Short Intramolecular N–hc(carbonyl) Interactions in Mixed-ligand Molybdenum Hexacarbonyl Derivatives

The syntheses and single-crystal structures of Mo(CO) 3 (phen)(dipy) (1), Mo(CO) 3 (biquin)(dipy) (2) and Mo(CO) 3 (dpme)(dipy) (3), (phen = 1,10-phenanthroline,) are described. In each case, distorted fac-MoC 3 N 3 octahedral coordination geometries arise for the metal atoms. Short intramolecular N–HC interactions from the dipy N–H group to a carbonyl carbon atom occur in each structure.


Introduction
Transition metal carbonyls are one of the most familiar families of organometallic compounds, and inorganic chemists have studied them intensively for some 120 years, since the serendipitous discovery OPEN ACCESS of nickel carbonyl, Ni(CO) 4 , in 1890 [1].Substituted carbonyls, in which one or more of the CO molecules are replaced by incoming ligand(s), represent an even larger family.Crystallographic studies have played a vital role in characterizing these compounds.The main focus has understandably been on determining the stereochemistry of the product (for example, whether a bi-substituted octahedral carbonyl adopts a cis or trans M(CO) 4 L 2 geometry [2] or whether a tri-substituted M(CO) 3 L 3 species forms the mer or the fac isomer [3]), and in detailed consideration of the metal-ligand, metal-carbon and C-O bond lengths in terms of electronic-structure models of back bonding [4].Despite this intensive study over many decades, it is notable that the crystal structures of some very simple substituted carbonyls, such as Mo(CO) 4 (bipy) (bipy = 2,2-bipyridine, C 10 H 8 N 2 ; see Figure 1 below) [5], and Mo(CO) 4 (biquin) (biquin = 2,2-biquinoline, C 18 H 12 N 2 ) [6] have been determined only very recently.These bidentate ligands naturally can only lead to cis isomers.
Further substitution of the carbonyl ligands is possible and Mo(CO) 2 (bipy) 2 [4] and Mo(CO) 2 (phen) 2 (phen = 1,10-phenanthroline, C 12 H 8 N 2 ) [7] both show a cis disposition of the two remaining carbonyl ligands.An interesting ligand in this area of chemistry is dipy (2,2-dipyridylamine, C 10 H 9 N 3 ): due to its flexible -NH-linking group between the pyridyl rings, it can bond as a bidentate ligand in a similar fashion to bipy as seen in Mo(CO) 4 (dipy) [8] or possibly facilitate the formation of polymeric species in which each of the aromatic N atoms bonds to a different metal atom [9].

Results and Discussion
The data collections, structure solutions and refinements were straightforward (see experimental section): all three compounds crystallize in the monoclinic space group P2 1 /n (No. 14) with Z = 4. Their crystal structures indicate that three CO molecules in the Mo(CO) 6 starting material have been replaced in each case.
The mean Mo-C bond distance of 1.932 (3) Å in 1 is significantly less that the corresponding value of 2.057 (2) Å for Mo(CO) 6 [12]: according to the classical back-bonding model for metal carbonyls, this can be rationalized in terms of a substantial transfer of electrons from the Mo 4d orbitals to the CO antibonding * orbitals [13], which enhances the double-bond character of the Mo-C bond and therefore shortens it.But because electrons are being "fed" into the CO antibonding orbitals, this effect should lengthen the C-O bond at the same time; the mean value of 1.165 (3) Å in 1 compares to 1.128 (4) Å in Mo(CO) 6 [12], i.e. the metal electrons are being "shared out" amongst fewer C-O antibonding orbitals in 1 than in Mo(CO) 6 , therefore they are lengthened to a greater extent.The fact that Mo1-C1 and Mo1-C2 (both trans to phen-N atoms) are slightly longer than Mo1-C3 (trans to the bonded dipy N atom) might correlate with the different (weak) -acceptor characters of the pyridine rings trans to the C atoms in question [4], although packing effects cannot be ruled out for the small differences seen here.
The phen ligand in 1 is close to planar (r.m.s.deviation = 0.022 Å for the 14 non-hydrogen atoms) and the metal atom deviates by -0.1905 (18) Å from its mean plane.The N3/C16-C20 ring of the dipy ligand makes a dihedral angle of 79.56 (9) with the phen ring and the Mo atom is displaced by −0.280 (3) Å from the N3/C16-C20 mean plane.The dihedral angle between the two rings of the dipy ligand is 31.79(10).The intramolecular C-HN interactions in 1 are discussed below.
In the crystal of 1, three weak C-HO interactions (Table 1) link the molecules (in which O3 accepts all three bonds) and an aromatic - stacking interaction occurs between inversion-symmetry related C7-C10/C15/C14 rings with a centroid-centroid separation of 3.6509 (15) Å and a slippage [11] of 0.358 Å.The next-shortest - centroid separation is greater than 4.0 Å.

