Iminoborylene complexes: evaluation of synthetic routes towards BN-allenylidenes and unexpected reactivity towards carbodiimides †‡

The synthetic and reaction chemistries of cationic iminoborylene complexes [L n M v B v N v CR 2 ] + , which feature a unique heterocumulene structure, have been systematically investigated. Precursors of the type CpFe(CO) 2 B(Cl)NCAr 2 (Ar = p -Tol/Mes, 5c / d ) have been generated by B-centred substitution chemistry using CpFe(CO) 2 BCl 2 and suitable lithiated ketimines – a reaction which is found to be highly sensitive to the steric bulk at both the metal fragment and the ketimino group. Carbonyl/phosphine exchange (using PCy 3 or PPh 3 ), followed by halide abstraction allows for the generation of the cationic iminoborylenes [CpFe(PR 3 )(CO)(BNCAr 2 )] + [BAr X4 ] − (R = Cy, Ar = p -Tol/Mes, 12c / d ; R = Ph, Ar = Mes, 13d ; Ar X = 3,5-X 2 C 6 H 3 where X = Cl, CF 3 ) which have been characterized spectroscopically and by X-ray crystallography. The reactivity of these iminoborylene systems towards a range of nucleophiles and unsaturated substrates has been investigated. The latter includes the ﬁ rst examples of M v B metathesis reactivity with a carbodi-imide, and results in Fe v B cleavage and formation of the isonitrile complexes [CpFe(PCy 3 )(CO)(CNR)] + - [BAr Cl4 ] − (R = i Pr/Cy, 16/


Introduction
The investigation of boron-transition metal complexes has attracted widespread attention in recent years.A number of novel classes of compounds featuring conventional 2-centre 2-electron metal-boron bonds have been studied, not only with respect to their structural and bonding properties, but also with a view to targeting new modes of reaction chemistry. 1Within this area, boryl complexes, L n M(BX 2 ), featuring a disubstituted boron-fragment coordinated at M were the first to be discovered, 2 and have subsequently been implicated in a number of unprecedented transformations, such as the borylation of unactivated hydrocarbon substrates. 3ore recently, reliable synthetic routes to subvalent transition metal borylene complexes, (L n M) x (BX), have also been developed. 4These species feature a mono-substituted boron fragment, and are of particular interest due to their close relationship with archetypal organometallic complexes. 5Along these lines, fluoroborylene (L n MBF) and aminoborylene (L n MBNR 2 ) species have been have been synthesized, representing isolobal analogues of classical carbonyl (L n MCO) 6 and vinylidene (L n MCCR 2 ) complexes. 7,8eactivity-wise the chemistry of many borylene complexes is dominated by the electrophilicity of the boron centre, which underpins their use in C-H activation 9 or cycloaddition reactions.7a,10 One possibility, with precedent in organometallic systems, to further broaden the scope of reactivity of transition-metal boron complexes is by the introduction of further elements of unsaturation into the boron ligand.Thus, for example, Braunschweig and co-workers have achieved this by use of boryl ligands containing B-X double or triple bonds (X = NR, O or CR 2 , Scheme 1). 11Taking this idea further, we have recently communicated 12 the first examples of cationic iminoborylene complexes [L n MvBvNvCR 2 ] + featuring an extended array of unsaturated bonds (Scheme 1). 13Such complexes can be viewed as hetero-analogues of well-known allenylidene complexes, 14 which show a highly versatile reaction chemistry resulting from their dual α, γ-electrophilicity and β-nucleophilicity.With this in mind, we set out to uncover new patterns of reactivity for iminoborylene complexes which are otherwise inaccessible to known alkyl-or aminoborylene systems. 4,7erein, we now report in full on synthetic approaches towards iminoborylene systems, and their reaction chemistry both with respect to anionic nucleophiles and unsaturated substrates.A key finding is the discovery of novel metathesistype reactivity towards carbodiimides, RNCNR.

