Group 11 Borataalkene Complexes: Models for Alkene Activation

Abstract A series of linear late transition metal (M=Cu, Ag, Au and Zn) complexes featuring a side‐on [B=C]− containing ligand have been isolated and characterised. The [B=C]− moiety is isoelectronic with the C=C system of an alkene. Comparison across the series shows that in the solid‐state, deviation between the η2 and η1 coordination mode occurs. A related zinc complex containing two [B=C]− ligands was prepared as a further point of comparison for the η1 coordination mode. The bonding in these new complexes has been interrogated by computational techniques (QTAIM, NBO, ETS‐NOCV) and rationalised in terms of the Dewar–Chatt–Duncanson model. The combined structural and computational data provide unique insight into catalytically relevant linear d10 complexes of Cu, Ag and Au. Slippage is proposed to play a key role in catalytic reactions of alkenes through disruption and polarisation of the π‐system. Through the preparation and analysis of a consistent series of group 11 complexes, we show that variation of the metal can impact the coordination mode and hence substrate activation.


Introduction
Alkene complexes of transition metals were among the first organometallic compounds reported. [1] Coordination of alkene ligands typically occurs through as ymmetric h 2coordination mode involving as ide-on approach to metal. Alkene binding is synonymous with the activation of this substrate during catalytic processes. [2][3][4][5][6] Forexample,linear d 10 of Cu I ,A g I and Au I complexes have been reported for aw ealth of synthetic transformations;i ncluding ring-expansions, [7] cyclisations, [8,9] hydroarylation, [10][11][12] hydroamination, [13,14] hydroalkyoxylation, [15] and carbonylation reactions. [16] These reactions are believed to share ac ommon mechanistic step in which binding of the alkene to the metal facilitates attack by anucleophile. [8,[17][18][19][20] An earlier theoretical analysis suggested that symmetrically bound alkene complexes are actually deactivated toward external nucleophiles and that slippage from an h 2 to h 1 coordination mode is crucial to activate the C = Cbond and facilitate orbital overlap in the transition state for nucleophilic attack. [21] Due to their relevance as potential catalytic intermediates,an umber of linear Au I alkene complexes have been isolated and structurally characterised, [22][23][24][25][26][27][28][29][30][31][32][33] as have related three-coordinate trigonal planar species. [34] In contrast, examples of crystallographically characterised linear Cu I and Ag I alkene complexes are extremely rare (Figure 1). [35,36] The paucity of data has meant that the comparison of bonding in ac omplete series of alkene complexes of the group 11 triad has not been possible.There is limited experimental evidence to show how modulation of the metal influences alkene binding,i ncluding the ability to adopt h 2 or h 1 coordination modes believed to be so important in key bond making steps during catalysis.
In this paper,weconsider the isoelectronic substitution of the C = Cb ond with a[ B = C] À bond and the binding of this fragment to as eries of group 11 and 12 metals.T he [B=C] À moiety is found within borataalkene compounds of the form [R 2 B=CR 2 ] À .T he synthesis of borataalkenes has been achieved by an umber of methods,t he principal approach involving the deprotonation of the parent boranes [R 2 B À CR 2 H]. [37][38][39][40] Crystallographically characterised examples of borataalkenes were first reported by Power and co-workers in the late 1980s,s hort B=Cb ond lengths of 1.4-1.5 were taken as an indication of double character. [41,42] More recently an umber of structurally characterised borataalkane compounds has been reported. [43][44][45][46] Coordination of [B = C] À moieties to transition metals is limited. Pioneering examples include those of Nçth and co-workers who described the coordination of af luorenylidene borane to transition metal carbonyl complexes, [47][48][49][50] and those of Piers and co-workers who reported methylidene borane complexes of tantalum and titanium along with their alkene-like reactivity. [51][52][53][54][55][56][57] Related coordination complexes of alkylidene boranes have also been reported. [58] To the best of our knowledge,g roup 11 complexes of borataalkene ligands are limited to asingle example involving coordination of a9 -borataphenanthrene anion to Au I . [59] Recently Braunschweig and co-workers reported the lithium boryl compound [cAAC·BH 2 ]Li (1)w hich can be isolated and is stable under ambient conditions. [60][61][62] An underappreciated characteristic of the anionic component [cAAC·BH 2 ] À (A)i si ts potential for B = C p-bonding and borataalkene character ( Figure 2). DFT calculations on this species (wB97xD/6-31G**/SDDAll) demonstrate that the B=Cb ond length (1.45 )i sc onsistent with its assignment as ab orataalkene.N atural bond orbital (NBO) calculations reveal that this is apolarised p-system. TheB = CWiberg bond index (WBI) is 1.64 and natural charges show localisation of more charge on the carbon atom (À0.32) relative to boron (À0.15). Key natural bond orbitals of the p-system are visualised in Figure 2. Despite the polarisation of the psystem, it should be noted that the contributions to the NBOs are relatively even:6 6% pAOc arbon;3 4% pAOb oron (bonding NBO) and 34 %p AO carbon;66 %p AO boron (anti-bonding NBO).
