounds 1 (1.838 Å), 2 (1.848 Å), and 3 (1.843 Å) is shorter than that in their precursors D1 (1.931 Å), D2 (1.946 Å), and D5 (1.942 Å), respectively. Similarly, the Ge–Y bond in compounds 1 (2.117(1) Å; Y = Cl), 2 (1.719(2) Å; Y = OSiPh₃), and 3 (1.820(4) Å; Y = NC₄H₄) is also shorter than that in compounds D1 (2.20(8) Å), D2 (1.767(3) Å), and D5 (1.892(3) Å), respectively. These differences are due to the electrophilicity of the oxygen atom in the Ge=O → B(C₆F₅)₃ moiety of compounds 1, 2, and 3 being higher than that of the oxygen atoms in the Ge(μ-O)₂Ge moiety of D1, D2, and D5, which makes the germanium atom in the former set of compounds more electrophilic than that in the latter set. Though these effects are observed in germanone IV, in comparison to the electron-donating i-Pr group bound to the germanium atom of germanone IV, the Cl, OSiPh₃, NC₄H₄, CCPh, and SPh atom/group bound to the germanium atom in germaacid chloride 1, germaester 2, N-germaacyl pyrrole 3, germaynone 4, and germaacyl thioester 10, respectively, exert electron-withdrawing (+I) effects and compete for the germanium atom's electron density, thus increasing the interaction between the germanium and oxygen atoms of the Ge=O bond. Therefore, the length of the formal Ge=O bond in compounds 1 (1.698(2) Å), 2 (1.696(2) Å), 3 (1.695(3) Å), 4 (1.708(2) Å), and 10 (1.698(3) Å) is shorter than that in germanones IV (1.718(2) Å), V (1.724(2) and 1.728(2) Å), VI (1.728(5) Å), and VII (1.718(2) Å).¹⁰ These data also reveal that relative to the polarisation of the Ge=O bond in germanone IV,¹⁰ the same bonds in germaacid chloride 1, germaester 2, N-germaacyl pyrrole 3, germaynone 4, and germaacyl thioester 10 are less polarised due to the electron-withdrawing effect of the Cl, OSiPh₃, NC₄H₄, CCPh, and SPh atoms/groups bound to the germanium atom, respectively. A consequence of the increased interaction between the germanium and oxygen atoms of the germaacyl bond in these compounds is the reduced Lewis basicity of the oxygen atom. This result is reflected in the interaction of this oxygen atom with the Lewis acid B(C₆F₅)₃, where the O → B bond in compounds 1 (1.493(5) Å), 2 (1.497(3) Å), 3 (1.494(6) Å), 4 (1.489(4) Å), and 10 (1.501(5) Å) is longer than the corresponding bond in germanone IV (1.473(4) Å).¹⁰ The O → B bond lengths observed in these compounds are similar to those observed in analogous silicon derivatives (VIII 1.503(3), IX 1.492(3), and X 1.493(3) and 1.488(3); M = Si) and boraacid chloride (XII 1.518(3); M = B) with an M=O → B(C₆F₅)₃ bond (Table 1).^{12g,12c,12d,21} However, in the monoalumoxane²⁰XI with an Al=O → B(C₆F₅)₃ bond, the O → B bond is shorter (1.444(3) Å) than those in compounds 1–4, 10, VIII, IX, X, and XII. All the bonding aspects discussed here are supported by theoretical studies (vide infra). Furthermore, the Ge=O bond (vide supra) in compounds 1–4 and 10 is slightly longer than the Ge=O bond in the base-stabilised germanones[L′′L^DGe=O] (L′′ = [CH{(C=CH₂)(CMe)(NAr)2}]; L^D = [{(Me)CN(Me)}₂C] (XIII), [{(Me)CN(i-Pr)}₂C] (XIV), 4-(Me₂N)–C₅H₄N (XV)) without an acceptor at an oxygen atom (1.646(2)-1.672(3) Å)⁸ and shorter than the Ge–O single bonds in germanium-μ-oxo dimers D1, D2, and D5 (1.848(2)–1.787(3) Å).

