On structure and stability of pyrimidine ylidenes and their homologues

Accepted Manuscript On Structure and Stability of Pyrimidine Ylidenes and Their Homologues Andrey A. Kirilchuk, Alexander B. Rozhenko, Jerzy Leszczynski PII: DOI: Reference:

S2210-271X(17)30032-4 http://dx.doi.org/10.1016/j.comptc.2017.01.024 COMPTC 2384

To appear in:

Computational & Theoretical Chemistry

Received Date: Revised Date: Accepted Date:

20 December 2016 23 January 2017 24 January 2017

Please cite this article as: A.A. Kirilchuk, A.B. Rozhenko, J. Leszczynski, On Structure and Stability of Pyrimidine Ylidenes and Their Homologues, Computational & Theoretical Chemistry (2017), doi: http://dx.doi.org/10.1016/ j.comptc.2017.01.024

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1

On Structure and Stability of Pyrimidine Ylidenes and Their Homologues Andrey A. Kirilchuka, Alexander B. Rozhenko*,a,b,c, Jerzy Leszczynskic a

Institute of Organic Chemistry, National Academy of Sciences of Ukraine, Murmans’ka Str. 5, 02660 Kyiv, Ukraine. b

Fakultät für Chemie der Universität Bielefeld, Universitätstr. 25, 33615 Bielefeld, Germany. c

Department of Chemistry and Biochemistry, Jackson State University, Jackson, MS 39217 Corresponding author: [email protected]

ABSTRACT: Structure of six-membered cyclic aminoiminocarbenes (pyrimidine ylidenes) and their group 13-15 homologues was studied by quantum chemical calculations. Isodesmic and dimerization reaction energies, NICS values, frontier molecular orbitals, as well as NBO, Bader’s “atoms in molecules” and Laplacian bond order (LBO) analyses were employed for estimating relative stability of the studied carbenes (carbene homologues). They show a low degree of aromatic delocalization and considerable variation of electronic structure depending on the nature of divalent element. The predicted stability for the series of interest is lower than that for the corresponding

five-membered

Arduengo

carbenic

structures,

but

comparable

bis(dimethylamino)carbene.

KEYWORDS: DFT, N-heterocyclic carbene, pyrimidine ylidene, isodesmic reaction, stability.

with

Alder’s

2

1. Introduction.

Since the discovery of stable carbenes by Bertrand [1] and Arduengo [2] these inquisitive species have gained a wide scope of applications as organocatalysts [3,4], ligands complexing with transition metals [3,5], and molecules stabilizing low-valent main-group elements [6,7]. A close examination of the field of N-heterocyclic carbenes allows discovering that main efforts of researchers have been focused on the five-membered cyclic carbenes. Although diverse “unconventional” types of N-heterocyclic carbenes are investigated [8], only few species with other cycle sizes have been studied [9,10]. In particular, few six-membered carbenes and their homologues synthesized to date are either saturated species [11,12] or unsaturated ones, namely pyridylidenes [8,13–15] and those, where the carbene ring is annealed with benzene moiety [16]. As the rule, six-membered cyclic carbenes (SMCC) are less stable than the five-membered congeners, but a variety of transition metal complexes based on such carbenes are known [17–19]. A number of publications have been devoted to theoretical investigations of SMCC and their homologues. A systematic study of SMCC stabilized by phosphorus has been carried out by Nyulászi et al [20]. One can also find several computational studies of SMCC in comparison with the group 13 and 14 homologues [21–25] and substituted (annealed) carbenes of this type [26–29]. However, to the best of our knowledge, almost all published works concerning nitrogen-containing carbenes and their homologues deal with diaminocarbenes of type I, whereas other interesting species, amino-imino carbenes II have somehow been overlooked by researchers. We have found only two examples of imino-substituted carbenes: for 1,3-azaphosphinin-2-ylidene (III) the ground state structures were determined by theoretical investigations [20]. While a number of diverse N-heterocyclic carbenes with reduced heteroatom stabilization, i.e. abnormal and remote carbenes, were synthesized to date [8,9], the amino-imino carbenes of type II are still absent in the large carbene family. It prompted us to fill this gap studying a new class of carbenes IV by means of quantum chemical calculations.