Results and discussion
The synthesis of terminal borylene complexes has been achieved using a variety of different approaches, including double salt elimination, 4f,g metal-to-metal borylene transfer, 15 and dehydrogenation of σ-borane complexes. 16Moreover, halide abstraction from haloboryl complexes has been shown to give access to cationic borylenes in a reliable fashion. 17ased on this approach, we envisaged the use of suitable imino-functionalized haloboryl complexes as precursors, which upon halide abstraction with sodium tetra-arylborates would give the desired cationic iminoborylenes (Scheme 2).

Synthesis of iminoboryl complexes
In order to put our synthetic efforts towards imino-substituted systems on a comparable basis to known complexes, we initially decided to target the [CpFe(CO) 2 ] unit as the metal fragment, given its successful use for the generation of related cationic aminoborylenes.7a,b For the construction of precursors featuring the necessary array of consecutive Fe-B-N-C bonds, we evaluated two synthetic approaches, differing in the order of formation of the relevant bonds to the boron centre (Scheme 3).
Mirroring existing synthetic routes to [CpFe(CO) 2 ] boryl complexes, 17c we initially attempted the generation of complexes of type 5 by reaction of the anionic [CpFe(CO) 2 ] − reagent 1 (as the sodium salt) with the corresponding dichloro-(imino)boranes 2, thus establishing the B-N connectivity prior to the formation of the Fe-B bond (Scheme 3, upper).While Cl 2 B(NvCPh 2 ), 2a, was readily synthesized according to Wade's original procedure, 18 it showed no reactivity towards ferrate 1.Assuming that the dimeric nature of 2a (indicated by its 11 B NMR shift of δ B = −7 ppm) is responsible for its low reactivity, we attempted to generate monomeric dichloro-(imino)boranes by the use of bulkier ketimino substituents (e.g.R = Mes or Trip).These syntheses were initially frustrated by a ligand redistribution reaction which apparently occurs on exposure to continuous vacuum [yielding ClB-(NvCR 2 ) 2 ], and which prevents isolation of the pure dichloro (ketimino)-boranes. 19This problem could be circumvented by in situ generation (see ESI ‡), which generates the corresponding monomeric compounds Cl 2 B(NvCR 2 ) (R = Mes/Trip (2d/e), δ B = 26/27 ppm).However, these systems do not show clean reactivity towards 1, with the starting borane being the predominant species in the reaction mixtures even under forcing conditions.
For this reason, we shifted our synthetic strategy towards a reversed order of bond formation reactions at boron, employing the known reaction of 1 with BCl 3 to generate the iron dichloroboryl complex 3 in situ (δ B = 91 ppm). 20Complex 3 was then treated with a series of ketiminolithium reagents LiNvCR 2 [R = t Bu/Ph/p-Tol/Mes/Trip (4a-e)], 21 to install the B-N linkage (Scheme 3, lower).Accordingly, the reactions with less bulky lithium salts (e.g.4a-d) lead to clean formation of the desired iminoboryl complexes 5a-d (as judged by 1 H and 11 B NMR spectroscopy), which could be purified by precipitation from hexane in case of the p-tolyl-and mesityl-substituted complexes (5c/d, 38-52%); the high solubility of complexes 5a/b, on the other hand, prevented their isolation as pure compounds.By contrast, the reaction of the lithium salt LiNvCTrip 2 (4e) with 3 gives a different type of boron-containing product, with the high field 11 B chemical shift (δ B = 27 ppm) arguing against Fe-B bond formation.The product is tentatively assigned as borane 2e, resulting from the nucleophilic displacement of the [CpFe(CO) 2 ] − anion (rather than chloride) from precursor 3.Such a transformation has recent precedent, 22 and is presumably induced by the large steric bulk of the bis(triisopropylphenyl)ketimino group.