The nature of the Ge=O bond in compounds 1–3 and 10 was analysed through natural bond orbital (NBO)^23,24 studies, and the details are provided in Table S6 (see the ESI‡). The Ge–O σ-bond in compounds 1 and 10 is formed by the overlap of the sp^2.59 and sp² hybrid orbitals of germanium with the sp^1.62 and sp^2.66 hybrid orbitals of oxygen, respectively (Fig. 5 and Table S6; see the ESI‡). In compounds 2 and 3, the sp^2.53 and sp^2.43 hybrid orbitals of germanium overlap with the sp^2.89 and sp^2.57 hybrid orbitals of oxygen to form the Ge–O bond, respectively (Fig. 5 and Table S6; see the ESI‡). MO calculations also reveal the presence of Ge–O bonds in compounds 1–3 and 10, and these bonds are deeply buried (Figure S63, see the ESI‡).

Fig. 5 NBO calculated Ge–O σ-bond in germaacid chloride 1, N-germaacyl pyrrole 3, and germaacyl thioester 10. The hybridisations of the germanium and oxygen orbitals involved in the overlap are mentioned along with the percentage contributions of the constituent atoms to the Ge–O bond.

NBO second-order perturbation theory analysis reveals that in germaacid chloride 1, the sigma bond between germanium and oxygen is formed by the donation of the lone pair of electrons on the oxygen atom to the σ* orbital of the Ge–Cl bond (Fig. 6a; 79.3 kcal mol⁻¹). The lone pair of electrons on the oxygen atom also interacts with the π* orbitals of the Ge–N_ATI bonds (Fig. 6b; 100.3 kcal mol⁻¹ and Fig. 6c; 52.8 kcal mol⁻¹). However, in addition to these interactions, there are two strong stabilising interactions between the sp^3.82 (Fig. 6d; 44.8 kcal mol⁻¹) and sp^0.29 (Fig. 6e; 43.6 kcal mol⁻¹) orbitals of oxygen and the π* orbital of the Ge–N₄ bond. Compounds 2, 3, and 10, instead of showing the aforementioned n (lone pair of electrons on oxygen) to σ*/π* orbital interactions, showed strong NBO donor–acceptor interactions from the s, p or sp^x orbitals of oxygen atoms to vacant s, p or sp^x orbitals of the germanium atoms [Fig. 6f–h (2), Fig. 6i–l (3), and Fig. 6m–p (10)]. However, in compound 10, a moderately strong NBO donor–acceptor interaction was found between the p orbital of oxygen and the σ* orbital of the Ge–S bond (27.9 kcal mol⁻¹) (Fig. 6q). In comparison, germanone IV showed three σ interactions: two O → Ge interactions and one O → σ*(Ge–C_i-Pr) interaction; these interactions result in a total stabilisation energy of 236.3 kcal mol⁻¹.¹⁰ Thus, the total stabilisation energy due to the donor–acceptor interactions in compounds 1 (320.8 kcal mol⁻¹), 2 (284.7 kcal mol⁻¹), 3 (303.7 kcal mol⁻¹), and 10 (329.2 kcal mol⁻¹) is higher than that in germanone IV, which is due to the difference in the nature of the atoms/moieties bound to germanium atom in these compounds (–Cl, –OSiPh₃, –NC₄H₄, and –SPh, respectively) instead of an i-Pr group. The Wiberg bond index (WBI) calculations for compounds 1, 3, and 10 also showed a slightly increased bond order for the Ge=O bond (0.74–0.76) relative to that in germanone IV (0.70)¹⁰ (Table S6; see the ESI‡). A similar bond order (0.7955) was calculated for silaaldehyde II (with BEt₃ as an acceptor bound to the oxygen atom); for silaacid chloride I and silaester III (without any acceptor bound to the oxygen atom), the calculated WBI values are 1.0993 and 1.0441, respectively.^12a In compounds 1, 2, and 10, the HOMO is localised on the phenyl ring of the B(C₆F₅)₃ moiety (Fig. S64; see the ESI‡), and in compound 3, it is localised on the pyrrole ring, which also reveals the stabilisation of the formal Ge=O bonds in these compounds (Fig. S64; see the ESI‡). Furthermore, NBO donor–acceptor interactions between oxygen and boron atoms can be observed in all these compounds (Fig. S65; see the ESI‡); the stabilisation energies due to these interactions are 280.3 kcal mol⁻¹, 315.6 kcal mol⁻¹, 296.3 kcal mol⁻¹, and 294.6 kcal mol⁻¹ in compounds 1 (Fig. S65a‡), 2 (Fig. S65b‡), 3 (Fig. S65c‡), and 10 (Fig. S65d‡), respectively. All these stabilisation energies are lower than that observed in germanone IV (334.9 kcal mol⁻¹),¹⁰ indicating the reduced electron donation from oxygen atoms to boron atoms in compounds 1–3, and 10.