3

The high stability of heterocyclic carbenes in the singlet ground state is traditionally referred to three main factors: a) a presence of π-donor and σ-acceptor atoms directly attached to the carbene carbon atom (Ccarb); b) an aromaticity inherent to the carbene heterocycle and c) a steric protection of the methylidene center by bulky groups [30,31]. Structures IV generally meet criteria a and b. In particular, it has previously been shown [32,33] that in some cases the X=N– imino-function (X= C, P(V)) is even more efficient -donor than the amino-group. The C=N double bond in IV should be strongly polarized towards nitrogen and the corresponding electron pair possesses a pure p-character. Additionally, a) the amino group in IV should additionally reinforce -electron density on Ccarb by the push-pull mechanism donating into the formally empty p-orbital on the divalent X atom; b) the resulting -system, similarly to the five-membered Arduengo congeners, still meets Hückel’s rule (6 -electron system). However, II and IV are slightly at a disadvantage from the standpoint of steric shielding, which should be ensured here by only one substituent R. It has been noted previously that by geometry optimization of structures of the parent carbenes, N-H proton often migrates to the carbene center [34]. Therefore, for the current study methyl-substituted species were chosen as model compounds. Herein, carbene 1c and its homologues 1a,b,d-f, as well as 4-amino-substituted model species 2a-f are studied computationally (DFT, MP2 and CCSD) with the aim to predict their thermodynamic stability within the series of the existing carbenes and outlooks of their possible synthetic preparation.

4

Me N B N

6

5

Me 1 N 2 C: N3

Me N + N N

4

R

R

R

1a, R=H 2a, R=NH2

1c , R=H 2c , R=NH 2

1e, R=H 2e, R=NH2

Me N

AlN

R 1b, R=H 2b, R=NH 2

Me N Si: N

Me N + P N

R

R

1d, R=H 2d, R=NH 2

1f, R=H 2f , R=NH2

2. Details of calculations.

All the structures were fully optimized without symmetry constraints (unless otherwise stated) using the TURBOMOLE program package [35,36] (version 6.4). The RI-SCS-MP2/cc-pVTZ approximation level [37,38] was used for geometry optimization of singlet state structures of compounds 1a-f, 2a-f. Vibration frequencies calculated numerically or analytically confirmed that the optimized structures were true minima on the potential energy surfaces. Corresponding energy values were used for calculations of the Gibbs energy magnitudes for isodesmic and dimerization reactions. To make sure that the selected approximation level was the appropriate choice for the systems of interest, the single point energy for isodesmic reactions for carbene 2c (Schemes 2, 3) was calculated at the coupled-cluster RI-CCSD and RI-CCSD(T) [39] approximation levels employing cc-pVDZ and cc-pVTZ basis sets [38] and using the structures optimized at the RI-SCS-MP2/cc-pVTZ level of theory (see the Supplementary material (SM), Table S3 for more detail). For determination of singlet-triplet energy gaps the

5

structures of singlet and triplet ground states of 1a-f, 2a-f were optimized at the RI-BP86/TZVP approximation level [40,41]. Cartesian coordinates for all optimized structures are collected in the SM. Charge density matrices based on single-point energy calculations, NICS(0) and NICS(1) values [42] were derived using the GAUSSIAN-03 program set [43] within the DFT approximation. The BP86 functional [35,36] was employed in combination with the TZVP basis sets [44]. The NBO charges [45–47] were determined for the structures optimized as local minima in energy as well as for those restricted within Cs symmetry using the Gaussian NBO module (ver. 3.1) implemented into the GAUSSIAN-03 [48]. Bader’s “atoms in molecules” [49] and Laplacian bond order (LBO) analyses were performed by means of the Multiwfn program [50]. Molecular orbitals were derived using the gOpenMol program [51]. Gabedit program [52] was used for structure rendering.

3. Results and discussion.

3.1. Structure. Similarly to Arduengo-type carbenes, the singlet ground state seems to be preferable for 1a-f and 2a-f as compared to the triplet state; the corresponding structures involve planar cyclic cores. Valence angles at X (where X represents a divalent atom) vary in a range 94 to 120°. N-X bond lengths depend on the X nature and vary from 1.29 to 1.97 Å. The NXN' bond angle decreases and N-X bond distance elongates while going from right to left and from up to down in the periodic table. Noteworthy, the structural parameters of unsubstituted (1) and amino-substituted species (2) differ only slightly. It is of interest to compare the structure of carbene 1c with those calculated for the known six- and fivemembered diaminocarbenes. The calculated N-C:-N bond angle in 1c (113.6°) is wider compared to the corresponding values in five-membered carbenes (101…102°) and similar to those found for I (114…116°) [9,28,29,53,54]. The =N–Ccarb bond (1.381 Å) is shorter than the >N–Ccarb one (1.389 Å). It supports the suggestion that, similarly to the amino-nitrogens in I, the imino-function is also an effective -donor into the formally vacant p-orbital of Ccarb.