Structure of Mo(CO) 3 (biquin)(dipy)
The structure of 2 has many similarities to that of 1 but also some important differences: again, a distorted fac-MoC 3 N 3 octahedral coordination geometry arises for the metal atom (Figure 3, Table 2).The bite angle for the N atoms of the biquin ligand in 2 is 71.80 (8) compared to 72.31 (6)° for the equivalent atoms in Mo(CO) 4 (biquin) [6].
The bond lengths about the Mo atom in 2 show similar trends to the equivalent data for 1: the mean Mo-C distance of 1.942 (3) Å is slightly longer than that in 1, whereas the mean C-O separation of 1.169 (3) Å in 2 is statistically indistinguishable from the equivalent value for 1.There is no obvious correlation between the Mo-C bond lengths in 2 and the identity of the ligand lying trans to the bond in question.
The dihedral angle between the N1-and N3-rings of the dipy ligand in 2 is 2.38 (10), indicating their near co-planarity, which is quite different to the situation in 1, where the same ligand is substantially twisted.Mo1 deviates from the N1-ring by 0.168 (4) Å.These large metal-atom-ring deviations in 2 are perhaps a reflection of significant steric crowding of the ligands.The intramolecular C-HN interactions in 1 are discussed below.
In the crystal of 2, two C-HO interactions to the same acceptor O atom occur (Table 2) and a weak aromatic - stacking interaction between the N4/C14/C19-C22 and the C14-C19 rings occurs [centroid-centroid separation = 3.7570 (17) Å] is seen.
The bond lengths about the Mo atom in 3 show similar trends to the equivalent data for 1 and 2: the mean Mo-C distance of 1.936 (3) Å in 3 is indistinguishable from that in 1, and very slightly shorter than that in 2. The mean C-O separation of 1.171 (3) Å in 3 is statistically indistinguishable from the equivalent values for 1 and 2. However, it is perhaps notable that all three C-O bond lengths in 3 are essentially identically, which does seem to correlate with the fact that the three trans ligands are all (electronically) isolated pyridine rings.The dihedral angle between the rings of the dipy ligand is 13.69 (13)°, indicating a conformation intermediate between those in 1 and 2. The molybdenum atom deviates from the N3-ring mean plane by 0.127 (4) Å.These data suggest that there is significantly less steric hindrance in 3 than in 2, which is hardly unexpected.The intramolecular interactions in 3 are discussed below.

Intramolecular interactions
The intramolecular interactions in 1, 2 and 3 listed in the Tables above are worthy of special attention.Each structure features a C-HN bond within the dipy ligand.This type of "non-classical" hydrogen bond has been accepted as a "real" interaction for 30 years [15] and a similar example within the dipy ligand also occurs in Mo(CO) 3 (dipy)(bipy) [16], which correlates with the approximate anti orientation of the N atoms of the aromatic rings in that compound, as also seen for 1, 2 and 3. Structure 1 also features a similar interaction from a dipy C-H donor to a phen N-atom acceptor.
Each structure also shows a far less expected interaction: a remarkably short N-HC contact, where the C atom is part of one of the carbonyl groups attached to the metal atom.In each case the position of the H atom was freely refined to achieve the best possible estimate for its geometry.The Bondi [17] contact distance for C and H is about 2.90 Å, thus the shortest such contact (in 1) of 2.36 (2) Å is some 0.54 Å shorter than the van der Waals' separation.The situation in 3 could be regarded as a bifurcated N-H(C,C) interaction.
A survey of the Cambridge Structural Database [18] (version 5.32 of November 2010 with two updates) was made to determine if other organometallic structures showed similar intramolecular N-HC(carbonyl) contacts (Table 4).The search cut-offs were an HC separation of less than 2.50 Å and an N-HC angle of greater than 120.These data indicate that short intramolecular N-HC(carbonyl) contacts occur in a wide range of structures, with many different ligands (containing the N-H grouping) and metals involved.This interaction, which appears to be more than a steric contact, is puzzling and theoretical analysis of the bonding might give further insights.