Fig. 6 Pictorial view of NBO donor–acceptor interactions between p or sp^x (x = 3.82, 0.29) orbitals of oxygen and the σ* orbital of the Ge–Cl bond/π* orbitals of the Ge–N_ATI bonds in compound 1 (a–e), s or p orbitals of oxygen and s or p orbitals of germanium in compound 2 (f–h), p or sp^x (x = 0.32, 4.59) orbitals of oxygen and s or p orbitals of germanium in compound 3 (i–l), s or p orbitals of oxygen and p or sp^1.45 orbitals of germanium in compound 10 (m–p), and p orbital of oxygen and σ* orbital of Ge–S bond in compound 10 (q). Energy values are given in kcal mol⁻¹. Hydrogen atoms are omitted for clarity. The cut–off interaction energies for LP → LP* and LP → BD* are ≥30 kcal mol⁻¹ and 20 kcal mol⁻¹, respectively.

As none of the monoanionic ligands, such as β-diketiminate and amidinate ligands, are known to stabilise compounds with formal Ge=O bonds, it is of interest to examine how the bulky monoanionic aminotroponiminate (ATI) ligand used in the present study helps to stabilise various compounds with formal Ge=O bonds. NBO second-order perturbation theory analysis reveals the existence of donor–acceptor interactions between (a) sp^x orbitals of nitrogen atoms of the ATI ligand to vacant s, p or sp^x orbitals of germanium in compounds 1–3 and 10 (Fig. S66a, b, S67a–d, S68a–d, and S69a–d; see the ESI‡); (b) N_ATI orbitals to the σ* orbital of the Ge–Cl bond in compound 1 (Fig. S66c and d; see the ESI‡) and N_ATI orbitals to the σ* orbital of the Ge–S bond in compound 10 (Fig. S69e and f; see the ESI‡); and (c) s or p orbitals of the chlorine atom to π* orbitals of Ge–N_ATI bonds in compound 1 (Fig. S66e and f; see the ESI‡). Owing to the interactions of types (b) and (c), the energies of the σ* orbital of the Ge–Cl bond in compound 1, π* orbitals of the Ge–N_ATI bonds in compound 1, and the σ* orbital of the Ge–S bond in compound 10 are lower, and these orbitals are available for accepting electrons donated by the O atom of the Ge=O bond. Further, energy decomposition analysis (EDA)²⁵ was performed using {Y–Ge=O → B(C₆F₅)₃} (Y = Cl (1), OSiPh₃ (2), NC₄H₄ (3), SPh (10)) as one fragment and the {ATI} ligand as another fragment with frozen geometries obtained from DFT calculations; the results are summarised in Table S7 (see the ESI‡). The large interaction energy (E_int) observed for these compounds arises essentially due to the favourable ΔE_orb term that describes the stabilising interaction between the ATI ligand and the Y–Ge=O → B(C₆F₅)₃ moiety (Y = Cl (1), OSiPh₃ (2), NC₄H₄ (3), SPh (10)).

Conclusions

Donor–acceptor-stabilised germaacyl chloride (i-Bu)₂ATIGe(O)(Cl) → B(C₆F₅)₃ (1), germaester (i-Bu)₂ATIGe(O)(OSiPh₃) → B(C₆F₅)₃ (2), and N-germaacyl pyrrole (i-Bu)₂ATIGe(O)(NC₄H₄) → B(C₆F₅)₃ (3) compounds were successfully isolated as stable species for the first time. Compounds 1, 2, and 3 can undergo nucleophilic substitution reactions without any disturbance to the Ge=O → B(C₆F₅)₃ moiety to afford germaynone (i-Bu)₂ATIGe(O)(CCPh) → B(C₆F₅)₃ (4), germaester (i-Bu)₂ATIGe(O)(Ot-Bu) → B(C₆F₅)₃ (5), germanone (i-Bu)₂ATIGe(O)(R) → B(C₆F₅)₃ (R = Ph 6, Me 7), and germaacyl thioester (i-Bu)₂ATIGe(O)(SPh) → B(C₆F₅)₃ (10) compounds in good yields. Interestingly, through the reactivity of 1 and 2, the feasibility to interconvert germaesters and germaacid chlorides is exposed. Attempts were also made to synthesise germaamides and germacarboxylic acids, and it is anticipated that the wisdom obtained during these endeavours will offer new directions to the isolation of these compounds as stable species in the near future.