Complexes Having established a viable synthetic route for the generation of complexes of type 5 by boron-centred substitution chemistry, and with the steric constraints of the ketimino nucleophile now apparent, we set out to investigate the scope of this approach by variation of the metal fragment.Thus, we generated the previously described tungsten dichloroboryl complex 6 (δ B = 91 ppm) alongside its bromo analogue 7 (δ B = 84 ppm) by reaction of the tungstate Na[CpW(CO 3 )] with the respective trihaloboranes. 23lthough 6 has previously been reported by Schmid and Nöth, it has not been structurally characterized, and given the dearth of structural data available for dihaloboryl systems we sought to investigate it crystallographically.Accordingly, the solid-state structure of 6 (Fig. 1) features a W-B bond [2.22(2)Å] which is considerably longer than in the corresponding CpFe(CO) 2 BCl 2 complex 3 [1.942(3)Å] (even taking into account the larger van der Waals radius of tungsten vs. iron: 2.10 vs. 2.05 Å), 20a,24 while the B-Cl bonds are in the expected range [e.g.1.78(1) and 1.79(1) Å for 6, cf.1.781( 6) and 1.783(4) Å for 3].20a Due to the presence of three carbonyl co-ligands, complex 6 is sterically rather congested when compared to 3, as can be seen from the close B-CO contacts [B(13)-C( 11 While 6 could be structurally characterized, its reactivityin terms of boron-centred substitution processesproves to be much less facile than the corresponding chemistry for 3. Thus, in contrast to the clean reactivity observed in the iron case, no M-B containing products could be observed upon reaction of the representative ketiminolithium salts 4a/d with either of the dihaloboryl-tungsten complexes 6 or 7.As judged by 11  These results further suggest that the boron-centred substitution reaction using a metal dihaloboryl complex is very sensitive to the steric bulk of the substituents both on the metal fragment and on the incoming nucleophile, with the partnership of the less sterically demanding iron boryl complex 3 and the less bulky iminolithium salts 4a-d uniquely bringing about substitution at boron without breakage of the metalboron bond.

Synthesis of iminoborylene complexes
With the iminoboryl-complexes 5c/d in hand, we next attempted the synthesis of the corresponding borylene complexes by halide abstraction.Reaction of 5d with Na[BAr f 4 ] [Ar f = 3,5-(CF 3 ) 2 C 6 H 3 ] leads to the formation of the corresponding cationic borylene [CpFe(CO 2 )(BNCMes 2 )] + , as indicated by a downfield shift in the 11 B NMR signal (δ B = 75 ppm, cf.47 ppm for 5d).While this borylene complex could be shown to be stable at −30 °C in solution over a period of several days, it decomposes rapidly at room temperature.This led us to investigate the use of more electron-rich metal fragments in order to generate borylene species stabilized by more efficient M → B π-backbonding.Thus, we attempted the photolytic displacement of the π-acidic carbonyl-ligands in 5c/d by strong σ-donor phosphine ligands.While attempts to substitute both carbonyl ligands by reaction with chelating bis-phosphines (dppe/dmpe for example) failed to yield the desired products, 25 reaction of 5c/d with monodentate donors cleanly gave the corresponding mixed phosphine/carbonyl complexes (Scheme 5). 26Assuming that bulky trialkylphosphines would lead to an additional kinetic stabilization of the corresponding borylene complexes, we first used PCy 3 in this substitution chemistry, leading to the formation of the desired complexes 10c/d in moderate yields (48-60%).In order to further investi-gate the influence of the steric/electronic properties of the phosphine ligands, we also employed PPh 3 in the reaction with 5d, giving the triphenylphosphine-substituted boryl complex 11d (43%).