Herein we describe the side-on coordination of the [B = C] À bond of [(cAAC)BH 2 ] À to linear d 10 metal fragments (M = Cu, Ag, Au and Zn). Thebonding has been interrogated by acombination of experimental ( 1 H, 11 B, 13 CNMR and IR spectroscopy) and computational techniques (QTAIM, NBO, ETS-NOCV). Comparison across the series of group 11 complexes shows that aspectrum of coordination between h 2 and h 1 is observed;w ith h 1 coordination becoming more favorable for Au > Ag > Cu. Thed ata potentially provide new insight into catalytically relevant isoelectronic alkene complexes of Cu, Ag and Au.
These measurements along with the close approach of the metal to the carbenic carbon (2.411(2)-2.68(1) )s trongly suggest the borataalkene approaches h 2 coordination. The coordination mode can also be inspected by the displacement of the metal centre along the BÀCa xis (Figure 3b). For classical h 2 -adducts (e.g. symmetric alkene-metal complexes), the metal is located at the mid-point along the C = Cbond axis (d/r = 0.5), whereas an h 1 -coordination mode (e.g. alkyl-metal complex) would result in the metal being found outside the CÀCb ond (d/r < 0). Thes tructures of 3a and 3b show the metal centre is displaced 0.29(5) and 0.16(3) from boron, respectively.Conversely,in3c the Au atom is only displaced 0.04(3) from B, implying the coordination is dominated by the B À Au interaction and an h 1 coordination mode.
Thezinc analogue 4 provides afurther benchmark for the h 1 coordination geometry.A sapost transition metal, the filled d-orbitals of zinc are low in energy and expected to play av ery limited role in bonding.T he structure of 4 was found have C 2 symmetry about an axis that passes through Zn1 and bisects the B1 À Zn1 À B1A angle (Figure 4). In the solid-state, 4 contains an ear linear two-coordinate Zn centre with a B À Zn À Ba ngle of 164.5(1)8 8.T he Zn À Bb ond length of 2.139(2) is consistent with those observed for the group 11 analogues,a si st he B=Cl ength of 1.505 (2) .F or comparison, homoleptic two-coordinate zinc boryl complexes reported by Yamashita and Nozaki contain Zn À Bbond lengths ranging from 2.052(3) to 2.087(3) . [69] In contrast to the Cu analogue 3a, 4 contains al ong Zn-C distance (> 2.8 ) which is well beyond the sum of the covalent radii and an open ZnÀBÀCa ngle (97. 38 8). [70] Both metrics are consistent with an h 1 coordination mode in which the principal interaction between the ligand and metal is through aZ n À B bond. Forc omparison, related borataalkene complexes in which [CH 2 = B(C 6 F 5 ) 2 ]i sc oordinated to titanocene and tantocene have been shown to adopt both h 2 and h 1 coordination modes. [57] But in this case the borataalkene coordinates as the h 1 -C and not h 1 -B isomer with the MÀC interaction dominating the bonding interaction, this differ-  QTAIM analysis of 3a-c identifies bond critical points (bcp) between boron and carbon, and boron and the coinage metal, but not from the coinage metal to carbon (Table 1). Nevertheless,the side-on binding mode and anon-negligible M-C interaction is supported by the curved bond paths identified in the contour plots from QTAIM analysis (Figure 5b). Thecurvature of the bond path is most marked for 3a (Cu) and least for 3c (Au). Virtually no curvature is seen in the bond paths for 4.Inthe free [(cAAC)BH 2 ] À anion (A), the electron density (1(r)) at the bcp between Band Cisfound to be 0.20. Upon complexation with ac oinage metal, 1(r) decreases to 0.19 (Cu, Ag) and 0.18 (Au) implying areduction in B = Cbond order.F or 3a-c,asmall 1(r) at the bcp between the Ma nd B( 0.06-0.08) is found, consistent with ac losed shell ionic interaction. Others have shown that ellipticity is not an accurate metric for evaluating the p-character of the B=Cbond. [71] Ac omplementary picture emerges from NBO calculations.T he WBIs are consistent with B = Cm ultiple bonding character throughout the entire series of compounds,w ith coordination to the metals reducing the B = CWBI (Table 2). Further,comparison of MÀBand MÀCWBIs reveals that the covalent component of the metal ligand bonding is dictated by the MÀBi nteraction as is expected for as ide-on but slipped bonding interaction. This M À Bvalue is largest for Au (compared with Ag and Cu) which could be interpreted as an indicator of astronger binding interaction and aconsequence of the greater radial extension of the AOso fA ur elative to its lighter congeners. [72,73] TheM -C interaction in the Zn analogue is the weakest of the series and consistent with very little interaction between the cAACc arbon atom and the metal. Based on the NPAcharges,the polarisation of the B = C