Author contributions

M. K. S. carried out all the experimental studies and drafted the manuscript. S. S. and P. M. helped M. K. S. with some of the experimental studies. The theoretical studies were carried out by G. M., who also wrote the theoretical section of the manuscript. B. P. assisted G. M. with some of the theoretical calculations/write-up. S. N. and G. R. corrected the experimental and theoretical write-ups of the manuscript, respectively.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

M. K. S., P. M. and S. S. thank the Indian Institute of Technology Delhi (IITD), New Delhi, India, and the University Grants Commission (UGC), New Delhi, India, for research fellowships. S. N. thanks the SERB, Department of Science and Technology (DST), New Delhi, India, for funding (EMR/2017/005519) and DST-FIST for establishing single-crystal X-ray diffraction (SR/FST/CSII-027/2014) and HRMS (SR/FST/CS-1-195/2008) facilities in the Department of Chemistry, IIT Delhi.

Notes and references

1. (a) Y. Xiong, S. Yao and M. Driess, Angew. Chem., Int. Ed., 2013, 52, 4302 and references cited therein; (b) M. Asay, C. Jones and M. Driess, Chem. Rev., 2011, 111, 354; (c) C. R. Fischer and P. P. Power, Chem. Rev., 2010, 110, 3877; (d) Y. Mizuhata, T. Sasamori and N. Tokitoh, Chem. Rev., 2009, 109, 3479; (e) S. Nagendran and H. W. Roesky, Organometallics, 2008, 27, 457; (f) R. Okazaki and N. Tokitoh, Acc. Chem. Res., 2000, 33, 625; (g) P. P. Power, Chem. Rev., 1999, 99, 3463; (h) J. Barrau and G. Rima, Coord. Chem. Rev., 1998, 178–180, 593.
2. (a) E. Bonnefille, S. Mazières, C. Bibal, N. Saffon, H. Gornitzka and C. Couret, Eur. J. Inorg. Chem., 2008, 4242; (b) L. Pu, N. J. Hardman and P. P. Power, Organometallics, 2001, 20, 5105; (c) M. Veith and A. Rammo, Z. Anorg. Allg. Chem., 1997, 623, 861; (d) P. Jutzi, H. Schmidt, B. Neumann and H.-G. Stammler, Organometallics, 1996, 15, 741; (e) N. Tokitoh, T. Matsumoto and R. Okazaki, Chem. Lett., 1995, 1087.
3. G. L. Wegner, R. J. F. Berger, A. Schier and H. Schmidbaur, Organometallics, 2001, 20, 418.
4. (a) R. Tacke, C. Kobelt, J. A. Baus, R. Bertermann and C. Burschka, Dalton Trans., 2015, 44, 14959; (b) K. Junold, M. Nutz, J. A. Baus, C. Burschka, C. Fonseca Guerra, F. M. Bickelhaupt and R. Tacke, Chem.–Eur. J., 2014, 20, 9319; (c) R. Azhakar, R. S. Ghadwal, H. W. Roesky, H. Wolf and D. Stalke, Chem. Commun., 2012, 48, 4561; (d) A. Jana, R. Azhakar, S. P. Sarish, P. P. Samuel, H. W. Roesky, C. Schulzke and D. Koley, Eur. J. Inorg. Chem., 2011, 5006; (e) S. S. Sen, G. Tavčar, H. W. Roesky, D. Kratzert, J. Hey and D. Stalke, Organometallics, 2010, 29, 2343.
5. (a) D. Ellis, P. B. Hitchcock and M. F. Lappert, J. Chem. Soc., Dalton Trans., 1992, 3397; (b) H. Wang and Z. Xie, Eur. J. Inorg. Chem., 2017, 4430.
6. T. Chlupatý, Z. Padělková, F. DeProft, R. Willem and A. Růzička, Organometallics, 2012, 31, 2203.