Spectroscopic characterization of 10c/d and 11d clearly signals the successful introduction of the phosphine coligands via 31 P NMR spectroscopy (δ P = 77.1/75.0/78.8ppm for 10c/10d/11d), while little change is observed in the respective 11 B spectra (δ B = 47/50/51 ppm).In addition, diastereotopic splitting is observed for the aryl substituents of the axially prochiral ketimino fragments, brought about by the formation of a chiral metal centre (e.g.p-CH 3 groups in 10c/10d/11c: δ H = 2.09, 2.06/2.11,2.09/2.12, 2. In comparison with their dicarbonyl-supported analogues, these complexes are more stable at room temperature, at least when handled under inert atmosphere conditions.Borylene formation can be followed by the downfield shifts in the respective 11 B signals (δ B = 82/85/85 ppm for 12c/12d/13d), while the shifts of the 31 P resonances are less informative (δ P = 84.9/75.0/69.3ppm).In the 1 H and 13 C NMR spectra, the two sets of distinct signals for the ketimino aryl substituents merge to give a single set of resonances, indicating fast rotation of the BNCAr 2 unit (e.g.δ H = 2.48/2.31/2.34ppm for the p-CH 3 signal in 12c/12d/13c), which is not frozen out even at low temperatures (down to −75 °C).The IR spectra of these new compounds are also informative.These feature not only a BNC stretch consistent with the analogous mode observed for allenylidenes [ν(BNC) = 1763/1753/1779 cm −1 ], but also blueshifted carbonyl stretching frequencies in comparison with their chloroboryl precursors [ν(CO) = 1962/1969/1984 cm −1 for 12c/12d/13c] consistent with weaker Fe → CO π-backbonding in the cationic systems.

Reactivity of the iminoborylene complexes
With the crystallographic and spectroscopic analysis of iminoborylene complex 12d hinting at a partial contribution from a carbo-cationic resonance form, we set out to determine experimentally whether selectivity for nucleophilic addition at either the αor γ-position would be observed.With this in mind, we further sought to compare the addition chemistry of both the mesityl-and p-tolyl substituted systems (12c/d, Scheme 6) in order to investigate the influence of the steric loading at the ketimino group.
In the first instance, we investigated whether reactions with a chloride source (e.The situation is slightly different, however, when using cyanide (KCN, 18-crown-6) as a nucleophile.In this case, boryl precursors 10c/d are completely resistant towards substitution at boron, so we investigated the reactivity of the corresponding borylenes 12c/d towards CN − .On mixing KCN and 18-crown-6 with 12c/d generated in situ by the reaction of 10c/d with Na-[BAr Cl 4 ], re-formation of the chloroboryls 10c/d is observed.This suggests that in the presence of NaCl (from the initial salt metathesis) and KCN, in conjunction with 18-crown-6 as a solubilizing agent, the addition of chloride is preferred over the addition of cyanide.Presumably such an observation reflects thermodynamic control due the more favourable B-Cl bond enthalpy (ca.128 vs. 107 kcal mol −1 ). 28The reaction of the pure complex 12d with KCN does, however, lead to addition of cyanide to the borylene.Once again, α-selectivity is observed, yielding the corresponding cyano-substituted borylcomplex 15d.Unfortunately, reaction of KCN with the less sterically encumbered borylene 12c gives only decomposition products, so that the influence of the aryl substituents on the regio-selectivity could not be fully investigated in this case.
Complexes 14c/d and 15d were fully characterized by spectroscopic, mass spectrometric and, in case of 14d, by crystallographic methods. 12The 11 B and 31 P resonances (δ B = 56/52/ 41 ppm, δ P = 76.3/74.1/73.3ppm for 14c/14d/15d) are similar to those of the corresponding chloroboryl complexes (δ B = 47/ 50 ppm, δ P = 77.1/75.0ppm for 10c/10d), which together with the CvN ketimino-resonances (δ C = 147.7/149.9/150.5 ppm) verify the postulated structures resulting from α-attack at boron.The observed high α-selectivity is presumably brought about by the high electrophilicity of the boron centre in each case, bearing in mind the fact that γ-selectivity has been observed in the addition of a variety of nucleophiles (including thiolate and cyanide) to cationic allenylidene complexes. 