6

3.2. Singlet-triplet gaps and frontier orbitals. Energy differences between singlet and triplet ground states ( EST) to some extent correlate with stability of carbenes and their homologues [55], albeit this relation is not straightforward [56]. EST values calculated for compounds 1a-f, 2a-f at the RI-BP86/TZVP level are given in Table 1. With the only exception of 1a and 2a, the EST values are large; hence no sizeable spin contamination should be expected as a result of applying the single-determinant DFT approach to calculations of the corresponding singlet and triplet ground states. TABLE 1. Calculated (RI-BP86/TZVP) singlet-triplet energy gaps ( EST) for structures 1a-f and 2a-f. Structure

X

EST

Structure

X

EST

Structure

X+

EST

1a

B–

1.6

1c

C

20.1

1e

N+

49.1

1b

Al–

37.5

1d

Si

55.6

1f

P+

52.8

2a

B–

6.2

2c

C

32.7

2e

N+

57.5

2b

Al–

40.7

2d

Si

61.4

2f

P+

51.5

Energies of the frontier molecular orbitals (FMOs) for structures 1a-f are listed in Fig. 1 (see also Table S4 for more detail), and their shapes are shown in Fig. 2. Derivatives of group 15 carry the total positive charge and this leads to lower orbital energy values, whereas for the anionic B and Al derivatives the opposite is observed. A close examination of FMOs for the studied model structures shows that the character of the frontier orbitals differs within the series. While the highest occupied molecular orbitals (HOMO) in the series 1a-e represent X’s lone pares (LPs), for phosphenium cation 1f the HOMO is the -type MO. Further, the considered series of compounds can be classified into two groups: the carbenes with anti-bonding orbitals mainly located on the N=C moiety (1a-d) and the other ones, in which LUMO is mainly the empty p-orbital on X (1e-f). In the first group, the high-lying unoccupied p-orbital on the low-electronegative X atom contributes to the *-type LUMO+1.

7

F FIG GUR RE 1. E Enerrgiees oof frronttier molecuularr orbbitaals ffor 11a-ff. C Charracteerissticss off thee FM MO Os foor thhe aamiino--subbstittuteed speccies 2a--f (see Taable S4) arre vveryy sim milaar too thhosee off 11a-ff wiith thee FM MO ennergies som mew whaat hiigheer ffor 2a--f. T The shapees oof M MOss foor comppouundss 2aa-f (Figg. 33) aalsoo cclossely ressem mble thoose of tthe corrrespponndinng coonggeneers iin thhe sseriies 11a-ff. O Onlyy in silyylenne 11d thhe H HOM MO O innvolves tthe nitrrogeen llonee paair, thee LUM MO reppresentss Sii=N Nm moiety aandd thee L LUM MO+ +1 m maiinlyy coonsiists of an em mptyy poorbiital on Si. Thee loone pairr off thee sillicoon aatom m siggnifficaantlyy coontrribuutes to tthe HO OMO O-11 prooviddingg foor siilyleene 1d d tthe ““nittrennium m caation” bbonndinng tyype of struuctuure. Itt haas beenn shhown pprevviouusly thaat singllet-tripplet spllittinng ffor satturated sixx-membbereed ccarbbene-like struuctuures ggeneerallly iincrreasses iin ggoinng ddow wnw wardds inn thee peerioodicc tabble [22,57]. W Whille EST valluess forr coomppounnds 1aa-d aandd 22a-d d meet m t thiis trrendd, thhis is nnot the casse ffor the am minoo-suubstiituted ggroup 15 hom molloguues: thee EST vvaluues forr 1ee aand 1f are verry cclose, aand thee sinngleet-trripleet ggap for 2e (X = N+) is eevenn larrgerr thhan tthatt for 2ff (X X = P+).. Thhis ccann bbe rreferrredd too thee high-lyinng -typpe H HOMO O off 2ff invvolvvingg p-orbitals att nittroggen andd caarboon aatom ms in thhe N N-C C=C m moiiety (Fiig. 33). Thee hiigh HO OMO O eenerrgy deccreaases thee EST m maggnituude forr 2ff. Thhe llow west unooccupied m mollecuularr oorbiital (LU UM MO) of 1f iis m mainnly tthe em mptyy p-oorbiital on thee phhospphorrus atoom. Theerefforee, foor aall m moddel sstructurres, eexceept 2f, thee higgheest -typpe ooccuupieed M MO O liees ddeepper andd the trripleet sttatee ariisess froom the sinngleet sttruccturee viia a pprom mottionn of onee eleectrron from m thhe -typpe H HO OMO O too thee -ttypee LU UM MO.