7. For selected references see: (a) I. Alvarado-Beltran, A. Rosas-Sánchez, A. Baceiredo, N. Saffon-Merceron, V. Branchadell and T. Kato, Angew. Chem., Int. Ed., 2017, 56, 10481; (b) M. M. Linden, H. P. Reisenauer, D. Gerbig, M. Karni, A. Schäfer, T. Müller, Y. Apeloig and P. R. Schreiner, Angew. Chem., Int. Ed., 2015, 54, 12404; (c) S. U. Ahmad, T. Szilvási, E. Irran and S. Inoue, J. Am. Chem. Soc., 2015, 137, 5828; (d) T. Muraoka, K. Abe, H. Kimura, Y. Haga, K. Ueno and Y. Sunada, Dalton Trans., 2014, 43, 16610; (e) A. C. Filippou, B. Baars, O. Chernov, Y. N. Lebedev and G. Schnakenburg, Angew. Chem., Int. Ed., 2014, 53, 565; (f) S. S. Sen, Angew. Chem., Int. Ed., 2014, 53, 8820–8822; (g) R. Rodriguez, T. Troadec, D. Gau, N. Saffon-Merceron, D. Hashizume, K. Miqueu, J. Sotiropoulos, A. Baceiredo and T. Kato, Angew. Chem., Int. Ed., 2013, 52, 4426; (h) Y. Xiong, S. Yao and M. Driess, Angew. Chem., Int. Ed., 2013, 52, 4302; (i) T. Muraoka, K. Abe, Y. Haga, T. Nakamura and K. Ueno, J. Am. Chem. Soc., 2011, 133, 15365; (j) Y. Gao, H. Hu and C. Cui, Chem.–Eur. J., 2011, 17, 8803; (k) Y. Xiong, S. Yao and M. Driess, Dalton Trans., 2010, 39, 9282; (l) S. Yao, Y. Xiong and M. Driess, Chem.–Eur. J., 2010, 16, 1281; (m) Y. Xiong, S. Yao, R. Müller, M. Kaupp and M. Driess, Nat. Chem., 2010, 2, 577; (n) J. D. Epping, S. Yao, M. Karni, Y. Apeloig and M. Driess, J. Am. Chem. Soc., 2010, 132, 5443; (o) Y. Xiong, S. Yao and M. Driess, J. Am. Chem. Soc., 2009, 131, 7562.
8. (a) S. Yao, Y. Xiong, W. Wang and M. Driess, Chem.–Eur. J., 2011, 17, 4890; (b) S. Yao, Y. Xiong and M. Driess, Chem. Commun., 2009, 6466.
9. L. Li, T. Fukawa, T. Matsuo, D. Hashizume, H. Fueno, K. Tanaka and K. Tamao, Nat. Chem., 2012, 4, 361.
10. S. Sinhababu, D. Yadav, S. Karwasara, M. K. Sharma, G. Mukherjee, G. Rajaraman and S. Nagendran, Angew. Chem., Int. Ed., 2016, 55, 7742.
11. A. V. Zabula, T. Pape, A. Hepp, F. M. Schappacher, U. C. Rodewald, R. Pöttgen and F. E. Hahn, J. Am. Chem. Soc., 2008, 130, 5648.
12. (a) D. C. H. Do, A. V. Protchenko, M. Ángeles Fuentes, J. Hicks, E. L. Kolychev, P. Vasko and S. Aldridge, Angew. Chem., Int. Ed., 2018, 57, 13907; (b) R. Rodriguez, D. Gau, T. Troadec, N. Saffon-Merceron, V. Branchadell, A. Baceiredo and T. Kato, Angew. Chem., Int. Ed., 2013, 52, 8980; (c) R. S. Ghadwal, R. Azhakar, H. W. Roesky, K. Pröpper, B. Dittrich, C. Goedecke and G. Frenking, Chem. Commun., 2012, 48, 8186; (d) R. S. Ghadwal, R. Azhakar, H. W. Roesky, K. Propper, B. Dittrich, S. Klein and G. Frenking, J. Am. Chem. Soc., 2011, 133, 17552; (e) Y. Xiong, S. Yao, R. Müller, M. Kaupp and M. Driess, J. Am. Chem. Soc., 2010, 132, 6912; (f) Y. Xiong, S. Yao and M. Driess, Angew. Chem., Int. Ed., 2010, 49, 6642; (g) S. Yao, M. Brym, C. Wullen and M. Driess, Angew. Chem., Int. Ed., 2007, 46, 4159; (h) S. Yao, Y. Xiong, M. Brym and M. Driess, J. Am. Chem. Soc., 2007, 129, 7268.