Angewandte Chemie
Forschungsartikel bond reverses on coordination to the metal, with charge localising on the boron atom and being depleted from the carbon atom in all cases (Table 2). Deviation toward h 1 coordination (Cu ! Ag ! Au)i sa lso accompanied by an increased polarisation of the B=Cb ond toward an ionic B À ÀC + structure as would be expected as the MÀBinteraction begins to become more important and the MÀCi nteraction (and electron transfer from MtoC)isdisrupted. At the same time the C À Nb ond of the cAACl igand shortens to compensate for the electron deficiencya tc arbon. TheC À N bond length of the cAACl igand takes values of 1.406(6), 1.413(5), 1.39(1), and 1.350(2) for 3a, 3b, 3c and 4 respectively. Inspection of the key orbitals involved in bonding in 3a-c reveals this interaction conforms to the Dewar-Chatt-Duncanson model (Figure 5c). Qualitatively the s-donation and p-backdonation components bonding can be inspected by second-order perturbation analysis.T his analysis shows that donation from the p(B=C) bonding orbital to the vacant s(M) metal acceptor orbital is far larger than back-donation from filled metal d-orbitals to the p*(B=C). Consistent with this finding,the calculated occupancy of the B = C p-orbital for 3a-3c is lower than A due to electron transfer occurring from the borataalkene to the metal (supplementary information, Table S3). Although the data are coherent across two different versions of NBO (v 6.0 and 3.1), the quantitative analysis of the donor-acceptor interactions by second order perturbation analysis is complicated by the very large and unrealistic energies (100-400 kcal mol À1 ).
We turned to ETS-NOCV calculations to further support the bonding model and quantify the bonding interactions. Inspection of the DE orb energies reveals that the orbital (covalent) interaction increases in magnitude across the series 3c (118.9 kcal mol À1 ) > 3b (71.1 kcal mol À1 ) > 3a (58.9 kcal mol À1 )suggestive of astronger binding of the [B = C] À moiety to Au over Ag and Cu. Thekey contributors to DE orb , D1 1 and D1 2 involve s-donation from the (p)B =Corbital to the metal (n)s orbital (Cu, 4s;Ag, 5s;A u, 6s) and p-backdonation from the metal (nÀ1)d orbital to the (p*) B=Co rbital. For example,f or 3a,t hese s-donation and p-back-donation components are quantified as contributing 28.4 and 5.3 kcal mol À1 to the total DE orb interaction (Figure 5d).
To gain ad eeper understanding of the potential energy surface that connects h 2 and h 1 coordination in 3a-c,scans of the MÀCbond between 1.5-3.4 were undertaken with DFT methods.T hese calculations revealed af lat potential energy surface about the equilibrium bond length. Only one energy minimum was located for each structure.C omparison of the series revealed that this minimum was displaced to longer M À Cbond lengths for Cu (2.44 ), Ag (2.62 )and Au (2.76 ) respectively.T he extent of displacement is beyond that expected for solely the difference in covalent radii of the group 11 metals.The calculations support the solid-state data and suggest that while deformation toward the h 1 coordination mode of the ligand is energetically accessible for the whole series,itoccurs more readily for the heavier analogues.
To assess the strength of binding of A to the coinage metal fragments and rationalize the ease of formation of 3a-c and 4 relative to their alkene analogues,a ni sodesmic equilibrium was considered (Scheme 2). There is aclear energetic driving force for the formation of the borataalkene complex over the alkene complex (DG8 8 rxn = À42-À52 kcal mol À1 ).

Conclusion
In summary,w er eport the preparation and analysis of ac omplete series borataalkene complexes (3a-c)o ft he coinage metals involving side-on coordination of a[ B = C] À ligand along with an analogous Zn complex (4). Calculations confirm that the bonding in these compounds is best described by the Dewar-Chatt-Duncanson model. The structural data and calculations also show the propensity of the ligand to adopt av ariety of coordination modes defined by as pectrum in between h 2 and h 1 coordination. Deformation toward h 1 coordination is aphenomenon that is expected to be accompanied by the polarisation of the p-system of the coordinated ligand and is greatest for Au > Ag > Cu. Due to the isoelectronic relation between [B=C] À and [C=C] moieties, 3a-c serve as isoelectronic models for catalytically relevant alkene complexes of group 11. Hence,o ur findings not only demonstrate new coordination chemistry of [B = C] À units they provide unique insight into catalytically relevant isoelectronic alkene analogues.