8

F FIG GUR RE 2. M Mollecuular orbbitall renndeeringg foor am minno-ssubsstituutedd speciees 11a-ff.

3..3. C Chaarge diistrribu utioon. T Thee ressults off thee Laaplaaciaan bbondd orrderr (L LBO O) annalyysiss [588] pprovvideed simiilar values ffor all derrivaativees 22a-ff (ssee thee SM M ddetaails)). T The bonnd ordder varriatiion in NC CCC CN mooietyy iss appproox. 0.55, itt eeviddencces low w arrom maticcityy for thhe sseriees oof innterrest. C55-C C6 bbondd possessees a suubsttantial douublee-boond chaaraccterr ((LB BO 11.5… …1.6). Caalcuulateed bbonnd eellippticiity vvaluues corrrobboraate thiss cooncllusioon (seee thhe S SM)). A Anallysis off LB BO O sshow ws thaat thhe bbond oordeer oof thhe > >N1–X X boondd inn 2aa-f is ggeneerallly low wer thaan tthat forr thhe = =N33-X onne. F Forr eexam mplle, ffor ccarbbenne 2cc (X X=C C) thhe L LBO Om magnituudess foor thhe > >N11–C2 annd = =N33–C C2 bbondds aare 0.79 annd oof 11.044. Itt ddem monsstraates thaat im minoo-niitroogenn prrovides moore effeectiive donnatioon tto thhe ccarbbeniic aatom m thhan thee am minoo-niitroggenn aatom m. L LBO Os oof thhe H2N-C4 N 4 boonds withiin thhe sseriees 22a-ff aree low w (00.722...11.233), w whiich iindiicattes a raatherr weeakk donorr eeffeect ffrom m thhe aaminno-ggroup attaacheed tto thhe C C=C C doubble bbonnd. T Thee strructturee is bettter reppresenteed bby tthe Lew wis sstructurres A-A A´ w withh the loocallizedd dooubble bbondds, thann w with connjuggateed sppeccies B oor C C.

9

F FIG GUR RE 3. M Mollecuular orbbitall renndeeringg foor coomppouundss 2aa-f.

M Me N+ X N

Me M N X N+

Me e N X N

Me e

NH 2

N NH 2

NH H2

H2 NH

A

A A'

B

C

N

X N

This conncluusioon iis ccorrroboorated by NICS vaaluess caalcuulated for compounnds 1a--f, 2a--f (T Tabble 2). Thhey aree ssignnificcanttly less nnegaativve tthann thhe NIC CS((0) maagniituddes (B33LY YP/66-31+G G*) prreviioussly repportted forr fiivem mem mbeeredd im midaazolle yylideene annd itts ssiliccon andd geerm maniium m deerivativves (caa. -110 pppm m) [[59]], evvideenciing low werr aarom matiicityy innherrentt too thhe seriees sstuddiedd heere. Foor X attom ms frrom m thhe ssam me pperiood oof tthe perrioddic table, thee ccalcculaated NIC CS valluess aree sliighttly m morre nnegaativee foor thhe hhighher ggrouup eelem mennts.

10

TABLE 2. NICS values (RI-BP86/TZVP) for structures 1a-f and 2a-f.