14,29oping to uncover more diverse patterns of reactivity, we targeted a study of the reactivity of the iminoborylenes towards unsaturated substrates.It has been shown that neutral borylene complexes undergo borylene transfer reactions with alkynes, 10 insertion reactions with isonitriles and carbodiimides, 30 and metathesis-type reactions with ketones, 30 while cationic borylenes oftentimes show contrasting reactivity, displaying hydride transfer reactivity towards ketones, 31 insertion reactions with carbodiimides, 32 and metathesis-type reactivity with isocyanates and phosphine sulfides.7b In order to investigate the reactivity of our iminoborylene complexes towards unsaturated substrates, we used the mesityl-substituted complex 12d which shows the highest resistance towards undesired hydrolysis and decomposition reactions.Mixing of 12d with non-polar substrates such as 2,3dimethyl-butadiene and trimethylsilyl-acetylene in dichloromethane leads to no conversion, even at 40 °C, and over prolonged periods of time.While this result is consistent with the fact that other cationic borylenes show little affinity for alkenes or alkynes, we were surprised to find that mixing of 12d with isopropyl-isocyanate also did not lead to any conversion (as judged from in situ 1 H and 11 B NMR measurements).This contrasts with the chemistry of cationic aminoborylenes, which react with isocyanates, RNCO, cleanly and under mild conditions to give the corresponding isonitrile complexes [CpFe(CO) 2 (NCR)] + via a metathesis-type reaction. 26y contrast, the reaction of 12d with an excess of either diisopropyl-or dicyclohexylcarbodiimide (RNvCvNR, R = i Pr/Cy) gives clean conversion within hours at room temperature, to a single 31 P containing species (δ P = 76.4/76.5 ppm, respectively) and a compound giving rise to a 11 B signal at δ B = 29/30 ppm.Rather than the carbodiimide insertion products found for related aminoborylene complexes 30,32 and organic boranes, 33 in situ spectroscopic analysis of the reaction mixture in this case supports an alternative pathway.Thus, as opposed to a characteristic low-field carbene 13  In case of 16, we were also able to isolate the metal-containing species by crystallization and unambiguously confirm its structure by X-ray crystallography (Fig. 3).In the solid state, complex 16 shows a piano-stool geometry, with the isonitrile unit featuring a linear geometry [∠ C( 26 This chemistry represents, to our knowledge, the first example of metathesis-type reactivity of a borylene complex towards a carbodiimide, and we therefore performed further investigations in order to better understand the reaction mechanism and to probe the fate of the boron-containing [BvNvCMes 2 ] heterocumulene fragment. Upon mixing of the 12d with the respective carbodiimide RNvCvNR at −60 °C in CD 2 Cl 2 , we observe the immediate formation of an intermediate (18/19 for R = i Pr/Cy), which is stable at temperatures below 0 °C.Accordingly, we were able to characterize these species by multinuclear NMR spectroscopy.
In the 1 H and 13 C NMR spectra we observe a splitting of the resonances for the ketimino aryl substituents (e.g.mesityl p-CH 3 in 18/19: δ H = 2.30, 2.25/2.31,2.25 ppm), as is also seen for the boryl precursor 10d.In addition, we also observe two sets of resonances for the carbodiimide i Pr/Cy substituents (e.g. for the N-CHR 2 protons in 18/19: δ H = 3.73, 3.01/3.24,2.54 ppm), consistent with desymmetrization of the RNCNR unit.These spectroscopic features are consistent with the formation of either a Lewis acid-base adduct between the electrophilic boron and one of the carbodiimide nitrogens, 32a,b,33b or with the formation of a [2 + 2]-cycloaddition product, 30a both of which have been observed as intermediates in the reactions of carbodiimides with borylene complexes.
Somewhat unexpectedly, the 11   Finally, we sought to establish the fate of the boron-containing fragment in the final reaction mixture.When the reaction is performed with a stoichiometric amount of either carbodiimide, the 1 H NMR spectra show the isonitrile complexes 16/17, together with a number of products containing mesityl-or isopropyl/cyclohexyl groups, respectively.Only in the presence of an excess of carbodiimide, could well-defined boron-containing products be isolated.