Experimental Section
Compounds 1, 2 and 3 were synthesized by standard techniques in organometallic chemistry [19].Stoichiometric quantities of Mo(CO) 6 , dipy and the appropriate second ligand were refluxed in toluene under a nitrogen atmosphere for six to seven hours and cooled to room temperature.Product recovery by vacuum filtration and rinsing with light petroleum ether gave single-crystal products in each case.
The single-crystal data for 1, 2 and 3 were collected on a Bruker SMART CCD diffractometer (graphite monochromated Mo K radiation,  = 0.71073 Å) at room temperature.Suitable crystals were selected and mounted on thin glass fibers with cyanoacrylate adhesive.After preliminary scans indicated good crystal quality and established initial unit cells, intensity data were collected with the aid of the SMART program.Data reduction with SAINT then proceeded in each case and empirical (multi-scan) absorption corrections were applied with SADABS, as summarized in the paragraphs below.The systematic absences uniquely identified the space group as P2 1 /n (No. 14) in each case and the structures were solved by direct methods with SHELXS and the atomic models refined against F 2 with SHELXL [20].The "observed data" threshold for calculating the R(F) residuals was set as I > 2(I).All the non-hydrogen atoms were routinely located during the refinements and modeled with anisotropic displacement factors and in final difference maps most of the H atoms were visible.The carbon-bound H atoms were relocated to idealized locations (C-H = 0.93-0.97Å) and further refined as riding atoms with the constraint U iso (H) = 1.2U eq (C) applied.The positions of the N-bound H atoms of the dipy ligands were freely refined from their difference-map locations and the constraint U iso (H) = 1.2U eq (N) was applied in each case.Full details are given in the deposited cifs.
Crystal data for 1: very dark red chunk, 0.

Conclusions
We have prepared and structurally characterized Mo(CO) 3 (phen)(dipy), Mo(CO) 3 (biquin)(dipy) and Mo(CO) 3 (dpme)(dipy).In each case, distorted fac-MoC 3 N 3 octahedral coordination geometries arise for the molybdenum atoms.The Mo-C and CO distances are broadly understandable in terms of electronic models of back bonding.As might be expected on steric grounds, Mo(CO) 3 (biquin)(dipy) shows the largest structural distortions.The crystal structures feature intermolecular C-HO bonds and aromatic - stacking interactions.Short intramolecular N-HC interactions from the dipy N-H group to a carbonyl carbon atom occur in each structure.A survey of the Cambridge Database indicated that these unusual interactions occur in a wide variety of organometallic compounds and they may be worthy of further theoretical analysis.

Figure 1 .
Figure 1.N-heterocyclic ligands used in this study.

Figure 2 .
Figure 2. The molecular structure of 1 showing 50% displacement ellipsoids for the nonhydrogen atoms.The short intramolecular C-HN and N-HC interactions are shown as double-dashed lines.

Figure 3 .
Figure 3.The molecular structure of 2 showing 50% displacement ellipsoids.The short intramolecular C-HN interactions and N-HC are shown as double-dashed lines.For clarity, H atoms not involved in these interactions are omitted.

Figure 4 .
Figure 4.The molecular structure of 3 showing 50% displacement ellipsoids.The short intramolecular C-HN and N-HC interactions are shown as double-dashed lines.For clarity, H atoms not involved in these interactions are omitted.