Structure

X

NICS(0) NICS(1) Structure

X

NICS(0) NICS(1) Structure

X NICS(0) NICS(1)

1a

B

-0.6

-4.5

1c

C

-4.0

-8.4

1e

N

-5.2

-9.7

1b

Al

0.1

-2.7

1d

Si

-2.3

-5.6

1f

P

-4.8

-8.0

2a

B

-2.2

-4.9

2c

C

-2.7

-6.2

2e

N

-2.3

-6.0

2b

Al

-0.3

-2.1

2d

Si

-1.8

-3.9

2f

P

-2.6

-4.9

Contour maps of the Laplacian of electron density for 2a-f (Fig. 4) reveal pronounced LP charge concentration for all studied species. It is most distinct for the anionic group 13 carbene homologues and less inherent to cationic species 2e and 2f. Since the equilibrium geometries for compounds 1a-f are almost perfectly planar in the single ground state, only negligible changes are observed by going to the Cs symmetrical structures. NBO charges derived for optimized Cs symmetrical structures of 1a-f, 2a-f are suitable for separating - and -charges (see the SM for more detail). The total charge on the divalent atom increases on going from left to right within the row of the periodic table, especially for the B – N series. Among other atoms, N1 and N3 demonstrate the biggest charge alteration. Surprisingly, flattening the amino-group in compounds 2a-f does not noticeably influence the total atomic charges. The replacement of the amino-group by hydrogen has significant effect only on the adjacent C4 ring atom; charges on other atoms change only slightly. This is corroborated by the NBO analysis of -electron population (Table 3, see also SM, Section 5): by the substitution of amino-group by hydrogen, the highest occupancy changes do not exceed 0.1 e; the imino nitrogen (N3) indicates the largest alteration. The nitrogen atom of the attached amino-group (N´) loses 0.2 to 0.4 e of its -population, but it is not accompanied by a visible increase of -charge on the X atom compared to the corresponding unsubstituted congener 1. Thus, the contribution of the direct polar conjugation (structure B) in the structures 2a-f is negligible even for the positively

11

charged species 2e,f. These facts along with the data on frontier molecular orbitals clearly show that the π-systems of studied compounds are split into the weakly interacting N–X:–N and C-C-C fragments.

FIGURE 4. Contour maps of the Laplacian of the calculated (RI-BP86/TZVP) electron density for 2a-f, projected onto the plane of the six-membered ring. Positive and negative values are marked by dashed and solid lines, respectively. The bond critical points are represented as blue circles; ring critical points are marked by orange circles; the bond paths are shown by solid blue lines.

12

TABLE 3. Selected NBO -orbital occupancies for Cs symmetrical structures of 1a-f and 2a-f.

6 5

Me 1 N 2 X: 3 N 4

'

N H2 Atom

1a

1b

1c

1d

1e

1f

N1

1.514

1.556

1.451

1.541

1.354

1.465

X2

0.254

0.131

0.565

0.336

1.034

0.704

N3

1.166

1.201

1.152

1.254

1.052

1.243

2a

2b

2c

2d

2e

2f

N1

1.536

1.575

1.498

1.580

1.430

1.538

X2

0.262

0.135

0.582

0.347

1.036

0.702

N3

1.263

1.311

1.257

1.360

1.140

1.338

N´

1.831

1.830

1.725

1.740

1.614

1.619

3.4. Relative stability. The relative stability of the studied series 1a-f and 2a-f will be analyzed using isodesmic reactions (Schemes 1-3). The corresponding calculated Gibbs free energy values ( G1-3) are given in Table 4. First we compare the studied series with the corresponding five-membered Arduengo-type counterparts (Scheme 1). The carbene model structures 1a-f, 2a-f are clearly less stable than the corresponding five-membered species 6a-f (strongly negative magnitudes of G1 are predicted). The largest differences in stability are predicted for carbenes 1c, 2c and nitrenium cations 1e, 2e; the smallest ones are expected for anions 1a,b, 2a,b, which is in line with the general trend of stability of carbene homologues of group 13 and 14 elements [60]. The lesser stability of 1 and 2 can be referred to the lower degree of the aromaticity in these molecules compared to fivemembered species 6a-f. However, it does not explain negative G2 values inherent to the isodesmic reaction of 1c,d and 2c,d with six-membered saturated diamino-substituted carbenes (silylenes) 8c,d (Scheme 2), though G2 values are significantly smaller than the corresponding G1 magnitudes (Table 4). Thus, our calculations evidence

13

that the donor effect of the imino-function, even reinforced by the attached amino-group, is not as effective as that provided by cyclic amino-nitrogen. And albeit amino-substituted compounds 2a-f are more stable than the corresponding unsubstituted species (1a-f), the predicted quantitative effect is rather modest. We suggest that the favorable polarization of π-system towards the formally vacant p-orbital at the carbenic carbon atom is partially compensated by a corresponding loss of ring aromaticity. This is supported by the more negative absolute NICS values predicted for series 1a-f as compared to those found for 2a-f. Me N X: N