Conclusions
Our investigation of the possible synthetic routes to iminoborylene complexes (12/13) has given insight into the scope of metal-fragments, ketimino substituents and ancillary ligands which allow for successful formation of the desired cationic heterocumulenes.For the synthesis of the iminoboryl-precursors, it is found that an optimal level of overall steric bulk, in combination with the correct order of bond formation (Fe-B prior to B-N bond formation), is required for the generation of the boryl complexes CpFe(CO) 2 {B(Cl)NCAr 2 } (Ar = p-Tol/Mes, 5c/d).The use of reagents with increased steric bulk on either the metal [CpW(CO) 3 vs.CpFe(CO) 2 ] or the ketimino side (Trip vs. Mes) leads primarily to products resulting from M-B bond breakage, illustrating the sensitivity of the boron-centred substitution reaction to steric factors.
While direct halide abstraction from complexes 5c/d leads to thermally unstable borylene species, the substitution of one carbonyl ligand for a tertiary phosphine drastically increases complex stability, leading to the isolation of the cationic heteroallenylidenes [CpFe(PR 3 )(CO)(BNCAr 2 )] + as borate salts (12c/d, 13d).The reactivity of these complexes towards nucleophilic substrates is dominated by the high electrophilicity of the boron centre, leading exclusively to α-attack, while the reactivity towards unsaturated substrates leads to unprecedented transformations.While no reactivity is observed towards isocyanates, we observe clean metathesis-type reactivity with carbodiimides.This contrasts with the insertion-type reactivity of closely related amino-and alkylborylene complexes towards the same substrates.Spectroscopic analysis of the reaction mixtures leads to identification of the boron-containing reaction products as the coordinatively trapped heteroallenes (20/21), with the metal-containing products being unambiguously identified as the isonitrile complexes (16/17).This reactivity is unprecedented and represents the first example of a productive metathesis-type reaction of a borylene compound with a carbodiimide.
g. [PPh 4 ]Cl) could be used to generate products of the type CpFe(CO)(PCy 3 ){BNC(Cl)Mes 2 }, thus allowing a formal α, γ-isomerization of the precursors 10c/d via iminoborylene intermediates (i.e. a formal reversal of the conversion of [L n MvCvC-CR 2 OH] + to [L n MvC(OH)CvCR 2 ] + via the corresponding allenylidene 27 ).However, exclusive α-attack led to the re-formation of the precursors 10c/d.In similar fashion, the reaction of 12c/d with sodium thiophenolate leads to the products of boron-centred nucleophilic attack, exclusively giving the B(SPh) complexes 14, independent of the steric bulk at the ketimino group.The syntheses of the thiolate-functionalized boryl complexes 14c/d could also be achieved directly by reaction of 10c/d with NaSPh in a boron-centred substitution reaction, thus providing independent verification of compound identity.
The 11 B resonances (δ B = 29/30 ppm for R = i Pr/Cy) indicate a three-coordinate boron centre without any metal-boron interaction, while the 1 H NMR spectra show the presence of three inequivalent i Pror Cy-groups [e.g.CHR 2 -units: δ H = 3.90/3.39/3.61for i Pr-(I/II/ III), 3.04/2.96/3.45 for Cy-(I/II/III), (for numbering see Scheme 8)].The 13 C-NMR and GHMBC-data indicate that all three alkylamino-substituents are bound to a central quaternary carbon (δ C = 152.4/155.9 ppm), with two of the alkylgroups (I and III, respectively) being in close proximity as seen from NOE difference spectra.Taken together, these observations suggest that in the presence of excess carbodiimide, R II NvCvNR III , coordinative trapping of the initial metathesis product [R I NvBvNvCMes 2 ] leads to the formation of a trialkyl-guanidinate, which is bound to the BvNvCMes heterocumulene fragment.The resulting triaminoboranes of the type RNvC(NR) 2 BNCMes 2 [with R = i Pr (20) and R = Cy (21), Scheme 8] thus resemble the metalla-amidinates [CpFe-(CO) 2 {C(NCy) 2 BNCy 2 }] + formed by mono-carbodiimide insertion in the case of aminoborylene systems. 32

Scheme 8
Scheme 8 Pathway of the reaction of borylene complex 12d with carbodiimides.Formation of the intermediates 18/19 and subsequent reaction to give the final products 16/17 and 20/21 (tentative structures).