13. (a) S. Sinhababu, R. K. Siwatch, G. Mukherjee, G. Rajaraman and S. Nagendran, Inorg. Chem., 2012, 51, 9240; (b) R. K. Siwatch, S. Kundu, D. Kumar and S. Nagendran, Organometallics, 2011, 30, 1998; (c) H. V. R. Dias, Z. Wang and W. Jin, Coord. Chem. Rev., 1998, 176, 67; (d) H. V. R. Dias and Z. Wang, J. Am. Chem. Soc., 1997, 119, 4650.
14. (a) D. Yang, J. Guo, H. Wu, Y. Ding and W. Zheng, Dalton Trans., 2012, 41, 2187; (b) S. M. I. Al-Rafia, P. A. Lummis, M. J. Ferguson, R. McDonald and E. Rivard, Inorg. Chem., 2010, 49, 9709; (c) M. Veith, S. Becker and V. Huch, Angew. Chem., Int. Ed., 1989, 28, 1237 ( Angew. Chem. , 1989 , 101 , 1287 ).
15. S. Karwasara, R. K. Siwatch, C. K. Jha and S. Nagendran, Organometallics, 2015, 34, 3246.
16. S. Karwasara, M. K. Sharma, R. Tripathi and S. Nagendran, Organometallics, 2013, 32, 3830.
17. (a) M. A. Beckett, D. S. Brassington, S. J. Coles and M. B. Hursthouse, Inorg. Chem. Commun., 2000, 3(10), 530; (b) M. A. Beckett and G. C. Strickland, Polymer, 1996, 37, 4629.
18. M. Hoshi, K. Shirakawa and M. Okimoto, Tetrahedron Lett., 2007, 48, 8475.
19. A. Bähr, L. C. Wilkins, K. Ollegott, B. M. Kariuki and R. L. Melen, Molecules, 2015, 20, 4530.
20. D. Neculai, H. W. Roesky, A. M. Neculai, J. Magull, B. Walfort and D. Stalke, Angew. Chem., Int. Ed., 2002, 41, 4294.
21. A. K. Swarnakar, C. Hering-Junghans, M. J. Ferguson, R. McDonald and E. Rivard, Chem.–Eur. J., 2017, 23, 8628.
22. CCDC 1564828–1564834, 1564836, and 1851011–1851015 contains the crystallographic data for this paper.‡.
23. F. Wheinhold and C. Landis, Valency and Bonding, Cambridge, 2005.
24. (a) A. E. Reed, L. A. Curtiss and F. Weinhold, Chem. Rev., 1988, 88, 899; (b) E. D. Glendening, A. E. Reed, J. E. Carpenter and F. Weinhold, NBO Version 3.1.
25. (a) S. I. Gorelsky, AOMix: Program for Molecular Orbital Analysis, version 6.6, University of Ottawa, Ottawa, 2010, http://www.sg-chem.net/; (b) S. I. Gorelsky, S. Ghosh and E. I. Solomon, J. Am. Chem. Soc., 2006, 128, 278; (c) T. Ziegler and A. Rauk, Theor. Chem. Acc., 1977, 46, 1; (d) K. Kitaura and K. Morokuma, Int. J. Quantum Chem., 1976, 10, 325.

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Footnotes

† Dedicated to Prof. V. Chandrasekhar on the occasion of his 60^th birthday.

‡ Electronic supplementary information (ESI) available: Experimental section, UV-vis spectra of compounds 1, 2, and 10; molecular structure determination of compounds D1, D3–D5, 1–7, 9, and 10; computational details (PDF). CIFs for compounds D1, D3–D5, 1–7, 9, and 10, are deposited with the Cambridge Structural Database (CSD). CCDC 1564828–1564834, 1564836, and 1851011–1851015. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8sc05380d

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