Me

Me N XH n N Me

+

R

N

+

R

3a-f

1a-f 2a-f

XH n N

Me N X: N Me

+ ΔG1

6a-f

4a-f 5a-f

a : X=B- , n=1 b: X=Al-, n=1 c: X=C, n=2 d: X=Si, n=2 e: X=N +, n=1 f : X=P+, n=1 1,4: R=H; 2,5: R=NH 2

SCHEME 1. Isodesmic reaction for estimating stability of 1a-f and 2a-f relative to five-membered species 6a-f. TABLE 4. Gibbs free energies ( G, kcal/mol, RI-SCS-MP2/cc-pVTZ) for isodesmic reactions (Schemes 1-3).

Item

X

G1

Item

X

G1

G2

G3

Item

X

G1

1a

B–

–5.0

1c

C

–30.8

-10.0

-1.5

1e

N+

–40.2

1b

Al–

–2.0

1d

Si

–15.0

-5.5

16.3

1f

P+

–29.1

-8.0

0.5

2a

B–

–8.1

2c

C

–28.7

2e

N+

–26.1

2f

P+

–17.7

2b a

Al–

–5.0

2d

Si

–15.7

(-6.6)a

(0.1)a

-6.2

15.6

E values derived at the CCSD(T)/cc-pVTZ//RI-SCS-MP2/cc-pVTZ level of theory.

14

Me N X: N

Me N XH 2 N Me

+

R

7c,d

1c,d 2c,d

Me N

XH2 N

+

R

Me N X N Me

+ ΔG 2

8c,d

4c,d 5c,d

1,4: R=H; 2,5: R=NH2 c: X=C; d: X=Si

SCHEME 2. Isodesmic reaction for comparing stability of 1c,d 2c,d with corresponding six-membered saturated congeners 8c,d. Me N X: N

+

R 1c,d 2c,d

Me Me N CH 2 Me N Me 9

Me N

XH2 N

R 4c,d 5c,d

+

Me Me N C Me N Me

+ ΔG 3

10

1,4: R=H; 2,5: R=NH2 c: X=C; d: X=Si

SCHEME 3. Isodesmic reaction for comparing stability of carbenes 1c, 2c and silylenes 1d, 2d with Alder’s bis(dimethylamino)carbene 10. At first sight, such a result makes structures 1 and 2 unattractive for experimentalists. However, let us to compare stability of carbenes 1c, 2c and silylenes 1d, 2d with the known acyclic Alder’s carbene 10 (Scheme 3, Table 4, G3). Accordingly to the calculation results, carbenes 1c and 2c are almost as stable as 10, whereas the G3 values calculated for silylenes 1d, 2d promise definitely higher stability for the latter compared to diaminocarbene 10. These results evidence that the studied species can generally be synthesized, or at least used as ligands in metal complexes. In addition, substitution of methyl and amino-groups in the studied carbene-like structures by bulky moieties can significantly enhance their stability (some examples of such calculated structures are presented in the SM).

15

3.5. Dimerization reaction. One of the most known aspects of chemistry of carbenes is their disposition to dimerize forming ethenes [61]. Stable dimers of saturated six-membered diaminocarbenes have been previously characterized [62,63]. We calculated free Gibbs energy values for the dimerization reaction for carbenes 1c, 2c and their homologues 1a,b,d-f, 2a,b,d-f (Scheme 4, Table 5). According to the calculations, E-dimers are more stable than their Z-stereoisomers by ca. 2 – 12 kcal/mol (with only exception of 2a, for which cis-dimer is 0.8 kcal/mol more stable). The reaction energies in Table 5 refer to trans-dimers. Values for Z-11 and Z-12 can be found in the SM, Table S5. Carbene dimers possess bent geometry; six-membered rings are turned relatively to each other (ca. 10°) because of steric repulsion between methyl groups and imino-nitrogen. On the contrary, flat structures with distortion from planarity less than 2° are inherent to charged dimers 11a,e, 12a,e. Disilenes 11d, 12d and dialanes 11b, 12b possess the typical “trans-bent” geometry [64], the cycles in disilenes are virtually parallel while in dialanes six-membered rings are turned on ca. 120° relatively to each other and their plains are inclined (Figure 5). The Si=Si bond lengths in 11d,12d are approximately 2.5 Å, the Al=Al distances in 11b,12b are ca. 2.2 Å. Authors were not able to localize the phosphorus-containing dimers 11f, 12f: they dissociated into 1f and 2f, respectively, during geometry optimization.

TABLE 5. Gibbs free energies (kcal/mol, RI-SCS-MP2/cc-pVTZ) for dimerization reactions (Scheme 4). ΔGd Structure X

ΔEd

X

ΔEd

ΔGd

-59.6 1e

N

106.1

118.0

-8.7

3.9 1f

P

-

-

C

-57.7

-42.1 2e

N

111.5

124.8

Si

0.4

14.8 2f

P

-

-

Structure

X

ΔEd

1a

B

-23.5

-12.1 1c

C

-73.2

1b

Al

44.8

52.7 1d

Si

2a

B

0.3

12.4 2c

2b

Al

50.6

60.7 2d

ΔGd Structure

16

e Me N X: N R 1 a-ff 2 2a-ff

R M Me N N X X N N Me e R

+

Gd

E-1 11a a-f E-1 12a a-f

a: X= =B-, b: X= =All-, c : X X=C C, d : X X=S Si, e: X=N X N +, f : X X=P P+ 1,11:: R= =H ; 2,,12: R R=N NH2 S SCH HEM ME E 4. Dim merrizationn reactiion of 11a-ff annd 22a-f. A As exxpeectedd, thhe ddispposiitionn too thee diimeerizaatioon raapiddly deccreaases byy moovinng ddow wnw wardds inn thhe perioodicc tabble. C Carbbennes 11c, 2c aree moostly innclinedd to dim merrize,, theeir Al-, N-, N aand P-cconttainningg hoomoologuess 1b b,e,ff, 2b b,e,,f haavee tthe leaast ddispposiitionn, w whiile silyylennes 1d,, 2d d annd aniionss 1aa, 22a pposssess innterrmediatte rreacctiviity. Thhesee faacts eempphassizee neecesssityy of bulky ssubsstituuentts too pprovvidee kinetiic sstabbilityy too sttruccturres 1c, 2c. N Neveerthhelesss, thee uunsuubsttituted com mpoounds aare signnificcanntly moore iincliinedd too dim merrizattionn than tthe amiino--subbstituteed ooness (by caa. 7 – 255 kccal/m moll ).

FIGU UR RE 55. Optim O mizzed sstruuctuures (RII-SC CS--MP P2/ccc-ppVT TZ) of ddim mers 11b b,d. 44. Conclu usioonss

17

Structures of six-membered amino-iminocarbene homologues of group 13-15 elements were analyzed using quantum chemical calculations, partition and visualization schemes in order to estimate the effect of the iminofunction and amino-group conjugated with the divalent carbenic moiety via C=N(X) double bond. The electronic structure of the studied model compounds varies noticeably depending on the nature of the divalent element. The studied species demonstrate significantly lower thermodynamic stability compared to their five-membered counterparts, strongly localized double bonds and rather low aromatic character. The stabilizing effect of the amino-group attached to the C=N(X) double bond does not exceed 2 kcal/mol. This can be due to the decreasing ring aromaticity caused by the amino group due to the additional C=N π-bond polarization. Nevertheless, the stability predicted for the studied structures is still comparable with that of the acyclic Alder's carbene. The amino-group attached to the C=N double bond lowers the disposition of the carbene (homologues) to dimerization. The introduction of bulky substituents into the discussed structures can make them isolable.

Acknowledgments A.B. Rozhenko thanks the Alexander von Humboldt Foundation (Germany) for generous donating the hardware for calculations and license to TURBOMOLE set of programs. Prof. U. Manthe and Dr. Thorsten Toensing are gratefully acknowledged for the technical support of the Gaussian-03 calculations.

Appendix A. Supplementary material Supplementary

data

associated

http://dx.doi.org/10.1016/j.comptc.

with

this

article

can

be

found

in

the

online

version

at

18

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TOC Image

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Highlights • The studied series of six-membered aminoiminocarbenes possess rather low aromaticity and strongly localized double bonds. • Stability of pyrimidine ylidenes is significantly lower than that of their five-membered congeners but is comparable with the isolable acyclic Alder’s carbenes. • The effect of an attached amino-group conjugated with the methylidene center, on stability of carbenes under consideration is negligible and does not exceed 2 kcal/mol. •

The six-membered aminoiminocarbenes substituted with bulky substituents can probably be isolated.